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

• Published in: Physical review letters Vol. 122; no. 13; p. 137801
• Main Authors: Zhang, Cuiyu, Gao, Min, Ribeiro de Almeida, R R, Weissflog, Wolfgang, Lavrentovich, Oleg D, Jákli, Antal
• Format: Journal Article
• Language: English
• Published: United States 05.04.2019
• ISSN: 1079-7114
• DOI: 10.1103/PhysRevLett.122.137801

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.