Fabrications, Characterizations And Optical Properties Of Metal-oxide-semiconductor Nanostructures

In the recent two decades, ZnO, SnO2, TiO2, Al2O3 as typical representatives of the metal oxide semiconductor nano-materials have attracted wide attention and have actively been explored because of the applications in ultraviolet lasing, photocatalytics, gas sensing and fluorescent biological labels. Thus, studies on fabrications and applications of these nanostructured materials are hot subjects of the current nanomaterials research. In this thesis, we describe the fabrications of SnO2 cubic and cubiod nanorods by hydrothermal route as well as spherical Al(OH)3 nanoparticles (NPs) by laser ablation in water. Based on the Raman and luminescence spectral analyses, we demonstrate that oxygen vacancies and its concentration in SnO2 nanostructure play very important roles in the Raman and photoluminescent property modifications. The obtained main results are as follows:1. Raman spectra acquired from the SnO2 nanocrystals (NCs) with spherical, cubic and cuboid nanorod morphologies show an almost morphology-independent Raman mode at～302 cm-1. The frequency of this mode has no obvious dependence on NC size, but its intensity increases with decreasing NC sizes. By considering the dipole changes induced by oxygen vacancies, we adopt the density functional theory and phonon confinement model to calculate Raman spectra with different morphology and size nanostructures. The results demonstrate that the presence of oxygen vacancies with a certain concentration plays a crucial role in the infrared-Raman active transformation and the intensity enhancement is a result of stronger phonon confinement.2. Bombarded with the excimer laser pulse in an aqueous solution of pure tin target, we fabricate pure SnO2 NP materials from the supernate. After annealed in oxygen atmosphere and vacuum respectively, we found that the NP sizes increase and the crystallinities become better. Raman spectral examinations show that the frequency of the A1g mode has no obvious dependence on NC size and its frequency shift to lower wave numbers is due to modified oxygen vacancies. Based on the presence of three knids of oxygen vacancies, we use tha density functional theory (DFT) to calculate the Raman spectra of the NCs with different oxygen vacancy concentrations and the obtained results clealy demonstrate that the bridging oxygen vacancy is responsible for the shift of the A1g mode. Through the photoluminescence (PL) spectral measurements and analysis, we reveal that the observed 413.6 and 617.2 nm PL peaks originate from bridge and in-plane oxygen vacancies, respectively. This result also implies that the SnO2 nanostructures prepared by laser ablation in water mainly contain bridge oxygen vacancy defects.3. Raman spectra acquired from SnO2 nanocrystals with different sizes show a size-independent Raman mode at～574 cm-1. The intensity increases as the nanocrystal size decreases and this tendency is contrary to that of the normal bulk Raman modes. By considering the existence of oxygen vacancies at the nanocrystal surface, we adopt the DFT to calculate the Raman spectra with different oxygen vacancy positions and concentrations. The results clearly demonstrate that the in-plane oxygen vacancy is responsible for the 574 cm-1 mode and the intensity enhancement is a result of the higher in-plane oxygen vacancy concentration.4. Broad full widths of half maxima (dampings) are observed from the low-frequency Raman spectra of the hydrothermally prepared SnO2 nanocrystal congeries. No matrix exists between these nanocrystals and the complex-frequency model is thus unable to explain the damping of these low-frequency Raman peaks. An alternative model in which damping is induced by the interaction between confined surface acoustic vibrations and localized electrons near the nanocrystal surface is proposed to explain the phenomenon. This model which suggests that damping is proportional to d3/2, where d is the average diameter of nanocrystals, very good explains our experimental results.5. Aluminum hydroxide nanocrystals consisting of an amorphous shell and crystalline core are fabricated by pulsed laser ablation of an aluminum target in water. The colloid consisting of nanocrystals with a uniform size exhibits a size-independent PL band at～383 nm. According to the PL excitation spectra and time-resolved PL decay analysis, this PL band originates from oxygen vacancies in the amorphous shell and Forster energy transfer occurs between the oxygen vacancy levels in the crystalline core and amorphous shell.