Gold nanoclusters of a size approaching the molecular limit ($<3\mathrm{n}\mathrm{m}$) were prepared on Si substrates in order to study alloy formation on the nanometer scale. For this purpose, indium atoms are deposited on top of the gold particles at room temperature and the formation of ${\mathrm{A}\mathrm{u}\mathrm{I}\mathrm{n}}_2$ is studied by x-ray photoelectron spectroscopy in situ. It is observed that the alloy formation takes place independent of whether the particles electronically are in an insulating molecular or in a metallic state. Most important, however, closed packed full-shell clusters containing 55 Au atoms are found to exhibit an outstanding stability against alloying despite a large negative heat of formation of the bulk Au-In system. Thus, ${\mathrm{A}\mathrm{u}}_{55}$ clusters may play a significant role in the design of nanoscaled devices where chemical inertness is of crucial importance.

• Received 14 October 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.016804