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

process is inhibited by rapidly removing the photogenerated holes from theQD core by trapping them on or near the QD surface, the electron coolingtime can be slowed down significantly.In contradiction to the above-discussed results, many other investigationsexist in the literature in which a phonon bottleneck was apparently notobserved. These results were reported for both self-organized S-K QDs[11,92,110–122] and II–VI colloidal QDs [116,118,120]. However, in severalcase [101,123,125], hot electron relaxation was found to be slowed, but notsufficiently to enable the authors to conclude that this was evidence of a phononbottleneck. For the issue of hot electron transfer, this conclusion may notbe relevant because in this case, one is not interested in the question of whetherthe electron relaxation is slowed so drastically that nonradiative recombinationoccurs and quenches photoluminescence, but rather whether the coolingis slowed sufficiently so that excited-state electron transport and transfer canoccur across the semiconductor–molecule interface before cooling. For thispurpose, the cooling time need only be increased above about 10 ps, becauseelectron transfer can occur within this timescale [36,126–128].The experimental techniques used to determine the relaxation dynamicsin the above-discussed experiments showing no bottleneck were all based ontime-resolved PL or transient absorption spectroscopy. The S-K QD systemthat were studied and exhibited no apparent phonon bottleneck includeIn x Ga l x As/GaAs and GaAs/AlGaAs. The colloidal QD systems wereCdSSe QDs in glass (750 fs relaxation time) [120] and CdSe [112]. Thus, thesame QD systems studied by different researchers showed both slowed coolingand nonslowed cooling in different experiments. This suggest a strongsample-history dependence for the results; perhaps, the samples differ in theirdefect concentration and type, surface chemistry, and other physical parametersthat affect carrier cooling dynamics. Much additional research is requiredto sort out these contradictory results.V. QUANTUM-DOT ARRAYSA major goal in semiconductor nanoscience is to form QD arrays andunderstand the transport and optical properties of these arrays. One approachto forming arrays of close-packed QDs is to slowly evaporate colloidalsolutions of QDs; upon evaporation, the QD volume fraction increases andinteraction between the QDs develops and leads to the formation of a selforganizedQD film. Figure 18 shows a TEM micrograph of a monolayer madewith InP QDs with a mean diameter of 49 A˚ and in which each QD isseparated from its neighbors by TOPO/TOP capping groups; local hexagonalorder is evient. Figure 19a shows the formation of a monolayer organized in aCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.