Lattice-site engineering in ZnGa2O4:Cr3+ through Li+ doping for dynamic luminescence and advanced optical anti-counterfeiting

Abstract

To develop reliable optical anti-counterfeiting materials, higher requirements are put forward for innovative anti-counterfeiting phosphors, which are hardly observed under natural light but recognizable under specified conditions. Herein, novel multimode luminescent phosphors of ZnGa₂O₄:0.005Cr³⁺, xLi⁺ (x = 0–0.08) have been successfully synthesized via a solid-state reaction. Li⁺ occupies the Zn sites and induces the appearance of oxygen vacancies. Oxygen vacancies contribute to the green-yellow emission at 520 nm, and Cr³⁺ contributes to the near-infrared (NIR) emission at 695 nm. The incorporation of Li⁺ enhanced green-yellow emission at 520 nm, thus leading to the emission color changing from red to yellow-green. Both, switching the excitation wavelength and varying the temperature resulted in a varied ratio of visible light to NIR light. The phosphor output showed intense NIR afterglow and weak visible afterglow in the absence of ultraviolet (UV) light irradiation. The incorporation of Li⁺ led to both improved NIR and visible afterglows. Because the prepared materials are sensitive to the composition, excitation wavelength, temperature, and duration time, they were successfully designed as components in a dynamic visual feast, including traditional patterns “Facial Makeup” and “Auspicious Cloud”, Morse code, and alphanumeric code, indicating that they are the potential intelligent materials for optical anti-counterfeiting with high-security level.