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Nanoseed Assisted PVT Growth of Ultrathin 2D Pentacene Molecular
Crystal Directly onto SiO2 Substrate
S. Atika Arabi,†,‡,# Ji Dong,†,# Misbah Mirza,† Peng Yu,† Liang Wang,† Jun He,§ and Chao Jiang*,†
†
CAS Key Laboratory of Standardization and Measurement for Nanotechnology, & CAS Center for Excellence in Nanoscience and
CAS Key laboratory of Nanosystem and Hierarchical Fabrication, National Centre for Nanoscience and Technology, No. 11
Zhongguancun Beiyitiao, Beijing 100190, China
‡
University of Chinese Academy of Science, No. 19A Yuquan Road, Beijing 100049, China
§
S Supporting Information
*
ABSTRACT: High order of molecular packing and perfect semiconductor/dielectric interface are two key factors to achieve
high performance for organic field-effect transistors (OFET). Moreover, the thin crystal offers an improved efficiency of carrier
injection for OFETs. To this aim, formation of thin and large single crystal directly on dielectrics is the basis to obtain the ideal
crystal OFETs. Herein, we report the controlled growth of ultrathin 2D Pentacene (Pn) crystal via nanoseed assisted physical
vapor transport (PVT) method grown directly on SiO2. The size, thickness, and density of Pn crystals are systematically studied.
Potentially effective parameters such as initially lowered Pn coverage and decreased supersaturation with the aid of carrier gas
flow were optimized to grow large, ultrathin 2D Pn crystalline flakes efficient for the fabrication of crystal OFETs. The typical size
and thickness of as-grown Pn crystalline flakes can be controlled to be large and thin enough. Device of ultrathin crystal with
bottom gate and top contact configuration showed mobility as high as 5.6 cm2 V−1 s−1, indicating that the proposed novel
architecture of organic molecular crystals may pave the way toward the application of large-sized single crystals of Pn in organic
electronics.
1. INTRODUCTION
Over past few years, single crystal field effect transistors
(SCFET) have become a subject of interest for understanding
the mechanism of charge transport in organic semiconductors.
Single crystals exhibit incomparable advantages over thin films
such as high purity, long-range-order molecular packing, and no
grain boundaries with minimized structural defects, establishing
the structure property relationship and ensuring higher device
performance.1,2 To date, it is generally acknowledged that the
SCFETs performance is highly sensitive to semiconductor/
dielectric interface. Many groups have reported the growth of
organic single crystal and their device fabrication techniques to
attain high mobility. By introducing a new technique of air flow
navigation, Zhengran fabricated a crystal of TIPS Pentacene.3
Onojima et al. used a simple method of electrostatic spray
deposition for organic crystal growth.4 Horowitz,5 Kloc,6 and
Laudise et al.7 are among the pioneers who developed a PVT
technique for the growth of several organic semiconductors by
© 2016 American Chemical Society
improving the design of high quality organic semiconductor
crystal devices. With emphasis on these PVT-based techniques,
Alborghetti et al. fabricated nanogap single-crystal organic
transistors8 while anisotropic measurements of fused-ring
thienoacene9 and Pn10 have also been reported. The high
performance single-crystal organic semiconductors are endowed with clarification of their intrinsic electronic properties
and significance for potential applications such as displays,11,12
large area electronics,11 and sensors.13
In most previously reported SCFET devices, grown organic
single crystals were usually thick and transferred onto other
substrates for the fabrication of SCFET. While in the solution
method, the solvent and impurities can inevitably deteriorate
the quality of crystal and the interfaces. The contaminated
Received: December 7, 2015
Revised: March 19, 2016
Published: March 28, 2016
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Figure 1. (a) Scheme of experiments during PVT growth. (b,c) AFM image (30 × 30 μm2) of ultrathin pentacene crystal with its height profile and
optical image, respectively. (d) SEM image of large-sized crystal obtained after PVT. (e) XRD of the corresponding crystal. (f,g) TEM image and
relative SAED pattern, respectively.
from several hundreds of nanometers to about 50 nm was
successfully synthesized. To the best of our knowledge, this is
the first report to come up with ultrathin Pn crystals by PVT
method. Furthermore, growth mechanism of different crystal
shapes manipulated by partly tailoring the deposition was
proposed. Significantly, the competing relations of surface
energy between the (110), (010), and (11̅0) surfaces lead
perfect hexagonal to rhombic shape. Moreover, by investigating
field effect behavior, the maximum mobility reaches up to 5.6
cm2 V−1 s−1 with average mobility 2.0 (±1.1) cm2 V−1 s−1. The
huge variation may be somewhat attributed to the anisotropy of
charge transport. Thus, our method provides an opportunity to
reveal the growth mechanism of Pn crystals and realize their
practical applications in future organic electronic technologies.
semiconductor/dielectric interface would result in poor device
performance and make it difficult to investigate the intrinsic
transport mechanism. Therefore, to achieve high device
performance, the best way to keep the entire interface is to
grow a thin organic single crystal in situ on a substrate with the
PVT method. However, the reduced thickness and interfacial
defects still exist as major challenges.14,15
The control of supersaturation during reaction time and in
situ growth of crystal may solve the problems addressed above.
In our previous work, we reported the in situ growth of organic
crystal using the seed growth method which promotes the
growth of crystal by reducing the Gibbs free energy of the
system with nanoseeds.16 This work explores the basis of
materials for the experimental research in obtaining large and
ultrathin 2D crystal and seeks the device applications for Pn
crystals. The main aim of this study is to get a better
understanding of the underlying mechanism of the formation of
ultrathin 2D Pn crystal via PVT process.
Here, the growth of large and ultrathin 2D Pn crystals with
specific facets is demonstrated via nanoseed assisted growth
method stimulated by supersaturation. The obtained Pn crystals
have been carefully characterized and a detailed study on the
growth of the crystallization mechanism is also provided. Pn
single crystal of size as large as 50 μm with thickness ranging
2. EXPERIMENTAL SECTION
2.1. Formation of Nanoseeds. Heavily doped Si wafers with
thermally grown SiO2 of 300 nm were chosen as substrates. The
substrates were cleaned consecutively with a solution of ethanol,
acetone, ammonia, and deionized water with a 1:6 volume ratio and in
the end with deionized water by using ultrasonic cleaner. Finally
substrates were blown with high purity dry nitrogen. Pn (SigmaAldrich used without further purification) monolayer (0.7 ML) was
deposited on substrates by thermal evaporator (AUTO 306, BOC
Edward Co.) at ambient substrate temperature under a vacuum
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pressure of 7 × 10−5 Pa. After deposition of thin film, substrates were
transferred to the glovebox having argon environment to protect them
from being oxidized in the air. Substrates with MLs were annealed at
120 °C to get seed crystal using a heating plate.
2.2. PVT Growth. A conventional horizontal furnace setup has
been used. For crystal enlargement, the annealed samples were shifted
to the furnace for subsequent PVT growth. Pn powder (97%, from
Aldrich Chemical Co.) was used as a source material while SiO2
substrates having nanocrystals (from annealing 0.7 ML) were placed
downstream in the furnace. The following procedure was used to grow
the Pn crystal as a reference throughout this report: the source was
heated at 285 °C, while substrate temperature maintained to 180 °C at
a reactor total pressure of 1 atm and 300 sccm flow. The growth time
was fixed to 30 min.
This procedure was modified in four series to determine the growth
conditions for nucleating the Pn crystal. Series A consists of substrates
with four Pn coverages of 0.3, 0.5, 0.7, and 1 ML. In the second series
(series B) the growth temperature of the Pn crystal was modified to
Tsub = 210, 180, 150, and 120 °C while otherwise following the
reference procedure. Series C was modified by using four different gas
flows as 100, 200, 300, and 400 sccm while maintaining other
conditions the same as the reference. In the fourth series (series D) the
growth time was varied to 10, 30, and 60 min while otherwise
following the reference procedure.
2.3. Device Fabrication and Characterization. Copper grid was
used as shadow mask for the device fabrication and a configuration
with top contact and bottom gate was adopted. Copper/gold
electrodes with 100 and 40 nm thickness were deposited respectively
directly on to the as grown crystal without disturbing its interface. The
channel length is 10 μm while the width is 20 μm. The electrical
property of the devices was carried out via a Keithley 4200 instrument
in air at room temperature. The morphology of the monolayer was
monitored by AFM,16 while Pn structure after PVT was characterized
by field emission scanning electron microscopy (FESEM) S4800
(Hitachi).
Their roles in the formation of Pn crystals are discussed in the
following section.
3.1. Coverage Effect. Nanoseed density provides control
over the nucleation of vapor-grown organic single crystals onto
the SiO2 surface Figure 2 displays the role of nanoseed density
Figure 2. (a1, b1, c1, d1) AFM images (5 × 5 μm2) of pentacene 1,
0.7, 0.5, 0.3 ML, respectively. (a2, b2, c2, d2) SEM images after
annealing of Pn ML with calculated nanoseed density. (a3, b3, c3, d3)
SEM images of single crystal after PVT growth.
3. RESULTS AND DISCUSSION
Schematic diagram based on experimental setup is given in
Figure 1a to depict the important stages of the molecular
transport process during PVT growth. The morphology of in
situ grown large Pn crystal was characterized by AFM as shown
in Figure 1b. The height of the Pn crystal indicates 48.5 nm
thicknesses while Figure 1c shows the optical image of the
corresponding large crystal. The SEM image of the ultrathin Pn
crystal fabricated using the reference conditions (see
Experimental section) is shown in Figure 1d. TEM
observations have shed light on the crystal upper surface
while selected area diffraction (SAED) patterns of Pn crystal in
Figure 1f,g unveil the single crystalline quality and growth
direction. The upper surface of the crystal is indexed to the
(001) plane. X-ray diffraction studies verify the high quality of
Pn crystals. Figure 1e is the XRD pattern of in situ grown large
Pn crystals; the strong diffraction peak at 6.24° corresponds to
(001) crystal plane. The calculated full width at half maxima of
the (001) peak is 0.18°.
The morphology of the final crystal structures depends upon
the nucleation, orientation, and reaction conditions. Although
there are reports on the PVT growth of Pn crystals, but no
systematic studies about the effect of carrier gas, growth time
measurements on the morphology have been carried out. The
following study shows that the density and thickness of the Pn
crystal can be systematically controlled. The determining
factors for the formation of large crystals are found to be
monolayer pentacene coverage and substrate temperature,
while for ultrathin crystal they are gas flow and growth time.
on the growth (series A). AFM images of ∼1, 0.7, 0.5, and 0.3
ML are shown in Figure 2 (a1, b1, c1, d1), respectively, for all
cases the morphology of Pn is disc-like comparable with the
reported work.17 Nanoseeds obtained after annealing of
respective coverages are presented in Figure 2 (a2, b2, c2,
d2). It is clear that density of nanoseeds increases with the
increase in coverage or vice versa. However, the phenomena of
this molecular redistribution vanished up to a certain thickness,
i.e., when coverage exceeds 5 ML. SEM images of large crystal
after subsequent PVT growth are shown in Figure 2 (a3, b3, c3,
d3).
On the basis of the experimental results, as density of
nanoseeds increases (up to 0.3/μm2), after subsequent PVT
growth, density of large crystals also increases. This shows that
nanoseeds promote the growth of crystals, as they provide
specific sites for the incoming molecules to accommodate. The
selectivity achieved in our system is more likely due to the
difference in thermodynamics on substrates having nanoseeds
or without nanoseeds. This view is supported by the following
explanation. For Pn, the critical island size was assumed to be 3
< i < 4 at substrate temperature between 25 and 70 °C.18 In the
case of bare SiO2 substrate, without nanoseeds, when Pn
molecule arrives, it is not easy for it to diffuse with another
molecule to become a stable nucleus, as its island size is below
the critical island size. Most of the rare Pn molecules lying
beyond the mean diffusion length will be desorbed from the
substrate before they collide with each other to form stable
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⎛ p⎞ ⎛
B⎞
ln⎜⎜ ⎟⎟ = ⎜A − ⎟
T⎠
⎝ po ⎠ ⎝
nuclei. As a result, crystals with very low density can be
observed. However, when identical Pn molecules arrived at the
substrate having nanoseeds, it is easy for them to diffuse to the
neighboring nanoseeds because the distance between these
species is limited by the mean diffusion length. This proposes
that there should be formation of a critical nucleus which
promotes nucleation. SEM images in Figure S1 show the
comparison of samples without nanoseeds and with nanoseeds.
Furthermore, thermodynamic analysis implies that the number
of molecules required for a stable island increases with
increasing substrate temperature19 which suggests that, for
our case, the value of critical island size may exceeds 4. So, in
this case nanoseed assisted growth helps incoming molecules
make a stable island. From the images in Figure 2, it can be
predicted that the density of large crystals depends on the
density of nanoseeds.
When monolayer coverage is beyond 0.7 ML, the nanoseed
density is so high that incoming molecules just take part in
increasing the density and thickness because they have less
space to enlarge the crystal size. Our experimental result
suggests that the low density of nanoseeds is indeed helpful for
achieving the large crystals.
3.2. Substrate Temperature. Temperature at the
substrate has a key effect on the crystal growth. For better
understanding of the formation of Pn crystal, we systematically
studied different growth temperature (series B) as illustrated by
SEM images in Figure 3. It is observed that growth is different
where p and po is equilibrium and ambient vapor pressure,
respectively, of Pn, while A and B are constants, and it is
recognized that B = ΔHsub/R, where “ΔHsub” defines enthalpy
of sublimation while “R” is the gas constant. By taking values of
A and B21 we can calculate the vapor pressure at source (285
°C) and different substrate temperatures. Graph (a) in Figure
S2 shows linear behavior of vapor pressure with respect to
temperature. It demonstrates that the vapor pressure of Pn also
decreases as temperature decreases from 210 to 120 °C. The
calculated vapor pressure at 120 °C is exp (−12) Pa, which
suggests that most of the Pn molecules will not desorb and help
in increasing the density of crystals.
At 210 and 180 °C, because of high substrate temperature,
desorption of Pn molecules occurs along with the absorption
and diffusion processes. At these temperatures supersaturation
is low, which results in lower nucleation probability. Due to
this, the density of crystals is less as shown by the SEM images
in Figure 3, while at 150 and 120 °C, because of lower vapor
pressure value ∼Psub = exp (−12) Pa desorption nearly ceases
and incoming molecules from the source will be immobile. The
increased supersaturation results in higher nucleation probability, and growth occurs only because of adsorption and
diffusion. This leads to high density crystals. Hence, there are
two important observations: (i) at higher substrate temperatures low density crystal is obtained because of lower
supersaturation; (ii) at lower substrate temperatures supersaturation is higher which results in high density crystals.
Calculated values of nucleation probability with varying Pn
vapor concentrations at different substrate temperatures is
illustrated in Figure S2(b). The observed trend of growth is in
consistent with the calculated values.
3.3. Gas Flow Rate Effect. Table 1 shows a series of SEM
images to demonstrate the detailed growth process of Pn crystal
with various gas flow rates from 100 to 400 sccm while keeping
other parameters as reference (series C). Corresponding AFM
images shown in Table 1 illustrated the thickness of Pn crystals
ranging from 700 nm down to 48 nm. At 100 sccm, we
obtained thick and large-sized crystals. In horizontal PVT, only
a small amount of vapor reverses flow either descending or
ascending in close proximity of the crystal and the heat source,
respectively.22 This results in less condensation of vapor on the
substrate through the boundary layer of gas. Thus, at lower gas
flow rate the crystals are large but with very low density. Lower
flow rates made the Pn vapors have sufficient residence time to
interact with the nanoseeds, which results in thick crystals. The
forced diffusion of vapor using carrier gas has more impact than
the molecular diffusion driven by concentration gradient as
proposed by Nam et al.23 Under this advection state, there
generally exists a maximum Pn vapor concentration and
supersaturation downstream from the source. As the flow rate
of N2 carrier gas increases, the supersaturation shifts further
downstream, as flow gives direction to the vapor, which means
by adjusting gas flow we can change supersaturation at a point.
At 300 sccm flow we got the best results. The typical thickness
of as-grown Pn crystalline flakes can be narrowed in a range
from 200 nm to around 50 nm. The upper surface of the crystal
is indexed to the (001) plane which has the lowest surface
energy.24 This results in offering the molecules to diffuse to the
side surfaces ((11̅0), (010), (110)), where the surface energies
Figure 3. SEM images of obtained single crystal at different substrate
temperatures (series A).
at different substrate temperatures. In our case, substrate
temperature of 180 °C shows the best results. These results can
be explained in terms of supersaturation and nucleation
probability of grown adatoms (here Pn molecules) given by
Sears20
⎡ ⎛ σ 2π ⎞⎤
PN ∝ exp⎢ −⎜ 2 2
⎟⎥
⎣ ⎝ k T ln α ⎠⎦
(2)
(1)
where σ is surface energy, T is substrate temperature, and α is
supersaturation. Assume that vapor pressure of Pn coming from
the source is steady over the entire tube with background
pressure of 1 atm. Vapor pressure can be calculated from the
integrated form of the Clausius−Clapeyron equation.
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Table 1. SEM and AFM Images of Obtained Pn Crystals with Different Gas Flow (Series B)
are higher; thus, the molecules diffuse much more easily into
the lattice planes on these surfaces. The faster the growth rates
of the side surfaces, the larger the area for the top surface. This
competitive behavior among different planes leads to the
extension of crystal size along the (010) plane. At the same
time accelerated speed of gas flow decreases the deposition rate,
which helps to obtain ultrathin crystals down to 48 nm. Our
PVT approach, therefore, resulted in very thin Pn crystals
endowing immense application in organic electronic.
Exceeding 300 sccm, flakes of Pn appeared which stand on
their edge onto the substrate. Because of high flow speed, most
of the volatilized molecules travel through the nanoseeds
without helping them with maturation.
These results suggest that to obtain ultrathin crystal, it is
necessary to control supersaturation which can be adjusted by
the aid of gas flow rate.
3.4. Time Effect. To further understand the formation of
Pn crystals, we also studied the time effect (series D). From the
images in Figure 4a, it is clear that size and thickness increase as
time extends. Different Pn shapes have also been observed. The
prominent shape of the crystal obtained is pentagonal and
rhombic, while the lifetime for the hexagonal crystal is very
small. The result indicates that the shape of Pn crystals with
distinct facets can be partially tailored by the deposition time.
In essence, the formation of different crystal shapes of Pn
crystals at different deposition times is governed by minimizing
the surface energy of the crystal system. An overall growth
mechanism has been proposed. Normally, during the
nucleation and growth, the vapor atoms prefer to fuse with
the high energy crystallographic planes in the crystal lattice
system. The high-energy planes preferentially disappear after
growth, and then the final crystals are terminated by low energy
planes.25 We have calculated the angles among the side faces for
large-sized crystals. The corresponding plane indexes of side
faces and the calculated values of angles which are consistent
with the reported work26 are presented in Figure 4a.
Based on the above analysis for Pn shapes, the evolution
process of their edge planes could be described as follows. For
hexagonal shape, large-sized crystal are confined by (110),
(010), and (110̅ ) surface energy planes. Due to the lower
surface energy, the growth rate of the (001) plane could be very
slow. Then, as time goes on, since the growth rate of the (010)
plane is high, so are those of (110) and (11̅0) planes. In our
Figure 4. (a) Crystal shape with corresponding angles between
different planes. (b) Schematic illustration of shape evolution with
respect to supersaturation and surface energy.
experiments, up to 60 min of growth, only (110) planes will be
left behind to become the final edge planes of Pn crystal and to
maintain the whole crystal with a minimum surface energy.
Above 60 min growth time, because of high supersaturation,
growth of the crystal along the (001) direction is high and
slower along other crystal planes. This is the limiting step
where the enhancement in size is diminished while thickness
increases. Over a growth period of 120 min it is observed that
the thickness of the crystal increases from a few tenths of
nanometers up to several micrometers. This infers that crystal
size and thickness are influenced by evolution of crystalline
planes. Growth mechanism of Pn crystal with increasing
supersaturation and surface energy is schematically illustrated in
Figure 4b.
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Figure 5. (a) Schematic of crystal device (inset: optical image of device). (b,c) Transfer and output curve of device, respectively. (d) Differential
output conductance of different thickness OSCFET.
(3)
gate voltage, the distribution of the gate-induced charges along
the channel becomes narrower as the VSD increases and at the
saturation regime a pinched off point occurs. This means the
channel resistance would increase with increasing VSD.
However, the differential conductance vertical to the substrate
in the source region has a positive correlation with the applied
voltage. It should increase with increasing the VSD from 0 in the
vicinity of VSD = 0 V. The total differential conductance is
dominated by the smaller differential conductance. When two
differential conductances are nearly the same, there will be a
turning point as shown in the Figure 5d. Compared to the thin
crystal (95-nm-thick) device, the turning point in the thick
crystal (360-nm-thick) device has a shift to higher VSD as shown
in Figure 5d. This indicates that the injection resistance leads to
much deterioration in the thick crystal device performance. We
can conclude that ON current increases with reduction in
crystal thickness. This suggests that it is meaningful to get an
ultrathin single crystal.
where IDS is the drain current, μ is the mobility, Ci is the
dielectric capacitance, W and L are the channel width and
length, respectively, VGS is the gate voltage, and Vth is the
threshold voltage. Our devices have good device performance,
and the highest hole mobility reaches up to 5.6 cm2/(V s).
We found that the device performance is influenced by the
crystal thickness because different crystal thickness results in
different injection barrier (hereafter called injection resistance)
from the top contact to the semiconductor/dielectric interface.
The effect of injection resistance (Ri) on the device
performance can be directly shown from the differential
conductance of the device output curves as displayed in Figure
5d. If we neglect Ri, the differential conductance in the channel
should monotonically decrease to 0 when increasing the sourcedrain voltage (VSD) from VSD = 0 V. This is because, at fixed
4. CONCLUSION
Our experiments have demonstrated the controlled growth of
large and ultrathin 2D Pn crystal via nanoseed assisted growth
through PVT method stimulated by supersaturation. By
optimizing the deposition parameters, we successfully illustrate
the synthesis of high quality, Pn single crystal with tens of
micrometers in size and around 50 nm in thickness. We
systematically monitor the effect of coverage and substrate
temperature on crystal size. We also demonstrated that use of
high gas flow rates decreases the deposition rate while at the
same time lowering the supersaturation, thus helping the
growth of ultrathin 2D crystal. Moreover, growth mechanism of
the shape evolution of as-grown Pn crystal has also been
explained with respect to different surface energies of various
Large scale optical images of crystals over a broad range of
time are shown in Figure S3. However, it is important to note
that if the heating of the furnace stops, the growth of the crystal
will also be terminated. Thus, besides the majority of crystals in
the final stage, there should some products staying in the
intermediate stage probably due to the late nucleation.
Furthermore, the growth condition for each nanoseed cannot
be precisely the same.
3.5. Device Characterization. In this study, the device
structure is shown schematically in Figure 5a; the interface
between single crystal and substrate is preserved during the
whole device fabrication. Figure 5b,c shows the transfer and
output curves of an organic single crystal field effect transistor.
The mobility of the device in the saturation region is derived
from the following equation
IDS =
μCiW
(VGS − Vth)2
2L
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planes. The interface between the crystal and the SiO2 dielectric
has been preserved during all processes of device fabrication,
which is important for the electrical transport study. Electrical
study based on crystals showed mobility as high as 5.6 cm2 V−1
s−1 revealed that the in situ growth of ultrathin crystals has a
significant result in transport performance. We believe that the
synthesis technique in this work manifests a new strategy for
the growth of large and ultrathin Pn crystals.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.5b01726.
SEM images for the comparison of growth on substrates
without nanoseeds and with nanoseeds, graphs showing
Pn vapor pressure in furnace and nucleation probability
of Pn at different substrate temperatures depending on
local supersaturation, and optical images of Pn crystal
after PVT at different times (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*Tel: 86-10-82545563. Fax: 86-10-62656765. E-mail: jiangch@
nanoctr.cn.
Author Contributions
#
S. Atika Arabi and Ji Dong contributed equally. The
manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China (Grants 21432005, 11374070,
61327009), and the “Strategic Priority Research Program” of
the Chinese Academy of Sciences (Grant No. XDA 09040201).
■
REFERENCES
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