Nuo Yang

Thermal transport in nanostructures

This review summarizes recent studies of thermal transport in nanoscaled semiconductors. Different from bulk materials, new physics and novel thermal properties arise in low dimensional nanostructures, such as the abnormal heat conduction, the size dependence of thermal conductivity, phonon boundary/edge scatterings. It is also demonstrated that phonons transport super-diffusively in low dimensional structures, in other words, Fouriers law is not applicable. Based on manipulating phonons, we also discuss envisioned applications of nanostructures in a broad area, ranging from thermoelectrics, heat dissipation to phononic devices.

Extreme Low Thermal Conductivity in Nanoscale 3D Si Phononic Crystal with Spherical Pores

In this work, we propose a nanoscale three-dimensional (3D) Si phononic crystal (PnC) with spherical pores, which can reduce the thermal conductivity of bulk Si by a factor up to 10,000 times at room temperature. Thermal conductivity of Si PnCs depends on the porosity, for example, the thermal conductivity of Si PnCs with porosity 50% is 300 times smaller than that of bulk Si. The phonon participation ratio spectra demonstrate that more phonons are localized as the porosity increases. The thermal conductivity is insensitive to the temperature changes from room temperature to 1100 K. The extreme-low thermal conductivity could lead to a larger value of ZT than unity as the periodic structure affects very little the electric conductivity.

A Revisit to High Thermoelectric Performance of Single-layer MoS2

Both electron and phonon transport properties of single layer MoS2 (SLMoS2) are studied. Based on first-principles calculations, the electrical conductivity of SLMoS2 is calculated by Boltzmann equations. The thermal conductivity of SLMoS2 is calculated to be as high as 116.8 Wm−1K−1 by equilibrium molecular dynamics simulations. The predicted value of ZT is as high as 0.11 at 500 K. As the thermal conductivity could be reduced largely by phonon engineering, there should be a high possibility to enhance ZT in the SLMoS2-based materials.

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.

Significant reduction of graphene thermal conductivity by phononic crystal structure

Abstract We studied the thermal conductivity of graphene phononic crystal (GPnC), also named as graphene nanomesh, by molecular dynamics simulations. The dependence of thermal conductivity of GPnCs (κGPnC) on both length and temperature are investigated. It is found that the thermal conductivity of GPnCs is significantly lower than that of graphene (κG) and can be efficiently tuned by changing the porosity and period length. For example, the ratio κGPnC/κG can be changed from 0.1 to 0.01 when the porosity is changed from about 21% to 65%. It is also shown quantitatively that there are more states available for Umklapp three-phonon scatterings in GPnCs. The phonon participation ratio spectra reveal that more phonon modes are localized in GPnCs with larger porosity. Our results suggest that creating GPnCs is a valuable method to efficiently manipulate the thermal conductivity of graphene.

Thermal Interface Conductance Between Aluminum and Silicon by Molecular Dynamics Simulations

The thermal interface conductance between Al and Si was simulated by a non-equilibrium molecular dynamics method. In the simulations, the coupling between electrons and phonons in Al are considered by using a stochastic force. The results show the size dependence of the interface thermal conductance and the effect of electron-phonon coupling on the interface thermal conductance. To understand the mechanism of interface resistance, the vibration power spectra are calculated. We find that the atomic level disorder near the interface is an important aspect of interfacial phonon transport, which leads to a modification of the phonon states near the interface. There, the vibrational spectrum near the interface greatly differs from the bulk. This change in the vibrational spectrum affects the results predicted by AMM and DMM theories and indicates new physics is involved with phonon transport across interfaces. Keywords:

Reduction of Thermal Conductivity by Nanoscale 3D Phononic Crystal

We studied how the period length and the mass ratio affect the thermal conductivity of isotopic nanoscale three-dimensional (3D) phononic crystal of Si. Simulation results by equilibrium molecular dynamics show isotopic nanoscale 3D phononic crystals can significantly reduce the thermal conductivity of bulk Si at high temperature (1000 K), which leads to a larger ZT than unity. The thermal conductivity decreases as the period length and mass ratio increases. The phonon dispersion curves show an obvious decrease of group velocities in 3D phononic crystals. The phonons localization and band gap is also clearly observed in spectra of normalized inverse participation ratio in nanoscale 3D phononic crystal.

Profiling Nanowire Thermal Resistance with a Spatial Resolution of Nanometers

We report a new technique to profile the thermal resistance along a nanowire with a spatial resolution of better than 20 nm. Using this technique, we mapped the thermal conductivity along a Si0.7Ge0.3/NiSi0.7Ge0.3 heterostructured nanowire. We also measured the interfacial thermal resistance (ITR) across the Si/NiSi2 interface embedded in Si/NiSi2 heterostructured nanowires. The ITR does not change even for adjacent interfaces as close as ∼ 50 atomic layers.

A Series Circuit of Thermal Rectifiers: An Effective Way to Enhance Rectification Ratio

A novel approachis proposed to enhance the thermal rectification ratio, namely, arranging two thermal rectifiers in series. Through theoretical analysis and molecular dynamics simulations on graphene/phononic crystal structures, the results show that the series thermal rectifiers enhance thermal rectification ratio significantly, compared to a single rectifier. Meanwhile, the results of theoretical prediction match well with simulation results.

Enhancing the Thermoelectric Figure of Merit by Low-Dimensional Electrical Transport in Phonon-Glass Crystals

Low-dimensional electronic and glassy phononic transport are two important ingredients of highly efficient thermoelectric materials, from which two branches of thermoelectric research have emerged. One focuses on controlling electronic transport in the low dimension, while the other focuses on multiscale phonon engineering in the bulk. Recent work has benefited much from combining these two approaches, e.g., phonon engineering in low-dimensional materials. Here we propose to employ the low-dimensional electronic structure in bulk phonon-glass crystals as an alternative way to increase the thermoelectric efficiency. Through first-principles electronic structure calculations and classical molecular dynamics simulations, we show that the π-π-stacking bis(dithienothiophene) molecular crystal is a natural candidate for such an approach. This is determined by the nature of its chemical bonding. Without any optimization of the material parameters, we obtained a maximum room-temperature figure of merit, ZT, of 1.48 at optimal doping, thus validating our idea.