Synthesis, characterization, and non-volatile memory device application of an n-substituted heteroacene

Article abstract of DOI:10.1002/asia.201301547

N-substituted heteroacenes have been widely used as electroactive layers in organic electronic devices, and only a few of them have been investigated in organic resistive memory devices. Here, a novel N-substituted heteroacene 2-(4'-(diphenylamino)phenyl)-4,11-bis((triisopropylsilyl)ethynyl)-1H-imidazo[4, 5-b]phenazine (DBIP) has been designed, synthesized, and characterized. Sandwich-structure memory devices based on DBIP have been fabricated and the devices show non-volatile and stable memory character with good endurance performance. Walk down memory lane: An N-substituted heteroacene 2-(4'-(diphenylamino)phenyl)-4,11-bis((triisopropylsilyl)ethynyl)-1H-imidazo[4, 5-b]phenazine (DBIP) has been designed, synthesized, and characterized. Sandwich-structure memory devices based on DBIP have been fabricated and the devices show non-volatile and stable memory character with good endurance performance. Copyright

Full text of DOI:10.1002/asia.201301547

FULL PAPER
DOI: 10.1002/asia.201301547
Synthesis, Characterization, and Non-Volatile Memory Device Application of
an N-Substituted Heteroacene
Chengyuan Wang,[a] Jiangxin Wang,[a] Pei-Zhou Li,[b] Junkuo Gao,[a] Si Yu Tan,[b]
Wei-Wei Xiong,[a] Benlin Hu,[a] Pooi See Lee,[a] Yanli Zhao,[b] and Qichun Zhang*[a]
Abstract: N-substituted heteroacenes have been widely used as electroactive
layers in organic electronic devices, and only a few of them have been investigated
in organic resistive memory devices. Here, a novel N-substituted heteroacene 2-
(4’-(diphenylamino)phenyl)-4,11-bis((triisopropylsilyl)ethynyl)-1H-imidazo[4,5-
b]phenazine (DBIP) has been designed, synthesized, and characterized. Sandwich-
structure memory devices based on DBIP have been fabricated and the devices
show non-volatile and stable memory character with good endurance performance.
Keywords: heteroacenes · nitrogen
heterocycles · non-volatile memory
device · organic electronics
AHCTUNGTRENNUNG
Introduction
Nitrogen-substituted heteroacenes have been widely ex-
plored for use in organic electronic applications, such as or-
ganic light-emitting diodes, organic field-effect transistors,
organic phototransistors, and organic solar cells.[8–12] N-sub-
stituted heteroacenes should have significant applications in
memory devices because the introduction of nitrogen into
the backbone of acenes[13] can improve the stability of the
molecules in both the ground state and charged state. More-
Organic resistive switching memories have been demonstrat-
ed to be promising alternative candidates to replace the con-
ventional silicon-based electronics,[1] owing to their advan-
tages such as simple processing, the flexibility of the devices,
and the low fabrication cost. Currently, several types of elec-
troactive systems such as hybrid organic–inorganic systems,[2]
donor–acceptor (D–A) composites,[3] conjugated polymers,[4]
and small-molecule organic semiconductors[5,6] have been
extensively explored as key layers in memory devices to
achieve high density data-storage memories. Due to their
well-defined structures, tunable band position, and more ac-
curate simulation-experiment matching, organic conjugated
small molecules have gained significant attention for the
comprehensive understanding of the charge carrier transport
mechanisms[7] to expand the organic resistive memories
from laboratory research to practical applications. Although
some organic small molecules have shown good performan-
ces in memory devices,[5,6] such systems are still rare and it
is thus still desirable to search for new organic compounds
with stable and good endurance performance in memory de-
vices.
À
over, both the extended p-conjugated structures and N H
hydrogen bond between neighbouring molecules will en-
hance the orderly stack of molecules in solid-state thin
films; this is most likely to facilitate the generated charge
carrier transport.[6a,b] Herein, we report a novel N-substitut-
ed heteroacene 2-(4’-(diphenylamino)phenyl)-4,11-bis((trii-
sopropylsilyl)ethynyl)-1H-imidazoACTHNUTRGNEU[GN 4,5-b]phenazine (DBIP).
DBIP has an imidazole-[4,5-b]phenazine moiety (acceptor)
that is conjugated with a triphenylamine unit (donor), which
has been widely used in memory materials owing to its abili-
ty to stabilize a charge-separated state in D–A molecules.
The novel N-substituted heteroacene demonstrates good
performances in non-volatile memory devices with good en-
durance and high stability.
Results and Discussion
Synthesis and Crystal Structure
[a] C. Wang, J. Wang, Dr. J. Gao, Dr. W.-W. Xiong, Dr. B. Hu,
Prof. P. S. Lee, Prof. Q. Zhang
DBIP was synthesized as a dark red powder (17% yield)
through a one-step condensation between 1,4-bis((triisopro-
pylsilyl)ethynyl)-2,3-diaminalphenazine and 4-(diphenylami-
no)benzaldehyde, as shown in Scheme 1.[14] DBIP has been
characterized by MALDI-TOF, high-resolution mass spectra
School of Materials Science and Engineering
Nanyang Technological University
50 Nanyang Avenue, 639798 (Singapore)
Fax : (+65) 67909081
E-mail: qczhang@ntu.edu.sg
(HRMS), H NMR and 13C NMR spectroscopy (Figure S1–
S4, see the Supporting Information), and single-crystal X-
ray diffraction.
1
[b] Dr. P.-Z. Li, S. Y. Tan, Prof. Y. Zhao
School of Physical and Mathematical Sciences
Nanyang Technological University
637371 (Singapore)
Red sheet-like single crystals of DBIP were grown in a so-
lution of methylene chloride by slow evaporation. The
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301547.
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Qichun Zhang et al.
Figure 2. a) Normalized UV/Vis absorbance spectrum of DBIP in aceto-
nitrile. b) Cyclic voltammograms curves of DBIP in a solution of acetoni-
trile with 0.1 mol·lÀ1 tetrabutylammonium hexafluorophosphate
(TBAPF6).
Scheme 1. Synthetic route towards DBIP.
single-crystal structure of DBIP in the front view is shown
in Figure 1a; this is determined from single-crystal X-ray
diffraction data. Because of the twisted configuration, the
triphenylamine unit is not in the same plane with the conju-
a localized aromatic p–p* transition and an intramolecular
charge transfer from the donor (triphenylamine unit) to the
acceptor (conjugated imidazole-[4,5-b]phenazine moiety),
respectively. The optical band gap (Eg) between the highest
occupied molecular orbital (HOMO) and the lowest unoccu-
pied molecular orbital (LUMO) energy levels of DBIP is
determined to be 2.17 eV from the onset of optical absorb-
ance wavelength, based on the equation Eg =hc/lonset, where
Eg is the optical band gap, h is the Planck constant (6.63ꢂ
10À34 m2 kgsÀ1), c is the speed of light (3ꢂ108 msÀ1), and
lonset is the onset optical absorbance wavelength of DBIP in
acetonitrile.
The electrochemical properties of DBIP were investigated
by cyclic voltammetry measurements (CVs) to determine
the HOMO and LUMO energy levels. As shown in Fig-
ure 2b, DBIP has a couple of reversible reduction peaks
and two irreversible oxidation peaks. The onset of the first
oxidation potential (Eonset(OX)) for DBIP was 0.28 V versus
FeCp2+/FeCp2. The HOMO energy level can be calculated
Figure 1. a) Single-crystal structure of DBIP; thermal ellipsoids drawn at
the 30% probability level. b) Molecular stacking of DBIP.
gated imidazole-[4,5-b]phenazine moiety. Figure 1b shows
the molecular stacking of neighbouring molecules. DBIP
adopts offset intersection face-to-face stacking with an inter-
layer distance of 3.29 ꢁ, which is shorter than that of van
der Waals bond, thereby indicating the strong p–p interac-
tions between adjacent molecules. Both the face-to-face
stacking and short interlayer molecular distance contribute
to the orderly arrangement of molecules in the thin film,
and this is to enable better charge carrier transport.
from the equation
E
HOMO =À(4.80+Eonset(OX)) to be
À5.08 eV, where EHOMO is the HOMO energy level,
Eonset(OX) is the onset oxidation potential of DBIP versus
FeCp2+/FeCp2. While the LUMO energy level is determined
to be À2.91 eV from the difference between the HOMO
energy level and the optical band gap.[15]
Optical and Electrochemical Properties
Memory Characteristics
Figure 2a shows the UV/Vis absorbance spectrum of DBIP
in acetonitrile. DBIP showed two maximum absorption
peaks at 278 nm and 467 nm; these peaks were ascribed to
Figure 3a shows the schematic diagram of the sandwich-
structure memory device. DBIP in the solution of cyclohex-
anone (10 mgmLÀ1) was spin-coated onto pre-cleaned
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Qichun Zhang et al.
and “ON” states for up to
twenty cycles. Figure 3c shows
the distribution of the resist-
ance of the “OFF” state (HRs)
and “ON” state (LRs) for
14 cycles. The resistance values
were obtained at 0.2 V in each
state. The ratios of the resist-
ance magnitude between the
“OFF” state and the “ON”
state are about 103 during the
14 switching cycles; this corre-
sponds to the ON/OFF ratio as
indicated in the I–V curves. The
good endurance performance of
the devices is possibly attribut-
ed to the highly ordered ar-
rangement in the thin film. The
average threshold voltages of
“Writing” and “Erasing” pro-
cesses for the 14 cycles are
2.60 V and À1.45 V, respective-
ly.
Figure 3. a) Schematic diagram of ITO/DBIP/Pt device. b) Current–Voltage (I–V) characteristics of the ITO/
&
*
DBIP/Pt device. c) Endurance performance of the devices; ( ) LRs, ( ) HRs. d) Retention performance of
The retention performance of
the device was evaluated by ap-
the devices; “ON” state (top line), “OFF state” (bottom line).
plying
a reading voltage of
indium–tin oxide (ITO) glass substrates with a thickness of
approximately 120 nm, which was measured by an Alpha-
Step IQ Surface Profiler. To avoid filamentary effect,[16] plat-
inum was used as the top electrode, which was sputter-
coated onto the DBIP layer through a shadow mask with
the diameter of 100 mm and thickness of 300 nm.
À0.2 V to either the “ON” state or “OFF” state under ambi-
ent conditions. As shown in Figure 3d, no current decay for
each state was observed after 4000 s during the test. The re-
tention performance suggested the potential practical appli-
cations of DBIP as a stable memory material.
The memory behavior of the ITO/DBIP/Pt devices were
demonstrated by the current–voltage (I–V) characteristics
(as shown in Figure 3b). When the voltage swept from 0 V
to +2.20 V, the device retained the high-resistance state
(“OFF” state). As the voltage reached +2.25 V, the current
was instantaneously increased to the magnitude of the com-
pliance current (5 mA), thereby indicating that the device
switched from a high-resistance state to a low-resistance
state (“ON” state). The transition of the device from the
“OFF” state to the “ON” state could act as a “Writing” pro-
cess in the memory device (sweep 1). The device retained
the “ON” state in the next sweep from +4.0 V to 0 V
(sweep 2) and even after removal of the power supply,
thereby suggesting typical non-volatile memory behavior. In
the following sweep (sweep 3) from 0 V to À2.0 V, a reverse
sweeping voltage of À0.95 V recovered the initial “OFF”
state. The recovery of the device from “ON” state to “OFF”
served as an “Erasing” process. The device was still in the
“OFF” state in the continuous sweep (sweep 4) form À2.0 V
to 0 V. When read at 0.2 V, the ON/OFF ratio of the device
was 103; such a high ON/OFF ratio ensures good readout ac-
curacy in potential practical applications.
Theory and Proposed Mechanisms
Molecular simulation was employed to describe the switch-
ing mechanisms and the generated charge carrier migration
processes between DBIP molecules and electrodes at each
state. As shown in Figure 4a, the LUMO energy level of
DBIP is mainly associated with the conjugated imidazole-
[4,5-b]phenazine moiety, whereas the HOMO energy level is
distributed across the whole molecule; electrons from the
HOMO could be excited to the LUMO energy level and
thus become more localized. The hole injection energy bar-
rier from the ITO electrode (with a work function of
À4.8 eV) to the DBIP film was estimated to be 0.12 eV, and
the electron injection energy barrier from the Pt electrode
(with a work function of À5.6 eV) to the DBIP layer is
2.74 eV, respectively. Therefore, the charge carrier migration
processes are dominated by the hole injection.
Figure 4b shows the electrostatic potential of DBIP. The
surface of DBIP has a continuous positive electrostatic po-
tential (in blue) with negative electrostatic potential regions
(in red) caused by electron-accepting groups, which serve as
“charge traps” to block the mobility of charge carriers.
Under a low sweeping voltage, holes and electrons are in-
jected into triphenylamine donors and conjugated imida-
zole-[4,5-b]phenazine acceptors from ITO and Pt electrodes,
Cyclic switching operations were carried out to further in-
vestigate the endurance performance of the device. General-
ly, the device could be repeatedly switched between “OFF”
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Experimental Section
Materials
The chemical reagents 2,3-phenazinediamine and 4-(diphenylamino)ben-
zaldehyde were purchased from Alfa Aesar or Aldrich (Germany) and
used as received. 1,4-bis((triisopropylsilyl)ethynyl)-2,3-diaminalphenazine
was prepared according to a literature procedure.[14] All solvents were
used without further purification.
Methods
MALDI-TOF mass spectrometric measurements were recorded on a Shi-
madzu Biotech AXIMA-TOF2. HRMS (ESI) spectra were recorded on
1
a Waters Q-Tof premier mass spectrometer. H NMR and 13C NMR spec-
tra were recorded on Bruker 300 MHz spectrometers. UV/Vis absorb-
ance was carried out on a Shimadzu UV-2501 spectrophotometer. Cyclic
voltammetry measurements were carried out on a CHI 604E electro-
chemical analyzer. Glassy carbon (diameter: 1.6 mm; area 0.02 cm2) was
used as a working electrode, platinum wires were used as counter elec-
trode and reference electrode, respectively. FeCp2+/FeCp2 was used as an
internal standard. Potentials were recorded versus FeCp2+/FeCp2 in a so-
lution of anhydrous acetonitrile with 0.1m tetrabutylammonium hexa-
fluorophosphate (TBAPF6) as a supporting electrolyte at a scan rate of
50 mVsÀ1, which has a HOMO energy level of À4.80 eV. The thickness
of the DBIP film and Pt electrode was measured with an Alpha-Step IQ
Surface Profiler. X-ray crystallographic data were collected on a Super-
Nova CCD diffractometer using graphite-monochromated CuKa radiation
(l=1.5418 ꢁ) at 293 K. The structure was resolved by direct methods
and refined by full-matrix least-squares cycles in SHELX-97. All the
non-hydrogen atoms were refined with anisotropic thermal parameters.
The hydrogen atoms were located at geometrically calculated positions
and were not refined. The current–voltage (I–V) characteristics of the Pt/
DBIP/ITO devices were measured under an ambient atmosphere using
a Keithley 4200 semiconductor characterization system in the voltage
sweeping model. Molecular simulation of the HOMO and LUMO energy
levels and electrostatic potential (ESP) surfaces of DBIP were performed
using the density functional theory (DFT) method of B3LYP with the 6-
31G (d) basis set.
Figure 4. a) HOMO and LUMO energy levels of DBIP. b) Electrostatic
potential of DBIP in both the “OFF” state and “ON” state.
respectively. Because of the energy barrier between the
electrodes and DBIP films and the “charge traps” induced
by acceptors, the film is in the “OFF” state and the current
is low. As the voltage is increased to the threshold voltage,
the electrons could exceed the energy barrier; this leads to
charge separation and charge carrier migration. As a result,
the molecular surface of DBIP becomes a continuous posi-
tive electrostatic potential and the traps are filled up, there-
by forming a stable environment in which charge carriers
could migrate through this open channel. The charge carri-
ers might not be easily released even without power supply,
thus resulting in a high-conductance state that can be main-
tained for a long period.[17] Thus, the device shows typical
non-volatile memory behavior.
Synthesis of 2-(4’-(diphenylamino)phenyl)-4,11-
bis((triisopropylsilyl)ethynyl)-1H-imidazo
ACHTUNGTRENNUNG
1,4-bis((triisopropylsilyl)ethynyl)-2,3-diaminalphenazine
0.96 mmol) and 4-(diphenylamino)benzaldehyde (283.62 mg, 1.04 mmol)
were mixed well in a 100 mL flask, and nitrobenzene (10 mL) was added
and the solution was heated at 1808C for 48 h. Then, the solution was
cooled down and nitrobenzene was distilled under reduced pressure. The
solid residue was purified by flash column chromatography with ethyl
acetate and hexanes (1:20) to afford DBIP as a dark red powder,
(140 mg, 17% yield). 1H NMR (300 MHz, CDCl3, 298 K): d=1.31 (m,
42H), 7.01 (s, 4H), 7.12 (m, 2H), 7.28 (m, 6H), 7.78 (m, 2H), 8.23 ppm
(m, 4H); 13C NMR (75 MHz, CDCl3, 298 K): d=11.62, 19.00, 100.18,
106.67, 106.72, 119.05, 119.14, 125.13, 129.63, 129.81, 130.03, 141.54,
142.67, 145.60, 151.79, 157.16 ppm; HRMS (MALDI-TOF): m/z calcd for
C53H61N5Si2 [M+H]+: 825.28; found: 826.7; HRMS (ESI): m/z calcd for
C53H61N5Si2: 824.4544; found: 824.4563. Single-crystals of DBIP were
grown in methylene chloride solution through slow evaporation.
Conclusions
In summary, a novel N-substituted heteroacene DBIP has
been successfully synthesized and characterized. The sand-
wich-structure memory devices based on DBIP showed
good endurable memory behavior with a high ON/OFF
ratio of 103 and good retention performance in each state.
Our results suggest that N-substituted heteroacenes could
have potential practical applications in non-volatile memory
devices.
CCDC 972421 (DBIP) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
Acknowledgements
Q.Z. acknowledges the financial support from AcRF Tier 1 (RG 16/12)
from MOE, MOE Tier 2 (ARC 20/12 AND arc 2/13), CREATE program
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Qichun Zhang et al.
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Received: November 18, 2013
Published online: January 2, 2014
Chem. Asian J. 2014, 9, 779 – 783
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