Electronic band structure change with structural transition of buckled Au 2 X monolayers induced by strain.

This study investigates the strain-induced structural transitions of η ↔ θ and the changes in electronic band structures of Au ₂ X (X = S, Se, Te, Si, Ge) and Au ₄ SSe. We focus on Au ₂ S monolayers, which can form multiple meta-stable monolayers theoretically, including η -Au ₂ S, a buckled penta-monolayer composed of a square Au lattice and S adatoms. The θ -Au ₂ S is regarded as a distorted structure of η -Au ₂ S. Based on density functional theory (DFT) calculations using a generalized gradient approximation, the conduction and the valence bands of θ -Au ₂ S intersect at the Γ point, leading to linear dispersion, whereas η -Au ₂ S has a band gap of 1.02 eV. The conduction band minimum depends on the specific Au-Au bond distance, while the valence band maximum depends on both Au-S and Au-Au interactions. The band gap undergoes significant changes during the η ↔ θ phase transition of Au ₂ S induced by applying tensile or compressive in-plane biaxial strain to the lattice. Moreover, substituting S atoms with other elements alters the electronic band structures, resulting in a variety of physical properties without disrupting the fundamental Au lattice network. Therefore, the family of Au ₂ X monolayers holds potential as materials for atomic scale network devices.