Growth of Self-Assembled Monolayer of Thiophene on Gold Surface: An Infrared Spectroscopic Study

The growth process of self-assembled monolayers (SAMs) of thiophene on Au(111) surfaces was revealed by Fourier-transform infrared reflection absorption spectroscopy (FT-IR-RAS). Thiophene spontaneously adsorbs from an ethanol solution onto the gold surface, and forms well-ordered SAM. FT-IR-RAS measurements and Langmuir adsorption isotherms revealed that thiophene SAMs possess two phases with different molecular orientations during SAM growth. In the primary phase, thiophene orients parallel to the gold surface. In the final phase, the molecular orientation changes from parallel to upright configurations with respect to the surface. This suggests that even molecules without alkyl-chain moiety can reorient during SAM growth. A transition of the molecular orientation is caused by a balance between thiophene-thiophene and thiophene-gold interactions.