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.

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.

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.