Breaking the balance of chemical ratio in ZnAl2O4 to regulate activators and traps for multimode luminescence – A new strategy

As a common transition metal activator, the non-rare earth element Mn has good luminescent properties, so it has been widely concerned. It is reported that Mn2+ and Mn4+ can co-exist in the matrix, and the luminescence of Mn with different valence states is usually controlled by the co-doping of other ions. In this work, we controlled the green light emission of Mn2+, the red light emission of Mn4+, and the NIR emission of defects by breaking the balance of chemical ratio in ZnAl2O4:Mn to achieve multimode luminescence. The spinel-structured ZnAl2O4 phosphors with deficiency of zinc were synthesized by high-temperature solid-state reaction, which were characterized by a series of techniques, including XRD, DFT calculation, PLE/PL spectroscopy, TL, persistent luminescence decay curves, and temperature-dependent PL spectra analysis. The deficiency of zinc results in the appearance of zinc vacancy (VZn) and oxygen vacancy (VO), and the increased oxygen vacancy defects inhibit the self-reduction of Mn4+. Under the excitation of 325 nm and 426 nm ultraviolet light, the phosphor showed green emission at 510 nm, red emission at 678 nm, and near-infrared emission at 767 nm. Due to the increase of vacancy defects, the higher concentration of zinc deficiency leads to stronger emission of green light, red light, and near-infrared light. The phosphor exhibited different light signals at different excitation wavelengths, and the thermal stability of Mn2+ and Mn4+ luminescence is inconsistent. The above characteristics show that the multimode luminescent phosphor synthesized in this work has broad application prospects in the field of fluorescence anti-counterfeiting.