Branch-shaped NaGdF 4:Eu 3+ nanocrystals: Selective synthesis, and photoluminescence properties

Abstract

The branch-shaped NaGdF 4:Eu 3+ 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 −/Ln 3+ molar ratio or the reaction temperature. The pure β-phase NCs were obtained at relatively high F −/Ln 3+ molar ratio and reaction temperature. In addition, the Eu 3+-doping concentration—dependent optical properties of the NaGdF 4:Eu 3+ NCs were investigated in detail. The result shows that the emissions from high energy level transitions (e.g., 5D 1, 5D 2, and 5D 3) are significantly impaired with increasing the Eu 3+-doping concentration due to the cross-relaxation process, and the emission at 612 nm is predominant since the doped Eu 3+ ions locate in the crystal fields without inversion center.