Structure of NaYF4 Upconverting Nanoparticles: A Multinuclear Solid-State NMR and DFT Computational Study

The exceptional upconverting properties of lanthanide-doped nanoparticles make them attractive systems with applications ranging from photovoltaics to biological labeling, imaging, and therapeutics. While they draw considerable interest, structural data, which are necessary to understand the upconversion process, remain scarce. In this work, we demonstrate the use of Na-23, F-19, and Y-89 solid-state NMR together with DFT calculations to characterize the structure of cubic NaYF4 nanoparticles with and without Er3+ doping. By measuring Na-23 MAS NMR spectra at various magnetic fields and 3QMAS spectra at ultrahigh field, we show that the spectra are characteristic of a solid solution in which cation sites are statistically occupied by Na+ or Y3+ ions. The Na-23 NMR spectra are broadened as a result of isotropic chemical shift distribution, whereas the extracted quadrupolar products appear to be small (<= 1.8 MHz), which is in good agreement with DFT calculations using CASTEP. The chemical shift distribution in F-19 NMR spectra is well-predicted by CASTEP calculations and shown to strongly depend on coordination by Y3+. Finally, the Y-89 NMR spectra consist of a single broad pattern, which also results from a chemical shift distribution that can be correlated to the coordination environment of the Y3+ cations. Our results show that the structure is a slightly distorted cubic phase and lanthanide doping has only a minor effect on the lattice parameters. The approach appears to be promising for gaining additional insight into the atomic level structure details to better understand properties that govern the upconversion process and its efficiency.