On-surface synthesis of heptacene and its interaction with a metal surface - Zugermeier, Malte; Gruber, Manuel; Schmid, Martin; Klein, Benedikt P.; Ruppenthal, Lukas; Mueller, Philipp; Einholz, Ralf; Hieringer, Wolfgang; Berndt, Richard; Bettinger, Holger F.; Gottfried, J. Michael (Nanoscale, 2017)
Heptacene was generated by surface-assisted didecarbonylation of an extgreeka-diketone precursor on a Ag(111) surface. Monitoring of the surface reaction and characterization of the adsorbed heptacene was performed with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT) calculations. The surface-assisted formation of heptacene occurs around 460 K. Both the heptacene and the precursor molecules are oriented along the high-symmetry directions of the (111) surface and their molecular extgreekp systems face towards the substrate. The interaction with the Ag(111) substrate is not laterally uniform, but appears to be strongest on the central part of the molecule, in line with the expectations from Clar's rule. In the STM images, heptacene shows a dumbbell shape, which may correspond to the substantial out-of-plane deformations of heptacene on Ag(111). As revealed by DFT, the center of the molecule is closer to the surface than the outer parts. In addition, the inner rings are most affected by charge redistribution between surface and molecule. Heptacene acts as an acceptor and receives a negative charge of -0.6e from the Ag(111) surface. Since vacuum-sublimable extgreeka-diketone precursors for even larger acenes are available, the approach is promising for the on-surface synthesis of higher acene homologues such as octacene and nonacene.
EAM Import::2019-03-08

On the accuracy of DFT-SAPT, MP2, SCS-MP2, MP2C, and DFT + Disp methods for the interaction energies of endohedral complexes of the C60 fullerene with a rare gas atom - Heßelmann, Andreas; Korona, Tatiana (Physical Chemistry Chemical Physics, 2011)
Selected points on the potential energy surface for the complexes Rg@C60 (Rg = He, Ne, Ar, Kr) are calculated with various theoretical methods, like symmetry-adapted perturbation theory with monomers described by density functional theory (DFT-SAPT), supermolecular Møller–Plesset theory truncated on the second order (MP2), spin-component-scaled MP2 (SCS-MP2), supermolecular density functional theory with empirical dispersion correction (DFT+Disp), and the recently developed MP2C method that improves the MP2 method for long-range electron correlation effects. A stabilization of the endohedral complex is predicted by all methods, but the depth of the potential energy well is overestimated by the DFT+Disp and MP2 approaches. On the other hand, the MP2C model agrees well with DFT-SAPT, which serves as the reference. The performance of SCS-MP2 is mixed: it produces too low interaction energies for the two heavier guests, while its accuracy for He@C60 and Ne@C60 is similar to that of MP2C. Fitting formulas for the main interaction energy components, i.e. the dispersion and first-order repulsion energies are proposed, which are applicable for both endo- and exohedral cases. For all examined methods density fitting is used to evaluate two-electron repulsion integrals, which is indispensable to allow studies of noncovalent complexes of this size. It has been found that density-fitting auxiliary basis sets cannot be used in a black-box fashion for the calculation of the first-order SAPT electrostatic energy, and that the quality of these basis sets should be always carefully examined in order to avoid an unphysical long-range behavior.
EAM Import::2019-03-08

On the adaptive finite element analysis of the Kohn-Sham equations: Methods, algorithms, and implementation. - Davydov, Denis; Young, Toby; Steinmann, Paul (International Journal for Numerical Methods in Engineering, 2015)
In this paper, details of an implementation of a numerical code for computing the Kohn–Sham equations are presented and discussed. A fully self-consistent method of solving the quantum many-body problem within the context of density functional theory using a real-space method based on finite element discretisation of realspace is considered. Various numerical issues are explored such as (i) initial mesh motion aimed at co-aligning ions and vertices; (ii) a priori and a posteriori optimization of the mesh based on Kelly's error estimate; (iii) the influence of the quadrature rule and variation of the polynomial degree of interpolation in the finite element discretisation on the resulting total energy. Additionally, (iv) explicit, implicit and Gaussian approaches to treat the ionic potential are compared. A quadrupole expansion is employed to provide boundary conditions for the Poisson problem. To exemplify the soundness of our method, accurate computations are performed for hydrogen, helium, lithium, carbon, oxygen, neon, the hydrogen molecule ion and the carbon-monoxide molecule. Our methods, algorithms and implementation are shown to be stable with respect to convergence of the total energy in a parallel computational environment.

On the critical role of the substrate: the adsorption behaviour of tetrabenzoporphyrins on different metal surfaces - Zhang, Liang; Lepper, Michael; Stark, Michael Werner; Menzel, Teresa; Lungerich, Dominik; Jux, Norbert; Hieringer, Wolfgang; Steinrück, Hans-Peter; Marbach, Hubertus (Physical Chemistry Chemical Physics, 2017)
The adsorption behaviour of 2H-5,10,15,20-tetraphenyltetrabenzoporphyrin (2HTPTBP) on different metal surfaces, i.e., Ag(111), Cu(111), Cu(110), and Cu(110)-(2 x 1)O was investigated by scanning tunnelling microscopy at room temperature. The adsorption of 2HTPTBP on Ag(111) leads to the formation of a well-ordered two-dimensional (2D) island structure due to the mutual stabilization through the intermolecular pi-pi stacking and T-type-like interactions of phenyl and benzene substituents of neighboring molecules. For 2HTPTBP on Cu(111), the formed 2D supramolecular structures exhibit a coverage-dependent behaviour, which can be understood from the interplay of molecule-substrate and molecule-molecule interactions. In contrast, on Cu(110) the 2HTPTBP molecules form dispersed one-dimensional (1D) molecular chains along the [1 (1) over bar0] direction of the substrate due to relatively strong attractive molecule-substrate interactions. Furthermore, we demonstrate that the reconstruction of the Cu(110) surface by oxygen atoms yields a change in dimensionality of the resulting nanostructures from 1D on Cu(110) to 2D on (2 x 1) oxygen-reconstructed Cu(110), induced by a decreased molecule-substrate interaction combined with attractive molecule-molecule interactions. This comprehensive study on these prototypical systems enables us to deepen the understanding of the particular role of the substrate concerning the adsorption behavior of organic molecules on metal surfaces and thus to tweak the ordering in functional molecular architectures.

On the effect of self-penalization of piezoelectric composites in topology optimization - Wein, Fabian; Kaltenbacher, Manfred; Kaltenbacher, Barbara; Leugering, Günter; Bänsch, Eberhard; Schury, Fabian (Structural and Multidisciplinary Optimization, 2011)
We investigate the occurrence of self-penalization in topology optimization problems for piezoceramicmechanical composites. Our main goal is to give physical interpretations for this phenomenon, i.e., to study the question why for various problems intermediate material values are not optimal in the absence of explicit penalization of the pseudo densities. In order to investigate this effect numerical experiments for several static and/or dynamic actuator and sensor objective functions are performed and their respective results are compared. The objective functions are mean transduction, displacement, sound power, electric potential, electric energy, energy conversion and electric power.

On the efficient solution of a patch problem with multiple elliptic inclusions - Schury, Fabian; Greifenstein, Jannis; Leugering, Günter; Stingl, Michael (Optimization and Engineering, 2015)
We present an asymptotic approach to find optimal rotations of orthotropic material inclusions inside an isotropic linear elastic matrix. We compute approximate optimal solutions with respect to compliance and a stress based cost functional. We validate the local and global quality of the candidate solutions by means of finite element based parametric optimization algorithms. In particular, we devise a lower bound algorithm based on the free material optimization approach. Several numerical experiments are performed for different traction scenario

On the energetics of conformational switching of molecules at and close to room temperature - Ditze, Stefanie; Stark, Michael Werner; Buchner, Florian; Aichert, André; Jux, Norbert; Luckas, Nicola; Görling, Andreas; Hieringer, Wolfgang; Hornegger, Joachim; Steinrück, Hans-Peter; Marbach, Hubertus (Journal of the American Chemical Society, 2014)
We observe and induce conformational switching of individual molecules via scanning tunneling microscopy (STM) at and close to room temperature. 2H-5,10,15,20-Tetrakis-(3,5-di-tert-butyl)-phenylporphyrin adsorbed on Cu(111) forms a peculiar supramolecular ordered phase in which the molecules arrange in alternating rows, with two distinct appearances in STM which are assigned to concave and convex intramolecular conformations. Around room temperature, frequent bidirectional conformational switching of individual molecules from concave to convex and vice versa is observed. From the temperature dependence, detailed insights into the energy barriers and entropic contributions of the switching processes are deduced. At 200 K, controlled STM tip-induced unidirectional switching is possible, yielding an information storage density of 4.9 × 10 bit/inch. With this contribution we demonstrate that controlled switching of individual molecules at comparably high temperatures is possible and that entropic effects can be a decisive factor in potential molecular devices at these temperatures. © 2014 American Chemical Society.

On the equilibrium of elastic bodies containing thin rigid inclusions - Leugering, Günter; Khludnev, A. M. (Doklady Physics, 2010)
The practice of production of composite materials with thin fibers requires a correct description of problems of the equilibrium of elastic bodies containing thin rigid inclusions. Of large importance is also an adequate description of the interaction of thin inclusions with cracks in an elastic body. As is known, inclusions can delaminate from the matrix, thereby forming a crack in the elastic body. A traditional approach to the description of cracks in elastic bodies consists in the use of equality-type boundary conditions at the crack faces. This approach frequently leads to unphysical results; for example, the corresponding solutions can yield a mutual penetration of the crack faces (see, e.g., [1, 2]). In the recent years, there was performed a wide cycle of investigations in the theory of cracks with boundary conditions corresponding to the mutual nonpenetration of the crack edges. In this case, the boundary conditions at the crack faces have the form of a set of equalities and inequalities. For acquaintance with such models, one can turn to [3, 4], where numerous appropriate references can be found. In this works, we suggest a model of an elastic body containing thin rigid inclusions with a property of delamination. For describing the delamination, we use inequality-type boundary conditions eliminating the mutual penetration of the crack faces.

On the experimental investigation of the electric and magnetic response of a single nano-structure - Banzer, Peter; Peschel, Ulf; Quabis, Susanne; Leuchs, Gerd (Optics Express, 2010)
We demonstrate an experimental method to separately test the optical response of a single sub-wavelength nano-structure to tailored electric and magnetic field distributions in the optical domain. For this purpose a highly focused y-polarized TEM10-mode is used which exhibits spatially separated longitudinal magnetic and transverse electric field patterns. By displacing a single sub-wavelength nano-structure, namely a single split-ring resonator (SRR), in the focal plane, different coupling scenarios can be achieved. It is shown experimentally that the single split-ring resonator tested here responds dominantly as an electric dipole. A much smaller but yet statistically significant magnetic dipole contribution is also measured by investigating the interaction of a single SRR with a magnetic field component perpendicular to the SRR plane (which is equivalent to the curl of the electric field) as well as by analyzing the intensity and polarization distribution of the scattered light with high spatial resolution. The developed experimental setup as well as the measurement techniques presented in this paper are a versatile tool to investigate the optical properties of single sub-wavelength nano-structures.
EAM Import::2019-03-08

On the Function of Saccharides during the Nucleation of Calcium Carbonate–Protein Biocomposites - Duchstein, Patrick; Kniep, Ruediger; Zahn, Dirk (Crystal Growth & Design, 2013)
The molecular mechanism of calcium carbonate nucleation in the presence of various types of collageneous proteins is unravelled from computer simulation of ion-by-ion association steps. Single calcium ions are incorporated in the triplehelix by formation of salt bridges to carbonyl and hydroxyl groups of collagen, while single carbonate ions tend to bind laterally to the biomolecule. However, upon multiple ion association, the self-organization of the forming aggregate strongly depends on the triple-helical collagenous strand. In absence of glycosylated lysine residues, we observed that carbonate ions bind to calcium ions that are already incorporated into the triple helix and eventually cause the unfolding of the protein. On the other hand, otolin-1, a specific, collagen-like protein found in biogenic calcite-based composites such as otoconia, comprises a particularly high degree of glycosylated amino acids which avoid such ``destructive'' calcium–carbonate contacts by providing alternative association sites more lateral to the backbone. This leads to the formation of a saccharide–calcium carbonate agglomerate that does not compromise the protein's triple helix and constitutes the organic–inorganic interface of the nucleating biocomposite.
EAM Import::2019-03-08

On the importance of a connected hard-phase skeleton for the creep resistance of Mg alloys - Amberger, Dorothea; Eisenlohr, Philip; Göken, Mathias (Acta Materialia, 2012)
The low density of Mg alloys renders them attractive for lightweight constructions. However, creep resistance remains an important limitation of Mg alloys for, for example, automotive power train applications. To gain a more detailed understanding of the correlation between microstructure and creep properties in die-castable Mg alloys, AZ91 alloys with nominal additions of 0, 1, 3, and 5 mass% Ca and the commercial alloy MRI 230D have been investigated. Creep tests show an increase in creep strength with increasing Ca addition and increasing cooling rate. Scanning electron microscopy reveals that this is correlated with an increasing interconnectivity of the intermetallic phase skeleton (in addition to the effect of small precipitates within the extgreeka-Mg matrix found in the case of MRI 230D). A simple isostrain composite analysis illustrates that a more interconnected skeleton shields more load from the matrix. Precipitation hardening can additionally strengthen the matrix, and thus the combination of both design approaches results in the highest observed creep resistance.
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On the influence of crack front curvature on the fracture behavior of nanoscale cracks - Möller, Johannes; Bitzek, Erik (Engineering Fracture Mechanics, 2015)
Atomic-scale fracture processes are traditionally investigated in quasi-2D models of straight, infinite cracks. This approach neglects crack front curvature effects, which might be particularly important for nanoscale crack nuclei in semi-brittle materials. Here we use 3D atomistic simulations to study penny-shaped cracks in body-centered cubic metals. Our results show extensive crack tip plasticity initiated by deformation twinning and followed by emission of screw dislocations which cross-slip along the crack front. Together with the interactions of dislocations and/or twins that are nucleated at differently oriented parts of the crack, these processes determine the fracture behavior of highly curved nanoscale crack
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Keywords: Cross-slip; Molecular dynamics simulation; Penny-shaped crack; Semi-brittle fracture; Twinning

On the influence of inhomogeneous stiffness and growth on mechanical instabilities in the developing brain - Budday, Silvia; Steinmann, Paul (International Journal of Solids and Structures, 2017)
The characteristic surface morphology of the mammalian brain is closely correlated with brain function and dysfunction. During development, the initially smooth surface evolves into an elaborately convoluted pattern. Growing evidence suggests that mechanical instabilities emerging from differential growth between a faster growing outer gray matter and a slower growing inner white matter play a major role in brain morphogenesis. Previous studies assume uniform growth and stiffness; yet, recent experiments indicate that the properties of brain tissue are highly inhomogeneous. Here, we hypothesize that regionally varying developmental pathways across the brain result in nonuniform material properties at the onset of cortical folding. We establish a computational model of brain growth to explore the effects of stiffness and growth variations in gray and white matter tissue to mimic cellular processes and evolving tissue microstructure. We present an effective approach to determine critical growth values from geometrical data and systematically study the effect of inhomogeneous material properties on growth-induced primary and secondary instabilities. Our results reveal that critical growth and wavelength strongly depend on the stiffness distribution in the developing brain. Regional variations in cortical growth affect secondary instabilities and evoke highly irregular folding patterns, but characteristic wavelength and critical growth remain relatively stable. The interplay of different influential factors including cortical thickness, brain geometry, stiffness, and growth explains how primary folds are highly preserved across individuals, whereas secondary and tertiary folds vary significantly. Our findings are directly applicable to imaging data of fetal brains and ultimately enable early diagnostics of cortical malformations to improve treatment of neurodevelopmental disorders including epilepsy, autism, spectrum disorders, and schizophrenia.
Keywords: Cortical folding; Growth; Instabilities; Period-doubling; Inhomogeneous properties

On the interaction of Mg with the (111) and (110) surfaces of ceria - Nolan, Michael; Lykhach, Yaroslava; Tsud, Nataliya; Skala, Tomas; Staudt, Thorsten; Prince, Kevin C.; Matolin, Vladimir; Libuda, Jörg (Physical Chemistry Chemical Physics, 2012)
The catalytic activity of cerium dioxide can be modified by deposition of alkaline earth oxide layers or nanoparticles or by substitutional doping of metal cations at the Ce site in ceria. In order to understand the effect of Mg oxide deposition and doping, a combination of experiment and first principles simulations is a powerful tool. In this paper, we examine the interaction of Mg with the ceria (111) surface using both angle resolved X-ray (ARXPS) and resonant (RPES) photoelectron spectroscopy measurements and density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U). With DFT + U, we also examine the interaction of Mg with the ceria (110) surface. The experiments show that upon deposition of Mg, Ce ions are reduced to Ce3+, while Mg is oxidised. When Mg is incorporated into ceria, no reduced Ce3+ ions are found and oxygen vacancies are present. The DFT + U simulations show that each Mg that is introduced leads to formation of two reduced Ce3+ ions. When Mg is incorporated at a Ce site in the (111) surface, one oxygen vacancy is formed for each Mg to compensate the different valencies, so that all Ce ions are oxidised. The behaviour of Mg upon interaction with the (110) surface is the same as with the (111) surface. The combined results provide a basis for deeper insights into the catalytic behaviour of ceria-based mixed oxide catalysts.

On the interaction of polycyclic aromatic compounds with graphene - Kozlov, Sergey M.; Vines, Francesc; Görling, Andreas (Carbon, 2012)
The adsorption and diffusion of benzene, hexafluorinated benzene, perylene, perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on graphene has been studied by density functional means on a generalized gradient approximation level, including a semi-empirical correction to account for dispersive forces. It is shown that for all considered molecules the adsorption strength is mainly due to the latter, with the electronic interaction being relatively small and repulsive. As a rule-of-thumb, the strength of the adsorption interaction is 11-13 kJ mol per C atom. The adsorption energies are large enough to avoid desorption at room temperature. The estimated diffusion and rotation barriers are remarkably small, thus allowing a rapid diffusion and self-arrangement even at cryogenic temperatures. Finally, the adsorption of benzene or perylene derivatives may act, depending on the molecule and nature of the substituents, as a source for n- or p-doping, achieving up to 0.2 electrons(holes)/molecule. The lowest unoccupied molecular orbitals of PTCDI and PTCDA are close in energy to the Dirac point of graphene and induce a conduction gap of ∼210-240 meV in the graphene band structure. Thus, they can be used for graphene band gap engineering and doping by the non-aggressive method of molecular adsorption. © 2012 Elsevier Ltd. All rights reserved.

On the inverse problem of the two-velocity tree-like graph - Avdonin, Sergei; Abdon, Choque Rivero; Leugering, Günter; Mikhaylov, Victor (ZAMM - Zeitschrift für angewandte Mathematik und Mechanik, 2015)
In this article the authors continue the discussion in [9] about inverse problems for second order elliptic and hyperbolic equations on metric trees from boundary measurements. In the present paper we prove the identifiability of varying densities of a planar tree-like network of strings along with the complete information on the graph, i.e. the lengths of the edges, the edge degrees and the angles between neighbouring edges. The results are achieved using the Titchmarch-Weyl function for the spectral problem and the Steklov-Poincare operator for the dynamic wave equation on the tree. The general result is obtained by a peeling argument which reduces the inverse problem layer-by-layer from the leaves to the clamped root of the tree. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: Inverse problem; planar tree of strings; Titchmarch-Weyl-matrix; dynamic Steklov-Poincare operator; leaf-peeling method

On the magnetic properties of iron nanostructures fabricated via focused electron beam induced deposition and autocatalytic growth processes - TU, Fan; Drost, Martin; Vollnhals, Florian; Späth, Andreas; Carrasco, Esther; Fink, Rainer; Marbach, Hubertus (Nanotechnology, 2016)
We employ Electron beam induced deposition (EBID) in combination with autocatalytic growth (AG) processes to fabricate magnetic nanostructures with controllable shapes and thicknesses. Following this route, different Fe deposits were prepared on silicon nitride membranes under ultra-high vacuum conditions and studied by scanning electron microscopy (SEM) and scanning transmission x-ray microspectroscopy (STXM). The originally deposited Fe nanostructures are composed of pure iron, especially when fabricated via autocatalytic growth processes. Quantitative near-edge x-ray absorption fine structure (NEXAFS) spectroscopy was employed to derive information on the thickness dependent composition. X-ray magnetic circular dichroism (XMCD) in STXM was used to derive the magnetic properties of the EBID prepared structures. STXM and XMCD analysis evinces the existence of a thin iron oxide layer at the deposit-vacuum interface, which is formed during exposure to ambient conditions. We were able to extract magnetic hysteresis loops for individual deposits from XMCD micrographs with varying external magnetic field. Within the investigated thickness range (2-16 nm), the magnetic coercivity, as evaluated from the width of the hysteresis loops, increases with deposit thickness and reaches a maximum value of ∼160 Oe at around 10 nm. In summary, we present a viable technique to fabricate ferromagnetic nanostructures in a controllable way and gain detailed insight into their chemical and magnetic properties.
EAM Import::2019-03-08

On the mechanism of Zn4O-acetate precursors ripening to ZnO: How dimerization is promoted by hydroxide incorporation - Segets, Doris; Milek, Theodor; Kirschbaum, Rolf; von Gernler, Marc; Lübbert, Christian; Drewello, Thomas; Peukert, Wolfgang; Zahn, Dirk (Journal of Chemical Physics, 2015)
We report on a combined experimental and molecular modelling study on Zn4O ion clusters stabilized by acetate molecules (OAc). In particular, ab initio calculations of acetate substitution by hydroxide ions are compared with mass spectrometry data. Though quantum calculations in the gas phase indicate strong energetic preference, no experimental evidence of stable Zn4O(OAc)6−x(OH)x clusters in ethanolic solutions could be observed. This apparent contradiction is rationalized by identifying the supportive role of hydroxide ions for the association of (OAc− → OH− substituted) Zn4O(OAc)6 and Zn4O(OAc)5+ clusters. Mass spectrometry and quantum calculations hint at the stability of (Zn4O)2(OAc)12−x(OH)x dimers with x = 1, 2. Therein, the hydroxide ions establish salt-bridges that allow for the formation of additional Zn3 motifs with the OH above the triangle center—a structural motif close to that of the ZnO-crystal. The association of Zn4O(OAc)6 clusters is thus suggested to involve OAc− → OH− substitution as an activation step, quickly followed by dimerization and the subsequent agglomeration of oligomer
Keywords: mechanism; particle; rod; nano; ripening

On the molecular mechanisms of the acid-induced dissociation of hydroxy-apatite in water - Hochrein, Oliver; Zahn, Dirk (Journal of Molecular Modeling, 2011)
The enamel/saliva interface is mimicked by the comparably much simpler model of (001) surfaces of hydroxy-apatite ( Ca10(PO4)6(OH)2 ) in contact with aqueous solution. At neutral pH, the dissociation of ions is penalized by more than 150 kJ mol-1 giving rise to very stable apatite-water interfaces. This picture changes drastically with decreasing pH, as the protonation of phosphate and hydroxide ions lowers the free energy of calcium ions dissociation. Our simulations suggest the mechanism of acid-induced apatite decomposition to i) require a considerable degree of protonation of the apatite surface. The first ion dissociation step ii) involves calcium ions which electrostatic binding has been locally destabilized through phosphate and hydroxide protonation. The depletion of calcium ions embedding the anions then allows iii) the dissociation of the anionic species. Along this line, the protective role of fluoride in caries prevention is related to the stabilization of the calcium triangles embedding the OH-/F- ions.
EAM Import::2019-03-08

On the Multiscale Computation of Defect Driving Forces - Mergheim, Julia; Steinmann, Paul; Ricker, Sarah (International Journal For Multiscale Computational Engineering, 2009)
In the present contribution, the computational homogenization scheme is extended toward the homogenization of configurational quantities like the Eshelby stress and material node point forces. Configurational mechanics is concerned with changes of the material configuration of continuum bodies and has numerous applications in defect mechanics, as, e. g., it can be shown that the material force at a crack tip corresponds to the J-integral and thus yields a criterion for crack propagation. In the theoretical part of this work, the differences between the homogenization of the direct and the inverse motion problem are elaborated. Therefore focus is put onto the influence of microscopic material interfaces and material body forces on the averaged field values. The theoretical results are illustrated by various numerical examples, which on one hand compare the homogenized configurational quantities for different microstructures, and on the other hand, point out which features of the microstructure influence the macroscopic configurational quantities. © 2009 by Begell House, Inc.
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Keywords: Configurational forces; Homogenization; Multi-scale

On the platinum-oxide formation under gas-phase and electrochemical conditions - Farkas, Attila; Fantauzzi, Donato; Mueller, Jonathan E.; Zhu, Tianwei; Papp, Christian; Steinrück, Hans-Peter; Jacob, Timo (Journal of Electron Spectroscopy and Related Phenomena, 2017)
We summarize our recent results on the oxidation of platinum surfaces, stressing the close connection between theory and experiment. The present theoretical approach is based on ab-initio quantum mechanics methods, such as density functional theory (DFT), ab-initio atomistic thermodynamics, Monte-Carlo methods and reactive molecular dynamics (RMD). Experimentally, photoelectron spectroscopy (XPS) under UHV and in-situ conditions (NAP-XPS) provides most of the discussed results. The oxidation of an extended variety of platinum surfaces is reviewed, starting from atomically flat Pt(111) over stepped surfaces to regularly shaped nanoparticles, under conditions ranging from UHV through ambient pressures up to electrochemical interfaces.
EAM Import::2019-03-08

On the Principles of Tweaking Nanostructure Fabrication via Focused Electron Beam Induced Processing Combined with Catalytic Growth Processes - Drost, Martin; TU, Fan; Vollnhals, Florian; Szenti, Imre; Kiss, Janos; Marbach, Hubertus (Small Methods, 2017)
Novel pathways to use a focused electron beam in ultrahigh vacuum in combination with catalytic growth protocols are explored for the fabrication of well-defined nanostructures. Thereby, the basic process is the local deposition of material from different precursor molecules (Co(CO)3 NO, Fe(CO)5). This can be realized with a focused electron beam by direct deposition of the precursor or by a chemical modification of the substrate, such that it becomes activated toward the decomposition of the precursor. For both methods, autocatalytic growth (AG) processes can occur, yielding the deposition of additional material. Interestingly, significantly different chemical selectivities for the two precursors are found, which can drastically change upon subtle changes of the surface. It is demonstrated that AG can be tweaked by the choice of the substrate/precursor combination. One possibility to quench catalytic activity of a substrate is the preparation of a thin layer of organic molecules, here porphyrins, which enables the fabrication of hybrid metal–organic nanostructures with line widths below 20 nm. Based on these findings, corresponding techniques are developed which exploit (auto)catalytic effects for the controlled fabrication of nanostructures. These results indicate that every substrate is applicable to electron beam induced surface activation by preparation of a thin porphyrin layer.
EAM Import::2019-03-08

On the regioselectivity of the insertion step in nickel complex catalyzed dimerization of butene: A density-functional study - Nikiforidis, Ioannis; Görling, Andreas; Hieringer, Wolfgang (Journal of Molecular Catalysis A-Chemical, 2011)
The nickel complex catalyzed 1-butene dimerization to yield linear and branched C olefins, a process of considerable industrial importance, is investigated with density-functional theory. Reaction energies and transition barriers have been calculated for the catalytic cycle of a nickel catalyst bearing two phosphine ligands. The data suggest olefin insertion as the rate determining step, and a hydrogen-transfer mechanism has been found as the most favorable chain termination step, in agreement with earlier studies on related systems. The olefin insertion step has subsequently been investigated in terms of its regioselectivity for three different ligands, i.e., the PH , PMe and the N-heterocyclic carbene ligand IMe. The activation barriers for the IMe ligands are mostly found to be lower than those for phosphine ligands, indicating a higher catalytic activity of carbene complexes in butene dimerization. The computed relative transition state energies suggest that the PMe and IMe ligands kinetically favor butene insertion into the primary Ni-butyl bonds, which leads to linear and singly branched C products. Energetic differences in π-complexes may under non-equilibrium conditions lead to changes in selectivity towards branched product olefins when the catalyst bears IMe ligands. © 2011 Elsevier B.V. All rights reserved.

On thermomechanical solids with boundary structures - Javili, Ali; Steinmann, Paul (International Journal of Solids and Structures, 2010)
This paper, in line with the previous works (Javili and Steinmann, 2009, 2010), is concerned with the thermomechanically consistent theory and formulation of boundary potential energies and the study of their impact on the deformations of solids. Thereby, the main thrust in this contribution is the extension to thermomechanical effects. Although boundary effects can play a dominant role in the material behavior, the common modelling in continuum mechanics takes exclusively the bulk into account, nevertheless, neglecting possible contributions from the boundary. In this approach the boundary is equipped with its own thermodynamic life, i.e. we assume separate boundary energy, entropy and the like. Afterwards, the derivations of generalized balance equations, including boundary potentials, completely based on a tensorial representation is carried out. The formulation is exemplified for the example of thermohyperelasticity. © 2010 Elsevier Ltd. All rights reserved.
UnivIS-Import:2015-03-09:Pub.2010.tech.FT.FT-TM.onther
Keywords: Boundary potentials; Nanomaterials; Surface tension; Thermoelasticity

On the short-range behavior of correlated pair functions from the adiabatic-connection fluctuation-dissipation theorem of density-functional theory - Heßelmann, Andreas; Görling, Andreas (Journal of Chemical Theory and Computation, 2013)
The short-range behavior of correlated pair functions from the adiabatic-connection fluctuation-dissipation theorem (ACFD) of density functional theory (DFT) employing local exchange-correlation kernels has been analyzed. It has been found that for large basis sets the pair function exhibits unphysical humps for small interelectronic distances if the adiabatic local density approximation kernel is used in the ACFD scheme (this method is termed ACFD/ALDA in this work). However, up to basis set sizes of quadruple-ζ type quality, the correlated pair function of ACFD/ALDA behaves physically correct and the method yields reasonable results for atomization energies, ionization potentials, and intermolecular interaction energies. In order to correct the deficiencies of the pair function of ACFD/ALDA for large basis sets, a short-range correction scheme has been devised on the basis of a combination of the ACFD/ALDA pair function for the large distance regime with a proper physically correctly behaving pair function at smaller distances. While this approach, termed as ACFD/ALDAcorr, practically yields results close to those of the ACFD/ALDA method for finite basis sets, it enables basis set extrapolation techniques and thus can take dynamic correlation effects fully into account in contrast to the ACFD/ALDA approach. This work also presents an efficient density-fitting algorithm to compute the ACFD correlation energies that enables the calculation of correlation energies of extended molecular systems. © 2013 American Chemical Society.

On the transition from plastic deformation to crack initiation in the high- and very high-cycle fatigue regimes in plain carbon steels - Bach, Jochen; Möller, Johannes; Göken, Mathias; Bitzek, Erik; Höppel, Heinz Werner (International Journal of Fatigue, 2016)
It is well known that fatigue life of metallic materials involves processes at different stages. Two main regimes can be distinguished: The first is governed by plastic deformation and related dislocation processes like cyclic saturation and strain localization. The second regime is mainly dominated by crack initiation and propagation. Whether an infinite fatigue life is obtained or not is therefore ultimately determined by the complex interplay of plastic deformation processes, the formation of small crack nuclei and their subsequent interaction with the surrounding material's microstructure. Thus, it is necessary to investigate the transition from plastic deformation to crack initiation in the high- and very high-cycle fatigue regime in more detail. Three plain carbon steels with different ferrite to pearlite ratios were investigated in order to determine the influence of different microstructures on the transition from cyclic deformation to crack initiation in the high and very high cycle fatigue (VHCF) regime. A new method was developed where the changes in the dissipated energy are used to account for irreversible plastic deformation during ultrasonic fatigue loading. At very small fatigue amplitudes, which still lead to VHCF-failure, the cyclic deformation regime covers the main part of the fatigue life. In this regime dislocation interactions lead to the accumulation of irreversible plastic deformation followed by small crack nuclei that initiate at phase or grain boundaries (GBs), marking the transition to the damage regime. In order to better understand the fundamental mechanisms at the transition between both regimes, idealized atomistic simulations were performed. To model fatigue-induced plastic deformation various defects, i.e. dislocations and vacancies, were placed in the vicinity of a GB. These defects generally decrease the globally applied critical stress for crack initiation whilst the local critical stress stays mainly unchanged in the same range as the theoretical GB strength. Based on the experimental results and on the general findings from the atomistic simulations it is concluded that an infinite fatigue life is obtained either when cyclic irreversible deformation is too small to build up the necessary stresses for crack initiation or when the formed crack nuclei cannot overcome the next microstructural barrier
Keywords: Atomistic simulations; Crack initiation; Steels; Ultrasonic fatigue; VHCF

On Timoshenko thin elastic inclusions inside elastic bodies - Khludnev, A. M.; Leugering, Günter (Mathematics and Mechanics of Solids, 2015)
The paper concerns the analysis of equilibrium problems for 2D elastic bodies with thin inclusions modeled in the framework of Timoshenko beams. The first focus is on the well-posedness of the model problem in a variational setting. Then delaminations of the inclusions are considered, forming a crack between the elastic body and the inclusion. Nonlinear boundary conditions at the crack faces are considered to prevent a mutual penetration between the faces. The corresponding variational formulations together with weak and strong solutions are discussed. The model contains various physical parameters characterizing the mechanical properties of the inclusion, such as flexural and shear stiffness. The paper provides an asymptotic analysis of such parameters. It is proved that in the limit cases corresponding to infinite and zero rigidity, we obtain rigid inclusions and cracks with the non-penetration conditions, respectively. Finally, exemplary networks of Timoshenko beams are considered as inclusions as well.
Keywords: Thin elastic inclusion; Timoshenko beams; crack; delamination; non-penetration boundary condition

On unified boundary conditions for improved predictions of near-wall turbulence - Jakirlic, S.; Jovanovic, Jovan (Journal of Fluid Mechanics, 2010)
A novel formulation of the wall boundary conditions relying on the asymptotic behaviour of the Taylor microscale λ and its relationship to the homogeneous part of the viscous dissipation rate of the kinetic energy of turbulence ∈h =5q2/λ2, applicable to near-wall turbulence, is examined. The linear dependence of λ on the wall distance in close proximity to the solid surface enables the wall-closest grid node to be positioned immediately below the edge of the viscous sublayer, leading to a substantial coarsening of the grid resolution. This approach provides bridging of a major portion of the viscous sublayer, higher grid flexibility and weaker sensitivity against the grid non-uniformities in the near-wall region. The performance of the proposed formulation was checked against available direct numerical simulation databases of complex wall-bounded flows featured by swirl and separation. © 2010 Cambridge University Press.
UnivIS-Import:2015-03-09:Pub.2010.tech.ITC.stmmec.onunif
Keywords: turbulence modelling; wall-bounded flows

Open top anodic Ta3N5 nanotubes for higher solar water splitting efficiency - Grigorescu, Sabina Georgiana; So, Seulgi; Yoo, Jeong Eun; Mazare, Anca Valentina; Hahn, Robert; Schmuki, Patrik (Electrochimica Acta, 2015)
In the present work we grow self-organized Ta2O5 nanotube layers in a H2SO4/NH4F electrolyte at various elevated temperatures. Under optimized conditions we obtain 4 extgreekmm long nanotubes that are well adherent to the substrate and can be grown very homogenous over large surface areas. Moreover, the key advantage of this approach is that an open top morphology (initiation layer free) is obtained. After a suitable conversion treatment in NH3 atmosphere Ta3N5 nanotubes are obtained, that after (Co-Pi+Co(OH)x) modification provide, under AM 1.5G illumination conditions, a photoelectrochemical current response of 4.7 mA cm-2 at +1.23 V vs RHE.
EAM Import::2019-03-08

Optical diametric drive acceleration through action–reaction symmetry breaking - Wimmer, Martin; Regensburger, Alois; Bersch, Christoph; Miri, Mohammad-Ali; Batz, Sascha; Onishchukov, Georgy; Christodoulides, Demetrios N.; Peschel, Ulf (Nature Physics, 2013)
Newton's third law of motion is one of the pillars of classical physics. This fundamental principle states that the forces two bodies exert on each other are equal and opposite. Had the resulting accelerations been oriented in the same direction, this would have instead led to a counterintuitive phenomenon, that of diametric drive. In such a hypothetical arrangement, two interacting particles constantly accelerate each other in the same direction through a violation of the action–reaction symmetry. Although in classical mechanics any realization of this process requires one of the two particles to have a negative mass and hence is strictly forbidden, it could nevertheless be feasible in periodic structures where the effective mass can also attain a negative sign. Here we report the first experimental observation of such diametric drive acceleration for pulses propagating in a nonlinear optical mesh lattice. The demonstrated reversal of action–reaction symmetry could enable altogether new possibilities for frequency conversion and pulse-steering applications.
EAM Import::2019-03-08

Optical Gap solitons and Truncated Nonlinear Bloch Waves in Temporal Lattices - Bersch, Christoph; Onishchukov, Georgy; Peschel, Ulf (Physical Review Letters, 2012)
We experimentally demonstrate the formation and stable propagation of various types of discrete temporal solitons in an optical fiber system. Pulses interacting with a time-periodic potential and defocusing nonlinearity are shown to form gap solitons and nonlinear truncated Bloch waves. Multi-pulse solitons with defects, as well as novel structures composed of a strong soliton riding on a weaker truncated nonlinear Bloch wave are shown to propagate over up to eleven coupling lengths. The nonlinear dynamics of all pulse structures is monitored over the full propagation distance which provides detailed insight into the soliton dynamics.
EAM Import::2019-03-08

Optically Monitored Catalytic Photonic Crystal Fibre Microreactor - Cubillas, Ana Maria; Schmidt, Matthias; Euser, Tijmen G.; Etzold, Bastian; Taccardi, Nicola; Unterkofler, Sarah; Wasserscheid, Peter; Russell, Philip St. John (European Conference on Lasers and Electro-Optics, 2013)
Here we show that by depositing metallic catalyst nanoparticles in the core of the fibre, we can turn the HC-PCF into a catalytically active microreactor. As a proof-of-principle experiment, we investigated the well-known catalytic hydrogenation of azobenzene in a kagomé HC-PCF, whose core wall has been decorated with rhodium (Rh) catalyst nanoparticles. The reaction was monitored online by in-fibre absorption spectroscopy.

Optically pure, monodisperse cis-oligodiacetylenes: Aggregation-induced chirality enhancement - Chernick, Erin; Börzsönyi, Gabor; Steiner, Christian; Ammon, Maximilian Michael; Geßner, David; Frühbeißer, Sabine; Gröhn, Franziska; Maier, Sabine; Tykwinski, Rik (Angewandte Chemie International Edition, 2014)
Conformational changes in the conjugated backbone of poly- and oligodiacetylenes (PDAs and ODAs) play an important role in determining the electronic properties of these compounds. At the same time, conformational changes can also result in a folded structure that shows helical chirality. Using D-camphor as a chiral building block, we have designed a high-yielding, iterative synthesis of monodisperse, optically pure cis-oligodiacetylenes (ODAs). cis-ODAs up to the tridecamer have been formed, which is the longest monodisperse cis-ODA reported to date. UV/Vis spectroscopy suggests a large effective conjugation length in THF, likely the result of a linear, planar conformation in this solvent. High-resolution STM/AFM measurements of the nonamer cast from THF onto HOPG show a linear structure. In iPrOH, circular dichroism (CD) spectra suggest the formation of chiral aggregates for ODAs with at least nine D-camphor units, based on a strong CD response. Conformation, conjugation: A series of optically pure, monodisperse cis-oligodiacetylenes (cis-ODAs) based on D-camphor building blocks has been synthesized up to the tridecamer length, which represents the longest cis-ODA reported to date. Optical spectroscopy and STM/AFM measurements indicate that the oligomers assume a linear, planar conformation in THF. Conversely, in iPrOH solution ODAs with n≥9 form chiral aggregates. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Optical polymers with tunable refractive index for nanoimprint technologies - Landwehr, J.; Fader, R.; Rumler, Maximilian; Rommel, Mathias; Bauer, Anton; Frey, Lothar; Simon, B.; Fodor, B.; Petrik, P.; Schiener, Andreas; Winter, Benjamin; Spiecker, Erdmann (Nanotechnology, 2014)
In order to realize a versatile high throughput production of micro-optical elements, UV-curable polymer composites containing titanium dioxide nanoparticles were prepared and characterized. The composites are based on an industrial prototype epoxy polymer. Titanium dioxide nanoparticles smaller than 10 nm were synthesized by the nonaqueous sol method and in situ sterically stabilized by three different organic surfactants. The composites exhibit high transparency. Distinct alteration of optical transmission properties for visible light and near IR wavelength range could be avoided by adaption of the stabilizing organic surfactant. Most importantly, the refractive index (RI) of the composites that depends on the fraction of incorporated inorganic nanoparticles could be directly tuned. E.g. the RI at a wavelength of 635 nm of a composite containing 23 wt% titanium dioxide nanoparticles is increased to 1.626, with respect to a value of 1.542 for the pure polymer. Furthermore, it could be demonstrated that the prepared inorganic-organic nanocomposites are well suited for the direct fabrication of low-cost micro-optical elements by nanoimprint lithography. A low response of the optical composite properties to temperature treatment up to 220°C with a shrinkage of only about 4% ensures its application for integrated micro-optical elements in industrial production.
UnivIS-Import:2015-04-14:Pub.2014.tech.IE.LEB.optica
Keywords: Composite materials; Refractive index matching; Substrate conformal imprint lithography; TiO; UV polymers

Optical Properties of Nanocrystal Interfaces in Compressed MgO Nanopowders - Mckenna, Keith P.; Koller, David; Sternig, Andreas; Siedl, Nicolas; Govind, Niranjan; Sushko, Peter V.; Diwald, Oliver (ACS nano, 2011)
The optical properties and charge trapping phenomena observed on oxide nanocrystal ensembles can be strongly influenced by the presence of nanocrystal interfaces. MgO powders represent a convenient system to study these effects due to the well-defined shape and controllable size distributions of MgO nanocrystals. The spectroscopic properties of nanocrystal interfaces are investigated by monitoring the dependence of absorption characteristics on the concentration of the interfaces in the nanopowders. The presence of interfaces is found to affect the absorption spectra of nanopowders more significantly than changing the size of the constituent nanocrystals and, thus, leading to the variation of the relative abundance of light-absorbing surface structures. We find a strong absorption band in the 4.0-5.5 eV energy range, which was previously attributed to surface features of individual nanocrystals, such as corners and edges. These findings are supported by complementary first-principles calculations. The possibility to directly address such interfaces by tuning the energy of excitation may provide new means for functionalization and chemical activation of nanostructures and can help improve performance and reliability for many nanopowder applications.
EAM Import::2019-03-08

Optical properties of vertical, tilted and in-plane GaN nanowires on different crystallographic orientations of sapphire - Tessarek, C.; Figge, S.; Gust, Arne; Heilmann, M.; Dieker, Christel; Spiecker, Erdmann; Christiansen, S. (Journal of Physics D-Applied Physics, 2014)
Self-catalysed and self-organized GaN nanowires were grown on c-, a-, m- and r-plane sapphire by metal-organic vapour phase epitaxy. In dependence on the crystallographic orientation of the sapphire substrate, vertical, tilted and in-plane GaN nanowires were achieved. The nanowire orientation is visualized by scanning electron microscopy and analysed by x-ray diffraction. The influence of the sapphire nitridation step on the nanowire formation is investigated. Spatially and spectrally resolved cathodoluminescence studies are carried out on the GaN nanowires to analyse the influence of the GaN nanowire orientation as well as the presence of both N- and Ga-polar sections in a single nanowire on the optical properties.
Keywords: GaN; nanowires; sapphire; growth; MOVPE; cathodoluminescence

Optics in Curved Space - Schultheiß, Vincent; Batz, Sascha; Szameit, Alexander; Dreisow, Felix; Nolte, Stefan; Tuennermann, Andreas; Longhi, Stefano; Peschel, Ulf (Physical Review Letters, 2010)
We experimentally study the impact of intrinsic and extrinsic curvature of space on the evolution of light. We show that the topology of a surface matters for radii of curvature comparable with the wavelength, whereas for macroscopically curved surfaces only intrinsic curvature is relevant. On a surface with constant positive Gaussian curvature we observe periodic refocusing, self-imaging, and diffractionless propagation. In contrast, light spreads exponentially on surfaces with constant negative Gaussian curvature. For the first time we realized two beam interference in negatively curved space.
EAM Import::2019-03-08

Optimal and approximate boundary controls of an elastic body with quasistatic evolution of damage - Kogut, Peter I.; Leugering, Günter (Mathematical Methods in the Applied Sciences, 2014)
In this paper, we study an optimal control problem for the mixed boundary value problem for an elastic body with quasistatic evolution of an internal damage variable. We suppose that the evolution of microscopic cracks and cavities responsible for the damage is described by a nonlinear parabolic equation. A density of surface traction p acting on a part of boundary of an elastic body Ω is taken as a boundary control. Because the initial boundary value problem of this type can exhibit the Lavrentieff phenomenon and non-uniqueness of weak solutions, we deal with the solvability of this problem in the class of weak variational solutions. Using the convergence concept in variable spaces and following the direct method in calculus of variations, we prove the existence of optimal and approximate solutions to the optimal control problem under rather general assumptions on the quasistatic evolution of damage.

Optimal Control in Matrix-Valued Coefficients for Nonlinear Monotone Problems: Optimality Conditions I - Kogut, Peter I.; Kupenko, Ol'Ga P.; Leugering, Günter (Zeitschrift für Analysis und ihre Anwendungen, 2015)
In this article we study an optimal control problem for a nonlinear monotone Dirichlet problem where the controls are taken as matrix-valued coefficients in L-infinity (Omega; R-NxN). For the exemplary case of a tracking cost functional, we derive first order optimality conditions. This first part is concerned with the general case of matrix-valued coefficients under some hypothesis, while the second part focuses on the special class of diagonal matrices.
Keywords: Nonlinear monotone Dirichlet problem; control in coefficients; adjoint equation

Optimal control of cracks in elastic bodies with thin rigid inclusions - Khludnev, Alexander M.; Leugering, Günter (ZAMM - Zeitschrift für angewandte Mathematik und Mechanik, 2011)
In this paper we consider the control of cracks in elastic bodies with rigid inclusion. We first describe the problem statement, provide an equivalent formulation as a variational inequality and prove existence and uniqueness of solutions. Furthermore, we consider this problem as a limiting problem when the elasticity parameters of the inclusion tend to infinity. Then we formulate the optimal control problems and derive an explicit formula for the crack sensitivity and for the energy release rate. We show existence of optimal solutions.

Optimal Control of Inclusion and Crack Shapes in Elastic Bodies - Khludnev, A.; Leugering, Günter; Specovius-Neugebauer, M. (Journal of Optimization Theory and Applications, 2012)
The paper is concerned with the control of the shape of rigid and elastic inclusions and crack paths in elastic bodies. We provide the corresponding problem formulations and analyze the shape sensitivity of such inclusions and cracks with respect to different perturbations. Inequality type boundary conditions are imposed at the crack faces to provide a mutual nonpenetration between crack faces. Inclusion and crack shapes are considered as control functions and control objectives, respectively. The cost functional, which is based on the Griffith rupture criterion, characterizes the energy release rate and provides the shape sensitivity with respect to a change of the geometry. We prove an existence of optimal solutions.

Optimal Design of Brittle Composite Materials: a Nonsmooth Approach - Prechtel, Marina; Leugering, Günter; Steinmann, Paul; Stingl, Michael (Journal of Optimization Theory and Applications, 2012)
Our goal is to design brittle composite materials yielding maximal energy dissipation for a given static load case. We focus on the effect of variation of fiber shapes on resulting crack paths and thus on the fracture energy. To this end, we formulate a shape optimization problem, in which the cost function is the fracture energy and the state problem consists in the determination of the potentially discontinuous displacement field in the two-dimensional domain. Thereby, the behavior at the crack surfaces is modeled by cohesive laws. We impose a nonpenetration condition to avoid interpenetration of opposite crack sides. Accordingly, the state problem is formulated as variational inequality. This leads to potential nondifferentiability of the shape-state mapping. For the numerical solution, we derive first-order information in the form of subgradients. We conclude the article by numerical results.
UnivIS-Import:2015-03-09:Pub.2011.tech.FT.FT-TM.optima
Keywords: Our goal is to design brittle composite materials yielding maximal energy dissipation for a given static load case. We focus on the effect of variation of fiber shapes on resulting crack paths and thus on the fracture energy. To this end; we formulate a shape optimization problem; in which the cost function is the fracture energy and the state problem consists in the determination of the potentially discontinuous displacement field in the two-dimensional domain. Thereby; the behavior at the crack surfaces is modeled by cohesive laws. We impose a nonpenetration condition to avoid interpenetration of opposite crack sides. Accordingly; the state problem is formulated as variational inequality. This leads to potential nondifferentiability of the shape-state mapping. For the numerical solution; we derive first-order information in the form of subgradients. We conclude the article by numerical results.

Optimal L1-Control in Coefficients for Dirichlet Elliptic Problems: H-Optimal Solutions - Kogut, Peter I.; Leugering, Günter (Zeitschrift für Analysis und ihre Anwendungen, 2012)
In this paper we study a Dirichlet optimal control problem associated with a linear elliptic equation the coefficients of which we take as controls in L (Ω). In particular, when the coefficient matrix is taken to satisfy the decomposition B(x) = ρ(x)A(x) with a scalar function ρ, we allow the ρ to degenerate. Such problems are related to various applications in mechanics, conductivity and to an approach in topology optimization, the SIMP-method. Since equations of this type can exhibit the Lavrentieff phenomenon and non-uniqueness of weak solutions, we show that the optimal control problem in the coefficients can be stated in different forms depending on the choice of the class of admissible solutions. Using the direct method in the Calculus of variations, we discuss the solvability of the above optimal control problems in the so-called class of H-admissible solutions. © European Mathematical Society.
Keywords: Degenerate elliptic equations; control in coefficients; weighted Sobolev spaces; Lavrentieff phenomenon; direct method in the Calculus of variations

Optimal L1-Control in Coefficients for Dirichlet Elliptic Problems: W-Optimal Solutions - Kogut, Peter I.; Leugering, Günter (Journal of Optimization Theory and Applications, 2011)
In this paper, we study a Dirichlet optimal control problem associated with a linear elliptic equation the coefficients of which we take as controls in the class of integrable functions. The coefficients may degenerate and, therefore, the problems may exhibit the so-called Lavrentieff phenomenon and non-uniqueness of weak solutions. We consider the solvability of this problem in the class of W-variational solutions. Using a concept of variational convergence of constrained minimization problems in variable spaces, we prove the existence of W-solutions to the optimal control problem and provide the way for their approximation. We emphasize that control problems of this type are important in material and topology optimization as well as in damage or life-cycle optimization.
Keywords: Degenerate elliptic equations; Control in coefficients; Weighted Sobolev spaces; Lavrentieff phenomeno; Variational convergence

Optimal reactor design and operation taking catalyst deactivation into account - Xie, Mingquan; Freund, Hannsjörg (Chemical Engineering Science, 2018)
In this work, a new approach for taking catalyst deactivation into account in the early stage of reactor design is derived and exemplified by the ethylene oxide synthesis as an industrial relevant case study. In this process, the catalyst lifetime is much larger than the residence time of reactants. The operating conditions over the whole catalyst lifetime and the reactor configuration are simultaneously optimized. The objective of the mathematical optimization problem is the overall reactor performance such as the average selectivity over the catalyst lifetime. An optimal detailed design could be derived with advanced temperature control, giving rise to a higher average selectivity and potentially longer catalyst lifetime compared to the reference case. (C) 2017 Elsevier Ltd. All rights reserved.
Keywords: Reactor design; Catalyst deactivation; Optimization; Process intensification; Modeling

Optimization of Solution-Processed Luminescent Down-Shifting Layers for Photovoltaics by Customizing Organic Dye Based Thick Films - Solodovnyk, Anastasiia; Kick, Christopher; Osvet, Andres; Egelhaaf, Hans Joachim; Stern, Edda; Batentschuk, Miroslaw; Forberich, Karen; Brabec, Christoph (Energy Technology, 2016)
We theoretically and experimentally determined the optimized film properties of luminescent down-shifting (LDS) layers for thin film solar cells. Based on the predictions from an adapted optical model, we coated thick (300–500 mm) and efficient luminescent down-shifting layers from environmentally friendly solvents and industrially scalable inks. LDS layers consisted of polyvinyl butyral (PVB) as binder and organic luminescent dyes as UV-converters. The luminescence quantum yields of the dyes were studied in solution (benzyl alcohol) and for solid thick films. Our data shows that the studied dyes retain luminescent efficiencies of approximately 90% in the solid state when processed from solution. We further apply the produced layers onto copper indium gallium diselenide (CIGS) solar cells to verify the theoretical predictions for enhancing the external quantum efficiency (EQE) in the UV region. For the best converters a remarkable enhancement of the EQE from 9% to 52% was recorded at 380 nm. These findings underline that printed LDS layers indeed have the potential to enhance the efficiency and the light harvesting capabilities of industrially relevant photovoltaic modules.

Optimization under uncertainty in chemical engineering: Comparative evaluation of unscented transformation methods and cubature rules - Maußner, Johannes; Freund, Hannsjörg (Chemical Engineering Science, 2018)
Keywords: Reactor optimization; Unscented transformation; Optimization under uncertainty; Reaction engineering; Cubature rule

Optimized layer architecture for an extended fatigue life of ultrafine-grained AA1050/AA5005 laminated metal composites - Kümmel, Frank; Tegtmeier, Thea-Simone; Höppel, Heinz Werner; Göken, Mathias (IOP Conference Series: Materials Science and Engineering, 2017)
The influence of interfaces on the fatigue life in AA1050/AA5005 ultrafine-grained laminated metal composites was investigated. At constant sheet thickness, the layer thickness and the number of material interfaces was varied by performing different cycles of accumulative roll bonding. It is found that crack deviation occurs if the crack approaches the soft to hard (AA1050/AA5005) material interface resulting in strongly enhanced fatigue lives compared to the mono-material. The most favourable layer architecture for enhanced fatigue life strongly depends on the applied stress amplitudes. At intermediate stress amplitudes a large layer thickness of 500 extgreekmm (N4) and 8 material interfaces gives the longest fatigue life. In contrast, at low stress amplitudes the sheets with 32 material interfaces and a layer thickness of 125 extgreekmm (N6) shows the longest fatigue life.
EAM Import::2019-03-08

Optimizing CuO p-type dye-sensitized solar cells by using a comprehensive electrochemical impedance spectroscopic study - Langmar, Oliver; Ganivet, Carolina R.; De La Torre, Gema; Torres, Tomas; Costa, Ruben Dario; Guldi, Dirk Michael (Nanoscale, 2016)
We introduce a novel and comprehensive approach for the evaluation and interpretation of electrochemical impedance spectroscopy (EIS) measurements in p-type DSSCs. In detail, we correlate both the device performance and EIS figures-of-merit of a series of devices in which, the calcination temperature, film thickness, and electrolyte concentration have been systematically modified. This new approach enables the separation of the different processes across the dye/semiconductor/electrolyte interface, namely the unfavorable charge recombination and the favorable electron injection/regeneration processes. In addition, studies on non-sensitized CuO and NiO electrodes provide insights into their affinity towards a reaction with the electrolyte - CuO is far less reactive towards the polyiodide species. Overall, this work underlines the superior features of CuO with respect to NiO for p-DSSCs and demonstrates a comprehensive optimization of the CuO-based DSSCs with respect to the device architecture by the aid of EIS analysis.
EAM Import::2019-03-08

Optimizing the number of multigrid cycles in the full multigrid algorithm - Thekale, A.; Gradl, Tobias; Klamroth, K.; Rüde, Ulrich (Numerical Linear Algebra With Applications, 2010)
Multigrid (MG) methods are among the most efficient and widespread methods for solving large linear systems of equations that arise, for example, from the discretization of partial differential equations. In this paper we introduce a new approach for optimizing the computational cost of the full MG method to achieve a given accuracy by determining the number of MG cycles on each level. To achieve this, a very efficient and flexible Branch and Bound algorithm is developed. The implementation in the parallel finite element solver Hierarchical Hybrid Grids leads to a significant reduction in CPU time.
EAM Import::2019-03-07

Optimizing TiO2 Nanotube Top Geometry for Use in Dye-Sensitized Solar Cells - Mir, Nooshin; Lee, Kiyoung; Paramasivam, Indhumati; Schmuki, Patrik (Chemistry - A European Journal, 2012)
For TiO2 nanotube-based dye-sensitized solar cells, the efficiency can be drastically enhanced by a synergetic effect that occurs when using nanowire-ended nanotubes in combination with an adequate nanoparticle decoration.
EAM Import::2019-03-08

Optimum between purification and colloidal stability of ZnO nanoparticles - Marczak, Renata; Segets, Doris; Voigt, Michael; Peukert, Wolfgang (Advanced Powder Technology, 2010)
Crystalline ZnO quantum dots have been synthesized by hydrolysis of zinc acetate dihydrate with lithium hydroxide in ethanolic solution. By varying different parameters of the synthesis process, the size of the ZnO particles can be controlled. Detailed investigation of the ripening of the nanoparticles evidenced that despite of the well-known influence of ageing temperature and time, the presence of the reaction byproduct lithium acetate strongly affects the ripening behaviour. In particular, the particle size can be almost completely arrested by the removal of this byproduct via reversible flocculation of the ZnO nanoparticles using heptane as an antisolvent. A closer analysis of the repeated washing process shows an initial improvement of the colloidal stability of the ZnO nanoparticles during the first purification cycle as it mainly removes the lithium acetate from the suspension and not the stabilizing acetate groups directly bound to the particle surface. With further washing the remaining acetate ligands are unable to maintain the stabilization against agglomeration of the ZnO nanoparticles. Thus, there exists an optimum between purification progress and colloidal stability. These findings are also confirmed by calculations according to the DLVO theory, which show that there exists nearly no primary minimum of small ZnO nanoparticles below 5 nm in the presence of stabilizing acetate ions whereas the decrease in acetate ions bound to the particle surface leads to a more and more pronounced primary minimum. The present work is of particular significance for the preparation of purified colloidal ZnO nanoparticles for studies of their electrical and optical properties with respect to their wide range of potential applications.

Optofluidic immobility of particles trapped in liquid-filled hollow-core photonic crystal fiber - Garbos, M. K.; Euser, T. G.; Russell, Philip St. John (Optics Express, 2011)
We study the conditions under which a particle, laser-guided in a vertically-oriented hollow-core photonic crystal fiber filled with liquid, can be kept stationary against a microfluidic counter-flow. An immobility parameter—the fluid flow rate required to immobilize a particle against the radiation force produced by unit guided optical power—is introduced to quantify the conditions under which this occurs, including radiation, viscous and gravity forces. Measurements show that this parameter depends strongly on the ratio of particle radius a to core radius R, peaking at an intermediate value of a/R. The results follow fairly well the theoretical estimates of the optical (calculated approximately using a ray optics approach) and numerically simulated drag forces. We suggest that the system has potential applications in, e.g., measurement of the diameter, refractive index and density of particles, synthesis and biomedical research.
EAM Import::2019-03-08

Optofluidic rotation of living cells for single-cell tomography - Kolb, Thorsten; Albert, Sahradha; Haug, Michael; Whyte, Graeme (Journal of Biophotonics, 2014)
Flow cytometry provides a high throughput, multi-dimensional analysis of cells flowing in suspension. In order to combine this feature with the ability to resolve detailed structures in 3D, we developed an optofluidic device that combines a microfluidic system with a dual beam trap. This allows for the rotation of single cells in a continuous flow, around an axis perpendicular to the imaging plane. The combination of both techniques enables the tomographic reconstruction of the 3D structure of the cell. In addition this method is capable to provide detailed 3D structural data for flow cytometry, as it improves the reconstructed z-resolution of a standard microscopy system to produce images with isotropic resolution in all three axes.
EAM Import::2019-03-08

Ordered arrays of epitaxial silicon nanowires produced by nanosphere lithography and chemical vapor deposition - Lerose, Damiana; Bechelany, Mikhael; Philippe, Laetitia; Michler, Johann; Christiansen, Silke (Journal of Crystal Growth, 2010)
Gold dot arrays on (1 1 1) Si substrates obtained through nanosphere lithography (NSL) combined with sputtering and annealing in Ar at 1000 °C are used to catalyze vapor liquid solid (VLS) epitaxial growth of silicon nanowires (Si NWs) using chemical vapor deposition (CVD) with SiH_4 in Ar. The NWs grow primarily epitaxially on the underlying (1 1 1) Si wafer following the four independent (1 1 1) directions. The diameter distribution of the wires reflects the diameter distribution of the catalyst gold dot arrays and is therefore predictable. The wire length depends on the size of the gold catalyst for the same CVD parameters. The wire position is foreseeable within the limits of the pattern geometrical quality, but one-to-one growth of NWs to gold dots is not always observed, probably due to (very locally) the remaining presence of silicon oxide. Overall, this inexpensive patterning method for obtaining high-quality crystalline VLS Si NWs by CVD fulfills the requirements of many device applications, where patterning control, quality and reproducibility of the nanostructures are crucial.
EAM Import::2019-03-08

Ordered Ferroelectric Lead Titanate Nanocellular Structure by Conversion of Anodic TiO2 Nanotubes - Macák, Jan; Zollfrank, Cordt; Rodriguez, Brian J.; Tsuchiya, Hiroaki; Alexe, Marin; Greil, Peter; Schmuki, Patrik (Advanced Materials, 2009)
The novel synthesis of ferroelectric perovskite PbTiO_3 layers is reported. For that, anodic self-organized TiO_2 nanotubes are used as a template for deposition of Pb inside the nanotubes. Upon thermal annealing, the filled template is converted to desired perovskite structure with nanocellular architecture. This approach could be advantageously used for synthesis of other piezoelectric or composite materials.
EAM Import::2019-03-08

Ordered "superlattice" TiO2/Nb2O5 nanotube arrays with improved ion insertion stability - Yang, Min; Yang, Guang; Spiecker, Erdmann; Lee, Kiyoung; Schmuki, Patrik (Chemical Communications, 2013)
Self-ordered TiO2/Nb2O5 "superlattice" nanotube arrays are successfully fabricated by electrochemical anodization from a commercially available Ti-Nb metal multilayer. The oxide nanotube walls then consist of alternating similar to 4 nm TiO2 and similar to 7 nm Nb2O5 rings. Such :superlattice" nanotube layers exhibit a much higher structural cycling stability for reversible Li-intercalation processes than pure TiO2 nanotubes.

Ordering and phase transitions in ionic liquid-crystalline films - Sobota, Marek; Wang, Xinjiao; Fekete, Melinda; Happel, Markus; Meyer, Karsten; Wasserscheid, Peter; Laurin, Mathias; Libuda, Jörg (ChemPhysChem, 2010)
Watching phase transitions: Ordering processes and phase transitions in thin films of the liquid-crystalline ionic liquid 1,3-didodecylimidazolium tetrafluoroborate [CCIM][BF] on Pt(111) (see picture) are studied by means of temperature-dependent IR reflection absorption spectroscopy. The method allows in situ monitoring of the structural properties of liquid-crystalline supported ionic liquid phase (SILP) catalysts under reaction conditions. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ordering aspects and intramolecular conformation of tetraphenylporphyrins on Ag(111) - Buchner, Florian; Kellner, Ina; Hieringer, Wolfgang; Görling, Andreas; Steinrück, Hans-Peter; Marbach, Hubertus (Physical Chemistry Chemical Physics, 2010)
A systematic scanning tunnelling microscopy study of the self-assembly, intramolecular conformation and supramolecular ordering of different tetraphenylporphyrins (xTPP) with or without a central metal atom (x = 2H, Fe, Co) on Ag(111) at room temperature is presented. The investigation covers a wide range, i.e. the adsorption behaviour from the low sub-monolayer up to the multilayer regime is described in detail and conclusively discussed in regard to molecule-molecule and molecule-substrate interactions. At monolayer coverage the molecules self-assemble in domains with a square unit cell caused by "T-type" intermolecular interactions, while the orientation of the domains along the symmetry axes is determined by adsorbate-substrate interactions. Interestingly for ordered monolayers domain boundaries always separate domains of different organizational chirality but same orientation of the square unit cell. This demonstrates that in the particular system chirality obviously restricts the long range order of the supramolecular TPP assemblies. In combination with DFT calculations it is also demonstrated that the previously reported intramolecular saddle shape deformation of TPPs upon adsorption is more pronounced for metallo-TPP than for 2HTPP. © 2010 the Owner Societies.

Organic and perovskite solar modules innovated by adhesive top electrode and depth-resolved laser patterning - Spyropoulos, George D.; Quiroz, Cesar Omar Ramirez; Salvador, Michael Filipe; Hou, Yi; Gasparini, Nicola; Schweizer, Peter; Adams, Jens; Kubis, Peter; Li, Ning; Spiecker, Erdmann; Ameri, Tayebeh; Egelhaaf, Hans-Joachim; et al. (Energy and Environmental Science, 2016)
We demonstrate an innovative solution-processing fabrication route for organic and perovskite solar modules via depth-selective laser patterning of an adhesive top electrode. This yields unprecedented power conversion efficiencies of up to 5.3% and 9.8%, respectively. We employ a PEDOT:PSS-Ag nanowire composite electrode and depth-resolved post-patterning through beforehand laminated devices using ultra-fast laser scribing. This process affords low-loss interconnects of consecutive solar cells while overcoming typical alignment constraints. Our strategy informs a highly simplified and universal approach for solar module fabrication that could be extended to other thin-film photovoltaic technologies.
Keywords: Indexed keywords Engineering controlled terms: Conducting polymers; Electrodes; Laminated composites; Organic lasers; Perovskite; Solar cells; Solar power generation Depth-resolved; Fabrication routes; Innovative solutions; Laser patterning; Photovoltaic technology; Power conversion efficiencies; Solar module; Universal approach Engineering main heading: Solar cell arrays GEOBASE Subject Index: efficiency measurement; electrode; film; innovation; laser method; perovskite; photovoltaic system

Organic-free synthesis of layer-like FAU-type zeolites - Inayat, Alexandra; Schneider, Christopher; Schwieger, Wilhelm (Chemical Communications, 2015)
The formation of layer-like FAU-type zeolites was facilitated in the absence of any organic template. Instead, the addition of simple inorganic salts turned out to be an effective and easy to handle alternative to organic additives to induce morphological and even structural changes during zeolite crystallisation.

Organic linkers on oxide surfaces: Adsorption and chemical bonding of phthalic anhydride on MgO(100) - Mohr, Susanne; Döpper, Tibor; Xu, Tao; Tariq, Quratulain; Lytken, Ole; Laurin, Mathias; Steinrück, Hans-Peter; Görling, Andreas; Libuda, Jörg (Surface Science, 2016)
To elucidate the adsorption behavior and interaction mechanisms of organic linker units on oxide surfaces, we have performed a model study under ultrahigh vacuum (UHV) conditions. We apply infrared reflection absorption spectroscopy (IRAS) in combination with density-functional theory (DFT), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Phthalic anhydride (PM) was deposited at temperatures between 100 and 300 K by physical vapor deposition (PVD) onto an ordered MgO(100) film grown on Ag(100). At 100 K, the first monolayer adsorbs molecularly with the molecular plane aligned parallel to the surface. Subsequent growth of a multilayer film at low temperature also occurs with preferential molecular alignment parallel to the surface. At 240 K, the multilayer desorbs without decomposition. At 300 K, a mixed monolayer of chemically modified ring-opened and intact phthalic anhydride exists on the surface. The chemically modified species binds in a strongly tilted geometry via opening of the anhydride ring to form a bis-carboxylate species. This species additionally stabilizes the coadsorbed molecular PM via intermolecular interactions. Finally, surface defects and hydroxyl groups are found to increase the amount of surface bis-carboxylate at 300 K, whereas the relative amount of coadsorbed molecular PAA decreases. (C) 2015 Elsevier B.V. All rights reserved.
Keywords: Phthalic anhydride; Magnesium oxide; Organic thin films; Infrared reflection absorption spectroscopy; Density-functional theory

Organic reactions in ionic liquids studied by in situ XPS - Niedermaier, Inga; Kolbeck, Claudia; Taccardi, Nicola; Schulz, Peter; Li, Jing; Drewello, Thomas; Wasserscheid, Peter; Steinrück, Hans-Peter; Maier, Florian (ChemPhysChem, 2012)
We demonstrate the application of in situ X-ray photoelectron spectroscopy (XPS) to monitor organic, liquid-phase reactions. By covalently attaching ionic head groups to the reacting organic molecules, their volatility can be reduced such that they withstand ultra high vacuum conditions. The applied method, which is new for the investigation of organic reactions, allows for following the fate of all elements present in the reaction mixtureexcept for hydrogenin a quantitative and oxidation-state-sensitive manner in one experiment. This concept is demonstrated for the alkylation of a tertiary amine attached to an imidazolium or phosphonium moiety by the anion 4-chlorobutylsulfonate ([ClC H SO ] ). In the course of the reaction, the covalently bound chlorine is converted to chloride and the amine to ammonium as reflected by the distinct shifts in the N 1s and Cl 2p binding energies. © 2012 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.

Origin of Granular Capillarity Revealed by Particle-Based Simulations - Fan, Fengxian; Ribeiro Parteli, Eric Josef; Pöschel, Thorsten (Physical Review Letters, 2017)
When a thin tube is dipped into water, the water will ascend to a certain height, against the action of gravity. While this effect, termed capillarity, is well known, recent experiments have shown that agitated granular matter reveals a similar behavior. Namely, when a vertical tube is inserted into a container filled with granular material and is then set into vertical vibration, the particles rise up along the tube. In the present Letter, we investigate the effect of granular capillarity by means of numerical simulations and show that the effect is caused by convection of the granular material in the container. Moreover, we identify two regimes of behavior for the capillary height Hc∞ depending on the tube-to-particle-diameter ratio, D/d. For large D/d, a scaling of Hc∞ with the inverse of the tube diameter, which is reminiscent of liquids, is observed. However, when D/d decreases down to values smaller than a few particle sizes, a uniquely granular behavior is observed where Hc∞ increases linearly with the tube diameter.

Origin of logarithmic resistance correction in graphene - Jobst, Johannes; Weber, Heiko B. (Nature Physics, 2012)
Chen et al.1 recently presented data of a logarithmic low-temperature anomaly in the resistance of graphene flakes and attributed this to Kondo physics. The samples were irradiated beforehand with ions, and the appearance of a logarithmic anomaly was linked to the presence of induced defect states with unpaired electrons. Here, we present evidence that this assignment to Kondo physics is wrong. The observed effect is due to electron–electron interaction (EEI)2, which in the diffusive regime also gives a logarithmic correction to the resistance (in two dimensions). It is an effect that is based on very general assumptions, and that has been observed in many two-dimensional materials in excellent agreement with theory. It has been pinpointed in graphene recently3.

Origins of barchan dune asymmetry: Insights from numerical simulations - Ribeiro Parteli, Eric Josef; Durán, Orencio; Bourke, Mary C.; Tsoar, Haim; Pöschel, Thorsten; Herrmann, Hans J. (Aeolian Research, 2014)
Barchan dunes - crescent-shaped dunes that form in areas of unidirectional winds and low sand availability - commonly display an asymmetric shape, with one limb extended downwind. Several factors have been identified as potential causes for barchan dune asymmetry on Earth and Mars: asymmetric bimodal wind regime, topography, influx asymmetry and dune collision. However, the dynamics and potential range of barchan morphologies emerging under each specific scenario that leads to dune asymmetry are far from being understood. In the present work, we use dune modeling in order to investigate the formation and evolution of asymmetric barchans. We find that a bimodal wind regime causes limb extension when the divergence angle between primary and secondary winds is larger than 90°, whereas the extended limb evolves into a seif dune if the ratio between secondary and primary transport rates is larger than 25%. Calculations of dune formation on an inclined surface under constant wind direction also lead to barchan asymmetry, however no seif dune is obtained from surface tilting alone. Asymmetric barchans migrating along a tilted surface move laterally, with transverse migration velocity proportional to the slope of the terrain. Limb elongation induced by topography can occur when a barchan crosses a topographic rise. Furthermore, transient asymmetric barchan shapes with extended limb also emerge during collisions between dunes or due to an asymmetric influx. Our findings can be useful for making quantitative inference on local wind regimes or spatial heterogeneities in transport conditions of planetary dune fields hosting asymmetric barchans. © 2013 Elsevier B.V.
Keywords: Barchan dunes; Dune asymmetry; Dune model; Sand transport; Wind erosion

Ortsaufgelöste Simulation des externen Stofftransports in komplexen Katalysatorträgergeometrien - Heidig, Tobias; Zeiser, Thomas; Schwieger, Wilhelm; Freund, Hannsjörg (Chemie Ingenieur Technik, 2014)
The design of tailor-made innovative catalyst support structures enables improved reactor performance by freely tunable fluid flow and mass transport characteristics. Numerical geometry generation, fluid flow simulation and particle tracking for mass transport investigation allow for the required understanding of the structure-transport relationship. Particle tracking, as simulation of the movement of many tracer particles, is costly. This article presents simulation methods including numeric and algorithmic challenges. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: CFD; Particle-Tracking; Stofftransport; Strukturierte Reaktoren; CFD; Mass transport; Particle tracking; Structured reactors

Overcoming Electrode-Induced Losses in Organic Solar Cells by Tailoring a Quasi-Ohmic Contact to Fullerenes via Solution-Processed Alkali Hydroxide Layers - Zhang, Hong; Shallcross, R. Clayton; Li, Ning; Stubhan, Tobias; Hou, Yi; Chen, Wei; Ameri, Tayebeh; Turbiez, Mathieu; Armstrong, Neal R.; Brabec, Christoph (Advanced Energy Materials, 2016)
It is shown that the performance of inverted organic solar cells can be significantly improved by facilitating the formation of a quasi-ohmic contact via solution-processed alkali hydroxide (AOH) interlayers on top of n-type metal oxide (aluminum zinc oxide, AZO, and zinc oxide, ZnO) layers. AOHs significantly reduce the work function of metal oxides, and are further proven to effectively passivate defect states in these metal oxides. The interfacial energetics of these electron collecting contacts with a prototypical electron acceptor (C) are investigated to reveal the presence of a large interface dipole and a new interface state between the Fermi energy and the C highest occupied molecular orbital for AOH-modified AZO contacts. These novel interfacial gap states are a result of ground-state electron transfer from the metal hydroxide-functionalized AZO contact to the adsorbed molecules, which are hypothesized to be electronically hybridized with the contact. These interface states tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer. Furthermore, the strong "light-soaking" effect is no longer observed in devices with a AOH interface. Solution-processed alkali hydroxides significantly reduce the work function of metal oxides, such as zinc oxide or aluminum zinc oxide (AZO), and are further proven to effectively passivate defect states in these metal oxides. The interface states with alkali hydroxide-modified AZO contacts tail all the way to the Fermi energy, providing for a highly n-doped (metal-like) interfacial molecular layer.
Keywords: alkali hydroxide; interfacial layers; organic solar cells; solution processing

Overcoming interface losses in organic solar cells by applying low temperature, solution processed aluminum-doped zinc oxide electron extraction layers - Stubhan, Tobias; Litzov, Ivan; Li, Ning; Salinas Batallas, Michael; Steidl, Matthias; Sauer, Gerhard; Forberich, Karen; Matt, Gebhard; Halik, Marcus; Brabec, Christoph (Journal of Materials Chemistry A, 2013)
Intrinsic zinc oxide (ZnO) is widely used as an electron extraction layer (EEL) for inverted polymer solar cells. Despite the excellent device performance, a major drawback for large area production is its low conductivity. Using microscopic simulations, we derived a technically reasonable threshold value of 10−3 S cm−1 for the conductivity required to overcome transport limitations. For conductivity values typical for ZnO we observed the interface layer thickness restriction at only a few tens of nanometers, either as a fill factor drop due to serial resistance, eventually accompanied by a second diode behavior, or by the need for light soaking. Higher conductive aluminum-doped zinc oxide (AZO), which was introduced earlier, meets the desired conductivity threshold, however, at the cost of high temperature processing. High annealing temperatures (>150 °C) significantly improve the electrical properties of ZnO, but prohibit processing on plastic substrates or organic active layers. Here we report on AZO layers from a sol–gel precursor, which has been already reported to give sufficiently high conductivities at lower processing temperatures (<150 °C). We investigate the influence of different precursor compositions on the electrical properties of the thin films and their performance in inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Low temperature AZO layers with thicknesses up to 680 nm maintained comparable performance to devices with thin AZO layers.
Keywords: Aluminum-doped zinc oxide; Annealing temperatures; High-temperature processing; Inverted polymer solar cells; Microscopic simulation; Organic active layers; Poly (3-hexylthiophene); Processing temperature Engineering controlled terms: Aluminum; Electric properties; Extraction; Processing; Sol-gels; Solar cells; Temperature Engineering main heading: Zinc oxide

Overcoming Interfacial Losses in Solution-Processed Organic Multi-Junction Solar Cells - Du, Xiaoyan; Lytken, Ole; Killian, Manuela; Cao, Jiamin; Stubhan, Tobias; Turbiez, Mathieu; Schmuki, Patrik; Steinrück, Hans-Peter; Ding, Liming; Fink, Rainer; Li, Ning; Brabec, Christoph (Advanced Energy Materials, 2016)
Organic solar cells are promising in terms of full-solution-processing which enables low-cost and large-scale fabrication. While single-junction solar cells have seen a boost in power conversion efficiency (PCE), multi-junction solar cells are promising to further enhance the PCE. In all-solution-processed multi-junction solar cells, interfacial losses are often encountered between hole-transporting layer (HTL) and the active layers and therefore greatly limit the application of newly developed high-performance donor and acceptor materials in multi-junction solar cells. Here, the authors report on a systematic study of interface losses in both single-junction and multi-junction solar cells based on representative polymer donors and HTLs using electron spectroscopy and time-of-flight secondary ion mass spectrometry. It is found that a facile mixed HTL containing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and MoO nanoparticles successfully overcomes the interfacial losses in both single- and multi-junction solar cells based on various active layers by reducing interface protonation, promoting better energy-level alignment, and forming a dense and smooth layer. Solution-processed single-junction solar cells are demonstrated to reach the same performance as with evaporated MoO (over 7%). Multi-junction solar cells with polymers containing nitrogen atoms as the first layer and the mixed PEDOT:PSS and MoO nanoparticles as hole extraction layer reach fill factor (FF) of over 60%, and PCE of over 8%, while the identical stack with pristine PEDOT:PSS or MoO nanoparticles show FF smaller than 50% and PCE less than 5%.
Keywords: Interfacial losses; Organic multi-junction solar cells; Organic photovoltaics; Recombination layer; Solution-processing

Overcoming the Interface Losses in Planar Heterojunction Perovskite-Based Solar Cells - Hou, Yi; Chen, Wei; Baran, Derya; Stubhan, Tobias; Luechinger, Norman A.; Hartmeier, Benjamin; Richter, Moses; Min, Jie; Chen, Shi; Ramírez Quiroz, César Omar; Li, Ning; Zhang, Hong; et al. (Advanced Materials, 2016)
An experiment was conducted to demonstrate that low-temperature processed NiO-based nanocrystal ink (LT-NiO) has the potential to form an almost loss-free hole selective interface for flat heterojunction perovskite-based solar cells. First, The patterned ITO substrates were ultrasonic cleaned with acetone and isopropanol for 10 min each. On cleaned ITO substrate, a dense and smooth layer of LT-NiO was deposited by spin coating and followed by annealing at 70?230°C for 10 min in air to remove organic components. The as-prepared perovskite precursor solution was filtered using 0.45 μm PTFE syringe filter and coated onto the ITO/LT-NiO substrate at a speed of 4000 r.p.m. for 35 s. J-V characteristics of all the devices were measured using a source measurement unit from BoTest. FTPS:FTPS-EQE measurements were carried out using a Vertex 70 from Brucker optics, equipped with QTH lamp, quartz beam splitter and external detector option. A major improvement in open circuit voltage is found by replacing PEDOT:PSS with LT-NiO. A detailed analysis reveals that LT-NiO significantly reduces non-radiative recombination at the PEDOT:PSS/perovskite interface and further enhances the radiative LED efficiency towards unity which brings open circuit voltage closer to the radiative limit.
Keywords: low temperature processing; nickel oxide; non-radiative voltage loss; perovskite solar cells

Overcoming the Thermal Instability of Efficient Polymer Solar Cells by Employing Novel Fullerene-Based Acceptors - Zhang, Chaohong; Mumyatov, Alexander; Langner, Stefan; Perea Ospina, Jose Dario; Kassar, Thaer; Min, Jie; Ke, Lili; Chen, Haiwei; Gerasimov, Kirill L.; Anokhin, Denis V.; Ivanov, Dimitri A.; Ameri, Tayebeh; et al. (Advanced Energy Materials, 2017)
Solution-processed organic solar cells with promising photovoltaic performance and extraordinary high thermal stability are achieved by employing novel fullerene-based acceptors in combination with two state-of-the-art polymer donors. The findings demonstrated in this work underline the necessity and importance of novel acceptor design rules for highly efficient organic solar cells with excellent device stability.
Keywords: Miscibility; Morphological analysis; Novel fullerene-based acceptors; Organic photovoltaics; Thermal stability

Overpotentials and Faraday Efficiencies in CO2 Electrocatalysis-the Impact of 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate - Neubauer, Sebastian; Krause, Ralf K.; Schmid, Bernhard; Guldi, Dirk Michael; Schmid, Guenter (Advanced Energy Materials, 2016)
The mixtures of room temperature ionic liquid 1-ethyl-3-methylimidazolium trifl uoromethanesulfonate ([EMIM]TFO) and water as electrolytes for reduction of CO2 to CO are reported. Linear sweep voltammetry shows overpotentials for CO2 reduction and the competing hydrogen evolution reaction (HER), both of which vary as a function of [EMIM]TFO concentration in the range from 4 × 10-3 M (0.006 mol%) to 4869 × 10-3 M (50 mol%). A steady lowering of overpotentials up to an optimum for 334 × 10-3 M is identifi ed. At 20 mol% and more of [EMIM]TFO, a signifi cant CO2 reduction plateau and inhibition of HER, which is limited by H2 O diffusion, is noted. Such a plateau in CO2 reduction correlates to high CO Faraday effi ciencies. In case of 50 mol% [EMIM]TFO, a broad plateau spanning over a potential range of 0.58 V evolves. At the same time, the overpotential for HER is increased by 1.20 V when compared to 334 × 10-3 M and, in turn, HER is largely inhibited. The Faraday effi ciencies for CO and H2 formation feature 95.6% ± 6.8% and 0.5% ± 0.3%, respectively, over a period of 3 h in a separator divided cell. Cathodic as well as anodic electrochemical stability of the electrolyte throughout this time period is corroborated in 1 H NMR spectroscopic measurements.
EAM Import::2019-03-08

Oxidation of stepped Pt(111) studied by x-ray photoelectron spectroscopy and density functional theory - Bandlow, Jochen; Kaghazchi, Payam; Jacob, Timo; Papp, Christian; Tränkenschuh, Barbara; Streber, Regine; Lorenz, Michael Peter Andreas; Fuhrmann, Thomas; Denecke, R.; Steinrück, Hans-Peter (Physical Review B, 2011)
In this comparative density functional theory and x-ray photoelectron spectroscopy study on the interaction of oxygen with stepped Pt(111) surfaces, we show that both the initial adsorption and oxidation occur at the steps rather than terraces. An equivalent behavior was observed for the oxide formation at higher chemical potentials, where, after the formation of a one-dimensional PtO-type oxide at the steps, similar oxide chains form on the (111) terraces, indicating the initial stages of bulk oxide formation. © 2011 American Physical Society.

Oxidative Depolymerization of Lignin in Ionic Liquids - Hildebrandt-Stärk, Kerstin; Taccardi, Nicola; Bösmann, Andreas; Wasserscheid, Peter (Chemsuschem, 2010)
Beech lignin was oxidatively cleaved in ionic liquids to give phenols, unsaturated propylaromatics, and aromatic aldehydes. A multiparallel batch reactor system was used to screen different ionic liquids and metal catalysts. Mn(NO_3)_2 in 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM][CF_3SO_3] proved to be the most effective reaction system. A larger scale batch reaction with this system in a 300mL autoclave (11g lignin starting material) resulted in a maximum conversion of 66.3 % (24h at 100 °C, 84×105Pa air). By adjusting the reaction conditions and catalyst loading, the selectivity of the process could be shifted from syringaldehyde as the predominant product to 2,6-dimethoxy-1,4-benzoquinone (DMBQ). Surprisingly, the latter could be isolated as a pure substance in 11.5 wt % overall yield by a simple extraction/crystallization process.

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles - Lykhach, Yaroslava; Bruix, Albert; Fabris, Stefano; Potin, Valerie; Matolinova, Iva; Matolin, Vladimir; Libuda, Jörg; Neyman, Konstantin M. (Catalysis: Science and Technology, 2017)
The concept of single atom catalysis offers maximum noble metal efficiency for the development of low-cost catalytic materials. Among possible applications are catalytic materials for proton exchange membrane fuel cells. In the present review, recent efforts towards the fabrication of single atom catalysts on nanostructured ceria and their reactivity are discussed in the prospect of their employment as anode catalysts. The remarkable performance and the durability of the ceria-based anode catalysts with ultra-low Pt loading result from the interplay between two states associated with supported atomically dispersed Pt and sub-nanometer Pt particles. The occurrence of these two states is a consequence of strong interactions between Pt and nanostructured ceria that yield atomically dispersed species under oxidizing conditions and sub-nanometer Pt particles under reducing conditions. The square-planar arrangement of four O atoms on 100 nanoterraces has been identified as the key structural element on the surface of the nanostructured ceria where Pt is anchored in the form of Pt2+ species. The conversion of Pt2+ species to sub-nanometer Pt particles is triggered by a redox process involving Ce3+ centers. The latter emerge due to either oxygen vacancies or adsorption of reducing agents. The unique properties of the sub-nanometer Pt particles arise from metal-support interactions involving charge transfer, structural flexibility, and spillover phenomena. The abundance of specific adsorption sites similar to those on 100 nanoterraces determines the ideal (maximum) Pt loading in Pt-CeOx films that still allows reversible switching between the atomically dispersed Pt and sub-nanometer particles yielding high activity and durability during fuel cell operation.

Oxide Nanotubes on Ti-Ru Alloys: Strongly Enhanced and Stable Photoelectrochemical Activity for Water Splitting - Roy, Poulomi; Das, Chittaranjan; Lee, Kiyoung; Hahn, Robert; Ruff, Tobias; Moll, Matthias; Schmuki, Patrik (Journal of the American Chemical Society, 2011)
The present work shows a significant enhancement of the photoelectrochemical water-splitting performance of anodic TiO2 nanotube layers grown on low concentration (0.01-0.2 at% Ru) Ti-Ru alloys. Under optimized preparation conditions (0.05 at% Ru, 450 °C annealing) the water splitting rate of the oxide tubes could be 6-fold increased. Moreover, the beneficial effect is very stable with illumination time; this is in contrast to other typical doping approaches of TiO2.
EAM Import::2019-03-08

Painting by numbers: Nanoparticle-based colorants in the post-empirical age - Klupp Taylor, Robin; Seifrt, Frantiek; Zhuromskyy, Oleksandr; Peschel, Ulf; Leugering, Günter; Peukert, Wolfgang (Advanced Materials, 2011)
The visual appearance of the artificial world is largely governed by films or composites containing particles with at least one dimension smaller than a micron. Over the past century and a half, the optical properties of such materials have been scrutinized and a broad range of colorant products, based mostly on empirical microstructural improvements, developed. With the advent of advanced synthetic approaches capable of tailoring particle shape, size and composition on the nanoscale, the question of what is the optimum particle for a certain optical property can no longer be answered solely by experimentation. Instead, new and improved computational approaches are required to invert the structure-function relationship. This progress report reviews the development in our understanding of this relationship and indicates recent examples of how theoretical design is taking an ever increasingly important role in the search for enhanced or multifunctional colorants. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Keywords: colloids; core/shell nanoparticles; designed nanostructures; functional coatings; structure-property relationships

Pairing fullerenes and porphyrins: Supramolecular wires that exhibit charge transfer activity - Wessendorf, Florian; Grimm, Bruno; Guldi, Dirk Michael; Hirsch, Andreas (Journal of the American Chemical Society, 2010)
A concept is elaborated of pairing electron donors and electron acceptors that share a common trait, wire-like features, as a powerful means to realize a new and versatile class of electron donor-acceptor nanohybrids. Important variables are fine-tuning (i) the complexation strength, (ii) the electron/energy transfer behavior, and (iii) the solubilities of the resulting architectures. In particular, a series of supramolecular porphyrin/fullerene hybrids assembled by the hydrogen bonding of Hamilton receptor/cyanuric acid motif has been realized. Putting the aforementioned variables into action, the association constants (K), as they were determined from H NMR and steady-state fluorescence assays, were successfully tweaked with values in the range of 10-10 M . In fact, our detailed studies corroborate that the latter reveal a dependence on the nature of the spacer, that is, p-phenylene-ethynylene, p-phenylene-vinylene, p-ethynylene, and fluorene, as well as on the length of the spacer. Complementary performed transient absorption studies confirm that electron transfer is indeed the modus operandi in our novel class of electron donor-acceptor nanohybrids, while energy transfer plays, if any, only a minor role. The accordingly formed electron transfer products, that is, one-electron oxidized porphyrins and one-electron reduced fullerenes, are long-lived with lifetimes that reach well into the time domain of tens of nanoseconds. Finally, we have used the distance dependence on electron transfer, charge separation and charge recombination, to determine for the first time a ß value (0.11) for hydrogen-bonding-mediated electron transfer. © 2010 American Chemical Society.

Palladium Activated Decoration of TiO2 Nanotubes by Copper Nanoparticles and Enhanced Photocatalytic Properties - Liu, Ning; Jha, Himendra; Hahn, Robert; Schmuki, Patrik (ECS Electrochemistry Letters, 2012)
The present work describes how a simple two step activation/deposition process can be used to uniformly decorate TiO2 nanotube layers with suitable nanoparticles. In a first step Pd-activation is used to create initiation sites for a subsequent chemical deposition, in our example, Cu. We show that the procedure for two different nanotube types, when optimized, can yield very homogeneous particle decoration of the tube walls down to the bottom of the nanotubes. In the present case deposition and air exposure lead to mixed Cu/Cu-oxide decoration. We illustrate that this combination can be used for effective visible light photocatalysis.
EAM Import::2019-03-08

Palladium-Mediated Ethylation of the Imidazolium Cation Monitored In Operando on a Solid Catalyst with Ionic Liquid Layer - Bauer, Tanja; Hager, Veit; Williams, Marc; Laurin, Mathias; Döpper, Tibor; Görling, Andreas; Szesni, Normen; Wasserscheid, Peter; Haumann, Marco; Libuda, Jörg (ChemCatChem, 2017)
The ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtOSO] was supported on Pd-AlO to yield a solid catalyst with ionic liquid layer (SCILL) material. During the hydrogenation of ethene, it was found that ethene was incorporated into the imidazolium moiety in a reaction catalyzed by the palladium surface. The formation of 1,2-diethyl-3-methylimidazolium ethylsulfate was confirmed ex situ by means of NMR spectroscopy. Moreover, the reaction itself could be monitored in situ by diffuse reflection infrared Fourier transformation (DRIFT) spectroscopy. A reaction mechanism involving surface carbene species was proposed on the basis of the analytical data.
Keywords: carbenes; heterogeneous catalysis; ionic liquids; palladium; reaction mechanisms

Panchromatic ternary/quaternary polymer/fullerene BHJ solar cells based on novel silicon naphthalocyanine and silicon phthalocyanine dye sensitizers - Ke, Lili; Gasparini, Nicola; Min, Jie; Zhang, Hong; Adam, Matthias; Rechberger, Stefanie; Forberich, Karen; Zhang, Chaohong; Spiecker, Erdmann; Tykwinski, Rik; Brabec, Christoph; Ameri, Tayebeh (Journal of Materials Chemistry A, 2017)
More efficient light harvesting throughout the whole solar spectrum by introducing third and fourth components offers a new pathway towards the development of high efficiency organic solar cells based on polymer/fullerene blends. Recently, dye molecules have been utilized as promising light harvesting photosensitizers in the near-IR region. Herein, we report the design, synthesis and application of a novel silicon naphthalocyanine (SiNC-1) as an efficient photosensitizer in single dye ternary devices as well as in multi-colored co-sensitized quaternary devices, incorporating a silicon phthalocyanine (SiPC-0 or SiPC-1) as the fourth component to complement the spectral absorption of the SiNC-1. The dominant complex charge transfer/transport mechanism behind the enhanced photosensitivity of the ternary blend has been investigated by means of electrical, optical, and advanced characterization techniques. External quantum efficiency (EQE) measurements on multi-colored dye sensitized devices covering the UV-vis as well as near-IR regions from 350 up to 900 nm outline apparent signal characteristics of each single dye, corroborating the effective contribution of both SiPC and SiNC dyes to enhancing the short-circuit current density (J). Our results further illustrate the potential of the multi-colored dye sensitization concept as a powerful approach to mitigate the non-ideal optical absorption normally encountered in organic-based optoelectronic devices.
Keywords: Engineering controlled terms: Charge transfer; Dye-sensitized solar cells; Efficiency; Electromagnetic wave absorption; Light absorption; Nitrogen compounds; Optoelectronic devices; Organic solar cells; Photosensitizers; Polymer blends; Polymer solar cells; Silicon; Solar cells Characterization techniques; Dye sensitization; External quantum efficiency; Light-harvesting; Naphthalocyanines; Phthalocyanine dyes; Signal characteristic; Spectral absorptions Engineering main heading: Silicon solar cells

Parallel Simulations of Self-propelled Microorganisms - Pickl, Kristina; Hofmann, Matthias; Preclik, Tobias; Köstler, Harald; Smith, Ana-Suncana; Rüde, Ulrich (Parallel Computing: Accelerating Computational Science and Engineering (CSE), 2014)
The scalability of a parallel simulation of self-propelled microorganisms in a fluid in the regime of low Reynolds numbers is investigated on current HPC clusters. For this, the lattice Boltzmann solver from the WALBERLA framework is coupled with the rigid body physics engine pe. The latter is enhanced with spring-like pair-wise interactions that can not only span over neighboring process domains but as well act as long-range pair-wise interactions between distant process domains. The communication is designed in such a way that only neighboring processes and the processes hosting the objects attached to the springs communicate. The suitability and efficiency of this approach is analyzed by a weak scaling of a representative setup for a swarm simulation on two supercomputers currently in the top ten of the TOP500 list, namely JUQUEEN and SUPERMUC. © 2014 The authors and IOS Press.
UnivIS-Import:2015-04-20:Pub.2014.tech.IMMD.lsinfs.parall_9
Keywords: computational fluid dynamics; MPI parallel simulation; particulate flow simulation; self-propelled microorganism

Parity-time synthetic photonic lattices - Regensburger, Alois; Bersch, Christoph; Miri, Mohammad-Ali; Onishchukov, Georgy; Christodoulides, Demetrios N.; Peschel, Ulf (Nature, 2012)
The development of new artificial structures and materials is today one of the major research challenges in optics. In most studies so far, the design of such structures has been based on the judicious manipulation of their refractive index properties. Recently, the prospect of simultaneously using gain and loss was suggested as a new way of achieving optical behaviour that is at present unattainable with standard arrangements. What facilitated these quests is the recently developed notion of `parity–time symmetry' in optical systems, which allows a controlled interplay between gain and loss. Here we report the experimental observation of light transport in large-scale temporal lattices that are parity–time symmetric. In addition, we demonstrate that periodic structures respecting this symmetry can act as unidirectional invisible media when operated near their exceptional points. Our experimental results represent a step in the application of concepts from parity–time symmetry to a new generation of multifunctional optical devices and networks.
EAM Import::2019-03-08

Particle based alloying by accumulative roll bonding in the system Al-Cu - Schmidt, Christian; et al. (Metals, 2011)
The formation of alloys by particle reinforcement during accumulative roll bonding (ARB), and subsequent annealing, is introduced on the basis of the binary alloy system Al-Cu, where strength and electrical conductivity are examined in different microstructural states. An ultimate tensile strength (UTS) of 430 MPa for Al with 1.4 vol.% Cu was reached after three ARB cycles, which almost equals UTS of the commercially available Al-Cu alloy AA2017A with a similar copper content. Regarding electrical conductivity, the UFG structure had no significant influence. Alloying of aluminum with copper leads to a linear decrease in conductivity of 0.78 µΩ·cm/at.% following the Nordheim rule. On the copper-rich side, alloying with aluminum leads to a slight strengthening, but drastically reduces conductivity. A linear decrease of electrical conductivity of 1.19 µΩ·cm/at.% was obtained.
WW1::895

Particle-based simulation of powder application in additive manufacturing - Ribeiro Parteli, Eric Josef; Pöschel, Thorsten (Powder Technology, 2016)
The development of reliable strategies to optimize part production in additive manufacturing technologies hinges, to a large extent, on the quantitative understanding of the mechanical behavior of the powder particles during the application process. Since it is difficult to acquire this understanding based on experiments alone, a particle-based numerical tool for the simulation of powder application is required. In the present work, we develop such a numerical tool and apply it to investigate the characteristics of the powder layer deposited onto the part using a roller as the coating system. In our simulations, the complex geometric shapes of the powder particles are taken explicitly into account. Our results show that increasing the coating speed leads to an increase in the surface roughness of the powder bed, which is known to affect part quality. We also find that, surprisingly, powders with broader size distributions may lead to larger values of surface roughness as the smallest particles are most prone to form large agglomerates thus increasing the packing's porosity. Moreover, we find that the load on the part may vary over an order of magnitude during the coating process owing to the strong inhomogeneity of inter-particle forces in the granular packing. Our numerical tool can be used to assist - and partially replace - experimental investigations of the flowability and packing behavior of different powder systems as a function of material and process parameters.
Keywords: Additive manufacturing; Discrete element method; Numerical simulation

Particle-Size-Dependent Interaction of NO2 with Pd Nanoparticles Supported on Model NOx Storage Materials - Desikusumastuti, Aine; Happel, Markus; Qin, Zhihui; Staudt, Thorsten; Lykhach, Yaroslava; Laurin, Mathias; Shaikhutdinov, S.; Rohr, F.; Libuda, Jörg (Journal of Physical Chemistry C, 2009)
Combining scanning tunneling microscopy, molecular beam methods, and time-resolved infrared reflection absorption spectroscopy, we investigate the structure and reactivity of Pd nanoparticles supported on single-crystal-based model NOx storage materials. The latter consist of barium aluminate-like nanoparticles supported on an Al2O3 film on a NiAl(110) substrate. On these surfaces, Pd deposition under ultra-high-vacuum conditions gives rise to the growth of three-dimensional Pd particles, nucleating at the predeposited barium aluminate aggregates. The reactivity of these systems toward NO2 is tested systematically as a function of NO2 exposure and Pd particle size. At room temperature, NOx generated by dissociative adsorption of NO2, sequentially covers the following sites on the Pd nanoparticles: (i) hollow sites on (111) facets, (ii) bridge sites at particles edges and particle defects, and, finally, (iii) on-top sites at particle corners. The occupation of different sites on the particles is monitored as a function of NO2 exposure and Pd particle size. Characteristic differences in the site occupation and the coverage dependence are observed as a function of particle size. At elevated NO2 exposures, all NO-related features disappear, indicating the onset of oxidation of the Pd particles. For the Pd-containing systems, several new vibrational bands are observed. These new features are assigned to surface nitrates adsorbed on oxidized Pd particles and, tentatively, to surface nitrates adsorbed on sites that arise from the interaction between barium aluminate and oxidized Pd particles. In the limit of high NO2 exposure, these new surface nitrates coexist with surface nitrates formed on the uncovered fraction of the barium aluminate nanoparticles.

Particle Tracking in Fluidized Beds with Secondary Gas Injection - Hensler, Timo; Tupy, Martin; Strer, Timo; Pöschel, Thorsten; Wirth, Karl-Ernst (Procedia Engineering, 2015)
Fluidized beds with secondary gas injection enjoy great popularity in process industry. Owing to their characteristic properties such as intense mixing of solids, excellent mass and heat transfer conditions as well as easy handling of solids, this type of apparatus is applied in various fields of process engineering nowadays. In the past decades research concerning fluidized beds with secondary gas injection has focused on understanding how solid particles and the injected gas are distributed within the apparatus. With the aid of invasive measurement techniques the region surrounding the injector nozzle was investigated with respect to the penetration depth of the gas jet above the nozzle orifice as well as the jet opening angle. A major drawback of the previously used measurement techniques consists in their invasive nature. Penetration of the injection zone by a probe can severely influence the local flow pattern and consequently has a detrimental effect on the reliability of the measured data. Therefore in the presented work for the first time the solids distribution as well as the motion of a single particle in a fluidized bed with secondary gas injection has been investigated by positron emission particle tracking (PEPT). This non-invasive technique is based on labeling one single particle, randomly selected from the bulk, radioactively, which allows for tracking its motion with high temporal and spatial resolution. The obtained data are compared with results derived from invasive measurements. Moreover PEPT-data have been used to perform investigations on the residence time behavior of particles within the jet region and the suspended phase. It could be found that the combination of invasive measurements and PEPT provide valuable information for the design and optimization of fluidized bed reactors with a well-defined injection zone.
EAM Import::2019-03-08

Pattern-fluid interpretation of chemical turbulence - Scholz, Christian; Schröder-Turk, Gerd; Mecke, Klaus (Physical Review E, 2015)
The spontaneous formation of heterogeneous patterns is a hallmark of many nonlinear systems, from biological tissue to evolutionary population dynamics. The standard model for pattern formation in general, and for Turing patterns in chemical reaction-diffusion systems in particular, are deterministic nonlinear partial differential equations where an unstable homogeneous solution gives way to a stable heterogeneous pattern. However, these models fail to fully explain the experimental observation of turbulent patterns with spatio-temporal disorder in chemical systems. Here we introduce a pattern-fluid model as a general concept where turbulence is interpreted as a weakly interacting ensemble obtained by random superposition of stationary solutions to the underlying reaction-diffusion system. The transition from turbulent to stationary patterns is then interpreted as a condensation phenomenon, where the nonlinearity forces one single mode to dominate the ensemble. This model leads to better reproduction of the experimental concentration profiles for the "stationary phases" and reproduces the turbulent chemical patterns observed by Q. Ouyang and H. L. Swinney [Chaos 1, 411 (1991)].

Pattern formation in a horizontally shaken granular submonolayer - Krengel, Dominik; Strobl, Severin; Sack, Achim; Heckel, Michael; Pöschel, Thorsten (Granular Matter, 2013)
We study the mechanism leading to the formation of stripe-like patterns in a rectangular container filled with a sub-monolayer of frictional spherical particles when it is subjected to horizontal oscillations. By means of Molecular Dynamics simulations we could reproduce the experimental results. Systematic simulations allow to identify friction to be responsible for the pattern formation, that is, the tangential interaction between contacting particles and between the particles and the floor of the container. When particles are in contact with the floor and other adjacent particles simultaneously, there emerges a frustrated situation in which the particles are prevented from rolling on the floor. This effect leads to local jamming and eventually to stripe-like pattern formation. In the long time evolution, the stripes are unstable. Stripes may merge as well as disintegrate. © 2013 Springer-Verlag Berlin Heidelberg.
Keywords: Frustration effects; Horizontal shaking; Pattern formation

Patterning of organic photovoltaic modules by ultrafast laser - Kubis, Peter; Li, Ning; Stubhan, Tobias; Machui, Florian; Matt, Gebhard; Voigt, Monika; Brabec, Christoph (Progress in Photovoltaics: Research and Applications, 2015)
In this paper, we demonstrate that laser patterning of organic solar cells by ultrafast laser systems (pulse length <350 fs) is an attractive process to produce photovoltaic modules with outstanding high geometric fill factors. Moreover, in terms of precision, registration, and debris generation and in terms of keeping the damage to the underneath layers at a minimum, ultrafast laser patterning with a pulse length of few hundreds of femtoseconds turns out to yield superior results. Ablation of all three different solar cell layers (electrodes (P1 and P3) and interfaces and semiconductor (P2)) is achieved with a single wavelength simply by a precise adjustment of the laser fluence and the patterning overlap. Camera positioning allows a precise registration between the various processing steps and a reduction of the width of the overall interconnection regime to the hundreds of micrometers dimension, resulting in high geometrical fill factors of over 90% for monolithically interconnected organic solar cell modules.
Keywords: Femtosecond pulses; Laser patterning; Organic solar cells; PV module

PDE-constrained optimization for advanced materials - Leugering, Günter; Stingl, Michael (GAMM-Mitteilungen, 2010)
In these notes we consider PDE-constrained optimization in the context of advanced materials. We give examples for optimization and control in the coefficients, free material optimization, topology optimization, shape optimization in elastic and piezo-electric materials. We explain some approaches to metamaterials, in particular for auxetic materials
Keywords: piezoelectricity; electromechanical interaction; shape sensitivity analysis

Pd-Ga model SCALMS: Characterization and stability of Pd single atom sites - Kettner, Miroslav; Maisel, Sven; Stumm, Corinna; Schwarz, Matthias; Schuschke, Christian; Görling, Andreas; Libuda, Jörg (Journal of Catalysis, 2019)
We have prepared model systems for Pd-Ga supported catalytically active liquid metal solutions (SCALMS) by physical vapor deposition of Pd and Ga in ultrahigh vacuum onto highly oriented pyrolytic graphite (HOPG) pre-modified by Ar+ bombardment. Combining AFM and IRAS with CO as a probe molecule, we investigate the growth behavior, the morphology, the surface chemistry, and the stability of Pd-Ga alloys on HOPG. Our results prove the single atom character of the active sites in Ga-rich Pd-Ga alloys. We show that upon interaction with CO, Ga is displaced from the surface and CO adsorbs in on-top position on surface Pd atoms. Experimental data on the vibrational frequencies and binding energies of CO are compared to the results of DFT calculations for CO adsorption on four different Pd-Ga models, i.e. PdGa(1 0 0), PdGa( 1 1 1), Pd-doped extgreeka-Ga(0 0 1), and Pd-doped fcc-Ga(1 0 0). The comparison suggests that widely separated Pd sites are formed in the Ga matrix.
EAM Import::2019-03-12

Pd model catalysts on clean and modified HOPG: Growth, adsorption properties, and stability - Kettner, Miroslav; Stumm, Corinna; Schwarz, Matthias; Schuschke, Christian; Libuda, Jörg (Surface Science, 2019)
Carbon is a common support material for noble metal nanoparticles in heterogeneous catalysis and electro-catalysis. Very few surface-science-based model studies have been performed, however, with noble metal nanoparticles on carbon supports. In this work, we present the first model study on carbon-supported noble metal nanoparticles using infrared reflection absorption spectroscopy (IRAS). The model catalyst is prepared on highly oriented pyrolytic graphite (HOPG) by physical vapor deposition (PVD) of Pd metal. We characterize the particle growth and morphology by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) and probe the adsorption properties by IRAS of adsorbed CO.PVD of Pd on clean HOPG leads to the formation of very large, weakly interacting particles. In order to increase the nucleation density upon Pd deposition, the HOPG surface was modified by Ar+ and O+ bombardment. The defect structures formed were characterized by STM and by IRAS during the ion bombardment. On the pretreated HOPG, Pd nucleates homogeneously and stable Pd nanoparticles (NPs) are formed. We investigated the adsorption properties of the Pd NPs as a function of the particle size and their thermal stability upon annealing. It is shown that carbon species migrate from the HOPG modified by ion bombardment onto the Pd NPs leading to modification of the CO adsorption properties.The work shows that Pd/HOPG is a very well-suited model system for in-situ adsorption and reaction studies by IRAS, in spite of the non-metallicity of the support.
Keywords: Infrared reflection absorption spectroscopy (IRAS); Highly oriented pyrolytic graphite (HOPG); Scanning tunneling microscopy (STM); Palladium; Carbon monoxide; Surface modification

Pd nanoparticle formation in ionic liquid thin films monitored by in situ vibrational spectroscopy - Mehl, Sascha; Toghan, Arafat; Bauer, Tanja; Brummel, Olaf; Taccardi, Nicola; Wasserscheid, Peter; Libuda, Jörg (Langmuir, 2015)
Ionic liquids (ILs) are flexible reaction media and solvents for the synthesis of metal nanoparticles (NPs). Here, we describe a new preparation method for metallic NPs in nanometer thick films of ultraclean ILs in an ultrahigh vacuum (UHV) environment. CO-covered Pd NPs are formed by simultaneous and by sequential physical vapor deposition (PVD) of the IL and the metal in the presence of low partial pressures of CO. The film thickness and the particle size can be controlled by the deposition parameters. We followed the formation of the NPs and their thermal behavior by time-resolved IR reflection absorption spectroscopy (TP-IRAS) and by temperature-programmed IRAS (TR-IRAS). Codeposition of Pd and [CCIm][OTf] in CO at 100 K leads to the growth of homogeneous multilayer films of CO-covered Pd aggregates in an IL matrix. The size of these NPs can be controlled by the metal fraction in the co-deposit. With increasing metal fraction, the size of the Pd NPs also increases. At very low metal content, small Pd carbonyl-like species are formed, which bind CO in on-top geometry only. Upon annealing, the [OTf] anion coadsorbs at the NP surface and partially displaces CO. Co-adsorption of CO and IL is indicated by a strong red-shift of the CO stretching bands. While the weakly bound on-top CO is mainly replaced below the melting transition of the IL, coadsorbate shells with bridge-bonded CO and IL are stable well above the melting point. Larger three-dimensional Pd NPs can be prepared by PVD of Pd onto a solid [CCIm][OTf] film at 100 K. Upon annealing, on-top CO desorbs from these NPs below 200 K. Upon melting of the IL film, the CO-covered Pd NPs immerse into the IL and again form a stable coadsorbate shell that consists of bridge-bonded CO and the IL.

P-doped multiwall carbon nanotube/perylene diimide derivative photoelectrochemical cells for photocurrent generation - Troeger, Anna; Ledendecker, Marc; Margraf, Johannes; Sgobba, Vito; Guldi, Dirk Michael; Vieweg, Benito; Spiecker, Erdmann; Suraru, Sabin-Lucian; Wuerthner, Frank (Advanced Energy Materials, 2012)
A perylene diimide (PDI) derivative bearing four chlorine substituents in the bay area is deposited together with pristine multiwall carbon nanotubes (MWNTs) and/or Nafion p-doped MWNTs (p-MWNTs) onto indium tin oxide (ITO) solid substrates by means of air-brushing. The resulting photoanodes are studied in photoelectrochemical cells and reveal highest photocurrent efficiencies when PDI and p-MWNT are combined as photoactive materials, indicating the beneficial effect of Nafion.

p-Doping of graphene in hybrid materials with 3,10-diazapicenium dications - Roth, Alexandra Marina; Schaub, Tobias; Meinhardt, Ute; Thiel, Dominik; Storch, Jan; Cirkva, Vladimir; Jakubik, Pavel; Guldi, Dirk Michael; Kivala, Milan (Chemical Science, 2017)
N,N′-Didodecyl-substituted 3,10-diazapicenium salts featuring bromide and hexafluorophosphate counterions have been designed as novel dopants to realize individualized graphene sheets in a series of cutting edge experiments and to intrinsically stabilize them via p-doping. Importantly, electrochemical studies revealed two consecutive irreversible one-electron reductions of the N,N′-didodecyl-substituted 3,10-diazapicenium salts to yield the corresponding radical cation and neutral quinoidal species. Formation of both species was accompanied by characteristic changes in the absorption spectra. The 3,10-diazapicenium bromide was found to be a potent dopant to produce hybrid materials with exfoliated graphene. Microscopy based on AFM and TEM imaging and spectroscopy based on Raman probing corroborated that, upon drying, the hybrid material consists of few layer (5–8 layers) turbostratic graphene sheets that are p-doped. Our findings identify the newly synthesized N,N′-dialkylated 3,10-diazapicenium salts as highly promising candidates for the fabrication of functional graphene materials with tailored properties.
EAM Import::2019-03-08

Pentacene Appended to a TEMPO Stable Free Radical: The Effect of Magnetic Exchange Coupling on Photoexcited Pentacene - Chernick, Erin; Casillas, Ruben; Zirzlmeier, Johannes; Gardner, Daniel M.; Gruber, Marco; Kropp, Henning; Meyer, Karsten; Wasielewski, Michael R.; Guldi, Dirk Michael; Tykwinski, Rik (Journal of the American Chemical Society, 2015)
Understanding the fundamental spin dynamics of photoexcited pentacene derivatives is important in order to maximize their potential for optoelectronic applications. Herein, we report on the synthesis of two pentacene derivatives that are functionalized with the [(2,2,6,6-tetramethylpiperidin-1-yl)oxy] (TEMPO) stable free radical. The presence of TEMPO does not quench the pentacene singlet excited state, but does quench the photoexcited triplet excited state as a function of TEMPO-to-pentacene distance. Time-resolved electron paramagnetic resonance experiments confirm that triplet quenching is accompanied by electron spin polarization transfer from the pentacene excited state to the TEMPO doublet state in the weak coupling regime.
EAM Import::2019-03-08

Periodic and chaotic psychological stress variations as predicted by a social support buffered response model - Field, Richard J.; Gallas, Jason; Schuldberg, David (Communications in Nonlinear Science and Numerical Simulation, 2017)
Recent work has introduced social dynamic models of people's stress-related processes, some including amelioration of stress symptoms by support from others. The effects of support may be ``direct'', depending only on the level of support, or ``buffering'', depending on the product of the level of support and level of stress. We focus here on the nonlinear buffering term and use a model involving three variables (and 12 control parameters), including stress as perceived by the individual, physical and psychological symptoms, and currently active social support. This model is quantified by a set of three nonlinear differential equations governing its stationary-state stability, temporal evolution (sometimes oscillatory), and how each variable affects the others. Chaos may appear with periodic forcing of an environmental stress parameter. Here we explore this model carefully as the strength and amplitude of this forcing, and an important psychological parameter relating to self-kindling in the stress response, are varied. Three significant observations are made: 1. There exist many complex but orderly regions of periodicity and chaos, 2. there are nested regions of increasing number of peaks per cycle that may cascade to chaos, and 3. there are areas where more than one state, e.g., a period-2 oscillation and chaos, coexist for the same parameters; which one is reached depends on initial conditions.
EAM Import::2019-03-08

Periodicity hubs and wide spirals in a two-component autonomous electronic circuit - Gallas, Jason; Cabeza, C.; Briozzo, Carlos A.; Garcia, Rodrigo; Freire, J. G.; Marti, Arturo C. (Chaos Solitons & Fractals, 2013)
We report an autonomous circuit containing periodicity hubs with surprisingly broad spirals. Knowledge of broad spirals is important because all presently known spirals are compressed along specific directions in parameter space making them difficult to study experimentally and theoretically. We characterize the performance of the circuit by computing stability diagrams for relevant sections of the control space. In addition, the alternation of chaotic and periodic spiral phases is contrasted with equivalent alternations obtained from an experimental implementation of the circuit.

Periodic open-cell foams: Pressure drop measurements and modeling of an ideal tetrakaidecahedra packing - Inayat, Amer; Schwerdtfeger, Jan; Freund, Hannsjoerg; Körner, Carolin; Singer, Robert; Schwieger, Wilhelm; Freund, Hannsjörg (Chemical Engineering Science, 2011)
The applicability of the new correlation for a large range of porosities was examined by comparing the experimental and simulated friction factors for the porous media with both high (foam structure) and low porosities (packed beds) for a large range of the Reynolds number. It was demonstrated that the correlation can successfully predict the pressure drop of foam structures as well as packed beds. (C) 2011 Elsevier Ltd. All rights reserved.
Keywords: Foam; Ideal structure; Pressure drop; Catalyst support; Tetrakaidecahedra model; SEBM

Periodic open cellular structures (POCS) for intensification of multiphase reactors: Liquid holdup and two-phase pressure drop - Lämmermann, Markus; Horak, Giulia; Schwieger, Wilhelm; Freund, Hannsjörg (Chemical Engineering and Processing, 2018)
Structuring concepts for catalytic reactors provide promising options for process intensification on the process unit level [1,2] as they offer a better control on the occurring transport processes, e.g. on the fluid flow, species distribution, pressure drop and heat transfer. Process intensification aims at the realization of processes that are only limited by reaction kinetics, but not anymore on hydrodynamics [3]. For this reason, in multiphase reactors it is desirable to achieve good liquid distribution, advanced heat transport and low pressure drop, just to mention a few transport characteristics. In this regards, the use of periodic open cellular structures (POCS) is a recent and very promising structuring concept, as packings of these structures offer superior and adjustable properties (e.g. low pressure drop, high surface area, good liquid distribution, advanced heat transfer) [4-8].In this contribution, static and dynamic liquid holdup as well as single- and two-phase pressure drop was measured in POCS. These kinds of structures can be used as catalyst support in packed bed reactors, and also as liquid distributor for trickle-bed applications. The POCS investigated in this study were fabricated with different additive manufacturing methods (fused deposition modeling, stereolithography and selective electron beam melting) to study the influence of different materials (ABS, resin, Ti6Al/4V). For the description of the static liquid holdup, a modified Eotvos correlation, which takes the contact angle of the different materials into account, is proposed. Dynamic liquid holdup and two-phase pressure drop was modelled using an extension of the concept of hydrodynamic tortuosity proposed by Inayat et al. [8], and a modification of the relative permeability model introduced by Saez et al. [9]. Furthermore, equations for the geometric characterization of POCS consisting of different unit cell types, namely Diamond, Kelvin and the recently presented DiaKel hybrid cell [5] were derived. The window diameter of the used unit cells as well as the porosity has a strong influence on the liquid holdup and the pressure drop. By taking the contact angle into account the static liquid holdup can be calculated with good precision (errors smaller than 20%). The concept of geometric tortuosity and the relative permeability model also is applicable to predict the (two-) phase pressure drop and the dynamic liquid holdup in stacked packings of POCS segments of different unit cells and materials. Both, pressure drop and liquid holdup can be predicted a priori (i.e., without further fitting) with good precision using the correlations established in this work solely based on geometric properties of the packing.
Keywords: Two-phase pressure drop; Liquid holdup; Structured catalysts; Relative permeability model; Additive manufacturing

Periodic open cellular structures with ideal cubic cell geometry: Effect of porosity and cell orientation on pressure drop behavior - Klumpp, Michael; Inayat, Amer; Schwerdtfeger, Jan; Körner, Carolin; Singer, Robert; Freund, Hannsjörg; Schwieger, Wilhelm (Chemical Engineering Journal, 2014)
Periodic open cellular structures (POCS) with ideal cubic cell geometry (icPOCS) were manufactured via the selective electron beam melting technique (SEBM). Depending on the cell dimensions (cell length, strut diameter) and cell orientation in respect to the main axis of the cylindrical structures, the icPOCS investigated in this work exhibit different porosities and/or tortuosities. The structures were characterized with respect to their morphological and geometrical parameters such as strut and cell dimension, specific surface area, and porosity using different techniques (e.g., micro-computed tomography and helium pycnometry). Additionally, a model for the calculation of the specific surface area is presented which takes surface roughness into account. The model enables the determination of the specific surface area on the basis of only two geometrical parameters, i.e., the strut diameter and the cell length, which are both easy to determine for such highly regular open cellular structures. The pressure drop of the icPOCS was measured with air as working fluid. A correlation allowing for the prediction of the pressure drop of icPOCS was developed based on the Ergun equation in its basic form. The presented correlation reflects the porosity and cell orientation and is solely based on the geometric properties of the structures. As the correlation does not rely on the use of empirical fitting parameters it is fully predictive. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: Open cellular structure; Pressure drop; Cell orientation; Specific surface area; SEBM

Periodic oscillations of the forced Brusselator - Gallas, Jason (Modern Physics Letters B, 2015)
We study the organization of stability phases in the control parameter space of a periodically driven Brusselator. Specifically, we report high-resolution stability diagrams classifying periodic phases in terms of the number of spikes per period of their regular oscillations. Such diagrams contain accumulations of periodic oscillations with an apparently unbounded growth in the number of their spikes. In addition to the entrainment horns, we investigate the organization of oscillations in the limit of small frequencies and amplitudes of the drive. We find this limit to be free from chaotic oscillations and to display an extended and regular tiling of periodic phases. The Brusselator contains also several features discovered recently in more complex scenarios like, e.g. in lasers and in biochemical reactions, and exhibits properties which are helpful in the generic classification of entrainment in driven systems. Our stability diagrams reveal snippets of how the full classification of oscillations might look like for a wide class of flows.

Permeability of Porous Materials Determined from the Euler Characteristic - Scholz, Christian; Wirner, Frank; Götz, Jan; Rüde, Ulrich; Schröder-Turk, Gerd; Mecke, Klaus; Bechinger, Clemens (Physical Review Letters, 2012)
We study the permeability of quasi-two-dimensional porous structures of randomly placed overlapping monodisperse circular and elliptical grains. Measurements in microfluidic devices and lattice Boltzmann simulations demonstrate that the permeability is determined by the Euler characteristic of the conducting phase. We obtain an expression for the permeability that is independent of the percolation threshold and shows agreement with experimental and simulated data over a wide range of porosities. Our approach suggests that the permeability explicitly depends on the overlapping probability of grains rather than their shape. © 2012 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2012.tech.IMMD.lsinfs.permea

Perovskite solar cells fabricated using dicarboxylic fullerene derivatives - Erten-Ela, Sule; Chen, Haiwei; Kratzer, Andreas; Hirsch, Andreas; Brabec, Christoph (New Journal of Chemistry, 2016)
Perovskite solar cells were first fabricated in dye sensitized solar cells. But also, perovskite hybrid solar cells were demonstrated to be among the most promising candidates within the emerging photovoltaic materials with their high power conversion efficiencies and low-cost fabrication. In this work, we design and synthesize a novel benzoic acid fullerene bis adduct material (BAFB) for use in perovskite hybrid organic-inorganic solar cells. The obtained maximum efficiency is reported to be 9.63% using a novel benzoic acid fullerene bis adduct (BAFB) for perovskite heterojunction solar cells.

Persistence of turbulent flow in microchannels at very low Reynolds numbers - Lammers, Peter; Jovanovic, Jovan; Delgado, Antonio (Microfluidics and Nanofluidics, 2011)
A fully resolved numerical simulation of a turbulent microchannel flow, with uniformly spaced twodimensional obstruction elements mounted at the wall and normal to the flow direction, was carried out at a very low Reynolds number ofRe 970 based on the centerline velocity and themicrochannel height.Employing the latticeBoltzmann numerical technique, all energetic scales of turbulence were resolved with about 19 × 10 6 grid points (1261 × 129 × 128 in the x 1, x 2, and x3 directions). The simulated results confirm the self-maintenance of turbulence at such a low Reynolds number. Turbulence persisted over more than 1,000 turnover times, which was sufficiently long to prove its self-maintenance. These findings support the conjecture that turbulence developing in microchannels having rough walls can not only be initiated but alsomaintained at very lowReynolds numbers. © Springer-Verlag 2011.
UnivIS-Import:2015-03-09:Pub.2011.tech.ITC.stmmec.persis

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface - Maier, Florian; Niedermaier, Inga; Steinrück, Hans-Peter (Journal of Chemical Physics, 2017)
This perspective analyzes the potential of X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV) conditions to follow chemical reactions in ionic liquids in situ. Traditionally, only reactions occurring on solid surfaces were investigated by X-ray photoelectron spectroscopy (XPS) in situ. This was due to the high vapor pressures of common liquids or solvents, which are not compatible with the required UHV conditions. It was only recently realized that the situation is very different when studying reactions in Ionic Liquids (ILs), which have an inherently low vapor pressure, and first studies have been performed within the last years. Compared to classical spectroscopy techniques used to monitor chemical reactions, the advantage of XPS is that through the analysis of their core levels all relevant elements can be quantified and their chemical state can be analyzed under well-defined (ultraclean) conditions. In this perspective, we cover six very different reactions which occur in the IL, with the IL, or at an IL/support interface, demonstrating the outstanding potential of in situ XPS to gain insights into liquid phase reactions in the near-surface region. (C) 2017 Author(s).

Perspectives of Nano-Carbon Based Engineering Materials - Greil, Peter (Advanced Engineering Materials, 2014)
Nano-carbon materials attained considerable scientific interest due to their unique physico-chemical properties. Much less reports can be found on transferring the unique properties of super-strong individual nanoparticles like carbon nanotubes and graphene nanoplatelets into load bearing engineering materials. After reviewing structure and properties of nano-carbon properties the size effect governing the reduction of inherent mechanical properties upon transfer into macroscopic engineering materials is considered. While the potential of mechanical property enhancement of composites with random orientation of elongated carbon nanoparticles is limited by very low percolation thresholds, manufacturing of aligned microstructures, and tailoring of nanoparticle/matrix interface offers plenty of space for optimizing the mechanical properties of composites subjected to tensile loads. Since compression is the more common loading situation for ultra low-density nano-carbon materials the collapse stress is important for deriving design limits of nanoporous carbon materials. At the same level of density materials with nanotube or sheet carbon allotropes forming the struts may be expected to achieve a compression strength orders of magnitude higher than the porous graphitic materials. Finally, economic aspects of nano-carbon manufacturing are discussed.
EAM Import::2019-03-08

Phantom Studies of Neutron Capture of Boron Containing Magnetic Nanoparticles - Tietze, Rainer; Dürr, Stephan; Lyer, Stefan; Taccardi, Nicola; Wasserscheid, Peter; Canella, Lea; Wagner, Franz M.; Petry, Winfried; Alexiou, Christoph (Biomedizinische Technik, 2013)
The main burden that hinders the translation of neutron capture therapy drafts to human trials is the difficulty to deposit a sufficient amount of boron in the targeted tumour areas. Magnetic nanoparticles containing B-10 isotopes attempt to be enriched in tumour tissues selectively by the means of external magnetic fields to overcome these restrictions. First pilot measurements of the neutron depth penetration investigated the neutron flux density in dependence of depth, presence of boron and additional presence of iron oxide nanoparticles. These irradiation experiments on different phantoms evidenced no disturbing attenuation effects of the applied boron and the co-present nanoparticles in the samples.
EVALuna2:19968

Phase Transition Behavior and Oriented Aggregation During Precipitation of In(OH)3 and InOOH Nanocrystals - Klaumünzer, Martin; Mackovic, Mirza; Ferstl, Pascal; Voigt, Michael; Spiecker, Erdmann; Meyer, Bernd; Peukert, Wolfgang (Journal of Physical Chemistry C, 2012)
The phase transition behavior and oriented aggregation (OA) during colloidal synthesis of In(OH)3 nanocrystals in water are investigated by TEM, SEM, X-ray diffraction, and density functional theory (DFT) calculations. Besides the cubic In(OH)3 phase, also orthorhombic InOOH is formed in a precipitation route using indium acetate as the In3+ source. Well-developed nano- and microcuboids are observed that consist solely of In(OH)3. In contrast, the InOOH phase remains semicrystalline even for long reaction (refluxing) times. The irregular growth of the InOOH phase is explained by proton transfers from hydroxyl groups to oxygen ions within the InOOH lattice that lead to OH disorder and lattice strain. DFT calculations of the surface energies of ideal and water-saturated low-index InOOH and In(OH)3 surfaces predict that the In(OH)3 phase becomes energetically more favorable than InOOH above a critical crystallite size. This explains why InOOH is formed before the In(OH)3 phase, which is an unusual pathway for a hydrothermal process. Once InOOH has transformed to In(OH)3 by incorporation of water, the crystallites can grow without restriction due to the disappearance of OH-disorder-induced strain. Finally, for the In(OH)3 cuboids a three-step formation process is suggested: In the first step, one-dimensional OA under formation of nanorods occurs. In the second step, parallel bundles are formed from the nanorods. In the third step these bundles merge into cuboids by three-dimensional O

Phononic band gaps in 2D quadratic and 3D cubic cellular structures - Körner, Carolin; Warmuth, Franziska (Materials, 2015)
The static and dynamic mechanical behaviour of cellular materials can be designed by the architecture of the underlying unit cell. In this paper, the phononic band structure of 2D and 3D cellular structures is investigated. It is shown how the geometry of the unit cell influences the band structure and eventually leads to full band gaps. The mechanism leading to full band gaps is elucidated. Based on this knowledge, a 3D cellular structure with a broad full band gap is identified. Furthermore, the dependence of the width of the gap on the geometry parameters of the unit cell is presented.
Keywords: Cellular materials; Dispersion relations; Finite element method; Metamaterials; Phononic band gaps

Phononic band gaps in periodic cellular materials - Liebold Ribeiro, Yvonne; Körner, Carolin (Advanced Engineering Materials, 2014)
The dynamic mechanical properties of finite two-dimensional periodic cellular materials are investigated by finite element eigenmode analysis for different architectures of the unit cell. Frequency band gaps are examined in quadratic and hexagonal lattice topologies with regular, inverted, and chiral architecture. Pronounced band gaps develop for chiral lattices. The formation of band gaps can be traced back to the resonance behavior of the elementary building blocks of the cellular structure for different boundary conditions (mode transition). Based on the findings of this work periodic lattice materials with specific band gaps can be designed. We show that periodic cellular materials with specific band gaps can be designed by means of FEM eigenmode analysis. The gap width and gap position strongly dependent on the geometry of the basic-cell. Our results reveal that the first band gap is associated with collective periodic rotation motions, and confirm that the band gap behavior is governed by the properties of the single strut. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Phosphonate- and Carboxylate-Based Self-Assembled Monolayers for Organic Devices: A Theoretical Study of Surface Binding on Aluminum Oxide with Experimental Support - Bauer, Thilo; Schmaltz, Thomas; Lenz, Thomas; Halik, Marcus; Meyer, Bernd; Clark, Timothy (ACS Applied Materials and Interfaces, 2013)
We report a computational study on the chemical bonding of phosphonates and carboxylates to aluminum oxide surfaces and how the binding properties are related to the amount of water in the experimental environment. Two different surface structures were used in the calculations in order to model representative adsorption sites for the phosphonates and carboxylates and to account for the amorphous nature of the hydroxylated AlOx films in experiment. For the phosphonates, we find that the thermodynamically preferred binding mode changes between mono-, bi-, and tridentate depending on the surface structure and the amount of residual water. For the carboxylates, on the other hand, monodentate adsorption is always lower in energy at all experimental conditions. Phosphonates are more strongly bound to aluminum oxide than carboxylates, so that carboxylates can be replaced easily by phosphonates. The theoretical findings are consistent with those obtained in adsorption, desorption, and exchange reactions of n-alkyl phosphonic and carboxylic acids on AlOx surfaces. The results provide an atomistic understanding of the adsorption and help to optimize experimental conditions for self-assembly of organic films on aluminum oxide surfaces.

Phosphonic Acids on an Atomically Defined Oxide Surface: The Binding Motif Changes with Surface Coverage - Schuschke, Christian; Schwarz, Matthias; Hohner, Chantal; Silva, Thais N.; Fromm, Lukas; Döpper, Tibor; Görling, Andreas; Libuda, Jörg (Journal of Physical Chemistry Letters, 2018)
We have studied the anchoring mechanism of a phosphonic acid on an atomically defined oxide surface. Using time-resolved infrared reflection absorption spectroscopy, we investigated the reaction of deuterated phenylphosphonic acid (DPPA, C6HsPO3D2) with an atomically defined Co3O4(111) surface in situ during film growth by physical vapor deposition. We show that the binding motif of the phosphonate anchor group changes as a function of coverage. At low coverage, DPPA binds in the form of a chelating tridentate phosphonate, while a transition to a chelating bidentate occurs close to monolayer saturation coverage. However, the coverage-dependent change in the binding motif is not associated with a major change of the molecular orientation, suggesting that the rigid phosphonate linker always maintains the DPPA in a strongly tilted orientation irrespective of the surface coverage.

Phosphonic Acids on Well-Ordered CoO Surfaces: The Binding Motif Depends on the Surface Structure - Schuschke, Christian; Schwarz, Matthias; Hohner, Chantal; Silva, Thais N.; Libuda, Jörg (Journal of Physical Chemistry C, 2018)
In this work, we investigate the interaction of deuterated phenylphosphonic acid (DPPA, C6H5PO3D2) with two different well ordered cobalt-oxide thin films, CoO(111) and CoO(100), grown on Ir(100). We applied coverage-dependent infrared reflection absorption spectroscopy (IRAS) and temperature-programmed (TP)-IRAS under ultrahigh vacuum conditions to study surface reactions, film growth, and thermal stability, both in monolayer and multilayer regimes. The results are compared to our previous studies on Co3O4(111). In the multilayer regime, amorphous multilayer films of DPPA are formed at a low temperature (180 K), which are very similar on all surfaces. At 250 K, the multilayer films crystallize, leading to changes in the molecular orientation relative to the surface. Above 340 K, the multilayer desorbs while only the chemically anchored monolayer remains on the surface. The monolayer films on all three surfaces are stable up to at least 400 K. Coverage and binding geometry, however, depend sensitively on the structure of the oxide surface. On CoO(100), DPPA undergoes complete deprotonation and anchors to the surface forming a fully deprotonated tridentate phosphonate, which dominates over the full range of coverages. On the oxygen terminated CoO(111), we find, in contrast, the formation of a bidentate species in the limit of zero coverage. At a higher coverage, we observe full deprotonation and the formation of tridentate species, which we attribute to the restructuring of the surface. The behavior differs from Co3O4(111), where DPPA forms a fully deprotonated tridentate phosphonate at a low coverage and a partially protonated bidentate phosphonate at higher coverage.(1) The different binding motifs are discussed in view of the arrangement of surface Co ions on the three oxide surfaces.

Photoactive self-assembled monolayers for optically switchable organic thin-film transistors - Salinas Batallas, Michael; Halik, Marcus (Applied Physics Letters, 2013)
We investigate the photoconductive and photovoltaic effects in organic thin-film transistors with thin hybrid dielectrics composed of aluminum oxide and self-assembled monolayers (SAMs). By using SAM molecules with an electro-optical functionality tuning of the photoinduced charge transfer at the interface of semiconductor and SAM upon illumination with laser light can be achieved. Control of the threshold voltage by the SAM composition enables the optical operation of the transistors without applying a gate voltage and affects the dynamics of photoinduced charge transfer.

Photobleaching and stabilization of carbon nanodots produced by solvothermal synthesis. - Peukert, Wolfgang; Wang, Wenshuo; Damm, Cornelia; Walter, Johannes; Nacken, Thomas (Physical Chemistry Chemical Physics, 2015)
In this work we performed a detailed investigation of the photostability of bottom-up produced carbon nanodots (CDs) prepared from citric acid and urea by solvothermal synthesis. Analytical ultracentrifugation (AUC) reveals that the CDs have a hydrodynamic diameter of <1 nm and a very narrow size distribution. In the community it is widely assumed that CDs are photo-stable. In contrast, we found that CDs exposed to UV-irradiation exhibit noteworthy fluorescence degeneration compared to freshly prepared CDs or CDs stored in the dark, indicating that fluorescence bleaching is caused by a photochemical process. We found that fluorescence intensity decay due to exposure to UV-irradiation is accelerated in the presence of oxygen and identified the surface status of CDs as the decisive factor of fluorescence bleaching of CDs. Based on a discussion on the underlying mechanisms we show how to avoid photobleaching of CDs.
Keywords: carbon; nanodots; analytical; centrifugation; size distribution; fluorescence

Photocatalysis vs. anodic-breakdown catalysis on TiO 2 layers - Paramasivam, Indhumati; Singh, S.; Moll, Matthias; Hauser, Christina; Meyer, Karsten; Schmuki, Patrik (Electrochimica Acta, 2012)
The present work compares two degradation approaches for organics molecules on TiO by (i) classical photocatalysis and (ii) anodic-breakdown (AB) catalysis. For this, TiO layers prepared by electrochemical anodization and annealing were used for the decomposition of crystal violet. AB-catalysis was performed at various applied potentials and the degradation kinetics and products were analysed. From the present results one can deduce that a valence band hole ejection mechanism is operative for AB catalysis and photocatalysis. We explain AB catalysis in terms of a Fowler-Nordheim type of junction breakdown model. © 2012 Elsevier Ltd. All rights reserved.

Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles - Paramasivalm, I.; Macák, Jan; Schmuki, Patrik (Electrochemistry Communications, 2008)
Ag and Au nanoparticles were found to significantly enhance the photocatalytic activity of self-organized TiO_2 nanotubular structures. The catalyst systems are demonstrated to be highly efficient for the UV-light induced photocatalytic decomposition of a model organic pollutant – Acid Orange 7. The metallic nanoparticles with a diameter of 10 ± 2 nm (Ag) and 28 ± 3 nm (Au) were attached to a nanotubular TiO_2 layer that consists of individual tubes of 100 nm of diameter, 2 extgreekmm in length and approx. 15 nm of wall thickness. Both metal particle catalyst systems enhance the photocatalytic decomposition significantly more on the nanotubes support than placed on a compact TiO_2 surface.
EAM Import::2019-03-12

Photocatalytic H2 production on self-decorated Au nanoparticles/TiO2 nanotubes under visible light - Lee, Kiyoung; Hahn, Robert; Schmuki, Patrik (Electrochemistry Communications, 2014)
In the present work we fabricate TiO2 nanotubes that are self-decorated with Au nanoparticles by anodizing low concentration (0.02 and 0.2 at.% Au) TiAu alloys. After formation, the TiO2 tube walls carry regularly dispersed self-decorated Au particles with 5 nm in diameter which cause significant surface plasmon resonances. This plasmon resonance effect can be exploited to achieve visible photoresponse and can be used for photocatalytic H2 production under visible light illumination.
EAM Import::2019-03-12

Photocatalytic properties of in situ doped TiO2-nanotubes grown by rapid breakdown anodization - Hahn, Robert; Stark, Martin; Killian, Manuela; Schmuki, Patrik (Catalysis: Science and Technology, 2013)
In the present work, we demonstrate an in situ electrochemical doping approach during the growth of TiO2 nanotubular layers formed by rapid breakdown anodization (RBA). For this, several oxygen containing metal anions, as doping species (WO42-, Al(OH)4-, SiO32-, MoO42-, SnO44-, CrO42- and MnO4-), were investigated as additives to the electrolyte and successfully incorporated into the nanotubular oxide during the ultrafast tube growth. A linear relationship between the concentration of anions in the electrolyte and the doping concentration in the oxide was found. The resulting doped layers were tested, after heat treatment, for their UV photocatalytic activity using the degradation of AO7 (an organic model compound routinely used for determining the kinetics of photocatalytic decomposition reactions).
EAM Import::2019-03-12

Photochemical Energy Storage and Electrochemically Triggered Energy Release in the Norbornadiene-Quadricyclane System: UV Photochemistry and IR Spectroelectrochemistry in a Combined Experiment - Brummel, Olaf; Waidhas, Fabian; Bauer, Udo; Wu, Yanlin; Bochmann, Sebastian; Steinrück, Hans-Peter; Papp, Christian; Bachmann, Julien; Libuda, Jörg (Journal of Physical Chemistry Letters, 2017)
The two valence isomers norbornadiene (NBD) and quadricyclane (QC) enable solar energy storage in a single molecule system. We present a new photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) experiment, which allows monitoring of the complete energy storage and release cycle by in situ vibrational spectroscopy. Both processes were investigated, the photochemical conversion from NBD to QC using the photosensitizer 4,4'-bis(dimethylamino)benzophenone (Michler's ketone, MK) and the electrochemically triggered cycloreversion from QC to NBD. Photochemical conversion was obtained with characteristic conversion times on the order of 500 ms. All experiments were performed under full potential control in a thin layer configuration with a Pt(111) working electrode. The vibrational spectra of NBD, QC, and MK were analyzed in the fingerprint region, permitting quantitative analysis of the spectroscopic data. We determined selectivities for both the photochemical conversion and the electrochemical cycloreversion and identified the critical steps that limit the reversibility of the storage cycle.

Photochemistry in a soft-glass single-ring hollow-core photonic crystal fibre - Cubillas, Ana M.; Jiang, Xin; Euser, Tijmen G.; Taccardi, Nicola; Etzold, Bastian; Wasserscheid, Peter; Russell, Philip St. John (Analyst, 2017)
A hollow-core photonic crystal fibre (HC-PCF), guided by photonic bandgap effects or anti-resonant reflection, offers strong light confinement and long photochemical interaction lengths in a microscale channel filled with a solvent of refractive index lower than that of glass (usually fused silica). These unique advantages have motivated its recent use as a highly efficient and versatile microreactor for liquid-phase photochemistry and catalysis. In this work, we use a single-ring HC-PCF made from a high-index soft glass, thus enabling photochemical experiments in higher index solvents. The optimized light-matter interaction in the fibre is used to strongly enhance the reaction rate in a proof-of-principle photolysis reaction in toluene.
EAM Import::2019-03-12

Photochemistry in hollow-core photonic crystal fiber microreactors - Euser, Tijmen G.; Cubillas, Ana Maria; Jiang, X.; Unterkofler, Sarah; Etzold, Bastian; Wasserscheid, Peter; Jones, Anita C.; Sadler, Peter J.; Russell, Philip St. John (Laser Applications to Chemical, Security and Environmental Analysis, 2014)
Hollow-core photonic crystal fiber uniquely allows low-loss propagation of light in liquid-filled microchannels, thus enabling highly efficient photochemistry, photo-switching, and photocatalysis at optical powers that are five orders of magnitude lower than in conventional systems.

Photochemistry in Photonic Crystal Fiber Nanoreactors - Chen, Jocelyn S. Y.; Euser, Tijmen G.; Farrer, Nicola J.; Sadler, Peter J.; Scharrer, Michael; Russell, Philip St. John (Chemistry - A European Journal, 2010)
We report the use of a liquid-filled hollow-core photonic crystal fiber (PCF) as a highly controlled photochemical reactor. Hollow-core PCFs have several major advantages over conventional sample cells: the sample volume per optical path length is very small (2.8nL cm-1 in the fiber used), long optical path lengths are possible as a result of very low intrinsic waveguide loss, and furthermore the light travels in a diffractionless single mode with a constant transverse intensity profile. As a proof of principle, the (very low) quantum yield of the photochemical conversion of vitamin B12, cyanocobalamin (CNCbl) to hydroxocobalamin ([H_2OCbl]+) in aqueous solution was measured for several pH values from 2.5 to 7.5. The dynamics of the actively induced reaction were monitored in real-time by broadband absorption spectroscopy. The PCF nanoreactor required ten thousand times less sample volume compared to conventional techniques. Furthermore, the enhanced sensitivity and optical pump intensity implied that even systems with very small quantum yields can be measured very quickly—in our experiments one thousand times faster than in a conventional cuvette.
EAM Import::2019-03-12

Photochemistry on soft-glass hollow-core photonic crystal fibre - Cubillas, Ana Maria; Jiang, X.; Euser, Tijmen G.; Taccardi, Nicola; Etzold, Bastian; Wasserscheid, Peter; Russell, Philip St. John (23rd International Conference on Optical Fibre Sensors, 2014)
Hollow-core photonic crystal fibre (HC-PCF) offers strong light confinement and long interaction lengths in an optofluidic channel. These unique advantages have motivated its recent use as a highly efficient and versatile microreactor for liquid-phase photochemistry and catalysis. In this work, we use a soft-glass HC-PCF to carry out photochemical experiments in a high-index solvent such as toluene. The high-intensity and strong confinement in the fibre is demonstrated to enhance the performance of a proof-of-principle photolysis reaction.

Photoelastic force measurements in granular materials - Daniels, Karen E.; Kollmer, Jonathan; Puckett, James G. (Review of Scientific Instruments, 2017)
Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local of stress. In this review paper, we summarize past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus.
EAM Import::2019-03-12

Photoelectrochemical and photocatalytic activity of tungsten doped TiO2 nanotube layers in the near visible region - Das, Chittaranjan; Paramasivam, Indhumati; Liu, Ning; Schmuki, Patrik (Electrochimica Acta, 2011)
In the present work we study the effect of WO3 doping on the photo-electrochemical behavior of self-organized TiO2 nanotube layers. Mixed oxide nanotubes were grown by anodization of Ti–W alloys containing 0.2 and 9% W, with a thicknesses of the oxide nanotube layers adjusted to about 1.1–1.2 extgreekmm. We show that by WO3 doping, the near visible photoresponse and photocatalytic performance can drastically be enhanced. While a content of 9 at% WO3 in photoresponse experiments is most beneficial, in long term experiments a higher efficiency is observed for the 0.2 at% W content. This is due to a gradual leaching of WO3 (dissolution into the electrolyte) for the higher WO3 content. This demonstrates that under optimized WO3 doping conditions a lasting visible light activation of TiO2 nanotubes can be achieved.
EAM Import::2019-03-12

Photoelectrochemical Poperties of Anodic TiO2 Nanosponge Layers - Sanchez-Tovar, Rita; Lee, Kiyoung; Garcia-Anton, Jose; Schmuki, Patrik (ECS Electrochemistry Letters, 2013)
In the present work we grow TiO2 nanosponge structures by anodizing Ti in a glycerol/water/NH4F electrolyte to thickness of some extgreekmm. We evaluate the photoelectrochemical behavior (bandgap, photocurrent-voltage characteristics) in presence and absence of methanol. Methanol drastically affects the photoresponse (due to hole capture and current doubling). The optimum thickness for photoelectrochemical applications of these nanostructures is dependent on the excitation wavelength. For applications such as solar light water splitting, anodic sponge structure of ≈500 nm thickness can be beneficially used to increase the photoresponse compared to compact TiO2 layers.
EAM Import::2019-03-12

Photoelectron spectroscopy of ionic liquid-based interfaces - Lovelock, Kevin R.J.; Villar-Garcia, Ignacio J.; Maier, Florian; Steinrück, Hans-Peter; Licence, Peter (Chemical Reviews, 2010)
Photoelectron spectroscopies (PES) are a class of surface analysis techniques that involve irradiating the sample in Vacuo with photons and measuring the kinetic energy of the electrons emitted. Ultrahigh vacuum (UHV) is generally required to analyze the emitted photoelectrons without interference from gas phase collisions. XPS can be used to identify and quantify the different chemical environments of the same element within a given IL. Each individual element signal is a function of the chemical environment of the atoms of interest. There are two contributions to the observed chemical shifts in XP spectra, namely initial and final state effects. Initial state effects are those that affect the charge of the atom before the photon strikes and are described by calculations of the ground state of a particular system.

Photoinduced degradation of methylammonium lead triiodide perovskite semiconductors - Tang, Xiaofeng; Brandl, Marco; May, Benjamin; Levchuk, Ievgen; Hou, Yi; Richter, Moses; Chen, Haiwei; Chen, Shi; Kahmann, Simon; Osvet, Andres; Maier, Florian; Steinrück, Hans-Peter; et al. (Journal of Materials Chemistry A, 2016)
Photoinduced degradation is a critical obstacle for the real application of novel semiconductors for photovoltaic applications. In this paper, the photoinduced degradation of CHNHPbI in a vacuum and air (relative humidity 40%) is analyzed by ex situ and advanced in situ technologies. Without light illumination, CHNHPbI films slowly degrade under vacuum and air within 24 hours. However, we find that CHNHPbI converts to metallic lead (Pb) when exposed to vacuum and light illumination. Further, a series of lead salts (e.g. PbO, Pb(OH) and PbCO) are formed when CHNHPbI is degraded under environmental conditions, i.e. under the combination of light, oxygen and moisture. Photoinduced degradation is found to be determined by the environmental atmosphere as CHNHPbI films remain very stable under nitrogen conditions. The results from vacuum conditions underpin that the high volatility of the organic component (CHNHI) is in conflict with reaching excellent intrinsic stability due to its role in creating ion vacancies. The degradation in air suggests that both oxygen and water contribute to the fast photodecomposition of CHNHPbI into lead salts rather than water alone. Given these basic yet fundamental understandings, the design of hydrophobic capping layers becomes one prerequisite towards long-term stable perovskite-based devices.
Keywords: Engineering controlled terms: Lead; Perovskite; Salts; Slow light Environmental conditions; Intrinsic stability; Light illumination; Organic components; Photo-decomposition; Photoinduced degradation; Photovoltaic applications; Real applications Engineering main heading: Photodegradation

Photoinduced release of active proteins from TiO2 surfaces - Song, Yanyan; Hildebrand, Helga; Schmuki, Patrik (Electrochemistry Communications, 2009)
The present work reports on enzyme attachment on and photoinduced release from TiO_2 surfaces. TiO_2 layers (amorphous and anatase) were modified with 3-aminopropyltriethoxysilane (APTES), followed by attachment of vitamin C and horseradish peroxidase (HRP). Using step by step XPS characterization and vis-spectroscopy we show that upon UV illumination the linker chain to the protein can be cut, releasing active HRP into the environment. The head silane group remains attached to the TiO_2 surface. The kinetics of this photoinduced release is significantly faster for the anatase form of TiO_2 compared with amorphous material. The results indicate that UV induced chain scission represents a very versatile tool for payload release from TiO_2 surfaces.
EAM Import::2019-03-12

Photoluminescence and energy transfer in Tb3+/Mn2+ co-doped ZnAl2O4 glass ceramics - Lakshminarayana, Gandham; Wondraczek, Lothar (Journal of Solid State Chemistry, 2011)
We report on Tb3+ as efficient sensitizer for red photoemission from Mn2+-centers in ZnO–B2O3–Al2O3–Si2O–Na2O–SrO glasses and corresponding gahnite glass ceramics. In comparison to singly or co-doped glasses, the glass ceramics exhibit significantly increased emission intensity. Structural considerations, ESR, and dynamic luminescence spectroscopy indicate partial incorporation of Mn2+ as well as Tb3+ into the crystalline phase, the former on octahedral Zn2+-sites. Interionic distance and charge transfer probability between both species depend on crystallization conditions. This enables control of the energy transfer process and, hence, tunability of the color of photoemission by simultaneous emission from Tb3+ and Mn2+ centers. Concentration quenching in Mn2+-singly doped materials was found at a critical dopant concentration of about 1.0 mol%. The energy transfer process was studied in detail by dynamic as well as static luminescence spectroscopy. Spectroscopic results suggest the application of the studied materials as single or dual-mode emitting phosphor for luminescent lighting.
EAM Import::2019-03-12

Photoluminescence of Mn2+ Centers in Chalcohalide Glasses - Yan, Qiqi; Liu, Yinyao; Chen, Guorong; Da, Ning; Wondraczek, Lothar (Journal of the American Ceramic Society, 2011)
We report on photoluminescence from Mn2+-doped chalcohalide glasses of the GeS_2–Ga_2S_3–CsCl system. Upon blue excitation at 447 nm, a broad emission band occurs in the green spectral range from 500 to 600 nm, indicating the presence of tetrahedrally coordinated Mn2+ species. When the Mn2+-concentration is increased up to 2 mol%, an increasingly intense secondary emission band evolves at about 610–660 nm. Meanwhile, the intensity of the green band decreases gradually and shifts toward the red, resulting a very flatten luminescence from 500 to 750 nm, which provides a promising white light emitting source. As for the origin of this unique luminescence behavior of Mn2+, it is proposed that the ligand coordination number of Mn2+ in the present chalcohalide glasses experiences a partial change from tetrahedral to octahedral coordination.
EAM Import::2019-03-12

Photoluminescence of samples produced by electroless wet chemical etching: Between silicon nanowires and porous structures - Voigt, Felix; Sivakov, Vladimir; Gerliz, Viktor; Bauer, Gottfried H.; Hoffmann, Bjoern; Radnoczi, Gyorgy Z.; Pecz, Bela; Christiansen, Silke (physica status solidi (a), 2011)
Samples containing silicon nanowires (Si-NWs) and highly porous structures (P-Si) were prepared by electroless wet chemical etching (EWCE) of crystalline silicon wafers using various etching parameters. Photoluminescence (PL) measurements were performed with excitation at 488 nm and a photon energy flux of 337mWcm (2). According to the diameters of the Si-NWs (> 10 nm), from quantum confinement (QC) theory no shift in PL peak energy compared to the bandgap of crystalline silicon is expected. However, PL measurements show peak emission energies ranging between 1.4 and 1.6 eV. After further treatment of the samples with HF, substantial PL emission was still detectable with the measured PL peak pinned at 1.4 eV irrespective of etching time. We explain the observations by the hypothesis that the persistent part of PL emission is generated by nanocrystals located at the rough sidewalls of the Si-NWs or residing within the porous sample structure. The part of the PL, which was present before HF treatment, but vanished after the treatment, is attributed to the presence of silicon suboxide surrounding the Si-NWs or covering other Si surfaces. This hypothesis is explored by means of three sample series, prepared with different preparation parameters. In the first series the time used during the initial metallization step in order to prepare an Ag nanoparticle layer on the top surface was varied, in the second series the etching time was the changed parameter and in the third series the HF to H2O2 concentration ratio was varied Strong visible orange colored PL of a sample produced by EWCE starting from a heavily doped wafer (n-type c-Si (111), As as dopant) and excited at 337 nm ( the sample was mounted on a glass substrate. Blue luminescence is due to the substrate).
EAM Import::2019-03-12
Keywords: nanowires; photoluminescence; porous silicon; silica; silicon; LUMINESCENCE; OXYGEN; NANOCRYSTALS; SILOXENE; ARRAYS

Photoluminescence properties of thermographic phosphors YAG:Dy and YAG:Dy, Er doped with boron and nitrogen - Chepyga, Liudmyla M.; Jovicic, Gordana; Vetter, Andreas; Osvet, Andres; Osvet, Andres; Brabec, Christoph; Batentschuk, Miroslaw (Applied Physics B-Lasers and Optics, 2016)
This paper investigates Dy-doped and Dy, Er-co-doped yttrium aluminum garnets (YAG) with the admixture of boron nitride with the aim of using them as efficient thermographic phosphors at high temperatures. The phosphors were synthesized using a conventional high-temperature solid-state method. The influence of two fluxes, BO and LiF/NHF, and the effect of activator and coactivator concentrations were investigated. Additionally, the effect of B and N substituting for Al and O ions, respectively, in the YAG:Dy co-doped with Er was studied for the first time. The changes in the host lattice led to a much stronger photoluminescence compared with the samples without B and N substitution. The admixture of BN also improves the thermal sensitivity of the YAG:Dy and YAG:Dy, Er thermographic phosphor
Keywords: Engineering controlled terms: Erbium; Light emission; Photoluminescence Coactivators; High temperature; High temperature solid state methods; Host lattice; Photoluminescence properties; Thermal sensitivity; Thermographic phosphors; Yttrium aluminum garnet Engineering main heading: Phosphors

Photoluminescent and storage properties of photostimulable core/shell type silicate nanoparticles - Osvet, Andres; Batentschuk, Miroslaw; Milde, Moritz; Lundt, Nils; Gellermann, Carsten; Dembski, Sofia; Winnacker, Albrecht; Brabec, Christoph (Physica Status Solidi (C) Current Topics in Solid State Physics, 2013)
We have prepared hybrid luminescent nanoparticles with amorphous SiO cores and crystalline ZnSiO:Mnshells by sol-gel technology. Under UV excitation the shells of the particles emit green Mn-related luminescence at 525 nm. In addition, under the UV excitation at least two kinds of traps are filled with electrons, contributing to a long afterglow and photostimulated luminescence. The paper presents the details of the charging and stimulation characteristics of the photostimulated luminescence. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Luminescent core/shell nanoparticles; Nanophosphors; Photostimulated luminescence; Zn2SiO4:Mn2+

Photon bunching reveals single-electron cathodoluminescence excitation efficiency in InGaN quantum wells - Meuret, Sophie; Coenen, Toon; Zeijlemaker, Hans; Latzel, Michael; Christiansen, Silke; Conesa-Boj, Sonia; Polman, Albert (Physical Review B, 2017)
Cathodoluminescence spectroscopy is a key analysis technique in nanophotonics research and technology, yet many aspects of its fundamental excitation mechanisms are not well understood on the single-electron and single-photon level. Here, we determine the cathodoluminescence emission statistics of InGaN quantum wells embedded in GaN under 6–30-keV electron excitation and find that the light emission rate varies strongly from electron to electron. Strong photon bunching is observed for the InGaN quantum well emission at 2.77 eV due to the generation of multiple quantum well excitations by a single primary electron. The bunching effect, measured by the g(2)(t) autocorrelation function, decreases with increasing beam current in the range 3–350 pA. Under pulsed excitation (p=2–100ns; 0.13–6 electrons per pulse), the bunching effect strongly increases. A model based on Monte Carlo simulations is developed that assumes a fraction extgreekg of the primary electrons generates electron-hole pairs that create multiple photons in the quantum wells. At a fixed primary electron energy (10 keV) the model explains all g(2) measurements for different beam currents and pulse durations using a single value for extgreekg=0.5. At lower energies, when electrons cause mostly near-surface excitations, extgreekg is reduced ( extgreekg=0.01 at 6 keV), which is explained by the presence of a AlGaN barrier layer that inhibits carrier diffusion to the buried quantum wells. The combination of g(2) measurements in pulsed and continuous mode with spectral analysis provides a powerful tool to study optoelectronic properties and may find application in many other optically active systems and devices.
EAM Import::2019-03-12

Photonic crystal fibres for chemical sensing and photochemistry - Cubillas, Ana Maria; Unterkofler, Sarah; Euser, Tijmen G.; Etzold, Bastian; Jones, Anita C.; Sadler, Peter J.; Wasserscheid, Peter; Russell, Philip St. John (Chemical Society Reviews, 2013)
In this review, we introduce photonic crystal fibre as a novel optofluidic microdevice that can be employed as both a versatile chemical sensor and a highly efficient microreactor.We demonstrate that it provides an excellent platform in which light and chemical samples can strongly interact for quantitative spectroscopic analysis or photoactivation purposes. The use of photonic crystal fibre in photochemistry and sensing is discussed and recent results on gas and liquid sensing as well as on photochemical and catalytic reactions are reviewed. These developments demonstrate that the tight light confinement, enhanced light-matter interaction and reduced sample volume offered by photonic crystal fibre make it useful in a wide range of chemical applications.

Photon Propagation in a Discrete Fiber Network: An Interplay of Coherence and Losses - Regensburger, Alois; Bersch, Christoph; Hinrichs, Benjamin; Onishchukov, Georgy; Schreiber, Andreas; Silberhorn, Christine; Peschel, Ulf (Physical Review Letters, 2011)
We study light propagation in a photonic system that shows stepwise evolution in a discretized environment. It resembles a discrete-time version of photonic waveguide arrays or quantum walks. By introducing controlled photon losses to our experimental setup, we observe unexpected effects like subexponential energy decay and formation of complex fractal patterns. This demonstrates that the interplay of linear losses, discreteness and energy gradients leads to genuinely new coherent phenomena in classical and quantum optical experiments. Moreover, the influence of decoherence is investigated.
EAM Import::2019-03-12

Photophysics of Molecular-Weight-Induced Losses in Indacenodithienothiophene-Based Solar Cells - Gasparini, Nicola; Katsouras, Athanasios; Prodromidis, Mamantos I.; Avgeropoulos, Apostolos; Baran, Derya; Salvador, Michael Filipe; Fladischer, Stefanie; Spiecker, Erdmann; Chochos, Christos L.; Ameri, Tayebeh; Brabec, Christoph (Advanced Functional Materials, 2015)
The photovoltaic performance and optoelectronic properties of a donor-acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3-bis(3-(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number-average molecular weight (Mn). Current-voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo-CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing Mn, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility-lifetime (μτ) product. By combining Fourier-transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing Mn reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with Mn = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10-4 cm2 V-1 s-1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.
Keywords: charge transport; energetic losses; indacenodithienothiophene; molecular weight; organic solar cells

Photovoltaic loss analysis of inkjet-printed polymer solar cells using pristine solvent formulations - Hoth, C. N.; Schilinsky, P.; Choulis, S. A.; Brabec, Christoph (Macromolecular Symposia, 2010)
We have recently reported inkjet-printed organic solar cells with a record power conversion efficiency of 3.5%. In this contribution, we present our first trials to process a functional active layer of a polymer:fullerene bulk-hetero junction solar cell by inkjet printing using tetralene as a solvent. Solar cells with the inkjet-printed active layer based on pristine tetralene show calibrated AM1.5 power conversion efficiency (PCE) of around 1.3% over comparable doctor bladed cells with PCE of 3.3%. Analysis in terms of one-diode equivalent circuit combined with current-voltage characteristics of the devices as a function of light intensity and voltage were performed to reveal the dominant loss mechanisms of the inkjet printed solar cells using tetralene solvent formulation. The loss analysis described in this paper helps us to identify the device performance limitations and to design methods to improve the performance of inkjet printed organic solar cells. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Device loss analysis; Fullerene; Inkjet printing; Morphology; Organic photovoltaics; Polymer; Polymer solar cells; Solvent formulations

Physical foaming of fluorinated ethylene-propylene (FEP) copolymers in supercritical carbon dioxide: single-film fluoropolymer piezoelectrets - Voronina, O.; Wegener, Michael; Wirges, Werner; Gerhard, Reimund; Zirkel, Larissa; Münstedt, Helmut (Applied Physics A-Materials Science & Processing, 2008)
Cellular polymer foams with significant piezoelectric activity were prepared from fluorinated ethylene-propylene (FEP) copolymers. The required void structure is obtained by saturation of FEP films with supercritical carbon dioxide and a subsequent heat treatment for controlled inflation. After bipolar electric charging of the voids and evaporation of electrodes, the FEP films show piezoelectric d33 coefficients up to 50 pC N. The present physical foaming process generates cellular efficients up to 50 pC/N. The present physical foaming process generates cellular fluoropolymer piezoelectret films with usually only one single void across the film thickness.
EAM Import::2019-03-12

Physical vapor deposition of [EMIM][Tf2N]: A new approach to the modification of surface properties with ultrathin ionic liquid films - Cremer, Till; Killian, Manuela; Gottfried, Michael; Paape, Natalia; Wasserscheid, Peter; Maier, Florian; Steinrück, Hans-Peter (ChemPhysChem, 2008)
Ultrathin films of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TfN], are prepared on a glass substrate by means of an in situ thermal-evaporation/condensation process under ultrahigh-vacuum conditions. By using X-ray photoelectron spectroscopy (XPS), it is demonstrated that the first layer of the IL film grows two dimensionally, followed by the three-dimensional growth of successive layers. The first molecular layer consists of a bilayer, with the [EMIM] cations in contact to the surface and the [TfN] anions at the vacuum side. The ultrathin IL films are found to be stable under ambient conditions. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Physics challenged by cells - Smith, Ana-Suncana (Nature Physics, 2010)
Cells are the building blocks of life. Ideas traditionally applied to physical problems are now helping to unravel their complex mysteries.
Keywords: FREE-ENERGY DIFFERENCES; FLUCTUATION THEOREM; ADHESION; RECONSTITUTION; EQUALITY; GROWTH; FORCE

Physics of cell adhesion: some lessons from cell-mimetic systems - Sackmann, Erich; Smith, Ana-Suncana (Soft Matter, 2014)
Cell adhesion is a paradigm of the ubiquitous interplay of cell signalling, modulation of material properties and biological functions of cells. It is controlled by competition of short range attractive forces, medium range repellant forces and the elastic stresses associated with local and global deformation of the composite cell envelopes. We review the basic physical rules governing the physics of cell adhesion learned by studying cell-mimetic systems and demonstrate the importance of these rules in the context of cellular systems. We review how adhesion induced micro-domains couple to the intracellular actin and microtubule networks allowing cells to generate strong forces with a minimum of attractive cell adhesion molecules (CAMs) and to manipulate other cells through filopodia over micrometer distances. The adhesion strength can be adapted to external force fluctuations within seconds by varying the density of attractive and repellant CAMs through exocytosis and endocytosis or protease-mediated dismantling of the CAM-cytoskeleton link. Adhesion domains form local end global biochemical reaction centres enabling the control of enzymes. Actin-microtubule crosstalk at adhesion foci facilitates the mechanical stabilization of polarized cell shapes. Axon growth in tissue is guided by attractive and repulsive clues controlled by antagonistic signalling pathways.
Keywords: INTERFERENCE CONTRAST MICROSCOPY; RECEPTOR-LIGAND BONDS; SUPPORTED MEMBRANES; GIANT VESICLES; FLUID MEMBRANES; IMMUNOLOGICAL SYNAPSE; GROWTH CONES; T-CELLS; IN-VIVO; ACTIVATION

Physics of Spatially Structured Materials - Mecke, Klaus (New Perspectives in Stochastic Geometry, 2009)
Modern imaging techniques with nanometre resolution open up the possibility to study the relationship between physical properties and geometric features of spatially structured materials.
EAM Import::2019-03-12

Picosecond dynamics in haemoglobin from different species: A quasielastic neutron scattering study - Stadler, Andreas M.; Garvey, Christopher J.; Embs, Jan Peter; Koza, Michael Marek; Unruh, Tobias; Artmann, Gerhard; Zaccai, Giuseppe (Biochimica Et Biophysica Acta-General Subjects, 2014)
General significance: The measured dynamic quantities showed significant and systematic variations between the investigated species, suggesting that they are the signature of an evolutionary adaptation process stimulated by the different physiological environments of the respective protein. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: Haemoglobin; Body temperature adaptation; Protein dynamics; Picosecond dynamics; Quasielastic neutron scattering

Planar defect formation in the gamma' phase during high temperature creep in single crystal CoNi-base superalloys - Eggeler, Yolita; Müller, Julian; Titus, M. S.; Suzuki, A.; Pollock, T. M.; Spiecker, Erdmann (Acta Materialia, 2016)
The structure and formation mechanism of extended planar defects in the extgreekg/ extgreekg′ microstructure of creep deformed CoNi-base single crystal superalloys have been studied by conventional and advanced transmission electron microscopy (TEM). Planar defects in numerous isolated as well as contiguous extgreekg′ precipitates on (111) planes reveal a characteristic configuration whereby superlattice intrinsic stacking faults (SISF) are fully embedded within antiphase boundaries (APB). Detailed analysis revealed that a leading 1/3[112] superpartial dislocation first creates an SISF. The SISF is then transformed into an APB by a trailing 1/6[112] partial dislocation. The partial is left inside the precipitate and remains as a dislocation loop. Thus, the entire shearing process constitutes a crystallographic slip of type 1/2[112]. A force balance analysis indicates that the initial APB energy exceeds the SISF energy. However, energy-dispersive X-ray spectroscopy (EDXS) indicates pronounced local reordering and diffusion processes near both types of planar defects. The APB qualitatively adopts the composition of the extgreekg phase whereas the SISF locally changes its composition towards that of the Co3W–D019 phase. We propose that these atomic diffusion processes determine the formation and shrinkage of the loops. A post mortem in situ TEM heating experiment shows that with increasing temperature the APBs exhibit complete faceting into [100] planes followed by coarsening, eventually leading to disintegration of the extgreekg′ precipitate. This indicates a detrimental impact of APBs as potential nuclei for fragmentation of the extgreekg/ extgreekg′ microstructure in CoNi-base superalloys.
EAM Import::2019-03-08

Plasmonic excitations in metallic nanoparticles: Resonances, dispersion characteristics and near-field patterns - Tatartschuk, Eugen; Shamonina, Ekaterina; Solymar, Laszlo (Optics Express, 2009)
Metamaterials acquire their functionality from the structuring of the small building blocks, ``artificial atoms''. Our paper provides a study of the resonant behaviour for a variety of metallic nanoparticles in the region of hundreds of THz. Resonant modes for nanorods of rectangular cross section are investigated numerically for different types of excitation and the set of resonant frequencies (fundamental and higher order) are determined for rods of various length. From that the dispersion relationship for surface plasmon-polaritons propagating along the rod is deduced. We analyse resonant-mode near-field distribution of the electric field, including the field lines, to emphasise the underlying physics. Resonant frequencies are also found and field distributions analysed when the rods are combined to form particles of L, U and O shapes. The similarities and differences between those particles, both in the values and in the number of resonances, are discussed. The results of this study may aid the design of nanostructured metamaterials with required properties in the IR and optical domain.
EAM Import::2019-03-12

Plastic anisotropy of ultrafine grained Al alloy AA6016 produced by accumulative roll bonding - Skrotzki, W.; Scharnweber, J.; Oertel, C. -G.; Höppel, Heinz Werner; Topic, Irena; Brokmeier, H. -G.; Jaschinski, J. (Solid State Phenomena, 2010)
In order to quantify the plastic anisotropy of the ultrafine grained aluminium alloy AA6016 produced by accumulative roll-bonding (ARB) the Lankford parameter is measured by tensile testing as a function of the number of ARB cycles. The experimental results are compared with those from texture-based Taylor simulations. Increasing differences between experiment and theory at higher number of ARB cycles may be attributed to highly oriented microstructural features.
WW1::853

Polarization features of optically pumped CdS nanowire lasers - Roeder, Robert; Ploss, Daniel; Kriesch, Arian; Buschlinger, Robert; Geburt, Sebastian; Peschel, Ulf; Ronning, Carsten (Journal of Physics D-Applied Physics, 2014)
High quality CdS nanowires suspended in air were optically pumped both below and above the lasing threshold. The polarization of the pump laser was varied while emission out of the end facet of the nanowire was monitored in a `head-on' measurement geometry. Highest pump-efficiency and most efficient absorption of the pump radiation are demonstrated for an incident electric field being polarized parallel to the nanowire axis. This polarization dependence, which was observed both above the lasing threshold and in the regime of amplified spontaneous emission, is caused by an enhanced absorption for parallel polarized optical pumping. Measured Stokes parameters of the nanowire emission reveal that due to the onset of lasing the degree of polarization rapidly increases from approximately 15 to 85%. Both Stokes parameters and degree of polarization of the nanowire lasing emission are independent of the excitation polarization. The transversal lasing mode is therefore not notably affected by the polarization of the pumping beam, although the supply with optical gain is significantly enhanced for an excitation polarization parallel to the nanowire axis.
EAM Import::2019-03-08

Polarization-induced extgreeks-holes and hydrogen bonding - Hennemann, Matthias; Murray, Jane S.; Politzer, Peter; Riley, Kevin E.; Clark, Timothy (Journal of Molecular Modeling, 2012)
The strong collinear polarizability of the A–H bond in A–H···B hydrogen bonds is shown to lead to an enhanced extgreeks-hole on the donor hydrogen atom and hence to stronger hydrogen bonding. This effect helps to explain the directionality of hydrogen bonds, the well known cooperative effect in hydrogen bonding, and the occurrence of blue-shifting. The latter results when significant additional electron density is shifted into the A–H bonding region by the polarization effect. The shift in the A–H stretching frequency is shown to depend essentially linearly on the calculated atomic charge on the donor hydrogen for all donors in which A belongs to the same row of the periodic table. A further result of the polarization effect, which is also expected for other extgreeks-hole bonds, is that the strength of the non-covalent interaction depends strongly on external electric fields.
EAM Import::2019-03-08

Polarized Raman scattering and SEM combined full characterization of self-assembled nematic thin films - Srikantharajah, Rubitha; Gerstner, Karin; Romeis, Stefan; Peukert, Wolfgang (Nanoscale, 2016)
Elongated particles are predestined for a fast transfer of optical and electronical signals in a preferred direction, which is mandatory for a quick response in optoelectronic devices. The performance of the material is based on the quality of defect less alignment of the particles. On this account we present an easy non-invasive methodology for characterization of both surface and bulk order. The characterization of bulk order is performed by orientation dependent variation of the polarized Raman scattering signal on large areas by mapping. Scanning electron microscopy and image analysis on the surface complete the characterization. New insights in dip coated nematic structures clearly show the interplay of evaporation induced and shear-induced self-assembly and reveal a comprehensive mechanistic picture for nanorod assembly: the shear force dominated regime orients the particle in direction of withdrawal. At low withdrawal velocity, however, shear forces and evaporation counteract to produce a three-layered film where the top and bottom layers are oriented perpendicular to each other. The middle layer gives a clear evidence for a reorientation by convective flow.

Polydirectional stability of granular matter - Zimber, Fabian; Kollmer, Jonathan; Pöschel, Thorsten (Physical Review Letters, 2013)
We investigate jammed granular matter in a slowly rotating drum partially filled with granular material and find a state of polydirectional stability. In this state, the material responds elastically to small stresses in a wide angular interval while it responds by plastic deformation when subjected to small stresses outside this interval of directions. We describe the evolution of the granulate by means of a rate equation and find quantitative agreement with the experiment. The state of polydirectional stability complements the fragile state, where the material responds elastically to small applied stresses only in a certain direction but even very small stresses in any other direction would lead to plastic deformations. Similar to fragile matter, polydirectionally stable matter is created in a dynamic process by self-organization. © 2013 American Physical Society.

Polyethylene-naphthalate (PEN) ferroelectrets: cellular structure, piezoelectricity and thermal stability - Fang, Peng; Qiu, Xunlin; Wirges, Werner; Gerhard, Reimund; Zirkel, Larissa (IEEE Transactions on Dielectrics and Electrical Insulation, 2010)
Cellular polyethylene-naphthalate (PEN) ferroelectrets are useful as soft and flexible electromechanical transducer materials. Improved cellular PEN foams are prepared by means of a + inflation + stretchingprocess and investigated with respect to their structure and their applications-relevant properties. It is found that most of the cellular voids have heights below 8 extgreekmm. The polymer walls do not allow sufficient gas exchange between the voids and the ambient atmosphere, when the cellular films are exposed to atmospheric pressures between a millibar and a few bars. As expected for ferroelectrets, a threshold voltage for charging is observed: A reasonable piezoelectric coefficient d_3_3 is only found when the charging voltage is higher than 4 kV. Furthermore, d_3_3 increases with charging voltage and reaches saturation at approximately 8 kV. Annealing after charging or charging at elevated temperatures may enhance the thermal stability of the PEN ferroelectrets. The d_3_3 of properly annealed samples is stable up to the respective annealing temperatures, but the annealing process reduces the piezoelectric activity of charged ferroelectret films to some extent. Samples charged at suitable elevated temperatures show much better thermal stability than those charged at room temperature, but the charging temperature should be limited to values below the material's glass-transition temperature Tg. Furthermore, the relevant elastic modulus c_3_3 of PEN ferroelectrets may decrease upon thermal treatment.
EAM Import::2019-03-08

Polyhydrogenated Graphene: Excited State Dynamics in Photo- and Electroactive Two-Dimensional Domains - Strauß, Volker; Schäfer, Ricarda; Hauke, Frank; Hirsch, Andreas; Guldi, Dirk Michael (Journal of the American Chemical Society, 2015)
Understanding the phenomenon of intense photoluminescence in carbon materials such as hydrogenated graphene, graphene nanoribbons, and so forth is at the forefront of investigations. In this study, six different types of hydrogenated graphene (phG) produced from different starting materials were fully characterized in terms of structure and optical spectroscopy. Comprehensive photoluminescence lifetime analyses of phGs were conducted by combining time-correlated single-photon counting with steady-state fluorescence spectroscopy and femtosecond transient absorption spectroscopy. The conclusion drawn from these assays is that graphene islands with diameters in the range from 1.1 to 1.75 nm reveal band gap photoluminescence between 450 and 800 nm. As a complement, phGs were implemented in hybrids with water-soluble electron accepting perylenediimides (PDIs). By virtue of mutual π-stacking and charge transfer interactions with graphene islands, PDIs assisted in stabilizing aqueous dispersion of phG. Implicit in these ground state interactions is the formation of 300 ps lived charge separated states once photoexcited.

Polymer-Based Spherical Activated Carbon as Easy-to-Handle Catalyst Support for Hydrogenation Reactions - Klefer, Heiko; Munoz Garcia, Macarena; Modrow, Antje; Böhringer, Bertram; Wasserscheid, Peter; Etzold, Bastian (Chemical Engineering & Technology, 2016)
The application of polymer-based spherical activated carbon (PBSAC) as catalyst support offers unprecedented advantages in the handling of the catalyst in all stages of its application due to its superior fluid dynamic properties and dust-free character, minimizing post processing times and facilitating catalyst recycling. The preparation of PBSAC-based Pd catalysts has been addressed using PBSAC of different particle sizes and degrees of activation. Prior to Pd deposition, the support was oxidized in nitric and sulfuric acidic media under different operating conditions in order to introduce surface charges for the ion exchange. This method was successfully applied as, even for the mildest functionalization conditions, full deposition of Pd was achieved. The synthesized Pd-PBSAC catalysts were successfully employed in the hydrogenation of cinnamic acid achieving its complete conversion at an optimum effective rate constant of 0.122mkg s.
Keywords: Cinnamic acid; Hydrogenation; Palladium catalyst; Polymer-based spherical activated carbon

Polymer-fullerene bulk-heterojunction solar cells - Brabec, Christoph; Gowrisanker, Srinivas; Halls, Jonathan J. M.; Laird, Darin; Jia, Shijun; Williams, Shawn P. (Advanced Materials, 2010)
Solution-processed bulk heterojunction organic photovoltaic (OPV) devices have gained serious attention during the last few years and are established as one of the leading next generation photovoltaic technologies for low cost power production. This article reviews the OPV development highlights of the last two decades, and summarizes the key milestones that have brought the technology to today's efficiency performance of over 7%. An outlook is presented on what will be required to drive this young photovoltaic technology towards the next major milestone, a 10% power conversion efficiency, considered by many to represent the efficiency at which OPV can be adopted in wide-spread applications. With first products already entering the market, sufficient lifetime for the intended application becomes more and more critical, and the status of OPV stability as well as the current understanding of degradation mechanisms will be reviewed in the second part of this article. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Polymerization related deformations in multilayer soft stamps for nanoimprint - Förthner, Michael; Papenheim, Marc; Rumler, Maximilian; Stumpf, Florian; Baier, Leander; Rommel, Matthias; Scheer, Hella Christin; Frey, Lothar (Journal of Applied Physics, 2017)
This paper presents a study concerning the macroscale deformation of soft stamps for nanoimprint lithography. Composite stamps consisting of two or three layers were prepared with varying process parameters, and the bow of the stamps was investigated with an optical profilometer. Deflection measurements were compared with results from a mechanical model in order to prove its validity. The composite stamps were prepared from polydimethylsiloxane (S-PDMS) and OrmoStamp® layers on different substrate materials (silicon or AF-32-glass). The influence of selected preparation parameters on the deformation was investigated, such as layer thickness, curing, and hardbake temperatures, as well as UV-exposure intensities. It is discussed how an appropriate choice of preparation parameters can reduce the deformation, too. By decreasing the curing intensity of a thin OrmoStamp layer, coated on top of a silicon wafer, from 200 mW/cm2 to 2 mW/cm2, the deformation was reduced from 6 μm to 0. It is shown that the mechanical model is capable of predicting the deformation of composite stamps consisting of OrmoStamp layers on top of a silicon substrate. Further on, deformation results for S-PDMS top layers are presented and discussed. As a promising process option for the reduction of stamp deformations, a new, room temperature curable PDMS recipe is introduced. For composite stamps prepared with this material, the deformation value was reduced below 10 μm, compared to values ranging from 10 μm up to 127 μm for thermally cured S-PDMS (50 °C or 70 °C). Finally, different two-layer stamps were coated with an additional third layer and measured again, in order to investigate the deflection for tri-layered cases. The research is of high relevance for the fabrication and application of substrate conformal imprint lithography stamps with minimized deflection. © 2017 Author(s).

Polymer/metal oxide hybrid dielectrics for low voltage field-effect transistors with solution-processed, high-mobility semiconductors - Held, Martin; Schießl, Stefan; Miehler, Dominik; Gannott, Florentina; Zaumseil, Jana (Applied Physics Letters, 2015)
Transistors for future flexible organic light-emitting diode (OLED) display backplanes should operate at low voltages and be able to sustain high currents over long times without degradation. Hence, high capacitance dielectrics with low surface trap densities are required that are compatible with solution-processable high-mobility semiconductors. Here, we combine poly(methyl methacrylate) (PMMA) and atomic layer deposition hafnium oxide (HfOx) into a bilayer hybrid dielectric for field-effect transistors with a donor-acceptor polymer (DPPT-TT) or single-walled carbon nanotubes (SWNTs) as the semiconductor and demonstrate substantially improved device performances for both. The ultra-thin PMMA layer ensures a low density of trap states at the semiconductor-dielectric interface while the metal oxide layer provides high capacitance, low gate leakage and superior barrier properties. Transistors with these thin (łeq70 nm), high capacitance (100–300 nF/cm2) hybrid dielectrics enable low operating voltages (<5 V), balanced charge carrier mobilities and low threshold voltages. Moreover, the hybrid layers substantially improve the bias stress stability of the transistors compared to those with pure PMMA and HfOx dielectrics.
EAM Import::2019-03-08

Polymer: Nonfullerene Bulk Heterojunction Solar Cells with Exceptionally Low Recombination Rates - Gasparini, Nicola; Salvador, Michael Filipe; Heumüller, Thomas; Richter, Moses; Classen, Andrej; Shrestha, Shreetu; Matt, Gebhard; Holliday, Sarah; Strohm, Sebastian; Egelhaaf, Hans Joachim; Wadsworth, Andrew; Baran, Derya; et al. (Advanced Energy Materials, 2017)
Organic semiconductors are in general known to have an inherently lower charge carrier mobility compared to their inorganic counterparts. Bimolecular recombination of holes and electrons is an important loss mechanism and can often be described by the Langevin recombination model. Here, the device physics of bulk heterojunction solar cells based on a nonfullerene acceptor (IDTBR) in combination with poly(3-hexylthiophene) (P3HT) are elucidated, showing an unprecedentedly low bimolecular recombination rate. The high fill factor observed (above 65%) is attributed to non-Langevin behavior with a Langevin prefactor (β/βL) of 1.9 × 10-4. The absence of parasitic recombination and high charge carrier lifetimes in P3HT:IDTBR solar cells inform an almost ideal bimolecular recombination behavior. This exceptional recombination behavior is explored to fabricate devices with layer thicknesses up to 450 nm without significant performance losses. The determination of the photoexcited carrier mobility by time-of-flight measurements reveals a long-lived and nonthermalized carrier transport as the origin for the exceptional transport physics. The crystalline microstructure arrangement of both components is suggested to be decisive for this slow recombination dynamics. Further, the thickness-independent power conversion efficiency is of utmost technological relevance for upscaling production and reiterates the importance of understanding material design in the context of low bimolecular recombination. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Charge recombination; Charge transport; Langevin; Nonfullerene acceptor; Organic photovoltaic devices

Polymorphic phase transitions: Macroscopic theory and molecular simulation - Anwar, Jamshed; Zahn, Dirk (Advanced Drug Delivery Reviews, 2017)
Transformations in the solid state are of considerable interest, both for fundamental reasons and because they underpin important technological applications. The interest spans a wide spectrum of disciplines and application domains. For pharmaceuticals, a common issue is unexpected polymorphic transformation of the drug or excipient during processing or on storage, which can result in product failure. A more ambitious goal is that of exploiting the advantages of metastable polymorphs (e.g. higher solubility and dissolution rate) while ensuring their stability with respect to solid state transformation. To address these issues and to advance technology, there is an urgent need for significant insights that can only come from a detailed molecular level understanding of the involved processes. Whilst experimental approaches at best yield time- and space-averaged structural information, molecular simulation offers unprecedented, time-resolved molecular-level resolution of the processes taking place. This review aims to provide a comprehensive and critical account of state-of-the-art methods for modelling polymorph stability and transitions between solid phases. This is flanked by revisiting the associated macroscopic theoretical framework for phase transitions, including their classification, proposed molecular mechanisms, and kinetics. The simulation methods are presented in tutorial form, focusing on their application to phase transition phenomena. We describe molecular simulation studies for crystal structure prediction and polymorph screening, phase coexistence and phase diagrams, simulations of crystal-crystal transitions of various types (displacive/martensitic, reconstructive and diffusive), effects of defects, and phase stability and transitions at the nanoscale. Our selection of literature is intended to illustrate significant insights, concepts and understanding, as well as the current scope of using molecular simulations for understanding polymorphic transitions in an accessible way, rather than claiming completeness. With exciting prospects in both simulation methods development and enhancements in computer hardware, we are on the verge of accessing an unprecedented capability for designing and developing dosage forms and drug delivery systems in silico, including tackling challenges in polymorph control on a rational basis.
EAM Import::2019-03-08

Porphyrin Antennas on Carbon Nanodots: Excited State Energy and Electron Transduction - Arcudi, Francesca; Strauß, Volker; Dordevic, Luka; Cadranel, Alejandro; Guldi, Dirk Michael; Prato, Maurizio (Angewandte Chemie International Edition, 2017)
We report the synthesis and electron donor-acceptor features of a novel nanohybrid, in which the light-harvesting and electron-donating properties of a meso-tetraarylporphyrin (TArP) are combined with the electron-accepting features of nitrogen-doped carbon nanodots (NCNDs). In particular, in an ultrafast process (>1012 s-1), visible-light excitation transforms the strongly quenched porphyrin singlet excited states into short-lived (225 ps) charge-separated states. On the other hand, ultraviolet light excitation triggers a non-resolvable transduction of singlet excited state energy from the NCNDs to the porphyrins, followed by the same charge separation observed upon visible light excitation.
EAM Import::2019-03-08

Porphyrin Metalation at the MgO Nanocube/Toluene Interface - Schneider, Johannes; Kollhoff, Fabian; Bernardi, Johannes; Kaftan, Andre; Libuda, Jörg; Berger, Thomas; Laurin, Mathias; Diwaid, Oliver (ACS Applied Materials and Interfaces, 2015)
Molecular insights into porphyrin adsorption on nanostructured metal oxide surfaces and associated ion exchange reactions are key to the development of functional hybrids for energy conversion, sensing, and light emission devices. Here we investigated the adsorption of tetraphenylporphyrin (2HTPP) from toluene solution on two types of MgO powder. We compare MgO nanocubes with an average size d < 10 nm and MgO cubes with 10 nm <= d <= 1000 nm. Using molecular spectroscopy techniques such as UV/vis transmission and diffuse reflectance (DR), photoluminescence (PL), and diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy in combination with structural characterization techniques (powder X-ray diffraction and transmission electron microscopy, TEM), we identified a new room temperature metalation reaction that converts 2HTPP into magnesium tetraphenyl-porphyrin (MgTPP). Mg2+ uptake from the MgO nanocube surfaces and the concomitant protonation of the oxide surface level off at a concentration that corresponds to roughly one monolayer equivalent adsorbed on the MgO nanocubes. Larger MgO cubes, in contrast, show suppressed exchange, and only traces of MgTPP can be detected by photoluminescence.
Keywords: porphyrin metalation; MgO cubes; solid-liquid interface; molecular spectroscopies; dye adsorption; hybrid materials

Positron emission particle tracking in fluidized beds with secondary gas injection - Hensler, Timo; Tupy, Martin; Strer, Timo; Pöschel, Thorsten; Wirth, Karl-Ernst (Powder Technology, 2015)
We apply positron emission particle tracking (PEPT) to a gas-solid fluidized bed with injection of a secondary gas through a centrally arranged nozzle and present a method to compute stationary fluid-dynamic characteristics of the system from the trajectories of a test particle. In order to evaluate this non-invasive method we compare the field of density obtained by PEPT with the density obtained by a traditional, well-established and approved, yet invasive, measurement technique to find good agreement. Besides the penetration depth of the jet region and the opening angle of the jet which are inferred from the density field, we use PEPT to measure quantities whose measurement using traditional methods is rather sophisticated, including the residence time of particles in the jet region and the suspended phase, the coefficients of axial and radial dispersion and the material flux across the jet boundaries. We conclude that PEPT is a reliable and at the same time versatile technique to measure stationary fluid-dynamic properties of dynamical particle systems at spatial resolution only limited by the duration of the measurement.
Keywords: Dispersion coefficient; Fluidized bed; Measurement technology; Positron emission particle tracking; Residence time; Secondary gas injection

Potential of two-line atomic fluorescence for temperature imaging in turbulent indium-oxide-producing flames - Münsterjohann, Bettina; Huber, Franz; Klima, Tobias; Holfelder, Sandra; Engel, Sascha; Miller, Joseph D.; Meyer, Terrence R.; Will, Stefan (Journal of Nanoparticle Research, 2015)
The applicability of two-line atomic fluorescence (TLAF) for temperature imaging in an indium-based flame spray pyrolysis (FSP) process is demonstrated using a single tunable optical parametric oscillator (OPO) to generate the required excitation wavelengths consecutively. Single-shot images of the detected fluorescence signals demonstrate that the signal levels in the flame are suitable for evaluation of temperature and verify the capability and potential of the measurement technique directly during particle formation without additional indium seeding. Qualitative averaged two-dimensional temperature distributions in the FSP flame are presented, showing the influence of varying sheath gas flow rates on the resulting temperature distribution. With the addition of a second OPO and detection system, the two fluorescence signals acquired consecutively in this work could be obtained simultaneously and enable spatio-temporally resolved single-shot temperature measurements in flame synthesis processes of indium-containing nanoparticles.
Keywords: Flame spray pyrolysis; Indium(III)-oxide nanoparticles; Two-line atomic fluorescence; Temperature measurement

Power Series Approximation for the Correlation Kernel Leading to Kohn-Sham Methods Combining Accuracy, Computational Efficiency, and General Applicability - Erhard, Jannis; Bleiziffer, Patrick; Görling, Andreas (Physical Review Letters, 2016)
A power series approximation for the correlation kernel of time-dependent density-functional theory is presented. Using this approximation in the adiabatic-connection fluctuation-dissipation (ACFD) theorem leads to a new family of Kohn-Sham methods. The new methods yield reaction energies and barriers of unprecedented accuracy and enable a treatment of static (strong) correlation with an accuracy of high-level multireference configuration interaction methods but are single-reference methods allowing for a black-box-like handling of static correlation. The new methods exhibit a better scaling of the computational effort with the system size than rivaling wave-function-based electronic structure methods. Moreover, the new methods do not suffer from the problem of singularities in response functions plaguing previous ACFD methods and therefore are applicable to any type of electronic system.

p-Phenyleneethynylene molecular wires: influence of structure on photoinduced electron-transfer properties. - Wielopolski, Mateusz; Atienza, Carmen; Clark, Timothy; Guldi, Dirk Michael; Martín, Nazario (Chemistry - A European Journal, 2008)
A series of donor-acceptor arrays (exTTF-oPPE-C ) containing π-conjugated oligo(phenyleneethynylene) wires (oPPE) of different length between π-extended tetrathiafulvalene (exTTF) as electron donor and fullerene (C ) as electron acceptor has been prepared by following a convergent synthesis. The key reaction in these approaches is the bromo-iodo selectivity of the Hagihara-Sonogashira reaction and the deprotecting of acetylenes with different silyl groups to afford the corresponding donor-acceptor conjugates in moderate yields. The electronic interactions between the three electroactive species were determined by using UV-visible spectroscopy and cyclic voltammetry. Our studies clearly confirm that, although the C units are connected to the exTTF donor through π-conjugated oPPE frameworks, no significant electronic interactions are observed in the ground state. Theoretical calculations predict how a simple exchange from C=C double bonds (i.e., oligo(p-phenylenevinylene) to C≡C triple bonds (i.e., oPPE) in the electron donor-acceptor conjugates considerably alters long-range electron transfer. Photoexcitation of exTTF-oPPE-C leads to the following features: a transient photoproduct with maxima at 660 and 1000 nm, which are unambiguously attributed to the photolytically generated radical-ion-pair state, [exTTF -oPPE- C ]. Both charge-separation and charge-recombination processes give rise to a molecular-wire behaviour of the oPPE moiety with an attenuation factor (β) of (0.2 ±0.05) Å . © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Präparationsprinzipien mikroporöser Materialien: Vom building block zum hierarchisch aufgebauten porösen System - Schwieger, Wilhelm; Klumpp, Michael; Al-Thabaiti, Shaeel A.; Hartmann, Martin (Chemie Ingenieur Technik, 2016)
Mikroporöse Materialien sind lange bekannt und werden aufgrund ihrer einzigartigen sorptiven und katalytischen Eigenschaften mit weiterhin steigendem Interesse untersucht, zielgerichtet synthetisiert und potenziellen Anwendungen zugeführt. Obwohl gerade die Mikroporosität entscheidend zu den herausragenden Eigenschaften in Katalyse und Sorptionsprozessen beiträgt, so bremst sie auch die Geschwindigkeit vieler solcher Prozesse. Deshalb rückt die Entwicklung von Materialien mit hierarchisch aufgebauter Porosität zunehmend in den Fokus der Forschung. Dieser Artikel spannt den Bogen von der grundsätzlichen Beschreibung mikroporöser Materialien und ihrer Baueinheiten, über grundsätzliche Präparationsprinzipien bei deren Herstellung bis hin zu den neueren Entwicklungen im Bereich der zeolithischen Materialien mit hierarchischer Porosität.
EAM Import::2019-03-08

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber - Euser, T. G.; Garbos, M. K.; Chen, J. S. Y.; Russell, Philip St. John (Optics Letters, 2009)
A great challenge in microfluidics is the precise control of laser radiation forces acting on single particles or cells, while allowing monitoring of their optical and chemical properties. We show that, in the liquid-filled hollow core of a single-mode photonic crystal fiber, a micrometer-sized particle can be held stably against a fluidic counterflow using radiation pressure and can be moved to and fro (over tens of centimeters) by ramping the laser power up and down. Accurate studies of the microfluidic drag forces become possible, because the particle is trapped in the center of the single guided optical mode, resulting in highly reproducible radiation forces. The counterflowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles, or cells.
EAM Import::2019-03-08

Precise control of intramolecular charge-transport: The interplay of distance and conformational effects - Schubert, Christina; Wielopolski, Mateusz; Mewes, Lars-Hendrik; De Miguel Rojas, Gustavo; Van Der Pol, Cornelia; Moss, Kathryn C.; Bryce, Martin; Moser, Jacques E.; Guldi, Dirk Michael; Clark, Timothy (Chemistry - A European Journal, 2013)
A new series of donor-bridge-acceptor (D-B-A) compounds consisting of π-conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron-donor and a fullerene (C) electron-acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono- and bi-fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p-phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy-matching between the Fc and the oFL units, which results in strong electronic-coupling. Hence, intramolecular charge-transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc-oFL-C conjugates results in transient radical-ion-pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge-separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge-separation rates, resulting in beta values between β 0.08 and 0.19 Å. In contrast, charge recombination depends strongly on the electron-donor-acceptor distance, but not at all on the linker. A value of β (0.35±0.01 Å) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D-B-A systems. Finely tuned transfer: Chemical modification of the oligo-fluorene linkers between Fc and C units enables fine-tuning of photoinduced charge-transfer processes in new donor-bridge-acceptor conjugates (see illustration). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Precise response functions in all-electron methods: Application to the optimized-effective-potential approach - Betzinger, Markus; Friedrich, Christoph; Görling, Andreas; Bluegel, Stefan (Physical Review B, 2012)
The optimized-effective-potential method is a special technique to construct local Kohn-Sham potentials from general orbital-dependent energy functionals. In a recent publication we showed that uneconomically large basis sets were required to obtain a smooth local potential without spurious oscillations within the full-potential linearized augmented-plane-wave method. This could be attributed to the slow convergence behavior of the density response function. In this paper, we derive an incomplete-basis-set correction for the response, which consists of two terms: (1) a correction that is formally similar to the Pulay correction in atomic-force calculations and (2) a numerically more important basis response term originating from the potential dependence of the basis functions. The basis response term is constructed from the solutions of radial Sternheimer equations in the muffin-tin spheres. With these corrections the local potential converges at much smaller basis sets, at much fewer states, and its construction becomes numerically very stable. We analyze the improvements for rock-salt ScN and report results for BN, AlN, and GaN, as well as the perovskites CaTiO , SrTiO , and BaTiO . The incomplete-basis-set correction can be applied to other electronic-structure methods with potential-dependent basis sets and opens the perspective to investigate a broad spectrum of problems in theoretical solid-state physics that involve response functions. © 2012 American Physical Society.

Predicting core level binding energies shifts: Suitability of the projector augmented wave approach as implemented in VASP - Pueyo Bellafont, Noelia; Vines, Francesc; Hieringer, Wolfgang; Illas, Francesc (Journal of Computational Chemistry, 2017)
Here, we assess the accuracy of various approaches implemented in Vienna ab initio simulation package code to estimate core-level binding energy shifts ( extgreekDBEs) using a projector augmented wave method to treat core electrons. The performance of the Perdew-Burke-Ernzerhof (PBE) and the Tao-Perdew-Staroverov-Scuseria (TPSS) exchange-correlation density functionals is examined on a dataset of 68 molecules containing Bi̊ghtarrowF atoms in diverse chemical environments, accounting for 185 different 1s core level binding energy shifts, for which both experimental gas-phase X-ray photoemission (XPS) data and accurate all electron extgreekDBEs are available. Four procedures to calculate core-level shifts are investigated. Janak-Slater transition state approach yields mean absolute errors of 0.37 (0.21) eV at PBE (TPSS) level, similar to highly accurate all electron extgreekDSCF approaches using same functionals, and close to XPS experimental accuracy of 0.1 eV. The study supports the use of these procedures to assign extgreekDBEs of molecular moieties on material surfaces of interest in surface science, nanotechnology, and heterogeneous catalysis.
EAM Import::2019-03-08

Predicting the Effects of Basepair Mutations in DNA-Protein Complexes by Thermodynamic Integration - Beierlein, Frank; Kneale, G. Geoff; Clark, Timothy (Biophysical Journal, 2011)
Thermodynamically rigorous free energy methods in principle allow the exact computation of binding free energies in biological systems. Here, we use thermodynamic integration together with molecular dynamics simulations of a DNA-protein complex to compute relative binding free energies of a series of mutants of a protein-binding DNA operator sequence. A guanine-cytosine basepair that interacts strongly with the DNA-binding protein is mutated into adenine-thymine, cytosine-guanine, and thymine-adenine. It is shown that basepair mutations can be performed using a conservative protocol that gives error estimates of  10% of the change in free energy of binding. Despite the high CPU-time requirements, this work opens the exciting opportunity of being able to perform basepair scans to investigate protein-DNA binding specificity in great detail computationally.

Predicting the Specific Surface Area and Pressure Drop of Reticulated Ceramic Foams Used as Catalyst Support - Inayat, Amer; Freund, Hannsjoerg; Schwab, Andreas; Zeiser, Thomas; Schwieger, Wilhelm; Freund, Hannsjörg (Advanced Engineering Materials, 2011)
Open-cell ceramic foams are attractive candidates as catalyst support owing to their remarkable heat and mass transfer characteristics. The accurate knowledge of the specific surface area and the pressure drop is a prerequisite for the reliable design of reactors with solid foam structures as internals. In this contribution, we propose a new correlation for the prediction of the pressure drop in reticulated ceramic foams using their predicted specific surface area. The correlation is based on an ideal tetrakaidecahedron geometry, which is the preferred representative geometry for reticulated foams. The validity of the correlation is demonstrated by experimental data of reticulated ceramic foams of different pores per inch (PPI), porosity, and material from the present study as well as from the open literature.

Preparation and Adsorption Properties of Pd Nanoparticles Supported on TiO2 Nanotubes - Honciuc, Andrei; Laurin, Mathias; Albu, Sergiu; Amende, Maximilian; Sobota, Marek; Lynch, Robert; Schmuki, Patrik; Libuda, Jörg (Journal of Physical Chemistry C, 2010)
Here we present preparation methods for a series of novel model heterogeneous catalysts consisting of Pd nanoparticles dispersed on self-organized TiO2 nanotube arrays (TiNTs). For a comprehensive comparative study of TiNTs' surface adsorption properties, different chemical and physical methods were employed for the deposition of the Pd nanoparticles: (i) e-beam deposition in high vacuum (HV) and (ii) in ultrahigh vacuum (UHV), (iii) impregnation, and (iv) metal precipitation. Following preparation, the Pd/TiNTs were exposed to several O-2 (oxidative) and CO (reductive) cleaning cycles at 500 K in UHV. After each cleaning cycle, the CO adsorption properties were systematically probed with time-resolved infrared reflection-adsorption spectroscopy to monitor the surface state of the catalyst. It is shown that clean and well-defined Pd/TiNT catalysts can be obtained, which are well suited for future adsorption and reactivity studies.

Preparation and characterization of ultrathin [Ru(CO) 3Cl 2] 2 and [BMIM][Tf 2N] films on Al 2O 3/NiAl(110) under UHV conditions - Sobota, Marek; Schernich, Stefan; Schulz, Hannes; Hieringer, Wolfgang; Paape, Natalia; Wasserscheid, Peter; Görling, Andreas; Laurin, Mathias; Libuda, Jörg (Physical Chemistry Chemical Physics, 2012)
Towards a better understanding of the interface chemistry of ionic liquid (IL) thin film catalytic systems we have applied a rigorous surface science model approach. For the first time, a model homogeneous catalyst has been prepared under ultrahigh vacuum conditions. The catalyst, di-μ- chlorobis(chlorotricarbonylruthenium) [Ru(CO) Cl ] , and the solvent, the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][Tf N], have been deposited by physical vapor deposition onto an alumina model support [Al O /NiAl(110)]. First, the interaction between thin films of [Ru(CO) Cl ] and the support is investigated. Then, the ruthenium complex is co-deposited with the IL and the influence of the solvent on the catalyst is discussed. D O, which is a model reactant, is further added. Growth, surface interactions, and mutual interactions in the thin films are studied with IRAS in combination with density functional (DFT) calculations. At 105 K, molecular adsorption of [Ru(CO) Cl ] is observed on Al O /NiAl(110). The IRAS spectra of the binary [Ru(CO) Cl ] + [BMIM][Tf N] and ternary [Ru(CO) Cl ] + [BMIM][Tf N] + D O show every characteristic band of the individual components. Above 223 K, partial decomposition of the ruthenium complex leads to species of molecular nature attributed to Ru(CO) and Ru(CO) surface species. Formation of metallic ruthenium clusters occurs above 300 K and the model catalyst decomposes further at higher temperatures. Neither the presence of the IL nor of D O prevents this partial decomposition of [Ru(CO) Cl ] on alumina. © 2012 the Owner Societies.

Preparation of carbide-derived carbon supported platinum catalysts - Hasse, Benjamin; Gläsel, Jan-Sebastian; Kern, Andreas; Murzin, D. Yu.; Etzold, Bastian (Catalysis Today, 2014)
Well defined carbon supported catalysts are of interest to deduce structure activity relationships and for catalyst optimization. Carbide-derived carbons (CDC) are high purity porous carbons with tunable pore structure. The deposition of platinum on silicon and titanium carbide based CDC was studied, using a three step process of acid treatment, ion-adsorption of platinum precursor and gas phase reduction. The influence of the steps on the carbon support properties and possibility to control the platinum loading and resulting cluster size was determined. Most crucial is the acid treatment step, where oxygen groups are introduced. At a too high degree of functionalization (non-carbon content above 30 wt%), which can result for nitric acid treatment, the pore structure is heavily attacked. For sulfuric acid treatments non-carbon contents below 15 wt% and only minor changes in the pore structure were observed. The ion-adsorption technique could be successfully transferred to the CDC supports. For medium to high degrees of functionalization (non-carbon contents above 20 wt%) the full deposition of 2.5 wt% of platinum was achieved. By the choice of platinum precursor, the pH of impregnation and the temperature of reduction the platinum cluster size could be varied from 2.5 to 5.3 nm. Thus, important knowledge how to prepare tuned Pt/C catalysts based on CDC and the ion-adsorption technique could be deduced.
EAM Import::2019-03-08

Preparation of electrochemically active silicon nanotubes in highly ordered arrays - Gruenzel, Tobias; Lee, Young Joo; Kuepper, Karsten; Bachmann, Julien (Beilstein Journal of Nanotechnology, 2013)
Silicon as the negative electrode material of lithium ion batteries has a very large capacity, the exploitation of which is impeded by the volume changes taking place upon electrochemical cycling. A Si electrode displaying a controlled porosity could circumvent the difficulty. In this perspective, we present a preparative method that yields ordered arrays of electrochemically competent silicon nanotubes. The method is based on the atomic layer deposition of silicon dioxide onto the pore walls of an anodic alumina template, followed by a thermal reduction with lithium vapor. This thermal reduction is quantitative, homogeneous over macroscopic samples, and it yields amorphous silicon and lithium oxide, at the exclusion of any lithium silicides. The reaction is characterized by spectroscopic ellipsometry for thin silica films, and by nuclear magnetic resonance and X-ray photoelectron spectroscopy for nanoporous samples. After removal of the lithium oxide byproduct, the silicon nanotubes can be contacted electrically. In a lithium ion electrolyte, they then display the electrochemical waves also observed for other bulk or nanostructured silicon systems. The method established here paves the way for systematic investigations of how the electrochemical properties (capacity, charge/discharge rates, cyclability) of nanoporous silicon negative lithium ion battery electrode materials depend on the geometry.
Keywords: atomic layer deposition; electrochemistry; lithium ion battery electrode; silica thermal reduction; silicon nanotubes

Preparation Principles of Microporous Materials: From Building Blocks Towards Hierarchically Ordered Systems - Schwieger, Wilhelm; Klumpp, Michael; Al-Thabaiti, Shaeel A.; Hartmann, Martin (Chemie Ingenieur Technik, 2016)
Microporous materials are well known and due to their unique properties in sorptive and catalytic processes also of still growing interest in research with regards to, e.g., targeted material synthesis and potential application in novel processes. Though, it is their microporosity which is in the first place the responsible property for the outstanding performance in sorption and catalysis, it is also its small pore size which thwarts the kinetic of such processes. This discrepancy is therefore the main driving force for the ongoing development of hierarchically porous materials. This article gives a basic description of microporous materials and their building blocks in general as well as the underlying principles of their preparation and additionally gives a review on the field of recent developments with special focus on hierarchically porous zeolites.
Keywords: Building blocks; Core-shell; Hierarchy; Porous materials; Preparation principles

Preventing mussel adhesion using lubricant-infused materials - Amini, Shahrouz; Kolle, Stefan; Petrone, Luigi; Ahanotu, Onyemaechi; Sunny, Steffi; Sutanto, Clarinda N.; Hoon, Shawn; Cohen, Lucas; Weaver, James C.; Aizenberg, Joanna; Vogel, Nicolas; Miserez, Ali (Science, 2017)
Mussels are opportunistic macrofouling organisms that can attach to most immersed solid surfaces, leading to serious economic and ecological consequences for the maritime and aquaculture industries. We demonstrate that lubricant-infused coatings exhibit very low preferential mussel attachment and ultralow adhesive strengths under both controlled laboratory conditions and in marine field studies. Detailed investigations across multiple length scales-from the molecular-scale characterization of deposited adhesive proteins to nanoscale contact mechanics to macroscale live observations-suggest that lubricant infusion considerably reduces fouling by deceiving the mechanosensing ability of mussels, deterring secretion of adhesive threads, and decreasing the molecular work of adhesion. Our study demonstrates that lubricant infusion represents an effective strategy to mitigate marine biofouling and provides insights into the physical mechanisms underlying adhesion prevention.

Principles for the Heart Treatment Layout of Ultrafine-Grain Aluminium Blanks with Locally Adapted Mechanical Properties - Merklein, Marion; Vogt, Uwe (Proceedings of the COMA 2010, 2009)
EAM Import::2019-03-08

Printable Dielectric Mirrors with Easily Adjustable and Well-Defined Reflection Maxima for Semitransparent Organic Solar Cells - Bronnbauer, Carina; Hornich, Julian; Gasparini, Nicola; Guo, Fei; Hartmeier, Benjamin; Luechinger, Norman A.; Pflaum, Christoph; Brabec, Christoph; Forberich, Karen (Advanced Optical Materials, 2015)
Building integrated semitransparent thin-film solar cells is a strategy for future eco-friendly power generation. Organic photovoltaics in combination with dielectric mirrors (DMs) are a potential candidate as they promise high efficiencies in parallel to the possibility to adjust the color and thus the transparency of the whole device. A fully solution processed and printable DM with an easily adjustable reflection maximum is presented that can be facilely attached to solar cells. The DM is optimized via optical simulations to the particular needs of the device with regard to photocurrent enhancement. The excellent agreement between experimental and theoretical results confirms the high optical quality of the printed layers with respect to homogeneity and surface roughness. The used inks are organic-inorganic nanocomposites with a large refractive index contrast of ≈0.7. The short-circuit current is enhanced by up to ≈24% for a semitransparent polymer solar cell.
Keywords: Dielectric mirrors; Organic photovoltaics; Printing; Refractive indices; Semitransparent

Printed Smart Photovoltaic Window Integrated with an Energy-Saving Thermochromic Layer - Guo, Fei; Chen, Shi; Chen, Zhang; Luo, Hongjie; Gao, Yanfeng; Przybilla, Thomas; Spiecker, Erdmann; Osvet, Andres; Forberich, Karen; Brabec, Christoph (Advanced Optical Materials, 2015)
A smart photovoltaic window is designed and constructed by solution-processing a layer of thermochromic VO2 nanoparticles on top of a semitransparent organic solar cell. The prepared smart window not only produces electricity using the visible part of the solar spectrum but also saves energy via intelligently modulating the amount of NIR radiation passing through the device in response to ambient temperature.

Printing high performance reflective electrodes for organic solar cells - Krantz, Johannes; Forberich, Karen; Kubis, Peter; Machui, Florian; Min, Jie; Stubhan, Tobias; Brabec, Christoph (Organic Electronics, 2015)
In this paper we solve a long term material problem of thin film organic electronics, namely the solution processing of an opaque electrode. Solution processing of opaque metallic top electrodes typically leads to severe shunting problems. We solve this issue by reversing the electrode sequence and print a highly conductive but opaque bottom electrode from metallic precursors. Devices based on these printed bottom electrodes are compared to reference stacks based on evaporated silver. The transparent top electrode is solution processed from silver nanowire inks, which results in highly transparent electrodes with excellent conductivity. The optical, mainly reflective properties of the opaque silver electrode are investigated in comparison to screen-printed silver pastes. The outstanding smoothness of the printed Ag electrode results in high reflectivity and poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based solar cells with η >. 2.5% and high fill factors performing on par with reference devices on evaporated silver electrode layers.
Keywords: Highly reflective electrode; Printed silver precursor ink; Solution processable electrode; Substrate architecture; Superstrate architecture

Probing a gas/liquid acid-base reaction by X-ray photoelectron spectroscopy - Niedermaier, Inga; Taccardi, Nicola; Wasserscheid, Peter; Maier, Florian; Steinrück, Hans-Peter (Angewandte Chemie International Edition, 2013)
A mechanistic study: In a Brønsted acid/base reaction, gaseous triflic acid (OTfH) transfers its proton to the basic Cl anion of the ionic liquid [CCIm]Cl forming volatile HCl and the IL [CCIm][TfO]. This anion exchange reaction is monitored in the near-surface region quantitatively by X-ray photoelectron spectroscopy as a function of time (see picture; BE=binding energy). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Probing charge transfer in benzodifuran-C60 dumbbell-type electron donor-acceptor conjugates: Ground- and excited-state assays - Li, Hui; Schubert, Christina; Dral, Pavlo; Costa, Ruben Dario; La Rosa, Andrea; Thüring, Jürg; Liu, Shi-Xia; Yi, Chenyi; Filippone, Salvatore; Martín, Nazario; Decurtins, Silvio; Clark, Timothy; et al. (ChemPhysChem, 2013)
Rigid electron donor-acceptor conjugates (1-3) that combine π-extended benzodifurans as electron donors and C molecules as electron acceptors with different linkers have been synthesized and investigated with respect to intramolecular charge-transfer events. Electrochemistry, fluorescence, and transient absorption measurements revealed tunable and structure-dependent charge-transfer processes in the ground and excited states. Our experimental findings are underpinned by density-functional theory calculations. Dumbbell arrays: A benzodifuran (BDF) donor is for the first time fused to two C molecules by different linkers to form triads 1-3 (see figure). Their redox behavior, absorption, fluorescence emission, and photoinduced intramolecular charge-transfer events have been investigated experimentally and theoretically. The lifetime of the resulting charge-separated state varies distinctly with the polarity of the solvents and the distance between the BDF and C units. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Probing colloidal interfaces by angle-resolved second harmonic light scattering - Schürer, Benedikt; Wunderlich, Sarina; Sauerbeck, Christian; Peschel, Ulf; Peukert, Wolfgang (Physical Review B, 2010)
We report angle-resolved second harmonic light scattering from bare colloidal particles in water. A nonlinear Mie model is used to quantitatively analyze the nonresonant second harmonic (SH) scattering profiles and to determine the elements of the local second-order surface susceptibility. The SH signal is attributed to oriented and electrically polarized water molecules at the charged particle interface. The results show how angle-resolved SH light scattering can be applied to probe not only resonant excited dye molecules but also nonresonant molecules such as water at the interface of colloidal particles.

Probing guided modes in a monolayer colloidal crystal on a flat metal film - Romanov, Sergei G.; Vogel, Nicolas; Bley, Karina; Landfester, Katharina; Weiss, Clemens K.; Orlov, Sergej; Korovin, Alexander V.; Chuiko, Gennady P.; Regensburger, Alois; Romanova, Alexandra; Kriesch, Arian; Peschel, Ulf (Physical Review B, 2012)
Two-dimensional slab hybrid metal-dielectric photonic crystals, which are prepared by assembling polymer colloidal spheres into closely packed monolayers of hexagonal symmetry on a gold-coated glass substrate, show an improved confinement of light compared with a colloidal monolayer on a glass substrate. We demonstrated that the optical response of such hybrid crystals consists of diffractively coupled waveguiding modes, Fabry–Perot resonances, and Mie resonances. Correspondingly, two major mechanisms, namely, band transport and hopping of localized excitations, participate in the in-plane light transport in such hybrid crystals.

Probing Mass Transfer in Mesoporous Faujasite-Type Zeolite Nanosheet Assemblies - Mehlhorn, Dirk; Inayat, Alexandra; Schwieger, Wilhelm; Valiullin, Rustem; Kärger, Jörg (ChemPhysChem, 2014)
Pulsed field gradient nuclear magnetic resonance (NMR) diffusion studies are performed by using cyclohexane to probe transport properties in a NaX-type zeolite with a hierarchical pore structure (house-of-cards-like assemblies of mesoporous nanosheets), which is compared with a purely microporous sample. With guest loadings chosen to ensure saturation of the micropores, and the meso- and macropores left essentially unoccupied, guest diffusion is shown to be enhanced by almost one order of magnitude, even at room temperature. Diffusivity enhancement is further increased with increasing temperature, which may, therefore, be unambiguously attributed to the contribution of mass transfer in the meso-and macropores.
Keywords: diffusion; hierarchical pore systems; nanostructures; nmr spectroscopy; zeolites

Probing the interaction of Rh, Co and bimetallic Rh-Co nanoparticles with the CeO2 support: catalytic materials for alternative energy generation - Varga, E.; Pusztai, P.; Ovari, L.; Oszko, A.; Erdohelyi, A.; Papp, Christian; Steinrück, Hans-Peter; Konya, Zoltan; Kiss, J. (Physical Chemistry Chemical Physics, 2015)
The interaction of CeO2-supported Rh, Co and bimetallic Rh-Co nanoparticles, which are active catalysts in hydrogen production via steam reforming of ethanol, a process related to renewable energy generation, was studied by X-ray diffraction (XRD), high resolution electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and low energy ion scattering (LEIS). Furthermore, diffuse reflectance infrared spectroscopy (DRIFTS) of adsorbed CO as a probe molecule was used to characterize the morphology of metal particles. At small loadings (0.1%), Rh is in a much dispersed state on ceria, while at higher contents (1-5%), Rh forms 2-8 nm particles. Between 473-673 K pronounced oxygen transfer from ceria to Rh is observed and at 773 K significant agglomeration of Rh occurs. On reduced ceria, XPS indicates a possible electron transfer from Rh to ceria. The formation of smaller ceria crystallites upon loading with Co was concluded from XRD and HRTEM; for 10% Co, the CeO2 particle size decreased from 27.6 to 10.7 nm. A strong dissolution of Co into ceria and a certain extent of encapsulation by ceria were deduced by XRD, XPS and LEIS. In the bimetallic system, the presence of Rh enhances the reduction of cobalt and ceria. During thermal treatments, reoxidation of Co occurs, and Rh agglomeration as well as oxygen migration from ceria to Rh are hindered in the presence of cobalt.

Probing the validity of an effective-one-particle description of granular dampers in microgravity - Sack, Achim; Heckel, Michael; Kollmer, Jonathan; Pöschel, Thorsten (Granular Matter, 2014)
We consider the attenuation of the oscillation of a flat spring due to the action of a granular damper. The efficiency of the damper is quantified by evaluating the position of the oscillator as a function of time using a Hall effect based position sensor. Performing experiments for a large abundance of parameters under conditions of microgravity, we confirm a recent theory for granular damping (Kollmer et al. in New J Phys 15:093023, 2013) and show that the theory remains approximately valid even beyond the limits of its derivation.
Keywords: Dissipation; Granular systems; Vibration damping

Process control strategies for the gas phase synthesis of silicon nanoparticles - Gröschel, Michael; Koermer, Richard; Walther, Maximilian; Leugering, Günter; Peukert, Wolfgang (Chemical Engineering Science, 2012)
In this contribution the identification of new reaction conditions for the production of nearly monodisperse silicon nanoparticles via the pyrolysis of monosilane in a hot wall reactor is considered. For this purpose a full finite volume model has been combined with a state-of-the-art trust-region optimisation algorithm for process control. Verified against experimental data, specific process conditions are determined accomplishing a versatile range of prescribed product properties. The main achievement of the optimisation is the possibility to control the different mechanisms in the particle formation process by mainly adjusting the temperature profile. Due to a successful separation of the nucleation and growth process, significantly narrower particle size distributions are obtained. Moreover, the presented optimisation framework establishes rate constants based on measured data.

Processing of Ceramic Foams with Hierarchical Cell Structure - Ceron-Nicolat, Bruno; Fey, Tobias; Greil, Peter (Advanced Engineering Materials, 2010)
Silicon carbide-based cellular ceramics characterized by a hierarchical pore structure were processed. An open-cellular PU foam template with a mean cell density of 10 pores per inch (ppi), equivalent to a cell diameter of 4 mm and a strut thickness of 250 mu m, was coated with a primary SiC slurry. Cross-linking of a polysiloxane binder at 190 h resulted in a SiC filled reticulated thermoset foam (first generation). Subsequently, this matrix foam was infiltrated with a second slurry of slightly different polysiloxane composition which upon heating to 290 degrees C caused bubble nucleation and formation of a second generation foam filling the cell space in the matrix foam skeleton. After pyrolysis at 1000 degrees C a SiC/Si-O-C reaction bonded composite foam with a hierarchical cell structure and a fractional density of 0.31 was obtained. SEM and X-ray mu-CT analyses confirm both generations of matrix as well as infiltrated foam to be open cellular but with a pronounced difference in cell size (matrix foam cell size 2.5 mm versus infiltrated foam cell size 0.29 mm). While the matrix foam skeleton provides control of macroscopic shape as well as density distribution in the component, single- or multistep foam infiltration may offer a high potential for improving the mechanical properties of hierarchical cellular materials. Thus, hierarchical structure formation offers a high potential to fabricate low density cellular ceramics which might be of interest for lightweight design of novel engineering materials.

Process specific catalyst supports-Selective electron beam melted cellular metal structures coated with microporous carbon - Knorr, Tilman; Heinl, Peter; Schwerdtfeger, Jan; Körner, Carolin; Singer, Robert; Etzold, Bastian (Chemical Engineering Journal, 2012)
Cellular support structures of Ti-6Al-4V produced by selective electron beam melting were coated with microporous carbon using a combined chemical vapor deposition and carbide-derived carbon approach. The production method allows for a process specific design of the open cellular metal structure. High thermal and electrical conductivity of the metal substrate are combined with the high surface area of the carbon coating within the tailored catalyst support. The study shows that for a successful deposition of the microporous carbon a protective oxide layer on top of the metal structure is necessary. Samples were impregnated with platinum as active metal and successfully tested in the hydrogenation of ethene. © 2011 Elsevier B.V..
Keywords: Carbide-derived carbon; Catalysis; Chemical vapor deposition; Foams; Microporous carbon; Selective electron beam melting; Silicon carbide; Structured catalyst support; Ti-6Al-4V

Production of nitrogen-doped graphene by low-energy nitrogen implantation - Zhao, Wei; Höfert, Oliver; Gotterbarm, Karin; Zhu, Junfa; Papp, Christian; Steinrück, Hans-Peter (Journal of Physical Chemistry C, 2012)
Nitrogen doping of graphene is a suitable route to tune the electronic structure of graphene, leading to n-type conductive materials. Herein, we report a simple way to insert nitrogen atoms into graphene by low-energy nitrogen bombardment, forming nitrogen-doped graphene. The formation of nitrogen-doped graphene is investigated with high resolution X-ray photoelectron spectroscopy, allowing to determine the doping level and to identify two different carbon-nitrogen species. By application of different ion implantation energies and times, we demonstrate that a doping level of up to 0.05 monolayers is achievable and that the branching ratio of the two nitrogen species can be varied. © 2012 American Chemical Society.

Production of well dispersible single walled carbon nanotubes via a "floating catalyst"-method - Toni, Franziska; Xing, Haichen; Walter, Johannes; Strauß, Volker; Nacken, Thomas; Damm, Cornelia; Wirth, Karl-Ernst; Guldi, Dirk Michael; Peukert, Wolfgang (Chemical Engineering Science, 2015)
In this paper, we present a unique process chain for the production of single-walled carbon nanotubes (SWCNT) and document the relationship between the process parameters in gas phase production, the dispersibility of SWCNTs in aqueous solutions of sodium cholate and their properties. SWCNTs were prepared by a "floating catalyst" method using a solution of ferrocene in ethanol as precursor. Production rates in the range from 5 to 20. mg/h were achieved by using an optimized riser reactor design, which enables reduction of undesired wall-sticking of the catalyst particles and SWCNTs. Products featuring iron catalyst contents less than 30. wt% were obtained for low precursor concentrations in combination with short residence time in the reactor. Transmission electron microscopy (TEM) and statistical Raman analyses of the products reveal that the SWCNTs exhibit a diameter distribution ranging from 0.5 to 2.0. nm. Processing conditions, including precursor concentration, residence time, etc. were found to have only a slight impact on the mean geometric SWCNT diameter. Sonication based post-processing of the as-prepared SWCNTs in aqueous solutions of sodium cholate leads to an effective individualization of SWCNTs as proven by atomic force microscopy (AFM), 3D fluorescence spectroscopy, and by multi-wavelength analytical ultracentrifugation (AUC). Overall, the dispersibility of the non-purified SWCNTs in a 2. wt% aqueous solution of sodium cholate increases as the ferrocene partial pressure is decreased and, in turn, the catalyst concentration is reduced in the gas phase. Statistical Raman spectroscopy performed with the SWCNTs reveals that more than 75% of the produced carbon species are SWCNTs and that about 2/3 are semiconducting.
Keywords: Analytical ultracentrifugation and Raman spectroscopy; CNTs; Dispersion; Floating catalyst

Projection Algorithms with Correction - Nicola, A.; Popa, Constantin; Rüde, Ulrich (Journal of Applied Mathematics & Informatics, 2011)
We present in this paper two versions of a general correction procedure applied to a classical linear iterative method. This gives us the possibility, under certain assumptions, to obtain an extension of it to inconsistent linear least-squares problems. We prove that some well known extended projection type algorithms from image reconstruction in computerized tomography fit into one or the other of these general versions and are derived as particular cases of them. We also present some numerical experiments on two phantoms widely used in image reconstruction literature. The experiments show the importance of these extension procedures, reflected in the quality of reconstructed images.
UnivIS-Import:2015-03-09:Pub.2011.tech.IMMD.lsinfs.projec