Anisotropic phonon transport and lattice thermal conductivities in tin dichalcogenides SnS2 and SnSe2

In recent years, layered semiconductor tin dichalcogenides, SnX2 (X = S and Se), have received great attention owing to their wide applications in numerous fields. However, theoretical studies on their phonon transport properties and lattice thermal conductivities are very rare. Herein, we carry out comprehensive investigations on the phonon transport properties and heat transfer phenomena in tin dichalcogenides using first-principles calculations combined with the Boltzmann transport theory, and compare them with the recent popular thermoelectric materials SnS (SnSe) and widely studied typical TMD MoS2 (MoSe2). The obtained total thermal conductivities of SnS2 and SnSe2 agree well with the experimental measured values. The ultralow out-of-plane thermal conductivities of both materials may be useful for thermoelectric applications. The contributions of different phonon branches to the total lattice thermal conductivities are evaluated, and the results suggest that it would be difficult for alloying to reduce kappa(L) because the contribution of the optical branches is rather small compared with SnS and SnSe. Subsequently, we investigated the size dependence of the thermal conductivities and found that nanostructuring may be efficient to further reduce kappa(L) since the mean free paths of the dominant phonon modes are relatively long. This conclusion is consistent with recent experimental findings, where the kappa(L) of SnS2 was found to evidently decrease with a decrease in the thickness of SnS2 films. We believe that this phenomenon may also exist in SnSe2, which may demonstrate a much better thermoelectric performance than that of SnS2 because of its higher anharmonicity than that of SnS2, leading to much lower thermal conductivity.