Low dimensional nano-materials, such as two-dimensional graphene, one dimensional carbon nanotubes, and zero dimensional C60 and carbon quantum dots, have made significant impacts in exploring novel materials properties, designing new functional materials, and enhancing materials performance. The basic atomic structural unit of graphite, graphene, CNTs, and C60 is the hexagonal carbon ring. Graphitization is defined as diffusion controlled process from disorder to order transformation in hexagonal networks. As the graphitization mechanism for three-dimensional graphite is well understood, many important fundamental issues for low-dimensional graphitization need to be investigated, such as kinetics and the atomic scale mechanism of low dimensional graphitization. It is known that oxygen and oxygen containing molecules affect growth of carbon nanostructures; low pressure oxygen and oxygen containing molecules increase the rate of graphitization of carbon, water vapor assisted process leads to a highly efficient “super-growth” of SWNTs, graphene grown in the presence of low pressure oxygen exhibits better morphology and electronic properties than in ultra-high vacuum, and CNTs grow on SiO2 nano-particles. In this talk, graphene/SiC and CNTs/SiC are used as the examples that various low dimensional carbon nanostructures could be grown on SiC by controlling growth environments. I will present that (a) graphene morphology is improved significantly when low pressure oxygen is present during graphene/SiC growth, and (b) graphene Pillared structures could be grown by controlling pressure and temperatures. Further analysis have revealed that oxygen exists in three forms: (a) oxygen in SiC lattice near the surface, (b) oxygen-carbon functional groups/compounds which decompose at low temperatures, and (c) oxygen on graphitic hexagonal networks which decompose at high temperatures, which is considered as oxygen atomic catalytic effects in low dimensional graphitization.

Transversal deformations of carbon nanotubes are those that modify their otherwise circular cross section. Such deformations can be induced to individual nanotubes by compression with atomic force microscope (AFM) tips, leading to mechanical [1] and electromechanical [2] responses. In the present work [3] we investigate, with a combination of experimental and theoretical techniques, the effect of transversal deformations on the Raman spectra of individual nanotubes. In the experiments, AFM is combined with a confocal Raman spectroscopy setup to follow, in situ, the evolution of the G-band feature of isolated single-wall carbon nanotubes (SWNTs) under transversal deformation. The SWNTs are compressed by a gold AFM tip against the substrate where they are sitting. Our theoretical simulations indicate two possible effects on Raman spectra resulting from such compression : First, the singularities of the nanotube electronic densities of states (DOS) are broadened and reduced due to the nanotube/Au interaction, leading to reduced Raman scattering intensities. Second, the vibrational spectra in the G-band region can be modified due to the nanotube symmetry breaking. The experimental results are consistent with such predictions. From eight deformed SWNTs, five exhibit an overall decrease in the Raman signal intensity, while three exhibit vibrational changes related to the circumferential symmetry breaking. Our results also reveal chirality dependent effects, which are averaged out in SWNT bundle measurements. [1] A. P. M. Barboza, H. Chacham and B.R.A. Neves, Phys. Rev. Lett. 102, 025501 (2009). [2] A. P. M. Barboza, A.P. Gomes, B.S. Archanjo, P.T. Araujo, A. Jorio, A.S. Ferlauto, M.S.C. Mazzoni, H. Chacham and B.R.A. Neves, Phys. Rev. Lett. 100, 256804 (2008). [3] P. T. Araujo, N. M. Barbosa Neto, H. Chacham, S. S. Carara, J. S. Soares, A. D. Souza, L. G. Canccedil;ado, A. B. de Oliveira, R. J. C. Batista, E. Joselevich, M. S. Dresselhaus, and A. Jorio, to be published.

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects. We aim to understand the cooperative mechanisms and kinetics of CNT self-organization and how this may limit density and alignment in vertically aligned CNT “forests”. We use synchrotron radiation in combination with a high-frame-rate (100 Hz) pixel array detector to uniquely study in situ the intrinsically rapid processes during nucleation and self-organization of CNT films made by chemical vapor deposition.[1] We calculate an order parameter from time-resolved, grazing-incidence small-angle X-ray scattering (GISAXS) images in order to quantify the evolution of CNT morphology as a function of growth time. Further, we elucidate the mechanisms of CNT self-organization by correlating the order parameter with real-time film height kinetics and qualitative CNT density measurements (from X-ray scattering intensity). We study a set of canonical CNT growth experiments (various thermal and chemical conditions) using a bilayer Fe/Al2O3 catalyst thin film on a Si wafer with C2H4/H2 precursors near atmospheric pressure. Our results show that the kinetics of CNT self-organization are proportional to the vertical growth rate of the forest. This suggests that the disorder-order transition is the result of excluded volume interactions,[2] which are in part governed by the lengthening rate of constituent CNTs, in conjunction with increasing number density of CNTs. We draw analogies with the isotropic-nematic (aligned) phase changes previously observed in CNT solutions and liquid crystals. Further, the beginning of upward forest growth marks the onset of vertical self-alignment, which occurs as the order parameter begins to saturate with time. However, both the final value of the order parameter and the time it takes to reach this value depend on processing conditions (e.g., catalyst annealing), indicating that inherent limits to the straightness (and thus defects) of the CNTs are linked to the nucleation conditions. Future work will relate the dynamics and mechanics of self-organization to the CNT diameter and attractive forces, the latter of which are implicitly essential to the morphology of the film and may influence the maximum density of CNTs. Practically, such knowledge will further engineering of CNT forest surfaces for applications where low interfacial resistance is essential, such as thermal and electrical interfaces. [1] E. R. Meshot, E. Verploegen, M. Bedewy, S. Tawfick, A. R. Woll, K. S. Green, M. Hromalik, L. J. Koerner, H. T. Philipp, M. W. Tate, S. M. Gruner, A. J. Hart, ACS Nano 2012. [2] L. Onsager, Annals of the New York Academy of Sciences 1949, 51, 627-659.

A novel ammonia synthesis strategy by using α-Fe₂O₃ nanowires-CNT supported nanocatalyst was developed in this research. When nitrogen and hydrogen gases are adsorbed on the nano catalyst, the wave function of nitrogen and hydrogen molecule and the wave function of the atoms of nanocatalyst (also nanowires) overlaps and electrons from the atoms of nanocatalyst are shifted to the antibonding states. By applying external magnetic field, energy levels of nanocatalysts are splitted according to Zeeman Effect and maximum overlapping of the energy states of adsorbate and adsorbent are obtained resulting in easiness in chemisorption process. α-Fe₂O₃ nanorods structure provides a new cutting edge in novel materials research for catalyst application of green chemical reaction. Further investigation of α-Fe₂O₃ nanorods was carried out to comprehend the underlying scientific fundamentals of this new generation catalyst for many valuable chemicals synthesis. Besides Zeeman interaction, hyperfine interaction must be considered to induce the singlet-triplet rephrasing. The preliminary works on the spontaneous growth of α-Fe₂O₃ nanorods through thermal oxidation method and the effects of substrate surface modification by the acid treatment to the catalytic activity of α-Fe₂O₃ nanorods supported by the CNTs are reported and evaluated. 2-level factorial design of experiment also had been using to study the interaction of the some chosen factors. By increasing the frequency and the quantity of Helmholtz Coils in the higher magnetic induction strength condition, significant increased of the ammonia yield were observed. The highest green ammonia yield that can be obtained in the trial-run experiment study was 219ppm. On the other hand the highest reaction rate by experiment was 253µmole/g.h by using the hybrid catalyst system of treated iron nanowires with the presence of CNT nanocatalyst. This method is the promising new route for green ammonia production at room temperature and ambient pressure.

An ability to control the basic materials structure and lattice configuration is essential for successful engineering applications of graphene and carbon nanotubes. In this talk, various fabrication techniques and nanoscale patterning processes for controlling the nanomaterials geometry such as graphene nano bridge width, pore array dimensions, and alterations of graphene lattice will be discussed. Carbon nanotube shape control such as various modified geometries of nanotube/nanocone arrays, bending of nanotubes, modification of carbon-based probe tip geometry will also be described. The implications of such geometry control on physical, electronic, mechanical, and bio-related properties will be discussed in relation to potential nanotech applications.

Carbon nanotubes offer a number of exceptionally unique properties, many of which are intimately related to their distinct one-dimensional concentrically nested structure. The ability to fabricate dense, horizontally aligned carbon nanotube arrays en masse offers a unique polarising material with properties hitherto unreported. However, as with more traditional optical polarising media, any polymeric or crystalline substrate which supports these aligned CNT, such as silicon, quartz or PET for example, introduce strong leading absorption edges thereby limiting the attainable spectral bandwidth. Here the ultra-broad band polarisability, in response to continuous wave excitation, of free-standing multi-walled carbon nanotubes is presented. Multi-walled carbon nanotubes, formed from 5±1 (±1 S.D.) walls and 25.4±13.3 (±1 S.D.) nm in diameter, have been synthesised in such a way as to permit mechanical pulling and the formation of aligned (Hermans orientation factor = -0.25) free-standing membranes. We have shown successful polarisation from the millimetre wave (lambda;=2.5 mm) to the ultra-violet near plasma frequency (6.5 eV / 190 nm). Anisotropic transport studies, DC and frequency opto-electronic response and Raman spectroscopy suggest the activation of distinct electron populations as a function of geometric alignment. Ab-initio effective medium and full numerical simulations confirm the measured continuous wave transmission. This broadband polarisation attributed to the CNTs multi-dimensional nature. CNT bundles polarise at longer wavelengths than individual CNTs, which themselves polarise at wavelengths longer than the individual graphene planes which comprise the CNT. Spatial transparency mapping reveals that membranes show varying degrees of porosity at differing energies which induce fundamental limitations to their polarisability range, though the optical transmission remains within the impressive 50-95% regime throughout the energy window reported.

The high chemical reactivity and natural abundance of atomic oxygen makes its interaction with carbon nanotubes (CNTs) of crucial importance for the understanding of their behavior in real environments. As a consequence several theoretical studies have been published on oxygen chemisorption, which are however limited in either the CNT model or the level of theory. Here we present large-scale density-functional-theory calculations of oxygen chemisorption on CNTs of different chiralities, aimed at building towards a systematic characterization of the thermodynamics and kinetics of the reactions and the clarification of the changes induced in their physico-chemical properties. In particular, our results (i) demonstrate the critical influence of the model on the relative stability of the adsorbates and (ii) provide an unprecedented understanding of the mechanisms that determine the chemisorption pattern and its dependence on concentration. Moreover the changes induced on the electronic and vibrational properties are examined using both calculations and experiments. Comparison with previous work on graphene demonstrates that the interaction with atomic oxygen is strikingly different from that of CNTs. _{This research was funded in part by Nano-Tera.ch, a program of the Swiss Confederation, evaluated by the SNSF. Part of the computational work was supported by a grant from the Swiss National Supercomputing Centre (CSCS) under project ID 245.}

Strips of graphene sheets, also known as graphene nanoribbons (GNRs), are 1 D graphene-based materials and exhibit different physicochemical properties when compared to carbon nanotubes. One way to modify the electronic and chemical properties of GNRs is by doping these structures with heteroatoms such as nitrogen. In this work we carried out the chemical oxidation of nitrogen-doped multi-walled carbon nanotubes (CNx-MWNTs) using potassium permanganate in order to obtain nitrogen-doped graphene nanoribbons. The oxidation reaction, its time, the oxidizer to nanotube mass ratio, and the temperature were parameters that were varied and carefully studied. The presence of nitrogen atoms makes CNx-MWNTs more reactive towards oxidation when compared to undoped multi-walled carbon nanotubes (MWCNTs). High-resolution transmission electron microscopy studies indicate that the oxidation of the graphitic layers within CNx-MWNTs results in the unzipping of large diameter nanotubes and the formation of a disordered coating on small diameter nanotubes. X-ray photoelectron spectroscopy show that the C 1s core-level spectra is similar between MWCNTs and CNx-MWNTs after oxidation. Nitrogen content within unzipped tubes decreased as a function of the oxidation time, temperature, and oxidizer amount. A comparative thermogravimetric analysis reveals a lower thermal stability of the (unzipped) oxidized CNx-MWNTs when compared to MWCNT samples. The oxidized graphene nanoribbons were chemically and thermally reduced and yielded to nitrogen-doped graphene nanoribbons (N-GNRs). This reported method represents a novel route to nitrogen-doped unzipped carbon nanotubes and provides a method to control the doping level in the resulting reduced GNR samples in bulk quantities.

What is the most suited oxidation state of the catalyst for the synthesis of carbon nanotubes (CNTs)? Although most agree that metallic catalyst favors optimal CNT growth, it is unclear which oxides are also suitable for CNT growth. We report a study of the impact of gas-phase pretreatment of supported iron-oxide catalyst in synthesizing tall carpets of crystalline and vertically aligned CNTs to understand the correlation between the catalyst oxidation state and the CNT forest growth. By varying the pretreatment conditions from a reducing environment (hydrogen), to a dry oxidizing environment (oxygen), and further to a wet oxidizing environment (oxygen and water vapor), notable changes were observed in both the collective CNT array growth behavior and the individual CNT characteristics. Although the greatest catalytic activity was observed following a full reduction to the zero-valent (metallic) Fe catalyst (carpet height of ~800 microns ), growth was also observed from a catalyst composed of both Fe3O4 and Fe2O3 particles (carpets height of ~500 and ~300 microns, respectively). The least effective catalyst was FeO, which led to clearly less-aligned CNT carpets only present at the edges of the sample. XPS core-level analysis, following pretreatment of the catalyst (no C2H4), emphasized the critical nature of the combined catalyst-underlayer interaction to achieve optimal catalyst activity during growth, and hence the most efficient catalyst reduction process. Additionally, CNT diameters during growth were strongly affected by the pretreatment process; 5 nm / 3 walls for the reducing anneal, 9 nm / 4 walls for the argon anneal and up to 17 nm / 10 walls for the oxidative (dry and wet) anneals. This is explained considering the impact of the surface tensions of the catalyst and catalyst-support interfaces for the different oxidation states of the iron catalyst, which evidently impacts the nucleation and growth of the CNTs. Overall, this work gives a collective picture of how the catalyst oxidation state affects the CNT growth based on the catalyst pretreatment environment and the nature of the catalyst-underlayer interactions. Such concepts are critical for the rational design of alternative catalyst-underlayer systems for efficient CNT synthetic processes.

Carbon nanotubes have been proposed as interconnects in future integrated circuits due to their extremely high current carrying capacity. However, this use requires CNTs to be grown in area densities of over ~ 3e13 cm-2. In fact, the area densities of most existing ‘high density&’ vertically aligned CNT forests grown by chemical vapor deposition (CVD) is only ~6e11 cm-2 [1,2], and they use only 5% of the cross sectional area. Density can only be increased by increasing the density of the original catalyst nanoparticles for the CVD process. We have derive three different methods to increase the catalyst and CNT density to 3e13 cm-2, and confirmed each value by a weight gain method. The weight gain method derives the area density by measuring the sample weight gain during growth, and dividing this by the weight per unit length of nanotube, as deduced from the CNT diameter and wall number as measured by TEM and the forest height as measured by SEM. The three processing methods that can be used to increase the catalyst density are (1) use a more diffusion resistant underlying support layer by plasma compaction to allow use of a thinner catalyst layer and thus the growth of smaller diameter SWNTs [3], (2) use a multi-cycle catalyst deposition process that increases the catalyst density cumulatively [4], and (3) use a short time carbon plasma to immobilise the catalyst particles before the growth step [5], as confirmed by us. The catalyst de-wetting process and the growth characterisation will be described in detail. References 1. K Hata, et al, Science 306 1362 (2004) 2. G Zhong, T Iwasaki, H Kawarada, Carbon 44 2009 (2006) 3. D N Futaba, et al, Nature Mat. 5, 987 (2006) 4. S Esconjauregui, et al, ACS Nano 4 7431 (2010) 5. G Zhong, et al ACS Nano 6 2893 (2012) 6. Y Yamazaki, et al, App. Phys. Exp 3 55002 (2010); C Zhang, unpublished 7. J Robertson, et al, JJAP 51 01AH01 (2012)

Recently, we and others have shown that oxide nanoparticles such as zirconia and silica can catalyze CVD growth of CNTs. Using in situ time-resolved XPS during CVD growth of CNTs employing zirconia nanoparticles, we have previously observed that at no point before, during, or after the CVD growth process does zirconia reduce to a metal or carbide. Point-localized EDS of nanoparticles attached to CNTs extracted from this process observed using TEM further verified the composition of these catalyst nanoparticles to be ~ZrO₂. Despite these observations, attempts at distilling CNT growth employing zirconia and other oxides to practice have been met with great difficulty. For example, areal yields of CNTs from processes employing silica- and silicon nitride-supported zirconia, silica, and titania are routinely orders of magnitude lower than comparable processes employing metallic catalysts and vary greatly each deposition. Despite a broad parametric study of numerous parameters (including, for example, particle size, particle synthesis method, concentration, CVD feedstock type, CVD pressure, CVD temperature), little progress was made at improving CNT growth yields using oxide nanoparticle catalysts on silicon wafer-based substrates. We report that high-surface-area carbon substrates activate zirconia and other solid-acid oxide nanoparticles for CVD growth of CNTs. Carbon-supported zirconia and titania nanoparticles have been shown to result in CNT yields within an order of magnitude of what is routinely obtained with iron and other metallic catalysts and with repeatable positive differentials against oxide-free control supports. The surface area, pore structure, and crystallinity of the carbon support is found to greatly influence the activity of the oxide nanoparticles. The role of the carbon support is studied through systematic variation of these parameters using specially-prepared carbon aerogels, xerogels, cryogels, solution-deposited thin films, and sputtered thin films. Using HR-TEM we have observed that upon heating of carbon-supported zirconia and other oxide nanoparticles, a fullerenic cage structure is formed around the nanoparticles via a solid-state graphitization process. It is hypothesized that this fullerenic cage serves as a protostructure for CNT formation and thus is a critical step in initializing oxide nanoparticles for CNT growth activity. Detailed observations of this mechanism are discussed along with implications for practical uses of oxide-based catalysts including growth of CNTs on challenging substrates and controlling CNT chirality and diameter.

The lack of reasonably homogeneous SWCNT materials is a major obstacle hindering their broad application. The main challenge on the way of overcoming this barrier remains the knowledge of carbon nanotube formation mechanism, which despite two decades of extensive efforts is not yet fully understood, and is still a hotly debated topic. One of the key groups of control parameters is the nature of the catalyst particle. Revealing and exploiting the proper features of the catalyst for controlling the growth of SWCNTs is extremely complex due to the wide variety of catalyst compositions, crystallographic and electronic structures, size, and adhesion to substrates [1]. Among these properties, the catalyst structure may be one of the more important, when we take into account some theoretical and experimental results that suggest the correlation between catalyst structure and grown tube chirality [2,3]. In this regard, the cap lift-off process becomes a crucial event in nanotube growth and requires deeper understanding, since it is in the very early stage of nucleation when the chirality is set. Here we report in situ ESTEM observation of the carbon nanotube nucleation process during CVD growth. We find that growth is accompanied by dynamic reconstruction of the catalyst particle morphology [4]. We observe a clear relationship between the carbon nanotube lift-off and reconstruction of corresponding constrained facets of the catalyst. An applied continuum model and detailed atomistic calculations identify chemisorbed carbon induced surface energy variation and the emergence of a pristine, high energy facet under the sp2-carbon dome as the critical factors in the lift-off process. It seems that CNT growth can be considered as another example of an oscillating chemical system. 1. A.R. Harutyunyan J. Nanosci. Nanotechnol. 9, 2480 (2009). 2. S. Reich, et al. Chem. Phys. Lett. 421, 469 (2006). 3. A.R. Harutyunyan et al. Science 326, 116 (2009). 4. E.M. Pigos et al., ACS Nano, 5, 10096, (2011).

Chiral-selective growth of single-walled carbon nanotubes (SWNTs) remains a great challenge that hinders their use in applications such as electronics and medicine. Recent experimental and theoretical reports have begun to address this problem by suggesting that selectivity may be achieved during nucleation by changing the catalyst composition or structure. Nevertheless, to establish a rational basis for chiral-selective synthesis, the underlying mechanisms governing nucleation, growth, and termination of SWNTs must be better understood. To this end, we report the first measurements of growth rates of individual SWNTs through in situ Raman spectroscopy and correlate them with their chiral angles. Our results show that the growth rates are directly proportional to the chiral angles, in agreement with recent theoretical predictions. Importantly, the evidence singles out the growth stage as responsible for the chiral distribution— distinct from nucleation and termination which might also affect the final product distribution. Our results suggest a route to chiral-selective synthesis of SWNTs through rational synthetic design strategies based on kinetic control.

The talk will focus on engineering of atomic layers of carbon and structurally related 2D layered materials. Graphene has become interesting in many ways but there are many ways to tailor these basic building blocks to create functional structures. Structures like quantum dots, ribbons etc. are some examples. In addition there are other materials that can treated similar to graphene structurally, hexagonal boron nitride and dichalcogenides being examples. Once these basic atomic layers can be extracted these can also be manipulated and engineered into interesting composite layered structures. Both stacked as well as in-plane engineered structures can be created allowing approaches to create new materials and structures. I will discuss some of these approaches and explore some of the properties of these hybrid structures. In addition, there is also a possibility of engineering 2D and 1D materials into new hybrids, for example those involving graphene and carbon nanotubes. The talk will discuss these new possibilities in materials science and engineering using unique nanoscale building blocks.

Our lab at MIT has been interested in how the 1D and 2D electronic structure of carbon nanotubes and graphene respectively influences their chemistry. We demonstrate a solution-phase technique for the production of large-area, bilayer or trilayer graphene from graphite, with controlled stacking and characterize transistors generated from such materials. We also show a stark difference in the rate of electron transfer chemistry with aryl diazonium salts on monolayer graphene supported on a broad range of substrates. Reactions proceed rapidly when graphene is on SiO2 and sapphire, but negligibly on alkyl-terminated and hexagonal boron nitride surfaces. Reactivity imprint lithography (RIL) is demonstrated as a technique for spatially patterning chemical groups on graphene by controlling the underlying substrate. We will also introduce several new concepts in sensor mechanisms utilizing carbon materials for the label free detection of protein-protein interactions, H2O2, NO and glucose, as well interfacing sensor arrays to live cells. Ion channels fabricated from the interior of a carbon nanotube have also been pioneered in our laboratory, as we have shown that single molecule, stochastic detection of individual ions is possible.

Current single-walled carbon nanotube (SWNTs) syntheses produce a heterogeneous mixture of nanotubes of varying lengths, diameters and helicities. Although SWNTs have outstanding (opto)electronic properties, the presence of a heterogeneous mixture of species with varying electronic character, lengths, diameters and helicities in as-grown materials has hindered their widespread use in practical applications. Post-synthetic sorting is essential in unlocking these remarkable properties, although most separation techniques rely on high-quality dispersions of individual SWNTs to function. The use of aggressive ultrasonication (often destructive) and repetitive ultracentrifugation (time-consuming) are required to make these dispersions - often suited only to small-scale studies. By exploiting processes that intrinsically aid the dispersion of SWNTs, we have developed a simple, scalable electrochemical technique that dissolves SWNTs in dry, air-free, non-aqueous electrolytes, without the need for ultrasonication, centrifugation, covalent modification, or strongly-bound wrapping agents. At significant reducing potentials, we observe the significant swelling of SWNT cathodes due to intercalation of solvent between nanotube bundles causing the exfoliation and debundling of the SWNT networks, generating nanotubide (SWNTn-) dispersions. Nanotubes appear to be dispersed individually from AFM and TEM analysis. In terms of the dissolved species, amorphous carbon and defective SWNTs are charged and dissolved preferentially, but prolonged reduction leads to the dissolution of purer material. The reductive dissolution process has the ability to generate concentrated SWNTn- solutions (> 2 mg/mL), suitable either for electrodeposition or surface modification, as required; both options are demonstrated.

Single-walled carbon nanotubes are a leading candidate to replace silicon as the active channel in future (sub-10nm technology node) logic devices. As the sub-10nm technology node will contain billions of transistors, sorting carbon nanotubes to several 9&’s purity is critical for the integration of the material. Column chromatography is employed to separate semiconducting from metallic carbon nanotubes with a purity of 99.5%. The process proceeds using a single surfactant; where both the metallic and semiconducting bands resolve and elute using the same surfactant system. This is a key advantage of this method as it allows for iterative processing and high levels of purity. The effect of the pore size of the column media was evaluated and found to have a large effect on the quality of the separation. Additionally, varying the pore-size demonstrates that both size-exclusion and selective adsorption play a role in the columns ability to resolve the carbon nanotubes by electronic type. Absorption spectroscopy was used to optically characterize the carbon nanotube solutions and evaluate the level of purity. After two iterations of chromatography, the metallic peaks (M11 transition) in the absorption spectra were completely attenuated making it difficult to quantify the level of purity for solutions where multiple iterations were performed. In order to quantify the purity level, a high throughput electrical testing method was developed. Several thousand short channel (~150nm) devices were fabricated where individual SWCNTs span the source and drain. The devices were measured using a semi-automated probe station and the number of semiconducting and metallic devices were simply counted. A semiconducting yield of 99.5 ± 0.2 % was achieved after the separation process. In addition to devices that employ a single CNT, thin-film transistors employing many, separated CNTs were also fabricated and tested. The devices employ varying channel lengths (1um to 80um) and widths (5um to 100um) on both flexible, plastic substrates and rigid, silicon substrates. As expected and indicative of the purity, trade-offs between channel length and Ion/Ioff and transconductance were observed. In both cases, local bottom gates where employed to avoid the need to grow dielectrics directly on the carbon nanotubes, allowing for the employment of thin (down to 10nm), high quality dielectrics. The devices exhibited excellent transport properties for TFTs with Ion/Ioff ratios of 106 and field-effect mobilities of ~ 20 cm2V-1s-1. The rigid substrates that employed 10nm HfO2/W embedded gates showed especially low subthreshold slopes of ~ 200 mv/dec. Generally speaking, the separation method described here allows for the isolation of high purity semiconducting CNTs for applications in thin-film technologies and provides one possible path forward to the extremely high purities necessary for high performance logic.

Carbon nanomaterials provide an excellent framework for catalyst support during heterogeneous catalysis.[1-2] Metal particles deposited on carbon nanotubes exhibit different behaviours over flat carbon supports due to the more strained nature of the support.[3] Different loadings of Fe catalyst on the surfaces of various carbon nanomaterials have been generated as a comparison. Fe deposited on carbon nanotubes ex-situ as well as Fe nanoparticles (NPs) on activated carbon and silica were tested to assess their activity. Bridged nanoparticles on supports have been shown to exhibit superior hydrogen spillover than non-bridged equivalents.[4-5] The bridging forms a physical pathway for hydrogen to travel from NP to support surface. This bridge is important in stabilizing the hydrogen after NP interaction. Fe particles have been synthesized which incorporate this physical bridge as a product of the NP@CNT formation. Using a novel process of forming catalyst nanoparticles on the surface of multi-walled carbon nanotubes (MWNT), a highly active Fischer-Tropsch (FT) and carbon dioxide reduction catalyst has been generated. The iron nanoparticles formed when catalyzing CNT growth also form discrete particles on the surface of the CNTs which have been used for carbon dioxide and carbon monoxide reduction. These particles are more active than analogous Fe particles deposited on the surface of purified nanotubes. This activity difference is due to an increased interaction between the formed particles and the surface on the nanotubes of the as grown Fe@CNTs over the Fe decorated CNTs. The increased interaction means the spillover of hydrogen from the nanoparticles onto the carbon surface is also greater leading to a more potent catalyst. References: 1. Serp, P.; Figueiredo, J. L., Carbon Materials for Catalysis. J. Am. Chem. Soc. 2009, 131 (28), 9856-9857. 2. Antolini, E., Formation of carbon-supported PtM alloys for low temperature fuel cells: a review. Materials Chemistry and Physics 2002, 78 (3), 562-573. 3. Zhang, Y.; Franklin, N. W.; Chen, R. J.; Dai, H., Metal coating on suspended carbon nanotubes and its implication to metal-tube interaction. Chem. Phys. Lett. 2000, 331 (1), 35-41. 4. Lachawiec, A. J.; Qi, G.; Yang, R. T., Hydrogen Storage in Nanostructured Carbons by Spillover: Bridge-Building Enhancement. Langmuir 2005, 21 (24), 11418-11424. 5. Li, Y.; Yang, R. T., Hydrogen Storage in Metalminus;Organic Frameworks by Bridged Hydrogen Spillover. J. Am. Chem. Soc. 2006, 128 (25), 8136-8137.

Overcoming growth termination remains a challenge for the synthesis of ultra-long carbon nanotubes (CNTs). As it has been proposed that the termination of growth is correlated with catalyst morphology evolution and subsurface diffusion of catalyst, we propose here the design of a catalyst stack composed of a thin oxide support under which resides a catalyst reservoir to replenish the catalyst during CNT synthesis, and hence enhance the lifetime and activity of the catalyst. Specifically, we compared samples with e-beam evaporated Fe thin film catalyst on Al2O3 underlayer and samples with an additional thin Fe sublayer reservoir beneath the alumina underlayer. With the inclusion of the Fe sublayer, we synthesized CNT carpets that were up to 100% taller compared to those samples without the Fe sublayer. The effectiveness of the Fe sublayer was observed for wide variations of process parameters such as growth duration and temperature. High-resolution imaging confirms the presence of carpets composed of crystalline and vertically aligned CNTs with lengths up to almost 2mm. Atomic force microscopy (AFM) carried out on samples using comparable process conditions showed a bimodal formation of Fe catalysts for the samples with the Fe sublayer, and subsequent etching experiments emphasize the presence of pinholes in the alumina support which enables the redistribution of Fe catalyst from the sublayer to the overlayer. A study of samples with varying thicknesses of the alumina underlayer further confirmed this hypothesis as thicker alumina layers exhibited shorter CNT growth due to the presence of fewer pinholes. This result is significant as it elucidates further mechanistic aspects of CNT growth and provides a new technique to extend the active lifetime of CNT growth catalyst, and thus enhance the CNT length, which is critically important for a number of applications based upon carbon nanotubes.

Nanocarbons have been under investigation for the past 40 years. The field has attracted increasing interest with each new discovery, from intercalation compounds to fullerenes to nanotubes to graphene, each enriching our large-scale understanding on nanocarbons. Starting with this background we review recent advances occurring in my group from the study of the Raman spectra of individual double wall and triple wall carbon nanotubes, the connections between graphite and few layer graphene, and transformations from one nanocarbon to another.

TBD

Graphene, a monolayer of sp²-bonded carbon atoms, has been attracting worldwide interests because of its unique two-dimensional structure, various fascinating properties and a wide range of intriguing potential applications. The graphene research in China is very active and has been developing very fast in the past several years, which covers nearly all the areas related to graphene including theories, synthesis, physical and chemical properties, and applications. Over 100 research institutions have been involved in graphene research with fast-growing project supports. Over 1,200 papers on graphene were published in 2011, and the total number of publications ranks second in the world. In this talk, I will give an overview about the status, funding and highlights of graphene research in China.

This talk summarizes current status and future prospect of atomically thin two-dimensional functional films such as graphene R&D in Japan. Japan's R&D strategy on graphene and its application to nanoelectronics has so far been less active relative to those of US, EU and the other East Asian countries. However, Japan is a leading country in the field of material science as well as physics and chemistry. Since cooperation and collaboration among physicists, chemists and material scientists is essential to research on atomically thin functional films, Japan's future efforts should include promotion of large-scale national projects and programs that not only are based on research integrated with related fields and "vertical" cooperation with applied fields such as next-generation electronic devices and systems, but also should include young researcher training and international cooperation. The goal of these projects is to realize ultra-low-power electronic devices and systems with new materials, to achieve energy and resource conservation, which is strongly needed to create future sustainable society and to meet societal expectations. The talk will cover funding status, the critical issues, some challenges and the highlights of graphene R&D in Japan.

In the area of graphene electronics, I will discuss applications involving very high frequency (>100 GHz) graphene transistors and on related device physics issues, such as the role of electrical contacts, current saturation, graphene topology and device size scaling. I will then review some optical properties of graphene with emphasis on the infrared, far-infrared and THz ranges of the spectrum. I will discuss ways of controlling graphene&’s absorption in these spectral ranges and applications of graphene in photonics. Optical excitation of graphene and graphene nanostructure plasmons, their properties, behavior and applications will be discussed.

Graphene-based materials are promising because of their electronic and thermal transport, mechanical properties, high specific surface area, that they can act as an atom thick layer, barrier, or membrane, among other reasons. (Our micromechanical exfoliation approaches [1,2] conceived of in 1998 yielded multilayer graphene and one paper described in detail how monolayer graphene could be obtained [1]). But in this 15-minute presentation I will discuss new materials as yet not made, that are important targets for materials synthesis: (i) the negative curvature carbons (see reference 3) and their likely applications, (ii) ultrathin very large area sp3-bonded sheets (we might call these ‘ultrathin large area diamond&’ but when very thin such as 1 nanometer, they will have properties strongly differing from bulk diamond; and some examples of what such large area (kilometers long, meter(s) wide) ultrathin diamond would be used for. This talk is based in part on a contribution to the special issue of the MRS Bulletin on graphene, nominally due out in November. Support of our work on graphene by the W. M. Keck Foundation, NSF, DARPA ‘iMINT&’, DARPA ‘CERA&’, ONR, SWAN NRI, ARO, AEC, and the SRC, is greatly appreciated. 1. Lu XK, Yu MF, Huang H, and Ruoff RS, Tailoring graphite with the goal of achieving single sheets, Nanotechnology, 10, 269-272 (1999). 2. Lu XK, Huang H, Nemchuk N, and Ruoff RS, Patterning of highly oriented pyrolytic graphite by oxygen plasma etching, Applied Physics Letters, 75, 193-195 (1999). 3. Zhu, Yanwu; Murali, Shanthi; Stoller, Meryl D.; Ganesh, K. J.; Cai, Weiwei; Ferreira, Paulo J.; Pirkle, Adam; Wallace, Robert M.; Cychosz, Katie A.; Thommes, Matthias; Su, Dong; Stach, Eric A.; Ruoff, Rodney S. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 332, 1537-1541 (2011).

Graphene electronic materials have been investigated for numerous end applications including rf transistors, spin logic chips, solar cells, flexible displays and a variety of sensors. These applications exploit graphene's unique combination of electrical, optical, thermal and mechanical properties. The nature of these diverse applications requires graphene materials in vastly different forms. Methods of producing graphene on large area platforms by sublimation, deposition, layer transfer and conductive inks are under development by various groups. Within this landscape of materials processing, Group 4 Development (G4D) will target specific graphene materials products for commercialization. In this talk, a description of technologies that are under development at G4D will be presented, as well as an update of its forthcoming commercial products.

TBD

Due to their excellent electrical and optical properties, carbon nanotubes have been regarded as extremely promising candidates for high-performance electronic and optoelectronic applications. However effective and efficient distinction and separation of metallic and semiconducting single-walled carbon nanotubes are always challenges for their practical applications. In our experiment, horizontally-aligned and high-density SWCNT arrays synthesized via chemical vapor deposition (CVD) on a stable temperature-cut (ST-cut) quartz substrate were used. And we show that these metallic and semiconducting single-walled carbon nanotubes on SiO₂ can have obviously different contrast in scanning electron microscope, and thus can be effectively and efficiently identified. We have demonstrated that semiconducting and metallic single-walled carbon nanotubes on SiO₂, probably a variety of insulator substrates which were positively or negatively charging in low-voltage SEM, can have obviously different contrast in SEM, and thus can be effectively and efficiently identified. The correlation between conductivity and contrast difference has been confirmed by using voltage-contrast scanning electron microcopy, Peakforce Tunneling Atom Force Microscopy, and field effect transistor testing. This phenomenon can be understood via a proposed mechanism involving the e-beam induced surface potential of insulators and the conductivity difference between metallic and semiconducting SWCNTs. This method demonstrates great promise to achieve rapid and large-scale distinguishing between metallic and semiconducting single-walled carbon nanotubes, adding a new function to conventional SEM.

We study the catalyst state during chemical vapor deposition (CVD) of carbon nanostructures by complementary in-situ grazing-incidence X-ray diffraction, in-situ X-ray reflectivity, in-situ X-ray photoelectron spectroscopy and environmental transmission electron microscopy. For near-atmospheric pressure carbon nanotube forest CVD, we find that typical oxide supported Fe catalyst films form mixtures of bcc and fcc phased Fe nanoparticles upon reduction and that depending on this phase composition different growth modes occur. For fcc-rich Fe nanoparticle distributions, we find metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For bcc-rich catalyst mixtures, Fe₃C formation more readily occurs and constitutes part of the CNT growth process. Our data indicates that metastable catalyst phases dominate the CNT growth. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron-carbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe₃C. In contrast, for low pressure CVD of single-wall nanotubes from Co catalysts we find that the catalyst state is purely metallic but that catalyst-support interactions (silicide formation) account for a remarkably narrow chiral distribution of the resulting tubes. Finally, we use the complementary in-situ metrology to compare the structural and chemical evolution of such nanoparticulate metals (1-d templates) to flat metal films (2-d templates) to elucidate similarities and differences between nanotube and graphene nucleation in CVD.

Carbon nanotubes (CNTs) are considered as prototypical “new” materials in nanoscience and nanotechnology. Indeed, CNTs provide two particularly exciting prospects. Firstly, the versatility of their properties according to their structure and their dimensions makes them objects of fundamental interest. Secondly, their outstanding mechanical strength, electrical and thermal conductivity and opto-electronic properties offer many opportunities for industrial applications. The combination of these exceptional and versatile properties might lead to the development of electronic systems where both active devices and interconnects are based on the same material. However, in spite of longstanding efforts, no major electronics application involving CNTs is available in the market yet. The main obstacle in the development of a CNT-based technology is that many aspects of their growth mechanisms remain obscure. In particular, the relationship between the nucleating/growing nanotube and the catalyst nanoparticle is not well understood. Despite some remarkable improvements in the control of CNTs features at the synthesis stage during the last decade [1-4], the optimization of CNT growth conditions remain mostly empirical and nanotube samples are frequently a mixture of different structures (number of walls, length, diameter and chiral angle) and morphologies (straight, bundled or entangled). Here, we have employed an in situ approach combining high resolution imaging and Electron Energy Loss Spectroscopy using an environmental scanning transmission electron microscope (ESTEM) in order to probe the relevant instants of a nanotube life at the atomic scale. The composition and the crystal structure of the catalytic nanoparticles were investigated as a function of the growth condition (temperature, pressure) and the nanoparticle size for various catalyst/carbon precursor combinations. The nature of the grown species (number of walls, diameter, crystallinity) were also systematically examined and correlated. Ex situ Raman measurements were used to characterize the in situ synthesized samples at a large scale. [1]. Harutyunyan AR, Chen G, Paronyan TM, Pigos EM, Kuznetsov OA, Hewaparakrama K, et al. Preferential growth of single-walled carbon nanotubes with metallic conductivity. Science. 2009;326(5949):116. [2]. Chiang W, Sankaran R. Linking catalyst composition to chirality distributions of as-grown single-walled carbon nanotubes by tuning NixFe1minus; x nanoparticles. Nat Mater. 2009;8(11):882-6. [3]. Bachilo S, Balzano L, Herrera J, Pompeo F, Resasco D, Weisman R. Narrow (n, m) Distribution of Single-Walled Carbon Nanotubes Grown Using a Solid Supported Catalyst. J Am Chem Soc. 2003;125(37):11186-7. [4]. Zheng L, O'connell M, Doorn S, Liao X, Zhao Y, Akhadov E, et al. Ultralong single-wall carbon nanotubes. Nat Mater. 2004;3(10):673-6.

We present a systematic study of the optical response of bilayer graphene as a function of misorientation angle using a combination of electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope. We find that an additional absorption peak (~4.40 eV), which has not been reported before, emerges in the ultraviolet region of the energy loss function of bilayer graphene for large misorientation angles (~30 degrees). The additional absorption peak is below the nominal π peak of bilayer graphene (~5.10 eV) and also at a different energy position of the π peak for monolayer graphene (4.95 eV). The analysis of the data also reveals that π peak as well as the π+σ peak (~15 eV) do not shift in energy as function of misorientation angle. The observations will be explained using total-energy first-principles calculations based on density functional theory within the random phase approximation. The results obtained in this study indicate that the misorientation angle between graphene layers can affect the optical properties, therefore suggesting that the misorientation angle between layers could also be used as a variable in the designing of nobel optoelectronic devices based on hybrid two-dimensional materials. This research was supported by Oak Ridge National Laboratory's Shared Research Equipment (ShaRE) User Facility (JCI), which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy, and by the National Science Foundation grant No. DMR-0938330 (WZ).

On graphite, GaN has been grown epitaxially by using pulsed laser deposition. Cleaving graphite layers after depositing nitrides by e.g. photoexfoliation should provide graphene as a two-dimensional substrate. Theoretical studies have suggested that the GaN-graphene interface has a well-defined 1x1 structure. However, the lattice mismatch between graphene and GaN is extraordinary large, c.a. 30%, encouraging to explore other stable interface structures. Here, we report first-principles predictions where the structure of graphene changes drastically with the epitaxial growth of GaN, by examining various non-1x1 superstructures. As the GaN epitaxy proceeds expanding graphene gradually, the tensile strain for graphene is released suddenly by partial breaking of the C-bond network. In contrast, AlN-graphene interfaces have lattice-matched structures as the most stable configuration. The interface exhibits spin polarization, even though ferromagnetic coupling is very weak in contrast with the AlN/MgB2 interface. Ref.: Y. Gohda and S. Tsuneyuki, Appl. Phys. Lett. 100, 053111 (2012).

One strategy to covalently functionalize single-walled carbon nanotubes (SWCNTs) is to oxidize the side-walls by an acidic treatment step to create COOH carboxylic functions and then convert these functions to COCl groups by reacting with SOCl2. The acid chloride functions can subsequently be reacted with many different grafting groups and offer great flexibility in terms of chemistry on tubes. However, the control of the functionalization process, i.e. the control of the number of covalent defects and the efficiency of the overall grafting process without destroying the intrinsic electronic and mechanical properties of CNTs, requires estimating the number of defects created at each step. In the present work, different oxidative conditions have been used to create SWCNTs with different kinds and densities of oxidized functions. A HiPco sample from NanoIntegris (SuperpureTM grade, highly purified sample), was used as one of the best starting materials commercially available on the market. The use of such a clean sample, with less than 2% of metallic impurities and less than 5% of carbonaceous impurities, is absolutely essential in order to obtain reliable results for quantitative analysis. This allowed us to quantify the number of created defects by spectroscopic, thermal and microscopic techniques, especially X-Ray photoelectron spectroscopy (XPS), thermogravimetric analysis coupled with mass spectrometry (TGA-MS) and high-resolution transmission electron microscopy (HR-TEM) associated with energy-dispersive X-ray spectroscopy (EDS). After the chlorination step with SOCl2, these techniques quantified the number of chlorine atoms grafted on the CNT side-walls. Finally, a grafting reaction with ferrocene derivatives and other electro-active groups was also successfully done on these samples. These functionalized CNTs were deposited on a glassy carbon electrode and cyclic voltammetry was used to evaluate the electrochemical activity of this modified electrode toward electron shuttle of biological interest such as nicotinamide adenine dinucleotide (NADH). The combination of techniques used in this study enabled estimating the level of functionalization at each step of the process, which gives a strong rationale for the optimization of such CNT treatment.

Carbon nanowalls comprising of a few graphene layers that are vertically aligned on silicon wafer and nickel foil substrates have been reproducibly produced by radio frequency (RF) ethanol-hydrogen-argon plasma-enhanced CVD. The reaction was carried out for 60 minutes at 850C at 300-400 torr and plasma power of 1.8 to 2.0 kW. Hydrogen ions from hydrogen play the role of catalyst for graphene growth. High temperature on the reactor walls and substrate, and the application of an RF plasma converts ethanol in the presence of hydrogen to carbon and OH radicals, and H+ ions. It is worth noting that the parameters that differentiate the growth of graphene nanowalls from the growth of carbon nanotubes is the high concentration of H+ ions in the plasma and the absence of transition metal catalysts on the substrates. The samples produced were characterized in detail by scanning electron microscopy and Raman spectroscopy. Further structural analyses by transmission electron microscopy (TEM) and electron diffraction were carried out on samples removed from the substrates by sonication and deposited on TEM grids. Electrochemical hydrogen adsorption measurements were carried out using the carbon nanowalls deposited on silicon as working electrodes in 6M KOH aqueous solution. The results are compared with similar hydrogen adsorption measurements on carbon nanotubes.

Controlling the uniformity of graphene over large areas requires simultaneous optimizations of the catalytic surface and the synthesis parameters. Application of combinatorial methods to optimize the catalytic substrate for graphene synthesis on 200mm wafers, will be discussed. The thin film stacks (Al/Ni or Ti/Ni) were prepared using magnetron sputtering in PVD chambers capable of depositing isolated spots of varying compositions<1> and the graphene synthesis was carried out in a low-pressure CVD chamber. In-situ TEM experiments have demonstrated that crystallization of carbon is preferred on Ni[111] as opposed to Ni[200].<2> Success in applying this principle to grow large area graphene has been limited by the growth of large area single-crystalline films of transition metals.<3,4> Our results indicate that an optimum substrate configuration consists of a thin-film stack that stabilizes the Ni[111] crystal face post-CVD and is a mixed phase as-deposited. While the [111] face stabilizes the sp2 carbon lattice, it is known that the solubility of carbon is higher in the [200] face. The XRD peak of C[002] was used to rapidly screen combinatorial samples for graphene growth, Raman microscopy and HR-TEM was used to analyze graphene samples. The overall results indicate a clear benefit of using combinatorial catalytic substrate discovery for graphene synthesis using CVD. This material is based on research sponsored by DARPA/Defense Microelectronics Activity (DMEA) under agreement number H94003-10-2-1003. The United States Government is authorized to reproduce and distribute reprints for Government purposes, notwithstanding any copyright notation thereon. References: 1. Hashim, I.; Mathur, M.; Phatak, P.; Kuse, R.; Malhotra, S.; Barstow, S.; Shanker, S.; Chinag, T.; Combinatorial PVD and ALD workflows for cost-effective R&D, Solid State Technology, 2009, 52(3), 26-31. 2. Helveg, S.; Lopez-Cartes, L.; Sehested, J.; Hansen, P. L.; Clausen, B. S.; Rostrup-Nielsen, J. R.; Abid-Pedersen, F.; Nerskov, J. S.; Atomic-scale imaging of carbon nanofibre growth, Nature, 2004, 427, 426-429. 3. Zhang, Y.; Gomez, L.; Ishikawa, F. N.; Madaria, A.; Ryu, K.; Wang, C.; Badmaev, A.; Zhou, C.; Comparison of Graphene growth on single-crystalline and polycrystalline Ni by chemical vapor deposition, J. Phys. Chem. Lett., 2010, 1, 3101-3107. 4. Thiele, S.; Reina, A.; Healy, P.; Kedzierski, J.; Wyatt, P.; Hsu, P-L.; Keast, C.; Schaefer, J.; Kong, J.; Engineering polycrystalline Ni films to improve thickness uniformity of the chemcia vapor deposition grown graphene films, Nanotechnology 2010, 21, 015601.

Nanostructured forms of graphene with well-defined structures, including graphene nanoribbons (GNRs) and graphene quantum dots, have attracted interest as high performance semiconductors and luminescent probes. Top-down approaches, including patterning graphene or unzipping carbon nanotubes (CNTs), have provided access to GNRs and allowed their electronic properties to be measured, but these methods do not provide atomically precise control over the GNR width and edge structure. Bottom-up approaches, in which GNRs are synthesized from specific molecular precursors, promise to provide such control but have so far not produced sufficiently long, dispersed ribbons deposited on insulating substrates. We developed a solution based bottom-up synthesis of GNRs, which relies on a benzannulation reaction at each carbon-carbon triple bond of poly(phenylene ethynylene)s (PPEs) followed by oxidative dehydrogenation for graphitization. PPEs are attractive GNR precursors because their polymerization is well established, tolerates a broad range of functional groups, and provides high molecular weight materials. The optical and electronic properties of these bottom-up GNRs will be discussed, as well as computational efforts to improve their dispersibility. The synthesis of several GNR model compounds will also be presented, which themselves are intriguing and previously unavailable polycyclic aromatic hydrocarbons.

We demonstrate a facile and stable chemical vapor deposition (CVD) method to synthesize graphene on Ni foil using ethanol and dimethyl ether (DME). An easy and non-chemical method was developed to transfer synthesized graphene from Ni foil with the assistance of polyvinyl alcohol (PVA). Effective Bubbling transfer method is also used to transfer big size graphene in this work. Raman scattering spectroscopy and scanning electron microscopy (SEM) were used to characterize both synthesized and transferred graphene. The graphene growth mechanism on Ni foil using ethanol CVD was investigated by exploring the effects of substrate temperature, cooling rate, and decomposition conditions of the two carbon sources. We clarified that graphene is synthesized from the decomposition byproducts together with the undecomposed ethanol. The uniqueness of ethanol with abundant dissociative OH- in CVD synthesis of graphene was also discussed for growth mechanism. Gas phase thermal decomposition of ethanol and DME at different temperatures and low pressure was simulated using chemical kinetic models to investigate the effect of carbon source decomposition conditions on the synthesis of graphene by CVD. Comparing the results of ethanol and DME, the appropriate CVD temperature for graphene synthesis from DME is lower than that for ethanol, in agreement with simulation results. Gas-phase thermal decomposition of ethanol and DME was also simulated under typical graphene growth conditions, both of ethanol and DME were decomposed into considerable amount of CH4, C2H4, CH2O, H2 and CO, concentration profiles of reaction species were compared to the predicted decomposition trend and discussed for understanding graphene growth mechanism.

Plasma enhanced chemical vapor deposition (PECVD) is a potential candidate to grow large-scale graphene. However it requires high temperature, high radio-frequency power and hydrogen to decompose methane. The mechanism of synthesizing graphene on nickel substrate is attributed to the dissolution of carbon atoms into nickel substrate at high temperature. Afterwards, the carbon dissolved nickel substrate is cooled down to induce the carbon precipitation on nickel surface. From the carbon nickel phase diagram, the nickel starts to absorb carbon and forms solid-solid solution at temperature as low as 550oC. In opposite to previous works, the synthesis of few-layer graphene is performed at this critical phase transition temperature by using a low-power PECVD with a hydrogen-free argon diluted methane recipe. The graphene quality is analyzed by transmission and Raman. The argon diluted methane with a fluence ratio of [CH4]/[Ar]=0.015 was employed in the PECVD system and the chamber vacuum was pumped down to 0.09 torr. With the use of a 50nm-thick nickel coated SiO2 on Si substrate heated up to 550oC, the carbon atoms decomposed at RF plasma power of 100 Watts was deposited on and then absorbed into the nickel host during a processing durations of 10, 100, 300 and 600 sec. Afterwards, the nickel absorbed carbon atoms were segregated from nickel layer and formed the graphene during the cooling process. The near-infrared optical transmittance of the synthesized graphene was increased from 21% to 90.5% when shortening the deposition time from 600 to 100 sec. The layer number of graphene with an absorption coefficient of 6.6×107 cm-1 can be obtained by reverse calculating the simplified transmission formula neglecting the reflectance from few-layer graphene surface. The graphene less than five layers can be obtained during 100-sec deposition in our case. The high-quality mono-layer graphene can be demonstrated by carefully controlling the deposition time ranged between 10 and 100 sec. The broadened D-mode Raman scattering peak at 1350 cm-1 indicates numerous defects existed in the low-temperature synthesized few-layer graphene. The G-mode at 1580 cm-1 caused by the sp2 C-C bond indicates the surface strain of the precipitated grapheme is significant, and the 2D-mode reveals a relatively weak intensity contributed by the second-order two-phonon mode of the disordered graphite sp2 materials. The small D/G band Raman intensity ratio and the narrow G-band linewidth represent good quality of the synthesized few-layer graphene. Lengthening the deposition duration to 10 min inevitably enlarges the ID/IG ratio from 0.2 to 1.8 and broadens the G-band linewidth from 37.2 to 67 cm-1, indicating that more carbon dangling-bonds, Stone Wales defects and multiple vacancies were incorporated in the graphene with increasing layers. No Raman scattering signal was obtained for the sample grown within 10 sec due to the slow carbon decomposition and dissolution processes.

Chemical vapour deposition (CVD) has been the most investigated method to grow large-scale (wafer scale) high-quality uniform graphene films. However, the CVD route requires mostly high growth temperature, typically 1000°C and researchers started to look for low temperature growth techniques - low cost and industry-friendly. In that prospect, graphene has been fabricated, recently, via solid-state dissolution of carbon across specific metals [1]. To do so, those metals must be able to store significant amount of carbon (to help the segregation). Consequently, metals like copper, platinum and iridium have been de facto excluded because they can store much less carbon in their bulk volume thus limiting any graphene formation reaction. Indeed, carbon solubility in platinum metal is very low (2.1 10-2 wt% 800°C), which explains why this metal has never been considered for such segregation experiments. Here, we report on the large scale graphene synthesis by segregation method on platinum metal (with surface coverage up to 98 % on 8 inch. wafer) [2, 3]. This result emphasizes an original demonstration that the segregation method is not limited to a restrictive sub-set of metals, but can be enlarged resulting in fast wafer scale graphene synthesis. Moreover, by appropriate choice of the solid carbon source we demonstrate the ability to tailor graphene shape into 1) suspended graphene above its substrate or 2) graphene in full-sheet contact with its substrate. In the first case, by using amorphous silicon carbide (a:SiCH) as source of carbon, an appropriate annealing can result in the suspension of graphene without any transfer or subsequent chemical treatment. This suspension arises via a kinetics of formation and dewetting of platinum silicide clusters. Suspension of graphene is highly desired for application requiring decoupling of graphene. Decoupling graphene from the substrate by fostering the interaction of the carbon source and the metallic substrate opens new perspective in the tailoring of graphene synthesis. This suspension can be suppressed by using a carbon source with less than 20% of silicon (either a:SiCH with Si < 20% or amorphous carbon (a:CH)). In this case, mono to few layer of graphene on full-sheet platinum is obtained (checked by I2D/IG ratio in Raman spectroscopy). In conclusion, we show that graphene could be successfully synthesized by segregation method on large wafer scale onto polycrystalline platinum thin film, thus extending this technique to the metals with very low carbon solubility. This technique can pave the way to new perspectives for graphene synthesis and for tailoring its shape on large wafer scale giving new opportunities for real-life applications. [1] M. Xu et al, ACS Nano. 5, 1522 (2011). [2] A. Zenasni et al, Appl. Phys. Lett. 100, 151907 (2012). [3] Segregation Method for Graphene Synthesis Extended to Metals with Low Carbon Solubility, A. Zenasni et al, Submitted.

Graphene has become one of the most researched nanomaterials recently. Besides being attracted by the unusual properties of graphene itself, researchers are increasingly interested in graphene derivatives, which offer versatility in physical and chemical properties and promise new applications, ranging from flexible electronics to energy storage. Linking heteroatoms of high electronegativity (such as fluorine, chlorine and iodine) to graphene can drastically alter its chemical reactivity, electrical conductivity, and capacitance. Fluorinated graphene (F-G), in particular, is a highly promising material for the fabrication of thin insulating films for nanoelectronics. Here, we report our progress in developing chemistry for the effective and controlled incorporation of fluorine onto graphene sheets. Using graphene oxide produced by Hummers method as the starting material, F-G will be generated through chemical conversion of the affluent oxygen-contaning groups (mainly hydroxyl, epoxide, and carboxylic) to fluorine. Chemical composition and electrical properties of the resulted F-G will be extensively characterized by spectroscopy methods (e.g. FTIR, scanning transmission x-ray, electron spin resonance, dielectric spectroscopy) and electrical measurements, and the results will be presented. We anticipate that this work will be significant in two aspects. First, the all liquid-phase chemistry involved can ultimately facilitate the mass production and easy formation of large-area F-G thin films. Second, this work will lead to better understanding of the molecular/atomic origins of the electrical properties of graphene derivatives in general.

We report a scalable preparation procedure for holey graphenes, a novel type of structure with randomly spaced holes through the layers of graphene. The procedure involves catalytic oxidation of reduced graphene oxide (rGO), resulting in rGO sheets with holes of controllable sizes in the range of a few nm to more than 100 nm. The method is highly scalable with multi-gram quantities of holey graphenes conveniently fabricated in our labs. These holey graphene materials showed interesting physical, electrical, and thermal properties as compared to rGO samples. Results from the applications of the holey graphene materials for both polymer composites and energy storage applications will also be presented.

Graphene, typically composed of one-atom-thick layer of carbon in a 2D hexagonal lattice, is a basic building block for graphitic materials of all other dimensionalities. The graphene has attracted tremendous worldwide attention because of their fascinating properties different from those of the carbon-based graphitic materials (e.g., extremely high charge carrier mobility, large specific surface area, thermal/electrical conductivity, and chemical/mechanical stability). Based on these superior characteristics, the graphene has emerged to be a valuable platform for a wide range of applications such as energy-storage materials, electronic device, polymer composites, liquid crystal devices and bio- and chemical sensors. Up to date, various synthetic methods for preparing graphene have been developed. The most straightforward way to fabricating graphene is the exfoliation method to ‘peel-off&’ from graphite. The graphene can be also formed by physical and chemical synthetic techniques, including chemical vapor deposition (CVD) and arc discharge, epitaxial growth, thermal decomposition of SiC, unzipping of carbon nanotubes (CNTs), and so on. However, most previous synthetic methods suffer from the precise control of the size, shape, edge, layer of graphene sheets. Consequently, it is still challenging to produce graphene with tailored morphology and diameters for various applications. In this presentation, two different ways in the synthetic methodology of graphene will be presented in the viewpoint of top-down approach and bottom-up approach. As a ‘top-down approach&’, the graphene sheets with well-defined shape are successfully fabricated using a simple oxidation and exfoliation process of high-crystalline carbon nanofibers (CNFs). Interestingly, the diameter and shape of the graphene sheets can be controlled by selectively designing the morphology of starting materials and optimizing the cutting method. As a ‘bottom-up approach&’, graphene sheets are formed using layer-by-layer (LbL) self-assembly approach with a metallic dopant. The LbL approach is used to form poly(allylamine)(PAA)/ poly(styrenesulfonate)(PSS) multilayer on a quartz substrate. During the carbonization process, the PSS layers can be transformed into graphene sheets due to its inherent aromatic and highly ordered structure. PAA layers served to protect the structural layers as well as prevent the agglomeration of graphene sheets. Most, importantly, these novel approaches can be used as an alternative tool for fabrication of various carbon-based nanomaterials with rational nanostructure design.

Large-scale synthesis of graphene of high quality at low cost and simple process is critical to the various applications of graphene. In this paper, we developed a facile chemical approach to the synthesis of graphene via a mild reduction of graphene oxide (GO) using self-reduced metal nanoparticals as a catalyst to the hydrolysis reaction of NaBH4 at room temperature. The morphology and structure of the graphene were characterized with atomic force microscopy and transmission electron microscopy. The reduction process and quality of graphene were followed and examined by UV-Vis absorption spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction, respectively. Compare to the existing methods, the advantages of this method are that graphene can be prepared in large quantity without using toxic reducing agents such as hydrazine or its derivates, making it environmental benign; the reaction is conducted at mild condition (room temperature), resulting in less defect formation during the process of reducing GO and easy operation; the metal catalyst can be recycled and the method can be easily scaled up. This approach opens up a new way to synthesis high quality graphene under a mild condition at low cost.

Binary composites of TiO2/graphene have been proved to have better photocatalytic activity than the commercial P25 TiO2 photocatalyst. Meanwhile, it is well known that the noble metal nanoparticles, such as Ag or Au show localized surface plasmon resonance (LSPR). Due to LSPR, Ag and Au nanoparticles can have strong and broad absorption in the visible region. Therefore, it is expected that the photocatalytic activity of titania could be further increased by incorporating Ag in the binary composites of TiO2 and graphene. However, recent literature lacks their study. To synthesize such a ternary composite, multistep reaction schemes involving synthesis of TiO2/graphene followed by Ag incorporation were performed. In this work, ternary nanocomposites of Ag/TiO2/graphene were synthesized in a single step by employing solvothermal reaction at 200 °C. Ethylene glycol was used for the reduction of AgNO3, while stoichiometric amount of water hydrolyzed titanium tetraisopropoxide. The synthesized nanocomposites were characterized by a number of microscopic and spectroscopic techniques. X-ray diffraction (XRD) showed that titania is present in both anatase and rutile phases. Notably, UV-visible spectroscopy confirmed a broad and strong absorption band centered at 390 nm due to the LSPR of Ag nanoparticles. The LSPR band proves successful reduction of AgNO3 under the solvothermal reaction conditions. The reduction of Ag+ was also confirmed by X-ray photoelectron spectroscopy (XPS). XPS further proves that in the composite TiO2 was present as Ti4+. The synthesized nanocomposites showed enhanced photocatalytic activity for the degradation of Rhodamine B (RhB) under visible light irradiation. Over 95% of RhB was degraded by the catalyst after 60 minute exposure of visible light. The rate constant of photocatalytic degradation of the catalyst showed an increase with an order of magnitude greater than that of commercial P25 TiO2 photocatalyst. Overall, this work suggests a versatile visible light photocatalyst, synergistically reinforced by the LSPR effect of Ag nanoparticles.

Organic chemistry has excelled at the formation of stable carbon-carbon bonds. A colorful variety of reactions create bonds in high yield, selectivity and often stereo specific. What happens, if we transfer such knowledge to carbon allotropes? Can we make organic synthesis on very large, two-dimensional planes? Industrially important products like black ink already consist of chemically modified carbon particles. And carbon black was just the beginning of a most remarkable successful story when in the last decades a plethora of carbon forms have become functionalized with small organic molecules by carbon-carbon bond forming reactions. Here, we present graphene functionalized with aryl diazonium ions and show the chemical changes by Raman spectroscopy(1), the surface changes by atomic force microscopy and kelvin probe measurements(2). In addition the electron transport is measured with increasing functionalization density showing that high doping levels can be achieved by the diazonium reaction(3,4). This primarily attached functional group opens the way to organic synthesis on functionalized graphene allowing the incorporation of biomolecules, polymers and catalysts.

Graphene nanoribbons (GNRs) with well-defined edges are predicted to exhibit interesting electronic and spintronic properties such as quantum-confined bandgaps and magnetic edge states for novel device applications. GNRs made by lithographic etching process have defects and dangling bonds along the edges, and their electron transport shows defect-dominated characteristics at low temperature. We show that pristine 1-3-layer GNRs can be produced by unzipping mildly gas-phase oxidized multi-walled carbon nanotubes using mechanical sonication in an organic solvent [1]. The nanoribbons are of very high quality as revealed by aberration-corrected transmission electron microscopy, Raman spectroscopy and electrical measurements [2]. The fabricated GNR devices have mobility up to ~3000cm2/Vs at room temperature. At low temperatures, they exhibited well-defined quantum transport phenomena, including Coulomb blockade, the Kondo effect, clear excited states up to sim;20 meV. Together with the signatures of intrinsic quantum-confined bandgaps and high conductivities, our data indicate that the nanoribbons behave as clean quantum wires at low temperatures, and are not dominated by defects [3]. Reference: [1] Liying Jiao, Xinran Wang, Georgi Diankov, Hailiang Wang and Hongjie Dai, “Facile synthesis of high quality graphene nanoribbons”, Nature Nanotechnology 5, 321 (2010). [2] Liming Xie, Hailiang Wang, Chuanhong Jin, Xinran Wang, Liying Jiao, Kazu Suenaga, and Hongjie Dai, "Graphene Nanoribbons from Unzipped Carbon Nanotubes: Atomic Structures, Raman Spectroscopy, and Electrical Properties", J. Am. Chem. Soc. 133, 10394 (2011). [3] Xinran Wang, Yijian Ouyang, Liying Jiao, Hailiang Wang, Liming Xie, Justin Wu, Jing Guo and Hongjie Dai, “Graphene nanoribbons with smooth edges behave as quantum wires”. Nature Nanotechnology, 6, 563 (2011).

The patterning of graphene has received a great attention, because it is essential to be utilized practically. However, when a top-down method is employed, the excellent property of graphene is lost due to the disordered structure in the edge. Here, we introduce a direct patterned growth of graphene on poly(styrene) (PS) film via UV irradiation. Because PS is crosslinked by UV-irradiation, a selective cross-linked region is prepared on the PS film/copper foil. When the sample is heated a high temperature under 1 Torr of Ar/H2 atmosphere, only non-crosslinked regions are converted into graphene, while cross-linked regions becomes amorphous carbon. The patterned grapheme is characterized by Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).

Graphene has attracted a great interest for its outstanding electrical, optical, magnetic and thermal properties since the first discovery. Chemical vapor deposition (CVD) method offers certain advantages over other ways due to its large-scale growth and reliable and compatible synthesis. Graphene can be grown by surface diffusion of carbon atoms which thermally dissociated by hydrocarbon source, but polycrystalline nature which originated from random nucleation is major bottleneck for an industrial application of graphene. Despite of numerous approaches based on controlling empirical parameters in graphene growth, typical domain size in chemical vapor deposited graphene is tens of micrometers. Also, fundamental understanding on growth mechanism is lacking and suppressing nucleation density at the very early stage of its growth is still needed. Thus, key issue in nucleation and growth of CVD graphene is that we should understand mean field for carbon diffusion which governs average domain size of graphene and find a way to increase it. Hydrocarbon source feeding control and understanding gas kinetics are highly required for nuclei density control, growth rate, and even for growth morphology evolution since lobed grains induce larger grain boundaries after coalescence. In this presentation, an effective way to decrease nucleation density in graphene growth and supersaturation ratio will be discussed. We used simple jig for better understanding of gas transport by mass transfer and diffusion through boundary layer in CVD system. We compared graphene grown in small jig gap and outer of jig in order to understand gas transport. Time and temperature dependent growth behavior will also be discussed.

Graphene is a very promising material for many microelectronic applications because of its unique electronic properties. Among the several proposed routes to fabricate layers of graphene, the growth of epitaxial graphene on 4H and 6H SiC(0001) appears to be particularly promising. In this talk, results from density-functional theory calculations will be presented for the potential energy surfaces and different diffusion rates of C atoms on the different polytype surfaces on 6H SiC(0001). We will then look at the early stages of growth of graphene on top of a carbon buffer layer on SiC(0001). Our results suggest that small clusters of carbon consisting of one, two, or three carbon rings are stable but prefer to be in an upright position, rather than flat on the surface. Only after the nucleus grows to 4 or more carbon rings does the lowest energy configuration become a graphene-like arrangement, where all atoms are flat on the surface.

The effects of applied external strain on the mechanical and electronic properties of graphene are of great interest for many future applications, such as highly sensitive strain sensors and electronic devices. Previous studies have shown that by applying strain to graphene the electronic structure can be altered and this can best be detected by Raman spectroscopy. However, previous studies on the effect of strain on graphene have focused primarily on exfoliated graphene, which consists of large domains, whilst future applications of graphene will more likely use graphene produced by chemical vapor deposition (CVD). CVD grown graphene, however, consists of many smaller domains and these domain boundaries are believed to affect how the strain alters the electronic structure of graphene. In this work we grow single-layer graphene by CVD on copper foil and transfer it onto flexible polymer (polydimethylsiloxane, PDMS and poly(methyl methacrylate), PMMA) substrates that allow us to apply strain and measure the resulting changes to the Raman spectrum. Unlike previous work, the use of CVD synthesized graphene demonstrates anomalous behavior regarding the shifting of the Raman peak positions when strain is applied and this is a combination of the doping affect from the contact with the substrate combined with the domain structure of the graphene. The causes of this strain behavior are also analyzed using theoretical modeling of the electronic structure, including phonon dispersion and shifting of the Dirac points. It was found that using a highly flexible polymer such as PDMS provided a highly biaxial stress resulting from applied uniaxial strain due to the high Poisson&’s ratio (asymp;0.5), whilst previous literature has focused on the use of more rigid polymers such as poly(methyl methacrylate) (PMMA) and the difference between the two is compared here. These results provide an important insight into the fundamental properties of graphene in real-world scenarios, and are of importance in future applications such as flexible electronic devices.

One of the primary reasons for the current interest in graphene is because of its unique electrical properties. For instance, carrier mobilities as high as 200,000 cm²/Vs have been measured for exfoliated graphene flakes. The electronic structure of graphene depends on the number of graphene layers and the stacking sequence between the layers. For instance, single-layer graphene is a semi-metal with a linear energy dispersion near the Dirac point, which results in a very small effective mass for the carriers. On the other hand, a band gap can be induced in Bernal stacked bi-layer graphene. For device applications it is crucial to maintain a low defect density and a uniform graphene thickness. A common technique used to fabricate large area graphene samples is by thermal decomposition of SiC in vacuum. One of the main advantages of this growth technique is that the graphene is formed directly on an insulating substrate, which is desirable for most device applications. The primary drawback is the high cost of the SiC substrate. Cu foils are low cost substrates for growing large area graphene films. Owing to the low solubility of C in Cu, these films typically self-terminate at monolayer coverage. However, the carrier mobility of these films is typically much lower than for films exfoliated from graphite. This is primarily from line defects that result from rotational domains in the graphene film. Another approach to forming uniform graphene films is to use Cu-Ni substrates. Since Ni is more catalytically active than Cu, graphene growth can proceed at lower temperatures and source pressures. In addition, the C solubility of the alloy can be adjusted by increasing the Ni alloy fraction, allowing the formation of graphene films with more than one layer. We have developed a technique using angle-resolved XPS to determine the average graphene thickness directly on metal foil and SiC substrates. Since monolayer graphene films can be grown on Cu substrates, these samples are used as a standard reference for a single layer of graphene. HOPG is used as a standard reference for bulk graphite. The electron mean free path of the C-1s photoelectron can be determined by analyzing the areas under the C-1s peaks of monolayer graphene/Cu and bulk graphite. With the measured electron mean free path, the graphene coverage of a film of arbitrary thickness can be determined by analyzing the area under the C-1s of that sample. Analysis of graphene coverages for graphene films grown on Cu-Ni substrates and of the thickness of both the graphene overlayer and intermediate buffer layer on SiC will be presented. In addition, Raman spectroscopy measurements of the graphene films were made and correlate well with the coverages determined by angle-resolved XPS. This research was supported in part by the NSF (#1006350/1006411).

Chemical vapor deposition (CVD) using Cu foil as the catalyst and substrate is a prevailing technology to grow large-area single-layer graphene films with virtually no size limit. In most applications, CVD-grown graphene is transferred onto substrates or devices via wet processes, where Cu etchant, deionized water, and other solvants are used to etch away the Cu foil and remove the polymer film that carries graphene during the transfer. However, this method is not suitable to transfer graphene onto liquid-sensitive substrates, such as MgO, or devices, such as tunnel junctions. We have developed a transfer method to first transfer the CVD-grown graphene using the traditional wet method onto a free-standing 4mu;m-thick Cu film, then transfer onto any substrate by a liquid-free process using PDMS. The Cu film was deposited on a Si wafer by DC magnetron sputtering and then exfoliated from the wafer. The integrity of the graphene film was largely preserved during the second transfer by PDMS owing to the smooth surface of the Cu film. Using this method, single-layer graphene films were transferred onto SiO2/Si, MgO, and Al2O3 substrates, showing good-quality in optical, scanning electron microscopy (SEM), and atomic force microscopy (AFM) images. Raman spectroscopy indicates that the transferred graphene film has single layer and has similar quality as a graphene film transferred by the tranditional wet method. Transport measurements of dry-transferred graphene films on Si/SiO2 substrate at room temprature show that their sheet resistance is 1.6~3.4 kOmega;/sq, charge carrier density is 4.1~5.3×1012 /cm2, and mobility is 460~760 cm2/Vs without doping, which are similar to the resuls of traditional wet-tranferred single-layer graphene. Single-layer graphene films were also transferred onto Al/Al2O3 strips by this dry-method to form graphene/AlOx/Al tunnel junctions, which exhibit good tunneling current-voltage characteristics at both 300 K and 4.2 K. This novel dry-transfer method can be applied to fabricating other electronic devices that prefer a liquid-free process.

Segregation method to synthesize graphene has been suggested and proposed in lots of metal and metal alloy system. Ni which has maximum 2.7 at% of carbon solubility is used as synthesis of graphene method by CVD and recrystallization method of a-C by annealing. However, large solubility of Ni compared with Cu, which produce 95% single layer graphene, causes few-layered graphene formation. Ni-Cu alloy system is then proposed to control resultant graphene layer thickness by controlling diffusion kinetic of Ni-Cu atoms and carbon atom. In recent study, by applying pressure and heat on carbon powder coated Ni film, graphene formed among Ni and dielectric substrate interface, which has no transfer process required. Here, we propose formation of graphene on dielectric substrate directly by different method, using diffusion barrier. When a-C deposited Ni is annealed, graphene formation is expected on surface in enough elevated temperature. However, depositing other material on surface which make carbide phase can prohibit diffusing out of carbon atom rather diffusing out in interface and prompt graphene formation in interface of Ni and substrate. By this method, after etching Ni and surface deposited material, direct graphene formation on target substrate can be achievable.

Understanding the growth of multilayered graphene via chemical vapor deposition (CVD) is vital for optimizing these materials for future applications. Multilayered graphene has applicability ranging from heat dissipation to energy storage to transparent conduction. Here, we present on controlling the uniformity of multilayered graphene using Ni(111) versus polycrystalline Ni films. In order to obtain Ni(111), we used direct current (DC) sputtering on an Al₂O₃(111) substrate, verifying the crystallinity of the Ni(111) using x-ray diffraction (XRD). To synthesize the polycrystalline Ni, we e-beam evaporated Ni onto Si/SiO₂(90 nm) substrates. We grew multilayered graphene on both the polycrystalline Ni and Ni(111) films via atmospheric pressure CVD using a CH₄ precursor, systemically varying temperature, CH₄ dose, and film annealing time, and optimized our cooling rate. We found that temperature and CH₄ dosage primarily controlled the thickness of our multilayered graphene whereas, the film annealing time and cooling rate predominantly controlled the quality and uniformity of multilayered graphene grown. We characterized the thickness and quality of the resulting multilayered graphene films via optical microscopy and scanning electron microscopy (SEM). Furthermore, we used Raman spectroscopy to verify the presence and quality of the grown multilayered graphene. Based on qualitative optical and scanning electron microscopy, we saw that the thickness-uniformity can be substantially improved on the Ni(111) compared with the polycrystalline Ni films. Additionally, we are continuing to elucidate the dependence of the thickness and uniformity of the multilayered graphene on substrate thickness and growth time. These findings represent an important step towards the fabrication of practical, large-scale, high quality multilayered graphene.

Despite countless studies of graphene growth on various substrates, improved understanding of the growth kinetics is necessary to manufacturing large-area graphene with high quality and a well-defined number of layers. Here we report growth of graphene using a cold-wall CVD system, which features rapid thermal control of the substrate (100 C/s) by resistive heating, along with in situ Raman spectroscopy. After a 10 second ramp to the desired temperature, graphene is observed for growth steps as short as 5 seconds using an acetylene carbon source. By incorporating Raman spectroscopy, we study the effects of heating and cooling rates, gas composition, and growth duration on the number of layers and quality of graphene while offering insight on the mechanism of growth. The capability to rapidly form graphene by local heating will be instrumental to continuous feed CVD methods.

For electronic applications, it is desired to grow graphene film with large grain sizes (high quality), monolayer, and wafer scale (large area). Large area and high quality graphene films were typically grown on metal catalysts through chemical vapor deposition (CVD) of hydrocarbon gases. Copper has been the most popular metal catalyst for wafer scale and single layer growth since low carbon solubility of copper tends to prevent to grow the second graphene layer underneath of or above the first layer. However, copper only catalyst is not a perfect solution for monolayer and high quality graphene films. Double layers were observed in small areas. Also, copper only catalyst might have the limit to control the grain sizes. Although grain size of graphene film was successfully enlarged by increase of the growing temperature and decrease of the feeding amount of hydrocarbon gas, it is still very difficult to control the grain size and position without controlling the catalyst. In this work, we studied the role of metal solute in metal catalysts to control the number of layers and the size of grains. As already known in catalyst research, the low coordinated atoms at the step edge sites are very reactive and governs the whole catalytic reactivity. Thus, blocking the step by intentionally poisoning with non-reactive atoms such as Au could be a way of controlling the graphene nucleation and carbon dissociation rate. We studied two different metal solutes in copper catalyst, Ni and Au, and compared with Cu only catalyst. Ni atoms at the step edge would serve as more reactive sites for graphene nucleation while, Au atoms would block the step edge of Cu. In our results, both of CuNi and CuAu alloy catalyst provide a single layer of graphene film, but nucleation behavior was significantly different and we believe this behavior is due to the reactivity difference between Ni and Au as we expected. We also fabricated graphene FET device using the graphene film grown from CuNi and CuAu alloy and measured the electron transport properties. We will discuss the difference between those two alloys during the presentations. Based on the results, we concluded that the quality graphene film can be controlled by the catalyst design.

Graphene is a 2-dimensional network of sp²-bonded carbon atoms and expected to be a material for the next generation of electronic devices. Formation methods of graphene films are roughly divided into mechanical exfoliation, chemical vapor deposition (CVD), and epitaxial growth on SiC. Among them, a wafer scale graphene film can be grown on a catalytic metal surface by CVD method. Recently, direct growth of graphene on insulating surfaces has also been realized [1]. However, a major drawback of this process is the high growth temperature, which is a serious problem in device applications. Our goal is direct synthesis of graphene films on insulating substrates at lower temperatures by CVD. We used sapphire (0001), (11-20) and (1-102) planes as substrates, CH₄ as a carbon source, and Ar and H₂ as carrier gases. Graphene films were grown at 1000°C at atmospheric pressure for 15~120 min. After the CVD, we observed their surface morphology by Atomic Force Microscopy (AFM) and characterized their properties by Raman spectroscopy. We used sapphire (0001) surfaces on which two kinds of domains formed: the domain A is a wide terrace region and the domain B single-stepped regions. We found that the topography and frictional force differ between the domains A and B after the growth for 120 min. Wrinkles, which are formed at the boundaries between independently grown graphene domains, were observed in the domain B, whereas no wrinkle appeared in the domain A. Frictional force on the domain B was smaller than that on the domain A. These results indicate that graphene films grew only in the domain B, suggesting that a self-catalytic property is generated in the domain B for direct growth of graphene. We observed time evolution of the surface morphology during the CVD growth. In the early stage from 5 to 15 min, a high density of pits appeared in the domain B. The pit size was smaller than 0.1mu;m. After growth for 30 min, the pit size was from 0.1mu;m to 0.2mu;m. After 45 min, the pit size was almost unchanged. After 60 min, wrinkles appeared in the domain B, and their density increased with time up to 120 min. We also measured growth time dependence of the Raman spectra. As the growth time increases, the D peak decreases. This suggests that the quality of the graphene films was improved and their domain enlarged with an increase in the growth time. We also used sapphire (11-20) and (1-102) surfaces and found that many hydrophobic carbon domains grew on the (11-20) surface. This sample exhibits a D peak intensity smaller than that of the (0001) or (1-102) surface. From these results, we can conclude that direct synthesis of graphene on sapphire surfaces has been realized through formation of self-catalytic surfaces in the early stage of growth. [1] M. Fanton et al., ACSNANO 5 (2011) 8062-8069.

Practical electronic device applications of two dimensional graphene highly depend on its quality and mass productions. Recent progress of its mass production method on the Cu or Ni foils shows that the quality graphene of large area is up to near the industrial applications such as transparent electrodes. Nonetheless, there are still many obstacles in case of electronic device applications. Pinholes, folding, and cutting of graphene are frequently appeared during the transfer processing to a substrate. Transfer method also is not suitable for conventional device fabrication scheme. Here we present the direct graphene wafer production with high quality, which is suitable for mass and easy electronic device production. Aligned patterned growth on a single crystal (111) plane of Cu grain with each size of about 10 micrometers is demonstrated. High-quality graphenes on transformed polycrystalline Cu (111) grains on a wafer were produced by chemical vapor deposition method with assisted Cu vapor catalysts. Deposited thin Cu film turned into large (111) orientation grains with average lateral size of 10~40 mu;m at high temperature. It is due to easy coordination of (111) planes at low dimension film. Growth is done by using the specially built thin Cu chamber that can hold 100 mm wafer. This method suppresses the evaporation of Cu layers on a wafer during the high temperature growth of high quality graphene which generally is needed for. Growth temperature can be elevated up to 950~1010 Celsius degree with maximum growth time of about 50 minutes depending on Cu film thickness. We probe graphene qualities using a partial etching method for optical visualization, scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The results showed that graphene consists of monolayers with high quality. In addition, Cu domains are enlarged more than ~30 micrometers in lateral size with mostly (111) crystal orientations, which manifest a critical condition of high-quality graphene growth [1]. It turned out that the quality of graphene grown on wafer scale Cu films is comparable with that of Cu foils and exfoliated one. Top gated FETs on a graphene wafer directly without graphene transfer show good device characteristics [2]. *[email protected] References 1.Li Tao, Jongho Lee, Harry Chou, Milo Holt, Rodney S. Ruoff, and Deji Akinwande, ACS Nano, 6, 2319 (2012). 2.Mark P. Levendorf, Carlos S. Ruiz-Vargas, Shivank Garg, and Jiwoong Park, Nano Letters, 9, 4479 (2009).

Chemical vapor deposition (CVD) offers an inexpensive, scalable route to produce large-area graphene. Until recently, most studies of graphene growth have been conducted on rough, polycrystalline metal foils. However, due to the crystallographic complexity of these surfaces, growth on polycrystalline substrates is difficult to understand and results in graphene with rotational grain boundaries and wrinkles that limit charge carrier and phonon transport. Here, we present a systematic study of the growth dynamics of graphene on copper (100), (110), and (111) thin films sputtered epitaxially onto sapphire and magnesium oxide substrates. These ultra-smooth, epitaxially-oriented thin films are better platforms for reproducible, high-quality graphene growth than their foil counterparts. We have explored growth on these copper films over a wide range of pressures (0.5 mTorr - 760 Torr) and temperatures (850 - 1050°C). We show that on these copper substrates, growth can be driven in either a planar or dendritic regime with controllable morphology. In the dendritic regime, the dendrites preferentially grow along the <100>, <111>, and <110> directions on copper (100), (110), and (111) surfaces, respectively, and this directionality is enhanced with increasing temperature. Furthermore, we illustrate that the lobed morphology observed in many low-pressure growths on copper foils transitions into dendritic growth if the inter-nucleation distance is sufficiently large. Both growth modes can be tuned to yield large-area, monolayer graphene. Raman spectroscopy of the graphene grown on all three substrates reveals 2D:G ratio ~ 4.0 and low D:G ratio after being transferred to silicon dioxide substrates. The graphene has optical transmittance between 96 and 98% over wavelengths of 400 to 1600 nm. Overall, the growth evolution studies presented here offer the possibility of controlling graphene crystal orientation, morphology, nucleation density, and anisotropic growth rate. In addition, this work provides a more rationally designed route to synthesize high-quality and high-performance graphene materials.

Graphene nanostructures such as nanoribbons and periodic rippled structures are expected to show unique electronic properties [1, 2]. Thermal decomposition of SiC is popular and importantly gives epitaxial graphene via the formation of the (6radic;3×6radic;3) buffer layer. However, the potential problems in this technique reside in collapse of surface morphologies and spatial thickness variation during growth. The former is a significant issue in fabricating nanostructures. We found periodic nanofacet formation after H2 gas etching of vicinal SiC surfaces [3,4]. This surface is useful as a template to achieve graphene nanostructures if the potential problems mentioned above are solved[5]. Chemical vapor deposition (CVD) is an alternative technique to grow epitaxial graphene and expected to avoid surface evolution.[6,7] A vicinal 6H-SiC(0001) surface was first etched by H2 gas and resulted in periodic nanofacets. CVD growth was performed using the C2H4/Ar gas mixture at 1200°C. AFM image analysis and Raman spectroscopy indicates monolayer graphene growth and its thickness of ~0.51nm, which may include the interface layer. A low-energy electron diffraction (LEED), however, shows a ring pattern due to randomly rotated domains and disordered (diffuse) (radic;3×radic;3)R30° spots. The radic;3 structure is probably originated in the SiO-related interface and affects crystallographic nature of graphene by CVD. Attempt to remove such interface structure was done by HF solution before growth, resulting in similar domain structures but interestingly no radic;3 interface structure was identified by LEED. We believe hydrogen termination of SiC surface takes place in this case and gives random nucleation of graphene. Conclusively, the interface structure is a key to realize single domain structures and further periodic graphene nanostructure. By excluding hydrogen termination on SiC surfaces single domain graphene is achievable. References [1] Y. H.Melinda et al., Phys. Rev. Lett. 98, 206805 (2007). [2] C. H. Park et al., Nature Physics 4, 213-217 (2008). [3] H. Nakagawa et al., Phys. Rev. Lett. 91, 226107 (2003). [4]M. Fujii and et al., Phys. Rev. Lett. 99, 016102(2007). [5] S. Tanaka et al., Phys. Rev. B81, 041406(R) (2010). [6] W. Strupinski et al., Nano Lett., 2011, 11 (4), pp 1786-1791 [7] A. Michon et al., Appl. Phys. Lett. 97, 171909 (2010)

Graphene, since its fascinating electrical properties have been well revealed, became one of the most attractive research topics in modern science. Its unique structure, two-dimensional honeycomb carbon lattice, enables it to have high chemical stability, mechanical strength, and low resistivity. This material is thus potentially expected to replace the silicon in electrical applications and arouses a great quantity of studies on the synthetical method of graphene to requisitely develop a large area with high quality growth with low cost for industrial manufactory. In this regard, we would like to propose a new method to synthesize graphene via Ni catalysed which takes advantages from the above mentioned techniques, the solid-state carbon source for effectively providing the feedstock, the microwave heating apparatus for fast synthesis, and the native protection of the carbon-doped Ni surface for reacting in the air environment and self-repairing. The pyrogenesis is from a rapid conversion of thermal power energy from the microwave via a SiC susceptor. By controlling input powers of the microwave, the annealing temperatures could be controlled, leading to a dissolving of the carbon atoms into Ni catalyzed layer. The carbon atoms in the Ni layer would play an important role to retard the formation of the NiO on the surface via a redox process and the graphene layers will be segregated during cooling. The produced graphene could be pre-patterned and the entire growth is in minutes in the atmosphere. The low equipment criteria and rapidity could allow this method to be promising for different application demands.

One-pot Synthesis of Graphene-ZnS Nanocomposites with Tween 20 as Growth Template Zonghua Wang*, Jianfei Xia, Feifei Zhang, Lishun Bi, Yanzhi Xia*, Yanhui Li Laboratory of Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory, College of Chemical and Environment Engineering, Qingdao University, Shandong, 266071, PR China Corresponding author: [email protected], [email protected]. Abstract: Graphene is a novel carbon nanomaterial, a flat monolayer of hexagonally arrayed sp2-bonded carbon atoms tightly packed into a two-dimensional (2D) nanosheets, has attracted much attention due to its excellent properties. Since its discovery in 2004, graphene has displayed as a fascinating material for a wide variety of potential applications in large range transistor fabrication, photovoltaics, catalyst supports, lion batteries, chemical sensors and drug delivery. To harness these properties for application, graphene has been utilized as matrix to prepare a wide range of composite materials. Zinc sulfide is an important semiconductor with bandage energy and has been studied extensively because of their wide applications in the fields of sensors, lasers, solar cells and light-emitting devices for many years. Graphene-ZnS (G-ZnS) not only can combine these advantages of graphene and ZnS but also may result in new properties. However, the surface of the semiconductor nanoparticles is highly defective, which directly influences the photocurrent generated by the nanoparticles. To solve the problem, it is essential to retard the recombination of electron-hole species in the semiconductors by molecular electron-relay semiconductor structures or electron-transport matrices. In this study, based on providing G-ZnS nanocomposite, in which more uniform and samller size of ZnS nanoparticles covered on graphene surface, we report an effective means on preparation of G-ZnS using Tween 20 as dispersing and stabilizing agent as well as growth template in the nucleation process of ZnS nanoparticles, GO as starting material, TAA as a homogeneous precipitation agent, via one-pot, an easy hydrothermal homogeneous precipitation method. Tween 20 constitutes with one hydrophobic aliphatic chain and three hydrophilic aliphatic chains with terminal hydroxyl groups. In the water-Tween 20 solution, Tween 20 boost the dispersion and stabilization of GO and ZnS, as well as control the nucleation process of ZnS nanoparticles in situ synthesis on GO surface. Compared with the kindred investigate, ZnS nanoparticles synthesized in this report have uniform smaller size and coated graphene nanosheets evenly. Acknowledgements. This work was financially supported by the National Natural Science Fundation of China (20975056 and 81102411); Shandong (ZR2011BZ004 and ZR2011BQ005); JSPS -NSFC Program (21111140014); and Taishan Scholar Program of Shandong Province (TS20070711); and the 973 special preliminary study plan (Grant no.: 2012CB722705).

Hexagonal boron nitride (h-BN) has emerged as a promising substrate for graphene electronic-devices. h-BN crystals are atomically thin (free from dangling bonds), contain low amount of charged impurities or surface charge traps and show small (~1.7%) lattice mismatch with graphene (reducing stress in graphene or ripple formation). Transferred CVD-grown graphene on h-BN has shown higher mobilities as compared with that on SiO2. However, the transfer process is either too complicated to apply for large scale transfer or ends up with contamination, wrinkling or breakage of graphene sheet. Direct growth of graphene on dielectric substrates provides the best solution. Here we present a two-step chemical-vapor-deposition (CVD) to grow graphene directly on h-BN thin layers. Raman spectra provide significant sharp 2D peak along with sharp D peak suggesting the growth of crystalline graphene islands. Atomic force microscopy confirms monolayer graphene islands. Study of variations in growth parameter suggests that atmospheric-pressure growth results in more nucleation sites with both thin lateral growth and thick vertical growth, whereas low-pressure growth favors lateral growth.

We have fabricated high soluble and high density-graphene ink by using chemical reaction of graphene oxide nanosheets. The reaction of graphene oxide nanosheet with amine-groups of organic compounds containing benzene-rings simultaneously induces the reduction of graphene oxide. The functionalized graphene nanosheets show a long-term stability in organic solvents and the organic compounds reacted with graphene nanosheets is easily desorbed at relatively low temperature. The graphene ink is well applied to ink-jet printing process with good dispersibility.

We predict that the magnetic moment induced by chemisorbed H atoms or any chemisorbed molecule via a single C-C bond on the top layer of a few-layer graphene system is tunable by an external electric field. Through first-principles electronic structure calculations, we show that this magnetoelectric effect is negligible in one-layer graphene, but becomes pronounced in bi-layer and tri-layer graphene, saturating in magnitude in quadri-layer graphene. The effect is due to shifting of the Dirac cone of the pure graphene layers relative to the bands of the hydrogenated layer, induced by the external field. The calculated magnetoelectric coefficient has values comparable to those found for ferromagnetic films or perovskites interfaces. All these results provide key information about formation and control of defect-induced magnetism in graphene materials.

Zigzag graphene nanoribbons (ZGNRs) are antiferromagnetic in the ground state with zero net magnetization due to the compensation of contributions from opposite edges. The uniform deformations (both shear and axial) do not produce magnetization due to the symmetry restrictions. However, one of the potential ways of inducing magnetism in GNR is through introduction of the strain gradient. We report the results of first-principles calculations that predict that the induction of net magnetization in the graphene nanoribbon upon non-uniform strain applied to the nanoribbon. Using density functional theory (DFT) method, we show that the bending or twisting of nanoribbon produces magnetization because in the presence of strain gradient the induced magnetization on opposite edges are not compensating each other. We estimate an average magnetization of ~ 3.3µB that produced from the bending of nanoribbon with the sinusoidal profile δx=Asin(2πz/L) with A= 3Å and L=87.4 Å (z=0..L/2, i.e. the half of the period). Our study suggests that the induced magnetization can be used for the control of magnetic structure in graphene including the trap of the domain walls.

Monolayer graphene has unique electronic and mechanic properties that attracted much attention of researchers for versatile applications of this material. Graphene in its pristine state is nonmagnetic. A magnetism can be induced in this sp² bonded carbon material by employing defects and impurities. It has been reported that zigzag edges of graphene nanoribbons show a characteristic magnetism due to localized electronic states. For practical applications, however, the graphene nanoribbons are inappropriate because two opposite zigzag edges are antiferromagnetically coupled so that the net magnetic moment is always zero. To overcome the difficulty, we introduced an odd number of zigzag edges into graphene by creating zigzag triangular holes with a specific size. Because of the frustration in antiferromagnetic paring of zigzag edges, graphene with zigzag triangular holes could always exhibit non-zero ferromagnetism. In this presentation, we discuss the stability, magnetization and electronic structure of graphene with zigzag triangular holes based on results of first-principles density functional theory calculations using Vienna Ab-Initio Simulation Package (VASP). We found that zigzag triangular holes with z₁- or z₁₁₂-terminated edges and hexagonal corners could be thermodynamically available in the experimental conditions. Magnetization of zigzag triangular holes strongly depends on edge and corner configurations as well as the size of triangle. The finite size effect was understood with electronic structure information, i.e., the location of the localized states and the Fermi level. Particularly, we proposed that zigzag triangular holes with z₁-terminated edges and hexagonal corners are most promising for designing thermodynamically stable spin-polarized graphene.

The potential of graphene for future electronic devices is dictated not only by its conductive but also its thermal properties. Overheating of charge carriers has important implications for devices. In graphene, significant overheating can occur because of the large difference between the heat capacity of the electron system with respect to the phonons [1] combined with a relatively weak electron-phonon coupling [2]. Therefore, it is important to accurately determine the electron temperature and understand how hot electrons influence conduction through the graphene device. We investigate the effects of hot electrons on carrier conduction through measurements of the quantum corrections to the classical (Drude) conductivity. It has been shown that these corrections [3] can be used as a direct measure of the electron temperature at thermal equilibrium [4]. We investigate how the electron temperature evolves with increasing electric field through detailed measurements of the weak localization (WL) and electron-electron interaction (EEI) corrections, and the universal conductance fluctuations (UCF). We measure the resistance of multi-terminal monolayer graphene devices on silica/silicon substrates at temperatures from 0.02 to 200K with applied AC and DC currents ranging from 100pA to 20mu;A. The WL and EEI corrections to the conductivity are determined from the temperature and magnetic field dependence of the resistance, and the UCF are measured as a function of carrier density. We determine the coherence length of the electrons from the WL and UCF measurements and show that it saturates at low temperatures. The saturation value is found to decrease linearly with increasing applied current. This is a direct demonstration of the overheating [2] and through analysis of the form of the WL [3] and statistics of the UCF [4] we determine the electron temperature far from equilibrium. We show that this temperature can exceed the lattice temperature by several orders of magnitude. The effect of the applied current on the EEI appears to be more complex and cannot be explained by a simple overheating model [5]. Our results indicate the possibility of a direct effect of the field on the interaction. [1] E. H. Hwang and S. Das Sarma, Phys. Rev. B 77, 115449 (2008) [2] A. S. Price, S. M. Hornett, A. V. Shytov, E. Hendry, and D. W. Horsell, Phys. Rev. B 85, 161411 (2012) [3] F. V. Tikhonenko, D. W. Horsell, R. V. Gorbachev and A. K. Savchenko, Phys. Rev. Lett. 100, 056802 (2007) [4] K. Kechedzhi, D. W. Horsell, F. V. Tikhonenko, A. K. Savchenko, R. V. Gorbachev, I. V. Lerner and V. I. Fal'ko, Phys Rev Lett. 102, 066801 (2009) [5] V. I. Kozub, A. M. Rudin, Phys. Rev. B 52,7853 (1995)

We experimentally study the effect of different scattering potentials on the 1/f noise observed in graphene devices on silica and silicon nitride substrates. The noise in nominally identical devices is seen to behave in two distinct ways as a function of carrier concentration, changing either monotonically or nonmonotonically. We attribute this to the interplay between long- and short-range scattering mechanisms. Water was found to significantly enhance the noise magnitude and change the type of the noise behaviour. By using a simple model, we show that water is a source of long-range scattering. Its presence on the graphene surface was found to increase the noise by an order of magnitude, yet cause a comparatively insignificant change in the resistance, which demonstrates that low-frequency noise and resistance in graphene can be determined by different scattering mechanisms. We have also shown that the 1/f noise at the Dirac point and at finite concentration originates from different sources of scattering and most likely from different fluctuation mechanisms.

Spintronics has already had a crucial impact on widely distributed applications such as magnetic memories (hard drives read heads, MRAMs) and sensors. Further propositions have been formulated toward potential post-CMOS logic architectures based on spin as the information vector. Efficient spin transport with large diffusion lengths in a lateral channel between spin polarized contacts is fundamental to this vision. However, despite several decades of intense research in particular with inorganic semiconductors, the required platform is still wanting. We will present magneto-transport experiments on epitaxial graphene multilayers on SiC connected to cobalt electrodes through alumina tunnel barriers. The spin signals are in the MOmega; range in terms of ΔR = ΔV/I [1]. This is well above the spin resistance of the graphene channel. The analysis of the results in the framework of drift/diffusion equations leads to spin diffusion length in graphene in the 100-300 mu;m range (as high as 285 mu;m) for a series of samples having different lengths and different tunnel resistances. Magnetoresistances up to 10% are observed. In the best case, the spin transport efficiency of epitaxial graphene is found to be of 75% of the ideal channel [1]. Our results show that graphene could be envisioned as a material of choice for transport/processing of spin information and unlock large scale logic circuits. Understanding the mechanism of the spin relaxation, improving the spin diffusion length and also testing various concepts of spin gate are the next challenges. [1] B. Dlubak, M.-B. Martin, C. Deranlot, B. Servet, S. Xavier, R. Mattana, M. Sprinkle, C. Berger, W. A. De Heer, F. Petroff, A. Anane, P. Seneor and A. Fert, Nature Phys., in press (2012). doi:10.1038/nphys2331

Being a true two-dimensional crystal, graphene has special properties. In particular, a point-like defect in graphene may have effects in the long range. This peculiarity questions the validity of using a supercell geometry in an attempt to explore the properties of an isolated defect. Still, this approach is often used in ab-initio electronic structure calculations, for instance. How does this approach converge with the size of the supercell is generally not tackled for the obvious reason of keeping the computational load to an affordable level. The present work addresses the problem of substitutional nitrogen doping of graphene. DFT calculations have been performed for different supercells. Although these calculations correspond to N concentrations that differ by 10%, the local densities of states on and around the defects are found to depend significantly on the supercell size. Fitting the DFT results by a tight-binding Hamiltonian makes it possible to explore the effects of a random distribution of the substitutional N atoms, in the case of finite concentrations, and to approach the case of an isolated impurity when the concentration vanishes. The tight-binding Hamiltonian is used to calculate the STM image of graphene around an isolated N atom. STM images are also calculated for graphene doped with few at% concentration of nitrogen.The results are discussed in the light of recent experimental data.

We report on the influence of UV-Ozone (UVO) treatment on the electrical properties of metal contacts formed to single layer graphene grown by CVD. Polycrystalline graphene was grown on copper foil and transferred onto a heavily doped silicon wafer with a 300 nm thick thermally grown SiO2 insulator using the method described previously [1]. E-beam deposited Ti (20 nm)/Au (80 nm) contacts were patterned by photolithography and a lift-off process. Just prior to depositing the contacts, the substrates were UVO treated in a commercial system for times ranging from 5 minutes to 25 minutes, where only the graphene surface in the lithographically-defined openings of the resist layer were exposed to UVO. The devices were completed by oxygen plasma etching the graphene in the field regions. For comparison, we fabricated test structures in parallel without UVO treatment. The electrical characteristics of the devices were measured in air at room temperature. All devices were found to have a high sheet carrier concentration at zero gate bias, i.e. the minimum in transconductance vs. gate-source voltage is shifted by many 10&’s of volts from VGS = 0V. We used un-gated TLM analysis to extract the sheet resistance from devices with L = 10.5, 16.5, 22.5, 28.5, 34.5, 40.5, and 46.5 mu;m and fixed contact width, W, of 10 mu;m. The combined contact resistance was taken from the extrapolated intercept at zero channel length. A linear regression fit the data well for long channel length, L > 10mu;m, where we assumed diffusive transport[2]. The extracted contact resistance showed a marked decrease from > 104 #8486;-mu;m without treatment to several hundred #8486;-mu;m with 25 minutes of UVO treatment. Sheet resistance remained relatively unchanged; Rs ~ 400 ohm/square. Raman spectroscopy was used to monitor the effect of UVO treatment, and from the minor increase in D-peak intensity we concluded that the graphene was not significantly degraded with UVO treatment. In summary, we found that contact characteristics were strongly dependent on interfacial conditioning prior to metallization. UVO was used as a simple process for cleaning resist residue and reducing the contact resistance. Using this approach, we obtained a contact resistance as low as 184±119 #8486;-mu;m that compares well with published record values [3]. [1] X. Liang et al., ACS Nano, 5, 9144 (2011), [2] A. Venugopal et al., J. Appl. Phys, 109, 104511 (2011), [3] A. D. Franklin et al., Electr. Dev. Lett., 33, 17 (2012)

We report on measurements of the Optical Hall effect [1-3] on epitaxial graphene samples, and present Landau Level transition features detected at 1.5 K as a function of magnetic field up to 8 Tesla. The epitaxial graphene samples, grown on C-face SiC, reveal Landau transitions which are detected in reflection configuration at oblique incidence for wavenumbers below, across and above the silicon carbide reststrahlen range. Small Landau transition features are enhanced across the silicon carbide reststrahlen range due to surface-guided phonon-polariton coupling with the electronic Landau transitions in the graphene layer. We analyze the spectral and magnetic-field dependencies of the coupled resonances, and compare our findings with previously reported Landau transitions measured in transmission. Additional features resemble transitions previously assigned to bilayer inclusion [4], as well as graphite [5]. We present and discuss a dielectric function tensor model description to account for the electromagnetic polarizability of the graphene layers, and which is sufficient for quantitative model calculation of the Optical Hall effect data. We present and discuss the transition energies and amplitude parameters. The optical-Hall effect measures the generalized ellipsometry parameters, i.e., the normalized Mueller matrix elements, of samples placed within external magnetic fields, as a function of wavenumber and magnetic field strength and direction. Our setup was described previously, and currently measures spectral data from 100 GHz to 210 THz [1]. We propose use of the Optical Hall effect for accurate characterization of the magnetooptic response of epitaxial graphene in reflection, and for quantitative analysis of Landau transition features. [1] T. Hofmann et al. Rev. Sci. Inst. 77, 063902 (2006); (Opt Hall) [2] T. Hofmann et al., Rev. Sci. Instrum. 81, 023101 (2010) (THz GSE) [3] T. Hofmann et al., Appl. Phys. Lett. 98, 041906 (2011) (graphene+RT OHE) [4] M. Orlita et al., PRB 83, 125302 (2011) [5] M. Schubert, Ann. Phys. (Leipzig) 15, No. 7 - 8, 480 - 497 (2006)

The electronic structure of graphene corresponds to a semi-metal with π-electrons at Fermi level, which are responsible of the unique electronic properties for this material. Graphene nanostructures show an energy gap resulting of the finite size, and are of current interest because of the potential applications in electronic and optoelectronic devices. Thus, we discuss some recent progress in the synthesis of graphene nanoflakes obtained from the reaction of polyaromatic hydrocarbons and from fullerenes or nanotubes. In this work, we are presenting ab-initio results for the electronic properties of graphene nanoflakes of different effective radius (R) and different shapes. We find that the, Kohn-Sham gap decreases with size as 1/R, as predicted by the simple confinement model, while the quasi-particle energy gap shows a deviation from that scaling rule. Secondly, the electronic structure of graphene nanoflakes with edge chemical modifications using different atoms and functional groups is analyzed. The results show the feasibility of tailoring the electronic and optical properties of graphene nanostructures by size, shape, and edge doping. Work supported by Conacyt-México under Grant No. 83604.

Graphene nanoribbons (GNRs) attract a lot of interests not only due to possible applications to future electronic devices but its unique electronic characteristics owing to the presence of edges. Depending on the edge character, zig-zag or arm-chair type, the electronic structure of GNRs shows different features from the one of 2D graphene [1]: a localized flat band and a band-gap, respectively. We have fabricated GNRs via epitaxial growth on vicinal SiC surfaces [2]. In this presentation we show the results of micro Raman spectroscopy of such GNRs in terms of electron-phonon interactions at edges. GNRs were grown on vicinal SiC surfaces (off toward <1-100>), which indicate ordered nanofacet structures with the typical (0001) terrace width of 10nm after high temperature hydrogen gas etching. Molecular beam epitaxy enables selective growth of a graphene buffer layer on (0001) terraces, resulting in nanoribbons with ~10nm width. The sample was then hydrogenated to transform the buffer layer to graphene [3]. Epitaxial relationship between SiC and graphene suggests the arm-chair edges of GNRs grown by MBE. A Raman spectrum shows typical graphene features including clear D-, G-, and G&’-bands. It is noticed that the D-band intensity is high compared to G-band, which implies the strong contribution of arm-chair edges acting as elastic scattering centers. This evidences the arm-chair edges (Raman active) present at the GNRs. Polarization angle dependence of the D-band intensity also supports this as it follows the cos4(theta;) law predicted by theories [4]. In comparison, GNRs with zig-zag edges grown on different vicinal SiC surfaces (off toward <11-20> ) is investigated and found no such polarization dependence. References [1] K. Nakada et al., Phys. Rev. B 54, 17954 (1996). [2] H. Nakagawa et al., Phys. Rev. Lett. 91, 226107 (2003). [3] C. Riedl et al., Phys. Rev. Lett. 103, 246804 (2009) [4] K. Sasaki et al., Phys. Rev. B 82, 205407 (2010).

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition, and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these collapsed wrinkle structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide folded wrinkles, we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics. *This work is published on-line in Nano letter, DOI: 10.1021/nl300563h, 2012

Topological defects (TD) in a material greatly influence various properties including its stability and deformation behavior, particularly so of a nano-material like graphene. Stone-Wales (SW) defect a commonly observed TD in graphene, is associated with a pair of pentagons and heptagons that arise from a 90^o rotation of its C-C bond. Here, we identify signatures of SW defects in the vibrational and electronic properties of graphene using first-principles Density Functional Theory (DFT) calculations. We find that the energy of interaction between SW defects exhibits a power law dependence on the separation between the defects, which we explain using continuum analysis. We determine the shifts in the G and D bands in the Raman spectra of graphene due to SW defects as a function of their concentration, which will be useful in interpretation of observed Raman spectra of graphene. We predict long wavelength rippling instabilities associated with a SW defect, consistent with earlier work ^[1], and present a quasi-continuum theory to show that the origin of these rippling instabilities is in the coupling of G band and strain. Our work shows that the effects of SW defect on electronic structure are quite anisotropic and subtle, which misled earlier works to conclude opening of band gap due to SW defects ^[2,3]. While a band gap opens up at K-point, we show that the Dirac cone (with vanishing gap) is shifted from K to K±δk in the Brillouin zone (BZ), in the direction dependent on the the orientation of a SW defect. Through a thorough study, we find that SW defects (just like the G band) lead to shift in the Dirac cones in the BZ from K to K±δk. References [1] Jie Ma, Dario Alfè, Angelos Michaelides and Enge Wang, Phys. Rev. B, 80, 033407 (2009). [2] Xiangyang Peng and Rajeev Ahuja, Nano. Lett. 8, 4464 (2008). [3] Valentin N. Popov, Luc Henrard and Philippe Lambin, Carbon 47, 2448 (2007). [4] Sharmila N. Shirodkar and Umesh V. Waghmare, in preprint.

Single atomic layer graphene, a two-dimensional network of sp2 carbon atoms, has already revealed a cornucopia of interesting physical and chemical properties. The electronic transport properties in graphene can be modified by a variety of ways. Charge transfer to and from absorbed molecules can shift the Fermi level with no substitutional impurities and thus has aroused growing interest nowadays. Few layer graphene exposed to Br2 and I2 were studied by Raman spectra in 2009, proving that single-layer graphene was doped by adsorbing I2 on the surface. However, up till now, no detailed electronic transport measurements of iodinated graphene at low temperatures or under magnetic fields has been reported. Recently, reversible fluorination, chlorination and bromination of few-layer graphene were also found. The halogenated graphene decomposes on H2/Ar annealing or on laser or electron beam irradiation. But the electronic transport properties of reduced fluorinated graphene were unsatisfied, which may limit the applications of this doping method. We report reversible charge transfer chemical doping of high quality single-layer graphene. The electron mobility of the pristine graphene devices was estimated to be more than 16,000 cm2 V-1 s-1. We first carried out an in-situ measurement of single-layer graphene under iodine doping at both room temperature and liquid helium temperature. The measurement results demonstrated that intrinsic graphene is greatly sensitive to different doping concentration of iodine vapor. At the highest doping concentration, iodine-doped graphene exhibits metallic characteristics as evidenced by linear relationship between resistance and temperature and by the positive magneto-resistance observed and can be fitted using Kohler's rule. The half-integer quantum Hall effect disappeared thoroughly after iodine doping. In Raman spectrum, two peaks at around 110 and 160 cm-1 were observed, proving that the iodine forms triiodide and pentaiodide on graphene sample. Then, a post immersing method was introduced. We put the iodine-doped graphene sample in alcohol at room temperature for 60 minutes. High solubility of iodine in alcohol helped cleaning the graphene automatically. The graphene samples completely recovered from the state of being doped after we took it out of alcohol. Clear half-integer quantum Hall effect can be seen again at B=8T at liquid helium temperature. Our observation opens a new way of iodination and deiodination of single-layer graphene without any permanent damages to it.

Graphene and few-layer graphene have an increasing number of applications in optoelectronics and photonics. Applications in lasers, optical modulators, switches and ultrafast photodiodes are only few of the latest developments. Performances are comparable if not superior to those attainable with typical inorganic semiconductors and promise graphene to be a material of choice. Device development have considered both mono- and multi-layer graphene, because of the interesting optical and electrical properties of bi- and tri-layer, originating from the more rich electronic band structure. Predicting the intrinsic limits of these devices in terms of modulation speed, sensitivity, saturation is an important task, which ideally involves the investigation of exfoliated mono- and multi-layer graphene with sophisticated optical probes. Recently, we have determined the carrier cooling rates in bi-layer graphene by pump-probe optical spectroscopy, unraveling the turnover between optical and acoustic phonon cooling and its dependence on carrier density [1]. Carrier cooling is very important for heat dissipation in devices, but it is not the only process influencing performance. For example, the switching speed in photodiodes depends on the intrinsic carrier recombination, but it is hardly accessed in real devices because of its convolution with carrier sweep out time. In this communication we determine the intrinsic carrier recombination time in exfoliated few-layer samples using femtosecond near-infrared (0.8-0.35 eV photon-energy) pump-probe spectroscopy. The spectra of the different single flakes show an evolving structure of photoinduced absorption bands superimposed on the bleaching caused by Pauli blocking of the inter-band optically coupled states. Supported by tight-binding model calculations of the electronic structure, we assign the photoinduced absorption features to inter-subband transitions as the number of layers is increased. Interestingly, the inter-subband photoinduced resonances show a longer dynamics than the inter-band bleaching, because of their independence from the absolute energy of the carriers with respect to the Dirac point. The dynamic of these inter-subband transitions depends only on the intrinsic carrier lifetime and provides an elegant method to access it in this important class of carbon nanostructures. We report lifetimes of 5 ps almost independently from the layer number up 4-layer graphene. Above such number of layers the carrier recombination time falls below 4 ps and is identical to what we observe in graphite. References [1] T. Limmer, A. J. Houtepen, A. Niggebaum, R. Tautz, E. Da Como, Applied Physics Letters 2011, 99, 103104.

The demand for large-scale continuous single crystal graphene is increasing rapidly. The fabrication of high-quality graphene has to be based on a deep understanding of the physicochemical parameters that control the chemical vapor deposition (CVD) process. Here we model the CVD growth of graphene by developing a modified Langmuir theory of surface adsorption and two-dimensional crystallization. The model provides criteria to select the adequate processing conditions to fabricate continuous, pin-hole free films. This model is combined with our experimental analysis to identify the range of parameters that define three different phases on the substrate surface: continuous graphene layer, partial graphene coverage, and carbon adsorbates with no graphene growth. The predictions of the model match well with our experimental results and with other experimental data from literature over a wide range of growth conditions. On the basis of our model and the experimental analysis of graphene nucleation and growth, novel input for engineering wafer scale continuous monocrystalline graphene films is provided.

Graphene growth on metal films in chemical vapor deposition (CVD) process is a promising method for production large-scale graphene films. The number of layers in graphene film can be determined by solubility of carbon in metal film and kinetics of graphene nucleation. In this study we perform combined atomistic and analytical study of graphene nucleation kinetics on the Ni(111) surface. Atomistic modeling is based on reactive empirical potentials for Ni-C system and molecular dynamics method. Analytical theory is based on reaction-diffusion model of carbon transport on Ni film, which includes carbon dissolution in Ni film and surface nucleation of graphene. The parameters of this model were determined from first-principles calculations. We find that carbon penetration in the nickel film strongly depends on deposition temperature. Only at low temperatures graphene nucleation can take place without significant dissolution of carbon in the nickel film. At typical CVD temperatures carbon prefer to fill subsurface layer before nucleation of first graphene layer. Thus temperature management can be used to control number of layers in growing graphene film.

Since the entangled excitation energy of the (TEAM) and the preserving of the quantum state of the topological position of the special carbon element is sensitive to the relaxation time , its duality position is suggested to be singlet which is investigated during relaxation time . The dynamical displacement of the divorce atom ( broken tripled bound ) could be detected (exist ) in each away the system stay on unstability where the zigzag edge is manifested , during the TEAM electron beam and the system transform to the zz57 edge during the relaxation time where there is no external beam . the armchair edge is stable until a carbon atom divorce from edge .Thus the beam divorce a carbon atom from the edge and leaving dangling atom , the dangling carbon atom try to participate in the edge structure so it move around the edge to find the place to take stability . Thereupon if there is no any external energy to the system the hole edge is stable with zz57 edge or armchair edge after a period of time.

Graphene, a mono-layer of sp2 hybridized carbon atoms, has seized great attention due to its extraordinary electrical and mechanical properties. Recently, chemical vapor deposition (CVD) method with CH4 or C2H2 gases has been demonstrated as most promising way among the various methods to produce a large-area and highly functional graphene. In the CVD method, however, additional transfer process must be included to step forward to practical applications and commonly used carbonaceous gases in the high temperature process are explosive. To overcome this obstacle, our group reported simple and scalable synthesizing process with polymers, which avoids physical transfer processes. In this work, we have obtained mono-layer and few-layer graphene directly converted from coal tar pitch films on a Si/ SiO2 substrate. The coal tar pitch, which contains approximately 93% of carbon as the form of condensed aromatic compound, is a residue produced by distillation or heat treatment of coal tar. Graphene converted from this waste oil contributes to the global environmental problem as well as reduces the related waste-disposal costs. The Ni capping metal on the coal tar pitch film plays both roles of a passivation layer on the evaporable coal tar pitch film and a metal catalyst for efficient graphene growth. The optimal annealing time is relatively short (< 5 min), compared with CVD method. After graphene growth, the metal layer can be removed easily by placing the sample directly in metal etchant. Furthermore, patterned graphene can be simply obtained by patterning the metal capping layer via shadow evaporation on the coal tar pitch films. The graphene recycled from coal tar pitch grown on dielectric substrate was directly used as electrodes for field-effect transistors. In conclusion, we demonstrate that coal tar pitch-derived graphene growth directly on inert substrates in active electronic devices will have great advantages because of its easy, safe, eco-friendly and scalable synthesizing process.

Graphene Oxide (GO) and reduced graphene oxides (RGO) have many outstanding physical and mechanical properties. Uniform and thickness controllable RGO films with large area have been prepared by evaporation-induced self-assembly at a liquid/air interface on glass substrates in combination with low temperature thermal reduction at 200 degree Celsius. The low-temperature reduction process has the advantage of complete compatibility with flexible or other substrates. The films are of centimeter scale and their thickness can be controlled. The structrual evolution and surface morphology of the films have been characterized. The obtained thermal reduction graphene oxide films exhibit excellent optical properties and a high elastic modulus of 76.18GPa and a hardness of 6.89GPa.

We will show that a low energy focused scanning electron beam can locally pattern graphene coated with a thin ice layer. The irradiated ice plays a crucial role in the process by providing activated species that locally remove graphene from a silicon dioxide substrate. After patterning the graphene, the ice resist is easily removed by sublimation to leave behind a clean surface with no further processing. The procedure is performed in-situ in a modified scanning electron microscope. Desirable structures such as nanoribbons are created using the method. Defects in graphene from electrons backscattered from the bulk substrate are identified. They extend several microns from the e-beam writing location. We will show that these defects can be greatly reduced and localized by using thinner substrates and/or gentle thermal annealing.

Nanometer-sized graphene, nanogrphene, has attracted much attention due to its unique characteristics and has been applied to nanoelectronics devices in the form of nanometer-sized interconnect and quantum dot. Recently, we propose the novel production methods for graphene using supercritical fluid. The starting materials are first immersed in supercritical fluids (SCFs) and then subjected to rapid depressurization (quenching). It is supposed that during such process, SCF diffusion between the graphene layers and/or thermal shock peel graphene sheets off. Using this method, high-quality, gram-scale processable graphene can be obtained without acid oxidation which leads to an introduction of undesirable oxygen functional groups and the degradation of ideal graphene/nanographene properties. In this study, we indicated that nanographene with a narrow distribution of the lateral size was obtained using SCF exfoliation, when we use platelet carbon nanofiber as a starting material [T. Tomai et al., Appl. Phys. Lett. 100, 233110 (2012).]. Moreover, it was observed that the lateral sizes of the thinner graphene sheets were distributed within a smaller range. This result suggests that during the exfoliation process, cutting of the basal plane took place. We also conducted Raman spectroscopic analysis for identifying the graphene quality and it was revealed that the defect density of nanographene can be reduced to less than that of the starting material by means of SCF exfoliation. In MRS Fall Meeting, the dependence of defect density of nanographene on the temperature protocol and on the starting material will be discussed in detail.

Single-layer graphene has emerged since 2004 and created a new research branch for high-speed electronics and ultrafast laser optics. The first graphene was obtained by exfoliating the graphite with sticky tapes, but this approach was lack of efficiency and reproducibility. Ultrasonic treatment of graphite was introduced later. However, the flaked graphene was small in size. The chemical-vapor deposition method was one efficient way to grow large-area and high-quality graphene, but this approach had high criteria on processing environment. Recently, the electrochemical exfoliation of graphite was proposed as a new-class technology, which could easily and rapidly produce high-quality few-layer graphene. Nevertheless, the effect of anodic oxidation to the quality of graphene was seldom discussed. In this work, the few-layer graphene obtained by electrochemical exfoliation are investigated to clarify the graphene oxidation phenomenon at changing biased conditions. By precisely controlling the bias with centrifugation, the quality and uniformity of few-layer graphene was analyzed. In experiment, the highly oriented pyrolytic graphite sheet was used as anode, and a Pt wire served as cathode. The electrolyte was prepared by adding 2.7 mL of 96% H2SO4 into 100 mL of DI water. Subsequently, both electrodes were dipped into the electrolyte and separated by 5 cm. The DC bias was applied to electrochemically exfoliate the graphite sheet at anode with its voltage varying from +3 to +10V. Afterwards, the graphite flakes floating on the surface of aqueous electrolyte solution was filtered by 1-mu;m porous filter, dispersed in acetone and then ultrasonic treated for 5 min. The supernatant was taken to fabricate the graphene film. The mixture was centrifugated at 550xg for 2 min. Despite the wide thickness distribution of as-exfoliated flakes, the graphene could be controlled with nearly identical size and layer number after centrifugation. With enlarging biased voltage, the electrochemical exfoliation rate of graphite was accelerated. The sulfate ions in electrolyte invade the graphite electrode violently to cause thicker flakes with heavier surface oxidation. The quality of centrifugated graphene was identified by comparing the Raman intensity at D-band (1350 cm-1), G-band (~1580 cm-1), and 2D-band (~2680 cm-1). The D/G band intensity ratio increased from 0.67 to 1.05 with increasing bias from +3 to +10V, indicating that the graphene with higher defect density due to enhanced oxidation was obtained at larger biased voltage. In opposite, the 2D/G mode intensity ratio was decreased from 0.33 to 0.2 with increasing bias, revealing that the graphene with reduced layer number and lower degree of oxidation can be obtained under low-voltage electrochemical exfoliation. This observation concludes the low-bias electrochemical exfoliation of graphite sheet facilitates few-layer graphenhe generation with suppressed oxidation.

Graphene nano-ribbons (GNRs) are one dimensional structures with high carrier mobility, high switching speed and width dependant band gap. These properties make them exciting materials in the fabrication of nanoelectronic devices. In recent years, there have been attempts to derive GNRs from carbon nano-tubes (CNTs). In this work, we introduce a novel sequential method for obtaining GNRs from CNTs. Multi-wall CNTs grow vertically on a Silicon substrate in a DC- Plasma enhanced chemical vapor deposition chamber at 700 C using a gas mixture of methane and hydrogen. By means of high precision photolithography we have obtained parallel lines of nickel as the seed for the growth of CNT followed by the vertical growh of CNTs in the same manner. Vertically-aligned CNTs are released from the substrate by the etching of underlying Si using SF6 palsma. Then, they are transferred onto another Si substrate in a horizontal manner through immersing in Di-methyl-formamide (DMF) solution. Horizontal CNTs are then exposed to a sequential passivation and etching process in a reactive ion etching unit. A mixture of hydrogen, oxygen and SF6 gases are used to result in proper unzipping of horizontal CNTs. While hydrogen plasma is believed to be responsible for the unzipping of the nanotubes, the passivation step is needed to avoid damaging the sidewalls of the previously ruptured walls. The formation of GNRs has been investigated using field-emission-SEM, transmission electron microscopy, Raman spectroscopy and atomic force microscopy. Graphene nanoribbons with a height of 2-3 nm have been obtained from CNTs with a diameter of 30-50 nm. For partially processed structures, we have observed half-zipped features where the central tube of CNTs is quite visible. Raman spectroscopy corroborates the conversion of nanotubes into graphene sheets by changing the intensity of the G-line with respect to D-line. In addition, we have used a high temperature treatment of nanotubes in a hydrogen plasma environment and observed an anomalous swelling of CNTs prior to unzipping. Further investigation of these structures using Raman and FTIR spectroscopy proves the incorporation of hydrogen in the CNTs and has caused a blue shift in the Raman spectrum.

We present a new method to extract the optical conductivity from infrared transmittance for large area chemical vapour deposited (CVD) graphene on silicon and apply it to experimental data in the photon energy range of 370-7000 1/cm. The optical conductivity does not only allow us to derive the carrier mobility and density but also gives information on the number of graphene layers and reveals the existence of polymer residue after transfer. Infrared transmittance combined with this method could be used in addition to Raman spectroscopy as a fast and easy standard non-invasive tool for assessing the quality of CVD graphene.

Ultra-thin flakes of layered materials have recently been attracting widespread research interest due to their exotic properties. In this work, we study the optoelectronic response of a hybrid of two such materials: graphene and MoS₂. Our devices consist of mechanically exfoliated graphene flakes transferred on top of similarly exfoliated MoS₂. The electrical response of the hybrid is studied in the presence of white light. We show that the resistance of the device is modulated in the presence of light and that the induced photoconductivity is persistent. We exploit this property to show a device design which can act as an optoelectronic switch. The effect of gate voltage on the responsivity of the system is also reported. We have also extended our studies to CVD (chemical vapor deposition) - grown graphene transferred onto MoS₂ which show qualitatively similar features, thereby attesting to the scalability of the device architecture.

Attaching fluorine atoms to the graphene surface is one possibility for controlling electronic properties and achieving new functions in graphene. We propose a well-controlled method of fluorinating both mono- and bi- layer graphene using Ar/F₂ plasma. Graphene samples on SiO₂ (285 nm) / Si substrates were prepared by mechanical exfoliation. Using a reactive ion etching system, samples are exposed to Ar/F₂ (90% / 10%) plasma at 0.1 Torr with RF power of 5 W and a 75 sccm gas flow rate. The reaction time ranged from 0.5 to 30 min. To prevent the plasma damage, substrates were placed face down on the RF electrode and then the graphene samples are mainly exposed to fluorine radicals. The characterization is based on Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), and carrier transport measurement. We can monitor fluorination degrees by Raman spectroscopy because F atoms form sp³ hybridized C bonds and act as point defects. We found controllability of fluorine contents by changing the reaction time and reactivity difference between mono- and bi- layer graphene, that is, changes of Raman spectra in bilayer graphenes are less drastic than monolayer graphenes. Reversibility checked by annealing evidenced successful fluorination. We annealed fluorinated graphene samples in Ar atmosphere for 90 min at 573 K. By annealing, D peak intensities of Raman spectra were significantly weakened in both mono- and bi- layer graphene, indicating reversible fluorination. To our knowledge, reversibility in plasma-fluorinated bilayer graphene has not been reported yet. XPS measurements were done to estimate fluorine content. For this experiment, a graphene sample was prepared by transfer of chemical vapor deposition (CVD)-grown graphene on Cu foil to the same Si substrate. This large area monolayer graphene sample was fluorinated for 3 min. The C1s spectrum consists of several largely shifted components which can be attributed to each types of carbon atom's bonding. The percentage of C atoms that form C-F bonding in graphene is estimated to 18 % by analyzing this C1s spectrum. We also measured transport properties of fluorinated graphene. Conventional graphene FET device was made by electron-beam lithography and evaporation of Ti/Au electrode. Firstly, we measured ρ-V_G (back gate) characteristics of pristine graphene device in vacuum. V_G of charge neutrality point (CNP) was around 0 V for this pristine device. After that the device were fluorinated for 0.5 min and measured again. By fluorination, graphene was largely p-doped and CNP was moved to V_G larger than 60 V, which was the limit of the measurement. Value of resistivity after fluorination was around twice of pristine device. This result indicates possibility of non-invasive p-type doping of graphene by relatively low F/C fluorination. More dramatic change of transport properties can be expected by further fluorination.

We demonstrate significant selective tip enhanced Raman spectroscopy (TERS) of graphene single and multiple layers on insulating silicon oxide substrates. A unique TERS probe with a gold nanoparticle embedded in a glass surrounding gives high dielectric contrast with a defined plasmonic resonance and without Raman background. Significant enhancement of the second-order phonon mode band of graphene flakes is observed. Such a probe acts under normal force sensing mode for mapping of fine structures such as graphene single layers, provides a complete free optical axis for top viewing of opaque samples and has considerable enhancement that does not require a conductive substrate for Raman enhancement through what has been defined as a gap mode. Specialized AFM scanning protocols of Raman difference spectroscopy have been implemented for TERS effectively discriminating between the near-field and far-field contribution of the scattered Raman signals. Such Raman difference protocols provide a selective enhancement of graphene single layer in-plane vibrational modes with lower effect of subsequent grapheme layers. Furthermore, such scanning protocols allow for maximizing Z polarization excitation even when using conventional Raman excitation with a Gaussian focused laser beam.

Monolayer Graphene, due to its high electron mobility (vs. Si) is identified to be an attractive material for transistor channels that can extend Moore&’s law. One of the key deliverables is achieving low source and drain parasitic contact resistance. This requires careful characterization of the surface of graphene in order to improve understanding of the metal to graphene interface. Far from the ideal picture of a 2-D crystal, TEM analysis has identified local corrugations called ripples, with height range from 3-5Å. Additionally, larger features, called wrinkles, with height up to 100Å have been reported for samples prepared by peeling Kish [1]. It is also shown through STM measurements, that these wrinkles are ubiquitous and that their conductance is lower than flat regions [1]. In this paper, for the first time, we employ conductive AFM to characterize the electrical and mechanical properties of these wrinkles as a function of the applied force. The graphene films in our study were prepared using CVD process followed by mechanical exfoliation on SiO2/Si substrate [2]. We apply a wide range of mechanical force (1.24 to 300 nN) to both wrinkles and flat regions. The IV curve is non-linear, we observe for both regions an increase in the current with applied force, followed by saturation for forces greater than 175 nN for the wrinkles and 125 nN for the flat regions. We calculate the ratio between the conductance (dI/dV) of the wrinkles and the flat regions. From the ratio vs. force curve we deduce that the wrinkles conductance is reduced by 14% in the non-plateau regime. The plateau conductance value for the wrinkles is reduced by only 6% compared to the flat regions. Our theoretical analysis of the IV data using a quantum mechanical tunneling model indicates the presence of a tunnel gap between the Pt/Ir coated AFM tip and the Graphene. It appears to be related to the Fermi level difference. The width of the wrinkles ranged from 31.5 to 140nm, while the height range was 1-9nm. We measure an average density of 8 wrinkles/um2. We also assessed the mechanical behavior of the wrinkles subjected to various force levels. The height of the wrinkle is reduced with increasing force, at 2X-fold increased force value the height is reduced by half. We further show that applying the same forces leads to a plastic deformation. In summary and based on the above mentioned results, we plan to investigate if such wrinkles impact the electrical contact resistance of large area structures, and how critical is the requirement of having an ideal 2-D sheet (wrinkle-free). The latter will be presented at the conference. [1] K. Xu, et al. Nano Lett, 9, 4446-4451 (2009) [2] X. Li et al., Science, 324 1312-1314 (2009)

Graphene has much potential for applications of nanoscale electronic devices and NEMS due to its notable intrinsic properties such as electron conductivity, mobility, flexibility, and mechanical strength. Because of its unique mechanical characteristics, frictional properties of graphene may be very different from other bulk materials. In some sense, graphene is an ideal model system to study the nanoscale friction because it is rigid within plane, but very soft for the out-of-plane deformation. Very recently, ultra-high vacuum friction force microscopy (FFM) experiment shows that nanoscale friction on graphene is 6 times increased after fluorination of the graphene surface. To understand the unusual enhancement of frictional properties, we calculated elastic properties of graphene and fluorinated graphene using first-principles density-functional theory (DFT) as implemented in Vienna Ab-Initio Simulation Package (VASP). Results of DFT calculations show that the out-of-plane bending stiffness of graphene increases up to 4-fold when fluorinated. Combined with the experimental fact, the theoretical finding indicates that the less compliant fluorinated graphene exhibits more friction. This tendency, however, contrasts to the convention in three-dimensional systems, that is, more compliant materials exhibit more frictions. Therefore, the mechanics of tip-to-graphene contacts would be characteristically different from that of conventional solid-on-solid contacts. Here, we devised a theory of nanoscale friction in the graphene-on-solid contact, analogous to the theory of nanoscale friction in solid-on-solid contacts [1]. Our theory suggests that the total lateral stiffness related to friction force is associated with the out-of-plane bending stiffness in two-dimensional systems. Therefore, the enhanced nanoscale friction on fluorinated graphene can be attributed to the increased bending stiffness of chemically modified graphene. On the basis of the FFM experiments and DFT calculation results about fluorinated graphene, we propose that frictional energy in two-dimensional systems is dominated by the out-of-plane deformation mode or the flexural phonons [2]. [1] R. W. Carpick, D. F. Ogletree and M. Salmeron, Appl. Phys. Lett., 70, 1548 (1997). [2] S. Kwon, J.-H. Ko, K.-J. Jeon, Y.-H. Kim and J. Y. Park, Nano Lett., submitted (2012).

Robust methods to chemically tune the unique electronic properties of graphene are in great demand due to the potential of the 2-D material to impact a range of device applications. We demonstrate substitutional doping of graphene by individual N atoms, B atoms, and B-N dopant pairs, respectively. Combining x-ray core level spectroscopy, scanning tunneling microscopy and density functional theory, we present a detailed picture of the diverse effects of different dopants and dopant bond type on the local electronic structure, carrier concentration and work function in chemically doped graphene. These findings have important implications for the potential of graphene as a transparent contact in organic electronics, especially photovoltaics, where a tunable work function material is highly desirable.

Chemical functionalization in graphene is a potential approach to engineer its electronic properties and make it more desirable and controllable for electronic device applications. We experimentally investigate the impact of chlorine adsorbates on the structure and properties of graphene. Raman spectra are compared before and after chlorine plasma treatment to characterize the change in defects and doping level in graphene. Subsequent XPS measurements are used to quantify the coverage percentage of chlorine on the surface. Furthermore, we conduct transport measurements on chlorinated graphene transistors to examine how chlorination modifies graphene properties, in terms of carrier concentration, mobility, sheet resistance and conductivity. It is found that exposure to chlorine plasma is an effective way to tune the Fermi level of graphene carriers toward the p-type direction, without sacrificing its high conductivity.

Graphene does not exhibit an electronic band gap which hinders its use for electronic applications. One way of addressing this problem is to attach functional groups to its graphitic backbone via chemical oxidation, which can be accomplished in a scalable and economical way. However, this chemical reaction is very difficult to control and reproduce, and it yields a very heterogeneous material, where different sheets are oxidized to different degrees, rendering the material impure with respect to its optoelectronic properties. We aim to address this problem by separating GO into fractions with narrowly-defined band gaps. We therefore employ molecules that selectively adsorb on graphene sheets with a certain degree of oxidation. This amplifies physical differences of the sheets, and they can be separated according to their electronic properties using centrifugation. We have recently demonstrated adsorption selectivity with simple ionic surfactants such as sodium dodecyl sulfate (SDS). Using liquid-cell atomic force microscopy, we visualize the adsorption pattern of the surfactants on GO with spatial resolution of about 5 nm. We find that SDS does not adsorb on strongly oxidized GO with an atomic carbon-to-oxygen (C:O) ratio of 2.0, which can be explained by electrostatic repulsion. As GO is reduced and its negative surface charge diminishes, we see how the GO sheets are populated by an astonishing pattern of micellar surfactant surface aggregates with diameters of 5-7 nm. In this regime of partial coverage, SDS preferentially adsorbs to the more reduced areas. We image this adsorption distribution at the solid/liquid interface by AFM, thus visualizing the variations in surface chemistry of the sheets with 5 nm resolution. Full coverage of the sheets is observed at a C:O ratio of 10.7. Cationic surfactants, in contrast, exhibit increasing coverage as the C:O ratio decreases. Fine-tuning of this band gap-selective adsorption can be achieved by varying the mixture and concentration of surfactants, as well as other solution parameters. We envision this approach will permit the tailor-design of GO with well-defined optoelectronic properties.

Oxygen as an impurity is known to degrade conductivity in graphene, but annealing at moderate temperatures has been found to reverse the effect. Here we report first-principles calculations of oxygen binding and reactions on graphene that elucidate the underlying physics. We find that two oxygen atoms can form an oxgyen dimer that can desorb from graphene with an overall activation barrier of 1.3 eV. However, it is energetically favorable to form even larger oxygen clusters. Once oxygen triplets form, oxygen can be removed in a more complicated reaction in which C atoms in graphene are consumed, leaving vacancies in graphene. Kinetic Monte-Carlo (KMC) calculations at finite temperature show that the concentration of both vacancies and equilibrated oxygen depend on the initial concentration of oxygen atoms and the annealing temperature, and an effective anneal of oxygen is possible only in case of a very high temperature and very low initial oxygen concentration. Overall, we find that thermal annealing to remove oxygen leads to etching of graphene. Existence of structural defects is also crucial in the oxygen dynamics and reaction. We find that structural defects such as Stone-Wales defects and grain boundaries attract and bind oxygen atoms due to the local strain. Thus, because of the small migration barrier of oxygen atoms on graphene, oxygen may migrate towards grain boundaries and aggregate to form oxygen clusters. Moreover, the strained C-C bonds at grain boundaries are easier to be broken, and indeed we find that the reaction of a triplet oxygen cluster has a lower energy barrier than on pristine graphene, indicating grain boundaries, which exist in most macroscopic graphene samples, facilitate the oxygen reactions and would be etched preferentially upon exposure to oxygen. If H atoms coexist with oxygen, an oxygen atom can bind to an H atom forming an OH group, which can also be removed by thermal annealing due to the weak binding, resulting in defect-free graphene. However, the newly formed OH groups may interact with surface oxygen atoms, forming various complex clusters, limiting the migration of oxygen species. This work was supported by DTRA Grant No. HDTRA1-10-1-0016 and the William A. and Nancy F. McMinn Endowment at Vanderbilt University. The calculations were performed at the DoD Air Force Research Laboratory Supercomputing Resource Center.

The potential of graphene, atomically thin layers with excellent mobility and strength, has motivated the research towards future electronics. However, implementation of graphene in practical electronic devices has been hindered owing to its semi-metallic behaviors or a zero-band gap nature. Here we describe the synthesis of graphene sheets decorated with ultra-thin Si-rich two-dimensional (2D) islands (Gr:Si sheets), in which the electronic property of graphene is modulated by coupling with the Si-islands. Analyses based on Transmission electron microscopy, atomic force microscopy, and electron and optical spectroscopies confirmed that the Si-islands with a thickness and lateral size. The field-effect transistors (FETs) based on Gr:Si sheets exhibit the enhancement of the transconductance and maximum to minimum current level compared with bare-graphene-FETs, and those magnitudes increase gradually with increasing the coverage of Si layers on the graphene. Moreover, temperature dependent current-voltage measurements show the increase in the resistance with decreasing temperature, which confirms the opening of substantial bandgap in the graphene by coupling with Si islands.

Ionic electroactive actuators has become attractive transduction materials due to their relatively large electromechanical actuations that can be generated under low voltages (a few volts) and many other advantages, such as flexibility and light weight. They can be directly integrated with microelectronic controlling circuits to perform complex actuation, which require an operation voltage of only a few volts. Graphene is a single layer of carbon atoms with sp2 carbon network structure. Graphene has an array of outstanding properties such as excellent mechanical, thermal, electrical and optical properties as well as a very high surface area. Due to these unique properties, graphene provides many potential applications including actuation materials that can offer better thermal, mechanical and electrical properties Recently, porous graphene has been introduced by activation of graphene using a concentrated solution of potassium hydroxide at 800 degree C. This material exhibits high surface area and outstanding electric and conductivity properties. This work suggests that by using porous graphene as the matrix and ionic liquids as the charge carrier, one can achieve high values of electromechanical strains at low voltages. The idea is based on using porosity with 2 nm in size in highly conductive graphene as channels to facilitate ion movement and hence to increase the actuation speed and magnitude. The actuators were made by infiltrating polytetrafluoroethylene (PTFE) as binder in graphene matrix and then introducing 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMI-Tf) as ionic liquid. Electrochemical characterizations revealed that this system exhibits high ionic conductivity and forms strong electric double layers of ions. Specific capacitance as high as 150 F/g was obtained and preliminary actuation analysis suggested that this system has the potential to show strains as high as 10%.

The absence of bandgap in pristine graphene is detrimental to the device function as post-silicon electronic devices. Bilayer graphene serves a unique alternative for bandgap engineering by breaking inversion symmetry of the graphene. Here, we used controllable molecular doping on both sides of bilayer graphene to break the inversion symmetry by doping-driven perpendicular electric field. The electrical transport behavior showed gradual increment of minimum conductivity (from 2.4 to 22kOmega;) according to the doping level, giving comparably high on/off ratio at the room temperature. The optical absorption also confirmed the formation of tunable bandgap by molecular doping. Moreover, in comparison to single molecular doping, the dual doping enabled the independent control of the carrier density and the bandgap opening of the bilayer graphene. The facile independent control of doping level and resultant performance enhancement will contribute significantly to the development of graphene based next generation electronics devices.

We present an in-depth study of the electronic transport and sensing properties of reduced graphene oxide (rGO)-based nanosensors. rGO devices are created by dielectrophoretic assembly of rGO platelets onto interdigitated electrode arrays, which are lithographically pre-patterned on top of SiO2/Si wafers. The availability of several types of oxygen functional groups allows rGO to interact with a wide range of organic dopants, including methanol, ethanol, acetone, and ammonia. We perform sensitive Scanning Kelvin Probe Microscopy (SKPM) and Scanning Impedance Microscopy (SIM) measurements on patterned rGO electronic circuits and show that both the electrical potential and charge distribution are significantly changed when the device is exposed to these organic dopants. We also demonstrate that SKPM and SIM experiments allow us to quantify the amount of charge transferred to the sensor during chemical doping, and to spatially resolve the active sites of the sensor where the doping process takes place.

Mechanical properties of defective mono-layer graphene sheets have been studied using experimental and computational tools. In experiments, elastic properties and breaking strength of free standing monolayer defective graphene membranes are measured by nanoindentation using an atomic force microscope. Defects have been introduced by exposure of membranes to oxygen plasma. Density of defects has been quantified using Raman and Auger electron spectroscopy, and also Transmission electron microscopy. Molecular dynamics simulations have been used to investigate the mechanical properties of free standing monolayer graphene membranes using reactive force fields. The effect of boundary conditions, as well as presence of defects in form of vacancies and bonded epoxide groups has been investigated and compared to experiments. Both experiments and simulations show decrease in Young&’s modulus and strength of graphene membranes by increasing defect density. However, the change in the elastic modulus is small below a certain defect density, which shows defective graphene membrane can still carry load and stay functional in different applications like decorated carbon based MOSFETs and graphene based nanocomposites.

In this paper, a method is presented to create and characterize mechanically robust, free standing, ultrathin, oxide films with controlled, nanometer-scale thickness using Atomic Layer Deposition (ALD) on graphene. Aluminum oxide films were deposited onto suspended graphene membranes using ALD. Subsequent etching of the graphene left pure aluminum oxide films only a few atoms in thickness. A pressurized blister test was used to determine that these ultrathin films have a Young&’s modulus of 154 ± 13 GPa. This Young&’s modulus is comparable to much thicker alumina ALD films. This behavior indicates that these ultrathin two-dimensional films have excellent mechanical integrity. The films are also impermeable to standard gases suggesting they are pinhole-free. These continuous ultrathin films are expected to enable new applications in fields such as thin film coatings, membranes and flexible electronics.

Graphene possesses excellent mechanical, thermal and electronic properties, not to mention its optical transparency and chemical stability. Much effort has been made towards the control of the graphene physical properties for technological applications. One way to achieve this control is through modifying graphene size and structure. In a search for the development of semiconducting graphene structures that can be easily produced at large scale, a new structure called graphene nanomesh, was recently derived by means of block copolymer lithography [1] and other methods [2-5]. Basically, a graphene nanomesh is a graphene structure made to possess a periodic array of nanoscale holes whose sizes and hole-to-hole distances can be used to control its overall electronic properties. Although the electronic properties of graphene nanomeshes are being intensively studied [1,2,4-6], their mechanical properties are still to be investigated. This work, then, presents the first study of mechanical, structural and thermal properties of graphene nanomeshes as function of hole and neck (i.e., smallest hole-to-hole distance) sizes, through atomistic molecular dynamics simulations. The second generation of the REBO potential will be employed to obtain the Young&’s modulus and coefficient of thermal expansion of graphene nanomeshes. [1] J. Bai, X. Zhong, S. Jiang, Y. Huang and X. Duan, Nature Nanotechnology 5, 190 (2010). [2] A. Sinitskii and J. M. Tour, J. Am. Chem. Soc. 132, 14730 (2010). [3] L. Zhang, S. Diao, Y. Nie, K. Yan, N. Liu, B. Dai, Q. Xie, A. Reina, J. Kong and Z. Liu, J. Am. Chem. Soc. 133, 2706 (2011). [4] G. Ning, Z. Fan, G. Wang, J. Gao, W. Qianc and F. Wei, Chem. Commun. 47, 5976 (2011). [5] L. Liu , Y. Zhang , W. Wang , C. Gu , X. Bai and E. Wang, Adv. Mater. 23, 1246 (2011). [6] H. Sahin and S. Ciraci, Phys. Rev. B 84, 035452 (2011).

With increasing CNT usage, it becomes desirable to develop the means for greater control of CNT growth and better utilization of feedstock and catalyst. By decoupling carrier gas flow from the carbon precursor, a novel reactor provides length and diameter modulation for tunable nanotube synthesis. The reactor design allows near 100% feedstock utilization and precise termination of reaction. In current CVD systems, while the catalyst facilitates the chemistry of CNT growth, it is part of the nanotube product, and hence not a true catalyst. A reactor design is considered to demonstrate multiple nanotube growths from a single catalyst bed.

Metal-carbon nanohybrid foams are a new family of nanostructured carbon materials synthesized by laser ablation of selected coordination compounds.[1,2] When irradiating organometallic precursors, the resulting foams consist of metal nanoparticles embedded within amorphous carbon nanoparticles, amorphous carbon nanoparticles, and graphitic nanostructures. Moreover, our laser ablation method enables the synthesis of metal-free carbon foams from aromatic molecular precursors.[2] The present work reports on important experimental parameters toward the controlled synthesis of gold-carbon nanohybrid foams. Thus, characterization studies indicate that the composition, gold nanoparticle dilution and crystallite size, and structure of the nanohybrid foams can be tailored by suitably tuning the laser parameters used and by choosing ligands of the irradiated targets.[2,3] Bottom-up growth mechanisms based on the recombination and assembly processes of high-temperature, ionized clusters taking place within the generated plasma plume are here proposed.[3] This "laser chemistry", based on the use of molecular precursors, would enable the facile production of multifunctional nanostructured carbon materials with a range of tunable properties. The study of the physicochemical properties (such as thermal- and textural properties) and the design of chemical processing strategies for these gold-carbon nanohybrids give promising hints of their potential technological applications. Thus, alternatively to our "laser chemistry" approach, wet chemistry strategies have been designed for the gold nanoparticle decoration of metal-free carbon foams, as well as of other carbon supports such as carbon nanotubes and graphene. The performance of the synthesized gold-carbon nanohybrids as heterogeneous catalysts will be also discussed.[3] [1] E. Muñoz, M. de Val, M. L. Ruiz-González, C. Loacute;pez-Gascoacute;n, M. L. Sanjuán, M. T. Martínez, J. M. González-Calbet, G. F. de la Fuente, M. Laguna, Chem. Phys. Letters 420 (2006) 86. [2] E. Muñoz, M. L. Ruiz-González, A. Seral-Ascaso, M. L. Sanjuán, J. M. González-Calbet, M. Laguna, G. F. de la Fuente Carbon 48 (2010) 1807. [3] A. Seral-Ascaso et al., submitted.

The stage transition, changing the number of graphene layers sandwiched between the two layers of intercalant was one of the most intriguing phenomena in structural studies of graphite intercalation compounds (GICs). While the question has been theoretically addressed, experimental studies revealed only macroscopic parameters. On the micro-scale the phenomenon remains elusive. Here we demonstrate direct real-time monitoring of the stage transitions using a combination of optical microscopy and Raman spectroscopy, which reveal the “mysterious” phenomenon with the striking clarity. The testing bed for our experiments were [(NH4)2S2O8-H2SO4]-GICs. They are characterized by the absence of the 2D band in their Raman spectra. The G band for different stage numbers was selectively enhanced by different excitation lasers. The stage transition begins on the edge of the flake and propagates toward the opposite edge. The transformation is accompanied by two distinct and independent phenomena: a) the structural deformations, and b) the color change. The structural deformation appears as a large wave moving across the sample surface, which we term the “Tidal Wave”; it indicates the leading edge of a transition front. Following the tidal wave, a front of bright rainbow colors (subfronts) propagates through the area. A new color appears when the intercalant is removed from a certain number of graphite galleries. The observations can be envisioned only in terms of the Daumas-Herold theory. Removal of intercalant from different graphite galleries occur independently to each other. While the tidal wave monotonously propagates in one direction, the subfronts can slow down for several seconds and then skip suddenly ahead or sideways. Often the subfront and even the tidal wave recede and then return and proceed in the original direction. These observations suggest that the driving force behind the stage transition is the electrochemical potential of surrounding solution. The lattice strain appears to have little influence over the kinetics of the process. The rate of the stage transition can be largely controlled by the concentration of the [(NH4)2S2O8-H2SO4] solution in which the flake is immersed. The typical rate values were in the range of 2 - 15 mu;m/s. The Raman spectroscopy shows that a large D band arises every time the tidal wave propagates through the acquisition spot. The stage transition proceeds through the intermediate state with the Raman spectrum very similar to that of amorphous carbon with a crystallite size of a few nm; thus we name it as a “pseudo-amorphous” state. Since the intercalation-deintercalation cycle is reversible (the graphene layer structure remains intact, no chemical bonds are broken), such dramatic change in the Raman spectra is caused just by transitional strain and mechanical deformations accompanying the stage transitions. Fascinating videos demonstrating stage transition will accompany the presentation.

Ultra-thin materials such as graphene are an important class of function materials. They also provide atomically simple and clean systems that are well-suited for transmission electron microscopy (TEM). With the latest improvement made by aberration correction, reliable interpretation of TEM images become more practical and can be well combined with image simulation and first-principles simulations to understand the configuration and dynamics of various atomic structures such as intrinsic ripples, development of defects, and chemistry at the edges. We demonstrate the usefulness of such combined study with results on various nanostructures in ultra-thin materials.

Graphene, a single atomic layer of graphite, has been provided physicists opportunities to explore an interesting analogy to the relativistic quantum mechanics. The unique electronic band structure of graphene lattice yields a linear energy dispersion relation where the Fermi velocity replaces the role of the speed of light. The exotic quantum transport behavior discovered in these materials including unusual half-integer quantum Hall effect and Klein tunneling effect. In this presentation I will discuss the exotic quantum transport behavior discovered in graphene nanostructures in the relation to the device applications beyond CMOS operation. In particular, I will present quantum carrier collimation both of which appear even at room temperature employing graphene lateral heterojunction. In addition, I will discuss the new type of material classes based on 2-dimensional van der Waal materials and their heterostructures extending the graphene based research into quasi 3-dimensional systems.

Direct formation of high quality and layer number-controlled graphene on dielectric substrates is one of the urgent issues to avoid transfer processes, by which the original graphene can be chemically and mechanically damaged. In this presentation, we report that high quality single layer graphene on SiO2/Si substrate has been successfully synthesized by chemical vapor deposition (CVD) process, during which minimal Cu vapor is remotely provided. The Cu vapors are generated by thermal diffusion from copper foil during the CVD process at designated temperature. The Cu vapors turn out to successfully catalyze the dehydrogenation of methane together with molecular hydrogen, leading to the nucleation for the graphene formation on amorphous SiO₂ substrate that is located underneath the copper foil. Micro Raman spectroscopy reveals that the graphene formed on SiO₂ substrate is single layer and has a comparable quality to the graphene grown on a copper foil. Raman mapping data shows the single layer graphene film covers > 95 % of graphene grown region (~ 1 mm²). With the graphene grown on a SiO₂ substrate, field effect transistor (FET) devices have been directly fabricated without any transfer steps. The ambipolar behavior with Dirac point is clearly observed in the resistance versus gate voltage curve. By attempting sequential double growths, bilayer graphene is formed without destruction of preformed single layer graphene. We also have proved that this method can be extended to various other non-metal substrates including fused quartz and hexagonal boron nitride

The fabrication of graphene with a controlled number of layers over large-areas is a key requirement for the exploitation of graphene in a number of applications. Chemical vapour deposition (CVD) on transition metal catalysts offers a low-cost method to address this need, however limited understanding of the underlying growth mechanism(s) means that process optimisation has thus far remained largely empirical. Large-area monolayer graphene (MLG) growth has been well demonstrated on polycrystalline Cu, however this commonly requires high temperatures (~1000°C) where catalyst sublimation becomes problematic. Attempts to grow on other polycrystalline catalysts which show larger C solubility, e.g. Ni, tend to produce inhomogeneous few-layer graphene (FLG). To address these unresolved issues we focus on Ni based catalysts which have been shown to be effective catalysts at much lower temperatures (450-600°C).¹ We use in situ, time-, and depth- resolved X-ray photoelectron spectroscopy (XPS) and in-situ X-ray diffraction (XRD) to directly observe graphene formation under realistic CVD conditions.² In particular we show that graphene growth occurs during isothermal hydrocarbon exposure and is not limited to a precipitation process upon cooling, and that (sub-surface) dissolved C plays a critical role. We consequently develop a coherent model for graphene formation and the role played by the catalyst support.³ Based on this understanding we develop a robust CVD process that for the first time yields complete MLG coverage on polycrystalline Ni catalysts at low temperatures (<600°C). We demonstrate the rational engineering of the catalyst,¹ and highlight the significant parameters involved in optimising this process. Through extensive characterisation with electron microscopy, raman spectroscopy, and electrical measurements we show the graphitic quality (average D/G peak ratio <8%) of the MLG produced is similar to optimised growth on Cu catalysts at much higher temperatures. We also demonstrate the growth of MLG of reasonable quality at CMOS compatible temperatures (<450°C) offering a potential pathway for integration of graphene with existing electronic architectures. Whilst this study focusses on achieving MLG coverage, we will also discuss processing routes for achieving coverage of a chosen number of graphene layers. (1) Weatherup et al. Nano Lett. 2011, 11, 4154-60. (2) Hofmann et al. J Phys Chem C 2009, 113, 1648. (3) Weatherup et al. ChemPhysChem 2012, (Early View).

While crumpling and unfolding a piece of paper is a simple act in daily life, the technology to reversibly crumple and unfold graphene sheets of atomic thickness does not exist. Yet, a precise control of crumpling and unfolding of large-scale graphene is essential to control the properties of graphene, and hence the key to advancing important graphene-based technologies for applications in electronics, energy storage, composites, and biomedicine. Here, we present a novel approach [J. Zang, S. Ryu, N. Pugno, Q. Wang, Q. Tu, M. Buehler, and X. Zhao, submitted (2012)] that addresses this challenge to control reversible crumpling and unfolding of large-area graphene by harnessing its mechanical instabilities. We transfer large-area graphene film on an elastomer substrate that has been highly pre-strained biaxially. We observe that the graphene film develops wrinkles and delaminated buckles when the substrate is relaxed uniaxially, and become crumpled when the substrate is relaxed biaxially. If the relaxed substrate is biaxially stretched, the crumpled graphene film can be unfolded. We explain the mechanical instabilities of graphene films on substrates with continuum theory and atomistic simulation. The reversible crumpling and unfolding of graphene films further enables us to fabricate large-area conductive coatings and electrodes capable of giant stretchability (over 450%), high transparency (up to 80%), super-hydrophobicity, and tunable wettability. We further demonstrate the use of novel graphene-polymer laminates as artificial muscles.

Graphene-based materials are promising because of their electronic and thermal transport, mechanical properties, high specific surface area, that they can act as an atom thick layer, barrier, or membrane, among other reasons. (Our micromechanical exfoliation approaches [1,2] conceived of in 1998 yielded multilayer graphene and one paper described in detail how monolayer graphene could be obtained [1]). Three main research areas of our group are: (i) Growth of large area graphene on metal substrates, characterization and physical properties, and studies of devices having graphene as a central component; (ii) Generation, study, and use of graphene-based platelets (typically derived from graphite oxide) including as dispersed in liquids, and powders derived from such colloids or generated by microwave or thermal treatment of graphite oxide; (iii) Generation and study of new types of carbon derived from graphene-based precursors, such as “activated microwave expanded graphite oxide”, or ‘aMEGO&’[3]. I will attempt to cover a variety of research projects underway in my group at this time and also use this opportunity to describe what I think are important new directions in carbon and first row element research, in the next 10-20 years. Support of our work by the W. M. Keck Foundation, NSF, DARPA ‘iMINT&’, DARPA ‘CERA&’, ONR, SWAN NRI, ARO, AEC, SRC, Graphene Materials LLC, and Graphene Energy, Inc., is appreciated. 1. Lu XK, Yu MF, Huang H, and Ruoff RS, Tailoring graphite with the goal of achieving single sheets, Nanotechnology, 10, 269-272 (1999). 2. Lu XK, Huang H, Nemchuk N, and Ruoff RS, Patterning of highly oriented pyrolytic graphite by oxygen plasma etching, Applied Physics Letters, 75, 193-195 (1999). 3. Zhu, Yanwu; Murali, Shanthi; Stoller, Meryl D.; Ganesh, K. J.; Cai, Weiwei; Ferreira, Paulo J.; Pirkle, Adam; Wallace, Robert M.; Cychosz, Katie A.; Thommes, Matthias; Su, Dong; Stach, Eric A.; Ruoff, Rodney S. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 332, 1537-1541 (2011).

The industrial exploitation of chemical vapour deposition (CVD) graphene crucially depends on the ability to generate large-area sheets with selected thicknesses, predefined domain orientations, edge geometries, and grain boundaries which govern the electronic structure, chemical activity, and mechanical strength. While attention has been directed at understanding the CVD growth mechanism on polycrystalline Cu, the precise control of CVD graphene remains unsolved. Here we show, both experimentally and theoretically, the implementation of structurally defined Cu substrates in conjunction with low-pressure CVD to generate high quality, tailored graphene with controlled shape, orientation, edge geometry, thickness, and quality. For example, the substrate&’s crystallographic orientation can be exploited to preferentially orient domain edges parallel to the <110> direction while generating high quality single layer graphene on Cu(111) and bilayer graphene on Cu(001). This research provides a new avenue towards the controlled production of high quality large-area graphene sheets for implementation in transparent conducting electrodes, sensors, and nanoelectronic device applications.

Graphene, a two-dimensional structure, consists of sp2 bonding with hexagonal lattice and has a lot of superior properties, such as low electrical conductivity, high thermal conductivity, strong mechanical property, and excellent optical property. Lots of approaches to synthesize grapehene had been employed, such as mechanical exfoliation via plastic tape, reduction of graphene oxide, epitaxial growth from SiC substrate. However, there still have some disadvantages and restrictions among these approaches, which limit practical application of graphene. In 2009, it was first time to propose the formation of graphene with large area on the Cu via CVD approach, which was considered as a clean, low-cost, and reliable process. However, detailed formation mechanism of graphene via the CVD process was still unknown, which motivates us to probe the mechanism and to path the way of improving the quality of graphene synthesized by the CVD process. In this study, we found the Cu vapor clusters, which are moved in the hot zone of furnace during the formation process, play an important role on the formation of grapheme. By taking the Cu vapor as the mediation layer, we are able to systematically investigat the formation process of graphene and provide a whole new and comprehensive view point at the physical and chemical phenomena. Finally, we can successfully form the graphene on the non-metal substrate such as Si/SiO2 wafer to device application without any transfer processes.

Graphene, a two-dimensional carbon allotrope, is often considered as a complement or even replacement for silicon in many electronics applications. However, the absence of an electronic bandgap in graphene prevents its use in logic devices. According to theoretical studies, a bandgap compared to that in silicon (1.1 eV) could be found in narrow graphene nanoribbons (GNRs) that have atomically precise armchair edges and widths less than 2 nm. Different top-down approaches, such as a combination of electron-beam lithography and dry etching, sonochemical method, nanowire lithography, and unzipping of carbon nanotubes, typically yield ribbons with widths > 10 nm and have limited control over the edges structure in GNRs. Here we report a novel bottom-up approach for gram quantities of narrow GNRs. The resulting ribbons are less than 2 nm wide, have atomically precise armchair edges, and could be up to a few µm long. These GNRs could be conveniently deposited from solution on any substrate, such as Si/SiO2, mica and Au(111), for further studies. The GNRs were characterized by different techniques, including NMR, UV-vis-NIR and Raman spectroscopy, XPS, EDX, SEM, AFM and STM. According to our preliminary data, these GNRs have a bandgap of about 1.4 eV, which is comparable to that in silicon (1.1 eV). These data suggest that GNRs obtained by this novel synthetic approach could be promising for applications in field-effect transistors with high on-off ratios. Also, since these GNRs could be synthesized in gram quantities and at a very high yield, they could be used for bulk applications, including coatings, composites and photovoltaic devices.

Chemical vapor deposition of graphene on copper has emerged as one of the most promising synthesis routes for a large scale production of high quality graphene. However, the electrical and mechanical properties of CVD graphene appear to be limited by its polycrystalline nature. Here, we study the physicochemical mechanisms underlying the nucleation and growth kinetics of graphene on copper [1], providing new insights necessary for the engineering synthesis of wafer-scale single crystals. The analysis at different growth temperatures has identified two distinct nucleation mechanisms that involve the competition between the capture of carbon adatoms by the growth of a supercritical nucleus, versus the desorption of carbon adatoms that leads to increase the graphene grain size. As the energetics of these phenomena varies with temperature, the nucleation activation energies can span over a wide range (1-3 eV) leading to a rational prediction of the individual nuclei size and density distribution. The growth-limiting step was found to be the attachment of carbon-adatom species to the graphene edges, and this was independent of the Cu crystalline orientation. [1] H.Kim, C.Mattevi, C.M. Reyes,et al. ACS Nano 2012, 6, 3614-3623.

Owing to its peculiar band dispersion, graphene can exhibit a wealth of anomalous transport phenomena, such as unconventional quantum Hall effect, Klein tunneling, and weak localization (WL) and weak antilocaliztion (WAL) of electrons. However, the feasibility of large-scale production of graphene sheet stands as a main obstacle between theoretical study and real-life application. Recently, Chemical Vapor Deposition (CVD) patterned large-area graphene sheet, associated with the technique transferring graphene onto the arbitrary substrates, is considered a significant improvement in graphene fabrication. We report the magneto-transport properties of very large-area graphene sheet fabricated by CVD method on Nickel layers and transferred to a stretchable PET substrate. The total area of the sample is substantially large up to few square centimeters (with a tested area up to tens of square millimeters), such epitaxial multilayer graphene sample is of obvious important to study for practical application. Negative longitudinal magnetoresistance (MR) effect caused by weak localization (WL) of electrons in graphene can be clearly seen at 2K, and a transition from negative to positive MR was observed as the magnetic field increases. While only positive MR effect was found at 300K. However, transverse resistivity is found to be linear to the external field, which indicates that the Hall Effect stays unaffected to the WL correction to the MR. Remarkable similarities and differences (such as lower carrier mobility) between our sample and microscopic ideal 2-dimensional graphene were also discussed. Furthermore, angular dependence of both longitudinal and transverse resistivity was measured in order to test the interplay of the WL and magnetoresistance effects in graphene sample. Such experimental was carreied out with a horizontal sample rotator along the current axis under a magnetic field up to 1 T. Quantificationally simulation with good agreement between theory and experiment was successfully achieved.

Due to their remarkable physical properties, such as high electronic and thermal conductivity along with excellent mechanical strength, graphene and graphene based hybrid materials have attracted a lot of attention in recent years. Graphene sheets decorated with metal nanoparticles are typical examples of emerging metal-carbon composites that currently attract special research efforts due to their enhanced potential for catalytic applications. Despite rapid advances, large quantities of high purity graphene with controlled thickness are not yet widely available. In this context, we present here, new results on the multi gram scale synthesis of few layer graphene with controlled thickness, its characterization using microscopic and spectroscopic techniques and the preparation of graphene-metal hybrid materials. Few layered graphene (FLG) was prepared by ethylene CVD at temperatures in the 600-750°C. TEM and Raman spectra showed that the sample contained few layered graphene and the thickness as determined by the broadening of the x-ray powder diffractograms ranged from 0.5-8 nm depending on the preparation conditions. Electron diffraction confirmed the presence of high quality crystalline layers of graphene. Metal-graphene composites were prepared by deposition from solution after suitable surface modification to anchor nanosized metal particles on the graphene layer. Using density functional theory, we have attempted to model the decomposition of the ethylene precursor and the initial steps of graphene nucleation. The synthesis, metal deposition and a possible growth mechanism for graphene by CVD will be discussed in the communication.

Graphene&’s two-dimensional structure exposes its entire volume to its surrounding, rendering highly amenable surface chemistry. Tremendous efforts have been placed on the chemical functionalization of pristine graphene by adsorbing foreign atoms, molecules and functional groups on its surface. Among various forms of chemical functionalization, hydrogenation of graphene (e.g., attaching atomic hydrogen to the carbon atoms in graphene) receives the most attention. Hydrogenation of graphene changes the hybridization of carbon atoms from sp2 into sp3. As a result, the 2D atomic structure of pristine graphene is distorted into 3D locally at each adsorbed hydrogen atom. As a result, when a pristine graphene is hydrogenated, it is found that the resulting graphene is rippled. The rippling can be explained by the local distortion of the atomic structure of graphene due to hydrogenation. These observations open up the possibility to tailor the morphologic properties of graphene in a bid to facilitate the formation of nanostructures of interest. In this work, we demonstrate the hydrogenation-assisted self-assembly of graphene. In particular, we focus on the hydrogenation-assisted formation of a carbon nanoscroll (CNS) from a freestanding graphene, using molecular dynamics simulations. We show that hydrogenation helps overcome the energy barrier to form an overlap in the non-hydrogenated part in the graphene. Once that energy barrier is overcome, the graphene can spontaneously roll up into a CNS. The successful formation of a CNS depends on the width of the free standing graphene and the hydrogenation aspect ratio. A phase diagram obtained from this study elucidates the key parameters that govern the successful scrolling of graphene into CNSs. With the ever maturing nanopatterning technique to hydrogenate graphene at high precision, the hydrogenation-assisted self-assembly of graphene holds great potential leading to a feasible fabrication technique of high quality and well controlled graphene-based novel nanostructures.

In the research of modern terahertz electronics, development of compact, tunable and coherent sources operating in the terahertz regime is one of the hottest issues. Graphene, a monolayer carbon-atomic honeycomb lattice crystal, has attracted attention due to its peculiar carrier transport properties owing to the massless and gapless energy spectrum. In this presentation we demonstrate that graphene is one of the best materials for new types of terahertz lasers as optical and/or injection pumping of graphene can exhibit negative-dynamic conductivity in the terahertz spectral range. We analyze the formation of nonequilibrium states in optically pumped graphene layers and in forward-biased graphene structures with lateral p-i-n junctions and consider the conditions of population inversion and lasing. The model used accounts for intraband and interband relaxation processes as well as deviation of the optical phonon system from equilibrium. Optical pumping suffers from a significant heating of both the electron-hole plasma and the optical phonon system, which suppresses the formation of population inversion. In the graphene structures with p-i-n junction, the injected electrons and holes have relatively low energies, so that the effect of cooling can be rather pronounced. The latter provides a significant advantage of the injection pumping in realization of graphene terahertz lasers. We benchmark graphene as a prospective material for injection-type terahertz lasers. In terms of compact solid-state lasers in the terahertz range, quantum cascade lasers (QCLs) are the most promising current-injection-type light-source devices providing sufficient output powers beyond 10&’s of mW with fine linewidth (~kHz) at frequencies over the entire terahertz frequency range. The QCL utilizes the intersubband transitions in the quantum wells of the superlattices for terahertz lasing. Carrier recycling schemes in its operation principle can multiply the quantum efficiency by the cascading number of the quantum wells. However it suffers from phonon-limited decoherency preventing room-temperature operation. Compared to such state-of-the-art terahertz lasers as QCLs, semiconductor Raman lasers, and/or p-Ge lasers, the graphene injection lasers may have substantial advantages serving room temperature operation as well as relatively high quantum efficiency (~1) and practically high output power (~mW), all of which owe to (i) the symmetric gapless linear band structure that does not require any specific structure for carrier depopulation that is mandatory in QCLs, (ii) extremely high optical phonon energy, resulting in ultrafast carrier energy relaxation making ease of population inversion, and (ii) extremely long carrier momentum relaxation times.

Graphene has shown the potential to significantly impact a number of different technologies, including energy storage. Properties such as high surface areas and electrical conductivity make it a promising material for hydrogen storage, battery, and ultra capacitor applications. One route to realizing the full potential of graphene in energy storage applications is the assembly of three-dimensional macroscopic graphene networks that retain the properties of individual graphene sheets. Herein we report the assembly of graphene sheets into a hierarchical architecture with length scales extending from the nanoscale to the macroscopic regime. These graphene macroassemblies are formed using sol-gel chemistry to cross-link suspensions of single layer graphene oxide (GO). The hierarchical structure possesses a number of novel properties including mechanical stiffness (>50 MPa) and electrical conductivities (~10² S/m) orders of magnitude higher than previously reported, surface areas that approach the theoretical values expected for a single graphene sheet (~2500 m²/g), and extraordinarily large mesopore volumes (greater than 5 cc/g). Finally, exceptional energy storage behavior was observed. The graphene-based electrode simultaneously exhibited high energy (~10² Wh/kg) and power densities (~10⁵ W/kg) in both organic and inorganic electrolytes. The details of the synthesis and characterization of these novel graphene macroassemblies will be presented. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DEAC52-07NA27344.

In this presentation, we report on our understanding of graphene/substrate interface properties and engineering of bottom-gated, large-scale triphenylene assembled graphene transistors by applying octadecyltrimethoxysilane (OTMS) self-assembled monolayers (SAM) onto the dielectric surface. To circumvent the challenge that at the initial stage of CVD growth of graphene, the isolated carbon monomers will form defective carbon clusters with pentagons, triphenylene (C₁₈H₁₂) (pentagon-free with only C and H), as the solid precursor, was used for high quality and large scale graphene synthesis. Transport measurements performed on back-gated graphene field-effect transistors (GFETs) with large channel lengths (~25µm) show a carrier mobility up to ~ 5090 cm²/(Vs) on SiO₂/Si substrate at room temperature under vacuum. Furthermore we show that, in virtue of the ultrasmooth SAM surface, reduced interfacial impurity scattering, and attenuated surface polar phonon scattering, the octadecyltrimethoxysilane (OTMS) passiviated SiO₂ interface consistently improves the graphene FET carrier mobility up to ~ 9080 cm²/(Vs), whose graphene active layer has been grown with triphenylene precursor. This makes it promising for practical applications. In addition, compared to the devices without interface engineering, triphenylene-derived graphene FETs with the OTMS-SAM exhibit a remarkable gate-dependent field effect mobility. Detailed analyses reveal that the different scattering mechanisms affect the mean free path of hole for graphene devices on bare SiO₂ substrates (charged impurity scattering) and OTMS functionalized SiO₂ substrates (resonant scattering), respectively. This renders the significantly different electrical performances of GFETs at room temperature. Acknowledgement: The work is in part supported by Research Grants Council of Hong Kong, particularly via Grants Nos. CUHK2/CRF/08 and CUHK4179/10E. J. B. Xu thanks the National Science Foundation of China (Grants Nos. 60990314 and 60928009) for support.

Chemical vapor deposition (CVD) has been a popular technique for growing carbon nanostrucutres such as carbon nanotubes (CNTs) and graphene, particularly for patterned and unpatterned growth on substrates to enable fabrication of devices, sensors etc. Low temperature plasma-enhanced CVD has also emerged as an alternative for carbon nanostructures growth in device applications. First, we report our recent results on vertical graphene layers grown by PECVD with details on growth parameters, characterization. These structures are readily grown on silicon, nickel, copper and several other substrates. Second, we report successful wafer-scale integration of PECVD-grown carbon nanofibers as nanoelectrodes in biosensors and neural interface. The author acknowledges Jessica Koehne, Emily Rand, Mike Marsh, Kendall Lee, Mike Oye, Keivan Davami, Mehrdad Shaygun and Jeong-Soo Lee for their contributions.

One of the most exciting and important research directions of graphene and other relevant atomically layered materials is to investigate how nanopatterning technologies can be adapted, developed, and refined to pattern 2D graphene films into nanostructures, aiming to obtain the desirable electronic characters and functionalies that could significantly leverage the unique property or potential of graphene.[1-4] In this work, we developed an upscalable nanolithographic method for patterning orderly arranged sub-10 nm half-pitch graphene nanoribbons (GNRs) and fabricated field-effect transistors (FETs) bearing such densely arranged GNRs. The nanofabrication method combines nanoimprint lithography (NIL), directed self-assembly of block copolymers, and plasma etching. Our work shows that the standard variation of the ribbon width (3σ value) among densely arranged GNRs strongly depends on the processing condition of directed self-assembly of block copolymers, and it can significantly affect the ON/OFF current ratio of the FETs bearing multiple GNR channels. A relatively large 3σ variation of the ribbon width can result in a poor ON/OFF current ratio, which is attributed to the non-synchronization of OFF states of multiple nonuniform GNRs. Through the processing optimization, we are able to create 8 nm hp GNRs with 3σ width variation less than 3 nm. The FET bearing 200 such GNRs exhibits a high ON/OFF current ratio > 60, which is well above that of the control FET with an unpatterned graphene channel. The nanofabrication method presented in this work could be further developed to realize nanomanufacturing of high-quality graphene nanostructures for future scale-up nanoelectronic applications. In addition, our work provides critical insights for the further optimizing the electronic properties of graphene nanostructures. [1] A. K. Geim, Science 2009, 324, 1530. [2] M. Y. Han, B. Ozyilmaz, Y. B. Zhang,P. Kim, Phys Rev Lett 2007, 98, 206805. [3] X. L. Li, X. R. Wang, L. Zhang, S. Lee,H. J. Dai, Science 2008, 319, 1229. [4] X. G. Liang, Y. S. Jung, S. W. Wu, A. Ismach, D. L. Olynick, S. Cabrini,J. Bokor, Nano Lett 2010, 10, 2454.

Intense research on graphene has been on the rise for last few years. Chemical Vapor Deposition (CVD) is a common method used for growing large area continuous graphene on a metal surface as a catalyst for graphene nucleation. This adds a necessary transfer step to the target substrate ultimately desired for graphene devices fabrication. Interfacing with graphene is one challenge in preserving the promising high mobility especially with gate dielectrics. This initiated the motivation for studying the effect of intermediate interfaces imposed by transfer processes. In this work, direct transfer, transfer using PMMA and transfer without PMMA are studied and compared in terms of the relative defectivity, discontinuities and surface roughness which correlates with degradation in mobility. Multilayer graphene was grown on 25 µm thick copper foils inside a high temperature furnace. The system was purged at ambient pressure in N₂ for 5 minutes. Then the temperature was ramped up to 1000 ^oC in 200 sccm Ar and 500 sccm H₂ for 30 minutes. Then the system was stabilized for 5 minutes at 1000 ^oC in 500 sccm H₂. 50 sccm CH₄ were added during the last 2 minutes for growth. Growth was terminated by N₂ quench to room temperature. Raman spectroscopy is performed on grown graphene sample to confirm few layers (>3) at widely spaced spots and mapping is done for a 100 x 100 µm² area. Afterwards the sample is cut into three pieces and transferred to 300 nm SiO₂ on Si. Piece ‘A&’ had PMMA in anisole (2.5% by mass) added. All three pieces are floated on HNO₃: DI water (1:3 concentrations by volume) for 3 minutes to etch graphene on the backside of the copper foil. Then all pieces are floated on Fe₃(NO₃)₃ granules: DI water (1:2 by mass) for 1 hour to etch the copper foil. The Iron Nitrate solution is then displaced with DI water. A vacuum holder is used to hold the SiO₂ wafer and brought in contact with the floating Graphene, sample ‘B&’, from the top side. For Samples ‘A&’ and ‘C&’, the SiO₂ wafers are immersed in the DI water container and carefully lifted capturing the floating Graphene sheet. All three samples are mildly titled to drain trapped DI water and help Graphene adhere to the SiO₂ substrate and left to dry for 5 hours. Finally, sample ‘A&’ is rinsed in Acetone to remove the PMMA. Raman spectroscopy is done for the three pieces and the comparison is carried out by comparing the ratio of D/G peaks over a mapping area of 20 x 20 µm² using 5 seconds exposure time, 4 averaging number, 50x lens and a 532.2 nm laser beam. SEM images at 1000x are taken for the three samples for a 200 x 200 µm² area to study discontuities. AFM measurements are taken for the three samples for a 50 x 50 µm² area to study surface roughness. The results are summerized in added table.

A nanohybrid of reduced Graphene Oxide grafted with polypyridylruthenium functionalized polystyrene was synthesized through a through a surface initiated ATRP method. Graphene is designed as the electron acceptor endcaping the polystyrene backbone, while pedant Ru complexes work as the light harvesting antennae and inject electrons to the Graphene upon irradiation. This nano-hybrid show good solubility in polar solvents, making it potential candidate for optoelectronic devices through a solution processable strategy. Moreover, due to a long lifetime of metal-to-ligand charge transfer state of Ru complex (~1 µs), it is able to facilitate the exciton transport and charge separation in active layer, which is a key characteristic for photovoltaic devices.

Graphene is the exemplary 2-D material with very attractive electronic, optical and mechanical properties. The huge interest in graphene led to rapid progress in understanding this material. However, obtaining high-quality graphene films remains a key challenge. Large-area chemical vapor deposition (CVD) on Cu foil shows most promise for growing high quality monolayer graphene. To isolate the graphene, water based wet transfer processes are usually used, but inevitably introduce contamination which degrades its performance. Efforts have been made to reduce the moisture or residuals trapped between graphene and substrate by using a thermal release tape based dry transfer. The dry transfer process requires uniform pressure to make a good contact of the comparatively rigid tape with the graphene grown on a microscopically rough surface of the copper substrate. Hence, tape residue and cracks are often seen on graphene after transfer. Here we report the development of a novel dry transfer of CVD graphene onto a variety of substrates. The transfer process starts with spinning PMMA on as-grown graphene, followed by applying a thermal release tape in which a window has been cut. The copper substrate is etched in ammonia persulfate. The suspended graphene/PMMA withinn the tape window is cleaned with DI water and dried. The complete stack is then applied onto the substrate. On a smooth surface such as thermal SiO2, graphene can be brought into intimate contact with SiO2after heating to 125 °C to release the tape. For other surfaces, such as deposited CVD oxide, the substrate was ramp heated on a hot plate slowly from 100°C to 150°C and kept at 150°C for 5 minutes. PMMA then was removed by acetone. Raman and AFM measurement were taken to confirm the high quality of graphene film. Electrical measurements of graphene devices show performance of dry-transferred graphene that is superior compared to wet transfer. Average mobilities of 6000 cm2V-1s-1 and high as 7500 cm2V-1s-1 were achieved with in-situ annealing at 115°C in vacuum of 10-5 Torr.

Microbial Fuel Cells (MFCs) are devices that harness the naturally occurring power generated by electrogenetic bacteria to produce energy in a clean and renewable way. Wastewater is used as the fuel source and the power is regenerated stably to peak power outputs at every insertion of liquid. Micro-sized MFCs can be used as portable power supplies, on chip power sources, or as testing vehicles to optimize materials and conditions for implementation into larger scale MFCs. We have developed a variety of micro-sized MFCs ranging from devices directly on a silicon chip to flexible mobile designs with volumes varying from 1-100 micro-liters (mu;l). The basic requirements of an MFC are a conductive biocompatible anode, typically carbon, where the bacteria will attach and a conductive cathode with an electron acceptor, often either ferricyanide or oxygen. Our previously published device had 1.25 mu;l volume with carbon nanotube anode separated into two chambers by a proton exchange membrane. The carbon cloth cathode chamber was filled with ferricyanide as the electron accepter (Mink et al, Nanoletters 2012). Recently, we have tested multi-layered graphene on copper foil as a thin anode in a 25 mu;l structure using oxygen as the electron acceptor. Graphene is expected to perform well as an anode due to its reported high electrical conductivity and large surface area. The graphene on copper foil device was tested alongside a control device with just copper foil as comparison. Both MFCs were fed 1g/L acetate solution in place of wastewater so that accurate comparisons could be made. Voltage was measured continuously with peaks occurring immediately after inoculation with acetate liquid. Polarization curves were measured with resistances ranging from 0 to 1Mohm resulting in peak power densities by volume of 12 W/m3 and corresponding current densities of 70 A/m3 (compared to 4 W/m3 and 43 A/m3 in the control copper foil.) Overall peak current densities for the graphene on copper foil (not corresponding to peak power) reached 164 A/m3 while the copper foil only achieved 44 A/m3. Interestingly, estimates of internal resistance show the graphene copper cell having 4 times larger resistance than the copper foil cell. As all conditions were kept equal, this is likely due to not having an optimal contact between the multi-layer graphene and the circuit. Contact engineering related to graphene is a challenging area requiring further attention. The graphene on copper foil anode is the first of its kind to use pure graphene as anode material instead of blending it with other anode materials (like carbon cloth) to produce a hybrid graphene anode. Producing 3 times larger power densities than the control copper foil, the use of graphene as an MFC anode holds excellent potential to improve the performance of future MFCs.

Reduced graphene oxide (rGO), a candidate for a high surface area carbon material with tunable nanoporosity, has been studied extensively. However, intercalation of ionic liquids (ILs) with rGO has not been explored yet to fabricate high performance electrochemical energy storage devices, namely ultracapacitors. This study therefore targets fabricating rGO-IL based ultracapacitors to search the feasibility and understand the effect of chemical interactions between rGO and ILs on the ultracapacitor performance. Impact of the chemical nature of synthesized ILs has been investigated for the effect of cations (ammonium, piperidinium, pyrrolidinium and imidazolium derivatives) using a common anion, bis(trifluoromethanesulfonyl)imide. This is to evaluate the role of chemical interactions tailored by the change in the interlayer chemistry for improving the electrochemical and thermal stability of ultracapacitors. To correlate the structure-property relation, we studied four different ILs, namely, (N-methyl-N-octylpyrrolidinium bis(trifluoromethanesulfonyl)imide), N-octyl-N,N,N-tributylammonium bis(trifluoromethanesulfonyl) imide, 1-butyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide and N-octyl-N-methylpiperidinium bis(trifluoromethanesulfonyl) imide). GO-IL composites were prepared by intercalating these ILs in the interlayer of as-synthesized GO in different solvent systems (deionized water or propylene carbonate).Powder X-Ray Diffraction (PXRD) analysis showed that original 2theta; of GO (11°) was lowered to 10.4-9.2° as a result of efficient IL intercalation. Infrared absorption spectroscopy (IRAS) studies revealed that covalent functionalization of GO by the ILs did not occur at room temperature indicating that these ILs intercalated non-covalently. The role of cations in GO-IL composites was monitored by IRAS during thermal annealing to investigate the chemical interactions between rGO and the ILs. IRAS and XRD studies showed that ILs with bulky and highly substituted ammonium-based cations promoted defect formation in the graphite basal plane during annealing treatment at 60-500°C. Therefore, ILs with long alkyl carbon chains are found not to play a role for a covalent functionalization during annealing and thus promoted thermal exfoliation. Indeed, N-methyl-N-octyl-pyrrolidinium and 1-butyl-3-methylimidazolium cations helped to stabilize GO&’s functional groups, which prevented removal of oxygen groups at <400°C. The anion and cation moieties of the ILs also influenced the decomposition temperatures studied by thermogravimetric analysis (TGA). For instance, N-methyl-N-octylpyrrolidinium bis(trifluoro-methanesulfonyl)imide and 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide expanded rGO layers upon annealing at 150°C and 300°C. Overall, optimization of chemical interactions in rGO-IL composites have been shown to improve the rGO-IL based ultracapacitor performance.

There has been a strong demand for finding the suitable gate dielectric which can optimize the excellent electric, mechanical and optical property of graphene. Despite the development of gate insulator materials that can improve the device performance, an excellent insulating property and flexibility were not compatible under high external stress with optical transparency. Here, we report the fabrication of high-performance, flexible graphene-based field-effect transistors using a graphene oxide (GO) as a gate insulator. The dielectric constant of GO film which was formed by Langmuir-Blodgett method was 3.1 at 77K and a low leakage current (17mA/cm2) below breakdown bias (1.5 × 106 V/cm). The graphene transistor using GO gate insulator shows a hole and electron mobilities of ~300 and 250 cm2/Vs, respectively. In addition, the stable performance under bending configuration (Rc : 4.4mm) of GO insulated graphene TFTs on the plastic substrate suggest promise for the use of this method to graphene flexible electronics.

Spintronics has a strong influence on the electronics market with applications in magnetic memories and sensors. In particular, the ubiquitous, optimized MgO-based spin valve has successfully established large tunnelling magneto-resistance (TMR) signals, enabling further miniaturisation and increased storage densities. Beyond these maturing technologies, the use of novel carbon-based materials provides numerous opportunities for spintronic devices with flexibility, improved performance and versatile functionalization. In particular, graphene is a promising material for potential spin-based beyond-CMOS architectures [1]. The integration of tunnelling barriers in graphene-based devices remains however challenging. Both the commonly reported lack of wetting of ultra-thin dielectrics and the resulting impact on the graphene lattice are critical issues in developing high performance devices. A promising solution involves the use of high quality sputtered 1nm thick Al2O3 tunnel barriers [1]. We illustrate in particular their potential through the characterization of graphene-based vertical spin-valves with the aim of studying nickel/graphene interface (NGI) spin filtering effects. Lithographed nickel lines were coated with graphene using a chemical vapour deposition step [2]. The passivation of the nickel surface by graphene prevents the growth of a surface oxide on exposure to air, as evidenced by X-ray photoelectron spectroscopy. Electrical measurements show evidence of charge transport and spin filtering through the NGI. While this study further underlines the pertinence of sputtered tunnel barriers on graphene, we will also discuss the potential of less intrusive atomic layer deposition of ultra-thin dielectrics on graphene [3]. [1] B. Dlubak, M.-B. Martin, C. Deranlot, B. Servet, S. Xavier, R. Mattana, M. Sprinkle, C. Berger, W. A. De Heer, F. Petroff, A. Anane, P. Seneor and A. Fert, Nature Phys., in press (2012) [2] R. S. Weatherup, B. C. Bayer, R. Blume, C. Ducati, C. Baehtz, R. Schlögl and Stephan Hofmann, Nano Lett. 11, 4154 (2011) [3] B. Dlubak, P. R. Kidambi, R. S. Weatherup, S. Hofmann and J. Robertson, Appl. Phys. Lett. 100, 173113 (2012)

We have fabricated efficient graphene cathode-based inverted organic solar cells using small molecules/conjugated polymers as photoactive materials and titanium oxide/zinc oxide as electron selecting/transporting layers. Amorphous titanium oxide/zinc oxide thin films were successfully coated on the graphene surface which was achieved by non-destructively modifying the graphene-layered sheets using simple polymeric interfacial layers that enabled effective interaction at the graphene/electron transporting layer junction. The observed power conversion efficiency was the highest reported for graphene cathode based devices and the device performance was approaching to that of the normal configuration. The advances demonstrated by this work using air stable graphene as the front electrode in the inverted geometry can provide valuable information in improving the device operational lifetime and long term stability, by avoiding commonly used ITO and PEDOT:PSS which are major sources of the degradation mechanism in organic photovoltaic solar cells.

A well-known chalcogenide phase change materials Ge2Sb2Te5 (GST) usually have two crystalline phase that exhibit vastly different electrical resistance and fast phase transition kinetics from amorphous to crystalline. But, to meet great demand for multi bit storage, we have investigated In-Sb-Te (IST) as a new candidate for multi level cell (MLC) phase change materaials, which shows multiple phase transformation kinetics. Then in this work, we have proposed the graphene as a stable inter-layer for multi stacking structure of GST and IST since we have found that the graphene not only maintains a thermal barrier between GST and IST, but also consist high thermal conductor during set/reset operation. Using high resolution TEM and atomic scale analysis method we have tried to investigate the microstructual characteristics of IST/Graphene/GST multilayers and atomic inter-diffusion between GST and IST through the graphene. As a result, it is found that the graphene layer preventing atomic inter-diffustion in each layer even at a upper crystalization temperature of GST and IST. Subsequently, this multi-stacking strurcture shows that the 4-different resistance levels are distinguishably seperated and the difference of each state is one order of magnitude due to the unique thermal conducting and interdiffusion properies of graphene layers.

Here we present room-temperature electrical conductivity measurement of wafer-scale, large-area suspended few layer graphene (FLG) membranes with areas up to 1000 mu;m2. Devices are built on a wafer with 300 nm low-stress silicon-nitride layer on front and backside. CVD (chemical vapor deposition) grown graphene films are used as the suspended membrane to cover the large-area openings on the nitride film on top of wafers. This represents areas two to orders of magnitude larger than prior transport studies on any suspended graphene device (single or few layer). We find a sheet conductivity of ~2500 e2/h (or ~10 ohms/square) of the suspended graphene, which is an order of magnitude higher than any previously reported FLG sheet conductance. I. FABRICATION Using hot KOH, silicon wafer was etched through 1 mm opening circles on the backside of the wafer. Nitride acts as an etch-stop on top of the wafer. Following that, the top nitride membrane was pattern and etched, using RIE, to introduce 5x5 mu;m2 up to 30x30 mu;m2 holes. Graphene was then transferred on the wafer covering these holes on the nitride membrane. Graphene films will be freely suspended over these holes making large-area (up to 30x30 mu;m2) graphene membranes. Consequently, two electrodes were deposited using conventional lithography followed by e-beam evaporation of metal and lift-off, on two sides of each graphene membrane. Device structure thus, contains electrodes connected by graphene which is partly suspended. II. MEASUREMENTS, RESULTS, AND CONCLUSION Electrical measurement to calculate conductivity of graphene films was performed showing a significant difference between the sheet conductivity of suspended and supported-on-substrate FLG graphene film. Suspended membrane shows sheet conductivity of ~2500 e2/h as opposed to same film on the substrate with conductivity of ~20 e2/h. Besides, we measured 3 devices with different membrane sizes (5x5, 20x20, 30x30 mu;m2) and realized that the largest membrane corresponds to the highest total conductance between electrodes. While there have been no other measurements in the literature of the conductivity of suspended FLG, we compare our results to that of supported FLG. The sheet resistance of our suspended graphene (~10 ohm/sq) is at least an order of magnitude higher than that of FLG graphene synthesized on Ni by CVD measured on solid substrates by other groups, who found varying sheet resistances typically around 250 ohm/sq or higher. The mechanism for such an increased conductance is believed to be related to decreased substrate related scattering, although detailed theoretical models for this for multi-layer graphene have yet to be developed. The ability to fabricate high conductance suspended graphene films can open the door to many potential applications. Of particular interest is the integration of graphene films with electrochemistry, nanopores, and DNA sequencing, a future application that holds much promise for the use of graphene films.

Graphene, a two-dimensional sheet of covalently bonded carbon atoms, has attracted widespread attention across many scientific disciplines due to its extraordinary electrical, mechanical, optical and chemical properties. With a high conductivity, a good transmittance and a low cost, graphene is suitable for energy storage, energy generation, electrochemical sensing and spectroelectrochemical applications. Despite the fast moving research in this field, there are still many challenges remains for these applications. Most of the research work focus on solution based graphene nanopallets produced from intercalation of graphite powder and reduction of graphene oxide. Recent fabrication process development in chemical vapour deposition (CVD) provides graphene of a large area, low defects and better uniformity, with many properties different from solution based graphene. In this work, we fabricated graphene electrodes by transferring CVD graphene on substrates of different termination and on infrared transparent substrates. The graphene electrodes were characterised by AFM, Raman spectroscopy, XPS, FTIR, electrochemical impedance spectroscopy and cyclic voltammetry. We found that the electrochemical properties of diamond were strongly influenced by the termination of supporting substrates. The implementation of graphene in spectroelelctrochemistry provides a new platform to study redox-dependent chemistry and protein transitions on this novel material.

Graphene nanomeshes (GNMs) are novel derivatives of graphene formed by the creation of a lattice of holes in a graphene sheet. Depending upon the hole shape, size and lattice constant, the GNM can be metallic, or semiconducting with a significant electronic band gap (~0.3 eV). In this work, we use first principles calculations to study the doping of semiconducting GNMs using an ion chelation mechanism. In particular we show examples for the passivation of the holes in the graphene lattice, the ion chelation they allow for, and their tunable p- or n-doping. Our calculations show that such mechanism leads to stable doping of the GNMs, with a negligible effect on their band structure. Such doped GNM structures can be the building blocks of graphene based nanoelectronics.

We herein report on transparent heterojunctions that consist of p-type CNTs and n-type ZnO films. The CNT/ZnO heterojunctions exhibited rectifying behaviors in the dark, indicating the formation of a p-n junction. In order to achieve photodiodes that are fully transparent, we fabricated CNT/ZnO heterojunctions on quartz substrates by transferring the CNT films onto ZnO films and using graphene as a transparent electrode to the CNT films. These devices showed an optical transmission over 80% in the visible region and an ideality factor of ~ 1. 46. Our results demonstrate that transparent photodiodes can be achieved by using CNT/ZnO heterojunctions and graphene electrodes.

Spin transport in graphene is a topic of growing interest for the conceivable development of spintronic devices. Graphene is a promising material for this application inasmuch as the intrinsic spin-orbit interaction does not mix states of different spin and intrinsic spin coherence times are therefore expected to be long. The spin decoherence mechanism investigated here theoretically is associated with the electric field of a charged impurity in the substrate (e.g. SiO2) beneath the graphene layer. The impurity is the source of an electric field in its vicinity with a non-vanishing component perpendicular to the graphene layer. The graphene may contain non-equilibrium charge carriers (electrons or holes), for example induced by an applied voltage between it and a gate beneath the substrate layer. Evidently these mobile charge carriers screen the Coulomb field associated with the impurity, however, a non-zero electric field in the graphene layer remains and its component perpendicular to the graphene layer breaks the reflection symmetry (graphene point group D6h). This enables a Rashba spin-orbit coupling mechanism. (The spatially varying perpendicular field near the impurity is an addition to the uniform field due to the gate, which also contributes to the Rashba-type spin-orbit coupling). The physical mechanism explored in this work is the spin-flip scattering due to this type of spin-orbit interaction. The principal interest focuses on charge carriers incident upon the region near the impurity and experiencing a spin flip in addition to the conventional screened Coulomb potential scattering event. This type of spin scattering is unique, inasmuch as it requires a spatially varying field due to a charged center (impurity) not located within the plane of the graphene. It is quite different from the usual case of Elliott-Yafet spin relaxation due to scattering. We calculate scattering cross sections in the framework of the Born approximation based on an 8x8 Dirac Hamiltonian for graphene and a non-linear Thomas Fermi screening model. The dependences of the scattering cross section on the distance of the impurity from the graphene layer and on the equilibrium carrier concentration in the graphene are discussed, and conclusions are drawn about the expected spin coherence times and spin diffusion lengths.

The discovery of chemical vapour deposition (CVD) routes for graphene synthesis have opened the opportunity to make films on the centimetre scale. The routes fall into two broad categories; the precipitation of a controlled amount of carbon from a super-saturated carbide and the CVD of a hydrocarbon on a surface which possesses a poor carbon solubility. Herein, we have studied the latter and focussed on the influence of the growth conditions (precursor balance, pressure and temperature) on the quality of the deposited film. Characterisation methods to determine the defect concentration and number of layers of the deposited graphene whilst on the growth substrate were considered. A thermodynamic interpretation of the influence of growth conditions on the deposited films is presented.

In this work, we perform first principles calculation within the Density Functional Theory (DFT) formalism [1,2] to study the symmetry breaking between hole and electron mobilities of graphene bilayer upon molecular oxygen adsorption. We predict that states mainly localized in the oxygen molecules and located above (and close to) the Fermi energy may act as ressonat scattering sources only for n-type carriers. We relate our calculations to experimental results in which this effect is indeed observed: Upon oxygen exposure, the hole mobility increases while the electron mobilty decreases. The behavior of the hole mobility is ascribed to the incorporation of molecular oxygen between the bilayer graphene and SiO2 substrate, weakening the long range Coulomb scattering and enhancing the mobilities (for both, electrons and holes). However, as shown in the calculations, the ressonant mechanism acts only on n-type gate, leading to an overall effect of decreasing electron mobilities. Our calculations are based on the SIESTA [3] implementation of the DFT formalism, and, in our models, we explicitly take into account the presence of the SiO2 substrate. [1] P. Hohenberg and W. Kohn, Phys. Rev.,136(3B):B864-B871, Nov 1964. [2] W. Kohn and L. J. Sham, Phys. Rev., 140(4A):A1133-A1138, Nov 1965. [3] José M Soler et al .Journal of Physics: Condensed Matter, 14(11):2745, 2002.

Conductive graphene inks find application in printed electronics, sensors, photovoltaics, and batteries/capacitors. We demonstrate the direct writing of highly concentrated graphene inks (~ 75 wt%) for patterning microelectrodes whose electrical conductivity is ~ 300 S/m after annealing at 200 degree C. Chemically reduced, graphene particles are prepared by solvothermal reduction in N-methyl-2-pyrrolidone (NMP). L-ascorbic acid is added not only to stabilize the particles to achieve the desired highly concentrated, printable inks, but also as a reducing agent during thermal annealing in an ambient condition at low temperature (~200 degree C). Ink formulation, printing behavior, and electrical properties as a function of particle size, binder composition, and annealing conditions will be discussed. In addition, representative printed electronic devices will be demonstrated.

The chemical properties of graphene are yet to be understood. We are investigating the chemical properties of graphene using electrochemical techniques. We fabricated single layer graphene electrodes derived from both mechanically exfoliated graphene and chemical vapor deposited (CVD) graphene. The rate of heterogeneous electron transfer of graphene was found to be one to two orders of magnitude higher than that of the basal plane of highly oriented pyrolytic graphite. The enhanced electrochemical activity is likely due to the presence of corrugations and imperfections that disrupt the sp2 conjugation of graphene which can alter its electrical, chemical, and mechanical properties. We further examined the rate of heterogeneous electron transfer of single layer CVD graphene using scanning electrochemical microscopy. It was found that the sites with a large concentration of defects are approximately one order of magnitude more reactive, compared to more pristine graphene surfaces, towards electrochemical reactions. In addition, we successfully passivated the activity of graphene defects by carefully controlling the electropolymerization conditions of o-phenylenediamine. The use of spatially resolved scanning electrochemical microscopy for detecting the presence and the “healing” of defects on graphene provides a strategy for in situ characterization and control of this attractive surface, enabling optimization of its properties for application in electronics, sensing, and electrocatalysis.

In recent years there has been tremendous interest in creating a new materials platform based on graphene/organic molecule interfaces. The molecule can either alter the electronic properties of graphene by doping or undergoing specific transformations in response to external stimuli such as light which is then read out electrically by graphene. We have reported a route to create a hybrid material by non-covalently latching dipolar molecules on graphene where molecular transformation can be used as an additional handle to reversibly modulate doping while retaining high mobilities. The interaction with graphene can also potentially alter the properties of the molecule itself. Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful analytical technique for ultrasensitive chemical or biochemical analysis as it can produce several orders of magnitude enhancement in Raman signals. Two mechanisms for SERS are widely accepted: electromagnetic mechanism (EM) and chemical mechanism (CM). It has been recently proposed that graphene a 2D layer of carbon atoms packed into a honeycomb crystal plane, can be used as a substrate for Graphene-enhanced Raman spectroscopy (GERS) study. In GERS, the Raman enhancement mechanism is not entirely clear as the enhancement could be due to CM, or strong quenching of fluorescence (FL) from the molecule by graphene. The systematic studies presented here on FL quenching in solution and on graphene in solid state as a function of chromophore concentration, comparison of wavelength dependent Raman signals and spatial mapping of Raman signal from the chromophore on graphene and bare SiO2/Si substrates provide valuable mechanistic insight the GERS effect. In this work, we use graphene as a substrate to probe Raman signal of a dipolar molecule namely pyrene tethered Disperse Red 1 (DR1P). DR1P was deposited homogeneously onto graphene on SiO2/Si and bare SiO2/Si substrates by spin coating. Upon comparison of the Raman signal of DR1P on single layer graphene with that on bare SiO2/Si substrate, we found that the Raman signal in the former was on an average 20 times stronger than the latter. The surface coverage of DR1P on graphene was determined by X-ray photoelectron spectroscopy and correlated with the Raman signal. As the surface coverage of DR1P on graphene increases, Raman intensity also increases and saturates at a certain high concentration. The fluorescence quenching of DR1P as a function of its concentration on graphene was also studied in solid state. In fact the FL quenching leads to reduction in the background and hence, enhancement of the Raman signals. We further show that the Raman enhancement occurs on single layer is much larger than on few layer graphene. However the saturation of Raman intensity at higher DR1P concentrations as well as shift in G-band for graphene indicate a possible contribution from charge transfer from the molecule to graphene.

In this presentation , we demonstrate an easy way to prepare a stable reduced graphene oxide (RGO) dispersion in aqueous or organic media by simple adjustment of the degree of reduction and pH of RGO dispersion, and a subsequent fabrication of transparent conductive RGO thin films on various substrates using a spray coating technique. RGOs were prepared using a hydrazine reducing agent from graphene oxide (GO), which was oxidized from graphite via a modified Hummers' method. The degree of reduction determined the surface properties, such as atomic composition, surface polarity and potential of RGO platelets. In addition, pH significantly affected the surface potential of graphene dispersion. The fine adjustment of degree of reduction and pH of RGO dispersion made production of fine RGO dispersions in aqueous, and organic media such as ethanol and DMF, possible without any aid of dispersing agents. The stable RGO dispersion using volatile ethanol medium provided a unique advantage to be spray-coated into uniform transparent conductive RGO thin films on various substrates including silicone wafer, flexible polycarbonate film, and glass bottle even at room temperature.

The Gprime; (or 2D) Raman band of AB stacked bilayer graphene comes from a double resonance Raman (DRR) process and is composed of four peaks (P11, P12, P21, and P22). In this work, the integrated areas (IA) of these four peaks are analyzed as a function of the laser power for different laser lines. We show that the dependence of the IA of each peak on temperature is different for each distinct laser excitation energy. This special dependence is explained in terms of the electronminus;phonon coupling and the relaxation of the photonminus;excited electron. In this DRR process, the electron is scattered by an iTO phonon from a K to an inequivalent Kprime; point of the Brillouin zone. Here, we show that this electron relaxes while in the conduction band before being scattered by an iTO phonon due to the short relaxation time of the excited electron, and the carrier relaxation occurs predominantly by emitting a low-energy acoustic phonon. The different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the Gprime; band. Some peaks show an increase of their IA at the expense of others, thereby making the IA of the peaks both different from each other and dependent on laser excitation energy and on power level. Also, we report that the IA of the Gprime; mode excited at 532 nm, shows a resonance regime involving ZOprime; phonons (related to the interlayer breathing mode in bilayer graphene systems) in which a saturation of what we call the P12 process occurs. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.

We report room temperature pre-reduction treatment for graphite oxide (GO) which produces hierarchical perforations in GO sheets. The pre-reduction treatment is room temperature stirring of GO with ludox solution followed by acid etching of silica nanoparticles. The technique introduces perforations with hierarchical pore dimensions in the GO sheets. The emergent material, termed as hierarchically perforated graphite oxide (HPGO), on subsequent reduction produces hierarchically perforated graphene nanosheets (HPGN). The perforations produced in HPGO are shown to be retained after reduction in HPGN, albeit with some modifications. It is known that symmetric supercapacitor (SC) made up of chemically produced graphene generally shows specific capacitance (Cs) in the range of 100 - 230 F g-1 for aqueous electrolyte systems but at low current rates. These Cs values are low and at high current density considerable decrease in the Cs value is observed. We tested SC properties of our perforated graphene (HPGN) as an electrode material in 1M H2SO4. At an applied current rate of 0.25 A g-1 the observed Cs was found to be remarkably higher, namely 450 F g-1 with an energy density of 62.5 Wh kg-1. The value of Cs was 204 F g-1 even at high current rate 20 A g-1. Additionally HPGN based SC showed good cycleability with small fading (7%) of Cs after 1000 cycles at a high current rate of 2 A g-1. We thus demonstrate that engineering perforations in graphene nanosheets is the key to realize very high value of specific capacitance in a supercapacitor.

Hydrogenation of carbon-based nano materials has been an attractive topic to researchers who develop new nanoscale engi-neering materials for electronic devices and hydrogen storage systems. As the hydrogenated graphenes have different properties, there have been many researches on the structure, thermodynamics and kinetics of chemisorbed hydrogen on graphene surface. Kinetics of hydrogen adsorbed on graphene surface is im-portant for hydrogen storage mechanism in carbon-based materials. Although taking into account the quantum effects in ionic motion it can be reduced, the diffusion barrier of a chemically adsorbed hydrogen atom on the pristine graphene surface has been estimated to be around 1 eV and a move from the edge of a stable hydrogenated island formed on the graphene surface needs to overcome around 3 eV barrier, therefore it was suggested that in order for the diffusion to take place with ease at room temperature, physisorbed hydrogen atoms or defect structures should be involved in the spillover process, but this issue is still debated. Besides the hydrogen storage, in order to model the realistic mechanisms of graphene hydrogenation and finely control the graphene properties, understanding the atomic hydrogen kinetics on graphene surface is indispensable. In this study, diffusion of hydrogen atom in the multilayered graphenes was investigated by density functional theory augmented with dispersion corrections. Inside the multilayered graphene with AB stacking, the most favorable pathways are mainly composed of the in-plane diffusion paths with the lower energy barrier (about1.1 eV for the bilayer graphene) and the interlayer jump paths have the higher energy barrier (about1.3 eV for the bilayer graphene). The glide defect made by shifting layer planes makes the energy barrier of the interlayer jump paths lowered to about 0.9 eV so that the adsorbed hydrogen atom diffuses through consecutive jumps from one layer to adjacent layer.

It has been demonstrated in several works that graphene has a high potential to be used as an extremely sensitive sensor in a wide variety of applications ranging from gas sensors to single molecules sensors. The remarkable sensitivity of these graphene based sensors is related to the fact that graphene is a truly bidimensional system and also because its electrical transport properties are profoundly changed when chemical species- simple and complex molecules - are absorbed or chemically bound to its surface, either by charge transfer or by an external electric field. Despite this great sensitivity, the selectivity of grapheme sensors to different chemical species is not very good. Some strategies have been used to overcome this hurdle and one good example is the use of biomarkers on antibodies to promote the selectivity in applications of graphene as biosensors. The final objective of this work is to develop highly sensitive and selective biosensors based on graphene. This will be accomplished by binding both natural and synthetic antibodies (aptamers) to the graphene surface. As we aim at producing devices that can be fabricated in a large scale, commercial graphene grown by CVD on copper foils is our starting point. In this work, we report on the results obtained on the transference of CVD graphene to SiO₂/Si substrates and on the characterization of the transferred material by optical microscopy, SEM and Raman spectroscopy. High quality films have been obtained which preserve a good morphology correlation with the original films on copper foils. Raman spectroscopy was used to characterize different regions of the transferred films, showing high quality monolayer (or decoupled bilayers/few layers in some regions). Several devices have been fabricated using direct laser writing pholithography and electrical transport characterization of regions with different morphologies and quality are underway.

Two dimensional (2D) carbon nanomaterials such as few graphite layers or graphene are extensively studied due to their unique properties suitable to be exploiting in a wide range of technological applications. Recently, it was reported in the literature the growth of high quality graphene monolayers using food, insects and waste as carbon precursors. This methodology opened a new way to convert the waste carbon into a high-value-added product. In the present work coconut coir dust, an agroindustrial biomass, was used as biotemplate for preparing carbonaceous materials. Carbon structures were synthesized through pyrolysis under nitrogen atmosphere (100mL/min) at 500, 750, 1000, 1250 and 1500°C during 2 hours. Starting materials were coconut coir dust in natura and coconut coir dust treated by hydrothermal method. The samples were characterized by X-ray diffraction, Raman Spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Raman spectra showed the D band for all samples, related to the presence of defects in sp² carbon structure and G band, indicative of graphite crystallites. It was also observed that the sample carbonized at 1500°C from coconut coir dust treated by hydrothermal method showed G&’ band at 2685cm ^-1 associated with the stacking order along the c-axis. X-ray diffraction analysis showed a broad peak around 2theta;= 22° related to the presence of amorphous carbon. By increasing the pyrolysis temperature it was observed changes in XRD profiles and the sample pyrolysed at 1500°C from coconut coir dust treated by hydrothermal method showed peaks at 2theta;= 26.5°, 43° e 45° assigned to (002), (100) (101) graphite plans, respectively. Scanning electron microscopy images showed the presence of overlapping sheets and plates. Transmission Electron Microscopy (TEM) images of coconut coir dust in natura unveiled the formation of amorphous sheet. While the use of coconut coir dust in natura and treated by hydrothermal methotd pyrolysed at 1500°C lead to the formation of some graphitic domains and few graphene layers. TEM analyses of the other samples are in progress.

Existence of ferromagnetism in recently discovered grapheme: one-atom-thick two-dimensional (2D) layers of sp2-bonded carbon are both fundamentally and technologically extremely important for many future applications, while it is not fully understood; several theoretical studies have proved the main cause of ferromagnetism in graphene due to various defects. The observed results established that these defects can be mended by treating the samples at elevated temperatures. Hence, temperature plays a crucial role in controlling the magnetism in graphene. Extensive and keen efforts are thus needed to have a fine control over magnetism in graphene. In this study, we report the experimental results highlighting the ferromagnetism in chemically reduced graphene by varying the temperature from 200-800 deg. at the step of 100 deg. under the inert atmosphere and observed the noticeable change in the degree of magnetization. Initially the magnetization enhanced systemically up to 500 deg. and further decreased with temperature. We can conclude that the temperature above 500 deg. is the starting stage to repair the defect mechanism, which results in the decrement of the magnetism leaving the graphene with lesser defects. Therefore, temperature is very crucial tool to mend the defects in grapheme to specific extent also having the magnetic properties. Hence, our study established a very simple way to enhance the ferromagnetism by creating defects in graphene by tuning the temperature, which is very useful tool to fabricate the future nanodevices based on graphene.

Graphene is widely regarded as one of the most promising materials for sensor applications. There have been tremendous efforts focusing on ways to develop more sensitive graphene sensors in recent years. Here we demonstrate that, without functionalizing or decorating graphene, a two-terminal pristine graphene based sensor can detect gas molecules at extremely low concentrations with detection limits down to 158 parts-per-quadrillion (ppq) for a range of vital gas molecules (NO, NO2, NH3, N2O, O2, SO2, CO2 and H2O) at room temperature. These unprecedented sensitivities were achieved by applying our recent reported concept of continuous in situ cleaning of the sensing material with ultraviolet light [1]. The simplicity of the concept, together with easy fabrication and graphene&’s transparency and flexibility to be used on various platforms, is expected to intrigue more investigations to develop pristine graphene based ultrasensitive sensors. [1] Chen, G.; Paronyan, T. M.; Pigos, E. M.; Harutyunyan, A. R., Scientific Reports 2012, 2, 343.

Recent researches on carbon nanomaterials such as graphene and carbon nanotubes are concerned with a great deal of their fundamental properties and applications. Kondo et al. (2008) experimentally manufactured a carbon nanostructure named ‘pillared-graphene&’. This nanostructure consists of two parallel graphene sheets and single-walled carbon nanotubes (SWNTs); the SWNTs connect the two graphene sheets as pillars whose axes are perpendicular to the plane of the graphene sheets. Since the thermal and electronic properties of pillared-graphene are superior, this nanostructure is expected to be applied to large-scale integrated (LSI) circuits. Loh et al. (2012) reported that the thermal boundary resistance, what is called the Kapitza resistance, of pillared-graphene varies under torsional and tensile loads. Therefore it is important to understand the mechanical behaviors of pillared-graphene under various loading conditions when its application to structural materials in LSI circuits is considered. In this paper, the deformation of pillared-graphene nanostructures is investigated using molecular-dynamics simulation. Two kinds of nanostructure models are prepared; one consists of two graphene sheets and a SWNT, while the other consists of two graphene sheets and two SWNTs. For both kinds of models, a nanoscale force in the direction within the plane of the graphene sheet is imposed on the edge of one graphene sheet, and the other sheet is fixed. For the first kind of models, the in-plane force is either in the zigzag or armchair direction of the graphene sheet. For the second kind of models, the distance between the two SWNTs is changed and their interaction effect is examined. The RAMMPS simulation code is employed. The integration of equations is performed using the Verlet algorithm. The Nose-Hoover thermostat is used to control temperature. The interactions between atoms are calculated using the adaptive interactive reactive bond-order (AIREBO) potential. How the shear strength of pillared-graphene depends on the direction of shear and the distance between the pillars is examined.

Graphene has the chemical structure consisting of two-dimensional layer of sp2-bonded carbon atoms, and have attracted considerable interest over the last ten years due to the extraordinary electrical, thermal, and mechanical properties arising from its unique structure. Polyaniline has attracted much attention in the past decades owing to its unique properties, such as facile synthesis, high environmental stability, and simple acid/base doping/dedoping. Graphene oxide was obtained using modified Hummer&’s method and was reduced by sodium borohydride. We prepared nitrogen doped graphene with melamine using Chemical Vapor Deposition and composite was obtained in the presence of chemically modified graphene and aniline monomer under acid conditions through direct electrochemical polymerization. We used a piece of indium-tin oxide glass sheet as the working-electrode and a platinum sheet as the counter-electrode. We investigated the effect of graphene and as filler on thermal and electrochemical properties of the composite. The surface morphology and structure of polyaniline, graphene and composites were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermo gravimetric analysis-differential scanning calorimetry (TGA-DSC) and Fourier transform infrared spectroscopy (FT-IR). TGA analysis of graphene oxide, graphene, polyaniline and the composite were made to compare the effects of thermal stability under a nitrogen atmosphere. The successful growth of polyaniline along the surface of carbon materials was verified by FT-IR and X-ray photoelectron spectroscopy (XPS) analysis. Electrochemical properties of graphene, polyaniline and composite were studied using cyclic voltammetry.

Graphene, truly two-dimensional (2D) atomic crystal which consists of sp2 bonded carbon atoms with honeycomb lattice, has been attracting tremendous interests due to its outstanding and novel electrical, mechanical, thermal, optical, and chemical properties. Thanks to its novel characteristics, graphene has a great potential to be a strong candidate for technologies that may go beyond the conventional Si based electronic devices. However, an undoped monolayer graphene acts as a zero-energy gap semiconductor, which currently limits its applications to electrostatic switchable devices. For these reasons, many research groups have been investigating various methods of opening a band gap in graphene. Those approaches include the fabrication of graphene nanoribbons with the widths of less than a few 10nm, chemical modification of graphene layer with chemically asymmetric doping in different charge-carrier densities, gap engineering of bilayer or multi-layered graphene, and uniaxial strain induced by tensile stress on graphene. Those results have demonstrated band gap openings up to a few 100meV but are still insufficient to meet a required electrostatic gating efficiency. One group theoretically predicted a significant gap (~1eV) by providing inhomogeneous strain condition to a graphene, which may potentially open up photovoltaic and electrostatic gating device applications of graphene. Here we investigate a strain-induced band gap of graphene on a hexagonal boron nitride (h-BN). The pure h-BN is used to synthesize a graphene layer by controlling the ratio of CH4 and Ar at a processing temperature of 1000°C. With the application of the periodic inhomogeneous strain to a graphene sheet, bandgap characteristics is evaluated along with the physical chacterizations. Scanning Transmission Electron Microscopy (STEM, JEOL-JEM-ARM200F) is used for imaging and structural analysis of graphene/h-BN layer. Sample's surface morphology was characterized by Field emission scanning electron microscopy (SEM, JEOL JSM-7100F). Epitaxially grown graphene on h-BN is analyzed by Raman spectroscopy (HORIBA , ARAMIS) and its topographic variations by Atomic force microscopy (AFM, Park Systems XE-100).

Recently, functionalized graphenes are spotlighted as next leading materials, such as metal nanoparticles (NPs) decorated graphene since their unique and enhanced properties. Among of metal NPs binding graphene, a lot of researchers are interested in Au NPs decorated graphene nano sheets (AuN-GrNS) because of their electrical property, raman scattering signal, catalytic behavior and surface plasmon resonance (SPR). In this study, we successfully prepared AuN-GrNS via phytochemical which are gallic acid and isoflavone as reductant under the r.t. The first synthesis step is intercalation of Au ions into graphene multi layers and next step is conversion from Au ions to Au NPs by phytochemicals on the graphene and at the same time, exfolication is occurred. AuN-GrNS were analyzed by TEM, SEM, AFM, UV/Vis, FT-IR spectroscopy, XRD, Raman spectroscopy, Semiconductor parameter analyzer, fluorescence spectrophotometer and fluorescence lifetime spectroscopy. From electron microscope images, Au NPs is observed that they are well attached on the graphene layer. And electrical conductivity and raman intensity is enhanced. Indeed PL intensity of LBL film which is composed with QD and AuN-GrNS is also enhanced about 2 fold. Therefore, from this methodology, AuN-GrNS can be simply prepared and this novel AuN-GrNS would have potential applications in various fileds.

Graphene offers superior chemical stability, large specific surface area , high electrical conductivity, and broad electro-chemical window. It is an ideal electrode material for electric double-layer capacitors (EDLCs) or supercapacitors. We have produced processed graphene for supercapacitors using a modified Hummers method and electrolytic exfoliation. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) have been applied to examine the structure of the produced graphene and confirmed the existence of both monolayer graphene and few-layer graphene sheets. To enhance its structural integrity as well as electrochemical performance, we also designed and fabricated graphene based composites with single-walled carbon nanotubes (SWCNTs) for applications as the electrodes of supercapacitors. In the graphene/SWCNT composite, the SWCNTs acted as a conductive additive, spacer, and binder. As the SWCNTs are inserted between graphene sheets, many wrinkles would be produced because graphene is topologically very flexible and the re-stacking of graphene is effectively avoided. Using the graphene-SWCNT composite as the electrodes of supercapacitors, an ultra high specific energy of about 155 Wh/kg has been achieved, offering great promise for industrial applications including the power supply system for electric and hybrid vehicles.

CVD graphene is a two-dimensional hexagonal lattice of carbon atoms with thickness of one or a few atomic layers. Due to its material stability and strength, absence of defects, and unique electronic band-structure, CVD graphene holds considerable promise for a number of applications in nanoscale electronics, optoelectronics, and mechanics, as well as being of fundamental interest in condensed matter physics. Many potential applications, such as high-speed field-effect transistors, require CVD graphene to be patterned at the nanoscale. In addition, it is difficult to control the etch depth precisely through the reactive ion etching due to the fluctuation of the etch process. The possibility of fabricating a full graphene device was investigated by utilizing atomic layer etching (ALET) technology. In this study, the etch characteristics of CVD graphene by ALET were investigated using an O₂ radical and Ar neutral beam. For the few layer CVD graphene deposited on the SiO₂ (300 nm)/ Si substrate, the monolayer etching condition of CVD graphene was observed by supplying O₂ radical at a pressure higher than the critical pressure during the adsorption step and by supplying an Ar beam at a dose higher than the critical dose. The removal of exactly one graphene layer per ALET cycle was detected through the increase of the transmittance by 2.3% after one ALET cycle. Full graphene field effect transistors were fabricated by reducing the channel layers using ALET, followed by annealing, and the electrical characteristics of the devices showed the possibility of fabricating fully functional graphene devices composed of an all graphene source/drain and graphene channel by utilizing ALET.

In this paper, a hydrothermal method has been presented for the first time to prepare a hemin-graphene nanosheet (HGN) based electrochemical biosensor for determining L-tyrosine levels. As-fabricated HGN biosensor was characterized by UV-visible absorption spectra, fluorescence spectra, fourier transform infrared spectroscopy (FTIR) spectra, and thermogravimetric analysis (TGA). The experimental results indicated that hemin was successfully immobilized on reduced graphene nanosheet through π-π interaction. TEM images and EDX result further confirmed the attachment of hemin on graphene nanosheet. The cyclic voltammetry (CV) tests were carried out for the bare glass carbon electrode (GCE), the graphene electrode (GN/GCE), and the hemin-graphene electrode (HGN/GCE). The HGN/GCE based biosensor exhibits a Tyrosine detection linear range from 5×10-7 M to 2×10-5 M with a detection limitation of 7.5×10-8 M at signal noise ratio (S/N) of 3. The sensitivity of this biosensor is 133 times higher than the bare GCE. In comparison with other works, electroactive biosensors are easy-fabricated, easy-controlled and cost-effective. Moreover, the hemin-graphene based biosensors demonstrate higher stability, a broader detection linear range, and better detection sensitivity. The study of oxidation scheme reveals that graphene enhanced the electron transfer between electrode and hemin, and the existence of hemin groups effectively electrocatalyzed the oxidation of tyrosine. This study contributes to a widespread clinical application of nanomaterial based biosensor devices with a broader detection linear range, improved stability, enhanced sensitivity, and reduced costs.

Vibrational properties and phonon spectra of graphene are of fundamental interest from which many physical properties such as thermal conductivity and heat capacity can be derived. A number of research groups have attempted to calculate the phonon spectra in graphite and graphene. However, these studies are limited on vibrational properties of graphite or pristine graphene only. In this work, the numerical analyses on the lattice vibration of the hydrogen passivated percolation network of graphene are performed. The forced vibrational method proposed by Williams et al. [1], which is based on the mechanical response to extract the pure vibrational eigenmodes for very complex and large systems by numerical simulation, has been applied to calculate the phonon density of states (PDOS) of percolation network of graphene with all dangling bonds are terminated by H atoms. The percolation networks in the present model are formed by the bond percolation procedures. Only interactions up to the fourth neighbor atom which is necessary to get accurate results is used. In case of H terminations, there appear high and low frequency region in the PDOS. The PDOS in the hydrogen passivated percolation network strongly depends on the bond probability and the decrease of PDOS at low frequency region induced by their percolation network structure. From the simulated PDOS, we have also observed that the high-frequency modes as dominantly H modes, as can be expected from the C-H stretching modes. In conclusions, the lattice vibrations in the hydrogen passivated percolation network of graphene show the remarkably different properties such as spatial localizations of lattice vibrations induced by the H atoms and percolation network structure. References: [1] M. L. Williams and H.J. Maris, Phys. Rev. B31, 4508-4515(1985)

Chemical doping to graphene is one of the important factors to control its electrical properties¹⁾. In particular, water molecules bound to graphene surfaces induce hole-doping in aqueous environment. In this paper, we show adsorption behaviors of protein molecules that were influenced by the chemical doping from the interfacial water layers. We also demonstrate that the force curve measurement on graphene surfaces is powerful to reveal the interaction between graphene and adsorbates. We deposited graphene flakes on single-crystalline sapphire (0001) and (1-102) surfaces by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). To reveal influence of a water layer at the interface between the sapphire surface and graphene, we annealed the sapphire surfaces at 700 °C for 1h just before graphene deposition. We observed blue shift of the G-peak and 2D-peak positions on the hydrophilic sapphire (0001) substrate. But, in the case of (1-102) surfaces, the G-peak and 2D-peak positions did not blue shift upon annealing. It has been reported that water layer coverage on sapphire surfaces depends on their plane directions²⁾. Therefore, the peak shifts were induced by the hole doping by the water layers that existed at the interfaces between the sapphire surfaces and the graphene flakes. We investigated adsorption of protein molecules to the same graphene/sapphire surfaces. We used ferritin molecules, which are negatively charged for adsorption to the graphene flakes. Ferritin molecules were found to be preferentially adsorbed to the graphene flakes that were supported by the hydrophilic (0001) surfaces. Amount of ferritin molecules adsorbed to the graphene flakes supported by the other substrate surfaces were dramatically small. These results indicate that adsorption behaviors of protein molecules to graphene are strongly affected by chemical doping owing to bound water molecules. We used atomic force microscopy (AFM) equipped with a colloidal probe to measure the force between the cantilever and the graphene surface. From force measurements of AFM in a buffer solution, we detected a long range repulsive force before tip jump into contact was observed during the approach. When ion strength in the buffer solution was high, the repulsive force was not observed. This indicates that the origin of repulsive force is an electrostatic interaction. In summary, we have shown the effect of chemical doping from the water layer to graphene flakes in aqueous environment through protein adsorption behaviors. By the force curve measurements using AFM, we can directly characterize surface chemistry of graphene in aqueous environment. 1) F. Schedin et al. Nat. Mater. (2007) 6, 652-655. 2) T. Tsukamoto et al. J. Phys. Chem. C (2012) 116, 4732-4737.

SnO₂ nanocrystals anchored on graphene oxide were synthetized by solvothermal route. The Graphene oxide composite was characterized by X-ray diffraction (XRD), Raman Spectroscopy, Fourier Transform spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). The samples were analyzed to investigate an interesting photocatalytic behavior. The presence of graphene contributes as an electron transporter of photocurrent generated charge carriers from tin dioxide nanoparticles.

The introduction of defects in graphene sheets is an alternative for manipulating the physical and chemical properties of this material. Several works have been performed[1-2] based on the introduction of defects in graphene, followed by the investigation of the ensuing changes in the material's electronic and structural properties. In this work, we investigate the introduction of four different types of grain boundaries on the top layer of bilayer graphene systems with a pristine bottom sheet. The introduction of these extended one-dimensional defects disrupts the Bernal stacking of the two layers. As a consequence, as in the case of graphene bilayers with a relative rotation between layers[3], that gives rise to Moirré patterns, we observe changes in the electronic structures of these defective bilayers: the character of AB stacked bilayer graphene is lost, giving rise to an electronic structure formed by the combination of the uncoupled states of the defective and pristine sheets. [1] J. Lahiri, Y. Lin, P. Bozkurt, I. I. Oleynik, and M. Batzill, Nat. Nano. 5, 326-329, (2010). [2] Correlated magnetic states in domain and grain boundaries in graphene; arXiv:1109.6923v1 [3] J. M. B. Lopes dos Santos, N. M. R. Peres, and A. H. Castro Neto, Phys. Rev. Lett. 99, 256802 (2007).

Epitaxial Graphene (EG) synthesized on the conventional polar planes of SiC substrates has shown great promise in high frequency and scalable graphene field effect transistors¹ despite challenges of step-bunch induced conductance anisotropy² and an interface layer (IFL)³ for growth on (0001) and wrinkled multi-layer EG on (000-1). In this work, we systematically grew EG on 6H-SiC non-polar (1-100) and (11-20) surfaces, as well as the semi-polar (01-14) surface, to elucidate and compare the resulting structure with typical EG on polar faces. EG was synthesized on 6H-SiC(1-100), (11-20) and (01-14) substrates with side dimensions 8-16 mm in size using a hot-wall Aixtron VP508 CVD reactor. Prior to growth, an in-situ H₂ etch step was used to produce a smooth morphology (RMS ~ 0.11 nm). After etching, the H₂ was evacuated, and subsequent EG growth was conducted at temperatures 1520 - 1620°C in a flowing Ar ambient of 10 slm and 100 mbar. Surface characterization techniques such as x-ray photoelectron spectroscopy (XPS), mu;-Raman spectroscopy (532 nm laser), atomic force microscopy (AFM) (tapping mode) and scanning electron microscopy were used to extract composition, presence, thickness, strain and surface morphology variations of EG across the samples. High resolution transmission electron microscopy was used to confirm layer thickness. The RMS roughness of a 10 x 10 mu;m² AFM scan of non-polar EG layers had values ranging from ~0.54 to 1.2 nm with no step-bunch and terrace morphology as seen for (0001) surfaces with typical roughness of ~2.3 nm. Growth on the semi-polar surface resulted in wrinkles as well as vertical ridges that are preferentially oriented parallel to each other, compared to the RMS values of 2.3 and 3.7 nm for EG on (0001) and (000-1) surfaces, respectively, all the EG films grown on non-/semi-polar orientations were markedly smoother. The Raman 2D mode position remains relatively constant over the mapped area (100 mu;m²) with an average of ~2708 ± 4.6 cm^-1, indicating uniform strain approaching values of strain-free exfoliated graphene 2684-2696 cm^-13. This contrasts with EG on polar substrates which show higher non-uniform strain, i.e., 2D-mode positions ranging from 2735 ± 3.7 cm^-1 on the terrace to 2747 ± 5.0 cm^-1 at step edge. XPS C-1s core-level data shows the splitting of the SiC (283.4 eV) and EG (285.0 eV) peaks similar to (000-1) grown layers⁴; this and other XPS features suggest that no interface layer is present on these non-polar surfaces unlike EG on the (0001) surface. The lack of an IFL for EG grown on non-polar orientations offers an attractive explanation for the recent favorable results for graphene nanoribbons formed on (1-10n) surfaces⁵. [1] Moon, J S., et al., IEEE El. Dev. Lett. 31, 260 (2010); [2] Lin, Y.-M. et al. El. Dev. Lett. 32, 1343 (2011); [3] A. Das, et al., Nat. Nanotech. 3, 210 (2008); [4] Jernigan G. et al., ACS Nano Lett. 9, 2605 (2009); [5] De Heer W. et al., nat. nanotech. 5, 727 (2010).

Recently the increased interest of carbon-based materials decorated with metal nanoparticles (NPs) for sensors and energy applications has generated a huge development of the research in this field. In the present research, we developed a simple synthesis method of metal NPs on graphene layer and we investigate the optical properties, the uniformity and the morphology of the adsorbed NPs on one layer graphene. Graphene prepared by CVD method on copper foil was decorated with gold or silver NPs, by using a simple and efficient method of solution plasma processing (SPP). SPP was developed in our laboratory as a general method for nanoparticles synthesis. The system graphene - metal NPs was transferred on different substrates as silicon or glass. The system graphene - metal NPs was characterized by micro Raman mapping spectroscopy, transmission electron microscopy (TEM) and UV-vis spectroscopy. As prepared, graphene was confirmed to be a single layer by using Raman spectroscopy. Micro-Raman mapping of 2D band at 2700 cm^-1 (two photon double resonance band) which has higher intensity than G band at 1583 cm^-1 (interlayer vibrations band of sp²-hybridized carbon) shows the quality and the uniformity of graphene layer on substrates. An enhanced Raman spectrum of graphene was detected when gold or silver NPs were adsorbed on the surface. The UV-vis spectra of graphene layer decorated with gold NPs which was transferred on a glass substrate shows a week absorption band of surface plasmon resonance (SPR), at 520 nm, due to the thin layer of NPs on surface. By changing the size of metal NPs we can tune the SPR band around 520 nm. TEM analysis showed that NPs are around 5 nm diameters and uniformly dispersed on graphene layer.

We studied the polarization dependence of the photocurrent in metal-graphene-metal photodevices [T. Muller et al, Nat. Photonics 4, 297 (2010)]. Graphene photodevices were fabricated by depositing Pd/Au and/or Ti/Au electrodes on single-layer graphene on Silicon dioxide-covered silicon substrates by using e-beam lithography. Photocurrent images were taken by scanning a focused laser beam across the photodevice. Raman spectra and photocurrent images were taken simultaneously in order to identify the exact position of the photocurrent. Polarization dependences were studied by varying the polarization of the laser beam. We found that the photocurrent is maximum when the polarization of the incident laser is parallel to the edges of electrodes. In order to make sure that the observed polarization dependence is not due to some artifacts of the experimental set up, we confirmed the result of by rotating the sample by 90 degrees. Furthermore, the polarization dependence was compared for different electrode metal combinations. This polarization dependence can be directly explained as being due to the anisotropic electron-photon interaction of Dirac electrons in graphene: the light absorption is known to be maximum when the polarization of the photon is perpendicular to the momentum of the electron being excited [A. Grueneis et al, Phys. Rev. B 67, 165402 (2003)]. The polarization dependence was examined as a function of the distance between the electrodes in order to investigate the effect of ballistic transport.

Graphene emerged as a reliable carbon based support system for Pd, Au and Pt nanoparticles in various applications viz. nanocatalyst for hydrogen storage, methanol oxidation for fuel cell and nanoelectronics. In such hybrids, the properties are largely controlled by the nature of the heterointerface. For instance, the efficiency of catalyst nanoparticles depends on the nature of the support and the catalyst/support interface in addition to the dispersion, size, shape and stability of the catalyst. However, synthesizing these hybrids remains a challenge in terms of controlling the particle size, distribution of the noble metals nanoparticles on the graphene sheets and the stability of the catalyst under different conditions. Typical approaches rely on surfactants to achieve size control and molecular linkers to attach the catalyst to graphene. We demonstrate a microwave assisted wet chemical route for rapid synthesis of graphene-based Pt nanoparticle hybrids without employing any surfactant and linkers. The method involves ethylene glycol as the solvent and reducing agent. A synergistic co-reduction mechanism describes the formation of the hybrid whereby the presence of the Pt ions leads to a faster reduction of GO and the the heterogeneous nucleation of Pt is facilitated by the presence of the defect sites on the reduced GO. Morphological characterisation and mechanistic investigation has been carried out using high resolution transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR) and x-ray diffraction (XRD). The resulting hybrid consists of reduced GO susbtrate uniformly decorated with ultrafine Pt particles. These hybrids shows exceptional activity for methanol oxidation with good long term stability. Besides, the hydrogen conversion efficiency of the catalyst is found to be excellent at room temperature conditions making it an ideal multifunctional hybrid based on graphene. The method enables excellent control over particle size, distribution on the support and the interface without the aid of functionalization and surfactant. Our approach is general and can be applied for other graphene-metal composite systems for a variety of applications.

Many theoretical and experimental investigations have indicated that the characteristics of finite-sized graphene are strongly affected by the edge type, such as armchair and zigzag. Recently, crystallographic etching of graphene/graphite has attracted much attention because of the high potential for the selective production of zigzag edge. Crystallographic etching of graphite surface is conducted by Ni, Co nanoparticle under reductive environment and by Ag nanoparticle under oxidative environment. In this study, we developed the facile production method for edge-controlled graphene using supercritical water, which can be act as oxidative agent. As a result, we succeeded in the efficient anisotropic etching of graphite surface under relatively low-temperature condition below 500 degreeC in supercritical water, whereas conventional gas-phase oxidative etching using Ag nanoparticle needed high temperature above 650 degreeC. Moreover, after supercritical treatment, suspended materials in the water are observed. By using transmission electron microscopy (TEM) and Raman spectroscopy, it is revealed that the graphene with zigzag edges which is introduced by etching in supercritical water are exfoliated. It is highly supposed that the anisotropically-etched graphene layer was isolated from the surface of graphite due to the exfoliation effect of supercritical fluid.

Graphene oxide (GO) and derivatives thereof hold promise to be exploited as a new class of carbon material for high performance electrical energy storage devices, such as ultracapacitors. GO, a nanonoporous electrode material, is an alternative to the commonly used activated carbon. However, it is important to optimize the supporting electrolyte and the carbon electrodes so that the energy storage performance can be maximized. Thus, GO is a suitable alternative material with its high surface area nature by tailoring its chemical functionality. Ionic liquids (ILs) have been explored as an alternative to the commonly used aqueous and organic electrolytes to improve electrochemical and thermal stability at high operating potentials and temperatures. To optimize the GO-based ultracapacitor performance, it is vital to understand the chemical interaction (covalent versus non-covalent) between reduced GO (rGO) and various ILs. The current study focuses on the interactions during reduction of GO by thermal annealing in the presence of intercalated four selected ILs (N-methyl-N-octylpiperidinium trifluoromethanesulfonate, N-methyl-N-octylpyrrolidinium trifluoromethanesulfonate, N-methyl-N-octylpyrrolidinium bromide, and N-methyl-N-octylpiperidinium bromide). Thermal reduction of GO provided exfoliation of these GO-IL composites in either deionized (DI) water or in propylene carbonate (PC). The physical and chemical interaction between ILs and rGO were further studied as a function of annealing temperature by in-situ infrared absorption spectroscopy (IRAS). Powder X-Ray Diffraction (PXRD) was also performed to analyze the interlayer distance of GO-IL composites both at room temperature and as a function of annealing temperatures. For instance, the d-spacing was observed to increase by about 3 Å after intercalating GO (9Å original d-spacing) with N-methyl-N-octylpyrrolidinium trifluoromethanesulfonate at room temperature. The loss of C-N and C-O species was identified by IRAS at ~200°C due to the disappearance of the peaks at ~1000-1500 cm-1 and 800-1200 cm-1, respectively, which was attributed to the removal of the IL. Thermogravimetric analysis indicated that the degree of rGO expansion was elevated so that further sheet exfoliation was possible with temperature. XRD analysis also confirmed an expanded d-spacing of the IL at ~150-300°C. IR studies were also helpful to understand the exfoliation characteristics based on the chemical nature of the individual ILs' cations and anions. Raman spectroscopy was also employed to evaluate the defect mechanism in rGO-IL composites. Overall, rGO-based ultracapacitors using pyrrolidinium and piperidinium-based IL systems have shown to vary the capacitor performance based on the nature of the anions (trifluoromethanesulfonate vs. bromide). This work therefore highlights the impact of optimizing chemical and physical interlayer interactions that improves the rGO-based ultracapacitor performance.

Gate modulated and laser dependent Raman spectroscopy have been widely used to study q = 0 zone center phonon modes, their self-energy and their coupling to electrons in graphene systems. In this work we use gate-modulated Raman of q ne; 0 phonons as a new technique to understand the nature of five second order Raman combination modes observed in the frequency range of 1700-2300 cmminus;1 of single layer graphene (SLG). Anomalous phonon self-energy renormalization phenomena are observed in all the five combination modes within this intermediate frequency region, which can clearly be distinguished from one another. By combining the double resonance Raman theory with the anomalous phonon renormalization effect, we show a new technique for using Raman spectroscopy to identify the proper phonon mode assignment for each combination mode. By using this new approach and comparing the experimentally obtained phonon dispersion, measured by using different laser excitation energies, with the help of both theoretical phonon dispersion relations and the angular dependence of the electron-phonon scattering matrix elements, we can assign each Raman peak to the proper phonon combination mode. This new approach will shed light on the understanding of more complex structures such as few-layer graphene (FLG) and stacking orders and other 2D-like materials.

Single-layer-graphene-assembled porous carbon (SLGAPC) is synthesized by a single step catalyst/template free pyrolysis of polymer poly (styrene 4-sulfonate-co-maleic acid) sodium salt. The as-synthesized SLGAPC is characterized by various characterization techniques such as X-Ray diffraction, Raman Spectroscopy, High Resolution Transmission Electron Microscopy and BET surface area measurements etc. SEM and TEM images of SLGAPC clearly show its highly micro-meso-porous nature. Interestingly, the morphology is well-defined with hexagonal pores of uniform size and shape. The Raman spectrum of as-synthesized SLGAPC confirms the nature of high quality single layer graphene as the primary building block of this material. It shows the presence of G band at 1585 cm-1 and presence of D band at 1350 cm-1, but more importantly the presence of 2D band at 2700 cm-1 attributable to single sheet graphene. The specific surface area of the as-synthesized SLGAPC is around 1720 m2/g as measured by BET technique. The SLGAPC is a highly conducting sample with specific conductivity of 100 S/m. Cyclic voltammetry (CV) measurements were performed by varying the scan rates between 10 and 200 mVs-1 with a potential window of 1.0 V in 1M H2SO4 solution. The CV curves exhibit a pure rectangular shape without any redox peaks (oxidation or reduction) with strong capacitive behavior which indicates efficient charge storage with fast ion transport in aqueous electrolytes. The charge-discharge studies of SLGAPC shows maximum capacitance of 154 F/g at a current density of 0.5A/g.

Among the several possible ways to functionalize materials, we investigate in this work one of large current interest; the Self-Assembled Monolayers - SAMs, on graphene and boron nitride. We suppose SAMs of phosphonic acids with a general chemical formula CH₃ (CH₂ )_nPO(OH)₂ ; specifically, we will suppose the octadecyl phosphonic acid (OPA, n =17), the tetradecyl phosphonic acid (TPA, n =13), and the octyl phosphonic acid (OcPA, n =7). In experimental works [1], it was observed that the phosphonic acids OPA, TPA and OcPA form similar structures when deposited on substrates - self-assembled bilayers (SABs, or bilayers). We investigated, by means of first principles calculations based on density functional theory (DFT), the energetics and the structural properties of graphene with adsorbed dimers of TPA, OPA and OcPA acids, and OPA on boron nitride substrate. We studied several possible configurations for dimers deposited along two distinct directions of graphene and boron nitride: armchair and zigzag. We also made calculations of these dimers deposited in a vertical orientation relative to the plane of graphene. Our calculations show that the preferred orientations of all dimers of the studied molecules atop graphene and boron nitride are along the zigzag direction, and that the dimers remove electrons from graphene (hole doping). For boron nitride the doping is much smaller, and with the opposite sign (n-type). Our predictions are consistent with experimental results with graphene as substrate [2]. [1]G. N. Fontes, R. L. Moreira, and B. R. A. Neves. Thermally induced stacking of octadecyl phosphonic acid self-assembled bilayers. Nanotechnology, 15:682, 2004. [2]M.C. Prado, R. Nascimento, L.G. Moura, M.J.S. Matos, M.S.C. Mazzoni, L.G. Cancado, H. Chacham, and B.R.A. Neves. Two-dimensional molecular crystals of phosphonic acids on graphene. ACS nano, 5(1):394 - 398, 2011.

Nonmagnetic graphene and micron size carbon particles in flakes form are immersed separately in a stable colloidal Fe3O4 nanoparticles suspension. The carbon particles behave like a platelets nonmagnetic volume inside a magnetized continuum medium; as a consequence, the carbon particles exhibit the characteristics of a magnetic dipole when the fluid is inside a magnetic field. In this case the carbon particles experiences dipole dipole interactions which results in the formation of a chain like structure in field direction. Effective thermal diffusivity of the smart fluid was measured taking into account the direction and intensity of an applied homogeneous magnetic field. Thermal wave cavity technique was used to measure the effective thermal diffusivity of the fluids. The thermal conductivity was obtained using the heat capacity of the constitutive materials of the liquid sample. A thermal conductivity enhancement is observed when heat flow is parallel to magnetic field in the case of the graphite flake particles; also, the thermal conductivity enhance due to the field is a function of the carbon powder concentration, presenting a maximum between 0.1 and 0.2 carbon volume fraction. Nevertheless for the graphene platelets the thermal enhancement due to magnetic orientation is not presented, due to the high Kapitza thermal resistance between grapheme particles.

Owing to the superiority of the mechanical and electronic properties of graphene, this carbon nanomaterial is expected to be applied to electronic devices. In its applications, graphene of large area and of rare defects must be supplied stably in great quantities. As a method of manufacturing graphene, thermal desorption of Si atoms from SiC substrates followed by reconstruction of C atoms into single- or multi-layered graphene is promising. In recent experiments by Fukidome et al. (2011), Si atoms in SiC layers epitaxially grown on Si substrates were thermally desorbed, and graphene on Si substrates was successfully realized. According to their results, however, all of the Si atoms in SiC cannot completely be desorbed, and graphene is created on SiC layers deposited on Si substrates. It was confirmed the crystallographic directions of the SiC layers affect the layer structure of graphene and therefore its band structure; graphene on (100) or (110) SiC is metallic, while graphene on (111) SiC is semiconducting. Controlling the crystallography of SiC leads to controlling the morphology and electronic properties of graphene created on it. In this paper, the mechanical properties of single- or multi-layered graphene on SiC substrates are investigated using molecular-dynamics. The equilibrated structures of single- or multi-layered graphene on SiC substrates are obtained. Next, the mechanical properties of such graphene are examined by peeling or indentation simulations focusing on the effects of the layer number of graphene and the crystallography of SiC.

3-Aminopropyltriethoxysilane (APTES) and 1H,1H,2H,2H-Perfluorooctyltriethoxysilane (B.PFES) were used to modify the interface between monolayer CVD graphene films and its supporting dielectric substrate to control the carrier type and density towards N-type and P-type graphene, respectively. Bottom gated graphene field effect transistors were fabricated to demonstrate the characteristics of the doped films. By using a lithography and patterning process, it was possible to dope adjacent regions of the CVD graphene film to produce a p-n junction. Characteristic I-V curves indicate the superposition of two separate Dirac points which confirms an energy separation of neutrality points within the complementary regions. This method minimizes doping-induced defects which can occur via other doping methods and results in a thermally stable graphene p-n junction for temperatures up to 200°C.

Coming device world will accept the bendable, stretchable, transparent system. In this regard, the advent of graphene has attracted researchers because of its outstanding properties such as electrical, optical, thermal and mechanical property. In this experiment, we exploited the possibility of the transparent strain sensor based on graphene film using its optical transparency and piezo-resistive characteristics. First, our works developed the piezo-resistive characteristic of graphene film together with mechanical strength and optical transparency. The optical transmittance of the strain sensor based on graphene films including substrates reached 75%. Additionally, the strain sensor can show durability and stability up to 7.1% strain In addition, we succeeded to demonstrate two types of strain sensor. One is a rosette type strain sensor and another is a glove-strain sensor. First, the rosette strain sensor takes a role in finding the direction of principle strain. That is, the rosette gauge correctly measured the direction of principle strain as well as the principle strain on the surface under the specific strain. In that case, Mohr&’s circle, common analysis tool for strain, was used to characterize the rosette strain sensor. Another type of strain sensor is the transparent glove strain sensor. It was exactly able to measure the strain of finger-motion. Therefore, we assure that it can be realized in forms of coming bendable and stretchable device world.

Various approaches to apply the unique large scale graphene in dye sensitized solar cells have been proposed. We achieve 3 times higher efficiency over graphene electrode by using CoS decoration on large scale CVD grown graphene film. Graphene-CoS (G-CoS) electrode is prepared using a chemical process named successive ionic layer absorption and reaction (SILAR). The HRTEM and AFM show that CoS nano-particles are grafted on the graphene film uniformly. The G-CoS electrode is found to be highly electro-catalytic towards iodine reduction with less charge transfer resistance (Rct~5.05 Omega;cm2) and high exchange current density (J0~2.50 mAcm-2). The enhanced performance is attributed to the increment of active catalytic sites of G-CoS triple-junction and high charge conducting pathway of graphene. The CoS decoration process of SILAR is a simple and scalable process which can easily adapted for a large scale G-CoS film fabrication for electro-chemical applications like oxygen reduction, fuel cells, hydrodesulphurization catalyst, super capacitors, Li ion battery anode and several others.

Recently, an ultrafast graphene photo-detector has been realized, which is based on the high carrier mobility of graphene and the junction property of graphene-metal electrode. However, at the graphene-metal junction, not only built-in potential due to the mismatch of Fermi level, but also thermoelectric junction due to the different density of state at the Fermi level or Seebeck coefficient can form. Therefore, if any photons are shined onto the junction, the junction is able to generate the current either by photo- or thermo-electric mechanism. What is important in the generation of the excited carriers is the graphene. The carrier multiplication inside graphene attributes the thermoelectric power (TEP) to dominate over the photoelectric current at this junction. Therefore, if these two junctions at the both end of graphene are placed at the different temperature, the net TEP can be used to measure temperature. In this letter, we fabricated a device in a way that incident IR can cause the temperature difference at both end of graphene. Our IR detector operates at room temperature without temperature control and can detect mid-IR range (7~14 micrometer). The responsivity and detectivity of IR sensor covered with SiOx absorber layer are observed to be 4.37 V/W and 1.0 x 107 cm Hz1/2/W at 5 Hz, respectively.

Graphene growth by chemical vapor deposition and following wet transfer process using polymer supporting layer are useful method to produce large area and high quality graphene on target substrate. We report a simultaneous wet transfer and doping method using hydrophobic polymers with various functional groups. The inverse transfer method was introduced to make graphene/polymer structure on target substrate, which leaves the polymer supporting layer between substrate and graphene as a dopant without leaving any involuntary polymer residues on top of graphene. The doping effect was confirmed by using Raman spectroscopy and ultraviolet photoelectron spectroscopy. Moreover, we also applied this method to demonstrate flexible organic field-effect transistors with graphene electrodes. Considering tunability and flexibility of polymers, this method would provide valuable directions for graphene-based organic electronics.

Due to high density integration of LSIs, thermal management becomes more and more important. Besides, graphene has received much attention as emerging research material for future nano-scale FETs and interconnects, because of its excellent electrical and thermal properties. In this paper, we report a series of calculations of the electrical and thermal properties of graphene nano-FETs for the first time, using Monte Carlo particle model of both electron and phonon transport. In the devices scaled below the size of electron mean-free-path, Monte Carlo particle method would be the most reliable and accurate method of quantitative evaluation of electron transport [1]. On the other hands, heat dissipation in the macro-scale devices has been analyzed as diffusion phenomena, while in nano-scale device, it cannot be treated the same way because the mean-free-path of the phonon becomes comparable to the device size. Therefore, we employed Monte Carlo particle method for not only electron, but also phonon transport simulations in graphene-channel nanometer-sized MOSFETs. Our simulation consists of two-steps; the first one is to simulate electron transport to obtain the spatial phonon distribution, the second step is to generate phonons based on the distribution, to simulate phonon transport, and to estimate local heating in the device. By using a different time-step for each step, (which is reasonable because the relaxation times of electrons and phonons are different), we succeeded in calculating both electron and phonon transport self-consistently for a reasonable computing time. In our model, phonon-phonon interaction rate is assumed to be a constant, and the phonon velocity is derived from the phonon dispersion curve of single layer graphene [2]. The local heating temperature is then being feedback to update the electron-phonon scattering rate. The peak of heat generation rate appears around a 20 nm separate position from the edge of the channel within the drain in case of channel length of 100 nm. We obtain the temperature rising rate of about 0.1 K/ps at the peak position at certain bias condition. Our method can be applied for power devices in which local heating is more crucial in determining device performance. Acknowledgment: The authors would like to thank Prof. K. Tomizawa of Meiji University and Prof. S. Murakami of Tokyo Institute of Technology for helpful suggestions and discussions. References: [1] N. Harada, M. Ohfuti, and Y. Awano, APEX 1 (2008) 024002, or A. Mohamad, N. Harada, and Y. Awano, APEX 5 (2012) 045102 [2] D. L. mafra, G. Samsonidze, L. M. Malard, D. C. Elias, J. C. Brant, F. Plentz, E. S.Alves, and M. A. Pimenta, Phys. Rev. B 76 (2007) 233407.

Functionalized Graphene has demonstrated exceptional potential for the development of nano-device technologies. Functionalizing graphene with oxygen produces graphene oxide. Structural investigations of graphene oxide has been made time and again, mostly on the basis of its electronic structure obtained using several experimental techniques. These experimental observations have been interpreted in light of several models. In crystals zone centered phonon can be probed using Raman spectroscopy. Raman spectrum of graphene oxide has been investigated in light of phonon dispersions and phonon density of states at the gamma point, obtained using ab-inito methods for previously proposed models. Functionalization of graphene in graphene oxide based on these phonon studies will be presented.

The thermal diffusivity of graphene nanosheet/polyamides (PA6, PA66 and PA12) composites were investigated. The polyamides/graphene nanosheet composites were prepared by an in-situ exfoliation melting process using a batch-type mixer and the thermal diffusivity of composites was measured by a laser flash method. The processing temperature of each polyamide grade was different and its effect on the thermal diffusivity was studied. The morphology of the composites was observed by field emission scanning electron microscopy. The thermal diffusivity of the PA/graphene nanosheet composites showed a significant improvement over those of PA/expanded graphite intercalated compound composites. Thermal diffusivity of the polyamides/graphene nanosheet composites was increased higher than 10 times compared to that of pure polyamides. It is suggested that the larger sheet size and high expansion ratio of the graphene nanosheet leads to 3-dimensional conductive pathways.

Recently nanopatterned graphenes have been fabricated with various lithographic methods, and their extraordinary properties have been widely studied. In addition, to obtain enhanced electrochemical activities, diverse metal or oxide nanoparticles were synthesized and attached on graphene. The bottom-up approaches utilizing molecular self-assemblies have obtained great attractions to generate nanopatterned graphene or nanoparticle-attached graphene because they effectively enable the fabrication of nanostructures and nanoparticles in large area. The diblock copolymer approach, a promising bottom-up technique to generate nanostructures and nanopatterns of various materials in large area, can be applied to an effective graphene nanopatterning. Diblock copolymers spontaneously assemble into periodic nanostructures, of which the size and morphology can be controlled by the molecular weight and composition of copolymers. In addition, diblock copolymer forms nanometer-sized micelles consisting of a soluble corona and an insoluble core in a selective solvent for one block of copolymers. The diblock copolymers and their micelles can be coated on solid substrates to form nanostructured thin films, which can be used as an etching mask to transfer patterns onto the substrate and as a nanotemplate to generate arrays of nanoparticles. In this work, we demonstrate the fabrication of arrays of graphene nanodots and the effective decoration of graphene with arrays of nanoparticles in large area by the diblock copolymer micellar approach. The hexagonal pattern of copolymer micelles was transferred to underneath graphene to generate arrays of graphene nanodots, and the size and inter-distance of graphene nanodots were controlled by the molecular weight of diblock copolymers. In addition, graphene was decorated with arrays of various nanoparticles using diblock copolymer micelles. The number density and size of nanoparticles on graphene were adjusted by the molecular weight of copolymers as well.

Several studies have recently found that single-layer graphene is chemically more reactive than few-layer graphene when supported on silicon dioxide substrates with sub-nm roughness. In this study, we have investigated the influence of substrates on chemical reactivity of graphene. Thermal oxidation and Ar-plasma etching of graphene on mica and SiO2/Si substrates were studied by atomic force microscopy (AFM) and Raman spectroscopy. Both reactions of single-layer graphene proceed more slowly on atomically flat mica than SiO2/Si substrates. Detailed analysis led to a conclusion that not only structural deformation but also interfacial interactions affect reactivity of graphene. These findings will be of great significance in manipulating various material properties of graphene in pursuit of its useful applications by choosing appropriate substrates.

Graphene oxide (GO) nanosheets have semiconducting π-conjugated sp2 domains with various sizes within an oxygenated sp3 domain. The chemical and physical properties can be widely tuned by adjusting the degree of oxidation. Recently, we have developed a simple photochemical reduction method of graphene oxide nanosheets. In this method, graphene oxide (GO) nanosheets are reduced by UV irradiation in H2 or N2 under mild conditions at room temperature. The photoreduction process can be extended to various applications of GO, as it can be performed without any additional chemicals and high reaction temperature. For example, the photoreduction route is very useful for micro-photopatterning a conducting sp2 part on insulating GO. In this paper, we report new photoreactions of graphene oxide nanosheets in an aqueous solution, in which H2 and CO2 are evolved from an aqueous suspension of GO nanosheets under UV irradiation. These reactions are based on the photoreactions of oxygen-containing functional groups and carbon. As a result, it was revealed that reduced GO (rGO) nanosheets exhibited many holes and defects, which potentially induce catalytic activities and/or magnetic properties. Furthermore, we revealed that electron or proton conductivities can be tuned by photochemical reduction. In photoelectrochemical measurements of a GO nanosheet electrode, we confirmed that a large anodic photocurrent was generated with the CO2 evolution, while a small cathodic photocurrent was generated by reduction of oxygen-containing functional groups and the H2 evolution reaction. The mechanism of the photoreaction between GO nanosheets and water is discussed in terms of band energy levels estimated from experimental results. Our research has demonstrated that GO and rGO-based materials with engineered morphologies, oxygenated functional groups, and conduction types can be easily/selectively fabricated by controlling the photoreaction conditions.

Formation of mono- and bi- layer epitaxial graphene on a SiC substrate by Si sublimation is one of the most important issues to obtain the high quality and large area single domain graphene with the controlled layer numbers. From the viewpoints of various applications, it is essential to transfer the epitaxial graphene with the large single domain from the SiC original surface onto the insulator layers such as SiO2 surface formed on the Si substrates. In our previous research, we reported a transfer process by using poly methyl methacrylate (PMMA)[1]. However, the process had included some problems for the reproducibility. In this report, we have presented an improved transfer process for the large area epitaxial graphene with good reproducibility. In our improved process, a Ti layer is deposited on graphene on a SiC substrate by the electron beam deposition and the Ti/graphene layer is peeled from SiC by using epoxy resin. After the peeling, the graphene surface on the Ti layer is covered by positive resist and the sample is annealed at 120°C. The sample surface is covered by a thermal release sheet (TRS) to support the resist/graphene layer. The Ti layer is etched away by hydrochloric acid at 50°C to separate the graphene/resist/TRS layer from epoxy resin. Finally, TRS and resist on SiO2 are removed by annealing at 120°C and acetone, respectively. We have successfully transferred 4.2mm × 2.7mm epitaxial graphene on SiO2/Si with good reproducibility. By the improved process, the graphene can be transferred to SiO2/Si with keeping the intrinsic stress induced by the thermal decomposition process for graphene formation[2]. The quality of transferred graphene was characterized by the microscopic laser Raman scattering spectroscopy. D band peak intensity of the typical Raman spectra from the transferred graphene is as very small as it from the pristine graphene on SiC. The results strongly indicate that the induced defects by our improved process are negligible. In conclusion, large single domain epitaxial graphene with negligible damage can be transferred from SiC to SiO2/Si by the present improved process with excellent reproducibility. [1] Y. Shimotsuji, et al, “New Damage-less Transfer Process for Epitaxial Graphene” IMAGINANO graphene 2011 [2] D. Tamakawa, et al, “Intrinsic stress of Transfer Epitaxial Graphene” IUMRS-ICEM 2012

Graphene is an excellent material for electronic and mechanical devices. However, single-layer graphene is a zero-gap semiconductor, and it is necessary to open its bandgap for application to switching devices. To open the bandgap of graphene, we focus on introduction of a periodic strain to graphene sheets. Although this method is theoretically predicted [1], it is difficult to incorporate the periodic strain to graphene. Then, we used porous alumina as a substrate suitable for bandgap opening. Porous alumina is self-organized by anodic oxidation of aluminum and has hexagonally arrayed nano-hole structure. We investigate a effects of the periodic nano-hole structure on the properties of graphene. Here, we show Raman spectra and atomic force microscope (AFM, Dynamic Force Mode) images of graphene flakes attached on porous alumina membranes. Raman spectra were obtained at 532 nm excitation. Graphene sheets were grown on copper substrates by CVD (Chemical Vapor Deposition) using CH₄. We confirmed growth of single-layer graphene on the copper substrates by Raman spectroscopy. Porous alumina was fabricated by the one-step or the two-step anodic oxidation process. The formed nano-hole diameter was 60 nm or 90 nm. Graphene sheets were transferred to the porous alumina surfaces or sapphire substrates (for comparison) by the standard PMMA transfer process. In the Raman spectra of the graphene sheets attached on the porous alumina membranes, the intensity of the D-, G-, and 2D- bands are 7~16 times enhanced compared with that on the sapphire substrate. It has been reported that this phenomenon is attributed to the presence of nano-hole arrays [2]. The G- band of the graphene sheets attached to the porous alumina substrate appeared at lower wavenumbers (1581~1582 cm^-1) than that on the sapphire substrate (1589 cm^-1). It was reported that the shift of G-band is caused by hole-doping to graphene [3]. Since the graphene sheets on the porous alumina are pseudo-free-standing due to the topography of the substrates, their doping effect is weakened compared with that on the sapphire surface. In the AFM images, graphene sheets attached to the porous alumina with pore diameter of 90 nm follow the shape of the porous alumina surface. The difference in heights of graphene sheets between the center and the edge areas of a porous alumina hole is about 16nm. Since the graphene sheets are subject to the porous alumina membrane surfaces, a periodic strain is generated in the graphene. In conclusion, use of porous alumina membrane as a substrate is a promising technique for bandgap opening of graphene. [1] F. Guinea et al., Nature Phys. 6 (2010) 30-33. [2] C. Zhang et al., J. Chem. Phys. 127 (2007) 044701. [3] S. Berciaud et al., Nano Lett. 9 (2009) 346-352.

Edge states of graphene and graphene nanoribbon are attracting a lot of attention in recent years. It is well known that two typical structures are formed at the graphene edges, which is so-called the armchair and the zigzag edges. In previous research of the edge structure for the highly oriented pyrolytic graphite (HOPG) by polarized microscopic laser Raman scattering spectroscopy, the D band peak intensity in the Raman spectra from the armchair edges depends on the polarization directions of both incident and scattered light [1]. In this report, we have performed the detailed analysis for the edge structures of the transferred epitaxial graphene by the polarized microscopic laser Raman scattering spectroscopy. The sample was a bi-layer epitaxial graphene transferred onto SiO2/Si from pristine epitaxial graphene formed on the Si-face SiC surface [2]. The polarized microscopic laser Raman spectroscopy was performed in the back scattering geometry at room temperature using Nd:YVO4 -SHG laser at 532 nm with the spatial resolution of about 1mu;m. The Raman spectra were obtained for two types of polarization direction of scattered light denoted by “parallel” and “perpendicular” to the polarization direction of the incident light [1]. There are two types of edges labeled by A and B in our graphene sample. The observed angle between the edge A and the edge B is about 90 degrees. Although the D band peak intensity observed from the edge A depended on the polarization direction, the D band peak intensity from the edge B did not depend on the polarization direction. The polarized Raman results strongly indicate that the edge A has linear mixed edge structures of the armchair and the zigzag edges. It is considered that the edge B has the structures combined with the 3 different directional armchair edges in an equivalent ratio, therefore, the polarization dependence of the D band peak intensity for the edge B seems to be canceled. In contrast to the configuration of the edge A with linear mixed edge structure, our present experimental results strongly indicate that the configuration of the edge B is regarded as a rugged structure. In addition, the results of atomic force microscope (AFM) measurements also supported our results for the edge structures. In conclusion, the polarized microscopic laser Raman scattering spectroscopy is very useful for the evaluations for the edge structures of transferred epitaxial graphene. [1] L. M. Malarda, M. A. Pimentaa, G. Dresselhaus, M. S. Dresselhaus, Phys. Rev. 473 (2009) 51_87 [2] Y. Shimotsuji, T. Ishida, S. Tanaka, A. Hashimoto, “New Damage-less Transfer Process for Epitaxial Graphene” IMAGINENANO graphene 2011

The chemical vapor deposition (CVD) of hydrocarbons appears as the most suitable graphene production method for large area applications such as flexible displays and photovoltaics. Recent developments demonstrate that graphene sheets can be uniformly deposited on copper foils over large areas and transferred to any arbitrary substrate. However, the CVD mechanisms have not been fully understood and therefore a complete control over the morphology of the produced sheets, including grain size and grain boundary control, has yet to be achieved. In this work, we report a systematic investigation of graphene CVD performed on a cold wall reactor system at very low methane partial pressure (10-5 torr) on copper substrates. No hydrogen or other diluting gases were used. A combination of optical microscopy images and Raman spectroscopy measurements was used to study graphene deposited at temperatures ranging from 730 to 950 °C. Monolayer graphene having fully connected grains can be easily obtained at very short deposition times (< 5 min) under these conditions. Moreover, the unwanted collateral growth of bilayer islands is seldom observed. The average grain size ranges from 2 to 50 µm as the temperature is increased, whereas grain morphology changes from rounded structures at low T to flower-like shapes at high T. Raman results show that the intensity of the D band and the ratio between the 2D and G bands are strongly influenced by the growth temperature (2D/G ratio ranges from 1 to 3.5 as temperature is increased). Such behavior is correlated to the increase in grain size as T is increased. In summary, it is demonstrated that CVD of graphene on Cu using very low pressure of methane in a cold wall reactor generates high quality pure monolayer sheets. Currently we are performing transport measurements on field-effect devices in order to determine the mobility of the produced graphene in an attempt to correlate it with grain morphology.

The performance of graphene FETs (GFETs) strongly depends on the interface between graphene sheet and the underlying substrate. In this work, we report that applying octadecyltrimethoxysilane (OTMS) SAMs onto conventional SiO2/Si substrate can consistently enhance the performance of coronene-derived large-area graphene FETs. The improved transport properties in terms of boosted carrier mobility (up to 10700 cm2/(Vs)), longer mean free path, nearly vanished hysteretic behavior and remarkably low intrinsic doping level are mainly attributed to the strong suppression of interfacial charge impurity scattering and remote interfacial phonon (RIP) scattering, less adsorption of dipolar species, as well as the attenuated charger transfer at the interface of graphene and the SiO2 dielectric. The intrinsic doping levels of graphene on OTMS and SiO2 have been quantitatively estimated and confirmed from the Dirac points of GFETs and the Raman mapping of G-peak positions. The facile graphene device fabricated over a large area provides an unprecedented combination of high performance and low cost for possible applications of all carbon-based nanoelectronics in future. Acknowledgement The work is in part supported by Research Grants Council of Hong Kong, particularly via Grants Nos. CUHK2/CRF/08 and CUHK4179/10E. J. B. Xu thanks the National Science Foundation of China (Grants Nos. 60990314 and 60928009) for support.

We present high quantum efficiency graphene/Si heterojunction devices for a variety of optoelectronic applications such as tunable photoswitching, photodetection, and photometry. Monolayer graphene/Si devices demonstrate photo-switching ON/OFF ratios exceeding 104 and photovoltage responsivities exceeding tens of kV/W making them highly suitable for low-power optoelectronics and weak-signal photodetection. Additionally, by layer-thickening and chemical doping, the photocurrent responsivity of graphene/Si heterojunction devices could be tuned to values high as 0.4 A/W, over two orders of magnitude better than that of previously-reported metal-graphene junctions. These devices can be also used as excellent photometers that remain linear over four decades of incident power, with broadband (400

Hybrid transparent conductors that integrate metal grids and graphene films are promising transparent conductors. Here, we demonstrate that the electrical and optical properties of these transparent conductors can be easily tailored by altering the metal grid design. We find that a thicker metal grid is more effective than broader metal gridlines to achieve low sheet resistance and high optical transmittance because it reduces the sheet resistance significantly without compromising the optical transmittance to a large extent. We also examined the thermal stability of the transparent conductor and found that it is thermally stable in air at 100C for an extended period of time. Graphene film transferred on top of the metal grid is also found to act as an effective barrier to retard the oxidation of the copper grid at 200C in ambient air.

Graphene are considered to be one of the best candidate transparent conductive films for optoelectronic devices due to its extraordinary electrical and mechanical properties. However, the reported performances of CVD-grown graphene is significantly limited by charged impurity scattering, extrinsic surface phonon scattering, resonant scattering on atomic-scale defects, and grain boundary defects. Here, we developed a metal/graphene hybrid transparent film to improve the performance as a transparent electrode. A systematic study of metal/graphene hybrid films was performed by control the shape and size of metal grid pattern on graphene. Optical, electrical and mechanical properties of metal/graphene hybrid films will be presented.

Boron-doped graphene/C composite fibers were synthesized by using electrospinning followed by heat treatment. Due to the strong electron-withdrawing capability, boron atoms are doped into graphene, forming boron doped graphene (BG) via a catalyst-free thermal annealing approach in the presence of boron oxide. Polyacrylonitrile(PAN) was used as electrospinning matrix and a carbon sources. Boron doped Graphene was dissolved in N,N-dimethylformamide and that was dispersed by ultrasonication. Dispersed graphene and PAN were mixed before electrospinning. Polypyrrole was coated on the electrospun BG/C composite fibers by in situ chemical polymerization in order to improve the electrochemical performance. The surface morphology and microstructure of the obtained boron doped graphene /C composite fibers were characterized using scanning electron microscopy (SEM) and elemental dispersive spectroscopy (EDS). X-ray diffraction (XRD) measurements were also carried out to determine the structure of composite fibers. Electrochemical performance of boron doped graphene/C composite fibers was evaluated in initial charge-discharge curve. Capacity of composite fibers was measured using cycling voltammetry (CV equipment).

Graphene is one of the materials that have attracted much attention in recent years. For switching device applications, bandgap opening is required because graphene is originally a zero-gap semiconductor. One of the methods for bandgap formation is making graphene nanoribbons (GNRs). To fabricate GNRs, catalytic etching using metal nanoparticles was reported [1]. In this process, the etching direction is controlled by the atomic structures of the substrate surface. To obtain a bandgap suitable for electronic devices, the etching width and spacing should be precisely controlled, which requires uniformly sized and positioned metal nanoparticles. In this paper, we propose a new etching technique using ferritin, which is one of the proteins that contain almost constant amount of Fe oxide in their inner core. Single-stepped sapphire (1-102) substrates were chemically cleaned using a H₂SO₄ and H₂O₂ mixed solution. Graphene films were attached to the sapphire surfaces by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). Ferritin molecules were adsorbed to the sapphire surfaces partially covered with graphene films in pure water or a buffer solution. To form metallic Fe nanoparticles, the ferritin-adsorbed substrates were annealed at 450°C in air in a furnace for 10 min to remove the protein shell. Then, the samples were annealed at 900°C in a hydrogen (320 sccm) and argon (600 sccm) mixed gas for 10 min. Surface morphology and vibrational properties of the etched graphene flakes were characterized by atomic force microscopy (AFM) and Raman spectroscopy, respectively. Graphene flakes were etched selectively in [1-10-1] direction under the present experimental conditions. In Raman spectra, the D and D' bands, which were not detected before the etching, are observed from the etched graphene flakes. They are originated in the edge states of GNRs. In AFM images, high-density parallel trenches are arranged because nanoparticles synthesized from ferritin are much small and their size distribution is very narrow. Density of the adsorbed molecules can be controlled by solution conditions, such as pH value and ionic strength. The GNR width depends on the spacing of ferritin adsorption. We found that the molecule spacing can be controlled by intermolecular interaction forces, such as the electrostatic repulsive force between the protein molecules, which can be also regulated by the solution conditions. These results indicate that the use of biologically synthesized metal nanoparticles is a promising technique to fabricate ordered arrays of graphene nanoribbons with an arbitrary width on arbitrary positions on insulating substrates. [1] T. Tsukamoto et al., J. Phys. Chem. C, 115 (2011) 8580-8585.

Poly-N-vinyl carbazole (PVK) was blended with graphene oxide (GO) to form PVK-GO polymer nanocomposite. The PVK-GO nanocomposite was characterized by XPS and ATR-IR confirming the successful distribution of GO into the PVK matrix due to the presence of functional carboxylic acid and hydroxyl groups on the nanocomposite. The heavy metal adsorption property of the PVK-GO nanocomposite was investigated by exposing the nanocomposite to a Pb (II) solution. It was observed that the adsorption capacity of Pb(II) onto PVK-GO nanocomposite increases proportionally with the increasing amount of GO, which can be attributed to the increase concentration of the GO nanomaterial and the presence of carboxylic sites on the GO surface. Furthermore, PVK-GO had higher adsorption capacity of Pb (II) at higher pHs. The adsorption of Pb (II) onto PVK-GO fits well with the Langmuir model. The adsorption capacity of Pb (II) was 887.98 mg/g and this was achieved using a 10%:90% wt ratio of PVK:GO at pH sim;7 within 90 minutes. This high removal efficiency only goes to show the applicability and effectiveness of using PVK - GO to remove heavy metals from aqueous solutions.

While graphene has been shown to possess exceptional electrical, thermal, and mechanical properties, processing and patterning graphene for large area applications remains a great challenge. Electrically insulating graphene-oxide (GO) is a promising precursor to graphene that can be processed from aqueous solution and subsequently converted to conductive reduced-graphene-oxide (rGO). GO reduction, however, typically relies on the use of hazardous chemicals, such as hydrazine, or energy-intensive high temperature treatment. We will present an alternative approach to GO reduction that enables the selective patterning of conductive rGO regions on insulating GO under ambient conditions. The process relies on a thin adsorbed water layer which forms on the GO surface under ambient conditions, and acts as a localized electrochemical cell when a voltage is applied between surface electrode pairs. We first demonstrated voltage-based GO reduction by applying a voltage between a fixed surface electrode and a moveable metal-coated atomic force microscope (AFM) tip, enabling micrometer-scale patterning of conductive graphitic pathways on GO films [1]. The same AFM setup can then also serve to map the local film topography and current carrying capacity. To further extend the voltage-induced reduction approach, rather than relying on a slowly rastering scanning probe tip, we employed micro- and nano-patterned electrode arrays [2]. By applying a voltage across source-drain pairs we selectively reduced the intervening GO in situ to produce ambipolar rGO field-effect transistors (FETs). Through the use of parallel electrode arrays, voltage-induced GO reduction potentially offers a low-cost, scalable approach to graphene processing for electronics. [1] J. M. Mativetsky, E. Treossi, E. Orgiu, M. Melucci, G.P. Veronese, P. Samorigrave;, V. Palermo, J. Am. Chem. Soc. 2010, 132, 14130-14136. [2] J.M. Mativetsky, A. Liscio, E. Treossi, E. Orgiu, A. Zanelli, P. Samorigrave;, V. Palermo, J. Am. Chem. Soc. 2011, 133, 14320-14326.

Graphene oxide (GO) is a chemically exfoliated form of carbon which contains sp2 and sp3 bonds as well as functional groups such as -OH and -COOH. This material has been attractive to the scientific community for many reasons, one among them is that it can be synthesized in large quantities at low cost [1]. Removal of oxygen functional groups allows tunability of the electrical and optical properties of graphene oxide. Several methods of reduction of GO have been reported, including chemical reduction, thermal annealing or a combination of these techniques [1-2]. In our present work, we used a modified hydrothermal treatment to reduce GO (rGO) [3]. Our rGO has been characterized by different techniques such as XPS, Raman, UV-vis, and FTIR. Raman spectroscopy has revealed two new peaks that have not been observed previously in rGO. We have investigated the origin of these peaks at various excitation energies and found that these peaks are resonant with maximum intensity around 514 nm excitation and also that they are first-order peaks. [1] Eda, G., & Chhowalla, M. (2010). Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics. Advanced materials (Deerfield Beach, Fla.), 2392-2415. doi:10.1002/adma.200903689 [2] Pei, S., & Cheng, H.-M. (2011). The reduction of graphene oxide. Carbon, 1-19. Elsevier Ltd. doi:10.1016/j.carbon.2011.11.010 [3] Zhou, Y., Bao, Q., Tang, L. A. L., Zhong, Y., & Loh, K. P. (2009). Hydrothermal Dehydration for the “Green” Reduction of Exfoliated Graphene Oxide to Graphene and Demonstration of Tunable Optical Limiting Properties. Chemistry of Materials, 21(13), 2950-2956. doi:10.1021/cm9006603

A systematic study on the parameter space of graphene CVD on polycrystalline Cu foils is presented aiming at a more fundamental level process rationale in particular regarding the choice of carbon precursor and mitigation of Cu sublimation. Complete monolayer graphene coverage of high quality (I2D/IG ~3.5 and ID/IG ~0.05, mobilities up to 3000 cm2/Vs, sheet resistances ~400-800 Ohm/square) is limited for methane as precursor to low pressure, H2 diluted exposures and most importantly to high temperatures (1000°C), at which Cu sublimation is at deleterious levels. In contrast, undiluted exposures of liquid aromatic compounds consistently give similar graphene quality at 100-150°C lower temperatures, enough to mitigate Cu sublimation. Graphene formation is not inherently self-limited to a mono-layer for any of the precursors. Rather the nucleation density, percentage of multi-layer nuclei and film uniformity/quality depend on the supplied carbon chemical potential and rate balance of contributing processes, as discussed in the context of basic heterogeneous catalysis and 2D crystal growth kinetics. Graphene formation is thereby significantly affected by the Cu crystallography, i.e. the evolution of microstructure and texture of the catalyst template form an integral part of the CVD process. References: Kidambi et al. The parameter space of graphene CVD on polycrystalline Cu. 2012 (submitted)

Selective graphene ribbon growth on copper twin crystals by chemical vapor deposition (CVD) has been realized. Graphene ribbons were formed selectively on narrow twin crystal regions with a (001) or high-index surface sandwiched between Cu crystals having (111) surfaces. The selective formation was sensitive to the partial pressure of CH₄ in Ar/H₂, which was used as the source gas for graphene growth. When the partial pressure of CH₄ was relatively low, graphene nucleation at steps on Cu (111) surfaces was suppressed and graphene was preferentially nucleated and formed on twin crystal regions. The preferential graphene nucleation and formation seem to be caused primarily by a difference in surface-dependent adsorption energies of reactants, which were estimated by first principles calculations. Concentrations of reactants on a Cu surface were also calculated by solving a diffusion equation to qualitatively explain the preferential nucleation observed in the experiments. The widths of graphene ribbons found in this study are as narrow as ~90 nm, which are basically the same as those of the twin crystals and can be narrower. Our findings may lead to self-organizing formation of graphene nanoribbons without relying on top-down approaches in the future. This work was supported by JSP through the “FIRST Program,” initiated by CSTP, Japan. This work was partly conducted at the Nano-Processing Facility supported by ICAN, AIST, Japan.

The incorporation of copper (Cu) - based interconnects into manufacturing has led to significant changes in silicon (Si) integrated circuit technologies compared to aluminum-based interconnects owing to its lower resistivity and higher electromigration resistance. However, the Cu diffusion into the Si layers degrades the reliability of the devices, forming Cu-Si compounds. Efforts to inhibit the diffusion of Cu into Si have developed various diffusion barrier materials such as W, Ta, TaN, TiN, TiW, and TaSi. Even though the conventional diffusion barriers smoothly carry on the role as diffusion barrier until the present, critical challenges are currently emerged with regard to thickness of conventional barrier materials. As the interconnect linewidth continuously decreases, the barrier thickness is also required to shrink to at least 5 nm, maintaining its barrier performance. As the diffusion barrier thickness is scaled down, barrier continuity and uniformity have become increasingly critical. However, the trade-off between barrier thickness and barrier performance makes certain dimensions unattainable so far. Therefore, novel barrier materials are required to reach near zero-thickness or zero-thickness, satisfying the following requirements: low resistivity, thermal stability, no interfacial reaction with Si, and preventing Cu diffusion at high temperature. Graphene, which is the monolayer form of carbon atoms arranged in two-dimensional hexagonal lattices, has attracted extensive interests owing to its outstanding electric and mechanical properties. In spite of one-atom thickness, a monolayer graphene is theoretically impermeable to standard gases including helium. Graphene is also thermally and chemically stable at extremely high temperatures, enduring at least 1500 °C. Conjecturably, the use of graphene as a zero-thickness diffusion barrier to protect Cu diffusion to Si can be realizable based on its unique properties that are suitable for necessities as a diffusion barrier. In this study, we demonstrate for the first time the ability of graphene films grown by CVD to prevent Cu atoms from penetrating into Si substrate at high temperatures. Two types of samples were prepared: Cu/single-layer graphene (SLG)/Si and Cu/multi-layer graphene (MLG)/Si. The performance of SLG and MLG as a diffusion barrier between Cu and Si were evaluated by annealing two types of samples in a vacuum chamber for 30 min at temperature range of 500 - 850 °C by flowing H2 gases or Ar gases. Surprisingly, it was observed that no copper silicide was formed below 800 and 850 °C, each for Cu/SLG/Si and Cu/MLG/Si structure samples according to x-ray diffraction and transmission electron microscopy analysis. These results indicate that both SLG and MLG can be operated as an effective zero-thickness diffusion barrier of which performance is competitive with that of conventional diffusion barrier materials.

Nowadays one of the most widely used nonvolatile memories is Si based memory. this faces limitations in its speed, operation power and density. The need to improve upon Si based memory devices triggered research into next generation nonvolatile memories which may be able to replace flash memory; memory such as Resistive Random Access Memory (ReRAM). In addition to the advantageous properties of oxide-based ReRAM including simple composition, feasible fabrication processes, and compatibility with contemporary semiconductor electronics, this resistance-concept memory has presented promising nonvolatile memory effects including fast operation speed and reliable data non-volatility with low power consumption. However, questions about the technical issues of resistive switching, which are expected to guide the actual performance features and the structure of ReRAM devices, have not been fully answered. Presence of an electric field across the surface between the electrode and the insulator leads to the random formation of conducting filaments resulting in a wide gap of resistive switching voltage distribution. In this paper, we have investigated a resistive switching system with a controlled electric field distribution induced by highly defective monolayer graphene. First, we controlled the amount of defects in monolayer graphene by using an Ar ion scattering assistant reactive system (IAR) and measured the concentration of defects by Raman spectroscopy. Second, we transferred the defective graphene on to the ReRAM oxide. The inhomogeneous distribution of electric field, due to the defective graphene regions, formed areas of high electric field concentration which strongly influenced the switching voltage distribution and the stability of the ReRAM devices. The Pt/defective-graphene/ReRAM-oxide/Pt structure is a good model system which can effectively control the electric field distribution in a resistive switching material.

Graphene nanoribbons (GNRs) have been predicted to reveal superior magnetic and electronic properties over graphene as the GNRs possess characteristic magnetic zig-zag edges and tunable band gap, which are apparently absent in pure graphene. Theoretical predictions1 envisage potential spintronic applications for GNRs due to weak spin-orbit coupling and negligible hyperfine coupling, however, this has not been confirmed by direct experimental observations using pure spin-sensitive techniques. That exactly forms the motivation of the current work, in which, we present and discuss our recent comprehensive experimental findings on the edge spin magnetism of unzipped (split) GNRs2. To accomplish this task, we have employed a combination of spin-sensitive multi-frequency electron spin resonance (ESR) spectroscopy (continuous wave, pulse and HYSCORE) coupled with static magnetization measurements. The salient features3,4,5 of the current work include: (a) unzipped GNRs have been shown to be used as a highly sensitive and reversible O2 sensors, and weak NH3 sensors (b) split GNRs exhibit room-temperature ferromagnetic-like properties (c) split GNRs reveal a strong temperature dependence of electron spin-lattice relaxation rate, and (d) split GNRs show remarkable dynamic magnetic features. We believe that, with this advanced knowledge of (edge) spin-based magnetism coupled with their room temperature semiconducting nature, GNRs may now be considered for the development of GNRs-derived sensor and spintronic devices. 1O.V. Yazyev, Phys. Rev. Lett. 101, 037203 (2008); 2D. V. Kosynkin et al., Nature (London) 458, 872 (2009); 3S.S.Rao et al., Nano Lett., 12,1210 (2012);4S.S.Rao et al., Appl. Phys. Lett., 98, 083116 (2011); 5S.S.Rao et al., New Journal of Physics 13 (2011) 113004

Graphene has been the focus of much recent research due to its unique electronic and optical properties, with potential for high performance electronics, tunable ultra-fast lasers, and transparent electrodes. Further development of graphene for commercial use requires effective large-area epitaxial production that maintains the desirable properties of exfoliated graphene. One such method of epitaxial graphene growth is thermal sublimation of Si from SiC. Sublimation of Si from the Si-face (0001) is the most controllable but produces a (6radic;3 × 6radic;3)R30° surface reconstructed carbon layer prior to graphene formation [1]. Also known as the “buffer” layer, this layer can be altered by subsequent hydrogen intercalation, resulting in quasi-free-standing (QFS) epitaxial graphene [2]. In order to determine the effect of hydrogen intercalation on the electronic and optical properties of graphene we performed spectroscopic ellipsometry experiments in a spectral range of 3 to 9 eV before and after hydrogen intercalation of buffer layer only carbon growth on 6H SiC (0001). Spectroscopic ellipsometry is a widely used technique for determining the optical properties of thin films, and can provide sensitivity to film quality, morphology, and strain. In the case of graphene sensitivity is obtained through the critical-point (CP) located at 4.5 eV [3, and references therein]. Analysis of CP absorption is achieved through a parameterized model dielectric function (MDF) which is varied until a best-match between model and experimental data is obtained. Best-match model results show drastic changes in the imaginary part of the MDF between previous measurements of epitaxial graphene on SiC, buffer layer only growth on SiC, and buffer layer growth after hydrogen intercalation. Buffer layer only growth exhibits a far greater absorption throughout the spectrum, with a CP energy that is shifted toward the infrared when compared to previous measurements of graphene. After hydrogen intercalation, the QFS graphene layer exhibits a lowered absorption and a CP with a higher energy than that of previous measurements, displaying an MDF closer to that of theoretical predictions for graphene by Yang et al., [4]. In conclusion, hydrogen intercalation of buffer layer carbon growth on SiC (0001) has been shown to produce QFS graphene with optical properties closest to that of theoretical predictions for graphene, further proving its effectiveness as a tool for large-area epitaxial graphene production. In addition, buffer layer carbon growth shows optical properties sufficiently different from that of graphene to allow spectroscopic ellipsometry to become a viable in-situ monitor for commercial production of hydrogen intercalated graphene on SiC. References: [1] Virojanadara et al., Phys. Rev. B 78, 245403 (2008) [2] Riedl et al., Phys. Rev. Lett. 103, 246804 (2009) [3] Boosalis et al., Appl. Phys. Lett. 101, 011912 (2012) [4] Yang et al., Phys. Rev. Lett. 103, 186802 (2009)

Toughening in carbon nanotube reinforced ceramic composites is achieved by crack deflection as with traditional fiber reinforced ceramics and also by a less known telescopic sliding of walls in the nanotubes1. The contribution of interwall sliding to the fracture energy absorption is thought to be dependent on a number of factors namely the number of walls, the diameter of the MWCNT and the density of the array. Experiments were designed to vary the properties of the nanotube arrays and understand the toughening behavior. Multi walled carbon nanotubes were used as reinforcements in silicon nitride coatings. Growth of carbon nanotubes is carried out by catalytic decomposition of acetylene over an iron catalyst layer. The properties of the MWCNT array are modified using a controlled heat treatment approach to attain variation in intertube diameter and number of walls in the nanotubes. A novel route2 of pulsed chemical vapor deposition is adopted to vary the density of the arrays. In the pulsing technique, different acetylene pressures combined with high gas flow velocities were used to vary the amount of carbon flux and correspondingly the density of the nanotube arrays. Optical extinction measurements were performed on the grown arrays to determine quantitative differences in the density of the arrays. The nanotube arrays are then vapor infiltrated with silicon nitride matrix using a low pressure chemical vapor deposition process with dichlorosilane and ammonia gases. The effectiveness of the multi walled nanotubes as toughening agents is qualitatively determined by studying the fracture surface and the pullout lengths in these composite coatings. Nanoindentation tests were also performed to estimate differences in hardness and Young&’s modulus as a function of nanotube array density and nanotube diameter. 1 Kothari et al., Journal of the American Ceramic Society, Volume 91 (8) 2008, p. 2743 2 Jackson et al., ACS Nano, Volume 4 (12) 2010, p 7573

Density functional studies are reported on the mechanisms and energy barriers for Li and Li+ diffusion through Stone-Wales (SW) and vacancy defects in graphene and graphite. The migration barriers for Li and Li+ diffusion through a graphene plane show that Li diffusion is aided by the presence of multiple vacancies. The initial Li charge state was investigated for the SW and di-vacancies (DV) and found to affect the apparent barrier for the diffusion for the later, with Li atom exhibiting a lower energy barrier for diffusion than Li+. This is a result of increased local charge transfer from Li atom to the DV in the graphene plane. The effect of the size and structure relaxation of the periodic model systems on the results will be presented.

Graphene transparent electrodes are flat, flexible and stable. Their work functions can be controlled. Nitrogen-substituted graphenes are expected for n-type negative electrodes. Reversed type solar cells or OLEDs using the transparent negative electrodes have been investigated instead of using unstable metal electrodes. Surface resistivity values of graphenes, however, are larger than those required for the electronic devices with current drive. Silver nanowire transparent electrodes prepared by printing show small surface resistivity values. Silver nanowires cause light scattering which is preferable for solar cells and lighting. They have apertures which are not suitable for electron exchange to an active device layer with a low electric conductivity. They are unstable to sulfur compounds in the atmosphere. In this study, we have investigated transparent conducting films composed of a reduced graphene oxide top layer and a silver nanowire under layer. The stacked layer structure compensates for their weak points. The reduced graphene oxide film can also be prepared by printing. Its electric resistance perpendicular to the film surface will be neglected if the thickness is very small. Graphene oxide was dip-coated from its aqueous suspension onto a half part of hydrophilic glass plate, and reduced by hydrazine vapor. A methanol suspension of silver nanowire with 111nm diameter was cast on the whole glass plate, and then PMMA solution was cast. The resulting film was removed from the glass plate in water. Surface resistivity was measured with a four-point probe method. The surface resistivity of the film was 2.5±0.2Omega;/sq, and its total transparency with the graphene layer at 550nm was 66±3% (specular transparency was 49±5%). The transparency of the graphene layer was 95.7%, indicating two-layer graphene with a <1nm thickness. SEM images of the film which had been put in a plastic case in the atmosphere for 3 months showed some corroded parts of the silver nanowire without the graphene layer, and a charge-up phenomenon was observed on the PMMA part. The film with the conducting graphene top layer did not give the charge-up and the corrosion. When the film without the graphene layer was exposed under the sulfur vapor, the surface resistivity rapidly increased to 10^6 Omega;/sq within 20 minutes after about a 10 minute induction period. The surface resistivity remained at about 10Omega;/sq after 170 minutes with the graphene layer. The ultra thin graphene layer prevented the sulfur vapor attack. The sulfur vapor treatment can be applied to conducting and non-conducting patterning and the short time treatment can decrease light scattering without increasing the resistivity very much. In place of PMMA, a fluorinated transparent polymer gave a more flexible film with 4.0±0.4Omega;/sq, and its total transparency was 65% at 300nm, 75% at 550nm, and 80% at 2000nm. The film was transparent from near ultraviolet to near infrared region.

One of the factors lowering the electric properties of CVD-graphene compared to mechanically exfoliated graphene is the existence of domain boundary that scatters electronic transport. Apart from efforts to have a domain size as large as possible, an easy method to measure the size of each domain is also important because the characterization of electric properties is only accessible by fabricating a device through a number of complicated processing steps. In this report, revealing nucleation sites and domain boundaries was attempted, based on the observation from the Raman spectroscopy that these regions are relatively defective with higher D/G ratio. This led to an assumption that the regions will be vulnerable to a simple chemical treatment, such as annealing in air. Samples with different domain size and coverage feature were prepared and tested upon an optimized annealing condition. As a result, etch trenches of graphene was observed in particular shape. The craters inside the flake were considered as nucleation sites, which were well matched with the prediction from the Raman spectroscopy. Furthermore, by measuring inter-nucleation sites distance, domain size of graphene could be estimated, which was also fit to anticipated values of the flakes. The location of bilayer was always on top of the nucleation sites where no creation of craters was observed. This confirmed the stability of bilayer predicted from the Raman spectroscopy as well. This simple and useful method provides a new route for evaluating the quality of graphene which is directly correlated to its electric property.

Defects play a crucial role in determining the materials properties and special defects can be extremely useful to generate novel materials and devices [1]. Additionally, Raman spectroscopy is one of the most important techniques for the characterization of graphitic materials. However, there is not yet a complete understanding of the signatures of the defects in the Raman spectra of graphene and low-dimensional carbon materials in general. We study some of the consequences of the defects on the Raman features based on the nature of the defects. Some of the defects act mainly on the phonon properties (for instance, isotope impurities [2]), while others also affect the electronic properties (for instance, vacancies and substitutional dopant atoms [3]). There are many mechanisms involved in determining the Raman features and they depend on several parameters like the nature of the defects, concentrations and temperature. Some of the phenomena involve the reduction of the lifetimes, double resonant processes [4], localization [2], etc. We make quantitative estimates of some of these Raman features and the responsible defects in order to better understand the physics of defects in graphene and their role on Raman spectroscopy. REFERENCES: [1] H. Terrones et al., Rep. Prog. Phys. 75, 062501 (2012) [2] J. F. Rodriguez-Nieva et al., Phys. Rev. B 85, 245406 (2012) [3] Y. A. Kim et al., DOI: 10.1021/nn301728j [4] P. Venezuela et al., Phys. Rev. B 84, 035433 (2011) ACKNOWLEDGEMENTS: JFRN and MSD acknowledge grant DMR-10-04147. RS acknowledge MEXT grant (No. 20241023).

The growth of epitaxial graphene on silicon carbide (SiC) surfaces is considered to pave the way for graphene-based electronic devices. Graphene can be grown on both polar {0001} surfaces of hexagonal SiC but with properties which are characteristic for the surface polarity. On the Si-face, monolayer graphene can be grown with good homogeneity in argon atmosphere. At low temperature (4 K), these layers show a carrier mobility of up to 29.000 cm2/Vs when the carrier density is tuned close to charge neutrality. However, the carrier mobility is strongly temperature dependent which is not expected from graphene and which therefore should be caused by the interaction with the substrate. In order to understand the role of interactions with the substrate we have investigated the properties of quasi-freestanding monolayer graphene (QFMLG) obtained by decoupling the buffer layer from the substrate by intercalation of various elements such as hydrogen or oxygen. In my presentation I will briefly summarize the growth and properties of epitaxial graphene on SiC before I survey recent work performed on quasi-freestanding graphene obtained by intercalation.

The utilization of graphene thin films in electronic device applications centers on the ability to control the graphene electronic properties. Substitutional doping of the graphene sheet with atoms that are electron-deficient or electron-rich relative to carbon represents an effective strategy for controllable fabrication of p-type or n-type graphene devices. Nitrogen and boron represent the logical n and p-type dopants for graphene, respectively. While a significant amount of recent research has demonstrated the successful incorporation of nitrogen into monolayer graphene sheets and corresponding modifications of graphene electronic structure, boron doping has received much less attention. Here, we present a novel synthetic route for boron-doped graphene, using a single-source (triethylborane) chemical vapor deposition process [1]. The substitutional boron doping is confirmed via an array of characterization techniques including x-ray photoelectron spectroscopy, near edge x-ray absorption fine structure, and mapping Raman spectroscopy measurements. Mapping Raman spectroscopy indicates that the boron-doped graphene is p-type relative to undoped graphene, and also demonstrates significant spatial variance in the dopant incorporation and final graphene structure. A unique feature of the triethylborane precursor is that slight modifications of the growth parameters afford control over the sp2 morphology, enabling the fabrication of unique hybrid films. The potential for these boron-doped graphene thin films in device applications will be discussed.

Controlling the surface topography of graphene is vital for exploiting its exceptional electronic properties. Therefore, understanding the peculiarities of graphene growth mechanism is essential. Currently, the origin of surface ripples is believed to be associated with factors including thermodynamic stability of two dimensional layers or membranes, presence of grain boundaries on the substrate and the difference between the thermal expansion coefficients of graphene and substrate. Recently, various groups have exploited liquid copper surface in order to overcome these obstacles and grow high quality graphene by CVD method [1-3], yet this approach comprises hidden challenges [4]. Indeed, in our recent paper [5] we experimentally demonstrated that the thermocapillary and solutocapillary forces can initiate planar or convection instabilities on a Cu-C binary system at elevated temperatures and thereby, the topographic pattern formation on the graphene is governed by dynamics of these instabilities. Analysis of these non-equilibrium processes is presented here based on Mullins-Sekerka and Benard-Marangoni instabilities in diluted binary alloys. It appears that the wavelength of the instability puts a new upper limit in the domain size of graphene grown from the melt. However, these analyses offer new control tools over the ripple texturing through synthesis parameters such as temperature, temperature gradient, quenching rate, diffusion coefficients of carbon in the metal catalyst and the miscibility gap of the metal catalyst-carbon system. 1. D Geng, et al., “Uniform hexagonal graphene flakes and films grown on liquid copper surface” P.N.A.S. doi/10.1073/pnas.1200339109 (2012) 2. Y.A. Wu “Large single crystals of graphene on melted copper using CVD “ ACS Nano doi/10.1021/nn3016629 (2012) 3. M. Terrones “Contrllong the shapes and assemblages of graphene” P.N.A.S. doi/10.1073/pnas 120524109 (2012) 4. A.R. Harutyunyan “Uniform hexagonal graphene film growth on liquid copper surface: Challenges still remain” P.N.A.S (in press) 5. T.M. Paronyan, et al., “Formation of ripples in graphene as a result of interfacial instabilities” ACS Nano 5, 9619 (2011)

The original concept of graphene electronics focused on carbon nanotube properties. Carbon nanotubes were known to be high mobility ballistic, phase coherent conductors and quantum confinement effects produced significant bandgaps. However, it turns out to be very difficult to develop nanotube electronics platform for a variety of reasons including fundamental physical constraints related to the quantum mechanical properties of the metal-to-nanotube contacts. Graphene electronics can in principle overcome the major problems because graphene structures can be patterned using conventional lithography and dissipation at contacts can be controlled. However, these developments rely on the premise that narrow, ballistic graphene ribbons can be produced. Experiments on conventionally patterned graphene structures produced from graphene that is deposited on insulating substrates have been discouraging. The graphene ribbon mobilities are so low due to edge roughness effects, to render this direction to be impracticable. On the other hand, graphene produced on silicon carbide turns has been found to be more immune to edge scattering problems. This effect is explained in terms of a topologically protected edge state that are in fact expected in essentially all graphene ribbons with reasonably straight edges. Moreover, recent developments of template grown graphene structures on silicon carbide are promising. Very narrow ballistic graphene ribbons that demonstrate electronic phase coherent properties, have been produced with these methods which again brings the original concept of graphene based nanoelectronics back into play. Recent developments in the science and technological implications of epitaxial graphene nanostructures will be reviewed.

CVD growth of graphene on Cu foil has been widely studied because it enables large-scale growth with low cost. However, CVD graphene grown on Cu foil is polycrystalline and has many domain boundaries [1], which are due to the formation of a number of graphene nuclei having different orientations. The domain boundaries limit the physical properties of graphene so that it is quite important to investigate the domain structure of CVD graphene [2,3]. We previously demonstrated the orientation-controlled growth of graphene by employing hetero-epitaxial Cu and Co films [4-6], but it is unclear how the developing graphene domains merge together to form a uniform graphene film. In this study, we investigated the interface of adjacent graphene domains based on optical microscope, SEM, AFM, and Raman mapping measurements. Large hexagonal-shaped graphene domains with two main relative orientations, 0 degree and 30 degree, grown on the heteroepitaxial Cu(100) film were studied. Only when the orientation of neighboring domains is identical (0 degree), we observe a defect and wrinkle-free connection between domains. However, in some cases, a clear boundary was observed even for the 0 degree case, as was usually observed for the 30 degree-rotated domains. These results suggest that unifying the orientation of graphene domains is a possible approach for the seamless atomic connection. We will also discuss graphene mobility related to the formation of domain boundaries. Our work may offer a new approach for the synthesis of a single-crystalline graphene free from domain boundaries. [1] P. Y. Huang et al., Nature 469, 389 (2011). [2] Q. Yu et al., Nat. Mater. 10, 443 (2011). [3] C. M. Orofeo et al., Carbon 50, 2189 (2012). [4] H. Ago et al., ACS Nano 4, 1414 (2010). [5] B. Hu et al., Carbon 50, 57 (2012). [6] Y. Ogawa et al., J. Phys. Chem. Lett. 3, 219 (2012).

Graphene nanoribbons (GNRs) attract a lot of interests not only due to possible applications to future electronic devices but its unique electronic characteristics owing to the presence of edges. Depending on the edge character, zig-zag or arm-chair type, the electronic structure of GNRs shows different features from the one of 2D graphene [1]: a localized flat band and a band-gap, respectively. So far several approaches have been performed to fabricate GNRs, e.g. electron-beam lithography, unzipping of carbon nanotubes, chemical synthesis and so on. We here demonstrate new approach to obtain GNRs, where we utilize SiC substrate surface nanostructure as a template and molecular beam epitaxy (MBE). We found that such SiC surface nanostructures (SiC nano-surfaces) are organized during hydrogen etching at high temperatures when vicinal SiC(0001) substrate is used. Periodic nanofacet structures with typical ordering distances of 10-20nm are achieved [2, 3]. By the used of solid carbon source GNRs (exactly a (6radic;3×6radic;3)R30° layer [4], which is transformed to a graphene monolayer after the hydrogenation process [5]) can be formed on such SiC nano-surface at the initial stage of MBE growth, monitored by In-situ RHEED. Resultant GNRs after hydrogenation are examined by LEED, AFM, STM, polarized Raman spectroscopy, and angle-resolved photoemission spectroscopy (ARPES). In-situ RHEED observation indicates that an initial (radic;3×radic;3)R30°-Si structure on the SiC(0001) terraces is gradually transformed into (6radic;3×6radic;3)R30° as C atoms are supplied. The diffraction intensity of (radic;3×radic;3)R30° streaks is slightly increased at the initial stage, implies that the nucleation of a buffer layer occurs at (radic;3×radic;3)R30°-Si sites. Finally, the (radic;3×radic;3)R30° and (6radic;3×6radic;3)R30° streaks show zero and maximum intensity, respectively, indicating the whole terraces are covered by the 6radic;3 buffer layer. After hydrogenation the sample shows clear graphene Raman spectrum featuring intense D-band, whose intensity exhibits polarization dependence of the incident laser. This behavior is understood by the presence of arm-chair edges at GNRs. ARPES shows the valence band maximum at ~0.1-0.2eV below the Fermi energy, indicative of a band-gap opening. References [1] K. Nakada et al., Phys. Rev. B 54, 17954 (1996). [2] H. Nakagawa, S. Tanaka, and I. Suemune, Phys. Rev. Lett. 91, 226107 (2003). [3] M. Fujii and S. Tanaka, Phys. Rev. Lett. 99, 016102 (2007). [4] S. Tanaka, K. Morita and H. Hibino, Phys. Rev. B 81, 041406(R)(2010). [5] C. Riedl et al., Phys. Rev. Lett. 103, 246804 (2009). [6] L. Canccedil;ado et al., Phys. Rev. Lett. 93, 5-8 (2004).

A comprehensive understanding of the fundamental growth mechanisms during CVD of 2D nanostructures (graphene and other 2D materials) is essential to realise potential applications. Growth mechanisms have so far been based on simplistic models of elemental solubility of the constituent elements (eg: C) in metallic catalysts eg: Cu (low solubility - surface reaction) and Ni (high solubility - precipitation from bulk). These models are speculated based on ex-situ experiments but in-situ experimental evidence remains elusive. Using a combination of in-situ X-ray diffraction (XRD) 1 and high-pressure time and depth resolved in-situ X-ray photoelectron spectroscopy (XPS)1 at realistic CVD conditions of pressure (~0.001 - 1 mbar) and extreme temperatures (700-1000oC) we analyse the behaviour of poly-crystalline metallic catalyst films (eg: Cu, Ni, Co) for 2D nanostructure growth during both gaseous and liquid precursor exposures. These measurements allow for a clear understanding of elemental incorporation in the nanostructure as it happens by identifying the catalyst state at any point of time during CVD. These coupled with ex-situ experiments allows for development of growth mechanisms based on experimental evidence, avoiding the need for speculation. Further, we study graphene growth directly on a dielectric (HfO2) and compare graphitic carbon growth mechanisms for metallic catalysts and novel oxide catalyst systems2,3 with an inherently low carbon solubility. 1. In Situ Characterization of Alloy Catalysts for Low-Temperature Graphene Growth, Weatherup R.S., Bayer B.C., Blume R, Ducati C., Baehtz C., Schlögl R., and Hofmann S. Nano Letters. 11, 4154 (2011) 2. Nanoscale Zirconia as a Non-Metallic Catalyst for Graphitization of Carbon and Growth of Single- and Multi-Wall Carbon Nanotubes, Steiner III S. A., Baumann T. F., Bayer B. C., Blume R., Worsley M.A., Moberlychan W. J., Shaw E. L., Schloegl R., Hart A. J., Hofmann S., Wardle B. L., JACS 131, 12144 (2009). 3. Hafnia nanoparticles - a model system for graphene growth on a dielectric, Kidambi P.R., Bayer B.C., Weatherup R.S., Ochs R., Ducati C, Szaboacute; D.V., Hofmann S., Physica Status Solidi - RRL. 5, 9, 341-343 (2011).

Due to its excellent electronic, thermal and mechanical properties, and its large surface area and low mass, graphene holds great potential for a range of applications. While extensive efforts have focused on enabling cost effective mass production of solution-processable graphene sheets, the dominant approaches rely upon Hummer&’s or modified Hummer&’s methods. These methods involve tedious and time consuming procedures. In brief, one must first oxidize graphite powder, exfoliate the oxidized product to form nonconductive graphene oxide (GO) suspensions, and finally reduce it to recover some fraction of its electrical conductivity. Here, we report a quick and scalable strategy to directly produce highly conductive graphene sheets with controlled porous structures and controlled sizes from a few nanometers to tens of micrometers. Our method is to lightly oxide the aromatic network using nitronium ions and microwave irradiation. By controlling the duration and power of the microwave irradiation and the nitronium ion concentration, graphene sheets with controlled sizes from a few nanometers to tens of micrometers can be directly fabricated without the requirement of a reduction process. These sheets are thermally stable, highly conductive, and are having higher photothermal efficiency. Even in the absence of polymeric or surfactant stabilizers, high concentrations of graphene dispersions with clean and well separated graphene sheets can be obtained both in aqueous and organic solvents. This greener, rapid and scalable approach produces high quality graphene sheets, enabling a broad spectrum of applications by low-cost solution processing techniques.

The broadband dielectric spectroscopy (BDS) technique (40 Hz to 10 GHz) is used here to measure the electronic transport across all observed size scales of composite materials for lithium batteries containing each an active material (e.g. carbon-coated LiFePO4) and a polymeric binder. Since all the polarizations at different scales are additive owing to their vectorial character, their contributions (relaxations) can thus be evidenced by a decomposition procedure of the Nyquist plots (ε” vs. ε&’). Data acquisitions as function of temperature were also carried out, in order to determine the activation energies of the conductivity and relaxation frequencies at the different scales of the materials architectures. In the case of the carbon-coated LiFePO4 (C-LiFePO4), the carbon coating thickness (2 to 4 nm) and the ratio sp2/sp3 (in the coating) modify the electron transfer in the material. Different electrical relaxations are evidenced, resulting from the polarizations at the different scales of the architecture: (a) intra-sp2 domains, (b) nanometric (electron hopping from sp2 to sp2 domains in carbon coating around LiFePO4 particles), (c) clusters, and (d) sample/current collector interface. The carbon-coating conductivity and the electron hopping frequency from sp2 domain to another increase with the ratio sp2/sp3. Moreover, the influence of the polymeric binder (sodium-carboxymethylcellulose) on the coating conductivity and electron hopping has been evidenced. These later parameters rise with the binder volume fraction. This phenomenon would be due to an electron trapping by the adsorbed species at the surface of the coating. Financial funding from UMICORE and the ANR program n° ANR-09-STOCK-E-02-01 is acknowledged.

This presentation reviews synthesis of epitaxial graphene by sublimation of silicon carbide (SiC) in a chemical vapor deposition (CVD) environment over the past several years. Characteristics of graphene grown in an Epigress VP508 reactor at Purdue are presented with emphasis on the SiC carbon face. The kinetics of thermal decomposition in vacuo or argon environments are examined to better understand the surface phase transformation of C-face 4H-SiC into graphene. Pressure-temperature-time plots are constructed which describe conditions under which the 4H-SiC C-face surface transforms into graphene. Data pertaining to processing conditions for graphene are used to explore structure/property relationships for C-face graphene. Graphene structure is examined using numerous analytical methods, including scanning probe microscopy, Raman spectroscopy, x-ray scattering and transmission electron microscopy. Results obtained from analyses using these methods provide information about the general quality of C-face graphene, and defects that are present in the material. Hall Effect measurements at 4 K are performed to determine carrier concentrations and mobility. Carrier mobility in excess of 18,000 cm2/Vs have been measured. Transport data are correlated with structural characteristics derived from the above-mentioned material analyses. Finally, prospects for direct deposition of graphene onto various substrates (without the use of layer transfer) are explored.

Diamond-like carbon (DLC) has gained much attention as a super-low friction material for automotive engines, aerospace equipments, and so on. The detailed understanding of the tribochemical reactions of the DLC is strongly required for clarifying its super-low friction mechanism and designing more efficient super-low friction materials. Here, classical molecular dynamics simulation is frequently employed to investigate the friction behaviors on atomic scale. However, the classical molecular dynamics method cannot simulate the chemical reaction dynamics. Therefore, we developed our original first-principles molecular dynamics simulator “Violet” [1] and tight-binding quantum chemical molecular dynamics simulator “Colors” [2] for the elucidation of the tribochemical reaction dynamics. First, we applied our first-principles molecular dynamics simulator to the investigation on the tribochemical reaction dynamics of the H-terminated DLC and clarified that the hydrogen-hydrogen repulsion at the interface leads to the super-low friction of the DLC. Our tight-binding quantum chemical molecular dynamics simulations on the larger models also support the above conclusion. Moreover, our tight-binding quantum chemical molecular dynamics simulations suggest the formation of H2 molecule at the friction interface by the tribochemical reaction of hydrogen atoms which terminate carbon atoms [3]. The super-low friction coefficient of 0.07 is realized after the formation of H2 molecule. Then we suggest that the formation of vapor phase at the DLC-DLC interface is the reason for the super-low friction of the DLC. Furthermore, we investigated the tribochemical reaction dynamics of the OH-terminated DLC. The friction coefficient of the OH-terminated DLC is significantly lower than that of the H-terminated DLC under a severe condition of 7GPa load. It suggests that the OH-terminated DLC can solve the instability problem of the H-terminated DLC under a very severe condition [4]. We also simulated the tribochemical reaction dynamics of the H-terminated DLC under methanol condition [5]. The simulation results show that the methanol molecules decomposed at the friction interface and then the OH-terminations are observed. Therefore, we concluded that the methanol condition is very effective to achieve the super-low friction because the OH-termination is realized by the tribochemical reactions of methanol molecules. [1] T. Shimazaki and M. Kubo, Chem. Phys. Lett., 503 (2011) 316. [2] M. Koyama, M. Kubo, A. Miyamoto et al., J. Phys. Chem. B, 110 (2006) 17507. [3] K. Hayashi, M. Kubo et al., J. Phys. Chem. C, 115 (2011) 22981. [4] K. Hayashi, M. Kubo et al., in preparation. [5] K. Hayashi, M. Kubo et al., Faraday Discuss., in press.

Lithium ion batteries (LIBs) are known to suffer from kinetic problems due to the low Li-ion/electron conductivity of the electrode. A 3D interconnected network of both electron and ion pathways is an ideal electrode architecture for tackling this problem owing to its ability for efficiently exchanging charge and mass that occur when charging/discharging the battery. However, the fabrication of these electrode structures usually requires complex procedures depending on the specific electrochemically active material that is used, and the electrochemically inactive material required for structural support significantly increases the mass and reduces the energy density of the electrode. It is now possible to fabricate highly electrically conductive and lightweight ultrathin graphite foam (UGF). The UGF can be readily integrated with different cathode materials that are commercially available by using a process close to that used in the LIB industry.¹ At a high charge/discharge current density of 1280 mA g^minus;1, the specific capacity of the lithium iron phosphate (‘LFP&’) loaded on UGF was 70 mAh g^minus;1, while LFP loaded on Al foil failed. Accounting for the total mass of the electrode, the maximum specific capacity of the UGF/LFP cathode was 23% higher than that of the Al/LFP cathode and 170% higher than that of the Ni-foam/LFP cathode. Using UGF, both a higher power and energy density can be achieved simultaneously, owing to its conductive (sim;1.3 × 10⁵ S m^minus;1 at room temperature) and three-dimensional lightweight (sim;9.5 mg cm^minus;3) graphitic structure. UGF also has excellent electrochemical stability compared to that of Al or Ni foils, which are generally used as conductive substrates in lithium ion batteries. The UGF electrode can be prepared in a facile and cost-effective way and it is compatible with various electrochemically active materials. We appreciate support from the U.S. Department of Energy (DOE) under award DE-SC001951 and ARPA-E contract DE-AR0000178. References [1] Hengxing Ji, Lili Zhang, Michael T. Pettes, Huifeng Li, Shanshan Chen, Li Shi, Richard Piner, and Rodney S. Ruoff, Nano Letters, 2012, 12, 2446-2451.

We demonstrate the synthesis of graphene wrapped anatase TiO2 nanofibers (GW TiO2 nanofibers) and electrochemical performance of the GW TiO2 nanofibers as anode materials in lithium ion batteries. The GW TiO2 nanofibers was assembled by functionalized TiO2 nanofibers and graphene oxide via electrostatic interaction, followed by heating in reduce gas. Structural and morphological properties were characterized by various techniques, such as X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and BET surface area analysis. Lithium insertion properties were evaluated by both galvanostatic and potentiostatic modes in half-cell configurations. Cyclic voltammetric study reveals the Li-insertion/extraction by two-phase reaction mechanism that is supported by galvanostatic charge-discharge profiles.

A doped graphene and multilayer graphenes may belong to a strongly correlated electron system whose tunneling conductance, i.e. the dI/dV lineshape, is not fully understood yet. We show that two side peaks and suppressed zero-bias peak observed in a doped graphene and multilayer graphenes are the characteristic features of the tunneling conductance of a sample with strong electron correlation, which does not reflect the sample density of states. We show that two side peaks are formed by elastic resonant tunnelling between sample and STM tip or lead and discuss when the zero-bias peak is suppressed. Our theoretical line shapes remarkably fit the experimental data provided by Brar et al. for doped graphene [1], Freitag et al. for bilayer graphene [2], and Bao et al. for trilayer graphene [3]. [1] Brar et al. Phys. Rev. Lett. Vol. 104, 036805 (2010). [2] Freitag et al. Phys. Rev. Lett. Vol. 108, 076602 (2012). [3] Bao et al. Nat. Phys. Vol. 7, 948 (2011).

We present the microscopic structure, physical properties and defects evolution in novel nanocarbon hybrids of single- and multi-walled carbon nanotubes (SW and MWCNTs) and detonation nano diamond forming truly tetragonal-trigonal nanocomposite ensemble that were subjected to 50, 100 and 103 kGy gamma ray doses. They were solution cast as thin films and characterized using various analytical tools to investigate hierarchical defects evolution much below threshold. This work is a summary of our recent work [Gupta et al. J. Appl. Phys. 107 (2010) and Gupta et al. J. Appl. Phys. 106 (2009)] where radiation-induced microscopic defects seem to be stabilized by UDD, especially in the case of SWCNTs as compared with MWCNTs when subjected to the maximum irradiation dose. Quantitative analyses of multi-wavelength Raman spectra assessed through variation in the prominent D, G and 2D Raman bands elucidated radiation-induced microscopic defects. A minimal change in the position of D, G and 2D bands and a marginal increase in intensity of the defect induced double resonant D band and Raman active 2D band are some of the implications suggesting combined radiation coupling and resilience. The in-plane correlation length (La) was also determined following TK relation from the ratio of D to G band (ID/IG) besides microscopic stress/strain. Moreover, we also suggest the following taking into account of intrinsic defects of the constituents: a) charge transfer arising at the interface due to the difference in electro-negativity of CNT C-sp2 and UDD core (C-sp3) leading to phonon and electron energy renormalization; b) mis-orientation of C-sp2 at the interface of MWCNT and UDD shell (C-sp2) resulting in structural disorder; c) softening or violation of the q ~ 0 selection rule leading to D band broadening and a minimal change in G band intensity; and d) normalized intensity of D and G bands with 2D band help to distinguish defect-induced double resonance phenomena. Furthermore, an attempt was made to identify the nature of defects (charged versus residual) and it seemed the electrical properties were more labile to irradiation than structural properties. This work was primarily conducted at the University of Missouri and MURR while the author (S.G.) was working there.

Lithium-sulfur batteries are very attractive candidates for power sources owing to the high theoretical capacity (1675 mAh/g) and energy density (2600 Wh/kg) of sulfur. Nevertheless, the utilization of sulfur cathode is limited due to fast capacity fading resulted from the dissolution of lithium polysulfides, large volumetric expansion of sulfur (80%), and the insulating nature of Li2S. Efficient trapping of polysulfides is highly desired to improve the cycle life of Li-S batteries. In this context, carbon materials such as porous carbon, hollow carbon sphere and carbon nanotube (CNT) are attractive to improve the sulfur utilization by restraining the solubility of lithium polysulfides and providing conductive paths for electrons. Porous carbon spheres are able to effectively constrain sulfur molecule into the pores due to its large pore volume and pore structure. The high aspect ratio of carbon nanotube reduces the random diffusion of polysulfides and the thin carbon wall allows fast transport of lithium ions. Combination of advantages of CNT and porous carbon may result in novel class of material for sulfur cathode. However, such a material has not been reported owing to difficulty of synthesis. Herein, we present a porous carbon/CNT-sulfur composite (particle size: ~1 µm, CNT diameter: 8~10 nm) with high capacity and cycling stability. The porous carbon-CNT with good electrical conductivity, high surface area and pore volume was prepared using a continuous and scalable aerosol-assisted process followed by calcination. A sulfur-carbon sphere composite (sulfur 70 wt%) was prepared by thermal infusion of sublimed sulfur into porous carbon-CNT spheres at 155 °C. The composite of sulfur embedded in porous carbon-CNT provides with 1) better inter-particle contact and resistance to volume change of sulfur during the charge/discharge process; 2) a short transport pathway for both electrons and Li ions to achieve high capacity; 3) a large conductive surface area for depositing lithium sulfide thereby minimizing the shuttle effect. The electrochemical performances of carbon/CNT-sulfur composite are evaluated and discussed. From galvanostatic discharge-charge process, the porous carbon/CNT-sulfur composite shows high reversible capacity of 630 mAh/g after 20 cycles between 1.5-3.0 V (vs. Li/Li+) at 0.1 C (1C: 1675 mA/g). Such a good electrochemical performance is attributed to unique pore structure and composition which prevents the aggregation of sulfur particle and enhances the electrical connectivity and integration of the active electrode materials. In addition, the highly conductive porous carbon and CNT scaffold provides fast ion transport pathway as well as buffers shuttling of lithium sulfide during charge and discharge. The unique properties of this composite and their simple fabrication make this class of materials attracnot;tive for further investigation for Li-S batteries applications.

We report significantly enhanced field-effect mobility of poly(3-hexylthiophene) (P3HT) field-effect transistors (FETs) by introducing boron-doped multi-walled carbon nanotubes (B-CNTs) into a P3HT semiconducting layer. Since B-CNTs have an electrostatic repulsion with neighboring B-CNTs due to the local positive charges of boron atoms, they can be homogeneously dispersed in solution, and also in the semiconducting layer after spin-casting, while undoped CNTs tend to be gathered forming undesired aggregates. In addition, because B-CNTs have excellent hole-selective transport property, they can further facilitate hole transport than undoped CNTs. In this work, organic FETs comprising a P3HT/B-CNT (1.0 wt%) composite layer show high field-effect mobility of 0.05 cm²/Vs, which is almost 20-fold greater than that of the pure P3HT FET (2.9 × 10^-3 cm²/Vs), and 4-fold greater than that of the P3HT/undoped CNT FET, respectively. The X-ray diffraction data indicates that the size of crystalline P3HT domain is gradually reduced with increasing B-CNT concentration. However, the mobility of the P3HT/B-CNT FETs continuously increases when the concentration of B-CNT reaches up to 10 wt%. This fact implies that the homogeneously dispersed B-CNTs in polymer matrix act as selective conduction bridges for hole transport between the crystalline P3HT domains.

A rational control of the structure of Single Wall Carbon Nanotubes during their synthesis is highly desirable, but currently limited by our poor understanding of nucleation and growth mechanisms and the lack of direct evidence on the actual state of the catalyst particle / nanotube interface. To progress towards an atomic scale understanding, we use a carefully assessed tight binding model for nickel and carbon [1, 2] to numerically investigate different aspects of the CCVD synthesis process. Owing to significant technical improvements of our grand canonical Monte Carlo code [2], we can extend our previous calculations [3] of carbon adsorption isotherms to nanoparticles (NPs) up to 807 Ni atoms, in a broad temperature range. We thereby study the carbon solubility and physical state of the metal catalyst as a function of size, temperature and carbon chemical potential conditions corresponding to nucleation and growth of SWNTs. Combining experimental information from Transmission Electron Microscopy and atomistic computer simulation, we try and understand the relation between the diameters of the tube and the metallic NP from which it grows [4]. We then study the wetting of the NPs with respect to sp2 carbon walls, that strongly depends on carbon concentration, and emphasize its role in the growth of tubes. This enables us to identify conditions leading to experimentally observed situations: aborted growth by encapsulation of the metal NP with carbon, growth termination by detachment of the tube from the NP and continuous growth under mild carbon chemical potential, temperature and feeding rate conditions. [1] H. Amara, J. M. Roussel, C. Bichara, J.-P. Gaspard and F. Ducastelle, Phys. Rev. B 79, 014109 (2009). [2] J. H. Los, C. Bichara and R. Pellenq, Phys. Rev. B 84, 085455 (2011). [3] H. Amara, C. Bichara and F. Ducastelle, Phys. Rev. Lett., 100, 056105 (2008). [4] M.-F. C. Fiawoo, A.-M. Bonnot, H. Amara, C. Bichara, J. Thibault-Pénisson and A. Loiseau, Phys. Rev. Lett. 108, 195503 (2012).

A facile methodology is developed to directly synthesize three-dimensional (3D) electrochemically supercapacitive arrays, consisting of multi-walled carbon nanotubes conformally covered by nanocrystalline vanadium nitride, firmly anchored to glassy carbon or Inconel electrodes. These nanostructures demonstrate a respectable specific capacitance of 289 F g-1, which is achieved in 1 M KOH electrolyte at a scan rate of 20 mV s-1. The well-connected highly electrically conductive structures exhibit a superb rate capability; at a very high scan rate of 1000 mV s-1 there is less than a 20% drop in the capacitance relative to 20 mV s-1. Such rate capability has never been reported for VN, and is highly unusual for any other oxide or nitride. These 3D arrays also display nearly ideal triangular voltage profiles during constant current charge-discharge cycling. Analysis of the post-electrochemically-cycled samples indicates negligible changes occurring in the VN nanocrystallite morphology, but a modification in the structure of the oxidized surface. We envision that the direct synthesis approach employed in this study may serve as a “drop-in” platform for large-scale commercial fabrication of a variety of carbon nanotube-supported functional materials that require excellent electrical conductivity to the underlying support.

We use a combination of theory (density functional calculations and simulations using classical potentials) and experiments (Z-contrast imaging & electron energy-loss spectroscopy using aberration-corrected scanning transmission electron microscopy) to probe the structure and electronic properties of grain boundaries in graphene and the interactions of grain boundaries with point defects. Grain boundaries exhibit an alternation of pentagonal and heptagonal rings where the local strain is compressive and extensive, respectively. Vacancies are attracted at pentagons and carbon interstitials are attracted at heptagons [1]. In both cases the point defects merge in the grain boundary and release strain. The grain boundaries also mediate vacancy-interstitial annihilation. Furthermore, grain boundaries attract O adatoms and bind them more strongly. Cooperative activity among O atoms can lead to preferential etching in grain boundaries. Z-contrast images obtained with an aberration-corrected scanning transmission electron microscope reveal twisted grain boundaries that undergo continuous evolution under the electron beam via conversion of pentagon-heptagon ring pairs into hexagon-hexagon ring pairs and vice versa. Molecular dynamics simulations using classical potentials probe the stability of straight boundaries and the activation energies for the observed evolution. Electron-energy-loss spectroscopy in the low-loss region (valence excitations) has been used to explore plasmon localization, a phenomenon that was recently observed to occur at impurities and grapheme edges [2]. The potential for grain boundaries acting as plasmonic wave guides will be discussed. An image of a highly symmetric grain boundary loop has been obtained and its structural, electronic, and magnetic properties have been explored by theory, including the possibility that single impurity atoms at the center of the loop may impart unique properties to this topological defect. This research was supported by NSF grant No. DMR-0938330 (WZ, J-CI), DOE grant DE- F002-09ER46554 (STP), by ORNL&’s Shared Research Equipment (ShaRE) User Facility, Oak Ridge National Laboratory, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences (J-CI), by the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. Department of Energy (SJP, JL,STP), and by the Defense Treat Reduction Agency grant HDTRA1-10-1-0016 (BW,YP,STP) [1] B. Wang, Y. Puzyrev and S. T. Pantelides, “Strain-enhanced defect reactivity at grain boundaries in polycrystalline graphene”, Carbon 49, 3983 (2011). [2] W. Zhou, J. Lee, J. Nanda, S. T. Pantelides, S. J. Pennycook, and J.-C. Idrobo, “Atomically localized plasmon enhancement in monolayer grapheme”, Nature Nanotechnol. 7, 161 (2012).

Energy storage phenomenon in supercapacitors (applied in high power demanding devices is generally attributed to electrical double layer charging, formed on electrode/electrolyte interface. Carbon materials, due to their well developed surface area, seem to be most promissing materials for supercapacitor electrodes. Unfortunately, capacitance revealed by these materials is still relatively small. However, if apart from double layer charging/discharging, some additional charge from faradaic reactions will be provided, the capacitance increases rapidly. This kind of additional capacitance, called pseudocapacitance, might be provided by enriching carbon materials in heteroatoms or applying carbon-transition metal oxides composites as electrode materials and assembling supercapacitor in asymmetric system. This study follows the completely novel insight on pseudocapacitance phenomenon, because an additional charge comes from electrolyte solution, not from electrode bulk. First study reported by our team concerns alkali metal iodides aqueous solution. Carbon electrodes, operating in these solutions can even reach the values of 2272 F/g (for 1 mol/L RbI solution), but supercapacitor capacitance does not exceed the value of 220 F/g, due to poor capacitance of counter electrode which determines the total capacitance of supercapacitor. Hence, huge capacitance of positive electrode remains unexploited. This work, focused on carbon electrodes with pseudocapacitive phenomena, reveals intriguing behavior of iodide/vanadyl system. These redox couples, when applied in solution as supercapacitor electrolyte, allows to exploit capacitance of positive electrode (operating in iodide solution) and significantly improve the capacitance of negative one (operating in vanadyl solution). Full electrochemical characterisation of activated carbon as well as activated carbon/multiwalled carbon nanotubes electrodes operating in conjugated redox couples solutions (1 mol/L KI and 1 mol/L VOSO4) reveals unusual capacitance values, being about 650 F/g for total system. This value significantly improves the energy of supercapacior, even to the level of 20 Wh/kg, which is the highest reported value for aqueous medium. Galvanostatic charge/discharge investigation, performed in wide range of current density (0.2 - 20 A/g) proved a good electrochemical behaviour of this capacitor. Cyclic voltammetry performed in range of scan rates 1-20 mV/s, reveals faradaic character of capacitance. Electrochemical impedance spectroscopy (100 kHz - 1 mHz) confirmed good charge propagation and small charge transfer resistance. These advantages are followed by very stable cyclability (about 10% of capacitance decay after 5000 charge/discharge cycles), small self-discharge and low leakage currents. Additionally, aqueous medium is much more environmental friendly and significantly cheaper than organic one, usually applied for high energy supercapacitors.

Graphene grown on SiC by thermal sublimation of Si atoms is considered a most promising route to obtain homogeneous large area graphene sheets on a semi-insulating substrate, suitable for development of graphene-based nanoelectronics. When grown in a furnace using a gas ambient, or confinement controlled sublimation, large sheets of monolayer graphene can be produced on the Si-terminated SiC(0001) surface. The carbon buffer layer, present on this surface, can be effectively decoupled by intercalation of hydrogen, or other atoms, so large area bi-layer graphene samples can be obtained. There is a general consensus concerning the experimental findings for graphene on Si-face SiC. This is not the case for graphene grown on the C-terminated SiC(000-1) surface. Whereas Si-face graphene exhibits sharp spots in LEED and the Bernal stacking, multilayer graphene on the C-face has been reported to stack in such a way that adjacent graphene layers are rotated with respect to each other. This rotational disorder was suggested to explain why multilayer graphene show single layer electronic properties, i.e. a single π-band. We recently prepared graphene in a furnace at higher temperatures, 1800-2000°C, than commonly utilized earlier for C-face SiC. We found formation of mu;m sized crystallographic domains, which exhibited sharp (1x1) spots in mu;-LEED and six Dirac cones in the Brillouin zone in recorded photoelectron angular distribution patterns. Adjacent domains showed different azimuthal orientations so macro-LEED patterns mimicked earlier published ring-like LEED patterns. Our findings clearly showed that within a domain adjacent layers are not rotationally disordered. Therefore we prepared graphene samples on C-face SiC at 1400-1600°C, i.e. at temperatures utilized by other groups, and investigated them using LEEM, XPEEM, XPS and ARPES. Also these showed the presence of mu;m sized crystallographic domains of few layers well ordered graphene. Effects of Na exposures on the electron band structure of C-face graphene were also studied using ARPES. These results will be presented and discussed in view of earlier findings.

Dye-sensitized solar cells are regarded as the next-generation solar cells. A dye-sensitized solar cell include a work electrode, a counter electrode and electrolyte between the two electrodes. In this paper, we report the application of graphene gels in all the three parts of dye-sensitized solar cells. The graphene gels lower the device fabrication cost by facilitating the device fabrication and replacing expensive materials. Our iodide/triiodide dye-sensitized solar cells with graphene as the counter electrode can have a photovoltaic efficiency of 7.2%, the highest for the iodide/triiodide dye-sensitized solar cells with graphene as the counter electrode.

We explore fundamental materials properties of hexagonal boron nitride (h-BN) as a new class of gate dielectric for high-performance graphene electronics. h-BN is an insulating isomorph of graphene with two-dimensional layered structure (weak van der Waals interactions between monolayers). While h-BN has been demonstrated as an ideal substrate material for graphene electronics, we here demonstrate that ultra-thin h-BN multilayers can also be used as highly functional, scalable, and robust gate dielectric material. Layered h-BN provides a smooth planar surface free of dangling bonds (therefore extremely low interface state density) and helps to avoid the formation of charge traps or electron-hole puddles in graphene channel. Ultra-low tunneling leakage current and very high breakdown electric field (up to 18 MV/cm) across the dielectric are measured, demonstrating the robust nature of the 2D layered dielectric material. Carrier mobility in graphene channel as high as ~20,000 cm2/V-s is measured at room temperature in a unique locally gated transistor architecture in which h-BN multilayer serves as both gate dielectric and supporting substrate.

Catalytic chemical vapor deposition (CCVD) is the most widely used technique for growing single-walled carbon nanotubes (SWCNTs). It is also the most promising one in terms of upscaling and structural control. However, it is commonly observed that CCVD-grown SWCNTs contain defects. Controlling the defect density of SWCNTs and graphene-like materials in general is a key issue for the control of their properties. We will report on the influence of the CCVD parameters on the defect density of SWCNTs using in situ and ex situ Raman analyses and HR-TEM observations [1,2]. In situ Raman monitoring was notably used to determine experimental conditions allowing the preparation of SWCNT samples free of pyrolytic carbon and not altered by air exposure. Using the Raman D band to quantify the SWCNT defect density brought two unexpected results. First, the evolutions of the G/D ratio with the synthesis temperature display two distinct Arrhenius-type regimes. Second, these Arrhenius plots systematically display a convex shape, that is a lower apparent activation energy with increasing temperature. HRTEM and Raman analyses support that long and straight SWCNTs with few -curvature-inducing defects are grown at high temperature while much shorter and more tortuous SWCNTs and carbon nanostructures are grown at low temperature. Kinetic models were tested to reproduce the experimental evolution of the G/D ratio as a function of the growth parameters. We found that two different models can account for the convex shape of the Arrhenius plots: (i) a model considering a change of intermediate states as a function of the temperature (for instance due to a phase transition of the catalyst particle) and (ii) a model considering a high-temperature process of defect creation (for instance by reaction with foreign gas species). The implications of both hypotheses for the growth mechanism of SWCNTs will be discussed. [1] Picher et al., Nanoletters (2009), 9, 542. [2] Picher et al., Carbon (2012), 50, 2407.

With extraordinary charge-accepting and conductivity properties, single wall carbon nanotubes (SWCNTs) are explored as donor-acceptor systems in light harvesting systems. Novel nanostructures are developed that employ SWCNTs in self-assembling, light-harvesting nanohybrids in conjunction with light-absorbing chromophores. The nanohybrid structures are characterized optically and electrochemically, demonstrating charge transfer reaction between photo-excited molecules and SWCNTs. This hybrid offers a facile manufacturing method for light harvesting nanomaterials, providing insight into photo-processes and current generation.

In realistic devices, graphene is often surrounded by dielectric materials, which strongly influence its electronic properties. Strong chemical bonding between graphene and surrounding materials drastically changes its electronic properties. However, it is now well known that graphene also bonds with several materials via weak van der Waals interaction. In such weakly bonded systems the electronic properties of graphene are affected by the dielectric screening due to the long-range Coulomb interaction with the surrounding materials. Here we investigate the effects of dielectric screening on the electronic properties of heterostructures of graphene and hexagonal boron nitride (hBN), where graphene binds weakly with hBN via van der Waals interaction. The first-principles density functional theory (DFT) combined with the many-body perturbation theory within the GW approximation are employed to accurately model the long range dielectric screening in weakly bound systems. A Bernal stacked graphene on the hBN susbtrate is expected to have a quasiparticle band gap on the order of 0.1 eV. The recent experiments, however, find no band gap in graphene deposited on the hBN substrate [1]. We show that misalignments (deviations from the perfect Bernal stacking) of graphene with respect to the hBN substrate present in experimental samples close the band gap in graphene [2]. Band gaps, which are robust to the external perturbations such as misalignment, can be opened up by chemical functionalization of graphene. Here we focus on the single-sided semi-hydrogenated graphene (graphone), which has a band gap larger than 2.5 eV. We find that the dielectric screening from hBN substrate lowers band gap of graphone by as much as 1 eV [2]. We also investigate the effects of dielectric screening on the conductivity of pristine graphene in graphene-hBN heterostructures [3]. The charged-impurity limited conductivity of graphene is found to increase due to the non-local dielectric screening of the charged-impurity scattering potential by the surrounding hBN layers. The results suggest that the dielectric screening by surrounding materials can be used to tune the electronic structure and transport properties of graphene-based nanostructures. [1] C. R. Dean et al., Boron nitride substrates for high-quality graphene electronics, Nat. Nanotechnol., 5, 722-726 (2010). [2] N. Kharche and S. K. Nayak., Quasiparticle Band Gap Engineering of Graphene and Graphone on Hexagonal Boron Nitride Substrate, Nano Letters, 11, 5274-5278 (2011). [3] N. Kharche, T. B. Boykin, and S. K. Nayak, Dielectric screening and conductivity of graphene in graphene-hBN heterostructures, To be submitted.

The network of grain boundaries that decorate graphene flakes offer a natural template for engineering their structural, morphological and transport characteristics. Quite like their three-dimensional counterparts, the overall response is expected to be dictated by an interplay between properties of the individual grain boundaries and tri-junctions, and microstructural parameters such as grain size and (in-plane) crystal orientation distribution. Here, we employ atomic-scale computations to investigate the stability of randomly oriented graphene flakes with respect to in-plane tension. We observe a wide variation in the boundary structures, from the classical 5-7 dislocated structure of symmetric boundaries to more open structures in random boundaries, a reflection of the available geometrical and morphological degrees of freedom. Small grain sizes (<10nm) lead to naturally buckled morphologies that are ironed out via soft deformation modes. The plastic response is invariably brittle and in some cases approaches that of pristine graphene. The eventual fracture takes place in an intergranular fashion. We also present experimental confirmation of the fracture mechanisms. Our results offer a basis for tailoring the structural rigidity as well as fracture of large-area graphene flakes via engineering of the constituent grain boundary nanostructure.

Among the interesting properties of graphene, its uniformly high transparency in the visible and near infrared regions along with good electrical conductivity and mechanical robustness are desirable for applications in optoelectronic devices, especially as transparent conducting electrodes in organic photovoltaic (OPV) solar cells or organic light-emitting diodes (OLED). Although there have been some initial demonstrations of graphene as transparent electrodes in OPV or OLED devices with promising results, the realization in real-life applications still remains challenging due to issues in device reproducibility or device yield, apart from their performance. In this work, we demonstrate that by engineering the interface between the graphene electrode and the organic hole transporting layer, the device performances and device yields become comparable to devices using indium tin oxide (ITO) as a transparent electrode for both OPV and OLED devices. Our study here further confirms that the key issue leading to poor performances or irreproducible results in OPV or OLED with graphene electrodes was the interface between the graphene and organic layers, which can be solved by a simple yet novel method introduced in this work. This represents an important step forward for the application of graphene as transparent electrodes.

We report the structural and electronic properties of doped semiconducting zigzag carbon nanotubes (CNTs) on the hexagonal boron nitride (hBN) sheet in the presence of external electric field obtained by first-principles calculations. We consider three different types of dopants (K, Cl and Ni) to study the dependence on doping polarity and calculate partial density of states to know whether the conduction or valence bands of the hBN substrate would be approach the Fermi level, causing some problems in CNT devices. Finally we show that though the electronic energy bands of the hBN sheet are shifted close to the Fermi level and some localized states occur in accordance of the dopant type and the field strength, the states do not touch the Fermi level under the field strength considered. It suggests that hBN as a substrate does not modify the electronic structure of the CNT improving the device performance, compared to conventional substrate materials such as silicon dioxide, whereas leakage current may occur for the doped-CNT on the hBN substrate when the high top-gate voltage is applied in the dual-gated field effect transistor.

Magnetic nanoparticles are used in a multitude of research areas. Comparably high and readily available specific surfaces combined with the ease of separation [1] due to very high saturation magnetizations make carbon-metal nanomagnets promising candidates as precious metal extraction agents [2] and in magnetic chemistry where a reagent is linked to a nanomagnet [3]. Furthermore, the feasibility as cleaning agents of complex liquids such as blood has been demonstrated [4]. The chemical stability in conjunction with a high saturation magnetization is decisive parameter for a broad range of applications. A systematical examination of the chemical stability of cobalt metal nanomagnets with a graphene-like carbon coating was used to study the otherwise rather elusive formation of nanometer sized physical defects in few layer graphene as a result of acid treatments [5]. A direct observation of these defects in carbon nanostructures like carbon nanotubes and graphene is challenging. The release of cobalt into the solutions over time offered a simple tool to monitor the degradation progress of such structures and therefore to determine the prevalent mechanism of physical defect formation. The suggested mechanistic insight was further confirmed by the covalent chemical functionalization of the particle surface with chemically inert aryl species, which leads to an additional thickening of the shells. The possibility to prepare well-defined shells or containers combined with the new findings may provide attractive tools in material sciences.

Homogeneous growth of vertical or spaghetti-like hollow carbon nanostructures were obtained on glass covered with transparent metallic Indium Tin Oxide (ITO), without any addiction of catalytic particles, by means of the chemical vapor deposition of C2H2 in the temperature range 800 - 940 K. Carbon nanostructures are filled by Sn metal. The carbon skin can be burned out in oxygen atmosphere living pillars of SnO2. This strategy is versatile and adaptable to many substrate materials where ITO can be deposited and treated at the nanostructure growing temperatures. The rational control of the simple growth process and the lack of external catalysts allows the fabrication over large areas with optimum, as well as tunable, electrical and optical characteristics. With the possibility to tailor ordered and eventually vertically aligned arrays of SnO2 pillars, or Sn covered by a carbon skin, it is envisaged that this approach will be a promising path to enhance the efficiency in devices for energy generation and storage, which could benefit from the improved charge transport in absence of grain boundaries or interface contacts, as well as from the here demonstrated increased good transparency of such nanostructured ITO electrodes at higher wavelengths (infrared part).

Although the graphene has demonstrated great promise as an appealing material for future electronic technology and fundamental physics applications with its superior properties, it is known that the graphene-based device performances are significantly influenced by the substrate material and the interface quality between the graphene and the substrate. There have been considerable amounts of experimental and theoretical studies showing that the degradation of graphene-based devices are caused by the substrate-induced charge traps and various scattering mechanisms, which result in the limited carrier mobility, hysteric behavior of I-V characteristics, and Dirac point shift. Therefore, for the improvement of graphene-device performance, it is critical to understand the dependence of graphene-base devices on the dielectric substrate materials and find the optimized material systems and process parameters. In this work, we performed a systematic investigation on the impact of various dielectric materials such as SiO2, Si3N4, Al2O3, and hexagonal boron nitride on the electrical characteristics of graphene transistors. By using DC and pulsed I-V measurements, the carrier transport properties of graphene transistors with various dielectric materials are comparatively studied. In addition, the charge trapping and hysteresis characteristics of these devices are quantitatively analyzed. Results show that the much improved carrier mobility and suppressed charge inhomogeneity are observed with both CVD and exfoliated graphene on hexagonal boron nitride substrate. Our results provide basic guidelines for further development and optimization of graphene/dielectric material systems and process integration for future high performance electronic device applications.

The high strength reported in pristine graphene (Lee et al., Science 321:385(2009)) stimulates great interest in utilizing high strength and stretchable graphene (Rogers et al. Nature 477:45(2011)) for biological structures and electronic devices. Current synthesis techniques for large-area graphene result in the appearance of grain boundaries (GBs) (Huang et al. Nature 469:389(2011)). Although there has been good understanding on how typical defects like dislocations and GBs influence the strength of three-dimensional polycrystals, how GB defects like pentagon-heptagon rings in the two-dimensional graphene influence its mechanical properties remains a big unknown. In this work, we address how and why pentagon-heptagon defects in tilt GBs may enhance or weaken the strength of graphene through both molecular dynamics simulations and theoretical analysis. 1. We have employed both molecular dynamics simulations and theoretical analysis to study the mechanical strength in graphene with GBs. We find that it is not just the density of defects that affects the mechanical properties, but the detailed arrangement of the GB defects. The strengths of tilt GBs are proportional to the square of their tilt angles if the pentagon-heptagon defects are evenly spaced, and the trend breaks down if pentagon-heptagon defects are distributed in other ways. 2. With the availability of a detailed simulation, we find that mechanical failure starts from the bond shared by hexagon-heptagon rings. 3. We have developed a theory to capture the interaction among hexagon-heptagon defects in two-dimensional graphene, which is capable of quantitatively predicting the observed mechanical behavior through molecular dynamics simulation. The present work provides fundamental guidance to understand how defects interact in two-dimensional crystals, which is important for utilizing high strength and stretchable graphene14 for biological and electronic applications.

There is a strong interaction between epitaxial graphene and silicon carbide (SiC) substrate through the buffer layer, resulting in strong n doping in graphene with a density of ~10¹³ cm^-2. Recently, it has been shown that the interaction can be reduced by decoupling the buffer layer from the substrate by H intercalation.[1] Our group has reported, from transport measurements, that quasi-free-standing bilayer graphene produced from epitaxial monolayer graphene by H intercalation is p-doped.[2] However, the adsorption characteristics of H have not been fully understood, which would strongly affect the electronic structures of quasi-free-standing graphene. In this work, we investigated the surface structures before and after H desorption and the H desorption process on quasi-free-standing monolayer graphene (QFMLG) by room-temperature and high-temperature in-situ scanning tunneling microscopy (STM) observations. A 4H-SiC(0001) substrate was used as a sample. The sample was annealed in Ar at 1700°C for 5 min to form a buffer layer, and then annealed in H₂ at 1000°C for 30 min for H intercalation and formation of QFMLG. The time evolution of the surface structure was observed by in-situ STM at 540-670°C in an ultrahigh vacuum. Above 630°C, a H-desorbed region preferentially grew from SiC step edges. The coverage of QFMLG region linearly decreased with time. This indicates zero-order desorption. The effective activation energy for H desorption from QFMLG was estimated to be 2.5±0.4eV. The desorption process includes the breaking of bonds between H and Si atoms and penetration of H atoms through a graphene sheet. The activation energy measured in our experiment is comparable to the Si-H bonding energy on Si(111)-7×7 (2.7±0.2 eV[3]) and smaller than the energy for H atom to penetrate through a graphene sheet (2.86[4] - 4 eV[5]). These results suggest that H adatoms diffuse at the graphene/SiC interface and desorb from the surface at energetically active sites like SiC step edges. [1] C. Riedl, C. Coletti, T. Iwasaki, A. A. Zakharov, and U. Starke, Phys. Rev. Lett. 103, 246804 (2009). [2] S. Tanabe, Y. Sekine, H. Kageshima, and H. Hibino, Jpn. J. Appl. Phys. 51, 02BN02 (2012). [3] M. L. Wise, B. G. Koehler, P. Gupta , P. A. Coon, and S. M. George, Surf. Sci. 258, 166 (1991). [4] M. Miao, Y. Liu, T. Wu, Q. Wang, and K. E. Gubbins, Diffusion-fundamentals.org 16, 1 (2011). [5] Y. Miura, H. Kasai, W. A. Diño, H. Nakanishi, and T. Sugimoto, J. Phys. Soc. Jpn. 72, 995 (2003).

Cubic silicon carbide (3C-SiC) is a metastable material, challenging to grow and therefore there are very few graphene studies using this polytype. Graphene on the hexagonal (111) oriented surface of cubic silicon carbide was demonstrated by sublimation growth and a uniform coverage of a single monolayer could be obtained on this orientation [1]. Due to the fact that (001) oriented 3C-SiC is not available in good quality so far the growth of graphene on this cubic surface could only be shown on small facetted areas [2] or on 3C-on-silicon with high defect densities [3]. Contrary to hexagonal and 3C (111), it is expected that no interface layer is formed between the silicon carbide (001) substrate and the epitaxial graphene layer. Our new technology for the preparation of 3C-SiC substrates allows us to study the formation of graphene on larger areas of (001) surfaces on free standing cubic silicon carbide. The (001) oriented cubic silicon carbide substrates were produced using a newly developed procedure based on the fast sublimation growth process (FSGP). A template of a thin 3C-SiC layer on silicon (provided by chemical vapor deposition) is used as a seed for the growth of a freestanding cubic silicon carbide sample of high structural quality. Using this new approach free standing substrates with a FWHM of less than 80 arcsec in the (002) reflection in XRD Rocking measurements with a large footprint of 1x1.5mm2 were produced [4]. In our research we were able to produce epitaxial graphene by a high-temperature atmospheric pressure sublimation process on (001) oriented free standing cubic silicon carbide. By low energy electron microscopy (LEEM) measurements a stack of three to four layers of graphene was found on (001) 3C-SiC. These structures were formed under the same conditions like normally used for the production of monolayer graphene on Si-terminated hexagonal surfaces, e.g. (0001) on 6H-SiC or (111) on 3C-SiC. This shows a faster formation rate of graphene on the non-polar (001) surface of cubic silicon carbide. Also the non-polar (11-20) surface of the hexagonal polytypes has shown a similar increase in the formation rate in work found in the literature which results in speeds between the slow Si-terminated (0001) and the fast C-terminated (000-1) face [5]. A modification and optimization of the process parameters in the graphene growth makes the fabrication of mono- or bilayer graphene on the cubic (001) surface of 3C silicon carbide possible. A different mechanism might make this formation more complex. Detailed characterization will reveal the nature of the interface structure and the doping of the graphene layer. [1] G.R.Yazdi, et.al. (to be published) [2] H.Handa,et.al. MSF 711 (2012), pp 242 [3] A.Ouerghi, et.al. Physical Review B 83, 205429 (2011) [4] P.Hens, et.al. Materials Letters 67, 1 (2012), pp 300 [5] B.K.Daas, et.al. MSF 717-720 (2012) pp 633

This paper investigated the effect of chromium underlayer on the structure, microstructure and composition of the nano-crystalline diamond films. Nano-crystalline diamond thin films were deposited at high temperature in microwave-induced plasma diluted with nitrogen, on single crystal silicon substrate with a thin film of chromium as an underlayer. The composition, structure and microstructure of the deposited layers were analyzed using non-Rutherford Backscattering Spectroscopy, Raman Spectroscopy, Near-Edge X-Ray Absorption Fine Structure, X-ray Diffraction and Atomic Force Microscopy. Nanoindentation studies showed that the films deposited on chromium underlayer have higher hardness values compared to those deposited on silicon without an underlayer. Diamond and graphitic phases of the films evaluated by x-ray and optical spectroscopic analysis determined consistency between sp2 and sp3 phases of carbon in chromium sample to that of diamond grown on silicon. Diffusion of chromium was observed using ion beam analysis which was correlated with the formation of chromium complexes by x-ray diffraction.

A carbon nanoscroll (CNS) is formed by rolling up a monolayer graphene into a spiral structure. A CNS is topologically open, distinct from a MWCNT. The unique topology of CNSs has inspired an array of novel applications. For example, CNSs have been shown to be a promising material for hydrogen storage. Enthusiasm for utilizing CNSs for hydrogen storage aside, one crucial but largely unexplored issue for the success of this potential application is an effective mechanism for molecular mass transport through CNSs. For example, it remains elusive how to shuttle the hydrogen molecules adsorbed inside CNSs. To address this unexplored issue, we demonstrate two effective mechanisms of molecular mass transport that leverage the unique topology of CNSs, using molecular dynamic simulations. The open topology of CNSs dictates their proneness of buckling instability. We show that buckling instability of CNSs under torsional deformation can be used to effectively transport the adsorbed hydrogen molecules inside the CNSs. The open topology of CNSs also allows for large tunability of their inner core size, which can also be leveraged for molecular mass transport. We demonstrate that electrical field induced change of carbon-carbon interlayer adhesion in a CNS can lead to the change of the CNS core size, which can effectively push the adsorbed hydrogen molecules out of the CNS. Both mechanisms demonstrated here are expected to be applicable to molecular mass transport of other materials as well, such as nanoparticles, water molecules, bucky balls, etc. The exceptional durability of the basal graphene also allows for reliable functionality of CNS-enabled molecular mass transport.

Graphene is an excellent 2D material for a range of potential electronic applications, owing to its high mobility and ultra-thin body. However, the performance of graphene field-effect transistors (FETs) is limited by the interfaces that are established to form a device—contacts yield high contact resistance and the gate dielectric yields reduced mobility. While many reports have shown techniques for reducing the contact resistance and/or scattering in the channel, this work establishes a unique approach to improving the overall performance in graphene FETs by taking advantage of the ability to transfer graphene to virtually any surface. Unlike traditional, bulk materials used to make FETs, graphene is 2D and can be transferred to a substrate of choice over large areas (e.g., 200 mm wafers). In this work, graphene FETs are demonstrated with a channel consisting of multiple, stacked layers of graphene that are electrically connected in parallel at the contacts and controlled by the same gate. This vertical stacking of graphene channels allows for the FET performance to be enhanced without altering the device footprint. Chemical vapor deposition (CVD) was used to synthesize single-layer graphene on copper foils. Each layer of graphene could then be transferred from the foils to the silicon substrate on which the devices were being assembled. This demonstration first involved the fabrication and testing of top-gated, single graphene layer FETs. Then, using the same gate and contact lithography masks, a second graphene layer was transferred to the substrate on top of the gate stack and contacts of the first layer. The material stack in the channel from bottom to top in this double-stacked graphene FET was then: SiO2 substrate, 1st graphene, gate dielectric (HfO2), gate metal, gate dielectric (HfO2), 2nd graphene. Adding the 2nd graphene layer consistently provided more than 2x increase in on-current and transconductance compared to the single-layer devices, yielding > 1 mA/mu;m at 1 V of drain-source bias. The primary reason for the > 2x performance improvement when going from one layer to two stacked layers is that the second layer had metal contacts below and above the graphene (double contacts), resulting in a lower contact resistance. It is also shown that further stacking can continue using the same mask set to achieve ultrahigh current and transconductance devices. Overall, this study demonstrates that making use of graphene&’s substrate independence and transferability can enable the fabrication of FETs with tunable performance using the same mask set and without enlarging the device footprint.

Recent theoretical work has shown that electronic properties of graphene sheets can be systematically modified by the partial hydrogenation of the sheets. Two possible perfect and distinct graphene/graphane interfaces (called zig-zag and armchair) have very different but potentially useful electronic properties which are nevertheless likely to be affected by the presence of defects. In an effort to evaluate their relative energetics and their potential for defects, the structure and energies of the zig-zag and armchair interfaces have been computed for infinite sheets of periodically alternating stripes of graphene and graphane ribbons of various widths. The presence of an interface causes significant strains in both the graphene and graphane regions, with both shear strains and area strains typically close to 1%. The associated large strain energies may lead to defects that relieve the strain but disrupt the lattice. The energies per unit length associated with the interfaces alone are approximately 0.12 eV/Å for the zig-zag interface and 0.11 eV/Å for the armchair. The large misfit strains and energies suggest that formation of strain-relieving defects at the interface should be highly favorable. This research has been sponsored by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy.

We report a fully bendable all-graphene modulator circuit with the capability to encode a sinusoidal carrier signal with quaternary digital information for the first time. By exploiting the ambipolarity and the nonlinearity in a graphene transistor, we demonstrated two types of quaternary modulation schemes: 4-ary amplitude-shift keying (4-ASK) and quadrature phase-shift keying (QPSK). Specifically, QPSK explores all four quadrants of the constellation, and it is the key building unit for highly efficient modulation techniques that are widely used in today&’s telecommunication standards. Remarkably, 4-ASK and QPSK can be realized with just 1 and 2 all-graphene transistors, respectively, representing a drastic reduction in circuit complexity when compared with conventional digital modulators. In addition, the devices are not only flexible but also highly transparent (~95% transmittance) owing to their all-graphene design with every component (channel, interconnects, load resistor, contact and gate electrodes) fabricated from graphene films. Taken together, these results represent a significant step toward achieving a monolithic system for high speed communication on a flexible and transparent platform.

Featured with high electron mobility and chirality-dependent wide-spectrum direct band gaps, single-walled carbon nanotubes (SWNTs) would facilitate photon absorption as well as charge separation and collection processes in photovoltaic energy conversion. In addition, the excellent mechanical properties of SWNTs can be exploited for reliable and even flexible solar cell. SWNT/n-Si heterojunction photovoltaic cell is the simplest photovoltaic system employing SWNTs. It was reported recently that the photovoltaic conversion efficiency (PCE) is improved by adjusting SWNTs morphology up to 2.4% yet still with the working mechanism and effect of SWNTs properties unclear. In this study, we have fabricated and characterized the SWNT/n-Si heterojunction photovoltaic cell. The heterojunction is built up by a SWNT thin film on top of the substrate which has a 3mm×3mm bare n-type silicon contact window in the center. The contact window is surrounded by SiO2 as insulating layer. The charge carriers are collected through SWNT thin film to silver paste electrode. Alcohol catalyst CVD method is used to grow the SWNT with the in-situ laser absorption method for the thickness control of SWNTs. The UV-vis-NIR spectrometer is used for the measurement of the absorbance and transmittance of SWNTs. The sheet resistance of SWNT film is measured by the standard four-probe method. The optimal properties of SWNT thin film for photovoltaic cell and the photocurrent generating mechanism for the solar cell will be discussed by comparing the PCE with different absorbance, transmittance and sheet resistance of SWNTs. Density gradient ultracentrifugation method is also used to separate semiconductor and metallic SWNTs to investigate the working mechanism.

As produced, raw carbon nanotubes are not soluble in many solvents necessary for printing applications. Standard methods for circumventing this problem involve sidewall functionalization and surfactants. Sidewall functionalization invariably destroys the π-network that gives carbon nanotubes their useful electronic properties, while surfactants deposit an insulating layer onto the carbon nanotube surface that must be washed off to regain the desired properties. Non-covalent functionalization offers the possibility to achieve solubility without the destruction of the π-network, but published methods have resulted in relatively low concentrations or substandard electronic performance. We have developed a scalable method to non-covalently functionalize long (> 3 mu;m) carbon nanotubes with simple pyrene derivatives. This method produces highly dispersed solutions with concentrations as high as 2.5 g/L that can be used to produce conductive coatings with sheet resistance as low as 350 Omega;/square with 85% transmittance at 550 nm without post-deposition washing or doping treatments. The functionalized carbon nanotubes can be formulated into solutions that can be printed by ink-jet deposition, Aerosol-Jet® deposition, screen printing, and spray coating for printed electronics fabrication, and the solutions are stable for months without signs of bundling.

Load transfer and mechanical strength of reinforced polymers are fundamental to developing advanced composites. This paper demonstrates enhanced load transfer and mechanical strength due to synergistic effects in binary mixtures of nano-carbon/polymer composites. Different compositional mixtures (always 1 wt. % total) of multi-wall carbon nanotubes (MWNTs) and single-layer graphene (SLG) were mixed in polydimethylsiloxane (PDMS), and effects on load transfer and mechanical strength were studied using Raman spectroscopy. Significant shifts in the G-bands were observed both in tension and compression for single as well binary nano-carbon counterparts in polymer composites. Small amounts (0.1 wt%) MWNT dispersed in 0.9 wt. % SLG/PDMS samples reversed the sign of the Raman wavenumbers from positive to negative values demonstrating reversal of lattice stress. A wavenumber change from 10 cm-1 in compression to 10 cm-1 in tension, and an increase in elastic modulus of ~103% was observed for MWNT0.1SLG0.9/PDMS with applied uniaxial tension. Presence of MWNTs in the matrix reduced the segmental polymeric chain length and provided limited extensibility to the chains. This in turn eliminated compressive deformation of SLG and significantly enhanced load transfer and mechanical strength of composites in tension. The orientation order of MWNT with application of uniaxial tensile strain directly affected the shift in Raman wavenumbers (2D band and G-band) and load transfer. It is observed that the cooperative behavior of binary nano-carbons in polymer composites resulted in enhanced load transfer and mechanical strength. Such binary compositions could be fundamental to developing advanced composites.

Monodispersed copolymer emulsions, each with a different polymer particle size, were used to investigate the effect of particle size on the electrical and thermomechanical properties of carbon black (CB)-filled segregated network composites. These emulsions were synthesized with equal amounts of methyl methacrylate (MMA) and butyl acrylate (BA), with latex particle size ranging from 83 to 771 nm. The electrical percolation threshold was found to decrease from 2.7 to 1.1 vol% CB as the latex particle size was increased. Microstructural images reveal diminished latex coalescence, and improved CB segregation, with increasing latex particle size. The glass transition temperatures for all systems were relatively unaltered, with changing CB concentration, while the storage moduli all increased. Furthermore, all systems exhibited a similar maximum electrical conductivity plateau of 0.7 S*cm-1, albeit at lower concentration for larger latex particle size. This ability to tailor percolation threshold with latex particle size provides an important tool for manipulating electrical and mechanical properties of polymer nanocomposites.

Single-walled carbon nanotubes (CNTs) are rolled-up graphene sheets with rigid one-dimensional structures. Evaluation of the interactions between molecules and CNTs has attracted much research. We have gotten quantitative evaluation of interactions between CNTs and solubilizers by the reaction of exchange of solubilizers on CNT. The exchange of solubilizers on SWNT has occurred by added oligo-DNA to the solution in which individually solubilized SWNT by sodium cholate (SC). This method was also useful to know microscopic environments of the solubilized CNTs, because the solubilizers on the CNT surfaces affected the spectroscopic properties of CNTs. Absorption spectra of CNTs were recorded at a certain temperature (15-40 °C). The peak intensities at 1080 nm, which represent chirality of (10,2), was plotted against the concentration of DNA and fit by the Hill equation to evaluate the equilibrium constants (Kα) of the solubilizers. Temperature dependence of the Kα was used to evaluate ΔH and ΔS of the exchanges of the solubilizers. These parameters indicated that the exchange of SC for DNA was entropy-driven. We also found that the thermodynamic parameters were dependent on chiralities of the CNTs and length of oligo-DNA. It was shown that the spectroscopic changes of the CNTs were useful to make quantitative evaluation of the exchanges of solubilizer molecules on CNTs.

Carbon nanomaterials and thin films are finding use in diverse applications such as energy generation/storage devices (eg. Solar cells, Battery and Supercapacitors), superhydrophobic coatings, field emission, biology etc. Carbon nanostructures have been till date synthesized only by established routes like CVD, PLD, Arc Discharge, Chemical synthesis etc. In our work we investigate the synthesis of carbon nanoparticles and thin films using a novel, facile, room temperature route of laser photochemistry. Pulsed laser irradiation of liquid haloaromatic compounds (e.g. chlorobenzene and o-dichlorobenzene) yield carbon nanospheres by a process that involves free radical generation and their subsequent ‘stitching&’ together to form carbonaceous networks. We term the process as Pulsed Laser-induced Photochemical Stitching (PLPS). This process when applied to various substrates yields films of carbon. We synthesized such carbon films on Glass, FTO-coated glass and cover slips. They display interesting carbon forest-like morphology which is highly superhydrophobic in character (contact angle with water droplet reaching up to appx 167o) accompanied by a laser induced tunability of the hydrophobic character. The carbon films being electronically conducting in nature (sheet resistance ranging from -2-10 kOmega;/sq), these were explored for application as catalytic counter electrodes in dye sensitized solar cells (DSSCs). They yield conversion efficiency which is only about 20% less as compared to the Pt-FTO as counter electrodes. We show that this photochemical method with minor modifications can also be used to generate transparent conducting carbon films on glass substrates at room temperature (sheet resistance = 60-80 kOmega;/sq, transparency of up to 70-75% in the spectral range of 400-800 nm).

Polytetrafluoroethylene (PTFE) is extensively utilized in medical devices because of its chemical and mechanical stability; however, it still has chance a risk of bacterial infection in the body. Recently, amorphous carbon (a-C) has attracted much attention as a coating material on medical devices owing to its outstanding properties. In addition, recent researches reported that a-C films might enhance antibacterial properties. However, direct deposition of a-C onto polymer surface leads to poor coating adhesion and film cracking. To get rid of this problem, many investigators have studied about surface modification of the polymer and interlayers to improve the adhesion strength and to solve the delamination between polymeric materials and a-C. The present work was thus aimed at improving the adhesive property of a-C-based film on PTFE by plasma-pretreatment with an appropriate interlayer. In this study, hydrogenated a-C (a-C:H) and hydrogen-free a-C (H-free a-C) were deposited onto the PTFE substrate using plasma deposition method. The durability and adhesion strength of a-C coated-PTFE using plasma pre-treatment with or without interlayers are presented. In addition, surface morphology and bacterial adhesion on a-C coated samples are presented. To enhance the adhesion strength of a-C to the PTFE substrate, a two-step procedure was applied: firstly, the PTFE substrates were pre-treated with Ar and O2 plasma. Secondly, fluorine-incorporated a-C:H (a-C:H:F) was introduced as an interlayer. The effect of the plasma pre-treatment on the surface chemical compositions and the surface morphology was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), respectively. A T-peel test was carried out to evaluate the adhesion strength of the a-C coated-PTFE and the a-C coated-PTFE with an interlayer. SEM study was conducted in order to determine the cracking or delamination after the stretch of a-C coated-specimens uniaxially to a certain strain. XPS analysis of plasma-pretreated PTFE indicated that substantial defluorination and oxygen bonding occurred by Ar plasma-pretreatment; however, slight defluorination occurred by O2-plasma pretreatment. SEM images showed that Ar plasma-pretreated surface exhibited wrinkle-like structure, while O2 plasma-pretreated surface exhibited deeply etched needle-like structure. In the T-peel test, Ar plasma-pretreatment improved adhesion strength more effectively than that of O2 plasma-pretreatment. The interlayer of a-C:H:F improved the a-C adhesion strength on PTFE, and also decreased the amount of film cracking. Bacterial adhesion of the coating was evaluated using Escherichia coli. H-free a-C coating exhibited a significantly higher inhibitory effect on bacterial adhesion than the a-C:H and non-coated PTFE. Thus, the present study indicates that the H-free a-C coated-PTFE with a-C:H:F interlayer can serve as a promising candidate for antibacterial medical devices.

Functional microporous conducting carbon with high surface area of about 1230 m2/g is synthesized by a single step pyrolysis of plant dead leaves (waste material) without any activation. Although we have used dead Neem (Azadirachta indica) leaves in this work, the process is generic and applicable to most forms of dead leaves. Various characterization techniques such as X-Ray diffraction, Raman Spectroscopy, High Resolution Transmission Electron Microscopy, EDAX Analysis and BET surface area measurements are employed to analyze the properties of the material. Electrochemical cyclic voltametry studies were performed in 0.5M H2SO4 solution between potential range 0-1V at various sweep rates. For quantitative considerations the specific capacitance was calculated from the galvanostatic charge discharge curves at different current densities ranging from 0.5A/g to 10A/g. The highest specific capacitance value 400 F/g is obtained at a current density of 0.5A/g. The concurrent high conductivity and microporosity realized in this carboneous material are key to the high energy supercapacitor application. Indeed, our synthesized functional carbon exhibits a high enengy density of 55Whkg-1 even at a current density of 0.5A/g. The areal capacitance value of the carbon derived from dead (Neem) plant leaves is also significantly higher which is 32µF cm-2. This performance is clearly superior to that of most commercially available and synthetically obtained activated carbon forms that have been used in ultracapacitor charge storage networks.

Carbon materials have been widely used in various forms such as fibers, mats, and thin film due to their unique chemical, electrical, and mechanical properties. Polyacrylonitrile(PAN) is one of the most suitable precursor for making high performance carbon materials. Most commercial PAN based carbon materials are produced by thermal stabilization and carbonization. Recently radiative stabilized processes such as γ-ray, UV and electron beam are attracted to enhance the properties of carbon materials. In this study, we measure and discuss the optoelectronic properties of electron beam irradiated thin film in comparison with the thermal processing. PAN was dissolved in N.N-Dimethylformamide(DMF) to prepare a 5 wt.% solution in room temperature for 12 hours. PAN/DMF solution was spin coated on the silicon wafer, and the electron beam was irradiated on these samples with 10 MeV accelerating voltage and 0.5 mA current at doses ranging from 100 to 500 kGy. Thin films were stabilized at 275°C in oxygen and nitrogen gas mixture atmosphere, and carbonized at 1,000°C in nitrogen atmosphere. The reflection spectra of stabilized thin films were measured by spectrophotometer from 360 nm to 740 nm. And carbonized thin films were analyzed by Raman spectroscopy. Conductivity was measured with a four point probe.

Fullerene (C₆₀) nanocrystals were assembled in two and/or three-dimension using a liquid-liquid interfacial assemble technic and electrophoretic deposition method. We have tried to crystallize C₆₀ nano and microcrystals by employing reprecipitation method1), and succeeded in preparing C₆₀ nanocrystals with various unexpected shapes and surprisingly monodispersed sizes2). The shapes and inner structures of the resulting C₆₀ nanocrystals were strongly dependent on the combination of good and poor solvents. In addition, we have succeeded in preparing assemble C₆₀ nanocrystals into two- and/or three-dimensional order employing liquid-liquid interfacial assemble technic and electrophoretic deposition method. Fabrication process of the nanocrystallization and layered thin film is as follows. For example, C₆₀nanocrystals, the diluted solution (1-7 mM, 200 mu;L) of C₆₀ was injected into the vigorous stirred 2-propanol as a poor solvent (10 ml) and kept for 2 hours to complete the nanocrystallization. Finally, C₆₀ nanocrystals were obtained as a dispersion state. The obtained C₆₀ nanocrystals dispersion were assembled at the water-hexane interface by addition of C₆₀ nanocrystals dispersion to the interface. The assembled film was transferred onto a solid substrate using the Langmuir-Blodgett deposition technique. The transferred film showed a monoparticle layer. Obtained thin films were characterized by SEM, TEM, powder XRD measurement and absorption spectra. As a result, we have demonstrated that thin film of C₆₀ nanocrystals can fabricate and control the thickness of their layered structure by changing the amount of nanocrystals dispersion, immersing times and so on. Details of fabricated condition and characterization of nanocrystals and thin films will be discussed. Reference 1) H. Masuhara, H. Nakanishi and K. Sasaki, Single Organic Nanoparticles. Springer, Berlin 2)A. Masuhara, FULLERENE NANOWHISKERS, K. Miyazawa Ed., Pan Stanford Publishing, Singapore, chapter 7 89-101 (2011).

The aim of this study is to determine the morphology development of porous carbon fiber webs using miscibility difference of binary blend system. The 10wt% Polyacrylonitrile (PAN) and 25wt% of Poly(acrylic acid)(PAA), Poly(ethylene glycol) (PEG), Poly(methyl methacrylate) (PMMA) and Polystyrene (PS) in DMF were prepared, respectively PAA, PEG, PMMA and PS solution was blended with the PAN solution as a weight ratio of 75:25. Carbon nanofiber webs were prepared by an electrospinning and a thermal treatment. The morphology and pore structure of the carbon nanofiber webs were investigated using a field emission scanning electron microscopy (FE-SEM). Specific surface area of the porous carbon nanofiber webs was calculated through the BET method investigated by Surface Area Analyzer. The porous carbon nanofiber webs having various pore structures were simply prepared by electrospinning process using binary blend system. As increasing the solubility parameter difference, two polymers are less miscible and the dispersed polymers take large portion in a blend system. Pore size was increased by blending large difference of solubility between two polymers. PAN/PAA blend was the smallest pore size as 1.66nm and PAN/PS blend was the largest as 18.23nm. The morphology and pore structures of blend fiber webs were controlled in various forms using miscibility of blends and it is useful to apply to air purification, gas separation and carbon electrode materials for appropriate purpose of uses.

Ferromagnetism in graphite without impurities is known to be related to structural misorientation, concentrated defect, or lattice defects, however, its origin is not yet fully understood. We have investigated graphite with structural disorder (ZYH grade) by employing electron spin resonance (ESR) measurements in comparison to highly-ordered (ZYA-grade) graphite without and with proton irradiation. Distinct characteristics of the conduction electron signals were thus observed, the g-values and linewidths being interpreted in terms of the carrier density and the magnetic susceptibility in relation to the defects in the systems.

We show how the structure, degree of sp2 hybridization and covalent bond length of hollow carbon cones [1] and discs respond to ex- situ heat treatment up to 2700°C. The structure was investigated by selected area diffraction and high resolution imaging in the transmission electron microscope (TEM). The sp2 character and covalent bond length was determined by analyzing carbon K ionization edge spectra acquired with an electron energy loss spectrometer attached to the TEM. Results from these measurements show that the graphitic structural order and bond character increased significantly with heat treatment temperature. The evolution of the bond character of the cones was found to be slightly more similar to non- graphitizing carbons [2] than that of the ‘graphitizing- carbon- like&’ discs [3]. This might be attributed to strain induced by the curved cone walls. [1] Krishnan et al., Nature 388 (1997) 451 [2] Zhang et al., Carbon 49 (2011) 5049 [3] Daniels et al., Philosphical Magazine 87 (2007) 4073

Raman nanoagent complex prepared from the covalently bonded Single-walled carbon nanotube (SWCNT) with the epithelial cell adhesion molecule (Epcam) has been used for early and fast detection of breast cancer cells (MCF-7). The Raman nanoagents have been well characterized and the Raman signals were collected from single cell mixture of both cancer and normal cells by using 633nm laser aided with Labspec software.We have been able to detect single cancer cell within first 30 min of exposure (Targeting process). The Raman mapping 3D images and Raman linearity scanning results showed that the nanoagents were targeting cancer cells specifically and thus revealed a very strong Raman signals, whereas there was no signal obtained from the normal cells.

It has been considerably researched on 2-dimensional materials such as graphene and boron nitride to realize nanoelectronic devices. Molybdenum disulfide (MoS2) has hexagonal 2-D structure and semiconducting behavior because of the band-gap from 1.29 to 1.9 eV corresponded with bulk and monolayer which indicates great potential to utilize them for 2-D transistor. However, it is necessary to develop a method to synthesize it in large-aera. Here, we report the large-area Molybdenum disulfide synthesized by chemical vapor deposition (CVD) at 750 degree Celsius. The number of MoS2 layers could be varied from monolayer to few layers by controlling thickness of molybdenum catalyst from 0.5 nm to 3 nm and synthesis time. The binding energies in X-ray photoelectron spectroscopy (XPS), 229.5 eV and 232.6 eV corresponded with Mo 3d5/2 and Mo 3d3/2 respectively, indicates the 2H-MoS2 structure showing semiconducting behavior. We also fabricated Molybdenum disulfide transistor with bottom gate structure. The device shows mobility of ~0.02 cm^2/V.s and on/off ratio of ~10^3 which can be improved by reducing contact potential and effective charged impurities at interface.

Recently, we have been reporting single-walled carbon nanotube (SWNT) growth by a gas source method in an ultra-high vacuum (UHV) chamber using ethanol gas [1, 2]. This growth technique enables SWNT growth in a high vacuum, and our group achieved SWNT growth at an ambient ethanol pressure of 1×10^-4 Pa with a Co catalyst. However, the SWNT yield decreased significantly, because the growth temperature had to be reduced to 400^#9675;C. In this study, we carried out SWNT growth at various temperatures using a Pt catalyst. Our results showed that the optimal growth pressure was considerably reduced in the growth from a Pt catalyst, while the yield was higher. SiO₂/Si substrates were used for the SWNT growth. After deposition of a Pt catalyst (0.2 nm in thickness) by a pulsed arc plasma gun in a UHV chamber, the substrate temperature was increased to the growth temperature. Then, ethanol gas was supplied to the substrate surface for 1 h. The growth temperature was varied between 400 and 700^#9675;C and the ethanol pressure was varied between 1×10^-5 and 1×10^-1 Pa. The resulting SWNTs were characterized by SEM, TEM and Raman spectroscopy. Raman and TEM results showed that most of grown nanotubes were SWNTs. SEM and Raman results showed that the SWNT yield reached its maximum at 800^#9675;C, at which the ambient ethanol pressure was 1×10^-2 Pa. As the growth temperature was reduced, both yield and optimal ethanol pressure decreased. At 700^#9675;C, the yield became its maximum at 1×10^-3 Pa, while it was fairly higher than the maximum yield obtained in the growth with Co at 1×10^-1 Pa. This indicates that Pt is a highly efficient catalyst for SWNT growth and that it is suitable for SWNT growth under low ethanol pressure. By optimizing the growth pressure, we could obtain SWNTs from a Pt catalyst even at 400^#9675;C. From the analysis for RBM peaks in the Raman spectra, the average diameter of the grown SWNTs and the diameter distribution were found to be much smaller and narrower, compared to those grown with Co. In addition, the ratio of semiconducting SWNTs was larger than those grown with Co. These results suggest that Pt catalyst is suitable for growth of SWNTs for device applications. [1] K. Tanioku, T. Maruyama and S. Naritsuka, Diamond Relat. Mater. 17 (2008) 589. [2] T. Maruyama et al. , J. Nanoscie. Nanotechnol. 10 (2010) 4095.

The thermomechanical properties and thermal stability of polymer nanocomposites, consisting of epoxy resin (ER) as matrix reinforced with carbon black (CB) nanoparticles or amine-modified multi-walled carbon nanotubes (MWCNTs) as filler, were investigated by Thermogravimetric/ Differential Thermal Analysis (TGA/DTA) and Dynamic Mechanical Analysis (DMA). Our interest was focussed on the effect of different filler concentration on the thermomechanical properties, thermal stability and degradation processes of the nanocomposites. The addition of carbon nanofillers enhances, generally, the thermal behavior of the neat epoxy matrix. TGA results reveal that the system shows, for all heating rates, good thermal stability for temperatures up to about 600 K with a maximum decomposition temperature higher than 630 K. For the samples with higher carbon concentration the degradation process of the nanocomposites is hampered by the increased filler content, because the filler nanoparticles form strong bonds with the epoxy matrix and, as a result, a complex network structure is created. The apparent activation energies during thermal decomposition process were calculated from measurements at different heating rates by using Kissinger method. The results demonstrate values of Eact in the range 216-237 kJ/mol for the filled ER/CB nanocomposites, higher than that of the neat epoxy resin (Eact= 201.15 kJ/mol).

Graphene oxide(GO) from graphite oxidation is water soluble due to fluent functional groups such as epoxide, carboxyl, hydroxyl. Water soluble GO is not only easily combined with water soluble polymer but also interlayer distance of GO is increased by intercalation of polymer. Original properties of polymer remains even intercalated in GO. With this GO/polymer composite has attracted to many researchers in capacitor, battery field. In capacitor, GO/polymer composite achieved almost 200 F/g in aqueous electrolyte. In this study, we used water soluble poly(4-sodium styrene solfonate) as a intercalated polymer and host material GO was as a backbone material. With this composite material we, investigated the hydrogen storage adsorption amount and hydrogen diffusion kinetics study with different film thickness of GO/PSS composite. GO was made by modified brodie method which interlayer distance around 6 #8491;. However, vigorous stirring with Go and PSS during 24 hr, the interlayer distance of GO/PSS was increased around 8 #8491; at the XRD. From the FT-IR we can confirm the original properties of GO and PSS were still remain in the GO/PSS composite material. Hydrogen storage capacity test was done at the quartz crystal microbalance at 5 atm, room temperature. The capacity of GO was less than 0.01 wt% however, PSS intercalated GO was shown the 1.3~1.4 wt% hydrogen capacity. Several reproducible experiment has similar results with a range of 1.3~1.4 wt% capacity. Also, we confirmed the hydrogen diffusion rate was changed by the GO/PSS composite material film thickness.

Carbon materials were prepared by heat treatment of cellulosic fibers such as henequen, cotton in the range of 500~1700°C, and characterized various properties of thermal stability, electrical properties and morphologies. Carbon fibers and carbon globule were formed depending on the heat treated temperature and carbon source. The carbon materials treated at below 1000°C had amorphous phase and the treatment at 1700°C induced the materials similar to glassy carbon. The higher treated temperature is, the electrical conductivity is improved in the range of micro scale. However, it is found that the existence lignin, inorganic elements in the natural fiber interferes to obtain the homogeneous carbon fiber.

We have investigated the magnetic properties of carbon powders which consist of nanodisks, nanocones and a small fraction of carbon black particles. Magnetization measurements were carried out using a supercomputing quantum interface device (SQUID) in magnetic fields -5 < B < 5 T for temperature 2 < T < 350 K. In the temperature range 2 < T < 10 K and after subtraction of the diamagnetic background, the magnetization shows a strong paramagnetic temperature dependence and this cannot be fitted to a Curie law. At T = 2 K, magnetization versus magnetic field shows a paramagnetic free spin behavior in the case when B/T > 1T/K, which could be a signature of electron localization in high magnetic fields at low temperature. This is in agreement with an electron g-factor close to the free electron value g=2.002 measured by EPR in these samples. Magnetization measurements also show an additional weak ferromagnetic contribution which is clearly higher than what can be attributed to a low level (< 100 ppm) of Fe impurities found in the samples.

Metal oxide materials such as SnO2, ZnO, TiO2, WO2, Ga2O3, and others have been examined for gas sensing applications and for control of industrial processes. In the present study, our aim has been to investigate bulk sensors via ceramic processing of nanoparticles of tin oxide under different pressing pressures, reinforced with nano-carbon fibers, and WO3 for their response to ethyl alcohol. The study was also performed to determine their detection capability of sensors for different concentrations of ethyl alcohol. The SnO2/CNF sensor&’s sensitivity for ethyl alcohol has increased by a factor of two, in comparison with that of pure SnO2 8-ton pressed sensor with lower response time. These results open the way towards further optimized lower cost CNF nanocomposite sensors as compared with expensive tin oxide/carbon nanotubes sensors.

Use of monolayer graphene is of great interest for transparent conducting film due to its remarkable properties such as sheet resistance, transparency and flexibility. Separated semiconducting carbon nanotubes have been widely utilized for high performance thin film transistors for display application. By combining these two unique materials and using transparent plastic, we fabricated fully transparent and flexible thin film transistors and logic circuits whose electrodes are monolayer graphene and channels are semiconducting carbon nanotubes. The obtained transistors showed various on/off ratio of up to 10^5 and mobility of up to 12 cm^2/Vs. The performance of transistors can be adjusted by changing the density of carbon nanotubes. The fabricated inverter showed extraordinarily high performance and gain of 45 with transistors that have high on/off ratio more than 10^5.

The purpose of this work is to study the structural and the surface morphology modification of the n-type diamond films electrodes as a function of nitrogen doping. The characterization of these films were made by Raman Spectroscopy, by X-ray diffraction and by Scan Electron Microscopy (SEM) and associates them to the electrochemical efficiency for the degradation of pesticides. The addition of nitrogen causes a distortion in the diamond structure increased its mechanical tensions and the number of defects (vacancy). By introducing these distortions, nitrogen behaves as a donor with high excitation energy, which characterizes it as a deep donor with a level at 1.7 eV within the forbidden energy band of the semiconductor [1]. Nitrogen may also influence the growth rate, quality and crystallographic direction of diamond growth. The nitrogen-doped nanodiamond (NDND) electrodes were produced using hot filament-assisted chemical vapor deposition method (HFCVD) from methane, hydrogen and nitrogen gas mixture. The methane and hydrogen flows (1 and 99 sccm, respectively were kept constant) and nitrogen flow was varied in the range between 1 to 15 sccm. The pressure in the reactor was 40 Torr and the temperature of the silicon substrate being maintained at about 800 °C. The results from Raman spectroscopy show that the undoped film presented only one peak centered at 1332 cm-1 characteristic of the diamond. The NDND presented the following peaks, at 1332 cm-1 from diamond, broad band at 1400-1700 cm-1 from graphite and strong band at 2100-2750 cm-1 from trans-polyacetylene. Through the images obtained from scanning electron microscopy (SEM), we find that the variation in the nitrogen doping influence the growth rate of film by promoting changes in the sizes of grains. This behavior supports the results obtained from X-ray diffraction analysis. It was possible to verify that a decreasing in the crystallite size from 50 to 20nm as a function of the nitrogen increase.

ATAC is a software tool that seeks to provide a model for simulating the annealing of carbon structures, for understanding current transformations, and also predicting them. By using molecular dynamics, Metropolis algorithms, and graph theory, large computational speed gains are achieved with the application of novel topological rules. The experimental transformation of the peapod structure to a double wall nanotube is successfully modeled, as well as the fusing of single wall nanotubes into junctions. Also discussed is a theoretical synthesis path for the creation of Haeckelite, as predicted by ATAC.

The development of synthesis methods for carbon nanotubes and nanosheets has lead to materials with interesting new properties. These however tend to be molecular materials, i.e. the syntheses produce single entities. For many applications in energy storage and catalysis these single entities need to be grown directly onto a scaffold, or attached onto a scaffold. There are however other approaches to the assembly of nanoscaled carbon materials. One of these is nanocasting of carbon into a mesoporous silica template. This approach yields arrays of 4-6 nanometer diameter carbon nanorods with hexagonal packing (among a number of potential geometries). We have taken this approach a step further by using hierarchically porous silica monoliths as templates. This produces arrays of carbon nanorods which form the walls of micrometer sized pores. TEM shows that the carbon nanorods form well ordered arrays, SEM, Small Angle XRD and nitrogen sorption show that the material is homogeneous throughout the entire sample volume (typically 1 cm3 volume). Such materials are of particular interest as catalyst supports and as current collectors for fuel cells and advanced batteries. For catalyst applications the very high surface area (>680 m2/g) coupled with the very low resistance to flow makes these materials very attractive for catalysis in general, particularly as flow through catalysts. For advanced energy utilization and energy storage, the bicontinuous nature of the materials makes for very efficient transport of electrons, ions and reactants within the materials.

SiC processes several unique chemical and physical properties that make it an attractive alternative to Si as a semiconductor material. Silicon carbide&’s properties make it a perfect candidate for applications such as high temperature sensors, x - ray emitters and high radiation sensors. The main objective of this thesis is to successfully grow silicon carbide nanowires on silicon substrates with the assistance of a metal catalyst, by the process of chemical vapor deposition (CVD). The nanowires grown were characterized by a variety of different methods including scanning electron microscopy (SEM), energy dispersive x - ray spectroscopy (EDS) and Raman spectroscopy. The effect of temperature, growth temperature, growth time and the catalyst type used are investigated to determine the most suitable conditions necessary for SiC nanowire growth. The results suggest that a growth temperature of 1050oC using a Ni catalyst (5 - 10 nm) and a silane flow rate of 35 sccm produces long nanowires densely grown over the areas of the substrate coated with the catalyst. EDS and Raman spectroscopy proved the nanowires were indeed silicon carbide and SEM analysis proves the nanowires grown had diameters of 50 - 100 nm.

Carbon-containing alumina and magnesia-alumina refractories are commonly used in steelmaking applications due to their excellent high-temperature strength and thermal shock resistance. Carbon is widely used in the form of natural flake graphite and at a high weight fraction ranging from 8 to 20 wt%. In recent years, one of the most important trends in refractory technology has been the development of low carbon-containing refractories. Besides the fact that flake graphite supply has become scarce, and graphite prices have started to rise, low carbon-containing refractories are needed to meet the most stringent requirements imposed by the steel industry carbon, avoiding the carbon contamination from refractory to liquid metal. Recent research has demonstrated that replacing graphite content with nanostructured forms of carbon allows considerable reduction of the total amount of carbon in graphite-containing refractories, resulting in materials with lower thermal conductivity (less heat loss in refractory brick and less damage to the furnace shell), and better performance for low carbon steel production. However, the nanostructured carbons have the disadvantage of being much more susceptible to oxidation at high temperatures due to their larger surface areas. In this work, seeking to reduce this effect, the influence of adding elements with high affinity to oxygen (Si, Ti, B) on the oxidation resistance of alumina-nanocarbon refractories were investigated through the factorial design of experiments. That is, the significance of the effect of three carbon sources (multiwall nanotubes, nanofibers and carbon black), two temperatures (750 and 1000 °C) and two burn-times (1 and 3 hours) on the carbon content was identified. Carbon losses were obtained using thermogravimetric analysis and oxidized areas of the bricks were examined using SEM and EDS. The synergistic effects of antioxidants and nanocarbons on refractory formulations were demonstrated.

Carbon nanomaterials have attracted much attention due to their unique properties and potential applications. In this presentation, we report the controllable synthesis of tubular carbon nanorods with controlled aspect ratios on a large scale by templating against rod-like nanocrystals. The rod-like nickel-hydrazine complexes were first prepared through a reverse micelle approach, followed by the subsequent coating of a polymer layer and selective etching of nanocrystal core, giving rise to tubular carbon nanorods of high uniformity and yield. The length of the tubular carbon nanorods is tunable in the range 30-500 nm and can reach as long as micrometers. Control of the length is conveniently achieved by tuning the hydrazine/nickel ratio, which affects the growth kinetics of the nanocrystal templates. The inner diameter of the tubular carbon nanorods can be adjusted in the range 10-20 nm by choosing different surfactants. The as-prepared carbon nanomaterials show excellent property in the electrochemical applications, which may open the door for its practical applications in many other fields, such as electrodes, batteries, and so on.

Raman spectroscopy is widely used to investigate epitaxial graphene grown on silicon carbide (SiC) where a strong emission due to two-phonon processes in the substrate overlaps the D and G line of the graphene overlayer. Commonly, that background is removed from the spectrum by subtracting the spectrum of a clean SiC substrate, a procedure which relies on the assumption that the Raman spectrum contains contributions from SiC and graphene only. It is very well known that graphene on SiC(0001) sits ontop of the buffer layer which has been discussed in terms of a strongly interacting layer of carbon atoms with graphene-like atomic structure. In order to identify a possible contribution of the buffer layer to the measured Raman spectrum of graphene on SiC we have carried out a careful comparison of the Raman spectra of monolayer (MLG) and bilayer graphene (BLG) on the buffer layer, the buffer layer alone, and quasi-freestanding graphene on H-terminated SiC(0001). The spectrum of the buffer layer, for which the 2D line is missing, contains two rather broad features, one centered at around 1350 1/cm and one at 1580 1/cm. The latter is accompanied by a smaller peak at the low-energy side situated at around 1485-1490 1/cm. It is evident that these features are also present in the spectra of MLG and BLG, but absent in the spectrum of QFMLG. This spectrum is therefore the intrinsic spectrum of the buffer layer and the origin of it will be discussed in the presentation.

Lithium encapsulated fullerene Li⁺@C₆₀, which is the only isolated and structurally well-determined endohedral fullerene as SbCl₆^- salt among all the C₆₀-based and alkaline-metal-encapsulated fullerenes, has attracted growing attention in the field of organic electronics and materials chemistry due to its strong electron accepting ability and great ionicity induced by the encapsulated lithium cation. On the other hand, hydroxylated fullerenes, so-called fullerenols C₆₀(OH)_n, have also attracted a considerable attention owing to their prominent hydrophilicity, bioactivity, and unique conductivity. Thus, the combination of these properties will lead to a new compound which can play a significant role in the field of materials chemistry. Here, we report the synthesis of lithium cation encapsulated fullerenol anion Li⁺@C₆₀(OH)₁₂ by means of hydroxylation of [Li⁺@C₆₀][PF₆^-] and its notable properties induced by the lithium cation and the outer hydroxyl groups. The hydroxylation of [Li⁺@C₆₀][PF₆^-] was carried out by the reported fuming sulfuric acid method under the optimized reaction conditions. The analytical data by IR, NMR, MALDI-TOF MS, and elemental analysis clearly showed the formation of lithium encapsulated fullerenol with eleven -OH groups and one fullerenoxide anion (C₆₀O^-) moiety. Very interestingly, each of hydroxyl groups was clearly detected by ¹H NMR spectroscopy, whereas the reference empty fullerenol synthesized by the same procedure showed the highly broadened OH signals due to the presence of a wide variety of isomers. The ¹³C NMR spectrum of the product showed 12 signals assigned for sp³ carbons at 70-80 ppm together with over 40 signals of sp² carbons at 130-160 ppm. This observation suggests that the encapsulated lithium cation induced the site-selective multi-hydroxylation in contrast to the case of the empty fullerene. Furthermore, it is noteworthy that a counter anion PF₆^- was not detected by ³¹P and ¹⁹F NMR spectroscopy. In ⁷Li NMR spectrum, the chemical shift of the encapsulated Li⁺ was found to be more shielded (ca. -15 to -19 ppm) than previously reported value of -10.5 ppm for [Li⁺@C₆₀][SbCl₆^-] salt. This abnormal upfield shift of our sample may be caused by the increased diamagnetic shielding by the attractive negative charge of fullerenoxide moiety. Therefore, our new compound could be considered as “cation-encapsulated anion nanoparticle”. The aggregation behavior and zeta potential of the product will also be discussed in our presentation.

Carbon NanoWalls (CNWs) is the one of the nano-carbon materials including graphene and carbon nanotubes(CNTs) and is possible to be grown at low temperature below 700°C without the catalysis by plasma enhanced chemical vapor deposition(PECVD)[1]. CNWs is also expected as the functional material as well as CNTs and graphene, several researchers have tried CNWs to many applications for example, the field emission [2], the template supported by Pt catalysts[3] and so on, however it have not been successful to find the useful applications compared with the CNTs and graphene. The one of the reasons was assumed that the grain size of CNWs are very small (about 20nm) including the amorphous regions[1], the crystallinity was poor. In this study, the effects of the pulsed green laser annealing (laser source: repetition rate:1kHz, wavelength: 527nm, pulsed width: 180ns, homogenized line beam: 7mm length, 0.1mm width) at ambient nitrogen to two different heights-CNWs grown on silicon substrate were investigated for the crystallinity and morphology using by Raman spectroscopy, scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy(XPS). For the 1mu;m height-CNWs, the peak of D band spectra which indicated the disorder[4] by Raman spectroscopy decreased as the laser energy density increased up to 1.3Jcm-2, the crystallinity of CNWs was improved by the laser irradiation. The morphology was not changed by the observation of SEM. On the other hand, for the 20mu;m height-CNWs, the peak of D band spectra decreased until the laser energy density increased up to 0.8Jcm-2, above 0.8Jcm-2, the peak of D band spectra rose again, and the profiles of D band and G band spectra were also broadening as the laser energy density increased. It is assumed that above 0.8Jcm-2, the CNWs melted to be recrystallized including the disorder or amorphous. By the analysis of XPS, sp3 orbitals which indicated the diamond structure irradiated at 1.5Jcm-2 increased compared with CNWs irradiated at 0.8Jcm-2 and as grown-CNWs. The morphology of CNWs irradiated at 1.5Jcm-2 was changed by the observation of SEM. As-mentioned above, the pulsed green laser annealing is effective for the improving the crystallinity of CNWs. [Reference] [1] K. Kobayashia et.al., J. Appl. Phys. 101, 094306 (2007) [2] Yihong Wu et.al., J. M a t e r. C h e m., 2004, 14, 473 [3] Seog Chul Shin et.al., J. Appl. Phys.110, 104308 (2011) [4] S. Kurita et.al., J. Appl. Phys. 97, 104320 (2005)

Among various carbon-coated magnetic nanoparticles that have been studied for potential biomedical applications, such as magnetic resonance imaging (MRI) and drug delivery, FeCo nanoparticles are promising candidates because of their high saturation magnetization and high Curie temperature. However, the ease of oxidation, dissolution in acidic environments, and potential toxicity of these materials in their native state prevent their use in biomedical applications. A graphitic coating on the surface of FeCo particles would address each of these issues and has been explored as a means to render them biocompatible. This report offers an efficient and tunable method for coating metal nanoparticles with a graphitic shell for practical applications. FeCo/graphitic carbon core/shell (FeCo/GC) nanoparticles were synthesized using a controllable salt-matrix-assisted mechanochemical method. Mechanically milled CoFe2O4 nanoparticles were reduced to FeCo and the resulting metallic nanoparticles were encapsulated into onion-like layered graphitic shells by a one-step salt-matrix-assisted carbon monoxide gas reductive annealing. The graphitic structure of the carbon shells was demonstrated through high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Raman spectroscopy. The synthesized FeCo/GC nanoparticles showed a lower magnetic saturation than our previous report (216.7 emu/g) for single phase FeCo nanoparticles produced by the mechanochemical method [A. Azizi et al. J Magn Magn Mater 322(2010) 3551-3554], which is attributed to the presence of the nonmagnetic carbon coating.

The cyclic transformations in shape and crystal structure of C ₆₀ microcrystals in dispersion are experimentally demonstrated by changing the dispersion solvent medium, and keeping the dispersion under supersonic irradiation. Rod-shaped C₆₀ microcrystal as a starting material was fabricated by the reprecipitation method as follows: 50 mu;l of m-xylene solution (2 mM) of C₆₀ was quickly injected into 10 ml of vigorously stirred 2-propanol as a poor solvent. The dispersion liquid was changed slowly from pale yellow to brownish-yellow, which indicates the formation of C₆₀ microcrystals, and should be further kept to stand for several hours to complete the crystal growth. To perform transformation experiments, the resulting rod-shaped C₆₀ microcrystals were filtrated, and then redispersed into 10 ml of another poor medium under supersonic irradiation at 42 kHz, 125 W. The similar procedure was repeated to cause further change by changing the dispersion solvent medium. The shape of C₆₀ microcrystals was changed from as-grown rod-shape to hexagonal disc-shape, then to belt-shape, and again back to rod-shape. The transformation processes were investigated by means of TEM, XRD and FT-IR measurements. As a result, the present transformation is regarded to be a crystal-to-crystal phase transition induced by the exchange of the dispersion medium, which is eventually ascribed to weak intermolecular interactions between C₆₀&’s inside crystal.

In this paper hydrothermal treatment effects on [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which is an electron acceptor material often used in plastic solar cells, were investigated in view of the underlying physico-chemical processes occurring in the aqueous solution. PCBM powder was hydrothermally treated by using proper aqueous solution of water. The resulting products were characterized by Fourier-transform infrared (FT-IR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and magnetic susceptibility measurements. Also, optical properties of the PCBM were investigated by the photoluminescence (PL) measurements. A strong change in the FT-IR spectra appeared in a five-days-treated sample; the four intramolecular F1u-derived modes exhibit softening and are split by the formation of the interfullerene bonds. Two vibration modes for the C60 moiety in PCBM each at 1470 and 1430 cm-1 with different intensities can be found in the IR spectra. Changes in the Raman spectra appeared at the frequency corresponding to the stretching mode of the fullerene cage, indicating that the fullerene cage was strongly affected by the treatment. The photophysical properties of PCBM were also investigated for the hydrothermally treated PCBM. The pristine PCBM powder showed an intense PL peak at 700nm, which has previously been assigned to emission from the S1 singlet excited state of PCBM by comparison with the C60 data. The decrease in the PL intensity was shown in a one-day treated and three-days-treated PCBM samples. The resulting products were characterized by using a superconducting quantum interference device. The one-day-treated PCBM sample showed quite a strong ferromagnetic ordering at 5 K. The XPS spectra showed strong changes in the chemical bonding in the hydrothermally treated PCBM. The peaks at 284.6 and 285.6 eV correspond to the sp2 and sp3 hybridized carbon atoms as in graphite, respectively. The formation of a mixed sp2-sp3 hybridization, which might be responsible for magnetism, will be discussed.

Research into organic-inorganic hybrid composites has become popular, particularly the development of new polymer nanocomposites. Compared to pristine polymers or conventional composites, these hybrid nanocomposites exhibit improved properties. Therefore, this method delivers new materials with significantly improved thermal and mechanical properties, while still permitting the use of existing commercial processes. Currently, a new class of materials has been used to investigate the behavior of nanoparticles, called polyhedral oligomeric silsesquioxanes (POSS). These are a class of three dimensional organic-inorganic hybrid silicon-oxygen particles with the generic formula of (RSiO1.5)n. These molecules contain an inner inorganic framework covered by inert and/or reactive organic substituents. POSS molecules with well defined shapes and sizes ranging from 1 to 3 nm have been described as the smallest version of colloidal silica. When there is covalent bonding between the POSS and polymeric matrix, reinforcement is favored. In this study, PU/POSS hybrid composites are prepared by a one-step PU reaction. The effect of incorporation of functionalized POSS molecules covalently or physically into PU matrix is investigated on mechanical reinforcement and thermal stability of these resulting PU/POSS hybrid composites. PU/POSS hybrid composites prepared with reactive POSS molecules exhibit the mechanical reinforcement, due to the fact that reactive POSS molecules chemically incorporated in PU are aggregated to crystallize, effectively working as a physical crosslinking in PU/POSS hybrid composites. Thermal degradation behavior of these hybrid composites in the early stage rather appeared to depend preferably on characteristics of POSS molecules incorporated, but after the major thermal decomposition in the range of 450 ~ 500 oC the composites with reactive POSS molecules can form a thermally stable ceramic char layer under thermooxidative condition. The ceramic char layer is clearly observed after the ablation test using oxyacetylene torch. It reveals compact multiple layers of spherical POSS particles with 30~50 nm diameter, which is consistent with size of POSS crystal domain formed in PU/POSS composites.

The difference of edge energy between nanometer material that its size is below dc. The surface energy quantifies the disruption of intermolecular bonds that occur when a surface is created. The traditional theory is incorrect for nanometer material. It not only needs the energy consumed by the cutting surface bond process, but also the energy to form quantum standing wave that opposite surface edge bond caused.The nanometer material form process need more energy than the size surpass dc material.The new theory can explain nanometer powder melting point decreasing phenomuna.

Carbon aerogel exhibits a novel low-dimensional dendritic growth pattern that results in a porous structure with a remarkably high surface area often exceeding 700 m2/g. This structurally strong material is electrically conductive which, coupled with the huge surface area to volume ratio, suggests the probability of dominant kinetically induced electronic surface scattering. Currently, a formal study of nanoscale kinetic surface scattering effects induced by a gaseous boundary has received little attention, despite the large surface area to volume ratio of many nanostructures. In this work, we consider the perturbing effects to the electron cloud or jellium of 500 µm thick monolithic carbon aerogel when submitted to airflow of varying interaction energies. A lab-grade wind tunnel capable of 40 m/s (Mach 0.11) is used to induce the kinetic scattering environment between the air and aerogel structure. The air impact energy is varied by changing the flow speed from stationary to high speed flow over the aerogel samples. The scattering effects of the gaseous boundary are clearly observable through electrical conductivity reductions as the air impact energy increases. This work reports the conductivity perturbations as a function of airflow. The perturbed transport properties of the aerogel are explained by kinetic surface scattering mechanisms that dominate the scattering landscape of high surface area to volume ratio materials as suggested by comparative measurements on bulk materials. Additional motivation for this work is to develop carbon aerogel velocimetry, a technologically important measurement capability that may be implemented discretely or in sensor “swarms.”

Mesoporous carbon nitrides with different structures, nitrogen contents and crystal structure have been prepared by a simple hard templating approach in which mesoporous silica templates with different pore diameters and different structures are used as templates. The amount of the nitrogen in the mesoporous carbon nitrides has been controlled by the simple adjustment of the nature of the nitrogen sources with different nitrogen content. The materials are thoroughly analyzed by sophisticated characterization techniques such as X-ray diffraction (XRD), N2 adsorption studies, high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy and CHN analysis. We also demonstrate that the basic character of the samples can be tuned by controlling the nitrogen in the carbon framework of the mesoporous carbon nitrides whereas the structure of the mesoporous carbon nitrides is controlled by varying the structure of the inorganic mesoporous silica templates. By this way, mesoporous carbon nitrides with uni, two and three dimensional porous structures are fabricated. The basic properties of the materials are investigated by the Temperature programmed desorption of CO2. It is found that the materials with a high nitrogen contents offers a high basicity. Finally, the basic catalytic performance of the mesoporous carbon nitride (MCN) for the transesterification of ethyl acetoacetate with various alcohols such as 1-butanol, 1-octanol, cyclohexanol, benzyl alcohol and furfuryl alcohol under heterogeneous reaction conditions without using any solvents is demonstrated. The catalytic results reveal that the mesoporous carbon nitride with three dimensional structure is found to be an efficient catalyst in transesterifying long and short chain primary alcohols, and cyclic and aromatic alcohols to afford their corresponding β-keto esters in high yields. More importantly, the catalyst is highly active when 1-butanol is used. The influence of various parameters such as temperature, reactant feed ratio, catalyst weight, and time-on-stream on the yield of the final product was studied in detail. In addition, the activity of the catalyst is also compared with pure mesoporous carbon and the results are discussed. The recyclability studies revealed that the MCN catalyst was highly stable under the rigorous reaction conditions and can be reused without loss of catalytic activity. The effect of structure and the nitrogen content affecting the catalytic activity of the materials is also investigated and the results will be discussed during the conference.

The anisotropic electrical conductivity of carbon nanotubes has stimulated worldwide interest in using nanotubes to enhance polymer conductivity. Generally, the conductivity of such composites increases drastically at a certain concentration of the conductive component called percolation threshold. Given the high aspect ratios and small diameters of carbon nanotubes, the percolation thresholds are often reported below 1 wt%; although nanotube dispersion and alignment strongly influence this value. In this research, we studied the fundamental effects of different nanostructures and processing conditions and host matrix as it relates to electrical conductivity in order to numerically derive the percolation threshold. Models typically account for filler volume fraction, component conductivity, compatibility between filler and polymer matrix, filler aspect ratio, and filler orientation. Control of the percolation property demonstrated novel piezo-resistive effect useful for pressure sensing applications.

Much work exists highlighting the percolation of onion-like carbon (OLC) in non-conductive polymer matrices for chemical resistance, enhanced mechanical properties and electromagnetic shielding applications. As with other carbon allotropes, dramatic results at low loadings are widely reported. However, little published work exists examining the effects of these same materials in inherently conductive polymers. This research examines the percolation threshold of un-modified OLC in conductive Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) thin films for electronic applications where maximum conductivity is of concern. Percolation at ~1% OLC loadings is achieved, with repeatable thin films exhibiting sheet resistances on the order of 1 Ohm/square are reported. Pristine, unmodified OLC is suspended in a water/PEDOT:PSS/organic solvent solution. Multiple organic solvents are investigated in this study. Thermal evaporation and annealing of films in an autoclave was carried out to form transmission lines. Scanning electron microscopy (SEM), SEM energy dispersive spectroscopy (EDS), atomic force microsopy (AFM) and optical profilometry are utilized to examine the resultant film morphology and filler distribution, and to demonstrate sufficient penetration of the carbon onions within the various films. A four-point probe and DC sourcemeter are used to measure surface and bulk DC conductivities. A network analyzer and anechoic chamber are employed to examine film AC response, including real and imaginary values for the transmission lines. The utilization of these materials in RF antenna applications is investigated.

Energy storage phenomenon in supercapacitors (applied in high power demanding devices as Hybrid Electric Vehicles, lifts, cranes, planes, etc.) is generally attributed to electrical double layer charging, formed on electrode/electrolyte interface. Carbon materials due to their well developed surface area seem to be most promising materials for supercapacitor electrodes. Unfortunately, capacitance revealed by these materials is still relatively small. On the other hand, if some additional charge originating from faradaic reactions will be provided, the capacitance increases rapidly. This type of additional capacitance, called pseudocapacitance, might be provided by enriching carbon materials in heteroatoms or applying carbon-transition metal oxides composites as electrode materials and assembling supercapacitor in asymmetric system. Another approach with pseudocapacitance originating directly from electrolyte solution based on iodide/iodine redox system was reported by our group. This work is focused on electrochemical behavior of the activated carbon supercapacitor electrodes electrochemically grafted by three different dihydroxybenzenes: hydroquinone, catechol and resorcinol. Electrochemical grafting with these agents diluted separately in sulfuric acid, potassium hydroxide and lithium sulfate strongly changes the carbon surface functionality and affects the capacitance values. The electrodes were functionalized in two electrode (carbon|carbon) cell during 100 cycles in voltage range 0.0 - 1.0 V at 5 mV/s sweep rate. Full electrochemical characterization of grafted activated carbons operating in acidic (1 mol/L H2SO4), alkaline (6 mol/L KOH) and neutral (1 mol/L Li2SO4) medium reveals significantly higher capacitance values, being about 350 F/g for total system with 1,4-dihydroxybenzene grafted electrodes operating in sulfuric acid, whereas for unmodified electrode it was only 130 F/g. This value significantly improves the energy of supercapacitor, even to the level of 12 Wh/kg, which is one of the highest reported values for aqueous medium. Galvanostatic charge/discharge investigation, performed in wide range of current density (0.2 - 20 A/g) proved a good electrochemical behavior of this capacitor. Cyclic voltammetry performed in range of scan rates 1-20 mV/s, reveals faradaic character of capacitance. Electrochemical impedance spectroscopy (100 kHz - 1 mHz) confirmed good charge propagation and small charge transfer resistance. These advantages are followed by very stable cyclability (about 15% of capacitance decay after 5000 charge/discharge cycles), small self-discharge and moderate leakage currents. Electrochemical results are correlated with physicochemical characterization of investigated system.

Due to their superior energy and power densities, Li-ion batteries are the state-of-the-art power sources for portable electronic devices and medium-to-large scale applications, such as electrical vehicles and stationary energy storage are on the horizon. From the studies conducted to present day, it is clear that carbon nanostructures hold great promise as potential anode material for a safer and more powerful Li-ion battery technology. However, in contrast to conventional anode materials, such as graphite, neither the SEI layer formation nor the charge storage mechanisms are yet fully understood. The lack of understanding is rooted in the difficulty to provide nanomaterial samples with well defined properties. Here we report on the synthesis and electrochemical testing of detonation nanodiamond-derived carbon onions for Li-ion battery applications. The use of carbon onions produced by thermal annealing of nanodiamond powder allows for unprecedented control over nanomaterial properties, particularly size, shape, and sample composition. Carbon onions are, therefore, the ideal “model” material for a thorough investigation of ion storage phenomena in nanostructures. By controlling both annealing temperature and annealing time, and thus the progression of the nanodiamond-to-carbon onion conversion, we are able to adjust the average number of graphitic shells, the shell curvature, and inter-shell spacing. These experiments led to a series of samples that contain carbon onions with a unique set of structural characteristics, allowing for an independent investigation of the contributions from the various structural parameters, and for the development of a comprehensive model describing the lithium storage mechanism in carbon onions and other carbon-based nanomaterials, particularly carbon nanotubes.

Crude glycerol is produced as a by-product of biodiesel production at about 10% by mass. However, crude glycerol has a myriad of impurities that dramatically reduces its value. Carbonization of glycerol could prove to be beneficial as carbon materials like coke, carbon fibers, activated carbon, and the different allotropes of carbon such as nanotubes and recently, graphene have various applications such as in supercapacitors, catalyst support, and in future electronics devices. In this work, we show for the first time that glycerol can be converted to carbon with high yields in a two-step process—oligomerization of glycerol and subsequent pyrolysis of the condensation product into carbon at temperatures which are generally lower than the decomposition temperatures used in the carbonization of biomasses (< 600 oC). Formation of the polycondensation products were confirmed by NMR and FTIR analysis while thermogravimetry revealed three decomposition temperatures. These were then used to design the pyrolysis protocol. Mass yields of 26.7 %, 10.1 %, 2.88% and 2.78% were obtained from the pyrolysis of the oligomerized products at 350, 400, 500, and 600 oC, respectively. SEM-EDS analysis shows that the pyrolyzed products consisted of mainly (> 70 %) carbon and oxygen, whereas TEM reveals graphitic structures. FTIR analysis showed the expected polar functionalities typical of pyrolytic and activated carbon. The pyrolyzed products were stable up to 800 oC based on TGA. The quality of the carbon products are shown in the Raman spectra where the relative profile of the characteristic D (~1350 cm-1) and G (~1580 cm-1) bands signify graphitic structures. The results show that at 350 oC pyrolysis, carbon can thus already be produced from direct pyrolysis of oligo-glycerol.

Carbon nanofibers (CNFs) have been thoroughly investigated as potential anode materials in Li-Ion battery owing to their exceptional properties such as the higher surface area to mass ratio, electrical conductivity and mechanical toughness. However, one of the major limitations of nano carbon materials is lower mass loading density. To address this issue, we have developed a novel anode system composed of CNFs directly grown on 3D Cu mesh current collector (hereafter mentioned as 3D CNFs) using a thermal catalytic chemical vapor deposition (CVD) method. Compared to CNF-based anodes on 2D Cu current collector, active material loading amount of the 3D CNFs was 400% higher. Owing to an increase of the active surface area, 3D CNFs demonstrated enhanced electrochemical performance of Li-Ion battery in terms of charge capacity (50% improvement), rate capability and cycling life. Interfacial contact between the CNFs and Cu could play a crucial role in promoting the electrochemical properties. The intermediate TiC thin layer, formed at high temperature 750°C, could function as an efficient electric conducting pathway and a strong bonding bridge between the CNFs and Cu. Furthermore, amorphous Si (a-Si)/3D CNFs hybrid anode system was synthesized using a sputter deposition method. It was found that the electrochemical properties of the a-Si/3D CNFs were superior to those of the pristine 3D CNFs.

The exfoliation and suspension of graphene is investigated via a combination of computational and experimental techniques. Molecular Dynamics, (MD) simulations show that an equimolar mixture of Benzene (C6H6) and Hexafluorobenzene (C6F6) has the highest affinity for graphene and forms an ordered periodic layer extending up to 30Å from the sheet surface. Quadrupolar interactions are seen as the driving force of the solvent structuring and result in stacks of alternating C6H6/C6H6 molecules with density oscillations of approximately 3.4Å, which correspond to the van der Waals diameter of carbon. The surface affinity of the solvent is verified using dynamic contact angle measurements. Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) techniques are used to demonstrate the quality of the mixed solvent graphene sheets after sonication in the mixed solvents. At high concentrations the mixture is freeze-dried at room temperature to produce sponge-like graphene structures held together by non-covalent interactions and reflect the structure of the graphene sheets in solution.

Phenyl boronic acids are important binding ligands to pendant diols useful for saccharide recognition. The aromatic ring can also function to anchor an otherwise hydrophilic polymer backbone to the surface of hydrophobic graphene or carbon nanotube. In this work, we synthesize a homologous series of seven phenyl boronic acid conjugated poly ethylene glycol 8 membered branched polymers using different phenyl boronic acids. We evaluate the resulting dispersion of single walled carbon nanotubes (SWNTs) in aqueous suspension, and the nIR fluorescence of these polymer-SWNT complexes. FTIR, UV-visible, NMR, and visible fluorescence spectroscopies were applied to characterize these polymers. The fluorescence of SWNTs wrapped by these polymers presents apparent variation in the intensity and the wavelength shift. An examination of the fluorescent excitation-emission profile demonstrated that the varied boronic acid structures have an ability to quench the fluorescence of the different SWNT species to the different level. Finally, the screening results of a library of sugar molecules demonstrated the success of the new design concept toward saccharide biosensor.

Amorphous selenium (a-Se) is a suitable photoconductor for imaging devices due to its low dark current and high spatial resolution, as well as general advantage of large-area, uniform film deposited by evaporation. Among various a-Se photodetectors, HARP (high gain avalanche rushing photoconductor) may be an ideal photoconductor for vacuum camera tube, because of its ultra high sensitivity enhanced by an internal signal multiplication. One of few drawbacks of HARP camera tube was that it requires a hot cathode for signal read-out. Although much smaller versions of HARP camera tubes have been developed by using field emitter array (FEA) [1][2], they still suffer from limited resolution and dynamic range. A diamond cold cathode is a promising choice to solve current problems of FEA-HARP. With a negative electron affinity surface and high thermal conductivity, diamond emitters should supply sufficient emission current to provide wide dynamic range. Furthermore, electron beam from diamond cold cathode has small dispersion angle due to its geometrical flexibility, which can prevent a resolution loss at the emitter. In our previous studies, a-Se based photodetector was assembled using nitrogen (N)-doped diamond cold cathode. This prototype detector showed a high sensitivity to both visible light and UV light. The next step is to apply signal multiplication in order to achieve reported high sensitivity [3]. In this study, we attempted to apply signal multiplication to a photodetector driven by diamond cold cathode. By applying a large voltage between anode and cathode, we obtained enough electric field within the a-Se film. Emission current (I)-applied voltage (V) characteristics showed that carrier multiplication in the a-Se film significantly increase the emission current under light illumination. No increment was observed in dark current. It was also found that the signal/noise (S/N) ratio of the detector decreased when the emission current grew to a certain point. The reduction of S/N ratio in high voltage region is due to a limited current supply from the emitter. The cause of current limitation is either large bulk resistance of the diamond, or limited carrier supply in diamond conduction band. Introducing a third electrode will help controlling the surface potential of the cold cathode, which enables signal multiplication for any operation voltage. [1] N. Egami et al., J. Vac. Sci. Technol. B 23, 2056 (2005) [2] Y. Honda et al., J. Vac. Sci. Technol. B 29, 04E104 (2011) [3] K. Tanioka et al., IEEE Electron Device Letters, EDL-8, 9, pp. 392-394 (1987).

Carbon nanomaterials exhibit some outstanding physical, chemical, and mechanical properties, such as light absorption, mechanical stiffness, and high conductivity, owing to the unique geometric arrangement of the carbon atoms and the extensive conjugated sp2-sp2 linkages and p-orbital electron delocalization in their structures. These structural features also enable carbon nanomaterials to serve as robust electrocatalysts, with most of their electrocatalytic activity associated to edge planes and dopants, such as boron or nitrogen. The edge planes exist only in a small area in carbon nanomaterials, such as at the tips of carbon nanotubes or at defect sites on the basal planes of graphitic nanostructures. A new class of graphitic material, so called carbon nanoneedle, is prepared and demonstrated here [1]. N-doped carbon nanoneedles (CNNs) are synthesized by self-assembling core-shell nanostructures and nanoreactors around cellulose nanocrystals, and subsequent graphitization. The resulting graphitic nanoneedles have well-organized graphitic multi-layers and large proportions of N-doped edge planes. The N-doped carbon nanoneedles showed to be highly efficient as a metal-free electrocatalysts for hydrazine oxidation, that is a very important reaction for liquid fuels cell. Reference: [1] R. Silva, T. Asefa. Edge-Plane-Rich Nitrogen-Doped Carbon Nanoneedles (CNNs) and Efficient Metal-Free Electrocatalysts. Angew. Chem. Int. Ed. 2012, 51, 1 - 6

Diazonium chemistry on sp2 carbon surfaces has allowed the formulation of printing inks from carbon black [1], has rendered carbon nanotubes soluble [2] and enabled the electrical modification of graphene [3]. The same chemistry can be applied to the surface of carbon coated nanomagnetic particles, thereby generating magnetic chemicals with complex surface functionalities. [4,5] In this presentation we will show the advantages of carbon based surface chemistry in the field of magnetic materials. These include true C-C bonding, high chemical inertia and optimal surface control. The versatility of the system for the anchoring of heavy metal ligands, homogeneous catalysts, polymers as well as biomolecules will be displayed by discussing illustrative examples from µl to m3 applications. [6-8] The rapid separation of the carbon coated nanomagnets from solution aids rapid experimentation and multi-step chemical synthesis procedures. By analogy, this may aid the development of more complex chemical procedures on the surfaces of solubilized nanotubes.

Carbon nanofoams (CNF) constitute a new form of carbon, which has some unique physical properties, such as ultra-low density, open porous structure and large surface area. This material attracted attention after reports of intrinsic magnetism and for various promising applications, such as in catalysis, energy storage, gas adsorption, tribology, biomedicine and magnetoelectronics. Although, little is known about the CNF microstructure, it is believed that the building blocks are schwarzites. These are composed of graphite-like sheets possessing sevenfold rings and having negative (hyperbolic) Gaussian curvature. Schwarzite fundamental units have low density, nealy half that of graphite. Here, we report results from atomistic simulations that shed light on the microstructure of CNF and predict their electronic and optical properties. We use classical Monte Carlo simulations to generate initial structures, which are subsequently fully relaxed with tight-binding molecular dynamics simulations. We investigate the interlinking of the units, the stability of the overall structure as a function of the density, the elastic moduli, the electronic density of states, and the dielectric function. A central result is that the present CNFs in bulk form, either composed of semiconducting or metallic schwarzite units, and despite their random and fractal network with many kinks and junctions between the units, are conductive, with no localized states near the Fermi level, contrary to other disordered carbons. Another notable result is that the CNF network does not resemble and can not be used as a model of the continuous random network of low-density amorphous carbon. We also find that the CNFs in bulk form, although soft, are rigid and not floppy, so they could be engineered for practical applications.

Among the possible ways to covalently functionalize the hydrophobic and inert surface of carbon nanotubes (CNT), oxidation by nitric acid or HNO3/H2SO4 mixtures is most frequently employed. HNO3/H2SO4 treatment being often more destructive, nitric acid treatment is usually preferred for the controlled formation of functional groups with minimum damage to the CNT structure. Recently, we have investigated in a combined experimental/theoretical work the early steps of oxidative treatment in nitric acid [1]. Thanks to density functional theory (DFT) calculations, we have proposed an efficient pathway leading to carboxylic acid functional groups on the surface. Starting from mono-vacancies, it ends up with the synergistic formation of dangling -COOH groups and the enlargement of the vacancies, as observed experimentally. Noncovalent doping of carbon nanostructures by charge transfer from/to donor/acceptor molecules or superacid like highly concentrated H2SO4, be it with holes or electrons, is usually thought as potentially interesting for many applications of carbon based nano-devices. However, from a theoretical point of view, little is known about such charge transfer processes. Employing DFT calculations, we have studied in details the interaction between a prototypical donor molecule the tetrathiafulvalene (TTF), a standard acceptor organic molecule, tetracyanoethylene (TCNE) and carbon nanostructures: grapheme layer and CNTs with various chiral indices, at several dopant concentrations [2]. Additionally, we have reported the adsorption properties of molecules present in superacid medium. Main results concern structural and thermodynamic aspects including dispersion forces effects, and evidently electronic structure modifications of the carbon nanostructures. Interestingly, we have shown that no charge transfer between pristine walls and H2SO4 molecules happens but only HSO4- molecules can p-dope the carboneous structures [3]. [1] I. Gerber et al, Chem. Eur. J., 17, 11467 (2011). [2] T. Hu et al, submitted to J. Phys Chem C. [3] P. Puech et al, Phys Rev B., 85, 205412 (2012).

“Covetics” are a new class of materials formed by the incorporation of high concentrations (> 6 wt%) of nanoscale carbon in a metal matrix. The carbon incorporates into the crystal structure of the host metal (Al, Cu, Ag and other metals) and remains dispersed after subsequent melting and re-solidification. The carbon is highly stable in these materials despite the absence of a predicted solid solution at such concentrations in the binary phase diagrams. This makes e-beam depositing covetic films with similar microstructure to the extruded metal possible. Covetics have been shown to exhibit enhanced electrical, mechanical and thermal properties when compared with non-covetic metals. We have performed energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), SEM, TEM, high angle annular dark field (HAADF)/STEM/electron energy loss spectroscopy (EELS), atomic force microscopy (AFM), and Raman spectroscopy to investigate the structure of Al, Cu, and Ag covetics. Both bulk samples and thin films are investigated. Carbon was detected in the form of nanoparticles 5 nm - 200 nm in diameter with an interconnecting carbon matrix. The carbon is detectable by EDS and XPS, but not by analytical methods such as LECO and GDMS due to its chemical affinity to the metal. Raman indicates a similar signal to that of CNTs in covetics. A detailed investigation of the morphology of the nanocarbon and the structure of several covetics will be presented. *Supported in part by NSF MRSEC under grant No. DMR 0520471 and ONR Code 332 # N0001410WX20992. We acknowledge support from the University of Maryland Nanocenter and the NISP Lab.

Heat conduction, in most applications, is generally governed by the Fourier&’s constitutive law. Although the heat flux in any direction, in general, depends on the complete thermal conductivity tensor, the off diagonal terms of the tensor are zero for isotropic materials from symmetry considerations. This symmetry is lost in the case of anisotropic materials such as graphite and graphene which leads to significant non-zero off-diagonal terms. We present alterations to the classical Green-Kubo relationship for thermal conductivity, and use the modified form to evaluate the complete conductivity tensor for graphite/graphene using molecular dynamics simulations with Tersoff bond-order potential for in-plane interactions, and a weak dispersive potential for interlayer interactions. We further verify that the anisotropic Green-Kubo form also satisfies the general tensorial properties.

As Cu interconnects for integrated circuits are downscaled, their performance and reliability degrade due to boundary scattering and electromigration. Instead, metallic carbon nanotubes (m-CNTs) and graphene nanoribbons (GNRs) have been proposed as potential Cu replacements. Such one-dimensional (1-D) conductors have strong sp2 bonds which limit electromigration and allow current densities over two orders of magnitude higher than Cu. However, an understanding of the fundamental limits of current flow in these 1-D conductors is still lacking. Here we study the upper limits of diffusive current flow in single-wall m-CNTs grown on SiO2/Si substrates and contacted with Ti/Pd electrodes. To select only single-wall CNTs, we carefully characterize their diameter both by atomic force microscopy (AFM) and Raman spectroscopy. We cross-check the “height” measured by AFM with the Raman G-peak splitting and, if available, the radial breathing mode (RBM) to ensure each m-CNT is a small-diameter (1.2 - 2.2 nm) single-walled device. We measure m-CNTs in vacuum to prevent breakdown by oxidation and, surprisingly, find that currents do not typically saturate, unlike previous observations which suggested a maximum current of ~25 mu;A [1]. In fact, at very high fields not previously probed before (>10 V/mu;m) the current continues to increase with a slope ~0.5-1 mu;A/V, allowing m-CNTs to reach currents well in excess of 25 mu;A. Subsequent modeling suggests that while electron-phonon scattering does play a strong role in limiting the maximum current, m-CNTs are cooled sufficiently by the underlying substrate to reach ~30-35 mu;A, which correspond to a current density of ~9 mA/mu;m for diameters of ~1.2 nm. We also compare the peak current density of m-CNTs [2] with GNRs [3] from previous studies performed in air (breakdown at ~600 oC). We find that as the “footprint” (width) between such device and the underlying substrate decreases, heat dissipation becomes more efficient (for a given width), allowing for higher current densities. We also note that m-CNTs do not suffer from edge scattering like GNRs do. Because of their smaller dimensions and lack of edges, m-CNTs can carry larger current densities than GNRs, up to ~16 mA/mu;m for an m-SWNT with a diameter of ~0.7 nm versus ~3 mA/mu;m for a GNR having a width of ~15 nm. Such current densities are the highest possible in any diffusive conductor, to our knowledge. [1] Z. Yao, C. L. Kane, and C. Dekker, Physical Review Letters 84, 2941 (2000). [2] A. Liao et al., Physical Review B 82, 205406 (2010). [3] A. D. Liao et al., Physical Review Letters 106, 256801 (2011).

The advance of nanotechnology has opened new opportunities to develop ever more sensitive sensors. Single-walled carbon nanotube (SWNT) is one of the most exciting materials in this regard due to its high surface-to-volume ratio and unique electronic structure. Ironically the ultrahigh sensitivity of SWNTs can be easily compromised by contaminations from the environment. Indeed, here we show that despite significant progress has been made in the last decade, we are still nowhere close to what a pristine SWNT based sensor can potentially offer [1]. Under continuous in situ ultraviolet (UV) light illumination, the electrical response of pristine SWNTs becomes extremely sensitive to gaseous molecules such as nitric oxide with a detection limit as low as 590 parts-per-quadrillion at room temperature. Gas sensing on NO2 and NH3 further confirmed that pristine nanotubes could have sensitivities orders of magnitude better than what previously had achieved. In addition, we observed the electrical response to reverse its direction upon NH3 exposure after applying in situ UV light illumination, which clearly explains the origin of ultrasensitivity reported here. Aiming for practical applications, we further illustrate how to address gas selectivity by introducing a gate bias. Because of its simplicity the concept of in situ cleaning can be deployed in existing sensor architectures. [1] Chen, G.; Paronyan, T. M.; Pigos, E. M.; Harutyunyan, A. R., Scientific Reports 2012, 2, 343.

Providing people worldwide an access to clean and safe water is one of the most motivating scientific and economic challenges of our modern society. Water purification and remediation can be afforded by membrane technologies, including inorganic carbon membranes. It is obvious that the manufacturing of materials for environmental issues must be considered in the framework of green and sustainable chemistry approaches. In this context, different bio-sourced nanocarbons were elaborated starting from natural polymer or by-products of food industry. Spherical carbon nanoparticles were elaborated by hydrothermal conversion of biomass. Colloidal solutions of carbon nanospheres have been used to prepare carbon membranes by several techniques including spin-coating and dip-casting. Some porosity agents have been added in order to adjust the porosity of the membrane and make them available to a wide range of potential applications. Carbon nanofibers were prepared by electrospinning of (bio)polymer followed by a convenient thermal treatment under controlled atmosphere. The electrospun fibers were deposited on convenient ceramic supports. Additives have been added in order to tune the intrinsic features of the fibers like surface roughness and porosity. The performances of the different nanocarbon-based membranes were comparatively evaluated in terms of water flow and molecular weight cut-off. The influence of the nature of the nanobuilding blocks on the properties of the membranes will be discussed. The intrinsic electrical conductive nature of these carbon membranes can also be exploited by using them as electrodes for electrochemical disinfection processes like the electro-Fenton processes. In-situ generation of oxidant species can provide a convenient disinfection process and limit the biofouling of the membrane.