Mean-intercept anisotropy analysis of porous media. II. Conceptual shortcomings of the MIL tensor definition and Minkowski tensors as an alternative - Klatt, Michael A.; Schroder-Turk, Gerd E.; Mecke, Klaus (Medical Physics, 2017)
Purpose Structure-property relations, which relate the shape of the microstructure to physical properties such as transport or mechanical properties, need sensitive measures of structure. What are suitable fabric tensors to quantify the shape of anisotropic heterogeneous materials? The mean intercept length is among the most commonly used characteristics of anisotropy in porous media, e.g., of trabecular bone in medical physics. Yet, in this series of two papers we demonstrate that it has conceptual shortcomings that limit the validity of its results. Methods We test the validity of general assumptions regarding the properties of the mean-intercept length tensor using analytical formulas for the mean-intercept lengths in anisotropic Boolean models (derived in part I of this series), augmented by numerical simulations. We discuss in detail the functional form of the mean intercept length as a function of the test line orientations. Results As the most prominent result, we find that, at least for the example of overlapping grains modeling porous media, the polar plot of the mean intercept length is in general not an ellipse and hence not represented by a second-rank tensor. This is in stark contrast to the common understanding that for a large collection of grains the mean intercept length figure averages to an ellipse. The standard mean intercept length tensor defined by a least-square fit of an ellipse is based on a model mismatch, which causes an intrinsic lack of accuracy. Conclusions Our analysis reveals several shortcomings of the mean intercept length tensor analysis that pose conceptual problems and limitations on the information content of this commonly used analysis method. We suggest the Minkowski tensors from integral geometry as alternative sensitive measures of anisotropy. The Minkowski tensors allow for a robust, comprehensive, and systematic approach to quantify various aspects of structural anisotropy. We show the Minkowski tensors to be more sensitive, in the sense, that they can quantify the remnant anisotropy of structures not captured by the mean intercept length analysis. If applied to porous tissue and microstructures, this improved structure characterization can yield new insights into the relationships between geometry and material properties.
EAM Import::2019-03-12

Measuring fast stochastic displacements of bio-membranes with dynamic optical displacement spectroscopy - Monzel, Cornelia; Schmidt, Daniel; Kleusch, C.; Kirchenbuechler, D.; Seifert, U.; Smith, Ana-Suncana; Sengupta, Kheya; Merkel, Rudolf (Nature Communications, 2015)
Stochastic displacements or fluctuations of biological membranes are increasingly recognized as an important aspect of many physiological processes, but hitherto their precise quantification in living cells was limited due to a lack of tools to accurately record them. Here we introduce a novel technique-dynamic optical displacement spectroscopy (DODS), to measure stochastic displacements of membranes with unprecedented combined spatio-temporal resolution of 20 nm and 10 mu s. The technique was validated by measuring bending fluctuations of model membranes. DODS was then used to explore the fluctuations in human red blood cells, which showed an ATP-induced enhancement of non-Gaussian behaviour. Plasma membrane fluctuations of human macrophages were quantified to this accuracy for the first time. Stimulation with a cytokine enhanced non-Gaussian contributions to these fluctuations. Simplicity of implementation, and high accuracy make DODS a promising tool for comprehensive understanding of stochastic membrane processes.

Measuring the Invisible: Determining the Size of Growing Nanodomains Using the "Inverse FCS" - Fenz, Susanne F.; Smith, Ana-Suncana; Monzel, Cornelia (Biophysical Journal, 2017)
Molecular recruitment into functional domains in the cell plasma membrane is an important prerequisite for many cellular processes, such as cell signaling, cell-matrix adhesion, or the formation of synapses. High precision measurements of the origin and evolution of these structures is key to their understanding; however, experimental methods with which one can detect molecular dynamics and aggregation, with microsecond temporal resolution and at nanometer scales, are scarce.One of the most developed approaches, which is up to the task, is fluorescence correlation spectroscopy (FCS). In FCS, fluorescent particles diffuse through a laser excitation beam and emit pulses of light proportional to the particle brightness.One way to circumvent the issue of relating probe intensities to their number is to use inverse fluorescence correlation spectroscopy (iFCS). Here, as opposed to conventional FCS, the molecule of interest is not labeled but diffuses in a homogeneously fluorescent environment, giving rise to negative stochastic intensity fluctuations.
Keywords: fluorescence correlation spectroscoppy; membranes

Measuring the Kerr-nonlinearity of Ag in plasmonic nano-circuits - Kriesch, Arian; Pfeifer, Hannes; Burgos, Stanley P.; Ploss, Daniel; Banzer, Peter; Atwater, Harry A.; Peschel, Ulf (Nonlinear Optics 2013, 2013)
We observe nonlinear switching of ultrashort (600fs) laser pulses in subwavelength plasmonic nanocircuits. This allows us to experimentally determine the third-order nonlinear coefficient of silver.
EAM Import::2019-03-12

Mechanical anisotropy of aluminium laminates produced by ARB - Chekhonin, P.; Beausir, B.; Scharnweber, J.; Oertel, C-G; Jaschinski, J.; Hausöl, Tina; Höppel, Heinz Werner; Brokmeier, H-G; Skrotzki, W. (Materials Science Forum, 2012)
The plastic anisotropy was studied on aluminium sheets with layers of different purity (A: 5N and B: 2N+) produced by accumulative roll bonding (ARB). Both material layers show a contrasting recrystallization behavior where A and B are discontinuously and continuously recrystallized, respectively. Global textures were measured by neutron diffraction. The mechanical anisotropy was measured by tensile testing after different numbers of ARB cycles. The planar anisotropy decreases with the number of ARB cycles while the normal anisotropy reaches a plateau after 4 cycles. Simulations of the Lankford parameters were carried out with the help of the viscoplastic self-consistent scheme (based on the global texture) and compared with the experimental data. Deviations of the simulated values from those of experiment are discussed with regard to through-thickness texture and material heterogeneities.
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Mechanical characterisation of a periodic auxetic structure produced by SEBM - Schwerdtfeger, Jan; Schury, Fabian; Stingl, Michael; Wein, Fabian; Singer, Robert; Körner, Carolin (physica status solidi (b), 2012)
We present a thorough investigation of the mechanical behaviour of a non-stochastic cellular auxetic structure. A combination of experimental and numerical methods is used to gain a deeper understanding of the mechanical behaviour and its dependence on the geometric properties of the cellular structure. The experimental samples are built from Ti-6Al-4V using selective electron beam melting, an additive manufacturing process giving the possibility to vary the geometry of the structure in a highly controlled manner. The use of finite element simulations and mathematical homogenisation allows us also to investigate off-axis properties of the cellular material. This leads to a more comprehensive understanding of the mechanical behaviour of the auxetics. Ultimately, the gained knowledge can be used to tailor auxetic materials to specific applications.
Keywords: auxetics; cellular materials; electron beam melting; finite element method; metals

Mechanical characterization of metallic thin films by bulge and scratch testing - Javed, Hassan; Merle, Benoit; Preiß, Eva; Hivet, Romain; Benedetto, Alessandro; Göken, Mathias (Surface & Coatings Technology, 2016)
The mechanical properties of 20-nm thick silver, titanium and nickel-chromium alloy films encapsulated between two silicon nitride ceramic layers are investigated by bulge and scratch tests. Bulge tests are used in order to measure the yield strength of the metallic films. An unexpected strain-softening behavior is observed for a silver sample, which is rationalized as a measurement artifact due to the delamination of the film from its substrate. An experimental workaround is proposed and shown to yield correct measurements. The adhesion strength of the metallic films is measured by scratch tests, and their failure mechanism is characterized by in-situ optical observations. All in all, the poorest mechanical properties are found in the silver. This is probably due to a combination of the low adhesion at the interface between the nitride and the metal and the low yield stress of the silve
Keywords: Bulge test; Scratch test; Silicon nitride; Silver; Thin film

Mechanical Properties of Carbon Fiber-Reinforced Aluminum Manufactured by High-Pressure Die Casting - Kachold, Franziska; Singer, Robert (Journal of Materials Engineering and Performance, 2016)
Carbon fiber reinforced aluminum was produced by a specially adapted high-pressure die casting process. The MMC has a fiber volume fraction of 27%. Complete infiltration was achieved by preheating the bidirectional, PAN-based carbon fiber body with IR-emitters to temperatures of around 750 °C. The degradation of the fibers, due to attack of atmospheric oxygen at temperatures above 600 °C, was limited by heating them in argon-rich atmosphere. Additionally, the optimization of heating time and temperature prevented fiber degradation. Only the strength of the outer fibers is reduced by 40% at the most. The fibers in core of fiber body are nearly undamaged. In spite of successful manufacturing, the tensile strength of the MMC is below strength of the matrix material. Also unidirectional MMCs with a fiber volume fraction of 8% produced under the same conditions, lack of the reinforcing effect. Two main reasons for the unsatisfactory mechanical properties were identified: First, the fiber-free matrix, which covers the reinforced core, prevents effective load transfer from the matrix to the fibers. And second, the residual stresses in the fiber-free zones are as high as 100 MPa. This causes premature failure in the matrix. From this, it follows that the local reinforcement of an actual part is limited. The stress distribution caused by residual stresses and by loading needs to be known. In this way, the reinforcing phase can be placed and aligned accordingly. Otherwise delamination and premature failure might occur.
Keywords: carbon fibers; composites; metallic matrix; modeling and simulation; residual stress; tensile testing

Mechanical properties of copper/bronze laminates: Role of interfaces - Ma, Xiaolong; Huang, Chongxiang; Moering, Jordan; Ruppert, Mathis; Höppel, Heinz Werner; Göken, Mathias; Narayan, Jagdish; Zhu, Yuntian (Acta Materialia, 2016)
Interfaces play a crucial role in mechanical behaviors of both laminated and gradient structured materials. In this work, copper/bronze laminates with varying interface spacing were fabricated by accumulative roll bonding and subsequent annealing to systematically study the interface effect on mechanical properties. Heterogeneities exist in chemical composition, grain size, hardness and texture across the interfaces. Simultaneous improvement of strength and ductility with decreasing interface spacing is found in tensile tests. Extra geometrically necessary dislocations (GNDs) are found to accumulate in the vicinity of interfaces, which is due to mechanical incompatibility across the interfaces. Importantly, an interface-affected zone spanning a few micrometers was found, which is not affected by interface spacing. These observations suggest the existence of an optimum spacing, which may produce the highest hardening capacity and ductility without sacrificing strength. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Interface; Laminate structure; Geometrically necessary dislocation (GND); Interface-affected zone; Back stress hardening

Mechanical properties of ultrafine-grained AlZnMg(Cu)-alloys AA7020 and AA7075 processed by accumulative roll bonding - Ruppert, Mathis; Strebl, Michael; Höppel, Heinz Werner; Göken, Mathias (Journal of Materials Science, 2015)
Ultrafine-grained AA7020 (AlZnMg) and AA7075 (AlZnMgCu) were produced by accumulative roll bonding (ARB) with up to 6 cycles. Different pre-heating treatments and their effect on the mechanical performance of the materials were investigated by means of hardness measurements and uniaxial tensile testing. It was found for AA7020 that by pre-heating at 230 degrees C for 2.5 min prior to each rolling step, an UTS of 550 MPa can be achieved, which is 51 % higher compared to the peak-aged T6 reference. For AA7075, pre-heating at 280 degrees C for 2.5 min leads to a very high UTS of 720 MPa after four ARB passes. A negative strain rate sensitivity was found for both alloys, which shifts toward zero with increasing number of ARB cycles. Post-ARB heat treatment was performed in order to overcome the reduced ductility after ARB. This leads to an enhanced strain hardening capacity after 4 cycles, resulting in an increase of the uniform elongation from 1.4 to 2.0 % for AA7020 and from 0.9 to 2.1 % for AA7075.

Mechanical Properties, Surface Structure, and Morphology of Carbon Fibers Pre-heated for Liquid Aluminum Infiltration - Kachold, Franziska; Kozera, Rafal; Singer, Robert; Boczkowska, Anna (Journal of Materials Engineering and Performance, 2016)
To efficiently produce carbon fiber-reinforced aluminum on a large scale, we developed a special high-pressure die casting process. Pre-heating of the fibers is crucial for successful infiltration. In this paper, the influence of heating carried out in industrial conditions on the mechanical properties of the fibers was investigated. Therefore, polyacrylonitrile-based high-tensile carbon fiber textiles were heated by infrared emitters in an argon-rich atmosphere to temperatures between 450 and 1400Â Â°C. Single fiber tensile tests revealed a decrease in tensile strength and strain at fracture. Young’s modulus was not affected. Scanning electron microscopy identified cavities on the fiber surface as the reason for the decrease in mechanical properties. They were caused by the attack of atmospheric oxygen. The atomic structure of the fibers did not change at any temperature, as x-ray diffraction confirmed. Based on these data, the pre-heating for the casting process can be optimized.
Keywords: advanced characterization; aluminum; casting and solidification; composites; heat treatment; metallic matrix

Mechanism of Sulfur Poisoning and Storage: Adsorption and Reaction of SO2 with Stoichiometric and Reduced Ceria Films on Cu(111) - Happel, Markus; Lykhach, Yaroslava; Tsud, Nataliya; Skala, Tomas; Prince, Kevin C.; Matolin, Vladimir; Libuda, Jörg (Journal of Physical Chemistry C, 2011)
We have performed a systematic synchrotron radiation photoelectron spectroscopy (SR-PES) study of the interaction of SO2 with stoichiometric CeO2(111) and reduced CeO2-x films supported on Cu(111). Resonant photoelectron spectroscopy (RPES) was utilized to monitor changes in the cerium oxidation states. Adsorption of SO2 at 150 and 300 K results mostly in formation of sulfite (SO32-) species on both reduced and stoichiometric ceria. Other species such as sulfate (SO42-), SO2 chemisorbed at cationic sites, anionic SO2-, atomic sulfur, surface sulfides, and bulk oxy-sulfides were detected. The origin of these species is discussed in relation to the changes in the oxidation state of ceria cations at the surface. The thermal evolution of the species formed upon adsorption at 150 and 300 K was studied systematically on both the stoichiometric and the reduced films. It was found that the sulfite partially desorbs to release SO2 and partially decomposes. In general, the reduced CeO2-x surface was found to be much more active toward cleavage of S-O bonds, as indicated by a higher amount of oxy-sulfide species produced. The decomposition pathways can be monitored via changes of the ceria oxidation state and are discussed in relation to the stoichiometry of the ceria surface.

Mechanisms of dislocation multiplication at crack tips - Bitzek, Erik; Gumbsch, Peter (Acta Materialia, 2013)
Whether a stressed material fractures by brittle cleavage or ductile rupture is determined by its ability to convert elastic strain energy to plastic deformation through the generation and motion of dislocations. Although it is known that pre-existing dislocations play a crucial role in crack tip plasticity, the involved mechanisms are unclear. Here it is demonstrated by atomistic simulations that individual pre-existing dislocations may lead to the generation of large numbers of dislocations at the crack tip. The newly generated dislocations are usually of different types. The processes involved are fundamentally different for stationary cracks and propagating cracks. Whereas local crack front reorientation plays an important role in propagating cracks, the multiplication mechanism at stationary cracks is connected with cross-slip in the highly inhomogeneous stress field of the crack. Analysis of the forces acting on the dislocations allows to determine which dislocations multiply and the slip systems they activate. These results provide the necessary physical link between pre-existing dislocations and the generation of dislocations at crack tips.
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Mechanisms of the water-gas shift reaction catalyzed by ruthenium pentacarbonyl: A density functional theory study - Schulz, Hannes; Görling, Andreas; Hieringer, Wolfgang (Inorganic Chemistry, 2013)
The mechanism of the water-gas shift reaction catalyzed by Ru(CO) is analyzed using density functional methods in solution within the conductor-like screening model. Four different mechanistic pathways have been considered. It turned out that the incorporation of solvent effects is very important for a reasonable comparison among the mechanistic alternatives. The explicit inclusion of a water solvent molecule significantly changes the barriers of those steps which involve proton transfer in the transition state. The corresponding barriers are either lowered or increased, depending on the structure of the corresponding cyclic transition states. The results show that protolysis steps become competitive due to solution effects. The formation of formic acid as an intermediate in another, alternative pathway is also found to be competitive. © 2013 American Chemical Society.

Mechanochemically induced sulfur doping in ZnO via oxygen vacancy formation - Daiko, Yusuke; Schmidt, Jochen; Kawamura, Go; Romeis, Stefan; Segets, Doris; Iwamoto, Y.; Peukert, Wolfgang (Physical Chemistry Chemical Physics, 2017)
Surface defects of ZnO nanoparticles were induced via mechanical stressing using a Turbula shaker mixer and a planetary ball mill, and the possibilities for surface modification and functionalization of the ZnO nanoparticles were exemplified by sulfur doping of activated ZnO. Raman spectroscopy reveals that the formation of oxygen vacancies (VO) does not only occur under high stressing conditions in a planetary ball mill but even upon rather 'mild stressing' in the shaker mixer. The temporal evolution of the vacancy concentration in ZnO stressed under different conditions can be described by a model that accounts for stress number and vacancy diffusion with diffusion coefficients of VO of 3.7 × 10-21 m2 s-1 and 2.4 × 10-20 m2 s-1 for stressing in the shaker and the planetary ball mill, respectively. The thickness of the VO layer was estimated to be about 1 nm. Thiourea was mixed with defective ZnO particles, and then heated at various temperatures for sulfur-doping. A linear relationship between the amount of induced VO and the level of sulfur doping was found. Remarkably, mechanical activation is indispensable in order to control the level of sulfur doping quantitatively. High-angle annular dark field scanning transmission electron microscopy (HAADF STEM) observations with energy dispersive X-ray spectroscopy (EDX) analysis clearly revealed that the doped sulfur atoms are concentrated at the particle surface. Thus, ZnO (core)/ZnS (shell) structures are obtained easily via mechanochemical activation and subsequent thermal treatment.

Membrane fluctuations mediate lateral interaction between cadherin bonds - Fenz, Susanne F.; Bihr, Timo; Schmidt, Daniel; Merkel, Rudolf; Seifert, Udo; Sengupta, Kheya; Smith, Ana-Suncana (Nature Physics, 2017)
The integrity of living tissues is maintained by adhesion domains of trans-bonds formed between cadherin proteins residing on opposing membranes of neighbouring cells. These domains are stabilized by lateral cis-interactions between the cadherins on the same cell. However, the origin of cis-interactions remains perplexing since they are detected only in the context of trans-bonds. By combining experimental, analytical and computational approaches, we identify bending fluctuations of membranes as a source of long-range cis-interactions, and a regulator of trans-interactions. Specifically, nanometric membrane bending and fluctuations introduce cooperative effects that modulate the affinity and binding/unbinding rates for trans-dimerization, dramatically affecting the nucleation and growth of adhesion domains. Importantly, this regulation relies on physical principles and not on details of protein-protein interactions. These omnipresent fluctuations can thus act as a generic control mechanism in all types of cell adhesion, suggesting a hitherto unknown physiological role for recently identified active fluctuations of cellular membranes.
Keywords: INTERFERENCE CONTRAST MICROSCOPY; CELL-ADHESION; ADHERENS JUNCTIONS; GIANT VESICLES; I CADHERINS; BINDING; DYNAMICS; DOMAINS; ACTIN; SPECIFICITY

Memory and neural networks on the basis of color centers in solids - Winnacker, Albrecht; Osvet, Andres (Biological Chemistry, 2009)
Optical data recording is one of the most widely used and efficient systems of memory in the non-living world. The application of color centers in this context offers not only systems of high speed in writing and read-out due to a high degree of parallelism in data handling but also a possibility to set up models of neural networks. In this way, systems with a high potential for image processing, pattern recognition and logical operations can be constructed. A limitation to storage density is given by the diffraction limit of optical data recording. It is shown that this limitation can at least in principle be overcome by the principle of spectral hole burning, which results in systems of storage capacities close to the human brain system.
Keywords: Color center; Neural network; Memory; Spectral hole burning; Optical storage; photostimulated luminescence

Memory Effect of Self-Assembled PS-b-PEO Block Copolymer Films with Selectively Embedded Functionalized TiO2 Nanoparticles - Kirschner, Johannes; Will, Johannes; Rejek, Tobias; Portilla Berlanga, Luis; Berlinghof, Marvin; Schweizer, Peter; Spiecker, Erdmann; Steinrück, Hans-Georg; Unruh, Tobias; Halik, Marcus (Advanced Materials Interfaces, 2017)
The memory effect in organic thin film transistors is described with a dielectric hybrid material composed of selectively localized charge trapping titania nanoparticles in a polystyrene-b-polyethyleneoxide (PS-b-PEO) diblock copolymer film. Upon solvent annealing in toluene atmosphere, the films readily form nanophase-separated features of PEO cylinders in a matrix of PS. X-ray reflectivity (XRR) investigations reveal that the TiO2 particles are selectively distributed in the top half of the hybrid layers. Applied in organic thin film transistor devices with 2-tridecyl-[1]benzothieno[3,2-b][1]benzothiophen (mono-C13-BTBT) semiconductor, the hybrid layers induce clockwise drain current hysteresis with promising memory ratio, long-term stability, and endurance. The occurrence of nonvolatile charge trapping directly relates to the hierarchically assembled nanostructure of the nanoparticles in the block copolymer since reference devices with PS homopolymers blended with TiO2 are found to be hysteresis free. Morphological characterization of the hybrid dielectric layers by atomic force microscopy and XRR experiments is presented to shed light on the internal electron density distribution. The layers are subsequently tested in capacitor stacks and transistors.

Mesoporous Organosilicas With Large Cage-Like Pores for High Efficiency Immobilization of Enzymes - Zhou, Zhou; Klupp Taylor, Robin; Kullmann, Simon; Bao, Huixin; Hartmann, Martin (Advanced Materials, 2011)
A group of hierarchically ordered PMOs with large cage-like pores are prepared with organosilica units including ethylene, ethenylene, and phenylene-bridged silanes. The structure and hydrophobicity of these materials are shown to offer significant advantages for the physical adsorption of lipase from aqueous media and also markedly enhance its activity.

Mesoscopic simulation of selective beam melting processes - Körner, Carolin; Attar, Elham; Heinl, Peter (Journal of Materials Processing Technology, 2011)
A 2D lattice Boltzmann model is developed to investigate melting and re-solidification of a randomly packed powder bed under the irradiation of a Gaussian beam during selective beam melting processes. Numerical simulation results are presented where individual powder particles are considered. This approach makes many physical phenomena accessible which cannot be described in a standard continuum picture, e.g. the influence of the relative powder density, the stochastic effect of a randomly packed powder bed, capillary and wetting phenomena. The proposed model, although still 2D, is able to predict many experimental observations such as the well known balling effect. A process map is used to demonstrate the effect of the process parameters, beam powder and scan speed. The simulation results are compared with experimental findings during Selective Electron Beam Melting (SEBM). The comparison shows good agreement between simulation results and experiments. © 2010 Elsevier B.V. All rights reserved.
Keywords: Additive manufacturing; Fluid dynamics; Heat transfer; Lattice Boltzmann method; Selective beam melting

Mesoscopic Structures of Triglyceride Nanosuspensions Studied by Small-Angle X-ray and Neutron Scattering and Computer Simulations - Schmiele, Martin; Schindler, Torben; Westermann, Martin; Steiniger, Frank; Radulescu, Aurel; Kriele, Armin; Gilles, Ralph; Unruh, Tobias (Journal of Physical Chemistry B, 2014)
Aqueous suspensions of platelet-like shaped tripalmitin nanocrystals are studied here at high tripalmitin concentrations (10 wt % tripalmitin) for the first time by a combination of small-angle X-ray and neutron scattering (SAXS and SANS). The suspensions are stabilized by different lecithins, namely, DLPC, DOPC, and the lecithin blend S100. At such high concentrations the platelets start to self-assemble in stacks, which causes interference maxima at low Q-values in the SAXS and SANS patterns, respectively. It is found that the stack-related interference maxima are more pronounced for the suspension stabilized with DOPC and in particular DLPC, compared to suspensions stabilized by S100. By use of the X-ray and neutron powder pattern simulation analysis (XNPPSA), the SAXS and SANS patterns of the native tripalmitin suspensions could only be reproduced simultaneously when assuming the presence of both isolated nanocrystals and stacks of nanocrystals of different size in the simulation model of the dispersions. By a fit of the simulated SAXS and SANS patterns to the experimental data, a distribution of the stack sizes and their volume fractions is determined. The volume fraction of stacklike platelet assemblies is found to rise from 70% for S100-stabilized suspensions to almost 100% for the DLPC-stabilized suspensions. The distribution of the platelet thicknesses could be determined with molecular resolution from a combined analysis of the SAXS and SANS patterns of the corresponding diluted tripalmitin (3 wt %) suspensions. In accordance with microcalorimetric data, it could be concluded that the platelets in the suspensions stabilized with DOPC, and in particular DLPC, are significantly thinner than those stabilized with S100. The DLPC-stabilized suspensions exhibit a significantly narrower platelet thickness distribution compared to DOPC- and S100-stabilized suspensions. The smaller thicknesses for the DLPC- and DOPC-stabilized platelets explain their higher tendency to self-assemble in stacks. The finding that the nanoparticles of the suspension stabilized by the saturated lecithin DLPC crystallize in the stable beta-tripalmitin modification with its characteristic platelet-like shape is surprising and can be explained by the fact that the main phase transformation temperature for DLPC is, as for unsaturated lecithins like DOPC and S100, well below the crystallization temperature of the supercooled tripalmitin emulsion droplets.

Metal-organic frameworks constructed with 1, 2, 4-triazol-1-ylacrylic acid ligand: Syntheses and crystal structures - Fischer, Nina; Inayat, Alexandra; Schwieger, Wilhelm; Burzlaff, Nicolai (Journal of Coordination Chemistry, 2010)
Synthesis of a triazole-based N, O ligand, 1, 2, 4-triazol-1-yl)acrylate (ta) is described. The coordination of this new ligand is studied by the reaction of K[ta] (3) with zinc(II), manganese(II), cobalt(II), iron(II), and magnesium(II). The ligand favors the formation of metal-organic frameworks (MOFs) apart from magnesium(II). Single-crystal X-ray diffraction analyses reveal MOF structures for [Mn(ta) ] (4), [Fe(ta) ] (5), [Co(ta)2(H O) ] (6), and [Zn(ta) ] (7). © 2010 Taylor & Francis.
Keywords: Ligand design; Metal-organic frameworks; Triazole; N; O ligand

Methane activation by platinum: Critical role of edge and corner sites of metal nanoparticles - Vines, Francesc; Lykhach, Yaroslava; Staudt, Thorsten; Lorenz, Michael Peter Andreas; Papp, Christian; Steinrück, Hans-Peter; Libuda, Jörg; Neyman, Konstantin; Görling, Andreas (Chemistry - A European Journal, 2010)
Complete dehydrogenation of methane is studied on model Pt catalysts by means of state-of-the-art DFT methods and by a combination of supersonic molecular beams with highresolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH and CH are specially stabilized at sites located at particles edges and corners by an amount of 50-80 kJ mol . This stabilization is caused by an enhanced activity of lowcoordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH and CH formation steps are endothermic on Pt(111) but markedly exothermic on Pt . An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJmol for first C - H bond scission and 40 kJ mol for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO film on Cu(111). It is shown that CH generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single-crystal surfaces in which CH was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Methane Oxidation Over Pd Supported on Ceria–Alumina Under Rich/Lean Cycling Conditions - Fouladvand, Sheedeh; Schernich, Stefan; Libuda, Jörg; Gronbeck, Henrik; Pingel, Torben; Olsson, Eva; Skoglundh, Magnus; Carlsson, Per-Anders (Topics in Catalysis, 2013)
Catalysts with highly dispersed palladium on alumina, alumina doped with 20 wt% ceria and ceria have been prepared, characterized and examined for net-lean methane oxidation. In particular, the activity and selectivity were investigated during rich/lean cycling of the feed. The ceria content is found to influence both the general and the instantaneous activity responses. The results indicate that the active phase of palladium changes between reduced and oxidised Pd during the rich/lean cycling, and that the process is influenced by the presence of ceria.

Methanol Steam Reforming Promoted by Molten Salt-Modified Platinum on Alumina Catalysts - Kusche, Matthias Manfred; Agel, Friederike; Ní Bhriain, Nollaig; Kaftan, Andre; Laurin, Mathias; Libuda, Jörg; Wasserscheid, Peter (Chemsuschem, 2014)
We herein describe a straight forward procedure to increase the performance of platinum-on-alumina catalysts in methanol steam reforming by applying an alkali hydroxide coating according to the "solid catalyst with ionic liquid layer" (SCILL) approach. We demonstrate by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed desorption (TPD) studies that potassium doping plays an important role in the catalyst activation. Moreover, the hygroscopic nature and the basicity of the salt modification contribute to the considerable enhancement in catalytic performance. During reaction, a partly liquid film of alkali hydroxides/carbonates forms on the catalyst/alumina surface, thus significantly enhancing the availability of water at the catalytically active sites. Too high catalyst pore fillings with salt introduce a considerable mass transfer barrier into the system as indicated by kinetic studies. Thus, the optimum interplay between beneficial catalyst modification and detrimental mass transfer effects had to be identified and was found on the applied platinum-on-alumina catalyst at KOH loadings around 7.5 mass%.
Keywords: catalyst characterization; heterogeneous catalysis; methanol reforming; molten salts; platinum

Method for extracting arbitrarily large orbital equations of the Pincherle map - Gallas, Jason (Results in Physics, 2016)
We report an algorithm to extract equations of motion for orbits of arbitrarily high periods generated by iteration of the Pincherle map, the operational kernel used in the so-called chaotic computers. The performance of the algorithm is illustrated explicitly by extracting expeditiously, among others, an orbit buried inside a polynomial cluster of equations with degree exceeding one billion, out of reach by ordinary brute-force factorization. Large polynomial clusters are responsible for the organization of the phase-space and knowledge of this organization requires decomposing such clusters.
EAM Import::2019-03-12

Metrology of laser-guided particles in air-filled hollow-core photonic crystal fiber - Schmidt, O. A.; Garbos, M. K.; Euser, T. G.; Russell, Philip St. John (Optics Letters, 2012)
Micrometer-sized particles are trapped in front of an air-filled hollow-core photonic crystal fiber using a novel dual-beam trap. A backward guided mode produces a divergent beam that diffracts out of the core, and simultaneously a focused laser beam launches a forward-propagating mode into the core. By changing the backward/forward power balance, a trapped particle can be selectively launched into the hollow core. Once inside, particles can be optically propelled along several meters of fiber with mobilities as high as 19 cm·s-1 W-1 (precisely measured using in-fiber Doppler velocimetry). The results are in excellent agreement with theory. The system allows determination of fiber loss as well as the mass density and refractive index of single particles.
EAM Import::2019-03-12

MFI-type (ZSM-5) zeolite-filled TiO2 nanotubes for enhanced photocatalytic activity - Paramasivam, Indhumati; Avhale, Abhijeet; Inayat, Amer; Bösmann, Andreas; Schmuki, Patrik; Schwieger, Wilhelm (Nanotechnology, 2009)
The present work demonstrates enhanced photocatalytic activity for zeolite-filled TiO2 nanotubes. ZSM-5 zeolite nanocrystals were grown on and into a TiO2 nanotubular skeleton (TiNT/ZSM-5) by multi-step hydrothermal synthesis consisting of in situ seeding and multiple in situ crystallization (MISC). The resulting zeolite nanocrystals were in the range of a few nanometers and they adhere well to the nanotubular inner walls. After crystallization, the photocatalytic activity of this zeolite-filled nanotube catalyst system was compared with neat anatase TiO2 nanotube (TiNT) and with calcined ZSM-5 powder. The results show for TiNT/ZSM-5 a highly enhanced efficiency for the decomposition of acetophenone (used as an aromatic model organic pollutant).

Microcantilever bending experiments in NiAl - Evaluation, size effects, and crack tip plasticity - Ast, Johannes; Przybilla, Thomas; Maier, Verena; Durst, Karsten; Göken, Mathias (Journal of Materials Research, 2014)
For a better understanding of the local fracture behavior of semi-brittle materials, we carried out bending experiments on notched microcantilevers of varying sizes in the micrometer range using NiAl single crystals. Smaller and larger beams were milled with a focused ion beam in the so-called soft <110> and hard <100> orientation and were tested in situ in a scanning electron microscope and ex situ with a nanoindenter, respectively. The measurements were evaluated using both linear-elastic fracture mechanics and elastic-plastic fracture mechanics. The results show that (i) the fracture toughness is in the same range as the macroscopically determined one which is around 3.5 MPa/m for the soft orientation and around 8.5 MPa/m for the hard orientation, that (ii) there is a strong influence of the anisotropic behavior of NiAl on the fracture toughness values, and that (iii) the J-integral technique is the most accurate quantification metho
Keywords: fracture; ion beam processing; scanning electron microscopy (SEM)

Microflow-based control of near-wall fluctuations for large viscous drag reduction - Jovanovic, Jovan; Srikantharajah, Rubitha; Lienhart, Hermann; Delgado, Antonio; Frohnapfel, Bettina; Jovanović, Djordje (Microfluidics and Nanofluidics, 2011)
The stabilizing effect of microgroove surface morphology on viscous drag reduction was studied experimentally in the inlet region of a plane channel flow. The stabilization is thought to be due to the ability of a microgrooved surface pattern to suppress the velocity fluctuations in the spanwise direction on a restricted portion of the wetted surface, which prevents vorticity development at the wall and consequently across the entire flow field. This smart microflow control strategy, which works successfully only under very particular circumstances, was implemented in a microgroove-modified channel flow in which the front part has a microgrooved surface topology. The results of pressure drop measurements indicate that microgrooved surfaces can effectively stabilize laminar boundary layer development, leading to a significant reduction in the viscous drag. In the rear flat part of the microgroove-modified channel test section, a maximum drag reduction of DR ≃ 35% was measured. This corresponds to an overall drag reduction of DR ≃ 16% at a length Reynolds number of Re x ≃ 10 6: The drag reduction effect persisted in a narrow range of flow velocities and for the reported experimental conditions corresponds to microgroove dimensions between 1.5 and 2.5 viscous length-scales. © Springer-Verlag 2011.
UnivIS-Import:2015-03-09:Pub.2011.tech.ITC.stmmec.microf
Keywords: microflow; drag; control

Microfluidic integration of photonic crystal fibers for online photochemical reaction analysis - Unterkofler, S.; Mcquitty, R. J.; Euser, T. G.; Farrer, N. J.; Sadler, P. J.; Russell, Philip St. John (Optics Letters, 2012)
Liquid-filled hollow-core photonic crystal fibers (HC-PCFs) are perfect optofluidic channels, based on their unique feature of low loss optical guidance in a liquid medium. Thereby, the overlap of the dissolved specimen and the intense light field in the micron-sized core is increased manifold as compared to conventional bioanalytical techniques, facilitating highly efficient photoactivation processes. Here we introduce a novel integrated analytical technology for photochemistry by microfluidic coupling of a HC-PCF nanoflow reactor to supplementary detection devices. Applying a continuous flow through the fiber, we deliver photochemical reaction products to a mass spectrometer in an online and hence rapid fashion, which is highly advantageous over conventional cuvette-based approaches.
EAM Import::2019-03-12

Microimprinting of nanocrystalline metals - Influence of microstructure and work hardening - Durst, Karsten; Backes, Björn; Müller, Johannes; Göken, Mathias; et al. (Journal of Materials Processing Technology, 2010)
The microimprinting behaviour of a wide variety of materials with grain sizes ranging from 25 nm up to a few mm was studied, using a flat punch indenter with a circular cavity with a diameter of 5 extgreekmm and a cavity line width of 350 nm. The flat punch die was fabricated by focussed ion beam machining. The imprinted ring geometry has been evaluated using atomic force microscopy in terms of extrusion height, relative to residual imprinting depth. The experimental work was complemented by Finite Element Analysis (FEA) of the forming process, using experimentally determined stress–strain data of the imprinted materials as input for the finite element simulations. Even so only macroscopic properties of the materials have been used for the modelling, both experiment and simulation yield comparable results. However, the relative extrusion height from FEA are much larger compared to the experimental results. It is found that grain size and the subsequent work hardening behaviour of the material has a strong influence on the local flow behaviour during imprinting. Nanocrystalline materials with nearly ideal-plastic deformation behaviour exhibit the best formability. The influence of the work hardening behaviour on the local flow of the material during microimprinting is moreover confirmed by imprinting of a pre-strained material. A reduction in the work hardenability of the material leads to an increasing microflow during microimprinting.
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Micro/Macroporous System: MFI-Type Zeolite Crystals with Embedded Macropores - Machoke, Albert; Beltran, Ana M.; Inayat, Alexandra; Winter, Benjamin; Weissenberger, Tobias; Kruse, Nadine; Guettel, Robert; Spiecker, Erdmann; Schwieger, Wilhelm; Spiecker, Erdmann (Advanced Materials, 2015)
Zeolite crystals with an embedded and interconnected macropore system are prepared by using mesoporous silica particles as a silica source and as a sacrificial macroporogen. These novel hierarchical zeolite crystals are expected to reduce diffusion limitations in all zeolite-catalyzed reactions, especially in the transformation of larger molecules like in the catalytic cracking of polymers and the conversion of biomass.

Microscopic insights into methane activation and related processes on Pt/Ceria model catalysts - Lykhach, Yaroslava; Staudt, Thorsten; Lorenz, Michael Peter Andreas; Streber, Regine; Bayer, Andreas; Steinrück, Hans-Peter; Libuda, Jörg (ChemPhysChem, 2010)
Ceria-based supported noble-metal catalysts release oxygen, which may help to reduce the formation of carbonaceous residues, for example during hydrocarbon reforming. To gain insight into the microscopic origins of these effects, a model study is performed under ultrahigh-vacuum conditions using single-crystal-based supported model catalysts. The model systems are based on ordered CeO (111) films on Cu(111), on which Pt nanoparticles are grown by physical vapor deposition. The growth and structure of the surfaces are characterized by means of scanning tunneling microscopy, and the electronic structure and reactivity are probed by X-ray photoelectron spectroscopy. Specifically, it is shown that the fully oxidized CeO thin films undergo slight reduction upon Pt deposition (CeO ). This effect is enhanced upon annealing (CeO ), thus indicating facile oxygen release and reverse spillover. The model system is structurally stable up to temperatures exceeding 700 K. The activation of methane is investigated using high-kinetic-energy CH (0.83 eV), generated by a supersonic molecular beam. It is shown that dehydrogenation occurs under rapid formation of CH or C species without detectable amounts of CH being formed, even at low temperatures (100 K). The released hydrogen spills over to the CeO support, which leads to the formation of OH groups. At 200 K and above, the OH groups start to decompose leaving additional Ce centers behind (CeO ). At up to 700 K, carbon deposits are quantitatively removed by reaction with oxygen, which is supplied by reverse spillover from the CeO film, thus leading to substantial reduction of the support ( CeO ). © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
Keywords: cerium oxide; heterogeneous catalysis; methane dehydrogenation; supported catalysts; platinum

Microscopic study on the interfacial strength of hydrogenated amorphous carbon coating systems - Spiecker, Erdmann; et al. (Surface & Coatings Technology, 2011)
Slight variation of the coatings in the ramp layer between Cr and a-C:H resulted in a good or poor adhesion behaviour. Both coatings exhibited in the ramp layer a quasi amorphous matrix structure with short-range ordered substructures. No crystalline chromium carbides could be detected in these regions. Focused investigations of the chemical gradients with Energy Filtered TEM (EFTEM) and Energy Dispersive X-Ray (EDX) spectrometry revealed a clear difference in the chemical composition of the ramp layers. Smooth gradients together with a nanocomposite structure seem to provide a high stability of the ramp layer against delamination. (C) 2010 Elsevier B.V. All rights reserve
Keywords: Diamond-like carbon; Interface; Adhesion strength; Cr adhesion layer

Microstructural evolution during deformation of tin dioxide nanoparticles in a comminution process - Armstrong, Patrick; Knieke, Catharina; Mackovic, Mirza; Frank, Gerhard; Hartmaier, Alexander; Göken, Mathias; Peukert, Wolfgang; Mackovic, Mirza (Acta Materialia, 2009)
Nanoparticles can be produced by wet grinding in stirred media mills if agglomeration is prevented by stabilization of the particles. Since the fracture mechanisms at the lower nanoscale are not yet understood, we studied the evolution of the microstructure within tin dioxide particles. Electrostatic stabilization allows the formation of tin dioxide with a mean particle size of 25 nm as measured by dynamic light scattering. High-resolution transmission electron microscopy (HRTEM) images show particles well below 10 nm and mean crystallite sizes of 9 nm were obtained from X-ray diffraction by applying the Rietveld refinement method. Additionally, TEM and HRTEM analyses were conducted to gain detailed insight into the microstructural effects governing the grinding process. Microscopy revealed surprisingly rich phenomena including the formation of shear bands, twinning and stacking faults that directly affect the grinding behavior. Interestingly the ceramic nanoparticles showed not only fracture patterns expected from brittle fracture but also many traces of plastic deformation. For comparison the uniaxial compression of particles up to 30 nm in diameter was simulated using molecular dynamics. The simulated particles shared microstructural details with the real samples, most importantly the shear bands which lead to significant plastic deformation. The internal microstructure produced during multiple particle stressing events in the mill and also observed in the simulations is directly linked to the fracture mechanism and the experimentally observed grinding limit. © 2009 Acta Materialia In
UnivIS-Import:2015-03-09:Pub.2009.tech.IW.LM.micros
Keywords: Molecular dynamics simulations; Nanomechanics; Nanostructure; Shear bands; Transmission electron microscopy (TEM)

Microstructure and Mechanical Properties of Accumulative Roll-Bonded AA1050A/AA5005 Laminated Metal Composites - Kümmel, Frank; Kreuz, Michael; Hausöl, Tina; Höppel, Heinz Werner; Göken, Mathias (Metals, 2016)
Laminated metal composites (LMCs) with alternating layers of commercial pure aluminum AA1050A and aluminum alloy AA5005 were produced by accumulative roll-bonding (ARB). In order to vary the layer thickness and the number of layer interfaces, different numbers of ARB cycles (4, 8, 10, 12, 14 and 16) were performed. The microstructure and mechanical properties were characterized in detail. Up to 8 ARB cycles, the ultrafine-grained (UFG) microstructure of the layers in the LMC evolves almost equally to those in AA1050A and AA5005 mono-material sheets. However, the grain size in the composites tends to have smaller values. Nevertheless, the local mechanical properties of the individual layers in the LMCs are very similar to those of the mono-material sheets, and the macroscopic static mechanical properties of the LMCs can be calculated as the mean value of the mono-material sheets applying a linear rule of mixture. In contrast, for more than 12 ARB cycles, a homogenous microstructure was obtained where the individual layers within the composite cannot be visually separated any longer; thus, the hardness is at one constant and a high level across the whole sheet thickness. This results also in a significant higher strength in tensile testing. It was revealed that, with decreasing layer thickness, the layer interfaces become more and more dominating.
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Keywords: Nanomaterials

Microstructure and mechanical properties of accumulative roll bonded AA6014/AA5754 aluminium laminates - Hausöl, Tina; Höppel, Heinz Werner; Göken, Mathias (Materials Science Forum, 2011)
Among the well-known methods of severe plastic deformation the accumulative roll bonding (ARB) process is most promising for producing ultrafine-grained (UFG) materials with extraordinary mechanical properties at an industrial scale. Besides, it has also been shown that the ARB process can be successfully used to produce multi-component materials with tailored properties by reinforcement or grading, respectively. In this work, laminates with alternating layers of the high strength aluminium alloy AA5754 and the AA6014 alloy, well-known for good formability and high surface quality, were produced by ARB at 230 °C. Microstructural and mechanical investigations were performed after 2, 4 and 6 ARB cycles by means of light and electron microscopy, nanoindentation experiments and tensile testing. After ARB processing an ultrafine-grained microstructure is obtained. The UFG microstructure as well as the local mechanical properties alter with the layer composition. With increasing number of ARB cycles the interfaces between the layers become more and more wavy by shear band formation. Compared to the pure accumulative roll bonded AA6014 the yield and ultimate tensile strength of the multi-component laminates are considerably higher and are only slightly reduced in comparison to the high strength AA5754. In terms of elongation to failure no reduction in ductility is found. The serrated yielding effect, clearly visible in AA5754, is shifted to higher strains or fully disappears, respectively, whereas in AA5754 the magnitude of serrations increases with increasing number of ARB cycles. Combining AA5754 and AA6014 sheets by ARB results in well bonded ultrafine-grained laminates which exhibit a combination of the beneficial properties of the single-component materials: high strength of AA5754 and good surface quality of AA6014.
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Microstructure and mechanical properties of accumulative roll bonded aluminium alloy AA5754 - Hausöl, Tina; Höppel, Heinz Werner; Göken, Mathias (Journal of Physics: Conference Series, 2010)
The aluminium alloy AA5754 is used for many technical applications. In this work, the accumulative roll bonding process is applied to this alloy in order to investigate the potential of an ultrafine-grained structure on the mechanical properties of this Al-Mg alloy. Sheets from AA5754 (AlMg3) were successfully processed by accumulative roll bonding in order to obtain an ultrafine-grained microstructure. The ARB process was performed at 230 °C or 250 °C up to 7 or 8 cycles respectively. Thus the grain size decreased from 10 extgreekmm (initial state) to approximately 80 nm (ultrafine-grained state, normal direction). The microstructural evolution and the mechanical properties have been investigated by means of scanning electron microscopy, hardness measurements and tensile testing. After one ARB cycle the samples showed an increase in hardness by a factor of almost 2 in comparison to the as-received material. Further processing causes a linear increase of hardness with each additional cycle. Yield strength and tensile strength of the roll bonded specimens are highly increased in comparison to the as-received samples whereas the ductility declined. A considerable increase in ductility is obtained by heat treatment of the ARB specimens at 250 °C, but on the expense of a moderate decreased strength. The deformation behaviour is also influenced by the ultrafine-grained structure. The occurrence of the Portevin-Le Chatelier effect is manifested by serrated stress-strain curves. The amplitude of serrations increases with increasing number of ARB cycles but can be reduced by the appliance of a higher strain rate. Lüders strain only occurs at the as-received, i.e. not strained, samples.
EAM Import::2019-03-12

Microstructure-dependent deformation behaviour of bcc-metals - indentation size effect and strain rate sensitivity - Maier, Verena; Schunk, Christopher; Göken, Mathias; Durst, Karsten (Philosophical Magazine, 2015)
In this work, the indentation size effect and the influence of the microstructure on the time-dependent deformation behaviour of body-centred cubic (bcc) metals are studied by performing nanoindentation strain rate jump tests at room temperature. During these experiments, the strain rate is abruptly changed, and from the resulting hardness difference the local strain rate sensitivity has been derived. Single-crystalline materials exhibit a strong indentation size effect; ultrafine-grained metals have nearly a depth-independent hardness. Tungsten as a bcc metal shows the opposite behaviour as generally found for face-centered cubic metals. While for UFG-W only slightly enhanced strain rate sensitivity was observed, SX-W exhibits a pronounced influence of the strain rate on the resulting hardness at room temperature. This is due to the effects of the high lattice friction of bcc metals at low temperatures, where the thermally activated motion of screw dislocations is the dominating deformation mechanisms, which causes the enhanced strain rate sensitivity. For the SX-materials, it was found that the degree of the indentation size effect directly correlates with the homologous testing temperature and thus, the material specific parameter of the critical temperature T-c. However, for the resultant strain rate sensitivity no depth-dependent change was found.
Keywords: nanoindentation; bcc metals; deformation behaviour; strain rate sensitivity; indentation size effect

Microstructure of veneered zirconia after surface treatments: A TEM study - Grigore, Alexandra; Spallek, Stefanie; Petschelt, Anselm; Butz, Benjamin; Spiecker, Erdmann; Lohbauer, Ulrich (Dental Materials, 2013)
Signcance. Regardless of the treatment protocol and produced roughness, the veneering ceramic perfectly sealed the zirconia surface. XRD showed an increased amount of monoclinic phase on the surface treated zirconia. However after thermal treatment, the monoclinic phase was re-transformed into the tetragonal polymorph. TEM/SAED analysis has found indication for a greater extend of the monoclinic transformation into the bulk zirconia compared to the treatment related defective zone depth. (C) 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Keywords: Y-TZP; X-ray diffraction; Martensitic transformation; Sandblasting; Bur drilling; Annealing

Micro-to-macro transition accounting for general imperfect interfaces - Javili, Ali; Steinmann, Paul; Mosler, Joern (Computer Methods in Applied Mechanics and Engineering, 2017)
The objective of this contribution is to establish a micro-to-macro transition framework to study the behavior of heterogeneous materials whereby the influence of interfaces at the microscale is taken into account. The term ``interface'' refers to a zero-thickness model that represents the finite thickness ``interphase'' between the constituents of the micro-structure. For geometrically equivalent samples, due to increasing area-to-volume ratio with decreasing size, interfaces demonstrate a more pronounced effect on the material response at small scales. A remarkable outcome is that including interfaces introduces a length-scale and our interface-enhanced computational homogenization captures a size effect in the material response even if linear prolongation conditions are considered. Furthermore, the interface model in this contribution is general imperfect in the sense that it allows for both jumps of the deformation as well as for the traction across the interface. Both cohesive zone model and interface elasticity theory can be derived as two limit cases of this general model. We establish a consistent computational homogenization scheme accounting for general imperfect interfaces. Suitable boundary conditions to guarantee meaningful averages are derived. Clearly, this general framework reduces to classical computational homogenization if the effect of interfaces is ignored. Finally, the proposed theory is elucidated via a series of numerical examples.
EAM Import::2019-03-12

Micro-to-macro transitions for continua with surface structure at the microscale - Javili, Ali; McBride, Andrew Trevor; Mergheim, Julia; Steinmann, Paul; Schmidt, Ulrike (International Journal of Solids and Structures, 2013)
A geometrically non-linear framework for micro-to-macro transitions is developed that accounts for the effect of size at the microscopic scale. This is done by endowing the surfaces of the microscopic features with their own (energetic) structure using the theory of surface elasticity. Following a standard first-order ansatz on the microscopic motion in terms of the macroscopic deformation gradient, a Hill-type averaging condition is used to link the two scales. The surface elasticity theory introduces two additional microscopic length scales: the ratio of the bulk volume to the energetic surface area, and the ratio of the surface and bulk Helmholtz energies. The influence of these microscopic length scales is elucidated via a series of numerical examples performed using the finite element method. © 2013 Elsevier Ltd. All rights reserved.
UnivIS-Import:2015-03-09:Pub.2013.tech.FT.FT-TM.microt
Keywords: Homogenization; Nanomaterials; Size effects; Surface elasticity

Mimicking the Growth of a Pathologic Biomineral: Shape Development and Structures of Calcium Oxalate Dihydrate in the Presence of Polyacrylic Acid - Thomas, Annu; Rosseeva, Elena; Hochrein, Oliver; Carrillo-Cabrera, Wilder; Simon, Paul; Duchstein, Patrick; Zahn, Dirk; Kniep, Ruediger (Chemistry - A European Journal, 2012)
The morphogenesis of calcium oxalate hydrates in aqueous solutions was investigated by varying the pH, oxalate concentration, and the concentration of the sodium salt of polyacrylate (PAA). With increasing amounts of PAA in solution, the shape of tetragonal calcium oxalate dihydrate (COD) changes from bipyramidal through elongated bipyramidal prisms to dumbbells and finally reverts to rodlike tetragonal bipyramidal prisms. PAA is incorporated into the prismatic zones of the growing COD crystals, thereby reducing the growth rate of the [100] faces along the <100> direction. Dumbbells start to develop through ``non-crystallographic'' branching from the prism faces and the formation of ``multiple head'' crystals. Adsorption of PAA on the rough surfaces of the splitting individuals supports the selection of new subindividuals and leads to the formation of core–shell patterns. The various shapes and structures of the biomimetic COD/PAA crystals and aggregates are closely related to the wellknown ``pathologic'' individuals observed in the urine of patients with urinary disease (including urinary stones).
EAM Import::2019-03-12

Miniature chiral beamsplitter based on gyroid photonic crystals - Turner, Mark D.; Saba, Matthias; Zhang, Qiming; Cumming, Benjamin P.; Schröder-Turk, Gerd; Gu, Min (Nature Photonics, 2013)
The linearly polarizing beamsplitter is a widely used optical component in photonics. It is typically built from a linearly birefringent crystal such as calcite, which has different critical reflection angles for s- and p-polarized light, leading to the transmission of one linear polarization and angled reflection of the other. However, the analogue for splitting circularly polarized light has yet to be demonstrated due to a lack of natural materials with sufficient circular birefringence. Here, we present a nano-engineered photonic-crystal chiral beamsplitter that fulfils this task. It consists of a prism featuring a nanoscale chiral gyroid network and can separate left- and right-handed circularly polarized light in the wavelength region around 1.615 µm. The structure is fabricated using a galvo-dithered direct laser writing method and could become a useful component for developing integrated photonic circuits that provide a new form of polarization control.
EAM Import::2019-03-12

Miniaturized bend tests on partially stabilized EB-PVD ZrO2 thermal barrier coatings - Pfeiffer, Carolin; Affeldt, Ernst; Göken, Mathias (Surface & Coatings Technology, 2011)
The elastic properties of thermal barrier coatings (TBCs) are important for modelling the lifetime of these coatings. A new test setup has been developed to measure the system modulus of electron-beam enhanced physical vapour deposited (EB-PVD) TBC coatings by miniaturized bend tests. Due to the brittleness, low stiffness and small thickness of the top coat and its complex microstructure, it is difficult to measure its Young's modulus by standard mechanical testing. For this reason, a special sample material has been prepared which consists of a 1 mm thick layer of EB-PVD TBC. This material was isothermally heat treated for different times at 950 °C, 1100 °C and 1200 °C and then tested in a specially developed miniaturized bend test. The bend test setup permits mechanical tests with a high resolution in stress and strain, where the strain is measured by digital image correlation. So the stiffness of the free-standing TBC samples could be measured with a high accuracy and the sintering behaviour of the EB-PVD TBC and the consequent rise of Young's modulus could be determined. The results show a significant increase of the system modulus with heat treatment time and temperature caused by sintering of the coating. An activation energy of 220 kJ/mol for the process has been determined. In addition, the material was tested by nanoindentation in order to measure Young's modulus on a local scale, and the porosity of the samples was determined by quantitative image analysis.
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Minimal surface scaffold designs for tissue engineering - Kapfer, Sebastian; Hyde, Stephen T.; Mecke, Klaus; Arns, Christoph H.; Schröder-Turk, Gerd (Biomaterials, 2011)
Triply-periodic minimal surfaces are shown to be a more versatile source of biomorphic scaffold designs than currently reported in the tissue engineering literature. A scaffold architecture with sheetlike morphology based on minimal surfaces is discussed, with significant structural and mechanical advantages over conventional designs. These sheet solids are porous solids obtained by inflation of cubic minimal surfaces to sheets of finite thickness, as opposed to the conventional network solids where the minimal surface forms the solid/void interface. Using a finite-element approach, the mechanical stiffness of sheet solids is shown to exceed that of conventional network solids for a wide range of volume fractions and material parameters. We further discuss structure property relationships for mechanical properties useful for custom-designed fabrication by rapid prototyping. Transport properties of the scaffolds are analyzed using Lattice-Boltzmann computations of the fluid permeability. The large number of different minimal surfaces, each of which can be realized as sheet or network solids and at different volume fractions, provides design flexibility essential for the optimization of competing design targets. (C) 2011 Elsevier Ltd. All rights reserved.
Keywords: Bone tissue engineering; Finite element analysis; Mechanical properties; Scaffold; Rapid prototyping

Minkowski Tensor Shape Analysis of Cellular, Granular and Porous Structures - Peukert, Wolfgang; Schröder-Turk, Gerd; Mickel, Walter; Kapfer, Sebastian; Klatt, Michael; Schaller, Fabian; Hoffmann, Matthias; Kleppmann, Nicola; Inayat, Amer; Armstrong, Patrick; Hug, Daniel; Reichelsdorfer, Martin; et al. (Advanced Materials, 2011)
Predicting physical properties of materials with spatially complex structures is one of the most challenging problems in material science. One key to a better understanding of such materials is the geometric characterization of their spatial structure. Minkowski tensors are tensorial shape indices that allow quantitative characterization of the anisotropy of complex materials and are particularly well suited for developing structure-property relationships for tensor-valued or orientation-dependent physical properties. They are fundamental shape indices, in some sense being the simplest generalization of the concepts of volume, surface and integral curvatures to tensor-valued quantities. Minkowski tensors are based on a solid mathematical foundation provided by integral and stochastic geometry, and are endowed with strong robustness and completeness theorems. The versatile definition of Minkowski tensors applies widely to different types of morphologies, including ordered and disordered structures. Fast linear-time algorithms are available for their computation. This article provides a practical overview of the different uses of Minkowski tensors to extract quantitative physically-relevant spatial structure information from experimental and simulated data, both in 2D and 3D. Applications are presented that quantify (a) alignment of co-polymer films by an electric field imaged by surface force microscopy; (b) local cell anisotropy of spherical bead pack models for granular matter and of closed-cell liquid foam models; (c) surface orientation in open-cell solid foams studied by X-ray tomography; and (d) defect densities and locations in molecular dynamics simulations of crystalline coppe
Keywords: geometric; statistic; spatial; anisotropy; shape; morphology; crystal

Mit Simulationen Nanokristallen und -kompositen auf der Spur - Milek, Theodor; Duchstein, Patrick; Zahn, Dirk (Nachrichten aus der Chemie, 2012)
Molekulardynamiksimulationen geben Aufschlüsse über die elementaren Schritte der Keimbildung und des Kristallwachstums. Langfristig soll dies der Steuerung von Keimbildungs- und Wachstumsprozessen dienen und neue Perspektiven zum Design nanokristalliner Systeme eröffnen.
EAM Import::2019-03-12

Mixed Layers of β-Lactoglobulin and SDS at Air-Water Interfaces with Tunable Intermolecular Interactions - Engelhardt, Kathrin; Weichsel, Ulrike; Kraft, Elena; Segets, Doris; Peukert, Wolfgang; Braunschweig, Björn (Journal of Physical Chemistry B, 2014)
Mixtures of extgreekb-lactoglobulin (BLG) and sodium dodecyl sulfate (SDS) were studied at pH 3.8 and 6.7 under equilibrium conditions. At these pH conditions, BLG carries either a positive or a negative net charge, respectively, which enables tunable electrostatic interactions between anionic SDS surfactants and BLG proteins. For pH 3.8, vibrational sum-frequency generation (SFG) and ellipsometry indicate strong BLG–SDS complex formation at air–water interfaces that is caused by attractive electrostatic interactions. The latter complexes are already formed in the bulk solution which was confirmed by a thermodynamic study of BLG–SDS mixtures using isothermal titration calorimetry (ITC). For acidic conditions we determine from our ITC data an exothermal binding enthalpy of -40 kJ mol–1. Increasing SDS/BLG molar ratios above 10 leads to a surface excess of SDS and thus to a charge reversal from a positive net charge with BLG as the dominating surface adsorbed species to a negatively charged layer with SDS as the dominating surface species. The latter is evidenced by a pronounced minimum in SFG intensities that is also accompanied by a phase change of O–H stretching bands due to a reorientation of H2O within the local electric field. This phase change which occurs at SDS/BLG molar ratio between 1 and 10 causes a polarity change in SFG intensities from BLG aromatic C–H stretching vibrations. Conclusions from SFG spectra are corroborated by ellipsometry which shows a dramatic increase in layer thicknesses at molar ratios where a charge reversal occurs. The formation of interfacial multilayers comprising SDS-BLG complexes is, thus, caused by cancellation of electrostatic interactions which leads to agglomeration at the interface. In contrast to pH 3.8, behavior of BLG–SDS mixtures at pH 6.7 is different due to repulsive electrostatic interactions between SDS and BLG which lead to a significantly reduced binding enthalpy of -17 kJ mol–1. Finally, it has to be mentioned that SFG spectra show a coexistence of BLG and SDS molecules at the interface for BLG–SDS molar ratios > 2.
EAM Import::2019-03-12
Keywords: surface; tension; intermolecular; interaction; air; water; interface

Mixed self-assembled monolayer of molecules with dipolar and acceptor character-Influence on hysteresis and threshold voltage in organic thin-film transistors - Jedaa, Abdesselam; Salinas Batallas, Michael; Jäger, Christof; Clark, Timothy; Ebel, Alexander; Hirsch, Andreas; Halik, Marcus (Applied Physics Letters, 2012)
In this report, we investigate the impact of the molecular dipole moment and redox active head groups (C ) in pure and mixed self-assembled monolayers (SAMs), which serve as an ultra-thin hybrid dielectric layer in low-voltage operating organic thin-film transistors. We show that the dipole of the SAM-forming molecules affects the threshold voltage, while the concentration of redox-active C moieties determines the hysteresis in devices with α,α′-dihexyl-sexithiophene and pentacene as organic semiconductors. © 2012 American Institute of Physics.

Mode-coupling theory of the glass transition for confined fluids - Lang, Simon; Schilling, Rolf; Krakoviack, Vincent; Franosch, Thomas (Physical Review E, 2012)
We present a detailed derivation of a microscopic theory for the glass transition of a liquid enclosed between two parallel walls relying on a mode-coupling approximation. This geometry lacks translational invariance perpendicular to the walls, which implies that the density profile and the density-density correlation function depends explicitly on the distances to the walls. We discuss the residual symmetry properties in slab geometry and introduce a symmetry adapted complete set of two-point correlation functions. Since the currents naturally split into components parallel and perpendicular to the walls the mathematical structure of the theory differs from the established mode-coupling equations in bulk. We prove that the equations for the nonergodicity parameters still display a covariance property similar to bulk liquids.
EAM Import::2019-03-12

Model-based control of dynamic frictional contact problems using the example of hot rolling - Werner, Stefan; Stingl, Michael; Leugering, Günter (Computer Methods in Applied Mechanics and Engineering, 2017)
We consider hot forming processes, in which a metal solid body is deformed by several rolls in order to obtain a desired final shape. To minimize cutting scrap and to ensure that this shape satisfies the required tolerances as precisely as possible, we formulate an optimal control problem where we use the trajectories of the rolls as control functions. The deformation of the solid body is described through the basic equations of nonlinear continuum mechanics, which are here coupled with an elasto-viscoplastic material model based on a multiplicative split of the deformation gradient. We assume that the deformations of the rolls can be neglected, thus we add unilateral frictional contact boundary conditions, resulting in an evolutionary quasi-variational inclusion. The associated control-to-observation map is non-differentiable due to changes of state between elastic and plastic material behavior, contact and separation and stick and slip motion, yet we still want to apply gradient-based methods to solve the optimal control problem and therefore have to make sure that derivatives of cost functional and constraints exist. To resolve this issue, we first regularize all non-differentiabilities and subsequently apply the direct differentiation method to obtain sensitivity information. Finally, we formulate a suitable algorithm and discuss numerical results for a real-world example to illustrate its capability. (C) 2017 Elsevier B.V. All rights reserved.
Keywords: Optimal control; Regularization; Sensitivity analysis; Nonlinear continuum mechanics; Frictional contact; Elasto-viscoplasticity

Model-Based Identification and Experimental Validation of the Optimal Reaction Route for the Hydroformylation of 1-Dodecene - Hentschel, Benjamin; Kiedorf, Gregor; Gerlach, Martin; Hamel, Christof; Seide-Morgenstern, Andreas; Freund, Hannsjörg; Sundmacher, Kai (Industrial & Engineering Chemistry Research, 2015)
Previously developed kinetic and dynamic models of the hydroformylation of 1-dodecene in a thermomorphic multicomponent solvent system (TMS), consisting of DMF, n-decane, and hydroformylation products, were experimentally validated applying various operation modes, such as batch, semibatch, and perturbed batch operation. On the basis of experimentally obtained data, which cover a broad range of physical conditions, the parameters of the reaction kinetics were refined to give reliable model predictions as basis for rigorous process optimization. The improved model was used for dynamic optimization to obtain optimal trajectories (e.g., temperature and gas dosing fluxes versus reaction time), which maximize the selectivity to the desired linear aldehyde product. The predicted optimal trajectories were successfully validated in semibatch reactor experiments.

Model-based prediction of optimal conditions for 1-octene hydroformylation - Hentschel, Benjamin; Peschel, Andreas; Xie, Mingquan; Vogelpohl, Christina; Sadowski, Gabriele; Freund, Hannsjörg; Sundmacher, Kai (Chemical Engineering Science, 2014)
After fitting the PC-SAFT parameters to pure component data the model was used to study the influence of product formation on the solubilities of H-2 and CO. Finally, the reaction conditions were optimized in order to maximize the differential selectivity to the desired product n-nonanal. It is shown that the performance of the hydroformylation reaction system can be significantly increased by optimally controlling temperature, total pressure, and the amount of H-2 and CO in the gas phase. (C) 2013 Elsevier Ltd. All rights reserved.
Keywords: PC-SAFT; Multiphase systems; VLE; Simultaneous optimization; Hydroformylation; Reactor design

Model catalytic studies of liquid organic hydrogen carriers: Dehydrogenation and decomposition mechanisms of dodecahydro-n-ethylcarbazole on pt(111) - Amende, Maximilian; Gleichweit, Christoph; Werner, Kristin; Schernich, Stefan; Zhao, Wei; Lorenz, Michael Peter Andreas; Höfert, Oliver; Papp, Christian; Koch, Marcus; Wasserscheid, Peter; Laurin, Mathias; Steinrück, Hans-Peter; et al. (ACS Catalysis, 2014)
Liquid organic hydrogen carriers (LOHC) are compounds that enable chemical energy storage through reversible hydrogenation. They are considered a promising technology to decouple energy production and consumption by combining high-energy densities with easy handling. A prominent LOHC is N-ethylcarbazole (NEC), which is reversibly hydrogenated to dodecahydro-N-ethylcarbazole (H -NEC). We studied the reaction of H-NEC on Pt(111) under ultrahigh vacuum (UHV) conditions by applying infrared reflection-absorption spectroscopy, synchrotron radiation-based high resolution X-ray photoelectron spectroscopy, and temperature-programmed molecular beam methods. We show that molecular adsorption of H-NEC on Pt(111) occurs at temperatures between 173 and 223 K, followed by initial C-H bond activation in direct proximity to the N atom. As the first stable dehydrogenation product, we identify octahydro-N-ethylcarbazole (H -NEC). Dehydrogenation to H-NEC occurs slowly between 223 and 273 K and much faster above 273 K. Stepwise dehydrogenation to NEC proceeds while heating to 380 K. An undesired side reaction, C-N bond scission, was observed above 390 K. H-NEC and H-carbazole are the dominant products desorbing from the surface. Desorption occurs at higher temperatures than H-NEC formation. We show that desorption and dehydrogenation activity are directly linked to the number of adsorption sites being blocked by reaction intermediates. © 2014 American Chemical Society.

Model Catalytic Studies of Novel Liquid Organic Hydrogen Carriers: Indole, Indoline and Octahydroindole on Pt(111) - Schwarz, Matthias; Bachmann, Philipp; Nascimento Silva, Thais; Mohr, Susanne; Scheuermeyer, Marlene; Späth, Florian; Bauer, Udo; Düll, Fabian; Steinhauer, Johann; Hohner, Chantal; Döpper, Tibor; Noei, Heshmat; et al. (Chemistry - A European Journal, 2017)
Indole derivatives were recently proposed as potential liquid organic hydrogen carriers (LOHC) for storage of renewable energies. In this work, we have investigated the adsorption, dehydrogenation and degradation mechanisms in the indole/indoline/octahydroindole system on Pt(111). We have combined infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS) and DFT calculations. Indole multilayers show a crystallization transition at 200 K, in which the molecules adopt a strongly tilted orientation, before the multilayer desorbs at 220 K. For indoline, a less pronounced restructuring transition occurs at 150 K and multilayer desorption is observed at 200 K. Octahydroindole multilayers desorb already at 185 K, without any indication for restructuring. Adsorbed monolayers of all three compounds are stable up to room temperature and undergo deprotonation at the NH bond above 300 K. For indoline, the reaction is followed by partial dehydrogenation at the 5-membered ring, leading to the formation of a flat-lying di-sigma-indolide in the temperature range from 330-390 K. Noteworthy, the same surface intermediate is formed from indole. In contrast, the reaction of octahydroindole with Pt(111) leads to the formation of a different intermediate, which originates from partial dehydrogenation of the 6-membered ring. Above 390 K, all three compounds again form the same strongly dehydrogenated and partially decomposed surface species.
Keywords: density functional theory; infrared reflection absorption spectroscopy; liquid organic hydrogen carrier; octahydroindole; X-ray photoelectron spectroscopy

Modeling charge transport in C60-based self-assembled monolayers for applications in field-effect transistors - Leitherer, Susanne; Jäger, Christof; Halik, Marcus; Clark, Timothy; Thoss, Michael (Journal of Chemical Physics, 2014)
We have investigated the conductance properties of C60-containing self-assembled monolayers (SAMs), which are used in organic field-effect transistors, employing a combination of molecular-dynamics simulations, semiempirical electronic structure calculations, and Landauer transport theory. The results reveal the close relation between the transport characteristics and the structural and electronic properties of the SAM. Furthermore, both local pathways of charge transport in the SAMs and the influence of structural fluctuations are analyzed.
UnivIS-Import:2015-04-14:Pub.2014.nat.dphy.ITP.ppem.modeli

Modeling martensic transformations in crystalline solids: validity and redesign of geometric approaches - Zahn, Dirk (Zeitschrift für Kristallographie, 2011)
Taking use of group theory, elegant mathematics allows transformations between unit cells and the estimate of intermediate structures. This, however, implies a (local) second order-type picture of what in reality is a first-order phase transition. Recent constant pressure molecular dynamics simulations combined with the transition path sampling approach opened a new level of detail for the mechanistic studies of pressure-induced phase transitions. For a selection of computationally favorable systems, we were able to observe local nucleation events, followed by subsequent phase growth and, provided large enough simulation cells, even the identification of phase growth from multiple centers leading to polycrystalline structures. On this basis, the validity of the long-established geometric approaches must be reconsidered. Phase interfaces are typically not smooth (which would justify a local second-order type modeling) but may be as sharp as a single layer of atoms. To account for this issue, a simple and generally applicable concept for the more appropriate design of the geometric modeling approaches to solid-solid transitions is outlined.
EAM Import::2019-03-12

Modeling of Metamaterials in Wave Propagation - Leugering, Günter; Rohan, Eduard; Seifrt, Frantiek (Wave Propagation in Periodic Media, 2010)
This chapter focuses on acoustic, electromagnetic, elastic and piezo-electric wave propagation through heterogenous layers. The motivation is provided by the demand for a better understanding of meta-materials and their possible construction. We stress the analogies between the mathematical treatment of phononic, photonic and elastic meta-materials. Moreover, we treat the cloaking problem in more detail from an analytical and simulation oriented point of view. The novelty in the approach presented here is with the interlinked homogenization- and optimization procedur
EAM Import::2019-03-12

Modeling of the effective work function instability in metal/high-κ dielectric stacks - Fet, A.; Haeublein, V.; Bauer, A. J.; Ryssel, Heiner; Frey, Lothar (Journal of Applied Physics, 2010)
This paper discusses the effective work function instability in high- extgreekk-based MOS gate stacks, which occurs after high temperature (1070 °C) processing. Theories which have been put forward to explain this effect are discussed and unified to a consistent phenomenological model. The Vfb roll-off effect is also discussed and can be described by the model.
EAM Import::2019-03-12

Modeling of the Plastic Characteristics of AA6082 for the Friction Stir Welding Process - Pellegrino, Sergio; Fratini, Livan; Merklein, Marion; Böhm, Wolfgang; Hung Nguyen, (Key Engineering Materials, 2015)
Focus of this paper is to model the plastic forming behavior of AA6082, in order to develop the numerical FE analysis of the friction stir welding processes and the simulation of subsequent forming processes. During the friction stir welding process, the temperatures reached can range up to 500 °C and have a fundamental role for the correct performance of the process, so the material data has to show a temperature dependency. Because of the tool rotation a strain rate sensitivity of the material has to be respected as well. In this context, the general material characteristics of AA6082 were first identified for different stress states. For the uniaxial state the standard PuD-Al used in the automotive industry was applied, for the shear state the ASTM B831-05 was used and for biaxial states the ISO 16842 was exploited. To characterize the plastic flow behavior of the AA6082 at elevated temperatures, tensile tests were performed according to DIN EN ISO 6892-2 from 25 °C to 500 °C with a strain rate from 0.1 s-1 up to 6.5 s-1.
EAM Import::2019-03-12

Modeling the dielectric function of degenerately doped ZnO: Al thin films grown by ALD using physical parameters - Latzel, Michael; Göbelt, Manuela; Brönstrup, Gerald; Venzago, Cornel; Schmitt, W.; Sarau, George; Christiansen, Silke (Optical Materials Express, 2015)
Transparent conductive thin films are a key building block of modern optoelectronic devices. A promising alternative to expensive indium containing oxides is aluminum doped zinc oxide (AZO). By correlating spectroscopic ellipsometry and photoluminescence, we analyzed the contributions of different optical transitions in AZO grown by atomic layer deposition to a model dielectric function (MDF) over a wide range of photon energies. The derived MDF reflects the effects of the actual band structure and therefore describes the optical properties very accurately. The presented MDF is solely based on physically meaningful parameters in contrast to empirical models like e.g. the widely used Sellmeier equation, but nevertheless real and imaginary parts are expressed as closed-form expressions. We analyzed the influence of the position of the Fermi energy and the Fermi-edge singularity to the different parts of the MDF. This information is relevant for design and simulation of optoelectronic devices and can be determined by analyzing the results from spectroscopic ellipsometry.

Modellbasierte Ermittlung der optimalen Reaktionsführung für integrierte Mehrphasenprozesse - Hentschel, Benjamin; Freund, Hannsjörg; Sundmacher, Kai (Chemie Ingenieur Technik, 2014)
Grundlage für eine optimale Reaktorauslegung ist die Kenntnis der optimalen Reaktionsführung. Modellbasierte Methoden sind dabei von besonderem Wert, da bei komplexen Systemen eine intuitive Erarbeitung der optimalen Reaktionsführung nicht mehr möglich ist. Am Beispiel der Hydroformulierung langkettiger Alkene wird hier eine auf modellbasierter Optimalsteuerung beruhende Methode beschrieben, die mögliche Stoffrückführungen bereits in einer frühen Phase der Prozessentwicklung berücksichtigt und damit neben der optimalen Reaktionsführung auch auf Anforderungen an die Trennoperationen schließen lässt.
EAM Import::2019-03-12

Modelling Aspects in Accumulative Roll Bonding Process by Explicit Finite Element Analysis - Fratini, Livan; Merklein, Marion; Böhm, Wolfgang; Campanella, Davide (Key Engineering Materials, 2013)
Accumulative Roll-Bonding (ARB) process is a severe plastic deformation (SPD) process, capable of developing grains below 1 extgreekmm in diameter and improving mechanical properties of the material. In this study, the authors compared two different FE-codes with respect of its applicability for numerical analysis of the ARB process. Modelling this process was achieved using the explicit code for Abaqus/CAE both in 2D and 3D. The proposed model was used to assess the impact of ARB cycles on the final material properties. The numerical results in 2D and 3D were compared and contrasted. The research work presented in this paper is focused on the simulation optimization based on CPU time minimization. The numerical simulations were also validated through a comparison with the experimental results.
EAM Import::2019-03-12

Modelling, Construction and Manufacture of a Lightweight Robot Arm - Hagenah, Hinnerk; Böhm, Wolfgang; Breitsprecher, Thilo; Merklein, Marion; Wartzack, Sandro (Proceedings of the 8th CIRP International Conference on Intelligent Computation in Manufacturing Engineering, 2012)
This contribution will show how modern materials as cellular titanium and nanocrystalline aluminium can be used to build advanced lightweight robot arms. Cellular titanium allowing a specific density distribution and the manufacture of fine structures with desirable properties is produced by selective electron beam melting while the nanocrystalline aluminium results from accumulative roll bonding. The specific properties of both materials will be highlighted. The paper will cover the definition of the product specification, the setting of a basic design and the optimisation of this by means of topology optimisation. This optimisation requires an intelligent modelling to be able to investigate different initial settings and boundary conditions. The optimised part will then be finished with regards to manufacturability. The paper will close with a brief report about the actual manufacture of one of the investigated variants.
UnivIS-Import:2015-04-16:Pub.2012.tech.FT.FT-KLMEFK.modell

Modelling haze and transmission of transparent filled systems in dependence of filler surface area, refractive index difference and wavelength - Wildner, Wolfgang; Drummer, Dietmar (SPE Proceedings ANTEC, 2015)
Glass-filled polymers with a matching refractive between polymer and glass might combine transparency. In this paper the transparency of glassfilled polymers transparency and haze, model-like specimens consisting out of a cuvette filled with oil and glass fillers were prepared. The haze of these specimens is strongly dependent on the refractive index difference between oil and filler material and is close to zero at a matching refractive index. On the basis of these measurements an empirical model for the calculation of transparency and haze is created. The results provided by this model show a good match with the measured values of the specimens and also with results of experiments described in literature. Based on the model, calculated graphs to determine the haze of glass-filled polymers in dependence of the specific filler surface area, the refractive index difference, the wavelength and the sample thickness.
UnivIS-Import:2015-10-26:Pub.2015.tech.FT.FT-KT.modell_3

Model NOx Storage Materials at Realistic NO2 Pressures - Desikusumastuti, Aine; Schernich, Stefan; Happel, Markus; Sobota, Marek; Laurin, Mathias; Libuda, Jörg (ChemCatChem, 2009)
The interaction of NO2 with single-crystal-based model NOx storage materials, consisting of barium aluminate nanoparticles on Al2O3/NiAl(110), are investigated by time-resolved infrared reflection absorption spectroscopy (TR-IRAS) at realistic NO2 partial pressures up to 1.75 mbar. The data is compared to spectra obtained under ultrahigh vacuum (UHV) conditions on the same model system. At 300 K, the NO2 uptake at pressures around I mbar proceeds through rapid initial formation of surface nitrites and nitrates, similar to that under UHV conditions. The vibrational spectra of the surface species formed at realistic NO2 pressures are comparable to those for species formed under UHV conditions. Beyond the formation of surface species, the formation of bulk nitrates occurs, but is kinetically strongly hindered. At a very low rate, the formation of a disordered barium bulk nitrate is detected. At 500K, this kinetic hindrance is overcome and the available Ba2+ is quantitatively converted to bulk Ba(NO3)(2). The IRAs spectrum of these Ba(NO3)(2) particles differs characteristically from those obtained for nitrate multilayers formed upon incomplete conversion under UHV conditions. In addition to the formation of bulk Ba(NO3)(2), a more weakly bound disordered nitrate species is formed. This species gives rise to a dynamic NO2 uptake and release well below the decomposition temperature of bulk Ba(NO3)(2). The experiments show that model studies under UHV conditions mainly provide information on the initial reaction mechanism, whereas the observation of actual bulk NOx storage phases requires experiments at realistic temperatures and pressures.
Keywords: heterogeneous catalysis; vibrational spectroscopy; nitrogen oxides; scanning tunneling microscopy; supported catalysts

Model Reduction, Structure-property Relations and Optimization Techniques for the Production of Nanoscale Particles - Gröschel, Michael; Leugering, Günter; Peukert, Wolfgang (International Series of Numerical Mathematics, 2011)
The production of nanoscaled particulate products with exactly pre-defined characteristics is of enormous economic relevance. Although there are different particle formation routes they may all be described by one class of equations. Therefore, simulating such processes comprises the solution of nonlinear, hyperbolic integro-partial differential equations. In our project we aim to study this class of equations in order to develop efficient tools for the identification of optimal process conditions to achieve desired product properties. This objective is approached by a joint effort of the mathematics and the engineering faculty. Two model-processes are chosen for this study, namely a precipitation process and an innovative aerosol process allowing for a precise control of residence time and temperature. Since the overall problem is far too complex to be solved directly a hierarchical sequence of simplified problems has been derived which are solved consecutively. In particular, the simulation results are finally subject to comparison with experiments.
Keywords: Population balance equations; optimal control; model reduction; parameter identification

Modified SFEM for computational homogenization of heterogeneous materials with microstructural geometric uncertainties - Pivovarov, Dmytro; Steinmann, Paul (Computational Mechanics, 2016)
In the current work we examine the application of the Stochastic Finite Element Method (SFEM) to the modeling of Representative Volume Elements for heterogeneous materials. Uncertainties in the geometry of the microstructure result in the random nature of the solution fields thus requiring use of the stochastic version of the Finite Element Method. For considering large differences in the material properties of matrix and inclusions a standard SFEM approach proves not stable and results in high numerical errors compared to a brute-force Monte-Carlo evaluation. Therefore in order to stabilize the SFEM we propose an alternative Gauss integration rule as resulting from a truncation of the probability density function for the random variable. In addition we propose new basis functions substituting the common Polynomial Chaos Expansion, resulting in higher accuracy for the standard deviation in the homogenized stress at the macro scale.
Keywords: Stochastic FEM; Computational Homogenization; Random Geometry

Molecular and electronic structure of PTCDA on bilayer graphene on SiC(0001) studied with scanning tunneling microscopy - Lauffer, Peter; Emtsev, Konstantin; Graupner, Ralf; Seyller, Thomas; Ley, Lothar (physica status solidi (b), 2008)
Epitaxial growth of graphene on SiC surfaces by solid state graphitization is a promising route for future development of graphene based electronics. In the present work we study the morphology and molecular scale structure of monolayer films of 3,4,9,10-perylene tetracarboxylic dianhydrid (PTCDA) on bilayer graphene on SiC(0001) by scanning tunneling microscopy (STM). First we discuss how the PTCDA molecules adsorb on bilayer graphene. We specially regard the density of PTCDA molecules at monolayer coverage. From the comparison with experiments on HOPG (Au(111) and Ag(110) from literature) we infer a non-planar adsorption geometry of PTCDA molecules on bilayer graphene. The electronic structure of the surrounding bilayer graphene substrate is investigated via scanning tunneling spectroscopy (STS) and reveals n-type doping of bilayer graphene during adsorption of PTCDA. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Molecular Dynamics Simulation of Phosphonic Acid–Aluminum Oxide self-organization and their evolution into ordered monolayers - Dietrich, Hanno; Schmaltz, Thomas; Halik, Marcus; Zahn, Dirk (Physical Chemistry Chemical Physics, 2017)
We outline an unprejudiced molecular dynamics simulation approach to study the mechanisms of self-organization encompassing the evolution of surfactant–surface interactions to the growth of self-assembled monolayers (SAMs). Therein, the time-length scale problem is tackled by combining an efficient docking-type procedure for implementing surfactant-by-surfactant association with detailed molecular simulations to explore structural relaxation. For this, nanosecond-scale molecular dynamics simulations unravel ordering processes during the gradual assembly of the monolayer. Along this line, different packing motifs of octadecyl phosphonic acid (ODPA) on the (0001) surface of extgreeka-alumina and implications for the final density and ordering of the resulting monolayers are elucidated. Moreover, the role of the solvent is discriminated by comparing SAM formation in 2-propanol, hexane and in a vacuum.

Molecular Dynamics Simulations of Liquid Phase Interfaces: Understanding the Structure of the Glycerol/Water–Dodecane System - Beierlein, Frank; Krause, Andreas; Jäger, Christof; Fita, Piotr; Vauthey, Eric; Clark, Timothy (Langmuir, 2013)
Modern spectroscopic techniques such as time-resolved second harmonic generation spectroscopy allow molecules to be examined selectively directly at phase interfaces. Two-phase systems formed by glycerol/water and alkane layers have previously been studied by time-resolved second harmonic generation spectroscopic measurements. In this molecular-dynamics study, a triphenylmethane dye was inserted at the glycerol/water-alkane interface and was used as a probe for local properties such as viscosity. We now show how extensive simulations over a wide range of concentrations can be used to obtain a detailed view of the molecular structure at the glycerol/water-alkane interface. Glycerol is accumulated in a double layer adjacent to the alkane interface, which results in increased viscosity of the glycerol/water phase in the direct vicinity of the interface. We also show that conformational ensembles created by classical molecular-dynamics simulations can serve as input for QM/MM calculations, yielding further information such as transition dipoles, which can be compared with spectroscopic measurements.

Molecular dynamics study of ferroelectric domain nucleation and domain switching dynamics - Boddu, Vishal; Endres, Florian; Steinmann, Paul (Scientific Reports, 2017)
Ferroelectric materials contain domains of ordered electric dipoles, separated by domain walls, that can undergo polarisation switching under externally applied electric fields. The domain switching dynamics in ferroelectric materials plays an essential role in their application to electronic and electro-optic de- vices. Previous studies suggest that the switching occurs largely through domain wall motion which is explained from the viewpoint of statistical physics on surface growth as the behaviour of a pinned elas- tic interface. We perform molecular dynamics simulations to investigate the domain switching process and quantitatively estimate the switching speed of anti-parallel 180° domains in ferroelectric, tetragonal BaTiO3 perfect single crystals at room temperature using the core-shell model. We observe an unprece- dented, non-linear increase in the domain switching speed caused by the nucleation of new domains within the switching domain. We determine the strength of the electric field to evoke nucleation of new domains and show that the nucleated domains diffuse into nearby favourable domains when the electric field is removed. Furthermore, we discuss the prominence of domain nucleations during ferroelectric switching.

Molecular excitation spectra by TDDFT with the nonadiabatic exact exchange kernel - Ipatov, Andrey; Heßelmann, Andreas; Görling, Andreas (International Journal of Quantum Chemistry, 2010)
A time-dependent density functional theory (TDDFT) method using the nonadiabatic exact-exchange kernel is discussed. Compared to previous TDDFT methods with orbital-dependent kernels, the presented time-dependent exact exchange (TDEXX) approach avoids a numerical unstable inversion of the Kohn-Sham response function. If the complete frequency dependency of the exact-exchange kernel is taken into account, then the proposed method yields the long-range distance behavior of charge-transfer (CT) excitation energies qualitatively correctly. The relation between time-dependent Hartree-Fock and TDEXX approaches is analyzed. This analysis shows by formal arguments why the TDEXX method is capable to correctly treat CT excitations and conversely shows that the problems to describe CT excitations with conventional TDDFT methods based on generalized gradient approximations (GGAs) has its origin in the shortcomings of the GGA exchange functionals. © 2010 Wiley Periodicals, Inc.

Molecular mechanics modeling of azobenzene-based photoswitches - Duchstein, Patrick; Neiß, Christian; Görling, Andreas; Zahn, Dirk (Journal of Molecular Modeling, 2012)
We present an extension of the generalized amber force field to allow the modeling of azobenzenes by means of classical molecular mechanics. TD-DFT calculations were employed to derive different interaction models for 4-hydroxy-4'-methyl-azobenzene, including the ground (S) and S excited state. For both states, partial charges and the-N=N-torsion potentials were characterized. On this basis, we pave the way to large-scale model simulations involving azobenzene molecular switches. Using the example of an isolated molecule, the mechanics of cyclic switching processes are demonstrated by classical molecular dynamics simulations. © Springer-Verlag 2011.

Molecular Mechanisms of [Bi6O4(OH)4](NO3)6 Precursor Activation, Agglomeration, and Ripening towards Bismuth Oxide Nuclei - Walther, Markus; Zahn, Dirk (European Journal of Inorganic Chemistry, 2015)
Molecular dynamics simulations have been employed to characterize the role of [Bi6O4(OH)4](NO3)6 cage structures in DMSO solution as precursors to larger bismuth oxide aggregates. We find that the nitrate ions play a twofold role: (i) the association of [Bi6O4(OH)4](NO3)6 clusters – which are fully coordinated by six nitrate ligands – is electrostatically disfavored giving rise to stable [Bi6O4(OH)4](NO3)6 solutions; (ii) in contrast, the dissociation of a single nitrate ligand results in attractive cluster–cluster interactions. This results in the formation of oligomers, which are initially bridged by one to three nitrate ions, but then form Bi–O contacts by sharing common edges and faces and eventually ripen into nuclei of bismuth oxide. Strikingly, this process may be induced by a single activated species, the [Bi6O4(OH)4](NO3)5+ cluster, which may bind several [Bi6O4(OH)4](NO3)6 clusters.
EAM Import::2019-03-12

Molecular Mechanisms of Solvent-Controlled Assembly of Phosphonate Monolayers on Oxide Surfaces - Dietrich, Hanno; Zahn, Dirk (Journal of Physical Chemistry C, 2017)
The explicit role of a series of solvents in octadecylphosphonic acid (ODPA) association and monolayer formation on the (0001) surface of extgreeka-aluminum oxide is explored from molecular dynamics simulations. For this purpose, molecule-by-molecule attachment and subsequent relaxation is studied in hexane, THF, and 2-propanol, respectively. From a purely structural viewpoint, the simulations show that the packing and ordering of the resulting SAMs closely resembles that of monolayers initially grown in vacuo, followed by immersion into the solvent afterward. In terms of the formation kinetics, we however find significant dissimilarities which result from solvent structuring at the interface and considerable hindering of surfactant association to the template surface at later stages of SAM growth. This leads to drastic deviation from diffusion-controlled kinetics and calls for a time-dependent picture of SAM formation mechanisms.
EAM Import::2019-03-12

Molecular Mechanisms of ZnO Nanoparticle Dispersion in Solution: Modeling of Surfactant Association, Electrostatic Shielding and Counter Ion Dynamics - Duchstein, Patrick; Milek, Theodor; Zahn, Dirk (PLoS ONE, 2015)
Molecular models of 5 nm sized ZnO/Zn(OH)2 core-shell nanoparticles in ethanolic solution were derived as scale-up models (based on an earlier model created from ion-by-ion aggregation and self-organization) and subjected to mechanistic analyses of surface stabilization by block-copolymers. The latter comprise a poly-methacrylate chain accounting for strong surfactant association to the nanoparticle by hydrogen bonding and salt-bridges. While dangling poly-ethylene oxide chains provide only a limited degree of sterical hindering to nanoparticle agglomeration, the key mechanism of surface stabilization is electrostatic shielding arising from the acrylates and a halo of Na+ counter ions associated to the nanoparticle. Molecular dynamics simulations reveal different solvent shells and distance-dependent mobility of ions and solvent molecules. From this, we provide a molecular rationale of effective particle size, net charge and polarizability of the nanoparticles in solution.
EAM Import::2019-03-12

Molecular Mie model for second harmonic generation and sum frequency generation - Wunderlich, Sarina; Schürer, Benedikt; Sauerbeck, Christian; Peukert, Wolfgang; Peschel, Ulf (Physical Review B, 2011)
A theoretical model to simulate second harmonic and sum frequency generation from stratified spherical particles of arbitrary material is presented and compared with the widely used Rayleigh-Gans-Debye approximation and to experimental results from polystyrene particles with adsorbed malachite green molecules. In this model, the nonlinear polarization is caused by individual dipoles placed in the vicinity of the sphere and is simulated on a molecular basis. This offers greater flexibility to model more sophisticated systems.

Molecular modeling of (1010) and (0001) zinc oxide surface growth from solution: islands, ridges and growth-controlling additives - Milek, Theodor; Zahn, Dirk (Crystengcomm, 2015)
The mechanism of (1010) and (0001) zinc oxide surface growth from ethanolic solution is investigated by molecular simulation. Growth steps are modelled at the maximum level of detail, i.e. by association of individual Zn2+ and OH- ions. Apart from structural relaxation, a mixed quantum/classical approach is used to explicitly study the proton-transfer reactions during crystal growth. Starting from idealized surfaces, we find that the (0001) face evolves into rough landscapes composed of small islands separated by 1 nm. On the other hand, the (1010) growth front shows the formation of ridges encompassed by analogous 1010 planes of the wurtzite structure. Contrary to idealized surface models, such rough surfaces obtained from explicit growth simulations enable us to identify considerable differences in both the binding site and energy for the association of growth-controlling additives. Using acetate and citrate ions as examples, we demonstrated the preferential association with peaks and kinks, respectively.
EAM Import::2019-03-12

Molecular modeling of amorphous, non-woven polymer networks - Krausse, Constantin A.; Milek, Theodor; Zahn, Dirk (Journal of Molecular Modeling, 2015)
We outline a simple and efficient approach to generating molecular models of amorphous polymer networks. Similar to established techniques of preparing woven polymer networks from quenching high-temperature molecular simulation runs, we use a molecular dynamics simulations of a generic melt as starting points. This generic melt is however only used to describe parts of the polymers, namely the cross-linker units which positions are adopted from particle positions of the quenched melt. Specific degrees of network connectivity are tuned by geometric criteria for linker-linker connections and by suitable multi-body interaction potentials applied to the generic melt simulations. Using this technique we demonstrate adjusting fourfold linker coordination in amorphous polymer networks comprising 10-20 % under-coordinated linkers.
EAM Import::2019-03-12

Molecular Orientation and Structural Transformations in Phthalic Anhydride Thin Films on MgO(100)/Ag(100) - Mohr, Susanne; Xu, Tao; Döpper, Tibor; Laurin, Mathias; Görling, Andreas; Libuda, Jörg (Langmuir, 2015)
Structural control of organic thin films on dielectric substrates is the key to tailoring the. physical properties of hybrid Materials, for example, for application in solar energy conversion, molecular electronics, or catalysis. In, this work, we investigate the molecular orientation of phthalic: anhydride (PAA) films on atomically well-defined mgO(100) on Ag(100) using temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS) in combination with density-functional theory (DFT). A robust procedure is presented to determine the orientation of the FAA Molecules, which relies on the intensity ratios of vibrational bands only We show that even at deposition temperatures of 110 K, the PAA multilayer grows with a-Specific molecular, orientation; that is the PAA Molecular plane is preferentially aligned parallel with the MgO surface,. No change of molecular orientation occurs up to a temperature of 145 K Between 145 and 160 K, the film restructures adopting a nearly flat lying molecular Orientation. Between 170 and 205 K, the film undergoes a second structural transition to a crystalline phase. This transition is associated with a pronounced molecular reorientation. The molecules adopt a tilted orientation and, simultaneously, rotate around their C-2 axes The reorientation behavior Suggests that the molecular orientation in the crystalline phase is controlled by the interaction with the MgO(100) substrate. At higher temperature, no further restructuring is observed until the PAA. multilayer desorbs at temperatures above 230 K.

Molecular Simulation of Ag Nanoparticle Nucleation from Solution: Redox-Reactions Direct the Evolution of Shape and Structure - Milek, Theodor; Zahn, Dirk (Nano Letters, 2014)
The association of Ag+ ions and the early stage of Ag nanoparticle nucleation are investigated from molecular dynamics simulations. Combining special techniques for tackling crystal nucleation from solution with efficient approaches to model redox-reactions, we unravel the structural evolution of forming silver nanoparticles as a function of the redox-potential in the solution. Within a range of only 1 eV, the redox-potential is demonstrated to have a drastic effect on both the inner structure and the overall shape of the forming particles. On the basis of our simulations we identify surface charge and its distribution as an atomic scale mechanism that accounts for creating/avoiding 5-fold coordination polyhedra and thus the degree of (multiple)-twinning in silver nanoparticles
EAM Import::2019-03-12

Molecular Simulation of Fundamental Processes in Nanoparticle - Polymer - Nanoparticle Systems Under Tensile Load - Zahn, Dirk (International Journal of Materials, Mechanics and Manufacturing, 2013)
We demonstrate molecular modeling of polymer (polyacrylate) association to ZnO nanoparticles by means of a recently developed molecule-by-molecule association approach. Upon multiple acrylate association steps, potential sites for connecting ZnO particles are elaborated and explored under tensile loading from molecular dynamics simulation. This offers molecular level insights into processes that account for elastic and plastic deformation, creep and self-healing in ZnO-polymer composite materials.
EAM Import::2019-03-12

Molecular wires - impact of π-conjugation and implementation of molecular bottlenecks - Schubert, Christina; Margraf, Johannes; Clark, Timothy; Guldi, Dirk Michael (Chemical Society Reviews, 2015)
In this review we highlight recent progress in the field of photochemically and thermally induced electron transport through molecular bridges as integrative parts of electron donor-bridge-acceptor conjugates. The major emphasis is hereby on the design and the modular composition of the bridges. To this end, we will demonstrate that control over attenuation factors and reorganization energies, on one hand, as well as electronic and electron-vibration couplings, on the other hand, enables tuning electron transport over distances as short as 3.5 Å and as large as 50 Å by up to nine orders of magnitude. In terms of electron transport, the maximum extreme is given by carbon-bridged oligo-p-phenylenevinylenes of different lengths, while a zinc tetraphenylporphyrin free base tetraphenylporphyrin dyad constitutes the minimum extreme.
EAM Import::2019-03-12

Molecular wires in single-molecule junctions: Charge transport and vibrational excitations - Ballmann, Stefan; Hieringer, Wolfgang; Secker, Daniel; Zheng, Q.; Gladysz, John A.; Görling, Andreas; Weber, Heiko B. (ChemPhysChem, 2010)
We investigate the effect of vibrations on the electronic transport through single-molecule junctions, using the mechanically controlled break junction technique. The molecules under investigation are oligoyne chains with appropriate end groups, which represent both an ideally linear electrical wire and an ideal molecular vibrating string. Vibronic features can be detected as satellites to the electronic transitions, which are assigned to longitudinal modes of the string by comparison with density functional theory data. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Molten o-H3PO4: A New Electrolyte for the Anodic Synthesis of Self-Organized Oxide Structures - WO3 Nanochannel Layers and Others - Altomare, Marco; Pfoch, Ole; Tighineanu, Alexei; Kirchgeorg, Robin; Lee, Kiyoung; Selli, Elena; Schmuki, Patrik (Journal of the American Chemical Society, 2015)
We introduce the use of pure molten ortho-phosphoric acid (o-H3PO4) as an electrolyte for self-organizing electrochemistry. This electrolyte allows for the formation of self-organized oxide architectures (one-dimensional nanotubes, nanochannels, nanopores) on metals such as tungsten that up to now were regarded as very difficult to grow self-ordered anodic oxide structures. In this work, we show particularly the fabrication of thick, vertically aligned tungsten oxide nanochannel layers, with pore diameter of ca. 10 nm and illustrate their potential use in some typical applications.
EAM Import::2019-03-12

Monitoring of catalytic reactions in photonic crystal fiber - Cubillas, Ana Maria; Schmidt, Matthias; Scharrer, Michael; Euser, T. G.; Etzold, Bastian; Taccardi, Nicola; Wasserscheid, Peter; Russell, Philip St. John (2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011, 2011)
Hollow-core photonic crystal fiber (HC-PCF) offers several advantages for chemical analysis, for example, long optical pathlengths, sub-L sample volumes, close to 100% overlap between light and sample, and more compact systems for sensing [1-2] and photochemistry [3-4]. Here we compare the performance of a conventional cuvette-based UV-VIS system for monitoring of catalytic reactions with a system based on liquid-filled HC-PCF. We study the photo-Fenton catalytic reaction, which under visible light irradiation induces degradation of methylene blue dye (MB) in the presence of Fe2+ / H2O 2 [5]. © 2011 IEEE.
CRIS-Team Scopus Importer:2023-08-15

Monitoring of liquid-phase organic reactions by photoelectron spectroscopy - Kolbeck, Claudia; Niedermaier, Inga; Taccardi, Nicola; Schulz, Peter; Maier, Florian; Wasserscheid, Peter; Steinrück, Hans-Peter (Angewandte Chemie International Edition, 2012)
There are strings attached: After linking the reacting groups to head groups of ionic liquids to drastically lower the vapour pressures of the reactants, ordinary liquid-phase organic reactions can be monitored by in situ X-ray photoelectron spectroscopy. This approach is demonstrated for the nucleophilic substitution of an alkyl amine and an alkyl chloride moiety, which are attached to the cation and anion of ionic liquids, respectively. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Monodispersed lysozyme-functionalized bioactive glass nanoparticles with antibacterial and anticancer activities - Zheng, Kai; Lu, Miao; Liu, Yufang; Chen, Qiang; Taccardi, Nicola; Hueser, Norbert; Boccaccini, Aldo R. (Biomedical Materials, 2016)
In this study, highly monodispersed spherical bioactive glass nanoparticles (BGS) with a particle size of 408 ± 36 nm were synthesized using a modified Stöber method. The BGS was then functionalized with lysozyme (LY) via a simple electrostatic interaction routine under selected conditions. The LY-functionalized BGS (LY-BGS) exhibited monodispersity, spherical morphology and homogeneity in size. The incorporated content of LY could be tailored conveniently by adjusting the initial concentration of the LY precursor for functionalization. Hydroxyapatite (HA) formed on the LY-BGS after soaking in simulated body fluid (SBF) for 7 d, but the formation was retarded compared to the non-functionalized BGS. The LY-BGS showed antibacterial activity towards Gram-positive B. subtilis and >90% of the bacteria was killed within 24 h after culture with the LY-BGS at a concentration of 1 mg ml-1. The LY-BGS also showed cytotoxicity towards the human hepatocellular carcinoma (HepG2) cell line. In addition, the relative cytotoxicity increased with an increase in the concentration of the LY-BGS in contact with the cells. As a comparison, the LY-BGS exhibited reduced or no cytotoxicity towards human umbilical vein endothelial cells (HUVECs) at the same concentration with respect to the HepG2 groups. Notably, the relative cell viability of HepG2 was 45.9% after exposure to the LY-BGS at a concentration of 10 extgreekmg ml-1 for 24 h, while no decrease in relative viability for the HUVECs was observed under the same conditions. This cytotoxicity window between cancerous cells and healthy cells could be expected for cancer treatment. Furthermore, the antibacterial properties and the bioactivity of LY-BGS make it a promising material for biomedical applications, particularly in the treatment of bone defects caused by tumors.
Keywords: antibacterial activity; anticancer activity; bioactive glasses; lysozyme; nanocomposites

Monte Carlo simulations of confined hard sphere fluids - Schröder-Turk, Gerd; Kuczera, S.; Hoerndlein, D.; Krach, Florian; Hiester, T.; Mecke, Klaus (Physics Procedia, 2010)
We describe a grand-canonical Monte Carlo simulation of a hard sphere fluid confined to porous media with arbitrary pore shape. The pore geometry is given by a triangulated surface representation of the solid-void interface. This algorithm is developed to study the dependence of thermodynamic quantities, such as the average density and adsorption, on the shape of the pore. Particular pore shape geometries that are analysed are the labyrinthine domains bounded by so-called triply-periodic minimal surfaces.
EAM Import::2019-03-12

MoO3 in Self-Organized TiO2 Nanotubes for Enhanced Photocatalytic Activity - Agarwal, Pankaj; Paramasivam, Indhumati; Shrestha, Nabeen Kumar; Schmuki, Patrik (Chemistry-An Asian Journal, 2010)
Self-organized nanostructures (nanotubes and nanopores) consisting of TiO_2-MoO_3 were grown on Ti-Mo substrates with different Mo contents (7 wt % Mo, 20 wt % Mo, and 35 wt % Mo) by electrochemical anodization. The photocatalytic activity of these oxide layers was compared with TiO_2 nanotubes. The results show clearly that MoO_3 doping of about 7 wt % (Ti7Mo in picture) strongly enhances the achievable photodegradation ability of titania nanotubes.
EAM Import::2019-03-12

Morphodynamic modeling of aeolian dunes: Review and future plans - Ribeiro Parteli, Eric Josef; Kroy, Klaus; Tsoar, Haim; Andrade, José S.; Pöschel, Thorsten (European Physical Journal - Special Topics, 2014)
Sand dunes are ubiquitous in deserts, on coasts, on the sea bottom, and on the surface of Mars, Venus and Titan. The quantitative understanding of dune dynamics is thus of relevance for a broad range of physical, geological and planetary sciences. A morphodynamic model for dunes, which combines an analytical description of the average turbulent wind field over the topography with a continuum saltation model, has proven successful to quantitatively reproduce the shape of aeolian dunes of different types. We present a short review on the physics of dune formation and the model development, as well as some future plans for further developments and applications.

Morphological and electrical control of fullerene dimerization determines organic photovoltaic stability - Heumüller, Thomas; Mateker, William R.; Distler, Andreas; Fritze, Urs F.; Cheacharoen, Rongrong; Nguyen, William H.; Biele, Markus; Salvador, Michael Filipe; von Delius, Max; Egelhaaf, Hans-Joachim; Mcgehee, Michael D.; Brabec, Christoph (Energy and Environmental Science, 2016)
Fullerene dimerization has been linked to short circuit current (J) losses in organic solar cells comprised of certain polymer-fullerene systems. We investigate several polymer-fullerene systems, which present J loss to varying degrees, in order to determine under which conditions dimerization occurs. By reintroducing dimers into fresh devices, we confirm that the photo-induced dimers are indeed the origin of the J loss. We find that both film morphology and electrical bias affect the photodimerization process and thus the associated loss of J. In plain fullerene films, a higher degree of crystallinity can inhibit the dimerization reaction, as observed by high performance liquid chromatography (HPLC) measurements. In blend films, the amount of dimerization depends on the degree of mixing between polymer and fullerene. For highly mixed systems with very amorphous polymers, no dimerization is observed. In solar cells with pure polymer and fullerene domains, we tune the fullerene morphology from amorphous to crystalline by thermal annealing. Similar to neat fullerene films, we observe improved light stability for devices with crystalline fullerene domains. Changing the operating conditions of the investigated solar cells from V to J also significantly reduces the amount of dimerization-related J loss; HPLC analysis of the active layer shows that more dimers are formed if the cell is held at V instead of J. The effect of bias on dimerization, as well as a clear correlation between PL quenching and reduced dimerization upon addition of small amounts of an amorphous polymer into PC60BM films, suggests a reaction mechanism via excitons.
Keywords: Engineering controlled terms: Amorphous films; Crystalline materials; Dimerization; Dimers; Fullerenes; High performance liquid chromatography; Liquid chromatography; Organic polymers; Polymer films; Polymers Amorphous polymers; Crystalline fullerene; Degree of crystallinity; Dimerization reactions; Electrical control; Operating condition; Organic photovoltaics; Polymer-fullerene system Engineering main heading: Solar cells

Morphological impact of zinc oxide layers on the device performance in thin-film transistors - Faber, Hendrik; Klaumünzer, Martin; Voigt, Michael; Galli, Diana; Vieweg, Benito; Peukert, Wolfgang; Spiecker, Erdmann; Halik, Marcus (Nanoscale, 2011)
Zinc oxide thin-films are prepared either by spin coating of an ethanolic dispersion of nanoparticles (NP, diameter 5 nm) or by spray pyrolysis of a zinc acetate dihydrate precursor. High-resolution electron microscopy studies reveal a monolayer of particles for the low temperature spin coating approach and larger crystalline domains of more than 30 nm for the spray pyrolysis technique. Thin-film transistor devices (TFTs) based on spray pyrolysis films exhibit higher electron mobilities of up to 24 cm² V−1 s−1 compared to 0.6 cm² V−1 s−1 for NP based TFTs. These observations were dedicated to a reduced number of grain boundaries within the transistor channel.

Morphological instability leading to formation of porous anodic oxide films - Hebert, Kurt R.; Albu, Sergiu; Paramasivam, Indhumati; Schmuki, Patrik (Nature Materials, 2011)
Electrochemical oxidation of metals, in solutions where the oxide is somewhat soluble, produces anodic oxides with highly regular arrangements of pores. Although porous aluminium and titanium oxides have found extensive use in functional nanostructures, pore initiation and self-ordering are not yet understood. Here we present an analysis that examines the roles of oxide dissolution and ionic conduction in the morphological stability of anodic films. We show that patterns of pores with a minimum spacing are possible only within a narrow range of the oxide formation efficiency (the fraction of oxidized metal atoms retained in the film), which should exist when the metal ion charge exceeds two. Experimentally measured efficiencies, over diverse anodizing conditions on both aluminium and titanium, lie within the different ranges predicted for each metal. On the basis of these results, the relationship between dissolution chemistry and the conditions for pore initiation can now be understood in quantitative terms.
EAM Import::2019-03-12

Morphological zeta-potential variation of nanoporous anodic alumina layers and cell adherence - Pedimonte, Birgit; Moest, Tobias; Luxbacher, Thomas; von Wilmowsky, Cornelius; Fey, Tobias; Schlegel, Andreas; Greil, Peter (Acta Biomaterialia, 2014)
Nanoscale surface modification of biomedical implant materials offers enhanced biological activity concerning protein adsorption and cell adherence. Nanoporous anodic alumina oxide (AAO) layers were prepared by electrochemical oxidation of thin Al-seed layers in 0.22 M C2H2O4, applying anodization voltages of 20-60 V. The AAO layers are characterized by a mean pore diameter varying from 15 to 40 nm, a mean pore distance of 40-130 nm, a total porosity of similar to 10% and a thickness of 560 +/- 40 nm. Zeta potential and isoelectric point (iep) were derived from streaming potential measurements and correlated to the topology variation of the nanoporous AAO layers. With decreasing pore diameter a shift of iep from similar to 7.9 (pore diameter 40 nm) to similar to 6.7 (pore diameter 15 nm) was observed. Plain alumina layers, however, possess an iep of similar to 9. Compared to the plain alumina surface an enhanced adherence and activity of hFOB cells was observed on the nanoporous AAO after 24 h culture with a maximum at a pore size of 40 nm. The topology-induced change of the electrochemical surface state may have a strong impact on protein adsorption as well as on cell adhesion, which offers a high potential for the development of bioactive AAO coatings on various biomaterial substrates. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights
Keywords: Nanoporous alumina; Zeta potential; Isoelectric point; Cell activity

Morphology analysis of near IR sensitized polymer/fullerene organic solar cells by implementing low bandgap heteroanalogue C-/Si-PCPDTBT - Ameri, Tayebeh; Khoram, Parisa; Heumüller, Thomas; Baran, Derya; Machui, Florian; Troeger, Anna; Sgobba, Vito; Guldi, Dirk Michael; Halik, Marcus; Rathgeber, Silke; Scherf, Ullrich; Brabec, Christoph (Journal of Materials Chemistry A, 2014)
In the current work, we have investigated the morphological aspects of the ternary solar cells based on host matrices of P3HT:PCBM and P3HT:ICBA, using the low bandgap polymer analogues of C- and Si-bridged PCPDTBT as near IR sensitizers, which show noticeably different performance. A direct comparison of these well-functional and poorly functional ternary blend systems provides insights into the bottlenecks of device performance and enables us to set up an initial set of design rules for ternary organic solar cells. Our study reveals the importance of surface energy as a driving force controlling sensitizer location and morphology formation of ternary blends. The interfacial surface energy results indicate that Si-PCPDTBT locates at amorphous interfaces and P3HT crystallites, while C-PCPDTBT tends to accumulate at amorphous interfaces and semi-crystalline (or agglomerated) domains of the fullerene derivatives. GIWAXS and SCLC results support this prediction where adding high content of C-PCPDTBT influences mainly the semi-crystallinity (aggregation) of the fullerene and reduces the electron mobility, but Si-PCPDTBT impacts mainly the P3HT ordering and, in turn, deteriorates the hole mobility. These findings show that the disruption of the fullerene semi-crystalline domains is more detrimental to the device performance than the disruption of the polymer domains.

Morphology and Linear-Elastic Moduli of Random Network Solids - Nachtrab, Susan; Kapfer, Sebastian; Arns, Christoph H.; Madadi, Mahyar; Mecke, Klaus; Schröder-Turk, Gerd (Advanced Materials, 2011)
The dependence of linear-elastic properties on effective density is studied for porous network solids, by voxel-based finite element methods. The same dependence is found for solid structures derived from Poisson-Voronoi processes and from confocal microscopy images of collagen scaffolds. We recover the power-law for the bulk modulus for low densities and suggest a functional form for the cross-over to a high-density porous solid.

Morphology-Controlled Organic Solar Cells Improved by a Nanohybrid System of Single Wall Carbon Nanotubes Sensitized by PbS Core/Perovskite Epitaxial Ligand Shell Quantum Dots - Soltani, Rezvan; Katbab, Ali Asghar; Sytnyk, Mykhailo; Amin, Atefeh Y.; Killilea, Niall Andrew; Berlinghof, Marvin; Ahmadloo, Farzaneh; Osvet, Andres; Unruh, Tobias; Heiß, Wolfgang; Ameri, Tayebeh (Solar RRL, 2017)
In the present work, a new solution processed nanohybrid system comprising of single-wall carbon nanotubes (SWCNTs) loaded by PbS quantum dots (QD) capped with an epitaxial ligand shell of methylammonium lead iodide perovskite clusters (MA4PbI6) is designed and fabricated. Attachment of PbS/PbI6 QDs on the surface of SWCNT is followed and evidenced by performing Fourier Transform Infrared Spectroscopy, X-ray photoelectron spectroscopy, and Field Emission Scanning Electron Microscopy. The steady state and dynamic photoluminescence results reveal efficient charge transfer from photo-excited PbS/PbI6 to SWCNTs. Very low amount (0.3 wt.%) of the as-synthesized PbS/PbI6-SWCNT is further incorporated into a polymeric solar cell containing P3HT and PC61BM and exhibits a power conversion efficiency improvement of around 15% compared to the P3HT:PC61BM bulk heterojunction reference solar cell. Significantly, loading perovskite capped PbS QDs on the surface of SWCNT works more efficient rather than incorporating PbS/PbI6 or SWCNT separately onto the composition of the photoactive layer. While PbS/PbI6 broaden the absorption window of photoactive layer and enhance the photon harvesting, their loading on the SWCNT has a significant influence on the faster exciton splitting by efficient electron transfer as well as keeping the desired crystallinity and nanoscale morphology of host matrix upon addition of QDs.

Morphology related photodegradation of low-band-gap polymer blends - Wang, Xiao; Egelhaaf, Hans-Joachim; Mack, Hans-Georg; Azimi, Seyed Hamed; Brabec, Christoph; Meixner, Alfred J.; Zhang, Dai (Advanced Energy Materials, 2014)
The morphology related photodegradation of low band-gap polymer blends is investigated using optical microscopy and scanning probe microscopy. Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (C-PCPDTBT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films without and with ODT, as well as poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diylalt-(2,1,3-benzothiadiazole)-4,7-diyl] (Si-PCPDTBT):PCBM blend films exposed to a focused 632.8 nm laser under ambient condition with and without inert gas protection are studied. The photodegradation of the polymer starts in the vicinity of the PCBM molecules (first sphere degradation), which effectively blocks the electron transfer processes. Stern-Volmer type kinetics are observed in the C-PCPDTBT:PCBM blend with ODT, which indicates that only a small number of photo-oxidized monomer units act as quenchers of the C-PCPDTBT polymer luminescence. Furthermore, in addition to the permanent damage of the polymer molecules, as witnessed from their Raman intensity decrease, the polymer photoluminescence demonstrates partial reversible recovery when inert gas protection is resumed, indicating the involvement of temporary polymer/O-charge transfer complexes in the photodegradation process.
Keywords: Nanoscale morphology; Phase separation; Photodegradation; Polymer blend films

Motif Reconstruction in Clusters and Layers: Benchmarks for the Kawska-Zahn Approach to Model Crystal Formation - Milek, Theodor; Duchstein, Patrick; Seifert, Gotthard; Zahn, Dirk (ChemPhysChem, 2010)
A recently developed atomistic simulation scheme for investigating ion aggregation from solution is transferred to the morphogenesis of metal clusters grown from the vapor and layers deposited on a substrate surface. Both systems are chosen as benchmark models for intense motif reorganization during aggregate/layer growth. The applied simulation method does not necessarily involve global energy minimization after each growth event, but instead describes crystal growth as a series of structurally related configurations which may also include local energy minima. Apart from the particularly favorable high-symmetry configurations known from experiments and global energy minimization, we also demonstrate the investigation of transient structures. In the spirit of Ostwald's step rule, a continuous evolution of the aggregate/layer structure during crystal growth is observed.
EAM Import::2019-03-12

Motion by stopping: Rectifying Brownian motion of nonspherical particles - Sporer, Susan; Goll, Christian; Mecke, Klaus (Physical Review E, 2008)
We show that Brownian motion is spatially not symmetric for mesoscopic particles embedded in a fluid if the particle is not in thermal equilibrium and its shape is not spherical. In view of applications to molecular motors in biological cells, we sustain nonequilibrium by stopping a nonspherical particle at periodic sites along a filament. Molecular dynamics simulations in a Lennard-Jones fluid demonstrate that directed motion is possible without a ratchet potential or temperature gradients if the asymmetric nonequilibrium relaxation process is hindered by external stopping. Analytical calculations in the ideal gas limit show that motion even against a fluid drift is possible and that the direction of motion can be controlled by the shape of the particle, which is completely characterized by tensorial Minkowski functionals.

Movers and shakers: Granular damping in microgravity - Bannerman, Marcus; Kollmer, Jonathan; Sack, Achim; Heckel, Michael; Müller, Patric; Pöschel, Thorsten (Physical Review E, 2011)
The response of an oscillating granular damper to an initial perturbation is studied using experiments performed in microgravity and granular dynamics simulations. High-speed video and image processing techniques are used to extract experimental data. An inelastic hard sphere model is developed to perform simulations and the results are in excellent agreement with the experiments. In line with previous work, a linear decay of the amplitude is observed. Although this behavior is typical for a friction-damped oscillator, through simulation it is shown that this effect is still present even when friction forces are absent. A simple expression is developed which predicts the optimal damping conditions for a given amplitude and is independent of the oscillation frequency and particle inelasticities. © 2011 American Physical Society.

Multi-component and multi-phase population balance model: The case of Georgeite formation as methanol catalyst precursor phase - Hartig, Martin Andreas Johannes; Jacobsen, Nikolas; Peukert, Wolfgang (Chemical Engineering Science, 2014)
A novel generally applicable multi-component and multi-phase population balance model is presented. The model classifies precipitation and crystallization systems into the number of components (i.e. building units of solid) and number of solid phases. The model includes reaction equilibria of metal ions in aqueous media, activity modeling, mixing, particle formation kinetics, population and mole balances. The new model is applied to the initial stages of copper catalyst precursor formation. The population balance model is simplified to a single-solid phase and multi-component precipitation. Simulations for several solid phases were performed at various experimental conditions. We show that the formation of the carbonate rich Georgeite phase is favored. The simulation results are verified by the measured global parameters pH and solid weight. Mixing plays a minor role in comparison to the variation of the reactant concentration. The timescale of precipitation including mixing, nucleation and crystal growth ranges from microseconds up to 1 s depending on the extent of secondary nucleation.
Keywords: multi-component; catalysis; solid; phase; mixing; aqeous; georgeite

Multidimensional Analysis of Nanoparticles with Highly Disperse Properties Using Multiwavelength Analytical Ultracentrifugation - Walter, Johannes; Löhr, Konrad; Karabudak, Engin; Reis, Wieland; Mikhael, Jules; Peukert, Wolfgang; Wohlleben, Wendel; Coelfen, Helmut (ACS nano, 2014)
The worldwide trend in nanoparticle technology toward increasing complexity must be directly linked to more advanced characterization methods of size, shape and related properties, applicable to many different particle systems in science and technology. Available techniques for nanoparticle characterization are predominantly focused on size characterization. However, simultaneous size and shape characterization is still an unresolved major challenge. We demonstrate that analytical ultracentrifugation with a multiwavelength detector is a powerful technique to address multidimensional nanoparticle analysis. Using a high performance optical setup and data acquisition software, information on size, shape anisotropy and optical properties were accessible in one single experiment with unmatched accuracy and resolution. A dynamic rotor speed gradient allowed us to investigate broad distributions on a short time scale and differentiate between gold nanorod species including the precise evaluation of aggregate formation. We report how to distinguish between different species of single-wall carbon nanotubes in just one experiment using the wavelength-dependent sedimentation coefficient distribution without the necessity of time-consuming purification methods. Furthermore, CdTe nanoparticles of different size and optical properties were investigated in a single experiment providing important information on structure-property relations. Thus, multidimensional information on size, density, shape and optical properties of nanoparticulate systems becomes accessible by means of analytical ultracentrifugation equipped with multiwavelength detectio
Keywords: nanoparticle; characterization; analytical; ultracetrifugation; CdTe; multidimensional

Multidisciplinary Free Material Optimization - Haslinger, J.; Kocvara, Michal; Leugering, Günter; Stingl, Michael (SIAM Journal on Applied Mathematics, 2010)
We present a mathematical framework for the so-called multidisciplinary free material optimization (MDFMO) problems, a branch of structural optimization in which the full material tensor is considered as a design variable. We extend the original problem statement by a class of generic constraints depending either on the design or on the state variables. Among the examples are local stress or displacement constraints. We show the existence of optimal solutions for this generalized free material optimization (FMO) problem and discuss convergent approximation schemes based on the finite element method.

Multi-Energy X-ray Imaging as a Quantitative Method for Materials Characterization - Firsching, M.; Nachtrab, Frank; Uhlmann, N.; Hanke, R. (Advanced Materials, 2011)
X-ray imaging is an established tool for materials characterization even though it usually has the drawback of not being a quantitative method. This drawback can be overcome by energy-sensitive X-ray imaging methods such as the basis material decomposition method presented in this paper, which can provide quantitative results for materials characterization.
EAM Import::2019-03-12

Multilayer Multiconfiguration Time-Dependent Hartree Study of Vibrationally Coupled Electron Transport Using the Scattering-State Representation - Wang, Haobin; Thoss, Michael (Journal of Physical Chemistry A, 2013)
The multilayer multiconfiguration time-dependent Hartree method is employed to study vibrationally coupled electron transport in models of single molecule junctions. To increase the efficiency of the simulation method, a representation of the Hamiltonian in terms of the scattering states of the underlying electronic Hamiltonian is used. It is found that with an appropriate choice of the scattering states the artificial electron correlation present in the original representation of the model is greatly reduced. This allows efficient simulation of the steady-state currents in a wide physical parameter space, which is demonstrated by several numerical examples. © 2013 American Chemical Society.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.multil

Multiscale approaches to protein-mediated interactions between membranes-relating microscopic and macroscopic dynamics in radially growing adhesions - Bihr, Timo; Seifert, Udo; Smith, Ana-Suncana (New Journal of Physics, 2015)
Macromolecular complexation leading to coupling of two or more cellular membranes is a crucial step in a number of biological functions of the cell. While other mechanisms may also play a role, adhesion always involves the fluctuations of deformable membranes, the diffusion of proteins and the molecular binding and unbinding. Because these stochastic processes couple over a multitude of time and length scales, theoretical modeling of membrane adhesion has been a major challenge. Here we present an effective Monte Carlo scheme within which the effects of the membrane are integrated into local rates for molecular recognition. The latter step in the Monte Carlo approach enables us to simulate the nucleation and growth of adhesion domains within a system of the size of a cell for tens of seconds without loss of accuracy, as shown by comparison to 10(6) times more expensive Langevin simulations. To perform this validation, the Langevin approach was augmented to simulate diffusion of proteins explicitly, together with reaction kinetics and membrane dynamics. We use the Monte Carlo scheme to gain deeper insight to the experimentally observed radial growth of micron sized adhesion domains, and connect the effective rate with which the domain is growing to the underlying microscopic events. We thus demonstrate that our technique yields detailed information about protein transport and complexation in membranes, which is a fundamental step toward understanding even more complex membrane interactions in the cellular context.
Keywords: theory; modeling and simulations; protein-ligand interactions; membranes

Multiscale Chemical Process Design Exemplified for a PEM Fuel Cell Process - Karst, Florian; Freund, Hannsjörg; Maestri, Matteo; Sundmacher, Kai (Chemie Ingenieur Technik, 2014)
To maximize the utilization of limited energy and material resources, a new design methodology for chemical processes is proposed based on a multiscale optimization approach. The different levels of the process hierarchy are presented and design aspects are discussed. The methodology is further exemplified by considering a methane steam reforming process for operation of a proton exchange membrane fuel cell (PEMFC) in an automotive application. The method delivers an optimal technically feasible process based on an integrated steam reforming reactor, operated in autothermal mode.
Keywords: Methane Reforming; Micro Kinetic Model; Multiscale Process Design; Process Optimization

Multiscale Modeling of Powder Bed-Based Additive Manufacturing - Markl, Matthias; Körner, Carolin (Annual Review of Materials Research, 2016)
Powder bed fusion processes are additive manufacturing technologies that are expected to induce the third industrial revolution. Components are built up layer by layer in a powder bed by selectively melting confined areas, according to sliced 3D model data. This technique allows for manufacturing of highly complex geometries hardly machinable with conventional technologies. However, the underlying physical phenomena are sparsely understood and difficult to observe during processing. Therefore, an intensive and expensive trial-and-error principle is applied to produce components with the desired dimensional accuracy, material characteristics, and mechanical properties. This review presents numerical modeling approaches on multiple length scales and timescales to describe different aspects of powder bed fusion processes. In combination with tailored experiments, the numerical results enlarge the process understanding of the underlying physical mechanisms and support the development of suitable process strategies and component topologies.
Keywords: Additive manufacturing; Cellular automaton method; Finite element method; Lattice Boltzmann method; Selective electron beam melting; Selective laser melting

Multi-Scale Modelling of Deformation and Fracture in a Biomimetic Apatite-Protein Composite: Molecular-Scale Processes Lead to Resilience at the μm-Scale - Zahn, Dirk; Duchstein, Patrick (PLoS ONE, 2016)
Fracture mechanisms of an enamel-like hydroxyapatite-collagen composite model are elaborated by means of molecular and coarse-grained dynamics simulation. Using fully atomistic models, we uncover molecular-scale plastic deformation and fracture processes initiated at the organic-inorganic interface. Furthermore, coarse-grained models are developed to investigate fracture patterns at the extgreekmm-scale. At the meso-scale, micro-fractures are shown to reduce local stress and thus prevent material failure after loading beyond the elastic limit. On the basis of our multi-scale simulation approach, we provide a molecular scale rationalization of this phenomenon, which seems key to the resilience of hierarchical biominerals, including teeth and bone.
EAM Import::2019-03-12

Multi-scale simulations of apatite–collagen composites: from molecules to materials - Zahn, Dirk (Frontiers of Materials Science, 2017)
We review scale-bridging simulation studies for the exploration of atomicto-meso scale processes that account for the unique structure and mechanic properties of apatite-protein composites. As the atomic structure and composition of such complex biocomposites only partially is known, the first part (i) of our modelling studies is dedicated to realistic crystal nucleation scenarios of inorganic-organic composites. Starting from the association of single ions, recent insights range from the mechanisms of motif formation, ripening reactions and the self-organization of nanocrystals, including their interplay with growth-controlling molecular moieties. On this basis, (ii) reliable building rules for unprejudiced scale-up models can be derived to model bulk materials. This is exemplified for (enamel-like) apatite-protein composites, encompassing up to 106 atom models to provide a realistic account of the 10 nm length scale, whilst model coarsening is used to reach extgreekmm length scales. On this basis, a series of deformation and fracture simulation studies were performed and helped to rationalize biocomposite hardness, plasticity, toughness, self-healing and fracture mechanisms. Complementing experimental work, these modelling studies provide particularly detailed insights into the relation of hierarchical composite structure and favorable mechanical properties.
EAM Import::2019-03-12

Nanocarbon hybrids: The paradigm of nanoscale self-ordering/self-assembling by means of charge transfer/doping interactions - Guldi, Dirk Michael; Costa Riquelme, Rubén Dario (Journal of Physical Chemistry Letters, 2013)
The scope of this Perspective is to highlight the potential of nanoscale self-ordering/self-assembling nanocarbons - fullerenes, single-wall carbon nanotubes, and graphene - en route toward novel charge transfer hybrids that unify several functions such as light harvesting, charge separation, and, eventually, catalysis. All of the given examples are fully characterized by a broad range of spectroscopic as well as microscopic techniques and, as such, document the success in tuning the electronic structure of functional nanocarbons by means of charge transfer/doping interactions. © 2013 American Chemical Society.

Nanochemie in der Schule - geht das überhaupt ? - Heinzerling, Peter (Nachrichten aus der Chemie, 2017)
Nanochemie ist ein aktuelles Forschungsgebiet, das auch im Chemieunterricht vorkommen sollte. Versuche dazu gab es bereits vor über 100 Jahren. Leicht abgewandelt eignen sie sich auch heute noch für die Schule.

Nanochemistry - A Split between 18th Century and Modern Times - Heinzerling, Peter; Oetken, Marco (World Journal of Chemical Education, 2018)
Nanoscience seems to be a main topic of this century. The chemical fundamentals are very old and named colloids since Graham in 1861. The acronym nano has been introduced in 1960 at a conference of measures and weights in Paris. In 1982 Binnig and Rohrer invented the scanning tunnel microscope(STM) and four years later Binnig the atomic force microscope(AFM). These microscopes are the main tools for the nanotechnology and from this times the number of publications exploded. In this article we'll focus on the chemical aspects of nanotechnology and how to implement experiments into school.
EAM Import::2019-03-12

Nanocrystalline Zeolite Tetraethylammonium-Beta Membrane for Preferential Sorption and Transport of CO2 over N2 - Singh, Puyam S.; Selvam, Thangaraj; Reuß, Stephanie; Avhale, Abhijeet; Lopez Orozco, Sofia; Schwieger, Wilhelm (Chemical Engineering & Technology, 2013)
CO2/N2 gas separation was performed over a nanocrystalline zeolite tetraethylammonium (TEA)-beta membrane prepared on a stainless-steel porous disc by repeated hydrothermal crystallization. Two to three consecutive hydrothermal syntheses were required to form a membrane comprised of a continuous and compact layer of zeolite beta nanocrystals on the support. The membrane TEA-BEA3 obtained by three consecutive syntheses, in which the membrane from two consecutive syntheses was used as support, exhibited the highest structural order. When the separation experiment was performed over this membrane without applying any external applied pressure, 100% selectivity of CO2 over N2 was observed. The separation was driven by differences in chemical potentials of the molecules generated only by the adsorption-desorption behavior of the gases into the membrane. The novel zeolite TEA-beta membrane provided promising results for the separation of small gas molecules due to the combined influence of diffusion and sorption selectivity.
Keywords: CO2/N-2 separation; Hydrothermal crystallization; Nanocrystalline zeolite; Tetraethylammonium-beta membrane

Nano Filler Materials for Transparent Polymer Composites: Influences on the Properties and on the Transparency — A review - Wildner, Wolfgang; Drummer, Dietmar (Polymer Composites, 2016)
EAM Import::2019-03-12

Nanofiller materials for transparent polymer composites: Influences on the properties and on the transparency—A review - Wildner, Wolfgang; Drummer, Dietmar (Journal of Thermoplastic Composite Materials, 2018)
Filler materials are widely used in combination with polymer materials. Conventional filler particles generally cause light scattering and absorption because of their optical characteristic or refractive index difference. With nanoparticles (NPs) as a filler material, it is theoretically possible to manufacture transparent compounds due to their small particle dimensions reducing the interaction with light. Nevertheless, the particles tend to build agglomerates and aggregates which reduce the composite's transparency considerably. This review gives an overview of the effect different particle materials have on the properties of transparent polymer composites with consideration of the composite's transparency. There are very few reports on highly transparent and thick (>1 mm) polymer nanocomposites with such an amount of particles that affect other properties of the polymer significantly. In the majority of cases, NPs lead to a significant lower transparency. This indicates that the homogeneous dispersion of the particles is still a major difficulty in producing transparent nanocomposites with enhanced properties.

Nanoforming behaviour and microstructural evolution during nanoimprinting of ultrafine-grained and nanocrystalline metals - Ast, Johannes; Durst, Karsten (Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing, 2013)
The influences of microstructure and the macroscopic material behaviour on the nanoforming behaviour of nickel, copper and aluminium with grain sizes ranging from single crystalline to nanocrystalline were studied using a flat punch indenter with a double ring cavity and with a wheel-shaped die. Of main interest in this work was the flow of crystalline materials in submicron sized cavities during imprinting. The ring cavities which have widths of 650 nm and 80 nm were fabricated by focused ion beam (FIB) machining. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to evaluate the imprinted ring geometries. The microstructure after imprinting was investigated in detail by FIB cross sections and electron back scatter diffraction (EBSD) as well as by using finite element analysis (FEA) of the forming process. SX-Ni showed the smallest extrusion height together with a sinking-in of the formed region. This is accompanied by strong orientation gradients up to 18° below the cavities. The UFG samples exhibited the best formability, with a subgrain formation inside and around the cavities. The plastic flow is confined to the surface and a pile-up formation occurs. For the nanocrystalline material only a slight elongation of the grains inside the cavity was found, yielding moreover a smooth and homogeneous extruded geometry. These findings can be explained by the grain size to cavity width ratio as well as the yield strength and the work hardening behaviour of the materials.
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Nanoindentation strain-rate jump tests for determining the local strain-rate sensitivity in nanocrystalline Ni and ultrafine-grained Al - Maier, Verena; Durst, Karsten; Müller, Johannes; Backes, Björn; Höppel, Heinz Werner; Göken, Mathias (Journal of Materials Research, 2011)
A nanoindentation strain-rate jump technique has been developed for determining the local strain-rate sensitivity (SRS) of nanocrystalline and ultrafine-grained (UFG) materials. The results of the new method are compared to conventional constant strain-rate nanoindentation experiments, macroscopic compression tests, and finite element modeling (FEM) simulations. The FEM simulations showed that nanoindentation tests should yield a similar SRS as uniaxial testing and generally a good agreement is found between nanoindentation strain-rate jump experiments and compression tests. However, a higher SRS is found in constant indentation strain-rate tests, which could be caused by the long indentation times required for tests at low indentation strain rates. The nanoindentation strain-rate jump technique thus offers the possibility to use single indentations for determining the SRS at low strain rates with strongly reduced testing times. For UFG-Al, extremely fine-grained regions around a bond layer exhibit a substantial higher SRS than bulk material.
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Nanomechanical behaviour of Al-Ti layered composites produced by accumulative roll bonding - Maier, Verena; Höppel, Heinz Werner; Göken, Mathias (Journal of Physics: Conference Series, 2010)
In this study Ti-foils were roll bonded together with commercially pure aluminium AA1050. The laminates were produced by using two 1 mm thick AA1050 sheets at the outer side of the stack combined with 100 extgreekmm thick Ti-foils as an intermediate layer for each accumulative roll bonding process. The samples were rolled up to 4 ARB cycles. Subsequently the sheets were post-process heat treated at 180°C, 400°C or 600°C, respectively, for 24 hours. The local mechanical behaviour of the Al/Ti intermetallic interfaces have been investigated using nanoindentation experiments. A strong dependence between annealing-temperature, – time and deformation grade is detected. While a heat treatment at 180°C only leads to a weak bonding between Al and Ti with a preservation of the UFG structure, temperatures up to 600°C are causing a complete recrystallisation of the microstructure and formation of diffusion layers with different Al and Ti concentrations.
EAM Import::2019-03-12

Nanometric thermal fluctuations of weakly confined biomembranes measured with microsecond time-resolution - Monzel, Cornelia; Schmidt, Daniel; Seifert, Udo; Smith, Ana-Suncana; Merkel, Rudolf; Sengupta, Kheya (Soft Matter, 2016)
We probe the bending fluctuations of bio-membranes using highly deflated giant unilamellar vesicles (GUVs) bound to a substrate by a weak potential arising from generic interactions. The substrate is either homogeneous, with GUVs bound only by the weak potential, or is chemically functionalized with a micro-pattern of very strong specific binders. In both cases, the weakly adhered membrane is seen to be confined at a well-defined distance above the surface while it continues to fluctuate strongly. We quantify the fluctuations of the weakly confined membrane at the substrate proximal surface as well as of the free membrane at the distal surface of the same GUV. This strategy enables us to probe in detail the damping of fluctuations in the presence of the substrate, and to independently measure the membrane tension and the strength of the generic interaction potential. Measurements were done using two complementary techniques - dynamic optical displacement spectroscopy (DODS, resolution: 20 nm, 10 ms), and dual wavelength reflection interference contrast microscopy (DW-RICM, resolution: 4 nm, 50 ms). After accounting for the spatio-temporal resolution of the techniques, an excellent agreement between the two measurements was obtained. For both weakly confined systems we explore in detail the link between fluctuations on the one hand and membrane tension and the interaction potential on the other hand.
Keywords: FLUORESCENCE CORRELATION SPECTROSCOPY; INTERFERENCE CONTRAST MICROSCOPY; RED-BLOOD-CELLS; FLUID MEMBRANES; LIPID-BILAYERS; BENDING STIFFNESS; MECHANICAL FLUCTUATIONS; PHOSPHOLIPID-VESICLES; UNILAMELLAR VESICLES; GIANT VESICLES

Nanomorphology and charge generation in bulk heterojunctions based on low-bandgap dithiophene polymers with different bridging atoms - Morana, Mauro; Azimi, Hamed; Dennler, Gilles; Egelhaaf, Hans-Joachim; Scharber, Markus; Forberich, Karen; Hauch, Jens; Gaudiana, Russell; Waller, David; Zhu, Zenghuo; Hingerl, Kurt; Van Bavel, Svetlana S.; et al. (Advanced Functional Materials, 2010)
Carbon bridged (C-PCPDTBT) and silicon-bridged (Si-PCPDTBT) dithiophene donor-acceptor copolymers belong to a promising class of low bandgap materials. Their higher field-effect mobility, as high as 10cm V s in pristine films, and their more balanced charge transport in blends with fullerenes make silicon-bridged materials better candidates for use in photovoltaic devices. Striking morphological changes are observed in polymer:fullerene bulk heterojunctions upon the substitution of the bridging atom. XRD investigation indicates increased π-π stacking in Si-PCPDTBT compared to the carbon-bridged analogue. The fluorescence of this polymer and that of its counterpart C-PCPDTBT indicates that the higher photogeneration achieved in Si-PCPDTBT:fullerene films (with either [C60]PCBM or [C70]PCBM) can be correlated to the inactivation of a charge-transfer complex and to a favorable length of the donor-acceptor phase separation. TEM studies of Si-PCPDTBT:fullerene blended films suggest the formation of an interpenetrating network whose phase distribution is comparable to the one achieved in C-PCPDTBT:fullerene using 1,8-octanedithiol as an additive. In order to achieve a balanced hole and electron transport, Si-PCPDTBT requires a lower fullerene content (between 50 to 60 wt%) than C-PCPDTBT (more than 70 wt%). The Si-PCPDTBT:[C70]PCBM OBH) solar cells deliver power conversion efficiencies of over 5%. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.

Nanoparticles size-dependently initiate self-limiting NETosis-driven inflammation - Munoz, Luis E.; Bilyy, Rostyslav; Biermann, Mona H. C.; Kienhoefer, Deborah; Maueröder, Christian; Hahn, Jonas; Brauner, Jan M.; Weidner, Daniela; Chen, Juan; Scharin-Mehlmann, Marina; Janko, Christina; Friedrich, Ralf P.; et al. (Proceedings of the National Academy of Sciences of the United States of America, 2016)
The critical size for strong interaction of hydrophobic particles with phospholipid bilayers has been predicted to be 10 nm. Because of the wide spreading of nonpolar nanoparticles (NPs) in the environment, we aimed to reveal the ability of living organisms to entrap NPs via formation of neutrophil extracellular traps (NETs). Upon interaction with various cell types and tissues, 10- to 40-nm-sized NPs induce fast (<20 min) damage of plasma membranes and instability of the lysosomal compartment, leading to the immediate formation of NETs. In contrast, particles sized 100-1,000 nm behaved rather inertly. Resulting NET formation (NETosis) was accompanied by an inflammatory reaction intrinsically endowed with its own resolution, demonstrated in lungs and air pouches of mice. Persistence of small NPs in joints caused unremitting arthritis and bone remodeling. Small NPs coinjected with antigen exerted adjuvant-like activity. This report demonstrates a cellular mechanism that explains how small NPs activate the NETosis pathway and drive their entrapping and resolution of the initial inflammatory response.
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Nanoporous Au: An Unsupported Pure Gold Catalyst? - Wittstock, Arne; Neumann, Björn; Schaefer, Andreas; Dumbuya, Karifala; Kübel, Christian; Biener, Monika M.; Zielasek, Volkmar; Steinrück, Hans-Peter; Gottfried, Michael; Biener, Jürgen; Hamza, Alex; Bäumer, Marcus (Journal of Physical Chemistry C, 2009)
The unique properties of gold especially in low temperature CO oxidation have been ascribed to a combination of various effects. In particular, particle sizes below a few nanometers and specific particle-support interactions have been shown to play important roles. In contrast, recent reports revealed that monolithic nanoporous gold (npAu) prepared by leaching a less noble metal, such as Ag, out of the corresponding alloy can also exhibit a remarkably high catalytic activity for CO oxidation, even though no support is present. Therefore, it was claimed to be a pure and unsupported gold catalyst. We investigated npAu with respect to its morphology, surface composition, and catalytic properties. In particular, we studied the reaction kinetics for low temperature CO oxidation in detail, taking the mass transport limitation due to the porous structure of the material into account. Our results reveal that Ag, even if removed almost completely from the bulk, segregates to the surface, resulting in surface concentrations of up to 10 atom %. Our data suggest that this Ag plays a significant role in activating of molecular oxygen. Therefore, npAu should be considered a bimetallic catalyst rather than a pure Au catalyst.

Nanoscale Characterization of Carrier Dynamic and Surface Passivation in InGaN/GaN Multiple Quantum Wells on GaN Nanorods - Chen, Weijian; Wen, Xiaoming; Latzel, Michael; Heilmann, Martin; Yang, Jianfeng; Dai, Xi; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Christiansen, Silke; Conibeert, Gavin (ACS Applied Materials and Interfaces, 2016)
Using advanced two-photon excitation confocal microscopy, associated with time-resolved spectroscopy, we characterize InGaN/GaN multiple quantum wells on nanorod heterostructures and demonstrate the passivation effect of a KOH treatment. High-quality InGaN/GaN nanorods were fabricated using nanosphere lithography as a candidate material for light-emitting diode devices. The depth- and time-resolved characterization at the nanoscale provides detailed carrier dynamic analysis helpful for understanding the optical properties. The nanoscale spatially resolved images of InGaN quantum well and defects were acquired simultaneously. We demonstrate that nanorod etching improves light extraction efficiency, and a proper KOH treatment has been found to reduce the surface defects efficiently and enhance the luminescence. The optical characterization techniques provide depth-resolved and time-resolved carrier dynamics with nanoscale spatially resolved mapping, which is crucial for a comprehensive and thorough understanding of nanostructured materials and provides novel insight into the improvement of materials fabrication and applications.
EAM Import::2019-03-12

Nanoscale Conducting Oxide PlasMOStor - Lee, Ho W.; Papadakis, Georgia; Burgos, Stanley P.; Chander, Krishnan; Kriesch, Arian; Pala, Ragip; Peschel, Ulf; Atwater, Harry A. (Nano Letters, 2014)
We experimentally demonstrate an ultracompact PlasMOStor, a plasmon slot waveguide field-effect modulator based on a transparent conducting oxide active region. By electrically modulating the conducting oxide material deposited into the gaps of highly confined plasmonic slot waveguides, we demonstrate field-effect dynamics giving rise to modulation with high dynamic range (2.71 dB/ extgreekmm) and low waveguide loss (∼0.45 dB/ extgreekmm). The large modulation strength is due to the large change in complex dielectric function when the signal wavelength approaches the surface plasmon resonance in the voltage-tuned conducting oxide accumulation layer. The results provide insight about the design of ultracompact, nanoscale modulators for future integrated nanophotonic circuits.
EAM Import::2019-03-12

Nanoscale Morphological and Structural Transformations of PtCu Alloy Electrocatalysts during Potentiodynamic Cycling - Khalakhan, Ivan; Waidhas, Fabian; Brummel, Olaf; Vorokhta, Mykhailo; Kus, Peter; Yakovlev, Yurii; Bertram, Manon; Dopita, Milan; Matolinova, Iva; Libuda, Jörg; Matolin, Vladimir (Journal of Physical Chemistry C, 2018)
PtCu bimetallic alloys are known to provide better activity than pure platinum in proton exchange membrane fuel cells. However, such catalysts undergo complex degradation processes during fuel cell operation, resulting in deterioration of their activity. By using in situ electrochemical (EC) atomic force microscopy combined with in situ EC infrared reflection absorption spectroscopy, we provide a comprehensive investigation of morphological and structural transformations of PtCu model thin film catalysts during accelerated degradation tests (ADTs). The ADTs consist of potentiodynamic cycling to three different upper potentials relevant for different modes of fuel cell operation. The results show that, depending on the upper potential limit, PtCu alloy electrocatalysts are subject to drastic changes in the surface composition, morphology, and structure.

Nanoscale Morphology of PTB7 Based Organic Photovoltaics as a Function of Fullerene Size - Roehling, John D.; Baran, Derya; Sit, Joseph; Kassar, Thaer; Ameri, Tayebeh; Unruh, Tobias; Brabec, Christoph; Moule, Adam J. (Scientific Reports, 2016)
High efficiency polymer:fullerene photovoltaic device layers self-assemble with hierarchical features from ångströms to 100's of nanometers. The feature size, shape, composition, orientation, and order all contribute to device efficiency and are simultaneously difficult to study due to poor contrast between carbon based materials. This study seeks to increase device efficiency and simplify morphology measurements by replacing the typical fullerene acceptor with endohedral fullerene LuN@PC BEH. The metal atoms give excellent scattering contrast for electron beam and X-ray experiments. Additionally, LuN@PC BEH has a lower electron affinity than standard fullerenes, which can raise the open circuit voltage of photovoltaic devices. Electron microscopy techniques are used to produce a detailed account of morphology evolution in mixtures of Lu 3 N@PC 80 BEH with the record breaking donor polymer, PTB7 and coated using solvent mixtures. We demonstrate that common solvent additives like 1,8-diiodooctane or chloronapthalene do not improve the morphology of endohedral fullerene devices as expected. The poor device performance is attributed to the lack of mutual miscibility between this particular polymer:fullerene combination and to co-crystallization of Lu 3 N@PC 80 BEH with 1,8-diiodooctane. This negative result explains why solvent additives mixtures are not necessarily a morphology cure-all.

Nanosculpt: A methodology for generating complex realistic configurations for atomistic simulations - Prakash, Aruna; Hummel, Martin; Schmauder, Siegfried; Bitzek, Erik (MethodsX, 2016)
Atomistic simulations have now become commonplace in the study of the deformation and failure of materials. Increase in computing power in recent years has made large-scale simulations with billions, or even trillions, of atoms a possibility. Most simulations to-date, however, are still performed with quasi-2D geometries or rather simplistic 3D setups. Although controlled studies on such well-defined structures are often required to obtain quantitative information from atomistic simulations, for qualitative studies focusing on e.g. the identification of mechanisms, researchers would greatly benefit from a methodology that helps realize more realistic configurations. The ideal scenario would be a one-on-one reconstruction of experimentally observed structures. To this end, we propose a new method and software tool called nanosculpt with the following features:The method allows for easy sample generation for atomistic simulations from any arbitrarily shaped 3D enclosed volume.The tool can be used to build atomistic samples from artificial geometries, including CAD geometries and structures obtained from simulation methods other than atomistic simulations.The tool enables the generation of experimentally informed atomistic samples, by e.g. digitization of micrographs or usage of tomography dat
Keywords: 3D materials science; Atom probe tomography; Digitized microstructure; Experimentally informed atomistic simulations; Molecular dynamics simulations; Ni-base superalloy

Nanostructured Encapsulated Catalysts for Combination of Fischer-Tropsch Synthesis and Hydroprocessing - Kruse, Nadine; Machoke, Albert; Schwieger, Wilhelm; Guettel, Robert (ChemCatChem, 2015)
Controlling the selectivity of Fischer-Tropsch synthesis in a single reaction step is highly desirable, but is a major challenge in heterogeneous catalysis. One approach is the application of bifunctional catalysts. However, to control the catalytic properties the morphology of the catalyst needs to be controlled on the nanoscopic scale. Herein, an innovative synthetic approach that allows the bottom-up construction of nanostructured bifunctional catalysts in a step-wise manner is described. The resulting material, which exhibits cobalt nanoparticles encapsulated inside a zeolite matrix, was proven to be active in the combined Fischer-Tropsch and cracking reaction, evidenced by a shift from waxy to liquid products. Consequently, the use of this novel approach was demonstrated in terms of material synthesis and catalytic applications.
Keywords: cracking; Fischer-Tropsch synthesis; heterogeneous catalysis; nanomaterials; zeolites

Nanostructured organosilicon luminophores in highly efficient luminescent down-shifting layers for thin film photovoltaics - Uekert, Taylor; Solodovnyk, Anastasiia; Ponomarenko, Sergei; Osvet, Andres; Levchuk, Ievgen; Gast, Jessica; Batentschuk, Miroslaw; Forberich, Karen; Stern, Edda; Egelhaaf, Hans-Joachim; Brabec, Christoph (Solar Energy Materials and Solar Cells, 2016)
We use a new type of solution processed luminescent molecules - nanostructured organosilicon luminophores (NOLs) - in printed luminescent down-shifting layers for photovoltaic light harvesting. We show that NOLs exhibit a high luminescent efficiency of 82-90% when added into two chemically different polymer matrices that are commonly used for photovoltaic encapsulation: polyvinyl butyral and ethylene-vinyl acetate. The coated layers are optimized to maximize both UV absorbance and visible light emission and transmittance. Attaching the luminescent layers onto a CIGS solar cell significantly improves the external quantum efficiency in the UV region: from 1% to 55% at 360 nm. As a result, the short circuit current density and power conversion efficiency increase by up to 3.2-4.3%. After experimental verification of our optical simulation model, we employ it to determine the ideal molecular structure of NOLs for luminescent down-shifting applied to CIGS.
Keywords: Absorbance; CIGS; Luminescence quantum yield; Luminescent down-shifting; Organic emitter; Organosilicon nanostructure; Photovoltaics

Nanotwinned silver nanowires: Structure and mechanical properties - Kobler, Aaron; Beuth, Thorsten; Klöffel, Tobias; Prang, Robby; Moosmann, Markus; Seherer, Torsten; Walheim, Stefan; Hahn, Horst; Kuebel, Christian; Meyer, Bernd; Schimmel, Thomas; Bitzek, Erik (Acta Materialia, 2015)
Ag nanowires have been manufactured with the ultrathin layer electrochemical deposition method. Transmission electron microscopy revealed a multiple twinned nanostructure that has not been reported in comparable structures using a non-template self-assembling preparation method. The [1 1 1] twin planes are oriented parallel to the 〈1 1 2〉 wire axis. The influence of the twin boundaries on the elastic and plastic properties of these wires has been investigated. Atomistic calculations revealed no influence of the twin boundaries on the Young's modulus of the wire for this geometry and along the 〈1 1 2〉 direction. However, the twin boundaries exhibited a strong orientation-dependent influence on the plastic deformation when indented orthogonal (〈1 1 1〉 direction) or parallel (〈1 1 0〉 direction) to the twin boundaries, reflecting the interaction between the glide systems active in fcc metals and the stress direction. Heating experiments showed a high heat resistance for the twin boundaries till sublimation in an in situ TEM heating experiment. Sublimation followed the twin boundaries, indicating the lower binding energy of hcp stacking in comparison to fcc stacking.
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Nanowire Arrays in Multicrystalline Silicon Thin Films on Glass: A Promising Material for Research and Applications in Nanotechnology - Schmitt, Sebastian W.; Schechtel, Florian; Amkreutz, Daniel; Bashouti, Muhammad; Srivastava, Sanjay K.; Hoffmann, Bjoern; Dieker, Christel; Spiecker, Erdmann; Rech, Bernd; Christiansen, Silke H. (Nano Letters, 2012)
Silicon nanowires (SiNW) were formed on large grained, electron-beam crystallized silicon (Si) thin films of only 6 pm thickness on glass using nanosphere lithography (NSL) in combination with reactive ion etching (RIE). Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) studies revealed outstanding structural properties of this nanomaterial. It could be shown that SiNWs with entirely predetermined shapes including lengths, diameters and spacings and straight side walls form independently of their crystalline orientation and arrange in ordered arrays on glass. Furthermore, for the first time grain boundaries could be observed in individual, straightly etched SiNWs. After heat treatment an electronic grade surface quality of the SiNWs could be shown by X-ray photoelectron spectroscopy (XPS). Integrating sphere measurements show that SiNW patterning of the multicrystalline Si (mc-Si) starting thin film on glass substantially increases absorption and reduces reflection, as being desired for an application in thin film photovoltaics (PV). The multicrystalline SiNWs directly mark a starting point for research not only in PV but also in other areas like nanoelectronics, surface functionalization, and nanomechanics.
Keywords: Silicon nanowires; multicrystalline; silicon thin film on glass; e-beam crystallization; photovoltaics; absorption; reactive ion etching; EBSD; TEM; XPS

Nanowire Interconnects for Printed Large-Area Semitransparent Organic Photovoltaic Modules - Guo, Fei; Kubis, Peter; Przybilla, Thomas; Spiecker, Erdmann; Hollmann, Andre; Langner, Stefan; Forberich, Karen; Brabec, Christoph (Advanced Energy Materials, 2015)
Semitransparent organic photovoltaic (OPV) cells promise applications in various transparent architectures where their opaque counterparts cannot contribute. Realizing practical applications of this technology requires the manufacturing of large-area modules without significant performance loss compared to the lab-scale devices. In this work, efficient semitransparent OPV modules based on ultrafast laser patterning on both glass and flexible substrates are reported. Solution-processed metallic silver nanowires (AgNWs) are used as transparent top electrodes. The efficient low-ohmic contact of the interconnects between the top AgNWs and the bottom electrode in combination with high-precision laser beam positioning system allow to fabricate semitransparent modules with high electrical fill factor of approximate to 63% and a remarkable geometric fill factor exceeding 95%, respectively. These results represent an important progress toward upscaling of high-performance OPV modules with reduced production costs.

Natural element analysis of the Cahn-Hilliard phase-field model - Rajagopal, Amirtham; Fischer, Peter; Kuhl, Ellen; Steinmann, Paul (Computational Mechanics, 2010)
We present a natural element method to treat higher-order spatial derivatives in the Cahn–Hilliard equation. The Cahn–Hilliard equation is a fourth-order nonlinear partial differential equation that allows to model phase separation in binary mixtures. Standard classical C0-continuous finite element solutions are not suitable because primal variational formulations of fourth-order operators are well-defined and integrable only if the finite element basis functions are piecewise smooth and globally C1-continuous. To ensure C1-continuity, we develop a natural-element-based spatial discretization scheme. The C1-continuous natural element shape functions are achieved by a transformation of the classical Farin interpolant, which is basically obtained by embedding Sibsons natural element coordinates in a Bernstein–Bézier surface representation of a cubic simplex. For the temporal discretization, we apply the (second-order accurate) trapezoidal time integration scheme supplemented with an adaptively adjustable time step size. Numerical examples are presented to demonstrate the efficiency of the computational algorithm in two dimensions. Both periodic Dirichlet and homogeneous Neumann boundary conditions are applied. Also constant and degenerate mobilities are considered. We demonstrate that the use of C1-continuous natural element shape functions enables the computation of topologically correct solutions on arbitrarily shaped domains.
UnivIS-Import:2015-04-14:Pub.2010.tech.FT.FT-TM.natura

Nb doping of TiO2 nanotubes for an enhanced efficiency of dye-sensitized solar cells - Yang, Mingjun; Kim, Dam-Hee; Jha, Himendra; Lee, Kiyoung; Paul, Jonathan; Schmuki, Patrik (Chemical Communications, 2011)
Nb-doped TiO2 nanotube (with CNb < 1 wt%) layers were successfully fabricated by self-ordered electrochemical anodization of Ti–Nb alloys. When used in dye-sensitized solar cells the efficiency enhanced by up to 30% compared to non-doped TiO2 nanotubes. IMVS measurements indicate the beneficial effect to be due to lower recombination losses.
EAM Import::2019-03-12

Nb-doping of TiO2/SrTiO3 nanotubular heterostructures for enhanced photocatalytic water splitting - Yang, Yang; Lee, Kiyoung; Kado, Yuya; Schmuki, Patrik (Electrochemistry Communications, 2012)
Ti alloys with low Nb contents (0.1 at.% and 0.5 at.%) were used to grow self-organized Nb-doped TiO2 nanotube arrays. These layers were partially converted to SrTiO3 using a hydrothermal treatment. Various factors, such as hydrothermal temperature, hydrothermal duration, and annealing conditions, were investigated in view of the photocatalytic water splitting performance of the formed Nb-doped TiO2/SrTiO3 heterostructure nanotubes. Photoelectrochemical characterization shows that Nb-doping efficiently enhances the photocatalytic water splitting performance of TiO2/SrTiO3 heterostructure nanotubes.
EAM Import::2019-03-12

Near ambient pressure XPS investigation of the interaction of ethanol with Co/CeO2(1 1 1) - Ovari, L.; Krick Calderon, Sandra; Lykhach, Yaroslava; Libuda, Jörg; Erdohelyi, A.; Papp, Christian; Kiss, J.; Steinrück, Hans-Peter (Journal of Catalysis, 2013)
Near ambient pressure X-ray photoelectron spectroscopy was applied to study the interaction of ethanol (CHCHOH) with a well-ordered CeO(1 1 1) film on Cu(1 1 1) and with a Co/CeO(1 1 1)/Cu(1 1 1) model catalyst. The oxidation state of the surface and the chemical nature of reaction intermediates were analyzed. At 300 K, the oxidation state of ceria decreased gradually with increasing ethanol pressure. At a constant pressure of 0.1 mbar, the reduction of Ce to Ce increased significantly between 320 and 600 K due to a higher mobility of oxygen or Ce centers at elevated temperatures. The main intermediate, ethoxide, was formed by dissociative adsorption of ethanol at room temperature. No coke formation was observed up to 600 K on CeO. Upon deposition of metallic cobalt, partial reduction of ceria was observed, leading to the formation of Co sites but still leaving metallic Co in the metal particles. During the reaction of ethanol with the Co/CeO(1 1 1) model catalyst, the amount of Co decreased drastically with increasing temperature, and at 600 K, the majority of Co was metallic. This process was accompanied by the severe reduction of ceria and the formation of significant carbon deposits. © 2013 Elsevier Inc. All rights reserved.

Near-Field Analysis of Bright and Dark Modes on Plasmonic Metasurfaces Showing Extraordinary Suppressed Transmission - Dobmann, Sabine; Kriesch, Arian; Ploss, Daniel; Peschel, Ulf (Advanced Optical Materials, 2014)
Plasmonic metasurfaces are investigated that consist of a sub-wavelength line pattern in an ultrathin ( 10 nm) silver film, designed for extraordinary suppressed transmission (EOST) in the visible spectral range. Measurements with a near-field scanning optical microscope (NSOM) demonstrate that farfield irradiation resonantly excites antenna-like (bright) modes that are localized on the metal ridges. The re-radiation of these modes into the far-field interferes destructively with the transmitted wave, thus suppressing transmission almost completely. In contrast, a second type of mode, bound (dark) surface plasmon polaritons (SPPs) launched from an NSOM tip, propagates well across the metasurface, preferentially perpendicular to the grating lines.
EAM Import::2019-03-12

Near IR sensitization of organic bulk heterojunction solar cells: Towards optimization of the spectral response of organic solar cells - Koppe, Markus; Egelhaaf, Hans-Joachim; Dennler, Gilles; Scharber, Markus C.; Brabec, Christoph; Schilinsky, Pavel; Hoth, Claudia N. (Advanced Functional Materials, 2010)
The spectroscopic response of a poly(3-hexylthiophene)/[6,6]-phenyl-C -butyric acid methyl ester (P3HT/PCBM)-based bulk heterojunction solar cell is extended into the near infrared region (NIR) of the spectrum by adding the low bandgap polymer polyp,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1 -b, 3, 4-b]-dithiophene)-alt-4, 7-(2, l, 3-benzothiadiazole)] [PCPDTBTJ to the blend. The dominant mechanism behind the enhanced photosensitivity of the ternary blend is found to be a two-step process: first, an ultrafast and efficient photoinduced charge transfer generates positive charges on P3HT and PCPDTBT and a negative charge on PCBM. In a second step, the positive charge on PCPDTBT is transferred to P3HT. Thus, P3HT serves two purposes. On the one hand it is involved in the generation of charge carriers by the photoinduced electron transfer to PCBM, and, on the other hand, it forms the charge transport matrix for the positive carriers transferred from PCPDTBT. Other mechanisms, such as energy transfer or photoinduced charge transfer directly between the two polymers, are found to be absent or negligible. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.

Negative coefficient of normal restitution - Müller, Patric; Krengel, Dominik; Pöschel, Thorsten (Physical Review E, 2012)
This paper shows that negative coefficients of normal restitution occur inevitably when the interaction force between colliding particles is finite. We derive an explicit criterion showing that for any set of material properties there is always a collision geometry leading to negative restitution coefficients. While from a phenomenological point of view, negative coefficients of normal restitution appear rather artificial, this phenomenon is generic and implies an important overlooked limitation of the widely used hard sphere model. The criterion is explicitly applied to two paradigmatic situations: for the linear dashpot model and for viscoelastic particles. In addition, we show that for frictional particles the phenomenon is less pronounced than for smooth spheres. © 2012 American Physical Society.

Neutron spectroscopic study of crystal-field excitations and the effect of the crystal field on dipolar magnetism in LiRF4 (R = Gd, Ho, Er, Tm, and Yb) - Babkevich, P.; Finco, A.; Jeong, M.; Piazza, B. Dalla; Kovacevic, I.; Klughertz, G.; Kraemer, K. W.; Kraemer, C.; Adroja, D. T.; Goremychkin, E.; Unruh, Tobias; Straessle, T.; et al. (Physical Review B, 2015)
We present a systematic study of the crystal-field interactions in the LiRF4 (R = Gd, Ho, Er, Tm, and Yb) family of rare-earth magnets. Using detailed inelastic neutron scattering measurements, we have been able to quantify the transition energies and wave functions for each system. This allows us to quantitatively describe the high-temperature susceptibility measurements for the series of materials and make predictions based on a mean-field approach for the low-temperature thermal and quantum phase transitions. We show that coupling between crystal field and phonon states leads to line-shape broadening in LiTmF4 and level splitting in LiYbF4. Furthermore, using high-resolution neutron scattering from LiHoF4, we find anomalous broadening of crystal-field excitations which we attribute to magnetoelastic coupling.

New basic insight into reductive functionalization sequences of single walled carbon nanotubes (SWCNTs) - Hof, Ferdinand; Bosch, Sebastian; Eigler, Siegfried; Hauke, Frank; Hirsch, Andreas (Journal of the American Chemical Society, 2013)
The reactivity of reduced single walled carbon nanotubes (SWCNTs) (carbon nanotubides), prepared under strict inert conditions in a glovebox with respect to the covalent functionalization with hexyl iodide and subsequent exposure to ambient conditions (air, moisture), was systematically investigated by Raman, absorption, fluorescence, and IR spectroscopy as well as by TG/MS measurements. We have discovered that the alkylation does not lead to a complete discharging of the tubes since follow-up reactions with moisture still take place leading to mixed functionalized carbon nanotube derivatives containing H- and OH-addends (but no carboxylates) next to the hexyl groups. This was confirmed by the exposure of carbon nanotubides to ambient conditions. The degree of hexylation determined both under strict inert (ic) and ambient (ac) conditions increases with an increasing K:C ratio of the reduced SWCNT starting material. The presence of OH-groups covalently attached to the nanotubes was also confirmed by postfunctionalization reactions with 2-thiophenecarbonyl chloride, leading to the corresponding esters. Control experiments with KO give rise to the formation of the same oxygen functionalities. These combined findings allowed for the suggestions of a plausible reaction mechanism, describing all the observed reactions on the SWCNTs side walls. The amount of subsequent side reactions after the treatment of reduced SWCNTs with electrophiles is strongly influenced by the reduction potential of the electrophile, which is responsible for the extent of reoxidation. Incomplete quenching of negative charges allows stronger oxidants/electrophile (e.g., O) to perform follow-up reactions. © 2013 American Chemical Societ

New insights into colloidal gold flakes: structural investigation, micro-ellipsometry and thinning procedure towards ultrathin monocrystalline layers - Hoffmann, B.; Bashouti, M. Y.; Feichtner, Thorsten; Mackovic, Mirza; Dieker, Christel; Salaheldin, A. M.; Richter, P.; Gordan, O. D.; Zahn, D. R. T.; Spiecker, Erdmann; Christiansen, S.; Hoffmann, Björn (Nanoscale, 2016)
High-quality fabrication of plasmonic devices often relies on wet-chemically grown ultraflat, presumably single-crystalline gold flakes due to their superior materials properties. However, important details about their intrinsic structure and their optical properties are not well understood yet. In this study, we present a synthesis routine for large flakes with diameters of up to 70 mu m and an in-depth investigation of their structural and optical properties. The flakes are precisely analyzed by transmission electron microscopy, electron backscatter diffraction and micro-ellipsometry. We found new evidence for the existence of twins extending parallel to the Au flake 111 surfaces which have been found to not interfere with the presented nanopatterning. Micro-Ellipsometry was carried out to determine the complex dielectric function and to compare it to previous measurements of bulk single crystalline gold. Finally, we used focused ion beam milling to prepare smooth crystalline layers and high-quality nanostructures with desired thickness down to 10 nm to demonstrate the outstanding properties of the flakes. Our findings support the plasmonics and nano optics community with a better understanding of this material which is ideally suited for superior plasmonic nanostructures.

New lattice Boltzmann method for the simulation of three-dimensional radiation transfer in turbid media - Mchardy, Christopher; Horneber, Tobias; Rauh, Cornelia (Optics Express, 2016)
Based on the kinetic theory of photons, a new lattice Boltzmann method for the simulation of 3D radiation transport is presented. The method was successfully validated with Monte Carlo simulations of radiation transport in optical thick absorbing and non-absorbing turbid media containing either isotropic or anisotropic scatterers. Moreover, for the approximation of Mie-scattering, a new iterative algebraic approach for the discretization of the scattering phase function was developed, ensuring full conservation of energy and asymmetry after discretization. It was found that the main error sources of the method are caused by linearization and ray effects and suggestions for further improvement of the method are made.
EAM Import::2019-03-12

New possibilities of accurate particle characterisation by applying direct boundary models to analytical centrifugation - Walter, Johannes; Thajudeen, Thaseem; Süß, Sebastian; Segets, Doris; Peukert, Wolfgang (Nanoscale, 2015)
Analytical centrifugation (AC) is a powerful technique for the characterisation of nanoparticles in colloidal systems. As a direct and absolute technique it requires no calibration or measurements of standards. Moreover, it offers simple experimental design and handling, high sample throughput as well as moderate investment costs. However, the full potential of AC for nanoparticle size analysis requires the development of powerful data analysis techniques. In this study we show how the application of direct boundary models to AC data opens up new possibilities in particle characterisation. An accurate analysis method, successfully applied to sedimentation data obtained by analytical ultracentrifugation (AUC) in the past, was used for the first time in analysing AC data. Unlike traditional data evaluation routines for AC using a designated number of radial positions or scans, direct boundary models consider the complete sedimentation boundary, which results in significantly better statistics. We demonstrate that meniscus fitting, as well as the correction of radius and time invariant noise significantly improves the signal-to-noise ratio and prevents the occurrence of false positives due to optical artefacts. Moreover, hydrodynamic non-ideality can be assessed by the residuals obtained from the analysis. The sedimentation coefficient distributions obtained by AC are in excellent agreement with the results from AUC. Brownian dynamics simulations were used to generate numerical sedimentation data to study the influence of diffusion on the obtained distributions. Our approach is further validated using polystyrene and silica nanoparticles. In particular, we demonstrate the strength of AC for analysing multimodal distributions by means of gold nanoparticles.

NH3 treatment of TiO2 nanotubes: from N-doping to semimetallic conductivity - Al-Thabaiti, Shaeel A.; Hahn, Robert; Liu, Ning; Kirchgeorg, Robin; So, Seulgi; Schmuki, Patrik; Basahel, Sulaiman N.; Bawaked, Salem M. (Chemical Communications, 2014)
In the present work we show that a suitable high temperature ammonia treatment allows for the conversion of single-walled TiO2 nanotube arrays not only to a N-doped photoactive anatase material (which is already well established), but even further into fully functional titanium nitride (TiN) tubular structures that exhibit semimetallic conductivity.
EAM Import::2019-03-12

N-Heterotriangulene chromophores with 4-pyridyl anchors for dye-sensitized solar cells - Meinhardt, Ute; Lodermeyer, Fabian; Schaub, Tobias; Kunzmann, Andreas; Dral, Pavlo O.; Sale, Anna Chiara; Hampel, Frank; Guldi, Dirk Michael; Costa, Ruben Dario; Kivala, Milan (RSC Advances, 2016)
A series of dimethylmethylene-bridged N-heterotriangulenes decorated with one, two, and three electron-withdrawing 4-pyridyls were synthesized. Their photophysical and electrochemical characteristics were examined and their successful application in n-type TiO2- and ZnO-based dye-sensitized solar cells demonstrated the ability of the 4-pyridyl moiety to act as an anchor.
EAM Import::2019-03-12

N-Heterotriangulenes: Fascinating Relatives of Triphenylamine - Hammer, Natalie; Schaub, Tobias; Meinhardt, Ute; Kivala, Milan (Chemical Record, 2015)
Carbonyl- and dimethylmethylene-bridged triphenylamines called N-heterotriangulenes are not only aesthetically pleasing extgreekp-conjugated scaffolds interesting on their own but also provide numerous possibilities for further synthetic modifications to serve as versatile precursors for the construction of functional organic molecules. In this Personal Account we give a historical synopsis depicting the long way from the initial synthesis of N-heterotriangulenes back in the 1970s to their derivatization followed by recent applications in organic electronics. As a part of our ongoing research on heteroatom-doped extgreekp-conjugated scaffolds we provide an overview of our synthetic efforts involving the N-heterotriangulene scaffolds and discuss the optoelectronic, redox, and self-assembly properties of the resulting molecular entities.
EAM Import::2019-03-12

Nickel catalyzed dimerization of propene in chloroaluminate ionic liquids: Detailed kinetic studies in a batch reactor - Eichmann, Marcus; Keim, Wilhelm; Haumann, Marco; Melcher, Berthold Ulrich; Wasserscheid, Peter (Journal of Molecular Catalysis A-Chemical, 2009)
The dimerization of propene in the presence of homogeneous nickel catalysts in acidic chloroaluminate ionic liquids has been investigated. The solubility of both substrate and products has been carefully determined in order to interpret kinetic findings. Detailed kinetic studies have indicated strong mass transport limitation of the overall reaction rate. The diffusion coefficient of propene in the ionic liquid has been determined from gas solubility measurements. This established method allowed the calculation of the Hatta number to be 4.9, indicating the severe mass transport limitation. For such fast biphasic reactions a loop reactor design is suggested.

Nickel Hydroxide Nanoparticle Activated Semi-metallic TiO2 Nanotube Arrays for Non-enzymatic Glucose Sensing - Gao, Zhi-Da; Guo, Jing; Shrestha, Nabeen Kumar; Hahn, Robert; Song, Yan-Yan; Schmuki, Patrik (Chemistry - A European Journal, 2013)
Semi-metallic TiO2 nanotube arrays (TiOxCy NTs) have been decorated uniformly with Ni(OH)2 nanoparticles without the aid of a polymer binder. The resulting hybrid nanotube arrays exhibit excellent catalytic activity towards non-enzymatic glucose electro-oxidation. The anodic current density of the glucose oxidation is significantly improved compared with traditional TiO2 nanotubes decorated with Ni(OH)2. Moreover, the Ni(OH)2/TiOxCy NT-based electrode shows a fast response, high sensitivity, wide linear range, good selectivity and stability towards glucose electro-oxidation, and thus provides a promising and cost-effective sensing platform for non-enzymatic glucose detection.
EAM Import::2019-03-08

Nickel oxide nanostructured electrodes towards perylenediimide-based dye-sensitized solar cells - Feihl, Sebastian; Costa Riquelme, Rubén Dario; Pflock, Stephan; Schmidt, Cordula; Schönamsgruber, Jörg; Backes, Susanne; Hirsch, Andreas; Guldi, Dirk Michael (RSC Advances, 2012)
In this work, we have realized nickel oxide (NiO) electrodes that serve as photocathodes in p-type dye-sensitized solar cells (p-DSSCs) sensitized by dendronized perylenediimides (PDIs). To this end, two different approaches in terms of preparing NiO nanoparticle pastes were pursued to fabricate mesoporous electrodes on conductive fluorine doped tin oxide (FTO) glass substrates. Firstly, commercially available NiO nanoparticles were dispersed in a mixture of ethanol and terpineol. Here, in order to obtain a mesoporous network two types of ethylcelluloses (EC), that is, EC 5-15 and 30-50 mPa s, were added in 1:1 weight ratios. Following the evaporation of ethanol, the resulting pastes were spread on FTOs by doctor blading and calcinated at different temperatures. Importantly, the calcination temperature evolved as a crucial aspect in developing efficient electrodes. Nevertheless, the visual appearance of these NiO electrodes prompts a fairly heterogeneous coverage. To circumvent the aforementioned problem, a second approach en route to homogenous electrodes was investigated. In that particular case, commercial NiO nanoparticles were mixed with a mixture of EC 5-15 and 30-50 mPa s at a 1:1 weight ratio, with triacetin as a plasticizer in ethanol. In doing so, pastes containing 7 wt% EC, 3 wt% triacetin, and 3 to 20 wt% NiO nanoparticles were prepared. Most importantly, scanning electron microscopy (SEM) images corroborated the fact that the resulting electrodes revealed a dense coverage on FTOs. In addition, further characterizations ranging from UV/Vis transmission spectroscopy and conductivity measurements to Barrett-Joyner-Halenda (BJH) pore size and volume analysis were carried out. In the final step, the applicability of the new NiO photoelectrodes for p-DSSCs was successfully demonstrated by utilizing two types of PDIs, namely a symmetric 1 and a non-symmetric dendronized 2. Key aspects such as time dependence of dye uptake, hole lifetime and resistance features of the electrodes under operation conditions were investigated. © 2012 The Royal Society of Chemistry.

Nitrite and nitrate formation on model NOx storage materials: on the influence of particle size and composition - Desikusumastuti, Aine; Qin, Zhihui; Happel, Markus; Staudt, Thorsten; Lykhach, Yaroslava; Laurin, Mathias; Rohr, F.; Shaikhutdinov, S.; Libuda, Jörg (Physical Chemistry Chemical Physics, 2009)
A well-defined model-catalyst approach has been utilized to study the formation and decomposition of nitrite and nitrate species on a model NOx storage material. The model system comprises BaAl2xO1+3x particles of different size and stoichiometry, prepared under ultrahigh-vacuum (UHV) conditions on Al2O3/NiAl(110). Adsorption and reaction of NO2 has been investigated by molecular beam (MB) methods and time-resolved IR reflection absorption spectroscopy (TR-IRAS) in combination with structural characterization by scanning tunneling microscopy (STM). The growth behavior and chemical composition of the BaAl2xO1+3x particles has been investigated previously. In this work we focus on the effect of particle size and stoichiometry on the reaction with NO2. Particles of different size and of different Ba2+ :Al3+ surface ion ratio are prepared by varying the preparation conditions. It is shown that at 300 K the reaction mechanism is independent of particle size and composition, involving initial nitrite formation and subsequent transformation of nitrites into surface nitrates. The coordination geometry of the surface nitrates, however, changes characteristically with particle size. For small BaAl2xO1+3x particles high temperature (800 K) oxygen treatment gives rise to particle ripening, which has a minor effect on the NO2 uptake behavior, however. STM shows that the morphology of the particle system is largely conserved during NO2 exposure at 300 K. The reaction is limited to the formation of surface nitrites and nitrates, which are characterized by low thermal stability and completely decompose below 500 K. As no further sintering occurs before decomposition, NO2 uptake and release is a fully reversible process. For large BaAl2xO1+3x particles, aggregates with different Ba2+ :Al3+ surface ion ratio were prepared. It was shown that the stoichiometry has a major effect on the kinetics of NO2 uptake. For barium-aluminate-like particles with high Al3+ concentration, the formation of nitrites and nitrates on the BaAl2xO1+3x particles at 300 K is slow, and kinetically restricted to the formation of surface species. Only at elevated temperature (500 K) are surface nitrates converted into well-defined bulk Ba(NO3)(2). This bulk Ba(NO3)(2) exhibits substantially higher thermal stability and undergoes restructuring and sintering before it decomposes at 700 K. For Ba2+-rich BaAl2xO1+3x particles, on the other hand, nitrate formation occurs at a much higher rate than for the barium-aluminate-like particles. Furthermore, nitrate formation is not limited to the surface, but NO2 exposure gives rise to the formation of amorphous bulk Ba(NO3)(2) particles even at 300 K.

Noble-Metal-Free Photocatalytic Hydrogen Evolution Activity: The Impact of Ball Milling Anatase Nanopowders with TiH2 - Zhou, Xuemei; Liu, Ning; Schmidt, Jochen; Kahnt, Axel; Osvet, Andres; Romeis, Stefan; Zolnhofer, Eva; Marthala, Venkata; Guldi, Dirk Michael; Peukert, Wolfgang; Hartmann, Martin; Meyer, Karsten; et al. (Advanced Materials, 2016)
Ball milling TiO2 anatase together with TiH2 can create an effective photocatalyst. The process changes the lattice and electronic structure of anatase. Lattice deformation created by mechanical impact combined with hydride incorporation yield electronic gap-states close to the conduction band of anatase. These provide longer lifetimes of photogenerated charge carriers and lead to an intrinsic cocatalytic activation of anatase for H2 evolution.
Keywords: Anatase; Ball milling; Hydrogen evolution; Photocatalysis; Titanium hydride

Noble Metals on Anodic TiO2 Nanotube Mouths: Thermal Dewetting of Minimal Pt Co-Catalyst Loading Leads to Significantly Enhanced Photocatalytic H2 Generation - Nguyen, Nhat Truong; Altomare, Marco; Yoo, Jeong Eun; Taccardi, Nicola; Schmuki, Patrik (Advanced Energy Materials, 2016)
A technique is introduced to strongly reduce Pt use for photocatalytic hydrogen generation from TiO2 nanotubes. By site selectively depositing thin layers of Pt only at the ``mouth'' of the nanotubes, and with a subsequent thermal dewetting step, an outstanding photocatalytic improvement with minimal amounts of cocatalyst is achieved.
EAM Import::2019-03-08

Nonchaos-Mediated Mixed-Mode Oscillations in an Enzyme Reaction System - Gallas, Jason; Hauser, Marcus J.B. (Journal of Physical Chemistry Letters, 2014)
We report numerical evidence of a new type of wide-ranging organization of mixed-mode oscillations (MMOs) in a model of the peroxidase−oxidase reaction, in the control parameter plane defined by the supply of the reactant NADH and the pH of the medium. In classic MMOs, the intervals of distinct periodic oscillations are always separated from each other by windows of chaos. In contrast, in the new unfolding, such windows of chaos do not exist. Chaos-mediated and nonchaos-mediated MMO phases are separated by a continuous transition boundary in the control parameter plane. In addition, for low pH values, we find an exceptionally wide and intricate mosaic of MMO phases that is described by a detailed phase diagram.

Nonclassical crystallization in vivo et in vitro (II): Nanogranular features in biomimetic minerals disclose a general colloid-mediated crystal growth mechanism - Rodriguez-Navarro, Carlos; Ruiz-Agudo, Encarnacion; Harris, Joseph; Wolf, Stephan (Journal of Structural Biology, 2016)
Recent research has shown that biominerals and their biomimetics (i) typically form via an amorphous precursor phase, and (ii) commonly display a nanogranular texture. Apparently, these two key features are closely related, underlining the fact that the formation of biominerals and their biomimetics does not necessarily follow classical crystallization routes, and leaves a characteristic nanotextural imprint which may help to disclose their origins and formation mechanisms. Here we present a general overview of the current theories and models of nonclassical crystallization and their applicability for the advance of our current understanding of biomineralization and biomimetic mineralization. We pay particular attention to the link between nonclassical crystallization routes and the resulting nanogranular textures of biomimetic CaCO mineral structures. After a general introductory section, we present an overview of classical nucleation and crystal growth theories and their limitations. Then, we introduce the Ostwald's step rule as a general framework to explain nonclassical crystallization. Subsequently, we describe nonclassical crystallization routes involving stable prenucleation clusters, dense liquid and solid amorphous precursor phases, as well as current nonclassical crystal growth models. The latter include oriented attachment, mesocrystallization and the new model based on the colloidal growth of crystals via attachment of amorphous nanoparticles. Biomimetic examples of nanostructured CaCO minerals formed via these nonclassical routes are presented which help us to show that colloid-mediated crystal growth can be regarded as a wide-spread growth mechanism. Implications of these observations for the advance in the current understanding on the formation of biomimetic materials and biominerals are finally outlined.
Keywords: Amorphous precursor; Biomimetics; Calcium carbonate; Dense liquid precursors; Nanogranular; Nanoparticle-mediated crystal growth; Nonclassical crystallization; PILP

Nonclassical crystallization in vivo et in vitro (I): Process-structure-property relationships of nanogranular biominerals - Wolf, Stephan; Böhm, Corinna; Harris, Joseph; Demmert, Benedikt; Jacob, Dorrit E.; Mondeshki, Mihail; Ruiz-Agudo, Encarnacion; Rodriguez-Navarro, Carlos (Journal of Structural Biology, 2016)
A distinct nanogranular fine structure is shared by a wealth of biominerals from several species, classes and taxa. This nanoscopic organization affects the properties and behavior of the biogenic ceramic material and confers on them attributes that are essential to their function. We present a set of structure-relationship properties that are rooted in the nanogranular organization and we propose that they rest on a common pathway of formation, a colloid-driven and hence nonclassical mode of crystallization. With this common modus operandi, we reveal the most fundamental and wide spread process-structure-property relationship in biominerals. With the recent increase in our understanding of nonclassical crystallization in vitro and in vivo, this significant process-structure-property relationship will serve as a source for new design approaches of bio-inspired materials.
Keywords: ACC; Amorphous calcium carbonate; Biomineralization; Nanoglobules; Nonclassical crystallization; Oriented attachment; PILP

Non-close-packed three-dimensional quasicrystals - Damasceno, Pablo F.; Glotzer, Sharon C.; Engel, Michael (Journal of Physics: Condensed Matter, 2017)
Quasicrystals are frequently encountered in condensed matter. They are important candidates for equilibrium phases from the atomic scale to the nanoscale. Here, we investigate the computational self-assembly of four quasicrystals in a single model system of identical particles interacting with a tunable isotropic pair potential. We reproduce a known icosahedral quasicrystal and report a decagonal quasicrystal, a dodecagonal quasicrystal, and an octagonal quasicrystal. The quasicrystals have low coordination number or occur in systems with mesoscale density variations. We also report a network gel phase.
EAM Import::2019-03-08

Non-covalent chemistry of graphene: Electronic communication with dendronized perylene bisimides - Kozhemyakina, Nina; Englert, Jan; Yang, Guang; Spiecker, Erdmann; Schmidt, Cordula; Hauke, Frank; Hirsch, Andreas; Spiecker, Erdmann (Advanced Materials, 2010)
Mutual attraction: π-π bonding promotes electronic interactions between graphene and a π-conjugated perylene in a liquid dispersion. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Non-invasive investigation of the cross-sectional solids distribution in CFB risers by X-ray computed tomography - Hensler, Timo; Firsching, Markus; Gomez Bonilla, Juan Sebastian; Woerlein, Thorsten; Uhlmann, Norman; Wirth, Karl-Ernst (Powder Technology, 2016)
We apply X-ray computed tomography for non-invasive investigation of the distribution of solids over the cross-section of a pilot plant scale circulating fluidized bed riser and present a novel method to derive volumetric solids concentrations from tomographic raw data. In this, the elimination of low-energetic radiation from the incident X-ray spectrum plays an essential role to avoid detrimental beam hardening effects, which are caused by the pipe material and the fluidized solids. The presented technique is applied for quantitative analysis of the radial solids concentration distribution in a gas-solid circulating fluidized bed riser. The examined operating range covers mean solids concentrations between 2.7 vol% and 37.2 vol% whilst the superficial gas velocity is varied between 1.5 m s-1 and 6.0 m s-1. The high spatial resolution of the measured data permits evaluation of the influence of the overall solids holdup and the superficial gas velocity on the radial solids concentration profile. Comparison of the cross-section averaged solids concentrations obtained by X-ray computed tomography with those inferred from differential pressure measurements confirms the high accuracy of the applied method. Based on a comprehensive set of measurement data a model is developed that allows for the prediction of the radial solids concentration distribution in vertical gas-solid upflow.
EAM Import::2019-03-08

Noninvasive manipulation of the optical response of opal photonic crystals - Romanov, Sergei G.; Regensburger, Alois; Korovin, Alexander V.; Romanova, Alexandra; Peschel, Ulf (Physical Review B, 2013)
The efficient manipulation with optical spectra of colloidal photonic crystals was achieved using the metal mirror positioned either immediately or at a distance from the crystal surface. If the mirror is close to the surface, the changes are mostly quantitative and relate to the improved light confinement. If the spacer provides a 2 extgreekp phase shift for round trip of the light at the wavelength of the diffraction resonance of the opal lattice, the defect band is formed in the photonic stop-band leading to 20–40 times increase of the transmission. The upper limit of the quality factor of the defect band was estimated by extrapolating the development of the defect transmission band as a function of the photonic crystal thickness. The specific features related to the photonic crystal dimensionality were revealed in the defect band spectra. The obtained results support the idea of achieving the control on the optical functionality of photonic crystals by recycling the transmitted light in the crystal volume.
EAM Import::2019-03-08

Nonuniformities in the angle of repose and packing fraction of large heaps of particles - Topic, Nikola; Gallas, Jason; Pöschel, Thorsten (Physical Review Letters, 2012)
We report a numerical investigation of the structural properties of very large three-dimensional heaps of particles produced by ballistic deposition from extended circular dropping areas. Very large heaps are found to contain three new geometrical characteristics not observed before: they may have two external angles of repose, an internal angle of repose, and four distinct packing fraction (density) regions. Such characteristics are shown to be directly correlated with the size of the dropping zone. In addition, we also describe how noise during the deposition affects the final heap structure. © 2012 American Physical Society.

Non-universal Voronoi cell shapes in amorphous ellipsoid packs - Schröter, Matthias; Schaller, Fabian; Kapfer, Sebastian; Hilton, James E.; Cleary, Paul W.; Mecke, Klaus; de Michele, Christiano; Schilling, Tanja; Saadatfar, Mohammad; Delaney, Gary; Schröder-Turk, Gerd (EPL - Europhysics Letters, 2015)
In particulate systems with short-range interactions, such as granular matter or simple fluids, local structure determines the macroscopic physical properties. We analyse local structure metrics derived from the Voronoi diagram of oblate ellipsoids, for various aspect ratios and global packing fractions φg. We focus on jammed static configurations of frictional ellipsoids, obtained by tomographic imaging and by discrete element method simulations. The rescaled distribution of local packing fractions φl, defined as the ratio of particle volume and its Voronoi cell volume, is found to be independent of the particle aspect ratio, and coincide with results for sphere packs. By contrast, the typical Voronoi cell shape, quantified by the Minkowski tensor anisotropy index β = β02,0, points towards a difference between random packings of spheres and those of oblate ellipsoids. While the average cell shape β of all cells with a given value of is similar in dense and loose jammed sphere packings, the structure of dense and loose ellipsoid packings differs substantially such that this does not hold true.

Novel Growth Regime of MDCK II Model Tissues on Soft Substrates - Kaliman, Sara; Jayachandran, Christina; Rehfeldt, Florian; Smith, Ana-Suncana (Biophysical Journal, 2014)
It is well established that MDCK II cells grow in circular colonies that densify until contact inhibition takes place. Here, we show that this behavior is only typical for colonies developing on hard substrates and report a new growth phase of MDCK II cells on soft gels. At the onset, the new phase is characterized by small, three-dimensional droplets of cells attached to the substrate. When the contact area between the agglomerate and the substrate becomes sufficiently large, a very dense monolayer nucleates in the center of the colony. This monolayer, surrounded by a belt of three-dimensionally packed cells, has a well-defined structure, independent of time and cluster size, as well as a density that is twice the steady-state density found on hard substrates. To release stress in such dense packing, extrusions of viable cells take place several days after seeding. The extruded cells create second-generation clusters, as evidenced by an archipelago of aggregates found in a vicinity of mother colonies, which points to a mechanically regulated migratory behavior.
Keywords: COLLECTIVE CELL-MIGRATION; STIFFNESS; APOPTOSIS

Novel nanographene/porphyrin hybrids-preparation, characterization, and application in solar energy conversion schemes - Kießling, Daniel; Costa, Ruben Dario; Katsukis, Georgios; Malig, Jenny; Lodermeyer, Fabian; Feihl, Sebastian; Roth, Alexandra Marina; Wibmer, Leonie; Kehrer, Matthias; Volland, Michel; Wagner, Pawel; Wallace, Gordon G.; et al. (Chemical Science, 2013)
Four novel nanographene/porphyrin hybrids were prepared, characterized, and probed in solar energy conversion schemes. Exfoliation of graphite by means of immobilizing four different porphyrins onto the basal plane of graphene is accompanied by distinct electronic interactions in both the ground and the excited states. In the ground state, a strong loss in oscillator strength goes hand-in-hand with a notable broadening of the porphyrin transitions and, as such, attests to the shift of electron density from the electron donating porphyrins to nanographene. In the excited state, a nearly quantitative quenching of the porphyrin fluorescence is indicative of full charge transfer. The latter is corroborated by femtosecond transient absorption measurements, which reveal the generation of the one-electron oxidized radical cation of the porphyrins with absorption maxima at 490 and 625 nm in the visible region and conduction band electrons in nanographene with features at 890 and 1025 nm in the near infrared region. We have demonstrated the applicability of the new nanographene/porphyrin hybrids in, for example, solar cells. In this regard, the presence of flakes is crucial in terms of influencing the injection processes, preventing aggregation, and reducing recombination losses, which are commonly encountered in porphyrin-based DSSCs. © 2013 The Royal Society of Chemistry.

Novel open cell aluminum foams and their use as reactive support for zeolite crystallization - Barg, S.; Soltmann, C.; Schwab, Andreas; Koch, D.; Schwieger, Wilhelm; Grathwohl, G. (Journal of Porous Materials, 2011)
Open cell aluminum (Al) foams are developed in this work by a novel direct foaming process in which a low concentrated alkane phase is emulsified in a stabilized Al powder suspension. In this versatile technique, the porosity parameters are adjusted during foaming of the emulsified suspensions and the final stability is achieved by a thermal treatment above the melting temperature of Al. The thin oxide layers surrounding the Al particles and the ascertained percolation of the molten metal are essential in this stage. The efficiency of the processing route is demonstrated with the essential requisites for successful foaming and the final structural stability. Furthermore, open cell Al-foams/zeolite composites exhibiting a hierarchical porous structure (nano-and macroscale) are produced by hydrothermal synthesis of Faujasite-like zeolite on the surface of the developed AI-foams. Due to the advantageous properties of zeolite X in N(2) adsorption, the application of the composites for separation of N(2) from air is specially envisaged.
Keywords: Al foams; Highly porous materials; Zeolite

Novel organic polymer for UV-enhanced substrate conformal imprint lithography - Fader, R.; Schmitt, H.; Rommel, Mathias; Bauer, A. J.; Frey, Lothar; Ji, R.; Hornung, M.; Brehm, M.; Vogler, M. (Microelectronic Engineering, 2012)
In this work, a novel kind of UV curing polymer is introduced as promising resist for UV-enhanced substrate conformal imprint lithography (UV-SCIL). This fully organic polymer is an epoxy based material and can crosslink via UV exposure to form a solid layer. The curing time of 17 s for this epoxy based resist is ten times shorter compared to commonly used resists for UV-SCIL. Imprints with this material are shown in this work as well as results of an HBr dry etch process of silicon, where the material served as etching mask. Using this polymer as resist for UV-SCIL enables shorter process times whereby the fidelity of the structures remains high. In this work, a novel kind of UV curing polymer is introduced as promising resist for UV-enhanced substrate conformal imprint lithography (UV-SCIL). This fully organic polymer is an epoxy based material and can crosslink via UV exposure to form a solid layer. The curing time of 17 s for this epoxy based resist is ten times shorter compared to commonly used resists for UV-SCIL. Imprints with this material are shown in this work as well as results of an HBr dry etch process of silicon, where the material served as etching mask. Using this polymer as resist for UV-SCIL enables shorter process times whereby the fidelity of the structures remains high.
Keywords: PDMS stamp; SCIL; Soft lithography; Substrate conformal imprint lithography; UV curing epoxides

Nucleation mechanism and kinetics of the perovskite to post-perovskite transition of MgSiO3 under extreme conditions - Zahn, Dirk (Chemical Physics Letters, 2013)
The atomistic mechanism of the perovskite i̊ghtarrow post-perovskite transformation of MgSiO3 is explored from molecular simulation mimicking extreme pressure and temperature conditions akin to planet Earth's interior (D'' region). The nucleation process is highly anisotropic and initiated by column-wise rotation of SiO6 octahedra around the [0 0 1] direction. The post-perovskite transition is found to be a rare event (i.e. one requiring substantial activation energy, ∼15 eV for the critical nucleus, but, once started, propagates very fast (103–104 m/s). The most stable phase front, the (0 1 0) interface, propagates at only 10% of the speed of the (0 0 1) phase front.
EAM Import::2019-03-08

Nucleation Mechanisms of a Polymorphic Molecular Crystal: Solvent-Dependent Structural Evolution of Benzamide Aggregates - Ectors, Philipp; Duchstein, Patrick; Zahn, Dirk (Crystal Growth & Design, 2014)
We report on molecular dynamics simulations dedicated to the aggregation and the early stage of nucleation of benzamide molecular crystals. As suggested by an earlier study of the molecular interactions in different benzamide polymorphs, we find the solvent to take a prominent role for polymorph control. The underlying driving forces are now rationalized by in-depth investigation of the formation of dimers, subsequent molecule association, and self-organization into aggregates of up to 100 benzamide molecules. Within the forming nuclei, we identify the development of ordered molecular motifs as characteristic for either the P1 or the P3 polymorph structure. From the different structural evolution related to the two nucleation scenarios, the preference of the stable P1 form is observed in aqueous solution, whereas precipitation from the vapor exhibits a tendency for the P3 polymorph of benzamide. Whereas benzamide aggregation from the vapor is mainly driven by hydrogen bonding between benzamide molecules, aggregation in aqueous solution is dominated by segregation of the hydrophobic moieties and extgreekp- extgreekp interactions of the benzene rings.
EAM Import::2019-03-08

Nucleation of Ligand-Receptor Domains in Membrane Adhesion - Bihr, Timo; Seifert, Udo; Smith, Ana-Suncana (Physical Review Letters, 2012)
We present a comprehensive model for the nucleation of domains in membrane adhesion. We determine the critical number of bonds in a nucleus and calculate the probability distribution of nucleation time from a discrete master equation. The latter is characterized by only four effective rates, which account for cooperative effects between bonds. We validate our model by finding excellent agreement with extensive Langevin simulations. In the range of parameters typical for cell adhesion, we find the critical number of bonds to be small. Furthermore, we find a characteristic separation between the bonds at which nucleation is particularly fast, pointing to potential regulatory mechanisms that could be used to control the cell recognition processes. DOI: 10.1103/PhysRevLett.109.258101
Keywords: GIANT VESICLES; CELLS; KINETICS; MECHANICS; DIFFUSION; STIFFNESS; DYNAMICS; GROWTH; SHAPE

Numerical investigation of the interphase forces and turbulence closure in 3D square bubble columns - Masood, Rao Muhammad Atif; Delgado, Antonio (Chemical Engineering Science, 2014)
3D time dependent numerical study has been performed to predict the flow hydrodynamics in bubble columns by employing explicit algebraic Reynolds stress (EARSM), re-normalization group (RNG) and RNG bubble induced turbulence (BIT) k-. ε models and the computational fluid dynamics (CFD) results are compared with experimental work of Deen (2001). This work shows the comparison between different drag force models (Ishii and Zuber, 1979; Schiller and Nauman, 1935; Grace et al., 1976) and drag coefficient). Moreover, the effects of the lift, virtual mass and suchlike other imperative non drag forces inclusive of wall lubrication as well as the turbulent dispersion have been examined. All the results show good quantitative agreement with experiments in case of turbulent kinetic energy, axial gas and liquid velocity profiles. EARSM k-. ε has found to perform better than both the RNG k-. ε modalities in estimating the turbulent kinetic energy profiles, but gave poor predictions of axial velocity profiles. Comparatively, RNG and RNG BIT models have successfully predicted the averaged flow field. Whereas the turbulent kinetic energy profiles at some locations, have been slightly under estimated by the RNG models. The inclusion of bubble induced turbulence led to slight improvement in estimation of the turbulent kinetic energy profiles. The advanced performance of RNG models stems in the better estimation of rate of turbulent dissipation and turbulent viscosity. Thus, both RNG and RNG BIT models can be readily utilized for the analysis of average flow fields and turbulent flow structures. © 2014 Elsevier Ltd.
UnivIS-Import:2015-03-09:Pub.2014.tech.ITC.stmmec.numeri
Keywords: Bubble column; Bubble induced turbulence; EARSM; Interphase forces; RNG; Turbulence models

Numerical investigation of three-dimensional bubble column flows: A detached eddy simulation approach - Masood, Rao Muhammad Atif; Delgado, Antonio (Chemical Engineering & Technology, 2014)
Numerical simulations were performed employing detached eddy simulation (DES) in a three-dimensional transient Euler-Euler framework for bubble columns, and all the computational fluid dynamics results were compared with a k-εlunate model and available experimental data. The numerical results are in good agreement with the experiments in predicting the time-averaged axial velocity and turbulent kinetic energy profiles. The flow-resolving capabilities of the DES model are highlighted, and it is shown that the DES turbulence model can be efficiently used for simulating flow field and turbulent quantities in the case of bubble columns.
UnivIS-Import:2018-02-22:Pub.2014.tech.ITC.stmmec.numeri_5
Keywords: Bubble column; Computational fluid dynamics; Detached eddy simulation; Turbulence models

Numerically exact, time-dependent study of correlated electron transport in model molecular junctions - Wang, Haobin; Thoss, Michael (Journal of Chemical Physics, 2013)
The multilayer multiconfiguration time-dependent Hartree theory within second quantization representation of the Fock space is applied to study correlated electron transport in models of single-molecule junctions. Extending previous work, we consider models which include both electron-electron and electronic-vibrational interaction. The results show the influence of the interactions on the transient and the stationary electrical current. The underlying physical mechanisms are analyzed in conjunction with the nonequilibrium electronic population of the molecular bridge. © 2013 American Institute of Physics.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.numeri

Numerical modeling of the wind flow over a transverse dune - Araújo, Ascânio D.; Ribeiro Parteli, Eric Josef; Pöschel, Thorsten; Andrade, José S.; Herrmann, Hans J. (Scientific Reports, 2013)
Transverse dunes, which form under unidirectional winds and have fixed profile in the direction perpendicular to the wind, occur on all celestial objects of our solar system where dunes have been detected. Here we perform a numerical study of the average turbulent wind flow over a transverse dune by means of computational fluid dynamics simulations. We find that the length of the zone of recirculating flow at the dune lee-the separation bubble-displays a surprisingly strong dependence on the wind shear velocity, u *: it is nearly independent of u * for shear velocities within the range between 0.2â€...m/s and 0.8â€...m/s but increases linearly with u * for larger shear velocities. Our calculations show that transport in the direction opposite to dune migration within the separation bubble can be sustained if u * is larger than approximately 0.39â€...m/s, whereas a larger value of u * (about 0.49â€...m/s) is required to initiate this reverse transport.

Numerical Simulation of Adsorption and Bubble Interaction in Protein Foams with the Lattice Boltzmann Method - Anderl, Daniela; Bauer, Martin; Rauh, Cornelia; Rüde, Ulrich; Delgado, Antonio (Food & Function, 2014)
The adsorption process and the resulting dynamic surface tension in the context of protein foams were studied. A diffusion-advection equation is solved using a lattice Boltzmann method (LBM) in order to simulate the adsorption of surfactants on a surface. With different adsorption isotherms, different surfactants can be modelled. The advection is driven by a flow field coming from the LBM. The phase transition is implemented with a free surface LBM approach where the liquid-gas two-phase flow is simplified to a single-phase free surface flow by using a volume of fluid approach. Looking at the different time scales for diffusion and advection, which are determined by the diffusion coefficient and the viscosity, respectively, the LBM is limited due to time and space resolution. The rates of protein transport to a surface by diffusion and by advection are investigated which indicate that diffusion is only relevant for modelling long-time studies. For those time ranges and low concentrations, the diffusion of proteins from a bulk to a surface of a droplet is simulated and compared with the literature. As a next step, situations as in protein foams are assumed. High concentrations of proteins, e.g. as in milk, result in a simplified scenario where neither diffusion nor advection is important. This is analysed theoretically which suggests an instantaneous change of surface tension. To examine the stability of foam lamellae, this is used for further simulations. Two bubbles rise close to each other with globally different surface tensions as for pure water and water with proteins. Depending on these surface tensions and the initial distance, the bubbles coalesce faster for high surface tensions and show less secondary motions for lower surface tension. It is concluded that bubbles in protein foams coalesce only at shorter distances than in pure water. This journal is © the Partner Organisations 2014.
UnivIS-Import:2015-03-09:Pub.2014.tech.IMMD.lsinfs.numeri

Numerical simulation of dense fluid-particle flows using computational fluid dynamics (CFD) and discrete element method (DEM) - Iqbal, Naveed; Rauh, Cornelia; Delgado, Antonio (Proceedings in Applied Mathematics and Mechanics, 2012)
Discrete Element Method (DEM) has been successfully coupled with Computational Fluid Dynamics (CFD) in the framework of OpenFOAM an open source CFD simulation code. In the current study, at first the model is validated with the simple test case of spherical particle comparing the results with the analytical solution. Then the simulation of a gaseous fluidized bed is considered. The coupled mass and momentum balance equations are used to calculate the flow behavior, particle fluidization and bubble formation. The dimensions of the simulation domain are similar to Link et al. (2005) but with different stiffness of particles. The higher velocity of gaseous fluid relative to particles entering through a jet causes the particles to fluidize. The particles behavior, fluidization, bubble formation and the velocity vectors of particles show a good agreement with the literature.
UnivIS-Import:2015-03-09:Pub.2012.tech.ITC.stmmec.numeri_1

Numerical simulation of heat transfer in the near-wall region of tubular reactors packed with metal open-cell foams - Bianchi, Enrico; Groppi, Gianpiero; Schwieger, Wilhelm; Tronconi, Enrico; Freund, Hannsjörg (Chemical Engineering Journal, 2015)
A numerical investigation of the thermal transport processes in the near-wall region of a reactor packed with metal open-cell foams is presented in view of the development of externally-cooled tubular reactors packed with catalytically active foams. Two different aluminium open-cell foams were scanned by X-ray micro-computed tomography in order to generate the 3D models used in this study. The conjugated solid-fluid heat transfer problem is solved by finite volume analysis (FVA) for different flow rates and fluid properties (Peclet number range from 1 to 105). Predictive correlations for the wall heat transfer coefficient are derived according to different thermal coupling scenarios between the open-cell foam bed and the reactor wall. Such kind of systematic study and the proposed correlations that account for the gap size represent a novelty in the field of published literature on open-cell foams. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: Metal open-cell foams; Sponges; Conjugated heat transfer; Wall heat transfer coefficient; Catalytic structured reactors; CFD

Numerical simulation of interfacial closures for 3D bubble column flows - Masood, Rao Muhammad Atif; Jovicic, Vojislav; Delgado, Antonio (Chemical Engineering & Technology, 2015)
Numerical investigation of flow hydrodynamics in a square cross-sectioned bubble column was conducted in a transient Euler-Euler environment by applying the simulation tool Ansys CFX 14.0. The influence of the drag coefficient (CD) was investigated and the results were also compared with drag force models. Furthermore, three different lift force models and a defined lift coefficient were studied. All results were compared with the available experimental data. All simulations were carried out for a single-hole sparger with given aspect ratio (H/D) and superficial gas velocity. Developing and improving computational fluid dynamics (CFD) tools for modeling of bubble column flows is of increasing interest. Numerical investigations of gas-liquid flow hydrodynamics in 3D square bubble columns were performed by means of a commercial CFD tool. The effect of the drag coefficient was determined and results were also compared to drag force models.
UnivIS-Import:2015-07-08:Pub.2015.tech.ITC.stmmec.numeri
Keywords: Bubble column; Computational fluid dynamics; Flow hydrodynamics; Interfacial closures; Numerical simulation

Numerical simulations of fluid dynamics in carrier structures for catalysis: characterization and need for optimization - Horneber, Tobias; Rauh, Cornelia; Delgado, Antonio (Chemical Engineering Science, 2014)
3D fluid dynamic numerical simulations of different structured open-cell foams are presented: a cubic lattice, the tetrakaidecahedron geometry (Kelvin cell geometry), and the diamond structure. The simulations are done with the open-source software OpenFOAM. Beneath the pressure drops of the different structures, which are compared using dimensionless numbers such as Reynolds number and Euler number, also the development of velocity and its frequency distribution, as an important parameter for chemical reactions, are evaluated. As a main result, the diamond structure provides good conditions with respect to dwell time distributions for the underlying chemical reaction. All the results give a deep knowledge in terms of the underlying principles and optimization approaches. © 2014 Elsevier Ltd.
UnivIS-Import:2015-03-09:Pub.2014.tech.ITC.stmmec.numeri_7
Keywords: Catalyst support; Computational fluid mechanics; Metal foam; Porous media; Pressure drop; Tetrakaidecahedron structure

Numerical simulations of particle-laden jet/spout flows using Eulerian-Lagrangian approach - Iqbal, Naveed; Rauh, Cornelia; Delgado, Antonio (Procedia Engineering, 2015)
Discrete Element Method (DEM) had been successfully coupled with Computational Fluid Dynamics (CFD) in the framework of OpenFOAM; an open source CFD simulation code. It is used to simulate particle laden flows using Eulerian approach for continuum phase and Lagrangian approach for solid phase. This numerical model is validated with simple test cases of spherical particle comparing the results with the analytical solution and is reported in earlier work. In current study, results of numerical simulation of spout-ﬂuid bed are compared against experimental and simulation results reported in literature. The pressure drop fluctuations and isosurface plots of solid volume fraction show a good qualitative prediction of different flow regimes. The particle velocity profiles in the vertical direction for different test cases are plotted and compared with the literature data. The comparison of results shows a good quantitative behavior of the model predictions.
EAM Import::2019-03-08

Numerical solution and experimental validation of the drawing process of six-hole optical fibers including the effects of inner pressure and surface tension - Luzi, Giovanni; Epple, Philipp; Scharrer, Michael; Rauh, Cornelia; Fujimoto Meinecke, Ken; Delgado, Antonio (Journal of Lightwave Technology, 2012)
Microstructured optical fibers (MOFs) achieve their desired performance via a pattern of holes that run trough the whole length of the fiber. The variation of the hole pattern allows the production of a variety of optical effects. However, the cross-sectional hole structure can be different from that designed in the preform, due to the combined effects of surface tension and internal pressure. The present paper focuses on the comparison between experiments and numerical calculation of a six hole-optical fiber taking into account the effects of surface tension and internal hole-pressure, since those are of essential importance during drawing. It is shown that the numerical computations deliver reliable results for practical applications and can be used as a predictive tool for fiber development, as long as the inner pressure or the temperature do not exceed too high values. © 2012 IEEE.
UnivIS-Import:2015-03-09:Pub.2012.tech.ITC.stmmec.numeri
Keywords: Hley fibers; microstructured optical fibers (MOFs); nmerical modelling; numerical methods; photonic crystal fibers

Numerical study of plasmonic absorption enhancement in semiconductor absorbers by metallic nanoparticles - Hornich, Julian; Pflaum, Christoph; Brabec, Christoph; Forberich, Karen (Journal of Applied Physics, 2016)
We are studying the influence of spherical silver nanoparticles (AgNP) in absorbing media by numerically solving the Maxwell's equations. Our simulations show that the near-field absorption enhancement introduced by a single AgNP in the surrounding medium is increasing with the growing particle diameter. However, we observe that the relative absorption per particle volume is on a similar level for different particle sizes; hence, different numbers of particles with the same total volume yield the same near-field absorption enhancement. We also investigate the effect of non-absorbing shells around the AgNP with the conclusion that even very thin shells suppress the beneficial effects of the particles noticeably. Additionally, we include AgNP in an organic solar cell at different vertical positions with different particle spacings and observe the beneficial effects for small AgNP and the scattering dependent performance for larger particles.

n- versus p-doping of graphite: what drives its wet-chemical exfoliation? - Wang, Bingzhe; Engelhardt, Verena; Roth, Alexandra Marina; Faust, Rdiger; Guldi, Dirk Michael (Nanoscale, 2017)
We have performed the syntheses of a novel pyrene-porphyrazine conjugate (ZnPzPy) and a reference porphyrazine (ZnPz) to promote the wet-chemical exfoliation of graphite based on the synergetic use of ultrasonication, centrifugation, and doping. ZnPzPy features, on one hand, a hydrophobic pyrene to anchor onto the basal plane of graphene, and, on the other hand, an amphoteric porphyrazine to either p- or n-dope graphene. To this end, we have characterized individual building blocks, that is, ZnPzPy and exfoliated graphite, and the resulting electron donor-acceptor nanohybrid, that is, ZnPzPy/graphene (ZnPzPy-G), by means of an arsenal of microscopic and spectroscopic techniques. From a full-fledged characterization we conclude that ZnPzPy facilitates the exfoliation of graphite affording suspensions featuring 9.5% of single- or few-layered ZnPzPy-G with a mean average size of 200 ± 140 nm. Importantly, a notable shift of charge density from graphene to ZnPzPy in the ground state of ZnPzPy-G corroborates the preference of exfoliated graphite to undergo p-doping rather than n-doping. As an immediate consequence, a full charge separation leads in the excited state to a 750 ± 150 ps lived charge separated state.
EAM Import::2019-03-12

O2 adsorption dependent photoluminescence emission from metal oxide nanoparticles - Gheisi, Amirreza; Neygandhi, Chris; Sternig, Andreas; Carrasco, Esther; Marbach, Hubertus; Thomele, Daniel; Diwald, Oliver (Physical Chemistry Chemical Physics, 2014)
Optical properties of metal oxide nanoparticles are subject to synthesis related defects and impurities. Using photoluminescence spectroscopy and UV diffuse reflectance in conjunction with Auger electron spectroscopic surface analysis we investigated the effect of surface composition and oxygen adsorption on the photoluminescence properties of vapor phase grown ZnO and MgO nanoparticles. On hydroxylated MgO nanoparticles as a reference system, intense photoluminescence features exclusively originate from surface excitons, the radiative deactivation of which results in collisional quenching in an O2 atmosphere. Conversely, on as-prepared ZnO nanoparticles a broad yellow emission feature centered at h extgreeknEm = 2.1 eV exhibits an O2 induced intensity increase. Attributed to oxygen interstitials as recombination centers this enhancement effect originates from adsorbate-induced band bending, which is pertinent to the photoluminescence active region of the nanoparticles. Annealing induced trends in the optical properties of the two prototypical metal oxide nanoparticle systems, ZnO and MgO, are explained by changes in the surface composition and underline that particle surface and interface changes that result from handling and processing of nanoparticles critically affect luminescence.
EAM Import::2019-03-08

Oblique impact of frictionless spheres: On the limitations of hard sphere models for granular dynamics - Müller, Patric; Pöschel, Thorsten (Granular Matter, 2012)
When granular systems are modeled by hard spheres, particle-particle collisions are considered as instantaneous events. This implies that while the velocities change according to the collision rule, the positions of the particles are the same before and after such an event. We show that depending on the material and system parameters, this assumption may fail. For the case of viscoelastic particles we present a universal condition which allows to assess whether hard-spheremodeling and, thus, event-driven Molecular Dynamics simulations are justified. © Springer-Verlag 2012.
Keywords: Coefficient of normal restitution; Event-driven molecular dynamics; Granular gases; Hard sphere model; Viscoelastic spheres

Observation of optical activity in dielectric biomimetic 8-srs networks - Turella, Fabio; Cumming, Benjamin P.; Schröder-Turk, Gerd; Gu, Min (Optics Letters, 2015)
We demonstrate the first experimental realization of the 8-srs network by direct laser writing in a dielectric medium. The optical properties of this photonic crystal are both numerically simulated and experimentally measured. The numerical simulations indicate a strong optical activity (OA) for regions of the spectrum close to the unit cell size (3 extgreekmm) and a high sensitivity to angular resolution. Experimentally, we observe 40° of OA for a region of the spectrum between 3 and 3.5 extgreekmm, confirming both simulation predictions.
EAM Import::2019-03-08

Observation of Plasmarons in Quasi-Freestanding Doped Graphene - Bostwick, Aaron; Speck, Florian; Seyller, Thomas; Horn, Karsten; Polini, Marco; Asgari, Reza; Macdonald, Allan H.; Rotenberg, Eli (Science, 2010)
A hallmark of graphene is its unusual conical band structure that leads to a zero-energy band gap at a single Dirac crossing point. By measuring the spectral function of charge carriers in quasi-freestanding graphene with angle-resolved photoemission spectroscopy, we showed that at finite doping, this well-known linear Dirac spectrum does not provide a full description of the charge-carrying excitations. We observed composite ``plasmaron'' particles, which are bound states of charge carriers with plasmons, the density oscillations of the graphene electron gas. The Dirac crossing point is resolved into three crossings: the first between pure charge bands, the second between pure plasmaron bands, and the third a ring-shaped crossing between charge and plasmaron bands.
EAM Import::2019-03-08

Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostructures - Schmitt, Sebastian W.; Sarau, George; Christiansen, Silke (Scientific Reports, 2015)
Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q 1300) and a low mode volume (0.2 < ( extgreekl/neff)3 < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.
EAM Import::2019-03-08

On an inverse problem for tree-like networks of elastic strings - Avdonin, Sergei; Leugering, Günter; Mikhaylov, Victor (ZAMM - Zeitschrift für angewandte Mathematik und Mechanik, 2010)
We consider the in-plane motion of elastic strings on tree-like network, observed from the ‘leaves’. We investigate the inverse problem of recovering not only the physical properties, i.e. the ‘optical lengths’ of each string, but also the topology of the tree which is represented by the edge degrees and the angles between branching edges. To this end we use the Boundary Control method for wave equations on graphs established in [4, 7]. It is shown that under generic assumptions the inverse problem can be solved by applying measurements at all leaves, the root of the tree being fixed.

On-Demand Coupling of Electrically Generated Excitons with Surface Plasmons via Voltage-Controlled Emission Zone Position - Zakharko, Yuriy; Held, Martin; Sadafi, Fabrizio-Zagros; Gannott, Florentina; Mandavi, Ali; Peschel, Ulf; Klupp Taylor, Robin; Zaumseil, Jana (ACS Photonics, 2016)
The ability to confine and manipulate light below the diffraction limit is a major goal of future multifunctional optoelectronic/plasmonic systems. Here, we demonstrate the design and realization of a tunable and localized electrical source of excitons coupled to surface plasmons based on a polymer light emitting field-effect transistor (LEFET). Gold nanorods that are integrated into the channel support localized surface plasmons and serve as nanoantennas for enhanced electroluminescence. By precise spatial control of the near-infrared emission zone in the LEFET via the applied voltages the near-field coupling between electrically generated excitons and the nanorods can be turned on or off as visualized by a change of electroluminescence intensity. Numerical calculations and spectroscopic measurements corroborate significant local electroluminescence enhancement due to the high local density of photonic states in the vicinity of the gold nanorods. Importantly, the integration of plasmonic nanostructures hardly influences the electrical performance of the LEFETs, thus, highlighting their mutual compatibility in novel active plasmonic devices.
Keywords: surface plasmon polariton; organic semiconductor; field-effect transistor; electroluminescence; Purcell effect; active plasmonics

On elastic bodies with thin rigid inclusions and cracks - Khludnev, A. M.; Leugering, Günter (Mathematical Methods in the Applied Sciences, 2010)
This paper is concerned with the analysis of equilibrium problems for two-dimensional elastic bodies with thin rigid inclusions and cracks. Inequality-type boundary conditions are imposed at the crack faces providing a mutual non-penetration between the crack faces. A rigid inclusion may have a delamination, thus forming a crack with non-penetration between the opposite faces. We analyze variational and differential problem formulations. Different geometrical situations are considered, in particular, a crack may be parallel to the inclusion as well as the crack may cross the inclusion, and also a deviation of the crack from the rigid inclusion is considered. We obtain a formula for the derivative of the energy functional with respect to the crack length for considering this derivative as a cost functional. An optimal control problem is analyzed to control the crack growth.

One-pot colloidal synthesis of plasmonic patchy particles - Bao, Huixin; Peukert, Wolfgang; Klupp Taylor, Robin (Advanced Materials, 2011)
The one-pot synthesis of patchy particles with morphologically tunable optical properties by the heterogeneous nucleation and growth of silver onto colloidal silica particles is described. Heat treatment of the silica and coating reaction conditions can be optimized in order to produce full yields of patchy particles. Dissolution of the cores leads to stable silver cages. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Keywords: asymmetric nanoparticles; colloids; core/shell nanoparticles; janus particles; silver

On-off switch of charge-separated states of pyridine-vinylene-linked porphyrin-C-60 conjugates detected by EPR - Kirner, Sabrina; Arteaga, Danny; Henkel, Christian; Margrat, Johannes T.; Alegret, Nuria; Ohkubo, Kei; Insuasty, Braulio; Ortiz, Alejandro; Martin, Nazario; Echegoyen, Luis; Fukuzumi, Shunichi; Clark, Timothy; et al. (Chemical Science, 2015)
The design, synthesis, and electronic properties of a new series of D-pi-A conjugates consisting of free base (H2P) and zinc porphyrins (ZnP) as electron donors and a fullerene (C-60) as electron acceptor, in which the two electroactive entities are covalently linked through pyridine-vinylene spacers of different lengths, are described. Electronic interactions in the ground state were characterized by electrochemical and absorption measurements, which were further supported with theoretical calculations. Most importantly, charge-transfer bands were observed in the absorption spectra, indicating a strong push-pull behavior. In the excited states, electronic interactions were detected by selective photoexcitation under steady-state conditions, by time-resolved fluorescence investigations, and by pump probe experiments on the femto-, pico-, and nanosecond time scales. Porphyrin fluorescence is quenched for the different D-pi-A conjugates, from which we conclude that the deactivation mechanisms of the excited singlet states are based on photoinduced energy-and/or electron transfer processes between H2P/ZnP and C-60, mediated through the molecular spacers. The fluorescence intensity decreases and the fluorescence lifetimes shorten as the spacer length decreases and as the spacer substitution changes. With the help of transient absorption spectroscopy, the formation of charge-separated states involving oxidized H2P/ZnP and reduced C-60 was confirmed. Lifetimes of the corresponding charge-separated states, which ranged from similar to 400 picoseconds to 165 nanoseconds, depend on the spacer length, the spacer substitution, and the solvent polarity. Interestingly, D-pi-A conjugates containing the longest linkers did not necessarily exhibit the longest charge-separated state lifetimes. The distances between the electron donors and the acceptors were calculated by molecular modelling. The longest charge-separated state lifetime corresponded to the D-p-A conjugate with the longest electron donor-acceptor distance. Likewise, EPR measurements in frozen media revealed charge separated states in all the D-p-A conjugates investigated. A sharp peak with g values similar to 2.000 was assigned to reduced C-60, while a broader, less intense signal (g similar to 2.003) was assigned to oxidized H2P/ZnP. On-off switching of the formation and decay of the charge-separated states was detected by EPR at 77 K by repeatedly turning the irradiation source on and off.

On Shape Optimization for an Evolution Coupled System - Leugering, Günter; Novotny, A. A.; Menzala, G. Perla; Sokolowski, J. (Applied Mathematics and Optimization, 2011)
A shape optimization problem in three spatial dimensions for an elasto-dynamic piezoelectric body coupled to an acoustic chamber is introduced. Well-posedness of the problem is established and first order necessary optimality conditions are derived in the framework of the boundary variation technique. In particular, the existence of the shape gradient for an integral shape functional is obtained, as well as its regularity, sufficient for applications e.g. in modern loudspeaker technologies. The shape gradients are given by functions supported on the moving boundaries. The paper extends results obtained by the authors in (Math. Methods Appl. Sci. 33(17):2118–2131, 2010) where a similar problem was treated without acoustic coupling.

On-Surface Synthesis and Characterization of Honeycombene Oligophenylene Macrocycles - Chen, Min; Shang, Jian; Wang, Yongfeng; Wu, Kai; Kuttner, Julian; Hilt, Gerhard; Hieringer, Wolfgang; Gottfried, J. Michael (ACS nano, 2017)
We report the on-surface formation and characterization of [30]-honeycombene, a cyclotriacontaphenylene, which consists of 30 phenyl rings (C180H120) and has a diameter of 4.0 nm. This shape-persistent, conjugated, and unsubstituted hexagonal hydrocarbon macrocycle was obtained by solvent-free synthesis on a silver (111) single-crystal surface, making solubility-enhancing alkyl side groups unnecessary. Side products include strained macrocycles with square, pentagonal, and heptagonal shape. The molecules were characterized by scanning tunneling microscopy and density functional theory (DFT) calculations. On the Ag(111) surface, the macrocycles act as molecular quantum corrals and lead to the confinement of surface-state electrons inside the central cavity. The energy of the confined surface state correlates with the size of the macrocycle and is well described by a particle-in-the-box model. Tunneling spectroscopy suggests conjugation within the planar rings and reveals influences of self-assembly on the electronic structure. While the adsorbed molecules appear to be approximately planar, the free molecules have nonplanar conformation, according to DFT.
EAM Import::2019-03-08