Study of energy transfer processes between the rare earth ions in tri-doped upconversion nanoparticles NaGdF4:Yb-Er-Tm and NaGdF4:Yb-Er-Ho

Study of energy transfer processes between the rare earth ions in NaGdF₄ nanoparticles tri-doped with rare earth ions Yb³⁺, Er³⁺ and Ho³⁺ or Tm³⁺ was carried out. The luminescence spectra in short-wave infrared and visible ranges were investigated. It was shown that Er³⁺ → Ho³⁺ energy transfer leads to Ho³⁺ luminescence increase. Both Er³⁺ and Ho³⁺ luminescence peaks were observed in short-wave infrared range. For Er³⁺ and Tm3+ co-doped nanoparticles it was hard to separate luminescence peaks in SWIR. However, both Er³⁺ and Tm³⁺ luminescence peaks were observed in visible range. We attribute this effects to Tm³⁺ → Er³⁺ energy transfer which occurs due to overlap of Er³⁺ and Tm³⁺ luminescence bands in short-wave infrared range which leads to Tm³⁺ luminescence decrease. This hypothesis was confirmed by study of β-NaGdF₄ tri-doped with Yb³⁺, Er³⁺ and Tm³⁺ luminescence spectra during heating. The intensity of Tm³⁺ luminescence increased during heating due to non-resonant nature of Yb³⁺ →Tm³⁺ energy transfer and the shape of spectra changed.