Prediction of 2D group-11 chalcogenides: insights into novel auxetic M2X (M = Cu, Ag, Au; X = S, Se, Te) monolayers

Abstract

Two-dimensional (2D) auxetic materials have recently attracted considerable research interest due to their excellent mechanical properties and diverse applications, surpassing those of three-dimensional (3D) materials. This study focuses on the theoretical prediction of mechanical properties and auxeticity in 2D M₂X (M = Cu, Ag, Au; X = S, Se, Te) monolayers using first-principles calculations. Our results indicate that the dynamically stable monolayers include low-energy α-Cu₂S, α-Cu₂Se, α-Cu₂Te, β-Ag₂S, β-Ag₂Se, α-Ag₂Te, β-Au₂S, β-Au₂Se and α-Au₂Te. These M₂X monolayers possess positive Poisson's ratios (PR) ranging from 0.09 to 0.52, as well as Young's moduli ranging from 19.92 to 35.42 N m⁻¹ in x and y directions. Specially, α-Cu₂S exhibits the lowest negative PR in θ = 45° × n (n = 1, 2, 3, 4) directions. The Poisson's function (PF) can be adjusted by increasing tensile strains. The β-phase monolayers exhibit positive PF with a linear change. Interestingly, the transition from positive to negative PF occurs in the α-Cu₂S and α-Ag₂Te monolayers at strains greater than +3% and +4%, respectively, while the α-Cu₂Se, α-Cu₂Te and α-Au₂Te monolayers maintain positive PF within the range of 0% to +6% strains. Furthermore, taking α-Cu₂S (α-Cu₂Te) as an example, the mechanism underlying negative (positive) PF is demonstrated to involve increased (decreased) bond angles, decreased thickness, and weakened (enhanced) d(M)–p(X) orbital coupling. The findings of this study not only enrich the family of 2D group-11 chalcogenides but also provide insights into their mechanical properties, thereby expanding their potential applications in mechanics.