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

successfully modeled the 2D island and stripe formation using a pairpotential with an attractive interaction and a longer-ranged repulsion [38].About the islands and stripes, they concluded that their formation resultsfrom attractive forces that bring the nanocrystals together and the longerrangedrepulsive forces that limit the aggregated domain size. About theisland-to-stripe transition upon increased surface coverage, they concludedthat a ‘‘spontaneous reorganization’’ occurs ‘‘as the repulsions between theaggregates become more important than those between the individualparticles within them.’’The LB approach to monolayer formation provides two technologicallyattractive features. The first is the ability to compress the nanocrystal monolayerto interparticle distances much closer than those allowed under normalsolution drop-cast conditions, thus providing greater control over the geometricproperties of nanocrystal monolayers that will be integral for future scientificinquiries and device applications. The second is the ability to transfer anearly exact copy of the nanocrystal film at the air–water interface to virtuallyany substrate. This is also an immensely important feature for the design andimplementation of nanocrystal-based electrical and optical devices.B. Evaporative Self-Assembly1. Hard-Sphere BehaviorIn situ SAXS measurements of an evaporating concentrated dispersion ofC 12 -coated silver nanocrystals reveal that the particles spontaneously selfassembleduring solvent evaporation provided that they are sufficiently sizemonodisperse and well stabilized from aggregation [11]. These nanocrystalsare sufficiently small that their diffusive characteristic time is comparable tothe characteristic time for solvent evaporation and they can order into theirthermodynamically lowest-energy phase [10]. This is qualitatively differentthan large 0.1–10- Am colloids that require days to settle slowly from solutionto form colloidal crystals due to their slow diffusion times. In a concentratednanocrystal dispersion, a 5-nm-diameter particle can diffuse up to 10 nmduring the time it takes for the solvent to evaporate 10 nm in thickness; thenanocrystals can sample their available phase space and it can be argued thatnanocrystal ordering can be treated as an equilibrium (or quasiequilibrium)problem. As a first approximation, the statistical mechanics of particles thatexperience hard-sphere interactions with interparticle potential A(r), definedas a function of center-to-center interparticle separation r, and the particleradius R, wherel if r V 2RA ¼0 if r > 2RCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.