(183j) Effect of Defective Microstructure on the Reflective Structural Color of Self-Assembled Colloidal Crystals

We report the relationship between measures of the crystal quality of self-assembled colloidal crystal films and the intensity of their structural color. Structural color arises from geometric diffraction; it has potential applications in optical materials because it is more resistant to environmental degradation than coloration mechanisms that are of chemical origin. Structural color can be produced from self-assembled films of colloidal size particles. To study the connection between crystal film quality and structural color intensity, we investigate the peak intensity and width of structural color reflection as a function of defect density, film thickness and impurity concentration using a combined experimental and computational approach. Polystyrene microspheres are self-assembled into defective colloidal crystals via solvent evaporation. Colloidal crystal growth via sedimentation is simulated with molecular dynamics, and the reflection spectra of simulated structures are calculated by using the finite-difference time-domain algorithm. We examine the impact of commonly observed defect types (vacancies, stacking fault tetrahedra, planar faults, and microcracks) on structural color peak intensity. We find that the reduction in peak intensity scales with increased defect density. The reduction is less sensitive to the type of defect than to its volume. In addition, the reflectance of structural color increases as a function of the crystal thickness, until a plateau is reached at thicknesses greater than about 9.0 µm. The maximum reflection is 78.8 ± 0.9%; this value is significantly less than the 100% reflectivity predicted for a fully crystalline, defect-free material. Finally, we manipulate the defect structures of colloidal crystals by introducing differently sized particles that function as impurities. As the concentration of the impurity is increased, the measured intensity of structural color reflection decreases by amounts that we correlate with the observed microstructure. These findings can guide the design of optical materials with variable structural color intensity.