Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

cooling time because a quantized transition from a higher quantized electronstate to the ground electron state was not present. Heitz et al. [101] also reportrelaxation times for holes of about 40 ps for stacked layers of S-K InAs QDsdeposited on GaAs; the InAs QDs are overgrown with GaAs and the QDs ineach layer self-assemble into an ordered column. Carrier cooling in thissystem is about two orders of magnitude slower than in higher-dimensionalstructures.All of the above studies on slowed carrier cooling were conducted onself-assembled S-K type of QDs. Studies of carrier cooling and relaxationhave also been performed on II–VI CdSe colloidal QDs by Klimov et al.[116,129] and Guyot-Sionnest et al. [95] and on InP QDs by Ellingson et. al.[130], and Blackburn et al. [131]. The Klimov group first studied electronrelaxation dynamics from the first-excited 1P to the ground 1S state usinginterband pump-probe spectroscopy [116]. The CdSe QDs were pumped with100-fs pulses at 3.1 eV to create high-energy electrons and holes in theirrespective band states and then probed with femtosecond white-light continuumpulses. The dynamics of the interband bleaching and induced absorptioncaused by state filling was monitored to determine the electron relaxation timefrom the 1P to the 1S state. The results showed very fast 1P to 1S relaxation,on the order of 300 fs, and was attributed to an Auger process for electronrelaxation, which bypassed the phonon bottleneck. However, this experimentcannot separate the electron and hole dynamics from each other. Guyot-Sionnest et al. [95] followed up these experiments using femtosecond infraredpump-probe spectroscopy. A visible pump beam creates electrons and holesin the respective band states and a subsequent infrared (IR) beam is split intoan IR pump and an IR probe beam; the IR beams can be tuned to monitoronly the intraband transitions of the electrons in the electron states and, thus,can separate electron dynamics from hole dynamics. The experiments wereconducted with CdSe QDs that were coated with different capping molecules(TOPO, thiocresol, and pyridine), which exhibit different hole trappingkinetics. The rate of hole trapping increased in the order: TOPO, thiocresol,and pyridine. The results generally show a fast relaxation component (1–2 ps)and a slow relaxation component (f200 ps). The relaxation times follow thehole trapping ability of the different capping molecules and are longest for theQD systems having the fastest hole trapping caps; the slow componentdominates the data for the pyridine cap, which is attributed to its faster holetrapping kinetics.These results [95] support the Auger mechanism for electron relaxation,whereby the excess electron energy is rapidly transferred to the hole, whichthen relaxes rapidly through its dense spectrum of states. When the hole israpidly removed and trapped at the QD surface, the Auger mechanism for hotCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.