Ca1-2xEuxLixMoO4: A novel red phosphor for solid-state lighting based on a GaN LED

Article abstract of DOI:10.1149/1.1856924

A novel red phosphor Ca_1-2xEu_xLi_xMoO ₄ with high performance is reported. Under the excitation of 254 run UV light, its luminescent intensity is comparable to that of the commercial red phosphor Y₂O₃:Eu³⁺. With high Eu³⁺ concentration, the phosphor has strong excitation due to f-f transitions of Eu³⁺ appearing around 395 nm. Thus, it is a promising material for solid-state lighting based on GaN light-emitting diode (LED).

Full text of DOI:10.1149/1.1856924

G186
Journal of The Electrochemical Society, 152 ͑3͒ G186-G188 ͑2005͒
0
013-4651/2005/152͑3͒/G186/3/$7.00 © The Electrochemical Society, Inc.
Ca1À2xEu Li MoO : A Novel Red Phosphor for Solid-State
x
x
4
Lighting Based on a GaN LED
Jiaguo Wang,a,b Xiping Jing,a,z Chunhua Yan, and Jianhua Lina
a
aThe State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry
and Molecular Engineering, Peking University, Beijing 100871, China
b
College of Chemistry and Materials Science, Wenzhou Normal University, Wenzhou 325027, China
A novel red phosphor Ca1Ϫ2xEu Li MoO with high performance is reported. Under the excitation of 254 nm UV light, its
x
x
4
3
ϩ
3ϩ
luminescent intensity is comparable to that of the commercial red phosphor Y O :Eu . With high Eu concentration, the
2
3
phosphor has strong excitation due to f-f transitions of Eu3 appearing around 395 nm. Thus, it is a promising material for
ϩ
solid-state lighting based on GaN light-emitting diode ͑LED͒.
©
2005 The Electrochemical Society. ͓DOI: 10.1149/1.1856924͔ All rights reserved.
Manuscript submitted June 25, 2004; revised manuscript received August 18, 2004. Available electronically January 24, 2005.
Since Nakamura et al.1 reported blue light emitting diodes
component for SSL.
16,17
Recently, Neeraj et al. also reported similar
1
8
͑
LEDs͒ suitable for commercial application in 1994, great progress
materials. In the present paper, we report the detailed results of
Ca1Ϫ2xEuxLixMoO4 phosphor.
2-8
has been achieved on GaN based LEDs, the emissions of which
cover from blue to near-UV region. Besides indicators, traffic lamps,
and full color displays, another important application for LEDs is
the assembly solid-state lighting devices ͑SSL͒. The devices are pre-
dicted to have many advantages, such as high efficiency, long life-
time, and long duration, over the daily used incandescent and fluo-
rescent lamps. There are several ways to assemble the SSL. A
conventional device is a combination of a blue LED with a yellow
Experimental
Reagents Eu O ͑99.99%͒, MoO ͑99.95%͒, CaCO ͑F.L͒, and
Li CO ͑A.R͒ were used for sample preparations. They were
weighed on stoichiometric ratio and ground by using an agate mor-
tar and pestle. To mix the raw materials homogenously, during
grindings, small amount of enthanol was added. After grinding, the
mixtures were preheated at 550°C for 3 h, and then reheated at
2
3
3
3
2
3
phosphor (YAG:Ce3 ), in which the phosphor absorbs blue light
ϩ
9
8
00°C for another 2 h in a muffle furnace. Finally, the samples were
and emits yellow light. Blending the blue light from the LED and
yellow light from the phosphor results in white light. This scheme is
relatively easy to be fulfilled and the device has been commercial-
ised. The degradation rates of LED and phosphor are not isochro-
nous, thus the scheme of blue LED ϩ yellow phosphor appears
ground into powder for characterizations. Phase purities were ana-
lyzed by using a Rigaku-2000 X-ray diffractometer ͑XRD͒. Excita-
tion and emission spectra were measured by using a Hitachi F4500
fluorescent spectrophotometer.
color shift. Another approach assembling the SSL is combining a
Results and Discussion
near-UV LED with tri-color phosphors,1
0-14
in which the near-UV
19
CaMoO belongs to the Scheelite structure with a tetragonal
4
LED emits UV/violet in the range 370-410 nm, then the UV/violet
lights excite the tri-color phosphors giving white light. Because the
visible emissions in this device are mainly from phosphors rather
than from LED and the variations of the emission features of the
tri-color phosphors with temperature are in similar rates, the color
shift can be reduced. Because most of the photoluminescent phos-
phors developed previously were for the fluorescent lamp applica-
tions, which are suitable for the excitation by 254 nm UV light,
phosphor that can be effectively excited by the near-UV light
is lacking, especially the red ones. The phosphors,
unit cell: a ϭ 0.5226 nm, c ϭ 1.1430 nm, Z ϭ 4, space group
2ϩ
2Ϫ
I4 /a. In this structure, Ca is eight-coordinated by O , and
1
6ϩ
2Ϫ
Mo
occupies the tetrahedral sites constructed by O . When
3ϩ
ϩ
doped with Eu , an equal amount of Li had to be added for the
charge compensation. Phase analysis indicated that all samples,
doped and undoped, have the same XRD pattern as CaMoO4
͑
Fig. 1͒, which suggested that the doped samples formed solid solu-
3ϩ
ϩ
2ϩ
tions. The doping mechanism should be Eu ϩ Li ϭ 2Ca
;
Sr (PO ) Cl :Eu ͑em 447 nm͒, MgBaAl O :Eu2ϩ ͑em 450
2
ϩ
10
4
6
2
10 17
15
2ϩ
2ϩ
2ϩ
nm͒, MgBaAl O :Mn , Eu ͑em 520 nm͒ and SrGa S :Eu
10
17
2 4
͑
em 530 nm͒ have a wide range of excitation spectra and their long
wavelength edge extends beyond 400 nm. Thus, they can be em-
ployed as the blue and green components for SSL. However, red
phosphors suitable for excitation by near-UV light are rare. Com-
3
ϩ
3ϩ
3ϩ
mercially used Y O :Eu , Y O S:Eu , and YVO :Eu do not
2
3
2
2
4
ϩ
2
have enough absorption in the near-UV region. Eu activated red
phosphors MS:Eu2 (M ϭ Ca,Sr,Ba) have absorptions in blue re-
gion rather than in near-UV region. Mn activated red phosphor
Mg O FGeO :Mn has wide absorption band, but its emission
ϩ
4
ϩ
4
ϩ
4
3.5
2
wavelength is at ϳ660 nm which is insensitive to the human eye.
Consequently, red phosphors are the bottleneck for the advancement
3
ϩ
of SSL. Our research showed that the red phosphors of Eu doped
molybdate have relatively strong absorption in the near-UV region
with good color purity, thus they are promising candidates as red
Figure 1. Typical XRD patterns of the sample in the system
z
E-mail: xpjing@chem.pku.edu.cn
Ca1Ϫ2xEu Li MoO .
x
x
4
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Journal of The Electrochemical Society, 152 ͑3͒ G186-G188 ͑2005͒
G187
Figure 4. Excitation spectrum of Ca0.5Eu0.25Li0.25MoO4 .
Figure 2. Variation of the unit cell volume with Eu3 content x in the
ϩ
system Ca1Ϫ2xEu Li MoO .
x
x
4
ponent of the tri-color phosphors, the color purity of this
phosphor is satisfied. Excited by 295 nm UV light ͑the optimised
wavelength for Ca1Ϫ2xEu Li MoO ), the emission intensity of
Eu3 and Li prefer to occupy the same sites of Ca2ϩ. Both Eu
͑
ϩ
ϩ
3ϩ
x
x
4
ϩ
2ϩ
20
106.6 pm͒ and Li ͑92 pm͒ have similar radii to Ca ͑112 pm͒,
thus a whole range of the solid solutions was formed in this system,
represented by the formula: (Ca1Ϫ2xEu Li )MoO , 0 р x р 0.5,
Ca0.84Eu0.08Li0.08MoO was about three times of that of commercial
4
3ϩ
phosphor Y O :Eu , while under the excitation of 254 nm
2
3
x
x
4
UV light, both phosphors gave identical intensities. Thus
see Fig. 2.
As a host matrix, CaMoO has been suggested as a laser material
3ϩ
Ca1Ϫ2xEu Li MoO may be used as a replacement of Y O :Eu
x
x
4
2
3
3ϩ
4
for fluorescent lamps. With the increase of the Eu concentration,
when doped with Nd3 or Dy
ϩ
3ϩ 21,22
.
Without doping, CaMoO
3ϩ
4
the intensity of the excitation peaks due to the f-f transitions of Eu
2
3
emits faint green light with an emission peak at 520 nm, where the
complex ion MoO4 is considered as the luminescent center. In the
present research, the excitation and emission spectra of CaMoO4
were also measured, see Fig. 3. Figure 3 also represents the excita-
increases, e.g., for the sample of x ϭ 0.25, these transitions be-
comes dominating ones ͑Fig. 4͒. Because the f-f transitions are lo-
cated in the wavelength range of blue and near-UV ͑370-410 nm͒,
this phosphor is suitable to be used for white lighting devices based
on GaN light emitting diodes.
2
Ϫ
tion and emission spectra of the low concentration doped sample
(
x ϭ 0.06). Eu3 and Li co-doped samples showed typical Eu
ϩ
ϩ
3ϩ
The variation of the luminescent intensity with Eu3 concentra-
tion was investigated. As the wide band excitation and f-f excitation
have different excitation mechanisms, when excited by UV lights
with different wavelengths, the samples performed different inten-
ϩ
red emissions with color coordinates x ϭ 0.665 and y ϭ 0.335. The
excitation spectrum may be divided into two regions: the strong and
wide excitation band appearing below 350 nm may be assigned to
the combination of the charge transfer transition of Eu -O and
MoO group; weak and narrow peaks above 350 nm may be as-
3
ϩ
2Ϫ
3ϩ
sity dependence on Eu concentration ͑Fig. 5͒. The intensity of the
2
4
Ϫ
samples excited by 295 nm UV ͑wide band excitation͒ approaches
maximum at x ϭ 0.1, then concentration quenching occurs, while
no concentration quenching appears when the samples were excited
by 395 nm light ͑f-f excitation͒, the intensity increases with the
3
ϩ
signed to the f-f transitions of Eu . The emission spectrum was
dominated by the red peaks at 613 and 616 nm due to the electric
5
7
3ϩ
dipole transition D → F , which indicates that Eu occupies a
0
2
15
3ϩ
3ϩ
site lacking inversion symmetry. The orange peak at 592 nm due
Eu concentration below x ϭ 0.3 and then saturates at higher Eu
5
7
to magnetic dipole transition D → F is weak. To be a red com-
concentration.
0
1
ϩ
Figure 3. Excitation and emission spectra of Ca0.88Eu0.06Li0.06MoO4 ͑solid
line, labeled as P͒ and host (ϫ30, dot line, labeled as H͒.
Figure 5. Variations of luminescent intensities with Eu3 content x in the
system Ca1Ϫ2xEu Li MoO .
x
x
4
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G188
Journal of The Electrochemical Society, 152 ͑3͒ G186-G188 ͑2005͒
cation. In particular, with high Eu3 concentration, the phosphor has
the strong f-f excitations in the near-UV region, thus it can be a good
candidate as a red component for solid state lighting based on GaN
LED.
ϩ
Acknowledgment
We are thankful to the financial support from the State Key Pro-
gram for Basic Research of China ͑G1998061306͒, the National
Nature Science Foundation of China ͑20221101͒, and the High-Tech
Research Program of Zhejiang ͑2003C31029͒.
Peking University assisted in meeting the publication costs of this article.
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3
ϩ 24
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Ca1Ϫ2xEu Li MoO is a high performing phosphor and may be
x
x
4
3
ϩ
used as a replacement of Y O :Eu for the fluorescent lamp appli-
2
3
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