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International audience; We report a scanning transmission X-ray microscopy (STXM) study of hematite nanorods, prototypical photoanode used in solar water splitting. Hematite nanorods were obtained by hydrothermal growth from aqueous solutions using FeCl3 as precursor. Potentials for onset of water splitting are smaller using this synthesis method, compared to values reported for hematite photoanodes obtained by epitaxial growth. STXM revealed the presence of a hexahydrate iron chloride phase at the surface of the nanorods, which is linked to the low onset potential values. We detail the quantification approach that revealed the specific microstructure of individual hematite nanorods.

The ongoing digitization of the world has led to a growing demand for energy- and cost-efficient storage and processing of data. As the existing memory and computing technologies run into fundamental physical limitations, novel devices and computing concepts are required to meet the demand. One of the most promising technologies for future non-volatile memory- and computing-applications are redox-based resistive switching devices. Such a device is composed of a metal-insulator-metal stack that allows the reversible and non-volatile switching of the electrical resistance through application of voltage signals. The resistance change is suspected to be caused by the formation and migration of ionic defects within a small filamentary region of the device. However, despite extensive research efforts the details of the microscopic redox processes that drive resistive switching are still under debate. The aim of this thesis is the elucidation of the switching mechanism in two classic resistive switching oxides, namely SrTiO3 and Ta2O5, by employing spectromicroscopic techniques. First, the influence of the SrTiO3 thin film properties on the switching characteristics is investigated. It is demonstrated that the film thickness and the crystallographic defect structure are decisive factors that determine the polarity of the resistance hysteresis. By combining spectromicroscopy and atomistic simulations, the role of extended defects as fast cation diffusion path is identified. Cation migration takes place along extended defects during device operation, which results in spatially confined phase separations. In operando transmission electron microscopy of defect rich SrTiO3 films provides evidence for a localized valence change from Ti4+ to Ti3+ caused by the electroforming process. In the high resistance state a homogeneous oxygen distribution is observed, whereas in the low resistance state nanometer sized oxygen-deficient regions are visible, which form at island boundaries. To characterize conductive filaments in Ta2O5 using photoelectron spectroscopy, a novel device layout is employed that combines a crossbar structure and graphene electrodes. Photoemission electron microscopy of functional Ta2O5 memristive devices reveals the presence of oxygen-deficient nanoscale filaments. The filament composition is quantified using core level spectroscopy and an oxygen vacancy concentration of 16% is measured in the low resistive state. The results further clarify the long debated question whether the filament is composed of a suboxide or metallic phase by demonstrating that no metallic tantalum component is present in the filament.

International audience; PB1-F2 is a non-structural protein of influenza A viruses (IAV) that modulates viral pathogenesis in a host-specific manner. In mammals, this protein has been shown to increase IAV virulence by delaying the early immune response and, eventually, exacerbating lung inflammation at the late stage of infection. PB1-F2 is a small protein, but displays very high sequence polymorphism and sequence length disparity depending on viral strain. These features result in strong variations in the cellular activity of PB1-F2. Studies have also reported that the effect of PB1-F2 is cell-type dependent. It has notably been shown that PB1-F2 can promote apoptosis in immune cells, but not in epithelial cells. This phenomenon appears to be partly related to the higher order structure of the protein, given that the presence of PB1-F2 β-aggregated structures in infected immune cells correlates with cell death induction. In this work, we evaluated, by synchrotron Fourier transform infrared spectromicroscopy, the impact of the transient expression of PB1-F2 on the biochemical composition of the human epithelial cell line HEK293T. Two PB1-F2 variants that are closely related to each other but derived from a strain with high [A/BrevigMission/1/1918 (H1N1)] or a low [A/WSN/1933 (H1N1)] virulence were studied here. Infrared spectra analysis revealed no specific enrichment of β-aggregated structures in PB1-F2-expressing cells. Nevertheless, this analysis suggested that there is a higher content of β-sheet secondary structures in the PB1-F2 from A/WSN/1933 than that from A/BrevigMission/1/1918. Our data also showed no change in membrane composition in the presence of PB1-F2, implying that PB1-F2 does not promote apoptosis in HEK293T cells. Finally, we found that the PB1-F2 from A/WSN/1933 interferes with adenosine triphosphate production, suggesting that this PB1-F2 variant may disturb the mitochondrial activity.

We review the historical and recent development of experimental techniques that are based on the combination of photoemission spectroscopy and high lateral resolution. The discussion specifically deals with the essential features of the two main classes of approaches in this domain-scanning spectromicroscopy and the techniques based on electron optics. Selected examples are presented to illustrate the exceptional capabilities made possible by the technological advances of recent years. The article is concluded by a short discussion of the foreseeable future of this fast-developing field. (C) 2009 Elsevier B.V. All rights reserved.

RWTH Aachen University, Diss., 2016; 206 pp., (2016)., An increasingly interconnected world creates a high demand for high-density and low-cost data storage. Redox-based memristive devices, which allow switching between high and low electrical resistances through the application of voltages, are highly attractive candidates for next-generation non-volatile memory. But their control and rational design is complicated by poorly understood switching and failure mechanisms.The complex nanoscale redox processes that are suspected to drive so-called resistive switching in these devices remain inadequately characterized. Especially, quantitative information about these processes, which is essential for further advances in the educated design, has been experimentally inaccessible so far.Therefore, spectroscopic tools with high spatial resolution are employed in this work to elucidate both switching and failure mechanism of memristive devices based on the model material SrTiO3. After thorough electrical characterization, two alternative photoelectron emission microscopy approaches are used. As photoemission is a surface sensitive process, the top electrodes of the devices are removed before investigation in the first approach. In the second step, thin graphene electrodes are employed,enabling in operando characterization. In combination with cross sectional, in operando transmission electron microscopy and spectroscopy, a clear evidence of a reversible, localized redox reaction is identified. In the low resistance state, a nanoscale filament in the SrTiO3 is oxygen-deficient, while it is nearly stoichiometric in the high resistance state, resulting in a valence change between Ti3+ and Ti4+. The carrier concentration modulation resulting from this valence change is quantified through comparison with calibration spectra. A carrier concentration change by a factor of two causes two orders of magnitude change in device resistance through a modulation of the effective Schottky barrier at the electrode/oxide interface. The microscopic origin of the polarity of the resistance hysteresis in these devices has long been debated, as it cannot be explained by the typically involved purely internal redistribution of oxygen vacancies. The spectroscopic results of this work reveal that instead, oxygen evolution and reincorporation reactions at the electrode/oxide interface are responsible for the valence change in the SrTiO3. Regarding the failure mechanism, it is found that fast reoxidation frequently results in retention failure in SrTiO3 devices, which can be inhibited by incidental, local phase separations. Mimicking this phase separation by intentionally introducing retention-stabilization-layers with slow oxygen transport is therefore derived as a design rule for retention-failure-resistant devices., Published by Aachen

Two powerful and complementary techniques for chemical characterisation of nanoscale systems are electron energy-loss spectroscopy in the scanning transmission electron microscope, and X-ray absorption spectroscopy in the scanning transmission X-ray microscope. A correlative approach to spectro-microscopy may not only bridge the gaps in spatial and spectral resolution which exist between the two instruments, but also offer unique opportunities for nanoscale characterisation. This review will discuss the similarities of the two spectroscopy techniques and the state of the art for each microscope. Case studies have been selected to illustrate the benefits and limitations of correlative electron and X-ray microscopy techniques. In situ techniques and radiation damage are also discussed.

The NO + H2 reaction on a potassium promoted Rh(110) surface is shown to sustain the formation of spatio-temporal periodic patterns leading to mass transport phenomena. The excitation of pulses and the mass transport mechanism are studied in the 10-7 and 10-6 mbar pressure range, with the potassium coverage varying between K = 0.05 and K = 0.12 ML. Using spectroscopic photoemission and spectroscopic low energy electron microscopy (SPELEEM) as well as related microprobe diffraction techniques, we show that the excitation mechanism comprises a cyclic structural transformation: K + O-coadsorbate → (2 × 1)-N → c(2 × 4)-2O,N → K + O coadsorbate. Laterally resolved spectroscopy demonstrates that potassium is accumulated in front of the nitrogen pulses, suggesting that adsorbed nitrogen acts as a diffusion barrier for potassium. © 2013 The Owner Societies.

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three- dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self- assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.

The present status of in-situ scanning transmission X-ray microscopy (STXM) is reviewed, with an emphasis on the abilities of the STXM technique in comparison with electron microscopy. The experimental aspects and interpretation of X-ray absorption spectroscopy (XAS) are briefly introduced and the experimental boundary conditions that determine the potential applications for in-situ XAS and in-situ STXM studies are discussed. Nanoscale chemical imaging of catalysts under working conditions is outlined using cobalt and iron Fischer-Tropsch catalysts as showcases. In the discussion, we critically compare STXM-XAS and STEM-EELS (scanning transmission electron microscopy-electron energy loss spectroscopy) measurements and indicate some future directions of in-situ nanoscale imaging of catalytic solids and related nanomaterials.

International audience; Originally developed for time-resolved X-ray absorption spectroscopy (XAS), energy-dispersive absorption spectroscopy offers new opportunities for applications such as fluorescence detection and microbeams for scanning probe spectroscopy, thanks to recent developments in both instrumentation and optics. In this context, this paper presents a first example of chemical mapping recorded at ID24, the energy-dispersive XAS beamline at the ESRF. Attributes of this geometry for microanalysis are addressed. Finally, present and future plans are discussed and developed in the light of the evolution of the focal spot on this instrument in the past ten years.

International audience; Microorganisms were searched for among the complex microstructures observed on the surface of a fragment of the previous termTatahouine meteoritenext term inherited from the Tunisian soil in which they were buried. In this view, the chemical compositions, particularly the nitrogen, phosphorus, and sulphur compositions, including the sulphur speciation, were investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) mapping, and scanning X-ray microscopy (SXM). A few 2-μm-thick filaments, partly covered by patches of calcite ensuring they were not deposited by a laboratory contamination, were observed by SEM. The EPMA maps show that the portions free of calcite of the filaments have low but constant contents of nitrogen, sulphur, and phosphorus. The SXM maps were recorded at 2473.5, 2478, and 2482.2 eV, which are respectively characteristic for amino acid linked sulphur, sulphite (SO32−), and sulphate (SO42−). The portions of the filaments detected by EPMA are also those that are enriched in amino acid linked sulphur. The calculated (N/S) elemental ratio is consistent with the one of the dehydrated Escherichia coli matter, contrary to the much lower (P/S) elemental ratio. In living cells, the bulk N and S elements are mainly located in large polymers by covalent bonds, whereas a significant amount of P belongs to small and reactive molecules. We thus can propose that the observed microstructures are dehydrated microorganisms, in which most of the elements that were composing the polymers were retained, whereas the small electrolytes and molecules were removed.

Spectromicroscopy with the imaging technique of X-ray photoelectron emission microscopy (X-PEEM) is a microchemical analytical tool installed in many synchrotron radiation laboratories, and which is finding application in diverse fields of research. The method of sample analysis, X-ray absorption spectroscopy, does not encounter the same problems as X-ray photoemission spectroscopy when sample charging occurs, hence even good insulators may often be analyzed without any apparent artifacts in images or spectra, We show, however, that charging effects cannot be neglected. We model the effect of surface charge formation on the secondary electron yield from uniform samples to demonstrate that surface charge primarily reduces the yield of electrons which may contribute to the detected signal. We illustrate that on non-uniform insulating samples, localized centers of charge may substantially affect microscope imaging and resolution as the electrostatic held close to the surface is distorted. Finally, in certain circumstances non-uniform surface charge may lead to unexpected lineshapes in X-fay absorption spectra causing, in some extreme cases, negative spectra, These negative spectra are explained, and several strategies are reviewed to minimize the impact of sample charging when analyzing poorly conducting samples of any nature. (C) 2000 Elsevier Science B.V. All rights reserved.

Photoelectron spectromicroscopy, the combination of conventional photoelectron spectroscopy and high lateral resolution, is a novel technique that exploits the high brightness of the most recent synchrotron sources. We briefly review its main implementation modes and present some examples of applications, with particular emphasis on semiconductor interface research.

We show that photoelectron spectromicroscopy with synchrotron radiation has been successfully exported from materials science to the life sciences. The transfer is well beyond mere feasibility tests. Photoelectron spectromicroscopy is in fact used for real, long-term studies in neurobiology. Specific examples are discussed, first to see in practice some applications to microchemical analysis of physiologic elements, and then to discuss present and future instrumentation development issues. This last point is illustrated by briefly presenting the MEPHISTO project and the CERVIN project for the Swiss Light Source. (C) 1997 Elsevier Science B.V.

WE present the first comparative study of the uptake of metal ions by neurones, performed for Zn, Cr, Co, Mo, Al, Ni, Mn and Cd. The study reveals substantial differences in the uptake of different metals, under similar exposure procedures. In particular, we found very large uptakes for aluminium and molybdenum. We also found significant effects of excitatory substances, in particular kainate, as stimulants of uptake of some of the metals.

Specimen charging under X-ray illumination is a well known phenomenon that can seriously obstruct the analysis of insulating samples. Synchrotron X-PEEM spectromicroscopy can reach a lateral resolution of 20 nm, 1-2 orders of magnitude larger than electron microscopies, but has the added capacity to probe oxidation state through total yield X-ray absorption near edge structure (XANES) spectroscopy. This capability may be compromised, however, if specimen charging restricts electron emission, as was encountered in the study of silicified bacteria from an Icelandic hot spring microbial mat. Bacteria living in an environment containing a high concentration of dissolved silica provide nucleation sites for amorphous silicate precipitation, a process which may lead to the preservation of the cellular structure, i.e. fossilization. TEM studies of bacteria in progressive stages of mineralization showed that mineral formation was initiated in the extracellular sheath, reaching the cell interior after death. Spectromicroscopy at the Si L-edge of sectioned mineralized bacteria encountered major charging difficulties, which were relieved by simultaneously illuminating the specimen with 325 nm HeCd laser light during the analysis. The low energy light excites mobile free electrons below the work function threshold, which can offset surface positive charge. This approach allowed spectroscopy to be performed from microscopic areas, and may be applicable to a wider range of insulating samples. (C) 2001 Elsevier Science B.V. All rights reserved.

Synchrotron-light-based microscopic interface characterization techniques have been a major factor in the progress towards the understanding control of interface properties and performances. Their role was recently enhanced by the advent of third-generation ultrabright sources. Such superior sources expand the performances and application domains of traditional spectroscopy and microscopy techniques. This point will be illustrated with a series of recent examples produced by ELETTRA in Trieste, Italy; the examples concern high-resolution spectroscopy, spectromicroscopy and time-resolved spectroscopy. Finally, we will discuss the exciting applications which exploit the source coherence to achieve superior imaging with X-rays. (C) 1998 Elsevier Science B.V. All rights reserved.