Energy exchange between vibration modes of a graphene nanoflake oscillator: Molecular dynamics study

We investigated the oscillatory behaviors of a square graphene-nanoflake (GNF) on a rectangular GNF via classical molecular dynamics simulations, and analyzed the energy exchange and the oscillation frequencies for three different modes. The simulation results using a model structure show that the GNF oscillator can be considered as a high frequency oscillator. As its initial velocity increases, its telescoping region increases, then its structural asymmetry along the axis due to own small rotation exerted asymmetric van der Waals (vdW) force on it, and finally, this asymmetric vdW force enhances its rotational motions during its axial translational motions. So the initial kinetic energy of the axial translational motion is changed into the energy of the orthogonal vibrational and the rotational motions. Its resonance frequencies are dependent on the aspect ratio of the bottom rectangular GNF, the difference between the lengths of the GNF oscillator and the bottom rectangular GNF, and the initial velocity.     

Developing accelerometer based on graphene nanoribbon resonators

We investigated an ultrahigh sensitive accelerometer based on graphene nanoribbon resonators. Sensing acceleration can be made by their resonance frequency shift and/or their capacitance change. Schematics and the static properties were introduced and the dynamic properties were investigated via classical molecular dynamics simulation. As the acceleration increased, the oscillations of the deflections were going dramatically faster and the mean deflections increased, then the capacitance continually varied with large amplitudes and the resonance frequencies linearly increased in a log–log scale by power regression. The energy loss decreased with increasing time, and the average quality factors were dramatically reduced with increasing acceleration.     

Heat-pulse rectification in graphene Y junctions: A molecular dynamics simulations

By using molecular-dynamics simulations, we demonstrate the existence of heat-pulse rectification in graphene Y junctions. Our results show that the heat pulse will separate into two parts when it flows from stem to branches. However, when it flows from branches to stem, substantial part of the heat pulse is reflected back into the branches with no separation. Moreover, we discuss rectification in different type of junctions and find that the preferred heat pulse transport direction depends on the structure of the junctions, which enable us to delicately control the heat rectification in graphene based structures.     

应力对石墨烯热输运性质影响的分子动力学模拟

本文建立了低维薄膜材料导热模型,运用非平衡分子动力学模拟的方法,利用lanmmps软件对单层石墨烯纳米带的导热特性进行仿真分析,根据Fourier定律计算热导率,再对石墨烯纳米带的原子施加一定耦合应力场,把应力耦合作用下的石墨烯热导率与正常的石墨烯纳米带进行了对比研究,模拟数据结果表明:在石墨烯纳米带上施加耦合应力时,会导致石墨烯纳米带热导率升高,且随应力增加而增大,模拟范围内热导率升高2.61倍,并且应力方向会对热导率变化产生一定影响,这个研究为纳米尺度上石墨烯相关研究和进一步提升热导率提供了新思路.     

导电石墨烯孔道内双电层结构的研究

为有效开发和利用新能源,人们迫切需要高性能的超级电容器提供能量的存储和转换.在超级电容器中双电层结构扮演着关键性的角色.本文利用分子动力学方法通过建立开放的石墨烯纳米孔道(1～2 nm),研究了KCl溶液在纳米孔道内的双电层结构,同时也比较了恒电量模拟(Q)和恒电势模拟法(U)下双电层结构的异同.结果表明在恒电势模拟法考虑了导电石墨烯壁的镜像作用使结果更符合实验中的材料系统.而石墨烯壁的镜像作用能额外吸附离子从而增强孔道内部的阴阳离子,这可能有助于电极电容的提升.通过对不同孔道高度的研究,本文发现水分子作为介电材料在水基超级电容器中发挥着决定性的作用.它能在很大程度上抵消不同离子和不同孔道高度下双电层的变化,从而在不同情况下获得了相似的电容.     

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.     

Phase transitions and kinetic properties of gold nanoparticles confined between two-layer graphene nanosheets

The thermodynamic and kinetic behaviors of gold nanoparticles confined between two-layer graphene nanosheets (two-layer-GNSs) are examined and investigated during heating and cooling processes via molecular dynamics (MD) simulation technique. An EAM potential is applied to represent the gold–gold interactions while a Lennard–Jones (L–J) potential is used to describe the gold–GNS interactions. The MD melting temperature of 1345 K for bulk gold is close to the experimental value (1337 K), confirming that the EAM potential used to describe gold–gold interactions is reliable. On the other hand, the melting temperatures of gold clusters supported on graphite bilayer are corrected to the corresponding experimental values by adjusting the ε_Au–C value. Therefore, the subsequent results from current work are reliable. The gold nanoparticles confined within two-layer GNSs exhibit face center cubic structures, which is similar to those of free gold clusters and bulk gold. The melting points, heats of fusion, and heat capacities of the confined gold nanoparticles are predicted based on the plots of total energies against temperature. The density distribution perpendicular to GNS suggests that the freezing of confined gold nanoparticles starts from outermost layers. The confined gold clusters exhibit layering phenomenon even in liquid state. The transition of order–disorder in each layer is an essential characteristic in structure for the freezing phase transition of the confined gold clusters. Additionally, some vital kinetic data are obtained in terms of classical nucleation theory.     

A nanoscale field effect data storage of bipolar endo-fullerenes shuttle device

We investigated the internal dynamics of an electro-fluid shuttle memory element, consisting of K⁺@C₆₀ and F⁻@C₆₀ encapsulated in a C₆₄₀ nanocapsule. Energetics and operating responses of bipolar endo-fullerenes shuttle memory device, (K⁺@C₆₀–F⁻@C₆₀)@C₆₄₀, were examined by classical molecular dynamics simulations under the external force fields.     

Nonvolatile memory devices based on self-assembled nanocrystals

Nonvolatile memory devices are one of the most important components in modern electronic devices. Many efforts have been made to fabricate high-density, low-cost, nonvolatile solid-state memory devices for use in portable/mobile electronic devices such as laptop computers, tablet devices, smart phones, etc. Among the many available nonvolatile memory devices, flash memory devices are of great interest to the electronics industry owing to their simple device structure, enabling high-density memory applications. Flash memory devices in which nanoparticles or nanocrystals are used as the charge-trapping elements have advantages over conventional flash memory devices because the charge-trapping layer and memory performance of the former can be readily optimized. Active research has recently been conducted to fabricate and characterize self-assembled-nanocrystal-based nonvolatile memory devices. We reviewed various strategies for fabricating nanocrystal-based nonvolatile memory devices and discussed the programmable memory properties and the device reliability characteristics of nanocrystal-based memory devices to possibly apply nanocrystal-based memory devices to those used in portable/mobile electronic devices. Finally, novel device applications such as printed/flexible/transparent electronic devices were explored based on nanocrystal-based memory devices.     

Self-assembly of graphene nanoribbon ring on metallic nanowires

Molecular dynamics simulations demonstrate that metallic nanowires (NWs) can activate and guide the self-assembly of graphene nanoribbon rings (GNR), allowing them to adopt a bilayered helical configuration on NWs. This unique technology attributes to the combined effects of the van der Waals force and the π–π stacking interaction. The size and chirality effects of GNR on the self-assembly of GNR–NW system are calculated. Diverse NWs, acting as an external force, can initiate the conformational change of the GNRs to form bilayered helical structures. The stability of the formed nanosystems is further analyzed for numerous possible applications.     

Diverse nanowires activated self-scrolling of graphene nanoribbons

Diverse nanowires (NWs) activating the self-scrolling of planar graphene (GN) nanoribbons have been studied by using molecular dynamics (MD) simulations. Once the NWs’ radiuses reach a threshold, all the seven NWs, acting as an external force, can initiate the conformational change of the GN nanoribbons, and finally form the core/shell composite NWs. Our simulation found that van der Waals (vdW) force plays an important role in the process of forming core/shell composite NWs. This preparation method of the core/shell composite NWs will open a further development of a broad new class of metal/GN core/shell composite NWs with enhanced properties. And these core/shell structures can be the building blocks of functional nanodevices with unique mechanical, electrical, or optical properties.     

金属Ni原子对石墨烯生长结构的影响

基于Material Studio软件平台,利用分子动力学方法,对Ni原子与石墨烯层状结构相互作用和晶体结构变化过程进行模拟分析,得到如下结论：低浓度Ni原子会吸附在石墨烯表面层沿边缘生长,活性从中心向边缘逐渐降低,高浓度的Ni原子会溶解到内层石墨烯中.当石墨烯层数增加,附着在表层石墨烯的Ni原子生长排列范围扩大,且在石墨烯表面形成的点阵排列被破坏,附着在内层石墨烯的Ni原子比表层石墨烯Ni原子排列更散乱,同时石墨烯生长结构逐渐出现弯曲;随着层数增多和Ni原子浓度增加,石墨烯的拉伸强度也随之增加,石墨烯生长缺陷的偏转角度也随之增大.通过计算以上结构的径向分布函数(RDF),验证了石墨烯长程有序到短程有序的结构变化过程.     

Vibration analysis of defected and pristine triangular single-layer graphene nanosheets

This paper investigates the vibration behavior of pristine and defected triangular graphene sheets; which has recently attracted the attention of researchers and compare these two types in natural frequencies and sensitivity. Here, the molecular dynamics method has been employed to establish a virtual laboratory for this purpose. After measuring the different parameters obtained by the molecular dynamics approach, these data have been analyzed by using the frequency domain decomposition (FDD) method, and the dominant frequencies and mode shapes of the system have been extracted. By analyzing the vibration behaviors of pristine triangular graphene sheets in four cases (right angle of 45-90-45 configuration, right angle of 60-90-30 configuration, equilateral triangle and isosceles triangle), it has been demonstrated that the natural frequencies of these sheets are higher than the natural frequency of a square sheet, with the same number of atoms, by a minimum of 7.6% and maximum of 26.6%. Therefore, for increasing the resonance range of sensors based on 2D materials, non-rectangular structures, and especially the triangular structure, can be considered as viable candidates. Although the pristine and defective equilateral triangular sheets have the highest values of resonance, the sensitivity of defective (45,90,45) triangular sheet is more than other configurations and then, defective (45,90,45) sheet is the worst choice for sensor applications.     

Nonvolatile unipolar resistive switching in ultrathin films of graphene and carbon nanotubes

We report unipolar resistive switching in ultrathin films of chemically produced graphene (reduced graphene oxide) and multiwalled carbon nanotubes. The two-terminal devices with yield >99% are made at room temperature by forming continuous films of graphene of thickness ∼20 nm on indium tin oxide coated glass electrode, followed by metal (Au or Al) deposition on the film. These memory devices are nonvolatile, rewritable with ON/OFF ratios up to ∼ 10⁵ and switching times up to 10 μs. The devices made of MWNT films are rewritable with ON/OFF ratios up to ∼400. The resistive switching mechanism is proposed to be nanogap formation and filamentary conduction paths.     

Molecular dynamics simulation study on mechanical responses of nanoindented monolayer-graphene-nanoribbon

We investigated the mechanical responses of the nanoindented graphene-nanoribbon (GNR)-resonator using classical molecular dynamics simulations. The nanoindented force in this work was applied to the GNR's local point and then, GNR-resonator's frequency could be tuned by a nanoindented depth. We found the hardening or the softening of the GNR during its nanoindented-deflections, and such properties were recognized by the shift of the resonance frequency. The linear elastic regime in low applied force is explicitly separated with the non-linear elastic regime in high applied force. In particular, at the threshold point, a very small change of the nanoindented depth can cause great change in the resonance frequency, and this property can enable the GNR to be applied to electromechanical relay switching devices and the quantum-computer in quantum-mechanical coupling as well as mass detectors, pressure sensors, accelerometers, and alarms.     

Mechanical properties of defective single-layered graphene sheets via molecular dynamics simulation

In this paper, the effects of two main types of structural defects, i.e. Stone–Wales and single vacancy, on the mechanical properties of single-layered graphene sheets (SLGSs) are investigated. To this end, molecular dynamics simulations based on the Tersoff–Brenner potential function and Nose–Hoover thermostat technique are implemented. The results obtained have revealed that the presence of defects significantly reduces the failure strain and the intrinsic strength of SLGSs, while it has a slight effect on Young’s modulus. Furthermore, the examination of loading in both armchair and zigzag directions demonstrated that SLGSs are slightly stronger in the armchair direction and defects have lower effect in this direction. Considering the fracture mechanism, the failure process of defective and perfect graphene sheets is also presented.     

Hexagonal-boron nitride substrates for electroburnt graphene nanojunctions

We examine the effect of a hexagonal boron nitride (hBN) substrate on electron transport through graphene nanojunctions just before gap formation. Junctions in vacuum and on hBN are formed using classical molecular dynamics to create initial structures, followed by relaxation using density functional theory. We find that the hBN only slightly reduces the current through the junctions at low biases. Furthermore due to quantum interference at the last moments of breaking, the current though a single carbon filament spanning the gap is found to be higher than the current through two filaments spanning the gap in parallel. This feature is present both in the presence of absence of hBN.     

Simulations of the bending rigidity of graphene

Molecular mechanics simulations for graphene bending rigidity are reported through calculations of the strain energy for graphene sheets subjected to a point loading. The rigidity is found to be dependent on the size and the shape of graphene sheets. Moreover, dependence of the rigidity on the deflection is found.     

不同尺度的蛋白质在纯石墨烯及氧化石墨烯表面的吸附比较

Using all-atom molecular dynamics (MD) simulations, we have investigated the adsorption stability and conformation change of different proteins on the surface of pristine graphene (PG) and graphene oxide (GO). We find that: (i) with the cooperation of the electrostatic interactions between proteins and oxygen-containing groups, GO shows better adsorption stability than PG; (ii) the peptide loses its secondary structure on both PG and GO surface, and the a-helix structure of the protein fragment is partially broken on PG surface, but is well preserved on GO surface, while the secondary structure of globular protein has no distinct change on both PG and GO surface. In general, GO presents better biocompatibility than PG. Our results are of significant importance to understand the interactions between proteins and PG/GO and the applications of PG/GO in biotechnology and biomedicine.     

Interfacial thermal resistance in multilayer graphene structures

The cross-plane thermal conductivities of multilayer graphene are investigated using nonequilibrium molecular dynamics simulation. It is found that the interfacial thermal resistance in multilayer graphene structures is strongly layer number dependent. It decreases with increasing layer number and reaches a limit as layer number is large enough. The interfacial thermal resistance for graphite and multilayer graphene has an anomalous relationship with temperature compared with that in superlattice structures. It increases with the temperatures above room temperature, which is attributed to phonon tunneling effects. Phonon tunneling probability is reduced due to the decreased phonon wavelength while temperature rises, which in turn causes the increased interfacial thermal resistance.