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 us ed to investigate contact-printed (CP), self-assembled monolayers (SAM s) of alkanethiols on gold. We studied the influence on order and orie ntation 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 conve ntional technique of immersing the substrate into solution. Our study shows that the CP process, when performed with a sufficiently concentr ated 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 cl assified as mainly disordered. As with SAMs prepared from solution, sh orter chains (dodecanethiol) lead to a lower degree of order compared to longer ones (hexadecanethiol), which can be attributed to their rel atively higher percentage of gauche conformations at room temperature. Nevertheless, with lateral force microscopy a slight contrast is stil l observable between immersed and microcontact-printed (mu CP) areas o f well-ordered films, as has been previously reported by other groups and attributed to slight differences in the packing density of the cha ins in the differently prepared regions of the final layer. On the bas is of our NEXAFS data the difference of gauche defects due to differen t average packing densities in the resulting layers can be estimated t o be less than 30%. (C) 1997 American Vacuum Society.