Relationship between oxygen defects and the photoluminescence property of ZnO nanoparticles: A spectroscopic view.

The present paper deals with the synthesis and characterization of ZnO nanoparticles obtained by coprecipitation method at three different pH conditions. Samples characterizations aimed at understanding their spectroscopic properties are carried out using a variety of experimental techniques. X-ray diffraction and transmission electron microscopic studies show significant increase in the particle size of the synthesized ZnO nanoparticles ranging from 3–40 nm with increasing pH values. Absorption spectra show particle size dependent blueshift in the energy band, which may be ascribed to quantum confinement effect. Fourier transform infrared (FTIR) spectroscopy reveals enhancement in the surface defects of the synthesized ZnO nanoparticles with increasing pH values. Electron paramagnetic resonance studies at room temperature (300 K) and several liquid helium temperatures (including the lowest temperature 13 K for all samples) indicate the presence of singly ionized oxygen vacancy defects (VO+) and O2- superoxide ions in the ZnO nanoparticles. These results are well supported by FTIR and photoluminescence spectroscopy. The emissions from these nanoparticles are also found to be size dependent, whereas the ratio of UV emission to oxygen vacancy related green emission is found to increase with increasing pH values. Furthermore, a plausible mechanism of the observed defect mediated green emission is proposed. [ABSTRACT FROM AUTHOR]