The upconversion luminescence properties of the Yb~(3+)-Ho ~(3+) system in nanocrystalline Y_2O_2S

摘要

Near-spherical Y_2O_2S:Yb~(3+),Ho~(3+) nanocrystals (NCs) with an average particle size of 40 nm were synthesized by the coprecipitation method followed by a solid-gas sulfuration technique. The effects of the Ho~(3+) ion doping concentration on the upconversion luminescence (UCL) property of the NCs was studied through the UCL spectra. Results show that the UCL intensity of Y_2O_2S:Yb ~(3+),Ho~(3+) NCs markedly changes with Ho~(3+) ion concentration, and that the Ho~(3+) ion concentration quench is observed at 0.25 mol%. This value is only half as much as that in micron Y _2O_2S prepared by a solid state reaction, which can be attributed to the distinct diffusion mechanism of activator ions in the coprecipitation process. In addition, strong red emissions can be observed in Y_2O_2S:Yb~(3+),Ho~(3+) NCs throughout all Ho~(3+) doping concentrations used. However, the Ho~(3+) in micron Y_2O_2S usually exhibits weak red UCL. Infrared spectra confirm that this result is related to the large vibrational quanta produced by OH~- and CO_3~(2-) groups adsorbed onto the surface of NCs. These large vibrational quanta can remarkably increase the probability of ~5S_2-~5F_5 and ~5I_6-~5I_7 multiphonon relaxation, leading to the enhancement of red emissions arising from ~5F _5 → ~5I_8 transitions. The UCL mechanism of the Yb~(3+)-Ho~(3+) system in nano- and microsized Y _2O_2S is also discussed.