David Alamarguy

Covalent grafting onto self-adhesive surfaces based on aryldiazonium salt seed layers

The chemistry of aryldiazonium salts has been thoroughly used in recent years to graft in a very simple and robust way ultrathin polyphenylene-like films on a broad range of surfaces. We show here that the same chemistry can be used to obtain “self-adhesive surfaces”. This target was reached in a simple way by coating various surfaces with chemisorbed organic films containing active aryldiazonium salts. These “self-adhesive surfaces” are then put into contact with various species (molecules, polymers, nanoparticles, nanotubes, graphene flakes, etc.) that react either spontaneously or under activation with the immobilized aryldiazonium salts. Our self-adhesive surfaces were synthesized following a simple aqueous two-step protocol based on p-phenylenediamine diazotisation. The first diazotisation step results in the robust grafting of thin polyaminophenylene (PAP) layers onto the surface. The second diazotisation step changed the grafted PAP film into a “poly-aryldiazonium polymer” (PDP) film. The covalent grafting between those self-adhesive surfaces and the target species was achieved by direct contact or by immersion of the self-adhesive surfaces in solution. We present in this preliminary work the grafting of multi-wall carbon nanotubes (MWCNTs), flakes of highly oriented pyrolytic graphite (HOPG), various organic compounds and copper nanoparticles. We also tested these immobilized aryldiazonium salts as electropolymerization initiators for the grafting-to process.

Characterization of graphene oxide reduced through chemical and biological processes

The study of new materials for transparent electrodes or new heterojunctions made of 2D materials combinations is a very active research topic. Challenges to overcome are the modulation of the optoelectronic properties of such materials to achieve competitive photovoltaic devices. In this work, graphene oxide was reduced into graphene through different chemical (hydrazine, ultraviolet photocatalysis) and biological (microorganisms) processes. W e benchmarked the reduction efficiency by probing materials characteristics using various physical characterization techniques. X-ray photoelectron spectroscopy (XPS) analyses were carried out to observe the effectiveness of the reduction processes through the sp2/sp3 content. In addition, the homogeneity of the reduction is investigated on micrometer scale sample with micro Raman mapping and extraction of the ID/IG ratio. Conductive-probe atomic force microscopy (CP-AFM) was employed to investigate the longitudinal conductivity of the different samples. The results show that hydrazine based reduction remains the most efficient. However, the bacterial procedure demonstrated partial reconstruction of the carbon network and reduced the amount of oxygenated functional groups.

Fretting Behavior of Nickel Coatings for Electrical Contact Applications

Fretting remains a major cause of connector failure and can impair reliability in complex systems. Oxidizable metals such as tin, copper and nickel are particularly prone to fretting degradation. We report here the first results of an investigation on fretting of nickel contacts with two types of deposits. Sulfate nickel layers are electrodeposited in different conditions and show very different behaviours during fretting tests. The characteristics of the layers are analyzed and show different compositions and microstructures. The compositions are measured by X-Ray Photoelectron Spectroscopy (XPS) which allows determining the chemical nature of the compounds formed during exposure to air. Topography is measured by AFM and the roughness and grain characteristics are assessed. Electrical properties at the micro/nanoscale are measured with the CP-AFM technique. Various loads are applied to the cantilever beam; the electrical characterization is performed versus the load. The results of fretting experiments are analyzed in terms of fretting regimes. The fretting regimes occurring during the test of the matte layers involve partial slip which delays the occurrence of contact resistance (Rc) increase. Gross slip in the interface is shown to create very poorly conducting wear debris leading to drastic increase of Rc. This study is part of a larger one aiming at tailoring coatings allowing the best tribological and electrical behaviors during fretting of nickel contacts.

Memristive and neuromorphic behavior in a Li x CoO 2 nanobattery

The phenomenon of resistive switching (RS), which was initially linked to non-volatile resistive memory applications, has recently also been associated with the concept of memristors, whose adjustable multilevel resistance characteristics open up unforeseen perspectives in cognitive computing. Herein, we demonstrate that the resistance states of LixCoO2 thin film-based metal-insulator-metal (MIM) solid-state cells can be tuned by sequential programming voltage pulses, and that these resistance states are dramatically dependent on the pulses input rate, hence emulating biological synapse plasticity. In addition, we identify the underlying electrochemical processes of RS in our MIM cells, which also reveal a nanobattery-like behavior, leading to the generation of electrical signals that bring an unprecedented new dimension to the connection between memristors and neuromorphic systems. Therefore, these LixCoO2-based MIM devices allow for a combination of possibilities, offering new perspectives of usage in nanoelectronics and bio-inspired neuromorphic circuits.

Electro-mechanical modelling of multilayer contacts in electrical connectors

Electrical contacts are an essential part of electrical circuits and many reliability problems are related to their failure. The present work uses numerical simulation in view of a better analysis of the electromechanical phenomena, in the case of multilayer electrical contacts. We study a ball/plane contact made of bulk CuZn alloy, protected by a thin Sn surface layer. A coupled finite element analysis is performed to calculate the contact resistance of the device: an elasto-plastic model is used to determine the geometry of the contact area, then an electrical model gives the resulting constriction resistance. Results of simulation are compared to experimental data. The respective contributions of the mechanical and electrical phenomena are analysed. The influence of a nickel underlayer is also examined.

Lubrication mechanisms of hot-dipped tin separable electrical contacts

Degradation phenomena occurring during the lifetime of tinned separable electrical contacts remain a factor of loss of reliability that can be minimised. In a previous paper, friction, wear and electrical properties of hot-dipped tin coatings on bronze substrates have been analysed with various techniques in order to show the improvement due to a well-suited fluorinated lubricant layer. Contacts were of the sphere on plane type obtained from strips of tinned CuSn/sub 4/ (as received for the flats, and formed by stamping for the dimples). Macroscopic friction cycles simulating insertion and withdrawal of separable contacts were performed as well as fretting cycles. Efficiency of the lubricant was strongly related to the film thickness in the case of macroscopic friction of dimple on flat contacts. For a sufficient film thickness, different behaviours were observed for different molecules. Here, the particularity of lubrication mechanisms of hot-dipped tin contacts is analysed. The evolution of static contact resistance values before friction and during the friction tests correlated to the wear behaviour shows the influence of some of the lubricant properties such as viscosity and chain structure.

Evaluation of the nanotube intrinsic resistance across the tip-carbon nanotube-metal substrate junction by Atomic Force Microscopy

Using an atomic force microscope (AFM) at a controlled contact force, we report the electrical signal response of multi-walled carbon nanotubes (MWCNTs) disposed on a golden thin film. In this investigation, we highlight first the theoretical calculation of the contact resistance between two types of conductive tips (metal-coated and doped diamond-coated), individual MWCNTs and golden substrate. We also propose a circuit analysis model to schematize the «tip-CNT-substrate» junction by means of a series-parallel resistance network. We estimate the contact resistance R of each contribution of the junction such as Rtip-CNT, RCNT-substrate and Rtip-substrate by using the Sharvin resistance model. Our final objective is thus to deduce the CNT intrinsic radial resistance taking into account the calculated electrical resistance values with the global resistance measured experimentally. An unwished electrochemical phenomenon at the tip apex has also been evidenced by performing measurements at different bias voltages with diamond tips. For negative tip-substrate bias, a systematic degradation in color and contrast of the electrical cartography occurs, consisting of an important and non-reversible increase of the measured resistance. This effect is attributed to the oxidation of some amorphous carbon areas scattered over the diamond layer covering the tip. For a direct polarization, the CNT and substrate surface can in turn be modified by an oxidation mechanism.

Nanocomposite thin films for surface protection in electrical contact applications

Increased demands on reliability, operations in harsh environments and miniaturisation of electrical contacts, justify research on totally innovating coatings. Such films require many specific properties such as proper conduction and mechanical behaviour. The approach reported here is based on the idea of associating a polymer matrix and a conducting charge. The polymer matrix was chosen to resist mechanical degradation while the charge allows good conductivity. The first results obtained by associating a carbon nanotube (CNT) network to a fluorinated polymer thin layer are presented here. Several characterisation techniques such as XPS, FT-IR and Raman spectroscopy have allowed the control of the various steps of the elaboration process. Films were deposited on cuprous coupons with a nickel underlayer and an electrodeposited gold final layer. Several matrixes were studied. The properties of these thin films were investigated in a ball plane contact configuration: first electrical and mechanical characterisations show low values of contact resistance and friction coefficient. The CNT network influence is investigated both at a macroscopic scale and at a microscopic one.

Multi-scale study of the electrical properties of organic layers grafted on gold surfaces

Many studies have been performed on the corrosion and fretting degradation mechanisms of gold plated contacts for low level applications. Reliability requirements constantly increase and finding new solutions able to postpone degradations is still a challenge. A large study has led to the elaboration of a fluorinated organic film grafted to the surface and allowing outstanding protection of the gold surfaces. These organic films involved new thiol functionalized perfluoro polyethers, deposited as mixed layers. Back-panel connector systems coated with various fluorinated organic films were submitted to a four gases corrosion test (Bellcore uncontrolled environment test). High values of the contact resistance Rc were found for the contacts coated with some of the films. The present study is aimed at understanding the electrical properties of these films. The electrical behavior of model contacts (dimple on flat) was investigated in static and in dynamic modes. Fretting experiments showed the combined influence of friction and of the surface roughness. Finally the electrical properties were investigated at a more microscopic scale. An experiment performed with a modified AFM microscope with a conducting probe (CP-AFM) showed the particular behavior of functionalized PFPE lubricant on gold. Results are discussed in order to have an insight into the conduction mechanisms involved.

Electronic properties of embedded graphene: doped amorphous silicon/CVD graphene heterostructures

Large-area graphene film is of great interest for a wide spectrum of electronic applications, such as field effect devices, displays, and solar cells, among many others. Here, we fabricated heterostructures composed of graphene (Gr) grown by chemical vapor deposition (CVD) on copper substrate and transferred to SiO2/Si substrates, capped by n‑ or p-type doped amorphous silicon (a-Si:H) deposited by plasma-enhanced chemical vapor deposition. Using Raman scattering we show that despite the mechanical strain induced by the a-Si:H deposition, the structural integrity of the graphene is preserved. Moreover, Hall effect measurements directly on the embedded graphene show that the electronic properties of CVD graphene can be modulated according to the doping type of the a-Si:H as well as its phase i.e. amorphous or nanocrystalline. The sheet resistance varies from 360 Ω sq(-1) to 1260 Ω sq(-1) for the (p)-a-Si:H/Gr (n)-a-Si:H/Gr, respectively. We observed a temperature independent hole mobility of up to 1400 cm(2) V(-1) s(-1) indicating that charge impurity is the principal mechanism limiting the transport in this heterostructure. We have demonstrated that embedding CVD graphene under a-Si:H is a viable route for large scale graphene based solar cells or display applications.