Two-dimensional Janus Si₂OX (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 Si₂OX (X = S, Se, Te) monolayers by first-principle calculations. Except for the Si₂OTe monolayer, both Si₂OS and Si₂OSe are found to be stable. Most importantly, both Si₂OS and Si₂OSe monolayers are predicted to be auxetic semiconductors with a large negative Poisson's ratio. The auxetic behavior is clearly observed in the Janus Si₂OS monolayer with an extremely large negative Poisson's ratio of -0.234 in the x axis. At the equilibrium state, both Si₂OS and Si₂OSe 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 Si₂OS and Si₂OSe 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 Si₂OS and Si₂OSe monolayers are new auxetic materials with asymmetric structures and show their great promise in electronic and nanomechanical applications.