Origin of thermal annealing-induced orange emissions from solution-grown ZnO nanocrystals

Abstract

ZnO nanocrystals were synthesised via solution route and then thermally annealed in air in the temperature range of 60–900°C. The correlation between the hydroxyl groups on the surface of ZnO nanocrystals and the orange photoluminescence (PL) of ZnO nanocrystals was investigated. The crystal structure, surface chemistry and luminescent properties of thermally annealed ZnO nanocrystals were examined with X-ray diffractometer, Fourier transform infrared (FTIR) spectroscope, PL spectrophotometer, transmission electron microscope (TEM) and selected area electron diffraction technique, respectively. It is found that an orange PL band appeared in the PL spectrum at about 600 nm only when the annealing temperature was higher than 600°C. By contrast, the hydroxyl groups were always present on the solution-grown ZnO nanocrystals as the annealing temperature varied in the range of 60–900°C. Our results demonstrate that the orange PL band has no direct correlation with the hydroxyl groups on the surfaces of the ZnO nanocrystals. Thermally induced defects are proposed to be responsible for the orange PL band.