Electron Beam-Driven Evolution in β-NaYF4 Nanoparticles with Non-uniform Cations: Phase Transitions and Luminescence

Despite remarkable progress in the synthesis of rare-earth nanomaterials, achieving precise control over the distribution of lanthanide ion (Ln3+) dopants remains a formidable challenge. Previous studies that utilized NaYF4 with mixed cation occupancy have offered valuable insights into nonuniform elemental doping and its influence on the emission properties of the materials. However, the complex interplay among cation distribution, structural transformation, and luminescent characteristics has not been fully understood and elucidated. This lack of comprehensive knowledge significantly hinders the optimization of the performance of nanomaterials. In this study, the structural transformation of individual β-NaYF4:Eu3+ nanoparticles under electron beam irradiation was systematically investigated. A discovery was made regarding the transition to a cubic phase. Elemental analysis of the pristine samples revealed that sodium was enriched at the edges of the nanoparticles. This sodium enrichment seemingly initiates the structural transition, which propagates from the surface toward the interior of the particles. Correlated cathodoluminescence electron microscopy (CCLEM) was employed in this research. The results confirmed that the emission intensity is highly correlated with the elemental distribution. Moreover, the structural disorder within the cubic phase leads to a decrease in the site symmetry compared to the theoretical Eu3+ luminescence. Overall, this study provides a more in-depth understanding of how elemental distribution impacts both the structural features and luminescent behavior of these materials. As a result, it makes a significant contribution to the development of optimized nanomaterials with enhanced performance.