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

mix the hole states with the angular momentum projections on the hexagonalaxis F3/2 and b1/2 [24]. The phonons with angular momentum 1 allow oneto flip the electron spin through the Rashba spin-orbital terms [12]. Thepolarization properties of the ground exciton state depend strongly on theangular momentum of phonons participating in the phonon-assisted opticaltransitions.The calculations of the relative strength of the phonon-assisted transitionsin NCs are unreliable because the strength of the exciton–opticalphonon coupling in NCs is a still a controversial subject (see, e.g., Ref. 23).This makes it difficult to predict the polarization properties of the darkexcitonstate. However, a single almost spherical CdSe NC [32] showscircularly polarized PL. The dark-exciton state in these crystals has total angularmomentum projection F = F2 (see Fig. 2d). This shows that thephonons with the angular momenta l = 2 and l = 1 can be responsible forthe phonon-assisted recombination of the dark-exciton state.The variation in the CdSe NC shape strongly affect their polarizationproperties. The individual CdSe NCs show a high degree of linear polarization(70%) when their aspect ratio changes from 1:1 to 1:2 [33]. Thisvariation of the NC shape changes the order of the exciton levels and theexciton state with the angular momentum projection F = 0 becomes theground state in elongated NCs, according to our calculations [3] (see Fig.2c). However, this state is also the dark exciton state with a zero dipoletransition matrix element. The linear PL polarization properties of the F = 0state can be due to the l = 0 phonon-assisted transitions that mix dark- andbright-exciton states with F = 0.The polarization properties of the individual nearly spherical CdSenanocrystals and CdSe nanorods suggest that the interaction of the phononswith holes is the major mechanism of phonon-assisted recombination of thedark excitons in NCs.B. Linear Polarization Memory EffectThe linear polarization memory effect in NC PL was observed by Bawendi etal. [20] for the case of the resonant excitation in the absorption band-edge tail.The sample was an ensemble of randomly oriented CdSe hexagonal NCs of 16A˚ radius and the sample did not have any preferential axis. The polarizationmemory effect in such a sample is due to the selective excitation of some NCswhich have a special orientation of their hexagonal axis relative to the polarizationvector of the exciting light. The same NCs emit light with polarizationcompletely determined by polarization properties of the ground exciton stateand the NC orientation.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.