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

B. Metal Nanocrystal SynthesisOrganic monolayer-coated metal nanocrystals can be synthesized at roomtemperature and high temperatures, depending on the material of interest.For example, gold and silver nanocrystals with highly crystalline cores arereadily produced at room temperature using alkanethiols as capping ligands[3]. Other materials, such as Co [1] and FePt [23], require higher temperaturesto drive the necessary metal reduction chemistry and ligand adsorption/desorption equilibria necessary to produce reasonably monodisperse stericallystabilized particles.Metal nanocrystal-arrested precipitation is conceptually simple. Metalsare precipitated from dissolved metal ions, or metallorganic precursors, in thepresence of capping ligands that bind to the nanocrystal surface to controlparticle growth. Without the capping ligands, the metal would precipitate intobulk material. In this synthetic process, a fundamental competition existsbetween particle nucleation and growth kinetics, and ligand adsorption/desorptionkinetics. Nucleation kinetics in the liquid phase and ligand adsorptionchemistry on small particles are both not well understood. Therefore, thesuccessful development of arrested precipitation procedures generallyrequires trial and error. The binding strengths of different functional groups,like thiols and amines, can vary dramatically for different metallic species. Ifthe ligands bind the metal too strongly, particles will not grow. In some cases,as with thiols and cobalt, the functional group stabilizes a molecular complexthat is extremely difficult to reduce to cobalt metal. Therefore, differentchemical routes and temperatures can be required to form nanocrystals withdiffering core composition and capping ligand chemistry. For example, goldand silver are easily reduced by NaBH 4 in the presence of thiolates [3],whereas copper will not reduce at room temperature upon the addition ofNaBH 4 due to the strong bonding between copper cations and thiolates. Infact, in the case of copper, the thermodynamic driving force toward metaloxidation by the thiol favors the thermolysis of copper thiolates into Cu 2 S atelevated temperatures (f150–200jC) [24]. Therefore, the chemical interactionbetween the chosen ligand and the metal must be considered in anysynthetic scheme explored. Amines and carboylated molecules have provenrelatively versatile for metal nanocrystal synthesis, as in the case of Co [1] andFePt [23] nanocrystals. Once the reduction chemistry proceeds, the ligandsmust provide sufficient binding and steric stabilization to prevent uncontrollablegrowth to very large sizes and high degrees of polydispersity. Acombination of ligands in a synthesis often provides the appropriate bindingstrength: One ligand binds weakly to provide particle growth under controlledconditions, and the other ligand provides strong binding to quench growth ata particular size.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.