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

The theory of the polarization memory effect for an ensemble ofrandomly oriented CdSe NC was developed in Ref. 12. Here, we will consideronly qualitative conclusions of Ref. 12 because it did not take the exchangeelectron–hole interaction into account.The linear polarized light selectively excites NCs with the hexagonalaxis predominantly parallel to the vector polarization of exciting light whenthe excitation frequency is in resonance with the F = 0 bright-exciton state.The emission of this nanocrystals is determined by the dark-exciton stateand emitted light polarization vector is perpendicular to the hexagonal axis.This leads to the negative degree of the PL polarization as was observed inRef. 20.If the exciting light is in resonance with F = F1 bright-exciton state,however, the degree of the linear polarization should be positive. This makesthe experiments on the linear polarization memory effect very sensitive to thenanocrystal size distribution and the frequency of optical excitation.C. Stokes Shift of the Resonant PL and Fine Structureof Bright-Exciton StatesThe strong evidence for the predicted band-edge fine structure has been foundin fluorescence line-narrowing (FLN) experiments [2,3]. The resonant excitationof the samples in the red edge of the absorption spectrum selectivelyexcites the largest dots from the ensemble. This selective excitation reduces theinhomogeneous broadening of the luminescence and results in spectrallynarrow emission, which displays a well-resolved longitudinal optical (LO)phonon progression. In practice, the samples were excited at the spectralposition for which the absorption was roughly one-third of the band-edgeabsorption peak. Figure 6 shows the FLN spectra for the size series consideredin this chapter. The peak of the zero-LO-phonon line (ZPL) is observedto be shifted with respect to the excitation energy. This Stokes shift is sizedependent and ranges from f20 meV for small NCs to f2 meV for large NCs.Changing the excitation wavelength does not noticeably affect the Stokes shiftof the larger samples; however, it does make a difference for the smaller sizes.This difference was attributed to the excitation of different size dots within thesize distribution of a sample, causing the observed Stokes shift to change. Theeffect is the largest in the case of small NCs because of the size dependence ofthe Stokes shift (see Fig. 7).In terms of the proposed model, the excitation in the red edge of theabsorption probes the lowest jFj = 1 bright-exciton state (see Fig. 2d). Thetransition to this state is followed by relaxation into the dark jFj = 2 state.The dark exciton finally recombines through phonon-assisted [2,12] ornuclear/paramagnetic spin-flip assisted transitions [2]. The observed StokesCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.