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Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots (QDs) were investigated by two-colour femtosecond (fs) pump-probe (400/800 nm) and picosecond time-resolved photoluminescence (ps TRPL) experiments.Pump-probe measurement results show that there are two components for the excited carriers relaxation,the fast one with a time constant of several ps arises from the Auger-type recombination,which shows almost particle sizeindependence.The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence.The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm.The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons,and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.
Size-dependence of optical properties and energy relaxation in CdSe / ZnS quantum dots (QDs) were investigated by two-color femtosecond (fs) pump-probe (400/800 nm) and picosecond time-resolved photoluminescence (ps TRPL) experiments. -probe measurement results show there there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle size independence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.