UNEXPECTED SQUARE SYMMETRY SEEN BY ATOMIC-FORCE MICROSCOPY IN BILAYERFILMS OF DISK-LIKE MOLECULES

Thin films of disk-shaped molecules are expected to display anisotropi c optical and transport properties, leading to applications in optical display or sensor technologies. Bilayer Langmuir-Blodgett films of mo nomeric triphenylene mesogens have been studied by atomic force micros copy. The triphenylene cores of the constituent molecules tend to prom ote the formation of columnar structures in the plane of the substrate and along the direction of deposition of the film. Atomic force micro scopy images of bilayer Langmuir-Blodgett films revealed two types of structure, one corresponding to an aligned columnar structure and the other to an unusual square lattice, which may result from the superpos ition of columnar structures in adjacent layers that intersect at near right angles. Annealing such bilayers near the melting point of the b ulk compound improved the structural ordering by reducing the angular spread of orientations associated with the well-developed columnar str ucture in some areas and by producing a more distinct square lattice i n other areas of the sample.