Layer Graphene Sheets Research Articles

Molecular simulations of stress wave propagation and perforation of graphene sheets under transverse impact

Transverse impact behaviour of graphene sheet (GS) has been studied using molecular dynamics (MD) simulations. One, two and three layers of GS have been impacted with a fullerene rigid projectile in the impact velocity range: 3.5–7.5 km/s. LAMMPS is used for MD simulations using the AIREBO force field. Kinetic energy, displacement, velocity and perforation resistance force of the projectile have been investigated to understand the perforation mechanics of GS. In-plane axial-wave and out-of-plane cone-wave speeds in the GS are determined. In-plane axial-wave speed is also determined using membrane theory where the modulus and Poisson's ratio used in that formula are derived from the MD simulations. Results show that the energy transmitted to the GS is equal to the work of perforation. At high impact velocities, two to three in-plane crack initiates in the GS along six different preferential crack angles at 60° interval in the form of bond breakage and propagates through the sheet during perforation which matches well with the lower bound of recent Lee et al. (2014) experiments. High axial-wave and cone-wave velocity of the graphene sheet indicates that GS will allow more momentum transfer offering the potential for improved ballistic protection per unit areal-density.

Ultrasonication assisted mild solvothermal synthesis and morphology study of few-layered graphene by colloidal suspensions of pristine graphene oxide

In the present work, a simple and systematic approach for the reduction of graphene oxide (GO), that converts to chemically modified graphene at low temperature (T = 200 °C) using ultrasonication assisted mild solvothermal method without the use of any surfactants/stabilizers has been employed. Graphene Oxide sheets are reduced in polar and non-polar organic liquid media under ultrasonication and thermal process conditions, which aids in active high yield chemically modified few layers graphene sheets with tunable C/O ratios dependent on the boiling point of the solvent. The reaction time, temperature, auto generated pressure in the reaction vessel and the reducing power of the solvents influenced the extent of reduction of graphene oxide sheets to modified graphene sheets. This study examined the dispersion behavior of graphene oxide in a polar solvent and non-polar, as well as the differences in the behavior in various solvents related to the solubility parameter. The chemically modified graphene produces a flexible morphology with a large surface area and micro porous distribution resulting in “sheets/flakes like” materials generated by very simple ultrasonication and thermal exfoliation of the reduced graphene oxide sheets in a wide variety of organic solvent.

Scanning microwave microscope imaging of micro-patterned monolayer graphene grown by chemical vapor deposition

Characterization of micro-patterned chemical vapor deposited monolayer graphene using a scanning microwave microscope has been presented. Monolayer graphene sheets deposited on a copper substrate were transferred to a variety of substrates and micro-patterned into a periodic array of parallel lines. The measured complex reflection coefficients exhibit a strong dependency on the operating frequency and on the samples' electrical conductivity and permittivity. The experiments show an extremely high sensitivity by detecting image contrast between single and double layer graphene sheets. Correlating the images recorded at the half- and quarter-wavelength resonant frequencies shows that the relative permittivity of the single layer graphene sheet is above 105. The results are in good agreement with the three dimensional numerical electromagnetic simulations. This method may be instrumental for a comprehensive understanding of the scanning microwave microscope image contrast and provide a unique technique to estimate the local electrical properties with nano-meter scale spatial resolution of two dimensional materials at radio frequency.

Influence of out-of-plane defects on vibration analysis of graphene: Molecular Dynamics and Non-local Elasticity approaches

Out-of-plane defects may exist in graphene inevitably or purposely. The present study aims at investigating the influence of out-of-plane defects on vibrational analysis of single layered graphene sheets (SLGSs) implementing both nonlocal elasticity and molecular dynamics (MD) simulations. In nonlocal elasticity analysis, the defect is considered as an initial curvature which is modeled by an analytical function having controllable parameters for the amplitude, extension, and location. In molecular dynamics analysis, defects are simulated by inserting inverse Stone–Wales defects in the perfect structure of SLGSs. Both nonlocal continuum and MD simulation results reveal that the defects increase the vibrational frequency. It is shown that classical elasticity overestimates frequencies with a considerable error while the nonlocal plate model can fit MD results by implementing a proper small scale parameter.

Buckling of hybrid nanocomposites with embedded graphene and carbon nanotubes

With the aid of atomistic multiscale modelling and analytical approaches, buckling strength has been determined for carbon nanofibres/epoxy composite systems. Various nanofibres configurations considered are single walled carbon nano tube (SWCNT) and single layer graphene sheet (SLGS) and SLGS/SWCNT hybrid systems. Computationally, both eigen-value and non-linear large deformation-based methods have been employed to calculate the buckling strength. The non-linear computational model generated here takes into account of complex features such as debonding between polymer and filler (delamination under compression), nonlinearity in the polymer, strain-based damage criteria for the matrix, contact between fillers and interlocking of distorted filler surfaces with polymer. The effect of bridging nanofibres with an interlinking compound on the buckling strength of nano-composites has also been presented here. Computed enhancement in buckling strength of the polymer system due to nano reinforcement is found to be in the range of experimental and molecular dynamics based results available in open literature. The findings of this work indicate that carbon based nanofillers enhance the buckling strength of host polymers through various local failure mechanisms.

Synthesis of Graphene with Noble Metals Nanoparticles on its Surface

Graphene sheets can be considered as a promising a support material for the catalytic nanoparticles. Here, we present a new approach for the preparation of graphene with noble metals nanoparticle on its surface using supercritical propanol - 2 as a reducing agent for nanocomposites graphene oxide- Pd or Pt nanoparticles. The prepared nanocomposites were characterized by X-ray diffraction analyses (XRD) and transmission electronic microscope (TEM). XRD reveals the face-centered cubic structure of Pd and Pt in the nanocomposites, TEM images show the good spatial distribution of metal nanoparticles on layered graphene sheets.

Theoretical analysis of transverse impact response in double layer graphene sheets

In designing future nanoscale devices or nanostructures as new element in structural mechanics, it is very important to predict the responses for these elements against various mechanical loading conditions. In this letter, an analytical solution of the impact response in double layer graphene sheets (DLGSs) is presented using a continuum mechanics theory. In this analytical model, the DLGSs are considered as a layer stack of two individual graphene sheet (GS) bound together by van der Waals (vdW) forces. The influence of impact velocity and mass on the impact response are predicted by using numerical simulation. The result shows that impact response of GSs subjected to nanomass has exceedingly short times with picoseconds order.

Modeling a multilayered graphene biosensing structure

Graphene are increasingly explored as possible novel sensor technology platform able to simultaneously perform multiple tasks within biocompatible functionalized nanostructures. High sensitivity, fast detection, affordability and reliability are just few of the characteristics envisaged by these emerging nanotechnology applications. The model of double-layered graphene sheet embedded in a biocompatible matrix has been investigated considering the phonon transport mechanism as the main stimuli carrier from and towards the biological support. Thus vibrations of single-layered and double layered graphene sheets embedded in a viscoelastic medium have been analyzed considering nonlinear van der Waals forcer within Kirchhoff plate theory and couple stress theory, with proper size effect considerations.

Facile Large Scale Production of Few-Layer Graphene Sheets by Shear Exfoliation in Volatile Solvent.

Few layer graphene sheets were synthesized from natural graphite through mechanical shear mixer in 1-butanol as solvent. The liquid phase exfoliation of graphite through the shear mixer generated incising forces for 20 minutes which changed the large amount of graphite's flake into few layer graphene. The removal of solvent from the deposited dispersion was performed immediately by keeping at the room temperature. The deposited graphene thin films were characterized by AFM, HR-TEM, XRD, FT-IR and Raman Spectroscopy. The HR-TEM results showed the formation of few layers and well dispersed graphene. The Raman spectroscopy and XRD characterization confirmed the good quality and non-oxidized state of graphene.

Synthesis and characterization of porous, mixed phase, wrinkled, few layer graphene like nanocarbon from charcoal

A technique to synthesis wrinkled graphene like nano carbon (GNC) from charcoal is reported in the current study. The charcoal produced by thermal decomposition and is intercalated by Hummers method. It is separated by centrifugation and sonication to get few layer graphene sheets. The structural and chemical changes of the nanostructure is elucidated by Raman spectroscopy, TEM, SEM-EDS and XPS. Raman spectra revealed the existence of highly graphitized amorphous carbon, which is confirmed by the appearance of five peaks in the deconvoluted first order Raman spectra. The SEM analysis reveals the formation of large area graphene sheets with nano-porous structure in it. The TEM/SAED analysis exhibits the presence of short range few layer graphene.

A modified molecular structural mechanics model for the buckling analysis of single layer graphene sheet

Abstract In this paper the classical molecular structural mechanics model of graphene is modified to improve its accuracy for the analysis of transverse deformations. To this aim, a sample graphene sheet under a uniform pressure is modeled by both molecular dynamics and molecular structural mechanics methods. The sectional properties of the beam element, by which the covalent bonds are modeled, are modified such that the difference between the results of the molecular mechanics model and molecular dynamics simulation is minimized. Using this modified model, the buckling behavior of graphene under a uniform edge pressure is investigated subjected to different boundary conditions for both zigzag and armchair chiralities. The results show that the obtained buckling loads are considerably less than those reported using the classical molecular structural models in the literature.

Giant enhancement of nanoscale thermal radiation based on hyperbolic graphene plasmons

Excitation of surface plasmons enables super-Planckian thermal radiation far beyond the blackbody limit. By patterning a single layer of graphene sheet into ribbons, the closed circular dispersion of graphene plasmons is opened to become hyperbolic, leading to broadband singularities of density of states. Extremely high-k evanescent waves can now couple with hyperbolic graphene plasmons. Consequently, a giant enhancement of the near-field radiative heat flux, by more than one order of magnitude, is demonstrated in this study using rigorous numerical simulations. The findings may open promising pathways for highly efficient thermal management, energy harvesting, and sub-wavelength thermal imaging.

Analytical investigation on free vibration of circular double-layer graphene sheets including geometrical defect and surface effects

Effects of a single vacancy defect or a pin hole on free vibration behavior of a double layer graphene sheet are investigated. Using the nonlocal continuum theory as well as the Gurtin–Murdoch theory, the nonlocality and surface effects are considered in equations of motion. Both of in-phase and anti-phase vibration modes are analytically analyzed. Employing the translational addition theorem for cylindrical vector wave functions, the geometrical defect as a circular hole in arbitrary size and location is modeled. The van der Waals interaction between the upper and lower layers is included using the Lennard–Jones pair potential. The computational efficiency and accuracy of results are validated by literature. Effects of boundary conditions, geometrical properties, nonlocality and surface effect parameters on in-phase and anti-phase vibrational modes are investigated. Results reveal that the fundamental natural frequency of an annular double-layer graphene sheet with a free eccentric circular defect is less affected by the size and location of the defect. Moreover, the surface effect parameters have more significant effects on the in-phase vibration modes than the anti-phase ones.

A Non-linear Static Equivalent Model for Multi-layer Annular/Circular Graphene Sheet Based on Non-local Elasticity Theory Considering Third Order Shear Deformation Theory in Thermal Environment

In this paper, it is tried to find an approximate single layer equivalent for multi-layer graphene sheets based on third order non-local elasticity theory. The plates are embedded in two parameter Winkler-Pasternak elastic foundation, and also the thermal effects are considered. A uniform transverse load is imposed on the plates. Applying the non-local theory of Eringen based on third order shear deformation theory and considering the van der Waals interaction between the layers, the governing equations are derived for a multi-layer graphene sheet. The governing equations for single layer graphene sheet are obtained by eliminating the van der Waals interaction. In this study, two different methods are applied to solve the governing equations. First, the results are obtained applying the differential quadrature method (DQM), which is a numerical method, and then a new semi-analytical polynomial method (SAPM) is presented. The results from DQM and SAPM are compared and it is concluded that the SAPM results are satisfactorily accurate in comparison with DQM. Since analyzing a multi-layer graphene sheet needs a time-consuming computational process, it is investigated to find an appropriate thickness for a single layer sheet to equalize the maximum deflections of multi-layer and single layer sheets. It is concluded that by considering a constant value of the van der Waal interaction between the layers, the maximum deflections of multi and single layer sheets are equal in a specific thickness of the single layer sheet.

Characteristics of thermally reduced graphene oxide and applied for dye-sensitized solar cell counter electrode

Graphene oxide (GO) was synthesized from a flake-type of graphite powder, which was then reduced to a few layers of graphene sheets using the thermal reduction method. The surface morphology, phase crystallization, and defect states of the reduced graphene were determined from an electron microscope equipped with an energy dispersion spectrometer, X-ray diffraction, Raman spectroscopy, and infrared spectra. After graphene formation, the intercalated defects that existed in the GO were removed, and it became crystalline by observing impurity changes and d-spacing. Dye-sensitized solar cells, using reduced graphene as the counter electrode, were fabricated to evaluate the electrolyte activity and charge transport performance. The electrochemical impedance spectra showed that increasing the thermal reduction temperature could achieve faster electron transport and longer electron lifetime, and result in an energy conversion efficiency of approximately 3.4%. Compared to the Pt counter electrode, the low cost of the thermal reduction method suggests that graphene will enjoy a wide range of potential applications in the field of electronic devices.

Engineering Vertical Aligned MoS2 on Graphene Sheet Towards Thin Film Lithium Ion Battery

Ultrathin MoS2 nanosheets vertically aligned on the single layer graphene sheet (MoS2-NS/G) were synthesized by a simple CVD method. It is found that the single layer graphene plays important roles not only in the synthesis process but also in the electrochemical performance. The favorable vertical design can offer remarkable advantages such as fast electron transport/collect ion and ion diffusion, reduced charge-transfer resistance, and facile accommodation of the strains caused by lithium intercalation and deintercalation. The MoS2-NS/G was calculated to show the highest capacity of 620mAhg−1 in these electrodes at a large current density of 4Ag−1 for 1000 cycles. It also delivers a good long-life cycling performance at high capacity of 1040mAhg−1. Additionally, the excellent rate performance of MoS2-NS/G may be due to the heterostructure of graphene and MoS2. It is also found that the charge storage of MoS2 contains capacitive process. Because electrons can directly transport from the current collector to the electrolyte along MoS2 basal planes, the need for carbon black and binder is eliminated. It is believed that the MoS2-NS/G also has immense potential for applications in supercapacitor.

Free‐Standing, Multilayered Graphene/Polyaniline‐Glue/Graphene Nanostructures for Flexible, Solid‐State Electrochemical Capacitor Application

Graphene/polyaniline multilayered nanostructures (GPMNs) are prepared using a straightforward process through which graphite is physically exfoliated with quaternary polyaniline (PANI)‐glue. This is only accomplished by sonication of the graphite flakes in an organic solvent to form continuous films with PANI. During the sonication, the conductive PANI‐glue is spontaneously intercalated between the graphene sheet layers without deterioration of the sp2 hybridized bonding structure. The resultant free‐standing, flexible films are composed of a network of overlapping graphene sheets and are shown to have a long‐range structure. The effects of different PANI content ratios and different interfacial energies (depending on the dispersion solvent) on the morphology and properties of the resulting GPMN are examined. It is found that GPMNs dispersed in water have a maximum specific capacitance of 390 F g−1 in a three‐electrode configuration. Importantly, the unique structural design of GPMNs enables their use as electrode materials for the fabrication of flexible, solid‐state electrochemical capacitors, which show an enhanced performance compared to graphene‐only devices. They exhibit a high specific capacitance of 200 F g−1, a cycling stability with capacitance retention of 82% after 5000 charge/discharge cycles, and, moreover, superior flexibility.

Effects of eccentric circular perforation on thermal vibration of circular graphene sheets using translational addition theorem

In this article, nonlocal thin plate theory of Eringen is employed to investigate effects of thermal environment on behavior of freely vibrating circular single layer graphene sheet containing a circular perforation of arbitrary size and location. In order to analytically solve equation of motion, the separation of variables method in conjunction with the translational addition theorem for Bessel functions is used. Accuracy and stability of results are verified by the literature. The behavior of sheets in low and high temperature conditions is considered. The influences of temperature change and various geometrical parameters on the natural frequencies are investigated by considering size-dependent material properties. In some cases, thermal buckling phenomenon was observed. Results show that the temperature changes play an important role in free vibration behavior of eccentric circular graphene sheets.

Passivation of CdSe Quantum Dots by Graphene and MoS2 Monolayer Encapsulation

The encapsulation of a monolayer of CdSe quantum dots (QDs) by one-to-three layer graphene and MoS2 sheets protects the QDs from oxidation. Photoluminescence (PL) from the QD cores shows a much slower decrease in core diameter over time due to slower oxidation in regions where the QDs are covered by van der Waals (vdW) layers than in those where they are not, for chips stored both in the dark and in the presence of light. PL mapping shows that the CdSe QDs under the central part of the vdW sheet age slower than those near its edges, because oxidation of the covered QDs is limited by transport of oxygen from the edges of the vdW sheets and not transport across the vdW layers. The transport of oxygen to the covered QDs is analyzed by coupling the PL results and a model of QD core oxidation.

Homogenized elastic properties of graphene for moderate deformations

This paper presents a simple procedure to obtain a substitute, homogenized mechanical response of single layer graphene sheet. The procedure is based on the judicious combination of molecular mechanics simulation results and homogenization method. Moreover, a series of virtual experiments are performed on the representative graphene lattice. Following these results, the constitutive model development is based on the well-established continuum mechanics framework, that is, the non-linear membrane theory which includes the hyperelastic model in terms of principal stretches. A proof-of-concept and performance is shown on a simple model problem where the hyperelastic strain energy density function is chosen in polynomial form.