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

limit given by radiative relaxation (f100 ns). In quantum wells, one-opticalphononrelaxation processes provided an effective mechanism for electronenergy relaxation because there is a continuum of electronic states. However,for strongly confined quantum dots in which the electronic energy separationis many times larger than the optical phonon energy, multiphonon processmust be invoked, and these processes are expected to be too slow to explainthe observed fast relaxation. This phenomenon is called the phonon bottleneck[30,31]. Alternative explanations involve coupling to specific LO F LAphonon combinations [32] or defect states [33] or to carrier-carrier scatteringwith electrons [34] or holes [35,36]. Intraband relaxation rates have beeninitially determined by fluorescence rise-time and transient interband spectroscopy.For the colloidal quantum dots, Klimov et al. were the first toobserve the subpicosecond relaxation of transient bleach features attributedto the 1P 3/2 –1P e transition [37]. This very fast relaxation, still observed in thelimit of a single electron–hole pair per nanocrystal, was explained by a fastAuger process involving scattering of the electron by the hole, as shown in theschematic in Fig. 6A. The electron–hole Auger relaxation is understood to beFigure 6 (A) Schematics of the electron–hole Auger-like relaxation process leadingto fast electron intraband relaxation. (B) If the hole is captured by a surface trap, theelectron–hole Auger coupling is reduced, leading to a slower intraband relaxation.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.