SUPERLATTICE STRUCTURE AT THE SURFACE OF A MONOLAYER OF OCTADECANETHIOL SELF-ASSEMBLED ON AU(111)

We report direct evidence of a unit mesh containing more than one hydrocarbon chain at the surface of a self-assembled monolayer of long-chain n-alkanethiols. Our helium diffraction measurements for a monolayer of n-octadecanethiol on Au(111) are consistent with a rectangular primitive unit mesh of dimensions 8.68 X 10.02 angstrom containing four crystallographically distinct hydrocarbon chains. This packing arrangement can also be described as a c(4 X 2) superlattice with respect to the fundamental simple hexagonal [square-root 3 X square-root 3)R30-degrees] array of lattice parameter 5.01 angstrom previously observed for monolayers of other n-alkanethiols on gold. No temperature-dependent phase behavior is observed in the temperature range where surface diffraction is measurable (30-100 K) and cycling up to temperatures as high as 50-degrees-C caused no observable change in the diffraction. It is proposed that this larger unit mesh is the result of a patterned arrangement of rotations of the hydrocarbon chains about their molecular axes. This patterned arrangement must be different than the herringbone structure expected by simple analogy to bulk n-alkanes.