title: Self-organized chiral colloidal crystals of Brownian square crosses.
abstract: We study aqueous Brownian dispersions of microscale, hard, monodisperse platelets, shaped as achiral square crosses, in two dimensions (2D). When slowly concentrated while experiencing thermal excitations, the crosses self-organize into fluctuating 2D colloidal crystals. As the particle area fraction φA is raised, an achiral rhombic crystal phase forms at φA ≈ 0.52. Above φA ≈ 0.56, the rhombic crystal gives way to a square crystal phase that exhibits long-range chiral symmetry breaking (CSB) via a crystal-crystal phase transition; the observed chirality in a particular square crystallite has either a positive or a negative enantiomeric sense. By contrast to triangles and rhombs, which exhibit weak CSB as a result of total entropy maximization, square crosses display robust long-range CSB that is primarily dictated by how they tile space at high densities. We measure the thermal distribution of orientation angles γ of the crosses' arms relative to the diagonal bisector of the local square crystal lattice as a function of φA, and the average measured γ (φA) agrees with a re-scaled model involving efficient packing of rotated cross shapes. Our findings imply that a variety of hard achiral shapes can be designed to form equilibrium chiral phases by considering their tiling at high densities.
authors.affiliation: Department of Physics and Astronomy, University of California-Los Angeles, Los Angeles, CA 90095, USA. Department of Chemistry and Biochemistry, University of California-Los Angeles, Los Angeles, CA 90095, USA.
authors.forename: Kun
authors.initials: K
authors.lastname: Zhao
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Mason
mesh_terms: D003102:Colloids / Q000737:chemistry*; D003460:Crystallization; D019277:Entropy; D008956:Models, Chemical; D008968:Molecular Conformation; D013816:Thermodynamics
chemical_list: D003102:Colloids