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

ing surface-to-volume ratio with diminishing particle size, surface trap statesexert an enhanced influence over photoluminescence properties, includingemission efficiency, and spectral shape, position, and dynamics. Further, it isoften through their surfaces that semiconductor nanocrystals interact with‘‘their world,’’ as soluble species in an organic solution, reactants in commonorganic reactions, polymerization centers, biological tags, electron-holedonors/acceptors, and so forth. Controlling inorganic and organic surfacechemistry is key to controlling the physical and chemical properties thatmake NQDs unique compared to their epitaxial QD counterparts. In SectionII, we discussed the impact of organic ligands on particle growth andparticle properties. In this section, we review surface modification techniquesthat utilize inorganic surface treatments.Overcoating highly monodisperse CdSe with epitaxial layers of eitherZnS [29,30] or CdS (Fig. 7) [20] has become routine and typically providesalmost an order-of-magnitude enhancement in PL efficiency compared to theexclusively organic capped starting nanocrystals [5–10% efficiencies can yield30–70% efficiencies (Fig 8)]. The enhanced quantum efficiencies result fromenhanced coordination of surface unsaturated, or dangling, bonds, as well asfrom improved confinement of electrons and holes to the particle core. Thelatter effect occurs when the bandgap of the shell material is larger than that ofthe core material, as is the case for (CdSe)ZnS and (CdSe)CdS (core) shellparticles. Successful overcoating of III–V semiconductors has also beenreported [31–33].The various preparations share several synthetic features. First, thebest results are achieved if initial particle size distributions are narrow, assome size-distribution broadening occurs during the shell-growth process.Because absorption spectra are relatively unchanged by surface properties,they can be used to monitor the stability of the nanocrystal core during andfollowing growth of the inorganic shell. Further, if the conduction bandoffset between the core and the shell materials is sufficiently large (i.e., largecompared to the electron confinement energy), then significant red-shiftingof the absorption band edge should not occur, as the electron wave functionremains confined to the core (Fig. 9). A large red shift in (core)shell systems,having sufficiently large offsets (determined by the identity of the core/shellmaterials and the electron and hole effective masses), indicates growth of thecore particles during shell preparation. A small broadening of absorptionfeatures is common and results from some broadening of the particle sizedispersion (Fig. 9). Alloying, or mixing of the shell components into theinterior of the core, would also be evident in absorption spectra if it were tooccur. The band edge would shift to some intermediate energy between theband energies of the respective materials comprising the alloyed nanoparticle.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.