arXiv subjects

Jin-Wu Jiang

Publications and source records attributed to Jin-Wu Jiang.

At least 73 records · Page 4Linked to original sources

arXiv2013

A Surface Stacking Fault Energy Approach to Predicting Defect Nucleation in Surface-Dominated Nanostructures

We present a surface stacking fault (SSF) energy approach to predicting defect nucleation from the surfaces of surface-dominated nanostructure such as FCC metal nanowires. The approach leads to a criteria that predicts the initial yield mechanism via either slip or twinning depending on whether the unstable twinning energy or unstable slip energy is smaller as determined from the resulting SSF energy curve. The approach is validated through a comparison between the SSF energy calculation and low-temperature classical molecular dynamics simulations of copper nanowires with different axial and transverse surface orientations, and cross sectional geometries. We focus on the effects of the geometric cross section by studying the transition from slip to twinning previously predicted in moving from a square to rectangular cross section for $\ <100\ > /\{100\}$ nanowires, and also for moving from a rhombic to truncated rhombic cross sectional geometry for $\ <110\ >$ nanowires. \hsp{We also provide the important demonstration that the criteria is able to predict the correct deformation mechanism when full dislocation slip is considered concurrently with partial dislocation slip and twinning. This is done in the context of rhombic aluminum nanowires which do not show a tensile reorientation due to full dislocation slip.} We show that the SSF energy criteria successfully predicts the initial mode of surface-nucleated plasticity at low temperature, while also discussing the effects of strain and temperature on the applicability of the criterion.

cond-mat.mtrl-sci↗

arXiv2013

Modulation of Thermal Conductivity in Kinked Silicon Nanowires: Phonon Interchanging and Pinching Effects

We perform molecular dynamics simulations to investigate the reduction of the thermal conductivity by kinks in silicon nanowires. The reduction percentage can be as high as 70% at room temperature. The temperature dependence of the reduction is also calculated. By calculating phonon polarization vectors, two mechanisms are found to be responsible for the reduced thermal conductivity: (1) the interchanging effect between the longitudinal and transverse phonon modes and (2) the pinching effect, i.e a new type of localization, for the twisting and transverse phonon modes in the kinked silicon nanowires. Our work demonstrates that the phonon interchanging and pinching effects, induced by kinking, are brand new and effective ways in modulating heat transfer in nanowires, which enables the kinked silicon nanowires to be a promising candidate for thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2013

Superior thermal conductivity and extremely high mechanical strength in polyethylene chains from {\it ab initio} calculation

The upper limit of the thermal conductivity and the mechanical strength are predicted for the polyethylene chain, by performing the {\it ab initio} calculation and applying the quantum mechanical non-equilibrium Green's function approach. Specially, there are two main findings from our calculation: (1). the thermal conductivity can reach a high value of 310 W/K/m in a 100 nm polyethylene chain at room temperature; (2). the Young's modulus in the polyethylene chain is as high as 374.5 GPa, and the polyethylene chain can sustain $32.85% \pm 0.05%$ (ultimate) strain before undergoing structural phase transition into gaseous ethylene.

cond-mat.mtrl-sci↗

arXiv2013

Mechanical Oscillation of Kinked Silicon Nanowires: a Natural Nanoscale Spring

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.

cond-mat.mes-hall↗

arXiv2013

Size-Sensitive Young's modulus of Kinked Silicon Nanowires

We perform both classical molecular dynamics simulations and beam model calculations to investigate the Young's modulus of kinked silicon nanowires (KSiNWs). The Young's modulus is found to be highly sensitive to the arm length of the kink and is essentially inversely proportional to the arm length. The mechanism underlying the size dependence is found to be the interplay between the kink angle potential and the arm length potential, where we obtain an analytic relationship between the Young's modulus and the arm length of the KSiNW. Our results provide insight into the application of this novel building block in nanomechanical devices.

cond-mat.mtrl-sci↗

arXiv2012

Enhancing the Mass Sensitivity of Graphene Nanoresonators Via Nonlinear Oscillations: The Effective Strain Mechanism

We perform classical molecular dynamics simulations to investigate the enhancement of the mass sensitivity and resonant frequency of graphene nanomechanical resonators that is achieved by driving them into the nonlinear oscillation regime. The mass sensitivity as measured by the resonant frequency shift is found to triple if the actuation energy is about 2.5 times the initial kinetic energy of the nanoresonator. The mechanism underlying the enhanced mass sensitivity is found to be the effective strain that is induced in the nanoresonator due to the nonlinear oscillations, where we obtain an analytic relationship between the induced effective strain and the actuation energy that is applied to the graphene nanoresonator. An important implication of this work is that there is no need for experimentalists to apply tensile strain to the resonators before actuation in order to enhance the mass sensitivity. Instead, enhanced mass sensitivity can be obtained by the far simpler technique of actuating nonlinear oscillations of an existing graphene nanoresonator.

cond-mat.mes-hall↗

arXiv2012

A comparative study of two molecular mechanics models based on harmonic potentials

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.

cond-mat.mtrl-sci↗

arXiv2012

How Does Folding Modulate Thermal Conductivity of Graphene

We study thermal transport in folded graphene nanoribbons using molecular dynamics simulations and the non-equilibrium Green's function method. It is found that the thermal conductivity of flat graphene nanoribbons can be modulated by folding and changing interlayer couplings. The analysis of transmission reveals that the reduction of thermal conductivity is due to scattering of low frequency phonons by the folds. Our results suggest that folding can be utilized in the modulation of thermal transport properties in graphene and other two dimensional materials.

cond-mat.mes-hall↗

arXiv2012

Why edge effects are important on the intrinsic loss mechanisms of graphene nanoresonators?

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 ($3\times10^{5}$) 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 reduce 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.

cond-mat.mtrl-sci↗

arXiv2011

Joule heating and thermoelectric properties in short single-walled carbon nanotubes: electron-phonon interaction effect

The electron-phonon interaction (EPI) effect in single-walled carbon nanotube is investigated by the nonequilibrium Green's function approach within the Born approximation. Special attention is paid to the EPI induced Joule heating phenomenon and the thermoelectric properties in both metallic armchair (10, 10) tube and semiconductor zigzag (10, 0) tube. For Joule heat in the metallic (10, 10) tube, the theoretical results for the breakdown bias voltage is quite comparable with the experimental value. It is found that the Joule heat can be greatly enhanced by increasing the chemical potential, while the role of the temperature is not so important for Joule heat. In zigzag (10, 0) tube, the Joule heat is smaller than the armchair tube, resulting from nonzero band gap in the electron band structure. For the electronic conductance $G_{e}$ and electron thermal conductance $σ_{el}$, the EPI has important effect at higher temperature or higher chemical potential. Compared with ballistic transport, there is an opposite tendency for $G_{e}$ to decrease with increasing temperature after EPI is considered. This is due to the dominant effect of the electron phonon scattering mechanism in the electron transport in this situation. There is an interesting `electron-drag' phenomenon for the phonon thermal conductance in case of low temperature and high chemical potential, where phonons are dragged by electrons from low temperature region into high temperature region through EPI effect.

cond-mat.mtrl-sci↗

arXiv2011

Graphene-based tortional resonator from molecular dynamics simulation

Molecular dynamics simulations are performed to study graphene-based torsional mechanical resonators. The quality factor is calculated by $Q_{F}=ωτ/2π$, where the frequency $ω$ and life time $τ$ are obtained from the correlation function of the normal mode coordinate. Our simulations reveal the radius-dependence of the quality factor as $Q_{F}=2628/(22R^{-1}+0.004R^{2})$, which yields a maximum value at some proper radius $R$. This maximum point is due to the strong boundary effect in the torsional resonator, as disclosed by the temperature distribution in the resonator. Resulting from the same boundary effect, the quality factor shows a power law temperature-dependence with power factors bellow 1.0. The theoretical results supply some valuable information for the manipulation of the quality factor in future experimental devices based on the torsional mechanical resonator.

cond-mat.mtrl-sci↗

arXiv2011

Bright and dark modes induced by graphene bubbles

Through a lattice dynamics analysis, it is revealed that the bubble plays a role of energy shield in the graphene, which helps to split the normal modes into two categories of distinct topological nature, namely the bright and dark modes. The topological invariants, Euler characteristic, of the bright and dark modes are 1 and 0, respectively. For bright modes, the energy is confined inside the bubble, so this type of modes are sensitive to the shape of the bubble; while opposite phenomenon is observed for the dark modes. The different behavior from these two types of normal modes is examined and verified in the process of phonon thermal transport. The bright and dark modes are expected to be distinguished in experiment with existing scanning force microscope techniques, and they should play significant roles in many other physical processes.

cond-mat.mtrl-sci↗

arXiv2011

Phonon modes at finite temperature in graphene and single-walled carbon nanotubes

The phonon modes at finite temperature in graphene and single-walled carbon nanotubes (SWCNT) are investigated by the mode projection technique combined with molecular dynamics simulation. It is found that the quadratic phonon spectrum of the flexural mode in graphene is renormalized into linear at finite temperature by the phonon-phonon scattering. Possible influence of this renormalization on the electron-phonon interaction and phonon thermal transport is analyzed. For the SWCNT, however the quadratic property of the phonon spectrum for the flexural mode can survive at finite temperature under protection of the two-fold degeneracy of the flexural mode in SWCNT due to its quasi-one-dimensional structure. The frequency and life time of optical phonon modes are also studied at different temperatures. In particular, the life time of the in-plane optical phonons in graphene and the axial optical phonon in SWCNT can be described by an uniform formula $τ(T)=560/T$, which coincides with the experimental results.

cond-mat.mtrl-sci↗

arXiv2011

Thermal expansion in multiple layers of graphene

In this file, we apply the nonequilibrium Green's function method to calculate the coefficient of thermal expansion (CTE) in multiple layers of graphene. We focus on the effect from different layer number $N$. Our main prediction is that: the increase of $N$ can either enhance or weaken CTE, depending on the strength of the substrate interaction $γ$. If $γ< ε$, where $ε$ is the inter-layer interaction, the CTE will increase with increasing $N$. Otherwise, if $γ> ε$, CTE will decrease with increasing $N$.

cond-mat.mtrl-sci↗

arXiv2011

Topological effect on thermal conductivity in graphene

The topological effect on thermal conductivity is investigated through the comparison among graphene nanoribbons, carbon nanotubes and the Mobius-like graphene strips (MGS), by molecular dynamics simulation. It is found that the thermal conductivity of MGS is less than one half of that of graphene nanoribbons. The underlying mechanism whereby MGS acquire such low thermal conductivity may be attributable to the enhanced phonon-phonon scattering and localization property, which are induced by the nontrivial topology of Mobius strip. Moreover by counting in the dimensions of MGS, a lower length/width ratio reduces its thermal conductivity, as the phonon-phonon scattering and localization within might be further elevated.

cond-mat.mtrl-sci↗

arXiv2011

A universal exponential factor in the dimensional crossover from graphene to graphite

A universal exponential factor, $γ_{c}=π/2$, is disclosed for the dimensional crossover of few-layer graphene (FLG) from two-dimensional graphene to three-dimensional graphite. $γ_{c}$ is found by analyzing available experimental data on different properties of FLG with varying thickness. A theoretical study on the phonon spectrum of the vertical acoustic mode in FLG is carried out to further check this exponential factor $γ_{c}$. Interestingly, the same exponential factor appears in the dimensional crossover of the phonon mode. It turns out that the exponential factor $γ_{c}$ is related to the homogeneous Helmholtz-like molal equation in the mass transfer with a first order chemical reaction. The finding should provide valuable information for experimentalists and theorists in the future investigation on thickness dependent properties of FLG.

cond-mat.mtrl-sci↗

arXiv2011

A nonequilibrium Green's function study of thermoelectric properties in single-walled carbon nanotubes

The phonon and electron transport in single-walled carbon nanotubes (SWCNT) are investigated using the nonequilibrium Green's function approach. In zigzag SWCNT ($n$, 0) with $mod(n,3)\not=0$, the thermal conductance is mainly attributed to the phonon transport, while the electron only has few percentage contribution. The maximum value of the figure of merit ($ZT$) is about 0.2 in this type of SWCNT. The $ZT$ is considerably larger in narrower SWCNT because of enhanced Seebeck coefficient. $ZT$ is smaller in the armchair SWCNT, where Seebeck coefficient is small due to zero band gap. It is found that the cluster isotopic doping can reduce the phonon thermal conductance obviously and enhance the value of $ZT$. The uniaxial elongation and compress strain depresses phonons in whole frequency region, leading to the reduction of the phonon thermal conductance in whole temperature range. Interestingly, the elongation strain can affect the phonon transport more seriously than the compress strain, because the high frequency $G$ mode is completely filtered out under elongation strain $ε>0.05$. The strain also has important effect on the subband edges of the electron band structure by smoothing the steps in the electron transmission function. The $ZT$ is decreased by strain as the reduction in the electronic conductance overcomes the reduction in the thermal conductance.

cond-mat.mtrl-sci↗

arXiv2011

First principle study of the thermal conductance in graphene nanoribbon with vacancy and substitutional silicon defect

The thermal conductance in graphene nanoribbon with a vacancy or silicon point defect (substitution of C by Si atom) is investigated by non-equilibrium Green's function (NEGF) formalism combined with first-principle calculations density-functional theory with local density approximation. An efficient correction to the force constant matrix is presented to solve the conflict between the long-range character of the {\it ab initio} approach and the first-nearest-neighboring character of the NEGF scheme. In nanoribbon with a vacancy defect, the thermal conductance is very sensitive to the position of the vacancy defect. A vacancy defect situated at the center of the nanoribbon generates a saddle-like surface, which greatly reduces the thermal conductance by strong scattering to all phonon modes; while an edge vacancy defect only results in a further reconstruction of the edge and slightly reduces the thermal conductance. For the Si defect, the position of the defect plays no role for the value of the thermal conductance, since the defective region is limited within a narrow area around the defect center.

cond-mat.mtrl-sci↗