Nathalie Brun

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.

Carbon nanotubes in macrophages: Imaging and chemical analysis by X-ray fluorescence microscopy

X-ray fluorescence microscopy (microXRF) is applied for the first time to study macrophages exposed to unpurified and purified single-walled (SW) and multiwalled (MW) carbon nanotubes (CNT). Investigating chemical elemental distributions allows one to (i) image nanotube localization within a cell and (ii) detect chemical modification of the cell after CNT internalization. An excess of calcium is detected for cells exposed to unpurified SWCNT and MWCNT and related toxicological assays are discussed.

Microstructure of opaque red glass containing copper

We have studied several ancient red opaque glasses: about 40 samples of Celtic red enamels, coming from Western and Central Europe and dating between the 4th and 1st century BC and Gallo-Roman tesserae (glass cubes used in mosaics) dating from the 4th century AD

Spectral mixture analysis of EELS spectrum-images

Recent advances in detectors and computer science have enabled the acquisition and the processing of multidimensional datasets, in particular in the field of spectral imaging. Benefiting from these new developments, Earth scientists try to recover the reflectance spectra of macroscopic materials (e.g., water, grass, mineral types…) present in an observed scene and to estimate their respective proportions in each mixed pixel of the acquired image. This task is usually referred to as spectral mixture analysis or spectral unmixing (SU). SU aims at decomposing the measured pixel spectrum into a collection of constituent spectra, called endmembers, and a set of corresponding fractions (abundances) that indicate the proportion of each endmember present in the pixel. Similarly, when processing spectrum-images, microscopists usually try to map elemental, physical and chemical state information of a given material. This paper reports how a SU algorithm dedicated to remote sensing hyperspectral images can be successfully applied to analyze spectrum-image resulting from electron energy-loss spectroscopy (EELS). SU generally overcomes standard limitations inherent to other multivariate statistical analysis methods, such as principal component analysis (PCA) or independent component analysis (ICA), that have been previously used to analyze EELS maps. Indeed, ICA and PCA may perform poorly for linear spectral mixture analysis due to the strong dependence between the abundances of the different materials. One example is presented here to demonstrate the potential of this technique for EELS analysis.

Self-sustained etch masking: A general concept to initiate the formation of nanopatterns during ion erosion

A material allowing for rapid and reliable formation of nanopatterned surfaces is an important issue in many areas of science today. Self-organized pattern formation induced by ion erosion is a promising bottom-up approach. In the case of the III-V semiconductors, this method can lead to several remarkable structure types even if the formation mechanism has yet to be found. Through high resolution chemical scanning, transmission electron imaging, and x-ray photo emission, we show through an investigation of GaSb that the capacity of III-V semiconductors to pattern under ion erosion is linked to the phase diagram of these materials. We suggest an original scenario to explain the specific behavior of III-V semiconductors, where one species segregates and acts as a continuously resupplied etching shield. This concept is at variance with the standard Bradley–Harper model and opens interesting perspectives for bottom-up patterning of compound materials.

Multi-dimensional and multi-signal approaches in scanning transmission electron microscopes

Developments in instrumentation are essential to open new fields of science. This clearly applies to electron microscopy, where recent progress in all hardware components and in digitally assisted data acquisition and processing has radically extended the domains of application. The demonstrated breakthroughs in electron optics, such as the successful design and practical realization and the use of correctors, filters and monochromators, and the permanent progress in detector efficiency have pushed forward the performance limits, in terms of spatial resolution in imaging, as well as for energy resolution in electron energy-loss spectroscopy (EELS) and for sensitivity to the identification of single atoms. As a consequence, the objects of the nanoworld, of natural or artificial origin, can now be explored at the ultimate atomic level. The improved energy resolution in EELS, which now encompasses the near-IR/visible/UV spectral domain, also broadens the range of available information, thus providing a powerful tool for the development of nanometre-level photonics. Furthermore, spherical aberration correctors offer an enlarged gap in the objective lens to accommodate nanolaboratory-type devices, while maintaining angström-level resolution for general characterization of the nano-object under study.

Intracellular fate of carbon nanotubes inside murine macrophages: pH-dependent detachment of iron catalyst nanoparticles.

BackgroundCarbon nanotubes (CNT) are a family of materials featuring a large range of length, diameter, numbers of walls and, quite often metallic impurities coming from the catalyst used for their synthesis. They exhibit unique physical properties, which have already led to an extensive development of CNT for numerous applications. Because of this development and the resulting potential increase of human exposure, an important body of literature has been published with the aim to evaluate the health impact of CNT. However, despite evidences of uptake and long-term persistence of CNT within macrophages and the central role of those cells in the CNT-induced pulmonary inflammatory response, a limited amount of data is available so far on the CNT fate inside macrophages. Therefore, the overall aim of our study was to investigate the fate of pristine single walled CNT (SWCNT) after their internalization by macrophages.MethodsTo achieve our aim, we used a broad range of techniques that aimed at getting a comprehensive characterization of the SWCNT and their catalyst residues before and after exposure of murine macrophages: X-ray diffraction (XRD), High Resolution (HR) Transmission Electron Microscopy (TEM), High Angle Annular Dark Field-Scanning TEM (HAADF-STEM) coupled to Electron Energy Loss Spectroscopy (EELS), as well as micro-X-ray fluorescence mapping (μXRF), using synchrotron radiation.ResultsWe showed 1) the rapid detachment of part of the iron nanoparticles initially attached to SWCNT which appeared as free iron nanoparticles in the cytoplasm and nucleus of CNT-exposed murine macrophages, and 2) that blockade of intracellular lysosomal acidification prevented iron nanoparticles detachment from CNT bundles and protected cells from CNT downstream toxicity.ConclusionsThe present results, while obtained with pristine SWCNT, could likely be extended to other catalyst-containing nanomaterials and surely open new ways in the interpretation and understanding of CNT toxicity.

Nanocomposite coatings codeposited with nanoparticles using aerosol-assisted chemical vapour deposition

Incorporating nanoscale materials into suitable matrices is an effective route to produce nanocomposites with unique properties for practical applications. Due to the flexibility in precursor atomization and delivery, aerosol-assisted chemical vapour deposition (AACVD) process is a promising way to synthesize desired nanocomposite coatings incorporating with preformed nanoscale materials. The presence of nanoscale materials in AACVD process would significantly influence deposition mechanism and thus affect microstructure and properties of the nanocomposites. In the present work, inorganic fullerene-like tungsten disulfide (IF-WS2) has been codeposited with Cr2O3 coatings using AACVD. In order to understand the codeposition process for the nanocomposite coatings, chemical reactions of the precursor and the deposition mechanism have been studied. The correlation between microstructure of the nanocomposite coatings and the codeposition mechanism in the AACVD process has been investigated. The heterogeneous reaction on the surface of IF-WS2 nanoparticles, before reaching the substrate surface, is the key feature of the codeposition in the AACVD process. The agglomeration of nanoparticles in the nanocomposite coatings is also discussed.

Microstructure and photoluminescence of porous Si formed on n-type substrates in the dark

Abstract The microstructure, from the general morphology to the lattice structure, of porous Si layers produced by electrochemical etching of highly doped n-type Si substrates in obscurity conditions was studied by scanning and transmission electron microscopies. Scanning electron microscopy revealed that the porous layer consists of two distinctive layers, characterised by the size of the observed structural features: an upper mesoporous layer and a lower macroporous layer. High resolution electron microscopy showed the presence of thin crystalline Si platelets held at various depths within the pores of the mesoporous layer, in contrast to the Si nanocrystallites and nanowires identified in previous studies. In spite of the different geometry of the nanoparticles resulting from the obscurity conditions, the observed photoluminescence was that of the typical red band.

Near-Field Electron Energy Loss Spectroscopy in Porous Silicon

First results of an Electron Energy Loss Spectroscopy in the Near Field (NFEELS) mode of n+ porous silicon are described here. Sequences of EELS spectra in the low loss energy range (0–30 eV) were recorded, using a scanning transmission electron microscope, as the e-beam was scanned across a nano-hole surrounded by Si platelets. This technique is shown to be very sensitive to spectral and spatial changes in the electromagnetic field distribution outside the surface of nanoparticles, governed by their local nature and shape.