Electric Field Directed Colloidal Self-Assembly, Crystallization and Annealing with Biomimetic Structural Color

Abstract

Self-assembly is an effective method for fabricating ordered microstructures with diverse functional properties, such as colloidal crystals. However, practical self-assembly techniques often incorporate defects into the structures produced that can compromise the functional performance. In this dissertation, we investigated defect structures created by the electric-field-induced self-assembly of colloidal spheres and the impact of these defects on the optical properties of the colloidal crystals produced. We characterized and modeled the microstructure and defects of colloidal crystals as well as their structural color properties. Additionally, we explored annealing strategies to reduce defects and enhance crystal quality.

Our first study explores the mechanism of coloration in colloidal crystals. Differing from the more widely investigated structural color, colloids with ~1 µm in size show prismatic coloration under conditions of off-axis transmission. We self-assemble colloidal polystyrene spheres arrays in a dispersed phase by application of direct current electric fields. The measured spectral peak wavelength agrees with the diffraction equation, with a diffraction efficiency is about 2.5-4%. In addition, we find less crystal layer thickness improves the color saturation. For 1 µm sized particle, the optimized color intensity is found at number of layer equal to five. In addition, we constructed a scattering model using Mie theory. This modeling specifies the building block size and layer number effects and allows full prediction of coloration spectrum with expanded design space.

We next engineer the level of defects incorporated into the self-assembled colloidal crystals and explore the effect of their abundance on the grating diffraction response. We report that the state behavior, grain morphology, and hexatic ordering are well controlled by the applied current density and salt concentration of the solution. Moreover, we find that the structural color intensity decreases defect abundance following two regimes. Compared to the defect concentrated regime, the color intensity increases about ten times faster than in the defect dilute regime, with turning point of the hexatic order parameter at ~0.87. Moreover, we demonstrate the structures simulated by molecular dynamics are consistent. The optical simulation reveals a unique trade-off between structural color intensity and the azimuthal uniformity.

In the following chapter, we look for effective ways to anneal defects in colloidal crystals. We design and fabricate a coplanar six-fold microelectrode device and operate it with a cyclically rotating electric field to dynamically resolve defects in a colloidal crystal. For various conditions, we characterize the evolution of the microstructure by the hexatic order parameter, number of grains, and Voronoi diagram. We report that the optimal condition is at AC field of 5VRMS, 5kHz and a cycling period of 15s, which removes >99% of defects and yields a hexatic order parameter of >0.98. Furthermore, we hypothesize and test a potential mechanism for the annealing. That is, the injected energy due to the AC field activates the defect rearrangement that progressively generates global annealing. This approach generates a master curve that collapses the annealing performance as a function of field amplitude, frequency, and cycle time. An active energy of ~65 kT per particle yielded successful annealing with the highest crystal quality.