The Effect of Random and Block Copolymerization with Pendent Carbozole Donors and Naphthalimide Acceptors on Multilevel Memory Performance

Article abstract of DOI:10.1002/asia.201701778

Polymeric materials have been widely used in the fabrication of data-storage devices, owing to their unique advantages and defined conduction mechanisms. To date, the most-functional polymers that have been reported for memory devices were synthesized through random copolymerization, whilst there have been no reports regarding the memory effect of block polymers. Herein, we synthesized a random copolymer (PMCz₈-co-PMBNa₂) and its corresponding block copolymer (PMCz₈-b-PMBNa₂) to study the effect of the method of polymerization on the memory properties of the corresponding devices. Interestingly, both devices (ITO/PMCz₈-co-PMBNa₂/Al and ITO/PMCz₈-b-PMBNa₂/Al) exhibited ternary memory performance, with threshold voltages of ?1.7 V/?3.3 V and ?2.7 V/?3.8 V, respectively. However, based on comprehensive measurements, the memory properties of PMCz₈-co-PMBNa₂ and PMCz₈-b-PMBNa₂ were found to be owing to the operation of different conduction mechanisms, which resulted from different molecular stacking in the film state. Therefore, we expect that this work will be helpful for improving our understanding of the conduction mechanisms in polymer-based data-storage devices.

Full text of DOI:10.1002/asia.201701778

DOI: 10.1002/asia.201701778
Full Paper
Polymers
The Effect of Random and Block Copolymerization with Pendent
Carbozole Donors and Naphthalimide Acceptors on Multilevel
Memory Performance
Qi-jian Zhang, Jia-hui Zhou, Hui Li, Jing-hui He, Na-jun Li, Qing-feng Xu, Dong-yun Chen,
[a]
Hua Li,* and Jian-mei Lu*
Abstract: Polymeric materials have been widely used in the
fabrication of data-storage devices, owing to their unique
advantages and defined conduction mechanisms. To date,
the most-functional polymers that have been reported for
memory devices were synthesized through random copoly-
merization, whilst there have been no reports regarding the
memory effect of block polymers. Herein, we synthesized a
random copolymer (PMCz -co-PMBNa ) and its correspond-
(ITO/PMCz -co-PMBNa /Al and ITO/PMCz -b-PMBNa /Al) ex-
8 2 8 2
hibited ternary memory performance, with threshold voltag-
es of ꢀ1.7 V/ꢀ3.3 V and ꢀ2.7 V/ꢀ3.8 V, respectively. Howev-
er, based on comprehensive measurements, the memory
properties of PMCz -co-PMBNa and PMCz -b-PMBNa were
8
2
8
2
found to be owing to the operation of different conduction
mechanisms, which resulted from different molecular stack-
ing in the film state. Therefore, we expect that this work will
be helpful for improving our understanding of the conduc-
tion mechanisms in polymer-based data-storage devices.
8
2
ing block copolymer (PMCz -b-PMBNa ) to study the effect
8
2
of the method of polymerization on the memory properties
of the corresponding devices. Interestingly, both devices
n
Introduction
corresponding storage density to be increased to 3 , hundreds
of millions of times greater than that of a binary system, which
n [23–27]
Owing to several advantages, such as excellent solution proc-
essability, low power consumption, structural tunability, and
offers a density of 2 .
In previous reports regarding OMDs,
[28]
several conduction mechanisms, such as redox, filament for-
[29,30]
[31–33]
[34–36]
3D stacking capabilities, polymeric materials have been widely
mation,
conformational change,
charge trapping,
[37–40]
applied in electronic devices, such as organic solar cells
and charge transfer,
have been successfully introduced to
[
1–6]
[7–10]
(
OSCs),
organic field-effect transistors (OFETs),
organic
and organic memory devices
However, to date, most polymer-based OMDs
interpret the properties of memory devices. Therefore, we
wondered whether ternary memory performance could be re-
alized through combining two individual conduction mecha-
nisms into a single polymer.
[
11–15]
light-emitting diode (OLEDs),
[
16–22]
(
OMDs).
have only exhibited the traditional binary memory behavior,
which cannot meet the ever-increasing global demands for
data storage. To solve this emergent problem, two main ap-
proaches have been put forward. The first approach involves
the application of new technologies to downscale memory
cells, thereby increasing their data-storage densities, which is
largely limited by their physical form factor and process tech-
nologies. The second approach involves the development of
multilevel OMDs (e.g., ternary devices), which would allow the
In our previous work, we designed a ternary OMD that was
based on polymer P4VPCz (poly(4-vinylpyridine) derivatives),
which contained pyridine and carbazole (Cz) groups in the side
chain. The conduction mechanism for the device based on this
polymer was found to involve a combination of conformation-
[41]
al-change and charge-trapping mechanisms.
Furthermore,
two pendent copolymers, poly(2-(naphthalen-1-yloxy)ethyl 5-
(2-(7-bromo-1,3-dioxo-4,5-dihydro-1H-benzo[de]isoquinolin-
2(3H)-yl)ethoxy)-2,2,4-trimethylhex-5-enoate)
(PMNB)
and
[
a] Dr. Q.-j. Zhang, J.-h. Zhou, H. Li, Prof. J.-h. He, Prof. N.-j. Li, Prof. Q.-f. Xu,
Prof. D.-y. Chen, Prof. H. Li, Prof. J.-m. Lu
College of Chemistry, Chemical Engineering and Materials Science
Soochow University
(poly(2-(naphthalen-1-yloxy)ethyl 5-(2-((2-butyl-1,3-dioxo-2,3-di-
hydro-1H-benzo[de]isoquinolin-6-yl)amino)ethoxy)-2,2,4-trime-
thylhex-5-enoate) (PMNN), which contained electron-donor
(
naphthalene) and electron-acceptor (1,8-naphthalimide; Na)
No.199 Renai Road
Suzhou
units in their side chains, were also synthesized. Through
tuning different connection sites on the 1,8-naphthalimide
framework, different steric effects and ternary memory per-
Jiangsu Sheng 215123 (P. R. China)
E-mail: lihuaw@suda.edu.cn
lujm@suda.edu.cn
[42]
formance were achieved for PMNB and PMNN. However, the
flexible chains were found to be different in the connection
sites of the naphthalimide moieties in polymers PMNB and
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201701778.
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1
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ꢀ
1
PMNN, which had a significant influence on the memory be-
havior of the corresponding devices. Thus, we wanted to
design an “ideal” polymer that could improve upon the indi-
vidual performances of PMNB and PMNN. In addition, our pre-
viously designed polymers were all obtained through random
copolymerization; thus, we also wanted to consider the effect
of other polymerization methods, such as block polymeri-
zation, on memory performance.
PMCz -b-PMBNa , was 12551 gmol , with a PDI of 1.33. Thus,
8 2
the relative ratio of MCz/MBNa in block polymer PMCz -b-
8
PMBNa was 8:2. Both polymers exhibited good thermal stabili-
2
ty, with a 5% decomposition temperature (T ) of about 2808C
and a glass-transition temperature (T ) of about 1008C, thus
implying good heat endurance for both polymers PMCz -co-
d
g
8
PMBNa and PMCz -b-PMBNa in their corresponding memory
2
8
2
devices.
Carbazole and 1,8-naphthalimide groups both possess a
[
43–45]
high degree of p-conjugated planarity,
which can induce a
Current–Voltage (I–V) Characteristics of PMCz -co-PMBNa
8
2
conformational change between the Cz and Na planes under
an external electrical field. Furthermore, the Cz group, which
contains a O=CꢀOꢀCꢀC spacer group in its side chain, has
been shown to exhibit binary “write once read many” (WORM)
memory behavior through a conformational-change mecha-
and PMCz -b-PMBNa2
8
Figure 2a shows the structure of the memory devices, in which
a thin film (thickness: ca. 80 nm) was sandwiched between an
aluminum (Al) electrode and an ITO electrode (for a cross-sec-
tional SEM image, see Figure 2b). Typical I–V characteristics of
[
33]
nism.
donors and Na chromophores are well-known electron accept-
In addition, Cz groups are also attractive electron
ꢀ
1
the memory devices were determined at a scan rate 0.1 Vs .
[
24]
ors, thereby readily allowing charge-transfer or charge-trap-
ping mechanisms in the side chains. Finally, the same flexible
Figure 2c shows the I–V characteristics of an ITO/PMCz -co-
8
PMBNa /Al memory device, which was initially in its low-con-
2
3
O=CꢀOꢀCꢀC chains are connected to nitrogen atoms with sp
ductivity state (OFF or “0” state). When a negative voltage
from 0 to ꢀ6 V was applied to the cell, two abrupt increases in
current were observed at threshold voltages of ꢀ1.7 and
ꢀ3.3 V, thus indicating the occurrence of two successive elec-
trical transitions from the OFF state to an intermediate conduc-
tance state (ON1 or “1” state) and from the ON1 state to a
high-conductivity state (ON2 or “2” state). The corresponding
OFF–ON1 and ON1–ON2 transitions could both serve as the
“writing” process in the memory device. The memory device
remained in the ON2 state under a subsequent voltage sweep
from 0 to ꢀ6.0 V. The long-term stability and performance of
the device was also evaluated from the retention time and a
stimulus effect test under the same conditions. As shown in
the Supporting Information, Figure S1, the PMCz -co-PMBNa -
hybridization at pendant Na moieties, which is theoretically fa-
vorable for the arrangement of Cz and Na moieties under an
external stimulus.
Herein, we report the synthesis of two polymers, PMCz -co-
8
PMBNa and PMCz -b-PMBNa , through random copolymeriza-
2
8
2
tion and block polymerization, respectively. I–V measurements
showed that both polymers exhibited ternary memory per-
formance, and the corresponding threshold voltages were
ꢀ
1.7 V/ꢀ3.3 V and ꢀ2.7 V/ꢀ3.8 V, respectively. However, ITO/
PMCz -b-PMBNa /LiF/Al (ITO=indium tin oxide) only exhibited
8
2
binary memory performance, whereas ITO/PMCz -co-PMBNa /
8
2
LiF/Al still exhibited the typical ternary memory performance.
Based on AFM, XRD, UV/Vis, and photoluminescence (PL) spec-
troscopy experiments, we attributed the ternary memory prop-
erties of PMCz -co-PMBNa and PMCz -b-PMBNa to the opera-
8
2
8
based device endured over 1ꢁ10 continuous read pluses of
ꢀ1.0 V, and no significant degradation in the current for any
state was observed after 100 min.
8
2
8
2
tion of different conduction mechanisms. We expect that this
work will guide the design of new polymers with improved
memory performance.
Subsequently, to exclude the possibility of aluminum parti-
cles penetrating the active layer during the electrical measure-
ments, a LiF layer (thickness: ca. 5 nm) was vacuum-deposited
onto the PMCz -co-PMBNa2 film as a buffer layer to prevent
8
Results and Discussion
direct contact with the Al electrode. Then, the electrical behav-
ior of the ITO/PMCz -co-PMBNa /LiF/Al device was measured
8
2
Synthesis and Characterization of PMCz -co-PMBNa and
8
2
under the same conditions as before. As shown in Figure 3a,
this device also exhibited ternary memory behavior, thus indi-
cating that this behavior originated from the intrinsic proper-
ties of the active layer itself. Thus, a PMCz -co-PMBNa -based
PMCz -b-PMBNa2
8
Scheme 1 shows our synthetic route to polymers PMCz -co-
8
PMBNa and PMCz -b-PMBNa ; for detailed procedures, see the
2
8
2
8
2
Supporting Information. The chemical structures of these poly-
memory device exhibited nonvolatile ternary WORM-type
memory behavior.
1
mers were confirmed by using H NMR spectroscopy (Figure 1).
Both polymers could be spin-cast into uniform thin films from
Similarly, the ITO/PMCz -b-PMBNa /Al device also showed
8
2
ꢀ
1
solutions in cyclohexanone (12 mgmL ). The relative molecu-
typical ternary WORM-type data-storage performance, as
shown in Figure 2d, and the threshold voltage for the OFF–
ON1 and ON1–ON2 transitions were ꢀ2.7 and ꢀ3.8 V, respec-
tively, much larger than the corresponding values for the
PMCz -co-PMBNa -based memory device. Then, the LiF layer
lar weight (M ) of random copolymer PMCz -co-PMBNa was
n
8
2
ꢀ
1
2
0205 gmol , with a polydispersity index (PDI) of 1.52, and
the ratio of MCz/MBNa segments was measured by using the
UV/Vis absorption of a solution of MBNa in DMF (see ref [41]).
8
2
Subsequently, the M of polymer R-PMCz [R=Reversible Addi-
was also applied to consider the possibility of metal-filament
formation in the electrical measurements. Unexpectedly, the
ITO/PMCz -b-PMBNa /LiF/Al device only showed binary
n
tion-Fragmentation Chain Transfer Polymerization (RAFT)] was
9
447, with a PDI of 1.26, whilst that of the final block polymer,
8
2
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Scheme 1. Synthesis of PMCz
8
-co-PMBNa
2
and PMCz
8
2
-b-PMBNa . AIBN=azobisisobutyronitrile, RAFT=reversible addition-fragmentation chain transfer.
transition from the ON1 state to the ON2 state was dominated
by the penetration of the Al filament.
Macrostructures of PMCz -co-PMBNa and PMCz -b-PMBNa
2
8
2
8
in the Solid State
To understand the Al-filament-penetration phenomenon in the
PMCz -b-PMBNa -based device, AFM analysis was performed to
8
2
investigate the surface morphologies of the PMCz -b-PMBNa
8
2
and PMCz -b-PMBNa films. As shown in Figure 4a, non-contin-
8
2
uous aggregation of PMCz -b-PMBNa in the solid state were
8
2
observed from the tapping-mode AFM profile images, whilst
D AFM topography images also confirmed the high root-
mean-square (RMS) surface roughness of the PMCz -b-PMBNa
3
8
2
film to be 8.6 nm, which offered the chance to form a conduct-
ing channel for the Al particles. In contrast, the AFM image of
1
Figure 1. H NMR spectra of a) PMCz
CDCl .
3
8
-b-PMBNa
2
and b) PMCz
8
2
-co-PMBNa in
the PMCz -co-PMBNa2 film showed a smooth surface (Fig-
8
ure 4b) with a RMS surface roughness of less than 1 nm (Fig-
ure 4d), which was beneficial for preventing the Al particles
from penetrating into the active layers. Clearly, the different
surface morphologies of the PMCz -b-PMBNa2 and PMCz -b-
memory behavior, and the threshold voltage was about
3.0 V, as shown in Figure 3b, thus illustrating that the second
ꢀ
8
8
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Figure 2. a) Schematic representation of the ITO/polymer/Al device; b) SEM image of a cross-section of an ITO/polymer/Al device; c) I–V characteristics of a
PMCz -co-PMBNa -based memory device; d) I–V characteristics of a PMCz -b-PMBNa -based memory device.
8 2 8 2
8 2 8 2
Figure 3. a) I–V characteristics of the ITO/PMCz -co-PMBNa /LiF/Al device; b) I–V characteristics of the ITO/PMCz -b-PMBNa /LiF/Al device.
PMBNa2 films was caused by their different polymerization
methods. For the block polymer, the naphthalimide fragment
possessed a relatively uniform arrangement and, thus, PMCz8-
b-PMBNa had a tendency to aggregate in the film state. In
2
contrast, the naphthalimide groups in PMCz -co-PMBNa were
8
2
randomly dispersed, which would increase its solubility and,
thus, decrease the aggregation of PMCz -co-PMBNa in the film
8
2
state.
Photophysical and Electrochemical Properties of PMCz -co-
8
PMBNa and PMCz -b-PMBNa
2
2
8
To understand the memory properties of the PMCz -co-
8
PMBNa - and PMCz -b-PMBNa -based devices in detail, UV/Vis
2
8
2
and CV measurements were performed (Figure 5). PMCz -co-
8
PMBNa and PMCz -b-PMBNa both exhibited a maximum ab-
2
8
2
sorption peak at l=298 nm and a shoulder absorption peak
within the range l=328–345 nm, which could be assigned to
p–p* transitions of the Cz pendant groups (Figure 5a). Further-
Figure 4. Tapping-mode AFM profile images (a,b) and 3D AFM topography
images (c,d) of thin films of PMCz -b-PMBNa and PMCz -co-PMBNa that
were spin-coated onto ITO substrates, respectively (scan size: 5 mmꢁ5 mm).
8
2
8
2
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Figure 5. a) UV/Vis absorption spectra of thin films of PMCz
8 2 8 2 8 2
-co-PMBNa and PMCz -b-PMBNa ; b) cyclic voltammetry (CV) curves of PMCz -co-PMBNa and
ꢀ
1
PMCz -b-PMBNa on ITO substrates in MeCN (scan rate: 100 mVs ).
8
2
more, the moderately intense broad absorption peak at l=
electric-field-induced conformational-change mechanism by
using the same O=CꢀOꢀCꢀC spacer group as a bridge be-
tween the framework backbone and the functional carbazole
3
62 nm was attributed to the n–p* and p–p* transitions of the
[24]
Na moieties (see the Supporting Information, Figure S2).
[33]
However, the UV/Vis spectrum of the PMCz -co-PMBNa2 film
groups.
Thus, we wonder whether this conformational-
8
was slightly broader than that of the PMCz -b-PMBNa2 film,
change phenomenon could also be the case in the PMCz -co-
8
8
which was mainly induced by the poor PDI, caused by the
random polymerization method. The absorption onsets for the
PMCz -co-PMBNa and PMCz -b-PMBNa films were both l=
PMBNa - and PMCz -b-PMBNa -based devices. First, we per-
2
8
2
formed XRD measurements to understand the degree of re-
gioregularity in the PMCz -co-PMBNa2 and PMCz -b-PMBNa
2
8
2
8
2
8
8
390 nm, with a corresponding optical band-gap of 3.18 eV.
films in their ground state. As shown in the Supporting Infor-
Next, CV measurements were performed to understand the
mation, Figure S3, a clear diffraction peak at 2q=9.238 was ob-
electrochemical properties of the PMCz -co-PMBNa2 and
served for the PMCz -b-PMBNa2 film, with a corresponding
8
8
PMCz -b-PMBNa2 films. Tetrabutylammonium perchlorate
d spacing of 9.56 ꢂ, which was consistent with long-range
order. Furthermore, based on the optimized structure that was
obtained from our theoretical calculations, the length of the
Cz plane was confirmed to be 9.01 ꢂ (see the Supporting Infor-
mation, Figure S4), which matched the long-range order very
well. Subsequently, a relatively weak diffraction peak at 21.248
was also detected, with a d space of 4.17 ꢂ, which could be at-
tributed to intermolecular p–p stacking of neighboring Cz
units. In conclusion, the Cz moieties formed a close face-to-
face conformation and ordered long-range channels in the
PMCz -b-PMBNa film, which could facilitate the movement of
8
(TBAP; 0.1m) was used as the electrolyte, whilst ferrocene was
used as a reference. As shown in Figure 5b, the PMCz -co-
8
PMBNa and PMCz -b-PMBNa films both exhibited an irreversi-
2
8
2
ble oxidation peak, with onsets at 1.46 and 1.22 eV, respective-
ly. Thus, the HOMO and LUMO energy levels could be calculat-
ed according to Equations (1) and (2).
EHOMO ¼ ꢀ½E þ4:80ꢀEferroceneꢁ
ð1Þ
ð2Þ
ox
ELUMO ¼ EHOMOþEg
8
2
charge carriers through the channels. Thus, we could rule out
The detailed HOMO and LUMO energy levels of the PMCz8-
the conformational-change phenomenon in the PMCz -b-
8
co-PMBNa and PMCz -b-PMBNa films are shown in the Sup-
2
8
2
PMBNa -based device, owing to the highly ordered stacking in
2
porting Information, Table S1. The energy barrier between the
the film state.
HOMO level of PMCz -co-PMBNa and the work function of the
8
2
We proposed a charge-transfer mechanism to explain the
conduction mechanism for the OFF–ON1 transition in the
ITO electrode (0.94 eV) was smaller than that between the
work function of the Al electrode and the LUMO energy level
PMCz -b-PMBNa2 device, because Cz groups are well-known
8
(
1.72 eV), thus indicating that PMCz -co-PMBNa was a p-type
8 2
electron donors, whilst Na groups have been widely applied as
electron acceptors. Owing to the highly ordered intermolecular
stacking in the PMCz -b-PMBNa polymer, the charge carriers
semiconductor, and that the memory performance was hole-
dominated. Similarly, PMCz -b-PMBNa was also found to be a
8
2
8
2
p-type active material and, thus, the PMCz -b-PMBNa -based
8
2
could be readily transferred from the Cz donors to the Na elec-
tron-deficient acceptors under an external voltage sweep.
Thus, a conduction channel could be formed for the move-
ment of charge carriers. To confirm the operation of this
charge-transfer mechanism, photoluminescence measurements
were performed (see the Supporting Information, Figure S5).
After a voltage sweep from 0 to ꢀ3.2 V, the intensity of the
photoluminescence was significantly quenched in the ON1
data-storage device was also governed by hole-transport.
Proposed Conduction Mechanism for PMCz -b-PMBNa -
8
2
Based Devices
Over the past decade, Kang and co-workers have demonstrat-
ed the generation of binary memory properties through an
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state; thus, the switch from the OFF state to the ON1 state
was attributed to a charge-transfer process. Therefore, the ter-
nary memory behavior of the PMCz -b-PMBNa polymer was in-
Subsequently, an intermolecular charge transfer (ICT) process
was also proposed for the subsequent transition from the ON1
state to the ON2 state. Because the pendent functional groups
adopted a partial or full face-to-face conformation at the
threshold voltage of about ꢀ1.7 V, a further increase in the
sweeping voltage would cause the charge carriers to transfer
from the Cz donors to the electron-deficient Na moieties to
form another conduction channel. To confirm the charge-trans-
fer mechanism, UV/Vis absorption spectroscopy was performed
8
2
duced by charge-transfer and filament-formation mechanisms.
Proposed Conduction Mechanism for PMCz -co-PMBNa -
8
2
Based Devices
In contrast, the degree of regioregularity was quite low for the
on the PMCz -co-PMBNa2 film before and after the voltage
8
PMCz -co-PMBNa copolymer in the ground state, as shown in
sweep (Figure 6b). Compared with the corresponding film in
the OFF state, the UV/Vis absorption spectrum of the PMCz8-
8
2
Figure 6a, which would hinder the mobility of charge carriers
through the active layer and, thus, the device would be in the
OFF state. Because the content of carbazole units was much
higher than that of 1,8-naphthalimide units, and because the
oxidation potential of carbazole was lower than that of 1,8-
naphthalimide (see the Supporting Information, Figures S6 and
S7), the holes would first inject from the ITO electrode into the
carbazole groups near the interface to form charged species
with an increase in the negative bias. Subsequently, the active
carbazole species would attract neighboring neutral carbazole
groups to form a partial or full face-to-face conformation.
To confirm the operation of a conformational-change mech-
co-PMBNa film was significantly broadened in the normalized
2
absorption spectrum after the voltage sweep, thus indicating
an increased polarity and dipole moment of PMCz -co-PMBNa ,
8
2
owing to charge-transfer interactions between the Cz electron
donor and the Na electron acceptor. Furthermore, photolumi-
nescence measurements were also performed to confirm the
charge-transfer process, as shown in the Supporting Informa-
tion, Figure S9. After the voltage sweep from 0 to ꢀ6 V, the in-
tensity was significantly quenched in the ON2 state, thus con-
firming the operation of a charge-transfer process under an ex-
ternal voltage. Therefore, the ternary memory property of the
PMCz -co-PMBNa -based device was attributed to conforma-
anism, XRD measurements were performed on the PMCz -co-
8
8
2
PMBNa film after the voltage sweep. Instead of depositing an
tional-change and charge-transfer mechanisms, as shown in
Figure 7b.
2
Al electrode on the active-layer, a liquid droplet of Hg was
placed on the PMCz -co-PMBNa film to serve as the top elec-
8
2
trode. The Hg droplet was removed immediately after an elec-
Conclusion
trical sweep from 0 to ꢀ2.5 V on the Hg/PMCz -co-PMBNa /ITO
8
2
device. As shown in Figure 6a, after the voltage sweep, a new
In summary, we synthesized two copolymers, PMCz -co-
8
diffraction peak at 2q=21.278 appeared, with a d spacing of
PMBNa and PMCz -b-PMBNa , that contained the same elec-
2
8
2
4
.17 ꢂ, which was also attributed to intermolecular p–p stack-
tron-donor (carbazole) and electron-acceptor (1,8-naphthali-
mide) units through random copolymerization and block poly-
merization, respectively. The corresponding sandwich-struc-
tured devices, ITO/PMCz -co-PMBNa /Al and ITO/PMCz -b-
ing of the Cz groups. Thus, an ordered face-to-face conforma-
tion of Cz units could be induced under an electric-field in the
PMCz -co-PMBNa film, and the charge carriers would migrate
8
2
8
2
8
through the Cz conformational channel to induce a switch in
conductivity from the OFF state to the ON1 state. AFM meas-
urements were also performed on the PMCz -co-PMBNa film
PMBNa /Al, for both copolymers exhibited ternary memory be-
2
havior. However, copolymer PMCz -co-PMBNa and block poly-
8
2
mer PMCz -b-PMBNa possessed completely different molecu-
8
2
8
2
before and after the voltage sweep to confirm the operation
of a conformational-change mechanism, as shown in the Sup-
porting Information, Figure S8.
lar stacking properties and surface morphologies in the film
state. Furthermore, the switching mechanisms for the polymer-
based memory devices were also different. For block polymer
Figure 6. a) X-ray diffraction patterns of PMCz
8 2
-co-PMBNa in different states (OFF and ON1); b) comparison of the ex situ UV/Vis spectra of thin films of
PMCz -co-PMBNa in the low-conductivity (OFF) and intermediate-conductivity (ON1) states.
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Figure 7. Schematic representation of the conductive switching processes in memory devices based on PMCz -b-PMBNa (a) and PMCz -co-PMBNa (b).
[
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The authors declare no conflict of interest.
[
[
[
Keywords: conducting materials · memory devices · molecular
stacking · nitrogen heterocycles · polymers
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Version of record online: && &&, 0000
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FULL PAPER
Thanks for the memories: Two copoly-
mers, PMCz -co-PMBNa and PMCz -b-
Polymers
8
2
8
Qi-jian Zhang, Jia-hui Zhou, Hui Li,
Jing-hui He, Na-jun Li, Qing-feng Xu,
Dong-yun Chen, Hua Li,* Jian-mei Lu*
PMBNa , were synthesized through
2
random and block copolymerization, re-
spectively. Their corresponding data-
storage devices exhibited different
memory properties, which was attribut-
ed to the operation of different conduc-
tion mechanisms.
&& – &&
The Effect of Random and Block
Copolymerization with Pendent
Carbozole Donors and Naphthalimide
Acceptors on Multilevel Memory
Performance
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ