Fabrication of flexible and robust CdS composite photonic crystal films through melt-compression

Abstract

The relatively high refractive index of CdS spheres makes them ideal building blocks for fabricating photonic crystals (PCs) with strong photonic bandgaps and brilliant structural colors. However, the high material density causes CdS colloidal spheres to suffer from rapid sedimentation and difficulty in assembling into highly ordered arrays. Besides, due to limited interaction between adjacent spheres, PC films are usually too fragile to meet the requirements of practical applications. Herein, CdS@SiO₂@P(MMA-BA) core–shell spheres were first synthesized through a semicontinuous and stepwise emulsion polymerization process and adopted as building blocks for composite PC films. Under uniaxial compression at elevated temperatures, polymer shells of adjacent core–shell spheres fused and formed a continuous flowing polymer melt, which yielded a strong shearing force driving CdS@SiO₂ cores assembly into ordered arrays from the pressing plates. Finally, due to the strong locking effect of the polymer shell matrix, flexible and robust free-standing PC films with vivid and reversible mechanically responsive structural colors could be obtained. This fast and versatile strategy is not only efficient and reliable for fabricating robust PC films using high refractive index spheres with large density for large-scale production but also valuable for designing complex multicolor PC patterns for practical applications.