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

lattice mismatch. Such a large lattice mismatch could not be tolerated in flatheterostructures, where strain-induced defects would dominate the interface.It is likely that the highly curved surface and reduced facet lengths ofnanocrystals relax the structural requirements for epitaxy. Indeed, two typesof epitaxial growth are evident in the (CdSe)ZnS system: coherent (with largedistortion or strain) and incoherent (with dislocations), the difference arisingfor thin (approximately one to two monolayers, where a monolayer is definedas 3.1 A˚ ) versus thick (>two monolayers) shells, respectively [30]. Highresolution(HR) TEM images of thin-shell ZnS-overcoated CdSe QDs reveallattice fringes that are continuous across the entire particle, with only a small‘‘bending’’ of the lattice fringes in some particles indicating strain. TEMimaging has also revealed that thicker shells (>two monolayers) lead to theformation of deformed particles, resulting from uneven growth across theparticle surface. Here, too, however, the shell appeared epitaxial, orientedwith the lattice of the core (Fig. 10). Nevertheless, wide-angle x-ray scattering(WAXS) data showed reflections for both CdSe and ZnS, indicating thateach was exhibiting its own lattice parameter in the thicker-shell systems.This type of structural relationship between the core and the shell was describedas incoherent epitaxy. It was speculated that at low coverage, theepitaxy is coherent (strain is tolerated), but that at higher coverages, thehigh lattice mismatch can no longer be sustained without the formation ofdislocations and low-angle grain boundaries. Such defects in the core–shellFigure 10 High-resolution TEMs of (a) CdSe core particle and (b) a (CdSe)ZnS(core)shell particle (2.6-monolayer ZnS shell). Lattice fringes in (b) are continuousthroughout the particle, suggesting epitaxial (core)shell growth. (From Ref. 30, reprintedwith permission.)Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.