Structure of Langmuir-Blodgett Films of Disk-Shaped Molecules Determined by Atomic Force Microscopy

Monolayer Langmuir-Blodgett films of a discotic mesogen have been studied with atomic force microscopy (AFM). These measurements confirm the "edge on" arrangement for the disk-shaped molecules suggested by surface pressure-area isotherms and show that the molecules form columns that are separated by 17.7 angstroms ± 10 percent. Column alignment is found to be predominantly along the film deposition direction, with an angular spread of 35°. The AFM images also show that the mean disk separation within the columns is 5.1 ± 1.3 angstroms, in good agreement with x-ray diffraction (XRD) results. Room-temperature XRD measurements on bulk samples of the same material indicate a disordered-hexagonal liquid crystalline mesophase, with a column-to-column spacing of 19.9 ± 0.2 angstroms.