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

These transients show a progressive increase in the delay (t d ) of the PLmaximum with a reduction in the detection energy (E) as indicated by the plotin Fig. 6a (solid circles). The derivative, dE/dt d , provides a measure of the holeenergy-loss rate. For 1.8-nm dots, the energy relaxation rate is nearly constant(1.5 eV/ps) between 2.3 and 3.1 eV, but reduces dramatically (down to f0.26eV/ps) below the 1S absorption peak [i.e., at the stage of final hole relaxationbetween ‘‘absorbing’’ and ‘‘emitting’’ fine-structure states (see schematics ofhole energy levels in Fig. 1a]. Two stages in hole relaxation are also observedfor dots with radii 1.2 and 3 nm (Fig. 6a, open squares and open triangles,respectively). The spectral onset of the ‘‘slow’’ relaxation region is sizedependent, closely following the position of the 1S absorption peak. The‘‘fast’’ and ‘‘slow’’ relaxation rates were in the range 1.3–1.8 eV/ps and 0.19–0.3 eV/ps, respectively.In Fig. 6b, we compare size-dependent relaxation rates observed forhole and electrons. In contrast to electron relaxation rates that increase withdecreasing dot size, hole rates show an opposite trend. In this case, both the‘‘fast’’ and ‘‘slow’’ rates decrease as the dot radius is decreased, indicating arelaxation mechanism that is different from the Auger-type energy transferresponsible for electron relaxation. The relaxation data obtained for sampleswith different surface passivations and in different solvents indicate that holeenergy-loss rates are independent of NQD surface/interface properties,suggesting that hole relaxation does not result from coupling to surface defectsor solvent molecules but rather is due to some intrinsic mechanisms suchas coupling to NQD lattice vibrations (phonons).The observed ‘‘fast’’ hole energy relaxation rates are close to thoseestimated for hole-LO phonon interactions in bulk CdSe (f1.4 eV/ps) [26],suggesting a ‘‘bulk’’-like relaxation process or, more specifically, a cascade ofsingle-phonon emission acts (one phonon per relaxation step). The fact thatphonon emission by ‘‘hot’’ holes is apparently not hindered by the discretecharacter of the energy levels implies that the valence-band states form a verydense spectrum (quasicontinuum) at spectral energies above the 1S absorptionpeak. Large effective hole masses, the existence of three valence subbandsstrongly intermixed by quantum confinement [19], and the fine-structuresplitting of valence band states [23] are all factors that can lead to a highdensity of hole states [the effect of fine-structure splitting is qualitativelyillustrated in Fig. 7 taking into account S- and P-type hole states; an evendenser spectrum is expected if the states of other symmetries (D, F, etc.) arealso taken into account]. The hole energy structures are further smeared outby broadening due, for examples, to dephasing induced by elastic carrier–phonon scattering [42]. Another factor that simplifies the process of meetingenergy conservation requirements in NQDs is the relaxation of momentumCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.