Cr³⁺-doped near-infrared (NIR) emitting persistent luminescence (PersL) materials with an emission range of 650-1000 nm are renewable by red light instead of ultraviolet (UV) light, which is highly promising for renewable tissue imaging in vivo and broad application prospects in biomedical diagnosis and treatment. We have recently reported a BaGa₂O₄∶Cr³⁺ (BGO∶Cr) NIR emitting PersL material with Cr³⁺ as the luminescent center and BaGa₂O₄ as the matrix. The BGO∶Cr PersL material exhibited UV excitation, light-emitting diode (LED) light restimulation, ultra-long PersL for more than 6 days, and excellent capability for information storage. This study aims to develop BaGa₂O₄∶Cr³⁺, Sm³⁺ NIR emitting PersL materials with stronger luminescence intensity and longer emission wavelengths than BGO∶Cr PersL materials by co-doping BGO∶Cr PersL materials with Sm³⁺ ions for the development of medical multimodal imaging, medical detection probes, integrated diagnosis, and treatment probes.

In this study, BGO∶Cr, Sm PersL materials were synthesized using a high-temperature solid-state synthesis method. The effects of Sm³⁺ doping concentration and calcination temperature on the luminescent properties and crystal structure of BGO∶Cr, Sm PersL materials were investigated, and the NIR luminescence mechanism was discussed. The surface shape, element distribution mappings, and thermal stability of BGO∶Cr, Sm PersL materials were observed and analyzed.

In PersL material characterization, all X-ray diffraction peaks of the BGO∶Cr, Sm PersL materials were consistent with those of the BGO plane crystals (PDF 46-0415), the energy dispersive X-ray spectrometry spectra and element distribution mappings of BGO∶Cr, Sm PersL materials indicated the presence of Ba, Ga, O, Cr, and Sm elements, and transmission electron microscopy images show that the average particle length of the PersL materials is 1.61 μm and the average width is 0.76 μm (Fig. 1). The BGO∶Cr, Sm PersL materials exhibit strong NIR luminescence at 734 nm, with the BGO∶Cr0.06, Sm0.004 sample exhibiting the highest luminous intensity. Furthermore, the PersL intensity of BGO∶Cr PersL materials was enhanced after co-doped with Sm3+ ion [Figs. 2(a), (b)], and the afterglow time of the BGO∶Cr0.06, Sm0.004 PersL material is 131.61 s. According to the thermoluminescence measurements, the electron trap energy-level depth is estimated to be approximately 0.553 eV by the half-width method, showing that BGO∶Cr, Sm PersL materials are suitable for providing PersL for a long time at room temperature [Figs. 2(d),(e)]. A schematic energy diagram of the PersL mechanism shows that the NIR luminescence of BGO∶Cr, Sm is produced by the spin-forbidden 2E(2G)→4A2(4F) transition of Cr3+ and that PersL is produced by the recombination of holes and charge carriers released from the trap after stopping UV irradiation [Fig. 2(f)]. Meanwhile, our studies have shown that BGO∶Cr, Sm PersL materials exhibit good thermal stability, and the maximum luminous intensity at 150 ℃ shows their potential as raw materials for red LEDs (Fig. 4). Finally, we found that BGO∶Cr, Sm PersL materials exhibit good crystallinity and NIR luminescence only when the calcination temperature reaches 1100 °C (Fig. 5).

In summary, BGO∶Cr, Sm PersL materials with an emission wavelength of 734 nm were prepared using a high-temperature solid-state synthesis method. The luminescence intensity reached the maximum when the composition of the PersL materials was BGO∶Cr_0.06, Sm_0.004, and the PersL intensity of BGO∶Cr was enhanced by Sm³⁺ doping. The calcination temperature has a significant effect on the luminescence properties and crystal structure of BGO∶Cr, Sm PersL materials. BGO∶Cr, Sm PersL materials with high purity can be obtained when the annealing temperature is 1100 ℃. The electron trap energy-level depth of BGO∶Cr, Sm PersL materials is approximately 0.553 eV when estimated using the half-width method. BGO∶Cr, Sm PersL materials exhibit strong and persistent luminescence by co-doping and have potential applications in night vision surveillance and medical imaging.