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

Slow, controlled precipitation of highly ordered colloidal crystals hasalso been achieved by the method of ‘‘controlled oversaturation’’ [91]. Here,gentle destabilization is induced by slow diffusion of a nonsolvent into a NQDsolution directly or through a ‘‘buffer’’ solution. The nonsolvent (e.g.,methanol), buffer (e.g., propan-2-ol, if present), and the NQD solution(toluene solvent) are carefully layered in reaction tubes (Fig. 32a). Colloidal–crystalnucleation occurs on the tube walls and in the bulk solution.Although both methods, not buffered and buffered, yield well-orderedcolloidal crystals from 100 to 200 Am in size, the latter process is relativelymore controlled and produces faceted, hexagonal platelets (Fig. 32c), ratherthan ragged, irregularly shaped colloidal crystals (Fig. 32b). The CdSe NQDbuilding blocks again yield (fcc) superlattices [91].In the self-assembly of large colloidal crystals from nano-sized crystals,the nanocrystals behave as artificial atoms. The self-assembly process islargely driven by the relative favorability of the interaction between theNQD surface ligands and the solvent. Without a favorable interaction,particle cores begin to attract via van der Waals forces. Nanoparticles canalso assemble into large crystals by a self-assembly process called ‘‘orientedattachment.’’ In contrast with self-assembly by ligand-stabilized colloids,oriented attachment entails direct interaction between ligand-free (or almostligand-free) nanocrystals [92]. The driving force for the assembly is the lack ofsurface-passivating ligands. The bare nanoparticles assemble in order tosatisfy surface dangling bonds, and the assembly process is sufficientlyreversible that oriented attachment dominates, leading to highly crystallineFigure 32 (a) Schematic illustrating the method of ‘‘controlled oversaturation’’ forthe preparation of CdSe NQD colloidal crystals. (b) Optical micrograph ofirregularly shaped colloidal crystals prepared without a ‘‘buffer–solvent’’ layer(i.e., faster nucleation). (c) Optical micrograph of faceted hexagonal colloidal crystalsprepared with a ‘‘buffer–solvent’’ layer (i.e., slower nucleation). (From Ref. 91,reprinted with permission.)Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.