Crystal structures and luminescence properties of NaYF₄ microcrystals at different pH values

Luminescence properties of materials are closely related to their crystal structures and growth characteristics. In this study, a series of β-NaYF₄:20% Yb³⁺, 2%Er³⁺ (20 and2%represent the reactant mole percentage) microcrystals with different morphologies and sizes were synthesized using a hydrothermal route. The crystal phase, morphology, size, and upconversion luminescence properties of the as-prepared samples were characterized by x-ray diffraction, field emission scanning electron microscopy and photoluminescence analysis, respectively. The effects of the precursor solution pHvalue on the upconversion luminescence properties of the β-NaYF₄:Yb³⁺, Er³⁺ crystals were investigated in detail. The results indicate that the preferential growth plane of the β-NaYF₄ crystals was significantly influenced by the precursor pHvalues. Crystal plane characteristics affected the luminescence behavior of β-NaYF₄ in the red and green spectral range. In addition, reasonable control of the β-NaYF₄ crystal growth has a significant effect on the fluorescence lifetime.