The upconversion luminescence properties of the Yb3+–Ho3+ system in nanocrystalline Y2O2S

Near-spherical Y2O2S:Yb3+,Ho3+ 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 Ho3+ 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 Y2O2S:Yb3+,Ho3+ NCs markedly changes with Ho3+ ion concentration, and that the Ho3+ ion concentration quench is observed at 0.25 mol%. This value is only half as much as that in micron Y2O2S 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 Y2O2S:Yb3+,Ho3+ NCs throughout all Ho3+ doping concentrations used. However, the Ho3+ in micron Y2O2S usually exhibits weak red UCL. Infrared spectra confirm that this result is related to the large vibrational quanta produced by OH− and CO32− groups adsorbed onto the surface of NCs. These large vibrational quanta can remarkably increase the probability of 5S2–5F5 and 5I6–5I7 multiphonon relaxation, leading to the enhancement of red emissions arising from 5F5 → 5I8 transitions. The UCL mechanism of the Yb3+–Ho3+ system in nano- and microsized Y2O2S is also discussed.