Tuning of crystal phase and luminescence properties of Eu³⁺ doped sodium yttrium fluoride nanocrystals

Abstract

We demonstrated an emulsion based wet chemical method for preparing Eu³⁺ doped cubic NaYF₄ and hexagonal Na(Y_1.5Na_0.5)F₆ nanocrystals. Here, we report the control of the crystal phase as a function of the Y³⁺/F^- ratio and pH of the solution. It was found that cubic NaYF₄ and hexagonal Na(Y_1.5Na_0.5)F₆ nanocrystals were prepared at 1:4 and 1:8 Y³⁺/F^- ratios, respectively. The hexagonal Na(Y_1.5Na_0.5)F₆ and cubic NaYF₄ phases appeared in the pH range of 6−7 and 2−3, respectively, indicating that the pH of the solution also plays an important role in tuning the crystal structure. The mechanism related to crystal phase control is proposed and discussed. It was also found that the emission intensity of the peak at 614 nm (⁵D₀ − ⁷F₂) for Eu³⁺ ions and decay time are sensitive to the crystal structure, which is again controlled by the Y³⁺/F^- ratio and pH of the solution. Again, it is seen that the interlayered water molecules strongly influence the photoluminescence of Eu³⁺ ions up to 300 °C, which was confirmed by XRD and TGA analysis.