TOF–SIMS study of alkanethiol adsorption and ordering on gold

A systematic study of alkanethiol self-assembly on gold was undertaken with the time-of-flight–secondary ion mass spectrometry (TOF–SIMS) technique. Following adsorption, the TOF–SIMS spectra exhibit sulfur-containing peaks (specially AuxSy clusters) and a rise of all hydrocarbon peaks (CxHy). But the most specific peaks are those directly related to the adsorbed molecules (M) such as deprotonated monomers (MH)−, thiolate (MAu−), dimers (M2Au−), trimers (M3Au2−) and even tetramers (M4Au3−). By examining the ion yield of the most relevant fragments as a function of adsorption time, it is possible to follow the self-assembly kinetics for all the alkanethiols chosen. The hydrocarbon and AuxSy peak intensities quickly rise and reach a saturation level after typically an adsorption period of a few minutes. However, the oligomer fragments exhibit a much slower rise and reach saturation only after a few hours. This observation corroborates the widely accepted theory of alkanethiol adsorption on gold, starting with a fast adsorption of a disordered layer followed by a slower phase of monolayer self-assembly leading to a high packing of the alkane chains. The oligomer ions clearly reflect, to some extent, the degree of organization of the monolayer, whereas hydrocarbon and AuxSy clusters just indicate the chemical changes at the surface.