A novel orange emissive phosphor SrWO4:Sm3+ for white light-emitting diodes

Article abstract of DOI:10.1016/j.jallcom.2010.07.138

A novel orange emissive phosphor, Sm³⁺-doped SrWO₄, was synthesized by high temperature solid-state reaction in air atmosphere. The excitation spectra show that the phosphors can be efficiently excited by ultraviolet and near-ultraviolet light, the optimized concentration is 4 mol%. Three emission peaks locate at 562, 596 and 642 nm, corresponding to CIE chromaticity coordinates of (x = 0.54, y = 0.46), which indicates the orange light emitting. The decay curves are well fitted with triple-exponential decay models. The quantum yield of the Sr_0.96Sm_0.04WO ₄ phosphor is about 70.65% under excitation of 377 nm. Furthermore, the temperature-dependent luminescence indicates the phosphor exhibits a small thermal-quenching property. So the phosphor is able to be applied to UV-LED chip-based white light-emitting diodes.

Full text of DOI:10.1016/j.jallcom.2010.07.138

Journal of Alloys and Compounds 507 (2010) 133–136
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
A novel orange emissive phosphor SrWO4:Sm3+ for white light-emitting diodes
Zheng-Hua Ju, Rui-Ping Wei, Jing-Xin Ma, Chao-Ran Pang, Wei-Sheng Liu∗
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
A novel orange emissive phosphor, Sm3+-doped SrWO4, was synthesized by high temperature solid-state
reaction in air atmosphere. The excitation spectra show that the phosphors can be efficiently excited by
ultraviolet and near-ultraviolet light, the optimized concentration is 4 mol%. Three emission peaks locate
at 562, 596 and 642 nm, corresponding to CIE chromaticity coordinates of (x = 0.54, y = 0.46), which indi-
cates the orange light emitting. The decay curves are well fitted with triple-exponential decay models. The
quantum yield of the Sr0.96Sm0.04WO4 phosphor is about 70.65% under excitation of 377 nm. Furthermore,
the temperature-dependent luminescence indicates the phosphor exhibits a small thermal-quenching
property. So the phosphor is able to be applied to UV-LED chip-based white light-emitting diodes.
© 2010 Elsevier B.V. All rights reserved.
Article history:
Received 4 August 2009
Received in revised form 18 July 2010
Accepted 20 July 2010
Available online 29 July 2010
Keywords:
Phosphor
SrWO4:Sm3+
Optical properties
White LEDs
1. Introduction
as an important activator for many different inorganic lattice pro-
6
ducing reddish orange light emitting due to its 4G5/2 → HJ (J = 5/2,
White light-emitting diodes (LEDs) provide great superiorities
such as high efficiency, energy-saving, environment-friendly, fast
response, long lifetime and good reliability, etc.; thereby they have
attracted much attention. Considering energy savings and envi-
ronmental friendliness, white LEDs are most potential solid-state
lighting sources to replace conventional incandescent and fluo-
rescent lamps [1,2]. Therefore, white LEDs are in highly demand
especially in the field of solid-state lightings. Since much effect
has been devoted to the research of white LEDs, much progress
has recently been made. For example, many researches focused
on the phosphors used for phosphor-converted white LEDs [3–30],
and quantum dots (QDs) have been highlighted as color-converting
components in the fabrication of white LEDs [31–33]; some reports
directed towards the phosphor-free white LEDs [34,35]. Even so,
exploring new materials and technology is still appealing to most
of researchers.
There are several ways to gain white light. One of them
is the combination of UV-LED with cyan- and orange-emitting
phosphor materials. In recent years, some rare-earth ions acti-
vated tungstates and molybdates [2,36–44] have been extensively
researched because of the special properties of WO24− and MoO42−
group. These phosphors show excellent thermal and hydrolytic sta-
bility, and have strong absorption within the ultraviolet region;
therefore, tungstates and molybdates are good choice as host mate-
rials for white LEDs. Among rare-earth ions, Sm3+ ion is well known
7/2, 9/2, 11/2) transitions. Yang et al. [45] prepared SrZnO2:Sm3+
and studied the luminescent properties. Li et al. [46] synthesized
Y2O2S:Sm3+ by combustion method, and investigated its lumi-
nescent properties. The luminescent characteristics of LiCaBO3:M
(M = Eu, Sm, Tb, Ce, Dy) phosphor for white LED was investigated by
Li et al. [47]. For all that, little attention has been paid to the lumi-
nescent properties of Sm3+-doped tungstates and molybdates, this
motivated us to design and fabricate phosphor SrWO4:Sm3+. Fortu-
nately, we prepared orange light-emitting phosphor SrWO4:Sm3+
successfully. The phosphor can be applied to the white LEDs field
above mentioned.
In the present work, the luminescent properties of Sm3+-doped
SrWO4 were studied, and the influence of the concentration of Sm3+
on the luminescent intensity were investigated. The chromaticity
coordinates of x = 0.54 and y = 0.46 have been calculated from the
emissional spectra gained by the CIE (Commission International de
l’Eclairage, France) system, it does not mean a monochromatic color
having a single peak in the spectra but a mixture of three peaks (562,
596 and 642 nm). Since the ability to withstand high temperature
is a basic requirement for phosphor applied in LEDs, the thermally
stable property of the phosphor was investigated, and it turned out
to be excellent. So the phosphor is a potential orange component
for white light-emitting diodes that based on UV-LEDs.
2. Experimental
2.1. Samples preparation
∗
Corresponding author. Tel.: +86 931 8915151; fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
The phosphors Sr1−xSmxWO4 (x = 0.02–0.06) were synthesized by solid-state
reaction method. Appropriate amount of SrCO3 (A.R.), WO3 (A.R.) and Sm2O3 (99.9%)
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.07.138

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Z.-H. Ju et al. / Journal of Alloys and Compounds 507 (2010) 133–136
Fig. 2. Excitation spectra of Sr1−xSmxWO4 (ꢀem = 596 nm) with different contents of
Sm3+: x = 0.02 (a); x = 0.04 (b); x = 0.06 (c).
Fig. 1. XRD patterns JCPDS No. 08-0490 database standard for SrWO4 (a) and
Sr1−xSmxWO4 with different contents of Sm3+: x = 0.02 (b); x = 0.04 (c); x = 0.06 (d).
The excitation spectra of Sr1−xSmxWO4 (x = 0.02–0.06) moni-
tored at 596 nm are shown in Fig. 2. It can be seen from Fig. 2
that the excitation spectra consist of a series of peaks in the range
of 315–510 nm. The strongest one at 405 nm and some peaks at
318, 346, 364, 377, 420, 441, 465 and 481 nm are ascribed to the
were thoroughly mixed in an agate mortar and then triturated with distilled ethanol
for an ideal mixing. Afterwards, the mixtures were put into alumina crucibles and
calcined in a muffle furnace at 1050–1100 ◦C for 3 h in air, followed by subsequent
cooling in air to ambient temperature. The obtained samples were ground lightly
into powders and taken to characterization.
transitions from the ground state to the excited states of Sm3+
.
2.2. Characterization
These samples with similar peaks, but the differences of the
fluorescent intensity can be clearly observed from Fig. 2. Along
with the content increase of Sm3+, the fluorescent intensity also
increased, and that reached the highest intensity when the content
of Sm3+ increased to 0.04. After that, the fluorescent intensity began
to decrease.
The synthesized products were characterized by X-ray diffraction (XRD) using a
RigakuD/MAX-2400 X-ray diffractometer with Cu K␣ (ꢀ = 1.5406 A) at 40 kV, 60 mA.
˚
The XRD patterns were collected in the 2ꢁ range from 10◦ to 90◦ with a step of
0.02◦. The photoluminescent (PL) spectra were measured by a Hitachi F-4500 fluo-
rescence spectrophotometer equipped with a Xe lamp as the excitation source. The
chromaticity coordinates have been calculated from the emissional spectra using
the CIE (Commission International de l’Eclairage, France) system.
Fig.
3
shows the emission spectra of Sr1−xSmxWO4
(x = 0.02–0.06) excited under 364 nm at ambient temperature.
It is can be obviously seen that these samples with the same
shape and positions, at least three strong peaks located at 562,
596 and 642 nm can be observed, and they can be assigned to the
Fluorescent lifetime and fluorescent quantum yield measurements were car-
ried out on an Edinburgh FLS 920 combined fluorescence lifetime and steady state
spectrometer. The FLS 920 spectrometer is equipped with an optical parametric
oscillator for lifetime measurement and a 450-W Xenon lamp for quantum yield
measurement. The quantum yield was measured by absolute method using an inte-
grating sphere. Firstly, no sample was placed on the sample holder, laser light alone
is detected by the spectrometer, and the integral value of this blank was labeled as
A. Secondly, the sample was placed on the sample holder and detected by the spec-
trometer, the integral value of sample of excitation was labeled as B, and the integral
value of sample of emission was labeled as C. The quantum yield was calculated with
the following formula: QY = C/(A − B) × 100%.
6
4G5/2 → HJ (J = 5/2, 7/2, 9/2) transitions of Sm3+ ion [47]. The peak
at 596 nm is the strongest one; two secondary peaks exist at 562
and 642 nm, respectively. The luminous colors of the phosphors
are orange as result of complex spectra. In general, luminous color
is represented by color coordinates and color ratios. At the present
work, the values of chromaticity coordinate of the phosphor
Sr0.96Sm0.04WO4 has been calculated using the CIE (Commission
Temperature-dependent luminescence of phosphor were measured at
20–200 ◦C using a Hitachi F-4500 fluorescence spectrophotometer with a Xe lamp as
the excitation source. The powder sample was heated to desired temperatures and
kept for 20 min for each temperature point to reach thermal equilibrium. To sim-
ply evaluation the temperature dependence, the normalized PL intensity was used
which was defined as relative PL intensity (T) = PL intensity (T)/PL intensity (20 ◦C).
All measurements were carried out at room temperature except for the
temperature-dependent luminescence.
3. Results and discussion
Fig. 1 shows the X-ray diffraction (XRD) patterns of the standard
pattern JCPDS No. 08-0490 and sample SrWO4:Sm3+. All the strong
peaks of the samples can be assigned to the tetragonal phase SrWO4
˚
˚
(a = 5.416 A and c = 11.95 A) reported in the literature (JCPDS 08-
0490).
It can be considered that the as-obtained samples have good
crystallinities and no other impurities were observed in the prod-
ucts. The strontium tungstate (SrWO4) belongs to sheelite structure
[48], and W6+ occupies the tetrahedral sites constructed by O2−
in this structure; composing WO24− anion complex. Sr2+ is eight-
coordinated by O2−, thus formed a distorted cube. The radius of
Sr2+ is 1.12 A, Sm 0.96 A and W 0.62 A, and it can be presumed
that the Sm3+ ions substitute for the Sr2+ sites.
3+
6+
˚
˚
˚
Fig. 3. Emission spectra of Sr1−x SmxWO4 (ꢀex = 364 nm) with different contents of
Sm3+: x = 0.02 (a); x = 0.04 (b); x = 0.06 (c).

Z.-H. Ju et al. / Journal of Alloys and Compounds 507 (2010) 133–136
135
Fig. 4. Fluorescent decay curves of Sr0.96Sm0.04WO4 emission at 596 nm excited by
377 nm (red line shows the fitting result). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
Fig. 5. Temperature-dependent luminescence of Sr0.96Sm0.04WO4 (ꢀex = 364 nm).
in Fig. 5. It can be seen that the phosphor shows unusual thermal-
quenching behavior. The fluorescent intensity increased as the
temperature was elevated. At a certain temperature (180 ◦C), the
fluorescent intensity approached to a maximum, and the decrease
as the temperature increased continuously. At 190 ◦C, the fluores-
cent intensity still maintains about 100% of that measured at the
temperature of 20 ◦C. Although the reasons need further study, nev-
ertheless, the results indicating that the phosphor SrWO4:Sm3+
has a small thermal-quenching property and good suitability for
application in white LEDs.
International de l’Eclairage, France) system. It has been found that
the sample has chromaticity coordinates of x = 0.54 and y = 0.46,
which shows orange light emission.
It also can be seen from Fig. 3 that the concentration of Sm3+ ions
has little affection on the position of emission peaks; but seriously
affected the fluorescent intensity. It was quite clear that the fluores-
cent intensity increase with the increasing concentration of Sm3+
ion up to x = 0.04, then decreased beyond the concentration due to
concentration quenching. The reason lies in that odds of complex
of non-radiative transition is increasing as the content of Sm3+ is
increasing, as a result, the fluorescent intensity of the phosphors
becomes lower.
The quantum yield of Sr0.96Sm0.04WO4 is 70.65% when the phos-
phor under excitation of 377 nm. The result indicates that the
phosphor can be effectively excited by ultraviolet light.
The emission spectra of Sr1−xSmxWO4 (x = 0.02–0.06) excited
by 346 and 377 nm show identical features with that excited by
364 nm, and the optimized concentration is 4 mol%, however, the
fluorescent intensity is lower than that under 364 nm excitation.
The result indicates that the phosphor can be more effectively
excited by ultraviolet 364 nm light. The chromaticity coordinates
of the phosphor Sr0.96Sm0.04WO4 have been calculated from the
emissional spectra (ꢀex = 377 nm) using the CIE system, it has chro-
maticity coordinates of x = 0.54 and y = 0.46, which also shows
orange light emission.
At room temperature, for phosphor Sr0.96Sm0.04WO4, the flu-
orescent decay curves corresponding to the emissional line at
562, 596 and 642 nm of the Sm3+ ion with identical features
(ꢀex = 377 nm). As an instance, the fluorescent decay curve of
Sr0.96Sm0.04WO4 for 596 nm emission and fitting curve are shown
in Fig. 4. The decay behavior can be expressed as follows:
4. Conclusions
In summary, a series of phosphors Sr1−xSmxWO4 (x = 0.02–0.06)
were prepared by solid-state method. The fluorescent intensities of
the phosphors excited by 364, 377 and 346 nm reach a maximum
at x = 4 mol%. The results of fitting the experimental data of fluo-
rescent decay curves revealed that Sm3+-doped SrWO4 phosphor
in terms of a triple-exponential decay model. The fluorescent life-
times of the phosphor emitted at 562, 596 and 642 nm is closer. The
quantum yield of the phosphor Sr0.96Sm0.04WO4 is about 70.65%
under excitation of 377 nm. In addition, the phosphor shows a small
thermal quenching, at 190 ◦C, the fluorescent intensity still main-
tains about 100% of that measured at the temperature of 20 ◦C.
Given good overall luminescent performances, this phosphor is a
potential candidate as orange-emitting component for white LED.
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
−t
−t
−t
Acknowledgements
I = I0 + A1 exp
+ A2 exp
+ A3 exp
ꢂ1
ꢂ2
ꢂ3
The authors thank the National Natural Science Foundation of
China (20771048, 20621091) for financial support.
where I and I0 are phosphorescence intensity, A1, A2 and A3
are constants and t is time: ꢂ1, ꢂ2 and ꢂ3 are decay time for
exponential components, respectively. They are well fitted with
triple-exponential function, and the fluorescent lifetimes of the
sample emitted at 562 nm is measured to be ꢂ1 = 0.0035 ms,
ꢂ2 = 0.11 ms and ꢂ3 = 0.75 ms. The values of fluorescent life-
time emitted at 596 nm are ꢂ1 = 0.0031 ms, ꢂ2 = 0.097 ms and
ꢂ3 = 0.75 ms; the data of fluorescent lifetime emitted at 642 nm are
ꢂ1 = 0.0037 ms, ꢂ2 = 0.096 ms and ꢂ3 = 0.74 ms, respectively.
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