Differential substitution of Ga3+ by Mn2+ and Cr3+ in LiGa5O8 to prepare multi-mode fluorescent materials as array elements for anti-counterfeiting

Traditional fluorescent anti-counterfeiting materials, excited by the near infrared (NIR) or ultraviolet (UV) light, typically tend to be homochromatic or occasionally bicolor. The luminescent characteristics of these materials are easily imitated by substitutes. It is very necessary to develop multi-mode fluorescent materials with high safety standards. The inverse spinel LiGa ₅O ₈ possesses more coordination (Ga ³⁺ site) and defects compared to ZnGa ₂O ₄, a material used in anti-counterfeiting applications, which require the performance of being easily identifiable and difficult to imitate. Here, a series of LiGa ₅O ₈:Cr ³⁺,Mn ²⁺ phosphors have been successfully prepared by the solid-state reaction. Mn ²⁺ and Cr ³⁺ ions were differentially substituted for Ga ³⁺ located at the tetrahedral site and the octahedral site, yielding a green emission at ∼512 nm (corresponding to the ⁴T ₁→ ⁶A ₁ transition of Mn ²⁺ ) in the absence of afterglow and a NIR emission at ∼718 nm (from the ²E→ ⁴A ₂ transition of Cr ³⁺ ) with an afterglow lasting more than 300 s. The emission intensity and lifetime of Mn ²⁺ decreased with the increasing of Cr ³⁺ concentration on account of the energy transfer from Mn ²⁺ to Cr ³⁺. Under UV excitation at 254 nm, changing emission colors ranging from green to red were found by increasing the Cr ³⁺ concentration. When exposed to UV light at 302 nm and 365 nm, the phosphors only emitted red and NIR signals. The phosphor also exhibited a heating-temperature dependent reproduction of NIR afterglow, attributed to the shallow and deep traps. The multi-mode fluorescence materials are sensitive to the composition, excitation wavelength, temperature, and duration time, making them suitable as the array elements for anti-counterfeiting.