Adjustment of charge trap number and depth in molecular backbone to achieve tunable multilevel data storage performance

Article abstract of DOI:10.1039/c3tc00745f

A series of conjugated donor-acceptor (D-A) molecules with the electron-rich unit triphenylamine (TPA) as donor and azobenzene chromophore and/or cyano group as acceptors were successfully synthesized. Acceptors varying in number and electron-withdrawing

Full text of DOI:10.1039/c3tc00745f

Journal of
Materials Chemistry C
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Adjustment of charge trap number and depth in
molecular backbone to achieve tunable multilevel data
Cite this: J. Mater. Chem. C, 2013, 1,
2320
storage performance†
*
Shifeng Miao, Yongxiang Zhu, Hao Zhuang, Xiaoping Xu, Hua Li, Ru Sun, Najun Li,
*
Shunjun Ji and Jianmei Lu
A series of conjugated donor–acceptor (D–A) molecules with the electron-rich unit triphenylamine (TPA) as
donor and azobenzene chromophore and/or cyano group as acceptors were successfully synthesized.
Acceptors varying in number and electron-withdrawing strength were incorporated into the molecular
backbone to investigate the impact on device switching behavior. It is found that the data storage
states and memory effects are highly dependent on the number and strength of the electron-deficient
groups. The devices based on the D–p–A1–p–A2 molecule with two electron-withdrawing groups
exhibited excellent ternary memory behavior, while those based on the D–p–A1 or D–p–A2 molecule
containing one electron-accepting moiety showed binary memory characteristics. In addition, the
bistable memory effects of TPAVC 2 and TPAAH 3-based data storage devices varied from WORM to
flash when the acceptor changed from a cyano group to an azobenzene chromophore. Therefore,
tunable multilevel memory device performance has been achieved by adjusting the number and
electron-withdrawing strength of acceptor moieties in the molecular backbone, offering guidance for
the rational design of superior D–A molecules for high density data storage (HDDS) applications.
Received 29th November 2012
Accepted 27th January 2013
DOI: 10.1039/c3tc00745f
www.rsc.org/MaterialsC
direction toward HDDS is the development of devices whose
switching may involve more than two levels.14 In a previous
Introduction
As a new type of digital storage, organic memory devices (OMDs)
have experienced intensied research effort over the past few
years due to their attractive advantages, including the ease of
materials processing, good scalability, low-cost potential, and
3D stacking capability.1–10 Various memory types, distinguished
as WORM (write-once, read-many-times),3 DRAM (dynamic
random access memory),4 and ash (rewritable),3d,4b,5 and
several kinds of switching mechanisms, such as charge transfer
(CT),6 conductive lament formation,7 trapping–detrapping of
charges,8 and conformational change,9 have been successfully
demonstrated. OMD performance has signicantly advanced
recently through the design and synthesis of superior electro-
active materials,10 optimized lm morphologies,11 and engi-
neered device architectures.12
study, a variety of organic material based ternary data storage
devices involving three distinct conductance states were real-
ized,13,15–17 offering a unique opportunity to store more than 2
bits in a single cell.13a,15,16 Additionally, a possible “charge
trapping” mechanism in which a molecule containing two
different depth charge traps18 may result in ternary data storage
characteristics was preliminarily proposed.15,16 Most efforts
have been focused on the design of building blocks for small
molecular backbones, however, less research has shed light on
the inuence of the number and electron-withdrawing strength
of acceptor moieties on device properties.13–17
Herein, three organic D–A small molecules (TPAAC 1, TPAVC
2, and TPAAH 3; see Fig. 1) were synthesized and characterized,
in which the TPA unit (D) serves as electron donor and the
azobenzene chromophore (A1) and/or cyano group (A2) serve as
electron acceptors. The TPA moiety, a large conjugated tertiary
amine system, can not only act as a strong electron donor, but
also as a good hole-transporter which could be introduced to
modify the electrical conductivity of the material effectively.19
The role of the donor and acceptor in the D–A molecule, espe-
cially the inuence of the number and strength of the acceptor
in the molecular backbone on the data storage properties, has
rarely been systematically investigated. In this paper, we
demonstrated that the data storage states and memory effects
are highly dependent on the number and strength of the
Currently, most data storage systems are conned to binary
and are based on electrical bistability in response to an external
bias.2–10 With the rapid progress of information technology,
HDDS are in intense demand.13 An effective step further in the
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
P. R. China. E-mail: lujm@suda.edu.cn; lihuaw@suda.edu.cn; Fax: +86 512
65880367; Tel: +86 512 65880368
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc00745f
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iodobenzene, copper powder, 18-crown-6, potassium carbonate,
benzyl bromide, triethylphosphite and sodium ethoxide were
purchased from Sigma-Aldrich, Alfa Aesar, TCI and Sinopharm
Chemical Reagent Co. Ltd. N,N-Dimethyl formamide (DMF) was
dried over calcium hydride and distilled prior to use. All other
reagents were used as received without further purication.
Compounds (4-cyanobenzyl)-phosphonic acid diethyl ester (6),20
4-(diphenylamino)benzaldehyde (7),21 (E)-N,N-diphenyl-4-(phe-
nyldiazenyl)aniline (TPAAH 3),22 and (E)-N,N-diphenyl-4-styry-
laniline (TPAVH 4)23 were prepared according to previously
reported procedures.
Synthesis of TPAAC 1. 4-Cyano aniline (0.24 g, 2 mmol) was
dissolved in a mixture of deionized water (1.5 mL) and
concentrated aqueous HCl (1.0 mL). The reaction mixture was
cooled down to 0–5 ꢀC in an ice bath; then a solution of sodium
nitrite (0.15 g, 2.2 mmol) in deionized water (2.0 mL) was added,
and the mixture became transparent. Aer vigorous stirring at
0–5 ꢀC for 1 h, an ice-cold solution of triphenylamine (0.49 g, 2
mmol) in DMF (5.0 mL) was added dropwise for about 30 min.
Aer additional stirring in an ice bath for 2 h, the mixture was
ltered and washed with water twice. Aer drying in a vacuum
oven overnight at 60 ꢀC, the crude product was puried by
column chromatography on silica gel using hexane/ethyl
Fig. 1 Chemical structures of four conjugated small molecules: TPAAC 1, TPAVC
2, TPAAH 3, and TPAVH 4.
acceptors. The device based on the D–p–A1–p–A2 molecule
TPAAC 1 with azobenzene and cyano as two electron-with-
drawing groups exhibited excellent ternary memory character-
istics, while that based on the D–p–A1 or D–p–A2 molecule
containing one electron-accepting moiety showed binary
memory behavior. Furthermore, the memory effects varied from
WORM to ash as the acceptor changed from the cyano group to
azobenzene chromophore. These disclose the underlying
structure–property relationship in the small-molecule multi-
level data storage devices, providing convictive proof for the
“charge trapping” mechanism, as well as offering denite
guidance for the design of superior D–A molecules for HDDS.
1
acetate as the eluent. Yield 0.69 g (92%). H NMR (300 MHz,
DMSO-d6) d (ppm): 8.02 (d, J ¼ 8.4 Hz, 2H), 7.92 (d, J ¼ 8.4 Hz,
2H), 7.84 (d, J ¼ 8.9 Hz, 2H), 7.43 (t, J ¼ 7.7 Hz, 4H), 7.26–7.20
(m, 6H), 6.98 (d, J ¼ 8.9 Hz, 2H). HRMS-ESI (m/z): [M + H]+ calcd
for C25H18N4, 374.1531; found, 374.1524. Anal. calcd for
C
25H18N4: C, 80.19; H, 4.85; N, 14.96. Found: C, 80.02; H, 5.07;
N, 14.63%.
Synthesis of TPAVC 2. 4-Cyanobenzyl bromide (1.97 g, 10
mmol) and triethylphosphite (2.0 g, 12 mmol) were placed in a
50 mL round bottom ask. A distillation apparatus was attached
to collect the poisonous ethyl bromide formed during the
reaction. The reaction mixture was heated to 150 ꢀC for 1 h and
then cooled to room temperature. The remaining triethylphos-
phite was removed in vacuo. Anhydrous THF (60 mL) was added
into the system. The solution was placed in an ice-bath for 15
min and then NaH (0.60 g (60% in mineral oil), 15 mmol) was
added. The mixture was stirred for 20 min and then a solution
of 4-(diphenylamino)benzaldehyde (2.73 g, 10 mmol) in anhy-
drous THF (10 mL) was slowly added. Aer the mixture was
stirred for 2 h in an ice-bath and for 6 h at room temperature,
8.0 mL of water was added and the solvent was evaporated. The
residue was dissolved in CH2Cl2 and washed several times with
water. The product was puried by column chromatography on
silica gel using hexane/CH2Cl2 as the eluent. Yield 2.53 g (68%).
1H NMR (300 MHz, CDCl3): d 8.13 (d, J ¼ 8.7 Hz, 2H), 7.52 (d, J ¼
8.7 Hz, 2H), 7.34 (d, J ¼ 8.5 Hz, 2H), 7.24–7.17 (m, 5H), 7.13–6.91
(m, 9H). HRMS-ESI (m/z): [M + H]+ calcd for C27H20N2, 372.1626;
found, 372.1623. Anal. calcd for C27H20N2: C, 87.07; H, 5.41; N,
7.52. Found: C, 87.12; H, 5.26; N, 7.44%.
Experimental section
The chemical structures of a series of TPA-based conjugated
small molecules (TPAAC 1, TPAVC 2, TPAAH 3, and TPAVH 4
where A stands for azobenzene chromophore, V vinyl group, C
cyano group, and H hydrogen) are shown in Fig. 1. The
synthesis of TPAAC 1 and TPAVC 2 is depicted in Scheme 1.
Materials and synthesis
4-Cyano aniline, sodium nitrite, triphenylamine, phosphorus
oxychloride, 4-cyanobenzyl bromide, aminoazobenzene,
Device fabrication
The indium-tin-oxide (ITO) glass was precleaned with water,
acetone, and alcohol sequentially in an ultrasonic bath for
Scheme 1 Synthesis of TPAAC 1 and TPAVC 2.
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20 min. The active organic lm was deposited under high ꢁ2.69 V (sweep 6) and remains in the ON2 state during the
vacuum (10ꢁ6 Torr). The lm thickness was about 80 nm, as following voltage sweep (sweep 7). This device, therefore,
shown in the SEM image in ESI.† An aluminum (Al) layer with a exhibits nonvolatile ternary WORM memory behavior. More-
thickness of about 100 nm was thermally evaporated and over, the three states of the ternary memory device are very
deposited onto the organic surface at about 5 ꢂ 10ꢁ6 Torr distinct, with the current ratio of “0”, “1” and “2” being about
through a shadow mask to form the top electrode. An active 1 : 102 : 104.
device area of 0.0314 mm2 was obtained.
The I–V characteristics of the ITO/TPAVC 2/Al device in
Fig. 2c show two distinct conductivity states. The device is
initially in the low-conductivity (OFF or “0”) state, with a current
of about 10ꢁ6 A at ꢁ1.0 V when a bias sweeping was applied
starting from 0 V. When the voltage applied was increased
further, the device switches from the OFF state to the high-
conductivity (ON or “1”) state at a threshold voltage of about
ꢁ2.1 V, as indicated by the abrupt increase of the current to
about 10ꢁ2 A (sweep 1). The high current density observed
during the subsequent voltage sweep indicates retention of the
ON state in the device (sweep 2). The ON-state I–V curves under
negative and opposite biases are symmetrical (sweep 3). The ON
state of the device could be distinguished from the OFF state by
an ON/OFF current ratio of about 104 when read at ꢁ1.0 V. Thus,
this device shows nonvolatile bistable WORM memory
behavior.
From the comparison of the electrical switching properties
of the two molecules, we have learned that the number of
electron-acceptors plays a crucial role in determining the data
storage states. The D–p–A1–p–A2 type molecule, TPAAC 1,
exhibited nonvolatile ternary memory behavior; whereas the D–
p–A type molecule, TPAVC 2, showed nonvolatile binary data
storage effects. In order to verify the results, another D–p–A type
molecule, TPAAH 3, was synthesized, and the electrical
switching behavior was investigated, as shown in Fig. 2c. In the
rst sweep from 0 to ꢁ4 V, the current increased progressively
Measurements
All electrical measurements of the device were characterized
under ambient conditions without any encapsulation using an
HP 4145B semiconductor parameter analyzer equipped with an
HP 8110A pulse generator. NMR spectra were obtained from an
Inova 300 MHz FT-NMR spectrometer. High-resolution mass
spectra (HRMS) were determined on a Micromass GCT-TOF
mass spectrometer with an ESI resource. UV-vis absorption
spectra were measured at room temperature by using a Shi-
madzu UV-3600 spectrophotometer. Cyclic voltammetry (CV)
was performed at room temperature using an ITO working
electrode, a Ag/AgCl reference electrode, and a counter elec-
trode (Pt wire) at a sweep rate of 100 mV sꢁ1 (CorrTest CS
Electrochemical Workstation analyzer). A 0.1 M solution of
tetrabutylammonium perchlorate (TBAP) in anhydrous DMF
was used. SEM images were taken on a Hitachi S-4700 scanning
electron microscope. Atomic force microscopy (AFM) measure-
ments were performed by using a MFP-3DTM (Digital Instru-
ments/Asylum Research) AFM instrument.
Results and discussion
Current–voltage (I–V) characteristics
The memory effects of the four molecules were revealed by I–V with the applied bias, and a sharp increase in the current
measurements on ITO/small molecule/Al sandwich-structure occurred at about ꢁ1.9 V, which indicates that the device
devices, as shown in Fig. 2. Fig. 2a presents the typical I–V switched from a low-conductivity (OFF or “0”) state to a high-
curves of TPAAC 1-based memory devices. The as-fabricated conductivity (ON or “1”) state. The transition from the OFF state
TPAAC 1 lms initially exhibit a low-conductivity (OFF or “0”) to the ON state is equivalent to the “writing” process in a digital
state. During a negative-voltage sweep from 0 to ꢁ4.0 V on one memory cell. The device exhibited good stability in this high-
cell of the device, the resulting current shows two sharp conductivity state during the subsequent negative sweep and it
increases at switching threshold voltages around ꢁ1.50 V and remained in the ON state even aer the power was turned off
ꢁ2.18 V (sweep 1), indicating that the storage cell undergoes (sweep 2). The I–V characteristics dened the electrical bist-
electrical transitions from an OFF state to an intermediate- ability of TPAAH 3 and also revealed the nonvolatile nature of
conductivity (ON1 or “1”) state and then to a high-conductivity the memory effect which is similar to TPAVC 2 devices. More
(ON2 or “2”) state. These OFF-to-ON1 and ON1-to-ON2 transi- importantly, the OFF state can be recovered by the simple
tions can be regarded as the “writing” process in a digital application of a reverse voltage at about 2.5 V (sweep 3). This is
memory cell.24,25 In the subsequent sweep from 0 to ꢁ4.0 V, the equivalent to the “erasing” process of a digital memory cell.26
storage cell of the device remains in its high-conductivity (ON2) Sweep 4 showed the I–V curve of the device right aer the
state (sweep 2). As the applied positive-voltage is swept from 0 to application of an erase, which indicates that the device
4.0 V, the device is still in a high-conductivity state (sweep 3). remained in the stable OFF state. In switching devices, two
Another cell of the device is measured under a voltage range of conducting states are observed and the conductance switching
0 to ꢁ2.0 V and it shows a switching threshold voltage at at a suitable negative voltage was associated with memory
ꢁ1.50 V (sweep 4), which suggests that this storage cell is effects. No bias was required to sustain the high-conductivity
switched from the OFF state to the ON1 state. Once the cell has state. Only a suitable positive bias could switch the high-
reached the ON1 state, it could be maintained during the conductivity state to a low one. This feature allows the appli-
subsequent scan from 0 to ꢁ2.0 V even aer removing the power cation of TPAAH 3 in a rewritable memory device. Thus, the ITO/
supply (sweep 5). When the voltage sweep was applied from 0 to TPAAH 3/Al device exhibits typical nonvolatile ash memory
ꢁ4.0 V, the cell undergoes an ON1-to-ON2 transition at about behavior with an ON/OFF current ratio of about 104.3. These
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Fig. 2 I–V characteristics of the four small molecule based memory devices: (a) TPAAC 1, (b) TPAVC 2, (c) TPAAH 3, and (d) TPAVH 4, respectively. The inset of (d) shows
a schematic diagram of the ITO/small molecule/Al sandwich-structure memory device.
results further indicated that the data storage states are highly
dependent on the number of acceptors in the backbone of the
D–A structured molecules.
Film morphology
To conrm the diffraction pattern and molecular ordering, the
microstructure of the three small molecules' thin lms was
studied using X-ray diffraction (XRD). For the XRD measure-
ments, the thin lms were thermally evaporated under high
vacuum with a thickness around 150 nm. As seen in Fig. S4,† the
diffraction patterns of the three molecules all showed sharp and
multiple diffraction peaks, demonstrating that the highly crys-
talline lms were formed in the solid thin lm state. A diffrac-
tion peak at 23.23ꢀ for TPAAC 1, a peak at 21.17ꢀ for TPAVC 2,
and a peak at 21.05ꢀ for TPAAH 3, correspond to p–p stacking
distances of 3.83 A, 4.17 A, and 4.22 A, respectively. The small
p–p stacking distance indicates stronger intermolecular inter-
actions. From the AFM height images shown in Fig. S5,† all the
molecules formed high-quality thin lms with low roughness,
good crystallinity and ordered packing, which can facilitate the
formation of highly efficient pathways for charge carrier trans-
port throughout the molecular lm,16a as well as rule out the
inuence of lm defects on the stability and switching effect of
the memory device.
Although both the TPAVC 2 and TPAAH 3 molecules
possessed D–p–A conjugated backbones and one electron-
withdrawing moiety, the strength of the electron-withdrawing
groups in the molecules was rather different. In the D–p–A1
molecule, the cyano group functions as the acceptor and
localizes the electron density due to its strong electron-with-
drawing ability, thus exhibiting WORM memory effects in a
storage device. However, in the D–p–A2 molecule, the azo-
benzene serves as the acceptor and hence shows ash memory
effects because of the weak electron-accepting capacity.
Therefore, by tuning the kind or strength of electron-with-
drawing groups in the molecule, the memory effect could be
effectively adjusted, which is vital for designing high-perfor-
mance multilevel data storage devices with various
applications.
˚
˚
˚
As can be seen in Fig. 2d, the device fabricated with TPAVH 4
as the active layer shows only a single moderately high
conductance state without any switching behavior, probably
because there are no available electron-acceptors as “traps” in
the material.
Optical and electrochemical properties
It is noteworthy that all the storage states of the memory
devices of TPAAC 1, TPAVC 2 and TPAAH 3 are highly stable for
up to 108 continuous read pulses at ꢁ1 V without any resistance
degradation (see Fig. S3a, c and e†). Moreover, the currents of all
the states in the three devices are stable under a constant
voltage stress of ꢁ1 V for up to 10 hours (see Fig. S3b, d and f†).
The UV-visible absorption spectra of the synthesized small
molecules, either in dilute tetrahydrofuran solution or in thin
lm forms on quartz substrates, are depicted in Fig. 3a, c, e and
S6a.† The main absorption bands at around 400–500 nm can be
assigned to the partial charge transfer (CT) between the donor
and acceptor moieties of the D–A molecules.27,28 The CT
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Fig. 3 UV-vis absorption spectra in dilute THF solution and thin film state (a, c and e) and cyclic voltammograms in 0.1 M TBAP/CH2Cl2 solution of TPAAC 1, TPAVC 2,
and TPAAH 3, respectively (b, d and f). The ferrocene/ferrocenium redox couple is used as a standard (ꢁ4.8 eV, the onset potential is 0.57 eV). A scan rate of 100 mV sꢁ1
was used.
transition in solid state shows an obvious red-shi compared an onset oxidation potential (Eooxnset) of 1.11, 1.17 and 1.01 V vs.
with that of the solution state. This red-shi might be attributed Ag/AgCl, respectively. The external ferrocene/ferrocenium (Fc/
to the formation of molecular aggregates and/or increased Fc+) redox standard potential was measured to be 0.57 V vs. Ag/
polarity of the thin lm,27,29 which would facilitate the occur- AgCl in dichloromethane. Assuming that the HOMO level for
rence of CT from the TPA to the acceptor moieties in neigh- the Fc/Fc+ standard is ꢁ4.80 eV with respect to the zero vacuum
boring molecules when an external electric eld is applied. The level, the HOMO levels for all materials are determined to be
optical band gaps (i.e., the difference between the highest ꢁ5.34, ꢁ5.40 and ꢁ5.24 eV, respectively. Thus, the LUMO levels
occupied molecular orbital (HOMO) energy level and the lowest are estimated to be ꢁ3.08, ꢁ3.13 and ꢁ2.77 eV, respectively. The
unoccupied molecular orbital (LUMO) energy level) of the three HOMO values vary from ꢁ5.24 to ꢁ5.34 eV, indicating that these
molecules from the absorption edges of the lms are estimated HOMO values are not only valuable for making memory devices
to be 2.26, 2.27 and 2.47 eV, respectively, as shown in Table 1. stable against oxygen oxidation, but are also useful to transfer
The electrochemical properties of these D–A molecules were holes easily from the ITO electrode due to their close work
investigated by cyclic voltammetry (CV) conducted in 0.1 M functions (ITO has a work function of ꢁ4.8 eV). In the ITO/small
nBu4NClO4 (TBAP)/dichloromethane (CH2Cl2) solution using molecule/Al sandwich device, the energy barriers of hole injec-
the lm deposited onto ITO substrate as the working electrode, tion from the electrode to the active layer were estimated to be
Ag/AgCl as reference electrode, and Pt wire as counter electrode. 0.54, 0.60 and 0.44 eV, respectively, from the work function of
The cyclic voltammograms for these small molecules are ITO and the HOMO of the active layer, and the energy barrier of
depicted in Fig. 3b, d, f and S6b.† The three molecules all electron injection from the electrode to the active layer was
exhibited an oxidation peak but no visible reduction peak, and estimated to be 1.20, 1.15 and 1.51 eV from the work function of
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Table 1 Optical and electrochemical properties of the three synthesized molecular films
FITO
ꢁ
ELUMO
FAl (eV)
ꢁ
Compound
lonset/nm
Eogpta/eV
Eoonx setb/eV
HOMOc/eV
LUMOd/eV
EHOMO (eV)
TPAAC 1
TPAVC 2
TPAAH 3
549
546
502
2.26
2.27
2.47
1.11
1.17
1.01
ꢁ5.34
ꢁ5.40
ꢁ5.24
ꢁ3.08
ꢁ3.13
ꢁ2.77
0.54
0.6
0.44
1.22
1.17
1.53
The data were calculated using the following equation: bandgap ¼ 1240/lonset of the three synthesized molecular lms. b vs. Ag/AgCl in CH2Cl2.
a
c
d
Calculated from cyclic voltammetry. Calculated from the HOMO energy level and optical bandgap.
Al (ꢁ4.3 eV) and the LUMO of the active layer. This indicates
In order to further understand the electrical switching
that hole injection from ITO into the HOMO is easier than behavior of the small-molecule-based ternary memory device,
electron injection from Al into the LUMO of the three mole- theoretical calculations were performed using the density
cules. Therefore, all three D–A molecules are p-type materials functional theory (DFT) method of B3LYP with the 6-31G (d)
and holes predominate in the conduction process.
basis set.30 DFT molecular simulation results (see Fig. 4)
showed that an open channel is formed from the molecular
surface throughout the conjugated backbone with continuous
positive molecular electrostatic potential (ESP, in red), through
which charge carriers can migrate smoothly. The negative ESP
regions (in blue) caused by the electron-decient groups, such
as the azobenzene chromophore and/or cyano moiety, can
function as “charge trapping” sites to reduce the mobility of
the charge carriers.18 The depth of these traps is in accordance
with the electron-withdrawing strength of the functional
groups (see Fig. S7†). Due to the highly ordered arrangement in
the thin lm, all the traps in the molecules have equal
opportunities to accept the injected charge carriers. In the
TPAAC 1-based data storage device, the shallower charge traps
arising from the azobenzene moiety will be lled quickly,
leading to the current transition from the OFF to ON1 state.
Data storage mechanism
To understand the electronic memory behavior of the D–A
molecules in this work, a wholly D type molecule, TPAVH 4,
composed of only one TPA unit and vinyl phenyl group without
any acceptor was prepared for comparison. The I–V character-
istics showed no conductance transition behavior for the ITO/
TPAVH 4/Al device, indicating that the electrical switching
property could not occur in the wholly D structured molecule,
and the acceptor is critical for the electrical switching of the
memory device. Thus, “charge trapping” arising from the
acceptors located in the molecular skeleton is probably
responsible for the conductance switching and the associated
memory behavior.
Fig. 4 DFT molecular simulation results (B3LYP/6-311G (d) level) of TPAAC 1, TPAVC 2, and TPAAH 3: molecular optimized geometry, ESP, and HOMO and LUMO
molecular orbitals.
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The deeper charge traps arising from the cyano group will be memory device performance. A correlation between the molec-
lled later, which results in the current transition from the ular structure and data storage properties of the synthesized D–
ON1 to ON2 state. The HOMO and LUMO orbital surfaces of A molecules is established, not only providing valid evidence for
TPAAC 1 are shown in Fig. 4. Upon the HOMO to LUMO the “charge trapping” mechanism of small-molecule ternary
transition, the electron density shis from the donor to OMDs, but also offering fundamental insight for the rational
acceptor side and becomes more localized. The trapped charge design of superior small molecules for HDDS.
carriers were stabilized by intra- and/or inter-molecular CT
which could form a charge-separated state. The trapped
carriers may not be easily de-trapped aer the power was
Acknowledgements
turned off or under non-degrading reverse elds, thus result- This work was nancially supported by the National Natural
ing in a high conductivity state that can be permanently Science Foundation of China (NSFC 21076134, 21176164 and
maintained. Consequently, the device shows nonvolatile 21206102), a project of the Department of Education of Jiangsu
ternary WORM-type memory characteristics due to the two Province (12KJB430011), Suzhou Nano-project (ZXG2012023)
charge traps with different electron-withdrawing strength and and the project supported by the Specialized Research Fund for
the formation of stable intramolecular CT.
the Doctoral Program of Higher Education of China (Grant no.
project funded by the Priority
When only one acceptor was located in the conjugated 20113201130003), and
a
molecular backbone as TPAVC 2 and TPAAH 3, however, there is Academic Program Development (PAPD) of Jiangsu Higher
also only one negative region in the ESP caused by the cyano Education Institution.
group and azobenzene chromophore, respectively, to act as a
charge trap to impede the carrier migration. Under an external
voltage bias, charge carriers with adequate activation energy
Notes and references
were injected into the electroactive layer and the charge traps
were fully lled, leading to the current transition from the OFF
to ON state. Upon HOMO to LUMO transition, the molecular
polarity of TPAVC 2 would be improved for the excited state
because of the considerable increase in charge separation. Since
strong molecular polarity and inter- or intra-molecular CT,
which could be attributed to the strong electron-withdrawing
ability, can facilitate the stability of the charge-separated state,
WORM-type bistable switching behavior was thus observed.
However, weak inter- and/or intra-molecular CT may result in
the poor stability of the charge-separated state. Under a reverse
bias, the trapped charge carriers could be de-trapped to the
initial states and the thin lm of TPAAH 3 returned to a low-
conductivity state, thereby ash bistable switching behavior was
achieved.
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In summary, we synthesized a series of conjugated D–A mole-
cules with the core unit TPA as donor and azobenzene chro-
mophore and/or cyano group as acceptors. The role of the donor
and acceptors, especially the inuence of the number and
electron-withdrawing strength of the acceptors in the molecular
backbone on the data storage properties, has been systemati-
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strength of the acceptors. The device based on the D–p–A1–p–
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effects varied from WORM to ash with the acceptor changing
from the cyano group to the azobenzene chromophore. Thus,
adjusting the number and electron-withdrawing strength of the
acceptors affords an effective way to ne-tune the multilevel
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