Adsorption of short-chain alkanethiols on Ag(1 1 1) studied by direct recoiling spectroscopy

Abstract

We use direct recoiling spectroscopy with time-of-flight analysis to study the adsorption of propanethiol on Ag(1 1 1) cleaned and polished in vacuum by cycles of grazing ion bombardment and annealing. We discuss the advantages and drawbacks of the technique to follow the growth of the organic film. In particular, the low damage imparted by the technique allows to follow in detail the evolution of the H, C and substrate recoiling peaks for a wide range of exposures ranging from 10 −1 to 2 × 10 4 L. The shape of the TOF spectra and the evolution of the recoiling intensities are consistent with a growth process in three stages: an initial fast one related to the density of defects at the surface, a second one where the surface is covered with a thin layer of organic molecules, presumably associated with lying-down molecules, and a final stage corresponding to a thicker layer that can be associated with a standing-up orientation of the molecules in the film. Annealing of the organic film to 250 °C produces complete depletion of C and H, leaving a small amount of S. The final S coverage after annealing depends on the initial roughness, being higher for rougher surfaces. We also observe an increase in the surface roughness after desorption of the thiol layer. Re-adsorption on this post annealed surface presents a marked enhancement of the initial sticking.