Molecular nanosciences

The bottom-up approach enabled by molecular chemistry provides an answer to the ever-increasing miniaturization that microelectronics must face. In this context, we explore the potential of electroactive POMs in the field of molecular memories by charge trapping or based on spin transport for new information processing devices. A particular effort is devoted to the shaping of these functional molecules on planar substrates: controlled thin films, monolayers, single molecule (A. Proust, F. Volatron). In nanoparticle assemblies, POMs (photo- and electro-reducible, magnetic…) can also play the role of functional coating agents to modulate the electronic or magnetic properties of the assembly by various stimuli (light, electric field…) (F. Volatron). Magnetic storage also underlies the design and study of dendrimers and multifunctional molecular assemblies incorporating high spin molecules, magnet molecules, photo-switchable molecules and/or magnetic switches that can also lead to magneto-caloric effects. (V. Marvaud)

Mots clés :
molecular memories, surface functionalization, coated nanoparticles, high spin molecules, photo-switchable molecules, magnetic switches, controlled assemblies, magnetic dendrimers.

Covalent shaping of polyoxometalate molecular films onto ITO electrodes for charge trapping induced resistive switching

R. Salles, W.-C. Poh, M. Laurans, F. Volatron, A. Miche, S. Alves, C. Carino, L. Tortech, G. Izzet, P.-S. Lee, * A. Proust*, Inorg. Chem. Front., 2024, DOI: 10.1039/d3qi01761c

Capitalizing on our experience in the handling of organic-inorganic POM hybrids, we have prepared bis-diazonium hybrids to harness the propensity of diazonium salts to form multi-layered materials upon electrochemical reduction. A few nanometers thick materials have thus been grown onto ITO electrodes and have shown to be potentially suitable for Write-Once-Read-Many (WORM) devices, with a low set voltage.

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

C. Huez, S. Renaudineau, F.Volatron, A.Proust*, D. Vuillaume, Nanoscale, 2023, 15, 10634-10614

The POM countercations have been found to have an influence on the electron transport properties of POM-based molecular junctions, formed by self-assembled monolayers (SAMs) of the Preyssler anion [Na⊂P5W30O110]14-electrostatically deposited on ultraflat gold surface prefunctionalized with a positively charged SAM of amine-terminated alkylthiol chains. The low-bias current, recorded by conductive atomic force microscopy (C-AFM) gradually increasing by a factor ∼100 by changing the counterion in the order K+, NH4+, H+ and TBA+, due to  the competitive effects of the energy position of the lowest unoccupied molecular orbital of the POM (LUMO) and the electrode coupling energy. 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.

Redox-controlled conductance of polyoxometalate molecular junctions

C. Huez, D. Guérin, S. Lenfant, F. Volatron, M. Calame, M. L. Perrin, A. Proust and D. Vuillaume*, Nanoscale, 2022, 14, 13790-13800

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 ∼ 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 ∼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 (∼ 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 ∼0.3 eV above the SOMO with a weak electronic coupling to the electrodes (∼ 50% of the dataset) or at ∼0.4 eV but with a better electrode coupling (∼ 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.

Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide : Insights from POMs Layers on Oxides

M. Laurans, K. Trinh, K. Dalla Francesca, G. Izzet, S. Alves, E. Derat, V. Humblot, O. Pluchery, D. Vuillaume, S. Lenfant, F. Volatron, and A. Proust*- ACS Appl. Mater. Interfaces 2020, 12, 42, 48109–48123.

Immobilization of polyoxometalates (POMs) onto oxides is relevant to many applications in the fields of catalysis, energy conversion/storage or molecular electronics. Optimization and understanding the molecule/oxide interface is crucial to rationally improve the performance of the final molecular materials. We herein describe the synthesis and covalent grafting of POM hybrids with remote carboxylic acid functions onto flat Si/SiO2 substrates. Special attention has been paid to the characterization of the molecular layer and to the description of the POM anchoring mode at the oxide interface through the use of various characterization techniques, including ellipsometry, AFM, XPS and FTIR. Finally, electron transport properties were probed in a vertical junction configuration and energy level diagrams have been drawn and discussed in relation with the POM molecular electronic features inferred from cyclic-voltammetry, UV-visible absorption spectra and theoretical calculations. The electronic properties of these POM-based molecular junctions are driven by the POM LUMO (d-orbitals) whatever the nature of the tether or the anchoring group.

Charge transport through molecular junctions based on the redox active [H7P8W48O184]33- polyoxometalate processed onto gold surfaces and gold nanodots

Dalla Francesca, K. ; Lenfant, S. ; Laurans, M. ; Volatron, F. ; Izzet, G. ; Humblot, V. ; Methivier, C. ; Guerin, D. ; Proust, A.* ; Vuillaume, D.* – Nanoscale 2019, 11, 1863-1878.

Polyoxometalates (POMs) are redox-active molecular oxides, which attract growing interest for their integration into nano-devices, such as high-density data storage non-volatile memories. In this work, we investigated the electrostatic deposition of the negatively charged [H7P8W48O184]33− POM onto positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs) preformed onto gold substrates or onto an array of gold nanodots. The ring-shaped [H7P8W48O184]33− POM was selected as an example of large POMs with high charge storage capacity. To avoid the formation of POM aggregates onto the substrates, which would introduce variability in the local electrical properties, special attention has to be paid to the preformed SAM seeding layer, which should itself be deprived of aggregates. Where necessary, rinsing steps were found to be crucial to eliminate these aggregates and to provide uniformly covered substrates for subsequent POM deposition and electrical characterizations. This especially holds for commercially available gold/glass substrates while these rinsing steps were not essential in the case of template stripped gold of very low roughness. Charge transport through the related molecular junctions and nanodot molecule junctions (NMJs) has been probed by conducting-AFM. We analyzed the current–voltage curves with different models : electron tunneling though the SAMs (Simmons model), transition voltage spectroscopy (TVS) method or molecular single energy level mediated transport (Landauer equation) and we discussed the energetics of the molecular junctions. We concluded to an energy level alignment of the alkyl spacer and POM lowest occupied molecular orbitals (LUMOs), probably due to dipolar effects.

Molecular signature of polyoxometalates in electron transport of silicon-based molecular junctions

M. Laurans, K. Dalla Francesca, F. Volatron, G. Izzet, D. Guerin, D. Vuillaume, S. Lenfant, Anna Proust, Nanoscale, 2018, 10, 17156-17165

Polyoxometalates (POMs) are unconventional electro-active molecules with a great potential for applications in molecular memories, providing efficient processing steps onto electrodes are available. The synthesis of the organic–inorganic polyoxometalate hybrids [PM11O39(Sn(C6H4)C≡C(C6H4)N2]3− (M = Mo, W) endowed with a remote diazonium function is reported together with their covalent immobilization onto hydrogenated n-Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers contacted with a mercury drop as a top electrode confirms their homogeneity. Adjustment of the current–voltage curves with the Simmon’s equation gives a mean tunnel energy barrier ΦPOM of 1.8 eV and 1.6 eV, for the Silicon–Molecules–Metal (SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M = Mo), respectively. This follows the trend observed in the electrochemical properties of POMs in solution, the polyoxomolybdates being easier to reduce than the polyoxotungstates, in agreement with lowest unoccupied molecular orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus clearly identifiable in the solid-state electrical properties and the unmatched diversity of POM molecular and electronic structures should offer a great modularity.

Photochromism and Dual-Color Fluorescence in a Polyoxometalate–Benzospiropyran Molecular Switch

A. Parrot, A. Bernard, A. Jacquart, S. A. Serapian, C. Bo, E. Derat, O. Oms, A. Dolbecq, A. Proust, R. Métivier, P. Mialane, G. Izzet, Angew. Chem. Int. Ed. 2017, 56, 4872 –4876

The photophysical properties of a Keggin-type polyoxometalate (POM) covalently bounded to a benzospiropyran (BSPR) unit have been investigated. These studies reveal that both closed and open forms are emissive with distinct spectral features (λem (closed form) = 530 nm, λem (open form) = 670 nm) and that the fluorescence of the BSPR unit of the hybrid is considerably enhanced compared to BSPR parent compounds. While the fluorescence excitation energy of the BSPR reference compounds (370 nm) is close to the intense absorption responsible of the photochromic character (350 nm), the fluorescence excitation of the hybrid is shifted to lower energy (400 nm), improving the population of the emissive state. Combined NOESY NMR and theoretical calculations of the closed form of the hybrid give an intimate understanding of the conformation adopted by the hybrid and show that the nitroaryl moieties of the BSPR is folded toward the POM, which should affect the electronic properties of the BSPR.

Surface Organization of Polyoxometalate Hybrids Steered by a 2D Supramolecular PTCDI/Melamine Network

A. Lombana, C. Rinfray, F. Volatron, G. Izzet, N. Battaglini, S. Alves, P. Decorse, P. Lang, A. Proust, J. Phys. Chem. C (2016), 120(5), 2837-2845

A 2D supramol. honeycomb network built on hydrogen bonding of perylene-3,4,9,10-tetracarboxylic acid diimide (PTCDI) and 1,3,5-triazine-2,4,6-triamine (Melamine) has been self-assembled on Au(111) by a soln. processed method. The ability of the porous network to host functional mol. oxides or polyoxometalates (POMs) has been investigated using a functionalized species [PW11O39Ge(p-C6H4-C≡C-C6H4-NHC(O) (CH2)4(-CH(CH2)2S-S-))]4- (KWGe[S-S]) : this inorg./org. hybrid built on a Keggin-type POM core and an org. tether incorporating a thioctic acid function has been designed to enhance the host-guest interaction by the formation of covalent Au-S bonds. XPS anal. confirmed the presence of the POMs that are covalently held onto the surface. Probed by STM operating under ambient conditions, the spatial organization of the POMs display some reminiscence of the org. template, while monitoring the POM deposition at various immersion times by PM-IRRAS showed that the POM raising at the substrate is fostered.

Binary Superlattices from Mo132 Polyoxometalates and Maghemite Nanocrystals : Long-Range Ordering and Fine-Tuning of Dipole Interactions

R. Breitwieser, T. Auvray, F. Volatron, Florence ; C. Salzemann, A.-T. Ngo, P.-A. Albouy, A. Proust, C. Petit, Small 2016, 12(2), 220-228

The successful coassembly of spherical 6.2 nm maghemite (γ-Fe2O3) nanocrystals and giant polyoxometalates (POMs) such as 2.9 nm Mo132 is demonstrated. To do so, colloidal solns. of oleic acid-capped γ-Fe2O3 and long-chain alkylammonium-encapsulated Mo132 dispersed in CHCl3 are mixed together and supported self-organized binary superlattices were obtained upon the solvent evapn. on immersed substrates. Both electronic microscopy and small angles x-ray scattering data reveal an AB-type structure and an enhanced structuration of the magnetic nanocrystals (MNCs) assembly with POMs in octahedral interstices. Therefore, Mo132 acts as an efficient binder constituent for improving the nanocrystals ordering in 3-dimensional films. In the case of didodecyldimethylammonium (C12)-encapsulated POMs, the long-range ordered binary assemblies were obtained while preserving the nanocrystals magnetic properties due to weak POMs-MNCs interactions. However, POMs of larger effective diam. can be employed as spacer blocks for MNCs as shown by using Mo132 capped with dioctadecyldimethylammonium (C18) displaying longer chains. In that case, POMs can also be used for fine-tuning the dipolar interactions in γ-Fe2O3 nanocrystal assemblies.

Electron transfer properties of a monolayer of hybrid polyoxometalates on silicon

F. Volatron, J.-M. Noël, C. Rinfray, P. Decorse, C. Combellas, F. Kanoufi, A. Proust, J. Mater. Chem. C, 2015, 3(24), 6266-6275

As electroactive molecules, polyoxometalates (POMs) have potential in charge trapping or resistive molecular memories, yet scarcely investigated until very recently. Since charge–discharge processes as well as transport properties are dependent upon the organization of the thin layers, we chose to explore a covalent approach and we prepared a diazonium post-functionalized Keggin-type polyoxometalate [PW11O39Ge(p-C6H4–CC–C6H4–N2+)]3− that was subsequently anchored on hydrogenated n-type Si(100) surfaces. A flat and homogeneous hybrid POM monolayer is obtained and characterized by AFM, ellipsometry and XPS techniques. Vertical and lateral electron transfers are studied by cyclic voltammetry and scanning electrochemical microscopy (SECM). If the electron transfer between the POM layer and the silicon surface is quite slow (kETvert = 5 s−1), SECM suggests that the monolayer displays a good lateral conductivity. Interestingly, SECM experiments evidence the influence of the organization of the layer on the lateral charge transfer and show the possibility to accumulate negative charges within the POM monolayer.

Photomagnetic molecular and extended network Langmuir–Blodgett films based on cyanide bridged molybdenum–copper complexes

N. Bridonneau, J. Long, J.-L. Cantin, J. von Bardeleben, D. R. Talham, V. Marvaud, RSC Adv., 2015,5, 16696-16701.

Two types of cyanide bridged molybdenum–copper photomagnetic films have been obtained : the first one is based on a molecular [MoCu6] complex, the other being a two-dimensional [MoCu2] coordination network. Both systems employ surfactant functionalized ligands and films were deposited on Melinex substrates using the Langmuir–Blodgett technique. All systems, including monolayer films, showed full retention of the intrinsic photomagnetic properties known for analogous solids as demonstrated by EPR spectroscopy.

Control of the Grafting of Hybrid Polyoxometalates on Metal and Carbon Surfaces : Toward Submonolayers

S. Gam Derouich, C. Rinfray, G. Izzet, J. Pinson, J.-J. Gallet, F. Kanoufi, A. Proust, C. Combellas, Langmuir, 2014, 30, 2287−2296.

A Keggin-type POM is attached to gold or glassy carbon surfaces by electro(chemical) or peptidic
coupling. In addition to demonstrating the robust attachment of the POMs (by electrochemistry, XPS, and IRRAS), the surface concentration, layer thickness, and rate constant for electron transfer from the surface to the POMs have been measured. The use of such complementary techniques is mandatory to characterize the modi !ed electrodes properly. Whatever the grafting method, experimental conditions are found to allow monolayer or submonolayer coverage. Besides covalently grafted species, additional electrostatically bonded POMs are present in the film. Cathodic polarization allows removing them to get a grafted !lm that is stable with time and potential, which is a requirement in the design of molecular memories.

Electron Transfer to Covalently Immobilized Keggin Polyoxotungstates on Gold

M. Yaqub, J. J. Walsh, T. E. Keyes, A. Proust, C. Rinfray, G. Izzet, T. McCormac, R. J. Forster, Langmuir 2014, 30, 4509−4516.

Spontaneously adsorbed monolayers have been formed on gold electrodes using a Keggin polyoxotungstate with covalently attached alkanethiol linkers of two different lengths. Films of both polyoxotungstates show two well- defined reduction processes associated with the polyoxotungstate centers where the ionic liquid, [BMIM][BF4], acts as supporting electrolyte. The surface coverages are both less than that expected for a close-packed monolayer. For the short and long linkers, the voltammetric response can be described in terms of the Butler− Volmer response involving a surface confined species using standard heterogeneous electron transfer rate constants of 170 and 140 s−1 for the first reduction and 150 and 100 s−1 for the second reduction processes, respectively. The rate of electron transfer to a solution phase redox probe, ferrocyanide, is significantly more sensitive to the length of the linker than the rate of electron transfer to the tungstate centers. This behavior probably arises due to potential-induced changes in the film structure.

Electrografting of Diazonium Functionalized Polyoxometalates : Synthesis, Immobilization and Electron Transfer Characterization from Glassy Carbon

C. Rinfray, G. Izzet, J. Pinson, S. Gam Derouich, J.-J. Ganem, C. Combellas, F. Kanoufi, A. Proust, Chem. Eur. J., 2013, 19, 13838 – 13846.

POMs at work. Polyoxometalates (POMs) are attractive candidates for the rational design of multi-level charge storage materials since they display reversible multi-step reduction processes in a narrow range of potentials. The functionalization of POMs allows for their integration in hybrid CMOS/molecular devices, provided that a fine control of their immobilization on various substrates can be achieved. Owing to the wide applicability of the diazonium route to surface modification, a functionalized Keggin-type POM [PW11O39{Ge(p-C6H4-CC-C6H4-N2+)}]3- bearing a pending diazonium group was prepared and subsequently covalently anchored onto a glassy carbon electrode. Electron transfer with the immobilized POM was thoroughly investigated and compared to that of the free POM in solution.

Organosilyl/germyl polyoxotungstate hybrids for covalent grafting onto silicon surfaces : towards molecular memories

N. Joo, S. Renaudineau, G. Delapierre, G.Bidan, L.-M. Chamoreau, R. Thouvenot, P. Gouzerh, A. Proust, Chem. Eur. J., 2010, 16, 5043-5051.

A hybrid organosilyl/-germyl Keggin polyoxotungstate was covalently grafted to an n-type silicon wafer (see figure) and the electrochemical behavior of the thus modified electrode was investigated.