Defect-induced new persistent cyan-emitting rare-earth-free phosphors for dynamic anti-counterfeiting and plant-growth LED applications

Abstract

Initially, a self-activated Sr2Zr(SiO3)4 phosphor was developed by substituting Sr2+ for Ca2+ in the reported Ca2Zr(SiO3)4. The XRD pattern of the synthesized Sr2Zr(SiO3)4 crystal matched that of monoclinic Sr3Y2Si6O18 (m-Sr3Y2Si6O18, cyclosilicate structure), and this discovery led to the development of a novel cyan-emitting Sr2Zr2(SiO3)6 phosphor with persistent luminescence (PersL). XRD and HRTEM analysis identified the presence of a secondary orthorhombic Sr3Zr2O7 phase. Under 284 nm excitation, the PL emission of the host exhibited broadband centred at 485 nm, originating from the Zr4+ ions and lattice defects. Additionally, the host showed cyan PersL with a duration of 60 s. The addition of an NH4Cl flux enhanced PersL intensity and duration of the host material (>60 s). Bi3+ doping and post-annealed vacuum treatment increased the oxygen vacancy compared to Sr2Zr2(SiO3)6:12 wt% NH4Cl phosphor, as evidenced by XPS analysis. After Bi3+ incorporation, an additional PL band centred at 620 nm was observed due to the oxygen-vacancy-induced electronic localisation around the Bi3+ ions. The optimised vacuum-treated phosphor Sr2Zr2(SiO3)6:12 wt% NH4Cl,0.02Bi3+ exhibited enhanced PersL intensity and duration (90 s) due to the presence of additional shallow electron traps. This vacuum-treated phosphor has been used in dynamic anti-counterfeiting applications by mixing it with red-emitting phosphors. Sr2Zr2(SiO3)6:12 wt% NH4Cl,0.02Bi3+ is the first cyan-emitting rare-earth-free PersL phosphor used in dynamic anti-counterfeiting and security ink applications. Furthermore, a Sr2Zr2(SiO3)6:12 wt% NH4Cl/Mg3Al2GeO8:0.005Mn4+,0.27Ba2+ mixture has been developed for use in plant growth LEDs, covering the absorption spectra of plant phytochrome (Pr) and chlorophylls.