Design and synthesis of conjugated polymers containing Pt(ii) complexes in the side-chain and their application in polymer memory devices

Article abstract of DOI:10.1039/c2jm16287c

We have synthesized conjugated polymers containing Pt(ii) complexes in the side-chain with different main-chains via a Suzuki coupling reaction. These polymers exhibit bistable properties and can be applied in memory devices, in which charge transfer and

Full text of DOI:10.1039/c2jm16287c

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PAPER
Design and synthesis of conjugated polymers containing Pt(II) complexes in
the side-chain and their application in polymer memory devices†
a
a
b
a
a
Peng Wang, Shu-Juan Liu, Zhen-Hua Lin, Xiao-Chen Dong, Qiang Zhao, Wen-Peng Lin,a
*
*
a
a
b
Ming-Dong Yi, Shang-Hui Ye, Chun-Xiang Zhu and Wei Huang
a
*
Received 1st December 2011, Accepted 14th March 2012
DOI: 10.1039/c2jm16287c
We have synthesized conjugated polymers containing Pt(II) complexes in the side-chain with different
main-chains via a Suzuki coupling reaction. These polymers exhibit bistable properties and can be
applied in memory devices, in which charge transfer and traps are responsible for the conductance
switching behavior. The devices could be defined as resistive random-access memory (ReRAM) with
a high ON/OFF current ratio, excellent stability and high read cycles (107). Furthermore, through the
study of the electrochemical properties and theoretical calculations of the polymers, we investigated the
significant effect of the polymer main-chain on the memory device performances. The device based on
the polymer with a polycarbazole main-chain exhibited a lower threshold voltage and a higher ON/
OFF current ratio than the device based on the polymer with a polyfluorene main-chain. Our
preliminary results indicate that this kind of material offers promising opportunities for the
development of polymer memory devices.
Hence, polymer memory materials and devices have become
Introduction
a very active research field and have attracted increasing research
interest. Most recently, many research groups have reported
various novel polymer materials for memory device applica-
tions.12–16 Their memory switching characteristics can be
assigned to metal-filament growth, space charge limited current,
nanocomposite redox, charge transfer (CT), conformational
change, ion motion, dynamic doping, etc.17–21 Furthermore,
various memory effects, such as resistive random-access memory
(ReRAM), write-once read-many-times (WORM) and dynamic
random-access memory (DRAM), have been realized success-
fully.22–24 In particular, ReRAM memory technology, namely
non-volatile and rewritable memory, is regarded as one of the
most important issues towards next generation high-density
universal memories.25
Nowadays, the most widely employed memories in information
technology are mainly fabricated from the traditional Si, Ge, and
GaAs inorganic semiconductors using complementary metal
oxide semiconductor technology. However, a number of physical
and economic factors have threatened the continuous down-
scaling of inorganic semiconductor-based memory devices.1,2
Thus, with the rapid development of modern information tech-
nology, it is very urgent to exploit new data-storage materials as
alternative or supplemental materials to traditional inorganic
materials.3,4 Polymer memories are regarded as one of the most
promising candidates for molecular-scale device applications due
to their attractive features compared with inorganic materials,
such as simplicity in structure, good scalability, stretchability and
flexibility, low-cost potential and large capacity for data
storage.5–8 In addition, compared to simple small organic mole-
cules, polymer memory materials also exhibit several merits, such
as high mechanical flexibility and ease of solution processing.9–11
Up to now, although there have been a large amount of
polymer memory materials reported, most of them are limited to
a pure organic skeleton.26–32 It is well-known that the material
structure is one of the pivotal factors in determining the
performances of memory devices. It will be very important to
develop new design principles and memory material systems.
Over the past decades, polymers containing transition-metal
complexes have gained considerable interest for the development
of functional materials with unique optoelectronic properties,
which can be tuned easily by changing the polymer backbones or
metal centers to endow them with rich properties. For example,
polymers containing Ir(III) complexes have been widely studied
and successfully applied in polymer light-emitting diodes
(PLEDs) due to their excellent phosphorescent emission.33–38
aKey Laboratory for Organic Electronics
& Information Displays
(KLOEID) and Institute of Advanced Materials (IAM), Nanjing
University of Posts & Telecommunications (NUPT), 9 Wenyuan Road,
Nanjing 210046, China. E-mail: iamsjliu@njupt.edu.cn; iamqzhao@njupt.
edu.cn; wei-huang@njupt.edu.cn; Fax: +86 25 8586 6008; Tel: +86 25
8586 6008
bNational University of Singapore, 10 Kent Ridge, 119260, Singapore
† Electronic supplementary information (ESI) available: figures of
UV-vis absorption and fluorescence spectra of Pt complex, iamP7 and
iamP8, as well as the TGA of iamP7 and iamP8. See DOI:
10.1039/c2jm16287c
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Conjugated polymers containing Pt(II) complexes, with rich
optoelectronic properties,39–41 are also a promising class of
optoelectronic materials. However, they have been rarely
exploited. For metal complexes, besides their excellent optical
properties, they also exhibit special electronic properties and
good redox reversibility, which are very beneficial for application
in memory devices. The excellent electric affinity capacity of
transition-metal complexes can improve the stability of the
conductivity states, and the reversible redox properties can make
the memory device more suitable for practical applications. In
addition, charge donors can be introduced easily by modification
of the polymer main-chain structure. Thus, polymers with on-
chain metal complexes show high potential for application in
memory devices.42,43 To the best of our knowledge, there are not
yet reports on the bistable properties and application in polymer
memory devices of conjugated polymers with Pt(II) complexes. In
this work, we synthesized a series of conjugated polymers con-
taining Pt(II) complexes in the side-chains by a Suzuki coupling
reaction. Their chemical structures are shown in Fig. 1. In
addition, their application in ReRAM memory devices with
a high ON/OFF ratio and good stability has been realized
successfully.
Theoretical calculations
All calculations were carried out with the Gaussian 03 package.44
The optimization of the complex structures was performed using
B3LYP density functional theory. The LANL2DZ basis set was
used to treat the Pt atom, and the 6-31G* was used to treat other
atoms.
Electrochemical measurements
All measurements were carried out in a one-compartment cell
under N2 gas, equipped with a glassy-carbon working electrode,
a platinum wire counter electrode, and an Ag/Ag+ reference
electrode. Measurements of oxidation and reduction were
undertaken in anhydrous solutions of CH2Cl2 and DMF,
respectively, containing 0.10 mol Lꢀ1 tetrabutylammonium
hexafluorophosphate (Bu4NPF6) as the supporting electrolyte.
The ferrocene/ferrocenium couple was added and used as the
internal standard. The scan rate was 50 mV sꢀ1
.
Materials
All reagents and chemicals were purchased from commercial
sources and used without further purification. 2-Phenylpyridine
and acetylacetone were purchased from Aldrich Chemical Co.
All other ligands and materials were purchased from either
Aldrich Chemical Co. or Acros. M3 was synthesized according
to the previous report.45
Experimental
Equipment
NMR spectra were recorded on a Bruker Ultra Shield Plus 400
MHz NMR instrument (1H, 400 MHz; 13C, 100 MHz). The gel
permeation chromatography (GPC) analysis of the polymers was
ꢀ
conducted on a Shimadzu 10 A with THF as the eluent and
poly(styrene) as the standard. The data were analyzed by using
the software package provided by Shimadzu Instruments.
Electrochemical measurements were performed with a CHI660E.
The UV-visible absorption spectra were recorded on a Shimadzu
UV-3600 UV-VIS-NIR spectrophotometer. Photoluminescent
Synthesis
Synthesis of monomer M2. KOH (2.86 g, 51 mmol) and M1
(1.98 g, 10.2 mmol) were added to ionic liquid (20 mL), and the
mixture was stirred magnetically for 5 min. Then, after 1,4-
dibromobutane (7 mL, 51 mmol) was added, the mixture was
stirred for another 5 h. Finally, the product was extracted with
Et2O (3 ꢁ 20 mL). The combined organic phases were evapo-
rated under reduced pressure and purified by column chroma-
tography on silica using ethyl acetate–petroleum ether (1 : 3) as
spectra were measured using
a
RF-5301PC spectro-
fluorophotometer. The thermogravimetric analysis (TGA) of the
polymers was performed with a WCT-2A instrument.
1
eluent to yield a colorless oil product. Yields: 60%. H NMR
(400 MHz, CDCl3) d: 7.84–7.82 (m, 1H), 7.72–7.69 (m, 2H),
7.56–7.52 (m, 3H), 7.44–7.42 (m, 1H), 7.35–7.30 (m, 2H), 4.30–
4.27 (t, 2H), 3.30–3.27 (t, 2H), 2.03–1.96 (m, 2H), 1.80–1.73 (m, 2H).
Synthesis of monomer M4. M2 (1.29 g, 4.0 mmol), M3
(4.0 mmol) and KOH (20 mmol) were added to DMSO (10 mL)
ꢂ
and the solution was stirred at 60 C for 5 h. After cooling to
room temperature, the mixture was poured into H2O, and then
extracted three times with ethyl acetate. The combined organic
layers were dried over anhydrous Na2SO4. The solvent was
evaporated under reduced pressure. The crude product was
purified by column chromatography on silica using ethyl
acetate–petroleum ether (1 : 3) as eluent to yield a white solid and
then recrystallized from dichloromethane and hexane. Yields:
72%. 1H NMR (400 MHz, CDCl3) d: 7.78–7.76 (d, 2H), 7.64–7.61
(m, 1H), 7.57–7.56 (d, 2H), 7.53–7.47 (m, 8H), 7.26–7.19 (m, 2H),
4.06–4.02 (t, 2H), 1.90–1.86 (t, 4H), 1.44–1.39 (m, 2H), 1.15–0.97
(m, 10H), 0.80–0.77 (t, 3H), 0.36–0.32 (m, 4H).
Fig. 1 Molecular structures of iamP7 and iamP8.
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Synthesis of monomer M5 and M6. A mixture of 1 equiv M4
and 1.5 equiv K2PtCl4 solution in 2-ethoxyethanol–water (3 : 1)
was heated to reflux for 24 h. After a large amount of water was
added, a large amount of yellow solid was precipitated. M5 was
obtained by filtration. The crude Pt(II) chloro-bridged dimer
(M5) was used for the next step directly. A solution of 1 equiv
M5, 3 equiv acetylacetone and 10 equiv of Na2CO3 in 2-ethoxy-
ethanol (30 mL) was heated to reflux for 16 h. The organic layer
was poured into H2O, and extracted three times with dichloro-
methane. The combined organic layers were concentrated under
reduced pressure. The obtained crude product was recrystallized
from dichloromethane and hexane. M6 was obtained as yellow
a vacuum chamber at 10ꢀ5 Torr and 50 ꢂC. The thickness of the
polymer layer was about 50 or 30 nm. A 390 nm thick aluminum
top electrode was thermally evaporated onto the polymer surface
at a pressure of around 10ꢀ7 Torr. The measurements were
carried out on devices of 0.4 ꢁ 0.4 mm2 in size in air. The devices
were characterized under ambient conditions, using a Hewlett–
Packard 4156A semiconductor parameter analyzer equipped
with an Agilent 16440A SMU/pulse generator.
Results and discussion
In this work, the copolymers iamP7 and iamP8 with a Pt(II)
complex in the side-chain were obtained by synthesizing the Pt(II)
complex monomer M6 with active groups first and then
copolymerizing with fluorene or carbazole active units (M7–
M10). The synthesis of monomer M6 with a Pt(II) complex in the
side-chain started by grafting the C^N ligand onto the C-9
position of fluorene through an alkyl chain. Next, the Pt(II)
chlorobridged dimer M5 was synthesized as shown in Scheme 1.
Then, the Pt(II) complex monomer M6 was obtained by bridge
splitting reactions of M5 and the subsequent complexation with
the diketonate ligand. Finally, the polymers iamP7 and iamP8
were readily synthesized in high yields by palladium-catalyzed
Suzuki coupling reactions using boric acid ester and dibromo
compounds as monomers (Scheme 1). Each polymer was
precipitated in methanol and purified by Soxhlet extraction with
acetone, and the polymers were characterized by 1H NMR
spectra, 13C NMR spectra and GPC analysis.
1
crystals. Yields: 55%. H NMR (400 MHz, CDCl3) d: 8.49–8.47
(d, 1H), 7.76–7.74 (d, 1H), 7.50–7.48 (d, 2H), 7.43–7.42 (d, 2H),
7.39–7.38 (d, 2H), 7.33–7.28 (m, 2H), 7.19–7.16 (t, 2H), 7.13–
7.11 (d, 1H), 7.08–7.03 (m, 1H), 5.52 (s, 1H), 4.23–4.19 (t, 2H),
2.06 (s, 4H), 2.00 (s, 6H), 1.90–1.86 (m, 2H), 1.25 (s, 2H), 1.21–
1.18 (t, 2H), 1.14–1.03 (m, 10H), 0.59–0.51 (m, 3H).
Synthesis of polymers iamP7 and iamP8. All polymers were
synthesized according to the same procedure. Herein, only the
synthesis of iamP7 is described in detail.
Polymer iamP7. A mixture of M8 (1 equiv), dibromo
compound (1 equiv), including M7 and M6, a small amount of
tetrabutylammonium bromide as the phase transfer catalyzer
and 2.0 mol% [Pd(PPh3)4] as the main catalyst was first added to
a round flask, and then a degassed mixture of toluene and
aqueous 2 M potassium carbonateꢂ(3 : 2 v/v) was injected under
N2. The mixture was stirred at 90 C for 72 h and then bromo-
benzene was added. After cooling to room temperature, the
mixture was washed with water and extracted with CH2Cl2. The
combined organic phase was concentrated and then added
dropwise to a mixture of methanol and deionized water (10 : 1 v/
v). A fibrous yellow–green solid appeared and was obtained by
filtration. The solid was dissolved in THF and then concentrated.
The solution obtained was dropped slowly into methanol again.
The fibrous yellow–green solid was filtered and then washed with
acetone in a Soxhlet apparatus for 3 days. Finally, the resulting
polymers were collected and dried under vacuum. Yields: 52%.
1H NMR (400 MHz, CDCl3) d: 7.83–7.49 (m, 6H), 2.27–1.88 (m,
4H), 1.25–0.71 (m, 30H); 13C NMR (100 MHz, CDCl3) d: 151.84,
140.52, 130.71, 126.19, 121.51, 120.00, 55.37, 40.40, 31.82, 30.07,
29.25, 23.94, 22.63, 14.10. Mn ¼ 8700, Mw ¼ 16 500 and PDI ¼
1.91.
The UV-visible absorption and photoluminescence spectra of
the Pt(II) complex, iamP7 and iamP8 were recorded in the solid
state (Fig. S1†). From these spectra, the characteristic bands of
the Pt(II) complex (M6) can be observed, indicating that the Pt(II)
complex has been successfully introduced into the polymer chain.
The thermal properties of the polymers were investigated by
TGA (Fig. S2†). The 5% weight-loss temperatures of both iamP7
and iamP8 were higher than 300 ꢂC, showing their good thermal
stability.
The electrochemical properties of the polymers were investi-
gated using cyclic voltammetry (Fig. 2). All of the electro-
chemical potentials were measured relative to an internal
ferrocene reference (Fc/Fc+). The EOx(onset) and ERed(onset) for
iamP7 are +0.96 and ꢀ2.61 V vs. Ag/Ag+, and those for iamP8
are +0.43 and ꢀ2.8 V vs. Ag/Ag+, respectively. The highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels can be deduced by the
equation EHOMO(LUMO) ¼ ꢀ(4.80 + E(onset)). Thus, the HOMO
and LUMO energy levels of iamP7 and iamP8 relative to the
vacuum level are estimated to be ꢀ5.76 eV, ꢀ2.16 eV and ꢀ5.18 eV,
ꢀ1.98 eV, respectively. The results suggest that iamP8 exhibits
a higher tendency than iamP7 to donate electrons.
1
Polymer iamP8. 65% yield. H NMR (400 MHz, CDCl3) d:
8.53–8.28 (m, 2H), 7.86–7.67 (m, 2H), 7.53–7.42 (m, 2H), 4.36–
4.18 (m, 2H), 1.96–1.70 (m, 2H), 1.45–1.16 (m, 2H), 0.93–0.74
(m, 2H). 13C NMR (100 MHz, CDCl3) d: 139.98, 133.213, 125.44,
123.70, 118.91, 109.03, 43.29, 31.63, 29.09, 27.02, 22.60, 14.08,
1.06. Mn ¼ 7100, Mw ¼ 10 400 and PDI ¼ 1.46.
For the two polymers, the polyfluorene and polycarbazole
main-chains can act as electron donors and the Pt(II) complexes
as electron acceptors. It is possible to form a CT state in two
polymers upon light or electrical excitation. Hence, their bistable
properties and application in electronic memory devices were
investigated. The devices were fabricated with an ITO/polymer
(50 nm)/Al sandwich structure as shown in Fig. 3.46–48 The top Al
electrode and bottom ITO electrode were connected to a varying
voltage source. A bias was applied to the ITO electrode with
Fabrication and characterization of polymer memory devices
The indium-tin-oxide glass substrate was precleaned with water,
acetone, and then 2-propanol in an ultrasonic bath (15 min each).
A toluene solution of iamP7 or iamP8 (10 mg mLꢀ1) was spin-
coated onto the ITO substrate, followed by solvent removal in
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Scheme 1 Synthetic routes for the monomers and polymers.
respect to the top Al electrode. The bottom ITO was grounded.
The current passing through the electrodes was monitored. The
memory capabilities of iamP7 were demonstrated by the current–
voltage (I–V) characteristics of the device, as shown in Fig. 4a. In
the first sweep from 0 to ꢀ5.0 V, the current was increased
progressively. When the voltage arrived at about ꢀ2.6 V, a sharp
increase in the injection current was observed, indicating that the
device switched from the ‘‘0’’ state (OFF state) to the ‘‘1’’ state
(ON state). This process was equivalent to a ‘‘writing’’ process.
The device exhibited good stability in the high-conductivity state
during the second sweep from 0 to ꢀ5.0 V and this process is
equivalent to a ‘‘read’’ process. In the third sweep from 0 to 5.0 V,
the device returned to the OFF state at about 3.0 V and this
process is equivalent to an ‘‘erasing’’ process. The OFF state can
also be read in the fourth sweep. Thus, the ‘‘write–read–erase–
read’’ cycle was completed and the behavior of the ITO/iamP7/Al
device shares some common characteristics with those of
ReRAM memory. In addition, an ON/OFF current ratio of
about 103 has been achieved in this memory device. In addition,
the device is stable at both ON and OFF states under a constant
stress of ꢀ1.0 V during the test as depicted in Fig. 4b. No
degradation in the current density was observed after more than
107 read cycles for both the ON and OFF states as depicted in
Fig. 4c. Thus, iamP7 exhibits excellent memory effects. The
memory performances of the device based on a different thick-
ness (30 nm) of the polymer iamP7 was also investigated
Fig. 2 Cyclic voltammogram of iamP7 and iamP8.
Fig. 3 A schematic diagram of the memory device.
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Fig. 4 Typical I–V characteristics (a) of the ITO/iamP7 (50 nm)/Al device in the ON and OFF states, effect of read cycles (b) on the ON and OFF states
and stability (c) of the ITO/iamP7 (50 nm)/Al device in either the ON or OFF state under a constant stress (ꢀ1 V).
(Fig. S3†). In the first sweep from 0 to ꢀ5.0 V, a switch-on
voltage at about ꢀ2.5 V was observed. The device was stable in
the ON state during the second sweep from 0 to ꢀ5.0 V. In the
third sweep from 0 to 5.0 V, a switch-off voltage at about 5 V was
observed. The OFF state can be read in the fourth sweep. Thus,
this device also behaved as a ReRAM, and the threshold voltages
were tuned by changing the thickness of the iamP7 layer. Because
the resistance of the active layer increases with decreasing
thickness, the currents for the ON state and the OFF state were
only about 10ꢀ3 and 10ꢀ6 A, respectively. The ON/OFF current
ratio of this device was about 103.
were studied by density functional theory (DFT) calculations. As
shown in Fig. 6a, the calculated HOMO and LUMO+1 iso-
surfaces for the model oligomer of iamP7 are located on the
fluorene moieties and the Pt(II) complexes, respectively, indi-
cating that a CT state from the polyfluorene main-chain to the
Pt(II) complex side-chain may be formed through the HOMO /
LUMO+1 transition. In addition, it can be seen that a channel is
formed from the molecular surface throughout the polymer
main-chain and the Pt(II) complex side-chain with continuous
molecular electrostatic potential (ESP, Fig. 6b), which is very
beneficial for the migration of charge carriers along the polymer
main-chain and between the main-chain and side-chain.
However, negative ESP regions (blue region) located at the Pt
center can be observed, further demonstrating that the Pt(II)
complexes can act as ‘‘traps’’ for charge carriers.51 Thus, the
memory mechanism may be assigned to the formation and
dissociation of a CT state induced by negative and positive
voltages (Fig. 7), respectively. The formation of a CT state is
responsible for the ON state. The application of an electric field
above the threshold value leads to charge transfer from the
polymer main-chain to the Pt(II) complex units on the side-chain.
Even after the driving power is turned off, the ON state can still
remain due to the stability of the CT complex. However,
a reverse bias of voltage applied to the polymer can dissociate the
CT state and the device returns to the original OFF state.
The CT state is determined by the charge donor and acceptor
groups.52 Therefore, we changed the polymer main-chain from
polyfluorene to polycarbazole to investigate the influence of the
polymer main-chain (as charge donor) on the performances of
device. Thus, iamP8 with polycarbazole as the polymer-main was
synthesized and the current–voltage characteristics of the ITO/
iamP8 (50 nm)/Al device were shown in Fig. 8a. Similar to
iamP7, the I–V curve of the ITO/iamP8/Al device also displayed
two conductivity states. In the first sweep from 0 to ꢀ5.0 V, an
abrupt increase in the current was observed at about ꢀ1.6 V. The
device exhibited good stability in the high-conductivity state,
which was proven during the second sweep from 0 to ꢀ5.0 V. The
‘‘erase’’ process was observed in the third sweep from 0 to 5.0 V,
and the device switched from the ON state to the OFF state at
about 2.88 V. The device remained in the OFF state during the
fourth sweep from 0 to 5.0 V. Thus, the device based on iamP8
behaved as a ReRAM. An ON/OFF current ratio of more than
103 was achieved in this memory device. In addition, the device
In order to investigate the memory mechanism of iamP7, the
redox properties of iamP7 were investigated first by cyclic vol-
tammetry (CV) as shown in Fig. 2. According to the redox onset
potentials of the CV measurements, the calculated HOMO and
LUMO energy levels are ꢀ5.76 and ꢀ2.16 eV, respectively.49
Thus, the lowest energy barrier between the work function (F) of
the ITO anode and the HOMO level of iamP7 is only 0.96 eV,
which is lower than the energy barrier between F of Al and the
LUMO of iamP7 (2.14 eV), suggesting that hole injection from
the ITO anode into the HOMO level of iamP7 (corresponding to
a negative bias) is a favored process and holes dominate the
conducting process (Fig. 5).50
In order to better understand the mechanism of the memory
device, the electronic properties of the model oligomer of iamP7
Fig. 5 Molecular simulation of the HOMO and LUMO energy levels for
iamP7 and iamP8 along with the work function of the electrodes.
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Fig. 6 Calculated HOMO, LUMO and LUMO+1 of model oligomers of iamP7 (a) and ESP surfaces of iamP7 (b).
HOMO of iamP8 is 0.38 eV, which is lower than the energy
barrier between F of Al and the LUMO of iamP8 (2.12 eV),
suggesting that hole injection from the ITO anode into the
HOMO level of iamP8 (corresponding to a negative bias) is
a favored process and holes dominate the conduction process.
In addition, the electronic properties of iamP8 were also
studied by DFT calculations as shown in Fig. 9. Similar to
iamP7, the calculated HOMO and LUMO+1 isosurfaces for the
model oligomer of iamP8 are located on the carbazole moieties
and the Pt(II) complexes, respectively, and negative ESP regions
(blue region) located at the Pt(II) center can be observed. Hence,
the formation and dissociation of a CT state induced by certain
voltages may be responsible for the memory behaviors of iamP8.
The device is switched from the OFF state to the ON state under
an electric field. Even after the driving power is turned off, the
memory device can steadily remain in the ON state until a reverse
voltage is applied.
It is noteworthy that the main-chain structures show great
influence on the threshold voltages of the devices. The switch-on
and switch-off voltages of the devices using iamP8 as active
materials (ꢀ1.6 and 2.88 V, respectively) are significantly lower
than those of the device based on iamP7 (ꢀ2.6 and 3.0 V,
respectively). Due to the same charge acceptor (Pt complex
moieties) for iamP7 and iamP8, their different threshold voltages
can be assigned to the difference in charge donors, namely poly-
fluorene for iamP7 and polycarbazole for iamP8. The lower
oxidation wave of polycarbazole (0.43 V) than polyfluorene
(0.96 V) made the charge transfer easier for iamP8 than iamP7 as
shown in Fig. 2. The lower energy barrier between the F of
the ITO anode and the HOMO level of iamP8 (0.38 eV) than the
counterpart of iamP7 (0.96 eV), as shown in Fig. 5, made the
Fig. 7 Memory mechanism of the device.
also displayed both stable ON and OFF state currents under
a constant stress of ꢀ1.0 V during the test, as shown in Fig. 8b.
No degradation in the current density was observed after more
than 107 read cycles for two states as depicted in Fig. 8c. Hence,
iamP8 also exhibits good memory effects. According to the redox
onset potentials of the CV measurements, the calculated HOMO
and LUMO energy levels are ꢀ5.18 and ꢀ1.98 eV, respectively.
Thus, the energy barrier between F of the ITO anode and the
Fig. 8 Typical I–V characteristics (a) of the ITO/iamP8 (50 nm)/Al device in the ON and OFF states, effect of read cycles on the ON and OFF states (b)
and stability (c) of the ITO/iamP8 (50 nm)/Al device in either ON or OFF states under a constant stress (ꢀ1 V).
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Fig. 9 Calculated HOMO, LUMO and LUMO+1 of the model oligomer of iamP8 (a) and ESP surfaces of iamP8 (b).
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than that of iamP7 (z103). Clearly, iamP8 exhibits a better
overall device performance than iamP7. Thus, our results
demonstrated that the memory behaviors of polymers containing
metal complexes can be optimized through the easy modification
of the polymer structures.
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In summary, we have synthesized two novel conjugated polymers
containing Pt(II) complexes on the side-chain, and studied the
current–voltage characteristics of the memory devices based on
them. The two devices exhibit the functionality of ReRAM
memory with a high ON/OFF current ratio and excellent
stability. In addition, the threshold voltage can be tuned by
changing the chemical structures of the polymer main-chains.
The excellent electron affinity and redox reversibility of the metal
complex moieties are responsible for the stability of the device.
Our research demonstrated that polymers containing metal
complexes exhibit some advantages for application in memory
devices, such as ultra-long retention times and endurance. As far
as we know, this is the first report about the memory behaviors of
polymers containing Pt(II) complexes. We believe that this work
will be very helpful for the further design and development of
excellent memory materials based on polymers containing metal
complexes.
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This work was financially supported by the National Basic
Research Program of China (973 Program, 2009CB930601 and
2012CB933300), the National Natural Science Foundation of
China (Project No. 21174064, 21171098 and 61076067), the
Natural Science Fund for Colleges and Universities in Jiangsu
Province (10KJB430010), the Ministry of Education of China
(No. IRT1148), A Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions,
and Nanjing University of Posts and Telecommunications
(NY210029 and NY211003).
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