Growth and ordering of Cu–O islands during oxygen adsorption on Cu(1 1 0) at 470 K

Abstract

Scanning tunneling microscopy was used to study the Cu–O island formation during oxygen adsorption on Cu(1 1 0) at temperatures around 470 K. In situ measurements allow a detailed analysis of the growth of the (2×1)O phase up to the saturation coverage of 0.5 ML. New islands nucleate below a coverage of 0.2 ML. Below this coverage no long-range ordering of the islands along the [1 1 0] direction takes place. Shortly after nucleation, very thin islands span the entire width of the terrace along the [0 0 1] direction suggesting a high mobility of the Cu–O compounds. The subsequent growth of the islands is linked to the changes of the terrace widths related to the underlying “added-row” reconstruction of the (2×1)O phase, i.e. with increasing terrace width there is an almost instant growth and/or rearrangement of the islands along the [0 0 1] direction. During this growth, step bunching of the initially mobile terrace edges occurs. The island growth along the [1 1 0] direction depends on the distance between neighboring islands, i.e. the larger these spacings, the faster the respective island grows. The growth along the [1 1 ̄ 0] direction finally stops at a critical spacing between the islands of about 20 Å. The remaining gaps are closed simultaneously starting at the terrace edge. When the islands reach a width of about 80 Å a splitting of the island ends can be observed in some cases. In the course of this process a partial rearrangement of the original island takes place. A diffusion coefficient D 0 0 1 of 5±2×10 −15 cm 2/s has been estimated for the average diffusion of Cu adatoms and/or Cu–O compounds along the gaps, i.e. along the [0 0 1] direction. Along the [1 1 ̄ 0] direction an estimate of 7×10 −15±1 cm 2/s for D 1 1 ̄ 0 is derived from fluctuations in structure at the island boundaries. Additional ex situ measurements show island grouping at higher temperatures. These and further observations are discussed on the basis of the complex adsorbate–adsorbate interactions of the Cu–O compounds and the added-row reconstruction process that rule the O/Cu(1 1 0) system on the one hand, and the influence of the high mobility of the adsorbed species observed at the temperature addressed here on the other hand.