Index-Matched Composite Colloidal Crystals of Core–Shell Particles for Strong Structural Colors and Optical Transparency

Colloidal photonic crystals show structural colors yet are generally opaque due to multiple scattering. To address this problem, composite colloidal crystals with a low index mismatch were prepared to demonstrate their selective reflection color and optical transparency, which, however, show relatively low reflection intensity. Thick composite colloidal crystals may enhance the reflection intensity, which, however, causes a significant loss in optical transparency as micrometer-sized defects also increase. Herein, we prepared composite colloidal crystal films of core–shell nanospheres in a polystyrene matrix, in which the refractive index is matched by adjusting the ratio of core-to-shell volume. Therefore, we demonstrate strong reflection colors in a thick colloidal film keeping high optical transparency. Furthermore, with no deterioration of light transmission in our index-matched composite colloidal crystals, bicolored reflective films were also successfully prepared by stacking two different colloidal crystal films. Finally, by introducing photopolymerizable resin inside colloidal crystals, we fabricated patterned composite photonic crystals through selective photopolymerization and repeated photopatterning process for multicolored films. These films may potentially be useful in reflective displays, encryption, and optical identification.