Signatures of quantum interference effects on charge transport through a single benzene ring

Article abstract of DOI:10.1002/anie.201207667

Molecular electronics: Charge transport through a single benzene ring is studied using the mechanically controlled break-junction technique. The low-bias conductance for a meta-coupled benzene ring is more than an order of magnitude smaller than that of a

Full text of DOI:10.1002/anie.201207667

.
Angewandte
Communications
DOI: 10.1002/anie.201207667
Quantum Interference
Signatures of Quantum Interference Effects on Charge Transport
Through a Single Benzene Ring**
Carlos R. Arroyo, Simge Tarkuc, Riccardo Frisenda, Johannes S. Seldenthuis,
Charlotte H. M. Woerde, Rienk Eelkema,* Ferdinand C. Grozema,* and
Herre S. J. van der Zant*
Until recently, single-molecule conduction experiments have
mostly been described in terms of a nonresonant tunneling
process where the molecule acts as a tunneling barrier.[1–3] In
this description, the width and height of the energy barrier,
and the electronic coupling between the molecule and the
electrodes are the main parameters that characterize the
efficiency of charge transport. Electron transfer through
a molecule then depends exponentially on the length of the
conductance pathway and this has indeed been observed in
many experiments.[1,3–7] However, such a simple tunneling
picture disregards quantum interference effects that can
strongly influence transport at the molecular scale.[8,9] There-
fore, these effects are a subject of growing scientific interest,
both theoretically[10–15] and experimentally.[16–20] One of the
simplest systems where quantum interference effects in
charge transport are expected is a single benzene ring.[8] It
has been shown theoretically that a benzene ring connected
between two electrodes in a para-configuration should have
a conductance that is several orders of magnitude higher than
that of a meta-configuration.[13,21] This can be understood in
terms of transport along different pathways through the
benzene ring. These pathways can be either spatial or orbital
pathways. Interference of transport components along these
different pathways can lead to constructive or destructive
interference.[10,11,13,22]
the first class, the molecule consists of an anthraquinone with
thiol anchoring groups.[16–18] In the second class, the coupling
between the molecule and the electrode is through either
a thiol[20,23] or a methyl thio-ether[19] that is directly connected
to a phenyl ring. This phenyl ring is coupled to the rest of the
molecule in either a para- or a meta-configuration, and large
differences in conductance between these two situations have
been observed. A disadvantage of the latter systems is that the
anchoring is very close to the place in the molecule where
interference effects occur. This can introduce alternative
conductance pathways that can obscure the quantum inter-
ference effects.[24]
Herein, the effect of quantum interference on charge
transfer through a single benzene ring is studied by consid-
ering a series of specifically designed molecules represented
in Figure 1a. The central part of the molecule is a benzene
ring to which anchoring units are connected either in para- or
meta-configuration. The anchoring groups consist of thienyl
rings that are connected through an ethynyl spacer to provide
some distance between the benzene ring, where the interfer-
Interference effects on charge transport have been
considered experimentally in two classes of molecules. In
[*] Dr. C. R. Arroyo,[+] R. Frisenda, Dr. J. S. Seldenthuis,
Prof. Dr. H. S. J. van der Zant
Kavli Institute of Nanoscience, Delft University of Technology
Lorentzweg 1, 2628 CJ Delft (The Netherlands)
E-mail: h.s.j.vanderzant@tudelft.nl
Homepage: http://vanderzantlab.tudelft.nl/
Dr. S. Tarkuc,[+] C. H. M. Woerde, Dr. R. Eelkema, Dr. F. C. Grozema
Department of Chemical Engineering
Delft University of Technology
Julianalaan 136, 2628 BL Delft (The Netherlands)
E-mail: r.eelkema@tudelft.nl
f.c.grozema@tudelft.nl
[+] These authors contributed equally to this work.
[**] We thank M. Perrin and J. M. Thijssen for discussions. This work has
been supported by FOM, the European Union seventh Framework
Programme (FP7/2007-2013) under the grant agreement no 270369
(“ELFOS”) and The Netherlands Organization for Scientific
Research (NWO) in the form of a VIDI grant to FCG.
Figure 1. a) Structures of molecules used in this study. The para-
(blue) and meta- (red) coupled benzene rings are coupled to the
thienyl anchoring units (green) by ethynyl spacers (black). b) Layout of
a mechanically controlled break-junction (MCBJ) setup. Inset: scan-
ning electron micrograph of a MCBJ device.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207667.
3152
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 3152 –3155

Angewandte
Chemie
ence occurs, and the electrodes.
To examine the effect of the
geometry of the anchor unit, the
linkage position of the thienyl
group (2- or 3-position) was also
varied.
The low-bias conductance
and formation of the single-
molecule
explored using the mechanically
controlled break-junction
junctions
was
(MCBJ) technique. The MCBJ
device, clamped in a three-point
bending configuration, is shown
in Figure 1b. By driving the
pushing rod against the bottom
part of the MCBJ device, the
gold constriction is stretched
until it breaks, leaving a pair of
sharp electrodes separated by
a nanometer-scale gap. Once
the bridge is broken, atomic-
sized gold contacts were repeat-
edly formed and broken by
moving the electrodes towards
Figure 2. Two-dimensional trace histograms constructed from 500 consecutive breaking traces taken at
ambient conditions and 0.1 V bias for junctions exposed to a 1 mm solution of a) 1, b) 3, c) 2, and d) 4
in chloroform. Regions of high counts represent the most probable breaking behavior of the contact. The
black curves are examples of individual breaking traces.
and away from each other at a speed of 9 nmsÀ1. Simulta-
neously, the conductance G = I/V was measured with a bias
voltage of 0.1 V applied across the electrodes. Details
concerning the experimental procedures are given in the
Supporting Information together with the synthetic proce-
dures for molecules 1–4.
denote the most probable evolution during the formation of
the molecular junctions.
The low-bias conductance of para-coupled benzene was
systematically studied using thienyl anchoring groups that are
coupled either through the 2- or the 3-position to provide
a quantitative assessment of the effect of the anchoring
groups. We found that the conductance value of 2 is slightly
lower than that of 1 and that the conductance plateaus of 2 are
more horizontal than those for 1 (see the Supporting
Information). This shows that for both anchoring groups
stable molecular junctions are formed with distinguishable
conductance values. The decrease in conductance by a factor
of three when going from 1 to 2 indicates that the conductance
is limited by the details of binding to the electrodes and the
corresponding charge injection into the molecule. A con-
ductance difference of a factor three in molecules 1 and 2 was
obtained by fitting a Gaussian function through the peaks in
the one-dimensional conductance histograms as described in
Supporting Information.
The individual breaking traces for the meta-coupled
molecules 3 and 4 show clear plateaus (see Figure S11 in
Supporting Information), but in a conductance region that is
at least one order of magnitude lower than for the para-
molecules (see the black individual breaking traces in Fig-
ure 2b,d). In the two-dimensional trace histograms, a slanted
region of high counts is found for 3 in the 10À5 G0 region,
whereas the conductance for molecule 4 (Figure 2d) is even
lower and approaches the limit of the resolution of the setup.
The slanted regions of high counts are assigned to the
averaging of individual traces, each showing distinct con-
ductance plateaus of different lengths. Remarkably, a recent
study[18] shows similar 2D histograms in which linear con-
jugated molecules yield clear conductance plateaus while
cross-conjugated molecules show slanted regions. Possible
In the experiments reported here, each molecule (1–4)
was deposited by pipetting a 2 mL droplet of a freshly
prepared 1 mm solution in chloroform onto the MCBJ
device. To exclude artifacts resulting from contaminant
species adsorbed on the gold surface, the characterization of
the MCBJ device was first performed for pure chloroform.
The breaking traces measured for pure chloroform do not
show any other conductance signatures other than observed
for an empty device in air (Figure S10). After characterization
of the pure solvent, a droplet containing molecules dissolved
in chloroform is added to the MCBJ device. This is
accompanied by the appearance of characteristic conductance
features that indicate the formation of molecular junctions
(see Figure S9 in Supporting Information). The spread of
conductance values found in this study is comparable to those
found for dithiol-terminated oligophenylene-ethynylene mol-
ecules,[25] confirming that the thienyl units act as anchoring
groups that are mechanically stable and allow the formation
of molecular junctions with clear conductance features.
Figure 2 represents two-dimensional trace histograms[26]
constructed from sets of 500 consecutive breaking traces
measured after the deposition of a solution containing the
molecules (1–4). In these trace histograms, the individual
breaking traces have been shifted along the horizontal
electrode displacement-axis to fix the rupture of the one-
atom gold contact at zero. The color scale indicates the
number of counts found at each conductance value for
a certain electrode displacement. The areas with high counts
Angew. Chem. Int. Ed. 2013, 52, 3152 –3155
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3153

.
Angewandte
Communications
constructive interference for 1 when the transmissions
through the HOMO and LUMO are combined. For 3, the
phase shift results in destructive interference between the
HOMO and LUMO transmission, as evident from the anti-
resonance in the full transmission plot (Figure 3a). It should
be noted that also the HOMO-1 and LUMO + 1 orbitals
contribute to the transmission within the HOMO–LUMO
gap, however the phase behavior of these orbitals is the same
as for the HOMO and LUMO; i.e., constructive and
destructive interference for 1 and 3, respectively (see
Supporting Information). This analysis therefore clearly
confirms the possibility of constructive and destructive
interference playing a role in the molecules studied exper-
imentally.
In a complementary study we have also performed similar
measurements of para- and meta-coupled (oligo-phenylene
vinylene) OPV3 derivatives covalently connected to gold
electrodes by thiol groups. The conductance histograms show
a similar difference in conductance between the para- and
meta-coupled central benzene ring as reported here. Theo-
retical calculations again assign the lower conductance for the
meta-coupled molecule to destructive interference between
LUMO- and HOMO-mediated transport. This observation
therefore confirms that the coupling to the central benzene
ring determines the occurrence of quantum interference
effects and not the spacers or anchoring groups.
In conclusion, we have shown that the conductance
through a single meta-coupled benzene ring is more than an
order of magnitude lower than through a para-coupled
benzene. Reproducible conductance traces were recorded
using thienyl anchoring units that are coupled to the benzene
ring through an ethynyl spacer. Non-equilibrium Greenꢀs
function calculations show that interference effects that are
related to the phase of the transmission function may be at the
origin of the observed large differences in conductance. The
results open up new opportunities for the design of single-
molecule devices based on quantum interference effects, for
instance switching devices that operate through a “chemical
gating” unit.[11]
Figure 3. a) Calculated transmission through the p-systems of mole-
cules 1 (solid line) and 3 (dashed line) in the wide band limit.
b) Amplitude (top) and phase (bottom) of the transmission through
the HOMO and LUMO of 1. c) Amplitude (top) and phase (bottom)
of the transmission through the HOMO and LUMO of 3.
explanations for this difference may be found in a stronger
energy dependence of the transmission function for molecules
with destructive quantum interference effects (see Figure 3)
combined with small changes in the work function associated
with (slightly) different binding configurations, or in a config-
uration dependence of the p-orbital overlap with the electro-
des.[27,28]
The dramatic reduction of the conductance through
a meta-coupled benzene ring (molecules 3 and 4) as compared
to para-coupled benzene (molecules 1 and 2) can be
interpreted as a form of quantum interference. Theoretically,
the charge propagation through molecules can be described
as a transmission through different molecular orbitals in the
Landauer formalism.[13] We have calculated the transmission
through molecules 1–4 using the non-equilibrium Greenꢀs
function method with a density functional theory (DFT)
calculation of the ground-state electron density. In Figure 3,
the calculated transmission through the p-systems of mole-
cules 1 and 3 in the wide band limit is shown. At energies
between the HOMO and LUMO energies, the transmission of
3 is more than an order of magnitude smaller than that of 1,
with an anti-resonance occurring at À4.38 eV, where the
transmission drops to zero. This anti-resonance is caused by
destructive interference between transmissions through dif-
ferent orbitals. In the non-equilibrium Greenꢀs function
formalism it is possible to separate the total transmission
into contributions from the individual molecular orbitals.
Since these contributions are complex (i.e., they have an
amplitude and a phase), an interference effect can arise when
transmission through different orbitals are combined.
Received: September 22, 2012
Revised: January 7, 2013
Published online: February 5, 2013
Keywords: break junctions · molecular electronics ·
.
quantum interference · single-molecule transport
[1] D. M. Adams, L. Brus, C. E. D. Chidsey, S. Creager, C. Creutz,
C. R. Kagan, P. V. Kamat, M. Lieberman, S. Lindsay, R. A.
Marcus, R. M. Metzger, M. E. Michel-Beyerle, J. R. Miller,
M. D. Newton, D. R. Rolison, O. Sankey, K. S. Schanze, J.
Yardley, X. Zhu, J. Phys. Chem. B 2003, 107, 6668 – 6697.
[2] P. P. Edwards, H. B. Gray, M. T. J. Lodge, R. J. P. Williams,
Angew. Chem. 2008, 120, 6860 – 6868; Angew. Chem. Int. Ed.
2008, 47, 6758 – 6765.
[3] R. L. McCreery, Chem. Mater. 2004, 16, 4477 – 4496.
[4] S. H. Choi, B. Kim, C. D. Frisbie, Science 2008, 320, 1482 – 1486.
[5] R. L. McCreery, A. J. Berggren, Adv. Mater. 2009, 21, 4303 –
4322.
The positions of the orbital resonances and the amplitudes
of the transmissions are approximately the same for both
molecules. However, the phase of the transmission through
the LUMO differs by p for 1 and 3, while the phase of the
HOMO is the same (see Figure 3b and c). This results in
3154
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 3152 –3155

Angewandte
Chemie
[6] L. Venkataraman, J. E. Klare, I. W. Tam, C. Nuckolls, M. S.
Hybertsen, M. L. Steigerwald, Nano Lett. 2006, 6, 458 – 462.
[7] B. Xu, N. J. Tao, Science 2003, 301, 1221 – 1223.
[8] P. Sautet, C. Joachim, Chem. Phys. Lett. 1988, 153, 511 – 516.
[9] M. Magoga, C. Joachim, Phys. Rev. B 1999, 59, 16011.
[10] A. A. Kocherzhenko, F. C. Grozema, L. D. A. Siebbeles, J. Phys.
Chem. C 2010, 114, 7973 – 7979.
[11] A. A. Kocherzhenko, L. D. A. Siebbeles, F. C. Grozema, J. Phys.
Chem. Lett. 2011, 2, 1753 – 1756.
[12] T. Markussen, R. Stadler, S. Thygesen, Nano Lett. 2010, 10,
4260 – 4265.
[19] S. V. Aradhya, J. S. Meisner, M. Krikorian, S. Ahn, R. Para-
meswaran, M. L. Steigerwald, C. Nuckolls, L. Venkataraman,
Nano Lett. 2012, 12, 1643 – 1647.
[20] S. Ballmann, R. Hartle, P. B. Coto, M. Mayor, M. Elbing, M. R.
Bryce, M. Thoss, H. B. Weber, arXiv:1203.4128v1 2012.
[21] S.-H. Ke, W. Yang, Nano Lett. 2008, 8, 3257 – 3261.
[22] D. M. Cardamone, C. A. Stafford, S. Mazumdar, Nano Lett.
2006, 6, 2422 – 2426.
[23] M. Mayor, H. B. Weber, J. Reichert, M. Elbing, C. Von Hanisch,
D. Beckmann, M. Fischer, Angew. Chem. 2003, 115, 6014 – 6018;
Angew. Chem. Int. Ed. 2003, 42, 5834 – 5838.
[13] G. C. Solomon, D. Q. Andrews, T. Hansen, R. H. Goldsmith,
M. R. Wasielewski, R. P. Van Duyne, M. A. Ratner, J. Chem.
Phys. 2008, 129, 054701.
[14] G. C. Solomon, D. Q. Andrews, R. P. Van Duyne, M. A. Ratner,
J. Am. Chem. Soc. 2008, 130, 7788 – 7789.
[15] G. C. Solomon, C. Hermann, T. Hansen, V. Mujica, M. A.
Ratner, Nat. Chem. 2010, 2, 223 – 228.
[16] C. M. Guꢁdon, H. Valkenier, T. Markussen, K. S. Thygesen, J. C.
Hummelen, S. J. van der Molen, Nat. Nanotechnol. 2012, 7, 305 –
309.
[17] D. Fracasso, H. Valkenier, J. C. Hummelen, G. C. Solomon, R. C.
Chiechi, J. Am. Chem. Soc. 2011, 133, 9556 – 9563.
[18] W. Hong, H. Valkenier, G. Meszaros, D. Z. Manrique, A.
Mishchenko, A. Putz, P. M. Garcia, C. J. Lambert, J. C. Hum-
melen, T. Wandlowski, Beilstein J. Nanotechnol. 2011, 2, 699 –
713.
[24] G. C. Solomon, D. Q. Andrews, M. A. Ratner, in Charge and
exciton transport through molecular wires, (Eds.: L. D. A.
Siebbeles, F. C. Grozema), Wiley-VCH, Weinheim, 2011,
pp. 19 – 52.
[25] V. Kaliginedi, P. Moreno-Garcia, H. Valkenier, W. Hong, V. M.
Garcia-Suarez, P. Buiter, J. L. H. Otten, J. C. Hummelen, C. J.
Lambert, T. Wandlowski, J. Am. Chem. Soc. 2012, 134, 5262 –
5275.
[26] C. A. Martin, D. Ding, J. K. Sorensen, T. Bjornholm, J. M.
van Ruitenbeek, H. S. J. van der Zant, J. Am. Chem. Soc. 2008,
130, 13198 – 13199.
[27] I. Diez-Perez, J. Hihath, T. Hines, Z. S. Wang, G. Zhou, K.
Mullen, N. J. Tao, Nat. Nanotechnol. 2011, 6, 226 – 231.
[28] J. S. Meisner, M. Kamenetska, M. Krikorian, M. L. Steigerwald,
L. Venkataraman, C. Nuckolls, Nano Lett. 2011, 11, 1575 – 1579.
Angew. Chem. Int. Ed. 2013, 52, 3152 –3155
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3155