Effect of Cd2+ and K+ Co-Doping on Upconversion Emission in Y2O3:Yb3+/Er3+ Phosphors and Application in Optical Thermometry

The Cd2+ and K+ co-doped Y2O3:Yb3+/Er3+ upconversion phosphors were synthesized via the combustion route and analyzed for their crystal structure, spectroscopic properties, upconversion behavior, and optical thermometric capabilities. Powder X-ray diffraction analysis confirmed that the crystal structure of host remained intact, with varying lattice parameters observed: decreasing for lower concentrations of Cd2+ and K+ (1 mol %) and slightly increasing at 3 and 5 mol % doping. Scanning electron microscopy reveals the fine particulates of sizes around 100 nm in 5 mol % Cd2+ and K+ co-doped Y2O3:Yb3+/Er(3+)while without Cd2+ and K(+)co-doping, it shows agglomerated form with spherical and rod shaped particles.Spectroscopic analysis across the UV-visible-NIR spectral range (200-2000 nm) indicated that Cd2+ and K+ dopants did not alter the spectral characteristics of prepared upconversion phosphor Y2O3:Yb3+/Er3+. Under excitation of 980 and 932 nm diode lasers, the upconversion emission exhibited green and red bands attributed to Er3+ ions. The intensity of the red band was predominant under 980 nm excitation, whereas the green band showed higher emission intensities under 932 nm excitation. Cd2+ and K+ doping significantly influenced the emission intensities of both bands but did not affect the emission wavelengths, with intensities decreasing as dopant concentrations increased. The temperature-dependent upconversion emission intensity variation was studied for optical thermometric applications, revealing enhanced relative sensitivity in Cd2+ and K+ doped samples compared to undoped phosphor. The maximum relative sensitivity of 0.01455 K-1 at 303.15 K has been achieved with the 5 mol % Cd2+ and K+ co-doped upconversion phosphor, suggesting its potential for optical thermometry. Furthermore, evaluation of CIE color coordinates derived from upconversion emission spectra showed that Cd2+ and K+ doping had negligible impact on color chromaticity.