Organic Electronics 14 (2013) 175–181
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Polymer composite of poly(vinyl phenol)-reduced graphene oxide
reduced by vitamin C in low energy consuming write-once–read-many
times memory devices
Messai A. Mamo a, Alan O. Sustaita a,b, Neil J. Coville c, Ivo A. Hümmelgen a,⇑
a
Departamento de Física, Universidade Federal do Paraná, Caixa Postal 19044, 81531-980 Curitiba, PR, Brazil
Advanced Materials Department, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a sección, San Luis Potosí 78216, Mexico
c
DST/NRF Centre of Excellence in Strong Materials and Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, PO Wits,
2050 Johannesburg, South Africa
b
a r t i c l e
i n f o
Article history:
Received 8 August 2012
Received in revised form 2 October 2012
Accepted 8 October 2012
Available online 16 November 2012
Keywords:
Reduced graphene oxide
Vitamin C
WORM memory
a b s t r a c t
We report the application of reduced graphene oxide, using vitamin C as reducing agent, to
make a composite with poly(vinyl phenol) as the active layer of write-once–read-many
times memory devices. These devices present a high ON/OFF current ratio of 105 when read
at 1 V, retain the information for a long time maintaining the ON/OFF current ratio constant, and require low energy for performing at 5 V the memory write (less than 108 J cm2 device active area) and read operations.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Optically readable bar codes are widely used in commercial and banking system. These bar codes present some
practical constraints because they are required to directly
expose the code to the reading laser beam and to the optical detector. In order to overcome these limitations, electronic readable equivalents to bar codes, i.e., memories
that permanently retain the information with a high electronic contrast between ON and OFF states are of high
interest. For wide application, such memories should be
inexpensive. Motivated by necessity and potential low
costs, several attempts have been made to develop writeonce–read-many times (WORM) memory devices derived,
for example, from carbon-structure based composite films
[1–6].
Graphene, a single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms, is a zero-gap semimetal
whose electronic band structure shows a linear dispersion
⇑ Corresponding author. Tel./fax: +55 41 33613645.
E-mail address: iah@fisica.ufpr.br (I.A. Hümmelgen).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.10.022
near the charge-neutral Dirac point. As a result, it has received significant scientific and technological attention in
the past few years owing to its extraordinary electronic
[7] and mechanical properties [8], thermal conductivity
[9], and high specific surface area [10]. These unique properties of graphene render it suitable for many applications
such as graphene-based electronics, composite materials,
molecular gas sensors, energy storage and conversion [11].
Intercalation–exfoliation of graphite by acid treatment,
thermal shock and intercalation of graphite have been
demonstrated to produce graphene sheets. A very simple
process to make high quality defect free graphene has been
achieved using a micromechanical cleavage technique
[12]. However, with this procedure graphene can only be
produced in small quantities. On the other hand, exfoliated
graphenes consisting of a few layers (of 2–3 and even up to
5) can be produced in large quantities.
Chemically modified graphenes, including graphene
oxide (GO), reduced graphene oxide (RGO) and their derivatives, have also attracted increasing attention [13,14]. A
GO sheet contains a great deal of epoxy, hydroxyl, carboxyl
and carbonyl groups [15]. The oxygenated groups of GO
176
M.A. Mamo et al. / Organic Electronics 14 (2013) 175–181
sheets can be partly removed by chemical, electrochemical
or thermal reduction process to prepare RGO [16]. More
importantly, Kudin et al. [17] reported that after deoxygenation the RGO conjugated structure was restored. In the
most successful cases, the chemical reduction of the GO
was conducted using hydrazine or hydrazine hydrate as
the reducing agent. A study by Long et al. [18] indicated
that when hydrazine is used as a reducing agent, nitrogen
atoms are incorporated into the structure of RGO. The N
atoms are expected to enhance the conductivity of the
RGO due to the lone pair of electrons associated with the
nitrogen atom. However, because hydrazine and hydrazine
hydrate are highly poisonous and explosive [19], researchers have sought alternative reagents. More recently, a
study conducted by Fernandez-Merino et al. [20] revealed
that ascorbic acid could be used as an alternative to hydrazine and is as efficient as hydrazine in the deoxygenation
of graphene oxide.
In this contribution we report on the development of
WORM memory devices based on RGO prepared using this
strategy, i.e., using vitamin C as a reducing agent. We have
made a composite containing RGO and poly(vinyl phenol)
(PVP) as organic medium. This composite was sandwiched
between metallic electrodes and electrically characterized,
demonstrating its suitability for high performance memory
devices.
2. Experimental section
All chemicals, except the carbon spheres, were obtained
from Sigma–Aldrich and used without further purification.
Graphite oxide was prepared in the form of both a thick
slurry and a fine dry powder starting from natural graphite
powder using the modified Hummers method, as previously reported [21]. A 9:1 mixture of concentrated
H2SO4/H3PO4 (360:40 mL) was added to a mixture of
graphite flakes (3.0 g, 1 wt equiv) and KMnO4 (18.0 g,
6 wt equiv). The reaction was then heated to 50 °C and stirred for 12 h. After the reaction mixture was cooled to room
temperature, distilled water (400 mL) with 30% H2O2
(3 mL) was added slowly. The solution was centrifuged
(7500 rpm for 20 min at 4 °C), and the supernatant was
decanted away. The remaining solid material was then
washed copiously with a total of 1000 mL of water and
200 mL of 30% HCl. Finally the solid material was then dissolved in 1000 mL of deionized water and centrifuged. The
material remaining after this extended, multiple-wash
process was coagulated with 200 mL of ether and the
resulting suspension was filtered through a 1 lm pore size
Teflon membrane filter. The solid obtained on the filter was
vacuum-dried overnight at room temperature, to give 4.6 g
of product.
A procedure for the reduction of the graphene oxide
was adapted from Fernandez-Merino et al. [19]. Deoxygenation of graphene oxide was conducted by dispersing the
synthesized GO at a concentration of 0.1 mg mL1 in distilled water. Then the pH of the dispersion was adjusted
to 9–10 with 25% ammonia solution to promote the colloidal stability of the graphene oxide sheets through electrostatic repulsion. After the solution was sonicated for
about 1 h, vitamin C (2 mmol) was added as a reducing
agent to the solution and the solution was heated at
95 °C under continuous stirring for 12 h. After 15 min the
brown mixture started to turn dark. The reduced graphene
oxide was filtered with a 1 lm pore size Teflon membrane
filter and repeatedly washed with warm distilled water. Finally the reduced graphene oxide was dried under vacuum
at room temperature for 24 h.
The graphene oxide and reduced graphene oxide were
characterized by transmission electron microscopy (TEM)
using a FEI Tecnai G2 Spirit electron microscope at
120 kV. Raman spectroscopy (J-Y T64000) was used to
measure the intensity of the D and G bands of the graphene
oxide and reduced graphene oxide.
The memory device was prepared by the simple construction of a 450 nm thick c-PVP-RGO composite film
(c-PVP: crosslinked PVP) sandwiched between a 100 nm
thick Al bottom and top electrode (Al/c-PVP-RGO/Al). The
device active area, corresponding to the overlap of the
top and bottom electrode, was 4 mm2. The thickness of
the PVP film was estimated by the capacitance measurement of the c-PVP film, assuming a relative dielectric constant equal to 3.5 [22].
For the fabrication of the devices, poly(4-vinylphenol)
(PVP) and methylated poly(melamine-co-formaldehyde)
(PMF) as a cross-linker, were dissolved in propylene glycol
monomethyl ether acetate, at concentrations equal to 7%
and 2.5%, respectively. RGO was then added to the solution
to prepare devices with different RGO to host polymer matrix ratios, specifically [RGO]/[PVP + PMF] w/w concentrations of 0.5%, 1%, 2% and 10%, as summarized in Table 1.
The composite mixtures were sonicated for ca. 3 h at
room temperature and 100 lL of suspension, in each case,
was then spin-coated onto an Al electrode at 2000 rpm.
The devices were then annealed in air at 200 °C for an hour
in order to cross-link the polymer [23,24]. The Al metal
was then thermally evaporated onto the composite surface
at 106 Torr base pressure using a shadow mask for device
patterning. The devices were then characterized using a
programmable Keithley 2602 source meter.
Table 1
Current density in both the OFF and ON state, ON/OFF current ratio and threshold voltage of the devices as a function of the
composition of the composite used in the memory devices.
Concentration of RGO in the c-PVP
matrix (%)
OFF current at 1 V (A/
cm2)
ON current at 1 V (A/
cm2)
ON/OFF
ratio
Threshold
voltage (V)
10
2
1
0.5
7.8 10–8
7.3 109
6.0 1010
4.0 1010
1.9 102
8.0 104
1.7 104
1.3 104
2.5 105
1.1 105
2.8 105
3.1 105
4.0
3.8
4.0
4.0
177
M.A. Mamo et al. / Organic Electronics 14 (2013) 175–181
3. Results and discussion
Graphite oxide was prepared in the form of either a
thick slurry or a fine dry powder. During deoxygenation,
the brown solution of graphene oxide slowly turned to
black (see Fig. 1). However, reduced graphene has low solubility in water. This low solubility of the RGO sheets in
water may be due to the strong p–p stacking tendency between RGO sheets that leads to the formation of irreversible agglomerates.
ATR-FTIR spectra of the oxidized and reduced graphene
samples are shown in Fig. 2. For unreduced graphene oxide
(Fig. 1a), the following features are observed: a broad, in-
tense band at 3000–3500 cm1 (OAH stretching vibrations) and narrower bands at about 1736 cm1 (C@O
stretching vibrations from carbonyl and carboxyl groups),
1624 cm1 (C@C stretching, skeletal vibrations from unoxidized graphitic domains), 1420 cm1 (OAH bending vibrations from hydroxyl groups), 1220 cm1 (breathing
vibrations from epoxy groups), and 872 cm1(attributed
to vibrations from epoxy, ether or peroxide groups)
[25,26]. For all the reduced materials (Fig. 1b), the intensities of the bands associated with oxygen functional groups
strongly decreased in relation to those of unreduced graphene oxide.
Moreover, the deoxygenated graphene oxide was also
confirmed by ultraviolet absorption spectra, as shown in
Fig. 3. The peak at 228 nm is related to the p–p electron
transition in the polyene-type structure of the graphene
oxide. After deoxygenation at 80 °C, only a peak centered
at 267 nm was observed, indicating that the electronic conjugation of GO was restored [21]. Moreover, an absorption
peak at 296 nm corresponding to the n ? p transition of
the C@O bond disappeared after reduction [21].
The microscopy images of the prepared graphenes (GO
and RGO) are presented in Fig. 4. The sample for TEM
was prepared by dispersing a small amount of the graphene in ethanol and then drop casting over a carbon coated
copper grid. The dried grid was used for TEM analysis. The
removal of OH groups and other functional groups during
deoxygenation results in a wrinkled structure (see Fig. 4),
which gives its fluffy physical appearance. These graphene
sheets have sizes in the order of a few square micrometres.
Fig. 5 represents the Raman spectra of pristine graphite,
GO and RGO. It was observed that the laser induces defects
and at high intensities even causes the burning of graphene. Hence, all the measurements were done at low laser
intensities.
The absence or low intensity of the D band in graphite
suggests that the graphite used is defect free or has a low
defect concentration. However, the overtone of the Dband, called the 2D band, is present and occurs at
2708 cm1. A highly intense G band, corresponding to
the optically allowed E2g phonons at the Brillouin zone
centre, occurs at 1586 cm1 [22]. The G band of GO is lo-
Fig. 1. (a) Graphene oxide and; (b) reduced graphene oxide in water.
1.05
a
b
2.7
1.00
228 nm
graphene oxide
0.90
absorbance
Transmittance
0.95
0.85
0.80
0.75
0.70
0.65
reduced graphene oxide
267 nm
1.8
296 nm
0.9
0.60
0.55
0.50
4000
3500
3000
2500
2000
1500
1000
wave number (cm-1)
Fig. 2. FT-IR spectra of (a) graphene oxide (b) reduced graphene oxide.
210
280
350
wavelenghth (nm)
Fig. 3. UV spectrum of graphene oxide and reduced graphene oxide.
178
M.A. Mamo et al. / Organic Electronics 14 (2013) 175–181
(a) 20000
18000
absorbance
16000
14000
12000
10000
8000
6000
4000
2000
1000
1500
2000
2500
3000
wavelength (cm-1)
(b)
30000
absorbance
25000
20000
15000
10000
5000
1000
1500
2000
2500
3000
wavelength (cm-1)
(c) 13000
12000
Fig. 4. TEM image of (a) graphene oxide and (b) reduced graphene oxide.
absorbance
11000
10000
9000
8000
7000
6000
cated at 1591 cm1, while that of RGO is shifted to
1583 cm1, close to the value of pristine graphite, indicating the reduction of GO. In addition a broadening of
the G band was observed in GO and RGO, and is attributed
to an increase in the graphene disorder. The chemical
treatments done to obtain GO and RGO induce defects in
the graphitic structure. As a result a broad D band with
an intensity comparable to that of the G band is obtained
in GO and RGO. The intensity of the D and G bands is used
as an indicator for the presence of defects in the samples
and also of the size of the in-plane sp2 domain. The ratio
of intensities of D and G-mode (ID/IG) peaks for RGO is
around 1.0 as compared to 1.1 for GO, thus indicating a
‘self-healing’ mechanism resulting in good restoration of
the p-conjugated structure [23].
The scanning electron microscopy image of the c-PVPRGO film deposited onto Al is shown in Fig. 6.
The electrical properties of the devices were investigated by carrying out current density–voltage (J–V)
5000
4000
500
1000
1500
2000
2500
3000
wavelength (cm-1)
Fig. 5. Raman spectra of (a) graphite (b) graphene oxide and (c) reduced
graphene oxide.
measurements. The J–V characteristics of devices made
from different ratios of RGO in the composites were similar. Each of the devices showed bistable electrical conductivity, corresponding to two distinct states that can be
associated with ‘‘0’’ and ‘‘1’’, allowing application to digital
circuits applying Boolean algebra. Measurements commenced with a low-conductivity (OFF) state (Fig. 7), and
as the applied voltage increased (top Al electrode positively biased) the current density increased gradually.
M.A. Mamo et al. / Organic Electronics 14 (2013) 175–181
Fig. 6. Scanning electron microscopy image of the c-PVP-RGO film
deposited onto Al; (a) 5% RGO and (b) 10% RGO.
The current density, J, remained in the low (ca. 108 A cm2 for the case of 0.5% and 1% RGO in the polymer matrix) level during sweep 1 until the threshold voltage
reached around 4 V. At the threshold voltage, J increased
suddenly to 102 A cm2, indicating that the device
switched from the low-conductivity OFF-state to the high
conductivity ON-state. Similarly, by applying a negative
applied voltage (not shown here) a similar OFF–ON transition occurred when the threshold voltage was attained.
During a backward sweep (Sweep 2), all the devices remained in the ON state (high conductivity) with an ON/
OFF current ratio about 105, when read at 1 V. The transition from the OFF state to the ON state corresponds to
the writing process for the memory device with the binary
‘‘0’’ and ‘‘1’’ representing the OFF and ON-state, respectively. The devices retain their ON states after a reverse
sweep (Sweep 3). This demonstrates that the information
which is written is non-erasable and that the devices are
a write-once–read-many times (WORM) memory.
A study of the relationship between the concentration
of RGO in the polymer matrix and the OFF and ON currents
of the devices, summarized in Table 1, indicates that as the
concentration of RGO increased in the polymer matrix both
the OFF and ON currents increased equally. As a result, the
179
ON/OFF current ratio is maintained approximately constant for all the devices (105). Reducing the RGO content
in the polymer matrix from 10% to 1%, lowers both the
OFF and ON currents by two orders of magnitudes. This
is very important when reduction of the device energy consumption during either writing or reading processes is required. This characteristic differs from what was observed
in devices prepared with graphene–poly(vinyl carbazole)
composites with a similar architecture [27]. On the other
hand, the increase in the concentration of RGO does not affect the threshold voltage of the devices, which remain
around 4 V.
The retention of both the OFF and the ON state for longer time is one of the most important properties in a WORM
memory device. In the ideal case it should retain the information ad eternum. Cycling stress tests were performed
with a Al/c-PVP-RGO/Al device in the OFF state and in
the sequence, in the ON state, by applying a continuous
read pulse (1 V, 1 s) every two minutes for more than
27 h (Fig. 8). The stability in both the OFF and the ON state
(which was attained after applying a 5 V pulse to the device) remained stable. After this cycling test no significant
degradation was noted after 105 cycles and the ON/OFF
current ratio remained constant and the memory device
presented excellent stability. The small reduction in both
the OFF and ON currents is due to Al path resistance increase due to oxidation, since all measurements were performed in air without any encapsulation. This
characteristic can be seen as an additional testimonial for
the robustness of these devices.
In a recent study on WORM memory devices based on
undoped carbon spheres a very short OFF to ON state transition time, of less than 700 ns, was measured [4]. The ON
state became permanent after a 5 V amplitude pulse applied for as short as a 10 ls duration, which was denoted
as the consolidation time [4]. It was proposed for such carbon-sphere composite based devices the ON transition occurs due to the occurrence of preferential current paths
through the carbon material that produce high current
densities between the particles [1,3,4]. This local high current density implies a high local power dissipation density.
It locally increases the temperature and degrades the polymer that separates the particles and the particles from the
electrodes, by this way producing permanent higher conductivity filaments.
Interestingly, our new RGO based devices also perform
similarly. Thus, taking the current flowing through the device during the writing operation it is possible to calculate
energy per device area E used in the write operation, i. e.,
Rt
E ¼ 0c J w ðtÞV w dt, where tc is the pulse time t required to
consolidate the ON state; Jw is the current density and Vw
is the voltage during the write operation, respectively.
Since the ON state current is achieved at tt 700 ns after
starting the write pulse and tt tc, as a first approximation, the energy can be estimated assuming Iw(t) = ION taken from Fig. 7, as an upper limit. For the data shown in
Fig. 7d, E JwVwtc 108 J cm2. Much less energy would
be consumed during the reading process, since the reading
process is performed at 1 V instead of 5 V in the case of the
writing process. Hence, these energy consumption values
M.A. Mamo et al. / Organic Electronics 14 (2013) 175–181
10
-1
10
-3
10
-5
10
-7
10
-9
10
-11
10
-13
2
Current density (A/cm )
2
Current density (A/cm )
180
10
-1
10
-3
10
-5
10
-7
10
-9
10
-11
10
-13
-5
-4
-3
-2
-1
0
1
2
3
(a)
(b)
(c)
(d)
4
5
-5
-4
-3
-2
-1
0
1
2
3
4
5
Voltage (V)
Voltage (V)
Fig. 7. Electrical characteristics of in Al/c-PVP-RGO/Al: (a) 10% (b) 2% (c) 1% and (d) 0.5% of RGO in c-PVP.
10
OFF current ratio constant, and require low energy for performing the memory write and read operations.
-2
ON
Current (A)
10
10
10
10
-3
Acknowledgements
-4
The authors would like thank CNPq (MAM, IAH), NRFSouth Africa (NJC) for Research Grants, CONACYT-Mexico
for CONACYT-mobility scholarships 290674(AOS), CT-Infra/Finep (MAM, IAH) for multiuser research facilities and
Lucieli Rossi and José P.M. Serbena for SEM microscopy.
-5
-6
OFF
10
References
-7
0.0
4
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
5
1.0x10
5
1.2x10
Time (s)
Fig. 8. Reading cycles for a Al/c-PVP-RGO/Al (1% RGO) device in both the
OFF and the ON state.
constitute an exceptional advantageous performance characteristic of these devices [28,29].
4. Conclusion
In summary, we report here the application of reduced
graphene oxide (using vitamin C as reducing agent) to
make a composite with poly(vinyl phenol) as the active
layer of write-once–read-many times memory devices.
The devices present a high ON/OFF current ratio of 105, retain the information for a long time maintaining the ON/
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