Hydrothermal synthesis and up-conversion luminescence of Ho3+/Yb3+ co-doped CaF2

CaF2:Ho3+/Yb3+ nano-particles with intense green up-conversion (UC) luminescence are successfully synthesized via a facile hydrothermal approach by using NH4F as the fluoride source and Na2EDTA as a chelating reagent. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and UC emission spectra are used to characterize the structures, shapes, and luminescent properties of the samples. The effects from fluoride sources and chelating reagents on the formations of CaF2 nano-particles are investigated, and the formation process is also deduced. Under the excitation of a 980-nm laser diode, the samples each show a green up-conversion emission centered at 540 nm corresponding to the S-5(2)/F-5(4) -> I-5(8) transitions of Ho3+. Moreover, the UC mechanisms of Ho3+/Yb3+ co-doped CaF2 nano-particles are also discussed.