Donor-acceptor oligoimides for application in high-performance electrical memory devices

Article abstract of DOI:10.1002/asia.201300335

Two new oligoimides, OI(APAP-6FDA) and OI(APAN-6FDA), which consisted of electron-donating N-(4-aminophenyl)-N-phenyl-1-aminopyrene (APAP) or N-(4-aminophenyl)-N-phenyl-1-aminonaphthalene (APAN) moieties and electron-accepting 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) moieties, were designed and synthesized for application in electrical memory devices. Such devices, with the indium tin oxide (ITO)/oligoimide/Al configuration, showed memory characteristics, from high-conductance Ohmic current flow to negative differential resistance (NDR), with corresponding film thicknesses of 38 and 48 nm, respectively. The 48 nm oligoimide film device exhibited NDR electrical behavior, which resulted from the diffusion of Al atoms into the oligoimide layer. On further increasing the film thickness to 85 nm, the OI(APAP-6FDA) film device showed a reproducible nonvolatile write once read many (WORM) property with a high ON/OFF current ratio (more than ×10⁴). On the other hand, the device that was based on the 85 nm OI(APAN-6FDA) film exhibited a volatile static random access memory (SRAM) property. The longer conjugation length of the pyrene unit compared to that of a naphthalene unit was considered to be responsible for the different memory characteristics between these two oligoimides. These experimental results suggested that tunable switching behavior could be achieved through an appropriate design of the donor-acceptor oligoimide structure and controllable thickness of the active memory layer.

Full text of DOI:10.1002/asia.201300335

FULL PAPER
DOI: 10.1002/asia.201300335
Donor–Acceptor Oligoimides for Application in High-Performance
Electrical Memory Devices
Yi-Cang Lai,[a] Tadanori Kurosawa,[b] Tomoya Higashihara,[b] Mitsuru Ueda,*[b] and
Wen-Chang Chen*[a, c]
Abstract: Two new oligoimides,
OI(APAP-6FDA) and OI(APAN-
6FDA), which consisted of electron-do-
nating N-(4-aminophenyl)-N-phenyl-1-
aminopyrene (APAP) or N-(4-amino-
phenyl)-N-phenyl-1-aminonaphthalene
(APAN) moieties and electron-accept-
nesses of 38 and 48 nm, respectively.
The 48 nm oligoimide film device ex-
hibited NDR electrical behavior, which
resulted from the diffusion of Al atoms
into the oligoimide layer. On further
increasing the film thickness to 85 nm,
the OI(APAP-6FDA) film device
85 nm OI(APAN-6FDA) film exhibit-
ed volatile static random access
a
memory (SRAM) property. The longer
conjugation length of the pyrene unit
compared to that of a naphthalene unit
was considered to be responsible for
the different memory characteristics
between these two oligoimides. These
experimental results suggested that
tunable switching behavior could be
achieved through an appropriate
design of the donor–acceptor oligoi-
mide structure and controllable thick-
ness of the active memory layer.
ing
4,4’-(hexafluoroisopropylidene)-
showed
a
reproducible nonvolatile
diphthalic anhydride (6FDA) moieties,
were designed and synthesized for ap-
plication in electrical memory devices.
Such devices, with the indium tin oxide
“write once read many” (WORM)
property with a high ON/OFF current
ratio (more than ꢀ104). On the other
hand, the device that was based on the
(ITO)/oligoimide/Al
configuration,
showed memory characteristics, from
high-conductance Ohmic current flow
to negative differential resistance
(NDR), with corresponding film thick-
Keywords: conjugation
·
donor–
acceptor systems · imides · oligo-
mers · thin films
Introduction
ies,[24–30] oligomers,[31–39] polymer/graphene complexes,[38–41]
polymer/fullerene blends,[44,45] and organic molecules that
are blended with nanoparticles.[46–48] Among these systems,
D–A imide compounds are among the most-promising ma-
terials for application in memory devices because they not
only show good electrical properties but also exhibit excel-
lent thermal stability, chemical resistance, and mechanical
strength.[6] Recently, devices that were based on polyimides
have been found to show various memory properties, such
as dynamic random access memory (DRAM),[6,11] static
random access memory (SRAM),[3,8] write once read many
(WORM)-type memory,[5,7] and flash memory.[9] In poly-
imide systems, field-induced charge transfer (CT) between
the D and A moieties plays an important role in the result-
ing memory characteristics. In particular, the stability of the
generated CT complexes after the removal of an electric
field is thought to determine the volatility of the memory
characteristics. If the CT complex is stable, the resulting
memory characteristics can be nonvolatile, such as
WORM[5,7] and flash,[9] whereas an unstable CT complex
leads to volatile memory characteristics, such as DRAM[6,11]
and SRAM.[3,8] Previously, several concepts have been pro-
posed for the stabilization of the CT complex.[3,5,7,9,16] One
of these concepts is conformational changes.[5,7] During CT,
chain twisting between the D and A units occurs, which gen-
erates an energy barrier to suppress the back-CT and stabil-
izes the CT complex, thus leading to nonvolatile memory
Data-storage devices that are based on organic materials
have received extensive recent investigation, owing to their
low cost, structural flexibility, and 3D-stacking capability.[1,2]
A wide range of organic materials that contain electron-do-
nating (D) and electron-accepting (A) groups have been re-
ported for applications in electronic and optoelectronic devi-
ces, including functional polyimides,[3–17] conjugated poly-
mers,[18–23] polymers with pendant-conjugated D/A moiet-
[a] Y.-C. Lai,+ Prof. W.-C. Chen
Institute of Polymer Science and Engineering
National Taiwan University
Taipei 106 (Taiwan)
Fax : (+886)2-23635230
E-mail: chenwc@ntu.edu.tw
[b] T. Kurosawa,+ T. Higashihara, Prof. M. Ueda
Department of Organic and Polymeric Materials
Tokyo Institute of Technology
Tokyo 152-8552 (Japan)
E-mail: ueda.m.ad@m.titech.ac.jp
[c] Prof. W.-C. Chen
Department of Chemical Engineering
National Taiwan University
Taipei 106 (Taiwan)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300335.
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characteristics. Another concept involves increasing the
value of the dipole moment.[3,5,9] The higher the dipole
moment is, the more stable the CT complex that is formed.
In addition, the introduction of large conjugated moieties or
moieties with high electron-affinities is also effective.[16]
Recently, Li and co-workers reported protonic-acid-doped
poly(Schiff base) memory devices with more than 1000 con-
secutive switching cycles and multilevel storage capability,
which constituted significant progress in polymer-based re-
sistive-switching memory devices.[49] The radical cation or
anion that were generated in the CT state could be stabi-
lized by such moieties through delocalization or a deep trap
of charges, thus leading to a stable CT complex. Although
the relationship between chemical structure and memory
characteristics in polymer systems has been widely investi-
gated, there are only a few reports on small-molecule sys-
tems.[31–39] Moreover, the application of single organic D–A
molecules to memory devices has rarely been studied.[34,35,38]
Song and co-workers employed conjugated molecules that
contained electron-donating triphenylamine and electron-ac-
cepting cyanovinyl moieties for application in memory devi-
ces, in which triphenylamine provided a better hole-trans-
porting ability for improved ON/OFF current ratios.[34,35]
The same group also reported that the architecture of conju-
gated D–A-type molecules clearly affected the ON/OFF
ratio and reversibility of the
nal groups on the transported carriers.
As described above, the effect of the arrangement of D
and A groups on the memory characteristics of conjugated
and non-conjugated systems has been clarified to some
extent. However, until now, the structural effects of D or A
moieties on the memory properties of small-molecule sys-
tems have not yet been studied. Furthermore, the challenge
of providing a structural design concept to express nonvola-
tile memory characteristic by stabilizing the CT complex is
of utmost importance in the development of small-molecule
memory. Herein, we report two new oligoimides that were
prepared from electron-donating N-(4-aminophenyl)-N-
phenyl-1-aminopyrene (APAP) or N-(4-aminophenyl)-N-
phenyl-1-aminonaphthalene (APAN) moieties and electron-
accepting 4,4’-(hexafluoroisopropylidene)diphthalic anhy-
dride (6FDA) moieties, as shown in Scheme 1. Taking our
previous results into account,[38] the D–A arrangement was
set to D–A–D. The CT effect between the APAP or APAN
moieties (D) and the 6FDA moiety (A) was expected to
lead to electronic switching of the memory device. More-
over, the long conjugation units in the APAP and APAN
moieties (pyrene and naphthalene, respectively) were antici-
pated to stabilize the CT complex by delocalizing the radical
cation, thus leading to relatively nonvolatile memory charac-
teristics. Various thicknesses of nanoscale oligoimide thin
memory characteristics.[35] Devi-
ces that were based on D–A–
D-structured molecules showed
high ON/OFF ratios and rever-
sible write/read/erase proper-
ties, whereas devices that were
based on A–D–A-structured
molecules exhibited low ON/
OFF ratios and irreversible
switching behavior. Herein, the
arrangement of D and A units
in
a molecule had a strong
effect on the conformation and
dipole moment, which resulted
in different memory properties.
Recently, we reported that simi-
lar arrangement effects of the
D and A units were also ob-
served in non-conjugated oli-
goimide system.[38] Devices that
were based on D–A–D-struc-
tured oligoimides exhibited
nonvolatile negative differential
resistance (NDR), whereas A–
D–A-structured
oligoimide
showed an insulating property.
The difference between the
memory properties of two types
of structured oligoimides (D–
A–D and A–D–A) resulted
from the influence of the termi- Scheme 1. Synthetic of OI(APAP-6FDA) and OI(APAN-6FDA).
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films were also fabricated for evaluating their memory char-
acteristics. This study indicates that the electrical switching
performance of devices could be tuned through the careful
design of functional D–A-type oligoimides and their film
thickness.
mation of OI(APAP-6FDA) and OI(APAN-6FDA)
(Figure 1). In the corresponding imide of 6FDA, the charac-
teristic peaks of the phthalimide unit are typically observed
at about d=7.8, 8.0, and 8.2 ppm as a singlet, a doublet, and
a doublet, respectively.[36] However, only the singlet of the
phthalimide unit in OI(APAP-6FDA) was observed at d=
1
7.8 ppm. Therefore, a 2D H,1H-COSY NMR spectrum was
Results and Discussion
recorded (Figure 1a). Considering the correlations to the
1
characteristic singlet (Figure 1a, #6) in the 2D H,1H-COSY
Synthesis of the Oligoimides
spectrum and the integral values, the two unassigned dou-
blets (#7 and 8) are covered by the doublet and multiplet
peaks of the pyrene unit (#9 and #10–17, respectively). In
The synthesis of oligoimides was carried out according to
the synthetic route shown in Scheme 1. A mono-nitro com-
pound that contained
a pyrene
moiety (NPAP) was synthesized
through a Buchwald–Hartwig reac-
tion between 1-aminopyrene and io-
dobenzene, followed by a nucleo-
philic aromatic substitution reaction
with 4-fluoronitrobenzene. Then,
NPAP was converted into mono-
amino APAP through palladium-
catalyzed reduction in quantitative
yield. The disappearance of a char-
acteristic IR absorption peak at
about 1300 cmÀ1, owing to the nitro
group, and the appearance of char-
acteristic peaks at about 3300 and
3400 cmÀ1, which were assigned to
the amino group, indicated the com-
plete conversion of NPAP into
APAP. The successful synthesis of
APAP was also confirmed by 1H
and 13C NMR spectroscopy and by
elemental analysis. An analogous
mono-amino compound that con-
tained
(APAN) was synthesized according
to
literature procedure.[50] By
using these obtained amino com-
pounds, APAP and APAN,
a
naphthalene moiety
a
OI(APAP-6FDA) and OI(APAN-
6FDA) were prepared by their con-
densation with an tetracarboxylic
acid dianhydride (6FDA) through
thermal imidization (Scheme 1).
The
desirable
structures
of
OI(APAP-6FDA) and OI(APAN-
6FDA) were confirmed by FTIR
spectroscopy (see the Supporting
Information,
Figure S1),
which
showed the characteristic peaks of
imide groups at about 1780 cmÀ1
(symmetrical stretching of carbonyl
groups), 1720 cmÀ1 (asymmetrical
stretching of carbonyl groups), and
1380 cmÀ1 (C N bond). Also, the
À
Figure 1. a) 1H,1H-COSY NMR spectrum of OI(APAP-6FDA) and b) 1H NMR spectrum of OI(APAN-
1H NMR spectra supported the for- 6FDA).
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the case of OI(APAN-6FDA), the characteristic peaks are
observed at about d=7.74, 8.01, and 8.13 ppm, without over-
lapping with any signals (Figure 1b). In addition, the suc-
cessful synthesis of OI(APAP-6FDA) and OI(APAN-
6FDA) was also confirmed by 13C NMR spectroscopy and
elemental analysis.
345 nm for OI(APAN-6FDA) arise from a CT transition
from the donor to the acceptor moieties. The optical band
gaps (Egopt) of OI(APAP-6FDA) and OI(APAN-6FDA), as
estimated from the onset optical absorbance, are 2.83 and
3.21 eV, respectively. The relatively smaller optical band gap
of OI(APAP-6FDA) is due to the longer conjugation length
of APAP.
Figure 4 shows the CV curves of the OI(APAP-6FDA)
and OI(APAN-6FDA) films on an ITO glass substrate in
a 0.1m solution of tetrabutylammonium perchlorate (TBAP)
in anhydrous MeCN. OI(APAP-6FDA) and OI(APAN-
Characterization of D–A Oligoimides
The optical, electrochemical, and thermal properties of
OI(APAP-6FDA) and OI(APAN-6FDA) are discussed
below. Figure 2 shows the UV/Vis absorption spectra of the
studied OI(APAP-6FDA) and OI(APAN-6FDA) films. The
absorption peak maxima (lmax) of OI(APAP-6FDA) and
OI(APAN-6FDA) films are observed at about 329 nm and
6FDA) exhibit reversible p-type doping behavior during the
ox
anodic scan. The oxidation onset (Eonset
) values of
OI(APAP-6FDA) and OI(APAN-6FDA) are about 0.89
and 1.00 V (versus Ag/Ag+), respectively; thus, the estimat-
Figure 4. Cyclic voltammograms of OI(APAP-6FDA) and OI(APAN-
6FDA) thin films on ITO glass in a 0.1m solution of TBAPF6 in MeCN;
Figure 2. Optical absorption spectra of OI(APAP-6FDA) and
OI(APAN-6FDA) as thin films on a quartz plate.
scan rate: 0.1 VsÀ1
.
300 nm, respectively. The energy gaps of 3.77 eV for
OI(APAP-6FDA) and 4.13 eV for OI(APAN-6FDA), as es-
timated from their lmax values, are in good agreement with
the calculated values for OI(APAP-6FDA) and OI(APAN-
6FDA) (3.29 and 3.76 eV, respectively). These absorption
ed HOMO energy levels are À5.21 eV for OI(APAP-
6FDA) and À5.80 eV for OI(APAN-6FDA). Moreover, the
LUMO energy levels of the oligoimides, as estimated from
the difference between the optical band gap and the
HOMO energy level, are À2.38 eV for OI(APAP-6FDA)
and À2.59 eV for OI(APAN-6FDA). The thermal properties
of the oligoimides were evaluated by using DSC (see the
Supporting Information, Figure S2). Both oligoimides
showed high glass-transition temperatures (Tg) at 182 and
1658C for OI(APAP-6FDA) and OI(APAN-6FDA), respec-
tively. The excellent thermal properties of the oligoimides
are expected to meet the requirements of heat resistance in
the electronics industry.
À
peaks are attributed to the p p* transition (from the
HOMO to the second LUMO (LUMO+1)) of the APAP
and APAN moieties (Figure 3). The absorption bands from
355 to 385 nm for OI(APAP-6FDA) and from 317 to
Memory-Device Characteristics of the Oligoimides
The memory effects of the studied oligoimides were deter-
mined by investigating the current–voltage (I–V) character-
istics of ITO/oligoimide/Al sandwich devices. The oligoi-
mide memory device stores data, based on high- (ON) and
low-conductivity (OFF) responses to external applied vol-
tages with grounded ITO electrodes. The film thickness was
shown to significantly affect the memory characteristics,
owing to metal diffusion.[12,51] If the film is too thin, a metal
filament may form in the device, thus resulting in high con-
Figure 3. Molecular orbitals of OI(APAP-6FDA) and OI(APAN-6FDA).
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ductance. Once the film becomes thicker, such a metal-fila-
ment effect can be avoided and the “real” chemical-struc-
ture/memory properties could be established. Thus, we se-
lected film thicknesses of 38, 48, and 85 nm to explore its
effect on the memory characteristics of the corresponding
devices. Figure 5 shows typical I–V curves of OI(APAP-
6FDA) and OI(APAN-6FDA) memory devices with various
film thickness (recorded device units of 0.5ꢀ0.5 mm2) at
a scan rate of 0.1 VsÀ1. In the case of a device that is based
on a 38 nm-thick OI(APAP-6FDA) film (Figure 5a), after
the first voltage scan, the device maintains an extremely
high conductance (ꢀ10À1 A) state during the positive or neg-
ative scans (Figure 5a, sweeps 2–5). This electronic behavior
was also observed in the device that was based on a 38 nm-
thick OI(APAN-6FDA) film (Figure 5d). When using
a film with a thickness of 48 nm, the devices that were based
on both oligoimides showed different switching behaviors to
devices that were based on 38 nm-thick films (Figure 5b,e).
Figure 5b shows I–V curves that were based on ITO/
OI(APAP-6FDA) (48 nm)/Al memory cells. First, the volt-
age is swept from 0 to À6.0 V and the related threshold
(Vth), maximum (Vmax), and minimum voltages (Vmin) are
À1.9, À3.0, and À4.7 V, respectively. These results indicate
that the Vth value is the transition point from the OFF state
to the ON state, after which the current increases abruptly
from Vth to Vmax and decreases with a subsequent applied
voltage from Vmax to Vmin. The overall first sweep is selected
as a “writing” process. There is a local current maximum
(Vmax) and also local current minimum (Vmin), which is de-
fined as the negative differential resistance (NDR) region.
The current density maintains the ON state (still with the
NDR) for the subsequent negative sweep from 0 to À6.0 V
(sweeps 2–4) and the positive sweep from 0 to 6.0 V (Fig-
ure 5b, sweep 5). The ON/OFF current ratio of the observed
memory behavior is 4ꢀ101 at À1 V. This result is also the
case for the device that is based on
a 48 nm-thick
OI(APAN-6FDA) film (Figure 5e). On further increasing
the film thickness to 85 nm, the OI(APAP-6FDA) memory
Figure 5. Current-voltage (I–V) characteristic of a) ITO/OI(APAP-6FDA) (38 nm)/Al; b) ITO/OI(APAP-6FDA) (48 nm)/Al; c) ITO/OI(APAP-6FDA)
(85 nm)/Al; d) ITO/OI(APAN-6FDA) (38 nm)/Al; e) ITO/OI(APAN-6FDA) (48 nm)/Al; and f) ITO/OI(APAN-6FDA) (85 nm)/Al devices.
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device exhibited unique switching behavior compared to de-
vices that were based on 38 or 48 nm-thick films. The device
was scanned through five voltage sequences: 0 to À6.0 V
(sweeps 1–4) and 0 to 6.0 V (sweeps 5) in steps of 0.1 V. In
the first sweep, 0 to À2.9 V (sweep 1), the current density
initially remains at low conductance (OFF state). Further
sweeping the voltage to À3.0 V, the current density abruptly
increases by several orders of magnitude and changes into
a high-conduction state, thus indicating a transition from the
OFF state to the ON state. The electrical transition of this
memory device serves as the writing process. The current
density maintains an ON state during the subsequent nega-
tive sweep from 0 to À6.0 V (sweeps 2–4) and positive
sweep from 0 to 6.0 V (Figure 5c, sweep 5). Once it is
switched ON, the memory device cannot return to the OFF
state, even after turning off the power or applying a reverse
bias, thus implying a nonvolatile characteristic. Figure 6a
shows the retention times and stress tests of both the ON
and OFF states of OI(APAP-6FDA). Under a constant
stress of À1 V, no obvious degradation in current is ob-
served for both the ON and OFF states for at least ꢀ104 s
during the readout test. The stimulus effect of read pulses
on the ON and OFF states was also investigated for the
long-time test (Figure 6b). The pulse period and pulse width
are 3 and 2 ms, respectively, as shown in Figure 6b, inset.
The OI(APAP-6FDA) memory device is stable for at least
ꢀ108 continuous read pulses of À1 V. Therefore, the switch-
ing behavior on the remnant stored data and the nonvolatile
nature of the memory device can explain the functionality
of a WORM-type memory characteristic with an observed
ON/OFF current ratio of ꢀ104 at À1 V. A device that was
based on an 85 nm-thick OI(APAN-6FDA) film also
showed different behavior to films with thicknesses of 38
and 48 nm (Figure 5 f). The device was scanned through six
voltage sequences, 0 to À6.0 V (sweeps 1–5) and 0 to 6.0 V
(sweep 6), in steps of 0.1 V. During the first negative sweep
from 0 to À2.7 V (sweep 1), the current density initially re-
mained at low conductance (OFF state). Further sweeping
the voltage up to À2.8 V, the current density abruptly in-
creased by several orders of magnitude and changed into
a high-conduction state, thus indicating a transition from the
OFF state to the ON state. The current density remained in
the ON state during the subsequent negative sweep from 0
to À6.0 V (sweeps 2 and 3). The third sweep was performed
after turning off the power for about 4 min. On turning off
the power for a longer time, the device returned to its OFF
state and could be reprogrammed into the ON state again,
with an accurate threshold voltage of À3.1 V and kept in the
ON state (sweeps 4 and 5). This switching behavior suggests
that the ON state could be retained for a short period of
time after the removal of power and would eventually relax
to its original OFF state. Similar positive switching phenom-
ena as in the negative sweep are also observed. After re-
maining in the ON state during the repeated negative sweep
(sweep 5), the memory cannot reset to the OFF state by
a continuous applied positive bias from 0 to 6 V (sweep 6).
Hence, we concluded that the memory device possessed
a “remnant”—yet volatile—nature of the ON state, because
the stored data were finally
lost. Figure 6c shows the reten-
tion times and stress tests of
both the ON and OFF states of
OI(APAN-6FDA).
Under
a constant stress of À1 V, no
obvious degradation in current
is observed for both the 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 6d).
The pulse period and pulse
width are 3 and 2 ms, respective-
ly, as shown in Figure 6d, inset.
The OI(APAN-6FDA) memory
device is stable for at least ꢀ108
continuous read pulses of À1 V.
Therefore, the switching behav-
ior on the remnant stored data
and the volatile nature of the
memory device can explain the
functionality of a SRAM.
Figure 6. Retention times in the “ON” and “OFF” states of the a) ITO/OI(APAP-6FDA) (85 nm)/Al and
c) ITO/OI(APAN-6FDA) (85 nm)/Al devices under a continuous readout voltage. Stimulus effect of read
pulses on the ON and OFF states of the b) ITO/OI(APAP-6FDA) (85 nm)/Al and d) ITO/OI(APAN-6FDA)
(85 nm)/Al devices. Inset shows the pulse shapes of the measurements.
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the CT complex. As a result the OI(APAN-6FDA) device
exhibits a volatile memory characteristic.
Proposed Switching Mechanism in Polymer Memory
Devices
Further detailed information regarding the switching mecha-
nism of fabricated devices that were based on OI(APAP-
6FDA) and OI(APAN-6FDA) with various thicknesses are
discussed below. In the thinnest (38 nm) OI(APAP-6FDA)
and OI(APAN-6FDA) films, once the devices are switched
ON, they remain in a very high-conductance state. Such
a high-conductance state can be observed when a conducting
path, such as a metal filament, is generated in the organic
layer.[10] On the other hand, the macroscopic electrical
switching of 48 nm-thick oligoimide films show nonvolatile
behavior with NDR properties, which is generally observed
in composites of metal nanoparticles and organic poly-
mers.[11,44–46] The presence of charge-trapping sites near the
electrodes may be attributed to this current-switching effect
and, perhaps, the interface traps occur during the deposition
of the upper Al electrode, owing to the rough surface. The
Al metal can diffuse into the oligoimide layer with sufficient
thermal energy and serve as the deep charge-trapping
center. Charge tunneling between the metal particles that
are entrapped in the oligoimide film and charge trapping/de-
trapping on the Al centers are responsible for the memory
characteristics in this device. Therefore, the switching behav-
iors as observed in the devices that were based on 38 and
48 nm-thick OI(APAP-6FDA) and OI(APAN-6FDA) films
did not come from the nature of the oligoimides but from
the diffusing metal inside the organic layer. However, the
85 nm-thick OI(APAP-6FDA) and OI(APAN-6FDA) films
provide a sufficiently thick memory layer to overcome the
metallic diffusion pathway and appear as the memory
nature of OI(APAP-6FDA) and OI(APAN-6FDA). The
mechanism of WORM for the OI(APAP-6FDA) memory
device and SRAM for the OI(APAN-6FDA) memory
device are further supported by a field-induced CT effect
from the theoretical calculations, as described below. The
molecular electronic properties were explored by using den-
sity functional theory at the B3LYP/6-31G(d) levels of
theory. The HOMOs are localized on the donor moieties of
the APAP or APAN groups, whilst the LUMOs are local-
ized on the acceptor moieties, that is, the phthalimide of
6FDA. This result implies that CT between the donor and
acceptor moieties occurs in the excited state of OI(APAP-
6FDA) and OI(APAN-6FDA). Under excitation with suffi-
cient energy, electrons are possibly transited from the donor
moieties to the electron-withdrawing group, whilst the con-
jugated donor moieties give a channel for hole transport,
thus improving the conductivity of the oligoimide. After the
CT, the generated radical cation of OI(APAP-6FDA) in the
donor moiety is stabilized through delocalization by the
pyrene unit, thus leading to a stable CT complex. The stabi-
lized radical cation still remains in the donor unit, even
after the applied voltage is turned off, thereby resulting in
a nonvolatile memory behavior. In comparison, the conjuga-
tion length of naphthalene unit is not enough to stabilize
Conclusions
In summary, we have successfully synthesized new D–A oli-
goimides from APAP or APAN (D) moieties and 6FDA
(A) moieties for application in resistive-type memory devi-
ces. Devices with the configuration ITO/oligoimide/Al ex-
hibited multi-memory characteristics, depending on their
film thickness. Memory devices based on 38 nm-thick
OI(APAP-6FDA) and OI(APAN-6FDA) films showed high
conductance with Ohmic current-flow behavior. Memory de-
vices based on 48 nm-thick OI(APAP-6FDA) and
OI(APAN-6FDA) films exhibited nonvolatile NDR charac-
teristic, presumably owing to diffused Al atoms forming an
impurity band of the charge-trapping level. Moreover,
memory devices based on 85 nm-thick OI(APAP-6FDA)
and OI(APAN-6FDA) films exhibited excellent WORM
and SRAM characteristics with high ON/OFF current ratios
of ꢀ104–105, a long retention time of 104 s, and reproducible
switching behavior. Based on the electrical characterization,
the switching behavior of these devices could mainly be con-
trolled through an electric-field-induced CT mechanism
from an “OFF” state to an “ON” state, as evidenced by the-
oretical calculations. Moreover, the conjugation length was
considered to be responsible for determining the volatility
of the observed memory characteristics. The larger conjuga-
tion structure of the pyrene segment in OI(APAP-6FDA)
provides a relative stable CT complex for nonvolatile
memory devices. This study provides new insight into the
design of functional oligoimides and their film-thickness-de-
pendent electrical switching characteristics for application in
advanced memory devices.
Experimental Section
Materials
The materials were used without further purification unless otherwise
noted. Dehydrated N,N-dimethylacetamide (DMAc), 1,3-dimethyl-2-imi-
dazolidinone (DMI), dehydrated THF, tri-tert-butylphosphine, palladium
on activated carbon (Pd/C, 10 wt.%), and NaH were purchased from
Wako, Japan. Dehydrated THF (Wako, stabilizer-free, 99.5%) was dis-
tilled from its solution in sodium benzophenone under a nitrogen atmos-
phere. 4-Fluoronitrobenzene was purchased from Aldrich. Tris(dibenzyli-
deneacetone)dipalladium (0) ([Pd2ACHTNUTRGNENUG(dba)3]), iodobenezene, N-phenyl-1-
aminonaphthalene, and 6FDA were purchased from TCI. 6FDA was puri-
fied by sublimation. All other compounds were purchased from Wako,
Japan. N-(4-Aminophenyl)-N-phenyl-1-aminonaphthalene (APAN) was
synthesized according to a literature procedure.[47]
Synthesis of N-Phenyl-1-aminopyrene (PAP)
The azeotropic dehydration of
a solution of 1-aminopyrene (2.17 g,
10.0 mmol) and sodium tert-butoxide (1.29 g, 13.4 mmol) in toluene
(10 mL) was performed by using a Dean–Stark apparatus for 1 h under
a nitrogen atmosphere. After cooling the system to RT, iodobenzene
(1.84 g, 9.00 mmol), [Pd2ACTHNUTRGEN(UNG dba)3] (41.2 mg, 0.0450 mmol), and tri-tert-butyl-
phosphine (32.7 mL, 0.135 mmol) were added and the mixture was stirred
Chem. Asian J. 2013, 8, 1514 – 1522
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Mitsuru Ueda, Wen-Chang Chen et al.
at 608C for 12 h. Then, the mixture was dropped into an aqueous solu-
tion of ammonium chloride, extracted with CHCl3, and dried over mag-
nesium sulfate. After removing magnesium sulfate by filtration, the solu-
tion was evaporated and the crude solid was purified by column chroma-
tography on silica gel (n-hexane) to give a cotton-like pink/brown solid
(0.795 g, 30% yield). 1H NMR (300 MHz, [D6]DMSO): d=8.60 (s, 1H;
NH), 8.38 (d, J=9.3 Hz, 1H; ArH), 8.18–7.93 (m, 8H; ArH), 7.27 (t, J=
Synthesis of 4,4’-Hexafluoroisopropylidenebis[4-(N-phenyl-N-naphth-1-
ylamino)phenyl phthalimide], OI(APAN-6FDA)
OI(APAN-6FDA) was synthesized according to a similar procedure as
that for OI(APAP-6FDA) with APAN in 84% yield. 1H NMR
(300 MHz, [D6]DMSO): d=8.13 (d, J=8.1 Hz, 2H; ArH), 8.01 (d, J=
8.1 Hz, 2H; ArH), 7.95–7.92 (m, 6H; ArH), 7.74 (s, 2H; ArH), 7.62–7.43
(m, 8H; ArH), 7.30–7.25 (m, 8H; ArH), 7.08 (d, J=7.5 Hz, 4H; ArH),
7.01 (t, J=7.5 Hz, 2H; ArH), 7.13 (d, J=7.8 Hz, 4H; ArH), 7.05–
7.00 ppm (m, 6H; ArH); 13C NMR (75 MHz, [D6]DMSO): d=166.06,
165.95, 147.67, 147.01, 142.17, 137.17, 135.64, 134.85, 132.80, 132.40,
130.59, 129.30, 128.44, 128.03, 127.38, 126.93, 126.67, 126.51, 126.22,
124.30, 124.06, 123.37, 123.18, 122.51, 122.16, 119.82 ppm; IR (KBr): n˜ =
7.2 Hz, 2H; ArH), 7.16 (d, J=7.8 Hz, 2H; ArH), 6.88 ppm (t, J=7.5 Hz,
À1
À
À
1H; ArH); IR (KBr): n˜ =3410 (N H stretch), 1301 cm (C N stretch).
Synthesis of N-(4-Nitrophenyl)-N-phenyl-1-aminopyrene (NPAP)
A solution of PAP (0.881 g, 3.00 mmol) in dehydrated DMAc (5 mL)
was added dropwise to a solution of sodium hydride (0.144 g, 6.00 mmol)
in dehydrated DMAc (5 mL) that had been cooled to 08C and the mix-
ture was stirred at 08C for 30 min. A solution of 4-fluoronitrobenzene
(0.847 g, 6.00 mmol) in dehydrated DMAc (5 mL) was added dropwise to
the above reaction mixture and the solution was heated at 1108C for 10 h
under stirring. After the reaction, the mixture was poured into ice water
and the precipitate was collected by filtration. The crude solid was puri-
fied by recrystallization from EtOH to give an orange/yellow solid
(0.782 g, 63% yield). 1H NMR (300 MHz, [D6]DMSO): d=8.42–8.01 (m,
11H; ArH), 7.45–7.39 (m, 4H; ArH), 7.26–7.20 (m, 1H;, ArH), 6.77 ppm
(d, J=9.6 Hz, 2H; ArH); 13C NMR (75 MHz, [D6]DMSO): d=153.8,
145.0, 139.0, 137.8, 130.5, 130.2, 130.1, 129.9, 128.8, 128.4, 127.8, 127.5,
127.0, 126.7, 126.4, 125.8, 125.6, 125.5, 125.2, 124.9, 123.7, 122.9, 121.8,
116.1 ppm; IR (KBr): n˜ =1308 cmÀ1 (NO2 stretch); elemental analysis
calcd (%) for C28H18N2O2: C 81.1, H 4.38, N 6.76; found: C 81.2, H 4.72,
N 6.59.
1781, 1720 (C=O stretch), 1377 cmÀ1 (C N stretch); elemental analysis
À
calcd (%) for C63H38N4F6O4: C 73.5, H 3.72, N 5.44; found: C 73.4,
H 4.00, N 5.43.
Characterization
FTIR spectra were recorded on a Horiba FT-120 Fourier transform spec-
trophotometer. NMR spectra were recorded on a Bruker DPX-300S
spectrometer at 300 MHz for 1H nuclei and at 75 MHz for 13C nuclei in
[D6]DMSO with tetramethylsilane as an internal reference. Elemental
analysis was performed on a Yanaco MT-6 CHN elemental analyzer.
Thermal properties were estimated on a TA Instruments DSC-Q100 dif-
ferential scanning calorimeter (DSC) under a nitrogen atmosphere at
a heating rate of 108CminÀ1. Cyclic voltammetry (CV) was performed by
using a three-electrode cell, in which indium tin oxide (ITO) was used as
a working electrode (film area: about 0.7ꢀ0.5 cm2), platinum wire was
used as an auxiliary electrode, and Ag/AgCl, KCl (saturated) was used as
a reference electrode. UV/Vis absorption spectra were recorded on a Hi-
tachi U4100 UV/Vis/NIR spectrophotometer. The thickness of the poly-
mer film was measured on a microFigure measuring instrument (Surfcor-
der ET3000, Kosaka Laboratory Ltd.).
Synthesis of N-(4-Aminophenyl)-N-phenyl-1-aminopyrene (APAP)
A mixture of NPAP (0.500 g, 1.21 mmol) and Pd/C (10 wt.%, 0.0500 g)
in EtOAc (5 mL) was stirred at RT for 24 h under a H2 atmosphere.
After the reaction, the mixture was filtered through Celite to remove the
Pd/C and the solution was evaporated to obtain a bright-yellow powder
(0.441 g, 95% yield). 1H NMR (300 MHz, [D6]DMSO): d=8.29–8.24 (m,
2H; ArH), 8.21–8.13 (m, 4H; ArH), 8.06–8.01 (m, 2H; ArH), 7.83 (d,
J=8.1 Hz, 1H; ArH), 7.10 (t, J=8.7 Hz, 2H; ArH), 6.96 (d, J=9.0 Hz,
2H; ArH), 6.76 (t, J=7.5 Hz, 1H; ArH), 6.66 (d, J=7.8 Hz, 2H; ArH),
6.57 (d, J=8.7 Hz, 2H; ArH), 4.94 ppm (s, 2H; NH); 13C NMR (75 MHz,
[D6]DMSO): d=149.8, 145.5, 141.0, 136.3, 130.7, 130.4, 128.8, 128.4,
127.3, 127.1, 126.9, 126.5, 126.3, 126.0, 125.4, 125.0, 124.8, 124.0, 123.1,
Fabrication and Characterization of Memory Devices
The memory device was fabricated on ITO-coated glass with the configu-
ration ITO/oligoimide/Al. Before deposition of the organic layer, the
ITO glass was pre-cleaned by ultrasonication with water, acetone, and 2-
propanol for 15 min each. The layers of the oligoimides were vapor-de-
posited at a rate of 0.5 ꢂsÀ1 under a pressure of about 1ꢀ10À6 Torr to
their target thickness, as determined in situ on a calibrated quartz crystal
microbalance (QCM). A 300 nm-thick Al electrode (0.5ꢀ0.5 mm2) was
À
119.0, 118.8, 117.7, 114.9, 114.8 ppm; IR (KBr): n˜ =3444, 3382 (N H
thermally evaporated through
a shadow mask at a pressure of 8ꢀ
stretch), 1331 cmÀ1 (C N stretch); elemental analysis calcd (%) for
À
10À7 Torr with a uniform deposition rate of 2 ꢂ·sÀ1. The electrical charac-
teristics of the memory device were recorded on a Keithley 4200 semi-
conductor parametric analyzer. All electronic measurements were per-
formed in a N2-filled glove box.
C28H20N2: C 87.5, H 5.24, N 7.29; found: C 87.6, H 5.48, N 7.13.
Synthesis of 4,4’-Hexafluoroisopropylidenebis[4-(N-phenyl-N-pyren-1-
ylamino)phenyl phthalimide], OI(APAP-6FDA)
6FDA (0.213 g, 0.479 mmol) was added to a solution of APAP (0.384 g,
1.00 mmol) in dehydrated THF (2 mL). The mixture was stirred for 1 h
at RT and THF was evaporated. The remaining solid was dissolved in
DMI (3 mL) and the mixture was heated at reflux for 4 h. After cooling
to RT, the solution was poured into MeOH to remove any excess APAP
and the precipitate was collected by filtration to obtain a yellow powder
(0.555 g, 98% yield). 1H NMR (300 MHz, [D6]DMSO): d=8.37 (d, J=
8.1 Hz, 2H; ArH), 8.32 (d, J=6.6 Hz, 2H; ArH), 8.26–8.20 (m, 6H;
ArH), 8.17–8.05 (m, 8H; ArH), 7.96–7.90 (m, 4H; ArH), 7.29 (t, J=
7.5 Hz, 8H; ArH), 7.13 (d, J=7.8 Hz, 4H; ArH), 7.05–7.00 ppm (m, 6H;
ArH); 13C NMR (75 MHz, [D6]DMSO): d=166.06, 165.94, 147.84,
147.26, 139.59, 137.15, 137.13, 135.59, 132.76, 132.37, 130.55, 130.29,
129.36, 128.12, 128.07, 127.67, 127.64, 127.15, 126.98, 126.42, 126.27,
125.39, 125.16, 124.44, 124.03, 123.82, 123.37, 122.58, 122.43, 122.33,
Computational Methods
Theoretical molecular simulations of the oligoimide were performed by
using the Gaussian 03 program package. The density functional theory
(DFT) method, by using Becke’s three-parameter functional with the
Lee, Yang, and Parr correlation functional (B3LYP) at the 6-31G(d) level
of theory, was exploited for the optimization of the ground-state geome-
try and the electronic properties.
Acknowledgements
120.12 ppm; IR (KBr): n˜ =1781, 1724 (C=O stretch), 1376 cmÀ1 (C N
Financial support of this work from the National Science Council is
highly appreciated.
À
stretch); elemental analysis calcd (%) for C75H42N4F6O4: C 76.5, H 3.60,
N 4.76; found: C 76.2, H 3.98, N 4.71.
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