Synthesis of Small Ce³⁺-Er³⁺-Yb³⁺ Tri-Doped BaLuF₅ Active-Core-Active-Shell-Active-Shell Nanoparticles with Strong...

Small fluoride nanoparticles (NPs) with strong down-conversion (DC) luminescence at 1.5 μm are quite desirable for optical fiber communication systems. Nevertheless, a problem exists regarding how to synthesize small fluoride NPs with strong DC emission at 1.5 μm. Herein, we propose an approach to improve 1.5 μm emission of BaLuF₅:Yb³⁺,Er³⁺ NPs by way of combining doping Ce³⁺ ions and coating multiple BaLuF₅: Yb³⁺ active-shells. We prepared the BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs through a high-boiling solvent method. The effect of Ce³⁺ concentration on the DC luminescence was systematically investigated in the BaLuF₅:Yb³⁺,Er³⁺ NPs. Under a 980 nm laser excitation, the intensities of 1.53 μm emission of BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺ NPs was enhanced by 2.6 times comparing to that of BaLuF₅:18%Yb³⁺,2%Er³⁺ NPs since the energy transfer between Er³⁺ and Ce³⁺ ions: Er³⁺:⁴I_11/2 (Er³⁺) + ²F_5/2 (Ce³⁺) → ⁴I_13/2 (Er³⁺) + ²F_7/2 (Ce³⁺). Then, we synthesized BaLuF₅:18%Yb³⁺,2%Er³⁺,2%Ce³⁺@BaLuF₅:5%Yb³⁺@BaLuF₅:5%Yb³⁺ core-active-shell-active-shell NPs via a layer-by-layer strategy. After coating two BaLuF₅:Yb³⁺ active-shell around BaLuF₅:Yb³⁺,Er³⁺,Ce³⁺ NPs, the intensities of the 1.53 μm emission was enhanced by 44 times compared to that of BaLuF₅:Yb³⁺,Er³⁺ core NPs, since the active-shells could be used to not only suppress surface quenching but also to transfer the pump light to the core region efficiently through Yb³⁺ ions inside the active-shells.