Graphene on hexagonal boron nitride (hBN) has been shown to have significantly improved mobility and charge inhomogeneity based on electrical transport measurements. Using scanning tunneling microscopy, we have observed that the surface roughness is reduced by one order of magnitude as compared to graphene on silicon oxide devices. The lattice mismatch between the graphene and hBN produces a moiré pattern in the topographic images which gives a direct measure of the relative orientation of the two lattices. Near the charge neutrality point, graphene breaks up into a series of electron and hole puddles due to potential fluctuations. Using scanning tunneling spectroscopy, we have shown that the potential fluctuations are also reduced by an order of magnitude on hBN [1]. The ultraflat and clean nature of graphene on hBN devices allows for the observation of scattering from buried step edges. The energy and spatial dependence of the scattering gives information about the dispersion relation of graphene and the chiral nature of the quasiparticles [2]. The underlying boron nitride substrate also produces a weak periodic potential for the electrons in graphene. This periodic potential is able to create new superlattice Dirac points at the wavevector of the potential. As the relative rotation angle between the graphene and hBN changes, the energy of this superlattice Dirac point changes. Using gate voltage dependent spectroscopy, we have observed the effect of the new Dirac point on the local density of states in graphene [3]. Our latest results on trilayer and twisted bilayer graphene will also be discussed.
[1] J. Xue et al., Nature Materials 10, 282 (2011).
[2] J. Xue et al., Phys. Rev. Lett. 108, 016801 (2012).
[3] M. Yankowitz et al., Nature Physics 8, 382 (2012).

Attention has turned increasingly to graphene bilayers and ultra-thin multilayers, because of their scientific richness and their promise in technological applications. Interlayer conductance is crucial in almost all such applications. However, most fabrication methods lead to "twisted" layer stacking, with a random angle of rotation between layers. As a result, momentum conservation suppresses interlayer conduction. Understanding the interlayer conduction for rotated stacking has proven surprisingly subtle and difficult, despite crucial advances by several groups. The interlayer conductance is predicted to vary by many orders of magnitude with rotation angle. To even define a conductance has required introduction of a phenomenological broadening parameter to account for decoherence and scattering. We find [1] that essentially all of the conceptual and computational problems of twisted bilayers are resolved by including phonon scattering explicitly in the calculation of interlayer transport. Then incoherent transport occurs naturally, no phenomenological broadening is required, and subtle computational issues are cleanly resolved. The phonon-mediated conductance arises primarily from the vibrational "beating" mode of the bilayer, in which out-of-plane acoustic (flexural) modes of the two layers are out of phase, so the layers vibrate against each other. We introduce a simple tight-binding Hamiltonian that can describe the electron scattering by this mode for arbitrary rotation. With phonons included, the calculated dependence on rotation angle is quite different than expected, and far more favorable for device applications. In particular, the extraordinary sensitivity to rotation angle disappears, replaced by a smooth and mild dependence, so the interlayer conductance is never very small at room temperature. Thus device behavior can be robust despite random rotation angles. Simple scaling relationships give a good description of the conductance as a function of temperature, doping, rotation angle, and bias. At low temperature we find a diode-like turn-on of the conductance with increasing voltage. [1] V. Perebeinos, J. Tersoff, and Ph. Avouris, Phys. Rev. Lett. (in press).

The increasing availability of two-dimensional crystals and the development of methods to stack them in a precise manner are promising advances towards a new class of hybrid solids. An outstanding fundamental question related to these materials is the extent to which individual layers of these materials interact electronically and whether such interactions would lead to emerging properties. We examine this question for the case of two azimuthally misoriented graphene layers (twisted bilayer graphene) using angle-resolved photoemission spectroscopy and density functional theory. Our finding of van Hove singularities and associated mini-gaps in the electronic spectrum prove unambiguously that the layers interact. Moreover, we find additional van Hove singularities at the boundaries of the moiré superlattice Brillouin zone formed by the twist. These observations illustrate how electronic dispersion can be modulated by moirés, structures ubiquitous in hybrid solids based on two-dimensional crystals.
This work is carried out in collaboration with J. T. Robinson at Naval Research Laboratory, P. J. Feibelman, T. E. Beechem, B. Diaconescu, G. L. Kellogg at Sandia National Laboratories, and A. Bostwick, E. Rotenberg at Lawrence Berkeley National Laboratory. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

By stacking various two dimensional (2D) atomic crystals on top of each other, it is possible to create multilayer heterostructures and devices with designed electronic properties [1-3]. However, various adsorbates become trapped between layers during their assembly, and this not only affects the resulting quality but also prevents the formation of a true artificial layered crystal upheld by van der Waals interaction, creating instead a laminate glued together by contamination. Transmission electron microscopy (TEM) has shown that graphene and boron nitride monolayers, the two best characterized 2D crystals, are densely covered with hydrocarbons (even after thermal annealing in high vacuum) and exhibit only small clean patches suitable for atomic resolution imaging [4]. This observation seems detrimental for any realistic prospect of creating van der Waals materials and heterostructures with atomically sharp interfaces.
We have employed focused ion beam milling to extract cross sectional slices with typical dimensions of 1 mu;m x 5 mu;m x 10 mu;m from the active region of several graphene - boron nitride heterostructures. Final ion polishing procedures allows the slice thickness to be reduced to 20-70nm, suitable for imaging by aberration corrected scanning transmission electron microscopy (STEM). By taking a side view of these complex heterostructures we find that the trapped hydrocarbons segregate into isolated pockets leaving the interfaces atomically clean. Our approach also allows comparison to the electrical device characteristics measured before slice extraction. We have analyzed the local interlayer separation and layer roughness was using line profiles extracted from our STEM images. We observe a clear correlation between interface roughness and the electronic quality of encapsulated graphene [5]. We also report on recent side view STEM imaging of graphene based electronic devices involving other 2D crystals such as tungsten disulphide. This work proves the concept of heterostructures assembled with atomic layer precision and reports their first TEM images.
1. Wang, H. et al. BN/Graphene/BN transistors for RF applications. IEEE Electron Device Lett. 32, 1209-1211 (2011).
2. Ponomarenko, L. A. et al. Tunable metal-insulator transition in double-layer graphene heterostructures. Nature Phys. 7, 958-961 (2011).
3. Britnell, L. et al. Field-effect tunneling transistor based on vertical graphene heterostructures. Science 335, 947-950 (2012).
5. Gass, M. H. et al. Free-standing graphene at atomic resolution. Nature Nanotech. 3, 676-681 (2008).
6. Haigh S. J. et al. Graphene-based heterostructures and superlattices: Cross-sectional imaging of individual layers and buried interfaces, Nature Mat. 11, 764-767 (2012)

Many thousands of papers have investigated graphene&’s remarkable structural, mechanical, optical, and electronic properties. Most of these studies, however, have focused on only one of graphene&’s attributes at a time, neglecting its unique combination of properties. In this talk we will look at these interdisciplinary boundaries, examining cases when a combination of graphene&’s properties, e.g. electrical and mechanical, are simultaneously important. For example, we will discuss experiments on graphene atomic membranes where electronic and optical signals can control the frequency, amplitude, and damping of a graphene drumhead resonance. We also present ultrafast measurements of photocurrent in graphene p-n junctions. At the boundary of graphene electronics and optics, these measurements probe the fundamental relaxation processes that are key to applications ranging from photodetectors to optical saturable absorbers. Finally, we explore the properties of a literal boundary that occurs in bilayer graphene. We present the first atomic resolution images of the soliton-like boundaries between bilayer graphene&’s different broken-symmetry structural ground states. True to form, these boundaries are predicted to dramatically influence both the mechanical and electronic properties of bilayer graphene.

A blueprint for producing scalable digital graphene electronics has remained elusive. Current methods to produce semiconducting-metallic graphene networks all suffer from either stringent lithographic demands that prevent reproducibility, process-induced disorder in the graphene, or scalability issues. Using angle resolved photoemission, we have discovered a unique one-dimensional metallic-semiconducting-metallic junction made entirely from graphene, and produced without chemical functionalization or finite size patterning. The junction is produced by taking advantage of the inherent, atomically ordered, substrate-graphene interaction on SiC; in this case when graphene is forced to grow over patterned SiC steps. This scalable bottom-up approach allows us to produce a semiconducting graphene strip whose width is precisely defined within a few graphene lattice constants, a level of precision entirely outside modern lithographic limits. The architecture demonstrated in this work is so robust that variations in the average electronic band structure of thousands of these patterned ribbons have little variation over length scales tens of microns long. The semiconducting graphene has a topographically defined few nanometer wide region with an energy gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet.[1] This work demonstrates how the graphene-SiC substrate interaction can be used as a powerful tool to scalably modify graphene's electronic structure to produce a wide band gap metal-semiconductor-metal nanostructure made entirely from graphene.
J. Hicks, A. Tejeda, A. Taleb-Ibrahimi, M.S. Nevius, F. Wang, K. Shepperd, J. Palmer, F. Bertran, P. Le Fèvre, J. Kunc, W.A. de Heer, C. Berger, and E.H. Conrad, Nature Phys. (in press).

Graphene offers unique scientific and technological opportunities as nanoelectromechanical systems (NEMS). Namely, graphene has allowed the fabrication of mechanical resonators that can be operable at ultra-high frequencies and that can be employed as ultra-sensitive sensors of mass and charge. In addition, graphene has exceptional electron transport properties, including ballistic conduction over long distances. Coupling the mechanical motion to electron transport in this remarkable material is thus highly appealing. Here, I will review some of our recent results on graphene NEMSs, including the mechanical coupling between 2 graphene resonators and nonlinear damping.

Large area, ultra-thin tensioned graphene and silicon nitride membranes are useful as optomechanical elements whose mechanical degree of freedom can be easily controlled using light due to low spring constants and resonator mass. Recently it has been observed that the Q of these membranes can be significantly improved by a combination of tensile stress, resonator geometry and optimized fabrication techniques. Here we fabricate CVD grown, electrostatically tunable graphene and graphene on silicon nitride drums of diameter up to 100 mu;m and use lasers to control the amplitude of mechanical vibrations using the back action provided by the photothermal effect. We can effectively cool (increase the effective damping) or heat (decrease the effective damping leading to self oscillation) the graphene and graphene on silicon nitride membranes in a Fabry-Perot cavity formed by the freestanding membrane, with cavity detuning provided by a highly reflective movable mirror placed in close proximity to the membrane. In the case of graphene on silicon nitride we electrically and optically read the photothermal back action of graphene on high Q silicon nitride membrane. The strong optomechanical coupling observed in these membranes is due to the low mass, high Q of these thin membranes and relatively strong absorption in the atomic monolayer. Experiments provide insight into optomechanical coupling and mechanical damping observed in these materials

Graphene-based materials are of interest because of their electronic and thermal transport, mechanical properties, high specific surface area, and 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 such CVD-grown graphene as a central component; (ii) Generation, study, and use of chemically modified graphene ‘platelets&’ (typically derived from graphite oxide) including as dispersed in liquids forming colloids, and powders derived from such colloids or separately 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 (‘aMEGO&’)[3]. I will cover a variety of current research projects underway in my group and also describe what I think are some important new in related carbon nanomaterial research, for 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, DOE, and the SRC, 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).

Despite its versatility and prevalence in the microelectronics industry, sputter deposition has seen very limited applications for graphene-based devices. We have systematically investigated the sputtering induced graphene disorder and identified the reflected high-energy neutrals of the sputtering gas as the primary cause of damage. In this talk, we will analyze the energetics of various species created during the sputtering process. We then introduce a new sputtering technique that dramatically reduces the impact of the fast neutrals, improves the structural integrity of the underlying graphene layer, and retains high deposition rates. Raman spectroscopy confirms that the graphene disorder modes can be suppressed to the level below that in the e-beam evaporated graphene. In-situ oxidation of the deposited 1nm Al film at elevated temperatures yields homogeneous, fully covered oxides with r.m.s. roughness much less than 1 monolayer, demonstrating the potential of using such technique for gate dielectrics, tunnel barriers, and multilayer fabrication in a wide range of graphene devices.

Bilayer graphene is an intriguing material in that its versatile electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Currently, twisted bilayer graphene is mainly obtained either by folding single-layer graphene or by the layer-by-layer stacking of transferred graphene at the cost of interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycrystalline structures. Here we study single-crystal bilayer graphene grown by ambient pressure chemical vapor deposition on Cu substrate. We control the growth by pre-patterned solid carbon source as pre-determined nucleation sites and synthesize spatially ordered arrays of single-crystal bilayer graphene. Close to 30% of the individual bilayer graphene grains are single-crystal in nature with high crystalline quality, as reflected in its having highest electron mobility ever reported for CVD graphene. We investigate the morphology and size of the graphene domains and the well-defined single-crystal hexagons can be synthesized under specific conditions. We also show new Raman active modes resulted from the twist, enabled by a combination of transmission electron microscopy and Raman spectroscopy. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.

The wide range of industrial applications possible for graphene has lead researchers to increase their efforts in growing larger area graphene that could be used in microelectronics, alternative energies, health science, etc. One of the first expected commercial applications of graphene - where it could compete with actual materials due to its high transparency, conductivity, and flexibility (20% stretchable) - is the transparent electrode for photovoltaic devices, touch screens and light electroluminescent diodes. These applications require the development of graphene processes compatible with industrial implementation.
The present work emphasizes a simple and quick route using a solid carbon source to grow graphene fully covered 8 in. wafers in industrial-compatible equipments. It is not limited to specific metals (even with very low carbon solubility) and does not require any gaseous carbon source and can be performed at relatively low temperature (700°C). The whole process flow has been operated using standard semiconductor-compatible clean room facilities. In our approach, we use the solubility of carbon in transition metals (here a thin film of nickel) that diffuses at high temperature from a carbon source underneath the metal to form graphene layers on the metal catalyst. The resulting quality of graphene and its number of layers are strongly dependent on the growth and annealing conditions. The resulting coverage of graphene upon the wafer is over 99% (fully interconnected) of the whole polycrystalline nickel surface and highly uniform throughout all the 8 in. wafer. Typical optical transmittance of ~90% at 550 nm and resistance of ~0.5-1 kOhm/sq have been obtained.
The graphene was implemented in different devices as a transparent electrode such as solar cells or OLED. It was also used as a top electrode in Resistive-RAM or as a floating gate in flash memories.

Chemical vapor deposition (CVD) is one of the most effective methods to produce high quality, large scale graphene. Various metals, Ni, Cu, Pt, Ru, etc., are used as catalysts to assist graphene growth. Pt is chosen as the catalyst since it can provide a stable and clean growth environment and the transfer process is more economical and time efficient. The atmospheric pressure CVD (APCVD) is used in this study. Graphene growth on Pt can be divided into two stages: carbon source (CH4 in this study) pyrolysis and graphene structure formation from dissociated carbon atoms. The first stage is controlled by the growth parameters such as gas flow rate, the deposition time and growth temperature, and the second stage is controlled by the cooling parameters such as cooling rate and gas environment. From our experiments, we found the cooling process plays a key role in the graphene growth, which is less studied in previous literature. A further understanding of the cooling kinetics is very important for better graphene growth control including thickness, uniformity, coverage, etc.. In this study, a series of experiments are carried out carefully to study various growth parameters (gas composition, gas flow rate, cooling rate, etc.) effects on graphene growth. Particularly, for the cooling effect investigation purpose, growth parameters in the first stage are fixed to produce the same amount of carbon atoms, while different cooling parameters are used in those experiments. The synthesized graphene films are examined by optical microscopy, Raman, AFM, and SEM. Based on our experiments results, we propose that there are three reactions happening during the cooling process: i). carbon atoms precipitate from Pt, bonding to each other to form graphene structure, ii). carbon atoms (from CH4 thermal decomposition) dissolved in Pt, and iii). carbon atoms react with H to form C-H bond. The synthesized graphene structure and properties are decided by the combined effects of those three reaction processes. Based on our understanding, the optimized growth conditions are proposed. High quality and large area graphene could be grown use such growth conditions. Hall devices and Field-effect transistors based on those high quality graphene are fabricated and excellent electronical performance results are obtained.

Single-wall carbon nanotubes (SWCNTs) possess superior electrical and optical properties and hold great promise for electronic and biomedical applications. Since the electronic property of a SWCNT strongly depends on its chirality, the lack of synthetic control in chirality has long been recognized as a fundamental impediment in the science and application of SWCNTs. Previous efforts to address this issue have resulted in significant progress in separation of synthetic mixtures, which has yielded predominantly single-chirality nanotube species. However, separation processes are limited by their small scale, high cost, and short length (< 500 nm) of the resulting chirality-pure nanotubes. They are therefore not viable for many, especially electronic device applications. Here we demonstrate a general strategy for producing carbon nanotubes with predefined chiralities by using purified single-chirality nanotubes as seeds for subsequent metal-catalyst-free growth, resembling vapor phase epitaxy (VPE) commonly used for semiconductor films. In particular, we have successfully synthesized a vast number of chiralities including (7, 6), (6, 5), (7, 7) etc, and used Raman characterization to confirm unambiguously the preservation of the original chirality of nanotube seeds. Furthermore, electrical measurements confirm the semiconducting nature of VPE grown individual (7, 6) and (6, 5) nanotubes. A Diels-Alder cycloaddition mechanism is proposed to interpret our metal-free nanotube growth process. The VPE approach is found be highly robust and should enable a wide range of fundamental studies and technological developments.

Graphene, a two dimensional material with honey-comb structures has drawn significant attention with its unique physical, mechanical and electrical properties. Tremendous efforts have been made to synthesize large-scale, high quality, single layer graphene. Here we report a novel vapor trapping method to synthesize graphene with grain size up to several hundreds of micrometers on copper foil. Controlled growth of graphene flowers with four lobes and six lobes has been achieved by varying the growth pressure and the methane to hydrogen ratio. Raman surface map of individual graphene grain indicated that the graphene flowers have high quality single-layer graphene as lobes and bilayer graphene as centers. Selected Area Electron Diffraction (SAED) also confirmed that each individual graphene island was a single grain. Surprisingly, electron backscatter diffraction (EBSD) study revealed that the graphene morphology had little correlation with the crystalline orientation of underlying copper substrate. Furthermore, field effect transistors were fabricated based on graphene flowers and the fitted device mobility could achieve sim;5200 cm2 Vminus;1 sminus;1 on Si/SiO2 and sim;20 000 cm2 Vminus;1 sminus;1 on hexagonal boron nitride (h-BN). The achieved high device mobility indicates that the large-grain, single-crystalline graphene is of great potential for graphene-based nanoelectronics.

Graphene exhibits unique properties, making it the material for the observation of novel quantum phenomenon and the development of future nano-devices. In spite of these remarkable properties, the use of graphene in nanoscale electronics devices requires a modification of its intrinsic semimetallic nature in order to open an energy gap. To open a gap in graphene, different techniques are used with a promising way consisting to synthesizing graphene ribbons. In fact, nano-Ribbons are characterized by a quantum confinement which induces a band gap formation, this one being inversely proportional to the nano-ribbons width. Among the different techniques investigated to fabricate graphene Ribbons, the thermal decomposition on SiC substrate is one of the most attractive approaches.
In the present work, we demonstrate the control of the formation of self-organized elongated graphene ribbons on Si-face SiC substrate. This approach offers the advantage to directly form ribbons, avoiding then the patterning of a pre-existing graphene layer, which leads to a degradation of the electronics properties.
Knowing that the surface graphitization begins from the step edge and then propagates toward the center of the terraces, we have established that the self-formation of ribbons strongly depends on two characteristics. The first one is the surface morphology of the SiC substrate. A surface rearrangement in well-defined steps which plays the starting point for graphene layers synthesis is a prerequisite. The second characteristic is the control of the early stages of the thermal decomposition of the Si atoms from stepped SiC substrate. A precise adjustment of the temperature and the duration of thermal annealing are essential.
By combination of different characterization methods, such as Low Energy Electron Diffraction, Atomic Force Microscope, Electric Force Microscope, Kelvin Force Microscope, Scanning Tunneling Microscope, Scanning Tunneling Spectroscopy and Raman Spectroscopy, we demonstrate that self-organized graphene ribbons are formed all along the substrate step edges. They are characterized by a thickness of 1 ML and by wideness in the 200-400 nm range and a length of several micrometers.
The formation of these ribbons constitutes a first step towards the synthesis of nano-ribbons and the opening of a bandgap, then changing the intrinsically semimetallic nature of the graphene.

Chemical vapor deposition (CVD) is the most promising route towards scalable graphene production and integration, key requirements for the commercial exploitation of graphene's unique properties. 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.
We have previously shown Ni based catalysts to be effective catalysts at much lower temperatures (450-600°C).¹ We have also used in situ, time-, and depth- resolved X-ray photoelectron spectroscopy (XPS) and in-situ X-ray diffraction (XRD) to monitor graphene formation under realistic CVD conditions.^1,2 In particular we reveal that graphene growth occurs during isothermal hydrocarbon exposure and is not limited to a precipitation process upon cooling, and that (sub-surface) dissolved carbon plays a critical role.
Here we use this understanding to realise scalable graphene CVD on Ni-based catalysts at ~600°C with complete monolayer coverage, and show that it is actually possible to achieve uniformity and quality that has hitherto only been reported for Cu-based CVD at >900°C.³ Raman mapping reveals a 2D/G ratio of >3.2, D/G ratio le; 0.08 and transport measurements show carrier mobilities of ge;3000cm²V^-1s^-1 on SiO₂ support.
Furthermore, we establish a kinetic growth model for graphene CVD based on flux balances,⁴ which is well supported by our systematic study of Ni-based polycrystalline catalysts. We highlight that a finite carbon solubility of the catalyst is a key advantage, as it allows the catalyst bulk to act as a mediating carbon sink while optimized graphene growth occurs by only locally saturating the catalyst surface with carbon. This also enables a route to the controlled formation of Bernal stacked bi- and few-layered graphene. The model is relevant to all catalyst materials and can readily serve as a general process rationale for optimized graphene CVD.
(1) Weatherup et al. Nano Lett. 2011, 11, 4154-4160.
(2) Weatherup et al. ChemPhysChem 2012, 13, 2544-2549.
(3) Kidambi et al. J. Phys. Chem. C (Article ASAP)
(4) Weatherup et al. ACS Nano 2012, (Article ASAP).

Recent progress in the production of large-area high-quality graphene by thermal CVD at the industrial scale is presented and discussed in the context of near-term commercial applications. We have produced continuous monolayer graphene films up to 60 inches in size, and can tailor the resistance to 150 Ohm per square or the mobility to 4500 cm^2/Vs. Working prototypes including touchscreens and flexible displays are presented with an emphasis on the benefits of graphene for these applications and global market outlook.

This talk will present our work on carbon nanotubes, graphene nanoribbons and graphene-metal oxide hybrid materials. Biological applications of carbon nanotubes will be discussed including a new fluorescence imaging method in the so called NIR-II region in the spectral window of 1000-1400nm. NIR fluorescence enhancement of carbon nanotubes and organic fluorophores will be presented on a novel plasmonic substrate. I will then talk about graphene nanoribbons, including several methods recently developed in our lab to form high quality graphene nanoribbons with narrow widths and smooth edges. Lastly, I will talk about our recent work on making nanoparticles and nanocrystals on graphene sheets for energy storage and photocatalytic applications.

For medical applications such as neuroprostheses and for fundamental research on neuronal communication, it is of utmost importance to develop a new generation of electronic devices which can effectively detect the electrical activity of nerve cells. Due to its maturity, most of the work with field effect transistors (FETs) has been done based on Si. However, the high electronic noise and relatively low stability associated to Si technology have motivated the search for more suitable materials. In this respect, the outstanding electronic and electrochemical performance of graphene holds great promise for bioelectronic applications. For instance, we have reported on arrays of CVD-grown graphene solution-gated FETs (SGFETs) for cell interfacing [1], demonstrating their ability to transduce with high resolution the electrical activity of individual electrogenic cells [2].
In this contribution, we will present a detailed discussion on the sensitivity of graphene SGFETs for in-electrolyte operation, together with a study of the electrolyte composition on the device performance. The sensitivity of SGFETs is dominated by two characteristic parameters: transconductance and electronic noise. The transconductance specifies how an initial gate voltage signal is converted to a current by the transistor. For graphene SGFETs, the transconductance is proportional to the interfacial capacitance at the graphene/electrolyte interface as well as the charge carrier mobility. We will present Hall-effect experiments performed in electrolyte demonstrating that both interfacial capacitance and carrier mobility in graphene are superior to other competing materials, including Si. A second relevant parameter to assess the performance of biosensors is the electronic noise of the device, as it defines the minimum signal that can be detected. We have investigated the electronic noise of graphene SGFETs and compared it to that of devices based on Si. It will be shown that, similarly to other semiconductors, 1/f noise is the dominant noise source in graphene devices. The origin of the 1/f noise will be discussed in this presentation, comparing the results of single-layer graphene and bilayer graphene devices. Finally, we will briefly report on the pH and ion sensitivity of graphene devices, and the influence of the chosen substrate for the device fabrication, as well as of surface contamination from the fabrication technology.
This work highlights the potential of graphene to outperform state-of-the-art Si-based devices for biosensor and bioelectronic applications [3].
[1] Dankerl et al., Adv. Funct. Mater., 20 (2010) 3117
[2] Hess et al., Adv. Mater., 23 (2011) 5045
[3] C. Schmidt, Nature 483 (2012) S37

Nanoelectronics-based detection schemes offer promising sensitive and label-free alternatives to bioanalysis. The large-scale synthesis, high carrier mobility and ambipolar transport make graphene potentially useful in the electrical detection of biomolecular targets. Here we report on the design, fabrication, and operation of novel oxide-on-graphene, bio-ready field effect sensors using graphene sheets synthesized by chemical vapor deposition. Our design uses thin layers of HfO2 and SiO2 films to isolate the graphene channel and electrodes from electrolyte and uses the top SiO2 surface for detection and further chemical functionalization. This design preserves the excellent transport characteristics of the graphene transducer while taking advantage of the well-established surface chemistry of SiO2 in facilitating specific biomolecular binding. The graphene transducer channel operates in solution with high stability and high carrier mobility of approximately 5000 cm2/Vs. By applying the solution gate voltage in pulse, we eliminate hysteresis in the transfer curve of the graphene channel, which is critical to achieving a high detection resolution of the sensor. We demonstrate the silanization of the SiO2 surface with aminopropyl-trimethoxysilane (APTMS), which can be further linked to biomolecular probes and targets. The pH sensitivity of the bare and APTMS-functionalized SiO2 is measured to be 46mV/pH and 43mV/pH respectively, in good agreement with literature results. With suitable linking chemistry, these graphene sensors can potentially be useful in the detection of biological events such as DNA hybridization, thus opening a new avenue for biosensing using nanoscale electronics.

Graphene, a one-atom thick sheet of sp2 carbon, offers many intriguing possibilities in the field of molecular sensing. Its unique combination of large areas with nanometer thickness and high electrical conductivity could enable small scale device sensitivity with large scale production methods. A major benefit of using graphene is the large toolbox of well-established chemistries for incorporating chemical functionalities or specific recognition elements at the device surface. Here, we will discuss our efforts to develop graphene-based biological field-effect transistors (BioFETs), which offer sensitivity comparable to sensors made with other nanoscale materials (carbon nanotubes, nanowires), but with greatly simplified production methods common in the semiconductor industry. Devices utilizing both graphene and graphene oxide will be covered, and surface spectroscopic studies of the material modification will be discussed. Successful results for the detection of specific DNA hybridization using graphene BioFETs will also be presented. We will further discuss our efforts to use graphene as a biofunctionalized interface for a number of materials, from polymers to dielectrics to semiconductors, of interest to the biosensing community. Graphene&’s ultrathin nature allows its inclusion in more traditional sensing platforms as a non-intrusive functionalization layer, discreetly lending its chemical flexibility to other, more inert materials without significantly impacting the sensing device.

Graphene has been identified as a promising material for different scientific disciplines due to its exceptional physiochemical, electronic and structural properties. The extremely high carrier mobility and capacity arouses enormous interest in the field of electronic sensor applications as well. However, it is a challenge to obtain constantly small size of graphene layers by mechanical exfoliation. In the present study, grapheme oxide (GO) was used as the transducer material for two different biosensor platforms:
In a first approach, interdigitated gold microelectrodes (IDEs) were fabricated with standard lithographical methods and used as a platform for label-free detection of specific DNA hybridization and denaturation events. GO flakes were dielectrophoretically immobilized onto the IDEs and reduced to conductive graphene oxide (r-GO) using a low-temperature, green fabrication route with L-ascorbic acid (Laa). These sensors were used for label-free, impedimetric detection of DNA hybridization and denaturation. This approach is very versatile and could be applied to different substrates such as polymeric materials, since this green fabrication route doesn&’t need harsh chemicals or elevated temperatures.
In a second approach, the ‘Micromolding in Capillaries&’ (MIMIC) technique was used to pattern GO lines with lengths up to 10 mm and width down to 10 µm on glass and Si/SiO2 substrates. The GO patterns were then reduced to r-GO using the same approach mentioned above. The lines were connected by evaporated gold contacts and encapsulated to be used with living cells for in vitro monitoring of cellular adhesion of tumor cells and for detection of extracellular field potential (exFP) of cardiac cells. We found that cells preferably aligned to the r-GO lines in contrast to the bare glass or SiO2 surfaces. The patterns can be used for the definition and stabilization of networks of neuronal cells on in vitro sensor platforms. Since in a typical application in this field, the transducer material of microelectrode arrays (MEAs) is made of metal and subsequently functionalized with cell growth supporting proteins such as laminin or fibronectin, our r-GO approach has the advantage that the transducer material and the cell adhesion promoting material is the same.
In conclusion it can be said that the r-GO material, even though it has distinctly reduced electrical performance compared the single sheets of graphene, has promising properties, which make it a favorable transducer material in biosensor applications.

Graphene&’s planar, conformal, and hydrophobic nature make it useful as an atomically thin coating which inhibits gaseous diffusion [1] and entraps liquids [2,3]. Most studies to date have only explored encapsulation of simple molecules with graphene, overlooking interactions between the hydrophobic sheet and other complex nanostructures like DNA and viruses.
We develop an atomically clean graphene transfer process using a poly(bisphenol A carbonate) (PC) scaffold for graphene grown by chemical vapor deposition (CVD). Large-area CVD graphene growth on Cu [4,5] allows the fabrication of large-area, conductive, encapsulating platforms between graphene and nanostructures of choice. Typically, graphene is transferred off the Cu growth surface in H2O [2] with a PMMA scaffold, and this scaffold is partially removed by a high-temperature anneal [6]. However, such high-temperature processing is incompatible with biomolecular nanostructures. PC transfers can be removed by dissolution at room-temperature, circumventing the annealing requirement. We confirm that the clean, PC-transferred graphene films have lowered residual doping by device transport and Raman spectroscopy. RMS roughness values, determined by atomic force microscopy (AFM), are 2-4 times lower (~0.6 nm) for PC- vs. PMMA-transferred films under the same growth and transfer conditions. Scanning tunneling microscopy (STM) of the PC-transferred graphene films reveals atomic resolution, despite degas temperatures lower than 100 °C.
Using the PC transfer process, we make graphene/biomolecule/graphene nanosandwiches on SiO2/Si and mica. We deposit a known rod-shaped biomolecule, the tobacco mosaic virus (TMV), on these substrates and examine the virions&’ heights before and after graphene-based encapsulation. With AFM, we find that the TMV heights, relative to the top-most graphene layer, decrease from 12.4 nm to 3.5 nm after encapsulation. These deformations occur despite the rigid TMV capsids. Here, the graphene conforms to the virions and exhibits a strong hydrostatic pressure on them, flattening and possibly dehydrating the capsid shell. Through AFM topographs, we determine that nanosandwiched TMV denatures at ~50 °C, much higher than the known value at ~42 °C [7]. The graphene/biomolecule/graphene system will elucidate fundamental fluid dynamics in this hydrophobic bilayer cell. Additionally, the conductive and electronically transparent character of graphene can allow biomolecular interrogation at the atomic-level by STM and by transmission electron microscopy.
[1] J. S. Bunch et al., Nano Lett. 8, 2458 (2008); [2] K. T. He et al., Nano Lett. 12, 2665 (2012); [3] J. M. Yuk et al., Science 336, 61 (2012); [4] X. Li et al., Science 324, 1312 (2009); [5] J. D. Wood et al., Nano Lett. 11, 4547 (2011); [6] Y.-C. Lin et al., ACS Nano 5, 2362 (2011); [7] M. Kelve et al., J. Biomol. Struct. Dyn. 5, 105 (1987).

Carbon nanotubes (CNT) are a promising biomimetic material, in part because smooth, narrow and hydrophobic inner pores of CNT are remarkably similar to the natural biological pores. Incorporation of nanotube channels into the biologically-relevant environments and measurements of ion transport in these assemblies would not only enhance out understanding of transport in these materials systems, but also open up ways to develop novel bioengineering applications. We will describe incorporation of carbon nanotube channels into a lipid membrane and measurement of osmotically-induced ion transport through these model nanopores using dynamic light scattering (DLS) experiments. We also discuss factors that govern ion rejection in these structures and compare the results with modeling results and measurements in macroscopic systems.

Nanometer-scale structures represent a novel and intriguing field, where scientists and engineers manipulate materials at the atomic and molecular levels to produce innovative materials for composites, electronic, sensing, and biomedical applications. Carbon nanomaterials, such as carbon nanotubes and graphene, constitute a relatively new class of materials exhibiting exceptional mechanical and electronic properties, and are also promising candidates for gas storage and drug delivery.
However, processing carbon nanotubes is severely limited by a number of inherent problems: purification from a variety of byproducts, difficult manipulation and low solubility in organic solvents and in water are only some of these problems. For these reasons, several strategies have been devised to make nanotubes “easier” materials. In particular, organic modification produces functionalized carbon nanotubes, which are much more processible and offer the possibility of introducing organic fragments useful for practical applications.
During this talk, we will discuss the use of functionalized carbon nanotubes and graphene as active surfaces for a number of practical applications. Glassy surfaces, covered with carbon nanotubes are ideal substrates for neuronal growth. Nanotubes are compatible with neurons, but especially they play a very interesting role in interneuron communication, opening possibilities towards applications in spinal cord repair therapy.
In addition, in combination with catalysts of different nature, carbon nanotube modified surfaces can serve for many scopes. Experiments aiming at the splitting of water to give oxygen, and therefore, molecular hydrogen, ideal for clean energy generation, will be described. Also, multiwalled carbon nanotubes, embedded inside mesoporous layers of oxides (TiO2, ZrO2, or CeO2), which in turn contain dispersed metal nanoparticles (Pd or Pt), result in nanocomposites with remarkable performance in catalytic reactions.
References
(1) Fabbro, A.; Bosi, S.; Ballerini, L.; Prato, M. ACS Chem. Neurosci. 2012, 3, 611-618.
(2) Toma, F. M.; Paolucci, F.; Prato, M.; Bonchio, M. et al. Nature Chemistry 2010, 2, 826-831.
(3) Cargnello, M.; Liz-Marzan, L. M.; Gorte, R. J.; Prato, M.; Fornasiero, P. et al. J. Am. Chem. Soc. 2012, 134, 11760-11766.

Functional electrical stimulation can be used to restore function in patients with a damaged nervous system. Electrical impulses delivered to the tissue can generate artificial action potentials (APs) that behave like APs naturally generated by a healthy nervous system. These APs will have the same effects and will propagate through neighboring neurons to induce motor function. Electrode morphology can impact the charge injection for neural stimulation as shown in literature reports on the electrochemical properties of vertically aligned multi-walled carbon nanotubes (CNTs), graphenated CNTs, and nanoribbons. For example, the roughness of carbon nanotubes increases the surface area of the electrode and in turn, decreases the impedance and power consumption.
In the present work, patterned CNTs were grown with the goal of improving the charge injection in two ways. The pores created by the roughness of the CNT structure may be too small for the ions in the electrolyte to penetrate. The patterns should increase the total surface area accessible by the ions. In addition, the activating function for neural cells is proportional to the spatial derivative of the current density in the tissue. When an electric potential is applied across a structure, the current density is higher at the edges. The patterns should also increase the total perimeter contributed by the array of pillars. By varying the dimensions of the pillared CNT structures, the total surface area accessible by the ions and the number of edges can be increased in order to achieve a maximum charge injection while simultaneously decreasing the impedance. A model of the pillared CNT structures was created in COMSOL. The optimal dimensions for the pillars were then chosen from the parameters that resulted in the most non-uniform current density. The electrochemical properties of the best structures were then characterized in vitro. Cyclic voltammetry was used to detect the presence of reactions with the electrolyte. Electrochemical impedance spectroscopy was used to measure the electrode-electrolyte interfacial impedance. Finally, potential transient measurements were used to measure the charge injection capacity of the electrode. It was observed that the charge injection of the CNT electrodes was not dependent on the total volume of the structure but was greatly affected by the surface area and the total edge contribution from the pillars. As both of these parameters were increased, the charge injection increased. The pillared structures with optimized aspect ratios had the highest charge injection compared to normal blanket CNT, graphenated CNT, and nanoribbon electrodes.

CVD Diamond thin-films are attractive as a material for construction of active bio-electronic devices. This is due to properties of Nano-crystalline diamond such as biocompatibility, wide potential window and substantially reduced bio-fouling or inflammatory reactions.
Here we present a novel approach for fabrication of all-carbon diamond Micro-electrode Arrays (MEAs) in which Nano-crystalline diamond (NCD) thin film represents the insulating layer and Boron-doped Nano-crystalline diamond (B-NCD) features conductive layer of the electrode as a replacement for conductive metals, such as Platinum or Titanium Nitride, with B-NCD showing better electrochemical performances[1][2]. The resulting MEAs are optimized and characterized in terms of their performances and impedance, and employed in vitro for recording/stimulation of neuronal electrical activity. The measurements are carried out using cultured dissociated neurons and compared with standard commercial MEAs to benchmark metallic MEA with full diamond MEA in the application of active neuron-device interface. The signal to noise ratio is evaluated and compared to results obtained by Dankerl et al. [2], showing higher signal to noise ratio in comparison to conventional metal MEAs.
[1] M. Bonnauron et al., phys. stat. sol. (a) 205, No. 9, 2126-2129 (2008).
[2] M. Dankerl, et al., Appl. Phys. Lett. 100, No. 2, 023510 (2012).

Carbon nanotubes (CNTs) can be grown on a variety of predefined sites as demonstrated by the catalytic chemical vapor deposition (CCVD) process which uses to thermally decomposed short-chain hydrocarbons over metal catalysts. This elaboration method ensures less unwanted carbon products as fibers or soot. It is well reported that the length, the diameter and the growth density of CNTs is related to the morphology of the catalyst particles, this approach needs a precise tunability of the particles morphology (size, shape, density). Apart from the commonly used catalytic growth, impregnated template-free supports with liquid metal-acetylacetonates (Me(acac)₂) can be performed. Thermal treatment of impregnation layer induces the decomposition of the adsorbed precursor on the surface. Nevertheless, the incorporation of Me(acac)₂ into organic solvent increases the global carbon feedstock present during the CNTs growth step. Furthermore the liquid impregnation method might be very challenging in the cases of hydrophobic surfaces. To overcome those issues, we propose to investigate the efficiency of metal-acetylacetonates vapor phase impregnation, with a particular focus on Co(acac)₂.
The effects of various processing parameters such as exposure time, pressure and temperature were particularly studied. From the hypothesis that the one-step CNTs growth is driven by the thermal decomposition of acetylene on Co-riched phases, the temperature, the time of reaction, the acetylene/hydrogen ratio and the H₂ annealing pre-step are critical parameters tailoring the Co-phases properties and their dynamic interactions with the short-chain hydrocarbons. The samples are deeply characterized to relate the gas-phase chemistry reaction to the density and the rate of nanotubes growth. CNTs and Co-phases are characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction (XRD).
In addition, the suitability of the gas-phase impregnation process to 3D substrates such as microparticles using fluidized bed vapor phase reactor is reported.

Carbon nanotubes (CNTs) are grown by thermal chemical vapor deposition, and their structure is studied as a function of the reduction and growth temperatures. During the reduction stage, the temperature must sufficient for oxygen to be removed from the catalyst layer by the reducing gas, and during the growth stage the temperature is used to crack the hydrocarbon feed gas to produce CNTs. Previously we demonstrated growth of straight, high crystalline quality CNTs on 2.5 nm thick Ni catalyst on W-coated Si (100) from 350 °C to 600 °C with a constant CO reduction at 600 °C. The highest temperature seen during the process appears to control the inner core diameter, which is ~ 1 nm for the above mentioned CNTs. This relationship is further explored in this work by lowering both the reduction and growth temperatures so that the highest temperature seen by the catalyst can be independently set. The resulting inner and outer wall diameters, number of walls, and crystalline quality are studied by high resolution transmission electron microscopy, and Raman spectroscopy is used to identify the chirality of single walled carbon nanotubes and the representative C-C bonding of multi-walled carbon nanotubes. The effects of the reduction and growth temperatures on CNT overall growth and structure are studied with different reduction gases including CO and NH3 and on different substrates including silicon and sapphire. The results are relevant to applications in which CNTs with specific properties are desired on substrates which cannot withstand the high temperatures normally required for high quality CNT growth.
This work is supported by the Laboratory Directed Research and Development Program at Sandia National Laboratories. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy&’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Growing graphene on a metal surface is one possible way to obtain a high quality graphene, with a controllable number of layers. The synthesis usually relies on a chemical vapor deposition of a carbon bearing gas on the surface of a metal such as Ir, Cu, or Ni. We investigate the latter case of graphene on Ni that is of particular interest because the role of carbon solubility in subsurface layers is both difficult to investigate experimentally and important to understand for the production of high quality graphene.
To study the interaction of carbon with nickel at the atomic level, we have developed a tight binding model [1] implemented in a Monte Carlo code. It has been used to study the nucleation of carbon nanotubes in CVD processes [2]. Using the same Grand Canonical algorithm as used in [1,3], but with significantly larger systems and better accuracy, we investigate the CVD synthesis of graphene on a Ni surface. Depending on the growth conditions, we show that variable amounts of C can be found in the subsurface layers and we correlate this to experimental data. Since the obtained graphene-like layer covering the Ni surface often presents defects (pentagons, heptagons, vacancies, hellip;), we also numerically study the healing mechanisms of these defects that are made more efficient in the presence of the metal surface layer [4]
[1] H. Amara, J.-M. Roussel, C. Bichara, J.-P. Gaspard and F, Phys. Rev. B 79, 014109 (2009)
[2] M.-F. C. Fiawoo, A.-M. Bonnot, H. Amara, C. Bichara, J. Thibault-Pénisson and A. Loiseau, Phys. Rev. Lett. 108, 195503 (2012)
[3] H. Amara, C. Bichara and F. Ducastelle, Phys. Rev. B 73, 113404 (2006).
[4] S. Karoui, H. Amara, C. Bichara and F. Ducastelle, ACS Nano 4, 6114 (2010)

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[5].
[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).
[5] M. Diarra, A. Zappelli, H. Amara, F. Ducastelle and C. Bichara, Phys. Rev. Lett. (accepted)

We report on the synthesis of graphene by means of molecular beam epitaxy (MBE). This technique is widely used in semiconductor device research to produce high-quality layers on different substrates at moderate temperatures (<1000 °C). The growth by MBE does not involve catalytic processes and could in principle be extended to a large variety of substrates. In addition, one of the main advantages of MBE is that it offers the benefit of accurate deposition rates and sub-monolayer thickness precision. Therefore, MBE might enable the growth of large area graphene films (mono- and few-layer) directly on different insulating or semiconducting substrates, which is of high technological relevance for many applications. We investigated the growth of graphene on a (6radic;3 × 6radic;3)R30° reconstructed SiC(0001) surface. This surface reconstruction, also known as buffer layer, is isomorphic to graphene (i.e. it possesses the same crystal structure and similar lattice constant), however with about 30% of its atoms covalently bond to the SiC substrate. Its use as a template enables the investigation of quasi-homoepitaxy of graphene by MBE, with the advantage over the use of epitaxial graphene that the results from different analysis methods are not disturbed by contributions originated from the substrate. Graphene films have been prepared on the buffer layer at a substrate temperature of 950 °C for different growth times. The source used for atomic carbon evaporation is a high-purity pyrolytic graphite filament heated by electric current. For the utilized growth conditions no concomitant surface graphitization due to Si atoms depletion takes place. Atomic force microscopy shows that the initial SiC surface morphology with atomically smooth terraces and steps in between them is not drastically altered after MBE growth. Raman spectroscopy reveals that the quality of the MBE-grown graphene films increases with growth time and that the average crystallite size exceeds 20 nm. X-ray photoelectron spectroscopy confirms that the thickness of the films increases as a function of the growth time and additionally proves that the buffer layer is preserved during the growth process. Grazing-incidence X-ray diffraction measurements were performed at the beamline ID10 of the European Synchrotron Radiation Facility in Grenoble. Signals from the buffer layer, as well as from the MBE-synthesized graphene, were detected. Interestingly, despite its nanocrystalline nature, the graphene films grown by MBE shows an in-plane alignment to the substrate, revealing that a conventional epitaxial growth on the buffer layer takes place. The results will be discussed in the context of MBE growth of graphene considering the most recent data reported in the literature.

Conjugated polymers have been proven to be able to selectively disperse semiconducting single walled carbon nanotubes from metallic tubes. The separation using this technique has as main advantage that the physical properties of the nanotubes can be maintained due to the non-covalent interaction between the polymer chains and the carbon nanotube walls. The current state of art of this technique only allows the separation of small diameter tubes (0.8-1.2 nm). Meanwhile, larger diameter tubes, which have more prospects for high performance optoelectronic device applications still cannot be separated using this technique. Here we demonstrated the selection of small and large diameter semiconducting SWNTs (0.8 to 1.6 nm) with very effective individualization by using polyfluorene-derivatives. Polyfluorene with long alkyl side-chains showed the ability to discriminate larger diameter nanotubes. Optical spectroscopy including photoexcitation lifetime measurements allows establishing the high quality of the SWNT dispersion in term of minimal metallic content and lower nanotubes bundling.
Molecular dynamics simulations allow rationalizing the nature and mechanism of the interaction between the polymer chains and SWNTs. The simulation demonstrated that the increasing of length of the polymer side-chains increases the coverage wrapping area and the binding energy between the polymer and the carbon nanotube walls. Using these highly enriched semiconducting nanotubes dispersions, we fabricated high-performance network field effect transistors. The field effect transistors showed ambipolar properties with charge-carrier mobility higher than 10 cm²/V.s and on/off ratio more than 10⁵ by using ion gel as the gate dielectric layer. Our finding offers a new approach to separate big diameter semiconducting carbon nanotubes allowing their use for high performance device applications.

Here we report real-time Raman spectroscopy, optical imaging, and reflectivity, combined with sub-second pulses of acetylene to measure the nucleation and growth kinetics of graphene on Ni films by pulsed chemical vapor deposition (CVD) and pulsed laser deposition (PLD). Despite many publications devoted to graphene growth by chemical vapor deposition (CVD) on different metals several major questions remain to be addressed. For example, in the case of metals which can dissolve carbon the contribution of surface growth versus precipitation during cool down is not clear. Also, crucial growth kinetics measurements required to understand the growth mechanisms have not been studied in real time, especially in the case of fast growth. Using pulsed gas delivery and real time optical measurements we demonstrate that a few layer, high quality graphene can be grown rapidly and isothermally at high temperatures. Using direct in situ Raman monitoring of the growth kinetics with 1s acquisition time at high temperatures (between 800-850 °C) graphene is found to grow within one second on Ni after exposure to a sub-second C2H2 pulse. Real-time Raman measurements were performed to reveal the fraction of graphene which appears during cool down as a function of the growth temperature and partial feedstock gas pressure. In addition, time-resolved reflectivity and direct video-imaging through a microscope were performed for comparison with the Raman kinetics measurements, revealing a 0.5 s delay in the onset of growth after the gas pulse. The combined approach involving pulsed feedstock delivery and real-time optical diagnostics opens new opportunities to understand and control the fast, sub-second growth of graphene on various substrates at high temperatures.
Research supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Science and Engineering Division, and performed in part at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy.

Water-assisted chemical vapor deposition (CVD) has become a standard synthesis method for high quality single-walled carbon nanotubes (SWCNTs). Some drawbacks of the water-assisted method, however, include good control of water concentrations in the feedstock and poor control of SWCNT diameters below 2.0 nm. Here, we describe a variation of water-assisted CVD that uses dry feedstocks with a small, controlled quantity of molecular oxygen. Reactions of oxygen with hydrogen in the reaction zone provide all the benefits of water-assisted growth at the substrate while maintaining dry valves and flowmeters. In addition, the oxygen-based technique allows water concentrations in the system to be varied precisely and with short time constants. Perhaps because of the improved control, we find that the SWCNT diameter can be easily tuned by changing the oxygen concentration during the growth phase. Changing the oxygen flow rate from 25 sccm to 60 sccm (in 5000 sccm of other feedstocks) varied the resulting SWCNT diameters from 1.5 to 0.5 nm, with typical diameter distributions less than +/- 30%. Control of SWCNT growth within this diameter range is ideal for probing opto-electronic properties of individual SWCNTs and SWCNT devices.

The use of graphene has the potential to improve the performance of a diverse range of advanced nanodevices, in applications ranging from nanoelectronics to ultra-sensitive environmental monitoring. Incorporation of graphene into devices requires the development of a large-scale method of fabrication of high-quality graphene, which remains a significant challenge despite years of effort. The method that shows the most promise is chemical vapor deposition (CVD), which, however, requires a high growth temperature (generally over 1100 K). Our work focuses on the development of a plasma-enhanced CVD approach to fabricate high-quality graphene at a low substrate temperature (as low as 525 K) without any external heating source. We have successfully deposited single-layer graphene on a flexible polycrystalline copper foil using an inductively-coupled plasma. Micro-Raman spectroscopy indicates that the films have thicknesses of up to three layers of graphene. Single-layer graphene crystals in the film were confirmed by imaging the graphene crystal lattice using high-resolution TEM. Further investigations revealed that the graphene grain size can be altered by controlling the delivery of carbon to the substrate surface. Graphene films transferred from copper foil to quartz exhibit transparency and resistance of ~80% and 8 kOmega;, respectively. The films were tested for their biosensing properties using a standard protein, bovine serum albumin. The method of fabrication of graphene at low temperature presented in this work has marked advantages over previous growth processes, and brings the production of graphene-based devices closer.

Large-scale monolayer graphene has been uniformly grown on a Cu surface at elevated temperatures by thermally processing a poly(methyl methacrylate) (PMMA) film in a rapid thermal annealing (RTA) system under vacuum. The detailed chemistry of transition from solid-state carbon to graphene on the catalytic Cu surface was investigated by performing in-situ residual gas analysis while PMMA/Cu-foil samples being heated, in conjunction with Raman spectroscopy after thermal processing. The data clearly show that the formation of graphene occurs with hydrocarbon molecules vaporized from PMMA, such as methane and/or methyl radicals, as precursors rather than by the direct graphitization of solid-state carbon. We also found that the temperature for vaporizing hydrocarbon molecules from PMMA and the time for maintaining the hydrocarbon gaseous atmosphere, which are dependent on both the heating temperature profile and the amount of a solid carbon feedstock are the dominant factors to determine the crystalline quality of the resulting graphene film. Under optimal growth conditions, the PMMA-derived graphene was found to have the carrier (hole) mobility as high as ~2,700 cm2V-1s-1 at room temperature, superior to common graphene converted from solid carbon.

Graphene is a major topic of research due to its structure, electronic, chemical, optical and mechanical properties [1]. The high electrical conductivity as well as the chemical stability in aggressive media of graphene makes it a candidate as electrode in electrochemical applications. We investigated the stability of single graphene layers (SGL) by in-situ NEXAFS at the Advance Light Source.
We used SGL grown on copper foil by CVD using methane [2] and transferred onto the back side of thin (~100nm) Si3N4 membranes transparent to the X-rays. The membranes are part of the electrochemical cell and separate the cell from the vacuum chamber connected to the Synchrotron ring. Pt and Ag wires were inserted in the cell body as counter and pseudo-reference electrodes. A 2 mM electrolyte solution flowed through inlet and outlet holes and the potential was controlled with a potentiostat. Using this experimental setup, it was possible to achieve real time monitoring of the element-specific electronic structure changes during electrochemical cycling.
X-ray absorption spectroscopy (XAS) from the graphene electrode material was performed in situ during cyclic voltammetry. The shape of the C K-level peak is sensitive to the chemical structure of the absorbing atom. Our spectra proved that at positive potentials formation of carboxyl COOH groups and CO is favoured, while at negative potentials hydroxyl groups C-OH are dominant. The electrochemical potential induces also the formation of point defects at potentials below 0.5 V, and fast degradation at higher potentials, as demonstrated by ex-situ studies with AFM. The formation of numerous point defects was also corroborated by Raman measurements of the D/G peak ratio. These observations are consistent with the progressive loss of graphitic character [4].
References:
[1] Y. Zhu, S. Murali, W. Cai, X. Li, J.W. Suk, J.R. Potts, and R.S. Ruoff, “Graphene and Graphene Oxide: Synthesis, Properties, and Applications”, Advanced Materials, Vol. 22, Iss. 35, pp. 3906-2924, 2010.
[2] A. Ismach, C. Druzgalski, S. Penwell, A. Schwartzberg, M. Zheng, A. Javey, J. Bokor, and Y. Zhang, “Direct Chemical Vapor Deposition of Graphene on Dielectric Surfaces”, Nano Letters, 10(5), pp. 1542-1548, 2010.
[3] P. Jiang, J.L. Chen , F. Borondics, P.A. Glans, M.W. West, C.L. Chang, M. Salmeron, J. Guo, “In situ soft X-ray absorption spectroscopy investigation of electrochemical corrosion
of copper in aqueous NaHCO3 solution”, Electrochemistry Communications 12, pp. 820-822, 2010.
[4] S. Stasio, and A. Braun, “Comparative NEXAFS Study on Soot Obtained from an Ethylene/Air Flame, a Diesel Engine, and Graphite”, Energy Fuels, 20 (1), pp. 187-194, 2006.

Several innovative techniques have been employed for the new generations of magnetoresistance (MR) devices including spin valves at heterojunctions (GMR), metal-insulator transitions driven by magnetic fields (CMR) and geometrical inclusions (EMR). We developed a very simple but effective method to fabricate large positive MR devices based on single layer graphene (SLG): an ultrathin layer of metal atoms is deposit on top of SLG and followed by oxidation on hotplate for 15 minutes. These SLG devices are promising candidates for magnetic sensors, which exhibit large positive MR up to 200% for B=2T and 600% for B=8T at cryogenic temperatures. Graphene&’s extraordinary electronic properties such as high mobility (~20000 cm2/Vs) and Landau level quantization have been well preserved even capped with these oxide layers. Strong temperature sensitivity and gate-voltage tunability are the two main features of these devices. Positive MR behavior maintains up to room temperature but with a smaller magnitude about 300% for B=8T. On the other hand, a quadratic dependence of the resistivity R on magnetic field B is clearly observed at high carrier density where graphene performs similar to ordinary metals; however R depends super-linearly on B at charge neutrality point (CNP) which mainly originates from massless Dirac-fermions in graphene. All MR behaviors at various temperatures and gate voltages can be well interpreted by the combination of classical PL model and the quantum model. The large magnitude of positive MR with flexible controlled parameters (i.e. temperatures and carrier density) suggests great potential for novel applications such as magnetic sensors and ultrahigh density memory storage.

Graphene/ferroelectric hybrid structures - while showing exciting performance in terms of high mobility and the accessibility of distinguishable, nonvolatile resistance states - have not yet shown their full potential: the direct control of the carrier density and type in graphene through variation of the ferroelectric polarization. This control could provide new means of manipulating electron transport in graphene through complex ferroelectric gate structures. However, hysteresis effects observed in previous studies of graphene on ferroelectric oxide thin films have been attributed to effects other than the ferroelectric polarization, presumably the charging of interface states or charge redistribution facilitated through polar molecules at the interface. Here, we will discuss graphene/PbZr_0.2Ti_0.8O₃ (PZT) hybrid structures that exhibit bidirectional interdependency between the graphene doping level and the ferroelectric polarization. This work was enabled by the development of a one-touch transfer process that allows for direct transfer of high-quality CVD grown graphene onto the ferroelectric film surface to ensure beneficial interfacial properties. Furthermore, as part of this work, we will discuss complete current-voltage and ferroelectric studies of graphene-contacted ferroelectric capacitors including evidence of symmetric current-voltage response and reversible ferroelectric polarization switching of epitaxial PZT thin films. Additionally we have completed detailed current-voltage studies of over 75 ferroelectric-gated graphene transistors and have observed that the manipulation of the polarization state of the ferroelectric gate oxide impacts both the carrier type and the density of carriers in the graphene. Additionally, we have established routes and gained insight into environmental factors that impact the process to the point where we can manipulate the carrier type from p- to n-type or even intrinsic as a response to the switching of the ferroelectric polarization. Detailed pulse-width- and time-dependent measurements have been used to separate the role of ferroelectric polarization effects from extrinsic charging or charge redistribution effects at the interface. This work is key to understanding prior studies and routes to utilization of ferroelectric-gated graphene for potential devices. Finally, we will also discuss our efforts on two-dimensional Raman mapping of graphene on ferroelectrics and photocurrent response in graphene devices on diverse ferroelectric domain structures.

Single-layer graphene can be easily located on the surface of a Silicon wafer with 300nm SiO₂ using optical microscopy. This may not be possible with other substrates of varying thicknesses. We have developed a technique for wet-etching the SiO₂, peeling the device from the surface, and transferring it to any arbitrary substrate. The device has electrical leads already patterned onto the flake via Electron Beam Lithography (EBL) and metal is deposited for good electrical contact. This technique eliminates the need to locate the graphene flake on the target substrate and align to it for patterning using EBL. The completed device is then ready for any electrical measurements and a direct comparison can be made between the electrical transport properties of graphene on SiO₂ and the target substrate. A device has been transferred from SiO₂ to another SiO₂ surface with no change in Dirac point position and only a small decrease in carrier mobility. This confirms that the transfer technique does not significantly degrade the quality the graphene flake. A graphene device has been transferred to a 250nm thick layer of Strontium Titanate (STO) that has been grown epitaxially on Nb-doped STO via Pulsed Laser Deposition (PLD). The STO layer, with a higher dielectric constant than SiO₂ (approximately 2 orders of magnitude at room temperature) has a higher capacitance and produces a more effective graphene FET. The Dirac point can be swept through with a much smaller gate voltage range. A higher carrier mobility is expected with a graphene device on the surface of a material with a higher dielectric constant if charged impurity scattering is a primary limiting factor. Graphene devices transferred to 200mu;m thick STO substrates display a qualitatively similar behavior. Possible reasons for the absence of the high dielectric substrate effect on graphene carrier mobility will be discussed.

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 via cross-linking reactions between single- and/or few-layer graphene oxide sheets in suspension. The hierarchical structure possesses a number of novel properties including mechanical stiffness (up to 10 GPa) and electrical conductivities (up to 10⁵ 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 (up to 5 cm³/g). Energy storage behavior for capacitor and Li-ion battery applications were evaluated. The graphene-based electrode simultaneously exhibited high energy (~10² Wh/kg) and power densities (~10² kW/kg) in aqueous electrolytes (symmetric cell), while metal oxide-coated graphene electrodes exhibited large Li-ion capacities (~1000mAh/g). The details of the synthesis and characterization of these novel graphene assemblies 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.

By layer-by-layer stacking of various two-dimensional atomic crystals, it is possible to form multilayer heterostructures with designed electronic properties. Double-layer graphene, especially, has attracted considerable interest because their electronic, optical, and mechanical properties provide useful characteristics not obtainable in single layer graphene. A well-known example is opening tunable band gaps in Bernal-stacked bilayer graphene by applying transverse electric field, which makes them promising candidates for nanoelectronic devices. Even though these properties of double-layer graphene are radically varied depending upon their structures, including the stacking order, rotation-angle and defects, their microstructural characteristics and properties have not yet been studied in depth.
We present quick, accurate and large-area mapping of microstructures in double-layer graphene, such as grains, defects and stacking rotation-angles. We use dark-field transmission electron microscopy (DF-TEM) combining with scanning electron diffraction with a nanoparallel electron beam, as well as high-resolution TEM in Cs corrected Titan Cube operated at a low kV. Grains and defects in double-layer graphene are identified and visualized, because DF-TEM images are sensitive to the alignment between the electron diffraction angle and the crystal orientation. Our comprehensive TEM study provides crucial structural information of double-layer graphene, which is sincerely demanded for their future development in optoelectronic and nanoelectronic devices.

P-N junction is a fundamental building block in the modern electronic circuits. Previously, we have fabricated graphene p-n junctions by a one step inhomogeneously thickness-dependent surface treatment of mono-/bilayer graphene sheets. Here, in this presentation, we systemically study the attributes across the graphene p-n junction interface by means of Kelvin probe force microscopy (KPFM) and transport measurements. Interestingly, we find the junction behaves distinctly for different surface treatments. Through organic surface charge transfer doping, the junction is sharply transformed from p-type (mono-layer) region to n-type (bi-layer) region in terms of surface potential distribution. The total channel resistance fits well with the sum of two individual GFETs. Although the rectification effect is not observed due to the absence of additional resistance in the junction, the output characteristic presents an abnormal negative differential resistance (NDR) behavior, whose origin and its deep impact to RF application will be discussed elsewhere. Here we find that, in the case of covalent bond doping, i.e. by oxygen plasma irradiation, the surface potential transition regions of the junction span up to several hundred nanometers. And the rectification effect is clearly observed in the two terminal measurements.
We ascribe the distinct behaviors to the carrier density variation in different graphene systems; namely, under the condition of charge transfer doping, since the graphene basal plane is largely reserved, the relatively large carrier density results in a metal-to-metal like junction, and an equivalent series resistance. However, in the case of the oxygen plasma irradiation, the trap states limit the carrier density and therefore a long space charge region is formed, which produces a metal-semiconductor-like junction. Our results not only unveil the detailed properties of graphene p-n junction interface, but also gain an insight into its great potential in nano-electronic applications.

The vanishing band gap of graphene severely restricts application in electronic and optoelectronic devices. Recently, graphene antidot lattices (GALs) have been suggested as a means of creating sizeable gaps. These structures are based on either periodic arrays of perforations [1,2] or patterned adsorption of hydrogen [3]. The properties of both types of GALs are analyzed based on atomistic simulations (tight-binding, DFT and DFT based tight-binding) as well as continuum approaches. The influence of superlattice geometry on band gap is discussed and simple scaling laws are explained. Also, optical, magnetic and transport properties will be presented. Calculations are compared to recent experiments and the feasibility of GAL based electronic and optoelectronic devices is discussed.
1. T. Garm Pedersen, C. Flindt, J. Pedersen, A-P. Jauho, N.A. Mortensen and K. Pedersen “Graphene antidot lattices - designed defects and spin qubits”, Phys. Rev. Lett. 100, 136804 (2008).
2. J. A. Fürst, J.G. Pedersen, C. Flindt, N.A. Mortensen, M. Brandbyge, T. Garm Pedersen, and A-P. Jauho “Electronic structure of graphene antidot lattices”, New J. Phys. 11, 095020 (2009).
3. R. Balog, B. Joslash;rgensen, L. Nilsson, M. Andersen, E. Rienks, M. Bianchi, M. Fanetti, E. Laelig;gsgaard, A. Baraldi, S. Lizzit, Z. Sljivancanin, F. Besenbacher, B. Hammer, T. Garm Pedersen, P. Hofmann, and L. Hornekaelig;r, “Band Gap Opening in Graphene Induced by Patterned Hydrogen Adsorption”, Nature Materials 9, 315 (2010).

One of the most promising applications for graphene is to enable next generation nanoelectronics. This requires a CMOS compatible wafer-scale synthesis, characterization and transfer of graphene film, which remain quite challenging. Previously, we had synthesized high-quality monolayer graphene on hydrogen-rich Cu (111) film. In this study, we will report our recent progress to address outstanding challenges in wafer-scale characterization, transfer of graphene and device fabrication with enhanced performance.
Raman mapping and statistical analysis were performed using our developed software (GRISP available on nanohub), indicating a high uniformity (>97% coverage) of monolayer graphene with immeasurable defects (>95% defect-negligible) across a 100-mm wafer. The transfer of wafer-scale graphene is not straight forward because: 1) direct etching of copper in ammonium persulfate (APS) at this scale typically results in corrugated film stack of polymer coated graphene before release; 2) two-step etching process demonstrated before is time inefficient (>24 hrs) even with highest concentration of buffered oxide etcher (BOE). We found out that a small addition of BOE in APS could release wafer-scale polymer coated graphene film without corrugation and in a substantially shorter time (<8 hrs). Raman inspection of transferred graphene film showed uniform monolayer obtained on standard 285 nm SiO₂/Si.
Over 40,000 graphene field effect devices (GFETs) were then fabricated on a 100-mm wafer covering an area of 70 ×70 mm², including 2- and 4- point probes for various electrical characterizations using back-gate. Typical Id-Vg curve of as fabricated GFETs exhibited severely positive Dirac voltage and asymmetric hole/electron transport with poor electron mobility. After applying a 15 nm thick passivation layer of H_fO₂ with atomic layer deposition, the Dirac point was back-shifted with improved symmetry for both hole and electron branches. An extracted mobility of ~3,500 (up to 5,700) cm²/Vs was observed under ambient conditions. Similar to fluoropolymer passivation yielding improved transistor performance, our study reveals that appropriate dielectric capping can restore the intrinsic properties of graphene that is often degraded during the transfer process as other literatures also observed.
This study demonstrates recent progress in wafer-scale Raman inspection, improved transfer of graphene as well as 104-scale fabrication of GFETs with enhanced performance via dielectric passivation, ensuring key essentials to very large-scale integrated systems.

Topological defects strongly influence the mechanical, electronic and even magnetic properties of low dimensional carbon-based systems. Taking advantage of the key role of defects in these systems, a unique route based on defect engineering is being developed to broaden the functionalities of graphene. In particular, vacancy-type defects are of an extraordinary importance as they are the key ingredient to understand the new properties shown by functionalized graphene after irradiation. While the role played by these vacancies as single entities has been extensively addressed by theory, experimental data available only refer to statistical properties of the whole heterogeneous collection of vacancies generated in the irradiation process. Scanning tunneling microscopy (STM) has great potential in this arena since it enables characterization of point defects at the atomic level.
In our work, we first created well characterized individual vacancies on graphene layers by Ar+ ion irradiation and then, using low temperature scanning tunneling microscopy and spectroscopy (UHV-LT-STM/STS), we individually investigated the impact of each type of such vacancies on the electronic, structural and magnetic properties of several graphene systems [1-3].
[1] M. M. Ugeda, I. Brihuega, F. Guinea and J. M. Goacute;mez-Rodríguez, Phys. Rev.
Lett 104, 096804 (2010).
[2] M. M. Ugeda, D. Fernández-Torre, I. Brihuega, P. Pou, A. J. Martínez- Galera,
R. Pérez and J. M. Goacute;mez-Rodríguez. Phys. Rev. Lett 107, 116803 (2011).
[3] M. M. Ugeda, I. Brihuega, F. Hiebel, P. Mallet, J. Y. Veuillen, J. M. Goacute;mez-
Rodríguez and F. Ynduráin Phys. Rev B,85, 121402 (R) (2012).

In this work, density functional theory
calculations are used to interpret new
X-ray photoelectron spectroscopy (XPS),
Infrared (IR) spectroscopy,
X-ray diffraction (XRD), and atomic force
microscope (AFM) measurements of the oxide
of epitaxial graphene. This layered carbon
material is obtained by Hummers oxidation
of 6- to 17-layer graphene films grown
epitaxially at high temperature
on a silicon carbide substrate.
Our measurements show that this form of
graphene oxide differs from the
conventional material obtained via the
standard oxidation, exfoliation, and
deposition method. In particular,
XRD shows that the film preserves the
layered structure and an excellent epitaxial
character, and shows interlayer distances
as large as 10 angstrom. XPS and IR
measurements concur in
showing a chemical structure based on
the presence of oxygen-functional groups
on the carbon basal planes and very little
amount of water molecules intercalated
in-between the layers. Further, AFM
measurements shows Young's moduli ranging
from 30 GPa up to 120 GPa. Extensive
density functional theory calculations
are carried out to address the complex
inverse problem posed by the aforementioned
measurements, attempting to elucidate the
molecular-scale feature of this novel
thin-film graphene-based material.
The calculations show that a most plausible
molecular structure for the oxide of
epitaxial graphene consists of mildly
oxidized graphene layers covalently bridged
by short polyoxymethylene chain.

Using in situ low-energy electron microscopy and density
functional theory, we studied the growth structure and work function
of bilayer graphene on Pd(111). Low-energy electron diffraction
analysis established that the two graphene layers have multiple
rotational orientations relative to each other and the substrate
plane. We observed heterogeneous nucleation and simultaneous growth
of multiple, faceted layers prior to the completion of second layer.
We propose that the facetted shapes are due to the zigzag-terminated
edges bounding graphene layers growing under the larger overlying
layers. We also found that the work functions of bilayer graphene
domains are higher than those of monolayer graphene, and depend
sensitively on the orientations of both layers with respect to the
substrate. Based on first-principles simulations, we attribute this
behavior to oppositely oriented electrostatic dipoles at the
graphene/Pd and graphene/graphene interfaces, whose strengths depend
on the orientations of the two graphene layers.

The discovery of graphene and the tremendous attention it received led to the discovery of graphene oxide (GO) that was obtained by exfoliating graphite oxide. To date, it is considered an easy method for large scale production of graphene. However, in the last few years, it has emerged as a “new old material” and has attracted tremendous attention from Material Scientists. This is largely because, GO provides an ideal platform to manipulate and control its chemical structure, optoelectronic properties and ionic conductivity for a wide range of applications for flexible electronics and optoelectronic devices such as photodetectors, photovoltaics, energy storage and biosensors. However, before its widespread incorporation for next generation applications, it is critical to understand the physical and electrical properties of GO that are highly dependent on the density and nature of functional groups on GO. Here, we have used electrostatic force microscopy (EFM) to inject and directly probe the migration of injected charge as the GO is progressively reduced to RGO. Our EFM results on GO flakes indicate that the injected charge is completely localized within the plane of GO. However, with increasing degree of reduction, the injected charge rapidly delocalizes over a few microns until it ends up at the edge of the flakes. The results suggest that as we go from GO to RGO, there are more percolating pathways of sp2 that are formed that act as conduits for charge migration. Our results are consistent with the quenching of fluorescence observed on individual flakes of GO measured as a function of increasing reduction of GO to RGO. We will combine EFM results with that of fluorescence imaging to monitor the preferential removal of each functional group on GO.

The growth mechanisms of chemical vapor deposited (CVD) graphene on copper are still not fully understood. Looking at the lattice orientations of a graphene sheet reveals a jigsaw of grain structure and grain boundaries. Maps of these features can give scientists clues as to what occurs during the synthesis of these two-dimensional structures.
In our study, we utilize a transmission electron microscope (TEM) to create orientation maps. The data for these maps are extracted through selected area diffraction (SAD) patterns created by the graphene lattice, allowing TEMs with electron energies above the graphene knock-on voltage to follow the method without damage to the structure. The rotation of the pattern correlates with the orientation of the graphene, thus measurement of the pattern&’s rotation obtains the lattice rotation. By combining the diffraction patterns taken across the whole area of the TEM sample, a map can be obtained. Increasing the number of diffraction patterns taken can increase the resolution of the maps and can also increase the size of the analyzed area to the millimeter scale. This results in a longer time to complete the process so the procedure was aided by custom software written to control the TEM for image acquisition and calculate the rotation of each pattern with minimal user interaction. The data is then plotted for visual representation.
The graphene samples that were mapped consist of a growth series that start as small, nucleated, cross-like islands that grow until their edges meet and the space between are filled in. The maps revealed that most of the islands are single-crystal with a few being bi-crystal which had lattices closely mis-oriented to each other by 30 degrees. The maps of the complete coverage samples expose the grain sizes and shapes. Statistics of the diffraction pattern rotations pulled from each sample and graphed show that the graphene lattice orientation has a bi-modal distribution preference on the copper&’s crystal substrate.
This process was also applied to double-layer graphene sheets transferred onto TEM grids. The SAD of these samples consisted of two diffraction spots, one set for each layer. The software was changed to determine and plot the mis-orientation between the two lattices. It also distinguished between areas of none, single-layer, and double-layer graphene.
With maps that can show the grain structure, lattice orientations and lattice mis-orientations, this method can become a tool for scientists to better understand the synthesis of graphene.
Acknowledgments: This work was supported by SWAN (GRC-NRI), AOARD-AFOSR (FA2385-10-1-4066) and the World Class University Program (by MEST through NRF (R31-10026)).

We study the effect of remote hydrogen plasma on graphene deposited on silicon oxide. We observe strong monolayer selectivity for reactions with plasma species, leading to isotropic
hole formation in the basal plane of monolayers and etching from the sheet edges. For few-
layer graphene and HOPG, we observe qualitatively different effects of hydrogen plasma, with
hexagonal etch pits that indicate that etching is highly anisotropic. The etch rate displays a
pronounced dependence on sample temperature for monolayer and multilayer graphene alike:
etching proceeds very slowly at room temperature, peaks at 400 °C and is suppressed entirely
at 700 °C. We observe that applying the same hydrogen plasma treatment to graphene de-
posited on the much smoother mica substrate leads to very similar phenomenology as on the
rougher silicon oxide, suggesting that a factor other than substrate roughness controls the reactivity of
monolayer graphene with hydrogen plasma species. We use hydrogen plasma treatment to produce graphene structures, such as nanoconstrictions and nanoscrolls, and investigate their properties using Raman spectroscopy, including tip-enhanced near-field Raman, as well as low-temperature transport measurements. The study suggests new directions for the controlled manipulation of graphene properties.

Graphene, a planar honeycomb lattice of close-packed carbon atoms, exhibits a linear electronic dispersion near Dirac K points where electrons behave as massless Dirac fermions [1]. In the case of double layer graphene, depending on relative orientation of the layers, interlayer interaction might lead to the electronic band structure splitting. The valence and conduction bands in a Bernal stacked graphene bilayer split into two parabolic branches near the K point. The splitting originates from the interaction of π electrons, leading to the opening of a band gap in bilayer graphene [2]. While a zero band gap limits the application of monolayer graphene FETs due to a low on/off ratio, Bernal stacked bilayer graphene with a tunable band gap is a promising candidate to improve efficiency of graphene FETs [3]. Deviation from Bernal stacked orientation is theoretically shown to affect band structure and consequently electronic properties. For industrial applications, the ability to map and control the misorientation angle between layers will be necessary to achieve the desirable electrical properties of the graphene devices. This work presents a method for systematically determining the misorientation between two layers on a wafer scale through the use of Raman spectroscopy. The validity of developed method and obtained misorientation maps will be confirmed by TEM study.
REFERENCES:
1. Y. Hao et al., Small 6 (2010)
2. L.M. Malarda et al., Physics Reports 473 (2009)
3. B. Fallahazad et al., Phsycal Review B 85 (2012)

Emerging as a promising and multifaceted material with fascinating electronic properties, such as high carrier mobility in the order of 105 cm2/Vs,[1] graphene has attracted an immense amount of interest from all related disciplines since the pioneering work of Novoselov et al. in 2004.[2] Fundamental knowledge of the physical properties of graphene and the physical mechanisms governing the electrical operation of graphene-based devices has grown dramatically.[3] With the recent success of large area chemical vapor deposition (CVD) growth of graphene,[4] industrial applications such as transparent electrodes, field effect transistors (FETs), and quantum well devices are becoming more promising. Surprisingly, there is little information on the intrinsic electronic band alignment of the graphene/oxide interface to date, despite its pivotal role in the design, fabrication, and characterization of graphene-based devices.
We report the first direct measurement of the Dirac point, the Fermi level, and the work function of graphene by performing internal photoemission measurements on a graphene/SiO2/Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO2/Si interfaces is established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO2 cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV ± 0.05 eV below the bottom of the SiO2 conduction band. When combined with the shift of the Fermi level from the Dirac point we are able to ascertain the position of the Dirac point at 3.6 eV ± 0.05 eV with respect to the bottom of the SiO2 conduction band edge, yielding a work function of 4.5 eV ± 0.05 eV which is in an excellent agreement with theory. The accurate definition of the work function of graphene is of significant importance to the engineering of graphene-base devices and the measurement technique we have demonstrated in this talk would find numerous applications in other 2-Dimensional material systems.
[1] Geim et al., Nature Materials 6 (3): 183-191.
[2] Novoselov et al., Science 2004, 306, 666-669.
[3] Das Sarma, et al., Rev. Mod. Phys. 2011, 83, 407-470.
[4] Kim, et al., Nature 2009, 457, 706-710.

A resonator is an essential component to modern electronics, most often used as a filter or an oscillator. One of the key requirements for a good resonator is high quality factor, which conventional electronic filters have failed to meet. Recently, stoichiometric silicon nitride resonators have become popular for their extremely high quality factor, which originates from the high stress they possess [1]. However, the insulating nature of the material has hindered its broader implementation. Attempts have been made to metalize the silicon nitride membrane by depositing thin layer of metal on top. However, it was found that such metal deposition results in degradation of the quality factor [2, 3] - by more than a factor of four when for only 5nm of chrome [2].
In this work, we show that graphene can be used as a conductive coating for SiN membranes without reducing the quality factor. We first demonstrate the fabrication of these Silicon Nitride - Graphene (SiNG) hetero-structure resonators. CVD graphene grown on copper foil is coated with PMMA on which PDMS is placed. After the copper etch, remaining graphene/PMMA/PDMS stack is placed on top of the silicon nitride resonators. Readily upon heating, PDMS delaminates, and final H2/Ar anneal removes the PMMA leaving only the silicon nitride and graphene.
Examining the quality factors of silicon nitride resonators with and without graphene on top using a piezo-drive - optical detection scheme, we have found that the quality factor degradation of silicon nitride resonator due to the added graphene layer is negligible, often within the margin of measurement errors. Furthermore, we have not only electrically actuated SiNG resonators using global silicon back-gate, but also tuned the resonators by changing the gate bias, while optical interferometry was used for detection. The highest quality factor measured on SiNG resonator using electrical drive was around 250,000 for the n1s1 mode of a 163µm diameter drum, while the tunability was about -2% per 10V. We have observed capacitive softening in the tuning curve, due to the highly stressed silicon nitride (~0.5%). We were also able to measure the displacement current-induced dissipation and found good agreement with an earlier study [4].
[1] “A megahertz nanomechanical resonator with room temperature quality factor over a million” S. S. Verbridge, et al., APL., 2008
[2] “Nanomechancial resonant structures in silicon nitride: fabrication, operation and dissipation issues”, L. Sekaric, et al., Sens. Actuators, A, 2002
[3] “Control of Material Damping in High-Q Membrane Microresonators”, P.-L. Yu, et al., PRL., 2012
[4] “Stamp Transferred Suspended Graphene Mechanical Resonators for Radio Frequency Electrical Readout”, X. Song, et al., Nano Lett., 2012

A new graphene transfer method using 'crystalbond' was devised by thermal recipes for bonding and releasing. Graphenes could be transferred to any specific location of the substrate in a large scale more simply and faster than the previous methods such as a scotch-tape method or the PMMA transfer method.
The transferred graphene was characterized by Raman spectroscopy and optical microscope image, confirming the good quality of the graphene during the transfer processes. Dirac points of transferred graphene were identified and characterized in a configuration of field effect transistor, indicating the good electrical property of the transferred graphene. The easy processes enable successive thermal stampings for local transfers of selected region of the graphene layer.

It is essentially important to develop suitable graphene patterning process for future industrial applications. Especially, transfer or patterning method of CVD-grown graphene has been studied. we eport simple soft lithographic process to develop easily applicable patterning method of large-scale graphene sheets by using chemically functionalized polymer stamp. Also important applications, capacitors with graphene electrode and commercial polymer dielectrics for the electrostatic-type touch panel are fabricated using the developed soft lithographic patterning and transfer process.

The electrochemical supercapacitor (SC) has received increasing attentions due to its high power density (10 kW kg-1), short charge/discharge duration (in seconds), long cycle life (over a million cycles), and environmental friendliness. Recently, boron-doped carbon has been researched as supercapacitor electrode materials because of the high specific surface area of carbon and the modified electronic structure which would result in the improved electrical double layer capacitance and pseudocapacitance by doping boron.
In this study, multi-walled carbon nanotubes (MWCNT) was doped with different concentrations of boron precursor, i.e., 0.5%, 2%, 10%, and 20%, by a chemical method. The bare MWCNT and boron-doped MWCNT with various concentrations were proposed as the materials for SC applications. The SC with 2% boron-doped MWCNT shows better specific capacitance and charge/discharge ability as compared with other cases. The specific capacitances of 9.75, 16.93, 66.49, 38.30, and 31.8 F/g were obtained from cyclic voltammetry at a scan rate of 5 mV/s for the SC with bare MWCNT, 0.5%, 2%, 10%, and 20% boron-doped MWCNT, respectively. Moreover, the SC electrode with 2% boron-doped MWCNT exhibits good stability and the capacitance retains even after 1,000 galvanostatic charge/discharge cyclings, suggesting its potential as the SC material.

Graphene, with a single layer plane structure, has amazing electrical and mechanical property, such as high electro conductibility, flexibility, stiffness. This unique structure has rendered graphene highly promising for various applications in energy storage, electronic device. To further explore new functions for graphene, the construction of macroscopic architectures using graphene as the building block has been well performed. 0-dimensional, two-dimensional and three-dimensional macrostructure have been constructed via vacuum filtration, layer-by-layer (LBL) assembly and chemical vapor deposition (CVD). Recently, great effort has been focused on porous graphene materials, Taking advantages of both the graphene composition and the pore structures, porous graphene materials have great application potentials in various fields. Three-dimensional porous graphene materials with high surface area have been prepared by LBL assembly. However, the design and preparation of graphene/ microporous structure is still empty.
In this paper, a porous graphene oxide/polystyrene (GO/PS) was designed and prepared as follows: At first, a molecularbrush of PS was prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) from GO surface. Then, the GO/PS plane molecularbrush obtained was crossrlinked by carbon tetrachloride and a graphene oxide supported-microporous polystyrene was obtained. The SI-ATRP and crosslinking conditions were optimized in this work. The structures of the molecularbrush of PS and the related crosslinking GO/PS were determined by FTIR, TG, SEM and nitrogen adsorption-desorption analysis. The experimental results showed that PS molecularbrush were successfully grown on to the surface of GO. After crosslinking, the PS component was crosslinked into many round nanoparticles with a diameter of 20-30 nm, and therefore the specific surface area of GO/PS obviously increased. This kind of porous GO/PS composite was promising for the application in adsorption-desorption energy storage areas.

We report a new approach for the fast and reproducible patterning of high-resolution reduced graphene oxide (rGO) microstructures over large area directly on rigid or flexible substrates. Such well-defined rGO micropatterns are created by modulating the surface energies of rGO thin films and pre-patterned elastomeric molds with the assistance of oxygen plasma. This technique offers the production of various shaped and sharp-edged rGO microstructures with a uniform thickness in the nanometer-scale. This plasma enhanced detachment patterning technique is extremely simple, effective, and easy to scale for large area rGO micropatterns without any pressurization and/or heating. The principle of our new patterning technique for the rGO microstructures is successfully explained based on the quantitative investigation of the work of adhesion between mold/rGO and rGO/substrate interfaces with the in-depth characterization of the high-resolution rGO micropatterns. As a demonstration of this patterning technique, we have successfully fabricated high performance top-contact organic field-effect transistors (OFETs) based on the rGO micropatterns as source-drain electrodes. Our patterning technique described in this manuscript shows great promise for making future graphene-based electronic devices

Graphene devices operated in ambient conditions are exposed to a variety of small molecules. These molecules act as dopant altering the electronic properties of graphene and interact strongly with defect sites.[1] For example on ruthenium, it was shown that line defects in graphene are extremely fragile toward chemical attack by water, which leads to splitting of the graphene film into numerous flakes, followed by water intercalation under the graphene.[2]
Here, we discuss the interaction of water molecules with epitaxial graphene on 6H-SiC(0001) based on low temperature scanning tunneling microscopy experiments. Epitaxial graphene on 6H-SiC(0001) allows us to compare water wetting with respect to the number of graphene layers. Distinct differences in the water adsorption structures of the interface layer compared to single and multiple graphene layers on SiC were observed. While the water clusters on graphene were influenced by the underlying graphene superstructure, larger water agglomerates were observed on the interface layer. Along defects, the graphene fractured occasionally into nanometer-sized flakes.
[1] F. Schedin et al. Nat. Mater., 6, 652-655 (2007).
[2] X. Feng, S. Maier, M. Salmeron, J. Am. Chem. Soc. 134 (12), 5662-5668, (2012).

Discussions of gauge fields on graphene are invariably given within continuum elasticity and there has been no discussion of when such approach breaks down. Expressions for gauge fields are then derived from a Hermitian electronic hamiltonian. This requirement translates into explicit conditions for atomic displacements that must be met at each unit cell. Following an approach where the atomic positions play the preponderant role, we develop an atomistic theory of strain on graphene, where all relevant quantities -including gauge fields- are directly expressed in terms of atomic displacements only. Using this approach we study suspended (rippled) graphene membranes under load by a sharp tip. In addition to the pseudo-magnetic field we also compute the deformation potential, which acts as an on-site potential energy. We observe that the deformation potential -neglected without proper justification in many published works- can modify the electronic spectrum dramatically in qualitative ways. Discussion of relevant experiments will also be given.

Chemical vapor deposition of graphene on copper is a promising method towards large area synthesis of high quality graphene. The growth of bi/few layer graphene can also be achieved on copper substrates and ontrol over coverage and stacking order can enable growth of AB versus AA stacked double layer graphene over large areas. Here we have investigated the stacking order of double layer graphene in correlation with growth conditions and the crystal orientations of copper. Toward this end, we have analyzed graphene grown on a variety of Cu crystal orientations [e.g. Cu(100), Cu(111), Cu(211), and Cu(310)] at different temperatures and methane/hydrogen gas pressure ratios. In-situ low-energy electron microscopy/diffraction (LEEM-LEED) characterization and Raman spectroscopy analysis reveal the presence of AA, AB stacking order as well as twisted bilayer graphene. Evidence for AA stacking has been also confirmed by micro- ARPES (angle resolved photoemission spectroscopy) performed on single and double layer graphene as well as by cross-sectional HRTEM imaging. Statistical distribution analysis shows that half of the double-layer graphene areas are AB stacked and correlation with the Cu crystal orientation is discussed.

Three-dimensional self-assembled graphene aerogels have been fabricated by mild chemical reduction method with L-ascorbic acid. The effect of graphene oxide concentrations, reduction agents and synthesis conditions (reduction temperature and reduction time) on the morphologies and electrical properties of the as-prepared graphene aerogels was systematically investigated. A simple but effective method controlling large specific surface area, pore volume, pore size and electrical conductivity of the graphene aerogels was achieved. After the annealing treatment at 400°C for 5h under Ar environment, the electrical properties of the developed graphene aerogels are significantly improved. The experimental results of this study are very useful for energy storage applications as the performance of graphene aerogel-based electrodes strongly depend on the morphology and electrical properties of the graphene aerogels.

We have studied, within density-functional theory in the LDA approximation, the structure of graphene placed on a Si(111) substrate to understand the atomic scale graphene-substrate interaction. We accomplished this using the SIESTA package on computational facilities at The University of Arkansas. Electronic wavefunctions are obtained from the code. We developed our own program to set an integration range and add up the densities for all points in a real-space mesh from each wavefunction at a dense k-point mesh. The program generates simulated STM images which are compared to experimental ones showing reasonable agreement.
This is an important study because of the widespread use of graphene and Si wafers in research and their potential use in large commercial scale operations. Understanding the electronic and topographic properties at this interface is important for integrating graphene into future nanoelectric devices.

Graphene has attracted extensive attention due to its exceptional thermal and electronic properties. Its gapless semi-metallic nature leads to significant electron-phonon (e-ph) interactions near the Dirac point at room temperature. Raman spectroscopy is typically used to characterize graphene in experiments and also to measure properties like thermal conductivity and optical phonon lifetime. The laser-irradiation processes underlying this measurement technique include coupling between photons, electrons and phonons. Recent experimental studies have shown that e-ph scattering limits the performance of graphene-based electronic devices due to the difference in their timescales of relaxation resulting in various bottleneck effects. Furthermore, recently published thermal conductivity measurements on graphene are sensitive to the laser spot size which indicates the existence of non-equilibrium between various phonon groups. These studies point to the need to study the spatially-resolved non-equilibrium between various energy carriers in graphene. In this work, we propose a diffusive multi temperature model which includes all significant energy carrier transport and interaction processes including diffusion, e-ph and ph-ph interactions, and apply the model to single layer graphene under laser irradiation. Electron cooling through phonon emission is computed from density functional theory (DFT) in terms of the net phonon generation rate, whereas the interactions between various phonon groups are modeled in terms of a relaxation time approximation using the parameters obtained from lattice dynamics (LD). Using our model, we obtained the spatially resolved temperature profiles of all relevant energy carriers throughout the entire domain; these are impossible to obtain through experiments. Our results clearly show the difference in temperature distribution of various carriers at steady state conditions within the hot spot. Our results indicate that experimental measurements could under-predict the lattice thermal conductivity if this non-equilibrium were disregarded.

A promising application of carbon nanostructures (CNS) - such as stacked cones, multi-walled, single-walled or entangled nanotubes, etc. - is in nanocomposites for high performance structural materials. One class of material suitable for nanocomposite production comprises CNS grown directly on micrometer-scale diameter glass or carbon fibers. Fibers coated with catalyst particles pass through a growth chamber and a forest of CNS forms on the surface of the fibers. Process parameters can be adjusted to produce CNS of different length, density and interconnectedness. These materials can be processed in the same way as conventional fiber-reinforced composites by infusing them with an epoxy matrix. Transmission electron microscopy (TEM) imaging is generally employed to obtain the structure and morphology of the CNS. However, statistical analysis of carbon nanotube “forests” can be challenging, especially when applications involve CNS which are not grown normal to the substrate fiber. The detailed arrangements - dispersion, clustering, bundling, networking, etc. - of the CNS within the composite can have a dramatic effect on the material&’s bulk properties.
In this study, two distinctly different networks of CNS were characterized: one was grown on carbon fibers, the other on glass fibers. The CNS grown on the carbon fiber substrates consisted predominantly of single, multi-walled nanotubes. The CNS grown on the glass fiber substrates had a tendency to form bundles of multi-walled carbon nanotubes. Detailed statistical analyses of the number of walls, tube diameters and the number and diameter of any CNS bundles formed were obtained from high resolution TEM. TEM tomography was then applied to further resolve the three-dimensional complexity and confirm the connectivity of the CNS. We show that bright-field images combined with tomography and energy-filtered TEM can be used to obtain information about the 3-D network of CNS in high-performance structural composites.

Low dimensional carbon nano-materials, such as two-dimensional graphene, one dimensional carbon nanotubes (CNTs), and zero dimensional C60, have made significant impacts in exploring novel properties, designing new functionalities, and enhancing performance of materials. Graphene is defined as one atomic layer of sp2 bonded carbon atoms arranged in a hexagonal lattice structure. CNTs are graphene sheets rolled-up into cylinders. Metal-free and well aligned CNTs and graphene are produced on SiC by surface thermal decomposition and graphitization at high temperatures under vacuum. However, the detailed atomic mechanisms of CNTs and graphene growths and their conversion are still subjects of investigation. Temperature effects on carbon nanostructural growth on SiC have been well studied, but the pressure effects are not well understood.
In this study, we have investigated the pressure effects on low dimensional carbon nanostructural growths on SiC surface at high temperatures. Generally, pressure reduction results in increasing surface graphitization rate of SiC, and conversion of lateral graphene to vertical CNTs structures. As the pressure is gradually reduced from ambient pressure to a few torr in Argon at 1700°C, the Raman scatterings at 292 and 388 cm-1 indicate the formation of single- walled carbon nanotubes (SWNTs) at a reduced pressure. Further, 2D bands shifts to low frequency at lower temperatures, indicating weak interactions between SiC lattices and carbon nanostructures on the surface. The kinetic behavior of SiC surface graphitization is analyzed. The study provides experimental evidences that graphene and CNTs growth and their interactions with SiC lattices could be kinetically controlled.

We present the characteristic electronic transport behavior of graphene field effect transistor (FET) devices due to electron irradiation with various electron dosages at room temperature and in high vacuum. Graphene FET devices were fabricated by mechanical exfoliation and transferred onto silicon wafers covered with a 285nm thick oxide. The pristine devices showed p-type doping behavior, and no hysteresis was measured after they were loaded in high vacuum. When electrons with energies ranging from 1keV to 30keV were irradiated on graphene, the electrical transport properties showed a hysteresis and the devices exhibited n-type behavior. It is known that carbon nanotube and graphene FET devices show hysteresis in ambient condition. Dipolar molecules (e.g. water) surrounding FET devices are considered as one of the causes of the hysteretic behavior because they generate trapped charge carrier states. The hysteresis is typically reduced or entirely disappears in vacuum. We find results in agreement with other groups that the hysteresis measured in ambient condition disappears in vacuum. Upon electron irradiation in high vacuum, however, we observed a reappearance of the hysteresis which was observed for extended times in high vacuum. In addition, we studied the dynamic behavior of a shift of the Charge Neutral Point (CNP) versus time. Immediately following irradiation, the CNP shifts to a negative value. As time progresses, it then increases slowly without reaching the value prior to irradiation. Furthermore, we will present the dependence of conductivity on irradiation dosage. When the electron dosage was comparable to typical values used in electron beam lithography, no significant reduction in electronic mobility was observed indicating that this level of irradiation is a benign processing technique for graphene FETs.

Epitaxial graphene on metal substrates has been demonstrated as a promising route for graphene synthesis. The graphene produced in this manner however is typically polycrystalline, with defects that can affect its properties. The impact of defects could be crucial when graphene interacts with adsorbed molecules due to their enhanced reactivity, so there is a need to understand the adsorption of environmentally abundant molecules, such as water and oxygen. Here we report a study of water adsorption on epitaxial graphene grown on Ru and Cu substrates using scanning tunneling microscopy (STM). We found that on Ru(0001), graphene defects are extremely fragile towards chemical attack by water, which splits the graphene film into numerous fragments at temperatures as low as 90 K, followed by water intercalation under the graphene. On Cu(111) water can also split graphene but far less effectively, indicating that the chemical nature of the substrate strongly affects the reactivity of the C-C bonds in epitaxial graphene.

Over the past few years, the physics of low dimensional electronic systems has been revolutionized by the discovery of materials with very unusual electronic structures. Among these, graphene has taken center stage due to its relativistic-like electron dynamics and potential applications in nanotechnology. Moreover, the recent discovery that hexagonal boron nitride (hBN) is a nearly-ideal substrate for high mobility graphene devices has enabled a new generation of quantum transport experiments in graphene. In this talk I will review our recent experiments on few layer graphene devices on hBN. In addition I'll review our optoelectronic experiments on graphene pn junctions, where we observe a hot-carrier mediated photothermoelectric intrinsic graphene optical response.

Graphene, a truly 2-dimensional gapless semiconductor, has been recognized as a revolutionary material for opto-electronic applications. The onset of interband transitions at optical frequencies, opens the possibility of ultrafast graphene-based photodetectors [1,2]. The low absolute absorption of graphene (2.3% of the incident light per layer) represents a limitation for the photocurrent efficiency [3]. Here we experimentally demonstrate a broadband enhanced light absorption in single- and multilayer graphene in the visible range of the electromagnetic spectrum. This enhanced absorption can reach values as large as 85% for a multilayer of 10 graphene sheets, and it is explained in the framework of the coherent absorption arising from interference and dissipation. The interference mechanism leading to the phenomenon of coherent absorption allows also for its precise control by varying the refractive index of the medium surrounding the graphene layers.
[1] F. Xia et al., Nature Nanotech., 4, 839 (2009)
[2] T. Mueller et al., Nature Photon., 4, 297 (2010)
[3] F. Bonaccorso et al., Nature Photon. 4, 611 (2010)

We report negative photoconductance along with ultra-high quantum gain of a completely new type of graphene-based photodetector. By interacting with photoexcited carriers from the adjacent silicon substrate, the photocurrent responsivity of graphene can exceed 10⁴ A/W at visible wavelengths (corresponding to quantum gain values exceeding 10⁴) with an external bias V=2V. The responsivity could be further tuned by adding a reverse-bias across the graphene/silicon junction or by modifying the device geometry. The devices can easily detect picoWatts incident power, and the fast transient response time (tens of milliseconds) makes them capable of many optoelectronic applications. Persistent photoconductance with response time of hours has also been observed under high incident light power (milliWatts), implying multiple mechanisms may cause this phenomenon. We will describe the fabrication and characterizations of the devices. The possible mechanisms of the negative photoconductance and ultra-high quantum gain will be discussed.

In this talk, we present an overview of our theoretical work on metamaterials and transformation optics using graphene. In [1] we showed that by deliberate control of graphene conductivity locally and inhomogeneously (which is possible by chemical doping and/or dc electric biasing of graphene); one can direct and manipulate surface plasmon polariton (SPP) surface waves within graphene. By creating inhomogeneous distribution of conductivity across the graphene layer, one may envision various optical elements across a platform that is only as thick as a carbon atom. We theoretically investigated prospect of several ultracompact plasmonic devices such as different types of ‘one-atom-thick&’ waveguiding elements [1] and functions such as Fourier transforming using ‘one-atom-thick&’ lenses [2].
In particular, we discuss our recent advances in this territory, including the design and synthesis of graphene-based cavities and scattering of SPPs from subwavelength graphene patches. We also address problem of coupling light to graphene SPPs and challenges faced in this regard. References: [1] A. Vakil, N. Engheta, Science 332, 1291 (2011); [2] A. Vakil and N. Engheta, Physical Review B 85, 075434 (2012).

Graphene, due to its atomically-thin nature, is an ideal translocation membrane for high resolution, high throughput, single-molecule DNA sequencing based on nanopores. Here, we report optofluidic graphene nanopores. We are able to create integrated nanoplasmonic pores on graphene membrane with optical antennae. Furthermore, we achieve tunability of both the nanopore dimensions and the optical characteristics of optical nanoantennae by controlling optical intensity and the dimension of nanoparticles. Finally, the optical function of our integrated plasmonic nanoantenna is manifested by multifold fluorescent signal enhancement during single lambda phage DNA translocation through a graphene nanopore. We believe our approach to forming an integrated optofluidic graphene nanopore with optical antenna could offer a new avenue and advances for nanopore-based simultaneous electrical and optical DNA sequencing in the future.

Graphene-based tunnel field effect transistors (TFETs) are under extensive study because of their predicted sub-60 mV/decade subthreshold swing, higher ON/OFF ratio compared to monolayer planar graphene FETs, etc. [1]. It has been theoretically shown that graphene TFETs can exhibit negative differential resistance (NDR), making these lucrative candidates for a variety of electronics applications [2]. However, NDR can be observed only when the two graphene layers are oriented such that their Dirac points are aligned perfectly [3]. An ON/OFF ratio of ~50 and ~10⁴ was obtained by Britnell et al., with h-BN and MoS₂, respectively, as the vertical tunnel barrier [1]. We have used conventional atomic layer deposited high-k dielectrics, Al₂O₃, TiO₂ and HfO₂ as vertical tunnel barriers to obtain ON/OFF ratio of 10⁷. We have obtained low subthreshold swing of ~100 mV/decade by engineering the tunnel barriers and the back gate dielectric. By using a stack of Al₂O₃/TiO₂ as our tunnel barrier, we minimize the equivalent oxide thickness (EOT) and off-state leakage simultaneously. We have also used a stack of SiO₂/HfO₂ as the back gate dielectric to control the graphene carrier concentration while keeping the EOT low. Thus, our transistors demonstrate the applicability of graphene in low-voltage logic applications. The drive current of these transistors are still low compared to the projected drive current metrics for end of the International Technology Roadmap for Semiconductors [4]. Temperature-dependent measurements of tunneling current are used to explain the nature of tunneling between the two graphene layers. Raman spectroscopy will be used to to map the misorientation angle of the two layers.
REFERENCES:
[1] L. Britnell et al., Science, 335 (2012)
[2] S.K. Banerjee et al., 70th Ann. DRC (2012)
[3] R. M. Feenstra et al., J. App. Phys., 111 (2012)
[4] International Technology Roadmap for Semiconductors, http://www.itrs.net/Links/2011ITRS/Home2011.htm (2011)

Recent interest in the development of flexible electronics operating at radio-frequencies (RF) requires materials that combine excellent electronic performance with the ability to withstand high levels of strain. Materials such as silicon nanomembranes (SiNMs), III-V metal-oxide-semiconductor thin-films and nanowires, indium-gallium-zinc-oxide, AlGaN/GaN heterostructures, carbon nanotubes (CNTs), and graphene have all been proposed for use in flexible RF electronics. However, enhancements to electronic performance have been achieved at the expense of device flexibility; to date, no flexible technology has achieved both unity-current-gain frequencies, f_T, and unity-power-gain frequencies, f_max, in the gigahertz regime at strains above 0.5%.
Graphene&’s unique electronic and mechanical properties make it a promising material for the fabrication of field-effect transistors (FETs) which require both high flexibility and high operating frequencies. While graphene has no band-gap, rendering it poorly suited for digital applications, its high carrier mobility, saturation velocity, and current-carrying capacity make it a promising candidate for high-frequency analog applications. Furthermore, the large-area films of graphene requisite for commercial fabrication of graphene-based technologies can be produced using low-cost, chemical vapor deposition (CVD) synthesis. Even on flexible substrates, FETs fabricated from CVD graphene films display excellent electronic properties and maintain stable DC characteristics to high levels of strain.
We fabricate graphene FETs (GFETs) on flexible, polyethylene naphthalate (PEN) substrates utilizing CVD graphene as the device channel material. Our GFETs demonstrate f_T and f_max up to 10.7 and 3.7 GHz, respectively, with strain limits of 1.75%. These devices represent the only reported technology to achieve gigahertz-frequency power gain at strain levels above 0.5%. As such, they demonstrate the potential of CVD graphene as a material to enable a broad range of flexible electronic technologies which require both high-flexibility and RF operation.

Graphene nanoribbons (GNRs) - narrow stripes of graphene [1] - are predicted to be semiconductors with an electronic band gap that sensitively depends on the ribbon width [2]. For armchair GNRs (AGNRs) the band gap is inversely proportional to the ribbon width. Furthermore, a quantum confinement-related periodic modulation of the band gap is added to the inverse scaling. This modulation has a period of three carbon dimers (ΔN=3) in ribbon width and becomes dominant for AGNRs narrower than ~3 nm [3]. This allows, in principle, for the design of GNR-based structures with specific and widely tunable electronic properties, but requires structuring with atomic precision. The inability to produce graphene nanostructures with the needed precision has so far hampered the experimental investigation of the electronic structure of narrow GNRs, which is in contrast to an impressive number of GNR-related computational studies.
In this presentation, I will report on our recently developed bottom-up approach to the fabrication of atomically precise GNRs [4]. It is based on a surface-assisted synthetic route using specifically designed precursor monomers, and has made available ultra-narrow GNRs and related graphene nanostructures for experimental investigations of their structural and electronic properties [4-10]. Using experimental methods such as scanning tunneling microscopy and spectroscopy, Raman and photoelectron spectroscopies, which are complemented by density functional theory calculations, we have studied the on-surface chemical reaction steps and the resulting graphene-related materials [4-10]. For the case of N=7 AGNRs, the electronic band gap and dispersion of the occupied electronic bands have been determined with high precision [10]. Finally, it will be shown that the surface-chemical route also gives access to substitutionally doped ribbons as well as heterostructures.
[1] A. K. Geim, Science, 324 (2009) 1530.
[2] V. Barone et al., Nano Lett., 6 (2006) 2748.
[3] L. Yang et al., Phys. Rev. Lett., 99, (2007) 186801.
[4] J. Cai et al., Nature, 466 (2010) 470.
[5] M. Bieri et al., Chem. Commun., (2009) 6919.
[6] M. Bieri et al., J. Am. Chem. Soc., 132 (2010) 16669.
[7] S. Blankenburg et al., Small, 6 (2010) 2266.
[8] M. Treier et al., Nature Chemistry, 3 (2011) 61.
[9] S. Blankenburg et al., ACS Nano, 6 (2012) 2020.
[10] P. Ruffieux et al., ACS Nano, 6 (2012) 6930.

In recent years graphene has attracted tremendous interest within the scientific community because of its exceptional electronic and optic properties. For example, high charge carrier mobility in connection with a high transparency makes it suitable for window electrodes. Moreover, doping of graphene with heteroatoms has further led to new materials suitable for catalysis and energy storage applications. Nitrogen doped graphene as a metal free catalyst material has proven to be a promising candidate for the oxygen reduction reaction (ORR) at the cathode in fuel cells.
In order to understand the catalytic properties in regard to the molecular structure of the catalytic centers we are currently working on a bottom-up synthesis of different nitrogen containing nanographenes and graphene nanoribbons. For this, we apply techniques of solution or surface assisted polymerization to gain polyphenylene polymers which upon ring-closure yield the desired graphene materials. In contrast to other protocols for example the treatment of graphene or graphene oxide with nitrogen sources like ammonia or the pyrolysis of nitrogen enriched precursors, the heteroatoms are at designated positions in graphene. In our approach the periphery thus can be designed by the synthesis of the monomers to be exclusive pyrimidine or pyridine type as one example.
It shall further be highlighted that nanographenes with nitrogen in a zig-zag periphery are accessible in our approach. This structure is a formal azomethine ylide and has been embedded in a polyaromatic framework for the first time. As it exhibits a dipolare structure its addition reaction towards oxygen is currently investigated. Oxygen tends to add to nitrogen doped graphene in a side-on fashion. These new nitrogen doped nanographenes shall help to model the reaction in order to understand the basic molecular mechanism of the oxygen reduction reaction.
In summary, we can extend our established method of bottom-up synthesis to a new class of nitrogen containing nanographenes and graphene nanoribbons.[1] [2] [3]
[1] M. G. Schwab, A. Narita, Y. Hernandez, T. Balandina, K. S. Mali, S. De Feyter, X. Feng, K. Müllen, Journal of the American Chemical Society 2012.
[2] Z.-S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Müllen, Journal of the American Chemical Society 2012, 134, 9082-9085.
[3] J. M. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. L. Feng, K. Müllen, R. Fasel, Nature 2010, 466, 470-473.

Graphene nanoribbons (GNRs) are strips of single atom thick sp2-hybridized carbon atoms with band-gap governed by their width. We demonstrate that GNRs with tapered morphology exhibit semiconducting-to-metallic continuum along its length with unique electrical behavior. These structures are synthesized via the “nanotomy” process (developed by our group) and the devices are fabricated using electron beam lithography. Further, devices were also fabricated via non-nanotomy process through electron beam lithography with Pd-Au electrode-deposition. We demonstrate the generation of Schottky internal barrier and diodic transistor transport in these novel graphene structures.

Graphene-based materials are promising candidates for a variety of applications in electronics. Graphene-based active layers in field-effect transistors are already making use of its excellent charge transport properties. However, before graphene can be deployed on a wide scale as the active electronic component in carbon-based integrated circuits, it will be critical to design and engineer heterojunctions with tunable, tailored functionality. Heterojunctions form the active components in many solid-state device applications. Therefore, in this study we use first-principles methods to explore the formation and properties of graphene-nanoribbon based heterojunctions for use in electronic componentry. Previous theoretical and experimental work indicates that, due to quantum confinement effects, armchair graphene nanoribbons are semi-conducting with an energy-gap scaling with the inverse of the GNR width. We use first-principles total-energy electronic-structure methods based on density functional theory to investigate the formation of periodic semiconductor heterojunctions (superlattices) composed of graphene nanoribbons of different width, connected by a smooth, continuous transition region. Our study reveals a number of unique properties of the heterojunctions. We establish that the interfacial transition regions must follow strict geometric rules with respect to compatible ribbon widths and tapering angles at the interfaces to avoid the formation of electronic states at the Fermi level that render the whole system to be metallic. When the semiconducting nature is preserved by a proper transition region, we find that our heterojunctions exhibit a type-I band alignment, in which the valence band maximum (VBM) and conduction band minimum (CBM) are localized to the wider regions of the heterostructured ribbon. In some cases when the lengths of these ribbons are appropriately engineered, we observe delocalization of the VBM and CBM to the entirety of the heterojunction, corresponding to the onset of tunneling. Further, varying the lengths of the two component nanoribbons in the heterojunctions introduces additional quantum confinement, which enables tuning of barrier and well size, and therefore offers additional degrees of freedom for tuning of band gaps. Our DFT predictions for the onset of tunneling (in which the VBM and CBM become delocalized) are in good agreement with a one-dimensional model to the Schrodinger equation invoking the envelope function approximation for superlattices. Finally, we also explore the effects of applying tensile and compressive strain on the quantum confinement and therefore the band gaps of the nanoribbons.

Fluorinated carbon materials are useful in a wide range of applications from batteries to catalysis. A current direction in the field of graphene research explores the potential of fluorinated graphene (FG) as ultrathin gate dielectric and tunnel barrier in carbon electronics. In this talk, I will describe our effort in synthesizing and understanding the properties of FG. We have taken two complementary paths to the synthesis of FG. In the first approach, we utilize established synthesis of bulk graphite fluoride and obtain FG through the exfoliation of high-quality stoichiometric graphite monofluoride (GMF) crystals. I will present structural, optical and electrical transport studies of exfoliated FG. We observe, for the first time, UV Raman active modes of GMF. Photoluminescence measurements show several emission modes from near IR to near UV, which are likely due to defects in GMF. Electrical transport measurements confirm the strongly insulating nature of this compound although the size of the band gap remains elusive. In the second approach, we fluorinate graphene sheets synthesized via chemical vapor deposition (CVD) using CF4 plasma. The resulting FG is systematically studied using a wide range of spectroscopic and microscopic probes. We find that the structural features of CVD graphene (wrinkles, folds, multi-layer patches) play a critical role in the spatial distribution of fluorine and dominate the charge transport of FG. XPS studies reveal the complexity and evolution of the carbon-fluorine bonds, where the defects and grain boundaries of CVD graphene are important. These studies highlight current challenges in realizing electronics-grade FG and point to the possible pathways forward.
In collaboration with: Bei Wang, Shih-Ho Cheng, Justin Sparks, Humberto Gutierrez, Ke Zou, Junjie Wang, Ning Shen, Qingzhen Hao, Youjian Tang, Peter Eklund, Vincent Crespi, Jorge Sofo (Penn State University) and Fujio Okino (Shinshu University, Japan).

Bilayer graphene is a diverse material system that can be extended by the introduction of an interlayer misorientation angle, the “twist angle” (theta;). By varying theta;, a range of electronic and optical features can be tailored in electronically coupled, twisted bilayer graphene (TBG).[1-2] TBG properties continuously vary from those of Bernal stacked (theta;=0°) bilayer graphene as the twist angle is increased. Extending this idea, we demonstrate the influence of theta; on the chemical properties of TBG. We fabricate large-area, electronically coupled TBG films in a multi-step transfer process starting from graphene grown by low-pressure CVD on Cu foil.[3] Using optical microscopy and Raman spectroscopy, we classify TBG domains into one of five twist angle categories, where individual TBG domains range in size from ~10 - 100 mu;m. The influence of fluorine and oxygen functional groups on these TBG films is subsequently characterized as a function of twist angle. As a control for non-selective defect introduction, we use ion-implantation to uniformly introduce vacancy-type defects into TBG films.
Following these treatments, Raman spectroscopy is used to probe both the defect structure of TBG and the influence of each process on interlayer coupling, which expresses a unique resonance signature in Raman spectra.[2] Due to the large domain size, together with the optical contrast provided by TBG, optical microscopy is a useful and efficient technique for characterizing the influence of twist angle on each process. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) provide further insight into the structure and bonding configuration of each system. Finally, we take advantage of the optical contrast provided by TBG to explore the intercalation of fluorine into artificial few-layer graphene structures. For a range of conditions, we find fluorination with XeF2 at room temperature is restricted to the outermost surface of few-layer graphene, while subsurface layers remain electronically coupled and chemically pristine.
[1] G. Li et al., Nat. Phys. 6, 109-113 (2010)
[2] R.W. Havener et al., Nano Lett. 12, 3162-3167 (2012)
[3] J.T. Robinson, S.W. Schmucker, et al., submitted (2012)

A consequence of doping a two-dimensional material like graphene is that the dopants are fully exposed to the atmosphere and the usual mechanisms for corrosion protection are not applicable. Here we show how elements are incorporated into graphene and explain how these reactive surface atoms remain non-oxidized even when stored in air. In the present work, annular dark-field (ADF) imaging in a scanning transmission electron microscope is used to locate and identify the impurity atoms in the graphene lattice. Electron energy-loss spectroscopy (EELS) is also used to confirm the impurity atom identification. We have found that silicon, chromium, iron, cobalt, nickel, and copper impurity atoms on graphene remain in elemental form even when the doped graphene is exposed to air for extended times. With this information, first principles calculations explain these observations are due to preferential bonding of O to non-incorporated atoms and H passivation effects. While we have only examined the microstructure and have not explored catalytic potential of impurities incorporated in graphene, it is anticipated that inert graphene can be transformed to a very active catalyst by embedding metal clusters and individual atoms in defects in graphene. This phenomenon can be used to employ the catalytic, electronic, and magnetic properties of more elements than are usually available for unprotected materials in air or other corrosive environments.*
*The experimental work was done by MFC and GD and was supported by the Materials Sciences and Engineering Division of the Office of Basic Energy Sciences, U.S. Dept. of Energy. The calculations were performed by WW sponsored by NSF Award Number DMR-0925529 and the Center for Emergent Materials at The Ohio State University, a NSF MRSEC (Grant DMR-0820414). W.W. also acknowledges support from the Ohio Supercomputer Center under project PAS0072.

Chemical doping is an efficient way to tailor the electronic, chemical and magnetic properties of graphene-like materials. We now report the synthesis of large-area, high-quality monolayer graphene sheets doped with different heteroatoms (N, B, Si, etc) on copper foils by a modified ambient-pressure chemical vapor deposition (AP-CVD) route. When compared to pristine graphene, the Raman spectra of heteroatom-doped graphenes show strong D-band caused by presence of non-carbon atoms and other structural defects contained within the lattice. In addition, significant changes in the intensity of the D&’-band were noted. Scanning tunneling microscopy (STM) and spectroscopy (STS) studies reveal that the defects in the doped graphene samples arrange in different geometrical configurations. For N-doped graphene, we observed dominant “peapod-like” features corresponding to double N substitution in the same graphene sublattice (N2AA). For the B-doped graphene sample, novel “croissant-like” features were detected, which could be attributed to B3-type doping configurations embedded in the hexagonal honeycomb lattice. These experimental results are in agreement with first principles calculations of local density of states (LDOS) of doped graphene and STM image simulations.

Graphene oxide (GO) is an attractive material as a precursor to graphene. Structurally, GO is a graphene sheet with oxygen-containing functional groups which are present in the form of carboxyl, hydroxyl and epoxy groups. GO is an electrical insulator due to disrupted conjugation of sp2 bonds in the basal plane but can be controllably converted to a conductor via reduction. The fundamental limit of GO as an electronic material is that the functional groups cannot be removed completely, thus leaving a highly defective material with carrier mobilities that are orders of magnitude lower in comparison to exfoliated or even CVD graphene. Despite its high degree of disorder, however, GO exhibits intriguing and unique properties that arise from “states” induced by the oxygen functional groups. One example is a tunable photoluminescence (PL), which can be tuned from blue to red by progressive chemical reduction. Our previous PL measurements have shown that the states at the boundary of sp2 and sp3 domains play a critical role. Here, using correlated scanning photocurrent microscopy (SPCM) and fluorescence with a diffraction limited spatial resolution, we report the optoelectronic properties of an individual GO flake as a function of controlled reduction. Progressive reduction of GO allows us to precisely control the lateral density of the sp2 (graphitic) and sp3 (oxygen bearing) domains. This provides a strong handle in monitoring the evolution of photocurrent and understanding the role of oxygen bearing functional groups that are involved in the PL. Furthermore, electric force microscopy (EFM) was used to investigate the of charge migration within the single nanosheet and to map the local potentials as we transition from GO to reduced GO (RGO).

We report here optical transmission spectra of CVD graphene integrated with Self Assembled Monolayers (SAMs) over wide frequency range from Far-IR to UV (4meV-6.2eV). Monolayers with different functional groups --CH3, NH2 and HMDS-- were fabricated on SiO2/Si and Al203 substrates by chemical reaction and then CVD-grown graphene was transferred on the SAMs-modified substrates into the Graphene/SAMs/Substrate structure.
In the Far-IR range, Drude absorption is observed from which we discuss change of plasma frequency, scattering rate, and their implication on the mobility of free carrier. In the Mid-IR range, interband absorption and Fermi level shift (Pauli-blocking) are controlled by the characteristics of SAMs. In ultraviolet-visible range, the excitionic absorption peak (at 4.6eV) exhibits changes in the peak position and strength depending on the SAMs-layers. Our result shows that optical response of CVD graphene can be modified by the insertion of the SAM-layers in wide frequency range which can be used in large scale optoelectronic device application.

Nonlinear optical materials (optical limiters and saturable absorbers) play a significant role in the field of optics by their unprecedented ability to modulate lasers pulses, as lasers have become an integral part of our lives with various potential applications. Graphene oxide (GO) thin films were found to display interesting broadband nonlinear optical properties. Their optical limiting activity for femtosecond laser pulses at 800 and 400 nm was investigated, which could be tuned by controlling the extent of reduction. The as-prepared GO films were found to exhibit strong broadband optical limiting behaviors, which were attributed to the nonlinear absorption (two-photon or three-photon absorption) of GO films. Their optical limiting performances were found to be significantly enhanced upon partial reduction by laser irradiation or chemical methods. The laser induced partial reduction of GO could result in enhancement of effective two-photon absorption coefficient at 400 nm by up to ~19 times, and enhancement of effective two- and three-photon absorption coefficients at 800 nm by ~12 and ~14.5 times respectively. The optical limiting thresholds of partially reduced GO films are much lower than those of various previously reported materials. Highly reduced GO films prepared by using chemical method displayed strong saturable absorption behavior. These thin films could be easily fabricated on glass and even plastic substrates by using solution processing methods on a large scale. Additionally, the good solubility of GO sheets enables them to easily mix with polyvinyl alcohol (PVA) to produce the flexible free-standing films. During the subsequent in-situ reduction of GO, the polymer matrix can prevent the re-aggregation of reduced GO sheets to retain a homogeneous suspension. Their nonlinear optical properties of femtosecond laser are well consist with the GO thin film on substrates. Low cost, easy preparation and excellent nonlinear optical properties make these GO materials promising candidates for practical applications as broadband femtosecond optical limiters or saturable absorbers.

We have demonstrated a novel transparent and flexible optoelectronic hybrid material based on reduced graphene substrates. The hybrid material created from the hydrothermal synthesis of ZnO nanowires on reduced graphene/PDMS substrates proved to be a promising candidate for flexible devices, such as flexible field emission devices. By Owing to the good mechanical and electrical contact between vertical ZnO nanowires and graphene film, the field emission of our hybrid material showed low turn-on voltages from 2.0 to 2.8 V µm-1, even in highly deformed geometries. Furthermore, the optimal doping of quaternary nitrogen via sequential hydrazine treatment and thermal reduction accomplished workfunction-tunable reduced graphene oxide film. The PLEDs employing N-doped reduced graphene cathodes exhibited a 7.0 cd/A electroluminescence efficiency for N-doped graphene at 17 000 cd/m2. Reduced barrier for electron injection from a workfunction-tunable, N-doped reduced graphene cathode offered remarkable device performance. We anticipate that the mechanical deformability and interesting electric and optoelectronic properties of graphene hybrid device would provide valuable insights for flexible applications, such as flexible displays and solar cells.

Transition metal oxide (MOx), such as MnO2, Co3O4, etc., has been proved to be promising candidates in electrochemical energy storage fields. However, the poor electrical and ionic conductivities and low stability of MOx hinder its applications. Forming composites with high surface area porous metal, carbon materials, or conducting polymers is a possible solution. Herein, we have developed facile and scalable asymmetric in situ deposition methods to incorporate MOx nanoparticles into conductive single-walled carbon nanotube (SWCNT) films. The high porosity of the SWCNT films accommodates MOx nanoparticles without sacrificing the mechanical flexibility and electrochemical stability of SWCNT films.
For example, we exposed one side of SWCNT films to acidic potassium permanganate (KMnO4) solution. The infiltrated KMnO4 solution partially etches SWCNTs to create abundant mesopores, which ensure electrolyte ions efficiently access deposited MnO2. Meanwhile, the remaining SWCNT network serves as excellent current collectors. The electrochemical performance of the SWCNT-MnO2 composite electrodes depends on the porosity of SWCNT films, pH, and concentration of KMnO4 solution, deposition temperature and time. Our optimized two-electrode electrochemical capacitor, with 1 M Na2SO4 in water as electrolyte, showed a superior performance with specific capacitance of 529.8 F g-1, energy density of 73.6 Wh kg-1, power density of 14.6 kW kg-1, excellent capacitance retention (99.9%) after 2000 charge and discharge cycles, and one of the highest reported frequency responses (knee frequency at 1318 Hz).
In another case, we first deposited the precursor of Co3O4 nanoparticles at the surface of SWCNTs. After calcination, necklace-like SWCNT-Co3O4 composite was produced. The conductive SWCNT matrix can efficiently reduce the resistance of the battery when the composite material is used as anode in lithium ion battery (LIB). The performance of the SWCNT-Co3O4 electrodes mainly depends on the mass loading of Co3O4 in the composite, reaction temperature and time. The specific capacity of the LIB equipped with the SWCNT-Co3O4 composite electrode reaches 1239 mAh/g at 0.1C, 750 mAh/g at 1C, and 180 mAh/g at 10C, respectively, and keeps 90.5%, 89.5%, and 83.3% of its original capacity at 1C, 5C and 10C after 1000 charge and discharge cycles.
The high performance flexible SWCNT-MOx composites have broad applications in portable electronics and electrical vehicles, especially where both high power and energy densities, and high frequency response are desired.

Dye-sensitized solar cells (DSSCs) represent a viable alternative to silicon-based solar cells due to their potential low cost, easy fabrication, and high efficiency (over 12%). In a typical DSSC operation, the iodide (I-) in electrolyte is oxidized at the dye-sensitized porous nanocrystalline TiO2 under illumination, while the triiodide (I3-) is reduced at the counter electrode (CE) for the purpose of dye regeneration. The counter electrode usually consists of a transparent conductive oxide (TCO) layer deposited with a layer of catalyst. Platinum is often selected for use as the catalyst because it has high catalytic activity toward I3- reduction and is sufficiently corrosion-resistant to iodine species present in the electrolyte. However, since platinum is a highly valued metal, much incentive exists to develop DSSC counter electrodes using cheaper, abundant materials that are equal or greater in their catalytic activity.
Carbon nanomaterials have been studied as a replacement counter electrode for platinum. Manipulating these nanostructures also allows for the control and use of defect-rich edge plane materials, which facilitate the electron kinetics associated with the reduction of I3-. Particularly, graphene has emerged as a potential catalyst for DSSC cathode, due to graphene&’s exceptional surface area and conductivity.
This study examines the use of vertical graphene (VG) as a CE of DSSCs. Based on our early research on VG, this material has many advantages over planer graphene, such as easy fabrication, highly porous structure, and abundant surface oxygen functional groups. The catalytic activity of VG for the reduction of I3minus; is studied using electrochemical techniques and density functional theory (DFT) calculations. By introducing different amounts of water vapor during the synthesis, the content of oxygen functional groups on VG can be tuned, which eventually leads to varying catalytic performance in DSSCs. The DFT calculation reveals that the oxygen functional groups play key roles for the I3minus; reduction. The VG electrode achieved a charge transfer resistance (RCT) of 7.3 × 10-3 #8486; cm2 versus 0.59 #8486; cm2 for platinized FTO glass (Pt/FTO). To the best of our knowledge, this is the lowest RCT for Iminus;/I3minus; redox couple of a DSSC CE. DSSCs fabricated with VG as CEs exhibit superior performance over that with Pt.

Hydride-terminated Si(111) photoelectrodes form a deleterious SiOx layer when anodic current is passed from the silicon electrode to an aqueous solution. This prohibits the use of silicon as a photoanode in a viable water splitting device. Graphene grown via chemical vapor deposition (CVD) was used to fabricate planar (111) n-type silicon photoanodes covered by monolayer graphene. Graphene-covered, H-terminated n-Si electrodes in contact with an aqueous K3[Fe(CN)not;6]/K4[Fe(CN)not;6] electrolyte exhibited increased stability under illumination. The short-circuit current (Jsc), open-circuit voltage (Voc), and fill factor (ff) decayed significantly less slowly for graphene-covered n-Si than for the corresponding bare H-terminated n-Si. The short-circuit current of the n-Si-H decayed to half its initial value in ~30 seconds while the same 50% decay in the short-circuit current for graphene-covered n-Si took ~75,000s. This behavior was attributed to a reduced rate of oxidation of the silicon surface when the silicon was covered by graphene. In non-aqueous electrolytes, graphene-covered n-Si electrodes demonstrated reduced Voc when compared to graphene-free H-terminated n-Si electrodes. Studies of Voc vs. solution redox potential for graphene-covered n-Si electrodes exhibited changes in Voc concomitant with redox solution potential. This Voc vs. solution potential relationship demonstrates that neither complete Fermi level pinning by graphene nor silicon bulk diffusion current limits the observed voltages.

We present flexible polymer light-emitting diodes (FPLEDs) using graphene oxide-modified single-walled carbon nanotube (GO-SWCNT) films as an anode on polyethylene terephthalate (PET) substrates. The electrode of GO-SWCNTs used in this study shows a quite low sheet resistance of ~75 Omega;/square at 65 % (at 550 nm) optical transparency with resistance to bending fatigue. The small-sized GO nanosheets onto the SWCNT network films are significantly effective in reducing the sheet resistance and the surface porosity of the SWCNT network films without sacrificing the transmittance. Moreover, the top layer of the large-sized GO nanosheets are crucial for high device efficiency to reduce the roughness of the SWCNT surface and enhance the wettability with PEDOT:PSS. The optimized FPLED using a GO-SWCNT electrode shows a maximum luminous efficiency of 5.0 cd/A (at 9.2 V), power efficiency of 2.4 lm/W (at 5.6 V), external quantum efficiency 1.9 % (at 9.0 V) and turn-on voltage (2.0 V), which is comparable to conventional PLEDs using an indium-tin-oxide (ITO) electrode (a maximum luminous efficiency of 6.2 cd/A (at 9.4 V), power efficiency of 2.6 lm/W (at 5.8 V), external quantum efficiency 2.3 % (at 9.0 V) and turn-on voltage (2.0 V)). This result confirms that a GO-SWCNT electrode can be efficiently used to replace ITO for flexible optoelectronic devices.

Chemical modification of carbon nanotubes (CNTs) is being the focus of much research, either to improve their solubility in various solvents or to add a functionality. Until now, most of the work dedicated to energy storage, with this approach, has been focussed on the development of pseudocapacitors based on activated carbon modified by a diazonium derivative of the electroactive anthraquinone (AQ) group. Even though activated carbon is currently the state of the art electrode material, it suffers from some drawbacks including its limited electrical conductivity and the need for a binder to ensure its expected cohesion. These drawbacks can be overcome when CNTs are used instead of activated carbon. Indeed, CNTs can be easily transformed into a nanotube network (bucky paper) by filtration of a solvent nanotube suspension leading to a self-supporting electrode with quite good electrical conductivity.
In this work, we report the preparation and the characterization of electrodes made of multiwalled carbon nanotubes (MWCNTs) chemically modified by a series of AQ derivatives. The covalent attachment of the AQ moiety to MWCNTs was performed via an azido group attached either to an alkyl spacer or to an aryl group, thus allowing to study the influence of the distance between the AQ moiety and the MWCNTs on the electrochemical properties of the resulting electrode. The modified MWCNTs (AQ-MWCNT) were characterized by thermogravimetric analysis and X-ray photoemission. Bucky papers obtained by filtration of a aprotic polar solvent AQ-MWCNT suspension, were characterized by cyclic voltammetry in 0.1M H2SO4, using the standard 3 electrode set-up. Thus, it has been demonstrated that a MWCNT electrode modified by an aryl azide AQ-MWCNT having a eight methylene groups spacer provides 54% higher specific capacitance, compared to a unmodified MWCNT electrode.

Gaphene is a “new rising star” in carbon-based materials due to its extraordinary properties at the nanoscale, such as high charge carrier mobility, electrical and mechanical properties. Now a days, there has been tremendous interest in graphene-metal nanocomposite materials because of their potential applications in field emission display devices, microelectronics, hydrogen sensing, high catalytic activity etc. In the present work metal (Pd, Ag, Au & Ti) nanoparticles are grafted by thermal evaporation and multilayer graphene (MLG) synthesized by microwave plasma enhanced chemical vapour deposition system, to form metal-MLG nanocomposites. A scanning and transmission electron microscopy study shows beautiful decoration of these metal nanoparticles of diameter 5 - 20 nm on graphene layers. During Raman spectroscopy studies, it was found that metal-MLG nanocomposites emit enhanced resonance in comparison with the as-deposited MLG and is related to surface enhanced Raman spectroscopy. The ID/IG ratio was found to decrease and is related to the passivation of defects, due to which the field emission is enhanced. Field emission studies of metal-MLG nanocomposites were carried out using diode type field emission set-up. Metal grafted MLG nanocomposites have low work function and higher field enhancement factor as compared to as deposited MLG. The enhancement in the field emission properties of metal grafted MLG nanocomposites is considered to be due to the modification in density of states near Fermi level. To investigate the microscopic origin for the enhanced field emission from metal grafted MLG, we carried out the first principles calculations using Mede A software. We investigate the density of states of these metal-MLG nanocomposites. It is found that the DOS are increased near the Fermi level after the decoration of metal nanoparticles over graphene. The theoretical results are found to be consistent with the observed experimental data.

Single-layer graphene was synthesized by using chemical vapor deposition, and transferred on SiO₂/Si substrates. For the formation of graphene p-n junction, a solution of benzyl viologen (BV) was first dropped and spin-coated on the 10 x 10 mm² graphene/SiO₂/p-type Si wafer, and then annealed at 100 ^oC for 10 min to make graphene uniformly n-type. Subsequently, a 5 x 5 mm² bare graphene was transferred on ~1/4 area of the n-graphene/SiO₂/p-type Si wafer, a solution of AuCl₃ was dropped and spin-coated on the surface of graphene, and similarly annealed. As a result, the graphene p-n vertical homojunction was formed on the ~1/4 area of the SiO₂/p-type Si wafer. 1-mm-diameter Ag electrodes were deposited on the top of both n- and p-graphene layers to complete the graphene p-n device. We prepared the graphene p-n vertical-homojunctions for various n doping concentrations at a fixed highest p-doping concentration. At lower n doping concentrations, the p-n junctions are ohmic, consistent with the Klein-tunneling effect. In contrast, at higher n doping concentrations, the p-n junctions show asymmetric rectifying behaviors. These results are discussed based on possible physical mechanisms.

The outstanding properties of carbon nanotubes (CNTs) and the steps forward on their controllable growth and surface functionalization, have boost research interest for their use as nano-additives in the development of novel polymer nanocomposites. Epoxy resins represent one of the most important classes of engineering polymers, with unique properties and wide variety of applications. The incorporation of carbon nanotubes in epoxy polymers can lead to significant improvement of their mechanical, thermal and conductivity properties. The successful preparation of nanocomposites strongly depends on the homogenous dispersion and interfacial interactions of CNTs with the polymer matrix. In order to promote those critical aspects organic modification and surface functionalization of CNTs is necessary.
Significant progress has been achieved so far in the development of nanocomposites using glassy epoxy polymers and organically modified carbon. In this study, we focus on the development of rubbery epoxy polymers reinforced with various types of multi wall carbon nanotubes, i.e. having varying length and width, being also surface modified with carboxyl groups (-COOH), various alkyl-amines (CxHy-NH2) or polyoxypropylene amines (Jeffamine D-2000). The effect of mixing conditions and percentage loading of CNTs were also investigated. The in-situ polymerization technique was used for the production of the epoxy-CNTs nanocomposites.
The characterization results of CNTs revealed a minor effect of chemical modification treatment on nanotubes&’ structural integrity and the successful grafting of the various functional organic groups. The mechanical properties of the epoxy polymer were significantly improved especially by the use of hexamethyl amine and polyoxypropylene D-2000 diamine functionalized MWCNTs. The respective nanocomposites exhibited simultaneous increase of stiffness and toughness. Short and thin carbon nanotubes also favor both the above properties to higher extent compared to larger size MWCNTs. The viscoelastic properties, such as storage modulus and glass transition temperature (Tg), were also improved with most of the CNT variants used as nano-additives, whereas, the thermal stability of the epoxy polymer was not sacrificed.
ACKNOWLEDGEMENTS. Co-funding of this research by EU-ESF and the Greek Ministry of Education, Lifelong Learning and Religious Affairs via the Program “Education and Lifelong Learning”, Action “Support of Postdoctoral Researchers/ESPA 2007-2013” is gratefully acknowledged.

Near-IR fluorescence is one of the unique properties of single-walled carbon nanotubes (SWCNTs) that has attracted significant attention for biomedical and optoelectronic applications. However, modification of SWCNTs by covalent functionalization often completely destroys exciton photoluminescence due to the introduction of sp3 defects that disrupt the pi-conjugated system of the pristine, sp2 carbon lattice. Here, we report evidence of photoluminescence from trions, or charged excitons, that may form in covalently functionalized SWCNTs as mobile excitons meet localized electrons or holes at sp3 defect sites. Two distinctly different chemistries, including the reductive Billups-Birch reaction, which injects excess electrons to the covalently functionalized SWCNTs, and oxidative, super-acid chemistry, which protonates the SWCNTs, produce similar trion photoluminescence features. This new trion emission is red-shifted by approximately 260 meV and is stable under ambient conditions, presumably due to the ability of sp3 defects to trap and stabilize the excess carrier population that enables charged excitons to form. Thin film transistors of the functionalized SWCNTs confirm the doping conditions which promote trion formation.

Electrode materials combining high electrical conductivity and optical transparency are crucial components for organic light emitting diodes (OLED). Graphene is thereby a highly promising alternative to commonly used Indium tin oxide (ITO), in particular considering that unlike ITO graphene is flexible and, when grown via Chemical Vapor Deposition (CVD), not constraint by limited natural resources. Critical challenges for graphene based OLEDs not only relate to the further improvement of large-area, controlled graphene CVD [1,2], but also to its integration, in particular to achieve efficient charge injection and graphene doping. Several dopants have recently been introduced, but most are chemically not stable or not applicable for organic electronic devices.
Here we show that transition metal oxides, such as MoO3 and WO3, are very efficient and stable p-type dopants for graphene leading to a more than three-fold reduction of sheet resistance. Our process is based on scalable graphene CVD [1,2] and the doping is carried out via thermal evaporation which can be fully integrated in the OLED fabrication process. With in-situ 4-point probe measurements we find that only a few nanometers of MoO3 are sufficient for efficient doping. Our systematic study of this doping process by ultra-violet and x-ray photoemission spectroscopy (UPS, XPS) shows a large interface dipole of 2.2 eV and band bending caused by an electron transfer from graphene to MoO3. The strong p-doping of graphene is enforced by the deep lying electronic states of MoO3 which exhibits a work function of >6.5 eV. Our XPS analysis also shows that the doping process leads to a metal oxide reduction from Mo 6+ to Mo 5+ states at the interface. The energy level alignment at the graphene/MoO3/organic interfaces as measured with UPS shows only very small energy barriers for hole-injection. Thus, MoO3 allows not only an efficient p-doping of graphene, but also provides a suitable matching for efficient hole-injection from graphene into the OLED layers. Based on this process, we demonstrate CVD graphene based OLEDs that show electro-optical performances similar to conventional ITO based devices.

Authors acknowledge funding from the EC project Grafol
[1] Kidambi et al., J Phys Chem C (2012) DOI 10.1021/jp303597m
[2] Weatherup et al., ACS Nano (2012) DOI 10.1021/nn303674g

In fuel cell technology, the development of efficient catalysts and method for catalyst deposition is crucial. Indeed, the efficiency of the catalyst will control the kinetics of the reaction by decreasing the activation energy. The activity and durability can be improved by optimizing the carbon support of the active catalyst[1,2]. A catalyst widely used in fuel cell applications is platinum (Pt), which is responsible for the cost of the fuel cell system. Our research focuses on improving the catalytic efficiency and durability of Platinum while reducing the amount needed for the development of a reliable fuel cell system. This goal is achieved by using carbon surfaces with very high surface area and by modifying the local electronic distribution of the surface using dopants. Therefore understanding and controlling the deposition and catalytic activity of the platinum is essential to develop a viable and reliable system. The paper will present the effect of dopant on the durability and efficiency of the catalyst for the oxygen reduction reaction, which represents the rate limiting factor in a proton exchange membrane fuel cell. This study was done using structural and dynamic ab initio methods.
[1] M. N. Groves, C. Malardier-Jugroot, M. Jugroot. J. Phys. Chem. C, 116 (19), 10548, 2012
[2] M. N. Groves, A.S.W. Chan, C. Malardier-Jugroot, M. Jugroot, Chemical Physics Letters, 481, 4-6, 2009

Reduced graphene oxide (rGO) is a promising solution processable material for a variety of thin-film optoelectronic applications. The two main barriers to widespread adoption of rGO are the lack of (1) fabrication protocols leading to tailored functionalization of the graphene sheet with oxygen-containing chemical groups, and (2) understanding of the impact of such functional groups on the optical properties and electronic structure of rGO. We carry out classical molecular dynamics and ab initio density functional theory calculations on a very large statistical set of disordered rGO structures to demonstrate the role of functional groups on the stability, work function and photoluminescence of rGO. Our calculations indicate the metastable nature of carbonyl-rich rGO structures at room temperature, and show the favorable energetics for their conversion to hydroxyl-rich structures via carbonyl to hydroxyl conversion near carbon vacancies and holes. We demonstrate a significant tunability in the work function of rGO by up to 0.5 eV by changing the oxygen content and by up to 2.5 eV in structures with a precise control of oxygen-containing functional groups. We show that the PL emission peak in rGO can be fine-tuned by modulating the fraction of epoxy and carbonyl groups. Taken together, our results guide the preparation of controlled rGO structures and pave the way to their application in the fields of optoelectronics and energy conversion.

Here we report on a scalable and direct growth of graphene micro ribbons on SiO2 dielectric substrates using a low temperature chemical vapor deposition. Due to the fast annealing at low temperature (750oC for 2-5 minutes) and dewetting of Ni, continuous few-layer graphene micro ribbons (2-10 µm in width, up to a few millimeters in length) grow directly on bare dielectric substrates through Ni assisted catalytic decomposition of hydrocarbon precursors. These high quality graphene micro ribbons exhibit low sheet resistance of ~700 Omega; -2100 Omega;, high on/off current ratio of ~3, and high carrier mobility of ~655 cm2V-1s-1 at room temperature, all of which have shown significant improvement over other lithography patterned CVD graphene micro ribbons. This direct approach can in principle form graphene ribbons of any arbitrary sizes and geometries. It allows for a feasible methodology towards better integration with semiconductor materials for interconnect electronics and scalable production for graphene based electronic and optoelectronic applications where the electrical gating is the key enabling factor.

Single and multilayer graphene and highly ordered pyrolytic graphite (HOPG) were exposed to pure hydrogen low temperature plasma (LTP).
In the first part, hydrogen LTP exposed HOPG is characterized with various experimental techniques such as photoelectron spectroscopy, Raman spectroscopy and scanning probe microscopy. Our photoemission measurement shows that hydrogen LTP exposed HOPG has a diamond-like valence band structure, which anticipates graphane formation. With the scanning tunneling microscopy technique, various atomic scale charge density patterns were observed, which might be associated with different C-H conformers of graphane. A very low defect density was observed by the scanning probe microscopy measurements, which enables a reverse transformation to graphene. Hydrogen LTP exposed HOPG possesses a high thermal stability, and therefore, this transformation requires annealing over 1000oC.
In the second part, a silicon stencil mask with a periodic pattern is used for hydrogen plasma microlithography of graphene supported on a Si/SiO2 substrate. Obtained patterns are imaged with Raman microscopy and Kelvin probe force microscopy, thanks to the changes in the vibrational modes and the contact potential difference (CPD) of graphene after treatment. A decrease of 60 meV in CPD as well as a significant change of the D/G ratio in the Raman spectra can be associated with a local hydrogenation of graphene, while the topography remains invariant to the plasma exposure.

Graphene nanoribbons (GNRs) attract great attentions in nano-electronic applications and solid state physics. Modification of electronic structures at K-point in GNRs has theoretically and experimentally been demonstrated: those are differed depending on the type of edge geometries. Armchair and zigzag edges indicate band-gap and a flat band at the K-points, respectively [1]. So far it is difficult to fabricate such dense GNRs as having a sole type of edges to macroscopically visualize those electronic structures at the K-points, which can be achieved by angle-resolved photoemission spectroscopy (ARPES). We have reported the growth of GNRs, possessing armchair edges, on unique SiC surfaces by molecular beam epitaxy (MBE)[2]. The GNRs were grown on vicinal SiC surfaces, consisting of periodic (0001) terraces/(1-10n) facets induced after high temperature H2-gas etching [3]. The massive arrays of GNRs, approximately 10 nm width and armchair edges, were obtained and characterized by polarized Raman spectroscopy and ARPES. Essentially those indicate apparent band-gap openings at the K-point of at least 0.14 eV. The edge type of the GNRs is determined by the vicinal direction of SiC substrate, which is [1-100] for armchair. Initially the buffer layer, which shows (6radic;3×6radic;3)R30 geometry to SiC(1×1), is epitaxially grown on SiC(0001) and as a result its edge should crystallographically be armchair type, as was confirmed by the polarized Raman spectroscopy [4]. Thus, the edge type of GNRs can be controlled by the use of different vicinal directions of off-axis SiC substrates. We here demonstrate the growth of both armchair and zigzag GNRs based on this strategy and show electronic structures.

References
[1] K. Nakada et al., Phys. Rev. B 54, 17954 (1996).
[2] T. Kajiwara et al., submitted. arXiv:1210.4304v1. (2012)
[3] H. Nakagawa, S. Tanaka, and I. Suemune, Phys. Rev. Lett. 91, 226107 (2003).
[4] K. Sasaki et al., Phys. Rev. B 82, 205407 (2010).

As a “surface-only” nanomaterial, the properties of monolayer or few-layer graphene structures are extremely sensitive to adsorbed ambient contaminants, with a correspondingly severe impact on the electrical characteristics and stability of graphene-based devices. We report on a simple, versatile functionalization method based on solution-phase self-assembly of alkane-amine layers on graphene.
Second-order Moller-Plesset (MP2) perturbation theory on a cluster model (methylamine on pyrene) yield a binding energy ~ 220 meV for the amine-graphene interaction, strong enough to enable formation of a stable aminodecane layer at room temperature. The MP2 calculations also indicate a weak charge transfer (0.01 electrons) from the amine to the pyrene. Atomistic molecular dynamics simulations on an assembly of 1-aminodecane molecules indicate that a self-assembled monolayer can form, with the alkane chains oriented perpendicular to the graphene basal plane. The calculated monolayer height (~ 1.7 nm) is in good agreement with Atomic Force Microscopy data acquired for graphene functionalized with 1-aminodecane, which yield a continuous layer with mean thickness ~1.7 nm, albeit with some island defects.
Passivation and adsorbate of graphene field-effect devices using 1-aminodecane yields with carrier mobilities in excess of 4000 cm2/(Vs) for electrons and holes at field-induced carrier concentrations of 1x10^12 (cm)^-2. Device mobilities are maintained, even after re-exposure to atmospheric conditions for up to 6 months. Molecule-passivated devices also show low hysteresis and excellent stability in terms of the (net) doping level.
Raman data also confirm that self-assembly of alkane-amines is a non-covalent process, i.e., does not perturb the sp2-hybridization of the graphene.
Further attractive features of this functionalization method include high-density binding of plasmonic metal nanocrystals for enhanced photoconductivity and seeded atomic layer deposition of inorganic dielectrics.

Graphene has attracted much attention due to its remarkable physical properties and potential applications in many fields. In special, the electronic properties of graphene are influenced by the number of layer, stacking sequence, edge state, and doping of foreign elements. Recently, many efforts have been dedicated to alter the electronic properties by doping of various species, such as hydrogen, oxygen, nitrogen, ammonia and etc.
Here, we report our recent results of plasma doping on graphene and its recovery behavior. We prepared mechanically exfoliated graphene, which contains mono-, bi- and multi-layered regions, and performed the plasma treatment using air and ammonia gas for oxygen and nitrogen doping, respectively. To compare the doping efficiency, we also performed air annealing. The recovery of the doped samples was carried out by high vacuum annealing at elevated temperatures. The doping level was estimated from the number of peak shift of G-band in Raman spectra. The downshift of G-band was observed after ammonia plasma treatment, which implies electron doping to graphene. On the other hand, the upshift of G-band was measured as a result of thermal annealing in air, which means hole doping. After the high vacuum annealing, we confirmed the recovery of the doped graphenes by replacement of G-band position in Raman spectra. Finally, the reversible process of doping and recovery can be achievable through controlled doping and recovery treatments.

Graphene has been regarded as a promising material in electronics, and recently, it is a rising star in photonics as well. It shows strong and broadband absorption per unit mass extending across a wide range from visible to infrared (IR). Together with its extremely high mobility, it is very potential in ultrafast broadband photodetector. Moreover, the electronic property of graphene is largely controlled by the chemical potential, which requires controlled doping.
Here, we report the use of 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (o-MeO-DMBI) as a strong n-type dopant for chemical-vapor-deposition (CVD) grown graphene. After doping, the CVD-grown graphene can be efficiently tuned from p-type to ambipolar and finally n-type transport behaviors, by simply varying the amount of o-MeO-DMBI applied. Also, if doped site-selectively, the CVD-grown graphene can be constructed to large amounts of p-n junctions which are composed by undoped and doped regions. By using photolithography, we can define the area ratio of p- and n- parts, which affects the ratio of hole-electron mobility. And by changing the amount of o-MeO-DMBI applied, we can tune the relative doping density between p- and n- parts, which affects the Fermi energy separations of p-n junctions. Photoresponse was tested on the FET devices which were fabricated by only n-doped graphene and p-n patterend one. Under a cold cathode fluorescent lamp, different photoresponses were observed for the only n-doped and p-n pattered graphene. Besides, the larger energy separations of p-n junctions lead to stronger photoresponse, which can be obtained from higher doping differences among the p- and n- parts. We also found that such photoresponse was attributed by junctions as well as bolometric effect. The chemical doping of CVD-grown graphene and its photoresponse demonstrated in this study is believed to be a feasible scheme in manipulating graphene chemical potential for future graphene-based optoelectroncis.

Since graphene grown from polymer by Sun et al. [Nature 468, 549, (2010)] firstly, using solid carbon source as potential feedstock for graphene synthesis has been proposed as a new environment benign approach. Unfortunately, due to the rapid evaporation of PMMA under high temperature, it was reported difficult to synthesize graphene converting from PMMA by conventional CVD. In this work, microwave plasmas were employed to synthesize single- or double-layer large scale graphene sheets on copper foils using solid carbon sources, polymethylmetacrylate (PMMA) and Polydimethylsiloxane (PDMS). The utilization of reactive hydrogen plasmas enable the graphene growth at reduced temperatures as compared to conventional thermal chemical vapor deposition processes. The effects of substrate temperature on graphene quality were studied based on Raman analysis, and a reduction of defects at elevated temperature was observed. Moreover, one facile approach to incorporate nitrogen into graphene by plasma treatment in a nitrogen/hydrogen gas mixture during the synthesis process was demonstrated. First principle calculations revealed that the incorporated nitrogen atoms with the Pyridinic-N and Pyrrolic-N formats in the graphene sheets would tune the band gap of graphene effectively which may provide a new step toward controlled graphene electronics, and this technique may lead to precise determination of graphene-based device characteristics in the future.

Currently, there is increasing interest in studying the properties of an electron donor atom doped-reduced graphene oxide (rGO) for potential application in electron rich field effect transistor (FET) (called n-type FET) devices and energy storage materials. Thermally doped nitrogen and phosphorous atoms on the sp2-carbon network of rGO enhance its structural and electrical properties because of its ability to donate an electron and form a de-localized structure due to the lone pair electrons of donor atoms.1-3
In this work, we report two kinds of applications using the electron donor atom doped-rGO. Firstly, we report new fabrication method and characterization of n-type graphene through thermal graphitization with nitrogen and phosphorous sources for n-type FETs. Especially, phosphorous doped graphene exhibits superior chemical and electronic stability on the FET device even under oxygen atmosphere environment.1 Furthermore, we like to report an identification on the changes of atomic structure of nitrogen doped graphene by STM. Atomic structural information of thermally annealed rGO provides an understanding on how the hetero-atomic doping could affect electronic property of rGO.4 Finally, we like to report the fabrication method and characterization of highly nitrogen doped porous rGO to use electrode materials for supercapacitor. This material with high surface area (~900 m2/g) seems to show high doping percentage (~20 %) in XPS.
References
1. S. Some, J. Kim, K. Lee, A. Kulkarni, Y. Yoon, SM. Lee, T. Kim and H. Lee, Adv. Mater. 24, 5481 (2012)
2. S. Some, P. Bhunia, EH. Hwang, K. Lee, Y. Yoon, S. Seo and H. Lee, Chem. Eur. J. 18, 7665 (2012)
3. P. Bhunia, EH. Hwang, Y. Yoon, E. Lee, S. Seo and H. Lee, Chem. Eur. J. 18, 12207 (2012)
4. Y. Yoon, S. Seo, G. Kim and H. Lee, Chem. Eur. J.18, 13446 (2012)

Graphene nanoscaffolds have shown excellent performance in catalysis, chemical sensing, and biomolecule sensing. Understanding the molecular adsorption mechanism and the molecular orientation effect on the graphene surface are very necessary for its application. Here, we report charge transport behaviors of a graphene channel in terms of the nature of two different graphene frameworks and the molecular orientation of the adsorbed functional molecules. Chemically synthesized n-doped graphene (e.g., nitrogen-doped reduced graphene oxide, n-RGO, labeled n-type prone graphene) and chemically vapor deposited graphene (CVD graphene, p-CVD-G, labeled p-type prone graphene)[1] are used as different graphene channels to interact with the adsorbed functional molecules. The functionalized benzene molecules and their diazonium salts are physically and chemically attached to the graphene channel surfaces, respectively. Interaction results of the adsorbed molecules at both graphene channel surfaces are measured by current-voltage transport curves of their field effect transistors. The physically (horizontally) bound molecules on both p- and n-type prone graphene channels produced p- and n-type doping effects depending on the functional group. Alternatively, the chemically (vertically) bound diazonium molecules did not produce molecular doping effects on changes of major charge carriers, but followed the nature of each p- and n-type prone graphene channel. Our novel findings promise a new molecular doping strategy of graphene for its applications.
References
1. Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C. J. Am. Chem. Soc. 2009, 131, 1336-1337.

Sensor-based detection of gases is an important task to improve the safety and quality of human life. Highly sensitive metal oxide based gas sensors are well established and widely implemented. However, they operate at high temperatures (250 - 600°C), which requires excessive energy and degrades their long-term stability. In this regard, graphene based gas sensors, which operate at low temperatures (25 - 85°C), could provide a solution. Upon gas adsorption the conductance changes quickly and with high sensitivity. Additionally these materials are scalable and easy to produce.
In this study, graphene was prepared by reduction of graphene oxide (GO), which was obtained by oxidation of graphite (Hummers method). The resulting reduced graphene oxide (rGO) is stable in water, enabling easy transfer to any substrate by spin coating or drop casting. To test the capability of rGO as a gas sensor material, the conductivity of rGO modified electrodes was studied in the presence of various gases (NO₂, CO₂, CH₄, H₂ and CO). Upon adsorption the change in resistivity of rGO coated electrodes was very sensitive, especially towards NO₂ and CO₂. Selectivity was developed by chemical modification. Electrochemical doping with Ni nanoparticles created selectivity towards CO. An increased sensitivity towards H₂ was accomplished by chemical doping with Pd and Pt nanoparticles. Doping with TiO₂ resulted in selective detection of NO₂.
Graphene based sensors provide a low-cost alternative for the detection of hazardous or explosive gases. The detection of CO₂ is particularly needed in many practical applications, e.g., for more energy efficient operation of air conditioning.

High electrical conductivity and optical transparency make graphene a potential material for use as a flexible transparent conductive electrode in organic electronic devices. For integration into these devices, the electronic energy level between the conductive graphene and the other active layers must be controlled. Here, an ultrathin layer of a polymer containing simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE), was deposited on graphene to lower its work function. Scanning Kelvin probe microscope (SKPM) techniques applied on graphene before and after PEIE deposition and demonstrated that the work function can be lowered over 1 eV. Electrical measurements performed on back-gated field effect graphene devices indicate that shifts in the Dirac point correlates well to the shift in the Kelvin Probe measurements, verifying the changes in graphene doping. Unlike destructive doping techniques of graphene such as substitutional doping or covalent functionalization, this method is defect-free as there is no discernible D peak in Raman spectra of graphene films after PEIE deposition. This method can be easily processed in air, from dilute solutions in environmentally friendly solvents such as water or methoxyethanol and does not suffer any change until a temperature of 190°C, making it compatible with the processing of printed electronic.

In recent years, there has been much interest in modelling graphene nanoribbons as they have
great potential for use in molecular electronics. We have employed the NEGF formalism to determine the conductivity of graphene nanoribbons in various configurations. The electronic structure calculations were performed withiin the framework of the Extended Huckel Approximation. Zig-zag graphene nanoribbons were considered with folded nanoribbon contacts and with silicon contacts to establish the importance of the contact geometry and chemistry on the conductance. The conduction pathways and their dependence on the geometry of the nanowires were also examined. It was found that the geometry of the contacts plays a crucial role in determining the electron transfer pathways. The results of our calculations are compared with that obtained from recent work carried out using tight-binding model Hamiltonians [1,2,3].
[1]. H Takahi and N Kobayashi, Physica E 43 (2011) 711.
[2] J.W. Gonzalex et al, Solid State Comm 152 (2012) 1400.
[3] M.Ashhadi and S.A.Ketabi, Phsica E (2012) in press.

Even though the solubility of Carbon Nanoforms in common solvents is really very low, the main methods for transforming them have so far involved the use of traditional chemical techniques, such as refluxing and sonication, in the presence of large amounts of strong inorganic acids or organic solvents, over long periods of time. The modification of carbon nanostructures in the absence of any solvent has many advantages over solution reactions: a) this methodology does not suffer the solubility problems; and b) the solid-state reactions have many benefits: reduced pollution, low costs, and simplicity in process and handling. These factors are especially important for scalable production.
There is an increasing number of publications demonstrating the advantages that technologies such as microwaves and ball milling, can offer for the modification of different compounds, in solvent-free conditions. In the present work we summarize our achievements in the modification of Carbon Nanotubes (CNTs) and Carbon Nanohorns (CNHs). It has been shown that these structures display strong microwave absorbing properties and this behaviour have been used for purification and functionalization. Another stimulating possibility is the ball milling approach where chemical bonds are activated by the presence of an external mechanical force. Both methods pave the way for green protocols and large-scale modifications while avoiding significant degradation of the structure. Finally, ball milling processes have also been applied to the exfoliation of graphite in order to form stable dispersions of graphene sheets.

All the spectacular properties of graphene are due to the confinement of electrons/holes in a plane of atomic thickness. This makes graphene based devices extremely sensitive to how it is grown on different substrates and how it is doped to obtain n-type and p-type conductivity in graphene layers. Though doping is extremely important for graphene based devices, but doping also introduces defects. Particularly this is extremely important for graphene, so it is desirable to achieve p-type and n-type conductivity in graphene without destroying monolayer induced properties such that high mobility due to linear dispersion relation can be retained. Here, we report the correlation between external defects incorporated due to supramolecular functionalization (doping) of graphene sheets. We have intercalated graphitic layers of HOPG by means of sonication to exfoliate graphene layers using different organic molecules, which also act as dopant. We have used toluene, acetone, propylene carbonate and DMF for this purpose. Further, graphene field effect transistors (g-FET) were fabricated by putting Au source-drain electrodes using electron beam lithography. SiO2 (300 nm)/Si(n++) was used as substrate. By analyzing the transfer characteristics (IDS-VDS), large shift in Dirac point (-20V to -40V) was observed in case of graphene exfoliated in acetone, propylene carbonate and DMF, whereas graphene exfoliated in toluene exhibits very small deviation in Dirac point (+0.3V). Position of the Dirac point clearly indicates that graphene layers exfoliated in acetone, DMF and propylene carbonate are p-type and in toluene, it exhibits pristine (undoped) behavior. We have observed a peak known as D band peak at 1350 cm-1 in Raman spectra in case of graphene exfoliated in acetone, propylene carbonate and DMF. This peak is absent in Raman spectra of graphene exfoliated in toluene. This allows us to correlate the extrinsic defects which could arise due to functionalization. DMF, propylene carbonate and acetone have oxygen atom, which attracts electron cloud from graphene sheet during sonication resulting supramolecular functionalization which can be correlated with D band in Raman spectra and the large shift in Dirac point, whereas toluene has no oxygen and also less polar compared to acetone, propylene carbonate and DMF. So, by choosing particular organic solvent for intercalation, it is possible to control doping and doping induced defects in graphene. Functionalization of graphene sheets was also analysed by monitoring shift in respective Raman peak positions, UV-Vis absorption spectroscopy and IR spectroscopy. Mobility value upto 10000 cm2/V-s was found for pristine graphene and with p-type doping it was reduced to 6000 cm2/V-s due to doping induced defects.

Due to the excellent electrical conductivity, reduced graphene oxide (RGO) may attract photoexcited electrons of semiconductor nanocrystals to improve the overall charge separation property. Such capability of electron trapping for RGO has rendered it promising potential in relevant photoconversion processes such as photocatalytic water splitting, photocatalysis and photovoltaics [1-3]. Here we presented the fabrication of CdSe/RGO nanoheterostructures and investigated the interfacial charge carrier dynamics using time-resolved fluorescence spectroscopy. The samples were prepared by depositing CdSe nanocrystals on the surface of RGO in the hydrothermal process. Note that RGO was obtained from typical Hummers&’ method. By applying the multiple chemical exfoliation treatment [4], the lateral size of RGO can be decreased to 50 nm and 5 nm. With the reduction of the characteristic size of RGO and thus the regulation of its work function, a further improved charge carrier separation for CdSe/RGO can be achieved. Time-resolved fluorescence spectra were measured to quantitatively analyze the electron transfer event between CdSe and RGO for CdSe/RGO and its dependence on modulation of the relative band structures. The correlation of charge carrier dynamics of CdSe/RGO with the result of their performance evaluation in photocatalysis may provide insightful information when using semiconductor/graphene nanoheterostructures in photoelectrochemical system for photoconversion applications.
[1] X. Y. Zhang, H. P. Li, X. L. Cui, and Y. Linb, J. Mater. Chem. 2010, 20, 2801-2806.
[2] Q. P. Luo, X.Y. Yu, B. X. Lei, H. Y. Chen, D. B. Kuang, and C.Y. Su, J. Phys. Chem. C. 2012, 116, 8111minus;8117.
[3] S. Sun, L. Gao, Y. Liu, and J. Sun, Appl. Phys. Lett. 2011, 98, 093112.
[4] B. D. Pan, J. Zhang, Z. Li, and M. Wu, Adv. Mater. 2010, 22, 734-738.

This work presents an innovative approach to fabricating controllable n-type doping graphene transistors with excellent air-stability by using self-encapsulated doping layers of titanium sub-oxide (TiOx) thin films, which are an amorphous phase of crystalline TiO2 and can be solution processed. The non-stoichiometry TiOx thin films consisting of a large number of oxygen vacancies is used as “active” n-type doping agent. A novel device structure consisting of both top and bottom coverage of TiOx thin layers on a graphene transistor exhibited strong n-type transport characteristics with its Dirac point shifted up to -80 V and an enhanced electron mobility with doping. The n-type doping density could be controlled with different concentrations of TiOx, and it can also be further tuned by illuminating ultra-violet light, which makes it possible to precisely control n-type doping level of graphene transistors. In addition to work as a controllable n-doping material, the TiOx film could simultaneously act as an effective encapsulated layer. An extended stability of the device without rapid degradation after doping was observed when it was exposed to ambient air for several days, which is not usually observed in other n-type doping methods in graphene. The technique of using an “active” encapsulated layer with controllable and substantial electron-doping on graphene provides a new route to modulate electronic transport behavior of graphene and has considerable potential for the future development of air-stable and large area graphene-based nano-electronics.
[1] “Self-encapsulated doping of n-type graphene transistors with extended air stability”, ACS Nano, Vol. 6, 6251, 2012

Tailoring the properties of carbon nanotubes (CNTs) to obtain materials suitable for various applications is an important subject that has attracted the attention of many researchers. One of the most effective ways to tune the physical and chemical properties of carbon nanotubes is to dope them with a foreign element, such as nitrogen. Nitrogen atoms that are incorporated into the CNTs structure can drastically change the properties of CNTs. The main effects are to change the number of defects in the structure and to alter molecular orbitals.
The properties of nitrogen-doped carbon nanotubes (N-CNTs) originate from its structure, thus it is practical to create a desired structure during synthesis. Here we have investigated the effect of the synthesis parameters, mainly temperature, on various characterizations of N-CNTs such as their morphologies, dimensions (diameter and length), defects, nitrogen inclusions and thermal stability. The N-CNTs were synthesized via Chemical Vapor Deposition (CVD) method using ammonia as the source of nitrogen and ethane as the source of carbon.
The results revealed strong correlations between the synthesis parameters and properties of synthesized N-CNTs. TEM images showed bamboo-like morphology at all temperatures, indicating that the growth mechanism was not altered by synthesis temperatures between 550°C and 950°C. As well, electron microscopy images revealed the presence of longer N-CNTs at higher synthesis temperatures. The diameter of nanotubes was also found to increase with increasing synthesis temperatures. XPS characterization indicated that the percentage of nitrogen inclusion decreases with increasing synthesis temperature. A TGA study demonstrated a trend of increasing thermal stability of N-CNTs with increasing synthesis temperature, up to 850°C. Based on these results by choosing the appropriate synthesis conditions, one can tune N-CNTs to control properties and obtain materials with the desired characteristics.

We report electronic measurements on high quality graphene nanoconstrictions (GNCs) fabricated in a transmission electron microscope (TEM), and the first measurements on GNC conductance with an accurate measurement of constriction width down to 1 nm. To create the GNCs, freely suspended graphene ribbons were fabricated using few-layer graphene grown by chemical vapor deposition. The ribbons were loaded into the TEM, and a current-annealing procedure was used to clean the material and improve its electronic characteristics. The TEM beam was then used to sculpt GNCs to a series of desired widths in the range 1-700 nm; after each sculpting step, the sample was imaged by TEM and its electronic properties were measured in situ. GNC conductance was found to be remarkably high, comparable to that of exfoliated graphene samples of similar size. The GNC conductance varied with width approximately as G(w) = (e2/h)w^0.75, where w is the constriction width in nanometers. GNCs support current densities greater than 120 mu;A/nm2, 2 orders of magnitude higher than that which has been previously reported for graphene nanoribbons and 2000 times higher than that reported for copper. This ranks GNC as a potential candidate for the interconnect application in the future electronics. (This work was supported by the National Science Foundation through Grant #DMR-0805136 and NIH Grant R21HG 004767 and R21HG 557611.Use of facilities of the Nano/Bio Interface Center at UPenn is gratefully acknowledged.)

Graphene has great interests because of its incredible electronic properties for the potential future electronic devices, such as high electrical mobility, high thermal conductivity and their transparency, from molecular-levelly thin two-dimensional materials. However, their intrinsical zero bandgap property is fairly hard to perform the high on-off current ratio of field effect transistor. For the overcoming this huddle, many researchers have studied opening bandgap of graphene with versatile methods. Among these attempts, graphene nano-ribbon with sub-10 nm structures for the quantum confinement was suggested and showed opening bandgap and high on-off ratio experimentally. However, to achieve a high driving current for the practical device application, densely aligned graphene nano ribbon array should be required.
A block copolymer is well-known material for the straightforwardly developing nano-sized pattern from sub-10 nm to 100 nm on large area through the self-assembly. Thus, this self-assembly based patterning materials has been intensively studied for the smaller sized pattern applied to semiconducting and hard disk industry. Our group particularly has investigated highly aligned sub-10 nm pattern formation from polystyrene-block-polydimethylsiloxane (PS-b-PDMS) block copolymer, which is representative for the high block copolymers, and their complex patterns to the application to practical devices.
In this presentation, we demonstrate sub-10-nm graphene nano-ribbon array field effect transistors which fabricated by cylindrical PS-b-PDMS block copolymer line patterns as a lithographical template on chemical vapor deposition (CVD) grown graphene monolayer sheets.

As the sibling of vertically aligned carbon nanotubes, vertical few-layer graphene network film (VGF), simply based on geometry consideration, can provide superior performance than laterally or randomly oriented graphene sheets based film (or graphene paper) for many applications, particularly for energy storage. Here we report our synthesis and material studies of VGFs, especially nitrogen-doped VGFs, for rapid supercapacitor applications.
VGFs and nitrogen in-situ doping were obtained by microwave plasma chemical vapor deposition. Nanostructure, electrical, and chemical properties of VGFs were studied. In particular, the electrochemical properties of doped VGF were investigated with the aim for energy storage. The supercapacitor performance was evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge measurements. Our results suggest that VGFs are of great potential to realize KHz supercapacitors. Terahertz (THz) time-domain spectroscopy was also used to determine the frequency dependent complex permittivity and conductivity of VGFs from DC up to THz.

Reduction of graphene oxide (GO) has recently generated intense research interest due to the possibility of using this method to inexpensively produce large quantities of graphene. Current reductive processes rely on thermal or chemical removal of oxygen functional groups from the surface. While reduction has been demonstrated, 10-12 atomic percent oxygen remains on the surface of GO after processing with current techniques. Furthermore, use of high process temperatures, ~1000 °C, in the reduction of GO leads to the generation of defects through the loss of carbon atoms in the basal plane. Therefore, the ultimate improvement in the electronic, optical, or mechanical properties of graphene that can be achieved through reduction of GO is limited by the defect formation and residual oxygen that remains after reduction. Here, we report the facile removal of oxygen functional groups from the surface through the reduction of GO in a carbon monoxide atmosphere. Common oxygen containing functional groups on the basal plane (epoxides, hydroxyls, ketone pairs) are removed from the surface of GO with CO₂ and H₂O being the only gas-phase products. We have used molecular dynamics and density functional theory calculations to elucidate the mechanisms for the reaction of CO with the functional groups on GO: these reactions are facile and proceed without degradation of the graphene sheet. The removal of oxygen functional groups from GO by CO is also confirmed experimentally using in situ attenuated total reflection Fourier transform infrared spectroscopy. Interestingly, we also observe that CO can heal defects in the basal plane of graphene. In a controlled set of experiments, we generated defects in a pristine graphene sheet grown on Cu foils and subsequently exposed it to CO: analysis of the D, G, and 2D Raman bands of graphene shows a near-complete healing of point defects such as monovacancies. Thus, our results indicate that CO-induced reduction of GO proceeds without damaging the underlying basal plane, producing reduced GO with superior properties.

Graphene&’s superior properties are expected to develop next generation electronic applications. However, a major challenge which still remains is its functionalization to incorporate it into different systems and applications without altering its superior properties. Generally, functionalization leads to the conversion of the planar sp2 hybridized state of carbon into the tetrahedral sp3 states. This conversion leads to increase in carrier scattering and reduction in carrier density and deterioration of graphene&’s properties. In this talk, we demonstrate an eta;6 functionalization route, where the d-orbital of a transition metal binds with the pi-cloud of the graphene. The result is preservation of the sp2 state of carbon atoms and the superior properties of graphene (high carrier density and low carrier scattering). Performance of several sensors based on eta;6 graphene will also be discussed. We envision that this functionalization route will significantly broaden graphene&’s scope of applications.

We have been studying graphene applications in satellite and launch vehicle technology. In particular we have examined the chemical vapor deposition of graphene from various hydrocarbon sources such as methane, ethane and propane and their effect on graphene properties. The larger molecules are found to more readily produce bilayer and multilayer graphene, due to a higher carbon concentration and different decomposition processes. However, single- and bilayer graphene can be grown with good selectivity in a simple, single-precursor process by varying the pressure of ethane. We will present data on graphene grown using this method and others along with graphene&’s potential applications including chemical sensors and thermal conduction.

Controlling the interaction of water with a solid surface is a classical and important problem in surface engineering. For a droplet resting on a solid, the angle formed between the solid-liquid interface and liquid-vapor interface in the side view is defined as the water contact angle. Higher the contact angle, the more water repellant a surface is. A material with a contact angle higher than 90° is termed hydrophobic while that above 150° is superhydrophobic. Low surface-energy materials and air-entrapping roughness features are typically used to achieve superhydrophobicity. High droplet mobility is also quintessential as it enables self-cleaning surfaces and lab-on-chip devices among other applications. When the droplet moves, the advancing front has a higher contact angle than the receding front and their difference is termed as contact angle hysteresis. Low hysteresis is desirable since it indicates lesser pinning of water droplets and hence greater mobility.
Here we explore the wetting behavior of graphene and investigate these properties in freestanding, three-dimensional, superhydrophobic graphene architectures. Template-directed chemical vapor deposition is used to grow micro-porous graphene foam, which has a lot of trapped air pockets that facilitate water droplets to stay suspended over them. In this Cassie wetting state, the droplet stays above the pores and exhibits low contact angle hysteresis. A fluoropolymer coating was used to prevent a Cassie to Wenzel state transition, wherein the droplet displaces air and completely wets the walls of the pores. An advancing angle of 163° and a receding angle of 143° were achieved. Even in rough situations such as droplet impact, the Cassie state is shown to be extremely stable and the droplet effectively rebounds off the surface without pinning to or damaging the structure. Lower hysteresis associated with the Cassie state allows droplets to roll off easily thereby removing contaminants. Such mechanically robust, superhydrophobic foams show potential for a variety of applications in self-cleaning surfaces, anti-contamination and low friction coatings.
Further, the wetting transparency of graphene was also studied and hysteresis significantly reduced on hydrophobic surfaces with nano roughness features. Monolayer to few layer graphene was used to drape the nanostructured surfaces and effect a hysteresis reduction from ~115° to as low as ~20°. This dramatic drop is once again attributed to prevention of a Cassie to Wenzel state transition. The mechanism here depends on the wetting transparency and impermeability of graphene rather than relying on a low surface energy material. The ability to minimize hysteresis on nanostructured materials without using a low surface energy material thus opens up new avenues for scalable and low cost production of lab-on-chip devices and anti-stick coatings.

Dye-sensitized solar cells (DSCs) have attracted considerable attention as the third generation solar cells because of their low fabrication cost and promising photon-to-current conversion efficiency (PCE). Two-dimensional reduced graphene oxide (rGO) and graphene oxide (GO) were incorporated into the (1) counter electrodes, (2) electrolyte and (3) photoanode of DSCs to improve their photovoltaic stability and fabrication efficiency. (1) Mesoporous rGO films fabricated by gel coating had been used as the platinum-free counter electrodes of DSCs. The rGO films could effectively catalyzed the electrochemical reduction of the triiodide ions, yielding a PCE of 7.19%, the highest efficiency for iodide/triiodide DSCs with graphene as the counter electrode. DSCs using the rGO counter electrode is more stable than that using the platinum counter electrode. (2) Amphiphilic GO having a hydrophobic core and hydrophilic side groups was used to gel 3-methoxyproponitrile (MPN). GO loading as low as 2.5wt.% was required for the gel formation. The 2.5wt.% GO-MPN organogel incorporated with iodine, 1-methyl-3-propylimidazolium iodide, guanidinium thiocyanate and 4-tert butylpyridine was used as the electrolyte in quasi-solid-state DSCs. DSCs with the GO-MPN organogel electrolyte exhibited a PCE of 6.37%, under AM1.5 illumination. DSCs with the gel electrolyte had better photovoltaic stability than its liquid electrolyte counterpart. (3) The optimal thickness of the mesoporous TiO2 photoanode is approximately 13-15µm. The fabrication of a thick and crack-free TiO2 film requires multiple printing of the TiO2 paste. A new TiO2 paste was formulated by incorporating GO into the paste. The GO served as an auxiliary binder, enabling the fabrication of a 13-15µm thick TiO2 film free of crack via a single print. DSCs fabricated from the GO-incorporated TiO2 paste had a PCE of 7.70%, similar to that fabricated from the conventional TiO2 paste without GO (7.76%). The discovery of this new TiO2 paste would greatly increase the fabrication efficiency of DSCs.

Transparent films of Pt nanoparticles on graphene were fabricated by solution casting and annealing process involving a home-made polymeric dispersant. The dispersant consisting of poly(oxyethylene)-segment and cyclic imide functionalities in the structure was prepared and used to stabilize the graphene dispersion in aqueous medium. In the second step, the in situ reduction of added dihydrogen hexachloroplatinate in ethanol afforded the homogenous dispersion of Pt nanoparticles (4.0 nm diameter) on graphene nanohybrids. Subsequently, the solution casting on polyimide substrate and annealing process allows the fabrication of large-scale dimension films that are most suitable for using as the counter electrolyte (CE) of dye-sensitized solar cells (DSSC). The characterization of the films by TEM showed the ultimate formation of Pt metal interconnected network on the surface of the 2D graphene aggregated roughness. The optimization of Pt/graphene at 5/1 w/w had led to the films that demonstrated a high counter-electrode catalytic activity and the DSSC efficiency of 8.00%, significantly outperformed the conventional platinum sputtered CE (7.14%). Moreover, the fabricated CE films appeared to be transparent and flexible for allowing the device of rear-illuminated DSSC. The graphene-Pt CE DSSC had exhibited a relatively high power efficiency of 7.01% in contrast to the merely 2.36% for the DSSC employing the conventional sputtered Pt CE, under the tests of rear-side illumination. The difference relies on the film transparency of the CE layer. Analyses of cyclic voltammetry, incident-photo-to-current efficiency and electrochemical impedance spectra were well correlated to the high efficiency of the cell performance. The use of polymeric dispersants and graphene-Pt nanohybrid is proven to be a viable approach for promoting the performances of DSSC.

Large-area (~4 cm2) and highly-crystalline monolayer nitrogen-doped graphene (NG) sheets have been synthesized on copper foils by ambient-pressure chemical vapor deposition (AP-CVD) method. As-grown graphene sheets could be easily transferred from copper foils onto other substrates (e.g. silicon/silicon dioxide wafers, TEM grids). Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the nitrogen dopants in as-synthesized NG samples are separated by one carbon atom and sit consequently on the same sub-lattice of graphene. Based on our first principles and tight binding calculations, this unbalanced distribution of dopants on one of the graphene sub-lattices will promote the opening of an electronic band gap. We control the synthesis parameters and use Raman spectroscopy and electrical transport measurements to monitor the nitrogen doping levels. Finally, we will demonstrate that NG behaves as an efficient molecular sensor, especially when performing graphene-enhanced Raman scattering (GERS) of various organic and bio-molecules.

The capability to detect ultra-low concentration of various substances vital for human activities is progressing rapidly owing to recent advances in nanomaterials. Carbon nanotubes (CNTs) and Graphene are promising nanomaterials for sensor applications. However, it is challenging to exploit their unique properties, particularly high surface to volume ratio, to achieve optimal sensitivity due to their vulnerability against contaminations. Here we show that despite considerable achievements have been made in the last several years, we are still far from what a pristine graphene or CNT can truly offer. By applying continuous in situ UV light illumination in the course of detection, we have observed 2 to 4 orders of magnitude better sensitivity than current state-of-the-art results for a range of gas molecules, and for the first time entered parts-per-quadrillion (PPQ) detection level at room temperature [1, 2]. We attribute this astonishing performance to UV light induced sensor surface cleaning during the detection. The result points out a route to exploit the intrinsic sensitivities of other nanomaterials.
[1] G. Chen, T. M. Paronyan, E. M. Pigos, and A. R. Harutyunyan, Scientific Reports 2, 343 (2012).
[2] G. Chen, T. M. Paronyan, and A. R. Harutyunyan, Appl. Phys. Lett. 101, 053119 (2012)

Hybrids of graphene with high charge carrier mobilities and semiconductor nanoparticles (NPs) with size-tunable absorption are ideal building blocks for optoelectronic devices. Coupling near-infrared absorbing PbSe NPs to graphene transforms the photoexcited states of the NPs into charge separated states. Holes are transferred to graphene, whereas electrons remain trapped in the PbSe NPs. Due to the fast charge transport in graphene a dramatic increase of photosensitivity is expected. Thus stable graphene - PbSe NP hybrids are promising materials for the fabrication of highly sensitive flexible near-infrared photodetectors.^1,2
Here we demonstrate a facile synthesis of few-layer graphene (FLG) - PbSe NP hybrids, their structural and chemical characterization (e.g. by transmission electron microscopy (TEM), Raman spectroscopy, absorption spectroscopy, infrared spectroscopy) and investigation of their response to light.
Growth of PbSe NPs on exfoliated FLG flakes was carried out using the hot-injection method. Adding FLG to a lead precursor solution leads to the coordination of the metallic precursor on the FLG flakes. The injection of a selenium precursor solution results in a direct growth of PbSe NPs on FLG without any molecular linker. Depending on the growth time, the NP size was controlled and the spectral responsivity of the hybrids was tuned. TEM images indicate that PbSe NPs with cubic crystal structure were selectively deposited on the FLG surface. Due to solvent exfoliation, FLG is free from oxidation/reduction induced defects and the high charge carrier mobility of graphene is not disturbed. Current-voltage characteristics of the FLG - PbSe NP hybrids are studied in the dark and under near-infrared illumination.

Graphene is a potential material for optoelectronics and photodetection owing to its high carrier mobility and transparence. However, its application is limited by the lack of spectral selectivity and its low photo-absorption of ~2.3%. We demonstrate a hybrid device that composes of monolayer graphene decorated with a layer of colloidal cadmium selenide quantum dots (CdSe QDs), exhibiting positive and negative photoconductivity upon irradiation with wavelength close to the quantum dots&’ excitonic absorption. The sign of the photoconductivity can be electrically controlled by modulating the charge carrier in graphene via back-gate voltage. We show that such positive/negative photoconductivity can be related to the charge transfer between QDs and graphene in the hybrid devices and we have studied the dependence of such charge transfer on QDs deposition, thermal activation and light illumination.

Graphene has been investigated for the applications in solar cells, transistors, sensors and light-emitting diodes due to its unique properties such as high mechanical, electrical, optical and thermal properties. 2D graphene film is one of the ideal materials for chemical sensors because of its high surface to volume ratio and facile integration with other electronic devices. 3D graphene foam can enhance the conductivity and mechanical durability further while maintaining the superior properties of the 2D graphene film. Also porous structure of 3D graphene foam is an extra advantage in chemical sensors. In this study, we demonstrated graphene-based NO2 chemical sensors by using 2D graphene film and 3D graphene foam on both flexible PET and paper substrates.
Chemical vapor deposition grown graphene film and foam were transferred to PET and clean room paper substrates, respectively. In the case of PET substrates, Au electrodes were sputtered with shadow mask before the transfer of graphene layer. In the case of paper substrates, graphene film and foam were turned over to expose graphene with the PMMA layer on back, followed by transfer to the paper substrate. Finally, two electrodes were formed by silver paste. Micro-Raman spectroscopy was conducted to characterize quality and thickness of 2D- and 3D graphene structures. Electrical behaviors under NO2 and air ambient were investigated under various strain conditions by monitoring I-V characteristics and time-dependent dynamic response. Our flexible graphene-based sensors on both PET and paper substrates showed rapid response under various strain conditions after the introduction of 200 ppm of NO2not; gas due to the charge transfer between graphene and NO2 molecules, which act as p-type dopants in graphene. Highly sensitive graphene sensors on flexible substrates were demonstrated on PET and paper substrates. Compared with monolayer graphene, the 3D graphene foam showed higher conductance and better mechanical stability. The details of our experiments and results will be presented.

We report our systematical studies on the structures and electrical properties of single layer graphene (SLG) top-gated devices using Y2O3 as dielectric layers. The ultrathin (~5nm) Y2O3 layers fabricated for the sandwiched device of SiO2/graphene/ Y2O3 are structurally uniform and dense, which is verified by the characterizations of AFM, TEM and Raman spectroscopy. Moreover, these SLG field-effect transistors (FETs) at cryogenic temperatures have excellent performance with transconductance up to 13000 mu;F/Vs (gmN=mu;Cox), which is among the highest in graphene FETs reported previously. The modification of electronic properties (due to insignificant underlying disorder) by Y2O3 layers on top of graphene have also been well studied including transport and quantum capacitance measurements. Based on Boltzmann transport theory concerning the change of dielectric environment, Coulomb scattering is confirmed quantitatively to be dominant in the sandwiched device and very few short-range impurities have been introduced by Y2O3. Both DC transport and AC capacitance measurements at high magnetic fields demonstrate that Landau levels have broadened more obviously than uncovered graphene on SiO2, which is caused by the additional charge impurities and enhanced inhomogeneity of carriers introduced by Y2O3 layers.

Graphene is an emergent candidate as transparent electrode material for next generation (opto-) electronic devices. Grown by chemical vapour deposition on copper foil results in large-area polycrystalline graphene sheets with excellent structural and electronic properties that can be transferred to virtually any other substrate [1]. The use of graphene as a transparent electrode in organic electronic devices, such as light emitting diodes and solar cells, requires matching its work function to the hole and electron transport levels of the organic semiconductors, in order to minimize charge injection barriers. This can, in principle, be achieved by depositing strong electron acceptor or donor molecules that undergo a charge transfer on the surface and thus induce surface dipoles. However, many potential electron acceptors, such as tetrafluoro-tetracyanoquinodimethane (F4TCNQ), are too volatile for practical applications. In this work, we show with photoelectron spectroscopy that the work function of graphene on glass can be continuously increased from 3.7 eV up to 5.5 eV with (sub-) monolayer coverage of the strong organic molecular acceptor hexaazatriphenylene-hexacarbonitrile (HATCN), which is practically relevant due to its high molecular weight (384 g/mol). A charge transfer type interaction between HATCN and graphene is verified by a low binding energy emission in the N 1s core level region and the observation of a presumably filled level derived from the lowest unoccupied level of HATCN. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy indicates a coverage-dependent re-orientation from flat-lying to vertically inclined HATCN in the monolayer regime, in analogy to what was reported for HATCN deposited on silver (Ag) surfaces [2]. This structure change may explain the sub-linear increase of the work function as function of HATCN coverage.
[1] Li, X. S. et al.; Science 324, 1312-1314 (2009)
[2] Bröker B.et al.; Phys. Rev. Lett., 104, 246805 (2010)

Lithium ion batteries (LIBs) have now become important power sources because of their high energy density and high voltage. Commercial anode materials in LIBs are usually graphitic due to their intrinsic stability and low cost. However, because graphitic materials have a relatively small specific capacity (372 mAh g-1), these materials cannot the performing demanding task of high capacity storage. Therefore, alternative anode materials with higher specific capacity such as metals, metal oxides, and metal sulfides should be substituted for the graphitic materials.
Among the metal oxides, SnO2 is an attractive material for the fabrication of negative electrodes for lithium ion batteries because of its high theoretical reversible capacity of 782 mA h g-1, which is more than twice that of graphite. Unfortunately, insertion of lithium ion induces a large volume change, which causes rapid degradation of the cycling performance and even cracking of electrode.
Graphene, an atomic single layer of honeycomb carbon lattice, has attracted great attention because of its superior electronic conductivity, high surface area (theoretical value 2600 m2 g-1), and physicochemical stability. Graphene used as substrate for metal oxide nanoparticles already showed to improve the mechanical stability and the electrochemical performances. Therefore, graphene-metal oxide composites are good candidate as anode for lithium ion batteries. However, generally, graphene-metal oxide composites are prepared in several steps before a final annealing aimed to improve the crystallinity of the oxide and to reduce graphene oxide. Therefore, known processes are commonly complicated and time-consuming.
In this study, we introduce a facile one-pot process for the fabrication of SnO2 nanoparticles-graphene nanocomposites based on the hydrothermal synthesis assisted by hydrazine, which control the SnO2 nanoparticles formation and crystallization as well as the reduction of graphene oxide to graphene. This strategy provides a highly crystalline oxide and more complete reduction of graphene oxide. The SnO2-graphene composite consists of 3-4 nm monodisperse SnO2 nanocrystals uniformly dispersed at the surface of graphene. When used as anode material for lithium ion batteries, it exhibits a first discharge capacity of 1662 mA h g-1, which rapidly stabilizes and still remains at 626 mA h g-1 even after 50 cycles, when cycled at a current density of 100 mA g-1. Even at the very high current density of 3200 mA g-1, the composite displays stable capacity of 383 mA h g-1 after 50 cycles. It is evident that the graphene framework has greatly improved the electron transfer and the stability of electrode compared to pure SnO2. The results prove that graphene-based heterostructures synthesized by this novel hydrothermal method are good candidates for anode materials in lithium ion batteries. Finally, this synthesis approach can be probably extended and generalized to large variety of metal oxides.

In addition to Graphene 2D transition metal chalcogenide, as for example MoS2 and WS2, nanostructures are promising materials for applications in electronics and mechanical engineering. Though the structure of these materials results in a highly inert surface with a low defect concentration, defects and edge effects can strongly influence the properties of these nanostructures. Therefore, a basic understanding of the interplay between electronic and mechanical properties and the influence of defects, edge states and doping is needed.
We demonstrate on the basis of atomistic quantum mechanical simulations of several types of MoS2 nanostructures how the topology, the edge structure, local defects, dislocations, doping and mechanical deformation can vary the mechanical behavior, the electronic properties, and also determine the electronic device characteristics of such systems.

In this talk, we will discuss the approaches to control electronic and optical properties of two-dimensional materials, graphene and monolayer molybdenum disulfide (MoS2).
First, we address the possibility of controlling the effective dielectric constant k of 2D materials by fabricating graphene or MoS2 devices that are suspended inside liquids ranging from hexane (k~1.9) to water (k~80). With increasing k, we observe robust signatures of screening of electrostatic interactions between charge carriers in both graphene and MoS2. For graphene, we observe a rapid increase of carrier mobility with k, with room temperature mobility reaching 60,000 cm2/Vs for high-k liquids. For MoS2, increasing k causes an upshift in photoluminescence energy, consistent with decreasing binding energy of excitons in this material.
Second, we investigate the influence of uniform mechanical strain fields on both monolayer MoS2 and graphene. In suspended graphene, the presence of strain quenches flexural phonons, a strong scatterer of charge carriers. We observe an experimental signature of this quenching -- room temperature resistance of graphene decreasing with strain. In monolayer MoS2, we observe large changes in intensity and positions of photoluminescence peaks. These changes are consistent with predicted transition from direct to indirect band gap character in this material at strain levels of ~2%.

It is well known that carbon-based nanomaterials such as graphene and carbon nanotubes exhibit remarkable mechanical, electrical, thermal and optical properties which has stirred a great deal of excitement for considering them for a wide variety of applications ranging from nanoscale transistors, interconnects, field-emission displays, photo-voltaics and nano-electro-mechanical-systems (NEMS). The investigation of graphene as a model 2D system has impacted a diverse array of fields spanning physics, chemistry, materials science, and engineering. While great strides have been made recently for applications that have stemmed from graphene&’s unique properties, in electronics applications, specifically digital electronics, the absence of a band-gap in graphene poses concerns for its attractiveness to enable high ON/OFF ratios. Although a band-gap in graphene is induced through quantum confinement by creating graphene nanoribbons, the band gaps nonetheless are small (few hundred meV) and it is challenging to maintain pristine edge chirality due to defects that are induced during nanofabrication of the ribbons. Recently, layered 2D crystals of other materials similar to graphene have been realized which include insulating hexagonal-BN (band gap ~5.5 eV) and transition metal di-chalcogenides which display properties ranging from superconducting NbS2 to semiconducting MoS2. In addition, it has been shown that bulk MoS2 films transform from an indirect band-gap semiconductor with a band gap of ~1.2 eV to a direct band gap semiconductor with a gap of ~1.8 eV for single atomic layers. The device applications of such systems show promising characteristics; for example transistors derived from 2D monolayers of MoS2 show ON/OFF ratios many orders of magnitude larger than the best graphene transistors at room temperature, with comparable mobilities. In this talk, I will provide an overview of the Electronics, Photonics and Magnetic Devices (EPMD) program in the Electronics, Communications and Cyber Systems (ECCS) division where graphene, as well as other layered 2D nanomaterials, are playing an important role for enabling innovative device applications in electronics, photonics and sensing.

Atomically thin layers can be formed by carbon, BN, metal chalcogenides, oxides and hydroxides, but no carbides have been reported to exist in 2-D form until recently. Transition metal carbides are known for many outstanding properties, but their strong bonds render their exfoliation into 2-D layers quite difficult. Nevertheless, we have synthesized a new family of early transition metals carbides in 2-D form, starting with the MAX phases. The latter is a large family (+60 members) of layered hexagonal ternary metal carbides and/or nitrides, where “M” stands for an early transition metal, “A” stands for a group 13 to 16 element, and “X” stands for carbon and/or nitrogen. Etching the “A” layer from MAX phase using hydrofluoric acid or other etchants at room temperature results in weekly bonded M_n+1X_n layers that can be separated by sonication yielding 2-D layers. To emphasize their similarity to graphene, we labeled them MXenes. The following compounds have already been synthesized: Ti₂C, Ti₃C₂, Nb₂C, Ta₄C₃, TiNbC, (V_0.5Cr_0.5)₃C₂, and Ti₃CN. A review of the published information about synthesis and properties of MXenes will be presented. DFT simulations showed that their band gaps can be tuned by changing their surface termination, and that their elastic properties along the basal planes are superior to their 3-D binary counterparts. MXene powders are ductile enough to be cold pressed into free-standing discs with conductivities comparable to graphite. Water contact angle measurements showed hydrophilic behavior due to oxygen or OH termination of MXene surfaces. Among a multitude of possible applications, herein we focus on electrical energy storage systems, such as lithium ion batteries, lithium ion capacitors and electric double layer capacitors. The simplicity of the synthesis process enables large-scale production of MXenes in quantities from grams to kilograms.

Besides graphene and hexagonal boron nitride, transition metal chalcogenides (TMC) such as MoS2, WS2, NbS2 and WSe2 also exhibit a layered structure in which the layers weakly interact via Van der Waals forces, and for this reason these materials exhibit excellent lubrication properties. For TMC, the layers are formed by the transition metal atom sandwiched by the sulfur atoms. MoS2 and WS2 in bulk are indirect band gap semiconducting materials. However, an isolated sheet of MoS2 or WS2 becomes a direct gap semiconductor [1]. This particular behavior makes them very attractive in terms of optical properties such as spin polarization, in which the lack of center of inversion of one layer plays a crucial role [2,3]. Therefore, it is important to study the properties of different configurations of bi-layer TMC systems, generated by displacing or rotating one layer with respect to the other, thus breaking the inversion symmetry of the bulk stacking. In order to first assess this issue, Density functional theory (DFT) calculations were carried out for different bilayer WS2 geometries considering different rotation angles and different displacements. It was found that for particular geometries of the bilayer systems, the indirect and direct band gaps “compete”, thus exhibiting different electronic and optical properties. As in graphene, Raman spectroscopy provides relevant information about the different layer stackings and twistings in these materials [4]. From the experimental standpoint, we will demonstrate that chemical vapor deposition (CVD) routes could result in the growth of TMC twisted layers, thus making this technique adequate to study the optical properties of new configurations experimentally. We will discuss these issues and also possible applications of these twisted bi-layers.
References
1.- Fai Mak, K., Lee, C., Hone, J., Shan, J., Heinz, T.F., Physical Review Letters, Vol. 105, 136805 (2010).
2.- Zeng, H., Dai, J., Yao, W., Xiao, D., Cui, X., Nature Nanotechnology, Vol. 7, 490-493 (2012).
3.- Fai Mak, K., He, K., Shan, J., Heinz, T.F., Nature Nanotechnology, Vol. 7, 494-498 (2012).
4.- Sato, K., Saito, R.,Cong,C., Yu, T., Dresselhaus, M.S., Physical Review B, Vol. 86, 125414 (2012).

Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. In this talk, we will discuss our recent results on the creation of two-dimensional single atom thick hydrogen-terminated and organic-terminated germanium (GeR) analogues of graphane (CH). Contrary to the indirect gap of Ge at 0.67 eV, these materials have direct band gaps centered at 1.5 eV for GeH, and are tunable depending on the nature of the surface ligand. These materials represent a new class of covalently-terminated single atom thick derivatives of Group 14 semiconductors, and have great potential for a wide range of optoelectronic and sensing applications.

SnS2 and SnS2/SnS superlattice nanotubes were synthesized in copious amounts using both sealed ampoules and a horizontal/vertical flow systems with Bi and Sb2S3 as catalysts [1]. The outer diameter of the tubes ranged from 13 to 165 nm, and their length varied from 90 nm to 3.2 micrometers. Careful examination in HRTEM revealed that most of the tubular structures consisted of layers showing periodic or almost periodic patterns which can be interpreted as a superstructure of SnS2 and SnS layers, whereas others exhibited evenly spaced fringes of SnS2. This observation was further confirmed by Raman measurements of individual nanotubes after the examination in HRTEM.
Chemical profiling showed a constant profile of the catalysts along the tube axis. It is believed that the high sulfur affinity of Bi causes partial decomposition of the pseudo hexagonal SnS2 precursor producing the sulfur deficient SnS and the misfit layered tubular structures of a type (SnS)n(SnS2)m. The scrolling process was promoted by adding miniscule amounts of Sb2S3 powder to the ampoules.
The tubular morphology of the SnS2/SnS ordered superstructure nanotubes is a result of the lattice mismatch between the two sublattices comprising the host-guest structure. This driving force comes in addition to the already established closure mechanism, i.e., annihilation of dangling bonds at the periphery of the layers of the inorganic nanotubes (INT) nanostructures.
The diversity of the structures manifests itself through different stacking orders of SnS2 and SnS layers along their common c-axis and their relative in-plane orientation. Folding vectors and chiral angles of both subsystems can be determined [2].
The scaling-up of the nanotubes synthesis was accomplished in a horizontal/vertical flow reactor. These reactors have produced 10-20 mg of either SnS2 or SnS/SnS2 ordered superstructure nanotubes phase in 30% yields.
1. G. Radovsky, R. Popovitz-Biro, M. Staiger, K. Gartsman, C. Thomsen, T. Lorenz, G. Seifert and R.Tenne, Angew. Chem, Intl. Ed. 50 (51), 12316-12320, (2011)
2. G. Radovsky, R. Popovitz-Biro, and R. Tenne, Chem. Mater. 24, 3004-3015, (2012).

Charged impurities, through their ability to deflect the motion of electrons, can alter the electronic properties of low dimensional materials in unexpected ways. We shed light on this process by using scanning tunneling spectroscopy to monitor the influence of an isolated impurity on the electronic distribution and Landau level spectrum of graphene in a magnetic field. Sweeping the energy from one Landau level to the next the observed spatial distribution of the wavefunction alternates between extended-states and states localized on the impurity. We find that the impurity strength, as measured by its effect on the Landau levels, depends on the fractional filling of extended states and can be controlled by a gate voltage. At low filling the impurity is screened becoming essentially invisible. Screening diminishes with filling until, for fully occupied states, the impurity is unscreened and significantly perturbs the spectrum. In this regime we report the first observation of Landau-level splitting due to lifting of the orbital degeneracy.

Seeking to understand the unique nature of the charge carriers in back-gated graphene devices, we performed scanning tunneling microscopy (STM) and spectroscopy (STS) experiments at low temperatures and in magnetic field. These techniques give access, down to atomic scales, to structural information as well as to the density of states.
In particular, we investigate the electronic properties of graphene in the quantum Hall regime by studying Landau quantization and its dependence on charge carrier density. Measurements were carried out on exfoliated graphene samples deposited on SiO2 as well as on BN. Upon changing the carrier density we find abrupt jumps in the Fermi level after each Landau level is filled. By performing spatially resolved STS/STM we demonstrate the true discrete quantum-mechanical electronic spectrum within the Landau level band near an impurity in graphene in the quantum Hall regime. Moreover, we show a way to tune the strength of the impurity by using a back-gate voltage.

The chemical functionalization of graphene can tailor its electronic, chemical, mechanical, and tribological properties. Importantly, these properties are often inextricably coupled together. Here, the coupling among the nanoscale chemical, structural, and frictional characteristics of fluorinated graphene (FGr) were probed with atomic force microscopy (AFM), photoelectron spectroscopy, Raman microscopy, and molecular dynamics (MD) simulations. FGr is made by exposing graphene to XeF2 gas, with longer exposures producing greater fluorination. Although bulk fluorinated graphite has a very low surface energy, our experiments and simulations both show that friction between nanoscale tips and FGr exceeds that for pristine graphene. This is a particularly strong effect that increases monotonically with greater fluorination, with friction on FGr > 10x higher than on pure graphene. The ability to resolve an ordered lattice in atomic stick-slip friction measurements also diminishes with greater fluorination, indicating that the fluorinated graphene is disordered. Quantitative AFM friction and Raman microscopy measurements also reveal a thickness dependence of the fluorination process. Finally, the simulations show substantial charge transfer from the tip to the highly electronegative fluorine groups. Collectively, these results provide important new insights into the atomic-scale effects of functionalization on graphene, showing how the structure, chemistry, electronic behavior, and frictional behavior are all coupled together. They also demonstrate how atomic scale friction is a sensitive probe of the local chemistry and structure of functionalized graphene.

One of the most fundamental properties of any electronic material is the nature of the interactions between quasiparticles. In graphene, when the chemical potential coincides with the Dirac point energy, the unscreened coulomb interaction has an important long-range contribution that leads to unusual logarithmic renormalizations. In addition, the electron-electron interaction is believed to share an important interplay with the electron-phonon interaction. The interplay between the two can lead to novel property of graphene. Here we address these properties by investigating graphene and doped graphene on different substrates with high-resolution angle-resolved photoemission spectroscopy (ARPES). We present the first direct measurements of the self-energy in graphene near the neutrality point. By examining the electron-electron interaction in different regimes, we show that the many-body physics in graphene differ from those of an ordinary metal, and can be properly tuned to give rise to novel correlated phases.

I will discuss transport spectroscopy measurements of Landau level gaps in dual-gated suspended BLG, which is achieved by using source-drain bias voltage V as a spectroscopic tool in the quantum Hall regime. Plotting the device&’s two-terminal differential conductance G=dI/dV as a function of V and carrier density n yields a series of distinct diamonds, which correspond to and evolve with QH plateaus. These diamonds arise from charge transport across graphene when the Fermi level is aligned to or detuned from Landau levels in the bulk of the device, and yield information on the bulk gap and edge channel transport. Using this technique, and exploiting our additional gate, we measure the evolution of nu; = ±4, -2 and -1 gaps as a function of magnetic field B (in the QH regime) in zero and finite out of plane electric field E. Our results provide insight into the E dependence of these QH states.

Understanding and manipulating the intrinsic properties of materials is crucial both from a scientific point of view and for practical applications. This challenge is particularly apparent in the case of atomically-thin materials like graphene, whose properties are strongly affected by interactions with adjacent substrates. For instance, the intrinsic mobility of electrons and holes in graphene is reduced by approximately a factor of ten [1,2], and the thermal conductivity by a factor of five when placed onto a typical substrate like SiO₂ [3-5]. Several studies have examined the intrinsic electrical and thermal properties of graphene by suspending it [2-4], however such reports have only focused on low-field and low-temperature transport.
In this work we study the intrinsic transport properties of suspended graphene devices at high fields (>1 V/mu;m) and high temperatures (>1000 K) for the first time. We find that at ~1000 K the average charge carrier saturation velocity is ~1.2×10⁷ cm/s (greater than in silicon) and the average thermal conductivity is ~300 W/m/K (similar to graphite and diamond). Combined with a detailed theoretical analysis, our study reveals salient features of charge and heat flow in graphene up to device breakdown under extreme operating conditions.
We fabricated over 20 devices using both mechanically exfoliated graphene and graphene grown by chemical vapor deposition (CVD) with Cr/Au contacts, initially on SiO₂(300 nm)/Si substrates. Then, we suspend the graphene by partially etching ~200 nm of the supporting SiO₂. We measure and model the suspended graphene current-voltage characteristics at high fields, up to electrical breakdown in both air and vacuum (~10^-5 torr) conditions. These comparisons enable us to estimate the breakdown temperature in vacuum to be 2230 +/- 720 K. We identify and discuss sample-to-sample variation from edges, defects (some introduced during fabrication), and uncertainty in device dimensions. Interestingly, the electrical transport appears entirely dominated by thermally-generated carriers at such high temperatures (>1000 K). The thermal conductivity is 300 +/-100 W/m/K at T = 1000 K for both exfoliated and CVD graphene. We also obtain a model of graphene thermal conductivity, finding it varies as ~T^-1.8, consistent with the behavior of graphite at such high temperatures.
[1] V. E. Dorgan et al., Appl. Phys. Lett. 97, 082112 (2010).
[2] K. I., Bolotin et al., Sol. St. Comm. 146, 351-355 (2008).
[3] S. Chen et al., Nat. Mat. 11, 203-207 (2012).
[4] W. Cai et al., Nano Lett. 10, 1645-1651 (2010).
[5] J. Seol et al., Science 328, 213-216 (2010).

We have used ultrahigh vacuum scanning tunneling microscopy to study graphene quantum dots (GQDs) deposited by dry contact transfer (DCT) [1] onto atomically clean semiconductor surfaces. GQDs on H-Si(100) exhibit the expected size-dependent energy gap, except for a number of sub-10 nm GQDs that show no gap. Careful analysis shows that these GQDs have predominantly zigzag edges with an associated metallic edge state that extinguishes the gap [2].
GQDs deposited by DCT onto UHV on cleaved GaAs(110) and InAs(110) substrates exhibit an electronic semitransparency effect in which the substrate electronic structure is visible through the graphene when the equilibrium graphene-substrate spacing is reduced by about 0.06nm [3]. We have also studied GQDs on H-Si(100) in which STM electrons are used to remove the hydrogen from beneath the graphene [4]. Simulations predict the experimentally observed electronic states at the Fermi level and modification of the graphene surface structure in accord with covalent bonds forming between graphene and clean Si(100).
We have also studied graphene grain boundaries for CVD grown (on Cu) graphene that has been transferred to silicon dioxide and mica substrates. STM images show graphene grain misorientation angles and standing wave patterns with ~1 nm decay lengths adjacent to the grain boundaries. For the mica case the graphene exhibits a much smaller rms roughness and there is clear evidence for multiple layers of solid water trapped beneath the graphene [5]. The ability to manipulate this water is also demonstrated.
We have used scanned probe microscopy extensively in our studies. This has benefitted recently by the development of an ion-based probe sharpening technique that we call field-directed sputter sharpening (FDSS) [6]. In FDSS, an ion beam is directed towards a biased probe. The bias establishes an inhomogeneous electric field that deflects the incoming ions to reduce the sputtering of the sharpest (highest electric field) region of the probe. This results in further sharpening of the probe, which enhances the effect, leading to probes with radii down to 1 nm. Furthermore, these probes can be coated with ultra-hard conductive coatings prior to FDSS.
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[2] K. A. Ritter, J. W. Lyding, Nature Materials 8, 235 (2009).
[3] K. He, et al. Nano Letters 10, pp 2446-3452 (2010).
[4] Y. Xu, et al. Nano Letters, in press (2011).
[5] K. T. He, et al. Nano Letters 12, 2665-2672 (2012).
[6] S. W. Schmucker, et al. Nature Communications 3, article 935 (2012).

AC electrical techniques, such as impedance spectroscopy, are useful for probing the different electrical processes present in a material being measured, resulting in the ability to separate the contributions of each process to the overall electronic behavior. Carbon nanotube thin films of different thickness were obtained by depositing 1-20 layers of multiwalled carbon nanotube solutions onto paper substrates. By not only varying the number of layers but also the carbon nanotube concentrations and the pore size of the paper substrates, it is possible to detect the evolution of all of the equivalent circuits that the carbon nanotube films demonstrate. Measurements were carried out in-plane and thru the thickness, allowing for the determination of the percolation threshold with respect to the number of layers in both directions. For films which are low density in plane, the resulting spectrum does not allow for the determination of the carbon nanotube properties separately as the insulating nature of the substrate overshadows the carbon nanotube conduction. For films which are sufficiently dense in plane as well as thick, bulk behavior of the carbon nanotubes are easier to detect.
However, for films which are sufficiently macroscopically dense but not overwhelmingly thick, previous in-plane impedance measurements on carbon nanotube thin films deposited on paper substrates found an equivalent circuit that contains three separate semicircles (referred to as the nested-RL equivalent circuit). At high frequencies, where short range interactions dominate, a smaller semicircle corresponding to carbon nanotube bundle resistance is found. At low to medium frequencies, a larger semicircle corresponding to bundle to bundle junction resistance appears. At very low frequencies, a third semicircle with a resistance greater than that of the bundle semicircle is found, corresponding to carbon nanotubes which are coiled around paper fibers.
Conversely, in the thru plane measurements, the impedance spectra do not show three distinct semicircles, and instead are constant in shape but different in magnitude as the number of layers is increased. For these films, percolation is a function of the number of layers deposited but is also a function of the carbon nanotube concentration of the solution used. For example, for the 1 mg/mL MWNT concentration, the thru-plane is already percolated at 1 layer. A secondary, smaller percolation occurs at higher layer numbers which can be found where the change in magnitude of the spectra is the largest. The first percolation threshold occurs with respect to concentration. While the equivalent circuit does not evolve in shape, as is the case for the in-plane measurements, it does change enough in size to track the secondary percolation behavior.

Recently, gas sensing polymer composites have broadened the applications of conductive polymer composites. Much attempt has been devoted to improve the sensitivity, response time and stability of gas sensors by modifying the sensing materials or fillers. The conductive fillers utilized to produce gas sensors have been concentrated on carbon blacks, carbon nanotubes and graphite but they still have limitations of sensitivity and/or cost. The novel nanoporous carbon, derived from polybenzoxazine, has been developed as an alternative conductive filler. In this study, conductive polymer composites fabricated by polyvinyl alcohol filled with nanoporous carbon were used as model vapor sensors. The porous structure can enhance the response sensitivity of the sensors. The effects of filler loading content were investigated in order to compare the selectivity and sensitivity towards different organic vapors.

Conductive aluminum paste electrodes are widely used in electronic devices, but the electric property is not as good as that of the bulk electrodes due to its inherent porous structures. In order to improve the electric conductivity of the aluminum paste electrodes, thin conductive layers were coated onto the surface of the porous electrodes to form the composite electrodes. In this study, the influences of different coating materials of polyaniline (PAni), graphene/polyaniline (G-PAni), and graphene oxide/polyaniline (GO-PAni) on the electric properties of the composite electrodes were investigated.
The PAni adopted in this work was prepared by emulsion polymerization by using sodium dodecyl sulfate as the dispersant. G-PAni and GO-PAni were synthesized by in situ polymerization using the chemically reduced graphene (CRG) and the graphene oxide (GO), respectively, as the additives. The organic solutions of PAni, G-PAni, and GO-PAni were prepared and coated separately onto the Al paste electrodes for fabricating composite electrodes.
The XRD spectra showed that, the intensity of characteristic peak of PAni at 2theta; = 20.1° reduced with the increasing of CRG and GO in G-PAni and GO-PAni, respectively, while the peak at 2theta; =13.3° increase inversely; it indicates that the PAni chains are arranged in a more ordered manner on the crystal plane when more CRG and GO were introduced. The TGA results showed that the decomposition temperature of PAni is around 550°C which was shifted to 580°C when CRG or GO were introduced. Both G-PAni and GO-PAni possessed better thermal stability which may result from their lower oxidation levels, that are containing more benzenoid rings, and more ordered structures.
The carrier mobilities in PAni, G-PAni, and GO-PAni films were measured using Hall Effect instrument, and the results showed that the G-PAni possesses the best performance with the value in the range from 4×10^-1 to 6×10^-1 cm^2/Vs. Furthermore, among the three composite electrodes, the G-PAni/Al paste electrode showed the lowest resistance of 10 mOmega;/sq. which is lower than that of the Al paste electrode of 12 mOmega;/sq. (both were tested with a thickness of 30 mu;m). The improvement in electric conductivity of Al paste electrodes by G-PAni coatings is ranging from 12 to 17 % depending on the CRG content in G-PAni, indicating that the G-PAni could be an efficient coating material for Al paste electrodes.

Carbon nanotubes (CNTs) are generally synthesized by chemical vapor deposition (CVD) using a carbon containing precursors, such as hydrocarbons, carbon monoxide or alcohol, and a transition metal catalyst, such as Ni, Co or Fe. A large body of literature describing CNT growth mechanism and the role of catalyst structure is available. Both ex situ and in situ high resolution images have shown that the shape of the metal catalyst nanoparticle changes during the CNT growth. However, the driving force for this shape change and its relationship to the CNT growth is still unclear. We have used an environmental scanning transmission electron microscope (ESTEM) to record video-rate images for in situ observation of catalyst nanoparticles during the CNT growth. Elongation, reshaping and breaking of the catalyst nanoparticles during the CNT growth was frequently observed. This common phenomenon was found to be associated with the CNT wall formation. For example, the number of walls changed during the catalyst elongation and contraction cycle. Moreover, in some cases, the catalyst not only elongated, but also broke into two parts, resulting in change in CNT growth direction or termination. Based on these observations, we present a model to explain the observed morphing behaviors of the catalyst nanoparticles during the CNT growth based on the interplay among the surface energies of carbon and metal, CNT/metal interfacial energies, and the stress exerted on the catalyst nanoparticle by the CNT. We will present the relationship between the CNT growth and the cyclic modification of the catalyst nanoparticle shape.

Recently, porous magnetic carbon nanofibers (CNFs) have attracted considerable research interest due to their unique magnetic responsivity, large surface area, high porosity and good chemical durability, which have shown various potential applications in a wide range of fields. The major challenge for porous magnetic CNFs is to development a robust and scalable strategy for the synthesis of Fe3O4 nanocrystals embedded CNFs with controllable porous structure and high porosity. In this contribution, we have demonstrated a novel strategy for the fabrication of hierarchical porous, magnetic Fe3O4@carbon nanofibers (Fe3O4@CNFs) based on polybenzoxazine precursors by a combination of electrospinning and in situ polymerization. The benzoxazine monomers could easily form thermosetting nanofibers by in situ ring-opening polymerization and subsequently be converted into CNFs by carbonization. The resultant fibers with an average diameter of 130 nm are comprised of carbon fibers with embedded Fe3O4 nanocrystals, and could have a high surface area of 1885 m2/g and a porosity of 2.3 cm3/g. Quantitative pore size distribution and fractal analysis were used to investigate the hierarchical porous structure using N2 adsorption and synchrotron radiation small-angle X-ray scattering measurements. The role of precursor composition and activation process for the effects of the porous structure is discussed, and the relative fraction of closed and open pores in CNFs is confirmed. The Fe3O4@CNFs exhibit efficient adsorption for organic dyes in water and excellent magnetic separation performance, suggesting their use as a promising adsorbent for water treatment, and also provided new insight into the design and development of a carbon nanomaterial based on a polybenzoxazine precursor.

We describe a facile fabrication of solid-state actuators by coating common polymer films with a layer of carbon nanotubes. The composite material actuators are multifunctional energy transducers and were powered by heat, light, or electricity. The maximum observed force produced by an actuator was 60 times its own weight. Actuators were also demonstrated to bend more than 90 degrees. The actuators were repeatedly activated for nearly 50,000 cycles without significant loss of performance for a sub-hertz actuator and 1,000,000 cycles in the case of a 30 Hz actuator. The utility of these devices was demonstrated by creating a walking robot.

The theoretical understanding of the linear optical response in carbon nanotubes (CNTs) is extremely well developed, both at the single(quasi)-particle and at the many-electron (exciton) level. The latter has proven crucial in obtaining good agreement between experimentally recorded and theoretically derived spectra, summarizing “excitonic effects” as an intensity increase and red-shift of the first E11 absorption peak of the single-particle spectrum.
However, the non-linear optical properties of CNTs remain scarcely investigated, and then only on the single particle level, even though both experimental and theoretical evidence support their ability to generate harmonic fields at efficiencies surpassing those of many bulk semiconductors[1].
Even considering the relatively simple effects of excitons on the linear spectrum, it is not immediately apparent how these would modulate the non-linear response function relative to the single-particle result due to its more complicated resonance structure. This questions the interpretation of non-linear experimental results based on single-particle models.
In this work, the second harmonic response in CNTs is investigated theoretically, applying single-particle methods in addition to a phenomenological continuum Wannier model and a tight-binding based Bethe-Salpeter approach. The excitonic response in chiral tubes is studied and compared to single particle results. Additionally, electric field induced second harmonic generation in achiral tubes is considered, again comparing single-particle and excitonic results. Based on the rather strict CNT optical selection rules and the model perturbative response function, the features (such as resonances, onsets of dispersive response, etc.) of the calculated spectra are explained, and the consequences of including excitonic effects is clarified.
Application of strong electrostatic fields transversal to the tube axis is shown to modulate the band structure, leading to changes in the linear optical response, already considered on the single-particle level elsewhere. This so called Stark-effect is considered on the excitonic level, and the effects of band-structure modulation on the second harmonic response is also considered. Finally, the possibility of applying second harmonic spectroscopy as a probe for the Stark effect is discussed.
1) T. Garm Pedersen and K. Pedersen, "Systematic tight-binding study of optical second harmonic generation in carbon nanotubes", Phys. Rev. B 79, 035422 (2009).

Despite the exceptionally high values for thermal conductivity reported for individual carbon nanotubes (CNTs), measurements on CNT based materials (buckypaper, CNT films and mats) have yielded conductivity values that are orders of magnitude lower than those of the constituent CNTs. Two major factors have been proposed as being responsible for the low conductivity of the CNT materials: The reduction of the intrinsic conductivity of individual CNTs due to inter-tube interactions and low thermal conductance at CNT-CNT interfaces that may be sensitive to the surrounding environment. There is disagreement on the degree to which physical parameters such as CNT length, interaction from neighboring CNTs, and contact area between interacting CNTs may affect these properties. In addition, there is wide discrepancy in the quantitative values of CNT conductivity and CNT-CNT conductance reported in the literature. Non-equilibrium molecular dynamics (MD) simulations are employed to investigate the effects the local physical structure has on intrinsic intra-tube conductivity and inter-tube conductance. Results show inter-tube conductance to be proportional to contact area and only weakly affected by direct inter-tube interactions from neighboring CNTs. Intrinsic intra-tube conductivity is shown to be independent of inter-tube interactions in coherent bundles characteristic of CNT network materials. In addition, the variability of data reported in literature is addressed by demonstrating that variation in simulation procedural parameters, such as boundary conditions, atomic potential, and implementation of the thermal baths regions, can have significant and unintentional effects on simulation results. Though rarely cited and possibly overlooked, the effects of differing procedural approaches can contribute significantly to the wide variation in computational predictions reported in the literature.

Growth of carbon nanotubes (CNTs) by catalytic chemical vapour deposition (CVD) commonly involves a process step for activation of the metal catalyst, transforming it from an oxidised to elemental state using a reducing gas. We have previously shown the use of tantalum as a sub-layer for the solid state reducing of iron catalyst by which we eliminate the need for such hazardous reducing gases in CVD reactions [1,2]. Here we investigate the effect of the relative thickness of catalyst and solid state reducing agent. By careful selection of this ratio we are able to maximise the CNT nucleation density for thermal CVD growth conditions, in particular we consider the combined effect of continuous catalyst oxidation by residual oxygen in the CVD chamber and of catalyst diffusion into the reducing layer. We also demonstrate the effectiveness of solid state reducing systems in laser-induced CVD where we observe low power thresholds for growth and achieve high resolution patterning based on localized growth of CNTs.
1. Bayer, B. Support-Catalyst-Gas Interactions during Carbon Nanotube Growth on Metallic Ta Films. The Journal of Physical Chemistry C 115, 4359-4369 (2011).
2. Co-Catalytic Solid-State Reduction Applied to Carbon Nanotube Growth
Bernhard C. Bayer, Martin Fouquet, Raoul Blume, Christoph T. Wirth, Robert S. Weatherup, Ken Ogata, Axel Knop-Gericke, Robert Schlögl, Stephan Hofmann, and John Robertson
The Journal of Physical Chemistry C 2012 116 (1), 1107-1113

Carbon-based materials of reduced dimensionalities have extraordinary properties that have been object of a great amount of research. Graphene, a single layer of sp²-hybridized carbon, is a very promising material for many applications, due to its exceptional thermal, mechanical and electrical properties. However, in its pristine form graphene is a zero band gap semiconductor, which limits its use in transistor applications.
Diverse approaches have been tried to solve this issue, some of which include: application of strain, quantum confinement in nanoribbons, chemical modifications etc. Ideally, the opening of the band gap should not compromise other desirable electrical properties of graphene, like the linear dependence of the energy of the conduction electrons with their momentum - that is, a Dirac cone would still be present at the band structure.
Recently [1], it has been suggested that graphynes - 2D allotropes in which sp² and sp carbons coexist - are promising candidates to replace graphene. Some of the graphynes exhibit distorted Dirac cones, with prefered directions for conductance, which could be exploited. Additionally, further Density-Functional Theory (DFT) calculations [2] have shown that the Dirac cone is retained in the presence of B, N and H heteroatoms, and that the presence of B and N may lead to a small opening of the band gap. Oxygen is an heteroatom that might be naturally present during syntheses and that is known to open the band gap of graphene.
In this work we have investigated, using fully reactive molecular dynamics [3], the structural and dynamical aspects of the oxidation mechanisms of graphyne structures. The simulations were carried out exposing both sides of the graphyne membranes to atomic oxygen atmospheres and investigating the chemical reactions in time. Our results showed that the existence of different sites for oxidation process, related to single, triple and resonant bonds, makes the process of incorporating oxygen much more complex than the graphene ones. Additionally to that, oxygen, as in graphene, can form several oxygen-containing functionalities like epoxy and alkoxy. In fact, in the beginning, some part of oxygen atoms attack preferentially the triple bonds, which is expected, but, as oxygen bonding continues, other sites, become more attractive. This leads to the formation of a very complex and disordered structure. The similarities and differences of graphene oxide and graphyne oxide are also addressed in terms of thermal and structural stability.
[1] Malko, Daniel et al., Physical Review Letters 108.8 (2012): 86804.
[2] Malko, Daniel, Christian Neiss, and Andreas Görling, Physical Review B 86.4 (2012): 045443.
[3] Van Duin, Adri CT et al., The Journal of Physical Chemistry A 105.41 (2001): 9396-9409.

Carbon nanotube (CNT) forests possess unique properties (e.g., high thermal and electrical conductivity, optical absorption, and mechanical compliance), which have rendered them as excellent candidates for applications such as field emitters, thermal and electrical interfaces, super dark absorbers, and through wafer interconnects. Coated CNTs are also attractive because of their potential applications as optical, electronic, catalytic, sensing, and magnetic materials or as reinforcement in composite materials. In this work, we investigate the relation between morphological differences between carbon nanotube (CNT) forests and their mechanical behavior. Deformation and failure mechanism of (1) uncoated CNT forests, (2) CNT forests coated with aluminum on the top surface, and (3) CNT forests conformally-coated with alumina were visualized using in-situ micro-indentation. The buckling behavior of the CNT forests were studied for the case of different growth induced morphological gradients along the height of the uncoated CNT forests. We find that CNT aerial density and tortuosity represent two key factors governing the overall mechanical response of the uncoated CNT forests to local compressive loading. Local buckles that determine a major part of the deformation mechanism of CNT forests form at the location of the best combination of low density and less tortuous CNTs along the height. Indentation of coated CNT forests reveals that both top and conformal coating of CNTs increase the stiffness. The buckle location could change along the height of the CNT forest in the case of coating due to the change of mechanical properties of the structure. Furthermore, failure mechanisms such as CNT delamination from the growth substrate and nanotube breakage occurred in the case of top and coformal coating respectively. This study elucidates the deformation and failure modes in uncoated and coated CNT forests which govern their performance in many applications.

Carbon nanotube networks show promise as a means to provide on a macroscopic size scale the excellent thermal, electrical, mechanical, and functionalization properties of carbon nanotubes. In such structures, the properties of nanotube junctions can heavily influence the performance of the entire network; for example, nanotube junctions are the primary bottleneck for heat and charge transfer in uncoated single-walled carbon nanotube (SWCNT) networks.[1,2] In addition to the intrinsic impedances of the individual junctions, collective effects arising from the interconnected nature of the network can contribute to the total impedance; Green&’s functions calculations that include such effects have been used to study the large increases observed in junction impedance for SWCNT networks relative to that measured at the junction of two isolated (i.e., not within a network) SWCNTs.[1]
In this work we present temperature-dependent (100K - 300K) thermal, electrical, and thermoelectric (i.e., thermopower) properties of SWCNT aerogels with ultralow density (~10 kg m-3). The aerogels are three-dimensional, isotropic networks of SWCNTs held together by van der Waals interactions at the junctions between nanotubes.[3] Their ultralow density leads to an interjunction spacing (mesh size) of 20nm,[4] which is much larger than previous SWCNT networks (mesh size ~2nm)[1] in which these transport properties have been studied. We measure dramatic (order of magnitude) improvements in the thermal and electrical conductances of the SWCNT junctions relative to previously reported junction conductances in SWCNT networks. The average junction conductances of the aerogel network are found to be close to the ideal thermal and electrical conductances at the junction of two isolated (non-network) van der Waals bonded SWCNTs,[2,5] suggesting the elimination of collective effects.
We further study how these properties are affected when the network is coated by graphitic layers in order to improve its mechanical properties.[3] We find that the improvements in mechanical properties come at the expense of electrical and thermal performance, as the junction impedances become outweighed by the increased impedances of the coated SWCNTs themselves. Reduction in the ballistic transport of electrical carriers is evidenced by both electrical conductivity and thermopower measurements.
[1] R. S. Prasher et al., Phys. Rev. Lett. 102, 105901 (2009).
[2] Y. Chalopin, S. Volz, and N. J. Mingo, J. Appl. Phys. 105, 084301 (2009).
[3] K. H. Kim, Y. Oh, and M. F. Islam, Nature Nanotech. 7, 562 (2012).
[4] L. A. Hough et al., Nano. Lett. 6, 313 (2006).
[5] M. S. Fuhrer et al., Science 288, 494 (2000).

Recently, flexible and stretchable features have been introduced in various devices such as sensor, transistor and OLED. Strain sensor can detect mechanical deformation of stretchable devices and therefore, has been widely used for detecting failure of structure Strain sensors were mainly fabricated by using crystalline silicon material but they have limitation in flexible and stretchable electronic applications. Furthermore, scaling up to large area is another main technical challenge for silicon-based strain sensor technology. Therefore, more adequate strain sensor devices with high sensitivity and high stability are needed for various purposes.
Among several promising candidates, single walled carbon nanotube (SWCNT) has gained a lot of attention due to its good electrical and mechanical properties. In fact, SWCNT has been widely used for conductor, transistor and sensor in stretchable electronic applications. Although SWCNT shows good performances, complicated high-cost fabricating methods have been used in many cases. Since SWCNT can be formulated into a solution ink and inkjet printing method has advantages of low material usage, easy patterning and low processing cost. Inkjet-printing of SWCNT-based electronic devices is considered one of the promising approaches for high-performance scalable sensor applications.
In this study, we fabricated SWCNT-based strain sensor on PDMS substrate by using inkjet-printing and sensing properties were measured. We used aqueous SWCNT solution which was fabricated by tip sonication and surfactant. First, the SWCNT solution was printed on PDMS substrate in form of specific patterns which affect cracks in SWCNT film and resistance. After pattern printing, UV ozone treatment was performed on the PDMS substrate for controlling wettability of PDMS substrate. And then SWCNT film was printed on PDMS substrate in form of dog-bone shape.
When electrical properties of SWCNT film without specific patterns were measured under mechanical deformation, resistance of the SWCNT film changed slightly. It is reason that crack density of the film is low and the cracks locate randomly in the film. The cracks affect resistance of SWCNT film under mechanical deformation, and it is represented by sensitivity of strain sensor. Positions of cracks and crack density can be easily controlled by specific patterns. Therefore, Inkjet-printing can provides various types of strain sensor due to advantage of easy patterning. In our test, the strain sensors can have various sensitivities which have 1~80 of gauge factor for strains up to 50%. In case of strain sensor with size of 16 X 1 mm2 has 71.24 of gauge factor. The strain sensors also have good linearity to the strain in various speeds (1, 5, 10, 50, 100 and 500 mm/min.) and the properties have no obvious changes in high cycle times. This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (2012K001368)

An alternative approach to the development of advanced structural composites based on engineered multi-scale carbon nanotube (CNT) fiber reinforcement has been examined. Electrophoresis technique was used to deposite multi-walled carbon nanotubes (MWCNTs) onto unidirectional carbon and glass fabric prior to the infusion with epoxy resin for the production of carbon/epoxy and glass/epoxy composites. CNTs were functionalized using ozone treatment for oxidation, followed by chemical reaction with polyethyleneimine (PEI) dendrimer to enable electrophoretic deposition (EPD) of the functionalized CNTs at the cathode. A novel recirculating system was used which enabled ozonolysis to be conducted on large-volume solutions of CNTs in the presence of high-powered sonication, facilitating preparation of stable dispersions suitable for EPD. These EPD treated composites were fabricated using vacuum assisted resin transfer molding (VARTM) and characterized under SEM. Increases in the in-plane shear strength of the composites with the CNT treatment have been measured and followed fracture analysis suggests that the improved strength is related to improved adhesion at the fiber-resin interface, relative to the untreated laminate. Additionally, this EPD treated composite leading to a significant improvement in electrical conductivity, especially higher in the in-plane fiber direction than the transverse fiber direction, indicating a preference for conducting CNT networks to form along the fibers. This research also reports an effective method for developing a fundamental understanding of the processing influence of carbon nanotubes on the elastic properties of this multifunctional hybrid composite. A combination of Halpin-Tsai equations and woven fiber micromechanics was used in hierarchy to predict the mechanical properties of
multi-scale composites, and the discrepancies between the numerical predicted and characterization experimental values are explained. The CNT coated glass laminate also exhibited clear changes in electrical resistance as a function of applied shear strain which could distinguish elastic and plastic regions for electrical damage sensing application.

Functional nanostructured carbon materials are relevant for applications in high performance composites, lithium storage, fuel cell technology, photovoltaics, or nanoelectronics. While the classical carbonization methods typically rely on high-energy processes, the development of novel low-temperature procedures promises to provide a pathway towards functional carbon materials with tailored nanoscopic morphology. In this context, we developed a novel ‘wet-chemical&’ carbonization approach that relies on the self-assembly and subsequent mild carbonization of designed, carbon-rich amphiphiles such as oligo(phenylene) or oligo(ethynylene) derivatives. In particular in the latter case, the metastable and carbon-only oligoyne segments exhibit an inherent reactivity to rearrange into other forms of carbon in the aggregated state. This reflects the fact that no other covalently bound elements need to be removed in the course of cabonization, which can hence proceed under benign conditions. The approach is versatile, as it can be performed in colloidal dispersions, monolayers, or liquid-crystalline phases of the carbon rich amphiphiles. This results in a variety of different nanostructured carbon materials such as capsules, tubes, sheets, or carbon-inorganic composites, templated and guided by the self-assembling properties of the amphiphilic precursors. Thus, glycosylated hexaynes as carbon-rich analogues of glycolipids self-assembled into vesicles with tailorable size in water. Subsequent carbonization by UV irradiation at 1°C in aqueous dispersion yielded carbon nanocapsules with a controlled diameter and a biofunctional surface decoration. Similarly, hexayne carboxylic acids were found to form stable monolayers at the air water interface that, upon UV irradiation at room temperature, gave rise to two-dimensionally extended carbon nanosheets. In all cases, the thus obtained carbon materials exhibited an amorphous graphite-like carbon (GLC) microstructure, as it would typically be formed only at temperatures well above 600°C in the case of hydrocarbon precursors. Furthermore, the general approach was extended towards smectic liquid-crystalline phases. An infiltration of the aqueous phase with precursors for silica, aluminosilicate, or titania enabled us to prepare the corresponding layered carbon-inorganic nanocomposites. Since the calcination of the inorganic domains at moderate temperatures served to fixate the pre-assembled carbon precursors, oligo(phenylene) derivatives could be employed in this case. In conclusion, our low-temperature carbonization approach constitutes a universal strategy for the rational preparation of tailored, functional carbon nanostructures and may, hence, open new possibilities for the use of carbon materials in emerging fields of technology.

This work reports a systematic comparative study of photosensitivity response of sulfur-doped nano- (NCD) crystalline diamond film and carbon nanomaterials composites (nano-SnO₂/CNT) surfaces, both synthesized by hot-filament chemical vapor deposition. The structure and composition of these products were characterized by raman spectroscopy, high resolution scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The photo-detection was possible by means of photo induced field emission based in the photosensitivity of the materials. NCD film show high sensitivity in the UV region which was studied using a steady state broad UV excitation source and two pulsed UV laser radiations, lambda;exc = 248 nm and lambda;exc = 193 nm respectively. Meanwhile nano-SnO₂/CNT shows photosensitivity response at lambda;exc = 658 nm radiation at room temperature. A possible mechanism model was applied to help understand the rol of carbon, defects and sulfur dopant concentration on the materials' photosensitivity.

We report the synthesis and electrical properties of diazonium functionalized double-walled carbon nanotubes (DWCNT). Correlated Raman and optical absorption spectroscopy unambiguously confirm that the covalent modification is outer wall-selective. Nearly 50% of the electrical conductivity is retained in thin films of covalently functionalized nanotubes due to the intact inner-tube conducting channels. Lacking of such channels, single-walled carbon nanotubes become insulators after similar functionalization. We further show that the covalently attached aryl groups can be cleanly removed by optical annealing. These results suggest the possibility of high performance DWCNT electronics with important capabilities of tailored surface chemistry on the outer walls while the inner tubes are chemically protected.

Catalytic chemical vapor deposition of CNTs is a popular fabrication method due its scalability, and means of controlling CNT diameter and length by catalyst size and process parameters. Catalyst nanoparticles, typically transition metal oxides, are created on a substrate by methods including deposition from suspension, precipitation from a solution, or dewetting of a thin film at elevated temperature. We demonstrate an ultrafast laser fabrication method for producing catalyst nanoparticles for CNT growth, via direct laser printing, without the introduction of a solvent or high temperature to the substrate. These nanoparticles are produced in air. The NiO nanoparticle size and distribution can be precisely controlled by fluence, geometry, and Ni film thickness. High resolution scanning transmission electron microscopy will be presented along with detailed nano-electron energy loss spectroscopy data. Additionally we will show evidence of CNT growth from these catalyst particles, as confirmed by electron microscopy and Raman spectroscopy. A model for the formation of the NiO nanoparticles will be presented.

Graphene is a very interesting material due to its exceptional properties including low resistivity and low absorption of light in the visible spectrum. This property makes graphene an excellent candidate for transparent conductors. However, a cost effective method for manufacturing large area devices has not yet been developed. Rapid scanning of a high intensity ultrafast laser may be selectively absorbed at the interface of even a single layer of material. Observations will be presented that show that single or double layer graphene can indeed be exfoliated by an ultrafast laser. These data will be presented as a function of intensity and repetition rate. The exfoliated graphene will be characterized with electron microscopy and Raman spectroscopy. Additional results from exfoliation of single/multi-layer graphene from other substrates will be presented with a physical model to explain our results.

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 always starts from the bond shared by hexagon-heptagon rings.
3. We have developed a theory based on disclination dipole interaction 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.
Our 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.

Polycyclic aromatic hydrocarbons (PAHs) as a type of chemically stable semiconducting molecules are extremely interesting materials in organic electronic devices such as organic photovoltaics (OPVs), organic thin film transistors (OTFTs), chemical sensors, and waveguides. Among the large family of PAHs, coronene (C₂₄H₁₂) has been received a lot of attention due to its strong π-π interaction between molecules and symmetrical structure to assist the formation and the stability of one-dimensional nano-architecture. In this work, vertically aligned coronene nanowires were grown on (100) Si substrate using thermal evaporation without any catalysts. The growth of coronene nanowires was dominated by a self-catalytic vapor-liquid-solid (VLS) mechanism. Namely, numerous dot-like coronene clusters were initially formed on the substrate, and then were grown alternatively to form nanowires. After growth time of 20 min, a dense forest of coronene nanowires held perpendicular to the (100) Si substrate and in good mechanical contact with the substrate. The coronene nanowires were approximately 50 - 120 nm in diameter and 1 - 2mu;m long. X-ray/ultraviolet photoelectron spectroscopy (XPS/UPS) was employed to investigate electronic structures of coronene nanowires and discuss how they might be grown to improve the charge transport as a cathode&’s buffer layer. Using atomic force microscopy (AFM) and transmission electron microscopy (TEM), the relationship between the surface morphology and the charge transfer was also allowed to be elucidated.

‘Green Chemistry&’ is the decent paradigm in organic chemistry in these days. The oxidation of alcohols to the corresponding carbonyl compounds under aqueous condition is of great importance, due to free of environmentally undesirable solvent and heavy-metal waste, and wide application of their oxidized products as various chemical intermediates. Especially, molecular oxygen, as a green oxidant, has been focused as an alternative stoichiometric oxidant; dichromate or dimanganate. However, due to low solubility of molecular oxygen under aqueous condition, it is hard to activate molecular oxygen for aerobic alcohol oxidation. Therefore, developing water compatible catalysts, excellently active to small amount of molecular oxygen in water, are inevitable for aqueous aerobic alcohol oxidation. Grphene oxide (GO), one of the water compatible nano materials, has been focused by many researchers, due to its unique thermal, mechanical, electronic, and chemical properties originated from its 2D structure and its potential to various applications. Particularly, it is a promising candidate as a 2D structured support for metals for aqueous oxidation reaction. Here, we demonstrate that the GO supported ruthenium catalyst shows excellent catalytic activities under aqueous condition for the oxidation of activated, non-activated, heterocyclic alcohols with molecular oxygen as a green oxidant. The catalyst is found very efficient, as revealed by TON value, which is up to 10000. As far as we know, the heterogeneously catalyzed aerobic selective alcohol oxidation has been rarely reported so far. The Ru/GO catalyst showed true heterogeneity and the catalyst could be reused without significantly loss of its catalytic activity.

The fabrication of conducting thin films from the carbon nanomaterial dispersion has been realized using various conventional techniques for colloidal assembly, including dip-coating, spray-coating, spin-coating, Langmuir-Blodgett (LB) assembly, layer-by-layer (LbL) assembly, and vacuum filtration. In some of these strategies for the deposition of thin films, however, it is difficult to control the film thickness precisely, due to the aggregation of the carbon nanomaterials, whereas others need a relatively large volume of suspension and are time consuming. Here, we present a new class of rapid solution process for fabricating highly uniform chemically derived graphene and carbon nanotubes (CNT) thin films with control over the thickness on the subnanometer-scale. The film deposition directly on rigid or plastic substrates is driven by dragging a meniscus of microliter graphene oxide (GO) or CNT suspension trapped between two plates - meniscus dragging deposition (MDD) technique. The fine tuning of the optoelectronic properties of the conducting thin films is achieved by simply varying the number of deposition, carbon nanomaterial concentration, dragging speed, and angle between two plates. This coating technique is simple, inexpensive, and easy to scale for large-area thin films used as transparent electrodes with a significant reduction of the material consumption. The deposition of highly uniform thin films enables the simple micropatterning of graphene and CNT with high resolution for its application to optoelectronic devices.

Self-assembly of highly conjugated organic molecules is an important reaction system that not only grows new geometrical structures useful for various applications, but more importantly grants in depth understanding about intermolecular interactions involved in the structural formation process. While there has been relatively clear understanding about molecular self-assembly on solid surfaces as well as self-assembly of π-conjugated molecules into free-standing nano- or microstructures, molecular self-assembly in vertical direction has been rarely realized. In this presentation, we report that C₆₀ molecules can be spontaneously self-assembled into vertical C₆₀ nanowires by ‘wetting-evaporation&’ process, which is a modified drop-drying process. During the drop-drying process, there are two competing forces applied to a solution droplet placed on a solid substrate: lateral force and vertical force while the droplet naturally dries off. In contrast that lateral force is the major force applied to a solution droplet during the general drop-drying process, resulting in laterally deposited self-assembled C₆₀ 1D wire or 2D disk depending on the type of solvent, the wetting-evaporation process is designed to maximize vertical force to a droplet so that C₆₀ containing solvent molecules are guided to move normal to substrate. For example, we obtained vertically grown C₆₀ nanowires by placing a C₆₀ film/Si substrate in a scintillation vial containing m-xylene as a carrier solvent. The morphology of vertically self-assembled 1D and 2D C₆₀ nanostructures have been confirmed by scanning electron microscopy (SEM), and their structural information by X-ray diffraction (XRD) and selected area electron diffraction (SAED). The details about the control experiments and the proposed mechanism of vertically self-assembled 1D and 2D nanostructures will be discussed.

The modification of SiO2 substrate surface has been known as one of many ways to improve the device performance of the carbon nanotube or the graphene transistors. In this work, we report our recent experimental and theoretical investigations on the pentacene-based transistors fabricated on the modified SiO2 surfaces. Pentacene field effect transistors (FET) were successfully fabricated on the SiO2 substrate with nanoscale grooves that were made with mechanical polishing technique. We found that the mobility of these pentacene FET increased about 10 times after using the grooved SiO2 substrate. It motivated us to carry out the first-principles calculations on the pentacene, graphene nano ribbon, and graphene on the model surfaces for the grooved SiO2 substrate. We considered the Si-terminated and the Si-rich surfaces of alpha-quarts SiO2. We performed density functional theory (DFT) calculations with plane wave basis for the periodic surface models. We compared the atomic and electronic structures of these aromatic carbon materials on the both surfaces. The differences of the electronic structures including charge transfers can explain the mobility increase of the pentacene transistor on the grooved SiO2 substrate. We demonstrate that our grooved SiO2 surface modification can improve the performance of the FET devices with the aromatic carbon materials.

Titanium oxide (TiO2) is widely used to pollution purification material and hydrolysis catalyst due to the photocatalytic ability under ultraviolet light. However TiO2 have some drawbacks such as low absorption efficiency of light and high recombination ratio of carriers. There are some researches to overcome these disadvantages. The doping of metal ion and anchoring with porous materials such as zeolite increased the photocatalytic ability. Recently, the research that the hybrid with carbon nanotube improved the photocatalytic property of TiO2 was published.
In this work, we synthesized TiO2 nanoparticles/graphene oxide composites using hydrothermal method and controlled the crystalline phase, size and distribution of TiO2 nanoparticles. Graphene oxide was obtained by modified Hummers method and titanium (IV) butoxide (TNBT) was used as TiO2 precursor. Firstly, graphene oxide and benzyl alcohol were suspended to organic solvent and TNBT was added to this suspension. After the sonication for 5 hours, the mixture was filtered and the composites were obtained by thermal treatment at 450 oC. Two types of solvent (ethanol and DMF) were used to observe the effect of ion density change at graphene oxide surface. Because DMF is more polar than ethanol, the ion density of graphene oxide is higher in DMF suspension. Therefore, TiO2 nanoparticles arise in places with the narrow size distribution. Whereas the large agglomerates of TiO2 were observed in ethanol suspension.
The Ti2p peaks of composites in X-ray photoelectron spectra showed that the TiO2 nanoparticles arose at graphene oxide surface. The intensity of O-C=O peak decreased after thermal treatment and this peak change showed that graphene oxide was reduced.
The photocatalytic ability was observed by decomposition of acid orange 7 (AO7) under ultraviolet light. The reference sample using only TiO2 nanoparticles decomposed 35% of AO7, whereas the sample of composites using ethanol and DMF decomposed 36% and 51% of AO7, respectively. The interaction with graphene oxide and surface area of TiO2 nanoparticles affected to the photocatalytic ability.

Recently, graphene, a single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms, promises a diverse range of applications from composite materials to quantum dots. While earlier synthetic methods for graphene production were challenging, there have been considerable advancements in the synthesis and processing methods, such as micromechanical exfoliation, oxidation/reduction protocols, epitaxial growth, and vapor deposition. Among various types of graphene and related carbon nanostructures, a stable suspension of graphene oxide (GO) is the common choice over pristine graphene owing to its facile synthetic nature in a controlled, scalable, and reproducible manner. The abundant oxygen-containing functional groups such as epoxide, alcohol, and carboxylic acids provide GO with excellent aqueous dispersity and also offer anchors for further chemical modifications.
Au clusters and Au nanoparticles (AuNPs), containing several to a few hundred Au atoms, have attracted much attention in recent years because of their unique properties, such as photoluminescence and ferromagnetism, and the dispersion of Au nanoparticles on graphene sheets provides a potential applications in sensing, bioimaging, catalytic, magnetic, and optoelectronic materials. Although various synthetic routes, such as the chemical reduction of Au precursors in the presence of pristine graphene or chemically modified graphene derivates, have been reported to prepare the graphene/AuNPs hybrid, few papers thus far have reported the site-selective self-assembly of AuNPs on graphene sheets. We have anchored the AuNPs on graphene edge or graphene basal plane selectively by site selective functionaliztion of graphene oxide or reduced graphene oxide. Furthermore, the electrocatalytic activity of the graphene/gold nanohybrid is evaluated.

Our standard large area (> m2) CNTSheettrade; manufacturing process produces anisotropic porous carbon nanotube sheets in the 10-200µm thickness range. They have electrical and thermal properties that are uniform in the “X” and “Y” direction (sheet plane) and are 1-3 orders of magnitude lower in the “Z” direction. This manufacturing platform, using mm long VASWCNT&’s or VAMWCNT&’s as the primary mechanically interlocking fiber material, allows us to create a large family of macroscopic 2D materials, whose porosity can be easily tuned. These can be used as nanofilters, ultra-light and thin EMI attenuation sheets, batteries/ultracapacitors/capacitative desalination electrodes, water purification filters, catalysts, etc. Due to the versatility of the manufacturing process, additional functional materials can easily be incorporated in the body or preferentially on the surface of the sheets, such as graphene, nano/micron size particle/filamentary/fiber material(s), etc. in a mechanically interlocked and electrically interconnected fashion. These 2D NanoToMacrotrade; sheets thereby allow us to greatly simplify the design, cost and manufacturing complexity of many next generation products enabled by nanomaterials.
In this paper, we demonstrate for the first time control of the directional properties of the sheets: from anisotropic to nearly isotropic in 3D. This directional tunability has a substantial effect on the 3D mechanical, electrical and thermal performances of the resulting materials. The availability of these additional process control options over our standard CNTSheettrade; manufacturing platform is expected to further increase the value proposition and accelerate the commercialization of a range of new nanomaterial enabled products.

The electrical resistance and transport properties of a Cu/CNT(5,5)+Cu/Cu junction has been calculated by employing non-equilibrium Green&’s functions and Density Functional Theory. The transport properties of Cu/CNT(5,5)/Cu junction are also calculated as a reference. Both the equilibrium and non-equilibrium conditions have been calculated. The results and the analysis show that the conductance of the Cu/CNT/Cu system has been largely enhanced by the incorporation of Cu due to the interaction between Cu and the CNT. The change of the I-V curve slope is also explained in terms of transmission spectrum.

Graphene has an excellent electronic conductivity, a high theoretical surface area of 2630 m2/g, and excellent mechanical properties and, thus, is a promising component for high-performance electrode materials. In searching for new tools to enhance photoactivity of semiconductors, the graphene-based nanocomposite systems has stood out as recent studies have shown its usefulness in electronics, catalysis, and photovoltaic devices. Owing to the abundance of delocalized electrons from the conjugated sp2-bonded carbon network, graphitic carbon enhances the transport of electrons photogenerated in semiconductor particles, leading to an increase in the photoconversion efficiency of the system. Graphene based metal or semiconductor nano-composites are generally synthesized using graphene oxide (GO) as the precursor, followed by its thermal reduction to reduced graphene oxide (R-GO). In this study, we will explore α-Fe2O3 as the catalyst material as it has been widely explored because of its optimal band gap (2.0-2.2 eV, capable of absorbing ~ 40% of the solar spectrum), low cost, abundance, nontoxicity and stability. Exceptional results of R-GO/Fe2O3 photoelectrode in PEC cell have been demonstrated. The reason for the high activity has been discussed on the basis of characterization reaction results and photoelectrochemical measurements.
A reduced graphene oxide/iron oxide (R-GO/Fe2O3) thin film structure has been successfully prepared on ITO substrates by directly growing iron oxide particles on the thermally reduced geaphene oxide sheets prepared from suspension of exfoliated graphene oxide. R-GO/Fe2O3 thin films were tested in PEC cell and found to be very economical way to improve the anodic performance of iron oxide in PEC cell. GO-Fe2O3 thin films offered ten times higher photocurrent density than pristine Fe2O3 thin film sample. XRD, SEM, EDS, UV-Vis and Raman spectroscopic studies were carried out to explore the structural, morphological and optical properties of the thin films. Mott-Schottky studies showed n-type semi conducting nature for GO and GO-Fe2O3 thin film samples. Studies revealed enhanced PEC performance of these photoelectrodes on account of its porous morphology, improved conductivity upon using graphene layer as substrate for iron oxide thin film deposition. A mechanism for flow of carriers in PEC cell across Fe2O3 and R-GO interface has also been proposed.
Keywords: Graphene oxide, iron oxide, Raman spectroscopy, photoelectrochemical.

A major goal in the nanosynthesis field is the fabrication of hybrid structures with multiple functions for use in smart materials, biomedical diagnostics, therapies, and theranostics. Desired functions may include combinations of photonic, magnetic, radiological, mechano-responsive, and controlled release behavior, and most structures possessing these abilities require complex multistep chemical synthesis. Recently, an aerosol-phase method has been demonstrated for encapsulating nanoparticles or macromolecules in graphene “nanosacks”. This process is simple, continuous, and scalable, and has the potential to become a general and flexible route to the creation of multifunctional nanostructures. To achieve this requires a better understanding the aerosol encapsulation process that determines the nature of the cargos that can be successfully wrapped, and which is governed by colloidal interactions between graphene oxide sheets and multiple nanoparticle cargos with the same or opposite surface charge. Also needed is information on sack sealing and solute release characteristics that are relevant to delivery and theranostics. Base titration is shown to be effective for creating stable colloids with uniform negative charge that self-segregate into uniform sack-cargo structures. Graphene nanosacks are also shown to be open structures that rapidly release soluble salt cargos when reintroduced into water, but can be partially sealed by filler addition during synthesis to achieve slow release profiles. One example application area for multifunctional probe is biomedical imaging, where nanoprobes are being developed for disease detection using MRI, CT, NIR fluorescence or other imaging modalities. In these cases, the probe typically combines particles of differing chemistry into a single structure through a series of chemical synthesis steps, which become increasingly difficult as the complexity of the probe increases. It would be advantageous to have a simple, widely applicable process to make multifunctional probes from simple, readily available unifunctional probes without the need for multiple chemical synthesis steps. We demonstrate the multifunctional probe concept by preparing Au/Fe3O4-filled graphene structures that are magnetically responsive and show excellent contrast enhancement in full scale MRI and full scale CT tests carried out on the same hybrid material.

Deposition of metal oxide nanoparticles on pristine graphene is expected to be difficult due to the lack of dangling bonds in the graphene plane. As a result, graphene oxide (GO) has been receiving increased attention due to its abundant functional groups to be potentially served as a support material to anchor metal oxide nanoparticles and easily reduced to graphene, referred to as reduced graphene oxide (r-GO). We describe here our work on synthesizing aluminium oxide nanoparticles into r-GO sheets by chemical vapor deposition (CVD) without any nanoparticle precursor and reducing agent. The nanoparticles are introduced by the commercial alumina membrane filter, which is widely used in vacuum filtration of GO dispersion to fabricate its free-standing paper-like materials. Moreover, by controlling the oxygen and argon gas flow ratio at different temperatures in CVD process, the nanoparticles can perfectly grow into graphene plane with uniform sizes ranging from 5 nm to 8 nm. The resulting functionalized r-GO paper shows a 20% higher electrical conductivity and wider band gap than conventional thermal annealed r-GO materials at the same temperature. We have also investigated the surface properties of the nanocomposites by contact angle measurement before and after functionalization. The nanoparticleminus;graphene composites surface exhibits hydrophilic behavior, whereas the thermal annealed r-GO paper is hydrophobic. Moreover, we have found that the surface photoluminescence (PL) intensity can be enhanced significantly compared to the non-nanoparticle r-GO based materials. These fascinating exceptional electronic, thermal, and surface properties show promise for a range of applications, especially in lithium-ion batteries (LIB). Therefore, we have evaluated the electrochemical performance of the nanocomposite paper as anode material in LIB. Electrochemical measurement results exhibit a lower irreversible capacity loss (ICL) during the first cycle than that of graphene-based anodes and an enhanced reversible capacity of about 500 mAh g-1 which is about two times higher than that of graphite-based anodes. This outstanding electrochemical behavior can be attributed to the unique microstructure and surface properties of the nanocomposites.

Nanoporous carbon-based membranes have significant potential in water desalination and filtration applications by enabling increased permeability, possessing superior strength characteristics, and exhibiting the potential for very high contaminant rejection [1]. In order to realize these benefits, experimental evidence of both the fabrication of nanopores and selective water transport is required. This study addresses the former; we demonstrate a method for nanopore creation and characterization in carbon thin films.
In order to qualify the thin film for use in water filtration, we explore the characteristics of pore size and shape, the role of interstitial spacing in the potential convective flow, limits of mechanical strength, and conceivable approaches to pore functionalization. Here we present preliminary results of these experiments and identify parameters that improve the features of thin graphitic carbon thin film as a water filtration membrane material.
[1] Cohen-Tanugi, D.; Grossman, J. Nano Lett. 2012, 12, 3602-3608.

Graphene Oxide (GO), the most widely studied functional derivative of Graphene, has broadband fluorescence and is electrically insulating, which limits its possible use in optoelectronic devices. Moreover, the role various functional groups play in inducing fluorescence in GO is unclear. Tailoring the functional groups present on GO is a pivotal step in using Graphene functional derivatives as active materials in optoelectronic devices. We present scanning confocal fluorescence spectroscopy and imaging of pristine GO sheets and under controlled reduction. In optimal reduction conditions, the observed fluorescence decay components may correspond to removal of different functional groups from the surface of GO. The emission intensity is dependent on the number of Graphene layers in the GO sheet as well as position on the sheet (edge vs. basal plane). This allows correlation of fluorescence intensity to the degree of reduction as well as identity of residual functional groups present on the surface of GO sheets. Such understanding of the decay of fluorescence and emergence of a conducting network during controlled reduction would be valuable in tailoring functional derivatives of Graphene for possible use in optoelectronic devices as well as other applications of GO dependent on its fluorescence, such as bio-imaging.

In this work we present a low temperature growth of carbon nanotubes on highly active graphene decorated structures. Iron nanoparticles were deposited on few-layer graphene sheets and the nano-structural system was heated in the presence of argon and hydrogen without adding an external hydrocarbon source. Samples were thoroughly characterized before and after synthesis by electron microscopy, surface area analysis and thermogravimetrical analysis. Nanotube growth was achieved at temperatures as low as 150 °C using two different heating methods (radio frequency generator or an electrical furnace). The synthesis temperature for nanotubes was found to depend on the diameter of the metal nanoparticles. Graphene sheets decorated with the smallest metal particles (5 nm) catalyzed multi-wall carbon nanotubes at the lowest temperature (150 °C), whereas the nanoparticles with diameter of 15 nm were found to initiate nanotube growth only at 400 °C or higher. The nanotube yield was found to increase as the synthesis temperature was varied between 150-500 °C. These novel nanoparticle/graphene-nanotube systems maybe used in a wide range of applications such as nano-electronics, catalysis, sensing, energy harvesting and bio-nano area.

We demonstrated nitrogen doped CNT (NCNT)/TiO2 core/shell nano-hybrid structure via an efficient and environmentally benign biomimetic mineralization of TiO2 on the surface of NCNT without any surface treatment and adhesive layer. The nitrogen site in NCNT played a role of initial nucleation site for biomineralization and uniformly thick TiO2 nanoshell was deposited on the surface of NCNT in neutral pH aqueous media at ambient pressure and temperature. This biomimetic mineralization method can be distinguished with previously reported organic biomineralization templates, such as proteins or peptides. The electroconductive and high-temperature-stable NCNT backbone enabled high-temperature thermal treatment and corresponding crystal structure transformation of TiO2 nanoshell into the anatase or rutile phase for optimized materials properties. The direct contact of NCNT and TiO2 nanoshell without any adhesive interlayer introduced a new carbon energy level in the TiO2 bandgap and thereby effectively lowered the band gap energy. A greatly enhanced photocatalysis over visible light region was observed with NCNT/TiO2 core/shell nano-hybrid structure.

Defects in the graphene lattice have been shown to create both semi-conducting and magnetic states¹, which could have an impact on the development of spintronics and novel transistor devices. However despite the current knowledge of intrinsic defects and their effects on these properties², defect formation and organization is still a largely uncontrolled process. The next step is understanding how to efficiently control the formation of these defects so as to obtain the desired functionality.
To this end, the structure, energy and properties of defect structures induced by a transmission electron microscope (TEM) beam in graphene³ is examined using hybrid exchange Density Functional Theory calculations. The variation in electronic and magnetic structure, along with related functional properties, will be discussed, the predicted structures compared directly to TEM images and where possible the computed properties are compared to detailed observations.
References
1. R. Singh and P. Kroll, Journal of physics. Condensed matter: an Institute of Physics
journal, 2009, 21, 196002.
2. F. Banhart, J. Kotakoski, and A. V. Krasheninnikov, ACS nano, 2011, 5, 26-41.
3. A. W. Robertson, C. S. Allen, Y. a Wu, K. He, J. Olivier, J. Neethling, A. I. Kirkland, and
J. H. Warner, Nature communications, 2012, 3, 1144.

PET {polyethylene terephthalate (C10H8O4)n}, usually present in refreshment and water bottles, are hard to be degraded. However, this material can be recycled and used to fabricate nanostructures. In this work, the objective was to obtain nanoparticles and carbon based nanostructures from the polymer type PET as a precursor in different conditions. Microwave irradiation was applied at the temperature range of 220-280°C at normal pressure and at 600 psi in the presence of acids and ethylene glycol with further calcination of products. The obtained nanoparticles were studied by means of scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), and Raman spectroscopy. It was established that the degradation of PET using of nitric acid at 1:16 and 1:8 and PET with water and nitric acid at ratios of 1:2, 1:1:3 and 1:0.75:1 allows the formation of ultrananocrystalline diamonds (with size about 10 nm or less) sometimes in agglomerates with a Raman band shift, at 1340 cmminus;1 and at 1600 cm minus;1.

Fracture of single atomic layers, especially graphene, has attracted increasing interests in mechanics and materials communicty over recent years. While existing studies are mainly focused on cases with individual cracks, fracture patterns in single atomic layers have been rarely explored. Here, we present a combined experimental and theoretical study on fracture and fragmentation of single-atomic-layer graphene on substrates. Our in situ observations show that deforming the substrates can crack large-area graphene films into patterns of long ribbons and rectangular fragments with controlled sizes. The large-area and controlled patterns of graphene ribbons and segments can be potentially used in many applications including composites and coatings. We further use the shear-sliding theory to characterize the stress and deformation of graphene on substrates and carry out Monte Carlo simulations of the fragmentation process. The theoretical model matches consistently with experimental results. In addition, our study provides a simple method to obtain large amounts of data for statistical strengths of graphene and graphene-polymer interfaces. These properties are of fundamental importance to graphene-based materials and devices, yet extremely challenging to be measured with existing methods.

In recent years, the rapid development of high quality graphene grown by chemical vapor deposition (CVD) has pushed graphene applications close to realization. High quality graphene is grown on metal catalysts with subsequent transfer to target substrates. This transfer process typically introduces issues with wrinkles, holes, and metal residues, causing irreproducibility. Hence, the growth of graphene directly on arbitrary insulating substrates is desirable.
We show that nanocrystalline graphene can be grown directly on a large variety of insulating and semiconducting substrates, including HfO2, Si3N4, Al2O3, SiO2, and GaN [1-4]. By using a significantly higher concentration of carbon precursor, typically CH4 or C2H2, a continuous film will form on essentially any high temperature (typical growth temperature is 1000 °C) compatible substrate. Nanometer-sized graphene platelets are formed in the gas phase and deposited on the flat substrate. In contrast with the typical growth of graphene on metals, this process is not self-limiting. The film thickness can be controlled from nominally single-layer to hundred nanometer thick graphite, by adjusting the process parameters.
This kind of graphene is denoted nanocrystalline since the crystal size is in the order of 10 nm, as determined from transmission electron microscopy, Raman spectroscopy, and electrical transport measurements. While the material is very uniform on an optical scale, with optical properties similar to those of pristine graphene, such disorder at the nanometer scale leads to poor charge carrier mobility in the order of tens of cm2/Vs. The conductivity is typically around a few kOmega; for films with optical transparency corresponding to pristine graphene.
Nanocrystalline graphene represent a possibility to grow a uniform, highly conducting graphene film with optical properties similar to those of pristine graphene directly on insulators without the need for transfer. Combined with high mechanical rigidity, such films show promise for several applications including transparent conductives and membranes.
[1] J. Sun, N. Lindvall, M. T. Cole, K. B. K. Teo, and A. Yurgens, Appl Phys Lett vol. 98, p. 252107, 2011.
[2] J. Sun, M. T. Cole, N. Lindvall, K. B. K. Teo, and A. Yurgens, Appl Phys Lett vol. 100, p. 022102, 2012.
[3] J. Sun, N. Lindvall, M. T. Cole, T. Wang, T. J. Booth, P. Boggild, K. B. K. Teo, J. Liu, and A. Yurgens, J Appl Phys vol. 111, p. 044103, 2012.
[4] J. Sun, M. T. Cole, S. A. Ahmad, O. Bäcke, T. Ive, M. Löffler, N. Lindvall, E. Olsson, K. B. K. Teo, J. Liu, A. Larsson, A. Yurgens, and Å. Haglund, IEEE Trans Semicond Manuf vol. 25, pp 494-501, 2012.

The talk will discuss efforts in manipulating graphene and creating graphene based structures, devices and materials. Graphene can be thought of as the ideal two dimensional building block using which several interesting structures, devices and materials can be constructed. We have used three dimensional constructs of graphene as supporting material for electrodes in energy storage applications, their planar sandwiches for hybrid electronic and optical devices and their quantum dots for building new materials. In addition to pure carbon graphene based materials, graphene can also be combined with other two dimensional materials to produce various stacked and in-plane hybrid architectures with multifunctional properties. The new directions in graphene science and technology and the future prospects and challenges of building graphene based functional materials will be discussed.

Thermal energy has been utilized to activate the decomposition and desorption of PMMA residues that are readily present on the transferred graphene surface.1-4 However, the detailed structural behavior of such polymer macromolecules during thermal annealing remains to be determined, considering the impact temperature, environmental, and substrate dependence. The lack of such knowledge hinders the development of an optimized recipe for better cleaning the residues. Using in situ Raman spectroscopy to study the annealing of the transferred graphene in neutral (N2) atmosphere, we have uncovered that polymer chains dehydrogenate and restructure with polymer chains linked by the inter-chain epoxide groups in the temperature range 150-200C. The interconnected chains are decomposed into varying size patches of amorphous carbon at higher temperatures, resulting in a broad Raman feature in the 1100 cm-1-1550 cm-1 range. A comparative study between annealing in reducing (10%H2/90%Ar) and oxidizing (pure O2) atmospheres indicates that the oxidizing atmosphere almost completely removes amorphous carbon residues at 500C, but also destroys the graphitic structure of graphene as evidenced by a clear increase of the Raman D peak intensity. Therefore, we have investigated the use of CO2, which represents a more gentle oxidizing environment, to facilitate the cleaning of PMMA residues without damaging the graphitic structure of graphene in any measureable way. This unique restructuring behavior of the PMMA residues appears to be exclusive on atomically flat 2D materials.
This work was supported partly by NSF (CHE-0911197) and the NRI SWAN Center.
[1] A. Pirkle, et al., Appl. Phys. Lett. 2011, 99, 122108.
[2] Y.-C. Lin, et al., ACS Nano 2011, 5, 2362-2368.
[3] J. Chan, et al., ACS Nano 2012, 6, 3224-3229.
[4] Y.-C. Lin, et al., Nano Lett. 2012, 12, 414-419.

The formation of aligned arrays of long, chemically pristine and electronically homogeneous single walled nanotubes is a persistent, grand challenge in the development of advanced electronic and sensor technologies based on nanotubes. In this presentation, we describe strategies that enable this outcome by combined use of orientationally controlled chemical vapour deposition growth of aligned nanotubes, followed by selective removal of either semiconducting or metallic tubes in a process that exploits nanoscale thermocapillary flows in thin film organic coatings (i.e, thermocapilliary resists). In addition to the process itself, we report detailed materials and physics aspects of fundamental thermocapillary mechanisms and their optimization for this application, through both experimental and theoretical studies.

Ordered mesoporous materials are a hot spot in materials science, because of their unique ordered mesoporous structure that is of great interest for a broad spectrum of applications including energy storage, absorption, separation, drug delivery and catalysis. Organic template directed self-assembly is one of the most promising approaches toward synthesis of ordered mesoporous materials, but it is facing challenges because of the weak non-covalent interactions between the organic templates and the building blocks. In this report, a reactive template was designed and introduced to induce a self-assembly with phenol/formaldehyde (PF) resol for construction of ordered mesoporous framework. The reactive template is synthesized by transformation of the chain ends of the commercial triblock copolymer EO106-PO70-EO106 (F127) from hydroxymethyl group to aldehyde group. The aldehyde end-group of this reactive F127 (R-F127) can react with PF resol to form a stable covalent bond during the aqueous self-assembly process. In this way, the greatly enhanced interaction resulting from a magnificent combination of the covalent bond and the hydrogen bond between the PF resol and the R-F127 leads to a successful self-assembly for formation of an ordered mesostructure. As a result, ordered mesoporous polymer and carbon materials with 3D caged cubic nanostructure can be synthesized using this reactive template-induced self-assembly approach in an aqueous solution.

The kinetics and mechanisms that govern the chemical vapor deposition of graphene on copper is the subject of active discussion. So far it has been claimed that the growth kinetics is dominated by crystallization from the initial supersaturation of carbon adatoms on the surface, which implies that the growth is independent of the hydrocarbon addition after the nucleation phase. However, this reasoning contradicts our observations that the growth depends on the hydrocarbon supply (duration and amount), rather than the initial supersaturation and that the secondary nucleations of graphene occur minutes after the initial nucleation phase. Here, we present an alternative growth model based on our detailed kinetics study of early-stage graphene growths on copper using an ethylene precursor. Our time-dependent graphene growth data displays a sigmoidal behavior described by the Gompertz model, indicating that the growth is controlled by the adsorption-desorption dynamics of the carbon reactants on the copper surface and the dispersive kinetic processes of catalytic dissociation and dehydrogenation. The associated time-dependent activation energy suggests that these processes of dissociative dehydrogenation of hydrocarbons can be the rate-limiting reaction with a maximum activation energy of 3.1 eV. Additional observation of the graphene flake morphology establishes the effect of copper sublimation that when sublimation is suppressed the flake morphology is dominated by sixfold lattice symmetry, devoid of fourfold copper lattice construction.

Electricity storage, the ability to capture and hold generated power during times of availability and retrieve it based on need and demand, is a growing challenge among a broad range of renewable energy sources. Compared with lithium-ion batteries, supercapacitors exhibit several advantages, such as long cycle life, short charging time, light weight, and high power density. Carbon nanotubes are extensively studied as electrode materials for supercapacitors because of their own superiority such as outstanding electrical conductivity for fast charge transferring compared to porous carbon materials. Graphene-based carbon materials, as newly discovered members of carbon family, have drawn extensive interests on various research areas including as electrode materials for supercapacitors, because of their own high specific area, exceptional electric-conductivity, facile synthesis, etc.
In this invited talk, we are going to discuss our most recent progress in the development of solid-state supercapacitors from reduced grapheme oxides (RGO) and our understandings on the charge storage mechanisms of the assembled supercapacitors. In our research, chemically exfoliated graphene oxides have been employed to form solid thin films, which are further reduced to form RGOs. All solid-state supercapacitors have been assembled to study their capacitive behavior. The specific capacitance is 3 times of that reported by Gao, et al. [1]; moreover, the corresponding volumetric capacitance reaches as high as nearly 150 times of the published results [1]. The underlying charge storage mechanism for this impressive capacitive behavior has been investigated using electrochemical impedance spectroscopy and will be discussed.
[1] W. Gao, et al. Nature Nanotechnology, 6, 496-500 (2011).

Electrocatalyst for oxygen reduction reaction (ORR) is crucial for a variety of renewable energy applications. The design and synthesis of highly active ORR catalysts with strong durability at low cost is highly desirable but remains to be difficult. Recently, we developed a series of reduced graphene oxide (GO) hybrids as efficient ORR catalysts by growing nanocrystals on graphene sheets. However, there is a dilemma for GO hybrids that the existence of oxygen functional groups on graphene sheets required for nucleating and anchoring nanocrystals could lower its electro-conductivity and limit the catalytic performance of the hybrid material.
We postulate that multi-walled CNT could overcome such problem as the inner graphitic walls could remain intact to provide highly conducting network while the outermost wall is oxidized and functionalized to afford chemical reactivity. Thus far, little was reported to control the CNT hybrid synthesis to avoid free-growth and to control the CNT oxidation degree, which is important for optimal interactions.
Here, we used a simple two-step method to synthesize cobalt oxide/CNT covalent hybrid as efficient ORR catalyst by directly growing nanocrystals on oxidized multi-walled CNTs. The mildly oxidized CNTs provided functional group on the outer walls to nucleate and anchoring of nanocrystals, while retaining intact inner walls for highly conducting network. The resulting cobalt oxide/CNT hybrid showed high ORR current density that outperformed graphene hybrid and commercial Pt/C catalyst at medium overpotential. It also afforded mainly 4e reduction pathway with excellent stability. The unique CoO/CNT hybrid structure is advantageous over the graphene counterpart in terms of charge transport and active sites. We also demonstrated that the CoO/CNT hybrid maintained high activity and good stability for ORR catalysis even at highly corrosive conditions, rendering it great potential for oxygen depolarized electrode in chlor-alkali electrolysis.

Small pieces of defect-free graphene obtained from graphite by mechanical exfoliation has been shown to exhibit intrinsic mechanical properties with the Young&’s modulus of ~1 TPa and a fracture strength of ~130 GPa.^[1] However, larger area graphene grown on metals by chemical vapor deposition (CVD) of hydrocarbons has grains with a variety of sizes, and thus also grain boundaries (GBs). It is anticipated that these GBs will have a strong effect on the mechanical, electrical, and thermal properties of graphene.^[2] We present the mechanical properties of single-crystal CVD-grown graphene as well as the effect of GBs on the fracture strength of polycrystalline graphene. To measure the fracture strength, CVD-grown graphene is transferred onto a substrate having circular holes. A graphene membrane is indented using an atomic force microscope (AFM) cantilever with a spherical tip in a scanning electron microscope (SEM) and the deflection of the AFM cantilever while indenting the membrane provides the fracture force. The GBs of each membrane are identified with isotope-labeling and Raman spectroscopy. The graphene is grown by sequential dosing of ¹³CH₄ and ¹²CH₄ and regions of pure ¹³C-graphene versus ¹²C-graphene are identified by Raman mapping.^[3] Thus, we can distinguish GBs within each membrane and also estimate the total length of the GBs. We describe the experimental techniques and results on the effect of GBs on the fracture strength of CVD-grown graphene. We appreciate support from the AEC (Advanced Energy Consortium) and the NSF (#0969106; CMMI: Mechanical Characterization of Atomically Thin Membranes).
References
[1] Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Science 2008, 321, (5887), 385-388.
[2] Grantab, R.; Shenoy, V. B.; Ruoff, R. S. Science 2010, 330, (6006), 946-948.
[3] Li, X. S.; Cai, W. W.; Colombo, L.; Ruoff, R. S. Nano Lett 2009, 9, (12), 4268-4272.

3D nanostructures and nanodevices have attracted tremendous interest in the past few years due to their unique properties, and present excellent functional properties than those of planar nanodevices. In this presentation, we developed a technique for the geometrical manipulation of freestanding nanowires using ion-beam irradiation with nanometer-scale resolution to fabricate 3D nanostructures. Such structures could integrate with conventional superconducting quantum interference devices to detect magnetic fields both parallel and normal to the substrate. We also designed and fabricated a 3D hierarchical structure of flower-like few-layer graphene nanosheets (GNSs) grown on silicon nanocone arrays (SNAs). The results indicated that the 3D hierarchical structure enhanced obviously the surface&’s wettability into a large contact angle state with ultrahigh adhesion, which provide a strategy to understand the ultra-adhesive mechanism of the “rose effect” and enhance the wettability of graphene for many practical applications.

Graphene has become one of the most widely studied nanomaterials due to many of its impressive properties and application potentials. The functionalities of graphene can be greatly expanded further by integrating graphene with other suitable materials to build graphene-based hybrid materials or structures. In this talk, I will discuss our recent work on graphene-semiconductor and graphene-metal hybrids and graphene composites for applications ranging from optoelectronics/plasmonics, radiation/photo detection and thermal management.

Graphene has significant potential for applications in electronics and various other related devices, but cannot be used for effective field-effect transistors operating at room temperature because it is a semimetal with a zero bandgap. Graphene also has many unique properties including high optical transmittance, and flexibility, which make graphene attractive for optoelectronic applications, including solar cells and transparent conductors. Graphene may provide a promising alternative to ITO as demonstrated by its high electrical conductivity, and remarkable optical transparency.
Once fabricated, such mesoscopic graphene structures can serve as a solution to open an energy bandgap in graphene, which is necessary for meaningful electronic applications (for example, switching transistors with a high ON/OFF current ratio). The fabrication of such structures demands efficient and enabling nanopatterning techniques to produce nanoscale periodic or quasiperiodic modulations (e.g., selectively etched meshes, doped areas, or topographically induced strain) over large areas. In principle, the edges of nanopatterned holes provide sites for chemical modification of graphene that could alter electrical properties, while maintaining the improved optical transmittance of the patterned samples. Graphene nanohole arrays (GNAs) may provide a unique scheme for improving both conductivity and transmittance. In addition to improving the transmittance by reducing graphene coverage, GNAs can provide large number exposed edges of holes for effective doping, allowing the possibility of tuning electrical transport properties of graphene.
Here, we demonstrate a successful fabrication of graphene nanomesh by using Poly(styrene-b-4-vinylpyridine) (PS-P4VP) type di-block copolymer template for the first time. A 20-nm thick silicon oxide film is first deposited onto graphene as the protecting layer and also as the grafting substrate for the subsequent block copolymer nanopatterning. The PSminus;P4VP block copolymer thin film with cylindrical domains normal to the surface is then fabricated and used as the etching template and a reactive-ion etch (RIE) process is used to punch holes into the graphene layer. The mesh structure, 5 - 30 nm in width, can be considered as a network of constrictions of graphene which leads to a bandgap opening. The band gap energy inversely scales with the neck width of the porous graphene structure. The availability of large number of nanohole edges also provides ideal sites for chemical attachment. Tthe electrical and optical properties of the nanopatterned graphene with various configurations will be described, and the interesting effects of altered doping species and amounts on the graphene properties will also be discussed.

Carbon nanotubes (CNTs) have been considered as one of the most promising materials for field emitters. In our study, we fabricate the FE diode and triode devices, utilizing the ultrathin CNT sheet and the lateral structure. CNT sheets are drawn and densified from CNT forest. The sheet has the thickness of about 100 nm and the uniform CNT alignment. Laser is utilized to cut the sheets to fabricate CNT emitter arrays. This method provides the simple and effective way to control the uniform geometry of the array. The device is fabricated with lateral structure, meaning both cathode and anode are positioned in the same plane. A pair of metals is produced to serve as the base for electrodes. The CNT array is attached onto one electrode with the tips uniformly facing the other metal. The devices are fabricated with and without metal-gated structure to behave as triode and diode, respectively. Under the applied electric field, the CNT tips act as the cathode, emitting electrons towards the other electrode, called anode. This sheet structure has the advantage of significantly relieved screening effect and highly focused potential energy. The threshold field (at which the current density reaches 1 mA/cm2) is as low as 0.44 V/µm and the current density is 8 A/cm2 at 0.84 V/µm. The diode and triode are connected into alternative signals to experiment their rectification and amplification capabilities, respectively. These vacuum devices have the advantages of high-frequency response, high-power operation, low-energy consumption, stable function under high temperature and high-radiation environment. In addition, the lateral structure makes the devices effectively integrated into electronic systems due to the flexibility of in-plane structure.

Graphene&’s capacity in nanomechanics goes well beyond the exceptional mechanical properties of a single layer of atoms arranged in a honeycomb lattice. The two-dimensional (2D) nature of graphene makes all carbon atoms within its backbone chemically accessible, allowing alteration of the carbon-carbon bonds for tuning mechanical, thermal and electrical properties. In this talk we describe our recent efforts to re-engineer sp2-bonds in order to “fuse” graphene layers together, which results in dramatic improvements in out-of-plane properties (e.g., shear/yield strength) and a structure with a fraction of bonds akin to diamond. Inter-layer crosslinking/bonding in ultra-thin chemically modified graphene (CMG) films was induced through chemical modification and laser annealing and was shown to increase the films Young's modulus and yield strength by an order of magnitude.
Using nanomechanical resonators, we monitor the mechanical properties of CMG films after various treatments and illustrate the benefits of the material's docility for nanomechanical devices. Structural transformations within multi-layer resonators results in high yield strengths (sustainable tensile stress up to 1 GPa) and in high resonator quality factors (Q ~ 30,000). More broadly, the ability to recrystallize layered 2D materials both in plane and out of plane, in a local and controlled manner, transforms CMG into a “material-by-design” with new capabilities in nanomechanics that are unavailable with traditional materials.
This work is carried out in collaboration with B.H. Houston in the Acoustics Division, E.S. Snow, J.T. Robinson, T.L. Reinekce, C.E. Junkermier, J.C. Culbertson in the Electronics Science and Technology Division, and R. Stine and P.E. Sheehan in the Chemistry Division at NRL. This work was supported by the Office of Naval Research.

All-carbon based photovoltaics comprised of n-type fullerenes and p-type single walled carbon nanotubes are tantalizing candidates for light-harvesting and storage because of the advantageous carrier transport properties and improved mechanical/chemical stability. However, fullerenes tend to slip away from the graphitic basal plane by virtue of the weak van der Waals force between dissimilar graphitic allotropes. Here, we report that the stability of these structures can be enhanced by wrapping them with “shaped” graphene nanoribbons, forming unprecedented photoconductive core-shell heterostructures. By precisely controlling the size and shape of the nanoribbons, various wrapping behaviors are observed and are integrated into photoelectric chemical cells for light-to-energy conversion. To rationally guide the synthesis and practically improve the overall power conversion efficiency, we have developed molecular dynamic simulations of C60-coated, graphene nanoribbon-wrapped carbon nanotubes that enable us to systematically construct a predictive model to explore the interplay between nanoribbon morphology and wrapping behavior. Theoretical modeling shows that the competition between the van der Waals and bending energies of the system has the profound implications on the wrapping direction and the morphology of final assembly, thus forming a tight feedback loop where device performance is optimized by improving assembly strategy and material control. A range of geometric effects including nanoribbon size, nanotube radius, and C60 coverage density are also explored.

Molecular recognition is central to the design of therapeutics, chemical catalysis and sensor platforms, with the most common mechanisms involving biological structures such as antibodies¹ and aptamers^2,3. The key to this molecular recognition is a folded and constrained heteropolymer pinned, via intra-molecular forces, into a unique three-dimensional orientation that creates a binding pocket or interface to recognize a specific molecule. An alternate approach to constraining a polymer in three-dimensional space involves adsorbing it onto a cylindrical nanotube surface^4-7. To date, however, the molecular recognition potential of these structured, nanotube-associated complexes has been unexplored. In this work, we demonstrate three distinct examples in which synthetic polymers create unique and highly selective molecular recognition sites once adsorbed onto a single-walled carbon nanotube (SWCNT) surface. The phenomenon is shown to be generic, with new recognition complexes demonstrated for riboflavin, L-thyroxine, and estradiol, predicted using a 2D thermodynamic model of surface interactions. The dissociation constants are continuously tunable by perturbing the chemical structure of the heteropolymer. The complexes can be used as new types of sensors based on modulation of SWCNT photoemission, as demonstrated using a complex for real time spatio-temporal detection of riboflavin in murine macrophages.
References:
1 Scanlan, C., Offer, J., Zitzmann, N. & Dwek, R. Exploiting the defensive sugars of HIV-1 for drug and vaccine design. Nature 446, 1038-1045 (2007).
2 Ellington, A. D. & Szostak, J. W. In vitro selection of RNA molecules that bind specific ligands. Nature 346, 818-822 (1990).
3 Cho, E. J., Lee, J. W. & Ellington, A. D. Applications of Aptamers as Sensors. Annu. Rev. Anal. Chem. 2, 241-264 (2009).
4 Nish, A., Hwang, J., Doig, J. & Nicholas, R. Highly selective dispersion of single-walled carbon nanotubes using aromatic polymers. Nature Nanotechnology 2, 640-646 (2007).
5 Tsyboulski, D. et al. Self-Assembling Peptide Coatings Designed for Highly Luminescent Suspension of Single-Walled Carbon Nanotubes. J. Am. Chem. Soc 130, 17134-17140 (2008).
6 Zheng, M. et al. DNA-assisted dispersion and separation of carbon nanotubes. Nature Materials 2, 338-342 (2003).
7 Tu, X., Manohar, S., Jagota, A. & Zheng, M. DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes. Nature 460, 250-253 (2009).

The determination and modeling of the electromagnetic (EM) characteristics of carbon nanotube (CNT) containing polymer composites is of much interest, with the objectives of obtaining (a) fundamental understanding of the influence of large aspect ratio electrical conductors, as well as for (b) practical applications. A large length to diameter aspect ratio, which could be as much as 106 (e.g., for a nanotube of 1 mm length and 1 nm diameter) enables pertinent characteristics, such as electrical percolation, to be obtained at much lower volume fractions, e.g., at < 0.01% nanotube filler concentrations. In the context of applications, CNT constituted composites have been advocated for a wide variety of uses, e.g., EM interference shielding, thermal management, energy conversion and electronic packaging applications, etc., in which characterization of EM properties would be important.
A model-based approach to characterize the electrical impedance of polymer matrices with carbon nanotube fillers, based on dielectric permittivity measurements in the sub-GHz regime, is proposed. In this context, an equivalent-series resistance (ESR) model, constituted of lumped resistances (R) and capacitances (C) is compared with distributed models such as a RC network and constant phase element (CPE) representations. It is shown, through detailed statistical analysis, that the CPE corresponding to a distribution/dispersion of relaxation times may best fit the experimental data.
This is the first time that the CPE model is being invoked to carbon dispersion in matrices and would likely have a large impact in future modeling. Its is then possible to assign acomplex conductivity,to carbon nanotube dispersions.

Photodetectors that are responsive in the infrared regime are highly desirable for various applications such as telecommunication, night-vision, gas detection, and motion sensors. Single-walled carbon nanotubes (SWNTs) are a promising candidate for infrared light detection due to its strong absorption in the infrared wavelengths and due to its long carrier diffusion lengths. In addition, SWNTs&’ solution processibility and mechanical flexibility makes them a viable material for future low-cost, light-weight, and flexible infrared sensors. Thus far, SWNT-based photodetectors have suffered from low sensitivity, likely due to the strong exiton binding energy and long channel lengths of the devices that make it difficult to collect the charges at the electrodes. In addition, the presence of metallic SWNTs can quench the photogenerated carriers, furthermore decreasing the charges collected at the electrodes. Herein, we present a novel infrared photodetector device in the form of a short-channel thin-film transistor, where the active channel material is composed of a thin-film of sorted semiconducting HiPCO SWNTs (previously developed by our group) with a thin layer of C60 on top. Interestingly, we have observed the p-type doping of SWNTs when infrared light was illuminated, causing a positive shift in the threshold voltage. Such a phenomenon which we term as ‘light induced doping of SWNTs&’ can be explained as C60 accepting the electrons that are excited to the conduction band of the SWNTs upon light absorption. When light is turned off, the electrons in the LUMO of C60 recombine back to the valance of SWNTs, resulting in a recovery of carriers back into the SWNTs. We have demonstrated that the sensitivity was drastically increased by the addition of C60, with external quantum efficiency of 14% and signal-to-noise ratio as high as 2 orders of magnitude. In addition, the photodetector responded immediately to the presence and absence of light, where the signal-to-noise ratio was constant during the entirety of our measurement of 20 minutes. Hence, the sensitivity, time response, and the stability of our infrared photodetector are comparable to some of the best reported photodetectors. Therefore, we believe that our novel SWNT/C60 based infrared photodetector can pave the way to a wide-variety of novel applications in the near future.

Single-walled carbon nanotubes (SWNTs) have attracted increasing attensions due to their extraordinary electrical properties, and easy processing availability for flexible electrical systems. For logic circuit applications, it is essential to utilize complementary metal oxide semiconductor (CMOS) architecture, which requires the development of both p-type and n-type semiconductors. However, SWNTs typically exhibit p-type transporting behavior so that the availability of high-performance and unipolar n-type behavior remains considerably limited. Selective, unipolar n-doping of intrinsically SWNT transistors is essential for its CMOS applications. In this work, regioregular poly(3-alkylthiophene)s (rr-P3ATs) were used to disperse sc-SWNTs to form the active layer of thin-film transistors on a flexible polyimide substrate. This highly effective sorting method yielded high-performance transistors with an on/off ratio of up to 10^6 and a mobility of up to 1.18 cm^2/Vs. N-doping of these transistors was done by evaporating 2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-MeO-DMBI-I) onto selected device channels which yielded devices with an on/off ratio of up to 10^7 and a mobility of up to 0.78 cm^2/Vs. The transistors were bent down to a 2.5 mm bending radius with no effect on device performance. The p- and n-type transistors were then used to fabricate CMOS inverters with high gains of up to 42. The effective dispersion of sc-SWNTs and the unipolar doping of selected transistors allow for the fabrication of CMOS technologies and demonstrates the viability of this platform in future flexible integrated circuits.

We report self-aligned T-shaped gate radio frequency (RF) transistor made of high-purity semiconducting carbon nanotubes. Here, we introduce a self-aligned fabrication method for carbon nanotube RF transistors, which incorporate a T-shaped (mushroom-shaped) aluminum gate, with oxidized aluminum as gate dielectric. Carbon nanotube RF transistors are predicted to offer good performance and high linearity when operated in the ballistic transport and quantum capacitance regime. In this way, the channel length can be scaled down to 140 nm which enables quasi ballistic transport, and the gate dielectric is reduced to 2-3 nm aluminum oxide, leading to quasi quantum capacitance operation. A current-gain cut-off frequency (ft) up to 22 GHz and a maximum oscillation frequency (fmax) of 10 GHz are demonstrated. Furthermore, the linearity properties of nanotube transistors are characterized by using the 1-dB compression point measurement with positive power gain for the first time, to our knowledge. Our work reveals that the importance and potential of separated semiconducting nanotubes for various RF applications.

Transparent electronics have attracted numerous research efforts in recent years due to its great potential to make significant commercial impact in a wide variety of areas such as transparent displays. High optical transparency as well as good electrical performance is required for this kind of applications. Pre-separated, semiconducting enriched carbon nanotubes are excellent candidates for this purpose due to their excellent mobility, high percentage of semiconducting nanotubes, and room-temperature processing compatibility. Here in this paper, we report fully transparent high-yield transistors based on separated carbon nanotube random network. High electrical performance is achieved by using large work function thin metal layer and indium-tin oxide (ITO) as contacts and all devices show excellent transparency (~82%). Furthermore, OLED control circuit has been demonstrated with transparent separated nanotube thin-film transistors and large range output light intensity modulation has been observed. Our results suggest the promising future of separated carbon nanotube based transparent electronics and can serve as the critical foundation for the next generation transparent display applications.

Designing robust superhydrophobic surfaces has been of interest for creating functional coatings such as self-cleaning, anti-condensation and ice-resistant surfaces. We developed an extremely robust superhydrophobic coating based on non-aligned mesh of carbon nanotubes (CNTs) grown on stainless steel surfaces using chemical vapor deposition (CVD) technique . The surfaces of the CNTs were chemically modified using plasma-enhanced CVD (PECVD) process to deposit a hydrophobic coating. The wetting studies of CNT based coatings showed that they are superhydrophobic with water contact angle of 156° and almost zero contact angle hysteresis. We demonstrated that the CNT mesh coatings, when exposed to steam condensation, are not wetted by condensed water. They are robust enough to retain superhydrophobicity and non-wetting property for prolonged exposure to corrosive steam environment [1]. We also showed that ice and frost formation is retarded on the surfaces of these CNT mats. The superhydrophobicity of the coating is not affected after multiple frosting-defrosting cycles. The CNT based coatings we developed thus exhibit stable water repellency. They also show anti-steam and anti-ice properties. The thermal and mechanical stability of CNTs plays a key role in engineering robust surfaces for such specific applications.
References:
[1] Badge, I.; Sethi, S.; Dhinojwala, A. Langmuir 2011, 27, 14726-14731.

Materials and processes capable of producing non-conventional fuels are critical to our future energy security. One such process, the Fischer Tropsch (FT) synthesis is a catalyzed chemical reaction that converts a mixture of carbon monoxide (CO) and hydrogen (H2) into clean burning fuels. Here we demonstrate that multi-walled carbon nanotubes (MWNTs) can be used to effect similar chemistry as FT synthesis (produce long chain hydrocarbons by CO hydrogenation) with orders of magnitude higher conversion efficiencies than the present state-of-the-art FT catalysts. Specifically, we report the successful production of synthetic fuels ranging from jet fuels to diesels using MWNTs. Density Functional Theory (DFT) calculations suggest that the enhanced catalytic activity is due to the presence of C-O groups (incorporated onto CNT surfaces during their synthesis) which are capable of catalyzing CO hydrogenation and subsequent chain growth, in order to produce long chain hydrocarbons. These findings open up a whole new paradigm of CNT-based catalysts and become a defining point for obtaining clean alternative fuels through simple and efficient chemical processes.
This research was made possible with support, in part, by the Illinois Department of Commerce and Economic Opportunity through the Office of Coal Development and the Illinois Clean Coal Institute and NSF-CBET-1133117. MT thanks JST-Japan for funding the Research Center for Exotic NanoCarbons, under the Japanese regional Innovation Strategy Program by the Excellence. BGS was supported by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy.