Hybrid nanotubes: Single step formation of homogeneous nanotubes of polypyrrolegold composites and novel switching transition of resistance beyond liquid nitrogen temperature

Hybrid nanotubes: Single step formation of homogeneous nanotubes of polypyrrole-gold composites and novel switching transition of resistance beyond liquid nitrogentemperatureAbhisakh Sarma, Milan K. Sanyal, Atikur Rahman, and Biswarup Satpati Citation: Journal of Applied Physics 112, 044304 (2012); doi: 10.1063/1.4746743 View online: http://dx.doi.org/10.1063/1.4746743 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/112/4?ver=pdfcov Published by the AIP Publishing

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Hybrid nanotubes: Single step formation of homogeneous nanotubesof polypyrrole-gold composites and novel switching transitionof resistance beyond liquid nitrogen temperature

Abhisakh Sarma, Milan K. Sanyal,a) Atikur Rahman, and Biswarup SatpatiSurface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 64, India

(Received 7 March 2012; accepted 17 July 2012; published online 22 August 2012)

Nanowires of polypyrrole have exhibited switching transition that reduces the resistance of the

wires by several orders of magnitude under certain bias around and below 30 K temperature. Here,

we have shown that by incorporating gold in these polypyrrole nanotubes using a cost effective

template based single-step chemical synthesis technique, this novel resistance switching transition

could be extended beyond liquid nitrogen temperature (>90 K) to make this phenomena

technologically relevant. The single step synthesis technique, reported here, provides us uniform

mixing of gold and polypyrrole during the formation of composite-nanotubes; with appropriate

choice of materials, this synthesis technique can be extended to form nanotubes of other metal-

polymer composites. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4746743]

I. INTRODUCTION

Conducting polymers have been widely applied in the

fabrication of electrical and optical devices in the past sev-

eral decades. With the development of nanotechnology,

nanostructures of various composites of conducting poly-

mers have been synthesized that exhibit unique physical and

chemical properties. These composite nanomaterials are suit-

able for wide range of potential applications in diverse areas

such as diagnosis, luminescent devices, light-emitting

diodes, and batteries.1–6 In the past ten years, impressive pro-

gress in the synthesis of nanostructures of composite materi-

als has been achieved through various routes such as

electrochemical deposition,7,8 self-assembly,9 blending,10

and reverse micelle guided growth approach.11 The concept

of “seeding” method has been developed for several years,

and was used for fabricating nanoparticle/polyethylene com-

posite materials.12 However, these techniques have not been

extended in the synthesis of metal-conducting-polymer com-

posites, where nanoparticles of metals are embedded uni-

formly in a tube/wire. The composites can manifest its

property better in the form of nanotubes or nanowires than in

the form of thin films due to significant increase in surface

that interacts with environment, to volume aspect ratio. This

has direct implications in the application fields of sensing de-

vice, fuel cell, medicine, etc. If the nanotubes are well

aligned and monodispersed over a substrate or template, one

can directly use them to fabricate appropriate devices with-

out further processing or lithography. For the synthesis of

nanotubes of composites, template based growth technique is

preferable, as it gives parallel, monodisperse, nonintercalated

nanotubes in a cost effective way. Polyaniline nanofibers

decorated with gold (metallic) nanoparticles13–15 have been

suitable for application as nonvolatile memory16 material,

polypyrrole nanotubes coated with gold nanocrystals,17 and

carbon nanotubes decorated with gold nanoparticle have also

been reported.18,19 All these template free synthesis proc-

esses provide nonaligned, intercalated, and polydisperse

nanoparticle-decorated nanowires. Nanoparticles incorpo-

rated in these techniques are also not well dispersed on the

wall of the tubes. Further in these techniques, the incorpora-

tion of metal nanoparticles in nanowires is done in multi-

stage process, and metal nanoparticles are generally get

attached only outside the walls of the nanowires/nanotubes.

The single step synthesis, reported here, provides us parallel

well aligned, nonintercalated, monodispersed nanocompo-

sites nanotubes, and this synthesis technique has all advant-

age of a template based growth technique.20 The single step

growth process for growing nanotubes of gold polypyrrole

composite, reported here, can be easily extended for other

metal-polymer composites. We have characterized the

obtained nanotubes using scanning electron microscopy

(SEM), secondary ion mass spectroscopy (SIMS), and trans-

mission electron microscopy (TEM) techniques. It is known

that polypyrrole nanotubes show resistance switching behav-

ior at low temperature, and this switching transition and

associated hysteresis can be used to develop memory device

provided this property survives at higher temperature.21 The

results reported here clearly show that this resistance switch-

ing behavior and associated hysteresis have increased sub-

stantially in gold-polypyrrole nanotubes and this switching

transition remains active above 90 K temperature to make

this phenomenon relevant technologically.

II. EXPERIMENTAL

A. Materials

Polycarbonate membranes of various pore diameters

(10, 15, 30, 50, 100, and 200 nm) were taken as template.

Membranes of various thicknesses were used in this study.

Membranes were purchased from Wattman (15, 30, 50, 100,

and 200 nm pore diameter) and Osmonics (10 nm diameter).

Pyrrole monomer (purchased from Merck, Germany) was

vacuum distilled and stored at �20 �C before use.a)Electronic mail: [email protected].

0021-8979/2012/112(4)/044304/6/$30.00 VC 2012 American Institute of Physics112, 044304-1

JOURNAL OF APPLIED PHYSICS 112, 044304 (2012)

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Chloroauric acid (HAuCl4) (purchased from Alfa Aser) was

used as gold source. Ferric chloride (FeCl3) (purchased from

Merck) was used as obtained.

B. Procedure

The growth method used here uses a salt that dissociates

into a metallic anion to act as an oxidizing agent for promot-

ing polymerization reaction. The formation of gold-

polypyrrole composite22 occurs in the pores of the membrane

and this process helps to disperse gold ions through out the

nanotubes, homogeneously. In this growth technique, the

membrane is placed between a two compartment glass cell

with rubber O-ring and clip. One compartment is filled with

0.1 M monomer, while in the other compartment 0.5 M ferric

chloride and 3 mM chlorauric acid solution was taken in a

volumetric ratio of 1:1 (refer in Figure 1).

When these chemicals are allowed to mix through the

nanopores of the membrane, the polymerization reaction

starts immediately and within few minutes polycarbonate

membrane becomes black. The chlorine (Cl�) and metal

chloride anion (AuCl�4 ) react in the pores of the template

with pyrrole monomer. Pyrrole reacting with ferric chloride

produces polypyrrole which first gets deposited on the sur-

face of the pore walls20 to form the nanotube. The reaction

of chloroauric acid and pyrrole produces gold22,23 that gets

embedded within the tube wall. The thickness of the mem-

brane used was about 10 lm which determines the length of

the tubes formed. We have observed that though chloroauric

acid alone reacts violently with pyrrole monomer in a pot,

this reaction does not occur properly in the pores of the poly-

carbonate membrane to form polypyrrole nanotubes. Gold

incorporated polypyrrole sedimentation primarily occurs in

the compartment containing pyrrole monomer. Mixture of

chloroauric acid and ferric chloride in certain molar ratio is

required to form nanotube of gold-polypyrrole composite

inside the pores of the polycarbonate membrane. As chlor-

oauric acid is a strong oxidizing agent and can oxidize pyr-

role quickly compared to ferric chloride, possible reason for

this observation is that chloroauric acid diffuses through the

pores and reacts with pyrrole monomer before it can diffuse

through the pores of the membrane, and reaction occurs out-

side the template. If molar concentration of the chloroauric

acid is reduced to have the reaction in the pores of the mem-

brane, we found that lack of oxidizing reagent prohibits good

tube formation. Only the reaction of a mixture of ferric chlo-

ride and chloroauric acid with higher ferric chloride molar

concentration produces the composite nanotubes reported

here (refer the schematic Figure 1).

C. Characterization

1. SEM

In order to study the form of nanoparticle incorporated

polymer nanotubes as grown within the polycarbonate tem-

plate, we have done SEM (Quanta200 FEG) measurement in

low vacuum. For these SEM measurements, we have

removed polycarbonate from one side of the membrane such

that embedded tubes can be exposed, but still the bottom of

those tubes remains embedded within polycarbonate mem-

brane. Figure 2 shows a typical scanning electron micros-

copy image of the nanotubes of gold-polypyrrole composite

formed by this single step preparation technique. The nano-

tubes are found to be grown over the entire area of the mem-

brane suggesting uniform polymerization reaction in all the

FIG. 1. Schematic diagram of the synthesis tech-

nique used here for the formation of gold incor-

porated polypyrrole nanotubes.

FIG. 2. Representative SEM micrograph of gold incorporated polypyrrole

nanotubes having an average diameter of 200 nm is shown. The micrograph

in the inset shows parallel, non-intercalated nanotube formation over large

area.

044304-2 Sarma et al. J. Appl. Phys. 112, 044304 (2012)

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pores, and incorporation of gold does not block the pores as

we get tubes of full length over entire membrane.

2. SIMS

In secondary ion mass spectroscopy measurement, the

sample is bombarded with cesium ion (Csþ) at 5 keV which

sputters the constituent materials from the sample, and the

obtained sputtered materials were analyzed using mass spec-

trometer to identify constituent. SIMS studies were performed

using a quadrupole mass spectroscopy-based instrument

(HIDEN Analytical Ltd., UK) with a high-performance triple

quadrupole filter and the base pressure during the measure-

ment was 6:8� 10�9 mbar. For the SIMS measurement, a pi-

ece of polycarbonate membrane having nanotubes (diameter

200 nm) was mounted with copper tape in such a way that we

can get compositional information along the length of the

tubes (depth of the membrane). It is known that polycarbonate

membrane is composed of carbon(C), oxygen(O), hydro-

gen(H) and polypyrrole is composed of C, N, H, and Cl (dop-

ant). Chlorine ion acts as doping as well as oxidizing agent in

polypyrrole formation. The SIMS data (Figure 3(a)) show that

count of N and Cl is constant over the length of nanotubes,

and Figure 3(b) shows that the ratio of Cl/N is constant, which

implies polymerization and doping occur uniformly along the

length of nanotube.24 However, the surface of the membrane,

which was facing the compartment containing ferric chloride

and chloroauric acid during chemical reaction shows higher

value of Cl and Au in the profile as reported earlier for poly-

pyrrole nanowires.24 Figure 3(a) shows the steady count of

gold apart from this end of tube indicating that the gold is

homogeneously dispersed along the length of the nanotubes.

Figure 3(b) clearly shows that the Au/Cl ratio remains con-

stant over entire length of the tubes as the gold profile follow

the chlorine profile.

3. TEM

To investigate the detailed microstructure and location

of gold within the nanotube, we have done TEM measure-

ments using FEI, Tecnai G2 20, S-TWIN microscope operat-

ing at 200 kV, equipped with a GATAN CCD camera. For

TEM analysis, the grown nanotubes were taken out from the

pores by dissolving the membrane in chloroform, then it was

sonicated and subsequently a drop of it was put on a carbon

coated copper grid for TEM study. Results of TEM study

will be presented in Sec. III.

4. Electronic transport property measurement

For electrical measurements, 2 mm diameter gold elec-

trodes were sputter deposited on both sides of the membrane,

to connect several nanotubes in parallel configuration and

polycarbonate membrane acts as insulating barrier to isolate

them from each other. Among various contact materials, we

have found that sputter-deposited gold contact gives low

contact resistance and small depletion capacitance as

obtained earlier from the dc current-voltage (I–V) character-

istics and capacitance measurements.25 All the measure-

ments reported here were done in two probe and pseudo-

four-probe configurations in a close-cycle refrigerator (Janis

Research Company, Inc.), where sample was in vacuum

(<10�3 mbar). I-V characteristics were measured using

Keithley 2400 source meter, Keithley 2000 multimeter, Agi-

lent 34420A nano-voltmeter, and Keithley 6517A electrome-

ter. Current biased measurements were done by driving

current using Keithley 2400 source meter and measuring

voltage using Agilent 34420A nano-voltmeter or Keithley

2000 Multimeter. Temperature was controlled by Lakeshore

340 temperature controller with the help of a manganin wire

heater (50 X) and a cernox sensor placed near sample. All

the instruments were interfaced with a computer via general

purpose interface bus (GPIB) interface. LABVIEW (National

Instruments Corp., Austin, TX) software was used for data

acquisition.

III. RESULTS AND DISCUSSION

The microstructures of gold-polypyrrole composite in

the nanotubes were investigated using high resolution TEM

and energy dispersive x-ray (EDX) spectroscopy measure-

ments in the TEM. Representative TEM images are shown in

Figures 4(a)–4(d) and typical EDX spectra are shown in Fig-

ure 4(e). Clearly one can see the presence of gold in EDX

spectra which was taken on a single nanotube. EDX data

were also taken from several portion of a single nanotube

and found to be almost constant over the entire length of the

nanotube, and this observation reconfirmed SIMS results that

the gold is homogeneously dispersed along the length of the

nanotubes. Cu peaks in EDX spectra are due to carbon

coated copper grid used for TEM study.

Figure 4(a) depicts a low-magnification bright-field trans-

mission electron micrograph showing individual nanotubes.

Gold nano-clusters that are embedded into the polypyrrole

nanotubes are not visible in this low-magnification micro-

graph. In order to explore the composite structure further in

particular to locate the presence of gold in the nanotube, we

expose a very small portion of the tube to the electron beam

for several minutes and due to which polypyrrole starts to

degrade (shown in Figure 4(b)) and embedded gold-

nanoparticles become exposed. A very small portion of this

exposed area is shown in high-magnification image in Figure

FIG. 3. (a) Representative SIMS data of different constituents along the

length of gold incorporated polypyrrole nanotubes. (b) Ratio of the counts of

Cl/N and Au/Cl was found to be constant over the length of the nanotubes.

044304-3 Sarma et al. J. Appl. Phys. 112, 044304 (2012)

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4(c) which clearly shows the presence of many crystalline

nanoparticles. Indeed, lattice images in Figure 4(d) shows a d

spacing (inter-planar spacing) of 2:07 6 0:04 A. This d

spacing is close to the (002) inter-planar spacing of bulk Au

(d002 for Au is 2:03A). From these TEM measurements and

associated EDX data (Figure 4(e)), we confirmed that the sig-

nal of gold is coming from within the tubes only and not from

any bare particles outside the tubes. We could not detect the

presence of any bare gold particle outside the nanotubes in

low-magnification as well as high-magnification TEM

images.

Now, we shall discuss the results of electronic transport

property measurements carried out at low temperatures.

These nanowires show a sharp transition to a highly conduct-

ing state above a certain threshold voltage (VTh). After the

switching transition, the low conducting state is recovered

when the bias voltage is reduced below a certain voltage

(VRe < VTh). Current-voltage (I-V) characteristics, thus, ex-

hibit hyteresis and was found to be symmetric for both posi-

tive and negative bias (Figure 5). In Figure 5, we have

shown a typical switching characteristic measured at 25 K

temperature of pure polypyrrole and gold incorporated poly-

pyrrole nanowires synthesized using a 200 nm pore diameter

membrane. The open circle data correspond to polypyrrole

(PPy) and filled circle corresponds to gold-incorporated pol-

ypyrrole (AuPPy) nanotube. For 200 nm diameter AuPPy

sample, if we increase the bias voltage above 18.99 V

(¼VTh) at 25 K temperature, the current through the nano-

wires increases abruptly and the current gets limited by the

current compliance of the sourcemeter, which is set to 10

mA. If we decrease the voltage below 1.1 V (¼VRe), then the

nanowire switches back to a low conducting state giving a

large hysteric I-V characteristics. The corresponding values

FIG. 4. (a) Low magnification TEM image. (b) The part of nanotube which

is exposed to the electron beam of 200 keV for several minutes. (c) High-

magnification image of the beam exposed part of the nanotube showing

nanocrystalline particles. (d) HRTEM images showing crystal plane of Au.

(e) EDX from single tube showing existence of gold.

FIG. 5. (a) Switching transition characteristics of 200 nm diameter nano-

tubes of polypyrrole-gold composites (filled circle) and polypyrrole ((open

circle) in the lower inset) are shown. Here, VTh corresponds to the threshold

voltage for switching and VRe corresponds to the return voltage from the

switched state. The current bias measurement of 200 nm gold incorporated

nanotube at 25 K is shown in the upper inset. (b) 200 nm gold incorporated

nanotube follows power law behavior I / V1þb in the region between VG

and VTh with b ¼ 10:1 fitted well at temperature 4.2 K and b ¼ 1:35 fitted at

90 K. Inset showing value of b decreases with the increase of temperature.

044304-4 Sarma et al. J. Appl. Phys. 112, 044304 (2012)

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of VTh and VRe for PPy nanowires are found to be 0.25 V

and 0.06 V, respectively. Due to this large-range bistability,

the nanotubes of AuPPy can act as efficient memory devices

if we use READ, WRITE, and ERASE voltage pulses that

take VRe � V � VTh, V > VTh, and V < VRe, respectively,

as shown for PPy nanowires earlier.26 Much higher hystere-

sis of AuPPy nanotubes as compared to nanowires of PPy

makes these nanotubes better materials for such device appli-

cation. Moreover, larger hysteresis allows the resistance

switching to survive at much higher temperature.

The area under the hysteresis was found to be independ-

ent of scan speed with which data were taken. The hysteresis

(VTh � VRe) was found to decrease with the increase of tem-

perature. The change in resistance due to switching was

found to be more than three orders of magnitude as shown in

Figure 5. Systematic measurement showed that the switching

threshold bias remains constant at any particular temperature

over a large number of voltage cycles. The switching thresh-

old VTh was found to be very sharp, and we have seen that if

the bias is kept few mV below the VTh switching never

occurs. But this sharpness decreases with the increase of

temperature. The sharpness of switching as well as hysteresis

is better in gold incorporated polypyrrole nanowires than

that obtained in PPy system, as shown in Figure 5(a). In the

upper inset of Figure 5(a), we have shown a typical hystere-

sis data of AuPPy sample obtained in current bias condition

that exhibit features of switched state in better clarity. The

nature of the obtained data is similar to the published27 data

of PPy nanowire. As the nanowires cross the VTh

(¼18.99 V), the value of resistance drops and measured volt-

age across the wires becomes 11.36 V (refer upper arrow in

the upper inset of Figure 5(a)). Further increase in current

reduces the voltage further giving a zone of negative differ-

ential resistance (NDR) expected in a charge density wave

system (CDW).21 Lower arrow in this figure indicates the

return path, the system remains in NDR state as the current

is reduced, and we obtain a hysteresis until VRe is reached.

From the voltage biased measurements, we have found

that current remains extremely small for both PPy and

AuPPy wires till a gap voltage VG is reached. The I-V curve

shows power law behavior (I / V1þb) in the region between

the gap voltage (VG) and threshold voltage (VTh), as shown

in Figure 5(b). At 4.2 K, the value of b was found to be 10.5

for AuPPy and corresponding value of b was found to be 4

for PPy. With the increase of temperature upto 50 K, the

value of b falls rapidly to 1 and then decreases slowly toward

b ¼ 0 (ohomic), as shown clearly in the inset of Figure 5(b).

The exponent b having value greater than two gives clear

evidence that the observed switching transition in resistance

is not due to space-charge effect. The switching transition

cannot be due to the formation of conducting filament and

thermal effect because by keeping the bias voltage few mV

below the VTh over several hours, the switching transition

was never observed. Joule heating cannot cause this switch-

ing either, as we have observed more than 3 orders of mag-

nitude change in resistance and could not found any change

in temperature. For such bias current (voltage), an increase

in temperature 100 K is needed to change the resistance by

two orders of magnitude.

The data shown in Figure 6(a) clearly show that with

increase of tube diameter threshold bias (VTh) decreases at any

particular temperature. It is to be noted here that the lengths of

nanotubes having 200 nm diameter were 1.2 times higher com-

pared to those having 100 nm diameter as used membrane of

different thicknesses. As a result, for a particular applied bias

voltage, the 200 nm nanotubes get lower field compared to

100 nm diameter tubes. If the switching was due to dielectric

breakdown, then for a particular temperature we should have

obtained higher threshold voltage in 200 nm diameter tube as

compared to that in 100 nm tube. The opposite behavior

observed here rules out the possibility of dielectric breakdown

as the cause of switching transition. From Figure 6(b), it is

clearly seen that with the increase of temperature the hysteresis

loop (VTh-VRe) decreases for both 200 nm and 100 nm diame-

ter tubes. The data in Figure 6(b) also show that switching

transition and corresponding hysteresis cannot be due to any

charge trap phenomenon, because in case of charge trap phe-

nomenon hysteresis loop should decrease with the decrease of

temperature. We could reproduce the hysteresis at each

FIG. 6. Comparison threshold bias (VTh) and hysteresis (VTh � VRe) of

200 nm (filled circle) and 100 nm (open circle) diameter gold incorporated

nanotubes at different temperatures. (a) The threshold voltage (VTh) of

200 nm diameter tube is always lower than 100 nm diameter tube at any tem-

perature, and switching threshold is inversely proportional to the tempera-

ture. (b) The hysteresis loop for 200 nm diameter tube is always lower than

100 nm diameter tube, and hysteresis loop for 200 nm and 100 nm vanishes

substantially after 90 K and 120 K, respectively.

044304-5 Sarma et al. J. Appl. Phys. 112, 044304 (2012)

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temperature over several cycles for all the nanotubes. Revers-

ibility in hysteresis confirms that the switching transition is not

due to any redox reaction. It is clear from the data (see Figure

5(a) and lower inset) that the hysteresis loop is much larger for

the gold-polypyrrole system compared to that in bare polypyr-

role nanotube. With increasing temperature, the threshold vol-

tages as well as hysteresis loop decrease (Figure 6). For pure

polypyrrole nanotubes of 200 nm diameter, the switching

behavior reduced substantially above 30 K, but for gold em-

bedded polypyrrole nanotubes of 200 nm diameter (reported

here) the switching is observed even above 90 K (refer Figure

6(a)). In the case of 100 nm diameter polypyrrole nanowires,

the switching remains upto 45 K. But gold incorporated nano-

tubes exhibit switching transition even above 120 K (shown

Figure 6(a)). The improvement of temperature domain over

which this switching transition occur is one of the major find-

ing reported here, and this result makes the nanotubes of

polypyrrole-gold composites important technologically.21

The observed switching transition reported here is con-

sistent with that predicted for charge density wave system.28

In structurally disordered materials (e.g., polymer and

polymer-metal hybrids), characteristics of coherent CDW

state can only appear due to the formation of an electron

crystal known as Wigner crystal (WC).27,29,30 The formed

WC remains pinned by the impurities present in the system

and under a sufficient bias sliding motion starts which takes

the system to a low resistive state through a switching transi-

tion,27 and the value of the threshold voltage depends on the

pinning strength. By introducing gold homogeneously in the

polypyrrole nanowires, we could effectively increase the pin-

ning strength in the system that results in the increase of

threshold voltage of switching transitions. The temperature

domain of WC state is expected to increase with increasing

pinning strength.31,32 The survival of switching transition

above 90 K in case of 200 nm and above 120 K for 100 nm

diameter nanowires clearly suggests that the presence of

gold in polypyrrole has increased the temperature domain of

WC state and associated resistance switching transition to

make it an applicable phenomenon.

IV. CONCLUSION

In conclusion, we have incorporated gold within the

wall of polypyrrole nanotube in a single step template based

growth technique, and these nanotubes show enhanced hys-

teresis of the resistance-switching-transition. Existence of

this switching behavior above liquid nitrogen temperature

enhances potential technological applications of these nano-

tubes. These nanowires can be used to design current/voltage

sensor or controller in low temperature nano-electronic cir-

cuits and also in high-density memory devices. The novel

switching transition at elevated temperature also opens up an

enormous scope for the basic research to understand the

interplay between interactions and disorder in quasi-one

dimensional system.27,31,32 By choosing proper metal com-

posites, these polymer nanotubes can be further developed

for various applications in solar cells, magnetic recording,

electromagnetic shielding, hydrogen storage materials, and

application in fuel cell technology.33,34

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