| Graphene has aroused great interest since its successful isolation in experiments, because of its excellent physical and chemical properties, such as Dirac fermion carriers, very high carriers mobility at room temperature, very high thermal conductivity and Hall effects at room temperature, etc. Nowadays, graphene nanoribbons (GNRs) with the width varying from several tens to few nanometers were achieved in experiments. Moreover, room temperature transistors have been built from the ribbons with very narrow widths. Many interesting behaviors have been achieved in zigzag GNRs, such as energy band-gap engineering, spin gapless semiconductors, half metals, etc. Compared to zigzag GNRs, the studies of armchair GNRs are quite few, which are non-magnetic semiconductors due to the lack of edge states located around the Fermi level. They are, however, energetically and thermodynamically more stable than zigzag GNRs. Therefore, exploring possible magnetism in armchair GNRs and related nanoribbons is full of importance.The excellent performance of graphene has aroused general curiosity about two-dimensional (2D) crystals of other materials. Boron nitride single layers, a III-V structural analogue of graphene, have attracted special interest due to its super thermal and chemical stabilities and intrinsic electrical insulation. Some interesting behaviors have been found in the single layer BN nanoribbons. For example, the variation in band gaps of BN nanoribbons with their widths, C-doping effect, and Stark effect. And half metallicity was obtained in zigzag BN nanoribbons when an external electrical field was applied or the B edge was hydrogenated. In our work, the electronic structures and spin gapless semiconductors in BN nanoribbons with vacancies are studied.Since the1980s, people have had great interest in synthesizing new allotropes of elemental carbon. Graphyne which is a two-dimensional structure of sp-sp2-hybridized carbon atoms, was predicted to be the most stable of the various diacetylenic nonnatural carbon allotropes in theory. Graphdiyne, a carbon allotrope, which has the same symmetry as graphene and has butadiyne linkages between its nearest-neighbor hexagonal rings has recently synthesized and fabricated on copper, showing experimentally the semiconductor property with conductivity of2.516*10-4 Sm-1,which is comparable to silicon. With the development of industry, the demands for fossil fuels have increased rapidly leading to many problems including environmental pollution and limited energy supply. Green energy sources are urgent to be developed. One promising alternative to fossil fuels is hydrogen. Nanostructured materials offer a host of promising routes for hydrogen storage due to the large effective surface area and the small volume. In our work, the hydrogen storage on graphyne is studied.In Chapter1, the lattice structures, electronic structures, and syntheis methods of graphene and GNRs are first briefly introduced. Then, some background of the BN single layers and BN nanoribbons are presented. Finally, the hydrogen storage is introduced.In Chapter2, as the theory foundation of the first-principles calculations, density functional theory (DFT) is presented. The pseudopotential method is also referred to simplify the DFT calculations. The Van der Waals correlation energy is considered to improve the DFT.In Chapter3, electronic structures and spin gapless semiconductors in BN nanoribbons with vacancies are presented. Based on DFT, we studied the electronic and magnetic structures of BN nanoribbons with vacancies. We found that the band gap could be tuned by changing the vacancies concentration, suggesting that there are possible transformations of band structures:from metal→spin gapless semiconductor→semiconductor. Thus, we found one possible approach to obtain spin gapless semiconductors, which is to seek the transition state between metal and semiconductor. Moreover, the obtained spin gapless semiconductors are insensitive to the width of the nanoribbons. The magnetism observed in the system is ascribed to dangling-bond states formed due to the vacancies, and could be verified by the Stoner criterion. The magnetism induced by B vacancies is stronger than that by N. We attribute the difference to the different degree of localization. These unique and interesting band structures observed in BN nanosystems have many potential applications for the future.In Chapter4, exploration of magnetism in armchair graphene nanoribbons with radical groups is presented. We investigated the magnetism of armchair graphene nanoribbons with edge saturations of O, CH2or NH radical groups within the framework of density functional theory. We obtained a variety of interesting band structures such as semimetals, spin gapless semiconductors, and semiconductors. Among the three radical groups considered, CH2can induce the strongest magnetism with2μB magnetic moments per group. The magnetism is found to come from the unsaturated states on the C atoms at the edges and in the radical groups. No spin polarization is triggered in the ribbons with NH radicals. Moreover, we proposed a concise model of chemical bonds to understand the magnetism in systems with different radical groups. Our study provides a possible way to carry out d0magnetism in armchair grapheme nanoribbons.In Chapter5, the hydrogen storage on graphyne is presented. We focus on the capacity of hydrogen storage on graghyne and the effect of the electric field on the hydrogen storage. Hydrogen gravimetric capacity would be up to20wt%, but the binding energy is only0.085eV, it is slightly smaller than the optimal adsorption energy for H2(0.1～0.2eV/H2). The binding energy per H2increases from0.08eV to0.27eV in the presence of an electric field of0.004a.u., and the corresponding gravimetric capacity is about11.1wt%. The mechanism is being investigated.In Chapter6, the research of Mn doped in topological insulators is presented. Through the first-principles calculations, electronic and magnetic structures of Bi2Se3with magnetic dopants Mn are studied. It’s found that the favorite doping site could change from Bi substitutional site to interstitial site, when both the spin orbit coupling and U are considered. And the energy difference between ferromagnetic state and antiferromagnetic state is also discussed. The value is very small. All the Bi2Se3with Mn doping are metallic. This work will be studied further.In Chapter7, a brief summary of the thesis is presented. |
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