Ordered (3 x 4) High-Density Phase of Methylthiolate on Au(111)

The formation of ordered phases of dimethyl-disulfide on the Au(111) surface has been investigated by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and state-of-the-art density-functional theory (DFT) periodic supercell calculations. The LEED diffraction pattern, obtained after a production method that includes two-step dosing and prolonged postdeposition annealing, unambiguously corresponds to a novel phase that consists of (3 x 4) domains coexisting with the as-deposited (root3 x root3)R30° structure. XPS measurements indicate that the coverage of the new (3 x 4) superstructure is the same as that of the (root3 x root3)R30° phase. In both phases, the binding energy of the S 2p3/2 core-level peak is found to be 162.2 eV, corresponding to the formation of a thiolate layer. The DFT calculations allow us to identify a viable metastable (3 x 4) structure where the S headgroups of the CH3S radicals select distinct adsorption sites; three quarters of them adsorb at bridge sites and one quarter at top sites. The relative energetics of the (3 x 4) and (root3 x root3)R30° configurations suggest that the two structures may coexist on the surface, in agreement with experimental data.