| The successful exfoliation of graphene has opened up a research upsurge in twodimensional materials.With the deepening of research for 2D materials,the types of 2D materials are increasing.A large number of 2D materials such as silicene,germanene,black phosphorus,transition metal chalcogenides,and hexagonal boron nitride have been theoretically predicted or even successfully synthesized.Among them,TMDCs not only have various structures such as 2H,1T and 1T’,but also have rich physical properties such as semiconductors,metals and superconductors.Various defects inevitably exist in monolayer 2H TMDCs,which will have certain influence on their structure and electronic properties.Manipulating these structural defects and exploiting their unique electronic properties can further broaden the application of TMDCs.In addition,the valley electron properties of TMDCs have also attracted extensive attention.The prediction and study of new transition metal twodimensional materials with valley electron properties will help to develop valley electrons with low power consumption,long transmission distance and high stability valley electronic devices.In this thesis,using first-principles calculations,we systematically investigate onedimensional metal quantum wires in TMDCs,the structural and electronic properties of defects in Janus MoSSe,and the electronic properties of monolayer and van der Waals bilayer TiClI.Chapter 1 outlines the development of 2D materials and the properties and research progress of two typical 2D TMDCs;Chapter 2 briefly introduces the theoretical knowledge and software of first-principles calculations;Chapter 3 studies the structures and electronic properties of 60°GBs in transition metal dichalcogenides;Chapter 4 predicts the possible defect species in Janus MoSSe,the stability of these defects and their influence on their electronic properties;Chapter 5 studies the electronic properties of monolayer and van der Waals bilayer TiClI and its control methods of valleytronics properties;Chapter 6 briefly summarizes the main research contents and innovations of this thesis,and provides the outlook for further research on transition metal 2D materials.The main research contents and conclusions of the thesis are as follows:(1)Defects usually adversely affect material properties,however large-angle(60°)GBs offer the possibility to engineer 2D TMDCs properties and form intrinsic heterojunctions in monolayer TMDCs.For 60° GBs in TMDCs,the bulk polarization of TMDCs is non-zero,and the polarization vector is reversed along the Armchair direction on both sides of the grain boundary,thus a polarization discontinuity appears at the GBs.To shield the polarized charges that generate the polarizing electric field,free electrons or holes(depending on the bonding at the GBs)are accumulated on the GBs,resulting in a dispersive state with metallic properties.In particular,the 60° GBs in 2D TMDCs behave as one-dimensional metallic quantum wires,which can be used as 1D channels for free carrier or exciton transfer.(2)2D TMDCs hold great promise in electronics and optoelectronics due to their novel electronic and optical properties.In TMDCs,structural defects are inevitable and might paly decisive role in device performance.In this work,point defects,line vacancies and 60° GBs are explored in 2D Janus MoSSe,a new member to the family of TMDCs,by means of the firstprinciples calculations.S and Se vacancies are found to be the most favorable point defects,and they tend to aggregate along the zigzag direction to form line vacancies.Comparing with isolated point defects,line vacancies induced in-gap states are more dispersive.In particular,60° GBs behave as one-dimensional metallic quantum wires,as a consequence of the polar discontinuity.Thus,effectively controlling the formation of defects at nanoscale brings new electronic characteristics,providing new opportunities to broaden the applications of 2D TMDCs.(3)The novel electronic properties of Janus TiClI monolayer and vdW bilayers have been demonstrated.Monolayer TiClI has strong SOC together with the inversion symmetry breaking,so a valley spin splitting of 62.67 meV appears at the K/K’ point.In monolayer TiClI doped with V and Cr magnetic atoms,valley polarizations of 36.70 and 45.35 meV are generated,respectively.The vdW bilayer TiClI has a typical type Ⅱ band arrangement with a considerable band offset.In addition,the interlayer polarization of bilayer TiClI is almost 100%,and the interlayer distance has little effect on the interlayer polarization.In this case,the interlayer valley excitons and valley polarization lifetimes can be extended,thus providing new research opportunities for light energy conversion and valley electronics.As the interlayer distance of vdW bilayer TiClI decreases,AB’ and AB stacked bilayer TiClI exhibit a transition from semiconductor to metal and behave as hole doping. |
