Polarization-Modulated Bent-Core Liquid Crystal Thin Films without Layer Undulation

Spatial confinement is known to affect molecular organizations of soft matter. We present an important manifestation of this statement for thin films of bent-core smectic liquid crystals. Prior freeze-fracture transmission electron microscopy (FFTEM) studies carried out on nitro-substituted bent-core mesogens (n-OPIMB-NO_{2}) revealed an undulated smectic layer structure with an undulation periodicity of ∼8  nm. We report cryogenic TEM measurements on ∼100  nm thick 8-OPIMB-NO_{2} films. In contrast to FFTEM results, our studies show only density modulation with periodicity b=16.2  nm, and no smectic layer undulation. We show that the discrepancy between the FFTEM and cryogenic transmission electron microscopy (cryo-TEM) results can be attributed to the different sample thicknesses used in the experiments. FFTEM monitors cracked surfaces of a relatively thick (5-10  μm) frozen sample, whereas cryo-TEM visualizes the volume of a thin (0.1  μm) film that was quenched from its partially fluid phase. These results have importance in possible photovoltaics and organic electronics applications where submicron thin films are used.