Two-dimensional Janus Si 2 OX (X = S, Se, Te) monolayers as auxetic semiconductors: theoretical prediction.

The auxetic materials have exotic mechanical properties compared to conventional materials, such as higher indentation resistance, more superior sound absorption performance. Although the auxetic behavior has also been observed in two-dimensional (2D) nanomaterials, to date there has not been much research on auxetic materials in the vertical asymmetric Janus 2D layered structures. In this paper, we explore the mechanical, electronic, and transport characteristics of Janus Si 2 OX (X = S, Se, Te) monolayers by first-principle calculations. Except for the Si 2 OTe monolayer, both Si 2 OS and Si 2 OSe are found to be stable. Most importantly, both Si 2 OS and Si 2 OSe monolayers are predicted to be auxetic semiconductors with a large negative Poisson's ratio. The auxetic behavior is clearly observed in the Janus Si 2 OS monolayer with an extremely large negative Poisson's ratio of -0.234 in the x axis. At the equilibrium state, both Si 2 OS and Si 2 OSe materials exhibit indirect semiconducting characteristics and their band gaps can be easily altered by the mechanical strain. More interestingly, the indirect-direct bandgap phase transitions are observed in both Si 2 OS and Si 2 OSe monolayers when the biaxial strains are introduced. Further, the studied Janus structures also exhibit remarkably high electron mobility, particularly along the x direction. Our findings demonstrate that Si 2 OS and Si 2 OSe monolayers are new auxetic materials with asymmetric structures and show their great promise in electronic and nanomechanical applications.