Direct Observation of Smectic Layers in Thermotropic Liquid Crystals

Abstract

We demonstrate subnanometer resolution cryo-TEM imaging of smectic layers in the smectic and nematic phases of two bent-core liquid crystals. Our results show perfect periodicity over several hundred layers in the smectic phase and also provide the first direct evidence of smectic clusters on length scales of 30-50 nm in a nematic liquid crystal. The results are corroborated with small angle x-ray scattering measurements. The observation of smectic clusters in the nematic phase is of special interest in bent-core liquid crystals, where the smectic clusters are stable over wide temperature ranges, in contrast to the well-known pretransitional "cybotactic" clusters that appear only in the vicinity of a bulk smectic phase. The means to characterize and manipulate this nanoscale molecular order could open up completely new liquid crystal-based technologies.

Helical nanofilaments of bent-core liquid crystals with a second twist

“…This indicates that the fringes mark the regions in which the smectic layers are close to the upright orientation with respect to the substrate. The contrast of fringes is provided by the electron density difference between the aromatic cores and hydrocarbon tails 18 . The parameters h and d for different homologues do not vary much: n ¼ 7: h ¼ 118 ± 3 nm and d ¼ 4.48 nm; n ¼ 8: h ¼ 130 ± 3 nm and d ¼ 4.60 nm; n ¼ 9: h ¼ 122 ± 3 nm and d ¼ 4.87 nm; n ¼ 12: h ¼ 107 ± 3 nm and d ¼ 4.64 nm.…”

“…Selected areas were then imaged at a dose level of 200 e À nm À 2 , which we found did not cause any radiation damage. Previous studies on B100 nm thermotropic bent-core liquid crystals films carried out with this instrument demonstrated the possibility to visualize smectic layers with a resolution better than 0.7 nm (Zhang et al 18 ) In contrast to FFTEM, which visualizes the topography of a replica in form of bright and shadowed areas with 2-3 nm resolution, cryo-TEM reveals a periodicity of layers when they are nearly parallel to the electron beam.…”

“…While FFTEM monitors the profile of a broken surface of a bulk (over micrometre thick) material, cryo-transmission electron microscopy (TEM) gives information about the inside of a thin (LB100 nm film) material; therefore, it can provide complementary information of the structures. Indeed, it was shown recently that cryo-TEM is suitable to visualize smectic layers of thermotropic liquid crystals 18 .…”

Direct mapping of local director field of nematic liquid crystals at the nanoscale

“…Although we have become adept at deducing textures indirectly through optical microscopies (1)(2)(3)(4)(5), probing the molecular-scale organization requires either a glassy material or rapid cooling of samples, providing metastable states that can be directly visualized through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) (6)(7)(8). Although these techniques are effective to study a variety of more complex LC phases, including smectic LCs (6,7,9), cholesteric and blue phases (10), and biological LC polymers (11,12), nonglassy, low molecular weight nematic LCs (NLCs) reorient during fast freezing. Polymer nematics can be quenched into metastable states but organization of static configurations through surface alignment is difficult.…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…4. This is based on the four-spot reflections off the meridian in small angle X-ray scattering studies [12][13][14][15][16][17][18][19][20][21]. It should be noted that the first interpretation of the four-spot reflections in small angle X-ray diffraction of bent-core molecules was ascribed to the form factor of a bent-core molecule [22].…”