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

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

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successfully repeated for up to three layers of well material. The ‘‘well’’ is thencapped with a redeposited layer of core metal ions to generate the full QD/QW structure. The thickness of the cladding layer could be increased <strong>by</strong>addition in several steps (up to five) of the metal and sulfur precursors [35].The nature of the QD/QW structure and its crystalline quality havebeen analyzed <strong>by</strong> HR TEM. In the CdS(HgS)CdS system, evidence has beenpresented for both approximately spherical particles, as well as facetedparticle shapes such as tetrahedrons and twinned tetrahedrons. In all cases,well and cladding growth is epitaxial, as evidenced <strong>by</strong> the absence of amorphousregions in the nanocrystals and in the smooth continuation of latticefringes across particles. Analysis of HR TEM micrographs also reveals thatFigure 12 High-resolution TEM study of the structural evolution of a CdS coreparticle to a (CdS)(core)shell particle to the final CdS(HgS)CdS nanostructure. (a1)Molecular model showing that all surfaces are cadmium terminated (111); (a2) TEMof a CdS core that exhibits tetrahedral morphology; (a3) TEM simulation agreeingwith (a2) micrograph. (b) Model of the CdS particle after surface modification withHg. (c1) Model of a tetrahedral CdS(HgS)CdS nanocrystal; (c2) a typical TEM of atetrahedral CdS(HgS)CdS nanocrystal. (d1) Model of a CdS(HgS)CdS nanocrystalafter twinned epitaxial growth, where the arrow indicates the interfacial layerexhibiting increased contrast due to the presence of HgS; (d2) TEM of a CdS(HgS)CdS nanocrystal after twinned epitaxial growth; (d3) simulation agreeing withmodel (d1) and TEM (d2) showing increased contrast due to presence of HgS; (d4)simulation assuming all Hg is replaced <strong>by</strong> Cd—no contrast is evident. (From Ref. 36,reprinted with permission.)<strong>Copyright</strong> <strong>2004</strong> <strong>by</strong> <strong>Marcel</strong> <strong>Dekker</strong>, <strong>Inc</strong>. <strong>All</strong> <strong>Rights</strong> <strong>Reserved</strong>.

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