Frequency upconversion and fluorescence intensity ratio method in Yb³⁺-ion-sensitized Gd₂O₃:Er³⁺-Eu³⁺ phosphors for...

Near infrared (NIR) to visible frequency upconversion emission studies in Er³⁺-Eu³⁺/Er³⁺-Eu³⁺-Yb³⁺ co-doped/tri-doped Gd₂O₃ phosphors prepared by the co-precipitation technique have been explored under 980 nm laser diode radiation. The developed phosphors were characterized with the help of XRD, FE-SEM and FTIR analysis. No upconversion (UC) emission was found in the Eu³⁺-doped Gd₂O₃ phosphor. UC emission from Eu³⁺ ions along with Er³⁺ ions was observed in Er³⁺-Eu³⁺ and Er³⁺-Eu³⁺-Yb³⁺ co-doped/tri-doped phosphors. The UC emission arising from the Er³⁺ and Eu³⁺ ions was enhanced several times due to the incorporation of Yb³⁺ ions. The processes involved in the UC emission were obtained on the basis of the effect of energy transfer/sensitization through the Yb³⁺ → Er³⁺ → Eu³⁺ process. The red/green intensity ratio was improved from 0.16 to 1.50 and 1.01 to 1.50 for Er³⁺-Eu³⁺-Yb³⁺ tri-doped phosphors as compared to the Er³⁺-doped and Er³⁺-Yb³⁺ co-doped phosphors, respectively, at a fixed pump power density. A UC fluorescence intensity ratio (FIR)-based temperature sensing study was performed in the prepared Er³⁺-Eu³⁺-Yb³⁺ tri-doped Gd₂O₃ phosphors for green upconversion emission bands in the 300 K-443 K temperature range. A maximum sensor sensitivity of about ∼0.0043 K^-1 at 300 K was achieved for the synthesized tri-doped phosphors upon excitation with a 980 nm laser diode. The colour coordinates lying in the green-yellow region are invariant, with variation in pump power density and temperature. The observed results support the utility of the prepared tri-doped phosphors in optical temperature sensing, display devices and NIR to visible upconverters.