Abstract
Porous anodic alumina (PAA), with large pore diameters (up to 100 nm), was used as a substrate for the growth of ZnO nanostructures. Almost unstrained ZnO nanoparticles of less than 8 nm in size were prepared by the spin-on method and annealed at 500 °C. Excitonic recombination is found to dominate the ultraviolet band. The ZnO nanoparticles have a high deep level emission at low excitation which becomes negligible at high excitation, as deduced from photoluminescence measurements. This large increase of the ultraviolet to visible luminescence ratio with excitation is ascribed to the variation of the ultraviolet and of the visible luminescence with excitation intensity. Non-radiative Auger recombination becomes dominant at high excitation. It is also shown that the PAA substrate improves the internal quantum efficiency of the ZnO nanoparticles achieved by the sol-gel method. At low excitation, the luminescence intensity drops by a factor of 3 between 13 and 300 K, leading to an estimate of the internal quantum efficiency as high as 30%.