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

een determined and quantitatively compared to that in InP quantum dots[46]. Preliminary results indicate that II–VI quantum wires can be grownsimilarly [48]. A similar approach was applied to growth of insoluble, but sizemonodispersedin diameter (4–5 nm), silicon nanowires. Here, reactions wereconducted at elevated temperature and pressure (500jC and 200–270 bar,respectively) using alkanethiol-coated gold nanoclusters (2.5 F 0.5 nmdiameter) as the nucleation and growth ‘‘seeds’’ [49].V. PHASE TRANSITIONS AND PHASE CONTROLNanocrystal quantum dots have been used as model systems to study solid–solid phase transitions [50–53]. The transitions, studied in CdSe, CdS, InP andSi nanocrystals [52], were induced by pressure applied to the nanoparticles in adiamond anvil cell by way of a pressure-transmitting solvent medium, ethylcyclohexane.Such transitions in bulk solids are typically complex anddominated by multiple nucleation events, the kinetics of which are controlledby crystalline defects that lower the barrier height to nucleation [50,53]. Innearly defect-free nanoparticles, the transitions can exhibit single-structuraldomainbehavior and are characterized by large kinetic barriers (Fig. 21). Incontrast to original interpretations which described the phase transition innanocrystals as ‘‘coherent’’ over the entire nanocrystal [50], the nucleation ofthe phase transition process was recently shown to be localized to specificcrystallographic planes [53]. The simple unimolecular kinetics of the transitionstill support a single nucleation process; however, the transition is nowthought to result from plane sliding as opposed to a coherent deformationprocess. Specifically, the sliding-plane mechanism involves shearing motionalong the (001) crystallographic planes, as supported by detailed analyses oftransformation times as a function of pressure and temperature.Because of the large kinetic barriers in nanocrystal systems, their phasetransformations are characterized by hysteresis loops (Fig. 21) [50,51,53]. Thepresence of a strong hysteresis signifies that the phase transition does notoccur at the thermodynamic transition pressure and that time is required forthe system to reach an equilibrium state. This delay is fortunate in that itpermits detailed analysis of the transition kinetics even though the system ischaracterized by single-domain (finite-size) behavior. As alluded to, theseanalyses were used to determine the structural mechanism for transformation.Specifically, kinetics studies of transformation times as a function of temperatureand pressure were used to determine relaxation times, or average timesto overcome the kinetic barrier, and, thereby, rate constants. The temperaturedependence of the rate constants led to the determination of activationenergies for the forward and reverse transitions, and the pressure dependenceCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.