Understanding the Role of Parallel Pathways via In-Situ Switching of Quantum Interference in Molecular Tunneling Junctions

Article abstract of DOI:10.1002/anie.202005047

This study describes the modulation of tunneling probabilities in molecular junctions by switching one of two parallel intramolecular pathways. A linearly conjugated molecular wire provides a rigid framework that allows a second, cross-conjugated pathway

Full text of DOI:10.1002/anie.202005047

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Understanding Role of Parallel Pathways via In Situ Switching of
Quantum Interference in Molecular Tunneling Junctions
Authors: Saurabh Soni, Gang Ye, Jeuting Zheng, Yanxi Zhang, Andika
Asyuda, Michael Zharnikov, Wenjing Hong, and Ryan C
Chiechi
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10.1002/anie.202005047
Angewandte Chemie International Edition
Understanding Role of Parallel Pathways via In Situ
Switching of Quantum Interference in Molecular Tunnel-
ing Junctions
1,2
1,2,6,¶
3,¶
1,2,4
5
5
Saurabh Soni , Gang Ye
, Jueting Zheng , Yanxi Zhang , Andika Asyuda , Michael Zharnikov ,
∗3
∗1,2
Wenjing Hong and Ryan C. Chiechi
1Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, the
Netherlands
2Zernike Institute for Advanced Materials, Nijenborgh 4, 9747 AG Groningen, the Netherlands
3State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China
4Present address: Microsystems, Department of Mechanical Engineering and Institute for Complex
Molecular Systems, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
5Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, D-69120, Heidel-
berg, Germany
6Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guang-
dong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen
518060, P. R. China
¶equal second co-authorship
*e-mail: whong@xmu.edu.cn r.c.chiechi@rug.nl
Abstract
This paper describes the modulation of tunneling probabilities in molecular junctions by
switching one of two parallel intramolecular pathways. A linearly-conjugated molecular wire
provides a rigid framework that allows a second, cross-conjugated pathway to be effectively
switched on and off by protonation, affecting the total conductance of the junction. This approach
works because a traversing electron interacts with the entire quantum-mechanical circuit
simultaneously; Kirchhoff’s rules do not apply. We prove this concept by comparing the
conductances of a series of compounds with single or parallel pathways in large-area junctions
1
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using EGaIn contacts and single-molecule break-junctions using gold contacts. We affect
switching selectively in one of two parallel pathways by converting a cross-conjugated carbonyl
carbon into a trivalent carbocation, which replaces destructive quantum interference with a
symmetrical resonance, causing an increase in transmission in the bias window.
Research in Molecular Electronics (ME) is aimed at developing systems in which function-
ality is derived from the intrinsic molecular features. Molecular tunneling junctions exploit the
quantum mechanical nature of charge-transport on the molecular scale, with the goal of developing
devices with new logic and memory/storage processing systems at molecular level.[1,2] Potentially
useful elements of ME include switches,[3,4] quantum interference-based molecular transistors,[5,6]
rectifiers,[7–9] and thermoelectric molecular junctions.[10]
Destructive quantum interference (QI) is a particularly interesting phenomenon in context
of ME because it can lead to changes in conductance by orders of magnitude that can be manip-
ulated via electrochemical gating, heteroatom substitution, change in bond-topology, substitution
effect, etc. without altering the width of the tunneling barrier.[1,6,11–21] The relationship between
bond topology, molecular orbitals and destructive QI features in transmission spectra is generally
understood,[22] but theory over-estimates the magnitudes of effects and is primarily useful for un-
derstanding trends in experimental data. Figure 1b schematically compares classical and quantum
intramolecular circuits comprising two parallel pathways. In the former, the total conductance is
the sum of individual contributions from individual pathways. In the latter, the individual pathways
give rise to an extra quantum mechanical term,[23] as each of them individually affect the electronic
2
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properties of junction due to QI.[24] Thus, a low-conductance cross-conjugated pathway in parallel
with a high-conductance linear-conjugated pathway can suppress the overall transmission. Trans-
lating such phenomena into observable changes in tunneling is the first step towards realizing static
devices. However, most QI studies involve single-molecule junctions, which are useful for fun-
damental studies but cannot be directly used in devices. In contrast, self-assembled monolayers
(SAMs) are useful for fundamental studies and are capable of forming static devices, for example,
proof-of-concept transistors based on gating QI features created by through-space intramolecular
interactions.[5]
In our previous studies involving QI effects on linear-conjugated anthracene (AC) and cross-
conjugated anthraquinone (AQ),[17,20] we showed that tunneling current for AQ is suppressed be-
cause of the presence of destructive QI feature near the Fermi level (Ef ) caused by the cross-
conjugated bond topology. We further isolated the effects of bond topology by modifying the
quinoid groups, finding that cross-conjugation leads to QI, but that specific functional groups can
shift QI features in or out of the bias window.[16,25] Here, we exploit the quantum mechanical na-
ture of molecular circuits to eliminate QI features reversibly in situ via a cross-conjugated pathway
that can be gated by protonation, but that is in parallel with a linear-conjugated pathway that pro-
vides structural rigidity. Gating occurs via the formation of trivalent carbon centers, which (like
heteroatoms) are functionally linearly-conjugated.[26,27]
To isolate the effects of the two pathways and control differences that arise between single-
molecule and SAM-based junctions, we studied four molecular wires shown in Figure 1a viz. BT
3
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a
b
Figure 1: (a) Self-assembled monolayers of molecular wires grown on AuTS form static
AuTS/SAM//EGaIn junctions (“/” and “//” denote covalent and van der Waals interfaces, respec-
tively). The four cores corresponding to BT, BPh, FH and FO. (b) Classical resistors in parallel,
based on the Kirchhoff’s law compared to an intramolecular, quantum circuit; e.g., two parallel
pathways in a tunneling junction in FO core, as indicated with the blue arrow for carbonyl and the
red arrow for C–C single bond.
(Bithiophene), BPh (Biphenyl), FH (Fluorene), and FO (Fluorenone), along with AQ as a ref-
erence.[16] The aromatic cores of these molecules, decorated with thiol anchoring groups, were
varied in the series to modify bond topology. Both BT and BPh are linearly-conjugated; FH
comprises parallel linear- and non-conjugated pathways; and FO comprises parallel linear- and
cross-conjugated pathways. We characterized tunneling charge-transport in large-area junctions
with EGaIn top-contact and in single-molecule break-junctions employing a Scanning Tunneling
Microscopy setup (STM-BJ). We support the experimental results with density functional theory
(DFT) via non-equilibrium green’s function (NEGF) simulations that examine the role of trivalent
carbon atoms in defining bond topology in the context of QI.
In our previous studies, simulations on single-molecule model junctions have agreed well with
experimental results from large-area junctions exhibiting bond-topology based QI,[16,25,28] transition
4
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a
b
1
0.1
0.01
0.001
1E-4
1E-5
1E-6
BT
BPh
FH
FO
FOH
1E-7
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
E-EF (eV)
Figure 2: (a) Simulated model junctions comprising an extended molecule (molecular wire con-
necting two leads made of the same material as the electrode) between semi-infinite periodic elec-
trodes. (b) Transmission probability versus electron energy w.r.t. E − Ef for four molecular wires
and FOH (protonated FO). As can be seen near Ef , the transmission trend is BT > FOH > FH >
BPh > FO. The discontinuities in the transmission spectra at E − Ef = 0.55eV are related to the
effects of the potential in the contacts, see Supporting Information for more details.
voltages,[16] molecular switches in SAMs[4] and the mechanical- and electrical-stability of SAMs.[29]
In this study, we used the model junction shown in Figure 2a, which comprises a finite central
molecule between semi-infinite, periodic bulk electrodes.
Figure 2b shows the transmission probability versus electron-energy, referenced to Ef . The
respective transmission spectra show sharp dips (signature of destructive QI) and give zero-bias
conductance values (G) as G = G0×T (Ef ), where G0 is the quantum of conductance (G0 = 2e2∕ℎ =
77.48mS) and T (Ef ) is the transmission probability at Ef . The results, which are summarized
5
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in Table 1, reveal five key features: 1) Linearly-conjugated, planar BT has the second smallest
optical band-gap (Eg) and the highest zero-bias conductance; 2) Planar FH, with a linear- and a
non-conjugated pathway in parallel, is the second-highest in conductance; 3) Linearly-conjugated,
twisted BPh (torsional angle = 40.3°) is less conductive than FH (in agreement with Venkataraman
et al.[30]), but when constrained in planar conformation (as happens in SAMs[31,32]), the conductance
of BPh planar overtakes FH, as shown in Figure S28; 4) Planar FO with a linear- and a cross-
conjugated pathway in parallel has a destructive QI feature around E − Ef = 1.5 eV and is the least
conductive, despite having the smallest Eg (Table 1); and 5) For FOH (the protonated form of FO),
the destructive QI feature near 1.5 eV disappears and a symmetrical feature appears near 0.5 eV,
leading to an increase in transmission at Ef . Simulations on FOH in the presence of TFA– anions
are shown in the Supporting Information.
We investigated the tunneling transport of the four molecules in large-area AuTS/SAM//EGaIn
junctions (where “/” and “//” denote a covalent and van der Waals interface, respectively, and AuTS
is ultraflat template-stripped Au substrate). Supporting Information details the SAM-formation
procedure and characterization via water contact angle, and synchrotron-based X-ray photoelec-
tron spectroscopy (XPS), the results of which are summarized in Table 1. As shown in Figure 3
(black dashed line), the order of current density in large-area junctions is BT ≥ BPh > FH > FO.
(Full scale J − V curves are shown in the Supporting Information.) The observation that BPh
is more conductive than FH reflects the participation of the non-conjugated pathway in FH; The
linearly-conjugated pathways in BPh and FH are identical because BPh is planarized in the SAMs,
but tunneling electrons simultaneously sample that pathway and the methylene bridge of FH be-
6
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-3.8 -2.0
EGaIn
32
28
24
20
16
12
8
STM-BJ
DFT
DFT (planar)
-2.2
-2.4
-2.6
-2.8
-3.0
-3.2
-3.9
-4.0
-4.1
-4.2
-4.3
4
-4.4 -3.4
BT
BPh
FH
FO
Figure 3: Experimental and simulated conductance data for BT ( ), BPh planar (2), BPh (twisted,
■), FH (▾), and FO (▴). Here, we compare the low-bias EGaIn conductance (black dashed line, left
axis), zero-bias STM-BJ conductance (red solid line, the red axis on the right), and zero-bias DFT
conductance (blue dashed and solid lines, the blue axis on the right). The DFT data connected by
dashed-blue and solid-blue lines are the simulations of EGaIn and STM-BJ data, respectively. The
molecular structures with type of conjugation are shown at the bottom. For sake of clarity, pathways
in FH and FO are labeled as non-conjugated and linearly-conjugated, and cross-conjugated and
linearly-conjugated, respectively.
7
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Table 1: Summary of the physical properties of BT, BPh, FH, FO, and FOH. Packing density is
in the order of 1014 molecules/cm2.
Compound
BT
BPh
3.27
16.10 ± 0.36
−4.35 ± 0.03
−3.04 (−2.70a)
20.5 ± 1
FH
3.19
FO
2.45
FOH
2.17
-
opt
UV-Vis Eg (eV)
2.73
EGaIn low-bias G (10−3 S cm−2) 28.20 ± 0.43
11.38 ± 0.17
4.15 ± 0.06
STM-BJ G (Log(G∕G0))
DFT G (Log(G∕G0))
XPS thickness (Å)
−3.92 ± 0.04
−2.11
18.3 ± 1
5.5 ± 0.55
80.0%
−4.16 ± 0.05 −4.26 ± 0.07 −3.89 ± 0.07
−2.94
20.4 ± 1
2.6 ± 0.26
82.4%
−3.22
18.2 ± 1
5.1 ± 0.51
85.7%
−2.81
-
-
-
Packing density
3.0 ± 0.3
78.9%
Yield of EGaIn junctions
a
for BPh planar structure
cause it is part of the ꢀ-bond framework. Replacing the methylene bridge with a carbonyl group
converts non-conjugation into cross-conjugation, thus FO is even less conductive than FH. This
counter-intuitive result is, again, a reflection of the nature of quantum mechanical circuits; cross-
conjugation leads to destructive QI, which suppresses conductance more than non-conjugation.
The trends in low-bias EGaIn conductance and zero-bias DFT conductance (black and blue dashed
lines, respectively, in Figure 3) are in good agreement, suggesting that our simulated, model junc-
tion captures the properties of large-area junctions, provided the correct molecular geometries are
used.
To exclude the possibility that intermolecular pathways play a significant role, we performed
single-molecule conductance measurements in STM-BJ setup, see the Supporting Information for
details). As shown in Figure 3, the conductance trend follows BT > FH ≥ FO > BPh (solid
red line), in agreement with DFT simulations (solid blue line). The trend in single-molecule con-
ductance also agrees with EGaIn results except that, as expected, BPh is much less conductive
because it can adopt its preferred, twisted geometry in solution. This discrepancy further supports
8
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our assertion that, in large-area junctions, conductance is dominated by transport through individual
molecules and that the only significant experimental variable is the conformation that the molecules
adopt in the two different measurement platforms.[28]
Protonation of FO is predicted to increase transmission. The increase in transmission prob-
ability near Ef in Figure 2b is modest, but is accompanied by the loss of a destructive QI feature
and the appearance of a symmetric feature near 0.5 eV. This feature resembles a Fano resonance,
implying that the trivalent carbon in FOH remains strongly coupled to the linearly conjugated
pathway, but is itself weakly coupled to the electrodes.[14] While the entire transmission spectrum
contributes to the overall tunneling probability, empirically, the observed conductance is dominated
by transmission near Ef .[16]
To test for the predicted increase in conductance upon protonation, we measured FO in single-
molecule junctions in the presence and absence of trifluoroaceticacid (TFA), using FH as a control.
Although FO is a weak base, in the presence of TFA there will be a small concentration of FOH
present at equilibrium. If FOH is more conductive than FO, it should shift the peak of the conduc-
tance histogram by a degree commensurate with the equilibrium concentration and the difference
in conductance between FO and FOH.
We prepared solutions of FO and FH with same concentrations as before, adding 30 % (v∕v)
TFA to effect protonation. As can be seen from Figure 4a, while the conductance histogram of
FH+TFA does not differ from that of FH, that of FO+TFA is shifted to higher conductances. In
our previous work on the inclusion of trivalent carbon centers in conjugated materials, they were
9
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b
a
24
Input
1E-4
1E-5
1E-6
1E-7
-4.0
-2.6
-2.8
-3.0
-3.2
20
-4.2
Predictions
-4.4
16
-4.6
FH
FH(TFA)
FO
FOH(TFA)
12
Molecule
8
FH
FH(TFA)
FO
4
STMBJ G
Superposition Model G
FO(TFA)
0
-6
-5
-4
-3
-2
-1
0
1
BPh AQ
AH
AC
FH
FO FOH
Conductance(log G/G0)
Molecule
Figure 4: (a) Histograms of counts versus conductance from STM-BJ measurements on FO and
FH with and without treatment with TFA. FH remains unaffected, however, the conductance of
FO increases upon protonation. Inset compares mean STM-BJ conductance (left axis) with DFT
predicted conductance trend (right axis). (b) STM-BJ conductance of sthe seven molecules (black
squares) and predicted conductance of FH, FO, and FOH (red circles), see the Supporting Infor-
mation for further details.
functionally linearly-conjugated through an empty p-orbital.[26,27] The trend in conductance in the
inset of Figure 4a suggests the same in single-molecule junctions and validates the DFT-NEGF
prediction of FOH > FH > FO.
Supported by experiments involving large-area and single-molecule junctions, DFT-NEGF
and an empirical superposition model (see Figure 4b and the Supporting Information), we show that
tunneling charge-transport across a molecular framework can be manipulated by altering conjugated
pathways in parallel with linearly-conjugated pathways. Although we have not yet succeeded in
switching the test molecular wire, FO, in SAMs, we are pursuing systems that are more readily
protonated for this purpose. Nonetheless, the parallel-pathway approach to switching allows the
in-place manipulation of QI-controlling bond topology without large structural changes, making it
a viable strategy for QI-switching in static, large-area molecular tunneling junctions.
10
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Acknowledgments
R.C.C. and Y.Z. acknowledge the European Research Council for the ERC Starting Grant 335473
(MOLECSYNCON). S.S. acknowledges the Zernike Institute for Advanced Materials. G.Y. ac-
knowledges financial support from the China Scholarship Council (CSC):NO.201408440247. We
thank the Center for Information Technology of the University of Groningen for their support and
for providing access to the Peregrine high performance computing cluster. A.A. and M.Z thank the
Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime at BESSY II and
financial support as well as A. Nefedov and Ch. Wöll for the technical cooperation during the exper-
iments at BESSY II. We thank E. Sauter for participation in some of the experiments in context of
this project. A.A. acknowledges the financial support by the DAAD-ACEH Scholarship of Excel-
lence. W. H. and J. Z. acknowledges the financial support from National Natural Science Founda-
tion of China (No. 21673195) and the National Key R&D Program of China (2017YFA0204902).
We thank Dr. Remco W. A. Havenith for insightful discussion and help with the ADF theoretical
simulations.
Keywords: Molecular Electronics ⋅ Quantum Interference ⋅ Self-assembled Monolayers ⋅
EGaIn ⋅ STMBJ
11
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Table of Contents graphic:
푮푻 = 푮ퟏ + 푮ퟐ + ퟐ 푮ퟏ × 푮ퟐ
Manipulating conductance in parallel molecular pathways is a crucial element in Molecular
Electronics. We explore intramolecular parallel pathways with different bond topology, in form of
a quantum circuit, in tunneling junctions comprising self-assembled monolayers. We employ QI
switching/gating with acid, without any structural changes, in single-molecular junctions.
15
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