Regulation Of The Number Of Layers On The Crystal Structure And Electronic Structure Of Two-dimensional SnX(X=S,Se)

By combine the crystal structure prediction methods based on particle swarm optimization algorithm with the first-principles calculations,we have systematically studied the crystal structures and the electronic properties of bulk Sn S and Sn Se,monolayer Sn S and Sn Se,bilayer Sn S and Sn Se,trilayer Sn S and Sn Se.The results show that due to the influence of the intermolecular forces between the layers,the most stable structures of monolayer,bilayer and trilayer Sn S at ambient pressure are all orthogonal structures,but they are different from the bulk Sn S(Pnma).The space group of monolayer Sn S is C2/m,both bilayer Sn S and trilayer Sn S are P4 mm.The calculation results of electronic properties show that the number of layers will effectively modify the electronic structures.For example,the band gaps of monolayer,bilayer and trilayer Sn S are 1.35 e V,0.24 e V,and 0.17 e V respectively;There are polar covalent bonds between Sn and S atoms in all structures.The electrons are mainly localized around S atoms.With the number of layers increasing,the amount of charge transferred from Sn to S atoms decreases and the degree of charge localization decreases,and the bonding is strengthened.Although the most stable structures of monolayer,bilayer and trilayer Sn Se under normal pressure are orthogonal structures,they are different from Pnma spatial group of block,the spatial group of monolayer Sn Se is C2/m,and the spatial group of bilayer and trilayer Sn Se is Pmm2.Further electronic properties studies have found that the number of layers also has a regulatory effect on the band gap of two-dimensional Sn Se.The band gap increases with the decrease of the number of layers.The band gaps of monolayer and bilayer Sn Se are 0.76 e V and 0.64 e V,respectively.However,the top of the trilayer Sn Se valence band and the bottom of the conduction band do not directly overlap,but the valence band crosses the Fermi surface and overlaps the conduction band,which clearly shows the characteristics of semi-metal.There are polar covalent bonds between Sn and Se atoms in all structures.The electrons are mainly localized around Se atoms.With the number of layers increasing,the amount of charge transferred from Sn to Se atoms decreases and the degree of charge localization decreases,and the bonding is strengthened.The research will provide theoretical guidance for the synthesis of few-layer Tin metal and its application in the field of solar cells and thermoelectrics.