Synthesis, characterization, and nonvolatile ternary memory behavior of a larger heteroacene with nine linearly fused rings and two different heteroatoms

Article abstract of DOI:10.1021/ja408208c

To achieve ultrahigh density memory devices with the capacity of 3 ⁿ or larger, organic materials with multilevel stable states are highly desirable. Here, we reported a novel larger stable heteroacene, 2,3,13,14-tetradecyloxy-5,11,16,22-tetraa

Full text of DOI:10.1021/ja408208c

Communication
pubs.acs.org/JACS
Synthesis, Characterization, and Nonvolatile Ternary Memory
Behavior of a Larger Heteroacene with Nine Linearly Fused Rings and
Two Different Heteroatoms
Pei-Yang Gu,†,‡ Feng Zhou, Junkuo Gao, Gang Li, Chengyuan Wang, Qing-Feng Xu,
†
‡
‡
‡
†
,
‡
,†
Qichun Zhang,* and Jian-Mei Lu*
†College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China
‡School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
*
S Supporting Information
organic light emitting diodes, organic solar cells, or even
memory devices. Yet, they have stability issues due to photo-
8
ABSTRACT: To achieve ultrahigh density memory
devices with the capacity of 3 or larger, organic materials
with multilevel stable states are highly desirable. Here, we
reported a novel larger stable heteroacene, 2,3,13,14-
tetradecyloxy-5,11,16,22-tetraaza-6,10,17,21-tetrachloro-
n
oxidation or dimerization via a Diels−Alder reaction. Such
problems could be solved through the replacement of CH
groups in the backbone of oligoacenes with heteroatoms (e.g.,
9
N, O, P, B, S). In fact, the type/position/number of
7
,9,18,20-tetraoxa-8,19-dicyanoenneacene (CDPzN),
heteroatoms will offer us more chances to tune the highest
occupied molecular orbital (HOMO)−lowest unoccupied
molecular orbital (LUMO) gap, HOMO/LUMO positions,
which has two different types of heteroatoms (O and N)
and nine linearly fused rings. The sandwich-structure
memory devices based on CDPzN exhibited excellent
ternary memory behaviors with high ON2/ON1/OFF
10
and the stability of as-prepared oligoheteroacenes. In past
decades, significant advancements have been witnessed in
6
.3
4.3
current ratios of 10 /10 /1 and good stability for these
three states.
11
synthesis, theoretical study, and applications of heteroacenes.
In this report, we are interested in larger heteroacenes with
two or more different types of heteroatoms because (1) more
heteroatoms might provide multilevel stable oxidation states,
he exponential growth of information communication
n
which is very important to realize 3 or larger data storage; (2)
T
poses a strong demand for developing next-generation
memory devices with high-density data storage, low cost,
simple structure, fast speed, lower power consumption, and
synthetic work should be more challenging; and (3) we will
have more chances to tune the stability and properties of as-
designed molecules. We reported here a novel larger
heteroacene (2,3,13,14-tetradecyloxy-5,11,16,22-tetraaza-
1
longer data retention time. Clearly, traditional inorganic-
semiconductor-based systems have reached their bottle necks
and face big challenges to further enhance their memory
6
,10,17,21-tetrachloro-7,9,18,20-tetraoxa-8,19-dicyanoennea-
2
cene, abbreviated as CDPzN, 1), which has two different types
of heteroatoms (O and N) and nine linearly fused rings. We
believe that CDPzN should have the following advantages: (1)
the introduction of an O atom could make CDPzN more stable
in both the ground and oxidation state; (2) the cyano group
may be useful for negative charge injection as suggested in
capacities. Recently, organic memory devices have evoked a lot
of research interest because they could realize ultrahigh density
data storage through either a 3D-stacked cross-bar array or
3
multilevel stable oxidation states. Currently, most resistive
organic memory devices are based on binary materials, which
have two conductivity states [i.e., ON (“1”, high conductivity)
and OFF (“0”, low conductivity) states] in response to the
12
previous reports; and (3) the memory device based on
CDPzN might exhibit multilevel stable conductivity states in
response to the applied voltage because the electron-with-
drawing abilities of cyano and pyrazine are different.
4
applied voltage. Such devices only can store 2-bits in a single
cell. Thus, in order to achieve ultrahigh density memory
devices, organic materials with multilevel stable states, which
n
The synthetic route for the preparation of CDPzN is
described in Scheme 1. The target compound was synthesized
in two steps: first, the commercially available chloranilic acid
was reacted with 1,2-bis(decyloxy)-4,5-diaminobenzene (0.9
equiv) in ethylalcohol to afford 1,4-dichloro-7,8-bis(decyloxy)-
phenazine-2,3-diol (2) in 55% yield. Then the as-prepared
intermediate 2 was condensed with tetrafluoroterephthalonitrile
can lead to an increasing capacity of 3 or larger, are highly
5
desirable.
Among all organic materials for data storage, small organic
molecules are more promising in approaching high-perform-
ance data storage because of their tunable properties and
6
designable structures. Although some molecules have been
demonstrated to show “0”, “1”, and “2” tristable states and
could store more than 2-bits in a single cell, such systems are
still rare and it is still desirable to search new organic
(
0.45 equiv) using potassium carbonate (5 equiv) as the base to
generate CDPzN in 85% yield.
n
7
compounds with a reliable storage capacity of 3 or larger.
Oligoacenes have been widely used as active layers in organic
Received: August 8, 2013
semiconductor devices such as organic field-effect transistors,
Published: September 11, 2013
©
2013 American Chemical Society
14086
dx.doi.org/10.1021/ja408208c | J. Am. Chem. Soc. 2013, 135, 14086−14089

Journal of the American Chemical Society
Communication
a
Scheme 1. Synthetic Route of CDPzN
a
(
i) 1.1 equiv of chloranilic acid, CH CH OH, N , reflux, 55%; (ii)
3 2 2
0
.45 equiv of tetrafluoroterephthalonitrile, 5 equiv of K CO , DMF,
2 3
100 °C, 85%.
The optical, thermal, and electrochemical properties of
CDPzN have been studied. Figure 1a shows the normalized
Figure 2. Memory-device characteristics of CDPzN. (a) Scheme of the
sandwich device and SEM image of a cross section of the device. (b)
I−V characteristics of the memory device fabricated with CDPzN. (c)
Stability of the device in three states under a constant “read” voltage of
−
1 V. (d) Stimulus effect of read pulse of −1 V on the ON,
Figure 1. Characterization of CDPzN. (a) Normalized optical
absorption spectra of CDPzN thin film on a quartz plate. (b) Cyclic
voltammogram curve of CDPzN thin films on ITO glass in a 0.1 M
intermediate, and OFF states. Inset shows the pulse shapes of the
measurements.
solution of TBAPF in acetonitrile solution. The scan rate: 100 mV
s .
6
These OFF-to-ON1 and ON1-to-ON2 transitions can be
regarded as a “writing” process. For the subsequent negative
sweep from 0 to −4 V (sweep 2) and the positive sweep from 0
to 4 V (sweep 3), the storage cell of the device maintains the
ON state. Another cell of the device was measured over a
voltage range of 0 to −3 V (sweep 4) and showed one STV at
−2.48 V, which suggested that this cell was switched from the
OFF state to the ON1 state. For the subsequent negative sweep
from 0 to −3 V (sweep 5) and the reverse voltage sweep
(sweep 6), the current density maintains the ON1 state. This
result indicated that once the cell reached the ON1 state, it
could still be maintained even when the power was shut off.
The storage cell underwent a transition from the ON1 to ON2
state at 3.31 V for the subsequent negative sweep from 0 to −4
V (sweep 7). Once it was switched to the ON2 state, the
memory device cannot return back to both the ON1 and OFF
states (sweeps 8−9). The three states of the ternary memory
cell are distinct, and the current ratio of the “OFF”, “ON1”, and
−1
optical absorption spectrum of the CDPzN thin film on a
quartz plate. The absorption spectrum of CDPzN exhibits three
prominent bands at 233, 266, and 379 nm, which can be
ascribed to a localized aromatic π−π* transition and intra-
molecular charge transfer from the donor (decyloxy) to the
acceptor (pyrazine and cyano) moieties, respectively. CDPzN
exhibits a very good thermal stability, with an onset
decomposition temperature of ∼367 °C (considering the 5%
weight loss temperature, Figure S6). The excellent thermal
property of CDPzN is expected to meet the requirements of
heat resistance in the electronics industry.
Figure 1b shows the cyclic voltammogram (CV) curve of the
CDPzN film on an indium−tin oxide (ITO) glass substrate in a
1
0
.1 mol L− solution of tetrabutylammonium hexafluorophos-
phate (TBAPF ) in anhydrous acetonitrile solution. The onset
6
4.3
6.3
reduction and oxidation potentials for CDPZN are −1.22, 0.94,
and 1.24 V, which correspond to the LUMO, HOMO, and
HOMO-1 energy levels of ∼ −3.20, −5.34, and −5.64 eV using
“ON2” states is 1:10 :10 . This device exhibits a typical write-
once read-many-times (WORM) behavior.
Figure 2c shows the retention times and stress tests of the
memory device for the OFF, ON1, and ON2 states. Under a
constant stress of −1 V, no significant degradation in current
for three different states could be observed for at least 10 000 s
during the readout test. Moreover, the stimulus effect of
continuous read pulses of −1 V on the three different states was
also investigated. As shown in Figure 2d (inset), the pulse
period and pulse width are 2 and 1 μs, respectively, which are
typical values in practical devices. The three different states in
onset
13
the equation E
= −e (4.40 + E
) eV. The
LUMO/HOMO
red/oxd
calculated band gap using CV data is 2.10 eV for CDPzN,
which was slightly smaller compared to the optical onset-edge
band gap result (2.43 eV).
Figure 2a shows the scheme of our prototype memory
device, which is a sandwich structure using ITO as bottom
electrodes, aluminum (Al, 100 nm thickness) as top electrodes,
and an organic layer of CDPzN molecules as an active layer.
The film thickness was about 100 nm, as measured by SEM
through a cross section of the film (Figure 2a).
The current−voltage (I−V) characteristics of the device were
measured by an HP4145B semiconductor parameter analyzer.
Figure 2b shows the typical I−V curve of a CDPzN-based
memory device. In the first sweep from 0 to −4 V, two sharp
transitions from the low-conductivity (OFF, “0”) state to an
intermediate-conductivity (ON1, “1”) state, to a high-
conductivity (ON2, “2”) state were observed at switching
threshold voltages (STVs) of −2.43 and −3.31 V, respectively.
7
memory devices are stable for at least 10 continuous read
cycles. Therefore, the switching behavior on the remnant stored
data and the nonvolatile nature of the memory device can
explain the functionality of a WORM-type memory character-
istic.
In order to understand the electrical switching behavior of
the CDPzN memory device, theoretical calculations have been
performed using the density functional theory (DFT) method
14
of B3LYP with the 6-31G (d) basis set. Figure 3a, 3b show
that the electron density distributions of the HOMO mainly
1
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Journal of the American Chemical Society
Communication
Figure 3c (OFF state) shows that the molecular surface has
continuous positive electrostatic potential (in red) along the
conjugated backbone, indicating that charge carriers can
migrate through this open channel. However, there are some
negative electrostatic potential regions (in blue) caused by
electron-acceptor groups. These negative regions can serve as
‘
‘charge traps’’ to block the mobility of charge carriers. As
shown in Figure 3d, the energy barrier (0.54 eV) between the
work function of ITO (−4.8 eV) and HOMO (−5.34 eV,
calculation based on CV) is lower than the energy barrier (1.1
eV) between the work functions of Al (−4.3 eV) and LUMO
(
−3.20 eV). Thus, holes might dominate the conduction
process in ITO/CDPzN/Al devices. The atomic force
microscopy (AFM) image (Figure S7) shows that the film is
smooth with good quality, which indicates that two charge traps
(
pyrazine and cyano groups) have equal opportunities to accept
the injected charge carriers. Possible charge carrier migration
processes are proposed in Figure 3c. Under low bias, the
CDPzN thin film is in a low-conductivity (OFF) state because
the energy barrier between the Al electrode and the CDPzN
layer is as large as 1.1 eV, which also blocks the electron
migration. Under high bias, the hole injection is easier and the
CDPzN has better conductivity at the intermediate-conductiv-
ity (ON1) state, which corresponds to the first oxidation
process. However, two traps are not filled at the same time: the
trap of pyrazine is filled and the trap of cyano is partly filled,
which could be attributed to the stronger electron-withdrawing
ability of cyano than pyrazine. With increasing bias, the trap of
cyano is also filled which leads to the current transition from
the ON1 to ON2 state, which corresponds to the second
oxidation process. Consequently, the device shows multilevel
memory characteristics due to the two charge traps with the
different electron-withdrawing abilities of cyano and pyrazine.
In summary, we have successfully synthesized a novel larger
stable heteroacene, CDPzN, containing two different types of
heteroatoms (N and O atoms) in their backbones and having
nine linearly fused rings. The sandwich-structure memory
devices based on CDPzN exhibited excellent ternary memory
6
.3
behavior with high ON2/ON1/OFF current ratios of 10 /
1
4.3
0 /1 and good stability for the three states. We believe that
our results could provide guidance for the design and synthesis
of new heteroacenes, which could be used as promising
candidates in nonvolatile memory devices.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Figure 3. DFT molecular simulation results and energy level diagram.
Experimental procedures, synthesis of CDPzN, fabrication and
measurements of the memory devices. This material is available
free of charge via the Internet at http://pubs.acs.org.
(
a, b) HOMO and LUMO of CDPzN. (c) Possible carriers’ migration
process based on ITO/CDPzN/Al devices. (d) Energy level diagram
of HOMO, HOMO-1, and LUMO for CDPzN along with the work
function of the electrodes.
AUTHOR INFORMATION
■
Corresponding Authors
qczhang@ntu.edu.sg
lujm@suda.edu.cn
locate on aromatic groups while the LUMO orbital is mainly
distributed on both pyrazine and cyano groups. Generally, the
conjugated molecules with such electron distribution indicate
the intramolecular charge transfer (ICT) property. Upon
undergoing the HOMO to LUMO transition, the trend of
the distribution on electron density displays two ways: one way
is a shift from the aromatic groups to the pyrazine groups, and
the other way is to move from the aromatic groups to the cyano
groups, which implies that the memory device based on this
molecule could exhibit two switching threshold voltages
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work is supported by the Singapore National Research
Foundation through the Competitive Research Programme
under Project No. NRF-CRP5-2009-04 (J.G., Y.Z., and C.X.).
Q.Z. acknowledges the financial support AcRF Tier 1 (RG 16/
(STVs), which is consistent with the experimental results.
1
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Journal of the American Chemical Society
2) and Tier 2 (ARC 20/12 and ARC 2/13) from MOE,
CREATE program (Nanomaterials for Energy and Water
Management) from NRF, and New Initiative Fund from NTU,
Singapore. P.-Y. Gu thanks the China Scholarship Council
Communication
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