Effects of Microstructural Properties on Structural Color of Self-Assembled Colloidal Crystals

This dissertation examines the relationship between colloidal microstructural properties and structural color, to guide the design of optical materials. Colloidal microstructures can interact with light to produce structural color, which is not prone to environmental degradation. The high stability of structural color provides great potential for optical applications such as coatings and displays. Previous studies in structural color from colloidal systems have primarily focused on controlling structural color wavelength by changing the dielectric periodicity of the material. However, the connection between colloidal microstructural properties and structural color reflectivity – particularly its magnitude – remains unclear. In this dissertation, we systematically and quantitatively investigate the effects of crystal thickness, defect density and structure, irregularity of particle size, and particle shape on structural color by experiment and simulation. The relationships can be applied to designing novel materials with tailored structural color properties. First, we report how film thickness, defect density, and defect type in colloidal crystals quantitatively affect their structural color reflectivity. Colloidal crystals with different thicknesses are fabricated by self-assembling monosized polystyrene microspheres via solvent evaporation. We find that the structural color reflectivity increases as a function of the crystal thickness, until a plateau is reached at 78.8 ± 0.9%. We also model crystals via molecular dynamics and simulate their reflection spectra by the finite-difference time-domain method. The simulation results show that the reduction in reflectivity scales with increased defect density and that stacking fault tetrahedra are most efficient in disrupting structural color. These findings can guide the efficient design of structural color materials and support defect engineering in colloidal crystals. Second, we evaluate the role of irregular-sized spherical particles in determining crystal quality and structural color reflectivity. By evaporative self-assembly and molecular dynamics simulation, we control the volume fraction of irregular-sized particles – by choosing particles that are either larger or smaller than the base colloids comprising the self-assembled crystals. Then we quantify crystal quality from analysis of diffraction patterns obtained by Fast Fourier transform of the scanning electron microscope images. We find that small irregular particles are more detrimental to crystal quality and structural color reflectivity than large irregular particles. When incorporated with 10 vol% of irregular particles, the reflectivity of crystal films with large (small) irregular particles decreases by 18.4% ± 5.6% (27.5 ± 5.8%), and crystal quality reduced by 40.0 ± 4.5% (48.8 ± 6.0%). This study can be applied to predict the level of irregular-sized particles that can be tolerated in structural color materials at a specified reflectivity. Finally, we explore the effect of particle anisotropy on structural color reflection from discoid packings. We prepare discoidal particles that vary in shape anisotropy and particle size by uniaxial compression of spheres. Discoids are self-assembled by evaporation into dense discoid packings, which exhibit non-iridescent structural colors. This coloration is a combination of backscattering and multilayer reflection. We find that the multilayer reflection displays progressively smaller peak height and broader bandwidth as the discoids become more anisotropic. In addition, Monte Carlo simulation is used to produce comparable discoid structures. The density profiles of the simulated structures in the wall-normal direction demonstrate that discoids with a higher shape anisotropy assemble into more disordered structures, which explains the less intense structural color. Our findings demonstrate that tunable geometries of discoids increase the opportunities for spectral control of non-iridescent structural color materials.PhDMacromolecular Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/170049/1/ltianyu_1.pd