Tuning the size and upconversion luminescence of NaYbF_4:Er^3+/Tm^3+ nanoparticles through Y^3+ or Gd^3+ doping

The upconversion emission tuning with size decrease has been achieved through Y3+ or Gd3+ ions doping in lanthanide-doped NaYbF4 nanoparticles prepared by a chemical coprecipitation method. With an increase of the Y3+ or Gd3+ doping concentrations (0−40 mol%), the nanoparticle size is continuously decreased from 250 nm down to 15.8 nm without phase transformation. Compared with Y3+ doping, the particle size decreases more rapidly via Gd3+ doping. Meanwhile, the intensity ratio of red emission to green emission in NaYbF4:2%Er3+ nanoparticles decreases monotonically with increasing Y3+ or Gd3+ doping concentrations, thus producing tunable upconversion emissions. For NaYbF4:2%Tm3+ nanoparticles, the luminescence intensity of the near-infrared upconversion emission at 800 nm was largely enhanced by about 36- and 17-fold for doping of 40 mol% Y3+ and 20 mol% Gd3+, respectively. These results offer an effective strategy to simultaneous control of the size and luminescence properties of lanthanide-doped upconversion nanoparticles.