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

Many features observed for superlattice growth are consistent with themodel developed by Burton et al. for the growth of crystals from atoms in thevapor phase that describes the growth of crystal layers based on the steps andkinks along the crystal surface that serve as adsorption sites for additionalatoms [49,50]. Slight differences in interparticle attraction can lead to ratherdramatic changes in superlattice film morphology. For example, the Hamakerconstant for gold in hexane is approximately 20% higher than for gold inchloroform due to the lower dielectric screening in hexane. Even though bothchloroform and hexane are considered good solvents for alkanethiol cappingligands with v c 0, the films deposited from hexane are very rough with roundsuperlattice domains, whereas the films from chloroform are relativelysmooth, as shown in Figs. 2E and 2F. HR-SEM images of films evaporatedfrom these two solvents, using the same nanocrystal size and capping ligands,show that the superlattice crystallizes preferably in the lowest energy [110] SLdirection, whereas superlattice growth from hexane is isotropic: The increasedinterparticle attraction in the solvent leads to similar crystal growth rates in allcrystallographic directions.Homogeneous superlattice nucleation can be induced by adding a relativelysmall amount of antisolvent to the dispersion. The antisolvent increasesthe supersaturation of the dispersion and promotes interparticle attractions.If too much antisolvent is added, the nanocrystals will simply flocculate. Theactivation barrier to homogeneous nucleation, however, is significant. Theexternal addition of energy, for example, by sonication can induce homogeneousnucleation as shown in Fig. 11, but the rapid evaporation of thesolvent under relatively poor solvent conditions results in a disordered nanocrystalfilm. Apparently, the balance between solvation and desolvation of thenanocrystals during the crystallization process does not provide sufficiententropic freedom for crystallization to occur in this timescale.In addition to isotropic van der Waals forces and steric repulsion betweenparticles, researchers have identified the potential importance of chain–chain interactions and shape effects [9,13,51]. For example, the superlatticestructure depends on the ratio of the capping ligand chain length hLi, to theradius of the nanocrystal core R: v = hLi/R. [9,13]. Whetten and co-workers[9] found that fcc packing is preferred below v c 0.73, whereas body-centeredcubic (bcc) order occurs above this value. As v increases, the energeticinterparticle attraction decreases, which induces the structural transition tothe more entropically favored bcc structure. Ligand–ligand interactions canalso lead to other deviations from fcc packing. For example, Luedtke andLandman performed molecular dynamics simulations, finding that dodecanechains adsorbed to gold nanocrystals could form interlocking bundles incontrast to shorter butyl groups which did not exhibit bundling [51]. Based onTEM investigations, nanocrystals have been found to form a variety ofCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.