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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.

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/MoS

The gas permeation through nanoscale membranes like graphene has been extensively studied by experiments and empirical models. In contrast to planar membranes, the single-walled carbon nanotube has a natural confined hollow structure, which shall affect the gas permeation process. We perform molecular dynamics simulations to investigate the effect of the nanotube diameter on the gas permeation process. It is found that the permeance constant increases with the increase of the nanotube diameter, which can not be explained by existing empirical models. We generalize the three-state model to describe the diameter dependence for the permeance constant, which discloses a distinctive confinement-induced adsorption phenomenon for the gas molecule on the nanotube’s inner surface. This adsorption phenomenon effectively reduces the pressure of the bulk gas, leading to the decrease of the permeance constant. These results illustrate the importance of the adsorption within the confined space on the gas permeation process.

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.

The continuous loading assumption is adopted in most present studies on the buckling of thin shells subjected to lateral loads, while the relationship between the finite number of loads and the lateral buckling remains unclear. In this work, we derive an analytic formula for the dependence of the critical buckling stress on the number of loads, which shows that the critical stress increases significantly with the increase of the load number and reaches a saturation value in the limit of large load number. Furthermore, the analytic formula reveals the dependence of the critical stress on the deviation of the radius in an imperfect shell. To verify the validity of the analytic formula, we perform molecular dynamics simulations to investigate the buckling of both perfect and imperfect single-walled carbon nanotubes under a finite number of lateral loads, where the analytic formula agrees with the simulation results. These results shall be valuable for understanding mechanical stability of elastic thin shells or nanoscale tubal structures subjected to discrete lateral loads.

Misfit strain is inevitable in various heterostructures like the graphene/MoS

The thermal expansion effect plays an important role in governing the thermal stability or the stable configuration of quasi-two-dimensional atomic layers, where the difference between the thermal expansion coefficient of different kinds of atomic layer in lateral heterostructure may cause strong thermal rippling of the atomic layer. We investigate the thermal expansion phenomenon in the WSe2-MoS2 lateral heterostructure. We find that the thermal expansion coefficient can be enhanced by more than a factor of two via varying the ratio between the WSe2 and MoS2 components in the heterostructure. The underlying mechanism is disclosed to be the buckling of the WSe2 region that is induced by the misfit strain at the coherent interface between WSe2 and MoS2. These findings shall be helpful in handling the thermal stability of functional devices based on the transition-metal dichalcogenide lateral heterostructures and other similar quasi-two-dimensional lateral heterostructures.

The cross-sectional shape of the nanotube is a key factor governing fundamental mechanical properties of the nanotube and the nanotube forest. In contrast to most circular nanotubes, in the present work, we demonstrate that the holey phenine nanotubes have polygonal cross sections with diameter-dependent number of sides. The non-circular cross section is attributed to the high twistability of the continuous C-C chains in the phenine nanotube. Consequently, the phenine nanotube forest has a square lattice structure rather than the regular hexagonal lattice of the carbon nanotube forest, resulting in a smooth buckling process under biaxial compression. The buckling pattern of the phenine nanotube forest is highly ordered with the orientation determined by the initial dislocation that frequently appears in the phenine nanotube forest.

We perform molecular dynamics simulations to investigate the energy dissipation of the resonant oscillation for the group IV monolayers of puckered configuration, in which the oscillation is driven with different actuation velocities. We find that, in the moderate actuation velocity regime, the nonlinear coupling between the resonant oscillation mode and other high-frequency modes will lead to the non-resonant motion of the system. For the larger actuation velocity, the effective strain generated during the resonant oscillating causes a structural transition from the puckered configuration into the planar configuration, which is a characteristic energy dissipation mechanism for the resonant oscillation of these group IV puckered monolayers. Our findings shed light on mechanical applications of the group IV monolayers in the nanomechanical resonator field.

Armchair and zigzag are two popular principal directions in many quasi-two-dimensional nanostructures such as graphene, hexagonal boron nitride, and black phosphorus. To date, nearly all published works discuss the anisotropy for physical quantities based on the comparison between quantity values in these two principal directions in single-layer black phosphorus. However, in the present work, by analyzing the directional dependence for the Young's modulus with an analytic formula, we explore the third principal direction with a direction angle φ(tp) = 0.268π, besides the armchair and zigzag directions. This new principal direction is determined by the puckered configuration of black phosphorus. The largest Young's modulus value is neither in the armchair, nor in the zigzag direction. Instead it is the third principle direction along which the Young's modulus has the maximum value. This work reveals that armchair and zigzag directions are not enough for the discussion of anisotropic properties in the single-layer black phosphorus, and it is crucial to include this new principal direction for anisotropy discussions.

We investigate the in-plane and cross-plane thermal conductivities of molybdenum disulfide (MoS2) using non-equilibrium molecular dynamics simulations. We find that the in-plane thermal conductivity of monolayer MoS2 is about 19.76 W mK(-1). Interestingly, the in-plane thermal conductivity of multilayer MoS2 is insensitive to the number of layers, which is in strong contrast to the in-plane thermal conductivity of graphene where the interlayer interaction strongly affects the in-plane thermal conductivity. This layer number insensitivity is attributable to the finite energy gap in the phonon spectrum of MoS2, which makes the phonon-phonon scattering channel almost unchanged with increasing layer number. For the cross-plane thermal transport, we find that the cross-plane thermal conductivity of multilayer MoS2 can be effectively tuned by applying cross-plane strain. More specifically, a 10% cross-plane compressive strain can enhance the thermal conductivity by a factor of 10, while a 5% cross-plane tensile strain can reduce the thermal conductivity by 90%. Our findings are important for thermal management in MoS2 based nanodevices and for thermoelectric applications of MoS2.

We show that the hexagonal structure of single-layer molybdenum disulphide (MoS2), 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 Young's modulus of the new phase along the zigzag direction is about 2.5 times higher than that of normal MoS2. 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 MoS2 at low temperatures.

We study the phonon modes in single-walled MoS$_{2}$ nanotubes via the lattice dynamics calculation and molecular dynamics simulation. The phonon spectra for tubes of arbitrary chiralities are calculated from the dynamical matrix constructed by the combination of an empirical potential with the conserved helical quantum numbers $(\kappa, n)$. In particular, we show that the frequency ($\omega$) of the radial breathing mode is inversely proportional to the tube diameter ($d$) as $\omega=665.3/d$ {cm$^{-1}$}. The eigen vectors of the first twenty lowest-frequency phonon modes are illustrated. Based on these eigen vectors, we demonstrate that the radial breathing oscillation is disturbed by phonon modes of three-fold symmetry initially, and the tube is squashed by the modes of two-fold symmetry eventually. Our study provides fundamental knowledge for further investigations of the thermal and mechanical properties of the MoS$_{2}$ nanotubes.

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 is 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 the ZnO nanoresonators with free polar surfaces., Nanotechnology, published. arXiv admin note: substantial text overlap with arXiv:1307.3072

This is a reply to the comment by Midtvedt and Croy on our article 2015 Nanotechnology 26 315706.

We propose to parametrize the Stillinger-Weber potential for covalent materials starting from the valence force field model. All geometrical parameters in the Stillinger-Weber potential are determined analytically according to the equilibrium condition for each individual potential term, while the energy parameters are derived from the valence force field model. This parametrization approach transfers the accuracy of the valence force field model to the Stillinger-Weber potential. Furthermore, the resulting Stilliinger-Weber potential supports for stable molecular dynamics simulations, as each potential term is at energy minimum state separately at the equilibrium configuration. We employ this procedure to parametrize Stillinger-Weber potentials for the single-layer MoS2 and black phosphorous. The obtained Stillinger-Weber potentials predict accurate phonon spectrum and mechanical behaviors. We also provide input scripts of these Stillinger-Weber potentials used by publicly available simulation packages including GULP and LAMMPS., Comment: 10 figures, 8 tables

The single-layer black phosphorus is characteristic for its puckered structure, which has leaded to highly anisotropy in its optical, electronic, and mechanical properties. We report, using the non-equilibrium Green's function approach and the first-principles method, that the anisotropy in the thermal conduction is very weak in the single-layer black phosphorus -- the difference between two in-plane directions is less than 4%. Our phonon calculations disclose that the two in-plane acoustic phonon branches have lower group velocities in the direction perpendicular to the pucker, as the black phosphorus is softer in this direction, leading to a weakening effect for the thermal conductance in the perpendicular direction. However, the out-of-plane acoustic phonon branch behaviors abnormally; i.e., the group velocity of this phonon branch is higher in the perpendicular direction, although the single-layer black phosphorus is softer in this direction. The abnormal behavior of the out-of-plane phonon branch is closely related to the highly anisotropic Poisson's ratio in the single-layer black phosphorus. As a result of the counteraction between the in-plane phonon modes and the out-of-plane phonon mode, the thermal conductance in the perpendicular direction is weaker than the parallel direction, but the anisotropy is pretty small., Nanotechnology, published

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.