First-principles Study Of Silicon Geometric And Electronic Structure Of Two-dimensional Organic Nano

Graphene is a monolayer of graphite and has a perfect two-dimensional honeycomb structure. With the discovery of various novel properties, it has attracted more and more attention of researchers all over the world. Since silicon and carbon belong to the same group in the Periodic Table and they have many similar properties, it has aroused great interest whether there may exist graphene-like structure for silicon? Although pure silicon nanosheet of graphene-like structure has not yet been prepared in experiment up to date, it is reported in 2010 that a new two-dimensional organosilicon nanosheet Si6H4Ph2 was successfully synthesized. A number of interesting scientific questions are to be answered with respect to this new material, such as the stability, the nature of its electronic properties and so on. A theoretical research is thus highly expected, and a detailed theoretical understanding will forward the development and application of such kind of materials. Thanks to the progress in quantum physics and chemistry as well as computational techniques, it is now possible to perform theoretical study on such kind of materials by first-principles methods based on density functional theory.In Chapter 1, we first make a brief summary on the progress in the study of silicon materials, including bulk silicon, silicon clusters, silicon nanowires and nanotubes. Then, we focus on the experimental and theoretical research progress of two-dimensional graphene-like silicon nanosheets.In Chapter 2, we briefly review the historical development and current status of quantum chemistry and outline the framework of density functional theory and its recent progress. Kohn established the foundation of density functional theory, who proved that any properties of a many-particle system in the ground state can be expressed as a functional of the ground-state electron density. The many-particle is transformed into an effective single-particle problem by incorporating many-particle interactions into the exchange-correlation energy. Therefore, the major target of density functional theory is to look for reasonable exchange-correlation energy functional. The chapter ends with a brief introduction to the scientific computer codes of density functional methods used for studies in this thesis.In Chapter 3, we investigate the stability and electronic structure of Si6H4Ph2. By a comparative study of pure silicon nanosheet Si6, hydrogen-passivated silicon nanosheet Si6H6 and phenyl-passivated silicon nanosheet Si6H4Ph2, we elucidate the mechanism on the stability of Si6H4h2. In addition, by examining the electronic structures of Si6H6 and 2, we find they both behave like an indirect gap semiconductor with a quite large gap, which is unfavorable for practical applications.In Chapter 4, aiming at the problem that hydrogen-passivated and phenyl-passivated silicon nanosheets Si6H6 and Si6H4Ph2 are both indirect-gap semiconductors with a quite large band gap, we tentatively carried out a preliminary research to fine tune the band gap of silicon nanosheet. Our research shows that, by fluorine-passivating silicon nanosheet, we can change the conduction band character from indirect-gap semiconductors (hydrogen-passivated and phenyl-passivated) with a big gap into a direct-gap semiconductor with a small gap, which would be favorable for potential applications as photovoltaic materials.