Beyond cadmium yellow: CdS photonic crystal pigments with vivid structural colors

Abstract

Monodisperse CdS spheres are considered ideal building blocks for fabricating photonic crystals (PCs) with a strong band gap and brilliant structural colors due to the high refractive index of CdS materials. However, the intrinsic strong yellow color from absorption and difficulty for assembly into PCs arising from the high density of CdS spheres both severely restrict the application of these promising high refractive index spheres. Besides, the possible release of poisonous Cd²⁺ ions is also an obvious safety concern towards applications. Herein, monodisperse CdS@SiO₂ spheres were adopted as building blocks and PC balls were fabricated through a microfluidic process. The presence of SiO₂ shells dramatically improves the chemical stability of CdS cores against acid etching and also increases the surface charge for effective self-assembly of building blocks into highly ordered PC balls. Through thermal treatment at appropriate high temperatures, the in situ formed carbon in CdS cores transformed PC balls into PC pigments with vivid and saturated size-dependent structural colors, which dominated over their intrinsic yellow colors. The vivid structural colors and angle independency from the symmetrical spherical geometrical morphology make PC pigments greatly valuable beyond the traditional cadmium yellow as totally new and promising safe pigments towards various practical applications.