An Alkyl Spacer Effect on the Structure of 4-Fluorobenzenethiol and 4-Fluorobenzenemethanethiol Self-Assembled Monolayers on Au(111)

Self-assembled monolayers (SAMs) formed by the adsorption of 4-fluorobenzenethiol (4-FBT) and 4-fluorobenzenemethanethiol (4-FBMT) on Au(111) were examined by scanning tunneling microscopy (STM) to understand the effect of a flexible methylene spacer between the sulfur head-group and phenyl group and the effect of solution temperature on the formation and structure of the SAMs. Although the adsorption of 4-FBT on Au(111) at room temperature for 24 h generated only disordered phase SAMs containing gold adatom islands, 4-FBT at 75 degrees C for 2 h formed mixed SAMs: disordered phases and ordered (2 x 12 root 2)R10 degrees superlattice with a rectangular unit cell containing six adsorbed molecules. On the other hand, SAMs formed from 4-FBMT, with a methylene spacer, at room temperature for 24 h on Au(111) had irregularly ordered phases containing uniformly distributed Vls with lateral dimensions of 2-5 nm; SAMs formed from 4-FBMT at 75 degrees C for 2 h were composed of slightly improved ordered phases and larger Vls with lateral dimensions of 5-12 nm as a result of Ostwald ripening. From this study, we found that the methylene spacer plays an important role in controlling the structure of SAMs formed from p-substituted fluorinated aromatic thiols.