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

decreased for the majority of NCs and a gradual growth of the maximumallowed polarization degree from 0.625 to 0.75 can occur. Precisely suchbehavior is observed in Fig. 9c, where the low-temperature polarization firstsaturates near f0.6 and then grows slowly to f0.73 at the highest fields.This model [Eq. (61)] provides an excellent fit to the measured PLpolarization data [36], as illustrated in Fig. 10, which displays experimentalresults of NCs of three different sizes (T = 1.45 K) along with results of themodeling for 57-A˚ NCs. Interestingly, all data are roughly equivalent; thisstems from the balancing role played by s nr and the exchange interaction g[see Eq. (51)] both of which decrease with increasing NC size. The best-fitvalues of g ex,2 and s nr are shown in the inset. The extracted exciton g factorsare close to 0.9, which is much smaller than the value calculated for thedark-exciton state.V. DISCUSSION AND CONCLUSIONSWe have shown that the dark/bright-exciton model presented in this chapterdescribes very well many important properties of the band-edge PL in CdSeNCs. The phenomena analyzed here include the fine structure of the PLexcitation spectra, a Stokes shift of the resonant PL, the shortening of theradiative decay time in a magnetic field, the polarization memory effect, thetransition from the circular polarized to the linear polarized PL induced bychanging the NC shape, and PL polarization in strong magnetic fields. Somequantitative disagreements between experimental data and the theory aremainly due to some oversimplifications in the theoretical approach. Forexample, the theory considered here does not take into account the nonparabolicityof the conduction and the valence bands. In small NCs, thenonparabolicity can strongly modify the level structure, the electron–holewave functions, the transition oscillator strength, the overlap integrals, theelectron–hole exchange interactions, g factors, and so forth. All of thesequantities, in principle, can be better described in the framework of a morerigorous approach.The dark/bright-exciton model described in this chapter can also beapplied to NCs with other than CdSe compositions. It was used, for example,to described the size-dependent Stokes shift in InAs NCs [41]. The penetrationof the electron wave function under the barrier was important in InAs NCs forquantitative description of this dependence.After a decade of studies and despite all the success of the dark/brightexcitonmodel, the PL in CdSe NCs still presents several unresolved puzzles.One of the unresolved issues is the temperature-dependent Stokes shiftCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.