Upconversion luminescence properties of Y2O2S:Er3+@Y2O2S:Yb3+,Tm3+ core-shell nanoparticles prepared via homogeneous co-precipitation

The Y2O2S:Er3+@Y2O2S:Yb3+,Tm3+ core-shell upconversion (UC) nanoparticles with average diameter of 95 nm were synthesized by the homogeneous co-precipitation method combining with the solid-gas sulfidation route. The increases of nanocrystaline size after the shell coating was observed both in the X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. This indicates the composition homogeneity core-shell Y2O2S nanocrystals. Meanwhile, the luminescence of both the Er3+ and Tm3+ ions are realized for the first time in the novel core-shell Y2O2S:Er3+@Y2O2S:Yb3+,Tm3+ nanoparticles under the excitations of both 980 and 1550 nm. When excited by a 980 nm laser diode, the Y2O2S:Er3+@Y2O2S:Yb3+,Tm3+ phosphor exhibits blue (approximate to 475 nm), green (approximate to 548 nm) and red (approximate to 670 nm) emissions in the visible region, which correspond to the (1)G(4) -> H-3(6) transition of Tm(3+)ions, S-4(3/2), H-2(11/2) -> I-4(15/2) and F-4(9/2) -> I-4(15/2) transitions of Er3+ ions, respectively. The very strong emission at the near infrared (NIR) region is mainly due to the H-3(4) -> H-3(6) transition of Tm3+ ions. The emission from both of Era} and Tm3+ ions under 980 nm excitation reveals the energy transfers of Yb3+ -> Tm3+ within the shell layer and Yb3+ -> Er3+ between the shell and the core. When pumping at 1550 nm, although only Er3+ ions can efficiently absorb the excitation energy, the strong UC emissions from Tm3+ ions were also observed. This is owing to the energy transfer between the core and the shell through Er3+ -> Yb3+ -> Tm3+ ions. (C) 2016 Elsevier B.V. All rights reserved.