X-ray photoelectron spectroscopy study of growth of thin cerium films on polypropylene

Abstract

From x-ray photoelectron spectroscopy (XPS) measurements of thermally evaporated cerium films on polypropylene (PP) we conclude that an ion bombardment pretreatment of the substrate significantly improves the deposited amount of cerium. The metal–polymer interface state is dependent upon the ion bombardment of the PP substrate. For a mild bombardment (Ar+ ions of kinetic energy less than 500 eV and doses less than 1015 ions/cm2) we observed the formation of Ce–C bonds, which are not observed if we apply a more energetic ion bombardment (ions kinetic energy 3 keV, doses≳1016 ions/cm2). The effect of the ion bombardment on the cerium film formation can be summarized in an increased coverage of the polymeric substrate for the same thickness of the cerium film. We find that the cerium film grows with a Volmer–Weber mechanism and the difference between treated and untreated samples consists in an increased coverage of the substrate by cerium for the treated samples. A new kind of plot of the XPS data is used to discriminate between different surface coverages. The XPS measurements have also shown that an oxidation of the cerium films takes place during the XPS measurements. Mass spectrometry of the residual gas composition of the ultrahigh vacuum system, where the XPS measurements are performed, shows that the x-rays induce an outgas of H2, water, OH, CO, and CO2 originating from the polypropylene substrate. Water in the PP is dissociated by x-ray influence and therefore is responsible for the oxidation of the cerium film.