Potential-Induced Structural Change in a Self-Assembled Monolayer of 4-Methylbenzenethiol on Au(111)

Potential-induced structural change in a self-assembled monolayer (SAM) of 4-methylbenzenethiol (4-MBT) on Au(111) is investigated by in situ scanning tunneling microscopy (STM). STM images in 0.1 M HClO 4 indicate that a 4-MBT SAM on Au(111) consists of multiple phases in electrochemical environments. These phases include ordered domains (α-phase), aggregated molecular patches (β-phase), and potential dependent structures (β‘-phase and γ-phase). At a potential positive of 0.3 V SCE, comparable to the potential of zero charge (pzc) of Au(111), Ostwald ripening of α-phase domains is observed. Aggregated molecular patches are annealed, by the applied potential, to form a structured β‘-phase. At a potential negative of 0.3 V SCE, the structure of 4-MBT SAMs transforms to a new phase (γ-phase), 2.0 ± 0.5 Å higher than the surrounding α-phase surface. This is shown to be an STM tip-enhanced structural change, and is a consequence of the weak binding of thiol to gold at a potential negative of the pzc. Stepping the substrate potential back from 0.2 to 0.4 V SCE transforms the γ-phase to the α-phase.