Formation of Self-Assembled Butanethiol Monolayers on Au Substrates: Spectroscopic Evidence for Highly Ordered Island Formation in Sub-Monolayer Films

Abstract

The initial stages of the formation of butanethiol self-assembled monolayers on well-prepared Au/mica substrates have been studied using infrared spectroscopic analysis of the CH2 and CH3 stretching vibrations and real-space STM methods. The projection of the molecular axes of the chemisorbed species onto the surface normal is initially less than found with the fully formed films, indicating that in the initial stages of deposition they adopt configurations more prone to the surface; the evolution to the well-packed near-vertical molecular conformation that is usually associated with monolayer structures is observed even for sub-monolayer films that have been equilibrated in solution for several days. We conclude that film formation under low-concentration conditions proceeds by island growth and that the evolution of these islands can be monitored by the spectral evolution of the infrared bands. We have also established that the infrared absorption of the symmetric methyl stretch (r+, 2876 cm-1) is linearly related to the overall surface coverage, while the absorption frequency is independent of coverage, thus supporting an island growth model in which the local molecular environment is unaffected by the macroscopic coverages. Our results indicate that thiolate desorption during film formation is not a significant process using methanol solvents and that the quality of the Au/mica substrate is crucial in determining the structural evolution of the growing film.