Carsten Nowak

Electric-field-induced low temperature oxidation of tungsten nanowires

Experiments on the room temperature oxidation of tip-shaped tungsten nanowires under an externally applied electric field are reported, yielding the controlled formation of tungsten oxide layers up to several tens of nanometers thick in the high field region at the tip of the nanowires. The initially very fast oxidation reaction is observed to virtually terminate at a field dependent state, defined by a critical field strength of 1.15(2)×109V∕m. Since electric fields of the order of 109V∕m are easily obtained, the observed effect may influence the chemical stability of nanoscaled systems significantly when used in ambient atmosphere.

Characterization of Nanoporous Materials with Atom Probe Tomography.

A method to characterize open-cell nanoporous materials with atom probe tomography (APT) has been developed. For this, open-cell nanoporous gold with pore diameters of around 50 nm was used as a model system, and filled by electron beam-induced deposition (EBID) to obtain a compact material. Two different EBID precursors were successfully tested-dicobalt octacarbonyl [Co2(CO)8] and diiron nonacarbonyl [Fe2(CO)9]. Penetration and filling depth are sufficient for focused ion beam-based APT sample preparation. With this approach, stable APT analysis of the nanoporous material can be performed. Reconstruction reveals the composition of the deposited precursor and the nanoporous material, as well as chemical information of the interfaces between them. Thus, it is shown that, using an appropriate EBID process, local chemical information in three dimensions with sub-nanometer resolution can be obtained from nanoporous materials using APT.

Reactive diffusion under Laplace tension.

Nanostructures such as nanospheres or nanowires may contain surfaces and interfaces of pronounced curvature. To investigate the impact of severe curvature on the kinetic of reactions, thin-film Al/Cu/Al and Cu/Al/Cu triple layers are deposited on tungsten tips of 25nm curvature radius. The thermal reaction of the layer structure is studied by atom probe tomography. Experiments demonstrate that the reaction rate depends significantly on the deposition sequence of metals. Interpretation of the observed reaction kinetics leads to the conclusion that under the influence of interfacial tension probably the two limiting cases of atomic transport, Darken and Nernst-Planck kinetics, are realized in dependence on the stacking sequence.

Stress Impact on Reactive Diffusion in Nano-Structures of Spherical Symmetry

To investigate the influence of stress on reactive diffusion, a remarkably clear experiment has been designed. Thin film Al/Cu/Al and Cu/Al/Cu triple layers are deposited on curved substrates of 25 nm radius and investigated by atom probe tomography. Due to the specific geometry, the excess volume of the reaction product induces compressive and dilatational stress on opposite sides of the product layer, even in the case of semi-coherent or incoherent interphase boundaries. The resulting stress gradient leads to additional driving force, which accelerates or decelerates the reaction rate in dependence on the stacking sequence of the layer material. By quantitative analysis, the induced level of stress can be quantified from the modified growth rate of the product.

In Situ Atom Probe Deintercalation of Lithium-Manganese-Oxide

Atom probe tomography is routinely used for the characterization of materials microstructures, usually assuming that the microstructure is unaltered by the analysis. When analyzing ionic conductors, however, gradients in the chemical potential and the electric field penetrating dielectric atom probe specimens can cause significant ionic mobility. Although ionic mobility is undesirable when aiming for materials characterization, it offers a strategy to manipulate materials directly in situ in the atom probe. Here, we present experimental results on the analysis of the ionic conductor lithium-manganese-oxide with different atom probe techniques. We demonstrate that, at a temperature of 30 K, characterization of the materials microstructure is possible without measurable Li mobility. Also, we show that at 298 K the material can be deintercalated, in situ in the atom probe, without changing the manganese-oxide host structure. Combining in situ atom probe deintercalation and subsequent conventional characterization, we demonstrate a new methodological approach to study ionic conductors even in early stages of deintercalation.

Artifacts Caused by Breast Tissue Markers in a Dedicated Cone-beam Breast CT in Comparison to Full-field Digital Mammography

RATIONALE AND OBJECTIVES The purpose of this ex vivo study was to investigate artifacts in a cone-beam breast computed tomography (CBBCT) caused by breast tissue markers. MATERIALS AND METHODS Breast phantoms with self-made tissue pork mincemeat were created. Twenty-nine different, commercially available markers with varying marker size, composition, and shape were evaluated. A dedicated CBBCT evaluation of all phantoms was performed with 49 kVp, 50 and 100 mA, and marker orientation parallel and orthogonal to the scan direction. The resultant images were evaluated in sagittal, axial, and coronal view with a slice thickness of 0.5 mm. Additionally, measurements of all markers in the same directions were done with full-field digital mammography. RESULTS All markers were visible in full-field digital mammography without any artifacts. However, all markers caused artifacts on a CBBCT. Artifacts were measured as the length of the resulting streakings. Median length of artifacts was 7.2 mm with a wide range from 0 to 48.3 mm (interquartile range 4.3-11.4 mm) dependent on composition, size, shape, weight, and orientation of the markers. The largest artifacts occurred in axial view with a median size of 12.6 mm, with a range from 0 to 48.3 mm, resulting in a relative artifact length (quotient artifact in mm/real physical length of the marker itself) of 4.1 (interquartile range 2.3-6.1, range 0-8.7). CONCLUSIONS Artifacts caused by markers can significantly influence image quality in a CBBCT, thus limiting primary diagnostics and follow-up in breast cancer. The size of the artifacts depends on the marker characteristics, orientation, and the image plane of reconstruction.

Reactive Diffusion under Laplace Tension in Spherical Nanostructures

Nanostructures are often distinguished by complex geometries different from a planar layer structure. In order to study the influence of a spherical symmetry, as it is e.g. found in core-shell nano-spheres, thin film Al/Cu/Al and Cu/Al/Cu triple layers are deposited on substrates of 25 nm curvature radius. The thermal reaction is studied by atom probe tomography. Due to the outstanding spatial resolution and the ability to analyse three dimensional structures without projection artefacts, the growth of reaction products at the curved interfaces can be studied even on the length scale of a few nanometers. The experiments demonstrate that the reaction rate depends significantly on the deposition sequence of the metals. This observation is naturally explained by Laplace tension.