Role of surface geometry and electronic structure in STM images of O/Ru(0001)

The Nanostructuring of Atomically Flat Ru(0001) upon Oxidation and Reduction

The O/Ru(0001) system is widely studied due to its rich phase variety of various stoichiometry and atomic arrangements, including the formation of a RuO₂/Ru(0001) oxide layer. Apart from homogeneous ruthenium surfaces in certain oxidation states, also strongly heterogeneous surfaces can exist due to oxidation state's variation at the nanoscale. We report on a scanning tunneling microscopy (STM) study of the nanostructuring of the oxidized Ru(0001) surface as a result of its interaction with molecular oxygen at elevated temperatures and subsequent reduction of a resulting RuO₂ film by CO or HCl molecules from the gas phase in high-vacuum environment.

Surface morphology of atomic nitrogen on Pt(111)

The surface morphology of chemisorbed N on the Pt(111) surface has been studied at the atomic level with low temperature scanning tunneling microscopy (STM). When N is coadsorbed with O on the surface, they form a mixed (2 × 2)-N+O structure. When the surface is covered with N atoms only, isolated atoms and incomplete (2 × 2) patches are observed at low coverages. In a dense N layer, two phases, (√3 × √3)R30°-N and p(2 × 2)-N, are found to coexist at temperatures between 360 and 400 K. The (√3 × √3)R30° phase converts to the (2 × 2) phase as temperature increases. For both phases, nitrogen occupies fcc-hollow sites. At temperatures above 420 K, nitrogen starts to desorb. The p(2 × 2)-N phase shows a honeycomb structure in STM images with three nitrogen and three platinum atoms forming a six-membered ring, which can be attributed to the strong nitrogen binding to the underlying Pt surface.

Understanding atomic-resolved STM images on TiO2(110)-(1 x 1) surface by DFT calculations

We present a combination of experimental STM images and DFT calculations to understand the atomic scale contrast of features found in high-resolution STM images. Simulating different plausible structural models for the tip, we have been able to reproduce various characteristics previously reported in experimental images on TiO(2)(110)-(1 x 1) under controlled UHV conditions. Our results allow us to determine the influence of different chemical and morphological tip terminations on the atomic-resolution STM images of the TiO(2)(110)-(1 x 1) surface. The commonest images have been properly explained using standard models for a W tip, either clean or with a single O atom located at the apex. Furthermore, a double transfer of oxygen atoms can account for different types of bizarre atomic-resolution features occasionally seen, and not conclusively interpreted before. Importantly, we discuss how typical point-defects are imaged on this surface by different tips, namely bridging O vacancies and adsorbed OH groups.

Reactivity of periodically rippled graphene grown on Ru(0001)

We report here the reactivity of epitaxial graphene islands and complete monolayers on Ru(0001) towards molecular oxygen and air. The graphene is prepared by thermal decomposition of ethylene molecules pre-adsorbed on an Ru(0001) surface in an ultra-high vacuum chamber. The graphene layer presents a periodically rippled structure that is dictated by the misfit between graphene and Ru(0001) lattice parameters. The periodic ripples produce spatial charge redistribution in the graphene and modifies its electronic structure around the Fermi level. In order to investigate the reactivity of graphene we expose graphene islands to a partial pressure of oxygen and following the evolution of the surface by STM during the exposure. For the exposure to air we removed the sample from the UHV chamber and we re-introduce it after several hours, taking STM images before and after. The surface areas not covered by the graphene islands present a dramatic change but the graphene structure, even the borders of the islands, remain intact. In the case of a complete graphene monolayer the exposure to oxygen or to air does not affect or destroy the rippled structure of the graphene monolayer.

Water Production Reaction on Rh(110)

By means of scanning tunneling microscopy and density functional theory calculations, we studied the water formation reaction on the Rh(110) surface when exposing the (2 x 1)p2mg-O structure to molecular hydrogen, characterizing each of the structures that form on the surface during the reaction. First the reaction propagates on the surface as a wave front, removing half of the initial oxygen atoms. The remaining 0.5 monolayers of O atoms rearrange in pairs, forming a c(2 x 4) structure. Second, as the reaction proceeds, areas of an intermediate structure with c(2 x 2) symmetry appear and grow at the expense of the c(2 x 4) phase, involving all the oxygen atoms present on the surface. Afterward, the c(2 x 2) islands shrink, indicating that complete hydrogenation occurs at their edges, leaving behind a clean rhodium substrate. Two possible models for the c(2 x 2) structure, where not only the arrangement but also the chemical identity is different, are given. The first one is a mixed H + O structure, while the second one resembles the half-dissociated water layer already proposed on other metal surfaces. In both models, the high local oxygen coverage is achieved by the formation of a hexagonal network of hydrogen bonds.