Virginie Serin

Ab initio study of bilateral doping within the MoS2-NbS2 system

We present a systematic study on the stability and the structural and electronic properties of mixed molybdenum-niobium disulphides. Using density-functional theory we investigate bilateral doping with up to 25% of MoS2 NbS2 by Nb Mo atoms focusing on the precise arrangement of dopants within the host lattices. We find that over the whole range of considered concentrations, Nb doping of MoS2 occurs through a substitutional mechanism. For Mo in NbS2 both interstitial and substitutional dopings can coexist depending upon the particular synthesis conditions. The analysis of the structural and electronic modifications of the perfect bulk systems due to the doping is presented. We show that substitutional Nb atoms introduce electron holes to the MoS2 leading to a semiconductor-metal transition. On the other hand, the Mo doping of NbS2 does not alter the metallic behavior of the initial system. The results of the present study are compared with available experimental data on mixed MoS2-NbS2 bulk and nanoparticles.

Experimental application of sum rules for electron energy loss magnetic chiral dichroism

We present a derivation of the orbital and spin sum rules for magnetic circular dichroic spectra measured by electron energy loss spectroscopy in a transmission electron microscope. These sum rules are obtained from the differential cross section calculated for symmetric positions in the diffraction pattern. Orbital and spin magnetic moments are expressed explicitly in terms of experimental spectra and dynamical diffraction coefficients. We estimate the ratio of spin to orbital magnetic moments and discuss first experimental results for the Fe L2,3 edge.

Plasmon as a tool for in situ evaluation of physical properties for carbon materials

Various carbon materials were studied using electron energy loss spectroscopy (EELS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The aim was to relate the information that can be extracted from EELS (plasmon energy values) to physical properties in order to obtain a new, simple and in situ tool that would allow the properties of carbon materials to be deduced routinely from EELS. The principle of the method is expected to be valid for any carbon material whatever the morphology (fibre, film), texture (anisotropic, isotropic), nanotexture, structure (turbostratic, graphitised) and also whatever the way the materials are processed. Provided the EELS spectroscopy is performed using appropriate acquisition parameters, the only limitation revealed so far is about the carbon material type, which has to derive from thermoplastic precursors. Reasons for both relations and limitations are discussed.

Thermal stability of carbon nitride thin films

The thermal stability of carbon nitride films, deposited by reactive direct current magnetron sputtering in N-2 discharge, was studied for postdeposition annealing temperatures T-A up to 1000 degre ...

Evidence for the solubility of boron in graphite by electron energy loss spectroscopy

We present the results of transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS) studies on two carbon–boron alloys both prepared by chemical vapour deposition at ca. 1000°C and differing in their [Boron]/[Carbon] atomic ratio as well as in their morphology. In both samples, impurity concentrations, principally oxygen and nitrogen, were found to be low relative to boron dopant levels. For low boron contents, typically around 5–10 at.%, the sample consisted of onion-like spherical particles approximately 10 nm in diameter which exhibited a non-homogeneous distribution of boron, concentrated at a level of 5–6 at.% in the centre. For this sample, studies of the B–K- and C–K-ELNES (electron energy loss near-edge structure) together with associated modelling of the unoccupied density of electronic states, indicate a substitution of boron atoms on threefold coordinated sp2-sites within the graphite network. For higher boron doping levels, typically 25 at.%, the sample consisted of homogeneous thin films. In this case, the change in shape of the B–K-ELNES indicates that boron has higher coordinations than planar trigonal together with possibly some residual sp2 sites. This study unambiguously demonstrates the presence of boron substitution solely within an sp2-bonded graphite network in the case of low boron contents and, when combined with other studies, gives an indication of the solubility limit for boron in graphite for the chemical vapour deposition (CVD) process.

A Complete Overhaul of the Electron Energy-Loss Spectroscopy and X-Ray Absorption Spectroscopy Database: eelsdb.eu.

The Electron Energy-Loss Spectroscopy (EELS) and X-ray Absorption Spectroscopy (XAS) database has been completely rewritten, with an improved design, user interface and a

Energy-loss magnetic chiral dichroism (EMCD): Magnetic chiral dichroism in the electron microscope

A new technique called energy-loss magnetic chiral dichroism (EMCD) has recently been developed [P. Schattschneider, et al. Nature 441, 486 (2006)] to measure magnetic circular dichroism in the transmission electron microscope (TEM) with a spatial resolution of 10 nm. This novel technique is the TEM counterpart of x-ray magnetic circular dichroism, which is widely used for the characterization of magnetic materials with synchrotron radiation. In this paper we describe several experimental methods that can be used to measure the EMCD signal [P. Schattschneider, et al. Nature 441, 486 (2006); C. Hebert, et al. Ultramicroscopy 108(3), 277 (2008); B. Warot-Fonrose, et al. Ultramicroscopy 108(5), 393 (2008); L. Calmels, et al. Phys. Rev. B 76, 060409 (2007); P. van Aken, et al. Microsc. Microanal. 13(3), 426 (2007)] and give a review of the recent improvements of this new investigation tool. The dependence of the EMCD on several experimental conditions (such as thickness, relative orientation of beam and sample, collection and convergence angle) is investigated in the transition metals iron, cobalt, and nickel. Different scattering geometries are illustrated; their advantages and disadvantages are detailed, together with current limitations. The next realistic perspectives of this technique consist of measuring atomic specific magnetic moments, using suitable spin and orbital sum rules, [L. Calmels, et al. Phys. Rev. B 76, 060409 (2007); J. Rusz, et al. Phys. Rev. B 76, 060408 (2007)] with a resolution down to 2 to 3 nm.

Magnetic properties of maghemite nanoparticles in a polyvinylpyridine matrix

Abstract Magnetic measurements have been carried out on nanocomposites consisting of individual maghemite (γ-Fe 2 O 3 ) particles distributed throughout a polyvinylpyridine matrix with a range of particle sizes and concentrations. Magnetic measurements and particle size measurements have been compared to investigate particle size effects upon magnetic properties. Magnetisation increases slowly with increasing iron content up to approximately 18%, at which point the increase becomes sharper. Magnetic moment, as derived from a modified Langevin equation, is found to be less than the value expected from the particle, indicating possible surface effects. Blocking temperatures have been observed to increase with increasing iron concentration. Assuming no significant particle interactions it is possible to consider the effective anisotropy of individual particles. Effective anisotropy has been observed to increase with decreasing particle size, implying an increase in contribution from surface effects, and increases with the presence of rod shaped particles, implying a contribution from shape anisotropy.

EELS investigation of FeCo/SiO2 nanocomposites.

The factors that determine the local magnetic properties of FeCo/SiO2 nanocomposite powders and films have been analysed by electron energy‐loss spectroscopy (EELS) and transmission electron microscopy (TEM). Attention has been given to the chemical composition, the local electronic structure and the atomic arrangement. The results show that the nanoparticles from sol‐gel prepared powders are generally Fe‐rich, whereas they are Co‐rich in sol‐gel prepared films. In addition, a subnanometre oxide layer at the surface of the FeCo nanoparticles has been clearly observed in the powder sample. It is found that the magnetic moment should be partly governed by alloying effects. Numerical values of the near‐surface magnetic moment have been obtained using the ab‐initio layer‐KKR method. These values should be helpful in understanding the layer‐by‐layer changes of the white line ratio close to the surface of the nanoparticles.

Nitrogen distribution in high tensile strength carbon fibres

It is already established that a nitrogen content in carbon fibres induces poor high tensile strength properties of the fibres. Up to now the nitrogen localization in the fibre, particularly along the diameter, was not known, due to the small size of the fibres (7 μm) and the insufficient sensitivity of the usual analytical techniques, using X-rays or ion beam for instance, for such low atomic number elements (nitrogen and carbon). By using electron Energy Loss Spectroscopy (EELS) in a Philips EM 400T microscope equipped with a V.G. electron spectrometer, we have determined this distribution for the Toray 300 carbon fibre. The measurements have been made by using the carbon and nitrogen K edge intensity when EEL spectra correspond to thin enough samples, thickness ≤ 500 A. The accuracy of this determination is on the order of 10%. In this way, we have measured a NC atomic concentration varying from 2 to 7%. For all the Toray 300 fibres a similar variation of the ratio is observed. The nitrogen content is low in the core of the fibre, about 2%. It remains practically constant in the whole center of the fibre and grows rapidly to 6–7% in the periphery. These measurements reveal a skin core concentration gradient effect in the high tensile strength fibres. They confirm that it is necessary to avoid this structure if one desires ultra-high mechanical properties for the fibres.