Triple-Mode Emissions with Invisible Near-Infrared After-Glow from Cr³⁺-Doped Zinc Aluminum Germanium Nanoparticles for Advanced Anti-Counterfeiting Applications

Materials exhibiting persistent luminescence (PersL) have great prospect in optoelectronic and biomedical applications such as optical information storage, bio-imaging, and so on. Unfortunately, PersL materials with multimode emission properties have been rarely reported, although they are expected to be very desirable in multilevel anti-counterfeiting and encryption applications. Herein, Cr³⁺-doped zinc aluminum germanium (ZAG:Cr) nanoparticles exhibiting triple-mode emissions are designed and demonstrated. Upon exposure to steady 254 nm UV light, the ZAG:Cr nanoparticles yield steady bluish-white emission. After turning off the UV light, the emission disappears quickly and the mode switches to transient near-infrared (NIR) PersL emission at predominantly 690 nm. The transient NIR PersL emission which arises from Cr³⁺ is induced by non-equivalent substitution of Ge⁴⁺. After persisting for 50 min, it can be retriggered by 980 nm photons due to the continuous trap depth distribution of ZAG:Cr between 0.65 and 1.07 eV. Inspired by the triple-mode emissions from ZAG:Cr, multifunctional luminescent inks composed of ZAG:Cr nanoparticles are prepared, and high-security labeling and encoding encryption properties are demonstrated. The results indicate that ZAG:Cr nanoparticles have great potential in anti-counterfeiting and encryption applications, and the strategy and concept described here provide insights into the design of advanced anti-counterfeiting materials.