Surface Eu3+ ions are different than “bulk” Eu3+ ions in crystalline doped LaF3 nanoparticles

Abstract

Distinct surface effects on the luminescence properties of highly crystalline LaF₃:Eu nanoparticles have been demonstrated, by incorporating different ligands on their surface as well as forming core–shell nanoparticles (i.e. doped LaF₃:Eu core with an undoped LaF₃ shell). These studies have established that the surface Eu³⁺ ions have a less symmetric crystal field than the “bulk”, and that they are responsible for the ligand induced changes in the asymmetric ratio, which is determined from the relative intensity ratio of the ⁵D₀ → ⁷F₂ (612 and 618 nm) and ⁵D₀ → ⁷F₁ (591 nm) transitions. This surface effect can very effectively be reduced by forming core–shell particles. The multi-exponential luminescent decay curves observed for these nanoparticles have been fitted using a three-parameter model involving the radiative decay constant of the nine inner shells (k_R), the radiative decay constant of the outermost shell (k_R¹⁰), and a parameter C, which describes the quenching, and is a function of the particular luminescent lanthanide ion, the type and amount of quenchers on or near the surface. The model accounts well for the observed changes in the asymmetric ratio of luminescence of these nanoparticles brought about by the ligands used to stabilise them.