Emergence of Unusual Cr ³⁺ Emission and Exceptional Thermal Stability in Ca ₃ Ga ₂ Ge ₃ O ₁₂ : Cr ³⁺ /Yb ³⁺ Garnet Phosphors for High-Performance Optical Thermometry and NIR pc-LEDs

Highly efficient near-infrared (NIR) phosphors are essential for the development of next-generation smart NIR light sources. Cr 3+ -activated NIR phosphors have garnered increasing attention due to their remarkable photoluminescence properties for NIR pc-LEDs. However, the scarcity of intense blue-light-pumped broadband NIR luminous materials and the low thermal stability of existing Cr 3+ activated phosphors are the main obstacles for the development of next-generation NIR phosphor-converted light-emitting diodes (pc-LEDs). Herein, a Ca 3 Ga 2 Ge 3 O 12:Cr 3+, Yb 3+ phosphor with NIR emission centered at about 1028 nm with an effective Cr 3+ →Yb 3+ energy transfer, used to achieve an excellent 14 times increase in emission under blue-light excitation (469 nm), is investigated. In addition, the NIR-I emission corresponding to the 2 F 5/2 → 2 F 7/2 transition of Yb 3+ shows excellent thermal stability, maintaining 91% of the room temperature intensity at 500 K. Furthermore, the Cr 3+ transition, 4 T 2 → 4 A 2, shows a sudden emergence at about 200 K following antithermal quenching. Owing to the diverse thermal quenching of the Yb 3+ and Cr 3+ emissions, the fluorescence intensity ratio (FIR) was measured, and a maximum relative temperature sensitivity of 2.53% K –1 at 200 K was observed, offering a wide range of thermometry between 200 and 500 K. The highly efficient and thermally robust luminescence of this material is mainly ascribed to the wide band gap of the host, extremely weak electron–phonon coupling effect with Huang–Rhys factor, S = 0.37, and high activation energy, Δ E = 0.23 eV. Further, Ca 3 Ga 2 Ge 3 O 12:Cr 3+,Yb 3+ phosphor was used to fabricate a NIR pc-LED with an output power of 3.7 mW under a driving current of 500 mA. This investigation successfully presents the Ca 3 Ga 2 Ge 3 O 12:Cr 3+,Yb 3+ phosphor as a highly efficient and thermally robust NIR emitter for realizing optical thermometers with high sensitivity and for next-generation blue-light-pumped pc-LEDs.