ALD: Atomic Layer Deposition BE: Binding Energy CPD: Contact Potential Difference CVD: Chemical Vapor Deposition DFT: Density Functional Theory fwhm: Full Width at Half Maximum GGA: Generalized Gradient Approximation GW: Gaussian width at half maximum IMFP: Inelastic Mean Free Path IP: Ionization Potentials KE: Kinetic Energy LDA: Local-Density Approximation LW: Lorentzian width at half maximum ML: Mono-Layer NAP-XPS: Near Ambient Pressure X-ray Photoelectron Spectroscopy RH: Relative Humidity SCLS: Surface Core-Level Shifts STM

Keywords: Scanning Tunneling Microscopy TEOS, TetraEthOxySilane TetraMethOxySilane UHV, Ultra High Vacuum VL, Vacuum Level XPS, X-ray Photoelectron Spectroscopy

Introduction

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. [START_REF] Bluhm | Photoelectron spectroscopy of surfaces under humid conditions[END_REF], [START_REF] Salmeron | Ambient pressure photoelectron spectroscopy: A new tool for surface science and nanotechnology[END_REF], [START_REF] Ogletree | Photoelectron spectroscopy under ambient pressure and temperature conditions[END_REF] X-ray Photoelectron Spectroscopy (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 10 5 m. UHV chambers (∼ 10 -10 mbar) help maximize the mean Contents 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 XPS experiments.

In order to make possible the use of XPS on a larger pressure range, a few groups around the world have designed a photoemission equipment that can operate under near 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. [START_REF] Ogletree | A differentially pumped electrostatic lens system for photoemission studies in the millibar range[END_REF] The Fritz Haber Institute has built a high pressure XPS based on the Berkeley prototype, installed at BESSY synchrotron (Berlin), since 2002. [START_REF] Pantforder | New setup for in situ X-ray photoelectron spectroscopy from ultrahigh vacuum to 1 mbar[END_REF] Several Ambient Pressure XPS (AP-XPS) instrument are in construction or already operational all around the world. [START_REF] Shavorskiy | Synchrotron-based ambient pressure X-ray photoelectron spectroscopy[END_REF] The building of AP-XPS analyzers of the Berkeley type, able to operate 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.

The first project deals with the Chemical Vapor Deposition process (CVD) for electronic applications. This was, indeed, a topic proposed in the research project (SAPRES) funded by the Agence Nationale de la Recherche. Generally speaking, organosilane molecules are used to functionalize the surface of (oxidized) silicon wafers. [START_REF] Ulman | An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly[END_REF], [START_REF] Sagiv | Organized monolayers by adsorption. 1. Formation and structure of oleophobic mixed monolayers on solid surfaces[END_REF], [START_REF] Sugimura | Organosilane selfassembled monolayers formed at the vapor/solid interface[END_REF] More specifically, tetraethoxysilane (Si[OC 2 H 5 ] 4 , TEOS), combined with an oxidizing agent (often water), is a popular precursor in the deposition of (multilayer) silica films by CVD or Atomic Layer Deposition (ALD). [START_REF] Hersam | Silicon-based molecular nanotechnology[END_REF], [START_REF] Vilan | Molecules on Si: electronics with chemistry[END_REF], [START_REF] Leskela | Comprehensive Materials Processing. Atomic-Layer-Deposited Thin Films for Silicon Nanophotonics[END_REF], [START_REF] George | Atomic layer deposition: an overview[END_REF], [START_REF] Emesh | Plasma-enhanced chemical vapor deposition of silicon dioxide using tetraethylorthosilicate (TEOS)[END_REF], [START_REF] Fujino | Silicon dioxide deposition by atmospheric pressure and Lowtemperature CVD using TEOS and ozone[END_REF], [START_REF] Klaus | Atomic layer deposition of SiO 2 using catalyzed self-limiting surface reactions[END_REF] On the other hand, its use as a surface modifier at a molecular level was much less explored.

In particular, much less is known on the elementary microscopic processes occurring between the organosilicon molecule and the silicon surfaces, including dissociation, adsorption of the dissociation fragments and further surface reaction of these products. Understanding TEOS reactivity on silicon surfaces, using advanced surface science techniques (like synchrotron radiation XPS and Scanning Tunneling Microscopy (STM)), is necessary for a better control of dielectric film growth.

As far as the NAP-XPS machine was not yet installed when I started my thesis work, we first examined the deposition of TEOS on clean Si(001)-2×1 at room temperature, up to pressures in the 10 -8 mbar range, using UHV XPS setups. These experiments were conceived as control experiments for the anhydrous deposition of the molecule on a well characterized clean surface, guaranteed by the low base pressures in the analysis chambers (low 10 -10 mbar). In fact, we were convinced that the NAP experiments involving mbars of TEOS and water would have led to more intricate situations (due to the relatively high background pressure of the reactive species expected in the 10 -8 mbar range). However, the outcomes of the UHV XPS experiments combined with STM (the observation of a self-assembly of the ethoxy units every second dimer row) were interesting and encouraged us to perform theoretical calculations of the electronic structure.

Chapter 2 presents the clean Si(001)-2×1 surface. Then, chapter 3 describes the adsorption of TEOS, at low coverage, on the Si(001)-2×1 surface at 300 K, studied through a joint experimental and theoretical approach, combining STM and synchrotron radiation XPS with first-principles simulations within the Density Functional Theory (DFT).

It shows that all Si -O bonds within the TEOS molecules are broken upon adsorption, releasing one Si atom per dissociated molecule, while the ethoxy (-OC 2 H 5 ) groups form new Si -O bonds with surface Si dimers.

The release of the silicon monomer by TEOS dissociation on the surface is analyzed, in more details, in chapter 4 by DFT calculations, in order to understand how TEOS fragments influence the diffusion of the monomer on the silicon surface. This work was realized in close collaboration with Dr. F. Finocchi at the Institut des NanoSciences de Contents Paris (INSP, UPMC). At higher coverage, the dissociation channel presented in chapter 3 is superseded by a second mechanism involving the breaking of C -O bonds. It leads to the attachment of alkyl moieties to the surface, probably, due to the presence of the silicon monomers produced during the initial regime. To understand these mechanisms, we compare, in chapter 5, the adsorption of tetraethoxysilane and tetramethoxysilane (TEOS, Si[OC 2 H 5 ] 4 and TMOS, Si[OCH 3 ] 4 ) on the Si(001)-2×1 and Si [START_REF] Denecke | Beamline I511 at MAX II , capabilities and performance[END_REF]-7×7 surface at 300 K.

Chapter 6 explores, using the NAP-XPS instrument, the main application of TEOS: its use as a precursor in ALD process for the deposition of thin silicon oxide layers on silicon surfaces. The NAP experiment with TEOS illustrates the paradigmatic shift with respect to UHV experiments. By increasing the pressure of reactants to the mbar range (TEOS and water) and measuring while dosing, we were confronted to a series an unusual issues. The concept of clean surface had to be abandoned, to be replaced by that of starting surface (the Si (001) surface, water-dosed under 10 -8 mbar). The decomposition of the gas phase, while measuring, leads us to find measurement conditions that eliminate (or at least mitigate) this effect. Finally, by increasing the water pressure and noticing (via the O 1s core-levels) the absence of liquid water layers on the surface, we finally obtained well controlled conditions to check whether the ethoxy units present at the surface can be hydrolyzed or not, a key point to discuss ALD processes.

The second project focuses on environmental molecular chemistry issues, more specifically water/minerals and vapor/liquid water interfaces. In the study of these systems, the NAP-XPS machine acquiring spectra under pressures in the 10 mbar range finds its "raison d'être", as liquid water needs to be condensed on cooled substrates (at

around 8 • C).
It is well known that ionic species, present at the surface of water droplets, are involved, in key chemical reactions in the troposphere. Chlorine atoms can significantly affect the tropospheric ozone balance in marine atmosphere and react, in atmospheric sea salt particles, with nitrogen oxides. Their behavior into the liquid and the possible interactions with the gas phase at the liquid surface need to be understood. [START_REF] Ghosal | Effect of water on the HNO 3 pressure dependence of the reaction between gas-phase HNO 3 and NaCl surfaces[END_REF] In chap-Contents 13 ter 7, we investigated the behavior of sodium halide salt at the liquid water surface and we tried to answer several questions. How are ions distributed at the surface? What is the influence of the anion polarizability on its behavior? And are anions and cations spatially separated?

Interactions between water and minerals were also studied in close collaboration with PHENIX laboratory (UPMC). Clay minerals have various industrial applications, beginning with their use in ceramic ware, paper-making, removal of impurities, decolorizing and more recently as molecular sieves, in catalysis. In relation with environmental issues, wind blown clay dusts are the place of key chemical reactions in atmospheric chemistry. [START_REF] Frinak | Infrared characterization of water uptake by low-temperature Namontmorillonite: Implications for Earth and Mars[END_REF] Clay repository are also used for CO 2 sequestration and geological storage of long half-time radioactive waste. [START_REF] Bradbury | Near-field sorption data bases for compacted MX-80 bentonite for performance assessment of a high-level radioactive waste repository in opalinus clay host rock[END_REF], [START_REF] Gaus | Role and impact of CO 2 rock interactions during CO 2 storage in sedimentary rocks[END_REF], [START_REF] Malikova | Dynamique de l'eau et des ions dans les argiles de type montmorillonite par simulation microscopique et diffusion quasi-elatique des neutrons[END_REF], [START_REF] Fowden | Clay Minerals: Their structure, behavior and use[END_REF], [START_REF] Velde | Introduction to clay minerals[END_REF] Therefore, the way water permeates the clay bands or soaks the dusts is of fundamental interest. In chapter 8, we present an example of the chemical characterization of clays by NAP-XPS, where we demonstrate that core-level XPS can monitor the hydration process of the interlayer counterion of swelling clays.

Chapter 1 X-ray Photoelectron Spectroscopy (XPS)

X-ray Photoelectron Spectroscopy (XPS)

Photoelectron Spectroscopy is one of the most suitable techniques used to investigate solid surfaces. It utilizes photo-ionization and analysis of the kinetic energy distribution of the emitted photoelectrons to study the composition, chemistry and electronic states of the surface region of a sample. It was developed by a Swedish scientist, Kai Siegbahn, who demonstrated the use of XPS as an analytical tool. His efforts in the domain were awarded with a Nobel Prize in 1981.

The concept of photoelectron spectroscopy is based on the photoelectric effect.

When a sample surface is irradiated with monochromatic photons of energy hν (generally 100 -2000 eV), electrons are emitted from the sample surface.

Depending on the energy, two types of spectroscopies are distinguished: the photoemission in the ultra violet region (Ultra violet Photoelectron Spectroscopy, UPS) that allows to eject the electrons from the valence band and photoemission by X-Rays (X-Ray Photoelectron Spectroscopy, XPS) that allows to eject electrons from the core level.

Photoemission in solid

The theoretical model of the photoemission process was developed by Berglund and Spicer in 1964. [START_REF] Berglund | Photoemission studies of copper and silver: theory[END_REF] The photoemission process is described as a three-steps model.

In this approach, optical excitation between two Bloch states, transport of the electron 1. X-ray Photoelectron Spectroscopy (XPS) A simple explanation of the three steps model can be given as follows:

1. Photon ionization.

An atom can absorb a photon with sufficient energy then it could be ionized causing an electron to escape from a bound state to a free state. The transition probability is related to the cross-section of the interaction process that depends on the photon energy, the atom from which the electron comes from and the electronic shell from which the photoelectron is ejected.

Photoelectron transport to the surface.

The surface sensitivity of this technique arises from the strong interaction of electrons with matter. Before escaping from a solid the electron can interact with the matter in different ways, giving rise to different photoemission structures Fig- • Elastically scattered electrons which have escaped from the solid without suffering energy loss. These lead to the main photoemission peak (case I).

• Inelastically scattered electrons which have lost kinetic energy on their way out of the solid. Such kinetic energy losses may arise due to electron-phonon (broadening), electron-electron (intra or interband transitions; few eV) or electron plasmon (few eV up to about 30 eV) transitions. These types of electrons give rise to a peak broadening or to satellites peaks (case II).

• Electrons that have undergone many inelastic scattering processes and lost the energy information of their original levels. They are termed as secondary electrons (case III).

• Electrons that have lost too much energy due to inelastic scattering processes and cannot escape from the surface (case IV).

Crossing the surface.

At the surface, the electrons have to overcome a potential barrier called the work function of the sample, φ s . The work function is the minimum energy required

X-ray Photoelectron Spectroscopy (XPS)

to move an electron from the Fermi level into vacuum. The energy of the emitted photoelectron is given by equation (1.1) where E K is the kinetic energy of the ejected electron, hν the photon energy, E B the binding energy of the core electron and φ s the work function of the sample:

E K = hν -E B -φ s (1.1)
The BE of the photoelectron is defined as the energy required to free electrons from their atomic orbits. In a metal it is conventionally measured with respect to the Fermi level. During a measurement the sample holder and the analyzer are placed in electrical contact and grounded to prevent the charging of the sample surface.

In this configuration, the Fermi level of the sample and the analyzer are aligned (Figure1.3). Then, the kinetic energy measured through the analyzer (E KM ) has to take into account only the work function of the analyzer, i.e., φ analyzer (we do not need to know the work function of the sample φ s ):

E KM = hν -E B -φ analyzer (1.2)
The kinetic energy of the emitted photoelectrons can be measured and the distribution of the emitted electrons are presented in an XPS spectrum as a function of kinetic energy (KE) or binding energy (BE).

Sampling depth

While probing surfaces with XPS, a useful signal is obtained from a depth of around 10 to 100 Å. The thickness of the layer that can be analyzed is called sampling depth which is related to the inelastic mean free path (IMFP) of the generated photoelectrons.

As described previously, electrons can undergo inelastic collisions before they escape the surface. The IMFP λ is defined as follows: the probability of suffering an inelastic scattering over an infinitesimal distance dx is dx/λ. λ is also the average distance traveled by the electron before it suffers an inelastic loss. The probability of the electron traveling a distance d through the solid without undergoing scattering is:

P(d) = exp(-d/λ) (1.3)
Figure 1.4 shows the IMFP as a function of KE. [START_REF] Seah | Quantitative electron spectroscopy of surfaces: A standard data base for electron inelastic mean free paths in solids[END_REF] Minimum IMFP observed for a photoelectron KE around 50 eV, can be readily obtained thanks to the synchrotron radiation tunability.

Chemical shift and Binding energy

The Binding Energy (BE) of an electron is the difference between the initial state (E initial (n), atom with n electrons) and the final state (E f inal (n -1), atom with n-1 elec-

X-ray Photoelectron Spectroscopy (XPS)

.4: IMPF (λ m ) as a function of KE for various solid elements. [START_REF] Seah | Quantitative electron spectroscopy of surfaces: A standard data base for electron inelastic mean free paths in solids[END_REF] trons (ion) and a free photoelectron).

BE = E f inal (n -1) -E initial (n) (1.4)
In the approximation of frozen orbitals, i.e. in the absence of relaxation due to electronic rearrangement following the photoemission process, it corresponds (Koopmans theorem) to the orbital energy (-ǫ i ).However, after the photoionization process, the surrounding orbitals relax around the core hole to screen it.

The BE of an electron depends on the chemical environment of the element. Indeed, the energies of the core electrons, which are measured with XPS, are influenced by the Coulomb interaction with the other electrons and the attractive potential of the nuclei. Changes in the chemical environment cause a spatial redistribution of the valence electrons, which influences the measured BE of the core electrons. For example withdrawal of valence electron charge causes an increase in BE while addition of valence electron charge causes a decrease in BE. The variation of BE results in the shift of the corresponding XPS peak. This change in BE of a core electron of an element due to a change in the chemical bonding of that element is called a chemical shift ∆ξ. If the

X-ray Photoelectron Spectroscopy (XPS)

Fermi level of the sample is taken as a reference then: ∆ξ = -∆ǫ i -∆E R + ∆E F (1.5) where:

• ∆ǫ i is the initial state contribution, and it is the real chemical shift. This term expresses potential modification due to the chemical bond between atoms, acting on the electrons. This could be analyzed using a simple physical model. We consider a point charge e (which represent the core hole) surrounded by the valence charge at a distance r (approximately the distance from the nucleus). Thanks to the Gauss theorem we have:

∆ǫ i ∝ Cδq r (1.6)
where C (constant) > 0 and δq is the charge transferred to the ligand. If δq > 0 then the atom loses charge by bonding to a more electronegative ligand, the initial state energy decreases, if δq < 0 then the atom gives charge by bonding to a less electronegative ligand, the initial state energy increases.

• ∆E R is the final state shift, it is due to the screening capacity of the core-hole by the environment (extra-atomic relaxation). In a solid, the extra-atomic relaxation energy is approximated by the electrostatic polarization energy around the hole.

Indeed, the creation of a point charge (hole) in a dielectric will polarize the surrounding volume (inducing an oriented array of dipoles). A scheme of the situation is a screening cloud that surrounds the positive charge, the energy associated with this extra-atomic screening is:

∆E R = 1 4πǫ 0 (1 -ǫ -1 r )
e 2 2r 0 (1.7) where ǫ r is the static relative dielectric constant, e is the electronic charge and r 0 is the effective screening radius. [START_REF] Egelhoff | Core-level binding-energy shift at surface and in solids[END_REF] 1. X-ray Photoelectron Spectroscopy (XPS)

• ∆E F takes into account the modification of the Fermi level position. In the case of a semiconductor, the Fermi level position in the gap depends on the doping (n or p), on the number of dopants and on the temperature. Moreover, the change of symmetry at the surface induces a modification of the band structure creating for example new gap states which determine the position of the Fermi level at the surface and then the band bending. The adsorption of a molecule can also induce modification in the band bending and in the Fermi level position in the gap.

Chemical shifts are readily observable and interpretable in XPS spectra as core levels are discrete and generally of a well-defined energy. Atoms of a higher positive oxidation state exhibit a higher BE due to the extra coulombic interaction between the photoemitted electron and the ion core. This ability to discriminate between different oxidation states and chemical environments is one of the major strengths of the XPS technique.

Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS)

Decades of surface science in ultra high vacuum (UHV) has shown the power of electron spectroscopy in its various manifestations. However, due to the strong interaction of electrons and ions with the gas, this surface science technique has been used almost on solid surfaces, exclusively in high vacuum environment. Indeed, UHV chambers (10 -9 mbar) help maximize the mean-free path, so that a high number of electrons will reach the detector/analyzer.

Experiments, designed for gaseous samples, [START_REF] Siegbahn | ESCA Applied to Free Molecules[END_REF] liquids, [START_REF] Siegbahn | ESCA applied to liquids[END_REF] and surfaces in a vapor atmosphere, [START_REF] Joyner | A "high-pressure" electron spectrometer for surface studies[END_REF] were developed, during the seventies, by the introduction of differentially pumped gas cells into the vacuum chamber in order to preserve the low pressure in the rest of the experiment.

Since then, and until the beginning of the new century, the field of Ambient-Pressure

Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS)

23

X-ray Photoelectron Spectroscopy (AP-XPS) was developed continuously. [START_REF] Siegbahn | Electron spectroscopy for chemical analysis of liquids and solutions[END_REF] [31] [START_REF] Ruppender | In situ X-ray photoelectron spectroscopy of surfaces at pressures up to 1 mbar[END_REF] [33]

All these systems shared a same "passive" design in the sense that the emitted photoelectrons were simply admitted and spatially filtered through apertures separating the sample in high-pressure environment from the detection chamber at lower pressure.

This was changed with the advent of a new design, developed by pioneering groups at the ALS and BESSY, where an electrostatic lens system was used to focus photoelectrons emitted through an aperture which significantly increases the transmission of the highpressure-adapted analyzers. [START_REF] Ogletree | A differentially pumped electrostatic lens system for photoemission studies in the millibar range[END_REF] [3] [START_REF] Bluhm | Methanol oxidation on a copper catalyst investigated using in situ X-ray photoelectron spectroscopy[END_REF] This novel instrumentation made possible the investigation of fluid and solid samples at higher pressures than ever before.

Moreover, it is with the advent of third-generation synchrotron radiation sources that AP-XPS experienced its breakthrough. These sources deliver a much higher photon flux than conventional X-ray and second-generation synchrotron radiation sources. The high photon flux implies a very large flux of photoelectrons, and AP-XPS measurements become much more feasible in spite of the large loss of photoelectron intensity due to to inelastic scattering in the ambient atmosphere.

Several instruments available to external users are now in use at the ALS, BESSY, MAXLAB, NSLS... and further instruments will be opened to external users in the future at synchrotron radiation facilities around the world (Soleil, Alba). The one presented here is the NAP-XPS, installed in February 2013, on the soft X-ray TEMPO Beamline of Synchrotron Soleil.

The electron spectrometer: SPECS electron spectrometer PHOI-BOS 150 NAP

As the electrons emitted by the sample travel through the gas phase they experience elastic and inelastic collisions with molecules. The mean free path λ, or average distance between collisions depends on their energy and gas pressure. For electrons of 400 eV kinetic energy, λ is about 4 mm when the pressure is 1 mbar. As a result of

X-ray Photoelectron Spectroscopy (XPS)

scattering, the count rate of detected electrons decreases exponentially with distance (Figure 1.5), indicating the need to shorten the travel distance in the high pressure region as much as possible. [START_REF] Salmeron | Ambient pressure photoelectron spectroscopy: A new tool for surface science and nanotechnology[END_REF] Figure 1.5: Signal vs pressure: The XPS signal decreases exponentially as the pressure increases.

All instruments are developed to make possible electron spectroscopy under environmental gases pressures and to maximize the mean free path of emitted electrons, so they can reach the detector. First, the sample is brought close to a small aperture at a distance equal or less than λ. Then, a differential pumping separate the analyze chamber from the analyzer in order to quickly lower the pressure and decrease the collision rate. As mentioned above, the key feature of the new design approach is the addition of electrostatic lens focusing into the differential pumping scheme. This makes possible the collection of a substantially larger fraction of electrons that would otherwise be lost due to the diverging nature of their trajectories and the small solid angle captured by the apertures separating the pumping stages as schematically illustrated in the diagrams of The analyzer owning the entire characteristic described previously was developed in a collaboration between the Advanced Light Source (Berkley, USA), the Fritz-Haber Institute (Berlin, Germany) and SPECS.

The PHOIBOS 150 NAP consists of a differentially pumped electrostatic pre-lens, The analyzer is a true 180 • hemispherical energy analyzer with 150 mm mean radius and is equipped with a 3D delay line detector system (one time and two lateral dimensions).

X-ray Photoelectron Spectroscopy (XPS)

Figure 1.7: The electron spectrometer: SPECS electron spectrometer PHOIBOS 150 NAP. [START_REF] Specs | Phoibos 150 NAP. Near ambient pressure hemispherical energy analyzer[END_REF] Figure 1.8: Sample holder configuration

Beamline and differential pumping

Several stages of pumping are also essential in order to preserve the vacuum level of the TEMPO beamline. Indeed, to prevent the attenuation of the X-ray beam intensity, the beamline is not protected by a membrane, usually made of Si 3 N 4 , Al, or Si, but by four stages of pumping keeping the pressure in the beamline in the low 10 -9 mbar. The beam, in the analysis chamber, has a diameter of 100 µm.

The whole NAP-XPS experiment

The system is designed with four chambers separated by gate valves, presented in Figure 1.9. Additionally, a load-lock allows for easy introduction of samples in the transfer chamber with minimal perturbation to the base pressure. The preparation chamber can be optionally equipped with surface science instruments, molecule evaporators, high temperature heating by laser, ion sputtering, LEED, etc. After preparation the sample is moved to the analysis chamber. Two possible manipulators can be installed in this chamber. The first manipulator and sample holder is designed to allow heating in the presence of gases at pressures of several mbar using a laser. The second one comprises a peltier cooler which coldest spot is the sample in order to condensate gases on the sample; for example, in the study of liquid water surface. The clean Si(001)-2×1 surface

X-ray Photoelectron Spectroscopy (XPS)

As described in the Introduction, the first project, developed during my thesis, deals with TEOS reactivity on the Si(001)-2×1 surface. To make the next chapters easier to understand, we will start by a precise description of this surface.

Structure of bulk silicon

Silicon is a group IV semiconductor with a bulk crystal structure known as the diamond structure, with a lattice spacing of 5.

The clean Si(001)-2×1 surface

Surface reconstruction

Truncation of the silicon bulk diamond structure along the (001) plan cuts through two Si -Si bonds for each surface silicon atom. Each surface atoms is left with two covalent bonds to the bulk and two dangling bonds holding one electron. Such a structure is thermodynamically unstable; therefore, the surface undergoes a reconstruction to achieve a lower energy state. [START_REF] Duke | Semiconductor surface reconstruction: The structural chemistry of two-dimensional surface compounds[END_REF] In order to reduce the dangling bond density, the surface atoms dimerize. The dimers form a 2×1 array, arranged in rows separated by trenches (Figure 2.2). The reported energy values of the weak π bond are approximately ∼28 kcal/mol. [START_REF] Wu | Adsorption of hydrogen atoms on the Si(100)-2x1 surface: implications for the H 2 desorption mechanism[END_REF], [START_REF] D'evelyn | π-bonded dimers, preferential pairing, and first-order desorption kinetics of hydrogen on Si(100)-(2x1)[END_REF], [START_REF] Nachtigall | Ab initio calculation of the energy of recombinative hydrogen desorption from the monohydride phase of silicon (100)[END_REF] It is notable that this bond energy is low compared to the bond energy of traditional alkenes which is reported to be 64 kcal/mol. [START_REF] Carey | Advanced Organic Chemistry[END_REF] Due to electronic effects, the dimers tilt out of the surface plane and make an angle of ∼ 20 • with it, as shown in Figure 2.2. The "down" atom moves closer to the plane of its three nearest neighbors, so its orbitals become more sp 2 -like. Its dangling orbital acquires more p-character, its energy increases, and its electronegativity decreases. The opposite happens to the "up" atom, which moves away from the plane of its neighbors.

Its "dangling" orbital becomes more s-like, its energy decreases and the silicon atom more electronegative. Therefore, the surface Si atoms in the buckled dimer differ chem-Figure 2.3: The highest occupied molecular orbital of a Si 9 H 12 dimer cluster. [START_REF] Filler | The surface as molecular reagent: organic chemistry at the semiconductor interface[END_REF] The up atom is nucleophilic and possesses the majority of the electronic charge.

ically. The "up" atom is negatively charged (nucleophilic silicon atom) and the "down" atom is electron deficient (electrophilic silicon atom) as shown in Figure 2.3.

Scanning Tunneling Microscopy investigation of the clean Si(001) surface

Using scanning tunneling microscopy (STM), the surface structure of Si(001) has been investigated in detail. Beyond a certain temperature, thermal motion allows dimers to cross a potential barrier, allowing them to oscillate rapidly between two inclined states. Molecular dynamics simulations demonstrate that silicon dimers tilt on the picosecond timescale at room temperature. [START_REF] Weakliem | Constant temperature molecular dynamics simulation of Si(001) and Ge(100): Equilibrium structure and short-time behavior[END_REF], [START_REF] Hamers | Atomically-resolved studies of the chemistry and bonding at silicon surfaces[END_REF] Since the time scale of the image collection by STM (millisecond-second) is much slower than the surface dynamics, at room temperature the asymmetry of the dimers is not distinguished as shown in Fig- ure 2.4(a). [START_REF] Yoshinobu | Physical properties and chemical reactivity of the buckled dimer on Si(100)[END_REF], [START_REF] Hamers | Scanning tunneling microscopy of Si(001)[END_REF], [START_REF] Hamers | Atomically-resolved studies of the chemistry and bonding at silicon surfaces[END_REF] However, at a temperature of 85 K (Figure 2.4(b)), the dimers movement is frozen and their asymmetry can be observed. It is possible to identify the up (bright protrusions) and down (dark protrusions) silicon atoms. [START_REF] Yoshinobu | Physical properties and chemical reactivity of the buckled dimer on Si(100)[END_REF], [START_REF] Wolkow | Direct observation of an increase in buckled dimers on Si(001) at low temperature[END_REF] The number of dangling bonds is reduced by the reconstruction, however, the Si(001)-2×1 surface is still highly reactive and sticking probabilities of many organic molecules are near unit at room temperature. [START_REF] Bent | Organic functionalization of group IV semiconductor surfaces: principles, examples, applications, and prospects[END_REF] In contact with air, a native oxide layer is readily formed on the surface and protects it from any reaction with other species that might be present in the environment. This oxide layer is often used as a masking agent and as a barrier layer to grow molecular thin films on the silicon surface and is also used in metal-semiconductor interfaces. [START_REF] Kollakowski | Silicon dioxide passivation of InP/InGaAs metalsemiconductor-metal photodetectors[END_REF] 2.4 XPS studies of clean Si(001)-2×1

Cleaning procedure

The first step, before obtaining a clean Si(001)-2×1, is to remove all the contamination from the substrate and the sample holder. This is achieved by degassing the sample at 600 • C by Joule effect for a period of 8 to 12 hours. This ensures that all contamination on the surface is removed from the surface before the oxide layer is taken off. To remove the native oxide layer, a rapid flash annealing at 1150 • C is performed while staying in the ultra-high vacuum (during this procedure in order to avoid contamination of the sample the base pressure must not exceed 10

XPS Characterization

Once the silicon surface is flashed it can be characterized by XPS to verify the cleanliness of the surface and to see if the oxide layer has been effectively removed. A photoemission spectrum of Si 2p core level of a clean surface (highly doped (phosphorous) n-type Si(001) wafer with a resistivity of 0.003Ω × cm) acquired at room temperature is shown in Figure 2.5 (XPS spectrum recorded at TEMPO beamline).

The photoelectrons were detected at 0 • from the sample surface normal n and at 46 • from the polarization vector E). In order to be surface sensitive, photon energy of 150 eV is used (overall resolution 70 meV).

The core-level spectra are fitted by sums of Voigt curves, i.e. the convolution of a Gaussian (of full width at half maximum GW) by a Lorentzian (of full-width at half maximum LW). The chosen LW is 45 meV. [START_REF] Bozek | Vibrationally resolved core-level photoelectron spectroscopy: Si 2p Levels of SiH 4 and SiF 4 molecules[END_REF], [START_REF] Prince | A critical comparison of selected 1s and 2p core hole widths[END_REF] The total full-width at half maximum (fwhm) of the Voigt profile is calculated using the formula from Ref. [START_REF] Olivero | Empirical fits to the Voigt line width: A brief review[END_REF]:

f whm = 0.5346 × LW + 0.2169 × LW 2 + GW 2 (2.1)
The fitting of the Si 2p spectra is done using a 2p 1/2 / 2p 3/2 doublet with a spinorbit splitting energy of 0.602 eV and an intensity ratio of 0.5 eV. [START_REF] Nagao | Nature of interface bonding of ethylene and benzene with Si(100)c(4x2): angle-dependent Si2p high resolution photoelectron spectroscopy studies[END_REF] In order to clarify the spectrum deconvolution, the Si 2p 1/2 and Si 2p 3/2 peaks of each components were summed.The choice of the surface components is inspired by previous XPS works on the clean and oxidized surface. [START_REF] Himpsel | Proceedings of the International School of Physics Enrico Fermi[END_REF], [START_REF] Landemark | Core-Level Spectroscopy of the Clean Si(001) Surface: Charge Transfer within Asymmetric Dimers of the (2x1) and c(4x2) Reconstructions[END_REF], [START_REF] Pasquarello | Si 2p core-level shifts at the Si(001) -SiO 2 interface: a first principles study[END_REF] The fitting parameters of the Si 2p spectra are collected in Table 2.1.

The attribution of the various Si 2p 3/2 components is based on the preceding Si 2p XPS works: [START_REF] Landemark | Core-Level Spectroscopy of the Clean Si(001) Surface: Charge Transfer within Asymmetric Dimers of the (2x1) and c(4x2) Reconstructions[END_REF], [START_REF] Olivero | Empirical fits to the Voigt line width: A brief review[END_REF], [START_REF] Bozek | Vibrationally resolved core-level photoelectron spectroscopy: Si 2p Levels of SiH 4 and SiF 4 molecules[END_REF], [START_REF] Koh | Reinvestigation of the Si 2p photoemission line shape from a clean Si(001)c(4x2) surface[END_REF]. The main peak (52.4 % of the Si 2p 3/2 spectral weight) is denoted B + SS, as it encompasses both the bulk component B and the positively charged down silicon dimer atom labeled SS. All components BE are referenced to the bulk Si

Chapter 3

Silicon monomers and selforganization following spontaneous dissociative adsorption of TEOS

Since the beginning of the silicon surface chemistry studies in the mid-1980s, [START_REF] Yoshinobu | Rehybridization of acetylene on the Si(111)-7x7 surface -a vibrational study[END_REF] the grafting of organic monomolecular arrays on silicon surfaces via the direct formation of Si -C bonds (both in UHV and wet conditions [START_REF] Buriak | Organometallic chemistry on silicon and germanium surfaces[END_REF], [START_REF] Filler | The surface as molecular reagent: organic chemistry at the semiconductor interface[END_REF]) has stimulated a considerable interest, essentially because of promising applications in molecular electronics. [START_REF] Hersam | Silicon-based molecular nanotechnology[END_REF], [START_REF] Vilan | Molecules on Si: electronics with chemistry[END_REF] Silanes are widely used to form self-assembled monolayers on silicon substrates. [START_REF] Ulman | An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly[END_REF], [START_REF] Sagiv | Organized monolayers by adsorption. 1. Formation and structure of oleophobic mixed monolayers on solid surfaces[END_REF], [START_REF] Sugimura | Organosilane selfassembled monolayers formed at the vapor/solid interface[END_REF] Within this class of molecules, alkoxysilanes (R n Si(OR) 4-n ) are not attached directly to clean silicon surfaces, but rather to native silicon oxide ones, either through the direct reaction of surface Si -OH with the hydrolysable alkoxy terminations of the molecules or via a more complex process involving the participation of molecular water. Fan and Lopinski [START_REF] Fan | STM and HREELS investigation of gas phase silanization on hydroxylated Si(100)[END_REF] investigated the reaction of an alkoxysilane, in the absence of molecular water, with the surface hydroxyls of the water-reacted surface (H,OH)-Si(001)-2×1. [START_REF] Gallet | Isolated silicon dangling bonds on a water-saturated n + -doped Si(001)-2x1 surface: an XPS and STM study[END_REF] On the other hand, the grafting of alkoxysilanes directly on clean Si(001)-2×1 is an emerging field of study.

The reaction of n-propyltriethoxysilane (PTES, Si

[OCH 2 CH 3 ] 3 [CH 2 CH 2 CH 3 ]
) on the clean Si(001)-2×1 was examined, at room temperature and UHV conditions by our group. [START_REF] Gallet | A synchrotron radiation X-ray photoemission spectroscopy study of npropyltriethoxysilane adsorption on Si(001)-2x1 at room temperature[END_REF] Using XPS, we observed that PTES adsorbs dissociatively at room temperature via the scission of Si -O bonds. However, the actual number of broken Si -O bonds in PTES after dissociative adsorption remained elusive due to the presence of

Silicon monomers and self-organization following spontaneous dissociative adsorption of TEOS

non-identical ligands (one propyl and three ethoxy groups). [START_REF] Gallet | A synchrotron radiation X-ray photoemission spectroscopy study of npropyltriethoxysilane adsorption on Si(001)-2x1 at room temperature[END_REF] The latter issue prompted us to examine how a simpler molecule with four identical ligands, tetraethoxysilane (TEOS, Si[OCH 2 CH 3 ] 4 ) reacts with Si(001)-2×1. Many unknowns remain about the microscopic processes governing the adsorption of TEOS on Si(001)-2×1 at room temperature and under UHV conditions (see e.g. the review by

Rauscher [START_REF] Rauscher | The interaction of silanes with silicon single crystal surfaces : microscopic processes and structures[END_REF]). In fact most studies focus on the deposition and electrical/structural characterization of SiO 2 films using TEOS as a precursor. [START_REF] Nishi | Handbook of Semiconductor Manufacturing Technology[END_REF], [START_REF] Ray | TEOS-based PECVD of silicon dioxide for VLSI applications[END_REF], [START_REF] Maeda | CVD TEOS/O 3 development history and applications[END_REF] The few published works devoted to the reaction of TEOS with the clean silicon surface date back to almost two decades ago. As an example, Bonzel et al. [START_REF] Bonzel | Low temperature growth of SiO 2 films on Si(100) using a hot molecular beam of tetraethoxysilane[END_REF], in a Mg Kα XPS and ultraviolet photoelectron spectroscopy (UPS) study, concluded that no adsorbed TEOS could be detected on the Si(001) surface after its exposure to 10 -7 -10 -3 mbar of TEOS at 300

K.

An earlier work, realized by Danner et al. and based on high resolution electron energy loss spectroscopy (HREELS), studied also the deposition of TEOS onto Si(001)-2×1 surface with different deposition conditions. [START_REF] Danner | Pathways and intermediates in the reaction of tetraethoxysilane on silicon (100)-2x1[END_REF] The adsorption of TEOS, made at 90 K, was followed by annealing at room temperature and above. There is a clear indication that TEOS is chemisorbed on the surface, but, in our opinion, the vibrational spectra do not provide a clear information on the nature of the bond breaking (Si -O or C -O bonds), since the Si -C stretching mode is indiscernible from the Si -O one.

In order to unravel the microscopic mechanisms governing the interaction of TEOS with clean Si(001), we combine chemical information delivered by synchrotron radiation XPS with structural ones produced by atomically-resolved STM. Electronic structure calculations based on DFT and STM image simulations gave a sound support to understand the experimental STM images.

XPS results

Si 2p core-level XPS analysis

We start with the determination of the nature of the chemical bonding of TEOS on Si(001), as provided by XPS. The measurements, presented in this chapter were performed at beamline I511 at Max-Lab synchrotron facility (Lund Sweden). In this experiment, the clean reconstructed surface is exposed to TEOS at 120 K and 300 K for doses of 6.75 L and 4.5 L respectively. The core-level spectra are plotted against a BE scale referenced to the Fermi level. The 2p [START_REF] Gallet | A synchrotron radiation X-ray photoemission spectroscopy study of npropyltriethoxysilane adsorption on Si(001)-2x1 at room temperature[END_REF] the number of remaining oxygen neighbors around Si T depends on the number of cleaved bonds, according to the scheme:

Si T (OC 2 H 5 ) 4 + n(≡Si S ) -→ Si T (OC 2 H 5 ) 4-n + n(≡Si S -OC 2 H 5 )
where ≡Si S is a tri-coordinated silicon of the surface and n an integer less than or equal to 4. The number of oxygen ligands i = 4n around the central atom Si T determines its oxidation state, denoted Si i+ . On the other hand, the number of oxygen ligands around Si T is not affected if the O -C bond is broken, i.e. the oxidation state of

Si T remains Si 4+ .
Therefore, the key spectroscopic feature of the Si 2p spectra is the BE position of the oxidation states. In the case of the Si/SiO 2 interface, each oxidation state is associated

Silicon monomers and self-organization following spontaneous dissociative adsorption of TEOS

with a well-defined Si 2p 3/2 surface core-level shifts (SCLS), referenced to the BE of the bulk Si 2p 3/2 component. With i being the oxidation state, the SCLS scales as ∼ 0.9 × i eV, the chemical shifts being additive. [START_REF] Himpsel | Microscopic structure of the SiO 2 /Si interface[END_REF], [START_REF] Jolly | Oxidized silicon surfaces studied by high resolution Si 2p core-level photoelectron spectroscopy using synchrotron radiation[END_REF], [START_REF] Pasquarello | Theory of Si 2p core-level shifts at the Si(001) -SiO 2 interface[END_REF], [START_REF] Yazyev | Origin of fine structure in Si 2p photoelectron spectra at silicon surfaces and interfaces[END_REF] XPS spectra themselves cannot give a definite answer on the extent of the dissociation.

C 1s core-level XPS analysis

The C 1s XPS spectrum (hν = 350 eV) of TEOS chemisorbed at room temperature, see Figure 3.2, is fitted with five Voigts components (LW is chosen at 80 meV). The very weak peak at lower BE (∼ 283.2 eV) can be ascribed to carbonaceous CHx species likely due to beam damage. [START_REF] Khaliq | Ene-like reaction of cyclopentene on Si(001)-2x1: An XPS and NEXAFS Study[END_REF] The BE of the four main components are interpreted based on straightforward initial-state considerations. [START_REF] Gallet | A synchrotron radiation X-ray photoemission spectroscopy study of npropyltriethoxysilane adsorption on Si(001)-2x1 at room temperature[END_REF] The substitution of a hydrogen (Pauling electronegativity = 2.20) bonded to a carbon by a more electronegative oxygen (Pauling electronegativity = 3.44) leads to a charge loss and to an increase of the C 1s core-level BE. Therefore, the α peak at 286. [START_REF] Nagao | Nature of interface bonding of ethylene and benzene with Si(100)c(4x2): angle-dependent Si2p high resolution photoelectron spectroscopy studies[END_REF] to Si). [START_REF] Khaliq | Ene-like reaction of cyclopentene on Si(001)-2x1: An XPS and NEXAFS Study[END_REF] The presence of ethyl groups needs the breaking of the O -C bond.

O 1s core-level XPS analysis

The O 1s XPS spectrum of the Si(001) surface exposed to TEOS at room temperature

STM measurements

While XPS proves that the dissociative adsorption of TEOS on Si(001) surface involves (essentially) the cleavage of the Si T -O bonds and the bonding of ethoxy groups to the surface. The nature of the adsorption sites needs to be determined by STM imagery.

STM imagery was performed using the Variable Temperature scanning tunneling microscope STM XA (Omicron NanoTechnology) of the UPMC campus (Paris). The occupied-states STM images were obtained in constant current mode (I = 100 pA), at a sample voltage (Vbias) of -2 V at room temperature. The chemically-etched tungsten tip was cleaned by heating in UHV prior to STM measurements. The silicon wafer was taken from the same batch as the one used for XPS (n-doped, 0.003 Ω × cm).

In order to examine the early stage of adsorption at 300 K, the surface was exposed to a low TEOS dose, 0.4 L, that is, an order of magnitude less than the exposure used in XPS. This eased the identification of the adsorption sites, as large portions of the surface remain clean. In Figure 3.4 (a), we show a filled state image of the surface. Two types of bright protrusions sitting on the dimer rows are observed. The minority adsorption feature (30 % of the dissociation products) appears as an isolated protrusion (blue circle fragments on surface buckling. On the dimer row bearing the ethoxy groups, the dynamic buckling is not affected: the dimer appear symmetric as the STM scanning time (ms to s) is much longer than the flip-flop buckling period (ps). [START_REF] Weakliem | Constant temperature molecular dynamics simulation of Si(001) and Ge(100): Equilibrium structure and short-time behavior[END_REF], [START_REF] Hamers | Atomically-resolved studies of the chemistry and bonding at silicon surfaces[END_REF] In contrast, where the silicon monomer sits, a static buckling is observed. The question of static/dynamic dimer buckling upon adsorption was addressed theoretically by Wilson et al., [START_REF] Wilson | Defect-induced dimer pinning on the Si(001) surface[END_REF] for the adsorption of phosphine (PH 3 ) on Si(001). When the two fragments (H and PH 2 ) decorate the two silicon dangling bonds of a dimer no static buckling occurs. This is a situation similar to the (fully symmetric) bonding of two ethoxy groups

Silicon monomers and self-organization following spontaneous dissociative adsorption of TEOS

on the same dimer. On the other hand, when PH 3 adsorbs datively (giving its lone pair to a down Si dimer atom), then static buckling is predicted by the calculation. Apparently, the adsorption of the silicon monomer causes a perturbation (e.g. a displacement of the pedestal silicon positions) comparable to the one due to the dative bonding of phosphine.

DFT calculations

To determine the exact position of the monomer on the surface a theoretical approach is needed.

The electronic structure and the adsorption energies of different adsorption configurations of TEOS on the silicon surface are computed within the Density Functional Theory, adopting the generalized gradient approximation (GGA) to the exchange and correlation energy. [START_REF] Perdew | Accurate and simple analytic representation of the electron-gas correlation energy[END_REF] The interaction between the ionic cores and the valence electrons was described through ultra-soft pseudo-potentials, as implemented in the VASP code. [START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF], [START_REF] Kresse | Ab initio molecular dynamics for liquid metals[END_REF] A Si slab consisting of 8 atomic layers, with lateral dimensions corresponding to a (8×4) two dimensional (2D) unit cell, is adopted in the simulations (Fig-

ure 3.6
). The lattice parameter was kept fixed at the computed theoretical value a = 546 pm. This 2D unit cell makes it possible to study the adsorption in the limit of low coverage. For one TEOS ad-molecule on the Si(001)-8×4 surface, the interaction energy between periodically repeated ad-molecules is negligible.

The slab is terminated by hydrogen atoms at the bottom side, in order to saturate bonds with unpaired electrons, and then relaxed. In all adsorption geometries, the two Si bottom layers and the H atoms are kept fixed, while all other degrees of freedom (Si atoms in the slab or atoms belonging to the ad-molecules) were relaxed until residual atomic forces do not exceed a few meV/ Å. At the surface of our slab, the relaxation results in the well-known buckling of the silicon dimers. The slab considered in our calculations comprises eight dimers that exhibit the experimentally observed alternate buckling expected at 0 K. The proper consideration of the buckling is important, because the reactivity of the Si(001)-2×1 strongly relies on the non-equivalent positions and charges of the two silicon atoms within the dimer.

A 1.7 nm thick void space separates periodically repeated slabs along the [001] direction, which prevents the adsorption of molecules from being biased by the presence of periodic images. This sampling would correspond to the gamma point in the Brillouin zone of the Si(001)-8×4 surface, which gives reasonably converged electronic DOS within the 50 meV smearing. Therefore, even if the precision of our calculations regarding the sampling of the Brillouin zone presented herein is not the best available, it offers far enough accuracy to calculate the LDOS and to simulate the STM images within the Tersoff-Hamann approximation.

TEOS fragments were positioned on the surface, four ethoxy groups on two dimers separated by an empty one and an adatom on the adjacent row. The adatom is bonded to two dimers, which belong to the same dimer row and roughly normal to a silicon on the second layer (position M in ref. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF], for more details see chapter 4). After configuration optimization the adatom distance from the two Si atoms within the dimers is 235 pm and from the silicon on the second layer 240 pm (the Si -Si bond length in the bulk is 235 pm), these values are in agreement with the calculated bonds lengths in ref. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF]. As the system is an open-shell system, spin polarized and non-spin polarized calculations

Silicon monomers and self-organization following spontaneous dissociative adsorption of TEOS

were performed; the resulting adsorption energies showed negligible differences. The corresponding adsorption energies were calculated for both configurations. After geometry optimization, the two configurations were almost degenerated (E ads (M1) = 1.97 eV; E ads (M2) = 2.01 eV). Apart from the position of the Si adatom, the most striking difference between the two configurations is the rotation of the ethoxy group in configuration M2 (by about 25 • around the surface normal from a symmetric configuration, as in M1), due to the effective repulsion between the ethoxy group and the neighboring Si adatom.

Using as an input the M1 and M2 optimized configurations, we computed the corresponding occupied-state STM images in the framework of the Tersoff-Hamann theory. [START_REF] Tersoff | Theory of the scanning tunneling microscope[END_REF] The intensity of the tunnel current is proportional to the integrated density of state in the energy range [ǫ F -∆V, ǫ F ], where ǫ F is the Fermi level and ∆V = 2 eV corresponds to the bias voltage of the experimental measurement. A density threshold ρ 0 was employed, which defines the height of the iso-density contour with respect to the atomic surface. ρ 0 was adjusted to a height of about 450 pm, which is a typical tipsurface distance in an experimental setup. The simulated STM images are presented in

Silicon monomers and self-organization following spontaneous dissociative adsorption of TEOS

Therefore, the adatom should sit on an M1 position (and not on M2). However, in the experimental image the "faint protrusion" is aligned with the depression between the two ethoxy-decorated dimers, corresponding to a bare dimer. This symmetric position of the "faint protrusion" is not accounted by the simulated image. In fact, we can hypothesize that the adatom moves back and forth between two equivalent M1 positions, and that the experimental image is time-averaged.

To our knowledge, this is the first time that a silicon monomer is identified by STM on a dimer row at room temperature. Adsorption and diffusion of silicon atoms on Si(001) was widely studied, in order to gain a better knowledge of the initial stages of homoepitaxial growth. [START_REF] Wolkow | Evidence of adsorbed atom pairing during homoepitaxial growth of silicon[END_REF], [START_REF] Zhang | An atomistic view of Si (001) homoepitaxy[END_REF], [START_REF] Wang | An atomically resolved scanning tunneling microscopy study of the thermal decomposition of disilane on Si(001)[END_REF], [START_REF] Vasek | Effects of hydrogen impurities on the diffusion, nucleation, and growth of Si on Si(001)[END_REF], [START_REF] Fehrenbacher | Interaction of SiH 4 with Si(100)-2x1 and with Si(111)-7x7 at 690 K: A comparative scanning tunneling microscopy study[END_REF] On cold enough substrates (T = 160 K), the diffusion of the monomer is frozen out, [START_REF] Wolkow | Evidence of adsorbed atom pairing during homoepitaxial growth of silicon[END_REF] but the precise geometrical configuration of monomers likely trapped in steps, islands, or clusters cannot be obtained precisely. [START_REF] Mo | Surface self-diffusion of Si on Si(001)[END_REF], [START_REF] Swartzentruber | Kinetics of Si monomer trapping at steps and islands on Si(001)[END_REF], [START_REF] Swartzentruber | Direct measurement of surface diffusion using atom-tracking scanning tunneling microscopy[END_REF], [START_REF] Bedrossian | Si binding and nucleation on Si(100)[END_REF] At room temperature, no isolated monomer on a terrace has ever been observed, mainly because its diffusion is so fast that it cannot be imaged and thus remains unnoticed. In our case, we propose that the presence of ethoxy groups helps trapping Si adatoms on the adjacent dimer row.

Summary

Using XPS and STM experiments in conjunction with DFT calculations and STM image simulations, we have shown that TEOS dissociates on the Si(001)-2×1 surface at room temperature. For exposures in the langmuir range, XPS shows that the majority dissociation product results from the cleavage of the Si -O bonds of the TEOS molecule.

The chemical adsorption sites of the majority species are better identified by STM at low coverage (0.4 L). The four ethoxy fragments decorate two surface dimers separated by an empty one. The remaining Si central atom moves to the adjacent dimer row, where it sits as an adatom.

In our DFT modeling, the adatom rests on three pedestal atoms, two silicon atoms from the top plane and one (hypercoordinated) silicon from the second plane. To ac-

Summary

53 count for the observed symmetries in the STM experimental image, we must assume that the monomer moves back and forth between two equivalent positions. We propose that the detection of the silicon adatom at room temperature is related to its hindered diffusion along the dimer row, due to the presence of ethoxy groups in the adjacent row and we will try to prove it in the next chapter.

Silicon monomers and self-organization following spontaneous dissociative adsorption of TEOS

esting pattern could be used to produce self-organized nano-structures on Si(001)-2×1.

Chapter 4

Silicon monomer diffusion on the clean and the TEOS covered Si(001)-2×1 surface

In chapter 3, we presented the observation of an isolated monomer by STM, on the Si(001)-2×1 surface after TEOS dissociation. In order to understand how TEOS fragments influence the diffusion of the monomer on the silicon surface, we decided to restart from the analysis of the monomer diffusion on the clean surface and afterward to extend our work to the ethoxy-covered surface.

Overview: Silicon monomer diffusion on the clean

Si(001)-2×1 surface

The diffusion of adatoms on semiconductor surfaces is a fundamental problem in surface physics, in particular, in relation to epitaxial growth. Two important questions to be answered are the identification of the binding sites for adatoms and the determination of the activation energies for surface diffusion. Two main approaches to atomistic simulations in materials science are possible, representing two extremes with respect to computational cost of the simulation.

The first widely used technique is ab-initio methods. They are adapted to reproduce a rich variety of experimentally observed effects. Ab-initio methods are computationally expensive, the largest systems studied so far involve geometry optimization for supercells with up to 600-800 atoms using parallel supercomputers. [START_REF] Milman | Ab initio total energy study of adsorption and diffusion on the Si(100) surface[END_REF] Due to the computational cost, dynamic simulations are limited to time scales of 10 to 100 ps.

The second technique is based on the use of empirical or semi-empirical interaction potentials to describe bonding between atoms. These potentials might be of different degrees of sophistication (e.g. two-body or many-body potentials), but they always rely on a specific parametrization of the total energy of the system as a function of atomic positions. Parameters for such schemes are usually selected so as to reproduce chosen experimental properties of the material in question. The advantages and drawbacks of this technique are directly opposite to those of ab-initio methods. This approach has the advantage of computational simplicity which allows large-scale computations to be performed at a relatively low cost and increases the time scale that it is possible to reach for dynamic simulations (1000 times higher than for ab-initio methods). However, these potentials for silicon, [START_REF] Stillinger | Computer simulation of local order in condensed phases of silicon[END_REF], [START_REF] Tersoff | New empirical model for the structural properties of silicon[END_REF] fail in reproducing the ground-state structure of the Si(001) surface. The problem can be traced back to the fact that the potentials are constructed to favor four-fold diamond structure coordination and the accuracy of modeling a system with dangling bonds using these potentials was never guaranteed.

In this chapter, we are interested in the electronic structure of the surface as a function of the monomer position and therefore, we decided to use ab-initio methods. dle of the slab was used to increase the computational efficiency. The authors simulated first the clean surface (without the ad atom) and in agreement with the results obtained by Roberts and Needs, they found that the p(2×2) alternating-buckled-dimer surface has the lowest energy. [START_REF] Roberts | Total energy calculations of dimer reconstructions on the silicon (001) surface[END_REF] However, the energy difference between the p(2×2) and the p(2×1) symmetric-dimer reconstruction was small (less than 0.1 eV/dimer); in order to reduce the size and thus the computational time, calculations were carried out on the p(2×1) symmetric-dimer reconstructed surface.

Ab initio methods

The total energy was mapped out as a function of the adatom position (x,y) in order to draw a potential energy surface E(x,y). For each (x,y) position, the z coordinate of the adatom is kept free, in order to determine the position minimizing the system total energy. Figure 4.2 shows the energy surface obtained by Brocks et al. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] On the interpolated energy surface, the binding position of the absolute minimum energy is identified to be position M. The geometry of the resulting equilibrium binding configuration of adatom and substrate (shown in Figure 4.3) may be understood in terms of the adatom making as many covalent bonds as possible without disrupting The first contour is at 0.1 eV with respect to the absolute minimum (M) and the contour spacing is 0.2 eV. The interval on the vertical energy axis is 1 eV and the zero of energy is chosen arbitrarily. The x axis is chosen so as to pass through a dimer.

the underlying substrate. The adatom, in position M, is bonded to two dimers which belong to the same dimer row (bond lengths are 2.35 Å) and to a silicon atom of the second layer substrate (bond lengths are 2.40 Å). In all ab-initio calculations, position M is recognized as the energy absolute minimum. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] [96]

The quasi-hexagonal site H corresponds to a local minimum on the energy surface. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] The adatom forms four long bonds of 2.48 Å to the substrate and stretches the dimer bonds to 2.42 Å. The total energy for this site is 0.25 eV higher than the absolute minimum. However, Smith et al. [START_REF] Smith | Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results[END_REF] considered, both the uniformly buckled asymmetric p(2×1) reconstructed surface and the alternating buckled p(2×2) surface; little difference was observed on the total energy map (less than 0.05 eV), except near the H site.

For the p(2×2) surface, H is a local minimum while in the case of the p(2×1) surface it is not; the local minimum is moved toward the M site on the C-M-H line. It shows that the substrate degrees of freedom play a role in the adsorption process, and that

Silicon monomer diffusion on the clean and the TEOS covered Si(001)-2×1 surface

Smith et al. [START_REF] Smith | Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results[END_REF] also reported closed values for these two path for both p(2×2) and p(2×1) surfaces. The energetic barrier value is modified between these two surface configurations of 0.2 eV from M to H (0.7 eV for the p(2×2) surface and 0.5 eV for the p(2×1) surface); in the reverse direction on the C-M-H line, an adatom in H position see a barrier of 0.2 eV for the p(2×2) surface and no barrier for the p(2×1) surface. For the M-S-C path, the calculated energy barrier is 0.6 eV for both reconstructions.

The comparison of various ab-initio calculations shows that qualitatively similar potential energy surfaces are obtained. There are, however, some quantitative differences. They can be due to the reconstruction surface used, the local configuration of dimers can modify how silicon adatoms are linked to the surface as described previously. A second reason is the various approximations that can be used for the calculation and more precisely the choice of the functional. This aspect will be discussed later.

Empirical or semi-empirical interaction potentials

The second possible approach for the understanding of silicon adatom diffusion is the use of empirical or semi-empirical interaction potentials. Many inter-atomic potentials can be used to obtain a correct description of equilibrium properties and microscopic energies of solids. Several types of such empiric (or semi-empiric) interatomic potentials have been developed for semi-conductors. Some of them correctly describe bulk elastic properties of the material, while others are more suitable for predicting surface properties. Consequently, the choice of the potential needs to take into account the characteristics of the material which are fundamental for the investigation. In particular, diffusion on Si(001) terraces was simulated [START_REF] Somasi | A density functional view of transition state theory: Simulating the rates at which Si adatoms hop on a silicon surface[END_REF], [START_REF] Toh | Diffusion of a Si atom on the Si(001)-2x1 surface: A Monte Carlo study[END_REF], [START_REF] Kharlamov | Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method[END_REF] with the help of the Stillinger-Weber potential, [START_REF] Stillinger | Computer simulation of local order in condensed phases of silicon[END_REF] or Tersoff potential; [START_REF] Tersoff | New empirical model for the structural properties of silicon[END_REF] molecular dynamics with the potential of an embedded atom (method MEAM [START_REF] Baskes | Calculation of the behaviour of Si ad-dimers on Si(001)[END_REF]) was also used for simulating individual silicon adatoms and ad-dimers on the Si(001) surface. [START_REF] Cai | Reconstruction of Si(001) and adsorption of Si adatoms and ad-dimers on the surface: Many-body potential calculations[END_REF] Zhang et al. [START_REF] Zhang | Adsorption and diffusion sites of a Si atom on a reconstructed Si(100)-2x1 surface[END_REF] and Smith et al. [START_REF] Smith | Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results[END_REF], using the Stillinger-Weber silicon potential, predicted the lowest-energy position to be position B and the second-lowest-energy site Diffusion path can also be compared. Same paths, as for first principle calculation are considered, two parallel to the dimer rows and one perpendicular to them. Zhang et al., [START_REF] Zhang | Adsorption and diffusion sites of a Si atom on a reconstructed Si(100)-2x1 surface[END_REF] using the Stillinger-Weber silicon potential, found that diffusion parallel to dimers rows occurs with an activation energy of 0.3 eV, and perpendicularly to them with 0.7 eV. Using MEAM [START_REF] Baskes | Calculation of the behaviour of Si ad-dimers on Si(001)[END_REF], the calculated activation energies are respectively 0.74 eV for parallel diffusion and 0.98 eV for normal diffusion. But using Tersoff potential, Simth et al. [START_REF] Smith | Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results[END_REF] gave the corresponding energies of 0.7 and 0.3 eV which are in conflict with previous results and first principles calculations. Finally, Cai et al. [START_REF] Cai | Reconstruction of Si(001) and adsorption of Si adatoms and ad-dimers on the surface: Many-body potential calculations[END_REF] conclude that the migration of an adatom occurs in parallel to the dimer rows between two rows.

We can conclude that available empirical potentials have been unable to predict, up to now, the most stable position for the adatom on the Si(001) surface and its most probable path for diffusion. These potential have been also unable to predict the asymmetric reconstruction structure of the surface in its ground state. This indicates that there is an intrinsic error in empirical treatment of dangling bonds. [START_REF] Milman | Ab initio total energy study of adsorption and diffusion on the Si(100) surface[END_REF] Finally, a different pathway also investigated, by Wang et al. [START_REF] Wang | Molecular dynamics simulation of single Si adatom diffusion on the Si(001) surface and across single-layer Si(001) steps[END_REF] and Ijima et al.. [START_REF] Iijima | Molecular dynamics study of adatom diffusion on Si(100) surface -importance of the exchange mechanism[END_REF], should be mentioned. Si adatom can diffuse along the direction perpendicular to the dimer row, involving exchanges between the adatom and surface atoms. They found that the Si adatom can diffuse on a Si(001) surface by switching position with a surface atom with a non-negligible frequency of occurrence at high temperature (1000 K to 1300 K). The exchange mechanism is not always a simple process, and complex

Silicon monomer diffusion on the clean and the TEOS covered Si(001)-2×1 surface

processes were observed where up to three surface atoms were involved. For diffusion perpendicular D ⊥ to the dimer row, the exchange events occurred slightly more frequently than hopping did (14 occurrences compared with 11) and reduce the energy barrier of diffusion. In the parallel direction D , the exchange has only a minor effect as the hopping occurs much more frequently.

The energy landscape of a Si adatom on the Si(001) surface

Looking at the various results described in the literature for the monomer diffusion on the clean Si(001) surface, and their dependance on the surface reconstruction and the calculation method, we decided to restart the investigation.

The energy surfaces, presented in this chapter, are computed within the Density Functional Theory (DFT), adopting the generalized gradient approximation (GGA) to the exchange and correlation energy. [START_REF] Perdew | Accurate and simple analytic representation of the electron-gas correlation energy[END_REF] The interaction between the ionic cores and the valence electrons is described through ultra-soft pseudo-potentials, as implemented in the VASP code. [START_REF] Kresse | Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[END_REF], [START_REF] Kresse | Ab initio molecular dynamics for liquid metals[END_REF]

The Si(001)-2×2 surface

The first slab consists of 10 atomic layers, with lateral dimensions corresponding to a (2×2) two dimensional (2D) unit cell. The lattice parameter, which is of 541.5 pm according to the experiment, is kept fixed at the computed theoretical value a = 546 pm which minimizes the energy of the Si bulk in the GGA approximation. A 1.7 nm thick void space separates periodically repeated slabs along the [001] direction, which prevents the adsorption of molecules from being biased by the presence of periodic images. The sampling corresponds to two k-points in the Brillouin zone of the Si(001)-2×2 surface (k-points are presented in Table 4.1). However, the energy was also calculated with eight k-points for the monomer in position M and D´of the surface, the energy

The energy landscape of a Si adatom on the Si(001) surface

63 barrier was unchanged (∆E < 0.05 eV).

Reconstruction Reciprocal space vectors

Number of kpoints

Coordinates of the kpoints The slab is symmetric and a monomer is present on each side. This approach maintain the symmetry of the cell and avoid the presence of an artificial dipole-dipole interaction between the periodic images in the z direction. In all adsorption geometries, the The dimer rows on the Si(001) surface are parallel to the y axis, and the direction of the dimer bond is along the x axis. An equidistant grid, with a spacing between the grid

2×2 b 1 = 2π √ 2a (1, 0, 0) 2 (1/4, 1/4) (1/4, -1/4) b 2 = 2π √ 2a (0, 1, 0) 8 (1/

Silicon monomer diffusion on the clean and the TEOS covered Si(001)-2×1 surface

points of 0.24 Å in the x and 0.54 Å in the y direction, was used to represent the surface.

The surface was interpolated using the Gaussian interpolation method of Igor Pro.

As shown in the overview, the proper consideration of the buckling is important, because the reactivity of the Si(001)-2×1 surface strongly relies on the non-equivalent positions and charges of the two silicon atoms within the dimer. In order to understand the buckling role, the results, obtained with the 2×2 surface cell, were compared with a 2×1 surface comprising two dimers with the same orientation. Both surfaces have the same dimer density. Negligible modifications are observed between the two surfaces (energy differences lower than 0.1 eV).

The binding position of the adatom at the absolute minimum (site M) can be identified on the interpolated energy surface. It corresponds to the configuration of the monomer described in the previous section and in chapter 3, where the adatom is bonded to two silicon atoms of two neighboring dimers and bonded to a silicon atom of the beneath layer.

Unlike Brocks et al. results, [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] the quasi-hexagonal site (site H) does not correspond to a local minimum on the energy surface. A local minimum is observed in position H´, the total energy for this site is 0.14 eV lower than the site H and 0.35 eV higher than the absolute minimum.

Both H and H´adsorption sites are described in Except for the H/H´local minimum difference, also raised up by Smith et al., [START_REF] Smith | Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results[END_REF] and which is a quite small quantitative difference, the energy surface is similar with the one obtained by Brock et al.. The energy barrier are also comparable, for the M-S-C line we obtain 0.75 eV vs 0.8 eV in ref. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] and for the M-D´barrier energy 0.6 eV vs 0.7 eV in ref. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF]. The major difference between these two calculations remains the surface buckling, indeed, in ref. [START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] the dimers are symmetric while in our calculations they are asymmetric.

The Si(001)-8×4 surface

A bigger slab was then investigated for two reasons:

• In the previous slabs, all the dimers of the surface are bonded to the monomer and this monomer can interact with its periodic images. A more realistic situation with an isolated monomer must be investigated.

• Because a bigger slab will be needed, in the next section, to add ethoxy groups on a clean row and investigate their role in the monomer diffusion. Therefore, to allow a direct comparison between the clean and the ethoxy covered surface, the same slab is used for both systems.

This slab is similar to the one presented in chapter 3 and corresponds to a 8×4 reconstructed surface. It is terminated by hydrogen atoms at the bottom side, in order to saturate bonds with unpaired electrons, and then relaxed. In all adsorption geometries, the two Si bottom layers and the H atoms are kept fixed, while all other degrees of freedom (Si atoms in the slab or atoms belonging to the ad-molecules) are relaxed until residual atomic forces do not exceed a few meV/ Å. An equidistant grid, with a spacing between the grid points of 0.34 Å in the x and 0.43 Å in the y direction, was used to represent the surface.

For the calculation, we used the gamma point in the Brillouin zone of the Si(001)-8×4 surface which gives a comparable density of k-points in the Brillouin zone relative to the 2×2 cell. We also run some additional test with a different k-point and the energy barriers were varied less than 0.05 eV.

The energy surface is presented in Figure 4.7. The asymmetry of the dimers has more important consequences on this energy surface with respect to the 2×2 surface.

The monomer at the vertical position of the upper silicon is less stable than the one at the vertical position of the lower silicon (∆E = 0.25 eV compared to 0.13 eV on the 2×2 surface). It induces an asymmetry of the energy surface, the axes (MD) and (DH) are not symmetry axes anymore; however, H is still a symmetry center. for the M-S U -C U path they have significantly increased (∼0.20 eV). This increase can be related to the proximity with the upper silicon atom, which position induces on increase in the z position of the adatom and so, weaker interactions are developed with underneath surface silicon atoms.

Silicon monomer diffusion on the clean and the TEOS covered Si(001)-2×1 surface

The second path we consider is presented Figure 4.9 (b). The energy barrier between H´and H is of 0.17 eV and between H´and D of 0.29 eV. They are similar to those measured for the 2×2 surface. But the stabilization, observed on H´position, is also significant at the transition state between M and H´, in position D´(see Figure 4.9 (c)), the energy barrier is of 0.29 eV (0.70 eV on the 2×2 surface).

In conclusion, the major difference between the 2×2 and 8×4 surface is the difference in the energy barrier between sites M and H´(0.70 eV and 0.29 eV respectively). This modification can considerably modify the possibilities of diffusion for the monomer.

Influence of the slab dimension

To verify that the dimensions of the slab are not responsible for these energy differences, we transformed the 2×2 slab into a 8×4 slab by replicating the 2×2 configuration four times along x and twice along y. In this new slab, only the atoms of one 2×2 cell are kept free during the calculation. The energy of the system was determined for sites M and D´, an energy difference between these two sites of 0.74 eV was obtained while, for the 2×2 slab, the barrier is of 0.70 eV.

Therefore, the energy difference is not due to an artifact of the calculation but to a physical effect linked to the slab dimension and to the adatom density on the surface. For the 2×2 slab, there are two dimers on the surface and one monomer between them, consequently, all the dimers of the infinite surface are interacting with a silicon monomer. On the contrary, on the 8×4 surface, there is one monomer for 16 dimers, and more precisely, the two dimers carrying the monomer are surrounded by free dimers.

The 8×4 slab simulates conditions that are more directly comparable with experiments, where few monomers are diffusing on the surface. The free dimers may be able to enforce the alternate buckling of a row (it explains the energy difference between sites C D and C U ) and to counterbalance the effect of the monomer. The comparison of the buckling angle for H and H´positions between the 2×2 and 8×4 surfaces (

GGA vs LDA

A delicate problem with DFT is the choice of the functional for exchange and correlation. The most widely used approximations are the local-density approximation (LDA), where the functional depends only on the density at the coordinate where it is evaluated and the generalized gradient approximation (GGA) which takes into account the gradient of the density at the same coordinate.

In order to control the impact of the functional choice on the potential surface, the energy barriers between several important monomer positions on the surface were calculated for both LDA and GGA functionals. The results are presented Table 4.2. We observe that the barriers are slightly higher in the GGA than in the LDA; however, the energy differences are small ≤ 0.5 eV and these differences do not modify the interpretation of the results. Another parameter must be discussed, in these two cases we used the theoretical value of the lattice parameter obtained by minimization of the silicon crystal structure energy. For the LDA calculation a = 540.6 pm and for the GGA a = 546 pm while the experimental value is of 541.6 pm. The choice of the theoretical lattice parameter seemed to us the most adapted as it minimizes the structure energy and avoid tensions on surface bonds.

Other functionals can also be used as Meta-GGA functional (includes the second derivative of the electron density, the Laplacian) or hybrid functional corresponding to non local-density approximations (NLDA) (include a component of the exact exchange energy calculated from Hartree-Fock theory). These functionals have been used in sev-

Silicon monomer diffusion on the clean and the TEOS covered Si(001)-2×1 surface

eral studies on the monomer diffusion on the Si(001) surface and some differences were observed compared to LDA and GGA functionals. [START_REF] Smith | Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results[END_REF], [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF], [START_REF] Lee | Development of the Colle-Salvetti correlationenergy formula into a functional of the electron density[END_REF] 4.3 The energy landscape of a Si adatom on the ethoxycovered Si(001) surface

In this section, the slab is similar to the previous one, it is a 8×4 reconstructed surface but four ethoxy groups are present on it, as it is the case after the dissociation of a TEOS molecule. Their positions are similar to the ones described in chapter 3, the four ethoxy groups are shared among two dimers of a row separated by a clean one.

The monomer is then positioned on the neighboring row in the area delimited by the red rectangle shown Consequently, the energy barriers, obtained for the ethoxy-covered surface, are very similar to those of the clean surface. Therefore, the observation of the monomer on the ethoxy-covered surface, which was not observed up to now on the clean surface, can not be simply explained by relying on the difference between static barriers.

An explanation can be proposed. The simulations performed in this chapter do not take into account the dynamic aspect of the dimers buckling. In contrast, the comparison of the 2×2 and the 8×4 surface have shown that the dimer configuration can have important consequences on the energy barriers. On the clean silicon surface, there is a dynamic buckling of the dimers on each row; while, on the ethoxy-covered one, the

Summary

75

TEOS dissociation fragments may modify the buckling dynamic of the adjacent rows and may create a trap for the monomer.

Summary

The analysis of the clean surface have shown the importance of the surface buckling for the determination of the energy landscape of a silicon monomer on the Si(001)

surface. All calculations presented in the literature, using ab-initio methods, are similar to our results with the 2×2 slab. However, we have shown, using a bigger slab (8×4 surface), more directly comparable with experiments, that the dimensions of the 2×2 slab do not allow to account for the role of neighboring free silicon dimers which tend to maintain the alternate buckling. This effect lowers significantly the required static barrier for the monomer to escape from its absolute minimum energy position (from 0.70 eV to 0.29 eV).

The surface buckling may also explain the absence of differences between the clean and the ethoxy covered surface, as the calculation is performed at 0 K the influence of the ethoxy groups on the dimer buckling dynamic of the ethoxy covered row and its neighboring rows can not be analyzed. Therefore, DFT-based molecular dynamics simulation could provide an answer to this question but would be computationally expensive for this type of large systems.

Chapter 5

Dissociation of Ethoxysilane on Si(001)-2×1 and Si(111)-7×7 at Room Temperature

In chapter 3, we examined the dissociation of TEOS on Si(001)-2×1 in the low coverage limit (∼ 1 L), combining STM, with XPS and periodic slab DFT calculations. [START_REF] Tissot | Silicon monomer formation and surface patterning of Si (001)-2x1 following TEOS dissociative adsorption at room temperature[END_REF] We found that the full dissociation of TEOS via Si -O bond cleavage is the majority channel. As shown in Figure 5.1, the ethoxy fragments attach to the four dangling bonds of two surface dimers, separated by a bare one, while the liberated Si atom (Si T ) moves to the dimer row adjacent to the ethoxy-decorated one. There, it sits as a triply coordinated adatom (a monomer), making two bonds with two adjacent silicon atoms and a third one with a silicon atom of the second layer. The release of silicon monomers leads to self-organized structures, with possible nano-patterning applications. Indeed, ethoxy-decorated rows alternate with Si adatom-occupied ones, non-reactive (or much less reactive), when compared to bare dimer rows (Figure 3.8).

However the reaction scheme is likely more complicated. Minority dissociation products are also detected by STM, probably non-fully dissociated molecules. Then, the XPS C 1s spectrum exhibits a weak component attributed to a Si -C bond, implying a C -O bond rupture (Figure 5.2), as it is believed to be the case for the TEOS/Si(111)-7×7 system. [START_REF] Spitzm Üller | Dissociative adsorption and site specificity in the initial stages of tetraethoxysilane (TEOS) interaction with Si(111)-(7x7)[END_REF], [START_REF] Rauscher | The interaction of silanes with silicon single crystal surfaces : microscopic processes and structures[END_REF] A possible non-selectivity in the products certainly impacts the patterning issue, and is crucial for the formation and quality of the interface in, e.g., an ALD (TEOS/water) process.

As different reaction channels, via Si -O and/or C -O bond cleavage (see Figure 5.2), could be opened as a function of molecular coverage, we wished to extend our preceding study on the TEOS/Si(001)-2×1 system to much larger exposures (tens of langmuirs). We present, in this chapter, synchrotron radiation XPS measurements performed at TEMPO Beamline, SOLEIL synchrotron facility (St Aubin, France) [START_REF] Polack | TEMPO: a new insertion device beamline at SOLEIL for time resolved photoelectron spectroscopy experiments on solids and interfaces[END_REF] and at beamline I511 at Max-Lab synchrotron facility (Lund Sweden). [START_REF] Denecke | Beamline I511 at MAX II , capabilities and performance[END_REF] Because the ethoxy (or ethyl) ligands bear two non-equivalent carbons, the spec-tral interpretation becomes complicated when various products with overlapping XPS components are present on the surface. Therefore, the TEOS adsorption experiments on the clean silicon surfaces were systematically duplicated by using tetramethoxysilane (TMOS, Si(OCH 3 ) 4 ). As the methoxy (or methyl) ligands present only one carbon, this leads to a considerable simplification of the C 1s spectral reconstruction and interpretation. This is of great help to interpret the C 1s spectrum of TEOS-covered silicon surfaces, assuming that the reactivity of both molecules is not affected by steric effects.

To our knowledge, no surface study of the dissociation of TMOS on silicon surfaces has ever been published.

The combined TMOS/TEOS study was extended to the Si(111)-7×7 surface. The reason was twofold.

• We believed that the pioneering works of Rauscher and coworkers needed to be reexamined in the light of photoemission measurements using synchrotron radiation. [START_REF] Rauscher | The interaction of silanes with silicon single crystal surfaces : microscopic processes and structures[END_REF] Indeed these authors used non-monochromatized Al Kα/Mg Kα XPS, whose major weakness with respect to synchrotron radiation XPS, besides a low spectral resolution, is the insufficient surface sensitivity that makes the observation of the interfacial oxidation states in the Si 2p spectrum practically impossible.

• The second point is related to the differences and similarities between the Si(111)- For large exposures, new peaks corresponding to oxidation states higher than Si + appear in the Si 2p spectrum of TMOS and TEOS. In particular, we see an oxide state peak with a SCLS of +3.22 eV (6.6 % of spectral weight) for TMOS (67.5 L) and a SCLS of ∼ +3.18 eV (3.8 % of spectral weight) for TEOS (13.5 L) and a fwhm ∼ 0.80 eV. We attribute this peak to a Si 4+ state. Similar SCLS (+3.2 eV) and fwhm (0.88 eV) were observed in the Si 2p spectrum of Si( 111)-7×7 exposed to water at room temperature, [START_REF] Jolly | Oxidized silicon surfaces studied by high resolution Si 2p core-level photoelectron spectroscopy using synchrotron radiation[END_REF] where the reaction of adatoms with water leads to (O 3 )Si -OH units. [START_REF] Jolly | Oxidized silicon surfaces studied by high resolution Si 2p core-level photoelectron spectroscopy using synchrotron radiation[END_REF], [START_REF] Wang | An atomically resolved scanning tunneling microscopy study of the thermal decomposition of disilane on Si(001)[END_REF] The observed fwhm is much smaller than the typical value of the thermally oxidized surfaces, ∼1.0 eV. As the SCLS is a function of the Si -O bond-length, [START_REF] Pasquarello | Si 2p core-level shifts at the Si(001) -SiO 2 interface: a first principles study[END_REF] the narrow width associated to the alkoxysilane suggests a greater static order than that of SiO 4 units in thermal amorphous silica. [START_REF] Jolly | Oxidized silicon surfaces studied by high resolution Si 2p core-level photoelectron spectroscopy using synchrotron radiation[END_REF] The Si 4+ state spectral weight corresponds to a surface coverage of ∼ 0.22 ML for TMOS and ∼0.13 ML for TEOS. The appearance of a Si 4+ state means that the C -O bond breaks and that Si T O 4 species are left on the surface according to reaction path (b), depicted in Figure 5.2.

For both molecules, intermediate states, Si 2+ and Si 3+ , are seen at ∼ +1.8 eV and ∼ +2.6 eV For TMOS, the Si 3+ and Si 2+ states represent 4.0 % and 1.9 % of the spectrum area. For TEOS, the sum of these two components represents less than 1.5 % of the whole spectrum. They could be attributed to partially dissociated molecules via Si -O bond breaking.

O 1s core-level XPS analysis

The corresponding O 1s spectra are given in Figure 5.4. The fitting parameters are collected in Table 5.2.

For low exposures to TMOS and TEOS, the spectra are essentially constituted by one component centered at a BE of 531.9 eV, that we attribute to an oxygen atom in a

(Si 3 )Si -O -C unit,

C 1s core-level XPS analysis

The C 1s spectra of both molecules after reaction of the molecules on Si(001)-2×1 surface are given in Figure 5.5. The fitting parameters are collected in Table 5.2.

First, we will start with the analysis of the TMOS spectra which is simpler, as discussed earlier. In Note that these two components have the same width and intensity. The BE difference between the two non-equivalent carbons is ∼1.4 eV. 5.5.

Dissociation of Ethoxysilane on Si(001)-2×1 and Si(111)-7×7 at Room Temperature

For the longest dose (13.5 L, Figure 5.5 (d)), the TEOS C 1s spectral shape is strongly modified. We need to introduce a component at 287. For the longest exposures, new peaks corresponding to higher oxidation state appear, see Figure 5.6 (b) and 5.6 (c). For TMOS, the Si 4+ state has a SCLS of ∼ +3.26 eV and a spectral weight of 4.1% (corresponding to a surface area of ∼0.17

O 1s core-level XPS analysis

The O 1s spectra are given in Figure 5.7. Similarly to the Si(001)-2×1 surface, the spectra measured for low TMOS/TEOS exposures, are constituted by one main component centered at 531.9 eV, that we attribute to (Si 3 )Si -O -C species resulting from the full dissociation of TMOS and TEOS, according to the path (a) of Figure 5.2. A smaller component is seen at higher BE, i.e. 532.6 eV, with a spectral weight of about 23 % for TMOS and 36 % for TEOS. Longer exposures, lead, for both molecules, to an increase of the high BE component intensity whose spectral weight reaches ∼39 % for TMOS and ∼68 % for TEOS. This increase is correlated to the appearance of the Si 4+ state in the Si 2p spectra, attributed to the C -O bond rupture mechanism (path (b) of Figure 5.2). We reach the same conclusion as that drawn in the Si(001)-2×1 case: the high BE peak in the O 1s spectra is due to oxygen atoms in Si T O 4 tetrahedra.

C 1s core-level XPS analysis

The C 1s spectra are given in Figure 5.8. The Fitting parameters are collected in Table 5.6. The spectra of TMOS, shown in

Reactions mechanisms of alkoxysilanes on silicon surfaces

As shown in the previous section, the studied alkoxysilane molecules are decomposed on Si(001)-2×1 surface via two reaction channels presented Si -O bond breaking of alkoxysilane was considered by Eng and Raghavachari.

[115] They performed DFT calculations on trimethoxysilane (SiH(OCH 3 ) 3 ) reaction over a Si dimer of the Si(001)-2×1 surface. The key feature is the zwitterionic nature of the silicon dimer, with a down electron-poor atom and an up electron-rich one. [START_REF] Yoshinobu | Physical properties and chemical reactivity of the buckled dimer on Si(100)[END_REF] In an bonds could break (the reaction could also be concerted), releasing a Si -OR species that could move away to further react (the nearest adatom on the half cell, a center adatom, is at ∼9 Å). This would lead to the attachment of three -OR moieties to three middle adatoms and to one corner adatom. For a full dissociation (4 Si -O broken bonds) the released silicon atom could move to a restatom site (in another half-cell) surrounded by three bare adatoms (making there four formal bonds). This adsorption site for a monomer was indeed observed by STM. [START_REF] Kohler | Scanning tunneling microscopy study of low-temperature epitaxial growth of silicon on Si(111)-(7x7)[END_REF] Therefore in the initial regime the dissociation of Si(OR) 4 molecules is similar to that of SiCl 4 that leads to the rupture of three to four Si -Cl bonds at room temperature. [START_REF] Whitman | The chemisorption of chlorosilanes and chlorine on Si(111)-7x7[END_REF] Consequently, we propose that alkoxysilane dissociation by C -O bond breaking channel occurs on the adatoms already present on the Si(111)-7×7 surface. The adatom backbonds are indeed strained, weakened single bonds. They break under exposure to hydrogen, [START_REF] Mantega | Spectroscopic characterization of a single dangling bond on a bare Si(100)-c(4x2) surface for nand ptype doping[END_REF] ammonia [START_REF] Chen | Silicon backbond strain effects on NH 3 surface chemistry: Si(111)-(7x7) compared to Si(100)-(2x1)[END_REF] and water. [START_REF] Jolly | Oxidized silicon surfaces studied by high resolution Si 2p core-level photoelectron spectroscopy using synchrotron radiation[END_REF] The other reaction sites for C -O bond breaking could be the silicon atoms released by the Si -O dissociation channel.

Summary

Synchrotron radiation XPS study gives crucial information on the reactivity of alkoxysilane molecules (TMOS and TEOS) on the Si(001)-2×1 and the Si(111)-7×7 surface at The presence of surface alkyl surface species, besides the expected alkoxy species,

Chapter 6

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

In the previous chapters, we attempt to describe the dissociation mechanism of TEOS on the clean Si(001)-2×1 surface. We explore different aspects of its reactivity, the dissociation mechanism and the distribution of fragments on the clean surface at low coverage (chapter 3), the future of the silicon monomer released on the surface (chapter 4), possible dissociation channels at high coverage and their probable link with the presence of the silicon monomer (chapter 5).

In this chapter, we will focus on one of the main application of TEOS: its use as a precursor in ALD process for the deposition of thin silicon oxide layer on the silicon surface.

Atomic layer deposition (ALD)

Several processes can be used to deposit silicon dioxide on a clean silicon surface, the most common are Chemical Vapor Deposition (CVD), Molecular Beam Epitaxy (MBE) or Atomic Layer Deposition (ALD). These techniques use gases as silane (dichlorosilane SiCl 2 H 2 or TEOS) and oxygen. [START_REF] Emesh | Plasma-enhanced chemical vapor deposition of silicon dioxide using tetraethylorthosilicate (TEOS)[END_REF], [START_REF] Fujino | Silicon dioxide deposition by atmospheric pressure and Lowtemperature CVD using TEOS and ozone[END_REF] Silane is used between 300 and 500 • C, dichlorosilane at around 900 • C, and TEOS between 650 and 750 • C. CVD oxides may contain impurities and byproducts of the deposition; indeed, TEOS produces a relatively pure oxide, whereas silane and dichlorosilane introduces hydrogen and chlorine

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

impurities respectively. [START_REF] Klaus | Atomic layer deposition of SiO 2 using catalyzed self-limiting surface reactions[END_REF] In this chapter, we will focus on ALD. Interest in ALD has increased in the mid-1990s and 2000s, in conjunction with the development of silicon-based microelectronics.

ALD is considered one deposition method with great potential for producing very thin, conformal films with control of the thickness and composition of the films possible at the atomic level. A major driving force for the recent interest is the perspective of using ALD in scaling down microelectronic devices. [START_REF] Leskela | Comprehensive Materials Processing. Atomic-Layer-Deposited Thin Films for Silicon Nanophotonics[END_REF], [START_REF] George | Atomic layer deposition: an overview[END_REF] ALD, a variant of CVD, is a repeated sequence of self-limited surface reaction. The main difference with CVD is that ALD uses chemical precursors that are introduced one at a time and separated by inert gas purging in order to avoid gas phase reactions. By repeating the process of growing individual layers, thin films can be applied to surfaces.

Some advantages of a self-limiting film include uniform surfaces, high conformity to surface features, high control and accuracy of atomic level thickness, and high reproducibility. ALD may have some drawbacks, such as incomplete surface reaction and slow rate. Surface science is summoned up to get a more in-depth knowledge of the surface reaction mechanisms, in order to improve the technological processes.

ALD process monitored by NAP-XPS

107

A schematic presentation of the ALD process is presented Figure 6.1. [START_REF] Leskela | Comprehensive Materials Processing. Atomic-Layer-Deposited Thin Films for Silicon Nanophotonics[END_REF] The precursor, which will be TEOS in our case, is added to the reaction chamber containing the water-covered Si(001) surface to be coated by ALD. A possible reaction path for this step is presented in Figure 6.

ALD process monitored by NAP-XPS

The ALD process is the alternance of the two steps described in the previous section, a TEOS pulse and a water pulse. To study this process with the NAP-XPS, we started from a water-covered silicon surface. A Si(001) sample (n-doped, 0.003 Ω × cm) was flashed into the preparation chamber (P = 1 × 10 -9 mbar) and exposed to water

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

(1 × 10 -8 mbar, 30 min, 13.5 L) in this same chamber to avoid carbon contamination during the transfer to the analysis chamber.

Due to the use of high pressures (mbar range) of water and TEOS in the analysis chamber during the acquisition, it was not possible to recover a good vacuum (∼ 1 × 10 -8 mbar) between two exposures. In these conditions, starting the study in the analysis chamber with an already water-covered silicon sample was the cleanest way to proceed.

First step: Exposure to TEOS

For the TEOS exposure, the pressure was increased to 1 mbar and kept at this value for 120 min (5.4 × 10 9 L). In the following section, we present the Si 2p, C 1s and O 1s core-level spectra acquired during the exposure and after the remove of the gas.

Influence of the gas phase on the Si 2p core-level position:

We present in Figure 6.3, the evolution of the Si 2p bulk peak as a function of TEOS pressure. We observe an increase of its BE of ∼ 0.2 eV between P = 2 × 10 -8 mbar and P = 0.5 mbar. After removing the gas from the chamber, the Si 2p bulk peak goes back to a position close to its initial one; the phenomenon is reversible.

On the clean Si(001)-2×1 surface, water dissociates into -H and -OH fragments that react with two silicon dangling bonds and then decorate these silicon atoms. [START_REF] Pierucci | The electronic structure and reactivity of tri-coordinated silicon atoms on water-covered Si(001) surface: time-resolved XPS and STM studies[END_REF],

[62] However, not all dangling bonds are capted with OH/H fragments. A few of them, amounting to 0.01 ML, remain unreacted. These isolated silicon dangling bonds are negatively charged (the dangling bond is doubly occupied) on highly doped n wafers.

The transfer of electrons from the bulk to the surface (due to the alignment of the bulk and surface Fermi levels) is accompanied by an upward band bending. Any change of the surface charge density involves a change in the band bending. When the electrically active surface dangling bonds are passivated, the band bending diminishes (i.e.

the bands go flatter). The BE changes of the Si 2p core-level reflect changes in the po-

ALD process monitored by NAP-XPS

109 sition of the Fermi level at the surface. For a n-type wafer, the increase in BE indicates that the band bending decreases and that the density of electrically active sites at the surface decreases. [START_REF] Hossain | Model for C defect on Si(100): The dissociative adsorption of a single water molecule on two adjacent dimers[END_REF], [START_REF] Hamers | Determination of the local electronic structure of atomic-sized defects on Si(001) by tunneling spectroscopy[END_REF] We tentatively propose that the TEOS molecules interact with the silicon dangling bonds, pushing up in energy the acceptor level. Hence the TEOS-Si dangling bond complex would be neutral. Interaction between oxygen lonepairs and silicon dangling bonds are expected (see for instance the case of aldehydes in ref. [START_REF] Pierucci | Benzaldehyde on water-saturated Si(001): reaction with isolated silicon dangling bonds versus concerted hydrosilylation[END_REF]) These defects may interact with a lone electrons pair of oxygen atoms present in the TEOS molecule; the non-dissociation of TEOS, in this case, would explain the reversibility of the phenomenon. The Si 2p spectra was fitted using the same components than those chosen in previous chapters. B is related to bulk component and S´to second layer silicon atoms. Before

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

filling the chamber with TEOS, we observe on the Si 2p spectra (Figure 6.4 (a)) a first oxidation state at + 0.9 eV (fwhm = 0.9 eV) and a fourth oxidation state at + 3.6 eV (fwhm = 3.2 eV) relative to the bulk component (at 99.62 eV) and corresponding to a spectral weight of 13.6 % and 0.9 % respectively.

To estimate the TEOS coverage of the surface, we can compare the intensity of the Si + and Si 4+ peak. On the water-saturated surface, half of the surface silicon atoms are Si -OH species, corresponding to a coverage of ∼ 0.5 ML can be deduced. During the reaction with TEOS, the number of Si + species on the surface is supposed to be constant; the comparison of the Si + and Si 4+ spectral weights gives a TEOS coverage of 0.03 ML.

On the C 1s (Figure 6.

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

ALD process monitored by NAP-XPS 113

During this first step, we observe mainly the appearance of ethoxy groups and an increase of the fourth oxidation state of silicon in agreement with the mechanism proposed in Figure 6.2, however it is not possible to determine how many bonds of the TEOS molecule are broken and replaced by a bond with surface hydroxyl groups. The origin of the Si -C bonds as minority species (pink components in Figure 6.5) remains unclear.

Second step: Exposure to water

The second step of ALD process consists in the introduction of water. During the exposure to water (120 min, up to 2 mbar, 1.1 × 10 10 L), we followed the evolution of C 1s and O 1s at hν = 640 eV, the spectra are presented in Figure 6.6 and Figure 6.7 respectively. After the remove of the water vapor, the Si 2p was measured at hν = 350 eV (to be compared with the Si 2p spectrum of Figure 6.4 (c)) and is presented in Figure 6.9.

The C 1s spectra are fitted similarly than previous ones, no modification of the spectra are observed during the increase of pressure, only a global decrease of the intensity due to the inelastic scattering of the photoelectron in the gas phase. (The component at ∼ 283 eV, ascribed to carbonaceous CH x species in the bulk, seems more intense than Figure 6.5, it is due to the photon energy. We choose to follow simultaneously the C 1s and O 1s spectra during exposure to water; indeed, at hν = 640 eV we are more sensitive to the bulk than at hν = 350 eV which exacerbate the peak associated to bulk contamination). The ratio between the ethoxy and ethyl groups components area remains constant. In presence of water, ethoxy groups may be hydrolyzed by water molecules, but no reaction can occur with ethyl groups. In consequence, the ratio between the ethoxy and ethyl groups components area should change. It is not the case here and no reaction are affecting the ethoxy groups of the silicon surface.

On the O 1s spectra Figure 6.7, we observe the emergence of a peak related to the water gas phase (at ∼ 536.2 eV and fwhm = 0.3 eV), the second peak at ∼ 532.7

eV, attributed to surface oxygen, is a very large structure (fwhm = 2.0 eV) containing various components and is, therefore, difficult to analyze. However, the shape and the fwhm of this peak do not change when RH increases from RH = 0 % (P ∼ 10 -8 mbar and T = 293 K) to 10 % (P ∼ 2 mbar and T = 293 K). In particular, we do not observe the emergence of a peak at ∼ 534 eV that could be attributed to liquid water on the surface.

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

Therefore, with a RH of 10 %, no water films are found on the (H,OH)-Si(001) surface. This is at odds with what is observed on the silica surface, for which NAP-XPS showed that a 1 nm thick film forms. [START_REF] Verdaguer | Growth and structure of water on SiO 2 films on Si Investigated by Kelvin probe microscopy and in Situ X-ray spectroscopies[END_REF] Figure 6.8 presents the O 1s spectra before and after the water dose. The spectra are normalized on the silicon auger LVV in order to evaluate the variation of oxygen quantities on the surface. The ratio between the area of the two O 1s peak measured after and before the water dose is about 3.

This observation implies that, despite the fact that the C 1s is not modified by the water exposure, some oxygen has been inserted into the surface. This is confirmed by the Si 2p spectrum presented Figure 6.9 where an increase of the Si 4+ component (at + 3.6 eV, fwhm = 0.9 eV, spectral weight after water dose = 6.2 %).

Therefore, the expected hydrolysis of the surface ethoxy group does not occur. As NAP-XPS demonstrates that at 2 mbar no liquid water is present on the surface, this suggests humidity ratios higher than 10 % should be explored. The presence of liquid water may be determining in the hydroxylation process; this point should be explored in the future. Nevertheless, the substrate undergoes an oxidation in presence of water molecules. It means that, in the present conditions, the direct oxidation of the substrate by water molecules is more favorable than an oxidation mechanism involving the hydrolysis of surface ethoxy groups by water molecules of the gas phase. As silicon wafer oxidation is an undesirable process, this interesting observation suggests

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

Beam damage on the silicon surface

During these experiments, we observed damages on our samples due to the X-ray beam. Similarly to UHV experiments, the sample was continuously moved during the acquisition to avoid damages induced by the X-ray beam on the surface. However, unlike UHV measurements, the damages directly induced by the beam on the surface are not the only one we have to take into account, the gas phase is also concerned. We present in this section two experiments illustrating beam induced reactions in the gas phase and their consequences on material deposition.

First experiment:

The water-covered surface was exposed to a TEOS pressure of 1 mbar during 30 min at room temperature. During the exposure, the Si 2p spectrum was continuously acquired at hν = 640 eV and the sample was scanned along x for a z value corresponding to position 1 on the sample scheme Figure 6.10. Then, the chamber was emptied and the Si 2p spectrum was acquired on several points of the surface. Three of them are presented Figure 6.10:

• The spectra acquired on position 1 (this position was exposed directly to the beam during the dosage) presents a huge oxide peak, unusual after TEOS dissociation.

• On position 2 (this position was not hit by the beam during the TEOS exposure), the spectrum contains very small component for the oxide, this spectra is similar to the ones presented in previous sections and chapters and shows what we should expect on the surface after reaction of TEOS in absence of the X-ray beam.

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

Consequently, scanning the surface during the acquisition is not sufficient, indeed, beam damage occurs rapidly (t ≤ spectrum acquisition duration ∼ 30 s) (We also observe during our experiments that this phenomenon was significant for a photon energy higher than the O 1s threshold at 530 eV as it is the case here.)

• On position 3, the spectrum contains a component relative to the oxide, smaller than on position 1 but still more significant than for position 2. This position is located 1 mm from position 1 while the beam has a diameter of 100 µm. It means that damages do not occur only where the X-ray beam strikes the surface but that TEOS fragments created by the beam in the gas phase are responsible for the oxidation of the surface on a large area.

The thickness of silicon oxide can be evaluated for positions 1 and 3, using the following equation:

I SiO 2 I Si = n SiO 2 σ SiO 2 λ SiO 2 n Si σ Si λ Si exp( d λ SiO 2 ) -1 = I ∞ I 0 exp( d λ SiO 2 ) -1 (6.1)
where:

• I SiO 2 and I Si are the intensity of the oxide and the bulk peak respectively,

• n SiO 2 and n Si are the density of silicon atoms of SiO 2 and Si respectively,

• σ SiO 2 and σ Si are the atomic photoionization cross sections of SiO 2 and Si respectively,

• λ SiO 2 and λ Si are the escape depths of SiO 2 and Si respectively,

• and d the oxide layer thickness.

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

The values of the different parameters are chosen in agreement with the work of Himpsel et al. [START_REF] Himpsel | Microscopic structure of the SiO 2 /Si interface[END_REF], I ∞ I 0 = 0.69 and λ SiO 2 = 11 Å.

For position 1,

I SiO 2 I Si = 1.

Second experiment:

The water-covered surface was exposed to a pressure of water of 5 mbar at room temperature. During the exposure the Si 2p spectrum was acquired at hν = 150 eV using a detuned gap. Figure 6.11 (a) shows the Si 2p spectra obtained in these conditions, the silicon oxide peak is present but small and there is no evolution of its shape with time. The gap was then tuned in order to increase the photon flux, modifications appear immediately as shown Figure 6.11 (b), the oxide peak increases gradually with time relative to the silicon bulk component.

To conclude, the sample analysis, in presence of a gas phase must be done with prudence, damages in the gas phase can not be neglected and may have huge consequences on the surface structure. In particular, we must be aware that damages may be not localized on the scanned area, that low photon flux should be used and that photon energies should be lower than important absorption threshold of elements present into the gas phase.

Summary

During the first step of the ALD process, we observe mainly the appearance of ethoxy groups and an increase of the fourth oxidation state of silicon due to the dissociation of TEOS on the water-covered Si(001) surface in agreement with the mechanism proposed in Figure 6.2, however it is not possible to determine how many bonds of the TEOS molecule are broken and replaced by a bond with surface hydroxyl groups. During the second step, the expected hydrolysis of the surface ethoxy group does not occur.

As NAP-XPS demonstrates that at 2 mbar no liquid water is present on the surface, this suggests humidity ratios higher than 10 % should be explored. Nevertheless, the substrate undergoes an oxidation at this RH. It means that, in the present conditions, the direct oxidation of the substrate by water molecules is more favorable than an oxidation mechanism involving the hydrolysis of surface ethoxy groups by the water vapor.

NAP-XPS allows to follow the ALD process in conditions similar to those used in industrial processes. However, we observe that the expected ethoxy group hydrolysis by water molecules does not occur and that direct oxidation of the substrate seems more favorable.

To come closer to industrial conditions, three experiments could be tried:

• the RH, during the second step of the ALD process, should be increased to higher values. To do that, the sample must be cooled down to reduce the water pressure needed for the analysis.

Atomic layer deposition on Si(001) surface using TEOS as a precursor and monitored by NAP-XPS

• the temperature, as mentioned in the introduction could be increased to ∼ 700 • C during the exposure to gases, the increase of temperature may allow the reaction of water on surface ethoxy groups.

• the use of a catalyst should be tried. Indeed, amine bases can catalyze the SiCl 4

and H 2 O reactions during SiO 2 ALD. With the amine base catalyst, the reaction requires much smaller reactant exposures and the reaction temperature can be lowered to ∼ 300 K. However, a problem with catalyzed SiO 2 ALD is that the amine catalysts readily react with the HCl reaction product to form a salt. For example, ammonium chloride forms when using NH 3 as the catalyst. Salt formation could be eliminated by using TEOS and would form ethanol instead of HCl and avoid salt formation. [START_REF] Ferguson | ALD of SiO 2 at room temperature using TEOS and H 2 O with NH 3 as the catalyst[END_REF] Chapter 7

Ion segregation at liquid water/gas interface

The NAP-XPS set up allows to increase the pressure in the analysis chamber to ∼ 20 mbar; it also permits, thanks to the Peltier cooler, to cool down the sample (as described in Chapter 1). In these conditions, solid/gas interface are no longer the only interface that it is possible to investigate. In particular, water droplets can be created in front of the electron analyzer at 283 K and ∼ 10 mbar. Therefore, the analysis of their surface is within reach.

In this chapter, we are interested in a particular aspect of the liquid/vapor interface, the behavior of ions at the liquid water surface. We present results obtained on saturated sodium-halide water solutions. For instance, ionic species, present at the surface water droplet, are involved, in key chemical reactions in the troposphere. Chlorine atoms can significantly affect the tropospheric ozone balance in marine atmosphere and react, in atmospheric sea salt particles, with nitrogen oxides. Their behavior into the liquid and the possible interactions with the gas phase at the liquid surface need to be understood. [START_REF] Ghosal | Effect of water on the HNO 3 pressure dependence of the reaction between gas-phase HNO 3 and NaCl surfaces[END_REF] The structure and chemistry of liquid water/air interface is, therefore, a topic of To understand the behavior of ions near the liquid water surface, we need first to look at its own structure. Indeed, the vapor/water interface has properties different from that of the bulk. Its structure was investigated using dedicated surface-sensitive techniques and molecular dynamics studies which will be briefly discussed in this section.

An early infrared-visible SFG study [START_REF] Du | Vibrational spectroscopy of water at the vapor/water interface[END_REF] concluded that 20 % of the surface molecules are orientated with one free OH bond pointing out of the liquid, at about 38 • relative to the surface. The second OH extending into the bulk, is involved in an hydrogen bond.

In this configuration, the permanent water dipoles are nearly contained within the surface plane which is consistent with the very small surface potential of water. [START_REF] Bader | Solvation and reorganization energies in polarizable molecular and continuum solvents[END_REF] More recent Vibrational Sum-Frequency Generation (VSFG) work indicates the presence of several types of species, at different depths. [START_REF] Raymond | Probing the molecular structure and bonding of the surface of aqueous salt solutions[END_REF], [START_REF] Tarbuck | Spectroscopic studies of solvated hydrogen and hydroxide ions at aqueous surfaces[END_REF] The topmost one is similar to the one previously mentioned. The other type of interfacial species are assumed to be completely surrounded by other water molecules (although not necessarily fully symmetrically tetrahedrally bonded), and are located a few angstroms from the top of the surface. [START_REF] Raymond | Probing the molecular structure and bonding of the surface of aqueous salt solutions[END_REF], [START_REF] Tarbuck | Spectroscopic studies of solvated hydrogen and hydroxide ions at aqueous surfaces[END_REF] A slightly different picture of water surface arises from XAS studies [START_REF] Wilson | Characterization of hydrogen bond acceptor molecules at the water surface using near-edge x-ray absorption fine-structure spectroscopy and density functional theory[END_REF]. An additional interfacial water species was identified, the "acceptor-only" configuration, where both OH bonds are directed towards the vacuum, and both electron lone pairs act as a hydrogen-bond donor, in agreement with ab-initio molecular dynamic studies by Kuo and Mundy. [START_REF] Kuo | An ab initio molecular dynamics study of the aqueous liquid-vapor interface[END_REF].

Photoelectron spectroscopy (PES) studies, using the liquid-water microjet technique, were reported for valence [START_REF] Winter | Full valence band photoemission from liquid water using EUV synchrotron radiation[END_REF] and core [START_REF] Winter | Hydrogen BIBLIOGRAPHY 205 bonds in liquid water studied by photoelectron spectroscopy[END_REF] electron emission. Ref. [START_REF] Winter | Full valence band photoemission from liquid water using EUV synchrotron radiation[END_REF] discusses the energy shifts and the broadening of the valence orbitals with respect to the gas phase when using 60-120 eV photon energies. A shift of about 1.4 eV is observed between the water vapor peak and the liquid one due to an increase of the relaxation 7.1. The liquid water/ vapor interface 125 energy in the liquid. Indeed, the screening contribution of the core-hole between the gas and the liquid phase is modified by an increase of the relative permittivity. The peak broadening is related to the statistical distribution of H-bonding configurations in the liquid.

The inner-shell PES [START_REF] Winter | Hydrogen BIBLIOGRAPHY 205 bonds in liquid water studied by photoelectron spectroscopy[END_REF] uses photon energies up to 1200 eV to probe the water O 1s core photoelectron emission. Depending on the photon wavelength the electrons are emitted either predominantly from H 2 O molecules at the surface or in bulk water, corresponding to about 6 Å or 20 Å probing depths. O 1s electron BE are identical in the two cases, equal to 538.1 eV, but a small additional peak at 536.6 eV appears when probing more in depth. The 536.6 eV peak was assigned to distorted tetrahedral configurations with an elongated hydrogen bond at one of the H-accepting sites.

[141]

Ions behavior at liquid water/ vapor interface

The traditional view of electrolytes surfaces is essentially an ion-free surface and is similar to the surface of neat water. However molecular simulations [START_REF] Padmanabhan | Specific ion adsorption and short-range interactions at the air aqueous solution interface[END_REF] and various spectroscopic techniques [START_REF] Winter | Full valence band photoemission from liquid water using EUV synchrotron radiation[END_REF], [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF], [START_REF] Gurau | On the mechanism of the hofmeister effect[END_REF], [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF] show a very different picture of these electrolytes surfaces. They suppose a segregation of ions to the surface. Small hard ions, such as alkali cations are repelled from the aqueous surface while large polarizable anions can be found (and even enhanced) at the surface.

Indeed, ions can be divided into two classes, kosmotropes and chaotropes, based on their binding strength with water molecules and their effect on water molecules organization. Generally, ionic behavior parallels the Hofmeister series which is a qualitative ordering of ions based originally on their propensity to salt-out proteins from aqueous solution. [START_REF] Hofmeister | Zur Lehre von der Wirkung der Salze. ZweiteMittheilung[END_REF] Large, polarizable, singly charged ions, with low charge density are chaotropes (for example, SCN -, I-, Cl -, NO - 3 , NH + 4 , Cs + , K + ions). They exhibit weaker interactions with water than water with itself and thus they interfere little in the hydrogen bonding of the surrounding water. Small or multiply-charged ions, with high charge density, are kosmotropes (for example, SO 2- 4 , Mg 2+ , Ca 2+ , Li + , Na + , H + , OH - ions). They create stronger interactions with water molecules than water with itself 126 7. Ion segregation at liquid water/gas interface and therefore are able to break water-water hydrogen bonds. Therefore, kosmotropes remain hydrated near the water surface, while chaotropes lose their hydration sheath [START_REF] Santos | Surface tensions, surface potentials, and the Hofmeister series of electrolyte solutions[END_REF].

In this chapter, we will focus on sodium halide salts behavior in water solution.

Na + is kosmotrope, it should create interactions with water and find its place into the water molecules network. Cl -, Br -and I -are chaotropes, furthermore their polarizability increases from Cl -to I -which may turn interactions with water increasingly difficult.

The results of MD slab simulations of 1.2 M aqueous solutions of the sodium halide series are presented Figure 7.1. [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF], [START_REF] Liu | Vibrational spectroscopy of aqueous sodium halide solutions and air-liquid interfaces: observation of increased interfacial depth[END_REF] In NaF solution, both ions are repelled from the air/water interface, leaving an almost ion-free top layer while chloride penetrates all the way to the interface and bromide and iodide even exhibit a surface concentration peak followed by subsurface depletion. Anions in the topmost layer are not completely solvated, but they do maintain a substantial solvation shell. They showed that in terms of getting an anion to the surface of a solution, polarizability seems to be the most important factor. The surface neutrality requires that for the more polarizable halide solutions, the sodium cations are dragged by the anions toward the interface and, consequently, exhibit a subsurface peak.

Liu et al. used vibrational sum frequency generation spectroscopy (SFG) to examine the liquid/vapor interface of sodium halide solutions and their results indicated that water structure at the interface of sodium bromide or sodium iodide is distorted more strongly than that of sodium chloride or sodium fluoride, suggesting that the role of halide anions at the interface increases with respect to increasing size and polarizability. [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF] The surface activity of the heavier halogen anions was the subject of several recent photoelectron spectroscopic studies. Weber et al. reported valence band photoemission data on liquid microjets of alkali metal iodide solutions spanning a wide range of concentration (0.5-12 M). [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF] Their data were consistent with a surface excess of the anion for NaI, but only one component is observed on the spectrum of all anions and no cation selectivity is shown. The absence of chloride and bromide enhancement on the surface Figure 7.1: Snapshots (top and side views) of the solution/air interfaces of 1.2 M aqueous sodium halides from the molecular dynamics simulations and density profiles (number densities) of water oxygen atoms and ions plotted vs distance from the center of the slabs in the direction normal to the interface, normalized by the bulk water density. [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF] appears to contradict the predictions of MD simulations. [START_REF] Ghosal | Electron spectroscopy of aqueous solution interfaces reveals surface enhancement of halides[END_REF] Using X-ray photoelectron spectroscopy operating at near ambient pressure, Ghosal et al. measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium iodide. [START_REF] Cheng | Ambient pressure X-ray photoelectron spectroscopy and molecular dynamics simulation studies of liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions[END_REF] In both cases, the surface composition of the saturated solution is enhanced in the halide anion compared with the bulk of the solution. The enhancement of anion concentration is more dramatic for the larger, more polarizable iodide anion. Cheng at al. also presented the Br 3d and Cl 2p XPS spectra for saturated solutions in surface and bulk sensitive conditions, two components are observed and attributed to symmetric solvation in the bulk and asymmetric solvation of the anion on the water surface. Only one peak is observed for the cation. [START_REF] Bauduin | Hofmeister effect on enzymatic catalysis and colloidal structures[END_REF] However, its behavior relative to anion segregation is not investigated; how are cations distributed below the surface to compensate the negative charge induced by anion segregation?

NAP-XPS analysis of the water surface: the ions behavior 7.2.1 Analysis conditions

Salts of high purity ( 99.9% -99.999%) purchased from Sigma-Aldrich were used.

NaCl and NaBr salts were roasted overnight before using them. A saturated salt solution was prepared with ultra pure water ( 18 MΩ × cm). Samples were, then, prepared by evaporation of the saturated salt water solution on a gold coated silicon surface. XPS measurements were performed between 278 K and 283 K and at a pressure between 8 mbar and 12 mbar, corresponding to RH = 100 % (P sat (water) = 12 mbar at 283 K)

Lowering the temperature of the sample was achieved by using a circulating chiller and a Peltier block attached to the sample holder. The salt solution on the sample surface could be visually observed, as shown For XPS measurement, the Au 4f peak was used as an energy reference. Indeed, no charge effects are observed on the water droplet which can be explained by the low resistance of the salt saturated solution. For example, a saturated sodium chloride solution has a resistivity of ∼ 5 Ω × cm. [START_REF] Barr | Nature of the use of adventitious carbon as a binding energy standard[END_REF]

Carbon contamination

Carbon contamination is a recurring problem in the analysis of this type of sample.

Organic molecules may come from the gold surface or the salt despite the fact that they were cleaned before use or from the chamber walls and the gas line.

Overall, the carbon contamination induces a C 1s XPS spectrum similar to the one presented in Figure 7.3 (b) and (c) which correspond to the C 1s spectrum of NaBr and NaI solutions taken at hν = 500 eV. The spectra are constituted by two peaks, a main one at ∼ 284.8 eV associated to alkyl chains and a second one at ∼ 286.2 eV corresponding 7. Ion segregation at liquid water/gas interface to ∼ 15 % of the spectral weight.

However, for the NaCl solution, we obtained a very different C 1s spectrum (see Figure 7.3 (a)). It contains components at high BE that can be associated to carboxylic acids. We suppose that this contamination comes from the salt as Aldrich uses organic molecules to purify it. To clean it, the salt was roasted before use but it was not enough to purify it entirely.

Cation behavior

If the behavior of the anion near the surface is known, there are still interrogations about the position of the counterion. Molecular dynamics shows that a cation plane should be present below the anion one near the liquid surface (see Figure 7.1). [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF]

NAP-XPS analysis of the water surface: the ions behavior 131

Thanks to the surface sensitivity of XPS, it is possible to investigate precisely the behavior of the cation in saturated salt solutions.

We acquired the Na 2s XPS spectra for the three saturated salt solutions. They are presented for two different photon energies in Figure 7.4. For each solution, one spectrum is acquired at a photon energy of 1200 eV corresponding to bulk sensitive conditions (E k ≈ 1140 eV, λ ≈ 40 Å) and one at 360 eV to surface sensitive conditions (E k ≈ 300 eV, λ ≈ 12 Å). [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF] The Na 2s BE is measured at 63.15 eV for all samples and for both photon energies, showing that the nature of the anion does not have influence on the Na 2s BE. The spectra are constituted by only one component attributed to the cation in a spherical solvation state into the bulk; therefore, no sodium cation is localized on the liquid water surface in direct interaction with the gas phase (it would induce the apparition of a second peak at higher energy similar to the one observe in the case of the anion as it will be discussed in the next section). For each electron KE, a probing depth is given in Figure 7.4. However, the evaluation of the probed thickness as a function of the electron KE is a sensitive issue. The inelastic mean free path (IMFP), which is the distance that photoelectrons travel before 7. Ion segregation at liquid water/gas interface they lose energy from inelastic collisions, depends upon the KE of the photoelectron: photoelectrons with higher KE can reach the detector from deeper in solution than photoelectrons with lower KE. However, the relation between the absolute IMFP values and photoelectron KE in aqueous solutions is still a subject of on-going current research and, for the analysis of aqueous solution surface, a source of error in the determination of concentration profiles from XPS data. At present, minimum IMFP range values for liquid water are accessible by calculation only. A minimum for the IMFP curve as a function of photoelectron KE is observed near 1.2 nm when different theoretical models are applied as shown in Figure 7.5. [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF], [START_REF] Michaud | Cross sections for lowenergy (1-100 eV) electron elastic and inelastic scattering in amorphous ice[END_REF], [START_REF] Emfietzoglou | The effect of model approximations on singlecollision distributions of low-energy electrons in liquid water[END_REF] Figure 7.5: Inelastic inverse mean free path of electrons in water in different phases calculated using different theoretical models. The squares are experimental data for amorphous ice. [START_REF] Emfietzoglou | The effect of model approximations on singlecollision distributions of low-energy electrons in liquid water[END_REF] The Na 2p core-level was also investigated for the three saturated salt solutions.

The Na 2p core-level is closed to the O 2s one; however, the O 2s peak contains two contributions, one from the gas phase and one from the liquid phase. In order to separate these two peaks, the sample was polarized of 40 eV as illustrated for NaI sample in figure 7.6. The polarization of the sample induces an increase of of 40 eV of the ki- The evolution of the ratio between Na 2p peak area and O 2s peak area after the correction by the cross sections is presented in Figure 7.9 for the three salts. These ratios 7.2. NAP-XPS analysis of the water surface: the ions behavior 135 are given as a function of the photoelectron KE which is characteristic of the probed depth. The ratio between Na 2s and O 2s peak area, corrected by cross sections, was also calculated in order to verify the accuracy of the cross section values. The same ratio was obtained (∆(Na/Oratio) ≤ 0.03). The depth profiles show that for NaCl, the concentration of Na relative to water is constant with depth. This is in agreement with density profiles presented Figure 7.1; for NaCl, the curve of the sodium follows the oxygen one. For NaBr and NaI, the behavior is different, the ratio with oxygen increases close to the surface, specially for NaI for which this ratio is multiplied by two between the bulk and the surface. These observations are in agreement with theoretical simulations which suppose the presence of an cation plan just below the surface. However, this is the first time that it is proved by an experimental measurement.

Anion behavior

The anion behavior analysis and more precisely the anion/cation ratio as a function of depth was already investigated for alkali halides, [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF], [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF], [START_REF] Ghosal | Surface segregation of bromine in bromide doped NaCl: Implications for the seasonal variations in Arctic ozone[END_REF], [START_REF] Cheng | Ambient pressure X-ray photoelectron spectroscopy and molecular dynamics simulation studies of liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions[END_REF] and will be discussed in this section.

The first spectra, presented in this section Figure 7.10, shows the valence band of the three saturated salt solutions acquired for hν = 500 eV. The energetic positions of the characteristic water valence orbitals are marked at the top of the Figure . The narrowwidth gas phase b1 (denoted b1gas) line appears at a BE of 7.6 eV, about 1.6-2 eV below the broad liquid water b1 (HOMO) line which is found at ∼ 5.8 eV below the Fermi level. Anion related features are found above the liquid water HOMO. [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF] The I 5p appears as a doublet at 2.6 and 3.4 eV. Br 4p is seen at about 4.1 eV, and Cl 3p at 4.9 eV. Th intensity of these peak relative to oxygen peaks present in the valence band confirms the high concentration of salt into the solution.

Figure 7.11 presents Cl 2p and I 4d XPS spectra of sodium chloride and sodium iodide solutions. In fact the 2p and 4d core-levels are spin-orbit split doublets, the energy difference between Cl 2p 3/2 and Cl 2p 1/2 is of 1.6 eV and between I 4d 5/2 and I 4d 3/2 of 1.7 eV [START_REF] Thomson | X-Ray Data Booklet[END_REF]. For both solutions, two doublets are necessary to fit the spectra properly, in agreement with the findings of Cheng et al.. [START_REF] Cheng | Ambient pressure X-ray photoelectron spectroscopy and molecular dynamics simulation studies of liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions[END_REF] The majority species (Cl 2p 3/2 at 197.6 eV and I 4d 5/2 at 49.0 eV) are attributed to anions in bulk water with a symmetrical solvation shell. The minority species found at higher BE (Cl 2p 3/2 at 199.8 eV and I 4d 5/2 at 50.6 eV) is assigned to anions having an asymmetrical solvation shell (i.e. surface anions). It can be expected that the relaxation energy is smaller for the anions at the very surface than for those in the bulk (shift of ∼ 1.5 eV). [START_REF] Cheng | Ambient pressure X-ray photoelectron spectroscopy and molecular dynamics simulation studies of liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions[END_REF] Both core-levels spectra are presented for two different photon energies, one sensitive to the bulk (E k ≈ 1100 eV, λ ≈ 40 Å) and one in surface sensitive conditions (E k ≈ 300 eV, λ ≈ 12 Å). In bulk sensitive conditions, the component at higher energy has almost disappeared while in surface sensitive conditions the ratio between the high energy peak area and the low energy peak area if of 0.54 in both cases. It confirms the presence of anions on the surface while only one cation environment attributed to the entirely solvated situation is A second interpretation of the two observed peaks must be considered. Due to the presence of organic molecules on the surface, as demonstrated in a previous section by the analysis of the C 1s spectrum, the peak at high energy may be attributed to an anion not only surrounded by water molecules but also by organic ones close to the surface. Figure 7.12 presents the Cl 2p and C 1s spectra acquired at hν = 500 eV for three different saturated NaCl solutions prepared in the same manner but containing, at the end, different quantities of carbon. The shape of the Cl 2p remains the same between these three different samples and seems independent of the carbon one; however, the carbon contamination amount are not very different between the three samples.

The Br 3d XPS spectra are also presented in Figure 7.13 and 7.14. Figure 7.13 shows the five scans of the Br 3d XPS spectrum acquired at hν = 200 eV, a significant decrease of the peak intensity with time is observed. The shape of the spectrum is also modified, the five scans are fitted in Figure 7.14. 3d core-level is spin-orbit split doublet, the energy difference between Br 3d 5/2 and Br 3d 3/2 is of 1.05 eV. [START_REF] Thomson | X-Ray Data Booklet[END_REF] On the first scan, the peak corresponding to the surface bromine represents 2.5 times the area of the bulk peak, then this ratio decreases with time to reach a value of 0.5 after ∼ 120 s. These observations are in agreement with an extraction of surface bromide by the photon beam.

The loss of halides in dry conditions has already been observed under X-ray irradiation; defects, such as anion vacancies, interstitials and color center in alkali halide may be created. [START_REF] Seitz | Color centers in alkali halide crystals[END_REF], [START_REF] Hersh | Proposed excitonic mechanism of color-center formation in alkali halides[END_REF], [START_REF] Itoh | Formation of interstitial-vacancy pairs by electronic excitation in pure ionic crystals[END_REF] Further experiments, using bulk crystals in presence of water vapor have reported an anion/cation average ratio of about 0.85 (for RHs below the deliquescence point). [START_REF] Ghosal | Electron spectroscopy of aqueous solution interfaces reveals surface enhancement of halides[END_REF] In particular, the work of Arima et al. [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF] described significant damages at the surface of a potassium bromide surface in presence of water.

An important decrease of the Br 3d peak intensity as a function of exposure time to the X-ray beam is presented in reference [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF] and the measured Br/Na ratio presents important variation when RH increases. It reaches a minimum of 0.2 for RH = 80 % and then increases to achieve almost 1 at RH = 100 %. The apparently lower loss of Br at higher humidity values is interpreted by an increased mobility of solvated Br -, which partially refills the irradiated spot by diffusing from the surrounding areas. Beam damage was found to be negligible for other elements, particularly for the counterion. This damage can be due to photochemical reactions involving water molecules. Soft X-ray photons can decompose water molecules in the gas phase or at the surface layer to create radicals, such as hydroxyl OH . , atomic oxygen O . , and hydroperoxyl O 2 H . which may oxidize aqueous anions to form molecular dianion, [START_REF] Mamou | Oxidation of aqueous bromide by hydroxyl radicals, studies by pulse radiolysis[END_REF], [START_REF] Thomas | Water adsorption by hydrophobic organic surfaces: Experimental evidence and implications to the atmospheric properties of organic aerosols[END_REF] and desorbs from the sample. This phenomenon may be significant in our experiment due to the high water vapor pressure present in the chamber (∼ 10 mbar compared to ∼ 1 mbar in reference [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF], [START_REF] Ghosal | Electron spectroscopy of aqueous solution interfaces reveals surface enhancement of halides[END_REF], [START_REF] Cheng | Ambient pressure X-ray photoelectron spectroscopy and molecular dynamics simulation studies of liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions[END_REF])

To evaluate the anion segregation near the surface, we attempt to determine the anion/cation ratio as a function of depth. Due to the shape and intensity evolution of the Br 3d with time, it was impossible to determine this ratio for NaBr solutions. The ratio value is lower than the stochiometric ratio of 1.0. A possible explanation 7. Ion segregation at liquid water/gas interface for the larger deviations from the expected value in the present experiments is a loss of I atoms due to beam damage at the solution surface as already guessed for the NaBr solution. However, the difference with stochiometric ratio is small and the difference can also be attributed to the imprecision of the cross sections used for the ratio calculation. As the sample can be polarized with respect to the ground, the work function can be measured via the KE cutoff of the secondary electron (i.e. the electrons with the slowest energy escaping from the liquid surface). Adding together the BE and the work function, we obtain the vertical ionization potentials (IP) of the valence level of liquid water and solutes core-levels (Na 2s, Cl 2p, Br 3d, I 4d).

Measurement of the secondary electron cutoff and determination of the work function

= hν -(E F K -E cuto f f K
). The procedure is illustrated by the NaBr solution case, see The cutoff of the secondary electrons from the liquid water is observed at a KE of ∼ 45 eV, over a strong background corresponding to secondary electrons from the gas phase (Figure 7.17 (a)). After a linear background subtraction, the liquid water secondary electron peak is presented in Figure 7.17 (b). For solid surfaces, it is a common practice to determine the cutoff by extrapolation, drawing a line on the step edge. We adopt also this method for liquid water, and the cutoff is determined as the intersection point of the horizontal baseline and the tangent to the step (green line in Figure 7.17 (b)).

Note that the edge is much steeper for the clean gold case than for the electrolyte solution, as shown in Figure 7.18. We can attribute this to the fact that the liquid/vapor interface is not that sharp, as it extends over ∼ 3 -4 Å, according to the simulated density profiles calculated by Jungwirth et al.. [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF] Therefore, the tangent method introduces for liquid an uncertainty in the cutoff determination estimated to ± 0.4 eV (while, for the gold surface it is < 0.1 eV)

Note that the low energy secondary electron peaks of NaI and NaCl solutions are practically superimposable (the 0.02 eV difference could be accounted by a mere background subtraction effect). The NaBr solution work function is found ∼ 0.2 eV greater than for NaI and NaCl solutions. We note that the step edge is also broader than that of the NaCl and NaI solutions, making a precise determination more difficult. In any Jungwirth et al. [START_REF] Jungwirth | Specific ion effects at the air/water interface[END_REF] calculated variations in surface potentials for KCl, KBr and KI solutions up to molalities of 2 mol/kg with respect to neat water. The trend is an increase of the surface potential (i.e. of the work function) with respect to that of pure water, in the order KCl < KBr < KI (big anions segregate more at the surface). Up to 7. Ion segregation at liquid water/gas interface m = 2 mol/kg the effect is relatively small (0.04 V for KI, less than 0.005 V for KCl). Unfortunately, the surface potentials are not calculated at saturation concentrations. Different surface organic contamination, with different molecular dipoles, may also affect the surface potential. The NaCl and NaI solutions (see Figure 7.3) contain different organic contamination on their surfaces, however the work functions are practically the same.

We would also add a comment on the concept of vacuum level. Through work function measurements, we measure the position of vacuum level VL, referenced to the Fermi level, "just outside the surface". [START_REF] Egelhoff | Core-level binding-energy shift at surface and in solids[END_REF], [START_REF] Trasatti | The absolute electrode potential: an explanatory note[END_REF], [START_REF] Cheng | Alignment of electronic energy levels at electrochemical interfaces[END_REF] This corresponds to a distance such that the image potential of the electron is zero, but this distance is much smaller than the size of the macroscopic sample. The work function we measure here is that of the metal/solution/vacuum system. Importantly, a link can be established with electrochemistry. For redox solutions in the standard state (infinite dilution), this quantity is equivalent to the absolute electrode potential, defined by Trasatti [START_REF] Trasatti | The absolute electrode potential: an explanatory note[END_REF] as "the difference in electronic energy between a point inside the metal (Fermi level) and a point outside the solution". This quantity is calculable by the (periodic) methods of computational surface science. [START_REF] Cheng | Alignment of electronic energy levels at electrochemical interfaces[END_REF] The vacuum level VL (that depends on the surface dipoles present at the surface) should be distinguished from the vacuum level at infinity VL ∞ , i.e. the ultimate reference level. VL is distinct from VL ∞ as it depends on the surface dipole barrier.

This discussion is necessary because of comparisons with liquid jet experiments, [START_REF] Winter | Electron binding energies of aqueous alkali and halide ions: EUV 208 BIBLIOGRAPHY photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations[END_REF], [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF] where VL ∞ is the reference level used to calibrate the BE scale, via the measurement of the KE position of the b1(gas) line, of known ionization potential (12.60 eV).

In Ref. [START_REF] Winter | Electron binding energies of aqueous alkali and halide ions: EUV 208 BIBLIOGRAPHY photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations[END_REF] , [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF] (pure water) as in Figure 7.10 of the present work, the b1(gas) line is found at lower BE than the HOMO of liquid water, 1.44 eV in Ref. [START_REF] Winter | Electron binding energies of aqueous alkali and halide ions: EUV 208 BIBLIOGRAPHY photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations[END_REF] , [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF] and 1.6-1.7

eV here. However, in contrast to the liquid jet case, in our experimental configuration, the sample and the spectrometer are in electrical contact, distant by ∼ 1 mm. The alignment of the Fermi levels of the two dissimilar materials (with different work functions) implies a charge transfer between the sample and the spectrometer, that leads to the appearance of a contact potential difference (CPD), [START_REF] Egelhoff | Core-level binding-energy shift at surface and in solids[END_REF] in which the gaseous molecules are plunged (see Figure 7.19 (a) and (b)) . This CPD induces a shift in the KE of the gas

Work function measurements and electronic level diagrams

Looking to the valence bands of 7.2, and compared to photoemission thresholds [START_REF] Delahay | Photoelectron emission spectroscopy of aqueous solutions[END_REF] already published.

Delahay [START_REF] Delahay | Photoelectron emission spectroscopy of aqueous solutions[END_REF] measured photoelectron thresholds making plots of the photoelectron yield Y n (with n = 0.4 or 0.5) against hν. As discussed by Coe, the onset of the photoelectron yield should occur at photon energies smaller than the vertical transitions (optimized overlap of the excited and ground-state wave-functions, according to the Franck-Condon principle). For the anionic series we do not discuss the absolute VDE (should we use the peak maximum or the low BE rising edge?) and the photoemission threshold values. We note simply that the trend and the energy differences are coherent for the two experiments. The difference can be ascribed to the choice of the vacuum level reference, as for the determination of the VBMw IP. In any case it is significantly smaller than the gas phase IP of Na + (80.07 eV [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF]), due to the solvation effect.

The O 1s BE of liquid water of 533.2 eV gives an IP (VL(cutoff)) of ∼ 537.4 eV, to be compared with the gas phase value of 539.79 eV. The 2.4 eV difference is also accounted by condensation. An illuminating discussion of solvation and condensation effects on the vertical IP/BE is given in Ref. [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF]. This point will be reexamined in Chapter 8 devoted to insertion of water in clays. A simple reasoning [START_REF] Maier | Origin of surface conductivity in diamond[END_REF], [START_REF] Chen | Surface transfer doping of semiconductors[END_REF] (assimilating work functions and redox potentials to the electrochemical potential) considers that the substrate (the solution) will transfer electrons to the solution (substrate) if (VL -E F ) Au is smaller (greater) than (VL -E F ) redox before contacting the two phases. The equilibrium will be reached when the two Fermi layers in the solid and the electrolyte align. Therefore with the considered range of H 2 concentrations of hydrogen in the solution, it is possible that electron exchange between the metal substrate and the water layer is governed by the H + /H 2 redox reaction.

Electronic level alignments

In conclusion, by growing macroscopic alkali halide droplets at saturation concentration onto a metallic substrate we have been able to draw the electronic diagram level of the solution, assuming that the Fermi levels of the metal and of the electrolyte are aligned. In particular we have estimated the position of the water valence band edge with respect to E F , and we have demonstrated that the measurement of the work function ("just outside the surface of the electrolyte solution") is feasible via the secondary cutoff method. We have examined what could pin the Fermi level in the solution, and considered the H + /H 2 redox couple. The fact that electron energy level diagrams at metal/electrolyte interfaces are obtainable via NAP-XPS is of prime importance in fields related to electrochemistry, [START_REF] Cheng | Alignment of electronic energy levels at electrochemical interfaces[END_REF], [START_REF] Maier | Origin of surface conductivity in diamond[END_REF] and photo-electrochemisty. [START_REF] Linsebigler | Photocatalysis on TiO 2 surfaces: principles, mechanisms, and selected results[END_REF], [START_REF] Zhang | Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces[END_REF]

Summary

In this chapter, we presented the first results we obtained, using the NAP-XPS instrument at the TEMPO beamline, on ion segregation at water solution surfaces.

Our results confirm a preferential presence of anions at the liquid water surface as suggested by previous experimental work and theoretical simulations. [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF] However, if we are sure of their presence due to the surface component observed on the anion core-level spectra, the reason of this presence is less clear. Indeed, this peak can be attributed to a pure assymetric water solvation shell of the anion or to an interaction with organic molecules (surface contamination). Furthermore, beam damage makes the analysis delicate.

For the cation the story is easier, particularly because beam damage was not observed. In agreement with Molecular dynamics results, [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF] cation are not present on the water surface (which may protect them from beam damage). But, when the anion polarizability increases, a cation plane is formed below the anion one near the liquid surface, to recover the electroneutrality of the solution. This is an important result, this is the first time that an experimental measurement prove the existence of an increase of cation concentration below the surface.

To clarify the role of carbon contamination on the surface, the use of surfactant that will create a layer on it, may be interesting. The influence of the chemical function (carboxylic acid, amine...) present on the surfactant skeleton on the ions behavior should be investigated. The interaction between anions and octadecylamine at a water droplet surface was already studied using Vibrational Sum Frequency Spectroscopy (VSFS). [START_REF] Gurau | On the mechanism of the hofmeister effect[END_REF] The vibrational spectra collected in the CH stretch range showed dramatic differences in the degree of ordering in the alkyl chains as a function of the anion nature in the subphase. This ordering seems to be modified by the ability of some anions to penetrate into the alkyl chain portion of the monolayer, thereby disrupting the hydrocarbon packing.

Furthermore, ions behavior is also fundamental at the liquid water/hydrophobic interface, for example at the interface with polymers and proteins. These interfaces, similarly to liquid water/vapor ones, are submitted to a reorganization of ions. Depending on the ion nature, this reorganization can be responsible for the solubility and stability of polymers and proteins in aqueous solutions (Hofmeister serie [START_REF] Hofmeister | Zur Lehre von der Wirkung der Salze. ZweiteMittheilung[END_REF]). [START_REF] Bauduin | Hofmeister effect on enzymatic catalysis and colloidal structures[END_REF]

The swelling properties of clays monitored by NAP-XPS

The hydration of clays has already been studied by several techniques: X-ray diffraction, infrared spectroscopy, several neutron induced techniques (neutron spin echo, Neutron Time of Flight and Backscattering). [START_REF] Gionis | On the structure of palygorskite by mid-and near-infrared spectroscopy[END_REF], [START_REF] Morodome | In situ X-ray diffraction study of the swelling of montmorillonite as affected by exchangeable cations and temperature[END_REF] X-ray diffraction (XRD) provides information about the effects of relative humidity (RH) on the interlayer spacing.

However, due to static and dynamic disorder in the interlayer, XRD does not allow to determine the specific atomic positions needed for detailed structural analysis, particularly with respect to the hydrogen positions, and gives essentially no information about dynamics. Neutron scattering also probes interlayer structure and water diffusion and has been recently used to study interlayer water diffusion in smectites. [START_REF] Malikova | Dynamique de l'eau et des ions dans les argiles de type montmorillonite par simulation microscopique et diffusion quasi-elatique des neutrons[END_REF] While much is known on the cristallographic, thermodynamic and dynamic aspects, very little is known about the electronic structure of these materials. The organization of water molecules around the interlayer cations must have a strong impact on their core-level ionization energy (BE), as suggested by recent NAP-XPS experiments on saline solutions. [START_REF] Winter | Electron binding energies of aqueous alkali and halide ions: EUV 208 BIBLIOGRAPHY photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations[END_REF] As natural clays are often complex non-homogenous systems (see below), the philosophy of our approach was to study the penetration of water molecules in a well defined, synthetic smectite, the hectorite, by monitoring the BE shifts of the counterions core-levels and of the silicate layer atoms. Besides its novelty as an application of NAP-XPS to clay minerals, our work enabled to establish a link between the BE of the counterion and the presence of water molecules around it. This can be considered as a first attempt to examine the electronic structure of ions in confined water via XPS. This chapter is organized as follows, it starts by a short presentation of clay structural characteristics and gradually zoom onto smectites and more precisely onto hectorite, highlighting their behavior under conditions of increased relative humidity (RH).

Then, we present the NAP-XPS results obtained for three hectorites containing different counterions and therefore, different behaviors relative to water sorption.

Clay Minerals

Clay layers: atomic structure and charge

The atomic structure of clay minerals, determined mainly from detailed X-ray studies beginning in mid-20th century ( [START_REF] Wyckoff | Crystal Structures[END_REF]) and more recently from electron microscopy and solid state NMR, [START_REF] Fowden | Clay Minerals: Their structure, behavior and use[END_REF] has a number of general features that span the entire clay family. At the same time, a continuum of possible atomic substitutions of the general structure exists and results in blurred boundaries in any attempt to classify clays. Gradual changes in atomic structure are an excellent framework for scientific purposes where the influence of a single parameter (e.g. clay layer charge density) and the resulting properties can be investigated. However, occurrence and purification down to strictly stochiometric phases in natural clays is impossible and most often we deal with mixtures.

To get a good control of the sample, we have examined a synthetic swelling clay, the hectorite, member of the smectite clay group (details of its preparation in section In the case of hectorite, the tetrahedra formed by oxygen atoms are occupied by Si atoms and octahedra by Mg 2+ . But some Mg 2+ ions are substituted by Li + creating a negatively charged clay layer. The charge is compensated by cationic species (counterions) on the layer surface or between two adjacent layers (interlayer region), see Fig- • For high tetrahedral layer charge (micas), the attraction between layers and abundant counterions is too strong for water to penetrate and for ions to become mobile and exchangeable.

• For zero-layer charge (pyrophyllites), particles do not absorb water. No counterions are present, the only forces holding the layers together are of Van der Waals type. It is thought to be due to the hydrophobicity of the uncharged layers and their large lateral size compared to their thickness. (The lateral size of clay particles ranges from 0.1 µm for smectites up to the macroscopic scale, order of cm for vermiculite and order of m for mica. There is a trend in the formation of laterally larger particles with increasing layer charge.)

• For the intermediate charge case (vermiculites, smectites), the swelling can occur.

Clay layers are highly anisotropic (thickness 1 nm, lateral size from nm to m). Hectorite platelets have been studied by Atomic Force Microscopy (AFM), their shape are irregular and the typical lateral dimension of a particle is about ∼ 1 µm as observed on 160

The swelling properties of clays monitored by NAP-XPS

Name

Charge (per unit cell)

Clay swelling/hydration

Swelling properties of clays and structure and dynamics of water and counterions confined between clay layers has been a subject of numerous investigations, [START_REF] Sposito | Structure of water adsorbed on smectites[END_REF] beginning with experimental techniques such as X-ray diffraction combined with water adsorption gravimetry [START_REF] Berend | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonites; 2, The Li + , Na + , K + , Rb + and Cs + -exchanged forms[END_REF], [START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite. 1. The sodium-exchanged form[END_REF], [START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite; 3, The Mg 2+ , Ca 2+ , and Ba 3+ exchanged forms[END_REF], NMR [START_REF] Fripiat | Thermodynamic and microdynamic behavior of water in clay suspensions and gels[END_REF], [START_REF] Delville | Structure of liquids at a solid interface: an application to the swelling of clay by water[END_REF], infrared spectroscopy [START_REF] Sposito | Infrared spectroscopic study of adsorbed water on reduced-charge Na/Li-montmorillonites[END_REF] and more recently microscopic simulation. [START_REF] Boek | Molecular modeling of clay hydration: a study of hysteresis loops in the swelling curves of sodium montmorillonites[END_REF], [START_REF] Chang | Monte Carlo and molecular dynamics simulations of interfacial structure in lithium-montmorillonite hydrates[END_REF], [START_REF] Chavez-Paez | Monte Carlo simulations of Ca-montmorillonite hydrates[END_REF], [START_REF] Marry | Microscopic simulation of structure and dynamics of water and counterions in a monohydrated montmorillonite[END_REF] For routine characterization of the hydration state of a clay sample, the combination of X-ray diffraction and water adsorption gravimetry has been widely used [START_REF] Berend | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonites; 2, The Li + , Na + , K + , Rb + and Cs + -exchanged forms[END_REF],

[182], [START_REF] Cases | Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite; 3, The Mg 2+ , Ca 2+ , and Ba 3+ exchanged forms[END_REF]. X-ray diffraction gives direct access to the layer spacing as a function of RH. Coupled with water adsorption gravimetry, the amount of adsorbed water for a given layer spacing can thus be determined. This approach is, however, subject to the following complications arising from the nature of real clay systems: inhomogeneity in the stacking of the clay layers, difficulty of distinguishing water adsorbed on internal (interlayer) and external surfaces of the clay particles, multiple porosities in the system (interlayer, mesopores, macropores etc... [START_REF] Faisandier | Structural organization of Na-and K-montmorillonite suspensions response to osmotic and thermal stresses[END_REF] The swelling characteristics of a clay, with a given charge location and density, is strongly dependent on the nature of the counterion. Na + , Cs + and Sr 2+ characteris-

The swelling properties of clays monitored by NAP-XPS

tics (size, charge and hydration enthalpy) are given table 8.2. Cs + and Na + have the same charge but Cs + has a bigger radius and its hydration enthalpy is, in absolute value, lower than for Na + . Sr 2+ has a radius close to the one of Na + and a double charge which increases its affinity for water, indeed, its hydration enthalpy is significantly higher. Consequently, the NAP-XPS experiments will be presented starting from the Cs-hectorite which should have the less affinity for water and then Na-hectorite and Sr-hectorite which should be easier to hydrate.

However, we should note that these enthalpies are bulk water thermodynamic constant and it is a clear evidence that the structure and dynamics of water are modified when water molecules are close to interfaces or confined in porous spaces. [START_REF] Michot | Water organisation at the solid-aqueous solution interface[END_REF] Salles et al. have shown that hydration energies for various interlayer cations, compare to bulk conditions, parallel well with the hydration energy of these cations in aqueous solutions, although the hydration energies for these cations are systematically smaller than those obtained in bulk water. Two effects may be invoked to explain the difference. First, the number of water molecules surrounding a cation in its hydration shell is smaller when it is located in the interlayer space compared to the situation in bulk water due to the confinement by the quasi 2D porosity. Secondly, an additional amount of energy is necessary to eventually separate the cation from the clay surface. [START_REF] Salles | Driving force for the hydration of the swelling clays: case of montmorillonites saturated with alkaline-earth cations[END_REF] In any case such effect cannot be quantified without the use of techniques such as conductivity measurements. [START_REF] Salles | Ionic mobility and hydration energies in montmorillonite clay[END_REF] Cation With its larger radius, its higher polarizability, lower charge density and thus lower hydration enthalpy, this transition is not favorable for Cs + ion and the monolayer state is formed with difficulties. [START_REF] Marry | Microscopic simulation of structure and dynamics of water and counterions in a monohydrated montmorillonite[END_REF], [START_REF] Hensen | Why clays swell[END_REF] The ambient temperature water adsorption and desorption isotherm for Na-hectorite clay (Figure 8.6) between its dry state (starting and final point of the sorption-desorption cycle) and 100 % RH shows distinctly two plateaus (in both branches) corresponding to the appearance of a pure monolayer and bilayer of water in the Na-hectorite interlayer space.

• For one water layer in the interlayer, cations are not residing in the middle of the interlayer space but are displaced towards the terahedral sheet enclosing the The fluorinated hectorite presents a substantial advantage with respect to its hydroxyl form that justifies its use. The isotherm sorption curve of the fluorinated Na-Figure 8.7: Schematic representation of the Na-hectorite structure after sorption of (a) one water layer and (b) two water layers into the interlayer. [START_REF] Kalo | Single crystal structure refinement of one-and two-layer hydrates of sodium fluorohectorite[END_REF] hectorite (F-hect) and of the hydroxylated one (OH-hect) are presented in Figure 8.8.

The fluorinated hectorite shows two clear plateaus which are not so well defined for the hydroxy-form and the transitions between the different domains are much sharper.

F-hect also contains less water than their hydroxylated equivalent possibly due to the hydrophobicity of basal surfaces; therefore, the water sorption lies only on the hydration of charge-compensating cations. [START_REF] Dazas | Smectite fluorination and its impact on interlayer water content and structure: A way to fine tune the hydrophilicity of clay surfaces?[END_REF] The ambient temperature water adsorption and desorption isotherm for Sr-hectorite clay (Figure 8.9) between its dry state (starting and final point of the sorption-desorption cycle) and 100 % RH shows a plateau from RH = 20 % and RH = 80 % corresponding to the presence of a bilayer of water in the Sr-hectorite interlayer space. The presence of a water bilayer since low relative humidity is in agreement with the high affinity of water 166

The swelling properties of clays monitored by NAP-XPS

.8: Comparison of the sorption isotherms of hydroxylated and fluorinated Nahectorite. [START_REF] Dazas | Smectite fluorination and its impact on interlayer water content and structure: A way to fine tune the hydrophilicity of clay surfaces?[END_REF] for Sr 2+ ions.

Sample preparation

The fluorinated hectorite was synthesized from gel precursors at high temperature as described elsewhere, [START_REF] Dazas | Smectite fluorination and its impact on interlayer water content and structure: A way to fine tune the hydrophilicity of clay surfaces?[END_REF] leading to a material with very high homogeneity of layer composition and charge density. All samples were initially sodium-saturated at room temperature by contact with aqueous solutions of NaCl. Cs-hectorite and Sr-hectorite were obtained by cation exchange in presence of the corresponding saline solution.

For XPS analysis, clays are deposited in thin layers on a gold coated silicon surface to limit the charge effect during the photoemission acquisition. The deposition is done by evaporation of a water solution containing clay platelets on the gold surface.

NAP-XPS measurements

The analysis of an isolating material as clays by XPS induces an important charge effect on the surface which weakens when the water pressure increases. [START_REF] Salmeron | Ambient pressure photoelectron spectroscopy: A new tool for surface science and nanotechnology[END_REF] Charging is observed as the conductivity of hectorite is too low to replenish the photoemitted electrons. The core-levels shift to lower KE is due to a dynamic equilibrium depending on the X-ray flux and on the stray electrons collected by the positively charged sample surface. In the NAP-XPS case, negative species (secondary electrons, anions resulting from the dissociation of water by the beam) coming from the gas phase participate to the compensation of the positive charge surface. 8.5 Na-hectorite

NAP-XPS measurements on Na-hectorite

The O 1s XPS spectra evolution with RH is shown in Figure 8.13. At RH = 0 %, similarly to Cs-hectorite, the O 1s spectrum contains only one peak at 532.7 eV attributed to the clay structure oxygen. When RH increases, two peaks are observed, one at 533.6 eV related to condensed water, [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF], [START_REF] Shchukarev | XPS with fast-frozen samples: A renewed approach to study the real mineral/solution interface[END_REF], [START_REF] Shchukarev | XPS study of the silica-water interface[END_REF] and a second one at 535.5 ± 0.1 eV (fwhm

Binding energy shift interpretation

The Na 2s peak shift to lower BE can be due to a variation of initial state effect and final state effect. The variation of the BE for the core-level i, also called chemical shift ∆ξ., can be decomposed into the sum of an initial state contribution, ∆ǫ i equals to the

The swelling properties of clays monitored by NAP-XPS

difference between final and initial Hartree-Fock total energies of the system, and a final state contribution corresponding to the total relaxation energy ∆E R (see chapter 1). [START_REF] Egelhoff | Core-level binding-energy shift at surface and in solids[END_REF] ∆ξ = -∆ǫ i -∆E R (8.1)

Initial state considerations:

Initial state effects are related to a change in the electrostatic potential felt by the core-level of the cation. Briefly, clays, formed of alternately positive and negative infinite charged planes, can be considered as condensers. The inter-lamellar hydration obviously changes the relative permittivity with respect to that of the dry case. The distance between the charged planes also increases. This in turn changes the capacitance, and affects the electrostatic potential within the clay platelets.

Simulations by classical molecular dynamic are underway at the PHENIX laboratory of the the Pierre et Marie Curie University (V. Marry and E. Dubois) in order to evaluate initial state effects on the cation between the dry and the hydrated state.

Final state considerations:

Final state effect corresponds to a modification of the relaxation energy of the core hole, recorded as ∆E R , between the dry and the hydrated clay case. The relaxation energy variation can be estimated by measuring the Auger parameter. The Auger parameter α is the sum of the auger KE with the photoelectron core-level BE. The energy associated with extra-atomic screening of a core hole is assumed to be given by the classical equation for the electrostatic polarization of a dielectric medium, treated as a continuum, by a point charge,

E R = 1 4πǫ 0 (1 -ǫ -1 )e 2 2r o , (8.2)
where e is the point charge, r o (an adjustable parameter corresponding to the radius of minimum screening distance) is the radius of the effective electron screening distance, and ǫ is the dielectric constant. [START_REF] Egelhoff | Core-level binding-energy shift at surface and in solids[END_REF] Since the photoelectron emission process is much faster than the time scale required to reorganize the initial solvation shell (the relaxation of the atomic positions surrounding the photoexisted atom does not have to be considered because relaxation times involved in such a process are of the order of the atomic vibration frequencies (∼ 10 -13 s). This is several orders of magnitude larger than the time involved in the photoemission process (∼ 10 -16 to 10 -17 s), the optical relative permittivity should be used. [START_REF] Weber | Photoemission from aqueous alkali-metal -iodide salt solutions using EUV synchrotron radiation[END_REF] However, for sodium cation solvated in the interlayer of hectorite and into water solution, we used for the interpretation of the BE shift, the static relative permittivity.

This choice is motivated by the theoretical observations made by Winter et al.. [START_REF] Weber | Photoemission from aqueous alkali-metal -iodide salt solutions using EUV synchrotron radiation[END_REF] They showed that, although the photoionization process is faster than the relaxation of the solvent dipoles, the favorable pre-orientation of water molecules around a cation allows using the static relative permittivity. Indeed, the cation presents a positive charge before and after ionization, which is not true for anionic ionization where the final state is neutral. Consequently, the static relative permittivity is adapted to cation relaxation while optical relative permittivity should be used for anion relaxation.

In absence of water , ǫ = ǫ clay ǫ 0 (with ǫ 0 the vacuum permittivity) with ǫ clay ≈ 2 (optical dielectric constant of silica) [START_REF] Pasquarello | Si 2p core-level shifts at the Si(001) -SiO 2 interface: a first principles study[END_REF]; after hydration ǫ = ǫ water ǫ 0 , for one water layer with ǫ water > ǫ clay (the static dielectric constant of one water layer can be estimated at ∼ 20 using the relation between the basal spacing and the dielectric constant described by Laird et al. [START_REF] Laird | Model for crystalline swelling of 2:1 phyllosilicates[END_REF]). The dielectric constant of the sodium cation environment increases with RH, according to equation 8.2, ∆E R > 0. Consequently we expect that the relaxation energy increases when water is inserted within the lamellae and that the BE decreases. Sr 2+ has an enthalpy of hydration in bulk water higher (in absolute value) than the one of sodium (see Table

Solvation energies

In this section, we will consider the relation between the vertical ionization or detachment energies we measure and solution thermodynamics. Indeed, the difference in electron BE of the Na 2s core level between liquid (E aq ) and gas-phase water (E g ) correlate with the free energy penalty for disrupting the local solvent structure, known as the solvent reorganization energy. [START_REF] Marcus | Ionic radii in aqueous solutions[END_REF] In order to extract, from XPS data, information concerning the solvation energy of sodium, we will use the Born model which treats the ion-solvent interaction. [START_REF] Marcus | Ionic radii in aqueous solutions[END_REF], [START_REF] Born | Dielectric solvation and ion size[END_REF], [START_REF] Lundholm | Core electron spectroscopy of water solutions[END_REF], [START_REF] Winter | Electron binding energies of aqueous alkali and halide ions: EUV 208 BIBLIOGRAPHY photoelectron spectroscopy of liquid solutions and combined ab initio and molecular dynamics calculations[END_REF] In this model, presented in Figure 8.17 For the present discussion, the numerical choice of R is not as relevant since we will relate electron BE, measured by photoemission, to experimental (thermochemical) values of the solvation free energy, as we will see below.

The free energy of solvation ∆G cav is:

∆G cav = -( z 2 e 2 8πǫ 0 R )(1 - 1 ǫ ) (8.3)
In any case, the continuum model used here does not assume any microscopic details of the solvation shell structure (discrete model Figure 8.17 (b)), and strictly speaking, the cavity (Born) model is applicable only for distances considerably larger than the ion radius. A microscopic description has to account for the specific interactions of a water molecule with ions. For cationic solvation, the O -H groups point away from the ion (then the oxygen atom points to the ion) in order to form hydrogen bonds with bulk water molecules at the solvation shell boundary.

Furthermore, equation 8.3 will be used to link measured vertical ionization energies and thermodynamic quantities in a simple and convenient manner in order to allow for a rough comparison with our experimental data.

To properly apply Equation 8.3 to the present experiment the relevant photoionization processes need to be considered in detail:

M + aq + hν → M 2+ aq + e -
1 8πǫ 0 R (1 - 1 ǫ ) = -3( e 2 8πǫ 0 R )(1 - 1 ǫ ) = -3∆G cav (8.6)
In the sodium photoemission process, z changes from +1 to +2. According to equation 8.5, the modification of z leads to two different solvation energies for Na + and Na 2+ . This situation, depicting the energies of the respective species involved, is illustrated Figure 8.18. Schematic energy level diagrams illustrates the liquid-phase BE shifts in terms of the solvation free energy, ∆G i, f (for initial and final state), relative to the respective gaseous ion. E g and E aq are the electron BE of the gas-phase species and the hydrated species, respectively.

The literature gives E g (Na + (2s)) = 80.07 eV (from NIST atomic spectra database)

and E aq (Na + (2s)) = 67.1 eV (taking the work function approximately equal to 4 eV). We deduce that ∆G cav (Na + ) = -4.32 eV ≈ -416.82 kJ/mol. This value can be compared to the tabulated solvation free energy of Na + (2s) which is -406 kJ/mol. A similar calculation can be done for the Na + surrounded by one water layer in the clay interlayer, we obtain a free energy solvation of 391.9 kJ/mol.

The calculated solvation free energy, obtained by this model, is in agreement with the tabulated value and reflect the fact that orientation changes of the solvation shell 8.8. Summary 183 are only small for ionization of a cation; in this case both the initial and final state corresponds to cationic solvation.

Summary

NAP-XPS brings a new approach with a surface sensitive technique on the adsorption of water by clays. We have compared the behavior of three hectorites containing Na + , Cs + or Sr 2+ counterions relative to water adsorption. O 1s XPS spectra analysis combined with counterion core-level XPS spectra analysis have shown that water adsorbs for Cs-hectorite only between clays particles while water is inserted into clay interlayer for Na-hectorite and Sr-hectorite. Indeed, for Cs-hectorite no modification is observed on the counterion core-level XPS spectrum while for Na-hectorite and Srhectorite, the counterion core-level peak shifts to lower BE.

However, in the case of Sr-hectorite, the behavior of the Sr 2+ is not clear, the presence of two components on the Sr 3d spectra in UHV conditions and until RH = 70 % remains misunderstood. To verify the hypothesis of a bulk and a surface Sr 2+ component on the spectra, we need to synthesize a sample where platelets will form a stack a flat layer and proceed to a depth analysis varying the photon energy in UHV conditions and in presence of water vapor.

Conclusion

This thesis highlights the contribution of photoelectron spectoscopy in the analysis of various interfaces. The first case study concerns the reactivity of the TEOS molecule with clean and water-covered Si(001) surfaces. It illustrates the transition from XPS used in UHV to XPS at near-ambient pressures. This project presents the precious information that can be obtained with UHV XPS, mainly when it is combined with STM and DFT calculations; and the new point of view introduced by the increase of pressure linked to the possibility to come closer to industrial conditions. We also describe the unusual difficulties, compared to UHV conditions, we met during the sample analysis and how we tried to avoid them.

The two following case studies do need the use of NAP-XPS, as we need to get high humidity ratios, up to 100 %, to form liquid water/vapor and solid/liquid interfaces. In all these experiments the use of NAP-XPS allows to perform experiments impracticable with conventional XPS such as the monitoring of clay hydration in situ or the analysis of a water droplet surface placed just in front of the analyzer's nose.

All the results we obtained for these three interfaces are summarized in the following lines.

The swelling properties of clays monitored by NAP-XPS

The first chapters are devoted to TEOS reactivity on the Si(001)-2×1 surface. In The ALD process is investigated using the NAP-XPS instrument in chapter 6. How-8.8. Summary 187 ever, the expected hydrolysis of the surface ethoxy group, that should occur during the second step of the ALD process when water is introduced, is not observed. Moreover, the substrate undergoes an oxidation in presence of water molecules. It means that, in a sequential exposure scheme, water is inefficient to form hydroxyls from ethoxy groups, and too oxidative as it reacts strongly with the substrate.

To understand the reasons why this reaction was not observed, several experiments should be performed. The RH, during the second step of the ALD process, should be increased to higher values, the temperature of the surface should be increased to ∼ 700 • C during the exposure to gases (the increase of temperature may allow the reaction of water on surface ethoxy groups); finally, the use of a catalyst should be tried (amine bases can catalyze the reaction and the reaction mechanism would be interesting to understand).

The second project was related to ion solvation into liquid solutions and confined systems as clays.

Unlike previous works (experimental work and theoretical simulations. [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF], [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF], [START_REF] Cheng | Ambient pressure X-ray photoelectron spectroscopy and molecular dynamics simulation studies of liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions[END_REF], [START_REF] Weber | Photoemission from aqueous alkali-metal -iodide salt solutions using EUV synchrotron radiation[END_REF]) on ion segregation at liquid/vapor interface which describe the behavior of anions at the liquid water surface and their propensity to segregate through the surface when their size and polarizability increases; our studies (chapter 7) are focused on the cation behavior. In agreement with Molecular dynamics results, [START_REF] Jungwirth | Ions at aqueous interfaces: from water surface to hydrated proteins[END_REF] cation are not present on the water surface. But, when the anion polarizability increases, a cation plane is formed below the anion one near the liquid surface, to recover the electroneutrality of the solution. This is an important result, this is the first time that an experimental measurement prove the existence of an increase of cation concentration below the surface.

As the solution droplets are formed on a metallic electrode, and as the conductivity is sufficiently large to avoid charging under the beam, we have been able to refer the BE with respect to the Fermi level of the metallic substrate and to the vacuum level of

Figure 1 . 1 :

 11 Figure 1.1: The steps of the three step model are (1) excitation of the electron to a Bloch final state inside the crystal, (2) transport of the electron to the surface, (3) escape of the electron to the vacuum

Figure 1 . 2 :

 12 Figure 1.2: (a) Photoelectron-matter interaction near the surface after X-ray or UV irradiation; (b) Schematic representation of a photoemission spectrum

Figure 1 . 3 :

 13 Figure 1.3: Schematic representation of the energy levels during a measurement. The sample and the spectrometer are electrically grounded in order to align the Fermi levels.

Figure 1

 1 Figure 1.6.

Figure 1 . 6 :

 16 Figure 1.6: Schematic representation of the NAP-XPS analyzer. Electrostatic lenses refocus the electron trajectories into the apertures between the differential pumping stages.[2]

Figure 1 . 9 :

 19 Figure 1.9: Overview of the NAP-XPS experiment

  5 Å and a two-atom basis. The chemical bond is purely covalent, each atom being tetrahedrally coordinated and its valence electrons occupying sp 3 hybrid orbitals. Silicon crystalline structure is presented in Figure 2.1.

Figure 2 . 1 :

 21 Figure 2.1: Bulk diamond structure of crystalline silicon with each atom tetrahedrally bonded to four nearest neighbors.

Figure 2 . 2 :

 22 Figure 2.2: Buckling of Si dimers. The dimer bonds make an angle of ∼ 20 • with the surface.[START_REF] Filler | The surface as molecular reagent: organic chemistry at the semiconductor interface[END_REF]

2 .

 2 Figure 2.4: Unoccupied-state STM images of Si(001)-c(4×2) at 85 K (left: Vs = 1.0 V, I t = 0.14nA) and Si(001)-2×1 at 300 K (right: V s = 1.5V, I t = 0.12nA)[START_REF] Yoshinobu | Physical properties and chemical reactivity of the buckled dimer on Si(100)[END_REF]

Figure 2 . 5 :

 25 Figure 2.5: XPS Si 2p spectrum (hν = 150 eV) of the clean Si(001)-2×1 surface. The takeoff angle of the photoelectrons, relative to the surface normal is 0 • .The experimental curves (dots) are fitted with sums of Voigt components (blue solid lines). The Si 2p 3/2 (Si 2p 1/2 ) fitting components colored lines. The fitting parameters are reported inTable 2.1.

Figure 3 . 1 :

 31 Figure 3.1: (a) Si 2p XPS spectrum (hν = 175 eV) of TEOS physisorbed on Si(001)-2×1 surface (n-doped, 0.003 Ω × cm) (clean surface exposed to 6.75 L of TEOS, 15 min, 10 -8 mbar, at 120 K). (b) Si 2p XPS spectrum of TEOS chemisorbed on Si(001)-2×1 surface (surface exposed to 4.5 L of TEOS, 10 min, 10 -8 mbar, at 300 K).The BE, spectral weights, and attributions of the different components are reported inTable 3.1.

( 4 . 5 L

 45 ) is shown in Figure3.3. It is fitted with a single component, at 532.1 eV (LW = 120 meV, fwhm = 1.1 eV). This BE is between that of surface Si -OH (532.3 eV) and that of Si -O -Si bridges (531.4 eV).[START_REF] Carniato | Characterization of hydroxyl groups on water-reacted Si(001)-2x1 using synchrotron radiation O 1s core-level spectroscopies and core-excited state density-functional calculations[END_REF] We attribute it to Si -O -C bonds, as the ethoxy groups are the main (80 %) carbon containing species at the surface. Oxygen in siloxane bonds cannot be detected.

Figure 3 . 2 :

 32 Figure 3.2: C 1s XPS spectrum (hν = 350 eV) of the TEOS chemisorbed on Si(001)-2×1 surface (n-doped, 0.003 Ω × cm) (surface exposed to 4.5 L of TEOS, 10 min, 10 -8 mbar, at room temperature). The BE is referenced with respect to Fermi level. The Si 2p 3/2 bulk component is 99.40 eV for the 4.5 L TEOS chemisorbed at 300 K. The BE, spectral weights, and attributions of the different components are reported in Table 3.2.

Figure 3 . 3 :

 33 Figure 3.3: O 1s XPS spectrum (hν = 640 eV) of TEOS chemisorbed on Si(001)-2×1 surface (n-doped, 0.003 Ω × cm) (clean surface exposed to 4.5 L of TEOS, 10 min, 10 -8 mbar, at 300 K). The BE is referenced with respect to Fermi level. The Si 2p 3/2 bulk component is 99.40 eV for the 4.5 L TEOS chemisorbed at 300 K.

Figure 3 . 4 :

 34 Figure 3.4: (a) Occupied-states STM image (U = -2.0V, I = 100 pA; 50 nm × 33 nm) of TEOS adsorbed at room temperature on Si(001)-2×1 (n-doped, 0.003 Ω × cm) after an exposure of 10 min under a pressure of 9 × 10 -10 mbar (0.4 L), (b) magnification image of the rectangular area of image (a) (8 nm × 8 nm), (c) topographical profile along the line drawn in (b).

Figure 3 .

 3 Figure 3.4 (a)) of apparent height ∼130 pm. It seems to encompass two adjacent silicon dimers in a row. We attribute it to a non-fully dissociated molecule.The majority of adsorption features is a pair of protrusions of height ∼109 pm (somewhat smaller than that of the preceding one). Four such pairs are enclosed in the white rectangle of Figure3.4 (a). Each pair extends over three dimers, as the protrusion maxima are separated by 2 × 384 pm, twice the inter-dimer distance in a row (Figure3.4 (b) and 3.4 (c)). The symmetry of the pairs of protrusions in the mirror planes[START_REF] Polack | TEMPO: a new insertion device beamline at SOLEIL for time resolved photoelectron spectroscopy experiments on solids and interfaces[END_REF] and (110) is a signature of the full TEOS dissociation where each bright protrusion is composed of two ethoxy moieties bonded to the same dimer. In contrast, the formation of three ≡Si S -OC 2 H 5 moieties plus one (Si S ) 3 Si T -OC 2 H 5 adduct would lead to asymmetric footprints. Because of the breaking of all four Si -O bonds, each bright protrusion is indeed composed of two ethoxy moieties bonded to the same dimer. Inasmuch as all Si -O bonds are broken in the TEOS molecule, the fate of the released silicon

Figure 3 . 5 :

 35 Figure 3.5: (a) Occupied-states STM image (U = -2.0 V, I = 100 pA; 3.3 nm x 4 nm) of one TEOS molecule adsorbed at room temperature on Si(001)-2×1 (n-doped, 0.003 Ω x cm), (b) 3D representation of the image.

Figure 3 . 6 :

 36 Figure 3.6: The 8×4 slab model within the unit cell used for the calculation.

Figure 3 . 7 :

 37 Figure 3.7: (a) Ball-and-stick representation of the adsorbed TEOS fragments obtained via DFT geometry optimization. (M1) corresponds to the configuration in which the monomer is sitting on the dimer side opposite to the ethoxy adsorption site, (M2) the monomer is pointing towards the ethoxy groups. (b) Simulated occupied-state STM images, computed within the Tersoff-Hamann approximation. Dangling bonds are denoted by DB.

Figure 3 .

 3 Figure 3.7 (b). As a preamble to any comparative discussion of the experimental and theoretical STM images, firstly, one must emphasize that calculations are carried out at 0 K. The simulated images correspond to an absolute minimum of energy; secondary energy minima that could be accessed at non-zero temperature via rotations of the ethoxy moieties around the Si -O and O -C axes (Figure 3.7) are not taken into account. In our simulation all tunneling currents are considered and represented in an intensity grey scale (the brightest corresponds to the most intense current). Therefore, we focus on the brightest spots in Figure 3.7 (b).The calculation gives a distinct spot for each ethoxy group, while the two ethoxy groups on a dimer are not resolved in the experimental STM images (Figure3.7). Rotations, unhindered at 300 K, may explain the difference.The calculation gives also a crucial information: the dangling-bond bearing Si atoms (denoted DB in Figure3.7 (a)), pertaining to the two adjacent dimers on which the adatom is sitting, give a spot fainter than that of the Si adatom. In the experimental image, the "faint protrusion" (green in the 3D image) is seen away from the ethoxy features (Figures 3.5(a) and 3.5 (b)).

Figure 3 . 8 :Figure 3 . 4 )

 3834 Figure 3.8: (a) Occupied-states STM image (U = -2.0 V, I = 100 pA; 40 nm × 40 nm) of a clean Si(001)-2×1 surface (n-doped, 0.003 Ω× cm) exposed, at room temperature, to TEOS (30 min, 9 × 10 -10 mbar), (b) magnification image of the rectangular area of image (a), (c) topographical profile along the line drawn in (b).

First-principles calculations 57 Figure 4 . 1 :

 5741 Figure 4.1: Schematic representation of the topmost three layers of the p(2×1) symmetric dimer reconstruction Si(001) surface. The large solid circles represent the top-layer atoms, the medium-sized open circles represent the second-layer atoms, and the small solid circles represent the third-layer atoms.

Figure 4 . 2 :

 42 Figure 4.2: Perspective view and contour plot of the total-energy surface E(x,y) obtained by Brocks et al.[START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] The first contour is at 0.1 eV with respect to the absolute minimum (M) and the contour spacing is 0.2 eV. The interval on the vertical energy axis is 1 eV and the zero of energy is chosen arbitrarily. The x axis is chosen so as to pass through a dimer.

Figure 4 . 3 :

 43 Figure 4.3: Perspective view of the equilibrium binding configuration obtained by Brocks et al.[81] showing the first-, second-, and third-layer substrate atoms as in Figure 4.2, and the bond lengths in angstroms. The adatom is shown as a large open circle.

4. 1 .

 1 Overview: Silicon monomer diffusion on the clean Si(001)-2×1 surface 61 to be site D. With Tersoff potential, Smith et al.[START_REF] Smith | Si adatom binding and diffusion on the Si(100) surface: Comparison of ab initio, semiempirical and empirical potential results[END_REF] gave identical conclusions. These results show that D is a local minimum while, with first-principles calculations, it is a saddle point. Using the modified embedded-atom method MEAM[START_REF] Baskes | Calculation of the behaviour of Si ad-dimers on Si(001)[END_REF], Baskes obtained two energy minimums M and B with close energies. Cai et al.[START_REF] Cai | Reconstruction of Si(001) and adsorption of Si adatoms and ad-dimers on the surface: Many-body potential calculations[END_REF] used an empirical potential able to correctly predict asymmetrical buckling of the Si(001) surface and the relative stability between them. They showed, with this potential, that the binding energies gradually decrease for site M, D, H, A, and B, i.e., therefore, site M is predicted to be the lowest-energy site and site D the second-lowest-energy position. The results from these empirical potentials are, therefore, conflicting with each other and also with those from first principles calculations.

  two central Si layers are kept fixed, while all other degrees of freedom are relaxed until residual atomic forces do not exceed a few meV/ Å. The slab considered in our calculations comprises two dimers that exhibit an alternate buckling, as expected at 0 K. The energy surface is presented Figure 4.4.

Figure 4 . 4 :

 44 Figure 4.4: Contour plot and perspective view of the 2×2 total energy surface.

Figure 4 . 5 .

 45 At the H´site, the monomer can make two bonds (∼ 2.3 Å) with two silicon atoms of a dimer. At the H site, only weak bonds are possible (∼ 2.55 Å) with the four silicon atoms of two dimers and may explain the obtaining of a less stable configuration. From the knowledge of the energy landscape, we can figure out how the silicon adatom diffuses. It can diffuse on the surface trough two different paths along the x direction. The first path connecting two adjacent cells is the path through the M-S-C line where S is a saddle point and C a local minimum (see Figure 4.6 (a)). The energy barrier is of 0.75 eV. The second path we consider supposes that the monomer in Hṕ osition cross a first saddle point in H to reach the second H´site of the cell and then a second saddle point in D to access to the next cell (see Figure 4.6 (b)). The energy

Figure 4 . 5 :

 45 Figure 4.5: Perspective view of the H and H´configurations on the 2×2 reconstructed surface.

Figure 4 . 6 :

 46 Figure 4.6: Energies along the possible diffusion paths on the 2×2 total energy surface.

Figure 4 . 7 :

 47 Figure 4.7: Contour plot and perspective view of the 8×4 total energy surface.

Figure 4 . 8 :

 48 Figure 4.8: Perspective view of the H and H´configurations on the 8×4 reconstructed surface (The values between brackets are the bond length obtained for the clean 2×2 surface).

Figure 4 .

 4 Figure 4.9 (a) presents the energy barrier for the path through the M-S-C line. The energy barriers for the M-S D -C D path are similar to the one on the 2×2 surface while

Figure 4 . 9 :

 49 Figure 4.9: Energies along the possible diffusion paths on the 8×4 total energy surface.

Figure 4 . 5 and

 45 Figure 4.8 respectively) shows that it is slightly more significant (∼ 5 • ) on the 8×4 surface.

  Figure 4.10.

Figure 4 . 10 :

 410 Figure 4.10: Schematic representation of the ethoxy-covered Si(001)-8×4 surface.

4. 3 . 73 Figure 4 .

 3734 Figure 4.11 compares the energy barriers for the adatom diffusion between two dimer rows. The energy barriers are similar to the ones of the 8×4 clean surface. However, the energy barrier the monomer has to cross to reach sites C 1 or C 2 from M 1 are higher than the ones to reach C 3 or C 4 from M 2 , certainly due to the presence of the ethoxy groups.

Figure 4 . 11 :Figure 4 .

 4114 Figure 4.11: Energies along the possible diffusion paths on a clean dimer row neighboring the ethoxy-covered row of the 8×4 ethoxy-covered energy surface. (The values between brackets are the energies obtained for the clean 8×4 surface)

Figure 4 . 12 :

 412 Figure 4.12: Energies along two possible diffusion paths (a) parallel to the dimer row, (b) perpendicular to the dimer row. (The values between brackets are the energies obtained for the clean 8×4 surface)

Figure 5 . 1 :

 51 Figure 5.1: TEOS fragments position (R = CH 2 CH 3 ) after dissociation on the Si(001)-2×1 surface at low coverage (∼ 1 L). The Si T denotes the silicon atom pertaining to the TEOS molecule.

Figure 5 . 2 :

 52 Figure 5.2: Possible dissociation channels of alkoxysilanes on silicon surfaces The Si T denotes the silicon atom pertaining to the alkoxysilane molecule.

11 5. 1 5 . 1 . 1

 111511 7×7 and the Si(001)-2×1 surfaces. The clean Si(111)-7×7 surface forms a wellcharacterized 7×7 reconstruction with fully occupied dangling bond on the restatom and partially filled dangling bond on adatom reactive sites; they are basically similar to the up (electron-rich) and down (electron-deficient) silicon atoms of the silicon dimer on the Si(001)-2×1 surface, respectively.[START_REF] Wong | Functionalization of Semiconductor Surfaces[END_REF] However the surface topology is different, especially the distance and layout of the dangling bonds, which can affect the nature of the reaction products, as it has been shown e.g. for acetonitrile.[START_REF] Carniato | Characterization of hydroxyl groups on water-reacted Si(001)-2x1 using synchrotron radiation O 1s core-level spectroscopies and core-excited state density-functional calculations[END_REF], For instance, pairs of silicon dangling bonds are at shorter distances on Si(001)-2×1 than on Si(111)-7×7. Moreover, for Si(001)-2×1, the reaction site on which TEOS fully dissociates encompasses six dimers of two adjacent 5. Dissociation of Ethoxysilane on Si(001)-2×1 and Si(111)-7×7 at Room Temperature dimer rows,[108] while nothing such exists on Si(111)-7×7, due to the different surface symmetries (see Figure 5.TMOS and TEOS reaction on Si(001)-2×1 at room temperature Si 2p core-level XPS analysis The Si 2p spectra of the Si(001)-2×1 surface exposed to TMOS (4.5 L and 67.5 L) and TEOS (4.5 L and 13.5 L) at room temperature are presented in Figure 5.3. For each spectrum, the Si 2p line shapes are reconstructed by sums of Voigt components.A dose of 4.5 L of TEOS and TMOS on the Si(001)-2×1 surface lead to the disappearance of the surface states (that have a SCLS of ∼ -0.5 eV with respect to the bulk peak B). The structure labeled S´(SCLS of ∼ 0.3 eV) remains, attributed to the second plane atoms.[START_REF] Karlsson | Atomic origins of the surface components in the Si 2p core-level spectra of the Si(111)-7x7 surface[END_REF] The first oxidation state Si + grows at a SCLS of ∼ +0.90 eV. It corresponds, for TMOS, to a silicon atom linked to a methoxy in a (Si) 3 Si -O -CH 3 unit (the ionized Si is underscored) and for TEOS to a silicon atom linked to an ethoxy groups ina (Si) 3 Si -O -CH 2 -CH 3 unit.The Si + structure represents 15.5 % of the spectral weight for TMOS and 13.2 % for TEOS. To estimate their surface area, we can make a comparison with the Si 2p spectra of the water-saturated surface (measured with same kinetic energy). Half the surface silicon atoms are Si -OH species and the Si + peak weight represents 15 % of the spectral area. Therefore, a coverage of ∼0.5 ML for TMOS dosage and of ∼0.45 % for TEOS dosage can be deduced. Note the absence of measurable oxidation states higher than Si + , in particular the Si 4+ component. The majority species are fully dissociated.By analogy with the TEOS case for which STM shows that the ethoxy fragments of one dissociated molecule occupy four dimers (Figure5.1), the dissociation pattern of TMOS should be alike at this same coverage.

Figure 5 . 3 :

 53 Figure 5.3: Si 2p XPS spectra (hν = 150 eV) of Si(001)-2×1 surface (n-doped, 0.003 Ω × cm) (a) exposed to TMOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (b) of 67.5 L (15 min, 10 -7 mbar) (c) exposed to TEOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (d) of 13.5 L (30 min, 10 -8 mbar). The BE is referenced with respect to the Fermi level. Fitting parameters are collected in Table 5.1

  resulting from the full dissociation of TMOS and TEOS, according to the path (a) on Figure5.2. A smaller component is seen at higher BE, 532.6 eV (about 18 % of the spectral weight for TMOS and TEOS). When the exposure increases, it leads, for both molecules, to an increase of the high BE component that reaches ∼34 % of5. Dissociation of Ethoxysilane on Si(001)-2×1 and Si(111)-7×7 at Room Temperature the spectral weight for TMOS and ∼ 26 % for TEOS. This increase is correlated to the appearance of the Si 4+ state in the Si 2p spectra due to the C -O bond rupture (path (b) in Figure5.2). Therefore, we attribute the high BE peak to oxygen atoms in a Si T O 4 tetrahedron.

  Figure 5.5 (a), the C 1s spectrum after the 4.5 L exposure is fitted by three components, at 284.10 eV, 286.73 eV and 287.28 eV. A fourth, weak component at 283.3 eV is attributable to beam damage.[108], [75] The small component 284.1 eV, corresponds to (Si 3 )Si -CH 3 . This suggests that the C -O bond can break. On the other hand, the main structures at higher BE, are related to a C -O bond, resulting from Si -O bond breaking. For the 4.5 L spectrum (low coverage limit), the main C -O component is at 286.73 eV and corresponds to an (Si 3 ) -O -Si -CH 3 unit. However, for the longest exposure (67.5 L), we need to introduce a component at 287.20 eV to fit properly the spectrum shown in Figure 5.5(b): the width of the C -O component, ∼1.6 eV becomes much wider than the fwhm of a single C 1s component (0.92 eV). The peak at 287.20 eV is attributed to (O 3 )Si -O -CH 3 on the basis of a second-neighbor effect chemical shift. Assuming that initial state effects dominate BE shifts, in a (O 3 )Si -O -CH 3 unit, the liganding of the silicon atom to four O atoms increases the charge loss of the carbon with respect to the (Si 3 )Si -O -CH 3 case, explaining the +0.6 eV BE shift. Note that Jolly and Bomben report a ionization potential shift of 0.67 eV between gaseous H 3 C -SiH 3 and H 3 C -SiCl 3 .[74] Such second second-neighbor effect in the BE shifts is not screened out by the presence of the silicon substrate, despite its high dielectric permittivity.

Figure 5 . 4 :

 54 Figure 5.4: O 1s XPS spectra (hν = 640 eV) of Si(001)-2×1 surface (n-doped, 0.003 Ω × cm) (a) exposed to TMOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (b) of 67.5 L (15 min, 10 -7 mbar) (c) exposed to TEOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (d) of 13.5 L (30 min, 10 -8 mbar). The BE is referenced with respect to the Fermi level. Fitting parameters are collected in Table 5.2

Figure 5 . 4 .

 54 The BE of the Si 2p 3/2 component (referenced to the Fermi level) is 99.40 eV. Naturally the (O 3 )Si -O -CH 3 component correlates with the appearance of the Si 4+ state in the Si 2p spectra, associated to species grafted after C -O bond rupture (path (b) in Figure 5.2). When one C -O bond is broken, one Si -O -Si T (OCH 3 ) 3 group and one Si -CH 3 group are formed. In such a case, the (O 3 )Si -O -CH 3 /(Si 3 )Si -O -CH 3 intensity ratio is 3:1. On the other hand, when two C -O bonds are broken, one (Si -O) 2 -Si T (OCH 3 ) 2 and two methyl groups are formed on the surface. Then the(O 3 )Si -O -CH 3 /(Si 3 )Si -O -CH 3 ratio is 1:1. At low coverage, the (Si 3 )Si -CH 3 componentrepresents 15 % of the spectral weight and the (O 3 )Si -O -CH 3 component 18.7 %, corresponding approximately to a ratio of 1:1. We deduce that two C -O bonds are broken. When the exposure increases, the (Si 3 )Si -CH 3 component represents 12.4 % of the spectral weight and the (O 3 )Si -O -CH 3 component 34.5 %, this ratio decreases to 3:1, pointing to the breaking of only one C -O bond. This coverage dependence can be explained by an increasing difficulty in finding free silicon dangling bonds on the surface as more and more adsorption sites are occupied. The C 1s spectra of TEOS shown in Figures 5.5 (c) and 5.5 (d) are more complex than that of TMOS because of the two non-equivalent carbons in the ethoxy. Besides a weak structure at 283.2 eV due to beam damage, we see three broad structures at ∼286.8 eV, ∼285 eV and ∼284 eV. The structures at ∼286.8 eV and ∼284 eV are related to carbon atoms bonded to oxygen and to silicon, respectively, as in the case of TMOS. With respect to TMOS, the additional feature at ∼285 eV is related to the terminal CH 3 of the ethoxy group. The low coverage (4.5 L) spectrum (Figure 5.5 (c)), for which ethoxy fragments directly grafted on the substrate dominate,[108] gives us the BE position of the two non-equivalent carbons, (Si 3 )Si -O -CH 2 -CH 3 , at 286.58 eV (close to the (Si 3 )Si -O -CH 3 component at 286.7 eV) and (Si 3 )Si -O -CH 2 -CH 3 at 285.23 eV.

Figure 5 . 5 :

 55 Figure 5.5: C 1s XPS spectra (hν = 350 eV) of Si(001)-2×1 surface (n-doped, 0.003 Ω × cm) (a) exposed to TMOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (b) of 67.5 L (15 min, 10 -7 mbar) (c) exposed to TEOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (d) of 13.5 L (30 min, 10 -8 mbar). The BE is referenced with respect to the Fermi level. Fitting parameters are collected in Table 5.3

5 . 2 . 1

 521 [START_REF] Klaus | Atomic layer deposition of SiO 2 using catalyzed self-limiting surface reactions[END_REF] eV we attribute to (O 3 )Si -O -CH 2 -CH 3 . Indeed, the BE is practically the same as that (O 3 )Si -O -CH 3 in TMOS (287.20 eV). To ensure self-consistency, a second component corresponding to(O 3 )Si -O -CH 2 -CH 3 isadded at BE of 285.74 eV with the same width and intensity as those of (O 3 )Si -O -CH 2 -CH 3 . The BE shift between the two non-equivalent carbons is ∼1.5 eV, in accordance with the one observed for the non-equivalent carbons in (Si 3 )Si -O -CH 2 -CH 3 units. Therefore, as for TMOS, higher doses lead to the appearance of a second dissociation mode of TEOS, via C -O bond breaking (path (b) in Figure 5.2). In the BE energy window 284 eV-285 eV, we see contributions from the ethyl moieties directly attached to the silicon substrate. We use a doublet of components, of equal widths and intensities for the sake of consistency, separated by ∼0.6 eV, (Si 3 )Si -CH 2 -CH 3 (at ∼284.15 eV) and (Si 3 )Si -CH 2 -CH 3 (at ∼284.75 eV).[108], [75] A supplementary component X, also at ∼285 eV, is introduced to improve curve fitting and is related to beam damage. Comparing Figure 5.5 (c) and 5.5 (d), we see that the increase of the (O 3 )Si -CH 2 -CH 3 components is correlated to the increase of the (Si 3 )Si -CH 2 -CH 3 components, which is expected when the number of dissociated fragments via C -O bond breaking increases.5.2 TMOS and TEOS reaction on Si(111)-7×7 at room temperature Si 2p core-level XPS analysisWe present, in Figure5.6, the Si 2p spectra of the Si(111)-7×7 surface after exposures to TMOS and TEOS. The fitting parameters are collected in Table5.4.In Figure5.6 (a) and 5.6 (c), we present the Si 2p spectra after a short exposure to TMOS (0.11 L) and TEOS (4.5 L), respectively. On the clean surface, components S´, C and SS, are attributed to the adatoms, the pedestal atoms and the restatoms, respec-5.2. TMOS and TEOS reaction on Si(111)-7×7 at room temperature 91 tively.[START_REF] Lay | Surface core-level shifts of Si(111)-7x7: A fundamental reassessment[END_REF] In particular, after the 0.11 L dose, we still observe the restatom component (SS). Similarly to the Si(001)-2×1 surface case, a first oxidation state Si+ grows with a SCLS of ∼ +0.90 eV. It is attributed to (Si 3 )Si -O -CH 3 (TMOS) and (Si 3 )Si -O -CH 2 -CH 3 (TEOS). The Si + structure represents 12.5 % (10%) of the spectrum area for TMOS (TEOS). It corresponds to an estimated Si 1+ coverage of 0.42 ML (0.33 ML) for TMOS (TEOS).

  ML). The intermediate states Si 3+ and Si 2+ , with SCLS of ∼ +1.89 eV and ∼ +2.60 eV, respectively, have also emerged. As a whole their spectral weight is 4.8 %. For TEOS, the Si 4+ state has a SCLS of ∼ +3.25 eV and a spectral weight of 7.1 % (corresponding to a Si 4+ areal density of 0.25 ML). The intermediate states Si3+ and Si 2+ at ∼ +1.8 eV and ∼ +2.70 eV as a whole have a spectral weight less than 1.5 %. As for the Si(001)-2×1 surface, the appearance of a Si 4+ state means that the Si -O bond breaks and that SiO 4 species are left on the surface according to the reaction path (b) of Figure5.2. As discussed earlier, the Si 3+ and Si 2+ states may be due to an incomplete dissociation via Si -O bond breaking.

Figure 5 . 6 :

 56 Figure 5.6: Si 2p XPS spectra (hν = 150 eV) of Si(111)-7×7 surface (n-doped, 0.003 Ω × cm) (a) exposed to TMOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (b) of 67.5 L (15 min, 10 -7 mbar) (c) exposed to TEOS for a coverage of 0.11 L (30 min, 5 × 10 -9 mbar); (d) of 4.5 L (10 min, 10 -8 mbar). The BE is referenced with respect to the Fermi level. Fitting parameters are collected in Table 5.4

Figure 5 .

 5 8 (a) and 5.8 (b), are fitted with the same components as the spectra of TMOS on Si(001)-2×1, see Figure 5.5 (a) and 5.5 (b). Similarly, the peak at 287.25 eV, corresponding to (O 3 )Si -O -CH 3 , increases for higher coverage and is correlated, to the appearance of the Si 4+ state in the Si 2p spectra due to the C -O bond rupture according to scheme (b) Figure 5.2. At low coverage, the spectral weight of the (Si 3 )Si -CH 3 component is 33 % and that of the (O 3 )Si -O -CH 3 component is 29 %. With an intensity ratio of about 1:1, we deduce that two C -O bonds are broken. When the exposure increases, the weight of the (O 3 )Si -O -CH 3 component is 42 % and that of the (Si 3 )Si -CH 3 component 17 %. The ratio between these two components is ∼ 2:1. This situation corresponds to the scission of only one C -O bond.

Figure 5 . 7 :

 57 Figure 5.7: O 1s XPS spectra (hν = 640 eV) of Si(111)-7×7 surface (n-doped, 0.003 Ω × cm) (a) exposed to TMOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (b) of 67.5 L (15 min, 10 -7 mbar) (c) exposed to TEOS for a coverage of 0.11 L (30 min, 5 × 10 -9 mbar); (d) of 4.5 L (10 min, 10 -8 mbar). The BE is referenced with respect to the Fermi level. Fitting parameters are collected in Table 5.6

Figure 5 . 8 :

 58 Figure 5.8: C 1s XPS spectra (hν = 350 eV) of Si(111)-7×7 surface (n-doped, 0.003 Ω × cm) (a) exposed to TMOS for a coverage of 4.5 L (10 min, 10 -8 mbar); (b) of 67.5 L (15 min, 10 -7 mbar) (c) exposed to TEOS for a coverage of 0.11 L (30 min, 5 × 10 -9 mbar); (d) of 4.5 L (10 min, 10 -8 mbar). The BE is referenced with respect to the Fermi level. Fitting parameters are collected in Table 5.6

Figure 5 . 8 .

 58 Similar trends are found for the C 1s spectra of TEOS shown in Figure5.8 (c) and5.8 (d). The fit procedure is similar to that used for the Si(001)-2×1. We see that the(O 3 )Si -CH 2 -CH3 peak intensity increases relatively to that of the (Si 3 )Si -CH 2 -CH 3 component from short to long exposures, in accord with a full dissociation via Si -O bond breaking (path (a) in Figure 5.2) at low coverage, and with C -O bond breaking (path (b)) at higher coverage. The weighing of the (Si 3 )Si -CH 2 -CH 3 is obscured by the component X related to beam damage.

Figure 5 . 2 .

 52 One involving the breaking of all four Si -O bonds (also clearly observed at low exposures by STM for TEOS), and one involving the rupture of the C -O bond (observed at large exposures).

"Figure 5 . 9 :

 59 Figure 5.9: Possible mechanism by Si -O bond breaking of Si(OR) 4 molecule on a zwitterionic Si pair of Si(001)-2×1. Si T denotes the alkoxysilane Si atom.

Figure 5 . 10 :CH 2 -Figure 5 . 9

 510259 Figure 5.10: Possible reaction path on Si(001)-2×1 leading to the full dissociation of a Si(OR) 4 molecule. Si T denotes the alkoxysilane Si atom.

5. 4 . Summary 103 Figure 5 . 11 :

 4103511 Figure 5.11: Scheme of the Si(111)-7×7 surface with its characteristic distances ( Å)

Figure 6 . 1 :

 61 Figure 6.1: Schematic representation of the Atomic Layer Deposition (ALD) process.

  2 (a), we suppose a reaction between the surface hydroxyl groups and TEOS. It creates a (Si S -O) n Si T (OC 2 H 5 ) 4-n adduct (where Si S indicates a surface silicon atom, Si T the silicon atom of the TEOS molecule and n the number of bonds created by one TEOS molecule with the surface) by the elimination of an ethanol molecule. On Figure 6.2 (a), n is arbitrary chosen equal to 2.After the TEOS adsorption on the surface, any excess of the molecule is removed from the reaction chamber by pumping it down. Then, water is added and hydrolyzes the ethoxy groups resulting from TEOS dissociation on the surface. The hydrolysis transforms them in hydroxyl groups as shown in Figure6.2 (b). Similarly to TEOS, water is then cleared from the reaction chamber and the freshly created hydroxyl groups are ready to react with TEOS again.

Figure 6 . 2 :

 62 Figure 6.2: Possible reaction path for the ALD process on the water-covered Si(001)-2×1 surface, (a) TEOS pulse and (b) Water pulse.

Figure 6 . 3 :Figure 6 . 4

 6364 Figure 6.3: Si 2p bulk position as a function the exposure of the water-covered Si(001) surface (n-doped, 0.003 Ω × cm) to TEOS (from P = 2 × 10 -8 mbar to P = 1 × 10 -2 mbar).

CH 2 -

 2 4 (b)), four components are used for the fit, two corresponding to ethoxy groups at ∼ 287.1 eV and ∼ 285.5 eV (fwhm = 1.2 eV) attributed to Si -O -CH 3 and Si -O -CH 2 -CH 3 respectively and two related to ethyl groups at ∼ 285.3 eV and ∼ 284.8 eV (fwhm = 0.9 eV) attributed to Si -CH 2 -CH 3 and Si -CH 2 -CH 3 respectively. A small component at ∼ 283 eV can be ascribed to carbonaceous CH x species likely due to beam damage. As mentioned previously, the pressure of TEOS is already of ∼ 10 -8 mbar in the chamber (which corresponds to pressures used for TEOS exposure in previous chapters) and, therefore, the reaction has already started.During the exposure, we observe an increase of the Si 4+ component (+ 3.6 eV, fwhm = 0.9 eV and spectral weight = 3.6 %) on the Si 2p spectrum (Figure6.4 (b)) associated with TEOS dissociation and oxide growth. However, this component also contains a contribution of the silicon atom from TEOS molecules of the gas phase or physisorbed on the surface. On the C 1s (Figure6.5 (b)), the components relative to ethoxy and ethyl groups are unchanged, but two new components associated to carbon atoms of TEOS molecules in the gas phase or physisorbed on the surface are present on the spectrum at higher energy (at ∼ 288.31 eV and ∼ 286.74 eV, fwhm = 0.7 eV)).After the exposure, the C 1s spectrum (Figure6.5 (c)) is similar to the one of the starting point; however, on the Si 2p spectrum (Figure6.4 (c)) the oxide component (at + 3.6 eV, fwhm = 0.92 eV, spectral weight = 2.4 %) has slightly increased compared to the one of the starting point spectrum (at + 3.6 eV, fwhm = 0.90 eV, spectral weight = 0.9 %) and the TEOS coverage can be estimated at 0.09 ML.

Figure 6 . 4 :

 64 Figure 6.4: Si 2p XPS spectrum (hν = 350 eV) of the water-covered Si(001) surface (ndoped, 0.003 Ω × cm) (a) at the starting point, (b) during the exposure to TEOS (120 min, up to 1 mbar, at room temperature), (c) after TEOS exposure. The BE is referenced with respect to the Fermi level.

Figure 6 . 5 :

 65 Figure 6.5: C 1s XPS spectrum (hν = 350 eV) of the water-covered Si(001) surface (ndoped, 0.003 Ω × cm) (a) at the starting point, (b) during the exposure to TEOS (120 min, up to 1 mbar, at room temperature), (c) after TEOS exposure. The BE is referenced with respect to the Fermi level.

Figure 6 . 6 :

 66 Figure 6.6: C 1s XPS spectra evolution (hν = 640 eV) before and during the exposure of the TEOS-covered silicon surface (n-doped, 0.003 Ω × cm) to water (120 min, up to 2 mbar, at room temperature). The BE is referenced with respect to the Fermi level.

Figure 6 . 7 :

 67 Figure 6.7: O 1s XPS spectra evolution (hν = 640 eV) before and during the exposure of the TEOS-covered silicon surface (n-doped, 0.003 Ω × cm) to water (120 min, up to 2 mbar, at room temperature). The BE is referenced with respect to the Fermi level.

Figure 6 . 8 :Figure 6 . 9 :

 6869 Figure 6.8: Si Auger LVV and O 1s core level peak recorded at hν = 640 eV from both the TEOS-covered silicon surface (n-doped, 0.003 Ω × cm) and the same surface after exposure to water. (P ∼ 10 -8 mbar)

Figure 6 . 10 :

 610 Figure 6.10: Si 2p XPS spectra (hν = 640 eV) of the water-covered surface (n-doped, 0.003 Ω × cm) after exposure to TEOS (30 min, P = 5 mbar), acquired on several position of the sample.

  8 and gives a value of d equal to 14.2 Å; for position 2,

6. 4 . Summary 121 Figure 6 . 11 :

 4121611 Figure 6.11: Si 2p XPS spectra (hν = 150 eV) of the water-covered Si(001) surface (ndoped, 0.003 Ω × cm) during exposition to water P = 5 mbar, acquired using (a) a low flux by detuning the gap, (b) a higher flux by tuning the undulator gap.

7 . 7 . 1 The liquid water/ vapor interface 7 . 1 . 1

 771711 fundamental interest. Many questions remain open. How are ions distributed? How do the water molecules rearrange at the water/vapor interface? Are anions and cations separated at the interface? What are the conditions for the formation of an electric double layer? How are ions accommodated in the solvent network, and what is the range of influence on the water network? 124 Ion segregation at liquid water/gas interface Liquid water surface structure

Figure 7 . 2 .

 72 Because the sample surface is the lowest temperature point in the sample chamber and that salts are hygroscopic, water vapor condenses on the salt first. (The concentration of saturated NaCl, NaBr and NaI solutions are of 35.8 g/100mL (6.1 mol/L), 85.2 g/100mL (8.2 mol/L) and 170 g/100mL (11.3 mol/L) respectively at 283 K.)

Figure 7 . 2 :

 72 Figure 7.2: Water droplets on the gold coated silicon surface in the analysis chamber during the acquisition (RH = 95 %, P = 12 mbar, T = 283 K).

Figure 7 . 3 :

 73 Figure 7.3: C 1s XPS spectra of saturated sodium chloride, sodium bromide and sodium iodide solutions surfaces measured with a photon energy of 500 eV at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate.

Figure 7 . 4 :

 74 Figure 7.4: Na 2s XPS spectra of saturated sodium chloride, sodium bromide and sodium iodide solutions surfaces measured with a photon energy of 1200 eV and 360 eV at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate.

7. 2 .

 2 NAP-XPS analysis of the water surface: the ions behavior 133 netic energy of all photoelectrons coming from the sample surface (which confirms the absence of charge effect at the solution surface) and allows to exclude the O 2s peak of the vapor from the acquisition window.

Figure 7 . 6 :

 76 Figure 7.6: Na 2p and O 2s XPS spectra of a saturated sodium iodide solution surface measured at hν = 600 eV (a) in the absence of polarization (b) with a polarization of 40 eV at RH = 95 % (P = 8 mbar, T = 278 K).

Figure 7 . 7 :

 77 Figure 7.7: Na 2p and O 2s XPS spectra of a saturated sodium iodide solution surface measured at several photon energies with a polarization of 40 eV at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate.

Figure 7 . 8 :

 78 Figure 7.8: Atomic calculation of cross section for (a) Na 2p and (b) O 2s as a function of photon energy.[START_REF] Yeh | WebCrossSections[END_REF]

Figure 7 . 9 :

 79 Figure 7.9: Na/O ratio as a function of depth after correction by the cross sections.

Figure 7 . 10 :

 710 Figure 7.10: Valence band of the three saturated salt solutions measured at hν = 500 eV at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate.

Figure 7 .

 7 Figure 7.11: (a) Cl 2p spectra of a saturated NaCl solution surface and (b) I 4d spectra of a saturated NaI solution surface measured with two photon energies at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate.

Figure 7 .

 7 Figure 7.12: C 1s and Cl 2p spectra of three different sodium chloride solutions prepared in the same manner, measured for hν = 500 eV at RH = 95 % (P = 8 mbar, T = 278 K).

Figure 7 . 13 :

 713 Figure 7.13: Br 3d spectrum evolution with time measured for hν = 200 eV at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate.

Figure 7 . 14 :

 714 Figure 7.14: Br 3d spectrum fit evolution with time measured for hν = 200 eV at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate.

However, Figure 7 .

 7 Figure 7.15). The I/Na ratio is found to be constant and equal to 0.6 which means that, for the minimum depth region we are able to probe (∼ 17 Å) the behavior of the anion is similar to the one of the cation.

Figure 7 . 15 :

 715 Figure 7.15: Atomic calculation of cross section for (a) I 4d and (b) Na 2s as a function of photon energy.[START_REF] Yeh | WebCrossSections[END_REF]

Figure 7 .

 7 Figure 7.16: (a) Na 2s and I 4d XPS spectra of a saturated sodium iodide solution surface measured with several photon energies at RH = 95 % (P = 8 mbar, T = 278 K). The BE is referenced with respect to Fermi level measured on the gold substrate. (b) I/Na ratio as a function of depth after correction by the cross sections.

7. 3 . 143 7. 3

 31433 Work function measurements and electronic level diagrams Work function measurements and electronic level diagramsThe fundamental characteristic of our set up, with respect to liquid jet experiments, is the fact that the conductive electrolyte solution is connected to the ground via the gold electrode. No charging effects are observed, and we expect the alignment of the Fermi levels of the gold substrate and of the electrolyte. This permits the measurement of the valence band energy levels (in particular the HOMO of liquid water) referenced to the Fermi level.

  By polarizing the sample by a negative voltage V bias , we increase the KE of the photoelectrons by +e |V bias | (e > 0) entering into the analyzer. The bias is used to increase the energy of the photoelectrons above the vacuum level of the analyzer and to get a better separation of the secondary electrons of liquid water from the background due to the gas phase.Knowing the exact photon energy hν, the KE of the cutoff, and the KE of the gold Fermi level E F K (pushed up in energy by e |V bias |) one obtains the work function WF

Figure 7 . 17 .

 717 Figure 7.17. V bias was measured precisely (40.1 eV) via the KE change (polarized vs nonpolarized) of the halogen core-levels. The photon energy was measured by making the difference between the 2 nd and 1 st order of Au 4f kinetic energies (hν = 500 eV).

Figure 7 . 17 :

 717 Figure 7.17: Secondary electron cut off measured at hν = 500 eV for the saturated NaBr solution with a polarization of 40 eV at RH = 95 % (P = 8 mbar, T = 278 K). (a) raw data spectrum, (b) spectrum after subtraction of a background.

Figure 7 . 18 :Table 7 . 1 :

 71871 Figure 7.18: Secondary electron cut off measured at hν = 500 eV for the saturated NaCl, NaBr, NaI solutions and for the gold surface with a polarization of 40 eV at RH = 95 % (P = 8 mbar, T = 278 K).

Figure 7 . 19 :

 719 Figure 7.19: Illustration of how electrons flow to the larger work function material A (a) when dissimilar conductive materials (A and B) are put into electrical contact (b), to establish the contact potential difference CPD. The sample A (spectrometerB) is negatively (positively) charged. At nozzle-spectrometer distances dns of ∼ 1 mm, the ∼ 0.1 mm diameter X-ray beam probes the solution A and the gas phase above it (c), hence the higher bound KE for gas phase O 1s. At very large dns values, the beam probes the gas phase molecules that see practically the same potential as that close to the spectrometer, hence the lower bound KE for gas phase O 1s. The absolute VL ∞ -E F is bracketed by the two extreme values deduced from the O 1s gas phase at d ns equal to ∼ 1 mm and "infinity".

Figure 7 .

 7 10, one can obtain the valence band maximum position of liquid water (VBMw) by linear extrapolation of the rising low BE. This can be clearly obtained in the case of the NaI solution, as the water b 1 HOMO is well separated from the I 5p level. VBMw is found at a BE of ∼ 4.15 eV. In Figure 7.20, the positions of the electronic levels in NaCl and NaI saturated solutions are summarized in an energy level diagram, taking either the Fermi level and the vacuum level VL ("just outside the solution") as a reference.

Figure 7 . 20 :

 720 Figure 7.20: Schematic energy diagram of the gold substrate/solution/vacuum system, referenced to the EF (left) and to VL(cutoff) (right). Note that the energies correspond to vertical transitions.

1. 2 )

 2 Figure 8.1: (a)Tetrahedral sheet (b) Octahedral sheet of a fluorinated hectorite layer.

ure 8 . 2 .

 82 The density and location of these substitutions together with the nature of the counterion are crucial factors. They determine the layer stacking within a clay particle, the response of the particle to increased humidity, the mobility and the potential exchange of counterions with an external system. In other words, charged clay layers, with compensating mobile cations in-between, are the key for all the widely-exploited clay properties such as swelling and water/ion retention.The direct link between swelling and ion exchange with the layer charge is confirmed Table8.1.[START_REF] Malikova | Dynamique de l'eau et des ions dans les argiles de type montmorillonite par simulation microscopique et diffusion quasi-elatique des neutrons[END_REF]

Figure 8 . 2 :

 82 Figure 8.2: Schematic representation of fluorinated Na-hectorite structure.

Figure 8 . 3 .

 83 Figure 8.3. The thickness of the platelets seen by AFM is approximately of 10-15 Å, in good agreement with the value expected for an isolated clay platelet.

Figure 8 . 3 :

 83 Figure 8.3: (a) AFM image of dissociated hectorite platelets. A dilute solution of hectorite particles was deposited on a mica substrate and evaporated. The vertical (color) scale ranges from 0 to 10 nm. (b) Topological profile along the line drawn on the AFM image

Figure 8 . 4 :

 84 Figure 8.4: Water adsorption phenomenon (a) at the microscopic scale and (b) at the mesoscopic scale.

Figure 8 . 5 ,

 85 Figure 8.6 and Figure 8.9 respectively.

Figure 8 . 5 :

 85 Figure 8.5: Water adsorption and desorption isotherm as a function of RH of Cshectorite

Figure 8 . 6 :

 86 Figure 8.6: Water adsorption and desorption isotherm as a function of RH of Nahectorite.[21]

Figure 8 . 9 :

 89 Figure 8.9: Water adsorption and desorption isotherm as a function of RH of Sr-hectorite

Figure 8 .

 8 10 presents the evolution of the C 1s peak position referenced to the gold one. The clean gold is grounded and therefore, does not charge, while the carbon contamination attached on the insulating clay platelets suffers the charging effect. Therefore, the variation of the C 1s BE position allows quantifying the charge effect on the sample surface as a function of RH. When RH increases in the chamber, the charge effect decreases and the BE moves towards a limit value of 285.20 eV. Moreover, the distance between the Si 2p and the C 1s, measured for different RH, remains constant. It confirms that both the carbon contamination and the clay platelets experience the same charge effect.

Figure 8 . 11 :

 811 Figure 8.11: Cs 4d XPS spectra (hν = 750 eV) of Cs-hectorite for RH = 0 % (UHV conditions), 10 % (P = 1.3 mbar, T = 283 K), 45 % (P = 5.4 mbar, T = 283 K) and 100 % (P = 12 mbar, T = 283 K). To overcome the charging of the sample, the C 1s peak at 285.20 eV was chosen as energy reference.

Figure 8 .

 8 Figure 8.12: O 1s XPS spectra (hν = 1200 eV) of Cs-hectorite for RH = 0 % (UHV conditions), 10 % (P = 1.3 mbar, T = 283 K), 45 % (P = 5.4 mbar, T = 283 K) and 100 % (P = 12 mbar, T = 283 K). To overcome the charging of the sample, the C 1s peak at 285.20 eV was chosen as energy reference.

= 1 .

 1 1 eV,) corresponding to water vapor. environment by the presence of water molecules. Their insertion in clay interlayers is in agreement with the water adsorption and desorption isotherm of Na-hectorite (see Figure 8.6), which shows that at RH = 50 % one water layer is already present in the clay interlayer. Unfortunately, the small photoionization cross section of Na 2s core-level makes the acquisition of a spectrum for two water layers in the interlayer impossible (at 283 K a pressure of 11 mbar should be reached to reach a RH close to 100 %).

Figure 8 . 14 :

 814 Figure 8.14: Na 2s and Mg 2p XPS spectra (hν = 750 eV) of Na-hectorite for RH = 0 % (UHV conditions), 10 % (P = 1.3 mbar, T = 283 K) and 50 % (P = 6 mbar, T = 283 K). To overcome the charging of the sample, the C 1s peak at 285.20 eV was chosen as energy reference.

  [START_REF] Wagner | Auger and photoelectron line energy relationships in aluminiumoxygen and silicon-oxygen compounds[END_REF] Its numerical value is unique to each chemical state and is independent of the charge referencing operation. It is related to the difference in extra atomic relaxation energy between the two chemical states. The variation of the Auger parameter was calculated between Nahectorite in UHV conditions and RH = 50 %, its value is ∆α = 0.70 eV.Wagner et al. showed that ∆α ≈ 2 × ∆E R .[START_REF] Wagner | Chemical shifts of auger lines, and the auger parameter[END_REF] Consequently, ∆E R ≈ 0.35 eV.

Figure 8 .

 8 Figure 8.16: O 1s XPS spectra (hν = 790 eV) of Sr-hectorite for RH = 0 % (UHV conditions), 40 % (P = 4.8 mbar, T = 283 K), 70 % (P = 8.4 mbar, T = 283 K) and 100 % (P = 12 mbar, T = 283 K). To overcome the charging of the sample, the C 1s peak at 285.20 eV was chosen as energy reference.

8

 8

. 2 )

 2 due to its double charge. It makes the insertion of water molecules possible and easier than with sodium. Water molecules are inserted into clay interlayer and solvate Sr 2+ additionally to the occupation of interaggregates holes. The ratio between liquid water and clay oxygen on the O 1s XPS sepctrum has8. The swelling properties of clays monitored by NAP-XPSstrongly increased comparing to the Cs-hectorite and confirms the adsorption of several water layers in the Sr-hectorite interlayer.

  (a), the cation is modeled by a spherical cavity with a charge ze and a radius R. This cavity is transferred from the vacuum (ǫ 0 ) to a medium placed into a dielectric medium (ǫ) representing the solvent.

Figure 8 .

 8 Figure 8.17: Possible models

vac ( 8 . 4 )Figure 8 . 18 :( 8 . 5 )

 8481885 Figure 8.18: Schematic energy level diagrams illustrating the liquid-phase BE shifts in terms of the solvation free energy.[START_REF] Weber | Photoemission from aqueous alkali-metal -iodide salt solutions using EUV synchrotron radiation[END_REF]

  The next step is the interpretation of this BE shift observed for both Na-hectorite and Sr-hectorite, classical molecular dynamis and DFT calculations are underway in collaboration with V. Marry and E. Dubois at the PHENIX laboratory of the the Pierre et Marie Curie University (UPMC) and R. Vuilleumier from the theoretical chemistry group of Ecole Normale Superieure (ENS).

chapter 3 ,

 3 using XPS and STM experiments in conjunction with DFT calculations and STM image simulations, we show that TEOS dissociates on the Si(001) surface at low coverage (∼ 1 L) via the cleavage of all its four Si -O bonds on a silicon dimer row, losing its Si central atom to form an adatom on the adjacent dimer row. This mechanism gives rise to ethoxy group pairs occupying a dimer out of two and to a structure alternating ethoxy-covered and Si adatom-occupied dimer-rows; therefore, TEOS could be used to produce self-organized nano-structures on Si(001)-2×1 .The Si adatom position, identified on the STM images at room temperature, coincides with the one theoretically predicted more than two decades ago for the clean Si(001)-2×1 surface,[START_REF] Brocks | Binding and diffusion of a Si adatom on the Si(100) surface[END_REF] providing an experimental confirmation which has been lacking so far. Our theoretical analysis of the monomer diffusion have shown the importance of the surface buckling for the determination of the energy landscape of a silicon monomer on the Si(001) surface. We have demonstrated, in chapter 4, that the dimensions of the slab surface are crucial for the understanding of the monomer surface diffusion. The Si(001)-8×4 surface provide results more directly comparable with experiments, than a smaller slab surface (Si(001)-2×2) which neglects the role played by neighboring free silicon dimers. However, our work does not answer the following question: why is the monomer observed on the ethoxy covered surface and not on the clean one? At high TEOS coverage ( ∼ 10 L), the initial regime corresponding to the full dissociation of the molecule, via Si -O bond breaking and emission of a silicon monomer, is in competition with a second regime involving the scission of the C -O bond. We proposed, in chapter 5, that the latter reaction occurs on the monomers produced by Si -O bond breaking. The consequence of this second reaction path is the creation of ethyl groups on the surface. The presence of surface alkyl surface species, besides the expected alkoxy species, has profound consequences for ALD deposition of silica on a clean silicon substrate. While alkoxy species can be hydrolyzed to silanols by exposure to water, surface ethyls species will remain unaltered.

  the solution. Interesting, but thorny questions are raised by our measurements. Two of them, are unescapable: what is the Fermi level of the saline solution? and what are the relationships between the vertical ionization or detachment energies we measure and solution thermodynamics?

Table 2 . 1

 21

	2 % of the Si

. 2p 3/2 component. The SS peak presents a surface core-level shift (SCLS) of only +0.06 eV. The peak labeled S, with a SCLS of -0.55 eV (13 % of the Si 2p 3/2 spectral weight), is related to the negatively charged up silicon dimer atom. The structure labeled C with a SCLS of -0.30 eV (9.4 % of the Si 2p 3/2 spectral weight), corresponds to subsurface silicons (3 rd plane); and finally the S' peak with a SCLS of +0.23 eV

(21.

Table 2 .

 2

1: Surface Core Level Shifts (SCLS) referenced to the bulk component binding energy BE = 99.35 eV. Widths and spectral weights of the Voigt components used to fit the Si 2p experimental spectrum shown in Figure

2

.5. The Lorentzian width was 0.045 eV for all components. a bake out). Curve fitting results and attributions are collected in Table

2

.1.

  3/2 BE of the bulk silicon component varies with coverage and substrate temperature. At room temperature, the 2p 3/2 BE of the bulk component is found at 99.35 eV. It shifts to 99.40 eV after TEOS adsorption at 300 K

	(due to a reduction of the upward band bending) and at 99.55 eV for TEOS physisorbed
	at 120 K, the increase in BE being largely due to a surface photovoltage effect [56] that
	flattens the bands at cryogenic temperature.
	The determination of the oxidation state of the central atom of the molecule, de-
	noted Si T , is crucial to conclude about the nature and the precise number of broken
	chemical bonds in the TEOS molecule. If we consider that dissociative chemisorption on
	the surface occurs via cleavage of Si -O bonds as for PTES,

Table 3

 3 Si) in SiO 2 . For instance, the BE shift per ethoxy ligand in gas phase ethoxysilaneSi[CH 3 ] n [OCH 2 CH 3 ] 4-n is reported to be only ∼ +0.4 eV,[START_REF] Jolly | Core-electron binding energies for gaseous atoms and molecules[END_REF] which suggests that the na-Si T -OC 2 H 5 adduct (plus three ≡Si S -OC 2 H 5 moieties) or to the release of an atomic silicon (plus four ≡Si S -OC 2 H 5 ). However, the Si 1+ component, due to ≡Si S -OC 2 H 5 moieties, cannot be distinguished from (Si S ) 3 Si T -OC 2 H 5 ones, the Si 2p

	.1.

  eV is attributed to carbons bonded to oxygen in ethoxy groups (Si -O -CH 2 -CH 3 ) and the β peak at 285.23 eV to terminal carbon atoms in those ethoxy groups (Si -O -CH 2 -CH 3 ). The ethoxy termination corresponds to ∼80 %

	of the carbon species on the surface. The detection of surface ethoxy groups, as major-
	ity products, is consistent with the observation of a single oxidation component (Si 1+ )
	in the Si 2p spectrum, confirming that Si T -O bond breaking is the main dissociation
	channel.
	Similarly, carbons (Pauling electronegativity = 2.55) bonded to silicon atoms (Paul-
	ing electronegativity = 1.90) are expected to be slightly more negatively charged than
	those bonded to other carbon atoms within ethyl groups; therefore the former ones are
	expected at lower BE. The minority product is the ethyl termination: the γ peak at 284.70
	eV is attributed to the terminal carbon (Si -CH 2 -CH 3 ) and the δ peak at 284.07 eV to the carbon atom directly bonded to silicon (Si -CH 2 -CH 3 ). A similar BE shift of 0.6
	eV is observed in the C 1s XPS spectrum of cyclopentene di-σ bonded on Si(001) that
	exhibits two components, at 284.2 eV (C bonded to Si) and at 284.8 eV (C non-bonded

Table 3

 3 ≡Si S , or the silicon of the TEOS molecule Si T .

	.2.

Table 4 .

 4 1: Description of the k-points used for the 2×2 reconstructed surface calculations (where a is the lattice parameter).

	8, 1/8) (1/8, 3/8) (3/8,
	1/8) (3/8, 3/8) (1/8, -
	1/8) (1/8, -3/8) (3/8, -1/8)
	(3/8, -3/8)

Table 4 .

 4

	Energy difference	GGA	LDA
	M-D´0.74 eV	0.69 eV
	M-H	0.48 eV	0.42 eV
	M-S	0.75 eV	0.75 eV

2: Energy difference between two sites of the surface obtained for two different approximations for exchange and correlation.

Table 5

 5

	.1

Table 5 .

 5 1: Si 2p 3/2 Surface core level shifts (SCLS, relative Si 2p 3/2 BE referenced to the bulk component one), spectral weights, full widths at half maximum (fwhm), and attribution of the Voigt components used to reconstruct the Si 2p XPS experimental spectra shown in Figure 5.3. The BE of the Si 2p 3/2 component (referenced to the Fermi level) is 99.40 eV.

Table 5

 5

	.2

3 )Si -O -CH 2 -CH 3

Table 5 .

 5 2: Si 2p 3/2 Surface core level shifts (SCLS, relative Si 2p 3/2 BE referenced to the bulk component one), spectral weights, full widths at half maximum (fwhm), and attribution of the Voigt components used to reconstruct the Si 2p XPS experimental spectra shown in

Table 5

 5

	.3

3 )Si -O -CH 2 -CH

Table 5 .

 5 3: C 1s Surface core level shifts (SCLS, relative Si 2p 3/2 BE referenced to the bulk component one), spectral weights, full widths at half maximum (fwhm), and attribution of the Voigt components used to reconstruct the Si 2p XPS experimental spectra shown in Figure

Table 5

 5

	.4

Table 5 .

 5 4: Si 2p 3/2 Surface core level shifts (SCLS, relative Si 2p 3/2 BE referenced to the bulk component one), spectral weights, full widths at half maximum (fwhm), and attribution of the Voigt components used to reconstruct the Si 2p XPS experimental spectra shown in Figure5.6. The BE of the Si 2p 3/2 component (referenced to the Fermi level) is 99.40 eV.

Table 5

 5

	.6

Table 5 .

 5 5: Si 2p 3/2 Surface core level shifts (SCLS, relative Si 2p 3/2 BE referenced to the bulk component one), spectral weights, full widths at half maximum (fwhm), and attribution of the Voigt components used to reconstruct the Si 2p XPS experimental spectra shown in Figure5.7. The BE of the Si 2p 3/2 component (referenced to the Fermi level) is 99.40 eV.

Table 5

 5

	.6

Table 7 .

 7 2: BE (Fermi level) and VDE (VL(cutoff)) of the anions valence levels. Comparison with photoemission thresholds from Ref.[START_REF] Delahay | Photoelectron emission spectroscopy of aqueous solutions[END_REF].If we consider now the core level BE, we note that the Na 2s BE in NaCl, NaBr, and NaI are identical for the three solutions. This can be understood if the Fermi level in the solutions does not depend on the nature of the anion. Considering the VL(cutoff), the IP of the cation Na + in the solution (67.35 eV for NaCl and NaI, 67.45 eV for NaBr) can be compared with those found with the liquid jet technique, 68.00 ± 0.15 eV.[START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF]

	Anion (bulk water)	Cl 3p	Br 4p	I 5p
	BE (peak maximum) VDE (peak maximum) BE (low BE rising edge) VDE (low BE rising edge)	∼ 4.9 ∼ 9.1 ∼ 3.4 ∼ 7.6	∼ 4.1 ∼ 8.3 ∼ 3.0 ∼ 7.2	∼ 2.6 ∼ 6.8 ∼ 1.4 ∼ 5.8
	Photoemission threshold[169]	8.81	8.05	7.19

Table 7 .

 7 E F in the solution is determined by redox couples. The redox level of the standard hydrogen electrode (SHE) H + /H 2 (standard electrode potentialE 0 = 0 V) is positioned 4.[START_REF] Hamers | Atomically-resolved studies of the chemistry and bonding at silicon surfaces[END_REF] eV below the vacuum level (at pH = 0). Therefore, the energy positioning (with respect to the vacuum level) of the other redox couples can be obtained via the following equation VL -E F (redox) = 4.44 + E 0 , with E 0 expressed in V.In the present electrolytes, the ionic species are H 3 O + (H + ), OH -(at a concentration of 10 -7 mol/L), Na + and the anions. Therefore we can consider the following redox couples: Na + /Na, H + /H 2 , the halogen redox couples, to which we can addH 2 O 2 /H 2 O.In fact X-ray induced radiolysis of the water layer can produce H 2 and H 2 O 2 .[START_REF] Allen | The radiation chemistry of water and aqueous solutions[END_REF],[START_REF] Mesu | Probing the influence of X-rays on aqueous copper solutions using time-resolved in situ combined video/X-ray absorption near-edge/ultraviolet-visible spectroscopy[END_REF] In Table7.3, we give the corresponding standard electrode potentials E 0 refereed to the SHE, and the deduced VL -E F values. 3: Values of standard potentials (E 0 ), VL -E F values for redox couples and gold metal surfaces (work function).

	At this point of the discussion a question should be raised. What fixes the Fermi
	level in the electrolyte solution at about 4.2 eV below VL(cutoff)? According to Trasatti
	[165] and others, [170]

Table 7

 7 .3 suggests that the electron transfer from the substrate to the solution to reduce Na + is merely impossible. Also, the oxidation of Cl -into Cl 2 and of Br-into Br 2 via electron transfer from the so-7. Ion segregation at liquid water/gas interface lution to the substrate is a priori not feasible. On the other hand, the reduction of H 2 O 2 (if present) to H 2 O could be envisaged. The reduction of H + is "on the border line" considering the metal substrate work functions (note the relatively low work function found for the degreased gold/Si substrate, likely due to surface contamination). This is also true for the oxidation of I -, but noticing that the NaCl and NaI solutions have practically the same work function, iodine redox couples should be excluded.In fact the work function of ∼ 4.2 eV is a value very close to the VL -E F of the H + /H 2 in standard conditions (4.44 eV). In fact the equilibrium value of the Fermi level depends on the pH and H 2 concentration (or partial pressure in the gas phase), Using the Q-pole inserted in the 2 nd stage of the differential pumping, we have measured the partial pressures of H 2 O, H 2 and O 2 of the sampled gas, in two cases: (i) for the residual gas (6 × 10 -9 mbar in the analysis chamber), (ii) for 5 mbar of water vapor in the analysis chamber. The data are collected in Table7.4.

	according to Nernst law:
	VL -E F = 4.44 +	0.058 2	2pH + log(	[H 2 ] [H 2 ] SHE	)	(7.1)
	Total pressure in		Q-pole partial pressures (mbar)
	analysis chamber
		H 2 O		H 2		O 2
	6 × 10 -9 mbar (resid-ual)	5 × 10 -11	6.5 × 10 -10	< 10 -13
	5 mbar (H 2 O)	2 × 10 -8		3 × 10 -9		2 × 10 -10

Table 7 .

 7 4: Partial pressures measured by the Q-pole in the second stage of the differential pumping system of the analyzer Table7.[START_REF] Ogletree | A differentially pumped electrostatic lens system for photoemission studies in the millibar range[END_REF] shows that H 2 gives the major contribution in the residual gas. H 2 is also the main pollutant in the water gas. The increase jump in H 2 pressure measured by the Q-pole by increasing the analysis chamber pressure from 6 × 10 -9 mbar to 5 mbar is 2.25 × 10 -9 mbar, about 1/10 of the H 2 O pressure. Assuming that the partial pressures measured by the Q-pole in the sampled gas are proportional to those in the 7.4. Summary 153 analysis chamber (no preferential pumping), the partial pressure of H 2 would be ∼ 0.5 mbar (5 × 10 -4 bar). This value seems very large, and we shall take it as an upper bound. A lower bound would be 6 × 10 -9 mbar, considering that almost all the residual is H 2 (see Table7.4), and that no supplementary H 2 is brought by introducing H 2 O. SHE > 6 × 10 -12 , the Nernst equation gives 4.13 eV < VL -E F < 4.35 eV. High concentrations of H 2 in the water layer could also be produced by radiolysis, but we have no direct visual indication, like bubbling,[START_REF] Mesu | Probing the influence of X-rays on aqueous copper solutions using time-resolved in situ combined video/X-ray absorption near-edge/ultraviolet-visible spectroscopy[END_REF] that it occurs.

	With pH = 7 and 5 × 10 -4 > [H 2 ] [H 2 ]

Table 8 .

 8 1: Classification of dioctahedral 2:1 clays as a function of layer charge. TETrefereed to tetrahedral substitutions and OCT-to octahedral substitutions

			Type of substi-	Counterion	Swelling, ion
			tution		exchange
	pyrophyllite	0	none	none	none
	smectites	0.4-1.2	TET, OCT	Ca 2+ , Mg 2+ ,	yes
				Na +
	vermiculites	1.2-1.8	TET, OCT	Mg 2+	yes
	micas	2.0-4.0	TET	K + (Ca 2+ )	none

The swelling properties of clays monitored by NAP-XPS

The behavior of cations was also investigated in a confined environment, we studied the hydration of counterion into clay interlayers in chapter 8. We proved that it is possible to follow the hydration of an hectorite system through counterion core-level XPS spectrum analysis and more precisely through its BE shift. The next step is the interpretation of this BE shift observed for both Na-hectorite and Sr-hectorite, classical molecular dynamics and DFT calculations are underway in collaboration with V. Marry and E. Dubois at the PHENIX laboratory of the the Pierre et Marie Curie University (UPMC) and R. Vuilleumier from the theoretical chemistry group of Ecole Normale Superieure (ENS).

has profound consequences for ALD deposition of silica on a clean silicon substrate.

While alkoxy species can be hydrolyzed to silanols by exposure to water, surface ethyls species will remain unaltered. Therefore, if the presence of Si -C bonds is not desired at the surface/interface, a first exposure of the clean surface to alkoxysilane molecules should be avoided. Instead, the silicon surface could be water-reacted first, to form surface silanols, [START_REF] Poncey | Adsorption of water on Si(001)-2x1 and Si(111)-7x7 surfaces at 90 and 300 K : a Si 2p core-level and valence band study with synchrotron radiation[END_REF], [START_REF] Gallet | Isolated silicon dangling bonds on a water-saturated n + -doped Si(001)-2x1 surface: an XPS and STM study[END_REF] and the chemisorption of alkoxysilane molecules, via an alcoholelimination reaction with surface silanols, [START_REF] Fan | STM and HREELS investigation of gas phase silanization on hydroxylated Si(100)[END_REF] could be explored. This reaction path will be investigated in the next chapter. phase photoelectrons, which is absent in the liquid jet configuration, as the b1(gas) KE does not change when the beam is moved away from the jet surface. [START_REF] Winter | Photoemission from liquid aqueous solutions[END_REF] The CPD effect on the gas-phase BE was studied more in detail with the O 1s corelevel of the water vapor. The gas phase core-level BE changes as the sample-nozzle distance is modified (as the irradiated volume is at different electrostatic potentials, according to the distance). We have measured the O 1s energy position, referenced to the Fermi level of the sample/analyzer, for increasing nozzle-sample distances d ns . At the optimized working distance d ns ∼ 1 mm (see Figure 7.19 (c)), the O 1s BE of the vapor is 535.30 eV (liquid water O 1s BE is at 533.20 eV). By retracting the sample several cm away from the nozzle (when the sample contribution vanishes) the gas phase O 1s BE is found at 535.84 eV. This BE (KE) energy increase (decrease) of 0.5 eV when d ns varies from ∼ 1 mm to "infinity" gives an estimate of the CPD. It indicates that electron charge is transferred from the analyzer to the sample (see Figure 7.19 (d)).

In fact VL ∞ should be measured with a sample freely floating in space, and initially electrically neutral, [START_REF] Egelhoff | Core-level binding-energy shift at surface and in solids[END_REF] which in practice cannot be achieved, because of charging.

Subtracting the "infinite" d ns O 1s BE to the gas phase IP of 539.79 eV [START_REF] Sankari | Vibrationally resolved O 1s photoelectron spectrum of water[END_REF] We note that VL(cuto f f ) -E F value of 4.2 eV is bracketed by these two value. VL and VL ∞ are expected to be close for pure water, as the surface dipole contribution should be of the order of 0.1 eV.

We consider now the use of the b1(gas) line in the valence band to position the vacuum level. For now, we neglect the CPD effect. In fact we note that the b 1 (gas)b 1 (liq) BE difference we find, only differs by 0.2 -0.3 eV from the value obtained by the Winter-Faubel group. Using the b 1 (gas) IP of 12.6 eV, and the b 1 (gas) BE of 7.4 -7.6 eV (see Figure 7.10), we find a VL ∞ (b 1 (gas)) -E F value of 5.0 -5.2 eV, about 1 eV larger than VL(cuto f f ) -E F . The discrepancy is large, contrary to the vacuum level determinations using the gas phase O 1s ionization energies and secondary cutoff that gave close values. At present we cannot provide any explanation.

Chapter 8

The swelling properties of clays monitored by NAP-XPS Clay minerals, rich in silicon and aluminum, are breakdown products of minerals formed in greater depths of the Earth's crust or mantel. At the Earth's surface, they come into contact with an acidic environment (carbon dioxide dissolved in water), active atmosphere and temperature fluctuations. This so-called weathering process is the main source of sediments and soils on the Earth's surface, on which the biosphere heavily relies. Clays, as layered, fine-grained (high surface area) materials with water retention and ionic exchange capacities, have crucial role in nutrient (both organic and inorganic) retention and pH buffering of soil. Furthermore, a vast number of applications of clays has been developed, beginning with their use in ceramic ware, paper-making, removal of impurities, decolorizing and more recently as molecular sieves, in catalysis, CO 2 sequestration and geological storage of long half-time radioactive waste. [START_REF] Bradbury | Near-field sorption data bases for compacted MX-80 bentonite for performance assessment of a high-level radioactive waste repository in opalinus clay host rock[END_REF], [START_REF] Gaus | Role and impact of CO 2 rock interactions during CO 2 storage in sedimentary rocks[END_REF], [START_REF] Malikova | Dynamique de l'eau et des ions dans les argiles de type montmorillonite par simulation microscopique et diffusion quasi-elatique des neutrons[END_REF], [START_REF] Fowden | Clay Minerals: Their structure, behavior and use[END_REF], [START_REF] Velde | Introduction to clay minerals[END_REF] With respect to these issues, it is crucial to establish a bridge between the microscopic scale (chemical bonding, ionic exchange, ion solvation, ion and matter transport) and the macroscopic scale (swelling, retention capacities). Clays are composed of micrometric particles formed by stacks of negatively charged alumina-silicate layers compensated by counterions such as Na + and Ca 2+ in between them, in the so called interlayer.

The origin of water adsorption and retention by clays lies in the hydration of these compensating interlayer cations and depends on the alumina-silicate layer negative charge and the nature of the counterions (charge, size). Consequently, the peak of C 1s was used as a reference for the energy calibration for all the spectra presented here. Its BE is fixed at 285.20 eV, corresponding to carbon in alkyl chains. [START_REF] Barr | Nature of the use of adventitious carbon as a binding energy standard[END_REF] This reference was chosen in order to ease the comparison between samples.

The swelling properties of clays monitored by NAP-XPS

The choice of this reference value is confirmed by the measurements of the C 1s spectrum of the adventitious carbon at the surface of a conductive sodium chloride solution ( ∼ 5 Ω × cm), its BE is also found at 285.2 eV, as described in the previous chapter.

Inhomogeneities in the surface potential (or work function according to Egelhoff) of the clay surface can appear when the surface state electronic population varies from place to place. This in turn induces a dispersion of the KE values of the photoelectrons generated at different locations of the surface, leading to peak broadening. These inhomogeneities disappear as soon as water adsorbs on the surface, [START_REF] Arima | Water adsorption , solvation , and Deliquescence of potassium bromide thin films on SiO 2 studied by ambient-pressure X-ray photoelectron spectroscopy[END_REF], [START_REF] Verdaguer | Correlation between charge state of insulating NaCl surfaces and ionic mobility induced by water adsorption : a combined ambient pressure X-ray Photoelectron Spectroscopy and Scanning Force Microscopy study[END_REF] as illustrated 8.4. Cs-hectorite 169 in Figure 8.10 (c), where the C 1s fwhm, exhibits a monotonic decrease when water pressure increases. The same effect is observed for the core-levels of the other elements present in the clay.

Cs-hectorite

We present, in this section, the results obtained with Cs-hectorite. XPS measurements were performed at 283 K and at pressures up to 12 mbar, corresponding to different RH values between 0 % and 100 % (P sat (water) = 12 mbar at 283 K). Hectorite swelling properties vary strongly with RH, however Tolbert et al. have shown that it is independent of the temperature, therefore, the comparison with the room temperature isotherms is relevant. [START_REF] Morodome | In situ X-ray diffraction study of the swelling of montmorillonite as affected by exchangeable cations and temperature[END_REF] Cs 4d XPS spectra evolution with RH is given in Figure 8.11. All Cs 4d XPS spectra can be fitted with a single d-orbital doublet at ∼76.2 eV (spin-orbit splitting energy of 2.31 eV and an intensity ratio of 0.66 eV). The absence of modification of both shape and BE of Cs 4d core-level peaks suggests that the Cs + chemical environment does not change when RH increases and so, that there is no water insertion into the clay interlayer. It is in agreement with the Cs-hectorite isotherm for which at RH = 100 %, less than one water layer is present in the clay interlayer.

The O 1s XPS spectra evolution with RH is shown in Figure 8.12. At RH = 0 %, only one peak is present at 532.7 eV attributed to the clay structure oxygen. When RH increases, the emergence and the increase of two peaks is observed, one at 533.6 eV attributed to condensed water, and a second one at 535.5 ± 0.1 eV (fwhm = 1.1 eV,) corresponding to water vapor. The position of the water vapor peak and its fwhm depends strongly on the distance between the analyzer and the sample. It is due to a variation of the gradient between the surface potential (vacuum level) of the material and the one of the analyzer. When the distance decreases the water vapor peak becomes larger and its BE increases.

The ratio between the clay structure oxygen peak, at 532.7 eV and the condensed

The swelling properties of clays monitored by NAP-XPS

The intensity of the condensed water oxygen peak relative to the clay structure oxygen peak slightly increases with RH; the ratio is of 1.2 at RH = 10 % and 1.35 at RH = 50 %. According to the water isotherm, there is no water into the interlayer at RH = 10 %, the peak of condensed water measured represents the condensed water in interaggregates holes. The increase of the ratio at higher RH corresponds to the insertion of water into the interlayer. RH increases from 0 % to 10 %, a shift of ∼ -0.1 eV of the Na 2s core-level is observed, then between 10 % to 50 %, the peak shift of -0.89 eV (relative to RH = 0 %). On the contrary, Mg 2p peak BE stays constant when RH increases; indeed, magnesium is located in the middle of a silica lamella and do not interact with water molecules. It confirms that the shift, observed for the Na 2s peak BE, is due to a modification of sodium cation

The swelling properties of clays monitored by NAP-XPS

Simulations by DFT are underway at the PHENIX laboratory of the Pierre et Marie Curie University (V. Marry and E. Dubois) in collaboration with the theoretical chemistry group of the Ecole Normale Superieure (R. Vuilleumier) in order to determine the BE difference of core-electrons of the cation between the dry and the hydrated state.

Sodium chloride solution

In order to confirm the final state effect interpretation of the Na 2s core-level BE shift, the results can be compared with those obtained for the saturated sodium chloride solution described in the previous chapter, where the BE of Na 2s entirely solvated in a sodium chloride water droplet was measured.

Sr-hectorite

In the studied serie (Cs + , Na + , Sr 2+ ) Sr 2+ has a the highest hydration energy in bulk water. Thus, it is expected that it will be easily hydrated in hectorite as confirmed by the sorption curves.

The Sr 3d XPS spectra evolution with RH is presented in

The swelling properties of clays monitored by NAP-XPS

Two different interpretations can be considered:

• The low BE component at 134.7 eV may be attributed to inter-lamella Sr 2+ , and the one at high BE to Sr 2+ at the surface of the stack of platelets, where the corehole is less well screened. To verify this hypothesis, a depth analysis was done varying the photon energy from 400 eV (surface sensitive conditions) to 1200 eV (bulk sensitive conditions) but slight modifications were observed due to the nonoriented clays particles deposition.

• An alternative explanation could be the following, the component at high BE 136.5

eV could be attributed to a counterion placed into the hexagonal cavity, and the component at 134.7 eV to Sr 2+ ions already surrounded by a few water molecules.

When RH increases to 45 %, two components are still observed at 134.4 eV and 135.9 eV (fwhm = 1.5 eV). It confirms the interaction of Sr 2+ with water molecules and similarly to the results obtained with Na-hectorite, the peak shifts to lower BE when RH increases.

At RH = 70 %, the BE of both peaks is constant, in agreement with the Sr-hectorite isotherm, the region from 20 % to 80 % corresponds to a bilayer configuration. If the hypothesis of two Sr 2+ , one in an hexagonal cavity and one partially hydrated at RH = 0 % was true, we should already observe, in this range of RH, only one component corresponding to the cation between two water layers into the interlayer.

Finally at RH = 100 %, we observe a second step in the clay hydration, only one component is present on the spectrum at 133.8 eV (fwhm = 1.4 eV). All Sr 2+ cations have an equivalent chemical environment due to the presence of several water layers.

We present, Figure 8.16, the O 1s XPS spectra evolution with RH. At RH = 0 %, only one peak is present at 532.7 eV corresponding to clay structure oxygen. In the same way as in Cs-hectorite case, when RH increases to 40 % and 70 %, the emergence of two peaks is observed, one at ∼ 536.2 eV corresponding to water vapor and a second one at 533.9 eV to liquid water. At RH = 100 %, the spectrum can also be fitted by three components but a shift of + 0.04 eV of the liquid water peak is observed and has a BE of