Tuneable blue-green co-activated CaAl₂O₄: 0.1 mol% Tb³⁺, x mol% Sm³⁺ (0 ≤ x ≤ 2) nanophosphor prepared by citrate sol-gel method

This study reports the analysis of tuneable blue-green phosphor of calcium aluminate (CaAl₂O₄) co-doped with terbium (Tb³⁺) and varying concentration of samarium (Sm³⁺) (0 ≤ x ≤ 2). The powder samples were synthesized by using the citrate sol-gel method and annealed at 900 °C for 2 h (h) for all samples. The X-ray powder diffraction (XRD) results revealed a developing amorphous monoclinic structure of CaAl₂O₄. Co-doping with Tb³⁺ and Sm³⁺ slightly affected the structure of synthesized CaAl₂O₄. Scanning electron microscope (SEM) results showed that co-doping the host material of CaAl₂O₄ slightly affected the morphology of the synthesized nanopowders. The transition electron microscope (TEM) images showed that the particle sizes of the host material were affected by doping. Ultraviolet–visible (UV–Vis) reflection spectroscopy suggested that the band gap of co-doped CaAl₂O₄:0.1 mol% Tb³⁺, x% Sm³⁺ (0 ≤ x ≤ 2) ranges between 5.05 and 5.20 eV. The photoluminescence (PL) results showed several emission peaks located at 420, 440, 484, 548, 558, 562, 584, 603 and 654 nm. Emission peaks at 420, 440, 484 and 558 nm were attributed to the host material. The peaks at 548 and 558 nm are originating from ⁵D₄→⁷F₆ and ⁵D₄→⁷F₅ transitions of Tb³⁺ while the peaks at 562, 603 and 654 nm seems to be originating from the dopant of Sm³⁺ which may be assigned to ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, and ⁴G_5/2 → ⁶H_9/2 transitions of Sm³⁺, respectively. CIE coordinates results suggested that the emission colour can be tuned from bluish to greenish as x% Sm³⁺ concentration is varied.