Nonvolatile electrical switching and write-once read-many-times memory effects in functional polyimides containing triphenylamine and 1,3,4-oxadiazole moieties

Article abstract of DOI:10.1021/ma1006446

This work reports the synthesis and characterization of a series of functional aromatic polyimides (OXTA-PI)s containing triphenylamine and 1,3,4-oxadiazole moieties. All the polyimides exhibit high glass transition temperatures of 309-319 °C. A resistive

Full text of DOI:10.1021/ma1006446

Macromolecules 2010, 43, 7159–7164 7159
DOI: 10.1021/ma1006446
Nonvolatile Electrical Switching and Write-Once Read-Many-Times
Memory Effects in Functional Polyimides Containing Triphenylamine
and 1,3,4-Oxadiazole Moieties
Kun-Li Wang,*,† Yi-Liang Liu,‡ Jian-Wei Lee,† Koon-Gee Neoh,‡ and En-Tang Kang‡
†Department of Chemical Engineering & Biotechnology, National Taipei University of Technology,
Taipei 10608, Taiwan, and ‡Department of Chemical & Biomolecular Engineering,
National University of Singapore, Kent Ridge, Singapore 119260
Received March 23, 2010; Revised Manuscript Received July 27, 2010
ABSTRACT: This work reports the synthesis and characterization of a series of functional aromatic
polyimides (OXTA-PI)s containing triphenylamine and 1,3,4-oxadiazole moieties. All the polyimides exhibit
high glass transition temperatures of 309-319 °C. A resistive switching device with the sandwich structure of
indium-tin oxide/polymer/Al was fabricated using the soluble polyimide from 4,40-hexafluoroisopropylidene-
diphthalic anhydride (OXTA-PIa). The device exhibits two conductivity states and can be switched from the
initial low-conductivity (OFF) state to the high-conductivity (ON) state at the threshold voltages of 1.8 V under
both positive and negative electrical sweeps, with an ON/OFF state current ratio in the order of 105. The ON
state of the device is nonvolatile and can withstand a constant voltage stress of -1 V for 6 h and 108 pulse read
cycles at -1 V under ambient conditions. Upon reversing the bias, the ON state cannot be reset to the initial
OFF states. The nonvolatile and inerasable nature of the ON state, as well as the ability to write, read, and
sustain the electrical states, fulfills the requirements of a write-once read-many-times (WORM) memory.
Introduction
1,3,4-oxadiazole moieties have been synthesized and character-
ized. An electronic device based on the triphenylamine and 1,3,4-
Conventional microelectronics is based, to a large extent, on
inorganic semiconductor integrated circuits. However, a number
of physical and economic factors have threatened the continuous
down scaling of silicon-based memory devices. These factors have
motivated research into memory devices based on organic
materials.1-3 Organic materials have certain attractive features
over the inorganic materials, including flexibility, miniaturized
dimensions, ease of processing, and the possibility for molecular
design through chemical synthesis. In comparison to memory
devices based on inorganic materials, polymer memories offer
advantages including low-cost potential, good processability,
good scalability, 3D stacking capability, large data storage
capacity, and device flexibility.4 In contrast to memory devices
based on inorganic materials, which store and retrieve data based
on the amount of charge stored in the device, organic memory
devices store data in another form, for instance, based on the
electrical bistability in response to an applied electric field. A
number of organic materials including conjugated molecules and
polymers,5-7 dopants,8 and complex systems9 have been investi-
gated for memory applications. Recently, polymer-based mem-
ory devices were reported to exhibit write-once read-many-times
(WORM),10,11 flash (rewritable),12-14 static random access,15,16
and dynamic random access17 memory effects. The materials,
devices, and mechanistic aspects of polymer electronic memories
have been reviewed recently.18
oxadiazole-containing polyimide derived from 4,40-hexafluoroi-
sopropylidenediphthalicanhydride (OXTA-PIa, structure shown
in Schemes 1 and 2) was fabricated and studied for its electrical
switching behavior under electrical sweeps. The device exhi-
bits nonvolatile and irreversible memory effects and fulfills the
requirements of a WORM memory.
Experimental Section
Materials. The materials hydroxylamine hydrochloride (from
Alfa Aesar), ammonium chloride (from Showa), sodium azide
(from Showa), 4-(diphenylamino)benzaldehyde (from Acros),
pyridine (from Echo), 3,5-dinitrobenzoyl chloride (from Acros),
hydrochloric acid (from Showa), hydrazine monohydrate (from
Alfa Aesar), 10% palladium on activated carbon (Pd/C, from
Merck), and tin(II) chloride anhydrous (from Sigma-Aldrich)
were used as received. The solvents N-methyl-2-pyrrolidinone
(NMP), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide
(DMSO), chloroform, and 1,4-dioxane were purchased from
Tedia, and tetrahydrofuran (THF), N,N-dimethylformamide
(DMF), methanol, ethanol, and acetic acid were purchased
from Echo. NMP was purified by distillation under reduced
˚
pressure over calcium hydride and stored over 4 A molecular
sieves. Acetic anhydride was purchased from Acros and used as
received. The aromatic tetracarboxylic dianhydrides, 4,40-hexa-
fluoroisopropylidenediphthalic anhydride (6FDA, from Chriskev),
4,40-sulfonyldiphthalic anhydride (SDPA, from TCI), 3,30,4,40-
benzophenonetetracarboxylic dianhydride (BTDA, from Chriskev),
4,40-oxidiphthalic anhydride (ODPA, from TCI), and pyromellitic
dianhydride (PMDA, from Chriskev) were sublimated before use.
Instrumentation and Measurements. Fourier transform infra-
red (FT-IR) spectra were recorded on a Perkin-Elmer GX FT-
IR spectrophotometer. NMR spectra were recorded using a
Bruker DRX-500 NMR (1H at 500.13 MHz and 13C at 125.76
MHz) spectrometer. Elemental analyses were performed on a
Over the years, polyimides have been of interest in microelec-
tronics because of their superior thermal and mechanical proper-
ties and low dielectric constants.19 Polyimides have also been
investigated for applications in photovoltaics,20,21 light-emitting
diodes,22,23 xerography,24 and polymer memories.25 In this work,
a series of functional polyimides containing triphenylamine and
*To whom all correspondence should be addressed: Tel 886-2-
27712171 ext 6411; e-mail klwang@ntut.edu.tw.
r 2010 American Chemical Society
Published on Web 08/12/2010
pubs.acs.org/Macromolecules

7160 Macromolecules, Vol. 43, No. 17, 2010
Wang et al.
Perkin-Elmer 2400 instrument. The inherent viscosity of poly-
imides was measured using a Tamson viscometer. Thermogravi-
metric analyses (TGA) were carried out on a Shimadzu DTG-
60/60H thermal analyzer under a nitrogen flowing rate of 50 cm3
min-1 and a heating rate of 10 °C min-1. Differential scanning
calorimetry (DSC) analyses were also performed on the Shi-
madzu DTG-60/60H thermal analyzer under a nitrogen flowing
rate of 30 cm3 min-1 and a heating rate of 10 °C min-1. Weight-
average (Mw) and number-average (Mn) molecular weights
were determined by gel permeation chromatography (GPC).
Four Waters (Ultrastyragel) columns (300 ꢀ 7.7 mm, guarded
and a platinum wire as the auxiliary electrode at a scan rate of
50 mV s-1 against a Ag/Agþ reference electrode in a 0.1 M
acetonitrile (CH3CN) solution of tetrabutylammonium per-
chlorate (TBAP).
Device Fabrication and Characterization. The ITO-coated
glass substrate was precleaned sequentially with deionized
water, acetone, and isopropanol in an ultrasonic bath for
15 min. A 100 μL DMAc solution of the functional polyimide
(10 mg mL-1) was spin-coated onto the ITO substrate at a
spinning rate of 1200 rpm, followed by solvent removal in a
vacuum oven at 10-5 Torr and 60 °C for 10 h. The thickness of
the polyimide film was about 50 nm, as determined by the AFM
edge profiling. Finally, Al top electrodes of about 400 nm in
thickness was thermally deposited onto the polymer surface
through a shadow mask at a pressure of about 10-7 Torr.
Electrical property measurements were carried out on devices
of 0.4 ꢀ 0.4 mm2, 0.2 ꢀ 0.2 mm2, and 0.15 ꢀ 0.15 mm2 in size,
under ambient conditions, using an Agilent 4155C semiconduc-
tor parameter analyzer equipped with an Agilent 41501B pulse
generator. The current density-voltage (J-V) data reported
were based on device units of 0.4 ꢀ 0.4 mm2 in size, unless stated
otherwise. ITO was maintained as the ground electrode during
the electrical measurements.
Synthesis of Monomers. N-(4-(5-(3,5-Dinitrophenyl)-1,3,4-
oxadiazol-2-yl)phenyl)-N-phenylbenzenamine (OXTAN). Com-
pounds 1 and 2 in Scheme 1 were prepared according to the
methods reported in the literatures.26,27 A mixture of 2.0 g
(6.38 mmol) of 4-tetrazolyltriphenylamine (2), 1.76 g (7.6 mmol)
of 3,5-dinitrobenzoyl chloride, and 20 mL of pyridine was
heated at 120 °C with stirring for 48 h in a 250 mL round-
bottomed flask. After cooling, the mixture was poured into
dichloromathane (100 mL), washed with water (30 mL ꢀ 3), and
dried over anhydrous MgSO4. The solvent was removed under
reduced pressure. The residue was recrystallized from chloro-
form to afford the pure product. The yield of the orange product
(OXTAN) was 2.0 g (65%); mp 193 °C. FT-IR (KBr, cm-1):
1537, 1340 (-NO2 stretch), 1592 (-CdN-). 1H NMR (CDCl3,
δ, ppm): 7.11 (d, J = 8.9 Hz, 2H), 7.20 (m, J = 7.5 Hz, 6H), 7.36
(t, J = 7.8 Hz, 4H), 7.96 (d, J = 8.9 Hz, 2H), 9.13 (t, J = 2 Hz,
1H), 9.2 (d, J = 1.9 Hz, 2H). Anal. Calcd for C26H17N5O5: C,
65.13%; H, 3.57%; N, 14.61%. Found: C, 0.65.19%; H, 3.64%;
N, 14.80%.
3
4
5
˚
and packed with 500, 10 , 10 , and 10 A porous gels in series)
were used for GPC analysis, using THF at a flow rate of 1 mL
min-1 as the eluent. The eluents were monitored with a RI
detector (JASCO Systems, RI-2031, Japan). Monodispersed
polystyrene samples were used as the molecular weight stan-
dards. UV-vis absorption spectra were measured on a Cary
UV 500 UV-vis-NIR spectrophotometer. Photoluminescence
(PL) spectra were measured on a Perkin-Elmer LS 55 lumines-
cence spectrophotometer. The thickness of the polymer film cast
on the indium-tin oxide (ITO)-coated glass substrate was
determined from the edge profile of the film, using the tapping
mode, on a Veeco multimode atomic force microscope (AFM)
equipped with a Nanosensors PPP-NCHR silicon tip. Cyclic
voltammetry (CV) was performed on a CHI model 619A
electrochemistry workstation with ITO as the working electrode
Scheme 1. Synthesis of the New Diamine Monomer OXTA
5-(5-(4-(Diphenylamino)phenyl)-1,3,4-oxadiazol-2-yl)benzene-
1,3-diamine (OXTA). A mixture of 1.0 g (2.08 mmol) of nitro
compound (OXTAN), 0.3 g of 10% Pd/C, 4 mL of hydrazine
monohydrate, and 40 mL of ethanol was refluxed with stirring
for 2 h in a 250 mL three-necked flask. The mixture was then
filtered to remove Pd/C, followed by solvent removed in a
rotary evaporator to afford the pure product. The yield of
Scheme 2. Synthesis of Polyimides (OXTA-PIs) via a Two-Step Condensation and Chemical Imidization Reaction

Article
Macromolecules, Vol. 43, No. 17, 2010 7161
the pale yellow product (OXTA) was 0.74 g (88%); mp 245 °C.
FT-IR (KBr, cm-1): 3446, 3380, 3323, 3213 (-NH2 stretch),
Table 1. Solubility of the OXTA-PI Functional Polyimidesa
solubilityb
1
1592 (-CdN-). H NMR (DMSO- d6, δ, ppm): 5.10 (s, 4H,
polymer code NMP DMAc DMSO DMF THF CHCl3 1,4-dioxane
NH2), 6.02 (d, J = 1.71 Hz, 1H), 6.52 (d, J = 1.8 Hz, 2H), 7.03
(d, J = 8.8 Hz, 2H), 7.17 (m, J = 6.2 Hz, 6H), 7.39 (t, J = 7.8
Hz, 4H), 7.87 (d, J = 8.8 Hz, 2H). 13C NMR (DMSO-d6, δ,
ppm): 102.46, 149.87, 100.79, 124.02, 164.51, 163.30, 115.58,
127.73, 120.47, 150.31, 146.09, 125.55, 129.88, 124.64. Anal.
Calcd for C26H17N5O: C, 74.44%; H, 5.05%; N, 16.70%.
Found: C, 74.47%; H, 4.96%; N, 16.73%.
Synthesis of Polymers (Scheme 2). To a stirring solution of
0.2 g (0.477 mmol) of OXTA in 2.0 mL of NMP, 0.21 g
(0.477 mmol) of 4,40-hexafluoroisopropylidenediphthalic anhy-
dride (3a) was gradually added. The mixture was stirred at
ambient temperature for 4 h to form the poly(amic acid)
(OXTA-PAA). Chemical imidization was carried out by addition
of 1.0 mL of acetic anhydride and 0.5 mL of pyridine into the
above-mentioned poly(amic acid) solution, followed by heating
at 120 °C for 3 h. The polymer solution was poured slowly into
300 mL of methanol. The precipitate (OXTA-PIa) was removed
by filtration, washed with hot methanol, and dried at 100 °C
under reduced pressure. FT-IR (film, cm-1): 1785 (asymmetrical
CdO stretch), 1730 (symmetrical CdO stretch), and 1351 cm-1
(C-N stretch). 1H NMR(DMSO-d6, δ, ppm): 7.07-6.93(d, 8H),
7.29 (s, 4H), 7.58 (s, 3H), 7.93-7.68 (m, 7H), 8.06-8.03 (t, 2H),
8.29-8.18 (d, 4H).
OXTA-PIa þþ
þþ
(
þþ
(
þþ þþ þþ
þþ
(
OXTA-PIb
OXTA-PIc
OXTA-PId
OXTA-PIe
þ
(
(
-
þ
(
-
-
-
(
-
-
-
(
-
-
(
(
-
-
-
-
-
-
-
a Abbreviations: NMP: N-methyl-2-pyrrolidinone; DMAc: N,N-di-
methylacetamide; DMF: N,N-dimethylformamide; DMSO: dimethyl
sulfoxide; THF: tetrahydrofuran. b Solubility: measure in the concen-
tration of 1 mg mL-1; þþ, soluble at room temperature; þ, soluble on
heating; (, partially soluble on heating; -, insoluble on heating.
Table 2. Thermal Properties of the OXTA-PI Functional Polyimides
Td at 5%
weight loss
in N2 (°C)b
Td at 10%
weight loss
in N2 (°C)b
char yield
in N2 (%)c
polymer code
Tga (°C)
OXTA-PIa
OXTA-PIb
OXTA-PIc
OXTA-PId
OXTA-PIe
317
317
319
309
442
440
428
431
440
476
472
464
463
471
27
43
35
12
29
d
-
a Glass transition temperature (Tg) measured by DSC at a heating rate
of 10 °C min-1
.
b Decomposition temperature recorded on DTG at a
c Char yield (%) at 800 °C. d Not observed.
Other polyimides, OXTA-PIb, OXTA-PIc, OXTA-PId, and
OXTA-PIe, in Scheme 2, were prepared by similar procedures
from OXTA and the corresponding dianhydrides, SDPA (3b),
BTDA (3c), ODPA (3d), and PMDA (3e).
heating rate of 10 °C min-1
.
ments of each proton are also given in the spectrum, and the
chemical shifts are consistent with the proposed polymer
structure. The polyimide OXTA-PIa obtained by chemical
imidization had an inherent viscosity of 0.25 dL g-1 in
DMAc. The number-average molecular weight (Mn) and
weight-average molecular weight (Mw) of OXTA-PIa were
2.7 ꢀ 104 and 5.5 ꢀ 104, respectively, giving rise to a poly-
dispersity index (PDI = Mw/Mn) of 2.03. The molecular
weights of other polyimides could not be measured by GPC
due to their poor solubility.
Polymer Properties. The solubility of the synthesized
polyimides in common organic solvents is shown in Table 1.
The solubility of polyimides is dependent on their chain
packing efficiency and intermolecular interactions, which in
turn are influenced by the rigidity, symmetry, and regularity
of the molecular backbone.28 Among these polyimides, those
derived from more rigid dianhydrides, OXTA-PIb, OXTA-
PIc, OXTA-PId, and OXTA-PIe, had poor solubilities. The
polyimide OXTA-PIa exhibits the best solubility in a variety
of solvents, such as NMP, DMAc, DMF, DMSO, THF,
chloroform, and 1,4-dioxane, at room temperature. Many
studies have reported that the presence of hexafluoroisopro-
pylidene groups in 6FDA introduces steric hindrance that
prohibits close packing of the chains and increases the affinity
of the chains for polar solvents.29
Thermal Properties. The thermal properties of the poly-
imides were evaluated by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA), and the
results are summarized in Table 2. The glass transition
temperatures (Tg) of the polyimides were found to range
from 309 to 319 °C. The polyimide OXTA-PId obtained
from ODPA (3d) showed the lowest Tg (309 °C) because of
the presence of a flexible ether linkage between the phthali-
mide units. OXTA-PIa exhibits a much higher glass transi-
tion temperature than that of the previously reported
P(BPPO)-PI15 (structure shown in Figure 1a) with a similar
molecular structure (317 °C for OXTA-PIa vs 250 °C for
P(BPPO)-PI), probably due to the more rigid molecular
chain in OXTA-PIa in the absence of the flexible phenoxyl
Results and Discussion
Monomer Synthesis. The synthesis route of 5-(5-(4-
(diphenylamino)phenyl)-1,3,4-oxadiazol-2- yl)benzenediamine
monomer (OXTA) is shown in Scheme 1. 4-Tetrazolyltriphenyl-
amine (2) reacts with 3,5-dinitrobenzoyl chloride to afford
the dinitro compound (OXTAN). The FT-IR spectrum of
OXTAN showed characteristic bands of nitro groups at 1340
and 1537 cm-1. Reduction of OXTAN in ethanol with
hydrazine monohydrate in the presence of a catalytic amount
of Pd/C at the reflux temperature produced the new com-
pound (OXTA). The FT-IR and NMR spectroscopic data
confirmed the structure of OXTA. The characteristic absorp-
tions of nitro groups at 1340 and 1537 cm-1 disappeared, and
new absorptions at 3239 and 3348 cm-1 (N-H stretching)
appeared in the FT-IR spectrum. The NMR spectra with
peak assignments, including 1H NMR, 13C NMR, and two-
dimensional COSY and HMQC spectra, of OXTA are shown
in Figure S1 (Supporting Information). When OXTAN was
reduced to a diamine, a chemical shift at 5.10 ppm, character-
1
istic of amino groups, appeared in the H NMR spectrum.
These results confirm that the compound (OXTA) synthe-
sized herein is consistent with the proposed structure.
Synthesis of Polymers. The functional polyimides OXTA-
PIa to OXTA-PIe in Scheme 2 were prepared via the con-
ventional two-step method by reacting equimolar of the novel
diamine (OXTA) with the respective aromatic dianhydrides
(3a-3e) to form the poly(amic acid)s at ambient temperature,
followedby chemical cyclodehydration (Scheme 2). Chemical
imidization of the poly(amic acid)s with a dehydrating agent,
such as a mixture of acetic anhydride and pyridine, was also
effective in preparing the polyimides. The polymer structures
were confirmed by FT-IR and NMR spectroscopies. All
the polyimides exhibited characteristic absorption bands
of the imide ring near 1785 (asymmetric CdO stretch), 1730
(symmetric CdO stretch), and 1351 cm-1 (C-N stretch).
1
The H NMR spectrum of the polyimide, OXTA-PIa, is
shown in the Supporting Information (Figure S2). Assign-

7162 Macromolecules, Vol. 43, No. 17, 2010
Wang et al.
gap energy (Eg) of OXTA-PIa is determined to be 3.05 eV.
The PL spectra of OXTA-PIa (6.7 ꢀ 10-7 mol L-1) and the
diamine monomer OXTA in chloroform are shown in
Figure 1b. The concentration of OXTA was adjusted to be
comparable to that of the repeat units of OXTA-PIa. The
emission spectra were obtained with an excitation wave-
length of 280 nm. As shown in Figure 1b, the OXTA
monomer exhibits an intense emission peak at 436 nm, which
is red-shifted by about 100 nm in comparison to the emission
of 2,5-diphenyl-1,3,4-oxadiazole.33 This large red shift arises
probably from the intense CT between the triphenylamine
and oxadiazole groups in OXTA. The fluorescent quantum
yield (ΦF) of the monomer (OXTA) in chloroform was
estimated by comparison with the PL of standard 9,10-di-
phenylanthracene. The fluorescent quantum yield of OXTA
was 23.6%. The fluorescence of OXTA-PIa, however, was
significantly quenched, indicating a considerable decay
of the excited singlet state of the OXTA moieties in the
polymer due to CT between the OXTA and phthalimide
moieties.34
Electrochemical Properties. The electrochemical proper-
ties of the polyimide OXTA-PIa were investigated by cyclic
voltammetry conducted in a 0.1 M dry acetonitrile solution
of TBAP under a nitrogen atmosphere for the film deposited
on a platinum disk electrode. The polymer exhibited an
oxidation peak at 0.60 V, with the onset located at 0.49 V.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels can be
calculated from the onset oxidation potential [EOx(onset)]
and the band gap of the copolymer based on the reference
energy level of ferrocene (4.8 eV below the vacuum level,
which is defined as zero)
HOMO ¼ - ½ðEOxðonsetÞ - EFOCÞ þ 4:8ꢁ ðeVÞ
LUMO ¼ HOMO þ Eg ðeVÞ
ð1Þ
Figure 1. (a) Normalized UV-vis absorption spectrum of OXTA-PIa
in chloroform with that of P(BPPO)-PI as the reference. (b) Emission
spectrum of OXTA-PIa in chloroform with that of the diamine mono-
mer, OXTA, as the reference.
According to these formulas, the HOMO and LUMO energy
levels for OXTA-PIa are calculated to be -5.18 and
-2.13 eV, respectively.
Electrical Switching Behavior of the Functional Polyimide.
Electrical switching and memory effects of OXTA-PIa were
tested by examining the current density-voltage (J-V)
characteristics of an ITO/OXTA-PIa/Al sandwich device,
as shown in Figure 2a. Initially, the as-fabricated device was
in the low-conductivity (OFF) state. The current density
remained low during the first negative sweep (with Al as the
cathode and ITO as the anode) from 0 to -3 V until the
switching threshold voltage of -1.8 V was reached. At this
voltage, the current density increased abruptly from 10-7 to
10-2 A cm-2, indicating the device transition from the initial
OFF state to the high-conductivity (ON) state. This electrical
transition is equivalent to the “writing” process in a digital
memory cell. The ON/OFF current ratio was in the order of
105 at -1 V. The device exhibited good stability in the ON
state during the subsequent negative and positive sweeps,
and it did not return to the OFF state even after turning off
the power (the second sweep) or upon applying a reverse
sweep (the third sweep). The nonvolatile and inerasable
nature of the ON state suggests that the OXTA-PIa device
exhibits WORM type memory.
group.30 The thermal stability of the polyimides was eval-
uated by TGA under a nitrogen atmosphere at a heating rate
of 10 °C min-1. The temperatures for 10% weight loss of the
polyimides in nitrogen range from 463 to 476 °C. The char
yields of the polyimides at 800 °C in nitrogen range from 12
to 43 wt %.
Optical Properties. The optical properties of the poly-
imide OXTA-PIa were investigated by UV-vis absorption
and photoluminescence (PL) spectroscopies in chloroform.
Figure 1a shows the UV-vis absorption spectra of OXTA-
PIa in chloroform, with P(BPPO)-PI15 as the reference. The
concentration of P(BPPO)-PI was about 5 ꢀ 10-7 mol L-1
.
The concentration of OXTA-PIa was adjusted to make the
number of its repeat units comparable to those of P(BPPO)-
PI. The absorption spectra are normalized to the maximum
absorption peak of P(BPPO)-PI for ease of comparison. As
shown in Figure 1a, the P(BPPO)-PI solution exhibits a
maximum absorption peak at 301 nm, which is attributed
to the π f π* transition of the π-electronic system deloca-
lized along the 2,5-bis(4-phenoxyphenyl)-1,3,4-oxadiazole
(BPPO) moieties.31 In comparison to P(BPPO)-PI, an addi-
tional triphenylamine group is incorporated into the OXTA-
PIa molecule, resulting in significant enhancement of the
intramolecular charge transfer (CT) and thus inducing a
CT absorption band at 364 nm.31 The absorption edge of
OXTA-PIa extends to about 407 nm, from which the band
In addition to the electrical switching effect, other para-
meters, such as stability and read cycles, are of equal
importance to the performance of a memory device. All of
these parameters were evaluated under ambient condi-
tions. Figure 2b shows the effect of operation time on the
memory device. Under a constant stress of -1 V, no obvious
degradation in current density was observed for both the ON

Article
Macromolecules, Vol. 43, No. 17, 2010 7163
Figure 2. (a) Current density-voltage (J-V) characteristics of OXTA-PIa in a 0.4 ꢀ 0.4 mm2 ITO/polymer/Al device with an initial negative electrical
sweep. The sequence and direction of each sweep are indicated by the respective number and arrow. (b) Effect of operation time on the ON and OFF
states of the memory devices under a constant stress of -1 V. (c) Effect of read pulse of -1 V on the ON and OFF states of the memory device.
(d) Current density-voltage (J-V) characteristics of OXTA-PIa in a 0.4 ꢀ 0.4 mm2 ITO/polymer/Al device with an initial positive electrical sweep.
and OFF states of the present device. The ON and OFF
states are also stable up to 1 ꢀ 108 read pulses of -1 V, as
shown in Figure 2c. These results indicate that these memory
devices exhibit good stability and performance. Devices with
different active areas of 0.4 ꢀ 0.4 mm2, 0.2 ꢀ 0.2 mm2, and
0.15 ꢀ 0.15 mm2 showed almost the same J-V character-
istics, indicating that the current density was independent of
the device area. The area-independent current density, good
reproducibility of the J-V characteristics, and stability of
the memory effects rule out the possibility of filament
conduction35 or polymer degradation effects in the present
device.
The memory device based on OXTA-PIa can also exhibit
electrical switching if the initial electrical sweep is positive. As
shown in Figure 2d, the device was initially swept positively
from 0 to 3 V. An abrupt increase in the current density was
observed at the switching threshold voltage of 1.8 V, which is
comparable in magnitude to the negative switching threshold
voltage of -1.8 V. The results indicate that the OXTA-PIa
device can be switched bidirectionally, with comparable posi-
tive and negative switching threshold voltages.
Figure 3. J-V characteristics of an ITO/OXTA-PIa/Al device. The
sequence and direction of each sweep are indicated by the respective
number and arrow. The 4th-6th sweeps were conducted sequentially
after heating the device at 150 °C for 10 min under vacuum.
Forrest et al. reported WORM memory devices based on
poly(ethylene dioxythiophene) (PEDOT):poly(styrenesulfonic
acid) (PSS) films.10,11 The switching of the PEDOT:PSS film
from a high to a low conductivitystate has been explained by a
simple undoping process. Thermal effects arising from Joule
heatingplaya significant role in destabilizing the PEDOT:PSS
complex formed by CT interaction.11 To further understand
the switching phenomenon in the present OXTA-PIa, at-
tempts were made, in vain, to heat a device in the ON state
to above Tg. At temperature above Tg, the polymer softened
and the device failed due to the difference in expansion
coefficients between the polymer and electrodes. To avoid
device degradation, a device in the ON state was heated at
150 °C for 10 min. The switching behavior of the device after
heating is shown in Figure 3. The device in the initial OFF
state was switched to ON state at -1.8 V and retained in the
ON state as described above (first to third sweeps). Interest-
ingly, after being heated at 150 °C for 10 min and then cooled
to room temperature, the device in the ON state was found
to return to its OFF state (the fourth sweep). The device could
be switched again to the ON state at the threshold voltage of

7164 Macromolecules, Vol. 43, No. 17, 2010
Wang et al.
-1.7 V and remained in the ON state during the subsequent
negative and positive sweeps. It could not be returned to the
OFF state even after turning off the power (the fifth sweep) or
upon applying a reverse sweep (the sixth sweep). The results
indicate that the turn-on phenomenon probably do not
originate from chemical reactions.
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while the triphenylamine group is a strong electron donor.37
Thus, under an electric field, electrons can transfer from
the triphenylamine donor to the phthalimide acceptor, via
the oxadiazole mediator group, to give rise to an abrupt
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This study reports the synthesis of a series of aromatic poly-
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groups. All the polyimides exhibit good thermal stability. The
polyimide from 4,40-hexafluoroisopropylidenediphthalic anhy-
dride (OXTA-PIa) also exhibits good solubility and thus proces-
sability. Bistable electrical switching devices based on the OXTA-
PIa thin films exhibit nonvolatile and irreversible WORM
memory effects with symmetric bidirectional switching thresh-
old voltages of (1.8 V. The electric field-induced charge
transfer from the triphenylamine donor to the phthalimide
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Acknowledgment. The authors thank the National Science
Council (NSC) of Taiwan for the financial support (NSC98-
2221-E-027-002) and Taipei Medical University and National
Taiwan Normal University for the NMR measurements.
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Supporting Information Available: Additional data, 1H
NMR, 13C NMR, and two-dimensional COSY and HMQC
spectra of the novel diamine OXTA (Figure S1), 1H NMR
spectrum of OXTA-PIa (Figure S2), and GPC elution curve of
the soluble polyimide OXTA-PIa (Figure S3). This material is
available free of charge via the Internet at http://pubs.acs.org.
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