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Abstract

Adsorption of Au and Pd over bilayer SiO2/Ru(0001) has been investigated using scanning-probe microscopy, x-ray photoemission spectroscopy (XPS), and theory. Low-temperature (∼5K) atomic-force (AFM) and scanning-tunneling microscopy (STM) measurements reveal small adsorption features after exposing the samples to small doses of either metal. For Pd, we note a homogeneous distribution of adsorbates across the surface, which consists of both amorphous and crystalline SiO₂ phases. By contrast, Au only adsorbs over amorphous areas and domain boundaries, which posses larger pores than can be found in the ordered portions of the film. Density functional theory (DFT) calculations reveal that this discrepancy is rooted in the pore-size-dependent barriers for diffusion of the two metals into the openings within the film, where they can then bind stably at the Ru interface. Auger parameter analysis of the Pd 3d and Au 4f orbitals from atoms binding in this manner show upward core-level-shifts, which theoretical calculations suggest originate from effects similar to those causing surface core-level-shifts for such metals. Further analysis of the computational results shows that such atoms actually donate electron density to the support, which is consistent with XPS results that show decreases in the work-function of the sample after adsorbing either metal. Additional features in the XPS studies suggest that a secondary binding mechanism, mediated by cluster formation over the SiO₂ film, becomes increasingly favorable as temperature and loading increases.