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

confined NQDs is not significantly slower than in bulk materials. These datafurther imply that the optical-gain buildup in NQDs is not inhibited by theefficiency of intraband energy losses.III.CARRIER TRAPPING AT INTERFACE STATES ANDEXCITED-STATE ABSORPTION IN NQDsA. Dynamics of Electron and Hole Surface TrappingAn important concern associated with light-emitting applications of NQDs isa high probability of carrier trapping at surface defects. For samples withincomplete surface passivation, surface trapping can lead to depopulation ofNQD quantized states on picosecond timescales leading to significantly reducedband-edge PL efficiencies [37]. We briefly review some results ofultrafast studies on electron and hole surface trapping. These studies wereperformed in the low-intensity excitation regime (less than one e–h pair perdot on average was excited) for which the role of fast multiparticle Augerrecombination (see Sec. IV. B) was insignificant.Electron trapping dynamics have been studied using primarily a femtosecondTA experiment [29,37]. At the stage following energy relaxation,bleacing of the lowest (1S) absorption peak is almost entirely due to thepopulation of the 1S electron state. The contribution of holes into thisbleaching is not significant because the hole state involved in the 1S transitionoriginates from a high-energy manifold of ‘‘absorbing’’ states that are notoccupied after energy relaxation is finished.The effect of the quality of surface passivation on electron dynamics isillustrated in Fig. 9, in which we compare the 1S bleaching decay for threesamples with distinctly different surface properties [37]. The samples arefreshly prepared NQDs passivated with organic molecules of trioctylphosphineoxide (TOPO) and trioctylphosphine (TOP) (TOPO-capped NQDs)(circles), the same NQDs but 8 months after preparation (squares), andfreshly prepared NQDs with ZnS overcoating (core–shell structures)(crosses). As indicated by the PL quantum yield measurements, the qualityof the ‘‘electronic’’ passivation of surface traps is progressively increased ingoing from the aged to the fresh TOPO-capped sample and then to the sampleovercoated with ZnS. For all samples, the 1S bleaching decay shows twodistinct regions: an initial region of fast sub-100-ps relaxation, which isfollowed by a slower nanosecond decay. The amplitude of the fast componentis sensitive to the NQD surface passivation, indicating that it is likely due tosurface-related electron relaxation. In freshly prepared TOPO-capped NQDs,the fast component is approximately 15% of the signal amplitude. Degradationin the surface passivation leads to an enhancement of this component upCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.