Influence of crystal phase and excitation wavelength on luminescence properties of Eu³⁺-doped sodium yttrium fluoride nanocrystals

Abstract

Here we report the preparation of Eu³⁺-doped cubic NaYF₄ and hexagonal Na(Y_1.5Na_0.5)F₆ nanocrystals and the crystal phase is modified by temperature of heating and varying the Y³⁺/F^- ratio. The volume fraction of the cubic phase decreases from 100 to 55% with increasing the temperature from 80 to 400 °C, and the lattice strain can be modified by changing the crystal phase. The PL intensity, decay time, and quantum efficiency are found to be sensitive to the crystal phase and excitation wavelength. The PL intensity of hexagonal phase sample is stronger than the cubic phase sample under direct excitation and reversal result is obtained under charge transfer (CT) excitation. Under 394 nm excitation, the decay time increases from 5.92 to 7.58 ms with changing the crystal phase from cubic to hexagonal. It is interesting to note that the decay time also varies with changing the excitation wavelength. The quantum efficiency value increases from 4.3 to 67.4% with changing from charge transfer excitation to direct excitation, and this value can be tuned with changing the crystal phase. Analysis suggests that the modification of the quantum efficiencies of the rare-earth doped sodium yttrium fluoride nanocrystals can be done by changing the crystal phase and excitation wavelength.