Influence of line defects on relaxation properties of graphene: A molecular dynamics study

The relaxation properties of single layer graphene sheets containing line defects were investigated using molecular dynamics simulation with AIROBE bond-order interatomic potential. The dynamic evolution of graphene sheets during relaxation condition was analyzed. The simulation results show that the single layer graphene sheets are not perfectly flat in an ideal state, and the graphene sheet shows a significant corrugations at the verge of sheet. The graphene sheet is bent with the line defects at the end of the sheet, and the extent of this bend also increases with the increase of the defect number. Furthemore, the graphene sheet transforms into a paraboloid with the line defects at the middle of the sheet.     

Graphene characterization: A fully non-linear spring-based finite element prediction

In the present study, a spring-based finite element model is formulated and utilized to predict the stress–strain behavior of single-layer graphene. Generalized force–generalized displacement behavior of the developed nonlinear springs follows the relation between the first derivative of the potential energy and the corresponding bond deformation, describing interatomic interactions. A number of different loading cases are examined in order to predict mechanical properties and characterize the graphene sheet. Predicted Young's and shear moduli, tensile and shear strength, tensile and shear failure strain, etc., under tension, compression and pure shear, are compared to results found in the literature, which are based on numerical, analytical or experimental methodologies. In all the above loading cases the graphene sheet is examined as a virtually orthotropic material, exhibiting different material properties in the armchair and zigzag directions. Different behaviors in tension and compression, as suggested by the modified Morse atomic bond stretching potential, are illustrated by the predicted stress–strain curves.     

A nitrogen-doped graphene film prepared by chemical vapor deposition of a methanol mist containing methylated melamine resin

The effect of nitrogen doping on the sheet resistivity of a graphene film is systematically studied by changing the doping concentration. The nitrogen-doped graphene film is grown on a Cu foil by chemical vapor deposition using an ultrasonically generated methanol mist containing methylated melamine resin (simply called ‘melamine’). Using this method, it is found that the magnitude of the sheet resistivity is controllable by changing the melamine concentration. Increasing the melamine concentration up to ～0.03 % causes a decrease of the sheet resistivity. We explain this by the substitutional doping of nitrogen atoms. A further increase in melamine concentration causes an increase of the sheet resistivity. This increase may be caused by the formation of pyridinic or pyrrolic N instead of substitutional N. Electron energy loss spectroscopy analyses for the carbon K-edge indicate a decrease of π ^? character with increasing melamine concentration up to 0.08 % and then it recovers for higher concentration. This is due to a separation of the graphitic region and the defective region at high melamine concentration.     

Concept of infrared photodetector based on graphene–graphene nanoribbon structure

We study the mechanisms of photoconductivity in graphene layer–graphene nanoribbon–graphene layer (GL–GNR–GL) structures with the i-type gapless GL layers as sensitive elements and I-type GNRs as barrier elements. The effects of both an increase in the electron and hole densities under infrared illumination and the electron and hole heating and cooling in GLs are considered. The device model for a GL–GNR–GL photodiode is developed. Using this model, the dark current, photocurrent, and responsivity are calculated as functions of the structure parameters, temperature, and the photon energy. The transition from heating of the electron–hole plasma in GLs to its cooling by changing the incident photon energy can result in the change of the photoconductivity sign from positive to negative. It is demonstrated that GL–GNR–GL photodiodes can be used in effective infrared and terahertz detectors operating at room temperature. The change in the photoconductivity sign can be used for the discrimination of the incident radiation with the wavelength 2–3 μm and 8–12 μm.     

Density functional theory study on the adsorption of alkali metal ions with pristine and defected graphene sheet

The graphene-based materials along with the adsorption of alkali metal ions are suitable for energy conversion and storage applications. Hence in the present work, we have investigated the structural and electronic properties of pristine and defected graphene sheet upon the adsorption of alkali metal ions (Li⁺, Na⁺, and K⁺) using density functional theory (DFT) calculations. The presence of vacancies or vacancy defects enhances the adsorption of alkali ions than the pristine sheet. From the obtained results, it is found that the adsorption energy of Li⁺ on the vacancies defected graphene sheet is higher (3.05?eV) than the pristine (2.41?eV) and Stone–Wales (2.50?eV) defected sheets. Moreover, the pore radius of the pristine and defected graphene sheets are less affected by metal ions adsorption. The increase in energy gap upon the adsorption of metal ions is found to be high in the vacancy defected graphene than that of other sheets. The metal ions adsorption in the defective vacancy sheets has high charge transfer from metal ions to the graphene sheet. The bonding characteristic between the metal ions and graphene sheet are analysed using QTAIM analysis. The influence of alkali ions on the electronic properties of the graphene sheet is examined from the Total Density of States (TDOS) and Partial Density of States (PDOS).     

Effects of defects on heat conduction of graphene/hexagonal boron nitride heterointerface

We investigate the effects of point defects on the Interface Thermal Resistance (ITR) of graphene/hexagonal boron nitride (G/h-BN) heterointerface with various stacking forms by ultrafast thermal pulse method. The results reveal that the ITR of different stacking forms presents a significant downward trend with the existence of point defects. This counterintuitive behavior is attributed to the defects increase the vibration intensity of out-of-plane phonons of graphene in low-frequency region, thus enhancing the phonons coupling between graphene and h-BN layer. ITR of G/h-BN is further reduced by 50% with the defect rate increases from 0% to 5% and that is reduced by 65% with the temperature rises from 200 K to 700 K. Besides, it is found that the defective G/h-BN has thermal rectification characteristic and that is positively related to temperature and defect rate. Our study provides a practical way for the application of defects in graphene and a new approach for the design of thermal rectifier devices.     

Universal optical conductance of graphite

We find experimentally that the optical sheet conductance of graphite per graphene layer is very close to (pi/2)e2/h, which is the theoretically expected value of dynamical conductance of isolated monolayer graphene. Our calculations within the Slonczewski-Weiss-McClure model explain well why the interplane hopping leaves the conductance of graphene sheets in graphite almost unchanged for photon energies between 0.1 and 0.6 eV, even though it significantly affects the band structure on the same energy scale. The f-sum rule analysis shows that the large increase of the Drude spectral weight as a function of temperature is at the expense of the removed low-energy optical spectral weight of transitions between hole and electron bands.     

Modulation of specific heat in graphene by uniaxial strain

The specific heat of uniaxially strained graphene was investigated in the present paper. A uniaxial strain can modulate specific heat depending on the amount and direction of the strain. Specific heat decreases with an increase in the amount of tension strain at a low temperature and increases with compression. Above 110 K, it varies as the amount of strain is reversed to that at a low temperature. These novel properties can be attributed to the strain-induced shift behavior of out-of-plane acoustic phonons, which is different from that of other phonons. When the strain shifts from zigzag to armchair direction, specific heat gradually decreases for a given temperature. However, the variation in specific heat with the strain direction is significantly less than that with the amount of strain. Further, the difference in specific heat between different strain directions decreases with an increase in temperature. This tunable specific heat may provide a new route for both the implementation of thermal memory and the thermal management of graphene nanoelectronic devices.     

Highly-doped p-type few-layer graphene on UID off-axis homoepitaxial 4H–SiC

In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H–SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 10¹⁵ cm⁻². We find that the material's electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.     

Elastic instability of bilayer graphene using atomistic finite element

In-plane elastic instability of bilayer graphene sheets is investigated using atomistic finite element approaches. The equivalent homogenised properties of graphene sheet are expressed in terms of the thickness, equilibrium lengths and force-field models used to represent the C–C bonds of the graphene lattice. The covalent bonds are represented as structural beams with stretching, bending, torsional and shear deformation, and the strain energies associated to affine deformation mechanisms. The overall mechanical properties and geometric configurations of the nano-structures represented as truss assemblies are then calculated minimising the total potential energy associated to the loading, thickness and average equilibrium lengths of the bonds. Different boundary conditions and aspect ratios are considered for both bilayer and single-layer graphene sheets. The bilayer graphene sheets are found to be offering remarkably higher buckling strengths as compared to single-layer sheets.     

载能氢原子与石墨(001)面碰撞过程中的能量传递行为的分子动力学研究

本文采用分子动力学方法研究了单一载能氢原子与石墨碰撞时氢原子被石墨反射、 吸附和石墨被氢原子穿透的发生系数以及碰撞中的能量传递机理. 研究发现: 与单层石墨相比, 多层石墨之间的长程相互作用增加了氢原子发生反射的能量范围, 尤其当入射能量大于20.0 eV时, 对反射过程的影响很明显; 当氢原子的入射能量大于25.0 eV时, 有一定的概率穿透四层石墨; 当氢原子入射能量高于28.0 eV时, 载能氢原子的能量传递给第二层石墨烯的比传递给第一层石墨烯的多. 这些结果对理解聚变反应中, 碳基材料的化学腐蚀及氚滞留有重要意义.     

Graphene susceptibility in Holstein model

We study the effects of the electron-phonon interaction on the temperature dependence of the orbital magnetic susceptibility of monolayer graphene. We use the linear response theory and Green's function formalism within the Holstein Hamiltonian model. The results show that the effects of the electron-phonon interaction on the susceptibility of graphene sheet have different behaviors in two temperature regions. In the low temperature region, susceptibility increases when the electron-phonon coupling strength increases. On the other hand, the susceptibility reduces with increasing the electron-phonon coupling strength in the high temperature region.     

Mechanical Properties of Ni-Coated Single Graphene Sheet and Their Embedded Aluminum Matrix Composites

The effects of Ni coating on the mechanical behaviors of single graphene sheet and their embedded Al matrix composites under axial tensionare investigated using molecular dynamics (MD) simulation method. Theresults show that the Young's moduli and tensile strength of grapheneobviously decrease after Ni coating. The results also show that the mechanical properties of Al matrix can be obviously increased by embedding asingle graphene sheet. From the simulation, we also find that the Young'smodulus and tensile strength of the Ni-coated graphene/Al composite isobviously larger than those of the uncoated graphene/Al composite. Theincreased magnitude of the Young's modulus and tensile strength ofgraphene/Al composite are 52.27 and 32.32 at 0.01 K, respectively,due to Ni coating. By exploring the effects of temperature on the mechanicalproperties of single graphene sheet and their embedded Al matrix composites, it is found that the higher temperature leads to the lower critical strain and tensile strength.     

Exciton states and excitonic absorption spectra in a cylindrical quantum wire under transverse electric field

The temperature dependence of the resistance in trilayer graphene is observed under different applied gate voltages. At small gate voltages the resistance decreases with increasing temperature due to the increase in carrier concentration resulting from thermal excitation of electron-hole pairs, characteristic of a semimetal. At large gate voltages excitation of electron-hole pairs is suppressed, and the resistance increases with increasing temperature because of the enhanced electron-phonon scattering, characteristic of a metal. We find that the simple model with overlapping conduction and valence bands, each with quadratic dispersion relations, is unsatisfactory. Instead, we conclude that impurities in the substrate that create local puddles of higher electron or hole densities are responsible for the residual conductivity at low temperatures. The best fit is obtained using a continuous distribution of puddles. From the fit the average of the electron and hole effective masses can be determined.     

不同基底石墨烯涂层的层间滑移减磨性能研究

石墨烯薄膜作为一种二维材料,是提高微/纳机电系统(MEMS/NEMS)摩擦力学性能的优异润滑剂.为了探究基底材料和石墨烯层数对其减磨性能的影响,本文通过在不同基底制备了不同层数的石墨烯涂层,利用原子力显微镜(AFM)实验和分子动力学(MD)仿真结合的方法,研究了石墨烯层数对减磨效应的影响.并且通过建立不同层数石墨烯涂层的摩擦性能分析模型,探究出石墨烯层间滑移是产生减磨的主要因素.结果表明:在不同载荷下,石墨烯涂层对硅基底和铜基底均有优异的减磨效果,摩擦力随着石墨烯层数的增加逐渐降低,当石墨烯层数大于10层时,达到最优99.3%的减磨效果.通过仿真分析发现,随着层数增加,石墨烯与基底的干摩擦转变为石墨烯的层间摩擦,并产生层间剪切滑移,石墨烯层间滑移是导致多层石墨烯优异减磨性能的主要因素.     

Saturable Absorption and Bistable Switching of Single Mode Fiber Core-Guided Light by a 6 nm-thick,Few Layers Graphene Coating on the Cladding Surface

The coupling of less than 80 µW of in-plane polarized near-infrared light in a 6 nm-thick graphene layer deposited on an optical fiber produces important permittivity changes leading to bistability and self-starting 50% modulation of over 1 W of continuous wave light in the core. These features arise from resonant coupling of core-guided light into the cladding by a 12-degree tilted, 1 cm long fiber Bragg grating via narrowband, polarization-dependent resonances that allow the selection of cladding modes with electric fields polarized in the plane of the graphene. The pulse repetition rate of the modulation increases from 10 to 269 Hz for input powers ranging from 0.3 to 1.33 W in the core, with no evidence of saturation. Investigations into the origin of these effects through physical modelling and different experimental conditions point to photo-induced Joule heating in the graphene layer giving rise to temperature increases of the order of 60 °C and corresponding permittivity changes in the graphene and underlying silica fiber. Those changes lead to shifts in the resonance positions which result in the equivalent of saturable absorption for light guided in the core without direct contact with the absorbing graphene layer.     

太赫兹波段石墨烯等离子体的增益特性

基于麦克斯韦方程组和物质本构方程对石墨烯表面等离子体进行了研究.从理论上探索了石墨烯表面等离子体激元在太赫兹波段的增益特性曲线,并且讨论了石墨烯表面等离子体增益与石墨烯中载流子浓度、石墨烯所处温度以及载流子动量弛豫时间的关系.研究结果表明:在太赫兹波段增益峰值随着石墨烯载流子浓度的增加而发生蓝移,并且在所讨论的温度范围内,由于增益峰所对应的频率都大于1 THz,因此温度的变化对增益峰值以及相应频率的影响不大,即在不同的温度下,相同载流子浓度所对应的增益曲线上峰值的位置和强度几乎相同;增益与石墨烯载流子动量弛豫时间相关,随着载流子动量弛豫时间的增加,使得激发态激励的电子增加,从而导致石墨烯表面等离子体增益变得更大,但这种动量弛豫时间的增加却因弛豫时间对受激辐射频率影响较小而并未对增益峰值位置产生影响.     

多层石墨烯的表面起伏的分子动力学模拟

运用经典分子动力学方法,研究了呈现不同堆积方式的多层石墨烯在不同温度下的表面起伏,并且和单层、双层石墨烯做对比,计算发现:室温下,多层石墨烯中存在着横向特征尺寸约为100 A的起伏,该尺寸会随着温度的升高而增大;同时,起伏的高度也随着温度的升高而增大,这些石墨烯的层内起伏高度关联函数都遵从幂指数标度行为G_h(q)αq~(-α),对于同一种石墨烯,温度越高幂指数越小;而在同一温度下,不同堆积方式的石墨烯的幂指数也不同,所有这些特征都来源于温度以及层间耦合作用引起的非谐效应。     

Nonlocal plate model for nonlinear bending of single-layer graphene sheets subjected to transverse loads in thermal environments

This paper investigates the nonlinear bending behavior of a single-layer rectangular graphene sheet subjected to a transverse uniform load in thermal environments. The single-layer graphene sheet (SLGS) is modeled as a nonlocal orthotropic plate which contains small scale effect. Geometric nonlinearity in the von Kármán sense is adopted. The thermal effects are included and the material properties are assumed to be size dependent and temperature dependent, and are obtained from molecular dynamics (MD) simulations. The small scale parameter e ₀ a is estimated by matching the deflections of graphene sheets observed from the MD simulation results with the numerical results obtained from the nonlocal plate model. The numerical results show that the temperature change as well as the aspect ratio has a significant effect on the nonlinear bending behavior of SLGSs. The results reveal that the small scale parameter reduces the static large deflections of SLGSs, and the small scale effect also plays an important role in the nonlinear bending of SLGSs.     

Detection of gas atoms via vibration of graphenes

The application of single-layered graphene sheets as mass sensors in detection of noble gases via a vibration analysis of graphenes is investigated using molecular dynamics simulations. An index based on frequency shifts of the graphenes attached by the distinct noble gas atoms is defined and examined to measure the sensitivity of the sensors. The dependence of number and location of gas atoms, size of graphene sheets, and type of restrained boundary of the sheets on the sensitivity is particularly studied. The simulation results indicate the resolution of a mass sensor made of a square graphene sheet with a size of 10 nm can achieve an order of ¹⁰−6 femtograms and the mass sensitivity can be enhanced with a decrease in sizes of graphenes.