Influence of annealing on the photoluminescence of nanocrystalline ZnO synthesized by microwave processing

Zinc oxide (ZnO) nanoparticles have been synthesized by a microwave heating method. The resulting powders were annealed at temperatures from 400°C to 800°C for 4 h. The annealed powders were characterized in terms of their structural, morphological, and optical properties by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and room-temperature photoluminescence (PL) spectroscopy, respectively. The XRD patterns of the samples revealed the formation of phase-pure ZnO, apparently without any impurities. However, FT-IR spectra revealed the presence of carboxylate impurities, the signals of which disappear as the annealing temperatures are increased from 400°C to 800°C. SEM pictures showed increasing size ZnO nanoparticles with increase in annealing temperature. The powder annealed at 800°C is identified as hexagonal wurzite ZnO. The improved PL spectral performance of the ZnO powder correlates well with the improved crystalline quality of the nanoparticles, as revealed by the XRD and PL. Sharp ultraviolet (UV) band edge luminescence has been observed at 375 nm, which is suitable for application in UV light emitting devices. The results also showed that the excitation wavelength dependence of the luminescence arises from quantum confinement effects.