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Cite this: J. Mater. Chem., 2011, 21, 10818
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BaZrSi3O9:Eu2+: a cyan-emitting phosphor with high quantum efficiency for
white light-emitting diodes
De-Yin Wang, Chien-Hao Huang, Yun-Chen Wu and Teng-Ming Chen*
Received 6th January 2011, Accepted 9th May 2011
DOI: 10.1039/c1jm00080b
In this paper, a cyan-emitting phosphor BaZrSi3O9:Eu2+ was synthesized and evaluated as a candidate
for white light emitting diodes (WLEDs). This phosphor shows strong and broad absorption in 380–
420 nm region, and the emission intensity of the optimized BaZrSi3O9:Eu2+ was found to be 90% and
198% of that of the commercial BaMgAl10O17:Eu2+ (BAM:Eu2+) under excitation at 405 nm and 420
nm, respectively. Upon excitation at 405 nm, the quantum efficiency of the optimized BaZrSi3O9:Eu2+ is
83% of that of BAM:Eu2+. The performance of this phosphor was further tested to fabricate white LED
lamps. By coating BaZrSi3O9:Eu2+ with a green-emitting (Ba,Sr)2SiO4:Eu2+ and a red-emitting
CaAlSiN3:Eu2+ on a near-ultraviolet (405 nm) LED chip, driven by a 350 mA forward bias current,
intense warm white light with a color rendering index of 90 has been produced.
1 Introduction
With the advantages of long operation lifetime and energysaving, white light emitting diodes (WLEDs) have been considered as the most promising light source for solid state lighting
and gained much attention in the past few years.1–5 White light
can be produced by a combination of a blue LED chip with
a yellow-emitting phosphor.4,6–11 However, such white light has
some disadvantages such as low color-rendering index (CRI) and
high correlated color temperature (CCT) due to the deficiency of
the red emission.6,7 An alternative way to produce white with
high CRI may be based on a combination of a near ultraviolet
(n-UV) LED chip (380–420 nm) with red, green, and blueemitting phosphors.6–9 Undoubtedly, phosphors play a crucial
role in producing high quality white light; as a result, the demand
for new phosphors in those spectral region has been rapidly
increasing.6–9
In this study, our attention is focused on BaZrSi3O9 (BZS) as
a host, which possesses a unique cyclosilicate mineral with two
formula units per unit cell.12–14 The crystal structure of BaZrSi3O9 consists of [Si3O9]6 rings with trigonally distorted BaO6
and ZrO6 octahedra,12–14 in which Ba2+ and Zr4+ ions are in
a parallel row along c axis as represented in Fig. 1. The Eu2+ ions
are expected to enter the Ba2+ sites in crystal lattice and they will
experience the negative charges of the nearest anion (O2 ions) in
addition to the positive charges of the neighboring cations
(Ba2+/Eu2+ ions) in the row direction. The positive charges can
orient one d orbital preferentially and will make Eu2+ emitting at
longer wavelength.15 Therefore, a blue or green even a red
Phosphors Research Laboratory and Department of Applied Chemistry,
National Chiao Tung University, Hsinchu, 30010, Taiwan. E-mail:
[email protected]; Tel: +886-35731695
10818 | J. Mater. Chem., 2011, 21, 10818–10822
emission from Eu2+ is expected in the host matrix of BZS. Based
on these considerations, the luminescence properties of BZS:Eu2+
were investigated and reported in this work with an aim to
explore new phosphors for n-UV LEDs and provide valuable
information for the application of this compound.
2
Experimental
2.1 Materials synthesis
Samples of BZS:xEu2+ (0.5 # x # 20%) were synthesized by
solid-state reactions. Stoichiometric amounts of BaCO3 (99.9%,
Aldrich), ZrO2 (99.5%, Aldrich), SiO2 (99.6%, Aldrich) and
Eu2O3 (99.9%, Aldrich) were ground in an agate mortar, then the
obtained mixtures were calcined at 1400 C for several hours
under a reducing atmosphere (5%H2/95%N2).
Fig. 1 Crystal structures of BaZrSi3O9.
This journal is ª The Royal Society of Chemistry 2011
WLED lamps were fabricated by integrating a mixture of
optical grade transparent silicon resin and phosphors blending of
cyan-emitting BZS:6%Eu2+, green-emitting (Ba,Sr)SiO4:Eu2+
commodity (Intematix-527) and red-emitting CaAlSiN3:Eu2+
commodity (Intemaix-R630) on a 405 nm n-UV GaN chip (AOT
Product No: C06HC, Spec: 405V09C, wavelength peak: 400–405
0.93 nm, chip size: 40 40 ml, forward voltage: 3.8–4.0 0.01
V, power: 80–90 3.85 mW).
of (Ba0.94Eu0.06)ZrSi3O9. The starting model for (Ba0.94Eu0.06)
ZrSi3O9 was built with the crystallographic data taken from
ICSD-70105 for the structure of Ba(Zr0.97Ti0.03)Si3O9 with space
group P6c2. The Rietveld analysis results indicate that the
weighted profile R-factor (Rwp) and the expected R factor (Re)
are 9.83% and 4.71%, respectively. Some selected crystallographic data of this compound obtained from Rietveld refinement are listed in Table 1.
2.2 Characterizations
3.2 Steady-state photoluminescence spectra of BZS:Eu2+
The phase purity of all BZS:xEu2+ samples was verified by using
powder X-ray diffraction (XRD) analysis with a Bruker AXS D8
advanced automatic diffractometer operated at 40 kV and 40 mA
The structure refinement
with Cu Ka radiation (l ¼ 1.5418 A).
was done by the general structure analysis system (GSAS)
program.16 The photoluminescence (PL) and PL excitation
(PLE) spectra of the samples were measured by using a Spex
Fluorolog-3 Spectrofluorometer equipped with a 450 W Xe light
source. The luminescence decay curves were measured on
a tunable nanosecond optical-parametric–oscillator/Q-switchpumped YAG:Nd3+ laser system (Ekspla). The quantum efficiency (QE) was measured by an integrating sphere whose inner
face was coated with Spectralon equipped with a spectrofluorometer (Horiba Jobin-Yvon Fluorolog 3–22 Tau-3). Thermal
quenching was tested using a heating apparatus (THMS-600) in
combination with PL equipment. The electroluminescence (EL)
spectra were recorded under a forward bias current 350 mA and
measured by using an integrating sphere with LED measurement
starter packages.
The PLE and PL spectra of BZS:xEu2+ (0.5% # x # 20%) are
shown in Fig. 3. The PLE spectra of BZS:xEu2+ (0.5% # x #
20%) show broad absorption bands from 300 to 420 nm, which
were attributed to the parity-allowed Eu2+ 4f7 / 4f65d1transition,15 making target samples interesting for application in n-UV
LEDs. It has been reported by Poort et al. that the Eu2+ 4f65d1
state would be in a lower energy state if Eu2+ ions are arranged in
a linear chain, resulting from the stabilization of d orbital.15 In
view of the ionic charges and ionic radii with 6 coordination
(rBa2+ ¼ 135 pm, rZr4+ ¼ 72 pm and rEu2+ ¼ 117 pm),17 the
incorporated Eu2+ ions in BZS are in the Ba2+ sites, which are in
a parallel row along c axis as mentioned above. These Eu2+ ions
3 Results and discussion
3.1 XRD refinement and crystal structure of (Ba0.94Eu0.06)
ZrSi3O9
All samples for BZS:xEu2+ (0.5% # x # 20%) were identified as
single phase. Fig. 2 shows the results of Rietveld refinement on
the XRD pattern of a powder sample with chemical composition
Table 1 Crystallographic data for (Ba0.94Eu0.06)ZrSi3O9
Formula
(Ba0.94Eu0.06)ZrSi3O9
Radiation type
2q range/deg
Temperature/K
Symmetry
Space group
a/A
b/A
c/A
a/deg
b/deg
g/deg
3
Volume/A
Z
Rp
Rwp
Re
c2
1.5418 A
10–100
295
Hexagonal
P6c2
6.7595(1)
6.7595(1)
10.0037(4)
90
90
120
395.841(2)
2
6.88%
9.83%
4.71%
10.27%
Atom coordinates
Ba
Zr
Si
O1
O2
Eu
x/a
y/b
z/c
Fraction
2
U/A
0.66667
0.33333
0.05863
0.23609
0.06968
0.66667
0.33333
0.66667
0.27823
0.18599
0.40887
0.33333
0.00000
0.00000
0.25000
0.25000
0.11968
0.00000
0.94000
1.00000
1.00000
1.00000
1.00000
0.06000
0.01642
0.00423
0.00745
0.01525
0.01381
0.01860
Seleted bond lengths/A
Fig. 2 Rietveld refinement of the powder XRD pattern of
Ba0.94Eu0.06ZrSi3O9 (observed—cross, calculated—red line, difference
between the observed and the calculated—bottom blue line, and Bragg
positions—vertical bars).
This journal is ª The Royal Society of Chemistry 2011
Ba–O2
Ba–O2
Ba–O2
Zr–O2
Zr–O2
Zr–O2
Si–O1
Si–O1
2.77922(7)
2.77924(7)
2.77928(7)
2.13083(5)
2.13086(5)
2.13083(5)
1.60483(4)
1.60075(4)
Ba–O2
Ba–O2
Ba–O2
Zr–O2
Zr–O2
Zr–O2
Si–O2
Si–O2
2.77928(7)
2.77924(7)
2.77922(7)
2.13086(5)
2.13080(5)
2.13080(5)
1.55533(4)
1.55533(4)
J. Mater. Chem., 2011, 21, 10818–10822 | 10819
Fig. 3 PLE (lem ¼ 475 nm) and PL (lex ¼ 405 nm) spectra of BZS:xEu2+
(0.5% # x # 20%).
along c axis experience negative charges from the nearest
O2 anions and positive charges of the neighboring Eu2+/Ba2+
cations; as a consequence, the crystal field orients the d orbital in
the chain direction preferentially. The preferred orientation
lowers the energy of d orbitals and will result in the photoluminescence emission of Eu2+ at longer wavelength. Accordingly, as can be seen in Fig. 3, BZS:xEu2+ (0.5% # x # 20%)
exhibits a cyan emission with a maximum wavelength at about
475 nm under 405 nm excitation. The cyan emission of the
phosphor is attributed to the 4f65d–4f7 transition of the Eu2+ ion.
The experimental optimal Eu2+ concentration (x) in BZS:xEu2+
was found to be 6%, from which the PL intensity of BZS:xEu2+
begins to decrease with increasing Eu2+ concentration due to the
concentration quenching effect.18 In the meantime, the Eu2+
emission and excitation spectra overlapped partially, and, as
a consequence, the high energy part of the emission (resonant
with the low energy part of the excitation spectra) is reabsorbed,
which resulted in Eu2+ emission with slight red shifting at high
Eu2+ concentration (i.e., x ¼ 20%). If we consider energy transfer
between two identical centers, the critical distance (Rc) is defined
as the distance for which the probability of energy transfer equals
the probability of radiative emission of Eu2+.19 There are two
common methods for the determination of Rc, one is the Blasse
eqn (1) and the other is the Dexter eqn (2) for energy transfer by
dipole–dipole interaction.19
1=
3
3V
Rc ¼ 2
(1)
4pXc N
Rc6 ¼ 0:63 1028
ð
4:8 1016 P
fs ðEÞfa ðEÞdE
E4
Lo ðlÞ Li ðlÞ
Lo ðlÞ
(3)
Ei ðlÞ ð1 4ÞEo ðlÞ
Le ðlÞ4
(4)
4¼
h¼
where Lo(l) is the integrated excitation profile when sample is
diffusely illuminated by the integrated sphere’s surface, Li(l) is
the integrated excitation profile when sample is directly excited
by the incident beam, Ei(l) is the integrated luminescence of
sample upon direct excitation, Eo(l) the integrated luminescence
of sample excited by indirect illumination from the sphere, and
Le(l) is the integrated excitation profile obtained from the empty
integrated sphere (without the sample present). Upon excitation
at 405 nm, the optical absorbance (4) of BZS:6%Eu2+ and BAM:
Eu2+ phosphor was calculated to be 47% and 42%, and the corresponding quantum efficiency (h) was 73% and 88%, respectively. These data indicate that BZS:Eu2+ has a relatively high
quantum efficiency and optical absorbance.
(2)
The definition of each symbols in these equations can be found
elsewhere.18–21 In the eqn (1) proposed by Blasse, the value of Rc
is derived from the critical concentration where concentration
quenching occurs18–21 while in Dexter eqn (2), the value of Rc is
determined from the spectral overlap.18–21 In the present case, by
3, N ¼ 2, and Xc ¼ 6%, Rc is
using the values V ¼ 395.8 A
determined to be 18 A from eqn (1). Furthermore, since the
allowed electric–dipole transitions are involved in the case of
10820 | J. Mater. Chem., 2011, 21, 10818–10822
Eu2+, P isÐ 102 for an allowed 4f–5d transition,18–20 and the values
E and fs ðEÞfa ðEÞd E were calculated to be 2.79 eV and
0.024 eV1 from the normalized excitation and emission spec is obtained
trum BZS:6%Eu2+; then, a corresponding Rc of 15 A
from eqn (2). We have observed a satisfactory agreement
between the values of Rc obtained from the two different
methods, showing energy transfer mechanism in this system is
governed by electrostatic interaction, but the presence of
exchange interaction cannot be excluded even, the latter is
between
dominant for short distance (a typical distance of 5 A)
the luminescent centers.
The luminescence intensity and quantum efficiency of a phosphor are important parameters to be considered for practical
application. As a reference, Fig. 4 gives a comparison of the PL
spectra of BZS:6%Eu2+ with that of BAM:Eu2+ under 405 nm
and 420 nm excitations, respectively. The PL intensity of BZS:6%
Eu2+ was found to be 90% and 198% of that of BAM:Eu2+ under
405 nm and 420 nm excitation, respectively. The optical absorbance (4) and quantum efficiency (h) were calculated by using the
following equations:22,23
Fig. 4 Comparison of PL spectra of BZS:6%Eu2+ with that of BAM:
Eu2+ under 405 nm and 420 nm excitations.
This journal is ª The Royal Society of Chemistry 2011
3.3 Time-resolved photoluminescence of BZS:Eu2+
2+
Fig. 5 presents the normalized decay curves for BZS:xEu (0.5%
# x # 8%) under pulse laser excitation at 430 nm. These decays
were analyzed at the maximum of Eu2+ emission at 475 nm and
the data were plotted as a semi-logarithmic plot. When BZS:xEu2+
(0.5% # x # 8%) is excited into Eu2+ 5d excited state under pulse
laser excitation at 430 nm, it was found that Eu2+ 5d–4f emission
decays exponentially with a lifetime 1.3 ms. The lifetime of Eu2+
in BaZrSi3O9:Eu2+ is somewhat longer than that usually
observed (0.4–1.2 ms).24 Eu2+ ions are arranged in linear chain in
the crystal lattice of BaZrSi3O9:xEu2+, and such a one-dimensional structure would orient one of the Eu2+ d-orbitals preferentially as reported previously.15 As a result, the Eu2+ excited
state would be delocalized, which may account for the observed
long Eu2+ decay lifetime. Similar rationalizations have been
described by Poort et al. in Ba2Mg(BO3)2:Eu2+, in which the long
Eu2+ lifetime (5.4 ms) was observed.24 The inset in Fig. 5 shows
the measured lifetime (s) of Eu2+ 5d–4f emission at various Eu2+
concentrations (x), from which it can be seen the lifetime of Eu2+
decreases with increasing Eu2+ concentration. In particular, the
lifetime of Eu2+ decreased from 1.390 ms to 1.224 ms when doped
Eu2+ concentration increased from 0.5% to 8%. The measured
lifetime is related to the total relaxation rate by:25,26
1 1
¼ þ Anr þ Pt
s s0
3.4 Thermal quenching properties of BZS:xEu2+
For the application in high power LEDs, thermal quenching
property of a phosphor is an important parameter to be
considered. Temperature dependent emission spectra for
BZS:6%Eu2+ under excitation at 405 nm were investigated and
shown in Fig. 6. The inset displays a comparison of the thermal
luminescence quenching of BZS:6%Eu2+ with that of BAM:Eu2+
and dependence of the full widths at half-maximum (FWHM) of
BZS:6%Eu2+ emission on temperature. As can be seen in Fig. 6,
the thermal stability of BZS:6%Eu2+ is inferior to that of BAM:
Eu2+, as supported by the observation that the PL intensity of
BAM:Eu2+ drops for only 20% when the temperature was
raised up to 300 C, while the PL intensity of BZS:6%Eu2+ has
dropped to 50% of its initial value at a temperature of 200 C and
drops more obviously with higher temperature. This observation
can be rationalized by the fact that increasing temperature has
increased the population of higher vibration levels, the density of
phonons, and the probability of non-radiative transfer (energy
migration to defects), and these factors explain why the FWHM
of the emission bands broaden while the emission intensities
decrease with increasing temperature.18
(5)
where s0 is the radiative lifetime, Anr is the nonradiative rate due
to multiphonon relaxation, and Pt is the energy transfer rate due
to energy transfer. With increasing Eu3+concentration, both the
energy transfer rate between Eu2+–Eu2+ and the probability of
energy transfer to killer sites (such as defects) increased and, as
a result, the lifetime shortened with increasing Eu2+ concentra in
tion. However, due to the longer Ba–Ba distance (5 A)
BaZrSi3O9:xEu2+, the low energy transfer rate between Eu2+ ions
may result and, thus, a large change in the decay time with
variation of the Eu concentration was not observed in BaZrSi3O9:xEu2+. As the concentration quenching results from energy
transfer processes, the observed decay investigation results also
Fig. 5 Normalized decay curves for BZS:xEu2+(0.5% # x # 8%) under
pulse laser excitation at 430 nm (lem ¼ 475 nm). The inset shows the
lifetime (s) of Eu2+ emission at various Eu2+ concentration (x).
This journal is ª The Royal Society of Chemistry 2011
further support that concentration quenching occurs in BZS:
xEu2+.
3.5 Electroluminescence (EL) of BZS:Eu2+ and fabrication of
LED lamps
In order to further investigate the potential of BZS:Eu2+ in the
application of n-UV LED-pumped WLEDs, shown in Fig. 7 is
the EL spectrum of a fabricated WLED lamp driven under a 350
mA forward bias current, and the inset in the upper-right shows
the photographs of the fabricated WLED lamp and its emission
color under the same forward bias. The Commission International de l’Eclairage (CIE) color coordinates (x, y), correlated
color temperature (CCT) and color rendering index (Ra) of the
generated white light were found to be (0.37, 0.41), 4450 K and
Fig. 6 Temperature-dependent PL spectra of BZS:6%Eu2+. The inset
shows a comparison of thermal quenching of BZS:6%Eu2+ with that of
BAM:Eu2+ and dependence of the FWHM of BZS:6%Eu2+ emission on
temperature.
J. Mater. Chem., 2011, 21, 10818–10822 | 10821
Acknowledgements
We gratefully thank the National Science Council of Taiwan for
financial support under Contract nos. NSC99-2811-M-009-052
(D.-Y.W.) and NSC98-2113-M-009-005-MY3 (T.-M.C.) and Dr
Li-Yang Luo for transient measurements.
Notes and references
Fig. 7 EL spectrum of the WLED lamp fabricated by coating a phosphor blending of BZS:6%Eu2+ (cyan-emitting), (Ba,Sr)2SiO4:Eu2+ (greenemitting) and CaAlSiN3:Eu2+(red-emitting) on a n-UV chip (405 nm)
driven by a 350 mA forward bias current. The inset photographs are the
fabricated WLED lamp and the emission color of the WLED.
Table 2 Full set of the 14 CRIs and the Ra of the fabricated WLED
lamp
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 Ra
95 94 89 85 94 93 86 83 87 89
87
91
94
94
90
90, respectively. Here Ra was determined from the full set of the
first eight CRIs shown in Table 2. Our WLED package exhibits
a higher Ra value (90) and a lower CCT value (4450 K) than
those of the WLED fabricated by combining a yellow YAG:Ce3+
phosphor with a blue InGaN chip (Ra ¼ 75, CCT ¼ 7756 K).27
4 Conclusions
In summary, we have synthesized and investigated a new cyanemitting phosphor BaZrSi3O9:Eu2+, which can be efficiently
excited over a broad spectral range from 300 to 420 nm. The
crystal structure of BaZrSi3O9:Eu2+ was determined by the
Rietveld refinement on powder sample. Upon excitation of
the optimized BaZrSi3O9:Eu2+ at 405 nm, we have observed that
its absorbance and quantum efficiency are as high as 112% and
83% of that of the BAM:Eu2+ commodity. The WLED lamp
fabricated with an n-UV chip, green/red-emitting phosphors and
cyan-emitting BaZrSi3O9:Eu2+ produced a white light possessing
a higher color rendering index (90) and a lower correlated color
temperature (4450 K).
10822 | J. Mater. Chem., 2011, 21, 10818–10822
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This journal is ª The Royal Society of Chemistry 2011