Identification and elimination of surface emission in lanthanide (co-)doped zirconia nanocrystals

Zirconia nanocrystals (ZrO2 NCs) are a stable host material for lanthanides, but their performance is lagging behind the leading NaYF4 nanomaterials. Here we leverage surface chemistry and core/shell architectures to uncover the contribution of dopants at the nanocrystal surface and of dopants in the nanocrystal bulk. We first assess the doping efficiency by ICP and find that, while Eu is almost quantitatively incorporated, the other lanthanides (La, Ce, Tb, Tm, Er, Yb) have about 50% incorporation efficiency over the studied doping range of 1-10%. We then determine the nanocrystal surface chemistry using NMR spectroscopy, despite the additional spectral line broadening caused by the paramagnetic lanthanide dopants. By varying the surface ligands and measuring the photoluminescence, we resolve the spectroscopic signals that are sensitive to a change in surface chemistry. Time-resolved emission spectra further reinforce the notion of a bulk component with a long luminescent lifetime and a surface component with a fast lifetime. Upon shelling Eu- or Tb-doped zirconia NCs with pure zirconia, the surface component disappears, and the photoluminescence quantum yield increases. We further functionalized the surface of the core/shell particles with oleylphosphonic acid ligands to obtain excellent dispersibility. These results show that lanthanide-doped zirconia NCs can be engineered to eliminate deactivation pathways.