L. Soriano

Thermal oxidation of TiN studied by means of soft x‐ray absorption spectroscopy

The oxidation of TiN in an oxygen flow at temperatures in the range 300–500 °C has been studied by means of soft x‐ray absorption spectroscopy. The analysis of the experimental results indicates that O progressively displaces N to form TiO2. The process appears to be controlled by the temperature dependence of the oxygen diffusion. Some oxidation is observed to take place even at room temperature. No evidence of oxynitride formation was found in thermally oxidized TiN, instead complete phase separation is observed. The interface between TiO2 and TiN seems to be very abrupt. The appearance of a sharp absorption peak in the N 1s spectrum of TiN is believed to be due to nitrogen atoms which are displaced during the oxidation process and remain unbounded within the TiO2 matrix. For temperatures above 400 °C, this peak disappears as the interstitial N atoms migrate to the surface and desorb.

The electronic structure of TiN and VN: X-ray and electron spectra compared to band structure calculations

We studied the electronic structure of TiN and VN by means of band structure calculations and spectroscopic techniques. The band structure calculations show that the bonding in these compounds is mostly covalent. The Fermi level intersects the transition metal 3d bands giving rise to the metallic conductivity observed in these nitrides. The particularly large stability of these compounds is due to filled metal 3d-nitrogen 2p bonding states. The X-ray photoemission (XPS) and the N 1s X-ray absorption (XAS) spectra are related to the occupied electronic states in the valence band and to the unoccupied electronic states in the conduction band respectively.

Adsorption and oxidation of K deposited on graphite

The bonding state of K intercalated in highly oriented pyrolitic graphite (HOPG) has been studied by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure/near-edge X-ray absorption fine structure (SEXAFS/NEXAFS) spectroscopies. Intercalation of potassium into HOPG occurs at room temperature after deposition on its surface by evaporation from a getter source. In this state XPS and angle-resolved NEXAFS at the K 1s edge show the existence of a transfer of charge from the potassium to the graphite layers and provide some clues about the hybridization of the p states of K with the σ∗ and Π∗ empty states of the graphite layers. Meanwhile, SEXAFS shows that the K atoms are placed at the centre of the hexagon units of an expanded and reordered graphite structure. Adsorption of 80 L of oxygen at 298 K on K-HOPG produces changes in the shape and intensity of the K 2p photoelectron peak, while the width and asymmetry of the C 1s peak decrease. The progressive incorporation of oxygen into the system is monitored by the development of an O 1s peak with a binding energy between 531.3 and 531.8 eV. In all cases, a rough estimation of the OK ratio calculated from the areas of the O 1s and K 2p peaks gives values around one. Further changes in the same direction at the K 2p and C 1s photoelectron peaks and an additional increase in the amount of incorporated oxygen occur after adsorption of oxygen at 373 K. Similarly, under these conditions the calculated OK ratio remains close to one. The SEXAFS spectra after adsorption of oxygen are similar to those obtained before adsorption, except for a general and small decrease of the intensity of the EXAFS oscillations. Fitting analysis of the spectra provides the same CK distance and N30N70 ratio (N30,70, apparent coordination numbers measured at incident angles of the radiation of 30° and 70° in respect to the surface normal) as before adsorption of oxygen. However, there is a decrease of ∼ 45% in the absolute values of N in respect to the original sample. NEXAFS spectra at grazing incidence are also similar but depict a new feature when recorded at normal incidence. The intensity of this feature increases with the amount of incorporated oxygen. The model proposed to explain these results assumes that adsorption of oxygen produces a depletion of the negative charge in the graphite layer and the formation of Oδ−2 species at the surface (δ likely to be two). Simultaneously, the adsorption of oxygen at 298 and 373 K induces the partial segregation of the intercalated potassium to the external surface of the HOPG crystal where it becomes associated to the adsorbed oxygen. In this state SEXAFS spectra are dominated by the potassium still intercalated within the layers, while XPS is more sensitive to the atoms segregated to the surface.

The Electronic-Structure of Zro2 - Band-Structure Calculations Compared to Electron and X-Ray-Spectra

Abstract We present and discuss band-structure calculations of ZrO2 in the cubic phase calculated by means of the Localized-Spherical-Waves (LSW) method with an extended basis set. The results obtained are in agreement with previous calculations and confirm a large covalent contribution to the bonding in ZrO2. The results are also compared to x-ray and electron spectra showing an overall good agreement between theory and experiment. In particular, the calculations reproduce well the distribution of the spectral weight and the magnitude of the band gap and the crystal-field splitting.

Correlation between N 1s core level x-ray photoelectron and x-ray absorption spectra of amorphous carbon nitride films

This work presents a comparative analysis of the N 1s core level spectra, as measured by x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS), of amorphous CNx films which gives evidence of the existing correlation between the different components that constitute the respective spectra. After annealing, the contribution of XPS at 399.3 eV and the components of XAS at 399.6 and 400.8 eV are clearly enhanced. They are assigned to sp2 with two neighbors and to sp states of nitrogen. In addition, the XPS component at 401.3 eV is related to the XAS feature at 402.0 eV and has been assigned to sp2 nitrogen bonded to three carbon neighbors.

Surface Functionalization of Nanostructured Porous Silicon by APTS: Toward the Fabrication of Electrical Biosensors of Bacterium Escherichia coli

Nanostructured porous silicon (nanoPS) basically consists in a network of silicon nanocrystals with high specific surface. Its intrinsic high surface reactivity makes nanoPS a very suitable material for the development of biosensors. In this work, the surface of nanoPS was functionalized by the use of (3-aminopropyl)triethoxysilane solutions in toluene. Escherichia coli (E. coli) antibodies were subsequently immobilized on the functionalized surfaces. Finally, fragments of this bacterium, which are specifically recognized by the antibodies, were immobilized. Moreover, devices with a metal/nanoPS/semiconductor/metal structure were fabricated aiming at the electrical biosensing of E. Coli bacterium. The experimental results showed a strong variation of the current as a function of the presence/absence of bacterium E. Coli and surface concentration.

Electronic interaction at the TiO2–Al2O3 interface as observed by X-ray absorption spectroscopy

The electronic structure of the TiO 2 -Al 2 O 3 interface has been studied using X-ray absorption spectroscopy. Special attention has been paid to the early stages of growth, i.e. the sub-monolayer regime (τ < 1). The Ti 2p spectra for coverages below I ML show significant changes with respect to those for large coverages and bulk TiO 2 indicating the presence of interfacial states. The spectra have been compared with atomic multiplet calculations reported in the literature. From this comparison it is concluded that strong electronic interactions occur at the interface, as deduced from the significant lowering of the crystal field of the Ti atoms at the interface (1.0 eV) as compared with bulk TiO 2 (1.8 eV). It is suggested that the important covalent character of the bonding of the Al 2 O 3 substrate is the responsible of this crystal field lowering.

Effects of Ni vacancies and crystallite size on the O 1s and Ni 2p x-ray absorption spectra of nanocrystalline NiO

We have studied the electronic structure of nanocrystalline NiO thin films, grown by radio-frequency magnetron sputtering under different experimental conditions, using x-ray absorption spectroscopy. The O 1s and Ni 2p spectra showed distinct changes as a function of O2 content in the plasma, which were reproduced with cluster model calculations. These changes are attributed to the incrementing of the surface contribution due to a decrease of the crystallite size as the O2 content in the plasma increases, and to the presence of induced nickel vacancies. Thus, the changes in the electronic structure can be related to the modification of structural and transport properties of these nanocrystalline films.

Electronic structure and chemical characterization of ultrathin insulating films

Abstract We report on the use of photoemission to probe the electronic structure of ultrathin films. The electronic structure of Zr 3 N 4 grown by low energy N 2 + implantation in polycrystalline zirconium has been investigated. The insulating characteristics of Zr 3 N 4 as well as the metal–insulator phase transition in ZrN x , when x approaches 1.33, can be nicely observed by photoemission during the implantation process. Other examples correspond to the understanding of oxide/oxide interfaces by in situ characterization of the growth of an oxide film on a dissimilar oxide. We have studied the electronic properties of the TiO 2 /SiO 2 interface by examining the O 2p valence band and the O 2s and Ti 3p core levels during growth of ultrathin films of TiO 2 on SiO 2 . We report evidence for the formation of cross linking oxygen ions to form Si O Ti bonds at the interface. In addition we use the high sensitivity of the Ni 2p XPS core level lineshape to the local co-ordination of the nickel atoms to study the interface NiO/MgO(100) by in situ characterization of the growth of ultrathin NiO films on MgO(100).

Study of the growth of ultrathin films of NiO on Cu(111)

We have studied the early stages of growth of the layers formed by Ni evaporation in an oxygen atmosphere at room temperature on a Cu(111) single crystal. The layers have been analysed by XPS and x-ray absorption spectroscopy as a function of the deposition time. A background analysis of the XPS spectra has also been performed. We have found a Stranski-Krastanov way of growth, i.e. the formation of a metallic Ni monolayer followed by the formation of defective NiO islands. Further deposition leads to the formation of stoichiometric NiO.