Robust and Flexible Colloidal Photonic Crystal Films with Bending Strain–Independent Structural Colors for Anticounterfeiting

Abstract

Colloidal photonic crystals (PCs) possess iridescent and metallic structural color, making them an attractive candidate for anticounterfeiting. However, traditional colloidal PC‐based anticounterfeiting materials usually have bending‐induced color‐switching characteristics or poor flexible stability, significantly affecting their color reproducibility and durability. Here, a novel robust colloidal PC film with bending strain–independent structural color and high flexible stability has been developed through the self‐assembly of SiO2 particles into the poly(ethylene glycol) diacrylate (PEGDA) matrix. The unique microstructure of the colloidal PC film contains ordered and disordered arrays of SiO2 nanoparticles embedded into the flexible PEGDA matrix, which is crucial to achieving bending strain–independent structural color. In fact, during the bending process, the colorless disordered arrays act as the buffer space, avoiding deformation of the colored ordered arrays and thus maintaining its original structural color. Remarkably, the film retains its structural and optical integrity after 10 000 times bending, supporting its high flexible stability and robustness. In addition, the film shows high transparency so that it can easily achieve an invisible and visible state transition under switch of weak and strong light. The film is potentially useful for applications in anticounterfeiting and encryption.

Dual Optical Information‐Encrypted/Decrypted Invisible Photonic Patterns based on Controlled Wettability

“…First, ordered arrays of SiO 2 nanoparticles obtained by evaporation-induced self-assembly process were immobilized in a PEGDA matrix by a UV photo-polymerization method, forming a SiO 2 -PEGDA composite PC films, where the volume fraction of silica is 30%, as previously reported (Figure 1a). [38] Next, the films were immersed in 2 wt% hydrofluoric acid solution to remove SiO 2 nanoparticles (Figure 1b). The obtained PEGDA films with IO structure were treated by trichloro(1H, 1H, 2H, 2H-tridecafluoro-n-octyl)silane (FOTS) to create hydrophobic properties (Figure 1c).…”

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

“…As discussed in previous finite-difference time-domain method simulations and experiments, the reflection peak positions in photonic crystal films depend primarily on particle size and lattice constant, while their intensity of reflection and transmission relies on the refractive index (RI) contrast of a particle matrix and film thickness. , When the RI contrast is reduced in colloidal crystal films, optical transparency can be significantly improved . For example, silica particles in a monomer resin were assembled into colloidal crystals by shear-induced crystallization and solidified by photopolymerization, producing composite colloidal crystals − in which the RI contrast was very small, indicating high transparency and sharp reflection peaks. Transparent composite PCs can also be prepared in various forms using hard-core/soft-shell particles; this is performed by melting low- T g polymers in soft shells during shear crystallization. , These transparent composite films have been intensively investigated in additional applications such as anticounterfeiting, , colorimetric sensors, contact lenses, and biomimetics. , The reflectance of thin films is relatively weak due to low RI contrasts.…”

Dual‐Modal Invisible Photonic Crystal Prints from Photo/Water Responsive Photonic Crystals

“…Photonic crystals (PCs) exhibit potential applications in the field of color display, [1][2][3][4][5][6][7] pigment, [8][9][10][11] printing/pattern, [12][13][14][15][16][17][18] sensing, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] chemical separation, [36,37] anti-counterfeiting, [38][39][40][41][42][43][44][45][46][47][48][49][50][51][...…”

“…Photonic crystals (PCs) exhibit potential applications in the field of color display, [ 1–7 ] pigment, [ 8–11 ] printing/pattern, [ 12–18 ] sensing, [ 19–35 ] chemical separation, [ 36,37 ] anti‐counterfeiting, [ 38–53 ] and photocatalysis, [ 54–57 ] benefiting from the unique periodic structures. The color of the PC comes from the diffraction, interference, and scattering of light by its ordered structures, which can be altered by the lattice constant, refractive index, and orientations of the crystal.…”