Design, Synthesis, and Characterization of a Novel Blue-Green Long Afterglow BaYAl3O7:Eu2+, Nd3+ Phosphor and Its Anti-Counterfeiting Application

Herein, a series of novel long afterglow nanophosphors BaYAl3O7:Eu2+, Nd3+ was synthesized by the combustion method. The investigation encompassed the characterization of X-ray diffraction, morphology, chemical valence, elemental composition, and photoluminescence behavior of BaYAl3O7:Eu2+ and BaYAl3O7:Eu2+, Nd3+ nanoparticles. Under 365 nm excitation, BaYAl3O7:Eu2+ and BaYAl3O7:Eu2+, Nd3+ show emission bands centered at 497 nm and 492 nm, which are attributed to the 4f65d→4f7 transition of Eu2+ ions. The optimal samples of BaYAl3O7:0.03Eu2+ and BaYAl3O7:0.03Eu2+, 0.02Nd3+ have average fluorescence lifetimes of 850 ns and 1149 ns, respectively. The co-doping of Nd3+ ions as the trap centers produced long afterglow luminescence properties, and the afterglow time could reach up to 8 min. Furthermore, the fluorescent powder can be mixed with polyacrylic acid to prepare anti-counterfeiting inks; a clover pattern and snowflake pattern have been successfully printed using screen printing technology, proving its potential application in the field of anti-counterfeiting.

The Charge Transfer Band as a Key to Study the Site Selection Preference of Eu3+ in Inorganic Crystals

On account of the strong oxidizing property of the europium(III) ion, its charge transfer band (CTB) can be easily formed in many inorganic compounds. In this work, the Eu³⁺ ions were singly doped into the K₃LuSi₂O₇ compound with a hexagonal structure, and two kinds of Eu³⁺-O^2- CTBs were detected by monitoring at specific wavelengths. The qualitative analysis of Eu³⁺ ion site occupation was illuminated by combining Eu³⁺-O^2- CTBs with the corresponding cell volume. Furthermore, the two kinds of Eu³⁺ sites are eventually assigned to the K(2) and Lu sites, which means that Eu³⁺ ions selectively occupy the site with a low coordination number, according to the calculated CT energy by the dielectric theory of complex crystals and the magnitude of CT energy in the excitation spectra. Meanwhile, at high temperatures, the CTB does not show the traditional thermal quenching like f-f transitions but demonstrates thermal enhancement; thus, by using this opposite variation in excitation spectra, a noninvasive optical thermometer is presented, and this opposite variation tendency is thought to be the difference of thermal stability of disparate excited energy states. When new luminescent phosphors are designed with interesting spectral properties, this work will give us an alternative approach to determine the site occupation preference of Eu³⁺, especially when there are more than two different sites in the compound.