Ronggui Yang

Ronggui Yang
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Ronggui Yang
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Physics - Materials Science (18)
 
Physics - Mesoscopic Systems and Quantum Hall Effect (6)
 
Physics - Soft Condensed Matter (1)

Publications Authored By Ronggui Yang

In this article, we first summarize and compare the phonon properties, such as phonon dispersion and relaxation time, of emerging pristine two-dimensional (2-D) materials with the single layer graphene to understand the role of crystal structure on their thermal conductivity. We then compare the phonon properties, between an idealized 2-D crystal, realistic 2-D crystals, and 3-D crystals, and present the physical picture on how the thermal conductivity of 2-D materials changes with sample sizes. The geometric effects, such as layer numbers and width, and other physical effects like defects, mechanical strains, and substrates, on the thermal properties of 2-D materials are discussed. Read More

It is challenging to characterize thermal conductivity of materials with strong anisotropy. In this work, we extend the time-domain thermoreflectance (TDTR) method with a variable spot size approach to simultaneously measure the in-plane (Kr) and the through-plane (Kz) thermal conductivity of materials with strong anisotropy. We first determine Kz from the measurement using a larger spot size, when the heat flow is mainly one-dimensional along the through-plane direction, and the measured signals are sensitive to only Kz. Read More

To date, the intrinsic thermal conductivity tensor of bulk black phosphorus (BP), an important 2D material, is still unknown, since recent studies focus on BP flakes not on bulk BP. Here we report the anisotropic thermal conductivity tensor of bulk BP, for temperature range 80 - 300 K. Our measurements are similar to prior measurements on submicron BP flakes along zigzag and armchair axes, but are >25% higher in the through-plane axis, suggesting that phonon mean-free-paths are substantially longer in the through-plane direction. Read More

Two-dimensional (2-D) transition metal dichalcogenides (TMDs) have shown numerous interesting physical and chemical properties, making them promising materials for electronic, optoelectronic, and energy applications. Tuning thermal conductivity of two-dimensional (2-D) materials could expand their applicability in many of these fields. In this paper, we propose a strategy of using alloying and nanodomains to suppress the thermal conductivity of 2-D materials. Read More

Thermal conductivity and interfacial thermal conductance play crucial roles in the design of engineering systems where temperature and thermal stress are of concerns. To date, a variety of measurement techniques are available for both bulk and thin film solid-state materials with a broad temperature range. For thermal characterization of bulk material, the steady-state absolute method, laser flash diffusivity method, and transient plane source method are most used. Read More

Recent success in synthesizing two-dimensional borophene on silver substrate attracts strong interest in exploring its possible extraordinary physical properties. By using the density functional theory calculations, we show that borophene is highly stretchable along the transverse direction. The strain-to-failure in the transverse direction is nearly twice as that along the longitudinal direction. Read More

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. Read More

Great success has been achieved in improving the photovoltaic energy conversion efficiency of the organic-inorganic perovskite-based solar cells, but with very limited knowledge on the thermal transport in hybrid perovskites, which would affect the device lifetime and stability. Based on the potential developed from the density functional theory calculations, we studied the lattice thermal conductivity of the hybrid halide perovskite CH3NH3PbI3 using equilibrium molecular dynamics simulations. Temperature-dependent thermal conductivity is reported from 160 K to 400 K, which covers the tetragonal phase (160-330 K) and the pseudocubic phase (>330K). Read More

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. Read More

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. Read More

We point out that the effective channel for the interfacial thermal conductance, the inverse of Kapitza resistance, of metal-insulator/semiconductor interfaces is governed by the electron-phonon interaction mediated by the surface states allowed in a thin region near the interface. Our detailed calculations demonstrate that the interfacial thermal conductance across Pb/Pt/Al/Au-diamond interfaces are only slightly different among these metals, and reproduce well the experimental results of the interfacial thermal conductance across metal-diamond interfaces observed by Stoner et al. [Phys. Read More

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. Read More

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. Read More

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. Read More

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. Read More

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. Read More

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. Read More

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. Read More

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. Read More

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. Read More