Controlling Structural Color in Colloidal Photonic Crystals through Binary Blends of Scattering and Absorbing Building Blocks

Structural color materials from colloidal photonic crystals (CPhCs) offer vivid, fade-resistant optics. While promising for large-area films, sensors, and anti-counterfeiting, their practical utility is hindered by intrinsic white scattering. This broadband scattering, arising from lattice defects and Rayleigh scattering, severely reduces visual contrast. Conventional mitigation involves adding light-absorbing secondary additives into the lattice. However, introducing these heterogeneous phases inevitably induces structural perturbations, disrupting long-range order and compromising photonic coherence. To overcome the trade-off between optical contrast and structural integrity, we propose a binary colloidal building block strategy. Instead of external additives, we engineered a light-absorbing building block by integrating polydopamine (PDA) within the shell of a scattering core-shell colloid via in situ oxidative polymerization. This preserves the exact geometric dimensions of the parent colloid. Consequently, the light-absorbing and scattering building blocks possess identical geometries but complementary optical functions. Through melt-shear assembly, we demonstrate that these binary colloids exhibit perfect packing compatibility. This geometry-matched blending prevents the loss of photonic crystallinity typical of additive-mixed systems. We show the optical characteristics and visual contrast of CPhC films can be precisely tailored by adjusting the blending ratio of the two colloids. Furthermore, applying this strategy across size-defined colloidal sets generates a comprehensive, high-contrast structural color palette without complex formulation optimizations. This approach establishes a scalable framework for engineering advanced structural color films while preserving lattice integrity.