Low-temperature phases of methylthiolate (CH3S) monolayers on Au(111) have been investigated by using cryogenic scanning tunneling microscopy. The methylthiolate monolayers have a structure with a (√3 × √3)R30° lattice at room temperature, while at 110 K it is transformed into a one-dimensional "chain structure" near step edges and a two-dimensionally ordered structure with an anisotropic lattice on terraces. The latter structure has an oblique commensurate (3 × 4) unit cell (8.6 × 11.5 Å) in which four methylthiolates are located as a "tetramer". This temperature-dependent phase transition is reversible. The formation of tetramers is interpreted as evidence of an important role of a clustering of sulfur species in forming the ordered structure of the thiolate monolayers.
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