Thermal treatment of n-alkanethiolate monolayers on gold, as observed by infrared spectroscopy

In the present work, we have used reflection-absorption infrared spectroscopy (RAIRS) to investigate the temperature behavior of n-alkanethiols adsorbed on gold substrates. A careful examination of the spectra (frequencies, integrated intensities, and link widths) reveals at least two different types of thermally induced disorder as the temperature is increased. These observations are discussed in light of other recent studies using complementary techniques. The overall picture that emerges is that an initial stage is characterized by a gradual untilting of the alkyl chain and the appearance of gauche conformational defects near the surface, and both of these effects are largely reversible. The subsequent nonreversible disordering of the monolayer is characterized by a large number of defects located below the surface. Infrared spectroscopy is particularly well suited for detecting these changes.