Radiative properties of Eu2+ in BaGa2S4

doi:10.1016/j.jpcs.2005.02.003

Journal of Physics and Chemistry of Solids

Volume 66, Issue 6, June 2005, Pages 1049-1056

https://doi.org/10.1016/j.jpcs.2005.02.003 Get rights and content

Abstract

A photoluminescence study of the blue-green emitting BaGa₂S₄:Eu²⁺ phosphor is reported. Diffuse reflectance, excitation and emission spectra were examined with the aim to enlarge the fundamental knowledge about the emission of the Eu²⁺ rare earth ion in this lattice. The thermal dependence of the radiative properties and the influence of the Eu²⁺ concentration were investigated. The Stokes shift, the crystal field splitting and the activation energy of the thermal quenching were determined. By combining these results with data available in literature, we discussed the radiative properties of the BaAl₂S₄:Eu²⁺ blue phosphor in relation with those determined in this study for the isostructural BaGa₂S₄:Eu²⁺ phosphor.

Introduction

Inorganic electroluminescent (iEL) devices are one of the most attractive candidates for full-colour flat panel displays. iEL devices present many attractive features, including self emission, stability, wide viewing angle, high contrast, fast response time and high resolution [1]. For full-colour iEL displays, red, green and blue emitting phosphors with suitable colour coordinates and high luminance are necessary. The lack of phosphor for the blue component was one of the main difficulties to commercialise full-colour devices. Blue emitting phosphors based on alkaline-earth binary compounds such as SrS:Ce and SrS:Cu have been investigated [2], [3]. In parallel, many studies have concerned the development of alkaline earth thiogallates with the formula M^IIGa₂S₄, where M^II=Ca, Sr, and Ba for EL applications [4], [5], [6]. More recently, Eu²⁺-activated barium thioaluminate, BaAl₂S₄:Eu²⁺, showing pure blue emission (x=0.12 and y=0.10) and high luminance (65 cd/m² at a driving voltage of 50 Hz) was presented as the most promising material for the blue component in full-colour iEL display [7], [8]. It was successfully used in 17″ full-colour iEL video prototype display by iFire Technology Inc. [9].

The radiative mechanisms of this phosphor are not completely understood and their fundamental knowledge needs to be enlarged. However, the difficulty to prepare BaAl₂S₄:Eu²⁺ in powder form, due to the high reactivity and hygroscopic character of the starting materials BaS and Al₂S₃, represents the main obstacle for a fundamental study [10]. In this paper we propose the study of the isostructural BaGa₂S₄:Eu²⁺ phosphor whose preparation needs less severe precautions than BaAl₂S₄. BaGa₂S₄:Eu²⁺ exhibits a blue-green emission with colour coordinates x=0.14 and y=0.48 and lumen equivalent of 325 Lm/W [11]. The diffuse reflectance, excitation and emission spectra and photoluminescence (PL) decay curves of powder BaGa₂S₄:Eu²⁺ samples were analyzed and the influences of the temperature and Eu²⁺ concentration on the luminescence process were investigated. BaGa₂S₄:Eu²⁺ and BaAl₂S₄:Eu²⁺ materials exhibit the same cubic structure with the space group Pa3 (Th⁶) [12] and in both host lattices Eu²⁺ ions replace Ba²⁺ ions. By supposing that the emission processes are similar in both compounds, the fundamental radiative characteristics of BaAl₂S₄:Eu²⁺ are discussed in relation with those of BaGa₂S₄:Eu²⁺.

Section snippets

Samples and experimental details

Polycrystalline BaGa₂S₄:Eu²⁺ sample were synthesised from BaS and Ga₂S₃ sulphide powders mixed in stoichiometric composition and annealed at 900 °C under a stream of H₂S+Ar for 4 h. The doping ions were introduced in the form of EuF₃. This preparation method is different from that reported by Peters et al. [11] and Davolos et al. [13] who had used barium carbonate (BaCO₃), gallium oxide (Ga₂O₃) and europium oxide (Eu₂O₃) as starting materials and H₂S gas as sulphuring agent. A third preparation

Emission spectrum

The emission spectrum of the BaGa₂S₄ powder doped with 1% of Eu²⁺ at 77 K under excitation at 360 nm consists of a broad band in the visible range (Fig. 1). The maximum wavelength of the band is located at 503 nm and the Full Width at Half Maximum (FWHM) is equal to 42 nm. No additional band due to the emission of Eu²⁺ in other barium thiogallates Ba_xGa_yS_x+3y/2[13] is observed, which indicates that no secondary phase is present in our samples.

The emission is ascribed to the dipole-allowed

BaGa₂S₄:Eu²⁺ radiative properties

The radiative properties of BaGa₂S₄:Eu²⁺ (1%) are discussed in this section and compared with the SrGa₂S₄:Eu²⁺ and CaGa₂S₄:Eu²⁺ phosphors. A scheme representing the electronic levels of Eu²⁺ was built by using the results presented here (Fig. 9).

This study has provided a better knowledge of the electron–phonon interaction in the BaGa₂S₄:Eu²⁺ phosphor. The Huang-Rhys parameter S, the Stokes shift ΔS and the phonon energy were determined. We observe the increase of the S parameter and Stokes

Conclusion

This work provides a deeper knowledge of the radiative processes of the blue-green emitting BaGa₂S₄:Eu²⁺ phosphor. The crystal field splitting was evaluated at about 12,000 cm⁻¹, the Stokes shift at about 4500 cm⁻¹ and the activation energy of the thermal quenching at 0.7 eV. Luminescence efficiency of Eu²⁺-doped BaGa₂S₄ is little affected by thermal quenching: the quenching temperature has been estimated at 420 K. The thermal quenching is described by a mechanism based on the energy transfer

Acknowledgements

This work was supported by a collaborative linkage grant No. 979350 from NATO and a CNRS/Academy of Sciences of Azerbaijan project No. 16994.

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Eu²⁺-doped bromophosphateapatite Sr₅(PO₄)₃Br phosphors were synthesized by the conventional high-temperature solid-state reaction. The crystal structure and luminescence properties of the phosphors, as well as their thermal stability and CIE chromaticity coordinates were systematically investigated. Photoluminescence spectra of Sr₅(PO₄)₃Br:Eu²⁺ exhibit a single blue emission at 450 nm under the excitation of 345 nm, which is ascribed to the 4f–5d transition of Eu²⁺. The phosphor shows very good thermal stability. The CIE color coordinates are very close to those of BaMgAl₁₀O₁₇:Eu²⁺ (BAM). All the properties indicate that the blue-emitting Sr₅(PO₄)₃Br:Eu²⁺ phosphor has potential application in white LEDs.
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The photoluminescence properties of Eu²⁺ and Ce³⁺ doped SrF₂ have been reported at room temperature and 77 K. The emission spectra of Eu²⁺ ions in SrF₂ consisted of a broad band assigned to the 5d-4f transition of the Eu²⁺ ion. Ce³⁺ ions in SrF₂ showed a weak emission intensity. The emission intensity of Eu²⁺ ion was greatly enhanced at 77 K. At room temperature and 77 K, the fluorescence decay of Eu²⁺ and Ce³⁺ ions in SrF₂ have been measured and the fluorescence decay curve have been reported.
Eu2+-doped Ba<inf>2</inf>GaB<inf>4</inf>O<inf>9</inf>Cl blue-emitting phosphor with high color purity for near-UV-pumped white light-emitting diodes
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Eu²⁺-doped borate fluoride Ba₂GaB₄O₉Cl was synthesized by the conventional high-temperature solid-state reaction. The crystal structure and luminescence properties of the phosphors, as well as their thermal luminescence quenching capabilities and CIE chromaticity coordinates were systematically investigated. Under the excitation at 340 nm, the phosphor exhibited an asymmetric broad-band blue emission with a peak at 445 nm, which is ascribed to the 4f–5d transition of Eu²⁺. It was further proved that energy transfer among the nearest neighbor ions is the major mechanism for concentration quenching of Eu²⁺ in Ba_2−xGaB₄O₉Cl:xEu²⁺ phosphors. The luminescence quenching temperature is 432 K. The CIE color coordinates are very close to those of BaMgAl₁₀O₁₇:Eu²⁺ (BAM). All the properties indicated that the blue-emitting Ba₂GaB₄O₉Cl:Eu²⁺ phosphor has potential application in white LEDs.
Thermal quenching of Eu2+ emission in Ca- and Sr-Ga<inf>2</inf>S<inf>4</inf> in relation with VRBE schemes
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Citation Excerpt :
Eu2+ has shown to be a suitable activator for realization of applicable LED phosphors [7]. Eu2+-doped thiogallates, in particular CaGa2S4, SrGa2S4 and BaGa2S4, have shown interesting luminescence properties which makes them attractive for blue-excited LED devices [8–15]. However, from an application point of view, the thermal quenching of Eu2+ emission in these compounds at the operation temperature of the LED chip, which can reach 150 ºC (∼420 K), is a major problem.
Structural and optical properties of MGa₂S₄ (M = Mg, Zn, Ca, Sr, Ba) compounds have been compared, and the vacuum referred binding energy (VRBE) schemes were constructed for the lanthanide ions in the iso-structural compounds CaGa₂S₄ and SrGa₂S₄ employing literature data. The VRBE of an electron in the 5d excited state of Eu²⁺ was found at 0.75 and 0.97 eV below the bottom of the conduction band (CB) in CaGa₂S₄:Eu and SrGa₂S₄:Eu, respectively. Such differences explains the unexpected higher thermal quenching temperature reported for Eu²⁺-doped SrGa₂S₄ (T_50% = ∼475 K) compared to Eu²⁺-doped CaGa₂S₄ (T_50% = 400 K) The significantly lower VRBE at the CB-bottom in CaGa₂S₄ versus SrGa₂S₄ may be explained by the shorter Ga-S bond lengths in SrGa₂S₄.
Photoluminescence of Ca<inf>x</inf>Ba<inf>1−x</inf>Ga<inf>2</inf>S<inf>4</inf>:Eu2+ solid solutions in wide excitation intensity and temperature intervals
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The peak position of the PLE spectra is almost constant at about 370 nm with different Ca content in CaxBa1−xGa2S4:Eu2+ solid solutions. The observed PL of CaxBa1−xGa2S4:Eu2+ solid solutions as well as BaGa2S4:Eu2+ and CaGa2S4:Eu2+ compounds is caused by 5d→4f electronic transitions in Eu2+ ions [7–14,23]. As was mentioned above, the PL band position of Eu2+ doped compounds is determined by rare-earth ion environment.
The detailed investigation of Ca_xBa_1−xGa₂S₄:Eu²⁺ solid solutions photoluminescence caused by 5d→4f electronic transitions in Eu²⁺ 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 Eu²⁺ 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 10⁷ W/cm² with a reversable emission efficiency droop only above 2·10⁴ W/cm².
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2016, Journal of Alloys and Compounds
Series of green emitting SrAl_2−xSi_xO_4−xN_x:Eu²⁺, Dy³⁺ phosphors have been synthesized via a high temperature solid-state method. The effects of (SiN)⁺ on phase structural, emission and excitation spectra and decay curves were investigated systematically. The X-ray diffraction (XRD) patterns show that the maximum amount of solubility is about x = 0.10. The emission wavelength can be red-shifted from 509 to 515 nm with increasing (SiN)⁺ concentration. Meanwhile, the average lifetime of samples are shortened from 845.86 to 765.34 ms, which can appropriately compensate for the AC time gap and the emission color of AC-LEDs will be improved. Finally, we use these phosphors and near UV-chips to fabricate LEDs, which show more stable luminescence properties accompanying with the decrease of the luminous efficiency as the (SiN)⁺ content increases.

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Journal of Physics and Chemistry of Solids

Abstract

Introduction

Section snippets

Samples and experimental details

Emission spectrum

BaGa₂S₄:Eu²⁺ radiative properties

Conclusion

Acknowledgements

References (26)

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Cited by (67)

Eu<sup>2+</sup>-doped Sr<inf>5</inf>(PO<inf>4</inf>)<inf>3</inf>Br blue-emitting phosphor with high color purity for near-UV-pumped white light-emitting diodes

Low temperature photoluminescence study of Ce<sup>3+</sup> and Eu<sup>2+</sup> ions doped SrF<inf>2</inf> nanocrystals

Eu<sup>2+</sup>-doped Ba<inf>2</inf>GaB<inf>4</inf>O<inf>9</inf>Cl blue-emitting phosphor with high color purity for near-UV-pumped white light-emitting diodes

Thermal quenching of Eu<sup>2+</sup> emission in Ca- and Sr-Ga<inf>2</inf>S<inf>4</inf> in relation with VRBE schemes

Photoluminescence of Ca<inf>x</inf>Ba<inf>1−x</inf>Ga<inf>2</inf>S<inf>4</inf>:Eu<sup>2+</sup> solid solutions in wide excitation intensity and temperature intervals

Optical properties of SrAl<inf>2-x</inf>Si<inf>x</inf>O<inf>4-x</inf>N<inf>x</inf>:Eu<sup>2+</sup>, Dy<sup>3+</sup> phosphors for AC-LEDs

Radiative properties of Eu2+ in BaGa2S4

Abstract

Introduction

Section snippets

Samples and experimental details

Emission spectrum

BaGa2S4:Eu2+ radiative properties

Conclusion

Acknowledgements

J. Alloys Compounds

Mat. Sci. Eng.

Mat. Res. Bull.

J. Solid State Chem.

J. Lumin.

J. Lumin.

J. Cryst. Growth

Electroluminescent Display

J. Electrochem. Soc.

J. Electrochem. Soc.

Jpn. J. Appl. Phys.

Radiative properties of Eu²⁺ in BaGa₂S₄

BaGa₂S₄:Eu²⁺ radiative properties