Branch-shaped NaGdF4:Eu3+ nanocrystals: Selective synthesis, and photoluminescence properties

The branch-shaped NaGdF4:Eu3+ nanocrystals (NCs) were synthesized by using polyvinylpyrrolidone (PVP) as a capping agent in ethylene glycol (EG) solution. The NCs were readily dispersed into water or ethanol to form a relatively stable suspension, which may facilitate their applications in biological fields. Meanwhile, the crystal structures of the NCs were tunable from the mixture of the α-(cubic) and β-(hexagonal) phases to the pure β-phase by varying the F−/Ln3+ molar ratio or the reaction temperature. The pure β-phase NCs were obtained at relatively high F−/Ln3+ molar ratio and reaction temperature. In addition, the Eu3+-doping concentration—dependent optical properties of the NaGdF4:Eu3+ NCs were investigated in detail. The result shows that the emissions from high energy level transitions (e.g., 5D1, 5D2, and 5D3) are significantly impaired with increasing the Eu3+-doping concentration due to the cross-relaxation process, and the emission at 612 nm is predominant since the doped Eu3+ ions locate in the crystal fields without inversion center.