Stéphane Lenfant

Molecular Rectifying Diodes from Self-Assembly on Silicon

We demonstrate a molecular rectifying junction made from a sequential self-assembly on silicon. The device structure consists of only one conjugated (π) group and an alkyl spacer chain. We obtain rectification ratios up to 37 and threshold voltages for rectification between −0.3 and −0.9 V. We show that rectification occurs from resonance through the highest occupied molecular orbital of the π group in good agreement with our calculations and internal photoemission spectroscopy. This approach allows us to fabricate molecular rectifying diodes compatible with silicon nanotechnologies for future hybrid circuitries.

Two‐Terminal Carbon Nanotube Programmable Devices for Adaptive Architectures

2010 WILEY-VCH Verlag Gm Devices based on nanoscale objects with well-defined structures and original electronic properties are of great interest for the development of innovative electronic circuits, in particular if they offer novel functionalities (such as memory or sensing) and are compatible with large-scale self-assembly techniques. Among these devices, two-terminal ones such as memristors are attracting intense interest due to their potential superior capabilities in terms of integration. However, at the nanometer scale, one faces the critical issue of variability among devices, both in terms of device-to-device performances and in terms of precise positioning of individual objects. It is, thus, very unlikely that conventional circuit architectures developed for silicon complementary metal oxide semiconductor (CMOS) devices will be ideally suited for these new devices. Adaptive architectures comprising a programming or a learning step are probably more reasonable candidates, as they are naturally tolerant to variability. Targeting adaptive circuits is a challenging approach, which requires the development of devices that combine several key properties among which a well-controlled memory effect is the most critical. In this context, single-walled carbon nanotubes (SWNTs) are of special relevance, as they combine nanometer-scale size and 1D character with exceptional electronic, mechanical, and chemical properties. In particular, carbon nanotube-based field-effect transistors (CNTFETs), when aggressively scaled, compete favorably with predictions of ultimate silicon-based devices of the same size. However, circuits based on such three-terminal devices do not present sufficient improvement in terms of scaling, performances, and functionality to compensate for the strong issues related to their integration. Indeed, despite important progress in the field, there is still no satisfactory solution for the implementation of a pure CNTFET-based technology that would be useful in terms of function and competitive and compatible with existing technologies to allow their co-integration. Recently, we showed that CNTFETs, coated with a thin film of photoconductive polymer, combine light sensitivity with a strong and well-controlled non-volatile memory effect. In this Communication, we show that such optically gated carbon nanotube transistors (OG-CNTFETs) can be used as two-terminal memory devices, i.e., programmable resistors, which have all the required characteristics to serve as building blocks for adaptive architectures. In particular, the nanotube channel resistivity can be precisely adjusted in a very large range, spanning several orders of magnitudes (without using the gate electrode), and then stored in a non-volatile way. We also establish the capability to handle the programming of multiple devices and demonstrate how this approach addresses the crucial issue of variability among devices. All together, it shows that such mixed nanotube–polymer devices have all the characteristics of artificial synapses that can provide an elegant solution to build neural network types of circuits. The conductivity of an OG-CNTFETat a fixed source/drain bias (VDS) can be controlled independently using either a gate potential (VGS) or illumination at a wavelength corresponding to an absorption peak of the polymer. Such transistor can be built from an individual SWNT, as in our previous study, or from networks of SWNTs. Constructing circuits based on single-nanotube devices presents severe fabrication difficulties in the present state of development of the field, in particular, due to the mixing of metallic and semiconducting nanotubes in available sources and to the need for complex steps to achieve the precise positioning of individual nano-objects. Conversely, the use of dense networks of randomly oriented SWNTs allows the efficient fabrication of multiple and interconnected devices. Moreover, the use of parallel stripes of SWNTs as illustrated in Figure 1a and 2a allows obtaining routinely on/off (ION/IOFF) current ratios of several decades. [8] As we showed in ref. [9], such use of nanotube networks has no impact on the optical-gating mechanism so that the conclusions of the present work can be extended to the single-nanotube-device case. The transistors are fabricated as described in the Experimental section. In the dark, the devices behave as p-type Schottky transistors. Under illumination, the conductivity of the device increases due to the accumulation and trapping of photogenerated electrons at the nanotube/dielectric interface.

The metal/organic monolayer interface in molecular electronic devices

The metal/molecules/metal is the basic device used to measure the electronic properties of organic molecules envisioned as the key components in molecular-scale devices (molecular diode, molecular wire, molecular memory, etc.). This review paper describes the main techniques used to fabricate a metal/molecules/metal device (or more generally electrode/ molecules/electrode junctions, with electrodes made of metal or semiconductor). We discuss several problems encountered for the metallization of organic monolayers. The organic/electrode interface plays a strong role in the electronic properties of these molecular devices. We review some results on the relationships between the nature of the electrode/molecule interface (physisorbed or chemisorbed, evaporated metal electrode, mechanical contact, etc.) and the electronic transport properties of these molecular-scale devices. We also discuss the effects of symmetric versus asymmetric coupling of the two ends of the molecules with the electrodes.

High anisotropic conductivity in organic insulator/semiconductor monolayer heterostructure

We demonstrate a highly anisotropic conductivity, with a 109 ratio, between the in-plane and perpendicular electrical transport in organic insulator/semiconductor heterostructures of monolayer thickness. These heterostructures are self-assembled monolayers made of alkyl chains and functionalized by various conjugated moieties at their ends. The high anisotropic conductivity is due to the close packing of the conjugated end groups. These structures might be the building blocks of molecular-scale devices.

Sub-ppm Detection of Nerve Agents Using Chemically Functionalized Silicon Nanoribbon Field-Effect Transistors†

Organophosphorus compounds (OPs) represent one of the most important and lethal classes of chemical warfare agents (e.g. sarin, tabun, soman). Highly active volatile OPs are powerful inhibitors of acetylcholinesterase, which is a critical enzyme of the nervous system. The ease of manufacturing OPs based on inexpensive starting materials makes these agents a weapon of choice for terrorist attacks. Thus, the rapid sensing of these nerve agents has recently become an increasingly important research goal. Various approaches have been reported for the detection of these chemical warfare agents including colorimetric and fluorimetric spectroscopies, enzymatic assays, piezoelectric devices, single-walled carbon nanotube resistors and capacitors. However, these systems are plagued by limitations such as slow response time, moderate selectivity, operational complexity, or limited portability. Field-effect transistors (FET) based on nanomaterials such as semiconducting nanowires, nanoribbons, or carbon nanotubes have been recently explored for chemical and biological detection. Their high effectiveness is mainly ascribed to an extreme sensitivity to electrostatic changes at the surface of the semiconductor and/or modifications of the Schottky barrier at the semiconductor/metal interface. A charge generation in the vicinity of the semiconductor of a FET is known to alter the electrical properties of the device. Several research groups have independently developed a series of small-molecule fluorescent sensors for OPs detection. They investigated organic moieties reactive towards OPs by formation of a phosphate ester intermediate and subsequent intramolecular nucleophilic substitution, which led to an ammonium salt and thus charge formation. We thought monitoring this charge generation with a functionalized FET could be a particularly promising approach. Herein, we report the development of an OPs chemical sensor based on highly sensitive silicon nanoribbon field-effect transistors (SiNR-FETs) functionalized with compound 1 (Scheme 1).

Nanotube transistors as direct probes of the trap dynamics at dielectric-organic interfaces of interest in organic electronics and solar cells.

The high sensitivity of nanotube transistors is used for the first time as a probe to study charge dynamics at a dielectric/polymer (polythiophene) interface, an inorganic/organic junction of particular importance for organic solar cells, and organic field effect transistors (OFETs). A carbon nanotube field effect transistor is coated with a thin film of a photoconductive polymer and photoexcited so as to generate carriers in the structure. Comparison between devices using SiO2 and TiO2 as gate dielectric reveals the critical role of the dielectric and clearly elucidates the relative contributions of the polymer and the dielectric layers on the separation, trapping, and relaxation of photogenerated charges.

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.

Self-assembled molecular monolayers as ultrathin gate dielectric in carbon nanotube transistors

We demonstrate the use of a self-assembled monolayer of octadecanethiol on gold as thin gate dielectric for a single-walled carbon nanotube field-effect transistor. P-type transistors display very steep subthreshold slopes, greatly reduced hysteresis, and band-to-band tunneling. The suppression of the gate efficiency for n-type transistors emphasizes the key role of the electrical dipole of the molecular layer in controlling the switching. Combining the versatility of organic dielectrics with the exceptional electronic and mechanical properties of carbon nanotubes opens interesting ways toward the realization of fully organic nanoscale transistors.

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.

Directed assembly for carbon nanotube device fabrication

Chemically and biochemically-directed assembly of nanotubes (NT) for electronics is reviewed. Examples of new field-effect devices prepared this way either for high frequency (40GHz) operation or for applications to an optoelectronics multilevel memory are presented. A route towards (bio)molecular interconnects for NTs is outlined