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 (P_r) and chlorophylls.