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

Figure 13 (A) Transmission electron microscopic image of a 1.6-Am dewetting ringformed from 35-A˚ gold nanocrystals in chloroform with an amorphous carbon film asthe substrate; (B) TEM image of hexagonal networks formed by Marangoni convectionof 35-A˚ diameter gold nanoparticles deposited using chloroform.Convective instability in the evaporating fluid eliminates the possibilityof hole formation due to dewetting, yet it can lead to the organization ofconvective cells and Marangoni flow [54,56]. Solvent evaporation at thedroplet surface leads to a vertical temperature gradient that can under certainconditions lead to convective fluid transport. The nanocrystals become sweptup in the convective currents. Marangoni flow occurs when the dimensionlessMarangoni number, Ma = r T DTd/qmj, exceeds a critical value of 80, where ris the solvent surface tension, r T = |dr/dT|, DT is the temperature differencebetween the upper and lower fluid boundaries, j is the thermal diffusivity, m isthe kinematic viscosity, q is density, and d is the film thickness [62]. The criticallattice parameter of the convective cells is a c = 2pd/L, where a c = 2 at theonset of Marangoni convection and L is the length of the hexagonal repeatingunit. Figure 13B shows an example of the hexagonal networks of ribbons ofnanocrystals with a 4.3-Am lattice spacing that have settled at the boundariesseparating the spatially organized convective cells [54].IV.PHYSICAL CONSEQUENCES OF SUPERLATTICEORDERThe tunability of superlattice structure with changes in nanocrystal composition,size, interparticle spacing, and symmetry provides unique opportunitiesto manipulate optical and electronic material properties. A number of recentstudies have focused on electron transport through monolayers of metalnanocrystal superlattices [2,63–66]. Although these studies have been conductedon different types of nanocrystals (Co [63], Au [2,64], and Ag [65,66]), afew general trends have been observed. Most importantly, single-nanocrystalcharging energies were found to dominate electron transport. Second, theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.