Time-resolved intraband relaxation of strongly confined electrons and holes in colloidal PbSe nanocrystals

The relaxation of strongly confined electrons and holes from higher excited states to their lowest excited states in colloidal PbSe nanocrystals has been time resolved using femtosecond transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor nanocrystals, both electrons and holes are strongly confined in PbSe nanocrystals. Despite the large electron and hole energy level spacings (at least 12 times the optical phonon energy), we consistently observe picosecond time-scale relaxation. Existing theories of carrier relaxation cannot account for these experimental results. We conjecture about mechanisms that could circumvent the expected phonon bottleneck in IV-VI quantum dots.