Approaching the Frontier Between Fiber Devices and Single Molecule Devices in Redox Gated Junction

Charge transport in two conducting polymer [poly­(bithiophene) (PBT) and poly­(ethylenedioxythiophene) (PEDOT)] nanojunctions was investigated using two microelectrodes, separated by micrometric gap. Such junctions are redox gated and exhibit conductance switching between low and high resistance states at potential of 1.2 and 0 V, respectively. Devices with conductance between 100 and 500 nS in the oxidized state were easily obtained, indicating control of the charge transport within the whole micrometric gap by a limited number of wires (less than 100 oligomeric strands). <i>I</i>/<i>V</i> characteristics and steady state conductance measurements, for various gate potential, indicate that measured on/off ratios can be as high as 1000 despite the small number of strands controlling the charge transport properties of the devices. Finally, we show that generating nanojunctions whose smallest diameter is below 4 nm on a length close to the size of a polaron, or its localization length, makes it possible to reach the frontier between fiber devices and single molecule devices

When Electron Transfer Meets Electron Transport in Redox-Active Molecular Nanojunctions

A scanning electrochemical microscope (SECM) was used to arrange two microelectrodes face-to-face separated by a micrometric gap. Polyaniline (PANI) was deposited electrochemically from the SECM tip side until it bridged the two electrodes. The junctions obtained were characterized by following the current through the PANI as a function of its electrochemical potential measured versus a reference electrode acting as a gate electrode in a solid-state transistor. PANI nanojunctions showed conductances below 100 nS in the oxidized state, indicating control of the charge transport within the whole micrometric gap by a limited number of PANI wires. The SECM configuration makes it possible to observe in the same experiment and in the same current range the electron-transfer and electron-transport processes. These two phenomena are distinguished here and characterized by following the variation of the current with the bias voltage and the scan rate. The electron-transfer current changes with the scan rate, while the charge-transport current varies with the bias voltage. Finally, despite the initially micrometric gap, a junction where the conductance is controlled by a single oligoaniline strand is achieved

Conducting Ferrocene Monolayers on Nonconducting Surfaces

The redox activity of a ferrocenyl monolayer grafted on an n-type Si(111) substrate was investigated by scanning electrochemical microscopy (SECM) in conditions where the substrate plays the role of an insulator. This approach permits the differentiation between the different possible electron-transfer and mass-transport pathways occurring at the interface. As an exciting result, the thin ferrocenyl monolayer behaves like a purely conducting material, highlighting very fast electron communication between immobilized ferrocenyl headgroups in a 2D-like charge-transport mechanism

Scanning Electrochemical Microscopy Investigations of Monolayers Bound to p-Type Silicon Substrates

p-Si type electrodes modified with different organic monolayers were investigated by reaction with radical anion and cation electrogenerated at a microelectrode operating in the configuration of a scanning electrochemical microscope. The method proves to be a convenient tool for investigating both the quality and the redox properties of the layer as previously demonstrated on metallic electrodes especially when the sample cannot be electrically connected. Approach curves recorded with the different mediators were used to investigate the electron-transfer rates across alkyl monolayers bound to p-type silicon substrates. Preliminary results indicate that the interfacial electron transfer occurs via electron tunneling through the organic layer as generally described for SAMs grafted on gold electrodes

Variations of Diffusion Coefficients of Redox Active Molecules in Room Temperature Ionic Liquids upon Electron Transfer

In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio γ = Dred/Dox, where Dred and Dox are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of γ have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest γ ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, γ tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The γ ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the γ ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation

Electrochemical Fabrication of Highly Stable Redox-Active Nanojunctions

Redox-gated molecular junctions were obtained starting with a relatively large gap between two electrodes, in the micrometer range, followed by electrochemical polymerization of aniline. Polyaniline (PANI) grows from the tip side until it bridges the two electrodes. The resulting junctions were characterized electrochemically by following the variation of the tip–substrate current as a function of the electrochemical gate potential for various bias voltages and by recording their I(V) characteristics. The two electrodes make contact through PANI wires, and microjunctions with conductances around 10–3 S were obtained. On the basis of a similar setup, PANI nanojunctions with conductances between 10–7 and 10–8 S were made, where the current appears to be controlled by fewer than 10 oligoaniline strands. Despite the small number of strands connecting the two electrodes, the junctions are highly stable even when several successive potential sweeps are performed. Comparison of the conductance measured in the oxidized and reduced states leads to an on/off ratio of about 70–100, which is higher than that reported for a single aniline heptamer bridging two electrodes, highlighting the interest of connecting a few tens of molecules using the scanning electrochemical microscopy (SECM) configuration. In some cases, the switching of the PANI takes place in several individual conductance steps close to that obtained for a single oligoaniline. Finally, starting with a microjunction and mechanically withdrawing the tip shrinks it down to the nanometer scale and makes it possible to reach the regime where the conductance is controlled by a limited number of strands. This work presents an easy method for making redox-gated nanojunctions and for probing the conductance of a few oligoanilines despite an initially large tip–substrate gap

Surface Initiated Immobilization of Molecules Contained in an Ionic Liquid Framework

A simple and general route for the immobilization of molecules containing ionic liquids framework was described. The proposed approach is inspired from the classical synthesis of ionic liquid and labeled surface-initiated synthesis of molecules bearing ionic liquid components. In the first step, bromide end layer was electrochemically grafted onto the electrode surface followed by its reaction with imidazole derivatives. The generated modified materials were characterized by electrochemistry and by X-ray photoelectron spectroscopy (XPS). As a result, molecule-based ionic liquids were successfully attached onto electrode material. The possibility to perform an anion-exchange reaction within the layer was demonstrated. Furthermore, the proposed surface functionalization approach was successfully performed without requiring the synthesis of any intermediate. The generated structures provide multifunctional systems containing ions, immobilized cation and mobile anion, and redox species

Reactivity of Platinum Metal with Organic Radical Anions from Metal to Negative Oxidation States

The reaction of platinum metal with an organic molecular radical anion leads to the formation of iono-metallic phases where Pt exists under negative oxidation states. This puzzling transformation of a “noncorrodible metal” was examined using localized electrochemical techniques in dimethylformamide containing different tetra-alkylammonium salts chosen as test systems. Our experiments demonstrate that the platinum metal is locally reduced as soon as the Pt faces relatively moderate reducing conditions, for example, when the Pt is used as a negative electrode or when the metal is in the presence of a reducing agent such as an organic radical anion. Scanning electrochemical microscopy (SECM) analysis, current−distance curves, and transient mode responses provide detailed descriptions of the reactivity of Pt to form negative oxidation states (the key step is the reaction of the metal with a molecular reducing agent), of the insulating nature of the “reduced” solid phases of the thermodynamics and kinetics conditions of the Pt conversion. The passage from the conductor to insulator states controlled the spatial development of the reaction that always remains in competition with the other “natural” roles of a metallic electrode. Formally, the phenomena can be treated by analogy with the C. Amatore's model previously developed for the mediated reduction of the poly(tetrafluoroethylene). Consequences of this general reactivity of Pt are discussed in view of a wide utilization of this metal in reductive conditions and the possible applications of such processes in the micropatterning of metallic surfaces

Surface and Electrochemical Properties of Polymer Brush-Based Redox Poly(Ionic Liquid)

Redox-active poly­(ionic liquid) poly­(3-(2-methacryloyloxy ethyl)-1-(N-(ferrocenylmethyl) imidazolium bis­(trifluoromethylsulfonyl)­imide deposited onto electrode surfaces has been prepared using surface-initiated atom transfer radical polymerization SI-ATRP. The process starts by electrochemical immobilization of initiator layer, and then methacrylate monomer carrying ferrocene and imidazolium units is polymerized in ionic liquid media via SI-ATRP process. The surfaces analyses of the polymer exhibit a well-defined polymer brushlike structure and confirm the presence of ferrocene and ionic moieties within the film. Furthermore, the electrochemical investigations of poly­(redox-active ionic liquid) in different media demonstrate that the electron transfer is not restricted by the rate of counterion migration into/out of the polymer. The attractive electrochemical performance of these materials is further demonstrated by performing electrochemical measurement, of poly­(ferrocene ionic liquid), in solvent-free electrolyte. The facile synthesis of such highly ordered electroactive materials based ionic liquid could be useful for the fabrication of nanostructured electrode suitable for performing electrochemistry in solvent free electrolyte. We also demonstrate possible applications of the poly­(FcIL) as electrochemically reversible surface wettability system and as electrochemical sensor for the catalytic activity toward the oxidation of tyrosine