Showing papers by "Marc Respaud published in 2016"

On-chip and freestanding elastic carbon films for micro-supercapacitors

Abstract: Integration of electrochemical capacitors with silicon-based electronics is a major challenge, limiting energy storage on a chip. We describe a wafer-scale process for manufacturing strongly adhering carbide-derived carbon films and interdigitated micro-supercapacitors with embedded titanium carbide current collectors, fully compatible with current microfabrication and silicon-based device technology. Capacitance of those films reaches 410 farads per cubic centimeter/200 millifarads per square centimeter in aqueous electrolyte and 170 farads per cubic centimeter/85 millifarads per square centimeter in organic electrolyte. We also demonstrate preparation of self-supported, mechanically stable, micrometer-thick porous carbon films with a Young’s modulus of 14.5 gigapascals, with the possibility of further transfer onto flexible substrates. These materials are interesting for applications in structural energy storage, tribology, and gas separation.

Concerted Growth and Ordering of Cobalt Nanorod Arrays as Revealed by Tandem in Situ SAXS-XAS Studies.

Abstract: The molecular and ensemble dynamics for the growth of hierarchical supercrystals of cobalt nanorods have been studied by in situ tandem X-ray absorption spectroscopy–small-angle X-ray scattering (XAS-SAXS). The supercrystals were obtained by reducing a Co(II) precursor under H2 in the presence of a long-chain amine and a long-chain carboxylic acid. Complementary time-dependent ex situ TEM studies were also performed. The experimental data provide critical insights into the nanorod growth mechanism and unequivocal evidence for a concerted growth–organization process. Nanorod formation involves cobalt nucleation, a fast atom-by-atom anisotropic growth, and a slower oriented attachment process that continues well after cobalt reduction is complete. Smectic-like ordering of the nanorods appears very early in the process, as soon as nanoparticle elongation appears, and nanorod growth takes place inside organized superlattices, which can be regarded as mesocrystals.

Enhanced magnetocrystalline anisotropy in an ultra-dense array of air-exposed crystalline cobalt nanowires

Abstract: The magnetic anisotropy of an ultradense array of crystalline cobalt nanowires is investigated by means of broadband ferromagnetic resonance and magnetic torque measurements. The array is grown epitaxially in solution on a Pt(111) film and consists of single crystalline metallic wires with a diameter of 6.2 nm and a center-to-center interwire distance of 9.6 nm. The shape anisotropy and the Co hexagonal compact structure with the c-axis along the wire axis combine with each other to impose a perpendicular magnetic anisotropy despite the high density of 8 × 1012 wires/in.2. The intrinsic uniaxial magnetocrystalline anisotropy constants K1 and K2 are extracted from the ferromagnetic resonance and torque measurements using a mean field approach accounting for the interwire dipolar interactions. At room temperature, and despite air exposure, an unexpected increase of K1 and K2 of more than 40% with respect to the bulk is evidenced.

Electrospray deposition of isolated chemically synthesized magnetic nanoparticles

Abstract: The deposition of isolated magnetic nanoparticles onto a substrate was performed using electrohydrodynamic spraying. Two kinds of nanoparticles were sprayed, 11 nm CoFe carbide nanospheres and 10.5 nm Fe nanocubes. By studying carefully the evolution of the sprayed charged droplets and the mechanism of nanoparticle dispersion in them, we could optimize the nanoparticle concentration within the initial nanoparticle solution (i) to reduce the magnetic interaction and therefore prevent agglomeration and (ii) to obtain in a relatively short period (1 h) a deposit of isolated magnetic nanoparticles with a density of up to 400 nanoparticles per µm2. These results open great perspectives for magnetic measurements on single objects using advanced magnetometry techniques as long as spintronics applications based on single chemically synthesized magnetic nanoparticles.

Structural and magnetic properties of He+ irradiated Co2MnSi Heusler alloys

Abstract: We have investigated the atomic disorder induced by a 150 keV He+ ion irradiation in a 40 nmthick Co2MnSi Heusler alloy. Disorder parameters on each atomic site are deduced from normal and anomalous X-ray diffraction measurements with Co and CuKasources. While the film grows mainly in the L21 phase with inclusion of B2 grains, we observe an increase of both theMn-Si and Co-Mn exchanges with the ion fluence. HAADF-STEM analysis demonstrates that the increase inMn-Si disorder corresponds to a growing size of the B2 grains while the Co-Mnexchange is accounted for a D03 disorder type in the L21 matrix. These structural modifications are shown to decrease the average magnetization of the alloy, which is due to D03 disorder and local defects induced by irradiation.

Magnetism and morphology in faceted B2-ordered FeRh nanoparticles

Abstract: – Whereas bulk equiatomic FeRh alloy with B2 structure is antiferromagnetic (AFM) below 370K, we demonstrate that surface configuration can stabilize the low-temperature ferro-magnetic (FM) state in FeRh nanoparticles in the 6-10 nm range. The most stable configuration for FM nanoparticles, predicted through first principles calculations, is obtained in magnetron sputtering synthesized nanoparticles. The structure, morphology and Rh-(100) surface termination are confirmed by aberration-corrected (scanning) transmission electron microscopy. The FM magnetic state is verified by vibrating sample magnetometry experiments. This combined theoretical and experimental study emphasizes the strong interplay between surface configuration, morphology and magnetic state in magnetic nanoparticles.

On-chip carbide derived carbon films for high performance micro-supercapacitors

Abstract: The integration of high performance energy storage systems on silicon chips is necessary to support the development of portable electronic devices. Electrochemical double layer capacitors (or supercapacitors) are a good candidate for this purpose as they provide high power densities and long cycle life. Nevertheless, the improvement of the energy densities delivered by supercapacitors is required. High areal and volumetric capacitance values could be reached while using carbide-derived carbons (CDCs), thanks to their tunable nanoporous structure. The fabrication of interdigitated micro-supercapacitors from chlorination of sputtered TiC films on silicon wafer is presented here. On-chip CDC film electrodes were successfully prepared and tested in aqueous electrolyte, delivering high volumetric capacitance of 410 F.cm−3. Moreover, the use of ionic liquids/solvent mixtures provided larger potential windows, leading to high energy densities up to 90 μWh.cm−2.

Nanotechnology practical teaching at school and university

Abstract: In the last two decades, the teaching of microelectronics and nanotechnologies has become more and more challenging. First, numerous fundamental sciences converge, which implies the introduction of more and complex theoretical concepts, from simple electronic to quantum mechanics, as well as from chemistry, biology, medicine, … Here, we present our teaching experience in short practical training programs adapted for secondary and university students where they confront their theoretical background to real life, i.e. by elaborating and testing their own micro/nano-system for a specified functionality.