Structure of c(4x2) superlattice in alkanethiolate self-assembled monolayers

Molecular dynamic simulations have been used to elucidate the c(4 x 2) superlattice structure of CH3-(CH2)(n-1)S- chains self-assembled on Au{111}. This superlattice of a (root 3 x root 3)R30 degrees 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 greater than or equal to 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.