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. In high-resolution images, two new structural features are resolved, which cannot be explained by the previously proposed (√3 × 2√3)-R30° 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°. 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.
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