Color-tunable and single-band red upconversion luminescence from rare-earth doped Vernier phase ytterbium oxyfluoride nanoparticles

Rare-earth upconversion (UC) nanophosphors that can convert near-infrared excitations into the visible light region are desirable for various applications such as photonics, photovoltaics and biological imaging. The choice of host lattices and the doping concentration of rare-earth ions are crucial for intense UC outputs and selective emission profiles. Here, we report the facile fabrication and UC performance of Vernier phase ytterbium oxyfluorides (V-YbOF) as a promising host lattice for the first time. Multiple doping sites with low symmetries (C-1 and C-s) in the V-YbOF structure and the layered Yb sublattice allowing high concentration of the sensitizer Yb3+ are expected to give novel and preferable UC performance. Experimentally, pure and rare-earth (Ho3+, Er3+, and Tm3+) doped V-YbOF nanoparticles were synthesized via a sol-gel process followed by subsequent fluorination treatment using polytetrafluoroethylene as the fluridizer. Tunable UC emissions with various red/green ratios were achieved in Ho3+ doped V-YbOF nanoparticles under 980 nm excitation. Single-band red (B660 nm) UC luminescence and strong near-infrared emissions (centred at 700 and 805 nm) were observed in Er3+ and Tm3+ incorporated samples, respectively. The energy transfer mechanism was investigated by combined P-I measurements, lifetime evaluation and associated with the crystal structure feature. This work illustrates that Yb-rich compounds with low-dimensional structure features are also promising candidates as UC host lattices for a variety of applications.