Luminescent properties and energy level structure of CaZnOS:Eu 2+

In this work it is shown that CaZnOS:Eu 2+ has no Eu 2+ emission even at low temperature. The observed and earlier reported red emission originates from a CaS:Eu 2+ impurity phase. By means of washing the as-prepared samples with diluted nitride acid, we were able to remove the CaS impurity phase and study the Eu2+ emission in the pure CaZnOS phase. A clear relation was found between the red emission intensity, the CaS XRD line intensities and the nitric acid solution washing time, with zero intensity after prolonged washing. A so-called VRBE (vacuum referred binding energy)-diagram was constructed showing the energy of the 4f n and 4f n-1 5d 1 states of the divalent and trivalent rare earth ions as dopants in CaZnOS with respect to the vacuum energy. This diagram shows that the 5d-levels of Eu 2+ are located in the conduction band , which explains the absence of 5d→4f emission. By comparing the VRBE diagram with diagrams of other related compounds like CaO, CaS, ZnO and ZnS it becomes clear that the Eu 2+ luminescence quenching is caused by a low lying conduction band , typical for Zn-based compounds. • We found that the well reported Eu 2+ red emission in CaZnOS:Eu 2+ originates from a CaS:Eu 2+ impurity phase and Eu 2+ dopant does not show any luminescence in CaZnOS by washing with nitric acid solution. • We constructed the vacuum-referred binding energy level (VRBE) diagram by measuring luminescence excitation spectra of Eu 2+ , Eu 3+ and Yb 3+ doped, phase pure CaZnOS samples. • From the VRBE diagram, we found that Eu 2+ excited states are all in the conduction band , which explains the above phenomenon that is no Eu 2+ emission in CaZnOS. • By comparing the VRBE diagram with diagrams of other related compounds like CaO, CaS, ZnO and ZnS it becomes clear that the Eu 2+ luminescence quenching is caused by a low lying conduction band , typical for Zn-based compounds.