David Guerin

An Organic Nanoparticle Transistor Behaving as a Biological Spiking Synapse

Molecule-based devices are envisioned to complement silicon devices by providing new functions or by implementing existing functions at a simpler process level and lower cost, by virtue of their self-organization capabilities. Moreover, they are not bound to von Neuman architecture and this feature may open the way to other architectural paradigms. Neuromorphic electronics is one of them. Here, a device made of molecules and nanoparticles-a nanoparticle organic memory field-effect transistor (NOMFET)—that exhibits the main behavior of a biological spiking synapse is demonstrated. Facilitating and depressing synaptic behaviors can be reproduced by the NOMFET and can be programmed. The synaptic plasticity for real-time computing is evidenced and described by a simple model. These results open the way to rate-coding utilization of the NOMFET in dynamical neuromorphic computing circuits.

Gold nanoparticle-pentacene memory-transistors

We demonstrate an organic memory-transistor device based on a pentacene-gold nanoparticles active layer. Gold (Au) nanoparticles are immobilized on the gate dielectric (silicon dioxide) of a pentacene transistor by an amino-terminated self-assembled monolayer. Under the application of writing and erasing pulses on the gate, large threshold voltage shift (22V) and on/off drain current ratio of ∼3×104 are obtained. The hole field-effect mobility of the transistor is similar in the on and off states (less than a factor of 2). Charge retention times up to 4500s are observed. The memory effect is mainly attributed to the Au nanoparticles.

A silicon nanowire ion-sensitive field-effect transistor with elementary charge sensitivity

We investigate the mechanisms responsible for the low-frequency noise in liquid-gated nanoscale silicon nanowire field-effect transistors (SiNW-FETs) and show that the charge-noise level is lower than elementary charge. Our measurements also show that ionic strength of the surrounding electrolyte has a minimal effect on the overall noise. Dielectric polarization noise seems to be at the origin of the 1/f noise in our devices. The estimated spectral density of charge noise Sq=1.6×10−2 e/Hz1/2 at 10 Hz opens the door to metrological studies with these SiNW-FETs for the electrical detection of a small number of molecules.

Electroactive Nanorods and Nanorings Designed by Supramolecular Association of π-Conjugated Oligomers

In investigations into the design and isolation of semiconducting nano-objects, the synthesis of a new bisureido pi-conjugated organogelator has been achieved. This oligo(phenylenethienylene) derivative was found to be capable of forming one-dimensional supramolecular assemblies, leading to the gelation of several solvents. Its self-assembling properties have been studied with different techniques (AFM, EFM, etc.). Nano-objects have successfully been fabricated from the pristine organogel under appropriate dilution conditions. In particular, nanorods and nanorings composed of the electroactive organogelator have been isolated and characterized. With additional support from an electrochemical study of the organogelator in solution, it has been demonstrated by the EFM technique that such nano-objects were capable of exhibiting charge transport properties, a requirement in the fabrication of nanoscale optoelectronic devices. It was observed that positive charges can be injected and delocalized all along an individual nano-object (nanorod and nanoring) over micrometers and, remarkably, that no charge was stored in the center of the nanoring. It was also observed that topographic constructions in the nanostructures prevent transport and delocalization. The same experiments were performed with a negative bias (i.e., electron injection), but no charge delocalization was observed. These results could be correlated with the nature of 1, which is a good electron-donor, so it can easily be oxidized, but can be reduced only with difficulty.

Electrochemical grafting of octyltrichlorosilane monolayer on Si

The octyltrichlorosilane (OTS) monolayer on hydrated Si (111) surfaces has been grafted by cyclic voltammetry (CV) using tetrabutylammonium perchlorate in dry methanol as supporting electrolyte. The percentage of OTS coverage, calculated from the current value at −1V, after 30 CV scans was found to be >97%, which is independently confirmed by atomic force microscopy. A mechanism of electrochemical grafting of OTS on Si via formation of Si–Si bonds is proposed. Current-voltage characteristics and impedance spectroscopic measurements on Al/OTS/Si structures reveal realization of a true OTS/Si interface.

Relaxation dynamics in covalently bonded organic monolayers on silicon

We study the dynamic electrical response of a silicon-molecular monolayer-metal junctions and we observe two contributions in the admittance spectroscopy data. These contributions are related to dipolar relaxation and molecular organization in the monolayer in one hand, and the presence of defects at the silicon/molecule interface in the other hand. We propose a small signal equivalent circuit suitable for the simulations of these molecular devices in commercial device simulators. Our results concern monolayers of alkyl chains considered as a model system but can be extended to other molecular monolayers. These results open door to a better control and optimization of molecular devices.

Fowler-Nordheim tunnelling and electrically stressed breakdown of 3-mercaptopropyltrimethoxysilane self-assembled monolayers

We report the confirmed occurrence of Fowler?Nordheim (FN) electron tunnelling in p+?Si (SiOx)/self-assembled monolayers of 3-mercaptopropyltrimethoxysilane (MPTMS)/Au structures. The statistically favoured values of the effective mass and energy barrier heights for electrons are determined to be in the ranges 0.15?0.18?me and 1.3?1.5?eV, respectively. The electrically stressed breakdown of the monolayers is observed to take place at very high fields, i.e.?16?50?MV?cm?1. Prior to the breakdown, switching of FN currents between different conduction states was observed; this is found to be related to a change in the electrical properties of monolayers owing to the creation of field-induced defects.

Access to Constrained Fluoropseudopeptides via Ring-Closing Metathesis of Fluoroalkenes

Bis-alkene substrates, containing one fluoroalkene and linked by an amide moiety, have been designed and synthesized to be subjected to ring-closing metathesis reactions. The substitution of fluoroalkene by a phenyl group enhanced the reactivity, and the resulting fluorinated lactams were obtained in high yields except when a hindered alkyl group was present in the molecule. The cycles were then subjected to ring opening in order to develop a new route to constrained fluoropseudopeptides bearing a fluoroalkene as a peptide bond mimic.

Synthesis and electrical properties of fullerene-based molecular junctions on silicon substrate

We report the synthesis and the electrical properties of fullerene-based molecular junctions on silicon substrates in which the highly π-conjugated molecule C60 (π quantum well) is isolated from the electrodes by alkyl chains (σ tunnel barriers). Initially, the Si/SiO2/σC60 architecture was prepared either by sequential synthesis (3 different routes) or by direct grafting of the presynthesized C60-σ-Si(OEt)3 molecule. We described the chemical synthesis of these routes and the physico-chemical properties of the molecular monolayers. Then the second σ tunnel barrier was added on the Si/SiO2/σC60 junction by applying a hanging mercury drop electrode thiolated with an alkanethiol monolayer. We compared the electronic transport properties of the Si/SiO2/σC60//Hg and Si/SiO2/σC60//σHg molecular junctions, and we demonstrated by transition voltage spectroscopy that the fullerene LUMO - metal Fermi energy offset can be tailored from ∼0.2 eV to ∼1 eV by changing the length of the alkyl chain between the C60 core and the Hg metal electrode (i. e. from direct C60//Hg contact to a 14 carbon atoms tunnel barrier).

Neuromorphic Time-Dependent Pattern Classification with Organic Electrochemical Transistor Arrays

Based on bottom-up assembly of highly variable neural cells units, the nervous system can reach unequalled level of performances with respect to standard materials and devices used in microelectronic. Reproducing these basic concepts in hardware could potentially revolutionize materials and device engineering which are used for information processing. Here, we present an innovative approach that relies on both iono-electronic materials and intrinsic device physics to show pattern classification out of a 12-unit bio-sensing array. We use the reservoir computing and learning concept to demonstrate relevant computing based on the ionic dynamics in 400-nm channel-length organic electrochemical transistor (OECT). We show that this approach copes efficiently with the high level of variability obtained by bottom-up fabrication using a new electropolymerizable polymer, which enables iono-electronic device functionality and material stability in the electrolyte. We investigate the effect of the array size and variability on the performances for a real-time classification task paving the way to new embedded sensing and processing approaches.