Comparison of two strategies to improve organic ternary memory performance: 3-Hexylthiophene linkage and fluorine substitution

Article abstract of DOI:10.1016/j.dyepig.2016.03.040

In this paper, two acceptor-donor-acceptor (A-D-A) type organic small molecules, 4,4'-(5,5'-(benzo[c] [1,2,5]thiadiazole-4,7-diyl)bis(3-hexylthiophene-5,2-diyl))bis(N-(4-nitrophenyl)-N-phenylaniline) (NTPA₂TBT) and 4,4'-(5,6-difluorobenzo[c] [1,2,5]thiadiazole-4,7-diyl)bis(N-(4-nitrophenyl)-N-phenylaniline) (NTPA₂BTF₂), were synthesized and fabricated into resistive random access memory devices. Compared with our previously reported molecule NTPA₂BT, NTPA₂TBT has 3-hexylthiophene linkages bridging its donor and acceptor, while there are two additional fluorine atoms in the benzothiadiazole moiety for NTPA₂BTF_2. Both the fabricated memory devices based on these two new molecules perform nonvolatile ternary memory characteristics with lower threshold voltages, higher reproducibility and better stability. The addition of 3-hexylthiophene bridges significantly promotes the planarity of the conjugation backbone and facilitates the molecular stacking, while the substitution hydrogen by fluorine improves the intermolecular interaction, and thus also induces an ordered molecular stacking. The insertion of 3-hexylthiophene bridges or fluorine atoms could be an effective strategy of tailoring organic molecules to improve data storage performance with lower power consumption.

Full text of DOI:10.1016/j.dyepig.2016.03.040

Accepted Manuscript
Comparison of Two Strategies to Improve Organic Ternary Memory Performance: 3-
hexylthiophene Linkage and Fluorine Substitution
Qing Bao, Hua Li, Yang Li, Jinghui He, Qingfeng Xu, Najun Li, Dongyun Chen, Lihua
Wang, Jianmei Lu
PII:
S0143-7208(16)30108-5
10.1016/j.dyepig.2016.03.040
DYPI 5165
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 February 2016
Revised Date: 18 March 2016
Accepted Date: 21 March 2016
Please cite this article as: Bao Q, Li H, Li Y, He J, Xu Q, Li N, Chen D, Wang L, Lu J, Comparison
of Two Strategies to Improve Organic Ternary Memory Performance: 3-hexylthiophene Linkage and
Fluorine Substitution, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.03.040.
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Comparison of Two Strategies to Improve Organic Ternary Memory Performance:
3-hexylthiophene Linkage and Fluorine Substitution
Qing Baoa, Hua Lia, Yang Lia, Jinghui Hea,*, Qingfeng Xua, Najun Lia, Dongyun Chena, Lihua
Wanga and Jianmei Lua,*
a College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation
Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R.
China. E-mail: jinghhe@suda.edu.cn; lujm@suda.edu.cn
Abstract
In this paper, two acceptor-donor-acceptor (A-D-A) type organic small molecules,
4,4'-(5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(3-hexylthiophene-5,2-diyl))bis(N-(4-nitrophenyl
)-N-phenylaniline)
(NTPA2TBT)
and
4,4'-(5,6-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)bis(N-(4-nitrophenyl)-N-phenylaniline)
(NTPA2BTF2), were synthesized and fabricated into resistive random access memory devices.
Compared with our previously reported molecule NTPA2BT, NTPA2TBT has 3-hexylthiophene
linkages bridging its donor and acceptor, while there are two additional fluorine atoms in the
benzothiadiazole moiety for NTPA2BTF2. Both the fabricated memory devices based on these two
new molecules perform nonvolatile ternary memory characteristics with lower threshold voltages,
higher reproducibility and better stability. The addition of 3-hexylthiophene bridges significantly
promotes the planarity of the conjugation backbone and facilitates the molecular stacking, while
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the substitution hydrogen by fluorine improves the intermolecular interaction, and thus also
induces an ordered molecular stacking. The insertion of 3-hexylthiophene bridges or fluorine
atoms could be an effective strategy of tailoring organic molecules to improve data storage
performance with lower power consumption.
Keywords
Ternary memory; conjugated small molecules; 3-hexylthiophene group; fluorine atom; lower
power consumption
1. Introduction
Recently, organic conjugated materials have received considerable attention for their potential
applications in the field of solar cells [1-3], field effect transistors [4, 5], data-storage devices [6-9]
and sensors [10]. Among them, small molecules were focused because of their tunable structures,
low costs, simple processing routes and flexibility [11-14]. Especially, organic resistive random
access memory (RRAM) has become one of the high-density information storage solutions due to
its simple structure, capability of layer-by-layer stacking and fast reading/writing speed [15-18].
Moreover, RRAM devices based on small organic molecules can achieve multilevel conductive
states per unit under external voltages by rational molecular design, leading to huge growth of
information density compared to traditional binary memories [19-22].
In order to effectively improve RRAM device performance, researchers have explored a series
of molecular engineering strategies, such as varying the types and numbers of electron acceptors
and/or donors in the molecular backbone, tailoring the molecular conjugated length, and
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modulating the length of side alkyl chain [23-26]. In our previous study, we demonstrated that
inserting alkynyl group as conjugated linkers could enhance the memory performance [27]. Such
an enhancement is attributable to the increase of intramolecular planarity, and in consequence the
better intermolecular stacking and facilitation of charge transportation. However, considering that
insertion of alkynyl can only be applied to limited categories of conjugated chains, it is necessary
to explore more efficient molecular design strategies to further promote storage performance.
Herein, we performed a comparative study to demonstrate two new effective molecular design
strategies: incorporation of thiophene ring as conjugation linkage or substitution of hydrogen by
fluorine atom. On one hand, inserting thienyl linkage into the conjugated backbone can attenuate
molecular torsion, facilitate the intermolecular π-π stacking, and thus optimize the device
performance [28-30]. On the other hand, organic molecules containing fluorine atoms possess
advantages of hydrophobicity, good solubility and film-forming ability [31-33]. Meanwhile,
fluorine atoms usually have influence on inter- or intramolecular interactions. Particularly, it has
been found that fluorine substitution on acceptor unit can ameliorate the efficiency of photovoltaic
devices dramatically [34-36]. However, such strategy was scarcely reported in RRAM devices
application [37].
We demonstrated these two strategies on the basis of our previous synthesized molecule
NTPA2BT with ternary memory performance [27]. Two new A-D-A type small organic conjugated
molecules (NTPA2TBT and NTPA2BTF2) were synthesized, respectively (Fig. 1). In NTPA2TBT,
3-hexylthiophene were inserted between benzothiadiazole and TPA moieties. The hexyl group of
thiophene was added to enhance the solubility of molecules. While in NTPA2BTF2, two hydrogen
atoms of benzothiadiazole group were replaced by fluorine. Both the two new molecule based
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devices show typical ternary WORM (Write-Once-Read-Many) memory behaviors. Compared
with NTPA2BT [27], both NTPA2TBT and NTPA2BTF2 based devices exhibit superior ternary
memory performance with lower average threshold voltages and higher reproducibility of
tri-stable conductance states. However, the mechanisms for memory performance enhancement
were distinct, as we discussed below.
2. Experimental section
2.1. Materials
3-hexylthiophene, tributylchlorostannane, triphenylamine, N-bromosuccinimide (NBS),
Cu(NO3)2·3H2O, bis(pinacolato)diboron (B2pin2), 4,7-dibromobenzo[c][1,2,5]thiadiazole and
4,7-dibromo-5,6-difluorobenzo[c][1,2,5]thiadiazole were all purchased from commercial sources
(Alfa Aesar, Aladdin, Tansoole and TCI). All other related reagents and solvents were used as
received without further purification.
2.2. Synthesis and characterization
The synthesis and characterization of the two target molecules (NTPA2TBT and NTPA2BTF2) are
shown as following. While the complete synthetic procedures of the intermediates and final
products are depicted in Fig. 2 (see the supplementary information for detailed experimental
processes).
4,4'-(5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(3-hexylthiophene-5,2-diyl))bis(N-(4-nitroph
enyl)-N-phenylaniline) (NTPA2TBT) [38]. A 100 mL three-neck round-bottom flask was filled
with 4-nitro-N-phenyl-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (1.2
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mmol, 0.5 g), 4,7-bis(5-bromo-4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (0.5 mmol, 0.31
g), potassium carbonate (2.17 mmol, 0.3 g), toluene (20 mL), H2O (20 mL) and ethanol (8 mL).
Under N2 protection, tetrakis (triphenylphosphine) palladium (0.04 mmol, 47 mg) was added to
the mixture quickly. The reactants were refluxed at 70 °C for 24 h until water addition. The water
phase was extracted with ethyl acetate and the organic phase was washed with water and brine.
The extract was dried over sodium sulfate. Column chromatography on silica gel using
dichloromethane-petroleum ether as eluent was used to purify the crude product. Purple solid
1
NTPA2TBT was obtained (0.29 g, 56%). H-NMR (400 MHz, CDCl3) δ (ppm): 8.10 – 8.07 (m,
4H), 8.03 (s, 2H), 7.87 (s, 2H), 7.51 (d, J = 8.6 Hz, 4H), 7.41 (t, J = 7.8 Hz, 4H), 7.24 (dd, J = 8.6,
1.8 Hz, 10H), 7.04 – 7.01 (m, 4H), 2.79 – 2.75 (m, 4H), 1.76 – 1.71 (m, 4H), 1.32 – 1.27 (m, 12H),
0.89 (d, J = 2.3 Hz, 6H).
4,4'-(5,6-difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)bis(N-(4-nitrophenyl)-N-phenylaniline)
(NTPA2BTF2) [38].
A
100 mL three-neck round-bottom flask was added with
4-nitro-N-phenyl-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (1.2 mmol, 0.5
g), 4,7-dibromo-5,6-difluorobenzo[c][1,2,5]thiadiazole (0.5 mmol, 0.17 g), potassium carbonate
(2.17 mmol, 0.3 g), toluene (20 mL), H2O (20 mL) and ethanol (8 mL). Under N2 protection,
tetrakis (triphenylphosphine) palladium (0.04 mmol, 47 mg) was added quickly to the mixture.
The reaction was refluxed at 70 °C for 24 h and then water was added. The water phase was
extracted with ethyl acetate before the organic phase was washed with water and brine. The
solvent was removed under reduced pressure after the extract dried over sodium sulfate.
Purification was done by column chromatography on silica gel using dichloromethane-petroleum
1
ether as eluent. Yellow solid NTPA2BTF2 was produced (0.20 g, 53%). H-NMR (400 MHz,
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DMSO) δ (ppm): 8.15 (d, J = 8.9 Hz, 4H), 7.90 (d, J = 8.0 Hz, 4H), 7.52 (t, J = 7.5 Hz, 4H), 7.44
(d, J = 8.2 Hz, 4H), 7.34 (d, J = 7.7 Hz, 6H), 6.99 (d, J = 8.9 Hz, 4H).
2.3. Fabrication of the memory devices
Indium-tin-oxide (ITO)-coated glass substrates were ultrasonicated in water for 20 min firstly.
Then they were pre-cleaned with detergent water and ultrasonicated in water, acetone, and ethanol
for 20 min sequentially. NTPA2TBT and NTPA2BTF2 were dissolved in chlorobenzene at a
concentration of 10 mg mL-1 respectively and the solutions were ultrasonicated for complete
dissolution. Then the solutions were filtered through syringe filter (pore size: 0.22 ꢀm). The
obtained filtrates were spin-coated onto each ITO-coated glass substrate at a speed of 380 RPM
(revolutions per minute) for 9 s, followed with 1700 RPM for 20 s. After the substrates were
placed into an evaporation chamber which was then pumped to a 1.0 × 10-6 Torr vacuum
environment, the top aluminum (Al) electrodes of about 100 nm in thickness were thermally
evaporated and deposited onto the active organic layer through a shadow mask. The active area of
each memory device cell was 0.0314 mm2.
2.4. Measurements and general methods
An Inova 400 MHz FT-NMR spectrometer unless otherwise noted was used to obtain all NMR
spectra in chloroform-d or dimethylsulfoxide-d6 containing 0.03% TMS as an internal reference.
UV-Vis absorption spectra were studied at room temperature with a Shimadzu UV-3600
spectrophotometer. Under the assistance of a reference electrode Ag/AgCl, a Pt wire counter
electrode and an ITO working electrode, a CorrTest CS Electrochemical Workstation analyzer at a
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100 mV/s sweeping rate collected the cyclic voltammograms of the solution of 0.1 mol/L
tetrabutylammonium hexafluorophosphate (TBAPF6) in anhydrous acetonitrile. A Shimadzu
XRD-6000 spectrometer determined X-ray diffraction patterns with a Cu KR monochromatic
radiation source at 40 kV and 30 mA. Atomic force microscopy (AFM) measurement was used to
record the surface morphology of the spin-coated thin films with a MFP-3DTM (Digital
Instruments/Asylum Research) AFM instrument. An HP 4145B semiconductor parameter analyzer
was employed to perform the current-voltage (I-V) measurements of the organic memory devices
(OMDs) under ambient conditions. Molecular simulations were conducted with the density
functional theory (DFT) on the level of GGA-BLYP-DNP using DMol3 code [39]. Scanning
Electronic Microscopy (SEM) images were investigated on a Hitachi S-4700 scanning electron
microscope.
3. Results
3.1. Optical and electrochemical properties
First, we demonstrate that the two molecules are of intramolecular charge transfer, faciliating the
memory switching as previously reported [27]. The UV-visible absorption spectra of the two
molecules NTPA2TBT and NTPA2BTF2 were collected in dilute dichloromethane solution and in
thin film state on quartz substrates as shown in Fig. 3a and 3b. The high-energy absorption bands
around 300 nm can be attributed to an intramolecular charge transfer (ICT) process mainly from
triphenylamine (TPA) skeleton to two nitro groups [27, 40]. While the peak in the region of
400-500 nm is associated with a π→π* transition with a minor part of ICT from the conjugated
backbones to the benzothiadiazole core [27, 41]. It is obvious that the CT transition of thin film
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shows a red-shift compared to that of the solution state, possibly resulting from the formation of
molecular aggregates and/or increased polarity of the thin film [41, 42]. For NTPA2TBT, the
absorption edge of the films is 615 nm while it is 518 nm for NTPA2BTF2. Correspondingly, the
optical band gaps of the two molecules are 2.02 and 2.39 eV, respectively.
Fig. 3c and 3d show the cyclic voltammograms demonstrating the electrochemical properties of
the two molecules. For calibration, the absolute redox potential of ferrocene/ferrocenium (Fc/Fc+)
is supposed to be −4.80 eV to vacuum, while under the identical condition, the external Fc/Fc+
redox standard potential is tested to be 0.426 eV vs. Ag/AgCl. The onset oxidation potentials
(Eoxonset) are 0.92 and 1.13 eV vs. Ag/AgCl for NTPA2TBT and NTPA2BTF2 respectively. While
the onset reduction potentials (Eredonset) are –0.89 and –0.92 eV, respectively. The highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) values are
onset
calculated according to the following equations: EHOMO = – [Eox
+ 4.8 – Eferrocene1/2] (eV),
ELUMO = – [Eredonset + 4.8 – Eferrocene1/2] (eV) [43, 44], in which Eoxonset is the onset potential of the
onset
oxidation process and Ered
is the onset potential of the reduction process. Therefore, as shown
in Table 1, the HOMO are calculated to be –5.29 and –5.50 eV, while the LUMO are determined
to be –3.48 and –3.45 eV, respectively. For the two molecule-based memory devices, hole
injection energy barriers between the work functions of ITO (–4.8 eV) and HOMO of the active
layer are found to be 0.49 and 0.70 eV, respectively. While electron injection energy barriers
between the work functions of Al (–4.3 eV) and LUMO of the active layer are estimated to be
0.82 and 0.85 eV. As shown in Fig. 4, an energy level diagram was drawn. Compared with the
work function of Al to the LUMO energy levels, the work function of ITO is more approaching to
the HOMO energy levels of both molecules. Obviously, hole injection from ITO into the HOMO
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level is more convenient than electron injection from Al into the LUMO level, thus hole injection
will have priority in the charge conduction process [45-47]. On one hand, the low HOMO levels
are favorable to make memory devices stable against oxygen oxidation; on the other hand, it is
advantageous for the hole transfer from the ITO electrode [48].
3.2. Film crystallinity and morphology
It has been widely reported that the crystallinity of the active materials has influence on the
performances of organic electro-resistive memories, such as the device yield and onset voltages
[17, 27]. Therefore, X-ray diffraction (XRD) was conducted to investigate the molecular stacking
of the two molecules in thin films (ca. 100 nm thickness). Fig. 5 shows the conspicuous diffraction
peaks of both thin films, indicating stacking ordering in the solid states. NTPA2TBT exhibits a
primary diffraction peak at 2θ = 6.27°, corresponding to an interlayer spacing of 14.1 Å, which is
smaller than that of NTPA2BT (14.6 Å) [27, 49]. Another diffraction peak appears at 2θ = 20.41°
(d = 4.3 Å), which likely corresponds to the third order Bragg reflection [50]. For NTPA2BTF2, the
XRD pattern shows a sharp peak at 2θ = 8.20°, which can be attributed to the interlayer spacing
peak. The corresponding value d = 10.8 Å is also smaller than that of NTPA2BT (14.6 Å) [27].
Another two diffraction peaks are observed at 2θ = 16.45° (d = 5.4 Å) and 24.78° (d = 3.6 Å). The
value d = 5.4 Å is half of d = 10.8 Å, and d = 3.6 Å is exactly one third of d = 10.8 Å. This
manifests that NTPA2BTF2 based thin films form a more ordered layer-by-layer lamellar packing
than NTPA2BT [27, 51], which is beneficial for achieving highly efficient charge transport and
hence better memory properties [17, 42, 52]. Furthermore, the intensity of diffraction peaks for
NTPA2BTF2 steeply increase, indicating an enhanced degree of crystallinity, which may be
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attributed to the promoted intermolecular stacking and the potential intra- and/or intermolecular
interactions (e.g., C-HꢀF interaction) originated from fluorine atoms [53-55].
The morphology surfaces of NTPA2TBT and NTPA2BTF2 films were investigated by atomic
force microscopy (AFM) as shown in Fig. 6. As both the NTPA2TBT and NTPA2BTF2 thin films
display even height images, they have smooth morphologies and thus good film-forming
processability for device fabrication, which are comparable with that of NTPA2BT (RRMS = 0.356
nm) [27]. For NTPA2TBT, the thin film exhibits a granular-like surface with the root-mean-square
roughness (RRMS) of 0.451 nm (Fig. 6a). While NTPA2BTF2 shows fibril-like structure with RRMS
= 0.332 nm (Fig. 6b). The densely-packed crystallinity and low roughness could lead to form
highly efficient pathways for charge carrier transport [39, 56].
3.3. Current-voltage (I-V) characteristics of the memory devices
I-V measurements on ITO/molecule/Al sandwich-structured devices reveal the memory effects of
these two molecules as shown in Fig. 7. Originally, all devices stay in a low-conductivity (OFF or
“0”) state. When one of the cells of the device is swept by a negative voltage from 0 to –4.0 V
(Sweep 1), two sharp jumps of the current occur at two threshold voltages, indicating electrical
transitions of the cell from the low-conductivity (OFF or “0”) state to an intermediate-conductivity
(ON1 or “1”) state and finally to the high-conductivity (ON2 or “2”) state. The “0”-“1” and “1”-“2”
transitions can be regarded as the “writing” process for practical memory storage [6, 57]. The
following scan from 0 to –4.0 V (Sweep 2) or to 4.0 V (Sweep 3) suggests that the
high-conductivity (ON2) state is maintained even after the power supply is removed. As a
negative voltage sweeps from 0 to –2.5 V, there is only the transition from OFF state to the ON1
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state achieved (Sweep 4). Subsequent sweep from 0 to –2.5 V on the same cell shows that the
memory cell stays in the ON1 state (Sweep 5). If a voltage sweep from 0 to 2.5 V is applied, the
cell exhibits in an intermediate-conductivity state (Sweep 6). Then, during the 0 to –4.0 V voltage
sweep, the cell goes through the ON1-to-ON2 transition (Sweep 7) and remains in the
high-conductivity state when the subsequent same voltage is swept (Sweep 8). When positive
voltage from 0 to 4.0 V is applied, the cell still retains in the ON2 state (Sweep 9). Thus, both
devices based on the two molecules exhibit typical nonvolatile ternary WORM
(Write-Once-Read-Many) memory behavior.
3.4. Comparison of memory performances
The molecular arrangements can greatly influence the reproducibility of the switching phenomena,
which is a significant parameter to evaluate the applicability of devices for practical use [56, 58].
In order to investigate the reproducibility of ternary behavior, statistical analysis on totally 100
memory cells for each molecule was developed. Table 2 shows that there are 43 out of 100 cells
performing electrical ternary behavior (43% yield) for NTPA2TBT, while the possibility of ternary
switching for NTPA2BTF2 based device is 44% yield (44 out of 100 cells), which are both higher
than that of NTPA2BT (38% yield). Hence, introducing 3-hexylthiophene bridge or fluorine atom
can be effective and encouraged to improve the device yield though current ratio of ternary
memory is still low. Moreover, compared with NTPA2BT, the average ON1 and ON2 threshold
voltages of NTPA2TBT based device (–1.87 and –2.53 V) and NTPA2BTF2 based device (–1.76
and –2.58 V) are both lower than those of NTPA2BT (–2.04 and –2.69 V), as seen in Table 2,
which could contribute to lowering the power consumption [41, 59].
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4. Discussion
4.1. Proposed storage mechanism
Here, “charge trapping” mechanism which has successfully guided our design of organic
molecules to perform ternary memory behaviors is proposed to explain the storage properties [6,
20]. As shown in Fig. 8, for NTPA2TBT and NTPA2BTF2, the calculated HOMO molecular
orbitals are mainly localized at the donor side (the triphenylamine moiety), while the LUMOs are
mostly localized at the acceptor sides. This suggests that there is intramolecular charge transfer
during migration from HOMO to LUMO. In addition, the electrostatic potential (ESP) of the two
molecules is also calculated. As shown in Fig. 8, the continuous positive ESP regions of the
molecular surface (in red and white) indicate that charge carriers can smoothly transfer throughout
the open channels of conjugated backbone. However, the negative ESP regions (in blue) can serve
as “charge traps” with distinct depths to block the migration of charge carriers, which are
functioned by the electron-deficient groups, such as benzothiadiazole and nitro groups [20, 60].
When there remains a low negative bias, as there is no enough energy for charge carriers to
overcome the injection barriers, the device stays in the OFF state [17]. Once an external voltage is
applied, because the “charge traps” depth is in accordance with the electron-withdrawing strength
of the acceptors, the shallower charge traps caused by the benzothiadiazole moieties will be firstly
fully filled, leading to the current transition from the OFF to ON1 state [61, 62]. As external
voltage continues to increase, the deeper charge traps from the nitro fragments are fully filled later,
realizing the current switching from the ON1 to ON2 state [15]. When the power is turned off or
under reverse bias, the separated-charges could not be de-trapped, as the trapped charges can form
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a stable charge-separated state due to the intramolecular charge transfer process [62, 63]. Thus, the
high-conductivity state can maintain for a long time (Fig. S6) [15]. Therefore, both devices exhibit
typical nonvolatile multilevel WORM memory performance.
4.2. The effect of 3-hexylthiophene and fluorine atom
By comparing molecules NTPA2BT and NTPA2TBT, their discrepancies on average threshold
voltages and ternary memory device yield suggest that 3-hexylthiophene bridges make a
contribution to tuning the device performance. From cyclic voltammograms data, it is apparent
that the hole injection barriers for NTPA2TBT (0.49 eV) are lower than that for NTPA2BT (0.63
eV) [27], partially contributing to the lower threshold voltages of NTPA2TBT based device [51,
64]. Moreover, the optimized geometry of NTPA2TBT as shown in Fig. 8 indicates that the torsion
angle of benzothiadiazole and triphenylamine groups is about 8o whereas that of NTPA2BT is 35o
[27]. Thus the introduction of 3-hexylthiophene bridges greatly decreases the steric repulsion
between these two groups. A better planarity could effectively extend the conjugation length,
enhance the conjugation degree, and thus facilitate the charge transport within the molecule.
Moreover, the increased planarity can contribute to the intermolecular stacking in thin film as
confirmed by the XRD pattern in Fig. 5a. These parameters are beneficial for improving the
memory device performance of lower threshold voltages and more reliable reproducibility [65,
66].
Similarly, fluorine atoms substitution also tunes the device performance such as decreasing the
average threshold voltages and improving ternary memory yield. However, the role of fluorine is
different from that of 3-hexylthiophene. The geometries of NTPA2BT and NTPA2BTF2 molecules
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are almost the same and the introduction of fluorine atoms does not increase the steric repulsion
between benzothiadiazole group and triphenylamine as revealed from the dihedral angle between
benzothiadiazole and TPA planes. However, in the view of molecular stacking, as shown in our
XRD pattern, NTPA2BTF2 appears in denser intermolecular packing and better crystallinity in thin
film than NTPA2BT, which facilitate the charge transfer [17, 27, 42, 52]. This could be attributed
to the highly ordered intermolecular stacking benefiting from the C-HꢀF interactions [54]. Hence,
lower threshold voltages for electrical switching can be achieved due to closer intermolecular
stacking, and thus the ternary memory yield is also improved. When the crystallinity is
particularly focused on, like in the case of organic electronic field [67], fluorine incorporation is
recommended.
5. Conclusion
In summary, two conjugated organic molecules (NTPA2TBT and NTPA2BTF2) were designed and
fabricated as the active layer of sandwich-structured RRAM devices. Both molecules contain the
same electron-withdrawing (benzothiadiazole and nitro groups) and electron-donating (TPA
fragment) moieties. The NTPA2TBT and NTPA2BTF2-based devices are of the lower switching
voltages and higher ternary yield than that of NTPA2BT. On one hand, the addition of
3-hexylthiophene bridges significantly promotes the planarity of the conjugation backbone and
facilitates the molecular stacking. On the other hand, the insertion of fluorine atoms also enhances
the film crystallinity, and thus induces an ordered molecular stacking. Our results show that both
insertion of 3-hexylthiophene bridges or substitution by fluorine atoms are effective strategies in
designing conjugated organic molecules to achieve superior organic memory performances.
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Acknowledgement
This work was financially supported by the NSF of China (21176164, 21336005),
Chinese-Singapore Joint Project (2012DFG41900), Suzhou Science and Technology Bureau
Project (SYG201524), and the Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
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Fig. 1 Sandwiched ITO/small molecule/Al memory device based on molecule NTPA2BT,
NTPA2TBT or NTPA2BTF2.
Fig. 2 Synthetic route of NTPA2TBT and NTPA2BTF2.
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Fig. 3 UV-vis absorption spectra of (a) NTPA2TBT and (b) NTPA2BTF2 in dilute DCM solution
and thin film on quartz substrate. Cyclic voltammograms in 0.1 M TBAP/ acetonitrile solution of
(c) NTPA2TBT and (d) NTPA2BTF2 in thin film on ITO-coated substrate.
Fig. 4 Energy level diagram for the ITO/ NTPA2BT, NTPA2TBT and NTPA2BTF2/Al memory
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devices [27].
Table 1. Electrochemical properties of NTPA2TBT and NTPA2BTF2 films
Eox(onset)a
(eV)
Ered(onset)b HOMOc LUMOd
Injection Barrier
Molecule
(eV)
–0.89
–0.92
(eV)
–5.29
–5.50
(eV)
–3.48
–3.45
Holee (eV) Electronf (eV)
NTPA2TBT
NTPA2BTF2
0.92
0.49
0.70
0.82
0.85
1.13
c
d
a ,b Vs. Ag/AgCl. Determined by the onset of the oxidation peak of cyclic voltammograms.
Determined by the onset of the reduction peak of cyclic voltammograms. e The difference between
f
the work functions of ITO and the HOMO energy level. The difference between the work
functions of Al and the LUMO energy level.
Fig. 5 XRD patterns of (a) NTPA2TBT and (b) NTPA2BTF2 films as-spun on ITO substrates.
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Fig. 6 AFM topography images of (a) NTPA2TBT and (b) NTPA2BTF2 thin films spin-coated on
ITO substrates. The scan size of both images is 2 ꢁm × 2 ꢁm.
Fig.
7
Typical I-V curves of memory devices: (a) ITO/NTPA2TBT/Al and (b)
ITO/NTPA2BTF2/Al.
Table 2. Statistic results of the three molecules-based memory devices
proportion of
proportion of
ON1 threshold ON2 threshold
Molecule
binary behaviora ternary behaviorb
voltagec (V)
voltaged (V)
NTPA2BTe
NTPA2TBT
62%
57%
38%
–2.04
–1.87
–2.69
–2.53
43%
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NTPA2BTF2
56%
44%
–1.76
–2.58
a, b Measured from totally 100 cells for each molecule-based memory device. c, d Average values
e
calculated from the cells of ternary behaviors for each type of device. Data for NTPA2BT from
reference [27].
Fig. 8 Molecular simulation results of NTPA2TBT and NTPA2BTF2: molecular conformation,
ESP, and HOMO and LUMO molecular orbitals (isovalue = 0.03 e/Å3).
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ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Two novel A-D-A type small organic conjugated molecules are synthesized.
The incorporation of 3-hexylthiophene group greatly decreases the steric repulsion.
The substitution of fluorine atom enhances crystallinity and molecular stacking.
Both the fabricated devices show better nonvolatile ternary WORM memory behaviors.
Both the two devices have higher ternary yield and lower threshold voltages.