Structural and optical properties of Cr ion-doped near-infrared long persistent luminescence silicogermanate phosphors with broad emission bands

Near-infrared persistent luminescence materials have recently attracted considerable attention in the field of bio-imaging, food analysis, anti-counterfeiting, etc. However, the limited number of efficient emitters restricts their further development. Herein, a novel broadband near-infrared long persistent phosphor Na2CaGe5SiO14:xCr3+ was synthesized using a high-temperature solid-phase method. Its emission range is mainly within 600-900 nm, and the persistent luminescence duration of the phosphor with the optimal doping concentration is more than 12 h. The optical properties of the samples and the relationship between the crystal field and chromium ion concentration were systematically investigated by UV diffuse reflectance spectra as well as fluorescence spectra, and the occupancy of Cr3+ ions in the Na2CaGe5SiO14 matrix was probed. The energy transfer process between Cr3+ ions at different temperatures and the decay behavior at different emission wavelengths were further analyzed in conjunction with low-temperature spectra and photoluminescence decay curves. Finally, Cr ion-doped silicogermanate near-infrared long persistent luminescent materials were tested for biological tissue penetration, demonstrating their potential applications in bio-imaging.