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

equires only minutes to complete. In other words, following particle formationbut prior to ligand stabilization, additional Cd and S (or Zn and S)precursor can be added, prompting ‘‘shell’’ growth that effectively encapsulatesthe isocrystalline core and the dopants. Structurally and optically, theshell–core particles behave like aged particles (better resolved hyperfine EPRsignals and improved Mn 2+ photoluminescence). Ligand-field electronicabsorption spectroscopy was also used to distinguish between surface-boundand lattice-bound dopant ions, regardless of whether incorporation of ionsinto the host matrix occurred as a result of aging or isocrystalline shell growth(Fig. 28) [74].Figure 28 (a) Ligand-field absorption spectra (300 K) for 3.0-nm-diameterCd 1 x Co x S (x = 0.023)-doped NQDs showing the CdS band-gap transitions (leftpanels) and the Co 2+ r 3 ligand-field transitions (right panels). The NQDs wereprepared by the standard coprecipitation method (see text) and subsequentlysuspended in pyridine. The spectra were taken 2 h (solid line) and 23 h (dashed line)after synthesis. The loss in Co 2+ signal over time suggests that Co 2+ is dissolvinginto the pyridine solvent. (b) Ligand-field absorption spectra (300 K) for 3.7-nmdiameterCd 1 x Co x S (x=0.009)-doped NQDs prepared by the isocrystalline(core)shell method, 2 h (solid line) and 28 h (dashed line) after synthesis. Thereproducibility over time suggests that the (core)shell doped NQDs are stable todissolution of Co 2+ into the pyridine solvent. The absorption band shape of (a) ischaracteristic of Co 2+ that resides on the CdS NQD surface, whereas that for (b) ischaracteristic of Co 2+ that is incorporated into the crystal lattice structure of thehost CdS. (From Ref. 74, reprinted with permission.)Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.