Pump power induced tunable upconversion emissions from Er ³⁺/Tm³⁺/Yb³⁺ ions tri-doped SrY ₂O₄ nanocrystalline phosphors

RE³⁺ (Er³⁺/Tm³⁺/Yb³⁺) ions tri-doped SrY₂O₄ nanocrystalline phosphors were synthesized by a citrate sol-gel method. After annealing at 1300 °C, the structure was identified to be pure orthorhombic and the particles were found to be nearly spherical in shape. The up-conversion emission spectra of the SrY₂O₄:1Er³⁺ phosphor revealed a bright green emission (⁴S_3/2 → ⁴I_15/2) around 551 nm. To enhance the red emission of the Er³⁺ ions, Yb ³⁺ ions were co-doped with Er³⁺ ions, leading to a greenish yellow emission under 980 nm excitation. In the case of the 1Er ³⁺/1Tm³⁺/3Yb³⁺ ions tri-doped phosphor, white-light emission was observed corresponding to the ¹G₄ → ³H₆ (Tm³⁺), ⁴S_3/2 → ⁴I_15/2 (Er³⁺) and ⁴F _9/2 → ⁴I_15/2 (Er³⁺) transitions at 487, 551 and 664 nm, respectively. It was interestingly noticed that, when the exciting pump power was changed from 100 to 900 mW, the blue emission band of the Tm³⁺ ions was enhanced by the suppression of the red and green emission bands of the Er³⁺ ions so that the chromaticity coordinates were also moved to the cyan emission region. This feature suggests that the Er³⁺ ion may also act as a sensitizer for the Tm³⁺ and Yb³⁺ ions and a possible energy transfer mechanism was also discussed.