Direct observation of liquid crystals using cryo‐TEM: Specimen preparation and low‐dose imaging

Abstract

Liquid crystals (LCs) represent a challenging group of materials for direct transmission electron microscopy (TEM) studies due to the complications in specimen preparation and the severe radiation damage. In this paper, we summarize a series of specimen preparation methods, including thin film and cryo‐sectioning approaches, as a comprehensive toolset enabling high‐resolution direct cryo‐TEM observation of a broad range of LCs. We also present comparative analysis using cryo‐TEM and replica freeze‐fracture TEM on both thermotropic and lyotropic LCs. In addition to the revisits of previous practices, some new concepts are introduced, e.g., suspended thermotropic LC thin films, combined high‐pressure freezing and cryo‐sectioning of lyotropic LCs, and the complementary applications of direct TEM and indirect replica TEM techniques. The significance of subnanometer resolution cryo‐TEM observation is demonstrated in a few important issues in LC studies, including providing direct evidences for the existence of nanoscale smectic domains in nematic bent‐core thermotropic LCs, comprehensive understanding of the twist‐bend nematic phase, and probing the packing of columnar aggregates in lyotropic chromonic LCs. Direct TEM observation opens ways to a variety of TEM techniques, suggesting that TEM (replica, cryo, and in situ techniques), in general, may be a promising part of the solution to the lack of effective structural probe at the molecular scale in LC studies. Microsc. Res. Tech. 77:754–772, 2014. © 2014 Wiley Periodicals, Inc.

On the structure of the Nx phase of symmetric dimers: inferences from NMR

“…Notably, chiral domains in the nematic phase of achiral compounds exhibiting dimerisation through hydrogen bonding were reported more than 15 years ago 19 ; however, such domains were found to exhibit macroscopic twist deformation of the director in the optical regime. Lastly, although the twist-bend model has been proposed 8,9,11,12,16,20 for the consistent interpretation of various experimental observations on the dimer N X phase, there is, to our knowledge, no direct identification of the heliconical structure and, furthermore, there are indications 21,22 that the dominant molecular arrangement in the N X phase could differ from that of the twist-bend model.…”

Nanoscale structures of polarisation-modulated bent-core materials in thin films

“…Previous studies on other thermotropic bent-core LC films carried out with this instrument demonstrated the possibility to visualise smectic layers with resolution better than 0.7 nm. [22,[24][25][26] The LC samples were dissolved in chloroform, and the solution was dispersed on a TEM grid coated with plasma-treated continuous carbon film. After evaporation of the chloroform, the thickness of the LC films was between 50 and 100 nm.…”

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

“…The manipulation of these nanometer-size molecules into coherent, centimeter-scale structures is now routine. 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.…”