Adrian Keller

Ion-Induced Nanoscale Ripple Patterns on Si Surfaces: Theory and Experiment

Nanopatterning of solid surfaces by low-energy ion bombardment has received considerable interest in recent years. This interest was partially motivated by promising applications of nanopatterned substrates in the production of functional surfaces. Especially nanoscale ripple patterns on Si surfaces have attracted attention both from a fundamental and an application related point of view. This paper summarizes the theoretical basics of ion-induced pattern formation and compares the predictions of various continuum models to experimental observations with special emphasis on the morphology development of Si surfaces during sub-keV ion sputtering.

Introducing artificial length scales to tailor magnetic properties

Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25–175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (≈60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 μm. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.

Probing Electron-Induced Bond Cleavage at the Single-Molecule Level Using DNA Origami Templates

Low-energy electrons (LEEs) play an important role in nanolithography, atmospheric chemistry, and DNA radiation damage. Previously, the cleavage of specific chemical bonds triggered by LEEs has been demonstrated in a variety of small organic molecules such as halogenated benzenes and DNA nucleobases. Here we present a strategy that allows for the first time to visualize the electron-induced dissociation of single chemical bonds within complex, but well-defined self-assembled DNA nanostructures. We employ atomic force microscopy to image and quantify LEE-induced bond dissociations within specifically designed oligonucleotide targets that are attached to DNA origami templates. In this way, we use a highly selective approach to compare the efficiency of the electron-induced dissociation of a single disulfide bond with the more complex cleavage of the DNA backbone within a TT dinucleotide sequence. This novel technique enables the fast and parallel determination of DNA strand break yields with unprecedented control over the DNAs primary and secondary structure. Thus the detailed investigation of DNA radiation damage in its most natural environment, e.g., DNA nucleosomes constituting the chromatin, now becomes feasible.

Self-organized metallic nanoparticle and nanowire arrays from ion-sputtered silicon templates

We demonstrate a production method for self-organized arrays of metal nanoparticles and aligned nanowires. Ion beam-sputtered Si∕SiO2 substrates are used as templates for metallic vapor deposition, forming aligned arrays of 5–20nm silver and cobalt nanoparticles with a period of 35nm. The 20nm diameter cobalt nanowires with lengths in excess of a micrometer are produced under appropriate conditions. All processing steps can be integrated into a single vacuum chamber and performed in a matter of minutes at mild temperatures. This inherently scalable technique can be extended to a range of substrate materials, array patterns, and nanoparticle materials.

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.

Simultaneous formation of two ripple modes on ion sputtered silicon

The amorphized surface of Si(100) sputtered with low energy ions at moderate temperature was found to develop two perpendicular ripple patterns overlaying each other. The evolution of these patterns was studied over a wide range of fluence. Coarsening of both ripple modes was observed, showing a similar time dependence with a coarsening exponent of n approximately 0.08. In the high fluence regime, the surface enters a steady state with both ripple modes still present.

Sequence dependence of electron-induced DNA strand breakage revealed by DNA nanoarrays

The electronic structure of DNA is determined by its nucleotide sequence, which is for instance exploited in molecular electronics. Here we demonstrate that also the DNA strand breakage induced by low-energy electrons (18 eV) depends on the nucleotide sequence. To determine the absolute cross sections for electron induced single strand breaks in specific 13 mer oligonucleotides we used atomic force microscopy analysis of DNA origami based DNA nanoarrays. We investigated the DNA sequences 5′-TT(XYX)3TT with X = A, G, C and Y = T, BrU 5-bromouracil and found absolute strand break cross sections between 2.66 · 10−14 cm2 and 7.06 · 10−14 cm2. The highest cross section was found for 5′-TT(ATA)3TT and 5′-TT(ABrUA)3TT, respectively. BrU is a radiosensitizer, which was discussed to be used in cancer radiation therapy. The replacement of T by BrU into the investigated DNA sequences leads to a slight increase of the absolute strand break cross sections resulting in sequence-dependent enhancement factors between 1.14 and 1.66. Nevertheless, the variation of strand break cross sections due to the specific nucleotide sequence is considerably higher. Thus, the present results suggest the development of targeted radiosensitizers for cancer radiation therapy.

The morphology of amorphous SiO2 surfaces during low energy ion sputtering

The morphology of different amorphous or amorphized SiO(2) surfaces, including thermally grown films, fused silica, and single crystalline quartz, during low energy ion sputtering has been investigated by means of atomic force microscopy. For all three materials, the formation of periodic ripple patterns oriented normal to the direction of the ion beam is observed at intermediate incident angles. At near-normal incidence, the SiO(2) surfaces remain flat, whereas a rotation of the ripple patterns is observed at grazing incidence. At intermediate angles, the patterns on the different surfaces exhibit wavelength coarsening of different strengths, which can be attributed to different amounts of near-surface mass transport by the surface-confined ion-enhanced viscous flow. In the framework of the recent hydrodynamic model of ion erosion, the observed differences in ripple coarsening are consistent with this interpretation and indicate that the surface energies of thermally grown SiO(2) and amorphized quartz are lower and higher than that of fused silica, respectively.

Molecular Processes Studied at a Single-Molecule Level Using DNA Origami Nanostructures and Atomic Force Microscopy

DNA origami nanostructures allow for the arrangement of different functionalities such as proteins, specific DNA structures, nanoparticles, and various chemical modifications with unprecedented precision. The arranged functional entities can be visualized by atomic force microscopy (AFM) which enables the study of molecular processes at a single-molecular level. Examples comprise the investigation of chemical reactions, electron-induced bond breaking, enzymatic binding and cleavage events, and conformational transitions in DNA. In this paper, we provide an overview of the advances achieved in the field of single-molecule investigations by applying atomic force microscopy to functionalized DNA origami substrates.

Reverse epitaxy of Ge: ordered and faceted surface patterns.

Normal incidence ion irradiation at elevated temperatures, when amorphization is prevented, induces novel nanoscale patterns of crystalline structures on elemental semiconductors by a reverse epitaxial growth mechanism: on Ge surfaces irradiation at temperatures above the recrystallization temperature of 250 °C leads to self-organized patterns of inverse pyramids. Checkerboard patterns with fourfold symmetry evolve on the Ge (100) surface, whereas on the Ge (111) surface, isotropic patterns with a sixfold symmetry emerge. After high-fluence irradiations, these patterns exhibit well-developed facets. A deterministic nonlinear continuum equation accounting for the effective surface currents due to an Ehrlich-Schwoebel barrier for diffusing vacancies reproduces remarkably well our experimental observations.