Quantum Dots Made of Metals: Preparation and Characterization
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Nanosize metal clusters occupy a position between the molecular and solid state and, because of the dominant role of their abundant surface area, provide a unique way to learn how metal–metal bonding, cluster shape, and packing are affected by ligands bound to the cluster surface. Such studies may give insights into complex issues in catalysis, such as selectivity of binding of substrates to vertex, edge, or face sites on a metal cluster, and how such binding affects the intermetal bond distances, essentially causing a surface reconstruction, mass redistribution, or shape change. Studies of the 3-D interface structure of nanosize metal clusters should yield information quite different from the extensive literature describing ligand interactions with extended, 2-D metal surfaces. Because clusters more closely resemble practical heterogeneous catalysts (e.g., hydrodesulfurization process, hydrogenation process), important new scientific and technical insights may be gained by their study.
The formation of metal colloids or clusters by the controlled reduction, nucleation, and growth from metal salts in aqueous solution has been investigated for over a century. In the case of gold colloids, the earliest scientific investigations were undertaken by Michael Faraday and spanned over a decade. Human interest in the colors of such colloidal systems dates back to the Roman times, as pointed out by Kreibig and Vollmer in their excellent review of the optical properties of metal clusters. An example cited therein is a Roman goblet from the fourth century in the British Museum whose fame is attributed to the shining colors generated by a composition of Ag and Au clusters. The purple colors of colloidal Au and Cu dispersions are thought to be the origin of the association of purple with royal colors.