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 (C₄SAMs) and sub-monolayers chemisorbed on Au(111) surfaces using infrared reflection-absorption spectroscopy. Our IR study follows the evolution of the adsorbates from a disordered phase of sub-monolayer C₄ at short deposition times, to the more ordered arrangement of butanethiol films following extended exposure to the thiol solution. From the variation of the absorbances of CH₃ vibrations with the deposition time, we find that the initial deposition of the imperfect monolayer from solution is rapid at high concentrations, and can last many hours in very dilute solutions; this rapid absorption is followed by a much slower process of additional absorption and self-organisation. From the evolution of the frequency and intensity of the CH₂ related absorption 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 modes 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 on the gold substrate.