Cross-plane thermal transport in MoS2

We study the cross-plane thermal transport in MoS2 using density functional theory by adequately accounting for the effect of widespread van der Waals (vdW) forces. We show that past studies report a large variation in the reported values of cross-plane thermal conductivity due to no or limited benchmarking of vdW functionals in computations and variations in sample sizes in experiments. In particular, we show that the validation of the considered vdW functionals based on the correct reproduction of harmonic properties (structural parameters, heat capacity) only is insufficient for the correct description of the thermal transport physics. Further, after having validated the suitability of different functionals, we find that the phonons contributing to cross-plane conductivity have an order of magnitude larger mean free path than that for the basal-plane thermal transport in MoS2 at room temperature. The cross-plane transport is quasiballistic with a more than 80% contribution coming from phonons having mean free paths larger than 500 nm. Furthermore, we demonstrate that four-phonon scattering lowers the cross-plane thermal conductivity by more than 35% at room temperature and is required for a correct description of cross-plane thermal transport physics in layered materials.