Stabilities, Electronic and Optical Properties of SnSe2(1–x)S2x Alloys: A First-Principles Study

First-principles investigations are performed on the stabilities and electronic and optical properties of SnSe2(1–x)S2x (x = 0.0625, 0.25, 0.5, 0.625, 0.8125, and 1.0) monolayer alloys by using density functional theory calculations. It is found that, above a critical temperature of 702 K, the mixing of SnSe2 and SnS2 is likely to form random alloys. The calculated negative substitution energy of S at the Se site of SnSe2 suggests an alternative strategy for the synthesis of the alloys, i.e., by the substitution of S for Se in SnSe2 monolayers. It is also shown that, due to the lattice mismatch and the pronounced charge transfer between SnSe2 and SnS2, the band-gap values of the alloys deviate strongly from the concentration-averaged values of the constituents. Moreover, the dielectric functions of the alloys are determined to be anisotropic, with optical properties along the xy plane being more susceptible to the S content than those along the z direction, and the alloying enhances the absorption strength in the visible spectral region. We hope that these insights will be useful for future applications of SnSe2(1–x)S2x alloys.