From micron to nano: Crystallization of colloid particles that mimic molecular crystals

The crystallization of micrometer-sized particles with the shapes of tetrabenzoheptacene (TBH) and 1,2:5,6-dibenzanthracene (DBT), both at polyacenes, in an electric field results in the formation of ordered 2D packings that mimic the plane group symmetries in their respective molecular crystal equivalents. The TBH-shaped and DBT-shaped particles assemble into the p2 plane group, which corresponds to the densest particle packing among the possible close-packed plane groups for these particles. The p2 symmetry observed in the TBH-shaped and DBT-shaped colloid crystal emulates the p2 symmetry of the (010) layers in their respective molecular crystals, which crystallize in monoclinic lattices. DBT-shaped particles also form ordered domains with pgg symmetry, replicating the plane group symmetry of the (100) layer in the orthorhombic polymorph of DBT.;Colloidal particles equipped with two, three, or four negatively charged patches, which endow the particles with two-fold, three-fold or tetrahedral symmetries, form 1D chains, 2D layers, and 3D packings when polarized by an AC electric field. Two-patch particles pack into the cmm plane group and 3D packings with I4mm space group symmetry. Three-patch particles form chains having a 21 screw axis symmetry and some chain pairs form double-helix regions that result from mutual twisting of the chains about each other. Larger 2D domains of the three-patch particles crystallize in the p6m plane group with alignment and packing densities that suggest random disorder in the domains.;Micron-sized spherical colloidal particles were self-assembled within circular or square wells, which were built in different sizes from 2 mum to 10 mum with a depth of 1.5 to 2.0 mum through photolithography on a silicon wafer, to demonstrate the densest packing of colloidal particles via confinement. The structures and average particle/well size ratios illustrated that three/six-fold axis were preferred in circular wells and four-fold axis were preferred in square wells.;These observations illustrate that the packing of colloid particles can mimic the crystallization of molecules with similar shapes, charge distribution and environment, and the structure and symmetry can transcend length scales from the molecular to the colloidal.