Development of NaYSiO:Yb phosphors with high thermal stability for NIR anti-counterfeiting: study of its crystal structure and luminescent properties.

Near-infrared (NIR) radiation has generated considerable industrial and research interest. However, NIR phosphors for this are limited by low quantum efficiency and broad spectra. Rare-earth-containing compounds doped with activators as host systems for NIR phosphors may resolve these limitations. Yb-doped NaYSiO phosphors were synthesized using a conventional solid-state reaction method. The main phase of the synthesized phosphor samples exhibited a hexagonal structure NaYSiO phase, and had an angular-shape with an average grain size of 1-3 μm. The NaYSiO:Yb phosphors showed a near-infrared emission from 950 to 1100 nm, which was attributed to the F → F transition of Yb ions under 270 and 920 nm excitation. The excitation spectra, recorded by monitoring the emission at 985 nm, showed two bands in the ultraviolet and infrared regions, which correspond to the charge transfer transition and the F → F transition of Yb ions. At 300 °C, the emission intensity of the NaYSiO:Yb phosphor remained constant at 82%. Furthermore, the thermal degradation was negligible after cooling, suggesting the possibility of application in advanced anti-counterfeiting applications.