Two-dimensional crystallization of hexagonal bilayer with Moiré patterns

Direct observation of crystallization dynamics in real space is of special interest to scientists in various disciplines. Although direct observation of transient structural transformation in a nanocrystalline system has been recently achieved using the state-of-the-art aberration-corrected transmission electron microscopy (AC-TEM), the small length scales of individual species in molecular systems still preclude routine observation of crystallization dynamics. Unidirectional packing of microbeads can serve as an experimental model system, as their dynamics can be observed and recorded readily in the laboratory due to their larger size and slower time scale. Herein, we present direct observation of a two-dimensional (2D) crystallization enabled by such a packing process. The direct imaging approach not only allows observation of the dynamics in a bead-by-bead fashion but also reveals intriguing phenomena, such as the formation of grain boundaries, disorder-order transitions, and the Moiré patterns which arise when two periodic monolayers are overlaid at certain angles. In addition, the imaging afforded by confocal microscopy facilitates a structural analysis of height-dependent polygonal tiling of the top monolayer, which has implication to the formation of 2D quasicrystals.