Pascal Martin

Comparison of a hair bundle's spontaneous oscillations with its response to mechanical stimulation reveals the underlying active process

Hearing relies on active filtering to achieve exquisite sensitivity and sharp frequency selectivity. In a quiet environment, the ears of many vertebrates become unstable and emit one to several tones. These spontaneous otoacoustic emissions, the most striking manifestation of the inner ears active process, must result from self-sustained mechanical oscillations of aural constituents. The mechanoreceptive hair bundles of hair cells in the bullfrogs sacculus have the ability to amplify mechanical stimuli and oscillate spontaneously. By comparing a hair bundles spontaneous oscillations with its response to small mechanical stimuli, we demonstrate a breakdown in a general principle of equilibrium thermodynamics, the fluctuation–dissipation theorem. We thus confirm that a hair bundles spontaneous movements are produced by energy-consuming elements within the hair cell. To characterize the dynamical behavior of the active process, we introduce an effective temperature that, for each frequency component, quantifies a hair bundles deviation from thermal equilibrium. The effective temperature diverges near the bundles frequency of spontaneous oscillation. This behavior, which is not generic for active oscillators, can be accommodated by a simple model that characterizes quantitatively the fluctuations of the spontaneous movements as well as the hair bundles linear response function.

Influence of the noise model on level set active contour segmentation

We analyze level set implementation of region snakes based on the maximum likelihood method for different noise models that belong to the exponential family. We show that this approach can improve segmentation results in noisy images and we demonstrate that the regularization term can be efficiently determined using an information theory-based approach, i.e., the minimum description length principle. The criterion to be optimized has no free parameter to be tuned by the user and the obtained segmentation technique is adapted to nonsimply connected objects.

Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions

In this work, we bridge the gap between short-range tunneling in molecular junctions and activated hopping in bulk organic films, and greatly extend the distance range of charge transport in molecular electronic devices. Three distinct transport mechanisms were observed for 4.5–22-nm-thick oligo(thiophene) layers between carbon contacts, with tunneling operative when d < 8 nm, activated hopping when d > 16 nm for high temperatures and low bias, and a third mechanism consistent with field-induced ionization of highest occupied molecular orbitals or interface states to generate charge carriers when d = 8–22 nm. Transport in the 8–22-nm range is weakly temperature dependent, with a field-dependent activation barrier that becomes negligible at moderate bias. We thus report here a unique, activationless transport mechanism, operative over 8–22-nm distances without involving hopping, which severely limits carrier mobility and device lifetime in organic semiconductors. Charge transport in molecular electronic junctions can thus be effective for transport distances significantly greater than the 1–5 nm associated with quantum-mechanical tunneling.

Giant Plasmon Resonance Shift Using Poly(3,4-ethylenedioxythiophene) Electrochemical Switching

Herein, we report the variation of localized surface plasmon resonance (LSPR) of gold nanoparticle (NP) arrays covered by poly(3,4-ethylenedioxythiophene) (PEDOT) as a function of the electronic state of the polymer. Giant shifts and fine-tuning of the LSPR of gold NPs surrounded by PEDOT/sodium docecyl sulfate have been achieved. The color variations of plasmonic/conducting polymer (CP) devices are given not only by changes of the optical properties of the CP upon doping but also by a close synergy of the optical properties of CP and NP. Such systems can considerably extend the field of CP-based electrochromic devices.

Grafting Oligothiophenes on Surfaces by Diazonium Electroreduction: A Step toward Ultrathin Junction with Well-Defined Metal/Oligomer Interface

The functionalization of electrode materials through diazonium electroreduction using a heteroaromatic compound, without phenyl groups, has been investigated for the first time. The electrochemical reduction of 2-aminoterthiophenyldiazonium cation, generated in situ, coats the electrode (glassy carbon (GC), gold or platinum) with an ultrathin organic layer, shown by X-ray photoelectron spectroscopy (XPS) of that deposited on gold to consist of terthiophene or oligothiophene. The coating is electroactive at potential close to that of terthiophene in solution. The electrochemical response of the modified GC electrode in the presence of various reversible redox couples shows that the attached layer acts as a conductive switch. It behaves as a barrier to electron transfer when the standard redox potential is below 0.5 V/SCE; in this case diode-like behavior is observed. However, for more oxidizing redox probes the layer can be considered as transparent and no barrier effect is observed. The layer deposited on a platinum ultramicroelectrode (UME) behaves similarly to that obtained on the large GC electrode. Scanning electrochemical microscopy (SECM) can be performed using this electroswitchable modified platinum UME which can act as a filter toward competitive redox exchange pathways.

Direct Observation of Large Quantum Interference Effect in Anthraquinone Solid-State Junctions

Quantum interference in cross-conjugated molecules embedded in solid-state devices was investigated by direct current-voltage and differential conductance transport measurements of anthraquinone (AQ)-based large area planar junctions. A thin film of AQ was grafted covalently on the junction base electrode by diazonium electroreduction, while the counter electrode was directly evaporated on top of the molecular layer. Our technique provides direct evidence of a large quantum interference effect in multiple CMOS compatible planar junctions. The quantum interference is manifested by a pronounced dip in the differential conductance close to zero voltage bias. The experimental signature is well developed at low temperature (4 K), showing a large amplitude dip with a minimum >2 orders of magnitude lower than the conductance at higher bias and is still clearly evident at room temperature. A temperature analysis of the conductance curves revealed that electron-phonon coupling is the principal decoherence mechanism causing large conductance oscillations at low temperature.

Ionic liquid viscosity effects on the functionalization of electrode material through the electroreduction of diazonium.

The electrochemical reduction of 4-nitrophenyl diazonium, NPD, in different ionic liquids presenting different viscosities has been investigated. The electrochemical studies show that the reduction of diazonium leading to the formation of its corresponding radical occurs whatever the viscosity of the grafting media. Following that, the presence of an organic layer attached to the electrode after electrochemical treatment was evidenced by cyclic voltammetry (CV) in acidic media thanks to the presence of nitro groups. Moreover, infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) confirm the presence of a nitrophenyl (NP) layer attached to the electrode material. Next, the examination of the electrochemical data through the measurement of the charge, corresponding to the reduction of the attached nitrophenyl (NP) moieties, shows that the surface concentration of NP, Γ(NP), decreases when the viscosity, η, of the grafting media increases. Additionally, in the case of the more viscous ionic liquid, N-tributyl-N-methylammonium bis(trifluoromethylsulfonyl)imide [Bu(3)MeN] [NTf(2)], a cosolvent has been added leading to fine decrease of the viscosity. The IR and CV investigations of the modified electrodes demonstrate the decrease of the amount of the attached molecules when the viscosity of the grafting media increases. In addition, a correlation between Γ(NP) as function of 1/η was observed. Finally, XPS and AFM experiments lead to an estimate of the thickness of the attached layer. As a result, both methods are in perfect agreement and thicknesses of 4 and 1 nm are measured after grafting in acetonitrile and in pure ionic liquid [Bu(3)MeN] [NTf(2)], respectively. By comparison with classical solvent, the use of viscous ionic liquid for the grafting leads to a decrease in the amount of the attached molecules and conduce to the formation of thinner or less dense layer.

Host−Guest Complexation: A Convenient Route for the Electroreduction of Diazonium Salts in Aqueous Media and the Formation of Composite Materials

Electrochemical grafting of a water-insoluble diazonium salt in aqueous media onto an electrode surface was achieved by host-guest complexation. 1-(2-Bisthienyl)-4-aminobenzene (BTAB) was solubilized in a water/beta-cyclodextrin solution (beta-CD). The corresponding diazonium salt was generated in situ then electroreduced. This process leads to the attachment of bithiophene or short oligothiophene groups to the electrode surface. The modified surfaces were analyzed by cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). The electrochemical investigations show that the water-based modified surface is similar to one generated in acetonitrile without beta-CD. Thus, the attached organic layer behaves like an electrochemical switch (above some threshold potential, a soluble external probe is oxidized, but the oxidized form cannot be reduced). The modified surfaces consist of grafted bisthienylbenzene (BTB) and cyclodextrins that can be removed from the surface. This procedure may be considered as a new means of creating a surface made of submicrometric holes in an organic semiconducting layer.

Micro/Nano-Structured Polypyrrole Surfaces on Oxidizable Metals as Smart Electroswitchable Coatings

Polypyrrole (PPy) films were electrodeposited from a pyrrole/sodium salicylate solution in water, through two-dimensional (2-D) polystyrene (PS) templates self-assembled on various oxidizable metals, after which the template was removed by dissolution in tetrahydrofuran (THF). The resulting PPy films were analyzed by scanning electron microscopy and atomic force microscopy. Two-dimensional PPy honeycomb structures are obtained on copper or mild steel by using PS spheres of various sizes. The morphology of these structures was controlled electrochemically, as an increase in the polymerization charge does not disturb the PPy honeycomb arrangement, leading instead to the formation of deeper pores accompanied by a change in their diameter. The hydrophobicity of the reduced micro-structured PPy surface is much greater than that of a bulk PPy film generated on the same metal. Reversible electro-switching of the wettability was obtained with marked variation of the apparent contact angle upon PPy oxido-reduction, and an important effect of film micro-structuration upon the wettability range.

Nonparametric statistical snake based on the minimum stochastic complexity

We propose a nonparametric statistical snake technique that is based on the minimization of the stochastic complexity (minimum description length principle). The probability distributions of the gray levels in the different regions of the image are described with step functions with parameters that are estimated. The segmentation is thus obtained by minimizing a criterion that does not include any parameter to be tuned by the user. We illustrate the robustness of this technique on various types of images with level set and polygonal contour models. The efficiency of this approach is also analyzed in comparison with parametric statistical techniques