Two-Terminal Molecular Memory through Reversible Switching of Quantum Interference Features in Tunneling Junctions

Article abstract of DOI:10.1002/anie.201807879

Large-area molecular tunneling junctions comprising self-assembled monolayers of redox-active molecules are described that exhibit two-terminal bias switching. The as-prepared monolayers undergo partial charge transfer to the underlying metal substrate (Au, Pt, or Ag), which converts their cores from a quinoid to a hydroquinoid form. The resulting rearomatization converts the bond topology from a cross-conjugated to a linearly conjugated π system. The cross-conjugated form correlates to the appearance of an interference feature in the transmission spectrum that vanishes for the linearly conjugated form. Owing to the presence of electron-withdrawing nitrile groups, the reduction potential and the interference feature lie close to the work function and Fermi level of the metallic substrate. We exploited the relationship between conjugation patterns and quantum interference to create nonvolatile memory in proto-devices using eutectic Ga–In as the top contact.

Full text of DOI:10.1002/anie.201807879

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Two-Terminal Molecular Memory via Reversible Switching of
Quantum Interference Features in Tunneling Junctions
Authors: Marco Carlotti, Saurabh Soni, Sumit Kumar, Yong Ai, Eric
Sauter, Michael Zharnikov, and Ryan C Chiechi
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10.1002/anie.201807879
Angewandte Chemie International Edition
Two-Terminal Molecular Memory via Reversible
Switching of Quantum Interference Features in
Tunneling Junctions
Marco Carlotti,†,‡ Saurabh Soni,†,‡ Sumit Kumar,†,‡ Yong Ai,†,‡ Eric Sauter,¶
Michael Zharnikov,¶ and Ryan C. Chiechi∗,†,‡
†Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands
‡Zernike Institute for Advanced Materials, Nijenborgh 4, 9747 AG Groningen, The
Netherlands
¶Applied Physical Chemistry, Heidelberg University, Im Neuenheier Feld 253, Heidelberg
69120, Germany
E-mail: r.c.chiechi@rug.nl
Abstract
This paper describes large-area molecular tunneling junctions comprising
self-assembled monolayers of redox-active molecules that exhibit two-terminal bias
switching. The as-prepared monolayers undergo partial charge-transfer to the
underlying metal substrate (Au, Pt or Ag) that converts their cores from a quinoid to
hydroquinoid form. The resulting rearomatization converts the bond topology from a
cross-conjugated to linearly-conjugated π system. The cross-conjugated form
correlates to appearance of an interference feature in the transmission spectrum that
vanishes in the linear-conjugated form. Due to the presence of electron-withdrawing
1
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10.1002/anie.201807879
Angewandte Chemie International Edition
nitrile groups, the reduction potential and the interference feature lie close the work
function and Fermi level of the metallic substrate. The relationship between
conjugation patterns and quantum interference is well-studied. We exploit this
relationship to create non-volatile memory in proto-devices using eutectic Ga-In as
the top-contact. The underlying switching mechanism reorders bond topology
without changing connectivity or altering the tunneling distance or thickness of the
monolayer. These results are a proof-of-concept for switching quantum interference
features on and off via in operando redox chemistry.
Keywords: EGaIn, Quantum Interference, Memory, Switching, Molecular
Electronics
2
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Quantum Interference (QI) is a collection of phenomena related to Fermions whose wave
functions can interfere with themselves; in molecular tunneling junctions, destructive QI
can lower the transmission probability between the electrodes, significantly lowering conduc-
tance without altering the tunneling distance.[1] Thus, compounds that produce destructive
QI could act as molecular switches, memory devices, or transistors.[2,3] Destructive QI effects
have been studied both theoretically and in multiple experimental platforms. In π-conjugated
molecules, they are generally ascribed to cross-conjugation,[4–7] meta-substitution,[8,9] or par-
ticular spatial arrangements.[10,11] Of particular interest are systems capable of toggling QI
effects via external inputs;[12–14] however, control over QI effects is currently limited to tran-
sient, single-molecule junctions and/or comparisons of different compounds in different en-
vironments,[15–17] for example, the ex operando (electro)chemical interconversion between a
cross-conjugated quinone and linearly-conjugated hydroquinone.[18,19] In this work we show
that self-assembled monolayers (SAMs) of a cross-conjugated compound incorporating a
tetracyanoquinodimethane (TCNQ) unit, TCNAQ (Figure 1), on different metal substrates
can be switched between, and addressed in, two conductance states (ON and OFF) in a two-
terminal proto-device using eutectic Ga-In (EGaIn) top-contacts. We ascribe the different
conductance states to the modulation of the bond topology of the molecule; TCNAQ—just
as TCNQ—can readily accept an electron (Figure S4) and form a stable (di)anion that con-
verts cross-conjugated pathways to linearly-conjugated pathways, altering the transmission
probability similarly to interconversion of quinones and hydroquinones (Figure 1).[20] A low-
lying LUMO brings the reduction potential of TCNAQ close to the oxidation potential of
Au, Ag and Pt electrodes, eliminating the need for a third electrode or redox agents.
3
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10.1002/anie.201807879
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B
A
Cross-conjugated
Quantum Interference
Linearly-conjugated
Quantum Interference
Figure 1: (A) A pure monolayer of TCNAQ in the cross-conjugated quinoid form is re-
versibly switched to a mixed-monolayer in which a fraction of the molecules are reduced
to a linearly-conjugated, hydroquinoid form; (B) Difference in conjugation pathways be-
tween the cross-conjugated neutral form (top), which give rise to destructive QI, and the
linearly-conjugated reduced form.
We prepared SAMs of TCNAQ on Au, Ag or Pt surfaces from the thioacetate precursor
via in situ deprotection.[21] X-ray photoelectron spectroscopy (XPS) spectra are consistent
with upright-standing molecules attached to the surface through a single thiolate bond (Fig-
ure 2). Synchrotron measurements provide additional evidence that the nitrile groups are
oriented predominantly parallel to the substrate (see Supporting Information for details).
The N 1s region of the XPS spectrum of the SAM features an additional peak at lower
binding energy (398.5 eV) that is not present in spectra of TCNAQ powder, which we as-
cribe to the spontaneous (partial) reduction of TCNAQ by the metal substrate. Similar
shifts are common in monolayers of TCNQ that are spontaneously reduced by the underly-
ing metal.[22] There, TCNQ is directly adsorbed to the metal substrate while, in SAMs of
TCNAQ, the redox-active core is bound through a phenylacetylene arm that is coupled to
the surface through a covalent S-Au bond. Thus, charge-transfer (redox) can still occur in
a geometry that is compatible with the formation of metal-molecule-metal junctions. In the
XPS spectra, about 14 % of TCNAQ molecules in the SAM are in a reduced state.
4
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10.1002/anie.201807879
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A
B
C
D
SAMs/ Au/mica
Figure 2: XPS spectra of N 1s and S 2p2/3 core levels for powder samples of TCNAQ (top)
and SAMs of TCNAQ on Au-on-mica (bottom). The peak at 398.5 eV, which is present only
in the monolayer, indicates the presence of a non-stoichiometric, reduced nitrogen-containing
species.
We investigated the electrical properties of TCNAQ in Au-on-mica/SAM//EGaIn junc-
tions (where ‘/’ and ‘//’ denoted covalent and Van der Waals interaction respectively).
EGaIn is a liquid metal that can be used to form stable, conformal, non-damaging contacts
with SAMs with a diameter of about 20 µm.[23–25] This methodology enables the formation
of junctions in multiple areas of a substrate rapidly and reproducibly, allowing the collec-
tion of statistically significant data and spectroscopic investigation of the SAM after J/V
cycling.[26] As controls, we measured junctions comprising hexadecanethiol (C16SH) and
analogs of TCNAQ bearing an anthraquinone core (AQ) or a linearly-conjugated, non-
redox-active anthracene core (AC) as shown in Figure S1.
Figure 3 shows forward and reverse J/V traces for junctions comprising TCNAQ (A),
AC (B), C16SH (C) and AQ (D). While the J/V traces of the latter three junctions are
perfectly overlapping, TCNAQ exhibits a hysteresis loop at negative bias; i.e., after being
biased at positive voltages, the conductance at negative bias decreases (OFF) and then
recovers its initial conductance (ON) after reaching −1.00 V. A maximum ratio of J between
5
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10.1002/anie.201807879
Angewandte Chemie International Edition
forward and reverse scan of 2.6 occurs at −0.65 V. As shown in Figure 4, the hysteresis
and magnitude of switching is reproducible across junctions comprising TCNAQ measured
on Au-on-mica and template-stripped[27] (TS) AuTS and AgTS. The effect is present but
diminished on PtTS (Figure S27-S28). No hysteresis or switching is present on any substrate
for AC, AQ or C16SH.
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
A
-3.5
-1.0
-0.5
0.0
0.5
1.0
-1.5
-2.0
-2.5
-3.0
-3.5
B
-4.0
-3.5
-1.0
-0.5
0.0
0.5
1.0
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
C
-1.0
-0.5
0.0
0.5
1.0
-2.0
-2.5
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
D
-1.0
-0.5
0.0
0.5
1.0
Potential (V)
Figure 3: Examples of log |J| vs. V of junctions comprising SAMs of TCNAQ on AuTS (A,
black), AC (B, blue), C16SH (C, red), AQ (on Au-on-mica, D, cyan). Solid dots represent
data acquired during five forward scans ranging from −1.00 V to 1.00 V (in 0.05 V steps,
acquired every 0.1 s), while open dots represent data acquired during five subsequent reverse
scans from 1.00 V to −1.00 V.
Given sufficient trace/re-retrace stability, hysteresis is a form of two-terminal bias-switching,[28–43]
but to translate it into a memory effect, the state of a static, two-terminal junction must
6
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10.1002/anie.201807879
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3.0
2.5
2.0
1.5
1.0
-1.0
-0.5
0.0
0.5
1.0
V
Figure 4: Plot of trace over retrace ratio for junctions comprising: (A) SAMs of TCNAQ
on AuTS (black), Au-on-mica (hollow), and AgTS (gray), C16SH on AgTS (red), AC on AuTS
(blue), AQ on Au-on- mica (cyan). Error bars represent confidence intervals with α = 0.05
(for TCNAQ on Au-on-mica they were omitted for clarity).
be switched reversibly between at least two conductance-states in operando. To characterize
the memory effects of Metal/SAM//EGaIn junctions, we performed Write operations (W)
by applying a −1.50 V bias, which puts the junction in the high-conductance ON state and
Erase operations (E) by applying 1.00 V, which puts it in the low-conductance OFF state.
We read the state at −0.50 V, measuring current-densities J of 0.10 Acm−2 to 0.01 Acm−2.
Figure 5A compares the resulting ON/OFF ratios for junctions comprising SAMs of TC-
NAQ, AC, AQ and C16SH on Au over four switching cycles. As expected, the ON/OFF
ratio for the controls (AC, AQ and C16SH) is 1, indicating no effect. However, TCNAQ
exhibits ratios as high as 6 ± 2. Figure 5A shows that the memory effect is identical for
Au-on-mica and AuTS, but the peak switching-ratio decreases and shifts to less negative
bias on AgTS. On PtTS, the peak decreases and shifts further and the hysteresis becomes
noticeable only over a larger bias window (see Supporting Information for details). This
trend is consistent with the proposed mechanism because the magnitude of the suppression
of conductance scales with the proximity of the destructive QI feature to the Fermi level.[44]
7
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A
B
8
8
6
4
2
0
6
4
2
0
1
2
3
4
1
2
3
4
5
10
15
20
Cycles
Cycles
Figure 5: ON/OFF Ratio per nth Write-Erase cycle, calculated as ratio of the current after
and before the nth Write operation for: (A) SAMs of TCNAQ on AuTS (black), Au-on-mica
(orange), and AgTS (gray), and (B) SAMs of C16SH on AgTS (red), AC on AuTS (blue), and
AQ on Au-on-mica (cyan). The error is calculated as confidence interval with α = 0.05.
As shown in Fig. 5, the ON/OFF ratio of TCNAQ slowly decreases, approaching 2
after about 10 cycles; however, even after 30 cycles, applying a −2.00 V bias restores the
initial ON/OFF ratio (Fig. S34). The conductance of TCNAQ decreases with each W-E
cycle (Table S3), which is typical with repeated switching cycles in molecular tunneling junc-
tions.[15] For junctions comprising TCNAQ, such damping could be the result of irreversible
reactions, structural modification of the SAMs or a change in the EGaIn contact. However,
because the conductance remains constant during J/V sweeps (Fig. 3, as opposed to R-E-W
cycles, i.e., Fig. 5), the underlying phenomenon is specific to large, rapid changes in bias.
In its pristine state, a destructive QI feature is clearly present in the transmission spec-
trum near Ef (Figure S44A).[45] In single-molecule STM break-junctions comprising TC-
NAQ, this QI feature manifests as a low conductance, comparable to that of AQ, which is
known to exhibit strong destructive QI effects in both single molecule and SAM-based junc-
tions (Figure S41).[6,46] However, ensemble junctions comprising SAMs of TCNAQ exhibit a
relatively high conductance, comparable to the linearly-conjugated analog AC (Figure S10),
which does not exhibit any QI features. We ascribe the difference between TCNAQ in SAMs
and in single-molecule junctions to the presence of reduced TCNAQ in the SAM (Figure
2C).[47] Tunneling charge-transport through SAMs is sensitive to the entire supramolecular
8
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10.1002/anie.201807879
Angewandte Chemie International Edition
structure of the monolayer, which comprises molecules in different conformations and, in
the case of TCNAQ, redox states.[11,48] Figure 1B shows the bond topology of TCNAQ
in the pristine and reduced states. The addition of one or two electrons converts the cross-
conjugated quinoid core into a linearly conjugated, fully aromatic hydroquinoid. (The driv-
ing force of rearomatization is the reason that TCNQ is an exceptional electron-acceptor.)
Treating each molecule in a SAM as a resistor in parallel, it follows from Kirchoff’s rules
that a small fraction of reduced TCNAQ molecules can dominate charge-transport through
the SAM due to the exponential difference in the conductance of TCNAQ in the cross-
and linearly-conjugated (quinoid/hydroquinoid) forms.[49] Specifically, if two pathways in a
SAM differ in conductance by two orders of magnitude (similar to AQ and AC at 0.50 V),
the presence of only 1 % to 2 % of the more conductive pathways is sufficient to dominate
the conductivity of the SAM.[50] Thus, the hysteresis and switching phenomenon are most
likely caused by a shift in the equilibrium between the low- and high-conductance states of
TCNAQ; applying a bias to the junction affects the fraction of molecules in the junction
that exhibit destructive QI.
If the proposed mechanism is correct, the ON state is metastable and should slowly relax
to lower conductance, since the thermodynamic minimum is the neutral, low-conductance
state. Indeed, the ON state current decreases in time with multiple read cycles, while
the OFF state only shows small, stochastic fluctuations, which are discussed further in the
Supporting Information. The initial ON/OFF ratio is restored after a new W cycle—i.e.,
applying a pulse at negative bias restores the SAM to the initial state in which a greater
fraction of TCNAQ is in the reduced state. We ascribe this observation to the slow kinetics
of the relaxation (re-oxidation) process. When a charge is placed in a SAM by the reduction
of a molecule, the local environment in the SAM reorganizes to minimize the free energy of
the system, which is a much slower process than the initial electron-transfer. After 5 min,
without any applied bias, the ON/OFF ratio decreases to 70 % of its initial value; after
20 min it decreases to 60 %.
9
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Metal/TCNAQ//EGaIn junctions are a form of non-volatile memory; their state is
retained when the power (bias) is removed. It is difficult to contextualize TCNAQ further.
There are examples of memory effects in molecular tunneling junctions, each demonstrating
a salient feature: some exhibit high switching ratios as single-molecules, but not in (proto-
)device platforms;[37,42] some require prohibitively complex fabrication;[35] some only switch
at low temperature;[28] some are resistant to fatigue when switched with light, but not with
bias.[51] In simple, two-terminal proto-devices, TCNAQ exhibits reasonably high ON/OFF
ratios that are stable for tens of minutes and that can be refreshed or re-written over at least
dozens of cycles. The switching mechanism is phenomenologically unique, exploiting the
coincidental alignment of a destructive QI feature and facile reduction with the Fermi level
and work function of Au to enable the shift of a dynamic equilibrium of molecules in high-
conductance states lacking QI features and low-conductance states with strong QI features
near the Fermi level. The switching effect is molecular in nature, and further investigation
and optimization can feasibly exploit this type of QI-based switching to achieve switching
ratios of orders of magnitude.
Acknowledgements
R.C.C. and F.A. acknowledge the European Research Council for the ERC Starting Grant
335473 (MOLECSYNCON). We thank the University of Groningen for access to the Pere-
grine Computing Cluster. M.Z. thanks the Helmholtz Zentrum Berlin for the allocation of
synchrotron radiation beamtime at BESSY II and A. Nefedov and Ch. W¨oll for the technical
cooperation during the experiments there; financial support of the German Research Society
(Deutsche Forschungsgemeinschaft; DFG) within the grant ZH 63/22-1 is appreciated.
10
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Graphical TOC Entry
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Table of Contents
A mixed-monolayer comprising cross- and linearly-
conjugated molecules exhibit high conductance.
When the linearly-conjugated species are reversibly
oxidized back to cross-conjugated species, destruc-
tive quantum interference is restored and the con-
ductance drops, causing a memory effect.
18
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