Probing the Influence of Disorder on Lanthanide Luminescence UsingEu-Doped LaPO4 Nanoparticles

摘要

Lanthanide-doped nanocrystals (NCs) differ from their bulk counterparts due to their large surface to volume ratio. It is generally assumed that the optical properties are not affected by size effects as electronic transitions occur within the well-shielded 4f shell of the lanthanide dopant ions. However, defects and disorder in the surface layer can affect the luminescence properties. Trivalent europium is a suitable ion to investigate the subtle influence of the surface, because of its characteristic luminescence and high sensitivity to the local environment. Here, we investigate the influence of disorder in NCs on the optical properties of lanthanide dopants by studying the inhomogeneous linewidth, emission intensity ratios, and luminescence decay curves for LaPO4:Eu³⁺ samples of different sizes (4 nm to bulk) and core–shell configurations (core, core–isocrystalline shell, and core–silica shell). We show that the emission linewidths increase strongly for NCs. The ratio of the intensities of the forced electric dipole (ED) and magnetic dipole (MD) transitions, a measure for the local symmetry distortion around Eu³⁺ ions, is higher for samples witha large fraction of Eu³⁺ ions close to the surface. Finally,we present luminescence decay curves revealing an increased nonradiativedecay rate for Eu³⁺ in NCs. The effects are strongest incore and core–silica shell NCs and can be reduced by growthof an isocrystalline LaPO4 shell. The present systematicstudy provides quantitative insight into the role of surface disorderon the optical properties of lanthanide-doped NCs. These insightsare important in emerging applications of lanthanide-doped nanocrystals.