Properties Of Different-phase Transition Metal Chalcogenides And Their Heterojunctions Based On The First-principles Calculations

Recently,two-dimensional(2D)transition metal chalcogenides(TMDs)have received extensive attention due to their peculiar crystal structures and good properties.Different bonding modes and configurations of transition group metal atoms and chalcogenide atoms in 2D TMDs will form different crystal phases,resulting in TMDs crystals with rich electronic structures.Furthermore,the weak van der Waals(vd W)interaction between the two layers of 2D TMDs enables the vertical stacking of different2D materials to form vd W heterostructures.Due to relatively weak interlayer coupling,vd W heterostructures can combine two or more different layered materials.The vd W heterostructures can retain most of the intrinsic properties from the components,with more novel properties and potential applications.Layered MoS₂ is a typical 2D TMDs material.It has stable 2H phase,as well as metastable metallic phase(T phase)and semi-metallic phase(T'phase).Not only that,2D MoS₂ can be modified by doping,electric field,stress and stacking with different substrates.It can be used as a unique functional device in electronics,photonics,spintronics,Silicon Valley electronics,quantum computing and other fields.This work is based on the two basic points of MoS₂ with different crystal phases and the weak van der Waals interaction between its layers.A vertical van der Waals semi-metal/semiconductor junction made of the semiconductor(2H)phase and the semi-metal(1T')phase of MoS₂ was designed.Based on density functional theory,the geometric structures of the two vertical stacks are predicted and their properties are systematically investigated.We find that 1T'/2H heterobilayer presents a type-I energy band arrangement,opening a direct bandgap of 31 me V under the spin-orbit coupling.Moreover,the heterobilayer combines the properties of the H-phase and T'-phase MoS₂,and its elastic constant and effective mass exhibit isotropy,while the size of the effective mass is similar to T'-phase.The carrier mobility can be on the order of 10~4 cm~2·V^-1·s^-1.Further,we also investigated the electronic properties changes of the 1T'/2H MoS₂heterobilayer under a vertical electric field.It is found that the band gap of the heterobilayer can be tuned by the direction of the electric field,and the type of band alignment can also be tuned from I-type to III-type by the electric field.On the other hand,the electric field can tune the energy gap size to vary in the range of 0-37 me V.Finally,the electrical properties of the 1T'/2H MoS₂ heterojunction under stress regulation are also investigated.It is found that stress can make the band gap of the heterojunction switch between direct and indirect band gap,the electrical properties change from semiconductor to metallic phase,and the band gap changes in the range of 0 to 0.2 e V.The work in this paper provides a theoretical basis for electronic devices based on van der Waals semimetal/semiconductor junctions,and provides theoretical guidance for low-dimensional logic devices and field-effect transistors based on 1T'/2H heterobilayer.