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

dopant emission signal occurs to the red of the NQD emission signal, or itoverlaps NQD PL if the latter is dominated by deep-trap emission. Itspresence has been cited as evidence for successful doping; however, therequired electronic coupling can exist even when the ‘‘dopant’’ is locatedoutside of the NQD [64]. Therefore, other methods are now preferred indetermining the success or failure of a doping procedure.Successful ‘‘core’’ doping was first achieved using low-temperaturegrowth methods, such as room-temperature condensation from organometallicprecursors in the presence of a coordinating surfactant [67] or roomtemperatureinverse-micelle methods [68–70]. Unfortunately, due to relativelypoor NQD crystallinity and/or surface passivation, photoluminescence fromundoped semiconductor nanoparticles prepared by such methods is generallycharacterized by weak and broad deep-trap emission. Thus, NQD quality isnot optimized in such systems. Other low-temperature methods commonlyused to prepare ‘‘doped’’ nanocrystals have been shown to yield only‘‘dopant-associated’’ nanocrystals. For example, the common condensationreaction involving completely uncontrolled growth performed at roomtemperature by simple aqueous-based coprecipitation from inorganic salts(e.g., Na 2 S and CdSO 4 , with MnSO 4 as the dopant source), in the absence oforganic ligand stabilizers, yields agglomerates of nano-sized domains andunincorporated dopant. 113 Cd- and 1 H-NMR (nuclear magnetic resonance)were used to demonstrate that Mn 2+ remained outside of the NQD in thesesystems [71]. Doping into the crystalline lattice, therefore, appears to requiresome degree of control over particle growth when performed at room temperature(i.e., excluding higher-temperature, solid-state pyrolysis reactions thatcan yield well-doped nanocrystalline, although not quantum confined (>20nm), material in the absence of any type of ligand control or influence [72]).The ability to distinguish between surface-associated and truly incorporateddopant ions is critical. Both can provide the necessary electronic coupling toachieve energy transfer and the resultant dopant emission signal, for example.Various additional characterization methods have been employed, such asNMR spectroscopy [64,71], electron paramagnetic resonance (EPR) [42,64–66,68,73], powder XRD, x-ray absorption fine-structure spectroscopy(XAFS) [73], chemical treatments (e.g., surface-exchange reactions andchemical etching–see below) [42,64], and ligand-field electronic absorptionspectroscopy (see below) [74].More recently, doped NQDs have been prepared by high-temperaturepyrolysis of organometallic precursors in the presence of highly coordinatingligands: Zn(Mn)Se at an injection temperature of 310jC [66] and Cd(Mn)Seat an injection temperature of 350jC [64]. The undoped NQDs prepared bysuch methods are very well size selected (f4–7%), highly crystalline, and wellpassivated [64,66]. However, the dopant is incorporated into the NQD at lowCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.