Design And Research On Novel Two Dimensional Topological Insulators Of Bi-base

Topological insulator is a new kind of exotic quantum state which possesses insulating bulk bands and nondispersive spin resolved edge states protected by the time reversal symmetry. Its various kinds of exotic characters can be widely used in spintronic devices, quantum calculation and energy conservation. Three dimensional topological insulator (3D TI) has been studied a lot but it is hindered when put into use due to several critical shortcomings:First,3D TI has numerous defects due to the size of the bulk and thus it is very difficult to keep the surface states from being annihilated in the bulk energy bands. Second, it is very difficult to modify the electronic property using electronic and magnetic field due to the extremely small penetrating depth. Although its electronic structure can be modified by atom doping, this method can also lower the fermi velocity on the edge states and material quality. Two dimensional topological insulator (2D TI), on the other hand, has many advantages over 3D TI. For example, the large bulk and surface ratio could be effectively reduced, the charge carrier density contributed from the bulk part, and a small amount of atom doping will modify the electronic structure in an appropriate manner. Also, the electronic and magnetic field can be used to change the electronic structure greatly due to the extremely small size in the limited direction, and it is very promising in the integrating electronic device. All these arguments illustrate that researches for two dimensional topological insulator are extremely necessary for the developing of the electronic device and for the further knowledge of the low dimensional nanoscale materials.Recently,2D TI has attracted much attention both in experimental and theoretical field. Until now, large numbers of 2D TIs have been proposed theoretically, examples including the quantum well structures represented by HgTe/CdTe and InAs/GaSb, the hexagonal honeycomb-like Bi(111) monolayers, III-V compound monolayers, Methyl-Functionalized Bi, Pb and Sb monolayers, two dimensional topological crystalline insulator (TC1) represented by the typical SnTe, exotic TIS and TISe which can be transferred between TCI and TI under the variation of external strain. Recently, the marking nontrivial topological phase has been found in the two-dimensional transition metal dichalcogenides (TMDs), for example, the X2CO2 (X=Cr,Mo,W), and transition metal dichalcogenides monolayers and their allotropes and derivatives Also, the quasi-two dimensional topological insulator has been found in the film structure of Bi2Se3. Recent researches have shown that the electronic band structure and topological character of Bi2Se3 film change a lot when the thickness is limited to several nanometers, which is quite different from the HgTe/CdTe quantum well. However, the vertical heterojunction is limited because it works via the van der waals force. Recently, lateral heterojunction has been proposed and realized in two dimensional materials. Different from vertical heterojunction, it works via the valence bond between the within the interface, which can change the character of the materials near the boundary dramatically. Using first principle, based on the previous researches, we proposed two kinds of two dimensional topological insulators, we also construct an exotic lateral heterojunction and the novel O bridge in the nanoribbon.This thesis is divided into five chapters: In chapter 1, a brief introduction to the mechanism of the topological insulator is presented. The research background and the problems encountered in research of 2D topological insulators are also given. In chapter 2, basic concept of density functional theory (DFT) is introduced. We also give the basis set of plane waves, pseudo-potential and the framework of the exchange correlation functionals. The general gradient approximation (GGA) is also introduced in this chapter.In chapter 3, using first-principles, we illustrate that GaBi3 and InBi3 monolayers are 2D topological insulators with bulk energy gaps of about 283meV and 247meV, respectively. As an example, GaBi3 shows that bulk effect and quantum confinement effect greatly influence the edge states. After a detailed research for the bulk energy bands and edge states with the external strain applied, we illustrate that while this kind structure has a robust topological insulating character against external strain, the strength of its topological character is dramatically changed, and this leads to appreciable variation to the edges states. The result shows that although compression does not threat the topological character of the material, it surely decelerates the electrons near the Fermi level, while we can get a much higher speed of these electrons if the tension is applied appropriately. All these come to the conclusion that these nested hexagonal structures are promising candidates for high-speed spintronics devices.In chapter 4, we proposed T-Ga2Bi2 monolayer which can be synthesized by two ordinary GaBi monolayers. The large range imaging frequency is eliminated in the T-Ga2Bi2 structure. Z2 invariance illustrates that T-Ga2Bi2 is a nontrivial topological insulator. The SOC induced band gap at Г point is 1.477 eV, and it is the largest gap at Г point up to now, which means that this kind of T-Ga2Bi2 structure has extremely large SOC strength. The zigzag nanoribbon and armchair nanoribbon are constructed to give a detailed research about the edge states. The results suggest that armchair edge states are more dispersive that zigzag edge states. To explore the topological strength of T-Ga2Bi2, nanoribbons of different width are constructed. The corresponding energy bands illustrate that the linear nondispersive edge states still remain when the width is limited to 1.7 nm, which corresponding to the extremely large topological SOC strength. To give a deep insight into the edge states, an exotic’atom bridge’ consist of O atoms was proposed. The symmetry and anti-symmetry types display totally different kinds of’bridge states’due to the interaction between the electrons via the O atom bridge. Finally, a novel two dimensional lateral heterojunction is constructed. The corresponding band structure and the electronic density distribution in real space indicate that topological phase transition takes place in the trivial T-Ga2Sb2. All these research suggest that T-Ga2Bi2 is a promising candidate for spintronic device.In chapter 5, the present works are summarized and a preview of the future research is drawn.