Electrochemical control of single-molecule conductance by fermi-level tuning and conjugation switching

Article abstract of DOI:10.1021/ja510335z

Controlling charge transport through a single molecule connected to metallic electrodes remains one of the most fundamental challenges of nanoelectronics. Here we use electrochemical gating to reversibly tune the conductance of two different organic molecules, both containing anthraquinone (AQ) centers, over >1 order of magnitude. For electrode potentials outside the redox-active region, the effect of the gate is simply to shift the molecular energy levels relative to the metal Fermi level. At the redox potential, the conductance changes abruptly as the AQ unit is oxidized/reduced with an accompanying change in the conjugation pattern between linear and cross conjugation. The most significant change in conductance is observed when the electron pathway connecting the two electrodes is via the AQ unit. This is consistent with the expected occurrence of destructive quantum interference in that case. The experimental results are supported by an excellent agreement with ab initio transport calculations.

Full text of DOI:10.1021/ja510335z

Communication
pubs.acs.org/JACS
Electrochemical Control of Single-Molecule Conductance by Fermi-
Level Tuning and Conjugation Switching
†
,#
‡,#
§,#
†
†
Masoud Baghernejad, Xiaotao Zhao, Kristian Baruel
̈
Ørnsø, Michael Fu
̈
eg, Pavel Moreno-García,
†
†
†
†
Alexander V. Rudnev, Veerabhadrarao Kaliginedi, Soma Vesztergom, Cancan Huang,
,
†
†
†
,§
Wenjing Hong,* Peter Broekmann, Thomas Wandlowski, Kristian S. Thygesen,*
,‡
and Martin R. Bryce*
†
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012, Bern, Switzerland
Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom
‡
§
Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
S Supporting Information
*
levels of the molecule, a large on/off conductance ratio can be
achieved by moving the frontier molecular level in and out of
resonance with the Fermi level EF.
ABSTRACT: Controlling charge transport through a
single molecule connected to metallic electrodes remains
one of the most fundamental challenges of nano-
electronics. Here we use electrochemical gating to
reversibly tune the conductance of two different organic
molecules, both containing anthraquinone (AQ) centers,
over >1 order of magnitude. For electrode potentials
outside the redox-active region, the effect of the gate is
simply to shift the molecular energy levels relative to the
metal Fermi level. At the redox potential, the conductance
changes abruptly as the AQ unit is oxidized/reduced with
an accompanying change in the conjugation pattern
between linear and cross conjugation. The most significant
change in conductance is observed when the electron
pathway connecting the two electrodes is via the AQ unit.
This is consistent with the expected occurrence of
destructive quantum interference in that case. The
experimental results are supported by an excellent
agreement with ab initio transport calculations.
Recently, the concept of quantum interference (QI) in
molecular transport junctions has been introduced theoretically
14
and verified experimentally. Destructive QI leads to very low
conductancemuch lower than anticipated from a simple
“
Lorentzian” model treating each molecular level as an
independent transport channel. It occurs as a result of a
nearly) complete cancellation of transmission probability due
(
to interference between different electron pathways, and it is
predicted to take place in organic molecules whenever the path
connecting the left and right electrodes via the molecule is cross
14a,15
conjugated.
For example, independent measurements
show that the conductance of a cross-conjugated anthraquinone
(
AQ) is ∼100 times lower than that of a linearly conjugated
14f,15,16
anthracene,
even though the molecular length, electronic
energy levels, and optical properties of the two molecules are
very similar.
In this work, two π-conjugated molecules, isomeric AQ-1,5
and AQ-1,4, with an anthraquinone core unit (Figure 1a), were
synthesized, and their electronic conductance was measured
under electrochemical gating using the scanning tunneling
microscopy break-junction (STM-BJ) technique. We demon-
strate gate potential control of the molecular energy levels over
almost 1 eV with conductance variations of >1 order of
magnitude. The origin of the conductance variation is a
combined effect of continuous Fermi level tuning (for
potentials outside the redox-active region) and abrupt changes
to the conjugation pattern of the molecule as a consequence of
oxidation/reduction of the molecule when the electrode
potential crosses the redox potential. These experimental
observations are complemented by density functional theory
(DFT)-based transport calculations. Plotting the calculated
transmission function of the reduced and oxidized molecule on
the electrode potential scale, we find excellent agreement with
experiments for the conductance versus gate potential in a 1 V
range around the AQ redox potential.
ontrolled switching between different charge-transport
states of single-molecule components is one of the most
C
essential and challenging aspects of molecular electronics.
Conductance switching in molecular junctions has been
demonstrated previously using various means, including
1
2
3
light, bias pulses, electrostatic gating, and electrochemical
4
gating. The concept of “electrochemical gating”, which
provides the opportunity to overcome the technical challenges
of incorporating a gate electrode in a solid-state molecular
device, has been employed in electrochemically active
5
6
molecular systems, including viologens, oligoaniline, ferro-
7
4b,8
9
cene, transition metal complexes,
perylenebisimides,
10
11
12
redox-active proteins, quinones, and tetrathiafulvalene,
13
as well as redox-inactive molecules. In the case of redox-
inactive molecules, or more generally when the electrode
potential does not overlap with the molecule’s redox potential,
the effect of the gate is simply to shift the molecular levels up or
down in energy relative to the Fermi level. In a simple picture
where tunneling through the molecule is described by
Received: October 8, 2014
“Lorentzian” transmission peaks centered at the discrete energy
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja510335z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 2. Calculated transmission function for the two charge states of
AQ-1,5 (a) and AQ-1,4 (b). The supercells used for the transport
calculations are also shown. The energy scale is relative to the Au
Fermi level.
states, constructed from 1100 conductance traces without any
data selection. A pronounced conductance peak centered at
−5.0
1
0
G is observed for the reduced state (rAQ-1,5), while no
0
clear peaks above the detection limit are found for the oxidized
form. The inset of Figure 1e shows the change in conductance
of AQ-1,5 over three consecutive cycles of the potential across
the redox potential. No significant decrease of the on/off ratio
is observed, which suggests that the manipulation of molecular
redox states is a highly reversible process.
Figure 1. (a) The molecules studied in this work. (b) CV of AQ-1,5
(
black) and AQ-1,4 (gray) assembly measured in the STM cell with
−1
sweep rate of 100 mV s in 0.1 M KClO aqueous electrolyte. Typical
Figure 1d,f shows individual conductance−distance traces
and histograms of AQ-1,4 in the reduced (blue) and oxidized
(red) states, respectively. For this molecule, clear conductance
4
conductance vs distance traces of rAQ-1,5 (c, blue, −530 mV), AQ-1,5
(
c, red, −280 mV), rAQ-1,4 (d, blue, −520 mV), AQ-1,4 (d, red, 280
plateaus are observed around 10− G and 10 G for the
4.0
−5.0
mV) relative to SCE. (e,f) 1D conductance histograms generated from
0
0
1100 individual curves without any data selection for rAQ-1,5 (e,
reduced and oxidized species, respectively, leading to peaks in
the conductance histogram at these values. We attribute these
peaks to the formation of rAQ-1,4 and AQ-1,4 single-molecule
junctions.
blue), AQ-1,5 (e, red), rAQ-1,4 (f, blue), and AQ-1,4 (f, red). (e,
inset) Three sequential in situ highly reversible switching cycles
between rAQ-1,5 and AQ-1,5 triggered by the electrode potential. The
data points are the most probable conductances of rAQ-1,5 (blue) and
AQ-1,5 (red). The noise level is indicated by the gray area in panels c−
The smaller difference in the conductance of the two redox
states of AQ-1,4, compared to AQ-1,5, can be explained by the
different electron pathways between the pyridyl units in the
cross-conjugated AQ-1,5 and linearly conjugated AQ-1,4
molecules. For AQ-1,4 the change in conductance results
primarily from a change in effective electron density of the
functional group attached as a “gating” unit to the OPE-type
backbone. In contrast, the electron pathway for AQ-1,5 goes
directly via the AQ unit, and thus the change from linear-
(reduced state) to cross-conjugation (oxidized state) directly
lowers the transmission probability.
f. The spike at log (G/G ) ≈ −2.2 is the switching of the amplifier.
0
Conductance measurements of both AQ-1,4 and AQ-1,5
were performed in an STM-BJ setup in 0.1 M KClO (pH
4
∼
5.8) using a cell with a four-electrode configuration, with Au
as tip and substrate, and Pt wires as counter-electrode and
quasi-reference electrode in an oxygen-free atmosphere (see
1
7
SI). Figure 1b shows the cyclic voltammogram (CV) of AQ-
,5 recorded in the STM cell. Two redox peaks are located
1
around −0.42 V for AQ-1,5 and −0.37 V for AQ-1,4 (Figure
In Figure 2, we show the transmission functions of AQ-1,5
(a) and AQ-1,4 (b) in reduced and oxidized forms calculated
from DFT. All the calculations were performed using the
1
b).
Figure 1c shows typical conductance−distance traces of AQ-
,5 in the reduced and oxidized states obtained from the STM-
19
1
GPAW electronic structure code using a double zeta plus
polarization (DZP) basis set. To overcome the well-known
problem of DFT to describe molecular energy levels we have
used the DFT+ Sigma scheme to correct the DFT eigenvalues
1
8
BJ experiment. For both redox states, the conductance curves
exhibit a short plateau at 1G due to atomic gold point contacts
0
existing just before rupture. In addition to the 1G plateau, the
0
2
0,21
blue curves show a more extended plateau, with conductances
as described in previous studies.
More details on the
ranging from 10− G to 10 G , which can be assigned to the
4.6
−5.4
computations are given in the SI. Our transport calculations
predict that the oxidized states generally have lower
conductance than the reduced states. This holds not only for
energies at the gold Fermi level but over the entire range of
energies within the HOMO−LUMO gap. Moreover, AQ-1,4 is
predicted to have higher conductance than AQ-1,5 in both
charge states. Finally, the change in conductance upon
changing the redox state is predicted to be larger for AQ-1,5
0
0
conductance of the reduced AQ-1,5 single-molecule junctions.
In contrast, no low-conductance plateaus were observed for
AQ-1,5 in the oxidized state (red curve). From this, we
conclude that the conductance of the oxidized form is below
the detection limit of our STM-BJ setup equipped with a linear
−
6
I−V convertor (10 G ). Figure 1e shows the one-dimensional
0
histograms of AQ-1,5 in its reduced (blue) and oxidized (red)
B
dx.doi.org/10.1021/ja510335z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
than for AQ-1,4. These findings are all consistent with the
experimental observations. The first two findings can be
explained by the variations in the HOMO−LUMO gap: for
both charge states the HOMO−LUMO gap is larger for AQ-
1,5 than for AQ-1,4, and for both molecules the HOMO−
LUMO gap is larger in the oxidized state than in the reduced
state. However, the third finding, namely that the conductance
change upon oxidation is larger for AQ-1,5 than for AQ-1,4,
cannot be explained simply from the size of the HOMO−
LUMO gap. Indeed, the increase in HOMO−LUMO gap upon
oxidation is very similar for the two molecules. Instead, the
larger suppression of the conductance in oxidized AQ-1,5 is a
result of destructive QI occurring because the electron pathway
connecting the left and right electrodes goes via the cross-
conjugated AQ unit. We note that the QI effect is not very
pronounced in oxidized AQ-1,5; in particular, the characteristic
transmission anti-resonance often observed in QI molecules is
not observed. Outside the redox-active region, the effect of the
gate potential should simply shift the molecular levels up or
down relative to the metal Fermi energy. Since the conductance
is proportional to the transmission function at the Fermi
energy, Figure 2 suggests that a significant gating effect should
be observable when the Fermi level is close to either the
HOMO or LUMO, and previous studies have shown that AQ-
Figure 3. Conductance histograms obtained at different potentials vs
SCE for (a) AQ-1,5 and (b) AQ-1,4. Black lines indicate the
conductance changes with electrode potential.
1
,4 and AQ-1,5, in the absence of gating potentials, conduct
22
through the LUMO.
To investigate the conductance versus gate potential, we
constructed conductance histograms for the two molecules at
different (fixed) electrode potentials. The histograms are shown
in Figure 3 for AQ-1,5 and AQ-1,4, and the conductance peaks
versus gate potential are plotted in Figure 4a,b. For potentials
higher than the redox potential, the molecules are in their
oxidized state. In this potential region, the peaks in the
conductance histograms shift to higher conductance as the
potential is moved toward more negative values (the Fermi
level is moved upward). This indicates that transport is indeed
mediated by the LUMO for the oxidized molecules. At a
potential around −0.4 V vs SCE, the conductance increases
sharply by approximately 1 order of magnitude due to the redox
process. For AQ-1,5 the conductance continues to rise as the
potential is decreased further, indicating that transport is also
LUMO-mediated in the reduced state. A significant con-
Figure 4. Experimental (a,b) and calculated (c,d) molecular
conductance as a function of the applied gating potential vs SCE for
AQ-1,5 (a,c) and AQ-1,4 (b,d). The vertical gray line indicates the
measured redox potential for the two molecules. The transmission
curves shown in the lower panels are the same as in Figure 2, but are
plotted here on the electrochemical energy scale relative to SCE.
scale relative to SCE in Figure 4c,d. Conversion of the
electrochemical energy scales is achieved based on the Fermi
level in the transport calculations corresponding to the
(negative) work function of Au(111), which is taken to be
24
5.3 eV, while the SCE is 4.68 eV relative to vacuum. The gray
line indicates the measured redox potentials. For potentials to
the left of the gray line, the molecule is in the reduced state and
the transmission is given by the blue curve. For potentials to
the right of the dashed line, the molecule is in the oxidized state
and the transmission is given by the red curve. The dots
indicate the predicted conductance at the potentials used to
produce the experimental conductance curve in Figure 4a,b.
The striking agreement between the calculations and experi-
ments strongly supports the interpretations of the conduction
mechanisms put forward in this paper and suggests that
(semi)quantitative modeling of single-molecule transport under
electrochemical control is possible using relatively simple
computational models. Finally, we note that although the
qualitative agreement between theory and experiments is
excellent, there are significant differences at the quantitative
level, in particular for the reduced states. We ascribe this to an
incorrect level alignment in the DFT+Sigma calculation for
reduced AQ-1,4 and a breakdown of the phase-coherent
transport mechanism close to the resonance in the case of
reduced AQ-1,5 (see SI for a more detailed discussion).
−6.1
−6.5
ductance change from 10 G (−330 mV vs SCE) to 10 G0
0
(−30 mV vs SCE) for AQ-1,5 in the oxidized state was
observed by carrying out the experiment in a complementary
STM-BJ with a developing logarithm I−V convertor with better
current sensitivity, which is in good agreement with the results
presented in Figure 1, showing that the oxidized state
−6
conductance of AQ-1,5 is <10 G . For AQ-1,4, it is found
0
that the conductance increases with positive potential in the
reduced state. At the potential close to redox potential (−420
mV vs SCE), we can still observe a less-pronounced
−
4.5
conductance peak around 10 G , which suggests the
0
existence of small amounts of molecules in the oxidized state
at the transition potential. We note that although the
conductance change induced by adjusting the electrode
potential is relatively small in the redox-inactive potential
region, as compared to the change taking place as the potential
crosses the redox peak potential, it is still comparable to those
4a,23
of several other previously studied molecular switches.
To enable a more precise comparison between the
experiments (Figure 4a,b) and the calculations, we plotted
the calculated transmission using the electrochemical energy
In summary, we have studied charge transport through
single-molecule junctions formed by two isomeric AQ-based
derivatives, AQ-1,5 and AQ-1,4, by employing an STM-BJ
C
dx.doi.org/10.1021/ja510335z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
technique under electrochemical control. For both AQ-1,5 and
AQ-1,4, the conductance has been controlled over 1 order of
magnitude by varying the electrode potential over a range of ∼1
V. In the redox-inactive potential region, the effect of the gating
is to shift the Fermi level relative to the molecular resonances,
leading to a modest change in conductance. At the redox
potential, large and reversible jumps of the conductance were
observed due to the change in redox state, which is
accompanied by changes in the conjugation pattern from
linear (in the reduced state) to cross-conjugated (in the
oxidized state). All these observations were supported by our
DFT-based transport calculations. In particular, we found an
excellent agreement between experiments and calculations for
the conductance versus gate potential both outside and inside
the redox-active regimes.
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ASSOCIATED CONTENT
Supporting Information
Synthetic procedures, computational, and single-molecule
measurement details. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
*
S
AUTHOR INFORMATION
Corresponding Authors
hong@dcb.unibe.ch
■
(15) Valkenier, H.; Guedon, C. M.; Markussen, T.; Thygesen, K. S.;
thygesen@fysik.dtu.dk
m.r.bryce@durham.ac.uk
van der Molen, S. J.; Hummelen, J. C. Phys. Chem. Chem. Phys. 2014,
16, 653.
(16) Hong, W.; Valkenier, H.; Meszaros, G.; Manrique, D. Z.;
Author Contributions
#
Mishchenko, A.; Putz, A.; Garcia, P. M.; Lambert, C. J.; Hummelen, J.
C.; Wandlowski, T. Beilstein J. Nanotechnol. 2011, 2, 699.
M.B., X.Z., and K.B.Ø contributed equally to this work.
Notes
(17) (a) Li, Z.; Pobelov, I.; Han, B.; Wandlowski, T.; Blaszczyk, A.;
The authors declare no competing financial interest.
Mayor, M. Nanotechnology 2007, 18, No. 044018. (b) Weibel, N.;
Grunder, S.; Mayor, M. Org. Biomol. Chem. 2007, 5, 2343. (c) Li, C.;
Mishchenko, A.; Li, Z.; Pobelov, I.; Wandlowski, T.; Li, X. Q.;
Wuerthner, F.; Bagrets, A.; Evers, F. J. Phys.: Condens. Matter 2008, 20,
No. 374122.
ACKNOWLEDGMENTS
■
We acknowledge discussions with Prof. Hans Siegenthaler and
Prof. Silvio Decurtins. This work was generously supported by
the Swiss National Science Foundation (200020-144471; NFP
(18) (a) Hong, W.; Manrique, D. Z.; Moreno-Garcia, P.; Gulcur, M.;
Mishchenko, A.; Lambert, C. J.; Bryce, M. R.; Wandlowski, T. J. Am.
Chem. Soc. 2012, 134, 2292. (b) Li, C.; Pobelov, I.; Wandlowski, T.;
Bagrets, A.; Arnold, A.; Evers, F. J. Am. Chem. Soc. 2008, 130, 318.
(19) Enkovaara, J.; Rostgaard, C.; Mortensen, J. J.; Chen, J.; Dulak,
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Kristoffersen, H. H.; Kuisma, M.; Larsen, A. H.; Lehtovaara, L.;
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T.; Petzold, V.; Romero, N. A.; Stausholm-Moller, J.; Strange, M.;
Tritsaris, G. A.; Vanin, M.; Walter, M.; Hammer, B.; Hakkinen, H.;
Madsen, G. K. H.; Nieminen, R. M.; Norskov, J. K.; Puska, M.;
Rantala, T. T.; Schiotz, J.; Thygesen, K. S.; Jacobsen, K. W. J. Phys.:
Condens. Matter 2010, 22, 253202.
6
2), the EC FP7 ITN “MOLESCO” project no. 606728, the
Scientific Exchange Programme NMSch (SciEx 13.060), FP7
project ACMOL (618082), and the University of Bern. K.B.Ø.
and K.S.T. thank the Danish Council for Independent
Research’s DFF Sapere Aude program (grant no. 11-
1
051390) for financial support.
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dx.doi.org/10.1021/ja510335z | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX