NaGdF4:Yb, Er, Tm Upconversion Nanoparticles for Bioimaging in Shortwave-Infrared Range: Study of Energy Transfer Processes and Composition Optimization

Upconversion nanoparticles are promising for many applications. For triple-doped nanoparticles (NPs), the luminescence intensity shows a non-linear dependence on the rare-earth ion concentration, making it difficult to obtain bright phosphors with high energy output. We investigated the energy transfer processes in β-NaGdF4:Yb-Er-Tm NPs and considered strategies for increasing the thulium luminescence intensity, in particular, the use of core–shell structures. The luminescence spectra were analyzed in the short-wavelength infrared (SWIR) and visible (VIS) regions. The Er3+ and Tm3+ luminescence lifetimes in the VIS region were measured to study the energy transfer processes between the active ions. The quenching of the Tm3+ luminescence in the SWIR region was observed. However, both Er3+ and Tm3+ luminescence bands were observed in the VIS range. We attribute these effects to energy transfer between Tm3+ 3F4 → 3H6 and Er3+ 4I13/2 → 4I9/2, which occurs due to overlap of Er3+ and Tm3+ luminescence bands, and also to competition between Er3+ and Tm3+ for energy transfer from Yb3+. For core–shell NPs, when Tm3+ and Er3+ are separated into adjacent layers, quenching cannot be avoided, likely due to the mutual diffusion of ions during shell synthesis. The most optimal strategy to obtain luminescence in the SWIR range is to use an inert intermediate shell between the layers containing Tm3+ and Er3+.