Ordered (3x4) High Density Phase of Methylthiolate on Au(111)

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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 a two-step dosing and prolonged post-deposition annealing, unambiguously corresponds to a novel phase that consists of (3x4) domains coexisting with the as-deposited (sqrt3xsqrt3)R30 structure. XPS measurements indicate that the coverage of the new (3x4) superstructure is the same as that of the (sqrt3xsqrt3)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 (3x4) 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 (3x4) and (sqrt3xsqrt3)R30 configurations suggest that the two structures may coexist on the surface, in agreement with experimental data.