Elisa Román

Synchrotron radiation studies of H2O adsorption on TiO2(110)

Synchrotron radiation photoemission has been used to study the interaction of H2O with defective and nearly-perfect TiO2(110) surfaces at temperatures between 160 and 300 K. Ti3+ 3d defect sites are implicated in the adsorption process, and by tuning the photon energy to 47 eV we find that a resonant photoernission process gives an enhanced photoermission sensitivity to the 3d defect states. Defects are produced on TiO2(110) by annealing to 1000 K in UHV; subsequent exposure to 104 L O2 produces nearly perfect surfaces, based on the suppressed Ti3d emission. Both nearly perfect and defective surfaces give rise to dissociative adsorption of H2O at 300 K. The saturation coverages are near 0.1 ML, independent of the initial defect concentration; however, the rate of dissociative adsorption (sticking probability) is higher on defective surfaces. The enhanced sensitivity to the Ti3+ defect states has allowed the observation of a surprising effect; the dissociative adsorption of H2O results in increased defect state intensity on the nearly perfect surfaces. This apparent charge-transfer to the substrate implies that a new model for the dissociation process on oxide surfaces is needed. At 160 K H2O adsorbs molecularly on both the nearly-perfect and the defective surfaces. Subsequent annealing experiments allow estimates of the interaction energies involved in the dissociation process.

A synchrotron radiation study of BaO films on W(001) and their interaction with H2O, CO2, and O2

The interaction of O2, CO2, and H2O with bulk BaO and BaO adlayers adsorbed on W(001) has been examined using ultraviolet photoelectron spectroscopy. H2O reacts with bulk BaO to form Ba(OH)2, while CO2 forms a surface layer of BaCO3. Water and carbon dioxide also react with a (√2×2)R45–BaO monolayer adsorbed on W(001) to produce adsorbed OH and CO3 species bound to the tungsten substrate. The interaction of O2 with W(001) is enhanced by the presence of a BaO monolayer on the substrate. The observations are compared with the results of previous studies.

Photoemission and HREELS study of K adsorption on TiO2(100)

The adsorption of K and coadsorption of O2 on TiO2(100) have been studied using photoemission and high resolution electron energy-loss spectroscopy (HREELS). We compare our results with those of earlier studies of potassium oxides. The energy separations of peaks appearing in photoemission spectra indicate the formation of a peroxide or superoxide at the surface following adsorption of > 1 monolayer of potassium. HREELS results are consistent with this conclusion.

Ionic implantation of N2+ in Ni(110) at 300 K

Abstract The present work gives results of a preliminary investigation, carried out by SES, AES, LEED and ELS, on the implantation of nitrogen ions in Ni(110) as a function of ion dose and subsequent surface heat treatment at different temperatures. The fine structure in the SES spectrum is the most sensitive to implantation: peaks at 9, 17.5 and 31.5 eV are shifted towards lower energies by E = 1 eV for the first two peaks and 2.8 eV for the last. At high nitrogen doses a disordered layer is observed by LEED. The p(2 × 3) structure is obtained when the crystal is heated to 750 K. The two electron loss peaks of 4.8 and 10 eV arise from an induced electron N 2p level situated 4.8 eV below the Fermi level.

UPS synchrotron radiation studies of NH3 adsorption on TiO2(110)

Abstract The interaction of NH 3 with perfect and defective TiO 2 (110) surfaces at 300 and 185 K has been studied using synchrotron radiation photoemission. To increase the sensitivity to Ti 3+ states, the Ti p-d resonance at an incident photon energy of 47 eV has been used. Ammonia adsorbs mainly molecularly at low coverages at 300 K, with some amounts of NH 2 and OH radicals. Upon annealing to high T both desorption and dissociation occurs. A maximum work function variation of −0.57 and −0.38 eV has been found for the perfect and defective surfaces, respectively. No significant differences in the electronic structure between adsorption at the two temperatures have been found, except for a higher coverage on the perfect surface.

Model of the effect of local frictional heating on the tribodesorbed gases from metals in ultra-high vacuum

In this paper, a model of thermal desorption of gases from the friction zone of metals in ultra-high vacuum is presented. Comparison of the model with the experimental data of tribodesorbed gases from AISI 304 steel shows that for mild friction the amount of desorbed gases owing to the local temperature increase is much lower than the experimentally obtained values. This finding supports the hypothesis that structural modification of material is the main driving process for tribodesorption. The study also shows that a significant volume of material around the friction zone contributes to the tribodesorption process.

Carbon based coatings for high temperature cutting tool applications

A new generation of carbon based composite coatings, with Cr and Ti as alloying elements and additions of B and/or N, was deposited by the unbalanced magnetron sputtering technique. The coatings were designed to produce a duplex layer with a hard coating of CrTiBCN as the intermediate layer and a carbon based tribological (solid lubricant) hard coating of C/CrTiB as the top layer in one deposition process. Chemical composition, physical properties and tribological behaviour of both the intermediate and the combined duplex layers were investigated for a wide range of temperatures (up to 600 °C). Hobs coated with these films were tested for cutting gears, using dry machining conditions. The test results under real conditions in production showed that this type of coating on hobs outperformed commercial TiN coatings.

Substrate/layer interface of amorphous-carbon hard coatings

A combined study of the crystalline structure, the chemical interaction, and diffusion processes of the substrate/layer interface of amorphous-carbon hard coatings is presented. The structure of the coatings and their gradient layer interface to a chromium buffer layer has been investigated on two substrates [Si(100) and tool steel] using x-ray diffraction (XRD). Chemical interaction and diffusion processes at the interfaces and within the layers were analyzed by Auger electron spectroscopy and x-ray photoemission spectroscopy depth profiles. The chromium buffer layer revealed similar textured structure on both substrates. The subsequent gradient layer was determined (within XRD limits) to be amorphous and composed of an amorphous-carbon and chromium-carbide composite. The chromium carbide maintains the same stoichiometry (Cr3C2), regardless of the gradually changing chromium content. No large-scale interdiffusion was measured, either between or within the layers.

Understanding the role of thiol and disulfide self-assembled DNA receptor monolayers for biosensing applications

A detailed study of the immobilization of three differently sulfur-modified DNA receptors for biosensing applications is presented. The three receptors are DNA-(CH)n-SH-, DNA-(CH)n-SS-(CH)n-DNA, and DNA-(CH)n-SS-DMTO. Nanomechanical and surface plasmon resonance biosensors and fluorescence and radiolabelling techniques were used for the experimental evaluation. The results highlight the critical role of sulfur linker type in DNA self-assembly, affecting the kinetic adsorption and spatial distribution of DNA chains within the monolayer and the extent of chemisorption and physisorption. A spacer (mercaptohexanol, MCH) is used to evaluate the relative efficiencies of chemisorption of the three receptors by analysing the extent to which MCH can remove physisorbed molecules from each type of monolayer. It is demonstrated that –SH derivatization is the most suitable for biosensing purposes as it results in densely packed monolayers with the lowest ratio of physisorbed probes.

Core-shell nanocrystalline structures in oxidized iron thin films prepared by sputtering at very low temperatures

We report on the effect of preparation temperature in the magnetic properties of oxidized iron thin films deposited by dc-magnetron sputtering below room temperature. Films prepared at 300 K show a typical thin film magnetic behavior, whereas samples prepared at 200 K present visible features of granular core-shell system formed by an oxide shell surrounding a ferromagnetic core and displaying exchange anisotropy. These differences are directly linked to the film microstructure and composition. We present results of microstructure, composition, and chemical analysis in order to discuss the observed magnetic behavior. Size and shape of iron crystallographic grains were characterized by transmission electron microscopy. Depth-resolved compositional characterization, obtained by ion-beam analysis techniques, has probed the penetration of oxygen along the thickness of the films and x-ray photoelectron spectroscopy was used to identify the different iron oxide species present in the structures.