Emission modification in ZnO nanosheets at thermal annealing

Photoluminescence (PL) and its temperature dependences, as well as the X ray diffraction (XRD), have been studied in the freshly prepared amorphous phase ZnO nanosheets, obtained by the electrochemical (anodization) method, and in the crystalline annealed ZnO nanosheets. The freshly prepared samples have been divided in two groups. One of these groups has been annealed at 400 °C for 2 hours in ambient air. Defect related PL bands with the peaks at 2.10-2.13, 2.42-2.46 and 2.65-2.69 eV are detected in amorphous state. Appreciable changes in the size of nanosheets as a function of thermal treatments have been revealed. XRD study has shown that annealing stimulates the Zn oxidation and the creation of ZnO with a wurtzite crystal lattice. In crystalline ZnO seven PL bands appeared with the PL peaks 1.46, 1.58, 2.02, 2.43, 2.70, 2.93 and 3.16 eV at 10K. The reasons of emission transformation and the nature of optical transitions related to the studied PL bands have been discussed. It is shown that the anodization method permits by a controllable way to obtain the wide range ZnO emission that is interesting for the future applications in room temperature white light-emitting diodes.