Dual center luminescence characteristics and multifunctional applications of Li2Ge7O15:Cr3+ near-infrared phosphors†

Abstract

Near-infrared (NIR) phosphors have been extensively studied in recent years due to their wide-ranging applications in plant growth lighting, night vision, and biomedical imaging. However, poor thermal stability has significantly limited their practical applications in many fields. This paper reports a NIR phosphor with dual emission centers, Li₂Ge₇O₁₅:Cr³⁺ (LG:Cr³⁺). Cr1 located at [GeO₆] produces narrow-line emission through the spin-forbidden ²E → ⁴A₂ transition, and as the temperature increases, electrons in the ²E level thermally migrate to the ⁴T₂ level, resulting in emission from the ⁴T₂ state. In contrast, Cr2 located at [LiO₆] generates broadband emission through the spin-allowed ⁴T₂ → ⁴A₂ transition, but this emission disappears due to thermal quenching effects as the temperature increases, making it undetectable at room temperature. The optical thermometry applications of the LG:Cr³⁺ phosphor were investigated using the fluorescence intensity ratio (FIR) technique, achieving a relative sensitivity (S_r) of 2.77% K⁻¹ at 100 K. Finally, a NIR phosphor-converted diode (NIR pc-LED) was fabricated using this phosphor and a 410 nm chip, demonstrating potential applications in biomedical imaging and night vision.