Room-Temperature Kinetics of Short-Chain Alkanethiol Film Growth on Ag(111) from the Vapor Phase

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Abstract

We have used time-of-flight (TOF) direct recoiling spectroscopy (DRS) to follow propanethiol adsorption at 300 K from the vapor phase on an Ag(111) surface, for exposures ranging from 10(-1) to 10(5) L. Results show that the adsorption proceeds with changes in the sticking coefficient, consistent with at least three phases. At low exposures, the alkanethiol molecules adsorb with high probability at defect sites, followed by a slower growth mode that essentially covers the whole surface. A third change in the sticking coefficient is associated with the final saturation stage, corresponding to a thicker layer related to molecules in a more upright orientation. The adsorption kinetics for hexanethiol is similar to that of propanethiol but taking place at higher rates, stressing the importance of the hydrocarbon chain length in the growth process. ISS-TOF measurements during thermal desorption show that most of the C, H, and S go away together, suggesting that the molecules adsorb and desorb from flat regions without S-C bond cleavage. Fitting the desorption maximum at 450 K with a first-order desorption curve gives a desorption energy of 1.43 eV. A small final S content that is correlated with the initial Ag(111) surface roughness is observed after desorption.

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