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

The organic monolayer-coated, sterically stabilized nanocrystals can bedispersed in various solvents and precipitated upon the addition of antisolvent(e.g., using solvent/antisolvent pairs such as hexane/ethanol or toluene/methanol). A miscible antisolvent increases the polarity of the solution, whichaggregates particles. Nanocrystal aggregation depends sensitively on particlesize; larger particles require lower antisolvent concentrations to aggregatethan the smaller particles [10]. The larger hydrophobic nanocrystals exposehigher surface areas to the solvent and exhibit greater interparticle interactionthan the smaller particles. Therefore, particles can be separated by size bytitrating organic dispersions with miscible polar solvents, using a processcalled size-selective precipitation. Nanocrystals differing in size by one latticeshell can be isolated using this technique [10,13]. The technique is generallypracticed by visual observation of the dispersion. If the dispersion becomescloudy, the solution has been overtitrated and particles of all sizes haveaggregated; the solution must be titrated to the point where opalescence justappears.Postsynthesis thermal annealing after size-selective precipitation hasproven to be very useful for further narrowing the size distribution of gold andsilver nanocrystals [6,27]. By refluxing in toluene for several hours, the sizeand shape distributions narrow to produce the highest-quality superlattices.Refluxing in solution prior to size-selective precipitation simply broadens thesize distribution; the distribution prior to annealing appears to be critical tothe success of this postsynthesis process [27]. It is not entirely clear whichphysical and chemical processes the heat treatment promotes; however, it appearsthat the nanocrystal shape becomes more consistent within the sampleand that the surface chemistry becomes more robust. In the case of gold andsilver nanocrystals coated with alkanethiol monolayers, the thiols can serve asetchants. Because the surface-bound ligands are in dynamic equilibrium withfree ligands, the particles may approach a preferred shape during solutionphaseannealing that aids superlattice formation.II.NANOCRYSTAL CHARACTERIZATIONElectron microscopy and small-angle x-ray scattering provide complementarymaterials characterization techniques for determining the average particlesize, size distribution, and superlattice structure. High-resolutionscanning and transmission electron microscopy provide direct imaging ofthe nanocrystals; the crystal structure can be closely examined and the shapeprobed. Microscopy, however, is practically limited to the examination ofonly a few hundred nanocrystals in a particular sample and is often highlydependent on the materials preparation procedures. Scattering techniques,Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.