Nonvolatile write-once read-many-times memory behaviors of polyimides containing tetraphenyl fluorene core and the pendant triphenylamine or carbazole moieties

Article abstract of DOI:10.1002/pola.29039

To better understand the structure–property relationships, two novel aromatic diamines containing tetraphenyl fluorene (TPF) moiety through triphenylamine (TPA) unit and carbazole (Cz) unit modification are designed and synthesized, respectively. Four thermally stable and excellent solubility polyimides are prepared and characterized. The excellent film-formation ability and thin film stability are investigated by X-ray diffraction (XRD) and atom force microscopy (AFM) measurements, respectively. The memory devices are fabricated, PIs films with low water uptakes sandwiched between indium-tin oxide (ITO) ground electrode and Al top electrode, and exhibit nonvolatile write-once read-many-times (WORM) memory behaviors with low threshold voltages, due to increasing the retention time through regulating the energy level. The current conduction mechanisms of all devices are linearly fitted by theoretical conduction model. Molecular simulations are used to demonstrate switching mechanism and the memory effects. The experimental results provide a sight for the design-adjustable switching voltage of memory devices.

Full text of DOI:10.1002/pola.29039

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Nonvolatile Write-Once Read-Many-Times Memory Behaviors of
Polyimides Containing Tetraphenyl Fluorene Core and the Pendant
Triphenylamine or Carbazole Moieties
Yanhua Yang,1 Zijun Ding,2 Jingcheng Xia,3 Youxuan Zheng,3 Shijin Ding,2
1
Yingzhong Shen
1Applied Chemistry Department, School of Material Science & Engineering, Nanjing University of Aeronautics & Astronautics,
Nanjing 210016, People’s Republic of China
2State Key Laboratory of ASIC and System, College of Microelectronics, Fudan University, Shanghai 200433,
People’s of Republic of China
3State Key Laboratory of coordination chemistry, College of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, People’s of Republic of China
Correspondence to: Y. Shen (E-mail: yz_shen@nuaa.edu.cn)
Received 3 February 2018; accepted 18 April 2018; published online in Wiley Online Libraryz
DOI: 10.1002/pola.29039
ABSTRACT: To better understand the structure–property rela-
tionships, two novel aromatic diamines containing tetraphenyl
fluorene (TPF) moiety through triphenylamine (TPA) unit and
carbazole (Cz) unit modification are designed and synthesized,
respectively. Four thermally stable and excellent solubility pol-
yimides are prepared and characterized. The excellent film-
formation ability and thin film stability are investigated by X-
ray diffraction (XRD) and atom force microscopy (AFM) meas-
urements, respectively. The memory devices are fabricated, PIs
films with low water uptakes sandwiched between indium-tin
oxide (ITO) ground electrode and Al top electrode, and exhibit
nonvolatile write-once read-many-times (WORM) memory
behaviors with low threshold voltages, due to increasing the
retention time through regulating the energy level. The current
conduction mechanisms of all devices are linearly fitted by the-
oretical conduction model. Molecular simulations are used to
demonstrate switching mechanism and the memory effects.
The experimental results provide
a
sight for the design-
C
V
adjustable switching voltage of memory devices. 2018 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56,
1630–1644
KEYWORDS: charge transfer; polyimide; synthesis; WORM
memory
random access memory (SRAM)) and nonvolatile memory
properties (write-once read-many-times (WORM), rewritable
(FLASH)), have been widely observed.1,8,9 For the sake of
improving storage performance, a great deal research con-
centrates on the modification of PIs structure.
INTRODUCTION In recent years, polymer-based resistive
memory materials have attracted tremendous attention due
to several attractive advantages, for instance, good process-
ability and scalability, well-tailored structure, three-
dimensional multi-stacking capability, high mechanical flexi-
bility, and thermal stability.1–6 Among, aromatic polyimide
(PI) is one of the most promising candidate polymeric mate-
rials, and numerous researches have been focused on the
application of PI in memory field. Since the first time about
PI-based memory device was reported,7 it was found that
PIs with memory performance generally contained both
electron-donor (D) and -acceptor (A) moieties within a
repeating unit, which could be beneficial to charge transfer
(CT) under the external applied electric field. Through the
introduction different D–A moieties into PI system, various
kinds of memory behaviors including volatile memory prop-
erties (dynamic random access memory (DRAM), static
As was well known, the increased the highest occupied
molecular orbital (HOMO) energy level would decrease the
energy barrier between the indium tin oxide (ITO) electrode
and the HOMO energy level of polyimides for the fabricated
ITO/polyimide/Al memory device, because the great majority
of the reported polyimides were p-type semiconductor mate-
rials and holes could be injected from the ground ITO elec-
trode to the HOMO energy level when an applied electric
field started from the negative scan. Moreover, the reduced
energy barrier would decrease the threshold voltage of the
fabricated memory device, increase the retention time and
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 (a) Chemical structure of PI(TPA-TPF PMDA). (b) Molecular optimized geometry of its basic unit. [Color figure can be
viewed at wileyonlinelibrary.com]
diminish power consumption, which was very important for
microelectronic devices application. By introducing suitable
functional moiety into the PIs chain could tune the HOMO
energy level and the lowest unoccupied molecular orbital
(LUMO) energy level, this caused that the synthesized PIs
possessed better CT ability and improved memory effect.
Thus, the devices could exhibit nonvolatile memory behavior.
According to the literature report, fluorene-based polymers
were widely used in organic emitting diodes (OLEDs),10 elec-
trochromic devices,11 and organic memory devices,12 due to
fluorene bearing high carrier mobility property. Meanwhile,
PIs containing a tetraphenyl fluorene (TPF) moiety exhibited
excellent memory behavior, due to nonplanar structure of
TPF moiety and the effect on conjugation system.13–15 Tri-
phenylamine (TPA) unit and carbazole (Cz) unit were as
promising candidate for memory devices due to their rich
electronic properties, low ionization potentials, and high
electron affinity.16 And the nitrogen atom of TPA and Cz
units shown positive electricity when them formed the cat-
ion radical, which improving the ability of electron-donating.
TPA-based and Cz-Based PIs, bearing excellent memory per-
formance and tune memory behaviors by adjusting energy
level, also often were reported when they as electron donor
and hole transporting material.17–24 Compared to the
reported literature, PIs containing TPA-TPF and Cz-TPF was
rare reported. After TPA and Cz were introduced into the
TPF core, it not only could improve hole transporting ability,
but also reduce the hole injection energy barrier between
the ITO electrode and the HOMO energy level of PIs. This
could make the memory devices of PIs containing TPA-TPF
and Cz-TPF show excellent nonvolatile memory behavior and
low power consumption.
dihedral angles of PI(TPA-TPF PMDA) were 42.68, 68.08,
75.18, and 77.08, respectively (Other PIs shown in Supporting
Information, Fig. S1, ESI†). PIs with such twist structure
shown excellent solubility in common solvents and high
thermal stability. For example, the temperature of 10% loss
weight could as up as 500 8C and a carbon residual was
beyond 62% in 800 8C. The amorphous nature and excellent
film-formation ability was investigated by X-ray diffraction
(XRD) measurements. And atom force microscopy (AFM)
measurements exhibited that the fabricated PIs films pos-
sessed smooth surface. In addition, in this PIs system, fluo-
rene unit, TPA, and Cz moiety acted as electron donor and
hole transporting channel, the ability of electron-donating
was very strong. As expected, the memory device, containing
PIs as electroactive layer sandwiched between ITO anode
and Al cathode, shown nonvolatile WORM-type memory
behavior. The threshold voltage was an order change from
21.48 to 21.74 V, with high ON/OFF ratio up 105. It indi-
cated the memory devices had low power consumption and
low misreading. This work demonstrated that by introducing
multi-electron-donors in PIs system could improve the
HOMO energy level and achieve high memory performance.
EXPERIMENTAL
Materials
N,N-Dimethylacetamide were distilled under reduced pres-
sure over KOH. (Beta-4)-platinum and 2,7-dibromo-9-flure-
none were synthesized according to the literature.25,26
3,30,4,40-biphenyltetracarboxylic dianhydride (BPDA), pyro-
mellitic dianhydride (PMDA), 4-(9H-carbazolyl)phenyl
boronic acid, and 4-(diphenylamino)phenyl boronic acid
were used as received. All the other reagents and solvents
were analytical grade, and then were used without further
purification.
In this article, two conjugated TPA-TPF and Cz-TPF diamine
were designed and synthesized by Suzuki coupling reaction.
And four PIs were obtained successfully through chemical
imidization method, using TPA-TPF and Cz-TPF reaction with
3,30,4,40-biphenyltetracarboxylic dianhydride (BPDA) and
pyromellitic dianhydride (PMDA), respectively. Then, PI(TPA-
TPF PMDA), PI(TPA-TPF BPDA), PI(Cz-TPF PMDA), and
PI(TPA-TPF BPDA) were characterized. Because tetraphenyl
fluorene (TPF) core was comprised of C9 atom with sp3
hybridized orbital on the fluorene unit by binding covalently
with two phenylamine ring, the synthesized PIs formed a
totally nonplanar structure. As shown in Figure 1, the
Measurements
The NMR spectra were recorded on a Bruker Advance 400
spectrometer at resonant frequency of 400 MHz for 1H and
101 MHz for 13C nuclei using CDCl3 or DMSO-d6 as the sol-
vent. Fourier transform infrared (FT-IR) spectra of intermedi-
ates and monomer were obtained from KBr pellets with a
Nicolet NEXUS-470 FTIR spectrometer. Elemental analysis
was performed on a Perkine-Elmer 240 C elemental analyzer.
The weight average molecular weight (Mw) and the number
average molecular weight (Mn) were determined by gel
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permeation chromatography (GPC) utilizing a Water 1515
pump and a differential refractometer. Tetrahydrofuran (THF,
1 mLꢀmin21) was used as the mobile phase and monodis-
persed polystyrene was used as the molecular weight stan-
dard. UV–vis absorption spectra were tested by
dichloromethane solution dissolving the synthesized polyi-
mides on a UV-2550 spectrophotometer. TGA was carried out
on a TGA 2050 (TA Instruments) under a nitrogen atmo-
sphere at a heating rate of 10 8Cꢀmin21 up to 800 8C. Melting
points were measured by WBR-1B melting point digital
apparatus. The solubility of polymers was determined by
immersing 10 mg polyimide in 1 mL various solvents at
25 8C or heating to 80 8C. Cyclic voltammogram (CV) was
recorded on a CHI 660b electrochemical workstation using a
three-electrode cell under a nitrogen atmosphere. Indium tin
oxide (ITO) electrode was coated PIs as working electrode,
and platinum plate electrode and Ag/AgCl electrode
employed as the counter electrode and the reference elec-
trode, respectively. A 0.1 M tetra-n-butylammonium perchlo-
rate (n-Bu4NClO4) in anhydrous acetonitrile was employed
also be rationally well predicted according to the LOI value
by Johnson equation:31
DHcomb:ðkJ=gÞ 5 8000=LOI
(4)
Monomer Synthesis
Synthesis of 909-Bis(4-Aminophenyl)-2,7-Dibromofluorene
(Br-TPF)
Br-TPF was prepared from 2,7-dibromo-9-flurenone (Br-Flu-
orenone) (5.1 g, 15 mmol), aniline hydrochloride (6.0 g, 45
mmol), and aniline (15 mL, 164.6 mmol) using a literature
procedure.32,33 The offwhite produce (4.6 g, 61%) was
obtained by recrystallization using EtOH. mp 250 8C. 1H
NMR (400 MHz, DMSO-d6, d, ppm): 7.86 (d, 2H, ArH), 7.53
(dd, 2H, ArH), 7.42 (d, 2H, ArH), 6.71 (d, 4H, ArH), 6.42 (d,
4H, ArH), 5.03 (s, 4H, -NH2). 13C NMR (400 MHz, DMSO-d6,
d, ppm): 155.06 (2 C-NH2), 149.03 (2 ArC), 137.96 (2 ArC),
131.66 (2 ArC), 130.75 (2 ArC), 129.28 (4 ArC), 128.59 (2
ArC), 123.08 (2 ArC), 116.16 (2 ArC), 114.39 (4 ArC), 64.49
(Ar-C-Ar).
as the supporting electrolyte at a scan rate of 0.05 mVꢀs21
.
Synthesis of 4,40-(9,9-Bis(4-Aminophenyl)-9H-Fluorene-2,7-
Diyl)-Bis(N,N-Diphenylaniline) (TPA-TPF)
The electrical measurement of the polymer sandwich mem-
ory device was performed at ambient conditions and without
the other encapsulation, using a Keithley 4200-SCS semicon-
ductor parameter analyzer. The compliance current was set
to 0.1 A to avoid breakdown of the device. Atomic force
microscopy (AFM) measurements were performed using a
Bruker Dimension AFM instrument. The cross-section scan-
ning electron microscopy (SEM) experiments were per-
A solution of Br-TPF (0.3 g, 0.59 mmol), 4-(diphenylamino)-
phenyl boronic acid (0.423 g, 1.42 mmol), Pd(PPh3)4
(0.0136 g, 0.0118 mmol), and K2CO3 (0.326 g, 2.36 mmol) in
toluene (16 mL) and water (8 mL) was heated at 110 8C
under a nitrogen atmosphere for 6 h. When the reaction was
completed, the reaction mixture was cooled to room temper-
ature and filtered. The crude product was extracted by tolu-
ene. After removing the solvent, the crude material was
purified by chromatography (ethyl acetate/n-hexane, 1/2,
v/v) to obtain a white powder (0.37 g, 75.5%). mp 313 8C.
FT-IR (KBr, thin film, t, cm21): 3427 (ANH2 stretching),
2925 (CAH stretching), 1618 (CAN bonding), 1103 (CAN
stretching). 1H NMR (400 MHz, DMSO-d6, d, ppm): 7.92 (d,
2H, ArH), 7.63 (d, 2H, ArH), 7.53 (t, 6H, ArH), 7.30 (t, 8H,
ArH), 7.05 (d, 16H, ArH), 6.85 (d, 4H, ArH), 6.42 (d, 4H,
ArH), 4.94 (s, 4H, ANH2). 13C NMR (101 MHz, DMSO-d6, d,
ppm): 153.19 (6 C-N), 147.69 (2 C-N), 147.03 (2 ArC),
144.84 (2 ArC), 139.84 (2 ArC), 138.54 (2 ArC), 136.14 (2
ArC), 135.45 (2 ArC), 129.25 (8 ArC), 129.21 (2 ArC), 127.76
(2 ArC), 125.80 (2 ArC), 124.34 (4 ArC), 124.25 (4 ArC),
123.96 (8 ArC), 122.86 (2 ArC), 120.23 (4 ArC), 114.93 (4
ArC), 64.32 (ArACAAr).
formed on
a SU8020 with field-emission-gun scanning
electron microscope at 3 kV. Water uptakes (WU) of the film
specimens were calculated using the following equation:27
ÂÀ
Á
Ã
WU ð%Þ 5
W
wet2 Wdry =Wdry 3 100%
(1)
where Wdry was the initial weight of the film after vacuum-
dried at 25 8C for 24 h. Wwet referred to the weight of film
samples after immersion in deionized water under the same
conditions. The water uptake of PIs was the mean data of
three parallel samples. And the weight of film specimens
was greater than 0.1 g to reduce the experimental errors.28
The X-ray diffraction (XRD) patterns of polyimides power
were taken from 58 to 908 (2h value) by Bruker AXS Panalyt-
ical using Cu radiation (45 kV, 200 mA, speed: 0.08). The
mean intermolecular distances (d-spacing) of polyimides
were determined based on the Bragg’s law:29
Synthesis of 4,40-(2,7-Bis(4-(9H-Carbazolyl) Phenyl)-9H-
Fluorene-9,9-Diyl)Dianiline (Cz-TPF)
nk 5 2d sinðhÞ
(2)
A solution of Br-TPF (0.3 g, 0.59 mmol), 4-(N-carbazolyl)-
phenyl boronic acid (0.41 g, 1.43 mmol), Pd(PPh3)4
(0.0136 g, 0.0118 mmol) and Na2CO3 (0.25 g, 2.36 mmol) in
toluene (16 mL) and water (8 mL) was heated at 110 8C
under a nitrogen atmosphere for 6 h. When the reaction was
completed, the reaction mixture was cooled to room temper-
ature and filtered. The crude product was extracted by tolu-
ene. After removing the solvent, the crude material was
purified by chromatography (ethyl acetate/n-hexane, 1/2, v/
v) to obtain a white powder (0.34 g, 69.4%). mp 271 8C. FT-
IR (KBr, thin film, t, cm21): 3432 (ANH2 stretching), 2925
where d was the d-spacing, k was the wavelength of radia-
tion (k 5 1.54 Å), h was the scattering, and n was an integer
number (1, 2, 3. . .). Limited oxygen index (LOI) could be cal-
culated using Van Krevelen and Hoftyzer equation:30
LOI ð%Þ 5 17:5 1 0:4 CR
(3)
where CR referred to char yield at 800 8C. When the LOI
value of polymer more than 28% was called self-
extinguishing material. Heat of combustion (DHcomb.) could
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(CAH stretching), 1610 (CAN bonding), 1058 (CAN stretch-
ing). 1H NMR (400 MHz, DMSO-d6, d, ppm): 8.26 (d, 4H,
ArH), 8.09 (d, 2H, ArH), 7.94 (d, 4H, ArH), 7.84 (d, 2H, ArH),
7.71 (t, 5H, ArH), 7.43 (q, 9H, ArH), 7.29 (t, 4H, ArH), 6.95
(d, 4H, ArH), 6.48 (d, 4H, ArH), 4.99 (s, 4H, ANH2). 13C NMR
(101 MHz, DMSO-d6, d, ppm): 154.27 (2 CANH2), 147.70 (2
ArC), 140.57 (4 CAN), 139.78 (2 ArC), 139.10 (2 ArC),
136.58 (2 ArC), 133.04 (2 C-N), 128.85 (2 ArC), 128.79 (4
ArC), 127.62 (4 ArC), 126.77 (4 ArC), 126.52 (4 ArC), 124.46
(2 ArC), 123.28 (4 ArC), 121.66 (2 ArC), 121.03 (4 ArC),
120.61 (4 ArC), 114.25 (4 ArC), 110.24 (4 ArC), 64.47
(ArACAAr).
FIGURE 2 (a) The structure of Al/polyimides/ITO sandwich
devices. (b) SEM image of a cross-section of PI (TPA-TPF
BPDA) film. [Color figure can be viewed at wileyonlinelibrary.
com]
Yield of PI(Cz-TPF BPDA) was 70%. The Mn and Mw values
were 26 800 and 41 600, respectively, with PDI at 1.55. FT-
IR (KBr, thin film, t, cm21): 1776 (C@O asymmetric stretch-
ing), 1716 (C@O asymmetric stretching), 1374 (CAN stretch-
ing), 788 (imide ring deformation). 1H NMR (400 MHz,
DMSO-d6, d, ppm): 8.39 (d, 2H), 8.05 (d, 20H), 7.73 (s, 4H),
7.57 (s, 4H), 7.47 (s, 10H), 7.30 (s, 4H). Anal. calcd. for
C77H44N4O4: C, 84.83; H, 4.16; N, 5.14; Found: C, 85.12; H,
4.34; N, 5.04.
Polymer Synthesis
The functional TPA-based and Cz-based PIs were synthesized
via a two-step procedure. Taking PI(TPA-TPF PMDA) as an
example, TPA-TPF (0.1 g, 0.120 mmol), PMDA (0.03 g, 0.138
mmol) and 3 mL of N,N-dimethylacetamide (DMAc) were
added in three-necked flask under a nitrogen atmosphere.
The reaction mixture was stirred at room temperature for
24 h to form polyamic acid (PAA). A mixture of acetic anhy-
dride (0.5 mL), pyridine (0.5 mL) and DMAC (2 mL) was
added to PAA solution and then heated to 100 8C for 8 h.
After cooling, the viscous polymer solution was poured into
methanol and collected by filtration. The resulting polyimide
was thoroughly washed with methanol and water, and then
dried at 100 8C for 24 h under vacuum.
Fabrication and Measurement of Memory Devices
The memory device was fabricated to the configuration of
Al/polymer/ITO, which ITO as the ground electrode and Al
as the top electrode during the slow voltage sweep, as
shown in Figure 2. Ahead of coating of the polymer film, the
ITO substrate was cleaned with deionized water, isopropanol
and acetone by ultrasonication for 30 min, respectively. Then
a DMAc solution with PI (20 mg/mL) was filtered through
polytetrafluoroethylene (PTFE) membrane syringe filters
with a pore size of 0.22 mm, and then was spin-coated onto
the precleaned ITO glass substrate at a low-speed rate of
500 rpm for 15 s then a high-speed rate of 3000 rpm for
60 s. And residual solvent was removed under a vacuum at
60 8C for 24 h.
Yield of PI(TPA-TPF PMDA) was 80%. The number average
molecular weight (Mn) and weight average molecular weight
(Mw) values tested by GPC were 31 000 and 46 700, respec-
tively, with the polydispersity indices (PDI 5 Mw/Mn) at 1.51.
FT-IR (KBr, thin film, t, cm21): 1776 (C@O asymmetric
stretching), 1715 (C@O asymmetric stretching), 1369 (CAN
stretching), 796 (imide ring deformation). 1H NMR (400
MHz, DMSO-d6, d, ppm): 8.32 (s, 2H), 8.06 (d, 4H), 7.82 (s,
2H), 7.76 (d, 2H), 7.63 (d, 4H), 7.55 (d, 2H), 7.43 (d, 6H),
7.29 (t, 8H), 7.04 (d, 14H). Anal. calcd. for C71H44N4O4: C,
83.76; H, 4.46; N, 6.29; Found: C, 74.48; H, 4.24; N, 7.18.
According to the previous experience.17,18 the thickness of
the obtained polymer films in the way was around 35 nm,
for example, PI (TPA-TPF BPDA) film was 33.0 nm from the
SEM image of a cross section, as shown in Figure 2(b), and
PI(TPA-TPF PMDA) film, PI(Cz-TPF PMDA) film and PI (Cz-
TPF BPDA) film were 34.7, 41.7, and 32.3 nm, respectively
(shown in Supporting Information, Fig. S9, ESI†). Finally, Al
top electrode (about 100 nm) with different areas of 0.5024,
0.1256, and 0.0314 mm2 was formed by thermal evaporation
onto the polymer surface through a shadow mask at a pres-
sure of about 1024 Torr with a uniform depositing rate of 1–
2 Å/s.
Yield of PI(TPA-TPF BPDA) was 84%. The Mn and Mw values
were 28 700 and 46 400, respectively, with PDI at 1.62. FT-
IR (KBr, thin film, t, cm21): 1780 (C@O asymmetric stretch-
ing), 1723 (C@O asymmetric stretching), 1370 (CAN stretch-
ing), 801 (imide ring deformation). 1H NMR (400 MHz,
DMSO-d6, d, ppm): 8.34 (d, 2H), 8.23 (d, 2H), 8.04 (t, 5H),
7.83 (s, 2H), 7.76 (d, 2H), 7.65 (d, 3H), 7.43 (t, 7H), 7.29 (d,
8H), 7.04 (d, 16H). Anal. calcd. for C77H48N4O4: C, 84.52; H,
4.51; N, 5.12; Found: C, 84.79; H, 4.14; N, 5.18.
Yield of PI(Cz-TPF PMDA) was 77%. The Mn and Mw values
were 25,200 and 42,700, respectively, with PDI at 1.69. FT-
IR (KBr, thin film, t, cm21): 1785 (C@O asymmetric stretch-
ing), 1716 (C@O asymmetric stretching), 1370 (CAN stretch-
ing), 796 (imide ring deformation). 1H NMR (400 MHz,
DMSO-d6, d, ppm): 8.44 (s, 2H), 8.19 (t, 6H), 8.06–7.90 (m,
8H), 7.65 (t, 6H), 7.56 (d, 2H), 7.46 (s, 11H), 7.29 (t, 5H).
Anal. calcd. for C71H40N4O4: C, 84.09; H, 4.08; N, 5.52; Found:
C, 83.45; H, 4.71; N, 5.10.
Molecular Simulations
Theoretical molecular simulations of the basic unit of the
synthesized polyimides were calculated through the Gaussian
03 program package.34 The density functional theory (DFT)
using the B3LYP functional with 6–31G(d) as the basic set
was exploited for the frontier orbitals, the geometry of elec-
tronic structures and the optimization of ground-state molec-
ular geometry.35
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SCHEME 1 Synthesis routes of TPA-TPF and Cz-TPF.
RESULTS AND DISCUSSION
in DMSO-d6, the proton from ANH2 group of TPA-TPF
appeared in the upfield of the spectrum (4.94 ppm), compar-
ing with appearing in the downfield of Cz-TPF (4.99 ppm).
The proton peaks of aromatic ring appeared at 6.42–7.92
and 6.48–8.28 ppm corresponding TPA-TPF and Cz-TPA,
respectively. For the 13C NMR spectra of TPA-TPF and Cz-TPF
(as shown in Supporting Information, Figs. S4 and S5), the
carbon peaks of aromatic ring appeared at 114.9–153.2 and
110.2–154.3 ppm, respectively. And the chemical shift of
64.3 and 64.5 ppm corresponded to the C9 carbon atom of
the fluorene, respectively. Besides, in FT-IR spectra of TPA-
TPF and Cz-TPF (as shown in Supporting Information, Fig.
S6, ESI†), the characteristic absorption peaks which origi-
nated from the ANH2 stretching vibration located at 3427
and 3432 cm21, respectively. The typical peak at 2925 cm21
was attributed to the CAH stretching, NAH bending vibra-
tion located at 1618 and 1610 cm21, and 1103 and
1058 cm21 were the peaks of the NAH stretching vibration,
respectively. All the spectroscopic data were in good agree-
ment with the expected structure.
Monomer Synthesis
The synthesized route of TPA-TPF and Cz-TPF were shown in
Scheme 1. First, 9-Flurenone underwent bromination reaction,
then the aniline was covalently bonded to the C9 carbon atom
of the fluorene. Finally, the TPA-TPF and Cz-TPF were obtained
by the Suzuki reaction with 4-(diphenylamino)phenyl boronic
acid and 4-(9H-carbazolyl)phenylboronic acid, respectively.
They contained nonplanar, large conjugated fluorene moieties
and excellent donor-electron units in good yield.
The Br-TPF was confirmed by 1H NMR and 13C NMR, as
shown in Supporting Information, Figures S2 and S3 (ESI†).
The TPA-TPF and Cz-TPF were confirmed by 1H NMR, 13C
1
NMR and FT-IR spectra. Figure 3 shown the H NMR spectra
of TPA-TPF and Cz-TPF. The chemical shifts of 4.94 ppm cor-
responded to ANH2 characteristics peak for TPA-TPF, how-
ever, it located at 4.99 ppm for Cz-TPF. It was probably that
TPA-TPF containing TPA moiety possessed stronger electron-
donating effect than Cz-TPF containing Cz moiety. As a result,
FIGURE 3 1H NMR spectra of TPA-TPF and Cz-TPF in DMSO-d6. [Color figure can be viewed at wileyonlinelibrary.com]
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SCHEME 2 Synthesis of TPA-based and Cz-based PIs. [Color figure can be viewed at wileyonlinelibrary.com]
Polyimides Synthesis
respectively. Then a mixture of acetic anhydride and pyridine
The functional TPA-based and Cz-based PIs were synthesized
effectively via the conventional chemical imidization reaction,
as shown in Scheme 2. The TPA-TPF and Cz-TPF were
reacted with PMDA and BPDA in DMAc at room temperature
for 24 h to form viscous polyamic acid (PAA) solution,
was added to PAA solution at 100 8C for 8 h.
The functional TPA-based and Cz-based PIs were confirmed
by elemental analysis, FT-IR and 1H NMR spectra. The 1H
NMR spectra of PI(TPA-TPF BPDA) and PI(Cz-TPF BPDA)
FIGURE 4 1H NMR spectra of PI(TPA-TPF BPDA) and PI(Cz-TPF BPDA) in DMSO-d6. [Color figure can be viewed at wileyonlineli-
brary.com]
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Solubility and Water Uptake
Table 1 listed the solubility of the synthesized PIs by dissolv-
ing 10 mg in 1 mL of solvent under stirring for 1 h. All the
polyimides were soluble in amide-type solvents, such as
NMP and DMAc, at room temperature. They also were solu-
ble completely or partially in DMF, DMSO, CH2Cl2 and tolu-
ene upon heating. The introduction of the aromatic pendant
groups (i.e., triphenylamine and phenyl carbazole) and non-
planar tetraphenyl fluorene (TPF) moieties resulted in twist
structure and increased intra-/intermolecular free volume,
this could be contributed to excellent solubility of the syn-
thesized PIs. It meets the requirements of the fabrication of
memory devices by spin-coating method.
Due to increasing free volume and the hydrophilic of the
imide ring C@O groups, some of the water molecules could
be entrapped in polymer system. The water uptakes of the
synthesized PIs films were in range of 0.72–0.81%, and it
was listed in Table 1. The water uptakes of films were much
lower than 2.9% of commercial Kapton-H film (PMDA/4,40-
ODA).28 The low water uptake might be ascribed to the fact
that the synthesized PIs contained the water proofing effect
of the large pendant moieties and the high content of phe-
nyl.27 The low water uptake could ensure excellent perfor-
mance of storage devices in microelectronic industry.
FIGURE 5 FT-IR spectra of TPA-based and Cz-based PIs. [Color
figure can be viewed at wileyonlinelibrary.com]
were shown in Figure 4, the amino and hydroxyl proton sig-
nal did not be observed in range of 4–5 and 10–11 ppm,
respectively, which indicated the successful polymerization
reaction between diamines and BPDA, and the complete
imidization reaction of PAA. The 1H NMR spectra of PMDA-
based polyimides were shown in Supporting Information,
Figures S7 and S8 (ESI†). Under the same conditions, the
proton peaks of aromatic ring of the TPA-based PIs appeared
in the upfield of the spectra (7.04–8.32 and 7.04–8.34 ppm),
comparing to the Cz-based PIs (7.29–8.44 and 7.30–8.39
ppm), due to TPA-TPF moieties bearing stronger electron-
donating effect. The FT-IR spectra of PIs were shown in Fig-
ure 5, the characteristics bands around 1780 and 1716 cm21
were attributed to the asymmetric and symmetric vibration
of the imide ring carbonyl groups, respectively, and
1370 cm21 was CAN stretching of imide ring, and 790 cm21
was confirmed the formation of imide ring structure. In gen-
eral, the results displayed that spectral data was approxi-
mately consistent with the target structure. According to Mn
data, the TPA-based PIs were composed of 30 and 26 repeat
units, respectively, and Cz-based PIs were 25. PDIs of PIs
were around 1.51–1.69 by GPC measurement, which were
well values for polycondensation. The high molecular
weights could afford good-quality thin films via spin-coating
for memory devices.36
Thermal Properties
The thermal stability properties of the synthesized PIs were
detected by TGA, as shown in Figure 6. The fluctuation could
be observed from TGA curves due to the degradation process
of various functional groups, the thermal degradation pro-
cess of the polymers backbone and the process of carboniza-
tion. As shown in Table 2, the 5% weight loss temperatures
(T5%) and the 10% weight loss temperatures (T10%) of the
synthesized polyimides were in range of 381–432 and 428–
506 8C, respectively. The concentration of carbonized residue
(CR) was more than 60% at 800 8C. The high CR could be
contributed to high aromatic content of PIs. The incorpora-
tion with twist TPF moiety and pendant unit could increase
the size and sharp anisotropy of polyimide and affect the
molecular packing, it could improve the processing charac-
teristics of relatively intractable polymers.37 According to the
experimental data and calculation, the limiting oxygen index
(LOI) were in range of 42–45%. Thus, the synthesized PIs
could be classified as self-extinguishing polymers. And the
values of heat of combustion (DHcomb.) were calculated in
range of 17.6–18.8 kJ/g. The excellent thermal properties
TABLE 1 Solubility Behavior and Water Uptake of the Synthesized PIs
Sample
WU (%)
DMAc
NMP
DMF
DMSO
CH2Cl2
Toluene
Acetone
PI(TPA-TPF PMDA)
PI(TPA-TPF BPDA)
PI(Cz-TPF PMDA)
PI(Cz-TPF BPDA)
0.72
0.78
0.77
0.81
111
111
111
111
111
111
111
111
11
1
1
2
2
1
2
1
1
1
2
2
2
2
111
11
11
1
111
1
111 5 soluble at room temperature; 11 5 soluble on heating; 1 5 partially soluble on heating; 2 5 insoluble even on heating.
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from 208 to 358 with d-spacing of 2.6–4.4 Å, indicating the
amorphous nature of films. The amorphous nature of the
synthesized PIs could be attributed to the introduction of
the bulky aromatic pendant moieties and TPF core with
totally nonplanar structure in the diamine units, resulting in
them loose chain packing and aggregation. This result is con-
sistent with their excellent organo-solubility. The amorphous
characteristic also made PIs possess excellent film-formation
ability; thus, the memory device could be fabricated through
convenient and conventional spin-coating process.38
The morphology of the active layer in the memory devices
played an important role for device performance, the surface
morphology and roughness of polymers films were charac-
terized through tapping-mode atomic force microscope
(AFM) measurements. The surface morphology images and
the corresponding typical cross-section profiles of AFM topo-
graphic image of polymer films on the ITO substrates were
shown in Supporting Information, Figure S10 (ESI†). The
root-mean-square (RMS) roughness was 2.81, 3.12, 3.43, and
3.27 nm for PI(TPA-TPF PMDA), PI(TPA-TPF BPDA), PI(Cz-
TPF PMDA), and PI(Cz-TPF BPDA), respectively. It indicated
that the fabricated PIs films through spin-coating process
possessed the relatively smooth surface, which facilitated the
hole injection process from ITO electrode to polyimides
active layer under applied electric field. Meanwhile, these
smooth surfaces could suppress the permeation of Al atom
into the polyimides layer during the vacuum deposition of
the Al electrode, which could prevent Al atom from disturb-
ing subsequently the memory behavior test.39 It suggested
that the fabricated PIs films possessed excellent thin film
stability.
FIGURE 6 TGA curves of the synthesized PIs under a nitrogen
atmosphere at a heating rate of 10.0 8Cꢀmin21. [Color figure
can be viewed at wileyonlinelibrary.com]
indicated that the synthesized PIs met to be used in memory
devices of microelectronic industry.
X-Ray Diffraction (XRD) and Atomic Force Microscope
(AFM) Measurements of the Polymers
The crystallinity of the polymers was analyzed by wide-angle
X-ray diffraction (XRD). As shown in Figure 7(a), the polyi-
mides powders exhibited broad without obvious peak fea-
tures in the 2h ranges from 188 to 358 with d-spacing of
2.6–4.9 Å, which indicated that they were all amorphous.
Meanwhile, all the PI films on the ITO glass substrate were
also broad without obvious peak features in the 2h ranges
TABLE 2 Thermal Properties of the Synthesized PIs
Sample
T5% (8C)
T10% (8C)
CR (%)
LOI (%)
DHcomb (kJ/g)
PI(TPA-TPF PMDA)
PI(TPA-TPF BPDA)
PI(Cz-TPF PMDA)
PI(Cz-TPF BPDA)
381
418
404
432
428
486
482
506
62.5
68.0
69.7
65.7
42.5
44.7
45.4
43.8
18.8
17.9
17.6
18.3
FIGURE 7 X-ray diffractograms of the (a) polyimides powders and (b) films. [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 8 (a) UV–vis absorption spectra of the synthesized PIs in CH2Cl2 solution. (b) Cyclic voltammetry sweeps of the synthe-
sized PIs measured in 0.1 M n-Bu4NaClO4/CH3CN (scan rate: 0.05 V s21). [Color figure can be viewed at wileyonlinelibrary.com]
Optical and Electrochemical Properties
than Cz-based PIs. And the energy band gaps were esti-
mated to 2.94, 2.95, 3.16, and 3.46 eV, respectively, as shown
in Table 3. The gradually increased energy band gap from
PI(TPA-TPF PMDA) to PI(Cz-TPF BPDA) was another evidence
that TPA-based PIs possessed better charge transfer effect
than Cz-based PIs. The above results show consistent with
our design expectation.
The UV–vis absorption spectra in CH2Cl2 solution about
1026 molꢀL21 were tested to characterize the optical prop-
erties of the synthesized PIs. As seen from the Figure 8(a),
the absorption bands of TPA-based PI originating from the
p–p* transition were wider than that of Cz-based PIs, and it
exhibited evidently bathochromic shift. This dominated that
TPA units possessed stronger donate-electron ability. And it
made the electron delocalization in the whole PI-system
and reduced p* orbital energy, which was conducive to
intra-/intermolecular charge transfer, thus, the UV–vis spec-
tra move to the low-energy absorption bands. The absorp-
tion maximum hypsochromic shifted from 391 to 385 nm
corresponding to PI(TPA-TPF PMDA) and PI(TPA-TPF BPDA)
for TPA-based PIs, and from 346 to 341 nm corresponding
to PI(Cz-TPF PMDA) and PI(Cz-TPF BPDA) for Cz-based PIs,
respectively. As the electron donor units was the same in
TPA-based PIs, the observed hypsochromic shift might be
attributed to the weaker intra-/intermolecular charge trans-
fer with decreasing electron-withdrawing ability from
PMDA units to BPDA units. It was the same for Cz-based
PIs. The onset edge wavelength (kedge) of PI(TPA-TPF
PMDA), PI(TPA-TPF BPDA), PI(Cz-TPF PMDA), and PI(Cz-TPF
BPDA) were extended to 422, 420, 393, and 387 nm,
respectively. The gradually decreased onset edge wave-
length from PI(TPA-TPF PMDA) to PI(Cz-TPF BPDA) also
suggested that TPA-based PIs had easier electron transition
The electrochemical property of the synthesized four PIs
was investigated by cyclic voltammetry (CV) measurements.
Figure 8(b) shown a p-doping (anodic) process from 0 to
13 V for PIs. Nevertheless, it displayed different electro-
chemical behaviors for TPA- and Cz-based PIs. Cyclic voltam-
mograms of TPA-based PIs exhibited quasi-reversible
oxidation behaviors during the p-doping scan in the range of
0 to 13 V. Two moderate ionization (oxidation) peaks
appeared at 2.01 and 2.39 eV for PI(TPA-TPF PMDA) versus
Ag/AgCl electrode, and two weak ionization peaks appeared
at 1.82 and 2.25 eV for PI(TPA-TPF BPDA). When the TPA-
based PIs were scanned from 13 to 0 V, a weak reduction
peak was observed at 0.58 and 0.31 eV for PI(TPA-TPF
PMDA) and PI(TPA-TPF BPDA), respectively. However, the Cz-
based PIs exhibited irreversible oxidation behaviors during
the p-doping scan in the range of 0 to 13 V. Two ionization
peaks were observed at 1.05 (moderate) and 2.25 eV (weak)
for PI(Cz-TPF PMDA), and 1.41 (weak) and 2.24 eV (weak)
for PI(Cz-TPF BPDA), respectively. In contrast, no reduction
TABLE 3 Optical and Electrochemical Data of the Synthesized PIs
ox
onset
Sample
UV–vis (nm)
E
(eV) (from CV)
Eg (eV)a
HOMO (eV)b
LUMO (eV)c
kmax
kedge
PI(TPA-TPF PMDA)
PI(TPA-TPF BPDA)
PI(Cz-TPF PMDA)
PI(Cz-TPF BPDA)
391
385
346
341
422
420
393
387
0.67
0.34
0.14
0.38
2.94
2.95
3.16
3.20
25.09
24.76
24.56
24.80
22.15
21.84
21.40
21.60
a
Estimated from the UV–vis absorption edge wavelength (kedge) using
vacuum level): HOMO 5 2[Eoonxset1 4.8 2 EFerrocene], where EFerrocene was
determined to be 0.38 V versus Ag/AgCl.
the Plank equation: Eg 5 1240/kedge
.
b
c
The HOMO energy level was determined from CV onset oxidation
potential using ferrocene as the external reference (4.8 eV below the
Determined from the equation LUMO 5 Eg 1 HOMO.
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FIGURE 9 (a) I–V characteristics of ITO/PI(TPA-TPF PMDA)/Al device. (b) Retention time of ITO/PI(TPA-TPF PMDA)/Al device. (c) I–V
characteristics of ITO/PI(TPA-TPF BPDA)/Al device. (d) Retention time of ITO/PI(TPA-TPF BPDA)/Al device. [Color figure can be
viewed at wileyonlinelibrary.com]
peak was observed for the Cz-based PIs during scanning
backwards. Above results demonstrated that TPA units and
Cz units possessed excellent electron-donating property and
acted as hole-transporting moieties in the PI-system. Mean-
while, PIs containing TPA units might be have stronger
donate electrons ability than that of Cz units. The energy lev-
els of HOMO and LUMO of the synthesized PIs were calcu-
lated from the UV–vis and CV data, as shown in Table 3.
Taking PI(TPA-TPF PMDA) as an example, the energy barrier
to hole injection between the ITO electrode (24.80 eV) and
the HOMO energy level (25.09 eV) was estimated to be 0.29
eV, the energy barrier to electron injection between the Al
electrode (24.28 eV) and the LUMO energy level (22.15 eV)
was estimated to be 2.13 eV. It indicated that hole injection
from ITO into HOMO of PI(TPA-TPF PMDA) was easier than
electron injection from Al into LUMO. Therefore, the PI(TPA-
TPF PMDA) was a p-type material and holes predominate
the conduction process. It was the same for PI(TPA-TPF
BPDA), PI(Cz-TPF PMDA) and PI(Cz-TPF BPDA).
of ITO/PIs/Al sandwich devices. The I–V characteristics of
ITO/TPA-based PIs/Al devices were measured under an
external electric field, as shown in Figure 9. The device of
ITO/PI(TPA-TPF PMDA)/Al initially maintained in the low-
conductivity (OFF) state during the first sweep from 0 to 23.0
V with ITO as anode and Al as cathode. When the external
voltage reached at 21.54 V, a sharp switching happened with
current increasing from 1028 to 1023 A, indicating the transi-
tion from the OFF state to the high-conductivity (ON) state, as
shown in Figure 9(a). This electrical transition could be
defined as “write” process. After this transition, the sandwich
device remained in the ON state and could not be reset to the
initial OFF state during the subsequent negative and positive
scan process. It demonstrated that the ITO/PI(TPA-TPF
PMDA)/Al memory device possessed the nonvolatile WORM-
type memory behavior with an ON/OFF current ratio up to
104 [shown in Supporting Information, Fig. S11(a), ESI†]. The
I–V characteristics of the fabricated with PI(TPA-TPF BPDA)
was measured in the same conduction, as shown in Figure
9(c), it also exhibited the nonvolatile WORM-type memory
effect with a threshold voltage of 21.60 V and an ON/OFF cur-
rent ratio up to 104 [shown in Supporting Information, Fig.
S11(b), ESI†]. The retention time of the PI(TPA-TPF PMDA)
and PI(TPA-TPF BPDA) were about 1100 s, respectively, as
shown in Figure 9(b,d).
Memory Performances of PIs-Based Memory Devices
Current–Voltage (I–V) Characteristics of the Memory
Devices
The memory effects of the synthesized PIs could be illus-
trated by measuring the current–voltage (I–V) characteristics
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FIGURE 10 (a) I–V characteristics of ITO/PI(Cz-TPF PMDA)/Al device. (b) Retention time of ITO/PI(Cz-TPF PMDA)/Al device. (c) I–V
characteristics of ITO/PI(Cz-TPF BPDA)/Al device. (d) Retention time of ITO/PI(Cz-TPF BPDA)/Al device. [Color figure can be viewed
at wileyonlinelibrary.com]
In the same conduction, the memory devices of the synthe-
sized Cz-based PIs were measured. As seen from Figure
10(a), the current of the PI(Cz-TPF PMDA) memory device
was initially in OFF state. When one cell of the device was
applied a voltage from 0 to 23 V, a sharp increase in current
was measured at threshold voltage of 21.72 V, with the cur-
rent switching from 1029 to 1024 A, indicating the device
transition from the OFF state to the ON state. Neither the
subsequent voltage sweeps or the reverse voltage sweeps,
the device could not return to original OFF state. Thus, the
ITO/PI(Cz-TPF PMDA)/Al device displayed the nonvolatile
WORM-type memory behavior with an ON/OFF current ratio
up to 105 [shown in Supporting Information, Fig. S11(c),
ESI†]. In addition, no degradation in current of the ON and
OFF states was observed in 1100 s under a constant stress
of 21 V, as shown in Figure 10(b). The memory device of
ITO/PI(Cz-TPF BPDA)/Al also exhibited nonvolatile WORM-
type memory behavior with a switching voltage of 21.74 V
and an ON/OFF current ratio of 5 3 102 [shown in Support-
ing Information, Fig. S11(d), ESI†], and retention times for
both ON and OFF states were for 1100 s, as shown in Figure
10(c,d), respectively.
based. It demonstrated that the devices of TPA-based PIs pos-
sessed lower energy consumption. And PIs containing TPA
moieties decreasing the energy band gap effect were better
than containing Cz moieties in this polymer system, it was in
accordance with the result of UV–vis absorption spectra, this
could be more beneficial to CT by microstructure.40 The devi-
ces of the fabricated PI(TPA-TPF PMDA), PI(TPA-TPF BPDA),
and PI(Cz-TPF PMDA) had a characteristics of low misreading
rate, due to their ON/OFF current ratios beyond 103.
Current Conduction Model of the Memory Devices
To further confirm the current conduction mechanisms of
the memory devices, the I–V characteristics of experimental
measures were fitted with different current conduction mod-
els. Figure 11 shows the double logarithmic plots and their
linear fitting of the I–V curves for set processes of WORM-
type memory behaviors. Figure 11(a) shown the I–V charac-
teristics of PI(TPA-TPF PMDA) memory device. It was found
that the OFF state was fitted to space-charge limited-current
(SCLC) conduction model, according to the following
equation:1
9EilV2
8d3
JSCLC
˚
(5)
Above results shown that the memory devices of the fabri-
cated by TPA-based and Cz-based PIs exhibited nonvolatile
WORM-type behaviors, and the switching threshold voltages
of the TPA-based device were lower slightly than that of Cz-
where J was the current density, V was the electric field, Ei
was the permittivity of the active layer, d was the film
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FIGURE 11 Linear fitting model and corresponding slopes of set process of the I–V curves for memory devices of (a) PI(TPA-TPF
PMDA), (b) PI(TPA-TPF BPDA), (c) PI(Cz-TPF PMDA) and (d) PI(Cz-TPF BPDA), respectively. [Color figure can be viewed at wileyonli-
nelibrary.com]
thickness, and l was the carrier mobility. In low-voltage por-
tion (from 0 to 20.26 V), the charge carriers generated grad-
ually at the interface between ITO electrode and the HOMO
energy level of PI(TPA-TPF PMDA), with a slope of 0.4. In
the second portion (from 20.28 to 20.64 V), the generated
charge carriers accumulated at the interface between ITO
and HOMO with a slope of 1.0, corresponding to the Ohmic
like conduction. The third portion (from 20.66 to 21.52 V)
was much steeper than the former portions, the generated
charge carriers were injected from ITO to HOMO with a
slope of 2.3, obeying the Child’s law. This result indicated
that the memory device underwent the restricted carrier
injection and transportation process at OFF state. When a
threshold voltage was reached, the concentration of carriers
increased enough to overcome the SCLC energy barrier, the
current density increased sharply to flow through polymer
layer. Then, the device switched to ON state, the current con-
duction was controlled by Ohmic conduction model, accord-
ing to the equation:1
indicating heterogeneous localized filament formation. There-
fore, the current conduction processes of the PI(TPA-TPF
PMDA) device were controlled by the SCLC conduction and
localized filament conduction. The current conduction proc-
essed of the PI(TPA-TPF BPDA), PI(Cz-TPF PMDA), and
PI(Cz-TPF BPDA) devices were dominant by the SCLC con-
duction and localized filament conduction, as shown in Fig-
ure 11(b–d), respectively. In Figure 11(d), the absolute value
of the slope of the fourth portion was 0.3, it was probably
attributed to larger energy band gap between the HOMO and
LUMO energy level than other PIs, according to the result of
UV–vis absorption spectra, it might cause that the repulsive
Coulomb interaction was generated between the numerous
charge carriers at the HOMO energy level before the charge
transfer.
Proposed Switching Mechanism Based on Quantum
Chemical Simulation
To better understand the charge transfer process and the
corresponding switching behaviors of the fabricated TPA-
based and Cz-based PIs memory devices, the electron cloud
density, molecular orbitals and energy levels of the synthe-
sized PIs were investigated by molecular simulations. To
model simplicity and practical availability for simulations,
the basic units (BUs) of PI(TPA-TPF PMDA), PI(TPA-TPF
JOhmic / qnlV=d
(6)
where n was the charge barrier density. A stable conduction
path was formed both the negative and positive voltage
sweeps. Meanwhile, it was found that the current levels of
ON state had nothing to do with the size of the device,
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FIGURE 12 Molecular simulation orbitals of the based units based on the B3LYP/6–31G(d) for PIs. [Color figure can be viewed at
wileyonlinelibrary.com]
BPDA), PI(Cz-TPF PMDA), and PI(Cz-TPF BPDA) were
selected as the model compounds.
solvent. In addition to LUMO, the two higher LUMOs (LUMO1
and LUMO2) were located on the dianhydride moieties, them
could serve as mediators to facilitate the intramolecular CT
process. Under excitation with sufficient energy, electrons in
the ground state could transit to the higher LUMO3 frontier
orbitals, because of overlapping of the HOMO with the
LUMO3. For PI(TPA-TPF PMDA), the energy barrier between
ITO electrode (24.80 eV) and HOMO energy level (24.812
eV) was lower than between Al electrode (24.28 eV) and
LUMO energy level (23.224 eV). Thus, the hole injection pro-
cess from ITO electrode was more convenient than the elec-
tron injection from Al electrode. And it was similarly for the
other polymers, as shown in Figure 13(a).
The simulation results for the BUs of PI(TPA-TPF PMDA),
PI(TPA-TPF BPDA), PI(Cz-TPF PMDA), and PI(Cz-TPF BPDA)
based on DFT/B3LYP/6–31G(d) using the Gaussian 09 pro-
gram shown in Figure 12. As observed, the HOMO energy
levels of the model compounds were highly conjugated and
spread over the TPF-fluorenyl and Cz-fluorenyl moieties,
which acted as the electron donors (D). However, the LUMO
was located on the PMDA and BPDA units, which served as
the electron acceptors (A). These features would facilitate
the charge transfer (CT) from D to A to form CT complexes
in PI system under an applied voltage. Taking the PI(TPA-
TPF PMDA) an example, the calculated HOMO energy of the
model compound was 24.812 eV, the LUMO energy was
23.224 eV and Eg was 1.588 eV. It was deviation with the
experimental results from the UV–vis absorption spectra and
CV measurements (HOMO: 25.09 eV, LUMO: 22.15 eV, Eg:
2.94 eV). This was caused that the theoretical calculation
only considered the basic unit of polyimide, experimental
measurement result was based on the entire polyimide mole-
cule and its test environment, containing the molecular inter-
action, the solvation effect between the film of polyimide and
When an applied voltage was applied to the memory device,
numerous holes were injected into the HOMO of PI(TPA-TPF
PMDA). When the threshold voltage was reached, some accu-
mulated enough energy holes at the HOMO could transit to
the LUMO3, forming a locally excited state. Under an applied
voltage, excitation of the TPA moieties could decrease the
ionization potential and promote intramolecular CT process.
Therefore, the locally excited state could easily transform to
the CT state by the relaxation of electrons from the LUMO3
to the LUMO through intermediate the LUMO2 and the
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FIGURE 13 (a) Energy level diagram for the ITO/PIs/Al sandwich devices. (b) The plausible electronic transition processes under an
electric filed. (c) Comparison of the UV–vis absorption spectrum of PI(TPA-TPF BPDA) thin film in the OFF and ON states. [Color
figure can be viewed at wileyonlinelibrary.com]
LUMO1, or from the intermediate LUMOs (LUMO2 and
LUMO1) to the LUMO, or directly from the HOMO to the
LUMO, as shown in Figure 13(b). These CT processes could
form a conductive CT complexes, generating a conduction
path for the charge carriers to migrate through. Therefore,
abrupt increase of the current was observed, the ITO/
PI(TPA-TPF PMDA)/Al memory device switched from OFF- to
ON-state. Therefore, holes were also excited to LUMO3.
Meanwhile, the memory devices of four PIs underwent simi-
lar electronic transition processes.
Meanwhile, the electric field induced “conformation-coupled
charge transfer” could be also used to explain the nonvolatile
WORM memory performance.41,42 Due to donor moieties
were covalently linked with acceptor moieties by single-
bonded, under an applied field, the formation the excited
state in the PI system could produce the conformation cou-
pled charge transfer process. Therefore, a large potential
energy barrier could arise in twisted molecular, and this
could produce a steric hindrance for the return of the elec-
trons and impede the conduction path dissociation. As a
result, the conduction path was stable and the device could
be maintained at ON state. An open channel for charge car-
riers existed continuously in both direction. Therefore, the
devices exhibited nonvolatile WORM-type memory behaviors.
With the further increase of bias, the injected holes could be
easily transported by the formed conduction path between
the ITO electrode and Al electrode. Therefore, the current
could flow through conduction path and the device could be
maintained at ON state. Due to the coplanar structure of the
synthesized PIs and the outstanding electron-donating prop-
erties of TPA and Cz units, which could increase the reten-
tion time. Therefore, the conductive CT complexes were
stable even if power down or reverse bias. When the con-
ductive CT complexes were formed under an applied electric
field, the formed conduction path could be kept to facilitate
charge carriers to move under both biases of the same mag-
nitude. The devices exhibited nonvolatile WORM memory
characteristics.
CONCLUSIONS
In summary, two novel aromatic diamines containing tetra-
phenyl fluorene (TPF) moiety, 4,40-(9,9-bis(4-aminophenyl)-
9H-fluorene-2,7-diyl)-bis(N,N-diphenylaniline) (TPA-TPF) and
4,40-(2,7-bis(4-(9H-carbazolyl) phenyl)-9H-fluorene-9,9-diyl)-
dianiline (Cz-TPF) were synthesized via a Suzuki coupling
reaction. And the new compounds were characterized by
spectroscopic means. The four functional polyimides, PI(TPA-
TPF PMDA), PI(TPA-TPF BPDA), PI(Cz-TPF PMDA), and
PI(TPA-TPF BPDA) were prepared by chemical imidization.
Then, them were characteristics by 1H NMR, FT-IR and ele-
mental analysis. The synthesized polyimides shown low
water uptake, excellent solubility and thermal stability. The
excellent film-formation ability was investigated by X-ray dif-
fraction (XRD) measurements. And atom force microscopy
(AFM) measurements exhibited that the fabricated PIs films
possessed excellent thin film stability. The memory devices
with a configuration of ITO/PIs/Al exhibited nonvolatile
WORM-type memory behaviors. The switching voltages were
21.54, 21.60, 21.72, and 21.74 V, the ON/OFF current
ratios were 104, 104, 105, and 5 3 102, respectively. The cur-
rent conduction processed of the memory devices were dom-
inant by the SCLC conduction and localized filament
conduction. In addition, molecular simulations were used to
explained the charge transfer processes and the electrical
The deduced memory mechanism could be supported by
UV–vis absorption spectra of the fabricated polymers films
with and without the applied electric field. A liquid Hg drop-
let acted as the top electrode in place of Al contact with ITO
substrate as the ground electrode. When an applied voltage
was applied to the memory devices, the Hg electrode was
removed and UV-vis absorption spectroscopy was measured.
PI(TPA-TPF BPDA) film was found to exhibit a slight increase
absorption intensity of the ON state relative to the pristine
state (OFF state), as shown in Figure 13(c) [other PIs shown
in Supporting Information, Fig. S12, ESI†]. The increase in
intensity of absorption band could be attributed to an
increase charge transition and subsequent CT induced by the
applied electric field. The increase of the conductive CT state
caused a switching from the OFF state to the ON state.
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2018, 56, 1630–1644
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21 K.-L. Wang, Y.-L. Liu, J.-W. Lee, K.-G. Neoh, E.-T. Kang,
Macromolecules 2010, 43, 7159.
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behaviors of the fabricated PI-based device could be affected
through electron-donating moieties adjusting the energy level
of polymers. The synthesized PIs had the potential application
for next generation high-density storage devices.
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