Inorganic Nanosheet Liquid Crystals: Self-Assembled Structures in Dispersions of Two-Dimensional Inorganic Polymers

Inorganic nanosheets obtained by exfoliation of inorganic layered materials are sheet-like nanocrystals with ultra high aspect ratio and are also classified as two-dimensional inorganic polymers. Under certain conditions, the nanosheets, dispersed in solvents, orient themselves and a liquid crystal phase is formed, which we name "inorganic nanosheet liquid crystal". Because only a few examples of liquid crystal systems of highly anistropic two-dimensional objects have been reported, nanosheet liquid crystals can serve as important model systems of complex liquids. On the other hand, nanosheet liquid crystals possess functionalities inherent in inorganic materials such as semiconducting and photocatalytic ones, therefore we expect a variety of applications that are not possible with conventional liquid crystals of organic molecules. In this manuscript, fundamental aspects and applications of inorganic nanosheet liquid crystals are reviewed. We first introduce basic researches that elucidate the phase transition behavior and structure of nanosheet liquid crystals. We then move to application studies such as macroscopic orientational control by external fields and fabrication of anisotropic nanosheet liquid crystal/polymer gel composites, followed by explanations on theoretical aspects.