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

imaged the internal colloidal opal structure by electron microscopy [16]. Theopals are superlattices of uniform-sized spherical silica colloids ranging from170 to 350 nm in diameter. These structural studies confirmed a century-oldhypothesis that the brilliant color produced by opals upon light exposureresults from the diffraction of light by the 3D colloidal array [16]. In 1966,Darragh et al. determined that the colloidal ‘‘building blocks’’ of the opal—previously considered to be amorphous silica—in fact consisted of aggregatedsmaller silica particles 30–40 nm in diameter [15].In 1989, Schmid and Lehnert [19] demonstrated Au nanocrystal superlatticeformation and Bentzon and co-workers [20] demonstrated hexagonalclose packing of iron oxide nanocrystals, some of the first examples of selforganizedmetal nanocrystal superlattices. Schmid and co-workers found that20-nm-diameter Au nanocrystals synthesized by sodium citrate reduction ofan aqueous gold salt solution and stabilization with phosphane [19] with apolydispersity of 10% [20] ordered into a lattice when evaporated onto asubstrate. Bentzon et al. used 6.9-nm-diameter iron oxide nanocrystals witha narrow size distribution [21]. These studies reiterated that size monodispersecolloidal particles with sufficient stabilization to aggregation at high-volumefractions organize into superlattice structures. Nanocrystals coated withorganic monolayers provide an ideal material for forming superlatticestructures, as the steric organic barrier prevents the metal cores from touchingeven after the solvent is removed. This is not the case with charge-stabilizedcolloids coated by ions. In a dispersion, these colloids are well stabilized byrepulsive electrostatic double-layer forces; however, upon removal of thesolvent, the particles aggregate irreversibly.Nanocrystal surface passivation by organic ligands was examined forCdSe nanocrystals by Steigerwald and co-workers in 1988 [22]. The use oforganic ligands to control particle size has proven very powerful for semiconductornanocrystals, and dramatic progress in the synthesis of semiconductornanocrystals ensued upon the development of high-temperaturesynthesis approaches in coordinating solvents. In 1994, Brust and co-workersdemonstrated the capping ligand approach to metal nanocrystal synthesis [3].As outlined in Fig. 3, their method relies on the room-temperature reductionof a gold salt in the presence of alkanethiol capping ligands under ambientconditions to yield crystalline particles stabilized with long-chain alkanes.Due to its simplicity, their approach has been reproduced and utilized bymany different research groups to study gold and silver nanocrystals, withvarious improvements in size control and narrowing of the size distribution[6,13]. Most recently, the high-temperature capping solvent approachesdeveloped for semiconductor nanocrystals have been applied to metal nanocrystalsby reducing metal precursors at high temperatures in variouscoordinating solvents (see Fig. 4) [1].Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.