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

are fully extended. Moving away from the surface, / decreases significantlydue to the curvature of the nanocrystal surface. This decrease is furtherexaggerated in smaller nanocrystals with greater surface curvature. The /curves can be calculated using the geometric equation for cylinders withligand cross-section area (SA thiol ) of 14.5 A˚ , extending radially from a curvedsurface with radius R p + z [26]:/ðzÞ ¼SA thiolR ph thiol ðR p þ zÞwhere h thiol is the surface area per thiol head group, which represents thebinding density, and z is the radial distance from the metal surface.The most important parameters determining the strength of the stericrepulsion are the ligand length l and the Flory–Huggins interaction parameterv between the ligand and the solvent. When v > 1 ⁄ 2 , A osm becomes attractivedue to the poor solubility of the ligands. For conventional solvents, the Flory–Huggins parameter can be estimated using solubility parameters or comparingcohesive energy densities [48]. Nanocrystal dispersibility depends on thebalance between the attractive van der Waals forces and the repulsive stericforces. At good solvent conditions, the ligands provide strong repulsiveforces, whereas a weak repulsion or even an attraction can exist under poorsolvent conditions, leading to flocculation of the nanocrystals.3. Superlattice CrystallizationEnergetic attraction between particles lowers the nanocrystal solubility andprovides a greater thermodynamic driving force for superlattice formation.Superlattice nucleation and growth, however, requires nanocrystals to desolvate,and the activation barrier to this process is much lower on a surfacethan in homogeneous solution. In the case of fast evaporative assembly from avolatile solvent, nucleation and growth of nanocrystal superlattices proceedsby a heterogeneous process. For example, nanocrystals drop-cast from aconcentrated dispersion in a good solvent, such as chloroform or hexane,under conditions of rapid evaporation crystallize primarily on the (111) SLplane (where the subscript SL denotes superlattice). Time-resolved SAXS hasbeen used to observe the formation of nanocrystal superlattice forms duringsolvent evaporation from concentrated nanocrystal dispersions and thescattering intensity profile shows the transition from the characteristics shapefactor for a dispersion of noninteracting particles to the organized superlattice,giving rise to the (111) SL diffraction peak from an fcc superlattice [11].The significant prominence of this peak relative to the others also reveals theheterogeneous crystallization and growth of the superlattice on the (111) SLlattice plane.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.