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

Figure 9 Absorption spectra for bare (dashed lines) and one- to two-monolayerZnS-overcoated (solid lines) CdSe NQDs. (Core)shell spectra are broader andslightly red-shifted compared to the core counterparts. Core diameters are (a) 2.3, (b)4.2, (c) 4.8, and (d) 5.5 nm. (From Ref. 30, reprinted with permission.)growth solution. Further, employing relatively low reaction temperaturesavoids growth of the starting core particles [20,30]. ZnS, for example, cannucleate and grow as a crystalline shell at temperatures as low as 140jC [30],and CdS shells have been successfully prepared from dimethylcadmium andbis(trimethylsilyl) sulfide at 100jC [20], thereby avoiding complications dueto homogeneous nucleation and core-particle growth. Additional strategiesfor preventing particle growth of the shell material include using organiccapping ligands that have a particularly high affinity for the shell metal. Thepresence of a strong binding agent seems to lead to more controlled shellgrowth; for example, TOPO is replaced with TOP in CdSe shell growth onInAs cores, where TOP (softer Lewis base) coordinates more tightly thanTOPO (harder Lewis base) with cadmium (softer Lewis acid) [33]. Finally, theratio of the cationic to anionic precursors can be used to prevent shellmaterialhomogeneous nucleation. For example, increasing the concentrationof the chalcogenide in a cadmium–sulfur precursor mixture hinders formationof unwanted CdS particles [20].Successful overcoating is possible for systems where relatively largelattice mismatches between core and shell crystal structures exist. The mostcommonly studied (core)shell system, (CdSe)ZnS, is successful despite a 12%Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.