Temperature and size dependence of time-resolved exciton recombination in ZnO quantum dots

ZnO nanocrystals with various sizes were prepared and characterized. Their photoluminescence dynamics has been investigated at low temperatures. For the smallest particles (3 nm), a defect-induced long-lived photoluminescence occurs around 2.5 eV which is slowed down at decay times longer than 3 ns when sample temperature T decreases. From thermal quenching of the 2.5 eV band, the exciton dissociation energy at defect centers is estimated around ∼11.8 meV. For larger crystallites (10 and 20 nm), the ultraviolet emission observed at 3.32 eV decays in less than 85 ps and follows a Varshni law [Y. P. Varshni, Physica (Amsterdam) 34, 149 (1967)].