Origin of polychromatic emission and defect distribution within annealed ZnO nanoparticles

Abstract

Defects distribution in ZnO nano-particles is introduced by high temperature annealing in air and corresponding visible emission are comprehensively analyzed. Grain size initially increases with increasing annealing temperature until 800°C and decreases after that. Higher temperature annealing alters defect distribution and initiates formation of OZn (Zinc at oxygen site). Formation of OZn, and presence of conventional defects is confirmed by Electron Paramagnetic Resonance spectra. Spatial distribution of polychromatic emission centers in annealed sample is estimated by diffusion mediated growth and confirmed by florescence imaging. Blue emission centre is found to be located on surface of particle/grain which gets modified by surface capping using PEG and PVA. These visible emissions exhibit strong excitation dependence, and can be excited below the bad gap energy. Based on XRD, EPR, emission by low energy excitation, and florescence imaging schematic of defect distribution within particle/grain and their corresponding emission is proposed.