THE STRUCTURE OF N-OCTADECANE THIOL MONOLAYERS SELF-ASSEMBLED ON AU(001) STUDIED BY SYNCHROTRON X-RAY AND HELIUM ATOM DIFFRACTION

We report on a new, more precise structural characterization of n-octadecane thiol monolayer self-assembled on the UHV-prepared Au(001) surface using a combination of helium atom diffraction, grazing incidence x-ray diffraction, and x-ray reflectivity measurements. Our results show that the equilibrated alkane chains form a two dimensional distorted hexagonal structure with lattice constants a=5.77±0.06 Å, b=4.81±0.02 Å, and α=53.1±0.46°. The alkane chains are tilted 33.5±1.0° from the surface normal and the tilt direction is ∼6.8±1.0° away from the elongated next-nearest-neighbor direction. The Au-thiol interface is found to have an unusual p(1x4) structure formed by an excess of Au atoms, which are believed to be produced in the dereconstruction process of the clean hexagonal Au(001) surface during the chemisorption of thiols. We propose that this Au interfacial layer modulates the height of the alkane chains resulting in a highly corrugated top surface, which forms a c(2x8) structure as observed by atom diffraction. These structural findings reveal a novel interplay between the head group/substrate interaction and the interchain van der Waals interaction which determines not only the alkane chain packing but also the structure of the interfacial layer of the substrate. Our studies also demonstrate that rich, complementary structural information can be obtained about the topmost surface, the chain packing, the interface arrangement, and the substrate of this and similar systems by the combination of atomic beam and synchrotron x-ray characterization techniques. © 1995 American Institute of Physics.