Structure and binding condition of thiophene self-assembled monolayers (SAMs) formed on Au(111) were investigated by using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The STM study revealed the first molecularly resolved image for closely packed thiophene SAMs having two domain structures: Disordered phases near domain boundaries and ordered domains consisting of pairs of molecules separated from each other by 4.35 Å, from the next row by 3.9 Å, and from the next columns by 8.8 Å. From the STM observation, it is considered that a cofacial interaction between thiophene molecules plays an important role in determining the surface structure of thiophene SAMs on Au(111). The XPS spectra in the S 2p region of thiophene SAMs on Au(111) were observed at 161 (S 2p3/2) and 162 eV (S 2p3/2), which are assigned to the gold-bound sulfur. In contrast to previous XPS and theoretical studies, our high-resolution STM and XPS measurements have clearly demonstrated that the sulfur headgroups in thiophene would chemically interact with gold in the self-assembly process.
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