Vacancy And Topological Defects And Their Physical Properties Of The Low-dimensional Silicon-based Nanomaterials Studied By Numerical Simulations

The successful fabrication of graphene stimulated greatly the researchers’ interests in the two-dimensional hexagonal materials, but its zero gap is the largest obstacle to apply it in the next generation of nano-electronic devices. Experimentally and theoretically, its electronic properties have been modified by different kinds of physical and chemical methods, such as the chemical modification, vacancy controlling, cutting it into the quasi-one-dimensional nanoribbons with defined edge. Moreover, at present many other two-dimensional hexagonal materials emerged, such as the single-layer h-BN, MoS2, silicene and germanene. These graphene-like nanostructured materials have also attracted significant attention of the researchers, working in the physics, chemistry and material sciences, which are expected to be used in the future electronic and spintronic devices.In this paper, we have studied by using the first-principles method the geometrical structures, electronic, magnetic and vibrational properties of the silicene with single-vacancy, the edge reconstruction of the one-dimensional zigzag-edge silicene nanoribbon and the chemically sulfur-functionalized edges of the zigzag graphene nanoribbons. In chapter1, we firstly give an introduction of the nanomaterials and nanotechnology, and the present theoretical and experimental investigations of the two-dimensional (2D) nanomaterials. At last, a brief introduction of the quasi-one-dimensional ribbons is given.In chapter2, we firstly give a brief introduction of the first-principles method in section2.1, and then in section2.2, we specially introduce the density functional theory and the Kohn-Sham equation, and how to solve it in practice by different methods. We also present the crystal symmetry operation and the methods to generate and integrate the k points in the Brillouin zone in practical calculations. In chapter3, we present our numerical calculations on the edge reconstruction of the zigzag edge silicene nanoribbons (SiNR) and the sulfur-functionalized edges of the zigzag graphene nanoribbons (GNR). In section3.1, we briefly review the present theoretical and experimental studies on the edge modification of the nanoribbons. In section3.2, we studied the edge reconstructions of the zigzag edge SiNRs, finding a new (2×1) reconstructed edge structure (edge-4). And a characteristic topological triangle-pentagon pair is found to exist at its edges, which is different from the two other reconstructed edges (edge-2and edge-3), found previously, and is the most stable with the lowest energy than them. Our numerical calculations show that they can transform into each other by applying a tensile strain or compression. The calculated Raman vibration modes of edge-4indicate that there are two new characteristic edge modes, which is helpful for finding the new edge-4in future experiments. The calculated electronic structure of edge-4SiNR shows its ground state is still an anti-ferromagnetic semiconductor. In section3.3, the stable geometric and electronic structures of the fully and half sulfur-edge-functionalized zigzag edge GNRs (ZGNRs) at their widths of four zigzag carbon chains (S-4-ZGNRs) are studied, and some important results have been obtained from our calculations:1) The two-dimensional plane structures are the most stable ground states in all possible isomers of the S-4-ZGNRs at both100%and50%terminations, which are all metallic;2) A much delocalized characteristic S-px lone-pair electron’s band crossing its Fermi level appears in the case of fully S-edge-termination, which is more extended in a large energy range of over8.0eV, in contrast to the much more localized S-px band, forming two almost flat bands in the case of half S-edge-termination.3) The ground state of50%S-edge-terminated ZGNRs is an anti-ferromagnetic semiconductor with an indirect gap of0.2eV.In chapter4, we investigate the silicene’s single-vacancy structure. Our numerical calculations found that there are three different single-vacancy structures (MV-1, MV-2and MV-3). Importantly, a new self-healing single vacancy (MV-1) has been found, which is different from those MV-2and MV-3, found previously in grphene and the other2D nanostructures. In the monovacancy MV-1, a center Si atom at its core form four covalent bonds with its four nearest-neighbor Si atoms, forming a plane-like sp3hybridization. The finite temperature first principles molecular dynamic simulations show that the MV-1is thermodynamically stable and it can stably exit at300K or even higher temperatures. But the MV-3is unstable, easily decaying into the MV-1, and the metastable MV-2could coexist with MV-1at low temperatures less than10K. The diffusion coefficient of MV-1is found to be much higher than that of the monovacancy in graphene. The calculated electronic structures of the defective silicene with MV-1and MV-2indicate that both of them are metal.In the last chapter, we investigate the effects of different substrates on the geometrical structures and electronic properties of the monovacancies in silicene. Here, two substrates are considered, including the single-layer h-BN and Ag(111) surface. For the h-BN substrate, two stable Si monovacancies are found to exit, among which one is the self-healing monovacancy (MV-1), and the other one is the (5|9) type one (MV-2) with a closed five-and nine-membered (5-9) pair of rings. The electronic structure of the composite system for the defective silicene on the single layer h-BN could be considered as a simple superposition of the single-layer h-BN and the silicene with monovacancy because of the weak van der Waals interaction between both of them. For the Ag(111) substrate, there exist three kinds of inequivalent Si atoms in the (3×3) reconstructed silicene, which are respectively denoted as Si-1, Si-2and Si-3, forming different MVs, because of the rather big coupling between the Ag(111) substrate and the silicene on it. For the Si-1monovacancy, there are possibly two stable monovacancy structures, among which one is the (5|9) type, MV-2and another one is the high-symmetric-like monovacany, MV-3. And for both the Si-2and Si-3monovacancy, only the high-symmetric-like MV-3is stable. Although there is a strong hybridization between both the silver and silicon atoms, the corresponding MV defect states could still be found to exist in the electronic structures of the composite system of the silicene with the MVs put on Ag(111) substrate.