Shells of crystal field symmetries evidenced in oxide nano-crystals.

By the use of a point charge model based on the Judd-Ofelt transition theory, the luminescence from Eu(3+) ions embedded in Gd(2)O(3) 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 an 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 the 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 Y (2)O(3) and Gd(2)O(3), these results also apply to Eu(3+) doped Y(2)O(3) nanoparticles.