Effect of different pseudopotentials on the phonon frequencies, dielectric constant, and Born effective charge of SnSe and SnSe2 nanostructures: A density functional perturbation theory study

The choice of the pseudopotentials (PPs) is a crucial step towards an accurate atomistic calculation of physical properties of 2D materials. In this study, based on Density-Functional Perturbation Theory (DFPT), we investigated the dependency of the phonon properties such as phonon frequencies, dielectric constant, Born effective charge, and vibrational modes of SnSe and SnSe2 monolayers to the different types of PPs. On the basis of Generalized Gradient Approximation (GGA) using PBE functional, we computed eight phonon calculations for both lattices in combinations with four different PPs including Normconversing (NC) FHI/HGH, Ultrasoft (USPP), and Standard Solid-State (SSSP), while other parameters like cut-off energy, k-points mesh, and convergence threshold were kept fixed. However, the phonon properties of acoustic and optical phonons were better predicted by USPP and SSSP for phonon dispersion of SnSe and SnSe2, respectively. Regarding the dielectric constant and Bohr effective charge calculations, our results showed that SnSe and SnSe2 structures possess the dielectric tensor of 8.9 and 6.6 along the in-plane direction by using USPP calculations. Our outcomes also revealed less sensitivity of SnSe2 polarization to the type of PPs by applying the electric field. This study recommends USPP and SSSP to further phonon analysis of other semiconductors.