Green HF-Free Synthetic Route to the High-Efficiency K2NaGaF6:Cr3+ Phosphor and Its NIR-LED Application toward Veins Imaging

Cr3+-activated phosphors with highly efficient broadband near-infrared (NIR) emission and superb thermal stability have been regarded as ideal candidates for next-generation NIR radiation sources. Nevertheless, the requirements of toxic and concentrated HF during syntheses of fluoride phosphors could lead to dramatic environmental and safety concerns. In this work, we developed a feasible HF-free green synthesis method to prepare K2NaGaF6:Cr3+ with high-efficiency NIR broadband emission using an aqueous solution instead of HF. The X-ray diffraction analyses and microscopy images all suggested the successful synthesis of the target materials without impurities. Theoretical simulations based on the density functional theory have also indicated a sufficient band gap of K2NaGaF6 as the host matrix for the Cr3+ activator. The luminescent properties of K2NaGaF6:Cr3+ were investigated in detail, where the optimum Cr3+ doping concentration was found at ∼6 mol %. Moreover, the temperature-dependent spectra revealed an excellent thermal stability of K2NaGaF6:Cr3+, where the luminescent intensity at 150 °C can retain as much as 91% of the initial value at the room temperature. These characteristics make K2NaGaF6:Cr3+ an ideal phosphor for phosphor-converted NIR light-emitting diodes. Therefore, device fabrication was carried out for venography demonstration, which shows superb imaging performance of human vessels and tissues.