Investigation of Upconversion, downshifting and quantum -cutting behavior of Eu³⁺, Yb³⁺, Bi³⁺ co-doped LaNbO₄...

This work presents the spectral conversion characteristics [upconversion (UC), downshifting (DS) and quantum-cutting (QC) optical processes] of Eu³⁺, Yb³⁺ and Bi³⁺ co-doped LaNbO₄ (LBO) phosphor samples synthesized by solid state reaction technique. The crystal structure and the pure phase formation have been confirmed by x-ray diffraction (XRD) measurements. The surface morphology and particle size are studied by scanning electron microscopy (SEM). The rarely observed intense red UC emission from Eu³⁺ ion has been successfully obtained in Eu³⁺/Yb³⁺ co-doped LaNbO₄ phosphor (on excitation with 980 nm) by optimizing the concentrations of Eu³⁺ and Yb³⁺ ions. The downshifting (DS) behavior has been studied by photoluminescence (PL) measurements on excitation with 265 nm wavelength from a Xe lamp source. A broad blue emission in the region 300-550 nm with its maximum ∼415 nm due to charge transfer band (CTB) of the host and large number of sharp peaks due to f-f transitions of Eu³⁺ ion have been observed. The energy transfer has been observed from (NbO₄)^3- to Eu³⁺ ion and the fluorescence emission has been optimized by varying the concentration of Eu³⁺ ion. An intense red emission has also been observed corresponding to ⁵D₀ → ⁷F₂ transition of Eu³⁺ ion at 611 nm in LBO: 0.09Eu³⁺ phosphor on excitation with 394 nm. The luminescence properties of Eu³⁺ ion are enhanced further through the sensitization effect of Bi³⁺ ion. The near infra-red (NIR) quantum cutting (QC) behavior due to Yb³⁺ ion has been monitored on excitation with 265 as well as 394 nm. The NIR QC is observed due to ²F_5/2 → ²F_7/2 transition of Yb³⁺ ion via co-operative energy transfer (CET) process from (NbO₄)^3- as well as Eu³⁺ ions to Yb³⁺ ion. This multimodal behavior (UC, DS and QC) makes this a promising phosphor material for multi-purpose spectral converter.