Photoemission spectroscopy of platinum overlayers on silicon dioxide films

Abstract

Soft x-ray photoelectron spectroscopy (SXPS) has been used to study ultrathin Pt films on silicon dioxide as model supported-catalyst materials. Using monochromatic synchrotron radiation. Pt 4f and Si 2p core level photoelectron peaks were measured as a function of platinum coverage in the range 0–10 ML. The bulk silicon and silicon dioxide film Si 2p peaks each show a binding energy decrease within the first ML of dosing. However, the effect is stronger for the silicon dioxide Si 2p peak, indicating an increased screening of the 2p electrons by the metal overlayer. We also observe a monotonic increase of the work function of the ultrathin film Pt/SiO2 system with coverage from 4.52 initially to 5.58 eV at ∼10 ML. The Pt 4f7/2 core level binding energy decreased from ∼72.2 to ∼70.9 eV between 0 and ∼10 ML coverage. This binding energy shift at low dose (⩽1 ML), the coverage dependence of the Pt line shape and intensity, and the large saturation coverage for the work function are each consistent with two-dimensional cluster-island growth. The metallic nature of the Pt overlayer with dose is quantitatively verified by nonlinear, least-squares numerical fitting of the Pt 4f SXPS peak line shapes with Gaussian-broadened Doniach–Sunjic functions.