Au-delà du gap de pression : étude par XPS d'interfaces à des pressions proches de l'ambiant

In many processes or technological objects, such as coating deposition, advanced material processing for electronics, magnetic or optical devices, electrochemical processes at an electrode, sensors and catalysis, etc. the interface between a surface of a solid and a liquid or a gas phase, plays a prominent role. Analogously, environmental sciences and sciences of the Living integrate into their models chemical reactions taking place at solid/liquid or liquid/gas interfaces.XPS is a powerful technique for interface analysis and has been widely use in the case of solid surface. The main advantage of XPS is its sensitivity to the material surface. Indeed, due to the low electron mean free path of electrons in a solid, only the photoelectrons at the extreme outer surface (1-10 nm) can escape the sample. However, XPS has traditionally been conducted under ultra-high vacuum (UHV) conditions. UHV conditions are utilized for two reasons. First, the analyzers are designed to work under UHV conditions. Second, the electrons must reach the detector and their mean free path is short at high pressures. For example at a pressure of 1 mbar, 100 eV electrons will travel 1 mm while under UHV conditions, the mean free path increases to 105 m. UHV chambers (10-10 mbar) help maximize the mean free path so that a high number of electrons will reach the detector/analyzer and the signal/noise ratio will increase making it possible to analyze the spectrum produced. This constraint makes UHV the standard environment of X-ray Photoelectron Spectroscopy (XPS) experiments.In order to make possible the use of XPS on a larger pressure range, a few groups around the world have designed photoemission equipment that can be operated under ambient pressure (up to 5 mbar). The Berkeley group (M. Salmeron LBNL-Materials Sciences Division, H. Bluhm LBNL-Chemical Sciences Division), who pioneered this field, has two such setups installed at the Advanced Light Source. The Fritz Haber Institute has built a high pressure XPS based on the Berkeley prototype, installed at BESSY synchrotron (Berlin), since 2002. The building of Ambient Pressure XPS (APXPS) analyzers of the Berkeley type, able to be operated at pressures in the range of 1 mbar, about 7 orders of magnitude higher than the pressure limit of conventional UHV equipment is both a technological and conceptual breakthrough. Differential pumping stages allow the sample to remain at environmental pressures, while maximizing the mean free path of emitted electrons, so they can reach the detector. Additionally, voltages are applied to electrostatic lenses in the unit to accelerate and focus the electrons onto the focal plane of the electron energy analyzer. A similar system Near-Ambient Pressure XPS, NAP-XPS), described in details in chapter 1, was delivered in December 2012 and installed at TEMPO beamline in February 2013. The first beamtime occurred in May 2013. During my thesis that started in October 2011 two different projects were developed, both related to interface analysis using the NAP-XPS instrument.Dans de nombreux procédés technologiques, tels que la fabrication de matériaux pour la microélectronique, l’étude des réactions chimiques à une électrode, ou encore la catalyse… L’interface entre la surface d’un solide ou d’un liquide avec un liquide ou une phase gaz joue un rôle fondamental. De façon analogue, les sciences de l’environnement ainsi que celles du vivant intègrent dans leurs modèles la réactivité aux interfaces solide/ liquide ou liquide/ gaz.L’XPS est une technique parfaitement adaptée à l’étude des interfaces et a été largement utilisée pour l’analyse des surfaces de solides. Le principal avantage de l’XPS est sa grande sensibilité aux surfaces. En effet, en raison du faible libre parcours moyen des électrons dans un solide, uniquement les photoélectrons provenant de l’extrême surface (1 – 10 nm) peuvent échapper à celle-ci. Cependant, l’XPS est traditionnellement utilisée dans des conditions d’ultravide (UHV) et cela pour deux raisons. La première est que les analyseurs d’électrons sont construits pour fonctionner en UHV. La seconde est que les électrons doivent pouvoir atteindre l’analyseur, or leur libre parcours moyen est faible dans un gaz à haute pression. Par exemple, à une pression de 1 mbar, des électrons possédant une énergie de 100 eV vont parcourir 1 mm alors qu’ en UHV ils pourront atteindre jusqu’ à 105 m.Dans le but de rendre possible l’utilisation de l’XPS à des pressions plus élevées, quelques groupes autour du monde, dont le groupe de Berkeley (sous la direction de M. Salmeron at de H. Bluhm) et celui du Fritz Haber Institute à Berlin, ont élaborés un équipement permettant d’atteindre des pressions proche de l’ambiant (5 mbar). La construction d’un analyseur d’électron capable de fonctionner à des pressions de l’ordre du mbar, c’est-à-dire à des pressions 7 ordres de grandeur supérieures à l’UHV, a été une avancée à la fois conceptuelle et technologique. Un système de pompage différentiel permet de maintenir l’échantillon dans des conditions dites environnementales tout en maximisant le libre parcours moyen des électrons de façon à ce qu’ils atteignent l’analyseur. De plus, des tensions sont appliquées à des lentilles électrostatiques dans le but d’accélérer et de focaliser ces électrons.Un système similaire (Near Ambient Pressure XPS, NAP-XPS) a été installé sur la ligne TEMPO du synchrotron Soleil en février 2013, le premier temps de faisceau ayant eu lieu au mois de mai suivant. Durant ma thèse, deux projets différents ont été développés, tous les deux liés à l’étude d’interfaces avec l’utilisation de la NAP-XPS.Le premier projet traite des procédés utilisés en micro-electronique pour déposer de fines couches d’oxydes : le dépôt chimique en phase vapeur (CVD) et la déposition de couches atomiques (ALD). En particulier, des molécules de la famille des silanes sont utilisées pour fonctionnaliser des surfaces d’oxyde de silicium ou comme précurseur, combiné à un agent oxydant comme l’eau pour le dépôt de films mince d’oxyde de silicium. Cependant, les mécanismes réactionnels des silanes sur les surfaces de silicium n’ont jamais été étudiés par des techniques telles que la microscopie a effet tunnel (STM) ou l’XPS et l’on sait peu de choses concernant leur mécanisme de dissociation et l’adsorption des divers fragments sur la surface

Particle size effect on the Langmuir-Hinshelwood barrier for CO oxidation on regular arrays of Pd clusters supported on ultrathin alumina films

International audienceThe Langmuir-Hinshelwood barrier (ELH) and the pre-exponential factor (νLH) for CO oxidation have been measured at high temperature on hexagonal arrays of Pd clusters supported on an ultrathin alumina film on Ni3Al(111). The Pd clusters have a sharp size distribution and the mean sizes are: 174±13, 360±19 and 768±28 atoms. ELH and νLH are determined from the initial reaction rate of a CO molecular beam with a saturation layer of adsorbed oxygen on the Pd clusters, measured at different temperatures (493≤T (K) ≤613). The largest particles (3.5 nm) give values of E LH and νLHsimilar to those measured on Pd (111) [2]. However, smaller particles (2.7 and 2.1 nm) show very different behavior. The origin of this size effect is discussed in terms of variation of the electronic structure and of the atomic structure of the Pd clusters

High-Density Isolated Fe 1 O 3 Sites on a Single-Crystal Cu 2 O(100) Surface

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Deciphering Radiolytic Oxidation in Halide Aqueous Solutions: A Pathway Toward Improved Synchrotron NAP-XPS Analysis

International audienceSynchrotron radiation near ambient pressure X-ray photoemission spectroscopy (SR NAP-XPS) has been an invaluable tool for examining gas/liquid and liquid/solid interfaces. Despite its benefits, concerns have emerged regarding beam damage in NAP-XPS experiments, particularly involving condensed liquid water, because of the high dose rates, greater than 105 Gy·s–1. This study investigates the radiolytic effects on the chemistry of concentrated NaX sodium halide solutions (X = Cl, Br, I) and Mg–Cl solution formed over the layered double hydroxide [Mg2Al(OH)6]+[Cl–]. The formation of oxidized species XO– as the radiolytic end product under soft X-ray irradiation is discussed in detail. We examine the impact of known parameters (such as the dose rate) on the abundance of XO–. The observed scatter in the data likely arises from still unrecognized or insufficiently controlled parameters (such as solute concentration or solution hydrodynamics). Deciphering these radiolytic effects in halide solutions allows us to propose guidelines for their better identification, understanding and control, ultimately improving the reliability of synchrotron NAP-XPS analysis for interfaces relevant to environmental chemistry and electrochemistry

Soft X-ray Heterogeneous Radiolysis of Pyridine in the Presence of Hydrated Strontium-Hydroxyhectorite and its Monitoring by Near-Ambient Pressure Photoelectron Spectroscopy

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Interaction of Atomic Hydrogen with the Cu 2 O(100) and (111) Surfaces

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Conductive TiN thin films grown by plasma- enhanced atomic layer deposition: Effects of N-sources and thermal treatments

International audienceThis work consists of optimizing TiN plasma-enhanced atomic layer deposition using two different N-sources: NH3 and N2. In addition to maximizing the growth per cycle (GPC) and to shorten the deposition duration, comprehensive in situ and ex situ physicochemical characterizations give valuable information about the influence of the N-source nature, their dilution in Ar, and the plasma power on layer?s final properties. N2 and NH3 dilutions within Ar are extensively investigated since they are critical to decreasing the mean free path (l) of plasma-activated species. A 1:1 gas ratio for the N-sources:Ar mixture associated with low flows (20 sccm) is optimal values for achieving highest GPCs (0.8 Å/cycle). Due to lower reactivity and shorter l of the excited species, N2 plasma is more sensitive to power and generator-to-sample distance, and this contributes to lower conformality than with NH3 plasma. The resistivity of the initial amorphous films was high ( ≥1000 Ωcm) and was significantly reduced after thermal treatment ( ≤400 Ωcm). This demonstrates clearly the beneficial effect of the crystallinity of the film conductivity. Though N2 process appears slightly slower than the NH 3 one, it leads to an acceptable film quality. It should be considered since it is nonharmful, and the process could be further improved by using a reactor exhibiting optimized geometry

Real-Time Study of CVD Growth of Silicon Oxide on Rutile TiO2(110) Using Tetraethyl Orthosilicate

The interaction of the ruffle TiO2(110) surface with tetraethyl orthosilicate (TEOS) in the pressure range from UHV to 1 mbar as well as the TEOS-based chemical vapor deposition of SiO2 on the TiO2(110) surface were monitored in real time using near-ambient pressure X-ray photoelectron spectroscopy. The experimental data and density functional theory calculations confirm the dissociative adsorption of TEOS on the surface already at room temperature. At elevated pressure, the ethoxy species formed in the adsorption process undergoes further surface reactions toward a carboxyl species not observed in the absence of a TEOS gas phase reservoir. Annealing of the adsorption layer leads to the formation of SiO2, and an intermediate oxygen species assigned to a mixed titanium/silicon oxide is identified. Atomic force microscopy confirms the morphological changes after silicon oxide formation