Oriented Graphene Sheets Research Articles

Synthesis and structural analysis of graphene nanosheets via microwave atmospheric pressure plasma

This study reports the effective synthesis of multilayered graphene sheets via microwave atmospheric pressure plasma. This innovative approach streamlines and expedites graphene production and other carbon nanostructures, eliminating the need for catalysts, solvents, or complex processing conditions. Ethanol is directly injected into a microwave-generated argon plasma plume, leading to the formation of graphene. Raman spectroscopy revealed characteristic peaks (2D, G, and D bands) confirming graphene composition, with defects indicated by the D band. X-ray diffraction analysis supported these findings, indicating a broad peak at 25∘ corresponding to the (002) plane, affirming a multi-layered graphene structure. Scanning electron microscopy exhibited crumpled, randomly oriented graphene sheets, albeit with uneven structures suggesting impurity incorporation. The presence of defects was quantified through the intensity ratio of the D to G band (ID/IG) in Raman spectroscopy, revealing a value of 0.80, signifying the presence of defects in the synthesized graphene. The 2D to G band intensity ratio (I2D/IG) suggested the existence of 7–10 graphene layers, highlighting the need for further optimization for enhanced graphene quality and purity.

Synthesis of Ag-decorated vertical graphene nanosheets and their electrocatalytic efficiencies

Herein we report the successful preparation of silver (Ag)-decorated vertically oriented graphene sheets (Ag/VGs) via helicon wave plasma chemical vapor deposition (HWP-CVD) and radiofrequency plasma magnetron sputtering (RF-PMS). VGs were synthesized in a mixture of argon and methane (Ar/CH4) by HWP-CVD and then the Ag nanoparticles on the prepared VGs were modified using the RF-PMS system for different sputtering times and RF power levels. The morphology and structure of the Ag nanoparticles were characterized by scanning electron microscopy and the results revealed that Ag nanoparticles were evenly dispersed on the mesoporous wall of the VGs. X-ray diffraction results showed that the diameter of the Ag particles increased with the increase in Ag loading, and the average size was between 10.49 nm and 25.9 nm, consistent with the transmission electron microscopy results. Ag/VGs were investigated as effective electrocatalysts for use in an alkaline aqueous system. Due to the uniquely ordered and interconnected wall structure of VGs, the area of active sites increased with the Ag loading, giving the Ag/VGs a good performance in the oxygen evolution reaction. The double-layer capacitance (C dl) of the Ag/VGs under different Ag loadings were studied, and the results showed that the highest Ag content gave the best C dl (1.04 mF cm−2). Our results show that Ag/VGs are likely to be credible electrocatalytic materials.

Sub-Nanometer Interfacial Oxides on Highly Oriented Pyrolytic Graphite and Carbon Nanotubes Enabled by Lateral Oxide Growth.

A new generation of compact and high-speed electronic devices, based on carbon, would be enabled through the development of robust gate oxides with sub-nanometer effective oxide thickness (EOT) on carbon nanotubes or graphene nanoribbons. However, to date, the lack of dangling bonds on sp2 oriented graphene sheets has limited the high precursor nucleation density enabling atomic layer deposition of sub-1 nm EOT gate oxides. It is shown here that by deploying a low-temperature AlOx (LT AlOx) process, involving atomic layer deposition (ALD) of Al2O3 at 50 °C with a chemical vapor deposition (CVD) component, a high nucleation density layer can be formed, which templates the growth of a high-k dielectric, such as HfO2. Atomic force microscopy (AFM) imaging shows that at 50 °C, the Al2O3 spontaneously forms a pinhole-free, sub-2 nm layer on graphene. Density functional theory (DFT) based simulations indicate that the spreading out of AlOx clusters on the carbon surface enables conformal oxide deposition. Device applications of the LT AlOx deposition scheme were investigated through electrical measurements on metal oxide semiconductor capacitors (MOSCAPs) with Al2O3/HfO2 bilayer gate oxides using both standard Ti/Pt metal gates as well as TiN/Ti/Pd gettering gates. In this study, LT AlOx was used to nucleate HfO2 and it was shown that bilayer gate oxide stacks of 2.85 and 3.15 nm were able to achieve continuous coverage on carbon nanotubes (CNTs). The robustness of the bilayer was tested through deployment in a CNT-based field-effect transistor (FET) configuration with a gate leakage of less than 10-8 A/μm per CNT.

Investigations on the effect of process parameters on the growth of vertically oriented graphene sheet in plasma‐enhanced chemical vapour deposition system

AbstractA multistage numerical model comprising the plasma kinetics and surface deposition sub‐models is developed to study the influence of process parameters, namely, total gas pressure and input plasma power on the plasma chemistry and growth characteristics of vertically oriented graphene sheets (VOGS) grown in the plasma‐enhanced chemical vapour deposition system containing the Ar + H2 + C2H2 reactive gas mixture. The spectral and spatial distributions of temperature and number densities, respectively, of plasma species, that is, charged and neutral species in the plasma reactor, are examined using inductively coupled plasma module of COMSOL Multiphysics 5.2 modelling suite. The numerical data from the computational plasma model are fed as the input parameters for the surface deposition model, and from the simulation results, it is found that there is a significant drop in the densities of various plasma species as one goes from the bulk plasma region to the substrate surface. The significant loss of the energetic electrons is observed in the plasma region at high pressure (for constant input power) and low input power (for constant gas pressure). At low pressure, the carbon species generate at higher rates on the catalyst nanoislands surface, thus enhancing the growth and surface density of VOGS. However, it is found that VOGS growth rate increases when input plasma power is raised from 100 to 300 W and decreases with further increase in the plasma power. A good comparison of the model outcomes with the available experimental results confirms the adequacy of the present model.

Synthesis of Vertically Oriented Graphene Sheets or Carbon Nanowalls-Review and Challenges.

The paper presents a review on the current methods for deposition of vertically oriented multilayer graphene sheets (often called carbon nanowalls—CNWs) on solid substrates. Thin films of CNWs are among the most promising materials for future applications in capacitors, batteries, electrochemical devices, and photovoltaics, but their application is currently limited by slow deposition rates and difficulties in providing materials of a desired structure and morphology. The review paper analyzes results obtained by various groups and draws correlations between the reported experimental conditions and obtained results. Challenges in this scientific field are presented and technological problems stressed. The key scientific challenge is providing the growth rate as well as morphological and structural properties of CNWs thin films versus plasma parameters, in particular versus the fluxes of reactive plasma species onto the substrate surface. The technological challenge is upgrading of deposition techniques to large surfaces and fast deposition rates, and development of a system for deposition of CNWs in the continuous mode.

Edge Planes Oriented Electrochemically Exfoliated HOPG for Electrochemical Capacitors

Increase in demand of flexible and portable electronic gadgets, created a market for similar energy storage devices. Freestanding, binder free electrode is the primary component to make such an energy storage device. Graphene, a 2D form of carbon has attracted much attention as an electrode material for supercapacitor applications, because of its properties, like, high surface area and excellent intrinsic electrical conductivity. Scalable, economical synthesis of graphene and its application as an electrode material is still in the development stage. Restacking and aggregation of graphene sheets during electrode fabrication decreases effective surface area for double layer formation. Electrochemical exfoliation of HOPG leads to growth of edge oriented graphene sheets on the surface of HOPG. Controlled electrochemical exfoliation of HOPG electrodes in different electrolytes shows different capacitive performance. This performance variation is related to pH of electrolyte. A maximum aerial capacitance of 760 mF cm-2 at current density of 1 mA cm-2 achieved in electrode exfoliated in 1 M H2SO4 electrolyte.

Plasma-electric field controlled growth of oriented graphene for energy storage applications

It is well known that graphene grows as flat sheets aligned with the growth substrate. Oriented graphene structures typically normal to the substrate have recently attracted major attention. Most often, the normal orientation is achieved in a plasma-assisted growth and is believed to be due to the plasma-induced in-built electric field, which is usually oriented normal to the substrate. This work focuses on the effect of an in-built electric field on the growth direction, morphology, interconnectedness, structural properties and also the supercapacitor performance of various configurations of graphene structures and reveals the unique dependence of these features on the electric field orientation. It is shown that tilting of growth substrates from parallel to the normal direction with respect to the direction of in-built plasma electric field leads to the morphological transitions from horizontal graphene layers, to oriented individual graphene sheets and then interconnected 3D networks of oriented graphene sheets. The revealed transition of the growth orientation leads to a change in structural properties, wetting nature, types of defect in graphitic structures and also affects their charge storage capacity when used as supercapacitor electrodes. This simple and versatile approach opens new opportunities for the production of potentially large batches of differently oriented and structured graphene sheets in one production run.

Dynamic application of novel electro-optic switchable device modulation by graphene oxide dispersed liquid crystal cell assembling CdS nanowires

In nanotechnology, the novel creation of nanostructures consistently feeds back into efforts to fabricate novel complex hybrid nanomaterials. Two-dimensional graphene oxide dispersed liquid crystalline materials (GDLC) module assembled with CdS nanowires(NW's) have received widespread unprecedented attention due to their exceptional mechanical, electrical, thermal properties. However, making macroscopic graphene oxide (GO) sheets with average diameter 2.1 μm, CdS nanowires (15–20 nm) requires novel technology to fabricate few layered graphene oxide (FGO)-sheets were uniformly distributed as macroscopically ordered structures. Aqueous GDLC-CdS nanowires are continuously twist to obtain macroscopic GO-sheets. Subsequently chemical reduction gave first macroscopic neat graphene sheets with high conductivity and good mechanical performance. Liquid crystal formation is the most viable approach to produce macroscopic, periodic self-assembled materials from oriented graphene sheets. We discovered well-soluble FLG-sheets can exhibit nematic liquid crystallinity into Dimethyl sulfoxide (DMSO), first established isotropic-nematic solid phase diagram demonstrated by optical microscopy textural evidences of switching and relevant spectroscopic characterizations. GO utilizes light absorption and nanoscale heat source to thermally induced phase transition with LC from homogeneous alignment to isotropic phase. Thus, volume contraction occurred over the surface area of the GDLC-nanowires hybrid complex module due to photothermal effect. Thus, an excellent conductor as well as high contrast electro-optic switchable cell also deserves most promising applications. These novel findings shed focus on microscopically assembled graphene-LC based semiconductor nanomaterial's textural phase behavior, which can only be realized as the field moves forward and makes more significant advances.

Ultratough cellular films from graphene oxide hydrogel: A way to exploit rigidity and flexibility of two-dimensional honeycomb carbon

The mechanical properties of hierarchically porous graphene films are unsatisfactory for the wide applications requiring toughness. Here, we report an innovative approach to construct anisotropic graphene-based cellular films with an oriented and interlinked microstructure, which can provide both high strength (2.4 ± 0.2 MPa) and extraordinary toughness (0.10 ± 0.01 MJ m−3). The specific strength (30 ± 2 MPa/(Mg m−3)) and specific modulus (8.4 ± 0.4 × 102 MPa/(Mg m−3)) of the porous films are even comparable to tough cancellous bone. The graphene oxide hydrogel films obtained from the shear and relaxation procedure are then reduced with hydroiodic acid/acetic acid, which solidifies the oriented and interlinked structures to give the final porous films. By this approach, the intrinsic features of an individual graphene sheet, such as high in-plane rigidity and excellent out-of-plane flexibility, are fully exploited. The great enhancement of strength and toughness is proved to be realized by a synergistic cooperation mechanism, where the unique architecture bears stress through tensile strain of oriented graphene sheets and dissipates energy through bending deformation of the bridged graphene sheets. The proof-of-concept design and deep understanding of the structure-property relationship will be of benefit to the fabrication and applications of graphene-based devices.

Optimization of biomass-based carbon materials for hydrogen storage

Hydrogen is considered as the most promising future fuel, as its combustion generates only water vapor besides energy. However, finding efficient, safe and low cost storage methods is the basic impediment for the adoption of hydrogen as fuel. For this purpose, five biomass-based carbon samples have been successfully prepared through KOH activation procedure. The microstructure of the prepared materials was tuned by varying KOH/precursor weight ratio from 1:1 to 5:1, in order to optimize their hydrogen storage behavior and to clarify the storage mechanism. A careful textural and morphological characterization of the prepared samples (N2 and CO2 adsorption, X-ray diffraction, electron microscopy observations) showed that by increasing the activation ratio the nature of the carbons changes from microporous to micro-mesoporous. Thus, an increase in the surface area was observed which enhanced H2 sorption capacity when high H2 pressures were adopted. At subatmospheric pressure, the role of active sites for H2 adsorption, located in the oriented graphene sheets, was clearly elaborated. Finally, the optimal carbon sample has shown a capacity of 6wt% and 1.22wt% at −196°C and 25°C respectively, and 200bar. These results make biomass-based carbons promising materials for H2 storage application.

Three-dimensionally bonded spongy graphene material with super compressive elasticity and near-zero Poisson's ratio.

It is a challenge to fabricate graphene bulk materials with properties arising from the nature of individual graphene sheets, and which assemble into monolithic three-dimensional structures. Here we report the scalable self-assembly of randomly oriented graphene sheets into additive-free, essentially homogenous graphene sponge materials that provide a combination of both cork-like and rubber-like properties. These graphene sponges, with densities similar to air, display Poisson's ratios in all directions that are near-zero and largely strain-independent during reversible compression to giant strains. And at the same time, they function as enthalpic rubbers, which can recover up to 98% compression in air and 90% in liquids, and operate between -196 and 900 °C. Furthermore, these sponges provide reversible liquid absorption for hundreds of cycles and then discharge it within seconds, while still providing an effective near-zero Poisson's ratio.

Combined CI-MD approach in formulation of engineering moduli of single layer graphene sheet

An evolutionary approach of multi-gene genetic programming (GP) is used to study the effects of aspect ratio, temperature, number of atomic planes and vacancy defects on the engineering moduli viz. tensile and shear modulus of single layer graphene sheet. MD simulation based on REBO potential is used to obtain the engineering moduli. This data is then fed into the paradigm of a GP cluster comprising of genetic programming, which was specifically designed to formulate the explicit relationship of engineering moduli of graphene sheets loaded in armchair and zigzag directions with respect to aspect ratio, temperature, number of atomic planes and vacancy defects. We find that our MGGP model is able to model the engineering moduli of armchair and zigzag oriented graphene sheets well in agreement with that of experimental results. We also conducted sensitivity and parametric analysis to find out specific influence and variation of each of the input system parameters on the engineering moduli of armchair and zigzag graphene sheets. It was found that the number of defects has the most dominating influence on the engineering moduli of graphene sheets.

High-Current Reliability and Growth Conditions of Multilayer Graphene Wire Obtained by Annealing Sputtered Amorphous Carbon

We fabricated multilayer graphene directly on SiO2 by annealing of sputtered amorphous carbon under a catalyst layer without complicated transfer processes, and investigated the effects of the catalysts and the annealing ambient gases on obtaining large-grain, multilayer graphene. As a result, it was found that annealing conditions with a Co catalyst layer in a nitrogen gas atmosphere are important for increasing the ratio of oriented graphene sheets, corresponding to a lower resistivity of the film. Furthermore, it was confirmed that the multilayer graphene wire obtained by optimizing the growth conditions can sustain a high current density of 107 A/cm2, that is, the lifetime of the multilayer graphene wire is over two orders of magnitude longer than that of a Cu wire with the same current density; this current density is over one order of magnitude higher than the current density that can be carried by a Cu wire for the same lifetime.

New insights into the microstructure of the friction surface layer of C/C composites

An attempt has been undertaken to obtain a better understanding of the friction and wear mechanisms of C/C composites by microstructural analysis of the friction surface layer and wear debris using scanning electron microscopy and transmission electron microscopy. The friction and wear properties of C/C composites of rough laminar pyrocarbon with added resin-derived carbon have been investigated with a home-made laboratory-scale dynamometer to simulate airplane normal landing. The results have shown that a friction layer with a thickness of several micrometers was formed on the bulk material. It was characterized as consisting mainly of amorphous carbon, but with a few microsized particles as well. This friction layer protected the bulk materials from serious degradation. In addition to the friction layer, a friction film with a thickness of less than 200 nm was observed for the first time. It partly covered the top surface of the friction layer. Transmission electron microscopy analysis has demonstrated that the friction film was characterized by a laminar structure, the belts of which were revealed to consist of highly oriented graphene sheets. The constant friction coefficient was attributed to this lubricant film.

Aqueous Liquid Crystals of Graphene Oxide

The formation of liquid crystals (LCs) is the most viable approach to produce macroscopic, periodic self-assembled materials from oriented graphene sheets. Herein, we have discovered that well-soluble and single-layered graphene oxide (GO) sheets can exhibit nematic liquid crystallinity in water and first established their isotropic-nematic solid phase diagram versus mass fraction and salt concentration. The zeta potential of GO dispersion is around -64 mV, and its absolute value decreases with increasing salt concentration, implying that the electrostatic repulsive force between negatively charged GO sheets is the dominant interaction in the system of GOLCs and also explaining the salt-dependent phase behavior. For single-layer GO sheets with average diameter of 2.1 μm and polydispersity index of 83%, the isotropic-nematic phase transition occurs at a mass concentration of ∼0.025%, and a stable nematic phase forms at ∼0.5%. Rheological measurements showed that GO aqueous dispersions performed as typical shear flows and confirmed the isotropic-nematic transition. The ordering of GO sheets in aqueous dispersions and the solid state is demonstrated by the characterizations of polarized-light optical microscopy, small-angle X-ray scattering, scanning electron microscopy, and transmission electron microscopy. The direct, real-time fluorescent inspections by confocal laser microscopy further reveal that the individually dispersed fluorescent GO sheets align with orientational directions along their long axes. These novel findings shed light on the phase behaviors of diversely topological graphenes and lay the foundation for fabrication of long-range, ordered nano-objects and macroscopically assembled graphene-based functional materials.

Patterning Vertically Oriented Graphene Sheets for Nanodevice Applications

Graphene has attracted growing interest in the past few years. Growing vertically oriented graphene sheets with a designed pattern is practically attractive for device applications based on graphene. Here we report a patterned synthesis of vertical graphene nanosheets using plasma-enhanced chemical vapor deposition. Both experimental and modeling results suggest that the electric field distribution above the substrate material plays a key role in the graphene coverage. Vertical graphene patterns can thus be designed through artificially designing the surface electric field distribution. A field-effect transistor (FET) sensor device has been demonstrated for detection of low-concentration gases using vertically patterned graphene sheets bridging a metal electrode gap.

Nanogold Spacing of Stacked Graphene Nanofibers for Supercapacitors

AbstractGraphene exhibits great potential for energy storage devices in electrochemical supercapacitors as it is a light‐weight, inexpensive material. However, as graphene nanomaterials are prone to stacking and therefore blocking electroactive sites, its real capacity is low. Recently, there has been a significant interest in quasi 1D structures of carbon nanofibers with radially oriented graphene sheets, called stacked graphene nanofibers graphite nanofibers or nanoribbons. Such nanofibers exhibit an unparalleled amount of edge sites on their surface. In this study, we use gold nanoparticles as nanospacers for separating stacked graphene nanofibers. We investigate the dependence of the capacitance of stacked graphene nanofiber/nanogold nanocomposites upon the size of gold nanoparticles (2–150 nm) and the amount of gold nanospacers in the composite. It was found that properly sized gold nanoparticles (of 40 nm) used as nanospacers of both 45 nm and 90 nm diameter stacked graphene nanofibers can increase the capacitance of nanofibers by three times.

Vertically oriented graphene sheets grown on metallic wires for greener corona discharges: lower power consumption and minimized ozone emission

Vertically oriented graphene-coated metallic wire is used as a corona discharge electrode. The discharge starts with a ‘graphene discharge stage’ and then transits to a ‘wire discharge stage’. Compared with conventional practice employing macro- or micro-sized electrodes, discharges from graphene sheets feature lower power consumption and significantly reduced ozone emission.

Hyperthermal atomic hydrogen and oxygen etching of vertically oriented graphene sheets

Carbon nanosheets have previously been shown to be promising high current field emission cathodes for a variety of potential applications. The vertically oriented planar sp2 carbon nanosheets grown by rf plasma-enhanced chemical vapor deposition terminate with one to seven graphene sheets and grow to ∼1 μm in height. High current field emission, Je∼0.15 mA mm−2 (8 V μm−1), conducted within an ultrahigh vacuum system in a diode configuration in line-of-sight to a mass spectrometer, shows that CH4, CO2, and CO are generated as a result of cathode bombardment by hyperthermal oxygen and hydrogen neutrals and ions generated by electron stimulated desorption at the Cu anode. Confirmation of the mechanism was achieved by repeating the experiments using a Au anode. Simultaneous acquisition of I-V data and the partial pressures of reaction products in the mass spectrometer have shown repeatable, sustained CH4, CO2, and CO production. As these hyperthermal atomic hydrogen and oxygen species impinge on the sidewalls and edges of the carbon nanosheets, they bond to various sites throughout the sp2 carbon array. Progressively, as further hydrogen and oxygen arrive, CH4, CO2, and CO are formed and desorbed, thereby etching the film. Raman spectroscopy has confirmed a corresponding increase in defect sites (ID/IG increased from 0.57 to 0.81) over the test interval. Scanning electron microscopy cross sections of carbon nanosheet cathodes before and after high current lifetime testing (>200 h) show the average etching rate to be ∼1.7×10−3 nm/s.

Constraints on Stellar Grain Formation from Presolar Graphite in the Murchison Meteorite

We report the results of isotopic, chemical, structural, and crystallographic microanalyses of graphitic spherules (0.3-9 μm) extracted from the Murchison meteorite. The spherules have12C/13C ratios ranging over 3 orders of magnitude (from 0.02 to 80 times solar), clearly establishing their presolar origin as stellar condensates. These and other isotopic constraints point to a variety of stellar types as sources of the carbon, including low-mass asymptotic giant branch (AGB) stars and supernovae. Transmission electron microscopy (TEM) of ultrathin sections of the spherules revealed that many have a composite structure consisting of a core of nanocrystalline carbon surrounded by a mantle of well-graphitized carbon. The nanocrystalline cores are compact masses consisting of randomly oriented graphene sheets, from PAH-sized units up to sheets 3-4 nm in diameter, with little graphitic layering order. These sheets probably condensed as isolated particles that subsequently coalesced to form the cores, after which the surrounding graphitic mantles were added by vapor deposition.