A NIR-to-NIR Ca2ZrSi4O12:Yb3+ phosphor security fiber with high thermal stability for anti-counterfeiting applications

Near-infrared (NIR) excitation to NIR emission (NIR-to-NIR) can be applied in diverse fields, specifically anti-counterfeiting technologies, owing to its security feature. In this study, Yb³⁺-doped Ca₂ZrSi₄O₁₂ was synthesized as a novel NIR-to-NIR phosphor via a conventional solid-state reaction. The synthesized Ca₂ZrSi₄O₁₂:Yb³⁺ phosphor exhibited a monoclinic structure, as the primary phase, and an angular morphology with an average particle size of 5–10 μm. It showed three excitation bands in the ultraviolet and infrared regions due to the charge-transfer and ²F_7/2 → ²F_5/2 transitions of the Yb³⁺ ions. The emission spectra obtained under 915 nm NIR excitation displayed a strong NIR emission band in the 950–1050 nm range, which correspond to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions. The Ca₂ZrSi₄O₁₂:Yb³⁺ phosphor exhibited excellent thermal stability, maintaining 84.5 % of its initial emission intensity at 375 °C, and showed minimal thermal degradation upon subsequent cooling. Finally, a masterbatch comprising the Ca₂ZrSi₄O₁₂:Yb³⁺ phosphor as an additive was manufactured for anti-counterfeiting applications, and its NIR-to-NIR emission characteristics were confirmed. Thus, Ca₂ZrSi₄O₁₂:Yb³⁺ phosphors are promising for sophisticated anticounterfeiting technologies.