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

Abstract

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 CO₃ ²⁻ 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.