Electronic band structure change with structural transition of buckled Au₂X monolayers induced by strain

This study investigates the strain-induced structural transitions of η ↔ θ and the changes in electronic band structures of Au₂X (X = S, Se, Te, Si, Ge) and Au₄SSe. We focus on Au₂S monolayers, which can form multiple meta-stable monolayers theoretically, including η-Au₂S, a buckled penta-monolayer composed of a square Au lattice and S adatoms. The θ-Au₂S is regarded as a distorted structure of η-Au₂S. Based on density functional theory (DFT) calculations using a generalized gradient approximation, the conduction and the valence bands of θ-Au₂S intersect at the Γ point, leading to linear dispersion, whereas η-Au₂S 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 Au₂S 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 Au₂X monolayers holds potential as materials for atomic scale network devices.