Scanning Tunneling Microscopy and Surface Science Investigation of Cesium on Gold(111)

Alkali metal adsorption on surfaces are of considerable research interest. A cesium-covered gold surface (Cs-Au(111)) has been investigated to elucidate its structure and chemical properties. Using scanning tunneling microscopy (STM) in ultra high vacuum (UHV) the surface structure of the system has been investigated. Supported by additional data from X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and Auger electron spectroscopy, the findings are as follows: the coverage of Cs on Au(111) that corresponds to the photoemission maximum and work function minimum was found to be in the range of 0.21--0.26 monolayers (ML). This is lower than the saturation coverage that was originally thought to correlate with the work function minimum. Cs was found to be mobile at sub-saturation coverages down to a temperature of 172 K. At coverages above 0.15 ML the surface has 2 coexisting phases. The more prevalent phase is the mobile two-dimensional gas that moves freely across the surface and is transparent to STM. The minority phase is more interesting. This phase is a condensed phase of islands of various structures, which grow more prevalent as the coverage is increased. The condensed phase begins as an incommensurate structure, which creates a moire pattern. This structure has a diamond-pattern unit cell that consists of 26 Cs atoms and corresponds to a surface coverage of 0.21 ML. As the coverage is increased a more close packed striped structure is observed with areas of p(2x2) and (31/2x31/2)R30°. The coverage associated with the striped phase was found to be in the 0.26--0.32 ML range. The saturation coverage was found to be >0.33ML. Multilayers are not stable at room temperature and will leave the surface in UHV conditions over the period of 3 days. The surface also shows indications of forming a thin insulating layer, but it is clear that sub-monolayer coverages of Cs do not form the well-known CsAu salt.