Computer modelling of the structure of 4-n-octyl-4′-cyanobiphenyl adsorbed on graphite

Abstract

We have developed a united atom model for the mesogenic molecule 8-CB and have performed energy minimizations of single and multiple molecule systems in the presence of a planar substrate. The largest isosteric energy of adsorption obtained with this model for an independent molecule on a smooth substrate is -183·2 kJ mol^-1. For this system, we have found eight stable configurations for pairs of roughly antiparallel molecules. A fifty-molecule strip based on two of these stable pairs is found to develop small, periodic fractures when minimized, consistent with scanning tunnelling microscopy studies of these systems. When a graphite substate potential is used, the largest single molecule energy of adsorption differs by only ⋍ 1% from the smooth substrate value. Although this suggests a weak coupling between the registry of the adsorbed monolayer of 8-CB and the underlying graphite, the adsorbed strip does appear to be commensurate with the surface. The strip normal is aligned with a lattice vector of the graphite. The molecular separations in this strip are almost identical to those obtained using a flat substrate.