High out-of-plane negative Poisson's ratios and strong light harvesting in two-dimensional SiS ₂ 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.