Up/down conversion luminescence properties of (Na0.5Gd0.5)MoO4:Ln3+ (Ln = Eu, Tb, Dy, Yb/Er, Yb/Tm, and Yb/Ho) microstructures: synthesis, morphology, structural and magnetic investigation

Abstract

Novel bi-pyramid-like (Na_0.5Gd_0.5)MoO₄:Ln³⁺ (Ln = Eu, Tb, Dy, Yb/Er, Yb/Ho, and Yb/Tm) hierarchical microstructures are synthesized via the EDTA-aided hydrothermal route. Time-dependent experiments were carried out and the morphological evolution was discussed, which signposted that the present method corresponded to a typical Ostwald ripening process for the formation of bi-pyramid-like structures. Elemental mapping analysis confirms the incorporation and uniform distribution of all the elements. Phase purity and crystal structure of the as-synthesized phosphor were analyzed by the X-ray diffraction (XRD) technique. Fourier transform infrared spectroscopy (FTIR) results confirm that for scheelite tetragonal symmetry, F₂ (ν₃) modes of vibrations are IR active. (Na_0.5Gd_0.5)MoO₄ was demonstrated to be a good host material for down- and up-conversion luminescence, which were strongly dependent on the hydrothermal reaction time interval. The down-conversion luminescence properties of (Na_0.5Gd_0.5)MoO₄ doped with Eu³⁺, Tb³⁺, Dy³⁺ were studied. Under 980 nm NIR excitation intense up-converted visible emissions were observed in (Na_0.5Gd_0.5)MoO₄:Yb³⁺/Ln³⁺ (Ln = Er, Tm, and Ho), which shows green (²H_11/2 → ⁴I_15/2 of Er³⁺ at 525 nm), blue (¹G₄ → ³H₆ of Tm³⁺ at 474 nm), and yellow (⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ of Ho³⁺ at 541 nm and 660 nm, respectively) luminescence. Also, Yb³⁺/Er³⁺-doped samples exhibit luminescence in the near infrared (combination of Er³⁺/Yb³⁺ at 977 nm, Er³⁺ at 1530 nm) region. The energy transfer process, fluorescence decay time, and colour coordinates were analyzed for both down- and up-conversion luminescence. Further, the magnetic properties of (Na_0.5Gd_0.5)MoO₄:Ln³⁺ (Ln = Eu and Yb/Er) as-prepared bi-pyramid-like structure exhibit typical paramagnetic behavior.