Monolayer Graphene Sheet Research Articles

A Green Chemistry of Graphene: Photochemical Reduction towards Monolayer Graphene Sheets and the Role of Water Adlayers

Clean sheets: Stable aqueous dispersions of graphene sheets (GSs) are obtained by exposing graphene oxide to irradiation with light at room temperature, without using any chemical additives. The photochemical reduction method is sustainable and scalable, repairs a majority of defects in the graphene layers, and can be used to fine-tune surface functional groups. Interestingly, the aqueous GS dispersions are stable without any added surfactant. The existence of a water layer that is strongly bound to GS is evidenced.

Localization behavior of Dirac particles in disordered graphene superlattices

Graphene superlattices (GSLs), formed by subjecting a monolayer graphene sheet to a periodic potential, can be used to engineer band structures and, from there, charge transport properties, but these are sensitive to the presence of disorder. The localization behavior of massless 2D Dirac particles induced by weak disorder is studied for both scalar-potential and vector-potential GSLs, computationally as well as analytically by a weak-disorder expansion. In particular, it is investigated how the Lyapunov exponent (inverse localization length) depends on the incidence angle to a 1D GSL. Delocalization resonances are found for both scalar and vector GSLs. The sharp angular dependence of the Lyapunov exponent may be exploited to realize disorder-induced filtering, as verified by full 2D numerical wave packet simulations.

Kapitza conductance of symmetric tilt grain boundaries in graphene

Non-equilibrium molecular dynamics simulations were employed to study the Kapitza conductance of symmetric tilt grain boundaries in the monolayer graphene sheet. Both armchair and zig-zag oriented bicrystal graphene were investigated. The Kapitza conductance of the interface shows length dependence up to 300 nm, which arises from the fact that long-wavelength phonons allowed in large-size graphene are able to transmit through the interface contributing to the Kapitza conductance. The Kapitza conductance exhibits monotonic increase with temperature, opposite to the trend of thermal conductivity of bulk graphene above room temperature. We found that the Kapitza conductance is inversely proportional to the number of dislocations per length of grain boundaries. The facts that the phonon density of states (DOS) shows no difference between the two crystals separated by the grain boundary and the vibrational DOS of grain boundary region atoms deviates from that of bulk atoms reveal that the interfacial thermal resistance arises from the structure defects, causing additional phonon scattering for the mismatched phonon spectrum of defects. The predicted length-independent Kapitza conductance ranges from 19 to 47 GW/Km2, which is larger than that of any other interfaces reported in the literature. Finally, theoretical analysis was carried out to explain why the thermal resistance scales with the number of defects per unit length.

Molecular dynamics simulation of temperature effect on mono-layer graphene sheets

The tensile and relaxation properties of mono-layer graphene sheets at different temperatures (0 K-3000 K) were investigated based on Molecular Dynamics(MD) simulation with AIREBO potential. The temperature effect on atomic structure of mono-layer graphene sheets during relaxation was analyzed as well as the relationship between temperature and mechanical properties of mono-layer graphene sheets under tensile stress/strain/load. The simulation results showed that the tensile properties and relaxation properties of mono-layer graphene sheets were all highly dependent on the temperature. When T =0 K, the relaxation of mono-layer graphene sheets was a dynamic equilibrium process with a complete atomic structure. The stability of mono-layer graphene sheets reduced when the temperature increased, the fluctuations of atoms during relaxation became somewhat irregular and violent with the increasing temperature. The tensile strength, ultimate tensile strain and Young’s modulus of mono-layer graphene sheets all decreased when the temperature rose from 0 K to 3000 K . The Young’s modulus of zigzag graphene sheets had a more temperature dependence than armchair graphene sheets, and the failure modes of mono-layer graphene sheets were different when the temperature changed.

Consideration of Bending and Buckling Behaviors of Monolayer and Multilayer Graphene Sheets

Graphene is a two-dimensional carbon based material. Remarkable mechanical, thermal and electrical properties of graphene make it as promising material for advanced applications; nevertheless, majority of its mechanical properties are still unknown. This research investigates buckling and bending behaviors of monolayer and multilayer armchair and zigzag graphene sheets. Bending stiffness, critical buckling force per unit length and critical strain of graphene sheets have been measured by molecular dynamic simulation method. Zigzag graphene sheet shows higher bending stiffness than armchair sheet. Van der Waals interaction between graphene sheets has an improving effect on the stability of middle layers. Cross-linkages reduce the buckling force per unit length and the buckling strain of multi layer graphene sheets.

First-Principles Study of Energy Band Gap of Graphene-Like B-C-N by Strain

We report a first-principles investigation on BN dopped monolayer graphene sheet and examined the electronic band structure and band gaps in equilibrium state and under strain. The obtained results reveal that the doping of B-N pairs on the hexagonal sheet can open the gap at the Dirac-like point. With heavy doping and more B-N bonds the energy bad gap is found to be larger. Upon tensile deformation, the dopped BCN monolayer sheet represents a strong anisotropic stress-strain relation. Detailed strain-gap relation investigation reveals that the energy band gap presents desperate variation trends for strain applied along and direction. Versatile band-gap modulation schemes can then be obtained through direction-dependent strain engineering of the BCN nanosheet..

Practical Molecular Dynamic Simulation of Monolayer Graphene with Consideration of Structural Defects

Molecular dynamic simulation method has been employed to consider the mechanical properties of pristine and defected armchair and zigzag monolayer graphene sheets under tension and compression. Effects of Stone Wales (SW), Cell deformed (CD) and cell vacancy (CV) defects on ultimate stress and strain of graphene sheets have been considered. Obtained results indicate that zigzag graphene sheet has higher elastic modulus and stiffness than armchair type. Three SW defects have the highest destructive effect on ultimate stress and strain of zigzag graphene sheet while CV defect reveals the highest reducing effect on those of armchair type.

Two-Step Hydrothermal Method for Synthesis of Sulfur-Graphene Hybrid and its Application in Lithium Sulfur Batteries

A two-step hydrothermal method is reported here for the synthesis of a sulfur-graphene hybrid material. The scanning electron microscopy, X-ray diffraction and Raman spectroscopy characterizations have confirmed that this method can produce a homogeneous sulfur coating on the graphene surface and prevent the graphene sheets from aggregation simultaneously to form a three dimensional carbon network. When tested as a cathode in lithium sulfur batteries, the hybrid material exhibits a high discharge capacity and good cycling stability at a current density of 1 A/g, even as the sulfur content reaches an extremely high level (75.2% wt). Its excellent electrochemical performance can be attributed to the stable carbon skeleton formed by the monolayer graphene sheets and the good contact between sulfur and graphene.

One-step synthesis of metal nanoparticle decorated graphene by liquid phase exfoliation

A novel method for the fabrication of a metal nanoparticle (NP)–graphene hybrid based on the pristine graphene from liquid-phase exfoliation in organic solvents is essential. In the present work, 1-cyanoethyl-2-ethyl-4-methylimidazole (2E4MZ-CN) was employed as a novel stabilizer for graphene sheets in an acetonitrile solution and simultaneously as a reducing agent for metal NPs deposited on the graphene sheets. A homogeneous pristine graphene dispersion is achieved by sonication of natural graphite in an acetonitrile solution of 2E4MZ-CN. Once exfoliated from natural graphite under sonication, the graphene sheets can be stabilized by 2E4MZ-CN through the π–π interaction. Atomic force microscopy, Raman spectroscopy and transmission electron microscopy (TEM) confirm the presence of monolayer and few-layer graphene sheets. The absence of oxidation or destruction of the sp2 character of the carbon is demonstrated by Raman spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Silver NP (AgNP) decorated graphene hybrid is in situ synthesized during the curing process of epoxy resin. Scanning electron microscopy, X-ray diffraction and TEM show that the small particle size and good distribution of AgNPs are realized due to the stabilization of epoxy network. The synthesized AgNP decorated graphene is used as nanoscale filler to prepare highly conductive epoxy-based composites (4 × 10−5 Ω cm). The low temperature sintering of AgNPs attached on the graphene constructs effective electrical network in the epoxy matrix. This method facilitates the preparation of graphene-based materials for various applications.

High-field magneto-transport at charge neutrality point in monolayer graphene

We have investigated the magneto-transport around filling factor ν = 0 in monolayer graphene sheet. Zero Hall plateau of Hall conductivity and minimum of longitudinal conductivity have been clearly observed despite the highly fluctuated behaviour of magneto-resistance above 10 T. These phenomena can be understood by the transport of counter-propagating electron and hole edge states at the charge neutrality point. We tried to explain the origin of remarkable fluctuations in magneto-resistance by the energy relaxations among the counter-propagating edge channels.

Optical absorption of zigzag carbon nanotubes at finite temperature

We investigate the rate of optical absorption of carbon nanotubes and graphene plane using full dispersion. Tight binding model Hamiltonian is used to calculate conductivity and optical absorption of the system. The contribution of both interband and intraband transitions on the optical spectra is considered. Our results are valid up to the ultraviolet region. The results for monolayer graphene sheet show a peak in energy E ≈ 2 eV . Furthermore, we investigate the effect of temperature and carbon nanotube radius on the optical spectra. Our results show that the peaks of optical conductivity shift to lower photon energy due to increase of the diameter of zigzag carbon nanotubes.

A Facile Approach to the Synthesis of Graphene Nanosheets Under Ultra-Low Exfoliation Temperature

High specific surface area graphene nanosheets have been obtained from graphite oxide by using an effective modified exfoliation method under vacuum, the exfoliation temperature (135 degrees C) is much lower than that conventionally applied (1050 degrees C) to obtain monolayer graphene sheets via rapid thermal shock. These products have fluffy and highly porous structure and with a lateral size typically of a few micrometers. Transmission electron microscopy (TEM) observation shows that it looks like a wrinkled transparent ultrathin film consisting of single or few-layer graphene sheets, and their Brunauer-Emmett-Teller surface area is as large as 750 m2/g. Simultaneously, X-ray photoelectron spectroscopy analysis revealed that considerable amount of oxygen-containing groups (C/O ratio, 5:1) retained on the graphene sheets after exfoliation process, which would provide convenience for further modification of the surface properties and chemistry of graphene sheets. This work offers a facile and scalable approach to fabricate graphene oxide and opens up a new vista of various potential applications electronics and composite materials.

Explicit analytical expressions for the critical buckling stresses in a monolayer graphene sheet based on nonlocal elasticity

Explicit expressions are given to study the biaxial buckling of monolayer graphene sheets. Based upon the continuum mechanics, a plate model is adopted in which the small length scale effect is incorporated into the governing equation through the nonlocal elasticity theory of Eringen. By employing the Galerkin method, analytical expressions are derived which allow quick and accurate calculation of the critical buckling loads of monolayer graphene sheets with various boundary conditions from the static deflection under a uniformly distributed load. The effectiveness of the present study is assessed by molecular dynamics simulations as a benchmark of good accuracy.

On the effective plate thickness of monolayer graphene from flexural wave propagation

We utilize classical molecular dynamics to study flexural, or transverse wave propagation in monolayer graphene sheets and compare the resulting dispersion relationships to those expected from continuum thin plate theory. In doing so, we determine that regardless of the chirality for monolayer graphene, transverse waves exhibit a dispersion relationship that corresponds to the lowest order antisymmetric (A0) mode of wave propagation in a thin plate with plate thickness of h=0.104 nm. Finally, we find that the achievable wave speeds in monolayer graphene are found to exceed those reported previously for single walled carbon nanotubes, while the frequency of wave propagation in the graphene monolayer is found to reach the terahertz range, similar to that of carbon nanotubes.

Synthesis of Large-Area and Monolayer of Graphene: Role of Transition Metal Support and Growth Time by CVD Method

Continuous monolayer graphene sheet with large area has been synthesized via chemical vapor deposition (CVD) method using liquid hydrocarbon as precursor. Synthesis parameters including growth substrate and growth time have been investigated to assess their influence on monolayer graphene synthesis. Raman spectroscopy and high resolution transmission electron microscopy (HRTEM) reveal that the number of layers and quality of graphene sheet depend greatly on the varied synthesis parameter. The study could be used to improve understanding the growth of graphene by CVD method in order to meet the needs of graphene in various electronic applications.

Easy and green synthesis of reduced graphite oxide-based hydrogels

We report an environmentally-friendly and easy to scale-up route to synthesize reduced graphite oxide (RGO) hydrogel by simple reduction of exfoliated graphite oxide (GO) with excess vitamin C (VC). Mono-layer graphene sheets self-assembling into a well-defined and interconnected 3D porous network through π– π interaction during gelation can be seen by scanning electron microscopy and atomic force microscopy images. The RGO hydrogels were further functionalized and the corresponding RGO/carbon nanotube or RGO/noble metal hybrid hydrogels were obtained after similar reduction or co-reduction when carbon nanotubes were added to and stabilized with GO sheets or when noble metal precursors were added and incorporated with GO sheets. Rheological performance and electrical conductivities of these RGO-based hydrogels were also investigated in this study. The residual VC retained in these hydrogels as a biofunctional component can be gradually released in a diffusion-controlled manner, which may endow these RGO-based hydrogels with a biofunctionality. Because encapsulated bioactive VC simultaneously occurs with the formation of these assemblies, the resulting RGO-based hydrogels may have great potential in use as transdermal systems for controlled delivery of VC, tissue engineering, biosensors, etc.

Fracture analysis of monolayer graphene sheets with double vacancy defects via MD simulation

Carbon nanostructures such as carbon nanotubes (CNTs) and graphene sheets have attracted great attention due to their exceptionally high strength and elastic strain. These extraordinary mechanical properties, however, can be affected by the presence of defects in their structures. When a material contains multiple defects, it is expected that the stress concentration of them superposes if the separation distances of the defects are low, which causes a more reduction of the strength. On the other hand, it is believed that if the defects are far enough such that their affected areas are distinct, their behavior is similar to a material with single defect. In this article, molecular dynamics (MD) is used to explore the influence of separation distance of double vacancy defects on the mechanical properties of single-layered graphene sheets (SLGSs). To this end, critical stress and strain of SLGSs containing double vacancy with different distances are determined and the results are compared with those of perfect SLGSs and graphene sheets with single vacancy defect. The results reveal that the ultimate strength of the SLGS with double vacancy tends to the one with a single vacancy when the separation distance becomes further. In this regard, the threshold distance beyond which double defects behave like a single one is examined. Finally, Young’s modulus of perfect, single and double vacancy defected graphene sheets with different separation distances is determined. It is concluded that this property is slightly affected by the separation distance.

Monolayer graphene from a green solid precursor

Monolayer and bilayer graphene sheets are synthesized by simple control pyrolysis of solid botanical derivative camphor (C 10H 16O), a green and renewable carbon source. Raman studies show much intense 2D peak than that of G peak, signifying presence of monolayer graphene. Transmission electron microscopic study shows predominately monolayer or bilayer graphene sheets, while trilayer graphene sheet were also observed. Synthesized graphene film on copper foil is transferred to poly(ethylene terephthalate) substrate to fabricate transparent electrode. Electrical and optical measurement shows a sheet resistance of 860 Ω/sq with a transmittance of 91% at 550 nm wavelength of the graphene film. The technique to fabricate monolayer or bilayer graphene based film from camphor is both viable and scalable for potential large area electronic applications.

Large-Scale Growth and Characterizations of Nitrogen-Doped Monolayer Graphene Sheets

In-plane heteroatom substitution of graphene is a promising strategy to modify its properties. Doping with electron-donor nitrogen heteroatoms can modulate the electronic properties of graphene to produce an n-type semiconductor. Here we demonstrate the growth of monolayer nitrogen-doped graphene in centimeter-scale sheets using a chemical vapor deposition process with pyridine as the sole source of both carbon and nitrogen. High-resolution transmission microscopy and Raman mapping characterizations indicate that the nitrogen-doped graphene sheets are uniformly monolayered. The existence of nitrogen-atom substitution in the graphene planes was confirmed by X-ray photoelectron spectroscopy. Electrical measurements show that the nitrogen-doped graphene exhibits an n-type behavior, different from pristine graphene. The preparation of large-area nitrogen-doped graphene provides a viable route to modify the properties of monolayer graphene and promote its applications in electronic devices.

Electrochemistry of Individual Monolayer Graphene Sheets

We report on the fabrication and measurement of devices designed to study the electrochemical behavior of individual monolayer graphene sheets as electrodes. We have examined both mechanically exfoliated and chemical vapor deposited (CVD) graphene. The effective device areas, determined from cyclic voltammetric measurements, show good agreement with the geometric area of the graphene sheets, indicating that the redox reactions occur on clean graphene surfaces. The electron transfer rates of ferrocenemethanol at both types of graphene electrodes were found to be more than 10-fold faster than at the basal plane of bulk graphite, which we ascribe to corrugations in the graphene sheets. We further describe an electrochemical investigation of adsorptive phenomena on graphene surfaces. Our results show that electrochemistry can provide a powerful means of investigating the interactions between molecules and the surfaces of graphene sheets as electrodes.