Upgrading Electroresistive Memory from Binary to Ternary Through Single-Atom Substitution in the Molecular Design

Article abstract of DOI:10.1002/asia.201601317

Herein, two molecules based on urea and thiourea, which differ by only a single atom, were designed, successfully synthesized, and fabricated into resistive random-access memory devices (RRAM). The urea-based molecule showed binary write-once-read-many (WORM) storage behavior, whereas the thiourea-based molecule demonstrated ternary storage behavior. Atomic-force microscopy (AFM) and X-ray diffraction (XRD) patterns show that both molecules have smooth morphology and ordered layer-by-layer lamellar packing, which is beneficial for charge transportation and, consequently, device performance. Additionally, the optical and electrochemical properties indicate that the thiourea-based molecule has a lower bandgap and may be polarized by trapped charges, thus the formation of a continuous conductive channel and electric switching occurs at lower bias voltage, which results in ternary WORM behavior. This study, together with our previous work on single-atom substitution, may be useful to tune and improve device performance in the future design of organic memory.

Full text of DOI:10.1002/asia.201601317

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Upgrading Electroresistive Memory from Binary to Ternary Through Single
Atom Substitution in Molecular Design
Authors: Xue-Feng Cheng; Er-Bo Shi; Xiang Hou; Shu-Gang Xia; Jing-Hui He; Qing-
Feng Xu; Hua Li; Na-Jun Li; Dong-Yun Chen; Jian-Mei Lu
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Upgrading Electroresistive Memory from Binary to Ternary
Through Single Atom Substitution in Molecular Design
Xue-Feng Cheng, [a] Er-Bo Shi, [a] Xiang Hou, [a] Shu-Gang Xia, [a] Jing-Hui He,*[a] Qing-Feng Xu, [a] Hua
Li, [a] Na-Jun Li, [a] Dong-Yun Chen[a] and Jian-Mei Lu*[a]
Abstract: In this paper, two molecules based on urea and thiourea
only differing by single atom were designed, successfully
Intensive studies were carried out to investigate the mechanism
of the RRAM storage behaviors through tuning the molecular
structure. The knowledge obtained could be used to further
guide the optimization of memory performance. Zhang et al.
changed the conjugated groups in the molecular backbone to
adjust the threshold voltages of the devices.4 Kang and co-
workers obtained different types of memory behavior through
substitution of terminal groups.5 Chen’s group used different
bridging bonds to adjusting the planarity of the molecular plane
and successfully optimized the memory performance.6 However,
in the above molecular tuning studies, an intrinsically
methodological flaw exists: in most of these work, the structural
change of molecule was too large, causing many electronic
(donor-acceptor numbers, HOMO-LUMO levels, and gaps),
geometric (planarity and conjugation length) and intermolecular
(stacking manner and strength) properties simultaneously.
These complex variations will cause unnecessary confusion
when correlating single structural effect with macroscopic device
performances. In this term, substituting the target molecule by a
single heteroatom, which only causes infinitesimal change to
molecular structure, is a rigorous way to study the structure-
performance relationship in RRAMs. Indeed, recently our group
successfully applied this single heteroatom substitution method
to decrease the threshold voltages7 and switch the storage type
from DRAM to SRAM 8. The mechanisms behind these are also
studied in depth, thanks to the simple structural variation.
However, switching the memory level through single atom
substation, for example, from binary to ternary, remains
challenging but very interesting.
a
synthesized and fabricated into resistive random access memory
devices (RRAM). Urea-based molecule shows binary WORM
storage behavior while thiourea-based molecule demonstrates
ternary storage behavior. The atomic force microscopy (AFM) and X-
ray diffraction (XRD) patterns show that both two molecules have
smooth morphology and ordered layer-by-layer lamellar packing,
which is be beneficial to charge transportation and in consequence
device performance. Meanwhile the optical and electrochemical
properties indicate that thiourea-based molecule has lower band-gap
and is feasible to be polarized by trapped charges, thus the
formation continuous conductive channel and the electric switching
occurs under lower bias voltage, result in ternary WORM behavior.
This study, together with our previous single atom substitution work,
may be useful to tune and improve the device performance in future
design of organic memories.
Introduction
With rapid development of modern information technology, the
society has already entered into an information explosion time.
The traditional optical and magnet storage technology have
almost been reaching downscaling limits1 while they still cannot
satisfy the increasing demands for high density data storage.
Under this circumstance, resistive random access memory
(RRAM)2 has received widely attention as one of the most
promising candidates for the next generation of high-density
storage solution. Of a metal/insulator (or semiconductor)/metal
configuration, RRAM is a type of resistive memory with simple
structures, layer-by-layer stack capability, low power
consumption and capability to store multilevel conductive state.
Once RRAMs store more than two states in each cell, the
information will increase from 2n to Xn (X is the state number of
each cell) under the original cell density. This dramatic increase
of the information density will hopefully solve the high density
memory problem. Among the potential materials which could
demonstrate resistive memory behavior, organic molecules
perform better than others because of low cost, flexibility, easy
purification and tunable structures.3
In this paper, we designed urea and thiourea based molecules
TFMCU and TFMCT (Figure 1), which only differ by one atom
(O versus S). We fabricate organic memory device using these
two molecules and successfully tune the devices from binary to
ternary memory behavior because of the easy polarizable of
sulfur atom and low band-gap of molecule TFMCT. This study,
together with our previous single atom substitution work, may be
useful to tune and improve the device performance in future
design of organic memories.
Results and Discussion
Synthesize and Characterization
TFMCU and TFMCT were successfully synthesized according to
Scheme 1. Through thermogravimetric analysis (TGA), TFMCU
and TFMCT were found exhibiting good thermal stability with
decomposition temperature (5% weight-loss) to be about 198 °C
and 230 °C, respectively (Supporting Information, Figure S1).
[a]
X. Cheng, E. Bo, X. Hou, S. Xia, Prof. J. He, Prof. Q. Xu, Pro. H. Li,
Prof. N. Li, Prof. D. Chen, Prof. J. Lu Department
College of Chemistry, Chemical Engineering and Materials Science,
Collaborative Innovation Center of Suzhou Nano Science and
Technology Institution
Soochow University
Suzhou 215123, P. R. China
E-mail: jinghhe@suda.edu.cn, lujm@suda.du.cn
Supporting information for this article is given via a link at the end of
the document
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Figure 1. (a) Molecular structures of TFMCU and TFMCT. (b) Scheme of the
TBAP/CH2Cl2 solution of (c) TFMCU and (d) TFMCT in thin film on ITO-coated
substrate at a sweep rate of 100 mV/s. The insert shows the CV curve of the
ferrocene standard swept in the same condition.
a)
λonset
Eg
HOMOb)
(eV)
LUMOc)
(eV)
Hole injectiond)
(eV)
Electron injectione)
(eV)
Molecule
(nm)
(eV)
TFMCU
TFMCT
302
345
4.11
3.59
-5.74
-5.78
-1.63
-2.19
0.94
0.98
2.67
2.11
a) Estimated from the onset of UV-vis absorption: Eg =1240/λonset; b) HOMO = -[Eonset
+ 4.8 – EFerrocence]; c) LUMO = HOMO + Eg; d) The difference
OX
between the work function of ITO and the HOMO level of molecule; e) The difference between the LUMO level of molecule and the work function of Al.
ITO/organic layer/memory device.
The electrical properties of TFMCU and TFMCT were
characterized by cyclic voltammetry (CV) in 0.1 mol/L
tetrabutylammonium
hexafluorophosphate
dried
Scheme 1. Synthetic routes of TFMCU and TFMCT.
dichloromethane solution. In Figure 2 (c) and (d), the onset
oxidation potentials of TFMCU and TFMCT are 1.35 eV and
1.39 eV. Assuming the absolute redox potential of
ferrocene/ferrocenium is -4.8 eV to vacuum. The external
ferrocene/ferrocenium redox standard potential measured under
the identical condition is 0.41 eV. The energy of the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) were obtained from the equation
Optical and electrical properties
To understand the optoelectronic properties of the two
molecules, UV-vis spectra of their films and solution were
recorded. As shown in Figure 2 (a) and (b), the absorption
peaks at 280 nm and 290 nm for TFMCU and TFMCT were
caused by the π-π* transition of urea and thiourea group. For
both TFMCU and TFMCT, compared with that in solution state,
the absorption peaks in film state become wider significantly,
and the peaks have 2 nm bathochromic-shift. These broadened
and red-shift of absorption peaks might be associated with the
accumulation of the molecules in the film state, orderly π-π
stacking or formation of molecular aggregation and increased
polarizability of the film, which will contribute to the improving of
the charge carrier mobility in the film9. From the UV-visible
spectra, λonset of TFMCU and TFMCT were 302 and 345 nm, and
the corresponding bandgap are 4.11 eV and 3.59 eV according
to the equation bandgap = 1240 / λonset, respectively.
OX
HOMO = - [Eonset + 4.8 – EFerrocence] and LUMO = HOMO +
Eg.10 As shown in Table 1, the energy of HOMO and LUMO for
TFMCU and TFMCT are -5.74 eV, -1.63 eV, -5.78 eV and -2.19
eV, respectively. There are two basic transportation model of the
charged species. One is hole injection model, the other one is
electron injection model. The hole injection barriers of TFMCU
and TFMCT are 0.94 eV and 0.98 eV while the electron injection
barriers of TFMCU and TFMCT are found to be 2.67 eV and
2.11 eV. Since the hole injection barriers are smaller than the
electron injection barriers for TFMCU and TFMCT, the hole
injection is the dominant charge transportation model in the
devices and both TFMCU and TFMCT are p-type materials.
Figure 2. UV-vis spectra of (a) TFMCU and (b) TFMCT in dilute CH2Cl2
solution and in the film on quartz substrates. Cyclic voltammograms in 0.1 M
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Table 1. Optical and electrochemical properties of the films fabricated with TFMCU and TFMCT
Morphology and accumulation of the film
In order to explore the molecular stacking in the active layer, we
investigated the X-ray diffraction patterns of the deposited film.
In Figure 3, three diffraction peaks appear at 2θ = 9.1°, 13.1°
and 18.3° in TFMCU-fabricated film, corresponding to the
spacing of 9.7 Å, 6.8 Å and 4.8 Å, respectively. For TFMCT film,
diffraction peaks at 2θ = 8.7°, 13.8° and 17.5°, were observed,
indicating ordered stacking with spacing of 10.2 Å, 6.4 Å and 5.1
Å. The primary diffraction peaks of these two molecules can be
contributed to the interlayer spacing.11 In TFMCU film, 9.7 Å is
approximately twice as 4.8 Å while in TFMCT film, 10.2 Å is a
doublet of 5.1 Å. These relationships indicate that both TFMCU
and TFMCT molecules form ordered layer-by-layer lamellar
packing in the film state while 9.7 Å and 10.2 Å are probably the
interlayer distances. This ordered layered stacking was widely
reported in both organic memory or solar cells devices, and such
construction is beneficial to charge transportation and in
consequence device performance.9d, 12
The surface roughness was characterized by atomic force
microscopy (AFM), as shown in Figure 4. The root-mean-square
roughness (RMS) of the films fabricated by TFMCU and TFMCT
are 1.59 nm and 2.22 nm, which indicate both films have smooth
morphology. TFMCU film has smoother surface and denser
accumulation, which are helpful to construct highly efficient
charge carrier transmission channel and improve device
performance.
Current–voltage (I–V) characteristics
Figure 5. Typical I–V curves of the ITO/ (a) TFMCU or (b) TFMCT/Al memory
device.
Figure 5 shows the current-voltage (I-V) characteristics of
ITO/TFMCU or TFMCT/Al sandwich structure devices. For
TFMCU, the device initially located at a low conductive state
(OFF state). When a voltage sweeps from 0 to -5 V (Sweep 1),
an abrupt jump occurred at -3.95 V from OFF state (10-7 A) to
high conductive state (ON state, 10-2 A). This kind of
transformation from the low to high conductive state can be
regarded as the “writing” process in memory devices. Such an
ON state could be sustained when a negative (Sweep 2, 0 to -5
V) or positive sweeps (Sweep 3, 0 to 5 V). This means the
reverse voltage cannot “erase” the data that had been stored.
The ITO/TFMCU/Al devices show typical nonvolatile binary
property (WORM) with a threshold voltage of -3.95 V and an
ON/OFF ratio as high as 105. While for ITO/TFMCT/Al device,
the device initially located at a low conductive state (OFF state)
as well. When a voltage sweeps from 0 to -5 V (Sweep 1), two
significant jump showed at -2.75 V and -4.30 V from OFF state
(10-9 A) to intermediate conductive state (ON1 state, 10-6 A) and
finally jumped to high conductive state (ON2 state, 10-2 A). Such
an ON2 state could be maintained when a negative (Sweep 2, 0
to -5 V) or positive sweeps (Sweep 3, 0 to 5 V). Afterwards,
when a voltage sweeps from 0 to -4 V (Sweep 4) on another
storage unit, only one jump occurred at -2.55 V from OFF state
to ON1 state and such an ON 1 state could be sustained when a
negative (Sweep 5, 0 to -4 V) or positive sweeps (Sweep 6, 0 to
4 V) as well. Next, when another sweep from 0 to -5 V (Sweep 7,
0 to -5 V) was applied to this storage unit again, another jump
occurred from ON1 state to ON2 state. This ON2 state could
also be maintained when a negative (Sweep 8, 0 to -5 V) or
positive sweeps (Sweep 9, 0 to 5 V). These experiments mean
the reverse voltage cannot “erase” the data neither stored by
ON1 state nor ON2 state. ITO/TFMCT/Al devices show typical
nonvolatile ternary property (WORM) with a threshold voltage of
-2.75 V and 4.30 V, and ON2/ON1/OFF ratio as high as 103 and
104.
Figure 3. X-ray diffraction patterns of TFMCU and TFMCT in evaporated films.
Furthermore, the retention stability of TFMCU and TFMCT
based memory devices under a constant voltage of -1 V
(Supporting Information, Figure S5). No obvious changes in
each conducting state for at least 103 s.
Figure 4. a, b) Tapping-mode (2 μm × 2 μm) AFM topography and typical
cross-section profile of AFM topography images of TFMCU and TFMCT,
respectively.
Proposed mechanism
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unknown nature, the types of traps existing in our organic films
were proposed related with the electron-acceptors, could be one,
two or three, corresponding to the binary, ternary and quaternary
RRAMs.17, 22 In our two molecules, the two electron-acceptors:
3,5-bis(trifluoromethyl)phenyl and 4-cyanophenyl groups in the
backbones play the roles of traps. According to the DFT
calculation, these two groups have different trap depths (in
Figure S4 in Supporting Information). When the external voltage
23
is applied, these two traps will be filled one by one.17b,
Therefore, theoretically, two sharply jumps should emerge and
exhibit ternary memory behaviors for both two molecules.
However, triggered by the substitution of “O” by “S”, only
TFMCT-based devices show ternary memory behaviors
whereas TFMCU ones are binary type.
The mechanism behind is attributable to the discrepancies in the
band gaps. In order to understand the mechanism of the electric
switching in these sandwiched devices, the energy level diagram
(Figure 6 (a)) of ITO/TFMCU and TFMCT/Al devices was drawn
to provide a visualized relationship between HOMO, LUMO
energy levels of TFMCU, TFMCT, and the work functions of ITO
and aluminum. The differences between the work function of
ITO and HOMO energy level (0.94 eV, 0.98 eV) are smaller than
that between LUMO energy and the work function of Al (2.67 eV,
2,11 eV) for both molecules, which indicates that hole injection is
the predominant way for the charge transfer under negative bias.
As shown in Figure 6 (b), when external electric fields were
applied to the two molecules from x, y and z directions, the
HOMO-LUMO gaps of TFMCU and TFMCT are narrowed as the
increase of electric filed, particularly from the chain direction.
The original band-gap of TFMCT is smaller than that of TFMCU.
Even though the band-gap of TFMCU is more sensitive to the
electric field, its gap is always large than that of TFMCT. With
similar crystalline structures, a narrower band-gap of molecule
would offer a significantly higher carrier-hopping rate, or namely
become more conductive, according to the Marcus transport
theory for organic materials.24 Additionally, the molecule TFMCT
has smaller dipole moment (1.79 a.u., for TFMCU is 2.24 a.u.).
Smaller dipole moment suggests that molecule TFMCT is easier
to be polarized, and the continuous conductive channel could be
formed under lower voltage which also contributes to the lower
threshold voltage of TFMCT based devices. That means
compared to TFMCU-based devices, TFMCT-based devices will
have lower threshold voltage.
Figure 6. (a) Energy level diagram for the ITO/TFMCU and TFMCT/Al devices.
(b) DFT-calculated variation in the HOMO-LUMO gap under an electric field
along different directions; (c) Molecular geometries of TFMCU, TFMCT and
the directions of the electrical field. Electrical field strengths are given in a.u. (1
a.u. =51.4 V Å-1).
In the previous research, many mechanisms have been applied
to explain the performance of organic storage behaviors, such
as metal-filament formation13, field-induced charge transfer14,
molecular redox15, conformational change16 and charge
trapping17. The charge trapping mechanism was originally
applied to describe the charge transportation in inorganic
insulators18. In previous study, we extended this theory into the
case of organic materials based on the positive-feedback effect
from the polarization of organic conjugated molecules. Here the
extend theory is invoked to explain the multilevel conductance
switching behaviors.
Since organic molecules stacking through relatively weak
intermolecular force, there are types of crystalline defects or
impurities.19 They are able to capture and immobilize charges
during charge transportations, functionalizing as charge traps.
When a charge injected into the active layer from the positive
electrode to fill a trap, the molecules around the trap will be
polarized due to the electric field. Even though polarization from
each trap just distributes in very small region, accumulation of
high density of injected charges and the filled traps will allow
more and more molecules rounding them be polarized. These
polarized molecules become more conductive and more readily
accept injected charges, leading to a positive feedback effect to
accelerate the polarization effect. Finally, sufficient conductive
regions overlap together to form the continuous conductive
channel which contributes to the sudden increase in conductivity
at specific voltage. Even when the external voltage removed, the
Since the threshold voltage of TFMCU based devices are larger,
we just observed one conductivity transition, which current level
had already arrived our testing limitation (Maximum compliance
current is 0.1 A). In a word, under our present instrument
limitation, we found that TFMCU based devices perform binary
WORM storage behavior while TFMCT based devices perform
ternary WORM storage behavior.
charge will mainly remain trapped, because the deep trapping
20
level prevents the detrapping process.17a,
Therefore the
polarized region and the conductive channel still remained. In
general preparation process, there are multiple types of traps,
including chemical impurities, crystal defects and domain
boundaries.19, 21 These traps are of distinctly energetic levels,
which would be filled sequentially under increasing voltages,
leading to multilevel electro-switching behaviors. Despite their
Conclusions
In conclusion, two molecules based on urea and thiourea
(TFMCU and TFMCT) were synthesized and the memory
devices were fabricated to investigate. For the TFMCT, because
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(m, 3H), 7.75 (t, J=2.0 Hz, 1H), 7.73 (t, J=2.0 Hz, 1H). HRMS: calcd for
of its lower band-gap and feasibility to be polarized, the
formation continuous conductive channel and the electron
transition will occur under lower bias voltage, contributing to the
ternary WORM behavior of TFMCT based device while under
present testing environment. In contrast, TFMCU has a large
band gap, even under a polarization field, therefore the as-
fabricated device shows binary WORM behavior at limited
voltages. Our work demonstrated that a low band gap of organic
molecules is critical to achieve multilevel electro-resistive
memory and such a low band gap could be tuned by simply
substitute the functional groups by a single atom while keep its
superior intermolecular stacking and other parameters intact as
much as possible. This study, together with our previous single
atom substitution work, may be useful to tune and improve the
device performance in future design of organic memories.
C16H9F6N3S [M + H] + 390.0455, found 390.0483
Fabrication of the devices
The indium tin oxide (ITO) glass was cleaned by detergent, and then
deionized water, acetone and ethanol were used to wash the glass by
ultrasonication for 20 min, respectively. After that, under the vacuum
degree of 10-6 torr, the organic materials were thermally deposited on the
ITO glass substrate by thermal evaporation. Next, a shadow mask was
covered on the organic film and Al film was evaporated with 100 nm
thickness and 0.2 mm diameter as top electrode. The area of the storage
unit is approximately 0.0314 mm2.
Measurements and general methods
All NMR spectra of organic molecules were obtained in
dimethylsulfoxide-d6 (DMSO-d6) solution containing 0.03 % TMS as an
internal reference using an Inova 400 MHz FT-NMR spectrometer.
Shimadzu UV-3600 spectrometer was used to record UV-visible
absorption spectra at room temperature. Cyclic voltammograms was
collected by CorrTest CS Electrochemical Workstation analyzer at a
sweep rate of 100 mV/s in 0.1 mol/L solution of tetrabutylammonium
hexafluorophosphate (TBAPF6) or in dried dichloromethane. X-ray
diffraction patterns were obtained from Shimadzu XRD-6000
spectrometer with a Cu Kα monochromatic radiation source at 40 kV and
30 mA. MFP-3DTM (Digital Instrument/Asylum Research) AFM
instrument was applied to characterize the surface morphology of the
fabricated film devices. The current-voltage (I-V) characteristics of the
memory devices were measured by Keithley 4200-SCS in voltage
sweeping mode. Thermogravimetric analysis (TGA) was performed on a
Perkin-Elmer Diamond TG/DTA instrument under a nitrogen atmosphere
with a heating rate of 10 °C min-1. SEM images were taken on a Hitachi
S-4700 scanning electron microscope. High-resolution mass spectra
Experimental Section
Materials
4-isocyanatobenzonitrile,
4-isothiocyanatobenzonitrile,
3,5-
bis(trifluoromethyl) aniline tetrabutylammonium hexafluorophosphate
were purchased from commercial sources (Alfa Aesar and TCI). All other
reagents and solvents were used as received without further purification.
Synthesis
Synthetic routes of molecules TFMCU and TFMCT were shown in
Scheme 1.
Synthesis of 1-(3,5-bis(trifluoromethyl)phenyl)-3-(4-cyanophenyl)urea
(TFMCU). 3,5-bis(trifluoromethyl)aniline (708.8 mg, 6 mmol) and dry
CH2Cl2 (60 mL) was added into a 100 mL three-neck round-bottom flask.
Then the nitrogen gas was blown into the solution for 20 min to remove
dissolved oxygen. After that, 4-isocyanatobenzonitrile (1212 μL, 6.6
mmol) was added dropwise to the mixture under the protection of
nitrogen gas at 0 °C. The reaction mixture was stirred at this temperature
for 5 h and at room temperature for two more hours. Then the reaction
mixture was extracted with water. Then the solvent in the organic phase
was evaporated under reduced pressure and the crude product was
purified by column chromatography (PE: EA = 3: 1) to give the target
compound TFMCU as white powder with a yield of 52% (1.2801 g, 3.43
mmol). 1H NMR (400 MHz, DMSO-d6): δ 9.56 (s, 1H), 9.52 (s, 1H) 8.15 (s,
2H), 7.77 (t, J=2.0 Hz, 1H), 7.75 (t, J=2.0 Hz, 1H), 7.72 – 7.65 (m, 3H).
HRMS: calcd for C16H9F6N3O [M + H] + 374.0683, found 374.0713
(HRMS) were acquired using
a
Micro mass GCT-TOF mass
spectrometer with an electrospray ionization source.
Acknowledgements
This work was financially supported by the NSF of China
(21176164, 21336005, 21603158), 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: Urea and thiourea • Ternary • Single atom
substitution • Charge trapping • Resistive random access
memory
Synthesis of 1-(3,5-bis(trifluoromethyl)phenyl)-3-(4-cyanophenyl)thiourea
(TFMCT). 3,5-bis(trifluoromethyl)aniline (708.8 mg,
6 mmol) was
dissolved in dry CH2Cl2 (60 mL) in a 100 mL three-neck round-bottom
flask. Blowing of nitrogen gas into the solution was followed to remove
the dissolved oxygen for 20 min. Then 4-isothiocyanatobenzonitrile (1212
μL, 6.6 mmol) was added under nitrogen atmosphere at 0 °C. After that,
the reaction mixture was stirred at 0 °C for 5 h and at room temperature
for two more hours. After completion of the, deionized water was poured
into the reaction system to extract and reserved the organic phase, which
was later evaporated. At last, column chromatography (PE: EA = 3: 1)
was used to purify the crude product and finally TFMCT was obtained as
white powder with a yield of 44% (1.1305 g, 2.90 mmol). 1H NMR (400
MHz, DMSO-d6): δ 10.64 (s, 1H), 10.55 (s, 1H), 8.25 (s, 2H), 7.86 – 7.80
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Chemistry - An Asian Journal
10.1002/asia.201601317
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Upgrading
the
electroresistive
Xue-Feng Cheng, Er-Bo Shi, Xiang
memory from binary to ternary
through single atom substitution for
the first time. The optical and
electrochemical properties indicate
that thiourea-based molecule has
lower band-gap and is feasible to be
polarized by trapped charges, thus
Hou, Shu-Gang Xia, Jing-Hui He,*
Qing-Feng Xu, Hua Li, Na-Jun Li,
Dong-Yun Chen and Jian-Mei Lu*
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Upgrading Electroresistive Memory
from Binary to Ternary Through
Single Atom Substitution in
Molecular Design
the
formation
of
continuous
conductive channel and the electric
switching occurs under lower bias
voltage, resulting in ternary WORM
behavior.
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