Nonvolatile bistable resistive switching in a new polyimide bearing 9-phenyl-9H-carbazole pendant

Article abstract of DOI:10.1039/c1jm13849a

A new polyimide bearing the functional pendant 9-phenyl-9H-carbazole moieties, poly[2,2-(4,4′-di(N-benzyloxycarbazole)-3,3′-biphenylene) propane-hexafluoroisopropylidenediphthalimide] (6F-BAHP-PC PI), has been designed as a functional material for resista

Full text of DOI:10.1039/c1jm13849a

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PAPER
Nonvolatile bistable resistive switching in a new polyimide bearing
-phenyl-9H-carbazole pendant†
9
ab
ab
ab
ab
ab
ab
ab
Benlin Hu, Fei Zhuge,* Xiaojian Zhu, Shanshan Peng, Xinxin Chen, Liang Pan, Qing Yan
ab
and Run-Wei Li*
Received 9th August 2011, Accepted 23rd September 2011
DOI: 10.1039/c1jm13849a
0
A new polyimide bearing the functional pendant 9-phenyl-9H-carbazole moieties, poly[2,2-(4,4 -di(N-
0
benzyloxycarbazole)-3,3 -biphenylene)propane-hexafluoroisopropylidenediphthalimide] (6F-BAHP-
PC PI), has been designed as a functional material for resistance memory devices. The ITO/6F-BAHP-
PC PI/Ag memory device exhibits nonvolatile resistive switching (RS) with a high ON/OFF ratio
6
4
(
>10 ), long retention time (>6 ꢀ 10 s), good endurance, and low power consumption (ꢁ100 mW).
In situ conductive atomic force microscopy measurements show that the RS of 6F-BAHP-PC PI
originates from the formation/rupture of nanoscale conducting filaments.
Introduction
PI containing both electron donor and acceptor moieties
within a repeating unit has been found to exhibit bistable RS
attributed to an electronic transition process between the ground
and excited states, which could be achieved by the induced charge
Over the past decade, resistance random access memory
(
RRAM) based on resistive switching (RS) has attracted much
attention as a next generation nonvolatile memory due to its
excellent properties, for instance, simple structure, mass storage,
high operation speed, and compatibility with traditional CMOS
16–28
transfer from donor to acceptor under applied electric fields.
However, the endurance behavior of the PI-based RRAM
devices has rarely been explored. Furthermore, no direct exper-
imental evidence has been given for the RS mechanism of PIs. On
the other hand, carbazole is a well-known hole-transporting unit
and electron donor unit. Carbazole derivatives exhibit high
charge mobility and a large band gap attributed to the improved
technology. Since the RS phenomenon was first discovered in the
1
/Al sandwiched structure in 1962, a large number of
Al/Al
2
O
3
inorganic materials have been reported to show RS, such as
2
chalcogenides, complex oxides, binary oxides, and amor-
9
phous carbon. Recently, some polymer systems have also been
3–5
6–8
47
planar biphenyl unit by the bridging nitrogen atom. Polymers
1
0–15
found to exhibit RS effect, including conjugated polymers,
with carbazole unit in the main chain or as a pendant group have
been widely investigated due to their unique properties for many
48
1
6–28
functional polyimides (PIs),
polymers with specific pendent
2
9–32
chromophores,
and polymer nanocomposites (embedded
30
fullerene, carbon nanotubes,
). Compared to chalcogenides and oxides
potential applications, such as photoconduction, electrolumi-
3
3–36
37–39
with metal nanoparticles,
48–50
17,29,44,51,52
53
and photorefraction materials.
nescence,
memory
4
0–42
or graphene oxide
17
Ree and co-workers reported a PI functionalized with 9H-
carbazole-9-ethanol, showing RS behavior. However, the
conjugate region of the functional pendant 9H-carbazole-9-
ethanol is not large enough and consequently can not support
a stable highest occupied molecular orbital (HOMO) energy.
Herein, we report a PI bearing 9-phenyl-9H-carbazole
pendant, a better hole-transporting unit with larger conjugated
confines and better oxidative stability compared to 9H-carba-
which have been investigated quite extensively as RS materials,
polymeric materials are more attractive for RRAM applications
due to significant advantages, such as high scalability and highly
tailorable properties. And also, polymeric materials possess the
advantages of rich structure modifications, easy solution proc-
essability, flexibility, low-cost, high mechanical strength and
11,43–46
three-dimensional stacking capability.
1
7
0
0
zole-9-ethanol,
biphenylene)propane-hexafluoroisopropylidenediphthalimide]
6F-BAHP-PC PI), which shows excellent nonvolatile RS char-
poly[2,2-(4,4 -di(N-benzyloxycarbazole)-3,3 -
(
a
Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of
Material Technology and Engineering, Chinese Academy of Sciences,
Ningbo, 315201, China. E-mail: zhugefei@nimte.ac.cn; runweili@nimte.
ac.cn
acteristics. This PI is soluble in common organic solvents so that
the PI thin films can be easily prepared by a conventional spin-
coating method. The RRAM devices with this PI as storage
medium exhibit good memory characteristics, including high
ON/OFF ratios, low operation voltages, low power consump-
tion, long retention times, and good endurance. Conductive
atomic force microscopy (C-AFM) measurements show that the
b
Zhejiang Province Key Laboratory of Magnetic Materials and
Application Technology, Ningbo Institute of Material Technology and
Engineering, Chinese Academy of Sciences, Ningbo, 315201, China
†
1
Electronic supplementary information (ESI) available. See DOI:
0.1039/c1jm13849a
5
20 | J. Mater. Chem., 2012, 22, 520–526
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ꢃ
RS of 6F-BAHP-PC PI originates from the formation/rupture of
nanoscale conducting filaments. This study suggests that 6F-
BAHP-PC PI is a promising material for RRAM applications.
110 C for about 36 h. Then the mixture was cooled to room
temperature, and poured into 700 mL ice water. The precipitate
was filtered and dried. The pale yellow compound was crystallized
1
from acetone/water (v/v ¼ 10 : 1), yield 60%. H NMR (DMSO-
d
6
, d, ppm): 10.12 (s, 1H), 8.26 (d, 2H, J ¼ 8.0 Hz), 8.19 (d, 2H, J ¼
Experimental
8
.4 Hz), 7.90 (d, 2H, J ¼ 8.4 Hz), 7.52 (d, 2H, J ¼ 8.0 Hz), 7.46 (t,
Materials
2H, J ¼ 7.6 Hz), 7.33 (t, 2H, J ¼ 7.2 Hz); IR (KBr): 3418 (s), 3050
(
(
w), 1595(s), 1510 (m), 1482 (w), 1445 (s), 1360 (m), 1337 (m), 1221
m), 1157 (m), 1125 (m), 832 (m), 753 (s).
Carbazole, 4-fluorobenzaldehyde, potassium tert-butoxide and
2
,2-bis(4- hydroxyphenyl)propane were obtained from Aladdin
0
reagent Co. and used as received. 4,4 -(Hexafluoroisopropy-
lidene)diphthalic anhydride (6FDA) was obtained from
Changzhou Sunlight Pharmaceutical Co., Ltd. and sublimated
before use. 2,2-Bis(3-amino-4-hydroxyphenyl)propane (BAHP)
was synthesized and purified according to the method reported in
ref. 54. All the other reagents and solvents were analytical grade
and without further purification.
Synthesis of (4-(9H-carbazol-9-yl)phenyl)methanol (CPM, 2 in
Scheme 1). A solution of sodium borohydride (0.196 g, 5.2 mmol)
in 1 M aqueous NaOH (10.5 mL) was added dropwise to
a suspension of 4-carbazol-9-ylbenzaldehyde (2.71 g, 10 mmol) in
ethanol (35 mL) under nitrogen. After stirring for 4 h, most of the
insoluble starting material was reacted. The reaction mixture was
poured into dichloromethane (42 mL) and washed with water
(
3 ꢀ 40 mL). The organic layer was dried over magnesium sulfate
Instrumentation
and most of the solvent removed under reduced pressure. The
pure product CPM was obtained by crystallization from
1
1
The H nuclear magnetic resonance ( H NMR) spectra were
measured at 400 MHz on a Bruker 400 AVANCE III spec-
1
dichloromethane/n-hexane (v/v ¼ 1 : 1), yield 74%. H NMR
(
DMSO-d , d, ppm): 8.24 (d, 2H, J ¼ 7.6 Hz), 7.61 (d, 2H, J ¼ 8.4
trometer, using dimethyl sulfoxide-d
thylformamide-d (DMF-d ) as solvent. The Fourier transform
infrared (FT-IR) spectra were obtained with a Thermo Nicolet
700 FTIR spectrometer, where the polymer samples were films
6
(DMSO-d
6
) or N,N-dime-
6
Hz), 7.56 (d, 2H, J ¼ 8.4 Hz), 7.42 (t, 2H, J ¼ 7.4 Hz), 7.35 (d,
7
7
2
4
1
H, J ¼ 8.4 Hz), 7.28 (t, 2H, J ¼ 7.2 Hz), 5.37 (t, 1H, J ¼ 5.6 Hz),
.65 (d, 2H, J ¼ 5.6 Hz); IR (KBr): 3200 ꢁ 3600 (br), 1607 (w),
595 (w), 1517 (m), 1477 (m), 1450 (s), 1226 (m), 750 (s), 726 (s).
6
with thickness ꢁ2 mm. The UV-vis absorption spectra were
recorded on a Perkin Elmer lambda 950 spectrophotometer at
room temperature. The inherent viscosities were measured with
Synthesis of 6F-BAHP PI (Scheme 2). 6FDA (0.8887 g, 2
ꢃ
mmol) and BAHP (0.5167 g, 2 mmol) were dissolved together in
dry NMP (6 mL) with isoquinoline (0.45 mL) as a catalyst. The
an Ubbelodhe viscometer at 30 ꢂ 0.1 C in N-methyl-2-pyrroli-
ꢄ1
done (NMP) at a concentration of 0.5 g dL . Thermogravi-
ꢃ
reaction mixture was gently heated to 70 C under stirring for 2
metric analysis (TGA) was performed on a Perkin-Elmer
ꢄ1
ꢃ
h, followed by refluxing for 5 h. The reaction solution was diluted
with 18 mL NMP and then poured into a mixture of 100 mL
methanol and 100 mL water under vigorous stirring. The
Diamond TG/DTA instrument at a heating rate of 10 C min
ꢄ1
under nitrogen atmosphere with a flow rate of 50 mL min .
Differential scanning calorimetry (DSC) measurements were
ꢃ
precipitate was filtered, washed with hot methanol and dried in
1
vacuum, giving the target PI product. H NMR (DMF-d , d,
carried out with Mettler Toledo DSC at a heating rate of 10 C
ꢄ1
ꢄ1
min under a nitrogen atmosphere with 50 mL min gas flow,
where the glass transition temperature was reported as temper-
ature in the middle of the thermal transition from the second
heating scan. Gel permeation chromatography traces of the
samples were recorded on a Viscotek T60A/LR40 GPC system,
where chloroform was used as the eluent, and linear polystyrene
was used as the standard. Cyclic voltammetry (CV) measure-
ments were carried out in a 0.1 M solution of tetrabutylammo-
7
ppm): 10.14 (s, 2H, Ar–OH), 8.21 (d, 2H, Ar–H), 8.08 (d, 2H,
Ar–H), 7.95 (d, 2H, Ar–H), 7.47 (d, 2H, Ar–H), 7.20 (d, 2H, Ar–
H), 7.02 (d, 2H, Ar–H), 3.51 (s, 6H, CH
br), 1785 (m), 1721 (s), 1618 (m), 1513 (s), 1432(m), 1383 (s),
256 (s), 1110 (m), 722 (m).
3
). IR (film): 3200 ꢁ 3600
(
1
Synthesis of 6F-BAHP-PC PI (Scheme 2). The obtained 6F-
BAHP PI further reacted with the CPM. 6F-BAHP PI (0.6690
g, 1.0 mmol), CPM (1.0933 g, 4 mmol) and triphenyl phosphine
4 6
nium hexafluorophosphate (n-Bu NPF ) in acetonitrile under an
argon atmosphere with a platinum gauze as counter electrode
(
1.0492 g, 4 mmol) were dissolved in dry tetrahydrofuran
and an Ag/AgCl (3.8 M KCl) reference electrode. A scan rate of
ꢄ1
(THF) (10 mL) under nitrogen atmosphere. Diisopr-
opyl diazocarboxylate (0.8088 g, 4 mmol) in dry THF (2.5 mL)
was then added dropwise into the reaction mixture. The reac-
tion mixture was stirred at room temperature for 24 h. Then
1
00 mV s was used during the CV measurements. Atomic force
microscopy (AFM) characterization was conducted by a Veeco
Dimension 3100 V scanning probe microscope in ambient
conditions using a tapping or CAFM mode.
Synthesis of monomers and polyimides
Synthesis of 4-carbazol-9-ylbenzaldehyde (1 in Scheme 1).
Carbazole (8.40 g, 0.05 mol) and potassium tert-butoxide (5.61 g,
0
.05 mol) were mixed in 200 mL anhydrous N,N-dime-
ꢃ
thylformamide (DMF) and heated to 110 C, and then 4-fluo-
robenzaldehyde (6.20 g, 0.05 mol) dissolved in DMF was dropped
into the mixture over 50 min. The mixture was maintained at
Scheme 1 Synthesis of CPM monomer.
J. Mater. Chem., 2012, 22, 520–526 | 521
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1
Fig. 1 H NMR spectra of (a) 6F-BAHP PI and (b) 6F-BAHP-PC PI.
Results and discussion
Characterization of polymer structures
A new functional compound bearing 9-phenyl-9H-carbazole
moieties, CPM, was synthesized with a yield of 74% (Scheme 1).
As shown in Scheme 2, 6F-BAHP-PC PI was synthesized in
a two-step method. Firstly, soluble 6F-BAHP PI was synthesized
directly from the polycondensation of the respective monomers
Scheme 2 Synthesis of 6F-BAHP PI and 6F-BAHP-PC PI polymers.
1
using isoquinoline as a catalyst. Fig. 1(a) shows the H NMR
spectrum of as-grown 6F-BAHP PI (the original NMR spectrum
of 6F-BAHP PI is described in Fig. S1 in the ESI†). The proton
peak of the hydroxyl side groups can be observed at 10.14 ppm,
and the characteristic peaks of the aromatic rings at 7.02–
the reaction solution was diluted with 21 mL THF and poured
into 220 mL methanol under vigorous stirring, giving the target
polymer product as precipitate. The precipitate was filtered,
washed with hot methanol, and dried in vacuum. The obtained
8
.21 ppm and the proton peak of the isopropyl groups at 3.51
0
polymer was identified as poly[2,2-(4,4 -di(N-benzyloxy-
0
ppm are also observed. No characteristic peaks of amino protons
1
are detected in the H NMR suggesting that the product contains
carbazole)-3,3 -biphenylene)propane-hexafluoroiso propylide-
1
nediphthalimide] (6F-BAHP-PC PI) by H NMR spectroscopy.
a negligible amount of partially imidized 6F-BAHP poly(amic
acid). It proves that the 6F-BAHP PI has been successfully
synthesized. Secondly, 6F-BAHP-PC PI was obtained by incor-
porating CPM moieties into the soluble 6F-BAHP PI. As shown
1
H NMR (DMF-d , d, ppm): 8.23–7.23 (m, 36H), 5.34 (s, 4H,
7
2 3
CH ), 3.50 (s, 6H, CH ); IR (film): 1786 (m), 1728 (s), 1610 (w),
1
7
598 (w), 1517 (s), 1452(m), 1373 (s), 1257 (m), 1110 (m),
50 (m), 723 (m).
1
in Fig. 1(b), the H NMR spectrum of 6F-BAHP-PC PI does not
show any hydroxyl peak originating from the 6F-BAHP PI (the
original NMR spectrum of 6F-BAHP-PC PI is described in
Fig. S2 in the ESI†). Fig. 2 shows FT-IR spectra of 6F-BAHP PI
and 6F-BAHP-PC PI. The formation of polyimides can be
Fabrication and characterization of the memory device
ꢄ1
confirmed by the peaks at ꢁ1780 and 1727 cm , which are
The ITO coated glass substrates were pre-cleaned sequentially
with ethanol, acetone and isopropanol in an ultrasonic bath for
assigned to the vibrating peaks of imide rings. The IR spectrum
of 6F-BAHP-PC PI (Fig. 2(b)) shows double peaks at 750 and
ꢄ1
2
6
0 min, respectively. A 0.2 mL cyclopentanone solution of the
F-BAHP-PC PI (1.0 wt % polymer) was spin-coated onto the
7
23 cm , which are the characteristic peaks of ortho-position
aromatic protons of two phenyl of carbazole moieties. The IR
ITO substrates at a spinning rate of 400 rpm for 12 s and 2000
ꢃ
rpm for 40 s, followed by drying in vacuum at 80 C for 8 h.
Before spin-coating, the solution was filtered through poly-
tetrafluoroethylene (PTFE) membrane micro-filters with a pore
size of 0.45 mm. The PI film thickness was about 80 nm, measured
using a spectroscopic ellipsometer (model M2000DI, Woollam).
In order to measure the electrical properties, Ag top electrodes of
about 150 nm in thickness were deposited at room temperature
by magnetron sputtering system with an in situ metal shadow
mask. The current–voltage (I–V) characteristics of ITO/6F-
BAHP-PC PI/Ag structures were measured in ambient atmo-
sphere using a Keithley 4200 semiconductor characterization
system with a voltage sweeping mode. The sweeping step is
0
.01 V. During the measurement, a bias voltage was applied
between the top (Ag) and bottom (ITO) electrodes with the latter
being grounded.
Fig. 2 FT-IR spectra of (a) 6F-BAHP PI and (b) 6F-BAHP-PC PI films.
The inset is the enlarged spectra.
5
22 | J. Mater. Chem., 2012, 22, 520–526
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spectrum of 6F-BAHP PI (Fig. 2(a)) shows a broad peak in the
ꢄ1
BAHP-PC PI is a thermally stable polymer, which can be used in
the microelectronic industry.
range 3600–3200 cm , while this peak disappears in Fig. 2(b).
The above analysis indicates that the hydroxyl groups of the 6F-
BAHP PI have been completely replaced by CPM moieties. The
ꢄ1
Optical and electrochemical properties
6
F-BAHP-PC PI shows an inherent viscosity of 1.54 dL g in
Fig. 4(a) shows the UV-vis absorption spectra of the 6F-BAHP-
PC PI in NMP solution and thin film state. The polyimide
exhibits absorption peak maxima at around 293 and 301 nm in
solution and thin film states, respectively. The absorption peaks
are attributed to the p–p* transition delocalized along the
p-electronic system. The optical band gap of 6F-BAHP-PC PI is
estimated to be 3.54 eV from the onset optical absorbance. Fig. 4
NMP. The weight-average molecular weight of the 6F-BAHP-
PC PI is 568916 and the polydispersity index is 4.9 by GPC
analysis. Such polyimide exhibits good solubility in common
solvents, such as DMF, THF, chloroform, cyclohexanone. Fig. 5
(
a) shows the AFM image of as-grown 6F-BAHP-PC PI films.
The polyimide thin films have a relatively flat surface with
a surface roughness of about 0.85 nm from AFM analysis.
Therefore, high quality 6F-BAHP-PC PI thin films have been
prepared by means of a conventional spin-coating method.
(
b) shows the cyclic voltammogram of 6F-BAHP-PC PI thin
films with platinum bottom electrodes using a 0.1 M solution of
tetrabutylammonium tetrafluoroborate (TBATFB) in anhy-
drous acetonitrile. The new functional polyimide exhibits
reversible p-doping behavior during the anodic scan. The onset
Thermal properties
ox
oxidation potential (Eonset ) of 6F-BAHP-PC PI is around
The thermal properties of the new functional polyimides were
evaluated by TGA and DSC under nitrogen atmosphere, as
shown in Fig. 3. The 6F-BAHP-PC PI is found to have a degra-
1
.26 V vs. Ag/AgCl, which is higher than other triphenylamine
20,26
(
TPA)-based PIs.
Therefore, the HOMO and lowest unoc-
ꢃ
cupied molecular orbital (LUMO) energy levels of 6F-BAHP-PC
PI are ꢄ5.74 and ꢄ2.20 eV, respectively. These results indicate
that the new functional polyimide could provide a lower and
more stable HOMO energy level and therefore a charge trapping
environment, which may result in more stable RS.
dation temperature (Td 10%) of 435 C and a glass transition
ꢃ
temperature (T
g
) of 245 C. The results indicate that the 6F-
Memory device characteristics of the polymers
The schematic structure of the sandwiched devices is depicted in
Fig. 5(b). The memory effects of 6F-BAHP-PC PI films are
demonstrated by the I–V characteristics of ITO/6F-BAHP-PC
PI/Ag memory devices. The polymeric memory devices store
data based on the switching between high (OFF) and low (ON)
resistance states in response to the external applied voltages.
Fig. 5(c) shows a typical I–V characteristics of bistable ITO/6F-
BAHP-PC PI/Ag memory devices. As can be seen in Fig. 5(c), the
as-fabricated device is initially in the OFF state (‘‘0’’ signal in
Fig. 3 (a) TGA and (b) DSC curves of 6F-BAHP-PC PI under argon
ꢃ
ꢄ1
atmosphere at a heating rate of 10 C min
.
ꢄ12
ꢄ9
data storage) with a current in the range of 10 –10 A as the
voltage sweeps from 0 to about 1.4 V. As the voltage further
increases, an abrupt increase in the current occurs in the device
around 1.5 V (with a current compliance of 0.1 mA), indicating
that the device undergoes a sharp electrical transition from OFF
state to ON state (‘‘1’’ signal in data storage). This OFF-to-ON
transition can function in a memory device as a ‘‘writing’’ or
‘‘SET’’ process. The device will remain in the ON state, even after
Fig. 4 (a) UV-Vis spectra of 6F-BAHP-PC PI in NMP solution and thin
the power is turned off or during sequential forward voltage
sweeping with the current compliance maintained at 0.1 mA. By
film states. (b) Cyclic voltammograms of 6F-BAHP-PC PI films.
2
Fig. 5 (a) AFM image of 6F-BAHP-PC PI films with a scanning size of 10 ꢀ 10 mm . (b) A schematic configuration of the ITO/6F-BAHP-PC PI/Ag
devices. (c) I–V characteristics of ITO/6F-BAHP-PC PI/Ag memory cells.
This journal is ª The Royal Society of Chemistry 2012
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applying a positive voltage with a higher current compliance
1 mA), an abrupt decrease in the current occurs in the device at
complicated. Fitting results suggest that the charge transport
behavior is in good agreement with a classical trap-controlled
space charge limited current (SCLC), which consists of three
(
around 0.4 V, indicating that the device is switched from the ON
state to OFF state. This ON-to-OFF transition can function in
a memory device as an ‘‘erasing’’ or ‘‘RESET’’ process. It is
2
portions: the ohmic region (I f V), Child’s law region (I f V ),
55
and the steep current increase region. Therefore, the dominant
conduction mechanism of the OFF state is trap-controlled
SCLC. The total different conduction behavior in ON and OFF
states suggests that the high conductivity in ON state cells should
be a confined, filamentary effect rather than a homogenously
6
found that an ON/OFF current ratio of the device as high as 10
can be achieved, which leads to a low misreading rate for
memory applications. The switching power is about 100 mW. The
results indicate that ITO/6F-BAHP-PC PI/Ag memory devices
exhibit nonvolatile unipolar RS behavior.
9
distributed one.
In order to investigate the endurance performance of ITO/6F-
BAHP-PC PI/Ag memory devices, cyclic switching operations
have been conducted. Fig. 6(a) shows the evolution of the
resistance of the two well-resolved states over more than 50
cycles. The resistance values were read out at 0.1 V in each
voltage sweep. Although there is a slight fluctuation of the
resistance in the ON and OFF states, the resistance ratios are
more than six orders of magnitude during RS cycles. Fig. 6(b)
shows the retention performance of ITO/6F-BAHP-PC PI/Ag
devices under 0.1 V durable stress. The measurement was carried
out in ambient atmosphere at room temperature using a reading
voltage of 0.1 V. As can be seen from Fig. 6(b), the variation of
Ag, as we know, is easy to be ionized, and Ag ions are apt to
diffusing and forming Ag filaments as applying a positive voltage
bias on Ag electrodes. Therefore, the RS of the ITO/6F-BAHP-
PC PI/Ag devices may originate from the forming/rupture of Ag
filaments in the 6F-BAHP-PC PI. To ascertain whether the
observed RS behavior of the 6F-BAHP-PC PI device is from the
polymer layer itself or Ag filaments, a negative bias was applied
to the device, namely, the positive bias was applied to ITO
electrodes. The devices are found to switch from OFF to ON
state at about ꢄ1.37 V and from ON to OFF state at about
6
ꢄ0.6 V with an ON/OFF ratio of more than 10 , as shown in
Fig. 8(a). Furthermore, using an inert metal (Pt) as top elec-
trodes, a similar RS phenomenon has also been observed, as
shown in Fig. 8(b). Therefore, it can be confirmed that for the
ITO/6F-HAB-DPC PI/Pt devices, the RS is from the polymer
layer itself rather than Ag filaments.
4
ON and OFF state resistance after 6 ꢀ 10 s under 0.1 V durable
stress is very little, confirming the nonvolatile nature of the
devices.
To understand the conduction and switching mechanisms of
ITO/6F-BAHP-PC PI/Ag devices, the I–V curves of both SET
and RESET processes were replotted in a log-log scale, as shown
in Fig. 7. The ohmic conduction model is found to satisfactorily
fit the I–V curve for the ON state, indicating that ohmic
conduction is dominant in the ON state. On the other hand, the
conduction mechanisms of the OFF state are much more
In the 6F-BAHP-PC PI chain, pendant 9-phenyl-9H-carbazole
moieties are electron donors and act as nucleophilic sites,
whereas trifluoromethyl groups and imide rings are electron
acceptors and act as electrophilic sites. These two parts are linked
through a nonconjugational chain, so the intramolecular charge
transfer is minimized. Fig. 9 shows the energy level diagram of
Fig. 6 (a) The endurance performance and (b) retention for the ON and
Fig. 8 I–V characteristics of (a) ITO/6F-BAHP-PC PI/Ag memory cells
where a negative bias was applied to the devices and (b) ITO/6F-BAHP-
PC PI/Pt memory cells where a positive bias was applied to the devices.
OFF states of the ITO/6F-BAHP-PC PI/Ag memory cells.
Fig. 7 I–V curves of the ITO/6F-BAHP-PC PI/Ag memory cells plotted
on a log-log scale and the linear fitting results in both ON and OFF states
in the processes of (a) SET and (b) RESET. Also shown are the corre-
sponding slopes for different portions.
Fig. 9 Energy level diagram of ITO/6F-BAHP-PC PI/Ag device.
This journal is ª The Royal Society of Chemistry 2012
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24 | J. Mater. Chem., 2012, 22, 520–526

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ITO/6F-BAHP-PC PI/Ag devices. The energy barrier for hole
injection from the bottom electrode to the active PI layer was
estimated to be 0.94 eV according to the work function of ITO
and HOMO level of the active PI layer. The energy barrier for
electron injection from the top electrode to the active layer was
estimated to be 2 eV according to the work function of Ag and
LUMO level of the active PI layer. Thus, for the 6F-BAHP-PC
PI, the energy barrier for hole injection from ITO electrode is
much lower than that for electron injection from Ag electrode.
Therefore, the conduction process in the device is likely to be
dominated by hole injection. With an increase in voltage, holes
are injected from the ITO into the polymer gradually. These
charges can move by a hopping process through the polymer
chain. Charge trapping sites in the polymer can lead to charge
accumulation and then redistribution of the electric field. At the
threshold voltage under a current compliance of 0.1 mA, nearly
all pendant 9-phenyl-9H-carbazole moieties are filled with holes,
and then the charges will have high mobility, namely, conducting
filaments form. The charges can flow through the filaments and
sustain the ON state. When the current compliance increases to
touched the 6F-BAHP-PC PI films. The schematic measurement
configuration is shown in Fig. 10(a). The voltage was applied to
the bottom electrode (ITO). C-AFM measurements were per-
2
formed in the same region of 2 ꢀ 2 mm in size under various
external voltage biases. The initial resistance of the films is very
high, no obvious leakage can be observed under a voltage of 0–
6 V (Fig. 10(b)). Therefore, the pristine 6F-BAHP-PC PI is in the
OFF state. As shown in Fig. 10(c), when applying an 8 V bias,
several highly conductive spots can be observed in the C-AFM
current map, with diameters of about 30–50 nm. It indicates that
the films undergo electrical transition from the OFF state to the
ON state. The number and current height of the conductive pots
increase with the voltage, as shown in Fig. 10(d). The C-AFM
results confirm that the RS of 6F-BAHP-PC PI originates from
the formation/rupture of conducting filaments.
Conclusions
In this study, we have successfully synthesized a novel polyimide
(6F-BAHP-PC PI) functionalized by a stable hole-transporting
and donor unit bearing 9-phenyl-9H-carbazole pendant. This
polyimide exhibits good solubility in common organic solvents
and good thermal stability. The optical band gap of 6F-BAHP-
PC PI is 3.54 eV and the onset oxidation potential is 1.26 V,
indicating that such polyimide could provide a lower and more
stable HOMO energy. The ITO/6F-BAHP-PC PI/Ag memory
devices show nonvolatile unipolar RS with an ON/OFF ratio of
1
mA, a large number of charges flow through the previously
formed filaments, and this process might produce an excess of
heat. In addition, along with the increase of current through the
filaments, the number of charges injected into the polyimide film
is also increased and the charges can be trapped in the PI layer
and the interface between the PI and electrode. When the voltage
bias exceeds a certain value, the number of injected charges will
rise more than the capacity of the filaments. Such excessive
current flow will also generate additional heat. Meanwhile, the
excessive current flow may lead to repulsive coulombic interac-
tions between the trapped charges gathering in the polyimide
layer and in the interface. The generated heat and repulsive
coulombic interactions will cause rupture of the filaments formed
in the switch-ON process. As a result, the device is switched back
to its initial OFF state.
6
10 , power consumption of ꢁ100 mW, good endurance and
retention performance. The dominant conduction mechanisms in
the ON and OFF states are ohmic behavior and trap-controlled
SCLC, respectively. The RS of ITO/6F-BAHP-PC PI/Ag devices
is attributed to the formation/rupture of nanoscale conducting
filaments. The high ON/OFF ratio, good endurance and reten-
tion, and low power consumption of ITO/6F-BAHP-PC PI/Ag
devices open the door to the possibility of mass production of
high performance nonvolatile memory devices at low cost.
To provide direct experimental evidence of the filamentary
mechanism of the RS in 6F-BAHP-PC PI, in situ C-AFM
measurements have been performed. During the measurements,
the Pt/Ir coated conductive tip was grounded and directly
Acknowledgements
The authors acknowledge financial support from the Chinese
Academy of Sciences (CAS), State Key Project of Fundamental
Research of China (973 Program), the National Natural Science
Foundation of China, Zhejiang Qianjiang Talent Project, Zhe-
jiang and Ningbo Natural Science Foundations.
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