Orientation and order in microcontact-printed, self-assembled monolayers of alkanethiols on gold investigated with near edge x-ray absorption fine structure spectroscopy

Near edge x-ray absorption fine structure spectroscopy (NEXAFS) was used to investigate contact-printed (CP), self-assembled monolayers (SAMs) of alkanethiols on gold. We studied the influence on order and orientation of the alkyl chains of both the chain length of the thiols and the concentration of the inking solution that was used for stamping. The results are compared to those for monolayers prepared by the conventional technique of immersing the substrate into solution. Our study shows that the CP process, when performed with a sufficiently concentrated solution of the thiol, can result in monolayers indistinguishable from those established by immersion into solution, independent of the thiol chain length. For lower concentrations, however, CP monolayers show a significant deviation from well-ordered films and have to be classified as mainly disordered. As with SAMs prepared from solution, shorter chains (dodecanethiol) lead to a lower degree of order compared to longer ones (hexadecanethiol), which can be attributed to their relatively higher percentage of gauche conformations at room temperature. Nevertheless, with lateral force microscopy a slight contrast is still observable between immersed and microcontact-printed (μCP) areas of well-ordered films, as has been previously reported by other groups and attributed to slight differences in the packing density of the chains in the differently prepared regions of the final layer. On the basis of our NEXAFS data the difference of gauche defects due to different average packing densities in the resulting layers can be estimated to be less than 30%.