Theoretical Studies On The Electronic Structure And Optical Properties Of Janus Two-dimensional Sulfide And Its Heterojunctions

Janus two-dimensional sulfides and their heterojunctions,as derivatives of two-dimensional sulfides,have exhibited excellent properties in recent years,making them promising candidates for high-performance nano-optoelectronic applications.In this dissertation,the electronic structure and optical properties of Janus two-dimensional sulfides and their heterojunctions are studied based on first-principles calculations,which are of great significance for the exploration of energy band engineering and optical properties of two-dimensional materials.This paper mainly studies two parts:（1）The effects of biaxial strain and spin-orbit coupling on the electronic structure of Janus two-dimensional sulfides are investigated.Take Ga₂Se Te as an example,due to the spin-orbit coupling and mirror symmetry breaking,Rashba type band splitting appears atΓpoint of the bottom conduction band and the band gap decreases significantly,Rashba spin splitting is anisotropic.The spin-orbit coupling strength and electronic structures can be greatly tuned by biaxial strain,which originate from the change of interaction between pz orbitals.As the lattice constant decreases,the interaction between pz orbitals is enhanced,leading to the decrease（increase）of the energy of corresponding bonding（anti-bonding）state,which is the origin of the indirect-direct-indirect transition of the band gap.In addition,when tensile strain is applied,the pz orbital component of Te atoms increases,which leads to the enhancement of the Rashba effect of Ga₂Se Te.（2）The effects of biaxial strain and applied electric field on the electronic structure and optical properties of Janus two-dimensional sulfide heterojunctions were investigated.Four stable heterostructures were constructed using two-dimensional Janus Ga₂Se Te and In₂Se₃.Compared with the pristine materials,such structures have higher optical absorption and can maintain high performance under the influence of electric field and strain.The polarization direction of In₂Se₃determines the band structure and band alignment.The direct-indirect band gap transition appears in the AA2 stacking configuration under the action of biaxial strain.The strain affects the electronic structure of the heterojunction mainly in the charge transfer between different material layers and the change of the electronic structure of the material within the layer.The electronic structure of the Janus Ga₂Se Te/In₂Se₃heterojunction is significantly modulated by the applied electric field,which is mainly due to the charge redistribution at the contact interface in the heterojunction.