Strain-driven thermodynamic stability and electronic transitions in ZnX (X = O, S, Se, and Te) monolayers

Semiconducting Zn chalcogenide monolayers are important members of the 2D family of materials due to their unique electronic properties. In this paper, we focus on strain-modulated electronic properties of monolayers of ZnX, with X being O, S, Se, and Te. ZnO and ZnS monolayers have a hexagonal graphene-like planar structure, while ZnSe and ZnTe monolayers exhibit slightly buckled silicene and germanene-like structures, respectively. Density functional theory calculations find the hexagonal ZnO monolayer to be dynamically stable. However, ZnS, ZnSe, and ZnTe monolayers are predicted to be less stable with small imaginary frequencies. The application of tensile strain to these monolayers, interestingly, yields stability of dynamically less stable structures together with the modification in the nature of the bandgap from direct to indirect. For a tensile strain of about 8%, a closure of the bandgap in ZnTe is predicted with the semiconductor-metal transition. The results, therefore, find strain-induced stability and modification in electronic properties of monolayers of Zn chalcogenides, suggesting the use of these monolayers for novel device applications.