First-principles Study On Electronic Structure Of Twisted Two-dimensional Transition Metal Dichalcogenides Materials

In recent years,research on two-dimensional materials has attracted much attention,especially twisted bilayer graphene,which has created a widespread research boom.Twisted bilayer graphene is usually observed in a variety of strongly correlated electronic states,such as superconducting and insulating states.Interactions between electrons play an important role in the appearance and disappearance of strongly correlated electronic states and the occurrence of phase transitions between different quantum states.However,in the conventional strongly correlated material system,the experimentally tunable parameters for the quantum phase transition are relatively limited,which limits the study of strongly correlated insulating states and their nearby superconducting states.The extensive researches of twisted bilayer graphene have accelerated the research on other two-dimensional materials,in particular,transition metal dichalcogenides,such as WTe₂,WSe₂,MoS₂,WS₂,etc.,in this thesis,the modulation of the electronic structure of transition metal dichalcogenides WTe₂bilayer and WTe₂/WSe₂heterojunctions by lattice structure twist angle（abbreviations twist angle）was studied using First-principles calculations method.The WTe₂/WSe₂heterostructures in three different stacking forms were investigated,namely AA,C7 and T stacking.In particular,we investigated the effects of twisting and biaxial tensile stresses on the electronic structure of the WTe₂/WSe₂heterojunctions in C7 stacking.First,we investigated the modulation of twisting on the electronic structure of the transition metal dichalcogenides WTe₂.Our results show that when the twisted WTe₂bilayer is turned to a specific angle,the band structure changes significantly,which leads to the change of band gap.The bandgap of the electronic structure of the twisted WTe₂bilayer shows a gradual decrease with the increase of the turning angle.Interestingly,the semiconductor-to-metal phase transition is found at a twist angle of 15 degree.In particular,the appearance of peaks near the Fermi level may influence the electron transport properties.Our results show that the electronic structure of WTe₂can be effectively tuned by twisting and can provide a reference for the regulation of two-dimensional band structure.These results are important for understanding the electronic properties of twisted systems and the application of twisted layered materials in future electronic devices.Secondly,the electronic structures of the non-twisted and twisted two-dimensional transition metal dichalcogenides WTe₂/WSe₂heterojunction bilayers were investigated.Our results show that for the twisted WTe₂/WSe₂heterojunction bilayer system,the bandgaps are all direct bandgaps,and the bandgap（K-K）shows a significant increasing trend when the twist angle is from 0 to 10 degree.However,when the twist angle is from11 to 14.2 degree,the bandgaps are all indirect bandgaps and the bandgap（G-K）shows a significant decreasing trend.The band structure of the twisted WTe₂/WSe₂heterojunction bilayer is significantly different from that of the non-twisted system.The band structure of the twisted WTe₂/WSe₂heterojunction bilayer has a transition between direct bandgap and indirect bandgap,and the bandgap varies.The band structure of the twisted WTe₂/WSe₂bilayer varies with the change of the twist angle.For example,a maximum of the bandgap is at the twist angle of 10.5 degree,while the minimum bandgap of 0.041eV is at 14.2 degree.The electronic structure of transition metal dichalcogenides can be effectively tuned by twisting.Finally,the heterostructures of WTe₂/WSe₂in three different stacking forms,namely,AA,C7 and T stackings were investigated.We studied the electronic structures of WTe₂/WSe₂in C7 stacking under twisting and biaxial tensile stresses.Our results show that the WTe₂/WSe₂heterojunctions show an indirect bandgap in their band structure at different twist angles and have different bandgaps for different twist angles.With the increase of the twist angle,the bandgap shows a gradually decreasing trend,which indicates that the bandgap is very sensitive to the change of the twist angle.Furthermore,our results show that the band structures of WTe₂/WSe₂heterojunctions are different under different tensile stresses,and they have different bandgaps for different tensile stresses.The band gap shows a gradual decrease with the increase of biaxial tensile stress,which indicates that the electronic structure of WTe₂/WSe₂is very sensitive to the change of tensile stress.In addition,the WTe₂/WSe₂heterojunction undergoes a transition from the indirect bandgap to the direct bandgap induced by biaxial tensile stress.Our results show that twisting and biaxial tensile stresses can effectively modulate the electronic structure of transition metal dichalcogenides.