KINETICS OF THE FORMATION OF BUTANETHIOL MONOLAYERS ON GOLD SUBSTRATES, AS STUDIED BY INFRARED-SPECTROSCOPY

We have studied the formation of butanethiol self-assembled monolayers (C(4)SAMs) and sub-monolayers chemisorbed on Au(111) surfaces using i nfrared reflection-absorption spectroscopy. Our IR study follows the e volution of the adsorbates from a disordered phase of sub-monolayer C- 4 at short deposition times, to the more ordered arrangement of butane thiol films following extended exposure to the thiol solution. From th e variation of the absorbances of CH3 vibrations with the deposition t ime, we find that the initial deposition of the imperfect monolayer fr om solution is rapid at high concentrations, and can last many hours i n very dilute solutions; this rapid absorption is followed by a much s lower process of additional absorption and self-organisation. From the evolution of the frequency and intensity of the CH2 related absorptio n bands, we observed that the angle of butanethiol molecular axis with respect to the surface normal is significantly greater in the initial state than for the final well-formed film; the intensities of CH2 mod es for fully equilibrated monolayers and sub-monolayers are extremely weak due to the nearly orthogonal orientation of the transition dipole moment with respect to the surface normal and to the polarisation of the incident radiation. Our data suggests that the initial deposition sites for individual butanethiol adsorbates are randomly distributed o n the gold substrate.