Flame-Made Cr³⁺-Doped Ga₂O₃ Nanoparticles Toward Cryogenic Thermometry and Near-Infrared LEDs

As an emerging ultrawide bandgap semiconductor, Ga₂O₃ nanoparticles (NPs) have garnered significant attention across various fields due to their high thermal and chemical stability. Conventional synthesis methods for Ga₂O₃ NPs, such as high-temperature solid-state reactions and hydrothermal processes, encounter challenges including complex procedures, limited scalability, and production of large particle sizes. In this study, we develop a straightforward approach to synthesize phase-pure, nanosized Ga₂O₃ NPs through flame spray pyrolysis, followed by an annealing treatment. By the incorporation of trivalent chromium ions (Cr³⁺) into the Ga₂O₃ NPs, it achieves intensive near-infrared (NIR) luminescence, specifically the R lines associated with the ⁴T₂ → ⁴A₂ and ²E → ⁴A₂ transitions. These R lines are sensitive to cryogenic temperatures due to their relatively small energy difference. The synthesized Ga₂O₃ NPs demonstrate excellent ratiometric thermometric performance within the temperature range of 83-213 K, achieving a maximum relative sensitivity of 3.35% K^-1 at 83 K based on the Boltzmann distribution of the luminescent thermometer. Furthermore, an efficient NIR phosphor-converted LED (pc-LED) device is fabricated by employing Cr³⁺-doped Ga₂O₃ NPs, which are excited by using a blue LED chip. This NIR pc-LED is subsequently applied in night vision illumination and anticounterfeiting detection.