Highly conducting π-conjugated molecular junctions covalently bonded to gold electrodes

Article abstract of DOI:10.1021/ja208020j

We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ, resulting in formation of a direct covalent θ bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated π system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the π system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G₀ = 2e²/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers.

Full text of DOI:10.1021/ja208020j

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pubs.acs.org/JACS
Highly Conducting π-Conjugated Molecular Junctions Covalently
Bonded to Gold Electrodes
Wenbo Chen,† Jonathan R. Widawsky,‡ Hꢀector Vꢀazquez,‡ Severin T. Schneebeli,† Mark S. Hybertsen,*,§
Ronald Breslow,*,† and Latha Venkataraman*,‡
Departments of †Chemistry and ‡Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
§Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973-5000, United States
S Supporting Information
b
ABSTRACT: We measure electronic conductance through
single conjugated molecules bonded to Au metal electrodes
with direct AuꢀC covalent bonds using the scanning
tunneling microscope based break-junction technique. We
start with molecules terminated with trimethyltin end
groups that cleave off in situ, resulting in formation of a
Figure 1. In situ formation of direct AuꢀelectrodeꢀC bonds starting
direct covalent σ bond between the carbon backbone and
from SnMe3 precursors.
the gold metal electrodes. The molecular carbon backbone
used in this study consist of a conjugated π system that has
one terminal methylene group on each end, which bonds to
highly conducting σ-bonded systems.7 For example, a conduc-
tance of 0.1 G0 through a butane backbone was demonstrated.
These direct AuꢀC bonded molecular junctions were created
starting with trimethyltin-terminated alkanes. The trimethyltin
end-groups cleaved off in situ, yielding direct AuꢀC bond
coupled junctions. Our density functional theory (DFT)-based
calculations showed that, in the limit of a single methylene group,
a conductance approaching G0 could be achieved, suggesting that
the direct AuꢀC link has near-ideal transmission characteristics.
While we succeeded in forming junctions with benzene, the
conductance was relatively low, consistent with calculations
indicating conduction occurred through the σ system.7,8
Here, we create single-molecule junctions using conjugated
backbones terminated with methylene groups that bind cova-
lently to gold metal electrodes, again through the use of SnMe3
groups that cleave off in situ. We find that the resulting junctions
have a conductance that is 100-fold higher than those of similar
junctions formed with conventional linkers.9 These junctions are
highly conducting because the AuꢀC bonds to the terminal
methylene units are well coupled to the conjugated π system.
This is in contrast to similar junctions created previously, where
Au was bound directly to a carbon on the benzene ring.7
Specifically, we find that the conductance of p-xylylene bonded
to gold electrodes approaches 1 G0. Our theoretical calculations
show that conductance occurs via near-resonant transmission.
For longer polyphenyls with 2ꢀ4 phenyl units, we find that the
conductance decreases exponentially with increasing length, with
a characteristic decay constant of 1.9/phenyl group.
the electrodes, achieving large electronic coupling of the
electrodes to the π system. The junctions formed with the
prototypical example of 1,4-dimethylenebenzene show a
conductance approaching one conductance quantum (G0 =
2e2/h). Junctions formed with methylene-terminated oligo-
phenyls with two to four phenyl units show a 100-fold
increase in conductance compared with junctions formed
with amine-linked oligophenyls. The conduction mechan-
ism for these longer oligophenyls is tunneling, as they
exhibit an exponential dependence of conductance on
oligomer length. In addition, density functional theory
based calculations for the AuꢀxylyleneꢀAu junction show
near-resonant transmission, with a crossover to tunneling
for the longer oligomers.
t is a great challenge to achieve electronically transparent
connections between metal electrodes and organic molecules,1
I
so as to minimize resistance introduced by the chemical linkers
normally used to form such interfaces.2 Typically, thiols2b,3 that
bind covalently to gold, or amines2a,b,4 that form donorꢀaccep-
tor bonds to under-coordinated gold, are used to electronically
couple organic backbones to metal electrodes. For each link group,
analysis of a series of single-molecule junctions as a function of
length has generally revealed a large contact resistance, signifi-
cantly larger than the ideal limit for a single channel of one con-
ductance quantum (G0 = 2e2/h).2a,c,5 A junction with conduc-
tance close to G0 has been demonstrated for H2 and benzene
molecules with platinum electrodes under high-vacuum conditions
at low temperatures.2d,6 However, the ability to create and control
transport through highly conducting molecularꢀmetal interfaces
remains a major challenge, especially under ambient conditions.
We have shown previously that direct AuꢀC covalent σ bonds
can be created in situ at the moleculeꢀgold interface, resulting in
We synthesized a series of trimethylstannylmethyl-terminated
polyphenyls and measured the conductance of single-molecule
junctions formed from these molecules using the scanning
tunneling microscope based break-junction (STM-BJ) method
(Figure 1).2a,3b In this technique, single-molecule junctions are
Received: August 24, 2011
Published: September 22, 2011
r
2011 American Chemical Society
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Figure 2. (a) Individual conductance traces measured in solutions of the SnMe3-terminated polyphenyl compounds P1ꢀP4. Measurements in solvent
alone are also shown for comparison (Au). The applied bias is 250 mV. (b) Conductance histograms of over 10 000 traces generated with linear bin size
of 0.0001 G0, shown on a logꢀlog scale. Inset: the same data on a linear scale. (c) Conductance versus number of phenylene units in the chain for
P1ꢀP4, and analogous measurements with a diamine series taken from ref 12. Dotted lines represent linear least-squares fit to P2ꢀP4 series. Note the
point for P1 above the line.
created by repeatedly forming and breaking Au point contacts3b
in a ∼10 mM 1,2,4-trichlorobenzene solution of the target tri-
methyltin-terminated molecules. Conductance (current/voltage) is
measured as a function of the relative tip/sample displacement to
yield conductance traces, which are used to generate conduc-
tance histograms. We synthesized 1,4-trimethylstannyl-terminated
xylylene using two different methods. In one, we converted 1,4-
bis-bromomethylbenzene (p-xylylene dibromide) to the dilithio
compound and reacted it with trimethylstannyl chloride.10 In the
other method, we reacted the p-xylylene dibromide with tri-
methylstannyl lithium.11 The latter procedure was used to attach
the trimethylstannyl groups for the polyphenyl compounds. The
details of these synthetic procedures and characterizations are
given in the Supporting Information (SI). We note here that these
compounds are toxic and should be handled with care.
Figure 2a compares individual conductance traces from mea-
surements of solutions of stannylated 1,4-dimethylenebenzene,
4,40-dimethylenebiphenyl, 4,400-dimethylene-p-terphenyl, and
4,40000-dimethylene-p-tetraphenyl (P1, P2, P3, and P4, re-
spectively). We see clear conductance plateaus at molecule-
dependent conductance values, although in the case of P1 it is
not straightforward to distinguish the molecular plateau from
that of the single-atom contact at a conductance around G0. These
plateaus are due to conduction through a molecule bonded in the
gap between the two Au point-contacts. These conductance
plateaus are seen in the measurements immediately after a
solution of the target molecule terminated with SnMe3 groups
is added, in contrast with measurements of 1,4-bis(trimethylstannyl)-
benzene,7 where conductance plateaus were seen in measure-
ments only after 2.5 h. This delay seen in measurements of 1,4-bis
(trimethylstannyl)-benzene indicated that conduction did not
occur through trimethylstannyl terminated molecules. In our
past work,7 this was confirmed by showing that the conductance
of auryltriphenylphosphine terminated compounds were the
same as those terminated by SnMe3 groups.
in situ dimerization of the target compounds after the SnMe3
linkers have been lost on the gold electrodes, in which the
conjugated systems are linked by a dimethylene bridge. Indeed,
some traces show two plateaus, one due to the monomer and one
from the dimer. However, we cannot determine, on the basis of
conductance alone, whether both are present during the entire
measurement. We will return to these features later in this paper.
Repeated measurements give a statistical assessment of the
junction properties. In Figure 2b, we show conductance histo-
grams for each of the compounds studied here. Each conduc-
tance histogram, generated from over 10 000 traces without any
data selection, reveals clear peaks at conductance values that
depend on the molecular backbone. The inset of Figure 2b shows
the same conductance histograms on a linear scale around 1 G0.
Here, we see a clear peak around 0.9 G0 for P1 that can be
distinguished from the peak near 1 G0 that is due to the conductance
through a single gold-atom contact. By fitting the peaks in these
conductance histograms with Lorentzians, we determine that
the conductance of P1 is about 0.9 G0, while the values for P2, P3,
and P4 are 0.1 G0, 0.014 G0, and 0.0022 G0, respectively. The
position of the highest conductance peak in each histogram is
plotted on a semilog scale against the number of phenyl rings in
the molecule in Figure 2c. We find that, for the series P2ꢀP4, the
conductance decays exponentially with increasing number of
phenyl groups, with a decay constant β = 1.9/phenyl or 0.43/Å.
For comparison, in Figure 2c, we also plot the conductance of
polyphenyls doubly terminated with amine linkers, which show a
similar decay in conductance with length.12
The histogram for P1, in Figure 2b, also shows a peak around
0.001 G0 due to the presence of dimer molecules formed in situ
during the measurement. To show that this is indeed due to
conduction through a dimer molecule, we synthesized the ditin
precursor of p-xylylene dimer (P1d, see Figure 3a) and measured
its conductance in the STM-BJ setup. The conductance histo-
gram for P1d shows a clear peak at 0.001 G0 but no feature other
than the gold conductance peak at 1 G0 (SI Figure 1). This clearly
demonstrates that the dimer molecule P1d is created in situ when
measurements of P1SnMe3 are carried out. That the conductance
In addition to plateaus seen at a high conductance, the traces
in Figure 2a show a second series of plateaus at 0.001 G0 for P1
(red) and 3 ꢁ 10ꢀ5 G0 for P2 (blue). These are attributed to
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Figure 4. (a) Calculated transmission spectrum for P1 bonded to Au
electrodes. Inset: isosurface plots of the real part of the transmitted
scattering states for energies at the vertical bars (ꢀ1.15 and ꢀ0.4 eV),
showing the even and odd combinations of the AuꢀC bonds coupled
through the π backbone. (b) Transmission spectra for P1ꢀP4. Bars
indicate the approximate position AuꢀC MO energies.
calculations for this system, we find indeed that the minimum
energy configuration has a 90° angle, and the barrier for rotation
is 10.4 kcal/mol, or 0.45 eV (SI Figure 4).
Figure 3. (a) Structures of additional compounds studied. (b) Indivi-
dual conductance traces measured in solutions of the SnMe3-terminated
compounds P1 (red) and 2MeP1 (black) at 25 mV applied bias. (c)
Conductance histograms of over 10 000 individual measurements,
generated with linear bin size of 0.0001 G0, shown on a logꢀlog scale.
Inset: same histograms on a linear scale.
The conductance of P2, the biphenyl analogue of P1, is 0.10 G0
(Figure 2b). This conductance is a factor of 9 lower than that of P1,
while in the case of amine-terminated polyphenyls, the difference
between benzene and biphenyl was a factor of only ∼6. To see if the
difference between P1 and P2 is partly due to an anomalous internal
twist angle at the central CꢀC bond, we synthesized a trimethyltin-
terminated analogue of fluorene with two methylene groups (P2a)
and measured its conductance using the STM-BJ setup. We find that
P2a has a conductance of 0.17 G0 (SI Figure 3), but the conductance
ratio between P1 and P2a (5.3) is still larger than the ratio for the
diamine analogues (4.3).9 This indicates that the lower conductance
of an AuꢀP2ꢀAu junction compared with that of the AuꢀP2aꢀAu
junction partly reflects a twist in the phenylꢀphenyl bond that
is absent in the fluorene analogue. However, we see that an
AuꢀP1ꢀAu junction still has a higher conductance than one
would expect, just extrapolating the exponential dependence
seen for the longer polyphenyl compounds investigated here.
To understand the origin of the high conductance observed
in these junctions, we carried out DFT-based first-principles
calculations14 with a gradient-corrected exchange-correlation
functional15 and a nonequilibrium Green’s function approach16
to calculate electronic transmission through these junctions
(see SI for details). Transmission curves for P1ꢀP4 junctions
are shown in Figure 4. The calculated zero-bias conductances
are 0.9 G0, 0.5 G0, 0.15 G0, and 0.05 G0 for P1ꢀP4, respectively.
Transmission at the Fermi level is derived from the mole-
cular orbitals (MOs) on the AuꢀC bonds that are very well
coupled to the molecular π backbone and to the gold electrodes.
For P1, this results in two distinct resonances, one for an even
combination of the AuꢀC bond MOs and one for the odd com-
bination, as seen from the isosurface plots of the transmitted
scattering state for each resonance (Figure 4a).17 Physically, the
AuꢀC bond MOs and the nearby π backbone MOs are nominally
fully occupied by electrons. The extent to which the Fermi level
for this junction falls within the nearest resonance depends on the
amount of charge transfer from the molecule to the electrodes. An
analysis of the Mulliken populations for the P1 junction shows net
positive charge, with the molecule having lost about 0.5 electron.
The electrostatic balance leads to the Fermi energy being placed
slightly above the highest MO resonance, resulting in conductance
of P1 being near-resonant, with a magnitude close to G0.
of the dimer is almost 3 orders of magnitude lower than that of the
monomer can be attributed to the saturated dimethylene bridge
between the two conjugated parts.
We also synthesized and measured the conductances of two
xylylene derivatives with trimethyltin terminations (see SI for
details). The first is a dimethyl-substituted xylylene (2MeP1),
and the second is a tetrafluoro-substituted xylylene (4FP1)
(Figure 3a). For 2MeP1, we see a conductance peak at ∼0.9
G0, very close to that of the unsubstituted P1, as shown in
Figure 3c. The fact that the methyl substituents do not affect
conductance significantly is consistent with a near-resonant
transport mechanism, as will be discussed further below. Thus
for both P1 and 2MeP1, we demonstrate near-resonance trans-
port across a molecular junction 0.8 nm in length.
The histogram for 2MeP1 in Figure 3c also shows a second
peak around 0.002 G0, due to the formation of the dimer molecule, as
in the case of P1. However, here we see a change in the conductance
of the P1 dimer when compared with that of the 2MeP1 dimer, as we
expect when the mechanism for transport involves tunneling through
the saturated ethano group. In contrast to the results with 2MeP1, we
find that 4FP1 does not show clear evidence for junction formation.
The conductance histogram generated from 10 000 measurements
does not show two clear peaks around G0 or a peak due to the
molecular dimer formed in situ (SI Figure 2). We attribute this to a
stronger SnꢀC bond in 4FP1SnMe3, making in situ cleavage of the
SnMe3 group more difficult.
In our past work with AuꢀC coupled alkanes and benzene,7
we found that the conductance of 1,4-didehydrobenzene cova-
lently bonded to Au electrodes was only 0.03 G0, significantly
lower than that of p-xylylene. For benzene, only the molecular σ
system, which is a rather poor conductor, was well coupled
through the AuꢀC bonds.7,8 In contrast, with xylylene the AuꢀC
bonds are very well coupled to the π system, yielding the high
conductances observed. In principle, this coupling will depend
on the angle between the AuꢀC bond and the phenyl plane and
will be maximum when this angle is 90°.13 On the basis of
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For the longer derivatives, the effective through-π-system
coupling between the AuꢀC MOs is reduced. The correspond-
ing energy splitting between the even and odd combinations of
these MOs also gets smaller, and the two distinct resonances seen
for P1 merge into a single, broad feature at ∼ꢀ0.5 eV, with
decreased transmission at the peak (Figure 4b). However, the
distinct even and odd combinations of the AuꢀC MOs can still
be clearly seen in the transmitted scattering states (SI Figures
5ꢀ7). The charge transfer from the molecule to the electrodes is
similar to that for P1, and the Fermi level is pinned at an energy
just above the highest resonance. The computed decay constant
β for P2ꢀP4 is 1.2/phenyl group.
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There are inherent errors in the use of the DFT MOs and
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the impact is minor for cases where the junction conductance is
close to G0, e.g., for metal point contacts,19 the calculated conduc-
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resonance with near-unit transmission close to the Fermi energy.
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larger for longer oligomers, where screening by the electrodes
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smaller than indicated by the DFT calculations, with an increase
in the effective β value for P2ꢀP4.
In conclusion, we have demonstrated a clear method to create
circuits with strong electronic coupling between gold electrodes
and conjugated molecules. We achieve a single-molecule junction
conductance close to one quantum across a length of ∼0.8 nm. This
remarkable result opens up new methods to create long and
highly conducting molecular junctions.
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’ ASSOCIATED CONTENT
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’ AUTHOR INFORMATION
Corresponding Author
rb33@columbia.edu; mhyberts@bnl.gov; lv2117@columbia.edu
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’ ACKNOWLEDGMENT
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Lett. 2005, 95, 4. (b) Evers, F.; Weigend, F.; Koentopp, M. Phys. Rev. B
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This work was supported in part by the Nanoscale Science and
Engineering Initiative of the NSF (award CHE-0641523), the
New York State Office of Science, Technology, and Academic
Research (NYSTAR), and NSF Career Award CHE-07-44185
(L.V.). J.R.W. was supported by the EFRC program of the U.S.
Department of Energy (DOE) under Award No. DE-SC0001085.
S.T.S. was the recipient of a Guthikonda Graduate Chemistry
Fellowship. Part of this work was carried out at the Center for
Functional Nanomaterials, Brookhaven National Laboratory,
which is supported by the DOE Office of Basic Energy Sciences,
under contract no. DE-AC02-98CH10886.
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