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

‘‘Teardrop-shaped’’ particles also arise from the tendency towardunidirectional growth. In this case, rod-shaped crystals are exposed to growthconditions favoring spherical particle shapes (i.e., equilibrium slow growthand low monomer concentrations), causing the rods to become rounded.Monomer concentration is then quickly increased to force elongation of the‘‘droplet’’ from one end into particles resembling tadpoles [38]. The growthregime governing the evolution of rods to spherical particles has been termed‘‘1D to 2D intraparticle ripening’’ [12]. Nanoparticle volumes and totalnumbers remain approximately constant (as do monomer concentrations),whereas nanoparticle shape changes dramatically. Intraparticle diffusion of c-axis atoms to other crystal faces may explain this transformation. The processis distinguished from ‘‘interparticle ripening,’’ or Ostwald ripening, which isobserved at even lower monomer concentrations. Intraparticle ripening isthought to occur when a ‘‘diffusion equilibrium’’ exists between the nanoparticlesand the monomers in the bulk solution [12]. Alternatively, it has alsobeen shown that nanodots can be used to ‘‘seed’’ the growth of nanorods.Here, the spherical particles are exposed to high monomer concentrationsthat promote one-dimensional (1D) growth from the template particles. Improvedshort-axis and aspect-ratio distributions have been reported for theserods (Fig. 16) [12].Rod-growth dynamics also depend on the identity of the phosphonicacid. The effectiveness of the phosphonic acid in promoting rod growthdepends critically on its steric bulk, or the length of its alkyl chain. Shorterchainphosphonic acids, such as HPA, more effectively promote rod growthcompared to longer-chain phosphonic acids, such as tetradecylphosphonicacid (TDPA). Combinations of longer- and shorter-chain phosphonic acidscan be used to readily tune rod aspect ratios [12] and control shape evolutiondynamics.The above morphologies reflect the underlying wurtzite crystal structureof CdSe. Occasionally, however, CdSe nucleates in the zinc-blende phase.When this occurs, a different type of morphology, the tetrapod, is observed.Here, the zinc-blende nuclei expose four equivalent (111) faces that comprisethe crystallographic equivalent of the wurtzite (001) faces (alternating planesof Cd or Se). From these (111) surfaces, four wurtzite ‘‘arms’’ grow unidirectionally.Further addition of monomer either lengthens the wurtzite arms,in the case of ‘‘purely’’ wurtzite arms, or generates dendriticlike wurtzitebranches, when zinc-blende stacking faults are present in the arm ends(Fig. 17) [38].CdSe rod QYs in PL are typically relatively low, f1–4%. Like theirspherical counterparts, however, rods can be overcoated with a higher bandgapinorganic semiconductor, increasing QYs to 14–20% [39,40]. Latticemismatch requirements for rods are somewhat more severe than for sphericalCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.