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

growth can proceed by Ostwald ripening. Here, sacrificial dissolution ofsmaller (higher-surface-energy) particles results in growth of larger particlesand, thereby, fewer particles in the system [8].Alternatively, supersaturation and nucleation can be triggered by a slowramping of the reaction temperature. Precursors are mixed at low temperatureand slowly brought to the temperature at which precursor reaction anddecomposition occur sufficiently quickly to result in supersaturation [11].Supersaturation is again relieved by a ‘‘nucleation burst,’’ after whichtemperature is controlled to avoid additional nucleation events, allowingmonomer addition to existing nuclei to occur more rapidly than new nucleiformation. Thus, nucleation does not need to be instantaneous, but it must bea single, temporally discreet event to provide for the desired nucleationcontrollednarrow size dispersions [10].Size and size dispersion can be controlled during the reaction, as wellas postpreparatively. In general, time is a key variable; longer reaction timesyield a larger average particle size. Nucleation and growth temperatures playcontrasting roles. Lower nucleation temperatures support lower monomerconcentrations and can yield larger-size nuclei, whereas higher growthtemperatures can generate larger particles as the rate of monomer additionto existing particles is enhanced. Also, Ostwald ripening occurs more readilyat higher temperatures. Precursor concentration can influence both thenucleation and the growth processes, and its effect is dependent on the surfactant/precursor-concentrationratio and the identity of the surfactants(i.e., the strength of interaction between the surfactant and the NQD or betweenthe surfactant and the monomer species). All else being equal, higherprecursor concentrations promote the formation of fewer, larger nuclei and,thus, larger NQD particle size. Similarly, low stabilizer/precursor ratios yieldlarger particles. Also, weak stabilizer–NQD binding supports growth of largeparticles and, if too weakly coordinating, agglomeration of particles intoinsoluble aggregates [10]. Stabilizer–monomer interactions may influencegrowth processes as well. Ligands that bind strongly to monomer speciesmay permit unusually high monomer concentrations that are required forvery fast growth (see Sect. III) [12], or they may promote reductive eliminationof the metal species (see below) [13].The steric bulk of the coordinating ligands can impact the rate of growthsubsequent to nucleation. Coordinating solvents typically comprise alkylphosphines,alkylphosphine oxides, alkylamines, alkylphosphates, alkylphosphites,alkylphosphonic acids, alkylphosphoramide, alkylthiols, fattyacids, and so forth of various alkyl chain lengths and degrees of branching.The polar head group coordinates to the surface of the NQD, and the hydrophobictail is exposed to the external solvent/matrix. This interaction permitssolubility in common nonpolar solvents and hinders aggregation of individ-Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.