Monika Fritzsche

Influence of hydrophobicity on the surface-catalyzed assembly of the islet amyloid polypeptide.

The islet amyloid polypeptide (IAPP) is a hormonal factor secreted by the β-cells in the pancreas. Aggregation of misfolded IAPP molecules and subsequent assembly of amyloid nanofibrils are critical for the development of type 2 diabetes mellitus. In the physiological environment, amyloid aggregation is affected by the presence of interfaces such as cell membranes. The physicochemical properties of the interface dictates the interaction of the peptide with the surface, i.e., electrostatic and hydrophobic interactions on hydrophilic and hydrophobic surfaces, respectively. We have studied the influence of hydrophobicity on the surface-catalyzed assembly of IAPP on ultrasmooth hydrocarbon films grown on ion-beam-modified mica surfaces by atomic force microscopy. The contact angle θ of these surfaces can be tuned continuously in the range from ≤20° to ∼90° by aging the samples without significant changes of the chemical composition or the topography of the surface. On hydrophilic surfaces with a θ of ∼20°, electrostatic interactions induce the assembly of IAPP nanofibrils, whereas aggregation of large (∼2.6 nm) oligomers is observed at hydrophobic surfaces with a θ of ∼90°. At intermediate contact angles, the interplay between electrostatic and hydrophobic substrate interactions dictates the pathway of aggregation with fibrillation getting continuously delayed when the contact angle is increased. In addition, the morphology of the formed protofibrils and mature fibrils at intermediate contact angles differs from those observed at more hydrophilic surfaces. These results might contribute to the understanding of the surface-catalyzed assembly of different amyloid aggregates and may also have implications for the technologically relevant controlled synthesis of amyloid nanofibrils of desired morphology.

Iron-assisted ion beam patterning of Si(001) in the crystalline regime

We present ion beam erosion experiments on Si(001) with simultaneous sputter co-deposition of steel at 660?K. At this temperature, the sample remains within the crystalline regime during ion exposure and pattern formation takes place by phase separation of Si and iron-silicide. After an ion fluence of F???5.9???1021?ions?m?2, investigations by atomic force microscopy and scanning electron microscopy identify sponge, segmented wall and pillar patterns with high aspect ratios and heights of up to 200?nm. Grazing incidence x-ray diffraction and transmission electron microscopy reveal the structures to be composed of polycrystalline iron-silicide. The observed pattern formation is compared to that in the range of 140?440?K under otherwise identical conditions, where a thin amorphous layer forms due to ion bombardment.

Nanohole pattern formation on germanium induced by focused ion beam and broad beam Ga+ irradiation

Hexagonally ordered nanohole patterns were produced on Ge(100) surfaces by focused Ga+ ion beam and broad Ga+ ion beam irradiations with 5 keV energy under normal incidence. Identical patterns were obtained by irradiations with a scanning focused ion beam under different irradiation conditions and with a broad Ga+ beam without scanning and five orders of magnitude smaller ion flux. Thus, we could demonstrate that nanohole pattern formation is independent of ion flux over several orders of magnitude and scanning of a focused ion beam under appropriate conditions is identical to broad ion beam irradiation.

Tuning the hydrophobicity of mica surfaces by hyperthermal Ar ion irradiation.

The hydrophobicity of surfaces has a strong influence on their interactions with biomolecules such as proteins. Therefore, for in vitro studies of bio-surface interactions model surfaces with tailored hydrophobicity are of utmost importance. Here, we present a method for tuning the hydrophobicity of atomically flat mica surfaces by hyperthermal Ar ion irradiation. Due to the sub-100 eV energies, only negligible roughening of the surface is observed at low ion fluences and also the chemical composition of the mica crystal remains almost undisturbed. However, the ion irradiation induces the preferential removal of the outermost layer of K(+) ions from the surface, leading to the exposure of the underlying aluminosilicate sheets which feature a large number of centers for C adsorption. The irradiated surface thus exhibits an enhanced chemical reactivity toward hydrocarbons, resulting in the adsorption of a thin hydrocarbon film from the environment. Aging these surfaces under ambient conditions leads to a continuous increase of their contact angle until a fully hydrophobic surface with a contact angle >80° is obtained after a period of about 3 months. This method thus enables the fabrication of ultrasmooth biological model surfaces with precisely tailored hydrophobicity.

Direct observation of antiferromagnetically oriented spin vortex states in magnetic multilayer elements

We report on the coupling of spin vortices in magnetic multilayer elements. The magnetization distribution in thin film disks consisting of two ferromagnetic layers separated by a nonmagnetic spacer is imaged layer-resolved by using x-ray microscopy. We directly observe two fundamentally different vortex coupling states, namely antiferromagnetic and ferromagnetic orientation of the flux directions. It is found that these states are predetermined for systems that involve a sufficiently strong interlayer exchange coupling, whereas for the case of a purely dipolar interaction both states are transformable into each other.

Persistent Current Reduction in Metal-Semiconductor FETs With a ZnCoO Channel in an External Magnetic Field

Transparent metal-semiconductor field-effect transistors (MESFETs) with a ZnCoO channel have been fabricated by pulsed laser deposition on c-plane sapphire substrates at a temperature of 550°C. The paramagnetic properties have been confirmed by magnetotransport measurements on undepleted ZnCoO films without Schottky gate contacts. The Au/AgxO Schottky gate contacts were processed by optical lithography and metallization. Below 50 K, the MESFET characteristics are persistently changed from a low resistance state (LRS) to high resistance state by an external magnetic field. The MESFET can be switched back into the LRS only by heating it up to room temperature.

III-V/Si on silicon-on-insulator platform for hybrid nanoelectronics

The unique properties of SOI wafers enable the integration of heterogeneous materials with distinct functionalities in different layers. In particular, III-V compound semiconductors are very attractive for low-noise and high-speed electronic and photonic components integrated on a single chip. We have developed a CMOS compatible and fully integrated solution for the integration of III-V compound semiconductors with silicon technology for optoelectronic applications. InAs compound semiconductor nanostructures are synthesized in SOI wafers using the combined ion beam implantation and millisecond liquid-phase epitaxial growth. Optoelectronic and microstructural investigations carried out on implanted, annealed, and selectively etched samples confirm the formation of high-quality III-V compound semiconductor nanostructures.