Fano-Engineered Au@Se Core-Shell Colloidal Pigments for Dual-Mode Structural Color.

Colloidal structural colorants are attractive because they are substrate-free and solution-processable, but single-component plasmonic or dielectric colloids usually exhibit broad visible resonances that limit chroma. Here we report Au@Se core-shell colloidal pigments in which a plasmonic Au core is embedded within a visible-range Mie-resonant Se shell. The key signature is not the gradual red shift expected from passive dielectric loading, but an asymmetric hybrid feature with compressed linewidth, consistent with Fano-like interference between the broad electric-dipole response of the Se shell and the narrower Au-core electric-dipole response. The hybrid mode yields Q factors up to ≈15 in freely dispersed colloidal pigments, producing narrow transmission windows and spectrally separated reflection bands from the same liquid dispersion. By independently tuning the Au core diameter and Se shell thickness in a benchtop synthesis, we achieve broad visible color tuning in transmission and reflection and enlarge the accessible international commission on illumination chromaticity (CIE) area relative to single-component Au or Se colloids. The particles are formulated into ultraviolet (UV)-curable resins and writing inks for 254-ppi color filters, freeform paper deposition, and polarization-selective readout. This work establishes a colloidal design rule in which the shell acts as an active visible resonator rather than a passive refractive-index coating.