【Domestic Papers】Flame-Made Cr³⁺-Doped Ga₂O₃ Nanoparticles Toward Cryogenic Thermometry and Near-Infrared LEDs

      Researchers from the National University of Defense Technology have published a dissertation titled "Flame-Made Cr³⁺-Doped Ga₂O₃ Nanoparticles Toward Cryogenic Thermometry and Near-Infrared LEDs" in ACS Applied Materials & Interfaces.

Project Support

      This work was supported by Huxiang Youth Talent Support Program (grant number2021RC3074).

Background

      Gallium oxide (Ga₂O₃), as an emerging ultrawide bandgap semiconductor, possesses high thermal stability and a high breakdown electric field, making it promising for power electronics and solar-blind photodetectors. Doping with Cr³⁺ ions enables broadband near-infrared emission and high-sensitivity temperature sensing, but conventional nanoparticle synthesis methods suffer from low efficiency, large particle size, and poor controllability. Flame spray pyrolysis (FSP) offers rapid high-temperature synthesis of high-purity, uniform nanoparticles, yet Cr³⁺-doped Ga₂O₃ NPs have not been reported. In this study, Cr³⁺-doped Ga₂O₃ NPs were synthesized via FSP, and their luminescence was significantly enhanced through high-temperature annealing, enabling applications in temperature sensing and near-infrared lighting.

Abstract

      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.

Conclusion

      In conclusion, Ga₂O₃:Cr³⁺ NPs were successfully synthesized by using the FSP method, and their NIR properties were significantly enhanced through high-temperature annealing. The underlying mechanisms were systematically investigated through a comparative analysis of TEM, XRD, and PLE results before and after annealing. The optical performance of Ga₂O₃:Cr³⁺ NPs at low temperatures was subsequently characterized, revealing an enhanced R-line emission intensity from the ²E → ⁴A₂ transition at cryogenic temperatures. The R₁ and R₂ emission lines, which originate from energy-level splitting, exhibited distinct temperature dependencies governed by the Boltzmann distribution. This phenomenon facilitated FIR-based temperature sensing, obtaining a high S_r value of 3.35% K⁻¹ at 83 K, indicating superior cryogenic sensing capability for Ga₂O₃:Cr³⁺ NPs. Additionally, these Ga₂O₃:Cr³⁺ NPs were integrated with commercial 450 nm LEDs to fabricate NIR pc-LEDs, which demonstrated the high performance of NIR luminescence as well as excellent performance in night vision and anticounterfeiting applications.