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Two-dimensional lateral heterostructures exhibit novel electronic and optical properties that are induced by their in-plane interface for which the mechanical properties of the interface are important for the stability of the lateral heterostructure. Therefore, we performed molecular dynamics simulations and developed a convolutional neural network-based machine learning model to study the fracture properties of the interface in a graphene/hexagonal boron nitride lateral heterostructure. The molecular dynamics (MD) simulations show that the shape of the interface can cause an 80% difference in the fracture stress and the fracture strain for the interface. By using 11 500 training samples obtained with help of high-cost MD simulation, the machine learning model is able to search out the strongest interfaces with the largest fracture strain and fracture stress in a large sample space with over 150 000 structures. By analyzing the atomic configuration of these strongest interfaces, we disclose two major factors dominating the interface strength, including the interface roughness and the strength of the chemical bond across the interface. We also explore the correlation between the fracture properties and the thermal conductivity for these lateral heterostructures by examining the bond type and the shape of the graphene/hexagonal boron nitride interface. We find that interfaces comprised of stronger bonds and smoother zigzag interfaces can relieve the abrupt change of the acoustic velocity, leading to the enhancement of the interface thermal conductivity. These findings will be valuable for the application of the two-dimensional lateral heterostructure in electronic devices. [ABSTRACT FROM AUTHOR]

This paper proposes a Whipple structure to enhance the impact resistance of graphene/aluminum alloy composites by varying the interlayer spacing between graphene and aluminum alloy. The increased interlayer spacing provides more deformation space for the graphene to absorb more deformation energy, and enables the formation of a debris cloud from the bullet fragments and graphene fragments, significantly reducing the impact energy per unit area of the next material. The impact limit serves as a critical metric for assessing the impact resistance of the Whipple structure. Based on molecular dynamics simulations, we developed a machine learning model to predict the protection of aluminum alloy, and quickly determined the impact limits of velocity, bullet radius, and interlayer spacing by using the machine learning model. An empirical equation for the impact limit of interlayer spacing was established. The results showed that non-zero interlayer spacing can significantly improve the impact resistance of the hybrid structure; to fully exploit the superior impact resistance of this Whipple structure, the number of graphene layers should be at least 3. Furthermore, at high impact velocities and large bullet radii, the impact limit of the interlayer spacing exhibits a substantial correlation with the number of graphene layers. These results provide valuable information for the design of the impact resistance of the graphene/aluminum alloy composites. [ABSTRACT FROM AUTHOR]

Thermal conductivity of porous graphene can be affected by defect concentration, nanopore shape and distribution, and it is hard to clarify the effects due to the correlation of those factors. In this work, molecular dynamics simulation is used to compare the thermal conductivity of graphene with three shapes of regularly arranged nanopores. The results prove the dominant role of defect concentration under certain circumstances in reducing thermal conductivity, while the coupling effect of nanopore shape should be noticed. When the atoms at the local phonon scattering area around each nanopore are properly removed, the abnormal increment of thermal conductivity can be detected with the increase of defect concentration. Heat flux vector angles can effectively characterize the local phonon scattering area, which can be used to describe the effect of nanopore shape. The coupling effect of defect concentration and pore shape with similar heat flux path is clarified according to this process. By adjusting vertex angle of triangle defect, there is a balanced state of the effect factors between the variation of defect concentration and the same phonon scattering area. It provides a possible way to describe the weighing factors of the coupling effect. The results suggest a feasible approach to optimize and regulate thermal properties of porous graphene in nanodevice. [ABSTRACT FROM AUTHOR]

Nanomotors serve as nanoscale engines by converting various energies into mechanical energy. In addition to the huge number of existing nanomotors, we propose a simple nanomotor based on the hollow carbon nanosphere (i.e., fullerene) that is full of gas. We investigate the acceleration of the nanosphere by leakage of gas through a nanopore by molecular dynamics simulations. The nanosphere can be driven to a high speed of 100 m/s under proper simulation conditions, which can be further tuned by temperature, gas density, and pore diameter. We observe rotation of the pore direction during the acceleration process for a nanosphere of different pore diameters. The acceleration process can be well described by the Meshchersky theory. We also simulate the deceleration process of the nanosphere due to the damping force of the gas, which can be analyzed in terms of the kinetic motion of gas molecules. The nanomotor proposed in this work shall be realizable in experiments and may be useful in driving the mechanic motion of fullerenes. [ABSTRACT FROM AUTHOR]

Few-layer graphene has been widely regarded as an efficient filter for gas separation, but the effect of the layer number on the gas permeation process is still unclear. To explore the layer number effect, we perform molecular dynamics simulations to investigate the gas permeation through a nanopore within the few-layer graphene. Our numerical simulations show that the permeation constant decreases with increasing layer number, which is analyzed based on the macroscopic Kennard empirical model. The macroscopic model is in good agreement with the numerical result in the limit of large layer number, but there are obvious deviations for the medium layer number. We generalize the macroscopic model by considering the nanoscale effect from the surface morphology of the nanoscale pore, which can well describe the layer number dependence for the gas permeation constant in the full range. These results provide valuable information for the application of few-layer graphene in the gas permeation field. [ABSTRACT FROM AUTHOR]

A key parameter for the performance of ceramic topcoats in a thermal barrier coating is the porosity, which is sensitive to many growing conditions. In the present work, molecular dynamics simulations are performed to investigate the microscopic mechanism for the dependence of the porosity on several experimental conditions in the atmospheric plasma spraying deposition process, including temperature, substrate roughness, and the thickness-to-radius ratio of the yttrium-stabilized zirconia shell. These factors can significantly modify the porosity through changing the buckling mode of the yttrium-stabilized zirconia shell. Our findings provide some microscopic mechanisms for improving the deposition quality of the atmospheric plasma spraying thermal barrier coatings. [ABSTRACT FROM AUTHOR]

Van der Waals heterostructures inherit many novel electronic and optical properties from their constituent atomic layers. Mechanical stability is key for realizing high-performance nanodevices based on van der Waals heterostructures. However, buckling instability is a critical mechanical issue for heterostructures associated with its two-dimensional nature. Using molecular dynamics simulations of graphene/MoS2 heterostructures, we demonstrate the relationship between buckling instability and the misfit strain that arises inevitably in such heterostructures. The impact of misfit strain on buckling depends on its magnitude: (1) A negative misfit strain causes a pre-compression of the graphene layer, which in turn initiates and accelerates buckling in this layer and reduces the buckling stability in the heterostructure as a whole. (2) A small positive misfit strain enhances the buckling stability of the graphene/MoS2 heterostructure by pre-stretching and hence decelerating the buckling of the graphene layer (where heterostructure buckling is initiated). (3) In the case of a large positive misfit strain, the graphene layer is pre-stretched while the MoS2 layer is significantly pre-compressed, so that heterostructure buckling is initiated by the MoS2 layer. Consequently, the buckling stability of the graphene/MoS2 heterostructure is reduced by increasing the large positive misfit strain. These findings are valuable for understanding the mechanical properties of van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2. [ABSTRACT FROM AUTHOR]

We apply united-atom (UA) nonequilibrium molecular dynamics simulations to predict the thermal conductivity for amorphous polyethylene at room temperature with chain length (CL) increasing from 4 to 1260. The thermal conductivity is determined by the competition of the two mechanisms: (1) The collision among the UA beads and (2) the phonon vibration. The collision mechanism mainly dominates CL≤7, while the phonon vibration mechanism mainly determines CL ≥ 140. The competition between the two mechanisms determines 12 ≤ CL ≤ 140. In particular, the thermal conductivity for 8 ≤ CL ≤ 11 cannot be obtained accurately due to the aggregation of the beads. [ABSTRACT FROM AUTHOR]

We show that the two molecular mechanics models, the stick-spiral and the beam models, predict considerably different mechanical properties of materials based on energy equivalence. The difference between the two models is independent of the materials since all parameters of the beam model are obtained from the harmonic potentials. We demonstrate this difference for finite width graphene nanoribbons and a single polyethylene chain comparing results of the molecular dynamics (MD) simulations with harmonic potentials and the finite element method with the beam model. We also find that the difference strongly depends on the loading modes, chirality and width of the graphene nanoribbons, and it increases with decreasing width of the nanoribbons under pure bending condition. The maximum difference of the predicted mechanical properties using the two models can exceed 300% in different loading modes. Comparing the two models with the MD results of AIREBO potential, we find that the stick-spiral model overestimates and the beam model underestimates the mechanical properties in narrow armchair graphene nanoribbons under pure bending condition. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed to investigate edge effects on the quality factor of graphene nanoresonators with different edge configurations and of various sizes. If the periodic boundary condition is applied, very high quality factors (3 × 105) are obtained for all kinds of graphene nanoresonators. However, if the free boundary condition is applied, quality factors will be greatly reduced by two effects resulting from free edges: the imaginary edge vibration effect and the artificial effect. Imaginary edge vibrations will flip between a pair of doubly degenerate warping states during the mechanical oscillation of nanoresonators. The flipping process breaks the coherence of the mechanical oscillation of the nanoresonator, which is the dominant mechanism for extremely low quality factors. There is an artificial effect if the mechanical oscillation of the graphene nanoresonator is actuated according to an artificial vibration (non-natural vibration of the system), which slightly reduces the quality factor. The artificial effect can be eliminated by actuating the mechanical oscillation according to a natural vibration of the nanoresonator. Our simulations provide an explanation for the recent experiment, where the measured quality factor is low and varies between identical samples with free edges. [ABSTRACT FROM AUTHOR]

Fullerene-based sandwiches have excellent controllable physical properties such as cross-plane thermal conductivity and charge transfer, which provide new possibilities for advancing the field of thermoelectricity. Because the pattern of the fullerene cluster impacts the physical properties of the sandwiches, it is important to understand the mechanisms for their structural transformations. In the present work, we find that the graphene/fullerene/graphene sandwich structure transforms among three configurations in an ideal environment, depending on the fullerene to graphene area ratio. Molecular dynamics simulations show that there are two critical values for the area ratio. The fullerene pattern transforms from circular to rectangular at the first critical area ratio of 1 π. The critical value of 1 π is successfully derived by comparing the geometrical perimeter of the circular and rectangular shapes for the fullerene cluster without any physical parameters. At the second critical area ratio, the graphene layers surrounding the fullerene cluster become separated due to the competition between the bending energy and the cohesive energy. The vacuum space is predicted to be 34 Å based on the analytic model. These findings provide fundamental insights into the mechanisms driving structural transformation of fullerene-based sandwiches, which will enable the selection of appropriate structures and materials to guide the design of future electronic and energy storage applications. [Display omitted] • The stable fullerene-based sandwich structure evolves among three typical configurations, which are governed by two critical ratios for the area ratio. • The fullerene pattern transforms from circular to rectangular at the first critical area ratio of 1 π. • At the second critical area ratio, the graphene layers surrounding the fullerene cluster are separated by the fullerenes. • It is demonstrated that the analytic models are universal and are applicable to other sandwich structures such as the MoS 2 /fullerene/MoS 2 structure. [ABSTRACT FROM AUTHOR]

Graphene has been expected to revolutionize the electronics, but suffered from its zero bandgap that will lead to an unacceptable leakage current in graphene-based transistors. Although a bandgap engineered graphene structure may have a bandgap, such bandgap is sensitively dependent on the geometry of the structure. Finding new two-dimensional (2D) semiconducting carbon allotropes is therefore extremely important for the new emerging all-carbon electronics. Based on first principles calculations, here we predict a new 2D semiconducting carbon allotrope, referred to as twin graphene. The allotrope has an intrinsic direct bandgap, very close to that of silicon and tunable by in-plane strain, indicating its great potential in nanoelectronics. The stability of twin graphene is carefully verified by both the first principles calculations and the molecular dynamics simulations. We determine also the mechanical properties of twin graphene, including the in-plane stiffness, Poisson’s ratio, shear stiffness and bending stiffness. Its excellent electronic and mechanical properties suggest that twin graphene may have a significant impact on many disciplines and fields related to nanotechnology. [ABSTRACT FROM AUTHOR]

We perform classical molecular dynamics simulations to investigate the mechanical compression effect on the thermal conductivity of the single-walled carbon nanotube (SWCNT) forest, in which SWCNTs are closely aligned and parallel with each other. We find that the thermal conductivity can be linearly enhanced by increasing compression before the buckling of SWCNT forests, but the thermal conductivity decreases quickly with further increasing compression after the forest is buckled. Our phonon mode analysis reveals that, before buckling, the smoothness of the inter-tube interface is maintained during compression, and the inter-tube van der Waals interaction is strengthened by the compression. Consequently, the twisting-like mode (good heat carrier) is well preserved and its group velocity is increased by increasing compression, resulting in the enhancement of the thermal conductivity. The buckling phenomenon changes the circular cross section of the SWCNT into ellipse, which causes effective roughness at the inter-tube interface for the twisting motion. As a result, in ellipse SWCNTs, the radial breathing mode (poor heat carrier) becomes the most favorable motion instead of the twisting-like mode and the group velocity of the twisting-like mode drops considerably, both of which lead to the quick decrease of the thermal conductivity with further increasing compression after buckling. [ABSTRACT FROM AUTHOR]

We study the phonon modes in single-walled MoS2 nanotubes via lattice dynamics calculation and molecular dynamics simulation. The phonon spectra for tubes of arbitrary chiralities are calculated from a dynamical matrix constructed by the combination of an empirical potential with the conserved helical quantum numbers (κ, n). In particular, we show that the frequency (ω) of the radial breathing mode is inversely proportional to the tube diameter (d) as ω = 665.3/d cm−1. The eigenvectors of the twenty lowest-frequency phonon modes are illustrated. Based on these eigenvectors, we demonstrate that the radial breathing oscillation is initially disturbed by phonon modes of three-fold symmetry, then eventually the tube is squashed by modes of two-fold symmetry . Our study provides fundamental knowledge for further investigations of the thermal and mechanical properties of MoS2 nanotubes. [ABSTRACT FROM AUTHOR]

We perform classical molecular dynamics simulation to investigate the mechanisms underpinning the unresolved, experimentally observed temperature-dependent scaling transition in the quality factors of graphene nanomechanical resonators (GNMRs). Our simulations reveal that the mechanism underlying this temperature scaling phenomenon is the out-of-plane migration of adsorbates on GNMRs. Specifically, the migrating adsorbate undergoes frequent collisions with the GNMR, which strongly influences the resulting mechanical oscillation, and thus the quality factors. We also predict a discontinuous transition in the quality factor at a lower critical temperature, which results from the in-plane migration of the adsorbate. Overall, our work clearly demonstrates the strong effect of adsorbate migration on the quality factors of GNMRs. [ABSTRACT FROM AUTHOR]

The temperature-dependent mechanical properties of single-layer molybdenum disulphide (MoS2) are obtained using molecular dynamics (MD) nanoindentation simulations. The Young's moduli, maximum load stress, and maximum loading strain decrease with increasing temperature from 4.2 K to 500 K. The obtained Young's moduli are in good agreement with those using our MD uniaxial tension simulations and the available experimental results. The tendency of maximum loading strain with different temperature is opposite with that of metal materials due to the short range Stillinger-Weber potentials in MoS2. Furthermore, the indenter tip radius and fitting strain effect on the mechanical properties are also discussed. [ABSTRACT FROM AUTHOR]

The extreme reduction of the thermal conductivity by defects and folds in carbon nanocoils (CNCs) are first demonstrated using nonequilibrium molecular dynamics simulations. The thermal conductivity for two different defect types with five different folds in the CNCs is extensively studied and the maximum reduction can be up to 70% at both room temperature and 600 K by comparison of the corresponding straight single-walled carbon nanotubes. We reveal that the phonon scattering by coupled defects and folds can govern the reduction of the thermal conductivity by calculating phonon polarization vectors. [ABSTRACT FROM AUTHOR]

We perform molecular dynamics simulations to investigate the effect of polar surfaces on the quality (Q)-factors of zinc oxide (ZnO) nanowire-based nanoresonators. We find that the Q-factors in ZnO nanoresonators with free polar (0001) surfaces are about one order of magnitude higher than in nanoresonators that have been stabilized with reduced charges on the polar (0001) surfaces. From normal mode analysis, we show that the higher Q-factor is due to a shell-like reconstruction that occurs for the free polar surfaces. This shell-like reconstruction suppresses twisting motion in the nanowires such that the mixing of other modes with the resonant mode of oscillation is minimized, and leads to substantially higher Q-factors in ZnO nanoresonators with free polar surfaces. [ABSTRACT FROM AUTHOR]

Abstract: Explicit solutions for the cohesive energy between carbon nanotubes, graphene and substrates are obtained through continuum modeling of the van der Waals interaction between them. The dependence of the cohesive energy on their size, spacing and crossing angles is analyzed. Checking against full atom molecular dynamics calculations and available experimental results shows that the continuum solution has high accuracy. The equilibrium distances between the nanotubes, graphene and substrates with minimum cohesive energy are also provided explicitly. The obtained analytical solution should be of great help for understanding the interaction between the nanostructures and substrates, and designing composites and nanoelectromechanical systems. [Copyright &y& Elsevier]

Weperform molecular dynamics simulations to investigate the reductionof the thermal conductivity by kinks in silicon nanowires. The reductionpercentage can be as high as 70% at room temperature. The temperaturedependence of the reduction is also calculated. By calculating phononpolarization vectors, two mechanisms are found to be responsible forthe reduced thermal conductivity: (1) the interchanging effect betweenthe longitudinal and transverse phonon modes and (2) the pinchingeffect, that is, a new type of localization, for the twisting andtransverse phonon modes in the kinked silicon nanowires. Our workdemonstrates that the phonon interchanging and pinching effects, inducedby kinking, are brand-new and effective ways in modulating heat transferin nanowires, which enables the kinked silicon nanowires to be a promisingcandidate for thermoelectric materials. [ABSTRACT FROM AUTHOR]

We perform classical molecular dynamics simulations to demonstrate the application of kinked silicon nanowires (KSiNWs) as nanoscale springs. The spring-like oscillation in gigahertz frequency range is successfully actuated using a similar procedure as the actuation of a classical mass spring oscillator. We detect the spring-like mechanical oscillation and some other low-frequency oscillations by the energy spectrum analysis, where a dimensional crossover phenomenon is observed for the transverse mode in KSiNWs with decreasing aspect ratio. Our findings shed light on the elastic properties of the KSiNW and open a way for its application in nanomechanical devices. [ABSTRACT FROM AUTHOR]

We conducted extensive molecular dynamics simulations to investigate the thermal conductivity of polycrystalline hexagonal boron-nitride (h-BN) films. To this aim, we constructed large atomistic models of polycrystalline h-BN sheets with random and uniform grain configuration. By performing equilibrium molecular dynamics (EMD) simulations, we investigated the influence of the average grain size on the thermal conductivity of polycrystalline h-BN films at various temperatures. Using the EMD results, we constructed finite element models of polycrystalline h-BN sheets to probe the thermal conductivity of samples with larger grain sizes. Our multiscale investigations not only provide a general viewpoint regarding the heat conduction in h-BN films but also propose that polycrystalline h-BN sheets present high thermal conductivity comparable to monocrystalline sheets. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed to investigate the buckling of single-layer MoS2 under uniaxial compression. The strain rate is found to have an important effect on the critical buckling strain, where higher strain rate leads to larger critical strain. The critical strain is almost temperature-independent for K, and it increases with increasing temperature for K owing to the thermal vibration assisted healing mechanism on the buckling deformation. The length-dependence of the critical strain from our simulations is in good agreement with the prediction of the Euler buckling theory. [ABSTRACT FROM AUTHOR]

We show that the hexagonal structure of single-layer molybdenum disulphide (), under uniaxial tension along a zigzag direction for large deformations, can transfer to a new quadrilateral structure by molecular dynamics (MD) simulations when the temperature is below 40 K. The new phase remains stable after unloading, even at room temperature. The Youngʼs modulus of the new phase along the zigzag direction is about 2.5 times higher than that of normal . Checking against density functional theory calculations shows that the new phase is preserved and displays excellent electrical conductivity. Our results provide physical insights into the origins of the new phase transition of at low temperatures. [ABSTRACT FROM AUTHOR]

[Display omitted] • Molecular dynamic simulations based on a reactive force field are performed to study the thermal stability of twin graphene. • Twin graphene transforms into amorphous graphene at high temperatures. • The initial structural evolution of twin graphene shows unique behavior that leads to a mesophase from 1500 to 1700 K. • v-Twin graphene possesses a higher transformation temperature of 2900 K than that of (h-) twin graphene. Twin graphene, a new phase of two-dimensional semiconducting carbon allotrope, is theorized to exist and show a great deal of potential applications due to its superior electronic and mechanical properties. In this investigation, the thermal stability of twin graphene is explored via molecular dynamic (MD) simulations based on a reactive force field (ReaxFF). We show that, before twin graphene totally transforms into amorphous graphene, the initial structural evolution of twin graphene shows unique behavior that leads to a mesophase consisting of amorphous graphene, carbon atom chains and complex structures from 1500 to 1700 K. In particular, a new twin structure, named v-twin graphene, is observed in the mesophase. Detailed simulations show that v-twin graphene possesses a more stable molecular structure than the original twin graphene. These findings provide fundamental understandings for the thermal stability and structural evolution of these two types of twin graphene. [ABSTRACT FROM AUTHOR]