Quantitative assessing of crystal field, nephelauxetic, and Stokes shift effects on the blue luminescence of Eu 2+ ions incorporated in ZnS films

A quantitative study of the crystal field, nephelauxetic, and Stokes shift effects on the 5d → 4 f levels of Eu 2+ ions incorporated in ZnS films is presented. The magnitude of the crystal effect was calculated using the simplest point charge model to express the crystal field splitting of Eu 2+ 5 d orbitals surrounded by four S 2− ligands in tetrahedral coordination symmetry. The shift of the centroid of the 5 d levels by nephelauxetic effect was analyzed using a model that involves the polarizability of the anion ligands, similar to that used to investigate this effect for Ce 3+ in many inorganic compounds. The Stokes shift was evaluated according to the Frank-Condon principle, under the idealized situation where the Stokes shift and the emission bandwidth can be expressed in terms of the Huang-Rhys parameter. We found that the redshift ϵ c f s = 0.32 eV of the 5d → 4 f transition for the free Eu 2+ ion, due to crystal field splitting was half of the redshift due to the nephelauxetic effect. This result is consistent with the large size, large polarizability, and low electronegativity of the sulfide ions. We also found a relatively large Stokes shift which can be attributed to the autoionization of the 5d electrons to the conduction band levels. The total redshift of the 5 d and 4 f energy levels with respect to the 5 d and 4 f energy gap for a free Eu 2+ in vacuum fits quite well with the blue luminescence peaked at 2.73 eV for Eu 2+ ions incorporated in the ZnS films. Based on the experimental results and the calculations for the crystal field splitting, nephelauxetic, and Stokes shift effects, we also propose a simplified energy band and levels diagram which helps to explain the excitation-emission mechanisms of the blue luminescence of the Eu 2+ ions incorporated in the ZnS films.