How Does a Wall Affect Crystal Formation?

Study offers new viewpoint to wall-induced single-crystal formation

crystal formation
Process of crystal transformation The elementary process of the crystal-to-crystal transformation from BCC to HCP via sliding of a crystal plane. © 2017 Shunto Arai and Hajime Tanaka.

Researchers at the University of Tokyo have revealed through experiments and numerical simulations the factors necessary for single-crystal (single-domain crystal) formation on a plane (wall) such as a substrate. The current outcome holds promise of providing useful guidelines for wall-induced single-crystal formation by demonstrating the role of matching between the substrate and liquid structure in forming a single crystal.

Formation of single crystals is key not only to the formation of colloidal photonic crystals (a periodic optical nanostructure scattering light like a semiconductor scattering electrons) but also for the growth of crystals and expression of functions in proteins and other molecules, and thus important in many applications. However, an understanding of the mechanism for crystal growth from a microscopic perspective, such as the role of the relation between liquid structures and crystal structures, was lacking.

Under these circumstances, the research group led by Professor Hajime Tanaka at the Institute of Industrial Science, the University of Tokyo, and then-graduate student Shunto Arai (currently assistant professor at the Graduate School of Engineering) successfully followed the crystallization process from a wall particle by particle in real time by using a charged colloidal system as one model for their study. The group also revealed through experiments and numerical simulations of single crystals formed from flat walls that the compatibility between the orientational order (order in the configuration of neighboring particles around a central one) in liquid and the structure of crystals formed on the walls, and matching between liquid structures favored by a wall and the orientational order formed in liquid play important roles. Furthermore, the researchers found conditions that lead to growth of crystals in more than one form, as opposed to those conducive to single-crystal formation.

“Thus far, it was unclear how local structural ordering of a liquid affects crystal formation. Our study shows that matching of local structuring of a liquid with liquid density modulation induced by a wall is crucial for formation of crystals with few defects.” says Tanaka. He continues, “We are hoping that our finding will contribute to formation of colloidal photonic crystals and growth of high-quality protein crystals.”