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Journal of Materials Science
Full Set - Includes `Journal of Materials
Science Letters'
ISSN 0022-2461
Volume 48
Number 3
J Mater Sci (2013) 48:1249-1255
DOI 10.1007/s10853-012-6866-4
Inkjet printed acrylic formulations
based on UV-reduced graphene oxide
nanocomposites
R.Giardi, S.Porro, A.Chiolerio,
E.Celasco & M.Sangermano
1 23
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Inkjet printed acrylic formulations based on UV-reduced
graphene oxide nanocomposites
R. Giardi •S. Porro •A. Chiolerio •
E. Celasco •M. Sangermano
Received: 6 July 2012 / Accepted: 3 September 2012 / Published online: 13 September 2012
ÓSpringer Science+Business Media, LLC 2012
Abstract This work reports the formulation of water-
based graphene oxide/acrylic nanocomposite inks, and the
structural and electrical characterization of test patterns
obtained by inkjet direct printing through a commercial
piezoelectric micro-fabrication device. Due to the presence
of heavily oxygenated functional groups, graphene oxide is
strongly hydrophilic and can be readily dispersed in water.
Through a process driven by UV irradiation, graphene
oxide contained in the inks was reduced to graphene during
photo-curing of the polymeric matrix. Printed samples of
the nanocomposite material showed a decrease of resis-
tivity with respect to the polymeric matrix. The analysis of
the influence of printed layer thickness on resistivity
showed that thin layers were less resistive than thick layers.
This was explained by the reduced UV penetration depth in
thick layers due to shielding effect, resulting in a less
effective photo-reduction of graphene oxide.
Introduction
Nowadays we are witnessing an increasing request for
electronic devices production characterized by high
demanding properties such as low manufacturing cost,
long-time endurance, environmental sustainable production
methods, recycling, low energy consumption, and high
efficiency. Polymeric materials seem to be the most
promising candidates to meet all these challenges [1].
Inkjet printing is one of the most promising manufacturing
technique which can be used to deposit polymers on a
variety of substrates [2]. Several reviews dealing with new
applications of inkjet printing technology are now available
[3,4]. In the inkjet printing, low viscosity should be
maintained in the polymer precursor and fast polymeriza-
tion needs to be performed soon after the deposition. For
these reasons, the use of UV curing process seems to be
very interesting because it is performed at room tempera-
ture, allowing the ink polymerization even on thermal
sensitive substrates such as paper, and in addition is a fast
overall manufacturing process [5]. In order to decrease the
surface resistivity of the dielectric polymer network, it is
necessary to disperse conductive fillers in the precursor, to
reach a conductive network (referring to the percolation
theory, the so-called infinite cluster [6], which could ensure
the requested electrical properties).
Printable inks based on conductive fillers are subjected
to several constraints to meet specific conditions, to be
ejected through nozzles of micrometric size (e.g.,
20–80 lm). These include the optimization of rheological
properties, ink viscosity, surface tension, and solvent
evaporation rate [7–11].
Several materials have been tested for use as conductive
inks [12], reporting different drawbacks. For instance,
conductive polymers presented the disadvantage of rela-
tively low conductivity [13], while metal nanoparticles
(NPs) based inks need to be sintered at temperatures gen-
erally too high for application on most flexible substrates
[14,15]. Temperatures as high as 300 °C may be poten-
tially sustained by poly-imide as high cost flexible sub-
strate [16]. Other examples of inkjet printable conductive
materials are silver nanocomposite inks, which exploit UV
curing to create the NPs in situ during the exposure,
R. Giardi S. Porro (&)A. Chiolerio E. Celasco
Istituto Italiano di Tecnologia, Center for Space Human
Robotics, c.so Trento 21, 10129 Turin, Italy
e-mail: Samuele.Porro@iit.it
M. Sangermano
Politecnico di Torino, Dipartimento di Scienza Applicata
e Tecnologia, c.so Duca degli Abruzzi 24, 10129 Turin, Italy
123
J Mater Sci (2013) 48:1249–1255
DOI 10.1007/s10853-012-6866-4
Author's personal copy
starting from a precursor [17,18]. In this case, there is no
need to transfer heat to the system [19,20], and cheap
substrates (e.g., PP, PET, paper) may be used, even though
the ultimate resistivity reached by those nanocomposites is
suitable for resistor applications only [21] and not for
conductive tracks, unless thermal sintering is performed.
As an alternative, carbon-based materials can be good
candidates for conductive inks, due to their low cost and
good electrical conductivity without the need of tempera-
ture treatments. This has been demonstrated for carbon
nanotubes printed thin films [22], graphene bi- and tri-layer
used as protective coating against oxidation on copper
NPs-based inks [23], and in graphene/water suspensions
[24].
Because of its high specific surface area, good chemical
stability, electrical and thermal conductivity, and high
charge carrier mobility (20 m
2
V
-1
s
-1
)[25,26], graphene
is actually the most suitable candidate to be dispersed in
photo-curable formulations to obtain a UV-cured conduc-
tive ink. So far, the manufacturing of graphene-based
polymer composites required not only that graphene sheets
were produced on a sufficient scale, but also that they were
incorporated, and homogeneously distributed, into various
polymeric matrices as single layers. However, graphite,
although inexpensive and available in large quantity,
unfortunately does not readily exfoliate to yield individual
graphene sheets. A widely investigated alternative is the
use of graphite oxide, a layered material produced by the
oxidation of graphite [27,28]. In contrast to pristine
graphite, the graphene derived sheets in graphite oxide
(graphene oxide sheets, GO) are heavily oxygenated,
bearing hydroxyl and epoxide functional groups on their
basal planes, in addition to carbonyl and carboxyl groups
located at the sheet edges [29,30]. The presence of these
functional groups makes GO sheets strongly hydrophilic,
which allows them to readily swell and disperse in water
[31]. Previous studies have shown that a mild ultrasonic
treatment of graphite oxide in water results in its exfolia-
tion to form stable aqueous dispersions that consist almost
entirely of one-nm-thick sheets [32]. Due to their structure,
GO sheets are not electrically conductive. Therefore, an
additional step is needed to reduce the oxide to graphene.
This has been achieved using several methods, e.g., by
thermal [33], chemical [34,35], or photo-thermal [36,37]
treatments.
This work explores the possibility of introducing aque-
ous dispersion of GO into acrylic resin matrices, such as
poly(ethylene glycol) diacrylate (PEGDA), thus fabricating
a conductive printable ink which is environmentally
friendly. The reduction of GO was performed using UV
light irradiation, which allowed the simultaneous photo-
polymerization of the polymeric matrix [38] that acted as a
binder. Structural and electrical characterization showed
the efficiency of the reduction method and promising val-
ues of conductivity of printed test patterns.
Materials and methods
Material
Commercial reagents were used: GO (thickness
0.7–1.2 nm) was purchased from Cheap Tubes Inc. (USA)
and used without further purification, PEGDA with
M
w
=575 g mol
-2
was purchased from Sigma-Aldrich
and DAROCUR
Ò
1173 radical photoinitiator (PI) from
BASF. PEGDA was chosen as polymeric matrix due to its
non-toxicity and water solubility, to fabricate an environ-
mentally friendly ink.
Samples preparation
Spin-coated GO aqueous dispersions (GOx)
GO aqueous dispersions were prepared by mixing GO
powder and PI in 1 g of deionized water. The GO con-
centration in water was varied between 1 and 4 per hundred
parts of resin (phr), while the PI content was varied
between 1 and 8 phr. The relative GO/PI content was
varied between 1/0.25 and 1/8 weight ratio, to evaluate the
PI content effect on the GO reduction (samples will be
referred to as GOx).
The GO aqueous dispersions were spin-coated on 1 cm
2
portions of single crystalline p-doped silicon wafer, pre-
cleaned by ultrasonic bath in isopropyl alcohol, rinsed with
water, and dried with nitrogen. The coated formulations
were irradiated with UV light for 2 min, with a light
intensity of 60 mW/cm
2
. After UV irradiation, samples
were dried at 80 °C under vacuum for 2 h to remove
residual water.
Inkjet printed GO aqueous dispersions (GOi)
A printable GO/water dispersion was formulated by mixing
0.02 g of GO powder in 4.5 g of deionized water (samples
will be referred to as GOi). In this formulation, a lower
concentration of GO was used to reduce the viscosity to a
value compatible with the use of the inkjet nozzle. High-
speed Ultraturrax was used for 5 min to obtain a homo-
geneous dispersion. Two-step ultrasonic bath (30 min at
40 kHz ?30 min at 59 kHz) was then used to further
grind and disperse the GO agglomerates. Finally, the dis-
persion was centrifuged at 14,000 rpm for 5 min to allow
residual large and heavy particles to precipitate at the
bottom of the test tube. The upper portion of the centri-
fuged dispersion was inserted into an ink reservoir, thus
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discarding the large precipitated particles. This and the
subsequent formulation were tested in a MicroFab Inkjet
Printer with automatic 3D position control, using an 80 lm
piezoelectric nozzle vibrating at a frequency of 250 Hz, at
room temperature. A test pattern of the GOi formulation
(prepared to assess the system printability) was printed on a
Si substrate for structural characterization.
GO aqueous dispersion/PEGDA UV-curable formulations
(GOp)
The formulations were prepared by adding 0.5 g of PEG-
DA and 0.08 g of PI to 4.5 g of DI water in which 0.02 g of
GO was previously dispersed (samples referred to as GOp,
and corresponding to the GO/PI ratio of 1/4). This GO/
PEGDA/water ink (Fig. 1a) was tested by inkjet spotting
straight line patterns with variable resolution (85–190 dots-
per-inch, dpi) and repetition of passes on the same track
(from 1 to 5) on microscope slides (Fig. 1b). The printed
thin films were irradiated with UV light for 2 min.
As a reference for bulk nanocomposite material, 100 lm
thick films of the GOp were obtained by deposition on a
microscope slide glass using a wire-wound bar and sub-
sequently exposed to UV light for 2 min. As a reference for
electrical characterization, a PEGDA/PI thick film was
similarly prepared without adding GO.
Characterization
The structural analysis and morphology of thick and thin
printed films were characterized by optical and scanning
electron microscopy (SEM). X-ray photoelectron spec-
troscopy (XPS) was performed using a monochromatic
X-ray beam with an Al K-asource with energy of
1486.6 eV. Before performing XPS, the homogeneity of
the samples was verified using an in situ secondary X-ray
imaging. Current/voltage (I–V) measurements were per-
formed on thick and printed films using a standard two-
point micro-contact setup of a Keithley 2635A multimeter.
The electrical characterization was performed on all sam-
ples at room temperature, in the range -200 to ?200 V.
Resistivity was computed comparing GO/PEGDA thick
films with printed thin films of several thicknesses (varied
with dpi resolution and repetition of printing on the same
track, measured by profilometry). These measurements
were planned to assess the variation in resistivity due to
GO reduction to graphene by UV irradiation, and any
potential collateral effects produced by the high strain rate
to which inks are submitted to in the printing nozzle.
Figure 1c shows a typical setup of the measurement for
inkjet printed thin films.
Results and discussion
We have investigated conductive printable inks, based on
aqueous acrylic UV-curable formulations containing GO.
The oxide can be easily dispersed in water and it can be
reduced during UV irradiation, with the simultaneous build
up of the crosslinking network. The polymer network acts
as a binder during printing and the in situ reduced GO
decreases the resistivity of the acrylic polymer, forming a
conductive percolative network. The preparation of elec-
trically conductive acrylic resins containing reduced
graphene oxide was previously investigated [38], showing
the occurrence of a single-step procedure starting from a
Fig. 1 GO/PEGDA/water ink (a), InkJet nozzle printing GO/PEGDA/water ink (b), and the two-point micro-contact setup for I–V
measurements used for inkjet printed GOp thin films (c)
J Mater Sci (2013) 48:1249–1255 1251
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homogeneous aqueous dispersion of GO, which undergoes
reduction induced by the UV radiation during photopoly-
merization of the acrylic resin.
This method can be used for the preparation of aqueous
acrylic formulations of variable viscosity, which are suit-
able for the fabrication of inkjettable UV-curable inks.
XPS analysis of GO reduction by UV irradiation
of GOx samples
In order to confirm the effectiveness of GO reduction by
UV irradiation, XPS spectra of GOx samples deposited on
Si wafer were compared before and after irradiation.
The GO/PI weight ratio was varied from 1/0.25 to 1/8
(S1 =1/0.25; S2 =1/0.5; S3 =1/1; S4 =1/2; S5 =1/4;
S6 =1/8 wt ratio). Figure 2reports an example of the
variation of XPS C1 s peaks after 2 min of UV irradiation
of an aqueous dispersion containing 1 phr of GO and 4 phr
of PI with respect to water. After irradiation, it is possible
to observe a significant decrease in intensity of the peaks
associated with carbonyl groups, evidencing the photoin-
duced GO reduction [39]. This deconvolution evaluation
was performed on different aqueous GO dispersions vary-
ing both GO and PI content, to evaluate the best
performing GO/PI ratio. The area and height of XPS peaks
associated with carbon–carbon and carbon–oxygen bond-
ing (carbonyl and carboxyl groups) have been analyzed.
Figure 3summarizes the analysis of the ratios of heights
and areas of peaks associated with carbon–oxygen and
carbon–carbon groups, proving that all samples show a
decrease of oxygen bonded to carbon after UV irradiation.
The measurements prove the reduction of GO, with resto-
ration of the extended conjugated sp
2
structure. According
to these results, the sample with GO/PI ratio of 1/4 gave the
highest value of GO reduction.
Inkjet printed tracks of GOi and GOp formulations
Several test patterns of the GOp and GOi formulations
were inkjet printed on transparent substrates (microscope
glass slides). Figure 4shows an image of a glass substrate
with inkjet printed straight lines of the GOp formulation
with various thicknesses. The thickness of printed films
was varied either by increasing the spotting resolution
(variation of dpi) or repeating the same track in multiple
passes, up to five times. In particular, the repeated tracks
showed a good uniformity and coverage of the substrate
(inset in Fig. 4). The analysis of the microstructure of the
Fig. 2 XPS spectra of pristine
GO (a) and a GOx sample
irradiated for 2 min with UV
light (b), showing the best fits
(red) to experimental data
(black) and the deconvoluted
peaks (blue) used for fitting. The
spectrum in (b) refers to the
formulation containing a GO/PI
ratio of 1/4 (Color figure online)
Fig. 3 Analysis of XPS C1s
peak deconvolution: height
(a) and area (b) ratios of C=O to
C–C contributions plus height
(c) and area (d) ratios of O–
C=O to C–C contributions,
for samples with several
concentrations of GO and PI in
water after 2 min of UV
irradiation (the formulations
correspond to the GO/PI ratio
reported as following:
S1 =1/0.25; S2 =1/0.5;
S3 =1/1; S4 =1/2; S5 =1/4;
S6 =1/8 wt ratio). For
comparison, pristine GO before
UV irradiation is also reported
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printed tracks demonstrates that GO flakes distribute uni-
formly on the substrate and form a continuous layer with
each other, ensuring continuity of electrical signal (Fig. 5).
Electrical properties
The electrical response of printed thin films (GOp) was
compared with one of the thick films (GOp and PEGDA
reference) fabricated by wire-wound bar. Figure 6shows
the raw (not normalized to layer resolution/thickness) I–V
characteristics of those samples. The I–V response of the
thin inkjet printed track features an absolute current which
accidentally appears in the same range of the pure matrix
thick film. This effect obviously depends on the difference
in size and thickness of thick and thin printed films.
Remarkably, the GOp thick film shows a non-linear
response, strongly different from the other materials: since
the electrical response (at least in the DC regime) may be
interpreted in terms of superposition of different contri-
butions, we may expect the nonlinear effect to come from
either the PEGDA matrix or the reduced GO filler. None-
theless, neither the pure PEGDA matrix nor the inkjet
printed sample show such a nonlinear trend, allowing us to
conclude that in the thick wire-wound bar-fabricated
sample there should still exist a fraction of unreduced GO,
possibly due to the complete absorption of UV radiation in
a few micrometers at the top of the sample. This phe-
nomenon, leading to inhomogeneous samples in the
direction perpendicular to the film plane, was already
observed on different classes of materials featuring UV in
situ reduction processes [26,29]. These phenomena are
thought to be due to the shielding effect originating by the
filler (in our case, the GO flakes) which limits the light
penetration depth, and therefore hinders the UV-induced
GO reduction.
Figure 7shows the resistivity of thin printed layer
samples as a function of sample thickness, compared to
thick films of GOp and pure PEDGA. Each experimental
point in the plot is given by a computation based on linear
fits to average I–V curves (error bars shown in the plot).
The addition of GO to PEGDA, after reduction by UV
irradiation, results in a decrease of resistivity by over an
order of magnitude (GOp TF versus PEGDA TF). For what
concerns printed samples, two trends may be evidenced,
both concurring to a resistivity decrease. By increasing the
number of passes and thus the track thickness, a small
decrease of resistivity is obtained (green dashed arrow in
Fig. 7); this fact is normally due to an increased volume
Fig. 4 Image showing inkjet printed test tracks of the GOp ink on a
microscope glass, after UV irradiation. Tracks with different thick-
ness are shown, corresponding to variation of dpi resolution or several
repetition of the printing. The inset shows an optical microscope
magnification of a printed track with 125 dpi and 3 passes
Fig. 5 SEM image showing the microstructure of an inkjet printed
track of GOi suspension (water was evaporated before inserting the
sample into SEM chamber). The inset shows a low-resolution image
of the printed track
Fig. 6 Raw I–V characteristics of pure PEGDA and GOp thick films
(TF), and GOp inkjet printed thin film (IjP)
J Mater Sci (2013) 48:1249–1255 1253
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available for electrons’ drift. Furthermore, by decreasing
the amount of ink spotted on a single-pass track (i.e.,
reducing the dpi resolution) and thus reducing the line
thickness, a strong reduction of GO is obtained (blue solid
arrow in Fig. 7). Those samples show a decrease of resis-
tivity by two orders of magnitude with respect to the pure
matrix. This counter-intuitive fact may be explained con-
sidering that in a thin track a higher fraction of GO is
reached and reduced by UV light than in a thick track, thus
better contributing to electrical conduction [26,29].
The analysis of coefficients of dispersion as a function
of the film thickness (R
2
, not shown here), computed by
fitting I–V curves with a linear function, demonstrates that
all samples ideally follow the Ohm’s law (within 0.4 %).
Only the GOp thick film sample appears out of scale, if
compared to thin inkjet printed samples and to pure
PEGDA matrix (Ohm’s law likelihood around 1.5 %). This
confirms the nonlinear behavior of thick films and the
shielding effect previously described for Fig. 6.
Conclusions
A route to obtain inkjet printable, environmentally friendly
inks based on graphene/acrylic nanocomposites was pre-
sented. The excellent rheological characteristics of the
formulations warranted printability with good repeatability.
The concurrent UV-driven polymerization of PEGDA
matrix and reduction of graphene oxide filler was verified
by XPS analysis. Thin printed samples of the nanocom-
posite showed a decrease of resistivity by two orders of
magnitude with respect to the pure matrix. In particular, it
was observed that the resistivity of thin layers was much
lower than one of the thick layers. This effect is ascribed to
the formation of free radicals, which may have a role in the
reduction of graphene oxide, from the photo-initiator used
to start the polymerization of the matrix. This reaction is
proportional to the amount of incoming UV light, therefore
it is more effective in thin layers, where the light pene-
tration is higher than in thick layers.
Suggested applications for the so-prepared inks are
devoted to flexible and organic electronics: the realization
of an electrode on top of a stacked structure (e.g., an active
device such as a transistor or a photovoltaic cell) incor-
porating organic semiconductors requires either conductive
polymers or high vacuum processes. This is important
since metal nanoparticle-based inks require a sintering
thermal treatment which is not compatible with organic
materials. A conductive ink ready to be structured by an
additive process like inkjet printing and needing only a fast
post-deposition treatment like UV curing is very interesting
also from an industrial point of view.
Future activity will be devoted to the incorporation of
metal nanoparticles or carbon nanotubes, to increase the
percolation and reduce the ultimate resistivity.
Acknowledgements The support by A. Chiodoni in helping with
SEM characterization is gratefully acknowledged.
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