We explore possible ways to manipulate the intrinsic edge magnetism in hexagonal graphene nanoflake with zigzag edges, using density functional theory supplemented with on-site Coulomb interaction. The effect of carrier doping, chemical modification at the edge, and finite temperature on the edge magnetism has been studied. The magnetic phase diagram with varied carrier doping, and on-site Coulomb interaction is found to be complex. In addition to the intrinsic antiferromagnetic solution, fully polarized ferromagnetic, and mixed phase solutions are obtained depending on the doped carrier concentration, and on-site Coulomb interaction. The complexity arises due to the competing nature of local Coulomb interaction and carrier doping, favoring antiferromagnetic and ferromagnetic coupling, respectively. Chemical modification of the edge atoms by hydrogen leads to partial quenching of local moments, giving rise to a richer phase diagram consisting of antiferromagnetic, ferromagnetic, mixed, and nonmagnetic phases. We further report the influence of temperature on the long-range magnetic ordering at the edge using ab initio molecular dynamics. In agreement with the recent experimental observations, we find that temperature can also alter the magnetic state of neutral nanoflake, which is otherwise antiferromagnetic at zero temperature. These findings will have important implications in controlling magnetism in graphene based low dimensional structures for technological purpose, and in understanding varied experimental reports.

Recent studies of thermal transport in nanostructured materials have demonstrated the breakdown of Fourier&’s heat conduction law through observations of ballistic transport. Despite its unique features, another instance of the breakdown of Fourier&’s law, hydrodynamic phonon transport, has drawn less attention because it has been observed only at extremely low and narrow temperature ranges in bulk materials. Here we show that hydrodynamic phonon transport can occur in suspended graphene at significantly higher temperatures and wider temperature ranges compared to bulk three-dimensional material cases. Using first principles calculation, the hydrodynamic transport in graphene is demonstrated with drift motion of phonons, phonon Poiseuille flow, and second sound. The significant hydrodynamic phonon transport in graphene is associated with graphene&’s two-dimensional features such as quadratic dispersion and large anharmonicity of long wavelength flexural acoustic phonon modes. The significant hydrodynamic phonon transport in graphene provides a new chance for understanding and manipulating heat flow in two-dimensional materials.
Acknowledgement: This work was supported by S3TEC, a US DOE EFRC, and AFOSR MURI through Ohio State Univ.

The very existence of isolated graphene is a challenge to the Mermin-Wagner theorem that forbid long-range (lattice) ordering in truly two-dimensional (2D) systems, which should be unstable against lattice fluctuations leading to crumpling, folding etc. In real materials, however, the anharmonic coupling between the in-plane lattice mode and the out-of-plane flexural modes was shown, in the high-temperature classical regime, to stabilize the system [1-4]. The generalization of these results in the low temperature quantum regime leads however to unphysical results signalizing that new physics is involved.
In this contribution we present a microscopic theory to investigate the thermodynamical properties of graphene, modeled as a crystalline membranes, in the zero/low-temperature regime. Using Quantum Field Theory, we generalize the self-consistent screening approximation (SCSA) approach [5] at the quantum level. A key role is played by the retarded nature of the anharmonic coupling between in-plane and out-of-plane lattice modes that, in the quantum limit, turns to have crucial different consequences than in the classical regime.
We identify a crossover temperature T* between classical and quantum regimes, which is T* ~ 70-79 K for graphene [6]. Below T* the heat capacity and thermal expansion coefficients results to decrease as power laws with decreasing temperature, vanishing at T=0,
reconciling the SCSA theory with the third law of thermodynamics [6].
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[5] K. V. Zakharchenko, R. Roldan, A. Fasolino, and M.I. Katsnelson, Phys. Rev. B 82, 125435 (2010).
[6] B. Amorim, R. Roldan, E. Cappelluti, A. Fasolino, F. Guinea, and M.I. Katsnelson, arXiv:1403.2637 (2014).

In the past decades, substantial efforts of experimentalists and theoreticians were dedicated to find optimal ways to tune the optical properties of semiconductors and graphene by application of external electric and magnetic fields. The motivation of this research lies in potential applications for integrated circuits in optical and quantum computers, for secure information transfer, and in new light sources. One of the promising approaches is in the use of polaritons, which are a quantum superposition of cavity photons and excitons in a nanometer-wide semiconductor layer or graphene. Since polaritons are interacting Bose particles, a polariton gas can transit to a superfluid state that, under certain conditions, propagates in a microcavity almost without dissipation. Owing to a small effective mass, 10^-4 of the free electron mass, the superfluid transition occurs at relatively high temperatures that can be comparable with the room temperature. However, one of the main problems to be solved in actual design of polariton-based optical devices is in weak response of polaritons to an external electric field owing to their net zero electric charge. To solve this issue, we propose a design of polariton-based, electrically controlled optical devices based on the Coulomb drag effect. It is known that if an electric current runs in a quantum well, a drag force is exerted on excitons located in a neighboring, parallel quantum well. Recently, it was also shown that drag from an electric current results in entrainment of polaritons and in formation of a persistent, directed polariton current. In our design we take advantage of a patterned microcavity to create a potential-energy landscape for polaritons in the form of one-dimensional channels, or optical wires. We consider various topologies of the channels, including optical splitters or switches, and interferometers. In all these cases, the polariton propagation in the channels is directed by an electrically controlled drag force. By considering the non-equilibrium polariton superfluid dynamics, we demonstrate the possibility to deliver optical signals in circuits to a desired location and switch the polariton paths by means of an external electric voltage. In our studies, we numerically integrate the nonlinear, non-equilibrium Gross-Pitaevskii equation for the wave function of a polariton condensate, in which a continuous pumping and polariton decay are taken into account. The nonlinearity in the Gross-Pitaevskii equation arises from repulsive scattering of polaritons. We also discuss the tunability of the system with an embedded graphene layer by dynamically changing the band gap in graphene by an external, normal electric field.

We investigate the structural evolution of reduced graphene oxide (RGO) sheets with carbon sp² fractions varying from 55 to 80 % using low temperature Coulomb blockade (CB) transport. At 4.2 K, all RGO sheets exhibit a complete suppression of current (CB) below a threshold voltage (V_t), the value of which decreased from 3.34 to 0.25 V with increasing carbon sp² fraction. From the temperature dependent V_t, we calculate an effective charging energy and individual graphene domain size of 160 meV and 1.34 nm at 55 % carbon sp² fractions, respectively. While these values are 20 meV and 4.18 nm at 80 % carbon sp² fractions, respectively. This implies that, with increasing reduction, newly formed sp2 domains increase the effective size of the graphene domain. For an applied voltage (V) >V_t, the current (I) follows a scaling law I ~ [(V-V_t)/V_t]^α where the scaling parameter α increases from 2.11 to 3.40 with increasing sp² fraction, suggesting that increasing sp² fraction creates more topological defects on the RGO. Our report provides a much desired insight of structural evolution of RGO sheets.

Two-dimensional (2D) layered materials host many interesting physical and chemical phenomena such as topological insulator and intercalation. Their nanostructures represent novel candidates to host those phenomena. Here we present our study on chemistry and physics of 2D layered nanomaterials. First, we have synthesized a range of morphologies including nanoplates, nanoribbons and their heterostructures. Second, we have developed a new method of zero-valent intercalation which allows unprecedented high levels of various metal intercalants inserted into the van der Waals gaps. The resulted optical properties and electrical conductance change drastically. Third, we have fabricated single nanostructure electrical transport devices. Using topological insulator nanostructures, we observed interesting physical phenomena including Aharonov-Bohm oscillations from topological surface electrons, ambipolar transport with effective control of the Fermi level into the bulk bandgap and across the Dirac Point, and localization effects emerging from the surface electrons in confined dimensions. Lastly, we also demonstrate the interesting tunable catalytic property.

The enormous impact of graphene on both fundamental science and potential device applications has rejuvenated interest in other layered materials as well, where individual atomic or molecular layers are weakly coupled through Van der Waals forces. A recent development in this field involves multi-component two-dimensional hybrids obtain via vertical stacking of different layered materials. Hybrid heterostructures of atomically thin membranes with clean interfaces promise devices that combine advantages of ultimate miniaturization and multiple functionality. In this work we demonstrate that metal dichalcogenides, such as Molybdenum disulphide (MoS₂), can be a natural partner to graphene for novel optoelectronic application because of the visible range bandgap, and gate tunable electrical transport in MoS₂. The responsivity of Graphene/MoS₂ hybrids can be as high as 10¹⁰ Ampere/Watt [1], which is nearly thousand times larger than other light-sensitive graphene hybrids. In addition, these devices display persistent photoconductivity that can be exploited to realize programmable optoelectronic switches. I shall outline this emerging concept in material engineering for optoelectronics from Van der Waals heterostructures with examples of different composition and stacking sequences.
References
[1] Kallol Roy, Medini Padmanabhan, Srijit Goswami, T. Phanindra Sai, Gopalakrishnan Ramalingam, Srinivasan Raghavan, and Arindam Ghosh, Nature Nanotechnology 8, 826 (2013).

The quest for novel two-dimensional (2D) materials has led to the discovery of hybrid heterostructures where graphene and other atomic layer films such as monolayer hexagonal boron nitride (hBN) form phase-separated domains or in-plane heterostructures [1-3]. Here, we report on a combined experimental and theoretical study of in-plane heterostructures of graphene-hBN. By implementing the concept of epitaxy to 2D space, we demonstrated a single-atomic layer, in-plane heterostructure: monolayer hBN grows from fresh edges of monolayer graphene with lattice coherence, forming a 1D boundary. More importantly, the crystallography of the hBN is solely determined by that of the graphene, forgoing configurations favored by the supporting Cu substrate [1]. Scanning tunneling microscopy and spectroscopy measurements reveal an abrupt 1D zigzag oriented boundary, which provides a rare opportunity to examine the spatial and energetic distributions of the 1D boundary states in real space. The polar-on-nonpolar 1D boundary between grpahene and hBN is expected to possess peculiar electronic states associated with edge states of graphene and the polarity of hBN. The revealed boundary states are about 0.6 eV below or above the Fermi energy depending on the termination of the hBN at the boundary, and are extended along but localized at the boundary. These results suggest that unconventional physical effects similar to those observed at 2D interfaces can also exist in lower dimensions, opening a route for tuning of electronic properties at interfaces in 2D heterostructures. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
[1] L. Liu, J. Park, D.A. Siegel, K.F. McCarty, K.W. Clark, W. Deng, L. Basile, J.C. Idrobo, A.-P. Li, and G. Gu, Science343, 163 (2014).
[2] Y. Gao, Y. Zhang, P. Chen, Y. Li, M. Liu, T. Gao, D. Ma, Y. Chen, Z. Cheng, X. Qiu, W. Duan, and Z. Liu, Nano Lett. 13, 3439 (2013).
[3] P. Sutter, R. Cortes, J. Lahiri, E. Sutter, Nano Lett. 12, 4869 (2012).

Recent progress in preparing well controlled 2D van der Waals heterojunctions has opened up a new frontier in materials physics. In this paper we address the intriguing energy gaps that are sometimes observed when a graphene sheet is placed on a hexagonal boron nitride substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behavior by accounting first for the structural relaxation of graphene&’s carbon atoms when placed on a boron nitride substrate and then for the influence of the substrate on low-energy π-electrons located at relaxed carbon atom sites. The methods we employ can be applied to many other van der Waals heterojunctions.

Graphene oxide films and fibers have attracted great attention due to their outstanding mechanical properties and the possibility to be converted into electrically conductive reduced graphene oxide by chemical or thermal reduction. Graphene oxide is a water dispersible 2D material easy to synthesize in large quantities from graphite by chemical oxidation. Macroscopic fibers made of this material have been recently prepared by wet -spinning. In spite of the remarkable mechanical properties, such as high modulus and tensile strength, the graphene oxide wet spun fibers usually possess low toughness, rough surfaces, and highly irregular cross- sections. Due to these characteristics, self-abrasion and failure due to fatigue and stress-concentration becomes a problem. Here, we developed an alternative fabrication method, using bar coating of water/GO dispersions, followed by drying to prepare large area GO thin films. These films displayed high toughness and excellent tear resistance. The films were cut into stripes that were subsequently scrolled to prepare macroscopic graphene oxide fibers with extremely large elongation to fracture and high toughness (up to 17 J/m³). The fibers also showed attractive macroscopic properties, such as circular cross section and smooth surface, resulting in great knottability and low self-abrasion. The fibers were reduced by chemical and thermal methods and became electrically conductive. After being annealed at high temperature, the fibers were also studied as electron field emitters. The scrolling method was easily modified to prepare hybrid graphene oxide fibers using several nanomaterials, such as polymer nanofibers, carbon nanotubes and transition metal chalcogenide nanosheets with fascinating architectures.

Graphene&’s two-dimensional nature makes it a perfect separating layer for liquid-solid interfaces. Combining its superior electrical properties, high mechanical strength and good chemical stability, graphene has shown extensive application potentials in smart sensors of gas molecular, PH, metal ions, biomolecules and some kinds of DNAs. In this study, single-layer graphene with area of several square centimeters was grown using chemical vapor deposition (CVD) techniques for use in sensing applications that could work in solutions or at solid/solution interfaces. We examined the effects of different solution environments on the CVD graphene and found that the Hall sensitivity and magnetoresistance for graphene in solution environments could be comparable with those for graphene in air. The measured Hall response is as large as 200 VA^-1T^-1 in water or some solutions, and the magnetoresistance can reach 13% under the magnetic field of 1.17 T in several different solutions. We also found that an electrical voltage which is proportional to the moving velocity can be produced as the interface between graphene, air and salt solution is moving. All these results indicate that the graphene is a potentially promising material for complex liquid environment sensors.

Graphene is proved to be one of the most exciting materials of great scientific and possibly technological importance. There are various ways to produce graphene. In terms of device applications, quality, and large scale production epitaxial growth of graphene on SiC looks most promising. Unfortunately, the first graphitic layer grown on SiC takes form of buffer layer and looses graphene-specific electronic properties owing to a strong graphene-substrate interaction. Subsequent layers regain the characteristic graphene linear electron dispersion; however, the influence of the interface is still effective in terms of doping and reduced carrier mobility. Recently, a number of attepmts have been realized to decouple graphene layers from substrate and tune its properties by intercalating different elements in graphene/SiC interface. Interface hydrogenation is the most popular procedure. There are also reports that Si may be intercalated into the interface region and decouple graphene layers. However, up to now there was no information on the interface structure induced by Si intercalation.
In the present work we present experimental and computational analysis on various structures unduced by Si intercalation in graphene/SiC interface depending on Si coverage. We have shown that Si may form structures at the interface identical to those formed by Si deposition on clean SiC surface, such as 3x3 reconstruction and metastable 2radic;3x4 structure. The latter is observed only in certain conditions and in limited areas on clean SiC, but is significantly stabilized by graphene overlayer. The calculations and experimental data show that graphene indeed is decoupled from the substrate by these interfacial structures formation and regains its linear electron dispresion, although exhibits some n-type doping. Some new interface specific superstructures have been also observed in our experiments and are discussed in the context of possibility of 2D material growth in the confined space of graphene/SiC interface. The Si intercalation is compartible with present semiconductor technology, easily performed, and the possibility to form a range of structures in graphene/SiC interface can prove to be useful in terms of tuning properties of graphene layer depending on interface structure.

The rising of Graphene based technologies opens a lot of new possibilities in the fields of electronics and sensing applications. The great challenge is to develop devices that could take the place of the silicon based ones. Recently, many studies on Graphene device fabrication have been reported, focusing on methods that exploit electron beam lithography as patterning technique [1][2]. However, in these approaches Graphene is exposed to many chemical and physical attacks that can damage it, leading to the detriment of its electronic properties.
To avoid these problems, we have developed a new technique that allows the direct deposition of Graphene samples with the desired shape on Cu thin films by Rapid Thermal Chemical Vapor Deposition (RT-CVD). First, we deposit on Cu surface a few hundred nanometers thick SiO₂-like layer (by plasma-CVD or Spin-On Glass), then, after the resist deposition, the windows of the developed pattern are opened down by means of optical lithography. After this step, we spread the geometries to the SiO₂ layer with a low concentrated HF based wet etching followed by the resist removal.
Graphene deposition on the Cu exposed areas of the sample is carried out by low (from 650°C to 800°C) temperature RT-CVD in an ethanol and hydrogen atmosphere without damaging the SiO₂ mask. Finally, we remove the SiO₂ hard mask and transfer the device onto a PMMA insulating substrate with our previously reported hot bonding method [3].
The designed devices are two cross-shaped Hall bars with micrometer width that can be applied as high-sensitivity sensors for the detection of strongly localized magnetic fields. Their sensing capabilities are currently investigated by means of a finite element based model able to simulate the spatial distribution of the electric potential inside the Hall plate, in the presence of magnetic fields generated by small magnetic particles (e.g. microbeads for biomolecular applications[4]).
[1] Zhengqing J. Qi ; Julio A. Rodríguez-Manzo ; Sung Ju Hong ; Yung Woo Park ; Eric A. Stach, et al.#8232;" Direct electron beam patterning of sub-5nm monolayer Graphene interconnects ", Proc. SPIE 8680, Alternative Lithographic Technologies V, 86802F (March 26, 2013); doi:10.1117/12.2013724.
[2] B Jabakhanji et al, Quantum Hall effect of self-organized Graphene monolayers on the C-face of 6H-SiC, 2014 J. Phys. D: Appl. Phys.47 094009
[3] G. Amato, E. Simonetto, L. Croin and E. Vittone (2014). A New Transfer Technique for Graphene Deposited by CVD on Metal Thin Films . MRS Proceedings, 1658, mrsf13-1658-rr15-95 doi:10.1557/opl.2014.506.
[4] A. Manzin, V. Nabaei and O. Kazakova, Modelling and optimization of submicron Hall sensors for the detection of superparamagnetic beads, J. Appl. Phys. 111, 07E513 (2012).

The exception stiffness and strength of graphene makes it a promising reinforcement in structural polymer composite materials [1]. We have studied the micromechanics of such graphene composites using Raman spectroscopy to map the strain in model composite systems comprising of single graphene flakes [2,3,4]. We have previously shown that the graphene behavior can be modelled using conventional composite theory despite being an atomic layer. For example, graphene follows the shear lag theory for short fibers, with a critical minimum flake length of 3 microns being required for good reinforcement. We have also shown that the modulus of graphene flakes reduces with the thickness of the flake due to poor internal stress transfer between the graphene layers [5]. Herein, we transfer this understanding of micromechanics to bulk graphene nanocomposites.
Polypropylene (PP) nanocomposites were prepared using electrochemically-exfoliated-few layer graphene [6] with two different flake diameters (5 microns and 20 microns). The crystallization temperature and degree of crystallinity of the PP was found to increase with the loading of few layer graphene, which suggests that the flakes act as crystallisation nucleation sites. Mechanical testing showed that the 5 µm flakes behaved as short fillers and reinforced the PP matrix poorly. The modulus of the 20 µm flake composites, however, increased linearly with loading up to 20 wt.%, without any of the detrimental aggregation effects seen in other graphene systems [7]. The performance of this high loading is found to be similar to that of chopped glass fibre composites but with a significantly lower weight due to the relatively low density of carbon to glass. The mechanical data were then compared with our previous work on graphene oxide and PMMA [8] and the apparent need to balance the degree of functionalization to improve matrix compatibility whilst not encouraging aggregation is discussed.
References
[1] RJ Young et al., Compos. Sci. Technol., 12, 1459-1476 (2012)
[2] L Gong et al., Adv. Mat., 24, 2694- (2010)
[3] RJ Young et al., ACS Nano, 4, 3079-3084 (2011)
[4] A. Raju et al., Adv. Functional Mat., 10.1002/adfm.201302869 (2014)
[5] L Gong et al., ACS Nano, 6, 2086-2095 (2012)
[6] AM Abdelkader et al., ACS Appl. Mater. Interfaces, 6, 1632minus;1639 (2014)
[7] C Valles et al., Faraday Discussions FD172 (2014)
[8] C Valles et al., Compos. Sci. Technol., 88, 158-164 (2013)

Graphene is two-dimensional semi-metal with a strong ambipolar electric field effect and very high carrier mobility at room temperature, which is also atomically thin yet remarkably strong yet mechanically with very good flexibility and stretchability, making it a promising candidate for electromechanical sensors and devices such as strain sensors. Although graphene based strain sensors have been reported they are utilized primarily as resistive materials where it&’s electrical resistance changes due to strain. In this work, we report a stretchable three-dimensional graphene foam based transistor for strain sensing. Here three-dimensional foam of monolayer graphene has been used as an active layer of the transistor. Using transistor as a sensor provides the opportunity of intrinsic amplification of strain response. Also graphene foam is inherently tolerant to defects due to its random network topology that provides carrier pathway especially at high strain when local defects could locally appear within the device.
The transistor has been fabricated on PDMS as a flexible and stretchable substrate. In the first step mono-layer graphene foam grown on copper using CVD was left in ferric chloride to etch away copper, then it is rinsed with DI water and transferred on PDMS substrate. In the next step Ti/Au was deposited through a shadow mask to form the source and drain contact. Finally Ionic liquid (1-Butyl-3-methylimidazolium hexafluorophosphate, Bmim PF6, 98%) was added at the top of the graphene to be used as a gate. The transistor length is 900 µm and width is 5mm. Our results show very high sensitivity of 0.374 mA change in drain current per percentage of strain. Due to high sensitivity and biocompatibility of this device, it has great potential to be used in biological and medical application.

Graphene has sparked a great deal of interest in the microelectronics field since the discovery of its excellent electronic transport properties. The main steps toward the development of high-performance graphene based devices are the synthesis of graphene and its transfer from the synthesis substrate onto a specific substrate, typically a dielectric material. Currently, chemical vapor deposition of graphene on Cu foil is considered as the most promising technique to produce large-scale high-quality single-layer graphene films. The challenge for device fabrication lies in the development of a reliable technique allow- ing the transfer of graphene without generating cracks, folds and ripples which are known to compromise graphene properties. It also emerged that Cu surface morphology on which graphene grows is of first importance to guaranty a successful defect-free transfer of graphene.
This work investigates the use of thin Cu film with varying thicknesses (from 1 micron down to 350 nm) as catalyst instead of Cu foil in order to improve graphene transfer. Wafer-scale high-quality single-layer graphene is produced by low pressure chemical vapor deposition (LPCVD) on the Cu film evaporated on a 300 nm-SiO2/Si wafer. Graphene grown on a thin Cu film does not present the rough morphology mimicking the deep Cu grain boundary grooves and ripples induced by the rolling process of Cu foils. The experimental results demonstrate that using a thin Cu film instead of a Cu foil effectively mitigates graphene morphology and results in graphene films with reduced defects after a conventional transfer procedure.
Starting from a Cu film instead of a Cu foil, we also established alternative transfer approaches. The flat and rigid Cu substrate can be exploited to directly process graphene by conventional thin film technologies. The Cu film can be used as a sacrificial layer after graphene growth to directly deposit graphene on the underlying SiO2/Si substrate. Alternatively, lift-off process can also be used to pattern the Cu thin film prior to CVD graphene growth in order to prevent residues induced by direct contact of resist on the top of graphene during lithography process. The patterned Cu film can be sublimated afterward to circumvent the use of wet etching which is known to cause an adverse impact on graphene quality. Scanning electron microscopy (SEM) and Raman spectroscopy are used at every single step of the transfer processes in order to systematically study their impact on physical properties of graphene such as doping and structural quality. The developed proximity transfer techniques based on the removal (wet etching or sublimation) of the thin Cu film demonstrated their efficiency to produce high-quality and uniform single-layer graphene on any final rigid substrate of interest.

Field effect transistors with graphene channels were interfaced with arrays of semiconductor quantum dots (QD) in a study of negative differential resistance. The electrical characteristics and opto-electronic behaviour of the elements were assessed. The dots&’ photoluminescence (PL) was assessed as a function of drain-source and gate-source biases.
Graphene - mono layer thick graphite - portrays high conductivity, chemical inertness, mechanical robustness and unusual dispersion relations. Characteristics of free-standing, mono or bi-layer graphene have been studied when deposited over nano-pore array of anodized aluminum oxide (AAO) substrates. Here, the pores were imbedded with core/shell CdSe/ZnS semiconductor quantum dots. One may postulate that this structure will have a profound effect on the resulting current and photoluminescence signals. Such arrangement has led to the realization of the first visible surface plasmon laser.
Negative differential resistance (NDR) is a nonlinear electronic response as a result of a competition between tunneling and thermal processes. Here we explored NDR for graphene channels when interfaced with well separated quantum semiconductor dots; the latter were imbedded in a periodic array of nano-pores. The graphene channels were part of field-effect transistors (FET). The channels were large, on the order of 1 cm². The contacts between the electrodes and the graphene were Ohmic. Silicon substrates were used as back-gate electrodes. The AAO was prepared using 2 step anodization process of 200 nm film of aluminum on Si/SiO₂ substrates. As a result of the anodization process, ca 50 nm thick perforated oxide layer was formed on the Si/SiO₂ substrate, with a 25-30 nm pore-diameter and pitch of 120 nm. Core-shell semiconductor CdSe/ZnS QDs were imbedded in the pores and the semi-transparent graphene was deposited on top of the alumina layer.
For the photoluminescence (PL) measurements, a 10 mW Ar ion laser at 514.5 nm was used in a confocal arrangement. Peak luminescence of the core-shell CdSe/ZnS QDs was measured as a function of Vds and Vgs. To obtain optimal resonance conditions, the sample was tilted and rotated. The effect of drain-source potential Vds on the peak PL intensity was also observed. The peak intensity has increased as a function of Vds and Vgs. The position of the transient has shifted in response to the gate voltage and in direct correlation with the appearance of NDR. Increased photo-sensitivity in carbon nanotube channels has been found to be correlated with NDR in the past.
In summary, by taking advantage of free-standing graphene channels in FET devices and by coupling the channels to quantum semiconductor dots, we have demonstrated novel opto-electronic devices.

We numerically investigated mechanical and electronic properties of graphene sheets in polycrystalline structures. Properties of polycrystalline structures are generally known to be dependent on grain size due to the competition between inter- and intra-granular strength, as known as Hall-Petch effect and reverse Hall-Petch effect. It is similarly suspected that the interfacial strength may be a key factor governing the properties of interlayer and intergranular strength in polycrystalline graphene sheets. Multiscale simulation approach was employed in two different length scales, nanometer scale and micrometer scale, in hierarchical manner. First principle calculations were performed to characterize interconnection morphology between the graphene layers. In addition, we employed molecular dynamics to study grain size effect on the interfacial properties and thermal effect as well. The effective properties of the polycrystalline graphene up to micrometer length scale were shown highly sensitive on the interlayer morphology. It is also shown morphology defects (such as vacancy) or metallic additives can significantly enhance the mechanical and electronic properties of the graphene bilayer.

Graphene is as an attractive filler material for polymers because of its excellent electrical, mechanical and thermal properties. In this paper, we report a massive increase in the electrical conductivity of a graphene nanoplatelet (GNP)/polystyrene composite by the addition of non-conducting silica particles. The non-conducting filler acts as a highly effective dispersion aid, preventing the sheet-like GNP from agglomerating during the solvent casting process used to fabricate the composite. The enhanced dispersion of the GNP leads to orders of magnitude enhancement in electrical conductivity compared to samples without this silica filler.

With thousands of patented applications currently anticipating its industrial scale production, graphene is bound to become one of the major materials of the 21st century in the fields of energy generation, storage, electronics, and semiconductors among many others. Single-layer-large-area (square meters) graphene is desirable for many applications. Current state-of-the-art method of producing such graphene and transferring it to an arbitrary substrate involves chemical vapor deposition (CVD) of graphene on copper foils and subsequent dissolution of the copper substrate in acidic media. This translates into 1:300,000 product to waste ratio and renders this process economically and environmentally not feasible. Current work presents a novel approach to transferring large-area single-layer graphene grown on copper via CVD onto Polyethylene terephthalate (PET) without destroying the copper substrate. Furthermore, the copper substrate is reusable indefinitely and has been shown to improve the quality of generated graphene with each consecutive synthesis/transfer. The copper etch-free transfer is made possible by the process of metal-assisted exfoliation (MAE), where a thin film (50 to 150nm) of physically deposited transition metal (nickel, cobalt, gold) is utilized as an adhesive layer. Supported by thermal-release tape or polydimethylsiloxane (PDMS), these metals demonstrate higher interaction potentials with graphene than exists between graphene and copper and can effectively exfoliate graphene from the copper foils. Graphene of 10x10 cm² area was grown on and transferred multiple times from the same copper foil onto PET rendering the later conductive. Although the lowest transferred graphene sheet resistance recorded was 163 Omega; sq^-1, the average sheet resistance bares anisotropic character and varies from 850 ± 250 Omega; sq^-1, when measured parallel to the cracks, to 8000 ± 2000 Omega; sq^-1 when measured across the cracks. The cracks are due to the tensile strain the metallic film is subjected to during the exfoliation process. Process alterations poised to alleviate this problem (such as rigid substrate exfoliation) have been proposed and will be examined in the future work.

Here, we have physically vapor deposited guanine onto four monolayers of graphene on silicon carbide in the construct of a test platform for a graphene-based field effect transistor for potential applications in electronics and biosensors. Guanine appeared to not decrease graphene charge carrier mobility. Relative humidity measurements of graphene stability on SiC were observed over a period of several days in which graphene charge carrier mobility did not change. This shows that guanine may be a suitable gate dielectric in a graphene-based field effect transistor.

The mechanical properties of a stacked, multilayer graphene sheet are characterized and compared to those of a crystalline, few-layer graphene sheet. The stacked sheet is assembled by manually combining single layer CVD-grown graphene monolayers, resulting in a turbostratic multilayer graphene with significant out-of-plane wrinkles and bubbles. Mechanical properties are characterized through the deflection of micron-scale cantilevers, prepared using focused ion beam milling, with an atomic force microscope. Cantilevers of varying geometric aspect ratios and number of graphene layers from stacked transferred graphene are fabricated and the mechanical properties are contrasted with those derived from the indentation of suspended graphene over a circular well. Atomistic simulations, and well-established analytical solutions are used to extract material and structural property information, and benchmark measured properties relative to complementary results from membrane indentation tests. Particular emphasis is placed on understanding the influence of the out-of-plane corrugations on the mechanical response of the stacked graphene sheet.

Layered hexagonal materials such as graphene and semi-conducing transitional metal dichalcogenides (TMDC) when stacked, either by mechanical stacking or epitaxial growth, form a misoriented superlattice exhibiting Moiré pattern [1, 2, 3]. This superlattice can serve as a prototypical material whose properties depend on the relative twist angle and interactions between the layers. Electrical, thermal and optical properties of the perfectly stacked (AB-stacked) few-layer graphene (FLG) obtained by mechanical exfoliation of graphite have been well established, but structure - property correlations in the misoriented bilayer graphene which deviates from the AB-stacked bilayer graphene (BLG) are not well understood. The misoriented stacking of the layers activates low energy phonon modes in the interior of the Brillouin zone, which are not present in the single-layer graphene and AB-stacked BLG [4]. Furthermore, a comprehensive and systematic study to synthesize misoriented bilayer graphene is still lacking.
We use ambient-pressure chemical vapor deposition (AP-CVD) to grow single-crystal graphene using polycrystalline copper as the catalyst. The relative misorientation angle between the graphene layers was determined using a combination of dark-field transmission electron microscopy (DF-TEM) and selected area electron diffraction (SAED). Furthermore, we characterized the misoriented BLG by employing Raman spectroscopy (measurements done with a 532 nm laser excitation). Finally, we compared the experimentally observed misorientation dependent vibrational properties with the large scale atomistic calculation performed using molecular dynamics simulation.
1. Luican, A., et al. "Single-layer behavior and its breakdown in twisted graphene layers." Physical review letters 106.12 (2011): 126802.
2. Kang, Jun, et al. "Electronic structural Moiré pattern effects on MoS2/MoSe2 2D heterostructures." Nano letters 13.11 (2013): 5485-5490.
3. Moon, Pilkyung, and Mikito Koshino. "Optical properties of the Hofstadter butterfly in the moiré superlattice." Physical Review B 88.24 (2013): 241412.
4. Kim, Kwanpyo, et al. "Raman spectroscopy study of rotated double-layer graphene: misorientation-angle dependence of electronic structure." Physical review letters 108.24 (2012): 246103.

Recently the hexagonal boron nitride (h-BN) has shown great promise as the substrate for graphene growth. The graphene fabricated using h-BN as the substrate is shown to have significantly less roughness and higher mobility compared to those grown on other, e.g., metal or SiO₂, substrates. However, one interesting aspect to note is that the graphene grown on the h-BN substrate might lead to B and N atoms from the substrate. In the present work, we will show that the interplanar defects may play a critical role in the impurity doping of graphene. In our study, stacked graphene/h-BN heterostructures are constructed to mimic the hybrid systems formed during growth, and first-principle calculations combined with transition states theory were performed to examine the evolution of defects in the heterostructure. Our results indicate that two single vacancies in neighboring (graphene or h-BN) sheets, when at close vicinity to each other, tend to coalesce into an interplanar defect. Subsequent defect evolution following the coalescence is shown to induce either B or N doping of graphene. Among all the possible crossplanar defects and the reaction path we identified, it has been further discoverred that this procedure can be controlled by manipulating the charge states, which provide a way for selecting B or N doping of graphene. Our findings provide mechanistic insights towards defect engineering and precise control of impurity doping when fabricating graphene on the h-BN substrate.

After the discovery of graphene and its remarkable properties in 2004,^[1] the covalent functionalization of graphene has been developed steadily for a better understanding of the chemistry of this exciting class of material as well as for the generation of new materials. Therefore, the graphene lattice has been chemically addressed by different synthetic pathways.^[2,3] All these approaches combine the functionalization of graphene, starting from pristine graphite, with an exfoliation and activation step. This effective exfoliation and activation step can be accomplished by an intercalation of the graphite with alkali metals, followed by the dispersion of the intermediately formed intercalate in a suitable solvent.^[4]
We have developed a novel reductive functionalization sequence, employing potassium as intercalating species. Subsequent to the exfoliation of reductively charged graphite, the respective graphenenide intermediates are covalently functionalized insitu by the addition of iodonium salts as electrophiles. We could demonstrate that this new functionalization reagent fits to the electrochemical potential of activated graphene and thus leads to higher degree of functionalization accompanied also by fewer amounts of side reactions in comparison to aryl iodine compounds.^[5]
In order to prove the successful introduction of sp³-anchoring atoms in the lattice of the functionalized material we used statistical Raman spectroscopy. Simultaneously, the product was characterized in detail by thermo gravimetric analysis coupled to a GC/MS system. This combination of analytical techniques enabled us to unambiguously qualify and quantify the functional groups attached to the carbon framework.
1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science, 2004, 306, 666.
2. J. M. Englert, C. Dotzer, G. Yang, M. Schmid, C. Papp, J. M. Gottfried, H. P. Steinrück, E. Spieker, F. Hauke and A. Hirsch, Nat.Chem., 2011, 3, 279.
3. R. A. Schäfer, J. M. Englert, P. Wehrfritz, W. Bauer, F. Hauke, T. Seyller and A. Hirsch, Angew. Chem. Int. Ed. 2013, 52, 754.
4. K.C. Knirsch, J. M. Englert, C. Dotzer, F. Hauke, A. Hirsch, Chem. Commun., 2013, 49, 10811.
5. F. Hof, R. A. Schäfer, C. Weiss, F. Hauke and A. Hirsch, Angew. Chem. Int. Ed. 2014submitted.

Development of new materials as electrodes for supercapacitors to achieve higher charge capacity has been a field of extensive research over the last few years. Graphene and graphene oxide (GO) with subtle structural modifications have been investigated as a potential material for this purpose due to their light weight, large surface area and high ionic conductivity. There are different methods reported in literature for synthesizing graphene/GO based materials yielding outstanding functional properties.
In this work, we demonstrate the synthesis and applications of highly porous, ultra-light, layered, 3D GO foams with dispersed nano zirconia particles (30 nm) using freeze drying starting from 2D GO building blocks. The GO based foams exhibit extremely low densities ~ 45-50 mg/cc and high BET surface areas ~ 85-120 m²/g. Interestingly, the morphology of the foam structure changes with zirconia content. SEM micrographs show layered flaky microstructures in pure GO with flake lengths le; 500 µm. The layered structures become prominent and the flake lengths increase to 8-10 mm with increase in zirconia content (upto 1:2 wt. ratio). Efforts are on to envisage whether addition of nanoparticles exfoliate the GO structures. Further increase in zirconia (3:4 wt %) leads to the flakes getting interconnected via zirconia threads; several zirconia T- and Y-junctions are also observed in the microstructure.
Efforts are on to evaluate the performance of these materials as potential electrodes for electrical double layer supercapacitors using different electrolytes for room temperature and high temperature applications. The foams with their unique morphologies are also being investigated as water purifying agents contaminated with heavy metal ions. Thus these materials open up avenues for novel applications using GO based composites.

There has been a surge of interest in holey graphene, which are graphene sheets with holes ranging from a few to tens of nm in average diameter. In addition to the intrinsic exotic properties of graphene, the nanoholes and their associated atomic sharp edges, holey graphene may open a new arena in the field of biotechnology; for ion and molecular separation, sensors, catalysts, battery, fuel cells, desalination and environmental decontaminations. However, most of the solution-phases holey graphene sheets rely on lengthy chemical oxidation via Hummers&’ method or other modified Hummers&’ methods to generate graphene oxide and then followed by KOH activation or extended HNO₃ oxidation. Based on the molecular oxidation mechanism of various oxidizing agents and microwave heating, for the first time, we will report a rapid, scalable and one step controllable synthesis of porous/nonporous graphene oxide directly from graphite particles with high product yield. As per our knowledge, this is the first chemical based microwave assisted synthesis method to controllably synthesize porous graphene oxide directly from graphite.
Chemical doping of heteroatom (N, B, S, etc.) in the graphene plane is important approach to tailor its electronic properties and thus have great impact on its application like supercapacitors, sensors, field effect transistors, etc. However, all the doping requires extreme experimental parameters like high temperature and/or pressure, which makes it time consuming, less cost effective and low energy efficient for industrial scale synthesis. Here in, we developed the innovative, simple, cost and time effective, scalable, solution based microwave assisted approach to synthesis N-doped porous reduced graphene from porous graphene oxide in one step. The N-doped holey graphene sheets demonstrated remarkable electrocatalytic capabilities to electrochemical reduction of oxygen.

High quality and wafer-scale graphene on insulating gate dielectrics is prerequisite for graphene electronic applications. For such applications, graphene is typically synthesized, and then transferred to a desirable substrate for subsequent device processing. Direct production of graphene on substrates without transfer is highly desirable for simplified device processing. However, graphene synthesis directly on substrates suitable for device applications, though highly demanded, remains unattainable and challenging. Here, we report a simple, transfer-free method capable of synthesizing graphene directly on dielectric substrates as low as 600 °C using polycyclic aromatic hydrocarbons (PAHs) as carbon source. Significantly, N-doping and patterning of graphene can be readily and concurrently achieved by this growth method. Remarkably, the graphene films directly grown on glass attained a small sheet resistance of 550 Omega;/square and a high transmittance of 91.2 %. Organic light-emitting diodes (OLED) fabricated on N-doped graphene on glass achieved a current density of 4.0 mA/cm² at 8 V compared to 2.6 mA/cm² for OLED similarly fabricated on Indium Tin Oxide (ITO)-coated glass, demonstrating that the graphene thus prepared may serve as a transparent electrode to replace ITO.

Since its discovery in 2004, graphene became a very promising candidate for future applications in nano electromechanical systems. Up to now, suspended graphene samples reach the highest reported mobilities. However, its electrical properties are very sensitive to mechanical disturbances, as graphene is only one atomic layer thick. This coupling theoretically gives rise to a so-called “pseudo-vector field”, and consequently to a “pseudo-magnetic field”.
The pseudo-magnetic field can, just as a real magnetic field, induce Landau levels in graphene devices, if the field is strong enough. In order to measure this effect, one needs (i) a high sample quality and (ii) a large homogeneity of the pseudo-magnetic field. A controlled way of inducing a pseudo-magnetic field in graphene devices is to strain suspended graphene, with for instance a comb drive actuator. However, large forces are needed to significantly stretch the graphene and induce a measurable pseudo-magnetic field. Even if these high forces can be reached, the question still remains how homogeneous and how large this pseudo-magnetic field will be.
Here we present numerical simulations of the pseudo-magnetic field in graphene with a hexagonal shape as a function of tri-axial strain. We find that for a hexagon with sides of 100 nm and a strain of 10%, the nearly homogenous pseudo-magnetic field goes up to 40 T. For a hexagonal sample size with sides of 1 micron, and the same amount of strain, the maximum pseudo-magnetic field is reduced to 4 T. In addition, we find that the relative orientation between the graphene lattice and the strain direction greatly influences the strength of the pseudo-magnetic field. By rotating the relative orientation the pseudo-magnetic field can even be tuned down to 0 T in the center region. This implies that one should take care of the lattice orientation with respect to the straining axis. Finally, we show that for a hexagon with sides of 100 nm and a strain of 10%, the pseudo-magnetic field is uniform over a length scale of 10 nm.

The electrochemical properties of nanocarbons used as supercapacitor electrodes are largely determined by the pore size and its distribution. Here, we report that high-mesopore-volume nanocarbons (HMC) derived by a decorated activation method considerably improve the capacitive behavior by providing the desired highly accessible surfaces. Two-electrode supercapacitors cells constructed with this HMC yielded high gravimetric capacitance of #65374;180 and #65374;200 F g^-1 with organic and ionic liquid electrolytes. Mesopore volume ratio above 70% of HMC facilitates fast ionic transport while preserving decent electronic conductivity and thus delivers both high energy and power density. This simple, cost-effective and scalable preparation technique and excellent capacitive behavior of HMC encourage its commercial use.

In organic chemistry, the so-called silylation is a versatile tool, to introduce silyl functionalities in organic compounds. Organosilanes serve as selective protecting-group reagents, derivatization reagents, reducing agents and reagents in cross-coupling chemistry inter alia.^[1] Recently, these functional groups were established by the covalent functionalization of carbon nanotubes due to their tuning of the electronic properties of SWCNTs.^[2]
The main focus of this work is based on a reductive silylation of graphene. Our reaction sequence for the silylation consists of a three-step conversion from graphite to silyl-functionalized nanomaterial via an activated graphite intercalation compound (GIC).^[3] After the initial solid state reduction via potassium and exfoliation in an inert solvent using ultrasonication, silylating agents are added to the dispersion in order to achieve the functionalization of the graphene framework.
The detailed analysis by optical microscopy, atomic force microscopy (AFM), Raman spectroscopy and thermal gravimetric analysis coupled with mass spectrometry (TGA/MS) reveals the pronounced impact of the starting material and the functionalization reagent with respect to the graphene yield and the degree of functionalization amongst others.
[1] A. E. Pierce, Silylation of Organic Compounds (Pierce Chemical Company, Rockford, Illinois, 1968).
[2] Y. Maeda, K. Saito, N. Akamatsu, Y. Chiba, S. Ohno, Y. Okui, M. Yamada, T. Hasegawa, M. Kako,
T. Akasaka, J. Am. Chem. Soc., 2012, 134, 18101-18108.
[3] K. C. Knirsch, J. M. Englert, C. Dotzer, F. Hauke, A. Hirsch, Chem. Commun., 2013, 49, 10811-10813.

It has been widely accepted that SERS enhancement results from a combination of electromagnetic mechanism (EM) and chemical mechanism (CM). Recently, the nanoparticle-film gap (NFG) system where metal NPs (supported localized surface plasmons LSPs) are separated from a bulk metal film (supported surface plasmon polaritons SPPs) by a spacer have been studied due to its strong local enhancement field. However, there are still some technical limitations in establishing effective and simple ways for reliable and precise control of sub-nanospacer with highly structural integrity for further development of NFG system. In addition, works on designing the nanospacer in NFG system for efficient interaction with target molecules for further improving SERS signals are rather limited. Here, we propose a novel NFG system by introducing ultrathin monolayer graphene as well-defined sub-nanospacer between Ag NPs and Ag film (named G(graphene)-NFG system). The new G-NFG system offers tremendous near-field enhancement with one of the highest enhancement ratio of 1700 reported to date in the graphene-metal plasmonic combination system. Our experiment and simulation reveal that this large enhancement is due to multiple couplings including the NP-NP couplings and NP-film couplings. Particularly, the additional CM enhancement from graphene nanospacer is also fully utilized in the as-proposed G-NFG system to further enhance the detection sensitivity, which finally achieves over 100-fold stronger signal in detecting (rhodamine 6G) R6G molecules compared with that of single metal NP-NP coupling system. Our results show that the single-layer graphene as a sub-nanospacer renders the proposed G-NFG system with particularly strong EM enhancement and additional CM enhancement in detecting some π-conjugated molecules to function as a powerful tool in analytical science and the related fields [1].
[1] X.H. Li, W.C. H. Choy, X. Ren, D. Zhang, and H.F. Lu, "Highly Intensified Surface Enhanced Raman Scattering by Using Monolayer Graphene as the Nanospacer of Metal Film- Metal Nanoparticle Coupling System", Adv. Funct. Mat. DOI: 10.1002/adfm.201303384.

Although large-area graphene films can be grown by thermal chemical vapor deposition (TCVD), the charge transport properties of TCVD graphene are dramatically impaired on the centimeter scale and above. As an alternative solution to this problem, we have achieved high-quality large-area bi-layer graphene by hot filament chemical vapor deposition (HFCVD). The graphene films obtained are uniform and with low defect density. The high quality of graphene films was confirmed by Raman spectroscopy mapping based on the ratio of 2D to G peak intensities over 200 mu;m × 200 mu;m areas, and by their optical transparency > 90%. The suitability of this bi-layer graphene for touch screens applications is confirmed by its relatively low sheet resistance. The optimized process parameters, i.e., growth time, annealing profile, and flow rates of various gases are discussed. Moreover, a transfer method using the electrostatic property of polyethylene terephthalate (PET) will be described that preserves the integrity of the as grown graphene films. The long term resilience that graphene can provide is a critical advantage in applications that require transparent conductive films, once the transparency and conductivity requirements similar to indium tin oxide (ITO) are met. In this context, the HFCVD process that we developed is suitable for industrial scaling of large TCFs that can replace ITO.

Pattern transferring from a self-assembled block copolymer (BCP) thin film to an underlying substrate, commonly referred to as BCP lithography, is a useful nanopatterning method to create nanostructures for a range of applications from bit-patterned media, memory devices, quantum dot arrays, to micro- and nano-electronics. Here, a pre-assembled block copolymer (BCP) thin film was floated, transferred, and utilized to effectively nanopattern Cu foil and graphene/Cu foil since they cannot be nanopatterned via conventional processes, i.e. spin-coating, due to the high surface roughness and susceptibility to harsh processing chemicals and etchants. Perpendicular hexagonal PMMA cylinder arrays in diblock copolymer P(S-b-MMA) thin films were pre-assembled on sacrificial SiO₂/Si substrates. The BCP thin film was floated in an aqueous solution of HF and collected with the target substrate, leading to well-defined nanoporous PS templates. The periodicity and the ordering of the domains were largely preserved after the floating transfer from the SiO₂/Si substrate to Cu foil or graphene/Cu foil.
We further show that the nanoporous template can be used for a subtractive process to fabricate nanoperforated (NP) graphene on Cu foil in sub-20 nm dimension. Verification of the NP graphene was conducted by Raman spectroscopy. Varying the oxygen plasma RIE etch times, the I_D/I_G values increased with increasing O₂ plasma RIE exposure and power. Comparing these I_D/I_G values to the literature values (correlating I_D/I_G ratio to constriction width), we estimated a w of 15 ~ 20 nm. We further characterized the electrical properties of NP graphene in a FET device geometry at room temperature. Upon nanopatterning with 20 W O₂ plasma for 3 sec, the NP graphene device had a ON/OFF conductance ratio to ~ 7 and mobility of ~ 40 cm²/Vmiddot;s. Further etching resulted in only a slight increase in the ON/OFF conductance ratio to ~ 9. However, the mobility decreased significantly to 8 cm²/Vmiddot;s and 2 cm²/Vmiddot;s for 35 sec and 45 sec etching times, respectively. In addition to the decreased mobility, the NP graphene devices demonstrated increased hole doping behavior as the Dirac point shifted from ~ 20 V to > 40 V.
The floated film was also transferred onto a Cu substrate to directly grow NP graphene from the catalytic substrate for an additive process. Once the BCP film was transferred to the Cu foil, the minority PMMA domains were degraded and ALO was deposited in the pores. To remove the PS template, and preserve the catalytic activity of the Cu, an effective stripping agent (AZ-300T) was utilized bypassing the use of strong acids or additional plasma etching. The process developed here has the potential to increase the utility of BCP lithography for atypical substrates while maintaining substrate integrity, specifically in the field of catalysis, and graphene electronics.

Carbon dots (C-dots) recently have attracted much attention owing to their fascinating physical properties and their wide range of potential applications, including bioimaging, optoelectronics, and catalysis [1]. In contrast to heavy-metal-based quantum dots (Q-dots), C-dots offer the advantages of absence of toxic heavy-metal elements, high chemical stability, high water dispersibility, and absence of blinking. In contrast to molecular fluorescent dyes, they show broader excitation spectra and low photobleaching. They consist of graphitic carbon with quasi-spherical structure [1].
This presentation addresses the synthesis of carbon dots (C-dots) via most simple heating of polyols (glycerol, diethylene glycol, PEG) to 180-230 °C [2]. Dehydration and carbonization can be supported by addition of Lewis-acidic metal salts (MgCl₂, ZnCl₂). The as-prepared C-dots are collected in quantities of 20 mg per 10 ml of polyol and are suitable for direct dispersion in water due to the inherently available polyol-type surface coating [3]. Aqueous suspensions show excellent colloidal stability and particle diameters of 3-5 nm at narrow size distribution and high quantum yields (up to 50 %) [3].
Upon modification with TbCl₃/EuCl₃, the C-dots show efficient energy transfer upon excitation (UV or blue-LED) to Tb³⁺/Eu³⁺ with characteristic line-type green/red emission and high quantum yields of 85 % (Tb³⁺) and 75 % (Eu³⁺) [3]. Thus, polyol synthesis as a most simple access to multicolored C-dots can be highly relevant in view of intense research on C-dots for molecular imaging and optoelectronics.
References
[1] Recent review: a) J. Shen, Y. Zhu, X. Yang and C. Li, Chem. Commun. 2012, 48, 3686. b) S. N. Baker and G. A. Baker, Angew. Chem. Int. Ed. 2010, 49, 6726. (Review).
[2] a) C. Feldmann, H. O. Jungk, Angew. Chem. Int. Ed.2001, 40, 359. b) C. Feldmann, Adv. Funct. Mater. 2003, 13, 101. c) P. Schmitt, N. Brem, S. Schunk, C. Feldmann, Adv. Funct. Mater.2011, 21, 3037.
[3] a) H. Dong, A. Kuzmanoski, D. M. Göszlig;l, R. Popescu, D. Gerthsen, C. Feldmann, Chem. Commun.2014, DOI: 10.1039/C4CC01715C. b) H. Dong, M. Roming, C. Feldmann, 2014, submitted.

We have synthesized high-quality large-area bilayer graphene by hot filament chemical vapor deposition (HFCVD). The graphene films obtained are uniform and with low defect density. The high quality of the graphene films was confirmed by Raman spectroscopy mapping. We also performed temperature-dependent Raman spectroscopy to analyze the phonon-phonon interactions in this bilayered material. We systematically studied the physical properties of the HFCVD bilayer graphene films, including: optical transmittance, sheet resistance, heat conductivity, bandgap, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. These properties will be discussed in terms of the degree of alignment or misalignment between the two graphene layers.

Origami pattern, for example, Miura-ori pattern, is a periodic array of artificially and geometrically folded mountains and valleys. With the predefined creases of origami, one can fold flat sheets to create three-dimensional deformable structures. The origami-based patterns have inspired some fascinating applications, such as deployable solar panels, self-folding membranes, origami-inspired stents, Miura-ori folded metamaterials, and origami lithium-ion batteries. Patterns generated by conventional origami methods usually require predefined creases. In that way, the produced origami patterns are in macroscale. The requirement of predefined creases and macroscale size patterns greatly limited the flexibility, stretchability and controllability of this otherwise promising technology. Here, we present a novel approach that overcomes these limits, to control reversible crumpling and unfolding of large-area graphene or graphene papers by harnessing the mechanical instabilities using soft materials. We transfer a large-area graphene or graphene paper on an elastomer substrate that is either uniaxially or biaxially stretched to 3~5 times of its original dimension. Self-organized origami patterns, such as wrinkles, delaminated buckles, or localized ridges, develop in graphene or graphene papers when the substrate is simply relaxed uniaxially or biaxially. The origami patterns of graphene or graphene papers can be unfolded by stretching the substrate back. Graphene nanocomposites or surfaces of the graphene or graphene papers exhibit an unprecedented combination of merits including high stretchability (e.g., linear strain ~400%, areal strain ~1500%), reliability (e.g., over 1000 stretch/relax cycles), transparent and conductive electrodes, on-demand superhydrophobicity, supercapacitors (e.g., specific capacitance ~196 F g^-1) and artificial muscle actuators (~100% areal strain). Our work not only reveals new modes of instabilities in graphene and graphene papers for studying fundamental properties of highly deformed and patterned graphene / grpahene papers in large-area, but also creates electrodes and coatings with unprecedented combined properties and tunability and provides a powerful tool for studying graphene-based nanocomposites and biomedical devices with changeable “on demand” functions.

Since its discovery, graphene has attracted huge attention because of its distinct electronic and optical properties which include thermal conductivity, high carrier mobility, optical transparency, robustness and wettability [1].Owing partly to its high surface-to-volume ratio, graphene has become a choice material well suited for sensor applications. In the last few years, graphene has emerged as a promising candidate for detection of gases with very low concentrations (ppb range) [2]. Ultrasensitive sensor response can be obtained due to the microstructure of graphene as every atom, being a surface atom, is capable of associating with a single molecule of the target vapour or gas.
In this study, we investigate the sensitivity and selectivity of graphene to gases like CO₂, CO, NH₃, NO₂ and inert gases for use as hermetic seal detection in MEMS based gyroscopes. This work is built upon ongoing C-based nano-material research in Masdar Institute where we explore hermetic seal monitors for MEMS based gyroscopes [3]. Graphene is being synthetized by 3 different techniques. CVD of graphene on metal substrate (Cu/Ni) and subsequent exfoliation onto Si substrate. Inkjet printing using the Fujifilm Dimatrix DMP 2831 and dispersion with a sonificator will also be used. The choice of substrate is an essential parameter for tuning properties of graphene based gas sensors [4]. Si and flexible substrates are being considered in this study. Due to lack of dangling bonds on its surface to enhance the chemisorptions of target molecules, the functionalization of graphene is used to tune the properties of the said films for target ion and gas detection. The functionalization of graphene with modifiers is key to enable interaction with specific molecules and improve selectivity. Ongoing work has already successfully oxidized graphene to graphene oxide that changed the properties of the graphene films. In this research, the optimized graphene film synthesized using the techniques at our disposal are integrated into a vacuum detector and will be reported in the meeting. The sensors fabricated involves etching deep Si channels using Bosch etch, then transferring the films to the said cavities, establishing microelectrical contacts to these films and then assessing the vacuum integrity through change in the electrical, mobility and electronic properties of the films.
References
Rumyantsev, S., et al. (2012). "Selective Gas Sensing with a Single Pristine Graphene Transistor." Nano Letters12(5): 2294-2298.
Yoon, H. J., et al. (2011). "Carbon dioxide gas sensor using a graphene sheet." Sensors and Actuators B: Chemical157(1): 310-313.
A. AlShehhi, et al. (2013). “Effect of graphene growth conditions on bulk electron mobility and conductivity of graphene on SiO₂ substrates.” 2013 MRS Fall Meeting and Exhibit, RR15.91.
R. Pearce, et al. (2011). “Epitaxially grown graphene based gas sensors for ultra-sensitive detection.” Sensors and Actuators B155 451-455.

Graphene oxide (GO) has attracted much attention as a material for exploring a variety of physical properties including electric transport, wide energy range of photoluminescence, and room-temperature ferromagnetism. The sp² and sp³ domains coexist in GO, and the sp²/sp³ domain fraction is crucially important for controlling the physical properties of GO. It should be possible to tune the various properties of GO in situ by adjusting the sp²/sp³ fraction, which is usually controlled chemically by thermal annealing, or by plasma treatment. Here, we report a method for in situ tuning of the sp²/sp³ fraction based on electrochemical reduction and oxidation (redox) reaction caused by ion migration in a solid-state electrolyte.[1] It uses an all-solid-state electric double layer (EDL) transistor with a multilayer GO and yttria stabilized zirconia (YSZ) proton-conductor. This method provides two-fold functions: in situ tuning of the sp²/sp³ fraction by application of dc bias and electrostatic carrier doping (ECD) to GO with extremely low gate voltage based on large capacitance of EDL. In addition to its effect on the transport property in an EDL transistor (EDLT), the reversible variation of the GO sp²/sp³ fraction was observed by ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy as a variation in the optical band gap. Furthermore, the operation mechanism was examined based on micro-X-ray photoemission and Raman spectroscopy to clarify the sp²/sp³ tuning behavior in the solid state ionics device.
[1] T. Tsuchiya, K. Terabe, M. Aono, Adv. Mater. 26, 1087-1091 (2014)

Graphene, a two-dimensional layer of carbon atoms in sp² bonding, has proven to be a versatile material with outstanding potential in applications such as transparent electrodes, strain and gas sensors, and high frequency transistors. Graphene has been successfully grown on a large area with high quality using high purity metal foil, preferable copper, as a catalyst. Still, the transfer procedure has become one of the major limitations of the overall operation. Direct development of graphene on insulating substrate remains on early stages and electrical properties are not sufficient to enable application in electronics where high quality graphene is required. Despite that, recent research shows great interest to develop new ways to synthesize graphene on insulators; the amount of defects is still a major issue to be accosted. In this work, we present a method to simultaneously design and synthesize graphene on insulating materials from the transformation of amorphous carbon into graphene by laser irradiation using nickel (Ni) as a catalytic interface layer. A Ni thin film, followed by a layer of amorphous carbon are deposited on the surface of the insulating target substrate. By laser irradiation on selected areas, the nickel layer is heated and carbon atoms are diffused and later crystallized upon cooling on the top of the insulating substrate. Raman spectroscopy confirms the presence of few layer graphene on the interface between Ni thin film and the insulating substrate. Additionally, the low intensity ratio between D and G peaks of the Raman spectra are the evidence of the quality of graphene. Furthermore, by using pre-pattened Ni, sharp edges and well defined patterns can be achieved. Laser assisted synthesis of graphene develops a new approach that ensures no polymer residues on the surface of graphene for demanding applications in photonics and electronics.

Gas-phase synthesis of nanoparticles provides materials with high purity at high production rates. For electrochemical applications, the formation of composites of the inorganic nanoparticles with a highly conductive matrix such as graphene is advantageous to improve the conductivity and to reduce the effect of mechanical stress during charging and discharging. A general, new and facile approach has been developed to fabricate nanoparticle/graphene nanocomposites. Gas-phase-synthesized nanoparticles and graphene oxide were used as raw materials which underwent a self-assembling process in the presence of selected reducing agents (ascorbic acid or ethylene diamine) under low-temperature hydrothermal conditions. As examples, Si nanoparticles and Fe₂O₃ nanoparticles synthesized in a hot-wall reactor and a flame reactor, respectively were processed towards Si/graphene and Fe₂O₃/graphene nanocomposites. Scanning (SEM) and transmission electron microscopy (TEM) showed that this method enables the formation of homogeneous nanocomposites. Characterization by XRD, FTIR, XPS, and other techniques revealed that the graphene oxide was well reduced. The graphene layers were well exfoliated and homogenously covered by nanoparticles (Si or Fe₂O₃) with high weight loadings of up to 69%. Electrochemical characterizations show that these nanocomposites are promising electrode materials for lithium-ion batteries and supercapacitors.

The oxidation of graphite to form graphite oxide (GO) is a very old process, and is currently utilized as a method to provide water suspendable graphitic material. There are numerous approaches to form GO, with reagents, temperatures, reaction times, and work up procedures being varied in different procedures. This variation results in the structure and chemistry of GO varying widely from group to group, and even vary within the same research group. This means that results can often not be repeated as different batches of GO have different properties.
To address this issue, we present a method of fractionation that separates GO based on the degree of oxidation. Using a mixed solvent system, GO sheets are segregated without the use of centrifugation or the addition of additives such as surfactants or salts. In a sample vial, highly oxidized sheets are found at the top of vial, while less oxidized sheets are found near the bottom. X-ray diffraction (XRD) analysis of the fractions shows a clear increase in the relative intensity of the pristine graphite peak as one samples from top to bottom. It is thus possible to obtain GO with varying degrees of oxidation, as well characterize different batches by creating a profile of the mass fraction of the GO as function of oxidation state. In addition to XRD, Raman spectroscopy and elemental analysis results are presented, and the properties of composites produced with different oxidation fractions are compared.

Compared to Si, III-V, and other semiconductor materials, graphene provides superb carrier mobility (~100,000 cm²/Vs), current density (~10⁹ A/cm²), thermal conductivity (~25 W/cmK), optical transmittance (~97.7% for monolayer), mechanical strength (Young&’s modulus ~ 0.5-1 TPa), and flexibility (~18%). Intrinsic graphene has a zero bandgap, and field effect transistors (FETs) made of intrinsic graphene have significant limitations in achieving minimal conduction at the charge neutrality point, resulting in poor pinch-off current (~100 mu;A/mu;m) and small ratios of I_on/I_off (<10). The bandgap of graphene, however, can be engineered using chemical doping or nanoribbon structures. Heterostructures of graphene and chemically doped graphene can enable normally-off enhanced-mode graphene FETs, which are crucial for high-performance RF switch applications.
Graphene heterostructures have been fabricated both vertically and laterally using wide bandgap barrier materials such as h-BN or GrF, resulting in graphene heterostructure FETs (HFETs) with a small a pinch-off current (~10 pA/mu;m). In this paper, we present our recent progress in lateral graphene HFETs consisting of graphene (Gr)/graphene fluoride (GrF)/graphene (Gr). The graphene lateral HFET operates with gate voltage modulation of the GrF barrier height to switch the HFET on and off. Our graphene lateral HFET uniquely takes full advantage of graphene&’s very high in-plane Fermi velocity (V_F~10⁶ m/s) and high source-injection velocity (2V_F/π) in the ballistic regime, which is greatly beneficial for achieving high cut-off frequency (f_T) for high-frequency (RF) operation.
With a 250-nm-thick GrF (lateral) barrier, our graphene lateral HFETs show normally-off enhancement-mode transistor operation with I_on/I_off ratio of ~100,000 at room temperature. The graphene lateral HFETs show excellent current-voltage saturation with an on-state current I_on of 6 mu;A/mu;m at V_ds = 1 V. Based on the channel resistance of the graphene lateral heterostructure measured as a function of temperature, we estimate the Gr/GrF barrier height to be <0.6 eV. The I_on can increase as the barrier thickness is reduced and the mobility of the GrF channel increases. We discuss the on-going scaling of GHFETs below 100 nm.

A new type of graphene device, switching Fowler-Nordhein tunneling current through vertically-stacked graphene/hBN/metal working at high voltage, will be presented in this talk. Graphene is a 2-dimensional plane of carbon with one-atom thick of graphite. Because of its low density of states near the Dirac point, its Fermi level can be modulated with the accumulated charge. For example, when 50 V of gate voltage is applied to monolayer graphene on 300nm SiO₂substrate, the Fermi level increases by around 0.2 eV [1]. Hexagonal boron nitride, the tunneling barrier, is composed of boron and nitrogen, and has the same structure with graphene. However, it is an insulator and breakdown voltage and dielectric constant are similar to those of SiO₂: ~ 0.8 V/nm and 4 respectively [3].
The current is modulated by tuning the barrier height between h-BN and Graphene similar to the barristor [3]. Interestingly, two types of tunneling have been switched between direct tunneling and Fowler-Nordheim tunneling depending on gate voltage. While the former depends on the density of states, the latter depends on the barrier height [2]. Therefore, the Fowler-Nordheim Tunneling current can be switched by the barrier-height modulation. In this work, metal/h-BN/graphene/h-BN/metal vertical structure was fabricated using monolayer graphene, 83.8-nm-thick hBN as a tunneling insulator and 21.5-nm-thick hBN as a gate insulator. When the gate voltage is chaged from 2V to 8V, the barrier height is modulated from 2.0 eV to 1.7 eV. While direct tunneling is changed to Fowler-Nordhiem tunneling, Ion/Ioff ratio was achieved to 10³ with 50 V of drain voltage.
References
1. Y. J. Yu, et al., Nano letters 9, 3430 (2009)
2. G.-H. Lee, et al., Appl. Phys. Lett. 99, 243114 (2011)
3. H. Yang, et al., Science 336, 1140 (2012)

Graphene quantum dots show Coulomb blockade, excited states and their orbital and spin properties have been investigated in high magnetic fields. Most quantum dots fabricated to date are fabricated with electron beam lithography and dry etching which generally leads to uncontrolled and probably rough edges. We demonstrate that devices with reduced bulk disorder fabricated on BN substrates display similar localized states as those fabricated on the more standard SiO2 substrates. For a highly symmetric quantum dot with short tunnel barriers the experimentally detected transport features can be explained by 3 localized states, 1 in the dot and 2 in the constrictions. A way to overcome edge roughness and the localized states related to this are bilayer devices where a band gap can be induced by suitable top and back gate voltages. By placing bilayer graphene between two BN layers high electronic quality can be achieved as documented by the observation of broken symmetry states in the quantum Hall regime. In addition we observe a Lifshitz transition indicating a tunable topology of the Fermi circle. This can be exploited to achieve smoother and better tunable graphene quantum devices. Work done in collaboration with D. Bischoff, P. Simonet, A. Varlet, Y. Tian, and T. Ihn.

In modern wireless communication system, signal amplification is critical for overcoming losses during multiple data transformations/processes and long-distance transmission. By utilizing the unique ambipolarity of graphene in novel device structures, the main limitations in traditional electronics can be successfully surpassed. Here we report a new type of double-gated graphene ambipolar device with the capability of operating under both common and differential modes to realize signal amplification. By exploiting the ambipolar transport property in single layer graphene, we further demonstrate the continuous controllability of the signal output in terms of amplitude and phase under suitable gate biases. In addition, our device has been shown to achieve a common mode rejection ratio (CMRR) of over 80dB without any external supporting circuitry, which is on par with most off-the-shelf commercial amplification systems and sufficient for the generic circuit applications. Combined with other circuit elements already demonstrated in literatures, such as mixers and modulators, the ultra-compact all-graphene wireless communication system for next generation can be successfully realized in the near future.

Flexible thin-film transistors (TFTs) are of central importance for diverse macroelectronic applications. The current TFTs using organic or inorganic thin film semiconductors are usually limited by either poor electrical performance or insufficient mechanical flexibility. Here we report a new design of highly flexible vertical TFTs (VTFTs) with superior electrical performance and mechanical robustness. By using the graphene as a work-function tunable contact for amorphous indium gallium zinc oxide (IGZO) thin film, the vertical current flow across the graphene-IGZO junction can be effectively modulated by an external gate potential to enable VTFTs with a highest on-off ratio exceeding 10⁵. The unique vertical transistor architecture can readily enable ultrashort channel devices with very high delivering current and exceptional mechanical flexibility. With large area graphene and IGZO thin film available, our strategy is intrinsically scalable for large scale integration of VTFT arrays and logic circuits, opening up a new pathway to highly flexible macroelectronics.

This talk will focus on the importance of interfacial reactions on the synthesis and applications of graphene nanocompiste materials. Graphene and functionalized graphene sheets are well defined two-dimensional materials. In addition to many useful properties that have been widely explored for different applications, graphene provides an ideal platform to study the fundamental properties and reactions occurring in such materials. The key questions include the location and the nature of the reaction sites and the fundamental pathways of the reactions. The reactions occurring at the interfaces determine how other materials are deposited and attached to the graphene surfaces, and the final overall architectures of the composite materials. In addition, many synthetic approaches have been investigated to manipulate the 2D and 3D architectures in graphene composites. The reactions and transport are affected by the architectures. Graphene and graphene nanocomposites are widely explored for energy conversion and energy storage, but each application has different requirements and the specific advantages and limitations need to be carefully investigated. The properties required for these applications are also determined by the interfacial reactions and the overall architectures of the materials.

Viscoelastic solids are valuable for passive vibration damping. Unprecedented solid-state viscoelasticity has been discovered in exfoliated graphite. The highly viscous behavior is due to the friction at the interface between the carbon layers in the graphite. The interfacial mechanism of mechanical energy dissipation is in contrast to the bulk viscous deformation mechanism, which is the case for polymers such as rubber. The loss tangent of the graphite reaches 35, compared to 0.7 for rubber. The incorporation of this graphite in a cement matrix results in microscale constrained-layer damping, with the loss modulus reaching the exceptionally high value of 7.5 GPa. In order for exfoliation to occur, the graphite layers that make up the wall of an intercalate island must be able to stretch greatly. The stretching of the wall enables an intercalate island to expand like a balloon. A wall consists of multiple layers of graphite, such that each layer does not necessarily extend all the way across the length of an island. There are about 60 graphite layers (on the average) in the cell wall of the exfoliated graphite used. The stretching of a wall is made possible by the sliding of the graphite layers with respect to one another within the wall. This sliding requires the overcoming of the van der Waals&’ forces between the graphite layers. The vapor-related driving force for exfoliation is adequate for overcoming these forces. For an irreversibly exfoliated graphite, the tremendous sliding of the graphite layers has already occurred during the completed exfoliation, so no further tremendous sliding occurs upon subsequent vibration. Nevertheless, the exfoliation process has irreversibly loosened the binding of the graphite layers and, as a consequence, a degree of sliding between the layers can easily occur upon subsequent vibration. This looseness is consistent with a very low modulus (~110 kPa) in the direction perpendicular to the wall, as shown by nanoindentation testing. The smoothness of the load vs. displacement curve during nanoindentation indicates that the indentation mechanism involves the stretching of the cell walls rather than the breakthrough of the walls. The deformation is mostly reversible upon unloading. With the displacement attributed to the sliding between the graphite layers in a cell wall (width ~20 nm), the maximum shear strain in the cell wall is ~39. This indicates elastomeric deformation, which has been previously reported in polymers only. The reversibility of the sliding is probably made possible by the ease of sliding of the graphite layers and the cellular structure, in which the extremities of a cell serve as pinning points that effectively link the graphite layers. Such links are akin to the crosslinks in rubber. Without exfoliation, the sliding is relatively difficult, due to the strong binding between the layers.

The term graphene covers a family of different materials and depending on the type of graphene, the corresponding properties and potential applications will vary. Graphene films synthesized via CVD will be more suitable to be applied in electronics, optoelectronics,^1,2 flexible batteries,³ and in the control and manipulation of light,^4,5 to mention a few. While the bulk graphene (including graphene oxide) will be more appropriate to be used in composite,⁶ coating and conductive ink applications where large scale graphene is required. As a consequence, depending on the type of graphene and the application complexity, the time to market will be very different.⁷
A few examples of the potential applications of graphene^1,3,4 will be covered such as some exciting results obtained using graphene nanostructures to achieve the electric control of light.⁵
In addition, the results obtained from functionalized graphene (graphene oxide, GO) based alumina nanocomposites will be explained; where the electrical conductivity and fracture strength of the alumina matrix could be improved by incorporating very small percentages of GO. In this case, the processing conditions required for the alumina material allowed the in-situ reduction of the GO restoring up to some extent the electrical conductivity of the starting material. Therefore, the processing method needed or selected to incorporate the graphene in order to make the final nanocomposite will be crucial in defining the “right/suitable” bulk graphene to be used.
[1] J. Meyer, P.R. Kidambi, B.C. Bayer, C. Weijtens, A. Kuhn, A. Centeno, A. Pesquera, A. Zurutuza, J. Robertson and S. Hofmann, Sci. Rep., 10.1038/srep05380 (2014).
[2] K. J. Tielrooij, J. C. W. Song, S. A. Jensen, A. Centeno, A. Pesquera, A. Zurutuza Elorza, M. Bonn, L. S. Levitov, and F. H. L. Koppens, Nat. Phys., 9, 248 (2013).
[3] D. Wei, S. Haque, A. Piers, J. Kivioja, T. Ryhänen, A. Pesquera, A. Centeno, B. Alonso, A. Chuvilin, and A. Zurutuza, J. Mat. Chem. A, 1, 3177 (2013).
[4] P. Alonso-González, A. Y. Nikitin, F. Golmar, A. Centeno, A. Pesquera, S. Vélez, J. Chen, G. Navickaite, F. Koppens, A. Zurutuza, F. Casanova, L. E. Hueso and R. Hillenbrand, Science 10.1126/science.1253202 (2014).
[5] J. Chen, M. Badioli, P. Alonso-González, S. Thongrattanasiri, F. Huth, J. Osmond, M. Spasenovicacute;, A. Centeno, A. Pesquera, P. Godignon, A. Zurutuza Elorza, N. Camara, F.J. García de Abajo, R. Hillenbrand and F.H.L. Koppens, Nature, 487, 77 (2012).
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[7] H. Alcalde, J. de la Fuente, B. Kamp, and A. Zurutuza, Proc. IEEE, 101, 1799 (2013).

An important current goal in graphene research is the generation of controlled band gaps. To investigate whether this can be achieved by chemical means, we have studied the changes in optical properties of reduced graphene oxide (RGO) in water suspension when exposed to ozone. The reduced graphene oxide, whose properties strongly resemble graphene, was produced by oxidizing graphite using the Hummers method and then treating with the reducing agent sodium borohydride. Exposure of the resulting RGO to ozone for periods of 5 to 35 minutes caused a dramatic bleaching of the sample&’s strong visible and ultraviolet absorption and the concurrent appearance of strong visible fluorescence in previously nonemissive samples. These observed spectral changes suggest a functionalization-induced band gap opening. The ozone treatment also caused solubilization of the RGO in water. The sample fluorescence enabled by ozonation was found to be highly pH-dependent with a pattern similar to that found previously for graphene oxide: sharp and structured emission features resembling the spectra of molecular fluorophores were present at basic pH values, but this emission reversibly broadened and red-shifted in acidic conditions. These findings are consistent with excited state protonation of the emitting species in acidic media. Oxygen-containing addends resulting from the ozonation treatment were detected by XPS and ATR FTIR spectroscopy and computational modeling was used to relate their presence to optical transitions in localized graphene oxide fluorophores. Further research on controlled ozonation-induced functionalization of graphene will be directed towards producing graphene-based optoelectronic devices with tailored and controllable optical properties.

We provide an overview of unique scanning probe technologies for nondestructive measurement of Graphene&’s mechanical, electrical and infrared properties. The data shown are acquire using the methods of PeakForce Quantitative Nanomechanical Mapping (PFQNM), PeakForce Kelvin Probe Force Microscopy (PF-KPFM), and infrared scattering Scanning Nearfield Optical Microscopy (IR-sSNOM) respectively. Together these modes of nanoscale characterization allow the user to quantitatively map properties such as adhesion, stiffness, work function, and carrier density - all with a spatial resolution below 20nm. We present two case studies as examples of applying these modalities to Graphene. In the first example, a multilayer exfoliated Graphene sample is characterized using co-localized PFQNM, PF-KPFM and IR-sSNOM. The methods are used in a complementary fashion to find and confirm defect rich regions, characterize the number of layers and estimate the nanoscale carrier density. In the second example, a single exfoliated layer of Graphene on a suspended Silicon Nitride window is characterized using the PFQNM and PF-KPFM techniques in ambient as well as a controlled Argon atmosphere environment. The combined methodology allows us to quantify Graphene&’s work function and study its spatial inhomogeneity. The demonstrated spatial resolution is below 20nm while the potential resolution is better than 10mV. We find that regions of high adhesion contrast appear to greatly influence the measured work function. Through the removal of water and oxygen, the controlled environment of the Glovebox is also shown to greatly reduce both the work function and the presence of inhomogeneities in Graphene.

Large-area graphene has been grown by catalytic chemical vapor deposition (CVD) on various metal substrates. However, the uniform growth of single-crystal graphene over wafer-scale areas remains a challenge toward the commercial realization of various electronic, photonic, mechanical, and other devices based upon the outstanding properties of graphene. Here we describe the growth of single-crystal monolayer graphene on hydrogen-terminated germanium (Ge) surface [1]. A single-crystal Ge substrate is a promising candidate for the growth of single-crystal graphene, because of (i) its catalytic activity for the catalytic decomposition of the formation of graphitic carbon on the surface; (ii) the extremely low solubility of carbon in Ge even at its melting temperature, enabling growth of complete monolayer graphene; (iii) the anisotropic atomic arrangement of single crystal Ge surface, enabling aligned growth of multiple seeds; (iv) the availability of a large area single-crystal surface via epitaxial Ge growth on Si wafers. We observed that well-defined atomic arrangement on the single crystal Ge surface enabled aligned growth of multiple seeds which can merge to single crystal graphene. Furthermore very weak van der Waals interaction between graphene and underlying Ge surface enabled facile dry transfer of graphene and recycling the Ge/Si wafer for continuing growth.
References
1) J. H. Lee et al., Science 344, 286 (2014)

In recent years, exceptional physical and chemical properties of graphene have been demonstrated and widely acknowledged. Graphene films synthesized by Chemical Vapor Deposition (CVD) with different configurations (monolayer, bilayer, multilayer, etc.) and grain sizes (up to cm size) were developed to offer a wide range of electrical and chemical properties facilitating their application in several fields. This led to numerous investigations on the potential use of CVD graphene for fabrication of electronic devices (sensors, flexible touch screens, etc.).
One of the remaining key challenges for the induction of CVD graphene as a commercially viable material for a wide range of applications, including consumer products, is the development of a low cost, high volume production method with high spatial control over its physical configuration (# of layers, size of individual grains, defect density, etc.). Numerous research groups have already demonstrated successful and reproducible synthesis of graphene on lab-scale with many different process conditions. However, only few attempts have been made thus far to scale up CVD graphene production to an industrially relevant scale or to incorporate these previously developed methods. Previous “scale-up” methods were based primarily on in-line roll-to-roll system designs. However this method is typical limited in total throughput, flexibility in reactor design, graphene cost per area and can only operate in a narrow processing window. Further roll to roll CVD systems typically have a high design/process and system development cost.
In this work we present first results of a novel batch process CVD system alternative that allows a quicker and less costly scale-up than previously developed CVD processing innovations. It also promises to require lower scale-up development costs and lower capital and consumable costs. For example, our process technology allows to CVD process a Cu foil roll (up to 30 m long) at a capacity of up to ~100 m²/day in our CVD EasyGraphene® 300 tool platform that has a 330 mm ID. First results of this new CVD graphene scale-up effort will be presented and compared with those of the CVD graphene generated on lab-scale in a traditional CVD tube furnace system.

Grain boundaries (GBs) have detrimental effects on electronic, thermal, and mechanical properties of graphene, including reduced electronic mobility, lower thermal conductivity, and reduced ultimate mechanical strength. For electronic applications, mobilities are of greatest concern. It is believed that large-area, single-crystal graphene or very large-grain, single-crystal-like graphene would revolutionize the field of flexible electronics. Scientists world-wide have been trying to achieve this holy-grail of fabricating such large-area, single-crystal graphene films. We have developed processes for realizing large-area, single-crystal or single-crystal-like, metallic substrates with (111) and (110) crystallographic orientation using thermo-mechanical processing and with suitable surfaces for hetereoepitaxial growth of graphene. We have also shown heteroepitaxial growth of graphene on such (111) and (110) surfaces. These developments would eventually result in a scalable, roll-to-roll process for fabrication of large-area, single-crystal or single-crystal-like graphene sheets for wide-ranging applications.

In this paper we discuss the role that the nucleation density plays in determining the mechanism of single-crystal graphene growth. To suppress the nucleation of graphene we supplement the already established methods of pretreatment of the Cu foils using oxidation and reduction by H₂ annealing with a new method for reducing the nucleation density that interacts directly with the growth process called transient reactant cooling [1]. Transient reactant cooling is performed by an Ar pulse to produce collisional deactivation of the active carbon growth species at the remaining nucleation sites. An important feature of transient reactant cooling is that in contrast to the Cu foil pretreatment methods it is capable to completely suppress nucleation of graphene. The two methods combined suppress the nucleation density by 3 orders of magnitude.
We use a kinetic model to show that in the extreme limits of nucleation density graphene growth occurs by distinctly different growth mechanisms. The kinetic model is described in terms of sticking coefficients for two competing adsorption steps representing random nucleation of graphene (s₀) and carbon incorporation at the edges of already existing islands (s₁). The resulting growth curves have a sigmoidal (S-shape) form characteristic of cooperative processes for the island growth dominated mode that occurs for s₁>>s₀ and change to a simple exponential form for the nucleation dominated mode s₀>>s₁. The shift of the growth mode has profound consequences not just on the number of the graphene islands (domains) that form, but also on the size and the crystallinity of the individual islands. At high nucleation densities graphene growth occurs by coalescence of individual islands, which results in undesirable grain boundary formation. In contrast, carbon incorporation at vanishing nucleation densities occurs by a cooperative island growth mode that intrinsically produces large-area single-crystal graphene. The concepts developed in this work generalized by the kinetic model serve as a framework for optimization of the graphene CVD growth process for large-area single-crystal growth. The model also reveals that the upper limit to the size of graphene is ultimately imposed by the stochastic nature of the nucleation process. The kinetic model is not material specific. It is applicable to two-dimensional crystallization of any material system subject to strongly attractive adsorbate-adsorbate interactions.
Work sponsored by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy.
[1] Gyula Eres, M. Regmi, C.M. Rouleau, J. Chen, I.N. Ivanov, A.A. Puretzky, and D.B. Geohegan Cooperative Island Growth of Large-Area Single-Crystal Graphene on Copper Using Chemical Vapor Deposition, ACS Nano DOI: 10.1021/nn500209d.

Pure graphene has limited practical applications due to its low carrier density, zero band gap, and chemical inertness. Local structures of graphene can be modified to obtain useful properties and applications such as semiconductor nanodevices, batteries, and chemical separation. Chemical treatment is one of the practical methods to functionalize graphene and to study mechanisms of their interaction with environment. The single-atom thickness nature of graphene makes it an ideal medium for revealing its own defect structure, bonding and dynamics on the atomic level. The ability to directly observe reactions on the atomic level, which has long been expected in many areas of science and technology, can significantly advance our understanding on the physical and chemical mechanisms controlling the formation of functional structures based on graphene.
In this talk, we will present direct atomic-level observations and density functional analyses on the atomic dynamics, incorporation of single Si atoms, electronic structures at topological defects in monolayer graphene. We monitored dynamic atomic processes during the formation of a rotary Si trimer in monolayer graphene using an aberration-corrected scanning-transmission electron microscope. An incoming Si atom competed with and replaced a metastable C dimer next to a pair of Si substitutional atoms at a topological defect in graphene, producing a Si trimer. Other atomic events including removal of single C atoms, incorporation and relocation a C dimer, reversible C-C bond rotation, and vibration of Si atoms occurred before the final formation of the Si trimer. Theoretical calculations indicate that it requires 2.0 eV to rotate the Si trimer. Density functional calculations showed that the heights of Si atoms did not change during the rotation of the Si trimer. The observed rotation of the Si trimer embedded in graphene is quite similar to the behavior of atomic/molecular rotors driven by current or light. Our real-time results provide insight with atomic precision for reaction dynamics during chemical doping at defects in graphene, which have implications for defect nanoengineering of graphene.

With the continuous improvement of in situ techniques inside transmission electron microscope (TEM), the capabilities of TEM extend beyond structurual characterization to high-precision nanofabrication and property measurement. Based on the idea of "setting up a nanolab inside a TEM", we present our recent progress in 2D Materials research including in situ growth, nanofabrication with atomic resolution, in situ property characterization, nanodevice construction and possible applications (e.g. a 5nm-diameter hole on graphene for third-generation gene sequencing, the spongy graphene as an ultra-efficient sorbent for oils and organic solvents, etc.). In addition, reconstructed point defects in graphene are created by electron irradiation and annealing. By applying electron microscopy and density functional theory, it is shown that the strain field around these defects reaches far into the unperturbed hexagonal network and that metal atoms have a high affinity to the non-perfect and strained regions of graphene. The electron irradiation induced trapping of metal atoms in strained areas and at defects in graphene may be used for engineering the local electronic and magnetic structure of graphene which is an alternative to substitutional doping.
References:
[1] L. Sun, F. Banhart, et. al., Science 312, 1199 (2006)
[2] J. R-Manzo, M. Terrones, et.al., Nature Nanotechnology 2, 307 (2007)
[3] L. Sun, A. Krasheninnikov, et.al., Physical Review Letters 101, 156101(2008)
[4] H. Bi, K. Yin, et al., Advanced Materials 24, 5124 (2012)
[5] X. Liu, T. Xu, et al., Nature Communications 4, 1776 (2013)
[6] H. Qiu, T. Xu, et al., Nature Communications 4, 2642 (2013)
[7] X. Li, X. Pan, et al., Nature Communications 5, 3688 (2014)
[8] X. Guo, G. Fang, et al., Science 344, 616 (2014)

Graphene growth on metal films via chemical vapor deposition (CVD) represents one of the most promising methods for graphene production. The realization of the wafer scale production of single crystalline graphene films requires an atomic scale understanding of the growth mechanism and the growth intermediates of CVD graphene on metal films. Here, we use in situ low-temperature scanning tunneling microscopy (LT-STM) to reveal the graphene growth intermediates at different stages via thermal decomposition of methane on Cu(111). We clearly demonstrate that various carbon clusters, including carbon dimers, carbon rectangles, and ‘zigzag&’ and ‘armchair&’-like carbon chains, are the actual growth intermediates prior to the graphene formation. Upon the saturation of these carbon clusters, they can transform into defective graphene possessing pseudoperiodic corrugations and vacancies. These vacancy-defects can only be effectively healed in the presence of methane via high temperature annealing at 800 °C and result in the formation of vacancy-free monolayer graphene on Cu(111).

Graphene has attracted huge attention for application because of its own optical transparency, mechanical stability and peculiar electronic property. For actual use, monolayer graphene with a large area domain is required. The graphene segregation from carbon atoms dissolved in Ni is one of candidates for large area graphene formation, because of a low nucleation probability.
Understanding the graphene growth mode is a key to control graphene growth. There are several reports tried to elucidate growth mechanism by real time low energy electron microscopy (LEEM). However, because graphene growth on Ni is so rapid, for example several tens mu;m per a second, it is hard to track nucleation sites and the graphene fronts.
In situ Scanning Electron Microscopy (SEM) is alternative method. It has a wide field of view up to a few millimeters. SEM also has potential to image monatomic layers with various imaging principals, such as charge contrast, step edge contrast, and so on. In this presentation, we show the feasibility of in situ SEM for direct observing the monolayer graphene growth. From the in situ observation, the property of nucleation of graphene segregation and growth mechanism on Ni is discussed.
By in situ SEM, we firstly found peculiar “edge contrast” at the front lines of graphene. The edge contrast occurred only at monolayer graphene edge, but did not at ones of multi-layer. Following the observation, it is confirmed that edge contrast of monolayer graphene is vanished by exposing the sample to air. By repeating cycles of heating with in situ SEM observations, the edge contrast was found to be diminished with cooled (room temperature) state and it recovered at nearly 400°C. Our results show only in situ SEM, not conventional ones, powerfully works to monitor monolayer graphene segregation process.
The in situ SEM images clearly showed the preferential nucleation sites of graphene segregation. We found two factors for graphene segregation. One is surface index of Ni, and another is surface morphology. Ni(111) is the most preferred surface to nucleate because of its binding energy for carbon atoms. Furthermore, step bunched region has tendency to nucleate because diffusion of carbon adatoms on surface is suppressed.

We investigate heat transport in graphene ribbons by solving the Boltzmann transport equation with ab initio scattering. Numerical solutions are obtained using a stochastic particle method that can simulate the ab initio scattering operator coupled to phonon dispersion relations and transition rates obtained from density functional theory calculations. This formulation enables, for the first time, ab initio simulation of transport at the device scale in graphene, for which the traditional relaxation-time approximation is inadequate and incorrectly predicts, among other things, a diverging thermal conductivity with device scale.
We perform simulations of long ribbons of finite width and compare our results with solutions of the homogeneous Boltzmann equation with the traditional approximation for boundary scattering. The comparison shows that the typical error introduced by modeling the effect of transverse diffuse boundaries by augmenting the homogeneous scattering rate is on the order of 10\%, with maximum deviations (as a function of the ribbon width) as large as 30\% at room temperature. Simulations of finite width and finite length ribbons show that significant additional kinetic effects are introduced when the ribbon length becomes comparable to the phonon mean free path.
The computational method used here can be straightforwardly extended to the simulation of three-dimensional materials and devices. Modifications required as well as other computational considerations will be discussed.

When graphene is etched to have specific nanostructures, its properties can be tuned or altered. To prepare such nanostructured graphene, numerous etching methods including an e-beam lithographic process have been developed. Recently, graphene etching by metallic nanoparticles have been studied, which takes advantage of catalytic activity of nanoparticles. However, this method suffers from the lack of morphological control because the conventional process to synthesize nanoparticles cannot accurately control the place of nanoparticles on graphene where the etching starts. The diblock copolymer approach is a promising bottom-up technique to generate nanostructures and nanopatterns of various materials in large area. Since metal precursors can be selectively loaded into one block of copolymers, nanopatterns of diblock copolymers and their micelles can be used as nanotemplates to generate various nanostructures of metallic materials on solid substrates. In this study, we synthesized Pt nanostructures from diblock copolymers and their micelles to tailor the morphology of graphene. The periodicity and feature size of graphene were effectively controlled by those of Pt nanostructures by tuning the molecular weights of copolymers. This diblock copolymer approach offers a facile way to fabricate diverse graphene nanostructures with high complexity in large area.

Edge-channel picture has been widely utilized to explain magnetotransport properties of two-dimensional electron systems. In graphene p-n junctions, the electron and hole modes of edge channels can mix at the p-n boundary . The current partitioning due to mixing between p and n edge channels have lead to the observation of quantized resistance of R = (h/e²)|nu;_n||nu;_p|/(|nu;_n|+|nu;_p|), where nu;_n and nu;_p are the Landau-level filling factors in the n and p regions, respectively.
In this work, we studied carrier transmission between co-propagating quantum Hall edge channels at p-n interfaces where the p and n edge channels were decoupled. The studied device was the high-quality dual-gated graphene n-p-n junctions encapsulated by hexagonal boron nitride. In the n-n'-n configuration, the two-terminal resistance R exhibited the quantum Hall effect with conventional quantization sequence. On the other hand, in the n-p-n configuration, the value of R became larger than the quantum resistance of h/e², indicating decoupling of p and n edge channels. In addition, the value of R exhibited oscillatory behavior as a function of magnetic field B. The oscillation period #8710;B differed from both the conventional Shubnikov-de Haas effect (#8710;B prop; B) and the Aharonov-Bohm effect with magnetic flux penetrating though the gated region (#8710;B = constant). The oscillatory behavior of R was well reproduced by our numerical calculation under the assumption that R oscillated as a function of the magnetic flux penetrating the insulating state formed between the p and n quantum Hall edge channels.

Graphite nanoplatelets (GNPs), consisting of small stacked graphene flakes, have attracted a lot of attention because of the combination of the layered structure and properties of single-layer graphene with cost-effectiveness and relatively easy production^[2]. However approaches followed to integrate GNPs in a thermoplastic have low yield and high production costs^[3] due to the large amount of solvents and/or energy required for the dispersion of nanofillers. Thus, there is a huge demand for a scalable fabrication technique.
In this work, we have used commercially available graphite nanoplatelets to produce polypropylene (PP) nanocomposites. For a comparative study of nanocomposite properties, carbon nanotubes were also used to prepare PP/CNTs. The processing method chosen for the production of nanocomposites was a two-step masterbatch technique. First, a standard extrusion process have been used to disperse the GNPs in the thermoplastic and produce PP/GNPs and PP/CNTs masterbatches containing 10 wt.% of GNPs and CNTs, respectively. Then, different weight fraction specimens were fabricated (0, 0.5, 1, 2.5 and 5% wt.) by diluting the masterbatch pellets via injection moulding.
Mechanical (three point bending and J-integral fracture toughness), thermomechanical characterization (DSC, TGA, DMA) and electrical characterization have been carried out in order to analyze the effect of the GNPs and CNTs on the thermoplastic material. Digital image correlation (DIC) analysis was performed in order to obtain the strain field around the crack tip during the fracture toughness test. An increase up to 25% and 50% in the J_IC fracture toughness was achieved for the 1 wt.% GNP and 0.5 wt.% CNT nanocomposites, respectively. The result obtained from the DIC analysis are in accordance with those obtained from the fracture toughness characterization. Electrical conductivity was achieved for nanocomposites with 5 and 10 wt.% CNT, having conductivities in the order of 10^-5 and 10^-2 S/cm, respectively.
Here we have demonstrated how using an industrial masterbatch compounding approach applied to a thermoplastic, nanocomposites with improved mechanical and electrical properties can be easily manufactured.
[1] A. K. Geim, K. S. Novoselov, Nat. Mater.2007, 6, 183.
[2] Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, R. S. Ruoff, Adv. Mater.2010, 22, 3906.
[3] B. Li, W.-H. Zhong, J. Mater. Sci.2011, 46, 5595.
[4] K. Prashantha, J. Soulestin, M. F. Lacrampe, P. Krawczak, G. Dupin, M. Claes, Compos. Sci. Technol.2009, 69, 1756.

Many researchers working on graphene assume that the material is inherently perfect. Gas effusion experiments, atomic resolution imaging and other techniques indeed suggest a high crystalline quality of graphene on the nano-scale. Potential applications of graphene, however, require information on the defectiveness of macroscopic graphene films. We have devised a new imaging technique (FIFE) that reveals a macroscopically significant concentration of structural defects in CVD grown graphene. These defects are caused by a fundamental limitation of current CVD processes. By employing a novel promoter-assisted graphene CVD, the density of defects can be minimized. The observed nanometer sized structural defects were found to establish a new fluid dynamical regime of extremely efficient mass transport. Control over defects by a passivation approach can significantly decrease the mass transport through graphene. This finding permits application of graphene-based barriers for corrosion protection of metallic surfaces.

Devices composed of few atomic building blocks represent the ultimate scaling of electronic devices. Challenges with traditional transistor layouts, especially the limitations of channel scaling, prevent the realization of this vision. Exploiting the unique properties of graphene we fabricated vertical field effect transistors where an external gate modulates the graphene/semiconductor interface. A facile processing scheme resulted in transistors with 4nm channel length. Electric transport through a single molecule was demonstrated and the carrier transport was studied. These results open up new exciting research areas for molecular electronics in practical and large scale applications.

Exfoliation of graphite in a liquid phase is a top down approach to produce graphene sheets suitable for further functionalization.¹ Organic solvents were screened in parallel with sonication conditions (time and power) in the search for new and more efficient strategies toward exfoliated graphene. We will present that stable dispersions of graphene can be formed especially in o-dichlorobenzene and N-methyl pyrrolidine.² However, careful spectroscopic examination by X-ray photoelectron spectroscopy (XPS), IR and Raman revealed the presence of defects on the graphene skeleton in the form of oxygen-based functionalities.³ Moreover, we will show that the number and nature of the species introduced onto the graphene lattice can be evaluated by high-resolution XPS and we will reveal how the sonication conditions applied critically affect the quality of the so-produced graphene.³ In the following step, we will present the immobilization of photoactive electron donors, such as porphyrins and others, onto graphene via non-covalent means (supramolecularly), toward the formation of novel electron donor-acceptor systems.⁴ Finally, solid evidence for the immediate utilization of these graphene-based ensembles as photocatalysts for the removal of dyes from wastewater and water splitting for hydrogen production will be given.⁵
Acknowledgments
Partial financial support from GSRT/NSRF 2007-2013 through action “ARISTEIA II” project FUNGRAPH 3150 is acknowledged.
References
S. P. Economopoulos, N. Tagmatarchis, Chem. Eur. J. 2013, 19, 12930.
T. Skaltsas, N. Karousis, H. J. Yan, C. R. Wang, S. Pispas, N. Tagmatarchis, J. Mater. Chem. 2012, 22, 21507.
T. Skaltsas, X. Ke, C. Bittencourt, N. Tagmatarchis, J. Phys. Chem. C2013, 117, 23272.
T. Skaltsas, S. Pispas, N. Tagmatarchis, Chem. Eur. J. 2013, 19, 9286.
T. Skaltsas, N. Karousis, S. Pispas, N. Tagmatarchis, Submitted, 2014.

Graphene device fabrication on large-area graphene typically involves several patterning steps using electron beam or optical lithography, followed by graphene etching and metallization. However, the resist films and lift-off chemicals used in lithography introduce compatibility issues, such as the difficulty of removing the resist from the graphene and undesired doping. The resist residue has a negative influence on the thermal and electrical properties of the graphene and interferes with functionalization of the graphene. This motivates the development of a ‘bottom-up&’, direct-write, lithography-free fabrication method.
In this work, a lithography-free fabrication method for graphene-based devices will be demonstrated. In the first step, large-area graphene is directly patterned by a focused ion beam (FIB) in order to isolate small strips of graphene from the bulk. An in situ Raman microscope allowed for direct observation of the graphene before and after the ion beam processing. The Raman spectra showed that a Ga-ion dose of 10 C/m² is sufficient to completely remove graphene. By optimizing the pattern design, the ion beam current and the background pressure damage to the graphene by scattered ions is prevented.
In the second step Pt contacts are formed by using our novel resist-free direct-write technique [1, 2]. This approach consists of the patterning of a thin seed layer of less than 0.5 nm Pt-containing material by electron beam induced deposition (EBID), followed by selective thickening of the seed layer by area-selective atomic layer deposition (ALD). This combined approach yields virtually 100% pure Pt (resistivity of 12 mu;Omega;.cm), while it allows for patterning of Pt line deposits of only 10 nm in width. This chemical approach to contact deposition is expected to yield lower contact resistances compared to conventional physical deposition techniques.
Preliminary electrical measurements on sub-optimal devices demonstrate contact resistances as low as (40 ± 30) Omega;.
[1] A.J.M. Mackus, S,A,F, Dielissen, J.J.L. Mulders, W.M.M. Kessels, Nanoscale4 (2012), 4477
[2] A.J.M. Mackus, N.F.W. Thissen, J.J.L. Mulders, P.H.F. Trompenaars, M.A. Verheijen, A.A. Bol, W.M.M. Kessels, J. Phys. Chem. C117 (2013), 10788

The excellent properties of graphene [1],[2] have driven the search for methods for its large-scale production. Graphene can be prepared by various methods [3] including micromechanical cleavage, epitaxial growth, chemical vapour deposition, exfoliation using graphite intercalation compounds and oxidation-reduction methods [4], [5]. These methods render high-quality graphene flakes although its low productivity makes them unsuitable for large-scale applications. The alternative strategy is the chemical oxidation of graphite or different carbon materials followed by chemical or thermal annealing.
Although the chemical oxidation of graphite is considered one of the most attractive methods to obtain graphene because it is cheaply scalable and versatile, it presents the disadvantage that the O-containing groups produced by chemical oxidation, which make graphene oxide (GO) non-conducting [6], cannot be completely removed by the thermal annealing reduction. Thus, the level of reduction of GO is directly related to the conductivity, which can increase several orders of magnitude through the reduction process [7], [8].
In this work, GO, produced using a slight modification of the Hummers oxidation method from natural graphite flakes [9], has been in situ reduced by Joule heating in a TEM with a STM holder. The reduction of GO has been measured qualitatively from the comparison of conductivity of the sample before and after the reduction, all in the same experiment. Besides, with this technique it is possible to reduce several individual GO nano-platelets during the same experiment and also to control the reduction of each GO nano-platelet from the measure of its conductivity and characterize them during the experiment (both through TEM observation and through current-voltage characteristic). Indeed, the results show how GO has been reduced by observing a decrease of the resistance of more than four orders of magnitude. Finally, it is planned to characterize GO nano-platelets before and after the reduction with EELS at the SuperSTEM.
References
[1] K. Novoselov et al, Science 306, 666-669 (2004)
[2] A. K. Geim, Science. 324 (5934), 1530-1534 (2009)
[3] Dilini Galpaya et al. Graphene 1, 30-49 (2012)
[4] Francesco Bonaccorso et al. Materials today 15, 12 (2012)
[5] Novoselov, K. S., et al. PNAS 102, 10451(2005)
[6] Inhwa Jung et al. Nano Lett., 8 (12), 4283-4287 (2008)
[7] Cristina Goacute;mez-Navarro et al. Nano Lett., 7 (11), 3499-3503 (2007)
[8] Akbar Bagri et al., nature chemistry. 2, 581 (2010)
[9] Beatriz Martín-García et al., ChemPhysChem 13, 3682 (2012)

Compression behaviors of graphite, reduced graphite oxide, and single and multilayered graphene have been reported in the literature. The published studies have suggested disparate behaviors under hydrostatic and/or non-hydrostatic compressions at ambient temperature. Some studies have suggested phase transition of the layered structure to a three-dimensional structure under pressure, while other studies, especially using graphene samples, have failed to observe any phase transitions. We have used both high-pressure angle-dispersive synchrotron x-ray powder diffraction and in situ Raman spectroscopic studies at room-temperature to investigate the hydrostatic compression of various layered carbon materials. Our results on the high-pressure behaviors and pressure-volume relations will be presented for the various samples investigated. Our results suggest that a phase transition may or may not be encountered at high pressures depending on the starting structure.

Carbon dots (CDs), comprising nanosized graphene flakes embedded in an amorphous carbon matrix, are a recent example of the many forms of carbon nanostructures which have attracted attention due to their unique properties. First synthesized in 2004, these highly fluorescent nanoparticles have already found widespread application in photocatalysis, optoelectronics, photovoltaics and bioimaging. Not only are carbon nanodots easy to fabricate, but they are also cheap, can be rendered water-soluble, chemically inert, have low cytotoxicity and a high resistance to photobleaching. While there have been several studies on the nature and mechanisms investigating the optical properties of CDs, the photophysics of CDs is still debated, especially concerning energy transfer mechanisms. In this presentation we focus on the energy transfer mechanisms occurring upon excitation of CDs and their effect on optical properties of the dots. Photoluminescence (PL) and absorption spectroscopy reveal two types of states: core states, located in the graphene nanodomains inside the CDS and surface states, located at passivated sites at the surface of the CDs. Photoluminescence excitation spectroscopy and time-resolved PL spectroscopy reveal energy transfer between core states and from core to surface states, but not between surface states. Determination of the rate constants of these transfer processes helps to understand the photophysics inside carbon dots, an essential step to specific, directed tailoring of the luminescent properties of such a highly luminescent material, enabling their use, among other things in lighting applications.

Noble metal on carbon supports, such as carbon black, Vulcan X and Ketjen Black, are generally used as the preferred catalysts in Proton Exchange Membrane Fuel Cells (PEMFCs). Nevertheless, these electrodes present some major disadvantages such as inadequate resistance to corrosion caused by electrochemical oxidation and/or loss of the metal active phase. Moreover, carbon is impermeable to gases and liquids and do not conduct protons thereby limiting the catalyst performances. To address the latter deficiency, Nafion an expensive protons conducting polymer is generally used. Graphene is been anticipated to hold great promise as an alternative for conventional carbon supports in PEMFCs because of its high surface area, resistance to corrosion and outstanding electronic, thermal, and mechanical properties. However, graphene synthesis methods such as mechanical or chemical exfoliation of graphite, involve either too many complex steps or use hazardous chemicals like hydrazine and often there is an accompanying loss of material conductivities. The deposition of metallic nanoparticles via chemical vapour deposition and chemical deposition suffers generally from weak interaction between the metal and graphene or the catalyst exhibits a low surface area. Here we report the electrochemical preparation of chemically charged graphene to replace the instable carbon and costly Nafion. Graphene was reproducibly exfoliated electrochemically at high yield and purity with low degree of oxidation and chemically modified with polystyrene sulfonate (PSS) preventing re-stacking and enhancing proton transport during PEMFC operation. Silver deposition was performed with a potentiostatic double-pulse technique to obtain narrow particle size, high surface area and homogenous spatial distribution. The electrocatalyst was tested for oxygen reduction reaction (ORR). A large increase in current density was observed for ORR that coincided with the decrease in particle size and increase in density. To the best of our knowledge this is the first time that Ag nanoparticles have been homogenously electrodeposited onto graphene at a dimension lower than 10nm. These methods led to the synthesis of defect free graphene with the subsequent ability to control, both the density and the size of metallic nanoparticles.

The advantages of incorporating graphene in nanocomposites are many, however these materials remain as yet too costly to produce for practical implementation. A large-scale alternative to single layer graphene that achieves similar properties at a greatly reduced cost is that of graphite nanoplatelets (multilayer graphene) embedded in a polymer matrix. Such composites are shown to attain sheet resistances (< 50#8486;) near the quasi-metallic regime. Among the challenges facing multilayer graphene nanocomposites are durability, flexibility, and maintaining the low-cost for direct translation into industrial applications. A unique approach of incorporating adhesive and durable acrylic polymers is presented to overcome these challenges. The method delivers two additional advantages along with the cost-point: 1) The flexibility is sufficient for applications in light-weight and foldable electronics; 2) the incorporation of n-doping nanoparticles in the composite makes them ideally suited for cathodic energy storage applications. The spray-cast method for applying the low electrical resistance composite is shown to be viable for large-area coatings, and is followed by a simple polishing step which enhances electron mobility. This substrate-independent, two-step process can be scaled for nearly any industrial requirements, and delivers uniform conductivities. The graphitic nanocomposite coating is evaluated for performance on both standard printer paper and glass slides to determine adhesion and conductivity under folding, while varying the platelet size and mass fraction of nanoplatelet content. At greater nanoplatelet loading in the composite, conductivity increases but we show that there is a trade-off in adhesion and therefore performance as the acrylic polymer content is reduced. Further characterization reveals a surprising homogeneity in the electric potential of the coating on the micro-scale, suggesting the low electrical resistance can be attributed to the lack of appreciable defects on the polished surface.

Graphene has remarkable mechanical, electronic, and thermal properties, which can be tuned by properly modifying graphene structure and composition through patterning and chemical functionalization. Graphene nanomeshes (GNMs) are graphene nanostructures consisting of a periodic arrangement of nano-scale holes or pores in the graphene lattice with neck widths less than 10 nm, mimicking dense arrays of ordered nanoribbons. Establishing rigorous structure-property-function relationships in such patterned graphene nanostructures is significant for their optimal design toward enabling a broad range of technological applications.
In this presentation, we report the results of a systematic study based on molecular-dynamics (MD) simulations of the thermal transport properties of GNMs as a function of the nanomesh architecture, as determined by the lattice arrangement of the GNM pores, as well as of the pore morphology, material density, and pore edge passivation. Scaling laws for the density dependence of the thermal conductivity are established. Specifically, we find that, for circular unpassivated pores, the inverse of the thermal conductivity scales linearly with the inverse of the density. For elliptical pores with aspect ratio (semi-major/semi-minor ellipse axis) greater than one, the thermal conductivity of the GNMs becomes anisotropic and this anisotropy becomes stronger as the GNM density decreases. Passivating the nanomesh pore edges with hydrogen atoms increases the GNM thermal conductivity. Further analysis of the phonon mean free paths based on the geometry of the GNMs shows that phonon transport in GNMs is dominated by phonon-pore edge scattering. The results of Monte Carlo sampling of phonon mean free paths explain the scaling rules derived from the MD simulations.

Graphene nanomeshes (GNMs) are graphene nanostructures consisting of a periodic arrangement of nano-scale holes or pores in the graphene lattice with neck widths less than 10 nm, mimicking dense arrays of ordered nanoribbons. Establishing rigorous structure-property-function relationships in such patterned graphene nanostructures is of utmost importance for their optimal design toward enabling a broad range of technological applications.
In this presentation, we report the results of a systematic computational study on the elastic response and fracture dynamics of GNMs based on molecular-statics and molecular-dynamics simulations of uniaxial tensile deformation tests. Both the elastic properties and the dynamical response to mechanical loading are determined as a function of the nanomesh architecture, namely, the lattice arrangement of the pores, as well as of the pore morphology, material density, and pore edge passivation. Scaling laws for the density dependence of the elastic modulus are established. We find that, for circular unpassivated pores, the elastic modulus scales with the square of the density. Deviations from this quadratic scaling are most strongly influenced by pore morphology and, to a lesser extent, by pore edge passivation and temperature. In addition, for the numerous GNM structures examined, stress-strain curves are generated and the ultimate tensile strength, fracture strain, and toughness are determined as a function of the GNM architectural, morphological, and physicochemical parameters. The structural responses of the GNMs during their fracture in the simulated mechanical deformation tests are analyzed in detail. We find that GNM fracture is characterized by a brittle-to-ductile transition as the GNM density decreases, determine the critical density for the transition, and characterize the key aspects of the ductile fracture mechanism.

Graphene has recently come to forefront of materials science as a potential material for use in semiconductor and spintronics applications. Graphene has seemingly ideal properties in many areas such as very high charge carrier mobility at room temperature (15,000 cm²/Vs), excellent thermal stability, tensile strength, as well as long spin diffusion length. Future research is aimed at developing spin-devices that could consume less power and have performance advantages over present technologies. However, graphene is a semi-metal with a closed electrical and optical band gap, and is therefore not suitable for semiconductor applications as an active material. Due to this, a large body of research has been focused on modifying graphene&’s electronic properties in a controllable way using such methods as chemical functionalization, quantum confinement, and symmetry breaking effects.
This work uses metal adatoms deposited onto single layer graphene by physical vapour deposition. The graphene-Co system was studied using X-ray spectroscopy techniques, Raman spectroscopy, atomic force microscopy, and density functional theory calculations. It is found that the Co tends to aggregate at impurity sites on the graphene, forming clusters when Co concentration is ~1 monolayer (ML) and below. Removing from vacuum and exposing Co/Graphene samples to oxygen at atmospheric pressure causes oxides of Co and C form in proportion to the Co coverage. X-ray spectroscopy was used to probe the occupied and unoccupied density of states and locate the band onset in each case. This treatment reveals an optical band gap in graphene when decorated with Co. The band opening is seen to coincide with the formation of non-destructive oxide groups on the graphene surface. The mechanism of band opening is motivated by minor charge transfer from the Co that results in enhanced graphene oxidation in the local area around Co. The chemical functionalization by oxygen causes opening of a band gap that can be tuned by the Co coverage.

The advent of chemical vapor deposition graphene in 2010 has allowed researchers to investigate large area graphene/n-type silicon heterojunction solar cells. The simplicity of the structure attracted attention from many groups and it was soon discovered that chemically doping the graphene improves the cell efficiency to approximately 9%. However, many devices reported in literature, particularly those that do not apply chemically-doped graphene show a distinctive S-shaped kink in the fourth quadrant of their I/V curves, resulting in reduced short-circuit current and fill factor.
We determine that the S-shaped kink is caused by a combination of the relatively low work function of graphene and the presence of a native oxide between the graphene and silicon. The oxide tunneling barrier presents a bias-dependent resistance to the collection of photo-generated carriers, resulting in significant recombination current under forward bias. However, the oxide also reduces forward current, which is advantageous for performance as it allows us to increase the open-circuit voltage in our solar cells. Thus, optimizing the thickness of this interfacial oxide is important to maximizing device performance. We find that by tuning the oxide thickness, we can achieve 11.3% efficiency in our devices, which is a 25% improvement over similar devices reported in literature.

Due to its atomic scale thickness, the deformation energy in a free standing graphene sheet can be easily released through out-of-plane wrinkles which, if controllable, may be used to tune the electrical and mechanical properties of graphene. Here we adopt a generalized von Karman equation for a flexible solid membrane to describe graphene wrinkling induced by a prescribed distribution of topological defects such as disclinations (heptagons or pentagons) and dislocations (heptagon-pentagon dipoles). In this framework, a given distribution of topological defects in a graphene sheet is represented as an eigenstrain field which is determined from a Poisson equation and can be conveniently implemented in finite element (FEM) simulations. Comparison with atomistic simulations indicates that the proposed model, with only three parameters (i.e., bond length, stretching modulus and bending stiffness), is capable of accurately predicting the atomic scale wrinkles near disclination/dislocation cores while also capturing the large scale graphene configurations under specific defect distributions such as those leading to a sinusoidal surface ruga² or a catenoid funnel.

Graphene represents a promising materials platform to enable the continued evolution of electronic and photonic technologies, well beyond the fundamental limits of traditional semiconductors. Here we argue that, by virtue of several distinctive properties (including the linear energy dispersion, large velocity and potentially ultra-high mobility), electrons in graphene are uniquely suited to radiation mechanisms that so far have been the exclusive domain of high-energy electron beams in vacuum. Specifically, we numerically investigate the use of sinusoidally corrugated graphene for the generation of THz light based on a radically new cyclotron-like radiation process, which does not require the application of any magnetic field. Instead, periodic angular motion under dc bias is simply produced by the graphene mechanical corrugation (obtained via conformal adhesion on a sinusoidally patterned surface), combined with its two-dimensional nature which ensures that the carrier trajectories perfectly follow the corrugation.
The radiation output of this basic geometry was investigated via rigorous electrodynamic simulations based on the finite difference time domain (FDTD) method, combined with a simple model of charge transport in graphene. The simulation results indicate that technologically significant room-temperature output power levels of several mW/mm² can be obtained at geometrically tunable THz frequencies, using graphene sheets deposited on sinusoidal gratings with periodicities of a few hundred nanometers [1]. Similar geometries (again involving the integration of graphene and nanoscale gratings) can also produce THz light based on other electron-beam radiation mechanisms such as the Smith-Purcell and Cherenkov effects, and comparable emission characteristics are predicted. These processes may open the way for a new family of THz optoelectronic devices, including graphene “free-electron” lasers potentially capable of room-temperature operation.
[1] K. Tantiwanichapan, J. DiMaria, S. N. Melo, and R. Paiella, “Graphene electronics for terahertz electron-beam radiation,” Nanotechnology24, 375205 (2013).
This work was supported by NSF under grant DMR-1308659.

Graphene, the two-dimensional (2D) form of graphite, has a thermal conductivity of up to ~5000 W m^-1K^-1, the highest measured value of any material at room temperature. The high thermal conductivity, which arises from extremely strong sp² bonding in the basal plane and unusually large phonon mean free path of the long-wavelength phonons, makes graphene attractive for nanoelectronic device applications such as transistors, interconnects, and heat spreaders. However, in these applications, graphene is in contact with other materials and the heat flow is suppressed not only through the graphene channel, but also across metal contacts, making it important to understand how interfaces affect both in-plane and cross-plane heat transfer in these configurations. Here, we demonstrate simultaneous measurements of the basal-plane thermal conductivity and the thermal boundary conductance (TBC) of graphene encased between titanium and silicon dioxide, using a frequency domain thermoreflectance (FDTR) microscope. Thermal phase images at six frequencies between 100 kHz and 50 MHz are collected and fitted to a thermal model to create maps of the in-plane thermal conductance and the TBC of several mechanically exfoliated graphene flakes encased between Ti and SiO₂. We find a room temperature thermal conductivity similar to that of graphene supported on SiO₂. In addition, we have investigated the effect of surface morphology on both in-plane and cross-plane heat transfer in graphene. Comparison of our samples suggests that the graphene-substrate adhesion energy plays a critical role in determining both thermal conductivity and the TBC, and that locally modified surface morphology could be used to optimize thermal transport in devices based on graphene and other 2D materials.

We demonstrate a solid carbon source based ambient pressure (AP) chemical vapor deposition (CVD) technique to synthesis high quality graphene crystal and large-area continuous film. We have used solid camphor, a botanical derivative and waste plastic as carbon sources in the CVD process. Large hexagonal and round shape crystals were synthesized on Cu foil by controlling the injection rate of carbon radicals produced during pyrolysis of precursor materials. Evolution of graphene crystals growth with the injection rate, gas atmosphere (Ar:H₂) and other growth parameters were explored. The individual crystal structures were investigated by optical microscopy, scanning electron microscopy and Raman mapping studies. Our findings show that controlling the growth conditions synthesis of large graphene crystal with a distinctive shape can be achieved in the solid precursor based CVD process. Similarly, continuous graphene films with controlled layer numbers, such as monolayer, bi-layer and few-layer graphene are synthesized on Cu foil by optimized growth parameters. We also revealed the structural transformation process of as-synthesized individual graphene crystals with oxidation of the Cu foil. We found that the transformation of a graphene crystal with Cu oxidation is significantly different for the thermal annealing and room temperature long-term atmospheric oxidation. Annealing created large cracks in an individual graphene crystal due to the thermal stress and strain created by rapid oxidation of Cu surface. On the other hand, in case of room temperature long-term atmospheric oxidation, oxygen diffusion occurred underneath of graphene crystal through the reactive edges and decoupled from Cu surface without any structural deformation.

Results from molecular dynamics simulations will be presented for the transformation of graphene nanoribbons (GNRs) with engineered structural defects into Carbon Nanotubes (CNTs). Several types of defects can initiate the bending of GNRs, a key factor for the formation of CNTs. Zigzag, armchair and chiral CNTs can be produced with this method. Detail studies of the dependence on the different type and distribution of the defects and the structure of the resulted CNT will be presented. The chirality of the CNTs can be controlled by properly manipulating the edges of the GNRs. The smallest CNT obtained in our simulations had a radius of about 0.5nm. Results for randomly distributed defects will be also presented.

Magnetic nanoparticle dispersions are usually made from superparamagnetic materials since in the absence of an external magnetic field, the lack of magnetic particle/particle attraction allows the creation of stable dispersions [1]. Common attempts to prepare stable dispersions of ferromagnetic particles failed since the strong magnetic particle/particle interaction can overcome repulsive effects from surfactants or steric stabilizers (typically polymers).
In the present work, we show graphene protected metallic nanomagnets. The graphene protects the metallic core from oxidative destruction and allows covalent functionalization of the particle surface. It is demonstrated how the direct, covalent attachment of highly charged polymers on the graphene layer can circumvent stabilizer detachment and permit the preparation of stable dispersions of ferromagnetic, graphene protected nanoparticles [2]. More specifically, graphene-coated metal nanoparticles, prepared by flame spray synthesis, are covalently functionalized with ionic polymer brushes (diazonium chemistry / ATRP). Particle size distributions with an average diameter of 24 nm provide a ferromagnetic fluid with unprecedented stability in water over months. This functional graphene-coated nanomaterial was tested for two different applications.
For inductive heating in medicinal chemistry or material science, the much higher magnetization of ferromagnetic metals over the currently used superparmagnetic oxides is attractive [3]. Measurements with inductive heating at different frequencies and various field strengths using stable dispersion of these graphene nanocoposite showed an average specific absorption rate of 360 W g^-1.
Deinking of printed paper is within the most expensive steps in paper recycling. The application of the stable dispersion as ferromagnetic ink simplifies this step and might face current issues. As printed parts of paper are rendered magnetically distinguishable from non-printed parts, a reduction of the paper mass, which has to be deinked, by magnetic pre-separation can be achieved.
[1] E. Kita, T. Oda, T. Kayano, S. Sato, M. Minagawa, H. Yanagihara, M. Kishimoto, C. Mitsumata, S. Hashimoto, K. Yamada and N. Ohkohchi, J. Phys. D. Appl. Phys., 2010, 43, 47401.
[2] M. Zeltner, R.N. Grass, A. Schaetz, S.B. Bubenhofer, N.A. Luechinger, W.J. Stark, J. Mater. Chem., 2012, 22, 12064.
[3] Q. A. Pankhurst, J. Connolly, S. K. Jones and J. Dobson, J. Phys. D. Appl. Phys., 2003, 36, R167.
[4] M. Zeltner, L. M. Toedtli, N. Hild, R. Fuhrer, M. Rossier, L. C. Gerber, R. A. Raso, R. N. Grass and W. J. Stark, Langmuir, 2013, 29, 5093.

Graphene scalability and mass production are important for future nano-carbon applications such as electronics, optoelectronics and spintronics. Very large scale integration (VLSI) interconnects require practical multi-layer graphene wires for the purpose of lowering resistivity as per the scaling law. Chemical vapor deposition (CVD) is a promising process for the incorporation of multi-layer graphene in the back end of line integration. In this work, we demonstrate the CVD-based growth of multi-layer graphene on a 300 mm wafer and discuss a correlation between CVD graphene crystallinity and electric properties.
Large area multi-layer graphene was grown by thermal CVD using C₂H₂, H₂, and Ar as source gases. Nickel, which is expected to facilitate low-temperature graphene growth, has presented high catalytic activity toward graphene growth and crystallization in its CVD-Ni form during metal organic CVD (MOCVD). Therefore, CVD graphene growth was achieved at the 300 mm wafer scale on CVD-Ni at 650 °C or less.
The crystallinity of the resulting graphene was evaluated using the ratio of G- and D-band signal intensities (G/D) in the Raman spectra. The graphene synthesized at low temperatures showed a G/D ratio ranging from 1 to 18. In general, graphene resistivity tended to decrease when this ratio increased. However, CVD graphene films presenting similar G/D ratio occasionally displayed different resistivity. An incident angle-dependent X-ray absorption fine structure (XAFS) analysis was conducted to elucidate the correlation between graphene crystal orientation and electric properties. The X-ray incident angle was varied from 15° to 90° (normal incidence). The incident angle-dependent C-K edge XAFS measurements revealed that graphene exhibited different crystal orientation for quasi-identical G/D ratios and tended to present lower resistivity when highly oriented, confirming the Raman results.
In conclusion, large area, high uniformity, and high crystalline multi-layer graphene was grown using a CVD-Ni catalyst. The incident angle-dependent XAFS analysis revealed that the resistivity of the CVD-formed graphene correlated with its crystal orientation. The elucidation of graphene electric properties hinges on a deep understanding of graphene fine structure and macroscopic characteristics.
This work is an “Ultra-Low Voltage Device Project” funded and supported by METI and NEDO. A part of the device processing was operated by AIST, Japan.

Demand of energy is growing dramatically due to increasing demand in developed as well as emerging economies. Improved technologies for the generation and storage of energy are vital to improve the way that society uses energy. As a result, large-scale research efforts are underway for production of electrical energy and develop supercapacitors for storage.
Here, we report the graphene oxide based supercapacitor which is good charge holding device according to our present study. Graphene oxide was synthesized using an eco-friendly improved method which emits O₂ molecule as a by-product of reaction. From this graphene oxide (GO), graphene was synthesized using two methods, viz., thermal treatment (GRH) and reduction with sugar (GRS). GO and GRS were characterized by using UV-Vis spectroscopy, X-ray diffraction, Raman spectroscopy and Fourier Transform Infrared spectroscopy. All samples were studied for their capacitive nature in aqueous electrolyte using cyclic voltammetry, galvanometric charge-discharge and galvanostatic impedance analysis. GO shows higher capacitive nature due to increase in number of oxygen containing functional group i.e. hydroxyl group and carbonyl group which help to increase capacitance of GO. Higher capacitance, low cost, and low preparation time makes GO a better choice than GRS and GRH.

We have developed a non-oxidative production route for graphene by the electrochemical intercalation of metal and organic ions into pristine graphite cathodes [1,2]. The intercalation process is found to exfoliate the graphite into few-layer graphene which is then collected from the electrolyte. Significantly it is found that no ultrasonic or centrifugation steps are required in order to get down to the few layer thickness. Furthermore, through the choice of electrode and electrolyte, the flake diameter can be controlled from 50 nm to 25 microns, with these wider flakes unusually large for exfoliated graphene (e.g. ultrasonication typically produces < 1 micron diameter flakes). These large flakes have shown promise in applications such as nanocomposites [3].
We have studied two different electrolyte compositions for the exfoliation process; (i) tetraalkylammonium cations in a NMP [1] and (ii) Li and triethylamine cations in a DMSO [2]. In both systems, the intercalation and exfoliation steps were studied using cyclic voltammetry. The flakes produced were then characterised by TEM, SEM, Raman spectroscopy and AFM. Our approach of using a reductive process of positive ions at the cathode is found to be particularly promising for producing material with a low oxygen content. (XPS finds a very similar atomic oxygen percentage in the starting graphite and final flakes.) Whereas, we have found that graphene oxide is produced when using an oxidative process at the anode. For example, we can make graphene oxide with a controlled oxygen content using a simple 0.2 M sodium citrate electrolyte [4].
[1] Single stage electrochemical exfoliation method for the production of few-layer graphene via intercalation of tetraalkylammonium cations, Cooper et al., Carbon 66, 340 -350 (2014)
[2] Continuous Electrochemical Exfoliation of Micrometer-Sized Graphene Using Synergistic Ion Intercalations and Organic Solvents, Abdelkader et al., ACS Appl. Mater. Interfaces 6, 1632minus;1639 (2014)
[3] Few layer graphene - polypropylene nanocomposites: the role of flake diameter, Valles et al., Faraday Discussions, FD173 in press
[4] High-yield electro-oxidative preparation of graphene oxide, Abdelkader et al., Chem Comm, Accepted, DOI: 10.1039/C4CC03260H

Graphene shows exotic electronic transport properties and coupling it with superconductors makes a very interesting system for understanding novel quantum phenomena associated with it. We report proximity induced superconductivity in superconductor-grapheme-superconductor (SGS) devices. FIB deposited superconducting tungsten electrodes have been used to induce proximity effects in monolayers of graphene. Electronic transport through these junctions has been investigated thoroughly by changing the junction length/ gap separating the two superconducting (source and drain) electrodes. SGS devices have been investigated under different magnetic fields (B=0-7 Tesla), currents (I=0-200 µA) and temperatures (T=1.8-10 K). The SGS devices show transport properties different in comparison to continuous superconducting wire deposited on graphene. Formation of a plateau in resistance R(T) at T~4 K, much below the tungsten electrode&’s critical temperature Tshy;_c (> 5 K), indicates superconductivity injection in graphene is a slow/weak process. The plateau may increase with the applied current and extend upto the lowest measured T=1.8 K. The measured voltage-current (VI) and RT curves indicate that electronic quantum transport through SGS device is a complex phenomenon, exhibiting multisteps, plateaus and knee structures, which need to be explored further in detail.

Graphene&’s remarkable combination of mechanical, electrical thermal and barrier properties have led to considerable interest in its use as a filler in polymer materials^[1], with the potential to provide consumer products with improved function. In a new collaborative project^[2] with industry, we are for the first time combining complimentary experimental techniques to explore the possibility of exploiting the benefits of graphene fillers in consumer goods technologies on a large scale.
Rheometry, small angle X-ray (SAXS) and neutron scattering (SANS), dielectric spectroscopy and tip enhanced Raman spectroscopy, enable us to tackle the challenges of utilising graphene as a filler in large scale consumer goods technologies: Understanding the crucial role of the interaction between the polymer and the graphene, its dispersion and orientation within the polymer matrix and the processing effects on the composite is essential to realize the potential of graphene as a filler.
Here we report our first experiments on graphene oxide (GO), a form of graphene whose functionalization can improve dispersion in polymer composites, within poly(methyl methacrylate) (PMMA) and polystyrene (PS) matrices. We have characterized a high level of processing induced orientation of the GO within our samples using SAXS, which can then be lost during post processing. This is also important for controlling the network properties, studied by rheometry, that the GO imparts on the different polymer systems. Controlling both the alignment and dispersion of the GO is crucial in determining the final material properties^[1,3]. The fascinating properties of polymer diffusion in carbon nanotube composites^[4] have begun to emerge in these systems and will be discussed with recently awarded SANS and quasielastic neutron scattering beamtime where we investigate the influence of the GO filler on the polymer chain conformations and dynamics. Such information is vital for understanding the observed processing effects and developing strategies for improving the compatibility between the polymer and the graphene, allowing us to unlock the potential of these nanocomposites.
References
[1] Kim, H.; Abdala, A.A.; Macosko, C.W. Macromolecules 2010, 43, 6515-6530.
[2] www.dur.ac.uk/grapol
[3] Young, R.J.; Kinloch, I.A.; Gong, L.; Novoselov, K.S. Composites Science and Technology 2012, 72 1459-1476.
[4] Mu, M.; Clarke, N.; Composto, R.C.; Winey, K.I. Macromolecules 2009, 43, 7091-7097.

Due to the growing need of device miniaturization, materials exhibiting high electrical and thermal conductivity at nanoscale are in high demand. Graphene is potential alternatives for applications as heat sink in a range of nanoscale devices. For more efficient thermal management, graphene-substrate contacts play an important junction interface in creating unusual resistivity and heat generation, which undoubtedly deteriorate the performance of nanosacle ultrafast low power devices. We investigate a relation between thermal transport property and crystal structure of graphene by considering surface and interface structure of the sample. We characterized the direct imaging of interfacial bonding of chemical vapor deposited (CVD)-graphene on substrate by changing the induced strain at the interface through heat treatment in vacuum. Scanning thermal microscopy (SThM) is carried out to study interfacial bonding of graphene/substrates in conjunction with its thermal and electrical conductance. These studies confirm the improved adhesion of annealed graphene on substrate by demonstrating its adhesion energy and thermal resistance. Quantification of the interfacial bonding energy and thermal conductance of graphene with substrates enables better design of the interface to manufacture future heat pad in nanoelectronic devices.

A scalable spray drying technique was used to crumple pristine graphene nanosheets for the first time. Graphene nanosheets were stabilized in water by pyrene derivatives and the final dispersions were atomized and dried in a spray dryer. During this process 2D graphene nanosheets are crumpled and transformed into 3D particles. This transition in sheet morphology was investigated by collecting samples at various heights in the spray dryer. Microscopy demonstrated that graphene nanosheets deformed as the droplet shrinkage induced capillary forces at the interface, and the nanosheets transformed into multi-faced crumpled particles with several dimples. Graphene oxide (GO) nanosheets were spray dried using the same procedure; however, their highly wrinkled final morphology was different than the crumpled pristine graphene nanosheets. Differences in the bending elasticity and surface chemistry of pristine graphene and GO nanosheets are the main reason for the difference in the final morphology. Spray drying parameters such as atomizer pressure, drying temperature and concentration of sheets in the dispersions were varied in order to observe their effect on the final morphology of crumpled particles. Also, in order to study the effect of second component on the final morphology other stabilizers such as polymers and surfactants were applied to stabilize pristine graphene nanosheets. Crumpled particles were redispersed into various solvents to assess their unfolding behavior. GO particles remained crumpled in the water; however, pristine graphene nanosheets immediately unfolded in the water due to the high affinity of pyrene derivatives for water. The GO behavior may be attributed to the formation of hydrogen bonds between GO nanosheets in a crumpled particle which prevent the unfolding in the solvent.

Tunable electrical/optical properties of plasma-functionalized graphene are explored for solar cell applications. We demonstrate tetrafluoromethane (CF₄)-based surface treatment on monolayer and multilayer graphene which results in fluorine-doped graphene with tunable electrical phases, depending upon the atomic % of fluorine in graphene lattice. We have achieved both insulating fluorinated graphene monolayer and semiconducting fluorinated graphene multilayer, as confirmed via electrical transport measurements. To demonstrate the applications of these graphene derivatives, the insulating phase of fluorinated graphene monolayer is explored as a recombination barrier layer and the semiconducting fluorinated graphene multilayer is exploited as a photoactive layer in Schottky barrier solar cell. Raman spectroscopic investigation on fluorinated graphene nanostructures suggests the intensity of D-peak can be benchmarked to identify the fluorine content in graphene lattice, which is confirmed by X-ray photoelectron spectroscopic measurements. Study on carrier transport & recombination effects and optical response behavior in fluorinated graphene nanostructures are also presented.

Global water crisis has lead to an increasing demand for the effective utilization of affordable water purification systems in third world countries. To mitigate increasing global water stress for rising energy demands, there is a need to improve the performance of thin film composite (TFC) reverse osmosis membranes. However, these membranes are prone to fouling (i.e., the deposition of materials on the membrane surface), which can significantly reduce the amount and quality of water produced. In order to effectively translate the use of these membranes to prevent fouling, we fabricated a multifunctional TFC membrane using graphene oxide as a template to dramatically reduce fouling of mineral scalants (calcium carbonate and sulfate), natural organic matter (humic acid), and microorganisms (Escherichia coli). The presence of graphene oxide creates a synergistic effect on improving multiple fouling resistances. Graphene oxide has bactericidal properties and is used as a template in the formation of gold nanostars for the photothermal ablation of bacteria. Photothermal heating of these membranes was effective in killing the E.Coli and maintaining the membrane at a temperature to prevent biofouling. The fully functionalized TFC membranes exhibited significant resistance towards calcium salt formation, humic acid deposition, as well as the ability to kill bacteria under exposure to an 808 nm laser. Our innovative membrane design and multi-functionality proves to have a synergistic effect and offers a potentially new and exciting improvement towards strengthening reverse osmosis effectiveness. Furthermore, these functionalized membranes could be used towards designing a low-energy reverse osmosis system in which the membrane surface is regenerated during operation which can drastically improve desalination processes.

Graphene has been attracting great interest due to its extraordinary electronic, thermal, and mechanical properties, resulting from its two-dimensional structure, and to its potential applications such as energy, wind turbines, construction, defence, automotive, aeronautics and aerospace applications. In last decades, there are numerous works about graphene synthesis and its applications in laboratory scale but mass production of graphene has still doubts and obstacles. One of the applicable methods is graphite oxidation to reduce the strong bonding between graphene sheets in graphite and obtain single layer graphene. In the present work, graphene nanosheets were produced in pilot scale by improved, safer and mild chemical route. The proposed technique provided to reduce the average number of graphene layers steadily from raw graphite to graphene nanosheets by stepwise chemical procedure. Additionally, graphene obtained from recycled carbon source was a scalable, cost effective and environmentally friendly. After the production of high quality and high quantity graphene, graphene based composites were produced by using different thermoset and thermoplastic polymers by melt-compounding process. With this fast and cost effective process, graphene sheets with controlled C/O ratio and structural properties were distributed homogeneously in polymer matrix. The mechanical and thermal properties of the materials were improved and significant weight reduction was provided by the addition of graphene in matrix material in low loadings changing from 0.1 to 10%. As a result, more reliable and long lasting composite production is possible by using graphene in polymer composites.

Photocurrent generated at a junction between ABA (Bernal) and ABC (Rhombohedral) stacking in tri-layer graphene has been observed. The Raman spectra of ABA- and ABC- stacked tri-layer graphene have been studied [1], but the photocurrent in tri-layer graphene has not yet been investigated. The photocurrent and the Raman spectra of the tri-layer graphene were measured simultaneously to identify the exact position of the photocurrent and the ABA/ABC junction. We investigated the mechanism of the photocurrent by measuring the back-gate bias dependence of the photocurrent in vacuum. In general, there are two mechanisms for photocurrent without an external bias: the photovoltaic effect from Fermi energy difference and the thermoelectric effect from the Seebeck coefficient difference. The dominant mechanism of photocurrent generated between the metal electrode and the graphene channel has been found to be due to the photovoltaic effect [2]. Here, we studied the dominant mechanism of the photocurrent in the ABA/ABC stacking junction in tri-layer graphene. We calculated the Fermi energy and the Seebeck coefficients of ABA and ABC stacked tri-layer graphene in order to explain the mechanism of the photocurrent at the junctions. The Fermi energy as a function of the number of electrons is calculated by the density functional theory. The Seebeck coefficient is calculated by the density functional theory and the Boltztrap program. In addition, we measured the photocurrent at junctions between single- and bi-layer graphene for comparison.
References
[1] T. A. Nguyen, J.-U. Lee, D. Yoon, H. Cheong, Scientific Reports 4, 4630 (2014).
[2] M. Kim, H. A. Yoon, S. Woo, D. Yoon, S. W. Lee, and H. Cheong, Appl. Phys. Lett. 101, 073103 (2012).

Technological progress is determined to a greater extent by developments of novel materials or new combinations of known materials with different dimensionality and diverse functionality are created. We investigate the development of ‘hybrid&’ nanomaterials by utilizing graphene-based systems coupled with transition metal oxides (e.g. nano-/micro- manganese oxides i.e. a-MnO₂ [Mn(IV)] and a-Mn₃O₄ [Mn (II, III)]) which lays the groundwork for high-performance electrochemical electrodes for alternative energy owing to their higher specific capacitance, wide operational potential window and stability through charge-discharge cycling, environmentally benignity, cost-effective, easily processable and reproducible on a larger scale. To accomplish this, we strategically designed and synthesized by direct anchoring or physical adsorption of a-MnO₂ and a-Mn₃O₄ on the variants of graphene namely, oxidized surface functional groups of graphene oxide (GO) and reduced graphene oxide (rGO) via mixing dispersions of the constituents under mild ultrasonication resulting in four different combinations. This facile approach affords strong chemical/ physical attachment and expected to have coupling between the pseudocapacitive transition metal oxides and supercapacitive nanocarbons leading to enhanced activity/reactivity and durability. We used a range of complementary analytical characterization tools to determine structure and physical properties such as scanning electron microscopy (SEM) combined with energy dispersive x-ray spectroscopy (EDS), atomic force microscopy (AFM), x-ray diffraction (XRD), resonance Raman spectroscopy (RRS) combined with elemental Raman mapping, and transmission electron microscopy in conjunction with selected-area electron diffraction (SAED). All of these techniques reveal surface morphology, local (lattice dynamical) and average structure, and surface charge transfer due to the physically adsorbed manganese of synthesized hybrids that helps to establish microscopic structure-property-function correlations to further investigate their electrochemical supercapacitor properties. The work is supported by the author's start-up (SG) and NSF-KY EPSCoR (EPS-0814194 and 3048108525-l4-046) grants.

A novel hybrid nanocomposite has been developed using graphene-multiwalled nanotubes (MWNT) coated with electrically conductive polypyrrole (PPy). The nanocomposite fabricated by electropolymerization technique was used as an electrode for a supercapacitor device. Scanning electron microscopy (SEM) image of the synthesized graphene-MWNT-PPy nanocomposite showed a layered structure morphology compared to classic “cauliflower” structure of control PPy. The nanocomposite electrodes have been electrochemically characterized using cyclic voltammetry (CV) to test their capability as an electrode in supercapacitors. The specific capacitance of the graphene-MWNT-PPy nanocomposite was ~500Fg^-1 at a scan rate of 5mVs^-1, whereas that of PPy was ~241 Fg^-1 at 5mVs^-1 in 0.5M Na₂SO₄ suggesting two fold increase in specific capacitance of nanocomposite hybrid compare to that of control PPy electrode . Interestingly, an increase in overall concentration of graphene-CNT in PPy solution resulted in an increase in specific capacitance. The MWNT-graphene forms an interface during electrodeposition that increased the overall specific surface area, which was higher than PPy alone. After performing CV on each nanocomposite for 100 cycles, it was observed that the PPy electrode degraded more as compared to the graphene-MWNT-PPy nanocomposite. SEM images of the PPy electrodes that have undergone 100 cycles of CV revealed a “nanorod” like structure. However, graphene-MWNT-PPy nanocomposite electrode developed a “nanocube” like structure. The average width of each nanorod was ~110nm and the length was ~1.2mu;m, while the average size of nanocube was ~100nm.

Thermal decomposition of a 4H-SiC(0001) surface has been utilized for the synthesis of a large-scale monolayer graphene (MLG) sheet. However, the electronic properties of MLG are different from those of a freestanding graphene (FSG). In particular, electron mobility is far smaller than an ideal value. In this context, there has been growing interest on the roles of the so-called interfacial buffer layer formed between the MLG sheet and the underlying subsurface. While this layer has a graphene-like 2D honeycomb structure, it does not electronically behave as a graphene layer. This is attributed to the understanding that a part of the carbon atoms in the buffer layer are covalently bonded to the top Si atoms of SiC. The substantial degradation of graphene's mobility is related to the presence of dangling bonds of the interfacial Si atoms, not covalently bonded to the carbon atoms in the buffer layer. Since the resulting charge states should affect the electronic properties of the MLG sheet, it is important to understand and control the charge states on the interface. Recently, hydrogen-intercalation method has been proposed to tailor quasi FSG by breaking and terminating the interfacial bonds [1, 2]. No experimental studies, however, have been made on the charge states of the interfacial buffer layer in a nanoscale. In this study, we have investigated the interfacial charge states with atomic precision by using noncontact scanning nonlinear dielectric microscopy (NC-SNDM). NC-SNDM can resolve topography, dipole moment distribution, and a potential of a surface with an atomic resolution through the measurement of the tip-sample capacitance [3].
We performed NC-SNDM imaging of graphenes formed on 4H-SiC(0001) with and without hydrogen-intercalation. Before the hydrogen-intercalation, the topography showed superperiodicity arising from a 6radic;3×6radic;3-R30° structure of MLG on a 4H-SiC(0001) surface. The observed potentials ranged from 0.3 V to 0.4 V. After intercalation, in sharp contrast, many flat islands arose in the topography. Their heights were 0.17nm higher than that of the coexisting superperiodic structure. This implies that the buffer layer was relaxed after the hydrogen-intercalation, because the height difference almost equals to the difference between the interlayer distance of graphene sheets and the length of the Si-C bonds. These flat areas have an almost null potential resulting from the termination of polar interfacial dangling bonds by atomic hydrogen. On the other hand, in a higher resolution image, we find tiny and shallow depressions on the flat islands. These depressions had slightly higher potentials than the surrounding flat areas, which indicate a small part of the interfacial bonds persisted even on the hydrogen-intercalated areas in our samples.
References:
[1] C. Riedl et al., Phys. Rev. Lett. 103, 246804 (2009).
[2] F. Maeda et al., Phys.Rev. B. 88, 085422 (2013).
[3] Y. Cho and R. Hirose, Phys. Rev. Lett. 99, 186101 (2007).

Graphene is an interesting materials system, which has been intensively focused on by researchers in the last decade. There are two isotropic Dirac cones in the electronic band structure, which lead to many novel electronic properties. The transport of Dirac carriers in graphene has been intensively studied. Many theories for the scattering mechanism of Dirac carriers for intrinsic and extrinsic graphene have been reported. There are measurements of the average scattering relaxation time of graphene Dirac carriers, as well as fast laser measurement of carrier relaxation time. However, few reports have been published regarding the measurement of carrier scattering relaxation time as a function of carrier energy, or the detection of carrier scattering mechanisms through analysis of measurements taken on transport of graphene Dirac carriers.
Our present work has developed a new general methodology to detect the carrier scattering relaxation time as a function of carrier energy, which can be then used to infer the carrier scattering mechanism for different temperature range. We have used this methodology to study the relaxation time as a function of carrier energy for the graphene substrated o SiO₂. We have then inferred the scattering mechanism at different temperatures using these results for the SiO₂ substrated graphene. We have found that in cryogenic temperature range, short-range interaction coming from point defects and vacancies are dominant; when the temperature is increased, the strength of long-range Coulomb interaction scattering coming from charged impurities on SiO₂ are increased. At room temperature, the short- and long-range scattering become almost equal. We have also found that the degree of asymmetry for the scattering time between electrons and holes, which is ~1 at about 20 K, increases with temperature, and becomes ~2 at about 300 K.
Our newly developed methodology can be applied not only in graphene, but also to other materials systems, especially to materials that have a simple band structure, including surface states of topological insulators, single layered MoS₂, WS₂, and black phosphorus.

Multi-layer graphene (MLG) is a promising functional material for mechanical applications due to its exceptionally anisotropic nano-structure comprised of the strongest carbon monolayers, graphene. Because most mechanical characterizations of graphene and MLG have been carried out under static or quasi-static conditions, the dynamic mechanical behavior of MLG largely unexplored. For deformation at a high strain rate (HSR) over 10³/s, various effects including dynamic stress localization become important, and the static mechanical characteristics cannot be extrapolated to the high strain rates. However, the lack of HSR evaluation techniques for nanomaterials including MLG has hindered the quantitative investigation of their intrinsic HSR characteristics. To address this challenge, we developed a micro-ballistic technique, so called advanced laser-induced projectile impact test (α-LIPIT). In this approach, a single 3.72 µm diameter silica sphere is propelled as a micro-bullet via laser ablation of gold, and impacts onto a micro-sample at a supersonic speed with a high aiming accuracy less than 1.1 degree deflection.
We present the dynamic responses of MLG membranes, in a range of thicknesses from 10 to 100 nm, observed from the high-speed penetration of the MLG membrane in α-LIPIT. The MLG membranes showed characteristic crack propagation, correlating to the two preferential directions, the zigzag and armchair directions. The dark-field TEM study of a thin MLG membrane (~10 nm thick) demonstrated fine-scale moiré fringes, resulting from extensive folding near the impact origin, due to rapid elastic snapback after penetration. By measuring the speed of a micro-bullet before and after penetration, we explicitly determined the energy absorbed during MLG penetration. In order to compare the α-LIPIT results with macroscopic ballistic results of steel, aluminum, and Plexiglas, we normalized the penetration energy by the MLG mass under the projected area of a projectile, which is the specific penetration energy. For the net specific penetration energy, excluding the kinetic energy transfer to the debris, MLG membrane could dissipate about 10 times more energy per mass than what steel does at a range of impact speeds (600-900 m/s). Our results revealed that MLG&’s superior speed of sound (~22 km/s) and the exceptional structural anisotropy enables MLG to exhibit excellent impact energy delocalization, which means material far beyond the direct impact region can also take kinetic energy from the projectile. We also confirmed the correspondence of our α-LIPIT results to macroscopic ballistic test results on typical isotropic materials: amorphous PMMA and polycrystalline gold.

With the correction of lens aberrations in scanning transmission electron microscopy (STEM) direct, real space imaging of defect configurations in graphene has become possible, below the threshold for most knock-on damage. Simultaneous electron energy loss spectroscopy (EELS) allows chemical bonding and optical properties to be probed [1]. While it has often been assumed that the spatial resolution of EELS in the optical regime would be many nanometers, several examples have been found of unexpectedly high, even atomic resolution [2]. A new theoretical framework has been developed to simulate images from individual valence excitations by combining density functional theory and dynamical scattering theory [3]. This combined experimental/theoretical technique is effectively an atomic-resolution variant of light-based spectroscopies, which have low spatial resolution, and promises therefore to be a powerful probe of the contribution of atomic-scale structures and defects to bulk optical and electronic properties.
[1] W. Zhou, M. Kapetanakis, M. Prange, S. T. Pantelides, S. J. Pennycook, and J.-C. Idrobo, "Direct Determination of the Chemical Bonding of Individual Impurities in Graphene," Phys Rev Lett 109 (2012) 206803.
[2] Wu Zhou, Jaekwang Lee, Jagjit Nanda, Sokrates T. Pantelides, Stephen J. Pennycook and Juan-Carlos Idrobo, Nature Nanotechnology, 7 (2012) 161.
[3] M. P. Prange, M. P. Oxley, M. Varela, S. J. Pennycook, and S. T. Pantelides, Phys Rev Lett 109 (2012). 246101.

The presence of long range surface plasmons near an emitter can enhance the radiative recombination rate in an emitter [1] resulting in the enhancement in photoluminescence emission from semiconductor light emitters [2]. Recently there has been active interest in the generation of light from graphene based materials. Graphene oxide in particular can be tailored to emit light over a wide range of visible to the near-infrared wavelength by controlling the reduction of graphene in the presence of oxygen and by controlling the ratio of the sp² and sp³ cluster [3]. In the present work we present the broadband emission from graphene oxide (GO) and reduced graphene oxide (rGO). We demonstrate that the photoluminescence from graphene oxide can be enhanced due to the coupling of the emitted photons with resonant surface plasmons.
The GO was prepared using modified Hummer&’s method [4]. Partial reduction of GO (rGO) was obtained chemically using hydrazine monohydrate. The rGO sample was purified by repeated filtration and washing with ethanol. The samples for optical property measurement were prepared by drop casting of GO dispersion on quartz substrate. This produces layered structure of GO and rGO thin film with normal orthogonal to the substrate.
The structure was characterized using transmission electron microscopy and corroborated using Raman Spectroscopy. Multilayer GO and rGO films were formed on the substrate with layer thickness varying from 235-200nm. The structural properties were further investigated using Raman spectroscopy which showed the presence of sp² and sp³ clusters in the graphene oxide composite. It has been observed that the crystallite size of sp² hybridized domains for GO is ~ 21 nm and the size is reduced to ~17 nm for the case of rGO.
The photoluminescence (PL) emission study of both graphene oxide (GO) and partially-reduced graphene oxide (rGO) have been investigated. It has been observed that the GO has broadband emission from green to near infrared range and upon reduction rGO shows blue PL emission. The broadband PL emission is due to the recombination of electron-hole pair in sp² domain embedded within sp³ matrix. The broadband PL emission also suggests the existence of various sizes of sp² domain within the same matrix. Further, PL emission from GO in the presence of Au metal thin film has been investigated. It has been observed that the entire broadband emission from GO in the green to near infrared wavelength region is enhanced significantly at room temperature. The Au-GO interface exhibits surface plasmon resonance in the visible wavelength region and is responsible for over 10 fold enhancement in the photoluminescence at ~2.36 eV.
[1] A. Neogi et al, Optics Letters, 30, 94 (2005)
[2] J. Lin, et al Appl. Phys. Lett., 97, 221102 (2012)
[3] G. Eda and Manish Chhowalla, Adv. Mater., 22, 2392 (2010).
[4] W. Hummers et al, J. Am. Chem. Soc., 80, 1339 (1958).

Information on carrier diffusion length is necessary to optimize electronic and optoelectronic devices constructed of graphene. Photoelectromagnetic (PEM) investigations are a known method of determining this parameter in semiconductors. The aim of this paper is to present applicability of contactless PEM method in investigations of graphene. When a graphene sample is illuminated by a circular spot of radiation, free electrons and holes are photogenerated in the illuminated spot and diffuse in all directions in the layer. In external magnetic field perpendicular to graphene surface the diffusing carriers are deflected by Lorentz force and the circulating PEM current flows. Under amplitude-modulated illumination of a sample the PEM current varies, and consequently the changing magnetic flux, caused by it, can induce a measurable voltage in suitably placed pick-up coils. The PEM signal depends on external magnetic field and on the gradient of concentrations of photogenerated carriers. It increases with the photogeneration rate, i.e., with illumination intensity and quantum coefficient of photogeneration of free carriers. PEM signal is also dependent on the carrier diffusion length, which is proportional to the square root of carrier mobility multiplied by their lifetime. As the carrier mobility in graphene is high, it is possible to measure PEM signal even in case of very short carrier lifetimes.
Theoretical calculations provide conclusion that there are at least three possible methods of using contactless PEM investigations for determining carrier parameters in graphene. Knowledge of inductive coupling between PEM current and the used pick-up coils is not necessary in these methods. They are based on fitting of experimental results with theoretical dependence of measured signal on the following experimental variables: external magnetic field, frequency of illumination chopping, and profile of laser beam incident upon a sample (changed e.g. by using refractive beam shaper). We present comparison of the results obtained for graphene films deposited on different substrates.
The presented contactless and nondestructive PEM method of investigations is suitable for rapid and simple inspection of graphene. It enables to avoid possible contaminations of the sample during contact preparation, as well as the difficulties in preparation of contacts. We anticipate our paper to be a starting point for mapping the recombination non-uniformity in graphene samples by scanning them with a light probe and performing the contactless PEM measurements.
This work was supported by the National Science Centre (Poland) project no. DEC-2012/05/B/ST7/01198.

Graphenide solutions, solutions of negatively charged graphenes were prepared from a variety of materials such as graphite, graphite nanofibers and carbon soot originating from food waste. Full exfoliation of the starting material, down to the single layer level is demonstrated by Raman and AFM with height distribution peaked on single layers. Conducting films from some of those solutions will be described, showing good performance.

In this study, we utilize graphene&’s unique combination of IR-transparency and electrical conductivity as an electrode material for pyroelectric infrared sensing. Our work takes a different approach from recent studies, all of which have considered ways to improve graphene's absorption for bolometric infrared detection. We integrate graphene electrodes with a polyvinylidene fluoride (PVDF) active layer to develop a flexible detector capable of achieving a specific detectivity comparable to other PVDF sensors in literature. More importantly, we show that the use of graphene as an IR-transparent electrode enables an order of magnitude improvement in operating frequency compared to other pyroelectric sensors. We further demonstrate that our device structure is compatible with pixelation, and capture several thermal images with minimal crosstalk. Our work thus highlights a new route for the utilization of graphene for faster and flexible thermal imaging applications.

Understanding and quantifying non-covalent interactions are of utmost importance for diverse disciplines like medicinal chemistry, supramolecular engineering, crystal packing, ligand-protein interaction, catalysis, molecular recognition to name a few. Nanoelectronic sensors based on one- and two-dimensional nanomaterials with their extremely large surface-to-volume ratios and unique electronic properties, are an ideal platform to study such non-covalent interactions in real-time. In particular, chemical vapor sensing can be a great tool to reveal fundamental molecule-nanomaterial binding affinities. However, nearly all existing nanoelectronic vapor sensors based on charge detection mechanism are plagued by intrinsically slow dynamics of interface trapped charges and defect-mediated chemisorptive charge transfer processes. As a result, sensor response is limited to tens to hundreds of seconds, which is a long known bottleneck for studying the dynamics of molecule-nanomaterial interaction and for many applications requiring rapid and sensitive response. Here, we report a fundamentally different sensing mechanism based on molecular dipole detection enabled by a pioneering graphene nanoelectronic heterodyne sensor. The dipole detection mechanism is confirmed by a plethora of experiments with vapor molecules of various dipole moments. Rapid (down to ~0.1 s) and sensitive (down to ~1 ppb) detection of a wide range of vapour analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronic sensors. Further, we demonstrate rapid real time detection of a mixture of vapor analytes with comparable performance to commercial flame-ionization-detector. More importantly, the fast, sensitive and reversible heterodyne sensor response allows us to study temperature dependent desorption rate kinetics for different organic vapor molecules on graphene, yielding corresponding binding energies. We show that the sensor response is related to the orientation and geometry of vapor molecule upon adsorption on top of graphene. Our work not only provides a fundamental understanding of molecule-nanomaterial interaction at high frequencies, but also leads to a platform technology for highly integrated, rapid, and sensitive biological/chemical sensors.

The use of membranes for gas permeation and phase separation offers many distinct advantages over other more energy dependent processes. The operational efficiencies of these membranes rely heavily on high gas permeability. Here, we report membranes with increased permeability, synthesized from a polymer nanocomposite that incorporates graphene and polydimethylsiloxane (PDMS). These graphene-PDMS nanocomposite membranes were able to significantly enhance the gas permeation of N₂, CO₂, Ar and CH₄ from pristine PDMS membranes. This is achieved by creating interfacial voids between the graphene flakes and polymer chains, which increases the fractional free volume within the nanocomposites giving rise to an increase in permeation. We show the enhanced nanocomposite membranes with their significant increase in gas permeation are able to provide substantial improvements to sensing and phase separation applications. This study offers a new area of research for graphene-based nanocomposites.

Graphene-enhanced Raman scattering (GERS), a new Raman enhancement phenomenon discovered in recent years that uses graphene as the substrate for enhancing Raman signals of adsorbed molecules, can produce clean and reproducible Raman signals of molecules. Compared to conventional Raman enhancement techniques such as surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS), whose Raman enhancement mechanisms are mainly due to the electromagnetic mechanism, GERS mainly relies on the chemical mechanism for Raman enhancement, and therefore obtains the unique property of molecular selectivity - the enhancement factors of different molecules are very different, ranging from ~1 to almost 100. In this work, we measured the GERS effect of molecules with different properties and discuss the main factors of molecular selectivity -molecular energy levels and molecular structures. In particular, the factor of molecular energy levels can be explained by the perturbation theory of Raman scattering. The factor of molecular structures, which include molecular symmetry, substituents and the configuration of rings in the molecules, can be explained by group theory and the charge-transfer between molecules and graphene. Both these factors of molecular energy levels and structures suggest that remarkable GERS enhancement requires strong molecule-graphene coupling and thus effective charge transfer between molecules and graphene, which is further experimentally supported by the light absorption change of molecules when in contact with graphene. This work is important for the fundamental study of molecule-graphene coupling and the chemical mechanism in Raman enhancement, as well as the applications in chemical detection, medical and biological technologies.

Layered number and stacking order of graphene are strongly related to their electrical and optical properties. Evaluations of these atomic-level characteristics are paid much attention in terms of both scientific and technology interests. Raman spectrophotometry is general method for evaluation of graphene, however, this technique obtains not the atomic-level local structure but an large-area information which is limited by wavelength of light. High-resolution transmission electron microscopy (TEM) is usually applied for observing the atomic-level structures. The graphene is, however, composed of carbon which is easily received electron-irradiation damage, especially the knock-on damage [1]. The knock-on damage shows characteristic energy of threshold and is reduced by applying the relatively low-voltage-electron beam. The atomic-level observation, however, is difficult at the low-voltage-electron beam because of some additional aberrations such as chromatic and high-order aberration. To realize the low-voltage atomic-level characterizing technique is needed.
Diffractive imaging is one of the high-resolution imaging techniques. A specimen image is calculated by a computer processing of a diffraction pattern [2, 3]. We developed the dedicated microscope which was based on a conventional scanning electron microscope (S-5500, Hitachi High-Technologies Corp.) for the diffractive imaging and demonstrated the atomic-level observation of carbon nanotube [4]. To obtain the quantitative diffraction pattern for the diffractive imaging, the diffraction pattern is recorded on the imaging plate by applying a film-loader system for the TEM below the objective lens.
The small-aperture stage (about 75 nm in diameter) is prepared for recording limited-area information of the graphene. By comparing the diffraction pattern to simulation quantitatively, approximately layered-number (2 or 3 layers) and tilt angle to an electron beam (3 to 4 degrees) become clear. By means of the diffractive imaging,
the atomic-level structure including stacking order of graphene is obtained.
We conclude that combining diffraction pattern with imaging based on dedicated microscope characterize the graphene at the atomic level. We will present out recent analyzing results obtained using this dedicated microscope.
Part of this work was supported by the Japan Science and Technology Agency.
REFERENCES
[1] L. Reimer and H. Kohl, Transmission Electron Microscopy, 5th ed. (Springer, New York. 2008)
[2] R. W. Gershberg and W. O. Saxton, Optik, 34 (1971) 275.
[3] J. R. Fienup, Appl. Opt., 21 (1982) 2758.
[4] O. Kamimura, et al., Appl. Phys. Lett. 98 (2011) 174103.

It is well-known that the orientation of the reinforcing elements in polymer-based composites plays a vital role in controlling mechanical properties. A number of different techniques can be employed to quantify the orientation of the reinforcement in composites. There is so far, however, no generally-accepted way of quantifying the orientation at the nanoscale of plate-like graphene flakes in a nanocomposite material. In this work, polarized Raman spectroscopy has been employed to characterize, first of all, the orientation of transverse sections of monolayer graphene on a copper substrate. Well-defined Raman spectra can be obtained from these transverse sections even though they are only one atom thick, because of the very strong resonance Raman scattering from graphene. It is found that the intensity of scattering of the Raman band is dependent of the axis of laser polarization when the laser beam is parallel to the surface of the graphene plane and it has been demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) can be used to quantify the orientation of this graphene monolayer. Based on this approach, polarized Raman spectroscopy was used to quantify the level of orientation of the graphene flakes in variety different graphene-based materials and nanocomposites. It is demonstrated further how it is possible to relate the degree of graphene orientation to stress transfer to the reinforcement in nanocomposites and, in particular, determine the Krenchel orientation factor for these plate-like fillers from the ODF. It is then shown how his approach can be employed to estimate the effective Young's modulus of the reinforcement in graphene-based nanocomposites and nanocomposites based upon other 2D materials.

Bulk graphene based and other sp² carbon materials prepared using different methods and with many exceptional properties have been studied for various green energy applications. In this talk, the studies for their various energy storage and conversion applications will be presented, and these include that for supercapacitor, actuator and photovoltaic application studies. 5 recent main references Y. Chen, et al, J. Am. Chem. Soc., 2013, 135, 8484-8487. Y Chen, et. al, Sci. Rep., 2013, 1408. Y. Chen, et al, Energy Environ. Sci., 2013, 6, 1623-1632. Y. Chen, et al, Adv Mater, 2013, 25, 2224-2228. Y Chen, et. al, Acc Chem Res, 2012, 45, 598-607.

Graphene has recently attracted great deal of attention by scientists due to its outstanding optical, electrical and mechanical properties and is therefore highly promising for various applications. Also, semiconducting quantum dots (QDs) with easily tunable optical and electrical properties, demonstrated their potential already in optoelectronic application such as photovoltaics.
In this work, the optical properties of CdSe QDs, as well as the transparency and high carrier mobility of graphene are combined in QD decorated reduced graphene oxide. The hybrid material is formed by dedicated thiol- functionalization of graphene followed by a direct chemical attachment of QDs [1, 2].
These hybrid nanocomposites have been successfully utilized in hybrid photovoltaic devices exceeding state of the art power conversion efficiencies (PCEs) and showing a PCE of 4.2% [3]. The graphene framework improves the charge extraction and transport within the inorganic-organic hybrid film as well as the open circuit voltage from 0.55V to 0.72V significantly. Utilization of graphene-quantum dot nanocomposites in further optoelectronic applications are very promising and are currently under investigation.
[1] Chuyen V. Pham, Michael Eck and Michael Krueger, Chem. Eng. J., 231, 146-154 (2013).
[2] Ch. V. Pham, M. Krueger, M. Eck, St. Weber, E. Erdem Appl. Phys. Lett. 104, 132102 (2014).
[3] Eck, Michael; Pham, Chuyen; Züfle, Simon; Neukom, Martin; Seszlig;ler, Martin; Scheunemann, Dorothea; Borchert, Holger; Erdem, Emre; Weber, Stefan; Ruhstaller, Beat, Krüger, Michael; Phys. Chem. Chem. Phys 16, 12251-12260 (2014).

Large-scale production of low-cost, highly-stable, metal-free two-dimensional (2D) crystals now become an important challenge for many applications including energy conversion and storage. Here, we demonstrate an efficient and eco-friendly method for the scalable synthesis of 2D crystals and their applications for energy conversion and storage. The first approach involves edge-selective functionalization of graphite into graphene nanoplatelets (GnPs) by a simple mechanochemical ball-milling process [1]. The approach is an important step toward high-yield exfoliation of three-dimensional graphite into two-dimensional crystals with minimal basal plane distortion. The second approach is scalable synthesis of 2D crystals with periodic holes and heteroatoms via polycondensation between A₃ and Bshy;₃ monomers. The resultant 2D crystals displayed outstanding electrochemical performance [2]. Our findings suggest that the 2D crystals can be simply prepared and conveniently used as base materials for a wide range of applications from wet chemistry to device applications [3].
References
[1] (a) Choi, et al., Chem. Commun.46, 6320-6322 (2010); (b) Bae, et al., ACS Nano 5, 4974-4978 (2011); (c) Jeon, et al., Proc. Nat&’l Acad. Sci., USA, 109, 5588-5593 (2012); (d) Jeon, et al., Adv. Mater. 25, 6238 (2013).
[2] (a) Jeon, et al., J. Am. Chem. Soc.135, 1386-1393 (2013); (b) Jeon, et al., Sci. Rept. 3, 2260 (2013); (c) Jeon, et al., Sci. Rept. 3, 1810 (2013); (d) Chang, et al., J. Am. Chem. Soc. 135, 8981-8988 (2013).
[3] (a) Ju, et al., Energy & Environ. Sci. 7, 1044-1052 (2014); (c) Ju, et al., Adv. Mater. Online ASAP (2014); (d) Kim, et al., ACS Nano 8, 2820-2825 (2014); (e) Jung, et al, Angew. Chem. Int&’l. Ed. 126, 2430-2433 (2014).

It has been envisioned that high-performance thermal energy storage technologies will play a broad and critical role in the sustainable use of energy in heating and cooling applications, including building and vehicle heating and cooling, solar energy harvesting, and thermal management of electrochemical energy storage and electronic devices. Thermophysical energy storage based on phase change materials (PCMs) is one of the technologies being actively pursued. One major barrier that is currently preventing the broad adoption of many PCM-based technologies, however, is the very low thermal conductivity of available PCMs, which significantly limits the power capacity. Increasing the PCM thermal conductivity without affecting other performance criteria such as energy density or thermal cycling stability has been the focus of much research in recent decades, but only incremental performance advancements have been realized.
Ultrathin-graphite foams (UGFs) made of pure sp2-hybridized carbon have a low volume fraction of ~1.2. vol% while the UGF struts retain the high basal-plane solid thermal conductivity of graphite (~1000 W/m-K), and the three-dimensional interconnected network overcomes the thermal interface resistance issue found in van der Waals-bonded graphene and CNT networks.^[1] Here we report an experimental investigation to show that embedding UGF with volume fractions as low as 0.8-1.2 vol% in a PCM can increase thermal conductivity by up to 18 times, with negligible change in the PCM melting temperature or mass specific heat of fusion.^[2] The thermal conductance enhancement is found to increase the rate of PCM melting even in an experiment setup with considerable parasitic loss. The electrical resistance of the UGF-PCM composites changed less than 2% over 100 solid-liquid phase transition cycles, indicating mechanical stability during thermal cycling. The increase in thermal conductivity, thermal cycling stability, and applicability to a diverse range of PCMs suggests that UGF composites are a promising route to achieving the high power capacity targets of a number of thermal storage applications, including building and vehicle heating and cooling, solar thermal harvesting, and thermal management of electrochemical energy storage and electronic devices.
References
[1] Michael T. Pettes, Hengxing Ji, Rodney S. Ruoff and Li Shi, Nano Lett., 2012, 12, 2959-2964.
[2] Hengxing Ji, Daniel P. Sellan, Michael T. Pettes, Xianghua Kong, Junyi Ji, Li Shi and Rodney S. Ruoff, Energy Environ. Sci., 2014, 7, 1185-1192.

The unique stereochemistry and functional groups in graphene oxide render its excellent mechanical strength, electronic structure, and molecular-level chemical sensing in graphene oxide based state-of-the art electronic devices. We report a unique fabrication technique to synthesize large scale reduced graphene oxide (RGO) thin film by pulsed laser deposition (PLD) process. The synthesized thin films show a remarkable charge carrier mobility of 655 cm² V^-1 sec^-1, which can be tuned by controlling the process parameters. X-ray diffraction, Raman spectroscopy, hall measurements, and resistance vs temperature studies of the thin film were employed to characterize the structural properties, optical-phonon vibrational modes, charge carrier mobility, and variable range hopping in RGO thin films. This research attempts to correlate the XRD and micro Raman spectroscopy to explain the factors influencing the electrical properties of functionalized RGO. We can conclude that the electrical properties are governed via three mechanisms: (a) conversion of the sp² -hybridized state to sp³, (b) intensity of the 2D scattering Raman mode, and (c) low temperature variable range hopping. The intensity and FWHM of the second order defect peak in Raman spectroscopy play an important role in charge carrier mobility of the thin films. The sp² conjugated graphene network was partially restored during the PLD process, which would result in the transition from insulator to semimetal in reduced graphene oxide. The charge transfer in RGO sheets might occur via a variable range hopping mechanism which involved the consecutive inelastic tunneling processes of charge carriers between two localized states. XRD analysis indicates an average interplanar distance of 0.39 nm in the grown thin films, which allowed the charge carrier hopping process between layers in RGO thin films. The highest charge carrier mobility achieved in some of the thin film samples can also be attributed to the reduced scattering centers by increased size of sp² clusters and thereby reducing the defect density. Due to the thermal generation of electronminus;hole pairs, the resistance of all the samples decreased under vacuum as the temperature was increased, which revealed their intrinsic semiconductor nature. The as-deposited RGO sheets could cover grain boundaries and wrinkles uniformly, thereby charge carriers underneath the film could effectively cross over the grain boundaries with the assistance of RGO sheet via variable-range hopping process through the highly conductive, nanometer-sized graphene islands thereby increasing its charge carrier mobility. This novel production of large area reduced graphene oxide thin film by PLD technique can prove beneficial for high mobility electronic devices and open the road map for further extensive research in this versatile material. This research was funded by National Science Foundation, Grant Numbers: DMR-1126375, DMR-0907037, and Missouri State University.

Single nanometer-sized pores in suspended graphene membranes have been used to detect and control DNA molecules, and could be the basis for next-generation single-molecule DNA sequencing devices. However, traditional fabrication of graphene nanopores via electron beam drilling is an expensive and low-yield process that has yet to reliably produce pores smaller than 3 nm in diameter. Here we demonstrate a novel method of creating pores in graphene membranes already wet in solution. A series of voltage pulses applied across the membrane nucleates and enlarges a nanopore with sub-nanometer control, allowing rapid, inexpensive, and precise fabrication of graphene nanopores that readily exhibit DNA translocations.

Graphene has gained considerable interest as a filler for composite materials because of its intrinsic mechanical, thermal, and electrical properties. Its lack of solubility in virtually all solvents has led to the common practice of either using graphene oxide or reduced graphene oxide in the place of pristine graphene sheets. Both of these graphene derivatives, however, have significantly reduced properties when compared to the pristine material. Here, instead of viewing the insolubility of pristine graphene as an obstacle to be overcome, we utilize it as the means to create oil/water emulsions, with graphene stabilizing the spheres as a two-dimensional surfactant. These emulsions are then used as the frameworks to make a wide variety of foam composites in a fundamentally new approach, as it does not require reduction, oxidation, chemical functionalization, surfactants, high boiling solvents, or the input of large amounts of heat or mechanical energy.
Many different monomers have been used in this system to impart different desired properties in the composites: from flexible samples made of polyisoprene to ultra-low density samples made of poly(butyl acrylate). The most investigated system, and the focus of this talk, is styrene/water. These composites have shown bulk conductivities up to ~7 S/m, as well as compressive moduli up to ~100 MPa and breaking strengths of over 8.3 MPa (1200 psi), all with densities as low as 0.25 g/cm³. Furthermore, studies have been performed to demonstrate the tunability of the spherical cavity size, conductivity, mechanical strength, and density. Potential applications of these materials, such as high performance construction materials, ultra capacitor electrodes, and catalyst supports, are also discussed.

A designer polysiloxane-based stabilizer for graphene was synthesized by a hydrosilylation reaction. This stabilizer was used as the polymer matrix to prepare a highly conductive polymer film. To prepare the stabilizer, 1-ethynylpyrene was grafted to the backbone of a poly(dimethylsiloxane)-co-(methylhydrosiloxane) random copolymer with a SiH:ethynylpyrene ratio of 1.0:1.3. Pyrene groups of the resulting copolymer stabilized graphene in chloroform through the π-π interactions. The graphene dispersion in chloroform was cast to form a graphene/polymer film. SEM and TEM images confirmed the homogeneous distribution of the graphene sheets in the film. The conductivity of this film containing 4 wt% of graphene was measured to be 220 S/m after the removal of unbound polymers using a simple separation technique. This is the first case of melt-processable conductive graphene/polymer film reported in the literature. Later, the ratio of SiH:ethynylpyrene was changed to 1.7:1.0 for the copolymer to self-crosslink at 110 #8304;C. This product was used for direct production of a conductive graphene/silicon elastomeric composite. The crosslinking reaction was observed by FT-IR spectroscopy and the network formation was confirmed by swelling and deswelling of the product.

The well known high intrinsic mobility of charge carriers in Ge would make it an excellent substitute for Si as a FET channel material, if only the Ge surface could be stabilized. The poor quality of the interfacial layer, caused by an unstable native oxide, has so far generally prevented this use of Ge. Ge surface passivation schemes with various high-k dielectrics have leaded to an improvement in the interfacial layer. Here we show that graphene can serves as an excellent passivator for Ge. High-quality graphene is grown directly on Ge (100), (111), and (110) using atmospheric-pressure CVD and methane precursors. The subsequent oxidation kinetics of Ge (001) are compared for graphene directly grown on Ge and for graphene transferred to Ge using an established transfer technique. X-ray photoemission spectroscopy studies show that for graphene grown on Ge(001) the interface is oxide-free and remains so over long periods of time. For graphene transferred to Ge(001) the interface contains stoichiometric and substoichiometric oxides. The thickness of these oxides increases with time, but quite slowly. Using spatially resolved XPS, we propose a model of diffusion limited oxidation initiated at edges of the graphene.
Supported by DOE

Recently, we succeeded in obtaining a cutting-edge graphene nanocomposite in which reduced graphene oxide (rGO) layers are sandwiched by periodic mesoporous silica (PMS) films with their mesochannels perpendicularly aligned toward rGO layers. Here we'll review and report the latest progress on the novel graphene composite regarding the control of film thickness, the pore size tuning, etc. and discuss their possible applications such as in environmental sensing.

The most common method for fabricating large area graphene is CVD growth on copper foils. Long term performance and stability of these sheets will be crucial to their applications to transparent conductors, liquid crystal displays, touch screens and solar cells. Herein we report on changes in the morphology and Raman spectra of such sheets on polycrystalline Cu foil. The Cu foil develops a thin semiconductor Cu₂O layer underneath the graphene and Cu. AFM probing of a sample over 4 years reveals that nano-scale vacancies/holes in the graphene have self-healing tendency due to relaxation produced by the buffering of Cu with Cu₂O. The Raman spectra for these samples show increases in intensity of both the graphene G and 2D bands by factor of 10³ after four years of aging. These results can help efficient characterize as-grown graphene directly on Cu and develop high quality large area single layer graphene sheets.

Recent progress in exfoliation techniques has set the foundations for the manufacturing of essentially any given layered bulk material in the monolayer limit. The "standard" production and manipulation of exfoliated two-dimensional (2D) monolayers has opened up the possibility to shuffle the different layers in composite materials with new emerging properties. Among 2D materials, graphene is highly promising for a wide variety of nanotechnological applications. Freestanding 2D materials spontaneously ripple inducing (local) changes into the material&’s properties. Consequently, porous supports such as 2D covalent organic frameworks (COFs) open up the possibility of creating graphene-COF composite nanodrums.
We studied the dynamics and electronic properties of graphene monolayers supported on different 2D COFs using tight-binding density functional theory (DFTB) and Born-Oppenheimer DFTB molecular dynamics. The COF pore size was increased with the appropriate choice of connectors and linkers thereby increasing the "freestanding" portion of graphene above the pore (connectors: boroxine, hexahydroxybenzene or hexahydroxytriphenylene; linkers: phenyl, biphenyl or triphenyl). Principal component analysis and power spectra were used to identify graphene&’s characteristic rippling modes and frequency at the COF pores as well as to guide the sampling of relevant structures for the evaluation of electronic properties.

As an earth abundant and second-best conductive metal, copper is expected to exhibit promising plasmonic properties. It is a good candidate to replace high cost metals such as silver and gold as a plasmonic material in many applications. However, copper devices quickly degrade as Cu₂O and CuO form at the surface when exposed to moist environment. Graphene, on the other hand, is a biocompatible and chemical stable two-dimensional semimetal. A hybrid system with the combined properties of copper and graphene is of great interest in the field of plasmonics. We report a two-step fabrication method to generate graphene-Cu structures with stable and intense localized surface plasmon resonance (LSPR) performance. Nanosecond pulsed laser was employed to generate ultra-high pressure for imprinting nanostructures of CVD grown single layer graphene on copper thin film. Arrays of holes and trenches with different dimensions down to 30 nm were instantaneously obtained after laser shock, and were imaged by scanning electron microscopy and atomic force microscopy. The intense local field of the hybrid system was probed by a Scattering near-field scanning optical microscopy (s-NSOM). Molecular sensing capability of the system was demonstrated by employing R6G as a probing molecule. The proposed method is capable of generating graphene-copper plasmonic structure with tunable gaps, which is convenient, ultra-fast and economic.

We study the Li clustering process on graphene and obtain the geometry, nucleation barrier and electronic structure of the clusters using first principles calculations. We estimate the concentration-dependent nucleation barrier for Li on graphene. While the nucleation occurs more readily with increasing Li concentration, potentially leading to the dendrite formation and failure of the Li-ion battery, the existence of the barrier delays nucleation and may allow Li storage on graphene. Our electronic structure and charge transfer analyses reveal how the fully-ionized Li adatoms transform to metallic Li during the cluster growth on graphene.

We investigate interaction of water droplets with different nanomaterials : Graphene and its allotropes - Biphenylene, Graphyne, Graphdiyne, Hexagonal and Cyclic graphene. Water microdroplets containing nanosheet are shown to spontaneously segregate into sack-cargo nanostructures upon drying. These cargo-filled nanosacks are promising for many potential applications where nanoscale materials should be isolated from the environment or biological tissue. Because of different topology of different nanomaterials, we observe different interaction. In addition, we investigate interface of nonmaterials with water droplet and its usage for water filtration.

Metal nanostructures exhibit unique combination of physical and chemical properties such as large absorption and scattering cross-section, high sensitivity to local dielectric environment and enhanced electric field at the surface that promises more effective diagnostics and therapy compared to conventional medicine for combatting against complex diseases such as cancer. Multifunctional hybrid nanomaterials with enhanced therapeutic efficiency at physiologically safe dosages for externally triggered, image-guided therapy are indispensable for nanomedicine. Here, we demonstrate a novel class of multifunctional hybrid nanopatches comprised of graphene oxide and gold nanostars for enhanced photothermal effect and image-guided therapy. The hybrid nanopatches with tunable localized surface plasmon resonance (LSPR) into the near infrared therapeutic window (650-900 nm) were realized using a biofriendly method that obviates the need for toxic shape-directing agents. The internalization of intact nanopatches into epithelial breast cancer cells was confirmed by Raman imaging, transmission electron microscopy and inductively coupled plasma mass spectrometry. It appears that the amphipathic nature and the large surface area of the graphene oxide acting as a soft, flexible, and biocompatible intracellular carrier for the in situ grown plasmonic nanostructures provided long term biocompatibility with extremely low cytotoxicity. Apart from remarkably improved photothermal efficiency compared to either of the components at very low dosages of the hybrids (10 µg/ml of GO) and using low laser power (0.75 W/cm^-2), the hybrid nanopatches exhibit strong Raman scattering making them excellent candidates for bioimaging, diagnostics and image-guided therapy applications.

Hydrogen plasma treatment of monolayer graphene suspended on silicon dioxide substrate has been previously shown to spontaneously produce holes with uniform dimensions. The patterning of such holes can introduce bandgaps in graphene, which is a prerequisite for its potential application as graphene transistors. In addition, the so called nanoporous graphene has potential applications in water desalination, DNA sequencing, and gas separation. However, these applications can only be realized if precise control over the hole size and distribution can be achieved, which necessitates a detailed understanding of the plasma-surface interaction mechanisms resulting in hole formation and growth. Here, we present results from our recent time-accelerated molecular dynamics (MD) simulations establishing the relationship between basic plasma parameters (plasma temperature, pressure, gas flow rate, etc), graphene microstructure (grain size, grain boundary characteristic) and the resulting nanoporous graphene structure. The plasma input parameters to the MD model are obtained from a first-principle-based thermo-chemical model. Our analysis confirms that there exists a range of plasma-parameters for which the hole creation process is active. The results demonstrate that the hole formation process is highly monolayer selective, and that nucleation of these holes originate from preexisting defects and grain boundaries rather than uniformly over the sheet. Additionally, the silicon dioxide substrate has been shown to play an active role in the hole formation reaction, confirming previous experimental results. Our simulations suggest the use of hydrogen plasma as a viable tool for tailoring monolayer graphene by controlled hole patterning.

Strategies for fabricating all-inkjet-printed, graphene-based electronic devices (such as transistors, supercapacitors, and gas sensors) on thin, flexible substrates are receiving increasing attention. Among commonly-used flexible substrates (paper, plastic, silk, etc.), polyethylene terephthalate (PET) and polyimide (Kapton^®) possess particularly attractive thermal and chemical stabilities. However, the hydrophobic nature of these plastic substrates inhibits coating with hydrophilic inks. While traditional UV, ozone, and plasma surface treatments can be used to enhance the printing of hydrophilic inks on these plastic substrates, the resulting inkjet printed films tend to be easily peeled away from the substrate. In this work, a facile surface modification process has been developed to tune the substrate surface properties to allow for the printing of adherent hydrophobic and hydrophilic inks. The substrate surface modification process involved the alternating, layer-by-layer deposition of a positively-charged polymer (polyethylenimine) and a negatively-charged polymer (poly(acrylic acid)). With proper control of the deposition process and selection of the top (final) polymer layer, the surface hydrophobicity, surface charge, and roughness of the substrate could be adapted for the printing of different inks. A means of tailoring the porosity of inkjet-printed graphene oxide thin films, via use of transient pore formers, has also been developed.
To demonstrate the utility of the aforementioned surface modification and porosity enhancement techniques for fabricating gas sensors, inkjet printing was used to apply a patch of graphene oxide and two silver electrodes onto surface-modified Kapton. Appropriate surface modification of the Kapton substrate resulted in excellent printability of both the hydrophobic silver nanoparticle inks and hydrophilic (water-based) graphene oxide inks. The porosity of the inkjet-printed graphene oxide films was enhanced by incorporating surface-modified polymer nanospheres within the graphene oxide ink, and then selectively removing these nanospheres after inkjet printing. The resulting inkjet-printed structure was sufficiently robust as to survive repeated bending (to a 1 mm radius of curvature) of the Kapton and exposure to reducing environments (e.g., incubation for 12 h at 95 °C in aqueous sodium borohydride or pyrrol solutions for the reduction of graphene oxide). The resulting all-inkjet-printed, highly-porous graphene-based sensor was able to detect ammonia vapor at a concentration of 1 ppm. The surface modification and porosity enhancement techniques employed in this work are versatile and may be extended to the inkjet-printing of other highly-porous functional thin films for a variety of electronic device applications.

Direct methanol fuel cells (DMFCs) are one of the most promising power source for low power applications and have attracted great attention. The basic operation principle of DMFCs involves methanol oxidation and oxygen reduction on the precious metal catalysts, which are loaded on the support surfaces. As is well-known, the dispersion of Pt-based alloys on carbon supports as well as catalyst particle size and shape plays a dominant role in the electrochemical performance for fuel cells. Carbon nanosheets (CNSs) can be described as graphite sheet nanostructures with a large number of edges that are composed of stacks of planar graphene sheets standing almost vertically on the substrate. Due to the exceptional properties such as high surface area, high conductivity and chemical inertness, CNSs is one of the candidate in the catalyst suport. In this study, we explore the CNSs by microwave plasma-enhanced chemical vapor deposition(MPECVD) in Ar-CH₄ system as a catalyst support for Pt-based nanoparticles by one-step electrodeposition. The relationship between the morphology, microstructure, deposition sites of Pt-based nanoparticles and electrochemical properties of the CNS-supported Pt-based nanoparticles were investigated with optimized electrodeposition conditions.
CNS films were deposited on Cu cubstrates by 2.45 GHz MPECVD method. The morphology of CNSs were controlled by changing the microwave power and the ratio of the partial pressure of the mixed gas. After deposition, CNS film was peeled from Cu substrate. The cleaned Pt electrode with some ethanol on its surface was covered completely by the as-synthesized CNS film. Electrodeposition of Pt-based alloy nanoparticles on modified Pt electrodes was prepared according to a one-step process by using cyclic voltammetry. The morphology, microstructure, chemical composition, and electrocatalytic activity and stability toward methanol oxidation of the CNS-supported Pt-based nanoparticles are investigated using SEM, TEM, EDX/XPS, and CV, respectively. We also investigated that the electocatalytic performance for methanol oxidation of Pt-based nanoparticles supported on HOPG or Single-walled carbon Nanotubes. The detailed results will be presented.
[1] Zhipeng Wang, Mao Shoji and Hironori Ogata, Appl. Surf. Sci.259(2012)219-224.

The recent trend towards replacing traditional fossil fuels with biofuels in applications ranging from transportation to power generation and home heating has also introduced new materials challenges. All these applications require materials with high strength, which could withstand large shear or compression stresses, combined with either thermal or electric conductivity and ductility for ease of processing. Traditionally metals, which could also withstand high processing or operating , temperatures have been better suited for most of these applications. In contrast to fossil fuels, biofuels also tend to have much higher acidity and higher combustion temperatures, making them very corrosive and difficult to store or operate in metal containers. Here we study a polymer nanocomposites system that is being developed as a viable alternative. Polypropylene is easily processed, stable at high temperatures, ductile, and most important highly resistant to corrosion. But before it can be developed as a functional substitute for metals, its flame retardant and thermal conductivity properties must be improved, while maintaining adequate mechanical properties for ease of processing. Here we show the results for graphene/ propylene nanocomposites where we measured the thermal conductivity, impact toughness, and cone calorimetric behavior as a function of the type of graphene and its concentration. We find that the thermal conductivity coefficients could be increased by more than an order of magnitude, with a concentration of graphene which also renders the composite flame retardant (according to the UL-94-V0 criteria). SAXS spectra indicate good exfoliation while WAXS spectra indicate that the graphene nanocomposites are composed of strongly adsorbed polymer chains, with a high degree of surface nucleated crystallinity. The large graphene lattice enables good phonon coupling with the polymer matrix thereby increasing the thermal conductivity.

In the recent decade, graphene has been intensively studied since its discovery and several graphene formation techniques were reported. We have utilized ion implantation and pulsed laser annealing to synthesize few layers of graphene on 6H and 4H-SiC substrate and systematically studied what parameters contribute to the graphene formation. Laser annealing SiC substrates to form graphene is an attractive approach because of fast processing, and the ability to control laser fluence, pulse duration, pulse number, rep rate, ambient conditions and SiC substrates in different poly-types. In a systematic study few layers of graphene have been synthesized by laser annealing 4H-SiC single crystals and we have identified what parameters could contribute to the graphene formation. Raman spectroscopy, SEM, cross-section TEM are used to characterize graphene quality. Especially in TEM images, we observe the presence of solid phase formation of 3C layers on 4H-SiC during pulse laser annealing and the evolution of graphene layers on top. With the help of thermal simulations, we estimate the surface temperature and better understand the mechanism of graphene formation.

Graphene-TiO₂ nanocomposites are a promising anode material for Li-ion batteries due to their inherent safety, mechanical and chemical robustness, as well as good high-rate capacity, and can be produced via a scalable surfactant-mediated process. However, despite a large number of scientific reports on the material, the mechanism of the enhanced high-rate Li⁺ storage capacity that results from the addition of graphene to TiO₂ - typically attributed to improved electronic conductivity - is still not well understood. Additionally, studies that target a better understanding of the influence of processing parameters on electrode properties are lacking. In this work, we investigate the influence of various processing parameters, in particular surfactant concentration, on the electrochemical performance of graphene-TiO₂. We show evidence that while graphene does improve e^- transport, Li⁺ transport in the nanocomposites and in the electrolyte may have a greater impact on high-rate Li⁺ storage capacity. We show that by optimizing the surfactant concentration, improved Li⁺ storage performance in the graphene-TiO₂ can be achieved. Furthermore, we suggest an approach for estimating power loss during charge/discharge cycling, which offers a succinct method for characterizing the high-rate performance of Li-ion battery electrodes.

Carbon-based materials, which are highly complex in nature, are at the heart of numerous technological applications including catalysis, fuel cells, super capacitors, batteries, and next-generation electronics. However, current technologies are marred with issues such as long-term durability under high potentials (e.g., fuel cells), low energy/power density (super capacitors), and metal intercalation issues (batteries). Research efforts worldwide have been focused on developing carbon and especially graphene/graphene oxide materials with superior structural and functional properties for fuel cell applications. State-of-the-art devices have typically been synthesized from highly heterogeneous precursors and amorphous carbon materials. Materials based on graphene and graphene-oxide (GO) are more homogeneous and possess properties, such as good chemical stability, excellent conductivity, and more importantly, can be functionalized in a controlled manner. These unique properties have led to explosion in research in areas of graphitic materials as catalyst support/material for ORR.
In this study, we report the synthesis of highly ordered GO based materials. We specifically target the control of functional groups and d-spacing of GO materials using low temperature drying and solvent treatments. XPS, FTIR, XRD and microscopy studies were used to unravel the structure-function correlation of the resulting materials. The synthesized graphene oxide structures were further subjected to reduction and nitrogen doping in different conditions to obtain an optimum ORR catalyst. In this report, we specifically target the the affect of pre-treatment on the structure and function of the catalyst systems and fundamental understanding is obtained about how each of the following factors affect the catalytic performance including (a) the role of metal impurities, (b) role of nitrogen, and (c) removal of water and (d) the content of graphitic carbon.
We report for the first time, highly active and durable graphene oxide based systems in acidic media. In a 0.5 M H₂SO₄ electrolyte, a huge shift in half wave potential and a significant decrease in peroxide production is obtained depending upon the graphene oxide treatment. These catalyst systems show a drop of less than 30 mV drop over 2000 cycles in acid electrolyte in presence of oxygen indicating the role of high graphitic content in providing stability.

In this study, we introduce the graphene GBs as an ideal structure for detection of toxic analytes. To this objective, we fabricated individual graphene GB based sensor and study their gas sensing characteristics by exposing against the dimethyl methylphosphonate (DMMP) or 1,2-dichlorobenzene (DCB) gas molecules. DMMP is an electron donor while DCB is electron acceptor molecules. We uncovered that an isolated graphene GB has ~300 times higher sensitivity to gas molecules than a single crystalline graphene grain. We also investigated the sensing response of a sequence of multiple GBs, naturally formed by the merging of the grains in the CVD growth. Our results show a clear sensing cross-over from a single GB toward a polycrystalline sensor, as the number of GBs are increased from 1 to more than 5. Our hybrid modeling of an electronic structure and transport in realistic GBs reveals that the ultra-sensitivity in GBs is caused by a synergetic combination of gas molecules accumulation at GB, together with the existence of ultrasensitive switchable transport channels in GBs. The discovered sensing platform opens up new pathways for the design of nanometer scale highly sensitive chemical detectors.
[1] P. Yasaei, B. Kumar, R. Hantehzadeh, M. Kayyalha, A. Baskin, N. Repnin, C. Wang, R. F. Klie, Y. P. Chen, P. Král, A. Salehi-Khojin, submitted.

Metal-graphene nanocomposites, in general, have high mechanical properties. Recent studies [1-2] on metal graphene nanocomposites have shown that Ni-Cu graphene systems show high strength during nanoindentation, while Al-graphene nanocomposites have showed mixed results [3, 4]. However, hardly any studies have focused on the fracture toughness of metal-graphene nanocomposites. The orientation of graphene and the presence of impurities in metal-graphene nanocomposites could play a critical role in the fracture properties of the nanocomposite.
In this talk, we will discuss a computational model to study fracture initiation and propagation mechanism in Al-graphene nanocomposites. We use a concurrent multiscale model [5, 6] that couples an atomistic region with continuum region. The atomistic region utilizes molecular dynamics whereas continuum region uses finite element method. We will discuss a bio-inspired design for the orientation and placement for the graphene platelets in the metal matrix to improve fracture toughness. Here, we determine the mechanism of a mode I crack initiation and propagation in Al-graphene nanocomposite and determine the fracture toughness K_IC. We will also discuss the change in fracture toughness due to orientation of graphene platelets.
Finally, we will discuss how to utilize the multiscale method to study the effect of impurities such as hydrogen at the crack tip that could change the plasticity behavior (dislocation nucleation mechanism) in the presence of graphene platelets. We will discuss the chemo-mechanics of dislocation interaction with graphene in the presence of impurities.
References
Chang, S., Nair, A.K. and Buehler, M.J. (2013) Nanoindentation of Nickel-Graphene nanocomposities, Philosophical Magazine Letters, 93 (4), 196-203.
Kim, Y., Lee, J., Shin, J.W., et al., (2013) Strengthening effect of single-atomic-layer graphene in metal-graphene nanolayered composites, Nature Communications, 4, 2114.
Bartolucci, S.F., Paras, J. et al. (2011), Graphene-aluminum nanocomposites, Material Science and Engineering A 528, 7933-7937.
Wang, J., Li Z., et al. (2012), Reinforcement with graphene nanosheets in aluminum matrix composites, Scripta Materialia, 66 (8), 594-597.
Nair, A.K., Warner, D.H. and Hennig, R.G. (2011) Coupled quantum-continuum analysis of crack tip processes in aluminum, Journal of Mechanics and Physics of Solids, 59.
Nair, A.K., Warner, D.H. and Hennig, R.G. and Curtin, W.A. (2010) Coupling quantum and continuum scales to predict crack tip dislocation nucleation, Scripta Materialia, 63 (12), 1212-1215.

Carbon nanotubes, graphene and various metallic nanowires such as silver nanowires have recently gained enormous interests for developing non-ITO (indium tin oxide) based TCEs (transparent conducting electrodes). While each of these nanomaterials has its advantages, they suffer from electrical transport bottlenecks giving especially low figure of merit at high optical transparencies and at 550nm wavelength of illumination. We have previously proposed a co-percolation transport model and experimentally demonstrated a silver nanowire wrapped graphene hybrid TCE (called ‘Hybrid graphene TCE&’) with a record figure of merit (namely, sheet resistance at 90% transmission of 550nm light). For using these hybrid graphene based TCEs in applications requiring flexible devices, it is important to study its properties on flexible substrates. In this work we show the development of the hybrid graphene TCE on flexible substrates such as PET. Structural and electrical properties of the hybrid graphene on PET substrate are reported. Change in resistance of the hybrid graphene is compared with pure graphene electrode on flexible substrate as the substrate is bent with various bending radius. The stability of the hybrid graphene TCEs on flexible substrates paves the way to apply them in transparent flexible electronics and optoelectronics.

In this work, we report on the surface properties (friction and work function) of micro-mechanically cleaved graphene layers placed on thermally gown thick insulating (~ 295 nm of SiO₂) films on commercial Si (001) substrates. By employing the atomic force microscopy (AFM) and scanning electron microscopy (SEM) with varying primary electron acceleration voltage (V_acc) in secondary electron imaging (SEI) mode, we determined coefficient of friction (m) and electronic work function (F), respectively, with graphene layers (n). The friction coefficient was deduced through line scan of friction maps simultaneously obtained wile measuring AFM topography. The findings show that the supported mono-, bi- and tri-layer graphene all yield similar results (~0.03) in contrast to multilayer (~0.027) and thicker graphite (~0.015) flakes. From SEI contrast variation, we obtained reproducible discrete distribution of SE intensity stemming from atomically thick graphene layers on a thick insulating substrate. It was made possible to uniquely relate the SE intensity contrast or SE intensity by itself to measure the number of graphene layers (i.e.n). Moreover, we found a distinct linear relationship between the relative SE intensity from graphene layers and the its&’ number provided relatively lower V_acc is used. The different contrasts in SEI micrographs at lower V_acc is attributed to the fact that the generation of secondary electrons emitted from the graphene was affected by the different work functions that corresponded to n (or thickness contrast; C). This simple facile method is superior to the conventional optical method in its capability to characterize graphene with sub-micrometer squares in area on various insulating substrates. These results are supplemented with optical microscopy, high-resolution transmission electron microscopy and Raman spectroscopy and Raman mapping yielding the structural quality (or disorder) of the graphene layers albeit semi-quantitatively.

Graphene transparent electrodes are flat, flexible and stable. Surface resistivity values of graphene, however, are larger than those required for the electronic devices with current drive such as solar cells or OLEDs. Silver nanowire (AgNW) transparent electrodes show small surface resistivity values. AgNWs cause light scattering that is preferable for solar cells and lighting. They have apertures that are unsuitable for electron exchange to an active device layer with low electrical conductivity. AgNWs are unstable against sulfur compounds in the atmosphere. The authors have reported a transparent conducting film composed of hydrazine-reduced graphene oxide (h-rGO)/AgNW/polymer stacked layers with 4 Omega;/sq sheet resistance (Rs) and 75% diffuse transmittance (DT) at 550 nm [1]. The film is transparent at a wavelength ranging from 200 to 2600 nm. Its surface roughness is about 3-4 nm. It is stable against sulfur vapor owing to the h-rGO protective layer. The work-function (WF) of pristine graphene is 4.5 eV, which is unsuitable for the cathode. The authors have also reported decreasing WF of the stacked films by electrochemical treatment [2]. The hydrazine-reduced GO was found to be notable for a marked decrease in WF after cyclic voltammetry (CV) in a negative potential region compared with other carbon materials. In this presentation, we will report that WFs of the films can be decreased by floating the films on aqueous borohydride solution.
Graphene oxide was dip-coated from its aqueous suspension onto a hydrophilic glass plate, and reduced by hydrazine vapor at 90°C. Methanol suspension of AgNW was cast on the glass plate, and then PMMA solution was cast. The resulting film was removed from the glass plate in water. Surface resistivity was measured with a four-point probe method. WFs were measured by ultra-violet photoelectron spectroscopy. X -ray photoelectron spectra were observed in order to measure the atom species.
During the reaction, hydrogen bubbles touched and left the film repeatedly. WF values of the films were decreased to 3.8-4.1 eV from 4.4 eV by the NaBH₄ treatment. These WF values of the films are smaller than the reported value (4.2 eV) of GO reduced by NaBH₄ [3]. Sheet resistance (Rs), diffuse transmittance (DT) and appearance of the films did not change. (Dimethylamino)borane also reduced the WF value. It is considered that the small WF values are attributable to the decrease of electron-attracting oxygen atoms, electron doping and electron-donating pyrrolic nitrogen.
1) K. Naito et al., Synthetic Metals 175(2013)42-46.
2) K. Naito et al., JSAP-MRS Joint Symposia, 17P-PM1-4 (2013)
3) H.-J. Shin et al., Advanced Functional Materials 19(2009)1987-1992.

Graphene, a one-atom-thick 2D single layer of sp²-bonded carbon, has been considered as an ideal supercapacitor electrode material due to its extremely large surface area, extraordinarily high electrical conductivity, good chemical stability, and high mechanical strength. However, graphene sheets have a tendency to restack themselves during their synthesis and during electrode preparation procedures, resulting in a loss of the effective surface area. Several approaches have been developed to prevent the restacking of graphene during processing, including the fabrication of highly corrugated and crumpled graphene sheets, the use of guest materials as spacers, and template-directed or controlled assembly of graphene sheets into three-dimensional porous structures. Among them, the use of solvated water as a spacer is considered as one of the best approaches. Reduced graphene oxide (RGO) hydrogel films, prepared by the filtration of RGO dispersions, have exhibited excellent supercapacitive performance with ultrahigh power density. However, the preparation of RGO hydrogel films by this filtration method is time-consuming. Moreover, the RGO hydrogel films have very low mechanical strength, which can easily experience degradation during electrode processing. In this study, we describe a simple method to prepare reduced graphene oxide hydrogel film on stainless steel substrates by the simultaneous electrodeposition and reduction of graphene oxide, and the subsequent use of the film as an electrode for supercapacitors. The obtained electrochemically reduced graphene oxide (ERGO) exhibited high specific capacitance of 147 Fg^-1 at a current density of 10 Ag^-1 and excellent electrochemical stability with capacitance retention of approximately 98 % after 2000 cycles. The specific capacitance of ERGO was significantly enhanced, up to 223 Fg^-1 at a current density of 10 Ag^-1, by using hydroquinone as a redox electrolyte.

Pillared-graphene is a three-dimensional (3D) nanostructure fabricated experimentally by Kondoh et al. (2008). This structure consists of parallel monolayer graphene sheets and single-walled carbon nanotubes (SWNTs); as pillars, the SWNTs connect the graphene sheets in their out-of-plane direction. Since the thermal conductivities of graphene and SWNTs in the in-plane and axial directions, respectively, are extremely large compared to those of conventional materials, such a 3D nanostructure is expected to be applied to thermal treetment in electronic devices. Vikas et al. (2010) investigated the thermal transport in pillared-graphene, and concluded that the length of pillars and the minimum interpillar distance are the two parameters governing its thermal transport. They constructed pillar-graphene junctions by combining 7-membered and 6-membered rings of carbon atoms. However, pillar-graphene junctions can also be constructed by combining 5-membered and 8-membered rings. Their conclusions might be altered if such an atomic configuration at junctions is adopted. Furthermore, they assumed only one kind of geometrical arrangement of pillars although there are many kinds of the arrangement. In this paper, we address the effects of atomic configurations at pillar-graphene junctions and geometrical arrangement of pillars on the thermal transport of pillared-graphene nanostructures by means of nonequilibrium molecular-dynamics simulation using the LAMMPS code. The velocity-Verlet algorithm is employed to integrate equations of motion. The interactions between carbon atoms are computed using the adaptive interactive reactive bond-order (AIREBO) potential. The effective thermal conductivities of pillared-graphene nanostructures in the in-plane and out-of-plane directions regarding the graphene plane are calculated. The outcome will be useful to thermal design of nanodevices.

To avoid the cumbersome transfer procedure of CVD graphene layers onto an insulating substrate, several investigators recently proposed to get rid, partially [1] or totally [2] of the metal catalyst substrate during its growth. We define Remote Catalysis as a process in which the metal catalyst faces the deposition substrate during CVD at a typical distance of 1 cm. Since graphene precursors are more likely formed onto the Cu surface rather than the destination substrate, their diffusion and arrangement mechanisms deserve deep investigation because the graphene quality is expected to deeply depend on them.
In our experimental set up, a Cu foil faces an oxidized Si substrate kept at 1000 °C at an average distance of 1 cm. To have efficient decomposition, we employed Ethanol (EtOH) as a C source, carried by H₂ bubbled into an EtOH reservoir and fluxed into the chamber.
The characterization as conducted by Raman spectroscopy indicates the formation of nano-domains [3] composed by a few layers graphene after 30 min deposition time. Atomic Force Microscopy and SEM have been used to characterize morphologically the samples observing a quasi-full coverage of SiO₂ surface. Preliminary four-probe electrical measurements have been also performed showing a sheet resistance of graphene in the order of a few kOmega;/#9633;.
The growth mechanism has been also investigated by combining oxygen plasma and AFM. In fact, by exposing the sample to repeated plasma processes we observe a progressive reduction of the graphene coverage of the sample. Since oxygen plasma erosion of graphene begins from island edges [4], we can extrapolate the nucleation density of the CVD method. The observation that only bigger domains survive after several mins. of plasma etching opens the possibility of using these domains as seeds for graphene re-deposition. The capabilities of this repeated etching/deposition cycles to produce graphene with domains in the mm range will be discussed.
[1] Teng P-Y et al, Nano Letters, 12, 1379-1384, (2012)
[2] Sun J., et al, Appl. Phys. Lett., 100, 022102 (2012)
[3] Martins Ferreira E. H. et al, Physical Review B, 82, 125429, (2010)
[4] Childres I. et al, New Journal of Physics, 13 (2011) 025008

Graphene, the two dimensional carbon atomic crystal emerged as a strong candidate for next generation electronics. One of the properties that makes graphene a major material of interest is its estimated mobility, µ which is exceedingly high due to higher conductivity combined with carrier modulation through the applied filed externally [Nature Mater, 6, 183 (2007)]. The modulation of carrier conduction in graphene transistor devices is achieved by using different gate electrodes metals, applying vertical field using dual gate structure, suspended graphene channel transistors and using high-k dielectrics. Graphene transfer process has a direct impact on contact resistance leading to a change in graphene channel resistance as we previously reported [Appl. Phys. Lett. 102, 183115 (2013)]. Hence, depositing different metal with clean transfer process would lead to change the total resistance without influencing the mobility. However, the utilization of second gate (top gate and back gate) is significantly degrading channel mobility. Suspended graphene channel transistors showed very high mobility [Solid State Communications 146 (2008) 351-355], but this approach is limited to small devices only and small devices lack stability in their performance. The choice of using high-k dielectrics is achieved via atomic layer deposition (ALD) deposition of high-k oxide dielectrics such as Al₂O₃ and HfO₂ has been successfully achieved [Appl. Phys. Lettt. 97, 043107, (2010)]. We previously reported ultra-thin 10 nm high-k Al₂O₃ dielectric with mobility exceeds 11000 cm² /V s [ACS nano 7.7, 5818 (2013)]. In this work, we report on a novel buried gate graphene device with ultra-scaled high-k titanium oxide (TiO₂) dielectric. The buried gate contact is deposited after 3000 Å thermally oxidation of heavily doped silicon wafers followed by metal lithography. Buffered oxide etchant (BOE-7:1 Transene) was used to recess the SiO₂ by approximately 1500Å. Then, TiO_{2 shy;}dielectric is deposited via ALD. Deposition of 20 nm titanium adhesion layer and 120 nm gold via physical vapor deposition (PVD) was followed by gentle sonication in acetone for the lift-off process and O₂ ashing to remove any additional photoresist residue. Graphene which was grown using chemical vapor deposition (CVD) at atmospheric pressure is transferred to the wafer via poly methyl methacrylate (PMMA) aqueous media is then transferred to the prepared wafer. Carrier injection manipulation in graphene conductance at dirac voltage is observed for V_ds ranges from -0.5V to 0.5V. It is found that resistance is reduced by 22% for V_ds> 0 while reduced by 16 % for V_ds< 0. The fabricated device indicating minimal trap charges observed in the small hysteresis voltage. Moreover, back gate biasing induces carriers in the channel which requires compensating negative bias from top gate to return to neutrality. The observed channel carrier modulation refutes the idea of induced carrier generation compensation.

It has recently been observed that AgBr, a conventional photographic material with a narrow band gap of 2.6 eV, serves as a catalyst for the various photocatalytic reactions under visible light irradiation. The main problem of AgBr, however, is the tendency to get reduced under visible light exposure. Graphene, especially nitrogen doped and/or amine functionalized graphene is shown to have enhanced catalytic and electronic properties. To increase the stability of AgBr, in the present work, composites of AgBr with both N-doped and amine functionalized reduced graphene oxide, GN, were synthesized. The synthesized composites, AgBr - GN, were well characterized by different spectroscopic and microscopic techniques. X-ray diffraction (XRD) excluded the presence of metallic silver in the hybrid materials. X-ray photoelectron spectroscopy (XPS) proved the presence of graphitic-, pyrrolic- and amino nitrogen in GN. The AgBr-GN composites were applied for the decomposition of methylene blue under solar light (ge;400 nm) illumination. Photocatalytic experiments showed that, the catalytic activity depends upon the concentration of GN in the composites. The composite with 0.5 wt% GN exhibited best catalytic activity. The dye degradation reactions followed first order kinetics. The stability test was carried out with the repeated use of the catalyst for the degradation of methylene blue. It showed that, minimal loss of the catalytic activity, a mere 1 at% of metallic Ag was generated as revealed by XPS analysis, after eleven catalytic cycles. The high stability of the catalyst was attributed to the fast removal of the photogenerated electrons in the conduction band of AgBr by the highly conducting GN. It was further proved by the photo current data. The efficiency of the catalyst was further revealed by the decomposition of 2-chlorophenol, a common colorless organic contaminant. Mechanism study explored that hydroxyl radicals played important role for the degradation of the dye.

Large-scale synthesis of graphene has been an important area in material development ever since this two-dimensional material was experimentally discovered in 2004. As simple as it is, the process of mechanical exfoliation can only yield graphene flakes of the order of a few hundred microns. Chemical Vapor Deposition (CVD), which generally employs mixing of a hydrocarbon gas and hydrogen at a high temperature (~1000 °C), has been widely used to reliably grow graphene over large areas. However, most works reported in literature up to date have focused on thermal CVD growth of graphene using catalytic approaches. Not only the high temperature of growth is not industry compatible, the requirement of catalysts imposes stringent conditions on substrate choices. The widely adopted post-synthesis transfer of graphene onto desired substrates is found to cause undesirable effects such as wrinkles/folds/cracks and unintentional doping. There have been a few reports on non-catalytic growth of graphene on SiO₂ and quartz, albeit at even higher temperatures. Accounts of low temperature non-catalytic growth are very limited, which employ hours&’ long durations of growth and lack a detailed investigation of the effects of different substrates. In this work, we successfully tackle the issues associated with high temperature, catalytic growth and report low temperature growth of graphene at 650 °C on a variety of substrates, non-catalytic (SiO₂, quartz) and catalytic (thin amorphous copper film), using a Plasma-Enhanced Chemical Vapor Deposition (PECVD) process with CH₄ as the precursor. The PECVD method developed by us gives complete graphene coverage over centimeters wide areas within a few minutes on all the substrates. Furthermore, to our knowledge, 650 °C is the lowest temperature reported so far for graphene growth on copper films.
The ability to control the growth process requires a deep understanding of the effect of process conditions. With the help of Raman spectroscopy, photospectroscopy, SEM, TEM and AFM, we carry out a systematic study to provide a thorough analysis of the impact of growth parameters, including the choice of substrate, the duration of growth and the surface roughness. These findings can enable graphene growth on different kinds of substrates including other 2D materials like boron nitride and MoS₂ with a higher degree of control, in turn making direct synthesis of 2D-layered heterostructures possible. As opposed to the thermal CVD growth on copper, which is a self-limiting process, the PECVD method developed here is easily scalable with time and can yield any desired number of graphene layers by simply increasing the duration of growth. These results are a big step towards free growth of graphene on any desired substrates at CMOS-compatible temperatures.

Graphene nanoribbons (GNRs) attract great interest as the band gap can be opened due to quantum confinement effect into 1D nanostructure [1]. Top-down approach utilizing lithography and plasma etching processes can produce GNRs, however they are heavily damaged due to oxidative etching process [2]. Thus, a bottom up approach without relying on lithographic techniques is promising to synthesize high quality GNRs.
We previously studied the domain structures of single-layer graphene grown on heteroepitaxial metal films in terms of crystalline plane dependence [3,4]. Based on these results, we have developed a novel bottom-up CVD approach to grow uniform and aligned GNRs on an epitaxial Cu(100) film with a controlled orientation [5]. We observed that a number of narrow trenches are formed on the Cu(100) surface with widths as narrow as ~40 nm, which are oriented in specific directions of the metal film. Interestingly, GNRs are found to grow inside these trenches selectively and grow simultaneously with the development of the trench structure. The Raman spectroscopy proves that the defect-related D-band is much weaker than the GNRs synthesized by top-down etching process, indicating our GNRs are high quality with low defects. Furthermore, low energy electron microscope (LEEM) revealed the selective growth of zigzag-edge GNRs, which should be interesting for nanoelectronics and spintronics applications. Our work offers a promising approach of growing 1D nanoribbon structure and useful for highly controlled growth of graphene nanostructures.
[1] X. Li et al., Science 319, 1229 (2008). [2] X. Jia et al., Nanoscale3, 86 (2011). [3] H. Ago et al., Appl. Phys. Exp., 6, 75101 (2013). [4] Y. Ogawa et al., J. Phys. Chem. Lett.3, 219 (2012). [5] R. M. Yunus et al., submitted.

Since the first isolation of graphene in 2004, a great progress has been made on its synthesis techniques from chemical exfoliation, chemical vapor deposition (CVD) to bottom-up organic synthesis. We have been working on the CVD growth of high-quality graphene and its 2D hybrid materials since 2008. By rationally designing the growth catalysts and the elementary steps in the growth process, we have been able to make a precise control of graphene layer number, stacking structures, doping, wrinkle structures and even bandgaps by hybridization with different 2D materials. This talk will present our recent progresses along this direction. A particular focus will be laid on growing graphene on groups IVB-VIB early transition metal foils and wide-gap semiconducting substrates such as h-BN and high k strontium titanate together with the designed growth of mosaic graphene, a periodic in-plane hybrid structure with other 2D materials. Since the first isolation of graphene in 2004, a great progress has been made on its synthesis techniques from chemical exfoliation, chemical vapor deposition (CVD) to bottom-up organic synthesis. We have been working on the CVD growth of high-quality graphene and its 2D hybrid materials since 2008. By rationally designing the growth catalysts and the elementary steps in the growth process, we have been able to make a precise control of graphene layer number, stacking structures, doping, wrinkle structures and even bandgaps by hybridization with different 2D materials. This talk will present our recent progresses along this direction. A particular focus will be laid on growing graphene on groups IVB-VIB early transition metal foils and wide-gap semiconducting substrates such as h-BN and high k strontium titanate together with the designed growth of mosaic graphene, a periodic in-plane hybrid structure with other 2D materials.

Practical implementation of graphene relies on affordable, reliable and scalable production of graphene with consistent quality. Wafer-scale production of graphene can be achieved via chemical vapor deposition (CVD), annealing SiC, reduced graphene oxides (R-GOs), and liquid exfoliation of graphite. Each demonstrates strengths and weaknesses. Liquid exfoliation and R-GOs suffer from high sheet resistance. Annealing SiC provides the highest quality graphene. However, the prohibitively high cost of SiC makes it unrealistic for practical applications. CVD of graphene on catalytic metal surfaces is by far the most promising technique. However, multiple deposition and transfer steps are required to deposit graphene on target surfaces, which are not desired for scalable device fabrications. Therefore, a synthetic technique with economic and technical viability is still absent. It is highly desired to develop an innovative synthetic technique providing high throughput and scalable production of graphene directly on dielectric surfaces with satisfactory conductivity and transmittance.
In this study, we report the development of a solid-state rapid thermal processing (RTP) technique depositing wafer-scale graphene directly on dielectric surfaces, such as SiO₂, fused silica, quartz, and sapphire. The Ni catalyst evaporates spontaneously, therefore, eliminating the post-growth Ni etching and graphene transfer procedures. By tuning Ni/C ratio, graphene of controlled number of graphitic layers can be obtained, including single-layer graphene (SLG), bilayer graphene (BLG), and multilayer graphene (MLG). SLG of high transmittance (~ 93 % at 550 nm) and low sheet resistance (~ 50 #8486;/sq) was obtained. According to temperature resolved Auger electron spectroscopy (AES) depth profiling studies, the graphene formation and Ni evaporation are ascribed to the formation and decomposition of nickel carbide (Ni₃C).

We present a cost-effective and solution-based method to prepare N-MEG used as Pt-free CE to achieve high-performance dye sensitized solar cells (DSSCs). The use of cyanamide solution as N-doping source at normal pressure and room temperature is suitable for mass production. A large area of N-MEG free of structural defects using this synthesis can be observed in TEM. XPS provides evidence that the pyrrolic-N and pyridinic-N in the carbon conjugated lattice are two main structures. High electrocatalytic activities can be reached based on electrochemical characterization. The curved structure of N-MEG creates a large porosity to facilitate electrolyte diffusion, leading to an improved Nernst diffusion resistance within the electrode pores. We also found that the air exposure time after completing N-MEG film has a significant effect on the degradation of active sites and surface wettability so that influence charge transfer process and electrolyte diffusion. FF can be effectively improved because of good electrolyte permeation within the pores resulting from N-doping effect and the control of air exposure time. Therefore, measurement of DSSC with N-MEG CE demonstrates a nearly comparable performance to that with Pt CE, which presents a great potential for the preparation of cost-effective and noble metal-free DSSCs. In addition, N-MEG can be employed as CE for Co(#1096;/#1087;) mediated YD2-o-C8 DSSCs, showing a significant improved long-term electrocatalytic reliability as compared with Pt CE.

We have demonstrated the functionalization of epitaxial graphene with nitrophenyl groups and by the application of the Kolbe reaction. The chemical formation of covalent carbon-carbon bonds involving the basal plane carbon atoms offers an alternative approach to the control of the electronic properties of graphene; the transformation of the carbon centers from sp² to sp³ introduces a barrier to electron flow by saturating the carbon atoms and opening a band gap which potentially allows the generation of insulating, semiconducting and magnetic regions in graphene wafers.¹ This raises the question of the role of covalent bonding in the interconnection of graphitic surfaces and the prospects for the use of such bonds in electronically conjugating neighboring carbon nanotube and graphene surfaces without saturating and destructively rehybridizing the carbon atoms at the point of attachment while simultaneously maintaining the band structures of the intact benzenoid nanostructures. In this talk I will discuss our recent results on the covalent modification of the electronic structure and properties of graphene, and the application of organometallic chemistry to facilitate the interconnection of single-walled carbon nanotubes and to increase the dimensionality of graphitic surfaces.^1-3
1. Bekyarova, E.; Sarkar, S.; Wang, F.; Itkis, M. E.; Kalinina, I.; Tian, X.; Haddon, R. C., Effect of Covalent Chemistry on the Electronic Structure and Properties of Carbon Nanotubes and Graphene. Acc. Chem. Res. 2013, 46, 65.
2. Moser, M. L.; Tian, X.; Pekker, A.; Sarkar, S.; Bekyarova, E.; Itkis, M. E.; Haddon, R. C., Hexahapto-Lanthanide Interconnects Between the Conjugated Surfaces of Single-Walled Carbon Nanotubes. Dalton Trans. 2014, 43, 7379.
3. Tian, X.; Moser, M. L.; Pekker, A.; Sarkar, S.; Ramirez, J.; Bekyarova, E.; Itkis, M. E.; Haddon, R. C., Effec of Atomic Interconnects on Percolation in Single-Walled Carbon Nanotube Thin Film Networks. Nano Lett. 2014, 14, in press.
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Issue of metal-graphene contacts in graphene-based electronic devices and transistors poses a serious limitation to the application of graphene as an alternative channel material to silicon despite the former&’s advantageous intrinsic carrier transport properties. Annealing has long been adopted as a post-processing treatment to improve metal-graphene contacts by assuming that the contamination sandwiched at the metal-graphene interface is reduced upon annealing. However, despite its plausibility, it is not clear that this assumption is incontrovertible, and indeed conflicting reports from various experimental studies suggest that there is a need to examine this matter closely. Here, we report a novel “soft shadow mask” technique that allows both resist-free and resist-patterned metal-graphene contacts to be fabricated on the same graphene flake with exactly the same dimensions. This enables a fair comparative study of the effects of resist residue, and the effect of annealing thereof. Comparable contact enhancements were observed as a result of annealing regardless of the presence or absence of resist residues ruling out the claim that removal of contamination is the sole contributing factor. Through systematic investigations, we found that a simple post-annealing treatment can actually establish many “end-contacted” metal-graphene contacts in the presence of defects or dangling bonds in graphene and by choosing the appropriate metal. [1]
Based on this knowledge, we introduce a facile contact treatment that creates many “end-contacted” graphene edges that are covalently bonded to metal. Our contact-treated graphene field-effect transistors exhibit contact resistance as low as 11 Omega;.mu;m in bilayer graphene transistors, and 100 Omega;.mu;m in single-layer graphene (SLG) transistors which, to the best of our knowledge, is the lowest reported value for SLG devices. Raman spectroscopy, SEM, AFM and TEM characterization results are presented to elucidate the physical changes to the graphene that give rise to the improvements observed. More importantly, the proposed contact treatment can easily be inserted into a complementary metal-oxide-semiconductor (CMOS) process flow for future integrated circuits incorporating graphene as an alternative channel material. [2]
References:
[1] W.S. Leong, C.T. Nai and J.T.L. Thong: What Does Annealing Do to Metal-Graphene Contacts? Nano Letters (2014). (DOI: 10.1021/nl500999r)
[2] W.S. Leong, H. Gong and J.T.L. Thong: Low-Contact-Resistance Graphene Devices with Nickel-Etched-Graphene Contacts. ACS Nano 8, 994 (2014).

We report on the fabrication and characterization of organic light emitting transistors (OLETs) with an indium-tin-oxide (ITO)-free platform, using graphene-based transparent conductive electrodes in place of ITO as gate electrode. A direct comparison between twin bottom-gate/top-contacts OLETs, where a standard ITO layer is replaced with a film made of a few graphene layers, shows that comparable electrical characteristics can be obtained along with a clear improvement in the electroluminescence generation characteristics. Our experimental observations pave the way to the exploitation of graphene-based transparent conductive electrodes within this class of emerging devices on flexible substrates, further promoting the novel era of flexible organic electronics.

Progress in developing next generation of OLED and photovoltaic devices is limited by the lack of suitable transparent electrode materials. CVD graphene lends itself as an ideal candidate with high optical transmission and flexibility. However it is limited as a transparent electrode due to the high intrinsic resistivity and poor work function matching required for efficient operation of solar cells and OLED devices.
We present the functionalization of multilayer CVD graphene by FeCl₃ intercalation to create a transferable and scalable alternative to current transparent conductors. The intercalation process has been previously shown to reduce the resistivity by 3 orders of magnitude while leaving the optical transmission nominally unchanged for exfoliated graphene [1]. We build on this technique and demonstrate up scaling to produce FeCl₃ intercalated CVD graphene with improved electrical properties, high uniformity and no significant change in the optical properties. Furthermore we observe a significant increase in the work function of intercalated graphene, therefore reducing the dependence on lossy work function matching hole injection layers required for efficient OLED and photovoltaic devices.
[1] Khraphach et al. Adv. Matt. 2012 DOI: 10.1002/adma.201200489

We have observed from sets of measurements on large scale monolayer planar graphene devices that by applying gate voltage from -100 to + 100 V, Fermi level of graphene can be tuned about 0.25 eV (2000 cm^-1). Numerical simulation also supports the experimental results. The graphene nano- ribbons devices (GNRs) of width ranges from 500 to 50 nm were also investigated experimentally and theoretically. The study on GNRs devices uncovers many aspects of Plasmonics properties in graphene which can be significant for Plasmonics applications. We performed Fourier Transform Infrared spectroscopy (FTIR) to measure the plasmons resonances of GNRs patterned (500-50 nm width) using electron beam lithography (EBL) on back gated graphene devices on intrinsic silicon substrate. Gate dependent FTIR transmission measurements on different widths of GNRs show several plasmons resonance peaks in between 0.08 -0.2 eV. It can be seen clearly from these results that as width of GNRs is decreased, the energy and line with of graphene plasmon peaks increased. This can be also seen from the fact that plasmon wave vector q is inversely proportional to width of GNRs (q prop;1/ (W)) where W is width of GNRs. Moreover, these plasmons wave vector, line width and intensity are also strongly gate tunable. These plasmons peaks are blue shifted with increasing in Fermi level with applied external gating. The shift in plasmon resonances peaks with applied external gate voltage was observed up to 100 cm^-1.

In the work, we have demonstrated the synthesis of graphene on copper enclosures with layer control. First, we showed that the two surfaces of the copper enclosures are coupled by carbon diffusion through the copper foil. By identifying the pathways for methane gas and active carbon, we found that carbon diffusing through the copper foil allows for the continual growth of graphene from underneath the monolayer graphene on the outside. Based on the inter-copper carbon diffusion process, we are able to grow bi-, tri- and multi-layer graphene films by tuning the thickness of the copper foil. Furthermore, we show the growth of uniform monolayer graphene by blocking the inter-diffusion pathways or terminating the carbon supply on the same copper structure, therefore, achieving layer control.

Large scale production of graphene materials is one of the critical problems in its basic research and for industrial application. Chemical methods such as the graphite oxide route [1-3] offer possibilities of producing graphene on a scale of tons. To further reduce the cost of produce graphene, we have developed a weak oxidation technique for producing low defect and high quality graphene materials.
During the development of the technique, the effects of parent graphite and oxidation conditions on the size of graphene oxide sheets upon exfoliation have been studied systematically. Parameters such as the crystalline size and orientation of the parent graphite, the amount of potassium permanganate and sulfuric acid, as well as the oxidation time were optimized. We discovered that, in contrast to the traditional graphite oxide routes[4,5], only half of the oxidants are required while maintain the integrity of π-π conjugated structure as much as possible, and a limited amount of oxidation groups can give rise to the expansion of graphene oxide. Based on such weakly oxidized graphite, the high degree of exfoliation and expansion of graphene oxide has been achieved with physical approaches, which significantly increased the production efficiency and decreased the energy consumption.
Since 2011, the Sixth Element (Changzhou) Materials Technology Co., Ltd has been devoted to the large-scale manufacture of graphene based power materials. We have been continuously improving the design of production equipment, developing fully-automated synthesis system to increase the efficiency and quality of the production. More importantly, the cutting-edge research and development has been focused on the innovation in the weak oxidation technique and in the downstream application of graphene materials. Currently, the capacity of graphene materials in Sixth Element reaches 100 tons per year and the materials have shown promising use in coatings, composite materials, lithium ion batteries, and super-capacitors.
[1] Park S, Ruoff RS. Nat Nanotechnol 2010,5(4):217-24.
[2] Gogotsi Y. J Phys ChemLett 2011, 2(19):2509-2510.
[3] Long Zhang, Yongsheng Chen. Carbon 2009, 47, 3365-3380
[4] D. A. Dikin , S. Stankovich , E. J. Zimney , R. D. Piner , G. H. B. Dommett , G. Evmenenko , S. T. Nguyen , R. S. Ruoff. Nature 2007, 448, 457 .
[5] H. Zhang, X. J. Lv, Y. M. Li, Y. Wang, J. H. Li, ACS Nano 2010,4, 380-386.

Pristine graphene has served an invaluable role in studies leading to fundamentals; yet, its utility in applications is often limited unless graphene is first functionalized to alter its structure and properties for a desired application. To do so, we work with defective and functionalized graphene which is produced through thermal splitting of graphite oxide to graphene oxide and by simultaneous and/or follow-on reduction to functionalized graphene. The graphene oxide approach also provides a major advantage in the development of bulk materials such as ultracapacitors, electrodes for batteries, electrochemical sensors, and graphene-based composites in general as it is suitable for the production of graphene in > tons/year quantities. I will summarize the production of functionalized graphene sheets (FGSs) by thermal exfoliation and the tuning of its properties by simultaneous thermal reduction processes. In spite of its rich history dating back to 1840, only recently it has been shown that this thermal exfoliation and reduction approach can indeed yield large fractions (> 80%) of single sheets. The challenge of single sheet production, however, leads into another challenge in the control of aggregate structures as single sheets readily collapse back to form multistacks due to van der Waals and capillary forces. Restacking is not only unavoidable but a certain degree of restacking in the presence of energy storage materials is also beneficial. I will provide examples related to supercapacitors and batteries.

Controllable growth of high quality, large area single-crystal graphene and bilayer graphene is critical for high-end applications in electronics and photonics. The discovery of graphene growth on copper by Chemical Vapor Deposition (CVD) has led to the polycrystalline large area films. In this talk, we presented two key mechanisms that are fundamentally important for graphene growth on Cu. (1) Oxygen (O) on the Cu surface substantially decreased the graphene nucleation density by passivating Cu surface active sites. Control of surface O enabled repeatable growth of centimeter-scale single-crystal graphene domains. O also accelerated graphene domain growth and shifted the growth kinetics from edge-attachment-limited to diffusion-limited. Correspondingly, the compact graphene domain shapes became dendritic. (2) We also established a new growth mechanism that can control the bernal-stacking ordering of two graphene layers, the synthesized single-crystal bilayer graphene domain can be as large as 500 micrometers, and more than 80% is of bernal stacking. Electrical transport property measurements also show carrier mobility over 60,000cm²v^-1s^-1 for both graphene and bilayer on Hexagonal Boron Nitride substrate. Particularly, we observed Bandgap ~100meV at a displacement field of 0.9V/nm in the CVD bilayer graphene, . Our works will meet the potentials of graphene in various applications.

Two-dimensional (2-D) materials represent a promising material platform for next-generation energy storage and conversion devices. Owing to their advantageous features of mechanical flexibility, large surface area and open ion transport pathway within 2D layers, highly electrochemically-active 2-D transition-metal oxides and phosphates based nanosheets materials show great promise in flexible high-density energy storage devices. Here I will present our recent progress on chemically integrated graphene/inorganic hybrid 2-D materials to build ultra-flexible, high-performance supercapacitors. One example is rationally-integrated hybrid vanadyl phosphate (VOPO₄)/graphene ultrathin-film solid-state pseudocapacitors, and the other is the MnO₂/graphene hybrid nanostructures based flexible planar supercapacitors. These hybrid structures not only take advantage of excellent electronic conductivity of graphene nanosheets but also introduce more electrochemically active surfaces for diffusion capabilities of electrolyte ions, which greatly facilitate charge transport during electrochemical processes.

An facile approach to densely packed freestanding hybrid films for supercapacitor electrodes is developed by self-assembling graphene oxide nanosheets with Ni(OH)₂ nanoplates intercalated. In the obtained graphene@Ni(OH)₂ hybrid films, Ni(OH)₂ nanoplates act as not only effective space inhibitors to prevent graphene restacking but also pseudocapacitors to improve the overall capacitance, while the highly conductive graphene nanosheets serve as expressways for efficient electronic transportation. The 3D expressway-like film electrodes exhibit superior supercapacitor performance including high gravimetric capacitance (~ 605 F/g), high volumetric capacitance (~ 677 F/cm³), excellent rate capability (only 27.7% capacitance loss of its initial value with a current density increased from 0.2 to 50 A/g) and superior cycling stability (167% capacitance increase of its initial capacitance after 5000 cycles). Moreover, such densely packed freestanding film electrodes are highly bendable, which have promising applications in flexible energy storage devices. An asymmetric supercapacitor based on such hybrid film electrodes was demonstrated, which exhibited competitive energy density (26.1 Wh/kg) even under high power density (40744 W/kg). The fabrication process of the graphene@Ni(OH)₂ films developed in this work is universe, which is applicable to many other composites with the combinations among 0-D, 1-D and 2-D materials.

One-dimensional carbon nanotubes and atomically thin two-dimensional graphene, as a brother in nanocarbon family, have themselves superior material properties such as high mechanical strength, high electrical conductivity, high surface area, and excellent chemical stability, which are certainly beneficial for energy storage. Nevertheless, when they are used for energy storage for instance, supercapacitor, battery, and hydrogen storage, their performance is still limited due to different processibilities. In spite of such different processibilities, the performance can still be maximized by appropriately forming carbon nanotube-graphene hybrid structures. The current sophistigated electronics industry and technology require often versatile types of devices for energy storage. I will demonstrate several examples of such hybrid structures and sometimes with other materials which could be useful for different purposes of energy storage devices.

The choice of electrolyte in designing electrochemical double layer capacitors (EDLC) or supercapacitors (SC) is crucial, since the nature of the electrolyte can substantially enhance the performance of these devices. An inherent technological bottleneck in EDLC&’s with aqueous and/or polymer electrolyte is limited operating voltage window as well as low specific energy densities. In this regard, ionic liquids capable of providing higher voltage window of operation and enhanced Specific energy densities are thought of as potential electrolytes for high performance EDLC devices. Here we show that EDLCs fabricated using liquid-phase exfoliated graphene as electrodes and ionic liquid 1-butyl-1-methylpyrrolidinium tris (pentafluoroethyl) trifluorophosphate [BMP][FAP] as electrolyte show remarkable properties compared to EDLC devices fabricated with aqueous potassium hydroxide KOH (6M) as well as widely used ionic liquid 1-Butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6]. We found that Graphene EDLC&’s with [BMP][FAP] as electrolyte possess specific capacitance values as high as 225 F/g, which is at least an order of magnitude higher than aqueous electrolyte based devices. A specific power density of ~ 50 kW/kg and high specific energy density of ~ 25 Wh/kg was obtained in these devices. Further, these devices indicated rapid charge transfer response even without the use of any binders or specially prepared current collectors and can operate smoothly over a voltage window of 5 - 6 V. A detailed electrochemical impedance spectroscopy analysis in order to understand the phenomenon of charge storage in Graphene based EDLC&’s with [BMP][FAP] as electrolyte will be presented. Our findings strongly indicate the suitability of [BMP][FAP] as choice electrolyte for graphene based EDLC devices.

Graphene nanoplatelets or their assemblies have been used in energy storage such as supercapacitors or batteries for years. With flexible ability of surface functionality, graphene nanoplatelets are able to improve the capacity of cathode materials including LiFePO₄ or Li₃V₂(PO4)₃ nanoparticles.to be close to their theoretical values. At the same time, the reasonably high electric conductivity of the three-dimensional (3D) networks of graphene platelets benefits to the excellent rate performance of the composite electrodes, which maintained the capacity of more than 80% theoretical values at rate of about 15 C. On the other hand, chemical activation has shown ability to generate 3D graphene materials with a BET surface area of above 3000 m²/g and nanometer-size pores by processing graphene nanoplatelets or fullerenes with KOH. The high porosity, coupled with the high electrical conductivity, makes such activated graphene materials candidates for high performance supercapacitor electrodes. Furthermore, it is found that the self-assembly of graphene quantum dots made chemical methods can be utilized to prepare 3D graphene networks. With additional post-processing, such graphene networks can demonstrate spherical or wire-like morphology. Recent processes show that the graphene networks are promising to lead to supercapacitor devices with high volumetric performances by designing and controlling the dimension at the microscopic scale in the carbons.
References:
1. Xianjun Zhu, Jing Hu, Wenyan Wu, Wencong Zeng, Huaili Dai, Yuanxin Du, Zhen Liu, Liang Li, Hengxing Ji, Yanwu Zhu, Sci Rep 2014, in press
2. Xianjun Zhu, Haidie Zhai, Wenyan Wu, Wencong Zeng, Yuanxin Du, Yu Zhong, Zan Yan, Hengxing Ji, Yanwu Zhu, J Mater Chem A 2014, 2, 7812
3. Shanthi Murali, Neil Quarles, Li Li Zhang, Jeffrey R. Potts, Ziqi Tan, Yalin Lu, Yanwu Zhu, Rodney S. Ruoff, Nano Energy 2013,
4. Yanwu Zhu, Shanthi Murali, Meryl D. Stoller, K. J. Ganesh, Weiwei Cai, Paulo J. Ferreira, Adam Pirkle, Robert M. Wallace, Katie A. Cychosz, Matthias Thommes, Dong Su, Eric A. Stach, Rodney S. Ruoff, Science 332, 1537, 2011
5. Ziqi Tan, Guangxiong Chen, Wencong Zeng, Yanwu Zhu, unpublished
6. Guangxiong Chen, Yanwu Zhu, unpublished

We have previously reported on a simple desktop method for producing high quality reduced graphene oxide sheets (RGO) which involved dispersing graphene oxide in an ethanol-water solvent and reducing it with sodium borohydride. Metal salts can also be potent reducing agents. Here we show that when these salts are incorporated into the reduction process, metalized graphene sheets can be formed. Several metallic salts were investigated to form Au, Ag, Pt, and AuPt alloy nanoplatelets incorporated into the graphene structure. The nature of these metalized graphene platelets was then examined using FTIR, TEM, HRTEM, Raman, and KPFM. By using a minimum of metal while maximizing the surface contact area of the graphene sheet, these nanoparticle-RGO composites have potential for use in energy-producing devices and/or as catalysts.

Graphene has been extensively studied for nanoelectronic application both theoretically and experimentally because of its characteristic electronic properties such as high carrier mobility. In particularly, graphene nanoribbons (GNRs) have attracted much attention in regard to interconnect applications. The electronic properties of GNRs are determined by the width and edge structure, i.e., zigzag-edge graphene nanoribbons (ZGNRs) or armchair-edge graphene nanoribbons (AGNRs). From an engineering point of view, controlling the atomic structures of GNRs is important for fabrication of low-resistance interconnects. In this study, we have examined how line defects containing pentagon-heptagon (5-7) pairs affect the transport properties of ZGNRs as well as AGNRs. We used the first-principles density functional theory to investigate the electronic properties of GNRs with 5-7 line defects. To calculate transport properties related to electronic structure, we used the non-equilibrium Green&’s function method. As a result, we have found that line defects arranged along the direction of electronic transport work as like an edge state and give conduction paths additionally. Our results suggest that such line defects can be effective for fabrication of low-resistance GNR interconnects. This work was conducted as part of the Ultra Low Voltage Device Project supported by the New Energy and Industrial Technology Development Organization (NEDO) and the Ministry of Economy, Trade and Industry (METI) of Japan.

While extensive efforts have focused on enabling cost effective mass production of solution processable graphene sheets, no approach has been reported to directly produce highly conductive and “clean” graphene sheets of controlled lateral sizes without involving any toxic regents or metal containing compounds and without generating toxic byproducts. With an aim to develop such an eco-friendly approach, this work reports that the synergy of piranha, intercalated molecular oxygen, and microwave heating enables controlled oxidation of graphite particles leading to rapid (60 seconds) and direct generation of highly conductive, clean graphene sheets without releasing any toxic gases and/or any potentially toxic aromatic byproducts, demonstrated by gas chromatography-mass spectrometry. These highly conductive graphene sheets have unique molecular structures, different from both reduced graphene oxide and pristine graphene sheets while combining many of their merits. They can be dispersed in both aqueous and commonly used organic solvents without surfactants/stabilizers and hence solution phase “clean” graphene sheets are obtained. “Paper-like” graphene films are generated with a conductivity of 22,790 S/m at their electrical percolation which is the highest conductivity of graphene films prepared via simple vacuum filtration from solution processable graphene sheets. After 2-hours low temperature annealing (300 °C), the conductivity further increased to 74,433 S/m. Most importantly, due to the short reaction time and open loop operation conditions (ambient conditions), this approach is scalable for mass production with a continuous flow microwave system. This totally eco-friendly, rapid approach for scalable production of high conductive and “clean” solution phase graphene sheets of different lateral sizes would enable a broad spectrum of applications from surface coating, energy storage to drug delivery, all by solution processing techniques of low cost.

Inspired by the strong near infrared (NIR) absorption, high photothermal conversion efficiency, and the exceptionally large surface area of graphene, graphene nanosheets have emerged as a new high-potential nanomaterial for biological applications, especially in the areas of photothermal therapy including photothermal enhanced drug and gene delivery systems. It would be highly desirable to monitor the in vivo distribution of multifunctional drug delivery systems, evaluate their post-treatment therapeutic outcomes in situ, and most importantly, to track the long term fate of graphene sheets in the human body. These capabilities could largely facilitate their application in practical multifunctional nanomedicine regimes, fighting various diseases.
Several strategies have been developed to fabricate nanosized graphene sheets. No strategy has been reported to directly fabricate graphene nanosheets (instead of GO nanosheets) in a one-pot reaction. Herein we report an unexpected discovery that monodispersed graphene nanosheets can be directly and rapidly (30s) fabricated via microwave assisted nitronium oxidation chemistry. It is noteworthy that this method of fabricating nanosheets involved no toxic metal compounds or reduction agents during the fabrication, and the product can be easily cleaned and purified, which is in highly contrast to those fabricated via Hummer&’s method or modified Hummer&’s methods. Another merit of the produced ME-LOGr nanosheets is that they can be directly dispersed into aqueous and other polar organic solvents without surfactants or stabilizing agents, allowing for the production of solutions of graphene nanosheets with “clean” surfaces. Most importantly, without the requirement for post-reduction processes, the fabricated graphene nanosheets exhibit strong NIR absorption, high photothermal, and photoacoustic conversion efficiencies. Therefore, they possess great potential as nanocarriers to develop multifunctional drug delivery systems with “on demand&’ release and in vivo photoacoustic imaging capabilities for in-situ evaluation of therapeutic effects and for tracking their long term fate.

As the mechanism under chemical vapor deposition (CVD) grown graphene continues to get revealed, it has become possible to selectively synthesis a more purpose qualifying graphene compared to its discovery in year 2004. Of understanding the CVD graphene growth mechanism, growing uniform bilayer graphene (BLG) seems to be more challenging compared to growing monolayer graphene (MLG). Yet clean growth of BLG brings attention as it offers distinguishable characteristics; mainly exhibiting band gap. In this study, we deposited Pt, Au, and Ni sources individually of varying thickness on thin copper foil to prepare heterogeneous metal catalysis. Metal alloys tend to serve as a better catalysis for BLG growth than of pure copper template because it is more reactive with carbon sources and has a desirable chemical activation energy. Such samples at under high temperature CVD growth process undergoes surface reorientation and adhesion-diffusion process of injected Ar, H₂, and CH₄ precursors. The amount of deposition for each metal were manoeuvred to find the most feasible condition of growing uniform BLG. In addition, the pre-washing and Ar annealing of the alloy before and after metal deposition showed a critical quality improving step in preparing uniform surface morphology of the alloy.
The BLG growth dynamics and its quality were identified using various analyzing methods such as Raman spectroscopy mapping, transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The scanning electron microscope (SEM) images showed BLG graphene domain growth progress in time. In particular, graphene grown on Cu/Au alloy show larger and uniform domain sizes while on Cu/Ni alloy, the domain sizes were smaller but more in number. The atomic force microscopy (AFM) was used to visualize the surface morphology and to measure the height of the adlayer. Also, the FETs were designed to measure the electrical properties such as sheet resistance, carrier mobility, and on-off ratio performance. These collected data was analyzed to define which heterogeneous metal conditions showed the most favorable for growing quality uniform BLG. Continuous study in proving the theory behind the CVD graphene growth mechanism is necessary for growing quality consistent graphene which can become competitively ready to be out in the applicable market.

Graphene oxide (GO) have a potential to trap carbon dioxides, according to recent experimental studies. We are interested in how a CO₂ molecule is trapped between GO layers, because of environmental concerns. We employed DFT calculations with PBE functional under periodic boundary conditions to investigate the energetics of CO₂ migration within hydrated or anhydrous GOs. Our supercell contains 64 carbon atoms attached by some epoxy and hydroxyl groups plus intercalated water molecules, abbreviated by C₆₄O₄(OH)₁₆bull;nH₂O. When anhydrous GO structures contain a CO₂ molecule, the CO₂ interacts repulsively with the GO layers to increase the interlayer spacing. The repulsive electrostatic interactions are reduced by the insertion of water molecules into CO₂-containing GO structures due to the occurrence of attractive water-layer interactions through hydrogen bonding. Consequently, the interlayer spacings in CO₂-containing hydrated structures are shortened compared with those in the anhydrous structures. The results indicate that the intercalated water molecules have the ability to connect the GO layers in the presence of CO₂. Furthermore, the DFT calculations indicated that the GO interlayer spacings, which are influenced by the intercalation of water molecules, control CO₂ migration within the GO layers. The importance of the interlayer spacings on the migration of CO₂ arises from the occurrence of repulsive interactions between CO₂ and oxygen-containing groups attached on the graphene sheets. We calculated the energetics of CO₂ migration within hydrated or anhydrous in GOs layers. We obtained the transition states of the CO₂@C₆₄O₄(OH)₁₆bull;nH₂O structures, respectively (n=0~2). The activation energy for CO₂ migration is sensitive to the number of water molecules. The energy values decrease from 44.0 to 8.4 kcal/mol when n decreases from 2 to 0. The results can be understand as follow. When the GO interlayer spacings are short due to the presence of intercalated water molecules, the repulsive interactions between CO₂ and the GO layers are strong enough to prevent CO₂ from migrating from its original position. Such repulsive interactions do not occur during the migration of CO₂ within anhydrous GO structures because of the relatively longer interlayer spacing. Accordingly, CO₂ migrates within anhydrous GO with a less significant barrier, indicating that CO₂ molecules are easily released from the GO. In terms of the repulsive interactions during CO₂ migration, water molecules play two roles. One role is that water molecules directly repel CO₂ through electrostatic interactions. The other role is that water molecules connect the two GO layers through hydrogen bonds. The two layers being closely separated strongly repel the intercalated CO₂. The DFT findings are helpful for understanding mechanisms of trapping CO₂ molecules in the interlayer space of hydrated GOs and for designing nanocarbon materials as promising candidates for capturing greenhouse gases.

Emerging electronics including bendable and rollable displays, and flexible sensors come closer to reality by showing the feasibility of industrial-level production of high quality graphene sheets by Chemical Vapor Deposition (CVD). However, transferring on the desired substrate and patterning for graphene device fabrication are still limited. The quality degradation is evitable during transferred on a desired flexible substrate, which is mainly incurred by the chemical damage and residues on removal of the support layer such as PMMA and the thermal damage by the use of a Thermal Release Tape (TRT). As for patterning, existing methods including lithographical methods and plasma etching are costly and hardly scalable as well as require complicated pre-defined masking and wet chemical etching processes.
Here we present a roll-to-roll patterning and transfer of graphene sheets capable of residue-free, no chemical treatment, and fast patterning. The graphene sheet attached to a Pressure Sensitive Film (PSF) is continuously patterned by applying pressure selectively with the pre-defined embossed roll. The patterned graphene sheet is adhered to the PSF with very low strength and can be easily transferred to the curved surface or a variety of flexible substrate without the aid of any heating mechanism. Compared to the transfer by the TRT and the PMMA support, the reduction in the occurrence of debris and defects was verified through Raman spectroscopy. The measurement of mobility and Dirac voltage by a Field Effect Transistor (FET) also confirmed that the doping effect caused by residues can be minimized with the PSF transfer.
The patterned graphene electrodes fabricated by the proposed method was incorporated into a fully functional touch-screen panel device. The width of the patterned film was 220 mm with the smallest line width of 20mu;m and the production rate of 0.3 m/min for patterning and transfer was achieved. Large-area uniformity was also confirmed by observing the variation of the sheet resistance of ~30% for 200 x 200 mm² and the representative value of ~300Omega;/sq before doping. We also demonstrated the renewable transfer process of graphene sheets that utilizes the lamination of the PSF followed by the pressure released delamination, showing only 15% decrease in the sheet resistance even after four times use.

Copper foil is the most common substrate to synthesize monolayer graphene by chemical vapor deposition (CVD). The surface morphology and conditions of the copper foil can be very different depending on the various suppliers or different batches. These surface properties of copper strongly affect the growth behavior of graphene, thus rendering the growth conditions irreproducible when different batches of Cu foils are used. Furthermore, the quality of the graphene is severely affected as well. In this work, we report a facile method of copper pre-cleaning to improve the graphene quality and reproducibility of the growth process. We found that the commercial Ni etchant (based on nitric acid) or nitric acid is the most effective cleaning agent among various acidic or basic solutions. The graphene grown on thus treated copper surface is very clean and mostly monolayer when observed under scanning electron microscopy (SEM) and optical imaging, as compared to the graphene grown on untreated copper foil. Different batches (but same catalog number) of copper foil from Alfa Aesar Company was examined to explore the effect of copper pre-cleaning, consistent growth results were obtained when pre-cleaning was used. This method overcomes a commonly encountered problem in graphene growth and could become one of the standard protocols for preparing the copper foil substrate for growing graphene or other 2D materials.

Today it is possible to routinely synthesize nanoscale building blocks with many excellent material properties, such as graphene or carbon nanotubes which have unique electrical conductivity, chemical structure, and mechanical resilience. However, support structure is required to utilize carbon nanomaterials for various applications. It seems natural therefore; the two building blocks in the new nanocomposite are both exciting materials in their own properties.
In previous methods, functional carbon nanomaterials are simply mixed with polymer solution or coated on polymer matrices. However, disordered fillers arrays, and inhomogeneous structure were generated by these methods. In addition, high dispersity and strong attachment of carbon nanomaterials on polymer matrices are still unsolved problems that still need to be solved.
In this work, we present that the polymer-mediated in situ BC-hybridization methods. Using amphiphilic comb-like polymer (APCLP) as a blending partner provides a green, easy, facile, efficient, and multi-functionalizing platform for generating new type of carbon materials incorporated scaffolds. In sharp contrast with the “ex-situ” hybrization method adopted before, in this study we take a “bottom-up” approach to design a new type of biomimetic materials incorporated functional nano-materials which shows several appealing biomimetic features. As building blocks for the self-assembly process we chose to use the bacterial cellulose from G.xylinus.
We first evaluated the effect of the factors and to find out the optimum concentration of each factor by comparing maximum BC production and BC yield. Five experimental factors, pH, temperature, culture time, ethanol concentration, and APCLP concentrations, were considered to have the most significant effect on the BC production of G.xylinus.
In a second set of experiments, to incorporate carbon nanomaterials with nanofibril, we propose a simple 1 step approach with synthesizing bacterial cellulose (BC), naturally formed microporous nanofibril networks, in a solution containing graphene or carbon nanotubes. As a result, the carbon nanomaterials are well incorporated not only on the surface of but also inside the microporous BC nanofiber scaffolds, which are evidenced by Raman spectroscopy, scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS).
Thus, this novel bio-inspired new concept strategy represents an efficient approach for controlling and improving the properties of bacterial cellulose and opens the avenue towards the biomimetic production of bacterial cellulose composite with demanded properties for successful regenerative medicine and tissue engineering applications.

We developed a general route to fabricate graphene based free standing, carbon black and binder free, flexible electrodes for high energy lithium ion battery. Various particles (element sulfur, element Tin, Tin oxide and Li_1.2Mn_0.5Ni_0.3Co_0.3O₂) were wrapped by graphene oxide through a simple solution phase assembly approach, no special interaction was needed. The as-prepared composite can be easily fabricated as free standing, flexible film and directly used as anode/cathode after recover graphene&’s conductivity through thermal annealing. A free standing, flexible electrode of SnO₂@Graphene was fabricated and used as an example for high energy lithium ion battery. The inter-connected graphene network functions as a conductive buffer matrix for the volume expansion of SnO₂ during charge and discharge. A high specific capacity of 830 mAh/g was retained after 100 cycles for SnO₂@Graphene free standing, flexible electrode at the current density of 200mA/g. The assembly method developed in this study is general, robust and easy to apply on other functional materials rather than battery material, which opens up an easy path to fabricate flexible devices.

Layer-by-Layer (LbL) assembly has been widely developed as one of the most powerful techniques to prepare multifunctional films for various energy storage and conversion devices. In spite of recent progress in the LbL-based electrochemical devices, the precise control of electrochemical behavior is still challenging in 3-dimensional LbL structure. In this study, we report electrocatalytic thin films for methanol oxidation by adjusting the assembly sequence of LbL films based on the graphene nanosheets and metal nanoparticles. To investigate the structural effect on the electrochemical behavior, the gold and palladium nanoparticles are assembled into the graphene oxide (GO) with three different sequence of LbL process: (GO/Au)_n(GO/Pd)_6-n, (GO/Pd)_n(GO/Au)_6-n, and (GO/Au/GO/Pd)_n. The electrocatalytic activity of (GO/Pd)_n(GO/Au)_6-n is higher than that of (GO/Au)_6-n(GO/Pd)_n in most of the cases, indicating that the electrocatalytic activity can be highly affected by the position of metal nanoparticle in the LbL structure. This phenomenon also represents the structural dependence of ionic transport between the electrode and the electrolyte. In the case of (GO/Au/GO/Pd)_n LbL structure, resulting in the increase of the contact layers between Au and Pd nanoparticles, the electrocatalytic activity shows higher than that of any other metal nanoparticle imbedded LbL structures. The X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the enhanced catalytic effect of the LbL films can be induced by the electronic modification of metal nanoparticles without an alloy or direct contact between the heterogeneous metal nanoparticles, owing to the electronic conduction of graphene nanosheets within the LbL structure. It is expected that this work provide insights into the modulation of intra-layer of LbL thin film.

Surface fluorination of carbon nanomaterials has been extensively studied, especially to utilize high energy density and high power density in energy storage devices. Most systems monitored high discharge current rates during the initial intercalation process of fluorinated species in the layered nanocarbons. Among all, fluorographenes (FGs) have shown enhanced charge storage properties, of which band gaps could be tuned by varying the fluorine content for the fabrication of high performance electrodes. FGs are also known as wide band-gap semiconductors. Their conductivity is controlled by the sp²/sp³ carbon-fluorine interactions. Surface or bulk fluorination of graphene-derived materials via edges or through the basal-plane defect sites has been also shown to control the degree of sheet resistance. Graphene surface or edge modification via fluorine functionalization is therefore hunting novel FGs for their implementations in graphene electronics and optoelectronics.
Major challenges have been the control and characterization of the surface properties in the presence of fluorinated functional groups, particularly involving oxyfluorination in the cases of fluorinated graphite/graphite oxide. Fluorine chemistry is very toxic which also limits the experimental trials and proper characterization by minimizing the environmental effects. Also high electronegativity of fluorine fosters the chemical reactivity with carbon leading both as a fluorinating agent and an oxidant. Therefore nature of the C-F bonds is crucial to investigate fluorination mechanisms and understand the fluorine-carbon interactions. For this purpose, we studied fluorination of GO (FGO) at either 10 or 15 psi for 24 hours using a pure fluorine/nitrogen gas mixture and compared with fluorographite. Structural investigations were performed by Infrared Spectroscopy combined with DFT calculations, Raman, X-Ray Photoelectron Spectroscopy techniques, and Powder X-Ray Diffraction to investigate both in-plane and out-of-plane C-F bonds and their oxygenated species.
We found that fluorination of GO occurred mainly on the surface and two new absorption bands appeared at ~ 743 cm^-1 and 482 cm^-1, that was correlated well with the presence of out-of-plane surface fluorine bonds only in FGO and could not be detected in fluorographite. Indeed introduction of oxyfluorinated species into the graphitic carbon could be explained by fluorination of epoxides. Based on elemental analyses, fluorination of GO also resulted in ~5-7 times more fluorine (4.57 at.% and 6.64 at.%, respectively) incorporation in bulk as compared to the fluorographite. Moreover the stability of the formed C-F bonds was studied by infrared spectroscopy in-situ in the presence of inert nitrogen flush or by exposing to the moisture. PXRD analyses indicated that the interlayer spacing of FGO expanded in the presence of intercalated C-F species and a defect formation was observed by the increase of I_D/I_G ratio from Raman analyses.

We investigate electrical conduction behaviors in dual-layer graphene/hexagonal boron nitride heterostructure. Graphene has been touted as a good conductor for a number of applications. Stacked graphene may overcome the intrinsic limit of monolayer layer which possesses extremely small conducting cross-sectional area. However, the inter-layer scattering between the two adjacent graphene layers should be minimized. In this work, dual-layer graphene heterostructures are demonstrated and characterized. Comparison of conduction properties is made among the new heterostructure, monolayer graphene, AB (Bernal)-stacked bilayer graphene, and stacked bilayer graphene. Significant improvement in current-carrying capability is observed in the graphene heterostructure. Improved reliability (under electrical stress) is also observed in terms of time-to-failure.

We present a novel method to achieve chemical functionalization between graphene and the substrates. In contrast to pristine graphene, the monolayer graphene placed on chemically activated SiO₂/Si substrates exhibits semiconducting behaviors, including low conductivity, high current on/off ratio, and large temperature dependence of transport properties. We observe nonlinear current-bias voltage curves and a transport gap at cryogenic temperature. Raman spectroscopy study shows evidences of sp³ hybridization of graphene, which indicates the chemical functionalization between graphene and the substrates. This study demonstrates formation of chemical bonding between graphene and the substrates, thus pointing toward an alternative method to engineering energy gap of graphene for future graphene-based electronic applications.

The development of portable, flexible, and lightweight energy-storage devices has been actively investigated in a wide range of emerging applications, including wearable electronics, electronic newspapers, and other devices. Supercapacitor, known as electrochemical capacitors, is one of the most promising energy-storage devices owing to their high power, high energy density, and long cycle life. For this reason, research on supercapacitor for portable electronics is in progress.
Conducting polymers (CPs) are widely recognized as promising electrode materials due to their high specific capacitance and rapid redox-based charge-discharge behavior. Among of those materials, Polythiophene (PT) is the most studied material because of their facile electrochemical synthesis and good chemical stability. However, few examples of its close analogue, polyselenophene (PSe), have been reported. PSe is expected to show various unique properties compared with other conducting polymers, particularly PT, such as lower bandgap, greater polarizability, planarity, and accommodating more effectively charge on doping. However, reported synthetic methods require multiple steps and tedious conditions, limiting the practical use of PSe in energy devices. Therefore, the development of a facile and simple synthetic method is desired to enable using PSe in energy devices.
Recently, graphene nanohybrids (NHs) with CPs have been studied as energy storage devices due to their synergetic effects, including fast electron/ion transport in electrodes, enhanced surface area, and electrochemical stability. However, the low energy and power densities of supercapacitors limit their practical application. Thus, design of novel hybrid materials with broad voltage window potential and rapid charge-discharge behavior is required to enable the practical development of high-performance supercapacitors.
In this work, we synthesized novel graphene-PSe NHs for use as electrochemical energy storage materials via in-situ polymerization method. The synthesized graphene-PSe NHs exhibited a broad potential window voltage, a large surface area, and high conductivity based on the synergetic influence of graphene-PSe NHs. The graphene-PSe NHs displayed enhanced specific capacitance, energy, and power densities compared with those of individual PSe or graphene layers. To evaluate the practical application of the graphene NHs in a flexible energy-storage system, all solid-state supercapacitors were fabricated based on graphene-PSe NHs using polymer hydrogel electrolyte. The fabricated solid-state supercapacitors were lightweight and exhibited high electrical performance. Furthermore, the solid-state graphene-PSe NHs could be folded and bent without introducing any defects in the device, maintaining the structural integrity even after several bending cycles.

Techniques to engineer the bandgap in graphene have attracted significant attention for applications in graphene-based electronics. To date, diverse strategies for the formation of a bandgap in graphene structures have been developed, including graphene quantum dots (GQDs) and graphene nanoribbons (GNRs). Among graphene nanostructures, GQDs have recently emerged as a potential candidate for fluorescent probes in bioimaging and semiconductor materials in electronic devices due to their unique characteristics, such as high surface area, large diameter, and enhanced surface grafting using the π- π conjugated network or surface groups. Interestingly, GQDs also exhibit upconversion photoluminescence (PL) properties, making them a valuable platform for photoelectrochemical cells.
To date, diverse synthetic strategies for GQDs have been developed, including electron-beam lithography, ruthenium-catalyzed C₆₀ transformation, and hydrothermal and electrochemical approaches. However, a dialysis process was required in most previous methods for the removal of additives or unreacted residual materials. Nevertheless, dialysis limits the size separation of GQDs with a diverse range of size. Furthermore, this technique typically takes several days, entails high cost, and requires additional steps for the removal of excess amount of solvent. Thus, it is necessary to develop an efficient and facile strategy for the synthesis of GQDs of various sizes.
In this presentation, we propose a simple and reliable approach for fabricating well-defined and low size distribution GQDs based on the oxidation of herringbone-type carbon nanofibers (HCNFs) and size-selective precipitation, which quickly separated nanometer-sized GQDs from the bulk GQD solution without a time-consuming dialysis process. This novel and facile approach describes the tailoring of PL emission of GQDs by varying the oxidation temperature and sedimentation velocity, which effectively produced GQDs of varying sizes. Additionally, the upconversion properties of GQD were determined and the response mechanism of the upconversion GQD-layer-modified working electrode in dye-sensitized solar cells (DSSCs) was investigated. In particular, the optimized GQD-layer modified DSSC led to ca. 9.4 % enhanced circuit density (J_sc) with long-wavelength absorption bands at 850 nm. The synthesized GQDs proved to be a feasible candidate for a phosphor in photovoltaic devices to enhance the light-harvesting ability in the long-wavelength range. This novel approach could be used as an alternative method for fabrication of various GQD based photovoltaic devices with rational nanostructure design.

Mercury (Hg) has been used as numerous industrial and domestic applications, including chemical additive, battery and thermometer, for decades. Hg can be extremely toxic, both to human health and to the environment. To improve safety, various types of mercury sensors have been invented to observe Hg²⁺ ions, including colorimetric analysis and photoelectrochemical methods. In particular, fluorescence method has been widely used owing to the simplicity. However, fluorescence sensors have shortcomings including the large pieces of equipment, which are not portable and time-consuming process. Other conventional Hg sensors have limitations, including a slow time response, low sensitivity and/or poor selectivity.
Graphene has superb thermal conductivity, excellent mechanical strength, flexibility, high conductivity and outstanding charge carrier mobility. Because of these properties, it has recently become the subject of considerable research interest for a wide range of applications, including supercapacitors, electronic devices, solar cells, biomedical applications and biosensors.
Transistor-based sensors have potential for the development of miniaturized and portable sensor systems with effective interfacing transfer. Graphene transistor can be integrated with liquid-ion gated field-effect transistor (FET) geometry. The liquid-ion gated FET system with a graphene transistor has shown low-voltage operation, rapid response and excellent stability in the liquid state.
In this presentation, we develop a straightforward fabrication methodology for flexible graphene-based aptasensors and demonstrate devices with high sensitivity and selectivity for Hg detection. Chemical vapor deposition (CVD)-grown single-layer graphene was successfully transferred onto a flexible substrate and integrated into the liquid-ion gated FET system via surface engineering. The graphene-based aptasensor had a rapid response time of < 1 s and a strong field-induced response, leading to a high sensitivity toward Hg²⁺ ions, with a detection limit of 10 pM, which is 2-3 orders of magnitude more sensitive than previously reported Hg sensors based on electrical measurements. The graphene-based aptasensor showed high selectivity toward Hg²⁺ ions in mixed solution containing numerous other metal ions and in real world samples derived from mussels. This is mainly because the specific binding of Hg²⁺ ions is facilitated by the interaction with the thymine base pairs in the aptamer, which forms a thymine-Hg²⁺ ion-thymine (T-Hg²⁺-T) complex. The flexible aptasensor system demonstrated excellent mechanical properties. Due to its high-performance, this graphene-based aptasensor has potential for detecting Hg exposure in human and in the environment.

Graphene has emerged as a potential material for optoelectronic applications due to its high electronic mobility and unique doping capabilities. We theoretically demonstrate the excitation of plasmons in large-area continuous graphene films using silicon gratings or graphene nanoribbons with the transparent substrate chosen potentially from a wide range of materials including insulators, semiconductors, polymers, and gels in mid-infrared regions. The plasmonic properties, e.g. the resonant wavelength, magnitude and Q-factor, can be tuned over a wide range via structure modulation and/or gating of graphene. The transmission dip is achieved over a wide angle range since the transverse magnetic (TM) polarization is independent of incident angles. Moreover, we found that the graphene/gratings hybrid structure has a high Q-factor (~66) and a sharp notch (with the full width at half maximum of ~122 nm) in the transmission spectra after adding a low-permittivity insulator underneath graphene. Numerical simulations demonstrate that the sensitivity of such a graphene plasmonic resonator can achieve a high value of up to 1697 nm/RIU (refractive index unit) when the wavelength shift at the plasmon resonance is detected. The excellent features of our proposed structures would make high performance graphene-based plasmonic sensors possible.
References
1.Y. Zhao, X. Hu, G. Chen, X. Zhang, Z. Tan, J. Chen, R. S. Ruoff, Y. Zhu and Y. Lu, Physical Chemistry Chemical Physics, 2013, 15, 17118-17125.
2. Y. Zhao, G. Chen, Z. Tao, C. Zhang and Y. Zhu, RSC Advances, 2014, DOI: 10.1039/C4RA03431G.

Trace gas detection at room temperature is important for a wide range of practical applications. In this study, Graphene oxide multilayer films were fabricated by a simple layer-by-layer assembly technique of oppositely charged graphene oxide followed by thermal reduction. With controlling the number of graphene multilayer, tunable selectivity toward electron-withdrawing and electron-donating gas molecules was attained. On the basis of X-ray photoemission spectroscopy, Raman spectroscopy and field-effect transistor analysis, we found that nitrogen atoms are incorporated into the reduced graphene oxide films and substituted carbon atoms during the thermal reduction. This reduced graphene oxide multilayer sensors showed selective detection of NO₂ and NH₃ down to very low concentrations. We attribute the observed high selectivity and sensitivity to well-known charge transfer mechanism of nitrogen doped graphene oxide multilayer sensor. This study shows the potential of highly selective gas sensor based on simple fabrication of graphene nanosheet multilayers.

In this work, the electrical and thermal properties of polyamide 6 (PA6) composites filled with different content of graphene nanoplatelets (GNPs) from 20 to 40 wt.% have been studied. The use of GNPs was found to be effective in increasing electrical and thermal conductivity of the polymer composites probably due to the enhanced connectivity offered by structuring filler with high aspect ratio in fillers.
The thermal conductivity increases from 0.25 to 7.20 W/mK for a #64257;ller content of 40 wt% of GNPs, whereas the electrical resistivity decreases more than seven orders from an insulator (pure polyamide 6, 10^13-10¹⁵Omega;/#9633;) to 100Omega;/#9633; (40 wt% of GNPs)

Effective thermal management is becoming increasingly important for high-power electronics and portable devices, e.g., smart phones and tablets, in which graphene and graphene-based paper (GP) structures may find applications due to their potentially high thermal conductivity and light-weight. However, the exceptional thermal properties of high quality graphene have not been realized for GPs upon assembly of individual graphene sheets prepared by chemical approaches, and the production of large area GPs also remains a challenge. Here, we demonstrate that large area free-standing GPs can be fabricated by direct electrospray deposition integrated with a continuous roll-to-roll process and separated by water exfoliation from highly hydrophilic substrates. Upon mechanical compaction and elimination of functional groups and defects by thermal annealing at the optimized temperature of 2200 °C, GPs can achieve a thermal conductivity of ~1238 Wm^-1k^-1 at a density of ~2.1 g/cm³, representing an over ~700% improvement as compared to GPs without annealing and ~400% improvement over the state-of-the-art GP materials. Higher temperature annealing at 2850 °C further increases the thermal and electrical conductivities to 1434 Wm^-1K^-1 and 1.83×10⁵ S/m, respectively. The highly thermally conductive, light-weight and flexible GPs display an outstanding heat spread ability and are more efficient in removing hot spots than Cu and Al foils, offering immense potential for superior thermal management and heat transfer.

Graphene, as a new generation carbon material, has attracted a great deal of attention in recent years. The two dimensional, one atom layer graphene exhibits exceptional electrical and mechanical properties. It is been reported that graphene&’s electronic properties (eg. band gap) and Raman behavior may vary with different under layer substrates. These influences may also transfer across graphene interface, and affect the electrocatalytic activities of graphene as well as other species above it.
The oxygen reduction reaction (ORR) is an important cathodic reaction in fuel-cells and metal-air batteries. Due to its electrochemically active basal plane and extreme thinness, SLG can be used as an ideal platform to explore new concepts in catalysis. For instance, by sandwiching SLG between an active ORR electrocatalyst and an under-layer materials, the effects of electronic coupling across this thin material can be studied. Scanning electrochemical microscopy (SECM) was used to characterize the graphene&’s electrocatalytical activity locally with high accuracy and spatial resolution.
Two kind of ORR catalysts, metal (Au, Pd, etc.) and molecular N-4 catalysts (porphyrins and phthalocyanines) are chose to study. Large-scale, single-layer graphene was synthesized by chemical vapor deposition (CVD) method and transferred onto patterned metal substrate. After absorbing molecular catalysts on top of graphene sheet, the catalytic activity difference with/without metal pattern under-layer was probed by SECM. This work might provide guidelines for design of metal and organometallic complex electrocatalysts system with a high catalytic activity and long-term operation stability.

The growth of semiconductors in nanowire structures has increased dramatically upon the identification of catalytic methods for preferential growth along a single axis. However, catalytic growth does have some limitations. Growth conditions are limited to temperature ranges compatible with the catalyst, and preparation of the catalyst dots requires additional steps. The question of the effect on luminescent efficiency of impurity-level incorporation of the metals into the semiconductor has not yet been addressed quantitatively. For this reason, some applications may require the use of catalyst-free growth methods.
So, We have grown GaN nanowires on graphene sheet with metal-organic vapor phase epitaxy (MOCVD). No metal catalysts were used. Direct deposition of graphene on Si(111) substrate is demonstrated using a catalytic chemical vapor deposition (CVD) process. Single-layer graphene is formed through surface catalytic decomposition of hydrocarbon precursors on thin copper foils pre-deposited on Si(111) substrates. And the growth of GaN nanowire with high quality of vertical nanowire was attempted by MOCVD on the single-layer graphene. By controlling the substrate temperature and the gallium source, the growth of highly vertical GaN nanowires can be achieved.
The graphene used catalyst of GaN nanowire was confirmed by Raman spectra, the surface morphology, structural and optical characterization of the grown GaN nanowires on graphene were studied by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), photoluminescence (PL) and X-ray diffraction (XRD) measurements.

The goal of this work is the development of advanced thermal interface materials (TIMs) for satellite applications. We are focusing on graphene materials and composites to take advantage of the large intrinsic thermal conductivity of graphene (~5,000 W/mK). If even a small fraction of graphene&’s thermal conductivity be realized in new composite TIMs for spacecraft, overall thermal conductivity pathways could be greatly increased impacting many parts of spacecraft.
We are developing synthetic methods with the goal of creating novel materials and composites with high thermal conductivities out of the plane with good coupling to interfaces. Approaches to create three dimensional graphene materials and composites are being pursued, specifically the formation of graphene foams by chemical vapor deposition on three dimensional metal networks. Porous three dimensional graphene, with high intrinsic thermal conductivity can be made by chemical vapor deposition on porous metal frameworks and shows great promise for electrical and thermal applications. Methods to enhance phonon coupling rely on better contact and bonding at the interfaces through the creation of chemical bonds and the formation of compliant composites with high thermal conductivity. This report will describe the chemical vapor deposition of graphene foams, the formation of composites materials for TIM applications and their characterization.

We report tremendous structural evolution of carbon nanosheets (CNSs) derived from isotropic pitch prepared by reforming commercially available naphtha cracking bottom oil. CNSs with thickness of 2 to 13 nm were prepared using spin-coating on quartz directly without a catalytic material, and following two successive heat-treatments consisting of stabilization at 270 °C in air and carbonization up to 1200 °C in H₂/Ar. The development of well-ordered graphitic layers with increasing carbonization temperature was observed around porous structure only in the CNS with thickness of 13 nm by transmission electron microscopy, whereas thin CNS did not revealed well developed fringe growth. In addition, Raman spectroscopy showed strong 2D band with only thick CNSs, indicating unusually well-developed graphitic layers of CNSs. We believe that the spin-coated pitch molecules evolve into highly ordered graphitic layers at relatively low carbonization temperature (1200 °C) due to polymerization of small pitch molecules and their self-assembled structure during carbonization.

Graphene has received much attention because of its remarkable properties such as high specific surface area and high electrical conductivity. These properties satisfy diverse requirements for electrochemical electrode such as fuel cell and metal-air batteries. In those applications, the oxygen reduction reaction (ORR) activity has been considered as one of significant factors. For this reason, various precious metal catalysts have been investigated for enhancing ORR activity. In recent years, research efforts have been focused on developing metal-free catalysts due to high costs of metal catalysts.
Most of carbon-based metal-free catalysts were synthesized by chemical vapor deposition (CVD) method, which is too expensive method for mass production compared to hydrothermal method. In this study, we synthesized nitrogen and boron co-doped graphene from graphene oxide by microwave-hydrothermal method. The process time for synthesizing samples can be significantly reduced result from application of microwave heating. The samples are prepared under different heating temperature conditions. For observing the morphology of samples, scanning electron microscopy (SEM) was carried out. Raman spectra were obtained at an excitation wavelength of 532nm, and we confirmed that intensity ratio (I_D/I_G) increased with heating temperature. The X-ray diffraction (XRD) patterns were recorded from Rigaku SmartLAb using Cu-Kα radiation. We investigated the chemical composition and bonding configuration of the samples by X-ray photoelectron spectroscopy (XPS). The XPS results demonstrated that nitrogen and boron atoms are successfully doped on graphene layers. Catalytic activities were characterized by three-electrode system. Linear sweep voltammetry (LSV) was measured in 0.1 M KOH electrolyte purged oxygen gas over 1 h, with a 1500 rpm electrode rotation speed and a 10 mVs^-1 scan rate. LSV results reveal that synthesized samples show higher ORR activity compared to that of non-doped sample. It may be concluded that the synthesized nitrogen and boron co-doped graphene by microwave-hydrothermal method has a great potential as an effective metal-free ORR catalyst.
Acknowledgement
This research was supported by the MSIP(Ministry of Science, ICT and Future Planning), Korea, under the “IT Consilience Creative Program” (NIPA-2014-H0201-14-1001) supervised by the NIPA(National IT Industry Promotion Agency)

Graphene attracts much attention due to its excellent properties, such as mechanical strength, high carrier mobility, surface sensitivity, and atomic-scale thickness. These properties are expected to open various new application fields, such as field-effect transistor (FET)-based high-frequency electronic devices, interconnects, transparent conductive electrodes and highly sensitive sensors. The synthesis of multi-layer graphene is also investigated to provide a great endurance of the current to an electrical wiring system. However, it is difficult to obtain uniform multi-layer graphene with large domains. Weatherup et al. reported that carbon diffusion barriers are useful to fabricate a uniform, high-quality graphene in the precipitation method [1]. In this study, the method is developed to provide multi-layer graphene with large domains by increasing the amount of the precipitated carbons with increasing both the time and the temperature in annealing.
After depositing Au (20nm)/ Ni (300nm)/ Al₂O₃ (2nm)/ a-C (40nm) on a sapphire substrate using e-beam deposition, the samples were annealed in vacuum at 900^oC for 5, 30, 60 and 120min, respectively. The processed samples were characterized on the morphology by optical and scanning electron microscopes, and on the quality by Raman spectroscopy.
The surface images shows that the coverage and the number of the graphene layers increased with the annealing time. The alternation of the contrast in the images shows that after the graphene started to cover the surface of the catalyst, the nucleation of graphene followed under the layer. The size of the islands increased with the time and finally reached to 15mu;m in a diameter at 120min. Judging from a strong, sharp G peak and a tiny D peak, which resulted in a very small D/G ratio of less than 0.01, the qualities of the graphene are excellent. The G&’/G ratios in the Raman signals also prove the special morphology of the layer, with 2 or 3 ML-thick graphene domains overlaid by 1ML-thick uniform graphene.
[1] R. S. Weatherup et al., Nano Lett., 13 (2013) 4624.
Acknowledgment: This work was supported in part by Specially Promoted Research (No.25000011) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Carbon related materials as typified by graphene and carbon nanotube are attracting interest from a wide range of fields, because these have the unique properties, such as the electron and thermal conductivity and the photo property, and are possible to make from the source reagent which is abundance of natural resources. In energy storage device and fuel cells, the carbon material is employed as electrode material, electrocatalysts or catalyst supporter. These devices require high electron conductivity and low mass transport resistance. Recently, an ordered-structure carbon material is developed as electrode material for lithium ion secondly-battery and super capacitor. When the mass transport is considered at the electrode in the electrochemical cell, the ordered macro pore acting as the aorta for mass diffusion, and the micro or meso pores functioning as the capillary vessel for electrolyte ions are needed ideally. Generally, ordered-structure carbon material only possesses either the macro pore or the meso pore. A porous material which has both pore of micro and meso is reported, but its preparation method is very complex. In this study, an ordered-structure carbon material is prepared at easy process using graphite oxide nanosheets and characterized about morphology, crystalline and electrochemical property.
An ordered-structure carbon material was prepared from Layer by Layer (LbL) method of graphite oxide nano-sheets and co-polymer made from polyvinyl alcohol and polyvinyl ammine. Quantity of layer stack was controlled by number of LbL times at polymer spheres. After that, graphite oxide nano-sheets on polymer spheres were reduced until the graphene using reductant agent, for example hydrazine and sodium boron hydrate. On the other hand, hollow graphene spheres were synthesized by the heat treatment of modified polymer spheres by graphite oxide nano-sheets. The morphological information of these materials was obtained by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Degree of crystallization in synthesized carbon materials was evaluated by raman spectroscopy and X-ray diffraction analysis. In addition, nitrogen content was estimated from data of X-ray photoelectron spectroscopy. Electrochemical analysis was conducted using a three-electrode electrolysis cell.
The morphology of graphene shell polymer core particle was sphere shape. This particle was assembled each other and formed opal structure. In hollow graphene spheres, the morphology depended on the number of graphene layer. In case of one layer, the graphene spheres shriveled like a deflated balloon. In contrast, a hollow sphere made from five graphene layers kept spherical form. Results of raman spectroscopy indicated possibility that kept presence of the function group or the nitrogen doping after reduction process.

We applied layer-by-layer (LbL) technique to fabricate transparent alternative graphene oxide (GO) and titanate nanotube (TNT) films of nanosized thickness. By appropriately choosing synthesis conditions, we are possible to fabricate uniform films with alternative TNT and GO layers which are transparent from visible through UV (lower to 200 nm) light ranges up to eight bilayers. The photocatalytic activity of these composite films was found dependent on the bilayer number and pollutant concentration. The increased activity with decreasing concentration of methylorange (MO) indicates their effectiveness toward trace organic pollutants.

Carbon black additives have been produced for several decades. These materials are
designed to enhance the properties of the host matrix and provide reinforcement in
elastomers, color in coatings, toners, plastics, and inks, and electrical conductivity in a
variety of applications. The morphology and surface chemistry of these products are
designed to break trade-offs and optimize performance; not all materials are the same.
Graphene based materials are a new family of carbon products that hold significant
promise. These materials can lead to ground-breaking new applications in electronics
and displays. Also, some of these materials may enable many of the same applications as
carbon black while bringing additional functionality. For these new materials to become
commercially successful they must be engineered to deliver superior performance and
must be produced safely at the right scale for these applications. Examples in polymer
composites and energy storage applications will be discussed.

Dye-sensitized solar cells (DSSCs) with ZnO nanostructures electrode layers have been extensively studied due to their reasonably high conversion efficiency with effective low cost. Recently, considerable research on DSSCs has focused on improving the electron transport and reducing the recombination rate by the use of alternative semiconductor materials or core-shell structures. The graphene is ideal for application in flexible electronic devices due to its high electrical conductivity, flexibility and transparency. Graphene can be used as support to fix nanoparticles on substrates, besides of modifier the morfology for increasing the intrinsic properties of ZnO. In this work, it was synthesized ZnO nanostructures in the presence of reduced grapheme oxide (RGO) on flexible substrate by chemical bath deposition for application in dye sensitive solar cells (DSSCs). Zinc nitrate and HMTA were used as raw materials. Micrographs obtained from Field Emission Gun Scanning Electron Microscopy (FEG-SEM), RAMAN and X-Ray Diffraction patterns confirmed the success in the synthesis of perpendicular aligned ZnO nanorods supported on RGO sheets or ZnO/RGO nanocomposites on ITO/PET substrates. The electron microscopy work has been performed with the Inspect F50 microscope of the LME/LNNano /CNPEM. The effect of using RGO as powder or RGO sheets thin film on ITO/PET during the ZnO nanostructures synthesis on the structure and morphology was analyzed. The use of RGO during the synthesis suppressed the formation of ZnO microrods and favored the formation of ZnO nanorods. Dye sensitized solar cells was assembled by growing ZnO nanorods/RGO films or ZnO/RGO nanocomposites on ITO/PET substrate. The high electric mobility of ZnO and the good electrical conductivity of the RGO sheets leaded to devices with better efficiency. DSSCs mounted by using ZnO/graphene nanocomposites films as counter electrodes showed efficiency 4 time higher than pure ZnO films.

The excellent physical, electronic and optical properties of graphene have driven the exploration of novel techniques for large-scale synthesis. Towards this end, solution-processing of chemically-derived graphene through reduction of large-area chemically exfoliated graphene oxide (GO) sheets represents a promising development. However, the current synthetic protocols result in the production of metastable, chemically inhomogeneous and spatially disordered GO structures, and consequently reduced graphene oxide (rGO) with poor optoelectronic properties. We present a scalable, one-step mild annealing technique which facilitates clustering of oxygen functional groups in the GO sheets, thereby leading to improved sheet characteristics, while preserving the oxygen content. This is manifested in the large increase in the electronic conductivity by four orders of magnitude, strong absorbance in the visible region, and a blue shift in the photoluminescence, compared to as-synthesized GO. Upon further downstream processing, we show that chemically-derived graphene produced from our pre-treated GO offers a marked enhancement in the electronic conductivity of rGO thin films, at significantly lower temperatures compared to conventional thermal reduction methods. Our atomistic calculations, combined with the experimental evidence, suggest that the one-step annealing process results in an energetically favorable and kinetically accelerated phase separation of the mixed sp²-sp³ hybridized GO phase into prominent oxidized and graphitic domains. The resultant improvements in the optoelectronic properties can be attributed largely to an increase in the sp² cluster size, offering control over the oxygen and carbon removal from GO sheets, and helping tune the residual oxygen content and porosity in graphene; thereby providing an additional lever to fine tune the rich and interactive oxygen framework in GO. This represents an important processing milestone with potential benefits in a vast array of applications such as filtration membranes, chemical/biological sensors, green catalysts, and optoelectronic functional devices.

Nowadays, graphene based nanocomposites are intensively studied because of their unique electrochemical properties, which make them useful in many technological areas such as biomedicine, electronics, energy storage and environmental remediation [1]. In this work, we report a simple method to prepare graphite-encapsulated Cu nanoparticle reduced-graphene oxide nanocomposite in a single annealing step. This interesting hybrid nanocomposite was obtained by annealing copper-foil-supported pristine graphene oxide under Ar atmosphere at 1000 °C. TEM and EDAX analyses showed that Cu nanoparticles develop a highly faceted morphology with large arm-like nanorods (1-4mu;m) emerging from the facets {111} of the fcc Cu crystal structure. These nanorods comprise carbon and silicon as their components. The graphite shell of Cu nanoparticles consists of arround 15 graphene layers (5 nm thick). The efficient reduction of pristine graphene oxide after the annealing process was assessed by Raman spectroscopy and FTIR. This nanocomposite could be used in catalysis, energy storage and ink-jet printing technology [2].
[1] doi: 10.1039/C1CS15078B
[2] doi: 10.1039/C1RA00260K

Surface-enhanced Raman scattering (SERS) has been shown to be highly promising for biological and chemical detection. Aggregated metallic nanoparticles (MNPs) have been studied as SERS nanoprobes to detect specific biological and chemical markers with high-sensitivity because of increased plasmonic hotspot junctions. Recently, a variety of SERS nanoprobes have been reported but there were problems related to signal reproducibility and colloidal stability for SERS-based detection. Therefore, an ideal SERS nanoprobe for biomedical applications should have high signal enhancement with consistency and generate uniform response for quantitative analysis of target molecules. We report herein that MNP aggregation were induced by PEGylated nano-graphene (PNG) sheets, resulting in the formation of a new class of SERS nanoprobes with controlled nano-size for biosensing and bioimaging applications. The MNPs with Raman dyes were modified with hydrophobic molecules in colloidal form, and then complexed with PNGs via hydrophobic and other noncovalent interactions. The PNGs were prepared via (1) oxidation of graphite, (2) intensive sonication, (3) PEGylation, and (4) subsequent chemical reduction. PEGylation of nano-graphene imparted colloidal stability, minimal non-specific binding of biological molecules, and biocompatibility. The PNG-induced MNP aggregates (PNG-MNPAs) showed significantly increased SERS signal higher than individual MNPs due to electromagnetic field enhancement in interstices of the MNPAs. A series of comparative studies showed that the degree of π-π stacking interactions and other molecular interactions between PNGs and MNPs were controlled as compared to the corresponding nanoscale graphene oxide (NGO), potentially due to substantial elimination of negatively charged oxygen-containing functional groups in the basal plane of NGO, and increase in aromatic rings with hydrophobic domains. This study has great potential to advance graphene-based SERS nanoprobes for a variety of biosensing and bioimaging applications.

Poly-aromatic hydrocarbons (PAH) can be found in soil, air, meat, fish and etc and were proposed to be used as sucreficial laerys for graphene transfer. In our communication we report teh compartative study for PAH nanostructure nucleation and growth mechanizms on a single and multi-layer graphene CVD grown on a copper substrate and suspended graphene. The PAH deposition was performed by PVD in vacuum and resultant morphology of a PAH was studied by means of SEM and micro-Raman as a function of time, rate, substrate temperature and graphene thickness. We found that that at high coverage PAH predominantly grows in a form of nanowires which have a good vertical alignment with respect to a graphene plane. It was shown that temperature of a substrate, deposition rate of PAH, and number of graphene layers were the key parameters to control the PAH morphology such as a nucleation density and diameter of PAH nanowires. We relate the orthogonal growth of PAH nanowires to the discotic nature of PAH molecules forming weak VDW interactions with graphene basal plane and lamella like structures due to favorable face-to-face intermolecular interaction. We envision PAH nanostructures grown on a graphene substrates may help to optimize the graphene transfer protocols and PAH filters .

The stacking of two-dimensional (2D) materials is a promising approach to explore novel materials because of predicted excellent properties [1]. Recently, we have demonstrated to grow large area and orientation controlled graphene as well as to create stacked bilayer graphene with a homogeneous interlayer rotation angle [2]. However, CVD-grown graphene is suffering from formations of either wrinkles or strain because of deference of thermal expansion efficient between graphene and Cu catalyst. Here, we have studied controlling ultra flat graphene for ideal stacking method.
For wrinkle formation, we found that cooling rate can modulate the wrinkles in graphene. In the case of slow cooling (0.3 degree C/sec), a density of wrinkle is five times less than that of fast cooling (~ 50 degree C/sec). On the other hand, regarding strain estimated by Raman spectroscopy, symmetry matching between graphene and Cu catalyst is effective factor because only Cu(111) give strain to graphene. However, the degree of strain is 0.2 - 0.7 % which is much smaller than lattice mismatch of graphene between graphene and Cu(111) (~4% ). These results suggest that wrinkles and strains in CVD-grown graphene are tunable optimizing CVD conditions. Also, we will present making artificial multi-stacked graphene using ultra flat graphene.
[1] A. Geim et al., Nature 499, 419 (2013).
[2] L. Brown, E. B. Lochocki et al., to be submitted (2014).

Graphene is one of the most exciting atomic materials. Applications include electronic devices were electrons can cover micrometer distances without scattering, even at room temperature. Graphene is a perfect thermal conductor, the carbon-carbon bond length in graphene is about 0.142 nanometer and thus more than 190 times stronger than steel. These are just some of the amazing properties of graphene. In this work, we report on the epitaxial growth of graphene grown on the cubic polytype of silicon carbide by the reduced pressure cvd process. The cubic polytype of SiC was grown on regular silicon substrate. We report on state of the art motilities, Raman results and AFM data. The graphene has been used to fabricate capacitive devices and nitric oxide detectors. This work is supported by the National Science Foundation under the PREM program

Transparent conductive electrodes (TCEs) are of an increasing importance for a wide range of technologies such as information (displays) and energy (solar cells, architectural and window glass) applications. Significant amount of research efforts has been dedicated to the replacement for the brittle and expensive indium tin oxide (ITO) coated films currently used as the TCEs. We present here a novel and universal method for the in-situ reduction of graphene oxide (GO) on various polymeric and inorganic substrates to prepare reduced graphene oxide (rGO)-coated materials suitable for potential applications as TCE&’s. The surface is chemically modified to permit the in-situ reduction of GO to rGO on the surface of the substrate. The method is universal and can be applied for a wide variety of polymeric and inorganic surfaces. In addition, the simplicity of this method provides a possible mean to fabricate large scale and robust transparent conductive electrodes suitable for some of the above-mentioned applications.

#8203;Recent advances in the development of nanoporous graphene (NPG) hold promise for the future of water supply by reverse osmosis (RO) desalination. But while previous studies have highlighted the potential of NPG as an RO membrane, the literature provides few clues as to whether NPG is strong enough to maintain its mechanical integrity under the high hydraulic pressures inherent to the RO desalination process.
We will introduce a new framework for elucidating the mechanical resilience of nanoporous graphene (NPG) under the high pressures required for RO desalination. Using molecular dynamics and continuum mechanics, we will show that an NPG membrane can maintain its mechanical integrity under RO pressures, but that the choice of substrate for graphene is critical to this performance.
We will also describe the relationship between graphene porosity, membrane stress, and substrate morphology, and suggest a surprising result: that NPG membranes with greater porosity may withstand higher pressures, due to the fact that its stress increases monotonically with Young&’s modulus.

Since its discovery, graphene has held great promise as a metal with massless carriers and thus extremely high mobility. This feature is the result of the two-dimensional character of the band structure due to the so-called Dirac cone for the ideal, perfectly ordered crystal structure. One of the implications of this ideal case is that the transport properties of this material should be immune to lattice disorder. In reality graphene, which is subject to varying amounts of disorder that depends on preparation method, environment, impurities and other extrinsic variables has been shown to exhibit an effective mass. Thus, metallic behavior with a wide range of mobilities has been reported. This situation contradicts the prediction of Abrahams, Anderson, Licciardello, and Ramakrishnan[i] that all two-dimensional systems must be insulating. Yet, the metallic behavior of graphene is consistent with earlier reports of metallic behavior in high mobility metal-oxide field effect transistors (MOSFET) and interface oxides. These systems are presumed to be two-dimensional due to their geometry but might have some three dimensional character since the charge region can extend over finite distances which could explain their metallic transport properties. In contrast, graphene, with its single layer structure, is a model system for studying the two-dimensional metal-insulator transition (MIT). In this work, we systematically increase the resistivity of epitaxial graphene via the introduction of chemical moieties using very low temperature plasmas. These results reveal the existence of a two dimensional MIT in epitaxial graphene consistent with a more recent general scaling model.[ii] In this case, the strongly localized state is separated from the metallic state by a weakly localized phase with s~logT. Magneto-transport and Hall effect measurements show that weak localization (WL) and enhanced electron-electron interaction effects both contribute to the observed behavior but in contrast to theoretical assumptions, the WL effects dominate as the system approaches the strongly localized phase.
[i] “Scaling theory of localization: Absence of quantum diffusion in two dimensions.,” Abrahams, E., Anderson, P. W., Licciardello, D. C. & Ramakrishnan, T. V., Phys. Rev. Lett.42, 673-676 (1979).
[ii] “Scaling Theory of Two-Dimensional Metal-Insulator Transitions,” Dobrosavljevic #769;, V., Abrahams, E., Miranda, E., and Chakravarty, S., Phys. Rev. Lett.79, 455 (1997).

We report the synthesis of heterostructures composed of graphene and boron nitride (BN) films. The BN was synthesized by short-pulse laser-plasma deposition while graphene was synthesized by hot filament chemical vapor deposition. The heterostructures were fabricated by two different sequences: graphene deposited on BN and BN deposited on graphene. The crystalline quality of the BN-graphene and graphene-BN heterostructures was evaluated by Raman spectroscopy mapping and temperature-dependent Raman spectroscopy to analyze the phonon-phonon interactions in these layered structures. The physical properties of the heterostructured films were carefully studied by: optical transmittance, sheet resistance, thermal conductivity, band-gap, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The advantages and disadvantages of each sequential deposition process are discussed in terms of the crystalline quality and physical properties of the BN-graphene and graphene-BN heterostructures.

Epitaxial growth of graphene on SiC is one of alternatives to the exfoliation of graphite crystals. The mainstream technology, suitable for a large area fabrication of graphene is based on Si sublimation from SiC. Recently, another graphene growth method on SiC, referred to as the chemical vapor deposition (CVD) technique, has been demonstrated. [1]
In this presentation the results of micro-Raman spectroscopy, atomic force microscopy (AFM), and contactless microwave transport spectroscopy are employed to discuss the influence of the interaction with the SiC substrate on the properties of as-grown as well as hydrogen intercalated quasi-free-standing graphene (QFSG) grown on SiC using sublimation as well as CVD method.[2,3,4]
The obtained results are discussed in terms of basic growth mechanism differences between graphene growth by Si sublimation, which is a “bottom-up” process and by CVD - a “top-down” process. The peculiar differences observed for graphene located on substrate macrosteps as compared to atomically flat terraces are shown. The role of hydrogen intercalation on the scattering mechanism observed by Raman spectroscopy as well as contactless microwave transport spectroscopy is discussed in context of different sensors applications.
References
[1] W. Strupinski et al. Nano Lett. 11, 1786 (2011).
[2] K. Grodecki et al., Appl. Phys. Lett. 100, 261604 (2012).
[3] K. Grodecki et al., J. Appl. Phys. 111, 114307 (2012).
[4] A. Drabi#324;ska et al., Phys. Rev. B 88, 165413 (2013).

Commercial exploitation of graphene's unique properties depends on the development of adequate graphene growth and integration technology. Catalytic techniques for producing graphene, particularly those based on chemical vapor deposition (CVD), are widely seen as most promising for achieving the requisite level of control over material structure and quality that is demanded by applications. However, the underlying mechanisms have yet to be fully understood, and control over the material structure and quality remains limited. Crucial to CVD growth control are the graphene-catalyst interactions at elevated temperatures during precursor exposure, where the catalyst surface is highly mobile with its the physical and chemical state affected by the process conditions and prior exposure history.
Here we investigate the dynamics of graphene-catalyst interactions during chemical vapor deposition using complementary in situ, time- and depth-resolved X-ray Photoelectron Spectroscopy[1-3], in situ scanning tunneling microscopy,[4] and grand canonical Monte Carlo simulations coupled to a tight-binding model. We focus on Ni(111) as a model catalyst surface and probe in-operando a wide range of hydrocarbon exposure pressures (10^-6-10^-1 mbar) as typically used in industrial CVD reactors.[4] The key atomistic mechanisms of graphene formation on Ni are observed and our data reveals a reciprocity between the carbon distribution close to the catalyst surface and the strength of the Ni-graphene interactions. Epitaxial graphene formation on Ni(111) leads to a depletion of carbon close to the Ni surface, which prevents the nucleation of further graphene layers and leads to a self-limiting growth behavior at low exposure pressures (10^-6-10^-3 mbar). A further hydrocarbon pressure increase (to ~10^-1 mbar) leads to weakening of the graphene-Ni(111) interaction accompanied by additional graphene layer formation, mediated by an increased concentration of near-surface dissolved carbon. We further show that growth of more weakly adhered, rotated graphene on Ni(111) is linked to an initially high concentration of near-surface carbon.
We relate these results to a simple kinetic growth model that we have previously established,[5] and thereby consistently explain previous graphene CVD results in the literature. The key implications of our findings for graphene growth control as well as their relevance to carbon nanotube growth are highlighted.
(1) Weatherup et al. Nano Lett. 2011, 11, 4154-4160
(2) Weatherup et al. ChemPhysChem 2012, 13, 2544-2549
(3) Weatherup et al. Nano Lett. 2013, 13, 4624-4631
(4) Patera et al. ACS Nano 2013, 7, 7901-7912
(5) Weatherup et al. ACS Nano 2012, 6, 9996-10003

It has been generally accepted that graphitic surfaces, including graphene and graphite, are hydrophobic and have water contact angle (WCA) of ca. 900. Here we show that this hydrophobic behavior is not the intrinsic property of the graphitic surfaces. Instead, our result demonstrated that a clean graphitic surface is hydrophilic and the previously reported hydrophobicity of graphitic surface was due to surface contamination by airborne hydrocarbon.
We found that a freshly prepared graphene/copper sample has a water contact angle of ca. 40°, significantly smaller than the previously reported value of 86°. The surface became more hydrophobic after exposure to ambient air, ultimately reaching a contact angle of ca. 90°. By using a combination of infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry, we demonstrated that this change of wettability results from the adsorption of airborne hydrocarbon contaminants on the graphene surface. Both XPS and IR measurement showed presence of hydrocarbon on the graphene surface upon its exposure to air and the amount of hydrocarbon correlates with the change of the WCA. Ellipsometry data showed that the surface adsorbed hydrocarbon reached monolayer coverage within 30 min. Most significantly, the same increase of WCA was also observed on as-grown graphene/nickel and freshly cleaved highly oriented pyrolitic graphite (HOPG) samples, suggesting that all graphitic surfaces are intrinsically hydrophilic and their wettability are affected by the same hydrocarbon contamination.
Being one of the most important properties of a surface, wettability reflects the various molecular scale interactions at the solid-liquid-gas interface. In this regard, our result echoes recent theoretical calculations that have predicted stronger-than-previously-expected water - graphite interactions. More importantly, wettability of a surface also defines many other surface properties, including but not limited to adhesion, charge doping, electrochemical activity, and interfacial heat transfer. Given that surface contamination was not considered in most of these studies, our result also calls for a revisit of these related investigations of graphitic materials.

Single atomic thin graphene represents a unique template for assembling small molecules to form graphene-molecule conjugates. Taking the advantage of large π-conjugating system in graphene, π-π interaction can readily explored for non-covalent functionalization of graphene with a variety of π-conjugating molecules. The formation of graphene-molecule conjugates can allow for engineering graphene electronic properties, modulating molecular chemical properties, or integrating the properties of the two to enable exciting new opportunities for molecular sensing, catalysis and energy storage. Here I will discuss a few examples to illustrate these various opportunities. First, we show that monomeric hemin can be conjugated with graphene through π-π interactions to function as a highly effective biomimetic catalyst for the oxidation of endogenous L-arginine. We also show that graphene can function as a common support to integrate the molecular hemin catalyst with an enzymatic catalyst (glucose oxidase) to create a graphene-hemin-glucose oxidase tandem catalyst that can enable biomimetic oxidation of endogeneous L-arginine for continuous generation of nitroxyl and can be used to create a long lasting antithrombotic coating for blood contacting medical devices. Furthermore, we show that the non-covalent functionalization through π-π interaction allows reliable immobilization of hemin molecules on graphene without damaging the graphene electronic properties to enable the creation of a highly sensitive and specific sensor for the detection of nitric oxide. Lastly, by exploring the high surface area and high conductivity of graphene and redox activity of small molecules, we show that graphene-hydroquinone conjugates can be explored for highly efficient capacitive energy storage.

We have developed a powerful new approach for working with graphene by treating it as an atomically thin sheet of paper to which we apply the principles of kirigami, the sculptural art of paper cutting. Using standard lithographic techniques to pattern a series of cuts into the graphene, we work in water so that we can pull the graphene along the surface or peel it up entirely. The elasticity of these kirigami springs is set by the pattern of cuts and by the bending stiffness (rather than the Young&’s modulus) of the graphene, allowing us to pattern graphene springs that can be strained by ~240% without tearing. We have used this technique to create incredibly resilient, flexible, and stretchable electrolyte-gated field effect transistors with electronic properties comparable to those of fixed electrolyte-gated graphene devices. Even after 1000 iterations of stretching and unstretching, the electronic properties of our devices remain excellent. This unusual way of interacting with graphene opens up a world of potential applications that we are just beginning to explore, including soft electrodes for biological experiments.

We report here an investigation of graphene field-effect transistors (G-FETs) in which the graphene channel is in contact with an electrolyte phase. In this work an ion gel, a mixture of poly(styrene-b-methyl methacrylate-b-styrene) triblock copolymer and ionic liquid 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)amide ([EMI][TFSA]), was employed as the electrolyte phase to simplify the device structure by eliminating the needs of electrolyte container and passivation layer that isolates metal contact from the electrolyte. The introduction of the electrolyte phase has an important advantage for fundamental measurements—the Fermi-level position E_F of the graphene channel can be tracked by measuring its electrochemical potential with respect to a reference electrode immersed in the electrolyte phase. Thus, the potential δ required to fill the energy band of graphene with charge carriers (i.e., electrons or holes) can be directly measured from the electrochemical potential change while the carrier density in the graphene channel is independently controlled with back-gate bias. In turn, the quantum capacitance C_Q (i.e., the DOS) of graphene can be estimated from this information. Furthermore, when the carriers in the graphene channel are induced through the electrolyte-gate (i.e., the counter electrode immersed in the electrolyte phase), the potential ΔPhi;_EDL required to charge the electric double-layer at the graphene/electrolyte interface can be conveniently separated from δ by comparing electrochemical potentials of graphene during back- and electrolyte-gating. As we will show, our dual gated devices allow systematic determination of four parameters — δ, C_Q, ΔPhi;_EDL, and double layer capacitance C_EDL — for graphene. The dual gated devices are generally useful testbeds for understanding transport and electronic structure, and the approaches we follow here should also be applicable for investigating other 2D materials such as MoS₂ and ultrathin layers of conventional semiconductors.

All carbon atoms of graphene belong to its surface, and thus electronic properties of graphene are sensitive to surface/interfacial phenomena. Interfaces with various materials are ubiquitously found in electronic devices and nanocomposites of graphene. Charge transfer (CT) across such interfaces is known to largely change a charge carrier concentration of graphene, and is of great importance in understanding electrical properties of the graphene-based systems. In the case of a graphene field-effect transistor (FET), peculiar features such as distorted transfer characteristics [1], a channel-length-dependent shift of the Dirac point [2], and apparently negative contact resistances [3] are the result of the interfacial CT from metallic electrodes. In this study, we further consider interfacial CT from adsorbates on a graphene surface. The interfacial CT is shown to reach as far as ca. 1 mu;m from the interface. Consequently, the CT from surface adsorbates is found to change the carrier concentration of graphene underneath the metallic electrodes where the adsorbates do not contact directly.
[1] R. Nouchi and K. Tanigaki, Appl. Phys. Lett. 96, 253503 (2010).
[2] R. Nouchi, T. Saito and K. Tanigaki, Appl. Phys. Express 4, 035101 (2011).
[2] R. Nouchi, T. Saito and K. Tanigaki, J. Appl. Phys. 111, 084314 (2012).

Transparent conductive electrodes (TCEs), which have transparency and conductivity, are of increasing significance and demand for diverse application areas such as information (displays, touch screen), energy (solar cell, architectural), and environment (sensors). Nowadays, indium tin oxide (ITO) deposited by sputtering process is used as the main TCE material. Although the ITO presents low sheet resistance (~ 30 Ohm/sq) and high transparency (~ 90 %), its fragility limits many potential applications in flexible and stretchable electronics. For these reason, many types of alternative materials such as conducting polymers, carbon nanotubes, graphene, and metal nanowires, have been studied as candidates of the flexible and stretchable TCEs. Although many of these candidates exhibit good mechanical flexibility and stretchability, none shows significantly higher conductivity and transparency together, compared to ITO. Here, we present a simple fabrication process of high-performance, stretchable TCEs based on the metal nanofibers which have long and continuous web geometries, hybrid with 2D material graphene. These TCEs show superb electric conductivity (~ 1 Ohm/sq) with high transparency (~ 91 %) as well as substantial flexibility and stretchability. Hybrid transparent electrode shows very stable electrical property under 500 mu;m radius bending and 50% stretching (sheet resistance changed less than 20%). Also hybrid film shows great uniformity, sheet resistance standard deviation reduced one-tenth compare to only metal nanofiber. Based on high mechanical property and electrical property, we demonstrated transparent and flexible TFT backplane. Single TFT consisted of AgNW-graphene hybrid film, Zirconium Aluminum Oxide, Indium Oxide, and metal nanofiber-graphene hybrid film. TFT show high electrical performance up to 100 cm²/Vbull;s, also represents ~90 % transparency (without substrate). TFT backplane can transfer to various substrates such as leaf, glass cup, glasses and human skin. We believe these TCEs based on the nanostructures present a promising strategy toward flexible and wearable electronics, and indicate the promise of future electronics beyond the limits of conventional ITO.

The interface between two immiscible electrolyte solutions (ITIES) can be electrically polarized to generate electrochemical potential gradients that are capable of promoting ion and electron transfer across the molecular boundary: this liquid#9474;liquid interface offers the possibility of incorporating, as well as constructing in situ at the interface, highly complex catalytic centers and supports, including photoactive oxides, metallic nanostructures and low dimensional carbon species, such as carbon nanotubes and graphene. Here we have grown graphene monolayers, on copper foil by chemical vapor deposition (CVD), and self-assembled the monolayer at the interface between an aqueous and a less dense organic solution. A simple, spontaneous metal deposition at the interface-assembled CVD single-layer was studied, for Au, Pd and Ag. The location of deposited metal nanoparticles (NPs) underneath the graphene monolayer was determined by the position and the intensity changes of the D, G, 2D bands using mapping mode Raman spectroscopy. Another electroless redox reaction for nucleation of the metal NPs was applied using the copper foil, which acts as an electron donor under the graphene monolayer, using that to transfer the electrons between the copper and the reduced metal. Considering the useful catalytic activities of the noble metals, gold, palladium and platinum NPs were deposited on the top of the CVD single-layer. The achieved graphene-based metal nanocomposites were confirmed using optical and electron microscopy. Furthermore, the combination of these two methods was applied to the asymmetric functionalization of the graphene layer, to prepare both sides decorated graphene layer with metal NPs of distinct metals. The methods for decoration either the bottom/top side or both sides of the single-layer graphene sheet with metal nanoparticles at the ITIES opens an alternative and useful way to prepare low dimensional carbon-based nanocomposites and electrode materials, using them for mainly catalytic applications, such as the conversion of carbon dioxide and other important electrocatalytic functions.

Effectively manipulating nanoscale graphene materials to assemble macroscopic, functional objects is a significant challenge for integrating graphene into next-generation devices. In recent years, graphene has been patterned at low concentrations onto substrates to create flexible and conducting features using 2D printing techniques. In addition, graphene has been employed as an additive in 3D printing polymer composites, enabling self-supporting structures on the multi-mm and cm scale which utilize its unique properties, although not as the primary component. We present a new 3D-printable, graphene-based liquid ink, as well as comprehensive characterization of the mechanical, electrical and biological properties of the printed graphene structures. The ink composed of majority graphene particles can be rapidly patterned at room temperature into self-supporting, user-defined constructs with feature sizes from 90-1000 µm. Although comprised primarily of graphene, the resulting objects are soft and robust due to the biocompatible elastomer minority component, which percolates, preferentially carries mechanical loads, while allowing graphene flakes to translate. The flexible 3D-printed graphene fibers and constructs have high electrical conductivity, through which is maintained after cyclic bending over many cycles. This conductivity is improved without compromising the mechanical properties following a low temperature thermal anneal. We also present what we believe to be the first in vitro and in vivo biological studies on graphene scaffolds, which is not in the form of two-dimensional patterns or low concentration graphene composites, such as doped hydrogels. The response of induced pluripotent stem cell (iPSC) derived human neurons as well as human mesenchymal stem cells (hMSCs) seeded onto 3D-printed graphene scaffolds is monitored over the course of several weeks, and shows that neurons attach and interact with the graphene environment much more intimately than with a biocompatible elastomer control. Additionally, hMSCs on 3D-printed graphene are highly viable, proliferate rapidly, and quickly develop morphologies reminiscent of neuronal cells without the inclusion of any differentiation-inducing factors in the media. To determine the biocompatibility of 3D-printed graphene, scaffolds were subcutaneously implanted into mice and analyzed at 7 and 30 days post implantation. Standard histology combined with “electron-histological” imaging techniques illustrate that host tissue readily integrates within the 3D-printed graphene, elicits no observable immune response, host tissue integration with 3D-printed graphene, and apparent vascularization within the graphene constructs. This suite of mechanical, electrical, and biological properties position this scalable3D-printingprintable graphene ink for significant potential impacts in the emergent the fields of flexible electronics, tissue engineering, and bioelectronics.

Graphene, as an outstanding representative of 2D crystals, offers unique physics and exciting functionalities. Unfortunately, the poor inter-sheet connections between isolated graphene flakes break the continuous pathway for electron or phonon transports, and suppress the intrinsically high conductivity; the unavoidable re-stacking and agglomeration in standard graphene products diminishes the accessible surface area. Hence the applications based on large-quantity graphenes (such as serving as adsorbents, supports, electrodes and additives) prefer a three-dimensional (3D) designed graphene architecture rather than a graphene powder. The 3D graphene is most desired to bring the extraordinary nanoscale properties of individual graphene flakes to macroscopic assemblies in the macroworld. Currently all 3D graphene products still suffer from poor electrical conductivity, low surface area, or insufficient mechanical properties; the self-supported reproducible 3D graphenes remain unavailable. Here, I talk about a novel chemical-blowing approach based on polymeric precursors to synthesize a new 3D graphene bubble-network, i.e. strutted graphene. The synthesis route is named as "sugar blowing" analogous to an ancient food art of “blown sugar”. Strutted graphene consists of mono-/few-layered graphitic membranes tightly "glued", rigidly fixed and spatially scaffolded by micrometer-scale graphitic struts. Such topological configuration provides intimate structural interconnectivities, freeway for electron/phonon transports, huge accessible surface area, as well as robust mechanical properties. It opens up a wide horizon for diverse practical usages, e.g. highest-power-density supercapacitors based on the strutted graphene.^[1] In addition, the "blowing" approach is also applied to synthesizing a serials of new nanosheets, e.g. BN nanosheets. The mass-produced low-cost BN nanosheets fundamentally ensures and promotes their studies and applications such as highly thermoconductive insulating polymeric composites filled with BN nanosheets for electronic packaging.^[2] References: [1] X.B. Wang, Y.J. Zhang, C.Y. Zhi, X. Wang, D.M. Tang, Y.B. Xu, Q.H. Weng, X.F. Jiang, M. Mitome, D. Golberg, Y. Bando, Nat. Commun. 4, 2905 (2013). [2] X.B. Wang, C.Y Zhi, L. Li, H.B. Zeng, C. Li, M. Mitome, D. Golberg, Y. Bando, Adv. Mater., 23, 4072 (2011).

The outstanding mechanical properties and high surface area of graphene presents a real opportunity to improve a large number of composite materials, structures and devices. Here, we will focus on graphene nanofiller reinforced carbon based micro-electro-mechanical systems (C-MEMS) and composites for marine hydrokinetic (MHK) power applications and advanced mechanical energy storage systems (i.e. flywheels). For example, graphene-based materials blended into a photo-resist prior to pyrolysis allow for fine-tuning of the electro-mechanical properties and response of C-MEMS devices. As a first step towards developing novel C-MEMS, nano-composite carbon cantilevers were fabricated for testing using 3 different lengths with fixed 10:1 length to width ratios. The devices investigated contained increasing amounts (from 0 wt. % to 2 wt. %) of reduced graphene oxide (RGO) blended into Novolac photo-polymer. An increase in conductivity, from 900 S/cm to 1700 S/cm, was observed. A piezoelectric actuator was used to randomly excite the substrate and a Laser Doppler Vibrometer (LDV) recorded the response for the first three bending modes of each beam. Increasing RGO wt. % was found to increase the natural frequency of the beam, indicating an altered stiffness. A model was generated to estimate the change in Young&’s modulus of the material. 2 wt. % RGO was found to increase the modulus from 41 GPa to 68 GPa, a factor of ~ 1.65. In the case of graphene reinforced epoxy composites, LDV data on test coupons indicates that as little as 1 wt. % loading increases the modulus from 15-35%, depending on the type of graphene nanofiller used. The effect that graphene can have on water uptake, important in MHK applications, and how water content effects the mechanical properties of the composite will also be discussed. Finally we will address our ongoing efforts to build graphene-reinforced carbon fiber (CF)/epoxy composites as well as build and test small industry relevant prototype mechanical flywheels utilizing these composite systems. Flywheels are rotating mechanical devices used to store rotational energy where the amount of energy stored is proportional to the square of the rotational speed. Stronger composites with increased inter-laminar strength are needed. To date, three point bend tests on arc samples indicate a 13% to 17% increase in ultimate strength for CF/epoxy/graphene nanofiller composites, when using a graphene filler material at 1 wt. % or 3 wt. % filler, respectively.
This work was supported by the Department of Energy&’s Office of Electricity Delivery and Energy Reliability, Office of Energy Efficiency & Renewable Energy and Sandia National Laboratories: Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy&’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

New carbon-based materials, such as graphene and carbon nanotubes, have received considerable attention in recent years for the excellent electrical, mechanical, and thermal properties that they exhibit. However, utilizing these characteristics on large, three-dimensional scales has proved difficult and prohibited many potential applications from becoming reality. Using a method of directional solidification known as freeze casting, we present a novel graphene oxide - carbon nanotube (GO-CNT) hybrid aerogel that extends the promising nano-properties of graphene and carbon nanotubes to a three-dimensional macrostructure. In the context of multifunctional hybrid materials, we have developed an aerogel that demonstrates an exceptionally high surface area, a high porosity, good mechanical properties, and a unique hierarchical pore network.

Doubly crosslinked microgels (DX MGs) are hydrogels composed of swollen polymer colloid particles which have intra- and inter-particle crosslinking. Recently, we have shown that DX MGs have potential to restore the mechanical properties of degenerated intervertebral discs. In this study, we used graphene oxide (GO) to prepare high modulus DX MG-GO composite gels for potential use in load-supporting soft tissue repair. The DX MG was prepared from poly(ethyl acrylate-co-methacrylic acid containing MG particles functionalised with glycidyl methacrylate. The influence of different loadings of GO on the mechanical properties of the DX MG-GO gels was investigated using dynamic rheology and static compression measurements. The storage and compression modulus values increased by a factor of 5 - 6 when only ca. 1.0 wt.% of GO was included within the DX MG-GO gels. Both moduli were proportional to the GO concentration, were tunable and much higher than modulus values for the GO-free DX MG control gels. Moreover, the yield strains for the composites were decoupled from the modulus values. The DX MG-GO gels could be formed at 37°C under physiological conditions within 15 min using an accelerator. Cytotoxicity experiments showed that the DX MG-GO gels were biocompatible, which suggests that the new composites studied here have potential biomaterial application. Because of their tunable modulus values with strong dependence on GO concentration, the injectable DX MG-GO composite gels introduced in this study show promise as a versatile injectable technology for repair of load bearing soft tissue.

Improving toughness of ceramics will have broad impact in their potential use in high temperature structural applications. Silicon nitride (Si₃N₄) is considered a structural ceramic with high temperature resistance, high hardness, and toughness due to a two phases, alpha- and beta-, forming an interlocking microstructure where beta- phase acts as rod reinforcements to equiaxed alpha- grains. Graphene additions to alpha-S_i3N₄ of less than 1.5 volume percent increase toughness up to 250% while maintaining high hardness and modulus compared to pure alpha-Si₃N₄. Unique fracture mechanisms found in alpha-Si₃N₄ graphene composites show nano scale graphene sheets acting as reinforcements that alter crack propagation path in three dimensions, compared to two-dimensions using one-dimensional fibers, over a range of length scales without sacrificing other mechanical properties. An analysis of experimentally measured bulk mechanical properties such as, flexural strength, micro hardness toughness, and chevron notch fracture toughness, reveal strength and toughening mechanisms contributions from nano scale graphene sheets and macro scale toughening from graphene platelets. Aqueous colloidal processing methods are shown to create homogenous and uniform graphene composites using few layer reduced and exfoliated graphene sheets within Si₃N₄ powder. Ceramic composites are consolidated under high pressure to high temperature (1600 °C) using a rapid direct current assisted heating method called, spark plasma sintering. Raman spectroscopy is used to map dispersion of graphene in sintered microstructure and to verify structure after consolidation. A review of processing science, sintering behavior of composites, and comparison between alpha- to preliminarily beta-Si₃N₄ composites mechanical properties and atomic resolution characterization of composite microstructure will be discussed.

Artificial van der Waals heterostructures have demonstrated both significant improvements of graphene's intrinsic properties and entirely new properties of their own. Early interest in these structures was based on nearly ideal carrier mobility in graphene on two-dimensional (2D) hexagonal boron nitride. Although exfoliation and reassembly of bulk vdW solids has yielded an impressive list of new systems, this method inherently limits the geometry and constituent materials of these structures. Growth of 2D heterostructures has been demonstrated, but mainly limited to the prototypical graphene/hBN system. Adding new constituent materials, particularly those with electronic heterogeneity, to these 2D heterostructures allows them to be engineered with a variety of new properties.
We present the growth and characterization of interfaces between an atomically thin silicon layer and graphene. First, graphene is grown on Ag(111) via atomic carbon deposition at temperatures from 600°C -700°C. Following the growth of graphene, atomic silicon is evaporated on the graphene-covered Ag(111) substrate at 320°C-360°C. The resulting silicon growth results in facetted domains capped with a honeycomb lattice with periodicity 6.4 Å; Raman spectroscopy reveals peaks at 520 cm^-1 and 900-1000 cm^-1 that coincide precisely with bulk diamond cubic silicon, indicating these domains are comprised of sp³ bonded crystalline Si. These 2D sheets of silicon demonstrate both semiconducting character and a honeycomb lattice is attributed to a silver-based reconstruction of the Si(111) surface. The resulting silicon domains grow in two different configurations with respect to the dendritic graphene: (1) silicon domains appear to grow directly on the Ag(111) surface and terminate at the graphene boundaries. These in-plane interfaces are atomically-precise and clearly resolved via scanning tunneling microscopy. Electronically, the density of states of both isolated constituent materials persist to these interfaces within the resolution of the measurement, indicating little interaction at the border. (2) The silicon growth is observed underneath the existing graphene flakes. The vertically stacked silicon graphene domains are identified via bias-dependent atomically resolved imaging, enabling imaging of the honeycomb silicon through the graphene domains at larger biases where graphene is transparent under STM and simultaneous imaging of both honeycomb lattices closer to the Fermi level. Furthermore, the vertical materials interfaces demonstrate distinct electronic signatures from either constituent material. The resulting interfaces represent atomically pristine interfaces between graphene and a two-dimensional, sp³ bonded, semiconducting Si film, demonstrating a significant step forward in the diversification of van der Waals heterostructures.

Chemical vapor deposition (CVD) on polycrystalline Cu catalysts has indeed emerged as the most widely used method to obtain high-quality, large-area graphene. However, the fundamental growth mechanisms, which are typically determined by the interaction between growing graphene and the catalyst during growth, remain largely unexplored specially for industrially relevant CVD conditions.
Here we report on the use of complementary in situ X-ray photoelectron spectroscopy (XPS), environmental scanning electron microscopy (ESEM) and X-ray diffractometry (XRD) at industrially relevant CVD conditions of pressure ~0.001-0.5 mbar and temperature ~900-1000^oC for several hydrocarbon sources and fingerprint the entire CVD process to address the current lack of understanding. [1,2] These techniques allow us to monitor the catalyst surface morphology, surface chemistry and bulk crystallography during the entire CVD process.
Graphene forms directly on metallic Cu during the high-temperature hydrocarbon exposure, where in we observe an upshift in the binding energies of the corresponding C1s XPS core level signatures. This is indicative of a charge transfer mediated coupling between the Cu catalyst and the growing graphene. Ambient air exposure post growth even at room temperature leads to a decoupling of the graphene from Cu due to an oxygen intercalation process.
We note that the graphene Cu interaction is strongly affected by even minor gaseous residue during CVD eg: oxygen contaminations, leading to a change in the graphene-Cu interaction during growth. Further our experiments show a minor carbon uptake into Cu which can under certain conditions manifest as carbon precipitation upon cooling. [1]
Finally we highlight the importance of our complementary in-situ approach to study the growth of other 2D nanomaterials during CVD and thereby elucidate growth mechanisms.
References:
1. Kidambi et al. Nano Letters 13 (10), 4769-4778 (2013).
2. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
3. Kidambi et al. PSS RRL. 5, 9, 341-343 (2011).

Membranes made of graphene oxide (GO) have demonstrated remarkable properties for water separation in that they allow unimpeded permeation of water vapor, but can be completely impermeable to other gases. This potentially provides an improved technology to address applications in water purification, desalination, and nanofiltration. In addition, GO membranes have great potential due to their relatively easy fabrication, low cost materials, and industrial scale-up opportunities. The narrow size range, nanoporosity and chemical functionality of these membranes all play key roles in the unique transport, however, much is still unknown about the fundamental properties of the materials and how those affect its functionality.
The GO membranes are a very complex stack of single GO layers, and the 3-D morphology of the stacking is thought to have a large impact on its function. Here we use electron tomography in the scanning transmission electron microscope (STEM) to image the 3-D structures. We compare membranes made with a range of GO materials both synthesized at PNNL and commercially sourced, and correlate the 3-D morphology with its macroscopic structure and permeability properties.

In-situ transport measurements of graphene during functionalization and doping can provide valuable information such as better understanding of influence of environmental effects. For example, vacuum-annealed graphene is n-type, but becomes p-type when exposed to ambient air. Here we present evidence that this effect arises from pinning of the Fermi energy by the four-electron oxygen redox couple in an adsorbed water film, a type of surface transfer doping. The pronounced electrical sensitivity to O₂ results from electron exchange between the redox couple and the graphene. This effect (performance volatility) must be considered when using graphene based devices for any application in humid air. Further, in-situ electrical properties (resistance, thermopower, and Hall mobility) of graphene subjected to controlled and sequential hydrogenation/fluorination using RF plasma will be discussed and correlated with ex-situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak-hydrogenation, the transport is seen to be governed by electron diffusion and low temperature transport properties show metallic behavior (conductance G remains non-zero as T approaches 0). For strong-hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (G reaches 0 as T approaches 0). Weak localization (WL) behavior is seen with a negative Magneto resistance (MR) for weakly-hydrogenated graphene and this WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly-hydrogenated graphene. In-situ transport study on fluorination of graphene shows clear evidence of band gap opening in graphene. Finally in-situ study of transport during doping with nitrogen using RF plasma of nitrogen/ammonia under pulsed laser irradiation will be discussed.

The combination of mono-atomic thickness, high transparency and low resistivity makes graphene a promising material for optoelectronics and photonics. Recently we realized a microcavity-controlled graphene transistor, acting as a spectrally selective and directional light detector and emitter [1]. In this work we discuss the substitution of graphene with nanocrystalline graphene (NCG) and characterize its electrical and optical properties. Furthermore we demonstrate NCG devices integrated in optical microcavities.
[1] M. Engel et al., Nature Commun 3 (2012) 906

Graphene is a strong Raman scatterer and this technique has long been used to determine quality and number of layers of such material; however it lacks the spatial resolution that is necessary to study engineered structures in detail. Scanning Probe Microscopy (SPM), and especially Atomic Force Microscopy (AFM) is a powerful technique to image physical properties of graphene, such as topography, conductivity or other electrical properties. Combining both techniques is challenging but extremely powerful, as it makes imaging of both chemical and physical properties possible, although conventional Raman only provides limited spatial resolution. The step beyond co-localized AFM and Raman is nanoRaman or nano-spectroscopy in general. In this talk, we will present the latest development in terms of Tip Enhanced Raman spectroscopy (TERS) that make possible nanoscale imaging of chemical and physical properties of graphene and other carbon species: innovative integration of technologies brings high-throughput optics and high-resolution scanning for high-speed imaging without interferences between the techniques. The latest developments in near-field optical probes now provide reliable solutions for academic and industrial researchers alike to easily get started with nanoscale spectroscopy.

Measurement of pH is important for environment, agriculture, food, beverage, cell culture, microbiology, and biomedical applications. As the human body is very sensitive to pH, pH sensors are used in the field closely related to the human life such as water quality and cosmeticse. Human body has different pH values in each body part including stomach, blood, skin surface. Graphene sheets have superior characteristics for bioapplications such as high mobility, flexibility, and biocompatibility. We fabricated flexible solution-gated field-effect transistors (SGFETs) on graphene sheet, which was transferred on polyethylene terephthalate (PET) substrate, to detect pH in real-time. The graphene SGFETs linearly detected pH from 2 to 10. The electrical characteristics and pH response of graphene SGFETs were evaluated by current-voltage (I-V) measurement in electrolyte solution. The drain-source current (I_DS) and the gate-source voltage (V_GS) curve of graphene SGFETs showed an ambipolar characteristics, both electron and hole conductions. The I_DS and the V_GS of graphene SGFETs depended on pH in electrolyte solution. The Dirac point of graphene SGFETs shifted linearly towards the positive voltage when pH was increased. The flexible graphene pH sensor has potential applying for the industrial field, bioscience and medical equipments.

We report a new strategy to manipulate systematically the sensitivity and selectivity of electrochemical sensors using a hybrid system composed of reduced graphene oxide (rGO) conformally coated around a three-dimensional carbon nanotube (CNT) framework. The rGO component in the hybrid is obtained by reduction of graphene oxide (GO) employing a facile, green and highly efficient electrochemical approach. More importantly, the electronic structure of the resulting rGO layer can be deliberately modulated by varying the electrochemical potential and the reduction time. The CNT component in the hybrid creates a highly conductive, nanoporous architecture that simultaneously facilitates electron transport and ion diffusion, which improves the electron transfer efficiencies with redox-active analytes. We find that using the rGO/CNT hybrid, we can achieve highly tunable electron transfer kinetics with a variety of redox probes. Next we demonstrate that the rGO/CNT hyrbids with different electronic structures exhibit dramatically different electrochemical sensitivities and selectivities toward detection of various neurochemicals. Remarkable synergistic effects for neurochemical detection are achieved through optimization of the composition and the electronic structure of the rGO/CNT hybrid. We also present a detailed study on the relationship between the electrochemical activities of the rGO/CNT hybrids and their electronic properties (band structure, work function, electron affinity, density of states, etc).

Graphene, a quasi-two dimensional atomic carbon lattice structure, has unique electronic and optical properties that make it potentially useful for applications where a combination of optical transparency, electrical current carrying capabilities, and chemical inertness is needed. A potential application of graphene is liquid sensing, where a graphene layer could enable optoelectronic functionalities beyond standard lab-on-chip technology. In this presentation, we introduce a graphene-based measurement platform for the combined electronic and optical characterization of liquids. We discuss the operation principle of the device and demonstrate that it can be used to electronically differentiate between water and decane. Also, we show that it is possible to combine electronic sensing and optical sensing in the same device by measuring in-situ optical spectra of the analyzed liquid. The results are relevant to lab-on-chip analytics, in particular to industrial domains such as oil & gas and healthcare.

Graphene, the high-strength, ultra-thin single sheet of carbon atoms, has received significant attention of late as the backbone material for next generation high-performance separation membranes applicable in many processes including water desalination and gas purification. Recent simulations have demonstrated that graphene perforated with subnanometer pores would result in a membrane that exhibits a permeability and selectivity orders of magnitude greater than current state-of-the-art polymer membranes. Though methods to create controlled, subnanometer pores in large areas of graphene have been presented, the presence of cracks and defects still limits usage in desalination and nanofiltration applications. Herein, we report a procedure to block tears, cracks, and intrinsic defects in monolayer graphene membranes and investigate osmotically-driven water and ionic transport through the fabricated subnanometer pores. We found the sealed graphene membrane exhibited high rejection of an organic dye molecule (~1 nm in diameter) while permitting a fast transport of water. These results represent the first demonstration of nanofiltration using porous, single-layer graphene and a significant advancement of the field of nanoporous two-dimensional materials.
Research funded by King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia through the Center for Clean Water and Clean Energy at MIT and KFUPM under project number R10-CW-09, and in part by the U.S. Department of Energy, Basic Energy Sciences, under award number DE-SC0008059. Research supported by ORNL's Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, the U.S. Department of Energy (JCI), and MRSEC Shared Experimental Facilities supported by the National Science Foundation under award number DMR-0819762 at MIT.

Graphene has emerged as a promising material for optoelectronic applications. However, as an active channel material for photodetectors, graphene suffers from a relatively low absorption coefficient, which is 2.3% due to its ultra-thin thickness. To address this issue, we integrated a three terminal graphene device on top of a silicon buried waveguide. By coupling a 35-um-long graphene to the evanescent field of the silicon waveguide, the absorption in graphene is greatly enhanced, giving more than one order enhancement of the responsivity of the graphene photodetector. Furthermore, with an additional gate terminal to tune the Fermi level in graphene, we directly increase the internal electric field of the metal/graphene interface to separate the photo-excited carriers more efficiently. The maximum responsivity of our device is 0.35 A/W, corresponding to an internal quantum efficiency of 18%. The spectral response of the photodetector is tested from 1300 nm to 2000 nm, and the speed of the device exceeds 40 GHz. Our work demonstrates the potential of graphene as an active material for high-speed, high-responsivity and broadband photodetectors in silicon photonics.

The determination of pH is ubiquitous from food and health care to environmental monitoring. Two most common ways to determine pH are pH meters and pH paper. pH paper measurement is only semi-quantitative by comparing colors to a chart and comparatively subjective. pH meter is large and lab-based and needs frequent calibrations and stringent storage conditions. A disposable paper-fluidic electrochemical pH sensor is thus promising as it combines advantages of both pH meters and strips and can be used in resource-limited conditions and point-of-care applications. Paper can serve as a filter to remove unwanted large particulates from samples while making low-volume measurements available and eliminating the need of external power supplies for sample delivering and handling by capillary pressures.
Graphene nanocomposites with metal oxides can have synergistic effects and are promising candidates as pH-responsive materials. Current production of graphene from graphene oxide and synthesis of nanocomposites from salt precursor solutions require reducing reagents such as hydrazine, NaBH4 and urea which are toxic, corrosive or even explosive, and thus there are serious safety and environmental issues. The process with reducing reagents is also undesirable due to their potential damage on electronic properties. Therefore a greener and more effective synthetic route of graphene nanocomposites is an unmet need.
Here we present a facile, controllable and inexpensive electrochemical synthesis of IrO₂-graphene nanocomposites and fabricate an easy-to-use integrated paper microfluidic electrochemical pH sensing platform in resource-limited settings, composed of PDMS-patterned paper strips with hydrophobic barriers, screen-printed carbon electrodes modified with IrO₂-graphene films and molded acrylonitrile butadiene styrene plastic holders. Repetitive cathodic potential cycling was employed for GO reduction which can completely remove unfavorable electrochemically unstable oxygenated groups and generate a 2D defect-free homogeneous graphene film with excellent stability and electronic properties. A uniform and smooth IrO₂ film in nanoscaled grain size is anodically electrodeposited onto the graphene film, without any observable cracks. The resulting IrO₂-graphene electrode showed a slightly super-Nernstian response from pH 2-12 in Britton-Robinson (B-R) buffer with linear responses, small hysteresis widths, fast response time and low sensitivities to different ionic species and dissolved oxygen. A digital portable pH meter was fabricated based on a multimeter. pH values of lake water at different locations of four major lakes in Madison, WI are measured both by a lab-based commercial pH meter with a glass electrode and our portable electrochemical paper-microfluidic pH sensor and the results are statistically consistent (R2=0.999).