Size dependences in the adsorptive properties of the surface of ytterbium nanofilms deposited on silicon: The CO–Yb–Si(111)7 $\times$ 7 system

This paper reports on a study of the adsorption of CO molecules on the surface of ytterbium nanofilms of different thicknesses, which were sublimed on Si(111)7 $\times$ 7 at room temperature. Dependences of two types were investigated: the surface concentration of adsorbed molecules vs. CO dose expressed in langmuirs and the work function of films vs. CO dose. It was shown that the behavior of these dependences is mediated by size effects and Friedel oscillations generated by the ytterbium-silicon interface. Both effects exert an influence on the binding of CO molecules to the surface. At low molecule concentrations, this binding is effected through lone electron pairs localized at the carbon ends of the molecules. These electrons form a donor-acceptor bond to the vacant 5d level of the metal, with the level dropping below the Fermi level. At high CO molecule concentrations, the pattern becomes more complex; indeed, the enhanced Coulomb interaction gives rise to a partial transfer of electrons from the 5$d$ level to the vacant 2$\pi^*$ orbital of CO molecules.