Tuning band gap of monolayer and bilayer SnS2 by strain effect and external electric field : A first principles calculations*

Recently many efforts have been paid to two-dimensional layered metal dichalcogenides (LMDs). Among them MoS$_2$ has become a prototype LMD, and recent studies show surprising and rich new physics emerging in other van der Waals materials such as layered SnS$_2$ [1-4]. SnS$_2$ is a semiconducting earth-abundant material and Sn is a group IV element replacing the transition metal in MoS$_2$. SnS$_2$ shows new possibilities in various potential applications. However, the knowledge on basic properties of layered SnS$_2$ is still not well understood. In this study, we consider two types of structures; 1T with $P\bar{3}m1$ (164) space group and 1H with $P6_{3}/mmc$ (194) space group. Our first principles calculations show that the 1T structure for SnS$_2$ is more stable than the 1H structure whereas latter is more stable for MoS$_2$. Moreover,in contrast to MoS$_2$,SnS$_2$ shows an indirect band gap both for 1T and 1H structures while 1T MoS$_2$ is metallic and 1H has a direct band gap. We also study strain effect in the range of 0-10\% on the band structure for monolayer and bilayer SnS$_2$ (both for 1T and 1H structures).We find significant change in their band gaps. We also investigate the bilayer SnS$_2$ with and without out-of-plane stress.