A broadband near-infrared phosphor: Solid-state synthesis, two Cr3+ sites occupation and enhanced thermal stability

The development of highly efficient, thermally stable Cr³⁺-activated near-infrared (NIR) phosphors has received significant attention in recent years. However, the synthesis of these phosphors often leads to environmental and safety concerns due to the presence of toxic and corrosive HF. So, we have successfully developed an HF-free synthetic route to synthesize a series of Cs₂NaGa_1-ySc_yF₆:Cr³⁺ phosphors. By changing the ratio of Ga³⁺/Sc³⁺, the Cs₂NaGa_0.5Sc_0.5F₆:0.11Cr³⁺ (CNGSF:0.11Cr³⁺) phosphor with the strongest luminescence intensity was obtained. Two sets of NIR emission bands due to two different Cr³⁺ sites can be observed under blue light excitation. The phosphor exhibits a high internal quantum efficiency (IQE) of 66.7 % and good thermal stability. The luminescence intensity at 423 K maintains 88.7 % of the intensity at room temperature. A NIR light-emitting diode (LED) composed of the CNGSF:0.11Cr³⁺ phosphor and a blue InGaN chip was used as a NIR light source, and clearly visible photographs of the physical object and veins of the human hand were obtained. These images show that the phosphor has great potential for application in NIR LEDs for night vision and vein imaging.