Zeitschriftenaufsätze

This article highlights the important role of scanning tunneling and atomic force microscopy in modern surface science experiments. Imaging with atomic resolution, manipulation of matter atom by atom, spectroscopy of confined electrons, molecular vibrational quanta, surface phonons, singleatom spin flips, and singlemolecule fluorescence photons are some of the diverse applications of the microscopes. The impact of the actual atomic or molecular termination of the tip is emphasized. A variety of examples presents the state of the art in quantum physics of surfaces and interfaces and demonstrates that scanning probe techniques significantly contribute to the understanding of matter at the atomic scale.

Full Matrix Capture is a multi-channel data acquisition method which enables flexible, high resolution imaging using ultrasound arrays. However, the measurement time and data volume are increased considerably. Both of these costs can be circumvented via compressed sensing, which exploits prior knowledge of the underlying model and its sparsity to reduce the amount of data needed to produce a high resolution image. In order to design compression matrices that are physically realizable without sophisticated hardware constraints, structured subsampling patterns are designed and evaluated in this work. The design is based on the analysis of the Cramér–Rao Bound of a single scatterer in a homogeneous, isotropic medium. A numerical comparison of the point spread functions obtained with different compression matrices and the Fast Iterative Shrinkage/Thresholding Algorithm shows that the best performance is achieved when each transmit event can use a different subset of receiving elements and each receiving element uses a different section of the echo signal spectrum. Such a design has the advantage of outperforming other structured patterns to the extent that suboptimal selection matrices provide a good performance and can be efficiently computed with greedy approaches.

Assessments of source reconstruction procedures in electroencephalography and computations of transcranial electrical stimulation profiles require verification and validation with the help of ground truth configurations as implemented by physical head phantoms. For these phantoms, synthetic materials are needed, which are mechanically and electrochemically stable and possess conductivity values similar to the modeled human head tissues. Three-compartment head models comprise a scalp layer with a conductivity range of 0.137 S/m to 2.1 S/m, a skull layer with conductivity values between 0.066 S/m and 0.00275 S/m, and an intracranial volume with an often-used average conductivity value of 0.33 S/m. To establish a realistically shaped physical head phantom with a well-defined volume conduction configuration, we here characterize the electrical conductivity of synthetic materials for modeling head compartments. We analyzed agarose hydrogel, gypsum, and sodium chloride (NaCl) solution as surrogate materials for scalp, skull, and intracranial volume. We measured the impedance of all materials when immersed in NaCl solution using a four-electrode setup. The measured impedance values were used to calculate the electrical conductivity values of each material. Further, the conductivities in the longitudinal and transverse directions of reed sticks immersed in NaCl solution were measured to test their suitability for mimicking the anisotropic conductivity of white matter tracts.

Lifecycle assessments suggest preventing halogenated solvents or solvent additives for environmentally friendly polymer solar cells. Thus, the active layers of polymer:fullerene bulk heterojunction solar cells based on poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl]-alt-[2-(20-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-CT) and the fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM) are cast from m-xylene solutions. Ortho-vanilline is used as a nonhazardous and nontoxic solvent additive. Completed photovoltaic devices are subjected to accelerated laboratory weathering tests. Photovoltaic parameters are periodically obtained from current-voltage recordings of the solar cells twice an hour under well-defined aging conditions following the International Summit on Organic Photovoltaic Stability (ISOS) protocols. An analysis of aging kinetics reveals the superposition of two individual degradation mechanisms, of which one is assigned to continued intermixing and the other one to the formation of a blocking layer by interfacial segregation.