Defect related emission of ZnO and ZnO Cu nanocrystals prepared by electrochemical method

Photoluminescence (PL), its temperature dependence, and SEM images have been applied for the comparative study of the ZnO and ZnO Cu nanocrystals (NCs). NCs were created by the electrochemical (anodization) method with different NC sizes and annealed at 400 °C for 2 hours in ambient air. PL spectra show a near band edge (NBE) emission and three defect-related PL bands in ZnO NCs, and four defect-related PL bands in ZnO Cu NCs. The defect related PL band with the peak at 2.60-2.70 eV is connected with the Cu complexes in ZnO Cu NCs. PL spectra of all PL bands in both NC types were studied in the temperature range 10-300 K. Their integrated PL intensities and the activation energies of PL thermal decays have been estimated. It is shown that the PL intensities of defect related PL bands have been stimulated essentially at doping by the small Cu concentration (2.2%) in ZnO Cu NCs. It is revealed additionally that the Cu doping leads to decreasing the activation energies of thermal decay for the defect related PL bands in ZnO Cu NCs in comparison with those in ZnO NCs. The physical reason of this effect and the mechanisms of optical transitions for defect related PL bands in ZnO NCs have been discussed.