Push-Pull archetype of reduced graphene oxide functionalized with polyfluorene for nonvolatile rewritable memory

Article abstract of DOI:10.1002/pola.25043

A solution-processable PFTPA-convalently grafted reduced graphene oxide (RGO-PFTPA) was synthesized by the 1,3-dipolar cycloaddition of azomethine ylide. Bistable electrical switching and nonvolatile rewritable memory effects were demonstrated in a sandwi

Full text of DOI:10.1002/pola.25043

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Push–Pull Archetype of Reduced Graphene Oxide Functionalized with
Polyfluorene for Nonvolatile Rewritable Memory
Bin Zhang,1 Yu Chen,1,2 Gang Liu,3 Li-Qun Xu,3 Junneng Chen,1 Chun-Xiang Zhu,4
Koon-Gee Neoh,3 En-Tang Kang3
1Shanghai Key Laboratory of Functional Materials Chemistry, Institute of Applied Chemistry, East China University of Science
and Technology, 130 Meilong Road, Shanghai 200237, China
2The State Key Laboratory of ASIC & System, Fudan University, 220 Handan Road, Shanghai 200433, China
3Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge, Singapore 119260,
Singapore
4Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge, Singapore 119260,
Singapore
Correspondence to: Y. Chen (E-mail: chentangyu@yahoo.com) or E.-T. Kang (E-mail: cheket@nus.edu.sg)
Received 25 June 2011; accepted 8 October 2011; published online 7 November 2011
DOI: 10.1002/pola.25043
ABSTRACT: A solution-processable PFTPA-convalently grafted
reduced graphene oxide (RGO-PFTPA) was synthesized by the
1,3-dipolar cycloaddition of azomethine ylide. Bistable electrical
switching and nonvolatile rewritable memory effects were
demonstrated in a sandwich structure of indium tin oxide/
RGO-PFTPA/Al. The switch-on voltage of the as-fabricated de-
vice was around ꢀ1.4 V, and the ON/OFF-state current ratio
was more than 103. The ON–OFF transition process is reversi-
ble because the application of a high enough positive voltage
can induce the reverse transfer of electrons, reducing the con-
ductivity back to its initial OFF state. Both the OFF and ON
states are accessible and very stable under a constant voltage
stress of ꢀ1 V for up to 3 h, or under a pulse voltage stress of
ꢀ1 V for up to 108 continuous read cycles (pulse period ¼ 2 ls,
C
V
pulse width ¼ 1 ls). 2011 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 50: 378–387, 2012
KEYWORDS: conjugated polymers; functionalization of poly-
mers; high performance polymers; polyfluorene; reduced gra-
phene oxide; rewritable memory effect; synthesis
INTRODUCTION Rather than encoding ‘‘0’’and ‘‘1’’ as a charge
stored in the cell of a silicon device, polymer memory stores
data in an entirely different form, such as in a high- and
low-conductivity response to an applied voltage.1 As alterna-
tives to the more elaborated processes of vacuum evapora-
tion and deposition of inorganic and organic molecular mate-
rials, solution processes, including spin-coating, spray-
coating, dip-coating, roller-coating, and ink-jet printing, can
be used to deposit polymers on a variety of substrates such
as plastics, inorganic wafers, glass, metal electrodes, and oth-
ers.1(b) The International Technology Roadmap for Semicon-
ductors has identified polymer memory as an emerging
memory technology since year 2005.2
cates that the mobilities for holes and electrons should be nearly
the same.4 Therefore, it would be very interesting to design and
synthesize graphene-based solution-processed organic/poly-
meric materials for molecular optoelectronic and photonic devi-
ces. In our previous work, we designed a series of soluble do-
nor–acceptor (D-A) type polymers for memory devices,5 among
which graphene oxide (GO)-covalently functionalized polymer
materials exhibit good bistable electrical switching and rewrit-
able memory effects.5(g–i) Some groups also reported the rewrit-
able or, the write-once-read-many-times memory properties of
polymer-grafted GO,6 and GO/reduced graphene oxide (RGO)-
doped polymer composites7–9 in recent years.
In contrast to graphene, its oxide form, GO, is an electrically
insulating material owing of the disrupted sp2 bonding net-
works, giving rise to a very low electron/hole transporting per-
formance, which is unfavorable for construction of the optoelec-
tronic devices such as organic light-emitting diodes (OLEDs),
polymer light-emitting diodes (PLEDs), solar cells, and electro-
active materials-based memory devices. On the other hand,
electrical conductivity and electron/hole transporting
When compared with silicon, graphene, which has a two-dimen-
sional planar geometry that enables multilayer device architec-
ture and simplifies future integration with complementary metal
oxide semiconductor technology, exhibits superior electron mo-
bility of up to 200,000 cm2/(V s), more than twice of that of the
highest mobility conventional semiconductor.3 Additionally, the
symmetry of the experimentally measured conductance indi-
C
V
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properties of RGO can be recovered by restoring the p-network.
Importantly, the preparation method for grafting organic or
polymeric moieties onto the surface of RGO retained structural
integrity of the RGO framework so that there is no loss of elec-
tronic structure. Up to the present moment, only fewer
reports10 investigating covalently chemical modification of RGO.
and with a flow of 3% H2/Ar stream for 12 h. 2,7-Dibromo-
9-phenyl-9-fluorenol (1),14 4,40-diformyltriphenylamine (2),15
and 9,9-bis(4-diphenylaminophenyl)-2,7-dibromofluorene (4)16
were synthesized according to the literatures.
Synthesis of 4,40-[4-(2,7-Dibromo-9-phenyl-9H-fluoren-9-
yl)phenylazanediyl]diben-zaldehyde (3)
It is well-known that the 1,3-dipolar cycloaddition reaction
To a stirred mixture of 1 (1.0 g, 2.4 mmol) and 2 (0.73 g, 2.4
mmol) in dichloromethane (100 mL), a solution of BF3ꢂEtO2
(0.38 mL, 3.0 mmol) was added dropwise in dichloromethane
(20 mL) under purified argon. Stirring was continued for 48 h
at room temperature, and then 50 mL of ethanol and 150 mL
of water were added to the reaction mixture. After phase-sep-
aration, the aqueous layer was repeatedly extracted with
dichloromethane, and the combined organic layers were
washed with water, dried over MgSO4, filtered, and the solvent
evaporated. The obtained crude product was further purified
by column chromatography (SiO2/petroleum ether–dichloro-
methane) to give compound 3 (0.94 g, 56%).
of azomethine ylides is one of the most versatile and widely
11
applied methodologies for the functionalization of C60
.
Fol-
lowing this basic concept, the reaction between sarcosine,
60, and the corresponding p-conjugated systems functional-
ized with one or two aldehyde groups, led to a variety of
60-based full-fledged dyads and triads.5(a),11,12 The same
C
C
approach can also be applied to the polymeric functionaliza-
tion of RGO. In this contribution, we first synthesized a new
polyfluorene (PF)-based conjugated polymer, namely
poly{[4,40-(4-(9-phenyl-9H-fluoren-9-yl)phenylazanediyl)diben
zaldehyde]-[4,40-(9H-fluorene-9,9-diyl)bis(N,N-diphenylbenze
namine)]-(9,9-dihexyl-9H-fluorene)} (hereafter denoted as
PFTPA-CHO), in which triphenylamine (TPA) acts as electron
donor and hole transporting material,13 and then this poly-
mer was directly used to react with RGO in the presence of
an excess of N-methylglycine to produce a highly soluble
RGO covalently functionalized with PFTPA (hereafter
denoted as RGO-PFTPA, Scheme 1). The varieties, excellent
optical and electronic properties, and high thermal and
chemical stability of PFs make them an attractive class of
materials for polymer-based optoelectronic devices. As a
result, this polymer exhibited excellent nonvolatile rewrit-
able memory effect, with a switch-on voltage of about ꢀ1.4
V and an ON/OFF current ratio in excess of 103.
1H NMR (CDCl3, 400 MHz): d/ppm ¼ 9.91 (s, 2H), 7.79 (d,
4H), 7.62 (d, 2H), 7.53 (d, 4H), 7.30 (d, 2H), 7.27 (s, 1H),
7.14 (m,8H), 7.04 (d, 2H).
Synthesis of Polymer PFTPA-CHO
A mixture of 3 (44.6 mg, 0.06 mmol), 4 (206.8 mg, 0.26
mmol), 5 (200 mg, 0.34 mmol), Pd(PPh3)4 (2 mg, 0.002
mmol), K2CO3 (276 mg, 2 mmol), toluene (4 mL), and distilled
ꢁ
water (1 mL) was stirred at 80 C for 36 h under purified ar-
gon. Then, a solution of sodium diethyldithiocarbamate trihy-
drate (22.5 mg, 0.1mmol) in deionized water (1 mL) was
added to the above system, and continued to stir at the same
temperature for 12 h. The collected precipitate from methanol
was further purified by Soxhlet extraction with acetone for 72
h to give a pale yellow solid (240 mg, 77%).
EXPERIMENTAL
General
1H NMR (400 MHz, CDCl3): d/ppm ¼ 9.87 (s, 1H), 7.80–6.96
(m, 95H), 2.10–1.99 (m, 10H), 1.20–0.94 (m, 30 H), 0.83–
0.62 (m, 25H). GPC (tetrahydrofuran (THF), linear polysty-
rene as reference): Mn ¼ 1.72 ꢃ 104, Mw/Mn ¼ 2.2.
Infrared (IR) spectra were recorded on a Nicolet Nagma-IR
550 spectrophotometer using KBr pellets. Molecular weights
[number-average (Mn) and weight-average (Mw)] were deter-
mined with a Waters 2690 gel permeation chromatography
(GPC) using a linear polystyrene standards eluting with tetra-
hydrofuran. Nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker 400 spectrometer at a resonance fre-
Functionalization of RGO with PFTPA (RGO-PFTPA)
A mixture of PFTPA-CHO (100 mg), RGO (5 mg), and
N-methylglycine (20 mg) in anhydrous N,N-dimethylforma-
mide (50 mL) was refluxed for 4 days. After termination of
reaction, 150 mL of methanol was added to the above mix-
ture. The crude product was collected by filtration using a
polycarbonate film (/ 0.45 lm), washed with ethanol and
dichloromethane, respectively, and then redissolved in DMF,
and filtered through a double-layer filter paper to remove
possible cross-linked byproducts. The filtration was poured
into methanol to give a gꢁray-black solid which was thor-
oughly vacuum-dried at 60 C over night. Yield: 89 mg.
1
quency of 400 MHz for H NMR in deuterated solution with a
tetramethylsilane as a reference for the chemical shifts. Raman
spectra were taken at room temperature with a MicroRaman
System RM3000 spectrometer and an argon ion laser operat-
ing at a wavelength of 785 nm as the excitation source. X-ray
photoelectron spectroscopy measurements were carried on a
Kratos AXIS HSi spectrometer with a monochromatized Al KR
X-ray source (1486.6 eV photons) at a constant dwell time of
100 ms and pass energy of 40 eV.
All chemicals were purchased from Aldrich and used without
further purification. Organic solvents used in this study were
purified, dried, and distilled under dry nitrogen. Purified nat-
ural graphite was purchased from Shanghai Yifan’s Graphite
Co. GO was prepared according to a method reported in our
previous work.5(i) RGO was obtained by reduction of GO by
Theoretical Calculations
Molecular simulations of the electronic properties, including
the optimized geometry, highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) of
the basic unit (BU) of RGO-PFTPA, were carried out using the
Gaussian 03 program package on a Hewlett-Packard Xeon
Two Sockets Quad-Core workstation with 8 CPUs and 16 GB
ꢀ1
ꢁ
ꢁ
thermal annealing to 900 C at a heating rate of 2 C min
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SCHEME 1 Synthesis of PFTPA-CHO and RGO-PFTPA.
memory. Orbital energy levels were calculated with density
function theory in B3LYP/6-31G(d) level. Vibrational frequen-
cies were calculated analytically to ensure that the optimized
geometries really correspond to the total energy minima.
water, acetone, and isopropyl alcohol. A 50-lL RGO-PFTPA
solution in toluene was spin-coated onto the precleaned ITO
substrate. The resultant film thickness is around 100 nm as
measured by a step-profiler. After drying, the film under
reduced pressure (10ꢀ5 Torr) and at room temperature
overnight, Al top electrodes were deposited onto the film
surface via thermal evaporation at 10ꢀ7 Torr through a
shadow mask. The J-V data reported were based on devices
Device Fabrication
The Indium tin oxide (ITO) glass substrates were precleaned
in ultrasonic bath for 15 min each in detergent, deionized
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the RGO-PFTPA structure, the observed IR spectrum of RGO-
PFTPA is in general similar to that of PFTPA and does not
give additional structural information.
The significant structural changes from GO to RGO, and then
to RGO-PFTPA are also reflected in their Raman spectra
(Fig. 2). Upon excitation with 768 nm laser, the Raman spec-
trum of GO displays two prominent bands at about 1,315
(D-band) and 1,581 (G-band) cmꢀ1 with the ID/IG of 0.52.
The D- and G-bands of RGO are located at 1,308 and 1,591
cmꢀ1, respectively, with a considerably increased D/G inten-
sity ratio (2.07) compared with that in GO. This change
implies a decrease in the average size of the sp2 domains
upon reduction of GO.18 Covalent graftings of the PFTPA
chains onto the surface of RGO led to the blue-shift of the D-
band and the red-shift of G-band, respectively, followed by
the increase of the G-band intensity. This result is not in ac-
cordance with the previous reports in the literature-
s.5(a,c,d),10,19 Usually functionalization of GO and RGO would
lead to the enhancement of the ID/IG ratio because the D-
band has been used to monitor the process of covalent func-
tionalization which transforms sp2 to sp3 sites, whereas the
G-band could be used to estimate the level and distribution
of modification. In this study, the PFTPA moieties grafted
onto the surface of RGO contain a large amount of sp2 aro-
matic carbon atoms (see the structure of PFTPA). Therefore,
although some sp2 carbon atoms in RGO were transformed
to sp3 carbon atoms, their amounts are still less than those
of the sp2 carbon atoms in the grafted polymer moieties.
This is the reason why the ID/IG ratio is decreased from 2.07
for RGO to 0.85 for RGO-PFTPA.
FIGURE 1 IR spectra of (a) GO, (b) RGO, (c) PFTPA-CHO, and
(d) RGO-PFTPA.
of 0.4 ꢃ 0.4 mm2 in size and about 300 nm in thickness.
The devices were characterized under ambient conditions,
using a Hewlett-Packard 4155B semiconductor parameter
analyzer with an Agilent 16440A SMU/pulse generator.
RESULTS AND DISCUSSION
PFTPA-CHO was synthesized through the Suzuki coupling
reaction of the monomers 3, 4, and 5 in the presence of the
tetrakis(triphenylphosphine)palladium(0) as the catalyst.
This polymer is soluble in many common organic solvents,
and by GPC analysis against a linear polystyrene standard
was found to have Mn of 1.72 ꢃ 104, and a polydispersity of
2.2. RGO was functionalized by the 1,3-dipolar cycloaddition
reaction between PFTPA-CHO, N-methylglycine, and RGO. It
should be noted that in this reaction multifunctionalized
PFTPA-CHO could have trouble forming network or cross-
linked structures. As a matter of fact, a small amount of in-
soluble cross-linked byproducts have indeed been observed,
but they can be easily removed from the crude product by
filtering, as described in the ‘‘Experimental’’ section. In addi-
tion, this problem can also be effectively overcome by carry-
ing out the addition reaction in the dilute solution. The re-
sultant polymer-grafted RGO is soluble in some common
organic solvents. The key to the development of RGO func-
tionalized polymers is to form selectively soluble polymer-
covalently grafted RGO hybrid materials under very mild
conditions.
The XPS spectra provide essential and useful information for
the reduction of GO and for the covalent attachment of the
PFTPA moieties onto the surface of RGO (Fig. 3). From Fig-
ure 3(a,c,e), it can be clearly seen that after reduction by
thermal annealing, a very weak O1s signal is observed in
both XPS spectra of RGO and RGO-PFPTA, this implying that
a trace of residual oxygen functionalities still exist in the
structure of RGO. The C1s XPS spectrum of GO [Fig. 3(b)]
As shown in Figure 1, the most characteristic features in the
IR spectrum of GO are the absorption bands corresponding
to the C¼¼O carbonyl stretching at 1733 cmꢀ1, the OAH de-
formation vibration at 1412 cmꢀ1, the CAOH stretching at
1226 cmꢀ1, and the CAO stretching at 1053 cmꢀ1 5(i),17
After
.
reduction by thermal annealing, the characteristic absorption
bands of GO nearly disappeared in the IR spectrum of RGO.
As expected, the characteristic stretching vibration of alde-
hyde groups in the IR spectrum of PFTPA-CHO at 1697 cmꢀ1
is absent in the IR spectrum of RGO-PFTPA, suggesting that
PFTPA was grafted onto the surface of RGO. Because of the
overwhelming contributions of multiple polymer chains in
FIGURE 2 Raman spectra of (a) GO, (b) RGO, and (c) RGO-
PFTPA. kex ¼ 785 nm.
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FIGURE 3 XPS wide-scan spectra and C1s core-level spectra of the samples. The inset of (f) is the N1s core-level spectrum of
RGO-PFTPA.
showed the peaks of carbon functionalities appeared at
283.9 (the C in C¼¼C bond), 284.6 (the C in CAC bond),
286.4 (the C in CAO bond), and 288.4 (the C in C¼¼O bond)
eV.20 The C1s XPS spectrum of RGO [Fig. 3(d)] showed three
peaks at 283.8, 284.8, and 286.1 eV, followed by a notable
enhancement of the sp2 C signal intensity in C¼¼C bond, and
a considerable decrease of the sp3 C intensity in CAO bond,
this implying the most of oxygen functionalities in GO have
been removed via thermal reduction. After covalent function-
alization with PFTPA, the C1s XPS spectrum of RGO-PFTPA
displayed the C signal of CAN bond at 285.5 eV. The N1s
XPS spectrum of RGO-PFTPA [see the inset of Fig. 3(f)]
showed two peaks of nitrogen functionalities at 399.8 (the N
in Ph3N) and 400.6 eV (the N in pyrrole ring).
The intramolecular D-A structure usually allows charge
transfer (CT) interaction or HOMO–LUMO electron transition
to occur, as is evident from the photoluminescence spectrum
of RGO-PFTPA (Fig. 4). By using 375-nm excitation light, the
fluorescence emission exhibited by RGO-PFTPA was appa-
rently quenched as compared with PFTPA-CHO. To confirm
this, we measured the absolute fluorescence quantum yields
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FIGURE 4 (a) UV/Vis absorption and (b) photoluminescence
(kex ¼ 375 nm) spectra of the samples in THF.
of the samples in dilute THF solution by using a Horiba-
Jobin-Yvon integrating sphere. The values are estimated as
51% for PFTPA-CHO and 40% for RGO-PFTPA. Such intramo-
FIGURE 6 Current density-voltage (J-V) characteristics of
a
0.16 mm2 Al/RGO-PFTPA/ITO device (a); stability of the device
in the ON and OFF states (b) under a constant stress of ꢀ1 V
and (c) under a continuous read pulse with a peak voltage of
ꢀ1 V, a pulse width of 1 ls, and a pulse period of 2 ls. The
inset of (b) shows the device structure used in the
measurement.
lecular quenching process may involve energy and/or CT
between PFTPA as electron donor and RGO as electron
acceptor. The UV/Vis absorption spectrum of PFTPA-CHO
shows two peaks at 312 and 382 nm, with a relatively inten-
sity ratio of 0.48. The main absorption peaks of RGO-PFTPA
are located at 307 and 358 nm, respectively, with a relatively
FIGURE 5 TGA profiles of the samples measured in nitrogen
(heating rate 10 ꢁC/min, N2 flow 100 mL/min).
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FIGURE 7 (Left) Calculated molecular orbitals of the BU of RGO-PFTPA, and (right) electronic transition from the ground state to
the CT state.
intensity ratio of 1.45. When compared with PFTPA-CHO, the
maximal absorption peak of RGO-PFTPA is shifted to the
blue by Dk ¼ 24 nm.
The nonvolatile rewritable memory effect was observed from
the characteristic J-V curves of the ITO/RGO-PFTPA/Al device
[Fig. 6(a)] under ambient conditions. The J-V curves clearly
displayed electrical bistable memory behaviors. The voltage
applied to the device was varied in a cycle from 0 ! ꢀ3 !
0 ! 3 ! 0 ! ꢀ3 V. Starting with the OFF state (low-con-
ductivity state) in the as-fabricated device, the current
increases slowly with the applied negative voltage sweep (0
! ꢀ3V). At the switch-on voltage of about ꢀ1.4 V, the cur-
rent density increases abruptly from 10ꢀ2 to 102 A/cm2
(Sweep 1), indicating device transition (writing process)
from the OFF state to the ON state (high-conductivity state).
After removing the power supply, the device is still in the
ON state (Sweep 2). These results indicated that ‘‘OFF’’ and
’’ON’’ can be encoded from the corresponding low-conductiv-
ity state (before writing) and high-conductivity state (after
writing), with an applied voltage (read voltage) in the range
of 0 to ꢀ1.4 V. After reading the ON state in the positive
TGA curves of the samples are shown in Figure 5. GO is ther-
mally unstable and suffers 15% weight loss upon heating to
ꢁ
100 C. The main mass loss (ꢄ30%) takes place around 200
ꢁC and is ascribed to the decomposition of labile oxygen func-
tional groups present in the material. A slower, steady mass
loss (ꢄ15%) of GO over the entire temperature range above
300 ꢁC can be attributed to the removal of more stable oxygen
functionalities. When compared with GO, RGO shows much
increased thermal stability. When heated to 800 ꢁC, about
8.12% weight loss was observed due to the removal of the re-
sidual oxygen functionalities. The thermal stability of RGO-
PFTPA is apparently higher than that of PFTPA-CHO. At 800
ꢁC, the amount of the PFTPA-CHO residue is 27.92%, whereas
that of the RGO-PFTPA residue is about 56.06%.
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workers on electron affinities of aromatic compounds.21 To
reduce the CPU time, a simplified model, in which the RGO
entity contains 18 aromatic rings and the polymer segment
only contains one repeat unit, was used to represent the BU
of RGO-PFTPA. From the optimized structures, the electron
distributions of both the HOMO and the LUMO were found to
be located on the RGO entity. In contrast to GO, in which the
ballistic charge carrier transport is significantly hindered due
to scattering at the surface or edge defects of epoxide and
carbonyl groups, the bandgap of graphene is theoretically
zero, this leading to an excellent but nonswitchable electrical
conductivity,9 and posing a major challenge for graphene elec-
tronics because the absence of a bandgap limits the ON/OFF
ratio of digital devices. Upon covalent functionalization with
the conjugated PFTPA polymer, the p-conjugation system of
RGO was effectively disturbed, giving rise to an increase of
energy bandgap. Both the HOMO-1 and HOMO-2 are located
on the PFTPA moieties. Without inputting voltage, the elec-
trons in the system are stable, this implying that the as-fabri-
cated Al/RGO-PFTPA/ITO device is in the OFF state. The OFF
state current can be fitted by a space-charge-limited current
model [Fig. 8(a)]:
ꢀ
ꢁ
V2
J ¼ Alee0 exp
d3
0:891 q3V
1=2
ð
Þ
(1)
kT pee0d
where J, A, l, ee0, d, V, k, T, and q are current density, positive
constant, the mobility of charge carriers, the absolute permi-
tivity of the complex, the thickness of the film, voltage, the
Boltzmann constant, the ambient temperature, and the abso-
lute value of the unit electronic charge (1.6 ꢃ 10ꢀ19 C),
respectively. At the switching threshold voltage, electrons
transit readily from HOMO to LUMO within the RGO moiety,
forming the locally excited state. However, the direct electron
transition from HOMO-1 and HOMO-2 to HOMO is restrained
due to the absence of overlapping between HOMO-1 or
HOMO-2 and HOMO (Fig. 7). HOMO-2 is distributed in the
molecular fragment directly bonded to the RGO nanosheets,
whereas the HOMO-1 is separated from the HOMO on the
RGO nanosheets by the fluorene moieties. Therefore, the spa-
tial hindrance will deter electron transition from HOMO-1 to
HOMO. Alternatively, electron transition from HOMO-2 to
HOMO will be easier to occur, filling the holes generated in
HOMO, followed by the spontaneous electron transition from
HOMO-1 to HOMO-2. In this case, the CT state is formed.
Along with the increase of CT interaction, dual-channel
charge transport pathways will form via inter-plane hopping
in RGO, and switch the Al/RGO-PFTPA/ITO device from the
OFF state to the ON state.1(c) The ON state current can also
be fitted by an Ohmic current model [Fig. 8(b)]
FIGURE 8 Experimental and fitted J-V characteristics of the Al/
RGO-PFTPA/ITO device in (a) the OFF state and (b) the ON
state.
sweep (Sweep 3), a positively biased sweep can program the
ON state back to the original OFF state at 1.65 V (erasing
process). The OFF state of the device can be read (Sweep 4)
and reprogrammed (Sweep 5) to the ON state again in the
subsequent negative sweep. Such a cycle is a typical ‘‘write-
read-erase-read-rewrite’’ process for a nonvolatile rewritable
memory device. Although there are some minor shift of the
write/erase voltages due to the possible influence of the
electrical stress on the inherent electrical relaxation of the
memory materials, as expected, our operation cycles can still
be repeated with fairly good accuracy. From Figure 6(b,c), it
can be clearly seen that both the OFF and ON states are ac-
cessible and very stable under a constant voltage stress of
ꢀ1 V for up to 3 h, or under a pulse voltage stress of ꢀ1 V
for up to one hundred million continuous read cycles (pulse
period ¼ 2 ls, pulse width ¼ 1 ls), with a ON/OFF state
current ratio of more than 103.
qnV
J ¼ B
(2)
d
To gain insights into the geometry and electronic structures of
RGO-PFTPA, computational studies were performed using den-
sity functional theory at B3LYP/6-31G level (Fig. 7). The accu-
racy of these methods was demonstrated by Schaefer and co-
where B is a positive constant and n is the density of charge
carriers. The intensive electron delocalization in RGO can
effectively stabilize the conductive CT state, resulting in the
nonvolatile nature of the ON state. However, application of a
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4 Charlier, J. C.; Eklund, P. C.; Zhu, J.; Ferrari, A. C. In Carbon
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reverse positive bias to the device can extract electrons from
the graphene, and program the device back to the OFF state.
CONCLUSIONS
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The reaction between sarcosine, RGO, and PFTPA-CHO, led to a
p-conjugated polymer-covalently grafted RGO memory mate-
rial (RGO-PFTPA). This material exhibited good solubilities in
several common organic solvents and good thermal stability.
When compared with PFTPA-CHO, the maximal absorption
peak of RGO-PFTPA is shifted to the blue by Dk ¼ 24 nm, and
the fluorescence emission exhibited by RGO-PFTPA was appa-
rently quenched due to the CT between PFTPA as electron do-
nor and RGO as electron acceptor. The J-V curves of the ITO/
RGO-PFTPA/Al device clearly displayed nonvolatile rewritable
memory performance. Both the OFF and ON states are accessi-
ble and very stable under a constant voltage stress of ꢀ1 V for
up to 3 h, or under a pulse voltage stress of ꢀ1 V for up to 108
continuous read cycles (pulse period ¼ 2 ls, pulse width ¼ 1
ls), with a ON/OFF state current ratio of more than 103. As
this material can be processed in solution, RGO-PFTPA pro-
vides great opportunities for device fabrication at low cost
and with ease. With the excellent electrical, mechanical, and
thermal properties of graphene, this bistable switching behav-
ior makes RGO-PFTPA a promising alternative or supplement
to present silicon-based semiconductor technology.
6 Li, G. L.; Liu, L. G.; Li, M.; Wan, D.; Neoh, K. G.; Kang, E. T. J.
Phys. Chem. C. 2010, 114, 12742–12748.
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M., II; Park, W.; Choi, W. K. Nano Lett. 2010, 10, 2441–2447.
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We are grateful for the financial support of the National Natural
Science Foundation of China (21074034), the Ministry of Edu-
cation of China (309013), the Fundamental Research Funds for
the Central Universities, the State Key Laboratory of ASIC & Sys-
tem of Fudan University (11KF007), the Shanghai Municipal
Educational Commission for the Shuguang fellowship
(08GG10), and the Shanghai Eastern Scholarship.
10 (a) Li, P. P.; Chen, Y.; Zhu, J.; Feng, M.; Zhuang, X.; Lin, Y.;
Zhan, H. Chem. Eur. J. 2011, 17, 780–783; (b) Lomeda, J. R.;
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