Mesoscopic-scale pattern formation induced by oxidation of Ru(0001)

Photoemission electron microscopy (PEEM) has been utilized for monitoring the modifications of a Ru(0001) surface induced by surface oxidation. The PEEM images of the initial oxidation stages reveal bright RuxOy regions surrounded by dark areas formed by the saturated oxygen chemisorption layer. For a narrow particular temperature interval the oxidation starts with the appearance of light nuclei or grains (<2 mum), which expand as long stripes along the main crystallographic directions of Ru(0001). Further oxidation proceeds via growth of linear as well as starshaped islands, which follow the hexagonal symmetry of the substrate and eventually form an ordered network. These mesoscopic-scale structures reflect the atomic-scale arrangement of the clean Ru substrate. The temperature region where the hexagonal arranged stripes exist shifts towards higher temperatures with increasing oxidation pressure. The stripes are thermally stable up to the onset of oxygen desorption. The amount of atomic oxygen incorporated in the micrometer-scale structures can be depleted by exposing the surface to CO at elevated temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.