Preparation of regular arrays of bimetallic clusters with independent control of size and chemical composition.

Regular arrays of bimetallic clusters have been prepared by atomic deposition, under UHV, on a nanostructured ultrathin alumina film. The alumina films are obtained by oxidation at 1000 K of a Ni3Al (111) surface. They present two regular hexagonal superstructures with lattice parameters of 2.4 and 4.1 nm. Pd clusters nucleate exclusively on the 4.1 nm superstructure forming regular arrays of clusters extending on the whole (1 cm2) substrate. Gold deposited on a previously formed Pd clusters array, condenses exclusively on the Pd clusters in forming a regular array of bimetallic AuPd clusters with a narrow size distribution. The size and the composition of the AuPd clusters can be controlled independently. Gold clusters nucleates also on the 4.1 nm superstructure but they can escape from the nucleation sites and coalesce with other gold clusters. By condensing Pd on the preformed Au clusters, PdAu clusters are formed together with pure Pd clusters nucleated on the free sites of the 4.1 nm superstructure of the alumina film.