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

match between InAs and ZnS of f11%. Otherwise, ZnS and ZnSe shouldbehave similarly as shells for InAs cores.Shell chemistry can be precisely controlled to achieve unstrained (core)shell epitaxy. For example, the zinc–cadmium alloy, ZnCdSe 2 was used forthe preparation of (InP)ZnCdSe 2 nanoparticles having essentially zero latticemismatch between the core and the shell [31]. High-resolution TEM imagesdemonstrated the epitaxial relationship between the layers, and very thickepilayer shells were grown—up to 10 monolayers, where a monolayer wasdefined as 5 A˚ . The shell layer successfully protected the InP surface fromoxidation, a degradation process to which InP is particularly susceptible (seeChapter 9).Optoelectronic devices comprising two-dimensional quantum-wellstructures are generally limited to material pairs that are well lattice matcheddue to the limited strain tolerance of such planar systems; otherwise, verythin well layers are required. In order to access additional quantum-welltypestructures, more strain-tolerant systems must be employed. As alreadyalluded to, the highly curved quantum-dot nanostructure is ideal for latticemismatchedsystems. Several quantum-dot/quantum-well (QD/QW) structureshave been successfully synthesized, ranging from the well latticematchedCdS(HgS)CdS [34–36] (quantum dot, quantum well, cladding) tothe more highly strained ZnS(CdS)ZnS [37]. The former provides emissioncolor tunability in the infrared spectral region, whereas the latter yields accessto the blue-green spectral region. In contrast to the very successful (core)shellpreparations discussed earlier in this section, the QD/QW structures havebeen prepared using ion-displacement reactions, rather than heterogeneousnucleation on the core surface (Fig. 11). These preparations have been eitheraqueous or polar-solvent based and conducted at low temperatures (roomtemperature to 77jC). They entail a series of steps that first involves thepreparation of the nanocrystal cores (CdS and ZnS, respectively). Core preparationis followed by ion-exchange reactions in which a salt precursorof the ‘‘well’’ metal ion is added to the solution of ‘‘core’’ particles. Thesolubility product constant (K sp ) of the metal sulfide corresponding to theadded metal species is such that it is significantly less than that of the metalsulfide of the core metal species. This solubility relationship leads to precipitationof the added metal ions and dissolution of the surface layer of coremetal ions via ion exchange. Analysis of absorption spectra during additionof ‘‘well’’ ions to the nanoparticle solution revealed an apparent concentrationthreshold, after which the addition of the ‘‘well’’ ions produced nomore change in the optical spectra. Specifically, in the case of the CdS(HgS)CdS system, ion exchange of Hg 2+ for Cd 2+ produced a red shift inabsorption until a certain amount of ‘‘well’’ ions had been added. Accordingto inductively coupled plasma–mass spectrometry (ICP-MS), which was usedCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.