High out-of-plane negative Poisson's ratios and strong light harvesting in two-dimensional SiS₂ and its derivatives

Two-dimensional (2D) materials with negative Poisson's ratios (NPRs) hold tremendous potential in diverse electronic devices. However, most 2D auxetic materials exhibit small out-of-plane NPRs and materials with bi-directional NPRs are rare. In this work, the SiS2 monolayer and its derivatives MX2 (M = Si, Ge, Sn and X = S, Se, Te) are systematically studied via first-principles simulation. We demonstrate that a SiS2 monolayer possesses a remarkable out-of-plane NPR with a value of -1.09 and an in-plane NPR (-0.13). Furthermore, a higher out-of-plane NPR (-1.79) can be achieved in a SnS2 monolayer by element substitution. Remarkably, SiS2 and its derivative MX2 monolayers exhibit excellent light harvesting over the ultraviolet and visible range, and the corresponding electronic properties show robustness against strains. Our results confirm that MX2 monolayers provide an ideal platform to explore auxeticity in two-dimensional limits.