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

the tetrahedral shapes are terminated by (111) surfaces that can be eithercadmium or sulfur faces [36]. The choice of stabilizing agent—an anionicpolyphosphate ligand—favors cadmium faces and likely supports the facetedtetrahedral structure that exposes exclusively cadmium-dominated surfaces(Fig. 12). In addition, both the spherical particles and the twinned tetrahedralparticles provide evidence for an embedded HgS layer in the presumed QD/QW structure. Due to differences in their relative abilities to interact withelectrons (HgS more strongly than CdS), contrast differences are evident inHR TEM images as bands of HgS surrounded by layers of CdS (Fig. 12).Size dispersions in these low-temperature, ionic-ligand-stabilized reactionsare reasonably good (f20%), as indicated by absorption spectra, butpoor compared to those achieved using higher-temperature pyrolysis andamphiphilic coordinating ligands (4–7%). Nevertheless, the polar-solventbasedreactions give us access to colloidal materials, such as mercurychalcogenides, thus far difficult to prepare using pyrolysis-driven reactions(Sect. II). Further, the ion-exchange method provides the ability to grow welland shell structures that appear to be precisely one, two, or three monolayersdeep. Heterogeneous nucleation provides less control over shell thicknesses,resulting in incomplete and variable multilayers (e.g., 1.3 or 2.7 monolayerson average, etc.). The stability of core–shell materials against solid-statealloying is an issue, at least for the CdS(HgS)CdS system. Specifically, cadmiumin a CdS–HgS structure will, within minutes, diffuse to the surface ofthe nanoparticle, where it is subsequently replaced by a Hg 2+ solvated ion[35]. This process is likely supported by the substantially greater aqueoussolubility of Cd 2+ compared to Hg 2+ , as well as the structural compatibilitybetween the two lattice-matched CdS and HgS crystal structures.IV.SHAPE CONTROLThe nanoparticle growth process described in Section II, where fast nucleationis followed by slower growth, leads to the formation of spherical orapproximately spherical particles. Such essentially isotropic particles representthe thermodynamic, lowest-energy shape for materials having relativelyisotropic underlying crystal structures. For example, under this growthregime, the wurtzite crystal structure of CdSe, having a c/a ratio of f1.6,fosters the growth of slightly prolate particles, typically exhibiting aspectratios of f1.2. Furthermore, even for materials whose underlying crystalstructure is more highly anisotropic, nearly spherical nanoparticles typicallyresult due to the strong influence of the surface in the nano-size regime. Surfaceenergy is minimized in spherical particles, compared to more anisotropicmorphologies.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.