Silicene, the silicon-based counterpart of graphene with a two-dimensional honeycomb lattice, has attracted tremendous interest both theoretically and experimentally due to its significant potential industrial applications. From the aspect of theoretical study, the widely used classical molecular dynamics simulation is an appropriate way to investigate the transport phenomena and mechanisms in nanostructures such as silicene. Unfortunately, no available interatomic potential can precisely characterize the unique features of silicene. Here, we optimized the Stillinger-Weber potential parameters specifically for a single-layer Si sheet, which can accurately reproduce the low buckling structure of silicene and the full phonon dispersion curves obtained from ab initio calculations. By performing equilibrium and nonequilibrium molecular dynamics simulations and anharmonic lattice dynamics calculations with the new potential, we reveal that the three methods consistently yield an extremely low thermal conductivity of silicene and a short phonon mean-free path, suggesting silicene as a potential candidate for high-efficiency thermoelectric materials. Moreover, by qualifying the relative contributions of lattice vibrations in different directions, we found that the longitudinal phonon modes dominate the thermal transport in silicene, which is fundamentally different from graphene, despite the similarity of their two-dimensional honeycomb lattices.

Strain engineering is one of the most promising and effective routes toward continuously tuning the electronic and optic properties of materials, while thermal properties are generally believed to be insensitive to mechanical strain. In this paper, the strain-dependent thermal conductivity of monolayer silicene under uniform bi-axial tension is computed by solving the phonon Boltzmann transport equation with force constants extracted from first-principles calculations. Unlike the commonly believed understanding that thermal conductivity only slightly decreases with increased tensile strain for bulk materials, it is found that the thermal conductivity of silicene first increases dramatically with strain and then slightly decreases when the applied strain increases further. At a tensile strain of 4%, the highest thermal conductivity is found to be about 7.5 times that of unstrained one. Such an unusual strain dependence is mainly attributed to the dramatic enhancement in the acoustic phonon lifetime. Such enhancement plausibly originates from the flattening of the buckling of the silicene structure upon stretching, which is unique for silicene as compared with other common two-dimensional materials. Our findings offer perspectives of modulating the thermal properties of low-dimensional structures for applications such as thermoelectrics, thermal circuits, and nanoelectronics.

Silicene, as a graphene-like two-dimensional material, now receives exceptional attention of a wide community of scientists and engineers beyond graphene. Despite extensive study on its electric property, little research has been done to accurately calculate the phonon transport of silicene so far. In this paper, thermal conductivity of monolayer silicene is predicted from first-principles method. At 300 K, the thermal conductivity of monolayer silicene is found to be 9.4 W/mK and much smaller than bulk silicon. The contributions from in-plane and out-of-plane vibrations to thermal conductivity are quantified, and the out-of-plane vibration contributes less than 10% of the overall thermal conductivity, which is different from the results of the similar studies on graphene. The difference is explained by the presence of small buckling, which breaks the reflectional symmetry of the structure. The flexural modes are thus not purely out-of-plane vibration and have strong scattering with other modes.

Grain refinement has been conducted to reduce the thermal conductivity and improve the thermoelectric performance of the (Zr,Hf)NiSn based half-Heusler alloys. Nanoscale in situ precipitates were found embedded in the matrix with submicron grains. The lattice thermal conductivity was decreased due to the enhanced boundary scattering of phonons. The increased carrier concentration and electrical conductivity were observed compared to the coarse-grained alloys, which is discussed in relation to the existence of nanoscale precipitates, the effect of antisite defects, and composition change. It is suggested that the nanoscale precipitates play a significant role in the observed electrical conductivity increase. © 2012 American Institute of Physics. [http://dx.doi.org/l 0.1 063/I.4730436] ·In past decades, thermoelectric (TE) materials have received rejuvenated interest due to their promising applications in direct thermal to electric energy conversion and solidstate refrigeration. The performance of a TE material is represented by the dimensionless figure of merit ZI = r:t.ZaT!K, where IX is the Seebeck coefficient, (J' is the electrical conductivity, K is the thermal conductivity mainly including the electron contribution Ke and the lattice contribution KL, and T is the absolute temperature. 1• 2The MNiSn-based (M = Zr, Hf, and Ti) half-Heusler compounds have recently been identified as potential high temperature TE materials for power generation up to llOOK. -5 Their combination of high Seebeck coefficients and moderate electrical conductivities gives rise to intrinsically high TE power factors. 6 • 7 However, the high thermal conductivity (K::::: 10Wm-1 K-1 ) makes this system less promising for TE applications.7 Several strategies for suppressing the lattice thermal conductivity by increasing phonon scattering via point defects or grain boundary scattering have been implemented. [4][5][6][7][8][9][10] High ZI values of >0.8 have been achieved in the Sb doped (Zr,Hf)NiSn based alloys. • 10Although boundary scattering is generally effective in suppressing the lattice thermal conductivity, the reduction of grain sizes will also induce a decrease in electrical conductivity due to the enhanced scattering for both electrons and phonons. Sharp eta/. proposed that for the MNiSn alloys, the lattice thermal conductivity would decrease faster than the carrier mobility below tens of micrometers of grain sizes, because the mean free path of phonons is larger than that of electrons in the alloys.11 In this work, the MNiSn based alloys with submicron grain sizes were prepared by melt spinning in order to enhance the phonon scattering and keep the electrical properties almost unchanged. Unexpectedly, an electrical conductivity increase was observed, although the lattice thermal conductivity was decreased to some extent •>corresponding author. Electronic mail: zhutj@zju.edu.cn. due to the reduced grain sizes. Possible causes are discussed and it is suggested that the nanoscale in situ precipitates in the matrix play a significan...

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The emergence of data-driven science has opened up new avenues for understanding the thermophysical properties of materials. For decades, alloys are known to possess very low thermal conductivity, but the extreme thermal conductivity that can be achieved by alloying has never been identified. In this work, we combine the Bayesian optimization with a high throughput thermal conductivity calculation to search for the lowest thermal conductivity atomic configuration of SiGe alloy. It is found layered structures are most beneficial for reducing the thermal conductivity among all atomic configurations, which is attributed to the strong branch-folding effect. Furthermore, the roles of interface roughness and layer thicknesses in producing the lowest thermal conductivity are investigated. Through another comprehensive search using Bayesian optimization, the layered structure with smooth interfaces and optimized layer thickness arrangement is eventually identified as the optimal structure with the lowest thermal conductivity.

Solving Peierls-Boltzmann transport equation with interatomic force constants (IFCs) from first-principles calculations has been a widely used method for predicting lattice thermal conductivity of three-dimensional materials. With the increasing research interests in two-dimensional materials, this method is directly applied to them but different works show quite different results. In this work, classical potential was used to investigate the effect of the accuracy of IFCs on the predicted thermal conductivity. Inaccuracies were introduced to the third-order IFCs by generating errors in the input forces. When the force error lies in the typical value from first-principles calculations, the calculated thermal conductivity would be quite different from the benchmark value. It is found that imposing translational invariance conditions cannot always guarantee a better thermal conductivity result. It is also shown that Grüneisen parameters cannot be used as a necessary and sufficient criterion for the accuracy of third-order IFCs in the aspect of predicting thermal conductivity.

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