Growth and characterization of ZnO nanoparticles by sol-gel process
Solid state lighting technology is of particular interest in application of semiconductors. To this end, ZnO nanostructures have gained great attention in the research community, in part because of its requisite large direct band gap. The stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize an optimized growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanoparticles by sol-gel method, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy), UV-Vis spectroscopy, X-ray diffraction (XRD) and other techniques. The influence of various growth parameters on the morphology, optical and electrical properties of the nanoparticles were also systematically studied. These include the growth temperature, volume ratios of water to ethanol solvent and different dopants effects. By controlling these parameters different shapes of nanoparticles, like spherical particles, nanorods and nanoflowers are demonstrated. XRD indicated that all the as-grown and annealed nanoparticles produced at temperatures between room temperature and 75 °C crystallize in the wurtzite structure and post growth annealing enhanced the crystalline quality of the materials while the band gap energy reduces. The crystallite size, obtained from XRD analysis, of as prepared ZnO nanostructures was found to decrease from 24 to 12 nm with the increase in volume ratio of ethanol in the solvent as peak intensities and sharpness increase with volume ratio of water. Thus in order to have smaller particles more volume ratios of ethanol solvent is favourable at growth temperature of 35 °C. The dopants were also observed to have slight effect on the grain sizes .No traces of zinc hydroxide were observed even in materials grown at lower temperature as reported by some authors. The optical quality of the nanostructures was investigated using PL. Both UV and defect related emissions have been observed for all as-grown and annealed samples of nanostructures. Photoluminescence spectra showed a strong ultra-violet emission, for annealed ZnO nanoparticles, which was centred on 385 nm and weak green emission at 550 nm confirming that the samples possess good optical properties with less structural defects and impurities. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. Annealing at a temperature of 600 °C enhances the UV emission and suppresses defect related deep level emission for all samples. The PL spectra showed strong, broad and intense emission in visible region for Ce-doped ZnO samples while other dopants suppressed this green emission. The reflectance spectra of the annealed products show that the percentage absorption in visible range increases with annealing temperature. UV measurements depict a shift in absorption edge confirming the changes in particle sizes with varying ratios of solvents (water and ethanol). The band gap decreased from 3.31 to 3.17 eV with an increase in the ethanol composition in the solvent, implying that the optical properties of these materials are clearly affected by the precursor compositions. The SEM micrograph of ZnO revealed that the surface aspect depends on both the dopant used and annealing temperature. The characterization of the nanoparticles with Scanning Electron Microscopy (SEM) showed that at low temperatures (35 °C and 45ºC) clearly defined spherical particles are formed while at higher temperatures agglomerated irregular and diminished nanoparticles were observed.