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

of the center frequency. The experimental observations for CdSe nanocrystalsare instead between 30% and 50% of the center frequency, and the linewidthincreases as the particle size becomes smaller. In addition, as shown in Fig. 2,the spectra obtained for n-type nanocrystals show a multiple-peak structure.For future applications, it will be useful to identify and control the conditionsnecessary to achieve the narrowest intraband linewidths given a finite amountof size polydispersity. The most likely explanation for the large linewidth isthe splitting of the 1P e states. In one experimental observation, zinc-blendeCdS nanocrystals exhibit a narrower linewidth than wurtzite CdSe or ZnOnanocrystals [14]. In order to narrow the linewidth, parameters such asnanocrystal shape and crystal symmetry can be investigated.IV.INTRABAND ABSORPTION PROBING OF CARRIERDYNAMICSBecause the IR absorption is directly assigned to electrons with littlecontribution from holes, it is a convenient probe of the electron dynamics.This is an advantage of intraband spectroscopy over transient interbandspectroscopy, because the latter yields signals that depend on both electronand hole dynamics. The combination of intraband spectroscopy with othertechniques that probe the combined electron and hole response, such asinterband transient spectroscopy, is a useful approach to analyaze the evolutionof the exciton and a way to study which specific surface conditionsaffect the trapping processes and the fluorescence efficiency [26].Figure 5 shows an example of the different time traces of the intrabandabsorption after the creation of an electron–hole pair in CdSe nanocrystalscapped with various molecules. Whereas TOPO (trioctylphosphineoxide)-capped nanocrystals exhibit strong band-edge fluorescence, both thiophenol-cappedand pyridine-capped nanocrystals have strongly reduced fluorescence.Intraband spectroscopy shows that the 1S electron relaxationdynamics differ dramatically depending on surface capping. For thiophenol,the electron in the 1S e state is longer lived than for TOPO-capped samples,indicating that it is hole trapping that quenches fluorescence. The hole trap ispresumably associated with a sulfur lone pair, which is stabilized by the conjugatedring. In contrast, for pyridine, which is also thought to be a hole trapbecause it is a strong electron donor, most of the excited electrons live only ashort time in the 1S e state. Therefore, it appears that pyridine strongly enhanceselectron trapping and/or fast (picosecond), nonradiative electron–holerecombination. Yet, there is a small percentage of nanocrystals (f5–10%)with a long-lived electron, and this must arise from a few of the nanocrystalsundergoing complete charge separation. In fact, thiophenol-capped nano-Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.