An investigation of bimetallic clusters by a combined use of electron microscopy and photoelectron spectroscopy: additive effects of alloying and cluster size on core-level binding energies

Bimetallic clusters of Ni-Pd, Cu-Ni, Cu-Au and Au-Ni deposited on amorphized graphite have been investigated by a combined use of photoelectron spectroscopy and electron microscopy. Metal core-level binding energies (Pd 3d, Au 4f and Ni/Cu 2p) of the bimetallic clusters deposited on amorphized graphite have been measured for different coverages or mean cluster sizes. When the cluster is large, the core-level binding energy of the major metallic component in the bimetallic clusters (e.g. Cu in Cu3Au or Cu7Ni3) is close to that of the bulk metal while that of the minor component (e.g. Au in Cu3Au or Ni in Cu7Ni3) shows the effect of alloying. The effect of alloying is found in the core-level energies of both Ni and Pd in the large clusters ofNi3Pd2. With the decrease in cluster size or coverage, the core-level binding energies of both the metals increase, just as in the case of monometallic clusters. The present results show the occurrence of parallel shifts in the core-level binding energy of metals due to alloying and cluster size effects, both the effects manifesting themselves in the small clusters. It is noteworthy that the core-level binding energy shifts in bimetallic clusters are distinctly different from those in bimetallic overlayers. Although alloy formation does not occur in the Au-Ni system in the bulk, the Au3Ni and Ni3Au clusters show variations in binding energies similar to the alloy clusters. It appears that alloying in the Au-Ni system may indeed occur in the nanometric regime of clusters.