The adsorption of O on Ru(0001) at a temperature of 400 K is studied in detail by means of scanning tunnelling microscopy (STM). With increasing O coverage, an ordered p(2 × 2) structure develops, followed by a p(2 × 1) structure. While the p(2 × 2) structure grows via island formation, the p(2 × 1) structure is abruptly formed by a disorder-order phase transition. After completion of the p(2 × 2) structure at a coverage of 0.25 ML, the surface develops a rough structure where the (2 × 2) units remain visible but appear with different heights. As the origin of this phenomenon, we propose additional O-O interactions and/or subsurface O due to the increase in O coverage. At coverages between 0.3 monolayer (ML) and 0.35 ML, different preformations of the p(2 × 1) structure are observed. First, small fragments of p(2 × 1) rows develop, which are randomly distributed over the surface and rotated by 120 • with respect to each other. They grow in one dimension and induce a criss-cross arrangement of linear chains of O atoms. Two-dimensional ordering starts via pairing of the p(2 × 1) rows. At a critical O coverage slightly below 0.40 ML, suddenly large p(2 × 1) domains are formed in three orientations (rotated by 120 • ), which coexist with remnants of the p(2 × 2) structure. At the saturation coverage of O (0.5 ML), the p(2 × 1) domains cover the surface completely.

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The film growth of Cu on clean and O-precovered Ru(0001) at different growth temperatures form 300 K to 450 K was investigated by scanning tunnelling microscopy (STM). Cu films on clean Ru(0001) grow in a multilayer mode at these temperatures. By using an O precoverage in the range of 0.1 monolayers (ML) up to a saturation coverage of 0.5 ML on clean Ru(0001), at 400 K different growth regimes are obtained. For ML a multilayer mode is preserved which changes into an O-induced two-dimensional (2D) growth for higher (0.2-0.5 ML). STM reveals the formation of an O/Cu surfactant structure on the surface due to migration of O initially located at the Ru surface. Its surface coverage rises linearly with O precoverage up to ML where it covers the surface completely. By increasing up to 0.5 ML, a drastic change in the morphology and density of the 2D islands occurs, which is accompanied by a change of the O/Cu surfactant structure. The O/Cu surfactant structure displays some order on a local scale for low , which changes into a disordered structure for ML. Structural similarities to oxidized surfaces of Cu(111) and the structures induced by postadsorption on Cu/Ru(0001) are discussed. Different models of surfactant mechanisms are presented to explain the observations. The locally ordered O/Cu surfactant structure (for ML) together with specific Cu film defects induce a heterogeneous nucleation of Cu with a high island density. Different mobilities of migrating Cu adatoms are established on top of the small islands and on the O/Cu structure resulting in enhanced interlayer diffusion explaining the observed 2D growth. The average island density only slightly changes within the temperatures investigated. In contrast, the saturated and disordered O/Cu surfactant structure (for = 0.4 - 0.5 ML) causes homogeneous nucleation. For this structure, the island density strongly depends on temperature and gives rise to an Arrhenius-like behaviour. The observed 2D growth is attributed to a reduction of the interlayer diffusion barrier. Cu growth on a formerly annealed Cu/O/Ru(0001) film system yields an almost perfect layer-by-layer growth caused by heterogeneous nucleation at periodically arranged Cu film defect sites. The relationship of the O/Cu surfactant structures to the ordered O/Cu bilayer on Ru(0001) - interpreted as a disrupted -like oxidized surface - was revealed.