Surface Engineering of ITO Substrates to Improve the Memory Performance of an Asymmetric Conjugated Molecule with a Side Chain

Article abstract of DOI:10.1002/asia.201700706

Organic multilevel random resistive access memory (RRAM) devices with an electrode/organic layer/electrode sandwich-like structure suffer from poor reproducibility, such as low effective ternary device yields and a wide threshold voltage distribution, and improvements through organic material renovation are rather limited. In contrast, engineering of the electrode surfaces rather than molecule design has been demonstrated to boost the performance of organic electronics effectively. Herein, we introduce surface engineering into organic multilevel RRAMs to enhance their ternary memory performance. A new asymmetric conjugated molecule composed of phenothiazine and malononitrile with a side chain (PTZ-PTZO-CN) was fabricated in an indium tin oxide (ITO)/PTZ-PTZO-CN/Al sandwich-like memory device. Modification of the ITO substrate with a phosphonic acid (PA) prior to device fabrication increased the ternary device yield (the ratio of effective ternary device) and narrowed the threshold voltage distribution. The crystallinity analysis revealed that PTZ-PTZO-CN grown on untreated ITO crystallized into two phases. After the surface engineering of ITO, this crystalline ambiguity was eliminated and a sole crystal phase was obtained that was the same as in the powder state. The unified crystal structure and improved grain mosaicity resulted in a lower threshold voltage and, therefore, a higher ternary device yield. Our result demonstrated that PA modification also improved the memory performance of an asymmetric conjugated molecule with a side chain.

Full text of DOI:10.1002/asia.201700706

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Surface engineering of ITO Substrate to improve memory
performance of asymmetric conjugated molecule with a side
chain
Authors: Xiang Hou, Xue-Feng Cheng, Xin Xiao, Jing-Hui He, Qing-
Feng Xu, Hua Li, Na-Jun Li, Dong-Yun Chen, and Jian-Mei Lu
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201700706
Link to VoR: http://dx.doi.org/10.1002/asia.201700706
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Chemistry - An Asian Journal
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Surface engineering of ITO Substrate to improve memory
performance of asymmetric conjugated molecule with a side
chain
Xiang Hou, Xue-Feng Cheng, Xin Xiao, [a] Jing-Hui He, * Qing-Feng Xu, Hua Li, [a] Na-Jun Li, [a]
[
a]
[a]
[a]
[a]
Dong-Yun Chen [a] and Jian-Mei Lu*[a]
Abstract: Organic multilevel random resistive access memories
RRAMs) of electrode/organic layer/electrode sandwich-like structure
structure and three-dimensional stackability, which are all able to
turbo the information density.3 In addition, RRAMs are able to
store more than two states per cell (for example, three states),
leading to the multilevel (ternary) RRAM, which can further
significantly increase the capacity.
(
suffer from poor reproducibility such as low effective ternary device
yield and wide threshold voltage distribution, and the improvement
by organic material renovation is rather limited. In contrast, surface
engineering of the electrode surfaces rather than molecule design
was demonstrated to effectively boost the performance of organic
electronics. In this work, we introduce surface engineering into
organic multilevel RRAMs to enhance their ternary memory
performance. A new asymmetric conjugated molecule containing
phenothiazine and malononitrile with a side chain (PTZ-PTZO-CN)
was fabricated into indium tin oxide (ITO)/ PTZ-PTZO-CN/Al
sandwich-like memory device. Modifying the ITO substrate by
phosphonic acids (PA) prior to device fabrication increases the
ternary device yield (the ratio of effective ternary device) and
narrows the threshold voltage distribution. The crystallinity analysis
reveals that PTZ-PTZO-CN growing on untreated ITO crystalize into
two phases. This crystalline ambiguity was eliminated and the sole
crystal phase was obtained as same as in the powder state after
surface engineering of ITO. The unified crystal structure and
improved grain mosaicity result in the lower threshold voltage and
thus higher ternary device yield. Our result demonstrated that PA-
modification has also improvement on the memory performance
based on the asymmetric conjugated molecule with a side chain.
Inspired by these advantages, various inorganic, organic,1a, 5
4
6
7
hybrid and bio- material systems were exploited as the active
materials of multilevel RRAMs. Among them, small conjugated
molecule8 has garnered scientific interest over the past few
years because of their low weights, flexibility, handy process and
structural tailorability. 3, 9 Several important parameters of the
devices such as the ternary device yield (the ratio of effective
ternary device among a batch of memory cells), threshold
voltage, ON/OFF current ratios could be partially improved
through molecular design by adjusting the terminal electron
acceptors,10 conjugation length,3b,11 planarity,12 and side chains13.
However, recent reviews commented that such improvement is
rather limited and not rationalized. 3a Therefore, other strategies
rather than molecular design to improve RRAM performance are
highly desirable.
Indium tin oxide (ITO) glass is widely used as the bottom
electrode of organic electronics, including RRAM, organic light
emitting diodes (OLEDs) and organic photovoltaics (OPVs).
Both surface energy matching and work function compatibility of
ITO have important roles in OLED and OPV performance after
modification.14 Recently, we demonstrated that surface
engineering of ITO substrates by phosphonic acids (PA) prior to
fabrication of organic ternary RRAMs produced the highest
ternary device yield and much better reproducibility, because the
deposited squaraine molecule (SA-Bu) with symmetrical butyl
Introduction
In recent years, the explosive growth of information urges new
information storage techniques to catch up the increasing
requirement of information density. As a new storage technique,
random resistive access memories (RRAM) work on the
resistance change of dielectric solid-state material switched by
external voltages. RRAMs has faster timescales (switching
tail/rigid core/butyl tail crystalize more orderly than on the
untreated ITO.15 To verify the general applicability of PA-
modification, conjugated molecules with asymmetric side chain
are worthy of testing.
In this paper,
a
new conjugated molecule containing
1
2
time< 10 ns), low power consumption, a simple and small cell
phenothiazine and malononitrile moiety with asymmetric side
chain, namely 2-((7-(4-(10H-phenothiazin-10-yl) phenyl)-10-
octyl-5-oxido-10H-phenothiazin-3-yl)
methylene)
[
a]
X. Hou, X.-F.Cheng, X. Xiao, Prof. J.-H. He, Prof. Q.-F.Xu, Prof. H.
Li, Prof. N.-J. Li, Prof. D.-Y.Chen, Prof. J.-M. Lu
College of Chemistry, Chemical Engineering and Materials Science
Collaborative Innovation Center of Suzhou Nano Science and
Technology Institution
malononitrile(PTZ-PTZO-CN) was designed, synthesized and
fabricated into RRAMs on PA-modified ITO. This T-shape
molecule has a large conjugated plane ended a loose alkyl chain
at the center position (Figure 2b). Meanwhile, its malononitrile
and sulfoxide groups are demonstrated to promising candidates
Soochow University
Suzhou 215123 (P.R.China)
E-mail: jinghhe@suda.edu.cn
for memory applications due to their outstanding properties as
electron accepters.16 Our result demonstrated that PA-
lujm@suda.du.cn
modification also has improvement on the memory performance
based on conjugated molecule with asymmetric side chain.
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
Figure 1. (a) The process of surface modification by phosphonic acids on the surface of ITO glass; (b), (c), (d) The wettability of the test in air; (e) The XPS
spectra of P(2p) of PA-modified ITO substrates.
We modified the ITO substrate by PAs self-assembly as shown
in Figure 1a. To obtain the highest surface coverages of
phosphonic acids on ITO surfaces, the tethering by aggregation
and growth procedure (T-BAG)17 was conducted. Two types of
devices. Synthesis of PTZ-PTZO-CN was on the basis of the
method as described in literature.18 The molecular structure of
PTZ-PTZO-CN is shown in Figure 2b and the detailed synthesis
and characterization of PTZ-PTZO-CN are described in the
Experimental Section as well as in Figure S2. NMR (Figures S3
and S4) and HRMS spectra were measured to ensure that the
target molecule is obtained.
ITO
substrates
were
obtained
by
reacting
with
phenylphosphonic acid and octylphosphonic acid, named ITO-P,
ITO-O, respectively. The surface of the ITO consists of indium
tin oxide, hydroxyl and carbonyl, which showed well wettability
The sandwich-structured organic memory devices were
fabricated through spin-coating of PTZ-PTZO-CN solutions onto
ITO substrates (ITO-O, ITO-P and original ITO), followed by
thermal deposition of Al electrodes as shown in Figure 2a. The
film thickness of the organic active layer and Al electrode are
both about 100 nm (Figure 2c). All the memory devices exhibit
binary and ternary Write-Once-Read-Many Times (WORM)
memory behaviors. The typical electrical switching and memory
effects of the binary (Figure 2d) and ternary devices (Figure 2e)
were revealed respectively by I-V characteristics. A voltage
sweep was applied from 0 to -6.0 V, and the compliance current
was set at 0.1 A. For the binary property, the device was initially
in the low conductive (“0” or OFF) state. The current of the
device increases slowly as the applied voltage increased, until
the voltage reaches -1.85 V and the current abruptly jumps to
10-2 A, which indicated the device transition from the low
conductive (“0” or OFF) state to the high conductive (“1” or ON)
state. Then the device retained its high conductive state in the
subsequent forward and reverse biased sweeps with bias up to
±6V, suggesting the devices showing the binary WORM electric
memory characteristics (Figure 2d).
o
for the hydrogen bond. The contact angle of ITO is about 18.3 ,
which corresponds well to the publication. After PAs treatment,
the wettability of ITO-P and ITO-O appears 67.4o and 87.3,
respectively (Figures 1c and 1d). This result confirms that the
surfaces of ITO-P and ITO-O present a typical hydrophobic
property for the octyl group and phenyl on the surface after
modification. Hence the change of wettability of these surfaces
proves the successful modification of ITO surfaces.
In order to confirm that PAs have been successfully modified to
the surface onto substrate, XPS and IR characterization are
conducted. The XPS P2p spectra of ITO-P, ITO-O and original
ITO are displayed in Figure 1e. For ITO-P and ITO-O, the P2p
peaks could be appears at about 133.2eV, whereas no distinct
feature of the original ITO is observed in this range. This peak
position corresponds to the oxidation state of P(V) and in good
agreement with the previous study,15, 17b confirming that the
existence of PAs(Figure 1a). In addition, the IR spectrum of
ITO-O (Figure S1) showed υantisym CH
2
=2926 cm-1 (indicative of
-1
-1
alkyl chain) and υ2sym CH = 2851 cm . 3007 cm , 1600~1450
cm-1 (1604, 1512 and 1449 cm ), 768 cm , 702 cm could be
attributed to the characteristic vibration peaks of the benzene
ring. These modes strongly imply the existence of PAs.17
-1
-1
-1
Rather than one jump, two abrupt jumps were observed at -1.1
and -2.7 V on a ternary device (in Figure 2e), when the voltage
was sweated from 0 to -6 V to. The current ratio of these three
Next, a new conjugated molecule with a side chain, 2-((7-(4-
2
5
(
10H-phenothiazin-10-yl)
phenyl)-10-octyl-5-oxido-
10H-
states is 1:10 :10 , indicating a transition from the low-
phenothiazin-3-yl) methylene) malononitrile (PTZ-PTZO-CN)
was synthesized and used as the active material in our RRAM
conductive (OFF) state to an intermediate-conductive (ON1)
state, then to a highly conductive (ON2) state. These OFF to
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ON1 and ON1 to ON2 transitions could be considered as
the ternary WORM electric memory characteristics, of potential
application in archive storage.3a, 5b, 19 For the devices with
WORM property, the stability of the devices was evaluated. The
retention time tests were performed under a constant stress of –
0.8 V for the ON or OFF states of the binary devices (Figure 2f)
and the ON2, ON1 or OFF states of the ternary devices (Figure
2g). The ON/OFF current ratios could be maintained and no
obvious degradation in current was observed after a testing
period of 6000 s.
“
writing” processes in memory operation (Sweep 1). The ON2
state was also maintainable during negative sweeps from 0 to -
.0 V (Sweep 2) and positive sweeps from 0 to 6.0 V (Sweep 3).
6
When a lower negative voltage was applied from 0 to -3 V to
another cell of the device, the OFF-to-ON1 transition occurs at
about -1.1V (Sweep 4). Furthermore, the ON1-to-ON2 transition
occurs in the same cell at -2.7 V by Sweep 5. Finally, this ON2
state could be sustainable during the negative and positive
sweeps (Sweeps 6 and 7), which indicate the devices showing
Figure 2. (a) The Diagram of the prototype sandwich device; (b) Molecular structure of PTZ-PTZO-CN; (c) SEM images of a cross-section view of the fabricated
RRAM devices; Typical Current-voltage (I–V) characteristics of the binary devices(d) and ternary devices(e). Characteristic retention stability of PTZ-PTZO-CN
based memory devices ((f) binary and (g) ternary) measured under a constant stress of -0.8 V at room temperature.
Although both binary and ternary memory switching are
observable on all three types of ITO substrates, the ratio of
effective ternary device (ternary device yield) and threshold
voltages are improved by PA-modification. As shown in Figure 3,
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1
50 cells were tested and the ternary yield on original ITO is as
low as 28%. After surface modification, the ratios raise to 38%
for ITO-P) and 40% (for ITO-O), respectively.
According to the cyclic voltammetry (CV) spectra (Figure S6b),
the HOMO/LUMO values were determined to be -5.02 /-2.97 eV.
The UV-visible absorption spectra were conducted to study the
optical properties of PTZ-PTZO-CN in solution and thin film
(Figure S6a). The maximum absorption peaks of the
(
The switching threshold voltages (Vths) distributions also
reflected the improvement of the device performance after
modification. The averaged first (Vth1) and second threshold
voltages (Vth2) of original-ITO devices are -3.40 V and -4.80 V,
respectively. After modification, both Vth1 and Vth2 of the devices
decrease progressively (-2.70/-4.80 V for ITO-P and -2.40/-3.60
V for ITO-O), as shown in Figure 3. Moreover, the distribution
histograms of the Vth1 and Vth2 were shown in Figure S5.
Compared to ITO and ITO-P, the Vth1 of the devices based on
ITO-O showed
reproducibility, which is a critical parameter for future application
of memory devices.
The above memory performance could be understood firstly
from the unique electronic properties of PTZ-PTZO-CN.
2 2
compounds in CH Cl and in film are at 424 nm and 442 nm,
respectively. The peak in the solution could be assigned to the
π−π* transition within the molecular backbone.20 In comparison,
the absorption peak in the film state shows shoulder peaks and
a 18 nm bathochromic shift, which indicates that the molecules
adopted a more regular arrangement or aggregation in the film,21
favoring the charge carrier transport between neighboring
molecules in the film. In addition, the onset of UV-absorbance is
604 nm, corresponding to an optical band gap of 2.05 eV. The
small band gap and proper HOMO/LUMO position, allowing
facile charge injection and carrier transport, which are necessary
condition for memory switching.
a
narrower distribution, implying better
Figure 3. The ternary yield and switching threshold voltages states of memory devices based on three types of substrates.
The intermolecular interaction partially revealed by the above
UV-vis spectra is believed to be a key factor to the memory
performance, which has been effectively tuned through PA-
modification in this work. Figures 4 and S7 show the X-ray
diffraction patterns of the molecular powder and all three films
on ITO, ITO-P and ITO-O. In the powder state, there are several
intensive features as well as their higher order diffractions, such
or three distinct peaks, indicating the crystalline preference in
several directions in the films induced by the substrate (Figures
4a and 4b). Particularly, on ITO substrate, three distinct peaks
are observable (Figure 4b). The first two peaks at 2θ = 5.02°
(d=17.61 Å) and its second order position 10.04° (d=8.81Å)
match well with the powder’s pattern. However, the last peak at
11.67° (d=7.58 Å) is not found from the powder. On ITO-P
substrate, these three peaks also exist, but the last peak at
11.67° become much weaker. In contrast, the film on ITO-O
substrate has left only the first two features without the third
peak. This discrepancy strong indicates that the third, which is
as
the
peak
at
2θ=5.00/10.00/15.25/20.06°
(
d=17.69/8.89/5.81/4.43 Å), 2θ=8.27/16.58° (d=10.68/5.39 Å)
and 2θ=12.11/24.38° (d=7.31/3.65 Å). Compared to the powder
state, most of peaks fade away and all three films only show two
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not found in the power state, is from an exotic crystal structure
different from the powder.
Figure 4. (a) XRD patterns of PTZ-PTZO-CN powder; (b) XRD patterns of PTZ-PTZO-CN films on ITO substrates, ITO-P substrates and ITO-O substrates.
Figure 5. (a), (b), (c) GISAXS patterns and the illustration of the PTZ-PTZO-CN film ((d), (e) and (f)) on ITO, ITO-P and ITO-O substrates, respectively.
We employ grazing-incidence small-angle X-ray scattering
GISAXS) analysis to resolve more details about these two
crystalline phases which may be blurred in the small angle range
(d~17.9 Å) is much lower (in Figures 5b and 5e). This peak
(
finally disappears on ITO-O substrate, leaving only the first and
-1
z
third peaks with d=19.3/9.3 Å (q =0.33/0.68 Å ) despite of trivial
in XRD patterns. In Figures 5a, 5b, 5c and S8, short arcs sit on
discrepancies. Since the first and second lattice spacing (19.8
and 17.9 Å) in GISAXS pattern, are quite close and larger than
the length of the molecule (about 17.3 Å in Figure S9), it is
further proved that these two spacing unlikely originate from the
same crystal phase.
In short, the films on ITO and ITO-P substrates have one more
arc at (d~17.9 Å) in GISAXS an exotic XRD peak at 2θ=11.4°
(d=7.75 Å) than on ITO-P substrate or the powder. The plausible
explanation is that two crystal phases (structures) of the only
molecule exist in the films on ITO and ITO-P substrate, while the
z
the q axis, confirming that all grains prefer to aggregate in an
out-of-plane preference, in accordance with our interpretation on
2
2
XRD pattern. On original ITO, three short arcs were shown in
Figures 5a and 5d, implying spacing distances of 19.8/17.95/9.5
-1
Å (q
first two peaks in XRD (19.25 Å/9.17 Å), while the first two arcs
19.8/17.95 Å) are in fact indistinguishable and merge into one
z
= 0.32/0.35/0.66 Å ). These three arcs correspond to the
(
peak at d=19.25 Å in XRD. Similarly, the film on ITO-P substrate
also has three arcs. However, the intensity of the second arcs
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similarly two crystal phases were also found in our previous
study15 (Figure 5d). After modification by phenylphosphonic acid,
the minor phase was gradually suppressed (Figure 5e) and
octylphosphonic acid modification can totally eliminate the minor
crystal phase (Figure 5f). Therefore, the degree of crystallite
ordering is promoted by unifying the crystal phases from two to
one, and to highly ordered, lamellar alignment. We suspected
that the exotic crystal phase originates from a few layer on the
ITO surface, which need to be verified in future but out of scope
of this work.
Materials
Octylphosphonic acid, phenylphosphonic acid, phenothiazine, p-
bromoiodobenzene, bis(pinacolato)diboron, 1-bromooctane, 2-
Pd(dppf)Cl and Terttrakispalladium were all purchased from commercial
sources (TCI, Alfa Aesar, and Sigma-Aldrich). 1,4- dioxane, ethanol and
acetone were all purchased from Sinopharm Chemical Reagent Co., Ltd.,
and used as received without further purification.
2
Synthesis
For organic ternary memory, charge-trapping mechanism is
widely invoked to explain the resistance switching. In the
previous work, we adapted the charge-trapping mechanism23 to
considering the hopping manner of charge transportation in
The detailed synthesis steps of PTZ-PTZO-CN are described in
supporting information.
Modification of ITO Surface with Phosphonic Acids
24
organic film. Briefly, a charge in organic film will polarize the
surrounding molecules due to the small dielectric coefficient of
organic materials. The surrounding will be more conductive, and
in turn allowing more charge to be injected in a positive
The ITO glasses of 2 cm × 2 cm size were washed with deionized water,
acetone and ethanol in ultrasonic 20 min, respectively. Then, these
glasses were dried with nitrogen gun. We use the following modification
steps: ITO glasses were placed horizontally on the sample holder,
immersed in 1 mmol / L phosphonic acids /tetrahydrofuran solution. The
surface of ITO glasses was submerged completely and placed in flowing
nitrogen. The solvent was evaporated, until the solution level to drop
below the upper surface of ITO in 30 °C. The treated ITO glasses were
cleaned with ethanol in ultrasonic and then annealed to 80 °C in nitrogen.
After 48 h, ITO glasses were washed with anhydrous ethanol,
trimethylamine/ethanol (5% by volume) and ethanol in ultrasonic 20 min,
respectively.
8
a, 24
feedback effect.
This finally creates a conductive channel
and cause a resistance abrupt switch. Caused by the positive
feedback effect, these channels generally are filamentary and
appear with random radius and locations, rather a uniform
dispersion on the cross-section as large as the electrode area.
The threshold voltages for onset these channels are not an
identical value, but of a certain distribution. Nevertheless, more
uniform crystal phase and higher lamellar alignment, the
conductive channels will be easier to create due to the
polarization. Therefore, the switching will occur at a lower
electric field (lower external voltages). In consequence, more
ternary switching will be found within a constant voltage (0 to -6
V). This explain while device on ITO-O substrate has higher
ternary yield if compared to that of ITO and ITO-P substrates.
Meanwhile, a uniform crystallinity limit the switching distributed
in a narrower range, as we found in Figure 3.
Fabrication of the Memory Device
The modified ITO glass substrates (2 cm × 2 cm) were transferred to the
glove box, in where moisture content and oxygen levels are less than 1
ppm. The organic layer was vacuum-deposited under a pressure of
around 8.0 × 10−6 Torr and the thickness was 100 nm. The aluminum
2
−6
electrodes (0.0314 mm ) were vacuum-deposited at 5.0 × 10 Torr
through a shadow mask to form top electrodes and the thickness was
100 nm. Both of the thickness was estimated from the SEM images.
Conclusions
Acknowledgements
In summary, we demonstrated that surface engineering of
indium tin oxide electrode is an effective strategy to improve the
performance of organic multilevel RRAMs. Surface engineering
of the ITO electrode by phosphonic acids (PA) is feasible to
conduct in a solution process and may work cooperatively with
other techniques such as molecular design. In this work, an
asymmetric and conjugated molecule grown on original ITO
crystalizes into two phases. Upon coverage of phenyl and octyl
moiety on ITO substrate, this molecule grows in only one crystal
structure. The unified crystallinity lowers down the necessary
voltage to onset the resistance switching, increasing the final
ternary device yield and narrowing the threshold voltage
distribution. Our results upgrade the understanding of how
intermolecular stacking affects the property of organic RRAMs
and reveal an effective strategy to achieve high-performing
organic memory devices.
This work was financially supported by the NSF of China
(
21336005, 21603158), the major research project of Jiangsu
Province office of Education (15KJA150008), Chinese-
Singapore Joint Project (2012DFG41900), Suzhou Science and
Technology Bureau Project (SYG201524) and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD).
Keywords: Ternary • phosphonic acid • Surface engineering •
ITO • Resistive random access memory
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Chemistry - An Asian Journal
10.1002/asia.201700706
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[
a]
[a]
Organic molecule was fabricated
onto the ITO substrate which was
modified by phosphonic acid. These
as-fabricated devices perform better
memory behaviors, such as higher
ternary device yield and lower
threshold voltage, compared with
those fabricated on unmodified ITO
substrate.
Xiang Hou, Xue-Feng Cheng, Xin
[
a]
[a]
Xiao, Jing-Hui He, * Qing-Feng Xu,
[
a]
Hua Li, [a] Na-Jun Li, [a] Dong-Yun
Chen [a] and Jian-Mei Lu*[a]
Page No. – Page No.
Surface engineering of ITO
Substrate to improve memory
performance of asymmetric
conjugated molecule with a side
chain
For internal use, please do not delete. Submitted_Manuscript
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