Theoretical Study On Electronic Structures Of Two-Dimensional Topological Insulators

Topological insulators (TIs) are newly discovered quantum state of matter in recent years, which are of much significance in the applications for spintronics and quantum computations, due to their novel physical properties. The bulk states in TIs are insulating, while the surface states or edge states are gapless conductor for three-dimentional (3D) and two-dimentional (2D) TIs, respectively. Such surface or edge states are protected by time-reversal symmetry against backscattering, which are quite promising for the realization of electronic devices with low-energy comsumption.Up to date, lots of TI materials have been theoretically predicted as topological insulators and some have been experimentally demonstrated. Among these TIs, HgTe/CdTe quantum wells and InAs/GaSb quantum wells are verified to be 2D TIs, namely quantum spin Hall (QSH) insulators, and the 3D TIs are realized in Bi1-xSbx, Bi2Se3, Bi2Te3 and Sb2Te3. Nevertheless, it still has a long way to achieve TI-based spintronics devices. One of the key factors that impede practical application of TIs is small bulk band gap, which limits the experiment must be operated at extremely low temperature. In addition, if the band gap is too small, the defects caused during the process of preparation and the thermal excitation under finite temperature will result in bulk charge carriers, which will interfere with the measurement and utilization of metallic states on the surface and edge. Therefore, searching for TI materials that are of large band gap has been a striving direction. Generally, spin-orbital coupling (SOC) plays a critical role in the bulk band gap of topological insulator, so increasing researches are mainly focused upon the systems that are composed of heavy elements with strong SOC, such as Bi, Sb and Sn elements.In present dissertation, based on density functional theory we propose several new TI materials with sizable bulk energy gap, and systematically investigate the modulation of their topological properties via strain and chemical adsorption. The dissertation is divided into seven chapters. In the first chapter, we give an introduction to the related theoretical and experimental studies on the TIs. The second chapter will introduce the density functional theory and give a brief description for software packages based on first-principle calculation. In the third chapter, electronic structures and topological properties of Bi(111) ultrathin films are studied, and then topological states modulation of Bi thin films by chemical adsorption are investigated. The fourth chapter is devoted to topological nontrivial properties of chemically functionalized arsenene films, and their robustness under strain. In the fifth chapter, the effect of substrate and external strain on electronic structures and topological properties of stanene film are studied. The sixth chapter will focus on the strain induced band inversion and topological phase transition in methyl-decorated stanene film, and proper substrate to realize QSH effect. The seventh chapter will investigate the effect of-SiH3 group on the electronic properties of two-dimensional materials composed of IV and V group elements. Finally, a summary of the constents and some outlook for TIs are presented. The main constents and conclusions are given as follows:(1) Based on the first-principles calculations, we systematically study the electronic structures and topological properties of bismuth (Bi) ultrathin films in (111) orientation. Considering the SOC, the Bi(111) films show the thickness dependent behavior of their band structures and topological properties. The single and two bilayers (BLs) Bi(111) films are confirmed to be nontrivial 2D TI system, which have large band gap of 0.5 eV, whereas for the films from three to five bilayers, a transition from semiconductor to semimetal is found. Moreover, under external electric field the topological properties of single bilayer Bi(111) thin film is very robust, but two bilayers film is not robust.In addition, we systematically investigated the topological surface states of Bi thin films of 1-5 bilayers in (111) orientation without and with H(F) adsorption, respectively. We find that compared with clean Bi films, a huge band gap advantageous to observe the QSH effect can be opened in chemically decorated single bilayer Bi films, where the band gaps are 1.08 eV and 1.04 eV for BiH and BiF, respectively. The band gap in 2 BL Bi film is enlarged by H adsorption, while the 3 BL Bi film is drived into QSH insulator by H adsorption with band gap of 0.33 eVo For 4 BL and 5 BL films, they still show metallic properties. For Bi films without and with chemical adsorption, the variation of electronic properties origins from different inter-BL coupling strength. Moreover, the orbital hybridization between adatoms and Bi atoms plays a significant role in the electronic properties.(2) Based on first-principles calculations, we propose one new category of 2D TIs in chemically functionalized (H,-CH3 and-OH) arsenene films. The results show that the pristine arsenene bilayer is normal insulator with trivial energy gap, while the surface decorated arsenene films are intrinsic 2D TIs with sizeable bulk gap. The bulk gaps are 0.236 eV,0.184 eV and 0.304 eV in AsH, AsCH3 and AsOH films, respectively. The nontrivial topology is further confirmed by nonzero Z2 topological invariant and the existence of helical edge states. Such helical edge states in these systems are desirable for dissipationless transport. Moreover, we find that the topological properties in these systems are robust against mechanical deformation by exerting biaxial strain.In addition, the topological properties of halogenated arsenene films are also investigated. We find that halogenated arsenene films including AsF, AsCl, AsBr and AsI films, are all quantum spin Hall insulators. The magnitude of the gap induced by effective SOC can reach 0.194 eV,0.232 eV,0.240 eV and 0.255 eV upon chemisorption of F, Cl, Br and I, respectively. Such large bulk gaps make them suitable to realize QSH effect in an experimentally accessible temperature regime. These novel 2D TIs with large bulk gaps are potential candidate in future electronic devices with ultralow dissipation.(3) From first-principles calculations, the effects of hBN and AlN substrates on topological nontrivial properties of stanene with different strains are studied. We find that QSH phase can be induced in stanene film on (?)×(?)h-BN substrate under tensile strain between 6.0% and 9.3% with stable state confirmed by phonon spectrum, while for (?)×(?) stanene on 5×5h-BN, QSH phase can be preserved without strain. However, for stanene on (?)÷(?) AlN substrate, QSH phase could not be found under compressive or tensile strain less than 10%, while for 2x2 stanene on 3×3 AlN, the needed compressive strain is just 2% to induce QSH phase, whose band gap is comparable to that of pure stanene. These theoretical results will be helpful to understand the effect of substrate and strain on stanene and further realize QSH effect in stanene on semiconductor substrate.(4) The researches for new QSH insulators with large bulk energy gap are of much significance for their practical applications at room temperature in electronic devices with low-energy consumption. By means of first-principles calculations, we proposed that methyl-decorated stanene (SnCH3) film can be tuned into QSH insulator under critical tensile strain of 6%. The nonzero topological invariant and helical edge states further confirm the nontrivial nature in stretched SnCH3 film. The topological phase transition originates from the s-pxy type band inversion at the Γ point with the strain increased. The SOC induces a large band gap of-0.24 eV, indicating that SnCH3 film under strain is a quite promising material to achieve QSH effect. The proper substrate,h-BN, finally is presented to support the SnCH3 film with nontrivial topology preserved.(5) By means of first-principles calculations the effect of-SiH3 adsorption on the electronic properties of stanene, arsenene, bismuthene and antimonene films is investigated. In the absence of spin-orbital coupling (SOC), SnSiH3 film is a semiconductor with the conduction bands near dominated by s orbitals and valence bands by pxy orbitals, whereas AsSiH3, SbSiH3 and BiSiH3 films are characterized by Dirac point at K point with the band near Fermi level are mainly contributed by pxy orbitals. When the SOC is included, SnSiH3 film is still a semiconductor with the band gap decreased, while AsSiH3, SbSiH3 and BiSiH3 films have relatively large band gap induced at K point, meaning that they may be quantum spin Hall insulators.The above research results reveal the effect of strain, electric field and substrates on the modulation of electronic structures and topological properties of TI materials, provide the effective ways to drive the topological phase transition in 2D films, and propose several 2D TIs with sizable band gap to realize QSH effect at room temperature. Our studies are not only helpful to deeply understand fundamental properties of TIs, but also provide the possibility of application in spintronics.