Single-Layer MX₂ (M = Zn, Cd and X = Cl, I): Auxetic Semiconductors with Strain-Tunable Optoelectronic Properties

The emergence of novel two-dimensional (2D) materials with multifunctional properties provides new possibilities for nanoscale applications. Here, based on first-principles calculations, we identify a class of 2D auxetic materials with a special structural buckling, namely, MX 2 monolayers (M = Zn, Cd and X = Cl, I). The calculated results indicate that four monolayers all exhibit an abnormal negative Poisson’s ratio (NPR) with a maximum value of −0.24, which is fairly rare in 2D transition metal halides (TMHals). The origin of NPR can be explained by the unique local corner-sharing tetrahedral structural motif of MX 2 monolayers under the low-dimensional effect. More importantly, ZnCl 2 monolayer can be exfoliated from its bulk counterpart with an ultralow exfoliation energy of 8.37 meV Å –2 (0.1341 J m –2 ), suggesting the feasibility of its experimental preparation. In addition, strain engineering studies on MX 2 monolayers show effective band gap modulation, indirect-to-direct band gap transitions, and significant enhancement of light absorption in the visible and near-ultraviolet regions. Our findings enrich the large family of versatile 2D TMHals and highlight the promising applications of MX 2 monolayers in nanomechanical and optoelectronic devices.