Structure of c(4×2) superlattice in alkanethiolate self-assembled monolayers

Molecular dynamic simulations have been used to elucidate the c(4×2) superlattice structure of CH₃-(CH₂)_n-1S- chains self-assembled on Au{111}. This superlattice of a (√3×√3)R30° overlayer has been observed in several experimental studies. Using experimental clues as guidelines, a systematic study of the phase-space of the orientations of the chain backbones and their directions of tilt has been performed. Three possible structures for monolayers formed from long chains (n ≥ 12) and one for short ones (n = 4,6) all of which are consistent with the symmetry suggested by X-ray diffraction experiments are identified. Two of the four structures proposed show the same surface topology as predicted by scanning tunneling microscopy. The simulations also show that a complex interplay exists among the azimuthal tilt direction, the polar tilt angle, and the chain backbone orientation.