Two-dimensional M2SD (M = Ge, Sn; D = Se, Te) monolayers with puckered structure: Electronic structure and optical properties

Abstract The anion substitution is an effectively method to modulate the physical properties of the two-dimensional (2D) semiconductors, which can facilitate the applications of 2D materials in the optoelectronic devices. Recently, the monolayer group-IV monochalcogenides have been paid much attention due to its unique electronic structure. However, few results on anion substitution in monolayer group-IV monochalcogenides are reported. Here, the electronic structure and optical properties of 2D puckered M2SD (M = Ge, Sn; D = Se, Te) monolayers are investigated by first-principles calculations. The phonon spectra and binding energy of 2D M2SD monolayers confirm its structure stability. The lattice constant, bond length and bond angle of Ge2SD and Sn2SD monolayers are related to the electronegativity and radius of Se and Te atoms. The 2D M2SD monolayers have an indirect band gap ranging from 0.70 to 1.23 eV, which can turn into direct band gap by applying the in-plane strains. Besides, a transition from semiconductor to metal in 2D Ge2STe and Sn2STe monolayers appears at a large compressive biaxial strain. Based on the dielectric functions, all of the 2D M2SD monolayers show a strong anisotropy in the low energy range (