Blue and NIR Upconversion in Tm ³⁺ /Yb ³⁺ Co‐Doped Yttrium Pyrogermanate Phosphor for High‐Sensitivity Optical Thermometry

ABSTRACT Tm 3+ and Yb 3+ co‐doped yttrium pyrogermanate (Y 2 Ge 2 O 7 ) phosphors were synthesized via the conventional solid‐state reaction method. The structural properties, optical properties for upconversion luminescence, and optical thermometric properties were studied. Phase analysis using powder x‐ray diffraction confirmed the formation of a single‐phase tetragonal structure without any secondary phases. The scanning electron microscopy revealed irregular, agglomerated grains of micrometer size range. The diffuse reflectance spectroscopy showed characteristic absorption bands for absorption from the ground state 3 H 6 of Tm 3+ ions to excited states 3 F 2,3 (684 nm) and 3 H 4 (797 nm), as well as the 2 F 7/2 → 2 F 5/2 transition of Yb 3+ ions (∼980 nm), confirming successful incorporation of dopant ions into the host lattice. Photoluminescence spectra under 355 nm excitation exhibited strong blue emission at ∼453 nm ( 1 D 2 → 3 F 4 ), alongside weaker blue, red, and near infrared emissions at 475, 650, and 792 nm, respectively. The upconversion luminescence emissions under 980 nm excitation produced characteristic blue (∼475 nm; 1 G 4 → 3 H 6 ) and near infrared (∼797 nm; 3 H 4 → 3 H 6 ) bands. Notably, the blue emission showed a stronger power‐dependent enhancement compared to the near infrared emission, attributed to multiphoton energy transfer upconversion involving Yb 3+ sensitizers. The maximum intensity was observed in the 2% Tm 3+ doped sample co‐doped with 5% Yb 3+ , confirming efficient Yb 3+ →Tm 3+ energy transfer. Further, temperature‐dependent upconversion luminescence studies demonstrated thermally modulated emission intensity, with calculated activation energies of 0.35 eV (blue emission) and 0.43 eV (near infrared emission). Optical thermometry based on the ratio of emission intensities for 797 and 475 nm exhibited excellent sensitivity, with absolute sensitivity (S a ) of 156.42 × 10 −2 K −1 and relative sensitivity (S r ) of 13.4% K −1 at 673 K. These results highlight Y 2 Ge 2 O 7 :Tm 3+ /Yb 3+ as a promising phosphor for blue upconversion‐based photonic and high‐sensitivity ratiometric thermometry applications.