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 HClO4 indicate that a 4-MBT SAM on Au(111) consists of multiple phases in electrochemical environments. These phases include ordered domains (alpha-phase), aggregated molecular patches (beta-phase), and potential dependent structures (beta'-phase and gamma-phase). At a potential positive of 0.3 V-SCE, comparable to the potential of zero charge (pzc) of Au(111), Ostwald ripening of alpha-phase domains is observed. Aggregated molecular patches are annealed, by the applied potential, to form a structured beta'-phase. At a potential negative of 0.3 V-SCE, the structure of 4-MBT SAMs transforms to a new phase (gamma-phase), 2.0 +/- 0.5 A higher than the surrounding alpha-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 gamma-phase to the alpha-phase.