High Photoluminescence Quantum Yield and Tunable Luminescence Lifetimes in the Sub‐Second Range of CaS:Eu²⁺ Phosphors for Tracer Based Sorting

Abstract 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.