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\DOI{10.5802/crchim.344}
\datereceived{2024-09-10}
\daterevised{2024-09-16}
\dateaccepted{2024-09-17}
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\begin{document}
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\title{Unveiling polyoxometalate redox properties at the nanoscale}

\alttitle{R\'{e}v\'{e}ler les propri\'{e}t\'{e}s d'oxydor\'{e}duction
des polyoxom\'{e}talates \`{a} l'\'{e}chelle nanom\'{e}trique}

\author{\firstname{Florence} \lastname{Volatron}\CDRorcid{0000-0002-1009-0412}}
\address{Institut Parisien de Chimie Mol\'{e}culaire (IPCM), CNRS,
Sorbonne Universit\'{e}, 4 Place Jussieu, F-75005 Paris, France}

\author{\firstname{Guillaume} \lastname{Izzet}\CDRorcid{0000-0002-9849-4939}}
\addressSameAs{1}{Institut Parisien de Chimie Mol\'{e}culaire (IPCM),
CNRS, Sorbonne Universit\'{e}, 4 Place Jussieu, F-75005 Paris, France}

\author{\firstname{Dominique} \lastname{Vuillaume}\CDRorcid{0000-0002-3362-1669}}
\address{Institute for Electronics Microelectronics and Nanotechnology
(IEMN), CNRS, University of Lille, Av. Poincar\'{e}, Villeneuve d'Ascq,
France}

\author{\firstname{Anna} \lastname{~Proust}\CDRorcid{0000-0002-0903-6507}\IsCorresp}
\addressSameAs{1}{Institut Parisien de Chimie Mol\'{e}culaire (IPCM),
CNRS, Sorbonne Universit\'{e}, 4 Place Jussieu, F-75005 Paris, France}
\email[A. Proust]{anna.proust@sorbonne-universite.fr}

\keywords{\kwd{Polyoxometalates}\kwd{Molecular oxides}\kwd{Redox-active
molecules}\kwd{Surface
functionalization}\kwd{Nanojunctions}\kwd{Electron
transport}\kwd{Resistive switching devices}}

\altkeywords{\kwd{Polyoxom\'{e}tallates}\kwd{Oxydes
mol\'{e}culaires}\kwd{Mol\'{e}cules r\'{e}dox actives}
\kwd{Fonctionnalisation de
surface}\kwd{Nanojonctions}\kwd{Transport
d'\'{e}lectrons}\kwd{Dispositifs de commutation r\'{e}sistifs}}

\begin{abstract}
Polyoxometalates (POMs) are molecular nano-oxides of early
transition metals. As molecular oxides, they are endowed with
remarkable redox properties, combining the reducibility of bulk metal
oxides and the high versatility of molecular species. They undergo
successive, reversible, and highly adjustable mono- (or multi-)
electronic reduction processes within a narrow range of potentials.
Furthermore, POMs are polyanions, with counter cations playing a
crucial role, beyond ensuring charge neutrality. As the missing link
between extended oxides, commonly found in microelectronics, and
conventional organic or organometallic molecules, POMs have attracted
ever-increasing interest in the field of nanoelectronics. They hold
promise as charge storage nodes in multilevel nonvolatile memories and
resistive switching devices, areas of interest currently boosted by the
development of neuromorphic computing. In this context, we have been
exploring various strategies to immobilize POMs onto electrodes with
the aim of improving the control of the molecules/electrode interface.
We have been investigating electron transport properties across
POM-based molecular nanojunctions to establish relationships among
the POM molecular structure, the electronic structure, and the
properties of POM devices (e.g., conductance, switching).
Additionally, we have demonstrated that we can commute the redox state
of a POM layer by exposure to light or by applying an electric field,
opening up new opportunities to stimuli-responsive devices.
\end{abstract}

\begin{altabstract}
Les polyoxom\'{e}tallates (POMs) sont des nano-oxydes mol\'{e}culaires
de m\'{e}taux de transition de la gauche du tableau p\'{e}riodique. En
tant qu'oxydes mol\'{e}culaires, ils sont dot\'{e}s de
propri\'{e}t\'{e}s d'oxydo-r\'{e}duction remarquables, combinant la
r\'{e}ductibilit\'{e} des oxydes m\'{e}talliques \'{e}tendus et la
grande polyvalence des esp\`{e}ces mol\'{e}culaires. Ils pr\'{e}sentent
des processus de r\'{e}duction mono- (ou multi-) \'{e}lectroniques
successifs, r\'{e}versibles et grandement ajustables, dans une gamme
\'{e}troite de potentiels. En outre, les POM sont des polyanions, les
contre-cations jouant un r\^{o}le crucial, au-del\`{a} d'assurer
l'\'{e}lectroneutralit\'{e}. En tant que cha\^{i}non manquant entre les
oxydes \'{e}tendus, que l'on trouve couramment en
micro\'{e}lectronique, et les mol\'{e}cules organiques ou
organom\'{e}talliques conventionnelles, les POMs ont suscit\'{e} un
int\'{e}r\^{e}t sans cesse croissant dans le domaine de la
nano-\'{e}lectronique. Ils sont prometteurs en tant que n\oe{}uds de
stockage de charges dans les m\'{e}moires non volatiles \`{a} plusieurs
niveaux et les dispositifs de commutation r\'{e}sistifs, des domaines
d'int\'{e}r\^{e}t actuellement stimul\'{e}s par le d\'{e}veloppement de
l'informatique neuromorphique. Dans ce contexte, nous avons explor\'{e}
diverses strat\'{e}gies pour immobiliser les POMs sur des
\'{e}lectrodes, dans le but d'am\'{e}liorer le contr\^{o}le de
l'interface mol\'{e}cule/\'{e}lectrode. Nous avons \'{e}tudi\'{e} les
propri\'{e}t\'{e}s de transport d'\'{e}lectrons \`{a} travers des
nano-jonctions mol\'{e}culaires \`{a} base de POM afin d'\'{e}tablir
des relations entre la structure mol\'{e}culaire du POM, la structure
\'{e}lectronique et les propri\'{e}t\'{e}s des dispositifs POM (par
exemple, la conductance, la commutation). En outre, nous avons
d\'{e}montr\'{e} que nous pouvions commuter l'\'{e}tat redox d'une
couche de POM en \'{e}clairant ou en appliquant un champ
\'{e}lectrique, ce qui ouvre de nouvelles perspectives pour des
dispositifs sensibles aux stimuli ext\'{e}rieurs.
\end{altabstract}

\shortrunauthors

\maketitle

%{\vspace*{1.5pc}}

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\section*{Introduction}

Polyoxometalates (POMs) are molecular nano-oxides.
Heteropolyoxometalates follow the \mbox{general} formula
$[\mathrm{X}_{{x}}\mathrm{M}_{{m}}\mathrm{O}_{{p}}]^{{n}-}$,
where M stands for an early transition metal in its highest oxidation
state  ($\mathrm{W}^{\mathrm{VI}}$, $\mathrm{Mo}^{\mathrm{VI}}$, 
$\mathrm{V}^{\mathrm{V}}$) and X is a heteroatom from the p (Al, Si, P, As,
Sb{\,\ldots}) or d group, which is missing in
isopolyoxometalates~\cite{1}. Some representative examples include the
Keggin-type  [{PW}\tsub{12}{O}\tsub{40}]$^{3-}$ and Wells--Dawson-type
[{P}\tsub{2}{W}\tsub{18}{O}\tsub{62}]$^{6-}$ anions, based on which
we have devised organic--inorganic hybrids. As molecular oxides, POMs
are endowed with remarkable redox properties, combining the
reducibility of bulk metal oxides and the high versatility of molecular
species. They undergo successive and reversible mono- (or multi-)
electronic reduction processes within a narrow range of
potentials~\cite{2}. These properties are highly tunable/adaptable,
according to the POM structure, its primo functionalization (see
below)~\cite{3}, the nature of the metallic addenda, and the nature of
the counter cations~\cite{4}. In their reduced forms, POMs are commonly
designated as electron reservoirs, and this has led to many
applications in photo/electrocatalysis and energy
conversion/storage~\cite{5,6}. POMs are not only nano-oxides but also
polyanions, and this makes them unique: their counter cations have
important effects on various properties and play a crucial role beyond
ensuring charge neutrality~\cite{7}. While POMs are traditionally
prepared in aqueous solution as alkali or acid salts, their association
with organic cations, such as alkylammonium, imparts their solubility in
nonaqueous polar solvents. The cations drive  POM organization up
to the crystallization of POM-based architectures and materials. The
redox potentials depend on the counter cations, the solvent, and other
environmental effects. Small cations like Li$^{+}$ and H$^{+}$
allow to increase the charge stored in POMs~\cite{8}.  In the solid
state, the counter cations contribute to the adjustment of the energy
levels of  POMs relative to the Fermi energy of an electrode,
modulating the conductivity~\cite{9}. 

As the missing link between extended oxides, commonly found in
microelectronics, and conventional organic or organometallic
molecules, POMs have attracted an ever-increasing interest in the field
of nanoelectronics. They have been implemented in
current-rectification devices~\cite{10}, POM complexes of lanthanide
single-ion magnets have been investigated as molecular spin
qubits~\cite{11}, and they have been introduced as charge storage nodes
in hybrid molecular/semiconductor capacitors~\cite{12} and multilevel
nonvolatile memories~\cite{13}. Crossbar arrays incorporating POMs
embedded in polymer films or deposited onto graphene oxide have shown
resistive switching behavior between a high resistance (OFF) state and
a low resistance (ON) state~\cite{14}, leading to memristive devices for
neuromorphic computing~\cite{15}. Because they are molecules,
implementation of POMs in molecular materials or devices requires a
shaping process~\cite{16}. Depositing POMs onto electrodes is not an
issue by itself since they readily physisorb on many. Yet in the
context of molecular electronics, the control of their packing density
and of their distribution on a substrate is a real issue that can
hardly be tackled using physisorbed deposition. Avoiding the formation
of aggregates, ensuring the uniformity of the layer to improve the
properties and limit the variability from one device to the other is
highly advisable. In this context, we have been \mbox{exploring} various
strategies to immobilize POMs onto electrodes, with the aim of
improving the control of the molecules/electrode interface that
underlies the ultimate electrical properties and hence the device
performance (the interface is the device)~\cite{17}. We have been
investigating electron transport properties across POM-based molecular
nanojunctions to figure out how the redox properties known from the
solution state studies translate to the solid-state device.
Additionally, we have demonstrated that we can commute the redox state
of a POM layer by exposure to light or by applying an electric field. Our
systematic approach will be revealed in the following sections. 

\begin{figure*}
{\vspace*{-1pt}}
\includegraphics{fig01}
\caption{\label{fig1}(a) Polyhedral representation of
[H\tsub{7}{P}\tsub{8}{W}\tsub{48}{O}\tsub{184}]$^{33-}$ with
WO\tsub{6} and PO\tsub{4} centers depicted by blue and green octahedra
and tetrahedra, respectively; W atoms and P heteroatom located at the
center and O atoms located at the apex of the polyhedra. (b) Scheme
(not at scale) of the POM electrostatic deposition. (c) AFM image of the
${}^{\mathrm{TS}}{\mathrm{Au}}$ substrate after POM deposition with the corresponding
height profile (following the blue line). (d) PM-RAIRS spectra before
(black) and after (red) the POM deposition; the POM fingerprints are
displayed in the highlighted region. Reproduced from Ref.~\cite{22} with
permission from the Royal Society of Chemistry.} 
{\vspace*{-2pt}}
\end{figure*}

\section{POM-shaping process onto electrodes}\label{sec1}

As POMs are polyanions, their sublimation cannot be performed. Many
examples of POM deposition onto electrodes rather deal with counterion
exchange or entrapment in polymers. This is also at the basis of the
elaboration of layer-by-layer materials~\cite{18,19} with photo- or
electrochromic properties~\cite{20,21}. This strategy is easy to
implement and we have used it to deposit the ring-shaped
[H\tsub{7}{P}\tsub{8}{W}\tsub{48}{O}\tsub{184}]$^{33-}$ onto
ultraflat template-stripped gold surfaces (${}^{\mathrm{TS}}{\mathrm{Au}}$)
functionalized with a self-assembled monolayer (SAM) of
8-amino-1-octane thiol. This POM has been chosen for its high charge
storage capacity (up to three reduction processes in the solution
state, each corresponding to the injection of eight electrons). The
experimental conditions of the dip-coating, incubation time of the
${}^{\mathrm{TS}}{\mathrm{Au}}$ substrate in a POM solution, and rinsing steps have been
adapted to favor the formation of a uniform POM monolayer. The quality
of the organic SAM on top of the substrate is also crucial to avoid the
formation of aggregates. The successive steps of the deposition have
been monitored by different techniques: (i) ellipsometry, which gives
macroscopic thicknesses that should be consistent with the formation
of monolayers (a few nanometers); (ii) atomic force microscopy (AFM) to qualify
the homogeneity of the layers, their roughness, and spatial extension;
(iii) polarization modulated reflection absorption infrared
spectroscopy (PM-RAIRS) to recover the vibrational fingerprints of the
organic SAM and that of the POM (Figure~\ref{fig1}). The whole procedure has been extended
to the deposition onto a large array of single-crystal Au
nanodots~\cite{22}. Charge transport across the molecular nanojunctions
has been probed by conducting AFM and the energetics of the molecular
junctions discussed (as will be exemplified for another system below).

We have also immobilized POMs onto carbon nanotubes (CNTs) through
$\rmpi$--$\rmpi$ interactions. This was achieved with a pyrene
derivative of the Dawson anion, which was detected along the sidewalls
of the single-walled CNTs by high-angle annular dark-field
scanning-transmission electron microscopy~\cite{23}. This
pyrene derivative belongs to the large family of covalent
organic--inorganic polyoxometalate hybrids that we have significantly
contributed to enrich~\cite{3,24}. Our group has indeed devised
systematic routes to the functionalization and post-functionalization
of POMs, which give organically augmented POMs. We have particularly
supplied organo-phosphonyl, -silyl, and -tin (or -germyl) derivatives
featuring remote and reactive chemical groups. The
primo functionalization provides simple and universal platforms, for
example, bearing iodo-aryl groups, that can be further engaged to give
more complex and bespoke post-functionalized species~\cite{25,26}.
Synthesis and subsequent reactions are generally carried out in organic
solvents. In our case, the POM solubility is ensured by the use of
tetrabutylammonium cations (TBA ${=}$\ 
N(C\tsub{4}{H}\tsub{9})\tsub{4}\tsup{+}). 

These POM-based hybrids offer a powerful alternative strategy for the
covalent grafting of POMs onto electrodes or functional materials.
Covalent bonding between the POM and the substrate indeed brings a
higher control for the formation of a POM monolayer. Furthermore, extra
POMs that would electrostatically or physically adsorb will be
eliminated by thorough rinsing, possibly under sonication. 

Covalent immobilization of POMs can be achieved either directly, thanks
to the remote reactive group on the POM hybrid, or in two steps. The
latter case involves a reaction between a POM hybrid and a substrate
decorated with a SAM bearing a complementary reactive function. It has
been exemplified with the classical peptide bond formation \mbox{between}
\mbox{carboxylic} acid terminated SAM and amine or aniline terminated POM
hybrid~\cite{27}. We have thus reported POM nanostructured gold
substrates for biosensing~\cite{28,29}. However, it is often hindered
by a poor efficiency of the peptide bond coupling and related strong
discrepancies in POM surface densities from one experiment to
another~\cite{30}. This led us to favor the direct grafting of POM
hybrids onto electrodes. Fine molecular engineering at the electrode
interface is then attainable through the thoughtful choice of the
remote/terminal anchor on the POM hybrid. We have reported that remote
diazonium groups allow the POM grafting on a wide variety of
substrates, ranging from carbonaceous materials~\cite{31,32} to
hydrogenated silicon~\cite{33}, gold~\cite{34}, or oxides~\cite{35}.
Thiol, protected or not~\cite{36}, and disulfide~\cite{37} are suitable
for gold substrates~\cite{38} while carboxylic acid groups open the
route to POM immobilization onto oxides, silicon oxide~\cite{39},
and indium tin oxide (ITO)~\cite{35}. Furthermore, we will see below
that given the choice of the anchor, the primo functionalization of the
POM also imparts the charge transport. Finally, the nature of the
covalent bond between the anchor and the electrode also impacts the
properties of the materials~\cite{35}.

Molecular chemists have thus plenty of room here to express their
creativity for a fine-tuning of the resulting properties of the
targeted functional \mbox{materials.}

\section{Retrieving the POM signature from electron transport
measurements in large-area molecular junctions}\label{sec2}

Once the POM layer is fixed at the electrode, charge transport across the
layer has been assessed by closing the nanojunction either with a
Hg-drop or the tip of an AFM set-up, in a vertical two-electrode
configuration, akin to that used in metal/insulator/metal
capacitors or memristive cells. Our first objective was to unravel the
role of the POM in the charge transport.

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}(a) The spontaneous grafting of
diazonium terminated POMs onto hydrogenated n-Si(100).  (b)~Current
versus voltage curves recorded with a Hg-drop  electrode
(semilogarithmic plot) on a monolayer of
$[\mathrm{PW}_{11}\mathrm{O}_{39}\{\mathrm{Sn}(\mathrm{C}_{6}
\mathrm{H}_{4})\mathrm{C}{\equiv}\mathrm{C}(\mathrm{C}_{6}
\mathrm{H}_{4})\ndash\}]_{4-}$
($\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}-}$, left and 
${\mathbf{K}}^{\mathbf{Mo}}_{\mathbf{Sn}-}$, right).
(c) Energy diagram showing the position of the LUMO of the POMs
relative to the Fermi energy of the silicon electrode. (d)~Histograms
of the extracted energy barrier $\rmPhi$ for the
${\mathbf{K}}^{\mathbf{W}}_{\mathbf{Sn}-}$ and
${\mathbf{K}}^{\mathbf{Mo}}_{\mathbf{Sn}-}$ layers in
blue and red, respectively. Reproduced from Ref.~\cite{40} with permission from
the Royal Society of Chemistry.}
\end{figure*}

\subsection{Effect of the nature of the metal addenda Mo versus
W~\cite{40}}\label{ssec21}

We have reported the spontaneous dediazonation of diazonium terminated
POM hybrids onto hydrogenated n-Si(100). The modified electrodes have
been characterized by ellipsometry to support the formation of a
monolayer and to assess its accurate thickness and by AFM to check the
homogeneity of the surface coverage. Two similar POM hybrids of the
Keggin type have been used, the only difference being in the nature of
the metal addenda W or Mo:
$[\mathrm{PM}_{11}\mathrm{O}_{39}\{\mathrm{Sn}(\mathrm{C}_{6}\mathrm{H}_{4})
\mathrm{C}{\equiv}\mathrm{C}(\mathrm{C}_{6}\mathrm{H}_{4})
\mathrm{N}_{2}\}]^{3-}$ (M ${=}$\ W or Mo, 
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}}$
[$\mathbf{N}_{{2}}^{{+}}$] and 
$\mathbf{K}^{\mathbf{Mo}}_{\mathbf{Sn}}$
[$\mathbf{N}_{{2}}^{{+}}$], K for Keggin), prepared as
tetrabutylammonium salts~\cite{40}.  We had also previously shown that
the signature of the POM immobilized at the modified Si-electrode was
recovered in its electrochemical response. Integration of the first
wave of the cyclic voltammogram (CV), recorded in a solution of pure
electrolyte, gave a packing density of about $10^{-10}$
mol${\cdot}$cm$^{-2}$ ($6\times 10^{13}$ POM${\cdot}$cm$^{-2}$)~\cite{33}, 
very close to the calculated value for a densely packed monolayer and
akin to the value usually determined for other molecules or molecular
complexes assembled onto surfaces~\cite{41}. The kinetics of the
electron transfer from the electrode to the POM layer has been
determined by varying the scan rate of the CV recording and compared to
the rates obtained with other electrodes and under different
conditions~\cite{31,42,43}. The next step was to characterize the
electron transport properties at the solid state by contacting the
molecular layer with a Hg-drop, acting as a top electrode and thus
closing the molecular Hg//POM/Si nanojunction. Current {versus}
voltage (I/V) curves have been recorded on several locations of the
devices and 2D-current histograms have been drawn for both the 
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}-}$ and
$\mathbf{K}^{\mathbf{Mo}}_{\mathbf{Sn}-}$ layers
(about 75 I/V curves). The curves look quite similar, with a low
dispersion, reflecting the homogeneity of the grafting. There is more
rectification in the case of the molybdate layer, but it is not really
significant since the rectifying ratios are weak (I(1~V)/I(${-}$1~V) ${=}$\ 
2.5). In the case of highly doped Si, there is no Schottky barrier
related to the semiconductor and the electron transport thus operates
by tunneling through the molecular layer. According to the Simmons
model, the tunneling current depends on the thickness of the molecular
layer, which can be determined experimentally by ellipsometry, and on
an energy barrier $\rmPhi$, which 
has been extracted from the experimental data
by fitting each I/V curve with the full Simmons
equation~\cite{44,45,46}. In an energy level diagram, the $\rmPhi$ value
gives the position of the molecular orbital involved in the transport,
here the LUMO of the POM, relative to the Fermi energy of the
\mbox{electrode.} The $\rmPhi$ value \mbox{histograms} have been drawn 
for each POM (Figure~\ref{fig2}).
There is a clear offset towards higher values for the tungstate
monolayer, with mean $\rmPhi$ values of 1.8 and 1.6~eV, for M ${=}$ W and
M ${=}$ Mo, respectively. This means that the LUMO of
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}}$ lies at a higher energy
than that of 
$\mathbf{K}^{\mathbf{Mo}}_{\mathbf{Sn}}$, which is 
fully consistent with the relative electron-acceptor abilities of the
POMs in solution. The redox potentials associated to the first
reduction process observed on the CV of POM solutions in acetonitrile
are ${-}$1.0 V/SCE and ${-}$0.5 V/SCE for M ${=}$ W and M ${=}$ Mo, respectively
(determined on
$\mathbf{K}^{\mathbf{M}}_{\mathbf{Sn}}$[\textbf{I}]). In
solution, molybdates are \mbox{easier} to reduce than analogous
tungstates~\cite{42,47}. The lower (more negative) the redox potential
in solution, the higher the LUMO energy level in the solid state and
the lower the tunneling current. Above all, detecting a POM effect
demonstrates the involvement of the POMs in the electron transport
%\break
mechanism.

\subsection{Effect of POM functionalization~\cite{39}}\label{ssec22}

To enlarge the scope of our investigations, we have\unskip\break prepared two POM
hybrids featuring remote carboxylic acid groups suitable for
immobilization onto oxides:
$\mathrm{TBA}_{4.4}[\mathrm{PW}_{11}\mathrm{O}_{39}
\{\mathrm{Sn}(\mathrm{C}_{6}\mathrm{H}_{4})\mathrm{C}{\equiv}
\mathrm{C}(\mathrm{C}_{6}\mathrm{H}_{4})\ubreak\mathrm{COOH}_{0.6}\}]$
($\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}}[\mathrm{COOH}]$) and 
$\mathrm{TBA}_{3.4}[\mathrm{PW}_{11}\mathrm{O}_{39}\ubreak\{\mathrm{O}
(\mathrm{SiC}_{2}\mathrm{H}_{4}\mathrm{COOH}_{0.8})_{2}\}]$
($\mathbf{K}^{\mathbf{W}}_{\mathbf{Si}}[\mathrm{COOH}]$). 
Both belong to the Keggin type and are tungstates but differ in their
functionalization scheme: mono-tin versus bis-silyl functionalization.
They also differ in their total charge, 4$^{-}$ versus 3$^{-}$ for
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}}$[COOH] and
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Si}}$[COOH], respectively,
which implies that the former will be more difficult to reduce than the
latter (${-}$1.01 V/SCE and ${-}$0.38 V/SCE in acetonitrile solution for
the first reduction process of
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}}$[COOH] and
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Si}}$[COOH], respectively.
The lower the charge, the easier the electron transfer to the POM.)
Both have been deposited onto the native SiO\tsub{2} layer covering
silicon wafers. The tin derivative can stand upright or be tilted and the
bis-silyl derivative can graft through one or both anchors, resulting
in a less organized layer. The POM grafting mode has been revealed by
FT-IR spectroscopy using a low-doped float zone silicon substrate,
which is transparent to IR radiation. We have assigned the bands of the
asymmetric and symmetric stretching modes of the carboxylate groups for
both monolayers. As described in the literature, the separation between
both bands gives an insight into the coordination mode of the
\mbox{carboxylate.} We were able to exclude the monodentate binding mode, but
we were not able to discriminate between the bidentate bridging and
bidentate chelating modes. As said above, the electron-acceptor ability
of  $\mathbf{K}^{\mathbf{W}}_{\mathbf{Si}}$[COOH] is higher
than that of $\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}}$[COOH] in the
solution state. This is supported by theoretical calculations and the
relative energy position of the POM LUMOs. Electron transport
properties across the Si/SiO\tsub{2}/POM molecular junctions have been
probed at the solid state with a Hg-drop electrode and the \mbox{experimental}
current--voltage curves have been analyzed as in the previous example,
using the Simmons \mbox{electron} \mbox{tunneling} model 
(Figure~\ref{fig3}). In this case, the extracted
energy barrier $\rmPhi$ includes both the energy barrier of the native
SiO\tsub{2} layer and the energy of the POM LUMO, denoted by
$\varepsilon_{\mathrm{POM}}$ and estimated as 1.1 and 1.7~eV (with
respect to Si) for the
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Si}-}$ and
$\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}-}$ layers, respectively.
Albeit a quantitative agreement between the values obtained in the
solution and in the solid state or by computation is not
expected, the trends are similar. The assessed energy difference
between both POM LUMOs is remarkably consistent from one technique to
the other. This fully validates our approach. \looseness=-1

\begin{figure*}
\includegraphics{fig03}
{\vspace*{-3pt}}
\caption{\label{fig3}(a) Grafting modes of
$\mathrm{TBA}_{4.4}[\mathrm{PW}_{11}\mathrm{O}_{39}
\{\mathrm{Sn}(\mathrm{C}_{6}\mathrm{H}_{4})\mathrm{C}{\equiv}\mathrm{C}
(\mathrm{C}_{6}\mathrm{H}_{4})\mathrm{COOH}_{0.6}\}]$ 
($\mathbf{K}^{\mathbf{W}}_{\mathbf{Sn}}$[COOH], top) and 
$\mathrm{TBA}_{3.4}[\mathrm{PW}_{11}\mathrm{O}_{39}\{\mathrm{O}
(\mathrm{SiC}_{2}\mathrm{H}_{4}\mathrm{COOH}_{0.8})_{2}\}]$
($\mathbf{K}^{\mathbf{W}}_{\mathbf{Si}}$[COOH], bottom)  onto
SiO\tsub{2}. (b) LUMO energy levels assessed from cyclic voltammetry in
the solution phase (CV) by theoretical calculations (theory) and by
analysis of the experimental current/voltage curves obtained by the
contact of solid-state self-assembled molecular layers with a Hg-drop
electrode, all referred to vacuum. Reprinted with permission from 
Ref.~\cite{39}. Copyright 2020 American Chemical Society.}
{\vspace*{-2pt}}
\end{figure*}

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}(a) Polyhedral representation of the Preyssler
anion [Na${\subset}$P\tsub{5}W\tsub{30}O\tsub{110}]$^{14-}$. (b) Mean
I--V curves obtained on
${}^{\mathrm{TS}}$Au--S(CH\tsub{2})\tsub{6}{NH}\tsub{2}/POM//c-AFM  tip
nanojunctions (semilogarithmic plot). Effect of the counter cation
type on the POM LUMO energy level position $\varepsilon_{0}$ (c) and
$\rmGamma_{1}$ and $\rmGamma_{2}$ electrode coupling energies (d) obtained
by simulation of the experimental I/V curves using the single energy
level model. Reproduced from Ref.~\cite{48} with permission from the
Royal Society of Chemistry.}
\end{figure*}

\subsection{Counterion effects~\cite{48}}\label{ssec23}

Most of the redox-active molecules involved in molecular junctions and
resistive switching materials/devices are neutral~\cite{49}, such as
porphyrins, ferrocene~\cite{50}, tetrathiafulvalene derivatives,
oligothiophenes, and donor--acceptor systems~\cite{51}. POMs constitute a
notable exception, and the associated cations offer a powerful lever to
tune not only the POM redox properties but also the performance of the
whole device. The introduction of coordination complexes in molecular
junctions is less common, and the role of possible counterions is
generally not assessed~\cite{52}. Yet, the presence of ionic species
modifies the dielectric constant and the conductance by modulation of the
internal electric field~\cite{53}. In redox-based random access
memory, it has been proposed that when the molecular film
becomes charged upon switching/variation of the redox state of its
constituents, image/mirror charges form in the metal electrodes to
maintain charge neutrality~\cite{54}. From the Marcus theory, in wet
electrochemical environments, it is also known that the kinetics of the
electron transfer is governed not only by the {thermodynamics} and the
redox potential driving force but also by the underlying
reorganization, including counterion rearrangement where applicable.
{Accordingly}, the migration of the counterions in the molecular film
was proposed to play a key role and to control the current--voltage (I/V)
hysteresis {characteristics} of electrical bistability and hence of
binary information storage capability~\cite{54}. The nature of the
counterions, their size, polarizability, or {mobility}, for instance, is thus
an important \mbox{parameter} in the molecular switch design~\cite{55}, which
has recently attracted attention.

We have thus prepared the Preyssler anion,
[Na${\subset}$P\tsub{5}{W}\tsub{30}{O}\tsub{110}]$^{14-}$, with
K$^{+}$, H$^{+}$, NH\tsub{4}\tsup{+}, and TBA$^{+}$ counter
cations. The POMs have been electrostatically deposited onto ultraflat
template-stripped ${}^{\mathrm{TS}}{\mathrm{Au}}$ electrodes
pre-functionalized with a positively charged SAM of
6-amino-1-hexane-thiol and the junctions have been closed with the tip
of a c-AFM to record I/V curves (400--500 I/V traces have been recorded
for reliable statistics). The mean {I}--V curves are plotted in
Figure~\ref{fig4}. At high voltages, the curves look very similar.
Conversely, there is a clear discrimination at low voltages (${-}$0.5 to
0.5~V), the current gradually increasing by a factor of 100 in the
order K$^{+}$\ ${<}$\ NH\tsub{4}\tsup{+}\ ${<}$\ H$^{+}$\ $<$\ TBA$^{+}$,
which is not predictable. Indeed, we could have expected the possibility
of proton-coupled electron transport to enhance the conductivity, which
is apparently not the case. Although it is commonly admitted that cations
have an overall effect on the stabilization of the POM energy
levels~\cite{9}, it still remains difficult to predict their relative
effect on the transport properties. Analyzing the I/V curves with the
single energy level (SEL) model~\cite{56,57,58} (allowing to assess the
energy position of the molecular orbital involved in the electron
transport) gives the same order for the energy level positioning of the
LUMOs, $\varepsilon_{0}$, K$^{+}$\ ${<}$\ NH\tsub{4}\tsup{+}\ ${<}$\ 
H$^{+}$\ ${<}$\ TBA$^{+}$.  This is once again puzzling since the
higher the $\varepsilon_{0}$, the lower the expected current. Another
parameter, the electrode coupling energy $\rmGamma$ has to be taken
into account (Figure~\ref{fig4}d, one at each electrode). $\rmGamma$
reflects the hybridization between the molecular and electrode energy
levels. These $\rmGamma$ parameters are included in the SEL model and
have also been retrieved from the experimental data. Accordingly, the
hybridization is stronger for the TBA$^{+}$ cation, explaining the
overall current increase trend. A full understanding of these
experimental findings clearly requires a sound theoretical support.
Meanwhile, we hypothesized that it could be discussed in terms of local
dipoles at the molecules/electrode interface. Bulky TBA$^{+}$ cations
would lie mainly on top of the POM layer and build a large global
dipole at the interface, which would induce a large vacuum level shift
and then result in a large LUMO offset from the Fermi level. On the
contrary, smaller cations---more in interstitial sites would create
weaker dipoles---randomly distributed around the POMs result in a
weaker, even negligible, average dipole at the interface, a lower
vacuum level shift, and LUMO offset. In the solution state, counterions
mediate the intermolecular electron transfer between two
POMs~\cite{59}. A similar mechanism operating in the solid state would
enhance the POM--cation--electrode coupling and explain the large
coupling energy parameter $\rmGamma$ obtained for TBA$^{+}$ cations.
The hypothesis that the POM/electrode interaction depends on the nature
of the cations is substantiated by a recent report of scanning
tunneling microscopy images of individual POMs deposited onto
Au(111) either by drop-casting (with TBA$^{+}$ counter cations) or by
soft landing from the gas phase with an electrospray ionization source
(without any cation). The size of the electron cloud density has been
found larger than the POM geometric size, disclosing some hybridization
with the electrode, the effect being stronger in the presence of the
TBA$^{+}$ cations~\cite{60}. Finally, the modulation of the
experimental conductivities we have observed probably reflects the
competitive effects of the energy position of the LUMOs and the
electrode coupling energies. This highlights the role of the 
counterion-dependent dipole at the POM/electrode interface and in 
molecule/electrode hybridization, which deserves further experimental
investigations and extensive theoretical studies. 

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}(a) POM electrostatic deposition on top of an
8-amino-1-octane-thiol SAM on ${}^{\mathrm{TS}}$Au. (b)~Evolution of
the mean current $\log(\overline{I}|)$ at ${-}$1~V following alternating
irradiation sequences and return to resting state (irradiated states
labeled 2, 4, and 7). (c) 2D histograms of the full current--voltage
curves (semilogarithmic plot, mean $\overline{\mathrm{I}}$/V curve in
black) recorded at each step by c-AFM. Reproduced from Ref.~\cite{61} with
permission from the Royal Society of Chemistry.}
\end{figure*}

\section{Effect of the redox state and redox
photoswitching~\cite{61}}\label{sec3}

Because of their multiple and readily accessible redox states, POMs are
fascinating molecular species. Reduced POMs can be prepared chemically,
photochemically, in the presence of an electron donor, or
electrochemically. Comparing the conductivity of nanojunctions
implementing the same POM but with different redox states seemed a
relevant supplement to the scope of our investigations. Furthermore,
manipulating the redox state of the POMs within a pre-formed monolayer
would open the way to stimuli-responsive nanojunctions and would hence
reinforce their attractiveness. It has been reported that the
individual molecular redox states of a Lindqvist-type hexavanadate
physisorbed onto Au(111) can be addressed by applying an external
electric field. Single point scanning tunneling spectroscopy
data have shown that the injection of up to four electrons resulted in
a staircase increase in conductivity~\cite{62}. On the other hand,
POM photoreduction is a well-known phenomenon that has been harnessed
for the development of photochromic materials~\cite{63}. Indeed, the
injection of electrons in the POM framework and their delocalization
result in metal-centered d--d transitions and metal-to-metal intervalence
charge transfer  transitions that are responsible for the deep
blue color displayed by most reduced POMs. However, because these
electronic transitions have rather low intensities, POM-based
photochromic materials generally involve thick or multilayer materials
obtained by layer-by-layer deposition. We have thus decided to disclose
the photocommutation of the POM redox state within a monolayer through
a change in its conductivity. We have selected the Keggin-type
polyoxomolybdate (TBA)\tsub{3}[PMo\tsub{12}O\tsub{40}] owing to the
well-known ability of this compound to undergo photoreduction reaction
under UV-irradiation. It is available in its fully oxidized state,
referred to as POM(0), or in its one-electron reduced state, which can
be chemically prepared to provide authentic samples of
(TBA)\tsub{4}[PMo\tsub{12}{O}\tsub{40}] (POM(I)). In a control
experiment, we checked that thick films of POM(0), obtained by
drop-casting on a quartz slide, indeed turned blue under
UV-irradiation. In the second step and to validate our approach, we have
electrostatically deposited a monolayer of POM(0) or POM(I) on top of a
Au substrate decorated by a 6-amino-1-hexanethiol SAM. This is a rare
example  of a molecular SAM built with open shell molecules~\cite{64}.
The conductivity of the resulting nanojunctions
Au-S(CH\tsub{2})\tsub{6}-NH\tsub{2}/POM(0)//c-AFM tip and
Au-S(CH\tsub{2})\tsub{6}-NH\tsub{2}/POM(I)//c-AFM tip has been
measured, and the mean current/voltage curves clearly showed an increase
in conductance by a factor of about 10 upon reduction (Figure~\ref{fig5}). The
individual I/V curves have been fitted with a simple analytical 
SEL model (vide supra). A statistical distribution of
the energy level values of the molecular orbital involved in the
transport, with respect to the Fermi energy of the gold electrode, has
been obtained for both junctions. The increase in the current observed
for the POM(I) junction is thus proposed to be due to a lowering of the
energy position of the main molecular orbital. The electron transport
would thus be mediated by the LUMO (at approximately  0.6--0.7~eV above the Fermi
energy, $\varepsilon_{\mathrm{F}}$) in the case of POM(0) and the
SOMO (single occupied molecular orbital) in the case of the radical
POM(I) species, roughly aligned with $\varepsilon_{\mathrm{F}}$.
Possible involvement of the SUMO (single unoccupied molecular orbital,
estimated at approximately  0.3--0.4~eV above $\varepsilon_{\mathrm{F}}$) in the
latter case should be supported by theoretical calculations. We then
monitored the change in conductivity upon successively turning on and
off an in situ UV-irradiation of the POM(0) layer. We expected the
POM(0) layer to be reduced under light. After turning the light off, we
let it in air, either at room temperature or under a moderate heating,
to favor its reoxidation. The I/V curves were measured after each
step. We observed a clear increase in the conductance upon irradiation,
followed by a return to a state of lower conductance, similar to the
initial one, after turning the light off. This is consistent with the
behavior displayed by the monolayers of POM(0) and POM(I) just
described above and leads to the conclusion that we were able to
commute the redox state of the POMs at the monolayer\break scale. 

\section{Towards resistive switching devices}\label{sec4}

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig06}
{\vspace*{-2pt}}
\caption{\label{fig6}(a) Polyhedral representation of the
bifunctionalized 
$\mathrm{P}_{2}\mathrm{W}_{17}\mathrm{O}_{61}
\{(\mathrm{SiC}_{6}\mathrm{H}_{4}{\ndash}\mathrm{C}{\equiv}
\mathrm{C}{\ndash}C_{6}\mathrm{H}_{4}\mathrm{NH}_{2})_{2}\mathrm{O}\}]$
Dawson-type POM $\mathbf{D}^{\mathbf{W}}_{\mathbf{Si}}
[\mathbf{ArNH}_{\mathbf{2}}]$.  (b) Pictorial
illustration of the molecular film formed on ITO electrode.  (c) Cyclic
voltammogram of a 
$\mathbf{D}^{\mathbf{W}}_{\mathbf{Si}}[\mathbf{Ar}]$
molecular film in acetonitrile solution (in 0.1~M TBAPF\tsub{6},
0.3~V${\cdot}$s$^{-1}$). (d) Field emission scanning electron microscopy 
image of the
$\mathbf{D}^{\mathbf{W}}_{\mathbf{Si}}[\mathbf{Ar}]$
film formed after a quadruple grafting. (e) Representative
current/voltage characteristics of a
$\mathbf{D}^{\mathbf{W}}_{\mathbf{Si}}[\mathbf{Ar}]$
molecular film in the device configuration displayed on the left side, 
showing a WORM-type behavior. Reproduced from Ref.~\cite{67} with permission
from the Chinese Chemical Society (CCS), Institute of Chemistry of 
Chinese Academy of Sciences (IC), and the Royal Society of Chemistry.}
{\vspace*{-3pt}}
\end{figure*}

In the studies previously described, a narrow bias window was
deliberately chosen to protect the thin POM monolayer from any damage.
In these conditions, we were not able to observe any resistive
switching. In the case of bulk oxides, it is commonly acknowledged that
the resistive switching mechanism is driven by the oxygen vacancies'
concentration and mobility~\cite{65}, with the formation and disruption
of conductive filaments. To emulate oxides and to allow the creation of
conductive channels and gradient of charges, thick POM layers might be
decisive. This prompted us to reconsider the use of POM functionalized
with remote diazonium groups. Indeed, it is well known that upon
electrochemical reduction, diazonium salts are prone to form
multilayers covalently bonded to electrodes~\cite{66}. The deposition
of POM monolayers from $[\mathrm{PM}_{11}\mathrm{O}_{39}
\{\mathrm{Sn}(\mathrm{C}_{6}\mathrm{H}_{4})
\mathrm{C}{\equiv}\mathrm{C}(\mathrm{C}_{6}\mathrm{H}_{4})
\mathrm{N}_{2}\}]^{3-}$ 
$\mathbf{K}^{\mathbf{M}}_{\mathbf{Sn}}
[\mathbf{N}_{\mathbf{2}}^{\mathbf{+}}]$
was proposed to be due to some steric hindrance brought about by the
POMs~\cite{31}.  To bypass it, our strategy was to introduce two
organic extensions. We have thus prepared
$(\mathrm{TBA})_{6}[\mathrm{P}_{2}\mathrm{W}_{17}\mathrm{O}_{61}\ubreak
\{(\mathrm{SiC}_{6}\mathrm{H}_{4}{\ndash}\mathrm{C}{\equiv}\mathrm{C}
{\ndash}\mathrm{C}_{6}\mathrm{H}_{4}\mathrm{NH}_{2})_{2}\mathrm{O}\}]$
$\mathbf{D}^{\mathbf{W}}_{\mathbf{Si}}[\mathbf{Ar}
\mathbf{N}\mathbf{H}_{\mathbf{2}}]$
with two remote aniline groups that have been transformed in
situ into diazonium by the addition of \textit{ter-}butyl-nitrite
\textit{t}-BuONO~\cite{67}.  After electrochemical activation, the
resulting radicals were expected to either attach to the electrode or
attack a neighboring aromatic cycle, with or without N\tsub{2}
release. This would result in a robust covalent
$\mathbf{D}_{\mathbf{Si}}[\mathbf{Ar}]$ molecular
film, thicker than a monolayer, albeit with poor organization but 
POM-to-POM communication potentially higher than in drop-casted
materials or in polymer embedding POMs. Indeed, molecular films have
been processed onto ITO from acetonitrile solution
and thoroughly washed under sonication. The CV
recorded at the modified electrode confirms the presence of the POMs
confined at the electrode, and an apparent packing density $\rmGamma$ was
extracted. The grafting procedure has also been repeated on the same
substrate several times to deposit more POMs, and it has been referred
to as monografting, double grafting, or quadruple grafting. The $\rmGamma$ values obtained by
integration of the first reduction wave on the CVs consistently
increase but remain low. They reflect the number of electroactive POMs
in the films and can be viewed as an indicator of the efficiency of the
grafting, but they cannot be directly correlated to the thickness of the
film. To assess the film thicknesses, we performed field emission
scanning electron microscopy, which confirmed the formation of
10--30 nm thick films on the ITO electrode. The thicknesses varied
significantly from one substrate to another, which is not surprising
given the experimental protocol but is a serious downside for
reproducibility. Even if the $\rmGamma$ values could not be directly
correlated to the thicknesses, a rough trend emerged: the higher the
$\rmGamma$, the higher the average thickness. A possible explanation for
the underestimation of $\rmGamma$ from the electrochemical studies is the
low mobility of the tetrabutylammonium cations inside the films. These
have been introduced as the POM charge balancing counterions, but they
were also present in the electrolyte. Using LiBF\tsub{4}, with the
smaller lithium cation, instead of TBAPF\tsub{6} as the electrolyte, both
for the grafting step and the modified electrode characterization,
 yielded a fourfold increase in  $\rmGamma$ value. Gold
electrodes were then deposited on top of the POM molecular films
by chemical evaporation to perform resistive memory tests. At 0~V, the
two-terminal device is in the OFF state (high resistance, low
conductance). When a negative bias was applied, the current slightly
and then abruptly increased at a threshold voltage between 
${-}$1 and ${-}$1.5~V to reach the ON state (low resistance, high
conductance). This corresponds to the writing process. An inverse
voltage was then \mbox{applied} from ${-}$2 V to 0~V following which the device
remained in the ON state, showing  nonvolatile behavior on removal
of the electric filed. However, the ON state could not be  erased
during further sweeping to ${+}$5~V, suggesting charge trapping and a
write once read many times (WORM)-type memory (Figure~\ref{fig6}). 
The SET voltage
variation on a given substrate and from one substrate to another were
quite important, probably in relation to the poor organization inside
the film. Charge trapping was confirmed by capacitance measurements.
The capacitance value indicated that less than 1\% of the estimated
electroactive POMs were involved. It is noteworthy that a reversible, rewritable
memory behavior (also referred to as a flash memory) was observed in
about 5\% of the tests, possibly ascribed to thinner films. Finally, it
is known from the literature that in a given family of molecules, the
manipulation of the electronic structure can change the electrical
behavior from a WORM-type to a flash-type memory~\cite{50,68}. There is
thus plenty of room to play with the molecular diversity of POMs and
their counter cations, the modularity of their redox properties, and the
morphology of the POM films to (i) tune the SET voltages and (ii)
facilitate charge detrapping on application of a reverse voltage.

Oxide-based valence change memories play an important part in the
development of memristive technologies for data storage, brain-inspired
neuromorphic computing, data security systems, and mobile
communications~\cite{69}. Crossbar arrays of substoichiometric
tungsten oxide $\mathrm{WO}_{3-x}$ memristors, viewed as artificial
synapses, have been investigated as reservoir computing (RC) featuring
internal dynamics and readout functions trained for handwritten digit
or speech recognition as well as other classification and forecasting
tasks~\cite{70}. Reservoir computing thus mimics the variability of
neural \mbox{connections} in the brain. The key features to implement RC are
variability, strong nonlinearity, and complex dynamic interactions.
These have also been observed in self-assembled networks of molecular
junctions, such as those built on azobenzene switches interconnected
by Au nanoparticles and deposited on a several-electrode
pattern~\cite{71}. The full potential of POMs for in-memory
computing~\cite{72} and RC~\cite{73} has not yet been realized. To
circumvent the intrinsically low in-plane conductivity of POM films, we
have combined $[\mathrm{PW}_{11}\mathrm{O}_{39}\{(\mathrm{SiC}_{3}
\mathrm{H}_{6}\mathrm{SH})_{2}\mathrm{O}\}]^{3-}$ POMs with Au
nanoparticles acting as electrode relays. We have shown that the resulting
POM/nanoparticle networks deposited on a six-electrode pattern display
current variability and higher harmonic generation ability. Nonlinear
interactions are also sources of low-frequency noise, which has been
proposed to be indicative of RC performance. The 2D
nanoparticle/molecule networks with the POMs displayed the highest
low-frequency source noise among various tested molecules. The random
telegram signal type noise  detected has been ascribed to electron
trapping/detrapping by the POMs~\cite{74,75}. Currently, work is
undergoing to improve POM-to-POM communication and  in-plane
conductivity of the molecular films. This will enlarge the scope of
applications of POMs.\looseness=-1

\section{Conclusion and outlook}\label{sec5}

Scaling down the size of components to increase the memory density of
electronic devices is no longer the main issue. The More-than-Moore
applications include the integration of new functionalities in
innovative/unconventional materials and a paradigm shift towards
in-memory computing that mimics brain function. In this context,
molecules  still have their rightful place because they represent a unique
combination of atomic structural precision, reproducibility, high
tunability, and solution processability. Among molecules, POMs hold
great promise because of their molecular diversity and the redox
properties they share with oxides. This has prompted us to engage in a
long-term project to underpin their potential in nanoelectronics. We
have favored a step-by-step fundamental approach, and we have shown
how to control the formation of densely packed POM monolayers on
various types of substrates. We have investigated electron transfer
kinetics to the POM monolayer and electron transport across the POM
monolayers, revealing the POM signatures. We have also exemplified that
POMs are stimuli-responsive molecules responding to light or to the
application of an electric field. At this point, our \mbox{experimental-based}
energy level diagrams outlining possible conduction mechanisms need to
be supported by theoretical calculations to move ahead. As POM
assemblies also involve their counter cations, the existence of
electric dipoles and their possible reorganization or reorientation under
an electrical bias or on POM charge trapping/detrapping and/or
cation migration should be taken into account to gain a better understanding
of the underlying mechanisms~\cite{76}. Polyanions thus offer a unique
opportunity to investigate the effect of counterions, which has seldom
and only recently been reported for other molecular junction/systems.
This is a powerful lever that we will investigate further.
Interestingly, artificial synapses based on peptide molecular junctions
have been shown to be sensitive to cation flux, mimicking the
behavior of Ca$^{2+}$ or K$^{+}$ in biosynapses. This has been
attributed to the influence of cations on the electronic states of
 peptide molecules~\cite{77}. 

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

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