Evolution of in-plane heat transport in tellurium from 2D to 3D

Abstract

Tellurene (Te) has attracted tremendous interest due to its outstanding electronic, thermoelectric, and optoelectronic properties. Recently, it is found that 2-layer Te exhibits an abnormal anisotropy of in-plane thermal conductivity. This inspires the current investigation, where the thickness dependence of in-plane thermal conductivities of tellurium from 2D to 3D is obtained by first-principles calculations and Monte Carlo simulations. Two intriguing phenomena are discovered: (1) Thermal conductivity rebound (TCR), i.e., the thermal conductivity first reduces and then increases with the increasing thickness. The predicted lowest point emerges between 6-layer to 15-layer; (2) Thermal anisotropy reversal (TAR), i.e., the in-plane thermal conductivity anisotropy reverses from abnormal (κ⊥/κ∥ > 1) to normal (κ⊥/κ∥ < 1) with increasing thickness. The predicted reversal point occurs at around 7-layer. To understand these phenomena, the frequency- and mode-dependent analyses on phonon group velocity and relaxation time are performed. As the thickness increases, the relaxation time almost monotonically increases, whereas the velocity of low-frequency optical (LFO) phonons shows the same varying trend with thermal conductivity, making it the main factor accounting for the TCR and TAR. The trend of the group velocity of LFO phonons can be attributed to the lattice expansion, which diminishes the covalent-like quasi-bonding (CLQB) in the cross-chain direction. The layer-dependent thermal transport of Te revealed in this work is expected to provide guidance for Te-based functional devices, for instance, the thermoelectric system where the lowest thermal conductivity is favorable.