arXiv subjects

Dominique Vuillaume

Publications and source records attributed to Dominique Vuillaume.

At least 19 recordsLinked to original sources

arXiv2026

Toward Polyoxometalate Nanoelectronics

Polyoxometalates form a large family of molecular oxide clusters of the early transition metals with unique and tunable properties (multi-redox, thermal and chemical robustness, magnetic). We review more than 30 years of experimental research on the electron transport properties of polyoxometalates devices, from thin films and self-assembled monolayers down to single-molecule junctions. We focus on the relationship between the polyoxometalate structures (structural type, nature of metals and heteroatoms, role of the counterions, redox states, electrode linkers and functional ligands) and the electronic structures of the polyoxometalate-based devices (energy positions of the molecular orbitals, energy offset at the interfaces). Then, we critically discuss the performances of polyoxometalates in nanoelectronics devices: capacitance and resistive switching memories, spintronics, quantum bits and neuromorphic devices. We conclude with a discussion about pending issues and perspectives.

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arXiv2026

Benzo-bis(imidazole) self-assembled monolayers molecular junctions in meta or para conformation: effects of protonation on the electrical and thermal conductances

We report the thermal conductances of molecular junctions made of self-assembled monolayers of benzo-bis(imidazole) molecules, without side groups or functionalized with two phenylamine side groups. In the two cases, when the molecules are connected to the electrodes by thiol anchoring groups in the meta-position, the thermal conductance is decreased compared to the same molecules connected in the para-position (ca. 16-29 nW/K and ca. 37-40 nW/K, respectively) in agreement with the theoretically predicted phonon interference effect in molecular junctions. Upon protonation, the thermal conductances of the meta-connected molecular junction increase by about 50% (reversible behavior upon deprotonation). The fact that only the thermal conductance of the meta-connected molecular junction is sensitive to the protonation/deprotonation is tentatively related to modifications of the structural organization of the molecules in the monolayer, which modifies the thermal conductance at the molecule/electrode interfaces. The electrical conductance is lower for the meta-connected molecule than for the para-connected one, due to destructive quantum interferences, as expected and reported for other molecular junctions. The conductance further decreases (reversibly) upon protonation. The energy position of the molecular orbital involved in the electron transport is not modified by the protonation and the decrease in current is related to changes in the molecule organization in the monolayer, which modulate the electronic coupling energy at the molecule/electrode interfaces.

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arXiv2025

Low-temperature cotunneling electron transport in photo-switchable molecule-nanoparticle networks

We report the temperature-dependent (4.2 - 300 K) electron transport properties (current-voltage) of photo-switchable two-dimensional arrays of gold nanoparticles (10 nm in diameter) functionalized by azobenzene derivatives. Under UV-light irradiation at 4.2 K, the azobenzene moieties are switched from the trans to cis isomers, leading to an increase of the current. In both conformations, at low temperature (< 77 K) and low voltage (< 1 V) the voltage- and temperature-dependent current behaviors show that electron cotunneling is the dominant transport mechanism. The number of cotunneling events Ncot slightly increases from ca. 1. 4 to 1.7 upon trans-to-cis isomerization of the azobenzenes. The nanoparticle Coulomb charging energy is not significantly modified (ca. 15 meV) by the azobenzene isomerization. This weak increase of Ncot is explained by the modest cis/trans current ratio (< 10) and the limited numbers of nanoparticle-molecule-nanoparticle junctions inserted between the two nanoscale electrodes (< 50 nm apart) connecting the network.

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arXiv2025

Electronic and Thermoelectric Properties of Molecular Junctions Incorporating Organometallic Complexes: Implications for Thermoelectric Energy Conversion

The electronic and thermoelectric properties of molecular junctions formed from iron and ruthenium metal-acetylide were studied using complementary experimental techniques and quantum chemical simulations. We performed physical characterizations of single-molecule and self-assembled monolayer junctions of the same molecules that allowed meaningful comparisons between the Ru and Fe adducts. In the case of the Fe-containing junctions, two distinct oxidation states are present. These junctions exhibit one of the highest Seebeck coefficients (S ca. 130 μV/K) reported to date for similar systems paired with broad electric conductance distribution and limited thermal conductance. As a result, the experimental thermoelectric figure of merit ZT for Fe-containing junctions reaches up to 0.4 for junctions with relatively high conductance. This is one of the highest ZT values reported for molecular systems at room temperature.

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arXiv2025

Dynamic switching of ferrocene and plasmonic interactions in Au/self-assembled monolayer/single Ag nanocube molecular junctions

We report the redox switching of ferrocene moieties embedded in a double tunnel barrier plasmonic cavity fabricated from a click-chemistry self-assembled monolayers of ferrocenyl-alkylthiol on ultra flat gold surfaces, connected to a single poly(vinylpyrrolidone) capped silver nanocube, AgNC, which is contacted by the tip of a conductive-AFM to study the electron transport properties in the dark and under light irradiation at the plasmonic resonance wavelengths. We observe a dual behavior in the current-voltage (I-V) characteristics in the dark: a large hysteresis loop at positive voltages and an hysteretic negative differential conductance (NDC) at negative voltages, due to the redox switching of ferrocene between its oxidized (Fc+) and neutral (Fc0) states. The I-V curves are analyzed by a generalized combined Marcus-Landauer model. We determine the highest occupied molecular orbital of the Fc+ and Fc0 states at 0.54 and 0.42 eV below the Fermi energy, respectively, with a weak reorganization energy < 0.1 eV upon switching. Under plasmonic excitation, the hysteresis and NDC behaviors are no longer observed and the I-V characteristics of the Au-ferrocenyl-alkylthiol/AgNC junctions become similar to Au-ferrocenyl-alkylthiol SAMs. A virtual molecular orbital due to the plasmon-induced coupling (fast electron transfer) between the two redox states of the Fc is determined at 0.46 eV. This dynamic behavior opens perspectives in artificial synaptic devices for neuromorphic computing with the additional function to turn on/off this synaptic behavior on-demand by light.

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arXiv2024

Low frequency noise in nanoparticle-molecule networks and implications for in-materio reservoir computing

We study the low-frequency noise (LFN), i.e. flicker noise, also referred to as 1/f noise, in 2D networks of molecularly functionalized gold nanoparticles (NMN: nanoparticle-molecule network). We examine the noise behaviors of the NMN hosting alkyl chains (octanethiol), fatty acid oleic acids (oleylamine), redox molecule switches (polyoxometalate derivatives) or photo-isomerizable molecules (azobenzene derivatives) and we compare their 1/f noise behaviors. These noise metrics are used to evaluate which molecules are the best candidates to build in-materio reservoir computing molecular devices based on NMNs.

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arXiv2024

Thermoelectric Properties of Benzothieno-Benzothiophene Self-Assembled Monolayers in Molecular Junctions

We report a combined experimental (C-AFM and SThM) and theoretical (DFT) study of the thermoelectric properties of molecular junctions made of self-assembled monolayers on Au of thiolated benzothieno-benzothiophene (BTBT) and alkylated BTBT derivatives (C8-BTBT-C8). We measure the thermal conductance per molecule at 15 pW/K and 8.8 pW/K, respectively, among the lowest values for molecular junctions so far reported (10-50 pW/K). The lower thermal conductance for C8-BTBT-C8 is consistent with two interfacial thermal resistances introduced by the alkyl chains, which reduce the phononic thermal transport in the molecular junction. The Seebeck coefficients are 36 μV/K and 245 μV/K, respectively, the latter due to the weak coupling of the core BTBT with the electrodes. We deduce a thermoelectric figure of merit ZT up to ca. 1E-4 for the BTBT molecular junctions at 300K, on a par with the values reported for archetype molecular junctions (olygo(phenylene ethynylene) derivatives).

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arXiv2024

Electronic Properties of Electroactive Ferrocenyl-Functionalized MoS2

The attachment of redox active molecules to transition metal dichalcogenides (TMDs), such as MoS2, constitutes a promising approach for designing electrochemically switchable devices through the control of the material charge/spin transport properties by the redox state of the grafted molecule and thus the applied electrical potential. In this work, defective plasma treated MoS2 is functionalized by a ferrocene derivative and thoroughly investigated by various characterization techniques, such as Raman, photoluminescence, X-ray photoelectron spectroscopies, atomic force microscopy (AFM) and electrochemistry. Furthermore, in-plane and out-of-plane conductive-AFM measurements (I-V and first derivative dI/dV-V curves) are measured to investigate the effect of the chemical functionalization of MoS2 on the electron transport properties. While the conduction and valence bands are determined at +0.7 and -1.2 eV with respect of the electrode Fermi energy for pristineMoS2, additional states in an energy range of ca. 0.45 eV below the MoS2 conduction band are measured after plasma treatment, attributed to S-vacancies. For ferrocene functionalized MoS2, the S-vacancy states are no longer observed resulting from the defect healing. However, two bumps at lower voltages in the dI/dV-V indicate a contribution to the electron transport through ferrocene HOMO, which is located in the MoS2 band gap at ca. 0.4-0.6 eV below the Fermi energy. These results are in good agreement with theoretical density functional theory (DFT) calculations and UV photoelectron spectroscopy (UPS) measurements.

cond-mat.mtrl-sci↗

arXiv2024

Molecular Junctions for Terahertz Switches and Detectors

Molecular electronics targets tiny devices exploiting the electronic properties of the molecular orbitals, which can be tailored and controlled by the chemical structure/conformation of the molecules. Many functional devices have been experimentally demonstrated; however, these devices were operated in the low frequency domain (mainly, dc to MHz). This represents a serious limitation for electronic applications, albeit molecular devices working in the THz regime were theoretically predicted. Here, we experimentally demonstrate molecular THz switches at room temperature. The devices consist of self-assembled monolayers of molecules bearing two conjugated moieties coupled through a non-conjugated linker. These devices exhibit clear negative differential conductance behaviors (peaks in the current-voltage curves), as confirmed by ab initio simulations, which were reversibly suppressed under illumination with a 30 THz wave. We analyze how the THz switching behavior depends on the THz wave properties (power, frequency), and we benchmark that these molecular devices would outperform actual THz detectors.

physics.app-ph↗

arXiv2023

Electronic Properties of Single Prussian Blue Analog Nanocrystals Determined by Conductive-AFM

We report a study of the electron transport (ET) properties at the nanoscale (conductive-AFM noted C-AFM thereafter) of individual Prussian Blue Analog (PBA) cubic nanocrystals (NCs) of CsCo(III)Fe(II), with size between 15 and 50 nm deposited on HOPG. We demonstrate that these PBA NCs feature an almost size independent electron injection barriers of 0.41 +/- 0.02 eV and 0.27 +/- 0.03 eV at the CsCo(III)Fe(II)/HOPG and CsCo(III)Fe(II)/C-AFM tip, respectively, and an intrinsic electron conductivity evolving from largely dispersed between ca. 5E-4 and 2E-2 S/cm without a clear correlation with the nanocrystal size. The conductivity values measured on individual nanocrystals are higher by up to 5 decades than those reported on PBA films.

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arXiv2023

Highly Conductive Tungsten Suboxide Nanotubes

We demonstrate a high electron conductivity (> 10^2 S/cm and up to 10^3 S/cm) of tungsten suboxide W18O(52.4-52.9)(or equivalently WO(2.91-2.94)) nanotubes (2 to 3 nm in diameter, ca. micrometer long). The conductivity is measured in the temperature range 120 to 300K by a four probe scanning tunneling microscope in ultra high vacuum. The nanotubes are synthesized by a low temperature and low cost solvothermal method. They selfassemble in bundles of hundreds of nanotubes forming nanowires (ca. micrometer long, few tens nm wide). We observe a large anisotropy of the conductivity with a ratio (longitudinal conductivity/perpendicular conductivity) of ca. 10^5. A large fraction of them (ca. 65 to 95%) shows a metallic like, thermal activation less, electron transport behavior. Few of them, with a lower conductivity from 10 to 10^2 S/cm, display a variable range hopping behavior. In this latter case, a hopping barrier energy of ca. 0.24 eV is inferred in agreement with the calculated energy level of the oxygen vacancy below the conduction band. This result is in agreement with a relative average concentration of oxygen vacancies of ca. 3%, for which a semiconductor to metal transition was theoretically predicted. These tungsten suboxide nanostructures are prone to a wide range of applications in nanoelectronics.

physics.app-ph↗

arXiv2023

Experimental observation of the role of countercations on the electrical conductance of Preyssler-type polyoxometalate nanodevices

Polyoxometalates are nanoscale molecular oxides with promising properties that are currently explored for molecule-based memory devices. In this work, we synthesize a series of Preyssler polyoxometalates (POMs), (Na-P5W30O110)14-,stabilized with four different counterions, H+, K+, NH4+ and tetrabutylammonium (TBA+), and we study the electron transport properties at the nanoscale (conductive atomic force microscopy, C-AFM) of molecular junctions formed by self-assembled monolayers (SAMs) of POMs electrostatically deposited on ultraflat gold surface prefunctionalized with a positively charged SAM of amine-terminated alkylthiol chains. We report that the electron transport properties of P5W30-based molecular junctions depend on the nature of the counterions, the low-bias current (in the voltage range -0.6 V to 0.6 V) gradually increasing by a factor ca. 100 by changing the counterion in the order K+, NH4+, H+ and TBA+. From a statistical study (hundreds of current-voltage traces) using a simple analytical model for charge transport in nanoscale devices, we show that the energy position of the lowest unoccupied molecular orbital (LUMO) of the P5W30 with respect of the Fermi energy of the electrodes increases from ca. 0.4 eV to 0.7 eV and that that electrode coupling energy also increases from ca. 0.05 to 1 meV in the same order from K+, NH4+, H+ to TBA+. We discuss several hypotheses on the possible origin of these features, such as a counterion-dependent dipole at the POM/electrode interface and counterion-modulated molecule/electrode hybridization, with, in both cases, the largest effect in the case of TBA+ counterions.

physics.app-ph↗

arXiv2023

Nanoscale electronic transport at graphene/pentacene van der Waals interface

We report a study on the relationship between structure and electron transport properties of nanoscale graphene/pentacene interfaces. We fabricated graphene/pentacene interfaces from 10-30 nm thick needle-like pentacene nanostructures down to two-three layers (2L-3L) dendritic pentacene islands, and we measured their electron transport properties by conductive atomic force microscopy (C-AFM). The energy barrier at the interfaces, i.e. the energy position of the pentacene highest occupied molecular orbital (HOMO) with respect to the Fermi energy of the graphene and the C-AFM metal tip, are determined and discussed with the appropriate electron transport model (double Schottky diode model and Landauer-Buttiker model, respectively) taking into account the voltage-dependent charge doping of graphene. In both types of samples, the energy barrier at the graphene/pentacene interface is slightly larger than that at the pentacene/metal tip interface, resulting in 0.47-0.55 eV and 0.21-0.34 eV, respectively, for the 10-30 nm thick needle-like pentacene islands, and in 0.92-1.44 eV and 0.67-1.05 eV, respectively, for the 2L-3L thick dendritic pentacene nanostructures. We attribute this difference to the molecular organization details of the pentacene/graphene heterostructures, with pentacene molecules lying flat on the graphene in the needle-like pentacene nansotructures, while standing upright in 2L-3L dendritic islands, as observed from Raman spectroscopy.

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arXiv2022

Redox-controlled conductance of polyoxometalate molecular junctions

We demonstrate the reversible in situ photoreduction of molecular junctions of phosphomolybdate [PMo12O40]3- monolayer self-assembled on flat gold electrodes, connected by the tip of a conductive atomic force microscope. The conductance of the one electron reduced [PMo12O40]4- molecular junction is increased by ca. 10, this open-shell state is stable in the junction in air at room temperature. The analysis of a large current-voltage dataset by unsupervised machine learning and clustering algorithms reveals that the electron transport in the pristine phosphomolybdate junctions leads to symmetric current-voltage curves, controlled by the lowest unoccupied molecular orbital (LUMO) at 0.6 - 0.7 eV above the Fermi energy with ca. 25% of the junctions having a better electronic coupling to the electrodes than the main part of the dataset. This analysis also shows that a small fraction (ca. 18% of the dataset) of the molecules is already reduced. The UV light in situ photoreduced phosphomolybdate junctions are systematically featuring slightly asymmetric current - voltage behaviors, which is ascribed to electron transport mediated by the single occupied molecular orbital (SOMO) nearly at resonance with the Fermi energy of the electrode and by a closely located single unoccupied molecular orbital (SUMO) at ca. 0.3 eV above the SOMO with a weak electronic coupling to the electrodes (ca. 50% of the dataset) or at ca. 0.4 eV but with a better electrode coupling (ca. 50% of the dataset). These results shed lights to the electronic properties of reversible switchable redox polyoxometalates, a key point for potential applications in nanoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Terphenylthiazole-based self-assembled monolayers on cobalt with high conductance photo-switching ratio for spintronics

Two new photo-switchable terphenylthiazoles molecules are synthesized and self-assembled as monolayers on Au and on ferromagnetic Co electrodes. The electron transport properties probed by conductive atomic force microscopy in ultra-high vacuum reveal a conductance of the light-induced closed (c) form larger than for the open (o) form. We report an unprecedented conductance ratio up to 380 between the closed and open forms on Co for the molecule with the anchoring group (thiol) on the side of the two N atoms of the thiazole unit. This result is rationalized by Density Functional Theory (DFT) calculations coupled to the Non-Equilibrium Green's function (NEGF) formalism. These calculations show that the high conductance in the closed form is due to a strong electronic coupling between the terphenylthiazole molecules and the Co electrode that manifests by a resonant transmission peak at the Fermi energy of the Co electrode with a large broadening. This behavior is not observed for the same molecules self-assembled on gold electrodes. These high conductance ratios make these Co-based molecular junctions attractive candidates to develop and study switchable molecular spintronic devices.

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arXiv2021

Molecular Electronics: electron, spin and thermal transport through molecules

This review presents recent results on the physics of electron transport in molecular devices. The review is organized as follows. A brief description of molecular junction (MJ) technology is first given followed by an introduction to the basic physics of electron transport through MJs from DC to about 20 GHz. Then, several sections review selected results on spin-dependent transport, plasmonics, quantum interferences, thermal transport and electronic noise in ME devices.

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arXiv2021

Conductance Switching of Azobenzene-Based Self-Assembled Monolayers on Cobalt Probed by UHV Conductive-AFM

We report the formation of self-assembled monolayers of a molecular photoswitch (azobenzene-bithiophene derivative, AzBT) on cobalt via a thiol covalent bond. We study the electrical properties of the molecular junctions formed with the tip of a conductive atomic force microscope under ultra-high vacuum. The statistical analysis of the current-voltage curves shows two distinct states of the molecule conductance, suggesting the coexistence of both the trans and cis azobenzene isomers on the surface. The cis isomer population (trans isomer) increases (decreases) upon UV light irradiation. The situation is reversed under blue light irradiation. The experiments are confronted to first-principle calculations performed on the molecular junctions with the Non-Equilibrium Green's Function formalism combined with Density Functional Theory (NEGF/DFT). The theoretical results consider two different molecular orientations for each isomer. Whereas the orientation does not affect the conductance of the trans isomer, it significantly modulates the conductance of the cis isomer and the resulting conductance ON/OFF ratio of the molecular junction. This helps identifying the molecular orientation at the origin of the observed current differences between the trans and cis forms. The ON state is associated to the trans isomer irrespective of its orientation in the junction, while the OFF state is identified as a cis isomer with its azobenzene moiety folded upward with respect to the bithiophene core. The experimental and calculated ON/OFF conductance ratios have a similar order of magnitude. This conductance ratio seems reasonable to make these Co-AzBT molecular junctions a good test-bed to further explore the relationship between the spin-polarized charge transport, the molecule conformation and the molecule-Co spinterface.

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arXiv2021

Thermal conductivity of benzothieno-benzothiophene derivatives at the nanoscale

We study by scanning thermal microscopy the nanoscale thermal conductance of films (40 to 400 nm thick) of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT-C8). We demonstrate that the out-of-plane thermal conductivity is significant along the interlayer direction, larger for BTBT (0.63 +/- 0.12 W m-1 K-1) compared to C8-BTBT-C8 (0.25 +/- 0.13 W m-1 K-1). These results are supported by molecular dynamics calculations (Approach to Equilibrium Molecular Dynamics method) performed on the corresponding molecular crystals. The calculations point to significant thermal conductivity (3D-like) values along the 3 crystalline directions, with anisotropy factors between the crystalline directions below 1.8 for BTBT and below 2.8 for C8-BTBT-C8, in deep contrast with the charge transport properties featuring a two-dimensional character for these materials. In agreement with the experiments, the calculations yield larger values in BTBT compared to C8-BTBT-C8 (0.6-1.3 W m-1 K-1 versus 0.3-0.7 W m-1 K-1, respectively). The weak thickness dependence of the nanoscale thermal resistance is in agreement with a simple analytical model.

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