Effect of biaxial strain on thermal transport in WS2 monolayer from first principles calculations

Recently, tungsten disulfide (WS2) monolayer has emerged as a two-dimensional material due to its outstanding physical properties. In this work, the thermal transports in strained WS2 monolayer are studied systematically by combining the first principles calculations and the Boltzmann transport equation. It can be found that the lattice thermal conductivity (k) decreases from 262.78 to 190.66 W m−1 K−1 for biaxial tensile strain (from 0% to 8%) and from 262.78 to 217.40 W m−1 K−1 for biaxial compressive strain (from 0% to −4%), respectively. The softened transverse/longitudinal acoustic phonon mode and the stiffened flexural acoustic phonon mode in strained WS2 monolayer change the phonon group velocities. We attribute the reduction of k in tensilely strained WS2 monolayer to the declining phonon group velocity, heat capacity and phonon lifetime of acoustic phonon modes. The comprehensive competition among the increasing phonon group velocity, heat capacity and the decreasing phonon lifetime suppresses the lattice thermal conductivity of compressively strained WS2 monolayer. Furthermore, the increasing phase space proves that the enhancing anharmonic phonon scattering, which results in the suppression of the k in tensilely strained WS2 monolayer. Additionally, the critical phonon mean free path (MFP) under different biaxial strain levels is calculated to demonstrate the contribution of the phonons with different MFPs to the total k of WS2 monolayer. This work provides a phonon behavior analysis of the thermal conduction in WS2 monolayer and paves the path for potential applications of WS2-based devices.