Different Steric-Twist-Induced Ternary Memory Characteristics in Nonconjugated Copolymers with Pendant Naphthalene and 1,8-Naphthalimide Moieties

Article abstract of DOI:10.1002/asia.201701044

Herein, novel random copolymers PMNN and PMNB were designed and synthesized, and the memory devices Al/PMNN and PMNB/ITO both exhibited ternary memory performance. The switching voltages of the OFF–ON1 and ON1–ON2 transitions for both memory devices are around ?2.0 and ?3.5 V, respectively, and the ON1/OFF, ON2/ON1 current ratios are both up to 10³. The observed tristable electrical conductivity switching could be attributed to field-induced conformational ordering of the naphthalene rings in the side chains, and subsequent charge trapping by 1,8-naphthalimide moieties. More interestingly, by adjusting the connection sites of 1,8-naphthalimide moieties to tune the steric-twist effect, different memory properties were achieved (PMNN showed nonvolatile write once, read many (WORM) memory behavior, whereas PMNB showed volatile static RAM (SRAM) memory behavior). This result will offer a guideline for the design of different high-performance multilevel memory devices by tuning the steric effects of the chemical moieties.

Full text of DOI:10.1002/asia.201701044

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Steric twist effect-induced different ternary memory
characteristics in non-conjugated copolymers with pendant
naphthalene and 1,8-naphthalimide moieties
Authors: Ming Wang, Zhuang Li, Hua Li, Jinghui He, Najun Li,
Qingfeng Xu, and Jian-Mei Lu
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10.1002/asia.201701044
Chemistry - An Asian Journal
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Steric twist effect-induced different ternary memory
characteristics in non-conjugated copolymers with pendant naphthalene
and 1,8-naphthalimide moieties
Ming Wang,[a] Zhuang Li,[a] Hua Li, *[a] Jinghui He,[a] Najun Li,[a] Qingfeng Xu,[a] and Jianmei Lu*[a]
Abstract: In this paper, novel random copolymer PMNN and PMNB
were designed and synthesized, and the memory devices Al/PMNN
or PMNB/ITO both exhibit the ternary memory performance. The
switching voltages of OFF-ON1 and ON1-ON2transitions for both
memory devices are around -2.0 V and -3.5 V, respectively, and the
ON1/OFF, ON2/ON1 current ratio are both up to 103. The observed
tristable electrical conductivity switching could be attributed to field-
induced conformational ordering of naphthalene ring in the side
chain, and subsequent charge trapping of 1,8-naphthalimide
moieties. More interestingly, through adjusting the connection sites
of 1,8-naphthalimide moieties to tune the steric twist effect, different
memory properties (PMNN with nonvolatile WORM memory
behavior, while PMNB with volatile SRAM memory behavior) were
achieved. This result will offer a guideline for the design of different
high-performance multilevel memory devices via tuning steric effects
of the chemical moieties.
the polymers containing naphthalene (PMN) have obvious
advantages, such as larger conjugated plane, excellent charge
liquidity and high ON/OFF current ratio.[6] Being structurally
similar to naphthalene group, 1,8-naphthalimide moiety is a well-
known electron-withdrawing group which can serve as the
charge traps to block the movement of the charge carriers.[7]
Furthermore, it is possible to utilize the different steric effects of
1,8-naphthalimide moieties through different connection sites to
7c, 8]
tune the memory behaviors.[7a,
Therefore, we hope to
introduce 1,8-naphthalimidemoieties in PMN to achieve different
ternary polymer memory devices.
Herein, we designed and synthesized two pendant copolymers,
PMNB and PMNN, with different connection sites of 1,8-
naphthalimide moieties, as shown in Figure 1a. The various
connection sites of 1,8-naphthalimide exhibited different steric
effects in polymer chains (Figure S1). Consequently, the
Al/polymer/ITO devices exhibit different ternary memory
behaviors. PMNB device shows volatile SRAM memory
behavior, while PMNN exhibits nonvolatile WORM memory
behavior. This result may offer a guideline for the design for
various high-performance multilevel memory performances via
tuning steric effects of the chemical moieties.
Introduction
With the rapid growth of information industry, polymeric binary
memory materials have attracted increasing attentions and have
potential as replacements for traditional silicon-based
semiconductors in the future due to the advantages of good
solution processability, ease of miniaturization and tailored
properties through molecular design.[1] Time till now, several
switching mechanisms such as charge transfer (CT) effect,
conductive filament formation, conformational change, and
trapping/detrapping mechanism have been illustrated.[1e, 1h, 2] To
meet the demand of high-density data storage (HDDS), one of
the strategies considered is to realize multilevel (eg. ternary)
memory devices, which can increase the data storage capacity
exponentially from 2n to 3n.[3] Recently, Kang et al.[4] have
reported a polymer-based ternary memory device through two
mechanisms. However, the ternary memory performance is
limited to nonvolatile memory, which is difficult to face the
practical demand for various ternary memory types.[5] Thus,
obtaining diversiform ternary polymeric memory devices are
highly desirable.
Results and Discussion
Among all the conformation-induced electrical bistability systems,
[a]
M. Wang, ZH.Li, Prof. H. Li, Prof. J. He, Prof. N. Li,Prof. Q. Xu, Prof.
J. Lu
College of Chemistry, Chemical Engineering and Materials Science,
Collaborative Innovation Center of Suzhou Nano Science and
Technology
Figure 1. (a) Molecular structures of PMNB and PMNN; (b)
A schematic diagram of the
ITO/polymer/Aldevice.
Soochow University
Suzhou 215123, P. R. China
E-mail: jinghhe@suda.edu.cn, lujm@suda.edu.cn
PMNN and PMNB were synthesized in cyclohexanone solution
by free radical copolymerization [Scheme 1; see the
Supporting Information (SI)]. The chemical structures of
Supporting information for this article is given via a link at the end of
the document.
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random copolymers and the relative ratios of electron
donor/acceptor segments were confirmed by the 1H NMR
spectra (Figure S2 and S3, SI). The relatively molecular weights
(Mw) of PMNB and PMNN, obtained by GPC using THF as the
eluent and methyl methacrylate as standards, were 11300 and
17500g·mol-1 with the polydispersity index (PDI) of 1.48 and
1.97separately.Both random copolymers exhibited good
thermalstability with an onset decomposition temperature of
top electrode (Figure S6). The typical I-V characteristics of
memory devices in steps of 0.1 V are shown in Figure 3. For the
case of PMNN (Figure 3a), the device initially starts with low-
conductivity state (OFF state) and the current increased slowly
until the turn-ON voltage. When a negative voltage is about –2.0
V, an abrupt increase in the current take place from 10-9 to 10-6
A, indicating that the device undergoes a sharp electrical
transition from
a low conductivity state(OFF state) to an
o
about 300 C The Tg data of PMNB and PMNN are 106oC and
intermediate conductance state (ON1 state). As the voltage
approaches –3.2 V, a second sharp leap in current is observed
from 10-5 to 10-2 A, indicating the transition from an intermediate
conductance state (ON1 state) to a high conductance state
(ON2 state)(sweep 1). The PMNN memory device exhibits the
distinctly tristable conductance state with ON1/OFF, ON2/ON1
current ratio both up to 103, and these OFF-ON1 and ON1-
ON2transitions that response to an external voltage can serve
as a “writing” process in the memory cell. After a subsequent
scan from 0 to –6.0 V, the memory device can be remained in
ON2 state even after removing the power supply (sweep 2). This
memory device, therefore, demonstrates typical non-volatile
WORM-type memory behavior.[12]
110oC calculated by DSC.(Figure S4, SI), implying that both
copolymers are capable to endure heat deterioration in memory
devices.[9]
Figure 2. (a) UV-vis absorption spectra of the PMNB and PMNN films on quartz plate. (b) Cyclic
voltammogram of PMNB and PMNN thin films spin-coated on ITO substrates in CH3CN containing
0.1 M TBAP at a scan rate of100 mV·s-1. The inset shows the CV curve of the ferrocene standard
swept in the same conditions as for the two polymers (E1/2 (ferrocene) was measured to be 0.48 eV
vs. Ag/AgCl in CH3CN).
UV-vis absorption spectra and cyclic voltammogram of PMNB
and PMNN films were measured to study photophysical and
electron chemical properties, respectively (Figure 2).The
characteristics data of PMNB and PMNN are summarized in
Table 1.The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of PMNB and
PMNN are calculated to be –5.05 eV/-2.32 eV and -5.07eV/-2.97
eV, respectively. Therefore, hole injection energy barriers of
ITO/HOMO are smaller than the electron injection energy
barriers of Al/LUMO, indicating that the conduction processes
are dominated by hole injection.[10] Atomic force microscopy
(AFM) images show that both films are quite smooth with root-
mean-square (RMS) roughness of about 1.5 nm (Figure S5, SI).
The observed smooth morphology of thin films is beneficial for
the charge injection from the electrode into the polymer films.[11]
Figure 3. Typical I-V characteristics of the ITO/PMNN/Al (a) and the ITO/PMNB/Al (b) in the OFF,
ON1 and ON2 state.
The electrical switching characterization of PMNB is shown in
Figure 3b, which is different from that fabricated with PMNN.
Similarly, the device is also initially at the OFF state. When a
negative voltage is applied, two abrupt increases in the current
can be observed around –1.96 V and –3.70 V, respectively.
However, the device can be reprogrammed from the OFF to
ON1 or ON2 state after removing the power about 5 min (sweep
3), indicating the device is rewritable. Besides, the voltage
sweeping from 0 to –3.0 V (sweep 5, 6) and 0 to –6.0 V (sweep7,
8) was successively performed. Thus, it can be concluded that
PMNB based memory device exhibits the volatile nature of a
static random access memory (SRAM).[13] The unstable ON2
state of the volatile memory device can be retained by a
refreshing voltage pulse (named as the rf trace) of –1.0 V within
1 ms duration every 5 s.
Table 1. optical and electrochemical properties of basic units and random copolymers in the DMF
Eox
Ered
HOMO
(eV)
LUMO
(eV)
ΦITO-HOMO
LUMO-
(eV)
(eV)
(eV)
ΦAl (eV)
PMNB
PMNN
0.88
-0.09
-5.05
-2.32
0.25
1.98
0.91
-0.11
-5.07
-2.97
0.27
1.33
The long-term stability and endurance performance are also
evaluated from the retention time and stimulus effect test under
the same atmosphere (Figure S7, SI). Figure S6 shows that the
PMNB and PMNN devices could endure over 108 continuous
read pluses of –1.0 V and no significant degradation in the
current for any state is observed for 100 min. It is noteworthy
that the writing voltages are all smaller than –4.0 V and the
solution.
Figure 1b is the schematic diagram of the ITO/polymer/Al
memory device, which contains a about 80 nm thick film that
sandwiched between an ITO bottom electrode and an aluminum
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Figure 5. UV-vis absorption spectra of the PMN, MBNa, MNNa, PMNB and PMNN in DMF.
reading voltage is as low as –1.0 V, which imply low power
consumption of the PMNB and PMNN device during practical
data-storage process.
The optical properties of basic units and PMNB or PMNN
(Figure 5) were measured to better understand the charge
trapping switching progress. The main absorption peaks of
PMNB and PMNN are identified by a linear superposition of the
absorption on PMN, MBNa and MNa.[18] Thus, the charge
transfer (CT) between the naphthalene donor and 1,8-
naphthalimide acceptor is relatively weak. Density functional
theory (DFT) molecular simulation results showed an open
channel with negative molecular electrostatic potential (ESP)
regions in blue, which were attributed to the 1,8-naphthalimide
groups (Figure S10). These negative regions can serve as
‘‘charge traps’’ to block the mobility of the charge carriers. Thus,
the second switching behavior is originated from the 1,8-
naphthalimide trapping instead of the CT effect. With the
increase of the voltage scan, the generated holes with sufficient
activation energy will inject into thin film layer to fill the 1,8-
naphthalimide traps. Consequently, the charge traps will full of
charges, thus forming another conducting channel to result in
high-conductivity state. [19]
Figure 4. the ex-situ UV-vis spectra of the PMNN (a) and PMNB (b) thin films in the low-
conductivity (OFF state) and high-conductivity (ON1 state). The ON1 state was induced using a
removable liquid-Hg droplet as the top electrode.
Huang and Ree al. once reported the conformational change of
a single polymer to realize binary memory behavior through
making disoriented pendant groups form oriented face-to-face
conformation under an electrical field.[14] X-ray diffraction (XRD)
analysis was first measured to ensure the low degree of
regioregularity in copolymer PMNN or PMNB film at the ground
state (Figure S8). When a negative bias (0 to –2.5 V) was
applied, the holes injected from electrode (ITO) to the
naphthalene groups near the interface (because the content of
naphthalene groups is significantly higher than that of 1,8-
naphthalimide) to form active species. Subsequently, attracted
by the active naphthalene species, the neighboring neutral
naphthalene or 1,8-naphthalimide groups tended to form a
partial or full face-to-face conformation, through which the holes
are delocalized to the neighboring naphthalene groups to form a
conducting channel.
The UV-vis and PL spectra measurements of PMNB or PMNN
thin film before and after the sweeping voltage from 0 to –2.5 V
were analyzed to verify conformational change mechanism,[14b,
Figure 6. Schematic diagram of the charge carrier transport process in the polymer PMNN and
PMNB based memory device.
15]
as shown in Figure 4. After the voltage sweep, UV-vis
showed a red shift (for PMNN film) because of the ordered π-π
stacking of naphthalene aromatic ring.[16] While PMNB film only
To better understand the switching behavior of these devices,
we diagramed the operational mechanism models,[1e] as
depicted in Figure 6. The ternary memory device fabricated with
PMNN or PMNB can be assigned to the conformation-induced
conductance switching of naphthalene rings (OFF to ON1 state)
and the subsequent charge trapping of 1,8-naphthalimide (ON1
to ON2 state). For PMNN, the flexible chain was connected to
1,8-naphthalimide group of the 4-site, after applying the voltage,
the region-random of naphthalene groups transitioned to a
region-regular ordering, and due to the similar arrangement of
naphthalene groups and 1,8-naphthalimide, the formed ordered
conformation was stable, thus it is difficult to return to the
original OFF state even shutting down the applied voltage. While
for PMNB, the flexible chain was connected to 1,8-
naphthalimide group of the N-site, which will cause the steric
hindrance of 1,8-naphthalimide groups in PMNB, thus leading to
conformational relaxation from the region-regular arrangement
to a region-random states of the pendant 1,8-naphthalimide
moieties.
exhibits slight broadening, indicating lower degree of
14a]
regioregularity compared to polymer PMNN.[10b,
High
sensitivity fluorescence spectrum (Figure S9) further confirmed
the conformational change phenomena of these two
polymers.[17]
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Conclusions
Keywords:pendent copolymer • tristable switching • double
mechanisms •steric effect
In conclusion, two new random copolymers PMNN and PMNB
containing
a flexible spacer and electron-accepting 1,8-
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high quality thin films but display different ternary memory
behaviors. According to the analysis, the memory behaviors
were achieved by combining dual mechanisms, conformational
change and charge trapping mechanism. The different memory
behaviors of PMNN and PMNB were caused by different
connection sites of 1,8-naphthalimide moieties thus leading to
different steric twist effects in polymers. This work offers
fundamental insight for the rational design of ternary memory
materials for polymer memory devices.
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Device Fabrication and Characterization: The indium tin oxide (ITO)
glass was precleaned sequentially with deionized water, acetone and
ethanol in ultrasonic bath each for 20 min, respectively. The copolymer
was dissolved in cyclohexanone (12 mg·mL-1) and filtered through micro
filters with a pinhole size of 0.22 mm, Thereafter, the solutions were spin-
coated onto ITO at 2000 rpm and the solvent was removedin a vacuum
chamber at 10-1 Pa and 50 oC for 12 h. The thickness of the polymer
layer was about 80 nm. Finally, the top Al electrode of 100 nm thick was
thermally evaporated through a shadow mask under a pressure of 5×10-
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a radius of 0.2 mm). All electrical measurements of the device were
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a
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The authors gratefully thank the NSF of China (21336005 and
21476152), and National Excellent Doctoral Dissertation funds
(201455).
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10.1002/asia.201701044
Chemistry - An Asian Journal
FULL PAPER
FULL PAPER
Due to the adjusting of the connection
sites of 1,8-naphthalimide moieties to
tune the steric twist effect, two novel
random copolymer PMNN and PMNB
exhibit different ternary memory
performance through conformational
Ming Wang, Zhuang Li , Hua Li,* Jinghui
He, Najun Li, Qingfeng Xu and Jianmei
Lu*
Page No. – Page No.
Steric twist effect-induced different
ternary memory characteristics in
non-conjugated copolymers with
pendant naphthalene and 1,8-
naphthalimide moieties
change
and
charge
trapping
mechanism.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.