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.
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