Structural effect on the resistive switching behavior of triphenylamine-based poly(azomethine)s

Article abstract of DOI:10.1039/c4cc05233a

Linear and hyperbranched poly(azomethine)s (PAMs)-based on triphenylamine moieties are synthesized and used as the functioning layers in the Ta/PAM/Pt resistive switching memory devices. Comparably, the hyperbranched PAM with isotropic architecture and semi-crystalline nature shows enhanced memory behaviors with more uniform distribution of the HRS and LRS resistances.

Full text of DOI:10.1039/c4cc05233a

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Structural effect on the resistive switching behavior
of triphenylamine-based poly(azomethine)s†
Cite this: Chem. Commun., 2014,
0, 11496
5
ab
c
a
b
c
Wenbin Zhang,‡ Cheng Wang,‡ Gang Liu,* Jun Wang, Yu Chen* and
a
Received 8th July 2014,
Accepted 30th July 2014
Run-Wei Li*
DOI: 10.1039/c4cc05233a
www.rsc.org/chemcomm
5
Linear and hyperbranched poly(azomethine)s (PAMs)-based on tri- hyperbranched polymers. With its propeller like geometry with a
phenylamine moieties are synthesized and used as the functioning dihedral angle of around 1201 between the phenyl rings connected to
layers in the Ta/PAM/Pt resistive switching memory devices. Com- the central nitrogen atom, triphenylamine (TPA) molecule is
parably, the hyperbranched PAM with isotropic architecture and considered as an interesting building block to construct hyper-
semi-crystalline nature shows enhanced memory behaviors with branched conjugated polymers for electronic applications.
more uniform distribution of the HRS and LRS resistances.
Aromatic poly(azomethine)s (PAMs) are a class of materials
composed of azomethine (CQN) unities and benzene rings
By providing inexpensive, lightweight, optically transparent and alternatively in the backbone and possess the merits of easy
CMOS compatible modules on mechanically flexible plastic sub- molecular design and synthesis, good thermal stability, excellent
strates, polymer memories have demonstrated great potential as mechanical strength, ability to form metal chelates, liquid crystalline
6
information storage components in current consumer digital properties and non-linear optical properties. In this work,
1
gadgets. Rather than encoding ‘0’ and ‘1’ as the amount of conjugated linear and hyperbranched PAMs with triphenylamine
charges stored in a silicon cell, resistive switching memory stores chromophores in the polymer backbone have been synthesized
data in a different matter, for instance, based on the high and via comparatively convenient one step condensation polymeriza-
low resistance states (HRS and LRS) of a metal/insulator/metal tion of aldehydes and amines (see Scheme 1, Scheme S1 and
2
structure in response to the external electric field.
ESI† for the detailed synthesis and characterization), and the
Various polymers, including conjugated polymers, polymers structural effect on their memory switching behaviors have been
with pendent chromophores, and donor–acceptor polymers, comparably investigated.
have been demonstrated to have memory switching character-
istics. Similar to the other organic electronic devices, most of are chemically identical, except for the spatial arrangement of the
the polymers used in memories are linear in structure. In triphenylamine chromophores. The structures of the as-synthesized
contrast to the linear counterparts, hyperbranched polymers PAMs were confirmed by H nuclear magnetic resonance ( H NMR)
exhibit unique three dimensional structures, good solubility, and Fourier transform infrared (FTIR). The H NMR signals with the
The building blocks of the linear and hyperbranched PAMs
3
1
1
1
low melting temperature and solution viscosity, and excellent
4
physiochemical properties. More importantly, the charge carrier
transport, which was found to be anisotropic and strongly
depends on the orientation and packing mode of the molecules,
can be efficiently enhanced in the isotropic architecture of the
a
Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key
Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of
Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo,
3
15201, P. R. China. E-mail: liug@nimte.ac.cn, runweili@nimte.ac.cn
b
c
Department of Physics, Ningbo University, Ningbo 315211, China
Institute of Applied Chemistry, East China University of Science and Technology,
Shanghai, 200237, China. E-mail: chentangyu@yahoo.com
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc05233a
‡
These authors contribute equally to this work.
Scheme 1 Molecular structures of the linear and hyperbranched PAMs.
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chemical shifts of 8.1–7.9 ppm (linear PAM) and 8.2–7.9 ppm
(hyperbranched PAM) correspond to the proton resonance of the
CQN groups in respective PAMs. The signals at 7.4–7.0 ppm are
ascribed to the protons on the aromatic rings of the triphenylamine
7
À1
moieties. In the FTIR spectra, the absorption peaks at 1618 cm
À1
and 1583 cm are ascribed to the formation of CQN groups
Fig. S1 of ESI†). Due to the presence of triphenylamine chromo-
7
b
(
phores in the main chains, both the linear and hyperbranched
PAMs show good solubilities in common polar organic solvents
of tetrahydrofuran, methylene chloride, chloroform, N-methyl
7b
pyrrolidone and dimethyl formamide. The number-average
molecular weight (M ) of the linear and hyperbranched PAMs
n
3
3
are 5.1 Â 10 and 3.6 Â 10 , respectively. GPC traces of the two
PAMs are shown in Fig. S2 of ESI.† In comparison to that of the
linear PAM, the GPC trace of the hyperbranched PAM is narrower,
which implies its more uniform molecular weight distribution. The
UV-Visible absorption spectra of PAMs, which show absorption
bands of the p–p* transition of the conjugated backbone at the
shorter wavelength region and the coupling between the n–p* and
Fig. 2 The current–voltage switching and endurance performance of the
Ta/PAM/Pt memory devices fabricated with the (a, c) linear and (b, d)
hyperbranched PAMs, respectively.
p–p* transitions of the arylamine moieties at the longer wavelength rod-like structure, which probably arises from the irregular
9
region, respectively, are displayed in Fig. S3 (ESI†). Both PAMs agglomeration of the polymer chains into wool balls. On the
exhibit excellent thermal stability with the onset decomposition other hand, the hyperbranched PAM, which possesses relatively
(10% weight loss) temperatures of 470 1C and 458 1C, respectively less polymer chain folding with larger steric hindrance displays a
(Fig. S4, ESI†). The onset oxidation potentials for the linear and uniform nanofilm. Meanwhile, the surface root-mean-square
hyperbranched PAMs are 0.86 V and 0.90 V, leading to the HOMO– roughness of the hyperbranched PAM film is smaller (0.76 nm)
LUMO energy levels of À5.28 eV/À2.85 eV and À5.32 eV/À2.93 eV, than that of the linear PAM film (1.23 nm).
respectively (Fig. S5 of ESI†).
The resistive switching performances of the two PAMs are
X-ray diffraction (XRD) techniques have been used to check the demonstrated by the current–voltage characteristics of the
crystallinity of the two PAMs (Fig. 1). In good agreement with the Ta/PAM/Pt structured devices (Fig. 2). A compliance current
À3
reported literature, linear PAM is amorphous with broad diffractive of 10 A has been preset to avoid device breakdown from
5
peaks in the 2y range of 51–901. In obvious contrast, the hyper- overstriking. It is found that the forming operations are always
branched PAM is semi-crystalline with diffractive peaks at 2y = 201, necessary for both polymer devices to be set to the LRS for the
241, and 301, respectively. Transmission electron microscopic first time (Fig. S6 of ESI†). Afterwards, a negative voltage of
(
TEM) images of the linear and hyperbranched PAMs are also À0.41 V can reset the linear PAM device to HRS, while a
shown in Fig. 1c and d, respectively. Apparently, the linear PAM is subsequent positive sweep of 1.40 V can set the linear PAM
non-crystalline, while the hyperbranched counterpart shows loca- device to the LRS again (Fig. 2a). The HRS to LRS transition can
lized crystalline regions. We also investigate the topographies of be defined as the ‘‘set’’ or ‘‘write’’ process, and accordingly, the
8
the PAM films by atomic force microscopy (AFM) technique in a LRS to HRS transition is defined as the ‘‘reset’’ or ‘‘erase’’
tapping mode before depositing top electrodes. Fig. 1e and f show process. Both the HRS and LRS can be read nondestructively and
2
mm  2 mm AFM images of the linear and hyperbranched PAMs, are stable after removing the power supply, thus completing the
respectively. The linear PAM clearly shows uneven morphology of a ‘‘forming-read-erasing-rewriting’’ cycles of a non-volatile memory
2
with a ON/OFF ratio over 10 . The linear PAM device behaves
similarly after being left in air for three months, except for a
notable change in the device resistance and the switching voltages.
In comparison, the hyperbranched PAM can be set and reset with
the sweeping voltages of 1.73 V and À0.53 V, respectively, and its
resistive switching behavior can be reproduced with good accuracy
after three months (Fig. 2b).
The endurance performance of the Ta/PAM/Pt was also evaluated
in ambient atmosphere by cyclic switching operations. Fig. 2c and d
show the evolution of the linear and hyperbranched PAM device
resistances in the first 128 cycles, respectively. The resistance
values in each DC sweep were read at 0.1 V. For the Ta/linear
PAM/Pt stacks, the HRS resistance rapidly decreases while the
Fig. 1 X-ray diffractive patterns of the (a) linear and (b) hyperbranched
LRS resistance fluctuates in less than one order of magnitude.
For the Ta/hyperbranched PAM/Pt memory devices, much more
PAMs powders. TEM and AFM images of the (c, e) linear and (d, f)
hyperbranched PAM films, respectively.
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aldehyde groups and nitrogen atoms of the azomethine and
triphenylamine chromophores may act as the nucleophilic and
7b,12a
electrophilic trapping site, respectively.
When a positive voltage
with sufficient amplitude is applied, the traps will become fully filled
with holes. The subsequently injected holes from the electrode can
migrate more freely in the polymer thin film and switch the device
from HRS to LRS. With the enhanced isotropic architecture in the
hyperbranched PAM thin films, charge carrier transport becomes
more efficient and stable, which accounts for the superior switching
11e
performance in the Ta/PAM/Pt devices. When the negative reset
voltage is applied to the devices, the filled traps are detrapped to
regenerate the potential well for charge carrier hopping and switch
12a
the device back to HRS.
In summary, linear and hyperbranched PAMs with identical
chemical structure but different molecular geometries and crystal-
line qualities have been successfully synthesized via condensation
polymerization and explored for resistive switching performance.
Both polymer exhibit smaller switching voltages of À0.41 V/1.4 V
Fig. 3 Weibull exponent (k) versus standard deviation to mean ratio (D/m)
for the HRS and LRS resistances of the PAM and other reported devices.
uniform distribution of the HRS and LRS resistances are and À0.53 V/1.73 V, ON/OFF ratios over 100, endurance capability
4
observed. We also count the resistance values of HRS and LRS for more than 5000 cycles and retention time exceeding 10 s. In
in all the switching cycles. As expected, a narrow distribution of comparison, the hyperbranched PAM thin film with isotropic
R
HRS (7.5 Æ 0.2 kO) and RLRS (270 Æ 15 O) are obtained in the architecture demonstrates uniform distribution of the HRS and
Ta/hyperbranched PAM/Pt memory, while the resistance values LRS resistances, which is beneficial for practical memory device
of Ta/linear PAM/Pt exhibit relatively fluctuated distribution applications.
range of 21 Æ 7 kO (RHRS) and 250 Æ 80 O (RLRS). We have also
The authors acknowledge the financial supports from the
used Weibull analysis to further quantify the uniformity of the State Key Project of Fundamental Research of China (973 Program,
PAM devices (details can be found in ESI†). Generally, the larger 2012CB933004), National Natural Science Foundation of China
Weibull exponent (k) and smaller standard deviation (D) to mean (51303194, 11274321, 61328402, 21074034), Ningbo Science and
(
m) ratio (D/m) correspond to superior uniformity of the para- Technology Innovation Team (2011B82004), Ningbo Natural
1
0
meters under evaluation. For the Pt/linear PAM/Ta stack, the Science Foundations (2013A610031).
D/m values of the HRS and LRS resistances are 0.3200 and 0.3333,
respectively. In contrast, the HRS and LRS resistances of the Notes and references
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