Comparative Study of Tetrahydrothiophene and Thiophene Self-Assembled Monolayers on Au(111): Structure and Molecular Orientation

Surface structure and molecular orientation of self-assembled monolayers (SAMs) formed by the spontaneous adsorption of tetrahydrothiophene (THT) and thiophene (TP) on Au(111) were investigated by means of scanning tunneling microscopy (STM) and carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. STM imaging revealed that THT SAMs have a commensurate (3 x 2 root 3) structure containing Structural defects in ordered domains, whereas TP SAMs are composed of randomly adsorbed domains and paired molecular row domains that call be described as all incommensurate packing structure. The NEXAFS Spectroscopy Study showed that the average tilt angle of the aliphatic THT ring and pi-conjugated TP ring in the SAMs were calculated to be about 30 degrees and 40 degrees, respectively, from the surface normal. It was also observed that the pi* transition peak in the NEXAFS spectrum of the TP SAMs is very weak, Suggesting that a strong interaction between pi-electrons and the Au surface arises during the self-assembly of TP molecules. In this study, we have clearly demonstrated that the Surface structure and adsorption orientation of organic SAMs oil Au(111) are strongly influenced by whether the cyclic ring is saturated or unsaturated.