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

NQD size. For small NQD sizes (R = 1.2 and 1.5 nm), excited-state absorptionoverwhelms bleaching before the crossover to optical gain. Thecritical NQD radius above which optical gain is observed is f2.3 nm.Excitation densities for which crossover to gain occurs (gain threshold)decrease as the dot size is increased. For dots of the ‘‘critical’’ 2.3-nm radius,the gain threshold is approached (but not crossed) at fhNi th = 8.5, whereas itis close to the theoretical limit of approximately one e–h pair per dot (see Sect.IV.A) for large 3.5-nm dots (hNi th = 1.4 for R = 3.5 nm). This strong sizedependence of the gain threshold for NQD hexane solutions is a direct resultof the size-dependent interplay between gain and PA.Analysis of TA data for NQDs with different surface passivationsindicates that the PA is sensitive to the identity of the capping layer. Forexample, in the case of hexane solutions, the magnitude of excited-stateabsorption can be reduced by overcoating NQDs with a layer of a wide-gapsemiconductor (e.g., using CdSe–ZnS core–shell structures). Excited-stateabsorption is also strongly dependent on the type of solvent/matrix material.Such commonly used solvents as toluene, chloroform, and heptamethylnonane(HTP) show a PA band comparable in intensity with that in hexane [46].However, the PA is almost completely suppressed in TOP, one of the NQDgrowth solvents, and in such solid-state matrices as polyvinyl butyral or solgeltitania [11], as well as in matrix-free, close-packed NQD films [46].Figure 13 displays pump-dependent TA data for CdSe NQDs withR = 2.3 nm dissolved in TOP. In hexane solutions, the TA pump dependencefor these dots only approaches the gain threshold but does not cross it (Fig.12b, crosses). However, in the case of TOP, these NQDs clearly show a gainband at the position of the PL band. The transition from absorption togain occurs at hNi th = 1.8 (circles in Fig. 13b), which is much lower than thegain threshold observed in hexane (f8.5). This difference provides additionalevidence of the size-dependent competition between gain and PA.Figure 14 displays gain data for close-packed, matrix-free NQD films.Shown are normalized nonlinear absorption spectra (spectra detected in thepresence of the optical pump) for three samples fabricated from NQDs withradii 2.1, 1.7, and 1.3 nm. For all samples, NQD radii are smaller than the‘‘critical’’ radius of 2.3 nm, and in the case of hexane solutions, these NQDsdo not show optical gain. In contrast to hexane-based samples, all solid-statesamples show optical gain at the position of the emitting transition, clearlyindicating suppression of PA. These data also provide an illustrative exampleof the size-controlled tunability of the gain band; it shifts by f300 meV as thedot radius changes from 2.1 to 1.3 nm.The strong effect of the identity of the solvent/matrix material on thestrength of PA indicates that this feature is not intrinsic to NQDs but israther a property of the NQD interface and/or matrix material [44]. OneCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.