François Rochet

Oxidized silicon surfaces studied by high resolution Si 2p core-level photoelectron spectroscopy using synchrotron radiation

Abstract The Si 2p soft X-ray photoemission study of various oxidized surfaces (thermal oxidation and room temperature (RT) oxidation) shows the benefit derived from the use of high-energy resolution (70 meV). Interesting structural information can be retrieved from an analysis of the line widths of the oxides, suboxides, and elemental Si peaks. In particular the binding energy (BE) of elemental silicon layers adjacent to the oxidized layer can be distinguished from that of deeper silicon layers (the two prominent `interfacial elemental Si lines are shifted by about +0.2 and −0.2 eV). These new data show a possible effect of oxygen second neighbors on elemental Si 2p binding energies. Moreover, changes in the oxide/suboxide line widths – as seen in a comparative study of the Si(1xa01xa01)-7×7 surface thermally oxidized in O2 and exposed at RT to O2 or H2O – may be attributed to static disorder, i.e., variations in Si–O bond lengths.

Pb1 defect study and chemical characterization of the Si(001)-SiO2 interface in oxidized porous silicon

In comparison with oxidized bulk crystals, we show that the morphology of oxidized porous silicon can offer a unique opportunity of measuring suboxide distribution by conventional XPS and to characterize thoroughly interfacial defects by EPR.

Oxidation of Small Supported Platinum-based Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen

AbstractnThe investigation of nanocatalysts under their working conditions of pressures and temperatures represents a real strategy toward a realistic understanding of their chemical reactivity and related issues. Additionally, the reduction of Pt load in the catalysts while maintaining their optimum performances is essential to large scale practical applications. Here, we show that small PtZn bimetallic nanoparticles (NPs) supported on the rutile and reduced TiO2(110)-(1xa0×xa01) surface can be prepared by a two step consecutive deposition process where Pt was deposited first and followed by Zn. In situ synchrotron-based near ambient pressure photoemission spectroscopy experiments are used to monitor the evolution of the oxidation states and surface elemental composition of pure Pt and PtZn NPs under high exposure to O2 pressure. The formation of stable Pt surface oxide was evidenced for both pure and PtZn NPs. While a sizeable encapsulation of pure Pt NPs by TiOx was seen after annealing at 440xa0K under 1xa0mbar of O2, no such effect was noticed for PtZn NPs. The formation of a zinc oxide layer on PtZn NPs enhances the stability of the NPs and induces a partial reduction of the TiO2(110) surface. Spontaneous formation of a Pt–Zn alloy phase at room temperature was seen in PtZn NPs.

Propanoate grafting on (H,OH)-Si(0 0 1)-2 × 1.

We have examined the reactivity of water-covered Si(0u20090u20091)-2xa0×xa01, (H,OH)-Si(0u20090u20091)-2xa0×xa01, with propanoic (C2H5COOH) acid at room temperature. Using a combination of spectroscopic techniques probing the electronic structure (XPS, NEXAFS) and the vibrational spectrum (HREELS), we have proved that the acid is chemisorbed on the surface as a propanoate. Once the molecule is chemisorbed, the strong perturbation of the electronic structure of the hydroxyls, and of their vibrational spectrum, suggests that the molecule makes hydrogen bonds with the surrounding hydroxyls. As we find evidence that surface hydroxyls are involved in the adsorption reaction, we discuss how a concerted or a radical-mediated reaction (involving the surface silicon dangling bonds) could lead to water elimination and formation of the ester.

Oxidation of the H-Si(111)-1×1 surface: high resolution Si 2p core-level spectroscopy with synchrotron radiation

Abstract The room temperature oxidation of H-terminated Si(111) surfaces by activated oxygen has been investigated using high resolution (70xa0meV) Si 2p core-level spectroscopy with synchrotron radiation. The data are compared with previous studies using photoemission spectroscopy and high resolution electron energy loss spectroscopy. The metastability of the oxidized layer formed at room temperature is also examined.

The Electronic Structure of Saturated NaCl and NaI Solutions in Contact with a Gold Substrate

The near ambient pressure X-ray photoelectron spectroscopy set up installed recently at SOLEIL synchrotron facility is used to study the electronic structure of NaCl and NaI saturated solutions formed on a gold substrate. The binding energies of the solution constituents are measured with respect to the Fermi level of the gold substrate. The C1s binding energy of the aliphatic contaminant floating at the surface of the solution is an evidence that the Fermi level in the metal and in the solution are aligned. The use of the Fermi level common energy reference is an added value with respect to previous works realized with micro-jets that were calibrated in energy with respect to vacuum level. We observe that the water valence molecular levels binding energies, and hence the Fermi positioning in the gap of the liquid, the Na+ 2s binding energy and even the work function are independent of the nature of the anions. The secondary electron energy distribution curves show that the work functions of the two solutions are equal within experimental uncertainty. We discuss this point considering the different ion distributions at the surface (related to the different size and polarizability of the anions), and the possible contribution of carbon contaminants. We compare the WF values extracted from the secondary electron edges to alternative measurements using the binding energy of the gas phase O1s or 1b1 spectra (referenced to the gold Fermi level). The ionization energies (referenced to the vacuum level), that we obtain by adding the work function to the measured binding energies, are in good accord with previously published works using micro-jets, obtained, however, at much lower solute concentration. Finally we discuss the origin of the Fermi level pinning in the liquid band gap and consider the possibility that the H+/H2 redox level is aligned with the metal Fermi level.

Temperature effects on the Si/SiO2 interface defects and suboxide distribution

Abstract Temperature effects on the Si/SiO 2 (0.5–5 nm thick) interface defects and suboxide distribution are studied by electron paramagnetic resonance and conventional X-ray photoelectron spectroscopy. The experiments are made on porous (66% porosity) silicon samples (10 μm thick) thermally oxidized in the 400–1000°C temperature range. Electron paramagnetic resonance measurements (∼9.6 GHz, 25°C) show a decrease of the density of broken bonds called P b1 when the oxidation temperature increases. However no change of the interfacial structure is observed through the suboxide distribution. We interpret our results in terms of viscoelastic relaxation.

Energy level alignment of a hole-transport organic layer and ITO: Towards applications for organic electronic devices

2,2,6,6-Tetraphenyl-4,4-dipyranylidene (DIPO-Ph4) was grown by vacuum deposition on an indium tin oxide (ITO) substrate. The films were characterized by atomic force microscopy as well as synchrotron radiation UV and X-ray photoelectron spectroscopy to gain an insight into the material growth and to better understand the electronic properties of the ITO/DIPO-Ph4 interface. To interpret our spectroscopic data, we consider the formation of cationic DIPO-Ph4 at the ITO interface owing to a charge transfer from the organic layer to the substrate. Ionization energy DFT calculations of the neutral and cationic species substantiate this hypothesis. Finally, we present the energetic diagram of the ITO/DIPO-Ph4 system, and we discuss the application of this interface in various technologically relevant systems, as a hole-injector in OLEDs or as a hole-collector interfacial layer adjacent to the prototypical OPV layer P3HT:PCBM.

NITRILES ADSORBED ON Si(001) AT 300 K STUDIED VIA SYNCHROTRON RADIATION CORE-ELECTRON SPECTROSCOPIES

This paper focuses on the use and value of XPS and NEXAFS spectroscopies to unveil the nature of the chemical bond of various bifunctional nitrile molecules adsorbed on Si(001) 2×1 at 300 K. The adsorption modes are also discussed in the light of recent theoretical publications devoted to optimized geometries and reaction paths of these molecules on Si(001).

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

The heterogeneous radiolysis of organic molecules in clays is a matter of considerable interest in astrochemistry and environmental sciences. However, little is known about the effects of highly ionizing soft X-rays. By combining monochromatized synchrotron source irradiation with in situ Near Ambient Pressure X-ray Photoelectron Spectroscopy (in the mbar range), and using the synoptic view encompassing both the gas and condensed phases, we found the water and pyridine pressure conditions under which pyridine is decomposed in the presence of synthetic Sr2+-hydroxyhectorite. The formation of a pyridine/water/Sr2+ complex, detected from the Sr 3d and N 1s core-level binding energies, likely presents a favorable situation for the radiolytic breaking of the O-H bond of water molecules adsorbed in the clay and the subsequent decomposition of the molecule. However, decomposition stops when the pyridine pressure exceeds a critical value. This observation can be related to a change in the nature of the active radical species with the pyridine loading. This highlights the fact that the destruction of the molecule is not entirely determined by the properties of the host material, but also by the inserted organic species. The physical and chemical causes of the present observations are discussed.