Construction of CaSnO₃:Bi²⁺ NIR Long-Persistent Luminescent Material with Trap Level Up-Conversion and Bi²⁺ Concentration Effect

This study focuses on synthesizing and characterizing CaSnO₃:Bi²⁺ near-infrared (NIR) long persistent luminescent materials. NIR persistent luminescent materials enable safe and efficient imaging due to their deep tissue penetration and reduced phototoxicity compared to ultraviolet (UV) excited materials. The CaSnO₃:Bi²⁺ materials were synthesized using a high-temperature solid-state method. The effects of varying Bi²⁺ doping concentrations (1, 2, 5, 7, and 10%) on the material’s properties were systematically investigated. The synthesis process was confirmed by X-ray diffraction (XRD) analysis, revealing a perovskite structure for all samples. Scanning electron microscopy (SEM) analysis indicated uniform particle sizes of approximately 1 μm, successfully incorporating Bi²⁺ ions confirmed by energy-dispersive X-ray spectroscopy (EDS). The luminescent properties of the CaSnO₃:Bi²⁺ materials were characterized using fluorescence spectroscopy and thermoluminescence spectroscopy. The excitation and emission spectra showed peaks at 260, 620, and 680 nm, corresponding to the transitions of Bi²⁺ ions. The samples exhibited NIR persistent luminescence under 260 nm excitation, with the CaSnO₃:5% Bi²⁺ sample demonstrating the highest phosphorescence intensity and longest decay time. This optimal performance was attributed to the highest trap concentration, confirmed by thermoluminescence spectroscopy. The persistent NIR luminescence of the CaSnO₃:Bi²⁺ materials was attributed to trap level up-conversion, a phenomenon where NIR excitation leads to NIR emission without the involvement of up-conversion materials. This mechanism arises from the reverse carrier transition from deep traps (DTs) to shallow traps (STs). Thermoluminescence spectroscopy further confirmed the occurrence of trap level up-conversion in the CaSnO₃:5% Bi²⁺ sample. The successful synthesis and characterization of CaSnO₃:Bi²⁺NIR long persistent luminescent materials with trap level up-conversion mechanisms opens up new avenues for their application in various fields.