Extended Range of Ratiometric Luminescence Codes with Ce$^{3+}$ Modified Gd$_2$O$_2$S:Er$^{3+}$, Yb$^{3+}$ Shortwave Infrared Phosphors

Lanthanide (Ln$^{3+}$) based luminescent materials, with their distinctive emission spectra, offer opportunities to develop novel coding patterns for various applications, including Tracer Based Sorting (TBS). TBS uses trace amounts of inorganic phosphors and is a promising method for sorting plastics based on criteria beyond plastic type. This study investigates the synergistic effect of co-doping with Ce$^{3+}$ to enhance Er$^{3+}$ luminescence at ≈1550 nm (with a maximum photoluminescence quantum yield (PLQY) of 5.7%) while significantly reducing Yb$^{3+}$ luminescence at ≈1000 nm. The underlying mechanisms of these properties are analyzed using absolute PLQY measurements in an integrating sphere, luminescence decay studies, and X-ray photoelectron spectroscopy. Hyperspectral shortwave infrared imaging reveals an extended range of unique tracer combinations based on ratiometric intensity measurements, particularly when the Yb$^{3+}$/Er$^{3+}$/Ce$^{3+}$ tracers are mixed with a single-doped Yb$^{3+}$ tracer, compared to simple mixtures of Ce$^{3+}$-free Yb$^{3+}$/Er$^{3+}$ and Yb$^{3+}$ tracers. Therefore, the use of the tri-doped Yb$^{3+}$/Er$^{3+}$/Ce$^{3+}$Gd$_2$O$_2$S tracer increases the diversity of available luminescent tracers that may be of potential interest in the identification and sorting of plastic waste using the TBS process.