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The investigation of nanocatalysts under ambient pressure by X-ray photoelectron spectroscopy gives access to a wealth of information on their chemical state under reaction conditions. Considering the paradigmatic CO oxidation reaction, a strong synergistic effect on CO catalytic oxidation was recently observed on a partly dewetted ZnO(0001)/Pt(111) single crystal surface. In order to bridge the material gap, we have examined whether this inverse metal/oxide catalytic effect could be transposed on supported ZnPt nanocatalysts deposited on rutile TiO2(110). Synchrotron radiation near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) operated at 1 mbar of O2 : CO mixture (4 : 1) was used at a temperature range between room temperature and 450 K. To tackle the complexity of the problem, we have also studied the catalytic activity of nanoparticles (NPs) of the same size, consisting of pure Pt and Zn nanoparticles (NPs), for which, moreover, NAP-XPS studies are a novelty. The comparative approach shows that the CO oxidation process is markedly different for the pure Pt and pure Zn NPs. For pure Pt NPs, CO poisoned the metallic surfaces at low temperature at the onset of CO2 evolution. In contrast, the pure Zn NPs first oxidize into ZnO, and trap carbonates at low temperature. Then they start to release CO2 in the gas phase, at a critical temperature, while continuously producing it. The pure Zn NPs are also immune to support encapsulation. The bimetallic nanoparticle borrows some of its characteristics from its two parent metals. In fact, the ZnPt NP, although produced by the sequential deposition of platinum and zinc, is platinum-terminated below the temperature onset of CO oxidation and poisoned by CO. Above the CO oxidation onset, the nanoparticle becomes Zn-rich with a ZnO shell. Pure Pt and ZnPt NPs present a very similar activity towards CO oxidation, in contrast with what is reported in a single crystal study. The present study demonstrates the effectiveness of NAP-XPS in the study of complex catalytic processes at work on nanocatalysts under near-ambient pressures, and highlights once more the difficulty of transposing single crystal surface observations to the case of nanoobjects.

International audience; We explore the gold functionalization of 2D CdSe nanoplatelets (NPL) as a possible way to tune their electronic and transport properties. We demonstrate that the size and location of the gold tip can be controlled using light and temperature. The Au tip–CdSe NPL hybrid presents a large rise of the conductance compared to the pristine semiconductor (i.e., without gold functionalization). The role of the semiconductor in this transport remains unclear and needs to be better understood. We hypothesize four mechanisms: (i) a reduction of the band gap energy due to the formation of a gold–selenium compound, (ii) a charge transfer between the metal and the semiconductor leading to an increase in carrier concentration, (iii) a change in the inter-nanoparticle tunnel barrier height, and (iv) a simple percolation process between the metallic grain. X-ray photoelectron spectroscopy (XPS) shows that the CdSe NPL are unaffected by oxidation and that gold is in the metallic state Au0. We consequently exclude the formation of a narrow band gap Au2Se phase as the possible mechanism leading to the observed rise of conductance. Moreover, Kelvin probe force microscopy and XPS give evidence for an increase in work function upon gold tipping, which can be interpreted in terms of a shift of the Fermi level toward the valence band maximum. As hole conduction in CdSe NPLs is very unlikely to occur, we rather favor the hypothesis that the strong increase in conduction is largely driven by percolation between the metallic tips as the main mechanism responsible for transport in this hybrid system.

International audience; The investigation of nanocatalysts under their working conditions of pressures and temperatures represents a real strategy toward a realistic understanding of their chemical reactivity and related issues. Additionally, the reduction of Pt load in the catalysts while maintaining their optimum performances is essential to large scale practical applications. Here, we show that small PtZn bimetallic nanoparticles (NPs) supported on the rutile and reduced TiO2(110)-(1 × 1) surface can be prepared by a two step consecutive deposition process where Pt was deposited first and followed by Zn. In situ synchrotron-based near ambient pressure photoemission spectroscopy experiments are used to monitor the evolution of the oxidation states and surface elemental composition of pure Pt and PtZn NPs under high exposure to O2 pressure. The formation of stable Pt surface oxide was evidenced for both pure and PtZn NPs. While a sizeable encapsulation of pure Pt NPs by TiOx was seen after annealing at 440 K under 1 mbar of O2, no such effect was noticed for PtZn NPs. The formation of a zinc oxide layer on PtZn NPs enhances the stability of the NPs and induces a partial reduction of the TiO2(110) surface. Spontaneous formation of a Pt–Zn alloy phase at room temperature was seen in PtZn NPs.

International audience; Organic layers chemically grafted on silicon offer excellent interfaces that may open up the way for new organic–inorganic hybrid nanoelectronic devices. However, technological achievements rely on the precise electronic characterization of such organic layers. We have prepared ordered grafted organic monolayers (GOMs) on Si(111), sometimes termed self-assembled monolayers (SAMs), by a hydrosilylation reaction with either a 7-carbon or an 11-carbon alkyl chain, with further modification to obtain amine-terminated surfaces. X-ray photoelectron spectroscopy (XPS) is used to determine the band bending (∼0.3 eV), and ultraviolet photoelectron spectroscopy (UPS) to measure the work function (∼3.4 eV) and the HOMO edge. Scanning tunneling microscopy (STM) confirms that the GOM surface is clean and smooth. Finally, conductive AFM is used to measure electron transport through the monolayer and to identify transition between the tunneling and the field emission regimes. These organic monolayers offer a promising alternative to silicon dioxide thin films for fabricating metal–insulator–semiconductor (MIS) junctions. We show that gold nanoparticles can be covalently attached to mimic metallic nano-electrodes and that the electrical quality of the GOMs is completely preserved in the process.

Etched and coated multilayer gratings and simple multilayers are investigated by means of x-ray scattering (XRS) and grazing x-ray reflectivity (XRR). Two types of gratings are studied: one lamellar grating coated by a La/B4C stack and two lamellar gratings etched in a Mo/B4C multilayer structure with different cyclic ratios. XRS measurements are presented as reciprocal space mappings or Q-mapping and the origin of the different structures observed in the Q-mapping are discussed in terms of typical coherent and diffuse scattering processes (Bragg spots, grating-Bragg spots, Bragg sheets, grating diffuse sheets, Yoneda wings, etc). From the XRS and XRR measurements, some structural parameters of the mirrors and gratings such as periods and roughness characteristics are estimated.

Artificially fabricated multilayers that exploit the effect of Bragg diffraction at long wavelengths are used as X-ray optical components. Periodic ultrathin bilayer stacks of, alternatingly, a metallic reflection layer and a non-metallic spacer layer prepared as @mm-scaled surface grating promise particularly high performance in applications as X-ray filters for high-resolution spectroscopy. Such gratings can be prepared by multilayer deposition onto Si(001) gratings or by coating flat Si(011) substrate surfaces with a multilayer, followed by subsequent etching of a grating structure. The structural quality of ultrathin multilayer gratings of both types has been characterized quantitatively combining TEM bright-field imaging of specimen cross-sections and applying a geometric phase method. The geometric phase method has been originally developed for the analysis of local displacement fields from HREM images and allows to obtain the relevant structure parameters. The application of this method to the characterization of Mo/B"4C and La/B"4C multilayer systems shows that the functionally decisive structure parameters, such as layer perfection, layer periodicity, and layer orientation, can be obtained with high precision from evaluation of TEM bright-field images. The essential role of such data analyses for a quantitative characterization of multilayer systems and for optimising the layer deposition techniques in the fabrication of X-ray optical layer systems will be demonstrated and discussed.

Multilayer interference mirrors (MIMs) are widely used as low-resolution monochromators for soft x-ray wavelength dispersive spectrometry (WDS). It is now possible to fabricate multilayer monochromators with medium resolution by etching MIMs with a large number of bilayers according to the profile of a lamellar grating. It will be shown both theoretically and experimentally that these multilayer gratings with appropriate characteristics can be used as monochromators for soft x-rays, whose spectral resolving power is considerably improved with respect to mere interference mirrors. Moreover, tunable soft x-ray sources based on the interaction of medium-energy electron beams with periodic multilayer structures are presently being developed in several countries. The physical processes involved in this type of sources belong to the class of quasi-Cerenkov emission. Among them are resonant transition radiation and parametric radiation. We present the basic concepts and the first experimental results concerning this new type of sources which should be very interesting for WDS.

We investigate on a Mo/Si multilayer interferential mirror (MIM) as a function of annealing at temperatures up to 600 °C. The physicochemical state of the silicon atoms within the MIM is characterized by X-ray emission spectroscopy (XES). A thin silicide layer is detected at the interfaces of the as-prepared system, whose thickness increases upon annealing. Measurements of the reflecting power at 1.33 nm show that both the reflectivity and the period markedly decrease as the annealing temperature increases, thus stressing on the interplay between the interface physico-chemical state and the optical properties. Our results shed light on the highest temperature regime, 600 °C, where the MIM loses its periodic structure as the majority of the silicon atoms are involved in the MoSi 2 compound. The diffusion coefficient is estimated from the evolution of the reflected intensity and the period. An activation energy of the diffusion process of 0.52 eV is deduced.

Publié dans The European Physical Journal - Applied Physics 31, 147 (2005); International audience; A new laboratory apparatus devoted to the characterization of various devices for the X-UV range (100-5000 eV), such as mirrors, diffraction gratings, spectrometers or detectors is described. The apparatus includes open x-ray tubes as x-ray sources, a two-crystal monochromator for wavelength selection and a goniometer. Various examples of its use are presented : dispersive mode where the radiation coming from the x-ray tube is dispersed by the two-crystal monochromator, spectrometric mode where the goniometer is used as a plane x-ray spectrometer and reflectometric mode where a selected wavelength is used to perform absolute reflectivity measurements.

The design of efficient lamellar multilayer amplitude grating (LMAG) monochromators in terms of bandwidth and peak reflectivity for the soft-X-ray domain, covering the K emission of low atomic-number elements: B to Si, is considered. A Mo / B 4 C LMAG suited for spectrometry in this spectral domain has been designed, fabricated using electron beam lithography together with reactive ion etching and tested with success.

Un spectrogoniometre de laboratoire utilisant comme source un tube emettant des radiations caracteristiques dans le domaine X mou et X-UV a ete developpe au Laboratoire de Chimie Physique (CNRS/UPMC) dans le cadre de l'Action Coordonnee Optique du CNRS. L'appareil comprend un monochromateur de type double-cristaux equipe de miroirs multicouches ou de cristaux de grande periode et un goniometre θ-2θ. Il permet des mesures de pouvoir reflecteur et de diffusion de dispositifs optiques, en particulier de miroirs et reseaux multicouches. Dans un futur proche il sera aussi destine a la caracterisation de nouvelles sources et de detecteurs X-UV. L'appareil peut aussi etre utilise comme spectrometre a cristal plan si l'anode est remplacee par l'echantillon a analyser. Differents exemples des possibilites offertes par cet instrument operant en mode reflectometrie ou en mode spectrometrie sont presentes.

An etched multilayer, a 2D structure fabricated by etching a periodic multilayer according to the pattern of a laminar grating, is applied in the soft X-ray range to improve the spectral resolution of wavelength dispersive spectrometers. The present article gathers all the successive stages of the development of such a device optimized to analyze the characteristic emission of light elements: design, structural and optical characterization and applications to X-ray spectroscopy. The evolution of the shape of the C Kα emission band of highly oriented pyrolytic graphite (HOPG), as a function of the angle between the emission direction and the (0 0 0 1) planes, is measured. These results, compared to those with a grating, demonstrate that the achieved spectral resolution enables disentangling σ → 1s and π → 1s transitions within the C K emission band.

A new soft-x-ray multilayer monochromator with an improved resolution and a low specular background is presented. The principles of the device, based on the use of a lamellar multilayer amplitude grating (LMAG), are summarized. The techniques of fabrication of the monochromator (multilayer deposition, UV lithography and reactive ion etching) are given. The performance of this new monochromator at photon energies around 1500 eV is demonstrated. Copyright © 2001 John Wiley & Sons, Ltd.

A new kind of xuv multilayer monochromator with a narrow spectral bandwidth is introduced. This monochromator is based on a Mo/Si multilayer mirror etched according to the profile of a lamellar grating. The fabrication of such a device involving multilayer deposition, UV lithography and reactive ion etching is presented. The monochromator has been characterized by means of the synchrotron radiation around the Si–L edge (100 eV). A reduction of the bandwidth has been observed with respect to the unpatterned mirror by a factor close to 3.

International audience; An x-ray multilayer monochromator with improved resolution and a low specular background is presented. The monochromator consists of a lamellar multilayer amplitude grating with appropriate parameters used at the zeroth diffraction order. The device is fabricated by means of combining deposition of thin films on a nanometer scale, UV lithography, and reactive ion etching. The performance of this new monochromator at photon energies near 1500 eV is shown.