Structure And Properties Of One-Dimensional Silica Nanomaterials

As a new rising interdisciplinary subject, nano science and technology has being of far reaching importance in the region of physics, chemistry, biology, material science, electronics and so on. The significance of nanophase materials rest with one fact, namely the physical and chemical property occur prominent change as decreasing size in the range of nanometer scale. Silica (SiO₂), which is the important component in glass, catalyst, Si-based microelectronic derives and optical fibers, is an increasingly important candidate to form 1D nano-materials. Silica nanotubes have attracted much attention in scientific communities because they play an importance in electronic and optical applications. Silica nanotubes or nanowires can be fabricated by many methods, such as excimer laser ablation, sol-gel, chemical-vapor-deposition (CVD), vapor-liquid-solid (VLS) and so on. We have calculated nanotubes and nanowires which have different cross section and length using the density-functional theory (DFT) at the 6-31G(d) level. The work mostly include three parts:(a)Stability and Size Effect of Silica NanotubeWe have carried out the structure optimization and frequency calculation of anhydrous structure (SiO₂-NTs) and hydrated structure (SiO₂-WNTs). The average binding energy (Eb), electronic structure and IR spectrum are discussed. Eb is equal when SiO₂-WNTs have the same layers or length. SiO₂-WNTs are energetically more favorable than 2MR- and 2MRW-Chain molecular chains structure reported elsewhere. Main contribution of HOMO come from oxygen atoms for the two tubular structures, while LUMO contribution is from silicon atoms. We have investigated infrared (IR) spectrum of 4MR-nanotubes and found the calculated data of vibration modes are well agreement with experiment data. Radial mode (RM) and tangential mode (TM) display strong size effect. Radial vibration's frequency shifts to high frequency while tangential vibration's frequency shifts to low frequency, namely, quantum size effect and anisotropic property of vibration in nanomaterial.(b)Structure and Property of Finite Length Silica NanowiresGeometry optimization and frequency calculation of silica single and double line structures composing of two-membered ring were investigated using the density-functional theory (DFT) at the 6-31G(d) level. Average binding energy (Eb), electronic structure, IR and Raman spectra are discussed. Eb monotonically changing as function of n and tend to a constant as nano line reaching infinity. Energy gaps of different kinds of structures presence distinct size effect. Frequency and intensity of RM and TM monotonically change as increasing n. Radial vibration's frequencies show red shift while tangential vibration's frequencies show blue shift. Raman spectra also have size effect and anisotropy. Eb, electronic structure and vibration frequency tend to convergence as increasing n. These results show the stability of silica nanowires when n trend to bigger number.(c)Theoretical Investigation of Infrared Vibration Spectra of the (SiInfrared vibration spectrum of nanosize (SiO^^O^Rt with line, ring and cage structures have been calculated using the density-functional theory (B3LYP) at 6-31G(d) level. As for quasi ID (one dimension) line and ring structure, frequencies of stronger infrared vibration modes are monotonically change as function of length, which shows strong size effect. The IR vibration modes in directions parallel and normal axis of nanoline or nanoring have opposite size dependence trend, of which reveal vibration anisotropy property of nano scale material. Due to constructional complexity, cage structure does not have quasi ID character. Morever, number of IR vibration modes increase as structural symmetry decreasing. Vibrations of two-membered ring, isolated hydroxyls of three structures and framework SiC>4 tetrahedron in cage structure are well agreed with experimental data. Theoretical calculation result of IR spectra can be used to guide characterization of silicon oxide nano scale materials which fabricated in experiment.