Nonvolatile electrical switching behavior and mechanism of functional polyimides bearing a pyrrole unit: Influence of different side groups

Article abstract of DOI:10.1039/c6ra11615a

To better understand the structure-property relationships and the mechanisms of the electronic switching behavior of functional polyimides, a new series of diamines containing a pyrrole core with different side groups were designed and synthesized. Three novel, thermally stable and aromatic polyimides (Py6FPIs) containing the as-designed diamines were prepared. They were fabricated into memory devices sandwiched between an Al top electrode and an indium-tin oxide (ITO) bottom electrode by spin-coating. The polyimides exhibited high thermal stability with glass transition temperatures (T_g) around 300 °C as determined by DMA. The memory devices were found to show nonvolatile bistable write-once-read-many (WORM) memory characteristics with diverse switching threshold voltages and ON/OFF ratios, depending on the electron affinity of the pendent groups. The low-conductivity state and the high-conductivity state could be sustained under a constant bias or a refreshing voltage pulse of 1.0 V. The switching mechanism and the memory effects were finely demonstrated with the aid of molecular simulations and electronic absorption spectra of these Py6FPIs.

Full text of DOI:10.1039/c6ra11615a

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Nonvolatile electrical switching behavior and
mechanism of functional polyimides bearing
Cite this: RSC Adv., 2016, 6, 52798
a pyrrole unit: influence of different side groups†
*
Zhuxin Zhou, Lunjun Qu, Tingting Yang, Jinglan Wen, Yi Zhang, Zhenguo Chi,
Siwei Liu, Xudong Chen and Jiarui Xu
To better understand the structure–property relationships and the mechanisms of the electronic switching
behavior of functional polyimides, a new series of diamines containing a pyrrole core with different side
groups were designed and synthesized. Three novel, thermally stable and aromatic polyimides (Py6FPIs)
containing the as-designed diamines were prepared. They were fabricated into memory devices
sandwiched between an Al top electrode and an indium-tin oxide (ITO) bottom electrode by spin-
coating. The polyimides exhibited high thermal stability with glass transition temperatures (Tg) around
300 ꢀC as determined by DMA. The memory devices were found to show nonvolatile bistable write-
once-read-many (WORM) memory characteristics with diverse switching threshold voltages and ON/OFF
ratios, depending on the electron affinity of the pendent groups. The low-conductivity state and the
high-conductivity state could be sustained under a constant bias or a refreshing voltage pulse of 1.0 V.
The switching mechanism and the memory effects were finely demonstrated with the aid of molecular
simulations and electronic absorption spectra of these Py6FPIs.
Received 4th May 2016
Accepted 24th May 2016
DOI: 10.1039/c6ra11615a
www.rsc.org/advances
voltage. Therefore, great efforts have been made for the devel-
opment of polymer materials with excellent electrical resistive
switching effects, such as vinyl polymers,2 poly(3,4-ethylenedioxy-
thiophene):poly(styrene sulfonate) (PEDOT:PSS),3 poly(9,9-bis(4-
diphenylaminophenyl)-2,7-uorene) (PDPAF),4 polymer/metal
hybrids5 and polymer/organic molecular blends.6
Introduction
Along with the dramatically increased capacity and drastically
decreased size of semiconducting memory devices in recent
decades, more and more new materials and advanced processes
have been in demand. Compared with the traditional inorganic
materials, organic and polymeric materials are promising
candidates for the next generation of memory applications. They
show notable advantages over inorganic materials, such as ease
of processability, high mechanical exibility, low fabrication cost,
property modulation by molecular design through chemical
synthesis, and three-dimensional (3D) multi-stacking capability
for achieving high density data storage.1 Furthermore, rather
than storing and retrieving data by encoding “0” (discharge) and
“1” (charge) in traditional memory devices like silicon- and metal-
oxide-based memory cells, memory devices based on electrically
bistable resistive switching organic or polymeric materials oper-
ate by the high and low conductivity response to the applied
Taking into account the requirements of the devices fabri-
cation process, aromatic polyimides (PIs) are greatly superior to
the polymer systems mentioned above and become promising
materials for device application, due to their outstanding
thermal and dimensional stability, high chemical resistance,
good mechanical strength and processability.7 More impor-
tantly, the macromolecular chain of polyimide naturally
possesses an electron-donor (D, diamine moiety) and electron-
acceptor (A, dianhydride moiety) structure within a repeating
unit, which may exhibit electrical bistability under certain
external voltage. In fact, functional PIs exhibiting various
memory behaviors have been extensively reported, from volatile
memory properties (like Dynamic Random Access Memory
(DRAM)8 and Static Random Access Memory (SRAM)8a,e,9), to
nonvolatile memory properties (like FLASH memory10 and
Write-Once-Read-Many (WORM) memory8a,b,11). However,
researches have been mainly focused on the exploration of
different diamine structures in functional polyimides with
better electrical properties for data storage. The role of the
donor and acceptor in these typical polymers, especially the
inuence of the electron affinities and the steric hindrance
effect of the diamine (donor) or dianhydride (acceptor) on the
PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for
High-performance Organic and Polymer Photoelectric Functional Films, State Key
Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and
Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China. E-mail:
ceszy@mail.sysu.edu.cn; Fax: +86 20 84112222; Tel: +86 20 84112222
† Electronic supplementary information (ESI) available: NMR results of all the
intermediate products; FTIR spectra, 1H NMR, WAXD patterns, thermal
properties of the Py6FPI lms; UV-vis absorption spectra of the diamines;
molecular simulation results in details; experimental and tted I–V curves for
PI-devices. See DOI: 10.1039/c6ra11615a
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Fig. 1 Chemical structure of the diamines containing a pyrrole moiety
and different side groups.
electronic switching behaviors, properties and mechanism, has
rarely been systematically studied and further perfected. A
deeper understanding of the structure–property relationship is
needed to help the design of new functional polyimides for
high-performance polymer memory devices.
In this work, we designed and synthesized a new series of
diamine molecules containing a pyrrole core with different side
groups (–CH3, –CF3 and –CPh3) (Fig. 1). Novel functional
aromatic PIs (Py6FPIs) containing the as-designed diamines
and dianhydride of 4,40-(hexauoroisopropylidene)diphthalic
anhydride (6FDA) were thus prepared. The incorporation of the
planar, conjugated, and electron-rich pyrrole moiety, together
with the strong electron-withdrawing triuoromethyl groups (in
dianhydride) into the polyimide backbone was expected to
enhance the electron donating-withdrawing property and the
charge-transporting capacity along the polymer backbone for
device application. We further explored the inuence of the D–A
interaction intensity to the device performance by nely regu-
lating the electron affinities and the steric hindrance effect of
the pendent groups, as it may have an impact on the frontier
molecular orbital of the polymers. The memory devices based
on the Py6FPIs exhibit nonvolatile bistable electrical switching
characteristics of a WORM type memory with long retention
time, varied ON/OFF ratios and diverse switching threshold
voltages. Moreover, theoretical calculations for molecular
orbital distribution, oscillator strength and dipole moment,
combined with the absorption spectra of polyimides in thin lm
state and solution state, exhibiting an exact correspondence
with each other, were applied to help better elucidate the
switching mechanisms and the memory effects.
Scheme 1 Synthesis routes for the diamines.
Fig. 2 Comparison for the 1H NMR spectra of the diamines (the blue
rounds indicate the Hd on the pendent phenyl rings).
pendent benzene demonstrate that different electron affinities
of the side groups do have a certain impact. For instance, as the
blue rounds indicated in Fig. 2, the Hd signal of PyCF3DA (7.64–
7.61 ppm) shied to lower eld as compared with that of
PyCH3DA (7.06–7.04 ppm) and PyCPh3DA (7.05–6.95 ppm),
referring the higher electron affinity of triuoromethyl group
(–CF3). This inuence might further affect the memory device
performance when the polyimides containing the correspond-
ing side group was fabricated as the active layer.
Results and discussion
Characterization of the diamine monomers
Characterization of the polyimides
The three novel diamines containing pyrrole and varied
pendent groups, PyCH3DA, PyCF3DA and PyCPh3DA were The Py6FPIs were prepared as lms by the conventional two-
synthesized via the well-known Paal–Knorr reaction, followed by step polymerization as shown in Scheme 2. The reaction
the reduction of the corresponding dinitro-compound (Scheme between the as-synthesized diamine and the commercially
1). The characterizations of the diamines are shown in the available dianhydride 6FDA in DMF gave a viscous precursor
Experimental section, Fig. S1 and S2.† The proton resonance poly(amic acid) solution, followed by programmed thermal
signals at around 5.00 ppm and the infrared absorptions at imidization on a glass plate. The resulting yellowish PI lms
about 3350 cmꢁ1 and 3450 cmꢁ1, which are the characteristics were tough and exible. The inherent viscosities of PyCH36FPI,
signals of amino groups, indicate a dinitro-compound was PyCF36FPI and PyCPh36FPI in NMP are 1.19, 0.94 and 1.06 dL
successfully reduced to a diamine. 1H NMR and 13C NMR gꢁ1, respectively. As shown in Fig. S3a,† the polyimides
spectra, mass spectra, IR and elemental analysis clearly conrm exhibited the characteristic infrared absorption bands of the
the diamino-compounds are fully consistent with the proposed imide ring at 1784 cmꢁ1 (asymmetric C]O stretching) and 1720
structures. Moreover, the different proton signals of the cmꢁ1 (symmetric C]O stretching). Fig. S3b† shows the 1H NMR
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char yields of the Py6FPIs (in the range of 55–63%) obtained
from the TGA curves at 900 ꢀC, the PyCPh36FPI ranked the
highest, resulted from its high aromatic contents. Besides, due
to the rigidity of the polymer backbones, the polyimide lms
showed relatively high dimensional stabilities with the coeffi-
cient of thermal expansion (CTE) at 45–50 mm mꢁ1 ꢀCꢁ1 in the
ꢀ
recorded range of 50–250 C. The soening temperatures (Ts)
gauged by TMA were in the range of 263–279 ꢀC, which proved
the high thermal stabilities of the polyimides as well.
Optical and electrochemical properties of the Py6FPIs
The optical properties of the polyimides were measured by UV-vis
spectroscopy. The key statistics are listed in Table 2. As shown in
Fig. 3a, the Py6FPIs solution exhibited a maximum absorption
abs
max1
abs
max2
peak (l
) at around 328 nm with a shoulder (l
) at about
306 nm. The former peak probably corresponds to the charge-
transfer transition from the pyrrole-containing diamine moie-
ties (donor) to the dianhydride moieties (acceptor), while the
Scheme 2 Synthesis routes for the Py6FPIs.
latter is originated from the n–p* and local p–p* transitions of
spectra of polyimides. The disappearance of amide bands and
the absence of carboxyl absorptions in the FT-IR spectra, as well
as the disappearance of the amide and carboxyl proton signals
indicated a virtually complete conversion of the precursor pol-
y(amic acid) to the polyimide. The wide-angle X-ray diffraction
(WAXD) patterns (Fig. S4†) show the amorphous nature in all
the as-prepared polyimide lms. These properties might be
attributed to the high steric hindrance effect of hexa-
uoroisopropyl group in the dianhydride moieties and the
pendent groups in the diamines moieties, which could avoid
the close packing of the macromolecular chain.
abs
the pyrrole moieties since the l
is well consistent with the
max2
absorption maxima of their corresponding monomers in solu-
tion (shown in Table 2 with parentheses, and in Fig. S6†). The
abs
absorption edges (l
) of all the polymers were observed in the
onset
abs
ranges of 371–377 nm. In this case, the order of l
principally
onset
correlated with the nucleophilicity of the pendent group. As
a strong electron acceptor group, –CF3 can reduce the nucleo-
philicity of the diamine moieties and weaken the CT intra-
molecular interaction, thus the CT absorption edge of PyCF36FPI
abs
onset
assumed a hypsochromic shi. The l
of PyCPh36FPI, yet
assumed a bathochromic shi due to the electron-donating
properties of triphenylmethyl group (–CPh3). For the solid thin
lms coated on the quartz plates, these polyimides showed
Thermal properties of the Py6FPIs
Thermal properties of the polyimides were investigated by DSC, a maximum absorption peak at around 332 nm with a shoulder
DMA, TMA and TGA, and the results are summarized in Table 1 at about 304 nm (Fig. 3b). Compared with the absorption spectra
abs
and Fig. S5.† The glass transition temperature (Tg) of all the measured in the solution state, the almost unchanged l
in
max2
ꢀ
polyimides were observed in the range of 276–287 C by DSC, the solid lm state further proves that this absorption is from the
and 296–306 ꢀC by DMA. All of the polyimides exhibited no local transitions of the pyrrole moieties. On the other hand, the
abs
ꢀ
obvious weight loss below 500 C in nitrogen atmosphere. The
l
red-shied and the absorption tails extended to a longer
max1
decomposition temperatures (Td) at 5% and 10% weight-loss wavelength due to the stronger intermolecular CT interaction in
abs
measured by TGA were recorded in the range of 515–543 ꢀC the solid lm state. The l
of PyCPh36FPI, however, located at
onset
and 537–565 ꢀC, respectively. These remarkable thermal a shorter wavelength than that of PyCH36FPI, which is different
stability properties allowed the Py6FPIs to undergo the high from the order in solution state. It could be ascribed to the closed
temperature electronic device fabrication processes. Among the packing of PyCH36FPI molecular chain in the solid thin lm
Table 1 Thermal properties of the Py6FPIs
e
Td (ꢀC)
Polyimide
CTEd (mm mꢁ1 ꢀCꢁ1
)
5 wt%
10 wt%
Char yieldf (wt%)
a
Tg (ꢀC)
b
Tg (ꢀC)
c
Ts (ꢀC)
PyCH36FPI
PyCF36FPI
PyCPh36FPI
276
287
277
296
306
298
264
279
263
50
49
45
515
522
543
537
543
565
58
55
63
a
b
c
Measured by DSC at a heating rate of 10 ꢀC minꢁ1 in nitrogen. Measured by DMA at a heating rate of 5 ꢀC minꢁ1
.
Soening temperature,
measured by TMA at a heating rate of 10 ꢀC minꢁ1
.
Calculated by TMA curves (50 to 250 ꢀC). Measured by TGA at a heating rate of 20 ꢀC
d
e
minꢁ1 in nitrogen. f Residual weight percentage at 900 ꢀC in nitrogen.
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Table 2 Optical and electrochemical properties of the Py6FPIs
In solutiona (nm)
As thin lmb (nm)
Oxidation potentialc (V)
abs
onset
abs
max1
abs
max2
abs
onset
abs
max1
abs
max2
OX
onset
OX
max1
OX
d
Polyimide
l
l
l
l
l
l
E
E
E
max2
Eg (eV)
HOMOe (eV)
LUMOf (eV)
PyCH36FPI
PyCF36FPI
PyCPh36FPI
374
371
377
329
325
331
307 (307)g
305 (300)
307 (305)
395
385
392
334
328
335
305
303
304
0.90
1.00
0.97
1.11
1.17
1.15
1.53
1.59
1.55
3.14
3.22
3.16
–5.32
–5.42
–5.39
–2.18
–2.20
–2.23
a
Measured in dilute solution in DMF (ꢂ2 ꢃ 10ꢁ5 M). b Spin-coated thin lm on quartz plate. c Obtained from cyclic voltammograms versus Ag/AgCl
in CH3CN. d Energy gap ¼ 1240/l
of the polyimide thin lm. e The HOMO energy levels relative to ferrocene (ꢁ4.80 eV) were calculated from
abs
onset
f
Eonset
.
LUMO ¼ HOMO + Eg. g Data in parentheses are the maximum UV-vis absorption wavelength measured in THF solution (10ꢁ5 M).
Fig. 3 Normalized UV-vis absorption spectra of Py6FPIs: (a) in DMF (ꢂ2 ꢃ 10ꢁ5 M); (b) PI films spin-coated on quartz plates.
state, while the bulky side group –CPh3 in PyCPh36FPI prevents molecular orbital (HOMO) energy levels were ꢁ5.32, ꢁ5.42 and
the dense packing thus alleviates the intermolecular interactions. ꢁ5.39 eV, respectively, based on the reference energy level of
Fig. 4 depicts the cyclic voltammograms (CV) of the poly- ferrocene (4.8 eV below vacuum level). Also note that the band
imide cast lms on the ITO-coated glass slides, which were used gap estimated from the absorption edge of polymer thin lms
as working electrodes in CH3CN containing 0.1 M solution of were 3.14, 3.22 and 3.16 eV, respectively. The lowest unoccupied
TBAP as supporting electrolyte and saturated Ag/AgCl as refer- molecular orbital (LUMO) energy levels were ꢁ2.18, ꢁ2.20 and
ence electrode. The related electrochemical properties are also ꢁ2.23 eV, respectively. These results indicate that the variation
summarized in Table 2. These PIs showed two oxidation peaks of the pendent groups with different electron affinities have
in the range of 1.11–1.17 V and 1.53–1.59 V, respectively. The a certain impact on the HOMO and LUMO energy levels. Hence,
onset oxidation potentials for PyCH36FPI, PyCF36FPI and the hole-injecting and electron-injecting properties of the
PyCPh36FPI were 0.90, 1.00 and 0.97 V respectively. Note that resulting memory devices, more or less, may be differed.
the oxidation potential of ferrocene (external standard) was 0.38
V versus Ag/AgCl measured by CV from a polymer-free ITO-
coated glass slide. Thus the estimated highest occupied
Electrical switching effects and memory performances of the
ITO/Py6FPI/Al devices
The memory behavior of Py6FPIs is demonstrated by the I–V
characteristics of ITO/Py6FPI/Al device. Fig. 5 shows the I–V curves
of the ITO/Py6FPIs/Al devices under an initially positive applied
voltage. The I–V curves of ITO/PyCH36FPI/Al device (Fig. 5a) are
used as an example in illustrating the memory effects. In the rst
sweep under the positive voltage from 0 to 4.1 V, the device was in
the low conductivity state, which was assigned as OFF state or “0”
signal in data storage. However, an abrupt increase in current from
10ꢁ10 to 10ꢁ5 A occurred at the continuous sweeping voltage
around 4.1–4.2 V, which was dened as the switching threshold
voltage, indicating the device transferred from a low conductivity
state to a high conductivity state (as-signed as ON state or “1” signal
in data storage). These distinct conductivity states allow the reading
process at a relatively low voltage (e.g., 1.0 V) for ON signal or OFF
Fig. 4 Cyclic voltammograms of Py6FPIs cast films on ITO-coated
glass slide in CH3CN.
signal without interference. During the continuous positive
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Fig. 5 Current–voltage (I–V) characteristics of the ITO/Py6FPI/Al devices using (a) PyCH36FPI, (b) PyCF36FPI or (c) PyCPh36FPI as active layer
under an initially positive applied voltage. The direction and sequence of each sweep are marked out by arrow and number, respectively.
Negative sweep (the 3rd and 4th sweeps) and positive sweep cycles (the 5th and 6th sweeps) were conducted sequentially after the power had
been turned off for about 3 min.
were observed. The I–V characteristics demonstrate that the ITO/
Py6FPI/Al devices exhibited nonvolatile WORM type memory effect.
To make further investigations on the performance of the
WORM type memory effects, the ON/OFF current ratio, the
retention time of the device and the read cycles were evaluated.
Fig. 6 shows that the pyrrole-containing polyimide devices had
varied ON/OFF current ratios from 103 to 5 ꢃ 105 depending on
the pendent groups.
The relatively low ON/OFF current ratio for PyCPh36FPI
probably results from the stronger p–p interaction between
molecular chains, considering three crowded benzene rings in
the triphenylmethyl group. Yet, they all exhibited high stabili-
ties during the measurement of the retention times for the ON
and OFF states, as shown in Fig. 7. By applying a constant
reading voltage of 1.0 V for the ON state and the OFF state, no
obvious degradation in current value was observed for an hour
or even longer. In addition, both the ON state and the OFF state
retained stable at read pulses of 1.0 V (each pulse lasts 0.5 ms)
without degradation (Fig. 8). These results suggest that the
memory devices possessed good reproducibility and stability.
Fig. 6 The ON/OFF current ratios of the ITO/Py6FPI/Al devices.
sweeping to 5.0 V, the device remained in the ON state with an ON/
OFF ratio over 105. This electronic transition from the OFF state to
the ON state corresponds to the “writing” process in a digital
memory cell. The device kept in ON state during the subsequent
reverse sweep (the 2nd sweep) from 5.0 V to 0.0 V. Even aer the
device was power-off for about 3 min, the ON state could be
preserved in the following negative sweeps from 0.0 to ꢁ5.0 V (the
3rd sweep) and ꢁ5.0 to 0.0 V (the 4th sweep), as well as the positive
sweeps from 0.0 to 5.0 V (the 5th sweep) and 5.0 to 0.0 V (the 6th
sweep), without relaxation to the OFF state (dened as the “erasing”
process). For the other two memory devices applying PyCF36FPI
and PyCPh36FPI as the active layer, similar switching properties
Switching mechanism
To better illustrate the transition process from the ON state to
the OFF state, the I–V curves in both states are analyzed in detail
using theoretical models. Under a low positive voltage (e.g., 0.5–
Fig. 7 The retention times for the ON and OFF states of the ITO/Py6FPI/Al devices using (a) PyCH36FPI, (b) PyCF36FPI or (c) PyCPh36FPI as active
layer under a constant bias of 1.0 V.
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Fig. 8 The effect of read pulses applying 1.0 V on the ON and OFF states of the ITO/Py6FPI/Al devices. (a) PyCH36FPI, (b) PyCF36FPI and (c)
PyCPh36FPI. The inset shows the pulses used in the measurement.
2.5 V, Fig. 9a), the I–V curves for PyCH36FPI of the ON state where A is the effective Richardson constant, fB is the barrier
follows the ohmic conduction model14 described by eqn (1).
height, 3i is the dynamic permittivity of the insulator, and d is
the lm thickness. It suggests that a Schottky barrier forms at
the ITO interface due to the work function differences between
ITO and the HOMO of PyCH36FPI. When the applied electric
eld reaches the threshold voltage, the electric eld-induced
charge transfer occurs. Electrons transit from HOMO to the
LUMOs of the PyCH36FPI, leaving holes that spread over the
pyrrole moieties. These holes give rise to an open channel for
charge carriers to migrate through. Therefore, abrupt increase
of the current in the device could be observed, indicating the
formation of the high conductivity state. Such I–V relationship
could also be found for PyCF36FPI and PyCPh36FPI in both ON
state and OFF state, as depicted in Fig. S7.† Thus, similar
electric eld-induced charge transfer mechanism in the
memory devices could be deduced.
J ¼ qnmV/d
(1)
where q is the electronic charge, n is the carrier density, m is the
carrier mobility, and d is the lm thickness. Note that the
relationship between current density (J) and current (I) is
a linear correlation. While in the OFF state, the I–V curves
(Fig. 9b) can be tted by the Schottky emission model15
described by eqn (2).
ꢀ
ꢁ
2
3
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢁq fB ꢁ qV=4p3id
kT
2
4
5
J ¼ AT exp
(2)
This may help better elucidate the variations in switching
threshold voltages for Py6FPIs with deferent side groups. As
mentioned above, the strongly electron-withdrawing tri-
uoromethyl groups signicantly lower the HOMO value, and
increase the energy barrier between the ITO electrode and the
HOMO of PyCF36FPI. Thus, under an insufficient applied
voltage, electrons are difficult to transit to the HOMO of
PyCF36FPI. While, a lower applied voltage can let the electrons
hop up to the HOMO of PyCH36FPI and PyCPh36FPI. As
a matter of fact, the order of barrier values relative to ITO (work
function – 4.7 eV), PyCH36FPI (0.62 eV) < PyCPh36FPI (0.69 eV) <
PyCF36FPI (0.72 eV), matched the order of switching threshold
voltages PyCH36FPI (4.15 V) < PyCPh36FPI (4.20 V) < PyCF36FPI
(7.55 V).
On the other hand, the nonvolatile WORM type memory
effects could be explained by the electric eld induced
“conformation-coupled charge transfer”.11a When the exited
states are formed under an electric eld, the conformation-
coupled charge transfer process may occur, leading to
a twisted molecular chain. Thus, a large potential energy barrier
for the dissociation of CT complexes arises due to the steric
hindrance for the return of the electrons, which eventually
stabilized the conductive CT state and retained the high
conductivity states. They offer continuous open channels for
charge carriers to move in both directions. Under these
circumstances, no matter which direction of external electric
eld is applied, the memory device will maintain at the ON state
Fig. 9 Experimental (dots) and fitted (solid line, for Schottky emission
model) I–V curves for the (a) ON state and (b) OFF state of the ITO/
PyCH36FPI/Al devices.
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In order to better understand the electrical switching
behaviors of the memory devices, molecular simulations were
used to demonstrate the electronic properties. The calculations
of the frontier orbitals and the dipole moment of the basic unit
(BU) were carried out by the DFT method at the B3LYP/6-31(d)
level with Gaussian 09 program package. Since the tri-
uoromethyl groups probably do not signicantly affect the
electronic properties of the BU,17 they were excluded in the
calculation of the mechanism clarication. In fact, we calcu-
lated the BU of PyCH36FPI with and without the triuoromethyl
groups at the same level, as can be seen in Fig. S8.† Without
disturbing the distribution, a slight difference could be found
for the energy levels of the molecular orbitals distributed on the
corresponding diamine moieties. For instance, the LUMO+2
and HOMO energy levels of the BU without triuoromethyl
groups were just about the LUMO+4 and HOMO energy levels of
the BU with triuoro methyl groups, respectively. Therefore, the
results of the simplied but reasonable molecular simulation
for the BU of PyCH36FPI are displayed in Fig. 10. The HOMO is
predominantly distributed on the pyrrole moiety, while the
LUMO is located on the phthalimide moiety, suggesting that the
pyrrole moieties serve as electron donor and the phthalimide
moieties serve as electron acceptor in PyCH36FPI. The higher
frontier orbitals, LUMO+1 and LUMO+2, are distributed over
the BU.
Fig. 10 The calculated frontier molecular orbitals of the basic unit of
PyCH36FPI (left) and the plausible electronic transitions induced by an
electric field (right).
instead of returning to the low conductivity OFF state, exhibit-
ing nonvolatile WORM type memory behaviors.
In this view, the more stable the CT complexes are, the more
possibility for the polyimide to possess nonvolatile WORM
properties. Under an applied eld, the dipole moment of
exciton changes and leads to a splitting of the original exciton
state.16 The evaluation of the dipole moment helps to under-
stand the memory characteristics. The theoretical dipole
moments for the BU of PyCH36FPI, PyCF36FPI and PyCPh36FPI
are 3.70, 2.32 and 3.82 debye, respectively. Herein, the reported
triphenylamine-containing TP6F-PI,8g whose BU dipole moment
was calculated as 2.07 debye in the study, is taken for compar-
ison. Since TP6F-PI possesses a lower dipole moment which
leads to a more unstable CT complex, it may have less possi-
bility for a WORM type memory than the reported polyimides
herein. In fact, the device based on TP6F-PI exhibited volatile
DRAM behavior, while our devices all exhibited nonvolatile
WORM behavior. Therefore, a WORM type memory can be
gained by introducing more nucleophilicity groups such as
pyrrole to the donor moieties of a polyimide chain.
The plausible electronic transitions induced by an electric
eld are presented in Fig. 10 as well, with the aid of further
theoretical calculations for the oscillator strengths and transi-
tion contributions by the time-dependent DFT (TD-DFT)
method (Table 3). As the HOMO
/ LUMO transition
possesses an oscillator strength of 0.0007 which is quite low,
electrons accumulated at HOMO are more inclined to hop up to
LUMO+1 and LUMO+2, because these transitions possess much
larger value of oscillator strength. When sufficient energy is
conducted, electrons at the HOMO of PyCH36FPI accumulate
and shi to LUMO+1 or/and LUMO+2 to result in an excited
state. As LUMO+1 and LUMO+2 delocalize over the BU, the
electrons at the excited states are capable of relaxing easily to
Table 3 Oscillator strengths and the assignment of S0 to Si transitions in the BU of Py6FPIsa
Polyimide
PyCH36FPI
Excited state
Oscillator strength
Orbitals
Contribution
1
2
7
0.0007
0.3321
0.3607
HOMO / LUMO
0.99
0.99
0.85
0.14
0.99
0.78
0.21
0.05
0.93
0.99
0.99
0.19
0.75
HOMO / LUMO+1
HOMO / LUMO+2
HOMO / LUMO+4
HOMO / LUMO
HOMO / LUMO+1
HOMO / LUMO+2
HOMO / LUMO+2
HOMO / LUMO+4
HOMO / LUMO
PyCF36FPI
1
2
0.0007
0.3565
9
0.6259
PyCPh36FPI
1
3
14
0.0007
0.2730
0.5544
HOMO / LUMO+1
HOMO / LUMO+2
HOMO / LUMO+3
a
Prohibited transitions are not listed.
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the LUMO which locates at a higher electron affinity phthali- the diamine moiety, while the LUMO+2 was widespread on the
mide moiety, via internal transition to from a CT state. Besides, pendent group, resulting in a higher contribution to the
the formation of the CT state may also derive directly from the excited state formation of the HOMO to LUMO+4 transition.
HOMO to the LUMO, but it's very unlikely due to the mentioned Taking on the into account the energy barriers (Fig. 11) in
low oscillator strength.
PyCF36FPI of the main transition with large oscillator
These processes are consistent with the absorption spectrum strength, 3.9 eV for HOMO to LUMO+1, and 4.63 eV for HOMO
of PyCH36FPI in solution. The onset absorption wavelength at to LUMO+4, the higher threshold voltage for PyCF36FPI-
374 nm (3.31 eV) arises from the HOMO / LUMO transition containing device could be well explained, since PyCH36FPI
(2.78 eV), and the two primary absorption peaks at 329 nm (3.77 possessed lower transition barriers, i.e. 3.76 eV for HOMO to
eV) and 307 (4.04 eV) nm correspond respectively to the HOMO LUMO+1, and 4.43 eV for HOMO to LUMO+2. Likewise, for
/ LUMO+1 transition (3.76 eV) and the HOMO / LUMO+2 PyCPh36FPI system, the barriers were 3.72 eV for HOMO to
transition (4.43 eV). These results are in absolute accordance LUMO+1, and 4.60 eV for HOMO to LUMO+3. On the other
with the conclusions in the optical properties section, where the hand, the lowest ranking for the threshold voltage of
absorption tail is assigned to the CT transition and the PyCPh36FPI could be chalked up to the comprehensive effects
absorption peak at 307 nm is assigned to the local transition of of the conjugated donor properties as well as the steric
the pyrrole moieties.
hindrance effect.
We subsequently found that the PyCF36FPI and PyCPh36-
FPI systems underwent similar electronic transition processes
with slight differences. As evidenced by the calculated oscil-
Conclusions
lator strength and transition assignment in Table 3, for Three novel polyimides Py6FPIs containing pyrrole moiety with
PyCF36FPI system, the accumulated electrons were more different pendent groups were synthesized. The yellowish PI
inclined to be excited to LUMO+4 rather than LUMO+2 under lms were tough and exible with highly thermal stability. The
sufficient energy. As the molecular distributions of PyCF36FPI simple memory devices with the conguration of ITO/Py6FPIs/
depicted in Fig. S9,† the LUMO+4 was mainly distributed on Al were fabricated by spin-coating. Nonvolatile bistable
WORM characteristics were observed under an applied electric
eld to the devices. They showed highly stable performances
with different turn-on threshold voltages and ON/OFF current
ratios depending on the pendent groups. It appeared that the
threshold voltages were tuneable trough modulating the elec-
tron affinity of the pendent group for these pyrrole-containing
polyimides. In addition, electronic absorption spectra and
molecular simulations were used to help clarify the CT
processes and understand the electrical switching behaviors.
These results suggest that the memory behaviors of the PI-based
devices can be signicantly affected and adjusted by the
nucleophilicity of the affiliated pendent groups of the PI chain
structure. The newly synthesized polyimides herein promise the
potential applications for the future alternative memory
technologies.
Experimental
Materials
Fig. 11 The energy diagram of the basic units for Py6FPIs.
Triuoroacetic acid, p-toluidine, triphenylmethyl chloride, 2-
bromo-1-(4-nitrophenyl)ethan-1-one, 4-(triuoro-ethyl)niline,
and hydroxymethanesulnic acid monosodium salt dihydrate
(rongalite) were purchased from Aladdin and used as received.
4,40-(Hexauoroisopropylidene)diphthalic anhydride (6FDA)
were purchased from National Pharmaceutical Group Chemical
ꢀ
Reagent Co., Ltd. and heated at 150 C under vacuum for 12 h
before use. Tetrabutylammonium perchlorate (TBAP) and
ferrocene was obtained from Alfa Aesar and used as received.
N,N-Dimethylformamide (DMF) purchased from Aladdin was
dried over calcium hydride for 12 h and distilled under reduced
pressure. All other reagents were analytical grade and used as
received from commercial sources, unless otherwise
mentioned.
Fig. 12 Schematic diagram of the memory device fabricated with
Py6FPI.
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to a previously reported procedure.12 Another intermediate 4-
tritylaniline was obtained by a one-step facile reaction between
aniline and triphenylmethyl chloride according to the ref. 13.
Instrumentation
1H NMR spectra of intermediates and monomers were recorded
on a Varian Mercury-plus 300 spectrometer, while 1H NMR
spectra of polymers and all 13C NMR spectra were recorded on
a Varian Unity Inova 500 NB spectrometer. All samples were
measured in a solution of deuterated dimethyl sulfoxide
(DMSO-d6) with tetramethylsilane (TMS) as an internal stan-
dard. Mass spectra were performed on a Thermo EI mass
spectrometer (DSQ II). Elemental analysis was run in a CHNS
elemental analyzer. Infrared spectra (IR) were analyzed by
a BRUKER TENSOR 27 Fourier-transform infrared (FT-IR)
spectrometer. The inherent viscosities (hinh) of polyimides
were obtained at a solid content of 0.5 wt% in N-methyl-2-
pyrrolidone (NMP) at 30 ꢀC on an Ostwald viscometer. Wide-
angle X-ray diffraction (WAXD) measurements were performed
on Rigaku SmartLab X-ray diffractometer at a scanning rate of
10ꢀ minꢁ1. Ultraviolet-visible (UV-vis) absorption spectra were
obtained on a Hitachi UV-Vis spectrophotometer (U-3900). For
the thin lm spectra, PIs were dissolved in NMP and ltered
through membrane microlters with a pore size of 0.22 mm,
spin-coated at a speed rate of 1500 to 2000 rpm for 10 s onto
quartz plates and baked at 150 ꢀC for 3 h under vacuum. The PI
solution spectra were measured at a concentration of approxi-
mate 2 ꢃ 10ꢁ5 M before the absorbance was normalized.
Thermogravimetric analyses (TGA) were carried out on a TA
thermal analyzer (Q50) under a nitrogen atmosphere at a heat-
ing rate of 20 ꢀC minꢁ1. Differential scanning calorimetry (DSC)
curves were obtained with a NETZSCH thermal analyzer (DSC
204) at a heating rate of 10 ꢀC minꢁ1 under a nitrogen ow. The
dynamic mechanical (DMA) spectra were determined by a TA
DMA 2980 analyzer in tensile mode at an amplitude of 20 mm,
a preload force of 0.01 N and a force track of 125% at a heating
rate of 5 ꢀC minꢁ1. Thermomechanical analysis (TMA) was
conducted with a TA TMA Q400 analyzer at a preload force of
Synthesis of 2,5-bis(4-nitrophenyl)-1-p-tolyl-1H-pyrrole
(PyCH3DN)
Monoamine p-toluidine (2.79 g, 26 mmol) was added to
a suspension of compound 2 (6.56 g, 20 mmol) in methanol (80
mL) and acetic acid (80 mL). The mixture was stirred at 90 ꢀC for
18 h under argon. Aer cooling to room temperature, the
resulting precipitate was ltered and washed by ethanol. The
crude product was subsequently puried by silica-gel column
chromatography using ethyl acetate/n-hexane (v/v ¼ 1/4) as an
eluent to afforded PyCH3DN as a light brown powder (6.60 g,
82.6%). 1H NMR (300 MHz, DMSO-d6, d, ppm): 8.06–8.03 (d,
4H), 7.29–7.26 (d, 4H), 7.23–7.20 (d, 2H), 7.10–7.07 (d, 2H), 6.82
(s, 2H), 2.34 (s, 3H). MS (EI, m/z): [M]+ calcd for C23H17N3O4:
399.12. Found: 399. IR (KBr, n, cmꢁ1): 1587 (C]N, stretching),
1520 (NO2, nas), 1333 (NO2, ns).
Synthesis of 2,5-bis(4-nitrophenyl)-1-(4-(triuoromethyl)
phenyl)-1H-pyrrole (PyCF3DN)
Monoamine 4-(triuoromethyl)aniline (4.19 g, 26 mmol) was
added to a suspension of compound 2 (6.56 g, 20 mmol) in
toluene (120 mL) and triuoroacetic acid (5.70 g, 50 mmol). The
mixture was stirred at 110 ꢀC for 24 h under argon. Aer cooling
to room temperature, the resulting precipitate was ltered and
washed by ethanol. The crude product was subsequently puri-
ed by silica-gel column chromatography using ethyl dichloro-
methane/n-hexane (v/v ¼ 1/2) as an eluent to afforded PyCF3DN
as a yellow powder (7.24 g, 79.9%). 1H NMR (300 MHz, DMSO-d6,
d, ppm): 8.10–8.07 (d, 4H), 7.79–7.76 (d, 2H), 7.43–7.40 (d, 2H),
7.29–7.23 (d, 4H), 6.85 (s, 2H). MS (EI, m/z): [M]+ calcd for
C23H14F3N3O4: 453.09. Found: 453. IR (KBr, n, cmꢁ1): 1589 (C]
N, stretching), 1522 (NO2, nas), 1331 (NO2, ns), 1167 (C–F, ns).
0.05 N and a heating rate of 10 C minꢁ1. Cyclic voltammetry
ꢀ
(CV) measurements were performed on a Bio-Logic VMP-300
multi-channel electrochemical workstation using a three-
electrode cell in 0.1 M solution of TBAP in acetonitrile
(CH3CN). The PI thin lm spin-coated onto an ITO glass
substrate was used as a working electrode and a platinum disk
was used as an auxiliary electrode. All cell potentials were taken
with the use of a Ag/AgCl, KCl (sat.) reference electrode. A
solution of ferrocene (5 mM) was used as an external reference
for calibration. All scans were run at a rate of 50 mV sꢁ1. Atomic
force micrographs (AFM) of the PI thin lms on the device
surface were obtained by a Bruker Multimode 8 microscope.
The thickness of the vacuum evaporated Al electrodes and the
spin-coated PI thin lms was measured with a Kosaka Labora-
tory Ltd. Surfcorder ET150 step proler.
Synthesis of 2,5-bis(4-nitrophenyl)-1-(4-tritylphenyl)-1H-
pyrrole (PyCPh3DN)
PyCPh3DN was synthesized by using 4-tritylaniline as mono-
amine in the similar procedure as PyCF3DN to give a yellowish
1
orange solid (9.95 g, 79.3%). H NMR (300 MHz, DMSO-d6, d,
ppm): 8.05–8.02 (d, 4H), 7.34–7.25 (m, 10H), 7.22–7.08 (m, 13H),
6.86 (s, 2H). MS (EI, m/z): [M]+ calcd for C41H29N3O4: 627.22.
Found: 627. IR (KBr, n, cmꢁ1): 1589 (C]N, stretching), 1514
(NO2, nas), 1333 (NO2, ns).
Synthesis of the diamines. The general procedure to
synthesis the monomer diamines is as follow: in a three-neck
ask, a mixture of dinitro compound (10 mmol), SnCl2$2H2O
(22.56 g, 100 mmol) and ethyl acetate (250 mL) was reuxed
with stirring for 4 h under argon. The resulting solution was
Synthesis of the monomers and polyimides
The general synthesis routes of the diamines and the corre- poured into saturated NaHCO3 solution (150 mL). The massive
sponding polyimides are outlined in Schemes 1 and 2, respec- pale white precipitate was ltered off, extracted with ethyl
tively. The intermediate 1,4-bis(4-nitrophenyl)butane-1,4-dione acetate for ve times, and dried. The crude products were
(2) was synthesized by the coupling reaction of 2-bromo-1-(4- puried by silica-gel column chromatography or recrystallized
nitrophenyl)ethan-1-one (1) with rongalite in DMF according from ethanol/isopropanol to give the pure diamines.
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4,40-(1-p-Tolyl-1H-pyrrole-2,5-diyl)dianiline (PyCH3DA)
asymmetrical stretching), 1717 (imide, C]O, symmetrical
stretching) and 1366 (C–N, stretching). Inherent viscosity, hinh
A yellow powder (2.78 g, 82.0%). 1H NMR (300 MHz, DMSO-d6, d,
ppm): 7.06–7.04 (d, 2H), 6.85–6.83 (d, 2H), 6.67–6.64 (d, 4H),
¼ 1.19 dL gꢁ1
.
6.34–6.31 (d, 4H), 6.10 (s, 2H), 4.96 (s, 4H, NH2), 2.27 (s, 3H). 13
C
NMR (125 MHz, DMSO-d6, d, ppm): 21.10, 107.84, 113.83,
121.42, 129.31, 129.52, 129.59, 135.78, 136.63, 137.24, 147.42.
MS (EI, m/z): [M]+ calcd for C23H21N3: 339.17. Found: 339. IR
(KBr, n, cmꢁ1): 3464, 3378 (N–H, n), 1620 (C]N, stretching),
1283 (C–N, stretching). Anal. calcd for C23H21N3: C, 81.38; H,
6.24; N, 12.38. Found: C, 80.94; H, 6.19; N, 12.30.
PyCF36FPI
1H NMR (500 MHz, DMSO-d6, d, ppm): 8.18–8.10 (d, 2H), 7.96–
7.88 (d, 2H), 7.80–7.74 (d, 2H), 7.73–7.68 (s, 2H), 7.48–7.41 (d,
2H), 7.35–7.28 (d, 4H), 7.24–7.11 (d, 4H), 6.69–6.57 (s, 2H). FT-IR
(n, cmꢁ1): 1784 (imide, C]O, asymmetrical stretching), 1721
(imide, C]O, symmetrical stretching) and 1364 (C–N, stretch-
ing). Inherent viscosity, hinh ¼ 0.94 dL gꢁ1
.
4,40-(1-(4-(Triuoromethyl)phenyl)-1H-pyrrole-2,5-diyl)
dianiline (PyCF3DA)
A light yellow crystalline solid (3.26 g, 75.3%). 1H NMR (300
MHz, DMSO-d6, d, ppm): 7.64–7.61 (d, 2H), 7.15–7.12 (d, 2H),
6.65–6.62 (d, 4H), 6.36–6.34 (d, 4H), 5.04 (s, 4H, NH2). 13C NMR
(125 MHz, DMSO-d6, d, ppm): 108.72, 113.90, 120.68, 126.08,
127.44, 127.69, 129.86, 130.25, 135.72, 143.32, 147.80. MS (EI,
m/z): [M]+ calcd for C23H18F3N3: 393.15. Found: 393. IR (KBr, n,
cmꢁ1): 3412, 3314 (N–H, v), 1620 (C]N, stretching), 1333 (C–F,
nas), 1178 (C–F, ns). Anal. calcd for C23H18F3N3: C, 70.22; H, 4.61;
N, 10.68. Found: C, 70.41; H, 4.58; N, 10.61.
PyCPh36FPI
1H NMR (500 MHz, DMSO-d6, d, ppm): 8.24–8.15 (d, 2H), 8.00–
7.92 (d, 2H), 7.79–7.71 (s, 2H), 7.54–6.86 (m, 27H), 6.68–6.54 (s,
2H). FT-IR (n, cmꢁ1): 1784 (imide, C]O, asymmetrical stretch-
ing), 1717 (imide, C]O, symmetrical stretching) and 1366 (C–
N, stretching). Inherent viscosity, hinh ¼ 1.06 dL gꢁ1
.
Fabrication and characterization of the memory devices
The guration of memory device fabricated with Py6FPI as
active layer is shown in Fig. 12. A glass substrate deposited with
ITO was precleaned by ultra-sonication with water, acetone, and
isopropanol each for 20 min. Homogenous Py6FPI solutions
were prepared in NMP and ltered via membrane microlters
with a pore size of 0.22 mm. The polymer lms were prepared
by spin-coated the solution onto the precleaned ITO glass
substrate at a speed rate of 1500 rpm for 12 s and baked at 150
ꢀC for 3 h under vacuum. The thicknesses of the smooth PI thin
lms were in the range of 65–70 nm with a root-mean-square
roughness (Rq) about 1.5 nm over an area of 1.0 ꢃ 1.0 mm2.
Finally, an aluminium top electrode with a thickness of 110 nm
was deposited onto the polyimide lm surface through
a shadow mask by thermal evaporation un-der vacuum. The
current–voltage (I–V) measurements of the memory device was
performed by a Keithley 4200-SCS semiconductor parameter
analyzer equipped with a Keithley 4205-PG2 arbitrary waveform
pulse generator.
4,40-(1-(4-Tritylphenyl)-1H-pyrrole-2,5-diyl)dianiline
(PyCPh3DA)
A light yellow crystalline solid (4.98 g, 87.7%). 1H NMR (300
MHz, DMSO-d6, d, ppm): 7.34–7.19 (m, 9H), 7.05–6.95 (m, 10H),
6.65–6.62 (d, 4H), 6.36–6.33 (d, 4H), 6.14 (s, 2H), 5.05 (s, 4H,
NH2). 13C NMR (125 MHz, DMSO-d6, d, ppm): 64.61, 107.67,
113.72, 121.10, 126.73, 128.21, 129.00, 129.39, 131.05, 131.41,
135.53, 137.96, 145.77, 146.48, 147.51. MS (EI, m/z): [M]+ calcd
for C41H33N3: 567.27. Found: 567. IR (KBr, n, cmꢁ1): 3454, 3379
(N–H, v), 1620 (C]N, stretching), 1290 (C–N, stretching). Anal.
calcd for C41H33N3: C, 86.74; H, 5.86; N, 7.40. Found: C, 86.43;
H, 5.86; N, 7.34.
Preparation of polyimides (Py6FPIs)
The synthesis of polyimide PyCH36FPI was used as an
example to illustrate the general synthetic procedure. To
a solution of PyCH3DA (0.3249 g, 0.9570 mmol) in 5 mL freshly
distilled anhydrous DMF, dianhydride 6FDA (0.4251 g, 0.9570
mmol) was added in one portion. The mixture was stirred at
room temperature under argon for 8 h to afford a viscous
poly(amic acid) solution. The poly(amic acid) was subse-
quently coated on a clean dry glass plate, followed by thermal
imidization in a vacuum oven at 80 ꢀC/1 h, 150 ꢀC/1 h,
230 ꢀC/1 h and 300 ꢀC/1 h. Aer cooling to room temperature,
the plate was soaked in warm water for a few minutes and the
polyimide lm (approximate 40 mm) was peeled off
automatically.
Molecular simulation
Molecular simulation was carried out with the Gaussian 09
program package. The molecular geometry, molecular orbitals
and dipole moment of the basic unit (BU) in the polyimide
molecular structure were calculated and optimized by means
of the density functional theory (DFT), using the Becke's three-
parameter hybrid density functional method in conjunction
with Lee–Yang–Parr's correction functional (B3LYP) method,
and the 6-31G(d) basic set. The time-dependent DFT (TD-DFT)
was additionally applied in the calculation of the oscillator
strengths and transition contributions. For all the simula-
PyCH36FPI
1H NMR (500 MHz, DMSO-d6, d, ppm): 8.23–8.08 (d, 2H), 7.96– tions, vibration frequencies were calculated analytically to
7.84 (d, 2H), 7.76–7.65 (s, 1H), 7.51–6.84 (m, 12H), 6.69–6.42 (s, ensure the minimum total energy of the optimized molecular
2H), 2.37–2.17 (s, 3H). FT-IR (n, cmꢁ1): 1782 (imide, C]O, geometry.
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(No. 2014CB643605), the National 863 Program of China (No.
2015AA033408), the National Natural Science Foundation of
China (No. 51373204, 51233008, and J1103305), the Science and
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