S. Zuber

Interaction of hydrogen with ultrathin titanium layers on tungsten

Abstract Hydrogen adsorption on titanium layers of an average thickness from 0.03 to 50 monolayers deposited on a tungsten substrate was investigated at liquid nitrogen temperature using the field electron emission microscopy techniques. It has been found that the interaction between titanium and hydrogen depends on the thickness and atomic structure of the Ti/W layer. For tightly arranged titanium layers isomorphous with the (011)W face the H–Ti interaction is weak and does not change the morphology and atomic structure of the layer. In the case of titanium layers isomorphous with the (001)W the H–Ti interaction dominates over the Ti–W interaction, beginning from the thinnest titanium layers deposited on the tungsten substrate. This results in a 3-dimensional nucleation of TiHx at the steps of the terraces of the (001)W face. For the titanium layers isomorphous with the (111)W and (112)W faces, the Ti–W interaction is stronger than the H–Ti interaction up to the thickness of 3 to 4 monolayers of titanium. Measurements of the average work function changes as a function of the H2 exposure indicate that, apart from the hydrogen adsorption state which raises the work function, there exists another one which decreases the work function. The latter state occurs in titanium layers isomorphous with the (111)W face for the coverage in the range from 0.65 to 2.7 titanium monolayers. Measurements of the local work function changes carried out for H2 adsorption on Ti/(111)W and Ti/(016)W indicate that this adsorption state cannot be recognized as the unstable β+ adsorption state observed during the hydrogen adsorption on a surface of bulk titanium [R. Duś, E. Nowicka, Z. Wolfram, Surf. Sci. 269/270 (1992) 545]. The one observed in this experiment is stable and is believed to result from the immersion of the adsorbed hydrogen atoms into the electron gas of the Ti–W interface.

Pd/GaN(0001) interface properties

This report concerns the properties of an interface formed between Pd films deposited onto the surface of (0001)-oriented n-type GaN at room temperature (RT) under ultrahigh vacuum. The surface of clean substrate and the stages of Pd-film growth were characterized in situ by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), ultraviolet photoelectron spectroscopy (UPS), and low energy electron diffraction (LEED).As-deposited Pd films are grainy, cover the substrate surface uniformly and reproduce its topography. Electron affinity of the clean n-GaN surface amounts to 3.1 eV. The work function of the Pd-film is equal to 5.3 eV. No chemical interaction has been found at the Pd/GaN interface formed at RT. The Schottky barrier height of the Pd/GaN contact is equal to 1.60 eV.

EELS investigation of Pd thin film growth on Nb substrate

Abstract Using multi-component catalysts offers the possibility to optimize the individual reaction steps. In the last decades, great attention has been focused on bimetallic systems, representing the next step from monometallic catalysts to multi-component ones. However, this approach requires detailed knowledge of both the influence of individual components and their mutual cooperation. In the study of these factors it is advantageous to use ultra-high vacuum conditions where well-defined-metal surfaces, so-called model catalysts, can be prepared and characterized by modern surface-science techniques. The Pd/Nb model catalysts were prepared by depositing a small amount of palladium on polycrystalline niobium substrates. The film growth was investigated via variations of intensity ratio of excited Pd and Nb plasmons. It has been found that Pd-Nb interface was formed by a continuous Pd layer growing on Nb substrate.

STM observation of steps and terraces on tungsten (2 1 1) surface

Thermally cleaned W(211) surface with 0.7 degrees miscut consists of (211) terraces separated by monoatomic steps. When the surface is exposed to oxygen and subsequently annealed at 1100-1900 K, the width of (211) terraces increases and multilayer steps are formed. Similar step bunching is observed during routine cleaning of the sample by annealing in oxygen and thermal flashing in ultra high vacuum. During such cleaning procedure islands of c(6 x 4) reconstruction are observed.

Temperature dependence of adsorbate induced work function changes of CuW(110) system — a Monte Carlo study

Abstract Temperature dependence of work function change 〈Δϑ〉 caused by adsorption of Cu on W(110) at very low coverages is examined. The phenomenon reported by Kolaczkiewicz and Bauer is explained by mutual depolarisation of nearest neighbours during formation of two-dimensional islands in the adlayer. It is assumed that Cu adatoms are polarisable and neutral. The work function change is taken to be equal to dipole potential barrier produced by the induced dipole moments of adatoms. Changes in arrangement of adatoms accompanying the formation of the islands are modelled by Monte Carlo simulations, using the continuous space model of submonolayer with a realistic intermodular potential given by Gollisch.

Preparation of TiO 2 thin films by reactive evaporation method

Thin films of TiO2 were prepared on glass substrates by reactive evaporation of TiO. Properties of the thin films were investigated under ultrahigh vacuum conditions by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). AFM topography exhibits a uniformity structure of the thin films and shows a high quality nanocrystalline structure of TiO2 with grain size ranging from 28 nm to 35nm. The XPS Ti-2p spectra confirmed the occurrence of the 4+oxidation state of Ti and the O-1s spectra revealed the presence of an O−2 photoelectron peak. The calculated O/Ti ratio in TiO2 thin films was in the range 1.85–2.15 depending on the samples. Additional analysis of the thin films was carried out by X-ray powder diffraction (XRD).