Fluorescence-line narrowing in CdSe quantum dots: Surface localization of the photogenerated exciton

The electronic properties of shallow band-edge surface traps in nanometer-size CdSe quantum dots are probed using fluorescence-line-narrowing spectroscopy. We find large changes in electron-hole-pair radiative lifetimes and couplings to LO phonons as the temperature is changed from 1. 75 to 10 K. We attribute these changes to the localization of the photogenerated hole at the surface of the dots, accompanied by thermally activated motion between these surface localized states. A simple model based on the observed exciton--LO-phonon couplings is constructed to estimate the extent of hole localization in the luminescing state. A size-dependent study (20--80 diameter) indicates that surface effects diminish rapidly with increasing size.