Sub-picosecond polychromatic photoluminescence studies of CdSe and PbSe nanodots

We present our studies of femtosecond photoluminescence of colloidal solns. of CdSe and PbSe nanocrystals, using polychromatic fluorescence upconversion. Ultrafast relaxation processes are obsd. in both cases upon excitation at 400 nm and 800 nm resp. Under moderately high excitation densities we studied the formation and dynamics of biexitons and triexcitons in CdSe nanocrystals. Contrary to earlier reports, all results could be understood without invoking the presence of charged particles. The dynamics of single excitons in PbSe nanocrystals is found to be similar to the case of CdSe, despite the high confinement in the former., The early-time spectra are characterized by emission from several low lying excited states. The kinetics point to a fast sequential cascade process between excited states, governed by energy gaps, and indicate the presence of addnl. dark states. The first direct measurement of the lowest excited states Huang-Rhys factors excludes that strong electron-phonon coupling mediates the intraband relaxation.