Study On The Growth, Structure And Doping Mechanism Of Bi-based Topological Insulators

Topological insulators (TIs), a new type of quantum matter insulated in the interior but conductive on the surface, have recently become an important topic in condensed-matter physics and material science. Especially, Bi-based chalcogenides as typical three dimensional TIs attract burgeoning attentions due to simple crystalline/electronic structure and fixed stoichiometry ratio. The unique properties of TIs make them one of the most promising candidates in fabrication of dissipationless devices and quantum computers and searching for exotic quasiparticle states. However, TI materials share a common problem that the bulk contribution usually overwhelms the surface states because of the presence of intrinsic or extrinsic defects. How to improve the quality of TI materials and understand the defects as well as the element doping mechanisms are the key challenges for the development of TIs.In this work, TI materials exemplified by nanoplates and films were grown by chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) methods. By the state-of-the-art transmission electron microscopy (TEM), high angle annular dark field (HAADF) and energy dispersive X-ray spectroscopy (EDS) techniques, we studied the nanostructures, defects and doping mechanisms of Bi-based TIs at the atomic scale, and established the relationship between defects and physical properties. The main work and achievements are listed as follows.(1) Well-aligned vertical Bi-based TI nanoplates were obtained. In our work, Bi-based TI nanostructures (nanoplates, nanowires and films) were synthesized by solvothermal synthesis, CVD and MBE methods. By controlling the growth conditions in CVD, we successfully synthesized TI nanoplates grown on/out-of the substrates. TEM analysis demonstrated Bi2O3 buffer layer and atom diffusion rate on the substrate surface determine the vertical growth. Furthermore, the vertical nanoplates tend to align in one dimensional array owing to the interface stress.(2) Frustum configurations of Bi-based TI nanopltates were directly revealed. The lateral surface of Bi2Te3 nanoplates were cross-sectionally fabricated to be observed by HAADF and EDS. In stark contrast to the general belief, the TI nanoplates are not triangular prisms with (100)/(110) as the lateral surface. They are mainly enclosed by the (001) and (01-4) facets, instead. First principles calculations found the oxygen adsorption energy:namely,0.4,0.9,2.1,1.5, and 3.5 eV per atom, respectively, for the (001), (015), (100), (01-4)-Te, and (01-4)-Bi surfaces. Hence, the observed faceting of the lateral surfaces was attributed to surface adsorption kinetics, rather than energetics in the equilibrium.(3) A useful method was developed to prepare TEM specimen with specific orientations. We fabricated TEM foils with a specific orientation of<100> (Bi2Te3) combining electron backscattering diffraction (EBSD) and FIB methods to secure a flat foil almost on the Bi2Te3<100> zone axis and ensure a maximum X-ray counts in TEM. With the state-of-the-art Super-X technique, atom-resolved EDS map was achieved to identify chemical information of Bi-based TIs.(4) Key structural puzzless in binary Bi-based TIs were unveiled. By the chemical identification at the atomic scale, a long neglected 7-layer lamella defect Bi3Te4 in Bi2Te3 was revealed with an atomic arrangement of Te-Bi-Te-Bi-Te-Bi-Te, which shows n-type conductivity but acts as acceptor for the whole film. Also, we studied the twin boundary in Bi2Te3, which was demonstrated to be located at the outer Te layer with a larger van der Waals gap.(5) Doping mechanism of Bi-based TIs were revealed by ChemiSTEM. Bi2Te3/Bi2Se3 doped by Se, Sb, Cr, Te, Fe and Ca were synthesized to tailor the properties of topological insulators. We demonstrated the central-layer substitution rule only applies to Se doped Bi2Te3, but not to Te doped Bi2Se3. Bond energies and formation energies by the first principle calculations supported the atom configuration. Also, we successfully manipulated the interior carriers by doping Sb with different concentrations, and induced magnetic properties by doping Cr and Fe into Bi2Te3. All the properties were demonstated to stem from X-on-Bi (X=Sb, Cr, Fe and Ca) defects.Our work on the growth, surface structure, intrinsic/extrinsic defects and the transport properties of Bi-based TIs may pave the way to more thoroughly understand and tailor the nature of the bulk by element dopants, and promote the topological insulator applications in spintronics and quantum computing.