Nonvolatile memory organic field effect transistor induced by the steric hindrance effects of organic molecules

Article abstract of DOI:10.1039/c0jm00854k

We report a nonvolatile memory organic field effect transistor (OFET) using ambipolar organic molecules as nano-interfaced semiconductor materials. Newly synthesized push-pull organic molecules (PPOMs) containing triarylamine as an electron donating group

Full text of DOI:10.1039/c0jm00854k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Nonvolatile memory organic field effect transistor induced by the steric
hindrance effects of organic molecules†
a
Mi-Hee Jung, Kyu Ho Song, Kyoung Chul Ko, Jin Yong Lee and Hyoyoung Lee
b
c
c
d
*
Received 28th March 2010, Accepted 13th July 2010
DOI: 10.1039/c0jm00854k
We report a nonvolatile memory organic field effect transistor (OFET) using ambipolar organic
molecules as nano-interfaced semiconductor materials. Newly synthesized push-pull organic molecules
(PPOMs) containing triarylamine as an electron donating group, thiophene as a spacer, and
malononitrile as an electron withdrawing group are proposed. The tilted PPOMs with side chains could
control the charge trapping of the conducting channel formed between pentacene and PPOMs.
Quantum chemical calculations were carried out to estimate the dihedral angle at the neutral and
electron charged anion molecules and to explain the charging effects for the memory OFET
performance (write-read-erase-read cycles). The charge mobilities of OFETs comprising the nano-
interfaced PPOMs were affected by the dihedral angles derived from the steric hindrance of the bulky
side chain of the spacer. The memory OTFT with higher dihedral angle material exhibited a wider
memory window as well as reduced current flow. The memory OFET devices showed a memory
window of ꢀ40 to +40 V, a memory ratio of 100 for ‘‘ON’’ to ‘‘OFF’’ currents, and long retention time
for the nonvolatile memory.
Singh et al. demonstrated OFET memory using poly(vinyl
alcohol) as a gate electret.4 Recently, Noh et al. reported
simple electrets using a polymer for the memory dielectrics.5
However, these devices still do not sufficiently satisfy the
criteria demanded in order to compete with other types of
memory devices with the electrets being generally limited to
polymer materials.
1. Introduction
The organic field effect transistor (OFET) has attracted
a great deal of attention given its potential application in low
cost, flexible, and large area electronics.1 Several OFET
devices containing organic semiconducting materials exhibit
high electronic performance comparable to or surpassing
those with amorphous hydrogenated silicon (a-Si:H).2
However, memory elements for OFET applications such as
parts of radio frequency identification devices, smart cards,
and disposable circuitry are less developed. Among organic
memory devices, a memory based on the OFET is especially
attractive because of its nondestructive readout and single
transistor applications. Initial attempts at nonvolatile organic
memory involved electrets, chargeable dielectrics, etc. Katz
et al. demonstrated OFET memory with a polarizable gate
insulator that induced floating-gate-like behavior,3 while
However, until now, there has been no report on nonvolatile
organic electrets using a nano-interfaced organic monomer layer
as a semiconductor material, even though the use of organic
monomer materials has become important for the development
of molecularly interfaced memory and logic elements. Further-
more, to increase the retention time of nonvolatile organic
memory devices, as well as to understand the intrinsic memory
properties, molecular design of the organic materials is becoming
an important issue.
In complement to demonstrate the concept of organic
nonvolatile memory devices, we report an OFET memory device
built on a silicon wafer and based on films of pentacene and
a SiO2 gate insulator that are separated by organic molecules
acting as a gate dielectric. We propose push-pull organic mole-
cules (PPOMs) containing triarylamine as an electron-donating
group (EDG), thiophene as a spacer, and malononitrile as an
electron-withdrawing group (EWG). The PPOMs were designed
for charge transport control through differences of the dihedral
angles induced by a steric hindrance effect of the side chains
within the molecules. Therefore, we expect that these tilted
PPOMs with potential energy barriers can save the charges that
are transported to the nano-interfaced conducting channel
formed between the semiconductor, pentacene, and SiO2 used as
the dielectrics. Finally, we also expect the charges to be
contributed to the memory capacity of the memory OFET
device.
aThin Film Solar Cell Technology Research Team, Advanced Solar
Technology Research Department, Convergence Components
&
Materials Research Laboratory, Electronics and Telecommunications
Research Institute (ETRI), 138 Gajeongno, Yuseong-gu, Daejeon,
305-700, Republic of Korea
bBioMed Research Team, BT Convergence Technology Research
Department, IT Convergence Technology Research Laboratory,
Electronics and Telecommunications Research Institute (ETRI), 138
Gajeongno, Yuseong-gu, Daejeon, 305-700, Republic of Korea
cDepartment of Chemistry, Sungkyunkwan University, 300 Cheoncheon-
dong, Jangan-gu, Suwon, 440-746, Republic of Korea
dNational Creative Research Initiative, Center for Smart Molecular
Memory, Department of Chemistry, Sungkyunkwan University, 300
Cheoncheon-dong, Jangan-gu, Suwon, 440-746, Republic of Korea.
E-mail: hyoyoung@skku.edu; Fax: +82-31-290-5934; Tel: +82-31-299-
4566
† Electronic supplementary information (ESI) available: Full synthetic
procedures of the PPOMs. See DOI: 10.1039/c0jm00854k
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EWG in the molecular backbone. Herein, two types of mole-
2. Experimental
cules:
R
¼
H
(2-((5-(4-(naphthalen-1-yl(phenyl)amino)phe-
CH3
Syntheses of the PPOMs (2-((5-(4-(naphthalen-1-yl(phenyl)-
amino)phenyl)thiophen-2-yl)methylene)malononitrile, QH, and
2-((4-methyl-5-(4-(naphthalen-1-yl(phenyl)amino)phenyl)thio-
phen-2-yl)methylene)malononitrile, QMe) as detailed in Sup-
porting Information (ESI†) were performed under a nitrogen
atmosphere in a dry box or by standard Schlenk techniques.
Solvents were distilled from appropriate reagents. Detailed
synthetic schemes and preparations of the PPOMs are provided
in Supporting Information.
nyl)thiophen-2-yl)methylene)malononitrile), QH:
R
¼
(2-((4-methyl-5-(4-(naphthalen-1-yl(phenyl)amino)phenyl)thiop
hen-2-yl)methylene)malononitrile), QMe, The intermediate
thiophene spacer of QH, 2-(5-(5,5-dimethyl-1,3-dioxan-
2-yl)thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,
was successfully synthesized from 5-bromothiophene-
2-carbaldehyde through protection of the aldehyde
group and borylation in 86% overall yield, and in
a similar manner the thiophene spacer of QMe, 2-(5-(5,5-
dimethyl-1,3-dioxan-2-yl)-3-methylthiophen-2-yl)-4,4,5,5-tet-
ramethyl-1,3,2-dioxaborolane, was synthesized from
2-bromo-3-methylthiophene in 77% overall yield. And then
the electron donor, N-phenylnaphthalene moiety, and
acceptor, malononitrile moiety, were added to the inter-
mediate thiophene spacer through Suzuki coupling and
condensation reactions in overall 25% and 29% yield for
QH and QMe, respectively. The detailed organic synthesis
and characterization of QH and QMe are reported in
the Supporting Information. This conclusion is also
consistent with Fourier transform infrared spectroscopy
(FTIR) spectra of the PPOMs (Fig. S1 in the Supporting
Information).
Top contact OFETs withthe PPOMs as depicted in Fig. 1b were
constructed on heavily n-doped Si substrates with a 300 nm SiO2
layer. The PPOMs were deposited at 4.9 ꢁ 10ꢀ7 Torr, maintaining
a substrate temperature of 70 ꢂC. PMMA was spin-coated as
a 60–80 nm thick film from a toluene solution onto the SiO2 layer.
The films were kept in an oven at 70 ꢂC overnight and subse-
quently annealed at 100 ꢂC for a period of 3 h. The PPOMs (20 nm
thickness) were vacuum deposited onto the PMMA/SiO2/Si
substrate. Pentacene films (50 nm thickness) were vacuum
deposited on the PPOM deposited substrates and kept at 30 ꢂC at
a deposition rate of 0.1 A/s and 10ꢀ7 Torr. Finally, gold electrodes
ꢀ
as source and drain contacts were deposited onto the organic
layers through a shadow mask. The channel lengthand width were
100 and 10 000 mm, respectively. The I–V curves were measured
using a Keithley 4200 semiconductor parameter analyzer. Write-
read-erase cycle tests, as well as switching time measurements,
were carried out using an Agilent pulse generator (model
HP81104A-80MHz or a homemade pulse generator) with a Tek-
tronix oscilloscope (model TDS-3014). All electrical measure-
ments were conducted at a constant temperature, 25 ꢂC, and
relative humidity, 50%, under air atmosphere without any device
encapsulation. Atomic force microscopy (AFM) measurements
were carried out on a Multimode XE-100 instrument (PSIA Inc.,
City, State, Country), operating in non-contact mode with silicon
cantilevers (resonance frequency in the range of 204–259 kHz and
an integrated Si tip with a typical curvature radius of 10 nm).
As mentioned earlier, the thiophene spacer was designed for
controlling the charge transport properties from triarylamine to
malononitrile and vice versa. The charge transport would be
affected by the dihedral angles derived from the steric hindrance
of the side chains of the thiophene spacer as shown in Fig. 1a.
For realization of the OFET memory device as shown in
Fig. 1b, a top contact OFET with the PPOMs was constructed on
the heavily n-doped Si substrates with a 300 nm SiO2 layer.
Poly(methyl methacrylate) (PMMA) was used as the dielectric
layer. PMMA is a hydroxyl-free polymer that decreases the
number of electron traps, providing the gate dielectric interface
buffered to the organic molecules.6 Gold electrodes, as source
and drain contacts, were deposited onto the organic layer
through a shadow mask (Device I). The channel length and width
3. Results and discussion
The newly synthesized PPOMs were composed of triarylamine
as an EDG, thiophene as a spacer, and malononitrile as an
Fig. 2 Potential energy surfaces for: neutral QH (a) and QMe (b);
anionic QH (c) and QMe (d), scanned at the B3LYP/6-31+G* level of
theory.
Fig. 1 (a) Molecular structure of the PPOMs (QH and QMe). (b)
Schematic of the OFET memory device.
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were 100 and 10,000 mm, respectively. An OFET device without
organic molecules (Device II) was prepared as a control for the
OFET memory device bearing organic molecules (Device I).
To understand the molecular structure of the OFET memory
devices at the molecular level, DFT calculations were performed
with a B3LYP exchange functional and 6-31+G* basis sets using
the Gaussian 03 suite of programs.7 Potential energy surface
(PES) curves were plotted by changing the dihedral angles
(C1-C2-C3-C4) from ꢀ90 to 90 degrees, with every 10 degree step
to predict the relationship between the charge variation and
dihedral angle as shown in Fig. 2. Fig. 2a shows that the neutral
QH molecule was the most stable state at a dihedral angle of
ꢀ20.8 degrees. The rotation energy barrier was relatively small
(0.4 kcal/mol) when the dihedral angle changed from negative to
positive. Nevertheless, for the neutral QMe molecule, the rota-
tion energy barrier was as large as 8.5 kcal/mol, due to the steric
hindrance effect of the methyl group attached to C4, for which in
the most stable dihedral angle reached up to ꢀ38.3 degrees as
depicted in Fig. 2b. When the molecules become anions, QH was
the most stable state at ꢀ0.7 degrees and had a symmetrical PES
curve as depicted in Fig. 2c. In contrast, the QMe molecule had
an unsymmetrical PES curve with ꢀ21.1 degrees in the most
stable position as shown in Fig. 2d. Based on the optimized
results of both neutral and electron charged anion states, the
dihedral angle of the QMe was about 20 degrees larger than that
of QH, due to the large steric hindrance of the C4 methyl group.
Therefore, when the molecules reached a ꢀ1 charge, the dihedral
angles of both the QH and QMe changed by about 20 degrees. As
a result, the QH turned out to be completely planar with QMe
also experiencing a change in conformation of ꢀ20 degrees closer
to the planar.
Fig. 4 AFM topographs of pentacene films on: a) SiO2; b) QH; c) QMe.
the QMe layer should be higher than that on the QH layer in
both neutral and electron charged anion states.
From XRD measurements, the intermolecular d-spacings of the
QH and QMe/PMMA/SiO2/Si substrates were 1.166 (2q ¼ 32.86)
ꢀ
and 1.179 A (2q ¼ 32.84) as shown in Fig. S2. Amazingly, the
intermolecular d-spacing of QMe was larger than that of QH due
to the larger dihedral angle. To improve OFET performance, an
understanding of the role of the organic semiconductor/gate
dielectric interface is crucial since the field effect mobility is largely
determined by the morphology of the semiconductor film at the
interface with the gate dielectric.11 In general, gate dielectric
surface properties have been modified with self assembled
monolayers (SAM). For example, SiO2 dielectrics treated with
octadecyltrichlorosilane (OTS) or hexamethyldisilazane (HMDS)
improve the mobility and decrease the gate leakage of most
OFETs.12 Pentacene shows a preferred orientation of the crys-
talline domain dependent on the chemical and physical properties
of the gate insulator controlled by the end functional group in the
SAM. A nonpolar surface modified by a CH3-terminated SAM
was beneficial for improving hole mobility by enabling effective
ordering of the pentacene crystalline lamellae.13 The topographs
of the pentacene films on the PPOM surfaces were observed by
AFM. No obvious orientation was detected with the pentacene
layer that formed on the SiO2 surface, as shown in Fig. 4a, which
indicates that the pentacene film consisted of either mostly
amorphous grains or crystalline grains with an in-plane texture,
where the layered stacks were perpendicular to the surface. In the
case of hydrocarbon functionalized PPOMs, every pentacene
nucleation site was sufficiently stabilized by hydrophobic inter-
actions with the PPOM surface to grow and form a large number
of small grains, as shown in Fig. 4b and c. The RMS roughness
was changed from 30.36 nm for the pentacene/SiO2 to 7.85 nm
and 7.28 nm for the pentacence/QH/SiO2 and pentacene/QMe/
SiO2 films, respectively. Consequently, the difference of the
memory OFET performance can be mainly attributed to the
chemical nature of the nano-interfaced PPOMs since the surface
roughness and gate leakage current were nearly identical regard-
less of surface modification.
The optimized structures and dihedral angles (C1-C2-C3-C4) of
the neutral and electron charged anion species of QH and QMe
are shown in Fig. 3. It is well known that a planar structure is
more efficient for electron transport than a tilted structure in
conjugated systems.8,9 Furthermore, as noted from the dihedral
angle, QH should give higher conductance than QMe, which is
10
exactly the same as in a previous study In other words, it is
expected that the charge mobility of the OFET device should be
reduced due to charge trapping on both QMe and QH layers.
Furthermore, according to the tilt angles, the charge trapping on
Fig. 5 shows transfer characteristics of the OFET devices. The
dielectric pristine SiO2 and PMMA polymer layers14 used as
control experiments herein have been shown to retain injected
charge. In our results, the pristine OFET with a SiO2 layer, as
depicted in Fig. 5a, or PMMA, as shown in Fig. 5b, shows only
weak hysteresis with a memory window (DVT) less than 5 V. The
memory window is defined as the difference between the threshold
voltages in the up and down steps of the gate voltages. Amazingly,
the transfer characteristics of the PPOM-FET devices show that
Fig.
3 B3LYP/6-31+G* optimized structures and dihedral angles
(C1-C2-C3-C4) of neutral and electron charged anionic QH and QMe.
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transfer characteristics of the devices comprising an additional
QH or QMe layer. It is believed that the hysteresis effect in pen-
tacene-based OTFTs is due to charge (electron) trapping and de-
trapping. In partial summary, the larger tilted structure caused by
the steric hindrance gives higher charge trapping and reduces the
charge mobility more and finally produces higher hysteresis.
Thus, the memory window of QMe is larger than that of QH.
For devices exhibiting memory performances, the ability for
write-read-erase-read pulse cycles, retention time, and reliability,
required for usage as nonvolatile memory devices, was further
investigated. Fig. 6 presents a representative result of write-read-
erase-read cycle tests of QH- and QMe-based organic molecular
devices. In the write-read-erase-read cycle, writing (ON state)
was achieved by applying a negative voltage pulse whose
amplitude (ꢀ60 V) was higher than Vc,ON, and erasing (OFF
state) through erasing the ON state by applying a positive voltage
pulse whose amplitude (60 V) was higher than Vc,OFF. A small
probe voltage (0 V) pulse was employed to read the states [ON
state (write) and OFF state (erase)] of the device. The QMe
device showed higher ON current and lower OFF current,
compared to the QH device.
Fig. 5 Transfer characteristics of the OFET with Vds ¼ ꢀ40 V
demonstrating the nonvolatile OFET memory device. Each measurement
was carried out with an integration time of 1 s.
Fig. 7 shows a representative result of the retention tests
carried out on the PPOM devices under ambient conditions.
Once the device was switched to the ON state by applying
a negative voltage pulse with an amplitude (ꢀ100 V) higher than
the Vc,ON, the ON state was retained without any degradation for
over 30 min, which is applicable for nonvolatile memory. When
the ON state was switched back to the OFF state by applying
a positive voltage pulse with an amplitude of 100 V, higher than
the Vc,OFF, the OFF state was retained without any degradation
for over 30 min. The QMe devices showed that the ON current
for the current decay was still more than one order of magnitude
larger than the OFF current, even after 30 min.
there is a clearly visible hysteresis between the two source–drain
currents for gate voltage sweep in the negative and positive
directions shown in Fig. 5c and d. In the transfer characteristics of
the OFET with QH (OFET-QH device) and QMe (OFET-QMe
device), the memory window of the OFET-QMe was wider than
that of OFET-QH and as expected, the drain current of OFET-
QMe decreased more than that of OFET-QH. For OFET-QMe,
the ON/OFF ratio was about 102 (two orders of magnitude
difference in current between the high conductance ON state and
low conductance OFF state). The ‘‘ON’’ current was 30 nA at
a gate voltage of ꢀ50 V. The ‘‘OFF’’ current appeared to be 0.6 nA
at the gate voltage of 50 V. This result corresponds to a memory
ratio of at least 50, a very competitive value, even when compared
to inorganic ferroelectric transistors.15 The ‘‘ON’’-‘‘OFF’’
switching phenomenon was achieved by the conversion of the
‘‘anion’’ and ‘‘neutral’’ states. When a gate voltage is applied to the
neutral QH and QMe, they undergo a gradual conformational
change from the neutral (‘‘OFF’’ state) to the anion (‘‘ON’’ state)
species. Then, when the gate voltage is turned off, the anionic
species (‘‘ON’’ state) of QH and QMe gradually change to neutral
(‘‘OFF’’ state) species. The difference in QH and QMe is the
response time as QH changes more quickly and the operation
voltage range of QMe is wider than that of QH. This property can
be explained by the vertical electron affinity (VEA) values of QH
and QMe. The calculated VEA values for QH and QMe were
ꢀ1.774 and ꢀ1.699 eV, respectively, implying that QH becomes
more stabilized than QMe by receiving one electron. Thus, QH
can be changed from the ‘‘OFF’’ to the ‘‘ON’’ state more easily
than QMe, which is consistent with the experimental observations
shown in Fig. 5. For the reverse change from ‘‘ON’’ to ‘‘OFF’’, the
reason for the faster change of QH is that the ‘‘ON’’ state of QH is
a more efficient charge transport medium than the ‘‘ON’’ state of
QMe. Therefore, it is assumed that due to charge trapping, the
main effect of the nano-interfaced QMe and QH reduced the
charge mobility of the conduction channel formed between pen-
tacene and the QMe (QH) layer, generating a hysteresis effect. It is
certainly true that the hysteresis becomes more obvious in the
The mechanism is proposed by charge trapping in the organic
molecules as charge tunneling through the semiconductor
Fig. 6 Switching performance of the OFET device based on the PPOMs
a) QH and b) QMe during continuous ‘‘write-read-erase-read’’ sequences.
The ON state was induced (‘‘write’’) by a ꢀ60 V pulse, and the OFF state
(‘‘erase’’) by +60 V. Between switching, the states were probed (‘‘read’’) by
measuring the current under a small voltage (0 V).
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been presented. The proposed electron donor-spacer-acceptor
PPOMs were successfully synthesized and characterized. The
observed memory effect occurred due to a charge trapping on
the conduction channel comprising PPOMs and charge de-
trapping with high potential energy barrier caused by the steric
hindrance effect of the nano-interfaced organic semiconductor,
PPOMs. As a matter of fact, the memory window of the OFET-
QMe was wider than that of the OFET-QH. The OFET
memory device presented herein may be applied to a variety of
potential applications, including novel, nonvolatile organic
memory, which would be useful for low-cost, lightweight, flexi-
ble, low duty data storage media, logic display driver and flash
memory.
Acknowledgements
Fig. 7 Retention times at the ON and OFF states of the OFET devices
based on QH and QMe.
This work was supported by Creative Research Initiatives
(Project Title: Smart Molecular Memory) of MEST/NRF.
organic molecules is possible. During the charge transfer process,
some of the carriers would be trapped due to the charge trapping
sites generated by the dihedral angle differences of the steric
hindrance effects of the PPOMs. Even when the gate field was
withdrawn, these trapped charge carriers remained in the organic
molecules because of the tunneling barrier of the organic mole-
cules. In addition, there was also an effect of the potential energy
barrier for the conformational change from the electron charged
anion to the neutral state, as noted from the potential energy
surface (Fig. 2). It is demonstrated that a higher potential energy
barrier gives longer time for the de-trapping of the injected
charge on the PPOMs. As a result, memory characteristics were
increased only when the PPOM layers existed, and their hyster-
etic I–V characteristics and memory window sizes were directly
related to the difference between the dihedral angles. Therefore,
it is contended that the memory behavior originated from the
modulation of the gate field by charges stored in the novel
organic molecules. The modulation of the nano-interfaced and
sterically hindered organic molecules between the semi-
conducting channel and dielectric materials may be considered to
be the cornerstone of the organic nonvolatile memory device.
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In conclusion, a nonvolatile memory OFET with a gate insu-
lator based on nano-interfaced organic molecular materials has
8020 | J. Mater. Chem., 2010, 20, 8016–8020
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