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

An additional factor that strongly influences NQD doping is ionic radiimismatch. The apparent relative ease with which a high-temperature solution-basedsynthesis method was used to prepare internally doped Zn(Mn)SeDMSs [66] may, in large part, be attributable to the size matching of the coremetal and dopant ionic radii: Zn 2+ (0.80 A˚ ) and Mn 2+ (0.74 A˚ ). In contrast,the relative difficulty of achieving even near-surface doping of Cd(Mn)Se bysimilar methods [64] may relate to the rather large size mismatch of thesubstituting cations: Cd 2+ (0.97 A˚ ) and Mn 2+ (0.74 A˚ ). Evidence frominverse-micelle preparations support this conclusion. Comparison of unagedCd(Co)S and Zn(Co)S DMSs, where the ionic radius of Co 2+ is 0.74 A˚ ,revealed that the dopant is well distributed throughout the core lattice in thelatter case, benefiting only minimally from an isocrystalline shell-growth step.VII.NANOCRYSTAL ASSEMBLY AND ENCAPSULATIONBecause of their chemical, size, shape, and properties tunability, NQDs havelong been considered ideal building blocks for novel functional materials.Many conceived device applications require that NQDs be controllablyassembled into organized structures, at a variety of length scales, and thatthese assemblies be macroscopically addressable. Even applications thatmake use of the optical properties of individual nanoparticles (e.g., fluorescentbiolabeling) require controlled assembly of bio–nano conjugates. For thesereasons, small- and large-scale assembly and encapsulation methods havebeen developed in which NQDs are manipulated as artificial atoms ormolecules. Encapsulation has typically involved incorporating NQDs intoorganic polymers [76–80] or inorganic glasses [81–83]. Either may simplyprovide structural rigidity to an NQD ensemble, as well as protection fromenvironmental degradation; or, the matrix material may be electronically,optically, or magnetically ‘‘active,’’ where the encapsulant then provides foradded functionality and/or device addressibility. Assembly approaches are asdiverse as the targeted applications and have emerged from each of thetraditional disciplines: physics/physical chemistry (e.g., self-assemblyapproaches), chemistry (chemical patterning of surfaces, e.g., dip-pen nanolithography[84,85], electric-field-directed assembly [86], etc.), biology (bioinspiredmineralization [87], DNA-directed assembly [85,88], etc.), andmaterials science (lithography-defined templating [89], etc.). The diversityof approaches suggests that the subject warrants a book of its own. Therefore,only a small subset of this field is reviewed here, namely self-assembly at thenanoscale.Particles uniform in size, shape, composition, and surface chemistry canself-assemble from solution into highly ordered 2D and 3D solids (Fig. 29).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.