ULTRAFAST CARRIER DYNAMICS IN SEMICONDUCTOR QUANTUM DOTS

The dynamics of band-edge photoluminescence (PL) in CdS nanocrystals ( NC's) dispersed in a glass matrix are studied with the femtosecond up- conversion technique. The time-resolved PL spectra exhibit several dis crete features (three of them are in the NC energy band gap) which are not pronounced in a cw PL spectrum. The initial stage of a PL decay i s governed by a depopulation of the lowest extended states due to carr ier trapping (localization) on the time scale of 1 ps. The low-energy bands originating from the extended-to-localized state transitions exh ibit extremely fast buildup dynamics (rise time is 400-700 fs) which i s explained by the preexisting occupation of the localized states.