Controlling the multicolor upconversion luminescence in CaF2 nanocrystals doped with Yb3+, Er3+ and Nd3+ ions under the excitation of a 808 nm laser

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Nd3+-sensitized nanoparticles have attracted attention recently due to their large absorption cross section and lower water absorption coefficient at 808 nm, which can potentially solve the laser-induced heating problem and is essential for labelling and imaging applications in living organisms. Here, we report a single-step hydrothermal synthesis of CaF2:Yb3+/Er3+/Nd3+ upconversion nanoparticles (UCNPs). The size of the as-prepared UCNPs decreases from ∼138 to ∼30 nm as the Nd3+ doping concentration increases from 0 to 3 mol%. Under the excitation of a 808 nm continuous-wave (CW) laser, these UCNPs exhibit typical UC emissions at 539 nm (green) and 656 nm (red) for Er3+, and the luminescence is strongest when Nd3+ doping concentration is 0.75 mol%. Moreover, with the increase of doped Nd3+ ions, the suppressing of green and the enhancing of red UC emissions can be observed, which leads to the luminescence color tuning from green to red. Further investigations suggest that the resonant cross-relaxations (CRs) between Er3+ and Nd3+ ions contribute to the UC luminescence color changing and the remarkable red UC emission for the highly Nd3+-doped UCNPs. These advances make the UCNPs potentially to be applied in vivo labelling, bioimaging and phototherapy.