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

Abstract 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.