Synthesis and Morphology of Two Carbazole-Pyrazoline-Containing Polymer Systems and Their Electrical Memory Performance

Article abstract of DOI:10.1002/cplu.201500188

A new atom-transfer radical polymerization (ATRP) initiator 4-[1-(2-dodecyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)-3-(4-nitrophenyl)-4,5-dihydro-1H-pyrazol-5-yl]phenyl 2-bromo-2-methylpropanoate (IN) as an electron acceptor (A) and a monomer 2-(9H-carbazole-9-yl)-ethyl methacrylate (MCz) as an electron donor (D) were simultaneously introduced into two different D-A polymer systems by using the end-functionalizing or blending method. The mass percentage of IN in the end-functionalized polymer PMCz-IN and the mixed polymer composite PMCz+IN were both controlled at approximately 1.0wt. The optical, electrochemical, and surface morphology properties of the two polymeric films prepared by means of spin-coating technology were comparatively investigated. Sandwich devices based on PMCz-IN and PMCz+IN demonstrated nonvolatile write-once-read-many-times memory (WORM) and volatile static random access memory (SRAM) characteristics, respectively, which were further verified by the Kelvin probe force microscopy (KPFM) measurements. The proposed memory mechanism could be attributed to the formation of a stable charge-transfer (CT) complex for PMCz-IN and an unstable CT complex for PMCz+IN. Furthermore, the different distribution of IN in the two polymeric films might be the main reason for the stability of the CT complex.

Full text of DOI:10.1002/cplu.201500188

DOI: 10.1002/cplu.201500188
Full Papers
Synthesis and Morphology of Two Carbazole–Pyrazoline-
Containing Polymer Systems and Their Electrical Memory
Performance
Cai-jian Lu,[a] Hua Li,[a] Qing-feng Xu,*[a] Qing-hua Xu,[b] and Jian-mei Lu*[a, c]
A new atom-transfer radical polymerization (ATRP) initiator 4-
[1-(2-dodecyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-
yl)-3-(4-nitrophenyl)-4,5-dihydro-1H-pyrazol-5-yl]phenyl
spin-coating technology were comparatively investigated.
Sandwich devices based on PMCz-IN and PMCz+IN demon-
strated nonvolatile write-once-read-many-times memory
(WORM) and volatile static random access memory (SRAM)
characteristics, respectively, which were further verified by the
Kelvin probe force microscopy (KPFM) measurements. The pro-
posed memory mechanism could be attributed to the forma-
tion of a stable charge-transfer (CT) complex for PMCz-IN and
an unstable CT complex for PMCz+IN. Furthermore, the differ-
ent distribution of IN in the two polymeric films might be the
main reason for the stability of the CT complex.
2-bromo-2-methylpropanoate (IN) as an electron acceptor (A)
and a monomer 2-(9H-carbazole-9-yl)-ethyl methacrylate (MCz)
as an electron donor (D) were simultaneously introduced into
two different D–A polymer systems by using the end-function-
alizing or blending method. The mass percentage of IN in the
end-functionalized polymer PMCz-IN and the mixed polymer
composite PMCz+IN were both controlled at approximately
1.0 wt%. The optical, electrochemical, and surface morphology
properties of the two polymeric films prepared by means of
Introduction
Over the past few decades, large numbers of resistive-type
memory devices based on organic or polymer systems for data
storage have been investigated.[1–10] In particular, polymer sys-
tems have attracted significant scientific interest owing to their
unique properties of good mechanical strength, flexibility, and
most important of all, ease of processing.[6–10] Until now, re-
ported polymer systems have been used for device applica-
tions such as static random access memory (SRAM), dynamic
random access memory (DRAM), write-once-read-many-times
memory (WORM), rewritable (Flash), and negative differential
resistance (NDR) memory behavior by means of an external
voltage bias or pulse.
vanced memory-device application.[11–34] By combining D moi-
eties with A moieties, two direct approaches are provided
during the fabrication of devices including incorporating and
blending methods. Thus, D–A polymer systems could also be
divided into two general groups: non-blended polymers (con-
sisting of conjugated[11–13] and non-conjugated[14–19] polymers)
and blended polymers (including metal nanoparticles,[20–22] ful-
lerenes,[23–25] carbon nanotubes,[26–28] graphene oxide,[29–31] and
organic dyes[32–34]). The incorporating method through the con-
nection of covalent bonds can proceed through various routes,
such as copolymerization,[12–15] grafting,[16,17] side-chain func-
tionalization,[18,19] and end functionalization. Owing to the clear
polymer structure of D–A polymers prepared by using the
end-functionalization approach, this novel method applied to
electrical memory devices has attracted more and more atten-
tion recently. For example, Ree et al. covalently incorporated
fullerene (A) with carbazole (D) into a single polymer system
through living anionic polymerization.[35] In addition, in our
previous study, we end-functionalized the carbazole-based
polymer chain using naphthalimide by means of atom-transfer
radical polymerization (ATRP).[36] Although using end-function-
alizing or blending methods for the development of high-per-
formance memory-device applications has been investigated
greatly, to the best of our knowledge the comparison between
them has been explored rarely. When we compared the mor-
phologies of polymeric films fabricated by using these two
methods, the end-functionalized polymeric film exhibited mi-
crophase separation, whereas the blended polymeric film ex-
hibited nanoparticle aggregation phenomena. It has been re-
ported that the morphology of the polymer composite will
Recently, polymer systems containing donor (D) moieties
and acceptor (A) moieties have been studied widely for ad-
[a] C.-j. Lu, Prof. H. Li, Prof. Q.-f. Xu, Prof. J.-m. Lu
College of Chemistry
Chemical Engineering and Materials Science
Soochow University
199 Ren’ai Road, Suzhou 215123 (P. R. China)
E-mail: lujm@suda.edu.cn
[b] Prof. Q.-h. Xu
Department of Chemistry
National University of Singapore (NUS) (Singapore)
[c] Prof. J.-m. Lu
State Key Laboratory of Treatments and Recycling for
Organic Effluents by Adsorption in Petroleum and Chemical Industry
199 Ren’ai Road, Suzhou 215123 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500188.
Part of a Special Issue to celebrate Singapore’s Golden Jubilee. To view
the complete issue, visit: http://dx.doi.org/10.1002/cplu.v80.8.
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1
affect the electrical memory behavior.[37–39] Thus, our current
study on the comparison of data storage performance be-
tween the end-functionalized polymer system and blended
polymer system is necessary.
which was characterized by H NMR spectroscopy and LC–MS
(see Figures S2 and S3 in the Supporting Information). With
the successful preparation of the monomer and the initiator,
polymers PMCz and PMCz-IN were prepared by using the ATRP
1
In this study, we chose a carbazole (MCz) as D and a pyrazo-
line (IN) as A. To combine MCz and IN into a single polymer
system, we utilized the end-functionalizing or blending
method, and obtained two polymeric films PMCz-IN and
PMCz+IN, respectively. Subsequently, these two electroactive
films were fabricated as sandwich memory devices. For com-
parison purposes, the mass percentage of IN in the two devi-
ces was kept at approximately 1.0 wt%. The fabricated indium
tin oxide (ITO)/PMCz-IN/Al device shows nonvolatile WORM
characteristics, whereas the ITO/PMCz+IN/Al device exhibits
volatile SRAM characteristics. It is concluded that the different
electrical characteristics of the two devices could be attributed
to the formation of a stable or unstable charge-transfer (CT)
complex between IN and MCz. Furthermore, the stability of
the CT complex might be associated with the distribution of IN
in the two films, and the formation of IN nanoparticles in the
PMCz+IN film could contribute to the volatile memory charac-
teristics. Therefore, we successfully achieved tunable electrical
memory performance by means of two different combined
methods based on the same electroactive substances.
method and their H NMR spectra are shown in Figure S4 in
the Supporting Information. The gel permeation chromatogra-
phy (GPC) analysis (see Figure S5 in the Supporting Informa-
tion) of PMCz and PMCz-IN indicated a number-average molec-
ular weight (Mn) of 12400 and 9200 gmolÀ1, respectively, and
a polydispersity index (PDI) of 1.24 and 1.23, respectively.
Thermal stabilities
Figure 1 illustrates the thermal stability of the two polymers
evaluated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The 5% weight-loss temperature
(Td5%) of PMCz and PMCz-IN were found to be 280 and 2908C,
respectively. In addition, polymer PMCz-IN was found to have
a glass-transition temperature (Tg) of 1408C, which was higher
than that of polymer PMCz (1248C). Therefore, the introduction
of an end-functionalized initiator led to an increase of Td5% and
Tg compared to polymer PMCz. Moreover, the good thermal
properties of the two polymers are expected to withstand heat
deterioration in the memory devices.[41]
Optical characteristics
Results and Discussion
First, we investigated the content of IN in the obtained poly-
mer PMCz-IN, which was determined as approximately
1.0 wt% by a UV/Vis absorption calibration curve as depicted
in Figure 2a. The absorption spectra of PMCz-IN and PMCz+IN
in the film state are shown in Figure 2b. The absorption peaks
at shorter wavelength (328 and 342 nm), which are consistent
with the absorption peaks of pure carbazole-containing poly-
mer PMCz (see Figure 2c), are at-
Synthesis and characterization
The monomer, 2-(9H-carbazole-9-yl)-ethyl methacrylate (MCz),
was synthesized according to previously reported proce-
dures[40] and characterized by using the H and 13C NMR spec-
1
tra as displayed in Figure S1 in the Supporting Information.
Scheme 1 illustrates the synthetic route of the initiator IN,
tributed to the p–p* electronic
transition of the carbazole moi-
eties.[40] However, the wide ab-
sorption band appearing at 350–
550 nm is associated with the in-
termolecular
(ICT) character of the initiator IN
end-functionalized in PMCz
(PMCz-IN) or embedded in PMCz
(PMCz+IN).[42,43]
Furthermore,
charge-transfer
the similar absorption values of
PMCz-IN and PMCz+IN further
demonstrate that we have en-
sured that the two polymeric
films have a similar mass per-
centage of IN.
To further demonstrate the in-
teraction between PMCz and IN,
the emission spectra of PMCz-IN
and PMCz+IN in the film state
were also investigated and are
displayed in Figure 2d. With an
Scheme 1. The synthetic routes to IN, PMCz-IN, PMCz, and the mixed polymer composite PMCz+IN.
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addition, the different emission peaks of PMCz-IN and PMCz+
IN at longer wavelengths must be associated with the phe-
nomenon of molecular aggregation. Compared with PMCz-IN,
the prepared PMCz+IN film using the blending method leads
more easily to the formation of initiator nanoparticles.[45]
Electrochemical characteristics
The electrochemical characteristics of the polymeric films were
investigated by cyclic voltammetry (CV) as shown in Figure 3
and summarized in Table 1. The oxidation and reduction be-
haviors of PMCz, PMCz-IN, and PMCz+IN on an ITO substrate
were measured in 0.1m TBAPF6/acetonitrile solution with Ag/
AgCl as reference electrode and Pt wire as counter electrode.
onset
Ox
The onset oxidation (E ) estimated from the CV curves for
PMCz, PMCz-IN, and PMCz+IN are 1.04, 1.05, and 1.08 V, re-
onset
Red
Figure 1. a) TGA and b) DSC thermograms of the polymers PMCz and PMCz-
IN, recorded under N2 at a heating rate of 10 and 208CminÀ1, respectively.
spectively. In addition, the onset reduction (E ) estimated
from the CV curves for PMCz-IN and PMCz+IN are À0.75 and
À1.00 V, respectively. Note that the oxidation half-wave poten-
tials for the external ferrocene/ferrocenium (E1/2, ferrocene) was
measured to be 0.42 V versus Ag/AgCl (see Figure 3a). The
HOMO and LUMO energy levels of the prepared polymeric
films were calculated from the CV tests with reference to ferro-
cene (4.80 eV below the vacuum level, which is defined as
zero) by applying Equations (1) and (2).[46,47]
Figure 2. a) Calibration curve for determining the mass percentage of IN in
PMCz-IN, using IN as standard. b) Normalized absorption spectra of PMCz-IN
and PMCz+IN thin films on quartz plate. c) Normalized absorption and
emission spectra of PMCz thin film on quartz plate (excitation wavelength:
340 nm). d) Normalized emission spectra of PMCz-IN and PMCz+IN thin
films on quartz plate (excitation wavelength: 340 nm).
Figure 3. Cyclic voltammograms of a) ferrocene in solution, b) PMCz,
c) PMCz-IN, and d) PMCz+IN films on an ITO substrate in 0.1m TBAPF6/ace-
tonitrile solution with Ag/AgCl as reference electrode and Pt wire as counter
electrode. A scan rate of 100 mVsÀ1 was used.
excitation wavelength of 340 nm, the emission maximum of
the carbazole chromophore of PMCz is at 376 nm, as shown in
Figure 2c. However, when we use the end-functionalizing or
blending method to introduce initiator IN into PMCz, the pre-
pared PMCz-IN or PMCz+IN films exhibit a decrease of the
emission peak of the carbazole chromophore and the appear-
ance of a new emission peak at 556 and 567 nm, respectively,
whereas the emission maximum of IN is at 626 nm (see Fig-
ure S6 in the Supporting Information). This phenomenon
might be associated with the occurrence of electron transfer
and/or energy transfer,[24,44] which is important for their
memory characteristics as discussed in the following section. In
Table 1. Electrochemical characteristics of the prepared polymeric films.
Onset
Red
Polymeric films
EOOnxset [V]
E
[V]
HOMO [eV][a]
LUMO [eV][a]
PMCz-IN
PMCz+IN
1.05
1.08
À0.75
À1.00
À5.43
À5.46
À3.63
À3.38
[a] The HOMO and LUMO energy levels were calculated from cyclic vol-
tammetry with reference to ferrocene (4.80 eV).
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onset
Ox
Electrical memory behavior
EHOMO ¼ À½ðE ÀE1=2, ferroceneÞ versus Ag=AgCl þ 4:80Š eV
ð1Þ
The electrical and switching characteristics of the ITO/PMCz/Al
device have been reported in detail by Kang’s group previous-
ly.[48,49] The device exhibits WORM memory behavior with the
switch-on voltage mainly in the range of À1.8 to À2.2 V (with
a yield of 56%), as illustrated in Figure S8 in the Supporting In-
formation.
onset
Red
ELUMO ¼ À½ðE ÀE1=2, ferroceneÞ versus Ag=AgCl þ 4:80Š eV
ð2Þ
Figure 5c illustrates the electrical and switching characteris-
tics of the ITO/PMCz-IN/Al device. During the first negative
voltage from 0 to À3.0 V (Sweep 1), the current density initially
remains at a low conductivity (OFF state). At the switch-on
voltage around À3.0 V, the current density increases abruptly
from 10À8 to 10À2 A, which indicates the device transition from
the initial OFF state to the high-conductivity ON state. The
transition from the OFF state to the ON state is equivalent to
the “writing” process in a digital memory cell. The current den-
sity maintains the ON state for the subsequent negative sweep
from 0 to À5.0 V (Sweep 2). Once the device had reached the
ON state, it could not return to the OFF state even after turn-
ing off the power or upon applying a reverse sweep (not
shown). The nonvolatility of the ON state suggests that the
ITO/PMCz-IN/Al device exhibits WORM memory behavior with
the switch-on voltage around À3.0 V, and the ON/OFF current
ratio is up to 106. The cell-to-cell uniformity for the ITO/PMCz-
IN/Al device was also investigated. Figure 5b shows the statisti-
Morphology of the thin films
To investigate the morphology of the thin films of PMCz-IN
and PMCz+IN, the tapping mode atomic force microscopy
(TM-AFM) height and phase images were obtained at room
temperature as shown in Figure 4. As shown in Figure 4a,b, we
can conclude that both the PMCz-IN and PMCz+IN thin films
have a smooth surface with root-mean-square (RMS) rough-
ness values of 1.02 and 1.50 nm, respectively. Hence, the modi-
fication of IN using long alkyl chains (such as nC12H49) is neces-
sary to achieve homogeneous films. In addition, as shown in
Figure 4b, the typical cross-section profile of the TM-AFM
height image of the PMCz+IN thin film clearly illustrates the
formation of IN nanoparticles (cf. images of PMCz-IN (Fig-
ure 4a) and PMCz (Figure S7 in the Supporting Information)
thin films). This result is consistent with the emission spectra
of the PMCz-IN and PMCz+IN thin films as shown in Figure 2d.
Interestingly, when comparing the TM-AFM phase images of
PMCz-IN shown in Figure 4c with that of PMCz+IN shown in
Figure 4d, the PMCz-IN thin film exhibits microphase separa-
tion with a close-packed structure.[39]
Figure 5. a) The schematic diagram of the fabricated memory devices.
b) The statistical data of reproducibility of the devices ITO/PMCz-IN/Al. Cur-
rent density–voltage (I–V) characteristics of the c) ITO/PMCz-IN/Al and
d) ITO/PMCz+IN/Al devices. Retention times of the ON and OFF states
under a continuous readout voltage of À1.0 V for e) ITO/PMCz-IN/Al and
f) ITO/PMCz+IN/Al.
Figure 4. The TM-AFM height images and typical cross-section profiles of
a) PMCz-IN and b) PMCz+IN; c,d) are the corresponding phase images of (a)
and (b), respectively. The scan size is 2”2 mm2.
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cal data obtained from 50 cells, as the switch-on voltage is
mainly between À2.6 and À3.0 V (with a yield of 64%).
voltage of 0 V. In Figure 6b,e, the black region corresponds to
a decrease of the surface potential after the charge was inject-
ed from the scan tip into the films. After waiting for 10 mi-
nutes, the same 2”2 mm2 areas were scanned at 0 V. As can be
seen from Figure 6c,f, compared to the PMCz-IN thin film, the
surface potential of the PMCz+IN thin film clearly recovered
to the previous level. Consequently, the evident contrast of the
KPFM images of the PMCz-IN and PMCz+IN thin films strongly
supports the nonvolatile and volatile memory characteristics,
respectively, and rule out the metallic filamentary conduction
mechanism.
In the case of the ITO/PMCz+IN/Al device, the electrical and
switching characteristics were also investigated, as shown in
Figure 5d. Starting with the low-conductivity OFF state of the
fabricated device, the current density increases slowly in the
range of 10À10 to 10À6 A with the applied negative voltage. The
current remains in the OFF state until the switch-on voltage of
about À3.5 V. When the voltage increases further, the current
density increases abruptly from 10À6 to 10À2 A (Sweep 1). The
ON/OFF current ratio is up to 104. The ON state was main-
tained during a subsequent sweep from 0 to À5.0 V (Sweep 2).
After reading the ON state in the negative sweep and turning
off the power for about five to eight minutes, the device was
reprogrammed from the OFF state to the ON state again with
a negative biased sweep (Sweep 3) and remains in the ON
state (Sweep 4). The ITO/PMCz+IN/Al device exhibited repeat-
ed operation from Sweep 5 and Sweep 6, and the ON state
could not be maintained after the removal of power and grad-
ually relaxed back to the OFF state. Therefore, the short reten-
tion ability based on the electrical switching concludes the vol-
atile nature of the static-random-access-memory (SRAM) on
the ITO/PMCz+IN/Al memory device.
Proposed memory mechanism
The two memory devices ITO/PMCz-IN/Al and ITO/PMCz+IN/
Al show WORM and SRAM characteristics, respectively. This can
be explained by the formation of a stable or unstable charge-
transfer (CT) complex[32,46] between the pendent carbazole
donor and pyrazoline acceptor moieties under an electric field.
To understand the charge-transfer mechanism, simulation re-
sults of the components in the carbazole–pyrazoline system,
including one unit of PMCz, one IN, and a representative unit
of PMCz-IN (MCz-IN), were carried out by using DFT/B3LYP/6-
31G(d) with the Gaussian 03 program, as illustrated in Table S1
in the Supporting Information. Monomer MCz with a lower
HOMO energy level (value) than IN indicates that the MCz
group has a stronger tendency to donate an electron, whereas
the high LUMO energy level of IN indicates its strong electron-
withdrawing ability. When these two components are covalent-
ly connected, the HOMO of MCz-IN is located on the MCz side,
whereas the LUMO of MCz-IN is located on the IN side. The
energy band gaps of IN as well as MCz-IN are much lower than
that of MCz, which indicates that the introduction of IN can
lead to a polymer system that is highly conductive once it is
charged under an electric field. Moreover, the total dipole mo-
ments of IN and MCz-IN (7.579 and 5.967 D) are much larger
than that of MCz (2.240 D), which can result in a high tendency
for charge separation in the carbazole–pyrazoline (D–A)
system.
The stability of the two memory devices at room tempera-
ture under ambient conditions was tested. Figure 5c,d show
the retention time tests under a constant stress of À1.0 V for
both the ON and OFF states of the ITO/PMCz-IN/Al and ITO/
PMCz+IN/Al memory devices. As shown in Figure 5, an ON/
OFF current ratio of approximately 106 (PMCz-IN) and approxi-
mately 104 (PMCz+IN) can be maintained and no apparent
degradation in the current is observed for both the ON and
OFF states after periods of at least 104 s.
To further verify the electrical bistable phenomenon of these
two devices, Kelvin probe force microscopy (KPFM)[50,51] was
conducted at room temperature as shown in Figure 6. First,
a 0.5”0.5 mm2 area of the ITO/PMCz-IN and ITO/PMCz+IN thin
films was scanned with a tip-applied bias voltage of +6 V, as il-
lustrated in the inset of Figure 6a,d, respectively. Second, a 2”
2 mm2 area was scanned immediately with a tip-applied bias
Figure 7 depicts the energy-level diagram for the two devi-
ces. In the measurements we used ITO as the anode and Al as
the cathode. The work functions of the two electrodes were
À4.80 and À4.28 eV for ITO and Al, respectively. For ITO/PMCz-
IN/Al, the energy barriers of the ITO/PMCz-IN (HOMO) and
Figure 6. a–c) Sequence of KPFM images showing the nonvolatile memory
characteristics of the PMCz-IN thin film on the ITO substrate; d–f) sequence
of KPFM images showing the volatile memory characteristics of the
PMCz+IN thin film on the ITO substrate. The scan size with a tip-applied
bias voltage of 0 V is 2”2 mm2, whereas the scan size of the area marked in
(a) and (d) with a tip-applied bias voltage of +6 V is 0.5”0.5 mm2.
Figure 7. Energy-level diagram for the ITO/PMCz-IN/Al and ITO/PMCz+IN/Al
devices.
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PMCz-IN (LUMO)/Al interfaces were 0.63 and 0.65 eV, respec-
tively. As for ITO/PMCz+IN/Al, the energy barriers of the ITO/
PMCz+IN (HOMO) and PMCz+IN (LUMO)/Al interfaces were
0.66 and 0.90 eV, respectively. PMCz with a pendent carbazole
group has been reported as a p-type material with the holes
as the dominant charge carriers.[48,49] Thus, due to the high
content of the carbazole moiety of these two polymer systems,
the conduction processes under an electric field are also domi-
nated by charge injection. The obtained I–V characteristics for
the two devices were further analyzed by fitting appropriate
conductance models, as shown in Figure 8. The I–V data for
moieties and pyrazoline electron-acceptor moieties. At this
stage, the devices are at low conductivity (OFF state). When
the bias voltage reaches the threshold voltage, electrons in the
ground state can transition to the various excited states and
a charge-transfer (CT) interaction can occur between MCz and
IN (illustrated in Figure 9a,c). Along with the increase of the CT
interaction, a channel for charge carriers is formed. Subse-
quently, the devices reach high conductivity (ON state). How-
ever, when the power is turned off, the stability of the formed
CT complex for these two devices is different. For the ITO/
PMCz-IN/Al device, the initiator IN is connected to the carba-
zole polymer chain by the covalent bond, which results in the
IN molecules being uniformly distributed and having a strong
interaction with MCz. Thus, once the polymer layer was con-
ductive in the ON state, the formed CT complex would be
stable as shown in Figure 9b.[19,35,36,46] As for the ITO/PMCz+
IN/Al device, the initiator in the polymer matrix aggregates to
form nanoparticles. In our previous study, we demonstrated
that the compound pyrazoline with the linkage -C=N-N- has
volatile memory characteristics.[52] Therefore, in this system, the
charges trapped by the IN nanoparticles probably redistributed
among the IN molecules as shown in Figure 9d,[32,38] which led
to the volatile nature of the ITO/PMCz+IN/Al device.
Conclusion
In this study, we successfully synthesized the monomer MCz
and the initiator IN, and further prepared the two electroactive
layers PMCz-IN and PMCz+IN by utilizing the end-functionaliz-
ing and blending methods, respectively. The fabricated ITO/
PMCz-IN/Al and ITO/PMCz+IN/Al devices had the same mass
percentage of IN, which showed WORM and SRAM memory
characteristics, respectively. This switching phenomenon was
further supported by KPFM measurements of the PMCz-IN and
PMCz+IN thin films. These phenomena were mainly caused
by the formation of a stable or unstable charge-transfer (CT)
complex between pendent carbazole donor and pyrazoline ac-
ceptor moieties under an electric field. Compared to the
PMCz-IN thin film with a microphase separation structure, the
formation of IN nanoparticles in the PMCz+IN thin film might
be the main reason for the unstable CT complex. Finally, we
provided a new approach to tune the electrical memory per-
formance using two different carbazole–pyrazoline-containing
polymer systems.
Figure 8. Experimental and fitted I–V curves for the ITO/PMCz-IN/Al device
and ITO/PMCz+IN/Al device in the OFF and ON states. Panels (a) and (b) are
the OFF state with the SCLC model and the ON state with the ohmic current
model for the ITO/PMCz-IN/Al device. Panels (c) and (d) are the OFF state
with the SCLC model and the ON state with the ohmic current model for
the ITO/PMCz+IN/Al device.
the OFF state of the PMCz-IN- and PMCz+IN-based devices
can be fitted by a trap-limited space–charge-limited conduc-
tion (SCLC)[13] model, whereas the ON state current can be
fitted by the ohmic model.
The proposed operational mechanism of these two devices
is depicted in Figure 9. Initially, under a low bias voltage,
charge carriers are blocked by the carbazole electron-donor
Experimental Section
Materials
Carbazole, chloroethanol, methacryloyl chloride, 4-bromo-1,8-naph-
thalic anhydride, dodecylamine, hydrazine hydrate (85%), 4-hy-
droxybenzaldehyde, 4’-nitroacetophenone, and 2-bromo-2-methyl-
propionyl bromide were used. CuBr (Sinopharm Chemical Reagent
Co., Ltd, 98.5%) was purified in acetic acid, washed with methanol,
and dried under vacuum to obtain a white powder. Cyclohexanone
and N,N,N’,N,’N’’-pentamethyldiethylenetriamine (PMDETA) were
distilled under vacuum. All other reagents and solvents were ana-
lytically pure and used as received without further purification.
Figure 9. a,b) Operating mechanism of the ITO/PMCz-IN/Al device; c,d) op-
erating mechanism of the ITO/PMCz+IN/Al device.
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5.37 Hz, 1H), 7.15–7.26 (m, 2H), 7.40–7.50 (m, 2H), 7.65 (dd, J=
Characterization
4.70, 10.23 Hz, 2H), 8.10–8.21 ppm (m, 2H).
1H NMR spectra were measured using INOVA 400 MHz NMR spec-
trometers, with CDCl3 or [D6]DMSO as solvent and tetramethylsi-
lane (TMS) as the internal standard at ambient temperature. Molec-
ular weights and polydispersity indexes (PDIs), relative to PMMA,
were measured using Waters 1515 GPC with THF as a mobile
phase at a flow rate of 1 mLminÀ1 and with a column temperature
of 308C. SEM images were recorded on a Hitachi S-4700 scanning
electron microscope. Thermogravimetric analysis (TGA) was con-
ducted on a TA Instruments Dynamic TGA 2950 at a heating rate
of 108C minÀ1 and under an N2 flow rate of 50 mLminÀ1. Differen-
tial scanning calorimetry (DSC) analysis was performed on a Shi-
madzu DSC 860A 85 instrument. UV/Vis absorption spectra were
determined on a Shimadzu RF540 spectrophotometer. Room-tem-
perature emission spectra were recorded using an Edinburgh-920
fluorescence spectrophotometer. Atomic force microscopy (AFM)
measurements were performed by using a Dimension Icon. Cyclic
voltammetry (CV) was performed at room temperature using an
ITO working electrode, a reference electrode Ag/AgCl, and a coun-
ter electrode (Pt wire) at a sweep rate of 100 mVsÀ1 (PGSTAT302N
Electrochemical Workstation analyzer). All electrical measurements
of the devices were characterized under ambient conditions with-
out any encapsulation using an Agilent Technologies B1500A semi-
conductor device analyzer equipped with a TTPX low and variable
temperature probe station.
2-(9H-Carbazole-9-yl)ethyl methacrylate (MCz): Methacryloyl chlo-
ride (7.95 g, 40.0 mmol) was added dropwise to a solution of Cz
(4.00 g, 19.0 mmol) in chloroform (50 mL) and triethylamine (Et3N)
(7.60 g, 75.0 mmol) at 0–58C and reacted for 12 h. The crude prod-
uct was isolated by evaporating the solvent and was purified twice
1
by recrystallization from 95% alcohol. Yield: 4.34 g, 82.1%. H NMR
(400 MHz, [D6]DMSO): d=8.15 (d, J=7.7 Hz, 2H), 7.66 (d, J=8.2 Hz,
2H), 7.46 (t, J=7.7 Hz, 2H), 7.21 (t, J=7.5 Hz, 2H), 5.74 (s, 1H),
5.53–5.50 (m, 1H), 4.75 (t, J=5.0 Hz, 2H), 4.48 (t, J=4.9 Hz, 2H),
1.65 ppm (s, 3H); 13C NMR (400 MHz, [D6]DMSO): d=167.02,
140.80, 136.03, 126.71, 126.28, 122.83, 120.88, 119.61, 110.08, 63.52,
41.84, 18.45 ppm.
Synthesis of the initiator (IN)
The synthetic procedure for the initiator IN is detailed in Scheme 1.
6-Bromo-2-dodecyl-1H-benzo[de]isoquinoline-1,3-(2H)-dione
(BrDBO): 4-Bromo-1,8-naphthalic anhydride (2.76 g, 10 mmol) and
dodecylamine (2.78 g, 15 mmol) were dissolved in ethanol
(100 mL). The reaction mixture was stirred and heated at reflux for
24 h. The resulting mixture was filtered and the resulting solid was
1
recrystallized from ethanol. Yield: 2.66 g, 61.4%. H NMR (400 MHz,
[D6]DMSO): d=8.53 (dd, J=11.9, 7.9 Hz, 2H), 8.30 (d, J=7.8 Hz,
1H), 8.19 (d, J=7.8 Hz, 1H), 7.97 (t, J=7.9 Hz, 1H), 4.01 (t, J=
7.2 Hz, 2H), 1.62 (dd, J=8.9, 4.6 Hz, 2H), 1.32–1.17 (m, 18H),
0.84 ppm (t, J=6.6 Hz, 3H).
Fabrication of devices
The schematic diagram of the fabricated memory devices is shown
in Figure 5a. The indium tin oxide (ITO)/glass (ITO: approximately
200 nm, see Figure S9a) was precleaned by sonication, with deion-
ized water, acetone, and ethanol for 15 min each. Samples
(10 mgmLÀ1) of PMCz, PMCz-IN, and PMCz+IN (PMCz blended
with approximately 1.0 wt% IN) shown in Scheme 1 in 1,2-dichloro-
ethane were prepared and filtered through microfilters with a pin-
hole size of 0.22 mm, after this the solution was spin-coated onto
the ITO/glass substrate at a rotational speed of 1500 rpm for 20 s.
These films were annealed at 908C in a vacuum oven for 12 h; the
films had a thickness of about 70–90 nm measured by SEM (shown
in Figure S9b–d). Finally, Al was thermally evaporated onto the film
surface at 5”10À4 Pa through a shadow mask to yield top electro-
des with a thickness of approximately 150 nm. The active area of
the fabricated device was 0.126 mm2 (a nummular point with
a radius of 0.2 mm). Thus, the three memory devices ITO/PMCz/Al,
ITO/PMCz-IN/Al, and ITO/PMCz+IN/Al were fabricated by the
above method. Herein, we focused on the investigation of the two
devices ITO/PMCz-IN/Al and ITO/PMCz+IN/Al.
2-Dodecyl-6-hydrazinyl-1H-benzo[de]isoquinoline-1,3-(2H)-dione
(DHBO): BrDBO (4.43 g, 10 mmol) and 85% hydrazine hydrate
(2.0 mL) were dissolved in 2-methoxyethanol (20 mL). The reaction
mixture was stirred and heated at reflux for 4 h. Then, the powder
was collected by filtration and washed with chloroform. Yield:
1
3.08 g, 78.0%. H NMR (400 MHz, [D6]DMSO): d=9.13 (s, 1H), 8.60
(d, J=8.4 Hz, 1H), 8.40 (d, J=7.3 Hz, 1H), 8.28 (d, J=8.6 Hz, 1H),
7.62 (t, J=7.9 Hz, 1H), 7.24 (d, J=8.6 Hz, 1H), 4.68 (s, 2H), 3.99 (t,
J=7.2 Hz, 2H), 1.62–1.54 (m, 2H), 1.31–1.19 (m, 18H), 0.84 ppm (t,
J=6.5 Hz, 3H).
3-(4-Hydroxyphenyl)-1-(4-nitrophenyl)prop-2-en-1-one (HNPE): 4-
Hydroxybenzaldehyde (2.44 g, 20 mmol), 4’-nitroacetophenone
(3.30 g, 20 mmol), and sulfuric acid (2 mL) were dissolved in acetic
acid (60 mL). The reaction mixture was stirred and heated at reflux
for 24 h. The resulting mixture was poured into a water/ice bath.
The white solid was obtained after filtering and was dried under
vacuum. Yield: 4.57 g, 85.0%. 1H NMR (400 MHz, [D6]DMSO): d=
10.20 (s, 1H), 8.35 (q, J=8.7 Hz, 4H), 7.80–7.73 (m, 4H), 6.86 ppm
(d, J=8.3 Hz, 2H).
Synthesis of the monomer (MCz)
2-Dodecyl-6-[5-(4-hydroxyphenyl)-3-(4-nitrophenyl)-4,5-dihydro-1H-
2-(9H-Carbazole-9-yl)ethanol (Cz): A mixture of carbazole (4.17 g,
25.0 mmol) and potassium hydroxide (2.10 g, 37.5 mmol) in N,N-di-
methylformamide (DMF) (50 mL) was stirred for 1 h and chloroe-
thanol (2.95 g, 37.5 mmol) was added dropwise. After stirring for
12 h at room temperature, the mixture was poured into deionized
water (500 mL) and the resultant white precipitate was removed
by filtration. The reactant was dissolved in aqueous alcohol
(70vol%, 20 mL) and insoluble matter was removed by filtration.
The filtrate was poured into deionized water (100 mL), and the
white flocculent precipitate was removed by filtration and dried
pyrazol-1-yl]-1H-benzo[de]isoquinoline-1,3-(2H)-dione
(DHHB):
HNPE (1.35 g, 5 mmol), DHBO (1.97 g, 5 mmol), and hydrochloric
acid (2.00 mL) were dissolved in ethanol (45 mL). The reaction mix-
ture was stirred and heated at reflux for 24 h. Then, the powder
was collected by filtration and washed with cooled ethanol. Yield:
1
2.65 g, 82.0%. H NMR (400 MHz, [D6]DMSO): d=9.51 (d, J=8.6 Hz,
1H), 9.45 (s, 1H), 8.49 (d, J=7.2 Hz, 1H), 8.30 (d, J=8.5 Hz, 2H),
8.21 (d, J=8.3 Hz, 1H), 8.04 (d, J=8.5 Hz, 2H), 7.83 (t, J=7.9 Hz,
1H), 7.21 (d, J=8.2 Hz, 2H), 7.13 (d, J=8.5 Hz, 1H), 6.67 (d, J=
8.3 Hz, 2H), 6.03–5.95 (m, 1H), 4.07–3.99 (m, 1H), 3.96 (d, J=
8.0 Hz, 2H), 3.30 (d, J=7.5 Hz, 1H), 1.57 (s, 2H), 1.23 (d, J=27.1 Hz,
18H), 0.83 ppm (t, J=6.4 Hz, 3H).
1
under vacuum. Yield: 4.01 g, 75.4%. H NMR (400 MHz, [D6]DMSO):
d=3.74–3.85 (m, 2H), 4.45 (dd, J=4.41, 7.07 Hz, 2H), 5.51 (t, J=
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the ice/water bath was removed and the reaction was allowed to
continue for 12 h at room temperature. The solution was filtered
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1
1.49 g, 75.0%. H NMR (400 MHz, [D6]DMSO): d=9.53 (d, J=8.8 Hz,
1H), 8.49 (d, J=7.0 Hz, 1H), 8.31 (d, J=8.1 Hz, 2H), 8.21 (d, J=
8.0 Hz, 1H), 8.04 (d, J=8.0 Hz, 2H), 7.84 (t, J=7.7 Hz, 1H), 7.53 (d,
J=8.0 Hz, 2H), 7.13 (d, J=7.5 Hz, 3H), 6.15 (t, J=10.1 Hz, 1H),
4.14–4.01 (m, 1H), 3.95 (t, J=6.5 Hz, 2H), 3.41 (dd, J=16.7, 7.4 Hz,
1H), 1.97 (s, 6H), 1.56 (s, 2H), 1.27–1.17 (m, 18H), 0.82 ppm (t, J=
6.0 Hz, 3H); LC–MS: m/z calcd. for C43H48BrN4O6+: 795.2752; found:
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Synthesis of the polymers PMCz and PMCz-IN
PMCz was synthesized by the ATRP method, using MCz (8 mmol)
as monomer, tert-butyl 2-bromoisobutyrate (0.02 mmol) as an ini-
tiator, CuBr (0.04 mmol) as a catalyst, and PMDETA (0.08 mmol) as
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Published online on June 15, 2015
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