Negative to Positive Magnetoresistance transition in
Functionalization ofCarbon nanotube and Polyaniline Composite
Krishna Prasad Maity1, a) and Narendra Tanty , Ananya Patra, V
Prasad1Department of Physics, Indian Institute of Science,
Bangalore 560012, India
Electrical resistivity and magnetoresistance(MR) in
polyaniline(PANI) with carbon nanotube(CNT) and func-tionalized
carbon nanotube(fCNT) composites have been studied for different
weight percentages down to thetemperature 4.2K and up to magnetic
field 5T. Resistivity increases significantly in composite at low
tem-perature due to functionalization of CNT compared to only CNT.
Interestingly transition from negative topositive magnetoresistance
has been observed for 10wt% of composite as the effect of disorder
is more infCNT/PANI. This result depicts that the MR has strong
dependency on disorder in the composite system.The transition of MR
has been explained on the basis of polaron-bipolaron model. The
long range Coulombinteraction between two polarons screened by
disorder in the composite of fCNT/PANI, increases the
effectiveon-site Coulomb repulsion energy to form bipolaron which
leads to change the sign of MR from negative topositive.
I. INTRODUCTION
In recent years PANI with CNT and fCNT com-posites have been
promising materials in applicationof gas sensing,1 DNA sensor,
electromagnetic shield-ing(EMI), electrode of supercapacitor,
printable circuitwiring, transparent covalent coating2,3etc.
Functional-ization of CNT prevents the nanotube aggregation,
im-proves interfacial interaction, dispersion and stabiliza-tion in
polymer matrix. However, it shortens the length,breaks C-C sp2
bonds, enhances the disorder creating de-fects on sidewall and
opening of the ends.4–6 It is reportedthat the disorder in the
system affects on the conductiv-ity which can be changed several
order of magnitude atlow temperature. The conductivity of CNT/PANI
com-posites change two order of magnitude by increasing thefiller
percentage due to strong coupling between CNTand lightly coated
polymer chain.7 The magnetoresis-tance study of doped PANI at high
magnetic field andlow temperature gives an idea of charge transport
on lo-cal molecular level ordering in the system.8 There is
atransition from negative to positive MR, observed withlowering
temperature, increasing magnetic field and de-creasing CNT
percentage in the composites.9 Also, Gu et.al has reported the
transition from positive to negativeMR in disordered PANI-silicon
nano composite around5.5T at room temperature10 and explained by
‘Wave-function shrinkage’, ‘Forward interference’ model. Ad-justing
the ‘dissociation’, ‘charge reaction’ mechanismof charge transport,
MR has been tuned between pos-itive and negative values.11 The
inversion of MR in or-ganic semiconductor device has been shown
depending onapplied voltage, temperature and layer thickness.12
Re-cently, the crossover of MR from positive to negative at100K in
doped polyaniline nanofibers has been reportedand explained by
‘Bipolaron’ model.13 Also Mamru et.alhas studied that polyaniline
composite shows MR tran-sition at room temperature by varying the
concentration
a)Electronic mail: [email protected]
of titania quantum dot due to suppression of polaron
atfunctionalized PANI and titania interface.14 Motivatedby these
works we have studied the conductivity andMR of fCNT/PANI and
CNT/PANI composites at lowtemperatures. To the best of our
knowledge the trans-port properties of fCNT and composites with
polymerhas not been explored much. Hence this study will helpus to
understand the effect of disorder in charge trans-port mechanism of
fCNT/PANI compare to CNT/PANIcomposite systems.
II. METHODS
We have synthesized polyaniline with multi-walledcarbon
nanotube(CNT) and functionalized carbon nan-otube(fCNT) composites
of different weight percent-ages(5,10 and 15wt%) by in-situ
chemical polymeriza-tion. Functionalization of CNT was done by
immersingCNT in the mixture of concentrated H2SO4 and concen-trated
HNO3 (3:1, volume ratio) for 24 hours at roomtemperature. Then fCNT
was washed several times withDI water, filtered and dried in
vacuum. We have followedthe well known procedure to synthesize PANI
compositewith CNT and fCNT.7,15,16 In this process we added
2mlaniline monomer with 40ml 1.0M HCl, required amountof CNT/fCNT
was added to make different weight per-centage comoposites and
sonicated for 1 hour to obtainwell dispersed suspensions, then
400mg Ammonium persulfate(APS) with 20ml 1.0 HCl solution was added
drop-wise into the above suspension at room temperature. To-tal
suspension was stirred at 450rpm for 24 hours. Af-ter filtering, it
was dried in vacuum oven for 48 hoursat temperature 600C. We
collected the sample in pow-der form. Making pellet we have
measured the resistiv-ity and magnetoresistance by using Van der
Pauw fourprobe method. We have done low temperature measure-ment
using Janis liquid helium cryostat equipped withsuperconducting
magnet.
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3
CNT/PANI composite as fCNT contributes more disor-der in the
composite system.
0 50 100 150 200 250 300 35010-210-1
100101102
103104105
106107
(hm
-cm)
T (K)
fCNT/PANI CNT/PANI
10wt% Composite
Figure 3. (a) The temperature dependence resistivity for10wt%
fCNT/PANI and CNT/PANI
Considering the coulomb interaction between the lo-calized
states which creates a small Coulomb gap ∆c inthe N(EF ), Efros and
Shklovskii(ES) showed that theN(EF ) should quadratically vanish at
the Fermi level21and equation 1 is modified to
ρ(T ) = ρ0exp[(TES/T )1/2] (2)
and the Coulomb gap
∆c = 0.905kBT−1/20 T
3/2ES (3)
in all dimensions. where TES is the Efros-Shklovskii tem-
0.24 0.32 0.40 0.48 0.56
301.4 95.4 39.1 18.8 10.2
-2
0
2
4
6
8
10
12
0.25 0.30 0.35 0.40 0.45 0.50
16 11 8 6 5 4
2
4
6
8
10
12
14
ln[
]
T-1/4 (K-1/4)
10wt% CNT/PANI
10wt% fCNT/PANI
T (K)
(a)
ln[
]
T-1/2 (K-1/2)
T (K)
10wt% fCNT/PANI
10wt% CNT/PANI
(b)
Figure 4. (a) Plot of lnρ vs T−1/4 above 15K fitted withMott’s
3D VRHmodel .(b) Plot lnρ vs T−1/2 below 15K fittedwith Efros and
Shklovskii(ES) model for 10wt% composite
perature. Figure 4(b) shows that resistivity data is wellfitted
with ES model below 10K and 15K for fCNT/PANI
Table I. Experimental values and VRH parameters for differ-ent
samples, fCNT/PANI, CNT/PANI. The parameters areT0, Mott
characteristic temperature; TES , Efros-Shklovskiitemperature; ∆c,
Coulomb gap energy
.
sample T0(K) TES(K) ∆c(meV )fCNT/PANI 3.42 × 106 38.8
0.010CNT/PANI 8.78 × 105 28.8 0.013
0 1 2 3 4 5-1.5-1.0-0.50.00.51.01.52.02.53.03.54.0
0 1 2 3 4 5
0
1
2
3
4
5
MR%
H (Tesla)
5wt%
10wt%
15wt%
4.2K(a)
MR%
H (Tesla)
5wt%
10wt%
15wt%
4.2K(b)
Figure 5. (a) and (b) Plot of MR% vs magnetic field fordifferent
wt% of CNT/PANI and fCNT/PANI composites at4.2K respectively.
and CNT/PANI composite respectively. It depicts thatthe
crossover from VRH to ES conduction took placefrom higher
temperature for CNT/PANI composite com-pared to fCNT/PANI.The
Coulomb energy gap decreasesin fCNT compared to CNT composite(see
Table I) whichattributes the screening of the long-range coulomb
inter-action increases due to more disorder in the system.
Thisresult is consistent with previously reported data.22,23
The variation of resistance with applied external mag-netic
field is known as MR [ defined as [R(H)-R(0)]/R(0),in percentage].
Recent years in many systems likeAl70Pd22.5Re7.5
24 and NixSi1−x25 thin films have shownmagnetoresistance changes
sign from negative to posi-tive by increasing magnetic field due to
high disorder inthe systems. In figure 5 we have plotted MR for
com-posites of different weight percent of CNT and fCNT.It is
observed that MR is always positive for all threewt% of fCNT/PANI
(ref. fig. 5(b)) and there is atransition from positive to negative
MR with increasein weight percentage of CNT from 5wt% to 10wt%
inCNT/PANI(fig. 5(a)). Interestingly the value of MR%decreases with
increasing weight percentage of fCNT. Forall the temperatures
(4.2K,10K,20K and 30K) measured,10wt% CNT/PANI composite shows
negative magne-toresistance and value of the MR% decreases with
in-creasing temperature but same wt% of fCNT/PANI com-posite shows
the positive MR and increases with decreas-ing temperature(fig.
6).
The transition of negative to positive MR due
tofunctionalization of CNT can be explained by bipolaron
5
0 5 10 15 20 25
0.0 2.2 3.2 3.9 4.5 5.0
0.000
0.005
0.010
0.015
0.020
ln[
]
H2 (T2)
H (Tesla)
10wt% fCNT/PANI Composite
4.2K
Figure 8. (a) Plot of ln[ρ(H)/ρ(0)] Vs H2 for 10wt%fCNT/PANI
composite at 4.2K
region, weak field MR can be expressed as follows:28,29
lnρ(H)
ρ(0)≈ t2(
e2a40~2
)(T0T
)34H2 (5)
where t2 = 52016 , and a0 is the Bohr radius, approxi-mately
equal to the localization length. The positive MRfor 10wt%
fCNT/PANI is well fitted with VRH modelH2law at 4.2K(fig. 8). We
have extracted the localizationlength 8.23nm using the equation
5.
V. CONCLUSION
We can conclude that functionalization of CNT andPANI composite
shows huge variation(seven order)in resistivity which is much
greater than same wt%CNT/PANI composite at low temperature. The
re-sistivity variation with temperature follows Mott VRHmodel and
below a certain temperature it follows ESmodel. There is a
transition of MR from negative topositive due to functionalization
of CNT in 10wt% com-posite with PANI. The fCNT contributes more
disorderin the composites compared to CNT confirmed by Ra-man
spectroscopy. The long-range Coulomb interactionis screened by
disorder, increases the on-site Coulombenergy which hinders to form
bipolaron. On the basisof ‘Bipolaron’ model, decrease of long-range
Coulomb in-teraction may be the cause of the MR transition
fromnegative to positive. At 4.2K the positive MR of 10wt%fCNT/PANI
composite has explained by ‘wavefunctionshrinkage’ model.
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Negative to Positive Magnetoresistance transition in
Functionalization of Carbon nanotube and Polyaniline
CompositeAbstractI IntroductionII MethodsIII CharacterizationA
Raman Spectroscopy
IV Result and DiscussionV Conclusion References