ZnO nanorod arrays were fabricated using a hydrothermal method. The nanorods were studied by scanning electron microscopy, photoluminescence (PL), time-resolved PL, X-ray photoelectron spectroscopy, and positron annihilation spectroscopy before and after annealing in different environments and at different temperatures. Annealing atmosphere and temperature had significant effects on the PL spectrum, while in all cases the positron diffusion length and PL decay times were increased. We found that, while the defect emission can be significantly reduced by annealing at 200 degrees C, the rods still have large defect concentrations as confirmed by their low positron diffusion length and short PL decay time constants.

Defects in three different types of ZnO nanostructures before and after annealing under different conditions were studied. The annealing atmosphere and temperature were found to strongly affect the yellow and orange-red defect emissions, while green emission was not significantly affected by annealing. The defect emissions exhibited a strong dependence on the temperature and excitation wavelength, with some defect emissions observable only at low temperatures and for certain excitation wavelengths. The yellow emission in samples prepared by a hydrothermal method is likely due to the presence of OH groups, instead of the commonly assumed interstitial oxygen defect. The green and orange-red emissions are likely due to donor acceptor transitions involving defect complexes, which likely include zinc vacancy complexes in the case of orange-red emissions.

ZnO commonly exhibits luminescence in the visible spectral range due to different intrinsic defects. In order to study defect emissions, photoluminescence from ZnO nanostructures prepared by different methods ͑needles, rods, shells͒ was measured as a function of excitation wavelength and temperature. Under excitation at 325 nm, needles exhibited orange-red defect emission, rods exhibited yellow defect emission, while shells exhibited green defect emission. Obvious color change from orange to green was observed for needles with increasing excitation wavelengths, while nanorods ͑yellow͒ showed smaller wavelength shift and shells ͑green͒ showed no significant spectral shift. Reasons for different wavelength dependences are discussed.

We report the preparation of highly monodisperse ZnO nanoparticles using poly(vinyl pyrrolidone) (PVP) as the capping molecules. The surface-modified ZnO nanoparticles were found to be remarkably stable. The optical absorption shows distinct excitonic features. Markedly enhanced near-band-edge ultraviolet photoluminescence and significantly reduced defect-related green emission were also observed. We attribute this observation to the nearly perfect surface passivation of the ZnO nanoparticles by the PVP molecules. The third-order nonlinear optical response of these PVP-capped ZnO nanoparticles in a dilute solution was found to be significantly larger (by at least two orders of magnitude) than that of the bulk ZnO.

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We have synthesized highly monodisperse wurtzite ZnO nanoparticles using poly-(vinylpyrrolidone) (PVP) as the capping molecules. The effect of surface modification on the size, structure, morphology, and optical properties of ZnO nanoparticles was investigated. It was found that many properties of the ZnO nanoparticles could be optimized by simply varying the molar ratio Zn(II)/PVP. The ZnO nanoparticles prepared under the optimum conditions are particularly stable, monodisperse, as well as small in size. The EXAFS result showed a structural contraction in the ZnO nanoparticles relative to the bulk. These nanoparticles exhibit distinct excitonic absorption features, markedly enhanced near-bandedge UV photoluminescence, and significantly reduced defect-induced green emission. The third-order nonlinear optical response of these PVP-capped ZnO nanoparticles in a dilute solution was found to be larger than that of the bulk ZnO by at least 2 orders of magnitude.

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