Fabrice Mathevet

Non-volatile organic memory with sub-millimetre bending radius

High-performance non-volatile memory that can operate under various mechanical deformations such as bending and folding is in great demand for the future smart wearable and foldable electronics. Here we demonstrate non-volatile solution-processed ferroelectric organic field-effect transistor memories operating in p- and n-type dual mode, with excellent mechanical flexibility. Our devices contain a ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) thin insulator layer and use a quinoidal oligothiophene derivative (QQT(CN)4) as organic semiconductor. Our dual-mode field-effect devices are highly reliable with data retention and endurance of >6,000 s and 100 cycles, respectively, even after 1,000 bending cycles at both extreme bending radii as low as 500 μm and with sharp folding involving inelastic deformation of the device. Nano-indentation and nano scratch studies are performed to characterize the mechanical properties of organic layers and understand the crucial role played by QQT(CN)4 on the mechanical flexibility of our devices.

Solution-growth kinetics and thermodynamics of nanoporous self-assembled molecular monolayers.

The temperature and concentration dependences of the self-assembly onto graphite from solution of a series of molecular building blocks able to form nanoporous structures are analyzed experimentally by in situ scanning tunneling microscopy. It is shown that the commonly observed coexistence of dense and nanoporous domains results from kinetic blockades rather than a thermodynamic equilibrium. The ripening can be favored by high densities of domain boundaries, which can be obtained by cooling the substrate before the nucleation and growth. Then ripening at higher-temperature yields large defect-free domains of a single structure. This thermodynamically stable structure can be either the dense or the nanoporous one, depending on the tecton concentration in the supernatant solution. A sharp phase transition from dense to honeycomb structures is observed at a critical concentration. This collective phenomenon is explained by introducing interactions between adsorbed molecules in the thermodynamic description of the whole system.

Tailored Single Crystals of Triisopropylsilylethynyl Pentacene by Selective Contact Evaporation Printing

Organic semiconductors have great potential for application in numerous emerging low-cost and disposable electronic devices such as organic thin fi lm transistors (OTFTs), solar cells, memories, sensors, and fl exible displays. [ 1–3 ] One of the most urgent demands for the realization of such devices is the need to develop new organic semiconductors with a high carrier mobility as well as good and cost-effective processibility for highperformance devices. Chemically-modifi ed pentacene derivatives such as triisopropylsilylethynyl pentacene (TIPS-PEN) have received much attention, in particular in OTFTs. [ 4 , 5–14 ]

Oligothiophene nanorings as electron resonators for whispering gallery modes.

Structural and electronic properties of oligothiophene nanowires and rings synthesized on a Au(111) surface are investigated by scanning tunneling microscopy. The spectroscopic data of the linear and cyclic oligomers show remarkable differences which, to a first approximation, can be accounted by considering electronic state confinement to one-dimensional boxes having, respectively, fixed and periodic boundary conditions. A more detailed analysis shows that polythiophene must be treated as a ribbon (i.e., having an effective width) rather than a purely 1D structure. A fascinating consequence is that the molecular nanorings act as whispering gallery mode resonators for electrons, opening the way for new applications in quantum electronics.

Periodic Positioning of Multilayered [2.2]Paracyclophane-Based Nanopillars†

The development of conjugated organic molecules as building blocks capable of ordering on well-defined surfaces is an active research area in view of future applications in molecular electronics, photonics, and nanomechanical devices. 4] Bottom-up approaches based on supramolecular chemistry are of particular interest for formation of preprogrammed organic self-assembled monolayers. Extensive efforts have been concentrated, independently, in two directions. The first, based on upright-oriented chemisorbed molecules such as thiols, consists of stacking functional moieties in the off-plane direction, which is usually detrimental for lateral order. The second, the objective of which is the mastering of increasingly complex lateral organizations, is based on two-dimensional molecules forming essentially planar structures. Combining complex in-plane self-organizations and off-plane protrusions in self-assembled molecular architectures is an important current challenge. This is a key requirement, for example, for the realization of biological molecular receptors or for decoupling electronically active molecules from a conducting substrate. The latter is currently achieved either through insulating layers or by introduction of molecular spacer units. However, such modifications strongly perturb the lateral organization. Moreover, complex combinations of in-plane and off-plane structures are precluded. A first step towards such combinations is the nanoscale positioning of rigid upstanding elements. We recently proposed a strategy to form “on-demand” noncovalent self-assemblies with predetermined 2D topologies on highly oriented pyrolitic graphite (HOPG) at the liquid/solid interface at room temperature. This method is based on a novel functional unit which acts as a cliplike noncovalent bond on HOPG. Here, we exploit this approach for the realization of 2D patterns of well-defined 3D nanostructures. Our strategy follows classical architectural paradigms based on the realization of a well-organized in-plane monolayer on HOPG and the emergence, perpendicular to the substrate, of a periodic array of standing organic nanopillars of tunable height. To validate this concept, we chose the multilayered [2.2]paracyclophane (PCP) moiety as a nanopillar of variable height. More precisely, we designed and synthesized a series of compounds (Scheme 1) bearing two

Narrow-line single-molecule transducer between electronic circuits and surface plasmons

A molecular wire containing an emitting molecular center is controllably suspended between the plasmonic electrodes of a cryogenic scanning tunneling microscope. Passing current through this circuit generates an ultranarrow-line emission at an energy of ≈1.5  eV which is assigned to the fluorescence of the molecular center. Control over the linewidth is obtained by progressively detaching the emitting unit from the surface. The recorded spectra also reveal several vibronic peaks of low intensities that can be viewed as a fingerprint of the emitter. Surface plasmons localized at the tip-sample interface are shown to play a major role in both excitation and emission of the molecular excitons.

OLIGOMERS CONTAINING ETHYNYLPYRIDAZINE MOIETIES : SYNTHESIS, FLUORESCENCE AND LIQUID CRYSTALLINE PROPERTIES. DIAZINES 50

Conjugated oligomers with ethynylpyridazine units have been synthetized by Sonogashira and Suzuki cross-coupling reactions. Some of them present interesting liquid crystals properties investigated by differential scanning calorimetry (DSC) and polarized light microscopy. Some of these oligomers are fluorescent.

Electronic energy and electron transfer processes in photoexcited donor-acceptor dyad and triad molecular systems based on triphenylene and perylene diimide units.

We investigate the photophysical properties of organic donor-acceptor dyad and triad molecular systems based on triphenylene and perylene diimide units linked by a non-conjugated flexible bridge in solution using complementary optical spectroscopy techniques. When these molecules are diluted in dichloromethane solution, energy transfer from the triphenylene to the perylene diimide excited moieties is evidenced by time-resolved fluorescence measurements resulting in a quenching of the emission from the triphenylene moieties. Simultaneously, another quenching process that affects the emission from both donor and acceptor units is observed. Solution ultrafast transient absorption measurements provide evidence of photo-induced charge transfer from either the donor or the acceptor depending upon the excitation. Overall, the analysis of the detailed time-resolved spectroscopic measurements carried out in the dyad and triad systems as well as in the triphenylene and perylene diimide units alone provides useful information both to better understand the relations between energy and charge transfer processes with molecular structures, and for the design of future functional dyad and triad architectures based on donor and acceptor moieties for organic optoelectronic applications.

Self-templating polythiophene derivatives: electronic decoupling of conjugated strands through staggered packing.

Whereas molecular electronics needs well-controlled 3D geometries for decoupling or interconnecting individual molecules, conjugated polymers form disordered structures when deposited on a substrate. We show that this trend can be overcome in polythiophene derivatives designed so as to exploit weak sulfur-bromine interactions. A self-template effect follows, leading to staggered organizations of well-aligned electronically decoupled conjugated strands, as observed in situ by scanning tunneling microscopy and spectroscopy on graphite.

Chemical engineering of donor–acceptor liquid crystalline dyads and triads for the controlled nanostructuration of organic semiconductors

Multi-segregated columnar structures provide a geometrically ideal scheme for ambipolar organic semiconductors, but are not easy to design. A set of novel materials with dyad and triad architectures based on 2 different discotic cores is reported and the conditions of emergence of such complex structures are investigated. The designed molecules associate together electron-donor triphenylene cores (D) and perylene or naphthalene diimides as acceptor moieties (A), both entities being linked via alkyl chain spacers. The evaluation in solution of their HOMO/LUMO energy levels by cyclic voltammetry demonstrates the preservation of the individual features of the D and A units. Their thermal and self-organization behaviors were studied by polarized-light optical microscopy, differential scanning calorimetry, temperature-dependent small-angle X-ray scattering and dilatometry, which permitted detailed investigation of the self-organization behaviour. These D–A compounds turned out to spontaneously self-organize into columnar mesophases at room temperature, with the D and A moieties segregated into either alternated stacks within mixed columns or in distinct columns, the latter providing an ideal configuration for 1D hole and electron transport pathways. In view of potential applications of the triad/dyad template, thin films of these self-organized materials were also probed by atomic force microscopy and grazing incidence X-ray scattering.