| 1. |
- Khanin, Vasilii, et al.
(författare)
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Exciton interaction with Ce3+ and Ce4+ ions in (LuGd)3(Ga,Al)5O12 ceramics
- 2021
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Ingår i: Journal of Luminescence. - : Elsevier BV. - 0022-2313. ; 237
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Tidskriftsartikel (refereegranskat)abstract
- Scintillators based on Ce-doped garnets are regularly co-doped with Mg2+ or Ca2+ to form Ce ions in 4+ state and reduce undesired afterglow. However overly high Ce4+ concentration leads to poor light yield performance. In order to understand the reason for variation in luminescence efficiency of Ce3+- and Ce4+-doped garnets we investigate the differences in energy conversion processes in complex LuGd2Ga3Al2O12:Ce3+/Ce4+ ceramics by means of VUV synchrotron irradiation. At first we have established via transmission spectroscopy and X-ray absorption spectroscopy that LuGd2Ga3Al2O12:Ce, Mg sample contains cerium in the 4+ state only. Then we show with VUV spectroscopy efficient interaction of excitons with Gd3+ and Ce3+, and lack of exciton absorption edge in LuGd2Ga3Al2O12:Ce4+ excitation spectrum. Instead, Ce4+ exhibits charge-transfer absorption band in the range of exciton emission. We suggest that when Ce4+ concentration becomes too high, the exciton → Gd3+ → Ce3+ energy transfer path is hindered. It leads to high intensity of Gd3+ luminescence in Lu1Gd2Ga3Al2O12:Ce, Mg ceramics, but lowered Ce3+ X-ray excited luminescence. Fine balance between 3+ and 4+ Ce concentrations is necessary to achieve the best performance of garnet scintillators.
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| 2. |
- Vrubel, Ivan I., et al.
(författare)
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Oxygen vacancy in ZnO-w phase : pseudohybrid Hubbard density functional study
- 2020
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 32:31
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Tidskriftsartikel (refereegranskat)abstract
- The study of zinc oxide, within the homogeneous electron gas approximation, results in overhybridization of zinc 3d shell with oxygen 2p shell, a problem shown for most transition metal chalcogenides. This problem can be partially overcome by using LDA + U (or, GGA + U) methodology. However, in contrast to the zinc 3d orbital, Hubbard type correction is typically excluded for the oxygen 2p orbital. In this work, we provide results of electronic structure calculations of an oxygen vacancy in ZnO supercell from ab initio perspective, with two Hubbard type corrections, UZn-3d and UO-2p. The results of our numerical simulations clearly reveal that the account of UO-2p has a significant impact on the properties of bulk ZnO, in particular the relaxed lattice constants, effective mass of charge carriers as well as the bandgap. For a set of validated values of UZn-3d and UO-2p we demonstrate the appearance of a localized state associated with the oxygen vacancy positioned in the bandgap of the ZnO supercell. Our numerical findings suggest that the defect state is characterized by the highest overlap with the conduction band states as obtained in the calculations with no Hubbard-type correction included. We argue that the electronic density of the defect state is primarily determined by Zn atoms closest to the vacancy.
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