Elucidating room-temperature optical transitions in annealed ZnO nanoparticles synthesized from an aqueous method

Abstract

ZnO nanoparticles (NPs), which were grown from chemical synthesis by an aqueous method and annealed, were studied. The annealed ZnO NPs have the hexagonal phase of wurtzite and no other phase was observed. The observed grains are spheroidal except for those annealed at 800 °C. The ZnO NP grain size increases and the strain reduces as annealing temperature increases. From the photoluminescence spectra it was also observed an increase in the emission of the defect band and a decrease of the emission at the near-bandgap-edge as the annealing temperature increases. Significant emission peaks at 2.63 eV and 3.19 eV show an increase and decrease respectively, while the peak at 2.96 eV remains constant as the annealing temperature increases. The origin of the 2.63 eV peak was identified as a transition between the corresponding levels of neutral O vacancies to monovalent Zn vacancies and the origin of the 2.96 eV peak was associated to a transition from conduction band to an interstitial O level. The origin of the 3.19 eV peak was associated to a transition from an interstitial Zn energy level to the valence band. The three transitions are the main ones that take place inside the NPs. The behaviors of these PL peaks agree with the analysis of the structural results. Analysis of the FTIR spectra reveals that only at about 400 °C and higher annealing temperatures, the precursor residues are completely removed.