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

Abstract

The carrier recombination processes in ZnO quantum dots ($\ensuremath{\sim}4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter), ZnO nanocrystals ($\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter) and bulk ZnO crystal have been studied using photoluminescence (PL) spectroscopy in the temperature range from $8.5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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 $(T<150\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. For higher temperatures $(T>150\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, 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.