Polymer light-emitting devices based on europium(III) complex with 11-bromo-dipyrido[3,2-a:2′,3′-c]phenazine

Article abstract of DOI:10.1007/s11426-015-5404-z

Polymer light-emitting diodes (PLEDs) containing Eu(DBM)3(BrDPPz) (DBM is dibenzoylmethane, and BrDPPz is 11-bromo-dipyrido[3,2-a:2′,3′-c]phenazine) doped in a blend of poly(9,9-dioctylfluorene) (PFO) and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) as the host matrix are reported. Eu(DBM)3(BrDPPz) exhibited high thermal stability and intense UV-Vis absorption. Narrow-bandwidth red emission at 612 nm with a full width at half-maximum (FWHM) of 14.0 nm was observed from Eu(DBM)₃(BrDPPz) in these double-layered PLEDs at dopant concentrations from 1 wt% to 8 wt%. For the PLED containing 1 wt% Eu(DBM)₃(BrDPPz), a maximum luminance of 829 cd/m² at 153.5 mA/cm², highest external quantum efficiency of 1.70% at 2.1 mA/cm² and maximum luminance of 0.74 cd/A at 4.31 mA/cm² were obtained.

Full text of DOI:10.1007/s11426-015-5404-z

SCIENCE CHINA
Chemistry
•
ARTICLES •
Ja nd uo ai :r y1 02 . 01 10 50 7/ sV1 o1 l4. 52 86 -0 1N 5 o- 5. 14 :0 14 –- 7z
doi: 10.1007/s11426-015-5404-z
Polymer light-emitting devices based on europium(III) complex
with 11-bromo-dipyrido[3,2-a:2′,3′-c]phenazine
1
1*
1
2
1
1
Qunping Fan , Yu Liu , Zhaoran Hao , Chun Li , Yafei Wang , Hua Tan ,
1
*
2
Weiguo Zhu & Yong Cao
1
Key Laboratory of Environment-Friendly Chemistry and Application of the Ministry of Education; College of Chemistry, Xiangtan University,
Xiangtan 411105, China
2
Institute of Polymer Optoelectronic Material and Devices, South China University of Technology, Guangzhou 510640, China
Received September 24, 2014; accepted November 6, 2014
Polymer light-emitting diodes (PLEDs) containing Eu(DBM)3(BrDPPz) (DBM is dibenzoylmethane, and BrDPPz is 11-bro-
mo-dipyrido[3,2-a:2′,3′-c]phenazine) doped in a blend of poly(9,9-dioctylfluorene) (PFO) and 2-tert-butylphenyl-5-biphenyl-
1
,3,4-oxadiazole (PBD) as the host matrix are reported. Eu(DBM)3(BrDPPz) exhibited high thermal stability and intense
UV-Vis absorption. Narrow-bandwidth red emission at 612 nm with a full width at half-maximum (FWHM) of 14.0 nm was
observed from Eu(DBM) (BrDPPz) in these double-layered PLEDs at dopant concentrations from 1 wt% to 8 wt%. For the
PLED containing 1 wt% Eu(DBM)
3
2
2
3
(BrDPPz), a maximum luminance of 829 cd/m at 153.5 mA/cm , highest external quan-
2
2
tum efficiency of 1.70% at 2.1 mA/cm and maximum luminance of 0.74 cd/A at 4.31 mA/cm were obtained.
europium(III) complexes, electroluminescence, optophysical properties, polymer light-emitting diodes
1
Introduction
based on the europium(III) complex Eu(FTA)
,4,4-trifluoro-1-phenyl-1,3-butanedione, Phen=1,10-phen-
anthroline) with an external quantum efficiency (EQE) of
.3%. Recently, we reported high-efficiency PLEDs based
on the europium(III) complex Eu(DBM) (DTPA-Phen)
DBM=dibenzoylmethane, DTPA-phen=3,8-bis[4-(diphen-
3
Phen (FTA=
4
Europium(ІІІ) complexes have been considered promising
red-emitting materials for use in full-color displays since
Kido et al. [1,2] initially reported the applications of euro-
pium(III) complexes. Compared to the high efficiency of
red-emitting iridium(III) and platinum(II) complexes [3–7],
europium(ІІІ) complexes exhibit considerable advantages
because of their extremely narrow emission spectral band-
width. Recently, spin-casting techniques have been used to
fabricate europium(III) complex-doped polymer light-emitt-
ing diodes (PLEDs) containing conjugated polymers as the
host matrix [8–20]. PLEDs formed by this method can be
simply fabricated by solution processing, and luminescence
quenching by concentration effects is prevented. For in-
stance, Zhang et al. [8] reported a high-efficiency PLED
4
3
(
ylamino)phenyl]-1,10-phenanthroline) with a maximum
2
brightness of 1333 cd/m and maximum EQE of 1.8%, and
3
another PLED based on Eu(DBM) (DPPz) (DPPz is dipyr-
ido[3,2-a:2′,3′-c]phenazine) with a maximum brightness of
2
1
783 cd/m and maximum EQE of 2.5% [9,11]. However,
devices formed by this method exhibited low luminous effi-
ciency and brightness compared with those of devices
formed by vacuum deposition [21–35]. For example, Liu et
al. [21] made an organic light-emitting device (OLED) us-
3
ing Eu(DBM) (BPhen) (Bphen=4,7-biphenyl-1,10-phenan-
throline) doped into triphenyldiamine with a high EQE of
2
7
.5%±0.5% at 0.02 mA/cm and Ma et al. [22] used tris-
(
thenoyltrifluoroacetone)(3,4,7,8-tetramethyl-1,10-phenant-
*
Corresponding authors (email: liuyu03b@126.com; zhuwg18@126.com)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

2
Fan et al. Sci China Chem January (2015) Vol.58 No.1
hroline)europium(ІІІ) as a dopant and (dicyanomethylene)-
-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran
as a host to fabricate OLEDs with a high brightness of 3000
2 Experimental
2
2
.1 Reagents and physical measurements
2
2
cd/m at 190 mA/cm and high luminance efficiency of 14.5
2
cd/A at 0.08 mA/cm .
Commercially available reagents and starting materials were
To acquire europium(III) complexes that produce good
device performance and charge transport characteristics, a
variety of functional europium(III) complexes have been
developed. Recently, the improved electroluminescence (EL)
performance of OLEDs containing fluorine- and chlorine-
functionalized europium(ІІІ) complexes has been reported
used to synthesize Eu(DBM) (BrDPPz). All reactions and
3
manipulations were carried out under an inert gas atmos-
1
phere. H NMR spectra were measured in CDCl solution
3
on a Bruker DPX (400 MHz) NMR spectrometer using tet-
ramethylsilane (TMS) as the internal standard. MALDI-
TOF mass spectrometric measurements were performed on
a Bruker Biflex III MALDI-TOF spectrometer (Switzer-
land). Elemental analyses (C, H, N) were performed with a
Perkin-Elmer 240 instrument (USA). Thermogravimetric
analyses (TGA) were performed under nitrogen atmosphere
at a heating rate of 10 °C/min using a Perkin-Elmer TGA-7
thermal analyzer (USA). UV-Vis absorption spectra were
recorded with a Shimadzu UV-265 spectrometer (Japan)
at room temperature. PL spectra were recorded on an
RF-5301 PC spectrometer (Perkin Elmer, USA) under
excitation with a He:Cd laser at 325 nm. PL quantum
[
27–32]. In contrast, PLEDs using bromine-functionalized
europium(III) complexes as emitters have seldom been in-
vestigated [17].
In this paper, we report the bromine-functionalized euro-
pium(III) complex Eu(DBM)
3
(BrDPPz) (BrDPPz=11-bro-
modipyrido[3,2-a:2′,3′-c]phenazine), the molecular stru-
cture of which is shown in Scheme 1. In this europium(III)
complex, BrDPPz is used instead of 3,8-dibromo-1,10-
phenanthroline (DBrPhen) [11] as the second ligand. This is
expected to improve the performance of devices because the
extended conjugation, heavy atom and steric bulk of BrD-
PPz should help to control concentration quenching
3 2
yields (ФPL) were determined using EuCl ·6H O as a
reference in solution [36]. Cyclic voltammetry was per-
formed on a CHI660A electrochemical workstation with a
scan rate of 50 mV/s at room temperature under argon. A Pt
disk, Pt plate, and Ag/AgCl electrode were used as the
working electrode, counter electrode, and reference elec-
[17,27–31]. DBM was chosen as the first ligand because of
its relatively high photoluminescence (PL) efficiency in
europium(III) complexes. The PL, electrochemical and
photophysical properties of devices fabricated containing
this europium(III) complex as a dopant at various concen-
trations from 1 wt% to 8 wt% in a blend of poly(9,9-dio-
ctylfluorene) (PFO) and 2-tert-butylphenyl-5-biphenyl-
4 6
trode, respectively, in n-Bu NPF (0.1 mol/L) in acetonitrile.
For calibration, the redox potential of the ferrocene/ferro-
+
cenium (Fc/Fc ) couple was measured under the same con-
1
,3,4-oxadiazole (PBD) (30 wt%) as a host matrix are also
ditions. Optical band gaps were estimated based on absorp-
tion edges. Reduction potentials were calculated from the
corresponding optical band gap and oxidation potential. On
the basis of the energy level of ferrocene (4.74 eV under
vacuum), and the oxidation and reduction potentials, the
highest occupied molecular orbital (HOMO) and lowest
investigated. Highly efficient monochromic red emission at
2
6
12 nm with a maximum luminance of 829 cd/m at 153.5
2
2
mA/cm and maximum EQE of 1.70% at 2.1 mA/cm are
achieved for the device containing 1 wt% Eu(DBM)
BrDPPz).
3
(
3
Scheme 1 Synthesis of the bromine-functionalized europium(III) complex Eu(DBM) (BrDPPz).

Fan et al. Sci China Chem January (2015) Vol.58 No.1
3
molecular orbital (LUMO) energy levels of the complex
were calculated according to an empirical formula [27,34].
PLEDs were fabricated as we reported previously [11]. The
devices had the following structure: indium tin oxide (ITO)/
poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid
(PEDOT:PSS, 50 nm)/poly(N-vinylcarbazole) (PVK, 40 nm)/
Eu(DBM) (BrDPPZ) in PFO-PBD (80 nm)/Ba (4 nm)/Al
3
(
150 nm). ITO was used as the anode, PEDOT:PSS as a
hole-injection layer and Ba/Al as a cathode. The weight
ratio of PBD in the PFO-PBD blend was 30 wt%. The dop-
ing concentration of Eu(DBM) (BrDPPZ) in the active layer
3
ranged from 1 wt% to 8 wt%.
2
.2 Synthesis of intermediates and target complex
Figure 1 TGA curve of Eu(DBM)
0 °C/min under nitrogen atmosphere.
3
(BrDPPz) at a heating rate of
1
Intermediates 1, 2, 3 and BrDPPz were synthesized accord-
ing to our previous work [37]. Eu(DBM) (BrDPPz) was
3
synthesized by reacting europium trichloride hydrate with
DBM and BrDPPz according to a conventional procedure
[
28]. The structures of the intermediates were confirmed by
1
H NMR spectroscopy and MALDI-TOF mass spectrometry.
Eu(DBM) (BrDPPz) was characterized by elemental analy-
sis and MALDI-TOF mass spectrometry.
3
2
1
.2.1 Synthesis of BrDPPz
1-Bromo-dipyrido[3,2-a:2',3'-c]phenazine (BrDPPz) was
synthesized according to a reported method [37]. Yield:
1
7
3
2%. m.p.: 249.0–251.0 °C, H NMR (CDCl , 400 MHz), δ
(
(
7
ppm): 9.47 (t, J=13.6 Hz, 2H); 9.26 (d, J=4.0 Hz, 2H); 8.43
s, 1H); 8.11 (d, J=8.8 Hz, 1H); 7.94 (dd, J=8.8 Hz, 1H);
+
Figure 2 Normalized UV-Vis absorption spectrum and PL spectrum
ex=349 nm) of Eu(DBM) (BrDPPZ) in dilute CHCl solution and PL
.76–7.73 (m, 2H). MALDI-TOF MS (m/z): 361.7 for [M ].
(
3
3
spectra in solution and a thin film of the PFO-PBD blend.
2
.2.2 Synthesis of Eu(DBM)
(BrDPPz) was synthesized according to a re-
ported procedure [31]. Yield: 53.0%. m.p.: 251.0–253.0 °C.
Anal. calcd for EuC63 : C, 63.97; H, 3.58; N, 4.74;
Found: C, 63.63; H, 3.45; N, 4.58. MALDI-TOF MS (m/z):
3
(BrDPPz)
Eu(DBM)
3
6
(
about 1×10 mol/L) and a thin film of the PFO-PBD (30
wt%) blend at 298 K. The UV-Vis absorption spectrum
contains two major absorption peaks at 274 and 369 nm, in
which the former is attributed to a ligand-centered -*
electron transition of BrDPPz and the latter is assigned to a
ligand-centered -* electron transition of DBM [31]. The
PL spectrum of the film contains a sharp peak at 613 nm
with a full width at half-maximum (FWHM) of 8 nm as
well as four weak peaks at 580, 596, 654 and 700 nm,
H
42BrN O
4 6
+
1
284.2 for [M ].
3
Results and discussion
3.1 Thermal stability
5
7
which correspond to the D
0
→ F
j
(j=2, 0, 1, 3, 4) electronic
The thermal properties of Eu(DBM)
3
(BrDPPZ) were stud-
3+
transitions of the Eu ion under photoexcitation, respec-
tively. No emission from the BrDPPz ligand was observed.
This indicates that complete energy transfer from BrDPPz
to the Eu ion occurs in this europium(ІІІ) complex, and
modifying the DPPz ligand with a heavy atom (bromine)
has no effect on the PL spectrum of its europium(III) com-
ied by TGA under nitrogen atmosphere, and the results are
presented in Figure 1. Eu(DBM) (BrDPPZ) exhibits high
thermal stability, with a decomposition temperature (T ) at
% weight loss of about 363.5 °C, which is favorable for
use in PLEDs.
3
3+
d
5
3
plex [14,15]. The absorption spectrum of Eu(DBM) (BrD-
PPz) (Figure 2) overlaps well with the PL spectrum of the
PFO-PBD host. As a result, efficient energy transfer from
3
.2 Photophysical properties
Figure 2 shows the normalized UV-Vis absorption and PL
the PFO-PBD to Eu(DBM) (BrDPPz) can be expected to
3
spectra of Eu(DBM) (BrDPPz) in dilute CHCl solution
3
3
occur via Förster energy transfer [15]. ФPL of Eu(DBM)3

4
Fan et al. Sci China Chem January (2015) Vol.58 No.1
(
(
(
(
BrDPPz) in CH
=0.73% in water) as a reference [36]. ФPL of Eu (DBM)
BrDPPz) is 28.3%, which is higher than that of Eu(DBM)
2 2 3 2
Cl was measured using EuCl ·6H O
Ф
f
3
3
DPPz) (27.1%) [9]. This exceedingly high PL efficiency
can be ascribed to the heavy atom effect of bromine, which
increases luminescent efficiency [27–29].
To investigate the efficiency of Förster energy transfer
between the host matrix and emissive dopant, PFO-PBD
films with various concentrations of Eu(DBM)
were prepared and excited by a He-Cd laser (325 nm). Fig-
ure 3 shows the PL spectra of the films of Eu(DBM)
BrDPPz) (1 wt%–8 wt%) in PFO:PBD (30 wt%). Two in-
tense emission peaks at 417 and 519 nm are observed for
these Eu(DBM) (BrDPPz)-doped PFO-PBD blend films,
which are assigned to PFO-PBD emission. Furthermore, a
series of weak emission peaks from the Eu ion at 612 nm
were observed in the doped films at doping concentrations
from 4 wt% to 8 wt%. The intensities of the peaks at around
3
(BrDPPz)
3
(
3
Figure 3 Normalized PL spectra of a PFO-PBD blend and Eu(DBM)
(BrDPPz)-doped PFO-PBD blend films with different dopant concentra-
tions from 1 wt% to 8 wt%.
3
3+
4
17 and 519 nm originating from PFO-PBD emission de-
creased with the increasing intensity of Eu(DBM) (BrDPPz)
emission at 612 nm with increasing concentration of
Eu(DBM) (BrDPPz). This indicates that Förster energy
transfer occurred from the PFO-PBD host to the Eu(DBM)
BrDPPz) guest [9,17]. Similar energy transfer from host to
3
3
3
(
guest has been observed in some other host-guest systems
of conjugated polymers and europium(III) complexes
[
19,20].
.3 EL properties
The EL spectra and Commission Internationale de
3
L’Eclairage (CIE) chromaticity diagrams of the Eu(DBM)
BrDPPz)-doped PFO-PBD PLEDs with different dopant
3
(
3
Figure 4 Normalized EL spectra of Eu(DBM) (BrDPPz)-doped PLEDs
concentrations are presented in Figure 4. Similar EL spectra
are observed for these doped devices, which are different
at different dopant concentrations from 1 wt% to 8 wt%. The inset is CIE
1
931 chromaticity diagrams.
3
from the PL profiles of the Eu(DBM) (BrDPPz)-doped
PFO-PBD films. An EL emission maximum at 612 nm with
a FWHM of 14 nm and three weak peaks at 574, 648 and
The EQE-current density (J) and brightness (B)-current
density (J) curves of these devices with different doping
7
02 nm are observed in these EL profiles [14,15]. The weak
concentrations are shown in Figure 5. The performance of
emission peaks at 400 to 550 nm also appear in the EL
spectra, and obviously decrease in intensity with increasing
dopant concentration. These minor high-energy emissions
probably correspond to exciplex/electroplex emission and
fluorenone emission from the PFO-PBD blend [12]. This
these Eu(DBM) (BrDPPZ)-based devices with different
dopant concentrations is listed in Table 1. The turn-on volt-
3
ages of the devices increase from 11.2 to 16.8 V with in-
creasing doping level from 1 wt% to 8 wt%. This further
indicates that the devices mainly operate by the carrier-
trapping mechanism rather than the energy-transfer mecha-
suggests that EL emission from the Eu(DBM)
3
(BrDPPz)-
2
doped PFO-PBD devices is mostly dominated by the dopant
rather than the PFO-PBD host at dopant concentrations
from 1 wt% to 8 wt% [8,12], which is the dominant mecha-
nism in OLEDs [12,35]. A dramatic difference was ob-
served between PL and EL spectra for iridium complex-
doped PLEDs, which was attributed to different emission
mechanisms [35]. The CIE coordinates of the PLEDs at
near-saturated red emission change slightly from (0.591,
nism [17,38]. A maximum brightness of 829 cd/m at 153.5
2
2
mA/cm and maximum EQE of 1.7% at 2.1 mA/cm were
obtained for the device with 1 wt% Eu(DBM) (BrDPPz).
3
The present results indicate that introduction of a large
-conjugated system is responsible for the improved device
performance [31]. However, when the doping concentration
was increased to 8 wt%, the EQE and brightness decreased
2
to 0.94% and 344 cd/m , which is caused by triplet-triplet
0
.347) to (0.607, 0.332) with increasing dopant concentra-
annihilation [25]. Figure 6 presents the dependence of lu-
tion from 1 wt% to 8 wt%.
L p
minance efficiency ( ) and power efficiency ( ) on current

Fan et al. Sci China Chem January (2015) Vol.58 No.1
5
Table 1 Performance of Eu(DBM)
3
(BrDPPz)-doped PFO-PBD devices with different dopant concentrations from 1 wt% to 8 wt%
2
Maximum EQE
EQE
Maximum B
J=100 mA/cm
a)
a)
Doping ratio Turn on
CIE
(X, Y)

L
(cd/A),

p
(lm/W),
J
J
2
2
2
2
(wt%)
voltage (V)
B (cd/m ) EQE (%) B (cd/m )
J (mA/cm )
J (mA/cm )
2
2
(
mA/cm )
(%)
1.70
1.14
0.94
(mA/cm )
1
4
8
11.2
16.2
16.8
2.1
153.5
111.2
92.2
829
439
344
0.76
0.72
0.29
742
435
332
(0.591, 0.325)
(0.613, 0.317)
(0.621, 0.309)
0.74, 4.31
0.39, 7.67
0.22, 30.59
0.18, 4.31
0.06, 7.67
0.04, 30.59
3.2
7.0
a) Maximum values of each device, 
L
p
=luminance efficiency,  =power efficiency.
Figure 6 Luminance efficiency-current density-power efficiency charac-
teristics of the Eu(DBM) (BrDPPz)-doped PFO-PBD devices with differ-
ent dopant concentrations.
Figure 5 External quantum efficiency-current density (EQE-J) and
brightness-current density (B-J) curves for the Eu(DBM) (BrDPPz)-doped
PFO-PBD devices with different dopant concentrations from 1 wt% to
wt%.
3
3
8
density (J) for Eu(DBM)
doping concentrations, and the data is listed in Table 1. The
highest  of 0.74 cd/A with a maximum  of 0.18 lm/W at
.31 mA/cm were obtained for the device with 1 wt%
Eu(DBM) (BrDPPz). With increasing doping concentration
from 1 wt% to 8 wt%, the maximum  and  gradually
3
(BrDPPZ) devices with several
L
p
2
4
3
L
p
decreased at high current density. This implies that effi-
ciency roll-off in the Eu(DBM) (BrDPPZ)-based devices is
3
suppressed with increasing dopant concentration and current
density.
3
.4 Dispersibility
Figure 7 Atomic force microscope (AFM) image of an Eu(DBM)
BrDPPz)-doped PFO-PBD film containing 8 wt% dopant.
3
-
To evaluate its dispersibility in a polymer matrix, a film of
(
8
wt% Eu(DBM) (BrDPPZ) doped in a blend of PFO-PBD
3
was prepared. The surface morphology of the film was ex-
amined by atomic force microscopy (AFM), and the result-
ing image is shown in Figure 7. An average surface rough-
were examined by cyclic voltammetry. The measured re-
versible onset oxidation/reduction potentials (Eox/Ered) for
ness R
PPZ)-doped PFO-PBD blend film. Compared to our report-
ed complex Eu(DBM) (DPPz) [11], Eu(DBM) (BrDPPZ)
a
of 1.463 nm is observed for the Eu(DBM)
3
(BrD-
Eu(DBM) (BrDPPz) were 0.80/2.08 V, respectively. Ac-
cording to an empirical formula [34], the LUMO and
3
3
3
HOMO energy levels for Eu(DBM) (BrDPPz) were calcu-
3
exhibits poor dispersibility in the same matrix. Therefore,
appropriate domain size and phase separation strongly in-
fluences the performance of the PLEDs [39].
lated to be 2.65 and 5.54 eV, respectively. Compared to
those of Eu(DBM) (DBrPhen) [17], Eu(DBM) (BrDPPz)
displayed a minor increase in HOMO level, which could
improve its hole-transporting properties [40]. Compared to
3
3
3 3
Eu(DBM) (DBrPhen), Eu(DBM) (BrDPPz) as a dopant can
trap carriers from PFO more efficiently, which results in
efficient EL in the corresponding devices [17,41]. However,
3
.5 Electrochemical properties
The electrochemical properties of Eu(DBM)
3
(BrDPPz)

6
Fan et al. Sci China Chem January (2015) Vol.58 No.1
8
9
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3 3
compared with Eu(DBM) (DPPz) [11], Eu(DBM) (BrDPPz)
possesses a higher LUMO energy level by 0.05 eV [42].
This implies that Eu(DBM) (BrDPPz) is poorer at trapping
electrons than Eu(DBM) (DPPz), which would decrease the
EL efficiency of the Eu(DBM) (BrDPPz)-doped PFO-PBD
devices [12,43]. Therefore, to obtain the better device per-
formance, further studies on the EL properties of Eu(DBM)
BrDPPz) to optimize polymeric host and device configura-
tion should be performed.
3
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3
1
1
1
0
1
2
3
(
4
Conclusions
In conclusion, we fabricated high-efficiency europium
complex-based PLEDs by doping Eu(DBM) (BrDPPz) into
3
a PFO-PBD blend. These devices exhibited a sharp emis-
sion at 612 nm, with a FWHM of 14 nm even at low dopant
13 Guan M, Bian ZQ, Li FY, Xin H, Huang CH. Bright red light-
emitting electroluminescence devices based on a functionalized euro-
pium complex. New J Chem, 2003, 27: 1731–1734
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concentration. A maximum EQE of 1.7% at 2.1 mA/cm ,
1
4
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This work was supported by the Major Program for cultivation of the Na-
tional Natural Science Foundation of China (91233112), the National
Natural Science Foundation of China (51273168, 21172187, 21202139),
the Ministry of Science and Technology of China (2010DFA52310), the
Innovation Group and Xiangtan Joint Project of Hunan Natural Science
Foundation (12JJ7002, 12JJ8001), and the key project of Hunan Provin-
cial Education Department (13A102, 12B123).
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