Ultrafast Carrier Dynamics in CdSe Nanocrystals Determined by Femtosecond Fluorescence Upconversion Spectroscopy

Femtosecond fluorescence upconversion has been utilized to study the band edge and deep trap emission dynamics of cadmium selenide (CdSe) nanocrystals (NC's) ranging in size from 27 to 72 Å in diameter. Both the band edge rise time and decay show a direct correlation to NC size, and a rise time 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 radiatively with the initial photogenerated hole. By this process, the large amplitude of the band edge emission that is attributed to direct electron/hole 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 lying just below the lowest electronic level consistent with the “Dark Exciton”. The extended deep trap emission arises from the relaxation of the excited-state conduction band electron to a surface-localized hole or vice-versa. A new model is presented which describes these mechanisms for exciton relaxation in CdSe quantum dots.