title: 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.
authors.affiliation: Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio, 44242.
authors.forename: Min
authors.initials: M
authors.lastname: Gao
authors.forename: Young-Ki
authors.initials: YK
authors.lastname: Kim
authors.forename: Cuiyu
authors.initials: C
authors.lastname: Zhang
authors.forename: Volodymyr
authors.initials: V
authors.lastname: Borshch
authors.forename: Shuang
authors.initials: S
authors.lastname: Zhou
authors.forename: Heung-Shik
authors.initials: HS
authors.lastname: Park
authors.forename: Antal
authors.initials: A
authors.lastname: Jákli
authors.forename: Oleg D
authors.initials: OD
authors.lastname: Lavrentovich
authors.forename: Maria-Gabriela
authors.initials: MG
authors.lastname: Tamba
authors.forename: Alexandra
authors.initials: A
authors.lastname: Kohlmeier
authors.forename: Georg H
authors.initials: GH
authors.lastname: Mehl
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Weissflog
authors.forename: Daniel
authors.initials: D
authors.lastname: Studer
authors.forename: Benoît
authors.initials: B
authors.lastname: Zuber
authors.forename: Helmut
authors.initials: H
authors.lastname: Gnägi
authors.forename: Fang
authors.initials: F
authors.lastname: Lin
keywords: CEMOVIS; freeze fracture; high-pressure freezing; lyotropic; thermotropic