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

significantly greater than the fast TA component indicates that the dominantmechanism leading to reduced band-edge PL efficiencies in samples withincomplete surface passivation is hole trapping.Carrier trapping at surface defects represents the most important mechanismfor nonradiative carrier losses in the regime of low excitation densities.However, recently developed methods for surface treatment using both inorganic(e.g., core–shell structures [9]) and organic [8] capping allow significantsuppression of this effect. These new methods provide routinefabrication of CdSe NQDs with PL quantum yields greater than f80% atroom temperature, strongly suggesting that yields near 100% are feasible.B. Competition Between Optical Gain and PhotoinducedAbsorptionIn addition to causing nonradiative carrier losses, surface trapping can deterioratethe lasing performance of NQDs because of excited-state absorption[i.e., photoinduced absorption (PA)] associated with carriers trapped at NQDinterfaces. [17,44] As discussed next, PA is strongly sensitive to both NQDsurface properties and the identity of the matrix material. Excited-stateabsorption, for example, develops in such commonly used solvents as hexaneand toluene, in which it can completely suppress optical gain, the effectparticularly pronounced in dots of small sizes.The competition between optical gain and PA was studied in Ref. 44using a femtosecond TA experiment. This experiment allows one to monitorthe absorption of the sample with (a) and without (a 0 ) a pump. In absorptionspectra, optical gain corresponds to a < 0 [i.e., to pump-induced absorptionbleaching (Da = a a 0 < 0) that is greater than a 0 ( Da/a 0 >1)]. PA correspondsto Da > 0 (i.e., to the situation for which the absorption of the excitedsample is greater than its ground-state absorption).In ultrafast, pump-dependent studies of NQDs, it is convenient tocharacterize carrier densities in terms of NQD average populations, hNi(i.e., in terms of the number of e–h pairs per dot averaged over an ensemble).The initial NQD average population, hNi 0 , generated by a short pump pulsecan be calculated using the expression hNi 0 = r a (tx p )J p , where J p is the perpulsepump fluence measured in photons/cm 2 and r a (tx p ) is the NQDabsorption cross section at the pump spectral energy, tx p [33]. For the caseof optical excitation well above the NQD energy gap, the NQD absorptioncross section can be estimated fromr a ðtxÞ ¼ 4p n 3 R3 bn AfðtxÞA2 a b ðtxÞð1Þwhere a b and n b are the absorption coefficient and the refractive index of thebulk semiconductor, respectively, n is the index of the NQD sample, and f isCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.