Arenethiols form ordered and incommensurate self-assembled monolayers on Au(111) surfaces

Self-assembled monolayers (SAMs) of 4-[4'-(phenylethynyl)-phenylethynyl]-benzenethiols on Au(111) surfaces are investigated by scanning tunneling microscopy (STM) under ultrahigh vacuum. STM images reveal long-range order in these SAMs with a rectangular unit cell containing two molecules. Ln high-resolution images, two new structural features are resolved, which cannot be explained by the previously proposed (root 3 x 2 root 3)-R30 degrees commensurate structure which consists of three equivalent domains. First, six equivalent domains are present, and the orientations of these domains with respect to the three [121] directions of Au(111) are +/-5 degrees. Second, superstructures are observed. Periodical ridges are observed as a modulation of the, STM imaging contrast within the ordered domains. A new model is proposed, which is very similar to the crystalline structure of p-terphenyl. The closest-packed row is aligned along the next-nearest neighbor or [121] direction of Au(111) with phenyl planes arranged in a herringbone fashion. The lack of simple commensurate structure of arenethiol SAMs is mainly attributed to intermolecular interactions.