Technical University of Berlin

About: Technical University of Berlin is a education organization based out in Berlin, Germany. It is known for research contribution in the topics: Quantum dot & Laser. The organization has 27292 authors who have published 59342 publications receiving 1414623 citations. The organization is also known as: Technische Universität Berlin & TU Berlin.

Nature of the Catalytic Centers of Porphyrin-Based Electrocatalysts for the ORR: A Correlation of Kinetic Current Density with the Site Density of Fe−N4 Centers

Abstract: In this work, it has been shown that structural changes of an as-prepared catalyst enable the assignment of the catalytic centers responsible for the direct and indirect oxygen reduction reaction, respectively, of porphyrin-based electrocatalysts. An iron porphyrin (FeTMPPCl)-based catalyst as well as a catalyst based on H2TMPP were prepared using the so-called foaming agent technique (FAT). The obtained iron catalyst was used as a generic material for the post-treatments. Structural changes were analyzed by 57Fe Mossbauer spectroscopy. The catalytic activity toward the oxygen reduction reaction (ORR) was determined using rotating (ring) disc electrode (R(R)DE) experiments. The catalysts exhibit a variation in the iron content between 2.9 and 4.5 wt % caused by the post-treatments. It has been found that the Mossbauer spectra of all catalysts can be fitted assuming two different ferrous Fe−N4 centers, a CFeN2 center (Fe2+, S = 2) and an Fe3C center (Fe0). After the intensities found in the Mossbauer spect...

Graph grammars with negative application conditions

Abstract: In each graph-grammar approach it is defined how and under which conditions graph productions can be applied to a given graph in order to obtain a derived graph. The conditions under which productions can be applied are called application conditions. Although the generative power of most of the known general graph-grammar approaches is sufficient to generate any recursively enumerable set of graphs, it is often convenient to have specific application conditions for each production. Such application conditions, on the one hand, include context conditions like the existence or non-existence of nodes, edges, or certain subgraphs in the given graph as well as embedding restrictions concerning the morphisms from the left-hand side of the production to the given graph. In this paper, the concept of application conditions introduced by Ehrig and Habel is restricted to contextual conditions, especially negative ones. In addition to the general concept, we state local confluence and the Parallelism Theorem for derivations with application conditions. Finally we study context-free graph grammars with application conditions with respect to their generative power.

Chirality distribution and transition energies of carbon nanotubes

Abstract: From resonant Raman scattering on isolated nanotubes we obtained the optical transition energies, the radial breathing mode frequency, and the Raman intensity of both metallic and semiconducting tubes. We unambiguously assigned the chiral index (n(1),n(2)) of approximately 50 nanotubes based solely on a third-neighbor tight-binding Kataura plot and find omega(RBM)=(214.4+/-2) cm(-1) nm/d+(18.7+/-2) cm(-1). In contrast to luminescence experiments we observe all chiralities including zigzag tubes. The Raman intensities have a systematic chiral-angle dependence confirming recent ab initio calculations.

Mining Subsidence Engineering

Abstract: If large, continuous cavities are created underground, as for example in coal mining, the layers of rock over the deposit generally break and subside. The subsidence can be reduced in amount and severity by filling in, or “stowing,” the mine cavity with waste material. In the course of subsiding, the land surface deforms into a generally flat, plate-shaped trough, the edge of which travels ahead of the mine working.

Radial breathing mode of single-walled carbon nanotubes: Optical transition energies and chiral-index assignment

Abstract: We present a comprehensive study of the chiral-index assignment of carbon nanotubes in aqueous suspensions by resonant Raman scattering of the radial breathing mode. We determine the energies of the first optical transition in metallic tubes and of the second optical transition in semiconducting tubes for more than 50 chiral indices. The assignment is unique and does not depend on empirical parameters. The systematics of the so-called branches in the Kataura plot are discussed; many properties of the tubes are similar for members of the same branch. We show how the radial breathing modes observed in a single Raman spectrum can be easily assigned based on these systematics. In addition, empirical fits provide the energies and radial breathing modes for all metallic and semiconducting nanotubes with diameters between 0.6 and 1.5 nm. We discuss the relation between the frequency of the radial breathing mode and tube diameter. Finally, from the Raman intensities we obtain information on the electron-phonon coupling.