Showing papers by "Rodney S. Ruoff published in 2012"

Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction

Abstract: We present two different ways to fabricate nitrogen-doped graphene (N-graphene) and demonstrate its use as a metal-free catalyst to study the catalytic active center for the oxygen reduction reaction (ORR). N-graphene was produced by annealing of graphene oxide (G-O) under ammonia or by annealing of a N-containing polymer/reduced graphene oxide (RG-O) composite (polyaniline/RG-O or polypyrrole/RG-O). The effects of the N precursors and annealing temperature on the performance of the catalyst were investigated. The bonding state of the N atom was found to have a significant effect on the selectivity and catalytic activity for ORR. Annealing of G-O with ammonia preferentially formed graphitic N and pyridinic N centers, while annealing of polyaniline/RG-O and polypyrrole/RG-O tended to generate pyridinic and pyrrolic N moieties, respectively. Most importantly, the electrocatalytic activity of the catalyst was found to be dependent on the graphitic N content which determined the limiting current density, while the pyridinic N content improved the onset potential for ORR. However, the total N content in the graphene-based non-precious metal catalyst does not play an important role in the ORR process.

Spongy Graphene as a Highly Efficient and Recyclable Sorbent for Oils and Organic Solvents

Abstract: In this work, spongy graphene (SG), a shape-mouldable and nanoporous material with a high specific surface area used as a versatile and recyclable sorbent material, is proposed and studied. SG shows highly efficient absorption of not only petroleum products and fats, but also toxic solvents such as toluene and chloroform (up to 86 times of its own weight), requiring no further pretreatment, which is tens of times higher than that of conventional absorbers. Moreover, SG can be regenerated (>10 times) by heat treatment, yielding the full release of adsorbates (>99%). The present work suggests SG a widespread potential for applications in industry as well as topics regarding environmental protection.

Thermal conductivity of isotopically modified graphene

Abstract: Among other exotic properties graphene exhibits the highest thermal conductivity observed so far. This is true at least for graphene composed of only 12C atoms. However, it is now shown experimentally that regions of 13C atoms can substantially reduce the thermal conductivity. Aside from their fundamental importance, these results suggest that thermal conductivity can be tailored by varying the relative amounts of carbon isotopes used.

Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors

Abstract: We present a novel method to prepare highly conductive, free-standing, and flexible porous carbon thin films by chemical activation of reduced graphene oxide paper. These flexible carbon thin films possess a very high specific surface area of 2400 m2 g–1 with a high in-plane electrical conductivity of 5880 S m–1. This is the highest specific surface area for a free-standing carbon film reported to date. A two-electrode supercapacitor using these carbon films as electrodes demonstrated an excellent high-frequency response, an extremely low equivalent series resistance on the order of 0.1 ohm, and a high-power delivery of about 500 kW kg–1. While higher frequency and power values for graphene materials have been reported, these are the highest values achieved while simultaneously maintaining excellent specific capacitances and energy densities of 120 F g–1 and 26 W h kg–1, respectively. In addition, these free-standing thin films provide a route to simplify the electrode-manufacturing process by eliminating...

Stretchable and highly sensitive graphene-on-polymer strain sensors

Abstract: The use of nanomaterials for strain sensors has attracted attention due to their unique electromechanical properties. However, nanomaterials have yet to overcome many technological obstacles and thus are not yet the preferred material for strain sensors. In this work, we investigated graphene woven fabrics (GWFs) for strain sensing. Different than graphene films, GWFs undergo significant changes in their polycrystalline structures along with high-density crack formation and propagation mechanically deformed. The electrical resistance of GWFs increases exponentially with tensile strain with gauge factors of ~103 under 2~6% strains and ~106 under higher strains that are the highest thus far reported, due to its woven mesh configuration and fracture behavior, making it an ideal structure for sensing tensile deformation by changes in strain. The main mechanism is investigated, resulting in a theoretical model that predicts very well the observed behavior.

Ultrathin graphite foam: a three-dimensional conductive network for battery electrodes.

Abstract: We report the use of free-standing, lightweight, and highly conductive ultrathin graphite foam (UGF), loaded with lithium iron phosphate (LFP), as a cathode in a lithium ion battery. At a high charge/discharge current density of 1280 mA g–1, the specific capacity of the LFP loaded on UGF was 70 mAh g–1, while LFP loaded on Al foil failed. Accounting for the total mass of the electrode, the maximum specific capacity of the UGF/LFP cathode was 23% higher than that of the Al/LFP cathode and 170% higher than that of the Ni-foam/LFP cathode. Using UGF, both a higher rate capability and specific capacity can be achieved simultaneously, owing to its conductive (∼1.3 × 105 S m–1 at room temperature) and three-dimensional lightweight (∼9.5 mg cm–3) graphitic structure. Meanwhile, UGF presents excellent electrochemical stability comparing to that of Al and Ni foils, which are generally used as conductive substrates in lithium ion batteries. Moreover, preparation of the UGF electrode was facile, cost-effective, and ...

Incorporation of manganese dioxide within ultraporous activated graphene for high-performance electrochemical capacitors.

Abstract: Manganese dioxide (MnO2) particles 2–3 nm in size were deposited onto a porous “activated microwave expanded graphite oxide” (aMEGO) carbon scaffold via a self-controlled redox process. Symmetric electrochemical capacitors were fabricated that yielded a specific capacitance of 256 F/g (volumetric: 640 F/cm3) and a capacitance retention of 87.7% after 1000 cycles in 1 M H2SO4; when normalized to MnO2, the specific capacitance was 850 F/g. Asymmetric electrochemical capacitors were also fabricated with aMEGO/MnO2 as the positive electrode and aMEGO as the negative electrode and had a power density of 32.3 kW/kg (for an energy density of 20.8 Wh/kg), an energy density of 24.3 Wh/kg (for a power density of 24.5 kW/kg), and a capacitance retention of 80.5% over 5000 cycles.

Chemical structures of hydrazine-treated graphene oxide and generation of aromatic nitrogen doping

Abstract: Park et al. use 13C and 15N solid-state NMR and X-ray photoelectron spectroscopy to study the chemical structure of hydrazine-treated graphite oxide. Hydrazine treatment is shown to lead to the incorporation of aromatic N2 moieties at the graphene edges and restore graphitic networks on the basal planes.

Nitrogen doping of graphene and its effect on quantum capacitance, and a new insight on the enhanced capacitance of N-doped carbon

Abstract: Many researchers have used nitrogen (N) as a dopant and/or N-containing functional groups to enhance the capacitance of carbon electrodes of electrical double layer (EDL) capacitors. However, the physical mechanism(s) giving rise to the interfacial capacitance of the N-containing carbon electrodes is not well understood. Here, we show that the area-normalized capacitance of lightly N-doped activated graphene with similar porous structure increased from 6 μF cm−2 to 22 μF cm−2 with 0 at%, and 2.3 at% N-doping, respectively. The quantum capacitance of pristine single layer graphene and various N-doped graphene was measured and a trend of upwards shifts of the Dirac Point with increasing N concentration was observed. The increase in bulk capacitance with increasing N concentration, and the increase of the quantum capacitance in the N-doped monolayer graphene versus pristine monolayer graphene suggests that the increase in the EDL type of capacitance of many, if not all, N-doped carbon electrodes studied to date, is primarily due to the modification of the electronic structure of the graphene by the N dopant. It was further found that the quantum capacitance is closely related to the N dopant concentration and N-doping provides an effective way to increase the density of the states of monolayer graphene.

Thermal transport in three-dimensional foam architectures of few-layer graphene and ultrathin graphite.

Abstract: At a very low solid concentration of 0.45±0.09 vol %, the room-temperature thermal conductivity (κGF) of freestanding graphene-based foams (GF), comprised of few-layer graphene (FLG) and ultrathin graphite (UG) synthesized through the use of methane chemical vapor deposition on reticulated nickel foams, was increased from 0.26 to 1.7 W m–1 K–1 after the etchant for the sacrificial nickel support was changed from an aggressive hydrochloric acid solution to a slow ammonium persulfate etchant. In addition, κGF showed a quadratic dependence on temperature between 11 and 75 K and peaked at about 150 K, where the solid thermal conductivity (κG) of the FLG and UG constituents reached about 1600 W m–1 K–1, revealing the benefit of eliminating internal contact thermal resistance in the continuous GF structure.

Processing–Morphology–Property Relationships and Composite Theory Analysis of Reduced Graphene Oxide/Natural Rubber Nanocomposites

Abstract: Dispersion of reduced graphene oxide (RG-O) into natural rubber (NR) was found to dramatically enhance the mechanical, electrical, and thermal properties of NR. However, property improvements were strongly dependent upon the processing history and nanocomposite morphology. Co-coagulating a stable RG-O suspension with NR latex afforded a weblike morphology consisting of platelet networks between the latex particles, while two-roll mill processing broke down this structure, yielding a homogeneous and improved dispersion. The physical properties of RG-O/NR vulcanizates with both morphologies were compared over a range of loadings; it was found that the network morphology was highly beneficial for thermal and electrical conductivity properties and greatly increased stiffness but was detrimental to elongation. A detailed comparative analysis of composite models found the Guth equation gave excellent fit to modulus data of the milled samples when taking the shape factor as equal to the platelet aspect ratio qua...

Improved electrical conductivity of graphene films integrated with metal nanowires.

Abstract: Polycrystalline graphene grown by chemical vapor deposition (CVD) on metals and transferred onto arbitrary substrates has line defects and disruptions such as wrinkles, ripples, and folding that adversely affect graphene transport properties through the scattering of the charge carriers. It is found that graphene assembled with metal nanowires (NWs) dramatically decreases the resistance of graphene films. Graphene/NW films with a sheet resistance comparable to that of the intrinsic resistance of graphene have been obtained and tested as a transparent electrode replacing indium tin oxide films in electrochromic (EC) devices. The successful integration of such graphene/NW films into EC devices demonstrates their potential for a wide range of optoelectronic device applications.

Preparation of supercapacitor electrodes through selection of graphene surface functionalities.

Abstract: In order to investigate the effect of graphene surface chemistry on the electrochemical performance of graphene/polyaniline composites as supercapacitor electrodes, graphene oxide (G-O), chemically reduced G-O (RG-O), nitrogen-doped RG-O (N-RG-O), and amine-modified RG-O (NH(2)-RG-O) were selected as carriers and loaded with about 9 wt % of polyaniline (PANi). The surface chemistry of these materials was analyzed by FTIR, NEXAFS, and XPS, and the type of surface chemistry was found to be important for growth of PANi that influences the magnitude of increase of specific capacitance. The NH(2)-RG-O/PANi composite exhibited the largest increase in capacitance with a value as high as 500 F g(-1) and good cyclability with no loss of capacitance over 680 cycles, much better than that of RG-O/PANi, N-RG-O/PANi, and G-O/PANi when measured in a three-electrode system. A NH(2)-RG-O/PANi//N-RG-O supercapacitor cell has a capacitance of 79 F g(-1), and the corresponding specific capacitance for NH(2)-RG-O/PANi is 395 F g(-1). This research highlights the importance of introducing -NH(2) to RG-O to achieve highly stable cycling performance and high capacitance values.

Toward the Controlled Synthesis of Hexagonal Boron Nitride Films

Abstract: Atomically smooth hexagonal boron nitride (h-BN) layers have very useful properties and thus potential applications for protective coatings, deep ultraviolet (DUV) emitters, and as a dielectric for nanoelectronics devices. In this paper, we report on the growth of h-BN by a low-pressure chemical vapor deposition (LPCVD) process using diborane and ammonia as the gas precursors. The use of LPCVD allows synthesis of h-BN with a controlled number of layers defined by the growth conditions, temperature, time, and gas partial pressure. Furthermore, few-layer h-BN was also grown by a sequential growth method, and insights into the growth mechanism are described, thus forming the basis of future growth of h-BN by atomic layer epitaxy.

Low-Temperature Chemical Vapor Deposition Growth of Graphene from Toluene on Electropolished Copper Foils

Abstract: A two-step CVD route with toluene as the carbon precursor was used to grow continuous large-area monolayer graphene films on a very flat, electropolished Cu foil surface at 600 °C, lower than any temperature reported to date for growing continuous monolayer graphene. Graphene coverage is higher on the surface of electropolished Cu foil than that on the unelectropolished one under the same growth conditions. The measured hole and electron mobilities of the monolayer graphene grown at 600 °C were 811 and 190 cm2/(V·s), respectively, and the shift of the Dirac point was 18 V. The asymmetry in carrier mobilities can be attributed to extrinsic doping during the growth or transfer. The optical transmittance of graphene at 550 nm was 97.33%, confirming it was a monolayer, and the sheet resistance was ∼8.02 × 103 Ω/□.

Selective-area fluorination of graphene with fluoropolymer and laser irradiation

Abstract: We have devised a method to selectively fluorinate graphene by irradiating fluoropolymer-covered graphene with a laser. This fluoropolymer produces active fluorine radicals under laser irradiation that react with graphene but only in the laser-irradiated region. The kinetics of C–F bond formation is dependent on both the laser power and fluoropolymer thickness, proving that fluorination occurs by the decomposition of the fluoropolymer. Fluorination leads to a dramatic increase in the resistance of the graphene while the basic skeletal structure of the carbon bonding network is maintained. Considering the simplicity of the fluorination process and that it allows patterning with a nontoxic fluoropolymer as a solid source, this method could find application to generate fluorinated graphene in graphene-based electronic devices such as for the electrical isolation of graphene.

Reducing extrinsic performance-limiting factors in graphene grown by chemical vapor deposition.

Abstract: Field-effect transistors fabricated on graphene grown by chemical vapor deposition (CVD) often exhibit large hysteresis accompanied by low mobility, high positive backgate voltage corresponding to the minimum conductivity point (Vmin), and high intrinsic carrier concentration (n0). In this report, we show that the mobility reported to date for CVD graphene devices on SiO2 is limited by trapped water between the graphene and SiO2 substrate, impurities introduced during the transfer process and adsorbates acquired from the ambient. We systematically study the origin of the scattering impurities and report on a process which achieves the highest mobility (μ) reported to date on large-area devices for CVD graphene on SiO2: maximum mobility (μmax) of 7800 cm2/(V·s) measured at room temperature and 12 700 cm2/(V·s) at 77 K. These mobility values are close to those reported for exfoliated graphene on SiO2 and can be obtained through the careful control of device fabrication steps including minimizing resist resi...

Low temperature casting of graphene with high compressive strength

Abstract: has attracted attention due to its fascinating properties such as high carrier mobility, [ 6–8 ] high thermal conductivity, [ 9 , 10 ] extraordinary elasticity and stiffness [ 11 ] and other properties. While mechanical exfoliation, [ 6 ] liquid exfoliation, [ 12 ] and epitaxial growth [ 13 ] can produce pristine graphene, graphene yields are currently too low for large-scale production of macrostructures. In contrast, chemical reduction of graphene oxide provides ‘graphene’ sheets in large scale for graphene macrostructures. [ 14–16 ] Graphene-based macrostructures prepared to date have been relatively weak mechanically, given their fl exible and often relatively porous or open structures, [ 17–26 ] particularly with respect to compressive strength when compared with commercial graphite products. [ 27–29 ] Achieving highly compacted and thus “fully dense” macrostructures based on graphene and measuring the physical properties of such material(s) is thus an important goal. Here, we report a pH-mediated hydrothermal reduction which is combined with moulding methods and allows controllable fabrication of compact high density graphene macrostructures with various shapes. The compact graphene (CG) product that is fabricated in this study shows great advantages over hitherto reported 3-D graphene products, [ 17–26 ] e.g. , a solid microstructure and a high density ( ∼ 1.6 g cm − 3 ) which is comparable to conventional graphite products [ 27–29 ] and an ultrahigh compressive strength ( ∼ 361 Mpa) which is 6 times higher than

Synthesis of High Quality Monolayer Graphene at Reduced Temperature on Hydrogen-Enriched Evaporated Copper (111) Films

Abstract: We report new findings on the chemical vapor deposition (CVD) of monolayer graphene with negligible defects (≥95% negligible defect-peak over 200 μm × 200 μm areas) on evaporated copper films. Compared to copper foils used in the CVD of graphene, several new unexpected results have been observed including high-quality monolayer synthesis at temperatures <900 °C, a new growth window using a hydrogen-free methane precursor for low-defects, and electron microscope evidence of commensurate growth of graphene grains on underlying copper grains. These thermal, chemical, and physical growth characteristics of graphene on copper films can be attributed to the distinct differences in the dominant crystal orientation of copper films (111) versus foils (100), and consequent dissimilar interplay with the precursor gas. This study suggests that reduced temperature, hydrogen-free synthesis of defect-negligible monolayer graphene is feasible, with the potential to shape and scale graphene grains by controlling the size and crystal orientation of the underlying copper grains.

Oxidation Resistance of Iron and Copper Foils Coated with Reduced Graphene Oxide Multilayers

Abstract: Protecting the surface of metals such as Fe and Cu from oxidizing is of great importance due to their widespread use. Here, oxidation resistance of Fe and Cu foils was achieved by coating them with reduced graphene oxide (rG-O) sheets. The rG-O-coated Fe and Cu foils were prepared by transferring rG-O multilayers from a SiO2 substrate onto them. The oxidation resistance of these rG-O-coated metal foils was investigated by Raman spectroscopy, optical microscopy, and scanning electron microscopy after heat treatment at 200 °C in air for 2 h. The bare metal surfaces were severely oxidized, but the rG-O-coated metal surfaces were protected from oxidation. This simple solution process using rG-O is one advantage of the present study.

Growth Mechanism and Controlled Synthesis of AB-Stacked Bilayer Graphene on Cu-Ni Alloy Foils

Abstract: Strongly coupled bilayer graphene (i.e., AB stacked) grows particularly well on commercial “90–10” Cu–Ni alloy foil. However, the mechanism of growth of bilayer graphene on Cu–Ni alloy foils had not been discovered. Carbon isotope labeling (sequential dosing of 12CH4 and 13CH4) and Raman spectroscopic mapping were used to study the growth process. It was learned that the mechanism of graphene growth on Cu–Ni alloy is by precipitation at the surface from carbon dissolved in the bulk of the alloy foil that diffuses to the surface. The growth parameters were varied to investigate their effect on graphene coverage and isotopic composition. It was found that higher temperature, longer exposure time, higher rate of bulk diffusion for 12C vs13C, and slower cooling rate all produced higher graphene coverage on this type of Cu–Ni alloy foil. The isotopic composition of the graphene layer(s) could also be modified by adjusting the cooling rate. In addition, large-area, AB-stacked bilayer graphene transferrable onto...

Activated graphene as a cathode material for Li-ion hybrid supercapacitors

Abstract: Chemically activated graphene (‘activated microwave expanded graphite oxide’, a-MEGO) was used as a cathode material for Li-ion hybrid supercapacitors. The performance of a-MEGO was first verified with Li-ion electrolyte in a symmetrical supercapacitor cell. Hybrid supercapacitors were then constructed with a-MEGO as the cathode and with either graphite or Li4Ti5O12 (LTO) for the anode materials. The results show that the activated graphene material works well in a symmetrical cell with the Li-ion electrolyte with specific capacitances as high as 182 F g−1. In a full a-MEGO/graphite hybrid cell, specific capacitances as high as 266 F g−1 for the active materials at operating potentials of 4 V yielded gravimetric energy densities for a packaged cell of 53.2 W h kg−1.

Thermoacoustic Sound Generation from Monolayer Graphene for Transparent and Flexible Sound Sources

Abstract: Transparent and flexible loudspeakers are realized with large-area monolayer graphene. The acoustic performances are characterized according to the supporting substrate effect and geometrical configurations. The substrate effect on the thermoacoustic sound generation from graphene is studied by controlling the surface porosity of various substrates.

Integration of the ferromagnetic insulator EuO onto graphene.

Abstract: We have demonstrated the deposition of EuO films on graphene by reactive molecular beam epitaxy in a special adsorption-controlled and oxygen-limited regime, which is a critical advance toward the realization of the exchange proximity interaction (EPI). It has been predicted that when the ferromagnetic insulator (FMI) EuO is brought into contact with graphene, an overlap of electronic wave functions at the FMI/graphene interface can induce a large spin splitting inside the graphene. Experimental realization of this effect could lead to new routes for spin manipulation, which is a necessary requirement for a functional spin transistor. Furthermore, EPI could lead to novel spintronic behavior such as controllable magnetoresistance, gate tunable exchange bias, and quantized anomalous Hall effect. However, experimentally, EuO has not yet been integrated onto graphene. Here we report the successful growth of high-quality crystalline EuO on highly oriented pyrolytic graphite and single-layer graphene. The epitaxial EuO layers have (001) orientation and do not induce an observable D peak (defect) in the Raman spectra. Magneto-optic measurements indicate ferromagnetism with a Curie temperature of 69 K, which is the value for bulk EuO. Transport measurements on exfoliated graphene before and after EuO deposition indicate only a slight decrease in mobility.

Thermal conductivity measurements of suspended graphene with and without wrinkles by micro-Raman mapping

Abstract: The thermal conductivity (κ) of suspended graphene membranes made by chemical vapor deposition (CVD) was measured by micro-Raman mapping. Cracks and wrinkles present in these suspended graphene membranes were identified by micro-Raman mapping, and κ values and their statistics were obtained on membranes free of such imperfections in a single mapping. Based on this new technique, an average κ value of 1875 ± 220 W m−1 K−1 at 420 K was measured on 26 suspended graphene membranes that were free of wrinkles, ~27% higher than the average value measured from 12 graphene membranes with wrinkles. These results suggest that the variation in published thermal conductivity values for suspended graphene samples could, at least in part, be due to the presence or absence of wrinkles.

Nanostructured hybrid transparent conductive films with antibacterial properties.

Abstract: Here, we demonstrate that the assembly of nanostructures with different dimensionalities yields “multicomponent hybrid” transparent conductive films (TCFs) with sheet resistance and optical transmittance comparable to that of indium tin oxide (ITO) films. It was shown that sheet resistance of single-component Ag nanowire (NW) films can be further decreased by introducing gold-decorated reduced graphene oxide (RG-O) nanoplatelets that bridge the closely located noncontacting metal NWs. RG-O nanoplatelets can act as a protective and adhesive layer for underneath metal NWs, resulting in better performance of hybrid TCFs compared to single-component TCFs. Additionally, these hybrid TCFs possess antibacterial properties, demonstrating their multifunctional characteristics that might have a potential for biomedical device applications. Further development of this strategy paves a way toward next generation TCFs composed of different nanostructures and characterized by multiple (or additional) functionalities.

Simultaneous Transfer and Doping of CVD-Grown Graphene by Fluoropolymer for Transparent Conductive Films on Plastic

Abstract: Chemical doping can decrease sheet resistance of graphene while maintaining its high transparency. We report a new method to simultaneously transfer and dope chemical vapor deposition grown graphene onto a target substrate using a fluoropolymer as both the supporting and doping layer. Solvent was used to remove a significant fraction of the supporting fluoropolymer, but residual polymer remained that doped the graphene significantly. This contrasts with a more widely used supporting layer, polymethylmethacrylate, which does not induce significant doping during transfer. The fluoropolymer doping mechanism can be explained by the rearrangement of fluorine atoms on the graphene basal plane caused by either thermal annealing or soaking in solvent, which induces ordered dipole moments near the graphene surface. This simultaneous transfer and doping of the graphene with a fluoropolymer increases the carrier density significantly, and the resulting monolayer graphene film exhibits a sheet resistance of ∼320 Ω/sq...

Uniform Wafer-Scale Chemical Vapor Deposition of Graphene on Evaporated Cu (111) Film with Quality Comparable to Exfoliated Monolayer

Abstract: This article demonstrated monolayer graphene grown on annealed Cu (111) films on standard oxidized 100-mm Si wafers with higher quality than existing reports. Large area Raman mapping indicated high uniformity (>97% coverage) of monolayer graphene with immeasurable defects (>95% defect-negligible) across the entire wafer. Key to these results is the phase transition of evaporated copper films from amorphous to (111) preferred crystalline, which resulted in subsequent growth of high quality graphene, as corroborated by X-ray diffraction and electron backscatter diffraction. Noticeably, such phase transition of the copper film was observed on a technologically ubiquitous Si wafer with a standard amorphous thermal oxide. A modified two-step etching transfer process was introduced to preserve the clean surface and electrical property of transferred monolayer graphene. The fabricated graphene field effect transistor on a flexible polyimide film achieved peak mobility over 4900 cm2/(V s) at ambient condition.

van der Waals Epitaxy of InAs Nanowires Vertically Aligned on Single-Layer Graphene

Abstract: Semiconductor nanowire arrays integrated vertically on graphene films offer significant advantages for many sophisticated device applications. We report on van der Waals (VDW) epitaxy of InAs nanowires vertically aligned on graphene substrates using metal–organic chemical vapor deposition. The strong correlation between the growth direction of InAs nanowires and surface roughness of graphene substrates was investigated using various graphene films with different numbers of stacked layers. Notably, vertically well-aligned InAs nanowire arrays were obtained easily on single-layer graphene substrates with sufficiently strong VDW attraction. This study presents a considerable advance toward the VDW heteroepitaxy of inorganic nanostructures on chemical vapor-deposited large-area graphenes. More importantly, this work demonstrates the thinnest epitaxial substrate material that yields vertical nanowire arrays by the VDW epitaxy method.

Selective surface functionalization at regions of high local curvature in graphene

Abstract: Monolayer graphene was deposited on a Si wafer substrate decorated with SiO2 nanoparticles (NPs) and then exposed to aryl radicals that were generated in situ from their diazonium precursors. Using micro-Raman mapping, the aryl radicals were found to selectively react with the regions of graphene that covered the NPs. The enhanced chemical reactivity was attributed to the increased strain energy induced by the local mechanical deformation of the graphene.