Modulation On Surface States Of Topological Insulators: First-principles Calculations

In this thesis, we have investigated theoretically the properties of the surface states oftopological insulators（TI） by using first-principles calculations based on the densityfunctional theory, which will provide a guidance for designing tunable TI-basednano-devices. The main results are summarized as follows:1. Strain induced topological phase transition: Based on first-principles calculations, westudy the dependence of topological phase on anisotropic interactions in Bi2Se3-typematerials. By applying different strains in order to vary interactions, we reveal that thetopological phase is insensitive to lateral interaction but can be effectively tuned bylongitudinal interaction. Longitudinal strain is inhomogeneous in the studied systems. Theinterquintuple interaction plays a dominant role in determining the topological phase. Theinter-quintuple separation in Sb2Se3is larger than that of Bi2Se3so that the spin-orbitcoupling of the former is weaker than the latter. Therefore Sb2Se3is a normal insulatorwhile Bi2Se3is a topological insulator. We explain the puzzling band-topology differencebetween Sb2Se3and Bi2Se3and propose an approach to tuning the topological phase bystrain. It is found that Sb2Se3can be converted into a topological insulator by applyingcompressive longitudinal strain to reduce the inter-quintuple separation and increase thespin-orbit coupling. A tensile strain will have an opposite tuning effect on theinter-quintuple separation and spin-orbit coupling and turn Bi2Se3into a normal insulator.We have studied thin films of Sb2Se3and Bi2Se3and also observed a strain-inducedtopological phase transition.2. The effects of the surface and interface on the TI films: Based on van der Waalsdensity functional calculations, we have studied few-quintuple-layer （QL） films of Bi2Se3and Bi₂Te₃. The inter-QL separation near the surface is found to have an up to about20%increase, while the inner QL separation is smaller and approaches the bulk value as thethickness grows. Accordingly, the surface Dirac cone of Bi2Se3film is evidently gappedfor small thickness （2⁴QLs） and the gap is reduced and finally closed with the increasingthickness, agreeing well with the experiments. We further studied the substrate effect byinvestigating the Bi2Se3/graphene system. It is found that the underlying graphene inducesgiant thickness-dependent Rashba splitting and Dirac point shift with respect to the Fermi level. Because Bi₂Te₃films have smaller relative inter-QL expansion and strongerspin-orbit coupling, the gapless topological surface states emerge in the film as thin as2QL, in good accord with the experiments.3. The tuning of the surface and interface states by semiconductor substrate: We haveinvestigated the heterostructures of Bi₂Te₃/Si and Bi₂Se₃/GaAs by first-principlescalculations. It is found that the semiconductor substrate plays an important role in tuningthe Fermi level with respect to the Dirac point of topological insulators （TIs）. Theworkfunctions of Bi₂Te₃and Bi₂Se₃are larger than that of the semiconductor substrate,leading to charge transfer from the substrate to TI and the upward Fermi level shift, whichis proportional to the difference of the workfunctions of the two interfaced materials.Thicker TI films have larger density of the states and hence the charge transferred to TIwill give rise to a smaller Fermi level shift. We also studied Bi₂Se₃on polarsemiconductor GaAs. It is found that different termination of GaAs at the interfacecorresponds to different work functions and will result in different Fermi level shift inBi₂Se₃.