Tunable Electronic Structure Of Two-Dimensional Transition Metal Dichalcogenides(Oxides) Heterostructure

Valley degree of freedom has been proved to exist in two-dimensional transition metal dichalcogenides and bilayer transition metal oxides.It is significant to investigate the valley characteristic in novel two-dimensional materials modified by interface recombination and biaxial strain.In this dissertation,the electronic structure of two-dimensional transition metal dichalcogenides and transition metal oxides are studied using density functional theory.Electric field and biaxial strain modulated physical properties of heterostructures are investigated systematically.Firstly,the electronic structure of WSe₂/BiIrO₃ heterostructures has been investigated.WSe₂ is a direct-band-gap semiconductor with valley characteristic.Different stacking patterns have different Ir-Se bond lengths and angles,which influence the spin splitting of monolayer WSe₂ in combined systems.The ferroelectric polarization direction of bilayer BiIrO₃ can switch the doping type and conductivity of monolayer WSe₂.By applying perpendicular electric field,the spin splitting of monolayer WSe₂ can be enhanced.However,the enhanced spin splitting of monolayer WSe₂ cannot be influenced by the change of electric fields,which shows the robust against electric fields.Secondly,the electronic structure and biaxial strain effect on WS₂/SrRuO₃heterostructures have been studied.Six different stacking patterns have been considered according to different atomic relative position.Monolayer WS₂ shows valley structure with valley polarization and spin splitting in all the patterns.Fermi level moves to a higher energy by applying tensile strains,yielding a n-type doping.The band gap of monolayer WS₂ decreases with increasing the tensile strains.By applying compression strains,valley disappears and the transition from a direct-band-gap semiconductor to an indirect-band-gap semiconductor appears in monolayer WS₂.Finally,the electronic structure of SrRuO₃/BiIrO₃ superlattices has been studied.When the ferroelectric polarization of bilayer BiIrO₃ points to the bilayer SrRuO₃,the band structure of SrRuO₃/BiIrO₃ superlattice is metallic without valley.When the ferroelectric polarization of bilayer BiIrO₃ points away from the bilayer SrRuO₃,valley characteristic both exit in bilayer BiIrO₃ and SrRuO₃.Bilayer BiIrO₃ shows spin splitting and valley polarization.Different stacking patterns can modulate magnitude of valley polarization and switch the p-or n-type doping of bilayer BiIrO₃.A spin-down polarized valley polarization appears in bilayer SrRuO₃.The calculations on the heterostructures with different atomic layers of BiIrO₃ demonstrate that the valley in transition metal oxides is limited to the bilayer structure.