Research On Design And Topological Properties Of Two-dimensional Materials Based On ?A Family

Two-dimensional?2D?topological insulators?TIs?,also known as quantum spin Hall?QSH?effect insulators,have insulating gap and metal surface states with considering the spin-orbit coupling?SOC?.These materials expected to make low-dissipation transmission devices.In this paper,using first-principles calculations,we have studied the topological properties of chemical functional group VA 2D materials and group VA compounds.The main researchs are as follows:Based on first-principles calculations,in chapter 3,we predict that the antimony?bismuthene?modified with hydroxyl?sulfydryl?are TIs,denoted as Sb/BiXH?XH=OH,SH?.The calculated band gaps of SbOH,SbSH,BiOH and BiSH are 330 meV,400 meV,820 meV and 850 meV,respectively.A Tight-binding model with SOC is proposed to describe the electronic characteristics of Sb/BiXH.The orbital filtering mechanism can explain the topological properties of Sb/BiXH:the XH group and Sb/Bi atom have orbital hybridization,which makes the p_z orbital of antimony and bismuth near the Fermi surface to be removed,and the Sb/Bi-pxy orbital near the Fermi surface to be preserved.This factor reduces the trivial6-band lattice band structures to a non-trivial 4-band lattice band structures.This is the mechanism which hydroxyl and sulfydryl-modified antimony and bismuthene are 2D TIs.We verify the topological properties of Sb/BiXH by calculating and analyzing the topological invariant Z₂=1 and the gapless edge state in the bulk band gap.The Sb/BiXH films with QSH effect and large energy gap expected to be an ideal material for manufacturing quantum devices operating at room temperature.The preparation of 2D TIs requires the support of substrate materials,so it is of great significance to design 2D TIs on traditional semiconductor surfaces.In chapter 4,we designed a theoretical model of antimony and leadene with hexagonal structure on Ge?111?surface covered by 1/3 Cl atoms,which is denoted as Sb/Pb@Cl-Ge?111?.The results show that Sb/Pb atoms form a stable hexagonal honeycomb lattice structure on the surface of Ge?111?,and the material has topological properties.The calculated band gap of Sb@Cl-Ge?111?is 810 meV,and the band gap of Pb@Cl-Ge?111?is 270 meV.We explain the topological property mechanism with the substrate orbital filtering mechanism:Cl-Ge?111?substrate effectively removes the p_z orbital of Sb/Pb away from the Fermi level so that only the p_x and p_y orbitals of Sb/Pb are left at the Fermi level.This mechanism is the key to produce the topological properties of Sb/Pb@Cl-Ge?111?.The theoretical research results of Sb/Pb@Cl-Ge?111?provide a feasible method for designing 2D TIs on the surface of traditional semiconductors.This makes spintronics and quantum computing devices at room temperature to be possible.Affected by the complex chemical environment,the defects of the 2D TIs is unavoidable during the preparation process,and these defects will break symmetry of space inversion.In chapter 5,based on first-principles calculations,we study the fluorinated methyl functionalized bismuth film(Bi₂C₂H_6-xF_x)can realize the coexistence of QSH effect,valley Hall effect and spontaneous polarization.The Bi₂C₂H_6-xF_x film has a topological band gap of 1.08 eV,and the topological properties are robust to the adsorption ratio of fluorine atoms.Due to the broken symmetry of space inversion,the strongest out-of-plane polarization is up to 0.25×10^-10 C/m,and some Bi₂C₂H_6-xF_x configurations have energy band splitting at the K and K'points in the Brillouin zone,which produce additional valley freedom,resulting in the coexistence of quantum spin and valley Hall effect.Doping and chemical adsorption on the surface often make the material magnetism.In chapter 6,we designed a set of 2D X₃Y₂?X=Na and K,Y=As,Sb?films with quantum anomalous Hall?QAH?effect,and analyzed the magnetism and magnetic exchange mechanism of the materials.Using first-principles calculations,the calculated topological band gap of Na₃As₂ reaches 112.7 meV,and it reaches 196.5 meV for Na₃Sb₂.We have proved the QAH effect of X₃Y₂ by studying the Chen number?C=1?and chiral edge states.X₃Y₂ is an out-of-plane spin-polarized ferromagnetic material with a magnetic moment of 3.0?B.The p orbitals of X⁺atoms act as a bridge to realize the double exchange magnetic coupling mechanism between Y anions.We also constructed the heterostructure of the X₃Y₂ films placed on the H-Si?111?substrate.The study found that H-Si?111?is a good substrate to support the thin film without destroying its QAH effect.The X₃Y₂ lattice is a ferromagnetic material with QAH effect,which indicates that it has broad application prospects in spintronic devices.