Electronic Properties Of Symmetry-breaking Two Dimensional Materials

The existence of quantum confinement effect results in more novel physical properties of low-dimensional materials,which provides unprecedented opportunities and realization ways for our continuous pursuit of device miniaturization,integration and low power consumption.However,researching on 2D materials is in full swing.Up to now,people’s work center mainly focuses on the exploration and utilization of the intrinsic degrees of freedom of materials,which mainly include electron,spin and valley degrees of freedom.As a new star after the electron and spin degrees of freedom,the valley degree of freedom leads to the emergence of the valley Hall effect and anomalous valley hall effect,which provides a new opportunity for the realization of high-speed,high-fidelity and low-power electronic devices.Therefore,2D materials valleytronics research came into being.In addition,considering that the appearance of the interface may have a significant impact on the material properties,the study of the interface properties of low-dimensional systems is also crucial.The construction of heterostructures,as one of the common means to introduce interfaces into systems,is favored by both theoretical and experimental researchers.Therefore,it is also crucial to theoretically explain the influence of heterointerface on the electronic properties and energy valley properties of the system.In this thesis,we investigate the electronic properties and valley electronic properties of novel 2D systems.We design 2D materials and predict the valleytronic properties in them,study the influence of interfacial pattern on the electronic properties of 2D heterostructures,as well as explore the effect of layer degrees of freedom on energy valley properties in vdW heterostructures composed by 2D Janus materials.We deeply analyze the physical origins of various novel properties by introducing physical models,which provide theoretical guidance for the experimentally verify and use of these physical properties.This thesis is divided into five chapters,arrangement is as follows.In Chapter 1,we first introduce the development history of valleyelectronics in 2D materials and the problems to be solved.Next,the important role of 2D heterostructures in material modification and the influence of 2D polar materials constituting heterostructures on the electronic and valley properties of the system are summarized.In Chapter 2,we first briefly introduce the first-principles calculation method,including how to use the density function theory to describe the electronic ground state properties and use the many-body perturbation theory to obtain the excited state properties.Next,the origin of the Berry phase/curvature and the construction of the maximum localized Wannier function are introduced,and finally the software packages used in this work are introduced.In Chapter 3,we delve into the valley electronic properties of novel 2D monolayer materials,and explain various novel properties through tight-binding models.In Chapter 4,we further investigate the effect of material polarity on the electronic properties of heterostructures and discuss the advantages of Janus TMDCs for realizing ohmic contacts and tuning valley excited states in heterostructures.Chapter 5 summarizes the full text and gives an outlook on future research work.The specific contents of this thesis are as follows:(1)2D monolayer energy valley material with out-of-plane polarity is designed with significant Berry curvature and Rashba effect,and it is verified that biaxial strain can regulate the Berry curvature.(2)The tight-binding model is used to verify that 2D materials with px,y orbital active hexagonal lattices have the potential to be ideal energy valley condidates,and we predict 2D BiSbC3 as a practical material to realize this guess based on the principle of orbital filtering.(3)We propose a valley-contrasted sublattice polarization mechanism suitable for 2D tetragonal lattices by using the tight-binding model and symmetry analysis,and predict that crystal symmetry can control the valley polarization of 2D LaOBiS2.(4)We deeply investigate the changes in the electronic properties of the nanotubes which rolled from lateral heterostructures,due to the in-plane polarization discontinuities of the components.(5)By constructing the vdW heterostructure and studying its electronic properties,we found that the functionalized MXenes are ideal contact electrodes for Janus MoSSe channel materials.(6)In the heterostructures composed by Janus TMDCs,we explore the effects of out-of-plane mirror symmetry breaking,interface stacking and alignment on the electronic and valley electronic properties of the system.(7)The effects of out-of-plane polarity and interface alignment on the interlayer coupling of heterostructures which composed by Janus TMDCs are discussed in detail,and the possible formation methods and types of interlayer excitons are predicted.