Jiang Jinwu

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phyjj@nus.edu.sg

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Thermal conductance of graphene and dimerite
(2009-05-01) Jiang, J.-W.; Wang, J.-S.; Li, B.; PHYSICS
We investigate the phonon thermal conductance of graphene regarding the graphene sheet as the large-width limit of graphene strips in the ballistic limit. We find that the thermal conductance depends weakly on the direction angle θ of the thermal flux periodically with period π/3. It is further shown that the nature of this directional dependence is the directional dependence of group velocities of the phonon modes in the graphene, originating from the D6h symmetry in the honeycomb structure. By breaking the D6h symmetry in graphene, we see more obvious anisotropic effect in the thermal conductance as demonstrated by dimerite. © 2009 The American Physical Society.
Elastic and nonlinear stiffness of graphene: A simple approach
(2010-02-12) Jiang, J.-W.; Wang, J.-S.; Li, B.; PHYSICS
The recent experiment C. G. Lee [Science 321, 385 (2008)] on the Young's modulus and third-order elastic stiffness of graphene are well explained in a very simple approach, where the graphene is described by a simplified system and the force constant for the nonlinear interaction is estimated from the Tersoff-Brenner potential. © 2010 The American Physical Society.
Self-repairing in single-walled carbon nanotubes by heat treatment
(2010-09-01) Jiang, J.-W.; Wang, J.-S.; PHYSICS
Structure transformation by heat treatment in single-walled carbon nanotubes (SWCNT) is investigated using molecular dynamics simulation. The critical temperature for the collapse of pure SWCNT is as high as 4655 K due to strong covalent carbon-carbon bonding. Above 2000 K, the cross section of SWCNT changes from circle to ellipse. The self-repairing capability is then investigated and two efficient processes are observed for the SWCNT to repair themselves. (1) In the first mechanism, vacancy defects aggregate to form a bigger hole, and a bottleneck junction is constructed nearby. (2) In the second mechanism, a local curvature is generated around the isolate vacancy to smooth the SWCNT. Benefit from the powerful self-repairing capability, defective SWCNT can seek a stable configuration at high temperatures; thus the critical temperature for collapse is insensitive to the vacancy defect density. © 2010 American Institute of Physics.
Why edge effects are important on the intrinsic loss mechanisms of graphene nanoresonators
(2012-03-01) Jiang, J.-W.; Wang, J.-S.; PHYSICS
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 × 10 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 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. © 2012 American Institute of Physics.
Isotopic effects on the thermal conductivity of graphene nanoribbons: Localization mechanism
(2010) Jiang, J.-W.; Lan, J.; Wang, J.-S.; Li, B.; PHYSICS
Thermal conductivity of graphene nanoribbons (GNR) with length 106 Å and width 4.92 Å after isotopic doping is investigated by molecular dynamics with quantum correction. Two interesting phenomena are found, (1) isotopic doping reduces thermal conductivity effectively in low doping region, and the reduction slows down in high doping region, (2) thermal conductivity increases with increasing temperature in both pure and doped GNR, but the increasing behavior is much more slow in the doped GNR than that in pure ones. Further studies reveal that the physics of these two phenomena is related to the localized phonon modes, whose number increases quickly (slowly) with increasing isotopic doping in low (high) isotopic doping region. © 2010 American Institute of Physics.
Young's modulus of graphene: A molecular dynamics study
(2009-09-23) Jiang, J.-W.; Wang, J.-S.; Li, B.; PHYSICS
The Young's modulus of graphene is investigated through the intrinsic thermal vibration in graphene which is "observed" by molecular dynamics and the results agree very well with the recent experiment. This method is further applied to show that the Young's modulus of graphene (1) increases with increasing size and saturates after a threshold value of the size; (2) increases from 0.95 to 1.1 TPa as temperature increases in the region [100, 500] K; (3) is insensitive to the isotopic disorder in the low disorder region (
Graphene-based torsional resonator from molecular-dynamics simulation
(2011-12) Jiang, J.-W.; Wang, J.-S.; PHYSICS
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 lifetime 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+0.004R2), 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 below 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. Copyright © EPLA, 2011.
Joule heating and thermoelectric properties in short single-walled carbon nanotubes: Electron-phonon interaction effect
(2011-12-15) Jiang, J.-W.; Wang, J.-S.; PHYSICS
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 the zigzag (10, 0) tube, the Joule heat is smaller than the armchair tube, resulting from nonzero bandgap 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. © 2011 American Institute of Physics.
Edge states induce boundary temperature jump in molecular dynamics simulation of heat conduction
(2009-08-07) Jiang, J.-W.; Chen, J.; Wang, J.-S.; Li, B.; PHYSICS
We point out that the origin of the commonly occurred boundary temperature jump in the application of Nose-Hoover heat bath in molecular dynamics is related to the edge modes, which are exponentially localized at the edge of the system. If heat baths are applied to these edge regions, the injected thermal energy will be localized thus leading to a boundary temperature jump. The jump can be eliminated by shifting the location of heat baths away from edge regions. Following this suggestion, a very good temperature profile is obtained without increasing any simulation time and the accuracy of thermal conductivity calculated can be largely improved. © 2009 The American Physical Society.
Thermal expansion in single-walled carbon nanotubes and graphene: Nonequilibrium Green's function approach
(2009-11-30) Jiang, J.-W.; Wang, J.-S.; Li, B.; PHYSICS
The nonequilibrium Green's function method is applied to investigate the coefficient of thermal expansion (CTE) in single-walled carbon nanotubes (SWCNT) and graphene. It is found that atoms expand about 1% from equilibrium positions even at T=0,K, resulting from the interplay between quantum zero-point motion and nonlinear interaction. The CTE in SWCNT of different sizes is studied and analyzed in terms of the competition between various vibration modes. As a result of this competition, the axial CTE is positive in the whole temperature range, while the radial CTE is negative at low temperatures. In graphene, the CTE is very sensitive to the substrate. Without substrate, CTE has large negative region at low temperatures and very small value at high-temperature limit, and the value of CTE at 300 K is -6× 10-6 , K -1 which is very close to a recent experimental result, -7× 10-6 , K-1. A very weak substrate interaction (about 0.06% of the in-plane interaction) can largely reduce the negative CTE region and greatly enhance the value of CTE. If the substrate interaction is strong enough, the CTE will be positive in whole temperature range and the saturate value at high temperatures reaches 2.0× 10-5 , K-1. © 2009 The American Physical Society.

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