EVIDENCE OF PRONOUNCED POSITIONAL ORDER AT THE GRAPHITE LIQUID-CRYSTAL INTERFACE OF A BULK NEMATIC MATERIAL

We present scanning tunneling microscopy images of a bulk nematic liquid crystal, viz., 4'-n-hexyl-4-cyanobiphenyl, on the surface of graphite. These images clearly show the existence of pronounced positional order at the solid-liquid crystal interface for a material with no positional order in the bulk. A feature of the molecular arrangement is the pronounced interdigitation of the aromatic cores. The unit cell consists of two opposing molecules, the aromatic core of one of them lying flat on the graphite surface, while the core of the adjacent molecule lies on its side. The packing arrangement also features large open spaces of uncovered substrate between the alkyl tails. The molecular arrangement is compared with that of the smectic alkylcyanobiphenyls.