Time-Resolved Intraband Relaxation of Strongly-Confined Electrons and   Holes in Colloidal PbSe Nanocrystals
  The relaxation of strongly-confined electrons and holes between 1P and 1S levels 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. Mechanisms that could possibly circumvent the phonon bottleneck in IV-VI quantum dots are discussed. 
Jeffrey M. Harbold, Frank W. Wise, Hui Du, Todd D. Krauss, Kyung-Sang   Cho, and Chris B. Murray