Concentration palette enabling temperature-responsive luminescence co-modulations of inorganic phosphors for a thermally triggered security tag

Abstract

Security tags (STs) based on temperature-responsive (TR) phosphors have become an emerging option for anti-counterfeiting and information security applications. However, state-of-the-art TR phosphors still incur drawbacks of inefficiency, instability, and especially, limited luminescence evolution. Herein, we demonstrate a “concentration palette” strategy to enable large-scale TR colour and intensity co-modulations in efficient and stable lanthanide-activated inorganic phosphors by coupling thermally stable/unstable colour units in one system with concentration management. Proof-of-concept experiments show that the designed Sr₃B₂O₆:Ce³⁺,Eu²⁺ phosphors possess unique colour (orange to blue) and orange intensity co-evolution in the temperature range of 78–500 K. Systematic characterizations reveal that the suitable crystal-field effect, moderate vibronic coupling and distinct thermal ionization of 5d electrons synergistically determine Ce³⁺ as a stable blue unit and Eu²⁺ as an unstable orange unit. The coupling of the Ce³⁺,Eu²⁺ units by managing the Eu²⁺ concentration further induces efficient energy transfer and unchanged thermal stability, together establishing the “concentration palette” and permitting unique colour and intensity co-modulation. Finally, our thermally triggered ST prototypes fabricated with the designed phosphors exhibit superior performance, with vivid TR properties, high information loads and good reliability in dual mode (colour and intensity) operation. These results pave a new avenue for exploring novel TR phosphors and promoting ST innovation.