Excitation-selective coexistence of upconversion and persistent luminescence in Pr3+-doped eulytite-type phosphates towards advanced anti-counterfeiting

Abstract

This work demonstrates the excitation-selective coexistence of blue-light-driven UVC upconversion (UC), X-ray-induced optical luminescence (XRL), and persistent luminescence (PersL) within a single Pr³⁺-doped host lattice. Pr³⁺-doped Ba₃Lu(PO₄)₃ and Sr₃Lu(PO₄)₃ phosphors exhibit multiple excitation-dependent luminescence modes and are studied as multifunctional materials for advanced anti-counterfeiting. When excited with blue light (444 nm), they efficiently produce blue-to-UVC upconversion via an excited-state absorption mechanism, while direct ultraviolet and visible excitation trigger characteristic Pr³⁺ photoluminescence. X-ray irradiation induces intense blue–red optical luminescence, followed by long-lasting red persistent luminescence detectable for several hours at room temperature. Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) measurements were employed to elucidate the nature of charge trapping processes that produce persistent emission. The TSL glow curves of all samples are dominated by a semi-broadband peak around 70 °C, indicative of a quasi-continuous distribution of trapping states. T_max–T_stop experiments combined with initial rise analysis reveal trap depths between 0.9–1.15 eV, supporting effective room-temperature PersL. EPR spectroscopy identifies radiation-induced phosphorus-related radical centers and shallow traps, which most likely correspond to F⁺-type centers, with thermal stability matching TSL results. These findings highlight the potential of Pr³⁺-doped eulytite-type phosphates as versatile, multi-level luminescent platforms for advanced anti-counterfeiting and encryption solutions.