Synthesis, characterization, and optical properties of zinc oxide nanostructures.

摘要

Chapter 1. A general introduction is presented that describes the new interdisciplinary field of nanoscience. The various unique properties of nanocrystals are highlighted, and different nanocrystal synthetic techniques are discussed. The importance of a technique that achieves control over nanocrystal size and morphology is stressed. Morphological control over zinc oxide nanocrystals using the "thermal decomposition of metal acetates" method is discussed. The numerous possible morphologies of nanocrystals of zinc oxide are described to illustrate the suitability of ZnO for a study of the relationship between nanocrystal morphology and properties.;Chapter 2. A study of the growth, structure, self organization properties, and photoluminescence, of ZnO nanorods with 2 nm diameter is presented. The disparity in relative intensity of X-ray diffraction peaks between the ZnO nanorods and bulk ZnO is modeled using XRD refinement software, and shown to arise from preferred orientation, which occurs due to the nanorod shape. The effect of various synthesis parameters---reaction time, and capping agent to precursor molar ratio---on the nanorod growth and structure is probed using synchrotron X-ray diffraction by monitoring the width and position of the (002) diffraction peak. The stacking properties of the nanorods are studied using small angle X-ray diffraction, which can probe larger scale ordering due to the small angles used. The photoluminescence properties are studied using solution photoluminescence measurements, and strong quantum confinement effects are observed, due to the small diameter of the nanorods.;Chapter 3. Morphological control of ZnO nanocrystals based on the coordinating power of the solvent used is presented. The various nano-shapes (nanotriangles, spherical nanoparticles, and nanorods) are studied by TEM and XRD. Using tilting TEM experiments and CrystalMaker models, the three dimensional nature of the nanotriangles is determined. Solution photoluminescence studies showed different PL properties for each nano-shape. The intensity of the green emission, attributed to defects, is found to correspond to the surface area to volume ratio of the nanocrystal, indicating that surface defects give rise to the emission. Control over nanocrystal shape is thus presented as a means to controlling green emission, which is usually unwanted in UV emitting applications.;Chapter 4. Spherical ZnO dots of similar diameter with modified surfaces are prepared by introducing different ligands into the synthesis. The PL properties of the nanocrystals are studied, and it is found that dots prepared with solvents that can fill in oxygen vacancies exhibit weaker green emission. Zeta potential measurements confirm that the dots with weaker green emission are less positively charged (less oxygen vacancies). A novel method to calculate the dot diameter based on the energy of the green emission is presented, and the results are in good agreement with TEM measurements. The advantage to this method is the calculation is straightforward, and can be employed regardless of the degree of confinement.;Chapter 5. The optical probing of single ZnO nanorods is presented. Near field scanning optical microscopy is employed to beat the diffraction limit of visible light (by placing the sample less than 10 nm from a pinpoint light source) and achieve optical images of individual nanorods. Synthesis of ZnO nanorods of the appropriate size scale for NSOM imaging is addressed. Although photoluminescence from single ZnO nanorods was not detected, in the resulting transmission mode optical images, in which light transmitted through the sample is collected, the nanorods appear brighter than the glass substrate. This anomalous contrast was found to vary with imaging wavelength and the presence of dopants in the nanorods. A model of NSOM transmission mode contrast based on refractive index is adopted to help explain the images, with materials of higher refractive index appearing brighter in transmitted NSOM images than those with a lower refractive index. The model predicts that NSOM contrast based on refractive index difference between sample and substrate is highly sensitive to even small differences in index of refraction. This model correlates well with the data, as the doped ZnO nanorods appear brighter in images than undoped nanorods due to a refractive index increase, and contrast varies with wavelength along with the variation of refractive index with wavelength.