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.