STM studies of ordered(31×31)R9°CO islands on Ag(111)

Abstract

The adsorption behavior of CO on Ag(111) is studied using low-temperature scanning tunneling microscopy. At submonolayer coverage, only single CO molecules are observed upon adsorption at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A further dosage leads to the formation of islands with various shapes and sizes. In addition, clusters with a diameter of about $11\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ are found that are mobile on the surface at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Though the position of the CO molecules within these clusters cannot be resolved, their size points to CO hexamers or heptamers. Well-ordered CO islands are observed only after heating the sample to $17\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, whereby two rotational domains with hexagonal $(\sqrt{31}\ifmmode\times\else\texttimes\fi{}\sqrt{31})R9\ifmmode^\circ\else\textdegree\fi{}$ structure are formed. A structural model is proposed in which CO hexamers represent the fundamental building blocks. The existence of two domains is explained with the alternate CO adsorption on the fcc and hcp places within the hexamers. The $(\sqrt{31}\ifmmode\times\else\texttimes\fi{}\sqrt{31})R9\ifmmode^\circ\else\textdegree\fi{}$ superlattice is the only well-ordered CO structure found in the temperature range between $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $35\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.