Structural and electronic properties of Ce overlayers and low-dimensional Pt-Ce alloys on Pt{111}

The structural, thermal, chemisorptive, and electronic properties of Ce on Pt{111} are studied by photoemission, Auger spectroscopy, scanning tunnel microscope (STM), and low-energy electron diffraction (LEED). Stranski-Krastanov-like growth of low-density Ce layers is accompanied by substantial valence charge transfer from Ce to Pt: in line with this, the measured dipole moment and polarizability of adsorbed Ce at low coverages are 7.2×10^-30 C m and ∼1.3×10^-29 m³, respectively. Pt-Ce intermixing commences at ∼400 K and with increasing temperature a sequence of five different ordered surface alloys evolves. The symmetry, periodicities, and rotational epitaxy observed by LEED are in good accord with the STM data which reveal the true complexity of the system. The various bimetallic surface phases are based on growth of crystalline Pt₅Ce, a hexagonal layer structure consisting of alternating layers of Pt₂Ce and Kagomé nets of Pt atoms. This characteristic ABAB layered arrangement of the surface alloys is clearly imaged, and chemisorption data permit a distinction to be made between the more reactive Pt₂Ce layer and the less reactive Pt Kagomé net. Either type of layer can appear at the surface as the terminating structure, thicker films exhibiting unit mesh parameters characteristic of the bulk alloy.