D. Guérin, S. Lenfant, D. Vuillaume

Electron transport in molecular nanostructures

We have designed and developed of a new approach for measuring conductance (up to one million molecular junctions) with a single C-AFM image. In this approach, nanodot-molecule junctions (NMJ) are fabricated from a large array of sub-10 nm single crystal Au nanodot electrodes. Each individual NMJ consists of less than one hundred molecules. Typically, 2000-4000 NMJs are measured in a few μm2 C-AFM image. Conductance histograms and 2D histograms of the current-voltage (I-V) curves are easily extracted from these measurements. This approach has led to several advances:
(i) Demonstration that the number and respective amplitudes of the conductance peaks depend on the molecular organization in the junctions and the atomic structure of the nanoelectrodes. Experimental determination of the energy position of molecular orbitals as a function of the applied C-AFM force, in good agreement with ab-initio calculations, and highlighting the role of interface dipoles (reorientation of the Au/molecule interface dipoles). [K. Smaali et al., Nanoscale (2015)].
(ii) Original determination of intermolecular interaction energies π-π from a detailed analysis of conductance histograms of the molecular junctions, combined with nano electrochemsitry, mesoscopic electron transport model and ab-initio DFT calculations [J. Trasobares et al., Nano Letters (2017)].
(iii) Considering the dimensions of these molecular nanojunctions inducing a reduction of parasitic capacitances, and a very good conductance induced by the reinforcement of intermolecular π-π interactions, first demonstration of a molecular diode operating at 17 GHz, and estimation of a cut-off frequency at 500 GHz. The combination of a large array of single-crystal gold nanoelectrodes and an interferometric scanning microwave microscope (iSMM) is the key feature to perform a statistical study of the microwave transport properties on hundreds of molecular diodes. [J. Trasobares et al. Nature Comm. (2016)]. This result opens the door for the development of rf-organic diodes for ambient electromagnetic energy harvesting, and spin excitations in molecular junctions made of magnetic molecules (see perspectives).

Main coll. : P. Leclere, J. Cornil (LCNM, U. Mons, Belgium); T. Martin (CPT, U. Marseille); T. Frederiksen, D. Sanchez-Portal (Donostia International Physics Center, San Sebastián, Spain); E. Levillain (CNRS, Moltech-Anjou, U. Angers), D. Théron (IEMN/NAM6).

Molecular switches

In the continuation of our work on the design and synthesis of photo-addressable switch molecules (azobenzene derivatives) with a high electrical conductance ratio between “cis” and “trans” isomers (7,000) [K. Smaali et al., ACS Nano (2010)], we have integrated these molecules, and also redox molecules (thiophene derivatives), in organized gold nanoparticle networks (NPSAN : Nanoparticles self-assembled networks) and demonstrated a non-volatile optoelectronic memory effect [Y. Viero et al., J. Phys. Chem. C (2015)], a negative differential resistance effect [T. Zhang et al., J. Phys. Chem. C (2017)]. When these NPSANs are connected with multi-electrodes (6 to 8), we have demonstrated that these systems allow the realization of optically reconfigurable logic functions and the implementation of unconventional calculation circuits such as “reservoir computing” a typical example of the “evolution in materio” concept. We have also demonstrated the performance improvements of these NPSANs using graphene nanoelectrodes [Y. Viero et al., Adv. Func. Mater. (2018)].

Main coll. : P. Blanchard, J. Roncali (CNRS, Moltech-Anjou, U. Angers); M. Calame (EMPA & Univ. Basel, Switzerland), F. Cleri, C. Krzeminski (IEMN/Physique/NAMASTE).

We have also reported the electron-transport properties of a new photoaddressable molecular switch. The switching process relies on a new concept based on linear π-conjugated dynamic systems, in which the geometry and, hence, the electronic properties of an oligothiophene chain can be reversibly modified by the photochemical trans−cis isomerization of an azobenzene unit fixed in a lateral loop. Electron-transport measurements through self-assembled monolayers on gold, in contact with an eGaIn top contact, showed switching with a conductance ratio of up to 1,000. Ab initio calculations were used to identify the most energetically stable conformations of the molecular switch; the corresponding calculated conductances qualitatively explain the trend observed in the photoswitching experiments [S. Lenfant et al., J. Phys. Chem. C (2017)].

Main coll.: : P. Blanchard, J. Roncali (CNRS, Moltech-Anjou, U. Angers); J. Cornil (LCNM, U. Mons, Belgium); C. van Dyck (Dept. Chem., Northwestern Univ., Evanston, USA).

Polyoxometalates

Polyoxometalates (POMs) as discrete nanoscale metal-oxo clusters able to incorporate magnetic centers in their structures offer a number of advantages for creating molecular spintronic devices, such as thermal and redox stability coupled with large structural diversity and tunability of the magnetic properties. We have explored a novel surface anchoring mode (organo- amino group−Au surface) in an approach to render magnetically functionalized POMs accessible to charge transport experiments in distinct environments, in solution as well as in surface-adsorbed monolayers. Using conducting tip atomic force microscopy (C-AFM) and transition voltage spectroscopy (TVS), the energies of {Co9(P2W15)3} frontier molecular orbitals in the surface-bound state were found to directly correlate with cyclic voltammetry data in aqueous solution, indicating a weak molecule-electrode electronic coupling [X. Yi et al., J. Am. Chem. Soc. (2017)].

We recently reported 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 together with their covalent immobilization onto hydrogenated n- Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers gives a mean tunnel energy barrier 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 [M. Laurans et al., Nanoscale (2018)].

Main Coll. : P. Kögerler (Inst. Inorg. Chem., Aachen univ. & Peter Grünberg Institute, Jülich, Germany), A. Proust (IPCM, CNRS, Sorbonne University).