Modulating Color Response in Polymeric Photonic Crystals through Refractive Index Tuning with Gold Nanoparticles

Responsive photonic crystals with high brightness, straightforward fabrication, and excellent tunability are in high demand for a wide range of potential applications, including sensors, printing, and display units. In this study, we present an innovative design of highly-ordered photonic crystals with tunable color response through gold (Au) deposition using a space-confined seeded growth method.<br/><br/>The pivotal aspect of modulating color response is controlling the deposition of Au nanoparticles (NPs) within the hollow polymeric shells. Benefiting from the strong interaction between AuNPs and resorcinol-formaldehyde (RF) polymer, even the addition of tiny Au NPs leads to an unexpected decrease in the effective refractive index of the core-shell NPs. Compared to other plasmonic materials, Au exhibits a unique ability to significantly reduce the effective refractive index.<br/><br/>Leveraging the continuous decrease in refractive index during AuNPs growth, we fabricate a series of composite films displaying blue-shifted colors. The structural color can be easily modulated over a broad spectral range by simply adjusting the hollow shells, resulting in more abundant colors with rich saturation. Moreover, the blue-shift effect can be eliminated through Au removal, allowing for reversible color changes. Simple fabrication, flexible adjustment, and outstanding optical properties of our composite film make it a promising choice for the development of functional colorimetric sensors in the future.