Study of the influences of molecular planarity and aluminum evaporation rate on the performances of electrical memory devices

Article abstract of DOI:10.1039/c4tc00311j

In this paper, two conjugated small molecules (CZ-BT and TPA-BT) were designed and successfully synthesized to investigate the influence of different electron donor moieties on thin-film morphology and their electrical memory device properties. Current-vo

Full text of DOI:10.1039/c4tc00311j

Journal of
Materials Chemistry C
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PAPER
Study of the influences of molecular planarity and
aluminum evaporation rate on the performances of
electrical memory devices†
Cite this: J. Mater. Chem. C, 2014, 2,
5709
a
a
a
a
a
a
*
Hongzhang Liu, Rongcheng Bo, Haifeng Liu, Najun Li, Qingfeng Xu, Hua Li,
ab
*
Jianmei Lu
and Lihua Wanga
In this paper, two conjugated small molecules (CZ-BT and TPA-BT) were designed and successfully
synthesized to investigate the influence of different electron donor moieties on thin-film morphology
and their electrical memory device properties. Current–voltage (I–V) curves indicated that both CZ-BT
and TPA-BT exhibited stable dynamic random access memory (DRAM) characteristics under appropriate
Al evaporation rate condition, but with different threshold voltages, the former lower than the latter due
to its better film morphology and closer intermolecular stacking. Furthermore, the performances of the
two devices under different Al electrode evaporation rates was also investigated, and the results showed
that the CZ-BT-based device still exhibited original DRAM characteristics, whereas the TPA-BT-based
device exhibited WORM characteristic when Al electrode evaporation rate was increased to 5 A˚ sꢀ1. The
variation of storage characteristics of TPA-BT could be mainly attributed to its poor film-forming
property, and the Al nano-particles would penetrate into the film during the quick evaporation process
to form filamentary conduction leading to WORM behavior.
Received 16th February 2014
Accepted 19th May 2014
DOI: 10.1039/c4tc00311j
www.rsc.org/MaterialsC
performance is yet a vital topic. So many factors that would have
an effect on device performance still remain unknown, such as
lm surface roughness,32 lm thickness33–36 and molecular or
Introduction
Microelectronics technology based on small organic molecules
and polymers have attracted increasing interest due to their
many advantages, such as low cost, large scalability, and
structure designable.1–15 Especially, the above-mentioned
materials have the potential to be fabricated as high density
data storage devices compared with traditional silicon-based
devices owing to their 3D-stacking ability, which could greatly
improve the number of memory cells in unit volume.1,3,16–20
Kang,21 Ree22–24 and Chen23,25–27 groups have put great effort into
studying the relationship between molecular structure and
device storage type. For example, they have achieved different
memory types such as WORM, Flash, SRAM and DRAM devices
through adjustment of strength and weakness of electron
donors and acceptors. Our group has been engaging in tuning
memory device characteristics through changing molecular
conjugated length,28 so-alkyl chain length29,30 and molecular
planarity.31 However, the stability of electric memory device
electrode deposition rate.37
Carbazole (CZ) and triphenylamine (TPA) are well-known
electron donors and oen used as the donor fragments of hole
transporting material.32,38–40 Compared with other electron
donors, triphenylamine compounds can form ammonium ion
radicals in the electric eld, leading to a good hole transport
property. Generally, such compounds have suitable glass tran-
sition temperature and excellent thermal and chemical
stability; these properties ensure a long-term and stable device.
In contrast, carbazole is also a well-known electron donor and
widely used for high thermal stability of hole transport material
due to its special rigid structure and a variety of functional
modications in positions 3, 6, and 9 of its molecular back-
bone.41 In this paper, we synthesized two molecules which
contain TPA and CZ moieties (CZ-BT and TPA-BT) as shown in
Scheme 1.
According to results revealed by AFM and XRD, CZ-BT has a
better lm-forming property, with RMS of about 2 nm, than that
of TPA-BT, with RMS of about 10 nm. I–V characteristics indi-
cated that both molecule-based devices exhibit stable DRAM
characteristics, and the switching threshold of CZ-BT is lower
than that of TPA-BT. This is due to CZ-BT's better planarity,
leading to a closer intermolecular p–p stacking. Furthermore,
the inuence of the different evaporation rate of aluminum
electrode was investigated, and it was found that when the
aCollege of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou, Jiangsu 215123, China. E-mail: lihuaw@suda.edu.cn; lujm@suda.edu.cn;
Fax: +86 512 65880367; Tel: +86 512 65880368
bState Key Laboratory of Treatments and Recycling for Organic Effluents by Adsorption
in Petroleum and Chemical Industry, Suzhou 215123, China
† Electronic supplementary information (ESI) available: TGA, 3D AFM images,
stimulus effect of read cycles, I–V characteristic. See DOI: 10.1039/c4tc00311j
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moiety. And UV-Vis spectra of TPA-BT in solution also generated
two absorption peaks (295 nm and 375 nm), and the generation
mechanism was similar to the former.42–44 We also measured
the UV-Vis absorption spectrum of the vacuum-deposited thin
lms of CZ-BT and TPA-BT. Compared with the absorption
spectrum in solution state, both absorption peaks in the lm
state experienced a red-shi (from 341 nm to 352 nm in CZ-BT
lm) or signicantly broadened (in TPA-BT lm), indicating
that they stacked closely and in an orderly fashion during
molecule deposition state,45 which is coincident with the results
revealed by AFM and XRD.
Scheme 1 Chemical structures and synthesis routes of two small
Fig.
2 shows the lm surface morphology of both
molecules.
compounds. The tapping mode AFM phase image of the CZ-BT
lm exhibits a smooth, featureless morphology, as shown in
Fig. 2a; root-mean-square (RMS) roughness does not exceed
5 nm (Fig. 2c). This type of at and featureless morphology of
the lm facilitates the charge injection from the electrode into
the organic molecule layer.32 Moreover, good surface
morphology can also prevent aluminum (Al) from penetrating
into the organic lm during the vacuum deposition of the
aluminum electrode. However, the regular and hill-like
morphology of the “counterpart” compound TPA-BT shows the
rougher morphology surface and the RMS is up to 10 nm
(Fig. 2b and d). 3D-AFM topography images of the two
compounds are shown in Fig. S2a and S2b (ESI†). In addition,
we investigated the tapping-mode AFM phase image of the TPA-
BT lm, which was prepared by vacuum vapor deposition using
evaporation rate of aluminum was increased to 5 A sꢀ1, the TPA-
˚
BT-based device showed WORM characteristic compared to its
original DRAM characteristic as the evaporation rate of
aluminum was about 0.5 A sꢀ1. However, the memory behavior
˚
of the CZ-BT-based device had nothing to do with the electrode
evaporation rate. We hope the concluded inuences of molec-
ular planarity and electrode evaporation rate on the device
performance could offer guidance in future materials design
and synthesis.
Results and discussion
ꢁ
Thermal properties of CZ-BT and TPA-BT were investigated by
TGA (Fig. S1, ESI†). All of the compounds exhibit good thermal
stability with thermal decomposition temperatures (5% weight
loss temperature) of 294 ꢁC (CZ-BT) and 272 ꢁC (TPA-BT),
respectively. It indicates that materials can endure heat deteri-
oration in memory devices and have a relatively wide range in
practice.
As shown in Fig. 1, the UV-Vis absorption spectrum of CZ-BT
in dilute dichloromethane solution exhibited an absorption
peak at 291 nm, which is attributed to the n–p* transition of the
carbazole moiety and a stronger absorption peak at 341 nm due
to the p–p* transition of the aromatic groups of the carbazole
a heated substrate (about 60 C), and we found the accumula-
tion of molecules tended to be worse as shown in Fig. S3.† XRD
patterns of the two compounds lms deposited on ITO glass are
shown in Fig. 3. There were three diffraction peaks in the CZ-BT
lm, which are located at 2q ¼ 6.1ꢁ, 10.2ꢁ and 20.4ꢁ (4.3 A),
˚
indicating the CZ-BT molecule adopts well p–p stacking in thin
ꢁ
˚
lm. Furthermore, the 2q ¼ 10.2 (8.6 A) is almost twice that of
ꢁ
˚
2q ¼ 20.4 (4.3 A), indicating that the CZ-BT molecule has good
layer-by-layer stacking in the lm. Similarly, in the TPA-BT lm,
ꢁ
ꢁ
˚
there were two diffraction peaks at 2q ¼ 9.0 and 18.1 (4.9 A),
respectively. The sharp diffraction peaks and layer-by-layer
Fig. 2 Tapping-mode height and a typical cross-section profile of
Fig. 1 UV-Vis absorption spectra of two compounds in CH2Cl2 AFM topographic images of compounds' thin film vacuum-deposited
solution and solid thin films on a quartz substrate.
onto ITO at room temperature: CZ-BT (a and c); TPA-BT (b and d).
5710 | J. Mater. Chem. C, 2014, 2, 5709–5716
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ELUMO ¼ EHOMO + Eg,
where EHOMO and ELUMO are HOMO and LUMO energy levels,
(onset)
ox
respectively, and E
is the onset oxidation potential of the
molecule. Eferrocene is the external standard potential of the
ferrocene/ferrocenium ion couple, 4.80 is the reference energy
level of ferrocene. HOMO and LUMO values were determined to
be ꢀ5.25 (or ꢀ5.36) and ꢀ2.11 (or ꢀ2.43) eV for CZ-BT and TPA-
BT in DCM, respectively. The energy barrier (F1) between work
functions of ITO (4.8 eV) and HOMO energy level for CZ-BT was
0.45 eV, which is much lower than the energy barrier (F2) of 2.17
eV between the work functions of Al (4.28 eV) and the LUMO
energy level. This indicates that hole injection from ITO into the
HOMO level was much easier than electron injection from Al
into the LUMO level for CZ-BT. TPA-BT had similar electro-
chemical properties. Therefore, both compounds are p-type
materials, and hole injection predominates the conduction
process of the memory devices.30,46 All the above data are listed
in Table 1.
A schematic sandwich structure comprising many top elec-
trodes (Al), a layer of small organic molecules and the bottom
electrode (ITO) are shown in Fig. 5a. The thickness of the
electroactive organic lm was about 200 nm, as measured by
scanning electron microscopy (SEM). The current–voltage (I–V)
characteristics of the ITO/CZ-BT/Al device are shown in Fig. 5b.
In the rst round scan from 0 to ꢀ4 V, the current increased
dramatically at a threshold voltage around ꢀ1.42 V, indicating a
transition from a low-conductivity state (OFF state) to a high-
conductivity state (ON state). The device remained in this ON
state during a subsequent scan followed by the rst scan from
0 to ꢀ4 V (sweep 2). However, about one minute of power off
aer the second scan, the current was back to the OFF state and
an abrupt current increase could be observed again at switching
threshold voltages around ꢀ1.46 V by the third voltage scan
(sweep 3). The device remained in this ON state during an
immediate subsequent scan from 0 to 4 V (sweep 4). The char-
acteristics of the device relaxing to OFF state aer power off
illustrated that the device based on CZ-BT was of a dynamic
random access memory (DRAM) type. Under a constant voltage
of ꢀ1 V, no signicant degradation was observed in the OFF and
ON states for over 100 min as shown in the inset in Fig. 5b.
Furthermore, the OFF and ON states were also stable up to 108
write–read cycles under the effect of continuous read pulses of
ꢀ1 V as shown in Fig. S4a (ESI†). Similar to CZ-BT, the device
based on TPA-BT showed DRAM memory characteristics, too, as
shown in Fig. 5c. However, the switching voltage of the TPA-BT-
based device was located at about ꢀ2.0 V, which is higher than
that of the CZ-BT-based device; this possibly was due to the
higher energy gap between the organic lm and the ITO
Fig. 3 X-ray diffraction spectra of CZ-BT and TPA-BT films deposited
on ITO glass substrates.
stacking of CZ-BT and TPA-BT lms illustrate that both mole-
cules self-assembled well into highly ordered crystalline lm.
Meanwhile, it can be concluded that the CZ-BT molecule packed
more closely and in a more orderly fashion than the TPA-BT
based on the results of AFM morphology and XRD spectrum.
The band gap of CZ-BT between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molec-
ular orbital (LUMO) energy levels estimated from the onset of
the absorption spectrum is 3.14 eV by the following equation: Eg
¼ hc/ledge. The electrochemical property of CZ-BT was measured
by cyclic voltammetry (CV) using a 0.1 M solution of tetrabuty-
lammonium perchlorate (TBAP) in anhydrous dichloromethane
as shown in Fig. 4. The onset oxidation E(ooxnset) of CZ-BT was 0.97
eV. Note that the Eferrocene was 0.52 eV from the CV measure-
ment. HOMO and LUMO energy levels were calculated by the
following equation:
(onset)
ox
EHOMO ¼ ꢀ[E
+ 4.8 – Eferrocene]
Table 1 Optical and electrochemical properties of both compounds
Fig. 4 Cyclic voltammetry (CV) curves of two compounds in anhy-
drous dichloromethane solution with 0.1 M of N+Bu4ClO4 as sup- Compound Eg (eV) HOMO (eV) LUMO (eV) F1 (eV) F2 (eV)
porting electrolyte. Inset of the figure shows CV curve of ferrocene
ꢀ
CZ-BT
TPA-BT
3.14
2.93
ꢀ5.25
ꢀ2.11
0.45
0.56
2.17
1.85
standard swept in the same conditions as that of the two compounds.
Scan rate was 100 mV sꢀ1
ꢀ5.36
ꢀ2.43
.
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Fig. 5 (a) (Top) Schematic diagram of sandwich device; (bottom) SEM image of a cross-section view of device; current–voltage (I–V) char-
acteristics of ITO/compound/Al device (evaporation rate of aluminum is about 0.5 A˚ sꢀ1) and its corresponding stability in two states under a
constant read voltage of ꢀ1 V, (b) CZ-BT, (c) TPA-BT; current–voltage (I–V) characteristics of memory device fabricated with compound film
(evaporation rate of aluminum is 5 A˚ sꢀ1), (d) CZ-BT, and (e) TPA-BT.
electrode. The inset in Fig. 5c and S4b (ESI†) exhibits that the storage type of the device transformed into a typical nonvolatile
device based on TPA-BT also had long-term stability. In addi- WORM memory behavior from DRAM memory behavior as
tion, the retention time test of the two compound-based devices shown in Fig. 5e, mainly due to the rapid evaporation rate,
was measured to clarify the volatile process of the device as leading to the aluminum penetration, which was substantiated
shown in Fig. S5.† It is noteworthy that the ON/OFF current via investigating the cross-section EDS (Fig. S6†) of the corre-
ratios of both compounds were almost up to 105, which lead to a sponding device. To further illustrate the above phenomenon,
low misreading rate for memory storage device applications; we deposited a layer of LiF on the TPA-BT lm with a thickness
additionally, switching threshold voltages were all lower than of about 5 nm. Then, aluminum electrode was evaporated onto
ꢀ2 V, and the reading voltage was only ꢀ1 V, which indicates the layer of LiF with a rate of 5 A sꢀ1. Thus, the formed device
˚
that the organic memory device has high stability and low exhibited original DRAM behavior as shown in Fig. S7.† In
power consumption.30,47
addition, we used metal Ag instead of the Al top electrode to
Above, we have concluded that the CZ-BT molecule packed demonstrate related metal lamentary mechanism; the I–V
more closely and in a more orderly fashion than the TPA-BT in curves are presented in Fig. S8.† In contrast, the CZ-BT-based
the deposited lm. We also explored that current–voltage device still exhibited initial DRAM behavior when the evapora-
ꢀ1
˚
characteristics of the TPA-BT-based device under the condition tion rate of the aluminum electrode was 5 A s as shown in
of an aluminum evaporation rate of 5 A sꢀ1 and found that the Fig. 5d. In addition, we found that the switching threshold
˚
5712 | J. Mater. Chem. C, 2014, 2, 5709–5716
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voltage (about ꢀ1.5 V) of the device based on CZ-BT was lower and the memory device rapidly returns to the original OFF state
than TPA-BT, which indicates that the memory storage device in a very short time. Therefore, the unstable CT states lead to
based on CZ-BT can transmit charge easier, which related well the DRAM behavior. Besides, the calculated dipole moment of
to the conclusion above.
CZ-BT is only 1.197 D and 1.913 D for TPA-BT, indicating that
To better understand the switching behavior of the devices the polarities of the two compounds are not strong enough to
based on the two compounds, the electronic properties of CZ-BT retain the charge transfer state and thus account for the short
and TPA-BT were explored through density functional theory retention time of the ON state.44,47 In addition, orderly and close
with the Gaussian 03 program. HOMO, LUMO and LUMO+1 molecular packing in the lm of the CZ-BT can facilitate
surfaces for the two molecules are shown in Fig. 6. Both HOMO formation of free carrier transport pathways and reduce the
and LUMO+1 tend to locate on the donor side (CZ, TPA), charge carrier injection barrier from the ITO to the CZ-BT layer
whereas LUMO locates on the acceptor (BT) and forms the (Fig. 6c).49 This means that the storage cell of the device based
distinct asymmetric distribution. Some electrons at HOMO may on CZ-BT switches from OFF to ON state under a relatively low
overcome the band gap via accumulating energy, then transit- voltage bias. In contrast, the storage cell of the device based on
ing to LUMO+1, resulting in an excited state. On the other hand, TPA-BT needs a higher turn-on voltage to reach ON state
electrons at HOMO could also be excited to LUMO with a lower because of its relatively poor intermolecular packing. When the
˚
energy barrier. So, this implies that CT can occur to form a evaporation rate of the aluminum electrode was increased to 5 A
conductive channel through the process either from LUMO+1 of sꢀ1, aluminum nanoparticles penetrated into the lm of TPA-BT
donor to the LUMO of acceptor or from the HOMO of donor to because of the rough surface and loose intermolecular packing
the LUMO of acceptor.9,47,48 This indicates that the excitation of of the lm, as well as the greater momentum of aluminum
the donor units (CZ, TPA) leads to the intra- or intermolecular nanoparticles. The penetrated aluminum nanoparticles inter-
CT state. With interaction between molecules in the lm, the acted with TPA-BT molecules and enhanced intramolecular or
open pathway will be formed by the increase in migrating intermolecular charge transfer in the excited state, resulting in a
carriers. Thus, the device can switch from the OFF state to the stable charge transfer state.50–54 When the device is powered off
ON state. When the power is turned off, the charge recombi- or under non-degrading reverse elds, the ON state of the
nation through the D–A connection due to the unstable CT state memory device can still remain due to the stable localization of
electrons in the acceptor fragment. Therefore, the performance
of the device is WORM behavior. When the TPA-BT layer was
covered with a layer of LiF, aluminum nanoparticles did not
have enough energy to penetrate to the LiF layer, and all the
aluminum nanoparticles were almost blocked outside of the LiF
layer. So, the device with the structure of ITO/TPA-BT/LiF/Al
showed initial DRAM behavior. In contrast, aluminum nano-
particles may not easily penetrate into the lm of CZ-BT due to
the close intermolecular packing and atter surface of the lm,
so the intramolecular or intermolecular CT of CZ-BT in lm was
hardly affected and still remain unstable. Therefore, the device
of CZ-BT still showed initial DRAM behavior.
Conclusions
In summary, two small molecules containing the same electron-
decient moiety with different electron-rich moieties were
synthesized and fabricated as the electroactive organic layers of
sandwich-structure memory devices. The devices based on the
two compounds demonstrated DRAM memory storage perfor-
mance because of the unstable CT states of the two compounds.
But the CZ-BT-containing carbazole fragment stacked more
closely and in a more orderly fashion in the thin lm than the
TPA-BT during vacuum-deposition, resulting in lower operating
voltage, which is highly desirable for low-power consumption
data-storage devices. In addition, the device based on TPA-BT
exhibited WORM memory storage performance when
aluminum evaporation rate was increased to 5 A sꢀ1. But the
˚
device based on CZ-BT still exhibited the original DRAM
behavior. Another conclusion is that the device based on TPA-
Fig. 6 LUMOs+1, LUMOs and HOMOs of (a) CZ-BT and (b) TPA-BT. (c)
Energy level diagram for the ITO/compound/Al devices. Work func-
tions of ITO and Al electrodes are ꢀ4.80 and ꢀ4.28 eV, respectively.
BT exhibited initial DRAM type behavior in the case of
ꢀ1
˚
aluminum evaporation rate of 5 A s when a layer of LiF was
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added between the TPA-BT lm and ITO layer. Molecular extracted repeatedly with chloroform. The organic layer was
structure, thin-lm morphology, and aluminum evaporation dried over anhydrous magnesium sulfate, the solvent was
rate were identied to play vital roles in the electroactive layer, removed under reduced pressure. The crude product was puri-
and device performance will be better by enhancing intermo- ed through silica gel column with ethyl acetate and petroleum
lecular interactions and the orderly accumulation of molecules. ether (volume ratio ¼ 1 : 10). Yield: 85%. 1H-NMR (400 MHz, d6-
We hope the results discussed here can provide a reference to DMSO) d (ppm): 9.75 (s, 1H), 7.70 (d, J ¼ 8.68 Hz, 2H), 7.40 (t, J ¼
the design and synthesis of electrical storage volatile material 7.72 Hz, 4H), 7.24–7.15 (m, 6H), 6.87 (d, J ¼ 8.71 Hz, 2H).
and stable memory device with low power consumption.
4-(Benzo[d]thiazol-2-yl)-N,N-diphenylaniline (TPA-BT).
Compound 4 (1.01 g, 3.7 mmol), 2-aminothiophenol (0.488 g,
3.9 mmol) and DMSO (50 mL) were added to a single-necked
Experimental section
ꢁ
ask. Aer stirring for 6 h at 160 C, the reaction mixture was
Materials
cooled to room temperature, and it was then poured into ice
water and isolated by ltration. The nal light yellow product
was obtained by column chromatography with dichloro-
methane and petroleum ether (volume ratio ¼ 1 : 3). Yield: 90%.
1H NMR (400 MHz, d6-DMSO) d (ppm): 8.09 (d, J ¼ 7.99 Hz, 1H),
7.98 (dd, J ¼ 8.46, 12.64 Hz, 3H), 7.50 (d, J ¼ 8.03 Hz, 1H), 7.40
(dd, J ¼ 7.48, 15.43 Hz, 5H), 7.17 (dd, J ¼ 7.68, 11.00 Hz, 6H),
7.01 (d, J ¼ 8.69 Hz, 2H). 13C NMR (400 MHz, CDCl3) d (ppm):
167.71, 154.18, 150.30, 146.79, 134.71, 129.43, 128.46, 126.44,
126.10, 125.31, 124.63, 123.95, 122.67, 121.62, 121.41. HRMS
Triphenylamine (95%), carbazole (CP), phosphorus oxychloride
(95%), 2-aminothiophenol (95%), 4-bromobenzaldehyde (98%),
18-crown-6 (CP) and o-dichlorobenzene (98%) were purchased
from TCI, Alfa Aesar, and Aladdin Co., Ltd, respectively. Other
solvents and drugs were all purchased from Sinopharm Reagent
Co., Ltd., and used as received without further purication.
Preparation of CZ-BT and TPA-BT
4-(9H-Carbazol-9-yl)benzaldehyde (3). Carbazole 1 (5.21 g, 30 (ESI, m/z) calcd. for C25H18N2S: 379.1269 (M + H+). Found:
mmol), 4-bromobenzaldehyde (8.5 g, 45 mmol), anhydrous 379.1267 (M+H+).
potassium carbonate (8.66 g, 0.135 mol), copper (4.77 g, 75
mmol), 18-crown-6 (1.19 g, 4.5 mmol) were added together into
Fabrication of memory device
250 mL three-necked ask, then 150 mL o-dichlorobenzene was
added to the above mixture. The mixture was heated to 180 C
The indium tin oxide (ITO) glass substrate was carefully pre-
cleaned sequentially with deionized water, acetone and alcohol
by ultrasonic bath for 30 min, respectively. The electroactive
organic lm was deposited under high vacuum (10ꢀ6 Torr). The
lm thickness was about 200 nm. An aluminum (Al) layer with
thickness of about 100 nm was thermally evaporated and
deposited onto the organic surface at about 2 ꢂ 10ꢀ6 Torr with
ꢁ
and stirred under argon atmosphere for 48 h. Then the mixture
was cooled to room temperature and the o-dichlorobenzene was
removed under reduced pressure. The yellow powdered solid
was isolated aer column chromatography (n-hexane–ethyl
acetate ¼ 4 : 1). Yield: 70%. 1H NMR (400 MHz, CDCl3) d (ppm):
10.12 (s, 1H), 8.16–8.13 (m, 4H), 7.80 (d, J ¼ 8.30 Hz, 2H), 7.51
(d, J ¼ 8.19 Hz, 2H), 7.44 (t, J ¼ 7.62 Hz, 2H), 7.34 (t, J ¼ 7.40 Hz,
2H).
2-(4-(9H-Carbazol-9-yl)phenyl)benzo[d]thiazole (CZ-BT).
Compound 3 (1.7 g, 6.27 mmol) and 2-aminothiophenol (0.788
g, 6.3 mmol) were added into DMSO, and the mixture was
heated to 150 ꢁC and stirred for 6 h. The mixture was cooled to
room temperature and poured into ice water, and then the
mixture was ltered and washed with deionized water. The
rate ranging from 0 to 5 A sꢀ1 through a shadow mask to form
˚
the top electrode. An active device area of 0.0314 mm2 was
obtained. In addition, the LiF layer with thickness of about 5
nm was formed at a rate of 2 A sꢀ1 under the same conditions.
˚
Devices with the structures of ITO/organic compound/Al and
ITO/organic compound/LiF/Al were obtained.
Measurement
crude product was puried by silica gel chromatography with 1H NMR spectra were obtained on Inova 400 MHz FT-NMR
ethyl acetate–petroleum ether (volume ratio ¼ 1 : 10) as the spectrometers. Thermogravimetric analysis (TGA) was con-
eluent to afford CZ-BT as white solid. Yield: 95%. 1H NMR (400 ducted on a TꢀA1Instruments Dynamic TGA 2950 at a heating rate
MHz, d6-DMSO) d (ppm): 8.39 (s, 2H), 8.27 (s, 2H), 8.21 (d, J ¼ of 10 C min under an N2 ow rate of 50 mL minꢀ1. UV-Vis
ꢁ
8.03 Hz, 1H), 8.13 (d, J ¼ 8.12 Hz, 1H), 7.88 (d, J ¼ 8.53 Hz, 2H), absorption spectra were recorded by a PerkinElmer Lambda-17
7.51 (d, J ¼ 24.39 Hz, 6H), 7.34 (s, 2H). 13C NMR (400 MHz, spectrophotometer in the 250–700 nm spectral region at room
CDCl3) d (ppm): 166.52, 153.77, 139.96, 139.68, 134.74, 131.86, temperature. Cyclic voltammetry was performed at room
128.62, 126.73, 126.11, 125.76, 125.00, 123.30, 122.94, 121.31, temperature using a platinum–carbon working electrode, a
120.03, 109.40. HRMS (ESI, m/z) calcd. for C25H16N2S: 377.1112 reference electrode Ag/AgCl, and a counter electrode (Pt wire) at
(M + H+). Found: 377.1107 (M+H+).
a sweep rate of 100 mV sꢀ1 on a Corr-Test CS Electrochemical
4-(Diphenylamino)benzaldehyde (4). DMF (8 mL) was added Workstation analyzer; a 0.1 M solution of tetrabutylammonium
into 100 mL three-necked ask and stirred in an ice bath for 30 perchlorate (TBAP) in anhydrous dichloromethane was used.
min, and then phosphorus oxychloride (4 mL) was added slowly Scanning electron microscopy (SEM) images were taken on a
into the mixture. Aer 1.5 h, the resulting viscous liquid was Hitachi S-4700 scanning electron microscope. Atomic force
poured into a solution of triphenylamine 2 (4.9 g, 20 mmol) in microscopy (AFM) measurements were performed using a MFP-
chloroform (50 mL) at 80 ꢁC and stirred for 7 h. The mixture was 3DTM (Digital Instruments/Asylum Research) AFM instrument
5714 | J. Mater. Chem. C, 2014, 2, 5709–5716
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Journal of Materials Chemistry C
to measure the surface morphology of lms. X-ray diffraction 16 D. I. Son, T. W. Kim, J. H. Shim, J. H. Jung, D. U. Lee,
(XRD) patterns were taken on an X'Pert-Pro MPD X-ray diffrac-
J. M. Lee, W. I. Park and W. K. Choi, Nano Lett., 2010, 10,
tometer. All electrical measurements of the devices were char-
2441.
acterized under ambient conditions without encapsulation 17 J. Lee, H. Chang, S. Kim, G. S. Bang and H. Lee, Angew.
using an Agilent Technologies B1500A Semiconductor Device Chem., Int. Ed., 2009, 48, 8501.
Analyzer equipped with a TTPX low and variable temperature 18 X. D. Zhuang, Y. Chen, G. Liu, P. P. Li, C. X. Zhu, E. T. Kang,
probe station.
K. G. Noeh, B. Zhang, J. H. Zhu and Y. X. Li, Adv. Mater.,
2010, 22, 1731.
19 T. W. Kim, H. Choi, S. H. Oh, G. Wang, D. Y. Kim, H. Hwang
and T. Lee, Adv. Mater., 2009, 21, 2497.
20 N. H. You, C. C. Chueh, C. L. Liu, M. Ueda and W. C. Chen,
Macromolecules, 2009, 42, 4456.
21 S. L. Lim, Q. Ling, E. Y. H. Teo, C. X. Zhu, D. S. H. Chan,
E. T. Kang and K. G. Neoh, Chem. Mater., 2007, 19, 5148.
22 W. Kwon, B. Ahn, D. M. Kim, Y. G. Ko, S. G. Hahm, Y. Kim,
H. Kim and M. Ree, J. Phys. Chem. C, 2011, 115, 19355.
23 B. Ahn, D. M. Kim, J. C. Hsu, Y. G. Ko, T. J. Shin, J. Kim,
W. C. Chen and M. Ree, ACS Macro Lett., 2013, 2, 555.
24 Y. G. Ko, W. Kwon, H. J. Yen, C. W. Chang, D. M. Kim,
K. Kim, S. G. Hahm, T. J. Lee, G. S. Liou and M. Ree,
Macromolecules, 2012, 45, 3749.
Acknowledgements
This work was nancially supported by the Chinese Natural
Science Foundation (21176164, 21206102 and 21336005), NSF
of Jiangsu Province (BE2013052), a project of the Department of
Education of Jiangsu Province (12KJB430011), Suzhou Nano-
project (ZXG2012023) and Project supported by the Specialized
Research Fund for the Doctoral Program of Higher Education of
China (Grant no. 20113201130003 and 20123201120005).
Notes and references
1 Q. D. Ling, F. C. Chang, Y. Song, C. X. Zhu, D. J. Liaw, 25 S. L. Lian, C. L. Liu and W. C. Chen, ACS Appl. Mater.
D. S. H. Chan, E. T. Kang and K. G. Neoh, J. Am. Chem.
Soc., 2006, 128, 8732.
Interfaces, 2011, 3, 4504.
26 Y. K. Fang, C. L. Liu and W. C. Chen, J. Mater. Chem., 2011,
2 Y. Yang, J. Ouyang, L. Ma, R. J. H. Tseng and C. W. Chu, Adv.
Funct. Mater., 2006, 16, 1001.
21, 4778.
27 H. C. Wu, C. L. Liu and W. C. Chen, Polym. Chem., 2013, 4,
3 B. Cho, S. Song, Y. Ji, T. W. Kim and T. Lee, Adv. Funct.
Mater., 2011, 21, 2806.
5261.
28 S. F. Miao, H. Li, Q. F. Xu, N. J. Li, J. W. Zheng, R. Sun,
J. M. Lu and C. M. Li, J. Mater. Chem., 2012, 22,
16582.
4 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.
5 Y. H. Chou, W. Y. Lee and W. C. Chen, Adv. Funct. Mater., 29 W. S. Ren, Y. X. Zhu, J. F. Ge, X. F. Xu, R. Sun, N. J. Li, H. Li,
2012, 22, 4352.
Q. F. Xu, J. W. Zheng and J. M. Lu, Phys. Chem. Chem. Phys.,
2013, 15, 9212.
30 Y. H. Zhang, H. Zhuang, Y. Yang, X. F. Xu, Q. Bao, N. J. Li,
H. Li, Q. F. Xu, J. M. Lu and L. H. Wang, J. Phys. Chem. C,
2012, 116, 22832.
6 J. Liu, Z. Yin, X. Cao, F. Zhao, L. Wang, W. Huang and
H. Zhang, Adv. Mater., 2013, 25, 233.
7 J. Liu, Z. Zeng, X. Cao, G. Lu, L. H. Wang, Q. L. Fan, W. Huang
and H. Zhang, Small, 2012, 8, 3517.
8 J. Lee, E. Lee, S. Kim, G. S. Bang, D. A. Shultz, R. D. Schmidt, 31 S. F. Miao, H. Li, Q. F. Xu, Y. Y. Li, S. J. Ji, N. J. Li, L. H. Wang,
M. D. E. Forbes and H. Lee, Angew. Chem., Int. Ed., 2011, 50,
J. W. Zheng and J. M. Lu, Adv. Mater., 2012, 24,
4414.
6210.
9 A. D. Yu, T. Kurosawa, Y. C. Lai, T. Higashihara, M. Ueda, 32 H. Zhuang, Q. J. Zhang, Y. X. Zhu, X. F. Xu, H. F. Liu, N. J. Li,
C. L. Liu and W. C. Chen, J. Mater. Chem., 2012, 22, 20754.
10 Y. W. Lin, C. J. Lin, Y. H. Chou, C. L. Liu, H. C. Chan and
W. C. Chen, J. Mater. Chem. C, 2013, 1, 5336.
Q. F. Xu, H. Li, J. M. Lu and L. H. Wang, J. Mater. Chem. C,
2013, 1, 3816.
33 K. Kim, Y. K. Fang, W. Kwon, S. Pyo, W. C. Chen and M. Ree,
J. Mater. Chem. C, 2013, 1, 4858.
11 S. J. Liu, W. P. Lin, M. D. Yi, W. J. Xu, C. Tang, Q. Zhao,
S. H. Ye, X. M. Liu and W. Huang, J. Mater. Chem., 2012, 34 S. Park, T. J. Lee, D. M. Kim, J. C. Kim, K. Kim, W. Kwon,
22, 22964.
Y. G. K. H. Choi, T. Chang and M. Ree, J. Phys. Chem. B,
2010, 114, 10294.
12 P. Wang, S. J. Liu, Z. H. Lin, X. C. Dong, Q. Zhao, W. P. Lin,
M. D. Yi, S. H. Ye, C. X. Zhu and W. Huang, J. Mater. Chem., 35 D. M. Kim, S. Park, T. J. Lee, S. G. Hahm, K. Kim, J. C. Kim,
2012, 22, 9576. W. Kwon and M. Ree, Langmuir, 2009, 25, 11713.
13 S. J. Liu, P. Wang, Q. Zhao, H. Y. Yang, J. I. Wong, H. B. Sun, 36 S. G. Hahm, T. J. Lee, D. M. Kim, W. Kwon, Y. G. Ko,
X. C. Dong, W. P. Lin and W. Huang, Adv. Mater., 2012, 24,
2901.
14 W. P. Lin, S. J. Liu, T. Gong, Q. Zhao and W. Huang, Adv.
Mater., 2014, 26, 570.
T. Michinobu and M. Ree, J. Phys. Chem. C, 2011, 115, 21954.
37 D. C. Wei, B. Wu, Y. L. Guo, G. Yu and Y. Q. Liu, Acc. Chem.
Res., 2013, 46, 106.
¨
38 B. X. Mi, Y. Q. Dong, Z. Li, J. W. Y. Lam, M. Haußler,
15 S. J. Liu, Z. H. Lin, Q. Zhao, Y. Ma, H. F. Shi, M. D. Yi,
Q. D. Ling, Q. L. Fan, C. X. Zhu, E. T. Kang and W. Huang,
Adv. Funct. Mater., 2011, 21, 979.
H. H. Y. Sung, H. S. Kwok, Y. Dong, I. D. Williams,
Y. Q. Liu, Y. Luo, Z. G. Shuai, D. B. Zhu and B. Z. Tang,
Chem. Commun., 2005, 3583.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5709–5716 | 5715

View Article Online
Journal of Materials Chemistry C
Paper
39 Q. D. Ling, E. T. Kang, K. G. Neoh, Y. Chen, X. D. Zhuang, 46 H. Li, N. J. Li, H. W. Gu, Q. F. Xu, F. Yan, J. M. Lu, X. W. Xia,
C. X. Zhu and D. S. H. Chan, Appl. Phys. Lett., 2008, 92,
143302.
40 X. D. Zhuang, Y. Chen, B. X. Li, D. G. Ma, B. Zhang and Y. Li,
Chem. Mater., 2010, 22, 4455.
41 K. Uchida, E. Tsuchida, Y. Aoi, S. Nakamura and M. Irie,
Chem. Lett., 1999, 28, 63.
42 C. Y. Huang, G. Q. Jiang and R. Advincula, Macromolecules,
2008, 41, 4661.
J. F. Ge and L. H. Wang, J. Phys. Chem. C, 2010, 114, 6117.
47 C. L. Liu, T. Kurosawa, A. D. Yu, T. Higashihara, M. Ueda and
W. C. Chen, J. Phys. Chem. C, 2011, 115, 5930.
48 T. Kurosawa, Y. C. Lai, T. Higashihara, M. Ueda, C. L. Liu
and W. C. Chen, Macromolecules, 2012, 45, 4556.
49 P. Cias, C. Slugovc and G. Gescheidt, J. Phys. Chem. A, 2011,
115, 14519.
50 J. Y. Ouyang, C. W. Chu, C. R. Szmanda, L. P. Ma and
Y. Yang, nature materials, 2004, 3, 918.
51 A. Prakash, J. Y. Ouyang, J. L. Lin and Y. Yang, J. Appl. Phys.,
2006, 100, 054309.
43 D. J. Liaw, K. L. Wang and F. C. Chang, Macromolecules,
2007, 40, 3568.
44 Y. L. Liu, Q. D. Ling, E. T. Kang, K. G. Neoh, D. J. Liaw,
K. L. Wang, W. T. Liou, C. X. Zhu and D. S. H. Chan, J. 52 L. Wang and E. K. Wang, Electrochem. Commun., 2004, 6, 49.
Appl. Phys., 2009, 105, 044501.
53 Z. H. Sun, C. X. Wang, J. X. Yang, B. Zhao and J. R. Lombardi,
¨
¨
¨
45 M. Gsanger, J. H. Oh, M. Konemann, H. W. Hoen,
J. Phys. Chem. C, 2008, 112, 6093.
A. M. Krause, Z. Bao and F. Wurthner, Angew. Chem., Int. 54 R. J. Tseng, C. O. Baker, B. Shedd, J. X. Huang, R. B. Kaner,
Ed., 2010, 49, 740.
J. Y. Ouyang and Y. Yang, Appl. Phys. Lett., 2007, 90, 053101.
5716 | J. Mater. Chem. C, 2014, 2, 5709–5716
This journal is © The Royal Society of Chemistry 2014