Synergistic interactions in trimetallic bonding: A comparison of the (NM = Cu, AgorAu) systems

The properties of Ag-Zn alloy films on Ru(001) have been examined using thermal desorption spectroscopy and photoemission. The results were compared to those previously reported for Cu-Zn and Au-Zn alloys on Ru(001). For the Zn-noble-metal alloys, the mechanism of decomposition was always the same: evolution of Zn into gas phase, with the noble metal remaining solid. In thick alloys with a low concentration of Zn, the decomposition temperature of Ag-Zn (600 K) was lower than that of Cu-Zn (640 K) or Au-Zn (670 K). Alloy formation induced only small shifts ( −0.2 to −0.3 eV) in the position of the Zn 2p, 3s and 3d levels. In contrast, the core and valence levels of the noble metals showed large shifts toward higher binding energy. For small amounts of Cu, Ag and Au dissolved in Zn multilayers, the shifts in the noble-metal core levels follow the sequence: Ag(3d52), 0.72 eV < Cu(2p32), 0.85 eV < Au(4f72), 1.40 eV. These shifts reflect a nd → (n + l)s,p rehybridization in the valence levels of the noble metal induced by the formation of bonds with Zn. Two-dimensional (2D) alloys of Zn and a noble metal in contact with Ru(001), θZn + θNM ⩽1, show Zn-Ru and Zn-NM bonds more stable than the corresponding bonds in ZnRu(001) or thick three-dimensional (3D) alloys. This phenomenon is caused by synergistic interactions that involve the Ru-Zn, Ru-NM and Zn-NM bonds. The strength of these interactions depends on the ability of the noble metal to form strong metal-metal bonds: Ag < Cu < Au. The supported 2D alloys show noble-metal core levels shifted toward lower binding energy with respect to those of thick 3D alloys. These shifts and the large thermal stability of the supported 2D alloys can be attributed to a cooperative charge transfer from Ru to Zn to the noble metal.