Adsorption stages of O on Ru(0001) studied by means of scanning tunnelling microscopy

Abstract

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.

Phase diagram of O/Ru(0001) from first principles

“…The discrepancy between the predicted and experimental upper phase boundary reflects the accuracy of current DFT calculations for adsorbates. Nevertheless, our simulations produce the observed coexistence regions between the lattice gas and the (2  2)-O structure, and the (2  2)-O with the (2  1)-O structure [10,11]. Finally, we find the presence of a ð ffiffi ffi 3 p  ffiffi ffi 3 p Þ Ã R30°structure at 2/3 ML and several coexisting structures above 0.5 ML, labeled B, C, D in Fig.…”

“…However, the absence of attractive interactions does not explain the presence of two regions of coexisting structures observed with STM [10,11]. The first region has both a (2 Â 2)-O structure and a lattice gas [10] and the second the (2 Â 2)-O and (2 Â 1)-O structures [11].…”

“…The first region has both a (2 Â 2)-O structure and a lattice gas [10] and the second the (2 Â 2)-O and (2 Â 1)-O structures [11]. Xiong and Li [9] have calculated the phase diagram from a model with hcp sites only having (an unspecified) first In this work we obtain the phase diagram from first principles.…”

Oxidation electronics: bond–band–barrier correlation and its applications

“…[ 56,257,85,189,[195][196][197][198][199][200] reaction at various stages, which enables the atomic valencies in the first two atomic layers to be specified. As will be discussed in detail subsequently, the phase transition originates from nothing more than the alteration of the oxygen valency (as assigned in Table 4) under a certain geometrical configuration at different oxygen coverage.…”

“…The 'oval' protrusions are observed in the Cu(110)-c(6Â2)-8O phase [48] and the 'honeycomb' protrusions composed of the 'dumbbells' are observed from the Cu(111)-O surface at higher temperature [49]. All the (Ag, Cu, Ni, Pt)(110) [53,54] and Ru(0001)-O surfaces [55,56]. STM studies should be able to reveal inherently common features caused by oxygen adsorption from all these many specific forms.…”