X-ray photoelectron spectroscopy study of CaS:Eu and SrS:Eu phosphors

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Abstract
The phosphors of alkali-earth sulfides SrS:Eux (x=0.05 and 0.10 mole per mole of SrS), SrSO:Eux (x=0.05), and CaS:Eux (x=0.0005, 0.05, and 0.10) were studied by x-ray photoelectron spectroscopy. The CaS:Eu phosphors were found to be stabilized against atmospheric water vapor and carbon dioxide by annealing at high temperature for several hours. The S 2p core level shows that the chemical state of sulfur changes abruptly from −2 to +6 between the annealing temperature of 900 and 1100 °C. The Ca core level, on the other hand, does not show much change depending on the annealing temperature. From the Eu 3d core-level spectra of SrS:Eux and CaS:Eux (x=0.10) phosphors, we confirm that the valence state of Eu in these phosphors is Eu3+.