Photoluminescence properties of green-emitting ZnGa2S 4:Eu2+ phosphor

Article abstract of DOI:10.1149/2.099205jes

Green-emitting ZnGa₂S₄:Eu²⁺ phosphors have been synthesized by solid state reaction. Their luminescence properties have been investigated by photoluminescence excitation and emission spectra, concentration quenching, thermal quenching and photoluminescence decay. The critical dopant concentration is found to be 0.05 mol of Eu²⁺ and the critical transfer distance of Eu²⁺ is calculated as 17 . The thermal quenching result suggests that the phosphor does not have good thermal-quenching property. Its chromaticity coordinates are very close to those of SrGa₂S₄:Eu²⁺. Because of their broadband absorption in the region 350-520 nm, the ZnGa₂S₄:0. 05Eu²⁺ phosphor can be a good green phosphor candidate for creating white light in phosphor-converted white LEDs, when combined with RB phosphors and a UV LED.

Full text of DOI:10.1149/2.099205jes

J188
Journal of The Electrochemical Society, 159 (5) J188-J192 (2012)
0
013-4651/2012/159(5)/J188/5/$28.00 © The Electrochemical Society
Photoluminescence Properties of Green-Emitting
2
+
ZnGa S :Eu Phosphor
2
4
Ruijin Yu,a,b Ruixin Luan, Caifeng Wang, Jingtao Chen, Zhenxing Wang,
a
a
a
a
b
b,∗,z
Byung Kee Moon, and Jung Hyun Jeong
a
College of Science, Northwest A&F University, Yangling, Shaanxi 712100, China
Department of Physics, Pukyong National University, Busan 608-737, South Korea
b
Green-emitting ZnGa2S4:Eu2 phosphors have been synthesized by solid state reaction. Their luminescence properties have been
+
investigated by photoluminescence excitation and emission spectra, concentration quenching, thermal quenching and photolumi-
2
+
2+
nescence decay. The critical dopant concentration is found to be 0.05 mol of Eu and the critical transfer distance of Eu is
calculated as 17 Å. The thermal quenching result suggests that the phosphor does not have good thermal-quenching property. Its
2
+
chromaticity coordinates are very close to those of SrGa2S4:Eu . Because of their broadband absorption in the region 350–520
nm, the ZnGa2S4:0.05Eu phosphor can be a good green phosphor candidate for creating white light in phosphor-converted white
LEDs, when combined with RB phosphors and a UV LED.
2
+
©
2012 The Electrochemical Society. [DOI: 10.1149/2.099205jes] All rights reserved.
Manuscript submitted December 23, 2011; revised manuscript received February 13, 2012. Published March 7, 2012.
II
Luminescence properties of the ternary compounds M M
2
was replaced by Ar to prevent the possible oxidation of the mixture at
temperatures below 700 C.
III(S,Se)
doped with various rare earth ions have been investigated
◦
4
1
–6
during the past decades. Nowadays, the compounds have been paid
more attention to another potential application for phosphor con-
verted light-emitting diodes (pc-LEDs) as solid-state lighting (SSL).
:Eu greenish-yellow phosphors showed a
higher luminescent efficiency (120, 110%, respectively) than com-
The structure of the final products was determined via X-ray
powder diffraction (XRD) using a Rigaku D/max 2200 vpc X-ray
diffractometer with Cu Kα radiation at 40 kV and 30 mA. The XRD
2
+
2+
◦
◦
CaGa
2
S
4
:Eu , Sr
2
Ga
2
S
5
patterns were obtained in the 15 ≤ 2θ ≤ 70 range. The photolu-
minescence (PL) and photoluminescence excitation (PLE) spectra of
3
+
3+
2+
mercial Y
3
Al
5
O
12:Ce (YAG:Ce ) phosphor and they also can be
ZnGa
2
S
4
:Eu were measured using a Fluorolog-3 spectrofluorome-
7,8
2+
fabricated with blue-chips to produce white light. SrGa
2
S
4
:Eu has
ter (Jobin Yvon Inc/specx) equipped with a 450 W Xe lamp and dou-
ble excitation monochromators. The decay curves and temperature-
a wide excitation band extending into the blue region, it is applicable
as a green phosphor for white LEDs in which blue LEDs are used to
2
+
dependent PL spectra of the phosphor ZnGa S :Eu were recorded
2 4
9,10
excite the phosphor.
SrGa
2
S
4:Sn,Re (Re = Ce, Gd) also has been
by a FLS920-Combined Fluorescence Lifetime and Steady State Spec-
trometer (Edinburgh Instruments), equipped with a 450 W xenon
lamp, a 150 W nF900 nanosecond flash lamp with a pulse width of 1
ns and pulse repetition rate of 40–100 kHz.
11
studied as possible phosphors of white LEDs. Yu et al. described the
2
+
structural and luminescent properties of Ca1−xSr
x
(Ga1−yAl ) S :Eu
phosphors. Changing the values of x and y, the emission peak shifts
y 2 4
almost linearly on the composition, which allows continuous tuning
of the peak emission peak from 496 nm (SrAl
2
S
4
:Eu) to 556 nm
:Eu). They all can meet the application requirements for
GaN based LEDs.
The ZnGa compounds have a bandgap at 3.18 eV and are ex-
Results and Discussion
12
(
2 4
CaGa S
Phase characterization.— The crystal structure of ZnGa
2
S
4
has
¯
been refined to be tetragonal, space group I42m with a = 5.297 Å,
2 4
S
3
15
13
c = 10.363 Å, and V = 290.77 Å . The as-prepared ZnGa S :
2+
pected as excellent phosphors. It has a defect chalcopyrite structure
by replacing a half of Zn site cations with vacancies and belongs to the
2 4
0
.01Eu sample was characterized by X-ray diffraction (XRD) pat-
¯
14,15
terns to verify the phase purity. As indicated in Fig. 1, we have ob-
I42m tetragonal structure.
Each metal atom is tetrahedrally sur-
2
+
served that the doped Eu ions have no obvious influence on the struc-
rounded by four sulfur atoms, while a sulfur atom is tetrahedrally sur-
2+
16
ture of the host, and all the diffraction peaks of the ZnGa S :0.01Eu
sample were in good agreement with the phase of ZnGa S (JCPDS
2 4
rounded by one Zn atom, two Ga atoms, and one vacancy. The pho-
2 4
2
+
toluminescence properties of ZnGa
2
S
4
:Eu have been investigated
1
7–20
2+
several years ago.
dicated that ZnGa
The excitation spectrum of ZnGa
2
S
4
:Eu in-
S
2 4
:Eu2 phosphor can be used in light-emitting
+
diode. However, its diffuse reflectance spectra, photoluminescence
decay, thermal quenching, and chromaticity coordinates properties as
a phosphor for LEDs have been never reported yet. In this work, we
2
+
2 4
synthesized Eu -doped ZnGa S thiogallate phosphors by high tem-
perature solid-state reactions, and described some new luminescent
properties of the phosphors.
Experimental
Ga
2
S
3
was prepared from Ga
2
O
3
(A.R.) under flowing H
2
S gas
◦
at 950 C for 2 h. EuS was prepared from Eu
reducing atmosphere at 1200 C for 3 h. Stoichiometric amount of raw
2
O
3
(99.99%) with CS
2
◦
materials, ZnO (A.R.), Ga
were mixed homogeneously and sintered for 2 h at 950 C in a flowing
2
S
3
(self-prepared), and EuS (self-prepared)
◦
2 2
H S stream. In the initial and the final heat-treatment stages, the H S
∗
Electrochemical Society Active Member.
E-mail: jhjeong@pknu.ac.kr
2
+
z
Figure 1. XRD pattern of ZnGa S :0.01Eu phosphor.
2 4
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Journal of The Electrochemical Society, 159 (5) J188-J192 (2012)
J189
Figure 2. Diffuse reflectance spectra of ZnGa2S4:xEu2 with different Eu
+
2+
concentrations.
4
0-1462). The lattice parameters a = 5.293 Å and c = 10.473 Å ob-
2+
2 4
Figure 3. Excitation (a, λem = 540 nm) and emission (b, λ = 395 nm; c,
ex
tained from the XRD pattern of the ZnGa
agreement with the literature data.
S :0.01Eu sample are in
2
+
.
15
λex = 460 nm) spectra of ZnGa2S4 :Eu
The diffuse reflectance spectra of the phosphors were measured and
are shown in Fig. 2. The absorption peaks of all the samples are similar
in their spectral features with a little difference in the relative intensity
2
+
The CIE chromaticity coordinates of ZnGa S :0.05Eu phosphor
2
4
are calculated in terms of the emission spectrum, and the value are
2
+
with Eu concentration increases. It can be seen that the strong
absorptions in the range 250–350 nm do not almost change with the
x = 0.299, y = 0.673. Since the excitation spectra are not well re-
2
+
solved, the position of the lowest 5d excited level of Eu (λ ) is
abs
2
+
increase of Eu concentration, which is attributed to the absorption of
the host. Another strong absorption occurs from the UV to the visible
generally estimated by using the mirror-image relationship between
21,22
the emission and the excitation spectra.
The Stokes shift (ꢀS) can
7
6
1
spectral region (370–550 nm), which is caused by the 4f →4f 5d
be estimated by taking twice the energy difference between the zero
phonon line and the energy of the emission maximum. The position of
the zero phonon line is taken to be the intersection point of the excita-
tion and emission spectra. In the present case, the lowest absorption
energy of Eabs is about 2.52 eV (492 nm) and the Stokes shift (ꢀS) is
transition of the Eu2 ion. With the Eu concentration increasing,
+
2+
this absorption becomes stronger, which yields the phosphor powder
23
color varying from pale green to dark green.
:0.05Eu2 are
+
The PL excitation and emission spectra of ZnGa
2
S
4
−
1
shown in Fig. 3. The excitation spectrum exhibits a broad absorption
from 350 to 520 nm, which are attributed to the host absorption and the
calculated to be 0.22 eV (1772 cm ).
2
+
Recently the PL properties of Eu -activated MGa S (M = Ca,
2 4
7
8
6
7
1
2+
7–10,24
4
f ( S7/2)→4f ( F)5d transitions of Eu ion, and consistent with the
Sr, Ba) thiogallate compounds for LEDs have been studied.
The
diffuse reflectance spectra as presented in Fig. 3. Because the broad
excitation matches well with Ga(In)N chip emission, ZnGa
crystallographic data of MGa S and the luminescence characteris-
2
4
2
+
2+ 2+
2
S
4
:xEu
tics of MGa S :Eu are summarized in Table I. Usually Eu ions
2 4
6
1
7
phosphors are suitable for n-UV or blue LED chip excited solid state
lighting. As it has a broad excitation, this phosphor can be excited
with different wavelengths. It can be seen from Figs. 3b and 3c,
exhibit broad-band emission, which is attributed to the 4f 5d –4f
transition, and the wavelength positions of the emission bands depend
very much on a host, changing from the n-UV to the red. Although
these compounds have similar chemical compositions, their crystal
structures are different. Previous works reported that the emission
wavelengths of MGa S :Eu shifted to longer wavelengths (red-
shift) as replacing M atoms with smaller ones due to larger crystal
2
+
ZnGa S :0.05Eu phosphor shows a green emission band peaking
2 4
at 540 nm under 395- and 460-nm excitations. No differences are
observed for the emission band shape and position for different exci-
tation wavelengths except for the luminescent intensity. The full width
at half maximum (fwhm) of the emission spectrum (curve c) is 47 nm.
These results coincide well with those of Yuta and Whit1 while the
maximum (565 nm) as well as the fwhm (about 100 nm) presented by
2
+
2
4
2
+
2+
field splitting. Such as BaGa S :Eu (λem = 504 nm), SrGa S :Eu
2
4
2 4
7
2+
(λem = 535 nm), and CaGa S :Eu (λem = 558 nm). The red-shift
2
4
and the emission wavelengths generally increase with decreasing the
18
Tagiev et al. show no agreement.
2+
2+
2+
size of ‘M’ cation (Ba = 1.36 Å, Sr = 1.26 Å, Ca = 1.12 Å).
Table I. Crystallographic data of MGa2S4 and the luminescence characteristics of MGa2S4:Eu2 (M = Ca, Sr, Ba, Zn).
+
Formula
CaGa2S4
SrGa2S4
BaGa2S4
ZnGa2S4
Crystal system
Space group
a (Å)
Orthorhombic
Orthorhombic
D2h -Fddd
Cubic
Th -Pa3
Tetragonal
I42m
5.293
5.293
10.473
293
540
47
1772
24
24
6
¯
D2h -Fddd
20.122
20.090
12.133
4904
20.855
20.511
12.213
5224
535
12.685
12.685
12.685
2041
504
b (Å)
c (Å)
3
V (Å )
Emission (nm)
fwhm (nm)
558
50
2079
49
2000
62
4000
Stokes shift (cm 1)
−
CIE
References
(0.410, 0.580)
(0.270, 0.690)
(0.143, 0.506)
(0.299, 0.673)
This work,
5
,9,12
5,9,12
5,21
15
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J190
Journal of The Electrochemical Society, 159 (5) J188-J192 (2012)
:Eu2 had to show the longer wavelength emis-
+
Theoretically, ZnGa S
2 4
2
+ 2+
sion than SrGa
was much smaller than Sr and Ca. However, in this experiment, the
2 4 2 4
S :Eu and CaGa S :Eu because Zn atomic size
2
+
emission wavelength of ZnGa S :Eu was almost same with that of
2 4
2+
2
+
2+
SrGa
2
S
4
:Eu . Even though Zn size was smaller than Sr , the lat-
2 4
was also small comparing with SrGa S ,
tice parameter of ZnGa
2
S
4
leading to the almost same crystal field splitting energy, and so emis-
sion bands of these two phosphors were located at the similar wave-
2
+
length. Therefore, the red-shift did not apply to ZnGa S :Eu in spite
2 4
2
+ 25
.
of the even smaller ionic radius of Zn
2
+
It is generally accepted that the Eu concentration plays an im-
portant role in the searching of optimal composition of phosphor.
Therefore, the emission spectra of ZnGa S :Eu at various concen-
trations excited by 460 nm light and the dependence of PL intensity
2
+
2 4
2
+
2+
of Zn1−xGa
2
S
4
:xEu on Eu concentration (x = 0.005–0.10) are
shown in Fig. 4. It is observed that the PL intensity increases with the
2
+
increasing of Eu content until reaches a maximum at x = 0.05, then
2
+
it falls steadily as the content of Eu further increases due to concen-
tration quenching, which is mainly caused by the non-radiative energy
Figure 5. The decay curves of Eu2 in ZnGa2S4:xEu at different concen-
trations (λex = 460 nm, λem = 540 nm) (x = 0.005, 0.01, 0. 03, 0.05, 0.07,
and 0.10).
+
2+
2
+
transfer among the identical Eu ions. The quench concentration is
20
very close to the reported value x = 0.04.
While discussing the mechanism of energy transfer in phosphors,
Blasse has pointed out that if the activator is introduced solely on Z
radiation re-absorption comes into effect only when there is broad
overlap of the fluorescent spectra of the sensitizer and activator and
ion sites, x
c
is the critical concentration, N the number of Z ions in the
2
+
unit cell and V the volume of the unit cell, then there is on the average
one activator ion per V/x
the critical concentration of Eu . The critical transfer distance (Rc) is
in the view of the emission and excitation spectra of ZnGa S :Eu
2
4
26
c
N. From Fig. 4, it is clear that 0.05 mol is
phosphor is unlikely to be occurring in the case. Since the fluorescent
2
+
2+ 2+
mechanism of Eu in ZnGa S :Eu phosphor is the 4f–5d allowed
2 4
approximately equal to twice the radius of a sphere with this volume:
electric-dipole transition, the process of energy transfer should be
controlled by electric multipole–multipole interaction according to
ꢀ
ꢁ
1
/3
3
V
29,30
Dexter’s theory.
R
c
≈ 2
[1]
2
+ 2+
4
πχ
c
N
The decay curves of Eu in the phosphor ZnGa S :xEu have
2 4
been measured at different concentrations (x = 0.005, 0.01, 0. 03, 0.05,
3
By taking the appropriate values of V, N and x
and 0.05, respectively), the critical transfer distance of center Eu in
c
(298.27 Å , 2,
0
.07, and 0.10) (λex = 460 nm, λem = 540 nm). The emission decay
2
+
2+
curves of Eu ion are well fitted with a second order exponential
Equation 2 :
2
+
ZnGa
2
S
4
:Eu phosphor was found to be 17 Å. Non-radiative energy
31
2
+
transfer between different Eu ions may occur by exchange inter-
2
7,28
I(t) = A
1
exp(−t/τ
1
) + A
2
exp(−t/τ
2
)
[2]
action, radiation reabsorption, or multipole-multipole interaction.
2
+
2+
Eu is an isolated emission center in ZnGa
2
S
4
:Eu phosphor. The
f 5d –4f transition of Eu is allowed while exchange interaction
is responsible for the energy transfer for forbidden transitions and
where I is the luminescence intensity, t is time, τ and τ
slow and fast components of the decay lifetimes, and A
the fitting parameters, respectively. The average lifetimes of Eu , τ¯ ,
could be calculated as Equation 3 :
1
2
are the
and A are
2
6
1
7
2+
4
1
2+
◦
29
typical critical distances are then about 5 A. This indicates that the
mechanism of exchange interaction plays no role in energy transfer
32
2
+
2+
2
2
2
between Eu ions in ZnGa
2
S
4
:Eu phosphor. The mechanism of
A
1
1
τ + A
2
2
τ
τ
1
τ¯ = A
[3]
τ
1
+ A
2
A representative pattern is shown in the inset of Fig. 5. The aver-
2
+
age lifetimes of Eu ions were determined to be 0.126, 0.115, 0.102,
2
+
0
0
.091, 0.085 and 0.079 μs for ZnGa
2
S
4
:xEu with x = 0.005, 0.01, 0.
3, 0.05, 0.07 and 0.10, respectively (Table II). With the concentration
increasing, the lifetime values decrease, due to the parity allowed elec-
2
+
tric dipole transitions of Eu ions with high transition probabilities.
The decay times of these transitions are submicroseconds. Generally,
2
+
at high concentrations of Eu , the migration of energy nonradiatively
2
+
between the Eu increases, resulting in a decrease in the lifetime.
In the solid-state lighting application, a lower-temperature quench-
ing effect is in favor of keeping the chromaticity and brightness of
white light output. The temperature dependence of the PL spectra
2
+
for the ZnGa
shown in Fig. 6. With an increase in temperature from 300 K to
80 K, the PL intensity decreases slowly. The thermal quenching
2 4
S :0.05Eu phosphor under excitation at 460 nm is
4
Table II. The lifetimes of Eu2 in ZnGa2S4:xEu at different
+
2+
concentrations (λex = 460 nm, λem = 540 nm) (x = 0.005, 0.01, 0.
0
3, 0.05, 0.07, and 0.10).
2
+
Figure 4. Dependence of PL intensities of ZnGa2S4:xEu (λex = 460 nm)
Concentration
0.005
0.126
0.01
0.03
0.05
0.07
0.10
2
+
with varying Eu concentrations. The upper inset represents the concentration
influence on the emission intensities of ZnGa2S4:xEu phosphors (x = 0.005,
2
+
τ¯ (μs)
0.115
0.102
0.091
0.085
0.079
0
.01, 0.03, 0.05, 0.07, and 0.10).
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Journal of The Electrochemical Society, 159 (5) J188-J192 (2012)
J191
Figure 6. Dependence of PL emission of ZnGa2S4:0.05Eu2 (λex = 460
+
nm) on temperature. The inset shows the relationship of relative intensity and
temperature.
temperature T50 defined as the temperature at which the emission in-
2
+
tensity is 50% of its original value, is 407 K for ZnGa
2
S
4
:0.05Eu
.
With heating the sample up to 420 K at which the LEDs usually work,
the emission intensity of it remains at about 44% of that measured at
room temperature. Dorenbos has proposed that the main mechanism
Figure 8. The CIE chromaticity diagram of MGa2S4:Eu2 (M = Ca, Sr, Ba,
+
Zn) phosphors.
2
+
responsible for the thermal quenching of 5d–4f emission of Eu in
solids is the ionization of the electron from the lowest energy level
2
+
6
1
of the relaxed Eu 4f 5d electronic configuration to the host lattice
3
0,33
conduction band level.
Following this suggestion, it is responsi-
ear regression, the thermal activation energy for quenching is found
to be ∼ 0.32 eV.
ble that the thermally activated ionization from the excited 5d state
2
+
of the Eu ion is for temperature quenching of the luminescence
The Commission International del’Eclairage (CIE) chromaticity
2
+
2+
in ZnGa
2
S
4
:Eu . The temperature dependence of the luminescence
coordinates of the MGa S :Eu (M = Ca, Sr, Ba, Zn) were depicted
2
4
34,35
intensity is described by the following Arrhenius equation
in Fig. 8 in form of four rectangles. The chromaticity coordinates of
2
+
2+
phosphors CaS:Eu (0.680, 0.310) and BaMgAl10O17:Eu (BAM)
(0.144, 0.072), which are two primary color phosphors used for flu-
orescent lamps, are also depicted in the figure. The chromaticity co-
ordinates of ZnGa S :Eu phosphor fall into the green region and
are very close to those of SrGa S :Eu . It is due to the similar-
ity of the photoluminescence spectra of both phosphors. As we can
see, if combined with other high efficiency blue and red phosphors,
ZnGa S :Eu should be a good green component phosphor for UV-
I
0
I(T ) = 1
[4]
+ c exp(−E/κT )
is the initial intensity, I (T) is the intensity at a given temper-
2
+
where I
0
2 4
2
+
ature T, c is a constant, E is the activation energy, which represents the
energy difference between the lowest excited state and the bottom of
the host lattice conduction band, and k is Boltzmann’s constant (8.62
2 4
−
5
2+
×
10 eV). Figure 7 plots ln[(I
0
/I)−1] vs. 1/T. When performing lin-
2
4
excited white LEDs.
Conclusions
Here, a green Eu2 -activated ZnGa
S thiogallate phosphor has
2 4
+
been synthesized by solid-state reaction and its luminescent properties
are investigated. The excitation and emission spectra of this phosphor
7
6
1
2+
show that all are broadband due to 4f –4f 5d transitions of Eu . The
PL emission spectrum shows a broad band peaking at 540 nm. The
2
+
luminescence characteristics of thiogallate phosphors MGa
M = Ca, Sr, Ba, Zn) have been compared in this paper. The optimal
S :Eu
2 4
(
2
+
2
2 4
concentration for Eu in ZnGa S is about 5 mol%. The critical
+
transfer distance of Eu is calculated as 17 Å and the mechanism of
concentration quenching is determined to be the multipole-multipole
interaction. The thermal quenching result suggests that the phosphor
does not have good thermal-quenching property. The energy barrier for
thermal quenching is confirmed as 0.32 eV by the Arrhenius equation.
Its chromaticity coordinates are very close to those of SrGa
2 4
S :Eu.
Because of their broadband absorption in the region 350–520 nm, the
2
+
ZnGa S :0.05Eu phosphor can be a good green phosphor candidate
2 4
Figure 7. Arrhenius plot of the temperature dependence of the PL emission
intensity of ZnGa2S4: 0.05Eu
for creating white light in phosphor-converted white LEDs, when
combined with RB phosphors and a UV LED.
2
+
.
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Journal of The Electrochemical Society, 159 (5) J188-J192 (2012)
Acknowledgments
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(
This work was supported by Chinese Universities Scientific Fund
QN2011119). And this research was supported by the Industrials
Strategic Technology Development Program (Project No: 10037416)
funded by the Ministry of Knowledge Economy (MKE, Korea).
(
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