Effect of Defective Microstructure and Film Thickness on the Reflective Structural Color of Self-Assembled Colloidal Crystals.

Abstract

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. While the relationship between the crystal structure and structural color reflection peak wavelength is well studied, the connection between assembly quality and the degree of reflective structural color is less understood. Here, we study this connection by investigating the structural color reflection peak intensity and width as a function of defect density and film thickness 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. Furthermore, we find that colloidal crystal films with small quantities of defects may be approximated as multilayer reflective materials. These findings can guide the design of optical materials with variable structural color intensity.