(334aq) Self-Assembly for Colloidal Crystallization and Biomimetic Structural Color

My PhD research focuses on quantifying the relationship between crystal quality of self-assembled colloidal crystal and their optical properties, especially structural color intensity. Structural color is of interest in artificial materials because it arises from the periodicity of crystal structures, and it has better environmental stability than dyes. I fabricate colloidal crystals with structural color by controlling crystallization of colloidal particles via evaporative self-assembly. To study the connection between crystal film quality and structural color intensity, I investigate the crystal reflectivity as a function of film thickness, defect density, and impurity concentration using a combined experimental and computational approach. In the experiments, I formulated a series of colloidal dispersions of submicron-sized polystyrene spheres. By controlling the initial volume fraction of the colloidal dispersion, I varied the final thickness of crystal films after solvent evaporation and measured the thickness profiles using a profilometer. By introducing differently sized particles that function as impurities, I manipulated the defective microstructures of colloidal crystals and characterized different kinds of defects by scanning electron microscopy (SEM). I further analyzed microscope images with MATLAB to quantify crystal quality. To evaluate reflective structural color, I measured the reflection spectra of crystal films using a UV-Vis spectrophotometer and analyzed the reflection peak intensity. In the simulation, I collaborated with a colleague to model crystal structures with different quality via molecular dynamics. Then I simulated the reflection spectra of modeled crystals using the finite-difference time-domain (FDTD) method. The results show that film thickness profoundly affects structural color intensity. The peak reflection of the structural color increases approximately linearly with the film thickness, ultimately reaching reflection saturation. Self-assembled colloidal crystals cannot reach 100% reflection because they contain a variety of defects. Reduction in structural color peak reflection scales with increased defect density and impurity concentration; vacancies have a particularly detrimental effect. These quantitative relationships I have developed serves as a specific platform to guide the design of optical materials and support defect engineering in colloidal crystals.