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Structural and optical characterizations of YAG:Eu3+ elaborated by the sol–gel process
Optical Materials 26 (2004) 101–105 www.elsevier.com/locate/optmat
Structural and optical characterizations of YAG:Eu3þ elaborated by the sol–gel process Damien Boyer *, Genevieve Bertrand-Chadeyron, Rachid Mahiou Laboratoire des Materiaux Inorganiques, UMR 6002 CNRS, Universite Blaise Pascal et ENSCCF, 24 Avenue des Landais, 63177 Aubiere Cedex, France Available online 23 January 2004
Abstract Sol–gel route has been successfully used to obtain pure Y3 Al5 O12 and Y3 Al5 O12 :Eu3þ powders. The formation process, the structure as well as the microstructure of the materials were analyzed by means of X-ray diffraction (XRD), thermal analysis (TG– TDA), infrared spectroscopy (IR) and scanning electron microscopy (SEM). It has been shown that sol–gel derived YAG powders crystallize around 900 C whereas a temperature much higher is necessary to obtain this compound by solid state synthesis (1500 C). Laser induced luminescence spectra, as well as luminescence decays of Eu3þ ions show undoubtedly the spectral features of a Eu3þ emitting center embedded in a unique site of D2 symmetry. 2003 Elsevier B.V. All rights reserved. Keywords: YAG; Sol–gel; Luminescence; Europium
1. Introduction Yttrium aluminates based on the system Y2 O3 –Al2 O3 are promising materials with regard to their optical properties [1,2]. The composition 3Y2 O3 –5Al2 O3 commonly called yttrium aluminum garnet (YAG) crystallizes in the cubic form [3] and exhibits outstanding properties when doped with a lanthanide or transition elements. As a result YAG is widely used as fluorescent and solid-state laser. Since the solid-state synthesis of YAG ceramics from aluminium and yttrium oxides requires high sintering temperatures (above 1600 C) [4,5], it is difficult to control both the homogeneity and purity of YAG powders through such synthesis conditions. Furthermore it has been shown that materials properties, such as the luminescence efficiency, depend strongly on the synthesis route [6]. Therefore, so as to provide a high quality material at lower temperature, we have undertaken to prepare Y3 Al5 O12 by a wet chemical way, the sol–gel process. This synthesis method enabled reducing drastically the *
Corresponding author. Tel.: +33-4-73-40-71-00; fax: +33-4-73-4071-08. E-mail address: [email protected] (D. Boyer). 0925-3467/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2003.11.005
crystallization temperature up to 900 C. Purity and morphology of powders were analyzed respectively by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Besides, the formation process has been analyzed by infrared (IR) spectroscopy and thermal analysis (TG–DTA). On the other hand, the optical properties of europium doped samples were investigated by fluorescence spectroscopy to determine the spectral distribution and the decay time arising from the doping ions distributed on the yttrium ions sites. YAG:Eu3þ is well-known as an efficient phosphor for field emission displays (FEDs) [7] so that it has given rise to a lot of studies. 2. Experimental 2.1. Materials and sample preparation Y3 Al5 O12 powders were prepared by the sol–gel process using alkoxides as precursors. The synthesis was carried out under anhydrous conditions in a dry argon atmosphere since moisture affects significantly alkoxides stability. Experimentally, two solutions were prepared: solution I consisted of anhydrous yttrium chloride (2.000 g, 3 eq., Aldrich) dissolved in isopropanol (i PrOH)
102
D. Boyer et al. / Optical Materials 26 (2004) 101–105
(anhydrous solvent, 25 ml, Aldrich), and solution II of potassium isopropoxide (1.200 g of metallic potassium, 9 eq., Aldrich––25 ml of i PrOH). Chunks of potassium need to be handled in a non-polar and highly anhydrous solvent such as distilled cyclohexane, and cleaned of any storage oil prior to the addition of i PrOH. Solution II is slowly added to solution I with vigorous stirring, the reaction is highly exothermic and KCl is immediately formed. After a 1 h reflux of the mixed solutions at 85 C, a powder of aluminum isopropoxide (3.487 g, 5 eq., Aldrich) was added directly to the solution. A homogeneous solution was obtained after further refluxing (85 C) and stirring vigorously for 4 h. After cooling, KCl was separated by centrifuge and a clear sol was isolated. Direct hydrolysis by adding water produced a transparent gel, which was further dried at 100 C (4 h) to yield a white xerogel. Several samples of powders were prepared by sintering the xerogel at 800, 1000 and 1500 C for 4 h.
C are shown in Fig. 1. No diffraction peak is observed for the sample heat treated at 800 C which indicates that the phase is still amorphous. At 1000 C (Fig. 1b), the diffraction pattern exhibits a single crystallized phase consistent with YAG compound. Further heating treatments give rise to a more significant crystallization characterized by much more defined diffraction pattern peaks. Indeed for the sample annealed at 1200 C (Fig. 1c), the diffraction pattern consists of higher intensity and lower half-width peaks. 3.2. Infrared spectra The FTIR spectra recorded from the YAG powders sintered at different temperatures are presented in Fig. 2. The xerogel spectrum (Fig. 2a) exhibits a broad band
(c) 1200°C (b) 1000°C
(a) 800°C 10
20
30
40
50
60
70
2θ (degree) Fig. 1. XRD patterns recorded for Y3 Al5 O12 samples sintered at (a) 800 C, (b) 1000 C and (c) 1200 C for 4 h.
694 724 462 788
Absorption intensity (a. u.)
XRD measurements were performed at room temperature on a Siemens D5000 diffractometer operating with Cu-Ka radiation. The infrared spectra were recorded on a Perkin Elmer 2000 FTIR spectrometer using the KBr pellet technique. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements were performed on a Mettler TGA/SDTA 851 thermogravimetric analyzer. Xerogel samples were heated in air between 20 and 1100 C at a rate of 5 C min1 . Micrographs were recorded using a Cambridge Scan360 microscope operating at 20 kV. Specimens were prepared by sprinkling a small amount of powder on the surface of an adhesive carbon film. The luminescence spectra were recorded with a monochromator Jobin-Yvon HR 1000 spectrometer, using a dye laser (continuum ND62) pumped by a frequency doubled pulsed YAG: Nd3þ laser (continuum surelite I). The dye solution was prepared by mixing Rhodamines 590 and 610. To achieve a resonant pumping in the blue wavelength range, i.e. in the 5 D2 manifold, the output of the dye laser was up-shifted to 4155 cm1 by stimulated Raman scattering in a high pressure gaseous H2 cell [8]. Fluorescence decays were measured with a LeCroy 400 MHz digital oscilloscope.
Intensity (a. u.)
2.2. Characterization techniques
3450 1532 1418 1638 (a) xerogel (b) 800°C (c) 1000°C (d) 1200°C
3. Results and discussion 500
3.1. Powder X-ray diffraction The X-ray diffraction patterns recorded from the asprepared xerogel sintered for 4 h at 800, 1000 and 1200
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber (cm ) Fig. 2. FTIR spectra corresponding to Y3 Al5 O12 synthesized by sol– gel process, (a) as a xerogel and after heat treatment at (b) 800 C, (c) 1000 C and (d) 1200 C for 4 h.
D. Boyer et al. / Optical Materials 26 (2004) 101–105
ranging from 2700 to 3800 cm1 consistent with O–H and C–H stretching frequencies in the isopropanol molecule and alkoxy groups. The peak situated at 1638 cm1 is attributed to the presence of water added in excess to hydrolyze the alkoxide solution. The bands observed at 1418 and 1532 cm1 are assigned respectively to C–O and CH3 , CH2 stretches bonds arising from organic groups. The peaks at about 462, 694, 724
103
and 788 cm1 are due to metal-oxygen bonds (Y–O and Al–O) vibrations [9,10]. As shown previously, the YAG crystallization occurs above 800 C. As a result the increase of the sintering temperature gives rise to sharper characteristic peaks and the removal of bands ascribed to the solvent and alkoxy groups (Fig. 2b and c). 3.3. Thermal behaviour
Weight Loss (%)
2
-10
1 -20
(b)
0
-30
-1 -2
-40
(a)
Heat Flow (µV/mass)
3
0
Exo
-3
-50
-4 -60 0
200
400
600
800
1000
-5 1200
TG and DTA curves were recorded so as to investigate the thermal decomposition process of the sol–gel derived YAG powder (Fig. 3) [10–12]. There are three stages of weight loss lying in the temperature ranges 25– 125 C, 125–400 C and 400–1000 C. The endothermic reaction up to 125 C results from the e alcohol vaporation and removal of absorbed moisture. The most significant weight loss observed between 125 and 400 C is attributed to the pyrolysis of the organic parts of the alkoxide molecules and more strongly adsorbed alcohol molecules. These reactions give rise to an exothermic phenomenon.
Temperature (°C)
Fig. 3. (a) TG and (b) DTA curves obtained from as-prepared Y3 Al5 O12 powder.
I (a. u.)
λ em = 610.2 nm 300 K
F 0→ D 2
7
465
466
(a)
5
λ (nm)
467
468
F 1→ D 0
7
5
I (a. u.)
λ em = 610.2 nm 300 K
7
580 (b)
Fig. 4. SEM images recorded with the same magnification from Y3 Al5 O12 powders annealed at (a) 800 C and (b) 1200 C for 4 h.
F0→ D0 5
585
590
595
λ (nm)
Fig. 5. Room temperature excitation spectra of the 5 D0 fi 7 F2 emission of Eu3þ in Y3 Al5 O12 : 2 mol% Eu3þ recorded in the vicinity of (a) 7 F0 fi 5 D2 and (b) 7 F0;1 fi 5 D0 .
104
D. Boyer et al. / Optical Materials 26 (2004) 101–105
The final weight loss is assigned to the departure of residual alkoxy groups trapped into the mineral matrix. Besides, the sharp exothermic peak located at 900 C indicates the onset of YAG crystallization. No signifi-
cant weight loss is observed subsequently, as a result it is assumed that the final product was obtained i.e. Y3 Al5 O12 as confirmed by XRD analysis. 3.4. Microstructure analysis
D0→ 7F1
5
SEM analysis was performed to study the morphology of YAG powder. The first micrograph (Fig. 4a), recorded from the sample annealed at 800 C for 4 h, shows irregular size blocks made of aggregated particles. The largest particle blocks can reach several micrometers. After sintering at 1200 C for 4 h, the densification process has occurred and, as a result, all aggregates are now much larger (Fig. 4b).
I (a. u.)
λex = 468.0 nm 300 K
D 0→ F4
5
7
D 0→ F2
5
7
3.5. Optical properties of YAG:Eu3þ
7 D 0→ F3
5
580
600
620
640
660
680
700
720
λ (nm) Fig. 6. Emission spectrum of Y3 Al5 O12 : 2 mol% Eu3þ at 300 K upon 5 D2 excitation.
Fig. 5 shows the 7 F0 fi 5 D2 and 7 F1 fi 5 D0 excitation spectra of the 5 D0 fi 7 F2 emission of Eu3þ (610.2 nm) in Y3 Al5 O12 : 2% Eu3þ . One counts 3 lines for each transition, i.e. 7 F0 fi 5 D2 and 7 F1 fi 5 D0 . Emission spectrum was recorded upon excitation in the 5 D2 level at 468 nm in the spectral region between
λ ex = 468.0 nm 300 K
0.01
λex = 468.0 nm λem = 591.0 nm 300 K
1E-3
D 0 → F1 7
Log (I) (a. u.)
I (a. u.)
5
1E-4
1E-5
τ = 4.3 ms 1E-6
585.0
587.5
590.0
592.5
595.0
597.5
600.0
λ (nm)
(a)
1E-7
λ ex = 468.0 nm 300 K
D 0→ F 2
610
7
615
620
625
5
7
5
630
40000
50000
λex = 468.0 nm λem = 610.2 nm 300 K
1E-4
1E-5
635
0
(b) 7
30000
τ = 4.4 ms
λ (nm)
(b)
20000
Time (µs)
605
10000
1E-3
Log (I) (a. u.)
I (a. u.)
5
0
(a)
Fig. 7. (a) D0 fi F1 and (b) D0 fi F2 high-resolution emission spectra of Eu3þ in Y3 Al5 O12 : 2 mol% Eu3þ at 300 K (Arrows show the different lines for each transition).
5000
10000
15000
20000
Time (µs)
Fig. 8. Luminescence decay curves of the (a) 5 D0 fi 7 F1 and (b) 5 D0 fi 7 F2 emissions of Eu3þ in Y3 Al5 O12 : 2 mol% Eu3þ at 300 K upon 5 D2 excitation.
D. Boyer et al. / Optical Materials 26 (2004) 101–105
570 and 720 nm (Fig. 6). All lines observed are due to the transitions 5 D0 fi 7 FJ (J ¼ 1–4) of the Eu3þ ions. By applying the selections rules for electric dipole (ED) and magnetic dipole (MD) transitions in D2 site symmetry, it can be determined which transitions are allowed. On this basis, the number of expected lines for the 5 D0 fi 7 FJ are the following: for 5 D0 fi 7 F0 zero, 5 D0 fi 7 F1 three (MD), 5 D0 fi 7 F2 three (forced ED). These are in good agreements with the number of lines observed in the high resolution 5 D0 fi 7 F1;2 emission spectra (Fig. 7a and b). Decay time of the 5 D0 emission was also investigated at 300 K. Both 5 D0 fi 7 F1 and 5 D0 fi 7 F2 transitions located at 591.0 nm and 610.2 nm have been monitored respectively. In both cases we observed a single exponential decay with a time constant close to 4.5 ms as shown in Fig. 8a and b). This result is in agreement with the fact that europium ions are distributed in a single site.
synthesized powders. Finally optical properties were investigated from Eu3þ doped samples. The results are consistent with the YAG crystallographic structure and as a result emphasize the quality of materials prepared by the sol–gel technique.
References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusion In the present work, we have succeeded in preparing YAG powders by a soft chemical route, the sol–gel process. On the other hand, we have analyzed the purity, the morphology as well as the formation process of
105
[9] [10] [11] [12]
K. Ohno, T. Abe, J. Electrochem. Soc. 134 (1987) 2072. B. Cockayne, B. Lent, J. Less-common. Met. 114 (1985) 199. H.S. Yoder, M.L. Keith, Am. Min. 7 (1951) 519. A. Ikesue, K. Kamata, K. Yoshida, J. Am. Ceram. Soc. 78 (1995) 2545. J.M. Yang, S.M. Jeng, S. Chang, J. Am. Ceram. Soc. 79 (1996) 1218. D. Boyer, G. Bertrand-Chadeyron, R. Mahiou, C. Caperaa, J.C. Cousseins, J. Mater. Chem. 9 (1999) 211. D. Ravichandran, R. Roy, A.G. Chakhovskoi, C.E. Hunt, W.B. White, S. Erdei, J. Lumines. 71 (1997) 291. G. Chadeyron, R. Mahiou, M. El-Ghozzi, A. Arbus, D. Zambon, J.C. Cousseins, J. Lumines. 564 (1997) 72. M. Veith, S. Mathur, A. Kareiva, M. Jilavi, M. Zimmer, V. Huch, J. Mater. Chem. 9 (1999) 3069. Y.H. Zhou, J. Lin, S.B. Wang, H.J. Zhang, Opt. Mater. 20 (2002) 13. R. Manalert, M.N. Rahaman, J. Mater. Sci. 31 (1996) 3453. S.K. Ruan, J.G. Zhou, A.M. Zhong, J.F. Duan, X.B. Yang, M. Su, J. Alloys Compounds 275 (1998) 72.
Structural and optical characterizations of YAG:Eu3þ elaborated by the sol–gel process Damien Boyer *, Genevieve Bertrand-Chadeyron, Rachid Mahiou Laboratoire des Materiaux Inorganiques, UMR 6002 CNRS, Universite Blaise Pascal et ENSCCF, 24 Avenue des Landais, 63177 Aubiere Cedex, France Available online 23 January 2004
Abstract Sol–gel route has been successfully used to obtain pure Y3 Al5 O12 and Y3 Al5 O12 :Eu3þ powders. The formation process, the structure as well as the microstructure of the materials were analyzed by means of X-ray diffraction (XRD), thermal analysis (TG– TDA), infrared spectroscopy (IR) and scanning electron microscopy (SEM). It has been shown that sol–gel derived YAG powders crystallize around 900 C whereas a temperature much higher is necessary to obtain this compound by solid state synthesis (1500 C). Laser induced luminescence spectra, as well as luminescence decays of Eu3þ ions show undoubtedly the spectral features of a Eu3þ emitting center embedded in a unique site of D2 symmetry. 2003 Elsevier B.V. All rights reserved. Keywords: YAG; Sol–gel; Luminescence; Europium
1. Introduction Yttrium aluminates based on the system Y2 O3 –Al2 O3 are promising materials with regard to their optical properties [1,2]. The composition 3Y2 O3 –5Al2 O3 commonly called yttrium aluminum garnet (YAG) crystallizes in the cubic form [3] and exhibits outstanding properties when doped with a lanthanide or transition elements. As a result YAG is widely used as fluorescent and solid-state laser. Since the solid-state synthesis of YAG ceramics from aluminium and yttrium oxides requires high sintering temperatures (above 1600 C) [4,5], it is difficult to control both the homogeneity and purity of YAG powders through such synthesis conditions. Furthermore it has been shown that materials properties, such as the luminescence efficiency, depend strongly on the synthesis route [6]. Therefore, so as to provide a high quality material at lower temperature, we have undertaken to prepare Y3 Al5 O12 by a wet chemical way, the sol–gel process. This synthesis method enabled reducing drastically the *
Corresponding author. Tel.: +33-4-73-40-71-00; fax: +33-4-73-4071-08. E-mail address: [email protected] (D. Boyer). 0925-3467/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2003.11.005
crystallization temperature up to 900 C. Purity and morphology of powders were analyzed respectively by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Besides, the formation process has been analyzed by infrared (IR) spectroscopy and thermal analysis (TG–DTA). On the other hand, the optical properties of europium doped samples were investigated by fluorescence spectroscopy to determine the spectral distribution and the decay time arising from the doping ions distributed on the yttrium ions sites. YAG:Eu3þ is well-known as an efficient phosphor for field emission displays (FEDs) [7] so that it has given rise to a lot of studies. 2. Experimental 2.1. Materials and sample preparation Y3 Al5 O12 powders were prepared by the sol–gel process using alkoxides as precursors. The synthesis was carried out under anhydrous conditions in a dry argon atmosphere since moisture affects significantly alkoxides stability. Experimentally, two solutions were prepared: solution I consisted of anhydrous yttrium chloride (2.000 g, 3 eq., Aldrich) dissolved in isopropanol (i PrOH)
102
D. Boyer et al. / Optical Materials 26 (2004) 101–105
(anhydrous solvent, 25 ml, Aldrich), and solution II of potassium isopropoxide (1.200 g of metallic potassium, 9 eq., Aldrich––25 ml of i PrOH). Chunks of potassium need to be handled in a non-polar and highly anhydrous solvent such as distilled cyclohexane, and cleaned of any storage oil prior to the addition of i PrOH. Solution II is slowly added to solution I with vigorous stirring, the reaction is highly exothermic and KCl is immediately formed. After a 1 h reflux of the mixed solutions at 85 C, a powder of aluminum isopropoxide (3.487 g, 5 eq., Aldrich) was added directly to the solution. A homogeneous solution was obtained after further refluxing (85 C) and stirring vigorously for 4 h. After cooling, KCl was separated by centrifuge and a clear sol was isolated. Direct hydrolysis by adding water produced a transparent gel, which was further dried at 100 C (4 h) to yield a white xerogel. Several samples of powders were prepared by sintering the xerogel at 800, 1000 and 1500 C for 4 h.
C are shown in Fig. 1. No diffraction peak is observed for the sample heat treated at 800 C which indicates that the phase is still amorphous. At 1000 C (Fig. 1b), the diffraction pattern exhibits a single crystallized phase consistent with YAG compound. Further heating treatments give rise to a more significant crystallization characterized by much more defined diffraction pattern peaks. Indeed for the sample annealed at 1200 C (Fig. 1c), the diffraction pattern consists of higher intensity and lower half-width peaks. 3.2. Infrared spectra The FTIR spectra recorded from the YAG powders sintered at different temperatures are presented in Fig. 2. The xerogel spectrum (Fig. 2a) exhibits a broad band
(c) 1200°C (b) 1000°C
(a) 800°C 10
20
30
40
50
60
70
2θ (degree) Fig. 1. XRD patterns recorded for Y3 Al5 O12 samples sintered at (a) 800 C, (b) 1000 C and (c) 1200 C for 4 h.
694 724 462 788
Absorption intensity (a. u.)
XRD measurements were performed at room temperature on a Siemens D5000 diffractometer operating with Cu-Ka radiation. The infrared spectra were recorded on a Perkin Elmer 2000 FTIR spectrometer using the KBr pellet technique. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements were performed on a Mettler TGA/SDTA 851 thermogravimetric analyzer. Xerogel samples were heated in air between 20 and 1100 C at a rate of 5 C min1 . Micrographs were recorded using a Cambridge Scan360 microscope operating at 20 kV. Specimens were prepared by sprinkling a small amount of powder on the surface of an adhesive carbon film. The luminescence spectra were recorded with a monochromator Jobin-Yvon HR 1000 spectrometer, using a dye laser (continuum ND62) pumped by a frequency doubled pulsed YAG: Nd3þ laser (continuum surelite I). The dye solution was prepared by mixing Rhodamines 590 and 610. To achieve a resonant pumping in the blue wavelength range, i.e. in the 5 D2 manifold, the output of the dye laser was up-shifted to 4155 cm1 by stimulated Raman scattering in a high pressure gaseous H2 cell [8]. Fluorescence decays were measured with a LeCroy 400 MHz digital oscilloscope.
Intensity (a. u.)
2.2. Characterization techniques
3450 1532 1418 1638 (a) xerogel (b) 800°C (c) 1000°C (d) 1200°C
3. Results and discussion 500
3.1. Powder X-ray diffraction The X-ray diffraction patterns recorded from the asprepared xerogel sintered for 4 h at 800, 1000 and 1200
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber (cm ) Fig. 2. FTIR spectra corresponding to Y3 Al5 O12 synthesized by sol– gel process, (a) as a xerogel and after heat treatment at (b) 800 C, (c) 1000 C and (d) 1200 C for 4 h.
D. Boyer et al. / Optical Materials 26 (2004) 101–105
ranging from 2700 to 3800 cm1 consistent with O–H and C–H stretching frequencies in the isopropanol molecule and alkoxy groups. The peak situated at 1638 cm1 is attributed to the presence of water added in excess to hydrolyze the alkoxide solution. The bands observed at 1418 and 1532 cm1 are assigned respectively to C–O and CH3 , CH2 stretches bonds arising from organic groups. The peaks at about 462, 694, 724
103
and 788 cm1 are due to metal-oxygen bonds (Y–O and Al–O) vibrations [9,10]. As shown previously, the YAG crystallization occurs above 800 C. As a result the increase of the sintering temperature gives rise to sharper characteristic peaks and the removal of bands ascribed to the solvent and alkoxy groups (Fig. 2b and c). 3.3. Thermal behaviour
Weight Loss (%)
2
-10
1 -20
(b)
0
-30
-1 -2
-40
(a)
Heat Flow (µV/mass)
3
0
Exo
-3
-50
-4 -60 0
200
400
600
800
1000
-5 1200
TG and DTA curves were recorded so as to investigate the thermal decomposition process of the sol–gel derived YAG powder (Fig. 3) [10–12]. There are three stages of weight loss lying in the temperature ranges 25– 125 C, 125–400 C and 400–1000 C. The endothermic reaction up to 125 C results from the e alcohol vaporation and removal of absorbed moisture. The most significant weight loss observed between 125 and 400 C is attributed to the pyrolysis of the organic parts of the alkoxide molecules and more strongly adsorbed alcohol molecules. These reactions give rise to an exothermic phenomenon.
Temperature (°C)
Fig. 3. (a) TG and (b) DTA curves obtained from as-prepared Y3 Al5 O12 powder.
I (a. u.)
λ em = 610.2 nm 300 K
F 0→ D 2
7
465
466
(a)
5
λ (nm)
467
468
F 1→ D 0
7
5
I (a. u.)
λ em = 610.2 nm 300 K
7
580 (b)
Fig. 4. SEM images recorded with the same magnification from Y3 Al5 O12 powders annealed at (a) 800 C and (b) 1200 C for 4 h.
F0→ D0 5
585
590
595
λ (nm)
Fig. 5. Room temperature excitation spectra of the 5 D0 fi 7 F2 emission of Eu3þ in Y3 Al5 O12 : 2 mol% Eu3þ recorded in the vicinity of (a) 7 F0 fi 5 D2 and (b) 7 F0;1 fi 5 D0 .
104
D. Boyer et al. / Optical Materials 26 (2004) 101–105
The final weight loss is assigned to the departure of residual alkoxy groups trapped into the mineral matrix. Besides, the sharp exothermic peak located at 900 C indicates the onset of YAG crystallization. No signifi-
cant weight loss is observed subsequently, as a result it is assumed that the final product was obtained i.e. Y3 Al5 O12 as confirmed by XRD analysis. 3.4. Microstructure analysis
D0→ 7F1
5
SEM analysis was performed to study the morphology of YAG powder. The first micrograph (Fig. 4a), recorded from the sample annealed at 800 C for 4 h, shows irregular size blocks made of aggregated particles. The largest particle blocks can reach several micrometers. After sintering at 1200 C for 4 h, the densification process has occurred and, as a result, all aggregates are now much larger (Fig. 4b).
I (a. u.)
λex = 468.0 nm 300 K
D 0→ F4
5
7
D 0→ F2
5
7
3.5. Optical properties of YAG:Eu3þ
7 D 0→ F3
5
580
600
620
640
660
680
700
720
λ (nm) Fig. 6. Emission spectrum of Y3 Al5 O12 : 2 mol% Eu3þ at 300 K upon 5 D2 excitation.
Fig. 5 shows the 7 F0 fi 5 D2 and 7 F1 fi 5 D0 excitation spectra of the 5 D0 fi 7 F2 emission of Eu3þ (610.2 nm) in Y3 Al5 O12 : 2% Eu3þ . One counts 3 lines for each transition, i.e. 7 F0 fi 5 D2 and 7 F1 fi 5 D0 . Emission spectrum was recorded upon excitation in the 5 D2 level at 468 nm in the spectral region between
λ ex = 468.0 nm 300 K
0.01
λex = 468.0 nm λem = 591.0 nm 300 K
1E-3
D 0 → F1 7
Log (I) (a. u.)
I (a. u.)
5
1E-4
1E-5
τ = 4.3 ms 1E-6
585.0
587.5
590.0
592.5
595.0
597.5
600.0
λ (nm)
(a)
1E-7
λ ex = 468.0 nm 300 K
D 0→ F 2
610
7
615
620
625
5
7
5
630
40000
50000
λex = 468.0 nm λem = 610.2 nm 300 K
1E-4
1E-5
635
0
(b) 7
30000
τ = 4.4 ms
λ (nm)
(b)
20000
Time (µs)
605
10000
1E-3
Log (I) (a. u.)
I (a. u.)
5
0
(a)
Fig. 7. (a) D0 fi F1 and (b) D0 fi F2 high-resolution emission spectra of Eu3þ in Y3 Al5 O12 : 2 mol% Eu3þ at 300 K (Arrows show the different lines for each transition).
5000
10000
15000
20000
Time (µs)
Fig. 8. Luminescence decay curves of the (a) 5 D0 fi 7 F1 and (b) 5 D0 fi 7 F2 emissions of Eu3þ in Y3 Al5 O12 : 2 mol% Eu3þ at 300 K upon 5 D2 excitation.
D. Boyer et al. / Optical Materials 26 (2004) 101–105
570 and 720 nm (Fig. 6). All lines observed are due to the transitions 5 D0 fi 7 FJ (J ¼ 1–4) of the Eu3þ ions. By applying the selections rules for electric dipole (ED) and magnetic dipole (MD) transitions in D2 site symmetry, it can be determined which transitions are allowed. On this basis, the number of expected lines for the 5 D0 fi 7 FJ are the following: for 5 D0 fi 7 F0 zero, 5 D0 fi 7 F1 three (MD), 5 D0 fi 7 F2 three (forced ED). These are in good agreements with the number of lines observed in the high resolution 5 D0 fi 7 F1;2 emission spectra (Fig. 7a and b). Decay time of the 5 D0 emission was also investigated at 300 K. Both 5 D0 fi 7 F1 and 5 D0 fi 7 F2 transitions located at 591.0 nm and 610.2 nm have been monitored respectively. In both cases we observed a single exponential decay with a time constant close to 4.5 ms as shown in Fig. 8a and b). This result is in agreement with the fact that europium ions are distributed in a single site.
synthesized powders. Finally optical properties were investigated from Eu3þ doped samples. The results are consistent with the YAG crystallographic structure and as a result emphasize the quality of materials prepared by the sol–gel technique.
References [1] [2] [3] [4] [5] [6] [7] [8]
4. Conclusion In the present work, we have succeeded in preparing YAG powders by a soft chemical route, the sol–gel process. On the other hand, we have analyzed the purity, the morphology as well as the formation process of
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