The structure of methylthiolate and ethylthiolate monolayers on Au(111): Absence of the (root 3 x root 3)R30 degrees phase

Surface structures of self-assembled methylthiolate and ethylthiolate monolayers on Au(111) have been imaged with STM. For saturation coverage of 0.33 ML at room temperature, the well-known (root 3 x root 3)R30 degrees phase routinely observed for longer chain alkanethiolates does not appear under any conditions for adsorbed methylthiolate and ethylthiolate. Instead, both thiolate species organize themselves into a well-ordered 3 x 4 structure. We thus conclude that the stable structure for saturation coverage of methylthiolate/ethylthiolate on Au(111) at RT is 3 x 4, not (root 3 x root 3)R30 degrees as generally believed. For coverage less than 0.33 ML, a striped-phase with short-range order is observed for methylthiolate. Fourier transform of the STM image from the striped-phase produces a clear (root 3 x root 3)R30 degrees "diffraction" pattern. This strongly indicates that the (root 3 x root 3) R30 degrees diffraction pattern for methylthiolate monolayers reported in literature is likely from the striped-phase, rather than from a true (root 3 x root 3)R30 degrees lattice in real space. Consequently, theoretical modeling that reproduces the (root 3 x root 3)R30 degrees structure for methylthiolate monolayers should be re-examined. (C) 2011 Elsevier B.V. All rights reserved.