Monolayer-Protected Clusters: Molecular Precursors to Metal Films

Monolayer-protected clusters (MPCs) are used to prepare solid, continuous metal films containing a single white metal or an alloy thereof. MPCs consist of nanoscopic metal cores coated with monolayers of thiolate ligands. In one method, multilayer films of carboxylate-functionalized alkanethiolate MPCs are assembled using Cu2+ coordinative bridges. The MPC film can be thermally decomposed at moderate temperature (<350 °C) to produce films of the core metal; the thiolate ligands escape as volatile disulfides. In another method, solutions of MPCs with alkanethiolate monolayers are cast or painted onto substrates, followed again by thermolysis to produce films of core metals. Films prepared from MPCs having metal alloy cores tend to exhibit metal segregation. A third method uses electrochemical generation of iodide at a Pt electrode to destabilize the MPC monolayer, an action that effectively coats the electrode with Au. The metal films are analyzed by stylus profilometry, atomic force microscopy, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and electrochemistry.