COMPUTER MODELING OF THE STRUCTURE OF 4-N-OCTYL-4'-CYANOBIPHENYL ADSORBED ON GRAPHITE

We have developed a united atom model for the mesogenic molecule 8-CB and have performed energy minimizations of single and multiple molecul e systems in the presence of a planar substrate. The largest isosteric energy of adsorption obtained with this model for an independent mole cule on a smooth substrate is -183.2 kJ mol-1. For this system, we hav e found eight stable configurations for pairs of roughly antiparallel molecules. A fifty-molecule strip based on two of these stable pairs i s found to develop small, periodic fractures when minimized, consisten t with scanning tunnelling microscopy studies of these systems. When a graphite substate potential is used, the largest single molecule ener gy of adsorption differs by only approximately 1% from the smooth subs trate value. Although this suggests a weak coupling between the regist ry of the adsorbed monolayer of 8-CB and the underlying graphite, the adsorbed strip does appear to be commensurate with the surface. The st rip normal is aligned with a lattice vector of the graphite. The molec ular separations in this strip are almost identical to those obtained using a flat substrate.