Benzothiazole derivatives containing different electron acceptors exhibiting totally different data-storage performances

Article abstract of DOI:10.1039/c3tc32008a

Two small conjugated molecules BTVCz-NO₂ and BTVCz, each incorporating an electron-donating carbazole group and a medium electron-withdrawing benzothiazole group, were both successfully designed and synthesized. Molecule BTVCz-NO₂ is

Full text of DOI:10.1039/c3tc32008a

Journal of
Materials Chemistry C
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Benzothiazole derivatives containing different
electron acceptors exhibiting totally different
data-storage performances†
Cite this: J. Mater. Chem. C, 2014, 2,
673
5
a
a
a
a
a
ab
Zhe Su, Hao Zhuang, Haifeng Liu, Hua Li,* Qingfeng Xu, Jianmei Lu*
b
and Lihua Wang
Two small conjugated molecules BTVCz–NO
carbazole group and a medium electron-withdrawing benzothiazole group, were both successfully
designed and synthesized. Molecule BTVCz–NO is an A1–D–A2 structure while BTVCz is a single D–A
structure. Both molecules were made into thin films by spin-coating. The two films differentiated over
their optical, electrochemical and morphological properties. And the fabricated device with BTVCz–NO
2
and BTVCz, each incorporating an electron-donating
2
2
as an active material showed non-volatile ternary WORM data-storage behavior with its switching
threshold voltages at ꢀ1.5 V and ꢀ2.5 V in tandom when an electrical field was applied, whereas its
“
counterpart” device with BTVCz as an active material exhibited volatile binary DRAM data-storage
Received 11th October 2013
Accepted 8th May 2014
behavior with its switching threshold voltage at ꢀ1.3 V. Combined with theoretical calculation of each
molecule, we conclude that different data-storage behaviors can be achieved by introducing different
DOI: 10.1039/c3tc32008a
2
electron acceptors and that molecule BTVCz–NO , as an A1–D–A2 structure, is more likely to achieve
www.rsc.org/MaterialsC
ternary data-storage performance.
which may result in the formation of conductive metal la-
ments and has a side effect on the performances of devices.
Organic and polymeric materials are poised to revolutionize the Therefore, synthesis of molecules with fewer azo components
1
. Introduction
40,41

eld of data storage due to their low-cost, structural exibility, has gained, and will continue to gain more attention from the
1–15
and three dimensional stacking capability.
Over the last scientic world. Furthermore, most of the molecules we
decade, it has been reported that sandwich-structured memory synthesized belong to either single Donor–Acceptor (D–A)
devices fabricated using polymeric materials can exhibit good molecules or Donor–Acceptor1–Acceptor2 (D–A1–A2) mole-
electrically bi-stable behavior with an ON/OFF current ratio of cules, while asymmetric Acceptor1–Donor–Acceptor2 (A1–D–A2)
4
6
16–28
1
0 to 10 .
Until quite recently, small molecules have also molecules have rarely been studied. Compared with single D–A
been widely investigated in memory devices because of their molecules, A1–D–A2 molecules have one more electron
designable molecular structures, high purity and excellent acceptor, A2. And compared with D–A1–A2 molecules, it can
batch-to-batch reproducibility in contrast to polymer counter- also prevent the overlapping of traps caused by their neigh-
29–36
37–39
parts.
In our previous study,
we studied a series of boring spatial locations, and lead to a well-proportioned
memory devices based on azo-containing small molecules, with distribution of the electron withdrawing groups, thus facili-
the results showing that these devices exhibited excellent tating the access towards multilevel data-storage performances.
ternary data-storage performances and increased data storage
2
Herein, two organic small molecules BTVCz–NO and BTVCz
density by a wide margin. On the other hand, the introduction have been successfully designed and synthesized. Each mole-
of azo groups largely increases the content of nitrogen atoms, cule incorporates a carbazole group and a benzothiazole group
linked with a carbon–carbon double bond. The carbazole
moiety is a large conjugated planar structure and has been
a
College of Chemistry, Chemical Engineering and Materials Science, Collaborative
widely used as a good hole transport moiety. In this case, it acts
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
42
as a donor (D). And the benzothiazole group, a thermally
Suzhou 215123, P. R. China. E-mail: lujm@suda.edu.cn; Fax: +86 512 65880367;
43
Tel: +86 512 65880368
b
stable and moderate electron-withdrawing moiety, acts as an
State Key Laboratory of Treatments and Recycling for Organic Effluents by Adsorption acceptor (A) in both molecules, which has rarely been studied in
in Petroleum and Chemical Industry, Suzhou 215123, P. R. China
the area of data-storage. Hence, molecule BTVCz is a single D–A
†
Electronic supplementary information (ESI) available: The compositions of the
structure. Within the molecule BTVCz–NO , a nitro group,
2
small molecules, TGA, retention characteristics of two devices, I–V curves of the
which can diminish the band gap for its strong electron-with-
drawing substitution effect, is also attached to the backbone of
BTVCz device at different intervals, and the stability test of BTVCz–NO
BTVCz. See DOI: 10.1039/c3tc32008a
2
and
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the carbazole moiety, forming an A1–D–A2 structure. Moreover, 8.67 (s, 1H), 8.44 (d, J ¼ 8.5 Hz, 1H), 8.12 (d, J ¼ 8.8 Hz, 1H), 7.58
both molecules can be fabricated into ITO/lm/Al sandwiched (d, J ¼ 8.3 Hz, 1H), 7.50 (d, J ¼ 8.5 Hz, 1H), 4.40 (d, J ¼ 6.9 Hz,
devices by spin-coating. The two molecules exhibit completely 2H), 1.91–2.01 (m, 2H), 1.31–1.5 (m, 6H), 0.88 (d, J ¼ 7.2 Hz, 3H).
1
3
different optical, electrochemical and morphological properties
C NMR (100 MHz, CDCl ) d 191.30, 145.18, 144.26, 141.63,
3
in their lms. And the fabricated devices exhibit totally different 130.00, 128.24, 124.57, 122.97, 122.72, 122.45, 117.52, 110.13,
data-storage performances. Therefore, we can achieve different 109.17, 44.02, 31.42, 28.91, 26.85, 22.48, 13.99.
data-storage behaviors by introducing different numbers of
2-p-Tolylbenzo[d]thiazole (5). A mixture of 4-methyl-benzal-
acceptors with different electron-withdrawing abilities. What is dehyde (6.0 g, 50 mmol), 2-aminothiophenol (7.5 g, 60 mmol),
ꢁ
more, A1–D–A2 structures are more likely to achieve ternary and DMSO (30 mL) was heated in an oil bath to 195 C, held at
data-storage performance. We hope this new design concept of that temperature for 2 h, and then poured into water. The
multi-level data storage materials can provide guidance for separated solids were recrystallized from ethanol to give 5 (8.8 g,
1
future realization of more superior high-density data storage 78%). H NMR (400 MHz, DMSO-d6) d 8.13 (d, J ¼ 8.0 Hz, 1H),
devices (HDDS).
8.04 (d, J ¼ 8.1 Hz, 1H), 7.99 (d, J ¼ 8.0 Hz, 2H), 7.60 (t, J ¼ 7.7
Hz, 1H), 7.51 (t, J ¼ 7.6 Hz, 1H), 7.40 (d, J ¼ 8.2 Hz, 2H), 4.55
1
3
(
3
s, 2H). C NMR (100 MHz, CDCl ) d 168.27, 154.15, 141.46,
2. Experimental section
1
34.95, 130.95, 129.74, 127.51, 126.27, 125.03, 123.06, 121.59,
21.55.
-(4-(Bromomethyl)phenyl)benzo[d]thiazole (6). Compound
(4.5 g, 20 mmol) and N-bromosuccinimide (NBS) (3.6 g, 20
2.1 Materials
2
Carbazole, fuming nitric acid, 1-bromohexane, 4-methyl-benzal-
dehyde, N-bromosuccinimide, sodium-t-butoxide, phosphorus
oxychloride, 2-aminothiophenol, N,N-dimethylformide (DMF)
and triethyl phosphite were purchased from commercial
sources (TCI, Alfa Aesar, and Sigma-Aldrich). All solvents were
purchased from Sinopharm Chemical Reagent Co., Ltd. All
chemicals were used as received without further purication.
5
mmol) were dissolved in 80 mL of CCl . The solution was heated
4
ꢁ
and reuxed at 75 C for 4 h. The precipitated succinimide was

ltered, and the solvent was evaporated from the solution. The
remaining gray oil was recrystallized from ethanol to afford
1
white crystalline powder of compound 6 (5.2 g, 86%). H NMR
(
400 MHz, DMSO-d6) d 8.13 (d, J ¼ 8.0 Hz, 1H), 8.04 (d, J ¼ 8.1
Hz, 1H), 7.99 (d, J ¼ 8.0 Hz, 2H), 7.62 (t, J ¼ 7.7 Hz, 1H), 7.53 (t,
2.2 Preparation of molecules
13
J ¼ 7.6 Hz, 1H), 7.45 (d, J ¼ 8.2 Hz, 2H), 4.55 (s, 2H). C NMR
The compounds BTVCz–NO2 and BTVCz were prepared as (100 MHz, CDCl ) d 167.18, 154.11, 140.59, 135.09, 133.61,
3
detailed in Scheme 1.
-Hexyl-6-nitro-9H-carbazole-3-carbaldehyde (2). The nitra-
129.73, 127.96, 126.46, 125.40, 123.34, 121.68, 32.64.
9
(E)-2-(4-(2-(9-Hexyl-6-nitro-9H-carbazol-3yl)vinyl)phenyl)-
tion was performed by dropping a small excess of fuming nitric benz[d]thiazole (8). Compound 6 (0.60 g, 2 mmol) and triethyl
ꢁ
acid (0.5 mL, 12 mmol) dissolved in acetic acid (3 mL) into a phosphite (0.5 mL, 3.1 mmol) were heated to reux at 185 C for
cooled, stirred solution made of 1 (2.8 g, 10 mmol) and acetic 3 h under stirring. The resulting clear brown oil was then
acid (20 mL). Aer ten minutes of further stirring, water was distilled under reduced pressure. Excess triethyl phosphite was
ꢁ
added, the solid precipitate was collected and washed thor- collected at 69 C (1.5 mm Hg) and compound 2 (0.58 g, 1.8
oughly with water, and recrystallized twice from acetic acid. The mmol) in dry DMF (5 mL) was added dropwise. NaOEt was
ꢁ
crude product was puried by silica gel column chromatog- added and the mixture was heated to 60 C for 2 h. The mixture
raphy (ethyl acetate–petroleum ether, 1/10, v/v) to give 2 (1.9 g, was cooled and quenched with EtOH. All solvents were then
1
5
0%). H NMR (400 MHz, CDCl
3
) d 10.14 (s, 1H), 9.08 (s, 1H), removed under reduced pressure. Solids were dissolved in
2 2
Scheme 1 (a) Chemical structure of BTVCz–NO and BTVCz. (b) Synthesis routes to BTVCz–NO and BTVCz.
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ꢀ
1
2 2 2 4
CH Cl , washed with distilled H O, dried over MgSO and the cyclohexanone solution of two molecules (10 mg mL ) was
ꢀ
1
solvent was removed. The residue was chromatographed on a spin-coated (2000 r s , 40 s) onto ITO and the solvent was
ꢁ
silica gel column (petroleum ether–dichloromethane, 2/1, v/v) removed in a vacuum chamber at 60 C for 12 h. The lm
to afford compound 8 (0.62 g, orange powder, 65%). m.p. 232.9– thickness was about 80 nm. Finally, a layer of Al was thermally
ꢁ
1
2
9
7
33.4 C. H NMR (400 MHz, CDCl
3
) d 9.09 (s, 1H), 8.43 (d, J ¼ evaporated and deposited onto the organic surface at about 5 ꢂ
ꢀ4
.1 Hz, 1H), 8.38 (s, 1H), 8.03–8.08 (m, 4H), 7.81–7.84 (m, 4H), 10 Pa through a shadow mask to form the top electrode and
2
.76 (t, J ¼ 7.6 Hz, 1H), 7.61 (d, J ¼ 16.2 Hz, 1H), 7.53 (d, J ¼ 8.5 so as to dene an active cross-sectional area of 0.0314 mm (a
Hz, 1H), 7.47 (d, J ¼ 9.2 Hz, 1H), 7.29 (d, J ¼ 16.2 Hz, 1H), 4.39 nummular point with a radius of 0.1 mm). All electrical
(
(
1
1
1
3
t, J ¼ 6.9 Hz, 2H), 1.87–2.01 (m, 2H), 1.32–1.51 (m, 6H), 0.88 measurements of the device were characterized under ambient
13
d, J ¼ 7.2 Hz, 3H). C NMR (150 MHz, CDCl ) d 167.71, 154.29, conditions, without any encapsulation, using a HP 4145B
3
43.92, 141.49, 140.97, 140.21, 134.95, 132.22, 130.43, 130.03, semiconductor parameter analyzer.
27.95, 127.73, 126.83, 126.46, 126.36, 126.25, 125.15, 123.33,
23.12, 121.85, 121.61, 119.29, 117.46, 110.00, 108.52, 43.79,
1.46, 28.94, 26.89, 22.50, 13.97. HRMS: calcd for C33H N O S
M + H] 532.1980, found 532.2073.
3
Results and discussion
29 3 2
+
[
3.1 Thermal properties
(
E)-2-(4-(2-(9-Hexyl-9H-carbazol-3yl)vinyl)phenyl)-benz[d]-
The thermal stabilities of BTVCz–NO and BTVCz were evalu-
2
thiazole (9). Compound 6 (0.60 g, 2 mmol) and triethyl phos-
phite (0.5 mL, 3.1 mmol) were heated to reux at 185 C for 3 h
under stirring. The resulting clear brown oil was then distilled
under reduced pressure. Excess triethyl phosphite was collected
at 69 C (1.5 mm Hg) and compound 1 (0.50 g, 1.8 mmol) in dry
DMF (5 mL) was added dropwise. NaOEt was added and the
mixture was heated to 60 C for 2 h. The mixture was cooled and
quenched with EtOH. All solvents were then removed under
ated by TGA under a nitrogen atmosphere at a heating rate of
ꢁ
ꢁ
ꢀ1
1
5
0 C min . It showed (Fig. S1, ESI†) that the temperature for
ꢁ
% weight loss of BTVCz–NO
while that of BTVCz was 210 C, demonstrating the good
2
in nitrogen was up to 220 C,
ꢁ
ꢁ
thermal stability of the materials.
ꢁ
3.2 Current–voltage (I–V) characteristics
reduced pressure. Solids were dissolved in CH
2 2
Cl , washed A schematic sandwich structure comprising a pair of electrodes
with distilled H O, dried over MgSO and the solvent was and a layer of small molecules is shown in Fig. 1a. And Fig. 1b is
2
4
removed. The residue was chromatographed on a silica gel the scanning electronic microscopic (SEM) image of one storage
column (petroleum ether–dichloromethane, 2/1, v/v) to afford cell from the direction of a cross-section view. From top to
ꢁ
compound 9 (0.66 g, yellow powder, 75%). m.p. 181.8–182.6 C. bottom, these layers are the aluminum electrode, BTVCz–NO2
1
H NMR (400 MHz, CDCl
3
) d 8.36 (d, J ¼ 7.6 Hz, 1H), 8.26 (s, 1H), nanolm, and ITO glass, respectively. The current–voltage (I–V)
8
1
.11–8.14 (m, 4H), 8.00 (d, J ¼ 8.0 Hz, 1H), 7.92 (d, J ¼ 7.9 Hz, characteristics of the ITO/BTVCz–NO /Al device are shown in
2
H), 7.80 (d, J ¼ 8.6 Hz, 1H), 7.68 (d, J ¼ 8.2 Hz, 1H), 7.52–7.40 Fig. 1c. In the rst sweep from 0 to ꢀ4 V, two abrupt current
(
(
(
1
1
1
2
4
m, 5H), 7.32 (d, J ¼ 16.2 Hz, 1H), 7.23 (d, J ¼ 16.7 Hz, 1H), 4.34 increases were observed at switching threshold voltages around
t, J ¼ 6.9 Hz, 2H), 1.87–2.01 (m, 2H), 1.37–1.48 (m, 6H), 0.88 ꢀ1.5 V and ꢀ2.5 V, indicating that the device underwent sharp
1
3
d, J ¼ 7.2 Hz, 3H). C NMR (150 MHz, CDCl ) d 167.93, 154.09, electrical transitions from a low-conductivity state (“0” or OFF
3
40.85, 140.49, 134.90, 131.79, 131.51, 128.00, 126.63, 126.34, state) to an intermediate-conductivity state (“1” or ON1 state)
25.90, 125.09, 124.84, 124.58, 123.22, 123.03, 122.81, 121.59, and then to a high-conductivity state (“2” or ON2 state). Aer a
20.44, 119.11, 119.06, 108.97, 108.92, 43.23, 31.57, 28.98, subsequent scan from 0 to ꢀ4 V, the storage cell remained at its
+
6.97, 22.55, 14.03. HRMS: calcd for C33
86.2130, found 486.2167.
H
30
N
2
S [M + H]
high-conductivity state (sweep 2). Even when a reverse scan
from 0 to 4 V was applied, the cell still remained at its high-
conductivity state (sweep 3). Sweep 4 was to measure another
cell of the device over a voltage range of 0 to ꢀ2 V and it showed
a switching threshold voltage at ꢀ1.5 V. During the subsequent
scan from 0 to ꢀ4 V, it showed a switching threshold voltage at
2
1
.3 Measurements
13
H NMR and C NMR spectra were obtained on an Inova 400
MHz FT-NMR spectrometer. UV-vis absorption spectra were
recorded using a Perkin-Elmer Lambda-17 spectrophotometer
at room temperature. SEM images were captured on a Hitachi S-
ꢀ
2.5 V (sweep 5). This device, therefore, exhibited typical
nonvolatile WORM (write-once, read-many-times) memory
behaviors.
4
700 scanning electron microscope. IR spectra were obtained
The transition from “OFF” to “ON1” state and then to “ON2”
state can be regarded as the “writing” process for the memory
device. It should also be emphasized that our device is highly
stable aer proper data writing and that a constant voltage
on a Nicolet iS10 FT-IR spectrometer. X-ray diffraction (XRD)
analysis was performed on lms using a Shimadzu XRD-6000
spectrometer with a Cu KR monochromatic radiation source at
ꢁ
ꢁ
4
0 kV and 30 mA. The 2q angle was scanned from 5 to 30 .
(
ꢀ1.0 V, for example) can be employed to read the “0”, “1” and
2” signals of the memory device (Fig. S2a, ESI†). There was no
signicant degradation in any of the three states for almost ten
“
2
.4 Fabrication of memory device
The memory devices consisting of lms of organic molecules hours. Furthermore, the effect of continuous read pulses of
sandwiched between an indium-tin-oxide (ITO) bottom elec- ꢀ1 V on the OFF, ON1 and ON2 states was investigated
trode and an Al top electrode were fabricated. The (Fig. S2b, ESI†). No degradation in the current was observed for
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Fig. 1 (a) Illustration of the sandwich device; (b) SEM image of a cross-section view of the device; (c) current–voltage (I–V) characteristics of the
memory device fabricated with the BTVCz–NO film; (d) I–V characteristics of the memory device fabricated with the BTVCz film.
2
8
8
all the three states aer more than one hundred million (10 ) hundred million (10 ) continuous read cycles. The switching
continuous read cycles, indicating that any of the states is probability of ITO/BTVCz/Al was 55 out of 64 cells (a device yield
insensitive to the read pulses.
of 86%) without encapsulation (Fig. S5, ESI†), which is also
The three states of the ternary memory cells were very reasonably acceptable for memory applications in the future.
distinct, with the current ratios of “0”, “1” and “2” being
2
.3
2.7
ꢃ1 : 10 : 10 . The switching probability of ITO/BTVCz–NO
2
/
3.3 Optical properties
Al was 51 out of 64 cells (a device yield of 80%) without
encapsulation (Fig. S4, ESI†), which is reasonably acceptable for
memory applications.
As shown in Fig. 2, the UV-vis absorption spectrum of BTVCz–
NO in dilute DMF solution displayed a major absorption peak
2
The ITO/BTVCz/Al device showed volatile binary DRAM
memory behaviors (Fig. 1d), and its switching threshold voltage
was at ꢀ1.3 V when an electric eld was applied. The DRAM
characteristics of the device are testied by the negative voltage
scan at the intervals of 0 s, 10 s, 20 s, 30 s, 40 s, 50 s and 60 s,
respectively, the OFF current decreased gradually and when the
interval time was 60 s the OFF current returned to the original
3
.8
state. The current ratio of “0” and “1” was 10 . The device is
also very stable aer proper data writing and that a constant
voltage (ꢀ1.0 V, for example) can be employed to read the “0”
and “1” signals of the memory device (Fig. S3a, ESI†). There was
no signicant degradation in any of the two states for almost
ten hours. Herein, the constant voltage means the interval of
time is innitely approximate to 0 s, which is different from the
big time gaps in Fig. S6.† Furthermore, the effect of continuous
read pulses of ꢀ1 V on the OFF and ON states was also inves-
tigated (Fig. S3b, ESI†). The stress of voltage of ꢀ1 V has a time
interval of 2 ms, a very small time gap which is innitely
approximate to 0 in this case. Therefore, no degradation in the
2
Fig. 2 The UV-vis absorption spectra of BTVCz–NO and BTVCz in
current was observed for the two states aer more than one the DMF solution state and in spin-coated films, respectively.
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Journal of Materials Chemistry C
at 365 nm. We also examined the UV-vis absorption spectrum of
LUMO (eV) ¼ HOMO + Eg
(eV) ¼ 1240/lonset
(2)
(3)
the spin-coated BTVCz–NO thin lms on ITO substrates. The
2
maximum absorption peak at 400 nm was determined to be
E
g
35 nm bathochromic-shied (from 365 nm to 400 nm) and
signicantly broadened. In contrast, molecule BTVCz in dilute
DMF solution exhibited a major absorption peak at 377 nm,
whereas its spin-coated lm was 8 nm red-shied and peaked at
As shown in Table 1, the energy barriers between the work
function of the ITO bottom electrode (ꢀ4.8 eV) and the HOMO
level for the two lms were 0.6 eV (BTVCz–NO ) and 0.23 eV
2
385 nm. The red-shi and broadened absorption peaks from
(
BTVCz) respectively. And the energy barriers between the
LUMO level and the work function of the Al (ꢀ4.3 eV) top
electrode for the two lms were 1.43 eV (BTVCz–NO ) and
.12 eV (BTVCz). This nding suggests that hole injection for
solution to solid state might be associated with the formation of
44,45
molecular aggregation or increased polarizability of the lm,
2
which is benecial for improving the charge carrier mobility of
the lms.
2
both lms from ITO into the HOMO level is more favorable than
electron injection from the Al top electrode into the LUMO level.
Thus, both lms are p-type materials and hole injection domi-
nates the conduction process.
3
.4 Electrochemical properties
Fig. 3 shows the cyclic voltammograms (CVs) of two lms
prepared by spin-coating. The onset oxidations (Eox ) were 1.06
onset
V and 0.69 V, respectively. The electrochemical potentials were 3.5 Morphology of the thin lms
measured using Ag/AgCl as a reference electrode. The external
Atomic force microscopy (AFM) was performed to characterize
+
ferrocene/ferrocenium (F /F ) redox standard potential (EFoc)
c
c
the surface morphology of the two lms. As shown in Fig. 4,
both lms exhibited two-dimensional nanograins in the solid
state with uniform size and minor roughness. This type of
nanometer-scale grain-like morphology was also observed on
other substrates with different polarities, such as glass, silicon
oxide, and quartz, indicating that the interaction between the
molecules rather than between the molecules and the substrate
drives the self-assembly of the molecules. The smooth
morphology of these two lms facilitates the charge injection
from the electrode into the organic molecules. Furthermore, it
can prevent the aluminium (Al) electrode from penetrating into
the lm during the vacuum deposition process. The XRD
was measured to be 0.46 V vs. Ag/AgCl in dichloromethane.
Assuming that the HOMO level for the F /F standard is 4.8 eV
c c
with respect to the zero vacuum level, the HOMO and LUMO
levels for both lms are estimated. The detailed results are
shown in Table 1.
+
onset
HOMO (eV) ¼ ꢀ[(Eox ) ꢀ EFoc + 4.8]
(1)
patterns of lms are shown in Fig. 5. The BTVCz–NO
2
lm
ꢁ
exhibited a conspicuous diffraction peak at 2q ¼ 15 . This value
˚
corresponds to a d-spacing of 5.9 A. In contrast, the BTVCz lm
ꢁ
exhibited a diffraction peak at 2q ¼ 7.5 which equals to a
˚
d-spacing of 11.8 A. Therefore, both molecules have good layer-
46–49
by-layer stacking in their corresponding lms.
3.6 Proposed data-storage mechanism
To determine the I–V performance, the structure of BTVCz–NO
2
was analyzed using density functional theory (DFT) molecular
simulations. DFT molecular simulation (Fig. 6a) results show
4 6
substrates in dichloromethane with 0.1 mol L n-Bu NPF
as the that an open channel is formed from the molecular BTVCz–NO2
Fig. 3 Cyclic voltammetry (CV) curves of the two films on ITO
ꢀ1
supporting electrolyte and the cyclic voltammetry (CV) curve of the surface throughout the conjugated backbone with continuous
ferrocene standard swept in dichloromethane solution.
positive molecular electrostatic potential (ESP, in red), through
Table 1 Optical and electrochemical properties of BTVCz–NO
2
and BTVCz films
Film (nm)
a
onsetb
ox
c
Molecules
l
max
l
onset
E
g
(eV)
E
(eV)
HOMO (eV)
LUMO (eV)
BTVCz–NO
BTVCz
2
400
385
490
435
2.53
2.85
1.06
0.69
5.40
5.03
2.87
2.18
a
b
c
The data were calculated by the following equation: E
g
¼ 1240/lonset of lms by two molecules. Against Ag/AgCl in dichloromethane. The
HOMO energy levels were calculated from cyclic voltammetry and were referenced to ferrocene (4.8 eV).
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Paper
Fig. 4 Tapping-mode AFM height images and typical cross-section profiles of AFM topographic images of two thin films on ITO substrates: (a
and c) for molecule BTVCz–NO ; (b and d) for molecule BTVCz.
2
deep a trap that the trapped electrons may not be easily
detrapped under non-degrading reverse elds, resulting in a
high conductivity state that can be maintained for a long time.
In sum, the ITO/BTVCz–NO /Al devices show non-volatile
2
ternary WORM-type characteristics.
Whereas only one acceptor (the benzothiazole group) is
located in the conjugated molecular backbone as BTVCz,
therefore, there is only one negative region in the ESP (Fig. 6b)
to act as a charge trap to impede the carrier migration, as
benzothiazole is a shallow trap. The trap is too shallow to
swallow all the charge carriers from the carbazole group and
52,53
then leads to limited delocalization
in the carbazole moie-
ties under the electric eld. What is more, the studied molecule
BTVCz has a weak theoretical dipole moment (3.9 D), which
helps in forming the limited delocalization. When the external
Fig. 5 X-ray diffraction (XRD) scans of two films on the ITO substrate. voltage bias halts, the trapped charge carriers can be de-trapped
to the initial states within one minute and the thin lm of
BTVCz returns to a low-conductivity state, thereby DRAM
bistable switching behavior is achieved. To clarify how these
which charge carriers can migrate. The negative ESP regions (in
blue) caused by the electron-decient groups, such as nitro
“traps” can give up the charge carriers aer the electric eld
disappeared, we studied the conduction mechanism of the
chromophores and benzothiazole moieties, can function as
devices at the “ON” state. Aer the electrical transition, a linear
50,51
“traps” to reduce the mobility of the charge carriers.
Because
1
/2
relationship was observed between log(I/V) and V in the ITO
bottom electrode device (Fig. 6c). This phenomenon follows the
Poole–Frenkel emission theory: the “traps” will become much
shallower under an electrical eld. The change in the depth of
of the orderly and close molecular packing, charges transfer
among the adjacent molecules easily, and all of the traps have
equal opportunities to accept the injected charges. When an
electric eld is applied, the traps arising from the benzothiazole
moiety will be lled up quickly, leading to the current transition
from the OFF to the ON1 state. The traps arising from the nitro
group will be lled up later, resulting in the current transition
54
“traps” is according to the following formula:
1
2
pffiffiffiffi
ꢀ
ꢁ
ꢂ
3
DE ¼ q p3
q
3
0
F
(4)
from the ON1 to the ON2 state. Upon the HOMO to LUMO where DE is the decrement of “traps”, q is the charge of an
transition, the electron density shis from the donor to the electron, 3
is the dielectric constant of the lm, 3 is the
0
q
acceptor side and becomes more localized. The nitro group is so permittivity of free space and F is the electric eld intensity.
5678 | J. Mater. Chem. C, 2014, 2, 5673–5680
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Journal of Materials Chemistry C
electric eld was applied, while its “counterpart” device based
on BTVCz lms exhibited volatile binary DRAM data-storage
behaviors with a switching threshold voltage at ꢀ1.3 V.
Acknowledgements
This work was nancially supported by the Chinese Natural
Science Foundation (21176164, 21206102 and 21336005), the
NSF of Jiangsu Province (BE2013052), a project of the
Department of Education of Jiangsu Province (12KJB430011),
Suzhou Nano-project (ZXG2012023) and the Project supported
by the Specialized Research Fund for the Doctoral Program of
Higher Education of China (Grant no. 20113201130003 and
2
0123201120005).
Notes and references
1
J. Lee, E. Lee, S. Kim, G. S. Bang, D. A. Shultz, R. D. Schmidt,
M. D. E. Forbes and H. Lee, Angew. Chem., Int. Ed., 2011, 50,
4414–4418.
2
3
4
J. S. Lee, J. Mater. Chem., 2011, 21, 14097–14112.
2
Fig. 6 HOMOs, LUMOs and ESP surfaces of (a) BTVCz–NO and (b)
J. C. Scott and L. D. Bozano, Adv. Mater., 2007, 19, 1452–1463.
N. H. You, C. C. Chueh, C. L. Liu, M. Ueda and W. Chen,
Macromolecules, 2009, 42, 4456–4463.
BTVCz in their optimized ground-state structures. (c) Linear relation-
1/2
ship between log(I/V) and V using ITO as the bottom electrode.
5
D. I. Son, T. W. Kim, J. H. Shim, J. H. Jung, D. U. Lee,
J. M. Lee, W. I. Park and W. K. Choi, Nano Lett., 2010, 10,
When an electrical eld is applied, the injection number of q
and the electric eld strength (F) increases, leading to a shal-
lower trap depth.
2
441–2447.
C. W. Chu, J. Ouyang, J. H. Tseng and Y. Yang, Adv. Mater.,
005, 17, 1440–1443.
6
7
8
2
To clarify how the shallow trap discharges when the external
bias halts, rst of all, we selected a brand-new cell on the device,
and then we applied a negative sweep from 0 to ꢀ4 V (sweep 1).
It was witnessed that the device underwent sharp electrical
transitions from the low-conductivity state (“OFF” state) to the
high-conductivity state (“ON” state). Aer that, we applied a
negative scan from 0 to ꢀ4 V at intervals of 0 s, 10 s, 20 s, 30 s, 40
s, 50 s and 60 s. Here, Dt ¼ 0 s means that we applied the same
sweep from 0 to ꢀ4 V. Immediately aer sweep 1 it was found
that the device was still at its “ON” state. As shown in Fig. S6,†
the “OFF” current decreased dramatically in the rst several
seconds, and then the momentum of its reduction slowed
down, but it did not return to its original “OFF” state until
Dt ¼ 60 s when its I–V curve almost overlapped with that of
sweep 1. Therefore, the shallow trap arising from the benzo-
thiazole group does not lose all the charge carriers it swallowed
before immediately when the external bias halts.
Q. D. Ling, D. J. Liaw, C. Zhu, D. S. H. Chan, E. T. Kang and
K. G. Neoh, Prog. Polym. Sci., 2008, 33, 917–978.
E. Kapetanakis, A. Douvas, D. Velessiotis, E. Makarona,
P. Argitis, N. Glezos and P. Normand, Adv. Mater., 2008, 20,
4568–4574.
9
J. Ouyang, C. Chu, C. Szmanda, L. Ma and Y. Yang, Nat.
Mater., 2004, 3, 918–922.
0 C. Ye, Q. Peng, M. Li, J. Luo, Z. Tang, J. Pei, J. Chen, Z. Shuai,
L. Jiang and Y. Song, J. Am. Chem. Soc., 2012, 134, 20053–
2
1 Y. Ma, Y. Wen, J. Wang, Y. Shang, S. Du, L. Pan, G. Li,
L. Yang, H. Cao and Y. Song, J. Phys. Chem. C, 2009, 113,
8
2 Y. Shang, Y. Wen, S. Li, S. Du, X. He, L. Cai, Y. Li, L. Yang,
H. Gao and Y. Song, J. Am. Chem. Soc., 2007, 129, 11674–
1
3 G. Li, K. Zheng, C. Wang, K. Leck, F. Hu, X. Sun and
Q. Zhang, ACS Appl. Mater. Interfaces, 2013, 5, 6458–6462.
4 P. Gu, F. Zhou, J. Gao, G. Li, C. Wang, Q. Xu, Q. Zhang and
J. Lu, J. Am. Chem. Soc., 2013, 135, 14086–14089.
1
1
1
0059.
548–8552.
1675.
1
1
Conclusions
In summary, two organic small molecules, BTVCz–NO
BTVCz, were synthesized and fabricated as the electroactive
2
and 15 R. Naber, K. Asadi, P. Blom, D. de Leeuw and B. de Boer, Adv.
Mater., 2010, 22, 933–945.
layers of the sandwich-structure memory devices. Both mole- 16 J. C. Scott, Science, 2004, 304, 62–63.
cules were stacked closely in their spin-coated lms with minor 17 K. Kim, Y.-K. Fang, W. Kwon, S. Pyo, W.-C. Chen and M. Ree,

uctuations. The device based on BTVCz–NO2 lms showed
J. Mater. Chem. C, 2013, 1, 4858–4868.
nonvolatile ternary WORM data-storage behaviors with switch- 18 C.-J. Chen, H.-J. Yen, Y.-C. Hu and G.-S. Liou, J. Mater. Chem.
ing threshold voltages at ꢀ1.5 V and ꢀ2.5 V in tandem when an
C, 2013, 1, 7623–7634.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5673–5680 | 5679

View Article Online
Journal of Materials Chemistry C
Paper
1
9 S. Park, K. Kim, D. M. Kim, W. Kwon, J. Choi and M. Ree, ACS 38 S. Miao, H. Li, Q. Xu, N. Li, J. Zheng, R. Sun, J. Lu and
Appl. Mater. Interfaces, 2011, 3, 765–773. C. M. Li, J. Mater. Chem., 2012, 22, 16582–16589.
0 S. G. Hahm, S. Choi, S.-H. Hong, T. J. Lee, S. Park, D. M. Kim, 39 S. Miao, H. Li, Q. Xu, Y. Li, S. Ji, N. Li, L. Wang, J. Zheng and
2
J. C. Kim, W. Kwon, K. Kim, M.-J. Kim, O. Kim and M. Ree, J.
Mater. Chem., 2009, 19, 2207–2214.
1 T. J. Lee, Y.-G. Ko, H.-J. Yen, K. Kim, D. M. Kim, W. Kwon,
J. Lu, Adv. Mater., 2012, 24, 6210–6215.
40 B. Lei, W. L. Kwan, Y. Shao and Y. Yang, Org. Electron., 2009,
10, 1048–1053.
2
S. G. Hahm, G.-S. Liou and M. Ree, Polym. Chem., 2012, 3, 41 W.-J. Joo, T.-L. Choi, J. Lee, S. K. Lee, M.-S. Jung, N. Kim and
276–1283. J. M. Kim, J. Phys. Chem. B, 2006, 110, 23812–23816.
1
2
2
2
2
2
2 W. Kwon, B. Ahn, D. M. Kim, Y.-G. Ko, S. G. Hahm, Y. Kim, 42 J. Lin and D. Ma, Appl. Phys. Lett., 2008, 93, 093505.
H. Kim and M. Ree, J. Phys. Chem. C, 2011, 115, 19355–19363. 43 H. Wang, G. Chen, X. Xu, H. Chen and S. Ji, Dyes Pigm., 2010,
3 Y.-H. Chou, H.-J. Yen, C.-L. Tsai, W.-Y. Lee, G.-S. Liou and
W.-C. Chen, J. Mater. Chem. C, 2013, 1, 3235.
4 L.-H. Xie, Q.-D. Ling, X.-Y. Hou and W. Huang, J. Am. Chem.
Soc., 2008, 130, 2120–2121.
5 C.-J. Chen, H.-J. Yen, W.-C. Chen and G.-S. Liou, J. Mater.
Chem., 2012, 22, 14085–14093.
86, 238–248.
44 M. Surin, P. Sonar, A. C. Grimsdale, K. M u¨ llen, S. De Feyter,
S. Habuchi, S. Sarzi, E. Braeken, A. V. Heyen, M. Van Der
Auweraer, F. C. De Schryver, M. Cavallini, J. F. Moulin,
F. Biscarini, C. Femoni, L. Roberto and P. Lecl `e re, J. Mater.
Chem., 2007, 17, 728–735.
6 Y. Lai, K. Ohshimizu, W. Lee, J. Hsu, T. Higashihara, 45 R. Schmidt, J. H. Oh, Y.-S. Sun, M. Deppisch, A.-M. Krause,
M. Ueda and W.-C. Chen, J. Mater. Chem., 2011, 21, 14502–
4508.
K. Radacki, H. Braunschweig, M. K ¨o nemann, P. Erk, Z. Bao
and F. W u¨ rthner, J. Am. Chem. Soc., 2009, 131, 6215–
6228.
1
2
7 J. Liu, Z. Yin, X. Cao, F. Zhao, A. Lin, L. Xie, Q. Fan, F. Boey,
H. Zhang and W. Huang, ACS Nano, 2010, 4, 3987–3992.
8 Q. D. Ling, F. C. Chang, Y. Song, C. X. Zhu, D. J. Liaw,
D. S. H. Chan, E. T. Kang and K. G. Neoh, J. Am. Chem.
Soc., 2006, 128, 8732–8733.
46 S. Loser, C. Bruns, H. Miyauchi, R. Ortiz, A. Facchetti,
S. Stupp and T. Marks, J. Am. Chem. Soc., 2011, 133, 8142–
8145.
47 L. Li, P. Gao, K. Schuermann, S. Ostendorp, W. Wang, C. Du,
Y. Lei, H. Fuchs, L. De Cola, K. M u¨ llen and L. Chi, J. Am.
Chem. Soc., 2010, 132, 8807–8809.
2
2
9 Y. Z. Lin, Y. F. Li and X. W. Zhan, Chem. Soc. Rev., 2012, 41,
4245–4272.
3
0 K. B. Burke, Y. Shu, P. Kemppinen, B. Singh, M. Bown, 48 T. Chen, X. Wu and R. Rieke, J. Am. Chem. Soc., 1995, 117,
I. I. Liaw, R. M. Williamson, L. Thomsen, P. Dastoor, 233–244.
W. Belcher, C. Forsyth, K. N. Winzenberg and G. E. Collis, 49 T. Prosa, M. Winokur, J. Moulton, P. Smith and A. Heeger,
Cryst. Growth Des., 2012, 12, 725–731. Macromolecules, 1992, 25, 4364–4372.
1 Q. Q. Shi, P. Cheng, Y. F. Li and X. W. Zhan, Adv. Energy 50 X.-D. Zhuang, Y. Chen, G. Liu, B. Zhang, K.-G. Neoh,
3
3
3
3
Mater., 2012, 2, 63–67.
E.-T. Kang, C.-X. Zhu, Y.-X. Li and L.-J. Niu, Adv. Funct.
Mater., 2010, 20, 2916–2922.
2 A. W. Hains, Z. Liang, M. A. Woodhouse and B. A. Gregg,
Chem. Rev., 2010, 110, 6689–6735.
3 B. Walker, C. Kim and T. Q. Nguyen, Chem. Mater., 2011, 23,
51 G. Liu, B. Zhang, Y. Chen, C.-X. Zhu, L. Zeng, D. S.-H. Chan,
K.-G. Neoh, J. Che and E.-T. Kang, J. Mater. Chem., 2011, 21,
6027–6033.
470–482.
4 Y. Li, Q. Guo, Z. Li, J. Pei and W. Tian, Energy Environ. Sci., 52 Q. Ling, Y. Song, S. Lim, E. Teo, Y. Tan, C. Zhu, D. Chan,
2
010, 3, 1427–1436.
D. Kwong, E. Kang and K. Neoh, Angew. Chem., 2006, 118,
3013–3017.
3
3
5 J. Roncali, Acc. Chem. Res., 2009, 42, 1719–1730.
6 H. X. Shang, H. J. Fan, Y. Liu, W. P. Hu, Y. F. Li and 53 Y. Fang, C. Liu, G. Yang, P. Chen and W. Chen,
X. W. Zhan, Adv. Mater., 2011, 23, 1554–1557. Macromolecules, 2011, 44, 2604–2612.
7 H. Li, Q. Xu, N. Li, R. Sun, J. Ge, J. Lu, H. Gu and F. Yan, J. Am. 54 G. Ottaviani, C. Canali, C. Jacoboni and A. Quaranta, J. Appl.
Chem. Soc., 2010, 132, 5542–5543. Phys., 1973, 44, 360.
3
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