Fabrication and Evaluation of Photonics Ball via Polymer Coating with High-Index Nanoparticles

Recently, structural color materials have garnered significant interest as environmentally friendly color materials. However, their application is limited due to their angle dependence when featuring a colloidal crystal structure. An isotropic spherical colloidal crystal (photonic ball) has been successfully fabricated using mono-disperse particles, such as SiO₂ or polystyrene. These photonic balls can exhibit vibrant colors and decrease the angle dependence of the observed hue, compared to flat colloidal crystals. However, the position of the reflection peak still changes when the light irradiation and the observation direction shifts, similar to traditional flat colloidal crystals.<br/>A practical solution to mitigate these issues could be to use a material with a higher refractive index, which could potentially lessen the angle dependence, even with this observation method. For example, with the [111] plane of the face-centered cubic (FCC) lattice formed on the photonic ball surface, the Bragg reflection formula is displayed as λ=1.633d(n²_effctive-sin²θ)^0.5, where n_effctive=n²_particleφ_particle+n²_air(1-φ_particle). When simulating the angle dependence of SiO₂ particles （n_SiO2=1.46） and CeO₂ particles（n_CeO2=2.10 ）, if θ changes from 10° to 50°, the reflection peak position for the SiO₂ particle photonic ball shifts by 17%. Meanwhile, for the CeO₂ particle photonic ball, the same angle change results in just about an 8% shift of the reflection peak position. Thus, by increasing the refractive index of the constituent materials, we can effectively suppress the angle dependence of photonic balls.<br/>In this study, we aimed to fabricate vibrant photonic balls with minimal angle dependence using CeO₂ particles, with high refractive index. The CeO₂ particles we used are coated with polyvinylpyrrolidone (PVP) on the surface, allowing them to disperse in water. However, the suspension method failed to fabricate photonic balls using these particles. In water, CeO₂(@PVP) particles maintain a certain distance even in high concentration areas due to the steric repulsion of polymer chains. As the water evaporates and the particles draw closer, the high Tg of PVP (80-130°C) hinders particle movement, making it difficult to arrange them regularly. Consequently, the dried particles arrange in an amorphous structure.<br/>To overcome this, we modified the surface to provide a repulsive force, ensuring a certain level of movement from evaporation to drying. We used polydopamine (PDA) to give a negative charge to the CeO₂(@PVP) particles by synthesizing CeO₂(@PVP)@PDA particles. The photonic balls made of CeO₂(@PVP)@PDA particles exhibited vivid colors, and when the light irradiation and the observation direction changed from 10° to 50°, the position of the reflection peak shifted by 10.2% . This indicates that the angle dependence is more suppressed than the photonic balls made of SiO₂ particles. To further reduce the angle dependence, we sintered the photonic balls in an oxygen-free environment. The sintered photonic balls shown not only a lower angle dependence (only about 7%) but also exhibited stronger coloration than the photonic balls made of CeO₂(@PVP)@PDA particles. We attribute this to the transformation of the polymer components on the particle surface into graphite carbon with a high refractive index after sintering.