Horst Niehus

Determination of surface reconstruction with impact-collision alkali ion scattering

Abstract The method of low energy ion scattering in the impact collision mode (ICISS) has been substantially improved by the use of alkali ions. Alkali ions extend the capabilities of ICISS towards the relative positioning of atoms lying in the first and deeper layers, which is a premise for the characterization of surface reconstruction. The self calibration procedure based on the experimental determination of an effective shadow cone is an important advantage. The small contribution of multiple scattering effects in ICISS allows a straightforward determination of the locations of surface atoms. On the basis of a large number of ICISS data the oxygen induced surface reconstruction of Cu(110) has been shown to be correctly described by the missing row model.

Atomic Force Microscopy Studies of Solid Lipid Nanoparticles

AbstractPurpose. Solid Lipid Nanoparticles (SLN) are an alternative carrier system for the controlled delivery of drugs. In most cases prednisolone loaded SLN show a biphasic release behaviour. The initial phase is characterised by a fast drug release, which is followed by a sustained drug release over several weeks. Methods. The particles are produced by high pressure homogenisation of a lipid (e.g. compritol, cholesterol) dispersed in an aqueous surfactant solution. In this study atomic force microscopy was used to image the original unaltered shape and surface properties of the particles. The crystallinity of the nanoparticles was investigated by differential scanning calorimetry. Results. The AFM investigations revealed the disc like shape of the particles. From differential scanning calorimetry data it can be concluded that the particle core is in the crystalline state. Additionally it was proven that the particles are surrounded by a soft layer. Conclusions. Thus it is conceivable that the fast initial drug release during in vitro dissolution tests takes place by drug release of the outer non-crystalline layers of the particles. The following sustained drug release can be assigned to the predisolone release of the inner crystalline particle layers.

Bulk dissolved Si as a cause of the “oxide” formation on Pt(111) surfaces

Abstract The influence of silicon atoms dissolved in the bulk of a Pt(111) crystal on the high temperature “oxidation” of Pt was studied by Auger spectrometry (AES) and low energy ion scattering (IS). The “oxidation” procedure was applied on clean and Si-prepared crystals. The preparation was performed by depositing a Si layer on the Pt(111) surface and thermally diffusing it into the bulk beyond the AES detectability. The results show that even deeply diffused silicon plays a decisive part in the “oxidation” process. This seems to explain the observations of other authors concerning the unexpected high stability of the “oxide” and the poor reproducibility of its rate of formation as well as of its saturation density. There is evidence that the “oxide” is primarily a silicon-oxygen bond.

Surface and subsurface oxygen adsorbed on Pt(111)

Low energy ion scattering (IS) was used to locate the oxygen adsorbed on Pt(111) in the “chemisorbed” and in the “oxide” state. The IS spectra show that while the “chemisorbed” oxygen is located in the topmost surface layer, this layer consists only of Pt atoms in the “oxide” case.

Dynamic plowing nanolithography on polymethylmethacrylate using an atomic force microscope

We present dynamic plowing nanolithography on polymethylmethacrylate films, performed with a scan-linearized atomic force microscope able to scan up to 250 μm with high resolution. Modifications of the surface are obtained by plastically indenting the film surface with a vibrating tip. By changing the oscillation amplitude of the cantilever, i.e., the indentation depth, surfaces can be either imaged or modified. A program devoted to the control of the scanning process is also presented. The software basically converts the gray scale of pixel images into voltages used to control the dither piezo driving cantilever oscillations. The advantages of our experimental setup and the dependence of lithography efficiency on scanning parameters are discussed. Some insights into the process of surface modifications are presented.

Growth of Al2O3 stripes in NiA(001)

Abstract Structure and growth mechanism of ultra-thin Al 2 O 3 films on NiAl(001) have been investigated with 180°-neutral impact collision ion scattering spectroscopy, high resolution spot profile analysis of low energy electron diffraction, and scanning tunnelling microscopy (STM). Ordered Al 2 O 3 films have been prepared by oxygen exposure at room temperature and subsequent annealing at T = 1200 K. The formation of epitaxial oxide films is unaffected by the initial composition or reconstruction of the substrate. The oxidized surface being covered by Al 2 O 3 appears to be microscopically rough, reversible stepped with step heights of about 3 A and consists of a network of elongated equally distributed oxide stripes along 〈100〉 and 〈010〉 directions of the NiAl(001) surface. The lateral anisotropy of the oxide is probably caused by the build up of internal stress in the growth process. After oxygen saturation exposure at room temperature and subsequent annealing, regions with rather regular arranged parallel oxide stripes show up in the STM images (mean width ≈ 27 A , period ≈ 54 A ). The corresponding low energy electron diffraction pattern shows the existence of orthogonal (2 × 1) antiphase domains with a (9 × 1) superstructure (period 26.02 A) close to the mean width of oxide stripes. Next to the oxide stripes, regrowing NiAl terraces and thin layers of amorphous Al 2 O 3 have been found.

Formation of subsurface oxygen at Ru(0001)

The conditions for formation of subsurface oxygen on the Ru(0001) surface have been studied using thermal desorption spectroscopy, low energy electron diffraction (LEED) and specular helium scattering. The incorporation of oxygen has been performed via dissociative chemisorption of three molecular carriers of atomic oxygen: NO2, O2 and N2O. The rates for oxygen dissolution can be related to the initial dissociative sticking probability of the molecules on the bare Ru surface. For sample temperatures below 800 K, oxygen penetration into the subsurface region starts only when oxygen molecules impinge on the saturated adsorbed layer characterized by the 1×1 O LEED pattern, indicative of 1 monolayer. A thermally induced transformation of this chemisorbed 1×1 O phase into subsurface oxygen could not be caused even at temperatures close to the onset of oxygen desorption. Oxygen incorporation into the subsurface region by passing through the 1×1 O adsorbed layer, however, shows Arrhenius-type behavior. For impin...

Ion scattering spectroscopy in the impact collision mode (ICISS): Surface structure information from noble gas and alkali-ion scattering

Abstract Low energy (2000 eV) ion scattering spectroscopy in the impact collision mode has been carried out in three modes: primary noble gas ions and detection of ions only (usual ICISS), detection of both ions and neutrals (NICISS) and use of primary alkali ions (ALICISS). The three modes have been compared by in situ measurements at a Pt(111) surface. Usual ICISS suffers from the high neutralization probability of the ions during scattering. This has a hardly predictable influence on the scattering patterns and limits the sensitivity to the first layer. In addition it asks for large primary ion doses which impair the state of the surface. In the case of ALICISS the neutralization is substantially reduced, while in NICISS it plays no role. These two ICISS modes supply a picture of the relative positions of the atom cores in real space making them ideally suited for the determination of surface reconstruction or relaxation.

Characterization of oxygen phases created during oxidation of Ru(0001)

Thermal desorption spectroscopy, ultraviolet photoelectron spectroscopy, low energy electron diffraction (LEED), and the reactive scattering of a CO molecular beam have been applied to determine the relationship between the formation of the subsurface oxygen phase and the growth of oxides during oxidation of Ru(0001). Emission of RuOx (x<4) molecules observed in the thermal desorption spectra during the heating of the oxygen-rich sample has been used as a simple measure for the presence of bulk oxides. When performing the oxygen exposure at a temperature lower than the onset for oxygen desorption (Tp<850 K) a mobile atomic oxygen species is predominantly formed in the subsurface region. The conversion of these subsurface oxygen atoms into a regular RuxOy phase takes place within the temperature region of 900–1150 K. The growth of oxide films becomes the dominating reaction channel when performing the oxidation at temperatures higher than the onset for oxygen desorption. The oxide formation is strongly red...

Spectral and spatial anisotropy of the oxide growth onRu(0001)

Scanning photoelectron spectromicroscopy has been used to study the onset and the initial stages of oxidation of Ru(0001) at three oxidation temperatures, 625, 675, and 775 K, and oxygen exposures of about 105 Langmuir. The lateral heterogeneity developed during oxide nucleation and growth and the local composition of the coexisting phases have been determined using as fingerprints the O 1s and Ru 3d spectra, thus combining chemical mapping with spectroscopy from selected features from the maps. The onset of oxide formation is characterized by the appearance of randomly distributed small islands (⩾0.5 μm) identified as germinal patches exhibiting some spectral features of bulk RuO2. The following anisotropic growth of the RuO2 phase and in particular the shape of the oxide islands shows a strong dependence on the oxidation temperature. The spectroscopic information obtained for the areas surrounding the oxide islands reveals an intermediate oxygen state characterized by distinct O 1s and Ru 3d features di...