Photoemission study of the adsorption of Cu on Pt(111)

Photoemission experiments have been performed to investigate the electronic structure of Cu adlayers on a Pt(111) surface. The evolution of the $3d$ states as a function of Cu coverage is observed at the Cooper minimum of the Pt5d emission (hν=150 eV), where the Cu3d emission is enhanced. For Cu coverages up to {~}0.6 of the first monolayer (ML), the $3d$ states give a symmetrical and resonancelike peak with a maximum at -2.6 to -2.7 eV with respect to the Fermi level EF, and a full width at half maximum of 1.4 to 0.5 eV. Between {~}0.6 and {~}1 ML, there is an increase in Cu3d emission around -3.5 eV, suggesting the onset of two-dimensional delocalization. The delocalization of the $3d$ states increases with increasing Cu coverage. Beyond {~}1 ML, Cu-Cu bonding states appear below -4 eV as well. The Cu adlayers studied, from submonolayer coverages up to {~}1.5 ML, show an interesting absence of Cu-derived emission near EF, in contrast with the flat emission at the corresponding energies shown by pure Cu. For small ({≤}0.6 ML) submonolayer coverage, the Cu2p3/2 core level, excited by MgKα radiation, is observed to be shifted (upward) by -0.60 to -0.65 eV relative to the bulk Cu2p3/2 core level. By 3 ML, the Cu2p3/2 binding energy is almost identical with that of a pure Cu sample. The adsorption of Cu "removes" the Pt4f7/2 surface core level, which then becomes bulklike. The adsorption of Cu also results in a work-function decrease. However, the Pt4f7/2 core-level data do not permit the work-function decrease to be explained by simplistic arguments based on charge transfer.