Rewritable multilevel memory performance of a tetraazatetracene donor-acceptor derivative with good endurance

Article abstract of DOI:10.1002/asia.201402899

A new tetraazatetracene derivative, 2,3-[4,4'- bis(N,N-diphenylamino)benzyl]-5,12-bis[(triisopropylsilyl)ethynyl]- 1,4,6,11-tetraazatetracene (TPAs-BTTT), displays rewritable multilevel memory behavior, which is probably induced by multielectron intramolecular charge transfer (CT).

Full text of DOI:10.1002/asia.201402899

DOI: 10.1002/asia.201402899
Communication
Charge Transfer
Rewritable Multilevel Memory Performance of a Tetraazatetracene
Donor–Acceptor Derivative with Good Endurance
Chengyuan Wang,[a] Benlin Hu,[a] Jiangxin Wang,[a] Junkuo Gao,[b] Gang Li,[a] Wei-Wei Xiong,[a]
Binghua Zou,[a] Mitsuharu Suzuki,[c] Naoki Aratani,[c] Hiroko Yamada,[c] Fengwei Huo,[a]
Pooi See Lee,[a] and Qichun Zhang*[a]
However, most of these materials only show typical write once,
Abstract: A new tetraazatetracene derivative, 2,3-[4,4’-
bis(N,N-diphenylamino)benzyl]-5,12-bis[(triisopropylsilyl)e-
read many times (WORM) behavior, or their performance
needs to be induced by various excitation sources,[4] which has
thynyl]-1,4,6,11-tetraazatetracene (TPAs-BTTT), displays re-
become a limitation for their practical application. Intramolecu-
writable multilevel memory behavior, which is probably
lar CT induces rewritable memory behavior in donor–acceptor
induced by multielectron intramolecular charge transfer
(D–A) molecules.[5] An interesting issue is whether multielec-
(CT).
tron intramolecular CT could occur, so that different charge in-
jection could diversify the resistance of one material, and
charge recombination could function as an “erasing” process,
Organic resistive random-access memories (RRAMs) could be
potential promising alternatives to traditional silicon-based
memories owing to their flexibility, nanoscaling ability, high
switching speed, and low power consumption.[1] Among vari-
ous memory materials employed in sandwiched RRAM devi-
ces,[2] organic small molecules have received much attention
owing to their well-defined structures; tunable band positions;
and more accurate simulation–experiment matching; which is
crucial to elucidate the structure–property relationship and
charge-carrier transporting mechanisms.[3] In particular, multile-
vel memory materials could be developed through a reasona-
ble molecule designing strategy to realize high-density data
storage (HDDS). It has been demonstrated that introducing dif-
ferent electron “traps” into molecules could generate multiple
resistive levels in one material when a potential is applied.
which would realize rewritable multilevel memory per-
formance. To achieve this concept, more D–A units in one mol-
ecule could be a solution. However, this type of material is
rare.
Recently, investigations into the synthesis and properties of
large N-heteroacenes have attracted much attention,[6] and
their potential applications in organic electronics, such as field-
effect transistors, phototransistors, light-emitting diodes, pho-
toelectrochemical cells, and photovoltaics, have been thor-
oughly investigated.[7] The electron-deficient pyridine or pyra-
zine units in N-heteroacenes could act as acceptors to realize
good electron injection, especially, multielectron injection. On
the other hand, the triphenylamine (TPA) unit has been widely
employed as a donor moiety in memory materials, owing to its
strong ability to stabilize a charge-separated state. By combin-
ing these two factors into one molecule, the synergistic effect
could lead to a novel material with multilevel memory proper-
ties. Herein, we report a new tetraazatetracene derivative,
TPAs-BTTT, which contains two TPA units as donors and 5,12-
bis[(triisopropylsilyl)ethynyl]-1,4,6,11-tetraazatetracene (BTTT)
as an acceptor. The memory devices based on TPAs-BTTT show
a typical rewritable multilevel behavior with good endurance.
Figure 1a shows the synthetic route for the preparation of
TPAs-BTTT. The starting material, 4,4’-bis(N,N-diphenylamino)-
benzil (1), was prepared from TPA and oxalyl chloride through
Friedel–Crafts acylation. TPAs-BTTT was obtained as a blue
powder in 13% yield by condensing 1,4-bis[(triisopropylsilyl)e-
thynyl]-2,3-diaminalphenazine (2) and 1 in acetic acid with
a catalytic amount of IBX.[8] Block single crystals were obtained
by slowly diffusing acetonitrile into a solution of TPAs-BTTT in
toluene. As shown in Figure 1b, in spite of the twisted confor-
mation of TPA, TPAs-BTTT adopts good offset intersection face-
to-face stacking among neighboring molecules. The interlayer
distance of 3.626 ꢀ is larger than that in common planar N-het-
eroacenes, but shorter than that of a van der Waals interaction;
[a] C. Wang, Dr. B. Hu, J. Wang, G. Li, Dr. W.-W. Xiong, B. Zou, Prof. Dr. F. Huo,
Prof. Dr. P. S. Lee, Prof. Dr. Q. Zhang+
School of Materials Science and Engineering
Nanyang Technological University
Singapore, 639798 (Singapore)
E-mail: qczhang@ntu.edu.sg
Homepage: http://dx.doi.org/10.1002/chemv.201400109
[b] Dr. J. Gao
College of Materials and Textile
Zhejiang Sci-Tech University
Hangzhou 310018 (P.R. China)
[c] Prof. Dr. M. Suzuki, Prof. Dr. N. Aratani, Prof. Dr. H. Yamada
Graduate School of Materials Science
Nara Institute of Science and Technology
Ikoma, 630-0192 (Japan)
[+] This author is one of the most prolific contributors to
Chemistry–An Asian Journal. In celebration of the 10th volume of
Chemistry–An Asian Journal in 2015, a short profile of the author in
the series “10 Years Ago and Now” is featured on ChemistryViews at:
http://dx.doi.org/10.1002/chemv.201400109.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/asia.201402899.
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Communication
sorption spectrum was measured as a solid film; this corre-
sponded to strong p–p interactions between adjacent mole-
cules. The onset of the absorbance wavelength (lonset) in the
solid film is l=721 nm, which is 26 nm redshifted relative to
that at l=709 nm in CH2Cl2. The optical band gap of TPAs-
BTTT is about 1.72 eV, as determined from lonset of the solid
film.
Cyclic voltammetry was performed in CH2Cl2/0.1m nBu4NPF6.
As shown in Figure 2b, TPAs-BTTT first undergoes a reversible
molecular reduction at a half-wave potential (E1/2 versus
[FeCp2]+/[FeCp2]; Cp=cyclopentadienyl) of ꢀ1.03 V, followed
by a less reversible reduction at E1/2 =ꢀ1.28 V. An irreversible
oxidation at an onset potential of 0.30 V (Eonset versus [FeCp2]+
/[FeCp2]) and a reversible oxidation at E1/2 =0.73 V can be ob-
served, respectively. The different redox behavior suggests
multielectron dissociation of TPAs-BTTT when excited by volt-
age potential, which could induce multilevel resistive states in
memory devices. The HOMO energy level can be determined
as ꢀ5.10 eV from the Eonset value of the first oxidation peak;
the LUMO energy level is calculated to be ꢀ3.38 eV from the
difference between the HOMO energy level and the optical
band gap.
Figure 1. a) Synthetic route to TPAs-BTTT. IBX=2-iodoxybenzoic acid. b) Mo-
lecular stacking of TPAs-BTTT.
The sandwich-structured memory devices were fabricated
on precleaned indium tin oxide (ITO) glass substrates. TPAs-
BTTT in cyclohexanone (10 mgmLꢀ1) was spin-coated as an
active layer with a thickness of 70 nm and high crystallization
could be observed by AFM (Figure S6 in the Supporting Infor-
mation). Gold was sputter-coated onto the TPAs-BTTT layer as
the top electrode through an in situ metal shadow mask with
a diameter of 100 mm and a thickness of 300 nm. The memory
behavior of the ITO/TPAs-BTTT/Au device is demonstrated by
current–voltage (I–V) characteristics (Figure 3a). In sweep 1, as
this indicates that there are p–p interactions between adjacent
molecules.
Figure 2a shows the UV/Vis absorption spectrum of TPAs-
BTTT. Two absorption peaks in the visible region (lmax =548
and 594 nm) are observed in CH2Cl2, which suggests strong in-
tramolecular CT. Redshifts of these absorptions occur (lmax
=
548!607 nm and lmax =594!658 nm) when the UV/Vis ab-
Figure 2. a) Normalized UV/Vis absorption spectrum of TPAs-BTTT. b) Normal-
ized cyclic voltammograms of TPAs-BTTT in CH2Cl2/0.1m nBu4NPF6.
Figure 3. a) Current–voltage (I–V) characteristics of ITO/TPAs-BTTT/Au device.
b) Endurance performance of the devices.
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the potential goes from 0 to +2 V, the current is maintained in
the low-magnitude region, the state of which is defined as
a high-resistance state (HR state). An abrupt increase in the
current can be observed with a threshold voltage at around
0.76 V; this indicates that the device switches to a low-resist-
ance state (LR2 state) by SET 1 process (Figure 3a). The device
can retain the LR2 state both in the dual sweep from +2.0 to
0 V and after removal of the power supply; this suggests typi-
cal nonvolatile behavior. In sweep 2, from 0 to ꢀ1.0 V, the cur-
rent drops and remains in an intermediate range, which is de-
fined as the LR1 state; the process can be called RESET 1 (Fig-
ure 3a). A positive sweep from 0 to 2 V (Figure 3a, sweep 3)
can further switch from the LR1 state to the LR2 state by
SET 2. Another sweep (Figure 3a, sweep 4) from 0 to ꢀ3 V can
recover the initial HR state. The recovery of the device from
the LR2 state to the HR state served as the RESET 2 process. Es-
sentially, the HR and LR1 states could be generated by RESET 2
and RESET 1 operations, respectively. Starting from the HR or
LR1 state, both SET 1 and SET 2 operations could result in the
LR2 state. The three distinguished resistive states suggest that
TPAs-BTTT has a multilevel memory performance. Each of
these states could be switched to others through SET 1/SET 2
or RESET 1/RESET 2 operations, which can be defined as writ-
ing and erasing processes, respectively, and indicate that the
as-fabricated memory device is rewritable. Cyclic switching op-
erations have been carried out to investigate the endurance
performance through the typical cycle of SET 1–RESET 1–
SET 2–RESET 2. Generally, the device can be repeatedly
switched between HR, LR1, and LR2 states for more than
70 cycles. Figure 3b shows the resistance distribution of the
three states for 76 cycles. The resistance values were read at
0.2 V in each state. The ON/OFF ratio between LR2 and LR1,
and LR1 and HR is about eight, which indicates that the device
would suffer much less from misreading. The SET 1 or SET 2
sweep from 0 to 2 V can switch the device to the LR2 state,
and a reverse RESET 1 sweep from 0 to ꢀ1.0–1.5 V can result
in the LR1 state. A larger reverse RESET 2 sweep of 0 to ꢀ3 V
can give the HR state, which suggests that the multiple-states
memory behavior is highly reproducible. The retention per-
formance of the device was evaluated by applying a reading
voltage of ꢀ0.2 V to the HR, LR1, or LR2 states under ambient
conditions (Figure S7 in the Supporting Information). No clear
current decay for each state was observed in 104 s during the
test, which suggested that the multilevel states were retained.
Molecular simulation is employed to understand the switch-
ing mechanism and charge-carrier migration. As shown in Fig-
ure 4a, the LUMO and HOMO energy levels are located at the
BTTT and TPAs moieties, respectively, which could be helpful
for intramolecular CT by combining good stacking with the
low LUMO energy level. Figure 4b shows the energy diagram
of the ITO/TPAs-BTTT/Au system. The energy barrier from ITO
(with a work function of ꢀ4.8 eV) to the TPAs-BTTT film is esti-
mated to be 0.3 eV, which is smaller than that (1.72 eV) of gold
(with a work function of ꢀ5.1 eV). Therefore, charge-carrier mi-
gration is dominated by hole injection. Interestingly, TPAs-BTTT
has very close HOMO (ꢀ5.100 eV) and HOMOꢀ1 (ꢀ5.093 eV)
energy levels, which suggests that double-electron excitation
Figure 4. a) Calculated HOMO and LUMO energy levels of TPAs-BTTT.
b) Energy diagram of the ITO/TPAs-BTTT/Au system. c) Schematic diagram of
the switching process of the device.
could happen in one excitation process. In SET 1 (Figure 4c),
the charge-carrier concentration is very low and the device is
in the HR state. As the voltage increases to the threshold volt-
age, double-electron excitation from the TPAs to the BTTT unit
happens in the active layer and more free charge carriers are
formed,[5a,9] and the device is set to the LR2 state. In the fol-
lowing RESET 1 sweep, a small reverse voltage (ꢀ1.0 to ꢀ1.5 V)
possibly recombines one electron with one hole and the
device is reset to the LR1 state. However, the LR2 state can be
recovered by another positive sweeping, which is the SET 2
process, whereas a higher reverse voltage in the RESET 2 pro-
cess can return the initial HR state.
In summary, a new tetraazatetracene derivative, TPAs-BTTT,
containing two donors and one acceptor has been successfully
prepared and characterized. The sandwich-like memory devices
with TPAs-BTTT as the active layer exhibited nonvolatile rewrit-
able multilevel memory behavior with good endurance and
stability, which was probably induced by multielectron intra-
molecular CT. Our success could provide a useful strategy to
develop novel organic small molecules for RRAMs.
Experimental Section
Data collection of crystals was performed on a Bruker APEX II CCD
diffractometer equipped with a graphite-monochromatized MoKa
radiation source (l=0.71073 ꢀ). Empirical absorption was per-
formed, and the structure was solved by direct methods and re-
fined with the aid of the SHELXTL program package.
CCDC 1014927 (TPAs-BTTT) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
Q.Z. acknowledges the financial support of AcRF Tier 1 (RG 16/
12) and Tier 2 (ARC 20/12 and ARC 2/13) from MOE, the
CREATE program (Nanomaterials for Energy and Water Man-
agement) from NRF, and the New Initiative Fund from NTU,
Singapore.
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Keywords: charge transfer
electrochemistry · heteroacenes · multilevel memory
·
donor–acceptor systems
·
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Published online on September 22, 2014
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