Imaging of liquid crystals using a tunnelling microscope

Abstract

Since its conception¹, the scanning tunnelling microscope (STM) has been used principally to explore surfaces of well ordered inorganic solids and graphite^2,3. We now report the first use of the STM to unambiguously image well defined organic adsorbates on surfaces. We have imaged individual organic molecules in a liquid crystal array on a graphite surface with the STM with nearly atomic resolution. Liquid crystals have previously been analysed by X-ray diffraction, neutron scattering, dilatometry and other means to determine molecular order. Here we identify a two-dimensional order in 4-n-octyl-4′-cyanobiphenyl, a smectic in the bulk, confirming and embellishing literature models. We have also imaged 4-(trans-4n-pentylcyclohexyl)benzonitrile, a nematic liquid crystal in the bulk at room temperature. We observe an increase in order in both liquid crystals with respect to their bulk phases. The bulk smectic appears crystalline on the graphite surface, as the smectic planes are perfectly registered with one another. The structure of the bulk nematic at the interface with the surface shows a strong conformity to the graphite substrate. In both cases the molecular axes lie parallel to the substrate, and the STM images a cross-section along the direction of the molecular axes of the classical bulk phases (Fig. 1).