New donor-acceptor oligoimides for high-performance nonvolatile memory devices

Article abstract of DOI:10.1021/cm201665g

We report the synthesis, optoelectronic properties, and electrical switching memory characteristics of three new donor-acceptor oligoimides consisting of the electron-donating moieties (triphenylamine or carbazole) and electron-withdrawing phthalimide moieties. The influence of different donor (D)-acceptor (A) arrangements, including D-A-D and A-D-A structures, on the electrical properties was explored. Devices based on D-A-D oligoimides revealed a reversible nonvolatile negative-differential-resistance (NDR) characteristic and excellent stability during operation. Without applying voltage stress, the on and off states of the devices showed no obvious degradation for an operation time of 10 s and 10⁸ read pulses. However, the devices prepared from the A-D-A oligoimide showed only the insulating properties. The different memory characteristic was probably because the terminal donor moieties in the D-A-D structure might facilitate the injection and transporting of the holes. Besides, the D-A-D oligoimide with triphenylamine groups exhibited an on/off ratio of 10⁴, 2 orders of magnitude higher than that with carbazole groups. The mechanism related to electrical switching properties was elucidated through molecular simulation. Thus the significance of D-A-D structure on tuning memory characteristics for memory device applications was revealed.

Full text of DOI:10.1021/cm201665g

ARTICLE
pubs.acs.org/cm
New DonorꢀAcceptor Oligoimides for High-Performance Nonvolatile
Memory Devices
||
Wen-Ya Lee,†,‡,|| Tadanori Kurosawa,§, Shiang-Tai Lin,† Tomoya Higashihara,§ Mitsuru Ueda,*,§ and
Wen-Chang Chen*,†,‡
†Department of Chemical Engineering and ‡Institute of Polymer Science and Engineering, National Taiwan University,
Taipei 106, Taiwan
§Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japan
S Supporting Information
b
ABSTRACT: We report the synthesis, optoelectronic properties, and electrical switching
memory characteristics of three new donorꢀacceptor oligoimides consisting of the electron-
donating moieties (triphenylamine or carbazole) and electron-withdrawing phthalimide
moieties. The influence of different donor (D)ꢀacceptor (A) arrangements, including D-A-
D and A-D-A structures, on the electrical properties was explored. Devices based on D-A-D
oligoimides revealed a reversible nonvolatile negative-differential-resistance (NDR) character-
istic and excellent stability during operation. Without applying voltage stress, the on and off
states of the devices showed no obvious degradation for an operation time of 10 s and 108 read
pulses. However, the devices prepared from the A-D-A oligoimide showed only the insulating
properties. The different memory characteristic was probably because the terminal donor
moieties in the D-A-D structure might facilitate the injection and transporting of the holes. Besides, the D-A-D oligoimide with
triphenylamine groups exhibited an on/off ratio of 104, 2 orders of magnitude higher than that with carbazole groups. The
mechanism related to electrical switching properties was elucidated through molecular simulation. Thus the significance of D-A-D
structure on tuning memory characteristics for memory device applications was revealed.
KEYWORDS: oligoimide, donorꢀacceptor, memory, NDR, resistor type
’ INTRODUCTION
mainly focused on the effects of the donor/acceptor chemical
structures of polyimides on the electrical switching behavior.
However, the imide molecules for the memory device application
are rarely studied.
Organic materials have been extensively investigated recently
for data storage devices due to low cost, structural flexibility, and
three-dimensional stacking capability.1 A wide range of organic
materials containing electron-donating (D) and electron-accept-
ing (A) groups, including functional polyimides,2 conjugated
polymers,3 conjugated diblock copolymers,4 nonconjugated
polymers with pendant conjugated donor/acceptor moieties,5
oligomers,6 polymerꢀgraphene complex,7 polymer/fullerene
blends,8 and organic molecules blended with nanoparticles9 have
been reported for the application of memory devices. Charge
transfer between donor and acceptor moieties of the polyimides
enhanced the conductivity of the thin films and was responsible
for the resulting memory characteristics.1a,2aꢀ2e Switching be-
tween high- and low-conductive state (on and off) of donorꢀ
acceptor polymers under applied voltages is important for data
storage devices.
Donorꢀacceptor polyimides are one of the most promising
candidates for memory device applications, since they not only
have good electrical properties for charge storage but also possess
excellent thermal stability (higher than 500 °C) and chemical
etching resistance. Various memory types based on the poly-
imides were demonstrated, including dynamic random access
memory (DRAM),2d,i static random access memory (SRAM),2a,f
write-once-read-many (WORM),2c,j and flash memory.2g Most
studies on the memory behaviors of imide-based materials
The electrical switching behaviors of several D-A oligomers
have been reported in the literature.6 Tetracyanoquinodi-
methane (TCNQ)ꢀCu complexes, in which TCNQ and Cu
were used as acceptor and donor moieties, respectively, exhibited
electrical switching.6a This switching was elucidated by the
electrochemical formation of the Cu filaments.6b In comparison
to the heterogeneous organicꢀmetal complexes, all organic D-A
molecules could provide more uniform properties.6cꢀe Song
and co-workers6d employed the molecules containing electron-
donating triphenylamine and electron-accepting cyanovinyl
moieties for the memory device applications, in which tripheny-
lamine provided a better hole transporting ability for improv-
ing the on/off current ratios. Later, they demonstrated that the
different architecture of the cyanide-substituted triphenylamine
molecules affected the on/off ratio and reversibility of their
memory devices significantly.6e The devices prepared from
the D-π-A-π-D molecules performed good write-read-erase
characteristics.6e In contrast, the A-π-D-π-A molecules exhibited
Received: June 13, 2011
Revised:
September 1, 2011
Published: September 21, 2011
r
2011 American Chemical Society
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Chemistry of Materials
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a relatively low on/off ratio and irreversible switching behavior. It
was suggested that the arrangement of the donor and acceptor
moieties affected the electrical switching characteristics
significantly.
poured into 1 M hydrochloric acid solution to give a pale greenish-
yellow solid. The crude product was purified by recrystallization from
toluene and hexane to give pale greenish-yellow crystals (4.50 g) in 78%
yield. IR (KBr) ν (cmꢀ1) 1592, 1330 (NO2 stretching). 1H NMR (300
MHz, DMSO-d6, δ, ppm) 8.48 (d, J = 8.7 Hz, ArH, 2H), 8.25 (d, J = 8.1
Hz, ArH, 2H), 7.93 (d, J = 9.0 Hz, ArH, 2H), 7.54 (d, J = 8.4 Hz, ArH,
2H), 7.46 (t, J = 6.9 Hz, ArH, 2H), 7.34 (t, J = 7.2, ArH, 2H).
Synthesis of 4,40-Dinitrotriphenylamine (DNTPA). DNTPA
was synthesized according to a similar procedure as NTPA, from N-
(4-nitrophenyl)aniline2f and 4-fluoronitrobenzene. The product was
purified byrecrystallizationfrom ethanol and water. Theyield of DNTPA
was 92%. IR (KBr) ν (cmꢀ1) 1581, 1342 (NO2 stretching). 1H NMR
(300 MHz, DMSO-d6, δ, ppm) 8.19 (d, J = 9.3 Hz, ArH, 4H), 7.51 (t, J =
7.5 Hz, ArH, 2H), 7.37 (t, J = 6.9 Hz, ArH, 2H), 7.27 (d, J = 7.5 Hz, ArH,
2H), 7.20 (d, J = 9.0 Hz, ArH, 4H).
Synthesis of 4-Aminotriphenylamine (ATPA). NTPA (1.45 g,
5.00 mmol) and palladium on activated carbon (Pd/C) (0.0725 g) was
dissolved in ethanol (10 mL) and the solution was refluxed. Hydrazine
monohydrate (1.5 mL) was added dropwise, and the mixture was stirred
at this temperature for 12 h. The reaction solution was immediately
filtered through Celite before cooling down to room temperature due to
the low solubility of ATPA in ethanol. The filtrate was concentrated and
poured into water. The precipitate was collected by filtration and
purified by recrystallization from ethanol to yield white crystals (0.726
g) in 56% yield. IR (KBr) ν (cmꢀ1) 3460, 3375 (NꢀH stretching). 1H
NMR (300 MHz, DMSO-d6, δ, ppm) 7.19 (t, J = 8.1 Hz, ArH, 2H),
6.91ꢀ6.85 (m, ArH, 6H), 6.80 (d, J = 8.4 Hz, ArH, 2H), 6.57 (d, J = 8.7
Hz, ArH, 2H), 5.09 (s, NꢀH, 2H).
In this paper, we report the synthesis, optoelectronic proper-
ties, and electrical switching memory characteristics of three new
donorꢀacceptor oligoimides, containing the electron-donating
moieties triphenylamine or carbazole and electron-withdrawing
phthalimide moieties. Both triphenylamine and carbazole groups
were explored intensively for data storage devices, since they
possessed excellent hole-transporting abilities. The rigid bridge
of carbazole may induce different molecular conformations and
electrical performance from the triphenylamine. The memory
device was fabricated on the indium tin oxide (ITO) coated
glass with the configuration ITO/oligoimides/Al. The influence
of two donorꢀacceptor arrangements (D-A-D and A-D-A
structures) on the electrical properties was also studied and
elucidated through molecular simulation. The experimental
results suggested that the D-A-D oligoimide-based devices
revealed the reversible nonvolatile negative-differential-resis-
tance (NDR) characteristics with on/off ratios of 102ꢀ104 and
excellent stability during operation. However, the devices pre-
pared from the A-D-A oligoimide showed only the insulating
properties. This study provided new strategies for the molecular
design of donorꢀacceptor oligomers for advanced memory
applications.
Synthesis of N-(4-Aminophenyl)carbazole (APC). To a
refluxed solution of NPC (1.44 g, 5.00 mmol) and Pd/C (0.0720 g)
in ethanol (10 mL) was added dropwise hydrazine monohydrate
(1.5 mL). The mixture was refluxed for 4 h and cooled to room
temperature. Pd/C was removed by filtration through Celite and the
filtrate was concentrated to give APC as a clear viscous liquid (1.24 g) in
96% yield. IR (KBr) ν (cmꢀ1) 3460, 3379 (NꢀH stretching). 1H NMR
(300 MHz, DMSO-d6, δ, ppm) 8.19 (d, J = 8.1 Hz, ArH, 2H), 7.40 (t, J =
7.5 Hz, ArH, 2H), 7.27ꢀ7.21 (m, ArH, 4H), 7.18 (d, J = 9.0 Hz, ArH,
2H), 6.81 (d, J = 8.4 Hz, ArH, 2H), 5.35 (s, NꢀH, 2H).
’ EXPERIMENTAL SECTION
Materials. 4-Fluoronitrobenzene and 4-trifluoromethylphthalic
acid were purchased from SigmaꢀAldrich Corp. and used as received.
Tetrabutylammonium perchlorate (TBAP, from TCI) was recrystallized
twice from ethyl acetate and then dried in vacuo prior to use.
4,40-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA, from
TCI) was purified by sublimation. Dehydrated tetrahydrofuran
(THF), dehydrated N,N-dimethylacetamide (DMAc), anhydrous po-
tassium carbonate, acetic anhydride, and cyclohexane were purchased
from Wako, Japan. Dehydrated THF (Wako, stabilizer-free, 99.5%) was
distilled from its sodium naphthalenide solution under nitrogen. All
other compounds were purchased from TCI and used without further
purification.
Synthesis of 4-Nitrotriphenylamine (NTPA). Diphenylamine
(1.69 g, 10.0 mmol) was added to a solution of sodium hydride (0.375 g,
15.6 mmol) in dehydrated DMAc (10 mL). The solution was stirred at
room temperature for 30 min and cooled to 0 °C. 4-Fluoronitrobenzene
(1.69 g, 12.0 mmol) in dehydrated DMAc (10 mL) was added dropwise
to this mixture. After the addition was complete, the mixture was stirred
at 100 °C for 1 h. Then the mixture was cooled to room temperature and
poured into a cold dilute hydrochloric acid solution. The crude product
was collected by filtration and recrystallized from 2-propanol and water
to give orange crystals (2.79 g) in 92% yield. IR (KBr) ν (cmꢀ1) 1581,
1315 (NO2 stretching). 1H NMR (300 MHz, DMSO-d6, δ, ppm) 8.07
(d, J = 9.3 Hz ArH, 2H), 7.45 (t, J = 7.8 Hz, ArH, 4H), 7.31ꢀ7.25
(m, ArH, 6H), 6.80 (d, J = 9.6 Hz, ArH, 2H).
Synthesis of 4,40-Diaminotriphenylamine (DATPA). Start-
ing from DNTPA, DATPA was synthesized by a similar procedure as
ATPA. The crude product was purified by recrystallization from ethanol
and water in 77% yield. IR (KBr) ν (cmꢀ1) 3428, 3347 (NꢀH
1
stretching). H NMR (300 MHz, DMSO-d6, δ, ppm) 7.04 (t, J = 8.7
Hz, ArH, 2H), 6.80 (d, J = 8.4 Hz, ArH, 4H), 6.65ꢀ6.57 (m, ArH, 3H),
6.53 (d, J = 8.7 Hz, ArH, 4H), 4.98 (s, NꢀH, 4H).
Synthesis of 4-Trifluoromethylphthalic Anhydride (3FA).
4-Trifluoromethylphthalic acid (7.02 g, 30.0 mmol) was dissolved in
acetyl chloride (15 mL) and the mixture was refluxed for 16 h. After
the reaction, acetyl chloride was distilled off and the residual solid
was purified by sublimation to yield a white crystal (6.17 g) in 95% yield.
IR (KBr) ν (cmꢀ1) 1867, 1774 (CdO stretching), 918 (CꢀCOꢀ
OꢀCOꢀC stretching). 1H NMR (300 MHz, DMSO-d6, δ, ppm)
8.51 (s, ArH, 1H), 8.37 (d, J = 7.8 Hz, ArH, 1H), 8.29 (d, J = 8.1 Hz,
ArH, 1H).
Synthesis of 4,40-Hexafluoroisopropylidenebis[4-(N,N-di-
phenylamino)phenyl phthalimide] (ATPA-6FDA). 6FDA
(0.555 g, 1.25 mmol) was added to a solution of ATPA (0.651 g, 2.50
mmol) in dehydrated THF (3 mL) under nitrogen. The solution was
stirred for 1 h and then THF was evaporated. The residual solid was
dissolved in acetic anhydride (5 mL) and refluxed for 4 h. After cooling
to room temperature, the precipitate was collected by filtration and
washed with water to give an orange powder (1.02 g) in 88% yield. The
obtained product was used without further purification. IR (KBr) ν
Synthesis of N-(4-Nitrophenyl)carbazole (NPC). A 100-mL
two necked flask equipped with a magnetic stirrer, a nitrogen inlet, a
DeanꢀStark trap, and a condenser was charged with carbazole (3.34 g,
20.0 mmol), K2CO3 (3.04 g, 22.0 mmol), dehydrated DMAc (40 mL),
and cyclohexane (15 mL). The mixture was heated to 120 °C for 4 h
under nitrogen to facilitate dehydration. After complete removal of
water, the residual cyclohexane was distilled off and 4-fluoronitroben-
zene (2.33 mL, 22.0 mmol) was then added. The mixture was heated to
160 °C and maintained at this temperature for 2 h. Then the mixture was
1
(cmꢀ1) 1781, 1720 (CdO stretching), 1381 (CꢀN stretching). H
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Scheme 1. Synthetic Routes for ATPA, APC, DATPA, and 3FA
NMR (300 MHz, DMSO-d6, δ, ppm) 8.17 (d, J = 8.7 Hz, ArH, 2H), 7.94
(d, J = 8.1, ArH, 2H), 7.74 (s, ArH, 2H), 7.37ꢀ7.30 (m, ArH, 12H),
7.12ꢀ7.04 (m, ArH, 16H). 13C NMR (75 MHz, DMSO-d6, δ, ppm)
167.0, 166.9, 148.5, 148.3, 148.2, 147.9, 133.9, 133.5, 130.5, 129.0, 128.8,
126.5, 125.5, 125.1, 124.5, 123.3, 94.95. Anal. Calcd for C55H34N4F6O4:
C, 71.1; H, 3.69; N, 6.03. Found: C, 70.7; H, 3.85; N, 5.98.
75 MHz for 13C nuclei with deuterated dimethyl sulfoxide (DMSO-d6)
as the solvent and tetramethylsilane as the reference. Fourier transform
infrared (FT-IR) spectra were measured by a Horiba FT-120 Fourier
transform spectrophotometer. Elemental analyses were performed on a
Yanaco MT-6 CHN recorder elemental analysis instrument. Thermal
properties were estimated from a Seiko TG/DTA 6300 thermogravi-
metric analysis system (TGA) and a TA Instruments DSC-Q100
differential scanning calorimeter (DSC) under a nitrogen atmosphere
at a heating rate of 10 and 6 °C/min, respectively. Cyclic voltammetry
was performed with the use of a three-electrode cell in which ITO
(polymer film area was about 0.7 ꢁ 0.5 cm2) was used as a working
electrode. A platinum wire was used as an auxiliary electrode. All cell
potentials were taken with the use of a Ag/AgCl, KCl (saturated)
reference electrode. UVꢀvis absorption spectra were measured with a
Hitachi U4100 UVꢀvisꢀNIR spectrophotometer. The thickness of the
polymer film was measured with a microfigure measuring instrument
(Surfcorder ET3000, Kosaka Laboratory Ltd.). Atomic force micro-
scopy (AFM) measurements were obtained with a NanoScope IIIa AFM
(Digital Instruments, Santa Barbara, CA) at room temperature. Com-
mercial silicon cantilevers (Nanosensors, Germany) with typical spring
Synthesis of 4,40-Hexafluoroisopropylidenebis[4-(carbazol-
9-yl)phenyl phthalimide] (APC-6FDA). APC-6FDA was synthe-
sized according to a similar procedure as ATPA-6FDA. The yield
of APC-6FDA was 74%. IR (KBr) ν(cmꢀ1) 1785, 1724 (CdOstretching),
1
1357 (CꢀN stretching). H NMR (300 MHz, DMSO-d6, δ, ppm)
8.26ꢀ8.23 (m, ArH, 6H), 8.03 (d, J = 7.8 Hz, ArH, 2H), 7.88 (s, ArH,
2H), 7.81 (s, ArH, 8H), 7.48ꢀ7.46 (m, ArH, 8H), 7.35ꢀ7.29
(m, ArH, 4H). 13C NMR (75 MHz, DMSO-d6, δ, ppm) 167.7, 166.9,
166.8, 157.2, 155.7, 151.0, 148.4, 141.6, 141.0, 137.6, 133.6, 131.6,
129.8, 128.0, 127.2, 123.9, 121.4, 121.1, 110.6. Anal. Calcd for
C55H30N4F6O4: C, 71.4; H, 3.27; N, 6.06. Found: C, 71.2; H, 3.34;
N, 5.99.
Synthesis of N,N0-[(Phenylimino)di-4,1-phenylene]bis-
(5-trifluoromethyl phthalimide) (APTA-3FA). To a solution of
DATP (1.11 g, 4.03 mmol) in dehydrated THF (20 mL) was added
4-trifluoromethylphthalic anhydride (1.94 g, 9.00 mmol). The mixture
was stirred for 1 h and then THF was evaporated. The residual solid was
dissolved in 1,3-dimethyl-2-imidazolidinone (DMI) (10 mL) and
refluxed for 4 h. After cooling to room temperature, the solution was
poured into water. The precipitate was collected by filtration and excess
3FA was removed by sublimation to give 2.61 g of an orange powder
(2.61 g) in 96% yield, mp 219 °C (DSC peak). IR (KBr) ν (cmꢀ1) 1782,
1724 (CdO stretching), 1384 (CꢀN stretching). 1H NMR (300 MHz,
DMSO-d6, δ, ppm) 8.28ꢀ8.26 (m, ArH, 4H), 8.17 (d, J = 8.4 Hz, ArH,
2H), 7.43ꢀ7.38 (m, ArH, 6H), 7.20ꢀ7.17 (m, ArH, 7H). 13C NMR (75
MHz, DMSO-d6, δ, ppm) 166.3, 166.2, 147.2, 135.5, 134.8, 134.4, 133.1,
132.0, 130.2, 128.7, 126.5, 125.6, 124.8, 123.7, 120.4. Anal. Calcd for
C36H19N3F6O4: C, 64.39; H, 2.85; N, 6.28. Found: C, 64.47; H, 3.03;
N, 6.28.
constants of 15 N mꢀ1 were used to operate the AFM in tapping mode.
3
Fabrication and Characterization of Memory Devices. The
memory device was fabricated on ITO-coated glass with the configura-
tion ITO/oligoimides/Al. Before deposition of the organic layer, the
ITO glass was precleaned by ultrasonication with water, acetone, and
2-propanol, each for 15 min. The layers of the oligoimides were vapor-
deposited at a rate of 0.5 Å sꢀ1 under a pressure of around 1 ꢁ 10ꢀ6
3
Torr to their target thickness (80 nm) as determined in situ by a
calibrated quartz crystal microbalance (QCM). A 300-nm-thick Al
electrode, 0.5 ꢁ 0.5 mm2, was thermally evaporated through the shadow
mask at a pressure of 8 ꢁ 10ꢀ7 Torr with a uniform depositing rate of
2 Å sꢀ1. The electrical characteristics of memory device were measured
3
with a Keithley 4200 semiconductor parametric analyzer. All electronic
measurements were performed in a N2-filled glovebox.
Computational Methodology. Theoretical molecular simula-
tion of the oligoimides were calculated through the Gaussian 03 program
package.10 Density functional theory (DFT) method, using Becke’s
Characterizations. NMR spectra were recorded on a Bruker DPX-
300S spectrometer at the resonant frequencies at 300 MHz for 1H and
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Scheme 2. Synthetic Routes for ATPA-6FDA, APC-6FDA, and ATPA-3FA and Their Memory Devices
three-parameter functional with the Lee, Yang, and Parr correlation
functional method (B3LYP) with 6-31G(d), was exploited for the
optimization of ground-state molecular geometry, electrostatic potential
(ESP), and electronic properties.
’ RESULTS AND DISCUSSION
Synthesis of Oligoimides. The synthesis of amino com-
pounds (ATPA, APC, and DATPA) is shown in Scheme 1. First
the precursory nitro compounds (NTPA, NPC, and DNTPA)
were prepared by aromatic nucleophilic substitution reactions
between 4-fluoronitrobenzene and diphenylamine, carbazole, or
N-(4-nitrophenyl)aniline.2f Then NTPA, NPC, and DNTPA
were converted to ATPA, APC, and DATPA, respectively, by
Pd-catalyzed reduction. The disappearance of characteristic
absorption peaks around 1580 and 1330 cmꢀ1 due to the nitro
groups and the appearance of characteristic peaks around 3450
and 3360 cmꢀ1 assigned to the amino groups in the FT-IR
spectra indicate complete reduction of NTPA, NPC, and
DNTPA to ATPA, APC, and DATPA, respectively. The success-
ful synthesis of these amino compounds was also confirmed by
1H NMR. The signals at 5.09, 5.35, and 4.98 ppm were assigned
to the amino groups of ATPA, APC, and DATPA, respectively.
3FA was synthesized by treatment of 4-trifluoromethylphthalic
acid with acetyl chloride, as shown in Scheme 1. Formation of
3FA was also confirmed by the FT-IR and 1H NMR spectra.
Figure 1. FT-IR spectra of ATPA-6FDA, APC-6FDA, and ATPA-3FA.
A series of imide compounds (ATPA-6FDA, APC-6FDA, and
ATPA-3FA) were prepared by the condensation reaction of
amino compounds (ATPA, APC, and DATPA) and acid anhy-
drides (6FDA and 3FA) in two steps, as shown in Scheme 2.
First, the amino compounds and acid anhydrides were mixed in
THF under nitrogen to yield amic acid solutions. Then, chemical
imidization in acetic anhydride was carried out for the prepara-
tion of ATPA-6FDA and APC-6FDA. Both imide compounds
were obtained as precipitates and used without further purifica-
tion. For preparation of ATPA-3FA, thermal imidization was
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performed in DMI because of high solubility of ATPA-3FA in
acetic anhydride, and excess 3FA was removed by sublimation.
The desirable structures of all imide compounds were confirmed
by FT-IR spectra, which showed the characteristic peaks of imide
groups at around 1780 cmꢀ1 (asymmetrical stretching of carbo-
nyl groups) and 1720 cmꢀ1 (symmetrical stretching of carbonyl
groups), and the CꢀN bond at around 1380 cmꢀ1, as presented
in Figure 1. Moreover, the formation of ATPA-6FDA, APC-
6FDA, and ATPA-3FA was confirmed by 1H NMR, 13C NMR,
and elemental analysis. The chemical shifts and peak integration
of NMR spectra shown in Figure 2 are consistent with the
proposed structure. Also, the experimental carbon, hydrogen,
and nitrogen contents are in a good agreement with the
theoretical contents. The above results suggested the successful
preparation of the proposed oligoimides.
Characterization of D-A Oligoimides. The thermal, photo-
physical, and electrochemical properties of D-A oligoimides
are listed in Table 1. The thermal decomposition temperatures
(Td, 5% weight loss temperatures) (Figure S1a, Supporting
Information) of ATPA-6FDA, APC-6FDA, and ATPA-3FA were
459, 526, and 381 °C, respectively, indicating good thermal
stability of the D-A oligoimides for memory device applications.
ATPC-6FDA exhibited the highest Td value, compared to the
other oligoimides, which was probably attributed to the rigid
rings within the carbazole moieties. The higher Td of ATPA-
6FDA than of ATPA-3FA was probably due to the former having
more triphenylamine moieties, enhancing the thermal stability.
Among these oligoimides, only ATPA-3FA showed the crystal-
line behaviors, as shown in Figure S1b in Supporting Informa-
tion. The crystalline and melting temperatures of ATPA-3FA
were observed at 176 and 219 °C, respectively, as shown in
Supporting Information. This crystallinity of ATPA-3FA might
arise from its bending structure. From the Gaussian calculation,
the dipole moments of ATPA-6FDA, APC-6FDA, and ATPA-
3FA were 1.54, 0.52, and 1.92 D. The bending structure of
ATPA-3FA possessed higher polarity and probably resulted in a
stronger intermolecular interaction, facilitating its molecular
packing.
Figure 2. 1H NMR spectra of (a) ATPA-6FDA, (b) APC-6FDA, and
(c) ATPA-3FA.
Figure 3. (a) UVꢀvis absorption spectra and (b) energy diagram of
D-A oligoimide thin films.
Table 1. Thermal, Optical, and Electrochemical Properties of the Prepared D-A Oligoimides
oligoimide
Td (°C)
Tc (°C)
Tm (°C)
absorption λmax (nm)
band gapa (eV)
HOMO (eV)
LUMOb (eV)
ATPA-6FDA
APC-6FDA
ATPA-3FA
459
526
381
300, 333
294, 318c, 333c
3.28
3.51
3.25
ꢀ5.21
ꢀ5.53
ꢀ5.32
ꢀ1.93
ꢀ2.02
ꢀ2.07
176
219
299,333
a Estimated from the onset of absorption. b LUMO = HOMO + band gap. c Absorption shoulders.
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Figure 4. Electrical switching characteristics of (a) ATPA-6FDA and (b) APC-6FDA, including IꢀV curves, retention time, write-read-erase-read
cycles, and pulse reading.
Figure 3a shows the UVꢀvis absorption spectra of the three
D-A oligoimides in thin films on quartz substrates. The band gaps
of ATPA-6FDA, APC-6FDA, and ATPA-3FA estimated from the
onset of the absorption spectra were 3.28, 3.51, and 3.25 eV,
respectively. The absorption maxima at 294 (4.21 eV) and
300 nm (4.13 eV) are respectively attributed to the πꢀπ*
transition of carbazole (HOMO f LUMO5) (4.66 eV) and
triphenylamine moieties (HOMO f LUMO4) (4.59 eV),
according to the theoretical calculation shown in Figures S2
and S3 (Supporting Information). The absorption band from
318 to 333 nm arose from the transition of charge transfer from
the donor to acceptor moieties. The absorption of the D-A-D
triphenylamine-based oligomers at 333 nm is stronger than that
of the A-D-A oligoimides, implying that the D-A-D arrangement
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provides a stronger probability of charge transfer than the A-D-A
one. Thus, the D-A-D molecules probably have a higher possi-
bility of charge transfer with neighboring molecules when the
external voltage is applied.
The electrochemical properties of the oligoimides were mea-
sured by cyclic voltammetry (CV) (see Figure S4 in Supporting
Information). The oxidation behaviors of the oligoimides were
obtained in anhydrous acetonitrile with tetrabutylammonium
perchlorate as electrolyte. The highest occupied molecular
orbital (HOMO) energy levels of the oligoimides were calcula-
ted from the cyclic voltammetry with reference to ferrocence
(4.8 eV) by the following equation: HOMO = ꢀ(Eonset + 4.8 ꢀ
Eferrocene), summarized in Table 1 and Figure 3b. The HOMO
levels of ATPA-6FDA, APC-6FDA, and ATPA-3FA are ꢀ5.21,
ꢀ5.53, and ꢀ5.32 eV, respectively. The lowest unoccupied
molecular orbital (LUMO) levels are not available from CV,
and thus the values of the LUMO levels were estimated from the
difference between the optical band gaps and the HOMO level.
Among the oligoimides, the HOMO level of ATPA-6FDA is the
closest to the work function of ITO glass. This indicates that the
barrier of the hole injection is the smallest, as compared to the
other molecules.
Memory Characteristics of the D-A Oligoimides. The
resistor-type memory of the D-A oligoimides was fabricated in
a simple sandwich configuration, Al/oligoimide/ITO. Figure 4
shows the IꢀV electrical switching of the memory devices based
on D-A-D oligoimides. The ATPA-6FDA device (Figure 4a) was
initially in a low-conductance state (off state) as the voltage swept
from 0 to 3 V. The current increased abruptly as a high-
conductance state (on state) when the voltage was around 3 V,
defined as the switching threshold voltage. As the voltage
increased continuously from the threshold voltage to 4.5 V
(Vmax), the current increased. When the voltage swept to a
high-voltage region around 8 V (Vmin), the current decreased
significantly as an off state. The writing and erasing process could
be manipulated by applied Vmax and Vmin to the devices in the
unipolar direction of voltage sweeping, which is defined as the
negative-differential-resistance (NDR) behavior. The NDR be-
havior is an electrical property of devices where increased
voltages induce a decrease in the current.4,9b This NDR switching
can produce repeatable switching many times in the same device.
At least 20 devices were measured from five different batches and
showed stable NDR characteristics. The stability of the memory
effect was evaluated by testing the retention time, pulse reading,
and write-read-erase-read (WRER) cycles, as shown in Figure 4.
The retention time of the devices based on the D-A-D oligomers
can maintain the on and off states longer than 104 s without
constant stress applied in the testing. This excellent electrical
stability indicates that the type of device based on the D-A-D
oligoimides is a nonvolatile memory. After pulse reading of 108
cycles at 0.5 V, no obvious degradation in the current was
observed for the on and off states. Thus, both on and off states
were stable in the long-term testing and repetitive operation
cycles of the reading pulse. Programmable cyclic duration was
tested by repeating the WRER cycles. The writing, reading, and
erasing were set to voltages of 4, 0.5, and 10 V, respectively. The
duration of each sequential step was about 4 s. The responding
on and off current showed good stability in the WRER testing.
The stable performance of the D-A-D oligoimides provided a
promising commercial potential for a wide variety of memory
applications, such as a portable disk, smart label, and radio-frequency
identification (RFID) tags.
Figure 5. Electrical switching performance of ATPA-3FA with thermal
treatment.
In comparison to ATPA-6FDA, APC-6FDA (Figure 4b) also
showed the NDR behaviors. However, it required a relatively
high voltage of 4.4 V to turn on the devices in the initial sweeping.
This is presumably related to its low-lying HOMO level of
5.53 eV, resulting in a large energy barrier of ca. 0.5 eV for the
hole injection. After the initial sweeping, the turn-on voltage of
APC-6FDA was reduced to 3.2 V, indicating that probably some
charges were trapped within the active layer, facilitating the
charge injection. Triphenylamine-based ATPA-6FDA exhibited
excellent unipolar electrical switching with an on/off ratio of up
to 104, while carbazole-based APC-6FDA showed only a poor
on/off ratio of 101ꢀ102. This is probably because the planar and
rigid structure of carbazole moieties of APC-6FDA facilitate
stacking with the adjacent carbazole moieties, resulting in en-
hanced charge hopping and an increase in the off current. The
stacking of carbazole units is evidenced by the photoluminescence
(PL) spectrum of APC-6FDA, as shown in Figure S5b of the
Supporting Information.
It should be noted that only the D-A-D molecules could
perform the electrical switching from a low-conducting state
(off) to a high-conductance state (on), while the A-D-A mol-
ecules could not show any electrical bistability (2 in Figure 5).
This suggests that there is a large barrier for charge injection in
the molecules with the A-D-A arrangement. The work function
of ITO (5.0 eV) is close to the HOMO levels of the oligoimides,
while Al electrode (4.3 eV) has a large mismatch of energy,
ca.2 eV, with the LUMO levels. Thus, hole injection occurs prior
to electron injection, when a driving voltage is applied on the
devices. In the case of A-D-A structure, it is hard for the donor to
transport charges with neighboring molecules owing to the end-
capping acceptor 6FDA, with a low-lying HOMO of ꢀ7.8 eV.2i
Therefore, it is difficult for the oligoimides with the A-D-A
structure to have a channel for hole carriers.
We found that thermal annealing significantly affected the
electrical properties and surface morphology of ATPA-3FA.
Thermal annealing can induce the crystalline behavior of
ATPA-3FA. The electrical properties of ATPA-3FA showed a
huge change from an insulator to a conductor after the device was
annealed, as shown in Figure 5. A very large current was
observed, and this may result from the formation of deep grain
boundaries. During the annealing, the ATPA-3FA molecules
aggregated to form a terrace morphology, as shown in Figure 6.
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Figure 6. AFM height image of ATPA-3FA: (i) without thermal treatment; (ii, iii) with thermal annealing at 180 °C for 30 min in different scales; (iv)
3-dimensional images for the surface with thermal annealing.
The boundaries of the terrace morphology provided an effective
pathway for the diffusion of the aluminum atoms. Therefore,
formation of the filaments induced a very high current when the
device was operating. The surface morphology of the vapor-
deposited thin films of ATPA-6FDA and APC-6FDA according
to AFM scanning (Figure S6, Supporting Information) showed a
smooth and continuous surface with root-mean-square rough-
ness of 0.41 and 0.39 nm, respectively. This suggests that the
electrical performance of the D-A-D oligoimides may be not
attributed to the formation of the aluminum filament, because
there is no effective pathway for diffusion of the aluminum atoms.
Besides, unlike the large influence of thermal annealing on
electrical switching of the A-D-A oligoimides, the D-A-D mol-
ecules did not exhibit significant change with thermal annealing
(Figure S7, Supporting Information). This is probably related to
the lack of crystalline or melting temperatures in the DSC scans.
Therefore, thermal annealing does not obviously affect the device
performance of the D-A-D materials.
B3LYP/6-31G(d) level. HOMO orbitals are localized at the
donor moieties regardless of triphenyamine or carbazole groups,
while LUMO orbitals are localized at the acceptor moieties,
6FDA. This implies that the charge transfer between donor and
acceptor moieties occurs in the excited state of the oligoimides.
Under excitations with sufficient energy, electrons are possibly
transferred from the donor moieties to the electron-withdrawing
6FDA group, while the conjugated donor moieties give a channel
for the hole transporting, improving the conductivity of the
oligoimides. Since the oligoimides are small molecules and not
fully conjugated, the intermolecular hopping of charge carrier
between neighboring donor groups dominates the charge trans-
port. It is found that the oligoimides have negative ESP regions
mainly arising from the oxygen atoms in phthalimide groups. ESP
is defined as the potential energy arising from charges at the
specific location.2a The negative ESP region means that this
region acts as a trap localizing the charge carrier, leading to charge
retention and memory effects.3b This supports that electrons
tend to transfer to phthalimide groups from the donor moieties
under excitation.
Molecular Simulation of the Oligoimides. The molecular
conformation and electronic properties were explored through
density functional theory. Figure 7 shows the optimized geome-
try, molecular orbitals and electrostatic potentials (ESP) of
ATPA-6FDA and APC-6FDA, which were calculated at the
The electron of the HOMO orbital of APC-6FDA is mainly
localized on the carbazole moieties and less localized on the benzene
ring between the carbazole moieties and the electron-accepting
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Figure 7. Molecular orbitals of the HOMO and LUMO levels and ESP surface of (a) ATPA-6FDA and (b) APC-6FDA.
Figure 8. Analysis of IꢀV characteristics of the off and on states of the devices based on (a, c) ATPA-6FDA and (b, d) APC-6FDA,
respectively.
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groups, suggesting that benzene may play a role of a spacer and
hinder the charge transfer. The torsional angle between the
carbazole segment and the adjacent benzene rings is 53.5°, much
larger than that between the phenyl rings of triphenylamine
(37°). This suggests that there is probably a large energy barrier
for electrons in the transition from the excited state to the ground
state of APC-6FDA. The difficult transition from LUMO to
HOMO levels may also result in the residual positive charges that
are still localized in the carbazole moieties when the applied
voltage is turned off. This leads to the occurrence of a large off
current. Besides, since ATPA-6FDA has a large conformational
freedom due to the rotation of the phenyl groups, the πꢀπ
interaction of ATPA-6FDA between the adjacent triphenylamine
groups is supposed to be weaker, as compared with APC-6FDA.
The high conformation freedom of the flexible phenyl groups
perturbed the packing of the triphenylamine groups, leading to a
low tunnel current in the off state.
’ CONCLUSIONS
We have successfully synthesized three new D-A oligoimides
and demonstrated the effects of the D-A arrangement on the
electrical switching behavior. The devices based on the D-A-D
oligoimide with the triphenylamine segment exhibited nonvola-
tile NDR behavior and a high on/off ratio of 103ꢀ104, while
A-D-A oligoimides with the same groups showed an insulating
property. The different memory characteristics were probably
due to the influence of terminal hole transporting groups
facilitating the injection of hole carriers. The memory devices
with D-A-D oligoimides revealed repeatable nonvolatile NDR
behaviors and excellent stability during operation. Moreover, the
D-A-D oligoimide with the triphenylamine groups exhibited a
lower off current than that with the rigid carbazole groups,
because of the difficulty of the transition between HOMO and
LUMO levels of the carbazole moieties. The residual positive
charges were probably localized in the carbazole moieties when
the applied voltage was turned off. The results provided a new
strategy for designing D-A oligomers for high-performance
memory device applications.
To further investigate the electrical behaviors of the memory
devices based on the oligoimides, the measured IꢀV curves were
analyzed by employing several theoretical models, including
ohmic contact, space-charge-limited conduction (SCLC),
PooleꢀFrenkel (PF) emission, and so on. Figure 8a,b shows
the linear fitting in the logarithmic plots of I versus V for the off
states of ATPA-6FDA and APC-6FDA with slopes of 2.8 and 2.4.
It means that a trap-limited SCLC dominates the electrical
properties in the off state because the slope is greater than 2.11
On the other hand, a linear relationship was observed in a plot of
log (I/V) versus V1/2 in the on state, as shown in Figure 8c,d. It
indicates that PooleꢀFrenkel emission dominates the high-
conductive state. The PF emission is probably attributed to
charge transport of organic materials filled with charge traps.12 In
the oligoimides, the electron-withdrawing 6FDA groups act as
deep charge-trapping sites. The injection barriers between the
ITO anode and the HOMO levels of the oligoimides are in the
range of 0.2ꢀ0.53 eV, which is much smaller than the barriers
between the Al cathode and LUMO levels (>2 eV). It indicates
that hole injection is easier than electron injection. In the off
state, hole injection occurs when the electrical field is larger than
the energy barrier. Close to the threshold voltages, the carriers fill
the trap sites and facilitate electron injection from the Al cathode,
leading to double-carrier injection and increasing abruptly the
conductivity of oligoimides. As the voltage bias is too high, the
charge-filled traps induce an opposite built-in electrical field
against the bias, resulting in the reduction of the conductivity.
As compared to the volatile memory properties of TP6FꢀPI2d
with a similar chemical structure, the oligomer showed nonvo-
latile NDR behavior. As compared to the solution-processed
polyimides, the oligoimides prepared from vapor deposition tend
to have denser stacking and increase the intermolecular interac-
tion with adjacent molecules, which easily induces the formation
of excimers and stabilizes the trapped charges. This also sup-
ported by the photoluminescence spectrum. ATPA-6FDA ex-
hibited a broad peak at 440 nm,13 as shown in Figure S5a
(Supporting Information), which is attributed to excimer
formation between the triphenylamine moieties. Similarly, the
APC-6FDA films showed a broad PL peak at 387 nm (Figure
S5b, Supporting Information), attributed to the emission of
excimeric carbazole moieties.5e These results indicate that
these oligoimides tend to form excimers in the excited
state. The intermolecular interaction within the oligoimides
enhances the charge trapping and leads to nonvolatile memory
characteristics.
’ ASSOCIATED CONTENT
S
Supporting Information. Seven figures showing TGA
b
and DSC profiles and CV curves of the oligoimides ATPA-6FDA,
APC-6FDA, and ATPA-3FA, molecular orbitals and AFM height
and phase images of ATPA-6FDA and APC-6FDA; PL spectra of
ATP-6FDA and APC-6FDA in the thin-film state; and electrical
switching characteristics of ATPA-6FDA and APC-6FDA thin
films after thermal annealing. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel 886-2-33665236; fax886-2-33665237; e-mailchenwc@ntu.
edu.tw (W.-C.C.) or ueda.m.ad@m.titech.ac.jp (M.U.).
Author Contributions
W.-Y.L. and T.K. contributed equally to this work.
’ ACKNOWLEDGMENT
Financial support from National Science Council of Taiwan,
Ministry of Economic Affairs of Taiwan, and National Taiwan
University Excellent Research program are highly appreciated.
W.-Y. L. highly appreciates Dr. Jung-Ching Hsu for the helpful
discussions of memory mechanisms.
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