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

We demonstrated an emulsion based wet chemical method for preparing Eu3+ doped cubic NaYF4 and hexagonal Na(Y1.5Na0.5)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(Y1.5Na0.5)F6 nanocrystals were prepared at 1:4 and 1:8 Y3+/F- ratios, respectively. The hexagonal Na(Y1.5Na0.5)F-6 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 (D-5(0) - 7F(2)) 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 degrees C, which was confirmed by XRD and TGA analysis.