Formation and domain structure of self-assembled monolayers by adsorption of tetrahydrothiophene on Au(111)

The formation and structure of tetrahydrothiophene (THT) self-assembled monolayers (SAMs) on Au(111) were examined using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). XPS measurements revealed that THT molecules, containing endo-sulfur aliphatic rings, can form chemisorbed SAMs, in contrast with the formation of physisorbed SAMs by dialkyl monosulfide, suggesting that the adsorption ability of monosulfide compounds on gold strongly depends on the structure of tail groups, such as an aliphatic ring or two alkyl groups attached to sulfur head groups. In addition, high-resolution STM imaging revealed, for the first time, that the adsorption of THT molecules on Au(111) results in long-range, two-dimensional, ordered SAMs, having a (3 x 2N/3) superlattice with many unique structural defects and a few vacancy islands. It is suggested that the unique surface structures of THT SAMs on Au(111) are mainly due to the weak van der Waals interactions between THT rings, as well as a dynamic structural variation of the THT ring in the SAMs. Our results from this study will be very useful in understanding SAM formation and the structures of organosulfur molecules containing an endo-sulfur ring.