The role of fluorine-substitution on the π-bridge in constructing effective thermally activated delayed fluorescence molecules

Article abstract of DOI:10.1039/c8tc01158c

Two thermally activated delayed fluorescence (TADF) emitters 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorophenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ-F) and 9′-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2,6-difluorophenyl)-9′H-9,3′:6′,9′′-tercarbazole (TCTZ-2

Full text of DOI:10.1039/c8tc01158c

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC01158C
Journal Name
ARTICLE
The role of fluorine-substitution on the π-bridge in constructing
effective thermally activated delayed fluorescence molecules
1
0
；Received 00th January 20xx,
a
a
a
c
a
b
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Yun Li, Jiao-Jiao Liang, Hong-Cheng Li, Lin-Song Cui, Man-Keung Fung, Stephen Barlow, Seth R.
b*
c
a*
a
Marder, Chihaya Adachi, Zuo-Quan Jiang, and Liang-Sheng Liao
Two thermally activated delayed fluorescence (TADF) emitters 9’-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2-fluorophenyl)-9'H-
,3':6',9''-tercarbazole (TCTZ-F) and 9'-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)-2,6-difluorophenyl)-9'H-9,3':6',9''-tercarbazole
TCTZ-2F) were synthesized and compared to the previously reported molecule 9'-(4-(4,6-diphenyl-1,3,5-triazin-2-
9
(
yl)phenyl)-9'H-9,3':6',9''-tercarbazole (TCTZ) to explore the possible effects of the fluorine atom(s) on the the molecular
conformation, electronic coupling between the donor and acceptor groups, and consequently the photophysical behavior
of the emitters. Specifically, the TCTZ-F molecule was calculated to have a smaller dihedral angle than TCTZ-2F and smaller
1 1
S -T gap than TCTZ, which led it to have the higest photoluminescence quantum yield (PLQY) among this series of
compounds. An OLED fabricated with TCTZ-F as the emitter achieved the highest external quantum efficiencies (22.5%) of
this series, indicating fluorination can play a useful role in TADF emitters.
ently, Adachi et al. found this method can also be beneficial for
minimizing the ΔE by increasing the CT component of the excited
Introduction
ST
2
2
state. In addition to a methyl group, the fluorine atom can also be
used to tune the geometry between either the donor or the
acceptor and the linker π-bridge. Fluoro substituents are both more
-9 electron-withdrawing and smaller than methyl groups (radii 1.35
and 2.0 Å, respectively). The former factor will modulate the various
pertinent energy levels in a manner distinct from the methyl group,
Thermally activated delayed fluorescence (TADF) materials have
attracted great attention in the field of organic light-emitting diodes
1
-4
(
OLEDs). A donor-acceptor architecture separated by a spacer
5
group is a common motif for constructing TADF materials.
Currently strategies involve reducing the spatial overlap of the key
orbitals involved in the lowest singlet (S
often assumed to be the HOMO and LUMO) in order to minimize
the exchange energy, leading to a small energetic separation of the
1 1
) and triplet (T ) excitations
and the latter one would be expected to lead smaller twist angles
when it is incorporated ortho to either the donor or acceptor. The
(
10-15
smaller twist angle may lead to increased S
leading to higher fluorescence quantum yields.
these interesting features of fluorine we synthesized two potential
TADF chromophores (TCTZ-F, TCTZ-2F) for systematic comparison to
1 0
-S oscillator strength,
lowest singlet (S
this must be implemented in a manner that preserves a significant
rate constant for radiative decay (fluorescence), krad, from S
One mechanism that has been used to tune the spatial overlap of
the ground and excited state wave function is the introduction of
torsions between the spacer group and the donor and/or the
acceptor in order to partially decouple the donor and acceptor
localized orbitals. There are several methods on modifying the
1
) and triplet (T
1
) excited states (ΔEST).
However,
23-25
Considering
1
to S0.
26-29
previously reported non-fluorinated analogue TCTZ.
These three
materials differ only from the number of fluorine atoms in the
phenylene bridge ortho to the donor. We found that the fluoro-
substituted phenylene spacer can also increase the twist angle and
minimize ΔEST. The TCTZ-F, with just one fluorine substituent, led to
a good compromise between achieving a small ΔEST and maintaining
a large oscillator strength. Consequently, it afforded the best OLED
device, which gave greenish-blue emission with an external
quantum efficiency (EQE) of 22.5%, while the TCTZ and TCTZ-2F
OLEDs exhibited EQEs of 17.1% and 19.6%, respectively.
1
6-18
general phenylene spacer group,
and a classic one is introducing
one or two methyl groups appended to a phenylene spacer in the
position(s) ortho to the donor to increase the donor-phenylene
dihedral angle. In previous studies, this approach was used to
construct host materials for blue phosphorescent OLEDs, because
1
9-21
molecules with high T
1
energies could be realized.
Rec
Results and discussion
a .
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of
Functional Nano & Soft Materials (FUNSOM), Joint International Research Laboratory
of Carbon-Based Functional Materials and Devices, Soochow University,
Suzhou 215123, P. R. China. E-mail: zqjiang@suda.edu.cn
Design and simulation
The target molecules utilize a tercarbazole group as the donor
moiety and triazine as the acceptor moiety (Scheme 1), and three
bridge groups are used as D-A linker, differing by the number and
position of the fluorine substitution. The TCTZ was previously
reported by Adachi et al., as part of a study for demonstrating that
extensive delocalization of molecular orbitals in structures with
b.
School of Chemistry and Biochemistry and Center for Organic Photonics and
Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United
States. E-mail: seth.marder@chemistry.gatech.edu
c .
Center for Organic Photonics and Electronics Research (OPERA), Kyushu University,
7
44 Motooka, Nishi, Fukuoka 819-0395, Japan.
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ARTICLE
Table 1. Thermal, photophysical, and electrochemical properties of TCTZ, TCTZ-F, and TCTZ-2F.
Journal Name
a)
b)
c)
d)
e)
f)
g)
h)
i)
T /T
λabs
(nm)
λmax
IP/EA
(eV)
HOMO/LUMO
(eV)
ET
∆EST
∆EST
f
d
g
Compound
o
(
C)
(nm)
444
465
489
(eV)
2.87
2.86
2.85
(eV)
0.130
0.095
0.078
(eV)
TCTZ
TCTZ-F
538/197
521/198
522/199
262,346
265,344
266,341
5.16/2.01
5.14/2.02
5.15/2.05
-5.68/-2.27
-5.68/-2.40
-5.68/-2.51
0.148
0.082
0.055
0.191
0.153
0.131
TCTZ-2F
a)
b)
d g
T obtained from TGA measurements and T obtained from DSC measurements UV–Vis absorption spectra measured in
dichloromethane solution; PL pspectra measured in toluene; IP and EA energy levels estimated from the redox potential in
dichloromethane solution in cyclic voltammetry (Fig. S4); HOMO and LUMO energy levels calculated from DFT;
the onset of phosphorescence spectra at 77 K; Energy splitting between S and T estimated from onset of the fluorescence and
c)
d)
e)
f)
T
E estimated from
g)
1
1
h)
i)
phosphorescence spectra; Calculated by TDDFT; f is the oscillator strength which is calculated by TD-DFT
well-separated HOMO and LUMO can suppress a decrease in k large sensibility of S to the change of CT state when the fluorine
f
1
3
0-36
while lowering ΔEST
.
Here we also use it as a reference material
1
atom is introduced in this system. Therefore, the S decreases much
with which to compare the effects of spacer fluorination.
1
more quickly from TCTZ through TCTZ-F to TCTZ-2F than the T ,
4
0
resulting in decreased of ΔEST. As for the distribution of natural
transition orbitals (NTOs) analysed using Multiwfn, all the
2
compounds have similar lowest unoccupied NTO but differ from
the highest occupied NTO, where the central carbazole gradually
4
1-42
participate in the distribution of highest occupied NTO.
difference in fluorine substitution, however, cannot fundamentally
change the T energy because the localized features on the central
bridges are still obvious. The dihedral angle can hardly vary the T
because the locally excited (LE) on the central bridge was
This
1
Scheme 1. Chemical structures for TCTZ, TCTZ-F and TCTZ-2F
1
4
3-44
Before the preparation of the target molecules, The ground state
insensitive to this factor.
Additionally, it was interesting to
(
S
0
) and excited state (S
1 1
, T ) geometries of TCTZ, TCTZ-F and TCTZ-
simulate the TCTZ constrained at the torsion angles of TCTZ-F and
TCTZ-2F. These two hypothetical “molecules” might help better
understand the influence of twist angle vs. electronegativity of F on
2
F were optimized using DFT/B3LYP/def2svp and TD-DFT/PBE0/
def2svp methods respectively to predict the effect of the fluoro
group(s) on the twist angle, frontier orbital distribution, and
1
the S energy. As shown in Table S1, the data indicate that it is the
3
7-39
calculated ΔEST.
The calculations suggest that the dihedral angle
introduction of fluorine atom(s) that obviously influences the LUMO
energy and ΔEST. On the other hand, it is also noted that the ∆EST is
nearly constant with the small change on torsion angle, thus we can
deduce the decrease trend of ∆EST is more related to the fluorine
feature.
between the phenylene ring and central carbazole increases from
o
o
o
5
3.23 , 56.65 to 57.32 with the addition of fluorine atom(s), which
is considerably less than the effect calculated for related methyl-
substituted species, consistent with the smaller size of fluorine
relative to that of methyl. As the dihedral angle increases, it led to
the oscillator strength (f) of transition from S0 to S1 weak. Synthesis and characterization
Calculated for the S -S transition decreased from 0.192, 0.153 to
.131 for TCTZ, TCTZ-F and TCTZ-2F, respectively. The S energies
for TCTZ, TCTZ-F and TCTZ-2F were 2.624, 2.497 and 2.390 eV
respectively; and the T energies for TCTZ, TCTZ-F and TCTZ-2F were
.477, 2.415 and 2.335 eV respectively. We note that both S and T
0
1
Given the results of the simulations, the three materials TCTZ, TCTZ-
F and TCTZ-2F were synthesized in two steps. Their chemical
structures are shown in Scheme 1. Suzuki-Miyaura coupling of 2-
chloro-4, 6-diphenyl-1, 3, 5-triazine with the appropriate aryl
boronic acids gave the three key intermediates refer to 2-(4-
0
1
1
2
1
1
showed decreased trends from TCTZ, TCTZ-F to TCTZ-2F, but the S1 fluorophenyl)-4, 6-diphenyl-1, 3, 5-triazine, 2-(3, 4-difluorophenyl)-
energy reduced much more quickly, and thus the calculated ΔEST 4, 6-diphenyl-1, 3, 5-triazine and 2, 4-diphenyl-6-(3,4,5-
trifluorophenyl)-1,3,5-triazine, in high yields (Detailed structures
are shown in SI). TCTZ, TCTZ-F and TCTZ-2F were then obtained by
values decreased from 0.148 (TCTZ), 0.082 (TCTZ-F) to 0.055 (TCTZ-
2
F) eV, respectively. Not surprisingly, given the relatively small
aromatic nucleophilic substitution with the tercarbazole moiety. In
these cases, the fluorine atom para to the triazine group is mostly
activated by its π-electron-withdrawing effect, allowing the desired
products to be obtained clearly. As the fluorine-substituted bridges
have only been rarely reported in previous work, we were
interested in whether the introduction of fluorine would negatively
impact the thermal stability. Thus, the thermal and morphologically
stabilities were examined by (Thermogravimetric Analysis) TGA and
differential scanning calorimetry (DSC), both at a heating rate of 10
changes in torsion angle, TCTZ-F and TCTZ-2F have similar frontiers,
with similar spatial separation of the HOMO and LUMO, to those
previously reported for TCTZ (Fig. S1).
To further understand how the fluorine atom affected the
excited states, charge transfer-locally excited (CT-LE) components
and CT strength are calculated and analysed, respectively. As shown
in (Fig. S1), percentage of charge transfer (CT) for S
TCTZ-F and TCTZ-2F are 78.63%, 80.20% and 81.08%, respectively.
The CT dominant property of S
in three molecules determines the determined from the second heating scan and showed very little
1
states of TCTZ,
o
-1
C min under nitrogen. The glass transition temperatures (T ) were
g
1
variation: T
g
values for TCTZ, TCTZ-F, and TCTZ-2F are 197, 198, and
2
| J. Name., 2012, 00, 1-3
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Table 2. PLQY, lifetime and transition constants of TCTZ, TCTZ-F, and TCTZ-2F.
ARTICLE
a)
a)
b)
b)
c)
τPF
τDF
ФPF
ФDF
(%)
Ф
kRISC
(10 s )
kPF
(10 s )
kDF
(10 s )
kISC
Compound
4
-1
6
-1
4
-1
6 -1
(10 s )
(
µs)
(µs)
(%)
(%)
TCTZ
TCTZ-F
TCTZ-2F
0.15
0.19
0.22
10.09
9.20
6.41
64.08
62.67
58.57
19.92
29.33
29.43
84
92
88
8.57
13.63
18.95
6.67
5.26
4.55
9.90
10.87
15.63
2.39
1.96
1.88
a)
τPF (the prompt lifetime) and τDF (the delayed lifetime) were obtained from transient PL decay of doped films (6% in DPEPO host was
b)
performed using a streak camera at room temperature);
to the prompt and delayed proportions in transient decay curves; Absolute PLQY of doped films measured with integrating sphere.
ФPF (the prompt PLQY) and ФDF (the delayed PLQY) were estimated according
c)
o
1
99 C, respectively. The temperature corresponding to 5% weight from the highest vibronic band in phosphorescence, reduced very
loss in the TGA was used as the decomposition temperature (T
values for TCTZ, TCTZ-F, and TCTZ-2F were 538, 521, and 522 C, with the calculation. This fact may arise from that the localized
respectively. These data indicate that the fluorine substitution has a triplet energy on the triazine part was insensitive to the fluorination
d
); T
d
slightly with the increase of the fluorination, which is consistent
o
minimal impact on thermal and morphological stabilities.
degree. Even so, we still observed the relative intensity of the
The steady-state UV-Vis absorption of three molecules in dilute shoulder peak (ν1→0) in Phos spectra of TCTZ, TCTZ-F and TCTZ-2F
3
2 2
CH Cl solution and photoluminescence (PL) spectra of three increase in sequence, which possibly mixed with the CT emission.
molecules in dilute toluene solution are presented in Fig. 1 and The estimated experimental ΔEST values were 0.130, 0.095 and
their related photophysical data are summarized in Table 1. All 0.078 eV for TCTZ, TCTZ- F and TCTZ-2F respectively. It is
these compounds exhibited very similar spectra. The stronger noteworthy that the decrease degrees from TCTZ through TCTZ-F to
higher energy absorptions at wavelengths below 300 nm are TCTZ-2F were larger than the theoretical ΔEST values 0.148, 0.082,
attributed mainly to the π–π* transitions on the identical 0.055 eV, which could be due to the DFT predicting a more planar
tercarbazole unit. The weak maxima at ca. 340 nm arise from the structure that what it may be the case in solution. The twist angle
lowest-energy tercarbazole-based transitions, while the weak increased slowly with one fluorine and two fluorine atoms
absorption tails extending beyond 350 nm are assigned to appended at the phenylene bridge, but even the largest twisted
tercarbazole-to-triazine charge-transfer. TCTZ, TCTZ-F and TCTZ-2F angle of TCTZ-2F was far away from a perpendicular configuration.
all had significant solvatochromic effects (Fig. S12), and the TCTZ-2F
displayed the largest red-shift (155 nm) from n-hexane to DMF
which was in line with the simulation results.
Transient PL measurement of TCTZ, TCTZ-F, and TCTZ-2F doped in
DPEPO (6 wt %) was performed using a streak camera at room
temperature. Transient PL decays of the three emitters are
described in Fig. 2 All the emitters exhibited obvious prompt and
delayed fluorescence components (PF and DF, respectively),
UV-Vis
FL
Phos
0
.8
.4
.0
.8
.4
.0
.8
.4
.0
consistent with the molecules behaving as TADF emitters. The
lifetime and the related photophysical PL quantum efficiency (PLQY,
Φ) are given in Table 2. According to the following equations
0
0
0
N
ꢃꢄꢅꢆ
F
N
N
N
N
1 − ꢀꢁꢂ =
0
ꢃꢁꢂ
N
TCTZ-F
0
ꢃꢁꢂꢃꢈꢂ ꢀꢈꢂ
0
ꢃꢇꢄꢅꢆ
=
ꢃꢄꢅꢆ ꢀꢁꢂ
0
0
300
400
500
600
700
5
4
3
2
1
Wavelength (nm)
10
TCTZ
TCTZ-F
TCTZ-2F
Fig. 1 UV-Vis absorption in dichloromethane, fluorescence (298 K),
and phosphorescence (77 K) spectra of TCTZ, TCTZ-F and TCTZ-2F
in toluene.
1
0
0
1
The comparison of room-temperature fluorescence (PL) spectra
and phosphorescence (Phos) spectra (obtained at 77 K) in toluene
afforded estimates of ΔEST that decrease with the addition of each
fluorine atom (Table 1). This shift is primarily attributable to
10
10
lowering of the S
1
energy (3.06, 2.96, and 2.90 eV estimated for
10
20
30
40
50
TCTZ, TCTZ-F and TCTZ-2F, respectively, from the high-energy onset
of the PL spectra), which is likely at least partly attributable to the
effect of the F atoms on the LUMO energy (consistent with the DFT
calculations and with electrochemical measurements, Table 1).
Lifetime (ꢀs)
Fig. 2 Transient PL decay spectra of TCTZ TCTZ-F and TCTZ-2F doped
into DPEPO films (6 wt %) at room temperature.
The PL peak positions also show increasing red-shifts with
fluorination (Table 1). However, the T energies (Table 1), estimated
1
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Table 3. The device's performance of TCTZ, TCTZ-F, and TCTZ-2F.
Journal Name
b)
c)
d)
-1
e)
f)
Device
V
Peak
(nm)
CIE
CEmax
PEmax
(lm W )
EQEmax
(%)
EQE
Roll-off
(%)
a)
-1
Emitters
(eV)
(cd A )
(%)
TCTZ
TCTZ-F
TCTZ-2F
4.45
4.35
4.69
472
500
512
(0.16,0.24)
(0.24,0.43)
(0.27,0.46)
28.00
56.50
52.50
19.73
40.79
35.14
17.1
22.5
19.6
8.6
16.2
14.9
50
28
24
a)
b)
-2
c)
Device structure ITO/HAT-CN/TAPC/mCP/DPEPO: 6% dopant /3TPYMB /Liq/Al ;
efficiency; Maximum power efficiency (PE); Maximum external quantum efficiency; external quantum efficiency (@ 1000 cd m )
Driving voltage (@ 0.2 mA cm ); Maximum current
d)
e)
f)
-2
R
The radiative decay rate constants of fluorescence (k ), internal
Conclusions
crossing (kISC), and reverse internal crossing (kRISC) are also
estimated and summarized in Table 2. As shown in Fig. 2, transient
PL measurements at room temperature clearly show delayed decay
components of TCTZ, TCTZ-F and TCTZ-2F with lifetimes of 10.09 µs,
In summary, by adding one or two fluorine atom(s) at the
phenylene bridge between donor (tercarbazole) and acceptor
(
triazine) could be used to modify ΔE . This trend in ΔE values
ST ST
may stem from multiple factors including the effect of the fluorine
9.20 µs and 6.41 µs, respectively. From these results, it is noted that on the twist angle, and its inductive effects on the LUMO energy
the value of kRISC increases from TCTZ through TCTZ-F to TCTZ-2F,
which can be ascribed to the gradually decreasing ΔEST. On the
other hand, the rate constant for decay of prompt fluorescence,
kPF,decreases along with the introduction of the fluorine atom,
which is in accordance with the calculated reduction in oscillator
strength. Thus, adding fluorine atoms at the phenylene bridge could
facilitate the RISC process but reduce the prompt PLQY, ΦPF,
through the lower oscillator strength associated with the slightly
enlarged dihedral angle. However, increased ΦDF can compensate
for the loss of PF; indeed TCTZ-F achieved the highest absolute
PLQY in this series.
-1
a)-2
EML
b)
-
2.0
-2.00-2.27
-
2.40
-2.40
-2.51
-3
-4
-5
-
3.32
Li/Al
-4.30
1
0
ITO
-5.10
-
6.00-5.60
-5.68
-6
-7
-5.90 -6.00 -5.68 -5.68
6% TCTZ
% TCTZ-F
% TCTZ-2F
6
6
-6.77
1
100
1000
cd A-1
-
9.90
Luminance
(
)
10000
Electroluminescent properties
c)1.0
d) 40
6
% TCTZ
6% TCTZ-F
0.8
0.6
0.4
0.2
0.0
6% TCTZ-2F
To investigate the electroluminescence properties of all the three
TADF emitting materials, we fabricated the devices with a
commonly used multi-layered structure: ITO/HAT-CN (10 nm)/TAPC
30
1000
20
10
0
TCTZ
TCTZ-F
TCTZ-2F
100
(35 nm)/mCP (10 nm)/DPEPO: 6% dopant (20 nm)/3TPYMB (40
nm)/Liq (2 nm)/Al (120 nm) (dopant=TCTZ for device 1, TCTZ-F for
device2, TCTZ-2F for device 3). As displayed in Fig. 3a), 1,4,5,8,9,11-
400
500
600
Wavelength (nm)
700
4
5
6
7
Voltage (V)
8
9
hexaazatriphenylene-hexacarbonitrile (HAT-CN) and Liq served as Fig. 3 a) Energy level diagrams of OLED devices. b) EQE–current
hole-and electron-injection materials, respectively; TAPC (1,1-bis[4- density characteristics of devices 1–3. c) EL spectra of devices 1–3.
N,N-di (p-tolyl) amino] phenyl] cyclohexane),having a suitable d) Current density- voltage-luminance characteristics of device 1-3.
[
estimated IE of 5.6 eV, was used as hole-transporting and electron-
blocking layer; mCP (N,N’-dicarbazolyl-3,5-benzene) as an exciton
blocking layer; 6 wt% TCTZ doped in DPEPO host was used as the
1
leading to lowering the S energy. We also observed that the
although the oscillator strength decreased with the introduction of
fluorine atom resulting in a decreased yield of prompt fluorescence,
the total PLQY can increase because of the enhanced RISC rate.
Thus, it is found that the use of a fluorinated phenylene bridge can
be an effective way to minimize the ΔEST without reducing the PLQY.
In OLEDs, the non-fluorinated reference material TCTZ showed
maximum EQE of 17.1%, comparable to the literature report. Both,
TCTZ-F and TCTZ-2F corresponding to mono- and di- fluoro-
substitution exhibited higher EQEs of 22.5% and 19.6%. These
results suggested that fluorine substitution of the bridge may be a
useful tool to tune the properties of TADF emitters.
emitting
layer
(EML);
and
3TPYMB
(tris-[3-(3-
pyridyl)mesityl]borane) acted as an electron-transporting material
(
(
see ESI for chemical structures). The emission peaks of devices 1-3
Table 3) are consistent with the PL peaks for neat thin films (Table
1). Key device performance parameters are compiled in Table 3.
The maximum EQEs for TCTZ-F and TCTZ-2F devices are both higher
than that for the TCTZ-based device (which shows similar
performance to previously reported TCTZ devices). TCTZ-F achieves
the best performance, even the external quantum efficiency
-
2
(
@ 1000 cd m ) that the TCTZ is also the highest one. And compare
to TCTZ the TCTZ-F and TCTZ-2F have low roll-off efficiency. Which
is consistent with the higher overall PLQY found for this compound
(
Table 2), which, as discussed above, may be a result of a good
Acknowledgements
compromise between ΔEST and f.
We acknowledge financial support from the National Natural
Science Foundation of China (Grant No. 61575136 and 21572152)
and the National Key R&D Program of China (Grant No.
2
016YFB0400700). This project is also funded by the Collaborative
This journal is © The Royal Society of Chemistry 20xx
4
| J. Name., 2012, 00, 1-3
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J oP ul er na as el od f oM na ot te ra i da lj su sC th me ma ri gs ti nr ys C
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DOI: 10.1039/C8TC01158C
Journal Name
ARTICLE
Innovation Center of Suzhou Nano Science and Technology (Nano-
CIC), by the Priority Academic Program Development of Jiangsu
Higher Education Institutions (PAPD), by the “111” Project of the
State Administration of Foreign Experts Affairs of China, and by the
Yunnan Provincial Research Funds on College-Enterprise
Collaboration (NO. 2015IB016). We also thank Yi Yuan and Dr.
Yadong Zhang for the helpful discussions in design and analysis of
the experiment.
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