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

Figure 29 Transmission electron micrographs and electron diffraction (ED)patterns for CdSe NQD superlattices (SLs) of different orientations: (a) Main:h111i SL -oriented array of 6.4-nm-diameter NQDs five layers thick; upper right: HR-TEM of a single NQD with its h110i axis parallel to the electron beam and its h002iaxis in the plane of the SL; lower right: small-angle ED pattern. (b) Main: facecentered-cubic(fcc) array of 4.8-nm-diameter NQDs (h101i SL projection); lowerright: small-angle ED pattern. (c) Main: fcc array of 4.8-nm-diameter NQDs (h100i SLoriented); lower right: small-angle ED pattern. (From Ref. 90, reprinted withpermission.)The process is similar for particles ranging in size from cluster molecules tomicron-sized colloidal particles [8]. It entails controlled destabilization andprecipitation from a slowly evaporating solvent (Fig. 30). As the solutionconcentrates, interactions between particles become mildly attractive. Particleassociation is sufficiently slow, however, to prevent disordered aggregation.Instead, ordered assembly dominates by a reversible process of particleaddition to the growing superlattice [10]. Fully formed ordered solids arecommonly called colloidal crystals (Fig. 31), where the cluster, NQD, orcolloid serves as the ‘‘artificial atom’’ building block. In the case of NQDs, theprocess is controlled by manipulating the polarity and the boiling point of thesolvent [10]. The solvent polarity is chosen to ensure that mild attractiveforces develop between the nanoparticles as the solvent evaporates. Theboiling point is chosen to ensure that the evaporation process is sufficientlyslow [10]. At the other extreme of very fast destabilization, by fast solventevaporation or nonsolvent addition, a rapid increase in the ‘‘sticking coefficient’’(particle attraction) and in the rate at which particles are added to thegrowing surface yields loosely associated fractal aggregates. Moderate destabilizationrates, implemented by using ‘‘moderate-boiling’’ solvents, produceclose-packed glassy solids having local order but lacking long-range order(Fig. 30a) [8]. For assembly of well-ordered NQD superlattices, the chosensolvent is typically a mixed solvent, involving both a lower-boiling alkane anda higher-boiling alcohol. The alkane evaporates more quickly than thealcohol, yielding relatively higher concentrations of the ‘‘destabilizing’’Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.