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Eu concentration dependence of luminescent properties of Sr1−xEuxGa2S4 phosphors synthesized by polymerized complex sulfurization method
Optical Materials 35 (2013) 1993–1996
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Eu concentration dependence of luminescent properties of Sr1 xEuxGa2S4 phosphors synthesized by polymerized complex sulfurization method Kouta Taniguchi, Tatsuya Honda, Ariyuki Kato ⇑ Department of Electrical Engineering, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka 940-2188, Japan
a r t i c l e
i n f o
Article history: Available online 1 December 2012 Keywords: Polymerized complex method Sulfurization Sr1 xEuxGa2S4 Concentration quenching Quantum efficiency Electroluminescence
a b s t r a c t Sr1 xEuxGa2S4 (x = 0, 0.05, 0.1, 0.3, 0.5, 0.7, 1) phosphors were synthesized by new synthesis method, polymerized complex sulfurization (PCS) method and usual solid state reaction (SSR) method. The structural and luminescent properties of synthesized phosphors were compared in term of Eu concentration dependence. The lattice constants calculated from XRD patterns were found to obey Vegard’s law for the samples synthesized by the PCS method. Quantum efficiency and luminance from EL elements prepared with the samples synthesized by PCS method were about twice as higher as those of SSR method in high Eu concentration range. These results suggest that uniform distribution of metal ions can be achieved by PCS method. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Alkaline earth thiogallates (MGa2S4, M = Ca, Sr, Ba) are excellent host materials for phosphors. These host material are activated with rare-earth ions as luminescent centers, and exhibit various emissions in visible region due to the 4f–4f or 5d–4f transitions in the rare-earth ions [1]. In particular, Ce3+ and Eu2+ in these host materials exhibit efficient blue and green emissions respectively [1–3], and have been studied for feasibility of realizing field-emission displays and inorganic electroluminescent displays [4]. Furthermore, optical gain and laser action were observed in Eu2+ and Dy3+-doped CaGa2S4, respectively [5–7]. Thus, the alkaline earth thiogallates are also attractive as host crystals of laser materials. Especially, the broad emission due to the 5d–4f transition in Eu2+ may allow alkaline earth thiogallates doped with Eu2+ to be used for tunable laser devices in visible region. Especially, Sr1 xEuxGa2S4 phosphor has a special feature showing concentration quenching hardly even if Sr sites are completely substituted by Eu, which is an advantage for the applications mentioned above [8,9]. However, in high Eu concentration samples synthesized by usual solid state reaction (SSR) method, non-uniform distribution of metal ions surrounding the luminescent centers affects their luminescent properties. Polymerized complex method (PCM) [10], another name of Pechini method [11], is expected to suppress the effect of the non-uniform distribution of metal ions on their luminescent properties. In this study, Sr1 xEux Ga2S4 phosphor materials were synthesized by polymerized com⇑ Corresponding author. E-mail address: [email protected] (A. Kato). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.10.039
plex sulfurization method, which is the combination of PCM and sulfurization in H2S atmosphere, hereafter abbreviated as PCS. Finally, dispersion-type electroluminescent (EL) elements were prepared to demonstrate the advantage of PCS method.
2. Experimental 2.1. Sample preparation As starting materials for the synthesis of Sr1 xEuxGa2S4 (x = 0, 0.05, 0.1, 0.3, 0.5, 0.7, 1.0) by PCS method, stoichiometric amounts of SrCO3, Ga2O3 and Eu2O3 were weighed to obtain 0.003 mol of Sr1 xEuxGa2S4. The starting materials were dissolved in diluted hydrochloric acid. As chelating agent, citric acid monohydrate was added. To dehydrate and polymerize the chelate complexes, propylene glycol was added, leading to the formation of a polyester resin. The resin was decomposed at 350 °C by a mantle heater and calcinated in air at 800 °C for 3 h to obtain a white oxide precursor. After subsequent sulfurization in H2S(5%) + Ar atmosphere at 800 °C for 3 h, Sr thiogallate phosphors with homogeneous metal ion distribution were obtained. For comparison, the samples were also synthesized by usual SSR method. The compounds of SrS, Ga2S3, EuS were used as starting materials and weighed in the stoichiometric ratio to obtain 0.005 mol of Sr1 xEuxGa2S4. Sr thiogallate phosphors were obtained by heating the mixtures of the starting materials at 1000 °C for 12 h in H2S (5%) + Ar atmosphere. Dispersion-type EL elements utilizing the obtained phosphors were prepared with spray coating. The phosphor layer and dielec-
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K. Taniguchi et al. / Optical Materials 35 (2013) 1993–1996
tric barrier layers were formed by spraying the obtained phosphors and commercially available BaTiO3 dispersed in acrylic resin using an airbrush, respectively. The ITO glass was used as the transparent electrode layer and another electrode of aluminum was formed by vacuum deposition. 2.2. Sample characterization The obtained samples were analyzed with an X-ray diffraction (XRD), electron probe micro analysis (EPMA), photoluminescence (PL) and Quantum efficiency (QE). XRD patterns were measured using Cu Ka radiation of X-ray diffractometer (Shimadzu, XRD7000). Atomic compositions were analyzed by wavelength dispersive X-ray spectroscopy using an electron probe micro analyzer (Shimadzu, EPMA-1600). PL spectra were measured using a 325 nm He–Cd laser as an excitation source (Omnichrome, 3056M10, 10 mW). Photoluminescence was dispersed by a monochromator (Nalumi, RM23) in conjunction with a photomultiplier (Hamamatsu, R943-02) coupled to a photon counter (Hamamatsu, C767). Absolute PL quantum efficiency measurements were done using an integration glass semi-sphere in conjunction with a photomultiplier (RCA, 7104) [2]. A 441.6 nm He–Cd laser (Omnichrome, 4056, 30 mW) was used as excitation source and signal from the photomultiplier was detected by a Pico ammeter (Takeda Riken, TR-8641). The EL elements were operated with AC voltage generated by a combination of a function generator (Metex, MS-9170), an audio amplifier (Sony, DHC-MD7) and an audio trance (Noguchi-trans, PMF-10WS). The luminance from the EL elements were measured by a luminance meter (Konica Minolta, LS-100) mounted with a close-up lens (Konica Minolta, 1804-743). 3. Results and discussion 3.1. XRD characterization Fig. 1a shows the XRD patterns of Sr1 xEuxGa2S4 (x = 0, 0.05, 0.1, 0.3, 0.5, 0.7, 1.0) synthesized by PCS. For comparison, that of SrGa2S4 synthesized by SSR is also shown. For the all SSR samples, all peaks correspond to the ICDD data of SrGa2S4 [12] or EuGa2S4 [13] and any other phase is not found, while some different peaks due to oxide precursor are appeared for the PCS samples of x = 0.3, 0.5 (shown by arrows in Fig. 1a). These residual oxides may result from insufficient sulfurization process. It was found that sulfurization becomes difficult for the precursor calcinated at high temperature in the process to determine the optimum calcination temperature, which is optimized to be 800 °C for x = 0.
(a) PCS
According to the phase diagram of Sr1 xEuxGa2S4 [14], the melting point of x = 0.3–0.5 is 20–30 °C lower than that of x = 0. Consequently, the calcination temperature of 800 °C might be relatively high for the PCS samples of x = 0.3, 0.5, leading to insufficient sulfurization. Fig. 1b shows the Eu concentration dependence of the lattice constant of a-axis calculated from the peak position of (0 2 2) and (4 2 2). The change of the lattice constant of the PCS samples is linear and obeys Vegard’s law [15], while that of the SSR samples is rapid. If Eu ions do not substitute Sr site uniformly, low Eu concentration domain (almost SrGa2S4) and high concentration domain (almost EuGa2S4) coexist. In this case, the XRD pattern roughly consists of an overlap of those of SrGa2S4 and EuGa2S4 and the peak position jumps from that of SrGa2S4 to EuGa2S4 with increasing Eu concentration. The rapid change of the lattice constant of the SSR samples in Fig. 1b may indicates non-uniform substitution of Eu. On the contrary, uniform substitution of Eu ions to Sr sites is successfully achieved for the PCS samples. 3.2. PL characterization Fig. 2a and b shows the PL spectra of Sr1 xEuxGa2S4 synthesized by PCS and SSR, respectively. All spectra are normalized by the respective peak intensity at x = 0.1 of the PCS and SSR samples. PL spectra of the all samples consist of a single broad band around 540 nm due to the 4f65d1 ? 4f75d0 transition in Eu2+. Eu concentration dependences of the peak intensity and peak wavelength of the PL band are shown in Fig. 2c and d respectively. In Fig. 2d, ratios of the residual oxygen to sulfur analyzed by EPMA measurements are also shown. According to the Eu concentration dependence of PL intensity of Sr1 xEuxGa2S4 grown from melt where Eu ions distributed uniformly [14], the intensity has a maximum at x = 0.1 and show concentration quenching above x = 0.1. The dependence for the SSR samples show the same tendency, however the intensity ratios of high Eu concentration samples to the maximum at x = 0.1 are much lower than those in the reference. This is probably due to the distortion of surroundings of Eu ions caused by the non-uniform substitution, resulting in increase of non-active Eu ions. On the other hand, rapid decrease of the intensity for the PCS samples is probably due to the residual oxides observed in XRD for x = 0.3–0.5. In higher Eu concentration range, the intensity of the PCS samples does not decease so much, that is, becomes higher than those of SSR, suggesting the uniform distribution of Eu ions. The peak wavelength becomes longer with increase of Eu concentration, which is due to decrease of the lattice constants, that is, increase of crystal field splitting in Eu2+. The red shift of the
(b)
Eu x=1.00 x=0.7 x=0.5
Solid
Intensity [a.u.]
x=0.3 x=0.1 x=0.05 PCS
x=0 (422)
(022)
10
20
SSR x=0
30
40
50
60
2θ [deg] Fig. 1. (a) XRD patterns of Sr1 xEuxGa2S4 samples prepared by PCS and SSR methods, (b) Eu concentration dependence of the lattice constant of a-axis.
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K. Taniguchi et al. / Optical Materials 35 (2013) 1993–1996
PL Intensity [a.u.]
1
PL
(b)
Sr1-xEu xGa 2 S4
Exc.325nm
PCS
R.T.
X=0.05 X=0.1 X=0.3 X=0.5 X=0.7 X=1.0
0.5
0 450
500
550
600
SSR
PLR.T.
X=0.05 X=0.1 X=0.3 X=0.5 X=0.7 X=1.0
0.5
0 450
650
500
550
600
650
Wavelength [nm]
Wavelength [nm]
(c)
(d) SSR PCS
SSR PCS
1
Peak Intensity [a.u.]
Sr1-xEu xGa 2 S4
Exc.325nm
1
PL Intensity [a.u.]
(a)
PCS SSR
0.5
0
0
0.1
0.2 0.3 0.4
0.5 0.6 0.7 0.8
0.9
1
Eu concentration X Fig. 2. PL spectra of Sr1 xEuxGa2S4 samples prepared by (a) PCS and (b) SSR methods, Eu concentration dependences of (c) the peak intensity and (d) peak wavelength. Ratios of residual oxygen to sulfur atom are also shown in (d).
PCS samples seems to be suppressed compared to the SSR samples in the range of x = 0.3–0.7. This suppression may be due to the residual of oxygen suggested by XRD (Fig. 1a) and EPMA (Fig. 2d). In Ce doped CaxSr1 xGa2S4, it is reported that the residual of oxygen causes a centroid shift of 5d levels in Ce3+, resulting in blue shift [16,17]. This centroid shift may suppress the red shift of the PCS samples. 3.3. QE and EL characterization Fig. 3 shows the Eu concentration dependence of absolute PL quantum efficiency of Sr1 xEuxGa2S4 synthesized by PCS and SSR. The quantum efficiency show a maximum around 20% at x = 0.1, and decrease with increasing Eu concentration for PCS and SSR. Monotonic decrease of quantum efficiency with increase of Eu con-
centration is clearly seen in SSR, while the quantum efficiency of the PCS samples decreases rapidly and stays almost constant value. In high Eu concentration range, the quantum efficiency of PCS samples is much higher than that of SSR. These dependences show the same tendency with those in Fig. 2c, indicating the uniform distribution of Eu ions in the PCS samples. Fig. 4 shows the applied voltage dependence of luminance from the EL elements. The waveform of the applied voltage is sine wave of 1.8 kHz. The luminance in PCS is more than twice as higher compared to SSR. Thus, the PCS method proposed in this study has a possibility to improve the luminance of EL elements. However, the luminance is very weak and the threshold voltages are as high as more than 600 V for PCS and SSR, which is due to poor film quality and low density of phosphor powders.
SSR PCS
Fig. 3. Eu concentration dependence of quantum efficiency of Sr1 xEuxGa2S4.
Fig. 4. Applied voltage dependence of luminescence from EL elements.
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K. Taniguchi et al. / Optical Materials 35 (2013) 1993–1996
4. Conclusions Sr1 xEuxGa2S4 phosphor materials were synthesized by polymerized complex sulfurization (PCS) method, which is the combination of polymerized complex method and sulfurization process, to compare the structural and luminescent characteristics with usual solid state reaction (SSR) method in term of Eu concentration dependence. As a result, the lattice constant became to obey Vegard’s law and an improvement in PL intensity, quantum efficiency and luminance from EL elements was achieved in high Eu concentration range owing to uniform distribution of metal ions. However, the residual of oxygen was found and results in blue shift of the emission. Therefore, further investigation of the synthesis condition is needed to decrease the residual of oxygen.
Reference [1] T.E. Peters, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230–236.
[2] A. Kato, M. Yamazaki, H. Najafov, K. Iwai, A. Bayramov, C. Hidaka, T. Takizawa, S. Iida, J. Phys. Chem. Solids 64 (2005) 1511–1517. [3] T. Takizawa, C. Hidaka, J. Phys. Chem. Solids 69 (2008) 347–352. [4] C. Chartier, C. Barthou, P. Benalloul, J.M. Frigerio, J. Lumin. 111 (2005) 147–158. [5] M.C. Nostrand, R.H. Page, S.A. Payne, W.F. Krupke, P.G. Schunemann, Opt. Lett. 24 (1999) 1215–1217. [6] S. Iida, T. Matsumoto, N.T. Mamedov, G. An, Y. Maruyama, A.I. Bairamov, B.G. Tagiev, O.B. Tagiev, R.B. Dzhabbarov, Jpn. J. Appl. Phys. 36 (1997) L857–L859. [7] A. Kato, S. Iida, M. Yamazaki, E. Yamagishi, C. Hidaka, T. Takizawa, J. Phys. Chem. Solids 66 (2005) 2076–2078. [8] S. Iida, A. Kato, M. Yamazaki, H. Najafov, H. Ikuno, J. Phys. Chem. Solids 64 (2003) 1815–1819. [9] A. Kato, M. Tanaka, H. Najafov, S. Iida, J. Phys. Chem. Solids 66 (2005) 2072– 2075. [10] M. Kakihana, J. Sol–Gel Sci. Technol. 6 (1996) 7–55. [11] M. Pechini, US Patent 3330,697 ,July 11, 1967. [12] ICDD No. 01-077-1189. [13] ICDD No. 01-071-0588. [14] C. Hidaka, T. Takizawa, J. Phys. Chem. Solids 69 (2008) 358–361. [15] A.R. Denton, N.W. Ashcroft, Phys. Rev. A 43 (1991) 3161–3164. [16] T.A. O’Brien, M.C. Zerner, P.D. Rack, P.H. Holloway, J. Electrochem. Soc. 147 (2000) 792–795. [17] E. Gambarov, A. Bayramov, A. Kato, S. Iida, Phys. Stat. Sol. (C) 3 (2006) 2726– 2729.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Eu concentration dependence of luminescent properties of Sr1 xEuxGa2S4 phosphors synthesized by polymerized complex sulfurization method Kouta Taniguchi, Tatsuya Honda, Ariyuki Kato ⇑ Department of Electrical Engineering, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka 940-2188, Japan
a r t i c l e
i n f o
Article history: Available online 1 December 2012 Keywords: Polymerized complex method Sulfurization Sr1 xEuxGa2S4 Concentration quenching Quantum efficiency Electroluminescence
a b s t r a c t Sr1 xEuxGa2S4 (x = 0, 0.05, 0.1, 0.3, 0.5, 0.7, 1) phosphors were synthesized by new synthesis method, polymerized complex sulfurization (PCS) method and usual solid state reaction (SSR) method. The structural and luminescent properties of synthesized phosphors were compared in term of Eu concentration dependence. The lattice constants calculated from XRD patterns were found to obey Vegard’s law for the samples synthesized by the PCS method. Quantum efficiency and luminance from EL elements prepared with the samples synthesized by PCS method were about twice as higher as those of SSR method in high Eu concentration range. These results suggest that uniform distribution of metal ions can be achieved by PCS method. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Alkaline earth thiogallates (MGa2S4, M = Ca, Sr, Ba) are excellent host materials for phosphors. These host material are activated with rare-earth ions as luminescent centers, and exhibit various emissions in visible region due to the 4f–4f or 5d–4f transitions in the rare-earth ions [1]. In particular, Ce3+ and Eu2+ in these host materials exhibit efficient blue and green emissions respectively [1–3], and have been studied for feasibility of realizing field-emission displays and inorganic electroluminescent displays [4]. Furthermore, optical gain and laser action were observed in Eu2+ and Dy3+-doped CaGa2S4, respectively [5–7]. Thus, the alkaline earth thiogallates are also attractive as host crystals of laser materials. Especially, the broad emission due to the 5d–4f transition in Eu2+ may allow alkaline earth thiogallates doped with Eu2+ to be used for tunable laser devices in visible region. Especially, Sr1 xEuxGa2S4 phosphor has a special feature showing concentration quenching hardly even if Sr sites are completely substituted by Eu, which is an advantage for the applications mentioned above [8,9]. However, in high Eu concentration samples synthesized by usual solid state reaction (SSR) method, non-uniform distribution of metal ions surrounding the luminescent centers affects their luminescent properties. Polymerized complex method (PCM) [10], another name of Pechini method [11], is expected to suppress the effect of the non-uniform distribution of metal ions on their luminescent properties. In this study, Sr1 xEux Ga2S4 phosphor materials were synthesized by polymerized com⇑ Corresponding author. E-mail address: [email protected] (A. Kato). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.10.039
plex sulfurization method, which is the combination of PCM and sulfurization in H2S atmosphere, hereafter abbreviated as PCS. Finally, dispersion-type electroluminescent (EL) elements were prepared to demonstrate the advantage of PCS method.
2. Experimental 2.1. Sample preparation As starting materials for the synthesis of Sr1 xEuxGa2S4 (x = 0, 0.05, 0.1, 0.3, 0.5, 0.7, 1.0) by PCS method, stoichiometric amounts of SrCO3, Ga2O3 and Eu2O3 were weighed to obtain 0.003 mol of Sr1 xEuxGa2S4. The starting materials were dissolved in diluted hydrochloric acid. As chelating agent, citric acid monohydrate was added. To dehydrate and polymerize the chelate complexes, propylene glycol was added, leading to the formation of a polyester resin. The resin was decomposed at 350 °C by a mantle heater and calcinated in air at 800 °C for 3 h to obtain a white oxide precursor. After subsequent sulfurization in H2S(5%) + Ar atmosphere at 800 °C for 3 h, Sr thiogallate phosphors with homogeneous metal ion distribution were obtained. For comparison, the samples were also synthesized by usual SSR method. The compounds of SrS, Ga2S3, EuS were used as starting materials and weighed in the stoichiometric ratio to obtain 0.005 mol of Sr1 xEuxGa2S4. Sr thiogallate phosphors were obtained by heating the mixtures of the starting materials at 1000 °C for 12 h in H2S (5%) + Ar atmosphere. Dispersion-type EL elements utilizing the obtained phosphors were prepared with spray coating. The phosphor layer and dielec-
1994
K. Taniguchi et al. / Optical Materials 35 (2013) 1993–1996
tric barrier layers were formed by spraying the obtained phosphors and commercially available BaTiO3 dispersed in acrylic resin using an airbrush, respectively. The ITO glass was used as the transparent electrode layer and another electrode of aluminum was formed by vacuum deposition. 2.2. Sample characterization The obtained samples were analyzed with an X-ray diffraction (XRD), electron probe micro analysis (EPMA), photoluminescence (PL) and Quantum efficiency (QE). XRD patterns were measured using Cu Ka radiation of X-ray diffractometer (Shimadzu, XRD7000). Atomic compositions were analyzed by wavelength dispersive X-ray spectroscopy using an electron probe micro analyzer (Shimadzu, EPMA-1600). PL spectra were measured using a 325 nm He–Cd laser as an excitation source (Omnichrome, 3056M10, 10 mW). Photoluminescence was dispersed by a monochromator (Nalumi, RM23) in conjunction with a photomultiplier (Hamamatsu, R943-02) coupled to a photon counter (Hamamatsu, C767). Absolute PL quantum efficiency measurements were done using an integration glass semi-sphere in conjunction with a photomultiplier (RCA, 7104) [2]. A 441.6 nm He–Cd laser (Omnichrome, 4056, 30 mW) was used as excitation source and signal from the photomultiplier was detected by a Pico ammeter (Takeda Riken, TR-8641). The EL elements were operated with AC voltage generated by a combination of a function generator (Metex, MS-9170), an audio amplifier (Sony, DHC-MD7) and an audio trance (Noguchi-trans, PMF-10WS). The luminance from the EL elements were measured by a luminance meter (Konica Minolta, LS-100) mounted with a close-up lens (Konica Minolta, 1804-743). 3. Results and discussion 3.1. XRD characterization Fig. 1a shows the XRD patterns of Sr1 xEuxGa2S4 (x = 0, 0.05, 0.1, 0.3, 0.5, 0.7, 1.0) synthesized by PCS. For comparison, that of SrGa2S4 synthesized by SSR is also shown. For the all SSR samples, all peaks correspond to the ICDD data of SrGa2S4 [12] or EuGa2S4 [13] and any other phase is not found, while some different peaks due to oxide precursor are appeared for the PCS samples of x = 0.3, 0.5 (shown by arrows in Fig. 1a). These residual oxides may result from insufficient sulfurization process. It was found that sulfurization becomes difficult for the precursor calcinated at high temperature in the process to determine the optimum calcination temperature, which is optimized to be 800 °C for x = 0.
(a) PCS
According to the phase diagram of Sr1 xEuxGa2S4 [14], the melting point of x = 0.3–0.5 is 20–30 °C lower than that of x = 0. Consequently, the calcination temperature of 800 °C might be relatively high for the PCS samples of x = 0.3, 0.5, leading to insufficient sulfurization. Fig. 1b shows the Eu concentration dependence of the lattice constant of a-axis calculated from the peak position of (0 2 2) and (4 2 2). The change of the lattice constant of the PCS samples is linear and obeys Vegard’s law [15], while that of the SSR samples is rapid. If Eu ions do not substitute Sr site uniformly, low Eu concentration domain (almost SrGa2S4) and high concentration domain (almost EuGa2S4) coexist. In this case, the XRD pattern roughly consists of an overlap of those of SrGa2S4 and EuGa2S4 and the peak position jumps from that of SrGa2S4 to EuGa2S4 with increasing Eu concentration. The rapid change of the lattice constant of the SSR samples in Fig. 1b may indicates non-uniform substitution of Eu. On the contrary, uniform substitution of Eu ions to Sr sites is successfully achieved for the PCS samples. 3.2. PL characterization Fig. 2a and b shows the PL spectra of Sr1 xEuxGa2S4 synthesized by PCS and SSR, respectively. All spectra are normalized by the respective peak intensity at x = 0.1 of the PCS and SSR samples. PL spectra of the all samples consist of a single broad band around 540 nm due to the 4f65d1 ? 4f75d0 transition in Eu2+. Eu concentration dependences of the peak intensity and peak wavelength of the PL band are shown in Fig. 2c and d respectively. In Fig. 2d, ratios of the residual oxygen to sulfur analyzed by EPMA measurements are also shown. According to the Eu concentration dependence of PL intensity of Sr1 xEuxGa2S4 grown from melt where Eu ions distributed uniformly [14], the intensity has a maximum at x = 0.1 and show concentration quenching above x = 0.1. The dependence for the SSR samples show the same tendency, however the intensity ratios of high Eu concentration samples to the maximum at x = 0.1 are much lower than those in the reference. This is probably due to the distortion of surroundings of Eu ions caused by the non-uniform substitution, resulting in increase of non-active Eu ions. On the other hand, rapid decrease of the intensity for the PCS samples is probably due to the residual oxides observed in XRD for x = 0.3–0.5. In higher Eu concentration range, the intensity of the PCS samples does not decease so much, that is, becomes higher than those of SSR, suggesting the uniform distribution of Eu ions. The peak wavelength becomes longer with increase of Eu concentration, which is due to decrease of the lattice constants, that is, increase of crystal field splitting in Eu2+. The red shift of the
(b)
Eu x=1.00 x=0.7 x=0.5
Solid
Intensity [a.u.]
x=0.3 x=0.1 x=0.05 PCS
x=0 (422)
(022)
10
20
SSR x=0
30
40
50
60
2θ [deg] Fig. 1. (a) XRD patterns of Sr1 xEuxGa2S4 samples prepared by PCS and SSR methods, (b) Eu concentration dependence of the lattice constant of a-axis.
1995
K. Taniguchi et al. / Optical Materials 35 (2013) 1993–1996
PL Intensity [a.u.]
1
PL
(b)
Sr1-xEu xGa 2 S4
Exc.325nm
PCS
R.T.
X=0.05 X=0.1 X=0.3 X=0.5 X=0.7 X=1.0
0.5
0 450
500
550
600
SSR
PLR.T.
X=0.05 X=0.1 X=0.3 X=0.5 X=0.7 X=1.0
0.5
0 450
650
500
550
600
650
Wavelength [nm]
Wavelength [nm]
(c)
(d) SSR PCS
SSR PCS
1
Peak Intensity [a.u.]
Sr1-xEu xGa 2 S4
Exc.325nm
1
PL Intensity [a.u.]
(a)
PCS SSR
0.5
0
0
0.1
0.2 0.3 0.4
0.5 0.6 0.7 0.8
0.9
1
Eu concentration X Fig. 2. PL spectra of Sr1 xEuxGa2S4 samples prepared by (a) PCS and (b) SSR methods, Eu concentration dependences of (c) the peak intensity and (d) peak wavelength. Ratios of residual oxygen to sulfur atom are also shown in (d).
PCS samples seems to be suppressed compared to the SSR samples in the range of x = 0.3–0.7. This suppression may be due to the residual of oxygen suggested by XRD (Fig. 1a) and EPMA (Fig. 2d). In Ce doped CaxSr1 xGa2S4, it is reported that the residual of oxygen causes a centroid shift of 5d levels in Ce3+, resulting in blue shift [16,17]. This centroid shift may suppress the red shift of the PCS samples. 3.3. QE and EL characterization Fig. 3 shows the Eu concentration dependence of absolute PL quantum efficiency of Sr1 xEuxGa2S4 synthesized by PCS and SSR. The quantum efficiency show a maximum around 20% at x = 0.1, and decrease with increasing Eu concentration for PCS and SSR. Monotonic decrease of quantum efficiency with increase of Eu con-
centration is clearly seen in SSR, while the quantum efficiency of the PCS samples decreases rapidly and stays almost constant value. In high Eu concentration range, the quantum efficiency of PCS samples is much higher than that of SSR. These dependences show the same tendency with those in Fig. 2c, indicating the uniform distribution of Eu ions in the PCS samples. Fig. 4 shows the applied voltage dependence of luminance from the EL elements. The waveform of the applied voltage is sine wave of 1.8 kHz. The luminance in PCS is more than twice as higher compared to SSR. Thus, the PCS method proposed in this study has a possibility to improve the luminance of EL elements. However, the luminance is very weak and the threshold voltages are as high as more than 600 V for PCS and SSR, which is due to poor film quality and low density of phosphor powders.
SSR PCS
Fig. 3. Eu concentration dependence of quantum efficiency of Sr1 xEuxGa2S4.
Fig. 4. Applied voltage dependence of luminescence from EL elements.
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K. Taniguchi et al. / Optical Materials 35 (2013) 1993–1996
4. Conclusions Sr1 xEuxGa2S4 phosphor materials were synthesized by polymerized complex sulfurization (PCS) method, which is the combination of polymerized complex method and sulfurization process, to compare the structural and luminescent characteristics with usual solid state reaction (SSR) method in term of Eu concentration dependence. As a result, the lattice constant became to obey Vegard’s law and an improvement in PL intensity, quantum efficiency and luminance from EL elements was achieved in high Eu concentration range owing to uniform distribution of metal ions. However, the residual of oxygen was found and results in blue shift of the emission. Therefore, further investigation of the synthesis condition is needed to decrease the residual of oxygen.
Reference [1] T.E. Peters, J.A. Baglio, J. Electrochem. Soc. 119 (1972) 230–236.
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