MX₂ Junctions （M=Mo,W;X=S,Se）:A Theoretical Study Of Their Classification,Structure And Property

By designing the inherent structure of transition metal dichalcogenide（TMDCs）nanosheets at the atomic scale,various TMDCs nanostructures with novel structural characteristics and unique electronic properties have been prepared.Construction of heterostructures is an effective means to improve the photoelectric characteristics of TMDCs nanostructures.In this thesis,the classification,stability and electronic properties of MX₂ junctions（M=Mo,W;X=S,Se）are studied by the first principle method based on density functional theory.The main research results of this thesis are as follows:Firstly,we propose the basic concepts of junction composed of different materials and different crystal phases,junction composed of different materials and the same crystal phase and junction composed of the same material and different crystal phases and focus on the structural characteristics,energetic stability and electronic property of1H,1T,1T′MX₂-M′X′₂ heterojunctions.Our calculation shows that the structural stability of 1H MX₂-M′X′₂ heterojunctions is higher than that of 1T and 1T’heterojunctions.The interlayer distance of 1H MX₂/M′X′₂ heterojunctions is smaller than that of 1T and 1T’heterojunctions,indicating that there is a strong interaction between the components of the former.We find that MX₂junctions with type-I band alignment can be used as a potential infrared laser emitting device and MX₂ junctions with type-II band alignment can be used as a good hydrogen evolution catalyst.MX₂junctions with metal characteristics show good electron transport characteristics,which can be used as potential low contact resistance material.Secondly,we construct（WS₂-Mo S₂）/Mo S₂ and WS₂/（WS₂-Mo S₂）/WS₂heterostructures and study their band alignments and photocatalytic mechanisms.The electrostatic potential analysis shows that there is synergistic effect and built-in electric field at the interface of the two heterostructures,which will affect the migration of photoinduced carrier.The results show that WS₂/(WS₂-Mo S₂)/WS₂ heterostructures has type-II band alignment and is a potential catalyst for hydrogen evolution.（WS₂-Mo S₂）/Mo S₂ heterostructure is type-I conductor,which can be used as candidate materials for infrared light emitting devices.Finally,we propose the 1H-1T’Mo S₂ heterophase junction with abrupt and gradual interface and investigate the influence of different interface structures on the geometric and electronic property of 1H-1T’Mo S₂ heterophase junction.We find that the maximum out-of-plane bending takes place at all interfaces.1H-and 1T’-phases still retain their respective electronic characteristics,although strong chemical bonds are formed at the interface of the abrupt interface model.For the gradient interface model,the S-vacancy defect at interface can not only reduce the magnitude of out-of-plane deformation of 1H-1T’Mo S₂ heterophase junction,but also increase the reactivity of1H-1T’Mo S₂ heterophase junction.The electronic state of the interfacial atoms are the key factor affecting the electronic properties of 1H-1T’Mo S₂ heterophase junction.