FORMATION OF SELF-ASSEMBLED BUTANETHIOL MONOLAYERS ON AU SUBSTRATES -SPECTROSCOPIC EVIDENCE FOR HIGHLY ORDERED ISLAND FORMATION IN SUBMONOLAYER FILMS

The initial stages of the formation of butanethiol self-assembled mono layers on well-prepared Au/mica substrates have been studied using inf rared spectroscopic analysis of the CH2 and CH3 stretching vibrations and real-space STM methods. The projection of the molecular axes of th e chemisorbed species onto the surface normal is initially less than f ound with the fully formed films, indicating that in the initial stage s of deposition they adopt configurations more prone to the surface; t he evolution to the well-packed near-vertical molecular conformation t hat is usually associated with monolayer structures is observed even f or sub-monolayer films that have been equilibrated in solution for sev eral days. We conclude that film formation under low-concentration con ditions proceeds by island growth and that the evolution of these isla nds 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 co verage, thus supporting an island growth model in which the local mole cular environment is unaffected by the macroscopic coverages. Our resu lts indicate that thiolate desorption during film formation is not a s ignificant process using methanol solvents and that the quality of the Au/mica substrate is crucial in determining the structural evolution of the growing film.