Fabrication of three-dimensional photonic crystals by colloidal templating

Abstract

Self-assembled colloidal crystallization routes to dielectric structures with face-centered cubic symmetry and submicrometer periodicity have been extensively studied in the past decade because of their potential applications as photonic crystals. However, inverse replicas cast from self-assembled polymer, silica, and emulsion spheres are generally polycrystalline and their structural coherence does not persist for more than a few hundred micrometers. Effort is therefore being made to find routes to structures which have well-defined macroscopic (larger than micrometer scale) morphologies as well as submicrometer periodicity. To this end, several approaches have been studied in this thesis. Basic concept is to use suspension droplets as confined geometry (i.e., liquid droplet containing a number of colloidal particles inside). When the suspension droplet is dried, the crystallization of submicrometer-sized colloidal particles occurs inside the droplet leading to a highly ordered colloidal assembly. Initial works in chapter and 2 have involved evaporation of droplet straddling at oil and air interface. At millimeter scale, controlled macroscopic spheroidal shape of suspension droplets was achieved by electrohydrodynamic force balance under the action of alternative current (AC) electric field. Furthermore, our colloidal crystals with a variety of shapes have replicated to ordered macroporous materials of entailed shapes by templating process. In addition, disk-like inversed opal structures have been demonstrated by templating colloidal crystals of which shapes were modified by adding surfactant. Small spherical colloidal crystals at micrometer scale, which are called supraballs, were generated uniformly by using microfluidics-based techniques in chapter 4 and 5. At a junction of flows of oil and water that contained submicrometer-sized particles, pico-liter suspension droplets were generated continuously by simple shear rupturing or break-off process and then co...