Radiative Properties of the Blue BaAl2S4:Eu2+ Phosphor

Journal of The Electrochemical Society, 153 共3兲 G253-G258 共2006兲

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0013-4651/2006/153共3兲/G253/6/$20.00 © The Electrochemical Society

Radiative Properties of the Blue BaAl2S4:Eu2+ Phosphor C. Barthou,a,z R. B. Jabbarov,b P. Benalloul,a C. Chartier,a N. N. Musayeva,b B. G. Tagiev,b and O. B. Tagievb a

Institut des Nano-Sciences de Paris, UMR CNRS 7588, Universités P. et M. Curie et Denis Diderot, Campus Boucicaut, F-75015 Paris, France b Institute of Physics, Azerbaijan National Academy of Sciences, Baku, Azerbaijan The optical properties of BaAl2S4 doped with Eu2+ were investigated. The synthesis was performed in evacuated and sealed quartz ampul at 1000°C during 1 h, then at 840°C during 20 h. With absorption, excitation, and emission spectra we have determined the crystal-field splitting, the redshift, the influence of the lattice vibration properties, and the thermal quenching of the luminescence. The crystal-field splitting of Eu2+ in BaAl2S4 was estimated on the order of 8000 to 10 000 cm−1 and the Huang-Rhys parameter S = 5 ± 1 corresponds to an intermediate electron-phonon coupling regime. The mean phonon energy for BaAl2S4 was evaluated by optical and Raman methods, and the obtained values were 32 ± 5 and 40 meV, respectively. These results confirm the very interesting properties of this blue BaAl2S4:Eu2+ phosphor, the most important being the very slight thermal quenching of the luminescence. © 2006 The Electrochemical Society. 关DOI: 10.1149/1.2164693兴 All rights reserved. Manuscript submitted June 29, 2005; revised manuscript received December 1, 2005. Available electronically February 1, 2006.

Inorganic thin-film electroluminescent 共iEL兲 devices are expected to be very interesting candidates for full-color flat display panels. The lack of a blue phosphor with suitable color coordinates, high enough luminance and efficiency was one of the main difficulties to commercialize full color devices. These last years, binary and ternary sulfide phosphors have been investigated, like SrS:Ce,1 共Ca,Sr兲Ga2S4:Ce,2 SrS:Cu,3 SrS:Cu,Ag,4 and CaS:Pb.5 Unfortunately, not one of these phosphors provides suitable performances for full-color display application. A breakthrough was made by Miura et al. in 1999, who have reported on a new thin-film blue EL phosphor, BaAl2S4 doped with Eu2+, which gives a higher brightness of 65 cd/m2 at 50 Hz and good color coordinates, x = 0.12 and y = 0.10.6 Greater brightness than 700 cd/m2 at 120 Hz has been obtained using the thick dielectric technology at iFire Company.7 This performance allows iFire Company to announce a full-color 34-in. iEL HDTV screen for the 2006 timeframe using the Color-By-Blue 共CBB兲 technique.8 The first report concerning this BaAl2S4:Eu2+ phosphor was presented by Donohue and Hanlon.9 The cubic structure with a lattice parameter of 1.2588 nm was established. At 300 K, a blue band emission peaked at 475 nm was obtained with a decay time on the order of 0.45 ␮s. In this pioneering paper, it was pointed that this phosphor will be efficient due to a wide bandgap leading to a low interaction between the excited states of Eu2+ and fundamental edge. This cubic structure was summarized by Eisenmann et al.10 and some luminescence properties were presented by Le Thi et al.11 The radiative mechanisms of this BaAl2S4:Eu2+ phosphor are not completely understood and their fundamental knowledge needs to be enlarged. In this paper diffuse reflectance, excitation, and emission spectra, and photoluminescence 共PL兲 decay curves of powder samples were analyzed and the influence of temperature on the luminescence process was investigated. The results are discussed in regards to the results recently presented for BaGa2S4:Eu2+ phosphor,12 which has the same cubic structure with the space group Pa3共Th6兲10 but is much less sensitive to moisture. More, the results obtained on powder will be useful to evaluate the quality of the thin films prepared for the iEL displays. Experimental In the past, Ba thioaluminate was synthesized by solid-state reaction from mixtures of Al, Ba, and sulfur, maintained for several days at temperatures between 800 and 1000°C,9 or by a reaction of BaAl2 intermetallic compound with sulfur at 1200°C.10 This compound has been also prepared with BaS and Al2S3 sulfide starting

z

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materials at a temperature not higher than 1000°C.11 For this synthesis, the difficulty comes from the high sensitivity of Al2S3 to moisture and reactivity with silica. We have chosen an intermediate solution, by using BaS, Al, Eu, and sulfur as starting materials. Eu amount was determined to prepare 2 and 5 mol % doped phosphor. Synthesis was carried out in an evacuated and sealed quartz ampul at 1000°C during 1 h, then at 840°C during 20 h. The ampul was opened in a glove box under Ar atmosphere. In this box, the powder was ground and put in a hermetic sealed box with a quartz plate on one side. The powder presents a white color. Diffuse reflectance spectra were obtained using an AVANTES AVS-S2000 spectrometer with a spectral range from 200 to 850 nm. The sample was illuminated with a UV deuterium lamp 共190–400 nm兲 and a halogen lamp 共360–1500 nm兲 simultaneously. The diffusion spectra at each illumination and detection angles were divided by the spectra of a white reference tile. This Teflon-based reference has a reflectance of 98% for the wavelength detection range and an isotropic diffusion. Emission and excitation spectra were recorded with a Jobin-Yvon Fluorolog-3 using a xenon lamp 共450 W兲. To correct the intensity of the excitation spectra, these spectra were compared to the excitation spectra of Rhodamine B dye, which is assumed as constant in the range 240–600 nm. The intensity of the xenon lamp was real-time monitored with a diode. For PL decay the samples were excited at 400 nm by a dye laser 共Laser Photonics LN 102, PBBO兲 pumped by a pulsed N2 laser 共Laser Photonics LN 1000, 1.4 mJ per pulse, pulse width 0.6 ns兲 with a repetition rate of about 1 Hz. The emitted light was collected from the same side as the excitation by an optical fiber with an aperture angle 20° and located at about 1 cm to the surface. The decays were recorded with a PM Hamamatsu R5600U and a Tektronix TDS 784A scope with a time constant on the order of 1 ns. Thermal dependence of the emission was analyzed from 77 to 550 K, which is the temperature limit of the setup, using a cold finger cryostat JANIS ST-100 under vacuum. X-ray diffraction patterns correspond to the lone cubic phase with a lattice parameter a = 1.265 nm in agreement with previous papers.9-11 Nevertheless, these patterns indicate a poorer crystalline quality of these samples compared to BaGa2S4 ones.12 The vibration properties of BaAl2S4 compound were determined by Raman spectroscopy. The comparison with the Raman spectrum of BaGa2S4 was performed in order to clarify the origins of the vibration modes in these cubic chalcogenides. Stokes and antiStokes spectra were recorded between −500 and +500 cm−1 in order to distinguish Raman lines from luminescence or satellite lines. Raman scattering spectra of BaGa2S4 and BaAl2S4 were measured by a Jobin-Yvon U1000 double monochromator with an S20 photomultiplier at room temperature in backscattering configuration. The 676.4 nm line of a Kr Spectra Physics laser was used as the excitation

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Figure 1. Spectral variation of the ratio between the absorption coefficient and the scattering coefficient 共determined from the Kubelka-Munk method兲 for the BaAl2S4:Eu2+ 共5%兲 powder at 300 K.

source at a power level of 140 mW. With a beam spot diameter of about 100 ␮m, the power density on the samples was about 300 W/cm2. The spectral resolution of the spectrometer is about 1 cm−1. Results and Discussion Absorption spectra.— Reflection spectra under different illumination and detection angles 共0 to 60°兲 were carried out to confirm the isotropic diffusion of our powder sample. In addition, the spectral dependence of the diffusion factor was neglected because the grain size of the powder is larger than the wavelength in the UVvisible range.13 The absorption spectra of BaAl2S4:Eu2+ at 300 K 共Fig. 1兲 was obtained from diffuse reflectance spectrum by using the Kubelka-Munk formula.14 The absorption factor K is linked to the diffuse reflectance rate R and to the diffusion factor Ssca 共1 − R兲2 K = Ssca 2R So, the ratio K/Ssca vs wavelength is a good representation of the absorption spectrum of the compound. The large absorption band located between 300 and 450 nm is ascribed to the direct absorption of the 4f7 → 4f65d Eu2+ ion transitions. The absorption band located in the UV range below about 295 nm 共above about 4.2 eV兲 is due to the absorption of the host matrix also with a possible contribution of Eu2+ absorption. This rough energy value for the optical gap is lower than the energy found by optical measurements on thin films by Peter et al., who propose a value of 5.05 and 4.85 eV, respectively, according with the direct or indirect bandgap nature of this compound.15 The much lower value of 3.98 eV found by Goh et al. on BaAl2S4 single crystal is difficult to consider as these authors have determined a surprising lattice parameter of 1.0248 nm for this cubic compound.16 Emission spectra.— In BaAl2S4 crystal, the Ba2+ ions occupy two different lattice sites with different symmetry and coordination number: 8 Ba2+ ions have a six-fold coordination and 4 Ba2+ have a 12-fold coordination number.10 Because the Eu2+ ions replace the Ba2+ ions in BaAl2S4:Eu2+, they are also distributed into these two different sites and one can consider in a first approximation that the six-fold-coordinated Eu2+ ions have an octahedral 共Oh兲 environment, and that the 12-fold-coordinated Eu2+ ions lie in icosahedral 共Ih兲 sites. In cubic and orthorhombic thiogallates, the emission spectrum of Eu2+ presents only one broad band. This is also the case for

Figure 2. Experimental 共black line兲 and calculated 共open circles兲 emission spectra of the BaAl2S4:Eu2+ 共5%兲 powder at 77 K under the excitation at 400 nm. Black square: SN vs analysis wavelength.

the emission spectrum of the BaAl2S4:Eu2+ 共5 mol %兲 powder at 77 K which exhibits under 400 nm excitation wavelength just a broad band centered at 478 nm with a full width at half-maximum 共fwhm兲 of 30 nm 共Fig. 2兲. This emission is ascribed to the dipole-allowed transition from the lower 4f65d共 7F兲 state to the 4f7共 8S7/2兲 fundamental state of the Eu2+ ions. Thereafter, we present only the results corresponding to the sample doped with 5%, this Eu2+ content presenting a compromise between the emission intensity and the chromaticity coordinates in the blue for iEL device applications. A decrease of the Eu2+ content leads to a shift of the emission toward high energy, a better blue, but to a decrease of the PL and EL intensities.6 The single configuration model was used to fit the emission band of BaAl2S4:Eu2+ which is characteristic of a phonon-broadened emission 共Fig. 2兲. In a first approximation we suppose that only the fundamental vibration level 共m = 0兲 of the excited electronic state is occupied at 77 K. Using this hypothesis, the intensity of the vibronic transition from the fundamental vibration level of the emitting electronic state to the n vibration level of the ground state, i.e., the m = 0 → n transition, is proportional to the expression13 exp共−S兲 ⫻

Sn n!

where S is the Huang-Rhys parameter and measures the interaction between the luminescent center and the vibrating lattice. The energy gap between two vibronic peaks is equal to the quantity h␯g, the mean lattice phonon energy for the 4f7共 8S7/2兲 ground electronic state. The best fit of the emission band was obtained with S = 5 ± 1 and h␯g = 32 ± 5 meV. This S value corresponds to an intermediate electron-phonon coupling and can be linked to the slight asymmetry of the emission band. By supposing that the ground-state parabola of the configurational coordinate model presents the same curvature as the excited state parabola, i.e., h␯g = h␯e = h␯, we can determinate the Stokes shift 共⌬S兲. ⌬S is related to the offset of the parabolas in the configurational coordinate diagram. ⌬S is equal to 共2S − 1兲h␯ = 288 meV 共S: HuangRhys coupling constant兲. Excitation spectra.— The excitation spectrum of BaAl2S4:Eu2+ 共5%兲 at 77 K for the emission at 490 nm is presented in Fig. 3. In Oh symmetry the 5d orbitals are split in two levels, t2g and eg, while they are still degenerated in Ih symmetry 共hg level兲. The more energetic observed band B is ascribed to the 4f7共 8S7/2兲 → 4f65d共 7F兲共eg兲, transition while the band C is the


Figure 3. Excitation spectra of the BaAl2S4:Eu2+ 共5%兲 powder at 77 K for the emission at 490 nm and emission spectra under 360 nm excitation wavelength. The excitation spectrum has been normalized with the value of the maximum of the 4f6共 7F0兲5d共t2g兲 excitation band determined by using the mirror-image relationship with the emission band.

superposition of the 4f7共 8S7/2兲 → 4f65d共 7F兲共t2g兲 and 4f7共 8S7/2兲 → 4f65d共 7F兲共hg兲 transitions. On the contrary, in the case of BaGa2S4:Eu the t2g and hg bands are well resolved.12 One can also see a trailing edge band located at about 5 eV, which is the limit of our setup to correct in energy, ascribed to the transitions between the valence band and the conduction band of the BaAl2S4 host matrix. This result is coherent with those presented on powder samples.11,17 As the excitation bands are not well resolved, it is difficult to estimate the crystal-field splitting. It must however lie between 8000 and 10,000 cm−1 共1 and 1.3 eV兲 and can be compared to the value of 10,000 cm−1 proposed by Djazovski.17 Versus temperature, the shape of the excitation spectra presents only very little changes between 78 and 300 K. Above this temperature, the B band shifts slightly to higher energy, whereas the C band presents only very little broadening. The weaker crystal-field splitting compared to barium thiogallate compound 共12,000 cm−1兲 is connected to the smaller size of Al3+ than Ga3+ and is still weaker than in BaS 共16 000 cm−117兲 due to the presence of trivalent ions. This is illustrated by the higher Ba-S mean value bond lengths in BaAl2S4: 共3.41 Å10兲 than in BaS 共3.2 Å兲. The Stokes shift ⌬S can be evaluated from excitation and emission spectra by using an almost mirror-image relationship. We have to consider the maxima of the excitation and the emission spectra corresponding to the lowest excited level 4f6共 7F0兲5d共t2g兲. As the structure of the excitation band is not well resolved, one cannot easily determine the position of 7F0. Therefore, we have used the mirror-image relationship where the maximum of the emission band is calculated to obtain the same mirror shape for the higher energy edge of the emission band and the lower energy edge of the excitation band 共Fig. 3兲. We have estimated Eabs = 2.90 eV and ⌬S = Eabs − Eemis, = 0.32 eV 共2580 cm−1兲. According to the formalism of Dorenbos,18 the knowledge of Eabs allows us to determine the redshift D or the energy lowering of the f-d transition in relation to the free ion 共Efree = 4.19 eV for Eu2+兲. We found D = 1.29 eV, which is weaker than in 共Ba,Sr兲Ga2S4 compounds, 1.3 and 1.64 eV, respectively.12,19 The difference of bond covalence is the main reason for this behavior. Due to the smaller size of the Al cation, Al-S bonds are shorter and more covalent than Ga-S ones. The replacement of Ga by Al leads to a decrease of the covalence of the Ba-S bond and consequently to the decrease of the nephelauxetic effect. The weaker redshift explains the higher energy of the f → d and

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Figure 4. Raman spectra of 共a兲 BaAl2S4:Eu and 共b兲 BaGa2S4:Eu 共b兲 recorded using the 676.4 nm Kr laser line. The vibration frequencies are given in cm−1. T = 300 K.

d → f transitions for BaAl2S4:Eu2+ compared to thiogallate compounds. The energy of the zero-phonon line, E0, at the intersection of the emission and excitation spectra was estimated to be 2.74 eV 共Fig. 3兲. Raman spectra.— Raman spectra of the polycrystalline BaAl2S4 and BaGa2S4 are given in Fig. 4a and b, respectively. The Raman spectrum of BaGa2S4 exhibits 29 clearly identified vibration lines. The Raman lines of the BaAl2S4 compound are hardly identified, except the most intense ones, due to a weaker signal-to-noise ratio that can be ascribed to the poorer crystalline quality of the sample. The spectra are dominated by two vibration modes at 312 and 320 cm−1 for BaAl2S4 and by one intense vibration line at 303 cm−1 for BaGa2S4. As shown for the orthorhombic chalcogallates,20 the Raman vibrations can be described by a molecular model. The orthorhombic MIIGa2S4 spectra were interpreted by considering the isolated GaS4 and MIIS8 vibrating units. The most intense vibration line was assigned to the totally symmetrical bond-stretching modes, where the S anions move together in the cation-anion direction in GaS4 or in MIIS8 units. By analogy and according to the XRD data,10 the vibrating units responsible for the BaAl共Ga兲2S4 Raman spectra may be the tetrahedral Ga共Al兲S4, the octahedral BaS6, and the square antiprismatic BaS8. The spectrum of BaGa2S4 is very similar to the Raman spectra of orthorhombic MIIGa2S4 compounds with MII = Pb, Sr, Eu, Yb, and Ca.20 The vibration lines are located in the same range of frequencies 共between 50 and 410 cm−1兲, indicating that the mass of Ba affects very slightly the spectrum and that no vibration involving this cation directly is present in the Raman spectrum. The higher number of vibration lines 共29兲 compared with the spectra of orthorhombic MIIGa2S4 共between 15 and 19兲 may be explained by the existence of three vibrating units in the structure of BaGa2S4 compared with only two for orthorhombic MIIGa2S4. The most intense line of the BaGa2S4 spectrum 共303 cm−1兲 is located at higher frequency than the most intense mode of the orthorhombic MIIGa2S4 spectra 共between 278 and 285 cm−1兲. Considering that the Ga-S lengths 共⬃2.27 Å10兲 are shorter and the Ba-S mean value lengths 共⬃3.39 Å10兲 are longer in cubic BaGa2S4 than in orthorhombic MIIGa2S4 共Ga-S between 2.26 and 2.29 Å, MII-S between 3.00 and 3.13 Å20兲, this suggests that the unit responsible for this mode is the GaS4 tetrahedrons and not the BaS6 or BaS8 units whose vibrations might lead to weaker frequency modes. This conclusion is useful to complete our previous study about the orthorhombic MIIGa2S4 compounds20 and to interpret without ambiguity the Raman vibrations: the most intense vibration mode can be ascribed to the totally symmetrical bond-stretching modes in GaS4 tetrahedrons.

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Figure 5. Emission spectra of the BaAl2S4:Eu2+ 共5%兲 powder at different temperatures from 77 to 550 K under excitation at 360 nm. Inset: Relative integrated PL intensity of the emission band vs temperature.

II

The spectrum of BaAl2S4 is similar to the spectra of M Ga2S4 but presents several significant differences. First, the frequency range, from 50 to 520 cm−1, is larger. Then, two vibration lines with very close frequencies 共312 and 320 cm−1兲 dominate the spectrum. These differences may be ascribed to the mass effect due to the change of the Ga3+ ion by the lighter Al3+ ion. The mass effect leads to the shift of the vibrations involving the trivalent cation, i.e., the vibrations in Ga共Al兲S4 units, to higher frequency. Nevertheless, the most intense Raman line in BaAl2S4 is little affected by the mass effect. It may be assigned to the bond-stretching vibrations in tetrahedral AlS4 units, as shown for BaGa2S4. The higher frequency of this mode compared with that of BaGa2S4 is coherent with the Al-S lengths 共⬃2.24 Å10兲 weaker than the Ga-S lengths 共⬃2.27 Å10兲. In conclusion, the most intense Raman mode of the BaAl2S4 spectrum is at 320 cm−1 and is due to totally symmetrical bondstretching vibrations in the AlS4 units. This frequency corresponds to a phonon energy of 40 meV. This value allows us to determine the Huang-Rhys parameter: S = 4.2. The results of this Raman investigation confirm also the poorer crystalline quality of the BaAl2S4 sample compared with the BaGa2S4, which may explain why the excitation band C is not as well resolved as in the excitation spectrum of BaGa2S4. Influence of the temperature and decays.— The emission spectra of the BaAl2S4:Eu2+ 共5%兲 powder sample are presented as a function of temperature from 78 to 550 K in Fig. 5 under 360 nm excitation wavelength. The wavelength of the peak emission decreases from 478 to 469 nm with the increasing temperature. The energy shift of the band is associated with the increase of the energy for the most occupied vibration level of the emitting electronic state. The fwhm of the band increases form 30 nm up to 46 nm with temperature due to the increase of the number of n → m vibronic transitions. At room temperature the emission band peaks at 473 nm and its fwhm is equal to 41 nm. The thermal quenching of the integrated PL intensity is presented in the inset of Fig. 5. Compared to 77 K, the thermal quenching is only 20% at room temperature and 35% at 550 K. This quenching is weaker than reported in Ref. 11, for which it was on the order of 30% at room temperature. The temperature dependence of the fwhm can be described using the configurational coordinate model and the Boltzmann distribution according to the following equation proposed in Ref. 21:

Figure 6. Experimental 共square兲 and calculated 共line兲 temperature dependence of the fwhm of the emission band measured under excitation at 360 nm for the BaAl2S4:Eu2+ 共5%兲 powder.

W共T兲 = h␯

冑

冉冊

8 ⫻ ln 2 ⫻ S ⫻ coth

h␯ 2kT

where h␯ is the mean phonon energy, S the Huang-Rhys factor, and k the Boltzmann constant. The best fit was obtained with S = 5.1 ± 0.2 and h␯ = 31 ± 2 meV, h␯ being the mean energy for the coupled phonons 共Fig. 6兲. These values enable us to evaluate the Stokes shift at 285 meV. Figure 7 presents the normalized PL decay curves at the peak maximum of the emission band for two temperatures, 78 and 550 K, under 400 nm excitation wavelength. The decays exhibit an exponential part with the characteristic lifetime ␶ of 360 ns at 78 K and 400 ns at 550 K values, close to the value of 342 ns found at 300 K in Ref. 17. These data indicate no influence of nonradiative transitions on the decay of Eu2+. At room temperature, the decay time of 400 ns is shorter than for thiogallate compounds like SrGa2S4:Eu 共480 ns兲19 and CaGa2S4:Eu 共590 ns兲,22 but longer than for BaGa2S4:Eu 共290 ns兲.12 To analyze the de-excitation process of Eu2+ excited ions we can calculate the normalized area under the decay curve SN = S/I0, which reflects the competition between radiative and nonradiative de-excitation processes of Eu2+ ions excited in their lowest en-

Figure 7. Decays of the Eu2+ emission in BaAl2S4:Eu2+ 共5%兲 powder for 78 and 550 K measured under excitation wavelength at 400 nm and analyzing wavelength at 480 nm. Inset: SN vs temperature.


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Table I. Characteristics of the Eu2+ emission in CaGa2S4, SrGa2S4, BaGa2S4, and BaAl2S4. The thermal quenching is given compared to the intensity emission at 77 K. Compound Reference Symmetry Peak wavelength fwhm x:y ␶ SN SN SN Thermal quenching Activation energy of the thermal quenching S hv

nm eV ns ns ns ns % eV

300 K 300 K 300 K 300 K 78 K 300 K 500 K 500 K

meV

ergy excited level when the decays are not strictly exponential.22 At 77 K, this normalized area varies with the analysis wavelength from 260 ns at 460 nm to 330 ns at 510 nm 共see Fig. 2兲. This behavior confirms that the emission band is the superposition of the emission due to different Eu2+ environments. For short wavelengths, the overlap of the absorption and emission bands favors the energy transfer from high energy sites towards lower energy sites and shortens the decay time. Due to this fact we can consider that the fit of the Eu2+ emission band 共Fig. 2兲 is not totally correct as the values of S and h␯. SN increases from 77 to 450 K. This behavior is comparable to the lengthening of ␶ observed for Eu2+ in CaGa2S4,22 but was not observed in BaGa2S4.12 Meijerink and Blasse have ascribed this behavior in the case of Eu2+ in the halosilicates Ba5SiO4X6 共X = Cl, Br兲 to the thermal population of a 4f65d state having a low probability of radiative transition.23 This can be a sextet state for which the transition to the 4f7共 8S7/2兲 ground state is spin-forbidden. At low temperature the emission originates from the lowest 4f65d state with an allowed transition to the ground state. With increasing temperature the sextet states become populated, lengthening the decay time. The increase may also be due to an octet state for which the transition to the ground state would be symmetry forbidden. Also in the chlorides MCl2 共M = Ca, Sr, Ba兲 the lifetime is longer at 300 K than at 78 K.24 In spite of the fact that the decay is quite exponential for the two first decades, it was not possible to characterize the thermal quenching with an activation energy related to 1/␶nr through the function 1/␶c共T兲 = 1/␶0 + 1/␶nr共T兲 as no significant decrease for SN was observed from 450 K up to 550 K under direct excitation into the Eu2+ absorption band. By comparing with the results obtained with BaGa2S4:Eu 共0.7 eV兲, we can say that this activation energy is higher for the BaAl2S4 host matrix as expected.12 To clarify the loss mechanisms responsible for decreasing luminescence intensity with temperature, it is necessary to know the location of the Eu2+ electronic levels relative to the valence and the conduction bands of the BaAl2S4 host matrix. With the results obtained in this work, it was not possible to determine these locations. But, due to a small concentration quenching in this ternary compound and a low probability of nonradiative relaxation to the ground state via the crossing between the fundamental and the excited parabolas, we have attended to a hole transfer from fundamental level of Eu2+ to the valence band of the host matrix. This mechanism was already proposed for the Eu2+ emission temperature quenching in SrGa2S4.19 This hypothesis is also confirmed with the electro-optical properties of iEL devices based on the BaAl2S4:Eu2+ phosphor where no ionization of Eu2+ occurs under high electric field applied to the semiconductor layer.17

CaGa2S4 21 Orthorhombic

SrGa2S4 18 Orthorhombic

BaGa2S4 12 Cubic

BaAl2S4 This work Cubic

562 0.20 0.43:0.56 610 480 590 200 90% 0.5 2 35

534 0.21 0.26:0.61 480 480 480 110 80% 0.6 3.8 35

490 0.30 0.14:0.48 305 300 310 100 70% 0.7 12 25

473 0.23 0.12:0.10 400 340 380 390 35% ⬎0.7 5 32

Conclusion In this study we have first determined some radiative properties very useful to describe the main characteristics of the Eu2+ emission in BaAl2S4: the crystal-field splitting, the redshift, the influence of the lattice vibration properties 共electron-phonon coupling, host mean phonon energy兲, the thermal quenching of the luminescence. The crystal-field splitting of Eu2+ in BaAl2S4 was estimated at about 9000 ± 1000 cm−1. We have evaluated the Stokes shift and we assume that the barium-thioaluminate compound has a wider bandgap material and a weaker thermal quenching effect is obtained. The Huang-Rhys parameters S and the phonon energy were determined. This value S = 5.1 ± 1 corresponds to an intermediate electronphonon coupling regime. We have evaluated the mean phonon energy for BaAl2S4 compound by two separate methods: optical and Raman methods; the obtained values were 31 ± 2 and 40 meV, respectively. This study allows the comparison of the properties of this blue phosphor with other ternary sulfide compounds doped with Eu2+ ions recently studied and is interesting for visualization application. The optical characteristics of the Eu2+ emission in four ternary compounds are listed in Table I. Compared to the three other ternary compounds, BaAl2S4 presents the lower temperature quenching at 500 K 共35%兲 with respect to 77 K, and the higher activation energy of the thermal quenching. This study confirms the very interesting properties of this blue BaAl2S4:Eu2+ phosphor, the most important being the very slight thermal quenching of the luminescence, for its application in inorganic electroluminescent devices, the sole issue being its high sensitivity to moisture, an issue which can be solved by the stack structure of the EL displays and their encapsulation. Acknowledgments This work was supported by a collaborative linkage grant no. 979350 from NATO and CNRS/Academy of Sciences of Azerbaijan project no. 16994. The authors thank M. Jouanne and J.-F. Morhange for the Raman spectra measurements. One author, Rasim Jabbarov, also thanks INTAS organization for fellowship grant no. 03-55-1051. References 1. I. Tanaka, Y. Izumi, K. Tanaka, and S. Okamoto, J. Lumin., 87–89, 1189 共2000兲. 2. K. Tanaka, S. Okamoto, Y. Izumi, Y. Inoue, and K. Kobayashi, Jpn. J. Appl. Phys., Part 2, 38, L1419 共1999兲. 3. S. S. Sun, E. Dickey, J. Kane, and P. N. Yocom, in 1997 International Display Research Conference Records, p. 301 共1997兲. 4. W. Park, T. C. Jones, and C. J. Summers, Appl. Phys. Lett., 74, 1785 共1999兲. 5. S. J. Yun, Y. S. Kim, J. S. Kang, K. Park, K. Cho, and D. S. Ma, in SID International Symposium Digest, p. 1142 共1999兲. 6. N. Miura, M. Kawanishi, H. Matsumoto, and R. Nakano, Jpn. J. Appl. Phys., Part

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