Spin State As a Probe of Vesicle Self-Assembly

International audienceA novel system of paramagnetic vesicles was designed using ion pairs of iron-containing surfactants. Unilamellar vesicles (diameter approximate to 200 nm) formed spontaneously and were characterized by cryogenic transmission electron microscopy, nanoparticle tracking, analysis, and light and small-angle neutron scattering. Moreover, for the first time, it is shown that magnetization measurements can be used to investigate self-assembly of such functionalized systems, giving information on the vesicle compositions and distribution Of surfactants between the bilayers and the aqueous bulk

Dramatic enhancement of double-walled carbon nanotube quality through a one-pot tunable purification method.

The purification process we propose is a one-pot gas-phase treatment; the CNT powder is simply submitted to a chlorine/oxygen atmosphere at around 1000 °C for 2 h. By varying the oxygen content in an excess of chlorine, the conditions were optimized in order to efficiently remove both metal (catalyst) and carbon impurities from DWCNT samples. Even if a high amount of sample is lost under the oxidative conditions used, a selective elimination of the carbon impurities obviously occurs and a metal impurity removal yield of 99% is obtained from thermogravimetry. Based on a multi-technique approach, we show that the purified DWCNTs are of high structural quality without any surface functionalization. This improvement of the wall quality through the chlorine/oxygen action is seen in particular with a division by 15 of the D over G band intensity of the Raman spectra. Among the existing procedures, the advantages of our purification method are indisputably its simplicity, low time-consuming and high efficiency combined with an enhanced quality of the purified CNTs. Such quasi-pure DWCNTs have high interest since they offer a unique opportunity to study the intrinsic properties and effects of the nanotubes themselves

Protecting Carbon Nanotubes from Oxidation for Selective Carbon Impurity Elimination

Purity of carbon nanotubes (CNTs) is essential to avoid a dramatic decrease in their performances. In addition to metallic impurities, carbonaceous impurities have been shown to be responsible for pronounced effects. However, they are highly difficult to be selectively removed from CNT samples because of the similar chemical reactivity of these two kinds of carbon species. The existing purification methods often lead to high CNT consumption (>90 wt %). The proposed method consists of a one-pot gas-phase treatment combining chlorine and oxygen. The CNT powder maintained in a chlorine stream is submitted to oxygen at moderate temperature [350 and 500 °C for single-walled CNTs (SWCNTs) and double-walled CNTs (DWCNTs), respectively], and the thermal treatment is then pursued at 900–1000 °C under chlorine alone. Our work reveals that this approach is able to significantly improve the selectivity of elimination of carbonaceous impurities. Thanks to the proposed purification treatment, only 19 and 11 wt % of carbon species (mainly carbon impurities) are lost for DWCNTs and SWCNTs, respectively. The mechanism proposed involves a protective effect by grafting of chlorine favored to the CNT walls. Because our simple one-pot purification method is also versatile and scalable, it opens new perspectives for CNT applications in high-added value fields

Superconductivity in Li3Ca2C6 intercalated graphite

In this letter, we report the discovery of superconductivity in Li3Ca2C6. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li3Ca2C6 is a ternary GIC, for which the intercalated sheets are very thick and poly-layered (five lithium layers and two calcium ones). It contains a great amount of metal (five metallic atoms for six carbon ones). Its critical temperature of 11.15 K is very close to that of CaC6 GIC (11.5 K). Both CaC6 and Li3Ca2C6 GICs possess currently the highest transition temperatures among all the GICs.Comment: 5 pages, 3 figure

Unraveling oxidation and spin states of single Fe based meso-macroporous silica catalyst in Fenton like reaction by magnetic measurements

Single-atom catalysts are acknowledged for their superior efficiency compared to nanoparticles or clusters, primarily because of the enhanced accessibility of the catalytic center. Essential parameters for assessing their performance are the metal loading degree and the oxidation state of the individual catalytic sites. This is particularly the case in Fe-based Fenton like reaction, in which both Fe2+ and Fe3+ are active but react with significantly different catalytic rates. While the elemental metal loading is easily assessed by elemental analysis, the determination of the oxidation degree is more challenging. To do so, we designed single Fe-based meso-macroporous silica materials as catalysts for the degradation of methylene blue, an organic dye serving as well-known model for the degradation of organic pollutants in wastewater. The silica materials were successfully synthesized by a sol-gel process through a combined templating mechanism with micelles and solid lipid nanoparticles of Fe-based surfactants. Magnetic measurements have revealed that half of the iron centers are in the Fe2+ state. The following of the Fenton like reaction through magnetic measurements agrees with a contribution of Fe2+ in the catalytic process. The magnetic response emerges as a valuable tool for quantifying and characterizing individual catalytic centers

Transport properties of Ti-Ni-Zr films grown by pulsed laser deposition

International audienceQuasicrystalline Ti41.5Ni17Zr41.5 thin films have been synthesized at different temperatures by pulsed laser deposition (PLD) from a Nd:YAG laser. Electrical resistivity, Hall coefficient, magnetoresistance and thermopower measurements have been conducted in the 4.4–300 K or 70–300 K temperature range. Ti41.5Ni17Zr41.5 quasicrystals are characterized by a high electrical conductivity, an order of magnitude higher than in other quasicrystals, independent of their morphology and microstructure. Hall measurements indicate that the films have a high carrier concentration. Thermoelectric powers in Ti41.5Ni17Zr41.5 have relatively small magnitudes and the values have been found to depend on the microstructure

Interfacial electronic transport phenomena in single crystalline Fe-MgO-Fe thin barrier junctions

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceSpin filtering effects in nano-pillars of Fe-MgO-Fe single crystalline magnetic tunnel junctions are explored with two different sample architectures and thin MgO barriers (thickness: 3-8 monolayers). The two architectures, with different growth and annealing conditions of the bottom electrode, allow tuning the quality of the bottom Fe/MgO interface. As a result, an interfacial resonance states (IRS) is observed or not depending on this interface quality. The IRS contribution, observed by spin polarized tunnel spectroscopy, is analyzed as a function of the MgO barrier thickness. Our experimental findings agree with theoretical predictions concerning the symmetry of the low energy (0.2 eV) interfacial resonance states: a mixture of Delta(1)-like and Delta(5)-like symmetries

Thermoelectric Properties of Nd_xCo_4-yNi_ySb₁₂ Skutterudite Compounds

In an effort to further understand the influence of Nd on the thermoelectric properties of the binary compound CoSb3 and try to optimize them through doping, we prepared and investigated NdxCo4-yNiySb12 compounds. The samples have been prepared by a conventional metallurgical route. Structural analysis have been carried out by X-ray diffraction. The chemical composition and micro-homogeneity have been checked by electron probe microanalysis. Measurements of the electrical resistivity, thermoelectric power, thermal conductivity and Hall coefficient have been performed. The influence of both Nd and Ni on the thermoelectric properties of the binary parent CoSb3 is presented and discussed. Nd plays the role of a dopant (n-type) and the presence of Ni contributes to decrease significantly the electrical resistivity

Magnetism for understanding catalyst analysis of purified carbon nanotubes

International audienceThe precise quantification of catalyst residues in purified carbon nanotubes is often a major issue in view of any fundamental and/or applicative studies. More importantly, since the best CNTs are successfully grown with magnetic catalysts, their quantification becomes strictly necessary to better understand intrinsic properties of CNT. For these reasons, we have deeply analyzed the catalyst content remained in nickel-yttrium arc-discharge single walled carbon nanotubes purified by both a chlorine-gas phase and a standard acid-based treatment. The study focuses on Ni analysis which has been investigated by transmission electron microscopy, X-ray diffraction, thermogravimetry analysis, and magnetic measurements. In the case of the acid-based treatment, all quantifications result in a decrease of the nanocrystallized Ni by a factor of two. In the case of the halogen gas treatment, analysis and quantification of Ni content is less straightforward: a huge difference appears between X-ray diffraction and thermogravimetry results. Thanks to magnetic measurements, this disagreement is explained by the presence of ions, belonging to NiCl2 formed during the Cl-based purification process. In particular, NiCl2 compound appears under different magnetic/crystalline phases: paramagnetic or diamagnetic, or well intercalated in between carbon sheets with an ordered magnetic phase at low temperature