Structure and emission transformations in ZnO nanosheets at thermal annealing

Photoluminescence (PL), scanning electronic microscopy (SEM), Raman scattering and X-ray diffraction (XRD) have been studied in freshly prepared amorphous phase ZnO nanosheets (NSs) and in thermal annealed crystalline ZnO nanosheets. ZnO NSs were created by the electrochemical (anodization) method using different technological regimes. ZnO NSs are characterized by the amorphous phase in a freshly prepared state. Defect related PL bands with the peaks at 1.90-2.03, 2.49-2.51 and 2.80-2.85 eV are detected in the amorphous ZnO phase. Then ZnO nanosheets have been annealed at 400°C for 2 h in ambient air. Raman and XRD studies have shown that annealing stimulates the ZnO oxidation and crystallization with the creation of ZnO nanocrystals with the wurzite crystal lattice. The oxidation and crystallization processes are accompanied by a transformation of the PL spectra. In crystalline ZnO NSs four PL bands appear with the PL peaks at 1.55-1.60, 2.06-2.10, 2.52 and 3.10 eV. The reasons for the emission transformation have been discussed. For some PL bands (1.90-2.03, 2.49-2.51 and 2.80-2.85 eV) the nature of radiative defects has been clarified from the comparative study of the ZnO NS structure and emission properties. It is shown that the anodization method permits to obtain the ZnO nanocrystals by a simple controllable way with emission in a wide spectral range that is interesting for future application in room temperature white light-emitting diodes.