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Abstract:
Adsorption of Au and Pd over bilayer SiO2/Ru has been investigated using scanning-probe microscopy, X-ray photoemission spectroscopy (XPS), and density functional theory (DFT). Low temperature (∼5 K) atomic force (AFM) and scanning tunneling microscopy (STM) measurements reveal the presence of small adsorption features after exposing the samples to small doses of either metal. In the case of Pd, we note a homogeneous distribution of adsorbates across the entire surface, which consists of both amorphous and crystalline phases. Au, however, adsorbs only over amorphous areas and domain boundaries, which possess larger pores than can be found in the ordered portions of the film. DFT calculations reveal that this difference is rooted in the pore-size-dependent barriers for diffusion of the two metals into the film, where they can then bind stably at the Ru interface. Auger parameter analysis of the Pd 3d and Au 4f core-levels from atoms binding in this manner show upward orbital-energy shifts, which, according to the results of theoretical calculations, originate from effects similar to those causing surface core-level-shifts for such metals. Further analysis of the computational results shows that such atoms donate electron density to the Ru support, which is consistent with XPS results that show band-bending effects related to 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 increase.
