arXiv subjects

Ronggui Yang

Publications and source records attributed to Ronggui Yang.

48 records · Page 3Linked to original sources

arXiv2016

Layer thickness-dependent phonon properties and thermal conductivity of MoS2

For conventional materials, the thermal conductivity of thin film is usually suppressed when the thickness decreases due to phonon-boundary scattering. However, this is not necessarily true for the van der Waals solids if the thickness is reduced to only a few layers. In this letter, the layer thickness-dependent phonon properties and thermal conductivity in the few-layer MoS2 are studied using the first-principles-based Peierls-Boltzmann transport equation approach. The basal-plane thermal conductivity is found to monotonically reduce from 138 W/mK to 98 W/mK for naturally occurring MoS2, and from 155 W/mK to 115 W/mK for isotopically pure MoS2, when its thickness increases from one layer to three layers. The thermal conductivity of tri-layer MoS2 approaches to that of bulk MoS2. Both the change of phonon dispersion and the thickness-induced anharmonicity are important for explaining such a thermal conductivity reduction. The increased anharmonicity in bi-layer MoS2 results in stronger phonon scattering for ZAi modes, which is linked to the breakdown of the symmetry in single-layer MoS2.

cond-mat.mtrl-sci↗

arXiv2015

Revealing the Anisotropic Thermal Conductivity of Black Phosphorus using the Time-Resolved Magneto-Optical Kerr Effect

Black phosphorus (BP) has emerged as a direct-bandgap semiconducting material with great application potentials in electronics, photonics, and energy conversion. Experimental characterization of the anisotropic thermal properties of BP, however, is extremely challenging due to the lack of reliable and accurate measurement techniques to characterize anisotropic samples that are micrometers in size. Here, we report measurement results of the anisotropic thermal conductivity of bulk BP along three primary crystalline orientations, using the novel time-resolved magneto-optical Kerr effect (TR-MOKE) with enhanced measurement sensitivities. Two-dimensional beam-offset TR-MOKE signals from BP flakes yield the thermal conductivity along the zigzag crystalline direction to be 84 ~ 101 W/(m*K), nearly three times as large as that along the armchair direction (26 ~ 36 W/(m*K)). The through-plane thermal conductivity of BP ranges from 4.3 to 5.5 W/(m*K). The first-principles calculation was performed for the first time to predict the phonon transport in BP both along the in-plane zigzag and armchair directions and along the through-plane direction. This work successfully unveiled the fundamental mechanisms of anisotropic thermal transport along the three crystalline directions in bulk BP, as demonstrated by the excellent agreement between our first-principles-based theoretical predictions and experimental characterizations on the anisotropic thermal conductivities of bulk BP.

cond-mat.mtrl-sci↗

arXiv2015

Phonon transport and thermal conductivity in two-dimensional materials

Two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have attracted increased interest due to their potential applications in electronics and optoelectronics. Thermal transport in two-dimensional materials could be quite different from three-dimensional bulk materials. This article reviews the progress on experimental measurements and theoretical modeling of phonon transport and thermal conductivity in two-dimensional materials. We focus our review on a few typical two-dimensional materials, including graphene, boron nitride, silicene, transition metal dichalcogenides, and black phosphorus. The effects of different physical factors, such as sample size, strain and defects, on thermal transport in Two-dimensional materials are summarized. We also discuss the environmental effect on the thermal transport of two-dimensional materials, such as substrate and when two-dimensional materials are presented in heterostructures and intercalated with inorganic components or organic molecules.

cond-mat.mtrl-sci↗

arXiv2015

Capillary rupture of suspended polymer concentric rings

We present the first experimental study on the simultaneous capillary instability amongst viscous concentric rings suspended atop an immiscible medium. The rings ruptured upon annealing, with three types of phase correlation between neighboring rings. In the case of weak substrate confinement, the rings ruptured independently when they were sparsely distanced, but via an out-of-phase mode when packed closer. If the substrate confinement was strong, the rings would rupture via an in-phase mode, resulting in radially aligned droplets. The concentric ring geometry caused a competition between the phase correlation of neighboring rings and the kinetically favorable wavelength, yielding an intriguing, recursive surface pattern. This frustrated pattern formation behavior was accounted for by a scaling analysis.

cond-mat.soft↗

arXiv2015

Phonon transmission across Mg2Si/Mg2Si1-xSnx interfaces: A first-principles-based atomistic Green's function study

Phonon transmission across interfaces of dissimilar materials has been studied intensively in the recent years by using atomistic simulation tools owing to its importance in determining the effective thermal conductivity of nanostructured materials. Atomistic Green's function (AGF) method with interatomic force constants from the first-principles (FP) calculations has evolved to be a promising approach to study phonon transmission in many not well-studied material systems. However, the direct FP calculation for interatomic force constants becomes infeasible when the system involves atomic disorder. Mass approximation is usually used, but its validity has not been tested. In this paper, we employ the higher-order force constant model to extract harmonic force constants from the FP calculations, which originates from the virtual crystal approximation but considers the local force-field difference. As a feasibility demonstration of the proposed method that integrates higher-order force constant model from the FP calculations with the AGF, we study the phonon transmission in the Mg2Si/Mg2Si1-xSnx systems. When integrated with the AGF, the widely-used mass approximation is found to overpredict phonon transmission across Mg2Si/Mg2Sn interface. The difference can be attributed to the absence of local strain field-induced scattering in the mass approximation, which makes the high-frequency phonons less scattered. The frequency-dependent phonon transmission across an interface between a crystal and an alloy, which often appears in high efficiency "nanoparticle in alloy" thermoelectric materials, is studied. The interfacial thermal resistance across Mg2Si/Mg2Si1-xSnx interface is found to be weakly dependent on the composition of Sn when the composition x is less than 40%, but increases rapidly when it is larger than 40% due to the transition of high-frequency phonon DOS in Mg2Si1-xSnx alloys.

cond-mat.mtrl-sci↗

arXiv2015

Steady State and Modulated Heat Conduction in Layered Systems Predicted by the Phonon Boltzmann Transport Equation

Based on the phonon Boltzmann transport equation under the relaxation time approximation, analytical expressions for the temperature profiles of both steady state and modulated heat conduction inside a thin film deposited on a substrate are derived and analyzed. It is shown that both steady state and modulated components of the temperature depend strongly on the ratio between the film thickness and the average phonon mean free path, and they exhibit the diffusive behavior as predicted by the Fourier law of heat conduction when this ratio is much larger than the unity. In contrast, in the ballistic regime when this ratio is comparable to or smaller than the unity, the steady-state temperature tends to be independent of position, while the amplitude and the phase of the modulated temperature appear to be lower than those determined by the Fourier law. Furthermore, we derived an invariant of heat conduction and a simple formula for the cross-plane thermal conductivity of dielectric thin films, which could be a useful guide for understanding and optimizing the thermal performance of the layered systems. This work represents the Boltzmann transport equation-based extension of the Rosencwaig and Gerko work [J. Appl. Phys. 47, 64 (1976)], which is based on the Fourier law and has widely been used as the theoretical framework for the development of photoacoustic and photothermal techniques. This work might shed some light on developing a theoretical basis for the determination of the phonon MFP and relaxation time using ultrafast laser-based transient heating techniques.

cond-mat.mes-hall↗

arXiv2014

First-Principles Prediction of Phononic Thermal Conductivity of Silicene: a Comparison with Graphene

There has been great interest in two-dimensional materials, beyond graphene, for both fundamental sciences and technological applications. Silicene, a silicon counterpart of graphene, has been shown to possess some better electronic properties than graphene. However, its thermal transport properties have not been fully studied. In this paper, we apply the first-principles-based phonon Boltzmann transport equation to investigate the thermal conductivity of silicene as well as the phonon scattering mechanisms. Although both graphene and silicene are two-dimensional crystals with similar crystal structure, we find that phonon transport in silicene is quite different from that in graphene. The thermal conductivity of silicene shows a logarithmic increase with respect to the sample size due to the small scattering rates of acoustic in-plane phonon modes, while that of graphene is finite. Detailed analysis of phonon scattering channels shows that the linear dispersion of the acoustic out-of-plane (ZA) phonon modes, which is induced by the buckled structure, makes the long-wavelength longitudinal acoustic (LA) phonon modes in silicene not as efficiently scattered as that in graphene. Compared with graphene, where most of the heat is carried by the acoustic out-of-plane (ZA) phonon modes, the ZA phonon modes in silicene only have ~10% contribution to the total thermal conductivity, which can also be attributed to the buckled structure. This systematic comparison of phonon transport and thermal conductivity of silicene and graphene using the first-principle-based calculations shed some light on other two-dimensional materials, such as two-dimensional transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2014

An Electrohydrodynamics Model for Non-equilibrium Electron and Phonon Transport in Metal Films after Ultra-short Pulse Laser Heating

The electrons and phonons in metal films after ultra-short pulse laser heating are in highly non-equilibrium states not only between the electron sub-system and the phonon sub-system but also within the electron sub-system. An electrohydrodynamics model consisting of the balance equations of electron density, energy density of electrons, and energy density of phonons is derived from the coupled non-equilibrium electron and phonon Boltzmann transport equations to study the nonlinear transport phenomena, such as the electron density fluctuation and the transient electrical current in metal films, after ultra-short pulse laser heating. The time-dependent temperature distributions is calculated by the coupled electron and phonon Boltzmann transport equations, the electrohydrodynamics model derived in this work, and the two-temperature model for different laser pulse durations, film thicknesses, and laser fluences. We find that the two-temperature model overestimates the electron temperature at the frontsurface of the film and underestimates the damage threshold when the nonlinear thermal transport of electrons is important. The electrohydrodynamics model proposedin this work could be a more accurate prediction tool to study the non-equilibrium electron phonon transport process than the two-temperature model and it is much easier to be solved than the coupled electron and phonon Boltzmann transport equations.

cond-mat.mtrl-sci↗

arXiv2014

Phonon Transport in Single-Layer Transition Metal Dichalcogenides: a First-Principles Study

Two-dimensional transition metal dichalcogenides (TMDCs) are finding promising electronic and optical applications due to their unique properties. In this letter, we systematically study the phonon transport and thermal conductivity of eight semiconducting single-layer TMDCs, MX2 (M=Mo, W, Zr and Hf, X=S and Se), by using the first-principles-driven phonon Boltzmann transport equation approach. The validity of the single-mode relaxation time approximation to predict the thermal conductivity of TMDCs is assessed by comparing the results with the iterative solution of the phonon Boltzmann transport equation. We find that the phononic thermal conductivities of 2H-type TMDCs are above 50 W/mK at room temperature while the thermal conductivity values of the 1T-type TMDCs are much lower, when the size of the sample is 1 μm. A very high thermal conductivity value of 142 W/mK was found in single-layer WS2. The large atomic weight difference between W and S leads to a very large phonon bandgap which in turn forbids the scattering between acoustic and optical phonon modes and thus resulting in very long phonon relaxation time.

cond-mat.mtrl-sci↗

arXiv2014

A new regime of nanoscale thermal transport: collective diffusion counteracts dissipation inefficiency

Understanding thermal transport from nanoscale heat sources is important for a fundamental description of energy flow in materials, as well as for many technological applications including thermal management in nanoelectronics, thermoelectric devices, nano-enhanced photovoltaics and nanoparticle-mediated thermal therapies. Thermal transport at the nanoscale is fundamentally different from that at the macroscale and is determined by the distribution of carrier mean free paths in a material, the length scales of the heat sources, and the distance over which heat is transported. Past work has shown that Fourier's law for heat conduction dramatically over-predicts the rate of heat dissipation from heat sources with dimensions smaller than the mean free path of the dominant heat-carrying phonons. In this work, we uncover a new regime of nanoscale thermal transport that dominates when the separation between nanoscale heat sources is small compared with the dominant phonon mean free paths. Surprisingly, the interplay between neighboring heat sources can facilitate efficient, diffusive-like heat dissipation, even from the smallest nanoscale heat sources. This finding suggests that thermal management in nanoscale systems including integrated circuits might not be as challenging as projected. Finally, we demonstrate a unique and new capability to extract mean free path distributions of phonons in materials, allowing the first experimental validation of differential conductivity predictions from first-principles calculations.

cond-mat.mes-hall↗

arXiv2014

Inhomogeneous Thermal Conductivity Enhances Thermoelectric Cooling

We theoretically investigate the enhancement of thermoelectric cooling performance in thermoelectric devices made of materials with inhomogeneous thermal conductivity, beyond the usual practice of enhancing thermoelectric figure of merit ZT. The dissipation of Joule heat in such thermoelectric devices is asymmetric which can give rise to better thermoelectric cooling performance. Although the thermoelectric figure of merit and the coefficient-of-performance are only slightly enhanced, both the maximum cooling power and the maximum cooling temperature difference can be enhanced significantly. This finding can be used to increase the heat absorption at the cold end. The asymmetric dissipation of Joule heat also leads to thermal rectification.

cond-mat.mes-hall↗

arXiv2013

Mechanics and Tunable Bandgap by Straining in Single-Layer Hexagonal Boron-Nitride

Current interest in two-dimensional materials extends from graphene to others systems like single-layer hexagonal boron-nitride (h-BN), for the possibility of making heterogeneous structures to achieve exceptional properties that cannot be realized in graphene.The electrically insulating h-BN and semi-metal graphene may open good opportunities to realize a semiconductor by manipulating the morphology and composition of such heterogeneous structures.Here we report the mechanical properties of h-BN and its band structures tuned by mechanical straining by using the density functional theory calculations.The elastic properties, both the Young's modulus and bending rigidity for h-BN, are isotropic.We reveal that there is a bi-linear dependence of band gap on the applied tensile strains in h-BN. Mechanical strain can tune single-layer h-BN from an insulator to a semiconductor, with a band gap in the 4.7eV to 1.5eV range.

cond-mat.mtrl-sci↗