Tuning Electronic Structure And Properties Of Novel Two-dimensional Material SnS₂ From Theoretical Studies

In this thesis, we have investigated the electronic structures and optical properties of two-dimensional?2D? SnS₂ nanosheets by using first-principles methods based on density functional theory. The obtained main results are given as follows.1) The electronic structures of 2D SnS₂ nanosheets are investigated. Numerical results show that when the layer numbers of SnS₂ nanosheets increase, the band gap values decrease, which is due to quantum size effects. In addition, our results also indicate that monolayer and bilayer SnS₂ nanosheets posess the characteristics of the indirect band structures, which are same as that Sn S2 bulk material. Moreover, to further study the electronic structures of 2D SnS₂ materials, we calculated the projected electronic states density of the bulk SnS₂, monolayer and bilayer SnS₂ nanosheets. The results show that the valence band edge mainly consists of the S-3p states, and the conduction band edge is hybridized by the S-3p and Sn-5s states. Also, when the monolayer and bilayer SnS₂ nanosheets are formed, the shoulders in the electronic states density are much sharp than that of SnS₂ bulk due to quantum confinement effects. Thus, we can conclude that the electronic structures can be tuned effectively by the layer number of 2D SnS₂ nanosheets.2) The influences of applied electric field on electronic structures of 2D SnS₂ nanosheets are investigated. In order to understand the physical reasons that the electric field effects on electronic structures of 2D SnS₂ nanosheets, we apply the electric field along the direction perpendicular to 2D SnS₂ nanosheets. Numerical results show that when the the strength of applied electric field increases, the band gap values of 2H-type and 4H-type SnS₂ nanosheets are decreased, and finally they possess the metallic properties. In addition, our results show that for 2D SnS₂ monolayer, the strong electric field is needed to tune the band gap of SnS₂ monolayer. Thus, these calculated results show that electric field can be used to tune effectively the band gap values of 2D SnS₂ nanosheets.3) The electronic structures and optical properties of 2D SnS₂-xSex alloys are studied. In order to understand the modulation on electronic structures and optical properties 2D SnS₂ nanosheets, we have investigated the influences of Se substituting S atom on the electronic states density and optical properties of 2D SnS₂ nanosheets. Numerical results show that when the Se concentration increases, the band gap values of the 2D SnS₂-xSex alloys are decreased. Moreover, the threshold values of optical absorption shift towards lower energy with increasing Se doping concentration. Thus, we can conclude that the gap values and optical properties of 2D SnS₂ nanosheets can be tuned effectively by Se substituting S atom.