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

Figure 2 FTIR spectra of n-type CdSe nanocrystals with the indicated diameters.(From Ref. 14.)Overall, the measured size dependence is in satisfactory agreement with thepredictions of the k p approximation, deviating more strongly at small sizes.The large oscillator strength of the 1S e –1P e transition leads to strongoptical changes upon photoexcitation of an electron–hole pair. Figure 3shows the interband pump fluence dependence of the IR transmission for asample of CdSe colloids. The intraband absorption cross sections derivedfrom such plots agree within 30% with results of estimations [12]. In accordwith expectations, the 1S e –1P e cross-section is similar to the interband crosssectionat the band edge. This similarity is obvious in Figure 4, which showsthe IR and visible spectral change upon electrochemical charge transfer inthin films (f0.5 Am) of CdSe nanocrystals [24]. In Fig. 4, the bleach of the firstexciton peak at 2 eV is complete (DODf0.5), arising from the transfer oftwo electrons to each nanocrystal in the film, and the intraband absorbanceat 0.27 eV is of the same magnitude (DODf0.8).For CdSe samples synthesized by organometallic methods [25], the sizedispersion, DR/R, is typically 5–10%. Using a 10% size dispersion as abenchmark, noting that the 1S e –1P e transition energy scales at most as R 2 ,and barring other broadening mechanisms, the overall IR inhomogenouslinewidth [full width at half-maximum (FWHM)] should be less than f23%Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.