Study On The First Principle Of Electronic Structure Of Nano - Topology Insulator (01ī5)

Recently, the study of topological insulators has attracted great attention in condensed-matter physics and material science. Topological insulators are a new quantum matter, which is not similar to the conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk and gapless edge or surface states in surface protected by time-reversal symmetry. These topological materials have been theoretically predicted and experimentally observed in a variety of material systems, including BiSb alloys, Bi2Te3 and Bi2Se3 crystals. The theoretical works based on density functional theory or tight-binding band calculations, focused on elucidating the structure-property relations, predicting new topological insulators, and mapping the band structures.A comprehensive theoretical study on the thin-film structures of these topological insulators is necessary to help better understand their properties and to help design future experiments to verify their effects.In this thesis, we use two different optimal methods to study the structures and properties of the close-packed planes{0115}H of Bi2Te3, Sb2Te3 and Bi2Te2Se nanoribbons with density functional theory in chapters three and four. Our studies will understand the topological surface states comprehensively. The contents of our works are as follows:(1) We study the electronic structures of close-packed planes (0115) thin films of Bi2Te3, Sb2Te3 and Bi2Te2Se employing spin-orbit coupling (SOC) by MedeA-VASP of first-principle with density functional theory. We chose to relax only the internal parameters, namely the atomic positions, keeping the lattice constants fixed. Such an approach has already been used previously. It was found that the gap width tend to decrease with increasing layer of the Bi2Te3 (0115) film at G point. The metallic state emerges when the layer of the Bi2Te3 (0115) film increases to five. The similar characters of the electronic structures were found in Sb2Te3 and Bi2Te2Se (0115) with 1-5 atom layers films. The energy gap at G point decreases to the magnitude of meV when the layer of the Bi2Te3 (0115) film increases to six. Therefore, we can predict the energy gap at G point will disappear with the increase in thickness of Bi2Te3, and Bi2Te2Se (0 1 1 5) films, which is consistent with Bi2Te3 (0 0 0 1) film.(2) We present the experiment of Bi2Te3 slab synthesized with hydrothermal method and electronic structure of the{0 1 1 5}H nanoribbons with first principles calculations. A lack of reconstruction or only relax the atomic positions agrees with the films on substrates due to the clamping effect, but our nanoribbons were grown in a free environment without substrates and the distance between atoms maybe change, resulting in a low symmetry. Therefore, we performed reconstruction to optimize the unit cell and then to fully relax atomic positions for our nanoribbons. From the experiment, we find that the interplanar spacings of Bi2Te3{0 1 1 5}H planes among these equivalent planes have a little change. By the theoretical calculation, the lattice parameter change will lead to variation of interplanar spacings and we confirm the experiment. Then we calculate the electronic properties and energy. We find that the electronic structures and the binding energy gets closer to the bulk with the increase of layers. The similar characteristic of the electronic structures and energy were found in (0 1 1 5) nanoribbons.