In this talk, I will introduce our recent progress on tuning the surface states of ultra-thin topological insulator films. The presentation includes two parts. (i) Using first-principles methods, we explain the puzzling band-topology difference between Sb2Se3 and Bi2Se3 and propose an approach to tuning the topological phase by strain . We demonstrate that Sb2Se3 can be converted into a topological insulator by applying compressive strain while the tensile strain can turn Bi2Se3 into a normal insulator. I will also show that the separation distance between quintuple layers (QL) in ultra-thin Bi2Se3 and Bi2Te3 films have a large increase after relaxation, leading to gap-opening at the surface Dirac cone, in good agreement with the experimental observation . I will further show that Pb adlayers on Bi2Se3 result in splitting of the Dirac cones and large Rashba spin splitting of the quantum well states . Most importantly, the quantum size effect of Pb adlayers leads to an oscillatory behavior of the Rashba splitting. (ii) Combining vapor-phase deposition method, Kelvin probe force microscopy, and first-principles calculations, we find that high-quality ultra-thin Bi2Se3, Bi2Te3, Bi2 (SexTe1-x)3, and Sb2Te3 nanoplates with triangular, truncated triangular, hexagonal and circular shapes can be grown on different substrates such as n-type and p-type Si (111), HOPG, SiO2, and MoS2 [4-10]. Various spiral structures can be also formed with the step height of one QL and large step width up to micrometers , providing an ideal platform to investigate the scattering of surface states off the spiral steps. The work functions and Fermi-level positions of the nanoplates can be tuned by the substrates via interfacial charge exchange [5-10].
1. W. L. Liu et al., Phys. Rev. B 84 , 245105 (2011).
2. W. L. Liu et al., Phys. Rev. B 87, 205315 (2013).
3. H. Yang et al., Phys. Rev. B 86, 155317 (2012).
4. G. L. Hao et al., Appl. Phys. Lett. 102, 013105 (2013).
5. G. L. Hao et al., Sol. Stat. Comm. 152, 2027 (2012).
6. G. L. Hao et al., J. Appl. Phys. 113, 024306 (2013).
7. G. L. Hao et al., RSC Advances 2, 10694 (2012).
8. G. L. Hao et al., AIP Adv. 2, 012114 (2012).
9. G. L. Hao et al., Sci. Adv. Mater. 4, 1001 (2012).
10. G. L. Hao et al., Sci. Adv. Mater. 6, 383 (2014).