Defect Structures in Colloidal Crystals and Their Effect on Grating Diffraction Structural Color

Colloidal crystals of micrometer-sized colloids create prismatic structural colors through the grating diffraction of visible light. Here, we develop design rules to engineer such structural color by specifically accounting for the effect of crystal defects. The local quality and grain size of the colloidal structure are varied by performing self-assembly in the presence of a direct current (DC) electric field. The deposition, self-assembly, and crystallization of these colloids results in polycrystals of variable size, as controlled by the dissolved ion concentration (from 0.01 to 10 mM) and the applied electric current (from 1.6-310 μA/cm²). Under these operating conditions, the global 6-fold crystal bond order parameter (ψ₆) of the self-assembled crystals varies from 0.45 ± 0.05 to 0.95 ± 0.01 and the crystal grain number density varies from about 5 to 100 per 0.01 mm². We find that the grating diffraction structural color intensity of these self-assembled materials is strongly correlated with the crystal quality and grain number, with the diffraction efficiency varying by a factor of ∼2.5 over the range of ψ₆ probed. Molecular dynamics (MD) simulation of the electrophoretic deposition reproduces the kinetics of the self-assembly as well as the final structures. It also extends the number and range of deposition conditions probed, thereby creating a library that can be used to study the relationship between defect properties and the grating diffraction structural color. Applying the finite-difference time domain (FDTD) method to solve for light-material interactions in the MD simulated structures yields calculated spectra that agree with experimental observations. The analysis also identifies a design trade-off between diffraction intensity and azimuthal uniformity as order parameter and grain density are varied, thereby demonstrating that the grating diffraction structural color of self-assembled crystals of micrometer-sized colloidal spheres may be controlled by means of their local crystal quality and polycrystallinity.