Ab-initio study of structural stability, electronic and optical properties of X2GeSe (X = P and Sb) monolayers

Abstract

This paper investigated the structural stability, electronic and optical properties of X2GeSe (X = P and Sb) monolayers in a hexagonal structure. The calculations are based on the density functional theory using the plane wave method in the Quantum-Espresso computational package for local density approximation (LDA) and the generalized gradient approximation (GGA). The results of phononic calculations indicate that P2GeSe and Sb2GeSe monolayers are dynamically stable in LDA and GGA approximations. The electronic section of X2GeSe (X = P and Sb) monolayers showed that both compounds are semiconductors. The P2GeSe and Sb2GeSe monolayers in the LDA approximation exhibit a direct bandgap at the Γ point with a quantity of 0.88 and 0.84 eV, respectively. Also, P2GeSe and Sb2GeSe monolayers have an indirect bandgap in the GGA approximation in the Г−K path whit amount of 1.07 and 1.02 eV, respectively. For X2GeSe monolayers (X = P and Sb), the optical gaps calculated from the absorption spectrum confirm the electronic bandgap results. The optical calculations showed that the static dielectric constant ε1(0) decreases in the x and z directions but increases in the y-direction with increasing constant atomic radius from P to Sb atom. According to the results, in both LDA and GGA approximations, the maximum reflections of the P2GeSe monolayer are in the ultraviolet domain of the electromagnetic spectrum and for the Sb2GeSe monolayer, the maximum of reflections occurs in the infrared domain. This result makes the studied monolayers a potential candidate for use in optoelectronic devices as a reflector layer of the electromagnetic beam. The calculated of the energy loss spectrum L(ω) diagram shows that the significant collective excitations of the electron charge density in the crystal, which is considered as the plasmonic peak, are in the y-direction. The values of the static refractive index n0, obtained from the squares of the real part of the dielectric function at zero energy, n0=ε1(0), are very close to that of the refractive index diagram at zero energy. For every examined monolayer, the static refractive index in the y-direction is larger than other x and z directions in both LDA and GGA approximations. Examination of the optical properties of the X2GeSe (X = P and Sb) monolayers in different x, y, and z-directions explains the anisotropy of the studied structures.