Fabrication of steady junctions consisting of α, ωbis(thioacetate) oligo(p-phenylene vinylene)s in nanogap electrodes

Article abstract of DOI:10.1021/ja062561h

For obtaining molecular devices using metal-molecule-metal junctions, it is necessary to fabricate a steady conductive bridge-structure; that is stable chemical bonds need to be established from a single conductive molecule to two facing electrodes. In th

Full text of DOI:10.1021/ja062561h

Published on Web 10/04/2006
Fabrication of Steady Junctions Consisting of
r,ω-Bis(thioacetate) Oligo(p-phenylene vinylene)s in Nanogap
Electrodes
Tien-Tzu Liang,†,‡,§ Yasuhisa Naitoh,*,†,‡ Masayo Horikawa,†,‡ Takao Ishida,†,‡ and
Wataru Mizutani†
Contribution from the Nanotechnology Research Institute, National Institute of AdVanced
Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, and
Synthetic Nano-Function Materials Project, National Institute of AdVanced Industrial Science
and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Received April 12, 2006; E-mail: ys-naitou@aist.go.jp
Abstract: For obtaining molecular devices using metal-molecule-metal junctions, it is necessary to
fabricate a steady conductive bridge-structure; that is stable chemical bonds need to be established from
a single conductive molecule to two facing electrodes. In the present paper, we show that the steadiness
of a conductive bridge-structure depends on the molecular structure of the bridge molecule for nanogap
junctions using three types of modified oligo(phenylene vinylene)s (OPVs): R,ω-bis(thioacetate) oligo-
(phenylene vinylene) (OPV1), R,ω-bis(methylthioacetate) oligo(phenylene vinylene) (OPV2), and OPV2
consisting of ethoxy side chains (OPV3). We examined the change in resistance between the molecule-
bridged junction and a bare junction in each of the experimental Au-OPV-Au junctions to confirm whether
molecules formed steady bridges. Herein, the outcomes of whether molecules formed steady bridges were
defined in terms of three types of result; successful, possible and failure. We define the ratio of the number
of successful junctions to the total number of experimental junctions as successful rate. A 60% successful
rate for OPV3 was higher than for the other two molecules whose successful rates were estimated to be
∼10%. We propose that conjugated molecules consisting of methylthioacetate termini and short alkoxy
side chains are well suited for fabricating a steady conductive bridge-structure between two facing electrodes.
nylene) (OPE),5-9,12-17 oligo(phenylene) (OP),18,19 and oligo-
(thiophene) (OT),20-22 have been reported. Since the OPV
Introduction
For the realization of molecular devices with metal-molecule-
metal junctions containing functional molecules, a number of
studies have been conducted previous to this study.1-4 In
particular, studies on potential ‘‘molecular nanowires” based
on dithiol terminated π-conjugated oligomers is proceeding at
a rapid pace. Typical Au-nanowire-Au junctions formed with
oligo(phenylene vinylene) (OPV),5-11 oligo(phenylene ethy-
backbone consists of a higher degree of planarity, and thus better
π-conjugation, than those for OPE or the other molecules,9,23-24
Au-OPV-Au junctions always show higher electronic con-
ductance than other conjugated oligomers.5-9 For example,
Blum et al. showed that the resistance of a cross-wire tunneling
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C.; Henderson, J. C.; Yao, Y.; Tour, J. M.; Shashidhar, R.; Ratna, B. R.
Nat. Mat. 2005, 4, 167.
† Nanotechnology Research Institute.
‡ Synthetic Nano-Function Materials Project.
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2004, 126, 4052.
§ Present address: Yokohama Oil & Fats Industry Co., Ltd., 1-1,
Minamisenngenncho, Nishiku, Yokohama, 220-0074, Japan.
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J. AM. CHEM. SOC. 2006, 128, 13720-13726
10.1021/ja062561h CCC: $33.50 © 2006 American Chemical Society

Fabrication of Steady OPV Junctions in Nanogap Electrodes
A R T I C L E S
Au-S-contacts by a methylene spacer; 1,4-diethoxy-2,5-bis-
[4′-(methylthioacetyl)styryl] benzene (OPV3), in which the
solubility in organic solvents is improved by introducing ethoxy
side chains.
In this paper, the average resistance and activation energy of
these three Au-OPV-Au junctions are estimated as reference
data for comparing the similar experimental results of other
papers.6,20 The molecular alignment on the Au surface is
investigated by X-ray photoelectron spectroscopy (XPS) analysis
to discuss the steadiness relationship between bridge-structures.
Experimental Section
Figure 1. Schematic illustration for IV measurements of Au-OPV2-Au
The fabrication procedure for sub-5 nm gap electrodes less than 5
nm on a silica chip is described in our previous paper10 and Supporting
Information. The basic experimental processing involves immersing a
clean gold surface of sub-5 nm gap electrodes into a solution containing
the OPV molecules with thioacetate (-SAc) or methyl thioacetate
(-CH2SAc) termini. The protective acetyl groups were removed just
prior to immersion of the gold electrodes into the molecule solution.
To 4 mL of 0.1 mM OPV2 or OPV3 solution in dichloromethane was
added 20 µL of 28% aqueous ammonia, and 1.5 mL of 2 mM
pyrrolidine solution in dichloromethane was added to 4 mL of 0.1 mM
OPV1 in dichloromethane. The solution was incubated for 10 min to
deprotect the thiol group, which converts thioacetate termini to thiolate
termini. A clean electrode chip was then immersed into the OPV
solution at room temperature for a period of 12-24 h. After removal
from the solution, the electrodes were thoroughly rinsed with dichlo-
romethane and then dried with nitrogen. The current through the
molecule was measured in a vacuum using an electrometer (Keithley
6517A) for a (1V bias.
junction.
Table 1. Molecular Structures, Spectroscopic Data for the OPV
Molecules
a The calculated length of the molecule from sulfur atom to sulfur atom.
b Absorption maximum in dichloromethane. c Emission maximum in dichlo-
romethane.
We characterized the monolayer formed from the OPVs on Au
surfaces (Au 50 nm/Cr 2 nm evaporated onto silicon oxide/Si), which
were fabricated using the same evaporation conditions to fabricate
nanogap electrodes, using XPS, sum frequency generation (SFG),
ellipsometry, and tapping-mode atomic force microscopy (AFM). The
XPS spectroscopy was observed using XPS (Theta Probe-System,
Thermo VG Scientific Inc.) with a monochromatic Al KR X-ray source,
1486.6 eV. The binding energy was calibrated using the Au(4f7/2) peak
energy with 84.0 eV as the energy standard. The X-ray power, the
pass energy of the analyzer, and the takeoff angle of photoelectrons
were respectively set at 45 W, 50 eV, and 90°. The energy resolution
of this system is less than 0.65 eV, as estimated by the Ag(3d5/2) peak
width under our measurement conditions.
junction consisting of self-assembled monolayers (SAMs) of
OPV with butoxy side-chain molecules was estimated to be 0.5
( 0.2 MΩ.6 They also showed that the cross-wire junction
contains ∼1000 molecules as calculated using a resistance of
0.6 ( 0.1 GΩ for single OPV molecules obtained from scanning
tunneling microscopy (STM) measurements.6 We also have
successfully measured current-voltage (IV) characteristics of
Au-polythiophene-Au25 and Au-OPV-Au junctions10 (using
the setup illustrated in Figure 1) and have confirmed an increase
in the conductance of the molecule-bridged junction. In addition,
we have measured the dependence of conductance on temper-
ature for the Au-OPV-Au junction yielding characteristic
hopping conduction behavior, and the activation energy Ea was
estimated to be ∼34 meV.10
Results and Discussions
1. IV Characteristics. Initially, we examined the change in
IV behavior between the bare junction and the molecule-bridged
junction for each of the experimental Au-OPV-Au junctions
to confirm whether the molecules were steadily bridged. The
IV behavior changes for the Au-OPV-Au junctions were
defined in terms of three types of behavior: successful, possible,
and failure. The resistance values of the experimental bare
junctions varied from 10 TΩ to 10 ΜΩ, as calculated from the
slopes of the line in the IV ohmic region from -0.05 to +0.05
V. Thus, to illustrate the three types of IV behaviors, we sorted
one set of Au-OPV2-Au junctions in which their bare
junctions had a similar resistance, ∼10 GΩ. The successful,
possible, and failure exemplifications of the Au-OPV2-Au
junction are shown in Figure 2.
However, for practical applications of molecular devices, it
is very important to fabricate a steady conductive bridge-
structure, wherein stable chemical bonds are established from
a single conductive molecule to two facing electrodes. We
consider that such steadiness of the conductive bridge-structure
is dependent on the molecular structure of the bridge molecule,
since the possibility of stable chemical bond formation is
strongly affected by such a conjugated molecular structure. We
have therefore compared the increase in steadiness of a
conductive bridge-structure junction using three types of R,ω-
bis(thiolate) oligo(phenylene vinylene)s (OPV) molecules as
shown in Table 1: 1,4-bis[4′-(thioacetyl)styryl] benzene (OPV1),
a fully conjugated molecule; 1,4-bis[4′-(methyl thioacetyl)styryl]
benzene (OPV2), in which the conjugation is broken near the
In Figure 2(a1) the overall amplitude of the current of the
successful molecule-bridged Au-OPV2-Au junction is larger
than that of its bare junction. In Figure 2(a2), all IV curves of
the successful bridged junction are almost linear lines in the
(25) Naitoh, Y.; Tsukagoshi, K.; Murata, K.; Mizutani, W. e-J. Surf. Sci.
Nanotechnol. 2003, 1, 41.
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A R T I C L E S
Liang et al.
Figure 2. Change in IV behavior between a bare junction (dashed line) and the molecule-bridged junction (solid line) shows three types of IV behavior for
an exemplificative Au-OPV2-Au junction: (a1) successful, (b1) possible, (c1) and (d1) failure. Their IV characteristics at ohmic regions are magnified in
(a2), (b2), and (c2) and (d2), respectively.
Figure 3. Temperature-dependent IV characteristics, the Arrhenius plots of temperature dependence of conductance, and the estimation of Ea for exemplificative
successful OPV1, OPV2, and OPV3 junctions are shown in (a1,a2), (b1,b2), and (c1,c2), respectively. The estimated activation energies Ea of the OPV1,
OPV2, and OPV3 junctions are 66.0, 41.1, 30.2 meV, respectively.
ohmic region, i.e., in the range of -0.05 to +0.05 V. This steady
IV behavior is interpreted as steady bridge-structure, wherein
stable chemical Au-S bonds are formed from a single bridge
molecule to two facing electrodes. The temperature-dependent
IV characteristics of this successful Au-OPV2-Au junction at
the ohmic region can be clearly observed in Figure 3(b1).
Analyzing the data in Figure 3(b1) using the hopping conduction
of the equation,10,13,20
I/V ) AH exp(-Ea/kBT)
(1)
yields Arrhenius plots of the temperature dependence of
conductance as shown in Figure 3(b2). AH is the proportionality
constant, and kB is the Boltzmann constant. The Arrhenius plots
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13722 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Fabrication of Steady OPV Junctions in Nanogap Electrodes
A R T I C L E S
Figure 5. Change in resistance of Au-OPV2-Au junctions between the
molecule-bridged junction and the bare junction. The bridging rate is 32/
52 (61.5%), and the successful rate is 7/52 (13.5%).
Figure 4. Change in resistance of Au-OPV1-Au junctions between a
molecule-bridged junction and a bare junction is defined in terms of three
types of results: successful, possible, and failure. The bridging rate is 19/
58 (32.8%), and the successful rate is 5/58 (8.6%). The resistance R is
calculated from the slope of the line in the IV ohmic region, from -0.05 to
+0.05 V.
show linear dependence around the high-temperature region
(J150 K >) and the activation energy Ea is estimated to be
41.1 meV. Since the temperature dependence in the low-
temperature region (j150 K) was gradually changed, the
conduction mechanism is not the simple hopping conduction.
However, because we could measure clear temperature depen-
dence in the high-temperature region, it can be considered that
the hopping conduction is dominant at least in the region.13
Similarly, the temperature-dependent IV characteristics, the
Arrhenius plots, and the estimation of Ea for the exemplificative
successful Au-OPV1-Au and Au-OPV3-Au junctions are
shown in Figure 3(a1,a2) and Figure 3(c1,c2), respectively. The
estimated Ea for the OPV1 and OPV3 junctions are 105.4 and
30.2 meV respectively.
Figure 6. Change in resistance of Au-OPV3-Au junctions between the
molecule-bridged junction and the bare junction. The bridging rate is 25/
27 (92.6%), and the successful rate is 16/27 (59.3%).
In possible cases, as shown in Figure 2(b1), the IV behavior
of the Au-OPV2-Au junction is similar to that of the
successful junction. However, the currents in these junctions
increase unstably in the high-voltage region. Also, a distorted
IV curve was obtained in the ohmic region as shown in Figure
2(b2). These unstable IV behaviors were interpreted as being
the result of an unsteady bridge-structure where either of the
Au-S bonds of the molecule might be unstable when a higher
voltage is applied.
2. Bridging Rate
The resistance of each of the experimental Au-OPV-Au
junctions was calculated from the slope of the line in the IV
ohmic region, from -0.05 to +0.05 V. The resistances of both
the bare junction and the molecule-bridged junction for each
of the Au-OPV1-Au junctions are shown in Figure 4. Among
the 58 Au-OPV1-Au junction samples examined, there were
19 bridging junctions that contained only 5 successful junctions
and 14 possible junctions. Thus, the bridging rate was 19/58 or
32.8%. After excluding 14 possible junctions from the 19
bridging junctions, the successful rate was down to 5/58, or
8.6%. In Figure 5, the bridging rate of the Au-OPV2-Au
junctions is 32/52 or 61.5%. After excluding 25 possible
junctions from the 32 bridging junctions, the successful rate is
only 7/52 or 13.5%. In Figure 6, the bridging rate of the Au-
OPV3-Au junction is 25/27 or 92.6%. After excluding 9
possible junctions from 25 bridging junctions, the successful
rate became 16/27 or 59.3%. The bridging-rate trend thus
obtained is OPV1 ) 32.8% less than OPV2 ) 61.5% less than
OPV3 ) 92.6%. The successful rate trend is OPV1 ) 8.6%
less than OPV2 ) 13.5% less than OPV3 ) 59.3%. The results
indicate that OPV3 is the optimum molecule for the fabrication
In failure cases, as shown in Figure 2(c1) for example, the
current amplitude of the molecule-bridged Au-OPV2-Au
junction is a slightly higher than that of its bare junction.
However, in the ohmic region, the amplitudes of some of the
IV curves of the failure junction are lower than those of its bare
junction, as shown in Figure 2(c2). The results indicate that a
bridge-structure was not fabricated in the nanogap; i.e., tunneling
is the dominant mechanism of the observed current. In Figure
2(d1), another kind of failure junction shows that the overall
amplitude of the current of the Au-OPV3-Au junction is lower
than that of its bare junction. The reverse result was clearly
observed in the ohmic region as shown in Figure 2(d2). The
reverse behavior was interpreted as due to a brittle bridge-
structure; i.e., either of the Au-S bonds of the bridging molecule
could separate together with Au atoms from the electrodes,
thereby damaging the structure of the electrode.
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A R T I C L E S
Liang et al.
the number of methylene groups in R-(CH2)m-SH.30,31 For m
) odd on gold or m ) even on silver, the molecules stand up
more vertically on the substrates. For m ) even on gold or m
) odd on silver, the molecular axes tilt onto the metal surface,
resulting in low surface coverage. The aligned molecular
morphologies of OPV1 and OPV2 on the gold surface as
obtained using XPS analysis are also in agreement with this
odd-even effect.
Table 2. Elemental Composition Estimated from XPS Peak Areas
for OPVs on Au Surfaces
S(2p3
)
2
S(2p3
)
2
S(2p3 )
/2
/
/
area ratio of
area ratio of
bound S
area ratio of
isolated S
at 161 eV
unbound S
sample
at 163−164 eV
at 162 eV
S(2p)/Au(4f)
C(1s)/Au(4f)
OPV1
OPV2
OPV3
33.3%
83.1%
40.3%
30.9%
16.9%
24.2%
35.6%
-
35.4%
0.08
0.14
0.15
0.74
1.94
0.81
In the case of OPV3, as shown in Table 2, the S(2p3/2) area
ratio of bound sulfur for OPV3. 24.2%, is larger than that of
OPV2, 16.9%; i.e., the photoelectrons emitted from the bound
sulfur atom of OPV3 which are shielded by the conjugated
molecular planes are fewer than those from the bound sulfur
atom of OPV2. Thus, the molecular axis of OPV3 is tilted onto
the gold surface, as shown in Figure 8c. Likewise, Seferos et
al. demonstrated that dithiol-terminated OPV molecules with
linear hexyloxy-substituted structures form less ordered SAMs
on an Au, surface compared to unsubstituted OPVs, and
generally yield low thicknesses.24
of steady Au-OPV-Au junctions compared to the other OPVs
examined in this study.
3. XPS Analysis
We clarify that the increase in the successful rate is decided
on the basis of the change in molecular alignment morphology
resulting from XPS analysis. We have attempted to discuss
molecular alignments on the gold surface based on detailed XPS
analysis of sulfur peaks, since the peak ratio of the sulfur peak
position reflects the molecular alignment and packing density.
For XPS measurements, since we only measure chemical states
of a molecular layer bound on one side, the detailed surface
situation is different from that inside nanogap electrodes.
However, we believe that such a measurement is not necessarily
meaningless.
To follow up on the molecular conformations which were
estimated using XPS on Au surfaces, we tried to estimate the
thicknesses of the OPV films using the following three methods,
sum frequency generation (SFG), ellipsometry, and atomic force
microscopy (AFM). In the case of SFG, we can expect to
evaluate the molecular angles with focusing on the peaks of
phenylenes around 1600 cm-1. However, since the structures
of OPVs are symmetrical and the coverage of OPV films is
poor, we could not detect clear peaks for all OPVs. In the case
of ellipsometry, the measurement of molecular thickness is too
sensitive with the light at 60° and 70° angles of incidence. (the
analysis of thickness was done using a polyethylene database).
Thus, we concluded that it is difficult to determine molecular
thickness of a very thin OPV film, even if other researchers
could estimate the thickness using this method. Finally, we
carried out the comparison of roughness on the Au surface which
was measured by AFM. This comparison of roughness was
recorded in Table 5 and Supporting Information in the study
by Seferos et al.24 The average roughness was estimated using
six root-mean-square values of 2 µm × 2 µm images which
were randomly positioned on surfaces. The average roughnesses
of bare Au and Au surfaces covered by OPV1, OPV2, and
OPV3 films were 0.50, 0.51, 1.03, and 0.56 nm, respectively.
The roughness of OPV2 is especially large among four surfaces.
The AFM results suggested that the OPV2 film was the thickest
and the molecular angle of OPV2 was the closest to vertical
among three OPVs as shown in Figure 8. The AFM results were
suitable to the thickness which was suggested by the discussion
of the shielding effect of sulfur peaks in XPS spectra.
It has been reported that the S(2p3/2) binding energies for
unbound alkane thiols and dialkyl disulfide are between 163
and 164 eV, and after self-assembly of these molecules onto a
gold surface, the S(2p3/2) binding energy deceases to 162
eV.26-29 We also found another sulfur species around 161 eV,
and this 161 eV peak is often seen in less densely packed
SAMs.28 It was considered that the peak at 161 eV is isolated
sulfur, which is assigned to “atomic sulfur produced by C-S
cleavage” or “another sulfur without C-S cleavage”.28
The S(2p3/2) peaks of OPV-Au surfaces observed are
collected in Table 2 and Figure 7. In the case of OPV1 both
the unbound sulfur peak at 163-164 eV and the bound sulfur
peak at 162.2 eV were observed. Both the S(2p3/2)/Au(4f) and
C(1s)/Au(4f) ratios of 0.08 and 0.74 for OPV1 were lower than
those for OPV2, which were 0.14 and 1.94, respectively. It is
considered that in the case of OPV1 a lower surface coverage
was formed than that for OPV2. Thus, the molecular axis of
OPV1 is tilted onto the gold surface, as shown in Figure 8a,
which resulted in low surface coverage. Seferos et al.24 have
also reported that the molecular axis of OPV1 is tilted up to
∼30° from the gold surface from ellipsometry measurements,
supporting our assumption concerning OPV1. On the other hand,
we considered that the molecular axis of OPV2 is vertical to
the gold surface as shown in Figure 8b, which resulted in high
surface coverage. Thus, the photoelectrons emitted from the
bound sulfur atom of OPV2 around 162 eV are partly shielded
by the conjugated molecular planes. As shown in Table 2, the
S(2p3/2) area ratio of bound sulfur is 16.9% for OPV2 and is
lower than that of OPV1 at 30.9%.
4. Steadiness of Bridge-Structures
We considered that the steadiness of the Au-OPV-Au
junctions was dependent on the alignment of OPV molecules
on the junction. We propose that after formation of OPV1
steadied alignment, a small number of the as-grown bridged
molecules were attracted by neighboring molecules and were
then tilted down for agreeing with the lying-down alignment
which is illustrated in Figure 8a. By the same token, although
In addition, the orientation of an aromatic moiety exhibits a
pronounced dependence on the length of the alkane spacer, i.e.,
(26) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083.
(27) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll,
W. Langmuir 1998, 14, 2092.
(28) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara,
H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799.
(29) Ishida, T.; Fukushima, H.; Mizutani, W.; Miyashita, S.; Ogiso, H.; Ozaki,
K.; Tokumoto, H. Langmuir 2002, 18, 83.
(30) Rong, H.-T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.;
Wo¨ll, C.; Helmchen, G. Langmuir 2001, 17, 1582.
(31) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L. Langmuir 1997, 13, 4018.
9
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Fabrication of Steady OPV Junctions in Nanogap Electrodes
A R T I C L E S
Figure 7. S(2p) XPS spectra of OPV-Au surfaces: (a) OPV1, (b) OPV2, and (c) OPV3.
Figure 9. Plot of activation energy of Au-OPV-Au junctions vs
absorption energy of the OPV molecules.
5. Conductivity and Activation Energy (Ea)
The estimated resistance trend of the Au-OPV-Au junctions
is OPV2 153 ( 91 MΩ > OPV3 92 ( 32 MΩ > OPV1 91 (
75 MΩ. Although all data show substantial margins of error,
we attempted to estimate the number of molecules within an
Au-OPV3-Au junction by using the resistance of a single
OPV3-like molecule (ethoxy side chains of OPV3 were
substituted by butoxy side chains) of 0.6 ( 0.1 GΩ.6
At this moment, it is difficult to identify the title angle of
the bridged OPV3 molecules as shown in Figure 8c, which
changes the conductance of Au-OPV3-Au junctions. How-
ever, the number of molecules bridged in an Au-OPV3-Au
junction is estimated to be about 10, which is quite small
compared to the 1000 molecules of a cross-junction with SAMs
of OPV3-like molecules.6
The activation energy Ea of hopping conduction for each
successful Au-OPV-Au junction was also estimated. The
average activation energy trend obtained is OPV1 94.5 ( 39.9
meV > OPV2 47.3 ( 8.6 meV > OPV3 33.7 ( 4.3 meV. The
estimated activation energies of the OPV junctions match that
of dithiol-terminated terthiophene of 10-100 meV.20
In addition, we tried to prove a bold assumption that the
change of the activation energy of successful Au-OPV-Au
junctions depended on the change of the band gap of OPV
molecules. Herein, Bre´das and Heeger indicated that the band
gap of molecules was usually in agreement with the value
deduced from optical absorption measurements.32 The absorption
energy trend of the OPV molecules is OPV2 3.41 eV > OPV1
3.37 eV > OPV3 3.17 eV as listed in Table 1. A plot of
activation energies of Au-OPV-Au junctions versus absorption
energies of OPV molecules is shown in Figure 9. Regrettably,
Figure 8. Schematic images of molecular alignments of OPVs on Au
surfaces: (a) OPV1, (b) OPV2, and (c) OPV3.
the stand-up alignment of OPV2 might be more likely to form
a steady structure than the lying-down alignment of OPV1, the
successful rate of OPV2 is also poor. We consider that both
the molecular angle and the variation of end groups do not
necessarily affect to the successful rate. However, by comparing
the above results of OPV2 and OPV3, it became clear that the
presence of the ethoxy parts of OPV molecules work for
improvements of the successful rate. The reason the ethoxy parts
work for the improvements might be explained by the following
model. In the bridging using OPV1 or OPV2, the OPVs
adsorbed on the gold surface may be tightly fixed by surround-
ing OPVs, and the orientations of the OPVs are limited. On the
other hand, in the case of Au-OPV3-Au junctions, we suggest
that the single OPV3 was not so influenced by the neighboring
molecules, resulting from the existence of ethoxy side chains.
It means that the molecular axis of OPV3 has higher orienta-
tional freedom than those of OPV1 and OPV2. The successful
rate of Au-OPV-Au junctions reflects the dependence on
steadiness of the bridge-structures.
(32) Bre´das, J. L.; Heeger, A. J. Chem. Phys. Lett. 1994, 217, 507.
9
J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13725

A R T I C L E S
Liang et al.
an explainable and reliable dependence is not demonstrated in
Figure 9 although the three kinds of OPV molecules have the
same backbone of 1,4-distyrylbenzene. However, it is clear that
the OPV3 molecule of the lowest absorption energy achieved
successful Au-OPV3-Au junctions of the lowest activation
energy. Especially, the difference in Ea between Au-OPV2-
Au and Au-OPV3-Au junctions reveals the effect of side
chains, wherein the donor, ethoxy side chains of OPV3,
increases the energy level of the HOMO with a reduction in
the band gap.32 It can be considered that the change of the energy
level effects the decrease of Ea.
structure. We propose that conjugated molecules consisting of
methylthioacetate termini and short alkoxy side chains are well
suited for fabricating a steady conductive bridge-structure
between two facing electrodes.
Acknowledgment. This work was supported by the New
Energy and Industrial Development Organization (NEDO) of
Japan under the Synthetic Nano-Function Materials Project. We
gratefully acknowledge Dr. Hideo Tokuhisa, Dr. Emiko Koya-
ma, and Dr. Yoshinobu Nagawa for their kind support for HPLC
purification and NMR identification in the synthesis of OPV,
and the measurement of UV-visible spectra. We also gratefully
acknowledge Dr. Tohru Nakamura, Dr. Toshitaka Kubo, Dr.
Miki Nakano, and Dr. Takayuki Miyamae for their kind support
for SFG, ellipsometry, and AFM observation and helpful advice
about the molecular angles on Au surfaces.
Conclusions
We have demonstrated that the steadiness of a conductive
bridge-structure depends on the molecular structure of the bridge
molecule. The successful rate trend of the Au-OPV-Au
junctions is OPV1 8.6% < OPV2 13.5% < OPV3 59.3%, which
is commensurate with the shift in steadiness of the conductive
bridge-structure: stable chemical bonds are established or
broken for a single conductive molecule from two facing
electrodes. The elevated successful rate of Au-OPV3-Au
junctions results from the isolation of the bridged molecular
wire between two facing Au surfaces. On the other hand, the
OPV1 and OPV2 molecular axes are fixed by the surrounding
molecules on the surfaces of electrodes, forming a brittle bridge-
Supporting Information Available: Synthesis of three types
of R,ω-bis(thiolate) oligo(phenylene vinylene) (OPV) molecules;
fabrication procedure and SEM images of nanogap electrodes;
the Arrhenius plots of temperature dependence of conductance
and the estimation of activation energy, Ea, for Au-OPV-Au
junctions. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA062561H
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13726 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006