Volatile static random access memory behavior of an aromatic polyimide bearing carbazole-tethered triphenylamine moieties

Article abstract of DOI:10.1016/j.polymer.2013.12.046

A functional polyimide (6F/CzTPA PI), 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA)/4,4′-diamino-4″-N- carbazolyltriphenylamine (DACzTPA), was synthesized in our present work for electrical resistive memory device applications. Semiconductor

Full text of DOI:10.1016/j.polymer.2013.12.046

Polymer 55 (2014) 1150e1159
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Volatile static random access memory behavior of an aromatic
polyimide bearing carbazole-tethered triphenylamine moieties
*
Lei Shi, Guofeng Tian, Hebo Ye, Shengli Qi , Dezhen Wu
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
A functional polyimide (6F/CzTPA PI), 4,40-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)/ 4,40-
diamino-400-N-carbazolyltriphenylamine (DACzTPA), was synthesized in our present work for electrical
resistive memory device applications. Semiconductor parameter analysis on the polyimide memory
devices indicates that the synthesized polyimide possesses a volatile static random access memory
(SRAM) characteristic with an ON/OFF current ratio of about 105 at the threshold voltage of around 1.5 V
and ꢀ1.8 V. In addition, the device using the 6F/CzTPA PI as the active layer reveals excellent long-term
operation stability with the endurance of reading cycles up to 108 under a voltage pulse and retention
times for at least 8 h under constant voltage stress (ꢀ1 V). The charge transfer mechanisms and the roles
of the donor and acceptor components in the PI macromolecules associated with the electrical switching
effect are elucidated on the basis of the experimental and quantum simulation results.
Article history:
Received 13 August 2013
Received in revised form
16 December 2013
Accepted 23 December 2013
Available online 2 January 2014
Keywords:
Polyimide
Electrical memory
SRAM
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
nanocomposites embedded with metal nanoparticles (NPs) [20e
22], fullerenes [23e25], carbon nanotubes (CNTs) [26,27],
In recent years, increasing attention has been paid to applica-
tions in memory devices based on polymeric materials possessing
bistable electrical resistive switching effects [1e7]. As one of the
most promising candidates for the next-generation memory de-
vices characterized by the high data storage density, fast write/read
speed, low power consumption and multilevel designs, such kind of
polymer electronic materials possesses unique advantages, such as
structural simplicity, good scalability, high mechanical flexibility,
low fabrication cost, ease of processability, and three-dimensional
(3D) multi-stacking capability for achieving high density data
storage [8,9]. And completely different from that of the traditional
silicon-based memory cells, which stores data by encoding each
device as “0” or “1”, polymer memory achieves information storage
based on the bistability of the materials, for instance, the high and
low electrical conductivity response to an applied voltage [10].
Many polymeric materials have been explored and some of
them have been demonstrated to exhibit excellent electrical
resistive switching effects with potentials to be applied in memory
devices, including conjugated polymers such as poly(9,9-bis(4-
diphenylaminophenyl)-2,7-fluorene) (PDPAF) [11], vinyl polymers
with specific pendent groups such as poly(N-vinylcarbazole) (PVK)
[12e15], functional polyimides (PIs) [16e19], and polymer
grapheme [28], and graphene oxide (GO) [29]. Among them, aro-
matic PIs have gained most widespread attentions in recent years
due to their excellent thermal and mechanical properties, high-
temperature dimensional stability, chemical inertness, and good
processability [30]. These functional electroactive PIs were usually
designed to contain both electron-donating and electron-accepting
groups within a repeating unit, which can contribute to electronic
transitions between the ground and excited states through the
induced charge transfer between the electron donor (D) and
acceptor (A) moieties under applied electric fields, and therefore
exhibit electrical memory behaviors [31e33]. By introducing
different electron donor and acceptor moieties into the polymer
chain or by designing different molecular structures, PIs have been
demonstrated to be able to exhibit non-volatile or volatile memory
effect [30,34e37]. This makes us believe that by combining suitable
functional groups into the polyimide chain through elaborate mo-
lecular design, it is possible to finely tune the highest occupied
molecular orbital (HOMO) energy level, the lowest unoccupied
molecular orbital (LUMO) energy level, the dipole moment as well
as the charge transport ability of the synthesized polyimides and
consequently lead to novel electrical memory effect [38e40].
Herein, we report our works on the synthesis and electrical
characterization of an aromatic polyimide, 4,40-(hexa-
fluoroisopropylidene)diphthalic anhydride/4,40-diamino-400-N-car-
bazolyltriphenylamine (6F/CzTPA PI), with structures in which the
carbazole-tethered triphenylamine units (CzTPA) function as the
* Corresponding author.
E-mail address: qisl@mail.buct.edu.cn (S. Qi).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.12.046

L. Shi et al. / Polymer 55 (2014) 1150e1159
1151
electron-donating moieties and the hexafluoroisopropylidene
phthalimide units (6FDA) serve as the electron-accepting species.
Both triphenylamine (TPA) and carbazole (Cz) are typical electron-
donating groups, and previous researches by combining them
individually with electron acceptor groups have revealed soluble
electroactive polyimides showing different memory behaviors such
as dynamic random access memory (DRAM) [5,8,36,41e46] and
static random access memory (SRAM) [35,36,45] behaviors. Here,
the integration of the TPA unit and the Cz unit to form the CzTPA
moiety and its subsequent utilization as the electron-donating
species is supposed to be able to enhance the electron donating
and charge transport capabilities of the resulting polyimide, and
correspondingly endow the synthesized material with stronger
potential to form electron donoreacceptor couples and conse-
quently novel memory effect. Electrical characterization results
suggest that the polymer possesses electrical bistability and the
memory devices using the synthesized polyimide as the active
layer (ITOjThin 6F/CzTPA PI LayerjAu) exhibit volatile electrical
switching effect, which could be swept both positively and nega-
tively from the low-conductivity (OFF) state to the high-
conductivity (ON) state with an ON/OFF current ratio of about 105
and a switching time less than 20 ns, and could be applied as static
random access memory (SRAM) in digital information storage.
Theoretical model analysis was carried out to elucidate the elec-
trical conducting process occurring in the synthesized polyimide.
Molecular simulation of the 6F/CzTPA model compound was con-
ducted to clarify the carrier transport process and memory mech-
anisms in the electroactive polyimide.
performed on a CHI660D Electrochemical Workstation (Shanghai
Chenhua Instruments Inc., China) using a three-electrode cell under
nitrogen environment. The polyimide films coated on a platinum
square electrode (working electrode) were scanned anodically and
cathodically (scan rate: 100 mV sꢀ1) in a solution of tetrabuty-
lammonium tetrafluoroborate (n-Bu4BF4) in dry acetonitrile (0.1 M)
with Ag/AgCl and a platinum net as the reference electrode and
auxiliary electrode, respectively. Ferrocene was used as the external
reference for calibration (0.38 V vs. Ag/AgCl). The thickness of the
polyimide films coated on the ITO substrate was measured using
atomic force microscopy (Digital Instruments Nanoscope IIIa, Veeco
Instrument) under contact mode. The currentevoltage (IeV) char-
acteristics of the sandwich devices were recorded by a Keithley
4200 SCS semiconductor parameter analyzer equipped with a
Micromanipulator PW-600 probe station (Advanced Technology
Co., limited, Hong Kong) in a clean and metallically shielding box in
ambient environment at room temperature. Molecular simulations
were carried out on the basic unit of the synthesized polyimide
using the Gaussian 09 program package. The molecular orbitals and
electronic properties were calculated on theory levels including
semi-empirical, ab initio and density functional theory (DFT).
2.3. Monomer synthesis and polyimide synthesis
Scheme 1 shows the synthetic route for the DACzTPA diamine
monomer and the final 6F/CzTPA polyimide. Details on their
preparation and characterization are given below. The 4,40-dia-
mino-400-N-carbazolyltriphenylamine (DACzTPA) was synthesized
by the condensation of N-(4-aminophenyl)carbazole (APCB) with
4-fluoronitrobenzene in the presence of cesium fluoride, followed
by the hydrazine monohydrate Pd/C-catalyzed reduction of the
intermediated dinitro compound (DNCzTPA) according to a similar
procedure reported previously [47].
2. Experimental
2.1. Materials
4-Fluoronitrobenzene, carbazole, hydrazine monohydrate, 10%
palladium on activated carbon (10% Pd/C), isoquinoline, and cesium
fluoride were purchased from J&K Scientific Co. Ltd. Anhydrous
potassium carbonate, anhydrous magnesium sulfate, sodium hy-
droxide (NaOH), tin(II) chloride dihydrate, 1-methyl-2-
pyrrolidinone (NMP), dimethylacetamide (DMAc), dimethylforma-
mide (DMF), tetrahydrofuran (THF), ethyl acetate, toluene, chloro-
form, acetone, isopropanol, methanol and ethanol were purchased
from Beijing Chemical Works, and they were all used as received.
m-Cresol was also bought from Beijing Chemical Works, and it was
purified by distillation over zinc powder and stored over 4 Å mo-
lecular sieves prior to use. The 4,40-(hexafluoroisopropylidene)
diphthalic anhydride (6FDA) was obtained from SigmaeAldrich
(Shanghai) Trading Co. and sublimated before use.
2.3.1. Synthesis of N-(4-nitrophenyl)carbazole (NPCB)
To a solution of 8.36 g (0.05 mol) of carbazole and 5.3 ml
(0.05 mol) of 4-fluoronitrobenzene in 100 ml of DMF, 4.14 g
(0.03 mol) of anhydrous potassium carbonate was added with
stirring at one portion. After heating at 150 ꢁC for 15 h under ni-
trogen atmosphere, the mixture was poured into 800 ml of distilled
water under stirring to obtain a yellow precipitate. The crude
product was then collected by filtration and recrystallized with
ethyl acetate to afford 11.5 g (80% in yield) of yellow crystals. DSC:
m.p. 211 ꢁC and FT-IR (KBr, cmꢀ1): 1594,1344 (NO2 stretch). 1H NMR
(DMSO-d6, 400 MHz),
J ¼ 7.53 Hz), 7.98 (d, 2H, J ¼ 9.05 Hz), 7.56 (d, 2H, J ¼ 8.24 Hz), 7.49 (t,
2H, J ¼ 7.70 Hz), 7.36 (t, 2H, J ¼ 7.86 Hz).
d
(ppm): 8.50 (d, 2H, J ¼ 9.04 Hz), 8.28 (d, 2H,
2.2. Characterization
2.3.2. Synthesis of N-(4-aminophenyl)carbazole (APCB)
To a solution of 5.76 g (0.02 mol) of NPCB in 40 ml ethanol, 15.8 g
(0.07 mol) of SnCl2$2H2O was added. After refluxing for 24 h, the
reaction mixture was condensed under reduced pressure to distill
off most of the ethanol followed by neutralization with 40 wt%
aqueous NaOH solution until the mixture became alkaline. The
resulting mixture was then extracted with toluene, dried over
anhydrous magnesium sulfate, and evaporated under reduced
pressure to get 4.5 g (85% in yield) of yellowish syrup. FT-IR (KBr,
cmꢀ1): 3375, 3459 (NeH stretch). 1H NMR (DMSO-d6, 400 MHz),
Fourier transform infrared (FT-IR) spectra were recorded under
ambient conditions on a Bruker Tensor 27 Fourier transform
spectrophotometer. KBr was used as a nonabsorbent medium.
Proton nuclear magnetic resonance (1H NMR, 400 MHz) spectra
were recorded on a Bruker AV400 spectrometer using DMSO-d6
and CDCl3 as the solvent. Differential scanning calorimetry (DSC)
measurements were performed on a TA Q20 system at a heating
rate of 10 ꢁC minꢀ1 under nitrogen atmospheres. Thermogravi-
metric analysis (TGA) was undertaken under nitrogen atmospheres
using a TA Q50 instrument at a heating rate of 10 ꢁC minꢀ1. Gel
permeation chromatography (GPC) analysis was carried out on a
Waters 515-2410 system using polystyrene standard as the mo-
lecular weight reference and THF as the eluent. Ultravioletevisible
(UV/vis) absorption spectra were recorded on a Shimadzu UV-2550
spectrophotometer. Cyclic voltammetry (CV) measurement was
d
(ppm): 8.19 (d, 2H, J ¼ 7.67 Hz), 7.40 (t, 2H, J ¼ 7.66 Hz), 7.26w7.17
(m, 6H), 6.79 (d, 2H, J ¼ 8.61 Hz), 5.43 (s, 2H).
2.3.3. Synthesis of 4,40-dinitro-400-N-carbazolyltriphenylamine
(DNCzTPA)
To a solution of 2.58 g (10 mmol) of APCB and 2.2 ml (21 mmol)
of 4-fluoronitrobenzene in 50 ml of DMF, 1.52 g (10 mmol) of

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L. Shi et al. / Polymer 55 (2014) 1150e1159
Scheme 1. Synthetic route for the DACzTPA monomer and the 6F/CzTPA polyimide.
cesium fluoride was added with stirring at one portion, and then
heated at 150 ꢁC for 24 h under nitrogen atmosphere. The resulting
mixture was then poured into 500 ml of methanol to yield a yellow
precipitate followed by filtration, washing thoroughly by ethanol
and drying in vacuum to afford 3.75 g (75% in yield) of yellow solid.
DSC: m.p. 296 ꢁC and FT-IR (KBr, cmꢀ1): 1579,1308 (NO2 stretch). 1H
number average molecular weight (Mn) and the weight average
molecular weight (Mw) of the polymer are 4.6 ꢂ 104 g molꢀ1 and
8.7 ꢂ 104 g molꢀ1, respectively, with the polydispersity index of 1.9,
as determined by GPC (polystyrene as standard, THF as solvent and
eluent). In addition, as evaluated by TGA and DSC measurements,
the 6F/CzTPA PI not only exhibits excellent thermal stability with
the 5% and 10% weight loss temperatures up to 510 ꢁC and 557 ꢁC
(see Fig. S3 for the TGA curve), but also possesses a high glass
transition temperature (Tg) of about 327 ꢁC (see Fig. S4 for the DSC
curve).
NMR (DMSO-d6, 400 MHz),
d (ppm): 8.28w8.25 (m, 6H), 7.75 (d,
2H, J ¼ 8.61 Hz), 7.53 (d, 2H, J ¼ 8.59 Hz), 7.53w7.46 (m, 4H), 7.36 (d,
4H, J ¼ 9.14 Hz), 7.32 (t, 2H, J ¼ 7.82 Hz).
2.3.4. Synthesis of 4,40-diamino-400-N-carbazolyltriphenylamine
(DACzTPA)
2.4. Sandwich memory device fabrication
In a 100 ml round-bottom flask equipped with magnetic stirring,
1.00 g (2 mmol) DNCzTPA and 0.3 g of 10% Pd/C were dissolved/sus-
pended in 30 ml of ethanol. The suspension solution was then heated
to reflux for 24 h under the dropwise addition of 5 ml hydrazine
monohydrate (finish in 2 h), followed by hot filtration to remove the
Pd/C, recrystallization from ethanol and drying in vacuum at 50 ꢁC
overnight to yield 0.70 g (80% in yield) of light-purple crystals. DSC:
m.p. 194 ꢁC and FT-IR (KBr, cmꢀ1): 3371, 3232 (NeH stretch). 1H NMR
The sandwich memory devices using the synthesized 6F/CzTPA
PI as the active layer were fabricated by spreading a 20 mg mlꢀ1
polyimide solution in DMAc onto a precleaned indium-tin oxide
(ITO) substrate using a spin coater set at 1500 rpm for 40 s. The
ITO conductive glass was precleaned sequentially with water,
acetone, chloroform and isopropanol by ultrasonication for 15 min
and then dried by a N2 gun. After keeping in a clean box pro-
grammed at constant temperature and humidity for ca. 4 h, the
spin-coated polyimide films were then baked at 100 ꢁC for 5 h
under vacuum to remove the residual solvent. 1H NMR measure-
ment was carried out to confirm that the solvent (i.e., DMAc) was
completely removed from the active layer. The thickness of the
polyimide films coated on ITO was around 50 nm, as determined
by atomic force microscopy measurement. Finally, the sandwich
devices were fabricated by vacuum evaporation of a thin Au layer
with the thickness of ca. 100 nm through a shadow mask onto the
polymer surface as the top electrode, the structure of which is
shown in Scheme 2. The sandwich memory devices were even-
tually prepared with the configuration of ITOjThin 6F/CzTPA PI
LayerjAu.
(DMSO-d6, 400 MHz),
d
(ppm): 8.20 (d, 2H, J ¼ 7.70 Hz), 7.41 (t, 2H,
J ¼ 8.19 Hz), 7.30 (d, 2H, J ¼ 8.18 Hz), 7.27w7.23 (m, 4H), 6.97 (d, 2H,
J ¼ 8.63 Hz), 6.78 (d, 2H, J ¼ 8.92 Hz), 6.60 (d, 2H, J ¼ 8.65 Hz), 5.06
(s, 2H).
2.3.5. Synthesis of the 6F/CzTPA polyimide
The 6F/CzTPA PI was synthesized from diamine DACzTPA and
dianhydride 6FDA by the conventional one-step method. To a so-
lution of 0.440 g (1 mmol) of DACzTPA in 8 ml of m-cresol, 0.444 g
(1 mmol) of 6FDA and 2 ml of isoquinoline were added in one
portion. The mixture was stirred for 2 h at ambient temperature
under N2 atmosphere and then heated to 190 ꢁC and maintained at
that temperature for 12 h. After cooling down to ambient temper-
ature, the viscous solution was poured slowly into 400 ml of
methanol with stirring to yield the polyimide as a precipitate. The
crude product was then collected by filtration, washed thoroughly
with hot methanol, and dried at 120 ꢁC overnight to give the final
polyimide (0.76 g, 86% in yield). FT-IR (KBr, cmꢀ1): 1784, 1725, 1370,
723 (imide ring) (see Fig. S1 for the IR spectrum). 1H NMR (CDCl3,
400 MHz),
d
(ppm): 8.12 (d, 2H, J ¼ 7.82 Hz), 8.04 (d, 2H,
J ¼ 7.98 Hz), 7.96 (s, 2H), 7.87 (d, 2H, J ¼ 7.73 Hz), 7.51w7.27 (m,18H)
(see Fig. S2 for the 1H NMR spectrum). The 6F/CzTPA PI exhibits
good solubility in common solvents, such as chloroform, THF, DMF,
DMAc, and NMP, at room temperature, due to the bulkiness and low
surface energy of the trifluoromethyl group in 6FDA. It can be cast
into transparent and uniform films from solutions by spin coating.
The inherent viscosity of the resulting polyimide was 0.73 dL gꢀ1
measured in DMAc at a concentration of 0.5 g dLꢀ1 at 30 ꢁC. The
,
Scheme 2. Illustrative structures for the ITOjThin 6F/CzTPA PI LayerjAu sandwich
device.

L. Shi et al. / Polymer 55 (2014) 1150e1159
1153
3. Results and discussion
(moderate) when scanning backward. Nevertheless, the predomi-
nant reduction peak at ꢀ1.40 V demonstrates the strong electron-
withdrawing ability of the 6FDA units. Besides, the polyimide ex-
hibits good CV reversibility during repeated cyclic scans between
ꢀ2.0 V and þ2.0 V, indicating the excellent electrochemical sta-
bility of the synthesized polyimide and the good adhesion at the
polymer-platinum interface. The quasi-reversible electron-
donating and electron-accepting behaviors observed here are
suggested to be partially responsible for the quasi-reversible
(delayed relaxation) electrical switching effect observed in the
sandwich memory devices shown later.
In addition, the energy levels of HOMO (the top of the valence
band) and LUMO (the bottom of the conduction band) of the syn-
thesized polyimide can be quantitatively calculated from the CV
onset oxidation potential (Eox (onset)) and the energy band gap (Eg)
obtained from the UV/vis absorption spectrum according to the
following equations: [41,49,50]
3.1. Optical and electrochemical properties of the 6F/CzTPA
polyimide
Fig. 1(a) shows the UV/vis absorption spectrum for the synthe-
sized 6F/CzTPA PI, as well as the 6FDA and DACzTPA monomers in
THF. A distinct absorbance band centered at around 326 nm (lmax
was observed on the spectrum of the 6F/CzTPA PI, with the ab-
sorption edge extending to a wavelength (ledge) of 372 nm, based
on which the optical energy band gap (Eg) of the material was
calculated to be 3.33 eV according to the Plank equation (Eg ¼ hc/
ledge). This strong UV absorption peak was supposed to be ascribed
)
to the n-
aromatic rings and nitrogen atoms that combine the characteristic
* transitions of triphenylamine and carbazole groups. Here, the
electron-withdrawing 6FDA species is believed to have promoted
the delocalization and conjugation of the current n- and p-p
p* transitions resulting from the conjugation between the
p-p
p
hꢀ
ꢁ
i
electronic system, since the UV/vis spectra in Fig. 1(a) indicate that
the absorption of the 6F/CzTPA PI at 326 nm was obviously
enhanced as compared to the individual CzTPA monomer.
HOMO ¼ ꢀ EoxðonsetÞ ꢀ Eferrocene þ 4:8 ðeVÞ
LUMO ¼ Eg þ HOMO
(1)
(2)
The electrochemical behavior of the synthesized polyimide was
investigated by cyclic voltammetry (CV) measurements conducted
using the thin PI film cast on a precleaned square platinum plate as
the working electrode to elucidate the relative ionization and
reduction potentials of our electroactive materials. Fig. 1(b) shows
the CV sweeps for both the p-doping and n-doping processes for
the synthesized 6F/CzTPA polyimide on platinum in the potential
range of ꢀ2.0 to þ2.0 V. The CV curve indicates that the 6F/CzTPA
polyimide exhibits a quasi-reversible p-doping behavior during the
anodical scan from 0 to þ2.0 V. A strong ionization (oxidation) peak
was observed at around 1.71 V (vs. Ag/AgCl electrode), whereas two
reduction peaks appear at about 1.14 V (weak) and 0.81 V (mod-
erate) when scanning backward. The results imply that the CzTPA
units consisted of carbazole and triphenylamine in the 6F/CzTPA
polyimide have a strong tendency to donate electrons and, as ex-
pected, readily function as hole-transporting sites in the material.
This is under expectation since carbazole and triphenylamine are
typical electron-donating moieties, and both of them have been
demonstrated to possess the ability to enhance the hole-injection
properties when being introduced into a material [46,48].
where Eferrocene is the potential of the external standard, ferrocenee
ferrocenium (Fc/Fcþ), which is determined to be 0.38 V vs. Ag/AgCl
under the same CV measurement conditions for the 6F/CzTPA PI.
The absolute energy level of ferrocene is 4.8 eV below the vacuum
level (i.e., 0 eV), and was used as the calibration reference.
As determined from the CV curve, the Eox (onset) for the poly-
imide is at around 0.89 V vs. Ag/AgCl, and as calculated previously
from the UV/vis absorption edge wavelength, the energy band gap
(Eg) is 3.33 eV. Thus, the HOMO and LUMO energy levels of the 6F/
CzTPA polyimide are estimated to be ꢀ5.31 and ꢀ1.98 eV, respec-
tively. In comparison, the onset reduction potential (Ered (onset)) of
the polyimide determined from the CV sweeps was ꢀ1.00 V, giving
a CV energy band gap of 1.89 eV, which deviates significantly from
the optical band gap (3.33 eV) estimated from the UV/vis absorp-
tion spectrum, which is common and often encountered in
literatures.
3.2. Memory effects and device performance
When scanning cathodically from 0 to ꢀ2 V, the polymer film
exhibits a distinct reduction peak at about ꢀ1.40 V (vs. Ag/AgCl
electrode), which is supposed to be related to the electrochemical
reduction of the 6FDA units that have a high tendency to adopt
electrons. However, the CV result indicates that this n-doping
process is also quasi-reversible, as verified by the observation of
two oxidation peaks at about ꢀ0.96 V (weak) and ꢀ0.33 V
The electrical switching effect of the synthesized PI was
demonstrated by the currentevoltage (IeV) characteristics of the
AujpolyimidejITO sandwich devices, as shown in Fig. 2(a). During
the first positive scan from 0 to 3 V (with ITO as cathode and Au as
anode), a progressive increase in the current occurring at a
threshold voltage of around 1.5 V was observed, indicating the
1.71 V
-0.33 V
-0.96 V
1.14 V
0.81 V
-1.40 V
Fig. 1. (a) UV/vis absorption spectrum of 6FDA, DACzTPA and 6F/CzTPA PI. (b) Cyclic voltammetry sweeps of the 6F/CzTPA PI measured in 0.1 M n-Bu4BF4/acetonitrile (scan rate:
100 mV sꢀ1).

1154
L. Shi et al. / Polymer 55 (2014) 1150e1159
(b)
(a)
(d)
(c)
Fig. 2. (a) Currentevoltage (IeV) characteristics of the ITOjThin 6F/CzTPA PI LayerjAu sandwich device; the sweep sequence and direction are indicated by the number and arrow.
The fourth, seventh and the tenth sweeps were conducted about 10 min after turning off the power. The ON state was sustained by a refreshing voltage pulse of ꢀ1 V (pulse
duration ¼ 1 ms) in every 5 s, as shown by the “rf” trace. (b) The ON- to OFF-state current ratios for both positive and negative sweeps as a function of applied voltage. (c) Effect of
operation time on the currents of the sandwich device at the ON and OFF states tested at ꢀ1 V bias under ambient conditions. (d) Effect of ꢀ1 V read pulse on the ON and OFF states
of the sandwich device. The inset shows the pulse shapes in the measurement (2 ms in period, 1 ms in duration width).
transition of the sandwich device from the low-conductivity state
(the OFF state) to the high-conductivity state (the ON state). The
ratio of the conductivities achieved between the two states is at
around 105. After this transition, the device remained in the high-
conductivity state, as observed during the following positive scan
(the second sweep). And the subsequent negative scan from 0 to
ꢀ3 V (the third sweep) did not turn the device from the ON state to
the OFF state, indicating that the memory device cannot be
retrieved to the initial OFF state by applying a reverse bias sweep,
and therefore is “unerasable”. However, the electrical character-
ization results indicate that the ON state of the device could only be
retained temporarily, and relaxed voluntarily to the OFF state after
turning off the power for a period of about 10 min, indicating the
actually “volatile” nature of the ON state. Nevertheless, as observed
from the fourth sweep (from 0 to 3 V) conducted over 10 min after
turning off the power, the device exhibits essentially identical
electrical OFF-to-ON switching behavior with the first sweep, and
retains the high-conductivity state during the followed two sweeps
(the fifth and the sixth sweeps), suggesting the “reprogrammable”
characteristics of the current memory device. The seventh sweep
was conducted about 10 min after turning off the power. The device
was found to be also able to switch from the OFF state to the ON
state when a reverse threshold voltage at around ꢀ1.8 V was
applied. And the device remained in the ON state when the nega-
tive sweep (the eighth sweep) and the positive sweep (the ninth
sweep) were repeated. The results observed here suggest that the
current polyimide memory device can be written bi-directionally
with almost no polarity at positive and negative threshold volt-
ages of comparable magnitude. The tenth sweep was carried out
about 10 min after turning off the power. The memory device was
found to have relaxed voluntarily to the OFF state without any
“erasing” process, whereas it could be reprogrammed to the ON
state when the threshold voltage at about ꢀ1.8 V was reapplied.
Besides, the volatile ON state could be permanently retained by
applying a refreshing voltage pulse of ꢀ1 V (1 ms duration) in every
5 s (the “rf” trace in Fig. 2(a)) or a continuous bias of ꢀ1 V (Fig. 2(c)).
The volatile, but reprogrammable nature of the ON state, and the
bi-directionally accessible ON and OFF states observed above sug-
gest that the current AujThin 6F/CzTPA PI layerjITO sandwich device
possesses electrical bistable characteristics that could serve as a
static random access memory (SRAM) in digital information storage
[37]. The OFF-to-ON electrical switching process is equivalent to the
“writing” process in a traditional memory cell. The distinct and bi-
directionally accessible bistable electrical states in the positive
voltage range from 0 to 1.5 V and in the negative voltage range from
0 to ꢀ1.8 V allow any constant voltage between 0 and 1.5 V or be-
tween 0 to ꢀ1.8 V (e.g. 1 V or ꢀ1 V) to “read” the “OFF” or “0” signal
(before switching on) and the “ON” or “1” signal (after switching on)
of the memory device in any order. The IeV characteristics of the
device are demonstrated to have good reproducibility during
repeated tests. And, further experiments on duplicated sandwich
devices revealed that the observed electrical switching effect was
repeatable, but with minor variations in the OFF-to-ON transition
voltage. To this point, it is concluded that the synthesized 6F/CzTPA
polyimide possesses unerasable but volatile, yet reprogrammable,
bi-directionally accessible and nonpolar electrical bistabilities and
the sandwich devices using the polyimide as the active layer could
be applied as volatile SRAM memory in digital information storage.
The low OFF-to-ON electrical transition voltage will endow the
prepared SRAM devices with low power-consuming characteristic.
However, it should be mentioned that, at a bias exceeding ꢀ5 V
or þ5 V, the device shows a sudden drop in the current to zero and
could not be operated any more, which is suggested to be due to
device failure at high electric field stress.

L. Shi et al. / Polymer 55 (2014) 1150e1159
1155
In addition to the SRAM memory effect and switching voltage,
(a)
there are also many other important parameters that play crucial
roles in determining the performance of a memory device and its
eventual practical applications, including the ON/OFF current ratio,
long-term retention stability, reading cycles and switching time.
Fig. 2(b) shows the ON/OFF current ratio of the memory device as a
function of the applied voltage. The results suggest that the sandwich
device based on the 6F/CzTPA PI achieves an ON/OFF current ratio as
high as around 105 during both the positive and negative switching-
on process. The high ON/OFF current ratio achieved here promises a
low misreading rate for the present devices during real memory ap-
plications. Furthermore, to evaluate the ability of the 6F/CzTPA pol-
yimide in retaining the two conductivity states, the ON and OFF
currents of the sandwich device were tested under ambient air con-
ditions, the results of which are shown in Fig. 2(c). Under a constant
bias stress of ꢀ1 V, the device shows excellent long-term operation
stability and no degradation in current was observed for both the ON
and OFF states overan 8 h period of measurement, demonstratingthe
excellent stability of the polyimide material and the observed elec-
trical memory effect. And, it is expected that, with proper electronic
encapsulation in practical applications, the retention time and the
stability of the device will be promoted further or at least be retained
intact. Fig. 2(d) shows the stimulus effect of the read pluses (ꢀ1 V in
(b)
magnitude, 2 ms in period, 1 ms in duration width) on the ON and OFF
states of the device. The result indicates no current degradation for
both the ON and OFF states after more than 108 read cycles under
continuous read pulses of ꢀ1 V, revealing the excellent information
storage and reading out stabilities of the current memory device.
Another key performance indicator for a memory device is the
switching time. Fig. 3 shows the transition response behavior of the
memory device below and above the switching-on threshold
voltage (i.e., 1.5 V as shown in Fig. 2(a)). The tested device was
verified to be in an initial OFF state, and a read pulse with magni-
tude of 1 V (below the threshold voltage) could not switch it to the
ON state, as shown in Fig. 3(a). Whereas, the device was immedi-
ately turned on when a read pulse of 2 V (above the threshold
voltage) was applied, as shown in Fig. 3(b). And no distinguishable
electrical delay was detected in Fig. 3b(b) within the resolution of
the Keithley 4200 (upper limit of the pulse rising time: 20 ns). Thus,
the switching time (i.e., response time, delay time) for the device is
supposed to be in the nanoseconds scale and could be further
confined to be faster than 20 ns. In addition to the ultrafast
Fig. 4. (a) IeV characteristics fitted with the SCLC conduction model (log I w log jVj)
for the memory device on the low-conductivity state (the OFF state) for both the
positive sweep (0 / þ1.5 V) and the negative sweep (0 / ꢀ1.8 V). (b) IeV charac-
teristics fitted with the Ohmic conduction model (I w V) for the memory device on the
high-conductivity state (the ON state) for both the positive sweep (þ1.5 / þ3.0 V) and
the negative sweep (ꢀ1.8 / ꢀ3.0 V).
response time, the result in Fig. 3(b) also indicates that the device
soon reaches its equilibrium ON state from the initial OFF state in a
period as short as ca. 90 ns. Here, it should be mentioned that both
the response time and the equilibrium time of the current memory
device are much faster than that (w1 ms) of a NAND (NOT/AND)
flash memory based on traditional semiconductors, and compara-
ble to the current widely-used dynamic random access memory
(DRAM) and SRAM inorganic semiconducting memory devices
(response time: <10 ns, access time: 60e100 ns or shorter)
[14,51,52]. On account of the above results, it is suggested that the
present polyimide constitutes a considerably promising candidate
material for the next-generation digital memory applications with
characteristics of mass storage capacity and ultrafast data access.
The ultrafast response time and the short equilibrium time will
ensure the fabricated memory device with the lowest misoperation
and the highest accuracy during the ultrafast read/write process. In
addition, as a further consideration, the fast switching-on behavior
observed in the current memory device implies that the charge
transfer occurring in the 6F/CzTPA donoreacceptor polyimide
system is considerably rapid under the applied electric field.
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
20
16
12
8
(b)
(a)
2.0 V pulse
ON current
1.0 V pulse
OFF current
y
a
turn on
90 ns
no del
4
0
0
400
800
1200
0
400
800
1200
Time (ns)
Time (ns)
Fig. 3. The transient response of the device current vs. time below and above the
switching-on threshold voltage. (a) The electrical current response of the device
initially in the OFF state. A voltage pulse (1.0 V) less than the switching-on threshold
voltage (1.5 V) is applied to confirm the low conductivity state of the device. (b) The
electrical current response of the same device under a voltage pulse (2.0 V) higher than
the switching-on threshold voltage. The device responses to the applied voltage
immediately with no distinguishable electrical delay and is transformed to its equi-
librium high conductivity state in ca. 90 ns.
3.3. Theoretical analysis, molecular simulation and switching
mechanism
3.3.1. Theoretical analysis
Many physical models have been proposed to explain the cur-
rent transition in a dielectric film, including Ohmic conduction,

1156
L. Shi et al. / Polymer 55 (2014) 1150e1159
Hopping conduction, Schottky emission, thermionic emission (TE),
FrenkelePoole emission, tunnel or field emission, direct tunneling,
FowlereNordheim tunneling, ionic conduction, and space-charge-
limited current (SCLC) transport [2,9,53]. However, for the pre-
sent case, it is found that any individual conduction mechanism
mentioned above cannot fit the characteristics for both the ON state
and the OFF state at the same time, implying the involvement of
probably different charge transport processes in the two accessible
states. Fig. 4(a) shows the IeV characteristics of the memory de-
vices in OFF states, presenting in the form of log I vs. log jVj. As can
be observed, during both the positive sweep (sweep 1 in Fig. 2(a),
0 / þ1.5 V) and the negative sweep (sweep 7 in Fig. 2(a),
0 / ꢀ1.8 V), the low conduction states have well been character-
ized by the space-charge-limited current (SCLC) model in which log
I is linearly proportional to log jVj (log I w log jVj). The power law of
the IeV result observed here suggests that the memory device
undergoes a restricted carrier injection and transportation process
at the initial stage of the voltage scan. And it is reasonable to deduce
that, when the applied bias exceeds the threshold value, numerous
charges will be generated and injected into the polyimide active
layer, and the ultrafast charge-transfer process (as elicited in Fig. 3)
will soon yield a conducting path with the fastest speed, triggering
the device to its high conduction state. The basically linearly-
correlated IeV characteristics shown in Fig. 4(b) for the ON states
of the device suggest that the high conductivity state is well fitted
to the Ohmic conduction relationship (I w V) in the range of þ1.5
V / þ3.0 V (sweep 1 in Fig. 2(a)) and ꢀ1.8 V / ꢀ3.0 V (sweep 7 in
Fig. 2(a)), indicating that the charge carriers in the polyimide active
layer overcome the SCLC energy barriers and eventually flow
through the device by the Ohmic conduction processes at the
higher biases during both the positive and negative voltage sweeps.
The laws of the charge transport by the SCLC and Ohmic models are
given by the following equations.
ꢀ
D
kT
Eae
J
OhmicfV exp
(4)
(J
¼
current density,
V
¼
electric field,
T
¼
temperature,
ε ¼ insulator dynamic permittivity, d ¼ insulator thickness, q is
charge, and is the carrier mobility.) [9].
m
3.3.2. Molecular simulation
With the purpose of getting a clear understanding of the charge
transfer process and the corresponding switching behavior of the
present polyimide memory device, molecular simulations were
carried out to elucidate the electronic structures, molecular or-
bitals, energy levels and dipole moments of our synthesized elec-
troactive donoreacceptor macromolecules. For the sake of model
simplicity and practical availability for computations, 4,40-diph-
thalimido-400-N-carbazolyltriphenylamine, with one DACzTPA
tethered via imide rings to two 6FDA fragments (see Table 1 for
model compound 1), which is essentially identical to the basic unit
of the 6F/CzTPA polyimide were selected as the model compound.
Different calculation methods ranging from semi-empirical
(including AM1 and PM3) to ab initio (including HF) and density
functional theory (including LSDA, B3LYP, and B3PW91) were
evaluated. The basis set was set as 6-31G(d), whenever necessary,
in all the first-principle calculations.
Fig. 5 summarized the calculated HOMO and LUMO energy
levels, and the related energy band gaps obtained from the mo-
lecular simulations conducted on different theory levels. To verify
the reliability of the simulation results, the corresponding experi-
mental HOMO, LUMO and band gap results determined from the
UV/vis absorption spectrum and the CV sweeps of the 6F/CzTPA
polyimide (see Fig. 1) were also shown. The results indicate that the
worst results come from the semi-empirical-based computations
(AM1 and PM3) and the ab initio (HF) calculations, with the
observation of considerably large deviations from the experimental
results either in energy levels or in energy band gaps, indicating
their unavailability in predicting the electronic structures of the
9εi
SCLCf
m
V2
8d3
J
(3)
Table 1
Molecular simulation results for the components in 6F/CzTPA PI based on B3LYP /6-31G(d) model chemistry.a,b
Components
HOMO level
DACzTPA
6FDA
Model compound 1
LUMO level
Energy gap
4.45 eV
5.33 eV
3.07 eV
Dipole moment
4.304 Debye
5.223 Debye
3.990 Debye
a
All the structures used for molecular orbital calculations were optimized firstly on AM1 level and then on HF/6-31G(d) level.
b
The structure of the 6F/CzTPA model compound 1 was constructed from the optimized 6FDA and CzTPA structure, and then optimized additionally on AM1 and HF/6-
31G(d) level.

L. Shi et al. / Polymer 55 (2014) 1150e1159
1157
orbitals, respectively, as illustrated in Table 1. The tethering of Cz to
TPA and the subsequent formation of the DACzTPA moieties are
supposed to significantly enhance the electron-donating charac-
teristics of the resulting material since the results indicate that the
HOMO, HOMO-1, HOMO-2, HOMO-3, HOMO-4 and HOMO-5 or-
bitals are all located on the CzTPA groups (see Fig. S5). Besides, the
reduced dipole moment (3.990 Debye) observed for the model
compound 1 in Table 1, as relative to the individual CzTPA
(4.304 Debye) and 6FDA (5.223 Debye), suggests that the synthe-
sized polyimide has a strong tendency for charge separation. These
features promise easier charge transfer from DACzTPA (HOMOs) to
6FDA (LUMOs), and the subsequent formation of charge-transfer
complexes in the polyimide system.
LUMO
HOMO
1.41
vacuum level
0
-4
-8
-1.25 -1.39
-1.98
-2.21 -2.32
-3.61
-3.42
-5.26
-5.39
-5.31 -5.31
-5.28
fr
om C
V
-7.39
-8.09
-8.30
CV
UV
AM1 PM3
HF
8.80
3.3.3. Switching mechanism
8
4
0
7.06
6.74
Fig. 6 shows the energy level diagram for the sandwich devices
with the configuration of ITOjThin 6F/CzTPA PI LayerjAu. The HOMO
and LUMO energy levels of the polyimide determined from CV and
UV/vis spectra are ꢀ5.31 eV and ꢀ1.98 eV, respectively, which are
associated in respective with the DACzTPA moieties and the 6FDA
units based on the quantum calculations. For the positive scan, ITO
serves as the cathode and Au as the anode. It is clear in Fig. 6 that
the electron-transfer energy barrier between the ITO electrode and
the LUMO level of the 6F/CzTPA PI (2.82 eV) is considerably higher
than that between the HOMO level of the 6F/CzTPA PI and the Au
electrode (0.21 eV). While for the negative scan, ITO is the anode
and Au is the cathode. Whereas, the electron-transfer energy bar-
rier of the Auj6F/CzTPA LUMO (3.12 eV) is still higher than that of
the 6F/CzTPA HOMOjITO (0.51 eV), and both of which are larger
than the energy barriers in the positive scan (i. e., 2.82 eV and
0.21 eV). In terms of these data, it is inferred that the 6F/CzTPA PI
most likely functions as a p-type material and hole injection from
metal electrodes into the HOMO of the polyimide is more favorable
during the charge injection process.
Taking the positive scanning process (0 / þ3.0 V) as an
example, theoretically speaking, an applied voltage exceeding
0.21 eV (the barrier between Au and the HOMO of the polyimide)
will provide enough energy for the holes on Au (or electrons on
HOMO) to overcome the energy barrier between Au and the HOMO
to inject into (for electrons, extract from) the active layer of the
sandwich device. Thus, during the voltage scanning process, holes
are readily loaded into the material under a low bias. Accordingly,
the device exhibits an instant increase in the current (although to
merely 10ꢀ8w10ꢀ7 A scale) at the very initial stage of the voltage
sweep, as observed in Fig. 2(a) (the first scan). However, it is
3.33
3.073.07
1.89
1.65
Simulation
Experiment
Fig. 5. Comparison of the energy band gaps (bottom) and the HOMO, LUMO energy
levels (top) of the 6F/CzTPA model compound 1 calculated from model chemistry on
different theory levels and those determined from the cyclic voltammetry (CV) and
UV/vis absorption spectrum of the polyimide. The basis set of 6-31G(d) was used in the
ab initio and DFT calculations.
current polyimide. DFT methods, especially B3LYP and B3PW91,
exhibit the best accuracy, in which the calculations based on the
B3LYP1/6-31G(d) theory level gave the most acceptable results. The
HOMO, LUMO, and band gaps calculated from the B3LYP/6-31G(d)
model chemistry are ꢀ5.28 eV, ꢀ2.21 eV, and 3.07 eV, respectively,
which are almost identical with the experimentally-determined
results, ꢀ5.31 eV, ꢀ1.98 eV, and 3.33 eV. Therefore, the B3LYP/6-
31G(d) model chemistry was eventually selected for all future
simulations and discussions.
Table 1 shows the corresponding B3LYP/6-31G(d)-based simu-
lation results for the DACzTPA unit, the 6FDA unit, and the model
compound 1 used to represent the 6F/CzTPA polyimide. The higher
HOMO energy level of DACzTPA (ꢀ4.76 eV) indicates that the
DACzTPA unit consisted of triphenylamine and carbazole groups
functions as the hole-accepting moieties (electron donor), while
the lower LUMO energy level of 6FDA (ꢀ2.27 eV) indicates the
strong electron-withdrawing ability of the 6FDA unit and its
availability to function as electron acceptor. The HOMO and LUMO
energy levels of the model compound 1 (representative of the
polyimide) are ꢀ5.28 eV and ꢀ2.21 eV, respectively, which are
basically consistent with the results obtained from the CV and UV/
vis spectra of the polyimide. These results indicate that when these
two components are covalently connected, the HOMO of the 6F/
CzTPA unit is located on the DACzTPA side with a decreased HOMO
energy level, whereas the LUMO of the 6F/CzTPA unit is located on
the 6FDA side with an increased LUMO energy level. The 6F/CzTPA
polyimide system is supposed to possess similar electronic struc-
tures, with DACzTPA functioning as the electron donor and 6FDA as
the electron acceptor. The two 6FDA fragments attached with the
DACzTPA unit are supposed to serve as two effective electron-
trapping sites since they correspond to the LUMO and LUMO þ 1
0
LUMO
p-type (hole injection)
hole
injection
LUMO(-1.98)
ITO
-2
-4
-6
Au
HOMO
the positive sweep
(from 0 to 3 V)
LUMO
hole
injection
ITO(-4.8)
Au(-5.1)
-5.31
HOMO
ITO
Au
HOMO
the negative sweep
(from 0 to -3 V)
6F/CzTPA PI
Fig. 6. Energy level diagram for the ITOjThin 6F/CzTPA PI LayerjAu sandwich device.

1158
L. Shi et al. / Polymer 55 (2014) 1150e1159
supposed that hole injection (or electron extraction) under bias
scans before switching on should be rather limited, as it has been
verified in Fig. 4(a) that the IeV characteristics of the device in the
low-conductivity state agree fairly well with the SCLC conduction
model. This is reasonable and expected, since the CzTPA unit in the
6F/CzTPA polyimide bears a triphenylamine group in the polymer
main chain and a carbazole unit as the pendent, both of which are
typical and efficient electron-donating moieties, and correspond-
ingly possess strong and definitely enhanced hole-trapping abilities
when combining together. Besides, simulation results in Table 1
suggest that the 6FDA moieties will also exhibit significant effect
in blocking the mobility of the injected holes, due to their consid-
erably low HOMO energy level (ꢀ8.20 eV). As a result, charge car-
rier migration in the active layer, which is responsible for the
electrical conductance of the device, is considerably limited.
Concomitantly, the injected holes will induce a countering space-
charge field in the polyimide matrix, which will screen the
applied external electric field, thereby hampering further charge
injection into the active layer [54]. Thus, the material exhibits a
high-resistance state and follows the SCLC conduction process,
accounting for the limited increase in the device current below
1.5 V.
With the increase of the applied external electric field, and the
continuous injection of the holes into the HOMOs (HOMO, HOMO-
1, HOMO-2, HOMO-3, HOMO-4, HOMO-5, see Fig. S5) located on
the CzTPA moieties and the corresponding electron injection into
the LUMOs (LUMO, LUMO þ 1, LUMO þ 2, LUMO þ 3, see Fig. S5)
located on the 6FDA parts, open channels for charge carriers were
finally generated between the charged HOMOs and charged LUMOs
through the charge-transfer interactions and the subsequent for-
mation of charge-transfer (CT) complexes. Consequently, a pro-
gressive increase in current was observed and the memory device
was suddenly triggered to its low-resistance state. The data-fitting
results shown in Fig. 4(b) suggest that the IeV behaviors since
switching on (i.e., after 1.5 V) have been well characterized by the
Ohmic conduction model, manifesting the change of the conduc-
tion mechanisms in the ON state. The device further remains in its
high-conductivity state during the subsequent positive and nega-
tive sweeps, which is suggested to be due to the strong electron
affinity of the 6FDA units, which could effectively hold the electrons
trapped in 6FDA moieties, and hamper their escapes through the
formation of CT complexes. Besides, the decreased dipole moment
(3.990 Debye) of the 6F/CzTPA basic unit, as compared to the in-
dividual 6FDA (5.223 Debye) and CzTPA (4.304 Debye) parts, as
listed in Table 1, implies that it will also be able to help to promote
the generation and the subsequent stabilization of the formed
charge separation structures. Therefore, the polymer could not be
reset to its initial OFF state during the followed bi-directional
voltage scans and the device manifests “unerasable” characteris-
tics in the ON state, as displayed in Fig. 2(a). However, the ON state
is turned out to be actually “volatile”. Over 10 min after turning off
the power, the device is automatically retrieved to the initial low-
conductivity OFF state, yet is “reprogrammable”. This indicates
that the CTcomplexes formed during the charge injection processes
are actually “temporarily stable” and “reversible”, which might be
partially verified by an indirect but explicit experimental evidence,
i.e., the quasi-reversible oxidation and reduction behaviors
observed in the CV test cycles depicted in Fig. 1(b). Mechanistically,
it is deduced that, although both the strong electron-withdrawing
ability of the 6FDA part and the decreased dipole moment of the
6F/CzTPA basic unit have positive effects in stabilizing the formed
CT complexes and in helping retaining the trapped electrons
through the increase of the back-charge-transfer energy barriers,
they could not inhibit their escapes and further neutralization with
the holes trapped at the CzTPA species in the polyimides, due to the
strong electron-withdrawing abilities of the CzTPA unit after hole
trapping and the important but easy-to-be-neglected fact that the
charge-trapping sites located on CzTPA are stoichiometrically
excess as compared to 6FDA (see Fig. S5). As a result, the device
shows a volatile SRAM memory effect.
The device is supposed to obey the same conduction mechanism
when starting with a negative voltage sweep process (0 / ꢀ3.0 V).
The slightly higher switching-on threshold voltage (i.e., ꢀ1.8 V)
observed during the negative sweep is suggested to be related to the
relatively higher energy barriers encountered during the negative
sweep (i.e., 0.51/3.12 eV vs. 2.82/0.21 eV), as depicted in Fig. 6.
4. Conclusions
In summary, an electrical resistive switching volatile static
random access memory (SRAM) behavior was realized in our pre-
sent work, based on an aromatic polyimide, 4,40-(hexa-
fluoroisopropylidene)diphthalic
anhydride/4,40-diamino-400-N-
carbazolyltriphenylamine (6F/CzTPA PI), with structures in which
the carbazole-tethered triphenylamine units (CzTPA) function as
the electron-donating moieties and the hexafluoroisopropylidene
phthalimide units (6FDA) serve as the electron-accepting species.
The fabricated memory device in a simple ITOjpolyimidejAu
sandwich structure exhibits two bi-directionally accessible con-
ductivity states during both the positive and negative voltage
sweeping processes with the achievement of an ON/OFF current
ratio as high as about 105, and displays no polarity except for a
trivial variation in the switching voltage. The device exhibits rather
fast response speed with a switching time less than 20 ns. More-
over, both the ON and OFF states were stable under a constant
voltage stress of ꢀ1 V and survived up to 108 read cycles with a
ꢀ1 V periodic read pulse. The IeV characteristic of the device on the
OFF state agrees fairly well with the space-charge-limited current
(SCLC) model, while it turns to the Ohmic conduction model on the
ON state. The roles of the donor and acceptor components in the
memory mechanism of the electroactive polyimide were elucidated
via molecular simulation. The strong electron-donating ability of
the carbazole-tethered triphenylamine moieties and their stoi-
chiometrically excess charge-trapping sites as compared to the
6FDA units are suggested to play an important role in realizing the
temporarily unerasable, but eventually volatile SRAM memory ef-
fect observed in the current system.
Acknowledgments
The authors thank the financial support from the National Key
Basic Research Program of China (973 Program, project No.
2014CB643600), Fundamental Research Funds for the Central Uni-
versities of China (project No. ZZ1306), National Natural Science
Foundation of China (project No. 50903006), the Specialized
Research Fund for the Doctoral Program of Higher Education (project
No. 20090010120008), and the Program for SCI@guoshi of CETV.
Appendix A. Supplementary material
Supplementary material related to this article can be found
online at http://dx.doi.org/10.1016/j.polymer.2013.12.046.
References
[1] Rath AK, Dhara K, Banerjee P, Pal AJ. Langmuir 2008;24:5937e41.
[2] Bandyopadhyay A, Pal AJ. Adv Mater 2003;15:1949e52.
[3] Stikeman A. Technol Rev 2002;105:31.
[4] Möller S, Perlov C, Jackson W, Taussig C, Forrest SR. Nature 2003;426:166e9.
[5] Hahm SG, Choi S, Hong SH, Lee TJ, Park S, Kim DM, et al. Adv Funct Mater
2008;18:3276e82.

L. Shi et al. / Polymer 55 (2014) 1150e1159
1159
[6] Wang KL, Liu YL, Lee JW, Neoh KG, Kang ET. Macromolecules 2010;43:7159e
64.
[32] Kim DM, Park S, Lee TJ, Hahm SG, Kim K, Kim JC, et al. Langmuir 2009;25:
11713e9.
[7] YouNH, ChuehCC, LiuCL, Ueda M, ChenWC. Macromolecules 2009;42:4456e63.
[8] Yang Y, Ouyang J, Ma L, Tseng RJH, Chu CW. Adv Funct Mater 2006;16:1001e14.
[9] Ling QD, Liaw DJ, Zhu C, Chan DSH, Kang ET, Neoh KG. Prog Polym Sci
2008;33:917e78.
[10] Pradhan B, Das S. Chem Mater 2008;20:1209e11.
[11] Ling QD, Kang ET, Neoh KG, Chen Y, Zhuang XD, Zhu C, et al. Appl Phys Lett
2008;92:143302.
[33] Chen CJ, Yen HJ, Chen WC, Liou GS. J Mater Chem 2012;22:14085e93.
[34] Lee TJ, Park S, Hahm SG, Kim DM, Kim K, Kim J, et al. J Phys Chem C 2009;113:
3855e61.
[35] Fang YK, Liu CL, Yang GY, Chen PC, Chen WC. Macromolecules 2011;44:2604e
12.
[36] Kuorosawa T, Chueh CC, Liu CL, Higashihara T, Ueda M, Chen WC. Macro-
molecules 2010;43:1236e44.
[12] Lai YS, Tu CH, Kwong DL, Chen JS. Appl Phys Lett 2005;87:122101.
[13] Lai YS, Tu CH, Dim-Lee K, Chen JS. IEEE Electron Device Lett 2006;27:451e3.
[14] Ling QD, Wang W, Song Y, Zhu CX, Chan DSH, Kang ET, et al. J Phys Chem B
2006;110:23995e4001.
[37] Liu YL, Wang KL, Huang GS, Zhu CX, Tok ES, Neoh KG, et al. Chem Mater
2009;21:3391e9.
[38] Kim K, Yen HJ, Ko YG, Chang CW, Kwon W, Liou GS, et al. Polymer 2012;53:
4135e44.
[15] Teo EYH, Ling QD, Song Y, Tan YP, Wang W, Kang ET, et al. Org Electron
2006;7:173e80.
[39] Yu AD, Kurosawa T, Lai YC, Higashihara T, Ueda M, Liu CL, et al. J Mater Chem
2012;22:20754e63.
[16] Tian GF, Qi SL, Chen F, Shi L, Hu WP, Wu DZ. Appl Phys Lett 2011;98:203302.
[17] Wang KL, Tseng TY, Tsai HL, Wu SC. J Polym Sci Part A Polym Chem 2008;46:
6861e71.
[18] Park S, Kim K, Kim JC, Kwon W, Kim DM, Ree M. Polymer 2011;52:2170e9.
[19] Li Y, Chu Y, Fang R, Ding S, Wang Y, Shen Y, et al. Polymer 2012;53:229e40.
[20] Tian GF, Wu DZ, Shi L, Qi SL, Wu ZP. RSC Adv 2012;2:9846e50.
[21] Tseng RJ, Huang J, Ouyang J, Kaner RB, Yang. Nano Lett 2005;5:1077e80.
[22] Lai Q, Zhu Z, Chen Y, Patil S, Wudl F. Appl Phys Lett 2006;88:133515.
[23] Qi SL, Iida H, Liu L, Irle S, Hu W, Yashima E. Angew Chem Int Ed 2013;52:
1049e53.
[40] Liu CL, Kurosawa T, Yu AD, Higashihara T, Ueda M, Chen WC. J Phys Chem C
2011;115:5930e9.
[41] Ling QD, Chang FC, Song Y, Zhu CX, Liaw DJ, Chan DSH, et al. J Am Chem Soc
2006;128:8732e3.
[42] Tian GF, Wu DZ, Qi SL, Wu ZP, Wang XD. Macromol Rapid Commun 2011;32:
384e9.
[43] Hsu JC, Chen Y, Kakuchi T, Chen WC. Macromolecules 2011;44:5168e77.
[44] Ko YG, Kwon W, Yen HJ, Chang CW, Kim DM, Kim K, et al. Macromolecules
2012;45:3749e58.
[45] Yen HJ, Chen CJ, Liou GS. Adv Funct Mater 2013. http://dx.doi.org/10.1002/
adfm.201300569.
[46] Kim DM, Ko YG, Choi JK, Kim K, Kwon W, Jung J, et al. Polymer 2012;53:1703e
10.
[24] Paul S, Kanwal A, Chhowalla M. Nanotechnology 2006;17:145e51.
[25] Ling QD, Lim SL, Song Y, Zhu CX, Chan DSH, Kang ET, et al. Langmuir 2007;23:
312e9.
[26] Liu G, Ling QD, Teo EYH, Zhu CX, Chan DSH, Neoh KG, et al. ACS Nano 2009;3:
1929e37.
[27] Hwang SK, Lee JM, Kim S, Park JS, Park HI, Ahn CW, et al. Nano Lett 2012;12:
2217e21.
[47] Chen YC, Huang GS, Hsiao CC, Chen SA. J Am Chem Soc 2006;128:8549e58.
[48] Chen F, Tian GF, Shi L, Qi SL, Wu DZ. RSC Adv 2012;2:12879e85.
[49] Djurovich PI, Mayo EI, Forrest SR, Thompson ME. Org Electron 2009;10:515e
20.
[28] Son DI, Kim TW, Shim JH, Jung JH, Lee DU, Lee JM, et al. Nano Lett 2010;10:
2441e7.
[50] Lee YZ, Chen X, Chen SA, Wei PK, Fann WS. J Am Chem Soc 2001;123:2296e
307.
[29] Zhuang XD, Chen Y, Liu G, Li PP, Zhu CX, Kang ET, et al. Adv Mater 2010;22:
1731e5.
[30] Ling QD, Liaw DJ, Teo EYH, Zhu C, Chan DSH, Kang ET, et al. Polymer 2007;48:
5182e201.
[51] Ma LP, Liu J, Yang Y. Appl Phys Lett 2002;80:2997e9.
[52] Kanwal A, Paul S, Chhowalla M. Mater Res Soc 2005;830:D7.2.1e7.2.5.
[53] Ling QD, Song Y, Lim SL, Teo EYH, Tan YP, Zhu C, et al. Angew Chem Int Ed
2006;45:2947e51.
[31] Kim K, Park S, Suk GH, Lee TJ, Kim DM, Kim JC, et al. J Phys Chem B 2009;113:
9143e50.
[54] Liu G, Ling QD, Kang ET, Neoh KG, Liaw DJ, Chang FC, et al. J Appl Phys 2007:
102. 024502.