Probing Structure and Molecular Conductance in Highly Ordered Benzyl Mercaptan Monolayers

The monolayer structure of alkanethiol and benzyl mercaptan (C 6 H 5 CH 2 SH) self-assembled monolayers on Au(111) is studied by scanning tunnelling microscopy in ultrahigh vacuum (UHV-STM) and photoelectron spectroscopy (XPS). Whereas alkanethiol monolayers exhibit the known c(4 × 2) overlayer structure, benzyl mercaptan (BM) monolayers show a novel reconstruction, resulting from thermal annealing at 368 K. Extended, striped phase domains having a commensurate, p(4 1 / 2 √3 × 2) overlayer structure with an oblique unit cell are observed. The tunnelling decay constant β of the molecular layer and the molecular conductance at the probe−monolayer contact point are obtained from tunnelling current-distance characteristics in nanoscopic junctions. Data from alkanethiols with different chain-lengths (octanethiol and decanethiol) yield a value of β = 0.95/Å, in good agreement with previous studies. BM monolayers show a lower decay constant (β = 0.55/Å), attributed to the π-conjugation in the phenyl ring. The measured currents are about 2 orders of magnitude lower than those of analogue dithiol derivatives that are chemically linked to both electrodes. This is related to the higher impedance of the physical contact between monolayer and STM probe compared to the covalent metal−molecule contact.