A strategy for synthesizing CaZnOS:Eu2+ phosphor and comparison of optical properties with CaS:Eu2+

Article abstract of DOI:10.1016/j.jallcom.2013.08.162

The red-emitting phosphor CaZnOS:Eu²⁺ was synthesized from CaCO₃, ZnS, Eu₂O₃ and CeCl₃ by controlling the sintering condition. It was found that Ce³⁺ ions can play a role of reductant to contribute to the formation of Eu²⁺ in CaZnOS matrix under inert protective atmosphere. While the gas flow changed to H₂/N₂, the product turned to CaS easily. XRD, photoluminescence spectra, UV-vis and IR absorption spectra were evaluated to investigate the origin of the distinctions of the optical properties and stabilities between the two divalent europium ions doped phosphors CaZnOS:Eu²⁺ and CaS:Eu²⁺. The similarities and differences between them were analyzed.

Full text of DOI:10.1016/j.jallcom.2013.08.162

Journal of Alloys and Compounds 583 (2014) 335–339
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
A Strategy for Synthesizing CaZnOS:Eu2+ Phosphor and Comparison
of Optical Properties with CaS:Eu2+
⇑
Zhongxian Qiu, Chunying Rong, Wenli Zhou, Jilin Zhang, Chengzhi Li, Liping Yu, Shubin Liu, Shixun Lian
Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education), Key Laboratory of Sustainable Resources Processing and Advanced Materials
of Hunan Province College, College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha, Hunan 410081, China
a r t i c l e i n f o
a b s t r a c t
The red-emitting phosphor CaZnOS:Eu2+ was synthesized from CaCO3, ZnS, Eu2O3 and CeCl3 by control-
ling the sintering condition. It was found that Ce3+ ions can play a role of reductant to contribute to the
formation of Eu2+ in CaZnOS matrix under inert protective atmosphere. While the gas flow changed to H2/
N2, the product turned to CaS easily. XRD, photoluminescence spectra, UV–vis and IR absorption spectra
were evaluated to investigate the origin of the distinctions of the optical properties and stabilities
between the two divalent europium ions doped phosphors CaZnOS:Eu2+ and CaS:Eu2+. The similarities
and differences between them were analyzed.
Article history:
Received 15 May 2013
Received in revised form 22 August 2013
Accepted 26 August 2013
Available online 6 September 2013
Keywords:
Red-emitting phosphors
CaZnOS:Eu2+
CaS
Ó 2013 Published by Elsevier B.V.
1. Introduction
Although a few CaZnOS phosphors have been found, the synthe-
sis of a single-phase CaZnOS phosphor still remains a challenge for
In order to enhance a rendering index (Ra < 80) for W-LED
based on a blue LED (GaN chip) precoated with a yellow phosphor
(YAG:Ce) that lacks sufficient red emission, red-emitting phos-
phors with blue–green broad excitation band, such as Ca1ꢀxSrx-
S:Eu2+ [1,2] and (Ca,Sr,Ba)2ꢀxSi5N8:xEu2+ [3–7], have been used in
the production process. To improve the sunlight conversion effi-
ciency of the photosynthetic process in agriculture, the red-emit-
ting phosphors Ca1ꢀxSrxS:Eu2+ have been employed to
manufacture sunlight-conversion plastic film to convert unused
portions (green component) of sunlight to red light [8]. However,
the main drawback of the alkaline earth sulfide-based phosphor
is the sensitivity to water, resulting in the production of H2S.
Meanwhile, the nitride-based red-emitting phosphor must be pre-
pared in high press process. So, novel blue–green to red-emitting
phosphor is still desirable.
Calcium zinc sulfide (CaZnOS) was first found as an intermedi-
ate product in recovery of zinc from its sulfide by carbothermal
reduction in the presence of lime [9] and has been shown to be a
semiconductor with a band gap of about 3.71 eV [10]. Later on,
CaZnOS was first investigated as a novel phosphor doped with
Mn2+ by Duan [11] and his coworkers who also proposed that
Pb2+, Eu2+, Ce3+ and Tb3+ would be proper activators for CaZnOS
host [12] in 2009. The next year, CaZnOS:Eu2+ was reported as a
high color rendering red-emitting phosphor applied for white LEDs
[13].
researchers to date because of the great difficulty in controlling the
phase purity as well as good property. As a quaternary oxysulfide,
the CaZnOS was not isolated in the individual state because of its
partial decomposition [10,14]:
ZnS þ CaO ꢀ CaZnOS ꢀ ZnO þ CaS
ð1Þ
As we know, zinc oxide (ZnO) will easily decompose to oxygen
gas (O2) and zinc-vapor when it is heated above 1205 K under
reducing atmosphere and the decomposition will accelerate with
the addition of a carbonate salt such as CaCO3 [15]. Besides, accord-
ing to the literature [16–18], extraction of zinc–metal from the
mixture of zinc sulfide (ZnS), carbon and calcium carbonate (or cal-
cium oxide) reacts drastically over 1180 K and the sulfur content is
captured by CaO as calcium sulfide (CaS). Furthermore, the yields
of Zn recovery from both ZnO and ZnS increase as the temperature
rises and the roasting time is prolonged. In the system of prepara-
tion of CaZnOS phosphors from ZnS, CaCO3 and H2, the similar pro-
cess of formation of zinc sublimation and CaS takes place during
heating. This process can be illustrated by the formulas A.1–A.5
(Formulas A, Supplemental material).
On the other hand, as reported, the decomposition of CaZnOS
commences beyond 1370 K under an inert atmosphere and, as a re-
sult, the CaS component forms substantially [14]. Consequently, it
is key to accurately control the H2/N2 flow rate, reaction tempera-
ture and heating time to make the CaZnOS-based phosphor the
dominate phase. In fact, it is very difficult to obtain pure CaZnOS
under reducing (H2/N2) atmosphere even if firing temperature
was controlled in lower level [13].
⇑
Corresponding author. Tel./fax: +86 73188865345.
E-mail address: shixunlian@gmail.com (S. Lian).
0925-8388/$ - see front matter Ó 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.jallcom.2013.08.162

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Z. Qiu et al. / Journal of Alloys and Compounds 583 (2014) 335–339
In this paper, we report a strategy to prepare the single-phase
CaZnOS phosphor doped with Eu2+, co-doping with Ce3+ as the
deoxidizer to promote the formation of Eu2+, rather than reducing
(H2/N2) atmosphere, followed by detailed characterization and
comparison of its red properties with CaS:Eu2+
.
2. Experimental
2.1. Sample preparation
CaZnOS host material and optimized nominal composition phosphors CaZ-
nOS:Eu2+,Ce3+ were synthesized by the conventional solid-state reaction method.
CaCO3 (A.R), ZnS (A.R), Eu2O3 (99.99%) and CeCl3 (99.95%) were used as raw mate-
rials. The nonedoped and doped CaZnOS, impure CaZnOS:Eu2+ and pure CaS:Eu2+
phosphors were designed as shown in Table 1. The relevant raw materials were
weighed out according to the stoichiometric molar ratio of the composition (the
doping concentrations were optimized as shown Fig. D1, Supplemental material),
and subsequently mixed and ground together in an agate mortar. The mixtures
were then sintered under a N2 flow atmosphere at 1270 K for 4 h. As for CaS:Eu2+
,
Fig. 1. The representative XRD patterns of samples: single phase CaZnOS (A) and
CaZnOS:Eu2+,Ce3+ phosphor (B) synthesized under N2 flow; CaZnOS:Eu2+,Ce3+
containing CaS (C) and CaS:Eu2+ phosphor (D) synthesized under H2/N2 flow.
the same starting materials except CeCl3 were used and kept at 1420 K for 8 h under
5%H2/95%N2 flow. In order to compare the optical properties of CaZnOS:Eu2+ to
CaS:Eu2+, a multi-phased phosphor CaZnOS:Eu2+,Ce3+ with a CaS impurity was syn-
thesized at some sintering condition at the present of 5%H2/95%N2 flow. Finally, the
as-synthesized phosphors were allowed to cool down naturally in the furnace.
Besides, only Ce3+ or Ce4+-doping CaZnOS samples were also prepared likewise in
which CeCl3 or CeO2 were used as direct dopant.
in CaS:Eu2+ phosphors made from CaCO3 and ZnS. And the mor-
phology of Eu2+-doped CaZnOS phosphor were examined by SEM
(Fig. 2). It can readily be seen that the phosphor powders are com-
posed of aggregated spherical particles with size smaller than
2.2. Characterization
5
l
m.
The crystalline phase of the prepared samples was examined by X-ray diffrac-
tometry with Cu Ka radiation at 40 kV and 30 mA (Rigaku D/MAX-2550 X-ray dif-
The results show that Eu2+ and Ce3+ codoped CaZnOS under
nitrogen flow was prepared and pure CaS can be made from CaCO3
and ZnS. And the contrast samples CaZnOS:Eu2+,Ce3+ with impurity
CaS were also obtained by controlling the roasting time and the
hydrogen flow rate.
fractometer, Tokyo, Japan). The elementary compositions were characterized by
energy dispersive X-ray spectroscopy (EDX). The microstructure and morphology
of the phosphors were observed by using JSM-5600LV scanning electron micro-
scope (SEM). The photoluminescence excitation and emission spectra of the sam-
ples were measured using an Hitachi F-4500 luminescence spectrofluorometer
equipped with a UV 390 nm filter, a 400 V photomultiplier tube voltage, and a
175 W Xenon lamp as the radiation source. And also solid-state UV–vis absorption
spectra were recorded by means of a Hitachi U-3010 spectrometer. Finally, infrared
spectroscopy was performed with a Fourier transform infrared spectrometer (AVA-
TAR 370, Nicolet Company, America). All measurements were carried out at room
temperature and in the air.
As shown in Fig. 3, host material CaZnOS primarily absorbs
ultraviolet light asymmetrically peaked at 378 nm assigned to host
lattice absorption and transfers to a wide green light emission at
about 500 nm. We could propose that Eu2+ and Ce3+ are both ex-
pected to occupy the Ca2+ sites preferably because the ionic radii
of Eu2+ (1.17 Å) and Ce3+ (1.03 Å) are close to that of Ca2+ (1.0 Å)
with six coordination [19]. With Ce3+ ions codoped, the red band
emission ascribed to 4f65d1 ? 4f7 transition of Eu2+ is detected. It
has to be mentioned that the major excitation band of CaZnOS:Eu2+
and emission band of CaZnOS host entirely overlap so as to energy
transfer from the host to Eu2+ to the utmost. Only a feeble
3. Results and discussion
Fig. 1 shows the representative X-ray diffraction (XRD) patterns
of pure CaZnOS (A) and doped CaZnOS samples. All of the diffrac-
tion peaks of the samples can be basically indexed to pattern card
PDF67-0788. For that synthesized under N2 flow, only a trace
impurity belonging to CaO is detected in the doped sample (B),
which can be scoured off by acetic acid [9,14] and is independent
of the optical property of CaZnOS phosphors. However, the com-
pound obtained under 5%H2/95%N2 flow is composed of CaZnOS
with CaS impurity (C) or single-phase CaS (D) due to excessive
reduction and increase of temperature. To further confirm the ele-
mentary compositions of the as-obtained samples, Energy Disper-
sive X-ray Spectroscopy (EDX) (Fig. B1 and Table B1, Supplemental
material) was used to analyze the elementary compositions of the
as-obtained samples, which revealed that zinc was not contained
Table 1
The nominal composition (mol%) and synthesized conditions of the samples.
No. Raw materials
Atomosphere Temperature Roasting Main
n
CaCO3:nZnS:nEu2O3:nCeCl3
(K)
time (h) phases
A
B
C
1:1(no Eu or Ce)
1:1:0.05%: 0.5%
1:1:0.05%: 0.5%
N2
N2
1270
1270
4
4
4
CaZnOS
CaZnOS
CaZnOS,
CaS
CaS
CaZnOS
CaZnOS
5%H2/95%N2 1270
D
E
F
1:1:0.05% (no Ce3+
1:1(: none): 0.5%
1:1(: none): 0.5% (CeO2) N2
)
5%H2/95%N2 1420
N2
8
4
4
1270
1270
Fig. 2. SEM image of CaZnOS:Eu2+ powder.

Z. Qiu et al. / Journal of Alloys and Compounds 583 (2014) 335–339
337
transition is impossibility. Consequently, it is believed that Ce3+ ions
are partially as deoxidizer, and partially as sensitizer for Eu2+ ions.
As we already know, with temperature rising, CaCO3 decom-
posed to CaO at first. Then CaZnOS was formed through a combina-
tion of CaO and ZnS. Keep the temperature climbing up beyond
1370 K and the equilibrium of formula (4) would be shifted toward
the right side substantially. Meanwhile, the drastic decomposition
of ZnO and zinc vaporization grew up under H2/N2. As a result,
phase-pure CaS was formed:
CaCO3 þ ZnS þ 0:05%Eu2O3 þ H2 ! CaS : 0:1%Eu2þ þ Zn
" þ1=2O2 " þCO2 " þH2O "
ð4Þ
where H2 was supplied by 5%H2/95%N2 flow and considered as the
reductant for Eu3+ ions as well as an assistant for decomposition of
ZnO. The excitation and emission spectra of the as-obtained
CaS:Eu2+ samples make no odds with those reported phosphors
[2,8,26]. It indicates that the method in this paper can provide a
new strategy to synthesize calcium sulfide based material.
Fig. 3. Excitation and emission spectra of CaZnOS (A), CaZnOS:Eu2+,Ce3+ (B),
It is interesting that similar spectra profiles emerge from both
CaS:Eu2+ and CaZnOS:Eu2+. However, due to the change of the crys-
tal structure and crystal field, some unique phenomena appear as
CaZnOS:Eu2+,Ce3+ containing CaS (C) and CaS:Eu2+ (D) phosphors.
for the two Eu2+ ion activated substances. Compared with CaS:Eu2+
,
absorption peak at 378 nm attributed to host lattice is found when
the Eu ions concentration is low. Besides, whether Ce3+ ions exist
or not, the luminescence spectrum profile of CaZnOS:Eu2+ has no
change. That is to say, Ce3+ ions play a role of deoxidizer to pro-
mote the formation of Eu2+ and also transfer energy to Eu2+ so that
no obvious emission of the sensitizer can be detected.
several differences of the spectra can be easily found. Firstly, the
ultraviolet excitation bands in Fig. 3 reveal the biggest variance.
CaS:Eu2+ possesses a prominent UV excitation band which locates
at about 327 nm attributed to 4f7(8S7/2) ? 4f65d1(t2g) [2,26] transi-
tion of Eu2+ ions, while the weak UV band of CaZnOS peaks at
378 nm attributed to host lattice (HL) absorption of CaZnOS. In
addition, the main excitation band of CaS:Eu ranges from 400 to
630 nm. For CaZnOS: Eu2+, the absorption at the visible region,
more abruptly, starts at about 420 nm. Nevertheless, the profiles
of the red emission bands of the two phosphors are nearly the
same except for the locations at 650 nm of CaS:Eu and 645 nm of
CaZnOS:Eu2+, respectively. On the other hand, the luminescence
Phosphors co-doped with Ce3+ and Eu2+ have been investigated
under reducing (H2/N2) atmosphere, such as YAG:Ce3+,Eu2+ [20],
Ba2ZnS3:Ce3+,Eu2+ [21], and Li2SrSiO4:Eu2+,Ce3+ [22]. As we know,
Eu can exist as Eu2+/Eu3+ and Ce as Ce3+/Ce4+ [23]. It is assumed
that Ce3+ ions undergo easy oxidation to induce the formation of
Eu2+/Ce4+ [23,24] during heating Eu3+ and Ce3+ co-existent mixture
so that energy transfer occurs from Ce3+ to Eu2+ resulting in the
enhancement of the emission of Eu2+. Hence, the redox reaction
would spontaneously happen in an enclosed system without
reducing gas. This process can be illustrated by the following
formulas [20,23,24]:
intensity of CaZnOS:Eu2+ is relatively
a quarter of that of
CaS:Eu2+. And because of this, once the impurity CaS forms, the
luminescence of CaS:Eu2+ would be absolutely dominant so that
the characteristic luminescence of CaZnOS:Eu2+ is difficult to be
detected as shown by phosphor C in Fig. 3.
Ce3þ ! eÅ þ Ce4þ
Eu3þ þ eÅ ! Eu2þ
ð2Þ
The UV–vis absorption spectra of the as-synthesized phosphors
are plotted in Fig. 4. The absorption band of CaZnOS host material
is concentrated in the near-UV region which well matches with the
excitation spectrum of pure CaZnOS as depicted in Fig. 3. By con-
trast, the absorption band of CaZnOS:Eu2+,Ce3+ stretches to the vis-
ible region so that a fairly wide excitation band of the phosphor
covered the whole green light region is observed. In comparison
with the pure CaZnOS:Eu2+,Ce3+, a remarkably uplift at a range of
430–630 nm turns up in impure CaZnOS: Eu2+,Ce3+ and CaS:Eu2+
phosphors. This result can support the phenomenon about higher
luminescence intensity of CaS than CaZnOS doped with Eu2+ ions.
Especially, the center of the UV absorption of CaS:Eu2+ shifts to
about 330 nm in accordance with the excitation band.
To better understand the differences of the optical spectra be-
tween CaZnOS:Eu2+ and CaS:Eu2+, a infrared (IR) test was also car-
ried out. Fig. 5 exhibits the IR spectra of different materials.
Comparison between pure CaZnOS and CaS (synthesized by the
method referred to Ref. [8]) suggests an assignment to a Ca–S
stretching model at 650, 931, 980, 1099, 1143 and 1213 cmꢀ1 as
the black arrows pointed which exist only in samples containing
CaS component. The atoms in CaZnOS are located in layers stacked
along the [001] irection so that Ca–O layers are inserted in the
aligned parallel [ZnS3O] layers attached to the O atom. In the polar
structure, the Ca–O layer and the other adjacent [ZnS3O] layer links
through van der Waals and consequently the interaction force be-
tween Ca and S atoms are quite weak. The peaks at 1414, 1501 and
ð3Þ
Therefore, we can infer from the above analysis that the red-emit-
ting phosphor (B) with composition CaZnOS:0.1%Eu2+, 0.1%Ce4+
,
0.4%Ce3+ was formed. In fact, CaZnOS materials just doped with
Ce3+ or Ce4+ were synthesized also under N2 flow when CeCl3 or
CeO2 was used as direct dopant. As shown in Fig. C1 (Supplemental
material), CaZnOS:Ce3+ phosphor shows similar optical property as
pure host material but the intensity of luminescence is obviously
enhanced. It is assumed that the doping level of Ce3+ is located at
the bottom of the conduction band of CaZnOS. The emission band
of CaZnOS:Ce3+ locates at the region of green light which almost
completely overlaps with the excitation band of Eu2+-doping CaZ-
nOS phosphor. Eu2+ and Ce3+ ions are f–d and d–d electron config-
urations, respectively. The energy transfer would occur between
activator/coactivator couples by efficient resonant type. Moreover,
when Ce3+ and Eu3+ ions co-exist in the matrix, Ce3+ can undergo
easy oxidation to Ce4+ during heating and the electron released dur-
ing the oxidation is used to convert Eu3+ to Eu2+ species. According
to the charge-transfer from Ce to Eu, energy transfer Ce3+ ? Eu2+
must happen and luminescence quenching of Ce3+ ions should take
place [25]. However, doping with Ce4+, nearly no obvious lumines-
cence phenomenon related to Ce4+ was detected in CaZnOS. The
electron configuration of Ce4+ is composed of 5d0 so that d–f

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Z. Qiu et al. / Journal of Alloys and Compounds 583 (2014) 335–339
Fig. 4. UV–vis absorption spectra of host CaZnOS, CaZnOS:Eu2+,Ce3+, CaZnOS:Eu2+,-
Ce3+ containing CaS and CaS:Eu2+ phosphors.
1628 cmꢀ1 as the blue arrows pointed would belong to S–S bend-
ing vibration in that them appear in both of CaZnOS and CaS.
Whereas, through a contrast with the spectra between CaZ-
nOS:Eu2+,Ce3+ and CaZnOS host, we can speculate the IR peaks at
872, 1398 and 2505 cmꢀ1 come from the Eu–O stretching vibration
as the red arrows pointed. Further support is the sharply drop of
these peaks in those samples containing CaS. Then, for all the
Eu2+ doped phosphors, the significant peak at 1480 cmꢀ1 ought
to be bound up with the distortion of crystal lattice after the re-
place of Ca2+ by larger Eu2+, which leads to the enhancement of
S–S bending viration. As the increase of the content of CaS impurity
within the phosphors, the peaks ranging from 650 to 1220 cmꢀ1
regularly gradually change with the decrease of Eu–O vibration
and increase of Ca–S stretching. And the intensities of the absorp-
tion decline progressively for the formation of cubic CaS with high
symmetry.
In addition, the crystal structure and coordination environment
of the two matrixes are different as well as the crystal field
strength. As shown in Fig. 6, CaZnOS consists of isotypic puckered
hexagonal ZnS and CaO layers and crystallizes in hexagonal space
group P63mc (No. 186) [10]. Strikingly, Ca is surrounded by three
O and three S atoms, where the O and S atoms are offered, respec-
tively, by two parallel adjacent ZnS3O layers. Looking at a close-up
of the environment of Ca site, we can easily find a distorted octa-
hedra formed around Ca with S1 and O1 as vertexes. It is assumed
that complex layered oxychalcogenides can offer scope for the
Fig. 6. The crystal structure CaZnOS.
realization of novel and unusual properties complementary to
those found in chalcogenides and oxides as a result of control of
the electronic communication between the chalcogenide and oxide
layers. As we know, the Ca site locates at the octahedral center po-
sition with 6 S around in cubic CaS crystal. As a consequence, when
Eu2+ ions displace the Ca2+ sites, the similar luminescence phenom-
ena arise up for both CaZnOS:Eu2+ and CaS:Eu2+
.
However, since the symmetry and the amount of covalency of
CaZnOS are relatively lower, the energy difference between the
4f7 and 4f65d1 configurations of Eu2+ ions increases so that the
emission band of CaZnOS:Eu2+ shifts to a shorter wavelength than
that of CaS:Eu2+ [27].
It is generally known that CaS compound will easily deliques-
cence exposed to the air. Although Ca ions are coordinated with
S atoms as well, the interatomic distance of Ca–S in CaZnOS crytal
lattice is about 3.0346 Å larger than that in CaS single crystal
(2.8445 Å, calculated by the data from Ref. [28]). In the oxygensul-
fide, Ca ions are more closer to O rather than S atoms and the inter-
action force between the Ca and S is quite weak. Hence, we can
speculate that the thermodynamic stability of CaZnOS would be
better than that of CaS. In order to investigate the chemical and
Fig. 5. Infrared spectroscopy of CaZnOS, pure CaZnOS:Eu2+,Ce3+
,
impure CaZ-
Fig. 7. PL and PLE spectra of CaZnOS with and without chemical and heat
treatment. The inset: XRD patterns of CaZnOS after chemical and heat treatment.
nOS:Eu2+, impure CaZnOS:Eu2+.Ce3+, CaS:Eu2+ and CaS.

Z. Qiu et al. / Journal of Alloys and Compounds 583 (2014) 335–339
339
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composition and optical property of CaZnOS had no change
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4. Conclusions
A new strategy for synthesizing CaZnOS:Eu2+ red-emitting
phosphor by co-doping with Ce3+ as deoxidizer rather than 5%H2/
95%N2 flow as reducing atmosphere is introduced in the present
work. The redox reaction (Ce3+ + Eu3+ ? Eu2+ + Ce4+) would sponta-
neously happen when the high-temperature solid-state reaction
takes place in the system of Eu3+ and Ce3+ co-existent host
(CaZnOS) without reducing gas, and also energy transfer occurs
from Ce3+ to Eu2+ resulting in the enhancement of the emission
of Eu2+. It has been discovered that the nominal composition
(mol%) CaZnOS:0.1%Eu2+ 0.5%Ce3+ phosphor has the optimal
,
green-to-red composition. Because CaZnOS has good thermal and
chemical stability we have evaluated its effectiveness in enhancing
sunlight harvesting for increasing agricultural production and
deemed it to be better than the CaS:Eu2+ phosphor.
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Acknowledgments
This work is partially supported by the National Natural Science
Foundation of China (Grant No. 20971042), Scientific Research
Fund of Hunan Provincial Education Department (No. 10A070), Hu-
nan Provincial Natural Science Foundation of China (No. 12JJ2029),
Scientific Research Fund of Hunan Provincial Education Depart-
ment (Grant No. 12K030) and aid program for Science and Technol-
ogy Innovative Research Team in Higher Educational Institutions of
Hunan Province.
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