ZnO:Cu nanorods with visible luminescence: copper induced defect levels and its luminescence dynamics

In this work, we report the synthesis of ZnO:Cu nanoparticles with rod-like morphology using co-precipitation method. X-ray diffraction analysis indicated that these ZnO: Cu nanoparticles has wurtzite structure with preferential growth along (1 0 1) crystal plane. The formation of additional defect levels in these particles on doping with Cu was investigated using Raman and fluorescence spectroscopy. The increase in intensity of E-1 (LO) mode observed at similar to 580 cm(-1) in the Raman spectra of ZnO: Cu nanoparticles, confirmed the formation of additional defect levels in these nanoparticles on doping with Cu. The doping concentration was evident in the intensity of the additional Raman mode observed at similar to 280 cm(-1) for the Cu doped nanoparticles. Photoluminescence spectra of Cu doped ZnO nanoparticles shows three visible emission peaks at 413, 435 and 531 nm along with a UV emission peak at 390 nm, whereas undoped ZnO nanoparticles showed only two peaks, at 389 and 582 nm. The shrinkage in band gap causing the emission of violet, blue and green colors on doping with Cu are attributed to the s-d and p-d exchange interactions between conduction band electrons of ZnO and localized d electrons of Cu ions, resulting in renormalization of band gap. The emission bands observed in these ZnO: Cu nanoparticles has been illustrated with a schematic energy level diagram.