Optimizing the short-wavelength infrared photoluminescence quantum yield and brightness of Er3+, Yb3+ co-doped yttrium orthophosphate phosphors for tracer-based sorting†

Development of efficient inorganic luminescent tracers is essential for various applications, including plastic sorting. In this study, yttrium orthophosphate (YPO₄) phosphors co-doped with Er³⁺ and Yb³⁺ lanthanide ions across a broad range of concentrations were synthesised via a solid-state reaction (SSR). The optimisation of SSR, and the structural and photoluminescence properties – including photoluminescence quantum yield (PLQY) and brightness (B) – of these new tracers are reported. The PLQY for the short-wave infrared (SWIR) emission of Er³⁺ under excitation with a 976 nm laser reaches a maximum of 67% for the sample with a nominal composition of 3 at% Er³⁺ and 3 at% Yb³⁺. The maximum brightness was achieved for a sample with a nominal composition of 3 at% Er³⁺ and 24 at% Yb³⁺. The brightest sample was printed using an industry-relevant process on a polyethylene pouch. Hyperspectral SWIR imaging under simultaneous irradiation of the pouch using a halogen lamp and 978 nm laser resulted in a strong luminescence signal peaking at 1540 nm. which was easily distinguishable against the reflectance background spectrum of polyethylene. This demonstration paves the way for future exploration of SWIR luminescence tracers in the next generation of plastic sorting approaches – tracer-based sorting.