On the nature of carrier relaxation and ion-ion interactions in ultrasmall β-NaYF4:Eu3+ nanocrystals - effect of the surface

The purely hexagonal phase of ultrasmall (similar to 10 nm) NaYF4 nanocrystals (NCs), containing different Eu concentrations, has been obtained by a modified co-thermolysis method. Detailed investigations of the excitation and relaxation mechanisms of the Eu ions in such NCs are reported. Based on the photoluminescence excitation, absorbance, photoluminescence and emission decay times measured as a function of the excitation wavelengths, it has been shown that two Eu sites with different excitation and relaxation characteristics are present in the case of ultrasmall NaYF4 NCs. It has been shown that, when the Eu concentration increases, strong ion-ion interactions influence the relaxation phenomena in Eu ions, changing their optical properties. Moreover, these ion-ion interactions enable connections between the surface ions and the internal ones via energy transfer from the surface to the NCs core. Furthermore, it has been proposed that the different kinetic properties of the surface Eu ions are mainly caused by the formation of a charge transfer state between the ions and ligand groups attached to the NCs surface.