An Investigation of Rigid p-Methylterphenyl Thiol Self-Assembled Monolayers on Au(111) Using Reflection−Absorption Infrared Spectroscopy and Scanning Tunneling Microscopy

We have prepared and characterized self-assembled monolayers (SAMs) of rigid p -methylterphenyl thiol (MTPT) on Au(111). According to ellipsometry and reflection−absorption infrared spectroscopy (RAIRS), MTPT forms densely packed monolayers on gold with the molecular axes slightly tilted away from the surface normal (∼17°). At room temperature, monatomically high islands are observed by scanning tunneling microscopy (STM) instead of the typical monatomically deep holes characteristic of alkanethiol films. Molecular resolution images of the MTPT monolayer reveal a (√3×√3)R30°-like packing with slightly larger lattice vectors ( a = 5.3 ± 0.3 Å, b = 5.3 ± 0.4 Å, γ = 60° ± 2°) than those of typical alkanethiol monolayers. This is probably due to the mismatch between the lowest energy packing configuration of the terphenyl monolayer (similar to that of crystalline terphenyl solid) and that of the underlying (√3×√3)R30° structure of the sulfur headgroups. As a result, small domains of order and a high density of defects are observed in the film.