Local Structure of Rare-Earth Fluorides in Bulk and
Core/Shell Nanocrystalline Materials
- Publication date
- Publisher
Abstract
NaREF<sub>4</sub> (rare-earth, RE = Y, Lu) nanocrystals (NCs) containing
Ln<sup>3+</sup>-dopants are of great interest due to their unique
ability to downshift, downconvert, and upconvert light. While the
luminescent properties and microscale structures of such NCs are well
understood, relatively little is known about their molecular-level
structures, the nature of the NC cores and shells, and the interactions
of the stabilizing ligands at the NC surface. To address these issues,
we present a comprehensive characterization of bulk β-NaYF<sub>4</sub>, one of the most commonly used hosts for Ln<sup>3+</sup>-dopants
in luminescent NCs. Using a combination of powder X-ray diffraction
(pXRD) and multinuclear (<sup>89</sup>Y, <sup>23</sup>Na, <sup>19</sup>F) solid-state nuclear magnetic resonance (SSNMR) spectroscopy, we
provide conclusive evidence that this compound crystallizes in a hexagonal <i>P</i>6<sub>3</sub>/<i>m</i> structure, resolving a
longstanding debate. The structure of this bulk form is related to
the structure of NaYF<sub>4</sub>/NaLuF<sub>4</sub> core/shell NCs.
From the similarities between the <sup>19</sup>F and <sup>23</sup>Na SSNMR spectra of the bulk and NC materials, it is concluded that
the NCs have the same β-NaYF<sub>4</sub> and β-NaLuF<sub>4</sub> phases as the bulk compounds. A series of cross-polarization
NMR experiments confirm the presence of oleates on the surface of
the particle via their proximity to surface Na<sup>+</sup> sites