Emerging negative Poisson's ratio driven by strong intralayer interaction response in rectangular transition metal chalcogenides

TL;DR

This study reveals a novel electronic-structure-driven negative Poisson's ratio in a specific 2D transition metal chalcogenide, R-Cu2Se2, challenging traditional geometry-based auxetic explanations.

Contribution

It uncovers an unconventional, structure-independent NPR driven by intralayer electronic interactions in R-Cu2Se2, expanding understanding of auxetic behavior in 2D materials.

Findings

R-Cu2Se2 exhibits a structure-independent anisotropic NPR.

The NPR arises from strong intralayer interaction response involving lone pair electrons.

Other R-TMCs do not show NPR, highlighting material-specific electronic effects.

Abstract

Auxetic behavior quantified by the negative Poisson's ratio (NPR) is commonly attributed to geometry evolution with re-entrant mechanism or other mechanical factors, which is thought to be independent of electronic structures. Thus, searching for electronic effect dominated auxetic behavior is challenging. Herein, from state-of-the-art first-principles calculations, by studying a class of two-dimensional (2D) transition metal chalcogenides (TMCs), namely X2Y2-type (X=Cu, Ag, Au; Y=O, S, Se) rectangular TMCs (R-TMCs), we identify that the monolayer R-Cu2Se2 unconventionally demonstrates a structure-independent anisotropic NPR. In contrast, the NPR is absent in other R-TMCs. The emerging NPR is attributed to the strong strain response of intralayer interaction in R-Cu2Se2, which can be traced to the lone pair electrons and weak electronegativity of Se atoms under multi-orbital hybridization. The emerging NPR would make R-Cu2Se2 a promising candidate in electronics protection, and our study would provide valuable clues and useful guidance for designing advanced auxetic materials.