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Bierwagen J.,University of Geneva | Yoon S.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Gartmann N.,LumiNova AG | Walfort B.,LumiNova AG | Hagemann H.,University of Geneva
Optical Materials Express | Year: 2016

SrAl2O4 doped with europium and dysprosium is a powerful and widely used afterglow material. Within this material strontium is found in two crystallographic different sites. Due to the similar ion radii and same charge, Eu2+-ions can occupy both sites, resulting in two different Eu2+- ions, one emitting in the blue and one in the green spectral range. The blue emission is thermally quenched at room temperature. In this paper we investigate the energy transfer between different Eu ions depending on the concentration and temperature using two different approaches: lifetime measurements and integrated intensity. We find an activation energy for the thermal quenching of the blue emission of 0.195 ± 0.023 eV and a critical radius for the energy transfer of 3.0 ± 0.5 nm. These results can help in designing better afterglow materials due to the fact that with energy transfer parts of the lost emission in the blue region at room temperature can be converted to the green site. © 2016 Optical Society of America. Source


Yoon S.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Otal E.H.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Maegli A.E.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Karvonen L.,Empa - Swiss Federal Laboratories for Materials Science and Technology | And 8 more authors.
Optical Materials Express | Year: 2013

The phosphor CaTiO3: Pr3+ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH3. Comparatively large off-center displacements of Ti in the TiO6 octahedra were confirmed for as-synthesized CaTiO3: Pr3 by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH3 treatment are explained as a result of the reduced concentration of oxygen excesses with simultaneous relaxation of the lattice strain. © 2013 Optical Society of America. Source


Otal E.H.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Maegli A.E.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Vogel-Schauble N.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Walfort B.,LumiNova AG | And 4 more authors.
Optical Materials Express | Year: 2012

Red emitting CaTiO3:Pr phosphors with a nominal composition of Ca0.998+xPr0.002TiO3+δ (0.02≤x≤0.04) were prepared by solid state reactions with different thermal post treatments and characterized by X-ray diffraction, transmission electron microscopy and photoluminescence. The Ca excess exhibited complete solubility up to 4% in the samples treated at 1400 °C but segregation in the form of Ruddlesden-Popper phases (Ca3Ti2O7 - Ca4Ti3O10) was observed in samples prepared at 1500 °C. The increase in temperature for stoichiometric samples showed a monotonic increase of decay time due to the reduction of non-radiative recombination defects. It was found that the Ca excess favored the formation of oxygen vacancies which are known to act as trap. In the samples treated at 1400 °C, 3% of Ca excess showed to be the best concentration to increase the decay time of persistent luminescence. For the samples treated at 1500 °C, the segregation of Ruddlesden-Popper phases left a constant amount of Ca soluble in all the CaTiO3 samples. This constant concentration of Ca caused the same density of defects and, consequently, the same decay time in all samples. © 2012 Optical Society of America. Source


Yoon S.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Otal E.H.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Maegli A.E.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Karvonen L.,Empa - Swiss Federal Laboratories for Materials Science and Technology | And 7 more authors.
Journal of Alloys and Compounds | Year: 2014

Fluorine-substituted CaTiO3:Pr phosphors were prepared by a solid-state reaction. Rietveld refinements of powder X-ray diffraction patterns revealed that increasing fluorine-substitution leads to the gradual shrinkage of the unit-cell. Enhanced afterglow intensities were observed with fluorine-substitution. Furthermore, the effect of annealing atmosphere was investigated by thermochemical treatment in different atmospheres (Ar, air and NH3). UV-Vis diffuse reflectance spectra and photoluminescence excitation spectra revealed that Pr4+ in the pristine CaTi(O,F) 3:Pr phosphor was partially reduced to Pr3+ under NH 3 flow leading to an intensity improvement of ca. 450% compared to CaTiO3:Pr. The substantial improvement of afterglow intensity by fluorine substitution and annealing in NH3 is considered to be connected with the generation of oxygen vacancies and the partial reduction of Pr4+ to Pr3+. © 2014 Elsevier B.V. All rights reserved. Source


Hagemann H.,University of Geneva | Lovy D.,University of Geneva | Yoon S.,Empa - Swiss Federal Laboratories for Materials Science and Technology | Pokrant S.,Empa - Swiss Federal Laboratories for Materials Science and Technology | And 3 more authors.
Journal of Luminescence | Year: 2016

The persistent phosphorescence and thermoluminescence of SrAl2O4:Eu2+:Dy3+ is reported for a variety of different excitation wavelengths and excitation temperatures, to provide new insights in the mechanism of the trapping and detrapping. These measurements reveal that the trapping is strongly dependent on the wavelength and temperature. First, with increasing loading temperature, the thermoluminescence peak shifts to lower temperatures which corresponds to a change of trap population. Secondly, the integrated thermoluminescent intensity increases with increasing loading temperature. All wavelength and temperature dependent experiments indicate that the loading of the traps is a thermally activated processes. Utilizing different wavelengths for loading, this effect can be enhanced or reduced. Furthermore excitation with UV-B-light reveals a tendency for detrapping the phosphor, reducing the resulting thermoluminescent intensity and changing the population of the traps. © 2015 Elsevier B.V. Source

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