A XAS study of the luminescent Eu centers in thiosilicate phosphors (original) (raw)
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Structure and luminescence of (Ca,Sr) 2 SiS 4 : Eu 2+ phosphors
Journal of Physics D: Applied Physics, 2010
Sr 2 SiS 4 :Eu 2+ , Ca 2 SiS 4 :Eu 2+ and the solid solution of both, europium-doped (Ca,Sr) 2 SiS 4 , were investigated as UV-VIS excitable green to red powder phosphors. Sr 2 SiS 4 :Eu 2+ shows two emission bands, peaking at 480 nm and 550 nm. By changing the ratio between Ca 2+ and Sr 2+ , the photoluminescent emission spectrum can be tuned. Using X-ray diffraction, the phase composition and lattice parameters of the thiosilicate compounds were determined. The material forms a single, monoclinic Sr 2 SiS 4 -like phase up to 40% substitution of Sr 2+ by Ca 2+ . From 50% to 90% of substitution by Ca 2+ , phase separation was observed, leading to more complex emission spectra.
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).
Optical Materials, 2010
Divalent europium is notorious for the tunability of its emission, depending on the host material in which it is used as a dopant. In europium-doped alkaline earth thiosilicates, two distinct emission bands can be observed for the alkaline earth metals Mg, Ca and Sr while only a single band is found for barium thiosilicate. In this work, we first complete the data with europium-doped magnesiumthiosilicate. Then, the solid solution of calcium and magnesium thiosilicate is presented. To conclude, the presence of multiple emission peaks in some compounds is explained on a structural basis, by analysing the possibilities for preferential orientation of the europium d-orbitals.
Valence states of europium in CaAl 2O 4:Eu phosphors
Optical Materials Express, 2012
Persistent luminescent CaAl 2 O 4 :Eu 2+ ,Nd 3+ powders were prepared by a non-aqueous sol-gel technique. The crystallization of calcium aluminate by heat-treatment of the sols is described in detail. After heat treatment in air, the europium dopant ions are mainly in a trivalent state. For the reduction to the divalent state post-annealing in a reducing nitrogen-hydrogen atmosphere is used. The reduction of europium ions is monitored by photoluminescence and x-ray absorption (XANES) spectroscopy. The degree of reduction is strongly dependent on the annealing temperature. Although for high temperature a strong enhancement of the Eu 2+ emission is observed, this also leads to powders with a gray body color.
Crystal structure and luminescence properties of SrxCa1-xAlSiN3:Eu2+ mixed nitride phosphors
Journal of Alloys and Compounds, 2009
LiYP 4 O 12 polyphosphate doped with Ce 3+ ions was prepared by the melt solution technique. The crystal structure, interatomic distances, and atom coordination numbers were determined using x-ray powder diffraction. A study of the spectral-kinetic luminescent properties was performed employing excitation with pulsed radiation from a synchrotron (UV-VUV range) and a laboratory x-ray source. The characteristics of Ce 3+ luminescence, namely the emission doublet maxima at 3.97 and 3.72 eV and the 4f-5d excitation maxima at 4.20, 5.11, 5.40, 5.65 and 6.55 eV, are discussed in terms of crystal field splitting in a low-symmetry site of the LiYP 4 O 12 host lattice. The location of the Ce 3+ energy levels with respect to the valence and conduction bands of the LiYP 4 O 12 host is estimated from the temperature dependence of the decay time measured for Ce 3+ 5d-4f luminescence.
Journal of Applied Physics, 2019
Theoretically probing the physics underlying the photoluminescence of phosphors and predicting their thermal quenching properties are significant issues in the field of phosphor research. The electronic ground states of a series of Eu(II)-doped alkaliearth sulfide phosphors, i.e., MS:Eu 2+ (M = Mg, Ca, Sr, Ba), have been analyzed using density functional theory calculations to characterize and analyze their photoluminescence properties in terms of quantum efficiency and its thermal decay tendency. Anderson's impurity model to MS:Eu 2+ enables devising a physical picture of how the electronic ground states jψ EuÀ5d i representing the Eu(II)-5d orbitals are mixed with those of the conduction bands (CBs) of host materials. The focus is on quantitatively deducing the electron delocalization nature of jψ EuÀ5d i over |CB〉, especially P k jψ MÀdk k i, which represents the bands formed by the d orbitals of M atoms. The ratio of the probability amplitudes of jψ EuÀ5d i and P k jψ MÀdk k i, i.e., C EuÀ5d =C MÀd , proves to be correlated with the electron localization nature of jψ EuÀ5d i, thereby suggesting that this ratio can be an effective parameter for evaluating the thermal quenching tendency of photoluminescence without more precise information on the electronic excited states. Energetically small gaps and large spatial overlaps between jψ EuÀ5d i and |CB〉 delocalize electrons in a hybridized state, which gives these electrons the tendency to dissipate without luminescence. The results explain the rankings of the quantum yield and its temperature dependence in the MS:Eu 2+ (M = Ca, Sr, Ba) systems, which follow the Dorenbos thermal quenching model, while MgS:Eu 2+ does not have the same mechanistic origin.
ACS Applied Materials & Interfaces, 2014
A new Ce 3+-activated thiosilicate phosphor, BaLa 2 Si 2 S 8 :Ce 3+ , was synthesized by using solid-state methods in a fused silica ampule and found to crystallize in the structure type of La 2 PbSi 2 S 8. The crystal structure has been characterized by synchrotron X-ray diffraction and refined with Rietveld methods. This novel cyan-emitting phosphor can be excited over a broad range from UV to blue light (380−450 nm) and generates a broadband emission peaking at 471 nm with a quantum efficiency of 36%. Nonradiative transitions between Ce 3+ ions in BaLa 2 Si 2 S 8 :Ce 3+ have also been demonstrated to be attributable to dipole−dipole interactions, and the critical distance was calculated to be 17.41 Å. When BaLa 2 Si 2 S 8 :Ce 3+ phosphor was utilized to incorporate with yellow-emitting (Sr,Ca) 2 SiO 4 :Eu 2+ phosphor and red-emitting CaAlSiN 3 :Eu 2+ phosphor on a 430 nm blue LED chip, a warm white light LED device with color rendering index of ∼96 was obtained. The results indicate that cyan-emitting BaLa 2 Si 2 S 8 :Ce 3+ can serve as a potential phosphor for incorporation in fabrication of solid-state lighting. The preparation, spectroscopic characterization, quantum efficiency, decay lifetime, thermalquenching behavior, and related LED device data are also presented.
Journal of Radiation Research and Applied Sciences, 2015
Europium doped di-strontium magnesium di-silicate phosphor namely (Sr 2 MgSi 2 O 7 :Eu 3þ) was prepared by the traditional high temperature solid state reaction method. The phase structure of sintered phosphor was akermanite type structure which belongs to the tetragonal crystallography with space group P42 1 m, this structure is a member of the melilite group and forms a layered compound. The EDX and FTIR spectra confirm the present elements in Sr 2 MgSi 2 O 7 :Eu 3þ phosphor. Photoluminescence measurements showed that the phosphor exhibited strong emission peak with good intensity, corresponding to 5 D 0 / 7 F 2 (613 nm) red emission and weak 5 D 0 / 7 F 1 (590 nm) orange emission. The excitation spectra monitored at 613 nm show broad band from 220 to 300 nm ascribed to OeEu charge-transfer band (CTB) centered at about 269 nm, and the other peaks in the range of 300e400 nm originated from fef transitions of Eu 3+ ions. The strongest band at 395 nm can be assigned to 7 F 0 / 5 L 6 transition of Eu 3þ ions due to the typical fef transitions within Eu 3þ of 4f 6 configuration.
Luminescence : the journal of biological and chemical luminescence, 2015
A europium (Eu)-doped di-calcium magnesium di-silicate phosphor, Ca2 MgSi2 O7 :Eu(2+) , was prepared using a solid-state reaction method. The phase structure, particle size, surface morphology, elemental analysis, different stretching mode and luminescence properties were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) with energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) and mechanoluminescence (ML). The phase structure of Ca2 MgSi2 O7 :Eu(2+) was an akermanite-type structure, which belongs to the tetragonal crystallography with space group P4̅21 m; this structure is a member of the melilite group and forms a layered compound. The surface of the prepared phosphor was not found to be uniform and particle distribution was in the nanometer range. EDX and FTIR confirm the components of Eu(2+) -doped Ca2 MgSi2 O7 phosphor. Under UV excitation...