Oxides formed by electrochemical treatment of metals frequently have properties and structures very different from the respective bulk oxides. Here, electronic structure and chemical composition were investigated for the oxide formed on polycrystalline zinc after electrochemical oxidation, and after subsequent reduction, in a Na2CO3 electrolyte. Photoluminescence and spectroscopic ellipsometry show the presence of states deep in the ZnO band gap in the oxidized sample, which consists of a highly disordered oxide. These states determine the absorption of light in the visible spectral range. After reduction, the characteristics of the ZnO electronic structure have disappeared, leaving a defect-dominated material with a band gap of ~1.8 eV. Complementary detailed analysis of the morphology of the resulting surfaces shows hexagon-shaped metallic Zn-"nanoplates" to be formed in the reduction step. The optical appearance of the surfaces is dark, because of their efficient extinction of light over a large part of the visible spectrum. The optical appearance is a result of changed surface morphology and electronic structure of the oxide film. Such materials may possess interesting applications in photocatalysis or photoelectrochemistry.

The native oxide layer formed spontaneously on the surface of metallic Zn was investigated by in situ spectroscopic ellipsometry (SE) and ex situ X-ray photoelectron spectroscopy (XPS). XPS analysis shows the coexistence of hydroxide and oxide in the oxide film. The growth kinetics of the oxide films in different atmospheres was measured in a specially designed optical cell. The growth in O 2 , Ar and air at high (>95%) and low (<10%) relative humidity was monitored for up to 72 h. Differences in the growth kinetics of the films are observed in different atmospheres. In argon, growth in layer thickness is limited. In oxygen, one obtains a growth with a fast initial phase followed by a linear second phase with a growth rate of %0.01 nm h À1 . The electronic structure of the layer changes rapidly at the beginning of exposure of the surface to the respective atmosphere, with no further changes observed during the experiment.

No abstract

The transient temperature evolution of ultrathin bismuth films, epitaxially grown on a silicon single crystal, upon femtosecond laser excitation is studied by time-resolved electron diffraction. The exponential decay of the film temperature is explained by phonon reflection at the interface, which results in a strongly reduced thermal conduction in the cross plane of the layered system. The thermal boundary conductance is found to be as low as 1273 W / ͑K cm 2 ͒. Model calculations, including phonon confinement effects, explain the linear relationship between the observed film-temperature decay constant and the film thickness. Even for 2.5 nm thin films the phonon transmission probability across the interface is given by bulk properties. Our simulations show that phonon confinement effects are negligible for bismuth-film thicknesses larger than 1 nm.

No abstract

Phosphating is a crucial process in the corrosion protection of metals. Here, activation and fluoride-assisted tricationic phosphating is investigated on aluminum-silicon (AS) coated steel surfaces. Dynamic light scattering results from the activation bath show a bimodal size distribution, with hydrodynamic radii of ~400 nm and ~10 μm. For the smaller particle fraction, static light scattering results are consistent with the interpretation of disklike particles as scattering objects. Particles of the larger fraction sediment with time. In the presence of electrolyte, the scattering intensity from the larger particle fraction increases. Coagulation with time is suggested to be related to the decrease in activity of the activation bath. Scanning Auger microscopy (SAM) shows a higher phosphorus concentration after titanium phosphate activation in the Al-rich areas compared to the Si-rich areas of the AS coatings. There is no correlation between the size of the species in the activation bath, and the size of the phosphate-containing regions on the activated surface. Phosphating was performed in the presence of hexafluorosilicic acid, H2SiF6, ammonium hydrogen difluoride, NH4HF2, and both, at an initial pH of 2.5. The absence of crystals after phosphating with H2SiF6 is an indication that SiF6(2-) is the final product of the oxide dissolution in the presence of fluoride. In the presence of NH4HF2, the Si-rich regions of the surface are phosphated before the Si-poor (Al-rich) regions. Hence, the phosphate distribution after activation and after phosphating are opposite. These results show that a high surface concentration of phosphate after activation is not sufficient for a high coverage with phosphate crystals after phosphating.

A focused ion beam system working with ions has been used to produce interconnects by means of ion beam synthesis. The capability of maskless patterning of the focused ion beam, its large depth of focus and the possibility of a dynamic focus control allow the fabrication of interconnects in three-dimensional devices with this method. Investigations have been performed using polycrystalline, amorphous and single-crystalline silicon substrates. The influence of implantation dose has been studied, the electrical resistance and thermal stability of the interconnects has been measured. For room temperature implantation and annealing at for 1 h resistivities between 34 and have been obtained for the various substrate materials. The interconnects have been found to be thermally stable up to . In order to demonstrate this method interconnection lines have been fabricated on the sloped walls of deep anisotropically etched grooves .

No abstract