Mesoscopic-scale growth of oxygen-rich films on Ru( [formula omitted]) investigated by photoemission electron microscopy

Abstract

Photoemission electron microscopy (PEEM) has been used for monitoring the modifications of the Ru(0 0 0 1) surface as induced by surface oxidation. The PEEM images of the initial oxidation stages exhibit a distribution of bright grains appearing on a dark background. The bright features are assigned to a Ru x O y region considered as a precursor of an extended RuO 2 phase. The dark areas reflect the presence of the saturated chemisorption layer of oxygen terminating the Ru surface. Increase of the oxygen load is reflected in the images by the increase of both brightness and density of individual grains. The growth of the oxide areas from nucleation centers up to a thick oxide film is found to strongly depend on the oxidation temperature. Three modes differing by characteristic morphologies are distinguished. For low temperatures, randomly distributed small-size patches dominate the surface. In the high-temperature oxidation regime, the oxidation produces an image, which consists of very bright discs randomly distributed over large dark areas. For moderate temperatures, bright stripes appear and grow along the main crystallographic directions of Ru(0 0 0 1). This mode starts with the formation of nuclei, which appear as small bright grains barely detectable by PEEM (<2 μm). Further oxidation occurs as a continuous growth of linear stripes, following the symmetry of the hexagonal substrate. Finally, the pattern resembles an ordered network of stripes. For very high oxygen exposures (>10 6 L), all the images become very bright with hardly distinguishable patterns. The saturated oxygen-rich layers created in the low- and moderate-temperature oxidation regimes are thermally unstable. The corresponding PEEM images become gradually darker when keeping the sample at temperatures below the desorption onset but higher than the respective oxidation temperatures thus revealing a depletion of the oxygen concentration within the topmost layer due to oxygen transport into the bulk.