Shells of crystal field symmetries evidenced in oxide nano-crystals

TL;DR

This study demonstrates that surface-induced symmetry breaking significantly influences the energy level splitting of Eu3+ ions in oxide nanocrystals, with observable effects predicted even considering size dispersion.

Contribution

It introduces a point charge model based on Judd-Ofelt theory to analyze surface effects on Eu3+ luminescence in oxide nanoparticles, highlighting the role of geometry without disorder.

Findings

Surface symmetry breaking affects energy level splitting.

Predicted observable modifications in realistic size dispersion conditions.

Estimated crystal field Stark effect extent of about 0.5 nm.

Abstract

By the use of a point charge model based on the Judd-Ofelt transition theory, the luminescence from Eu3+ ions embedded in Gd2O3 clusters is calculated and compared to the experimental data. The main result of the numerical study is that without invoking any other mechanisms such as crystal disorder, the pure geometrical argument of the symmetry breaking induced by the particle surface has influence on the energy level splitting. The modifications are also predicted to be observable in realistic conditions where unavoidable size dispersion has to be taken into account. The emission spectrum results from the contribution of three distinct regions, a cluster core, a cluster shell and a very surface, the latter being almost completely quenched in realistic conditions. Eventually, by detailing the spectra of the ions embedded at different positions in the cluster we get an estimate of about 0.5 nm for the extent of the crystal field induced Stark effect. Due to the similarity between Y2O3 and Gd2O3, these results apply also to Eu3+ doped Y2O3 nanoparticles.