Theoretical Studies Of Topological Electronic States And Electron Spin-polarization In Low Dimensional Materials

With the rapid development of spintronics, more and more attention was paid on the spin-orbit coupling (SOC) effect in materials, which can induce many intriguing phenomena, such as quantum spin Hall (QSH) effect, etc. These effects play important roles in spin-field-effect transistors and spin-quantum-computers. Without the presence of external magnetic field and magnetic ordering, through SOC effect, one can manipulate spins by a way of the full electrical solution, which provides a versatile theoretical platform for nanoelectronic device applications. Topological insulators (TIs), as a newly discovered quantum class of materials with topological order, have become an exciting subject of intensive research not only in fundamental and applied branches of condensed matter physics, but also in solid-state chemistry and materials science. TIs are the materials with a bulk band gap generated by strong SOC effect and topologically protected metallic surface or edge states. It is demonstrated that there are closely relationships between TIs, QSH effect and quantum abnormal Hall (QAH) effect, which will have wide applications in spintronics devices.Introducing electronic spin-polarization into nanomaterials is also an important issue in the field of spintronics. Compared with inorganic materials, organic nanomaterials can be synthesized and processed easily. More importantly, spintronics devices based on organic nanomaterials usually have good flexibility, excellent mechanical properties, and even relatively rich electrical, magnetic and optical properties. Additionally, spintronics devices with low energy consumption can greatly improve the speed of information processing and the density of information storage. So in order to meet the performance requirements of spintronics devices, inducing and controlling electronic spin-polarization of organic nanomaterials become highly desivable.In this thesis, two-dimension (2D) materials, such as gallium bismuthide, graphitic carbon nitride, boron nitride are considered. Based on first-principles calculations, we systematicly investigated the topological properties induced by SOC effect and the spin-polarization of p-orbital electrons. The main results are summarized as follows:(a) Using first-principles calculations, we demonstrate that 2D cubic-diamond-like (2DCD) GaBi bilayer with the surface atoms passivated by hydrogen atoms (HGaBi) is a stable 2D TI with the gapless edge states of the HGaBi nanoribbons. The topological aspects are mainly related to the px,y-orbital of the sp3-hybridized atoms in the lattice. The bulk band gap can be as large as 0.320 eV, which is implementable for achieving quantum-spin-Hall states at room temperature.(b) We performed first-principles calculations on the already-synthesized honeycomb lattice of s-triazines with a chemical formula of C6N6 (g-C6N6). We demonstrated that it has topologically nontrivial electronic states characterized a Ruby model. The SOC strength of px,y-orbitals is stronger than those in graphene and silicene. The band gaps due to SOC are 5.50 meV (K points) and 8.27 eV (Γ point), respectively, implying that the QSH effect could be achieved at a temperature lower than 95 K.(c) We proposed a new family of graphene-like carbon natrides with fractal frameworks (C4N3-H). We demonstrated that they have stable ferromagnetism accompanied by half-metallicity, which are highly dependent on the fractal orders. The ferromagnetism increases gradually with the increase of fractal order. The spin-polarized electronic states come mainly from the pz-orbitals of the carbon and nitrogen atoms, which can be described the Lieb’s theorem. The Curie temperature of these metal-free systems estimated from Monte Carlo simulations is considerably higher than room temperature (Tc-1105K). These carbon nitrides with fractal structure are quite promising for the applications in spintronic devices, and open an avenue for the design of d0 magnetic materials with exotic properties.(d) We demonstrated that the electron zero-energy states (ZESs) of BN/Graphene core-shell quantum dots (QTs) in triangular shapes can be well tuned by varying the size and topology of the graphene segment, independent of the BN segment. The net spin of the systems is dominated by the graphene segment, which can be described by a Lieb’s theorem:S=|NA-NB|/2, NA and NB being the number of atoms belonging to the two sublattices of bipartite lattices. We also proposed a π-electron Hubbard model within a mean-field approximation to deal with the electron spin-polarization of BN/C hetero-structured graphene-like materials.(e) We proposed that the chemisorption of fluorine (F) atoms on the boron (B) atoms of hexagonal boron nitride (h-BN) can induce spontaneous magnetization, and form long-range ferromagnetic ordering. The electronic spin-polarization due to F adsorption arises mainly from the pz-orbitals of the nitrogen (N) atoms nearest to the sp3-hybridized B atoms and the F atoms. Virtual hopping is allowed in the ferromagnetic arrangement under the condition of exchange mechanisms, which give rise to stable ferromagnetic ordering. In addition, N-terminated triangle-shaped antidote defects have large magnetic moments and interact with F-adsopaton defect in a ferromagnetic way. In cooporation with experiment group, we development an efficient one-step fluorination-assisted method and a facile one-step solid-state reaction method to synthesis BN nanosheeets with ferromagnetism. The results not only verify the theoretical predictions, but also provide the theoretical and experimental basis for the synthesis of BN nanomaterials with stable ferromagnetism.