Nonvolatile rewritable memory effects in graphene oxide functionalized by conjugated polymer containing fluorene and carbazole units

Article abstract of DOI:10.1002/chem.201100807

A new polymer, poly[{9,9-di(triphenylamine)fluorene}(9,9-dihexylfluorene) (4-aminophenylcarbazole)] (PFCz) was synthesized and used in a reaction with graphene oxide (GO) containing surface-bonded acyl chloride moieties to give a soluble GO-based polymer

Full text of DOI:10.1002/chem.201100807

DOI: 10.1002/chem.201100807
Nonvolatile Rewritable Memory Effects in Graphene Oxide Functionalized
by Conjugated Polymer Containing Fluorene and Carbazole Units
[
a]
[b]
[a]
[b]
[a]
Bin Zhang, Yi-Liang Liu, Yu Chen,* Koon-Gee Neoh, Yong-Xi Li,
[
c]
[d]
[b]
Chun-Xiang Zhu, Eng-Soon Tok, and En-Tang Kang*
Dedicated to Professor Dr. Dr. h.c. Michael Hanack on the occasion of his 80th birthday
Abstract: A new polymer, poly
di(triphenylamine)fluorene}(9,9-dihex-
ylfluorene)(4-aminophenylcarbazole)]
A
H
U
G
R
N
U
G
exhibited two accessible conductivity
states, that is, a low-conductivity (OFF)
state and a high-conductivity (ON)
state, and can be switched to the ON
state under a negative electrical sweep;
it can also be reset to the initial OFF
state by a reverse (positive) electrical
sweep. The ON state is nonvolatile and
can withstand a constant voltage stress
of ꢀ1 V for 3 h and 10 read cycles at
8
ꢀ1 V under ambient conditions. The
nonvolatile nature of the ON state and
the ability to write, read, and erase the
electrical states, fulfill the functionality
of a rewritable memory. The mecha-
nism associated with the memory ef-
fects was elucidated from molecular
simulation results and in-situ photolu-
minescence spectra of the GO–PFCz
film under different electrical biases.
(
PFCz) was synthesized and used in a
reaction with graphene oxide (GO)
containing surface-bonded acyl chlo-
ride moieties to give a soluble GO-
based polymer material GO–PFCz. A
bistable electrical switching effect was
observed in an electronic device in
which the GO–PFCz film was sand-
wiched between indium–tin oxide
Keywords: graphene oxide · molec-
ular devices · polyfluorene · poly-
mers · rewritable memory
(
ITO) and Al electrodes. This device
[3,10]
Introduction
epoxy and hydroxyl groups on the basal planes.
Both
small molecules and polymers have been covalently at-
tached to the GOꢀs highly reactive oxygen functionalities, or
non-covalently attached on the graphitic surfaces of chemi-
cally modified graphenes, for potential use in polymer com-
posites, paperlike materials, sensors, photovoltaic applica-
Graphene, which was discovered most recently, has attracted
considerable interest owing to the long-range p-conjugation
yielding extraordinary thermal, mechanical, and electrical
[1–9]
properties.
Its two-dimensional (2D) network is the fun-
[3]
damental building block of other carbon-based materials,
such as 0D fullerene, 1D carbon nanotubes, 3D graphite and
tions, and drug-delivery systems.
Our current interet in the chemical modification and func-
tionalization of GO mainly involves the GO-based polymer
memories. In comparison to inorganic-materials-based
memory devices, molecular and polymeric memories exhibit
low-cost potential, simplicity in structure, good scalability,
potential for high-density data storage in 3D arrays, and
3
D diamond. The chemistry of graphene reported in the lit-
eratures mainly concerns the chemistry of graphene oxide
GO) with chemically reactive oxygen functionalities, in-
cluding carboxylic acid groups at the edges of GO, and
(
[11]
possibility of fabricating flexible devices.
Reversible
[
a] B. Zhang, Prof. Dr. Y. Chen, Y.-X. Li
Key Laboratory for Advanced Materials
Institute of Applied Chemistry
switching characteristics of organic nonvolatile memory
transistors (ONVMTs) using GO nanosheets as a charge-
[12]
East China University of Science and Technology
trapping layer have been reported by Jen et al. The trans-
fer curves of GO-based ONVMTs showed large gate-bias-
dependent hysteresis with threshold voltage shifts over 20 V.
In our previous work, we designed and synthesize a novel
1
30 Meilong Road, Shanghai 200237 (P. R. China)
E-mail: chentangyu@yahoo.com
[
b] Dr. Y.-L. Liu, Prof. Dr. K.-G. Neoh, Prof. Dr. E.-T. Kang
Department of Chemical & Biomolecular Engineering
National University of Singapore, Kent Ridge
Singapore 119260 (Singapore)
conjugated
triphenylamine-based
polyazomethine
(TPAPAM), covalenty grafted onto graphene oxide (GO–
TPAPAM), and placed the resulting films within aluminum
E-mail: cheket@nus.edu.sg
[13]
[
c] Prof. Dr. C.-X. Zhu
and indium–tin oxide contacts. The GO–TPAPAM-based
memory device exhibited the bistable electrical switching
and nonvolatile rewritable memory effect. A soluble GO
functionalized with poly(N-vinylcarbazole) (GO–PVK) was
also prepared by covalent coupling of GO–TDI (TDI=tolu-
ene-2,4-diisocyanate) with end-functionalized PVK (PVK–
SNDL, Department of Electrical & Computer Engineering
National University of Singapore, Kent Ridge
Singapore 119260(Singapore)
[
d] Prof. Dr. E.-S. Tok
Department of Physics, National University of Singapore
Kent Ridge, Singapore 117542(Singapore)
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FULL PAPER
DDAT; DDAT=S-1-dodecyl-S’-(a,a’-dimethyl-a’’-acetica-
[14]
cid)trithiocarbonate). This material could also be used as
a two-state electronic memory element. We used model cal-
culations to explain these observations, and found that the
polymer coats the GO sheets and acts as a tunneling barrier
for electrons moving from one GO sheet to another. The
“ON–OFF” transition process is perfectly reversible, as the
application of a high enough positive voltage can induce the
reverse transfer of electrons, reducing the conductivity back
to its initial value. Very recently we prepared a GO–poly-
mer complex, consisting of GO nanosheets with covalently
grafted poly(tert-butyl acrylate) (PtBA) brushes, by means
of surface-initiated atom-transfer radical polymerization
[15]
(
ATRP). The grafted hydrophobic polymer brushes sub-
stantially enhance the solubility of GO in organic solvents,
and the GO–g-PtBA nanosheets can form a uniform and
stable dispersion in toluene. The functionalized GO nano-
sheets can also be integrated into an electroactive polymer
matrix. Bistable electrical-conductivity switching behavior
and a nonvolatile electronic memory effect were demon-
strated in a composite thin film of poly(3-hexylthiophene)
(
P3HT) containing 5 wt% GO–g-PtBA in an Al/GO–g-
PtBA +P3HT/ITO sandwich structure. The molecular-struc-
ture-dependent memory effects demonstrate that it is possi-
ble to tune the electrical switching behavior in a GO–poly-
mer complex by tailoring molecular structures of the at-
tached polymer segments.
In this contribution, we first synthesized a new conjugated
polymer, poly ACHTUNGTRENNUNG[ {9,9-di(triphenylamine)fluorene}(9,9-dihexyl-
fluorene)(4-aminophenylcarbazole)] (PFCz), and then this
polymer was used in a reaction with GO containing surface-
bonded acyl chloride moieties to give a soluble GO-based
polymer material GO–PFCz (Scheme 1). This polymer also
shows a nonvolatile rewritable memory effect, with a turn-
on voltage of about ꢀ1.3 V and an ON/OFF state current
Scheme 1. Synthesis of GO–PFCz.
3
ratio of more than 10 .
Results and Discussion
The covalent attachment of PFCz onto GO through the
amide linkage was confirmed by the IR spectra (Figure 1)
and XPS spectra (Figure 2). The main characteristic absorp-
tion bands in the IR spectrum of GO are located at 1733
(
C=O carbonyl stretching of -COOH), 1412 (OꢀH deforma-
ꢀ1
tion vibration), 1226 (CꢀOH stretching) and 1053 cm (Cꢀ
O stretching). Upon treatment with PFCz, the bands at 1732
ꢀ
1
and 1412 cm disappeared, followed by the appearance of a
Figure 1. FTIR spectra of a) GO, b) PFCz, and c) GO–PFCz.
ꢀ
1
new vibration band at about 1667 cm , which is assigned to
the stretching mode of the amide linkage. The vibration at
responding to N bound to the carbonyl C (i.e., NHꢀC=O)
was observed in the N1s XPS spectrum of GO–PFCz.
ꢀ
1
1
590 cm is attributed to CꢀNH stretching, implying that
2
residual unreacted NH groups still remain in the resulting
As shown in Figure 3, the main absorption peak of PFCz
is located at 302 nm. After covalent attachment onto the
surface of GO, the largest absorption peak is shifted to the
blue by Dl=5 nm as a result of the possible electron trans-
fer from PFCz, as electron donor, to GO, as electron accept-
or. The absorption shoulder of GO–PFCz is significantly re-
2
GO–PFCz material. The N1s XPS spectrum of PFCz clearly
indicated that the peaks of nitrogen functionalities appeared
at 399.93 (the N in Ph N) and 401.17 eV (the N in NH ). In
3
2
contrast to PFCz, an additional component at 398.46 eV cor-
Chem. Eur. J. 2011, 17, 10304 – 10311
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10305

Y. Chen, E.-T. Kang et al.
Figure 4. Steady-state fluorescence spectra of the samples in THF; lex
70 nm.
=
3
Figure 2. The N1s XPS spectra of a) PFCz and b) GO–PFCz.
Figure 5. Cyclic voltammogram of the GO–PFCz film on Pt with
Bu NClO as the supporting electrolyte at ambient temperature. Scan
rate: 15 mVs
4
4
ꢀ
1
.
quantum yields of the mixture of GO and PFCz in THF,
with different GO concentrations. With the increase in GO
concentration from 1 to 2 to 5 to 10 wt%, the fluorescence
quantum yield of the mixture exhibits a significant decrease
(
from 0.30 to 0.28 to 0.23 to 0.17).
The electrochemical properties of the GO–PFCz film
coated on a Pt disk electrode, as shown in Figure 5, were
measured by cyclic voltammetry (CV) in deaerated acetoni-
trile containing recrustallized Bu NClO (0.10m) at room
Figure 3. UV/Vis absorption spectra of the samples in THF.
4
4
+
temperature. All potentials (vs Ag/Ag ) were converted to
[18]
duced, probably owing to the fact that incorporation of the
carbazole group reduces the effective conjugation of the flu-
values versus SCE by adding 0.29 V.
Its first oxidation
and reduction potentials were found to be +1.23 and
[16]
+
orene backbone. The weak and featureless absorption in
the 420–600 nm region of the GO–PFCz indicates presence
ꢀ0.85 V versus Ag/Ag , respectively, which correspond to
+1.52 and ꢀ0.56 V versus SCE, respectively. The HOMO/
LUMO values of GO–PFCz were experimentally calculated
by the onset of the redox potentials taking the known refer-
ence level for ferrocene, 4.8 eV below the vacuum level, ac-
[17]
of the GO group.
The fluorescence measurements of PFCz and GO–PFCz
were carried out in THF by applying 370 nm as excitation
wavelength (Figure 4). The intensity of the emission band of
GO–PFCz at 415 nm was a little bit quenched when com-
pared to that of PFCz suggesting energy and/or electron
transfer between the excited singlet states of the PFCz
moiety and the GO moiety. In addition, the absolute fluo-
[19]
cording to the Equation (1):
HOMO=LUMO ¼ ꢀðEonsetꢀEoxðferroceneÞÞꢀ4:8 eV
ð1Þ
In our electrochemical experiments ferrocene exhibited
an oxidation peak with an onset of 0.38 V versus Ag/AgCl.
The calculated HOMO and LUMO values were ꢀ5.65 and
ꢀ3.57 eV, respectively. The calculated HOMO–LUMO
rescence quantum yields (f ) of PFCz and GO–PFCz are
PL
0.31 and 0.24, respectively. To further study this fluorescence
quenching effect, we measured the absolute fluorescence
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Rewritable Memory
FULL PAPER
CV
bandgap (Eg ) is found to be 2.08 eV. The values of the ioni-
zation potential (IP) and the electron affinity (EA) can also
be estimated from these onset potentials (vs. SCE) by using
[20]
Equations (2) and (3), in which the constant 4.39 is the re-
lationship between IP, EA, and the electrochemcial poten-
[21]
tials.
The calculated IP and EA values are 5.91 and
3.83 eV, respectively.
IP ¼ Efirst oxidation þ 4:39 eV
EA ¼ Efirst reduction þ 4:39 eV
ð2Þ
ð3Þ
The electrical switching and memory effects of GO–PFCz
are demonstrated by the J–V characteristics of an electronic
device with the GO–PFCz film sandwiched between the
ITO and Al electrodes, as shown in Figure 6a. The device
was initially in the low-conductivity (OFF) state. During the
first negative sweep (with Al as the cathode and ITO as the
anode) from 0 to ꢀ4 V, the device exhibited an abrupt in-
crease in current density at ꢀ1.3 V, indicating the electrical
transition from the initial OFF state to the high-conductivity
(
ON) state. This switch-on process is equivalent to the “writ-
ing” process in a digital memory cell. The ON/OFF current
3
ratio is more than 10 at ꢀ1 V. The ON state is stable and
can be retained even after turning off the power supply (the
second sweep), indicating the nonvolatile nature of the ON
state. When a positive sweep from 0 to 4 V was applied (the
thrid sweep), the ON state was reset to the initial OFF state
at +3.3 V. This switch-off process is equivalent to the “eras-
ing” process of a digital memory cell. The device is thus
erasable and allows for application in a rewritable data stor-
age system. The subsequent fourth sweep was applied after
removing the power supply. The device was found to retain
the low-conductivity state, indicating its good stability in the
OFF state. The ability to write, read, and erase the electrical
states, as well as the nonvolatile nature of the ON and OFF
states, fulfill the functionality of a flash-type memory. As in-
dicated by the subsequent fifth to twelfth sweeps, the above
write–read–erase–read switching cycles can be repeated with
good accuracy, except for slight variations in the switching
threshold voltages, associated probably with inherent relaxa-
[22]
tion property of the polymer chain. Figure 6b shows the
effect of operation time on the stability of the GO–PFCz
device. Under a constant stress of ꢀ1 V, no evident degrada-
tion in current density was observed for both the ON and
OFF states over a 3 h period. In addition, the two electrical
Figure 6. a) Current-density–voltage (J–V) characteristics of
a
0.4ꢂ
2
0
.4 mm ITO/GO–PFCz/Al device. The sequence and direction of each
sweep are indicated by the respective number and arrow. b) Effect of op-
eration time on the ON and OFF state currents of the ITO/GO–PFCz/Al
device under a constant stress of ꢀ1 V. c) Effect of read pulses of ꢀ1 V
on the ON and OFF state currents of the ITO/GO–PFCz/Al device. The
insets of c) show the pulse used for measurement.
8
states were also able to withstand 10 read pulses of ꢀ1 V,
as shown in Figure 6c.
To gain insights into the above electrical switching effects,
computational studies have been performed on the GO–
PFCz using density functional theory (DFT) method at the
B3LYP/6-31G(d) level. The accuracy of these methods for
the calculation of electron affinities of aromatic compounds
has been demonstrated by Schaefer et al. To save comput-
ing resources, a simplified GO–PFCz model was employed
for the calculation. In the model, the GO moiety contains
tionalities, that is, epoxide, hydroxyl, carbonyl and carboxyl,
and the polymer segment contains one repeat unit, including
all the functional moieties. The first three highest occupied
molecular orbitals (HOMO, HOMO2, and HOMO3) and
the lowest unoccupied molecular orbitals (LUMO) of the
GO–polymer model, as well as the plausible electronic tran-
sitions under excitation, are depicted in Figure 7. The
LUMO of GO–PFCz is located on the GO moiety, indicat-
[23]
16 aromatic rings together with four kinds of oxygen func-
Chem. Eur. J. 2011, 17, 10304 – 10311
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10307

Y. Chen, E.-T. Kang et al.
Figure 7. Calculated molecular orbitals of the basic unit and the plausible electronic transitions under the excitation of GO–PFCz.
ing that the GO moiety may act as the electron acceptor.
The first three highest occupied molecular orbitals (HOMO,
HOMO2, and HOMO3) are located on the triphenylamine,
GO and fluorene moieties, respectively, indicating that all
the functional moieties are able to donate electrons.
switched from the OFF state to the ON state. The above
electric-field-induced charge-transfer (CT) in GO–PFCz is
consistent with the previously reported photoinduced CT
[26]
between graphene and functionalized polyfluorene
or
[17,27]
CdSe nanoparticles.
Due to the giant p-conjugation
In the as-cast GO–PFCz film, the oxygen functionalities
introduce a number of defect sites composed of sp -hybrid-
plane, the electrons can delocalize throughout the GO nano-
sheet, making the conductive CT state stable even after re-
moving the applied bias. The flexible amide linker between
GO and polymer segment can also inhibit dissociation of
the CT complex. A nonvolatile ON state is thus observed.
However, a reverse positive bias with sufficient energy can
extract electrons from the graphene nanosheet, returning
the device back to the initial OFF state. In addition, as elu-
cidated by the UV/Vis absorption and fluorescence spectra,
incorporation of the carbazole group interrupts the effective
conjugation and intrachain ordering of the fluorene backbo-
3
ized carbon atoms; these defects limit the in-plane charge
[24]
transportation in the GO nanosheet. In addition, the at-
tached polymer segment tends to intercalate between the
neighboring GO nanosheets and acts as a tunneling barrier
[25]
for the transverse charge transportation. As a result, the
as-fabricated GO–PFCz device remained in the low conduc-
tivity (OFF) state during the initial electrical sweep. At the
switching threshold voltage, electrons transit readily from
HOMO2 to the LUMO within the GO moiety, forming a lo-
cally excited state. Electron transition from the HOMO and
HOMO3 to the LUMO, however, is inhibited, due to the
absence of overlapping between LUMO and the two
HOMOs. Alternatively, electrons in HOMO3 can overcome
the energy barrier between HOMO3 and HOMO2
[16b,28]
ne.
Therefore, charge migration along the polymer
backbone is limited in the pre-excited GO–PFCz molecule,
ꢀ
2
ꢀ2
resulting in a lower OFF state current (3.4ꢂ10 Acm for
GO–PFCz at ꢀ1 V).
To elucidate the above electric-field-induced CT process,
in situ fluorescence spectra of the GO–PFCz film in an ITO/
polymer/Al sandwich device under electrical biases were
studied (Figure 8). The electrical bias was applied on the Al
electrode, with ITO as the ground electrode. All the emis-
sion spectra obtained at the excitation wavelength of
320 nm display a maximum fluorescence peak at 396 nm and
a shoulder peak at around 430 nm, arising probably from
(
0.33 eV) and fill the generated holes in HOMO2, followed
by spontaneous electron transition from HOMO to
HOMO3. As a result, the excited electrons in LUMO can
reside in the GO nanosheet and delocalize among the giant
p-conjugation system. Due to the strong reducibility, these
electrons can probably reduce the GO to graphene. In the
partially reduced GO nanosheet, the defect sites associated
with oxygen functionalities are significantly eliminated, al-
lowing a better electron delocalization and transportation in
both in-plane and transverse directions. The device is thus
[29]
the conjugated backbone of the polymer segments. When
a negative bias (ꢀ4 V) larger than the switch-on threshold
voltage (ꢀ1.3 V) is applied on the GO–PFCz device, electri-
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Rewritable Memory
FULL PAPER
memory effects. A device based on this material can be
switched “ON” and “OFF” during the negative and positive
electrical sweeps, respectively. Electric-field-induced charge-
transfer (CT) from the polymer donor to the GO acceptor
gives rise to a conductive CT state, resulting in the electrical
transition from the initial OFF state to the ON state. How-
ever, a reverse bias can dissociate the CT state and reset the
device back to the initial OFF state. The electric-field-in-
duced CT process is supported by fluorescence quenching
and recovery in the in situ fluorescence spectra of the GO–
PFCz film under electrical biases. Incorporation of carbazole
group in the GO–PFCz molecule can result in a larger
switch-off threshold voltage and a higher ON/OFF current
ratio.
Figure 8. In situ PL spectra of the GO–PFCz film in an ITO/polymer/Al
sandwich device under electrical biases. OFF-1 denotes the emission
spectrum before applying any electric bias, while ON-1 to OFF-3 denote,
respectively, the emission spectra after applying the respective electrical
bias indicated in the bracket (with Al as the working electrode and ITO
as the ground electrode).
Experimental Section
General: All chemicals were purchased from Aldrich and used without
further purification. Organic solvents were purified, dried, and distilled
under dry nitrogen. Purified natural graphite was purchased from Shang-
hai Yifan Graphite Co. GO was prepared from graphite by the Hummers
cal switching from the OFF state to the ON state is trig-
gered, as shown in Figure 6a. The resultant fluorescence
spectrum (ON-1 spectrum) shows a decrease in the emission
intensity in comparison to the initial fluorescence spectrum
[
14b]
method, and dried for 10 days over P
2 5
O in a vacuum oven before use.
2
,7-Dibromo-9,9- di(triphenylamine)fluorene (4) was prepared according
[
32]
to the literature. The UV/Vis absorption spectral measurements were
carried out with a Shimadzu UV-2450 spectrophotometer. FTIR spectra
were recorded on a Nicolet Nagma-IR 550 spectrophotometer using KBr
pellets. Steady-state fluorescence spectra were measured on a HORIBA
Jobin Yvon FluoroMax-4 spectrofluorometer, and the absolute fluores-
cence quantum yields were measured by integrating sphere method on
the same instrument. The samples for fluorescence measurement were
dissolved in dry THF, filtered, and transferred to a long quartz cell, and
then capped and bubbled with high purity argon for at least 15 min
before measurement. NMR spectra were recorded on a Bruker 400 spec-
(
OFF-1 spectrum). This partial fluorescence quenching
arises probably from CT from the polymer segment to the
GO moiety. As a result, the conductive CT state is formed,
and the device is switched to the ON state. When a positive
electrical sweep is applied on the GO–PFCz device, for ex-
ample, the 3th sweep in Figure 6a, electrons in the graphene
nanosheet are extracted and the CT state is dissociated. The
device is thus switched back to the initial OFF state. The
erasing of the ON state is supported by recovery of the ini-
tial fluorescence spectrum corresponding to the OFF state
after applying a positive bias of +4 V (OFF-2 spectrum).
The slight decrease in the OFF-2 spectrum in comparison to
the OFF-1 spectrum is probably associated with a delay in
conformational change of the polymer chain in response to
1
trometer at a resonance frequency of 400 MHz for H NMR spectra in
deuterated solution with a tetramethylsilane (TMS) as a reference for the
chemical shifts. XPS 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.
n w
Molecular weights [number-average (M ) and weight-average (M )] were
determined with a Waters 2690 gel-permeation chromatograph (GPC)
using a polystyrene standards eluting with tetrahydrofuran. Cyclic vol-
tammetry was performed on an ALS630B electrochemical analyzer in
deaerated benzonitrile containing recrystallized tetrabutylammonium
[30]
change in the electrical field. The “ON–OFF” changes in
the fluorescence spectra match the write–erase processes in
the J–V characteristics of the GO–PFCz device, and can be
repeated with good accuracy, as indicated by the subsequent
ON-2, OFF-3, and ON-3 spectra.
4 4
perchlorate (Bu NClO , 0.1m) as the supporting electrolyte at 298 K. A
conventional three-electrode cell was used with a platinum working elec-
2
trode (surface area of 0.3 mm ) and a platinum wire as the counter elec-
trode. The Pt working electrode (Bioanalytical System ACHUTNGRENNUG( BAS), Inc.) was
routinely polished with BAS polishing alumina suspension and rinsed
with acetone before use. The measured potentials were recorded with re-
spect to the Ag/AgNO (0.01m) reference electrode. All electrochemical
3
measurements were carried out under an atmospheric pressure of argon.
As we discussed above, the working mechanism in our
memory device is due to charge transfer. The charge-trans-
fer processes and the switching characteristics are independ-
[31]
ent of the choice of electrodes. Although the turn-on volt-
age of the device may vary only slightly with the electrode
work functions, the memory characteristics (worm/flash/
dram), the on/off ratio and the device stability are unrelated
to the used electrodes.
Device fabrication: The indium–tin oxide (ITO) glass substrate was care-
fully precleaned sequentially with deionized water, acetone, and 2-propa-
nol in an untrasonic bath for 15 min, and then treated with oxygen
plasma.
10 mgmL ) in N,N-dimethylacetamide (DMAc) was spin-coated onto
the ITO substrate at a spinning rate of 1000 rpm, followed by solvent re-
A solution (100 mL) the functionalized graphene derivative
ꢀ
1
(
ꢀ
5
moval in a vacuum oven at 10 Torr and 508C for 10 h. The thickness of
the polymer layer was about 50 nm, as determined by the AFM edge
profiling. Finally, an Al top electrode of about 400 nm in thickness was
thermally deposited onto the material surface through a shadow mask at
Conclusion
ꢀ
7
a pressure of about 10 Torr. Electrical property measurements were car-
A new solution-processable GO–PFCz polymer was synthe-
sized and characterized for its electrical switching and
2
2
2
ried out on devices of 0.4ꢂ0.4 mm , 0.2ꢂ0.2 mm , and 0.15ꢂ0.15 mm in
size, under ambient conditions, using an Agilent 4155C semiconductor
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10309

Y. Chen, E.-T. Kang et al.
parameter analyzer equipped with an Agilent 41501B pulse generator.
water was added to the above reaction mixture to remove the triethylam-
monium salts formed during the reaction. The crude product was collect-
ed by filtration, re-dissolved in a small amount of DMF, and then precipi-
The current-density–voltage (J–V) data reported here were based on the
2
device unit of 0.4ꢂ0.4 mm in size, unless stated otherwise. ITO was
maintained as the ground electrode during the electrical measurements.
2
tated from MeOH/H O (v/v=1:1) solution (this procedure was repeated
at least 3 times). The resulting material (124 mg) was dried in vacuum at
658C for 2 days.
Molecular simulation: Molecular simulation of GO–PFCz was carried
[
33]
out with the Gaussian 03
program package on a Hewlett–Packard
Xeon Two Sockets Quad-Core workstation with 8 CPUs and 16 GByte
memory. Molecular orbitals and their energy levels were calculated with
density function theory (DFT) in B3LYP/6–31G(d) level. Vibrational fre-
quencies were calculated analytically to ensure that the optimized geo-
metries really correspond to the total energy minima.
Acknowledgements
Synthesis of 9-(4-nitrophenyl)-9H-carbazole (1): A stirred solution of 1-
bromo-4-nitrobenzene (14.47 g, 72 mmol), carbazole (10.02 g, 60 mmol),
The authors are grateful for the financial support of the National Natural
Science Foundation of China (21074034), the Ministry of Education of
China (309013), the Fundamental Research Funds for the Central Uni-
versities, the Shanghai Municipal Educational Commission for the Shu-
guang fellowship (08GG10) and the Shanghai Eastern Scholarship.
K
2
CO
3
(8.29 g, 60 mmol), 1,10-phenanthridine (2.16 g,12 mmol) and CuI
(
2.27 g,12 mmol) in DMF (100 mL) was heated at 1558C for 24 h. After
cooling down to room temperature, ice-water (500 mL) was poured into
the reaction mixture. The collected yellow soild was washed with distilled
water and methanol for several times, respectively, and dried at 508C for
1
6
5
(
h
under reduced pressure. Yield: 15 g (88%). H NMR (CDCl
3
,
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00 MHz): d=8.46 (m, 2H), 8.14 (d, J=8 Hz), 2H, 7.78 (m, 2H), 7.49
d, J=8 Hz, 2H), 7.44 (m, 2H), 7.34 ppm (m, 2H); EI-MS: m/z: 288.1
+
[
M ].
Synthesis of 3,6-dibromo-9-(4-nitrophenyl)-9H-carbazole (2): Comopund
(8.67 g, 30 mmol) and N-bromosuccinimide (NBS, 11.74 g, 66 mmol)
were stirred in anhydrous DMF (50 mL) at 08C for 48 h; then ice-water
500 mL) was added to the system, The collected yellow precipitate was
washed with water and methanol for several times, respectively, and
dried at 508C for 6 h under reduced pressure. Yield: 10 g (75%).
3
H NMR (CDCl , 500 MHz): d=8.51–8.49 (m, 2H), 8.21 (d, J=8 Hz,
H), 7.76–7.73 (m, 2H), 7.57–7.54 (m, 2H), 7.34–7.32 ppm (d, J=8 Hz,
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2
2
+
H); EI-MS: m/z: 445.9 [M ].
[
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Synthesis
ene)(4-nitrophenylcarbazole)] (PFNCz):
.17 mmol), 3 (200 mg, 0.341 mmol), 4 (137 mg, 0.17 mmol), [Pd
2 mg, 0.002 mmol), K CO (276 mg, 2 mmol), toluene (4 mL), and dis-
of
poly
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[{9,9-di(triphenylamine)fluorene}(9,9-dihexylfluor-
mixture of (76 mg,
(PPh
2
04.
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277–2289.
A
2
[
[
0
(
A
H
U
G
R
N
U
G
3 4
) ]
2
2
3
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tilled water (1 mL) in a Schlenk tube was stirred at 808C for 36 h. Then
an excess phenylboronic acid (200 mg) and bromobenzene (200 mg) were
added as end-capping reagents sequentially in 12 h interval. The mixture
was extracted with chloroform (10 mL) three times, and the combined or-
ganic extracts were washed with water, brine, and dried over sodium sul-
fate. The salt was filtered off, and the filtrate was concentrated into a
small volume. The polymer solution was added dropwise into stirred
methanol. After filtration, the collected solid was purified by reprecipi-
tating into methanol and then Soxhlet extraction with acetone. The poly-
[
[
13] X. D. Zhuang, Y. Chen, G. Liu, P. P. Li, C. X. Zhu, E. T. Kang, K. G.
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mer was then dried under vacuum to give a pale yellow solid (213 mg,
1
yield 78%). H NMR (CDCl
(
0
3
, 400 MHz): d=8.0–8.3 (m, 4H), 7.3–7.9
m, 20H), 6.7–7.2 (m, 34H), 1.91–2.10 (m, 8H), 0.92–1.10 (m, 24H),
.51–0.77 ppm (m, 20H).
Synthesis of poly[{9,9-di(triphenylamine)fluorene}(9,9-dihexylfluor-
ene)(4-aminophenylcarbazole)] (PFCz): A mixture of PFNCz (200 mg)
and SnCl ·2H O (250 mg) in THF (40 mL) was refluxed for 6 h. The mix-
ture was concentrated under reduced pressure and treated with H
[
ACHTUNGTRENNUNG
[
2
2
2
O
[
(
40 mL) and the pH was adjusted to 9 by treatment with 10% aqueous
NaOH. The mixture was extracted with CHCl
layers were washed with saturated brine and H
3
(60 mL) the organic
O and dried. After re-
moval of the solvent and dried under vacuum, a yellow solid (178 mg)
3
038.
2
[
[
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.10 (m, 24H), 0.51–0.77 ppm (m, 20H); GPC (THF, linear polystyrene
3
, 500 MHz): d=8.0–8.3 ACHTUNTGENRNUG( m, 2H), 7.3–7.9-
ACHTUNGTRENNUNG
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50 mL) at 1308C for 3 days under argon in the presence of triethylamine
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(
(
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Received: March 16, 2011
Published online: July 29, 2011
Chem. Eur. J. 2011, 17, 10304 – 10311
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10311