Photoluminescence investigation of the carrier recombination processes in ZnO quantum dots and nanocrystals

The carrier recombination processes in ZnO quantum dots (similar to 4 nm in diameter), ZnO nanocrystals (similar to 20 nm in diameter) and bulk ZnO crystal have been studied using photoluminescence (PL) spectroscopy in the temperature range from 8.5 to 300 K. The obtained experimental data suggest that the ultraviolet PL in ZnO quantum dots originates from recombination of the acceptor-bound excitons for all temperatures. In the larger size ZnO nanocrystals, the recombination of the acceptor-bound excitons is the dominant contribution to PL only at low temperature (T < 150 K). For higher temperatures (T > 150 K), PL is mostly due to recombination of the donor- bound excitons. Recombination processes in ZnO quantum dots and nanocrystals differ from those in bulk ZnO mainly because of the large surface-to-volume ratio in both types of nanoparticles and, consequently, a large number of acceptor defects near the surface. No strong inhomogeneous broadening has been observed in ultraviolet PL from ZnO quantum dots. Our results shed light on the carrier-recombination processes in ZnO quantum dots and nanocrystals, and can be used for the ZnO nanostructure optimization for the proposed optoelectronic and spintronic applications.