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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)]. [ABSTRACT FROM AUTHOR]

In the quantum confinement regime, the time-resolved photoluminescence mapping of CdSe nanocrystals reveals unusual size-dependent characteristics in solute or solid-state environments. The former case typifies long-lived intradot exciton recombinations, while in the latter energy-dependent Kohlrausch relaxations quench luminescence via interdot energy transfer. The observed energy transfer rate increases for larger size (1.5×108 s-1 for d=2.5 nm and 6.4×108 s-1 for d=10 nm). [ABSTRACT FROM AUTHOR]