First-Principles Study On Physical Properties Of Two-Dimensional Hexagonal Boron Nitride And Heterostructures

In the past two decades,the discovery of graphene and the subsequent development of other graphene like layered materials together constitute the history of 2D material research.The electrical,magnetic and photoelectric properties of 2D materials have been widely studied.Modern electronic technology is developing rapidly towards the trend of smaller,faster and cheaper.According to Moore’s law,many 2D materials are considered as candidate materials that are expected to break through the theoretical limit of silicon transistors.At the same time,the research of 2D materials is also accompanied by great challenges,such as defect engineering in large-scale 2D layered sample synthesis.Structural defects have a decisive impact on the electronic properties of 2D materials.The study of the influence mechanism of defects on the physical properties of 2D materials and their heterostructs is helpful to develop specific defect engineering to accurately control the electronic properties of 2D materials.In this paper,the effects of point defects on the geometric and electronic structure of hBN monolayer and the single photon emission produced by local defect states are investigated.At the same time,the point defects in the heterostructure interface composed of hBN and TMDCs and their effects on electronic structure,charge transport and photocatalytic properties are discussed.The main research contents are summarized as follows:1.By constructing vacancy defects（such as single boron atom vacancy,V_B）in hexagonal boron nitride monolayer and substituting carbon atoms for nitrogen atoms（CN）3VB.The geometric,electronic and optical properties of defects are systematically investigated using first-principles calculations.The results show that the introduced local defect state can regulate the electronic structure of hexagonal boron nitride monolayer.Firstly,the configuration of V_B defect and the atomic bond length around it are very sensitive to the charged state of the defect itself,and the highest negative charged state depends on the electronic state at the minimum of the conduction band.Secondly,（CN）3VB defects can change from symmetrical metastable state to asymmetric ground state structure through atomic structure relaxation.During the relaxation process,the defect energy levels are divided interactively,resulting in some defects suspended by defectsσlocal defect states contributed by bonds and reconstructedπbonds.Finally,vacancy and carbon doping defects significantly improve the absorption intensity of hBN to visible light,and there is the possibility of internal optical transition.There is an internal transition of visible light with an energy threshold of about 2.58 e V in（CN）3VB.2.Two dimensional hexagonal boron nitride and TMDCs（MoS₂,MoSe₂,WS₂,and WSe₂）are combined to form heterostructure hBN/MX₂（M=Mo,W,and X=S,Se）.The Vacancy defects and elements doping defect exist in hBN.The effects of interface defects on the electronic structure,interfacial charge transfer and photocatalytic properties of hBN/MX₂（M=Mo,W,and X=S,Se）were studied by using the first principle.The results show that after introducing vacancy and element doping,the n-type doped hBN/MX₂（M=Mo,W,and X=S,Se）heterostructure maintains the I-type band arrangement,while the p-type doping changes the I-type band arrangement into the II type band arrangement.Compared with the intrinsic hBN/MX₂（M=Mo,W,and X=S,Se）,n-type doping greatly enhances the interface interaction of heterojunction.Through defect engineering technology,two Z-type photocatalytic reactions with opposite transfer directions of photogenerated carriers are realized,which can effectively separate and transmit electron hole pairs generated by light,and improve the stability of photocatalytic system.