A. Galtayries

Structural properties of Al/Mo/SiC multilayers with high reflectivity for extreme ultraviolet light

We present the results of an optical and chemical, depth and surface study of Al/Mo/SiC periodic multilayers, designed as high reflectivity coatings for the extreme ultra-violet (EUV) range. In comparison to the previously studied Al/SiC system, the introduction of Mo as a third material in the multilayer structure allows us to decrease In comparison to the previously studied Al/SiC system with a reflectance of 37% at near normal incidence around 17 nm, the introduction of Mo as a third material in the multilayer structure allows us to decrease the interfacial roughness and achieve an EUV reflectivity of 53.4%, measured with synchrotron radiation. This is the first report of a reflectivity higher than 50% around 17 nm. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and x-ray photoelectron spectroscopy (XPS) measurements are performed on the Al/Mo/SiC system in order to analyze the individual layers within the stack. ToF-SIMS and XPS results give evidence that the first SiC layer is partially oxidized, but the O atoms do not reach the first Mo and Al layers. We use these results to properly describe the multilayer stack and discuss the possible reasons for the difference between the measured and simulated EUV reflectivity values.

Electrochemical deposition of Co3O4 thin layers in order to protect the nickel-based molten carbonate fuel cell cathode

Abstract In order to protect the state-of-the-art nickel cathode used in the molten carbonate fuel cell, electrochemical potentiostatic deposition, a cheap and low-temperature technique, has been used to produce a cobalt oxide coating. This process was optimised on nickel substrates, analysing thoroughly the effect of the imposed potential, electrolyte composition, pH, electrolysis duration and the pre-oxidation of the substrate. Thin and homogeneous cobalt oxide layers were elaborated, in particular, in nitrate media at pH 7.4 on a nickel substrate previously oxidised. The structure and morphology of the deposits were characterised either directly or after an annealing treatment, by X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy. A cubic Co3O4 structure was identified, which is in agreement with the results obtained by cyclic voltammetry.

XPS analysis and microstructural characterization of a Ce/Tb mixed oxide supported on a lanthana‐modified transition alumina

In the framework of a research project aimed at developing alternative materials for application in three-way catalysis, this work reports on the preparation and characterization of a ceria-terbia mixed oxide supported on a lanthana-modified transition alumina. This complex multicomponent oxide system has been characterized by combining x-ray diffraction (XRD), high-resolution electron microscopy (HREM), temperature-programmed desorption mass spectrometry (TPD-MS), Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). Following a dry impregnation procedure, the lanthana-containing phase was deposited first, the ceria-terbia oxide being dispersed on the modified alumina in a subsequent step. The structural and chemical characterization studies carried out on the La 2 O 3 /Al 2 O 3 binary system provide no evidence of a crystalline lanthana-containing phase. Neither lanthanum oxide nor the phases resulting from its aging in air could be identified. Likewise, no lanthana-alumina mixed oxide phases could be deduced from analysis of the HREM micrographs, selected-area electron diffraction (SAED) or XRD patterns. As deduced from the H 2 O and CO 2 TPD studies, the chemisorptive properties of the La 2 O 3 /Al 2 O 3 are significantly different from those of the alumina and the bulk lanthana. We conclude, accordingly, that the supported lanthana consists of a highly dispersed oxide in strong interaction with the alumina support. By contrast, after deposition and further calcination of the cerium-terbium nitrate precursors, the presence of fluorite microcrystals of average size 5 nm could be identified both by XRD and HREM. The XPS studies carried out on the alumina support, La 2 O 3 /Al 2 O 3 and CeTbO x /La 2 O 3 /Al 2 O 3 , as well as a series of reference systems consisting of aged-in-air lanthana, CeLaO x and CeTbO x mixed oxides, have revealed the presence in the ternary system (CeTbO x /La 2 O 3 /Al 2 O 3 ) of at least two different types of chemical environments for the lanthanum ions. With the help of the La 3d 5/2 spectra recorded for the different reference systems, we conclude that these two environments correspond to highly dispersed lanthana strongly interacting with OH- and/or CO 3 2- groups, and to La 3+ species incorporated in the Ce/Tb mixed oxide phase.

XPS study of the adsorption of NH3 on nickel oxide on Ni(111)

The aim of the present work was to investigate the initial stages of NH3 interaction with nickel oxide layers formed on well-defined Ni(111) surfaces. The selected technique was x-ray photoelectron spectroscopy. Ammonia adsorption was performed at 10−7 mbar and room temperature in a preparation chamber attached to the analysis chamber. After each adsorption, the N 1s core level was analysed and its intensity was plotted as a function of exposure (at the same pressure) to obtain the adsorption kinetnetics. The XPS peak shapes and the chemical shifts were related to the nature of the adsorption products. In our experimental conditions, we observed that the more reactive initial surface was the one covered by the O-adsorbed phase corresponding to the precursor state of the NiO formation. The NiO alone does not exhibit significant reactivity, whereas a mixture of NiO and the O-adsorbed phase shows a similar behaviour to that of the pure O-adsorbed phase. From the first adsorption until the last one, two nitrogen adspecies were detected: one with an N 1s binding energy (BE) of 397.8 eV and the other at 399.7 eV. The data indicate that the surface reactivity is structure-sensitive for the NiO layer and the mechanisms of adsorption seem to involve NH3 dissociation into NHx adspecies, along with OH formation. Copyright

Oxygen buffering capacity (OBC) of praseodymium-modified CeO2: influence of the Pr distribution in the ceria host lattice

This work reports on the redox behaviour of different ceria-praseodymia mixed oxides with two different chemical compositions (Ce/Pr molar ratios of 4:1 and 9:1), The samples consist of bulk oxides and ceria-supported samples with varying mixed oxide loading (equivalent to 7 or 12 mixed oxide layers). As revealed by x-ray diffraction and high-resolution electron microscopy, the samples consist of homogeneous fluorite-like mixed oxides. Their redox properties were characterized by means of the oxygen buffering capacity (OBC) technique, which quantitatively evaluates the capacity of the samples for attenuating fast oscillations of the oxygen partial pressure in the He stream flowing through the sample. These oscillations are induced by injecting with a high frequency pulses of oxygen into the inert gas stream. Although the XPS analyses performed on both a bulk and a supported sample showed Ce/Pr molar ratios very close to each other, the bulk mixed oxide exhibits significantly better OBC values. Also remarkable is that an ageing treatment at 1223 K improves the OBC response of the supported mixed oxides. In accordance with the XPS data, the enhancement of their redox properties runs parallel to an increase in the Ce/Pr surface ratio. These results are discussed in terms of the role played by the depth profile distribution of Pr in the different oxide samples.

Adsorption of NH3 on oxygen pre-treated Ni(1 1 1)

The adsorption of NH3 on oxygen pre-treated Ni(1 1 1) surfaces has been studied at room temperature using X-ray photoelectron spectroscopy (XPS). Oxygen pre-treatments have been performed at 650 K. This protocol leads to a ‘‘two-phase domain’’ (O-adsorbed phase + NiO islands) over a large range of oxygen exposures. The investigation of the surface reactivity towards NH3 shows that ammonia is adsorbed provided that the O-adsorbed phase is present; the surface reactivity increases with the O-adsorbed phase coverage. Two N-adspecies have been detected from the N 1s core level peaks at 399.8 � 0.2 and 397.8 � 0.2 eV and assigned to molecular NH3 and dissociated NH2 species, respectively. The molecular adsorption results from direct impingement of the NH3 molecules, whereas the dissociated one results from the dissociation of a part of the preadsorbed molecular species. At saturation, the dissociated species is the more abundant one (about 4/5 of the total N 1s peak) whatever the initial coverage (>0) of the surface by the O-adsorbed phase. The XPS data indicate that this dissociation is correlated to the formation of OH from oxygen of the adsorbed phase and hydrogen abstraction from the molecular ammonia. When the O-adsorbed phase is absent on the surface, i.e. for clean Ni(1 1 1) or the complete NiO layer, none of these surface reactions with ammonia occurs, under the same adsorption conditions. 2003 Elsevier Science B.V. All rights reserved.

Location of trivalent lanthanide dopant energy levels in (Lu0.5Gd0.5)2O3

The location of Ln3+ dopant energy levels relative to bands in (Lu0.5Gd0.5)2O3 was studied. A several-steps analysis of XPS measurements on heavy lanthanides sesquioxides Ln2O3 (Ln = Gd, Tb, Dy, Er, Tm, Yb, Lu) and on Sc2O3 and Y2O3 reference materials were used to locate Ln3+ dopant ground state relative to the top of the valence band in (Lu0.5Gd0.5)2O3 within an error bar of ±0.4 eV. The agreement between XPS data and model was found improved relative to previous studies. When compared to XPS analysis, prediction based on optical absorption shows a slight underestimation attributed to the lack of precision in Ce4+ charge transfer band measurement.

ToF-SIMS Imaging Study of the Early Stages of Corrosion in Al-Cu Thin Films

The pitting corrosion of Al-Cu thin film alloys was investigated using samples that were heat treated in air to form through-thickness Al 2 Cu particles within an Al-0.5% Cu matrix. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) analysis revealed Cu-rich regions 250―800 nm in lateral extent near the metal/oxide interface. Following exposure that generated pitting corrosion, secondary electron, secondary ion, and AFM images showed pits with size and density similar to those of the Cu-rich regions. The role of the Cu-rich regions is addressed.

One-Step Formation of Bifunctionnal Aryl/Alkyl Grafted Films on Conducting Surfaces by the Reduction of Diazonium Salts in the Presence of Alkyl Iodides

The formation of partial perfluoroalkyl or alkyl radicals from partial perfluoroalkyl or alkyl iodides (ICH2CH2C6F13 and IC6H13) and their reaction with surfaces takes place at low driving force (∼-0.5 V/SCE) when the electrochemical reaction is performed in acetonitrile in the presence of diazonium salts (ArN2(+)), at a potential where the latter is reduced. By comparison to the direct grafting of ICH2CH2C6F13, this corresponds to a gain of ∼2.1 V in the case of 4-nitrobenzenediazonium. Such electrochemical reaction permits the modification of gold surfaces (and also carbon, iron, and copper) with mixed aryl-alkyl groups (Ar = 3-CH3-C6H4, 4-NO2-C6H4, and 4-Br-C6H4, R = C6H13 or (CH2)2-C6F13). These strongly bonded mixed layers are characterized by IRRAS, XPS, ToF-SIMS, ellipsometry, water contact angles, and cyclic voltammetry. The relative proportions of grafted aryl and alkyl groups can be varied along with the relative concentrations of diazonium and iodide components in the grafting solution. The formation of the films is assigned to the reaction of aryl and alkyl radicals on the surface and on the first grafted layer. The former is obtained from the electrochemical reduction of the diazonium salt; the latter results from the abstraction of an iodine atom by the aryl radical. The mechanism involved in the growth of the film provides an example of complex surface radical chemistry.

Observation of an asymmetrical effect when introducing Zr in Mg/Co multilayers

We have developed Mg/Co, Mg/Zr/Co, Mg/Co/Zr, and Mg/Zr/Co/Zr periodic multilayers and measured at 25.1 nm a reflectivity (R) highly sensitive to the material order within the period. To understand why Mg/Co/Zr is a more efficient mirror (R=50%) than Mg/Zr/Co and Mg/Zr/Co/Zr (∼40%), we have probed the interface quality through time-of-flight secondary ion mass spectrometry and nuclear magnetic resonance measurements. The Zr-on-Co interface is found quite sharp while a strong intermixing process is evidenced between the upper Co and lower Zr layers, responsible for the decrease in optical contrast and subsequent R loss.