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

ligand concentrations yield a reduced rate of growth from the {111} facescompared to the {100} faces, which experience enhanced relative growth rates.Further, alkylthiols are more effective in controlling relative growth ratescompared to alkylamines. The latter, a weaker Pb binder, consistently leads tolarge, thermodynamically stable cubic shapes. Finally, the reaction temperatureplays a key role in determining particle morphology. The lowesttemperatures (140jC) yield the metastable rod-based morphologies, withintermediate star shapes generated at moderate temperatures (180–230jC)and truncated octahedra isolated at the highest temperatures (250jC).Interestingly, the rod structures appear to form by preferential growth of{100} faces from truncated octahedra seed particles. For example, the ‘‘tadpole’’-shapedmonopods are shown by HR TEM studies to comprise truncatedoctahedra ‘‘heads’’ and [100]-axis ‘‘tails,’’ resulting from growth from a(100) face. The star-shaped particles that form at 180jC are characterized bysix triangular corners, comprising each of the six {100} faces. The {100} faceshave ‘‘shrunk’’ into these six corners as a result of their rapid growth, similarto the replacement of the (001 ) face by slower growing faces during theformation of arrow-shaped CdSe particles (discussed earlier). The isolation ofstar-shaped particles at intermediate temperatures suggests that the relativegrowth rates of the {100} faces remain enhanced compared to the {111} facesat these temperatures. Further, the overall growth rate is enhanced as a resultof the higher temperatures. The star-shaped particles that form at 230jC arerounded and represent a decrease in the differences in relative growth ratesbetween the {100} faces and the {111} faces, the latter, higher-activationbarriersurface benefiting from the increase in temperature. A definitive shiftfrom kinetic growth to thermodynamic growth is observed at 250jC (or atlong growth times). Here, the differences in reactivity between the {100} andthe {111} faces are negligible given the high-thermal-energy input thatsurmounts either face’s activation barrier. The thermodynamic cube shapeis, therefore, approximated by the shapes obtained under these growthconditions. In all temperature studies, the alkylthiol : precursor ratio wasf80 : 1 and monomer concentrations were kept high, conditions supportingcontrolled and kinetic growth, respectively.The III–V semiconductors have proven amenable to solution-phasecontrol of particle shape using an unusual synthetic route. Specifically, themethod involves the solution-based catalyzed growth of III–V nanowhiskers[44]. In this method, referred to as the ‘‘solution–liquid–solid mechanism,’’ adispersion of nanometer-sized indium droplets in an organometallic reactionmixture serves as the catalytic sites for precursor decomposition and nanowhiskergrowth. As initially described, the method afforded no control overnanowhisker diameters, producing very broad diameter distributions andmean diameters far in excess of the strong-confinement regime for III–VCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.