Synthesis and photoluminescence properties of Ce3+ and Eu2+-activated Ca7Mg(SiO4)4 phosphors for solid state lighting

Abstract

Ce3+ and Eu2+ singly doped and Ce3+/Eu2+-codoped Ca7Mg(SiO4)4 phosphors are synthesized by the conventional solid state reaction. The Ce3+ activated sample exhibits intense blue emission under 350 nm excitation, the composition-optimized Ca7Mg(SiO4)4 : 4%Ce3+ shows better color purity than the commercial blue phosphor, BaMgAl10O17 : Eu2+ (BAM : Eu2+) and exhibits superior external quantum efficiency (65%). The Ca7Mg(SiO4)4 : Eu2+ powder shows a broad emission band in the wavelength range of 400-600 nm with a maximum at about 500 nm. The strong excitation bands of the Ca7Mg(SiO4)4 : Eu2+ in the wavelength range of 250-450 nm are favorable properties for applications as light-emitting-diode conversion phosphors. Furthermore, the energy transfer from the Ce3+ to Eu2+ ions is observed in the codoped samples, the resonance-type energy transfer is determined to be due to the dipole-dipole interaction mechanism and the critical distance is obtained through the spectral overlap approach and concentration quenc

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This journal is c the Owner Societies 2012
Phys. Chem. Chem. Phys.
Citethis: DOI: 10.1039/c2cp23343f
Synthesis and photoluminescence properties of Ce3+and Eu2+-activated
Ca7Mg(SiO4)4phosphors for solid state lightingw
Yongchao Jia,abHui Qiao,abYuhua Zheng,abNing Guoaband Hongpeng You*a
Received 22nd October 2011, Accepted 20th December 2011
DOI: 10.1039/c2cp23343f
Ce3+and Eu2+singly doped and Ce3+/Eu2+-codoped Ca7Mg(SiO4)4phosphors are synthesized
by the conventional solid state reaction. The Ce3+activated sample exhibits intense blue emission
under 350 nm excitation, the composition-optimized Ca7Mg(SiO4)4:4%Ce3+shows better color
purity than the commercial blue phosphor, BaMgAl10O17:Eu2+(BAM:Eu2+) and exhibits
superior external quantum efficiency (65%). The Ca7Mg(SiO4)4:Eu2+powder shows a broad
emission band in the wavelength range of 400–600 nm with a maximum at about 500 nm.
The strong excitation bands of the Ca7Mg(SiO4)4:Eu2+in the wavelength range of 250–450 nm
are favorable properties for applications as light-emitting-diode conversion phosphors.
Furthermore, the energy transfer from the Ce3+to Eu2+ions is observed in the codoped
samples, the resonance-type energy transfer is determined to be due to the dipole–dipole
interaction mechanism and the critical distance is obtained through the spectral overlap
approach and concentration quenching method.
1.Introduction
Due to the advantages of high efficiency, long lifetime, low
power consumption and environment-friendly characteristics,
solid state light (SSL) using phosphor converted white light-
emitting-diodes (pc-WLEDs) to generate white light is the
current research focus not only in scientific areas, but also in
technical industry.1–5To generate white light from LEDs,
the most frequent and simple method is to combine an
InGaN-based blue diode with the yellow phosphor material
Y3Al5O12:Ce3+. However, the limitations of the application
of the single yellow phosphor are high color temperature
(greater than 4500 K) and low color-rendering index (CRI)
which is due to the deficiency of red light in the LED spectrum,
meaning that this method cannot meet the requirements of
applications needing high color rending properties.6–8Another
approach to realize white emission is to use tricolor broadband-
emitting phosphors with a near-UV LED (n-UV). Compared
with the above ‘‘Blue+Yellow’’, the ‘‘n-UV +blue/green/red’’
would allow the application of many more types of luminescent
materials and exhibit high color rending properties and low color
point variation against the forward-bias currents.9–14With the
development of efficient LEDs that emit light in the n-UV range,
most research interest has been paid to this approach to attain
high color rendering and wide range of color temperatures for
meeting the requirement of various applications.
Customers require new phosphors that offer good color and
durability at low cost for white LEDs. Silicate matrixes are the
most attractive candidates as mother structures due to several
merits, such as facile synthesis, high chemical stability, good
thermal quenching, and exhibiting intense luminescence when
doped with the proper luminescent center.7,15–20As a member
of the silicate family of hosts, the bredigite Ca7Mg(SiO)4
(CMSO) attracted our attention. The crystal structure of
bredigite has been determined by Moore and Araki (1976)
and the corresponding biological applications have been
reported.21–23As far as we know, however, there are few
reports on the luminescent properties of rare-earth ion activated
CMSO, and further study about this system is urgently
needed.24Ce3+and Eu2+ions are the most important acti-
vators for LED applications. Their emissions, arising from
orbitally allowed d - f electronic transitions, are wavelength
tunable because they are sensitive to the crystal field splitting
and nephelauxetic effects. For the above reasons, we choose
the CMSO as the host, and Ce3+and Eu2+as the activators in
the present work, and give detailed information on three
aspects: (1) We studied the luminescence behavior of Ce3+
ion doped CMSO samples carefully, and compared it with that
of the commercial blue phosphor (BaMgAl10O17:Eu2+) in
terms of color purity. (2) The concentration effect on the
optical properties of CMSO:Eu2+was discussed. (3) Efficient
energy transfer between the Ce3+and Eu2+ions was observed
in the host lattice, and related results were investigated to shed
light on the energy transfer mechanism.
aState key Laboratory of Rare Earth Resource Utilization,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, P. R. China. E-mail: hpyou@ciac.jl.cn
bGraduate University of the Chinese Academy of Sciences,
Beijing 100049, P. R. China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cp23343f
PCCP
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2.Experimental procedures
2.1 Syntheses
The phosphors with the composition of CMSO:xCe3+
(x = 0.005, 0.01, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09),
CMSO:yEu2+(y = 0.01, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08)
and CMSO:0.04Ce3+, zEu2+(z = 0.01, 0.02, 0.04, 0.05, 0.06,
0.07, 0.08) were synthesized by a solid state reaction approach
using MgO [analytic reagent (A.R.)], CaCO3[analytic reagent
(A.R.)], SiO2[analytic reagent (A.R.)], and Eu2O3(99.99%) as
the starting materials. The stoichiometric amounts of the raw
materials were weighed out and thoroughly mixed by grinding
in an agate mortar, and subsequently the mixture was pre-fired
at 873 K for 2 h. After slowly cooling to the room temperature,
the pre-fired samples were thoroughly re-ground and then
calcined at 1623 K for 3 h in the CO reducing atmosphere.
2.2Characterization
X-Ray powder diffraction measurements were performed on a
D8 focus diffractometer (Bruker) at a scanning rate of 0.21 min?1
in the 2y range from 101 to 601, with graphite-monochromatized
Cu-Ka radiation (l = 0.15405 nm) at 40 kV and 40 mA. The
photoluminescence (PL) and photoluminescence excitation
(PLE) spectra of the obtained powders were recorded with a
Hitachi F-4500 spectrophotometer equipped with a 150 W
xenon lamp as the excitation source. The luminescence decay
curve was obtained from a Lecroy Wave Runner 6100 digital
oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns,
gate = 50 ns) as the excitation source (Continuum Sunlite
OPO). Photoluminescence quantum yield (QY) was measured
using an absolute PL quantum yield measurement system
C9920-02. All the measurements mentioned above were performed
at room temperature. The temperature-dependence properties
of the phosphors were measured by anFLS-920 combined
fluorescence lifetime and steady state spectrometer (Edinburgh
Instruments), and the excitation sources used include a 450 W
xenon lamp.
3.Results and discussion
3.1Phase formation and structural characteristics
Fig. 1 presents the XRD patterns of the same CMSO host by
using optimal activator conditions of (a) x = 0.04; (b) y =
0.04; (c) z = 0.04 as the typical samples. All the diffraction
peaks of the samples can be basically indexed to the standard
data of Ca7Mg(SiO4)4(JCPDS card no. 36-0399). No other
phase is detected, indicating that the obtained samples are
single phase and the activator ions have been successfully
incorporated in the host lattices by replacing the Ca2+due
to their similar ionic radii.25From the single crystal X-ray data
reported by Moore and Araki,21Ca7Mg(SiO4)4crystallizes in
the orthorhombic space group Pmnn with unit cell dimensions
of a = 10.909 A˚, b = 18.34 A˚, c = 6.739 A˚, V = 1348.28 A˚3,
and Z = 4. There are three crystallographic sites for Ca2+ions
of which the coordination numbers are 12, 10 and 9, respectively.
When the Ce3+and Eu2+ions substitute the Ca2+sites,
interesting luminescent properties of the corresponding acti-
vators are expected.
3.2Luminescence properties of Ce3+-doped materials
The luminescence properties of the Ce3+ions in CMSO are
shown in the Fig. 2. The emission spectrum of CMSO:Ce3+
exhibits a broad nonsymmetrical band in the wavelength range
of 350–550 nm with a peak centered at about 420 nm, and it
can be fitted into two sub-bands peaking at about 400 nm and
434 nm, roughly corresponding to the transitions of the 5d
excited state to its two ground-state configurations of the2F7/2
and2F5/2of the Ce3+ions, respectively (see Fig. S1 in the
Supporting Informationw). The energy difference between 400
and 434 nm is about 1960 cm?1, which is in good agreement
with the theoretical value of B2000 cm?1.26From above
emission spectra, it can be concluded that there is only one
kind of Ce3+luminescent center in the host lattice. As pointed
out above, three Ca2+crystallography sites exist in the crystal
structure; the single kind of emission center is inconsistent
with the fact. One possible explanation for the appearance of
Fig. 1
tions of (a) x = 0.04; (b) y = 0.04; (c) z = 0.04. Where x is the Ce3+
concentration in CMSO:xCe3+, y is the Eu2+concentration in
CMSO:yEu2+and z in the Eu2+concentration in CMSO:0.04Ce3+,
zEu2+.
XRD profiles for the typical phosphors in the optimal condi-
Fig. 2
420 nm for excitation and lex= 350 nm for emission); the inset shows
the dependence of emission intensity on the Ce3+concentration.
PLE and PL spectra of Ca7Mg(SiO4)4:Ce3+phosphor (lem=
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Phys. Chem. Chem. Phys.
the phenomenon is that even though the coordination numbers
of the Ca2+crystallography sites are different, the crystal-field
environments are similar in the overall. Another possibility is
that the Ce3+ions occupy one kind of crystallography site of
the Ca2+ions. The PLE spectrum of the CMSO:Ce3+
monitored at 420 nm shows a broad band from 310 to
400 nm, which can match well with the InGaN-based LEDs,
indicating the potential of the phosphor for WLED applica-
tions. The concentration effect was studied to optimize the
composition of the CMSO:Ce3+and the result is shown in
the inset of Fig. 2. The optimum concentration of the Ce3+
ions in CMSO:xCe3+is x = 0.04. When x exceeds the critical
value, a drop in emission intensity is observed due to concen-
tration quenching.
During the application of the WLED technology, the blue
phosphor should possess high efficiency in order to counter the
reabsorption by the green or red phosphor in the blue region,
and high color purity aiming to get a high color rendering
index. Much attention has been paid to these two aspects in
the development of blue phosphors.27Following the trend, we
are interested in the corresponding information about the
CMSO:Ce3+. The CIE coordinates of CMSO:0.04Ce3+
and BAM are depicted in Fig. 3. The CIE coordinates of
CMSO:0.04Ce3+and BAM are (0.156, 0.044) and (0.142, 0.107),
respectively, indicating the wonderful property in the color
purity for CMSO:0.04Ce3+. The quantum efficiency of the
optimum sample is 65% under 350 nm excitation, and it can
be improved by optimizing the composition and preparation
process. These primary results indicate the potential of the
powder as a UVLED conversion phosphor.
3.3Optical behavior of CMSO:Eu2+samples
The PLE and PL spectra of CMSO:0.04Eu2+are shown in
Fig. 4. The PLE spectrum of CMSO:0.04Eu2+shows broad
absorption bands ranging from 250 to 450 nm. The broad
bands are attributed to the parity-allowed Eu2+4f7- 4f65d1
transition and can match well with the n-UV chips, indicating
the interest for application in the WLED area. As pointed out
for the PL spectrum, the CMSO:0.04Eu2+exhibits a green
emission with a maximum wavelength at 500 nm. One kind of
Eu2+luminescence center can be concluded by two experi-
mental phenomena: firstly, the emission spectra are similar
with different excitation wavelengths of 300, 365, 400 and
430 nm; secondly, the emission spectra exhibit high symmetry
profiles. The single kind of luminescence center can also be
explained by the similarity of the crystal field environments of
the Ca2+crystallography sites. Under 350 nm excitation, the
quantum efficiency of CMSO:0.04Eu2+is measured to be 18%.
This value is lower than that of the CMSO:0.04Ce3+sample,
andthepotentialreasonisthemoreseriousradiationreabsorption
in the CMSO:Eu2+system, which has a negative effect on the
luminescence efficiency of the phosphor.
The effect of the Eu2+concentration y on the emission
intensity of Eu2+doped CMSO is shown in Fig. 5. The
emission intensity for 365 nm excitation increases with increasing
Eu2+concentration, maximizing at about y = 0.04. When the
Eu2+concentration is beyond the critical value, concentration
quenching occurs. Concentration quenching is mainly caused
by the non-radiative energy transfer among Eu2+ions, and the
possibility of this increases as the concentration of Eu2+increases.
Since there is one kind of Eu2+luminescent center in the host
lattice, the critical energy transfer distance (Rc) between Eu2+ions
can be calculated by the following equation:28
RC? 2
3V
4pxcZ
??1=3
ð1Þ
Where V is the volume of the unit cell, xc is the critical
concentration of the activator ion, and Z is the number of
formula units per unit cell. For the CMSO host, Z = 4, V =
1348.28 A˚3and the critical concentration of Eu2+is found to
be 0.04. Therefore, Rcof Eu2+is determined to be 25.47 A˚.
Non-radiative energy transfer among Eu2+ions usually
occurs as a result of an exchange interaction, radiation
Fig. 3
BAM and Ca7Mg(SiO4)4:0.04Ce3+, zEu2+excited at 365 nm.
CIE chromaticity diagram for the commercial blue phosphor
Fig. 4
500 nm for excitation and lex= 300, 365, 400 and 430 nm for emission).
PLE and PL spectra of Ca7Mg(SiO4)4:Eu2+phosphor (lem=
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reabsorption or a multipole–multipole interaction.29The
exchange interaction is responsible for the energy transfer
for forbidden transitions and a typical critical distance is
approximately 5 A˚, which can be excluded in the case of
the allowed-transition and the calculated Rc of Eu2+ion,
respectively. The mechanism of radiation reabsorption comes
into effect only when there is broad overlap of the fluorescence
spectra. In view of the emission and excitation spectra of
CMSO:Eu2+phosphor, the radiation reabsorption mecha-
nism should not occurr in this case. Therefore, the multipole–
multipole interaction dominated the concentration quenching
mechanism of Eu2+emission. Besides the concentration-
dependence of the intensity, a shift in the emission band to
longer wavelength upon raising the Eu2+concentration is
observed in Fig. 5, which is due to the energy transfer between
the Eu2+ions.
3.4Energy transfer between Ce3+and Eu2+
The energy transfer between the Ce3+and Eu2+ions has been
studied by many groups to realize color point tuning, which is
an important challenge for improving white light LEDs.30–33
Through appropriately controlling the activators content, the
color hue can be tuned to meet the requirements in different
areas. From the optical properties of Ce3+/Eu2+singly doped
samples, a conspicuous spectral overlap between the emission
band of Ce3+ions and the excitation band of the Eu2+ions
can be observed, as shown in Fig. 6. Therefore, it is expected
that an efficient transfer can occur from the Ce3+to Eu2+
ions. The Ce3+and Eu2+ions act as energy donor and energy
acceptor, respectively.
In order to further investigate the energy transfer process
between the Ce3+and Eu2+ions in the host lattice, a series of
samples were prepared. The concentration of Ce3+was fixed
at the optimal value x = 0.04 and the content of Eu2+was
varied in the range of 0–0.08. Fig. 7 shows the emission spectra
of CMSO:0.04Ce3+, zEu2+phosphors. Under the excitation
of 365 nm, the emission spectra appear not only as a strong
blue band of the Ce3+ions but also as a strong green band of
the Eu2+ions. With increasing Eu2+content, the emission
intensity of the Eu2+reaches a maximum as z equals about
0.04 (also the critical concentration of the singly-doped samples)
and then begins to decrease due to concentration quenching.
For the emission intensity of the Ce3+ions, a remarkable drop
is seen from z = 0.01 to 0.08. The above result preliminarily
affirms that there is an energy transfer from the Ce3+to Eu2+
ions. To further identity the phenomenon, PL decay curves of
the Ce3+ions in CMSO:0.04Ce3+, zEu2+were measured
with excitation at 355 nm and monitored at 440 nm. As shown
in Fig. 8, the decay curve for singly Ce3+-doped CMSO can be
well fitted into a single-exponential function:
I = I0exp(?t/t)(2)
where I0and I are the luminescence intensities at time 0 and t,
respectively, and t is the decay lifetime. On the basis of eqn (2),
the lifetime values were determined to be 37.4 ns for the
sample CMSO:0.04Ce3+. The doping of Eu2+ions signifi-
cantly modifies that the fluorescence dynamics of Ce3+ions.
The results indicate that the fluorescence decays deviate
Fig. 5
0.03, 0.04, 0.05, 0.06, 0.07, 0.08).
PL spectra (lex= 365 nm) for Ca7Mg(SiO4)4:yEu2+(y = 0.01,
Fig. 6
Eu2+and the PL spectrum of Ca7Mg(SiO4)4:Ce3+.
Spectral overlap between the PLE spectrum of Ca7Mg(SiO4)4:
Fig. 7
zEu2+(z = 0.01, 0.02, 0.04, 0.05, 0.06, 0.07, 0.08).
PL spectra (lex = 365 nm) for Ca7Mg(SiO4)4:0.04Ce3+,
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slightly from a single-exponential rule, indicating the presence
of a non-radiative process. The effective lifetime is defined as:
R1
t ¼
0tIðtÞdt
R1
0IðtÞdt
ð3Þ
From eqn (3), the effective lifetime values were determined to
be 29.7, 29.2, 27.6, 8.7, 6.5, 2.9, and 1.4 ns for CMSO:0.04
Ce3+, zEu2+with z = 0.01, 0.02, 0.04, 0.05, 0.06, 0.07 and
0.08, respectively. The decay lifetime for the Ce3+ions was found
to reduce with increasing Eu2+doping content, which strongly
demonstrated the energy transfer from the Ce3+to Eu2+ions.
The energy transfer efficiency (ZT) from Ce3+to Eu2+can be
expressed by:34
ZT¼ 1 ?t
t0
ð4Þ
where t and t0are the decay lifetime of the sensitizer (Ce3+)
ion with and without activator (Eu2+) ion present, respectively.
The energy transfer efficiency from Ce3+to Eu2+ions in
CMSO:0.04Ce3+, zEu2+is calculated as a function of the
Eu2+concentration (z) and exhibited in Fig. S2.w It is
observed clearly that the energy transfer efficiency (ZT) increases
gradually with increasing Eu2+dopant concentration.
According to Dexter’s energy transfer expressions of multi-
polar interaction and Reisfeld’s approximation, the following
relation can be given:35,36
I0
I/ Cn=3
ð5Þ
Where C is the content of Eu2+and n = 6 and 8 corres-
ponding to electric dipole–dipole and dipole–quadrupole
interactions, respectively. The I0/I ? Cn/3plots are further
illustrated in Fig. 9a–b, and the relationship is observed when
n = 6 and 8. A liner relation is observed when n = 6. This
clearly indicates that the energy transfer mechanism from
Ce3+to Eu2+ions is a dipole–dipole interaction. For the
electric dipole–dipole mechanism, the critical distance (Rc) for
energy transfer from Ce3+to Eu2+can be expressed by:37
R6
c¼ ð3 ? 1012Þfd
Z
FSðEÞFAðEÞ
E4
dE
ð6Þ
where fdE 0.02 is the electric dipole oscillator strength for
Eu2+ions.RFS(E)FA(E)/E4dE represents the spectral overlap
Eu2+excitation FA(E), and it is calculated to be about
0.00304 eV?5. Therefore, the Rc of the energy transfer is
calculated to be about 23.8 A˚. This is approximately the same
as for our previous work Sr3Al2O5Cl2:Ce3+, Eu2+.30Further-
more, the critical distance was estimated according to eqn (1).
In the case of Ce3+/Eu2+-doped samples, the critical concen-
tration is 0.76, which can be obtained from the total concen-
tration of the Ce3+and Eu2+ions where the energy transfer
efficiency ZT is 0.5. Through the concentration quenching
method, the Rcwas calculated to be 20.4 A˚. This value is
consistent with the result of the spectral overlap approach,
which justified the theory that the dipole–dipole interaction
dominates the energy transfer mechanism.
During the discussion about the application of silicate phos-
phors to white-LEDs, the problem of stability and the thermal
quenching properties should be taken into consideration. The
main problem with the alkaline earth orthosilicate phosphors is
their strong alkaline character and thus reactivity towards electro-
philic attack, e.g. by H2O, CO2or H+, as a result of the rather
high electron density on the oxygen atoms of the SiO44?. We infer
that our samples also will suffer the above problem, and the
relativestabilityagainstmoistureandCO2wastestedbymeasuring
the luminescence intensity of the sample (CMSO:0.04Eu2+),
which was exposed in air for different times. To our surprise,
we did not observe the apparent decrease of the luminescence
intensity (seen in Fig. S3w), which indicates that our sample is
relatively stable in air for several days. However, it is worth noting
that the result is very preliminary and some errors may be
involved during the process of measurement. From the viewpoint
of longtime application, an additional measure, e.g. a particle
coating, should be processed for the improvement of the stability.
In addition, the temperature dependence of the emission intensity
of CMSO:0.04Eu2+phosphor between 25 1C and 200 1C is
shown in Fig. S4.w It can be seen that the normalized emission
intensity decreased to 63.0% of the initial value with increasing
temperature up to 150 1C. The quenching temperature T50, for
which the intensity is half of the maximum intensity, can be
evaluated as about 175 1C. The above analysis indicates that
between the normalized shapes of Ce3+emission FS(E) and
Fig. 8
0.04Ce3+, zEu2+(excited at 355 nm, monitored at 440 nm). Round
circles: experimental data, Red solid line: fitting results.
Photoluminescence decay curves for Ce3+in Ca7Mg(SiO4)4:
Fig. 9Dependence of I0/I of Ce3+on (a) C6/3and (b) C8/3.
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the thermal quenching property of CMSO:0.04Eu2+is com-
parable to that of the previously reported silicate phosphor.38
The Commission International de L’Eclairage (CIE) chro-
maticity coordinates for CMSO:0.04Ce3+, zEu2+are repre-
sented in Fig. 3. With increasing Eu2+content, the chromaticity
coordinates (x, y) vary systematically from (0.156, 0.044) to
(0.179, 0.275) and ultimately to (0.195, 0.427). The corres-
ponding color tone of the samples changes gradually from blue
to cyan and eventually to green. Thus, through simply changing
the doping content for Eu2+, the color point tuning has been
realized to meet the needs of different illumination applications.
Furthermore, in order to compare to the single-doped sample,
the quantum efficiency of CMSO:0.04Ce3+,0.04Eu2+was also
measured. The obtained value is 16% under 350 nm excitation.
It is worth noting that the value is lower than those for
CMSO:0.04Ce3+andCMSO:0.04Eu2+,whichcanbeattributed
to there being many more defects in the crystal structure of the
codoped samples.
4.Conclusions
In summary, we have synthesized a series of Ce3+/Eu2+acti-
vated Ca7Mg(SiO4)4phosphors by solid state reaction. For the
Ce3+activated sample, an intense blue light is emitted under
350 nm excitation. The related properties of the optimal
sample were compared with the commercial blue phosphor
BaMgAl10O17:Eu2+, the result exhibited by the obtained sample
showed better color purity and the quantum efficiency is 65%.
For the Eu2+activated sample, the effect of concentration on the
luminescence properties was studied. The optimal concentration
was 0.04 and the red shift of the emission with increasing the
Eu2+content was observed in the experiment. For the codoped
samples, a tunable blue-to-green light can be emitted by appro-
priately tuning the relative proportion of Ce3+/Eu2+. The energy
transfer has been demonstrated to be a resonant type via an
electric dipole–dipole mechanism and the critical distance has
been estimated to be consistent by the spectral overlap approach
and the concentration quenching method.
Acknowledgements
This work is financially supported by the Fund for Creative
Research Groups (Grant No. 20921002), the National Natural
Science Foundation of China (Grant No. 20771098), and the
National Basic Research Program of China (973 Program,
Grant No. 2007CB935502).
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