Violet luminescence and its origin in ZnO nanoparticles prepared by co-precipitation method

The Excitation wavelength dependent photoluminescence (PL) is employed to understand the origin of rarely reported violet emission observed from ZnO nanoparticles prepared by co-precipitation method. These ZnO nanoparticles showed a very weak and broad emission peak spread over the violet and blue region when excited with 320 nm. Interestingly, a sharp and intense violet emission at 316 nm is evolved for 330 nm excitation wavelength and its intensity enhanced tremendously as the excitation wavelength increased from 320 nm to 350 nm. When ZnO nanoparticles excited with 360 nm excitation wavelength, the sharp violet peak changed to a violet-blue doublet with peak positions at 406 nm and 434 nm. With further increase of excitation wavelength to 390 nm, a sharp blue emission is observed but with less intensity compared to the violet emission. By using the excitation wavelength dependent PL and PL excitation (PLE) spectra of ZnO nanoparticles, the violet emission is assigned to the transition of electrons from the conduction band to the zinc vacancy (V_Zn) and blue emission to the transition from zinc interstitial (Zn_i) defect level to the zinc vacancy (V_Zn) defect level. This study helps to understand the defect levels responsible for violet and blue emission from ZnO and benefit to the light emitting diodes and biological fluorescence applications.