We study and report the relationship between self-assembled colloidal crystal quality and the brilliance of structural color displayed by these crystals. Structural color has potential applications in optical materials because this color arises due to light diffraction from periodic arrays of particles; it is less prone to environmental degradation than other ways to produce color. To establish the relationship between colloidal crystal quality and the brilliance of structural color, we studied two variables using a combined experimental and computational approach. First, we quantified the relationship between the thickness of self-assembled colloidal crystals and the brilliance of structural color as measured by reflection. Second, we incorporated dopants into the self-assembly process to probe how impurities and the defect microstructures they produce affected crystal quality and structural color brilliance. In the experiments, we assembled polystyrene microspheres into colloidal crystals with different thickness via solvent evaporation. The thickness profiles were characterized by cross-sectional scanning electron microscopy and profilometry. The spectral response of colloidal crystals measured with a white light spectrophotometer shows how the brilliance of structural color increases as a function of the crystal thickness. Additionally, we manipulated the defect structures of colloidal crystals by introducing differently sized particles as impurities. As the concentration of the impurity spheres increased, the brilliance of structural color reflection decreased. We found that impurity size determined the sensitivity of structural color brilliance to impurity concentration. We simulated colloidal crystal growth via sedimentation at various impurity concentrations using HOOMD-Blue. The reflection spectrum of defect structures was calculated using the Finite Difference Time Domain (FDTD) algorithm as implemented in Lumerical™. The simulation results were used to validate and explain the experimental results. These findings can guide the design of optical materials to tune structural color intensity.

Effect of crystal quality on the brilliance of structural color from self-assembled colloidal crystals

2019 Spring National Meeting

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