2014, Buchwald, J., Sarmanova, M., Rauschenbach, B., Mayr, S.G.

The mechanical properties of surfaces and nanostructures deviate from their bulk counterparts due to surface stress and reduced dimensionality. Experimental indentation-based techniques present the challenge of measuring these effects, while avoiding artifacts caused by the measurement technique itself. We performed a molecular dynamics study to investigate the mechanical properties of a GaN step of only a few lattice constants step height and scrutinized its applicability to indentation experiments using a finite element approach (FEM). We show that the breakdown of half-space symmetry leads to an "artificial" reduction of the elastic properties of comparable lateral dimensions which overlays the effect of surface stress. Contact resonance atomic force microscopy (CR-AFM) was used to compare the simulation results with experiments.

2014, Lorenz, P., Ehrhardt, M., Zimmer, K.

The defect-free laser-assisted structuring of thin films on flexible substrates is a challenge for laser methods. However, solving this problem exhibits an outstanding potential for a pioneering development of flexible electronics. Thereby, the laser-assisted delamination method has a great application potential. At the delamination process: the localized removal of the layer is induced by a shock wave which is produced by a laser ablation process on the rear side of the substrate. In this study, the thin-film patterning process is investigated for different polymer substrates dependent on the material and laser parameters using a KrF excimer laser. The resultant structures were studied by optical microscopy and white light interferometry (WLI). The delamination process was tested at different samples (indium tin oxide (ITO) on polyethylene terephthalate (PET), epoxy-based negative photoresist (SU8) on polyimide (PI) and indium tin oxide/copper indium gallium selenide/molybdenum (ITO/CIGS/Mo) on PI.

2011, MacKo, S., Frost, F., Engler, M., Hirsch, D., Höche, T., Grenzer, J., Michely, T.

Pattern formation on Si(001) through 2 keV Kr+ ion beam erosion of Si(001) at an incident angle of # = 30° and in the presence of sputter codeposition or co-evaporation of Fe is investigated by using in situ scanning tunneling microscopy, ex situ atomic force microscopy and electron microscopy. The phenomenology of pattern formation is presented, and experiments are conducted to rule out or determine the processes of relevance in ion beam pattern formation on Si(001) with impurities. Special attention is given to the determination of morphological phase boundaries and their origin. Height fluctuations, local flux variations, induced chemical inhomogeneities, silicide formation and ensuing composition-dependent sputtering are found to be of relevance for pattern formation.

2012, Mayr, S.G., Arabi-Hashemi, A.

Fe-Pd-based ferromagnetic shape memory alloys constitute an exciting class of magnetically switchable smart materials that reveal excellent mechanical properties and biocompatibility. However, their application is severely hampered by a lack of understanding of the physics at the atomic scale. A many-body potential is presented that matched ab inito calculations and can account for the energetics of martensite ↔ austenite transition along the Bain path and relative phase stabilities in the ordered and disordered phases of Fe-Pd. Employed in massively parallel classical molecular dynamics simulations, the impact of order/disorder, point defects and severe plastic deformation in the presence of single- and polycrystalline microstructures are explored as a function of temperature. The model predictions are in agreement with experiments on phase changes induced by ion irradiation, cold rolling and hammering, which are also presented.

2014, Hennes, M., Lotnyk, A., Mayr, S.G.

Magnetically anisotropic as well as magnetic core-shell nanoparticles (CS-NPs) with controllable properties are highly desirable in a broad range of applications. With this background, a setup for the synthesis of heterostructured magnetic core-shell nanoparticles, which relies on (optionally pulsed) DC plasma gas condensation has been developed. We demonstrate the synthesis of elemental nickel nanoparticles with highly tunable sizes and shapes and Ni@Cu CS-NPs with an average shell thickness of 10 nm as determined with scanning electron microscopy, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements. An analytical model that relies on classical kinetic gas theory is used to describe the deposition of Cu shell atoms on top of existing Ni cores. Its predictive power and possible implications for the growth of heterostructured NP in gas condensation processes are discussed.

2014, Zimmer, K., Wang, X., Lorenz, P., Bayer, L., Ehrhardt, M., Scheit, C., Braun, A.

The optimization of laser scribing for the interconnection of CIGS solar cells is a current focus of laser process development. In addition to the geometry of the laser scribes the impact of the laser patterning to the electrical properties of the solar cells has to be optimized with regards to the scribing process and the laser sources. In-process measurements provide an approach for reliable evaluation of the electrical characteristics. In particular, the parallel resistance Rp that was calculated from the measured I-V curves was measured in dependence on the scribing parameters of a short-pulsed ns laser in comparison to a standard ps laser at a wavelength of 1.06 μm. With low pulse overlap of ∼ 20% a reduction of Rp to 2/3 of the initial value has been achieved for ns laser pulses. In comparison to ps laser slightly more defects were observed at the investigated parameter range.

2012, Jakob, A.M., Müller, M., Rauschenbach, B., Mayr, S.G.

Located beyond the resolution limit of nanoindentation, contact resonance atomic force microscopy (CR-AFM) is employed for nano-mechanical surface characterization of single crystalline 14M modulated martensitic Ni-Mn-Ga (NMG) thin films grown by magnetron sputter deposition on (001) MgO substrates. Comparing experimental indentation moduli-obtained with CR-AFM-with theoretical predictions based on density functional theory (DFT) indicates the central role of pseudo plasticity and inter-martensitic phase transitions. Spatially highly resolved mechanical imaging enables the visualization of twin boundaries and allows for the assessment of their impact on mechanical behavior at the nanoscale. The CR-AFM technique is also briefly reviewed. Its advantages and drawbacks are carefully addressed.

2014, Asenova, I., Manova, D., Mändl, S.

In this paper we investigate the possibility of incorporating nitrogen into amorphous, photocatalytic TiO2 thin films, prepared at room temperature, during the growth process. The aim is to reduce the bandgap of the UV active thin films. Physical vapor deposition experiments employing a titanium vacuum arc with gas backfill ranging from pure oxygen to pure nitrogen, are carried out. The resulting films are characterized for chemical composition, phase composition, optical properties and hydrophilicity in order to determine a correlation between gas composition and thin film properties. The experimental results point that a visible change in the band structure of the deposited layers is achieved.

2011, Claussen, I., Mayr, S.G.

We report on vibrating reed measurements combined with density functional theory-based calculations to assess the elastic and damping properties of Fe-Pd ferromagnetic shape memory alloy splats. While the austenite-martensite phase transformation is generally accompanied by lattice softening, a severe modulus defect and elevated damping behavior are characteristic of the martensitic state. We interpret the latter in terms of twin boundary motion between pinning defects via partial 'twinning' dislocations. Energy dissipation is governed by twin boundary drag, primarily due to lattice imperfections, as concluded from the temperature dependence of damping and related activation enthalpies.

2012, Macko, S., Grenzer, J., Frost, F., Engler, M., Hirsch, D., Fritzsche, M., Mücklich, A., Michely, T.

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 ≈ 5.9×10 21 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 140-440K under otherwise identical conditions, where a thin amorphous layer forms due to ion bombardment.