Strain stabilized alloying of immiscible metals in thin films

We report on the structure and composition of codeposited thin Ag and Cu films on a Ru(0001) substrate. All three metals are immiscible in their bulk forms, however, monolayer films of Ag and Cu are observed to alloy on Ru(0001). In the dilute Cu case, preferential formation of the alloy is found. With increasing Cu concentration, domains of the alloy are formed with characteristic shapes. These data point to a mechanism of alloy formation driven by strain relief of the overlayer film. Details of the structure and composition of the film can be understood within this picture.