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Luais B.,University of Lorraine | Luais B.,CNRS Laboratory for Photonics and Nanostructures
Chemical Geology

Chemical separation and isotopic measurement of germanium using hexapole-collision cell-MC-ICPMS were developed in various Fe-Ni, ZnS and silicates matrices in order to investigate the potentiality of Ge as an isotopic tracer of planetary differentiation and rock-forming processes. Analytical procedures are described for the critical step of silicate dissolution in HF+HNO3 medium, as well as for Ge chemical purification using a single cationic-exchange resin step for Fe-Ni and ZnS matrices, and two anionic and cationic resin steps for silicate matrices. Germanium isotopic measurements using MC-ICPMS were performed with appropriate Ar+H fluxes in the collision cell to eliminate argide interferences on Ge masses. Three methods of mass bias correction, including sample standard bracketing, external Ga mass bias correction using the exponential law, and the empirical "regression method", give similar results and demonstrate the use of Ga as an appropriate element for mass bias correction of Ge. Results are presented as delta values with respect to JMC Ge standard, and NIST3120a Ge standard for comparison. We show a long-term 2SD reproducibility of less than 0.24% on the δ74Ge.These analytical methods have been applied to Fe-meteorites, sphalerite (ZnS) deposits, and geostandard silicates ranging from ultramafic to basaltic to granitic compositions, and to an iron formation composition. Fe-meteorites and terrestrial silicate samples display small variations of δ74GeJMC=+1.77±0.22% and +0.89±0.16% (2SD reproducibility), respectively. This contrasts with the large variations seen in low-temperature rocks, such as the ZnS ores (δ74GeJMC=-0.37 to -1.69%), and banded iron formations (IF-G Isua, δ74GeJMC=+1.38%). A slight δ74GeJMC-NBO/T negative tendency in silicate samples indicates that polymerisation of silicate melt would control the small Ge isotope fractionation among mantle silicates.A comparison of δ74Ge values of iron meteorites and Earth silicate mantle opens new perspectives in deep Earth processes. On the basis of theoretical metal-silicate isotopic equilibrium processes, the low δ74Ge of silicate Earth cannot reconcile one-stage process of core-mantle segregation. It is proposed that the δ74Ge(JMC) value of silicate earth samples of +0.89±0.16% (or δ74Ge(NIST3120a)=+0.53%) represents the composition of the accessible Earth modern mantle. The δ74Ge of the silicate mantle in equilibrium with the core at time of core formation would be distinct to that of the present mantle in result of distinct thermodynamic parameters, e.g. fO2, pressure, inducing changes in coordination and valence state of Ge in the silicate crystallographic structure. In addition, the light isotopic composition of the Earth's mantle could result from reverse diffusive processes induced by an increase in oxidation state at the end of core formation. This would have some implications on core formation modelling and the use of Ge isotopes for tracing the origin of deep mantle plumes. © 2012 Elsevier B.V. Source

Collin S.,CNRS Laboratory for Photonics and Nanostructures
Reports on Progress in Physics

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays. © 2014 IOP Publishing Ltd. Source

Glas F.,CNRS Laboratory for Photonics and Nanostructures
Journal of Applied Physics

For use in quantitatively modeling the growth of gold-seeded semiconductor nanowires in the vapor-liquid-solid mode, we calculate the difference of chemical potential between a liquid melt formed of group III and group V atoms dissolved in gold and the corresponding solid III-V binary compound. Fits to our results are provided for seven compounds as simple polynomials of the concentrations in the III-V-Au liquid and temperature. We find that the difference of chemical potential increases with the group III and group V concentrations, decreases with increasing temperature, and can easily reach several hundreds of meV per III-V pair. We discuss these values and variations in the light of published experimental results, in particular as regards the crystalline structure adopted by the nanowires during growth. © 2010 American Institute of Physics. Source

Armitano J.,CNRS Laboratory for Photonics and Nanostructures
Research in microbiology

Bacteria, and in particular marine bacteria, can be found in environments that are poor in nutrients. To survive, they are able to move toward more favorable niches by a mechanism called chemotaxis, whose first step consists in the detection of substrates by chemoreceptors. We developed a chemotactic assay enabling rapid testing of several hundred different solutes and we identified several molecules eliciting a chemotactic response from two aquatic Shewanella species. We propose that this assay be used for other bacteria to determine the repertoire of chemotactic molecules, generally not clearly elucidated. Copyright © 2011 Institut Pasteur. Published by Elsevier Masson SAS. All rights reserved. Source

Dubrovskii V.G.,RAS Ioffe Physical - Technical Institute | Cirlin G.E.,RAS Ioffe Physical - Technical Institute | Sibirev N.V.,St Petersburg Academic University | Jabeen F.,CNRS Laboratory for Photonics and Nanostructures | And 2 more authors.
Nano Letters

We report on the new mode of the vapor-liquid-solid nanowire growth with a droplet wetting the sidewalls and surrounding the nanowire rather than resting on its top. It is shown theoretically that such an unusual configuration happens when the growth is catalyzed by a lower surface energy metal. A model of a nonspherical elongated droplet shape in the wetting case is developed. Theoretical predictions are compared to the experimental data on the Ga-catalyzed growth of GaAs nanowires by molecular beam epitaxy. In particular, it is demonstrated that the experimentally observed droplet shape is indeed nonspherical. The new VLS mode has a major impact on the crystal structure of GaAs nanowires, helping to avoid the uncontrolled zinc blende-wurtzite polytylism under optimized growth conditions. Since the triple phase line nucleation is suppressed on surface energetic grounds, all nanowires acquire pure zinc blende phase along the entire length, as demonstrated by the structural studies of our GaAs nanowires. © 2011 American Chemical Society. Source

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