PHOTOLUMINESCENCE OF CaxBa1–xGa2S4 SOLID SOLUTIONS ACTIVATED BY Eu2+ IONS (original) (raw)
We have studied the photoluminescence (PL) of CaxBa1–xGa2S4 solid solutions (x = 0.1, 0.2, 0.3, 0.4, and 0.5) activated by ions of the rare-earth element Eu2+ in the temperature range 10–300 K, due to 4f65d → 4f7 transitions in the Eu2+ ions. The photoluminescence spectra of CaxBa1–xGa2S4:Eu are located in the blue-green – green regions, and their maxima are shifted from 506 nm for x = 0.1 to 531 nm for x= 0.5. We show that the long-wavelength shift as Ca content increases is due to the increasing crystal fi eld, in the resence of which the 5d orbitals of the europium ions are split and consequently the energy of the recombination transitions decreases. We observed high temperature stability of the position of the photoluminescence spectra and the integrated intensity (decreasing by 13% for all x) in the temperature range 10–300 K. We have established that an increase in the Ca content in CaxBa1–xGa2S4:Eu leads to a decrease in the thermal shift of the photoluminescence, and for x = 0.5 there is no shift.
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The detailed investigation of CaxBa1-xGa2S4:Eu2+ solid solutions photoluminescence caused by 5d-4f electronic transitions in Eu 2+ ions with x value from 0.1 to 0.9 in wide excitation intensity and temperature intervals was performed. It is shown that the change of x from 0.1 to 0.9 appears in a tune of emission spectrum peak position from 506 nm to 555 nm with minimal losses in the integral intensity not more than by 8%. The long wavelength shift of the emission spectra was determined as a result of the rise in the crystal field splitting energy of 5d-orbitals of Eu 2+ ions in thiogallate host with increasing Ca content, which decreases the energy of the 5d-4f emission transition. The energies of the zero phonon line, red shift and the Stokes shift were defined for x value from 0.1 to 0.9. High temperature stability of the integral emission intensity with a decrease at only 25–40% depending on x value was found in the range from 10 K to 400 K. It was revealed that the increase in Ca content leads to a reduction of the temperature shift of the emission spectrum whereas at x=0.5 the shift is absent. The photoluminescence decay time constants were found to be from 305 ns to 470 ns for x value from 0.1 to 0.9. The extreme stability of the emission band shape and spectral position was found in a wide range of excitation power density from 10 -3 W/cm2 to 10 7 W/cm2 with a reversable emission efficiency droop only above 2*10 4 W/cm2 .
The structural and luminescence properties of chalcogenide semiconductor CaxBa1-xGa2S4 solid solutions (x = 0.1 – 0.9) doped with 7 at.% of Eu2+ ions were studied at room temperature. It was found, that the crystal structure of CaxBa1−xGa2S4 solid solutions varies with the amount of Ca2+ cations and phase transition from cubic to orthorhombic takes place with increase of x value. CaxBa1-xGa2S4:Eu2+ solid solutions exhibit intense photoluminescence in cyan to yellow spectral region depending on x due to 5d → 4f electron-dipole transitions in Eu2+ ions. The peak position of the emission band shifts from 506 nm for x = 0.1 to 555 nm for x = 0.9 and the full width at half maximum of the emission band varies from 62 nm to 72 nm depending on the symmetry of the crystal lattice. The PL excitation spectrum of CaxBa1-xGa2S4:Eu2+ covers the range at half maximum from 310 nm to 480 nm for x = 0.1 and to 520 nm for x = 0.9. It was shown that long-wavelength shift is caused by influence of the growing crystal field strength on Eu2+ ions.
Luminescence of Ca0.5Ba0.5Ga2S4 Crystals Activated by Eu2+ and Er3+ Ions
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Japanese Journal of Applied Physics, 2002
Thermoluminescence of Eu and Ce co-doped CaGa 2 S 4 has been investigated for the first time. All co-doped samples were found to exhibit a long afterglow of the Eu emission having a peak energy of 2:20eV.Analysesofmeasuredtransientthermoluminescencecurvesshowedtwoactivationenergies,2:20 eV. Analyses of measured transient thermoluminescence curves showed two activation energies, 2:20eV.Analysesofmeasuredtransientthermoluminescencecurvesshowedtwoactivationenergies,0:2 eV and $0:94 eV, for most co-doped samples. These two activation energies are shown to be explainable by the model which includes one trap level with two activation processes, and hole release from the terminal state of the Eu ion. Experimentally observed thermoluminescence curves are shown to be well reproduced by the theoretically formulated model which takes into account the above-mentioned hole release process.
Journal of The Electrochemical Society, 2008
Ba 1−x−2y Eu x Ce y Li y Ga 2 S 4 phosphor samples were synthesized by a solid-state reaction method. The lattice parameter of the Ba 1−x Eu x Ga 2 S 4 phosphor samples was linearly decreased from 12.654 to 12.492 Å, and the main emission wavelength was shifted from 501 to 530 nm with increase of the Eu 2+ ion concentration ͑x͒ from 0.05 to 0.50. The relative photoluminescence ͑PL͒ intensity under 450 nm excitation was increased with increase of the x from 0.05 to 0.25 due to the excitation band extended to the longer wavelength. However, when x Ͼ 0.30, the PL intensity was decreased due to nonradiative transitions among the Eu 2+ ions and the formation of EuGa 2 S 4 impurity phase. The Commission International de l'Eclairge chromaticity coordinates were also largely shifted from the bluish-green emitting region ͑x = 0.143, y = 0.506͒ to the green emitting one ͑x ϭ 2.414, yϭ 0.658͒ by increasing the x from 0.05 to 0.50. The energy transfer from the Ce 3+ ion to the Eu 2+ ion enhanced the PL intensity of the Eu 2+ ion emission under near ultraviolet and blue excitation. The PL intensity of the Ba 0.900 Eu 0.050 Ce 0.025 Li 0.025 Ga 2 S 4 phosphor sample was higher by 24% than that of the Ba 0.95 Eu 0.05 Ga 2 S 4 phosphor sample under 450 nm excitation.
Materials
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Structure and photoluminescence of (Ca,Eu) 2 SiS 4 powders
Journal of Physics: Condensed Matter, 2007
The photoluminescence of Ca 2 SiS 4 :Eu powders was investigated in detail as a function of europium concentration (from 0.1% Ca substitution to the fully substituted Eu 2 SiS 4 ). At low europium dopant concentration (<10%) the powders crystallize in an orthorhombic structure and the emission spectrum is dominated by two broad emission bands, at 564 and 660 nm. The emission can be tuned from yellow (CIE x = 0.46, y = 0.53) to red (CIE x = 0.65, y = 0.35) by variation of the Eu concentration. An energetic coupling exists between both bands, leading to a broad excitation wavelength range. Powders with high europium concentration (>40%) crystallize in a monoclinic structure, details of which were determined by Rietveld refinement of x-ray diffraction data. For the composition CaEuSiS 4 (i.e. 50% substitution), the luminescence peaks at 614 nm, shifting to shorter wavelengths upon further substitution of Ca by Eu. Although considerable thermal quenching is present at room temperature in the fully Eu-substituted compound, Eu 2 SiS 4 is still photoluminescent, with a peak emission wavelength of 577 nm. A strong correlation is found between the crystallographic and luminescent properties of the (Ca, Eu) 2 SiS 4 powders. The broad emission and excitation bands make this phosphor a good candidate for use in phosphor-converted light-emitting diodes (pcLEDs).
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Journal of Physics and Chemistry of Solids, 2003
CaGa 2 S 4 based luminophors have recently attracted significant attention due to their high efficiency. Very little is known now about the surface and near surface luminescence of these luminophors. The ion-luminescence of CaGa 2 S 4 :Eu excited by low energy hydrogen ions (up to 3 keV) and the radical-recombination luminescence excited by neutral hydrogen atoms of thermal energies have been studied. The latter is due to chemical energy released at the surface during heterogeneous recombination of atoms into molecules (, 4 eV per recombination event). The radical-recombination luminescence is likely the most surface kind of luminescence, while the ion-luminescence is the luminescence of near-surface layers. In this regard the photoluminescence is the bulk one of with which the characteristics of the surface luminescence might be compared.
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