Stability and electronic, phononic, thermal and optical properties of SrX (X = S, Se) nanosheets based on AIMD and DFT computations.

We study the electronic structure, stability, and thermal and optical properties of hexagonal SrS and SrSe monolayers using first-principles calculations. PBEsol generally improves the calculated equilibrium properties of solids and their surfaces, predicting a wider band gap of the monolayers, which is closer to the results obtained with HSE06. Phonon dispersion relations and the ab initio molecular dynamics (AIMD) method confirm the dynamic and thermal stability of both structures. The phonon modes of a SrSe monolayer are lower in energy than those of SrS, and the gap between the acoustic and optical modes is larger, leading to weaker thermal properties such as heat capacity. Additionally, both SrS and SrSe monolayers show an active optical response in the far-visible light region, with some differences in how they behave optically depending on the direction of polarization. SrS exhibits a slightly higher static dielectric constant than that of SrSe, and the primary optical conductivity also follows the same trend. Another important finding is that an SrS monolayer exhibits plasmonic excitations, but SrSe does not.