W. Maus-Friedrichs

Study of the surface electronic structure of MgO bulk crystals and thin films

The electronic structures of the surfaces of MgO single crystals, oxidized Mg polycrystals and oxidized Mg films grown by molecular beam epitaxy on Si(100) surfaces were studied using several techniques. These include metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (He I)), and X-ray photoelectron spectroscopy (XPS). Spectra of oxidized Mg layers on Si(100) show additional features to those obtained for cleaved MgO crystals. These spectral features are attributed to dissociative adsorption of oxygen at bulk oxygen sites. Weak heating of the oxidized Mg layers removes these features and the electronic spectra for all three studied systems become similar. However, the experimental MIES and UPS spectra, both arising mainly from the ionization of the O 2p orbitals, have different structures. They are interpreted on the basis of ab initio Hartree-Fock and density functional calculations of the electronic structures of the ideal MgO(100) surface. It is shown, that the differences in the spectra can be understood by taking into account that UPS spectra reflect the density of electronic states within several surface layers, whereas MIES probes the surface states which are the most extended into the vacuum.

Coadsorption of Cs and hydrogen on W(110) studied by metastable impact electron spectroscopy

Abstract The adsorption of hydrogen and Cs alone as well as the coadsorption of Cs and hydrogen on W(110) was studied by metastable impact electron spectroscopy (MIES) and supplemented by UPS, AES, and work function measurements. The main conclusions are: (a) ionization of the Cs(6s) level is not observed for Cs coverages θ Cs ⩽ 0.4 ML, (b) the formation of a H(1S)-metal bond occurs at a binding energy of E B = 4.7 eV , (c) upon Cs and hydrogen coadsorption an independent layer of hydrogen is formed between the substrate and the Cs adlayer. The charge density of the Cs adlayer is lowered by about 25% upon the formation of the hydrogen intermediate layer.

Sr diffusion in undoped and La-doped SrTiO3 single crystals under oxidizing conditions

Strontium titanate SrTiO3(100), (110), and (111) single crystals, undoped or donor doped with up to 1 at% La, were isothermally equilibrated at temperatures between 1523 and 1773 K in synthetic air followed by two different methods of Sr tracer deposition: ion implantation of 87Sr and chemical solution deposition of a thin 86SrTiO3 layer. Subsequently, the samples were diffusion annealed under the same conditions as before. The initial and final depth profiles were measured by SIMS. For strong La-doping both tracer deposition methods yield similar Sr diffusion coefficients, whereas for weak doping the tracer seems to be immobile in the case of ion implantation. The Sr diffusivity does not depend on the crystal orientation, but shows strong dependency on the dopant concentration supporting the defect chemical model that under oxidizing conditions the donor is compensated by Sr vacancies. A comparison with literature data on Sr vacancy, Ti, and La diffusion in this system confirms the concept that all cations move via Sr vacancies. Cation diffusion is several orders of magnitude slower than oxygen diffusion.

CO2 chemisorption at Mg and MgO surfaces: a study with MIES and UPS (He I)

Abstract The interaction of CO2 with Mg and MgO films (on Si substrates) is studied with MIES in conjunction with UPS (He I) at room temperature. On Mg surfaces the presence of a carbonate (CO2−3) species is detected on top of an oxide layer. On MgO surfaces chemisorption does not take place at regular sites, but presumably at low-coordinated O2− ions as found at step sites. Again a CO2−3 species can be identified with MIES. Coadsorbed alkali atoms (Li) increase the rate of carbonate formation on MgO by about a factor of three at room temperature.

Alkali-metal-affected adsorption of CO on W(110) studied by metastable impact electron spectroscopy

Abstract The adsorption of CO on clean and alkali-covered W(110) is studied by Metastable Impact Electron Spectroscopy (MIES) and UPS. Moleculary adsorbed CO is not observed for adsorption on clean W(110) neither by MIES nor by UPS at room temperature, but its existence cannot be excluded by the present results. From a comparison with data for the coadsorption of oxygen and K it is concluded that for K precoverages up to 0.8 ML features from the molecular adsorption of CO are seen in the MIE spectra. K precoverages beyond 0.8 ML, however, show spectral features corresponding to oxygen adsorption only. The exposure of a CO saturated W(110) surface to various amounts of K confirms that the complete disappearance of CO induced molecular spectral features occurs around 0.8 ML K coverage, and is mediated by a direct transfer of the K(4s) electron to the CO molecule.

Excitation of Li(2p) in slow collisions of Li+ ions with cesiated W(110) surfaces

The excitation of the Li(2p) state in low-energy collisions of Li+ ions with low work function surfaces is studied for impact energies below 1.0 keV. The yields of excited atoms and electrons are measured for the scattering from cesiated and oxidized cesiated W(110) surfaces characterized by AES, LEED, and Δφ. It is concluded that Auger deexcitation of the 2p state populated by resonant charge transfer between the Li+ projectile and the partially cesiated surface strongly influences the photon yield. For the oxidized surface it is shown that resonant electron exchange between the solid and the Li(2p) state is not the mechanism for projectile excitation. It is proposed that projectile excitation is caused by a direct transition between the 2s and 2p states of the Li+ projectile which is neutralized on its way towards the surface.

Electron spectroscopy on plasma treated lignin and cellulose

Abstract Cellobiose and lignin were plasma treated in synthetic air and argon using a dielectric barrier discharge at atmospheric pressure. Changes due to the plasma modification of the surfaces were studied by the techniques of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, and metastable impact electron spectroscopy. The combination of these techniques makes it possible to distinguish between hydroxyl and carbonyl groups at the surface, on the one hand, and conjugated and non-conjugated carbon bonds, on the other hand. This type of differentiation would not be easily possible based on XPS alone, even at very high resolution. The plasma treatment in oxygen containing atmospheres oxidizes the lignin surface by the generation of hydroxyl, carbonyl, and carboxyl groups and reduces cellulose surfaces by the degradation of hydroxyl groups and the formation of double bonds between carbon and oxygen. The plasma treatment in argon leads to the reduction of both lignin and cellulose by the formation of double bonds under degradation of hydroxyl groups.

Alkali adsorption on W(110) studied by metastable impact electron spectroscopy

The adsorption of alkali atoms (Li, Na, K, Cs) on W(110) at room temperature has been studied by Metastable Impact Electron Spectroscopy (MIES) and UPS as a function of the alkali coverage. In the coverage range up to 0.5 ML (in units of the saturation coverage at room temperature) emission via the Auger de-excitation process involving substrate electrons is observed. Emission which can be associated with the alkali s-levels can be identified only at alkali coverages exceeding 0.4 ML. This conclusion is corroborated by results obtained for the W(110) surface exposed to NaCl and by the oxidation of alkali layers. An explanation is offered for the fact that the interaction of the He(21S) and He(23S) metastables with the surface is different. An attempt is made to analyse and interpret the line shapes of the alkali induced features.

Surface electronic structure of pure and oxidized non-epitaxial Mg2Si layers on Si(111)

Abstract Non-epitaxial magnesium silicide (Mg 2 Si) films of 100 A thickness were grown on Si(111). The formation of Mg 2 Si is identified by characteristic shifts of the Mg 2p and Si 2p peaks in X-ray photoelectron spectroscopy (XPS). Information on the electronic structure of this surface is obtained by metastable impact electron spectroscopy (MIES). The surface density of states obtained with MIES is dominated by strong emission below the Fermi level ( E F ) displaying a characteristic double peak structure. The oxidation of these silicide surfaces is performed at room temperature. The electronic structure of these surfaces is investigated with MIES, UPS (HeI) and XPS. From the comparison with oxidized Mg films it is concluded, that the surface is terminated by an insulating MgO layer. Subsurface oxidation of the silicide does not take place. Furthermore, no formation of silicon oxides is observed.

Electron spectroscopic analysis of the human lipid skin barrier: cold atmospheric plasma-induced changes in lipid composition.

The lipids of the stratum corneum comprise the most important components of the skin barrier. In patients with ichthyoses or atopic dermatitis, the composition of the skin barrier lipids is disturbed resulting in dry, scaly, itching erythematous skin. Using the latest X‐Ray Photoelectron Spectroscopy (XPS) technology, we investigated the physiological skin lipid composition of human skin and the effects of cold atmospheric plasma treatment on the lipid composition. Skin lipids were stripped off forearms of six healthy volunteers using the cyanoacrylate glue technique, plasma treated or not and then subjected to detailed XPS analysis. We found that the human lipid skin barrier consisted of 84.4% carbon (+1.3 SEM%), 10.8% oxygen (+1.0 SEM%) and 4.8% nitrogen (+0.3 SEM%). The composition of physiological skin lipids was not different in males and females. Plasma treatment resulted in significant changes in skin barrier lipid stoichiometry. The total carbon amount was reduced to 76.7%, and the oxygen amount increased to 16.5%. There was also a slight increase in nitrogen to 6.8%. These changes could be attributed to reduced C‐C bonds and increased C‐O, C=O, C‐N and N‐C‐O bonds. The moderate increase in nitrogen was caused by an increase in C‐N and N‐C‐O bonds. Our results show for the first time that plasma treatment leads to considerable changes in the human skin lipid barrier. Our proof of principle investigations established the technical means to analyse, if plasma‐induced skin lipid barrier changes may be beneficial in the treatment of ichthyotic or eczematous skin.