Dr. Bernhard Walfort Profile

Dr. Bernhard Walfort

at RC Tritec AG

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Proceedings Article | 1 April 2020 Paper

Characterization and afterglow of SrAl2O4:Eu,Dy for various phosphor applications

T. Delgado, J. Bierwagen, N. Gartmann, B. Walfort, I. Kinski, M. Pollnau, H. Hagemann

Proceedings Volume 11357, 113571C (2020) https://doi.org/10.1117/12.2558394

KEYWORDS: Europium, Dysprosium, Ions, Luminescence, Strontium, Energy transfer, Oxygen, Magnesium, Temperature metrology, Crystals

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SrAl₂O₄ that is optically activated by Eu²⁺, often additionally co-doped with Dy³⁺, is a non-radioactive persistent phosphor which is known for its excellent afterglow properties. It has found various applications, e.g. in the watch industry, for security signs, in medical diagnostics, and in photovoltaics. The monoclinic SrAl₂O₄ was synthesized in polycrystalline form and structurally characterized. Its luminescence and afterglow properties were studied. Wavelength-dependent thermoluminescence experiments were performed on SrAl₂O₄:Eu and SrAl₂O₄:Eu,Dy polycrystalline samples. Substitution of Sr²⁺ by Eu²⁺ on two different Sr sites in the crystal is associated with blue and green Eu²⁺ emission. Excitation at 445 nm allows to selectively excite one of the two different Eu²⁺ ions, whereas excitation at 375 nm excites both Eu²⁺ ions. Incorporation of dysprosium increases significantly (by a factor of about 4 to 8) the total number of traps involved in the afterglow of this persistent phosphor. Increasing the temperature at which the samples are irradiated (loaded) from 173 K to 248 K reveals that many new traps can only be occupied or activated at higher temperatures, leading to a strong increase of the integrated thermoluminescence intensity, in particular for the Dy-codoped samples. The results of this study reveal that the diversity of traps leading to the long afterglow is much larger than previously reported in the literature. We propose that the presence of dysprosium induces an excitation-induced charge-transfer reaction Eu²⁺ + Dy³⁺ → Eu³⁺ + Dy²⁺. However, the principal traps responsible for the efficient afterglow are temperature-activated and appear to be associated with the green-emitting Eu²⁺ ion on the Sr2 site coupled to a nearby dysprosium ion.

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