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

where tech-TOPO performance was batch-specific due to the relatively randompresence of adventitious impurities [15]. More recently, tech-TOPO hasbeen replaced with ‘‘pure’’ TOPO to which phosphonic acids have been addedto controllably mimic the presence of the tech-grade impurities [17]. In addition,TOPO has been replaced with various fatty acids, such as stearic andlauric acid, where shorter alkyl chain lengths yield relatively faster particlegrowth. The fatty acid systems are compatible with the full range of cadmiumprecursors, but they are most suited for growth of larger NQDs (>6 nm indiameter), compared to the TOPO–TOP system, as growth proceeds relativelymore quickly [16]. The cadmium precursor is typically dissolved in thefatty acid at moderate temperatures, converting the Cd compound to cadmiumstearate, for example. Alkyl amines were also successfully employed asCdSe growth media [16]. Incompatible systems are those that contain theanion of a strong acid (present as the surfactant ligand or as the cadmiumprecursor) and thiol-based systems [16]. Perhaps the most successful system,in terms of producing high quantum yields (QYs) in emission and monodispersesamples, uses a complex mixture of surfactants: stearic acid, TOPO,hexadecylamine, TBP, and dioctylamine [18].High QYs are indicative of a well-passivated surface. NQD emission cansuffer from the presence of unsaturated, ‘‘dangling’’ bonds at the particlesurface which act as surface traps for charge carriers. Recombination oftrapped carriers leads to a characteristic emission band (‘‘deep-trap’’ emission)on the low-energy side of the ‘‘band-edge’’ photoluminescence (PL)band. Band-edge emission is associated with recombination of carriers inNQD ‘‘interior’’ quantized states. Coordinating ligands help to passivatesurface trap sites, enhancing the relative intensity of band-edge emissioncompared to the deep-trap emission. The complex mixed-solvent system,described earlier, has been used to generate NQDs having QYs as high as 70–80%. These remarkably high PL efficiencies are comparable to the bestachieved by inorganic epitaxial-shell surface-passivation techniques (see Sect.III). They are attributed to the presence of a primary amine ligand, as well asto the use of excess selenium in the precursor mixture (ratio Cd : Se of 1 : 10).The former alone (i.e., coupled with a ‘‘traditional’’ Cd:Se ratio of 2 : 1 or 1 : 1)yields PL QYs that are higher than those typically achieved by organicpassivation (40–50% compared to 5–15%). The significance of the latterlikely results from the unequal reactivities of the cadmium and seleniumprecursors. Accounting for the relative precursor reactivities by using suchconcentration-biased mixed precursors may permit improved crystallinegrowth and, hence, improved PL QYs [18]. Further, in order to achieve thevery high QYs, reactions must be conducted over limited time spans from 5 to30 mins. PL efficiencies reach a maximum in the first half of the reaction anddecline thereafter (Fig. 4). Optimized preparations yield rather large NQDs,Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.