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

Abstract

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 κ_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 κ_L since the mean free paths of the dominant phonon modes are relatively long. This conclusion is consistent with recent experimental findings, where the κ_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.