Phonon transport in Janus monotayer MoSSe: a first-principles study

Transition Metal Dichalcogenide (TMD) monolayers are very widely studied due to their unique physical properties. Recently, Janus TMD monolayer MoSSe, with a sandwiched S-Mo-Se structure, has been synthesized by replacing the top S atomic layer in MoS2 with Se atoms. In this work, we systematically investigate the phonon transport and lattice thermal conductivity (kappa(L)) in MoSSe monolayers using first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). The calculated results show that the kappa(L) of MoSSe monolayers is much lower than that of MoS2 monolayers, and higher than that of MoSe2 monolayers. The corresponding thermal sheet conductance of MoSSe monolayers is 342.50 W K-1 at room temperature. This can be understood by studying the phonon group velocities and lifetimes. Compared to MoS2 monolayers, the smaller group velocities and shorter phonon lifetimes of MoSSe monolayers give rise to a lower kappa(L). The larger group velocities of MoSSe compared to those of MoSe2 monolayers are the main reason for the higher kappa(L). The elastic properties of MoS2, MoSSe and MoSe2 monolayers are also calculated, and the order of the Young's modulus is identical to that of the kappa(L). The calculated results show that isotope scattering leads to a 5.8% reduction of the kappa(L). The size effects on the kappa(L) are also considered, and are usually used in device implementation. When the characteristic length of the MoSSe monolayer is about 110 nm, the kappa(L) reduces to half. These results may offer perspectives on thermal management of MoSSe monolayers, for applications in thermoelectrics, thermal circuits and nanoelectronics, and may motivate further theoretical or experimental efforts to investigate thermal transport in Janus TMD monolayers.