High Photoluminescence Quantum Yield and Tunable Luminescence Lifetimes in the Sub‐Second Range of CaS:Eu$^{2+}$ Phosphors for Tracer Based Sorting

The shift toward a circular economy for plastics requires innovative sorting technologies that improve recycling efficiency and material purity. Tracer-based sorting (TBS) is a promising approach that employs luminescent tracers with distinct optical properties for the precise identification of plastics. This study presents the synthesis and characterization of europium-doped calcium sulfide (CaS:Eu$^{2+}$) luminescent materials as potential tracers for TBS. Synthesized via a flux-assisted solid-state reaction, these phosphors achieve photoluminescence quantum yields (PLQYs) of up to 65% with tunable luminescence decay lifetimes ranging from 0.05 to 0.3 seconds. Long-term chemical stability under ambient conditions is confirmed over 18 months. The application of CaS:Eu$^{2+}$ in a TBS prototype is demonstrated using a standard CMOS video camera to differentiate materials based on decay times. These results highlight the suitability of CaS:Eu$^{2+}$ phosphors as stable, high-performance tracers, offering a viable way to improve plastics recycling processes.