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

Under a different growth regime, one that promotes fast, kineticgrowth, more highly anisotropic shapes, such as rods and wires, can be obtained.In semiconductor nanoparticle synthesis, such growth conditionshave been achieved using high precursor, or monomer, concentrations in thegrowth solution. As discussed previously (Sect. II), particle-size distributionscan be ‘‘focused’’ by maintaining relatively high monomer concentrationsthat prevent the transition from the fast-growth to the slow-growth (Ostwaldripening) regime [14]. Even higher monomer concentrations can be used toeffect a transition from thermodynamic to kinetic growth. Access to theregime of very fast kinetic growth allows control over particle shape. Thesystem is essentially put into ‘‘kinetic overdrive,’’ where dissolution ofparticles is minimized as the monomer concentration is maintained at levelshigher than the solubility of all of the particles in solution. Growth of allparticles is thereby promoted [14]. Further, in this regime, the rate of particlegrowth is not limited by diffusion of the monomer to the growing crystalsurface, but, rather, by how fast atoms can add to that surface. In this way,the relative growth rates of different crystal faces have a strong influence overthe final particle shape [38]. Specifically, in systems where the underlyingcrystal lattice structure is anisotropic (e.g., the wurtzite structure of CdSe),simply the presence of high monomer concentrations (kinetic growth regime)at and immediately following nucleation can accentuate the differences inrelative growth rates between the unique c axis and the remaining lattice directions,promoting rod growth. The monomer-concentration-dependenttransition from slower-growth to fast-growth regimes coincides with a transitionfrom diffusion-controlled to reaction-rate-controlled growth and fromdot to rod growth. In general, longer rods are achieved with higher initialmonomer concentrations, and rod growth is sustained over time by maintaininghigh monomer concentrations using multiple-injection techniques.Finally, the relative rates of different crystallographic faces can be furthercontrolled by the judicious choice of organic ligands [12,17].In order to more precisely tune the growth rates controlling CdSe rodformation, high monomer concentrations are used in conjunction with appropriateorganic ligand mixtures. In this way, a wide range of rod aspectratios has been produced (Fig. 13) [12,17,38]. Specifically, the ‘‘traditional’’TOPO ligand is supplemented with alkyl phosphonic acids. The phosphonicacids are strong metal (Cd) binders and may influence rod growth by changingthe relative growth rates of the different crystal faces [38]. CdSe rods formby enhanced growth along the crystallographically unique c axis (taking advantageof the anisotropic wurtzite crystal structure). Interestingly, the fastgrowth has been shown to be unidirectional—exclusively on the (001) face[38]. The (001) facets comprise alternating Se and Cd layers, where the Cdatoms are relatively unsaturated (three dangling bonds per atom). In contrast,Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.