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

Abstract

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.

Reversible Photochromic Photonic Crystal Device with Dual Structural Colors

“…A photonic crystal (PhC) provides unique optical properties for thorough control and manipulation of photons within a specific wavelength range not possible otherwise. , Three-dimensional (3D) PhCs with 3D photonic band gap (PBG) have attracted increasing interest in sensors, , lasers, , displays, , anticounterfeiting, , broadband reflectors, , light emitters, and so forth, for their abilities to reflect light and manipulate light emission in all the three dimensions. Nowadays, various two-dimensional assembly methods focusing on core–shell and multicomponent hybrid colloidal nanospheres, , such as self-assembly, , spin coating, , photolithography, deposition, , and viscoelastic shear processing, , have been used to enhance the optical properties of two-dimensional PhC films. However, the 3D assembly fabrication methods for 3D PhC devices based on nanospheres are missing research, limited by intrinsic drawbacks of poor mechanical behavior and microstructural defects (dislocation, point/line defects, and crack).…”

Solvent-Responsive Invisible Photonic Patterns with High Contrast for Fluorescence Emission Regulation and Anti-Counterfeiting

“…Photonic crystals (PCs) are optical materials formed by periodic arrangements of media with different refractive indices, showing a photonic band gap (PBG). − When the PBG is located in the visible range, the light overlapping with this wavelength range cannot be propagated in the PCs, and accordingly, the PCs present the color visible to the naked eye, namely, the structural color. − Responsive photonic crystals (RPCs), by combining colloidal crystals with responsive polymers, can change their lattice spacing or refractive index in response to external stimuli (e.g., mechanical force, magnetic field, temperature, humidity, pH, solvent, etc.) to change their structural colors. − Among these RPCs patterned PCs with different responsiveness in patterns and background regions to external stimuli can be obtained by local modification of RPCs.…”

Self-Assembled Honeycomb Lattices of Dielectric Colloidal Nanospheres Featuring Photonic Dirac Cones

“…Two-dimensional photonic crystals with a honeycomb lattice have Dirac cones in the photonic band structure and thus show interesting wave-transport properties. − Because a topological edge mode in the vicinity of Dirac points is easily obtained by geometric perturbations, the honeycomb lattice has recently attracted substantial attention, displaying unique properties such as low-loss unidirectional light propagation , and topologically protected lasing. , So far, in experimental reports, a honeycomb lattice has been fabricated primarily in top-down methods, − which are, however, time-consuming and costly. Alternatively, bottom-up approaches produce uniform nanostructures of colloidal particles in a large area at low cost, − but specially designed colloidal particles such as tripatch particles or DNA-coated particles , are needed as building blocks for those honeycomb lattices. As reported, the synthesis of submicrometer-sized tripatch particles requires elaborate experimental control and is time-consuming, while DNA-mediated assembly requires accurate control of the experimental conditions, including salt concentration and temperature .…”