Multinuclear solid-state NMR spectroscopy of doped lanthanum fluoride nanoparticles

Multinuclear solid-state NMR spectroscopy and powder X-ray diffraction (XRD) experiments are applied to comprehensively characterize a series of pure and lanthanide-doped LaF3 nanoparticles (NPs) that are capped with di-n-octadectyldithiophosphate ligands (Ln(3+) = diamagnetic Y3+ and Sc3+ and paramagnetic Yb3+ ions), as well as correlated bulk microcrystalline materials (LaF3, YF3, and ScF3). Solid-state La-139 and F-19 NMR spectroscopy of bulk LaF3 and the LaF3 NPs reveal that the inorganic core of the NP retains the LaF3 structure at the molecular level; however, inhomogeneous broadening of the NMR powder patterns arises from distributions of La-139 and F-19 NMR interactions, confirming a gradual change in the La and F site environments from the NP core to the surface. La-139 and F-19 NMR experiments also indicate that low levels (5 and 10 mol %) of Ln(3+) doping do not significantly change the LaF3 structure in the NP core. Similar doping levels of paramagnetic Yb3+ ions severely broaden F-19 resonances, but only marginally effect La-139 powder patterns, suggesting that the dopant ions are uniformly distributed throughout the NP core and occupy vacant La sites. Measurements of La-139 T-1 and T-2 relaxation constants are seen to vary between the bulk material and NPs and between samples with diamagnetic and paramagnetic dopants. Sc-45 NMR experiments confirm that the dopants are integrated into the La sites of the LaF3 core. Solid-state H-1 and P-31 magic-angle spinning (MAS) NMR spectra aid in probing the nature of the capping ligands and their interactions at the NP surface. P-31 cross-polarization (CP)/MAS NMR experiments identify not only the dithiophosphate head groups but also thiophosphate and phosphate species which may form during NP synthesis. Finally, F-19-P-31 CP/MAS and H-1 MAS experiments confirm that ligands are coordinated to the NP surface.