Are physicists afraid of mathematics? - Kollmer, Jonathan; Pöschel, Thorsten; Gallas, Jason (New Journal of Physics, 2015)
A recent study claimed that heavy use of equations impedes communication among biologists, as measured by the ability to attract citations from peers. It was suggested that to increase the probability of being cited one should reduce the density of equations in papers, that equations should be moved to appendices, and that math training among biologists should be improved. Here, we report a detailed study of the citation habits among physicists, a community that has traditionally strong training and dependence on mathematical formulations. Is it possible to correlate statistical citation patterns and fear of mathematics in a community whose work strongly depends on equations? By performing a systematic analysis of the citation counts of papers published in one of the leading journals in physics covering all its disciplines, we find striking similarities with distribution of citations recorded in biological sciences. However, based on the standard deviations in citation data of both communities, biologists and physicists, we argue that trends in statistical indicators are not reliable to unambiguously blame mathematics for the existence or lack of citations. We digress briefly about other statistical trends that apparently would also enhance citation success.
Keywords: citation strategies; science communication; social physics

A review of models for single particle compression and their application to silica microspheres - Paul, Jonas; Romeis, Stefan; Tomas, Jürgen; Peukert, Wolfgang (Advanced Powder Technology, 2014)
We report on the deformation behavior of single silica microspheres. For the first time a detailed discussion on the quantitative evaluation of force–deformation data in the elastic and plastic regime is given. The microspheres are compressed between two flat plates of a custom built manipulation device supported by a scanning electron microscope. The device allows a high sample throughput enabling full statistical evaluation of force–deformation data of single microspheres. Existing theories describing the deformation behavior of single spheres are discussed and applied to the elastic and plastic deformation regime of silica microspheres. The results obtained from the theories are compared between each other and values reported in literature. The silica microspheres exhibit a significantly different deformation behavior than expected from bulk fused silica, i.e. a distinct plastic deformation behavior. Furthermore, a significant decrease in Young’s modulus and hardness was observed caused by high porosity and reduced crosslinking of silicon atoms, respectively, that is inherent to silica produced by Stöber synthesis. However, the calculated contact pressures exceed the compressive strength of bulk fused silica by at least a factor of two as an effect of a reduced amount of material flaws in microstructures.

A Review of Photocatalysis using Self-organized TiO2 Nanotubes and Other Ordered Oxide Nanostructures - Paramasivam, Indhumati; Jha, Himendra; Liu, Ning; Schmuki, Patrik (Small, 2012)
Photocatalytic approaches, that is the reaction of light-produced charge carriers at a semiconductor surface with their environment, currently attract an extremely wide scientific interest. This is to a large extent due to the high expectations: i) to convert sunlight directly into an energy carrier (H2), ii) to stimulate chemical synthetic reactions, or iii) to degrade unwanted environmental pollutants. Since the early reports in 1972, TiO2 has been the most investigated photocatalytic material by far; this originates from its outstanding electronic properties that allow for a wide range of applications. Not only the material, but also its structure and morphology, can have a considerable influence on the photocatalytic performance of TiO2. In recent years, particularly 1D (or pseudo 1D) structures such as nanowires and nanotubes have received great attention. The present Review focuses on TiO2 nanotube arrays (and similar structures) that grow by self-organizing electrochemistry (highly aligned) from a Ti metal substrate. Herein, the growth, properties, and applications of these tubes are discussed, as well as ways and means to modify critical tube properties. Common strategies are addressed to improve the performance of photocatalysts such as doping or band-gap engineering, co-catalyst decoration, junction formation, or applying external bias. Finally, some unique applications of the ordered tube structures in various photocatalytic approaches are outlined.
EAM Import::2019-03-13

A robust comparison of dynamical scenarios in a glass-forming liquid - Vispa, Alessandro; Busch, Sebastian; Tamarit, Josep Lluis; Unruh, Tobias; Fernandez-Alonso, Felix; Pardo, Luis Carlos (Physical Chemistry Chemical Physics, 2016)
We use Bayesian inference methods to provide fresh insights into the sub-nanosecond dynamics of glycerol, a prototypical glass-forming liquid. To this end, quasielastic neutron scattering data as a function of temperature have been analyzed using a minimal set of underlying physical assumptions. On the basis of this analysis, we establish the unambiguous presence of three distinct dynamical processes in glycerol, namely, translational diffusion of the molecular centre of mass and two additional localized and temperature-independent modes. The neutron data also provide access to the characteristic length scales associated with these motions in a model-independent manner, from which we conclude that the faster (slower) localized motions probe longer (shorter) length scales. Careful Bayesian analysis of the entire scattering law favors a heterogeneous scenario for the microscopic dynamics of glycerol, where molecules undergo either the faster and longer or the slower and shorter localized motions.

Aryl substitution of pentacenes - Waterloo, Andreas; Sale, Anna Chiara; Lehnherr, Dan; Hampel, Frank; Tykwinski, Rik (Beilstein Journal of Organic Chemistry, 2014)
A series of 11 new pentacene derivatives has been synthesized, with unsymmetrical substitution based on a trialkylsilylethynyl group at the 6-position and various aryl groups appended to the 13-position. The electronic and physical properties of the new pentacene chromophores have been analyzed by UV-vis spectroscopy (solution and thin films), thermoanalytical methods (DSC and TGA), cyclic voltammetry, as well as X-ray crystallography (for 8 derivatives). X-ray crystallography has been specifically used to study the influence of unsymmetrical substitution on the solid-state packing of the pentacene derivatives. The obtained results add to our ability to better predict substitution patterns that might be helpful for designing new semiconductors for use in solid-state devices.
EAM Import::2019-03-13

A self-assembled metamaterial for Lamb waves - Khanolkar, Amey; Wallen, Samuel; Ghanem, M. Abi; Jenks, J.; Vogel, Nicolas; Boechler, N. (Applied Physics Letters, 2015)
We report the design and characterization of a self-assembled, locally resonant acoustic metamaterial for Lamb waves, composed of a monolayer of 1.02 extgreekmm polystyrene microspheres adhered to a 1.27 extgreekmm thick free-standing silicon membrane. A laser-induced transient grating technique is used to generate Lamb waves in the metamaterial and to measure its acoustic response. The measurements reveal a microsphere contact resonance and the lowest frequency spheroidal microsphere resonance. The measured dispersion curves show hybridization of flexural Lamb waves with the microsphere contact resonance. We compare the measured dispersion with an analytical model using the contact resonance frequency as a single fitting parameter, and find that it well describes the observed hybridization. This study may lead to an improved understanding of microscale contact mechanics and to the design of new types of acoustic metamaterials.

A SEQUENTIAL CONVEX SEMIDEFINITE PROGRAMMING ALGORITHM WITH AN APPLICATION TO MULTIPLE-LOAD FREE MATERIAL OPTIMIZATION - Stingl, Michael; Kocvara, Michal; Leugering, Günter (SIAM Journal on Optimization, 2009)
A new method for the efficient solution of a class of convex semidefinite programming (SDP) problems is introduced. The method extends the sequential convex programming (SCP) concept to optimization problems with matrix variables. The basic idea of the new method is to approximate the original optimization problem by a sequence of subproblems, in which nonlinear functions (defined in matrix variables) are approximated by block separable convex functions. The subproblems are semidefinite programs with a favorable structure which can be efficiently solved by existing SDP software. The new method is shown to be globally convergent. The article is concluded by a series of numerical experiments with free material optimization problems demonstrating the effectiveness of the generalized SCP approach. © 2009 Society for Industrial and Applied Mathematics.
UnivIS-Import:2015-03-09:Pub.2009.nat.dma.lama1.aseque
Keywords: Material optimization; Semidefinite programming; Sequential convex programming; Structural optimization

A Series of Pyrene-Substituted Silicon Phthalocyanines as Near-IR Sensitizers in Organic Ternary Solar Cells - Ke, Lili; Min, Jie; Adam, Matthias; Gasparini, Nicola; Hou, Yi; Perea, Jose Dario; Chen, Wei; Zhang, Hong; Fladischer, Stefanie; Sale, Anna Chiara; Spiecker, Erdmann; Tykwinski, Rik; et al. (Advanced Energy Materials, 2016)
An attractive method to broaden the absorption bandwidth of polymer/fullerene-based bulk heterojunction (BHJ) solar cells is to blend near infrared (near-IR) sensitizers into the host system. Axial substitution of silicon phthalocyanines (Pcs) opens a possibility to modify the chemical, thermodynamic, electronic, and optical properties. Different axial substitutions are already designed to modify the thermodynamic properties of Pcs, but the impact of extending the π-conjugation of the axial ligand on the opto-electronic properties, as a function of the length of the alkyl spacer, has not been investigated yet. For this purpose, a novel series of pyrene-substituted silicon phthalocyanines (SiPc-Pys) with varying lengths of alkyl chain tethers are synthesized. The UV-vis and external quantum efficiency (EQE) results exhibit an efficient near IR sensitization up to 800 nm, clearly establishing the impact of the pyrene substitution. This yields an increase of over 20% in the short circuit current density (J ) and over 50% in the power conversion efficiency (PCE) for the dye-sensitized ternary device. Charge generation, transport properties, and microstructure are studied using different advanced technologies. Remarkably, these results provide guidance for the diverse and judicious selection of dye sensitizers to overcome the absorption limitation and achieve high efficiency ternary solar cells.
Keywords: Absorption limitations; Charge transport; Near-IR sensitizers; Silicon phthalocyanine; Ternary solar cells

A shape-topological control problem for nonlinear crack-defect interaction: The antiplane variational model - Leugering, Günter; Kovtunenko, Viktor (SIAM Journal on Control and Optimization, 2016)
We consider the shape-topological control of a singularly perturbed variational inequality. The geometry-dependent state problem that we address in this paper concerns a heterogeneous medium with a micro-object (defect) and a macro-object (crack) modeled in two dimensions. The corresponding nonlinear optimization problem subject to inequality constraints at the crack is considered within a general variational framework. For the reason of asymptotic analysis, singular perturbation theory is applied, resulting in the topological sensitivity of an objective function representing the release rate of the strain energy. In the vicinity of the nonlinear crack, the antiplane strain energy release rate is expressed by means of the mode-III stress intensity factor that is examined with respect to small defects such as microcracks, holes, and inclusions of varying stiffiness. The result of shape-topological control is useful either for arrests or rise of crack growth.
Keywords: Antiplane stress intensity factor; Crack-defect interaction; Dipole tensor; Nonlinear crack with nonpenetration; Shape-topological control; Singular perturbation; Strain energy release rate; Topological derivative; Variational inequality

A significant cathodic shift in the onset potential of photoelectrochemical water splitting for hematite nanostructures grown from Fe-Si alloys - Wang, Lei; Lee, Chong-Yong; Kirchgeorg, Robin; Hildebrand, Helga; Müller, Julian; Spiecker, Erdmann; Schmuki, Patrik (Materials Horizons, 2014)
Thermal oxidation of Fe to nanostructured hematite (wires, flakes) is currently widely investigated to produce efficient photoanodes for photoelectrochemical water splitting. The process carried out on pure iron, however, has the key drawback that not only hematite but a layered structure of Fe2O3-Fe3O4-FeO is formed where the thick suboxide layer underneath the Fe2O3 is highly detrimental to the photoresponse. In the present work, we show that suboxide formation can be largely suppressed if hematite nanowires/nanoflakes are thermally grown on Fe-Si alloys. For hematite structures grown on a Fe-Si alloy with 5 at% Si, a photocurrent onset potential as low as 0.6 V-RHE can be reached (under AM 1.5 illumination and 1 M KOH). We believe that the results represent a key finding towards the formation of optimized hematite nanostructures using a thermal oxidation method.

A simple method for residual stress measurements in thin films by means of focused ion beam milling and digital image correlation - Krottenthaler, Markus; Schmid, Christoph; Schaufler, Jens; Durst, Karsten; Göken, Mathias (Surface & Coatings Technology, 2013)
Hydrogenated amorphous carbon (a-C:H) coatings contain substantial residual stresses which particularly influence the failure behaviour of the coating system. However, measuring their residual stresses with conventional methods is not trivial. In this work, a simple method is presented to determine residual stresses of thin films locally by using stress relaxation tests by means of focused ion beam (FIB) milling and digital image correlation (DIC). It is shown, that the H-bar geometry, as it is used for TEM lamella preparation, is especially suitable for such measurements. Displacements due to relaxation of residual stresses are tracked using DIC and the stresses are obtained by correlation with finite element analysis. The method is applied to determine residual stresses of two a-C:H coatings processed with different deposition processes and parameters. It was found, that the coatings differ in their mechanical properties as well as in their residual stress state. The proposed method offers therefore a simple way for analysing residual stresses in thin amorphous coatings.
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A Simple QM/MM Approach for Capturing Polarization Effects in Protein-Ligand Binding Free Energy Calculations - Beierlein, Frank; Michel, Julien; Essex, Jonathan (Journal of Physical Chemistry B, 2011)
We present a molecular simulation protocol to compute free energies of binding, which combines a QM/MM correction term with rigorous classical free energy techniques, thereby accounting for electronic polarization effects. Relative free energies of binding are first computed using classical force fields, Monte Carlo sampling, and replica exchange thermodynamic integration. Snapshots of the configurations at the end points of the perturbation are then subjected to DFT-QM/MM single-point calculations using the B3LYP functional and a range of basis sets. The resulting quantum mechanical energies are then processed using the Zwanzig equation to give free energies incorporating electronic polarization. Our approach is conceptually simple and does not require tightly coupled QM and MM software. The method has been validated by calculating the relative free energies of hydration of methane and water and the relative free energy of binding of two inhibitors of cyclooxygenase-2. Closed thermodynamic cycles are obtained across different pathways, demonstrating the correctness of the technique, although significantly more sampling is required for the protein-ligand system. Our method offers a simple and effective way to incorporate quantum mechanical effects into computed free energies of binding.

A solution-processed barium hydroxide modified aluminum doped zinc oxide layer for highly efficient inverted organic solar cells - Zhang, Hong; Stubhan, Tobias; Li, Ning; Turbiez, Mathieu; Matt, Gebhard; Ameri, Tayebeh; Brabec, Christoph (Journal of Materials Chemistry A, 2014)
Inverted organic solar cells (iOSCs) with air stable interface materials and top electrodes and an efficiency of 6.01% are achieved by inserting a barium hydroxide (Ba(OH)) layer between the aluminum doped zinc oxide (AZO) electron extraction layer and the active layer. A low bandgap diketopyrrolopyrrole-quinquethiophene alternating copolymer (pDPP5T-2) and phenyl-C61-butyric acid methyl ester (PCBM) were chosen as the active layer compounds. Compared to the control device without Ba(OH), insertion of a few nm thick Ba(OH) layer results in an enhanced V of 10%, J of 28%, FF of 28% and PCE of 80%. Modification of AZO with a solution processed low-cost Ba(OH) layer increased the efficiency of the inverted device by dominantly reducing the energy barrier for electron extraction from PCBM, and consequently, reduced charge recombination is observed. The drastic improvement in device efficiency and the simplicity of fabrication by solution processing suggest Ba(OH) as a promising and practical route to reduce interface induced recombination losses at the cathode of organic solar cells.
Keywords: Aluminum doped zinc oxide layers; Barium hydroxide; Inverted organic solar cells; Solution-processed

Aspects of Computational Homogenization at Finite Deformations: A Unifying Review From Reuss' to Voigt's Bound - Saeb, Saba; Steinmann, Paul; Javili, Ali (Applied Mechanics Reviews, 2016)
The objective of this contribution is to present a unifying review on strain-driven computational homogenization at finite strains, thereby elaborating on computational aspects of the finite element method. The underlying assumption of computational homogenization is separation of length scales, and hence, computing the material response at the macroscopic scale from averaging the microscopic behavior. In doing so, the energetic equivalence between the two scales, the Hill–Mandel condition, is guaranteed via imposing proper boundary conditions such as linear displacement, periodic displacement and antiperiodic traction, and constant traction boundary conditions. Focus is given on the finite element implementation of these boundary conditions and their influence on the overall response of the material. Computational frameworks for all canonical boundary conditions are briefly formulated in order to demonstrate similarities and differences among the various boundary conditions. Furthermore, we detail on the computational aspects of the classical Reuss' and Voigt's bounds and their extensions to finite strains. A concise and clear formulation for computing the macroscopic tangent necessary for FE2 calculations is presented. The performances of the proposed schemes are illustrated via a series of two- and three-dimensional numerical examples. The numerical examples provide enough details to serve as benchmarks.
EAM Import::2019-03-13

Aspects of implementing constant traction boundary conditions in computational homogenization via semi-Dirichlet boundary conditions - Javili, Ali; Saeb, Saba; Steinmann, Paul (Computational Mechanics, 2016)
In the past decades computational homogenization has proven to be a powerful strategy to compute the overall response of continua. Central to computational homogenization is the Hill–Mandel condition. The Hill–Mandel condition is fulfilled via imposing displacement boundary conditions (DBC), periodic boundary conditions (PBC) or traction boundary conditions (TBC) collectively referred to as canonical boundary conditions. While DBC and PBC are widely implemented, TBC remains poorly understood, with a few exceptions. The main issue with TBC is the singularity of the stiffness matrix due to rigid body motions. The objective of this manuscript is to propose a generic strategy to implement TBC in the context of computational homogenization at finite strains. To eliminate rigid body motions, we introduce the concept of semi-Dirichlet boundary conditions. Semi-Dirichlet boundary conditions are non-homogeneous Dirichlet-type constraints that simultaneously satisfy the Neumann-type conditions. A key feature of the proposed methodology is its applicability for both strain-driven as well as stress-driven homogenization. The performance of the proposed scheme is demonstrated via a series of numerical examples.
EAM Import::2019-03-13

Assemblies of Mesoporous FAU-Type Zeolite Nanosheets - Inayat, Alexandra; Knoke, Isabel; Spiecker, Erdmann; Schwieger, Wilhelm (Angewandte Chemie International Edition, 2012)
Holey sheets: Nanosheet assemblies of FAU-type zeoliteX with intracrystalline mesopores are prepared (see picture). This material is synthesized by soft-templating and combines micro, meso, and macropores in a hierarchically interconnected manner.
Keywords: hierarchy; mesoporous materials; nanosheets; soft templating; zeolites

Assessment of Periodic Open Cellular Structures for Enhanced Heat Conduction in Catalytic Fixed-Bed Reactors - Schwieger, Wilhelm; Freund, Hannsjörg; Bianchi, Enrico (Advanced Engineering Materials, 2015)
In this paper, the effective heat conductivity of periodic open cellular structures (POCS) is investigated by means of a systematic numerical study. 3D geometries of different configurations were considered, representing the unit cell volume of the modular POCS. Special emphasis is put on the strut morphology, i.e. the strut cross sectional shape and the distribution of the solid material along the strut. At the same porosity, the optimal designed structures show a higher effective heat conductivity than conventional open-cell solid foams with random cell geometry. This possibility to exceed the effective thermal conductivity of state-of-the-art open-cell foams by optimization of their geometrical properties is an interesting novel aspect of high relevance regarding their application in catalytic reactors.

Assessment of stress relaxation experiments on diamond coatings analyzed by digital image correlation and micro-Raman spectroscopy - Ahmed, Furqan; Krottenthaler, Markus; Schmid, Christoph; Durst, Karsten (Surface & Coatings Technology, 2013)
The analysis and control of residual stresses are important for understanding the fracture and delamination behavior of coatings and thin films. In this work, the stress relaxation in microcrystalline diamond coatings after focused ion beam (FIB) milling was assessed by micro-Raman spectroscopy and digital image correlation. Using the FIB, the so called H-Bar geometry was milled in the diamond coating. The cutting introduced a strain relief in the coating perpendicular to the longer sides of the bar. The relaxation strains were recorded from high resolution scanning electron microscopy images of the H-Bar before and after FIB cutting. Using finite element modeling of the milled geometry, the residual stress level in the coating is evaluated from the relaxation strains. Furthermore, extgreekm-Raman spectroscopy was used to assess the biaxial residual stress level in the coating before and after FIB cutting. The extgreekm-Raman signal inside the bar showed strong incidence for stress relaxation and confirmed to some extent the applicability of the FIB–DIC method.
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Assigning Electronic States in Carbon Nanodots - Strauß, Volker; Kahnt, Axel; Zolnhofer, Eva; Meyer, Karsten; Maid, Harald; Placht, Christian; Bauer, Walter; Nacken, Thomas; Peukert, Wolfgang; Etschel, Sebastian; Halik, Marcus; Guldi, Dirk Michael (Advanced Functional Materials, 2016)
Luminescent carbon nanodots exhibit enormous potential to be used as functional building blocks in energy conversion devices. Their intrinsic heterogeneity, however, hinders a thorough characterization and subsequently their applicability. In this work, the spectroscopic and electrochemical properties of prototype carbon nanodots were thoroughly studied in solution. Spectro-electrochemical experiments revealed information on the spectral features of several oxidized and reduced species of our carbon nanodots. Therefore, these experiments were complemented with pulse radiolysis where the spectral signature of the one-electron oxidized and reduced forms were identified. Both, electrochemically and radiolytically reduced and oxidized products show high reactivity in solution which correlates with an open-shell character, detected by paramagnetic resonance spectroscopy. The herein presented studies help to understand the complex nature of carbon nanodots and serve to overcome difficulties in device fabrication.

Assignment of near-edge x-ray absorption fine structure spectra of metalloporphyrins by means of time-dependent density-functional calculations - Schmidt, Norman Anja; Fink, Rainer; Hieringer, Wolfgang (Journal of Chemical Physics, 2010)
The C 1s and N 1s near-edge x-ray absorption fine structure (NEXAFS) spectra of three prototype tetraphenyl porphyrin (TPP) molecules are discussed in the framework of a combined experimental and theoretical study. We employ time-dependent density-functional theory (TDDFT) to compute the NEXAFS spectra of the open- and closed-shell metalloporphyrins CoTPP and ZnTPP as well as the free-base 2HTPP in realistic nonplanar conformations. Using Becke's well-known half-and-half hybrid functional, the computed core excitation spectra are mostly in good agreement with the experimental data in the low-energy region below the appropriate ionization threshold. To make these calculations feasible, we apply a new, simple scheme based on TDDFT using a modified single-particle input spectrum. This scheme is very easy to implement in standard codes and allows one to compute core excitation spectra at a similar cost as ordinary UV/vis spectra even for larger molecules. We employ these calculations for a detailed assignment of the NEXAFS spectra including subtle shifts in certain peaks of the N 1s spectra, which depend on the central coordination of the TPP ligand. We furthermore assign the observed NEXAFS resonances to the individual molecular subunits of the investigated TPP molecules. © 2010 American Institute of Physics.

Association Rates of Membrane-Coupled Cell Adhesion Molecules - Bihr, Timo; Fenz, Susanne; Sackmann, Erich; Merkel, Rudolf; Seifert, Udo; Sengupta, Kheya; Smith, Ana-Suncana (Biophysical Journal, 2014)
Thus far, understanding how the confined cellular environment affects the lifetime of bonds, as well as the extraction of complexation rates, has been a major challenge in studies of cell adhesion. Based on a theoretical description of the growth curves of adhesion domains, we present a new (to our knowledge) method to measure the association rate k(on) of ligand-receptor pairs incorporated into lipid membranes. As a proof of principle, we apply this method to several systems. We find that the k(on) for the interaction of biotin with neutravidin is larger than that for integrin binding to RGD or sialyl Lewis(x) to E-selectin. Furthermore, we find k(on) to be enhanced by membrane fluctuations that increase the probability for encounters between the binders. The opposite effect on k(on) could be attributed to the presence of repulsive polymers that mimic the glycocalyx, which points to two potential mechanisms for controlling the speed of protein complexation during the cell recognition process.
Keywords: GIANT VESICLES; BINDING; BONDS; KINETICS; MODEL; DYNAMICS; SELECTIN; LIFETIME; SYSTEM; FORCE

A Stable Crystalline Triarylphosphine Oxide Radical Anion - Schaub, Tobias; Zolnhofer, Eva; Halter, Dominik; Shubina, Tatyana; Hampel, Frank; Meyer, Karsten; Kivala, Milan (Angewandte Chemie International Edition, 2016)
Triarylphosphine oxides (Ar3P=O) are being intensely studied as electron-accepting (n-type) materials. Despite the widespread application of these compounds as electron conductors, experimental data regarding the structural and electronic properties of their negatively charged states remain scarce owing to their propensity for follow-up chemistry. Herein, a carefully designed triarylphosphine oxide scaffold is disclosed that comprises sterically demanding spirofluorenyl moieties to shield the central phosphoryl (P=O) moiety. This compound undergoes chemical one-electron reduction to afford an exceptionally stable radical anion, which was isolated and characterized by X-ray crystallography for the very first time. The experimental data, corroborated by computational studies, shall allow for the construction of phosphine oxide materials with enhanced stability.
EAM Import::2019-03-13

A star-shaped D-π-A small molecule based on a tris(2-methoxyphenyl)amine core for highly efficient solution-processed organic solar cells - Min, Jie; Luponosov, Yuriy N.; Solodukhin, Alexander N.; Kausch-Busies, Nina; Ponomarenko, Sergei A.; Ameri, Tayebeh; Brabec, Christoph (Journal of Materials Chemistry C, 2014)
A new star-shaped D-π-A small molecule, 2,2′,2′- nitrilotris[(3-methoxy-4,1-phenylene)-2,2′-bithiene-5′, 5-diyleth-1-yl-1-ylidene]trimalononitrile N(Ph(OMe)-2T-DCN-Me), with high efficiency potential for photovoltaic applications was synthesized. Introducing a soluble tris(2-methoxyphenyl)amine (TPA-MeO) core unit can significantly improve the solubility of star-shaped small molecules without negatively impacting intramolecular conjugation and intermolecular solid state arrangements. Solution-processed organic solar cells based on a blend of N(Ph(OMe)-2T-DCN-Me) and PCBM show an initially high power conversion efficiency of over 4% under simulated AM 1.5. © the Partner Organisations 201
Keywords: Engineering controlled terms: Molecules; Stars; Synthesis (chemical) High power conversion; Organic solar cell; Photovoltaic applications; Small molecules; Solution-processed; Star-shaped Engineering main heading: Solar cells

A supramolecular approach for the facile solubilization and separation of covalently functionalized single-walled carbon nanotubes - Bosch, Sebastian; Zeininger, Lukas; Hauke, Frank; Hirsch, Andreas; Hauke, Frank (Chemistry - A European Journal, 2014)
Through a combination of an electronic-type selective diazonium-based attachment of a Hamilton receptor unit onto the carbon nanotube framework and a supramolecular recognition approach of a cyanuric acid derivative, we herein introduce a highly promising strategy for the tuning of carbon nanotube solubility and, directly related to that, a solution-based easy and straightforward separation of covalently functionalized carbon nanotube derivatives with respect to their unfunctionalized counterparts. The supramolecular complexation of the cyanuric acid derivative provides the driving force for the dramatically increased dispersibility and for the long-time stability of the individualized single-walled carbon nanotube derivatives in chloroform. The selective covalent functionalization of metallic carbon nanotubes can easily be analyzed with the aid of scanning Raman microscopy techniques. The functional derivatives have furthermore been characterized by UV/Vis-NIR and fluorescence spectroscopy as well as by mass spectrometric coupled thermogravimetric analysis. The investigation of the supramolecular complexation is based on an in-depth UV/Vis-NIR analysis and atomic force microscopy investigations. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A Surfactants Walk to Work: Modes of Action of Citrate Controlling (10-10) and (000-1) Zinc Oxide Surface Growth from Solution - Milek, Theodor; Zahn, Dirk (Zeitschrift für Anorganische und Allgemeine Chemie, 2016)
Molecular dynamics simulations unravel the association of citrate to (10–10) and (000–1) growth fronts of zinc oxide and demonstrate an unexpected mobility of the surfactant. Citrate association to perfectly planar (10–10) and (000–1) ZnO-ethanol interfaces was found to be favored over dissociation by 1.5–2 eV hence suggesting strongly bound, immobilized surfactants. However, intramolecular stress prevents binding of all three carboxyl groups to planar surfaces and the typical arrangement is that of two carboxyl-Zn contacts (including two salt bridges each) whilst the remaining carboxyl group is dangling into the solvent. As a consequence, the surfactant exhibits a 'walking' mechanism to move along the surface by exchanging the role of its carboxyl groups. This finding has strong implications for the role of citrate during crystal growth as illustrated by a recently developed simulation scheme based on hundreds of individual Zn2+ and OH– ion association steps. In particular, for the (10–10) surface – which grows via formation of ridges embedded by (10–10) faces – these simulations show how citrate ions move towards steps and bind to the growth front by additional 4 eV per surfactant molecule.
EAM Import::2019-03-13

A switch for epitaxial graphene electronics: Utilizing the silicon carbide substrate as transistor channel - Krach, Florian; Hertel, Stefan; Waldmann, Daniel; Jobst, Johannes; Krieger, Michael; Reshanov, S.; Schoner, A.; Weber, Heiko B. (Applied Physics Letters, 2012)
Due to the lack of graphene transistors with large on/off ratio, we propose a concept employing both epitaxial graphene and its underlying substrate silicon carbide (SiC) as electronic materials. We demonstrate a simple, robust, and scalable transistor, in which graphene serves as electrodes and SiC as a semiconducting channel. The common interface has to be chosen such that it provides favorable charge injection. The insulator and gate functionality is realized by an ionic liquid gate for convenience but could be taken over by a solid gate stack. On/off ratios exceeding 44000 at room temperature are found. © 2012 American Institute of Physics.
UnivIS-Import:2015-04-14:Pub.2012.nat.dphy.IAP.LAP.aswitc

Asymmetric accumulative roll bonding of aluminium-titanium composite sheets - Hoi Pang Ng,; Przybilla, Thomas; Schmidt, Christian; Lapovok, Rimma; Orlov, Dmitry; Höppel, Heinz Werner; Göken, Mathias (Materials Science and Engineering A-Structural Materials Properties Microstructure and Processing, 2013)
Aluminium–titanium (Al/Ti) composite sheets were fabricated via asymmetric accumulative roll bonding (AARB), which capitalises on additional shear to enhance plastic deformation. Multi-layers of Al alloy (AA1050) and commercially-pure Ti sheets were alternatively stacked and rolled-bonded with varied roll diameter ratios (dr) ranging from 1 to 2, for up to four passes. Annealing of selected composite sheets was subsequently carried out at 600 °C for 24 h to compare the rates of solid-state diffusion reactions between Al and Ti components. Mechanical tests revealed that both tensile strength and ductility of the sheets increase systematically with dr. The microstructures and the Al/Ti interfaces of the sheets were analysed in detail using TEM, SEM and FIB techniques. It is shown that not only does AARB lead to a more refined grain size of the Al matrix but also it promotes the development of a nanostructured surface layer on Ti that comprises crystallites of 50–100 nm in size, which is otherwise absent in the case of symmetric ARB (i.e. dr=1). The AARB-processed sheets exhibit a larger thickness of the interdiffusion layer at the Al/Ti interfaces than the counterparts processed via the symmetric ARB route, the difference being in excess of 15%. The effects and the implications of AARB processing on mechanical behaviour and diffusion kinetics are discussed with respect to the microstructural evolutions.
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Asymptotic analysis of 3-D thin piezoelectric rods - Leugering, Günter; Nazarov, Sergei A.; Slutskij, Andrey S. (ZAMM - Zeitschrift für angewandte Mathematik und Mechanik, 2014)
We derive an asymptotic one-dimensional model of a thin piezoelectric rod by means of a dimension reduction procedure. The rod is made from a heterogeneous material with a possibly varying cross-section and distorted ends. Asymptotically exact error estimates are derived. Representation formulas for effective moduli are established and, for a concrete piezoelectric material, we provide an explicit form.

Asymptotic analysis of flow processes at drawing of single optical microfibres - Luzi, Giovanni; Epple, Philipp; Scharrer, Michael; Fujimoto Meinecke, Ken; Rauh, Cornelia; Delgado, Antonio (International Journal of Chemical Reactor Engineering, 2011)
Microstructured optical fibres (i.e. fibres that contain holes) have assumed a high profile in recent years, and given rise to many novel optical devices. The problem of manufacturing such fibres by heating and then drawing a preform is considered for both the cases of annular microfibres and annular capillaries. A fluid-mechanics model suggested in literature that uses asymptotic analysis based on the small aspect ratio of capillaries is analysed and revised. The leading-order equations are examined in some asymptotic limits, many of which give valuable practical information about the control parameters that influence the drawing process. Additionally, the solution obtained for a single capillary provides a suitable basis for describing more complicated fibre structures. Copyright © 2011 The Berkeley Electronic Press. All rights reserved.
UnivIS-Import:2015-03-09:Pub.2011.tech.ITC.stmmec.asympt
Keywords: asymptotic analysis; holey fibre; microstructured optical fibres; numerical efforts; photonic crystal fibres

Asymptotic expansions of the hypergeometric function with two large parameters-application to the partition function of a lattice gas in a field of traps - Cvitkovic, Mislav; Smith, Ana-Suncana; Pande, Jayant (Journal of Physics A: Mathematical and Theoretical, 2017)
The canonical partition function of a two-dimensional lattice gas in a field of randomly placed traps, like many other problems in physics, evaluates to the Gauss hypergeometric function F-2(1)(a, b; c; z) in the limit when one or more of its parameters become large. This limit is difficult to compute from first principles, and finding the asymptotic expansions of the hypergeometric function is therefore an important task. While some possible cases of the asymptotic expansions of 2F1(a, b; c; z) have been provided in the literature, they are all limited by a narrow domain of validity, either in the complex plane of the variable or in the parameter space. Overcoming this restriction, we provide new asymptotic expansions for the hypergeometric function with two large parameters, which are valid for the entire complex plane of z except for a few specific points. We show that these expansions work well even when we approach the possible singularity of 2F1(a, b; c; z), |z| = 1, where the current expansions typically fail. Using our results we determine asymptotically the partition function of a lattice gas in a field of traps in the different possible physical limits of few/many particles and few/many traps, illustrating the applicability of the derived asymptotic expansions of the HGF in physics.
Keywords: hypergeometric function; asymptotic expansion; method of steepest descent; large parameters; special functions; partition function

A systematic approach to identify cellular auxetic materials - Körner, Carolin; Liebold Ribeiro, Yvonne (Smart Materials and Structures, 2015)
Auxetics are materials showing a negative Poisson's ratio. This characteristic leads to unusual mechanical properties that make this an interesting class of materials. So far no systematic approach for generating auxetic cellular materials has been reported. In this contribution, we present a systematic approach to identifying auxetic cellular materials based on eigenmode analysis. The fundamental mechanism generating auxetic behavior is identified as rotation. With this knowledge, a variety of complex two-dimensional (2D) and three-dimensional (3D) auxetic structures based on simple unit cells can be identified.
Keywords: auxetic material; negative Poissons ratio; periodic cellular structure

A test of improved force field parameters for urea: molecular-dynamics simulations of urea crystals - Ozpinar, Gul Altinbas; Beierlein, Frank; Peukert, Wolfgang; Zahn, Dirk; Clark, Timothy (Journal of Molecular Modeling, 2012)
Molecular-dynamics (MD) simulations of urea crystals of different shapes (cubic, rectangular prismatic, and sheet) have been performed using our previously published force field for urea. This force field has been validated by calculating values for the cohesive energy, sublimation temperature, and melting point from the MD data. The cohesive energies computed from simulations of cubic and rectangular prismatic urea crystals in vacuo at 300 K agreed very well with the experimental sublimation enthalpies reported at 298 K. We also found very good agreement between the melting points as observed experimentally and from simulations. Annealing the crystals just below the melting point leads to reconstruction to form crystal faces that are consistent with experimental observations. The simulations reveal a melting mechanism that involves surface (corner/edge) melting well below the melting point, and rotational disordering of the urea molecules in the corner/edge regions of the crystal, which then facilitates the translational motion of these molecules.

A thermo-mechanical cohesive zone model accounting for mechanically energetic Kapitza interfaces - Esmaeili, Ali; Javili, Ali; Steinmann, Paul (International Journal of Solids and Structures, 2016)
Interfaces can play a dominant role in the thermo-mechanical response of a body. The importance of interfaces is more pronounced as the problem scale decreases since the interface area to the bulk volume ratio grows. The objective of this contribution is to study computational aspects of modeling thermo-mechanical solids containing mechanically energetic, geometrically non-coherent Kapitza interfaces under cyclic loading. The interface is termed energetic in the sense that it possesses its own energy, entropy, constitutive relations and dissipation. To date, classical thermomechanical cohesive zone models do not account for elastic interfaces. Therefore we propose a novel interface model that couples the classical cohesive zone formulation to the interface elasticity theory under the Kapitza assumption within a thermo-mechanical framework. In other words, such an interface model allows for discontinuities in geometry, temperature and normal stress fields, while not permitting a jump in the normal heat flux across the interface. The equations governing a fully non-linear transient problem are given. They are solved using the finite element method. The results are illustrated through a series of three-dimensional numerical examples for various interfacial parameters. In particular, a comparison is made between the results of the classical thermo-mechanical cohesive zone model and our novel (cohesive + energetic Kapitza) interface formulation.

Atomically Defined Co 3 O 4 (111) Thin Films Prepared in Ultrahigh Vacuum: Stability under Electrochemical Conditions - Faisal, Firas; Bertram, Manon; Stumm, Corinna; Cherevko, Serhiy; Geiger, Simon; Kasian, Olga; Lykhach, Yaroslava; Lytken, Ole; Mayrhofer, Karl J. J.; Brummel, Olaf; Libuda, Jörg (Journal of Physical Chemistry C, 2018)
We have explored the stability, the structure, and the chemical transformations of atomically defined Co3O4(111) thin films under electrochemical conditions. The well-ordered Co3O4(111) films were prepared on an Ir(100) single crystal under ultrahigh vacuum (UHV) conditions and subsequently transferred and characterized in the electrochemical environment by means of cyclic voltammetry (CV), scanning flow cell inductively coupled plasma mass spectrometry (SCF-ICP-MS), and electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). We have found that the Co3O4(111) films are stable in phosphate buffer at pH 10 at potentials between 0.33 to 1.33 VRHE. In the corresponding potential range, the corrosion rates established by means of SCF-ICP-MS were well below 0.1 monolayer per hour. Additionally, low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) studies have shown that the long-range order, the thickness, and the composition of the films were preserved under electrochemical conditions. Disintegration of the film and formation of holes after repeated potential cycling within the stability window were ruled out by EC-IRRAS using CO as a probe molecule. In general, the stability of the Co3O4(111) films depends critically on both the pH and electrode potential. Increasing the pH from 10 to 12 compromised the structural stability of the Co3O4(111) films due to faster redox processes at the surface. In particular, we observed accelerated oxidation of cobalt followed by the formation of oxyhydroxide during the anodic scan and accelerated reduction to cobalt hydroxides during the cathodic scan. Decreasing the pH from pH 10 to pH 8, on the other hand, led to faster dissolution, in particular at potentials below 0.2 VRHE, where the dissolution rate increased rapidly due to formation of soluble Co(II) species. Our studies demonstrate that thin well-ordered oxide films prepared in UHV can be transferred into the electrochemical environment while preserving their atomic surface structure if the conditions are chosen carefully. This opens a surface science approach to atomically defined oxide–electrolyte interfaces.
EAM Import::2019-03-13

Atomically-defined model catalysts in ultrahigh vacuum and in liquid electrolytes: particle size-dependent CO adsorption on Pt nanoparticles on ordered Co3O4(111) films - Faisal, Firas; Stumm, Corinna; Bertram, Manon; Wähler, Tobias; Schuster, Ralf; Xiang, Feifei; Lytken, Ole; Katsounaros, Ioannis; Mayrhofer, Karl; Schneider, M. Alexander; Brummel, Olaf; Libuda, Jörg (Physical Chemistry Chemical Physics, 2018)
We have studied particle size effects on atomically-defined model catalysts both in ultrahigh vacuum (UHV) and under electrochemical (EC) conditions in liquid electrolytes. The model catalysts were prepared in UHV by physical vapour deposition (PVD) of Pt onto an ordered Co3O4(111) film on Ir(100), yielding nanoparticles (NPs) with an average size from 10 to 500 atoms per particle (0.8 to 3 nm). The model systems were characterized in UHV using surface science methods including scanning tunnelling microscopy (STM), before transferring them out of the UHV and into the electrolyte without contact to ambient conditions. By X-ray photoelectron spectroscopy (XPS) we show that the model surfaces are stable in the EC environment under the applied conditions (0.1 to 1 M phosphate buffer, pH 10, 0.33 to 1.03 V-RHE). As a reference, we study Pt(111) under identical conditions. In UHV, we also investigated the adsorption of CO using infrared reflection absorption spectroscopy (IRRAS). Under EC conditions, we performed equivalent experiments using EC infrared reflection absorption spectroscopy (EC-IRRAS) in combination with cyclic voltammetry (CV). Characteristic differences were observed between the IR spectra under EC conditions and in UHV. Besides the red-shift induced by the interfacial electric field (Stark effect), the EC IR bands of CO on Pt(111) show a larger width (by a factor of 2) as a result of local variations in the CO environment and coupling to the electrolyte. The CO IR bands of the Pt NPs are even broader (by a factor of 5), which is attributed to local variations of the interfacial electric field at the NP surface. Further pronounced differences are observed between the spectra taken in UHV and in the electrolyte regarding the site occupation and its dependence on particle size. In UHV, adsorption at on-top sites is preferred on Pt(111) at low coverage and similar adsorption ratios of on-top and bridge-bonded CO are formed at saturation coverage. In sharp contrast, on-top adsorption of CO on Pt(111) is partially suppressed under EC conditions. This effect is attributed to the competitive adsorption of anions from the electrolyte and leads to a clear preference for bridge sites at higher potentials (>0.5 V-RHE). For the Pt NPs, the situation is different and an increasing fraction of on-top CO is observed with decreasing particle size, both under EC conditions and in UHV. For the smallest particles (10-20 atoms) we do not detect any bridge-bonded CO. This change in site preference as a function of particle size is attributed to stronger on-top adsorption on low-coordinated Pt atoms of small Pt NPs. The effect leads to a clear preference for on-top adsorption in the electrolyte even at low CO coverage and over the full potential range studied.

Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts: Principal Differences in Stability and Reactivity - Neitzel, Armin; Figueroba, Alberto; Lykhach, Yaroslava; Skala, Tomas; Vorokhta, Mykhailo; Tsud, Nataliya; Mehl, Sascha; Sevcikova, Klara; Prince, Kevin C.; Neyman, Konstantin M.; Matolin, Vladimir; Libuda, Jörg (Journal of Physical Chemistry C, 2016)
We have investigated the stability and the reactivity of atomically dispersed Pt, Pd, and Ni species on nanostructured CeO2 films by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. All three metals reveal specific similarities associated with the high adsorption energy of atomically dispersed Pt2+, Pd2+, and Ni2+ species that exceeds the corresponding cohesive energies of the bulk metals. The corresponding Pt-CeO2, Pd-CeO2, and Ni-CeO2 model catalysts have been prepared in the form of thin films on CeO2(111)/Cu(111) substrates and investigated experimentally under ultrahigh vacuum conditions. The atomically dispersed Pt2+, Pd2+, and Ni-2 species were formed exclusively at low concentrations of the corresponding metals. High concentrations resulted in the presence of additional metal oxide phases and emergence of metallic particles. We found that under the employed experimental conditions the Pd-CeO2 films closely resemble the Pt-CeO2 system with respect to the redox behavior upon reaction with hydrogen. Unlike Pt-CeO2, the Pd-CeO2 system shows a strong tendency to stabilize Pd2+ not only at the surface but also in the ceria bulk. In sharp contrast to both Pt-CeO2 and Pd-CeO2, the Ni-CeO2 system does not exhibit the redox functionality required for hydrogen activation due to the remarkably high stability of Ni2+ species.

Atomic and electronic structure of few-layer graphene on SiC(0001) studied with scanning tunneling microscopy and spectroscopy - Lauffer, Peter; Emtsev, Konstantin; Graupner, Ralf; Ley, Lothar; Seyller, Thomas; Reshanov, Sergey; Weber, Heiko B. (Physical Review B, 2008)
Epitaxial growth of graphene on SiC surfaces by solid state graphitization is a promising route for future development of graphene based electronics. In the present work, we have studied the morphology, atomic scale structure, and electronic structure of thin films of few-layer graphene (FLG) on SiC(0001) by scanning tunneling microscopy and spectroscopy (STS). We show that a quantitative evaluation of the roughness induced by the interface is a tool for determining the layer thickness of FLG. We present and interpret thickness dependent tunneling spectra, which can serve as an additional fingerprint for the determination of the layer thickness. By performing spatially resolved STS, we find evidence that the charge distribution in bilayer graphene is inhomogeneous. © 2008 The American Physical Society.
UnivIS-Import:2015-04-14:Pub.2008.nat.dphy.IAP.LAP.atomic

Atomic layer deposited aluminum oxide films on graphite and graphene studied by XPS and AFM - Speck, Florian; Ostler, Markus; Röhrl, Jonas; Emtsev, Konstantin; Hundhausen, Martin; Ley, Lothar; Seyller, Thomas (Physica Status Solidi (C) Current Topics in Solid State Physics, 2010)
Epitaxial graphene (EG) grown by solid-state decomposition of SiC is a promising material for future graphenebased electronics. On EG, the fabrication of a field-effect device requires the deposition of suitable gate insulator. Atomic layer deposition (ALD) of aluminum oxide might be useful for this purpose. Therefore, we investigated the growth of ALD-Al 2O 3 on highly oriented pyrolytic graphite (HOPG) and EG by atomic force microscopy (AFM), photoelectron spectroscopy (XPS) and Raman spectroscopy. Water as oxidant in the ALD process leads to inhomogeneous nucleation, whereas ozone leads to the formation of closed oxide layers. However, significant degradation of the EG takes place at higher temperatures and ozone exposure as witnessed by XPS and Raman spectroscopy. Careful adjustment of the process parameters allows reducing the damage to a level undetectable in XPS while providing enough nucleation centers for the formation of a closed Al 2O 3 film potentially suitable as a gate insulator. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
UnivIS-Import:2015-03-09:Pub.2010.nat.dphy.IAP.exphys.atomic

Atomic Layer Deposition from Dissolved Precursors - Wu, Yanlin; Döhler, Dirk; Barr, Maissa K. S.; Oks, Elina; Wolf, Marc; Santinacci, Lionel; Bachmann, Julien (Nano Letters, 2015)
We establish a novel thin film deposition technique by transferring the principles of atomic layer deposition (ALD) known with gaseous precursors toward precursors dissolved in a liquid. An established ALD reaction behaves similarly when performed from solutions. "Solution ALD" (sALD) can coat deep pores in a conformal manner. sALD offers novel opportunities by overcoming the need for volatile and thermally robust precursors. We establish a MgO sALD procedure based on the hydrolysis of a Grignard reagen
Keywords: Atomic layer deposition; LBL; SEAR; thin films; magnesium oxide; microfluidic

Atomic Layer Deposition of i-Sb2S3/p-NiO Thin Layers into Anodic Alumina Membranes for Photoelectrochemical Water Splitting - Barr, M. K.S.; Assaud, Loic; Wu, Yanlin; Bachmann, Julien; Santinacci, L. (ECS Transactions, 2015)
A nanostructured photoelectrode was fabricated by atomic layer deposition (ALD) of NiO and Sb2S3 in nanoporous alumina. The fabrication, i.e. the successive ALD of NiO and Sb2S3, and the characterization of the arrays of coaxial NiO/Sb2S3 junction by electron microscopies are first reported. The optical and photoelectrochemical preliminary testing of the combination of such extremely thin absorber, Sb2S3, with a p-type semiconductor, NiO, are proposed in a second part. Although the photoconversion performances should be improved, the present work demonstrates validity of the concept.
EAM Import::2019-03-13

Atomic layer deposition on 2D transition metal chalcogenides: Layer dependent reactivity and seeding with organic ad-layers - Wirtz, Christian; Hallam, Toby; Cullen, Conor Patrick; Berner, Nina C.; O'Brien, Maria; Marcia, Mario; Hirsch, Andreas; Duesberg, Georg S. (Chemical Communications, 2015)
This commmunication presents a study of atomic layer deposition of Al2O3 on transition metal dichalcogenide (TMD) two-dimensional films which is crucial for use of these promising materials for electronic applications. Deposition of Al2O3 on pristine chemical vapour deposited MoS2 and WS2 crystals is demonstrated. This deposition is dependent on the number of TMD layers as there is no deposition on pristine monolayers. In addition, we show that it is possible to reliably seed the deposition, even on the monolayer, using non-covalent functionalisation with perylene derivatives as anchor unit.

Atomic Ordering and Sn Segregation in Pt-Sn Nanoalloys Supported on CeO2 Thin Films - Neitzel, Armin; Kovacs, Gabor; Lykhach, Yaroslava; Kozlov, Sergey M.; Tsud, Nataliya; Skala, Toma; Vorokhta, Mykhailo; Matolin, Vladimir; Neyman, Konstantin M.; Libuda, Jörg (Topics in Catalysis, 2017)
The stability and atomic ordering in Pt-Sn nanoalloys supported on CeO2 thin films have been studied by means of synchrotron radiation photoelectron spectroscopy and density functional calculations. Using CO molecules as a probe, we explored the development of the surface structure of supported Pt-Sn nanoalloys with respect to a reference Pt/CeO2 model system. We found a significant decrease in the density of CO adsorption sites on supported Pt-Sn nanoalloys caused by Sn segregation to the surface upon annealing. Additionally, we found that atomic ordering in Pt-Sn nanoalloys is driven by the balance between the surface segregation energy of Sn atoms and the energy of heteroatomic bond formation. Our calculations demonstrate a clear tendency for Sn segregation to the nanoalloy surface. For Pt105Sn35 and Pt1097Sn386 nanoparticles, we calculated a surface stoichiometry of Pt2Sn which is only slightly dependent on temperature in thermodynamic equilibrium. The analysis of Bader charges in Pt-Sn nanoalloys revealed a strong correlation between the charge and the coordination number of Sn atoms with respect to Pt neighbors. In particular, the magnitude of the charge transfer from Sn to Pt increases as a function of the Sn coordination number.
Keywords: Pt-Sn nanoalloy; CO tolerant catalyst; Model catalyst; Synchrotron radiation photoelectron spectroscopy; Density functional theory

Atomistic aspects of fracture - Bitzek, Erik; Kermode, James R.; Gumbsch, Peter (International Journal of Fatigue, 2015)
Any fracture process ultimately involves the rupture of atomic bonds. Processes at the atomic scale therefore critically influence the toughness and overall fracture behavior of materials. Atomistic simulation methods including large-scale molecular dynamics simulations with classical potentials, density functional theory calculations and advanced concurrent multiscale methods have led to new insights e.g. on the role of bond trapping, dynamic effects, crack-microstructure interactions and chemical aspects on the fracture toughness and crack propagation patterns in metals and ceramics. This review focuses on atomistic aspects of fracture in crystalline materials where significant advances have been achieved over the last ten years and provides an outlook on future perspectives for atomistic modelling of fracture.
WW1::1020

Atomistic In Situ Investigation of the Morphogenesis of Grains during Pressure-Induced Phase Transitions: Molecular Dynamics Simulations of the B1-B2 Transformation of RbCl - Zahn, Dirk; Tlatlik, Harald (Chemistry - A European Journal, 2010)
The mechanistic details of the pressure-induced B1–B2 phase transition of rubidium chloride are investigated in a series of transition path sampling molecular dynamics simulations. The B2i̊ghtarrowB1 transformation proceeds by nucleation and growth involving several, initially separated, nucleation centers. We show how independent and partially correlated nucleation events can function within a global mechanism and explore the evolution of phase domains during the transition. From this, the mechanisms of grain boundary formation are elaborated. The atomic structure of the domain–domain interfaces fully support the concept of Bernal polyhedra. Indeed, the manifold of different grain morphologies obtained from our simulations may be rationalized on the basis of essentially only two different kinds of Bernal polyhedra. The latter also play a crucial role for the B1̊htarrowB2 transformation and specific grain boundary motifs are identified as preferred nucleation centers for this transition.
EAM Import::2019-03-13

Atomistic modeling of apatite-collagen composites from molecular dynamics simulations extended to hyperspace - Duchstein, Patrick; Zahn, Dirk (Journal of Molecular Modeling, 2011)
The preparation of atomistic models of apatitecollagen composite mimicking enamel at length scales in the range of 1–10 nanometers is outlined. This biocomposite is characterized by a peculiar interplay of the collagen triplehelix and the apatite crystal structure. Structural coherence is however only obtained after drastic rearrangements, namely the depletion of protein-protein hydrogen bonds and the incorporation of calcium triangles which are stabilized by salt-bridges with the collagen molecule. Starting from an isolated collagen triple helix and a single-crystalline apatite structure, a composite model is obtained by gradually merging the two components via an additional (hyperspace) coordinate. This approach allows smooth structural relaxation of both components whilst avoiding singularities in potential energy due to atomic overlap.
EAM Import::2019-03-13

Atomistisches Verständnis der Keimbildung und des Kristallwachstums durch molekulare Simulationen - Anwar, Jamshed; Zahn, Dirk (Angewandte Chemie, 2011)
Die Modellierung von Keimbildungsprozessen durch molekulare Simulation ermöglicht nicht nur die Berechnung kinetischer und thermodynamischer Eigenschaften, sondern bietet zugleich mechanistische Einblicke auf atomarer Ebene. Die vielfältigen Möglichkeiten, Experimente durch Modellsimulationen zu ergänzen, sind jedoch mit der Überwindung einer Reihe von technischen Schwierigkeiten verbunden. Keimbildungsprozesse sind seltene Ereignisse, die direkten Brute-Force-Molekulardynamiksimulationen nur in Ausnahmefällen zugänglich sind. Erst durch Fortschritte bei der Überbrückung von Zeit- und Längenskalen konnten kürzlich entwickelte Ansätze auch solche Simulationen ermöglichen, die lange Zeit als undurchführbar galten. Dabei können einerseits generische Modelle zur Erarbeitung eines allgemeinen Verständnisses genutzt werden. Zum anderen sind aber auch detaillierte Einblicke in realitätsnahe Systeme möglich, sofern die Kristallbildungsprozesse in handhabbare Module unterteilt werden. So können die Assoziation einzelner Ionen in der Lösung, die Bildung von geordneten Motiven, Reifungsreaktionen, Keimbildung und postkritisches Wachstum von Nanokristallen auf molekularer Ebene untersucht werden. Durch die Analyse des Wechselspiels mit additiven Molekülen lassen sich zudem Einblicke in funktionalisierte Nanopartikel und Kompositmaterialien gewinnen.
EAM Import::2019-03-13

Attractive particle interaction forces and packing density of fine glass powders - Ribeiro Parteli, Eric Josef; Schmidt, Jochen; Blümel, Christina; Wirth, Karl-Ernst; Peukert, Wolfgang; Pöschel, Thorsten (Scientific Reports, 2014)
We study the packing of fine glass powders of mean particle diameter in the range (4-52)μm both experimentally and by numerical DEM simulations. We obtain quantitative agreement between the experimental and numerical results, if both types of attractive forces of particle interaction, adhesion and non-bonded van der Waals forces are taken into account. Our results suggest that considering only viscoelastic and adhesive forces in DEM simulations may lead to incorrect numerical predictions of the behavior of fine powders. Based on the results from simulations and experiments, we propose a mathematical expression to estimate the packing fraction of fine polydisperse powders as a function of the average particle size.

Auf dem Weg zu größtmöglicher Effizienz bei der katalytischen Nutzung von Edelmetallen: Atomar dispergiertes Oberflächen-Platin - Bruix, Albert; Lykhach, Yaroslava; Matolinova, Iva; Neitzel, Armin; Skala, Tomas; Tsud, Nataliya; Vorokhta, Mykhailo; Stetsovych, Vitalii; Sevcikova, Klara; Myslivecek, Josef; Fiala, Roman; Vaclavu, Michal; et al. (Angewandte Chemie, 2014)
Platin ist das am vielseitigsten eingesetzte Element in der Katalyse. Allerdings begrenzt der hohe Preis des Edelmetalls die Verwendung in vielen Bereichen, z. B. in Katalysatormaterialien für Brennstoffzellen. Trotzdem nutzen konventionelle Katalysatoren oftmals nur einen Bruchteil ihres Pt-Gehaltes, nämlich diejenigen Atome, die sich auf der Oberfläche des Katalysators befinden. Eine effizientere Edelmetallnutzung setzt somit eine höhere, bevorzugt atomare Dispersion der Pt-Atome auf der Oberfläche voraus. Tatsächlich ist es möglich, solche atomar dispergierten Pt-Spezies mit sehr hoher Stabilität auf einer Katalysatoroberfläche herzustellen. Mithilfe von DFT-Rechnungen identifizieren wir ein spezifisches Strukturmerkmal solcher Systeme, eine sogenannte Cerdioxid-“Nanotasche”, die Pt2+-Ionen so stabilisiert, dass diese sowohl thermischem Sintern als auch einer Diffusion in tiefere Katalysatorebenen widerstehen. Die theoretisch vorhergesagte außerordentliche Stabilität wurde experimentell an Modellkatalysatoren verifiziert. In realen Pt-CeO2-Nanomaterialien, die tatsächlich eine sehr hohe katalytische Aktivität in Brennstoffzellen zeigen, identifizieren wir diese Bindungsstellen ebenfalls. Die so erhaltenen neuen Materialien können dazu beitragen, den Bedarf an Edelmetallen für Katalysatormaterialien erheblich zu reduzieren.

A universal interface layer based on an amine-functionalized fullerene derivative with dual functionality for efficient solution processed organic and perovskite solar cells - Azimi, Seyed Hamed; Ameri, Tayebeh; Ameri, Tayebeh; Zhang, Hong; Hou, Yi; Ramírez Quiroz, César Omar; Min, Jie; Hu, Mengyao; Zhang, Zhi-Guo; Przybilla, Thomas; Matt, Gebhard; Spiecker, Erdmann; et al. (Advanced Energy Materials, 2015)
The successful application of an effective dipolar interface layer based on an amine functionalized fullerene derivative (DMAPA-C60) is reported for the perovskite and organic photovoltaic devices. The incorporation of DMAPA-C60 facilitates a favorable energy level alignment, and results in enhanced mobility-lifetime (μt) produc
Keywords: energy level alignment; mobility-lifetime products; organic photovoltaic devices; perovskite solar cells

A universal method to form the equivalent ohmic contact for efficient solution-processed organic tandem solar cells - Li, Ning; Stubhan, Tobias; Krantz, Johannes; Machui, Florian; Turbiez, Mathieu; Ameri, Tayebeh; Brabec, Christoph (Journal of Materials Chemistry A, 2014)
The highly transparent, conductive and robust intermediate layer (IML) is the primary challenge for constructing efficient organic tandem solar cells. In this work, we demonstrate an easy but generic approach to realize the fully functional, solution-processed IMLs. In detail, solution-processed silver-nanowires are packed at low concentration between hole- and electron-transporting layers to convert an otherwise rectifying interface into an ohmic interface. The IMLs are proven to be of ohmic nature under applied bias, despite the unipolar charge selectivity of the single layers. Ohmic recombination within IMLs is further proven in organic tandem solar cells fabricated by doctor-blading under ambient conditions. The tandem solar cells based on PCDTBT:[70]PCBM as the bottom cell and pDPP5T-2:[60]PCBM as the top cell give a power conversion efficiency of 7.25%, which is among the highest values for solution-processed organic tandem solar cells fabricated by using a roll-to-roll compatible deposition method in air. © the Partner Organisations 2014.
Keywords: Engineering controlled terms: Hole concentration; Nanowires; Ohmic contacts Ambient conditions; Deposition methods; Electron-transporting layers; Intermediate layers; Low concentrations; Power conversion efficiencies; Solution-processed; Tandem solar cells Engineering main heading: Solar cells

Automated synthesis of quantum dot nanocrystals by hot injection: Mixing induced self-focusing - Salaheldin, Ahmed M.; Walter, Johannes; Herre, Patrick; Levchuk, Ievgen; Jabbari, Yasaman; Kolle, Joel; Brabec, Christoph; Peukert, Wolfgang; Segets, Doris (Chemical Engineering Journal, 2017)
The hot injection technique for the synthesis of quantum dots (QDs) is a well-established and widely used method in the lab. However, scale-up rules do not exist. One reason is that in particular the role of process parameters like mixing on particle formation is largely unknown, as systematic examination of the latter is impossible for the laborious and complex manual synthesis. Herein we studied the mixing induced self-focusing of particle size distributions (PSDs) of CdSe QDs using automation in combination with a defined stirrer geometry. Basis for our study is a platform that allows parallelization with inline temperature monitoring, defined injection rate, accurate sampling times as well as controlled stirring. Reproducibility in terms of optical product properties was analyzed by absorption and emission whereas reproducibility in terms of the PSD was verified by deconvolution of UV/Vis absorbance spectra and especially by analytical ultracentrifugation (AUC) complemented by transmission electron microscopy (TEM). In line with previous results, AUC confirmed that even QDs made by hot injection in an automated setup are polydisperse with multimodal size distributions. Finally, reproducibility in combination with early stage sampling and controlled mixing allowed us for the first time to analyze the influence of stirring on focusing and defocusing of PSDs, that has been expressed in terms of the evolution of the relative standard deviation (RSD). Our work paves the way to gain in-depth understanding of often forgotten process-structure relationships of colloidal nanoparticles which eventually is a first step in the direction of the development of scalable synthesis and reliable application of high-quality QDs in technical application

Automatic Determination of Fiber-Length Distribution in Composite Material using 3D CT Data - Teßmann, Matthias; Mohr, Stephan; Gayetskyy, Svitlana; Hassler, Ulf; Hanke, Randolf; Greiner, Günther (EURASIP Journal on Advances in Signal Processing, 2010)
Determining fiber length distribution in fiber reinforced polymer components is a crucial step in quality assurance, since fiber length has a strong influence on overall strength, stiffness, and stability of the material. The approximate fiber length distribution is usually determined early in the development process, as conventional methods require a destruction of the sample component. In this paper, a novel, automatic, and nondestructive approach for the determination of fiber length distribution in fiber reinforced polymers is presented. For this purpose, high-resolution computed tomography is used as imaging method together with subsequent image analysis for evaluation. The image analysis consists of an iterative process where single fibers are detected automatically in each iteration step after having applied image enhancement algorithms. Subsequently, a model-based approach is used together with a priori information in order to guide a fiber tracing and segmentation process. Thereby, the length of the segmented fibers can be calculated and a length distribution can be deduced. The performance and the robustness of the segmentation method is demonstrated by applying it to artificially generated test data and selected real components. Copyright © 2010 Matthias Temann et al.
UnivIS-Import:2015-03-09:Pub.2010.tech.IMMD.IMMD9.automa

Auxetic cellular structures through selective electron-beam melting - Schwerdtfeger, Jan; Heinl, Peter; Singer, Robert; Körner, Carolin (physica status solidi (b), 2010)
This paper is concerned with the build up and characterization of well-defined auxetic structures (negative Poisson ratio) from Ti-6Al-4V through selective electron-beam melting (SEBM). SEBM is a rapid prototyping/manufacturing technique allowing for the direct translation of CAD models to real world objects. Using SEBM we are able to produce structures of arbitrary geometry in a well-defined manner. Here, we introduce a self-designed 3D-auxetic structure and determine its mechanical properties. We also address the dependence of Young's modulus on relative density. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Axial coordination of pyridyl-containing pentacenes to porphyrins - Waterloo, Andreas; Lippert, Rainer; Jux, Norbert; Tykwinski, Rik (Journal of Coordination Chemistry, 2015)
A pair of pentacenes that are functionalized in the 6-position with either a 3- or 4-pyridyl group via a triazole linker have been used to form complexes with a tetra(aryl)ruthenium(II) porphyrin through axial coordination. The pentacene–porphyrin dyads 5 and 6 have been structurally characterized through a combination of spectroscopic techniques. UV–vis spectroscopy shows that the absorption profiles of the two chromophores, the porphyrin and the pentacene, are complementary, providing absorptions throughout the UV and visible regions. While the dyads are reasonably stable in the solid state under ambient conditions they are, unfortunately, only stable in solution for hours when exposed to light and air.
EAM Import::2019-03-13

Band gap effect on the photocatalytic activity of supramolecular structures obtained by entrapping photosensitizers in different inorganic supports - Cojocaru, Bogdan; Neatu, Stefan; Parvulescu, Vasile I.; Dumbuya, Karifala; Steinrück, Hans-Peter; Gottfried, Michael; Aprile, Carmela; Garcia, Hermenegildo; Scaiano, J. C. (Physical Chemistry Chemical Physics, 2009)
Different metallophthalocyanines and 2,4,6-triphenylpyrylium (TP ) ions were entrapped in different inorganic supports such as Y zeolite, mesoporous MCM-41, TiO-SiO and SiO following a specific protocol. The resulting supramolecular structures were characterized by chemical analysis and diffuse reflectance UV-Vis measurements. The determination of the band gap on the basis of UV-Vis measurements showed that the host is not a spectator in this process and an electronic interaction occurs that lowers the band gap of the support. The XPS measurements were performed with an in situ X-ray photoelectron spectrometer, which can be operated at pressures of up to 1 mbar at the sample. They indicated that the formation of the supramolecular structure generates a stable environment, in which the oxidation state of the metal can still be influenced by reaction of the metal center with gas molecules. The photocatalytic tests carried out in photodecomposition of dipropyl sulfide showed a good correlation between the band gap values and the photocatalytic activity for metallophthalocyanine complexes. It is remarkable that the triphenylpyrylium ion follows the same trend observed for metallophthalocyanines, although it is a different type of photosensitizer. © the Owner Societies 2009.

Bandgap engineering of graphene by physisorbed adsorbates - Kozlov, Sergey M.; Vines Solana, Francesc; Görling, Andreas (Advanced Materials, 2011)
Adsorption of PTCDI induces a bandgap opening on graphene, due to the specific electronic structure of PTCDI, characterized by a LUMO energetically located in the vicinity of the Dirac point. The approach is highly versatile for bandgap engineering, tuning the bandgap by modifying the adsorption geometry, the active adsorbate coverage, or by chemical modifications of the adsorbate. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Band gap formation and Anderson localization in disordered photonic materials with structural correlations - Froufe-Perez, Luis S.; Engel, Michael; Jose Saenz, Juan; Scheffold, Frank (Proceedings of the National Academy of Sciences of the United States of America, 2017)
Disordered dielectric materials with structural correlations show unconventional optical behavior: They can be transparent to long-wavelength radiation, while at the same time have isotropic band gaps in another frequency range. This phenomenon raises fundamental questions concerning photon transport through disordered media. While optical transparency in these materials is robust against recurrent multiple scattering, little is known about other transport regimes like diffusive multiple scattering or Anderson localization. Here, we investigate band gaps, and we report Anderson localization in 2D disordered dielectric structures using numerical simulations of the density of states and optical transport statistics. The disordered structures are designed with different levels of positional correlation encoded by the degree of stealthiness extgreekq. To establish a unified view, we propose a correlation-frequency ( extgreekq– extgreekn) transport phase diagram. Our results show that, depending only on extgreekq, a dielectric material can transition from localization behavior to a band gap crossing an intermediate regime dominated by tunneling between weakly coupled states.
EAM Import::2019-03-13

BaO Clusters on MgO Nanocubes: A Quantitative Analysis of Optical-Powder Properties - Sternig, Andreas; Mueller, Markus; Mccallum, Mark; Bernardi, Johannes; Diwald, Oliver (Small, 2010)
Uniformly sized and shaped nanoparticles are well suited for the quantitative characterization of optical-powder properties. For the first time, quantum yields related to photoluminescence emissions that originate from the photoexcitation of MgO nanocube corners and edges are measured. In addition, the surfaces of these nanoparticles are doped with submonolayer barium, which oxidizes during adsorption onto the MgO nanocrystal surfaces and transforms in O_2atmosphere into BaO. UV–Vis diffuse reflectance and photoluminescence spectroscopy is employed to explore whether 10-3 monolayer equivalents of these dopants affect the MgO specific optical properties. Surface-admixed BaO produces additional absorption and photoluminescence emission features but does not significantly affect those specific to MgO nanocubes. On this basis the number of optically active sites that can be sampled inside a powder of alkaline earth oxide nanoparticles using a standard spectrometer system is estimated.
EAM Import::2019-03-13

Barrier inhomogeneities limited current and 1/f noise transport in GaN based nanoscale Schottky barrier diodes - Kumar, Ashutosh; Heilmann, Marcus; Latzel, Michael; Kapoor, Raman; Sharma, Intu; Göbelt, Manuela; Christiansen, Silke H.; Kumar, Vikram; Singh, Rajendra (Scientific Reports, 2016)
The electrical behaviour of Schottky barrier diodes realized on vertically standing individual GaN nanorods and array of nanorods is investigated. The Schottky diodes on individual nanorod show highest barrier height in comparison with large area diodes on nanorods array and epitaxial film which is in contrast with previously published work. The discrepancy between the electrical behaviour of nanoscale Schottky diodes and large area diodes is explained using cathodoluminescence measurements, surface potential analysis using Kelvin probe force microscopy and 1ow frequency noise measurements. The noise measurements on large area diodes on nanorods array and epitaxial film suggest the presence of barrier inhomogeneities at the metal/semiconductor interface which deviate the noise spectra from Lorentzian to 1/f type. These barrier inhomogeneities in large area diodes resulted in reduced barrier height whereas due to the limited role of barrier inhomogeneities in individual nanorod based Schottky diode, a higher barrier height is obtained.

Basal-Plane Functionalization of Chemically Exfoliated Molybdenum Disulfide by Diazonium Salts - Knirsch, Kathrin; Nerl, Hannah C.; Cucinotta, Clotilde S.; Berner, Nina; Gholamvand, Zahra; McEvoy, Niall; Wang, Zhenxing; Abramovic, Irena; Vecera, Philipp; Halik, Marcus; Sanvito, Stefano; Duesberg, Georg S.; et al. (ACS nano, 2015)
Although transition metal dichalcogenides such as MoS2 have been recognized as highly potent two-dimensional nanomaterials, general methods to chemically functionalize them are scarce. Herein, we demonstrate a functionalization route that results in organic groups bonded to the MoS2 surface via covalent C-S bonds. This is based on lithium intercalation, chemical exfoliation and subsequent quenching of the negative charges residing on the MoS2 by electrophiles such as diazonium salts. Typical degrees of functionalization are 10-20 atom % and are potentially tunable by the choice of intercalation conditions. Significantly, no further defects are introduced, and annealing at 350 °C restores the pristine 2H-MoS2. We show that, unlike both chemically exfoliated and pristine MoS2, the functionalized MoS2 is very well dispersible in anisole, confirming a significant modification of the surface properties by functionalization. DFT calculations show that the grafting of the functional group to the sulfur atoms of (charged) MoS2 is energetically favorable and that S-C bonds are formed.
Keywords: 1T-polytype; basal-plane; chemical exfoliation; covalent functionalization; layered materials; molybdenum disulfide; transition metal dichalcogenides

Basic aspects of phase changes under high pressure - Delgado, Antonio; Kulisiewicz, Leszek; Rauh, Cornelia; Benning, Rainer (Annals of the New York Academy of Sciences, 2010)
Some substances of technological importance reveal phase change phenomena in the pressure and temperature range typically applied in biotechnology and food processing. For example, media with high molar volumes like edible oils and fats undergo liquid-solid phase transition at pressure increases up to several hundred megapascals. This article is concerned with theoretical considerations of the line of coexistence of solid and liquid phases in the pressure and temperature domain that corresponds to the phase boundary as a function of temperature and pressure. A universal model equation based on the equilibrium thermodynamics allowing prediction of the phase transition line of homogeneous media with high molar volume is presented. Approximate solutions of the model equation are discussed, which allow the phase boundary of substances with high molar volume to be described by a linear relation at least sequentially. The methods of experimental determination of the phase boundary under high pressure are presented and an attempt is made to validate the theoretical model with respect to the experimental data. © 2010 New York Academy of Sciences.
UnivIS-Import:2015-03-09:Pub.2010.tech.ITC.stmmec.basica
Keywords: Edible oils; High pressure; Phase boundary; Phase change; Thermodynamic modeling

Basic Insights into Tunable Graphene Hydrogenation - Schäfer, Ricarda; Dasler, Daniela; Mundloch, Udo; Hauke, Frank; Hirsch, Andreas (Journal of the American Chemical Society, 2016)
The hydrogenation and deuteration of graphite with potassium intercalation compounds as starting materials were investigated in depth. Characterization of the reaction products (hydrogenated and deuterated graphene) was carried out by thermogravimetric analysis coupled with mass spectrometry (TG-MS) and statistical Raman spectroscopy (SRS) and microscopy (SRM). The results reveal that the choice of the hydrogen/deuterium source, the nature of the graphite (used as starting material), the potassium concentration in the intercalation compound, and the choice of the solvent have a great impact on the reaction outcome. Furthermore, it was possible to prove that both mono- and few-layer hydrogenated/deuterated graphene can be produced.

BDA: A novel method for identifying defects in body-centered cubic crystals - Möller, Johannes; Bitzek, Erik (MethodsX, 2016)
The accurate and fast identification of crystallographic defects plays a key role for the analysis of atomistic simulation output data. For face-centered cubic (fcc) metals, most existing structure analysis tools allow for the direct distinction of common defects, such as stacking faults or certain low-index surfaces. For body-centered cubic (bcc) metals, on the other hand, a robust way to identify such defects is currently not easily available. We therefore introduce a new method for analyzing atomistic configurations of bcc metals, the BCC Defect Analysis (BDA). It uses existing structure analysis algorithms and combines their results to uniquely distinguish between typical defects in bcc metals. In essence, the BDA method offers the following features: Identification of typical defect structures in bcc metals. Reduction of erroneously identified defects by iterative comparison to the defects in the atom's neighborhood. Availability as ready-to-use Python script for the widespread visualization tool OVITO [http://ovito.org
Keywords: Atomistic simulations; bcc materials; Defectanalysis; Molecular dynamics

Beneficial effects of liquid crystalline phases in solid-state dye-sensitized solar cells - Costa, Ruben Dario; Werner, Fabian; Wang, Xinjiao; Grönninger, Philipp; Feihl, Sebastian; Kohler, Florian; Wasserscheid, Peter; Hibler, Susanne; Beranek, Radim; Meyer, Karsten; Guldi, Dirk Michael (Advanced Energy Materials, 2013)
Two novel double-alkyl functionalized imidazolium ionic liquid crystals have successfully been utilized to demonstrate the benefits of the liquid crystalline phase on the ssDSSC performance. In particular, a good balance between dye regeneration and hole transport is only realized in the liquid crystalline phase. Devices that employ a single component ionic liquid based electrolyte show a remarkably stable efficiency during 1000 h under outdoor operation temperature conditions and 1 sun illumination. Two novel double-alkyl functionalized imidazolium ionic liquid crystals have successfully been utilized to demonstrate the benefits of the liquid crystalline phase on the ssDSSC performance. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Benz-bisimidazole-bridged perylenes - Linearly expanded chromophores - Schönamsgruber, Jörg; Hirsch, Andreas (European Journal of Organic Chemistry, 2015)
The synthesis and characterization of a new type of very large perylene-based molecules 9 are reported. The extension of the conjugated extgreekp system was accomplished by the facile condensation of two bay-functionalized perylene moieties with 1,2,4,5-tetraaminobenzene. The resulting chromophore in 9 consists of 35 conjugated extgreekp-electron pairs and, therefore, is comparable to pentarylenes and hexarylenes containing 31 and 36 conjugated extgreekp-electron pairs, respectively. The unsubstituted imine N atoms of the benz-bisimidazole bridges in 9 can be readily and reversibly protonated to give the dications 9aH22+. The twofold protonation of 9 is accompanied by a bathochromic shift of the main absorption band and pronounced fluorescence quenching. The experimental results were corroborated by quantum mechanical calculations.
Keywords: Chromophores; Dyes; NIR dyes; Perylene; Rylene dyes; π Conjugation

Benzo[a]pyrene-induced metabolic shift from glycolysis to pentose phosphate pathway in the human bladder cancer cell line RT4 - Verma, Nisha; Pink, Mario; Boland, Stefan; Rettenmeier, Albert W.; Schmitz-Spanke, Simone (Scientific Reports, 2017)
Benzo[a]pyrene (B[a]P), a well-known polyaromatic hydrocarbon, is known for its lung carcinogenicity, however, its role in bladder cancer development is still discussed. Comparative two-dimensional blue native SDS-PAGE analysis of protein complexes isolated from subcellular fractions of 0.5 µM B[a]P-exposed cells indicated a differential regulation of proteins involved in carbohydrate, fatty acid, and nucleotide metabolism, suggesting a possible metabolic flux redistribution. It appeared that B[a]P exposure led to a repression of enzymes (fructose-bisphosphate aldolase A, glucose-6-phosphate isomerase, lactate dehydrogenase) involved in glycolysis, and an up-regulation of proteins (glucose-6-phosphate 1-dehydrogenase, 6-phosphogluconolactonase) catalyzing the pentose phosphate pathway and one carbon metabolism (10-formyltetrahydrofolate dehydrogenase, bifunctional purine biosynthesis protein). Untargeted metabolomics further supported the proteomic data, a lower concentration of glycolytic metabolite was observed as compared to glutamine, xylulose and fatty acids. The analysis of the glutathione and NADPH/NADP+ content of the cells revealed a significant increase of these cofactors. Concomitantly, we did not observe any detectable increase in the production of ROS. With the present work, we shed light on an early phase of the metabolic stress response in which the urothelial cells are capable of counteracting oxidative stress by redirecting the metabolic flux from glycolysis to pentose phosphate pathway.
EVALuna2:9763

Benzo[a]pyrene-mediated toxicity in primary pig bladder epithelial cells: A proteomic approach - Verma, Nisha; Pink, Mario; Rettenmeier, Albert W.; Schmitz-Spanke, Simone (Journal of Proteomics, 2013)
The studies described in this paper deal with a sequence of cellular events induced by the environmental toxicant benzo[a]pyrene (B[a]P) that were investigated in primary urinary bladder epithelia cells (PUBEC) from pigs by using a proteomic approach. Two-dimensional (2DE) gel electrophoresis unveiled the differences in protein expression between cells exposed to 0.5 extgreekmM B[a]P for 24 h and control cells. Twenty-five differentially expressed proteins involved in DNA repair, mitochondrial dysfunction, and apoptosis were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). These findings were supported by the concentration-dependent increase in olive tail moments as determined by the comet assay and by a time-dependent increase in histone H2A.x (H2AX) phosphorylation upon B[a]P exposure. On the other hand, the expression of voltage-dependent anion channel 2 (VDAC2), cathepsin D (CTSD), heat shock protein 27 (HSP27), and heat shock protein 70 (HSP70) hinted to apoptosis occurring through the intrinsic apoptotic mitochondrial pathway. Taken together, these data suggest that B[a]P is capable of inducing DNA damage in urinary bladder epithelial cells at low concentrations during a short exposure period, thus eventually leading to cell death by apoptosis. Biological significance Epidemiological studies have indicated PAHs as potential candidates for initiating bladder cancer development, although the precise risk is still unknown (Kaufman et al. (2009) [1]). In recent years, the understanding of the metabolic capacity of urothelial cells has broadened continuously; i.e. a wide range of xenobiotic metabolizing cytochrome P450 enzymes (CYP) were detected in urothelial cells from humans and animals (Roos et al., 2006; Guhe et al., 1996 [2] and [3]), thus indicating that urothelial cells are not only passively exposed to reactive metabolites but also actively by intracellularly producing reactive intermediates that can induce cancer. Moreover, small quantities of non-metabolized B[a]P and its hydroxylated derivatives have been identified in blood and urine (Rossella et al. (2009) [4]). Thus, it appears plausible that B[a]P, a highly lipophilic compound, is taken up by the urothelium and metabolically activated to carcinogenic intermediates in these cells. In our previous studies with primary uroepithelial cells isolated from freshly slaughtered pigs we demonstrated the ability of these cells for a strong uptake of B[a]P and its conversion to the oxidative metabolite (3-OH–B[a]P) (Verma et al. (2012) [5]). The present study is a continuation of this previous work exhibiting the effects of B[a]P exposure on cellular functions of PUBEC. The results indicated caspase-dependent apoptosis induced by B[a]P due to DNA damage (possibly lethal double-strand breaks as indicated by H2AX phosphorylation). Taken together, these studies provide strong evidence for the ability of B[a]P to act as a bladder carcinogen.
EAM Import::2019-03-13

Benzoic Acid and Phthalic Acid on Atomically Well-Defined MgO(100) Thin Films: Adsorption, Interface Reaction, and Thin Film Growth - Xu, Tao; Mohr, Susanne; Amende, Maximilian; Laurin, Mathias; Döpper, Tibor; Görling, Andreas; Libuda, Jörg (Journal of Physical Chemistry C, 2015)
To better understand the interaction and the growth of thin films of functionalized organic molecules on oxide surfaces, we have studied the adsorption, reaction, and desorption of benzoic acid (BA) and phthalic acid (PA) on a well-ordered MgO(100) thin film grown on a Ag(100) single crystal surface. We have applied isothermal time-resolved infrared reflection absorption spectroscopy (TR-IRAS) and temperature-programmed IRAS (TP-IRAS) under ultrahigh-vacuum (UHV) conditions. BA is dosed using a supersonic molecular beam (SSMB) source while PA is deposited by physical vapor deposition (PVD). For both molecules we have explored the film growth as a function of temperature, both in the monolayer and in the multilayer regime. We have also investigated structural transitions and desorption by temperature-programmed experiments in the range from 100 to 400 K. In addition, we carried out density-functional (DF) calculations. We find that both molecules BA and PA bind through the carboxyl groups to the MgO (100) surface. Upon adsorption at 100 K BA binds in an asymmetric bidentate geometry which exhibits a small tilting angle between the aromatic plane and the surface. Beyond the monolayer, a disordered multilayer film grows, which crystallizes under formation of dimers at around 180 K as indicated by a characteristic splitting of the IR bands. The BA multilayer desorbs at 240 K. Upon adsorption at 300 K, only a BA monolayer forms. Again, BA forms an asymmetric bidentate but with a larger tilting angle compared to low-temperature adsorption. For PA adsorption at 100 K, the adsorption mechanism is observed to change with coverage. At low coverage, both carboxyl groups are deprotonated, and the molecule forms an asymmetric bis-bidentate carboxylate with the aromatic plane nearly perpendicular to the surface. At high coverage, only one carboxyl group binds to the surface and forms an asymmetric bidentate carboxylate while the molecules maintain an upright standing orientation. During PVD of PA, a small fraction of phthalic anhydride (PAA) is formed which coadsorbs at low temperature. Upon annealing, the PAA. desorbs around 250 K, triggering a structural transformation of the PA multilayer during which the PA adopts a more flat lying orientation. The PA multilayer itself desorbs around 310 K. Therefore, only monolayer of PA is stable around 300 K. Again, the adsorption mechanism is coverage dependent, changing from a bis-bidentate carboxylate at lower coverage to a monobidentate carboxylate at higher coverage.

Benzoporphyrins: Selective Co-sensitization in Dye-Sensitized Solar Cells - Lodermeyer, Fabian; Costa, Ruben Dario; Malig, Jenny; Jux, Norbert; Guldi, Dirk Michael (Chemistry - A European Journal, 2016)
A novel class of dyes, namely benzoporphyrins, was synthesized and implemented into dye-sensitized solar cells. They feature complementary absorptions compared to N719, which renders them promising candidates for co-sensitization in DSSCs. Notably, metallated benzoporphyrins reveal a TiO2-nanoparticle attachment that is size and aggregation dependent. Therefore, unproductive energy-transfer events between the selectively attached dyes can be prevented. In light of the latter, an efficiency improvement of 39 % has been achieved upon selective adsorption of benzoporphyrins and N719 onto different layers of TiO2 photoelectrode.
EAM Import::2019-03-13

Bestimmung kritischer Radien von Tropfen auf Oberflächen mit einem Gradienten der Benetzbarkeit - Zielke, Paul; Szymczyk, Janusz A.; Delgado, Antonio (Proceedings in Applied Mathematics and Mechanics, 2008)
No abstract available for this article
UnivIS-Import:2018-09-05:Pub.2008.tech.ITC.stmmec.bestim_3

Beyond the percolation universality class: the vertex split model for tetravalent lattices - Nachtrab, Susan; Hoffmann, Matthias; Kapfer, Sebastian; Schröder-Turk, Gerd; Mecke, Klaus (New Journal of Physics, 2015)
We propose a statistical model defined on tetravalent three-dimensional lattices in general and the three-dimensional diamond network in particular where the splitting of randomly selected nodes leads to a spatially disordered network, with decreasing degree of connectivity. The terminal state, that is reached when all nodes have been split, is a dense configuration of self-avoiding walks on the diamond network. Starting from the crystallographic diamond network, each of the four-coordinated nodes is replaced with probability p by a pair of two edges, each connecting a pair of the adjacent vertices. For all values 0 <= p <= 1 the network percolates, yet the fraction f(p) of the system that belongs to a percolating cluster drops sharply at p(c) = 1 to a finite value f(p)(c). This transition is reminiscent of a percolation transition yet with distinct differences to standard percolation behaviour, including a finite mass f(p)(c) > 0 of the percolating clusters at the critical point. Application of finite size scaling approach for standard percolation yields scaling exponents for p -> p(c) that are different from the critical exponents of the second-order phase transition of standard percolation models. This transition significantly affects the mechanical properties of linear-elastic realizations (e.g. as custom-fabricated models for artificial bone scaffolds), obtained by replacing edges with solid circular struts to give an effective density phi. Finite element methods demonstrate that, as a low-density cellular structure, the bulk modulus K shows a cross-over from a compression-dominated behaviour, K(phi) alpha phi(k) with kappa approximate to 1, at p = 0 to a bending-dominated behaviour with kappa approximate to 2 at p = 1.
Keywords: percolation; networks in phase transitions; mechanical properties of solids

Bias-controlled selective excitation of vibrational modes in molecular junctions: a route towards mode-selective chemistry - Volkovich, Roie; Härtle, Rainer; Thoss, Michael; Peskin, Uri (Physical Chemistry Chemical Physics, 2011)
We show that individual vibrational modes in single-molecule junctions with asymmetric molecule-lead coupling can be selectively excited by applying an external bias voltage. Thereby, a non-statistical distribution of vibrational energy can be generated, that is, a mode with a higher frequency can be stronger excited than a mode with a lower frequency. This is of particular interest in the context of mode-selective chemistry, where one aims to break specific (not necessarily the weakest) chemical bond in a molecule. Such mode-selective vibrational excitation is demonstrated for two generic model systems representing asymmetric molecular junctions and/or scanning tunneling microscopy experiments. To this end, we employ two complementary theoretical approaches, a nonequilibrium Green's function approach and a master equation approach. The comparison of both methods reveals good agreement in describing resonant electron transport through a single-molecule contact, where differences between the approaches highlight the role of non-resonant transport processes, in particular co-tunneling and off-resonant electron-hole pair creation processes. © the Owner Societies 2011.
UnivIS-Import:2015-03-09:Pub.2011.nat.dphy.ITP.ppem.biasco

Bicanonical ab Initio Molecular Dynamics for Open Systems - Frenzel, Johannes; Meyer, Bernd; Marx, Dominik (Journal of Chemical Theory and Computation, 2017)
Performing ab initio molecular dynamics simulations of open systems, where the chemical potential rather than the number of both nuclei and electrons is fixed, still is a challenge. Here, drawing on bicanonical sampling ideas introduced two decades ago by Swope and Andersen [ J. Chem. Phys. 1995 , 102 , 2851 - 2863 ] to calculate chemical potentials of liquids and solids, an ab initio simulation technique is devised, which introduces a fictitious dynamics of two superimposed but otherwise independent periodic systems including full electronic structure, such that either the chemical potential or the average fractional particle number of a specific chemical species can be kept constant. As proof of concept, we demonstrate that solvation free energies can be computed from these bicanonical ab initio simulations upon directly superimposing pure bulk water and the respective aqueous solution being the two limiting systems. The method is useful in many circumstances, for instance for studying heterogeneous catalytic processes taking place on surfaces where the chemical potential of reactants rather than their number is controlled and opens a pathway toward ab initio simulations at constant electrochemical potential.
EAM Import::2019-03-13

Bicontinuous minimal surface nanostructures for polymer blend solar cells - Kimber, Robin G. E.; Walker, Alison B.; Schröder-Turk, Gerd; Cleaver, Douglas J. (Physical Chemistry Chemical Physics, 2010)
This paper presents the first examination of the potential for bicontinuous structures such as the gyroid structure to produce high efficiency solar cells based on conjugated polymers. The solar cell characteristics are predicted by a simulation model that shows how the morphology influences device performance through integration of all the processes occurring in organic photocells in a specified morphology. In bicontinuous phases, the surface defining the interface between the electron and hole transporting phases divides the volume into two disjoint subvolumes. Exciton loss is reduced because the interface at which charge separation occurs permeates the device so excitons have only a short distance to reach the interface. As each of the component phases is connected, charges will be able to reach the electrodes more easily. In simulations of the current–voltage characteristics of organic cells with gyroid, disordered blend and vertical rod (rods normal to the electrodes) morphologies, we find that gyroids have a lower than anticipated performance advantage over disordered blends, and that vertical rods are superior. These results are explored thoroughly, with geminate recombination, i.e. recombination of charges originating from the same exciton, identified as the primary source of loss. Thus, if an appropriate materials choice could reduce geminate recombination, gyroids show great promise for future research and applications.
EAM Import::2019-03-13

Binary Indium-Zinc Oxide Photoanodes for Efficient Dye-Sensitized Solar Cells - Peukert, Wolfgang; Stanzel, Melanie; Kunzmann, Andreas; Guldi, Dirk Michael; Costa Riquelme, Rubén Dario (Advanced Energy Materials, 2016)
The benefits of incorporating binary metal-oxide electrodes en route toward efficient dye-sensitized solar cells (DSSCs) have recently emerged. The current work aims at realizing efficient indium-doped zinc oxide based DSSCs by means of enhancing charge transport processes and reducing recombination rates. Electrochemical impedance spectroscopic assays corroborate that low amounts of indium reduce charge transport resistances and increase electron recombination resistances. The latter are in concert with a remarkable enhancement of the charge collection efficiency from 33% to 83% for devices with ZnO and InZnO photoanodes, respectively. Going beyond 15 mol% of indium, an effective electron trapping increases the charge transport resistance and, in turn, dramatically reduces charge collection efficiency. Upon implementing InZnO into an electron cascade photoanode architecture featuring an InZnO bottom layer and a ZnO top layer, a device efficiency of 5.77% and a significantly high current density of 20.4 mA cm in binary ZnO DSSCs are achieved.
Keywords: binary oxides; charge collection efficiency; flame spray pyrolysis; indium-zinc oxide photoanodes; n-type DSSCs

Bioactive glass (type 45S5) nanoparticles: in vitro reactivity on nanoscale and biocompatibility - Mackovic, Mirza; Hoppe, Alexander; Detsch, Rainer; Mohn, Dirk; Stark, W. J.; Spiecker, Erdmann; Boccaccini, Aldo R. (Journal of Nanoparticle Research, 2012)
Bioactive glasses represent important biomaterials being investigated for the repair and reconstruction of diseased bone tissues, as they exhibit outstanding bonding properties to human bone. In this study, bioactive glass (type 45S5) nanoparticles (nBG) with a mean particle size in the range of 20–60 nm, synthesised by flame spray synthesis, are investigated in relation to in vitro bioreactivity in simulated body fluid (SBF) and response to osteoblast cells. The structure and kinetics of hydroxyapatite formation in SBF were investigated using transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) revealing a very rapid transformation (after 1 day) of nBG to nanocrystalline bone-like carbonated HAp. Additionally, calcite is formed after 1 day of SBF immersion because of the high surface reactivity of the nBG particles. In the initial state, nBG particles were found to exhibit chain-like porous agglomerates of amorphous nature which are transformed on immersion in SBF into compact agglomerates covered by hydroxyapatite with a reduced size of the primary nanoparticles. In vitro studies revealed high cytocompatibility of nBG with human osteoblast cells, indicated through high lactatedehydrogenase (LDH) and mitochondrial activity as well as alkaline phosphatase activity. Hence, this study contributes to the understanding of the structure and bioactivity of bioactive glass (type 45S5) nanoparticles, providing insights to the phenomena occurring at the nanoscale after immersion in SBF. The results are relevant in relation to the understanding of the nanoparticles’ bioreactivity required for applications in bone tissue engineering.
UnivIS-Import:2015-03-09:Pub.2013.tech.IW.LM.bioact
Keywords: Nanoparticles; Bioactive glass; TEM; Hydroxyapatite; Osteoblast

Biogenic formic acid as a green hydrogen carrier - Preuster, Patrick; Albert, Jakob (Energy Technology, 2018)
Formic acid (FA) is an important platform chemical that has numerous applications in the chemical, agricultural and leather industry. So far, FA is exclusively produced from fossil raw materials using either carbonylation of methanol or acidolysis of formates. In this contribution, sustainable and environmentallyfriendly approaches to produce biogenic FA from biomass (e.g. OxFA-process) are presented and combined with different approaches for the catalytic decomposition of bio-based FA solutions. Several homogeneous and heterogeneous approaches for the catalytic decomposition of aqueous FA to hydrogen and CO2 are compared for their applicability. Moreover, their technical application in fuel cells or as energy storage vector respectively hydrogen storage molecule are discussed.

Bioinspired Hybrid White Light-Emitting Diodes - Weber, Michael; Niklaus, Lukas; Pröschel, Marlene; Brana Coto, Pedro; Sonnewald, Uwe; Costa Riquelme, Rubén Dario (Advanced Materials, 2015)
The first bioinspired hybrid white-light-emitting diodes (bio-HLEDs) featuring protein cascade coatings are presented. For easy fabrication a new strategy to stabilize proteins in rubber-like material was developed. The synergy between the excellent features of fluorescent proteins and the easily processed rubber produces bio-HLEDs with less than 10% loss in luminous efficiency over 100 hours.

BIOPHYSICS Alive and twitching - Smith, Ana-Suncana (2016)
Living systems are constantly being driven out of equilibrium by consuming energy. Studying fluctuations can tell us how they do so while maintaining order — and what this teaches us about non-equilibrium processes in general.

Bistability in a nonequilibrium quantum system with electron-phonon interactions - Wilner, Eli Y.; Wang, Haobin; Cohen, Guy; Thoss, Michael; Rabani, Eran (Physical Review B, 2013)
The existence of more than one steady state in a many-body quantum system driven out of equilibrium has been a matter of debate, both in the context of simple impurity models and in the case of inelastic tunneling channels. In this paper, we combine a reduced density matrix formalism with the multilayer multiconfiguration time-dependent Hartree method to address this problem. This allows us to obtain a converged numerical solution of the nonequilibrium dynamics. Considering a generic model for quantum transport through a quantum dot with electron-phonon interaction, we prove that a unique steady state exists regardless of the initial electronic preparation of the quantum dot, consistent with the converged numerical results. However, a bistability can be observed for different initial phonon preparations. The effects of the phonon frequency and strength of the electron-phonon couplings on the nonequilibrium dynamics and on the emergence of bistability is discussed. © 2013 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.bistab

Black TiO2 Nanotubes: Cocatalyst-Free Open-Circuit Hydrogen Generation - Liu, Ning; Schneider, Christopher; Freitag, Detlef; Hartmann, Martin; Venkatesan, Uma Maheswari; Müller, Julian; Spiecker, Erdmann; Schmuki, Patrik (Nano Letters, 2014)
Here we report that TiO2 nanotube (NT) arrays, converted by a high pressure H-2 treatment to anatase-like "black titania", show a high open-circuit photocatalytic hydrogen production rate without the presence of a cocatalyst. Tubes converted to black titania using classic reduction treatments (e.g., atmospheric pressure H-2/Ar annealing) do not show this effect. The main difference caused by the high H-2 pressure annealing is the resulting room-temperature stable, isolated Ti3+ defect-structure created in the anatase nanotubes, as evident from electron spin resonance (ESR) investigations. This feature, absent for conventional reduction, seems thus to be responsible for activating intrinsic, cocatalytic centers that enable the observed high open-circuit hydrogen generation.
Keywords: TiO2 nanotube arrays; water-splitting; hydrogenation; photocatalysis; cocatalyst; black titania

"Black" TiO2 Nanotubes Formed by High-Energy Proton Implantation Show Noble-Metal-co-Catalyst Free Photocatalytic H2-Evolution - Frey, Lothar; Osvet, Andres; Zhou, Xuemei; Haeublein, Volker; Schmuki, Patrik; Frey, Lothar; Spiecker, Erdmann; Mackovic, Mirza; Liu, Ning; Hartmann, Martin; Nakajima, Tomohiko; Venkatesan, Uma Maheswari (Nano Letters, 2015)
We apply high-energy proton ion-implantation to modify TiO nanotubes selectively at their tops. In the proton-implanted region, we observe the creation of intrinsic cocatalytic centers for photocatalytic H-evolution. We find proton implantation to induce specific defects and a characteristic modification of the electronic properties not only in nanotubes but also on anatase single crystal (001) surfaces. Nevertheless, for TiO nanotubes a strong synergetic effect between implanted region (catalyst) and implant-free tube segment (absorber) can be obtained.
Keywords: ion-implantation; Nanotubes; photocatalysts; self-organization; titania; water-splitting

Blending Through-Space and Through-Bond π-π-Coupling in [2,2']-Paracyclophane-oligophenylenevinylene Molecular Wires - Wielopolski, Mateusz; Molina-Ontoria, Agustin; Schubert, Christina; Margraf, Johannes; Krokos, Evangelos; Kirschner, Johannes; Gouloumis, Andreas; Clark, Timothy; Guldi, Dirk Michael; Martin, Nazario (Journal of the American Chemical Society, 2013)
A series of ZnP-pCp-oPPV-C60 conjugates covalently connected through [2,2′]-paracyclophane-oligophenylenevinylene (pCp-oPPV) bridges containing one, two, and three [2,2′]-paracyclophanes (pCps) has been prepared in multistep synthetic procedures involving Horner–Wadsworth–Emmons olefination reactions and/or Heck type Pd-catalyzed reactions. Molecular modeling suggests that charge transfer is effectively mediated by the pCp-oPPVs through a predominant hole-transfer mechanism. Photophysical investigation supports molecular modeling and reveals two major trends. On one hand, C60 excitation of 1, 2, and 3 leads exclusively to charge transfer between pCp and C60 to afford a ZnP-(pCp-oPPV)•+-C60•– radical ion pair state without giving rise to a subsequent charge shift to yield the ZnP•+-pCp-oPPV-C60•– radical ion pair state. On the other hand, ZnP excitation of 1, 2, and 3 results in a rather slow charge transfer between ZnP and C60, after which the ZnP•+-pCp-oPPV-C60•– radical ion pair state evolves. In temperature-dependent ZnP fluorescence experiments, which were performed in the temperature range from 273 to 338 K, two domains are discernible: low and high temperature behaviors. In the low temperature range (i.e., below 30 °C) the rate constants do not change, suggesting that a superexchange mechanism is the modus operandi. In the high temperature range (i.e., >30 °C) the rate constants increase. Moreover, we find rather strong distance dependence for 1 and 2 and weak distance dependence for 2 and 3. A damping factor of 0.145 Å–1 is derived for the former pair and 0.012 Å–1 for the latter.
EAM Import::2019-03-13

Blindness of the exact density response function to certain types of electronic excitations: Implications for time-dependent density-functional theory - Heßelmann, Andreas; Görling, Andreas (Physical Review Letters, 2009)
Examples of electronic excitations are presented and discussed that do not contribute to the sum-over-state expression of the frequency-dependent density-density response function and thus do not lead to poles of the latter. As a consequence, these excitations principally cannot be described by time-dependent density-functional theory (TDDFT) response methods or any other method relying on the poles of the density-density response function. For these excitations, the DFT response approach commonly used in molecular physics or quantum chemistry is shown to not properly represent a DFT method and to yield differences of Kohn-Sham eigenvalues as excitation energies. The discussed examples are simple valence-valence excitations in the neon atom and the nitrogen molecule. © 2009 The American Physical Society.

Bonding mechanisms of graphene on metal surfaces - Kozlov, Sergey M.; Vines, Francesc; Görling, Andreas (Journal of Physical Chemistry C, 2012)
The basic bonding mechanisms of graphene on transition-metal surfaces leading to chemisorption and physisorption are identified and studied in the example of graphene adsorption on Ni(111) by means of density functional theory at the general gradient approximation level with semiempirical corrections for dispersive interactions. In the more stable chemisorbed graphene, relatively strong Pauli repulsion between graphene and the substrate is compensated by donation/back-donation bonding of the same magnitude. In this case, the electronic interactions with the substrate significantly perturb the electronic structure of graphene, but the adsorption energy is still dominated by van der Waals (vdW) interactions. In physisorbed graphene, weak Pauli repulsion equilibrates the vdW attraction without affecting the electronic structure of graphene. The relative stability of physisorbed and chemisorbed graphene is shown to be changed by carbidic C impurities in the subsurface region. © 2012 American Chemical Society.

Boolean reconstructions of complex materials: Integral geometric approach - Arns, C. H.; Knackstedt, M. A.; Mecke, Klaus (Physical Review E, 2009)
We show that for the Boolean model of random composite media one can, from a single image of a system at any particle fraction, define a set of parameters which allows one to accurately reconstruct the medium for all other phase fractions. The morphological characterization is based on a family of measures known in integral geometry which provides powerful formulas for the Boolean model. The percolation thresholds of either phase are obtained with good accuracy. From the reconstructions one can subsequently predict property curves for the material across all phase fractions from the single three-dimensional image. We illustrate this for transport and mechanical properties of complex Boolean systems and for experimental sandstone samples.
Keywords: Boolean algebra; granular materials; integral equations; percolation; random media

Boosting performance of low temperature fuel cell catalysts by subtle ionic liquid modification - Zhang, Guirong; Munoz Garcia, Macarena; Etzold, Bastian (ACS Applied Materials and Interfaces, 2015)
High cost and poor stability of the oxygen reduction reaction (ORR) electrocatalysts are the major barriers for broad-based application of polymer electrolyte membrane fuel cells. Here we report a facile and scalable approach to improve Pt/C catalysts for ORR, by modification with small amounts of hydrophobic ionic liquid (IL). The ORR performance of these IL-modified catalysts can be readily manipulated by varying the degree of IL filling, leading to a 3.4 times increase in activity. Besides, the IL-modified catalysts exhibit substantially enhanced stability relative to Pt/C. The enhanced performance is attributed to the optimized microenvironment at the interface of Pt and electrolyte, where advantages stemming from an increased number of free sites, higher oxygen concentration in the IL and electrostatic stabilization of the nanoparticles develop fully, at the same time that the drawback of mass transfer limitation remains suppressed. These findings open a new avenue for catalyst optimization for next-generation fuel cells.
EAM Import::2019-03-13

Boosting the Activity in Supported Ionic Liquid-Phase-Catalyzed Hydroformylation via Surface Functionalization of the Carbon Support - Weiß, Alexander; Munoz, Macarena; Haas, Alexander; Rietzler, Florian; Steinrück, Hans-Peter; Haumann, Marco; Wasserscheid, Peter; Etzold, Bastian (ACS Catalysis, 2016)
Supported ionic liquid phase (SILP) catalysts are comprised of a thin ionic liquid film containing a homogeneous transition metal complex that is supported on a highly porous substrate. The application of functionalized activated carbon as a support material for SILP catalysts in a continuous gas-phase propene hydroformylation reaction reveals that the surface properties of the carbon support have a significant influence on the catalytic performance. The catalytic activity of the pristine activated carbon-based SILP catalysts was found to be negligibly small (TOF4h = 3.5 h(-1)) in contrast to that of common oxidic supports such as silica (TOF4h = 15 h(-1)) or alumina. By amine functionalization of the carbon support surface, a dramatic increase in catalytic activity could be observed outperforming the tested oxidic supports by an order of magnitude. A distinct relationship between increased nitrogen content and the point of zero charge of the support and the catalytic activity could be observed. The largest activity increase (TOF4h = 105 h(-1)) was found for functionalized activated carbon SILP catalysts featuring a high point of zero charge and a high nitrogen content. Despite the fact that aldol formation is catalyzed by a base, the respective alcohol was the only detectable byproduct in the reactions.
Keywords: hydroformylation; supported ionic liquid phase; surface functionalization; amino groups; activated carbon

Boron containing magnetic nanoparticles for neutron capture therapy - an innovative approach for specifically targeting tumors - Tietze, Rainer; Unterweger, Harald; Dürr, Stephan; Lyer, Stefan; Canella, Lea; Kudejova, Petra; Wagner, Franz M.; Petry, Winfried; Taccardi, Nicola; Alexiou, Christoph (Applied Radiation and Isotopes, 2015)
The selective delivery of (10)B into the tumor tissue remains to be further improved for successful and reliable Boron Neutron Capture Therapy applications. Magnetic Drug Targeting using intraarterially administered superparamagnetic nanoparticles and external magnetic fields already exhibited convincing results in terms of highly efficient and selective drug deposition. Using the same technique for the targeted (10)B delivery is a promising new approach. Here, systematic irradiation experiments of phantom cubes containing different concentrations of boron and nanoparticles as well as varying three-dimensional arrangements have been performed.
EVALuna2:19923

Boron-containing MFI-type zeolites with a hierarchical nanosheet assembly for lipase immobilization - Marthala, Venkata; Urmoneit, Lynn; Zhou, Zhou; Machoke, Albert; Schmiele, Martin; Unruh, Tobias; Schwieger, Wilhelm; Hartmann, Martin (Dalton Transactions, 2017)
Hierarchical nanosheet assemblies of boron-containing MFI-type zeolites were successfully synthesized using bifunctional surfactant molecules under static conditions. With the optimized Si/B = 50 in the synthesis gel, a material with high specific surface area (483 m2 g(-1)) and a maximum pore size of 6.5 nm suitable for lipase encapsulation was obtained.

Bottom-gated epitaxial graphene - Waldmann, Daniel; Jobst, Johannes; Speck, Florian; Seyller, Thomas; Krieger, Michael; Weber, Heiko B. (Nature Materials, 2011)
High-quality epitaxial graphene on silicon carbide (SiC) is today available in wafer size. Similar to exfoliated graphene, its charge carriers are governed by the Dirac-Weyl Hamiltonian and it shows excellent mobilities. For many experiments with graphene, in particular for surface science, a bottom gate is desirable. Commonly, exfoliated graphene flakes are placed on an oxidized silicon wafer that readily provides a bottom gate. However, this cannot be applied to epitaxial graphene as the SiC provides the source material out of which graphene grows. Here, we present a reliable scheme for the fabrication of bottom-gated epitaxial graphene devices, which is based on nitrogen (N) implantation into a SiC wafer and subsequent graphene growth. We demonstrate working devices in a broad temperature range from 6 to 300-K. Two gating regimes can be addressed, which opens a wide engineering space for tailored devices by controlling the doping of the gate structure. © 2011 Macmillan Publishers Limited. All rights reserved.
UnivIS-Import:2015-04-14:Pub.2011.nat.dphy.IAP.LAP.bottom

Bottom sealing and photoelectrochemical properties of different types of anodic TiO2 nanotubes - Lee, Kiyoung; Schmuki, Patrik (Electrochimica Acta, 2013)
Over the past years, the photoelectrochemical behavior of anodic TiO2 nanotubes has intensively been investigated. However, comparably little attention has been paid to the large influence of growth parameters on the phtoelectrochemical tube properties. In the present work we investigate nanotubes of comparable geometry but using either aqueous electrolytes or different organic electrolytes. We show that very drastic differences in their photoresponse can be obtained. Moreover, we show that a recently introduced tube bottom sealing treatment has a very different effect on different tube types and strongly affects the performance of nanotube layers in dye sensitized solar cells.
EAM Import::2019-03-13

Bottom-Up Assembly of Silica and Bioactive Glass Supraparticles with Tunable Hierarchical Porosity - Egly, Steffen; Fröhlich, Christina; Vogel, Stefanie; Grünewald, Alina; Wang, Junwei; Detsch, Rainer; Boccaccini, Aldo R.; Vogel, Nicolas (Langmuir, 2018)
We investigate the formation of spherical supraparticles with controlled and tunable porosity on the nanometer and micrometer scales using the self-organization of a binary mixture of small (nanometer scale) oxidic particles with large (micrometer scale) polystyrene particles in the confinement of an emulsion droplet. The external confinement determines the final, spherical structure of the hybrid assembly, while the small particles form the matrix material. The large particles act as templating porogens to create micropores after combustion at elevated temperatures. We control the pore sizes on the micrometer scale by varying the size of the coassembled polystyrene microspheres and produce supraparticles from both silica- and calcium-containing CaO/SiO2 particles. Although porous supraparticles are obtained in both cases, we found that the presence of calcium ions substantially complicated the fabrication process since the increased ionic strength of the dispersion compromises the colloidal stability during the assembly process. We minimized these stability issues via the addition of a steric stabilizing agent and by mixing bioactive and silica colloidal particles. We investigated the interaction of the porous particles with bone marrow stromal cells and found an increase in cell attachment with increasing pore size of the self-assembled supraparticles.

Bounds on size-dependent behavior of composites - Saeb, Saba; Steinmann, Paul; Javili, A. (Philosophical Magazine, 2017)
Computational homogenisation is a powerful strategy to predict the effective behaviour of heterogeneous materials. While computational homogenisation cannot exactly compute the effective parameters, it can provide bounds on the overall material response. Thus, central to computational homogenisation is the existence of bounds. Classical first-order computational homogenisation cannot capture size effects. Recently, it has been shown that size effects can be retrieved via accounting for elastic coherent interfaces in the microstructure. The primary objective of this contribution is to present a systematic study to attain computational bounds on the size-dependent response of composites. We show rigorously that interface-enhanced computational homogenisation introduces two relative length scales into the problem and investigate the interplay between them. To enforce the equivalence of the virtual power between the scales, a generalised version of the Hill–Mandel condition is employed, and accordingly, suitable boundary conditions are derived. Macroscopic quantities are related to their microscopic counterparts via extended average theorems. Periodic boundary conditions provide an effective behaviour bounded by traction and displacement boundary conditions. Apart from the bounds due to boundary conditions for a given size, the size-dependent response of a composite is bounded, too. The lower bound coincides with that of a composite with no interface. Surprisingly, there also exists an upper bound on the size-dependent response beyond which the expected `smaller is stronger' trend is no longer observed. Finally, we show an excellent agreement between our numerical results and the corresponding analytical solution for linear isotropic materials which highlights the accuracy and broad applicability of the presented scheme.
EAM Import::2019-03-13

Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals - Levchuk, Ievgen; Osvet, Andres; Tang, Xiaofeng; Brandl, Marco; Perea, Jose Dario; Högl, Florian; Matt, Gebhard; Hock, Rainer; Batentschuk, Miroslaw; Brabec, Christoph (Nano Letters, 2017)
In the past few years, hybrid organic-inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15-25 nm formamidinium CH(NH2)2PbX3 (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415-740 nm, full width at half-maximum (fwhm) of 20-44 nm, and radiative lifetimes of 5-166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI3 nanocrystals paves the way for perovskite quantum dot photovoltaics. © 2017 American Chemical Society.
Keywords: Chelation; Energy gap; Hybrid materials; Ligands; Metal halides; Nanocrystalline materials; Optoelectronic devices; Perovskite; Photoluminescence; Semiconductor quantum dots; Surface treatment; Colloidal nanocrystals; Formamidinium; Hybrid organic-inorganic; Optoelectronic applications; Optoelectronic measurements; Photoluminescence quantum yields; Re-precipitation; Room temperature synthesis; Nanocrystals

Broadband, Lensless, and Optomechanically Stabilized Coupling into Microfluidic Hollow-Core Photonic Crystal Fiber Using Glass Nanospike - Zeltner, Richard; Xie, Shangran; Pennetta, Riccardo; Russell, Philip St. John (ACS Photonics, 2017)
We report a novel technique for launching broadband laser light into liquid-filled hollow-core photonic crystal fiber (HC-PCF). It uniquely offers self-alignment and self-stabilization via optomechanical trapping of a fused silica nanospike, fabricated by thermally tapering and chemically etching a single mode fiber into a tip diameter of 350 nm. We show that a trapping laser, delivering ∼300 mW at 1064 nm, can be used to optically align and stably maintain the nanospike at the core center. Once this is done, a weak broadband supercontinuum signal (∼575–1064 nm) can be efficiently and close to achromatically launched in the HC-PCF. The system is robust against liquid-flow in either direction inside the HC-PCF, and the Fresnel back-reflections are reduced to negligible levels compared to free-space launching or butt-coupling. The results are of potential relevance for any application where the efficient delivery of broadband light into liquid-core waveguides is desired.
EAM Import::2019-03-13

Broadband NIR photoluminescence from Ni2+-doped nanocrystalline Ba-Al titanate glass ceramics - Gao, Guojun; Reibstein, Sindy; Spiecker, Erdmann; Peng, Mingying; Wondraczek, Lothar (Journal of Materials Chemistry, 2012)
Nanocrystalline Ba-Al titanate precipitates from supercooled TiO2-BaO-SiO2-Al2O3 melts by catalyzed volume nucleation in the presence of Ni2+, forming a BaAl2Ti6O16 hollandite-type lattice. Ni2+-species are incorporated into the crystalline environment in octahedral coordination. Hollandite formation is accompanied by precipitation of tetrahedrally distorted BaTiO3 as a secondary crystal phase, where crystal species and habitus can be clearly distinguished by dark-field transmission electron microscopy. Resulting photoluminescence due to spin-allowed relaxation of T-3(2g)(F-3) to (3)A(2g)(F-3) in VI Ni2+ occurs from three distinct emission centers. It spans the spectral range of 1.0 to 1.6 mu m and exhibits a lifetime of about 60 mu s, which suggests applications in tunable lasers and broadband optical amplifiers. Besides red and IR laser excitation, NIR photoemission can be excited with conventional near UV light sources, i.e. in the spectral range of 350-420 nm. Decay kinetics as well as position and shape of the emission band can be adjusted by dopant concentration and synthesis conditions.

Broadband Omnidirectional Diversion of Light in Hybrid Plasmonic-Photonic Heterocrystals - Ding, Boyang; Bardosova, Maria; Pemble, Martyn E.; Korovin, Alexander V.; Peschel, Ulf; Romanov, Sergei G. (Advanced Functional Materials, 2011)
Broadband, omnidirectional, and polarization-independent diversion has been achieved of more than 90% of the light flow intensity off its incidence direction using hybrid metal–dielectric plasmonic-photonic heterocrystals. These architectures were prepared by depositing metal film on the interface between two photonic crystals of different parameters. The magnitude of light losses was extracted from angle-resolved measurements of transmission and reflectance spectra. Comparing these data for different stages of constructing the complex architecture, the diffraction in colloidal crystals, the excitation and radiative decay of short-living surface plasmon polaritons in a corrugated metal film and the eigenmode mismatch at the interface between two different photonic crystals were identified as corroborating physical mechanisms behind the light diversion.
EAM Import::2019-03-13

Broadband Optomechanically Stabilized Coupling to Liquid-Filled Hollow-Core Fiber Using Silica Nanospike - Zeltner, Richard; Xie, Shangran; Pennetta, Riccardo; Russell, Philip St. John (Optical Trapping Applications 2017, 2017)
An optomechanically self-aligned glass nanospike, fashioned from tapered single-mode fiber, is used to launch broadband light into liquid-filled hollow-core PCF. Stable, efficient and close to achromatic coupling is obtained over a 500 nm bandwidth.
EAM Import::2019-03-13

Broadband UV-to-green photoconversion in V-doped lithium zinc silicate glasses and glass ceramics - Gao, Guojun; Meszaros, Robert; Peng, Mingying; Wondraczek, Lothar (Optics Express, 2011)
We report on photoluminescence of vanadium-doped lithium zinc silicate glasses and corresponding nanocrystalline Li_2ZnSiO_4 glass ceramics as broadband UV-to-VIS photoconverters. Depending on dopant concentration and synthesis conditions, VIS photoemission from [VO4]3- is centered at 550-590 nm and occurs over a bandwidth (FWHM) of 250 nm. The corresponding excitation band covers the complete UV-B to UV-A spectral region. In as-melted glasses, the emission lifetime is about 34 extgreekms up to a nominal dopant concentration of 0.5 mol%. In the glass ceramic, it increases to about 45 extgreekms. For higher dopant concentration, a sharp drop in emission lifetime was observed, what is interpreted as a result of concentration quenching. Self-quenching is further promoted by energy transfer to V4+ centers (^2Гt4i̊ghtarrow^2Гt3). Partitioning of vanadium into V5+ and V4+ was examined by electron paramagnetic resonance and X-ray photoelectron spectroscopy. Suppression of V5+-reduction requires careful adjustment of the optical basicity of the host glass and/or synthesis conditions.
EAM Import::2019-03-13

Bulge fatigue testing of freestanding and supported gold films - Merle, Benoit; Göken, Mathias (Journal of Materials Research, 2014)
The bulge test was used to investigate the fatigue properties of gold thin films with a thickness between 100 and 300 nm. The membranes were pressurized at a rate of 0.2 Hz up to 105 times, during which their stress and strain states were continuously recorded. Gold films on a silicon nitride substrate were cyclically loaded into tension and compression. Due to the presence of the substrate, no membrane failure was observed, but the residual stress shifted from an initially tensile state to an increasingly compressive one. Typical fatigue damage mechanisms consisting of extrusions were found in some large grains. Freestanding films were cyclically loaded in pure tension until failure occurred. The data acquired during the fatigue tests show a strong ratcheting of the films, which is indicative of cyclic plastic creep. Microstructural investigations clearly show grain boundary sliding in very thin films with columnar grains extending through the thickness.
WW1::987

Bulk and Surface Excitons in Alloyed and Phase-Separated ZnO-MgO Particulate Systems - Zhang, Huanjun; Gheisi, Amirreza; Sternig, Andreas; Mueller, Knut; Schowalter, Marco; Rosenauer, Andreas; Diwald, Oliver; Maedler, Lutz (ACS Applied Materials and Interfaces, 2012)
The rational design of composite nanoparticles with desired optical and electronic properties requires the detailed analysis of surface and bulk contributions to the respective overall function. We use flame spray pyrolysis (FSP) to generate nanoparticles of the ternary Zn–Mg–O system the compositions of which range from solid solutions of Zn2+ ions in periclase MgO to phase separated particle mixtures which consist of periclase (cubic) MgO and wurtzite (hexagonal) ZnO phases. The structure and composition of the composite ZnxMg1–xO (0 łeq x łeq 0.3) particles are investigated using X-ray diffraction and high-resolution transmission electron microscopy, whereas UV diffuse reflectance and photoluminescence (PL) spectroscopy are used for the investigation of their optical properties. Vacuum annealing has been carried out to track the effects of stepwise elimination of surface adsorbates on the photoexcitation and PL emission properties. We demonstrate that for Zn0.1Mg0.9O particles, the admixed ZnO suppresses the MgO specific surface excitons and produces a PL emission band at 470 nm. Although gaseous oxygen partially reduces the emission intensity of hydroxylated particles, it leads to entire quenching in completely dehydroxylated samples after vacuum annealing at 1173 K. Consequently, surface hydroxyls at the solid–gas interface play a significant role as protecting groups against the PL-quenching effects of O2. The obtained results are relevant for the characterization of ZnO-based devices as well as for other metal oxide materials where the impact of the surface composition on the photoelectronic properties is usually neglected.
EAM Import::2019-03-13

Burn-in Free Nonfullerene-Based Organic Solar Cells - Gasparini, Nicola; Salvador, Michael Filipe; Strohm, Sebastian; Heumüller, Thomas; Levchuk, Ievgen; Wadsworth, Andrew; Bannock, James H.; de Mello, John C.; Egelhaaf, Hans Joachim; Baran, Derya; Mcculloch, Iain; Brabec, Christoph (Advanced Energy Materials, 2017)
Organic solar cells that are free of burn-in, the commonly observed rapid performance loss under light, are presented. The solar cells are based on poly(3-hexylthiophene) (P3HT) with varying molecular weights and a nonfullerene acceptor (rhodanine-benzothiadiazole-coupled indacenodithiophene, IDTBR) and are fabricated in air. P3HT:IDTBR solar cells light-soaked over the course of 2000 h lose about 5% of power conversion efficiency (PCE), in stark contrast to [6,6]-Phenyl C61 butyric acid methyl ester (PCBM)-based solar cells whose PCE shows a burn-in that extends over several hundreds of hours and levels off at a loss of ≈34%. Replacing PCBM with IDTBR prevents short-circuit current losses due to fullerene dimerization and inhibits disorder-induced open-circuit voltage losses, indicating a very robust device operation that is insensitive to defect states. Small losses in fill factor over time are proposed to originate from polymer or interface defects. Finally, the combination of enhanced efficiency and stability in P3HT:IDTBR increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene-based acceptor instead. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Butyric acid; Defects; Degradation; Fullerenes; Open circuit voltage; Organic solar cells; Enhanced efficiency; Light soaking; Nonfullerene acceptor; Open circuit voltage loss; P3HT; Poly-3-hexylthiophene; Power conversion efficiencies; [6; 6]-phenyl-C61-butyric acid methyl esters; Solar cells

Butterfly gyroid nanostructures as a time-frozen glimpse of intracellular membrane development. - Wilts, Bodo D.; Apeleo Zubiri, Benjamin; Klatt, Michael A.; Butz, Benjamin; Fischer, Michael G.; Kelly, Stephen T.; Spiecker, Erdmann; Steiner, Ullrich; Schroeder-Turk, Gerd E. (Science Advances, 2017)
The formation of the biophotonic gyroid material in butterfly wing scales is an exceptional feat of evolutionary engineering of functional nanostructures. It is hypothesized that this nanostructure forms by chitin polymerization inside a convoluted membrane of corresponding shape in the endoplasmic reticulum. However, this dynamic formation process, including whether membrane folding and chitin expression are simultaneous or sequential processes, cannot yet be elucidated by in vivo imaging. We report an unusual hierarchical ultrastructure in the butterfly Thecla opisena that, as a solid material, allows high-resolution three-dimensional microscopy. Rather than the conventional polycrystalline space-filling arrangement, a gyroid occurs in isolated facetted crystallites with a pronounced size gradient. When interpreted as a sequence of time-frozen snapshots of the morphogenesis, this arrangement provides insight into the formation mechanisms of the nanoporous gyroid material as well as of the intracellular organelle membrane that acts as the templat

Calcium thin film growth on polyfluorenes: Interface structure and energetics - Sharp, J. C.; Feng, X. F.; Farrner, J. A.; Guo, Y. X.; Bebensee, Fabian; Baricuatro, J. H.; Zillner, Elisabeth; Zhu, J. F.; Steinrück, Hans-Peter; Gottfried, Michael; Campbell, C. T. (Journal of Physical Chemistry C, 2014)
The adsorption of Ca vapor on two polyfluorenes, poly(9,9-di-n-hexyl-2,7- fluorene) (PDHF) and poly(9,9-di-n-hexyl-2,7-fluorene vinylene) (PDHFV), has been studied by adsorption microcalorimetry, low-energy He ion scattering spectroscopy (LEIS), and ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS) at surface temperatures between 130 and ∼400 K. At 300 K, the initial heat of adsorption of Ca on PDHF is 250 kJ/mol and on PDHFV is 315 kJ/mol. We ascribe this initial heat to Ca reacting with impurities or defects in the polymers. The heat of adsorption for both polymers decreases nearly exponentially to the sublimation enthalpy of bulk Ca solid (178 kJ/mol) by 0.75 ML. The observed behavior is attributed to the increasing probability of Ca finding and adding to solid three-dimensional Ca islands which nucleate and grow on the polymer surface and eventually grow into a continuous solid Ca film on top of the polymer. LEIS, which shows only a slow increase of the signals related to solid Ca, supports this model. Incoming Ca atoms are subject to a kinetic competition between diffusing into the polymer to react with subsurface defects or impurities versus adding to the three-dimensional Ca clusters on the surface. For studies done at 140 K, a similar behavior is observed, with the heat of adsorption starting lower and decreasing more quickly to the heat of sublimation, already by 0.1 ML Ca coverage. This behavior, along with the quicker growth of Ca on the surface of the polymer, is attributed to the slower diffusion of Ca adatoms to subsurface defects/impurities at 140 K than at 300 K. The opposite effect is observed when studied at 400 K. Calcium induces a downward band bending and work function decrease on both polymers, consistent with transfer of electrons from Ca to the polymers. © 2014 American Chemical Society.

Calculating CD Spectra of Flexible Peptides: An Assessment of TD-DFT Functionals - Brkljaca, Zlatko; Malis, Momir; Smith, David M.; Smith, Ana-Suncana (Journal of Chemical Theory and Computation, 2014)
Calculations of CD spectra can provide important structural information for peptide systems. Although TD-DFT is an attractive method for these calculations, recent studies have pointed to problems with modeling charge transfer excitations. Motivated by these problems, we benchmarked the performances of CAM-B3LYP, wB97X-D, M06-2X, B3LYP, and PBE0 against high level ab initio RICC2 calculations for selected peptide structures. Furthermore, we compared the performance of the functionals with the experimentally available data. Our results show that long-range corrected functionals (CAM-B3LYP and wB97X-D) correlate relatively well with RICC2 calculations, as does the meta-hybrid M06-2X, while the global hybrid functionals (B3LYP and PBE0) exhibit the aforementioned charge-transfer artifacts. On the other hand, PBE0 and even more so M06-2X and B3LYP produce spectra in better agreement with the experimental data. We have clarified this apparent discrepancy by finding that the surplus charge-transfer excitations, exhibited by B3LYP and PBE0, seem to have a negligible contribution to the final spectra, once appropriate structural averaging is performed.
Keywords: CIRCULAR-DICHROISM SPECTRA; TRANSITION-METAL-COMPLEXES; HYBRID DENSITY FUNCTIONALS; MOLLER-PLESSET THEORY; IDENTITY APPROXIMATION; MOLECULAR-DYNAMICS; ABSORPTION-SPECTRA; AQUEOUS-SOLUTION; EXCITED-STATES; MODEL CC2

Calculation of the CD Spectrum of a Peptide from Its Conformational Phase Space: The Case of Met-enkephalin and Its Unnatural Analogue - Brkljaca, Zlatko; Condic-Jurkic, Karmen; Smith, Ana-Suncana; Smith, David M. (Journal of Chemical Theory and Computation, 2012)
We have investigated the conformational phase spaces of both Met-enkephalin and Ada-enkephalin in 2,2,2-trifluoroethanol in order to connect them to their respective CD spectra. To this end, we have characterized the conformational preferences of the zwitterionic and neutral forms of Met-enkephalin and of both the R- and the S-epimers of Ada-enkephalin, as obtained by classical molecular dynamics. The CD spectrum for each peptide was subsequently obtained with a procedure of successive averaging, which accounts for the behavior of the solvent, the side chains, and the backbone variations of the peptides. To make an appropriate comparison with experimental results, we have produced composite spectra that account for the appropriate contributions of the zwitterionic and neutral forms of the peptides as well as the expected epimeric ratio. Such a procedure results in theoretically obtained CD spectra that show significant promise in terms of reproducing their experimentally measured counterparts.
Keywords: DENSITY-FUNCTIONAL THEORY; ELECTRONIC CIRCULAR-DICHROISM; EXCHANGE MOLECULAR-DYNAMICS; AQUEOUS-SOLUTION; METHIONINE-ENKEPHALIN; ABSORPTION-SPECTRA; HELICAL PEPTIDES; EXCITED-STATES; FORCE-FIELD; AMINO-ACIDS

Can we obtain the coefficient of restitution from the sound of a bouncing ball? - Heckel, Michael; Glielmo, Aldo; Gunkelmann, Nina; Pöschel, Thorsten (Physical Review E, 2016)
The coefficient of restitution may be determined from the sound signal emitted by a sphere bouncing repeatedly off the ground. Although there is a large number of publications exploiting this method, so far, there is no quantitative discussion of the error related to this type of measurement. Analyzing the main error sources, we find that even tiny deviations of the shape from the perfect sphere may lead to substantial errors that dominate the overall error of the measurement. Therefore, we come to the conclusion that the well-established method to measure the coefficient of restitution through the emitted sound is applicable only for the case of nearly perfect spheres. For larger falling height, air drag may lead to considerable error, too.

Capture of Carbon Dioxide at the Gas-Liquid Interface Elucidated by Surface Science Approaches - Maier, Florian (Angewandte Chemie International Edition, 2011)
New methods of photoelectron spectroscopy and the application of ionic liquids offer insight into fundamental processes in CO2–liquid systems for carbon dioxide capture and storage, for example, the selective absorption of CO2 in aqueous monoethanolamine.
EAM Import::2019-03-13

Carbon-atom wires: 1-D systems with tunable properties - Casari, C. S.; Tommasini, Matteo; Tykwinski, Rik; Milani, Alberto (Nanoscale, 2016)
This review provides a discussion of the current state of research on linear carbon structures and related materials based on sp-hybridization of carbon atoms (polyynes and cumulenes). We show that such systems have widely tunable properties and thus represent an intriguing and mostly unexplored field for both fundamental and applied sciences. We discuss the rich interplay between the structural, vibrational, and electronic properties focusing on recent advances and the future perspectives of carbon-atom wires and novel hybrid sp–sp2-carbon architectures.
EAM Import::2019-03-13

Carbon Dioxide Capture by an Amine Functionalized Ionic Liquid: Fundamental Differences of Surface and Bulk Behavior - Niedermaier, Inga; Bahlmann, Matthias; Papp, Christian; Kolbeck, Claudia; Wei, Wei; Krick Calderon, Sandra; Grabau, Mathias; Schulz, Peter; Wasserscheid, Peter; Steinrück, Hans-Peter; Maier, Florian (Journal of the American Chemical Society, 2014)
Carbon dioxide (CO2) absorption by the amine-functionalized ionic liquid (IL) dihydroxyethyl dimethylammonium taurinate at 310 K was studied using surface- and bulk-sensitive experimental techniques. From near-ambient pressure X-ray photoelectron spectroscopy at 0.9 mbar CO2, the amount of captured CO2 per mole of IL in the near-surface region is quantified to similar to 0.58 mol, with similar to 0.15 mol in form of carbamate dianions and similar to 0.43 mol in form of carbamic acid. From isothermal uptake experiments combined with infrared spectroscopy, CO2 is found to be bound in the bulk as carbamate (with nominally 0.5 mol of CO2 bound per 1 mol of IL) up to similar to 2.5 bar CO2, and as carbamic acid (with nominally 1 mol CO2 bound per 1 mol IL) at higher pressures. We attribute the fact that at low pressures carbamic acid is the dominating species in the near-surface region, while only carbamate is formed in the bulk, to differences in solvation in the outermost IL layers as compared to the bulk situation.

Carbon Nanodots: Supramolecular Electron Donor-Acceptor Hybrids Featuring Perylenediimides - Strauß, Volker; Margraf, Johannes; Dirian, Konstantin; Syrgiannis, Zois; Prato, Maurizio; Wessendorf, Cordula; Hirsch, Andreas; Clark, Timothy; Guldi, Dirk Michael (Angewandte Chemie International Edition, 2015)
We describe the formation of charge-transfer complexes that feature electron-donating carbon nanodots (CND) and electron-accepting perylenediimides (PDI). The functionalities of PDIs have been selected to complement those of CNDs in terms of electrostatic and -stacking interactions based on oppositely charged ionic head groups and extended -systems, respectively. Importantly, the contributions from electrostatic interactions were confirmed in reference experiments, in which stronger interactions were found for PDIs that feature positively rather than negatively charged head groups. The electronic interactions between the components in the ground and excited state were characterized in complementary absorption and fluorescence titration assays that suggest charge-transfer interactions in both states with binding constants on the order of 8x10(4)M(-1) (25Lg(-1)). Selective excitation of the two components in ultrafast pump probe experiments gave a 210ps lived charge-separated state.
Keywords: carbon nanodots; charge transfer; donor-acceptor compounds; perylenediimides; transient absorption

Carbon Nanodots: Toward a Comprehensive Understanding of Their Photoluminescence - Strauß, Volker; Margraf, Johannes; Dolle, Christian; Butz, Benjamin; Nacken, Thomas; Walter, Johannes; Bauer, Walter; Peukert, Wolfgang; Spiecker, Erdmann; Clark, Timothy; Guldi, Dirk Michael (Journal of the American Chemical Society, 2014)
We report the characterization of carbon nanodots (CNDs) synthesized under mild and controlled conditions, that is, in a microwave reactor. The CNDs thus synthesized exhibit homogeneous and narrowly dispersed optical properties. They are thus well suited as a testbed for studies of the photophysics of carbon-based nanoscopic emitters. In addition to steady-state investigations, time-correlated single-photon counting, fluorescence up-conversion, and transient pump probe absorption spectroscopy were used to elucidate the excited-state dynamics. Moreover, quenching the CND-based emission with electron donors or acceptors helped shed light on the nature of individual states. Density functional theory and semiempirical configuration-interaction calculations on model systems helped understand the fundamental structure-property relationships for this novel type of material.

Carbon Nanohorn-based electrolyte for Dye-Sensitized Solar Cells - Lodermeyer, Fabian; Costa Riquelme, Rubén Dario; Casillas Pacheco, Ruben; Kohler, Florian; Wasserscheid, Peter; Prato, Maurizio; Guldi, Dirk Michael (Energy and Environmental Science, 2014)
For the first time, carbon nanohorns were implemented into solid-state electrolytes for highly efficient solid-state and quasi-solid-state DSSCs. They feature an effective catalytic behavior towards the reduction of I3- and enhance the I3- diffusivity into the electrolyte. In a final device, solar cells with 7.84% efficiency at room temperature were achieved. As a matter of fact, this is the highest reported efficiency for nanocarbon-based electrolytes up to date.

Carbon Nanohorns as Integrative Materials for Efficient Dye-Sensitized Solar Cells - Costa Riquelme, Rubén Dario; Feihl, Sebastian; Kahnt, Axel; Gambhir, Sanjeev; Officer, David L.; Wallace, Gordon G.; Lucio, Mara Isabel; Herrero, Mara Antonia; Vzquez, Ester; Syrgiannis, Zois; Prato, Maurizio; Guldi, Dirk Michael (Advanced Materials, 2013)
Different nanocarbons, that is, single-wall carbon nanotubes, graphene, single-wall carbon nanohorns (SWCNHs), and their respective oxidized analogs have been used to fabricate novel doped TiO2 electrodes for DSSCs. Our results indicate that all of the nanocarbons significantly enhance the device characteristics when compared to standard TiO2 electrodes. Overall, our most outstanding finding is that SWCNH derivatives are also a plausible material for developing highly-efficient DSSCs.

Carbon nanotube sidewall functionalization with carbonyl compounds-modified birch conditions vs the organometallic reduction approach - Gebhardt, Benjamin; Syrgiannis, Zois; Backes, Claudia; Graupner, Ralf; Hirsch, Andreas; Hauke, Frank (Journal of the American Chemical Society, 2011)
Covalent addition reactions turned out to be one of the most important functionalization techniques for a structural alteration of single walled carbon nanotube (SWCNT) scaffolds. During the last years, several reaction sequences based on an electrophilic interception of intermediately generated SWCNT carbanions, obtained via Birch reduction or by a nucleophilic addition of organometallic species, have been developed. Nevertheless, the scope and the variety of potential electrophiles is limited due to the harsh reaction conditions requested for a covalent attachment of the functional entities onto the SWCNT framework. Herein, we present a significant modification of the reductive alkylation/arylation sequence, the so-called Billups reaction, which extends the portfolio of electrophiles for covalent sidewall functionalization to carbonyl compounds-ketones, esters, and even carboxylic acid chlorides. Moreover, these carbonyl-based electrophiles can also be used as secondary functionalization reagents for anionic SWCNT intermediates, derived from a primary nucleophilic addition step. This directly leads to the generation of mixed functional SWCNT architectures, equipped with hydroxyl or carbonyl anchor groups, suitable for ongoing derivatization reactions. A correlated absorption and emission spectroscopic study elucidates the influence of the covalent sidewall functionalization degree onto the excitonic transition features of carbon nanotubes. The characterization of the different SWCNT adducts has been carried out by means of Raman, UV-vis/nIR, and fluorescence spectroscopy as well as by thermogravimetric analysis combined with mass spectrometry and X-ray photoelectron spectroscopy analysis. © 2011 American Chemical Society.

Carbon nanotubes-reinforced copper matrix composites produced by melt stirring - Hippmann, Sophie; Li, Qianqian; Addinal, Raphael; Volk, Wolfram (Proceedings of the Institution of Mechanical Engineers, Part N: Journal of Nanoengineering and Nanosystems, 2013)
Due to their geometry and extraordinary mechanical properties, carbon nanotubes have been envisioned as promising enhancements for metal matrix composites. In this article, we report on our melt stirring approach for incorporation of carbon nanotubes into a copper alloy matrix and on the experimental results obtained by assessing the material properties. The existence of carbon nanotubes in the copper matrix after processing was proven by Raman analysis.We found that the small amounts of carbon nanotubes (0.1wt %) in the composites influenced mechanical and abrasive wear properties.
EAM Import::2019-03-13

Carbon on platinum substrates: From carbidic to graphitic phases on the (111) surface and on nanoparticles - Vines, Francesc; Neyman, Konstantin; Görling, Andreas (Journal of Physical Chemistry A, 2009)
The formation of carbonaceous deposits on Pt(111) surfaces and Pt nanoparticles has been studied using suitable models and density-functional calculations. The study addresses a broad range of processes, from the very first stage of carbon deposition up to a final building of graphene monolayers (ML) defined as a 1:1 ratio of the number of C atoms to surface Pt atoms. A carbidic phase is formed below a coverage of ∼0.3 ML, when negatively charged carbon atoms are strongly adsorbed preferentially on fcc hollow sites. On Pt nanoparticles, the adsorption of carbon atoms seems to be enhanced near particle edges due to the special flexibility of defect sites. Above a coverage of ∼0.3 ML, the formation of small C., aggregates becomes possible. Interestingly, thermodynamics favors the formation of C trimers at a coverage of 0.33 ML, whereas the formation of C dimers requires a higher coverage of 0.5 ML. The covalently bonded C species is supposed to be the key fragment for the formation of benzene-like rings at coverages above 0.6 ML. These rings are expected to be the building blocks for the graphene monolayer. However, the typical electronic structure of graphene is not observed until a coverage above ∼1.8 ML is reached. We corroborated the experimentally suggested carbon double-layer to be stable. It is proposed to consist of a monolayer of carbidic atoms C adsorbed on Pt with a graphene layer adsorbed on the carbidic layer. Some of the carbidic atoms serve as anchors for the graphene layer, with noticeably strong covalent bonds formed. This double-layer model would imply a much higher adhesion of the graphene layer than in the single-layer model. © 2009 American Chemical Society.

Carboxylate Ion Pairing with Alkali-Metal Ions for β-Lactoglobulin and Its Role on Aggregation and Interfacial Adsorption - Beierlein, Frank; Clark, Timothy; Braunschweig, Björn; Engelhardt, Kathrin; Glas, Lena; Peukert, Wolfgang (Journal of Physical Chemistry B, 2015)
We report a combined experimental and computational study of the whey protein extgreekb-lactoglobulin (BLG) in different electrolyte solutions. Vibrational sum-frequency generation (SFG) and ellipsometry were used to investigate the molecular structure of BLG modified air-water interfaces as a function of LiCl, NaCl, and KCl concentrations. Molecular dynamics (MD) simulations and thermodynamic integration provided details of the ion pairing of protein surface residues with alkali-metal cations. Our results at pH 6.2 indicate that BLG at the air-water interface forms mono- and bilayers preferably at low and high ionic strength, respectively. Results from SFG spectroscopy and ellipsometry are consistent with intimate ion pairing of alkali-metal cations with aspartate and glutamate carboxylates, which is shown to be more effective for smaller cations (Li(+) and Na(+)). MD simulations show not only carboxylate-alkali-metal ion pairs but also ion multiplets with the alkali-metal ion in a bridging position between two or more carboxylates. Consequently, alkali-metal cations can bridge carboxylates not only within a monomer but also between monomers, thus providing an important dimerization mechanism between hydrophilic surface patches.
EAM Import::2019-03-13
Keywords: electrolyte; solution; molecular; dynamics; surface

Carbyne: The Molecular Approach - Tykwinski, Rik (Chemical Record, 2015)
For the last 60+ years, the synthesis and study of cumulenes and polyynes have been the focus of a small, but dedicated, group of researchers. Many of the remarkable electronic, optical, and structural properties of cumulenes and polyynes had already been identified in the earliest reports. The molecular lengths achievable by the initial syntheses were, unfortunately, somewhat limited by synthetic methods available. For the past 15 years, we have worked toward expanding on the synthesis of cumulenes and polyynes through the development of new methods and stabilization motifs. As new compounds have become available, homologous series of cumulenes and polyynes have then been examined as a function of molecular length. While we are not yet there, we would like to eventually provide a general description of the sp-carbon allotrope carbyne, and this account presents some of our efforts toward this goal.
EAM Import::2019-03-13

Catalyst recycling in monophasic Pt-catalyzed hydrosilylation reactions using ionic liquids - Taccardi, Nicola; Fekete, Melinda; Berger, Markus; Stanjek, Volker; Schulz, Peter; Wasserscheid, Peter (Applied Catalysis A-General, 2011)
The technical relevant Pt-catalyzed hydrosilylation of allyl chloride with trichlorosilane was investigated in a monophasic reaction system containing an ionic liquid (IL) additive. ILs screening allowed us to identify the liquid 1-decyl-3-methylimidazolium bis(trifloromethanesulfonyl)amide ([DMIM][NTf2]) as the best additive for such process. This IL combined complete miscibility with reactants as long as chemical and thermal robustness. After the reaction, the hydrosilylation product (3-chloropropyl)trichlorosilane was distilled off and the residual IL/catalyst solution could be reused. In such a way, a final TON of 14,200 was achieved over 10 reaction/distillation cycles without any loss of catalytic activity. The ability of [DMIM][NTf2] to prevent catalyst decomposition was deeper investigated and it was found that even with a Pt loading as high as 9500 ppm, the IL acted as protective/stabilizing agent for the catalyst during the thermally stressing product distillation.

Catalytically Triggered Energy Release from Strained Organic Molecules: The Surface Chemistry of Quadricyclane and Norbornadiene on Pt(111) - Bauer, Udo; Mohr, Susanne; Döpper, Tibor; Bachmann, Philipp; Späth, Florian; Düll, Fabian; Schwarz, Matthias; Brummel, Olaf; Fromm, Lukas; Pinkert, Ute; Görling, Andreas; Hirsch, Andreas; et al. (Chemistry - A European Journal, 2017)
We have investigated the surface chemistry of the polycyclic valence-isomer pair norbornadiene (NBD) and quadricyclane (QC) on Pt(111). The NBD/QC system is considered to be a prototype for energy storage in strained organic compounds. By using a multimethod approach, including UV photoelectron, high-resolution X-ray photoelectron, and IR reflection-absorption spectroscopic analysis and DFT calculations, we could unambiguously identify and differentiate between the two molecules in the multilayer phase, which implies that the energy-loaded QC molecule is stable in this state. Upon adsorption in the (sub)monolayer regime, the different spectroscopies yielded identical spectra for NBD and QC at 125 and 160K, when multilayer desorption takes place. This behavior is explained by a rapid cycloreversion of QC to NBD upon contact with the Pt surface. The NBD adsorbs in a η:η geometry with an agostic Pt-H interaction of the bridgehead CH subunit and the surface. Strong spectral changes are observed between 190 and 220K because the hydrogen atom that forms the agostic bond is broke. This reaction yields a norbornadienyl intermediate species that is stable up to approximately 380K. At higher temperatures, the molecule dehydrogenates and decomposes into smaller carbonaceous fragment
Keywords: Catalysis; DFT calculations; IR spectroscopy; Photoelectron spectroscopy; Surfaces and interfaces

Catalytic production of hydrogen from glucose and other carbohydrates under exceptionally mild reaction conditions - Taccardi, Nicola; Assenbaum, Daniel; Berger, Markus; Bösmann, Andreas; Enzenberger, Florian; Woelfel, Rene; Neuendorf, Stephanie; Göke, Volker; Schödel, Nicole; Maaß, Hans-Jürgen; Kistenmacher, Hans; Wasserscheid, Peter (Green Chemistry, 2010)
A catalytic reaction system for the production of hydrogen from sugars and even water-insoluble biomass like cellulose is presented. The reaction system is based on an ionic liquid that has the role to dissolve the carbohydrate feedstock and a ruthenium catalyst. As hydrogen dissolves in this media at very low level, hydrogen consuming side reactions have been hindered, leading to a gaseous product mixture consisting mainly of hydrogen and carbon dioxide. Investigations with isotopic labelling suggest a reaction sequence in which glucose first thermally decomposes to formic acid followed by Ru-catalyzed decomposition of the latter to hydrogen and CO_2.

Catalytic SILP Materials - Riisager, Anders; Fehrmann, Rasmus; Haumann, Marco; Wasserscheid, Peter (Topics in Organometallic Chemistry, 2008)
The principle of catalytic SILP materials involves surface modification of a porous solid material by an ionic liquid coating. Ionic liquids are salts with melting points below 100°C, generally characterized by extremely low volatilities. In the examples described in this paper, the ionic liquid coating contains a homogeneously dissolved Rh-complex and constitutes a uniform, thin film, which itself displays the catalytic reactivity in the system. Continuous fixed-bed reactor technology has been applied successfully to demonstrate the feasibility of catalytic SILP materials for propene hydroformylation and methanol carbonylation.

Catalytic test reactions for the evaluation of hierarchical zeolites - Hartmann, Martin; Machoke, Albert; Schwieger, Wilhelm (Chemical Society Reviews, 2016)
Hierarchical zeolites have received increasing attention in the last decade due to their outstanding catalytic performance. Several types of hierarchical zeolites can be prepared by a large number of different techniques. Hierarchical zeolites combine the intrinsic catalytic properties of conventional zeolites and the facilitated access and transport in the additional meso-or macropore system. In this tutorial review, we discuss several test reactions that have been explored to show the benefit of the hierarchical pore system with respect to their suitability to prove the positive effects of hierarchical porous zeolites. It is important to note that positive effects on activity, stability and less frequently selectivity observed for hierarchically structured catalysts not necessarily are only a consequence of the additional meso-or macropores but also the number, strength and location of active sites as well as defects and impurities. With regard to these aspects, the test reaction has to be chosen carefully and potential changes in the chemistry of the catalyst have to be considered as well. In addition to the determination of conversion, yield and selectivity, we will show that the calculation of the activation energy and the determination of the Thiele modulus and the effectiveness factor are good indicators of the presence or absence of diffusion limitations in hierarchical zeolites compared to their parent materials.

Cationic Two-Coordinate Complexes of Pd(I) and Pt(I) Have Longer Metal-Ligand Bonds Than Their Neutral Counterparts - Macinnis, Morgan C.; Demott, Jessica C.; Zolnhofer, Eva; Zhou, Jia; Meyer, Karsten; Hughes, Russell P.; Ozerov, Oleg V. (Chem, 2016)
One-electron oxidation of known (tBu3P)2M (1, M = Pd; 2, M = Pt) with [Ph3C][HCB11Cl11] leads to two-coordinate, monovalent cations of the formula [(tBu3P)2M][HCB11Cl11] (3, M = Pd; 4, M = Pt), which also possess linear geometry but with elongated M–P bonds. Spectroscopic and computational studies consistently show that the unpaired electron of the d9 configuration of 3 and 4 belongs to largely non-bonding orbitals: the s/dz2 hybrid for Pd and the degenerate dx2-y2/dxy pair for Pt. We show that molecular-orbital-based arguments alone are incapable of predicting or rationalizing the observed M–P bond lengthening on oxidation; correct prediction and rationalization are achieved only by inclusion of electrostatic and Pauli effects. This emphasizes the dangers of interpreting any perturbative changes in bond metrics solely on the basis of energies and occupancies of molecular orbitals; the inclusion of electrostatic and Pauli components is essential to providing a more complete picture.
EAM Import::2019-03-13

Cellular polyethylene-naphthalate films for ferroelectret applications: foaming, inflation and stretching, assessment of electromechanically relevant structural features - Fang, Peng; Wirges, Werner; Wegener, Michael; Zirkel, Larissa; Gerhard, Reimund (e-polymers, 2008)
Cellular polymer films are frequently employed for packaging, insulation, and printing. They can also be used as ferroelectrets in transducer applications. Here, we propose a preparation process for cellular polyethylene-naphthalate (PEN) films with the following steps: (1) foaming by means of supercritical carbon dioxide (CO2), (2) controlled inflation through gas diffusion and expansion, and (3) biaxial stretching. We describe and assess the cellular structure that is formed under suitable processing conditions. For foaming, the PEN films are saturated with supercritical CO2 at room temperature for a few hours, at a pressure as high as 100 bar. The subsequent temperature treatment is very critical for controlling the sample density. Additional inflation can improve the cellular geometry, and higher stabilization temperatures during the inflation process cause stronger inflation and thus lower densities. Stretching may be employed in order to achieve a regular cellular structure with lens-like voids. High electromechanical responses, i.e. large piezoelectric thickness coefficients are found only on samples within the proper density range and with optimal cellular structures.
EAM Import::2019-03-13

Cellular Ti-6Al-4V structures with interconnected macro porosity for bone implants fabricated by selective electron beam melting - Heinl, Peter; Müller, Lenka; Körner, Carolin; Singer, Robert; Müller, Frank (Acta Biomaterialia, 2008)
Selective electron beam melting (SEBM) was successfully used to fabricate novel cellular Ti-6Al-4V structures for orthopaedic applications. Micro computer tomography (μCT) analysis demonstrated the capability to fabricate three-dimensional structures with an interconnected porosity and pore sizes suitable for tissue ingrowth and vascularization. Mechanical properties, such as compressive strength and elastic modulus, of the tested structures were similar to those of human bone. Thus, stress-shielding effects after implantation might be avoided due to a reduced stiffness mismatch between implant and bone. A chemical surface modification using HCl and NaOH induced apatite formation during in vitro bioactivity tests in simulated body fluid under dynamic conditions. The modified bioactive surface is expected to enhance the fixation of the implant in the surrounding bone as well as to improve its long-term stability. © 2008 Acta Materialia Inc.
Keywords: Bioactivity; Porous structures; Selective electron beam melting; Surface modification; Titanium alloys

Cellulose-Based Ionogels for Paper Electronics - Thiemann, Stefan; Sachnov, Swetlana; Pettersson, Fredrik; Bollström, Roger; Österbacka, Ronald; Wasserscheid, Peter; Zaumseil, Jana (Advanced Functional Materials, 2014)
A new class of biofriendly ionogels produced by gelation of microcellulose thin films with tailored 1-ethyl-3-methylimidazolium methylphosphonate ionic liquids are demonstrated. The cellulose ionogels show promising properties for application in flexible electronics, such as transparency, flexibility, transferability, and high specific capacitances of 5 to 15 extgreekmF cm-2. They can be laminated onto any substrate such as multilayer-coated paper and act as high capacitance dielectrics for inorganic (spray-coated ZnO and colloidal ZnO nanorods) and organic (poly[3-hexylthiophene], P3HT) electrolyte-gated field-effect transistors (FETs), that operate at very low voltages (<2 V). Field-effect mobilities in ionogel-gated spray-coated ZnO FETs reach 75 cm2 V-1 s-1 and a typical increase of mobility with decreasing specific capacitance of the ionogel is observed. Solution-processed, colloidal ZnO nanorods and laminated cellulose ionogels enable the fabrication of the first electrolyte-gated, flexible circuits on paper, which operate at bending radii down to 1.1 mm.

CFD simulation of bubble column flows: an explicit algebraic Reynolds stress model approach - Masood, Rao Muhammad Atif; Rauh, Cornelia; Delgado, Antonio (International Journal of Multiphase Flow, 2014)
CFD study of flow hydrodynamics has been conducted in transient Euler-Euler environment by using commercial simulation software Ansys CFX 14.0. First, three different wall lubrication force models are compared. Afterwards, the influence of turbulent dispersion force models has been analyzed. Finally, the performance of Explicit Algebraic Reynolds Stress Model (EARSM) combined with k - ε and Baseline (BSL) models has been tested. Three different combinations of EARSM were included with two cases incorporating the Bubble Induced Turbulence (BIT) as well (i.e. EARSM k - ε BIT, EARSM BSL, EARSM BSL BIT). All simulations were compared for average velocity and turbulent kinetic energy profiles with experiments. The performance of these models was also compared with Re-Normalization Group (RNG) and k - ε models. EARSM k - ε model was unable to capture the axial liquid as well as gas phase velocities close to experimental values. On contrary, EARSM BSL and EARSM BSL BIT were more successful in predicting the velocity profiles and showed good agreement with experiments, with EARSM BSL slightly less accurate in predicting axial velocity profiles. All models predicted the turbulent kinetic energy profiles reasonably good both quantitatively as well as qualitatively, while near the bottom of the column; EARSM k - ε slightly under predicted the turbulent quantities. Thus, EARSM combined with BSL and BIT can be effectively exploited for simulating flow fields and turbulent quantities. © 2014 Elsevier Ltd.
UnivIS-Import:2015-03-09:Pub.2014.tech.ITC.stmmec.cfdsim
Keywords: BSL; Bubble column; Bubble induced turbulence; EARSM; Interfacial closures; Turbulence models

Challenges and Perspectives of the Polymer-Induced Liquid-Precursor Process: The Pathway from Liquid-Condensed Mineral Precursors to Mesocrystalline Products - Wolf, Stephan; Gower, Laurie (New Perspectives on Mineral Nucleation and Growth, 2017)

Challenging the scope of continuous, gas-phase reactions with supported ionic liquid phase (SILP) catalysts-Asymmetric hydrogenation of methyl acetoacetate - Öchsner, Eva; Schneider, Martin; Meyer, Carolin; Haumann, Marco; Wasserscheid, Peter (Applied Catalysis A-General, 2011)
In the last years Supported Ionic Liquid Phase (SILP) catalysis has attracted growing interest for continuous gas phase reactions. The concept combines in a unique manner the advantages of traditional homogeneous and heterogeneous catalysis by making use of the extremely low vapor pressure of an ionic liquid film on support. In this way, continuous gas phase reactions with macroscopically solid but microscopically homogeneous catalyst materials have been successfully realized. However—so far—continuous gas-phase SILP catalysis has only been demonstrated for reactions with relatively simple selectivity problems (such as, regioselectivity, chemoselectivity). This study demonstrates the successful application of a SILP catalyst in continuous gas phase asymmetric hydrogenation. The hydrogenation of methyl acetoacetate (MAA) is presented using an immobilized dibromo[3-(2,5-(2R,5R)-dimethylphospholanyl-1)-4-di-o-tolylphosphino-2,5-dimethylthio-phene]-ruthenium (Ru1Br2) catalyst.

Changes within the stabilizing layer of ZnO nanoparticles upon washing - Schindler, Torben; Schmutzler, Tilo; Schmiele, Martin; Lin, Wei; Segets, Doris; Peukert, Wolfgang; Appavou, Marie-Sousia; Kriele, Armin; Gilles, Ralph; Unruh, Tobias (Journal of Colloid and Interface Science, 2017)
In detail, it was observed that the diffuse acetate shell shrinks to the size of a monolayer upon washing but the acetate content of this monolayer is higher than within the diffuse shell of the particles of the native dispersion. A second washing cycle reduces the acetate concentration within the stabilizing shell and the stability of the dispersion drops accordingly. After another (third) washing cycle strong agglomeration was observed for all investigated samples. (C) 2017 Elsevier Inc. All rights reserved.
Keywords: Small angle scattering; Stabilization; Quantum dots; Purification; Stabilizing shell

Chaotic dynamics of a magnetic nanoparticle - Gallas, Jason; Bragard, Jean; Suarez, Omar J.; Laroze, David; Pleiner, H.; Vargaz, P. (Physical Review E, 2011)
We study the deterministic spin dynamics of an anisotropic magnetic particle in the presence of a magnetic field with a constant longitudinal and a time-dependent transverse component using the Landau-Lifshitz-Gilbert equation. We characterize the dynamical behavior of the system through calculation of the Lyapunov exponents, Poincaré sections, bifurcation diagrams, and Fourier power spectra. In particular we explore the positivity of the largest Lyapunov exponent as a function of the magnitude and frequency of the applied magnetic field and its direction with respect to the main anisotropy axis of the magnetic particle. We find that the system presents multiple transitions between regular and chaotic behaviors. We show that the dynamical phases display a very complicated structure of intricately intermingled chaotic and regular phases.

Characteristics of large threedimensional heaps of particles produced by ballistic deposition from extended sources - Gallas, Jason; Pöschel, Thorsten; Topic, Nikola (Philosophical Magazine, 2013)
This paper reports a detailed numerical investigation of the geometrical and structural properties of three-dimensional heaps of particles. Our goal is the characterization of very large heaps produced by ballistic deposition from extended circular dropping areas. First, we provide an in-depth study of the formation of monodisperse heaps of particles. We find very large heaps to contain three new geometrical characteristics: they may display two external angles of repose, one internal angle of repose, and four distinct packing fraction (density) regions. Such features are found to be directly connected with the size of the dropping zone. We derive a differential equation describing the boundary of an unexpected triangular packing fraction zone formed under the dropping area.We investigate the impact that noise during the deposition has on the final heap structure. In addition, we perform two complementary experiments designed to test the robustness of the novel features found. The first experiment considers changes due to polydispersity. The second checks what happens when letting the extended dropping zone to become a point-like source of particles, the more common type of source.

Characteristics of solution gated field effect transistors on the basis of epitaxial graphene on silicon carbide - Ristein, Jürgen; Zhang, Wenying; Speck, Florian; Ostler, Markus; Ley, Lothar; Seyller, Thomas (Journal of Physics D-Applied Physics, 2010)
A solution gated field effect transistor has been fabricated on epitaxial single-layer graphene on 6H-SiC(0 0 0 1). Output and transfer characteristics were systematically studied as a function of electrolyte pH. The transfer characteristics of the device show a pH dependent shift of 19 ± 1 mV/pH. From the minimum sheet conductivity observed, an average charge carrier mobility of 1800 ± 100 cm 2 V -1 s -1 at room temperature has been inferred. It turns out that the Fermi level in the graphene layer is strongly pinned in the vicinity of the Dirac point. The analysis of the transfer characteristics is consistent with a concentration of 4 × 10 14 cm -2 interface states at 0.1 eV below the Dirac energy that is presumably associated with the (6√3 × 6√3)R30°-reconstruction at the interface between SiC(0 0 0 1) and graphene. © 2010 IOP Publishing Ltd.
UnivIS-Import:2015-03-09:Pub.2010.nat.dphy.IAP.exphys.charac

Characterization of the stability of semiconductor lasers with delayed feedback according to the Lang-Kobayashi model - Gallas, Jason A.; Junges, Leandro; Pöschel, Thorsten (European Physical Journal D, 2013)
We report a numerical characterization of the stability of semiconductor lasers with delayed feedback under the simultaneous variation of the delay time τ and the pump current P. Changes in the number of External Cavity Modes are studied as a function of the delay time while the Regular Pulse Package regime is characterized as a function of the pump current. In addition, we describe some remarkable structures observed in the τ ×P control plane, delimiting where these and other complex regimes of laser operation exist.

Characterization of thickness variations of thin dielectric layers at the nanoscale using scanning capacitance microscopy - Yanev, V.; Rommel, Mathias; Bauer, A. J.; Frey, Lothar (Journal of Vacuum Science & Technology B: Microelectronics Processing and Phenomena, 2011)
In this work, the applicability of scanning capacitance microscopy (SCM) for film thickness characterization and its sensitivity to the surface roughness on nanoscale were examined experimentally. SiO_2 layers with different film thicknesses (between 5 and 19 nm) were analyzed by conventional capacitance-voltage (C-V) measurements and using SCM in the scanning capacitance spectroscopy (SCS) mode. The influence of the film thickness on the SCM signal was studied in detail by comparison of modeled data with experimental data. The dC/dV-V characteristics measured by SCS at the nanoscale could be correlated with derivatives of conventionally measured C-V curves as well as simulated C-V characteristics for the different film thicknesses. Quantitatively comparing their peak areas, it was found that the dC/dV signal of SCS correlates with the change in the insulator thickness. The sensitivity of SCM for the detection of local variations of dielectric-layer thicknesses at the nanoscale was demonstrated by SCM mapping of crystalline high-k layers, where spatial differences of the SCM signal could be directly correlated with changes in the topography caused by film thickness variations.

Characterization of thin CeO2 films electrochemically deposited on HOPG - Faisal, Firas; Toghan, Arafat; Khalakhan, Ivan; Vorokhta, Mykhailo; Matolin, Vladimir; Libuda, Jörg (ELSEVIER SCIENCE BV, 2015)
Electrodeposition is widely used for industrial applications to deposit thin films, coatings, and adhesion layers. Herein, CeO2 thin films were deposited on a highly oriented pyrolytic graphite (HOPG) substrate by cathodic electrodeposition. The influence of the deposition parameters on the yield and on the film morphology is studied and discussed. Morphology and composition of the electrodeposited films were characterized by in-situ atomic force microscopy (AFM), scanning electron microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). By AFM we show that the thickness of CeO2 films can be controlled via the Ce3+ concentration in solution and the deposition time. After exposing the films to ambient air, cracking structures are formed, which were analyzed by AFM in detail. The chemical composition of the deposits was analyzed by XPS indicating the formation of nearly stoichiometric CeO2. (C) 2015 Elsevier B.V. All rights reserved.
Keywords: Fuel cell catalyst; Electrodeposition; Cerium oxide; Thin films; AFM; Cracking

Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin-Film Morphology and Device Performance - Ou, Kai-Lin; Ehamparam, Ramanan; Macdonald, Gordon; Stubhan, Tobias; Wu, Xin; Shallcross, R. Clayton; Richards, Robin; Brabec, Christoph; Saavedra, S. Scott; Armstrong, Neal R. (ACS Applied Materials and Interfaces, 2016)
This report focuses on the evaluation of the electrochemical properties of both solution-deposited sol-gel (sg-ZnO) and sputtered (sp-ZnO) zinc oxide thin films, intended for use as electron-collecting interlayers in organic solar cells (OPVs). In the electrochemical studies (voltammetric and impedance studies), we used indium-tin oxide (ITO) over coated with either sg-ZnO or sp-ZnO interlayers, in contact with either plain electrolyte solutions, or solutions with probe redox couples. The electroactive area of exposed ITO under the ZnO interlayer was estimated by characterizing the electrochemical response of just the oxide interlayer and the charge transfer resistance from solutions with the probe redox couples. Compared to bare ITO, the effective electroactive area of ITO under sg-ZnO films was ca. 70%, 10%, and 0.3% for 40, 80, and 120 nm sg-ZnO films. More compact sp-ZnO films required only 30 nm thicknesses to achieve an effective electroactive ITO area of ca. 0.02%. We also examined the electrochemical responses of these same ITO/ZnO heterojunctions overcoated with device thickness pure poly(3-hexylthiophehe) (P3HT), and donor/acceptor blended active layers (P3HT:PCBM). Voltammetric oxidation/reduction of pure P3HT thin films on ZnO/ITO contacts showed that pinhole pathways exist in ZnO films that permit dark oxidation (ITO hole injection into P3HT). In P3HT:PCBM active layers, however, the electrochemical activity for P3HT oxidation is greatly attenuated, suggesting PCBM enrichment near the ZnO interface, effectively blocking P3HT interaction with the ITO contact. The shunt resistance, obtained from dark current-voltage behavior in full P3HT/PCBM OPVs, was dependent on both (i) the porosity of the sg-ZnO or sp-ZnO films (as revealed by probe molecule electrochemistry) and (ii) the apparent enrichment of PCBM at ZnO/P3HT:PCBM interfaces, both effects conveniently revealed by electrochemical characterization. We anticipate that these approaches will be applicable to a wider array of solution-processed interlayers for "printable" solar cells.
Keywords: charge selective interlayers; impedance spectroscopy; organic solar cells; phase separation; voltammetry; zinc oxide

Characterization of ZnO structures by optical and X-ray methods - Petrik, P.; Pollakowski, B.; Zakel, S.; Gumprecht, Thomas; Beckhoff, B.; Lemberger, M.; Labadi, Z.; Baji, Z.; Jank, M.; Nutsch, A. (Applied Surface Science, 2013)
ZnO thin films doped by Ga and In as well as multilayer structures of ZnO/Al2O3 have been investigated by X-ray fluorescence, Raman spectrometry, spectroscopic ellipsometry and vacuum ultra violet reflectometry. Systematic changes in the optical properties have been revealed even for Ga concentrations below 1%. The Raman active phonon mode of Ga doping at 580 cm-1 shows a correlation with the Ga concentration. Optical models with surface nanoroughness correction and different parameterizations of the dielectric function have been investigated. There was a good agreement between the dielectric functions determined by the Herzinger–Johs polynomial parameterization and by direct inversion. It has been shown that the correction of the nanoroughness significantly influences the accuracy of the determination of the layer properties. The band gap and peak amplitude of the imaginary part of the dielectric function corresponding to the excitonic transition changes systematically with the Ga-content and with annealing even for low concentrations.
EAM Import::2019-03-13

Charge carrier dynamics in a ternary bulk heterojunction system consisting of P3HT, fullerene, and a low bandgap polymer - Koppe, Markus; Egelhaaf, Hans-Joachim; Clodic, Ewan; Morana, Mauro; Lueer, Larry; Troeger, Anna; Sgobba, Vito; Guldi, Dirk Michael; Ameri, Tayebeh; Brabec, Christoph (Advanced Energy Materials, 2013)
The photoresponse of P3HT:PC BM based organic solar cells can be enhanced by blending the bulk heterojunction with the low band gap polymer Si-PCPDTBT. Organic solar cells containing the resulting ternary blend as the photoactive layer deliver short circuit currents of up to 15.5 mA cm. Morphological studies show modest phase separation without the perturbation of the crystallinity of the P3HT:PC BM matrix, in accordance with the measured acceptable fill factors. Picosecond time-resolved pump-probe spectroscopy reveals that the sensitization of P3HT:PC BM with Si-PCPDTBT involves the transfer of photogenerated positive polarons from the low band gap polymer to P3HT within few hundreds of picoseconds. Intensity dependent experiments in combination with global fitting show that the charge transfer from Si-PCPDTBT to P3HT competes with non-geminate charge carrier recombination of the holes in the Si-PCPDTBT phase with electrons in the PC BM phase, both processes being of diffusive nature. At excitation densities corresponding to steady state conditions under one sun, modelling predicts hole transfer efficiencies exceeding 90%, in accordance with IQE measurements. At higher pump intensities, bimolecular recombination suppresses the hole transfer process effectively. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Charge distribution analysis in Agn m+ clusters: Molecular modeling and DFT calculations - Milek, Theodor; Döpper, Tibor; Neiß, Christian; Görling, Andreas; Zahn, Dirk (Journal of Molecular Modeling, 2014)
Neutral and charged silver clusters Ag and Ag (m=0,...,20) are investigated by simulated annealing molecular dynamics simulations. With increasing charge to size ratio the clusters undergo deformation to better accommodate surface charges. The latter are heterogeneously distributed and reflect the interplay of overall electrostatic repulsion and different degrees of local under-coordination of surface atoms. By comparison with atomic charges from natural population analysis based on density-functional calculations we demonstrate the suitability of the embedded-atom force-field in combination with the charge equilibrium approach. © 2014 Springer-Verlag Berlin Heidelberg.
Keywords: DFT calculations; Metal nanoparticles; Molecular mechanics; Partial charges analysis

Charge-transfer excitation energies with a time-dependent density-functional method suitable for orbital-dependent exchange-correlation kernels - Heßelmann, Andreas; Ipatov, Andrey; Görling, Andreas (Physical Review A, 2009)
A time-dependent density-functional (TDDFT) response equation for the effective Kohn-Sham potential instead of the electron densities is presented that enables the use of orbital-dependent exchange-correlation kernels. In combination with the frequency-dependent exact-exchange kernel the present approach describes long-range charge-transfer excitations qualitatively correct in contrast to standard TDDFT methods, as shown by formal analysis and applications to molecular systems. Even charge-transfer excitations obtained with the adiabatic frequency-independent exact-exchange kernel exhibit a long-range dependence with the distance of the charge transfer, which, however, is too weak by a factor of 2. This indicates that it is not the frequency dependence of the kernel alone that leads to a correct description of charge-transfer excitations. © 2009 The American Physical Society.

Charge Transport in C60-based Single-Molecule Junctions with Graphene Electrodes - Leitherer, Susanne; Brana Coto, Pedro; Ullmann, Konrad; Weber, Heiko B.; Thoss, Michael (Nanoscale, 2017)
We investigate charge transport in C60-based single-molecule junctions with graphene electrodes employing a combination of density functional theory (DFT) electronic structure calculations and Landauer transport theory. In particular, the dependence of the transport properties on the conformation of the molecular bridge and the type of termination of the graphene electrodes is investigated. Furthermore, electron pathways through the junctions are analyzed using the theory of local currents. The results reveal, in agreement with previous experiments, a pronounced dependence of the transport properties on the bias polarity, which is rationalized in terms of the electronic structure of the molecule. It is also shown that the edge states of zigzag-terminated graphene induce additional transport channels, which dominate transport at low voltages. The importance of the edge states for transport depends profoundly on the interface geometry of the junctions.

Charge Transport in Organic Materials: Norm-Conserving Imaginary Time Propagation with Local Ionization Energy as the External Potential - Kriebel, Maximilian; Sharapa, Dmitry; Clark, Timothy (Journal of Chemical Theory and Computation, 2017)
An additional charge carrier described as its wave function is propagated in imaginary time using stepwise matrix multiplication and a correction to ensure that the simulation is norm-conserving. The propagation Hamilton operator uses the local ionization energy of a rubrene single crystal, calculated with semiempirical molecular orbital theory, as an external potential for holes to model the interaction with the underlying molecular structure. Virtual electrodes are modeled by setting the potentials in the appropriate areas to constant values with the difference corresponding to the source-drain voltage. Although imaginary time cannot be interpreted directly as time, the simulated gate-dependent imaginary transfer rate is in acceptable qualitative agreement with the experimentally measured gate-dependent hole-transfer rate through a rubrene single crystal.

Charge Transport in Pentacene–Graphene Nanojunctions - Pshenichnyuk, Ivan; Brana Coto, Pedro; Leitherer, Susanne; Thoss, Michael (Journal of Physical Chemistry Letters, 2013)
We investigate charge transport in pentacene-graphene nanojunctions employing density functional theory (DFT) electronic structure calculations and the Landauer transport formalism. The results show that the unique electronic properties of graphene strongly influence the transport in the nanojunctions. In particular, edge states in graphene electrodes with zigzag termination result in additional transport channels close to the Fermi energy, which deeply affects the conductance at small bias voltages. Investigating different linker groups as well as chemical substitution, we demonstrate how the transport properties are furthermore influenced by the molecule-lead coupling and the energy level lineup. © 2013 American Chemical Society.
UnivIS-Import:2015-04-14:Pub.2013.nat.dphy.ITP.ppem.charge
Keywords: Energy and Charge Transport; Energy Conversion and Storage

Chemical and (Photo)-Catalytical Transformations in Photonic Crystal Fibers - Schmidt, Matthias; Cubillas, Ana Maria; Taccardi, Nicola; Euser, Tijmen; Cremer, Till; Maier, Florian; Steinrück, Hans-Peter; Russell, Philip St. John; Wasserscheid, Peter; Etzold, Bastian (ChemCatChem, 2013)
The concept of employing photonic crystal fibers for chemical and (photo)-catalytical transformations is presented. These optofluidic microdevices represent a versatile platform where light and fluids can interact for spectroscopic or photoactivation purposes. The use of photonic crystal fibers in chemistry and sensing is reviewed and recent applications as catalytic microreactors are presented. Results on homogeneous catalysis and the immobilization of homogeneous and heterogeneous catalysts in the fiber channels are discussed. The examples demonstrate that combining catalysis and the excellent light guidance of photonic crystal fibers provides unique features for example, for photocatalytic activation and quantitative photospectroscopic reaction analysis. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chemical fingerprints of large organic molecules in scanning tunneling microscopy: Imaging adsorbate-substrate coupling of metalloporphyrins - Buchner, Florian; Warnick, Karl-Georg; Wölfle, Thorsten; Görling, Andreas; Steinrück, Hans-Peter; Hieringer, Wolfgang; Marbach, Hubertus (Journal of Physical Chemistry C, 2009)
In this combined experimental and theoretical work we demonstrate at the example of tetraphenylporphyrins on Ag(111) how differences in individual adsorbate orbitals and their interaction with the substrate can be exploited to switch the appearance of the adsorbate in scanning tunneling microscopy (STM) experiments, such that electronically and chemically very similar large molecules become distinguishable in STM. In particular, an intermixed layer consisting of 2HTPP (TPP ) tetraphenylporphyrin), CoTPP, and FeTPP molecules on Ag(111) was investigated, and it is demonstrated that STM images acquired with different bias voltages constitute fingerprints of the different molecules within the intermixed layer. By means of density functional calculations the observed features could be explained in detail and traced back to a direct orbital interaction of the adsorbed molecule with the surface. The explicit consideration of the surface in the calculations therefore turned out to be decisive to achieve good agreement with experiment. © 2009 American Chemical Society.

Chemical functionalization and characterization of graphene-based materials - Bottari, Giovanni; Angeles Herranz, Ma; Wibmer, Leonie; Volland, Michel; Rodriguez-Perez, Laura; Guldi, Dirk Michael; Hirsch, Andreas; Martin, Nazario; D'Souza, Francis; Torres, Tomas (Chemical Society Reviews, 2017)
Graphene-based materials (GBMs), with graphene, their most known member, at the head, constitute a large family of materials which has aroused the interest of scientists working in different research fields such as chemistry, physics, or materials science, to mention a few, arguably as no other material before. In this review, we offer a general overview on the most relevant synthetic approaches for the covalent and non-covalent functionalization and characterization of GBMs. Moreover, some representative examples of the incorporation into GBMs of electroactive units such as porphyrins, phthalocyanines, or ferrocene, among others, affording electron donor-acceptor (D-A) hybrids are presented. For the latter systems, the photophysical characterization of their ground- and excited-state features has also been included, paying particular attention to elucidate the fundamental dynamics of the energy transfer and charge separation processes of these hybrids. For some of the presented architectures, their application in solar energy conversion schemes and energy production has been also discussed. Graphene-based materials (GBMs), with graphene, their most known member, at the head, constitute a large family of materials which has aroused the interest of scientists working in different research fields such as chemistry, physics, or materials science, to mention a few, arguably as no other material before. In this review, we offer a general overview on the most relevant synthetic approaches for the covalent and non-covalent functionalization and characterization of GBMs. Moreover, some representative examples of the incorporation into GBMs of electroactive units such as porphyrins, phthalocyanines, or ferrocene, among others, affording electron donor-acceptor (D-A) hybrids are presented. For the latter systems, the photophysical characterization of their ground- and excited-state features has also been included, paying particular attention to elucidate the fundamental dynamics of the energy transfer and charge separation processes of these hybrids. For some of the presented architectures, their application in solar energy conversion schemes and energy production has been also discussed.

Chemistry at surfaces - Steinrück, Hans-Peter; Libuda, Jörg; King, David A. (Chemical Society Reviews, 2008)
No abstract is available for this content:

Chemistry with Graphene and Graphene Oxide-Challenges for Synthetic Chemists - Eigler, Siegfried; Hirsch, Andreas (Angewandte Chemie International Edition, 2014)
The chemical production of graphene as well as its controlled wet chemical modification is a challenge for synthetic chemists. Furthermore, the characterization of reaction products requires sophisticated analytical methods. In this Review we first describe the structure of graphene and graphene oxide and then outline the most important synthetic methods that are used for the production of these carbon-based nanomaterials. We summarize the state-of-the-art for their chemical functionalization by noncovalent and covalent approaches. We put special emphasis on the differentiation of the terms graphite, graphene, graphite oxide, and graphene oxide. An improved fundamental knowledge of the structure and the chemical properties of graphene and graphene oxide is an important prerequisite for the development of practical applications.

Chirality Transfer in Imidazolium Camphorsulfonate Ionic Liquids through Ion Pairing Effects - Schneiders, Karola; Bösmann, Andreas; Schulz, Peter; Wasserscheid, Peter (dvanced Synthesis & Catalysis, 2009)
The paper describes our studies on ion pair interactions in ionic liquids (IL) using an asymmetric hydrogenation reaction as probe. Three different ionic liquids carrying prochiral keto-functionalized cations were hydrogenated in the presence of their chiral, enantiomerically pure counter-ion using an achiral heterogeneous ruthenium catalyst. For the hydrogenation of N-(3′-oxobutyl)-N-methylimidazolium camphorsulfonate (2), N-(3′-oxobutyl)imidazolium camphorsulfonate (4) and N-(5′-oxohexyl)-N-methylimidazolium camphorsulfonate (6) we found a strong dependency of the enantiomeric excess (ee in the cation) on the polarity of the solvent, the concentration of the IL and the structure of the IL. The highest ee values of up to 94% were found for the hydrogenation of 2 in ethanol. Interestingly, we observed that the ee (and consequently the strength of ion pair interaction) had a pronounced maximum for a certain concentration of the IL in the solvent depending on the nature of the solvent and on the substrate. Remarkably, the concentration leading to the maximum ee could be rationalized by independent determination of the degree of dissociation which was obtained by a combination of diffusion-ordered NMR spectroscopy and conductivity measurements.

Chirality transfer in mandelate ionic liquids through ion pairing effects - Sachnov, Swetlana; Schneiders, Karola; Schulz, Peter; Wasserscheid, Peter (Tetrahedron-Asymmetry, 2010)
Ion pairing in [N-(3′-oxohexyl)-N-methylimidazolium][(R)-mandelate] was probed as a function of its concentration in ethanol and compared to the corresponding [(S)-camphorsulfonate] ionic liquid. The applied methodologies comprised asymmetric hydrogenation with ee monitoring as well as independent diffusion-ordered NMR and conductivity measurements.

Chiral Photonic Crystals with 4-fold Symmetry: Band Structure and S-Parameters of Eight-Fold Intergrown Gyroid Nets - Saba, Matthias; Turner, Mark D.; Gu, Min; Mecke, Klaus; Schröder-Turk, Gerd (Proceedings of SPIE, 2013)
The Single Gyroid, or srs, nanostructure has attracted interest as a circular-polarisation sensitive photonic material. We develop a group theoretical and scattering matrix method, applicable to any photonic crystal with symmetry I432, to demonstrate the remarkable chiral-optical properties of a generalised structure called 8-srs, obtained by intergrowth of eight equal-handed srs nets. Exploiting the presence of four-fold rotations, Bloch modes corresponding to the irreducible representations E- and E+ are identified as the sole and non-interacting transmission channels for right- and left-circularly polarised light, respectively. For plane waves incident on a finite slab of the 8-srs, the reflection rates for both circular polarisations are identical for all frequencies and transmission rates are identical up to a critical frequency below which scattering in the far field is restricted to zero grating order. Simulations show the optical activity of the lossless dielectric 8-srs to be large, comparable to metallic metamaterials, demonstrating its potential as a nanofabricated photonic material.
EAM Import::2019-03-13

Chiral water-soluble perylenediimides - Schmidt, Cordula; Boettcher, Christoph; Hirsch, Andreas (European Journal of Organic Chemistry, 2009)
For the first time the synthesis of covalently linked very water-soluble chiral perylenediimides (PDIs) is presented. The PDIs carry the amino acids alanine or lysine as imide substituents, respectively, which are coupled to Newkometype dendrimers acting as hydrophilic groups. The tert-butylprotected precursors 5 as well as its water-soluble free acid compounds 6 were investigated by absorption, fluorescence and circular dichroism (CD) spectroscopy. Except for the sterically most crowded 2 generation alanine compounds 5f and 6f all chiral dyes show bisignate CD effects which indicate helical aggregation. The 1 generation alanine compound 6b shows the greatest CD effects in buffer solution and its aggregation behaviour was also directly proven by the use of cryogenic transmission electron microscopy. Furthermore, the formation of chiral superstructures of 6b was found to be pH- and concentration-dependent. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

Chlorination of titanium carbide for the processing of nanoporous carbon: A kinetic study - Becker, Philipp; Glenk, Friedrich; Kormann, Martina; Popovska, Nadejda; Etzold, Bastian (Chemical Engineering Journal, 2010)
Nanoporous carbon synthesized via the chlorination of titanium carbide at temperatures above 400 °C shows a good performance, e.g. for gas storage or electrochemical applications. In this work the influence of the temperature and of the chlorine concentration on the reaction rate and the development of the porous structure during the course of the reaction was studied. The synthesis follows a Langmuir–Hinshelwood approach regarding chlorine, which can be simplified to a power law rate with the order of 0.71, and the specific surface area is created mainly in the last 10% of conversion.
EAM Import::2019-03-13

Chloroalkylsulfonate ionic liquids by ring opening of sultones with organic chloride salts - Paape, Natalia; Wei, Wei; Bösmann, Andreas; Kolbeck, Claudia; Maier, Florian; Steinrück, Hans-Peter; Wasserscheid, Peter; Schulz, Peter (Chemical Communications, 2008)
An efficient route to prepare ionic liquids with chloroalkylsulfonate anions is presented; the synthesis proceeds in a one-step ring-opening reaction of sultones with an organic chloride salt and provides a very attractive access to new anion functionalised ionic liquids. © The Royal Society of Chemistry.

Choosing the right nanoparticle size-designing novel ZnO electrode architectures for efficient dye-sensitized solar cells - Pfau, Markus; Kunzmann, Andreas; Segets, Doris; Peukert, Wolfgang; Wallace, Gordon G.; Officer, David L.; Clark, Timothy; Costa, Ruben Dario; Guldi, Dirk Michael (Journal of Materials Chemistry A, 2017)
A novel concept for constructing optimized ZnO-based photoanodes as integrative components of dye-sensitized solar cells (DSSCs) is realized by deploying differently sized nanoparticles, ranging from 2 to 10 nm, together with commercially available 20 nm nanoparticles. The 2 nm nanoparticles were used to construct an efficient buffer layer for transparent electrodes based on 10 nm nanoparticles, resulting in a relative increase of device efficiency from 1.8 to 3.0% for devices without and with a buffer layer, respectively. A mixture of 10 and 20 nm nanoparticles was optimized to maximize the diffuse reflection and to minimize the charge transport resistance in a light-scattering layer. This optimization resulted in a homogenous layer of more than 15 mm that provided a device efficiency of 3.3%. The buffer layer, transparent electrode, and light-scattering electrode, were then combined to give an overall efficiency of around 5%. Thus, this work demonstrates that varying the electrode architecture with nanoparticles of different diameters is a powerful strategy for improving the overall efficiency of ZnO-based DSSCs.

Circular Dichroism in Biological Photonic Crystals and Cubic Chiral Nets - Saba, Matthias; Thiel, M.; Turner, M. D.; Hyde, S. T.; Gu, Min; Grosse-Brauckmann, K.; Neshev, D. N.; Mecke, Klaus; Schröder-Turk, Gerd (Physical Review Letters, 2011)
Nature provides impressive examples of chiral photonic crystals, with the notable example of the cubic so-called srs network (the label for the chiral degree-three network modeled on SrSi2) or gyroid structure realized in wing scales of several butterfly species. By a circular polarization analysis of the band structure of such networks, we demonstrate strong circular dichroism effects: The butterfly srs microstructure, of cubic I4(1)32 symmetry, shows significant circular dichroism for blue to ultraviolet light, that warrants a search for biological receptors sensitive to circular polarization. A derived synthetic structure based on four like-handed silicon srs nets exhibits a large circular polarization stop band of a width exceeding 30%. These findings offer design principles for chiral photonic devices.

Circular ratchets as transducers of vertical vibrations into rotations - Heckel, Michael; Müller, Patric; Pöschel, Thorsten; Gallas, Jason (Physical Review E, 2012)
Granular ratchets are well-known devices that when driven vertically produce a counterintuitive horizontal transport of particles. Here we report the experimental observation of a complementary effect: the striking ability of circular ratchets to convert their vertical vibration into their own rotation. The average revolution speed shows a maximum value for an optimal tooth height. With no special effort the rotation speed could be maintained steady during several hours. Unexpected random arrests and reversals of the velocity were also observed abundantly. © 2012 American Physical Society.

Classification of Additives for Organic Photovoltaic Devices - Machui, Florian; Maisch, Philipp; Burgues-Ceballos, Ignasi; Langner, Stefan; Krantz, Johannes; Ameri, Tayebeh; Brabec, Christoph (ChemPhysChem, 2015)
The use of additives to improve the performance of organic photovoltaic cells has been intensely researched in recent years. However, so far, no system has been reported for the classification of additives and their functions. In this report, a system for classifying additives according to the fundamental mechanism by which they influence microstructure formation for P3HT:PCBM is suggested. The major parameters used for their classification are solubility and drying kinetics. Both are discussed in detail and their consequences on processing are analyzed. Furthermore, a general mechanism to classify the impact of additives on structure formation is suggested and discussed for different materials relevant to organic photovoltaic devices.
Keywords: additives; fullerenes; ink formulation; organic photovoltaics; polymers

Classification of Zinc Sulfide Quantum Dots by Size: Insights into the Particle Surface–Solvent Interaction of Colloids - Peukert, Wolfgang; Lutz, Christian; Yamamoto, Kyoko; Komada, So; Süß, Sebastian; Mori, Yasushige; Segets, Doris (Journal of Physical Chemistry C, 2015)
We present a detailed study on the classification of ZnS quantum dots (QDs) by size selective precipitation (SSP). SSP allows the postsynthetic narrowing of a given feed distribution and is usually realized by titration of a poor solvent into a suspension of dispersed particles. Thereby preferred flocculation of larger structures is induced. Our results confirm that SSP is a highly robust process, following a law of mass action. That means a certain solvent composition always leads to the same ratio between coarse and fines particles with respect to a specific particle size xi. This ratio is independent of the particle size distribution (PSD) of the feed, the washing history of the particles, and the solid concentration of the particles. Regarding the illustration of our findings, we established a combined approach that takes Hansen solubility parameters (HSP) of solvent mixtures as well as changing van der Waals interactions into account. Relating both to each other, a size-dependent region of enhanced solubility is clearly identified. Our concept allows a differentiation between volume-related effects like van der Waals interactions and surface-related effects like the interaction of a ligand with a solvent mixture. A comprehensive interpretation of classification results obtained with different good solvents and poor solvents enables to deduce a general strategy for the demanding determination of HSP for small colloids. Our work makes an important contribution to the design of an appropriate colloidal postprocessing which is applicable to larger quantitie
Keywords: ZnS; quantum; dot; selctive; precipitation; flocculation; structure; hansen; solubitlity; size

Clathrate colloidal crystals - Lin, Haixin; Lee, Sangmin; Sun, Lin; Spellings, Matthew; Engel, Michael; Glotzer, Sharon C.; Mirkin, Chad A. (Science, 2017)
DNA-programmable assembly has been used to deliberately synthesize hundreds of different colloidal crystals spanning dozens of symmetries, but the complexity of the achieved structures has so far been limited to small unit cells. We assembled DNA-modified triangular bipyramids ( 250-nanometer long edge, 177-nanometer short edge) into clathrate architectures. Electron microscopy images revealed that at least three different structures form as large single-domain architectures or as multidomain materials. Ordered assemblies, isostructural to clathrates, were identified with the help of molecular simulations and geometric analysis. These structures are the most sophisticated architectures made via programmable assembly, and their formation can be understood based on the shape of the nanoparticle building blocks and mode of DNA functionalization.
EAM Import::2019-03-13

CNT reinforced light metal composites produced by melt stirring and by high pressure die casting - Li, Qianqian; Rottmair, Christian A.; Singer, Robert (Composites Science and Technology, 2010)
Light metal matrix composites are of great interest due to their potential for reducing CO emission through lightweight design e.g. in the automotive sector. Carbon nanotubes can be considered as ideal reinforcements, due to their high strength, high aspect ratio and thermo-mechanic properties. In this research, CNT reinforced light metal composites were produced by melt stirring and by high pressure die casting, which can be both easily scaled up. The light metal composites showed significantly improved mechanical properties already at small CNT contents. The influence of CNT concentration on the composites was also studied. © Elsevier Ltd.
Keywords: A. Carbon nanotubes (CNTs); A. Metal matrix composites (MMCs); A. Nano composites; B. Mechanical properties; E. Casting

CO2 activation on single crystal based ceria and magnesia/ceria model catalysts - Lykhach, Yaroslava; Staudt, Thorsten; Streber, Regine; Lorenz, Michael Peter Andreas; Bayer, Andreas; Steinrück, Hans-Peter; Libuda, Jörg (European Physical Journal B, 2010)
Novel multifunctional ceria based materials may show an improved performance in catalytic processes involving CO activation and reforming of hydrocarbons. Towards a more detailed understanding of the underlying surface chemistry, we have investigated CO activation on single crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully stoichiometric CeO (111) films on a Cu(111) substrate. Samples with different structure, oxidation state and compositions are generated, including CeO /Cu(111) (reduced), MgO/CeO /Cu(111) (reduced), mixed MgO-CeO /Cu(111) (stoichiometric), and mixed MgO-CeO /Cu(111) (reduced). The morphology of the model surfaces is characterized by means of scanning tunneling microscopy (STM), whereas the electronic structure and reactivity is probed by X-ray photoelectron spectroscopy (XPS). The experimental approach allows us to compare the reactivity of samples containing different types of Ce , Ce , and Mg ions towards CO at a sample temperature of 300 K. Briefly, we detect the formation of two CO -derived species, namely carbonate (CO ) and carboxylate (CO ) groups, on the surfaces of all investigated samples after exposure to CO at 300 K. In parallel to formation of the carbonate species, slow partial reoxidation of reduced CeO /Cu(111) occurs at large doses of CO . The reoxidation of the reduced ceria is largely suppressed on MgO-containing samples. The tendency for reoxidation of Ce to Ce by CO decreases with increasing degree of intermixing between MgO and CeO . Additionally, we have studied the stability of the formed carbonate species as a function of annealing temperature. © 2010 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.

Coating of Pd/C catalysts with Lewis-acidic ionic liquids and liquid coordination complexes – SCILL induced activity enhancement in arene hydrogenation - Lijewski, Martin; Hogg, James M.; Swadzba-Kwasny, Malgorzata; Wasserscheid, Peter; Haumann, Marco (RSC Advances, 2017)
New solid catalysts with ionic liquid layer (SCILLs) were prepared by the coating of heterogeneous Pd/C catalysts with a thin film of acidic liquid: a known chloroaluminate ionic liquid (IL), [BMIM]Cl/AlCl3, and a recently developed liquid coordination complex (LCC), urea/AlCl3. Both coatings improved the activity of Pd/C catalyst against toluene hydrogenation; at lower temperatures (40 °C) the IL-SCILL performed better, whereas at 80 °C the LCC-SCILL gave higher conversion. A thin film of ionic liquid proved to be sufficient to increase activity, while larger volumes of ionic liquid resulted in a decrease in activity due to pore blocking and hence mass transport limitations. Kinetic investigations in batch mode revealed a zero order with respect to toluene concentration, a first order in hydrogen pressure, and activation energy of 64.7 kJ mol-1 for the Pd/C catalyst. The SCILL catalysts had slightly lower values of 53.1 kJ mol-1 for [BMIM]Cl/AlCl3 and 61.4 kJ mol-1 for urea/AlCl3, respectively.
EAM Import::2019-03-13

Coefficient of restitution as a fluctuating quantity - Montaine, Marina; Heckel, Michael; Kruelle, Christof; Schwager, Thomas; Pöschel, Thorsten (Physical Review E, 2011)
The coefficient of restitution of a spherical particle in contact with a flat plate is investigated as a function of the impact velocity. As an experimental observation we notice nontrivial (non-Gaussian) fluctuations of the measured values. For a fixed impact velocity, the probability density of the coefficient of restitution, p, is formed by two exponential functions (one increasing, one decreasing) of different slope. This behavior may be explained by a certain roughness of the particle which leads to energy transfer between the linear and rotational degrees of freedom. © 2011 American Physical Society.

Coefficient of restitution for viscoelastic spheres: The effect of delayed recovery - Schwager, Thomas; Pöschel, Thorsten (Physical Review E, 2008)
The coefficient of normal restitution of colliding viscoelastic spheres is computed as a function of the material properties and the impact velocity. From simple arguments it becomes clear that, in a collision of purely repulsively interacting particles, the particles lose contact slightly before the distance of the centers of the spheres reaches the sum of the radii, that is, the particles recover their shape only after they lose contact with their collision partner. This effect was neglected in earlier calculations, which leads erroneously to attractive forces and thus to an underestimation of the coefficient of restitution. As a result we find a different dependence of the coefficient of restitution on the impact rate. © 2008 The American Physical Society.

Coefficient of restitution of aspherical particles - Glielmo, Aldo; Gunkelmann, Nina; Pöschel, Thorsten (Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2014)
We consider the motion of an aspherical inelastic particle of dumbbell type bouncing repeatedly on a horizontal flat surface. The coefficient of restitution of such a particle depends not only on material properties and impact velocity but also on the angular orientation at the instant of the collision whose variance is considerable, even for small eccentricity. Assuming random angular orientation of the particle at the instant of contact we characterize the measured coefficient of restitution as a fluctuating quantity and obtain a wide probability density function including a finite probability for negative values of the coefficient of restitution. This may be understood from the partial exchange of translational and rotational kinetic energy.

Coefficient of tangential restitution for the linear dashpot model - Becker, Volker; Schwager, Thomas; Pöschel, Thorsten (Physical Review E, 2008)
The linear dashpot model for the inelastic normal force between colliding spheres leads to a constant coefficient of normal restitution, εn =const, which makes this model very popular for the investigation of dilute and moderately dense granular systems. For two frequently used models for the tangential interaction force we determine the coefficient of tangential restitution, εt, both analytically and by numerical integration of Newton's equation. Although εn =const for the linear-dashpot model, we obtain pronounced and characteristic dependences of the tangential coefficient on the impact velocity, εt = εt (g). The results may be used for event-driven simulations of granular systems of frictional particles. © 2008 The American Physical Society.

Coefficient of tangential restitution for viscoelastic spheres - Schwager, Thomas; Becker, Volker; Pöschel, Thorsten (European Physical Journal E, 2008)
The collision of frictional granular particles may be described by an interaction force whose normal component is that of viscoelastic spheres while the tangential part is described by the model by Cundall and Strack (G´ eotechnique 29, 47 (1979)) being the most popular tangential collision model in Molecular Dynamics simulations. Albeit being a rather complicatedmodel, governed by 5 phenomenological parameters and 2 independent initial conditions, we find that it is described by 3 independent parameters only. Surprisingly, in a wide range of parameters the corresponding coefficient of tangential restitution, εt, is well described by the simple Coulomb law with a cut-off at εt = 0. A more complex behavior of the coefficient of restitution as a function on the normal and tangential components of the impact velocity, gn and gt, including negative values of εt, is found only for very small ratio gt/gn. For the analysis presented here we neglect dissipation of the interaction in normal direction.

Coexistence of both gyroid chiralities in individual butterfly wing scales of Callophrys rubi - Winter, Benjamin; Butz, Benjamin; Dieker, Christel; Schroeder-Turk, Gerd E.; Mecke, Klaus; Spiecker, Erdmann (Proceedings of the National Academy of Sciences of the United States of America, 2015)
The wing scales of the Green Hairstreak butterfly Callophrys rubi consist of crystalline domains with sizes of a few micrometers, which exhibit a congenitally handed porous chitin microstructure identified as the chiral triply periodic single-gyroid structure. Here, the chirality and crystallographic texture of these domains are investigated by means of electron tomography. The tomograms unambiguously reveal the coexistence of the two enantiomeric forms of opposite handedness: the left- and right-handed gyroids. These two enantiomers appear with nonequal probabilities, implying that molecularly chiral constituents of the biological formation process presumably invoke a chiral symmetry break, resulting in a preferred enantiomeric form of the gyroid structure. Assuming validity of the formation model proposed by Ghiradella H (1989) J Morphol 202(1): 69-88 and Saranathan V, et al. (2010) Proc Natl Acad Sci USA 107(26): 11676-11681, where the two enantiomeric labyrinthine domains of the gyroid are connected to the extracellular and intra-SER spaces, our findings imply that the structural chirality of the single gyroid is, however, not caused by the molecular chirality of chitin. Furthermore, the wing scales are found to be highly textured, with a substantial fraction of domains exhibiting the < 001 > directions of the gyroid crystal aligned parallel to the scale surface normal. Both findings are needed to completely understand the photonic purpose of the single gyroid in gyroid-forming butterflies. More importantly, they show the level of control that morphogenesis exerts over secondary features of biological nanostructures, such as chirality or crystallographic texture, providing inspiration for biomimetic replication strategies for synthetic self-assembly mechanisms.
Keywords: electron tomography; chirality; crystallographic texture; photonic crystal; butterfly wing scales

Coexistence of dilute and densely packed domains of ligand-receptor bonds in membrane adhesion - Schmidt, Daniel; Bihr, Timo; Seifert, Udo; Smith, Ana-Suncana (EPL - Europhysics Letters, 2012)
We analyze the stability of micro-domains of ligand-receptor bonds that mediate the adhesion of biological model membranes. After evaluating the effects of membrane fluctuations on the binding affinity of a single bond, we characterize the organization of bonds within the domains by theoretical means. In a large range of parameters, we find the commonly suggested dense packing to be separated by a free energy barrier from a regime in which bonds are sparsely distributed. If bonds are mobile, a coexistence of the two regimes should emerge, which agrees with recent experimental observations. Copyright (C) EPLA, 2012
Keywords: CELL-ADHESION; FLUCTUATIONS; JUNCTIONS; VESICLES

Collective displacements in a molecular crystal polymorphic transformation - Zahn, Dirk; Anwar, Jamshed (RSC Advances, 2013)
A long-standing question is whether concerted molecular displacements can occur during first-order phase transformations in molecular crystals. We have investigated the molecular level mechanism of the temperature-induced extgreekb i̊ghtarrow extgreeka phase transition in the molecular crystal DL-norleucine using molecular dynamics (MD) simulations. The simulations have employed the transition path sampling MD approach coupled with quenching to ascertain the mechanism of transformation close to the coexistence temperature, which is more representative of laboratory conditions than conventional brute-force MD that involves excessive superheating to induce the transformation. The study reveals that on length scales below 10 nm the extgreekb ̊htarrow extgreeka transformation in DL-norleucine crystals occurs by a concerted layer displacement mechanism. This finding is significant since collective structural transitions could be the mechanisms of choice in molecular crystals with hydrogen-bonded layers or chains, which are considered to be particularly amenable to control and have the potential to form the basis for molecular machines that transform heat into mechanical work.
EAM Import::2019-03-13

Collective granular dynamics in a shaken container at low gravity conditions - Kollmer, Jonathan; Sack, Achim; Heckel, Michael; Zimber, Fabian; Müller, Patric; Bannerman, Marcus; Pöschel, Thorsten (7th International Conference on Micromechanics of Granular Media: Powders and Grains 2013, 2013)
We investigate the collective dissipative behavior of a model granular material (steel beads) when subjected to vibration. To this end, we study the attenuation of the amplitude of an oscillating leaf spring whose free end carries a rectangular box partly filled with granulate. To eliminate the perturbing influence of gravity, the experiment was performed under conditions of microgravity during parabolic flights. Different regimes of excitation could be distinguished, namely, a gas-like state of disordered particle motion and a state where the particles slosh back and forth between the container walls in a collective way, referred to as collect-and-collide regime. For the latter regime, we provide an expression for the container size leading to maximal dissipation of energy, that also marks the transition to the gas like regime. Also for systems driven at fixed amplitude and frequency, we find both the gas regime and the collect-and-collide regime resulting in similar dissipative behavior as in the case of the attenuating vibration. © 2013 AIP Publishing LL
Keywords: damping; Granular material

Collective Intermolecular Motions Dominate the Picosecond Dynamics of Short Polymer Chains - Morhenn, Humphrey; Busch, Sebastian; Meyer, Hendrik; Richter, Dieter; Petry, Winfried; Unruh, Tobias (Physical Review Letters, 2013)
Neutron scattering and extensive molecular dynamics simulations of an all atom C100H202 system were performed to address the short-time dynamics leading to center-of-mass self-diffusion. The simulated dynamics are in excellent agreement with resolution resolved time-of-flight quasielastic neutron scattering. The anomalous subdiffusive center-of-mass motion could be modeled by explicitly accounting for viscoelastic hydrodynamic interactions. A model-free analysis of the local reorientations of the molecular backbone revealed three relaxation processes: While two relaxations characterize local bond rotation and global molecular reorientation, the third component on intermediate times could be attributed to transient flowlike motions of atoms on different molecules. The existence of these collective motions, which are clearly visualized in this Letter, strongly contribute to the chain relaxations in molecular liquids.

Collision of viscoelastic spheres: Compact expressions for the coefficient of normal restitution - Müller, Patric; Pöschel, Thorsten (Physical Review E, 2011)
The coefficient of restitution of colliding viscoelastic spheres is known analytically as a complete series expansion in terms of the impact velocity where all (infinitely many) coefficients are known. While being analytically exact, this result is not suitable for applications in efficient event-driven molecular dynamics (eMD) or direct simulation Monte Carlo (DSMC) methods. Based on the analytic result, here we derive expressions for the coefficient of restitution that allow for application in efficient eMD and DSMC simulations of granular systems. © 2011 American Physical Society.

Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies - Vogel, Nicolas; Utech, Stefanie; England, Grant T.; Shirman, Tanya; Phillips, Katherine R.; Koay, Natalie; Burgess, Ian B.; Kolle, Mathias; Weitz, David A.; Aizenberg, Joanna (Proceedings of the National Academy of Sciences of the United States of America, 2015)
Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, we show that a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal's curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies-potentially as more efficient mimics of structural color as it occurs in nature.
Keywords: Colloids; Hierarchy; Photonic crystal; Self-assembly; Structural color

Coloring Semitransparent Perovskite Solar Cells via Dielectric Mirrors - Ramírez Quiroz, César Omar; Bronnbauer, Carina; Levchuk, Ievgen; Hou, Yi; Brabec, Christoph; Forberich, Karen (ACS nano, 2016)
While perovskite-based semitransparent solar cells for window applications show competitive levels of transparency and efficiency compared to organic photovoltaics, the color perception of the perovskite films is highly restricted because band gap engineering results in losses in power conversion efficiencies. To overcome the limitation in visual aesthetics, we combined semitransparent perovskite solar cells with dielectric mirrors. This approach enables one to tailor the device appearance to almost any desired color and simultaneously offers additional light harvesting for the solar cell. In the present work, opto-electrical effects are investigated through quantum efficiency and UV-to-visible spectroscopic measurements. Likewise, a detailed chromaticity analysis, featuring the transmissive and reflective color perception of the device including the mirror, from both sides and in different illumination conditions, is presented and analyzed. Photocurrent density enhancement of up to 21% along with overall device transparency values of up to 31% (4.2% efficiency) is demonstrated for cells showing a colored aesthetic appeal. Finally, a series of simulations emulating the device chromaticity, transparency, and increased photocurrent density as a function of the photoactive layer thickness and the design wavelength of the dielectric mirror are presented. Our simulations and their experimental validation enabled us to establish the design rules that consider the color efficiency/transparency interplay for real applications.
Keywords: Bragg mirror; dielectric mirror; perovskite photovoltaics; room temperature crystallization; semitransparent

Combined Computational Approach Based on Density Functional Theory and Artificial Neural Networks for Predicting the Solubility Parameters of Fullerenes - Perea Ospina, Jose Dario; Langner, Stefan; Salvador, Michael Filipe; Kontos, Janos; Jarvas, Gabor; Winkler, Florian; Machui, Florian; Görling, Andreas; Dallos, Andras; Ameri, Tayebeh; Brabec, Christoph (Journal of Physical Chemistry B, 2016)
The solubility of organic semiconductors in environmentally benign solvents is an important prerequisite for the widespread adoption of organic electronic appliances. Solubility can be determined by considering the cohesive forces in a liquid via Hansen solubility parameters (HSP). We report a numerical approach to determine the HSP of fullerenes using a mathematical tool based on artificial neural networks (ANN). ANN transforms the molecular surface charge density distribution (σ-profile) as determined by density functional theory (DFT) calculations within the framework of a continuum solvation model into solubility parameters. We validate our model with experimentally determined HSP of the fullerenes C, PCBM, bisPCBM, ICMA, ICBA, and PCBM and through comparison with previously reported molecular dynamics calculations. Most excitingly, the ANN is able to correctly predict the dispersive contributions to the solubility parameters of the fullerenes although no explicit information on the van der Waals forces is present in the σ-profile. The presented theoretical DFT calculation in combination with the ANN mathematical tool can be easily extended to other π-conjugated, electronic material classes and offers a fast and reliable toolbox for future pathways that may include the design of green ink formulations for solution-processed optoelectronic devices.
Keywords: Engineering controlled terms: Computation theory; Design for testability; Fullerenes; Molecular dynamics; Neural networks; Optoelectronic devices; Solubility; Van der Waals forces Computational approach; Continuum solvation models; Electronic materials; Environmentally benign; Hansen solubility parameters; Molecular dynamics calculation; Numerical approaches; Solubility parameters Engineering main heading: Density functional theory

Combined photoemission and scanning tunneling microscopy study of the surface-assisted ullmann coupling reaction - Chen, Min; Xiao, Jie; Steinrück, Hans-Peter; Wang, Shiyong; Wang, Weihua; Lin, Nian; Hieringer, Wolfgang; Gottfried, J. Michael (Journal of Physical Chemistry C, 2014)
The adsorption and reaction of 4,4-dibromo-para-terphenyl (DBTP) and 1,3,5-tris(4-bromophenyl)benzene (TBB) on Cu(111) surface were studied with X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. In addition, complementary scanning tunneling microscopy (STM) data are presented. At submonolayer coverage, scission of C-Br bonds occurs between 170 and 240 K. The estimated activation energy for this process is considerably lower than the C-Br bond energy, indicating that bond scission is assisted by Cu atoms of the substrate. The remaining molecular backbones undergo linkage by C-Cu-C bonds to form organometallic oligomers. Annealing of these oligomers leads to the formation of C-C bonded covalent two-dimensional networks. Above monolayer coverage, complete C-Br cleavage requires higher temperature, confirming the role of the Cu surface in the reaction. The results provide insight into the C-Br bond scission as the initial step of the surface-assisted Ullmann reaction. © 2014 American Chemical Society.

Combining atomistic simulation and x-ray diffraction for the characterization of nanostructures: A case study on fivefold twinned nanowires - Niekiel, Florian; Bitzek, Erik; Spiecker, Erdmann (Acs Nano, 2014)
Recent progress in achieving high degrees of monodispersity in chemical synthesis of complex nanostructures creates the unique situation in which individual nanostructures become representative for the whole ensemble. Under these conditions, atomistic simulations can play a completely new role in interpreting structural data obtained from averaging techniques. We apply this approach to fivefold twinned Ag nanowires for which the existence of an ambient-stable tetragonal phase in the nanowire core has been recently proposed. Quantitative comparison of experimental X-ray diffraction data with atomistic calculations unequivocally shows that the diffractograms can be fully explained by the complex strain state and defect structure of fivefold twinned Ag nanowires with fcc crystal structure. In addition, our approach enables rapid and accurate determination of wire diameters by a modified Scherrer analysis which uses a database generated by atomistic simulations. © 2014 American Chemical Societ
Keywords: atomistic simulation; disclination; fivefold twinned nanowires; X-ray diffraction

Combining Bottom-Up Self-Assembly with Top-Down Microfabrication to Create Hierarchical Inverse Opals with High Structural Order - Schaffner, Manuel; England, Grant; Kolle, Mathias; Aizenberg, Joanna; Vogel, Nicolas (Small, 2015)
Colloidal particles can assemble into ordered crystals, creating periodically structured materials at the nanoscale without relying on expensive equipment. The combination of small size and high order leads to strong interaction with visible light, which induces macroscopic, iridescent structural coloration. To increase the complexity and functionality, it is important to control the organization of such materials in hierarchical structures with high degrees of order spanning multiple length scales. Here, a bottom-up assembly of polystyrene particles in the presence of a silica sol-gel precursor material (tetraethylorthosilicate, TEOS), which creates crack-free inverse opal films with high positional order and uniform crystal alignment along the (110) crystal plane, is combined with top-down microfabrication techniques. Micrometer scale hierarchical superstructures having a highly regular internal nanostructure with precisely controlled crystal orientation and wall profiles are produced. The ability to combine structural order at the nano- and microscale enables the fabrication of materials with complex optical properties resulting from light-matter interactions at different length scales. As an example, a hierarchical diffraction grating, which combines Bragg reflection arising from the nanoscale periodicity of the inverse opal crystal with grating diffraction resulting from a micrometer scale periodicity, is demonstrated.
Keywords: hierarchy; inverse opals; photolithography; photonic crystals; self-assembly; structural color

Combining Electron-Accepting Phthalocyanines and Nanorod-like CuO Electrodes for p-Type Dye-Sensitized Solar Cells - Langmar, Oliver; Ganivet, Carolina R.; Lennert, Annkatrin; Costa, Ruben Dario; De La Torre, Gema; Torres, Tomas; Guldi, Dirk Michael (Angewandte Chemie International Edition, 2015)
A route is reported for the synthesis of two electron-accepting phthalocyanines featuring linkers with different lengths as sensitizers for p-type dye-sensitized solar cells (DSSCs). Importantly, our devices based on novel nanorod-like CuO photocathodes showed high efficiencies of up to 0.191 %: the highest value reported to date for CuO-based DSSCs.
EAM Import::2019-03-13

Communication: Mode-selective vibrational excitation induced by nonequilibrium transport processes in single-molecule junctions - Härtle, Rainer; Volkovich, Roie; Thoss, Michael; Peskin, Uri (Journal of Chemical Physics, 2010)
In a nanoscale molecular junction at finite bias voltage, the intramolecular distribution of vibrational energy can strongly deviate from the thermal equilibrium distribution and specific vibrational modes can be selectively excited in a controllable way, regardless of the corresponding mode frequency. This is demonstrated for generic models of asymmetric molecular junctions with localized electronic states, employing a master equation as well as a nonequilibrium Green's function approach. It is shown that the applied bias voltage controls the excitation of specific vibrational modes by tuning the efficiency of vibrational cooling processes due to energy exchange with the leads. © 2010 American Institute of Physics.
UnivIS-Import:2015-03-09:Pub.2010.nat.dphy.ITP.ppem.commun

Communication via electron and energy transfer between zinc oxide nanoparticles and organic adsorbates - Gnichwitz, Jan-Frederik; Hirsch, Andreas; Guldi, Dirk Michael; Peukert, Wolfgang; Werner, Fabian; Marczak, Renata (Journal of Physical Chemistry C, 2009)
Stable ZnO nanoparticles suitable for further surface functionalization were synthesized in the liquid phase from homogeneous ethanolic solutions of the precursors lithium hydroxide and zinc acetate. It was found that the growth of the particles was governed by temperature as well as the presence of the reaction byproduct lithium acetate during the aging process. In particular, the reaction could be almost completely arrested by removal of this byproduct. The washing consisted of repeated precipitation of the ZnO particles by addition of alkanes such as heptane, removal of the supernatant, and redispersion in ethanol. Furthermore, the surface of the colloidal ZnO nanoparticles was successfully modified by catechol-anchoring group containing dye molecules, i.e., 5-(N-(3,4-dihydroxyphenethyl)-2-phenoxyacetamide)-10,15,20-(p-tert- butyltriphenyl)porphyrinatozinc (DOPAZ) and 5-(3,4-dihydroxy-N-phenylbenzamide)- 10,15,20-tris(4-tert-butylphenyl)porphyrinatozinc (CAMIZ), for the study of photochemical properties. Thermogravimetric analysis proved the stability of the catechol anchor groups. Steady-state absorption spectroscopy as well as steady-state and time-resolved emission studies confirmed the electronic communication between the ZnO nanoparticles in their excited state and both of the porphyrins. More than 96% emission quenching of ZnO can be achieved by addition of the porphyrins, proving that the visible emission of the ZnO is caused by surface states, since only the surface of the particles was altered by the grafting experiments. Moreover, with increasing porphyrin concentrations the lifetimes changed from 46.0 to 15.3 ns. The shortened lifetimes prompt a new deactivation pathway, namely, through the electronic coupling of the porphyrins to the ZnO nanoparticle. Assuming that the decrease in lifetime is entirely due to electron transfer to the porphyrins, a rate constant of 0.35 × 10 s could be determined for this process. When testing the excited state of the porphyrin in comparative assays between ZnO and Al O , we conclude a similar electron transfer deactivation. © 2009 American Chemical Society.
Keywords: Nanoparticle; electron; functionalization

Comparative computational analysis of the Cahn–Hilliard equation with emphasis on C1-continuous methods - Käßmair, Stefan; Steinmann, Paul (Journal of Computational Physics, 2016)
The numerical treatment of the fourth-order Cahn–Hilliard equation is nonstandard. Using a Galerkin -method necessitates, for instance, piecewise smooth and globally C1-continuous basis functions or a mixed formulation. The latter is obtained introducing an auxiliary field which allows to rephrase the Cahn–Hilliard equation as a set of two coupled second-order equations. In view of this, the formulation in terms of the primal unknown appears to be a more intuitive and natural choice but requires a C1-continuous interpolation. Therefore, isogeometric analysis, using a spline basis, and natural element analysis are addressed in the present contribution. Mixed second-order finite element methods introducing the chemical potential or alternatively a nonlocal concentration as auxiliary field serve as references to which both higher-order methods are compared in terms of accuracy and efficiency.
EAM Import::2019-03-13

Comparative study between conventional macroscopic IV techniques and advanced AFM based methods for electrical characterization of dielectrics at the nanoscale - Yanev, V.; Erlbacher, Tobias; Rommel, Mathias; Bauer, A. J.; Frey, Lothar (Microelectronic Engineering, 2009)
High-k dielectrics exhibit strong variations in their electrical characteristics on the nanometer scale due to morphology alteration. Therefore, electrical measurement techniques with nanometer resolution based on atomic force microscopy (AFM) like conductive AFM (cAFM) and tunneling AFM (TUNA) are essentially very well applicable to complement conventional macroscopic current–voltage (IV) techniques. Comparative experiments between conventional IV, cAFM and TUNA measurements on SiO2 films and on high-k/SiO2 stacks prove the capability and the accuracy of cAFM/TUNA as advanced methods for electrical characterization of thin dielectric films. Especially TUNA provides the spatial resolution and the current sensitivity required for the characterization of local electrical properties at the nanoscale allowing for the characterization of dielectric layers at high current densities.
Keywords: Conductive atomic force microscopy (cAFM); High-k dielectrics; Tunneling atomic force microscopy (TUNA)

Comparative study of embedded atom potentials for atomistic simulations of fracture in α-iron - Möller, Johannes; Bitzek, Erik (Modelling and Simulation in Materials Science and Engineering, 2014)
Atomistic simulations play a crucial role in advancing our understanding of the crack-tip processes that take place during fracture of semi-brittle materials like extgreeka-iron. As with all atomistic simulations, the results of such simulations however depend critically on the underlying atomic interaction model. Here, we present a systematic study of eight extgreeka-iron embedded atom method potentials used to model cracks subjected to plane strain mode-I loading conditions in six different crystal orientations. Molecular statics simulations are used to determine the fracture behavior (cleavage, dislocation emission, twinning) and the critical stress intensity factor KIc. The structural transformations in front of the crack tips, and in particular the occurrence of [1 1 0] planar faults, are analyzed in detail and related to the strain-dependent generalized stacking fault energy curve. The simulation results are discussed in terms of theoretical fracture criteria and compared to recent experimental data. The different potentials are ranked according to their capability to model the experimentally observed fracture behavior.
EAM Import::2019-03-13
Keywords: atomistic simulation; EAM potential; fracture; iron

Comparative study of the carbide-modified surfaces C/Mo(110) and C/Mo(100) using high-resolution x-ray photoelectron spectroscopy - Gleichweit, Christoph; Neiß, Christian; Maisel, Sven; Bauer, Udo; Späth, Florian; Höfert, Oliver; Vollnhals, Florian; Drost, Martin; Marbach, Hubertus; Görling, Andreas; Steinrück, Hans-Peter; Papp, Christian (Physical Review B, 2015)
The preparations of single (monolayer) and bulk carbides on the Mo(110) and Mo(100) single crystals are followed in situ at 1200 K using synchrotron-based high-resolution x-ray photoelectron spectroscopy of the C 1s and core Mo 3d(5/2) levels. By comparing the experimental results to first principles calculations using density functional theory, we suggest real-space surface structures for the carbide-modified surfaces. For a monolayer carbide on Mo(110), carbon dimers adsorb in the long-bridge site, most likely at a coverage of 3/8 ML carbon atoms per Mo surface atom. For the bulk carbide, we find a coverage of similar to 0.5ML on the surface, and the calculations show that single carbon atoms are more stable than dimers on the surface. The monolayer carbide on Mo(100) exhibits a coverage of similar to 1ML and agrees with previous studies, while the bulk carbide preparation probably leads to a faceting of the surfac

Comparing Different Synthesis Procedures for Carbide-Derived Carbon-Based Structured Catalyst Supports - Knorr, Tilman; Schwarz, Aaron; Etzold, Bastian (Chemical Engineering & Technology, 2014)
Different approaches were analyzed to combine the tuneable micropore structure of carbide-derived carbons with a foam-like secondary porosity. The resulting structured catalyst supports were characterized in detail and applied in the model reaction of ethene hydrogenation. Preparation methods studied were dip-coating using polytetrafluoroethylene as binder on cellular metal structures, a chemical vapor deposition coating of the metal structures with thin carbide layers and subsequent conversion to carbide-derived carbon, and the partial or full conversion of carbide foams to carbon/carbide composites. For the binder method, optimal parameters for stable slurry preparation as well as for calcination of the slurry were obtained. It could further be demonstrated that the conversion of carbide foams into carbon/carbide composites leads to an appreciation between decreasing mechanical strength and increasing specific surface area.
EAM Import::2019-03-13

Comparison between an analytical asymptotic fiber drawing model with full Navier-Stokes solution taking into account the effects of inner pressure and surface tension - Luzi, Giovanni; Epple, Philipp; Scharrer, Michael; Fujimoto Meinecke, Ken; Rauh, Cornelia; Delgado, Antonio (Journal of Lightwave Technology, 2011)
A fluid-mechanics model that make use of asymptotic analysis based on small aspect ratio of capillaries has been compared with the full 3-D set of the N.-St. equations, for modelling the fabrication of capillary tubes. The final asymptotic equations are solved numerically and then compared with the N.-St. solutions, obtained with a commercial finite elements solver. The present paper focuses on the comparison of the solution of the two methods taking into account the effects of surface tension and internal hole pressure, since those are of essential importance during drawing. It is shown that the analytical asymptotic method delivers reliable results for practical applications, as long as the inner pressure or the temperature does not exceed too high values. © 2011 IEEE.
UnivIS-Import:2015-03-09:Pub.2011.tech.ITC.stmmec.compar
Keywords: Asymptotic analysis; holey fibres; mathematical modelling; microstructured optical fibres (MOFs); numerical methods; photonic crystal fibres

Comparison between optimized effective potential and Kohn-Sham methods - Heßelmann, Andreas; Görling, Andreas (Chemical Physics Letters, 2008)
It is shown that, in general, Gaussian basis set xOEP methods cannot yield the HF total energy because, in general, products of occupied and unoccupied orbitals are effectively linearly dependent. xOEP methods can yield the HF total energy only if the quality of the employed orbital basis set is insufficient with respect to the computational accuracy. Furthermore it is shown that the basic equation underlying the xOEP method can be obtained via a weighted minimization of the difference between the occupied-virtual block of the xOEP and the HF exchange matrix, with weight factors given by inverse orbital eigenvalue differences. © 2008.

Comparison of Anodic TiO2-Nanotube Membranes used for Frontside-Illuminated Dye-Sensitized Solar Cells - Mohammadpour, Fatemeh; Moradi, Mahmood; Cha, Gi; So, Seulgi; Lee, Kiyoung; Altomare, Marco; Schmuki, Patrik (ChemElectroChem, 2015)
In the present work we compare TiO2-nanotube lift-off strategies for the construction of frontside-illuminated dye-sensitized solar cells (DSSCs). Anodic nanotube layers were detached from the metallic back contact by using different techniques and transferred onto a fluorine-doped tin oxide substrate. We show that if we use an optimized potential step treatment to fabricate membranes, DSSC efficiencies can be significantly increased ( extgreekh>8 %). The improved efficiency is ascribed to higher specific dye loading and enhanced electron-transport properties of optimally fabricated TiO2-nanotube membranes.
EAM Import::2019-03-13

Comparison of different methods for determining the size of a focal spot of microfocus X-ray tubes - Salamon, M.; Hanke, R.; Krueger, P.; Sukowski, F.; Uhlmann, N.; Voland, V. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2008)
The EN 12543-5 describes a method for determining the focal spot size of microfocus X-ray tubes up to a minimum spot size of 5 extgreekmm. The wide application of X-ray tubes with even smaller focal spot sizes in computed tomography and radioscopy applications requires the evaluation of existing methods for focal spot sizes below 5 extgreekmm. In addition, new methods and conditions for determining submicron focal spot sizes have to be developed. For the evaluation and extension of the present methods to smaller focal spot sizes, different procedures in comparison with the existing EN 12543-5 were analyzed and applied, and the results are presented.
EAM Import::2019-03-13

Comparison of different methods for image lag investigation of indirect converting flat-panel detectors - Hofmann, Th.; Burtzlaff, S.; Voland, V.; Salamon, M.; Nachtrab, Frank; Sukowski, F.; Uhlmann, N. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2009)
To measure the origins and dependencies of image lag in indirect converting flat-panel detectors, different methods were implemented. According to ASTM E 2597-07, the detector will be irradiated for a defined period with a constant flux. Then shutting off the tube, one can determine the image lag by monitoring the detector signal. We developed a different method, using a triggered shutter, to enable exactly defined illumination of the detector. Integration times longer than 1 s allow an exposure only during the delay time of the detector. This way, comparable results for image lag for decreasing intensity can be obtained. To investigate the image lag for increasing intensity, the triggered shutter stays open. By opening the shutter for more than one frame, then closing, it is possible to observe the build-up and decay of the image lag in dependence on the illumination time. Using these methods image lag was investigated with respect to irradiation intensity, time, incident X-ray spectrum, gain capacity of the detector and lag homogeneity of the detector surface.
EAM Import::2019-03-13

Comparison of different sources for laboratory X-ray microscopy - Ebensperger, T.; Stahlhut, P.; Nachtrab, Frank; Zabler, S.; Hanke, Randolf (Journal of Instrumentation, 2012)
This paper describes the setup of two different solutions for laboratory X-ray microscopy working with geometric magnification. One setup uses thin-film transmission targets with an optimized tungsten-layer thickness and the electron gun and optics of an electron probe micro analyzer to generate a very small X-ray source. The other setup is based on a scanning electron microscope and uses microstructured reflection targets. We also describe the structuring process for these targets. In both cases we show that resolutions of 100 nm can be achieved. Also the possibilities of computed tomography for 3D imaging are explored and we show first imaging examples of high-absorption as well as low-absorption specimens to demonstrate the capabilities of the setups.
EAM Import::2019-03-13

Comparison of electroluminescence intensity and photocurrent of polymer based solar cells - Hoyer, Ulrich; Pinna, Luigi; Swonke, Thomas; Auer, Richard; Brabec, Christoph; Stubhan, Tobias; Li, Ning (Advanced Energy Materials, 2011)
The reciprocity theorem for solar cell predicts a linear relation between electroluminescence emission and photovoltaic quantum efficiency and an exponential dependence of the electroluminescence signal on the applied voltage. Both dependencies are experimentally verified for polymer based solar cells in this paper. Furthermore it is shown, that electroluminescence imaging of organic solar cells has the potential to visualize the photocurrent distribution significantly faster than standard laser beam induced current mapping (LBIC) techniques.

Comparison of various sol-gel derived metal oxide layers for inverted organic solar cells - Oh, Hyunchul; Krantz, Johannes; Litzov, Ivan; Stubhan, Tobias; Pinna, Luigi; Brabec, Christoph (Solar Energy Materials and Solar Cells, 2011)
Inverted bulk-heterojunction solar cells have recently captured high interest due to their environmental stability as well as compatibility to mass production. This has been enabled by the development of solution processable n-type semiconductors, mainly TiO and ZnO. However, the device performance is strongly correlated to the electronic properties of the interfacial materials, and here specifically to their work function, surface states as well as conductivity and mobility. It is noteworthy to say that these properties are massively determined by the crystallinity and stoichiometry of the metal oxides. In this study, we investigated aluminum-doped zinc oxide (AZO) as charge selective extraction layer for inverted BHJ solar cells. Thin AZO films were characterized with respect to their structural, optical and electrical properties. The performance of organic solar cells with an AZO electron extraction layer (EEL) is compared to the performance of intrinsic ZnO or TiO EELs. We determined the transmittance, absorbance, conductivity and optical band gap of all these different metal oxides. Furthermore, we also built the correlations between doping level of AZO and device performance, and between annealing temperature of AZO and device performance. © 2011 Elsevier B.V.
Keywords: Al doped ZnO (AZO); Charge selective extraction layer; Inverted Organic Solar cells; Sol-Gel synthesis; TiOx; ZnO