Ultra-low lattice thermal conductivity induces high-performance thermoelectricity in Janus group-VIA binary monolayers

TL;DR

This study demonstrates that Janus STe₂, SeTe₂, and Se₂Te monolayers have ultra-low lattice thermal conductivity and high thermoelectric efficiency, making them promising materials for thermoelectric applications.

Contribution

The paper introduces three new Janus monolayers with exceptionally low thermal conductivity and high thermoelectric figure of merit, supported by first-principles calculations.

Findings

Ultra-low lattice thermal conductivities at 300 K.

High ZT values indicating excellent thermoelectric performance.

Strong phonon coupling and anharmonicity contribute to low thermal conductivity.

Abstract

In this paper, the electrical transport, thermal transport, and thermoelectric properties of three new Janus STe$_{2}$, SeTe$_{2}$, and Se$_{2}$Te monolayers are systematically studied by first-principles calculations, as well as the comparative with available literature's results using different methods. It is found that the Seebeck coefficient and conductivity have opposite dependence on temperature, and we illustrate this phenomenon in detail. The decrease of the thermoelectric power factor (PF) with temperature originates from the decrease in conductivity. To obtain accurate and convergent lattice thermal conductivity, the root mean square (RMS) is calculated to obtain a reasonable cutoff radius for the calculation of third-order forces. Janus STe$_{2}$, SeTe$_{2}$, and Se$_{2}$Te monolayers exhibit ultra-low lattice thermal conductivity of 0.2, 0.133, and 4.81$\times10^{-4}$ W/mK at 300 K, which result from the strong coupling effect between the acoustic mode and the low-frequency optical branch, low phonon group velocity, small phonon lifetime, and large anharmonicity. Consequently, ultra-high $\textit{ZT}$ values of 2.11 (2.09), 3.28 (4.24), and 3.40 (6.51) for n-type(p-type) carrier doping of STe$_{2}$, SeTe$_{2}$, and Se$_{2}$Te are obtained, indicating that they are promising thermoelectric materials.