Complete Structural Phases for Self-Assembled Methylthiolate Monolayers on Au(111)

The structure of self-assembled methylthiolate monolayers on Au(111) as a function of surface coverage has been investigated using scanning tunneling microscopic imaging, and complete structural phases are obtained. A full layer (1/3 ML) of methylthiolate organizes into a long-range ordered (3 × 4) phase with chiral domains. The domain boundary has a local (3 × 4√3)-rect structure. Gradual removal of methylthiolate by partial thermal desorption leads first to the formation of a local (7 × √3)-rect phase with coverage equal to 0.286 ML. This (7 × √3)-rect phase is found to coexist with the (3 × 4) phase and a disordered phase. As the coverage is reduced to below 0.28 ML, stripes parallel to the ⟨112⟩ directions appear. All the phases are made from the basic, CH3S–Au–SCH3, structural motif. Cis–trans transformation of the CH3S–Au–SCH3 unit is observed. For an isolated CH3S–Au–SCH3 unit, both methyl groups have equal probability of contributing to the cis–trans transformation at 77 K. Within the stripe phas...