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

Figure 1 Left: High-resolution transmission electron microscopic image showingthe crystalline structure of a gold nanoparticle. Right: Dodecanethiol molecules chemisorbto the gold surface, providing a passivation layer that prevents aggregation ofthe gold cores. The capping ligand length can be varied to control the interparticlespacing in a superlattice.Metal nanocrystal superlattices provide the opportunity to develop newtechnologies. These granular materials have controllable domain size, separation,and interparticle spacing. Magnetic and superconductor nanocrystalsoffer particularly unique opportunities [1,12]. Sterically stabilized nanocrystalsalso provide model systems to study molecular-phase behavior [13]. Theinteraction energies can be tuned by particle size and capping ligand chemistryand the materials are readily characterized by electron microscopy and x-rayscattering techniques. Furthermore, due to their small size, the mass transferlimitations common to larger colloidal systems do not pose technical obstacles.Therefore, these particles provide a test bed for simple fluid models ofphase behavior.A. Early DevelopmentsResearch on submicrometer-scale colloidal superlattices (or colloidal crystals)began as early as the 1930s with x-ray studies by Levin and Ott on thecrystallinity of natural deposits of opal [14]. These researchers were primarilyinterested in understanding the phenomena responsible for the color, or ‘‘fire,’’seen in opal specimens obtained from different areas of the world [15–17]. In1946, opals were described by Copisarow and Copisarow [18] as being composedof colloidal particles, as directly confirmed in 1964 by Sanders, whoCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.