Effect of co-doping of Ce3+ and alkaline metals on the photoluminescence in CaGa2S4 and SrGa2S 4 hosts

Article abstract of DOI:10.1002/pssa.200669619

Ce³⁺ doped alkaline-earth thiogallates represented as M ^IIGa₂S₄ (M^II= Ca, Sr, Ba) are known as blue phosphors with high brightness and good purity of color. However the Ce concentration is limited bel

Full text of DOI:10.1002/pssa.200669619

phys. stat. sol. (a) 203, No. 11, 2718–2722 (2006) / DOI 10.1002/pssa.200669619
3+
Effect of co-doping of Ce and alkaline metals
on the photoluminescence in CaGa2S4 and SrGa2S4 hosts
*
Chiharu Hidaka , Kazutaka Miura, Shinya Oikawa, and Takeo Takizawa
Department of Physics, College of Humanities and Sciences, Nihon University, 3-25-40 Sakura-josui,
Setagaya-ku, Tokyo 156-8550, Japan
Received 24 February 2006, revised 14 June 2006, accepted 26 June 2006
Published online 31 August 2006
PACS 42.70.Hj, 78.55.Hx, 78.60.Fi
3+
II
II
Ce doped alkaline-earth thiogallates represented as M Ga S (M = Ca, Sr, Ba) are known as blue phos-
2
4
phors with high brightness and good purity of color. However the Ce concentration is limited below about
1 mol% in the process of the single crystal growth, presumably due to the charge imbalance of the dopant.
3+
Here in order to increase Ce concentration, alkaline metals and Ce ions are codoped in the alkaline earth
thiogallates and the effect of co-doping on the photoluminescence is studied. More than 15 mol% Ce dop-
ing is achieved, and PL intensity in Na codoped Ca and Sr thiogallates becomes three and five times
3+
stronger than that of the only Ce 1 mol% doping case, respectively.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
Rare earth elements doped alkaline earth thiogallates exhibit various visible light emission and are ex-
pected as materials suitable for EL devices [1, 2]. Especially, as the blue emission of Ce3+ doped com-
pounds shows high brightness and a broad spectrum, their application to laser devices is also expected
[3]. In order to investigate their possibility, we have so far grown single crystals of the compounds and
measured photoluminescence (PL) spectra. It was shown in our study that Ce ions could be incorporated
in single crystals of the alkaline earth thiogallates up to a concentration of 1 mol%. We have also re-
ported that excess Ce doping caused deterioration of the crystalinity of the compounds and reduction of
the emission intensity. When Ce ions are incorporated in the compounds, the divalent alkaline earth
metal ions are substituted by the trivalent Ce ions. The violence of the charge balance in the substitution
is considered to be a reason why the overdoped Ce3+ leads to the poor crystalinity and the decrease in the
emission intensity. Here, co-doping of Ce3+ with monovalent alkaline metals is performed to keep the
charge balance. Actually, the luminescence spectra of the mixed compounds of Sr Ce Na Ga S was
1–2x
x
x
2 4
already been reported [4], where it was shown that the solid solution was formed from x = 0 to 0.5 and
the emission reached its maximum at x = 0.1. In the case of CaGa S co-doped with Na+ and Ce3+, we
2
4
have reported that Ce3+ could be introduced up to the concentration of 30 mol% and the PL intensity of
the co-doped samples was enhanced three times in comparison to the case of only Ce doping samples
(1 mol%) [5].
In this paper, three alkaline metals (Li, Na, K) are selected as co-dopants of Ce3+ and the effect of co-
doping on the photoluminescence spectra will be discussed.
*
Corresponding author: e-mail: komaz@phys.chs.nihon-u.ac.jp, Phone: +81 3 5317 9772, Fax: +81 3 5317 9772
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
Paper
phys. stat. sol. (a) 203, No. 11 (2006)
2719
2 Sample preparation
Compounds of CaS, SrS, Ga S , Ce S , Li S, Na S and K S were used as starting materials. These
2 3 2 3 2
2
2
compounds were weighed to about 0.4 g in Ar atmosphere at an appropriate composition of
a
II
M1−2xCe M Ga S (MII = Ca, Sr, Ma = Li, Na, K). About twenty samples were prepared by varying x from
x
2 4
x
0.0 to 0.5. The starting materials were sealed under a vacuum of 1 × 10–4 Pa in a quartz ampoule
(7 mm × 40 mm), the inner surface of which was coated with carbon film by burning acetone gas.
The quartz ampoule was put in a furnace and heated up to 1120 °C a little above the melting point of
Ga S (1105 °C) for 1 h. The compounds were synthesized by the solid state reaction. The products after
2
3
synthesis were identified by the powder X-ray diffraction analysis and the crystalline phases were identi-
fied. The measurements of powder X-ray diffraction (XRD) were performed by using a Cu K line
α
(40 kV, 30 mA) at 300 K. It should be noted here that all the products were synthesized in evacuated
quartz ampoules so that they were considered as being synthesized according to the scheduled composi-
tions.
a
Figures 1 (a)–(c) show XRD patterns of Sr Ce M Ga S (Ma = Li, Na, K) between x = 0.1
1–2x
x
2 4
x
(10 mol%) and x = 0.5 (50 mol%). At low concentration up to x = 0.1 (10 mol%) a single phase belong-
24
ing to the space group of D -Fddd clearly appeared for all co-dopants. However unidentified lines
2h
emerged in the diffraction patterns according to the concentrations of Li, Na and K above x = 0.3
(30 mol%), x = 0.2 (20 mol%) and x = 0.15 (15 mol%), respectively. The diffraction lines do not shift
within the accuracy of our measurement and stay constant up to these x-values. In the case of
a
Ca Ce M Ga S (Ma = Li, Na, K), the similar tendency was also observed, though undefined lines
1–2x
x
2 4
x
appeared above x = 0.3 (30 mol%), x = 0.3 (30 mol%) and x = 0.4 (40 mol%) for Li, Na and K
co-dopants, respectively. The result dose not agree with that of the Sr Ce Na Ga S case of
1–2x
x
x
2 4
Ronot-Limousin et al. [4] where they showed the variation in the lattice constants with Na concentration.
They indicated, however, that this was not the case when Li was used as a co-dopant, presumably owing
to its small ionic radius. They did not describe the case of K co-dopant. The discrepancy between our and
their results may arise from the different synthetic methods used, and further detailed study should be
made on this point.
50mol%
(Ce,K)
50mol%
(Ce,Li)
CeGa4NaS8[1]
50mol%
(Ce,Na)
40mol%
(Ce,K)
40mol%
(Ce,Li)
40mol%
(Ce,Na)
30mol%
(Ce,K)
30mol%
(Ce,Li)
30mol%
(Ce,Na)
20mol%
(Ce,K)
20mol%
(Ce,Li)
15mol%
(Ce,K)
20mol%
(Ce,Na)
10mol%
(Ce,Li)
10mol%
(Ce,K)
10mol%
(Ce,Na)
SrGa2S4[1]
SrGa2S4[1]
SrGa2S4[1]
10
15
20
25
30
35
40
45
10
15
20
25
30
35
40
45
10
15
20
25
30
35
40
45
Diffraction angle 2theta [deg.]
Diffraction angle 2theta [deg.]
Diffraction angle 2theta [deg.]
a)
b)
c)
a
a
Fig. 1 Powder X-ray diffraction patterns of Sr Ce M Ga S (M = Li (a), Na (b), K (c)) in the x-value between 0
1–2x
x
x
2
4
and 0.5 together with the reported data of SrGa S and CeGa NaS . Unidentified lines are shown by the solid circles
2
4
4
8
except for (a) where most of the lines cannot be assigned owing to the large noises.
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3+
2720
C. Hidaka et al.: Effect of co-doping of Ce and alkaline metals on photoluminescence
1.0
1.0
0.8
0.6
0.4
0.2
0.0
SrGa2S4
CaGa2S4
Ce,Na
0.8
Ce,Li
Ce,Na
Ce, K
0.6
0.4
Ce,Li
0.2
Ce,K
a)
b)
0.0
1.5
1.5
2.0
2.5
3.0
3.5
4.0
2.0
2.5
3.0
3.5
4.0
Photon energy [eV]
Photon energy[eV]
a
a
Fig. 2 Photoluminescence spectra of Ca Ce M Ga S (a) and Sr Ce M Ga S (b) for varying
0.8
0.1
0,1
2
4
0.8
0.1
0.1
2
4
a
M as Li, Na and K at 300 K.
3 Photoluminescence spectra
a
3.1 Ca Ce M Ga S
1–2x x 2 4
x
The powdered samples were used in order to remove the effect of the shape of samples on the emission
intensity. A 325 nm He–Cd laser line was used as an excitation light. Photoluminescence spectra were
detected using a spectrometer equipped with a CCD detector (HAMAMATSU, PMA11) at room tem-
perature. Figure 2 shows the typical photoluminescence spectra showing the effect of co-doping on Ca
and Sr thiogallates. Two peaks of the emission spectra at 2.5 and 2.8 eV are due to the 5d–4f electronic
transitions of the Ce3+ ions in the host crystals. The integrated intensities of the emission spectra were
obtained and plotted as a function of the concentration of Ce and alkaline metal (Fig. 3(a)). The maxi-
a
mum intensity of Ca Ce M Ga S (M = Li, Na, K) show little dependence on the co-doping concentra-
1–2x x 2 4
x
tion of the alkaline metal ions, but becomes three times stronger than that of the only Ce3+ 1 mol% dop-
ing case.
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Ce, Li
Ce, Na
Ce, K
Na
Li
K
only Ce 3+1mol%
a)
b)
0
10
20
30
40
50
0
5
10
15
20
25
30
Concentration [mol%]
Concentration [mol%]
a
a
Fig. 3 Integrated intensity of photoluminescence spectra of Ca Ce M Ga S (a) and Sr Ce M Ga S
4
1–2x
x
x
2
4
1–2x
x
x
2
a
3+
(b) (M = Li, Na, K) as a function of x. The emission intensity of an only Ce doped sample was marked
as a cross. Solid lines are shown as guide for eye.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
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phys. stat. sol. (a) 203, No. 11 (2006)
2721
The peak energy of the emission shifts toward the low energy side with increasing x-value. It is con-
sidered that this is caused by the crystal field strengthened with increasing the number of Ce ions. When
the crystal field becomes stronger, the split of the excited states of 5d levels broadens and the energy
difference between the lowest excited state of 5d levels and 4f levels decreases.
a
3.2 Sr Ce M Ga S
1–2x x 2 4
x
a
The photoluminescence spectra of Sr Ce M Ga S were also measured, some examples of which are
1–2x x 2 4
x
shown in Fig. 2(b). The emission spectra have two peaks having the same origin as stated in the section
above. Figure 3(b) shows the integrated intensities as a function of Ce, Ma concentration in this case.
They have a maximum at 5–10 mol% for all compounds co-doped. In comparison to that of SrGa S
2
4
doped with only Ce3+ at 1 mol%, the maximum PL intensity was almost the same as that of K co-doped
compounds, and two and five times stronger in the case of Li co-doped and Na co-doped samples, re-
spectively. The Na co-doping was found to be the best. Since the charge compensation will be achieved
by the two ions having the same mean valence as the host ion, both of the separation and the ionic radii
of the two substituting ions should be made as close as possible to strengthen the effect. Thus the close-
ness of the ionic radii of Na+ (1.18 Å [6]) and Ce3+ (1.14 Å [6]) might work very well in this case, which
is expected to be more effective than to make smaller the difference in the ionic radii of alkaline
earth ions (Ca or Sr) and the alkaline metal ions (Li, Na, K). Ronot-Limousin et al. reported that
Sr Ce Na Ga S shows high external luminescence efficiency in the range of 0.05 ≦x ≦0.2 though
1–2x
x
x
2 4
our emission intensity decreases in the concentration above x = 0.1 as shown in Fig. 3(b). This may be
due to the undefined phase being included for x above 0.15 in our case.
The emission peak energies were determined only by Ce concentration independent of the kind of
co-dopants and shifted to the low energy side with increasing it. The reason is the same as described
previously.
4 Summary
a
a
We have prepared the compounds of Ca Ce M Ga S and Sr Ce M Ga S (M = Li, Na, K) and
1–2x x 2 4 2 4
1–2x
x
x
x
investigated the photoluminescence spectra in order to find the appropriate value of the Ce content
for obtaining the maximum emission intensity. From the powder X-ray diffraction analysis, it is
a
expected that the solid solution was formed up to 30 mol% for Ca Ce M Ga S and 15 mol% for
1–2x x 2 4
x
a
Sr Ce M Ga S . By incorporating alkaline metals as co-dopants, the content of Ce3+ in the thiogallates
1–2x
x
2 4
x
can be extremely increased above 1 mol%, the upper limit of Ce incorporation without co-doping.
The photoluminescence of these compounds has been measured at 300 K. Spectra due to the 5d–4f
a a
electronic transitions were obtained. In the both case of Ca Ce M Ga S and Sr Ce M Ga S , the
1–2x x 2 4 1–2x
x
2 4
x
x
emission intensity estimated from the spectra has a maximum approximately at 5 mol% for the Li and K
co-doped compounds and at 10 mol% for the Na co-doped ones. The difference in the maximum of the
a
emission intensity according to host materials was found. The compounds of Ca Ce M Ga S have
1–2x x 2 4
x
almost the same maximum emission intensity independent of the doped alkaline metals. On the other
a
hand, the PL intensity of the Sr Ce M Ga S compounds was strikingly affected by alkaline metals.
1–2x x 2 4
x
The reason for the difference in the two host compounds is not yet clarified and further detailed study
should be required. However, the co-doping of rare earth element ions with alkaline metals is considered
to be very effective to enhance the photoluminescence in (Ca, Sr) thiogallates.
Acknowledgement This work was partly supported by Research Grant from Nihon University.
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3+
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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