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Liquid phase mass production of air-stable black phosphorus/phospholipids nanocomposite with ultralow tunneling barrier

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Few-layer black phosphorus (FLBP), a recently discovered two-dimensional semiconductor, has attracted substantial attention in the scientific and technical communities due to its great potential in electronic and optoelectronic applications. However, reactivity of FLBP flakes with ambient species limits its direct applications. Among various methods to passivate FLBP in ambient environment, nanocomposites mixing FLBP flakes with stable matrix may be one of the most promising approaches for industry applications. Here, we report a simple one-step procedure to mass produce air-stable FLBP/phospholipids nanocomposite in liquid phase. The resultant nanocomposite is found to have ultralow tunneling barrier for charge carriers which can be described by an Efros-Shklovskii variable range hopping mechanism. Devices made from such mass-produced FLBP/phospholipids nanocomposite show highly stable electrical conductivity and opto-electrical response in ambient conditions, indicating its promising applications in both electronic and optoelectronic applications. This method could also be generalized to the mass production of nanocomposites consisting of other air-sensitive two-dimensional materials, such as FeSe, NbSe2, WTe2, etc.
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Liquid phase mass production of air-stable black
phosphorus/phospholipids nanocomposite with ultralow
tunneling barrier
Qiankun Zhang#1, Yinan Liu#2, Jiawei Lai2, Shaomian Qi2, Chunhua An1, Yao Lu1,
Xuexin Duan1, Wei Pang1, Daihua Zhang1, Dong Sun2,3, Jianhao Chen*2,3 and Jing Liu*1
1State Key Laboratory of Precision Measurement Technology and Instruments, School of
Precision Instruments and Opto-electronics Engineering, Tianjin University, NO. 92
Weijin Road, Tianjin, China, 300072
2International Center for Quantum Materials, School of Physics, Peking University, NO.
5 Yiheyuan Road, Beijing, China, 100871
3Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
*Corresponding authors:
Jian-Hao Chen (chenjianhao@pku.edu.cn) and Jing Liu (jingliu-1112@tju.edu.cn)
# These authors contributed equally to this work
ABSTRACT
Few-layer black phosphorus (FLBP), a recently discovered two-dimensional
semiconductor, has attracted substantial attention in the scientific and technical
communities due to its great potential in electronic and optoelectronic applications.
However, reactivity of FLBP flakes with ambient species limits its direct applications.
Among various methods to passivate FLBP in ambient environment,
nanocomposites mixing FLBP flakes with stable matrix may be one of the most
promising approaches for industry applications. Here, we report a simple one-step
procedure to mass produce air-stable FLBP/phospholipids nanocomposite in liquid
phase. The resultant nanocomposite is found to have ultralow tunneling barrier for
charge carriers which can be described by an Efros-Shklovskii variable range
hopping mechanism. Devices made from such mass-produced FLBP/phospholipids
nanocomposite show highly stable electrical conductivity and opto-electrical
response in ambient conditions, indicating its promising applications in both
electronic and optoelectronic applications. This method could also be generalized to
the mass production of nanocomposites consisting of other air-sensitive two-
dimensional materials, such as FeSe, NbSe2, WTe2, etc.
Keywords: Black Phosphorus; nanocomposite, liquid phase mass production; air
stability; opto-electrical response.
Introduction
Liquid-phase processing is a widely used method in industry for material processing
which is easily scalable and potentially low cost [1-4]. It enables mass fabrication of thin
film and composite devices for numerous applications, including flexible electronics [5-
10], energy harvesting [11-13], environmental monitoring [14-16], water purification [17]
and catalysts developments [18]. However, strategies to improve the environmental
stability of liquid processed materials are seldom studied, which become crucially
important due to the emergence of a series of high performing low-dimensional materials
with low environmental stability [19-23]. Black phosphorus (BP) is one of such
important but air-sensitive materials. It is expected to have great potential in electronic
and optoelectronic applications, because of its high carrier mobility [24], tunable direct
bandgap [25-27], and electrical/optical/mechanical anisotropy [27-30]. Thus liquid-phase
exfoliation/ processing of few-layer BP (FLBP) has attracted considerable attentions [31-
32]. High quality exfoliation and suspension has been successfully achieved in both
organic and aqueous solvents [3,4,33]. However, it remains a challenge to achieve air
stability for the mass production, since currently available passivation techniques [34, 35]
are not directly compatible with the liquid phase process. Consequently, the existing mass
production techniques are not yet suitable to be used for industrial-scale applications.
In this work, we accomplished FLBP mass exfoliation and passivation in a single
step by exfoliating bulk BP in chloroform with phospholipids (1,2-dipalmitoyl-sn-
glycero-3-phosphocholine, shortened as DPPC) dissolved in. DPPC is the main
component of cell membrane and its molecular structure contains a hydrophilic tail and a
hydrophobic head. The amphiphilic nature enables these molecules to intercalate with
bulk BP crystals to improve exfoliation efficiency of the later (shown in Fig. 1a), and
then self-assemble a thin layer on the surface of exfoliated FLBP to protect it from
ambient environment (as shown in Fig. 1b). No substantial morphological changes, as
measured by atomic force microscopy (AFM), are observed for these DPPC passivated
FLBP kept in air for more than two weeks. More importantly, the electrical resistance of
the FLBP/DPPC composite remained almost the same after exposure to ambient
conditions for one month (Fig. 2f). We also found that the insulating DPPC [36]
passivation produces a surprisingly weak barrier for charge carrier transport.
Temperature-dependent measurement reveals an Efros-Shklovskii variable range hopping
(ES-VRH) mechanism for the FLBP/DPPC composite which exceeds the most optimistic
expectation from the rigid BP/DPPC/DPPC/BP tunneling model.
Methods
Preparation of DPPC exfoliated FLBP. The DPPC solution was prepared by adding 30
mg DPPC (in powder form, purchased from Sigma-Aldrich Inc., Part No. P0763) into
100 mL chloroform. Then, 30 mg BP powder was added in 100 mL of the prepared DPPC
solution and exfoliated in a kitchen blender for 30 min. Then, the exfoliated suspension
was centrifuged for 30 min at 1000 rpm. The supernatant was formed on the top of the
suspension. Top 80% of the supernatant containing DPPC wrapped FLBP was collected
and lyophilized for further analysis. All the above procedures were conducted under
ambient conditions. The concentration of FLBP was estimated to be 0.28 mg/mL by
measuring the mass of the lyophilized powder of the top 80% supernatant after
centrifugation.
Sample characterization. HR-TEM and EELS images were taken with a Tecnai G2 F20
(FEI, USA). HR-TEM and EELS samples were prepared by pipetting several microliters
of DPPC/FLBP dispersion onto the holey carbon grid. Raman spectra were acquired by
DXR microscopy (Thermo Scientific, USA) with a 532 nm laser. The sample was
prepared by drop-casting FLBP/DPPC onto Si/SiO2 substrate. AFM measurements were
performed using a Dimension Icon (Bruker, German) instrument operating in tapping
mode. A mechanically exfoliated FLBP was transferred onto Si/SiO2 substrate followed
by dip-coating a layer of DPPC on its surface. All the sample characterizations were
performed under ambient conditions.
FLBP/DPPC composite device fabrication. Interdigitated electrodes were pre-fabricated
on lithium phosphate (300 μm) substrate for transport and photocurrent measurements
(photocurrent data is shown in supplementary information), by standard UV lithography
and lift-off processes. The electrodes were made of 20 nm Cr and 180 nm Au. The width
and length of each electrode finger were 5 μm and 500 μm, respectively, with a lateral
spacing of 15 μm. Then, FLBP/DPPC dispersion was drop casted onto the interdigitated
electrode by a micropipette. After the solvent was dried in ambient condition, a thin film
of FLBP/DPPC composite was formed on the top of electrodes.
Temperature dependent resistance measurement. Transport measurements were
performed with a probe station in ambient conditions and with a Quantum Design
Physical Property Measurement System (PPMS) at low temperatures. Electrical
excitation was applied by a Keithley 2400 source meter controlled by a Labview program
written in-house. No gate voltage was applied during transport measurements.
Results and Discussion
Sample characterization. FLBP was exfoliated in chloroform with and without
DPPC by a kitchen blender. The blender was accelerated up to 24000 revolutions per
minute (rpm) to generate a fully developed turbulent flow within the container, which
provides much higher exfoliation efficiency than standard sonication method [2,4].
Centrifugation was then performed at 1000 rpm for 30 min to disperse FLBP uniformly.
Top 80% of the supernatant was collected and used for further testing. Figure 1c shows
the exfoliation results of 30 mg BP dissolved in 0.3 mg/mL DPPC chloroform solution
(right) and pure chloroform (left), respectively, under the same exfoliation power and
duration. The one exfoliated in 0.3 mg/mL DPPC chloroform solution showed much
darker color, and thus had much higher concentration of FLBP in the dispersion,
compared to the one exfoliated in pure chloroform. The concentration of FLBP/DPPC
dispersion was measured to be around 0.28mg/ml, which is the same order of magnitude
as the concentration exfoliated by other organic solvent [2,3,4].
Furthermore, several standard techniques were applied to characterize the DPPC
exfoliated FLBP flakes. The successful exfoliation of BP in DPPC chloroform solution
was confirmed by high resolution transmission electron microscopy (HR-TEM), as
shown in Fig.1d and 1e. The TEM image shows that the lattice of the crystal is intact over
a 400-nm by 400-nm region, indicating that high quality FLBP flakes can be obtained
using DPPC assisted liquid-phase exfoliation. Raman spectroscopy was used to further
confirm the crystal structural integrity of the exfoliated FLBP nanosheets (Fig. 1h). The
Raman spectrum presents the characteristic
,  and
phonon vibration modes
at wavenumbers of ~359, ~436, and ~463 cm1, respectively.. These peak positions are
consistent with the Raman spectra of the mechanically exfoliated FLBP flakes [2,3,4],
suggesting that the flakes are crystalline after the liquid phase exfoliation. In order to
verify the coverage of FLBP with DPPC, electron energy loss spectroscopy (EELS) was
performed to characterize the DPPC exfoliated FLBP nanosheets. Figure 1f and g are the
EELS images of a DPPC exfoliated FLBP flake presented in the inset of Fig. 1f. The
images show the spatial distribution of phosphorus and nitrogen element, respectively.
The EELS micrographs present the homogeneous distribution of both phosphorus and
nitrogen elements (from DPPC), confirming that DPPC uniformly covers the FLBP.
Figure 1i, j and k exhibit the controlled size and thickness distribution of the
exfoliated FLBP by centrifuging the FLBP dispersion under different speeds of revolution.
Three DPPC/FLBP dispersion solutions were prepared by performing the centrifugation
at revolutions of 6000, 4000 and 2000 rpm, respectively. After centrifugation, a droplet
collected from top 80% of each supernatant was drop casted on Si/SiO2 substrate for
further characterization. The diameters and thicknesses of the FLBP flakes were
measured by atomic force microscopy (AFM), the distributions of which both followed
the Gaussian function. For the samples centrifuged at 6000, 4000 and 2000 rpm, the
distributions of the diameters were centered at 1.3 nm, 200 nm and 1100 nm, respectively.
The thickness distributions were centered at 4 nm and 9 nm for samples centrifuged at
4000 rpm and 2000 rpm, respectively. This capability to control the size and thickness
distributions is vital to applications such as advanced batteries, sensors, light emitting
diodes, biological therapy products, when FLBPs with suitable size ranges are required.
The passivation effect of DPPC on FLBP was examined by investigating the stability
of morphology and electrical resistance of FLBP/DPPC under long-term air exposure.
The morphology was measured by AFM on a mechanically exfoliated FLBP flake
transferred onto a Si/SiO2 substrate which was then covered by a DPPC layer through a
dip-coating process (Since additional DPPC molecules randomly spread on top of the
self-assembly layer, the surface of FLBP flakes obtained directly by liquid exfoliation
technique is very rough and hence hard to determine the effect of DPPC on FLBP air
stability by morphology). Figure 2a-c show the AFM images of this flake exposed in air
for 0, 7 and 16 days, respectively. The flake does not show obvious sign of oxidation, in
stark contrast to the bare FLBP flake (Fig. 2d and e). We selected a square area of 1 μm ×
1μm in each AFM image as marked by white broken lines in Fig. 2a-e and measured the
surface roughness of each of them. The surface roughness of DPPC passivated FLBP
flake increases slightly after more than two-week air exposure [37], while the surface
roughness of non-passivated FLBP exhibits an eight-fold increase in just 3 days [38], as
presented by red lines in Fig. 2f. To study the effect of DPPC on the electrical resistance
of FLBP/DPPC under ambient conditions, the device was prepared by drop-casting the
liquid-phase exfoliated FLBP/DPPC dispersion on the interdigitated electrode pre-
fabricated on lithium phosphate substrate (as shown in Fig. 2f). Remarkably, the
resistance of the composite remains almost constant (only ~7% changes) for twenty-eight
days, with a value around 100k, except it decreased by about 25% for the first two days
[39]. The change within the first two days was likely due to the evaporation of organic
solvent. In contrast, the resistance of the device fabricated by bare FLBP dispersion in the
same manner never decreases, but rather, increases almost nine orders of magnitude
within 5 days (Fig. 2f). Both the morphology analysis and electrical resistance
measurement demonstrate the capability of DPPC layer to isolate BP from water and/or
oxygen in the air.
Besides the excellent passivation capability, another interesting property of the DPPC
layer was observed after careful examination of the electrical conductivity of the
FLBP/DPPC nanocomposite: the DPPC layer does not electrically insulate the
encapsulated FLBP. Typically, for electrical transport between two conductors separated
by a single rigid insulating layer (barrier), either an increase in the barrier thickness at
constant bias or a decrease in the bias voltage at constant barrier thickness leads to an
exponential increase in the resistance. If the number of barriers increases in a device,
under constant bias voltage the effective bias across each barrier will decrease. Based on
this model, devices made from the FLBP/DPPC composite should be completely
insulated, given the thickness of the DPPC molecule that define the barrier thickness and
the average size of the exfoliated BP flakes that determine the number of barriers in the
device (see supplementary information for details). However, the resistance of the
FLBP/DPPC composite is only 100 k and remains conductive down to 100 K as
discussed below in the Transport mechanisms. This result indicates that the effective
barrier for charge carriers to tunnel through the DPPC layer is low. Thus, the DPPC layer
cannot only drastically increase the stability of exfoliated FLBP in ambient environment,
but also preserve its electrical conductivity, which is crucial for electronics applications
based on mass production of FLBP.
As the main component of cell membrane, phospholipid layer maintains the integrity
of cells and prevents exotic molecules from entering into the cell because of their
amphiphilic nature and self-assembly properties. Therefore, it is not surprising that DPPC
can cover the FLBP surface uniformly, and prevent water and/or oxygen molecules from
reaching BP surface. Additionally, as the interconnection among the phospholipid
molecules is very weak, the phospholipid layer can keep the cell fluidity. As a result, the
DPPC layer may not be a rigid insulating layer, instead, it allows carriers easily transport
between two BP flakes and from BP flakes to the electrodes. This could explain the
exceedingly low resistance of the FLBP/DPPC composite in contrast to the estimation
from the rigid tunneling barrier model (more details in supplementary information).
Transport mechanism. We carried out temperature dependent resistance
measurement of the FLBP/DPPC composite to determine its carrier transport mechanism.
Figure 3a shows the ID-VD curves of the device fabricated by depositing FLBP/DPPC
dispersion on interdigitated electrode which was measured at temperatures T ranging
from 100 K to 200 K. The two-probe resistance of the FLBP/DPPC composite is around
10 GΩ below 100 K, which decreases rapidly as temperature increases.
In order to reveal the nature of charge transport in such composite, we investigated ID
vs. T curves at several VD ranging from 2V to 7V (see Fig. 3b). It shows that ID exhibits
an exponential activation behavior as the temperature increases. Based on the fact that the
device consists of multiple FLBP flakes overlapping each other, the charge carriers will
need to hop from one FLBP flake to another during the transport process. Thus we adopt
the variable range hopping (VRH) conduction model which generally describes charge
carrier conduction in disordered systems:
 
(1)
where R0 is the pre-factor, T0 and p is the characteristic temperature and exponent,
respectively. p is the important parameter whose value helps to distinguish different
charge carrier hopping mechanisms, as described below. In the more ordinary case of the
Mott-VRH model, the density of states near the Fermi surface is taken as a constant and
the value of p in Eq. (1) is given by p=1/(D+1), where D is the dimension of the system
[40-42]. In a two dimensional (2D) system, p=1/3, and the characteristic temperature T0 is:
  
(2)
where N(EF) is the density of states near the Fermi energy EF, and ζ is the localization
length. In the case of strong Coulomb [43, 44] interactions or in systems with multiple
overlapping semiconductor nanoflakes [45, 46], the behavior of the charge transport can
be described by the Efros-Shklovskii (ES) VRH model. In such case, the density of states
vanishes linearly with charge carrier energy approaching EF and a Coulomb gap appears
at the Fermi level. The exponent p in the ES-VRH is 1/2 in all dimensions [47, 48]. The
characteristic temperature in 2D is then given by:
    
 (3)
where ε0 and ε are the permittivity of vacuum and relative dielectric constant of the
material, respectively.
In lightly doped semiconductors, the Coulomb gap is the same order of magnitude
with the measured temperature range. When temperature becomes higher, thermal effects
will smear out the Coulomb gap and a crossover from ES- to Mott-VRH with increasing
temperature is often observed. On the other hand, for   , the Coulomb
interaction plays a dominant role, and only ES’s law is expected [49].
Here, we first determine the key exponent p. From Eq. (1), one can obtain the
reduced activation energy W [50, 51]:
   
    
(4)
The value of p can be obtained from lnW = A - p lnT, where A is a constant. Figure 4a, b
and c show lnW vs. lnT plots at bias voltage VD = 3V, 4V and 5V, respectively. From the
linear fits, the value of p is found to be 0.471, 0.454, and 0.484 for VD = 3, 4 and 5V,
respectively. These p values match very well with the ES-VRH model. Hence in the
following discussion, we use the ES-VRH model to analyze the temperature dependent
resistance data.
Figure 4d shows a semi-log scale plot of R vs. T-1/2 for the device at different bias
voltages. The linearity of the log resistance with inverse square root of temperature for all
the bias voltages in Fig. 4d shows that R(T) fits very well with the T-1/2 behavior. By
linearly extrapolating the solid lines in Fig. 4d to high temperature limit, R0 for all bias
voltages can be obtained and the range of R0 is from 320.5 to 78.0 Ω.
From the slopes of Fig. 4d, the characteristic temperature TES for the device at
different bias voltages can be obtained. Using an effective relative dielectric constant ε =
8.3 for BP flakes [52-54], the value of TES is determined to be approximately 3 × 104 K
for the bias voltages ranging from 2V to 7V in one volt interval. TES is much higher than
room temperature, which satisfies the criterion    [49], and is consistent
with the fact that Mott-VRH is not observed in our devices [50, 51]. From the values of
TES and using Eq. (3), the localization length (ζ) is determined to be between 0.17 nm to
0.19 nm, which is a description of the tunneling process between the BP flakes and does
not depend on the size of the flakes [52]. A summary of the results obtained from our
measurements is presented in Table 1.
Conclusions
In conclusion, we accomplished single-step mass production of FLBP/DPPC
nanocomposite in liquid phase. The self-assembled DPPC layer on the surface of FLBP
during liquid exfoliation prevents water and/or oxygen from reaching FLBP, and thus
greatly improves the air-stability of the passivated FLBP. On the other hand, DPPC
exhibits a surprisingly low tunnel barrier for charge transport, resulting in relatively low
resistance of the FLBP/DPPC composite. Temperature dependent resistance measurement
of the FLBP/DPPC composite reveals an Efros-Shklovskii variable range hopping
mechanism. We anticipate the investigations and characterizations performed on
FLBP/DPPC composite in this work help to promote the mass production of thin film
devices based on BP and other air-sensitive nanomaterials.
Acknowledgement
This project has been supported by the National Basic Research Program of China (973
Grant Nos. 2013CB921900, 2014CB920900), the National Natural Science Foundation of China
(NSFC Grant Nos. 21405109, Nos. 11374021 and Nos. 11674013), and Seed Foundation of
State Key Laboratory of Precision Measurement Technology and Instruments (Pilt No. 1710).
Author Contributions
J.L. and J.C. conceived the experiment with help of D.S. Q.Z. exfoliated the BP thin flakes,
completed TEM, EELS, Raman and AFM measurements, fabricated and tested the devices under
the supervision of J.L.. C.A., Y.L. performed dip-coating process. X.D., W.P. and D.Z. provided
experimental instruments for dip-coating and device fabrication and testing. Y.L. performed the
transport measurements under the supervision of J.C. J.L. performed the photocurrent
measurements under the supervision of D.S. Y.L., Q.Z., D.S., J.L. and J.C. discussed the results,
analyzed the data and wrote the manuscript. All the authors commented on the paper.
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TABLE 1. Summary of ES-VRH fitting results with various biases.
VD(V)
2
3
4
5
6
7
R0(Ω)
320.5
178.1
137.0
102.9
84.8
78.0
T0(K)
29433
30772
30867
31400
31826
31769
ζ(nm)
0.191
0.183
0.182
0.179
0.177
0.177
CAPTIONS
Figure 1. Characterizations of liquid exfoliated BP with DPPC. a) Schematic diagram
of DPPC intercalating bulk BP crystal during liquid exfoliation. b) Schematic diagram of
DPPC self-assembly layer on the surface of FLBP; c) Dispersions of FLBP nanoflakes
exfoliated in chloroform in the absence (left) and presence (right) of DPPC under the
same conditions. d) HR-TEM image of a FLBP exfoliated by DPPC in chloroform (scale
bar is 50 nm) and e) FFT pattern of the selected area represented by red box in (d) (Scale
bar is 5 nm). f) and g) EELS images of the FLBP flake presenting nitrogen and
phosphorus elements, respectively, are obtained from HR-TEM image shown in inset of g
(scale bar is 20 nm). h) Raman spectrum of exfoliated FLBP nanoflake, with signature
peaks observed at ~359, ~436, and ~463 cm1, respectively. i) Size distribution histogram
of BP dispersion centrifuged at 2000 rpm, 4000 rpm, and 6000 rpm, respectively. j) and k)
Thickness distribution histogram of BP dispersion centrifuged at 2000 rpm and 4000 rpm,
respectively.
Figure 2. Air stability of DPPC wrapped FLBP. a), b) and c) AFM images of the same
mechanically exfoliated FLBP flake covered by DPPC layer kept in ambient condition
for 0 day, 7 days and 16 days, respectively. d) and e) AFM images of mechanically
exfoliated bare FLBP flake kept in ambient condition for 0 day and 3 days, respectively.
The square areas marked by white broken lines in Fig. 2a-e (with size of 1μm × 1μm)
were selected for surface roughness measurement. All scale bars are 1μm. f) Time-
dependent evolutions of surface roughness and resistance. Surface roughness of FLBP
with and without DPPC passivation was measured in the square areas in Fig. 2a-e
(presented by red lines in upper and right axis); resistance measurements consist of bare
FLBP and FLBP/DPPC nanocomposite devices (presented by black lines in lower and
left axis). The diameter and thickness of the composite (FLBP) film was around 25 (15)
m and 2 (2) m, respectively. Inset shows the cartoon of the device used for resistance
measurements.
Figure 3. Temperature dependent resistance measurement of the FLBP/DPPC
nanocomposite. a) Current vs. voltage curves of the FLBP/DPPC nanocomposite device
in the temperature range of 100-200 K. Inset: Photo of FLBP/DPPC film deposited on
interdigitated electrodes. b) Current vs. temperature curves for the device at voltage bias
ranging from 2V to 7V.
Figure 4. Efros-Shklovskii Variable ranging hopping of the FLBP/DPPC composite.
(a)(c) Reduced activation energy (W) plotted vs. temperature (T) in a log-log scale of the
device with voltage bias set at 3, 4 and 5V respectively. The squares are the experimental
data points and the solid red lines are linear fit of the data. From the slopes of the plots,
we obtain p = 0.454, 0.448, and 0.451 for VD = 3, 4 and 5V, respectively. All correspond
to the ES-VRH model. (d) Semi-log scale plot of R vs. T1/2 for the device at different
voltage biases. From the slopes, we obtain TES =29433, 30772, 30867, 31400, 31826 and
31769 K for the voltage bias ranging from 2V to 7V, respectively.
FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
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