Chemisorption of alkylsilanes, RSiH 3 (R hexyl, octyl, octadecyl), occurs via cleavage of the three SiÀH bonds and subsequent formation of three SiÀAu bonds. [1] The resulting novel class of alkylsilane-based monolayers on Au(111) has been characterized by X-ray photoemission spectroscopy (XPS) and reflection ± absorption infrared spectroscopy (RAIRS). Variable energy XPS studies suggest alkylsilane coverage and orientation on Au( 111) is similar to that found for alkanethiol monolayers. [2,3] Oxidation of the chemisorbed alkylsilane monolayer generates two novel and surprising results: 1) upon oxidation, the resulting alkylsiloxane layer becomes transparent to scanning tunneling microscopy (STM) imaging and 2) the oxidative conditions induce the gold surface to return to the 23 Â 3 p surface reconstruction typical for clean gold surfaces.
Results and Discussion
An STM image of clean Au(111) 23 Â 3 p is displayed in Figure 1 a. The image displays the parallel striped features of the Au(111) 23 Â 3 p surface reconstruction. [4±6] Exposure to a saturating dose of octylsilane in ultrahigh vacuum (UHV) dramatically alters the surface topography (Figure 1 b). The octylsilane monolayer surface is characterized by an interwoven pattern of elevated sinuous ridges surrounding small, depressed regions. Adjacent ridges frequently combine to form looped or knotted features. The ridges are % 0.8 ä high, relative to the depressed regions. In addition, there are numerous interstitial island features 20 ± 40 ä in diameter with apparent heights of 2.5 ± 3.0 ä relative to the darkly contrasted regions. The apparent height of an island feature is equivalent to and indistinguishable from that of a single-step Au terrace. The percentage of the area covered by Au islands in Figure 1 b (6.5 %) is consistent with the ejection of one to two atoms per (1 Â 23) primitive unit cell. [7] The presence of the islands indicates relaxation of the 23 Â 3 p surface reconstruction and is consistent with formation of Au(111) 1 Â 1. [7, 8] of the force volume data, and to H.-J. Butt (MPI-P) for helpful discussions. A.-S.D. and U.J. thank Prof. H. W. Spiess for his continued support.