High performance volatile polymeric memory devices based on novel triphenylamine-based polyimides containing mono- or dual-mediated phenoxy linkages

Article abstract of DOI:10.1021/ma902574n

Two novel functional polyimides (PIs), PI(AAPT-TPA) and PI(APT-TPA), consisting of electron-donating 4-ammo-4′-(p-aminophenoxy)-triphenylamine (AAPT) or 4,4′-bis(p-aminophenoxy)-tri-phenylamine (APT) and electron-accepting phthalimide moieties, were prepa

Full text of DOI:10.1021/ma902574n

1236 Macromolecules 2010, 43, 1236–1244
DOI: 10.1021/ma902574n
High Performance Volatile Polymeric Memory Devices Based on Novel
Triphenylamine-based Polyimides Containing Mono- or Dual-Mediated
Phenoxy Linkages
Tadanori Kuorosawa,†, Chu-Chen Chueh,‡, Cheng-Liang Liu,‡ Tomoya Higashihara,†
Mitsuru Ueda,*,† and Wen-Chang Chen*,‡,§
†Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguro-ku, Tokyo 152-8552, Japan, ‡Department of Chemical Engineering, National Taiwan University,
Taipei, Taiwan 106, and §Institute of Polymer Science and Engineering, National Taiwan University, Taipei,
Taiwan 106. These authors equally contributed to this work.
Received November 23, 2009; Revised Manuscript Received December 25, 2009
ABSTRACT: Two novel functional polyimides (PIs), PI(AAPT-TPA) and PI(APT-TPA), consisting of
electron-donating 4-amino-40-(p-aminophenoxy)-triphenylamine (AAPT) or 4,40-bis(p-aminophenoxy)-tri-
phenylamine (APT) and electron-accepting phthalimide moieties, were prepared for the memory device
applications. The TPA moieties as electron donor are expected to enhance the electron donating and charge
transport ability with phthalimide moieties (electron acceptor). The monophenoxy linkage PI(AAPT-TPA)
had a higher Tg and a lower band gap than the dual-phenoxy linkage PI(APT-TPA). It suggested the more
rigid backbone of the former and led to different memory characteristics. The memory devices with the
configuration of ITO/PI/Al exhibited two conductivity states and could be swept positively or negatively with
a high ON/OFF current ratio of 108-109. The PI(AAPT-6FDA) device relaxed from the ON state to the OFF
state quickly after the applied voltages was removed, whereas the ON state of the PI(APT-6FDA) device
could remain for around 4 min after the power was turned off. It suggested that dynamic random access
memory (DRAM) was exhibited for the PI(AAPT-6FDA) device and that static random access memory
(SRAM) was for the PI(APT-6FDA) device. The volatile memory characteristics were probably attributed to
the unstable charge transfer (CT) complex based on the weak theoretical dipole moments of the studied PIs.
The dual-mediated phenoxy linkage of PI(APT-6FDA) led to the more twisted conformation compared to
the monosubstituted PI(AAPT-6FDA) based on the theoretical analysis by the density functional theory
(DFT) method. It thus produced a potential barrier for delaying the back CT process by the electric field and
explained the SRAM characteristic. The present study suggested the importance of the TPA structure for the
memory characteristics. The fast switching and high ON/OFF characteristics also indicated the new TPA
based polyimides for advanced memory technology.
Introduction
excellent physical and chemical properties besides the electrical
switching behavior.8-16 Functional PIs containing both electron
Resistive switching behavior has been primarily studied in
amorphous silicon-based materials for the last 30 years. It was
found that the digital data could be stored by applying or
discharging electrical charges through a simple device configura-
tion consisting of two electrode layers sandwiching the storage
medium. As compared to the traditional inorganic semiconductor-
based memories, polymeric memory devices are extensively attrac-
tive nowadays because of the advantages of rich structure flexi-
bility, low-cost, solution processability, and three-dimensional
stacking capability.1-3 The recently studied polymer systems
exhibit memory switching characteristics, including conju-
gated polymers,4-7 functional polyimides (PIs),8-16 polymers
with specific pendent chromophores17-20, and polymer nano-
composites (metal nanoparticle21-23 and fullerene24-26
embedded). The widely reported electrical switching pheno-
mena in polymers and the corresponding polymer electronic
memories as an emerging area in organic electronics have been
reviewed by Kang and co-workers recently.2,3
donor and acceptor moieties within a repeating unit contribute to
electronic transition between the ground and excited states, which
could be manipulated by the induced charge transfer (CT) from
donor (D) to acceptor (A) under applied electric fields.8-11
Among the studied functional PIs, the triphenylamine (TPA)-
based PIs attract significant attention.8,9,14-16 For example, the
memory device of a TPA-functionalized polyimide PI(TP6F)
exhibited dynamic random access memory (DRAM) behavior
while a device based on PI(PYTPA) (containing combined donor
effects from both TPA and diphenylpyridine) showed the im-
proved DRAM performance as reported by Kang and co-
workers.8,9 Meanwhile, Ree and co-workers also propose a series
of TPA-based PIs for memory characteristics.14-16 They found that
PI(6F-2TPA) (two connected TPA rings) showed stable digital
nonvolatile write-once-read-many-times (WORM) and volatile
DRAM memory characteristics depending on film thickness.14
Besides, a flash-type memory of PI(6F-HAB-TPAIE) bearing
two pendent TPA groups and a unipolar WORM memory of
PI(6F-HTPA) having an attached hydroxyl group on TPA were
also reported recently.15,16 However, Kang and co-workers
also found that the conformational changes induced by the
incorporated mediators on polymer backbone would result in
Among all the studied polymer systems, PIs are promising
candidates for memory device applications because of their
*To whom correspondence should be addressed. E-mail: ueda.m.ad@
m.titech.ac.jp (M.U.); chenwc@ntu.edu.tw (W.-C.C.)
pubs.acs.org/Macromolecules
Published on Web 01/07/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 3, 2010 1237
Scheme 1. Syntheses of PI(AAPT-6FDA) and PI(APT-6FDA)
increased torsional displacement between the two monomer
moieties and then produce a potential barrier for the back
electron transfer.11 However, the relationship between the poly-
mer structure and memory device characteristics requires further
exploration.
Experimental Section
Materials. p-Iodophenol, p-fluoronitrobenzene, aniline, trit-
butylphosphine (96þ%), copper powder (99.5þ%), 18-crown-
6-ether (98þ%), palladium-activated carbon (Pd 10 wt %),
DMAc, and toluene were purchased from Wako, Japan and
dimethyl sulfoxide (DMSO) was purchased from Aldrich. Tris-
(dibenzylideneacetone)dipalladium (0) (Pd2(dba)3) (>75%)
and 4,40-(hexafluoroisopropylidene)diphthalicanhydride(6FDA)
were purchased from TCI and were used as received. The
detailed syntheses and characterization of 4-iodo-40-nitrodiphe-
nyl ether (INE), N-(4-nitrophenyl)aniline, 4-nitro-40-(p-nitro-
phenoxy)triphenylamine (NNPT), and 4,40-bis(p-nitropheno-
xy)triphenylamine (NPT) are described in the Supporting In-
formation.
Synthesis of 4-Amino-40-(p-aminophenoxy)triphenylamine
(AAPT). Hydrazine monohydrate (10 mL) was added dropwise
to a solution of NNPT (2.14 g, 5.00 mmol) and palladium-
activated carbon (0.0224 g) in ethanol (15 mL), and the mixture
was refluxed for 24 h. The reaction solution was filtered through
Celite to remove palladium-activated carbon and washed with
THF. The filtrate was concentrated and poured into water. The
precipitate was collected by filtration and purified by recrystal-
lization from chloroform and hexane to yield 1.26 g (69% in
yield) of white crystals. IR (KBr), ν (cm-1): 3460, 3379 (N-H
stretching). 1H NMR (300 MHz, δ, ppm): 7.13 (t, J = 8.0 Hz,
ArH, 2H), 6.94 (d, J = 9.0 Hz, ArH, 2H), 6.80-6.73 (m, ArH,
9H), 6.59-6.55 (m, ArH, 4H), 4.92 (s, N-H, 2H), 4.82 (s, N-H,
2H). 13C NMR (75 MHz, DMSO-d6, δ, ppm): 154.83, 149.49,
147.13, 146.67, 145.86, 142.86, 136.42, 129.63, 128.43, 125.81,
121.16, 120.44, 120.06, 118.43, 115.79, 115.69. Anal. Calcd for
C24H21N3: C, 78.5; H, 5.76; N, 11.4; found, C, 78.4; H, 5.90; N,
11.2.
In this study, we report the syntheses and memory device
characteristics of two new TPA-based PIs, such as PI(AAPT-
6FDA) derived from 4-amino-40-(p-aminophenoxy)triphenyl-
amine (AAPT) and 4,40-(hexafluoroisopropylidene)diphthalic
anhydride (6FDA) and PI(APT-6FDA) from 4,40-bis(p-amino-
phenoxy)triphenylamine (APT) and 6FDA (Scheme 1). The TPA
moieties as electron donor are expected to enhance the electron
donating and charge transport ability with phthalimide moieties
(electron acceptor) via phenoxy linkage (L) for device applica-
tions. The thermal, optical, and electrochemical properties of
PI(AAPT-6FDA) and PI(APT-6FDA) were also characterized.
These PIs could be well dissolved in common organic solvents
and easily processed into a smooth thin film layer. The memory
behavior was conducted by a simple sandwich device configura-
tion consisting of polymer films between indium-tin oxide (ITO)
bottom electrode and Al top electrode. The mono- or dual-
mediated phenoxy linkages between the TPA and phthalimide
moieties, represented as (D-L-A)n (PI(AAPT-6FDA)) or (L-D-
L-A)n (PI(APT-6FDA)), was found to significantly affect the
volatile memory behaviors. PI(AAPT-6FDA) exhibited DRAM
performance, while PI(APT-6FDA) showed the static random
access memory (SRAM) behavior. In order to clarify the switch-
ing mechanism of the memory devices, theoretical calculations
under the density functional theory (DFT) method at the B3LYP
level with the 6-31G(d) basic set was applied to analyze the
geometry and electronic transitions of the studied polymers. The
obtained electrical characteristics not only suggested that PI-
(AAPT-6FDA) and PI(APT-6FDA) were excellent candidates
for a volatile type memory but also presented anunderstanding of
the TPA structural effects on the memory device performance.
Synthesis of 4,40-Bis(p-aminophenoxy)triphenylamine (APT).
APT was prepared by a similar procedure according to the
synthesis of AAPT, starting from NPT. APT was obtained as
white crystals in 90% yield. IR (KBr), ν (cm-1): 3460, 3375

1238 Macromolecules, Vol. 43, No. 3, 2010
Kuorosawa et al.
(N-H stretching). 1H NMR (300 MHz,CDCl3, δ): 7.19 (t, J =
7.8 Hz, ArH, 2H), 7.03-6.98 (m, ArH, 6H), 6.89-6.81 (m, ArH,
9H), 6.73 (d, J = 8.1 Hz, ArH, 4H), 3.53 (s, N-H, 4H); 13C
NMR (75 MHz, CDCl3, δ, ppm): 154.87, 149.46, 148.80, 142.90,
142.83, 129.46, 126.37, 122.37, 121.74, 121.19, 118.64, 116.67.
Anal. Calcd for C30H25N3: C, 78.4; H, 5.48; N, 9.14; found, C,
78.3; H, 5.67; N, 8.96.
Fabrication and Measurement of the Memory Device. The
memory device was fabricated on the indium-tin oxide (ITO)-
coated glass, with the configuration of ITO/polyimides/Al.
Before the fabrication of the polymer layer, the ITO glass was
precleaned by ultrasonication with water, acetone, and isopro-
panol each for 15 min. A 25 mg/mL of well-dissolved PI(AAPT-
6FDA) or PI(APT-6FDA) polymer solution in DMAc was first
filtered through 0.45 μm pore size of PTFE membrane syringe
filter. Then, the filtered solution was spin-coated onto the ITO
glass at a speed rate of 1000 rpm for 60 s and annealed at 150 °C
for 30 min under nitrogen. The thickness of the thin film was
determined to be around 40-50 nm. Finally, a 300-nm-thick Al
top electrode was thermally evaporated through the shadow
mask (recorded device units of 0.5 ꢀ 0.5 mm2 in size) at a
Synthesis of Poly[4-amino-40-(p-aminophenoxy)triphenylami-
ne-hexafluoroisopropyl-idenediphthalimide] (PI(AAPT-6FDA)).
6FDA (0.444 g, 1.00 mmol) was added into a solution of AAPT
(0.367 g, 1.00 mmol) in dehydrated DMAc (solid contents 15
wt %) under nitrogen. The reaction mixture was stirred at room
temperature for 24 h to give a poly(amic acid) (PAA) solution. A
reaction mixture of acetic anhydride (0.1479 g, 1.49 mmol) and
pyridine (0.1146 g, 1.49 mmol) was added to the above PAA
solution. The reaction mixture was stirred at room temperature
under a nitrogen atmosphere for 24 h. The resulting solution was
poured into methanol, and the precipitate was collected by
filtration and washed with methanol. The final product was
dried at 200 °C under vacuum for 10 h, and the polymer yield
was 92%. The number average molecular weight (Mn) and
weight average molecular weight (Mw) values estimated from
GPC were 4.49 ꢀ 104 and 7.90 ꢀ 104, respectively, with the
polydispersity index (PDI = Mw/Mn) at 1.76. IR (KBr), ν
(cm-1): 1786, 1724 (CdO stretching), 1381 (C-N stretching).
1H NMR (DMSO-d6, δ, ppm): 8.20-8.00 (m, ArH, 4H), 7.80 (s,
ArH, 2H), 7.49 (d, J = 8.7 Hz, ArH, 2H), 7.36 (d, J = 8.4 Hz,
ArH, 4H), 7.22-7.09 (m, ArH, 11H). Anal. Calcd for
C43H23N3: C, 66.6; H, 2.99; N, 5.42; found, C, 66.2; H, 3.30;
N, 5.17.
pressure of 10-7 torr with a uniform depositing rate of 3-5 A/s.
˚
The electrical characterization of the memory device was per-
formed by a Keithley 4200-SCS semiconductor parameter ana-
lyzer equipped with a Keithely 4205-PG2 arbitrary waveform
pulse generator. ITO was used as the cathode (maintained as
common), and Al was set as the anode during the voltage sweep.
The probe tip used 10 μm diameter tungsten wire attached to a
tinned copper shaft with a point radius <0.1 μm (GGB In-
dustries, Inc.). All of the electronic measurements were per-
formed in a glovebox.
Computational Methodology. Molecular calculations studied
in this work have been performed with the Gaussian 03 program
package.27 Equilibrium ground state geometry and electronic
properties of basic unit of PI(AAPT-6FDA) and PI(APT-
6FDA) were optimized by means of the density functional
theory (DFT) method at the B3LYP level of theory (Beckes-
style three-parameter density functional theory using the Lee-
Yang-Parr correlation functional) with the 6-31G(d) basic set.
Synthesis of Poly[4,40-bis(p-aminophenoxy)triphenylamine-
hexafluoroisopropyl-idenediphthalimide] (PI(APT-6FDA)). Using
APT as a diamine, PI(APT-6FDA) was synthesized by a proce-
dure similar to that use for PI(AAPT-6FDA). The yield of
PI(APPA-6FDA) was 93%. Mn, Mw, and PDI values of PI-
(AAPT-6FDA) were 3.17 ꢀ 104, 6.27 ꢀ 104, and 1.98, respec-
tively. IR (KBr), ν (cm-1): 1786, 1728 (CdO stretching), 1377
Results and Discussion
Syntheses and Characterization of Monomers and Polymers.
The monomer synthesis of AAPT was described in the
Experimental Section and also shown in the Scheme S1(a)
of the Supporting Information. First, the precursory N-(4-
nitrophenyl)aniline and 4-iodo-40-nitrodiphenyl ether (INE)
were prepared from 4-fluoronitrobenzene and aniline, and
from 4-fluoronitrobenzene and 4-iodophenol, respectively.
Then, the Ullman reaction of N-(4-nitrophenyl)aniline and
INE produced 4-nitro-40-(p-nitrophenoxy)triphenylamine
(NNPT), which was further hydrogenated to AAPT. Similar
to AAPT, APT was prepared in three steps as shown in
Scheme S1(b) (Supporting Information). The palladium-
catalyzed N-arylation reaction of aniline with two equiva-
lents of INE produced 4,40-bis(p-nitrophenoxy)triphenyl-
amine (NPT), which could be converted to APT with the
catalytic reduction. The characteristic nitro absorp-
tion peaks of all nitro compounds at around 1580 and
1340 cm-1 in the FT-IR spectra disappeared, and the new
absorption peaks at around 3450 and 3370 cm-1 appeared
after the reduction of NNPT and NPT, which were properly
assigned to amino groups. The 1H NMR spectra of these two
diamine compounds are presented in Figure 1 with the
assignments of all peaks. The signals at 4.92 and 4.82 ppm
were assigned to the asymmetric amino groups of AAPT,
while those at 3.53 ppm were assigned to the amino groups of
APT. In addition, the 13C NMR and elemental analysis
clearly supported the formation of all monomers. The above
results suggest the successful preparation of the AAPT and
APT monomers.
1
(C-N stretching). H NMR (DMSO-d6, δ, ppm): 8.12 (d, J =
8.1 Hz, ArH, 2H), 7.93 (d, J = 8.7 Hz, ArH, 2H), 7.76 (s, N-H,
1H), 7.43 (d, J = 8.7Hz, ArH, 4H), 7.27 (t, J = 7.8 Hz, ArH, 2H),
7.15-6.96 (m, ArH, 15H). Anal. Calcd for C49H27N3: C, 68.2; H,
3.70; N, 4.68; found, C, 67.3; H, 3.35; N, 4.69.
Characterization. NMR spectra were recorded on a BRU-
KER DPX-300S spectrometer at resonant frequencies of
1
300 MHz for H and 75 MHz for 13C nuclei using CDCl3 or
DMSO-d6 as the solvent and tetramethylsilane as the reference.
FT-IR spectra were measured by a Horiba FT-120 Fourier
transform spectrophotometer. Number- and weight-average
molecular weights (Mn and Mw) were evaluated by GPC on a
JASCO PU-2080 Plus with two polystyrene gel columns (TSK
GELS GMHHR-M). DMF containing 0.01 M LiBr was used as
a solvent at a flow rate of 1.0 mL min-1 calibrated by poly-
3
styrene standard samples. The UV-visible optical absorption
spectra were recorded on a Hitachi U-3210 spectrophotometer
at room temperature. The absorbance of polymer solutions was
evaluated in the wavelength range of 280-800 nm. Elemental
analyses were performed on a Yanaco MT-6 CHN recorder
elemental analysis instrument. Thermal properties were esti-
mated from a Seiko TG/DTA 6300 thermal analysis system
(TGA) and a TA Instruments DSC-Q100 differential scanning
calorimetry (DSC) under a nitrogen atmosphere at a heating
rate of 10 and 6 °C/min, respectively. Cyclic voltammogram was
performed at room temperature using a working electrode (ITO,
polymer film area of about 10 ꢀ 30 mm2), a homemade reference
electrode Ag/AgCl, and a counter electrode (Pt wire) at a sweep
rate of 0.1 V/s (CHI 611B electrochemical analyzer). A 0.1 M
solution of tetrabutylammonium perchlorate (TBAP) in anhy-
drous acetonitrile was used as an electrolyte. The thickness of
the polymer film was measured with a Microfigure Measuring
Instrument (Surfcorder ET3000, Kosaka Laboratory Ltd.).
The polymers, PI(AAPT-6FDA) and PI(APT-6FDA),
were prepared by the polycondensation of diamines
(AAPT and APT) with a dianhydride (6FDA) in two steps,
as shown in Scheme 1. First, diamine and 6FDA were mixed
in N,N-dimethylacetamide (DMAc) (solid content 15 wt %)

Article
Macromolecules, Vol. 43, No. 3, 2010 1239
Figure 1. 1H NMR spectra of monomer (a) AAPT and (b) APT in CDCl3.
methylformamide (DMF), DMAc, N-methylpyrrolidone
(NMP), and chloroform. The high solubility may be attrib-
uted to the introduction of flexible ether linkages and hexa-
fluoroisopropyl units in the polymer structure. The thermal
stability of these two polyimides was evaluated by thermo-
gravimetric analysis (TGA) under a nitrogen atmosphere
(Figure S1, Supporting Information). The 1% weight-loss
temperatures (Td1%) of PI(AAPT-6FDA) and PI(APT-
6FDA) were determined to be around 517 and 514 °C,
respectively. These PIs not only showed excellent thermal
stability but also possessed high glass transition tempera-
tures (Tg) of 276 and 252 °C for PI(AAPT-6FDA) and
PI(APT-6FDA), respectively (see Figure S2, Supporting
Information). The lower Tg of PI(APT-6FDA) than that of
PI(AAPT-6FDA) also indicated the more twisted polymer
backbone of the former. The excellent thermal stability of
PIs is expected to meet the requirement of heat resistance in
the electronics industry.
Figure 2. FTIR absorption spectra of PI(AAPT-6FDA) and PI(APT-
6FDA).
under nitrogen to yield a viscous PAA solution. Then,
chemical imidization was carried out by adding acetic anhy-
dride and pyridine into the PAA solution to produce the
polyimide. The desired structures of the polyimides were
confirmed by FT-IR spectra, which showed the character-
istic absorption peaks of the imide group at 1780 cm-1
(asymmetrical stretching of carbonyl) and 1720 cm-1
(symmetrical stretching of carbonyl), and the C-N bond
at around 1370 cm-1, as presented in Figure 2. Moreover, the
Optical and Electrochemical Properties. Figure 4 shows the
UV-vis absorption spectra of the studied polyimides in
THF solution and thin film state. PI(AAPT-6FDA) exhibits
absorption peak maxima (λmax) around 300 and 308 nm in
solution and thin film states, respectively, while those of
PI(APT-6FDA) are around 301 and 303 nm, respectively.
The above absorption peaks are attributed to the π-π*
transition delocalized along the π-electronic system. Besides,
PI(AAPT-6FDA) apparently exhibits more intense absorp-
tion intensity in the wavelength above 350 nm as compared
to PI(APT-6FDA) in thin film state. It indicates that the
monomediated phenoxy linkage could have a better copla-
narity than the dual-mediated one, in which the charges are
transferred from the APT/AAPT to 6FDA via the linkage(s),
when both polymer structures are alike.11 As a result, the
1
desired formations of all polymers were confirmed by H
NMR, shown in Figure 3, and elemental analysis. The Mn
values of PI(AAPT-6FDA) and PI(APT-6FDA) were 44900
and 31700 as determined by Gel Permeation Chromatogra-
phy (GPC). Both polymers were readily soluble in common
organic solvents such as tetrahydrofuran (THF), N,N-di-

1240 Macromolecules, Vol. 43, No. 3, 2010
Kuorosawa et al.
Figure 3. 1H NMR spectra of (a) PI(APT-6FDA) and (b) PI(AAPT-6FDA) in DMSO-d6.
Figure 4. Optical absorption spectra of PI(AAPT-6FDA) and PI-
(APT-6FDA) in THF solution and as thin films.
Figure 5. Cyclic voltammograms of PI(AAPT-6FDA) and PI(APT-
6FDA) on an ITO electrode in 0.1 M TBAP/acetonitrile solution with
Ag/AgCl as reference electrode and Pt wire as counter electrode.
optical band gap (Egopt) of PI(AAPT-6FDA) and PI(APT-
6FDA), estimated from the onset optical absorbance, are
3.15 and 3.55 eV, respectively. Therefore, the larger Egopt of
PI(APT-6FDA) than that of PI(AAPT-6FDA) probably
resulted from the more twisted conformation by introducing
one more phenoxy linkage, which will be further discussed in
the theoretical analysis section.
Figure 5 shows the cyclic voltammagram (CV) of PI-
(AAPT-6FDA) and PI(APT-6FDA) films on ITO glass

Article
Macromolecules, Vol. 43, No. 3, 2010 1241
using a 0.1 M solution of tetrabutylammonium perchlorate
(TBAP) in anhydrous acetonitrile. Both PI(AAPT-6FDA)
and PI(APT-6FDA) exhibit reversible p-doping behavior
during the anodic scan. The onset oxidation (Eonsetox) of
PI(AAPT-6FDA) and PI(APT-6FDA) is around 0.96 and
0.90 V vs Ag/Agþ, respectively, and thus, the estimated
highest occupied molecular orbital (HOMO) level is -5.32
1/2
and -5.26 eV, respectively. Note that the Eferrocene is
0.44 V from the CV measurement without any polymer film
coated on ITO glass. Besides, the lowest occupied molecular
orbital (LUMO) energy levels of PI(AAPT-6FDA) and
PI(APT-6FDA) estimated from the difference between
the optical band gap and HOMO energy are -2.17 and
-1.71 eV, respectively. The result suggests that the frontier
orbitals of PI(AAPT-6FDA) are more stabilized as com-
pared to PI(APT-6FDA), and the additional twisted phe-
noxy linkages increased the LUMO energy level significan-
tly, probably due to the nonlinear or twisted conformation of
the polymer backbone.
Memory Device Characteristics of the Polymers. The
memory effects of PI(APT-6FDA) and PI(AAPT-6FDA)
are demonstrated by the current-voltage (I-V) character-
istics of an ITO/polymer/Al sandwich device. The polymeric
memory devices store data based on the high- (ON) and low-
conductivity (OFF) response to the external applied vol-
tages.1 Figure 6 exhibits the typical I-V curves of the
memory devices fabricated with PI(APT-6FDA) and PI-
(AAPT-6FDA). For the case of PI(APT-6FDA) (see
Figure 6a), the device is initially in the OFF state (“0” signal
in data storage) with a current in the range of 10-12-10-14
A
as the voltage swept from 0 to -3.5 V. When the voltage
increases further, it induces an abrupt increase in current at a
threshold voltage of about -3.5 V, as indicated by the
transition from the OFF state to ON state (“1” signal in
data storage). The ON/OFF current ratio of the studied
memory device is as high as 109, which leads to a low
misreading rate for memory applications. This electronic
transition from OFF state to ON state in the first sweep
serves as the “writing” process. In addition, the device could
be kept in the ON state during the subsequent second sweep
from 0 to -6 V (second sweep). The third sweep is conducted
after turning off the power for about 4 min. The device can be
reprogrammed, starting from the OFF state, to the ON state
again with an accurate threshold voltage of -3.6 V and kept
in the ON state (the third and fourth sweeps). It suggests that
the ON state could be retained for a short period of time after
the removal of power and would relax to the original OFF
state eventually. Similar positive switching phenomena as in
the negative sweep are also observed. After remaining in the
ON state during the repeated negative sweep (the fourth
sweep), the memory cannot reset to the OFF state by the
continuous applied positive bias from 0 to 6 V (fifth sweep)
and thus shows nonerasable switching behavior. However,
after turning off the power for 4 min, the device can be
reprogrammed to the ON state repeatedly with a threshold
voltage of 3.8 V and similarly retain the ON state (the sixth
and seventh sweeps). Hence, it could be concluded that the
memory device possessed a “remnant”, yet volatile, nature of
the ON state because the stored data are finally lost.11 This
unstable ON state of the volatile memory device can be
maintained by a refreshing voltage pulse of -1 V of 1 ms
duration every 5 s (named the rf trace). The switching
behavior indicates that the device is rewritable (over
20 sweeps) with small variations on threshold voltage and
also can be written symmetrically and bidirectionally. The
I-V curves exhibiting the volatile property can be measured
in at least 10 devices with reproduced data. The stability of
Figure 6. Current-volatge (I-V) characteristics of (a) PI(APT-6FDA)
and (b) PI(AAPT-6FDA) memory device.
Figure 7. Retention times on the ON and OFF states of the ITO/
PI(APT-6FDA)/Al device under a continuous readout voltage.
the memory effect was also evaluated under the same condi-
tions.
Figure 7 shows the retention times and stress tests of both
the ON and OFF states of PI(APT-6FDA). Initially, the
memory device starts to turn ON or OFF to a high or low
conductivity state. Under a constant stress of -1 V, no
obvious degradation in current is observed for both ON
and OFF states for at least 104 s during the readout test. The
stimulus effect of read pulses on the ON and OFF state was
also investigated for the long time test (Figure 8). The pulse
period and width are 3 and 2 μs, respectively, as shown in the
inset of Figure 8. The PI(APT-6FDA) memory device is

1242 Macromolecules, Vol. 43, No. 3, 2010
Kuorosawa et al.
stable for at least 108 continuous read pulses of -1 V.
Therefore, the switching behavior on remnant stored data
and the volatile nature of memory device can explain the
functionality of a SRAM.11
increase (the first or third and fifth sweep). Besides, the turn
ON threshold voltage and ON/OFF current magnitudes in
both electrical sweeps are also comparable. The third sweep
is performed after turning the power for off for usually less
than 30 s. The OFF state can be further recovered to a stored
state again when the switching voltage was reapplied indicat-
ing that the memory device is rewritable. The short retention
ability of the ON state determines the DRAM type of
PI(AAPT-6FDA). This field-induced electronic transition
is similar to the previous device based on the PI(TP6F).8 The
volatile process can be repeated many times for 10 different
cells with a reproducible switching as well. Different from the
memory characteristics of PI(TP6F), PI(AAPT-6FDA) with
a bidirectional writing property could not relax to the OFF
state by a reverse bias. It suggests the effect of the conforma-
tional change induced by the phenoxy linkage on the mem-
ory characteristic. However, one phenoxy linkage between
the TPA and 6FDA moieties cannot efficiently stabilize the
CT complexes and thus exhibit the DRAM type memory
behavior. In order to clarify the detailed converting mechan-
ism, theoretical analysis was applied and discussed in the
following section.
Meanwhile, the electrical behavior of the PI(AAPT-
6FDA) (only one phenoxy linkage between the TPA and
6FDA moieties) device was also studied as shown in
Figure 6b. The I-V curve observed during the positive and
negative sweep still keeps the volatile characteristics. As
compared to the PI(APT-6FDA) device (two phenoxy lin-
kages between the TPA and 6FDA moieties), the main
difference is the short period of time for the retained ON
state of the PI(AAPT-6FDA) device. Initially, the device
is in the OFF state with quite a low current level of
10-11-10-14A. The transition from the OFF state to the
ON state was observed with a bidirectional abrupt current
Proposed Switching Mechanism by Theoretical Analysis.
Molecular simulation on the basic unit of PI(AAPT-6FDA)
and PI(APT-6FDA) was carried out by DFT/B3LYP/6-
31G(d) with the Gaussian 03 program.27 The sketch map
of the structures for the basic unit (BU) of PI(AAPT-6FDA)
and PI(APT-6FDA) and optimized structures is plotted to
the left of Figure 9. The trifluoromethyl groups of PIs are not
included in the BU since these fragments do not have
significant differences in electronic properties.8-11 The PI-
(APT-6FDA) (represented as (L-D-L-A)n) shows the more
twisted conformation relative to the PI(AAPT-6FDA)
(represented as (D-L-A)n) due to the incorporation of the
additional mediated noncoplanar geometry of phenoxy link-
age. It also contributes to the blue-shifted absorbance of
PI(APT-6FDA) than PI(AAPT-6FDA), as discussed above.
Figure 8. Stimulus effect of read pulses on the ON and OFF states of
the ITO/PI(APT-6FDA)/Al device. The insert shows thepulse shapes in
the measurement.
Figure 9. Electronic density contours of molecular orbitals and electrostatic potential surface (ESP) of the basic units: PI(AAPT-6FDA) and PI(APT-
6FDA) memory devices.

Article
The right part of Figure 9 shows the charge density
Macromolecules, Vol. 43, No. 3, 2010 1243
relative to PI(APT-6FDA). The only one-mediated phenoxy
linkage on PI(AAPT-6FDA) cannot induce enough poten-
tial barriers for the back CT since the other side can still
contribute to the charge recombination through the D-A
connection. Therefore, the unstable CT states lead to the
DRAM behavior, which is the same as that of the memory
device based on PI(TP6F)8 and PI(PYTPA).9 The positive
and negative charges are segregated from the CT process and
localized in the donor and acceptor under the electric bias.
When the power is turned off, the unstable CT states rapidly
return to the original OFF state. Combined with the remark-
able difference in thin film state UV absorption, it suggests
that the dual-mediated phenoxy linkages PI(APT-6FDA),
similar to that reported for PI(BPPO), would more possibly
form temporary electric field induced CT due to the larger
inter- and intrachain twisting effects, while the phenoxy
linkage of PI(AAPT-6FDA) with a less twisted conforma-
tion still makes it behave as PI(TP6F).8 Moreover, PI(APT-
6FDA) and PI(AAPT-6FDA) show close theoretical
dipole moments of 2.87 and 2.65 D, respectively. The smaller
dipole moment compared to those of the sulfur containing
PIs (around 6.00 D) probably leads to the unstable conduc-
tive complex. Both contributions make PI(APT-6FDA) a
less volatile (but still with a volatile nature) memory device
than PI(AAPT-6FDA). As a result, the memory devices
based on PI(APT-6FDA) and PI(AAPT-6FDA) show
SRAM and DRAM behaviors, respectively, and the
CT mechanism can also be supported by our theoretical
calculation.
isosurfaces of the basic unit of PI(AAPT-6FDA) and PI-
(APT-6FDA) with the most energetically favorable geome-
try. The relative ordering of the occupied and virtual
molecular orbitals gives a reasonable indication of the
excited properties and charge transport ability. Both the
HOMO and LUMOþ2 isosurfaces tend to locate on triphe-
nylamine (D), while the LUMO and LUMOþ1 located on
phthalimide (A). Besides, the HOMO and higher excited
state (LUMOþ1 and LUMOþ2) are slightly distributed on
the phenoxy linkages on PI(AAPT-6FDA) or PI(APT-
6FDA). The electronic transition corresponds to promoting
an electron from the ground state to the excited state. As the
applied bias at threshold voltage, electrons at the HOMO
with sufficient energy transit to LUMOþ2 within TPA unit
to give rise to excited state. CT can be observed to form the
conductive complex through processes such as indirectly
from the LUMOþ2 to the LUMOþ1 and then to the LUMO
and also directly from the HOMO to LUMO. This indicates
that the electron transfer subsequent to the excitation of the
AAPT or APT moiety leads to the intra- or intermolecular
CT state. Besides, the device switching to the ON state with a
symmetrically comparable threshold voltage of either polar-
ity also supports the switching mechanism of electric-field
induced CT.9,11 As compared to the previous PI(BBPO)
memory device11 (oxadiazole moieties as relative donor
and phthalimide as relative acceptor with two phenoxy
linkages, (L-D-L-A)n type and PI(APT-6FDA)) (replace-
ment of the oxadiazole moieties by TPA) this also shows a
similar SRAM performance. The slight difference is the
threshold voltage since the overlapping of the HOMO with
LUMOþ1 and LUMOþ2 at the oxadiazole units contri-
butes more possibilities to be excited except for the CT from
HOMO to the highest energy of LUMOþ2. In the two
distinct heterocyclic ring units, the action of the perpendi-
cular electric field on the dihedral angle is shown to increase
because of the inclusion of suitable functional groups.28 CT
is sensitive to the dihedral angle that creates a potential
barrier along the molecules as the geometry varies to the
more twisted one. Therefore, the CT occurs in the PI(APT-
6FDA) device from TPA (D) to phthalimide(A) through the
phenoxy linkages (L) followed by the modulation of con-
formation with an increased dihedral angle between the D
and A units. The enhanced dihedral angle between the
ground and excited states was also verified by theoretical
calculation,11 which agrees with the increase in torsional
angle based on the action of a static electric field perpendi-
cular to the ring-ring bond.28 The phenoxy linkages are
considered to be the suitable moiety on the whole systems as
a mediator for the CT process with conformation changes of
the molecules, which can increase the back CT energy barrier
and possibly prevent the recombination of segregated
charges. This electrical switching under the applied negative
and positive threshold voltage is suggested to form a stable
environment temporarily and store the transferred charges.
It can prevent the charge from going back to the initial state
even after the application of reverse bias.28,29 Finally, the
PI(APT-6FDA) still relaxes to the initial conformation after
the removal of the applied electric field for a short period of
time. The dihedral angles between the ring moieties reduce to
the original state, and also, the potential barriers of back CT
disappear. Hence, the back CT occurs from the phthalimide
to TPA (reset to the OFF state) and confirms the volatile
switching behavior.
Conclusions
We have successfully synthesized two new TPA-based poly-
imides, PI(AAPT-6FDA) and PI(APT-6FDA), for memory
device application. The memory devices with the configuration
of ITO/PI(APT-6FDA) or PI(AAPT-6FDA)/Al exhibited dis-
tinct volatile memory characteristics of SRAM and DRAM,
respectively. Moreover, each memory device shows a high
ON/OFF current ratio of 109, a long retention time of 104 s,
and device-to-device switching uniformity. The theoretical ana-
lysis suggests that the CT mechanism could be used to explain the
memory characteristics of the polyimides studied here. The weak
dipole moment of the PI(AAPT-6FDA) and PI(APT-6FDA)
provide an unstable CT complex for the volatile memory device.
The dual-mediated phenoxy linkges of PI(APT-6FDA) led to the
more twisted conformation compared to the monosubstituted
PI(AAPT-6FDA) on the basis of theoretical analysis. It thus
produced a potential barrier for delaying the back charge
transfer (CT) process by the electric field and explained the
SRAM characteristic. However, the PI(AAPT-6FDA) device
makes a more volatile ON state because of the possible lack of
the back CT barriers. Thus, the result provides the strategies
for the design of functional PIs for advanced memory device
applications.
Acknowledgment. Financial support of this work from the
National Science Council, the Ministry of Economic Affairs, and
the Excellence Research Program of National Taiwan University
is highly appreciated.
Supporting Information Available: Detailed syntheses and
characterization of 4-iodo-40-nitrodiphenyl ether (INE), N-(4-
nitrophenyl)aniline, 4-nitro-40-(p-nitrophenoxy)triphenylamine
(NNPT), and 4,40-bis(p-nitrophenoxy)triphenylamine (NPT),
and TGA and DSC curves of PI(AAPT-6FDA) and PI(APT-
6FDA) under nitrogen atmosphere. This material is free of
charge via the Internet at http://pubs.acs.org.
However, the PI(AAPT-6FDA) device provides only
monomediated phenoxy linkage (L) between the D and A
units indicating a more rigid and also less twisted structure

1244 Macromolecules, Vol. 43, No. 3, 2010
Kuorosawa et al.
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