Granular magnetoresistance in cobalt/poly (3-hexylthiophene, 2, 5-diyl) hybrid thin films prepared by a wet chemical method

Article abstract of DOI:10.1063/1.3213561

Cobalt/poly (3-hexylthiophene, 2, 5-diyl) (P3HT) hybrid thin films were prepared by a wet chemical method. Their microstructure consists of a nanoscale mixture of a crystalline P3HT matrix, interspersed with amorphous P3HT regions containing the cobalt na

Full text of DOI:10.1063/1.3213561

Granular magnetoresistance in cobalt/poly (3-hexylthiophene, 2, 5-diyl) hybrid thin
films prepared by a wet chemical method
Tianlong Wen, Dan Liu, Christine K. Luscombe, and Kannan M. Krishnan
Citation: Applied Physics Letters 95, 082509 (2009); doi: 10.1063/1.3213561
View online: http://dx.doi.org/10.1063/1.3213561
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APPLIED PHYSICS LETTERS 95, 082509 ͑2009͒
Granular magnetoresistance in cobalt/poly „3-hexylthiophene, 2, 5-diyl…
hybrid thin films prepared by a wet chemical method
Tianlong Wen, Dan Liu, Christine K. Luscombe, and Kannan M. Krishnana͒
Department of Materials Science and Engineering, University of Washington, Box 352120, Seattle,
Washington 98195-2120, USA
͑Received 1 June 2009; accepted 8 August 2009; published online 27 August 2009͒
Cobalt/poly ͑3-hexylthiophene, 2, 5-diyl͒ ͑P3HT͒ hybrid thin films were prepared by a wet chemical
method. Their microstructure consists of a nanoscale mixture of a crystalline P3HT matrix,
interspersed with amorphous P3HT regions containing the cobalt nanoparticles. Magnetic and
transport measurements are consistent with this microstructure and the temperature dependence of
the resistance of these hybrid systems is well-fitted ͑R2=0.9993͒ to the fluctuation induced tunneling
model. Moreover, by applying a magnetic field, a magnetoresistance ratio of 3% was observed in 17
vol % Co hybrid films at 10 K. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3213561͔
The interplay between electrical transport and an applied
magnetic field in various inorganic, ferromagnetic materials
systems, and morphologies, as demonstrated by the observa-
tions of anisotropic magnetoresistance,1 tunneling magne-
toresistance ͑TMR͒,2 and giant magnetoresistance,3 is now
well-established. These phenomena have significant meaning
both in understanding the fundamental physical principles of
spin transport4 and in practical applications.5 Specifically,
magnetoresistance can be exhibited in magnetic/nonmagnetic
thin film hetrostructures6 as well as in systems where granu-
lar magnetic entities are embedded in a nonmagnetic
matrix.7,8
Complementing the above, spin transport in organic
semiconductors has recently emerged as a viable alternative
for spintronics9 largely because of the potentially long spin-
relaxation time in organic materials10 arising from their in-
herently weak spin-orbit coupling. This is also consistent
with the current trend of employing organic semiconducting
materials in various electronic devices including solar cells,11
diodes,12 transistors,13 and flexible displays14,15 with poten-
tial advantages that include flexibility of the material system,
ease of processability, and the potential to make light-weight
and low-cost devices.9 However, the development of an all-
organic, physically flexible, magnetoresistance device is cur-
rently not possible because, to the best of our knowledge, no
suitable ferromagnetic organic material has been found so
far. Nevertheless, preliminary TMR ͑Ref. 16͒ results using
an organic semiconducting ͑OSC͒ material, i.e., tris͑8-
hydroxyquinolinato͒ aluminum ͑Alq3͒, as a nonmagnetic
spacer in a ferromagnet/OSC/ferromagnet sandwich struc-
ture, are promising. In this context, we report magnetoresis-
tance measurements in a hybrid system comprised of cobalt
magnetic nanoparticles ͑Co MNPs͒ embedded in a semicon-
ducting poly͑3-hexylthiophene, 2,5-diyl͒ ͑P3HT͒ polymer
matrix. Since the semiconducting polymer matrix is flexible,
this composite device can potentially be a good candidate for
a structurally flexible magnetoresistance device. Unlike other
magnetoresistance devices that were prepared by physical
methods and require extreme conditions, these composite de-
vices were prepared by a wet-chemical method, which can
largely reduce the cost and simplify the processing. Further-
more, because cobalt nanoparticles passivated with organic
surfactant can be mixed uniformly with P3HT with any ratio
in an organic solvent, composite films with different cobalt
concentration can be easily obtained. Finally, we contribute
to the ongoing debate on the underlying mechanism of spin
transport in organic materials and demonstrate a good fit of
the temperature-dependent resistivity data to a fluctuation in-
duced tunneling ͑FIT͒ model for our sample.
Cobalt nanoparticles were synthesized by the well-
established thermal decomposition procedure by injecting
cobalt carbonyl ͓Co2͑CO͒ ͔ into a hot solution of 1,2-
8
dichlorobenzene in the presence of surfactant.17 The as-
synthesized cobalt nanoparticles, coated with oleic acid as
stabilizer to prevent the particle from aggregating and oxi-
dizing, are then washed, precipitated with ethanol, and dried.
Then 6 mg P3HT and various amount of dried cobalt was
codissolved by sonicating in 600 ␮l toluene, and stirred for
4 h at 45 °C to make P3HT/Co nanoparticles solutions with
different ratios. Films were made by drop casting 63 ␮l of
the mixed solution onto an indium tin oxide glass substrate
to yield a Co/P3HT hybrid film. The film was annealed in
5 vol % H2/95 vol % Ar atmosphere for 2 h at 150 °C,
and then a gold contact strip was deposited on top of the film
by evaporation to perform transport measurements in a quan-
tum design physical property measurement system ͑PPMS͒.
Slices of the composite films were made by microtome and
put on Cu grids and their microstructure characterized by
scanning transmission electron microscopy ͑STEM͒ using a
FEI Tecnai G2 F20 microscope operating at 200 kV. The
crystalline structure of the composite film, the pure P3HT
film and cobalt nanoparticle powders were characterized by
␪-2␪ x-ray diffraction ͑XRD͒ with the Cu K␣ radiation
͑␭=1.54 Å͒.
P3HT is
a
well-known semiconducting ͑␴ϳ10−6
–10−7 S/cm for pressed pellet,18 and ␴ϳ10−8–10−9 S/cm
for cast film19͒ and semicrystalline20,21 ͑a mixture of crystal-
line lamella and amorphous domains on the nanometer
length scale͒ polymer widely used in organic electronics.22
Further, the hexyl group on the polymer chain of P3HT
makes it possible to process it easily in an organic solvent
environment.22 In addition, because the cobalt nanoparticles
are stabilized by a layer of surfactant with a hydrophobic tail,
it is possible to completely codisperse P3HT and cobalt
a͒
Electronic mail: kannanmk@u.washington.edu.
0003-6951/2009/95͑8͒/082509/3/$25.00
95, 082509-1
© 2009 American Institute of Physics
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082509-2
Wen et al.
Appl. Phys. Lett. 95, 082509 ͑2009͒
6
5
4
3
2
1
0
Ο
(a)
(d)
Co MNP
∆
∆
∆
expt.
P3HT
fitting curve
100 nm
12vol.%Co
0
1000 2000 3000 4000 5000
H (Oe)
15vol.%Co
17vol.%Co
(e)
(b)
10 20
40 50 60 70 80
302
θ (deg)
0
2
4
6
8
1 0
1 2
15
FIG. 2. ͑Color online͒ X-ray ␪-2␪ diffraction scans of Co MNPs, P3HT, and
their composite film. Peaks ͑᭝͒ at 5.4°, 10.8°, and 16° correspond to ͑100͒,
͑200͒, and ͑300͒ a-axis orientations of P3HT; and the peak ͑᭺͒ at 44.2°
corresponds to the ͑221͒ crystallographic planes of ␧-Co.
r(n m )
Co
500 nm
Co (f)
C
O
Cu
Co
(c)
Si
S
Cu
10
5
over, combined with our STEM observations, we can infer
that the cobalt nanoparticles are distributed in the amorphous
region of P3HT polymers. Further, addition of cobalt nano-
particles into the P3HT matrix unfold some P3HT crystalline
lamella nanodomains to accommodate these cobalt
nanoparticles27 so that as many crystalline regions of the
P3HT as possible can be preserved to minimize the free en-
ergy of the whole system. This in turn will reduce the num-
ber of the crystalline nanodomains of the P3HT per unit vol-
ume, and thus reduce the intensity of the ␪-2␪ XRD peak at
5.4°.
To investigate the conduction mechanism in the P3HT
polymer as well as in the hybrid system, temperature depen-
dent resistance, ␳͑T͒, of both films were measured by a stan-
dard four-terminal technique in the PPMS system. Pure
P3HT with the same concentration has a resistance beyond
the measurement limit of the PPMS system. Thus we can say
that the conduction observed for the hybrid film is via elec-
tron tunneling between the highly conducting regions con-
taining cobalt nanoparticles distributed in the amorphous re-
gions of the P3HT. Moreover, ␳͑T͒ of the hybrid system is
best-fitted ͑R2=0.9993͒ to the fluctuation induced tunneling
͑FIT͒ model,28,29 used to describe charge transport in carbon-
poly͑vinyl chloride͒ systems, i.e., ␳͑T͒=␳o exp͓T1/͑T+To͔͒
at low temperature, where kBT1 is the energy required for an
electron to tunnel across the polymer gap between conduct-
ing particle aggregates and for TӶT0 the resistivity is
temperature-independent.28 In this FIT model, the highly
conducting regions containing small particle aggregates are
separated by a less conducting polymer matrix, which is
analogous to the observed microstructure in these hybrid
samples. The temperature-dependence of the resistivity of
the 12 vol % Co hybrid film without applied magnetic field
͓Fig. 3͑a͔͒, shows good agreement with the FIT model below
100 K. The upturn in resistivity at low temperatures, were
also fitted to a number of other mechanisms, from localiza-
Energy (KeV)
C
(g)
A
B
200 nm
O
Co
S
Co
Cu
10
15
5
Energy (KeV)
FIG. 1. ͑Color online͒ ͑a͒ TEM image of as-synthesized Co MNPs; ͓͑b͒ and
͑c͔͒ the STEM image of Co/P3HT hybrid film ͑15 vol % Co͒ ͑d͒ Chantrell’s
fitting with experiment data of the composite film, the fitting gives the
magnetic diameter of 7.6 nm with standard deviation of 0.6 nm; ͑e͒ the size
distribution extracted from Chantrell’s fitting ͑Ref. 24͒ by assuming log-
normal distribution; and ͓͑f͒ and ͑g͔͒ representative EDX spectra collected
at points A and B in ͑c͒, respectively.
nanoparticles in an organic solvent, and make a composite
film with any ratio of cobalt and P3HT. The physical diam-
eter, do, of the cobalt nanoparticles, determined by TEM ob-
servation of the as-synthesized cobalt nanoparticles
͓Fig. 1͑a͔͒ is doϳ8 nm, which is within the regime of
superparamagnetism.23 Figures 1͑b͒ and 1͑c͒ shows the dark
field image of the hybrid film obtained by STEM observa-
tion. Due to the atomic-number or Z-contrast, aggregates of
cobalt nanoparticles ͑bright areas͒, separated by the dark ar-
eas of P3HT can be clearly resolved. The magnetic size, dm,
of the cobalt nanoparticles in the hybrid film were measured
by fitting the M-H curve of the composite film with
Chantrell’s method ͓Fig. 1͑d͔͒,24 assuming a log normal size
distribution of the cobalt nanoparticles, giving dmϳ7.6 nm
with standard deviation of 0.6 nm ͓Fig. 1͑e͔͒. The x-ray dif-
fraction of the composite film with different concentrations
of cobalt is shown in Fig. 2. The peak at 44.2° corresponds to
the ͑221͒ crystallographic planes of ␧-Co;25 and the peaks at
5.4°, 10.8°, and 16° correspond to ͑100͒, ͑200͒, and ͑300͒
a-axis diffraction peaks of P3HT.26 The peak intensity of
P3HT at 2␪=5.4°, observed to be inversely related to the
concentration of cobalt nanoparticles in the hybrid films, is
proportional either to the number of the nanodomains of
crystalline P3HT per unit volume26 or to the number of do-
mains with a-axis oriented normal to the substrate surface.21
The films cast from solution prefer an alignment with a-axis
normal to the substrate21 and this does not change with the
addition of cobalt nanoparticles into the films since no peak
at 2␪=23° corresponding to the b-axis and c-axis is
observed.26 Hence, we conclude that the addition of cobalt
nanoparticles results in the unfolding of some P3HT crystal-
line lamella nanodomains rendering them amorphous. More-
29
tion ͑␳ϳT−p/2,␳ϳln T,␳ϳTp/2, in different dimensions͒
to variable range hopping ͓ln͑␳͒ϳT−1/͑1+d͔͒,28 but all these
fittings were inferior ͑only fits to VRH is also shown in
Fig. 3͒ to the FIT model. Finally, the increase in resistance
with temperature above 100 K is consistent with the
interpretation28 that the energy barrier between cobalt aggre-
gates will be reduced by thermal fluctuation, but will be wid-
ened by the rapid physical expansion of the polymer as tem-
perature increase; the competition between these two effects
will result in a minimum resistance28 and the barrier for tun-
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082509-3
Wen et al.
Appl. Phys. Lett. 95, 082509 ͑2009͒
-1/2
-1/3
-1/4
(a)
ln
ln
ln
(
ρ
ρ
ρ
)
)
)
~T
~T
~T
MR ratio is still small and cannot be retained at room tem-
perature. Further enhancement of the device performance can
be envisioned by optimizing the material system and the mi-
crostructure as well as by improving electrode contact. Such
work is in progress.
(b)
10.80
0
-1
-2
-3
12vol.%Co
15vol.%Co
17vol.%Co
50k
49k
48k
47k
(
10.79
10.78
10.77
(
0.1 0.2 0.3 0.4 0.5 0.6
-1/(1+d)
T
Expt.
Fitting
This project is partially supported by National Science
Foundation DMR Grant Nos. 0501421, 0120967, and
0747489 and the Murdock Foundation. Part of this work was
conducted at the University of Washington NanoTech User
Facility, a member of the NSF National Nanotechnology
Infrastructure Network ͑NNIN͒.
-8 -6 -4 -2
0
H (T)
2
4
6
8
0
50 100 150 200 250 300
T (K)
FIG. 3. ͑Color online͒ ͑a͒ The dependence of resistance of the 12 vol % Co
hybrid film on temperature is well-fitted to the fluctuation-induced tunneling
͑FIT͒ conduction model ͑adjusted R2 =0.9993͒ at low temperature ͑nonlinear
curve fitting using ORIGIN software͒, giving T1 =2.2 K and To =31 K. The
turning point is due to the differential thermal expansion between cobalt and
P3HT matrix. ln͑␳͒ϳT−1/͑1+d͒ curves ͑d=1, 2, and 3͒ for variable range
hopping at TϽ100 K are also plotted ͑inset͒ and these curves are a poor fit
with significant deviation from the expected straight line. ͑b͒ The resistance
change at 10 K of Co/P3HT hybrid composite film, for different nanopar-
ticle concentrations, as a function of the externally applied magnetic field.
1T. R. McGuire and R. I. Potter, IEEE Trans. Magn. 11, 1018 ͑1975͒.
2J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Phys. Rev.
Lett. 74, 3273 ͑1995͒.
3M. N. Baibich, J. M. Broto, A. Fert, F. N. Vandau, F. Petroff, P. Eitenne, G.
Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. 61, 2472 ͑1988͒.
4J. F. Gregg, I. Petej, E. Jouguelet, and C. Dennis, J. Phys. D: Appl. Phys.
35, R121 ͑2002͒.
5S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. V.
Molnar, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Science
294, 1488 ͑2001͒.
neling at different temperatures is as depicted in the inset of
Fig. 3͑a͒.
6R. L. White, IEEE Trans. Magn. 28, 2482 ͑1992͒.
7A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. P. Young, S. Zhang, F. E.
Spada, F. T. Parker, A. Hutten, and G. Thomas, Phys. Rev. Lett. 68, 3745
͑1992͒.
The magnetoresistance of several Co/P3HT hybrid com-
posite films with different composition, and up to an applied
field of 8 Tesla, was measured for the temperature range
from 10 to 300 K. Figure 3͑b͒ shows magnetoresistance ratio
of Co/P3HT hybrid composite film of three different compo-
sitions with respect to applied magnetic field at 10 K, in
which the magnetoresistance is defined as MR%=͓␳͑H͒
−␳͑0͔͒/␳͑0͒, where ␳͑H͒ and ␳͑0͒ are resistivity of the com-
posite film at the applied field of H and 0, respectively. As
shown in Fig. 3͑b͒, the resistivity of the composite film de-
creases as the external magnetic field increases, and contin-
ues to decrease slowly even at an external magnetic field of 8
Tesla. Meanwhile, if the concentration of the cobalt magnetic
nanoparticles increases from 12 vol % Co to 17 vol % Co,
the magnetoresistance ratio of the hybrid composite film at
8 T increase from 1.8% to 2.9%. As the temperature in-
creases, the magnetoresistance ratio of these hybrid films de-
creases rapidly, which is trivial at 200 K and disappears at
room temperature. As discussed above, the conduction of
electron in these hybrid composite films is via the tunneling
between the highly conducting region containing cobalt
nanoparticles in the P3HT amorphous region through the
crystalline region of P3HT. Thus we propose that when elec-
trons tunnel between these conduction regions, the tunneling
probability for the spin-up and spin-down electrons is differ-
ent and leads to different resistances when the magnetization
configuration of these nanoparticles are altered from random
to being parallel to the applied magnetic field.30
8J. Q. Xiao, J. S. Jiang, and C. L. Chien, Phys. Rev. Lett. 68, 3749 ͑1992͒.
9W. J. M. Naber, S. Faez, and W. G. van der Wiel, J. Phys. D: Appl. Phys.
40, R205 ͑2007͒.
10A. R. Rocha and S. Sanvito, J. Comput. Theor. Nanosci. 3, 624 ͑2006͒.
11K. M. Coakley and M. D. McGehee, Chem. Mater. 16, 4533 ͑2004͒.
12G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J.
Heeger, Nature ͑London͒ 357, 477 ͑1992͒.
13F. Garnier, R. Hajlaoui, A. Yassar, and P. Srivastava, Science 265, 1684
͑1994͒.
14C. J. Drury, C. M. J. Mutsaers, C. M. Hart, M. Matters, and D. M. de
Leeuw, Appl. Phys. Lett. 73, 108 ͑1998͒.
15G. H. Gelinck, H. E. Huitema, E. V. Veenendaal, E. Cantatore, L. Schr-
ijnemakers, J. B. P. H. V. D. Putten, T. C. T. Geuns, M. Beenhakkers, J. B.
Giesbers, B. H. Huisman, E. J. Meijer, E. M. Benito, F. J. Touwslager, A.
W. Marsman, B. J. E. V. Rens, and D. M. Deleeuw, Nature Mater. 3, 106
͑2004͒.
16T. S. Santos, J. S. Lee, P. Migda, I. C. Lekshimi, B. Satpati, and J. S.
Moodera, Phys. Rev. Lett. 98, 016601 ͑2007͒.
17V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, Science 291, 2115
͑2001͒.
18T. A. Chen, X. Wu, and R. D. Rieke, J. Am. Chem. Soc. 117, 233 ͑1995͒.
19S. A. Chen and C. S. Liao, Macromolecules 26, 2810 ͑1993͒.
20S. Malik and A. K. Nandi, J. Polym. Sci., Part B: Polym. Phys. 40, 2073
͑2002͒.
21H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard,
B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W.
Meijer, P. Herwig, and D. M. de Leeuw, Nature ͑London͒ 401, 685
͑1999͒.
22Z. Bao, A. Dodabalapur, and A. J. Lovinger, Appl. Phys. Lett. 69, 4108
͑1996͒.
23K. M. Krishnan, A. B. Pakhomov, Y. Bao, P. Blomqvist, Y. Chun, M.
Gonzales, K. Griffin, X. Ji, and B. K. Roberts, J. Mater. Sci. 41, 793
͑2006͒.
In conclusion, Co/P3HT composite films were prepared
by a wet chemical method for the purpose of electron trans-
port investigation. The electron transport in this hybrid
system is via tunneling between regions containing cobalt
nanoparticles aggregate in the composite film. A magnetore-
sistance ratio of 3% was observed for the 17 vol % Co
sample at 10 K. Compared to ferromagnet/OSC/ferromagnet
sandwich structures investigated by many groups,9,16 the Co/
P3HT hybrid system allows us to investigate spin transport
in a hybrid organic granular system. In contrast to devices
with inorganic/organic sandwich structures, the granular/
hybrid system can be made physically flexible, though the
24S.-H. Yoon, M. Gonzales-Weimuller, Y.-C. Lee, and K. M. Krishnan,
J. Appl. Phys. 105, 07B507 ͑2009͒.
25V. F. Puntes and K. M. Krishnan, Appl. Phys. Lett. 78, 2187 ͑2001͒.
26T. Erb, U. Zhokhavets, G. Gobsch, S. Raleva, B. Stühn, P. Schilinsky, C.
Waldauf, and C. J. Brabec, Adv. Funct. Mater. 15, 1193 ͑2005͒.
27S. Yeh, T. Wu, K. Wei, Y. Sun, and K. Liang, J. Polym. Sci., Part B:
Polym. Phys. 43, 1220 ͑2005͒.
28E. K. Sichel, J. I. Gittleman, and P. Sheng, Phys. Rev. B 18, 5712 ͑1978͒.
29P. A. Lee and T. V. Ramakrishnan, Rev. Mod. Phys. 57, 287 ͑1985͒.
30M. Holdenried, B. Hackenbroich, and H. Micklitz, J. Magn. Magn. Mater.
231, 13 ͑2001͒.
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