Silica Nanostructure: Theoretical Design And Property Prediction From First-principles Calculations

Silica nanostructures have been the focus of much research over the past few decades, because of its potential applications in microelectronics, optical communications and catalysis. Searching for the size-dependent stable configurations and fascinating properties resulting from quantum-confinement effect are quite crucial for the synthesis and application of silica nanomaterials. In this thesis, we performed first-principles calculations based on density functional theory (DFT) to reveal the atomic structures, energetic stability, mechanical and electronic properties of silica nanostructures, including the nanorings (OD), nanochain/nanotube (ID), and nanosheet (2D). We focus on the size-dependent phase transition, the plausibility of these nanostructures, and the relationship between silica nanosheet and nanotubes. The results presented in this thesis are useful not only for synthesis of novel silica nanomaterials in future experiments, but also for understanding the growth mechanisms of quasi-one-dimensional silica nanomaterials, such as nanowires and nanotubes.This thesis includes the following aspects:(1) We presented a new family of silica nanoclusters, including one-dimensional nanochain (NC) and nanoring (NR) structures via the assembly of two-(2MR), three-(3MR), four-(4MR), and six-membered ring (6MR) in different ways. The energetically most favorable configurations are predicted to be size-dependent which possess different structural features at different size ranges. For small-size silica NRs (SiO2)n with n<12, the configurations with 2MR-3MR hybrid structures (2-3MR-NRs) are energetically most stable, followed by the structures containing 3MRs. For 1222, the configurations composed of uniformly hybrid 2MRs and 4MRs (2-4MR-NRs) are the most stable structures. The 2-6MR-NRs are the most unstable configurations in the range of sizes under study.(2) We performed first-principles calculations to study the energy, electronic structures, and IR spectra of silica nanorings (NR) consisting of two-and four-membered ring (2-4MR) units. The results show that 2-4MR-NRs have several peaks at the vicinity of 1000-1150 cm-1 regions, clearly different from the 2MR-NR (<1000 cm-1). The outcome could be recognized as the spectroscopic fingerprints of 4MR and provide good signatures for the experimental detection of silica nanoclusters with specific structures. (3) On the basis of recent experimental findings, we proposed a novel silica nanosheet with a graphene-like structure consisting of 6MRs. The equilibrium configuration, energetic stability, mechanical and electronic properties of this new structure was investigated systematically. We predicted that silica nanosheet has high energetic stability, anisotropic mechanical properties and excellent electrical resistivity. For instance, the energy of a single nanosheet with respect to that of a-quartz crystal is only 0.144 eV/SiO2. For the three-dimensional structure consisting of multi-nanosheet with AB stacking sequence, the relative energy is further reduced to 0.096 eV/SiO2. The Young's modulus along the zigzag direction is about 43% higher than that along armchair direction. The energy gap of silica nanosheet is wider than 8 eV. Under certain conditions, silica nanotubes can be formed via curling up a silica nanosheet along different directions. Armchair silica nanotubes are more stable than the zigzag ones.