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

To avoid this problem, many groups have utilized another opticaltechnique, photoluminescence excitation (PLE) spectroscopy [8,9,54,74–76].PLE is similar to TDA in that it selects a narrow subset of the sampledistribution to obtain absorption information. However, in PLE experiments,one utilizes the emission of the nanocrystals. Thus, this technique is particularlysuited to the efficient fluorescence observed in high-quality samples.PLE works by monitoring a spectrally narrow emission window within theinhomogenous emission feature while scanning the frequency of the excitationsource. Because excited nanocrystals always relax to their first excitedstate before emission, the spectrum that is obtained reveals absorptioninformation about the narrow subset of nanocrystals that emit.An additional advantage of this technique is that emission informationcan be obtained during the same experiment. For example, fluorescence linenarrowing(FLN) spectroscopy can be used to measure the emission spectrumfrom a subset of the sample distribution. In particular, by exciting thenanocrystals on the low-energy side of the first absorption feature, only thelargest dots in the distribution are excited.Figure 4 demonstrates all of these techniques. In Fig. 4a, absorption andemission results are shown for a sample of CdSe nanocrystals with a meanradius of 1.9 nm. On this scale, only the lowest two excited electron–hole pairstates are observed in the absorption spectrum (solid line). The emissionspectrum (dashed line) is obtained by exciting the sample well above itsfirst transition so that emission occurs from the entire sample distribution.This inhomogeneously broadened emission feature is referred to as the fullluminescence spectrum. If, instead, a subset of the sample distribution isexcited, a significantly narrowed and structured FLN spectrum is revealed.For example, when the sample in Fig. 4 is excited at the position of the downwardarrow, a vibrational progression is clearly resolved (due to longitudinaloptical phonons) in the emission spectrum. Similarly, by monitoring theemission at the position of the upward arrow, the PLE spectrum in Fig. 4breveals absorption features with higher resolution than in Fig. 4a. Further,additional structure is observed within the lowest absorption feature. As wediscuss later, these features (labeled a and h) represent fine structure presentin the lowest electron–hole pair state and have important implications forquantum-dot emission. However, before discussing this fine structure, we firsttreat the size dependence of the electron structure in CdSe nanocrystals.C. Size Dependence of the Electronic StructureAlthough the absorption and PLE spectra in Fig. 4 show only the two lowestexciton features, high-quality samples reveal much more structure. For example,in Fig. 5, PLE results for a 2.8-nm-radius CdSe sample are shown alongwith its absorption and full luminescence spectra. These data cover a largerCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.