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

of the stabilizing ligand, and the steric interactions between particles.Polydisperse nanocrystals (r typically >10%) do not form superlattices[13,34] and the coordination number is lower than in close-packed orderedsolids because long-range order does not exist. The radial distributionfunctions, however, for these films exhibit regular oscillations characteristicof the average particle diameter and interparticle separation, as in the case formolecular liquids [13]. Scattering from these dense fluids has enabled thecalculation of pair interparticle potentials in the absence of solvent withreasonable accuracy [13]. Monodisperse nanocrystals form lattices that giverise to sharp diffraction peaks. These peaks are readily indexed to determinethe superlattice crystal structure and lattice parameters [9,13].III.SUPERLATTICE FORMATIONNanocrystal superlattices require size-monodisperse nanocrystals that arewell stabilized by organic capping ligands. Although this is a necessarycondition for superlattice formation, it is not sufficient. The formation processis also extremely important in determining the dimensionality and the extentof order. Superlattices have been formed by essentially two different approaches:(1) Langmuir–Blodgett processing in which the density of thenanocrystal dispersion is carefully controlled through the disorder–orderphase transition in two dimensions [35] and (2) evaporative crystallization,which yields superlattices in either two or three dimensions [10,36].A. Two-Dimensional Order in Langmuir–BlodgettTechniquesNanocrystal dispersions can be drop cast onto liquid surfaces to form twodimensionalsuperlattices, or monolayers, at an air–liquid interface. Depositionon a Langmuir–Blodgett (LB) trough provides the ability to controlthe surface density of the nanocrystal layer by compressing the trough. Twodetailed studies have been reported for CdSe nanocrystals [37] and Agand Au nanocrystals [36]. Bawendi and co-workers [37] used monodisperseCdSe nanocrystals capped with triotylphosphine oxide with nanocrystals thatvaried in size from 25 to 53 A˚ in diameter. Surface pressure versus areaisotherms showed strong hysteresis, indicating attractive interactions betweenthe nanocrystals in the rigid monolayer. TEM analysis of films transferredonto glass substrates and carbon-coated TEM grids showed short-range translationalorder and long-range nearest-neighbor orientational order—characteristics of a hexatic phase. It was also determined that the lack oflong-range translational order was caused by dislocation defects, not theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.