Addressing Multiple Resistive States of Polyoxovanadates: Conductivity as a Function of Individual Molecular Redox States

Article abstract of DOI:10.1021/jacs.8b08780

The sustainable development of IT-systems requires a quest for novel concepts to address further miniaturization, performance improvement, and energy efficiency of devices. The realization of these goals cannot be achieved without an appropriate functiona

Full text of DOI:10.1021/jacs.8b08780

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Article
Addressing Multiple Resistive States of Polyoxovanadates:
Conductivity as a Function of Individual Molecular Redox States
Oliver Linnenberg, Marco Moors, Almudena Notario-Estévez, Xavier López,
Coen de Graaf, Sophia Peter, Christoph Bäumer, Rainer Waser, and Kirill Yu. Monakhov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08780 • Publication Date (Web): 12 Nov 2018
Downloaded from http://pubs.acs.org on November 13, 2018
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Addressing Multiple Resistive States of Polyoxovanadates:
Conductivity as a Function of Individual Molecular Redox
States
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Oliver Linnenberg,1 Marco Moors,2 Almudena Notario-Estévez,3 Xavier López,3
Coen de Graaf,3,4 Sophia Peter,1 Christoph Bäumer,2 Rainer Waser,2,5 and
Kirill Yu. Monakhov1,‡,*
1
Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074
Aachen, Germany
2 Peter Grünberg Institute, JARA-FIT, Forschungszentrum Jülich GmbH, Wilhelm-Johnen-
Straße, 52425 Jülich, Germany
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Universitat Rovira i Virgili, Departament de Química Física i Inorgànica, c/Marcel·lí
Domingo 1, 43007 Tarragona, Spain
4 ICREA, Passeig Lluís Companys 23, 08010, Barcelona, Spain
5
Institut für Werkstoffe der Elektrotechnik 2, RWTH Aachen University 52056 Aachen,
Germany
‡ Author present address: Leibniz-Institute of Surface Engineering (IOM), Permoserstr. 15,
04318 Leipzig, Germany
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ABSTRACT: The sustainable development of IT-systems requires a quest for novel
concepts to address further miniaturization, performance improvement and energy
efficiency of devices. The realization of these goals cannot be achieved without an
appropriate functional material. Herein, we target the technologically important electron
modification using single polyoxometalate (POM) molecules envisaged as smart
successors of materials that are implemented in today’s complementary metal-oxide-
semiconductor (CMOS) technology. Lindqvist-type POMs were physisorbed on the
Au(111) surface, preserving their structural and electronic characteristics. By applying an
external voltage at room temperature, the valence state of the single POM molecule could
be changed multiple times through the injection of up to 4 electrons. The molecular
electrical conductivity is dependent on the number of vanadium 3d electrons, resulting in
several discrete conduction states with increasing conductivity. This fundamentally
important finding illustrates the far-reaching opportunities for POM molecules in the area of
multiple-state resistive (memristive) switching.
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INTRODUCTION
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Memory elements based on reversible switching of the internal electrical resistance as a
function of an external voltage and current are considered as a serious competitor for the
actually dominating charge based memories such as Flash and DRAM1,2. The simple
“electrode/insulator/electrode”-cell setup of these resistive switching memories offers a
typical low power consumption, high scalability and low switching time in combination with
CMOS compatibility, which makes them very appealing for the usage in the next-
generation information technology, including neuromorphic computing3. However, when
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exploiting common transition-metal-oxide-layers (e.g. TiO2 , HfO2 or SrTiO3 ) as switching
units with dimensions significantly lower than the actual possible limit of 10 nm, growing
technical issues emerge due to quantum physical effects that can only be eased by
tremendous efforts in case of big industrial production lines. Elaboration of chemical and
physical approaches at the level of molecules can therefore help reaching the ultimate
scalability, also granting access to not-yet-developed functionalities.
Metal-oxo clusters (so-called POMs7,8,9,10) and their inorganic–organic hybrid
derivatives11 meet in principle the requirements necessary for a possible implementation12
in resistive switching devices. The as-yet-undeveloped utilization of single, spatially clearly
defined POM molecules13 remains, however, the key challenge to achieving the
fundamental breakthrough in the resistive memory-based computer technology. It allows
first to circumvent the problem of <10 nm electron tunneling effects known from
microelectronics, i.e. to transit from top down to bottom up approach and second to
implement discrete energy levels of POMs instead of the band structure of a dielectric
solid-state material. These points are beneficial for developments in the area of
neuromorphic computing where the fundamentally important memristive functionality
operates with multiple resistive states (MRS) that are manifested by changes in the many
stable, metal-centered redox states. Due to the unique interplay between the molecular
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charge, the redox properties (associated to VIV/V) and the spin degrees of freedom
(existence of {VIVO}2+ with spin-½), polyoxovanadates14,15 — a subfamily of POMs — have
emerged as the most promising prototypes for molecular memory devices. Furthermore,
some polyoxovanadate structures such as Lindqvist-type hexavanadates16,17 allow altering
these characteristics without significant geometric perturbations18.
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Herein, the resistive switching cell is represented by embedding fully-oxidized
hexavanadate molecules into a model setup, in which the bottom electrode is represented
by the Au(111) surface and the top electrode is the platinum tip of a scanning tunneling
microscope (STM). Although STM in resistive switching studies is a rarely used technique,
it has already been successfully applied both for the mechanistic elaboration of classical
thin film systems19,20 as well as for new approaches such as the atomic switch21 and
superionic conductive solids22. The choice of the Lindqvist-type hexavanadate was also
prompted by its low molecular charge (2–), solubility in organic solvents and
thermodynamic stability in air that is all precious for immobilization procedures on
electrode surfaces. Especially, the charge of this POM enables to circumvent the highly
relevant problem of agglomeration illustrated e.g. for the [VIV10VV O42(I)]5– polyoxoanion23
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on Au(111). Using different experimental and computational techniques, we show that the
individual hexavanadate molecule displays MRS on Au(111) under unprecedented
conditions, room temperature and low operating voltage. This is so far a remarkable
observation at the level of single molecules and thus a promising way to establish a new
kind of molecular resistive switching cells.
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RESULTS AND DISCUSSION
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Materials Design. The target hexavanadates [V6O13{(OCH2)3CCHR’SR}2]2– (1–7) were
prepared from pentaerythrithol-based ligands that were augmented with thioether –SR
groups at the molecular periphery (R = Me (L1, L2), Pr (L3), C5H9 (L4), Ph (L5), 2-C10H7
(L6), CF3 (L7), R’ = H (L1, L3–L7), Me (L2)) to provide a certain degree of stabilization on
gold surfaces. The molecular structures of 1–3 and 5–7 were revealed by single-crystal X-
ray diffraction and display the hexavanadate core capped by two (tris)alkoxo ligands in a
transoid (anti-) fashion, as illustrated in Figure 1 (see Supporting Information for details of
synthesis and X-ray diffraction structural analysis). The bond valence sum analysis and
51V-NMR spectroscopy (signal at ca. –495 ppm24) show that the hexavanadate unit in
these compounds is fully oxidized ({VIV VV }).
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Figure 1. Conceptual chart and absorption/photoemission spectra.
a
Schematic
representation of hexavanadate single-molecule 5 adsorbed on Au(111) with the above
spotted STM tip. The DFT geometry optimizations of 5 on Au(111) give the Au–S distances of
3.36/4.14 Å (face) and 4.80/5.03 Å (edge). The phenyl rings in DFT-optimized 5 cap the gold
surface with Au···Ph contacts of ca. 3.72/4.89 Å (face) and ca. 4.04/5.14 Å (edge). The
Au···Ph contacts are expected to provide additional stabilization as implied by the IRRAS
data. b,c IRRAS (b) and IR spectra (c) of 5 with correlated stretching vibrations. d,e,f,g XPS
spectra of 1 (d: V 2p; f: S 2p) and 5 (e: V 2p; g: S 2p) on Au(111).
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Proof-Of-Concept Adsorption Properties. The infrared reflection-absorption
spectroscopy (IRRAS) was first used to assess the propensity of thermally stable 1–7 to
retain the single-molecule structural integrity when adsorbed on the gold surface (see
Supporting Information for details). The IRRAS spectra exhibit typical (tris)alkoxo (ca.
1110, 1050 cm–1) and terminal / bridging V–O (ca. 950, 810–790 cm–1) stretching
vibrations of the hexavanadate cage which are, in comparison with the polycrystalline bulk
materials, not significantly influenced by adsorption of 1–7 on Au (Figure 1). A weakening
of the arylic C–H stretching vibrations in the 3080–3010 cm–1 region, as observed for 5,
points out that the phenyl rings are planarly-oriented to the Au surface due to dispersive
interactions. From these data it can be concluded that the hexavanadate structures remain
remarkably unaffected by the contact with the metallic surface.
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To shed light on their adsorption geometries and the corresponding energetics, density
functional theory (DFT) calculations were performed for two representative cases of 1–7,
that is for 1 (R = Me) and 5 (R = Ph). The DFT geometry optimizations indicate that the
edge-orientation is slightly more favorable than the face-orientation for 1 and 5 by 0.15 eV
and 0.23 eV, respectively (see Supporting Information for details). In line with the
calculations of the Keggin-type polyoxoanion [SiW12O40]4− on Ag(111)25 and Ag(100)26, the
main properties of 1 and 5 are neither strongly impacted by the adsorption site nor by
orientation of the hexavanadate structure with respect to the surface.
Molecular Valence State and Adsorption Type On the Surface. Further insights into the
immobilization and electronic properties of 1 and 5 on Au(111) were gained by X-ray
photoelectron spectroscopy (XPS) and STM (see Supporting Information for details). The
XPS measurements (Figure 1) were performed on samples with a low submonolayer
coverage exhibiting isolated single molecules, as evidenced by STM. For 1 and 5, only the
electron population {VIV VV } could be detected (1: 98 %, 5: 99 %), which is in accordance
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with the fully-oxidized nature of these compounds in solution and in the solid state. The
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DFT results also indicate that the chemical integrity of the hexavanadate dianion is
preserved in its adsorbed form. The Bader analysis of the adsorbed molecules assigns a
charge of –1.9 to the {V6} unit and the projected density of states (PDOS) of 1 and 5 also
reveal only minor changes in the electronic structure when comparing the isolated
molecules to their face- and edge-oriented forms. The PDOS of the POM–Au hybrid
system is nearly independent of the adsorption mode.
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Fitting of the S 2p peaks27,28 in the XPS spectra suggests that for 1 and 5, the sulfur
atoms of the (tris)alkoxo ligands are predominately oriented to the gold surface (EB = ca.
163.3 eV). Signals attributed to non-coordinated (free) thioethers (ca. 164.7 eV) or
covalently grafted sulfur species as a consequence of a homolytic C–S bond cleavage (ca.
161.7 eV) were not or only marginally observed (< 5%). These XPS data provide evidence
that the hexavanadate molecules are weakly physisorbed, fully oxidized and lay flat on
Au(111).
Single-Molecule Imaging. Next, high resolution STM images of 1 (R = Me) and 5 (R =
Ph) on Au(111) were recorded. From the deposition of a 7.5·10–3 mM acetonitrile solution
on the substrate it can be concluded that only a submonolayer coverage with discrete
single molecules was attained. The measured three-dimensional electron densities (EBIAS
= 1.00 V, p = 1·10–9 mbar) of the discrete hexavanadates show a homogeneous
distribution with no long-range ordering effects. The mean lateral dimensions of 1 (4.3±0.3
x 3.8±0.3 nm) differ significantly from those of 5 (5.5±0.4 x 4.9±0.5 nm) with the higher
steric nature. The measured height of the deposited hexavanadates (0.80±0.05 nm) is in
good agreement with the measured edge-to-edge electron-density (ca. 0.8 nm) of the
polyoxoanion cage in crystal structures of 1 and 5 as well as with the calculated height (ca.
0.76 nm) of the energetically favored edge-orientation indicated by DFT. Notably, both
POMs are characterized by a continuous parabola- (and not “hilly”-like) height profile and
estimated ellipsoid dimensions (Figure 2). In consistency with a study of POMs on highly
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ordered pyrolytic graphite (HOPG)29, the length and width of 1 and 5 in the STM images is
significantly greater than the measured electron density of their crystal structures. This is
due to the pronounced overestimation30 of charged molecules on a polarizable surface31,32
by the STM tip. Here, the repulsive interactions between the negatively charged tip and the
polyoxoanion with its enlarged electronic density extension provoke van der Waals
interactions of the single molecules with the substrate. These polarization and charge-
balancing effects propagate parallel to the surface with an insignificant influence on the
molecule height and they are only present directly around the charged adsorbate. This is
also supported by the fact that the physical dimensions of the immobilized hexavanadates
are independent of their concentration (7.5·10–2 to 7.5·10–3 mM) and the deposition
procedure (drop- or dip-coating). As a result, both investigated 1 and 5 are discretely
immobilized on Au(111) as single molecules.
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Figure 2. Immobilized single molecules and conductivity response. a,b,c,d STM images
of drop-coated (7.5·10–3 mM) 1 (a) and 5 (c) recorded on Au(111) with EBIAS = 1.0 V (Iset
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100 pA) at room temperature and the corresponding line profiles for 1 b) and 5 (d). e,f,g
Experimental single-point STS data of single scans (gray) of 1 (e) and 5 (f) at room
temperature and of 5 (g) at 100 K on Au(111), shown with the corresponding electron
population for each transition; fitted data (dose-response function) are indicated in red. f,h
The dI/dE of the I–V curve (h) is depicted for fitted data of 5 (f).
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Molecular Current–Voltage Characteristics. The stable scanning with positive bias
voltage and the convenient STM depiction of single hexavanadate molecules of 1 and 5
allowed us to assess their current–voltage (I–V) characteristics via single-point scanning
tunnelling spectroscopy (STS) at room temperature (~ 295 K), which is a favored operating
condition in the field of molecular electronics and spintronics33. In addition, the I–V curves
at 100 K were acquired for compound 5 with improved stabilization attributes on Au(111).
The substrates with immobilized 1 and 5 were scanned continuously by employing the
STM measurement parameters (EBIAS = 1.0 V, Iset = 100 pA, scanning range: 100x100 nm)
to localize the deposited single molecules. The single-point I–V measurements were
performed during the ongoing scanning procedure by spotting the STM tip above one of
the heretofore localized single molecules. It was found that the best I–V curves could be
recorded for a positive bias potential in the range of 0.0–2.0 V with a linear scan rate of
dE/dt = 1.0 V·s–1. At 100 K the tip positioning of 5 became more fragile due to a setup-
specific temperature gradient between the tip and the cooled sample. To clarify the
spectra, the I–V data were fitted by using a dose-response function for compounds 1 and
5. In contrast to classical step functions, this function type clearly represents the non-
infinite slopes and the horizontal parts of all steps. The I–V curves presented in Figure 2
show a Coulomb staircase-like potential-dependent increase of the current with clearly
distinct current steps for 1 at 0.66, 1.08 and 1.48 V and for 5 at E = 0.61, 1.09 and 1.55 V.
Further implied steps are observed at ca. 1.90 and 1.97 V for 1 and 5, respectively. The
reduction potentials of 5 are only slightly shifted (E = 0.57, 0.97, 1.33 and 1.86 V) upon
lowering the temperature (100 K).
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To elucidate the distinct resistive states, the deviations of the fitted I–V data (dE/dI)
were calculated, resulting in four distinct electron transitions for 1 and 5. As 1 and 5 remain
fully oxidized when adsorbed on gold, each step in the recorded I–V curves can be
attributed to a single-electron reduction of the hexavanadate core, that is {VIV VV } 
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{VIV VV }  {VIV VV }  etc. Substantial statements about the retention time of the
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simulated switching cells cannot be given because the direct contact of the POM with the
metallic substrate prevents the stabilization of the tip induced charge within an accessible
time scale.
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Figure 3. Projected density of states (PDOS). a PDOS of isolated molecules 1 and 5. b
PDOS of molecules 1 and 5 adsorbed on Au(111) in the face- and edge-orientation. c 1-, 2-
and 3-electron reduced molecule 5 adsorbed in the edge-orientation. d Adsorbed molecule 5
with an applied field of Eext = 0.5 eV Å–1. All energies refer to the Fermi level. The highest
occupied band, located between −1.0 and 0.0 eV below the Fermi level (EF), corresponds to
S and C atoms of the ligands, whereas the polyoxovanadate (POV) oxo band (red line) lies
lower in energy, E < −1.0 eV. Empty d-vanadium bands (blue line), essential for the
molecular conductivity, appear slightly above EF.
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Upon one-electron reduction, the partial filling of the vanadium d-levels in PDOS (Figure
3) is corroborated by the Mulliken atomic spin densities (ASDs). If an external electric field
Eext = 0.5 eV Å–1 is applied to 1 and 5 on Au(111), the lowest vanadium d-band downshifts
in energy, fully hitting the Fermi level and accepting electronic density from gold. Bader
analysis proves the reduction of adsorbed molecules, with an increase of the molecular
charge of roughly one unit, up to –2.72 for 5 in the face orientation and a concomitant
oxidation of the gold slab by 0.94 e. The Mulliken ASDs also indicate that the extra
electron transferred upon application of the electric field is accepted by vanadium atoms
(see Supporting Information for details). This illustrates the fundamental role of the
vanadium d-levels in the conductivity of the herein explored POMs on Au.
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CONCLUSIONS
We showcased the potential of a molecular system to display MRS at room temperature
and low bias voltages in a technologically relevant resistive switching model setup. This
remarkable finding thus paves the way for neuromorphic computing at the level of single
molecules. Although, it is noteworthy that our experiments and computations demonstrate
presently the fundamental effect of detecting several discrete and stable reduction states
of a single POM molecule. The eventual industrial implementation will indeed require the
electrical contacting of individual molecules which remains a great challenge in the fields
of molecular electronics and spintronics. Further studies will in particular have to focus on
an increased charge stabilization by molecular and/or interface tuning in order to realize
non-volatile switching.
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ASSOCIATED CONTENT
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Supporting Information. Details of synthesis and analytical characterization of organic
ligands L1–L7 and hexavanadate compounds 1–7. The crystal data and details of
structure refinements are described in the CIF files: CCDC 1854538–1854543. Data of
IRRAS, XPS and STM/STS measurements at hybrid molecule–surface interfaces. Details
of DFT calculations. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
* E-mail: kirill.monakhov@iom-leipzig.de
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was supported by the Emmy Noether program of the Deutsche
Forschungsgemeinschaft (DFG). We are also grateful to the Spanish government
(Projects CTQ2014-52774 and CTQ2017-83566-P) and the Generalitat de Catalunya
(2017-SGR629) for funding. We thank U. Simon (RWTH Aachen University) for access to
the IRRAS equipment and R. Borowski (Forschungszentrum Jülich) and T. Pössinger
(RWTH Aachen University) for technical assistance.
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