Morphology-Tuned Photoluminescence in Eu ³⁺ Doped β-NaYF ₄ Microcrystals Prepared by a Facile Soft-Templated Route

NaYF4 is commonly recognized as a superior host lattice for luminescent materials due to its low phonon energy, high chemical stability, and polymorphic flexibility. In particular, the hexagonal β-NaYF4 phase is preferred for both up- and down-conversion optical applications. Here, we report a facile microemulsion-based, low-temperature (ranging from 60 to 90 °C) synthesis of β-NaYF4 doped with Eu3+ ions. Unlike conventional high-temperature methods or ligand-assisted low-temperature techniques, our approach enables phase-pure β-NaYF4 formation without requiring postsynthetic treatments. The synthesis temperature significantly influences the resulting morphology, with the microcrystals transitioning from spindles to ellipsoids and, finally, hexagonal rods. These morphological changes are systematically correlated with the photoluminescence characteristics of Eu3+, including emission intensity, asymmetry ratio (5D0 → 7F2/5D0 → 7F1), and emission lifetime. The results demonstrate how the synthesis temperature in the microemulsion reaction media can effectively tailor anisotropic growth and local asymmetry in β-NaYF4, which in turn affects the radiative and nonradiative energy transfer from the Eu3+ sites.