Ultrafast carrier dynamics in CdSe nanocrystals determined by femtosecond fluorescence upconversion spectroscopy

Femtosecond fluorescence upconversion has been utilized to study the band e dge and deep trap emission dynamics of cadmium selenide (CdSe) nanocrystals (NC's) ranging in size from 27 to 72 Angstrom in diameter. Both the band e dge rise time and decay show a direct correlation to NC size, and a rise ti me that depends on excitation energy. Surface-oxidized and non-oxidized NC' s display the same band edge fluorescence decay kinetics, but the relative amplitudes of the short and long components differ. The deep trap emission that appears within 2 ps is attributed to ultrafast relaxation of a surface selenium;dangling bond electron to the valence band where it combines radi atively with the initial photogenerated hole. By this process, the large am plitude of the band edge emission that is attributed to direct electron/hol e recombination is attenuated within the initial 2-6 ps. The long lifetime of the band edge emission originates from a triplet state, with an energy l ying just below the lowest electronic level consistent with the "Dark Excit on". The extended deep trap emission arises from the relaxation of the exci ted-state conduction band electron to a surface-localized hole or vice-vers a. A new model is presented which describes these mechanisms for exciton re laxation in CdSe quantum dots.