Photoluminescence properties of the Eu³⁺-doped ZnS nanocrystals and the crystal-field analysis

The Eu³⁺-doped ZnS nanocrystals prepared by the chemical method have a cubic structure. All the transitions ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4, 5, 6) of Eu³⁺ were observed in ZnS nanocrystals. In particular, the forbidden transitions of ⁵D₀ → ⁷F₀, ⁵D ₀ → ⁷F₃ and ⁵D₀ → ⁷F₅ were all observed and their observed values of energies were found to match well with those predicted from a complete diagonalization of the crystal-field Hamiltonian for Eu³⁺. The photoluminescence of the transition ⁵D₀ → ⁷F₁ confirms the crystal-field analysis that in the first order approximation the crystal field has no effect on the multiplet ⁷F₁. The crystal-field analysis shows that the multiplet ⁷F₂ has a doubly degenerate sublevel and a triply degenerate sublevel, which also agrees with the photoluminescence results very well. Although not all sublevels of crystal levels of multiplet ⁷F₄ were observed in the photoluminescence of the transition ⁵D₀ → ⁷F₄, the crystal-field parameter was estimated from the width of the emission band of this transition. In accordance with the superposition model, the crystal-field parameters and were fitted to be -59.8 meV and 41.6 meV, respectively.