The surface structures, adsorption conditions, and thermal desorption behaviors of
benzenethiol (BT) and benzenemethanethiol (BMT) self-assembled monolayers (
SAMs) on Au(111) were examined by means of scanning tunneling microscopy (STM), X-ray photoelectron microscopy (XPS), and thermal desorption spectroscopy to understand the effects of the alkyl spacer between the phenyl group and the
sulfur atom. Although XPS spectral shapes in the S 2p region for both
SAMs are similar, the surface structures and thermal desorption behaviors differ significantly. BT
SAMs on Au(111) were composed of disordered phases, whereas BMT
SAMs have well-ordered phases containing vacancy islands. The strong desorption peak for parent mass species (m/z=110,C(6)H(5)SH(+)) was observed in BT
SAMs at about 500K, whereas no desorption peak (m/z=124,C(6)H(5)CH(2)SH(+)) was observed from BMT
SAMs. Interestingly, the dominant TD peak for the benzyl fragments (m/z=91,C(6)H(5)CH(2)(+)) formed via C-S bond cleavage was observed in BMT
SAMs at around 400K. From this study, we clearly revealed that the small modification in chemical structure by inserting a methylene spacer between the phenyl group and the
sulfur atom affects 2D SAM structures, adsorption conditions, and thermal desorption behaviors and stability. The results obtained here will be very useful in designing and fabricating aromatic
thiol SAMs for further applications.