Rewritable ternary data storage devices based on polymethacrylate containing pendent azobenzene-naphthalene with the combined effects of conformation change and charge traps

Article abstract of DOI:10.1039/c7tc02473h

In this paper, we combined shallow electron trap azobenzene with planar naphthalene in the side chains of polymethacrylate. The sandwich-structured ITO/polymer/Al device shows different memory characteristics with the different side-chains of the polymer. When the alkyl length is 0, the device could only be turned to a moderate OFF state from the ON state, while when the alkyl length is 2 carbons the device can be turned to a moderate OFF state first and then to the original OFF state as the electric field is increased. It has been rarely reported that one device based on pendent polymers shows three different current states under a continuous electric field, which could be corresponding to "0", "1" and "2" for ternary data storage. The conductance switching can be explained in terms of field-induced conformational ordering of the side-chains in the polymer, and the "traps" of azobenzene impeding the mobility of the charge carriers. X-ray diffraction patterns were carried out to confirm the conformation change of the pendent groups during the OFF to ON and then back to OFF state processes.

Full text of DOI:10.1039/c7tc02473h

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Rewritable ternary data storage device based on
polymethacrylate containing pendent azobenzene-naphthalene
with combined effects of conformation change and charge traps †
Received 00th January 20xx,
Accepted 00th January 20xx
Zhuang Lia, Ming Wanga, Hua Li*a, Jinghui Hea, Najun Lia, Qingfeng Xua and Jianmei Lu*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this paper, we combined the shallow electron trap azobenzene with the planar naphthalene in the side chain of
polymethacrylate. The sandwich-structured device ITO/polymer/Al shows different memory characteristics as the different
side-chains of the polymers. When the alkyl length is 0, the device could only be turned to a moderate OFF state from the
ON state, while the alkyl length is 2 carbon the device can be turned to a moderate OFF state first and then to the original
OFF state as the electric field increased. It has been rarely reported that one device based on pendent polymers shows
three different current states under continuous electric field, which could be corresponding to “0”, “1” and “2” for ternary
data storage utilizes. The conductance switching can be explained in terms of field-induced conformational ordering of the
side-chains in the polymer, and the “traps” of azobenzene impede the mobility of the charge carriers. X-ray diffraction
patterns were carried out to confirm the conformation change of the pendent groups during the OFF to ON and then back
to OFF state processes.
length between the carbazole group and the polymer
backbone is increased. They ascribed the mechanism to the
conformation change of carbazole groups in the side chains
Introduction
Recently, the electric memory devices based on organic and
polymer materials have been received great interests for their
low-cost, rich structure flexibility, 3D stacking capability and
large capacity for data storage. They are potential alternatives
to traditional silicon-based semiconductors in the application
under continuous electric field. In addition, there are other
many examples that the different memory performances
based on the charge trapping-detrapping mechanism due to
the pull-push electron groups in the molecular backbone.
Azobenzenes have been extensively applied in electric
memories with electric bi-stability owing to the good charge
transport capability through the p-π backbone17-19. Our group
has reported a series of memory devices based on the
polymers containing azobenzene groups. We found the azo
groups could act as a shallow electron trap during the electric
switching process20-22. Herein, we combine the azobenzene
with the moderate π-planar naphthalene in the side chain of
polymethacrylate and expect that the synthesized polymers
with different alkyl length between the side functional groups
and the polymer main-chains could act different memory
performances under electric field.
of memory devices1-8
. The pendant polymers containing
functional groups in the side chains have extra advantages in
memory device duo to the easy monomer design, simple
synthesis methods and good film-forming abilities. Through
designing and synthesis different π-conjugated or donor-
accepter groups in the side chains, different memory devices’
performances could be achieved9-16
.
The conductive
mechanism was believed due to field-induced conformation
change or charge trapping-detrapping process during the OFF
to ON state switch. The Kang group15 has reported that the
electrical behavior of the memory device changes from a single
conductivity state (PVK, no spacer) to the nonvolatile bi-stable
states (PMCz, spacer is -(CH2)2-) and then to the volatile bi-
stable states (PVBCz, spacer is -CH2-O-(CH2)2-) when the spacer
Experimental
Material
1-aminonaphthalene, triethylamine, phenol, Sodium nitrite,
Potassium carbonate, 2-chloroethanol methacryloyl chloride
and 2,2’-azobis (isobutyronitrile) (AIBN) were all purchased
from Shanghai Chemical Reagent Co., Ltd without further
purification. Tetrahydrofuran (THF), N, N-dimethylformamide
and cyclohexanone were purified by vacuum distillation. Other
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solvents were obtained from commercial source and used as Characterization of the polymers
ARTICLE
received.
The whole synthesis procedure of PMANA0 and PMANA2 was
shown in Scheme 1. The successful polymerization was
confirmed by the 1H-NMR spectra as shown in Figure 1. The
vinyl proton signal of the methacrylate group at δ= 6.41, 5.82
ppm in MANA0 and δ= 6.17, 5.61 ppm in MANA2 totally
Synthesis Route
The synthesis routes were shown in Scheme 1.
1
disappeared in the corresponding polymer’s H-NMR spectra.
Both polymers have good thermal stability with onset
decomposition temperature of 309 oC and 285 oC, respectively
(Figure 2).
Scheme 1 The synthetic route of the polymer PMANA0 and PMANA2.
Synthesis procedure of the polymers
Free-radical polymerization of PMANA 0. PMANA0 were prepared
by free radical method using AIBN as initiator. The compound 3 (1 g,
3.2 mmol) and AIBN (4 mg, 0.024 mmol) were dissolved in 2 mL
cyclohexanone and stirred at 80 oC for 24 hours under nitrogen
atmosphere. The resulting polymer solution was cooled and poured
into ethyl acetate to precipitate the polymer. The chemical
structure of the polymer was confirmed by 1H-NMR (Figure 1a). The
number-average molecular weight of PMANA0 determined by GPC
is 18000 with a polydispersity index of 1.40.
Fig. 1 1H-NMR spectra of polymer (a) PMANA0 and (b) PMANA2, the insets
are the vinyl proton signal of the methacrylate group in the monomer.
Free-radical polymerization of PMANA2. The synthesis of PMANA2
is similar to PMANA0. The chemical structure of PMANA2 was
confirmed by 1H-NMR (Figure 1b). The GPC indicates the number
average weight of PMANA2 is about 13100 and PDI is 1.58.
Fabrication and measurement of the memory devices
The indium-tin oxide (ITO) glasses were pre-cleaned with
water, acetone, and alcohol sequentially in an ultrasonic bath
for 25 min, respectively. The polymer solution was prepared in
cyclohexanone (10 mg/mL) and filtered through microfilters
with a pinhole size of 0.22 µm. The solutions were then spin
coated onto ITO. The residue solvent was removed in a
vacuum chamber at 10-3 torr and 60 oC for 16 hours. The
polymer film thickness was measured to be ~80 nm. A layer of
Al, ~200 nm in thickness, was thermally evaporated and
deposited onto the polymer film surface at about 10-6 torr
Fig. 2 TGA curves of polymer (a) PMANA0 and (b) PMANA2 measured in
nitrogen atmosphere at a heating rate of 10 oC/min.
Photophysical and Electrochemical properties
The UV-Vis spectrum of PMANA0 in DMF solution is shown in Figure
3a, the maximum absorption peak at 376 nm is due to π-π*
electronic transitions of the aromatic ring. The band gap between
through a shadow mask to form the top electrode. The active the HOMO and LUMO level of PMANA0 can be estimated to be
device area was about 0.0314 mm2.
about 2.88 eV (Figure 3a). The HOMO energy levels can be
calculated from the onset oxidation potential based on the
reference energy level of ferrocene (Figure 3b), acoording to the
following equations.
Results and discussion
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Fig. 3 (a) and (c) are UV-vis spectra of PMANA0 and PMANA2 in THF solution; (b) and (d) are cyclic voltammetry (CV) curves of PMANA0 and PMANA2 film on
ITO substrate in acetonitrile with 0.1 M n-Bu4NPF6 as the supporting electrolyte. The insets are the CV curves of the ferrocene standard, swept in the same
conditions as that for polymer films.
HOMO (eV) = – (EOX + 4.8 – EFOC);
LUMO (eV) = HOMO – Eg,
3), the current was abrupt decreased to a moderate OFF2 state at a
threshold voltage of about 1.1 V. The device remained in this OFF2
state during the subsequent scan from 0 to 3V (sweep 4). This
electrical transition represents the “erase” process for the memory
device. The erased state (“2”) can be further written to the stored
state (“1”) when the switching threshold voltage was applied
(sweep 5), and also allowed a voltage (e.g., -1.0 V) to read the “2”.
The device showed good repeatable write-erase-write processes
(Figure 4a, cell2). This indicates that the device is a rewritable
memory device and exhibits tri-stable electrical conductivity states
which correspond to “0”, “1” and “2” for ternary data storage
utilities.
where EOX is the onset oxidation potential and EFOC is the external
standard potential of the ferrocene/ferricinium ion couple. The
HOMO and LUMO energy levels are determined to be -4.98 eV and -
2.10 eV, respectively. Similarly, the HOMO and LUMO energy levels
of PMANA2 were determined to be -5.64 eV and -2.74 eV,
respectively, from the UV-Vis spectrum and cyclic voltammetry
curves (Figure 3c and d).
Memory effect of the device based on PMANA0 and PMANA2
Figure 4d shows the memory effects of the
ITO/PMANA2/Al sandwiched device. In the first voltage scan
from 0 to -3V (sweep 1), a sharp current increase occurred at a
threshold voltage of around -2.20 V, indicating a transition
Memory devices of an ITO/PMANA0/Al sandwich structure were
fabricated mentioned above. The memory effects of PMANA0 are
demonstrated by the typical I-V curves of this sandwich device, as
shown in Figure 4a. In the first voltage scan from 0 to -3 V, a sharp
current increase occurred at a threshold voltage of around -1.79 V,
indicating a transition from a low-conductivity state (OFF1 state) to
a high-conductivity state (ON state). This electrical transition serves
as the “writing” process for the memory device. The device
remained in this ON state during the subsequent scan from 0 to -3 V
(sweep 2), where the ON/OFF current ratio was up to 104, allowing
a voltage (e.g., -1.0 V) to read the “0” or “OFF” signal (before
writing) and “1” or “ON” signal (after writing) of the memory
device. But in a voltage sweep of opposite bias from 0 to 3 V (sweep
from
a low-conductivity state (OFF1 state) to a high-
conductivity state (ON state). This electrical transition serves
as the “writing” process for the memory device. The device
remained in this ON state during the subsequent scan from 0
to -3 V (sweep 2). However, once a reverse voltage from 0 to 4
V was applied (sweep 3), an abrupt decrease in the current
was observed from 10-2 to 10-3 A (OFF2 state) at a threshold
voltage of about 3.11 V and then to 10-6 A (OFF1 state) at a
threshold voltage of 3.77 V. Other cells in the device
performed repeatable write-erase-write memory behavior,
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which indicates that the PMANA2 based device is also traditional binary device storage density which is 2^50.
rewritable type characteristics. The device based on PMAN0 Both of the devices have the good stability. No degradation
and PMANA2 displayed totally different IꢀV performance: the in current was observed after more than 15 minutes under a
device based on PMAN0 can only be reversed to the moderate constant stress of -1 V or surpass one hundred million (108)
OFF state while the device based on PMANA2 can be totally read cycles under a continuous read pulses of -1 V (Figure 4b, c,
turned to the original OFF current state. Each of our memory e, f). This characteristic promises a low misreading rate due to
devices has 50 same ternary memory cells, the device's the precise control of the tri-current states.
storage density of 3 ^ 50 is 600 million times compared to the
Fig. 4 (a) Current–voltage (I–V) characteristics of the memory device based on PMANA0; (b) is the effect of retention time on the ON and OFF states of the
memory device under a constant stress of -1.0 V; (c) is the effect of read cycles on the ON and OFF state under a read pulse voltage of -1.0 V; (d) Current–
voltage (I–V) characteristics of the memory device based on PMANA2; (e) is the effect of retention time on the ON and OFF states of the memory device
under a constant stress of 1.0 V; (f) is the effect of read cycles on the ON and OFF state under a read pulse voltage of 1.0 V.
barriers between the Φ of Al (-4.28 eV) and LUMO energy
Proposed Memory Mechanism
levels (2.18 eV for PMANA0 and 1.54 eV for PMANA2). This
Firstly, the relative energies of HOMO and LUMO energy levels
of the two polymers and the work functions (Φ) of Al top and
ITO bottom electrodes play an important role in the different
switching behavior in the respective devices. The energy
barriers between the Φ of ITO (-4.8 eV) and HOMO energy
levels are 0.18 eV for PMANA0 and 0.84 eV for PMANA2,
which are much lower than their corresponding energy
indicates that hole injection from ITO into the HOMO of the
polymers (corresponding to ITO as the anode) is easier than
electron injection from Al into the LUMO of the polymers.
Thus, PMNA0 and PMANA2 are both p-type materials and
holes predominate the conduction process. Additionally, the
hole injection barrier for PMANA0 is 0.18 eV, which is much
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lower than that for PMANA2 with 0.84 eV, this may explain the reverse voltage was applied, hence the decrease in current
level can only recover to 10-3 A (Figure 6c).
lower onset voltage of PMANA0 based memory device.
Fig. 6 The conformation change and electron trap filling state of the side-
chain groups under electric filed, (a) is the OFF state film, (b) is the ON state
film and (c) is the OFF1 state film.
The conductive process of ITO/PMANA2/Al device is similar
to that of ITO/PMANA0/Al device. When a reverse voltage was
applied, the trapped charges in the azo groups could be easily
detrapped, causing the current back to 10-3 A at a threshold
voltage around 3.11 V. However, the device of ITO/PMANA2/Al
can be further erased to the original OFF state (10-6 A). We
believe that this is because the longer flexible spacer between
the pendant naphthalene group and the main-chain backbone
in PMANA2 allows a larger free volume and a greater degree
of conformational freedom for the pendant naphthalene
group24. Therefore, the device based on PMANA2 is easy to
return to the random state when a reverse voltage is applied
(Figure 6c to 6a).
Fig. 5 X-ray diffraction patterns of PMANA0 and PMANA2 in different states
(OFF-film and ON-film).
To interpret the different memory properties of PMANA0 and
PMANA2 based devices, the XRD (Xꢀray diffraction) patterns
before and after voltage sweeping were measured, as shown in
Fig. 5. In comparison with OFFꢀfilm state, the diffraction peak
of PMANA0 and PMANA2 in ONꢀfilm at 2θ= 21.2° (d = 4.2
Å) has been strengthened. In addition, a new diffraction peak of
PMANA0 in ONꢀfilm at 2θ= 26.3° (d = 3.4 Å) appeared and
the diffraction peak of PMANA2 in ONꢀfilm at 2θ= 11.7° (d =
7.56 Å) has also been strengthened. This indicates that the
molecular arrangement of PMANA0 and PMANA2 in ONꢀfilm
is partly or fully regular because the azonaphthalene pendant
groups in PMANA0 and PMANA2 tend to form faceꢀtoꢀface
ordered structures during voltage application.
Conclusions
In summary, two simple sandwiched structure devices based
on PMANA0 and PMANA2 with the film thickness of 80 nm
were fabricated and their electronic characteristics were
investigated. Both devices showed a special rewritable memory
with tri-current states, which correspond to “0”, “1” and “2”
for ternary data storage. The rewritable ternary device is
potential for ultra-high density data storage utilizes. The
conductance switching mechanism was demonstrated to arise
from conformation-induced and electronic trap influence in
the side chain polymers, which were unambiguously
supported by X-ray diffraction patterns. Hopefully the present
study could provide guidance for efficient synthesis of more
polymers for ultra-high density memory device usage.
To better understand the mechanism of PMANA0 and
PMANA2 based devices, a mechanism model was proposed, as
shown in Fig. 6. The naphthalene pendant groups in PMANA0
are initially in random orientations. Under an applied field,
hole injection from ITO oxidizes to the naphthalene groups
near the interface, forming positively charged species, which
would drive the nearby neutral naphthalene groups to form
partial or full face-to-face conformation with the neighboring
naphthalene groups (Figure 6b), resulting in enhanced charge
transport through neighboring polymer chains23. Meanwhile,
the trap arising from Azo would be full of injected electrons to
form the conducting channels. For PMANA0, the
azonaphthalene acts as a rigid pendant groups in polymer side Acknowledgements
chain, it is difficult to return to its initial random state even if a
The authors gratefully thank the NSF of China (21336005 and
reverse voltage is applied. However, the Azo moieties possess a
relatively shallow electron trap depth. The captured charges
would be easily evicted out from the shallow traps when a
21476152), and National Excellent Doctoral Dissertation funds
(201455).
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Rewritable ternary data storage device based on polymethacrylate
containing pendent azobenzene-naphthalene with combined effects of
conformation change and charge traps
Zhuang Lia, Ming Wanga, Hua Li*a, Jinghui Hea, Najun Lia, Qingfeng Xua and Jianmei Lu*a
Table of Contents
Novelty: We combined the shallow electron trap azobenzene with the planar naphthalene
in the side chain of polymethacrylate. The sandwich-structured device ITO/polymer/Al shows
different memory characteristics as the various alkyl lengths in the side-chains of the
polymers, and the rewritable ternary device is potential for ultra-high density data storage
utilizes.