Solubility Based Identification of Green Solvents for Small Molecule Organic Solar Cells - Burgues-Ceballos, Ignasi; Machui, Florian; Min, Jie; Ameri, Tayebeh; Voigt, Monika M.; Luponosov, Yuriy N.; Ponomarenko, Sergei A.; Lacharmoise, Paul D.; Campoy-Quiles, Mariano; Brabec, Christoph (Advanced Functional Materials, 2014)
Replacing halogenated solvents in the processing of organic solar cells by green solvents is a required step before the commercialization of this technology. With this purpose, some attempts have been made, although a general method is yet to be developed. Here, the potential of the Hansen solubility parameters (HSP) analysis for the design of green ink formulations for solution-processed active layer in bulk heterojunction photovoltaic devices based on small molecules is demonstrated. The motivation of moving towards organic small molecules stems from their lower molecular weight and more definite structure which makes them more likely to be dissolved in a wider variety of organic solvents. In the first step, the HSP of selected active materials are determined, namely, the star-shaped D-π-A tris[4-[5′′-(1,1-dicyanobut-1-en-2-yl)-2,2′-bithiophen-5-yl]pheny]amine N(Ph-2T-DCN-Et)3 small molecule and fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM). Secondly, computer simulations based on HSP allow the prediction of suitable green solvents for this specific material system. The most promising green solvents, according to the simulations, are then used to fabricate solar cell devices using pristine solvents and two solvents mixtures. These devices show power conversion efficiencies around 3.6%, which are comparable to those obtained with halogenated solvents. This good performance is a result of the sufficient solubility achieved after a successful prediction of good (green) solvents.
Keywords: Hansen solubility parameters; ink formulations; non-halogenated solvents; organic photovoltaics; small molecule

Soluble graphene: Generation of aqueous graphene solutions aided by a perylenebisimide-based bolaamphiphile - Englert, Jan; Röhrl, Jonas; Schmidt, Cordula; Graupner, Ralf; Hundhausen, Martin; Hauke, Frank; Hirsch, Andreas (Advanced Materials, 2009)
(Figure Presented) Single-layer graphene (SLG) is deposited onto Si/SiO substrates from aqueous dispersions using a scalable and quick detergent-based method (see figure). The deposits are analyzed using absorption and Raman spectroscopy and atomic force and optical microscopy. Evaluation of the two-phonon defect-induced Raman peak of individual particles on the substrate is used to confirm exfoliation into graphene monolayers. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers - Zirzlmeier, Johannes; Casillas, Ruben; Reddy, Rajagopala; Coto, Pedro B.; Lehnherr, Dan; Chernick, Erin; Papadopoulos, Ilias; Thoss, Michael; Tykwinski, Rik; Guldi, Dirk Michael (Nanoscale, 2016)
We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 1012 s-1, respectively, in room temperature solutions.
EAM Import::2019-03-13

Solution-Processed Metallic Nanowire Electrodes as Indium Tin Oxide Replacement for Thin-Film Solar Cells - Krantz, Johannes; Richter, Moses; Spallek, Stefanie; Spiecker, Erdmann; Brabec, Christoph (Advanced Functional Materials, 2011)
Solution processed silver nanowire (Ag NW) films are introduced as transparent electrodes for thin-film solar cells. Ag NW electrodes were processed by doctor blade-coating on glass substrates at moderate temperatures (less than 100 degrees C). The morphological, optical, and electrical characteristics of these electrodes were investigated as a function of processing parameters. For solar-cell application, Ag NW electrodes with an average transparency of 90% between 450 and 800 nm and a sheet resistivity of 10 O per square were chosen. The performance of poly(3-hexylthiophen-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells on Ag NW electrodes was found to match the performance of otherwise identical cells on indium tin oxide. Overall, P3HT:PCBM solar cells with an efficiency of 2.5% on transparent Ag NW electrodes have been realized.
Keywords: silver nanowires; electrodes; organic electronics; indium tin oxide replacement; solar cells

Solution-Processed Parallel Tandem Polymer Solar Cells Using Silver Nanowires as Intermediate Electrode - Guo, Fei; Kubis, Peter; Li, Ning; Przybilla, Thomas; Matt, Gebhard; Stubhan, Tobias; Ameri, Tayebeh; Butz, Benjamin; Spiecker, Erdmann; Forberich, Karen; Brabec, Christoph (Acs Nano, 2014)
Tandem architecture is the most relevant concept to overcome the efficiency limit of single-junction photovoltaic solar cells. Series-connected tandem polymer solar cells (PSCs) have advanced rapidly during the past decade. In contrast, the development of parallel-connected tandem cells is lagging far behind due to the big challenge in establishing an efficient interlayer with high transparency and high in-plane conductivity. Here, we report all-solution fabrication of parallel tandem PSCs using silver nanowires as intermediate charge collecting electrode. Through a rational interface design, a robust interlayer is established, enabling the efficient extraction and transport of electrons from subcells. The resulting parallel tandem cells exhibit high fill factors of similar to 60% and enhanced current densities which are identical to the sum of the current densities of the subcells. These results suggest that solution-processed parallel tandem configuration provides an alternative avenue toward high performance photovoltaic devices.
Keywords: parallel-tandem; polymer solar cells; solution-processed; silver nanowires

Solution-processed single-crystalline organic transistors on patterned ultrathin gate insulators - Tsurumi, J.; Amin, Atefeh Y.; Okamoto, T.; Mitsui, C.; Takimiya, K.; Matsui, Hiroyuki; Halik, Marcus; Takeya, Jun (Organic Electronics, 2014)
Single-crystalline organic transistors of 3,11-didecyl-dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C10-DNBDT-NW) and 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) were fabricated by solution processes on top of the patterned hybrid ultrathin gate dielectrics consisting of 3.6 nm-thick aluminum oxide and self-assembled monolayers (SAMs). Due to the excellent crystallinity of the channel films, bottom-gate and top-contact field-effect transistors exhibited the average field-effect mobility of 3.7 cm2/V s and 4.3 cm2/V s for C10-DNBDT-NW and C10-DNTT, respectively. These are the first successful devices of solution-processed single-crystalline transistors on ultrathin gate dielectrics with the mobility above 1 cm2/V s, opening the way to develop low-power-consumption and high-performance printed circuits.

Solvation structure and dynamics of Ag+ in aqueous ammonia solutions: A molecular simulation study - Sansotta, Stefano; Zahn, Dirk (Journal of Chemical Physics, 2017)
We present an ab initio-based force-field for silver ion interactions with water and ammonia. Compared to quantum calculations, our model allows for rather large-scale molecular dynamics simulations of silver solutions of aqueous ammonia. For a series of NH3:H2O ratios ranging from 1 to 20 mol. %, Ag+ ions were mainly found as octahedral [Ag(NH3)x(H2O)]6-x+ coordination complexes with preferential values of x ranging from 0 to 3. In the first coordination structure, water łeftrightarrow ammonia exchanges occur within a 1-3 ps time scale and, depending on the NH3 concentration, imply significant fluctuations of x covering the whole range from 0 to 6. Based on ns-scale molecular dynamics simulations, chemical potentials are derived for all Ag+ coordination species as functions of temperature and ammonia concentration. Moreover, we compare the diffusion constants of the [Ag(H2O)6]+ to [Ag(H2O)3(NH3)3]+ coordination complexes, based on the solutions of the corresponding ammonia content.
EAM Import::2019-03-13

Solving stress constrained problems in topology and material optimization - Kocvara, Michal; Stingl, Michael (Structural and Multidisciplinary Optimization, 2012)
This article is a continuation of the paper Kovara and Stingl (Struct Multidisc Optim 33(4-5):323-335, 2007). The aim is to describe numerical techniques for the solution of topology and material optimization problems with local stress constraints. In particular, we consider the topology optimization (variable thickness sheet or "free sizing") and the free material optimization problems. We will present an efficient algorithm for solving large scale instances of these problems. Examples will demonstrate the efficiency of the algorithm and the importance of the local stress constraints. In particular, we will argue that in certain topology optimization problems, the addition of stress constraints must necessarily lead not only to the change of optimal topology but also optimal geometry. Contrary to that, in material optimization problems the stress singularity is treated by the change in the optimal material properties.
Keywords: Topology optimization; Material optimization; Stress based design; Nonlinear semidefinite programming

Some critical factors for photocatalysis on self-organized TiO2 nanotubes - Liu, Ning; Paramasivam, Indhumati; Yang, Mingjun; Schmuki, Patrik (Journal of Solid State Electrochemistry, 2012)
In the present work, different intrinsic and extrinsic parameters are investigated that affect the photocatalytic activity of self-organized TiO2 nanotube layers. Particularly, the influence of annealing temperature and annealing atmosphere, the influence of different gas purging in the electrolyte, and the effect of applied voltage on the photocatalytic degradation rates of acid orange (AO7) are discussed. We find that the effect of the reducing gas atmosphere dominates over the anatase/rutile ratio in activating the nanotube layers. Moreover, we show that the effect of different gas purging (Ar and O2) of the electrolyte affects the reaction rate twofold: (1) by providing electron acceptor states and also by (2) a different change in the red–ox potential, i.e., the band bending in TiO2. By an external anodic voltage, the reaction rates can be increased drastically due to increased band bending. Nevertheless, the magnitude of the effect is also affected by the presence or absence of O2 in the electrolyte.
EAM Import::2019-03-13

Spatially-controlled laser-induced decoration of 2D and 3D substrates with plasmonic nanoparticles - Bashouti, M. Y.; Povolotckaia, A. V.; Povolotskiy, A. V.; Tunik, S. P.; Christiansen, S. H.; Leuchs, Gerd; Manshina, A. A. (RSC Advances, 2016)
We demonstrate a new approach which can be used for targeted imparting of plasmonic properties for a wide range of different substrates (transparent and non-transparent) which may have any 2D or 3D topological structure created independently in a prior step with some other technology.
EAM Import::2019-03-13

Spatially resolved flame zone classification of a flame spray nanoparticle synthesis process by combining different optical techniques - Kilian, Daniel; Engel, Sascha; Borsdorf, B.; Gao, Yi; Kögler, Andreas; Kobler, S.; Seeger, Thomas; Will, Stefan; Leipertz, Alfred; Peukert, Wolfgang (Journal of Aerosol Science, 2014)
Flame spray synthesis of silica nanoparticles is characterized by a set of complementary optical techniques. By means of laser-sheet based Mie scattering imaging, 2D-chemiluminescence imaging and coherent anti-Stokes Raman spectroscopy (CARS) local information on spatial droplet distribution, combustion zone, nucleation zone and temperature in the flame could be obtained. In addition, the outcomes from optical metrology are validated by thermophoretic sampling at different flame heights and the synthesized powders were analyzed by N2 gas sorption. By comparing these results the flame can be quantitatively classified into three distinct zones: (i) the droplet zone where precursor atomization and evaporation take place, (ii) the nucleation zone indicated by SiO*/Si* radicals as a preliminary species before SiO2 particle formation and (iii) the sintering zone characterized by the highest temperatures in flame. In addition the spatial spreading of the nucleation zone as a function of precursor concentration is investigated. Theoretical calculations and experimental results show an extended nucleation regime for the lowest precursor concentration compared to higher concentrations. Although this study is performed with hexamethyldisiloxane (HMDS) precursor to synthesize silica nanoparticles as a model system, dimensionless analysis shows that the results, concerning the spray formation, can be transferred to the synthesis of other materials as well.

Spectra library: An assumption-free in situ method to access the kinetics of catechols binding to colloidal ZnO quantum dots - Lin, Wei; Haderlein, Michael; Walter, Johannes; Peukert, Wolfgang; Segets, Doris (Angewandte Chemie International Edition, 2016)
Assumption-free and in situ resolving of the kinetics of ligand binding to colloidal nanoparticles (NPs) with high time resolution is still a challenge in NP research. A unique concept of using spectra library and stopped-flow together with a "search best-match" Matlab algorithm to access the kinetics of ligand binding in colloidal systems is reported. Instead of deconvoluting superimposed spectra using assumptions, species absorbance contributions (ligand@ZnO NPs and ligand in solution) are obtained by offline experiments. Therefrom, a library of well-defined targets with known ligand distribution between particle surface and solution is created. Finally, the evolution of bound ligand is derived by comparing in situ spectra recorded by stopped-flow and the library spectra with the algorithm. Our concept is a widely applicable strategy for kinetic studies of ligand adsorption to colloidal NPs and a big step towards deep understanding of surface functionalization kinetics. Well-established libraries of target spectra that are derived by means of careful offline analysis and identification of equilibrium data within larger kinetic datasets can be used for any particle-ligand system. Kinetics of ligand binding to nanoparticles can be derived free of assumption, in situ, and with high time resolution.
Keywords: colloids; kinetics; ligand exchange; nanoparticles; surface analysis

Spectroscopic and electrochemical characterization of heteropoly acids for their optimized application in selective biomass oxidation to formic acid - Albert, Jakob; Lüders, Daniela; Bösmann, Andreas; Guldi, Dirk Michael; Wasserscheid, Peter (Green Chemistry, 2014)
Different Keggin-type polyoxometalates have been synthesized and characterized in order to identify optimized homogeneous catalysts for the selective oxidation of biomass to formic acid (FA) using oxygen as an oxidant and p-toluenesulfonic acid as an additive. Applying the optimized polyoxometalate catalyst system H[PVMoO ] (HPA-5), a total FA-yield (with respect to carbon in the biogenic feedstock) of 60% for glucose within 8 h reaction time and 28% for cellulose within 24 h reaction time could be achieved. The transformation is characterized by its mild reaction temperature, its excellent selectivity to FA in the liquid product phase and its applicability to a very wide range of biogenic raw materials including non-edible biopolymers and complex biogenic mixtures. © 2014 The Royal Society of Chemistry.

Spectroscopic Observation and Molecular Dynamics Simulation of Ga Surface Segregation in Liquid Pd-Ga Alloys - Grabau, Mathias; Erhard, Jannis; Taccardi, Nicola; Krick Calderon, Sandra; Wasserscheid, Peter; Görling, Andreas; Steinrück, Hans-Peter; Papp, Christian (Chemistry - A European Journal, 2017)
Liquid binary Pd-Ga alloys with low Pd contents of 0.8, 1.8, and 4.7at% of Pd were examined as a function of sample temperature in ultra-high vacuum by using angle-resolved XPS. Upon cooling from 750 to 400K, a pronounced temperature-dependence of the Pd concentration in the liquid phase was observed, which was explained by the transition from the pure liquid phase to a two-phase system, consisting of a solid Ga5Pd phase and a Pd-depleted liquid Pd-Ga alloy. In the liquid Pd-Ga alloy, Pd is always depleted from the topmost interface layer, as deduced from angle-resolved XPS at 0 and 80 degrees emission, independent of temperature and Pd concentration. This observation is interpreted as an inhomogeneous depth distribution function of Pd, that is, the segregation of Ga to the surface of the liquid phase. The results of a DFT-based molecular dynamics simulation (MD) independently show interfacial stratification of Ga and an inhomogeneous Pd distribution along the surface normal. The evaluation of the experimental data with a rigid layer model based on the MD calculations leads to excellent agreement with the simulation.
Keywords: DFT MD; gallium; liquid-metal catalysis; palladium; XPS

Spiking Systematics in Some CO2 Laser Models - Gallas, Jason (Academic Press Inc., 2016)
The CO laser is a complex dynamical system that has been investigated extensively both experimentally and through numerical simulations. As a result, a number of models exist for this laser, famed for providing satisfactory agreement between numerical and experimental observations. But the laser involves a large number of freely tunable control parameters whose impact on its performance and stability is not known in detail. The spontaneous emergence and organization of laser stability phases are also poorly understood. Here, we review recent progress in the classification of laser spiking, periodic or nonperiodic self-pulsations, predicted for CO lasers with modulated parameters and with feedback, instantaneous or delayed. The unfolding of spiking is classified with the help of numerically obtained high-resolution stability charts for experimentally accessible control parameters. Such stability charts display novel regular and irregular features, suggesting that the laser control parameter planes harbor remarkable symmetries not yet accounted for theoretically but which are experimentally within reach. High-resolution stability charts put stringent tests on the reliability and accuracy of current models in forecasting laser dynamics.
Keywords: CO2 laser stability; Laser spiking organization; Laser stability charts

Splitting of the Universality Class of Anomalous Transport in Crowded Media - Spanner, Markus; Hoefling, Felix; Kapfer, Sebastian; Mecke, Klaus; Schroeder-Turk, Gerd E.; Franosch, Thomas (Physical Review Letters, 2016)
We investigate the emergence of subdiffusive transport by obstruction in continuum models for molecular crowding. While the underlying percolation transition for the accessible space displays universal behavior, the dynamic properties depend in a subtle nonuniversal way on the transport through narrow channels. At the same time, the different universality classes are robust with respect to introducing correlations in the obstacle matrix as we demonstrate for quenched hard-sphere liquids as underlying structures. Our results confirm that the microscopic dynamics can dominate the relaxational behavior even at long times, in striking contrast to glassy dynamics.

Spontaneous Growth of Magnesium Hydroxide Fibers at Ambient Conditions - Gheisi, Amirreza; Sternig, Andreas; Rangus, Mojca; Redhammer, Günther; Hartmann, Martin; Diwald, Oliver (Crystal Growth & Design, 2014)
Spontaneous transformation paths of nanomaterials point to guiding, principles for synthesis. We describe the room temperature transformation of MgO nanocubes into Mg-3(OH)(5)Cl center dot 4H(2)O nanofibers in air and investigated the underlying formation mechanism using electron microscopy, X-ray diffraction, and solid-state NMR spectroscopy. Upon contact with water vapor, the magnesium hydroxide needles were found to grow out of agglomerates of highly dispersed MgO nanocubes with preadsorbed SiCl4. Corresponding one-dimensional nanostructures do not form on low surface area materials. The presented growth approach is potentially extendable to other hydrolyzable metal oxides at ultrafine dispersion.

Spray-coatable ionogels based on silane-ionic liquids for low voltage, flexible, electrolyte-gated organic transistors - Thiemann, Stefan; Sachnov, Swetlana; Gruber, Mathias; Gannott, Florentina; Spallek, Stefanie; Schweiger, Manuel; Krückel, Johannes; Kaschta, Joachim; Spiecker, Erdmann; Wasserscheid, Peter; Zaumseil, Jana (Journal of Materials Chemistry C, 2014)
We introduce a new type of silane-based ionogels that are produced by gelation of the ionic liquid 3-methyl-1-(3-(triethoxysilyl)propyl)-imidazolium bis(trifluoromethylsulfonyl)imide ([(EtO)3SiPMIM][TFSI]) with tetramethylorthosilane and formic acid. In the obtained ionogels the cations are involved in the network formation while the anions can move freely. The ionogels show advantageous properties for application in flexible electronics, such as low modulus, solution processability and high specific capacitance. Spray-coated ionogels were used as high capacitance gate dielectrics for organic (poly[3-hexylthiophene], P3HT) electrolyte-gated transistors (EGTs) that operated at very low voltages (<2 V) with high on/off ratios in air over weeks. Devices fabricated on polymer foil remained functional during repeated bending cycles with strains up to 2.3%.

Spray-Coated Silver Nanowires as Top Electrode Layer in Semitransparent P3HT:PCBM-Based Organic Solar Cell Devices - Krantz, Johannes; Stubhan, Tobias; Richter, Moses; Spallek, Stefanie; Litzov, Ivan; Matt, Gebhard; Spiecker, Erdmann; Brabec, Christoph (Advanced Functional Materials, 2013)
Silver nanowire (Ag NW) thin films are investigated as top electrodes in semitransparent inverted organic solar cells. The performance of semitransparent poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) organic solar cells with Ag NW top electrode layers is found to match very closely the performance of reference devices based on thermally evaporated, highly reflective metal silver top electrodes. The optical losses of the semitransparent electrodes are investigated in detail and analyzed in terms of transmission, scattering, and reflection losses. The impact on an external back reflector is shown to increase the light harvesting efficiency of optically thin devices. Further analysis of transparent devices under illumination from the indium tin oxide (ITO) backside and through the Ag NW front electrode open the possibility to gain deep insight into the vertical microstructure related devices performance. Overall, Ag NW top electrodes are established as a serious alternative to TCO based electrodes. Semitransparent devices with efficiencies of over = 2.0% are realized.
Keywords: silver nanowires; electrodes; semitransparent photovoltaic devices; organic electronics; spray deposition

Stability and Photoelectronic Properties of Layered Titanate Nanostructures - Riss, Alexander; Elser, Michael J.; Bernardi, Johannes; Diwald, Oliver (Journal of the American Chemical Society, 2009)
Layered titanate nanostructures offer promising photoelectronic properties that are subject to surface chemistry-induced morphology changes. For a systematic evaluation of the bulk and surface contributions to the photoactivity of these structures, we investigated their photoelectronic properties and in particular their dependence on the condition of the gas-solid interface. We comprehensively explored the stability of Na_2Ti_3O7 nanowires and scrolled up H_2Ti_3O7 nanotubes by means of transmission electron microscopy, Raman, and FT-IR spectroscopy and subjected both titanate sheet-based structures to controlled thermal activation treatment under high vacuum conditions. We found that throughout thermal annealing up to T = 870 K the structure and morphology of Na_2Ti_3O7 nanowires are retained. Consistent with the significant photoluminescence emission that is attributed to radiative exciton annihilation in the bulk, UV-induced charge separation is strongly suppressed in these structures. H_2Ti_3O7 nanotubes, however, undergo transformation into elongated anatase nanocrystals during annealing at temperatures T ≥ 670 K. Photoexcitation experiments in O_2 atmosphere reveal that these structures efficiently sustain the separation of photogenerated charges. Trends in the abundance of trapped holes and scavenged electrons were characterized quantitatively by tracking the concentration of paramagnetic O- and O_2- species with electron paramagnetic resonance spectroscopy EPR, respectively. An incisive analysis of these results in comparison to those obtained on airborne anatase nanocrystals underlines the critical role of surface composition and structure on charge separation and, in consequence, on the chemical utilization of photogenerated charge carriers.
EAM Import::2019-03-13

Stability conditions for exact-exchange Kohn-Sham methods and their relation to correlation energies from the adiabatic-connection fluctuation-dissipation theorem - Bleiziffer, Patrick; Schmidtel, Daniel; Görling, Andreas (Journal of Chemical Physics, 2014)
The occurrence of instabilities, in particular singlet-triplet and singlet-singlet instabilities, in the exact-exchange (EXX) Kohn-Sham method is investigated. Hessian matrices of the EXX electronic energy with respect to the expansion coefficients of the EXX effective Kohn-Sham potential in an auxiliary basis set are derived. The eigenvalues of these Hessian matrices determine whether or not instabilities are present. Similar as in the corresponding Hartree-Fock case instabilities in the EXX method are related to symmetry breaking of the Hamiltonian operator for the EXX orbitals. In the EXX methods symmetry breaking can easily be visualized by displaying the local multiplicative exchange potential. Examples (N-2, O-2, and the polyyne C10H2) for instabilities and symmetry breaking are discussed. The relation of the stability conditions for EXX methods to approaches calculating the Kohn-Sham correlation energy via the adiabatic-connection fluctuation-dissipation (ACFD) theorem is discussed. The existence or nonexistence of singlet-singlet instabilities in an EXX calculation is shown to indicate whether or not the frequency-integration in the evaluation of the correlation energy is singular in the EXX-ACFD method. This method calculates the Kohn-Sham correlation energy through the ACFD theorem theorem employing besides the Coulomb kernel also the full frequency-dependent exchange kernel and yields highly accurate electronic energies. For the case of singular frequency-integrands in the EXX-ACFD method a regularization is suggested. Finally, we present examples of molecular systems for which the self-consistent field procedure of the EXX as well as the Hartree-Fock method can converge to more than one local minimum depending on the initial conditions. (C) 2014 AIP Publishing LLC.

Stabilization of Small Platinum Nanoparticles on Pt-CeO2 Thin Film Electrocatalysts During Methanol Oxidation - Brummel, Olaf; Waidhas, Fabian; Faisal, Firas; Fiala, Roman; Vorokhta, Mykhailo; Khalakhan, Ivan; Dubau, Martin; Figueroba, Alberto; Kovacs, Gabor; Aleksandrov, Hristiyan A.; Vayssilov, Georgi N.; Kozlov, Sergey M.; et al. (Journal of Physical Chemistry C, 2016)
Pt-doped CeOx thin film electrocatalysts have recently been shown to exhibit high activity and stability at the anode of proton exchange membrane fuel cells (PEM-FC). To identify, the role of the Pt dopant and the origin of the high stability of Pt-CeOx films, we applied electrochemical in situ IR spectroscopy on Pt-CeOx model thin film catalysts during methanol (1 M methanol) oxidation. The model catalysts were prepared by magnetron cosputtering of Pt (9-21 atom %), and CeO2 onto clean, and carbon-coated Au supports, All samples were characterized by scanning electron microscopy (SEM), energy-dispersive, X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) before and after reaction. At pH 1 (0.1 MHClO4) the Pt-CeOx dissolves partially during potential cycling, whereas the films: are largely stable at pH 6 (0.1 M phosphate buffer). Electrochemical IR spectroscopy of the adsorbed CO shows that Metallic Pt is formed on all Pt-CeOx samples during methanol oxidation. In comparison to Pt(111), Pt aggregates on Pt-CeOx show a CO on-top signal, which is red shifted by at least 25 cm(-1) and suppression of the bridging CO signals. Whereas the Pt particles on Pt-CeOx,films with high Pt concentration (>20 atom %) undergo rapid sintering during the potential cycling, small metallic Pt aggregates are stable under the Same conditions on films with lbw Pt concentration (<15 atom % Pt). By means of density functional theory (DFT) calculations we analyzed the spectral shifts of adsorbed CO as a function of nanoparticle size both on free and ceria-supported Pt particles, Comparison with the experiment suggests the formation of "subnano"-particles, i.e., particles with up to 30 atoms (<1 nm particle diameter), which do not expose regular (111) facet sites. At sufficiently low Pt loading) these subnano-Pt particles are efficiently stabilized by the interaction with the ceria support under conditions of the dynamically changing electrode potential.

Stable algorithm for event detection in event-driven particle dynamics - Bannerman, Marcus; Strobl, Severin; Formella, Arno; Pöschel, Thorsten (Computational Particle Mechanics, 2014)
Event-driven particle dynamics is a fast and precise method to simulate particulate systems of all scales. In this work it is demonstrated that, despite the high accuracy of the method, the finite machine precision leads to simulations entering invalid states where the dynamics are undefined. A general event-detection algorithm is proposed which handles these situations in a stable and efficient manner. This requires a definition of the dynamics of invalid states and leads to improved algorithms for event-detection in hard-sphere systems.

Stable Co-Catalyst-Free Photocatalytic H 2 Evolution From Oxidized Titanium Nitride Nanopowders - Zhou, Xuemei; Zolnhofer, Eva; Nguyen, Nhat Truong; Liu, Ning; Meyer, Karsten; Schmuki, Patrik (Angewandte Chemie, 2015)
A simple strategy is used to thermally oxidize TiN nanopowder (∼20nm) to an anatase phase of a TiO2:Ti3+:N compound. In contrast to the rutile phase of such a compound, this photocatalyst provides activity for hydrogen evolution under AM1.5 conditions, without the use of any noble metal co-catalyst. Moreover the photocatalyst is active and stable over extended periods of time (tested for 4months). Importantly, to achieve successful conversion to the active anatase polymorph, sufficiently small starting particles of TiN are needed. The key factor for catalysis is the stabilization of the co-catalytically active Ti3+ species against oxidation by nitrogen present in the starting material.
EAM Import::2019-03-13

Stable free surface flows with the lattice Boltzmann method on adaptively coarsened grids - Thürey, Nils; Rüde, Ulrich (Computing and Visualization in Science, 2009)
In this paper we will present an algorithm to perform free surface flow simulations with the lattice Boltzmann method on adaptive grids. This reduces the required computational time by more than a factor of three for simulations with large volumes of fluid. To achieve this, the simulation of large fluid regions is performed with coarser grid resolutions. We have developed a set of rules to dynamically adapt the coarse regions to the movement of the free surface, while ensuring the consistency of all grids. Furthermore, the free surface treatment is combined with a Smagorinsky turbulence model and a technique for adaptive time steps to ensure stable simulations. The method is validated by comparing the position of the free surface with an uncoarsened simulation. It yields speedup factors of up to 3.85 for a simulation with a resolution of 4803 cells and three coarser grid levels, and thus enables efficient and stable simulations of free surface flows, e.g. for highly detailed physically based animations of fluids. © 2008 Springer-Verlag.
UnivIS-Import:2015-03-09:Pub.2009.tech.IMMD.lsinfs.stable
Keywords: Adaptive grids; Free surface flows; Lattice Boltzmann method; Physically based animation

Stacking Energies for Average B-DNA Structures from the Combined Density Functional Theory and Symmetry-Adapted Perturbation Theory Approach - Fiethen, Annamaria; Jansen, Georg; Heßelmann, Andreas; Schuetz, Martin (Journal of the American Chemical Society, 2008)
Stacking energies for the ten unique tetramers composed of two complementary base pairs in an average B-DNA arrangement are calculated with the DFT-SAPT variant of intermolecular perturbation theory and compared to spin-component scaled second-order Møller-Plesset theory. The well-defined decomposition of the interaction energy available in DFT-SAPT suggests that at least the first-order electrostatic and exchange and the second-order total dispersion energies have to be accurately modeled to reproduce the different stacking energies of the various base pair steps, while the induction contributions can effectively be accounted for through a scaled dispersion energy.
EAM Import::2019-03-13

Star-shaped D-π-A oligothiophenes with a tris(2-methoxyphenyl)amine core and alkyldicyanovinyl groups: synthesis and physical and photovoltaic properties - Luponosov, Yuriy N.; Min, Jie; Solodukhin, Alexander N.; Bakirov, Artem V.; Dmitryakov, Petr V.; Shcherbina, Maxim A.; Peregudova, Svetlana M.; Cherkaev, Georgiy V.; Chvalun, Sergei N.; Brabec, Christoph; Ponomarenko, Sergei A. (Journal of Materials Chemistry C, 2016)
Synthesis of a series of star-shaped oligomers having a novel electron donating tris(2-methoxyphenyl)amine (m-TPA) core, which is linked through a bithiophene or terthiophene extgreekp-bridge with electron-deficient alkyldicyanovinyl (alkyl-DCV) groups, is described. A comprehensive study of the oligomers revealed significant dependence of their physical properties, including absorption, molecular frontier energy levels, crystal packing, and melting and glass transition temperatures, upon the chemical structure. A comparison of their photophysical properties to the nearest analog having the common dicyanovinyl (DCV) groups demonstrated a number of benefits to use alkyl-DCV units for the design of donor–acceptor small molecules: higher solubility, increased electrochemical stability, better photovoltaic performance, and possibility to control the relative physical and photovoltaic properties by a simple adjustment of alkyl and extgreekp-bridge lengths. Modification of the well-known triphenylamine (TPA) core in the star-shaped oligomers by methoxy groups increases not only solubility, but also crystallinity of the oligomers, whereas their photovoltaic performance stays on a similar level as their analogs with a TPA core. The study demonstrates that these design strategies represent interesting and simple tools for the effective modulation of properties of star-shaped molecules.
EAM Import::2019-03-13
Keywords: Engineering controlled terms: Crystal structure; Glass transition; Molecules; Oligomers; Solubility Design strategies; Electrochemical stabilities; Electron-deficient; Electron-donating; Photophysical properties; Photovoltaic performance; Photovoltaic property; Star-shaped molecules Engineering main heading: Stars

Static and Frequency-Dependent Dipole–Dipole Polarizabilities of All Closed-Shell Atoms up to Radium: A Four-Component Relativistic DFT Study - Bast, Radovan; Heßelmann, Andreas; Salek, Pawel; Helgaker, Trygve; Saue, Trond (ChemPhysChem, 2008)
We test the performance of four-component relativistic density functional theory by calculating the static and frequency-dependent electric dipole–dipole polarizabilities of all (ground-state) closed-shell atoms up to Ra. We consider 12 nonrelativistic functionals, including three asymptotically shape-corrected functionals, by using two smooth interpolation schemes introduced by the Baerends group: the gradient-regulated asymptotic connection (GRAC) procedure and the statistical averaging of (model) orbital potentials (SAOP). Basis sets of doubly augmented triple-zeta quality are used. The results are compared to experimental data or to accurate ab initio results. The reference static electric dipole polarizability of palladium has been obtained by finite-field calculations using the coupled-cluster singles, doubles, and perturbative triples method within this work. The best overall performance is obtained using hybrid functionals and their GRAC shape-corrected versions. The performance of SAOP is among the best for nonhybrid functionals for Group 18 atoms but its precision degrades when considering the full set of atoms. In general, we find that conclusions based on results obtained for the rare-gas atoms are not necessarily representative of the complete set of atoms. GRAC cannot be used with effective core potentials since the asymptotic correction is switched on in the core region.
EAM Import::2019-03-13

Statistical raman microscopy and atomic force microscopy on heterogeneous graphene obtained after reduction of graphene oxide - Eigler, Siegfried; Hof, Ferdinand; Enzelberger-Heim, Michael; Grimm, Stefan; Müller, Paul; Hirsch, Andreas (Journal of Physical Chemistry C, 2014)
Graphene oxide can be used as a precursor to graphene, but the quality of graphene flakes is highly heterogeneous. Scanning Raman microscopy (SRM) is used to characterize films of graphene derived from flakes of graphene oxide with an almost intact carbon framework (ai-GO). The defect density of these flakes is visualized in detail by analyzing the intensity and full width at half-maximum of the most pronounced Raman peaks. In addition, we superimpose the SRM results with AFM images and correlate the spectroscopic results with the morphology. Furthermore, we use the SRM technique to display the amount of defects in a film of graphene. Thus, an area of 250 × 250 μm of graphene is probed with a step-size increment of 1 μm. We are able to visualize the position of graphene flakes, edges and the substrate. Finally, we alter parameters of measurement to analyze the quality of graphene in a fast and reliable way. The described method can be used to probe and visualize the quality of graphene films. © 2014 American Chemical Society.

Steepest descent ballistic deposition of complex shaped particles - Topic, Nikola; Pöschel, Thorsten (Journal of Computational Physics, 2016)
We present an efficient event-driven algorithm for sequential ballistic deposition of complex-shaped rigid particles. Each of the particles is constructed from hard spheres (typically 5.1000) of variable radii. The sizes and relative positions of the spheres may mutually overlap and can be chosen such that the surface of the resulting particle appears relatively smooth. In the sequential deposition process, by performing steps of rolling and linear motion, the particles move along the steepest descent in a landscape formed by the boundaries and previously deposited particles. The computer time for the simulation of a deposition process depends on the total number of spheres but only weakly on the sizes and shapes of the particles. The proposed algorithm generalizes the Visscher-Bolsterli algorithm [1] which is frequently used for packing of spheres, to non-spherical particles. The proposed event-driven algorithm allows simulations of multi-million particle systems using desktop computers.
Keywords: Complex shaped particles; Event-driven algorithm; Non-spherical particles; Steepest descent ballistic deposition

Steering the formation of supported Pt-Sn nanoalloys by reactive metal-oxide interaction - Neitzel, Armin; Kovacs, Gabor; Lykhach, Yaroslava; Tsud, Nataliya; Kozlov, Sergey M.; Skala, Tomas; Vorokhta, Mykhailo; Matolin, Vladimir; Neyman, Konstantin M.; Libuda, Jörg (RSC Advances, 2016)
The formation of a supported Pt-Sn nanoalloy upon reactive metal-oxide interaction between Pt nanoparticles and a Sn-CeO2 substrate has been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional modeling. It was found that Pt deposition onto a Sn-CeO2 substrate triggers the reduction of Sn2+ cations yielding Pt-Sn nanoalloys at 300 K under ultra-high vacuum conditions. Three distinct stages of Pt-Sn nanoalloy formation were identified associated with the growth of (I) ultra-small monometallic Pt particles on a Sn-CeO2 substrate, (II) Pt-Sn nanoalloys on a Sn-CeO2 substrate, and (III) Pt-Sn nanoalloys on a stoichiometric CeO2 substrate. These findings suggest the existence of a critical size of monometallic Pt particles above which the formation of a Pt-Sn nanoalloy becomes favorable. In this respect, density functional modeling revealed a strong dependence of the formation energy of the PtxSn nanoalloy on the size of the Pt particle. Additionally, the thermodynamically favorable bulk and surface Pt/Sn stoichiometries were identified as two parameters that determine the composition of the supported Pt-Sn nanoalloys and limit the extraction of Sn2+ from the Sn-CeO2 substrate. Primarily, the formation of a bulk Pt3Sn alloy phase drives the growth of the Pt-Sn nanoalloy upon Pt deposition at 300 K. Upon annealing, Sn segregation on the surface of the Pt-Sn nanoalloy promotes further extraction of Sn2+ until the thermodynamically stable Pt/Sn concentration ratios of 3 for the bulk and approximately 1.6 for the surface are reached.

Step-by-step self-assembled hybrids that feature control over energy and charge transfer - Grimm, Bruno; Schornbaum, Julia; Jasch, Hannelore; Trukhina, Olga; Wessendorf, Florian; Hirsch, Andreas; Torres, Tomas; Guldi, Dirk Michael (Proceedings of the National Academy of Sciences of the United States of America, 2012)
In the current work, we have documented the use of two complementary supramolecular motifs, namely multipoint hydrogen bonding and metal complexation, as a means to control the step-by- step assembly of a panchromatically absorbing and highly versatile solar energy conversion system. On one hand, two different perylenediimides (1a/1b) have been integrated together with a metalloporphyrin (2) by means of the Hamilton receptor/cyanuric acid hydrogen bonding motif into energy transduction systems 1a•2 or 1b•2. Steady-state and time-resolved measurements corroborated that upon selective photoexcitation of the perylenediimides (1a/1b), an energy transfer evolved from the singlet excited state of the perylenediimides (1a/1b) to that of the metalloporphyrin (2). On the other hand, fullerene (3) and metalloporphyrin (2) form the electron donor-acceptor system 2•3 via axial complexation. Photophysical measurements confirm that an electron transfer prevails from the singlet excited state of 2 to the elec-tron- accepting 3. The correspondingly formed radical ion pair state decays with a lifetime of 1.0 ± 0.1 ns. As a complement to the aforementioned, the energy transduction features of 1a•2 were combined with the electron donor-acceptor characteristics of 2•3 to afford 1a•2•3. To this end, time-resolved measurements reveal that the initially occurring energy-transfer interaction (53 ± 3 ps) between 1a/1b and 2 is followed by an electron transfer (12 ± 1 ps) from 2 to 3. From multiwavelength analyses, the lifetime of the radical ion pair state in 1a•2•3 - as a product of a cascade of light-induced energy and electron transfer - was derived as 3.8 ± 0.2 ns.

Stern-Brocot trees in spiking and bursting of sigmoidal maps - Freire, J. G.; Pöschel, Thorsten; Gallas, Jason (EPL - Europhysics Letters, 2012)
We study the global organization of oscillations in sigmoidal maps, a class of models which reproduces complex locking behaviors commonly observed in lasers, neurons, and other systems which display spiking, bursting, and chaotic sequences of spiking and bursting. We find periodic oscillations to emerge organized regularly according to the elusive Stern-Brocot tree, a symmetric and more general tree which contains the better-known asymmetric Farey tree as a sub-tree. The Stern-Brocot tree provides a natural and encompassing organization to classify nonlinear oscillations. The mathematical algorithm for generating both trees is exactly the same, differing only in the initial conditions. Such degeneracy suggests that the wrong tree might have been attributed to locking phenomena reported in some of the earlier works. © Copyright EPLA, 2012.

Stochastic behavior of the coefficient of normal restitution - Gunkelmann, Nina; Montaine, Marina; Pöschel, Thorsten (Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2014)
We consider the collision of a rough sphere with a plane by detailed analysis of the collision geometry. Using stochastic methods, the effective coefficient of restitution may be described as a fluctuating quantity whose probability density follows an asymmetric Laplace distribution. This result agrees with recent experiments by Montaine [Phys. Rev. E 84, 041306 (2011)]PLEEE81539-375510.1103/PhysRevE.84.041306. © 2014 American Physical Society.

Strain and Charge in Epitaxial Graphene on Silicon Carbide Studied by Raman Spectroscopy - Röhrl, Jonas; Hundhausen, Martin; Speck, Florian; Seyller, Thomas (Materials Science Forum, 2010)
The phonon frequencies of epitaxial graphene on silicon carbide (SiC) depend on mechanical strain and charge transfer from the substrate to the epitaxial layer. Strain and doping depend on the preparation process and on the number of graphene layers. We measured the phonon frequencies by Raman spectroscopy and compare the results between epitaxial layers fabricated by high temperature annealing and by hydrogen intercalation of the covalently bound graphene layer of the 6√3 × 6√3 reconstructed SiC surface. Only the latter graphene layer shows tensile strain, which can partly be explained by lattice mismatch between substrate and epitaxial graphene. © (2010) Trans Tech Publications.
UnivIS-Import:2015-03-09:Pub.2010.nat.dphy.IAP.exphys.strain
Keywords: Epitaxial graphene; Raman spectroscopy; Strain

Stress and doping uniformity of laser crystallized amorphous silicon in thin film silicon solar cells - Agaiby, R. M. B.; Becker, M.; Thapa, S. B.; Urmoneit, U.; Berger, A.; Gawlik, A.; Sarau, George; Christiansen, S. H. (Journal of Applied Physics, 2010)
Simultaneous and locally resolved determination of the mechanical stress variation and the free hole concentration using Raman spectroscopy is demonstrated in laser crystallized amorphous silicon layers. Such layers are often used for the fabrication of thin film solar cells, e.g., on borosilicate glass substrates. The combined effects of stress and doping on the Raman signal can be separated based on the use of three wavelengths in the visible. The results show that the free hole concentration in the samples investigated varies between 1×1018 and 1.3×1019 cm-3. Stress as well as the free hole concentration vary substantially within the sample. The stress level varies between 575 and 850 MPa (±12 MPa). Cross-sectional transmission electron microscopy images show the presence of extended lattice defects such as dislocations and grain boundaries in the crystallized Si layer which could account for the lateral stress variations detected by Raman spectroscopy. The impact of film inhomogeneity in terms of stress and doping on the performance of a solar cell will be discussed.
EAM Import::2019-03-13

Stress evolution and cracking of crystalline diamond thin films on ductile titanium substrate: Analysis by micro-Raman spectroscopy and analytical modelling - Rosiwal, Stefan; et al. (Acta Materialia, 2011)
In this study, the cracking of a brittle diamond coating on a titanium substrate under tensile straining was investigated by micro-Raman spectroscopy and analytical modelling. The in-situ micro-Raman investigations showed that the fracture strength of the coating is ∼1.5 GPa, and it contains an initial compressive residual stress of ∼-5.4 GPa which can be reduced by the application of uniaxial tensile strain. At higher strains, the compressive stress was transformed into the tensile regime and cracks appeared in the coating, followed by a reduction in crack spacing and finally delamination. The stress measurements across different cracked coating segments, using Raman spectroscopy, indicated tensile stresses at the middle and compression near the edges of the segment under tensile load. Coating fragmentation leads to a relaxation of the stress within the cracked coating segment. Further cracking of the smaller segments requires larger strains. Here, the classical shear lag model was extended to derive the stress distribution in the coating bonded to the substrate, considering both residual stress and cracking using a fracture criterion. The effect of substrate plasticity on the evolution of residual stress and on the cracking behaviour of the coating is also introduced. A good agreement is found between the modelling approach and the measured failure behaviour as well as the stress distributions in the fractured diamond segments. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserve
Keywords: Analytical methods; Fracture; Raman spectroscopy; Residual stresses; Thin films

Strong phonon-plasmon coupled modes in the graphene/silicon carbide heterosystem - Koch, R. J.; Seyller, Thomas; Schaefer, J. A. (Physical Review B, 2010)
We report on strong coupling of the charge-carrier plasmon extgreekwPL in graphene with the surface-optical phonon extgreekwSO of the underlying SiC(0001) substrate with low-electron concentration (n=1.2×1015 cm-3) in the long-wavelength limit (qII->0). Energy-dependent energy-loss spectra give clear evidence of two coupled phonon-plasmon modes extgreekw± separated by a gap between extgreekwSO(qII->0) and extgreekwTO(qII->0), the transverse-optical-phonon mode, in particular, for higher primary electron energies (E0≥20 eV). A simplified model based on dielectric theory is able to simulate our energy-loss spectra as well as the dispersion of the two coupled phonon-plasmon modes extgreekw±. In contrast, Liu and Willis Phys. Rev. B 81 081406 (2010) postulate in their recent publication no gap and a discontinuous dispersion curve with a one-peak structure from their energy-loss data.
EAM Import::2019-03-13

Structural basis for the shielding function of the dynamic trypanosome variant surface glycoprotein coat - Bartossek, Thomas; Jones, Nicola G.; Schafer, Christin; Cvitkovic, Mislav; Glogger, Marius; Mott, Helen R.; Kuper, Jochen; Brennich, Martha; Carrington, Mark; Smith, Ana-Suncana; Fenz, Susanne; Kisker, Caroline; et al. (Nature Microbiology, 2017)
The most prominent defence of the unicellular parasite Trypanosoma brucei against the host immune system is a dense coat that comprises a variant surface glycoprotein (VSG). Despite the importance of the VSG family, no complete structure of a VSG has been reported. Making use of high-resolution structures of individual VSG domains, we employed small-angle X-ray scattering to elucidate the first two complete VSG structures. The resulting models imply that the linker regions confer great flexibility between domains, which suggests that VSGs can adopt two main conformations to respond to obstacles and changes of protein density, while maintaining a protective barrier at all times. Single-molecule diffusion measurements of VSG in supported lipid bilayers substantiate this possibility, as two freely diffusing populations could be detected. This translates into a highly flexible overall topology of the surface VSG coat, which displays both lateral movement in the plane of the membrane and variation in the overall thickness of the coat.
Keywords: membrane biophysics; membrane trafficking; parasite immune evasion; SAXS

Structural Characterization of Lecithin-Stabilized Tetracosane Lipid Nanoparticles. Part I: Emulsions - Schmiele, Martin; Busch, Sebastian; Morhenn, H.; Schindler, Torben; Schmutzler, Tilo; Schweins, R.; Lindner, P.; Boesecke, P.; Westermann, M.; Steiniger, F.; Funari, Sergio S.; Unruh, Tobias (Journal of Physical Chemistry B, 2016)
The structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-stabilized colloidal tetracosane emulsions was investigated by photon correlation spectroscopy and small-angle X-ray and neutron scattering, using emulsions with different neutron scattering contrasts. Special emphasis was placed on the structure of the DMPC stabilizer layer covering the emulsion droplets. A monolayer, structurally similar to a half DMPC bilayer, with a thickness of 16 å is found. Thereby, the phosphocholine headgroups arrange flat at the oil-water interface. A deep penetration of the tetracosane oil into the stabilizer layer can be ruled ou

Structural Characterization of Lecithin-Stabilized Tetracosane Lipid Nanoparticles. Part II: Suspensions - Schmiele, Martin; Busch, Sebastian; Morhenn, H.; Schindler, Torben; Schmutzler, Tilo; Schweins, R.; Lindner, P.; Boesecke, P.; Westermann, M.; Steiniger, F.; Funari, Sergio S.; Unruh, Tobias (Journal of Physical Chemistry B, 2016)
Using photon correlation spectroscopy, transmission electron microscopy, microcalorimetry, wide-angle X-ray scattering (WAXS), and small-angle X-ray and neutron scattering (SAXS, SANS), the structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-stabilized colloidal tetracosane suspensions was studied from the molecular level to the microscopic scale as a function of the temperature. The platelike nanocrystals exhibit for tetracosane an unusual orthorhombic low-temperature crystal structure. The corresponding WAXS pattern can be reproduced with a predicted orthorhombic unit cell (space group Pca2), which usually occurs only for much longer even-numbered n-alkanes. Special emphasis was placed on the structure of the DMPC stabilizer layer covering the nanocrystals. Their structure was investigated by SAXS and SANS, using suspensions with different neutron scattering contrasts. As for the emulsions in Part I, the crystallized nanoparticles are covered by a DMPC monolayer. Their significant smaller thickness of 10.5 å (for the emulsions in Part I: 16 å) could be related to a more tilted orientation of the DMPC molecules to cover the expanded surface of the crystallized nanoparticle

Structural characterization of the phospholipid stabilizer layer at the solid-liquid interface of dispersed triglyceride nanocrystals with small-angle x-ray and neutron scattering - Schmiele, Martin; Schindler, Torben; Unruh, Tobias; Busch, Sebastian; Morhenn, Humphrey; Westermann, Martin; Steiniger, Frank; Radulescu, Aurel; Lindner, Peter; Schweins, Ralf; Boesecke, Peter (Physical Review E, 2013)
Dispersions of crystalline nanoparticles with at least one sufficiently large unit cell dimension can give rise to Bragg reflections in the small-angle scattering range. If the nanocrystals possess only a small number of unit cells along these particular crystallographic directions, the corresponding Bragg reflections will be broadened. In a previous study of phospholipid stabilized dispersions of β-tripalmitin platelets, the x-ray powder pattern simulation analysis (XPPSA) was developed. The XPPSA method facilitates the interpretation of the rather complicated small-angle x-ray scattering (SAXS) curves of such dispersions of nanocrystals. The XPPSA method yields the distribution function of the platelet thicknesses and facilitates a structural characterization of the phospholipid stabilizer layer at the solid-liquid interface between the nanocrystals and the dispersion medium from the shape of the broadened 001 Bragg reflection. In this contribution an improved and extended version of the XPPSA method is presented. The SAXS and small-angle neutron scattering patterns of dilute phospholipid stabilized tripalmitin dispersions can be reproduced on the basis of a consistent simulation model for the particles and their phospholipid stabilizer layer on an absolute scale. The results indicate a surprisingly flat arrangement of the phospholipid molecules in the stabilizer layer with a total thickness of only 12 Å. The stabilizer layer can be modeled by an inner shell for the fatty acid chains and an outer shell including the head groups and additional water. The experiments support a dense packing of the phospholipid molecules on the nanocrystal surfaces rather than isolated phospholipid domains. © 2013 American Physical Society.

Structural commonalities and deviations in the hierarchical organization of crossed-lamellar shells: A case study on the shell of the bivalve Glycymeris glycymeris - Böhm, Corinna; Demmert, Benedikt; Harris, Joseph; Fey, Tobias; Marin, Frederic; Wolf, Stephan (Journal of Materials Research, 2016)
The structural organization of the palliostracum—the dominant part of the shell which is formed by the mantle cells—of Glycymeris glycymeris (Linné 1758) is comprised of five hierarchical levels with pronounced structural commonalities and deviations from other crossed-lamellar shells. The hierarchical level known as second order lamellae, present within other crossed-lamellar shells, is absent highlighting a short-coming of the currently used nomenclature. On the mesoscale, secondary microtubules penetrate the palliostracum and serve as crack arrestors. Moreover, the growth lamellae follow bent trajectories possibly impacting crack propagation, crack deflection, and energy dissipation mechanisms whilst circumventing delamination. Finally, at least two structural elements are related to external circatidal and circaanular stimuli. This emphasizes that endogeneous rhythms may contribute and (co-)control the self-organization of a complex mineralized tissue and that it is insufficient to rely fully on a reductionistic approach when studying biomineralization.
Keywords: crystal growth; toughness; biomimetic (assembly)

Structural factors controlling size reduction of graphene oxide in liquid processing - Nacken, Thomas; Halbig, Christian E.; Wawra, Simon; Damm, Cornelia; Romeis, Stefan; Walter, Johannes; Tehrani, Mohammad J.; Hu, Yichen; Ishii, Yoshitaka; Eigler, Siegfried; Peukert, Wolfgang (Carbon, 2017)
Graphene oxide materials can be prepared in manifold synthesis approaches and allow for further post-functionalization to adopt the properties of the desired application. Small sized sheets with lateral dimensions on the 100 nm scale are e.g. beneficial for cell applications. Breakage of sheets by liquid phase processing is an ideal way to generate such nano-sized sheets. However, until now it is unknown how functionalization influences the breakage behavior. We have chosen single layered oxo-functionalized graphene (oxo-G) derivatives with lattice defects < 1% to evaluate their size reduction rate upon ultrasonication of flakes with respect to the type and degree of functionalization of the graphene sheets. Lateral dimensions of the processed sheets were quantitatively determined in solution by analytical ultracentrifugation. The highest size reduction rate is observed for pristine oxo-G, which bears the highest degree of functionalization of around 60% and the negatively charged organosulfate group. Partial hydrolysis of organosulfate, or conversion of surface-functional groups to hydroxyl-groups in majority or partial defunctionalization lead to size-reduction rates which are up to 25% lower in comparison with the initial oxo-G. Moreover, the size reduction rate can be correlated in first approximation also with UV–vis spectroscopy.

Structural Investigations of Self-Assembled Monolayers for Organic Electronics: Results from X-ray Reflectivity - Khassanov, Artöm; Steinrück, Hans-Georg; Schmaltz, Thomas; Magerl, Andreas; Halik, Marcus (Accounts of Chemical Research, 2015)
Self-assembled monolayers (SAMs) have been established as crucial interlayers and electronically active layers in organic electronic devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), organic thin film transistors (OTFTs), and nonvolatile memories (NVMs). The use of self-assembling functionalized organic molecules is beneficial due to mainly three advantages compared with common thin film deposition approaches. (1) Molecular self-assembly occurs with surface selectivity, determined by the interaction between the functional anchor group of the organic molecules and the target surface. (2) The film thickness of the resulting layers is perfectly controllable on the angstrom scale, due to the self-terminating film formation to only a single molecular layer. And finally, (3) the wide variability in the chemical structure of such molecules enables different SAM functionalities for devices, ranging from electrical insulation to charge storage to charge transport. The SAM approach can be further expanded by employing several functionalized molecules to create mixed SAMs with consequently mixed properties. The function of SAMs in devices depends not only on the chemical structure of the molecules but also on their final arrangement and orientation on the surface. A reliable and nondestructive in-depth characterization of SAMs on nonconductive oxide surfaces is still challenging because of the very small thickness and the impracticality of methods such as scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In this Account, we illustrate how X-ray reflectivity (XRR) provides analytical access to major questions of SAM composition, morphology, and even formation by means of investigations of pure and mixed SAMs based on phosphonic acids (PAs) of various chain structures on flat alumina (AlOx) surfaces. XRR is an analytical method that provides access to spatially averaged structural depth profiles over a relatively large area of several square micrometers. The key outcome of XRR, the surface-normal electron density profile of the SAMs, leads to precise information on the SAM thickness with subangstrom resolution and allows for the determination of molecular tilt angles and packing densities. We have systematically increased the chemical complexity of PA molecules and the resulting SAMs, utilizing XRR to provide insight into the SAM structures. In SAMs composed of functionalized molecules or complex chain structures, the distribution of electron rich and electron poor signatures is detected and thus the molecular order within the SAM is determined. In mixed SAMs of two different molecular species, electron density profiles reveal the morphology and how the surface-normal structure changes if one component of the mixed SAM is altered. Furthermore, XRR was applied to investigate in situ the self-assembly of functionalized PA molecules from solution by tracking the monolayer growth over time. Even though the results provided by XRR on in-plane molecular arrangement are limited, it presents excellent information on the molecular scale along the surface normal and in addition allows for drawing conclusions on the intermolecular interactions within the SAM.

Structural quantities of quasi-two-dimensional fluids - Lang, Simon; Franosch, Thomas; Schilling, Rolf (Journal of Chemical Physics, 2014)
Quasi-two-dimensional fluids can be generated by confining a fluid between two parallel walls with narrow separation. Such fluids exhibit an inhomogeneous structure perpendicular to the walls due to the loss of translational symmetry. Taking the transversal degrees of freedom as a perturbation to an appropriate 2D reference fluid we provide a systematic expansion of the m-particle density for arbitrary m. To leading order in the slit width this density factorizes into the densities of the transversal and lateral degrees of freedom. Explicit expressions for the next-to-leading order terms are elaborated analytically quantifying the onset of inhomogeneity. The case m = 1 yields the density profile with a curvature given by an integral over the pair-distribution function of the corresponding 2D reference fluid, which reduces to its 2D contact value in the case of pure excluded-volume interactions. Interestingly, we find that the 2D limit is subtle and requires stringent conditions on the fluid-wall interactions. We quantify the rapidity of convergence for various structural quantities to their 2D counterparts.
EAM Import::2019-03-13

Structure and interactions in benzamide molecular crystals - Ectors, Philipp; Ectors, Dominique; Zahn, Dirk (Molecular Simulation, 2013)
In this study, we resolved discrepancies concerning the experimentally determined structure of benzamide molecular crystals from dispersion-corrected density functional calculations. A clear energy ranking was obtained for the two candidates of the stable (P1) modification of benzamide. This was rationalised by subtle differences of the molecular interactions in the molecular crystal. The potential energy of the different structures was dominated by the interplay of intermolecular attraction and molecular torsion/deformation to accommodate favourable hydrogen-bonded networks. Using suitable proxies arranged in pseudo-crystalline set-ups, we discriminated the contribution of electrostatics, π-π interactions and intra-molecular interactions to the lattice energies. © 2013 Taylor and Francis.
UnivIS-Import:2015-03-09:Pub.2013.nat.dgeo.IGM.LM.struct
Keywords: crystal structure; density-functional theory calculations; molecular interactions; polymorphism

Structure-Dependent Anchoring of Organic Molecules to Atomically Defined Oxide Surfaces: Phthalic Acid on Co3O4(111), CoO(100), and CoO(111) - Xu, Tao; Schwarz, Matthias; Werner, Kristin; Mohr, Susanne; Amende, Maximilian; Libuda, Jörg (Chemistry - A European Journal, 2016)
We have performed a model study to explore the influence of surface structure on the anchoring of organic molecules on oxide materials. Specifically, we have investigated the adsorption of phthalic acid (PA) on three different, well-ordered, and atomically defined cobalt oxide surfaces, namely 1)Co3O4(111), 2)CoO(111), and 3)CoO(100) on Ir(100). PA was deposited by physical vapor deposition (PVD). The formation of the PA films and interfacial reactions were monitored in situ during growth by isothermal time-resolved IR reflection absorption spectroscopy (TR-IRAS) under ultrahigh vacuum (UHV) conditions. We observed a pronounced structure dependence on the three surfaces with three distinctively different binding geometries and characteristic differences depending on the temperature and coverage. 1)PA initially binds to Co3O4(111) through the formation of a chelating bis-carboxylate with the molecular plane oriented perpendicularly to the surface. Similar species were observed both at low temperature (130K) and at room temperature (300K). With increasing exposure, chelating mono-carboxylates became more abundant and partially replaced the bis-carboxylate. 2)PA binds to CoO(100) in the form of a bridging bis-carboxylate for low coverage. Upon prolonged deposition of PA at low temperature, the bis-carboxylates were converted into mono-carboxylate species. In contrast, the bis-carboxylate layer was very stable at 300K. 3)For CoO(111) we observed a temperature-dependent change in the adsorption mechanism. Although PA binds as a mono-carboxylate in a bridging bidentate fashion at low temperature (130K), a strongly distorted bis-carboxylate was formed at 300K, possibly as a result of temperature-dependent restructuring of the surface. The results show that the adsorption geometry of PA depends on the atomic structure of the oxide surface. The structure dependence can be rationalized by the different arrangements of cobalt ions at the three surfaces.
Keywords: adsorption; carboxylic acids; cobalt; IR spectroscopy; thin films

Structure-Dependent Dissociation of Water on Cobalt Oxide - Schwarz, Matthias; Faisal, Firas; Mohr, Susanne; Hohner, Chantal; Werner, Kristin; Xu, Tao; Skala, Tomas; Tsud, Nataliya; Prince, Kevin C.; Matolin, Vladimir; Lykhach, Yaroslava; Libuda, Jörg (Journal of Physical Chemistry Letters, 2018)
Understanding the correlation between structure and reactivity of oxide surfaces is vital for the rational design of catalytic materials. In this work, we demonstrate the exceptional degree of structure sensitivity of the water dissociation reaction for one of the most important materials in catalysis and electrocatalysis. We studied H2O on two atomically defined cobalt oxide surfaces, CoO(100) and Co3O4(111). Both surfaces are terminated by O2- and Co2+ in different coordination. By infrared reflection absorption spectroscopy and synchrotron radiation photoelectron spectroscopy we show that H2O adsorbs molecularly on CoO(100), while it dissociates and forms very strongly bound OH and partially dissociated (H2O)(n)(OH)(m) clusters on Co3O4(111). We rationalize this structure dependence by the coordination number of surface Co2+. Our results show that specific well-ordered cobalt oxide surfaces interact very strongly with H2O whereas others do not. We propose that this structure dependence plays a key role in catalysis with cobalt oxide nanomaterials.

Structure, morphology and interface properties of ultrathin SnTTBPP(OH)2-films adsorbed on Ag(100) - Schmidt, Norman Anja; Hub, Christian; Gnichwitz, Jan-Frederik; Hieringer, Wolfgang; Hirsch, Andreas; Fink, Rainer (Physical Chemistry Chemical Physics, 2011)
The molecular interaction of dihydroxo[5,10,15,20-tetrakis(4-tert-butyl- phenyl)porphyrinato]-tin(iv) (SnTTBPP(OH)), the structural order and growth of ultrathin films on Ag(100) have been studied by means of low-energy electron diffraction (LEED) and synchrotron based photoelectron spectroscopy, i.e., X-ray photoemission (XPS) and near-edge X-ray absorption fine structure (NEXAFS/XANES) spectroscopy. For the first time, monolayer adsorption of a metalloporphyrin with octahedral coordination of the metal center by two additional axial hydroxo ligands is investigated in a multi-technique study. The delicate balance of molecule-substrate interactions and intermolecular interactions leads to the formation of a densely-packed organic monolayer which is commensurate with the Ag(100) substrate. From NEXAFS linear dichroism an almost coplanar orientation of the porphyrin system is derived. XPS and NEXAFS clearly indicate that the axial hydroxo ligands are cleaved in monolayer films, i.e., upon adsorption to the Ag substrate. With increasing film thickness orientational order gets lost and leads to polycrystalline growth for thicker films as confirmed by scanning X-ray transmission microscopy (STXM). © 2011 the Owner Societies.

Structure of a three-dimensional nano-powder subjected to repeated fragmentation and sedimentation - Topic, Nikola; Weuster, Arnulf; Pöschel, Thorsten; Wolf, Dieter (New Journal of Physics, 2015)
We revisit the problem of the structure of a nano-powder subjected to repeated fragmentation and sedimentation, and extend the analysis to the more relevant three-dimensional (3D) case. One important question not addressed previously is how the fractal dimension and dust exponent depend on space dimension. We find that the qualitative behavior of the nano-powder in three dimensions is similar to that in two dimensions. But the fractal dimension changes from 1.6 ± 0.1 in two dimensions to 2.1 ± 0.1 in three dimensions. The scaling relation between the fractal dimension and the dust exponent characterizing the fragment size distribution is the same as in two dimensions. The universality of these exponents is addressed by comparing the results with a much simpler lattice model. Although the different settling kinetics of the fragments leads to different anisotropies, the fractal properties are not affected.
Keywords: fractals; granular systems; macroscopic aggregates; pourous materials; powders

Studying the dynamic behaviour of porphyrins as prototype functional molecules by scanning tunnelling microscopy close to room temperature - Marbach, Hubertus; Steinrück, Hans-Peter (Chemical Communications, 2014)
Scanning tunnelling microscopy (STM) enables us to directly observe the dynamic behaviour of organic molecules on surfaces. White imaging atoms and molecules using STM is certainly fascinating by itself, corresponding temperature-dependent measurements allow for the quantitative determination of the energetics and kinetics of the underlying molecular surface processes. Herein, we review recent advances in the STM investigation of the dynamic behaviour of adsorbed porphyrins at and close to room temperature. Three different case studies are discussed, providing insight into the dynamics of diffusion, rotation, reaction, and molecular switching at surfaces, based on isothermal STM measurements. The reviewed examples demonstrate that variable temperature STM can be a suitable toot to directly monitor the dynamic behaviour of individual adsorbed molecules, at and close to room temperature. Free base porphyrins on Cu(111) proved to be particularly suitable for these studies due to the strong bonding interaction between the iminic nitrogen atoms in the porphyrin macrocycle and the Cu substrate atoms. As a consequence, the corresponding activation energies for surface diffusion, self-metalation reaction and conformational switching are of a magnitude that allows for monitoring the processes at and around room temperature, in contrast to most previous studies, which were performed at cryogenic temperatures. The kinetic analysis of the surface diffusion and setf-metalation was performed using an Arrhenius approach, yielding the corresponding activation energies and preexponential factors. In contrast, the conformational switching process was analysed in the framework of transition state theory, based on the Eyring equation. This approach provides a more detailed insight into interpretable thermodynamic potentials, i.e., the enthalpic and entropic contributions to the activation barrier. The analysis shows that at room temperature the adsorption and switching behaviour of the investigated free base porphyrin on Cu(111) is dominated by entropic effects. Since the entropic energy contribution vanishes at low temperatures, the importance of experiments conducted at temperatures close to room temperature is emphasized.

Study of the chemical Stability of Cu-3(btc)(2) (HKUST-1) due N-2-absorption, x-ray powder diffraction and EPR spectroscopy - Jee, Bettina; Himsl, Dieter; Icker, Maik; Hartmann, Martin; Pöppl, Andreas (Chemie Ingenieur Technik, 2010)

Study of the effects of inner pressure and surface tension on the fibre drawing process with the aid of an analytical asymptotic fibre drawing model and the numerical solution of the full N.-St. equations - Luzi, Giovanni; Epple, Philipp; Rauh, Cornelia; Delgado, Antonio (Archive of Applied Mechanics, 2013)
Micro-structured 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 the case of annular capillaries. A fluid mechanics model suggested in the literature that uses asymptotic analysis based on the small aspect ratio of capillaries has been compared with the full 3D set of the N.-St. equations, for modelling the fabrication of capillaries. The final asymptotic equations, analysed in some asymptotic limits, are solved numerically and then compared with the N.-St. solutions, obtained with a commercial finite elements solver. These asymptotic limits give valuable practical information about the control parameters that influence the drawing process, taking into account the effects of surface tension and inner pressure, since those are of essential importance during drawing. It is shown that the asymptotic method delivers reliable results as long as the inner pressure does not exceed too high values. © 2013 Springer-Verlag Berlin Heidelberg.
UnivIS-Import:2015-03-09:Pub.2013.tech.ITC.stmmec.studyo
Keywords: Asymptotic analysis; Holey fibres; Mathematical modelling; Micro-structured optical fibres (MOFs); Numerical methods; Photonic crystal fibres

Study on the deformation mechanics of hard brittle coatings on ductile substrates using in-situ tensile testing and cohesive zone FEM modeling - Rehman, Hamad Ur; Ahmed, Furqan; Schmid, Christoph; Schaufler, Jens; Durst, Karsten (Surface & Coatings Technology, 2012)
In the present work, the mechanics of the stress transfer from a ductile substrate to a brittle coating and the failure of coating segments by normal cracking and delamination are studied using in-situ tensile testing and cohesive zone finite element modeling. The in-situ tensile tests were performed in a scanning electron microscope (SEM) on a diamond coated Ti-substrate and hydrogenated amorphous carbon (a-C:H) coatings on two different steel substrates, with different work hardening behavior. These brittle coatings form cracked coating segments during the in-situ tensile test. The stress transfer from the substrate to the coating segments is dependent on the elastic and elastic/plastic properties of coating and substrate. Furthermore, the segmentation of the coating together with the delamination depends strongly on the film and substrate properties. Using cohesive zone finite element modeling, the influence of substrate and coating properties on the stress transfer, segmentation of the films and on the delamination is discussed. It was found that the saturation in spacing of the cracked coating segments is either cause by a localization of the plastic strain in the substrate or by a delamination of the coating segments. Delamination only occurs, when the flow stress of the substrate is about √3 times larger than the interfacial shear strength.
WW1::945

Sub-bandgap photon harvesting for organic solar cells via integrating up-conversion nanophosphors - Chen, Wei; Hou, Yi; Osvet, Andres; Guo, Fei; Kubis, Peter; Batentschuk, Miroslaw; Winter, Benjamin; Spiecker, Erdmann; Forberich, Karen; Brabec, Christoph (Organic Electronics, 2015)
Lanthanide-doped up-conversion nanophosphors were employed as up-conversion converters (UC-converters) combined with organic solar cells (OSCs). The optical properties of the external up-conversion layers were investigated, and the optimized layers were laminated with OSCs. Strong photocurrents were observed when OSCs with an UC converter were illuminated by a NIR laser. An improvement of photocurrent and efficiency were also observed under AM 1.5 G sun irradiation. Our investigations show a proof-of-concept that organic solar cells could utilize sub-bandgap photons via integrating up-conversion nanophosphors, which provide us a promising approach to exceed the efficiency limit of single junction organic solar cells.
Keywords: IR light harvesting; Nanophosphors; Organic photovoltaics; Up-conversion

Subharmonic instability of a self-organized granular jet - Kollmer, Jonathan; Pöschel, Thorsten (Scientific Reports, 2016)
Downhill flows of granular matter colliding in the lowest point of a valley, may induce a selforganized jet. By means of a quasi two-dimensional experiment where fine grained sand flows in a vertically sinusoidally agitated cylinder, we show that the emergent jet, that is, a sheet of ejecta, does not follow the frequency of agitation but reveals subharmonic response. The order of the subharmonics is a complex function of the parameters of driving.

Substituting TiCl4-Carbon Nanohorn Interfaces for Dye-Sensitized Solar Cells - Casillas Pacheco, Ruben; Lodermeyer, Fabian; Costa, Ruben Dario; Prato, Maurizio; Guldi, Dirk Michael (Advanced Energy Materials, 2013)
An easy-to-implement strategy to introduce carbon nanohorns as interlayers in the photoelectrode of a dye-sensitized solar cell without affecting its overall performance is presented. This provides a clean, easy, and ecofriendly alternative to achieve highly efficient cells.

Substrate-mediated phase separation of two porphyrin derivatives on Cu(111) - Buchner, Florian; Zillner, Elisabeth; Röckert, Michael; Gläßel, Stefanie; Steinrück, Hans-Peter; Marbach, Hubertus (Chemistry - A European Journal, 2011)
Sorting molecules: Effective phase separation of two porphyrin species, namely CoTPP and 2HTPP (TPP= tetraphenylporphyrin, 2HTPP=2H- tetraphenylporphyrin), on Cu(111) is reported. The separation process can be conclusively explained by the domination of molecule-molecule interactions for CoTPP, whereas the adsorption behavior of 2HTPP is dominated by molecule-substrate interactions (see figure). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Sulfur oxidation on Pt(355): It is the steps! - Streber, Regine; Papp, Christian; Lorenz, Michael Peter Andreas; Bayer, Andreas; Denecke, Reinhard; Steinrück, Hans-Peter (Angewandte Chemie International Edition, 2009)
Steps in the right direction: The role of steps in the removal of sulfur from a Pt model catalyst via metastable SO and SO species, was demonstrated by in situ high-resolution X-ray photoelectron spectroscopy. From isothermal experiments the activation energy of the rate-determining step was deduced to be 34 kJmol. © 2009 Wiley-VCH Verlag GmbH &. Co. KGaA.

Sulfur species in graphene oxide - Eigler, Siegfried; Dotzer, Christoph; Hof, Ferdinand; Bauer, Walter; Hirsch, Andreas (Chemistry - A European Journal, 2013)
The structure of graphene oxide (GO) is of crucial importance for its chemical functionalization. However, the sulfur content present in GO prepared by Hummers' method has only been addressed by a few authors so far. It has been reported that hydrolysis of sulfur species takes place and that stable sulfonic groups are present in graphite oxide. In this manuscript, in contrast to earlier reports, sulfate species are identified that are covalently bound to GO and still present after extensive aqueous work-up. Additionally, we exclude the possibility that sulfonic groups are present in GO as major species after aqueous work up. Our results are based on bulk characterization of graphene oxide by thermogravimetry and subsequent analysis of the decomposition products using mass spectroscopy and infrared spectroscopy. Up to now, the combustion temperature between 200 and 300°C remained almost unaddressed. In a temperature dependant experiment we reveal two main decomposition steps that differ in temperature and that are closely related to the sulfur species in GO. While the decomposition, between 200 and 300°C, is related to the degradation of organosulfate, the other one, between 700 and 800°C, is assigned to the pyrolysis of inorganic sulfate. Furthermore, organosulfate is to some extent responsible for the reactivity of GO. Therefore, the structural model of GO was extended by adding organosulfate in addition to epoxy and hydroxyl groups, which are predominantly covalently bound above and below the carbon skeleton. Furthermore, the identification of organosulfate groups beneath epoxy groups makes new molecular architectures feasible and can be used to explain the properties of GO in various applications. Sulfur on the surface: Organosulfate was found to be an essential part of the structure of graphene oxide (see figure), which was prepared according to Hummers' protocol. These organosulfate groups play an important role in the immobilization of adsorbed species and influence the chemical reactivity of the graphene oxide. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Supported homogeneous catalyst makes its own liquid phase - Kaftan, Andre; Schönweiz, Andreas; Nikiforidis, Ioannis; Hieringer, Wolfgang; Dyballa, Katrin M.; Franke, Robert; Görling, Andreas; Libuda, Jörg; Wasserscheid, Peter; Laurin, Mathias; Haumann, Marco (Journal of Catalysis, 2015)
2,2'-((3,3'-di-tert-butyl-5,5'-dimethoxy-[1,1'-biphenylj-2,2'-diyl)-bis(oxy))bis(4,4,5,5-tetraphenyl-1,3,2-dioxaphospholane) modified rhodium complexes molecularly adsorbed onto porous silica powder show surprisingly high activity and regioselectivity in the gas-phase hydroformylation of propene to but-anal, with no sign of deactivation. Operando IR investigations combined with density functional theory calculations confirm a side reaction: the aldol condensation of the butanal products. These heavier by-products accumulate inside the pores of the catalytic material. IR and gas chromatography show a direct relation between formation of enones, products of the aldol condensation, performance, and stability of the catalytic system. This demonstrates that the aldol condensation products generated in situ act as a solvent providing an ideal environment to the impregnated homogeneous catalyst. (c) 2014 Elsevier Inc. All rights reserved.
Keywords: Hydroformylation; Supported homogeneous catalysis; Operando spectroscopy; Infrared spectroscopy; Density functional theory; Silicon dioxide; Rhodium

Supported ionic liquid phase (SILP) catalyzed hydroformylation of 1-butene in a gradient-free loop reactor - Haumann, Marco; Jakuttis, Michael; Werner, Sebastian; Wasserscheid, Peter (Journal of Catalysis, 2009)
The supported ionic liquid phase (SILP) catalysis technology was applied to gas-phase hydroformylation of 1-butene using sulfoxantphos 1 modified rhodium complexes. Kinetic experiments were performed in a fixed bed reactor and compared to a gradient-free gas-phase loop reactor (Berty type). The influence of substrate concentration, temperature and syngas pressure was determined. Data from fixed bed and Berty reactor were found to be in good agreement with respect to activation energy and reaction order. Ex-situ NMR studies of fresh and used SILP catalysts confirmed that the ligand remained intact after prolonged time on stream.

Supported ionic liquids on non-porous and porous inorganic materials-A topical review - Selvam, Thangaraj; Machoke, Albert; Schwieger, Wilhelm (Applied Catalysis A-General, 2012)
A topical review of the use of non-porous and porous inorganic materials as supports for the immobilization of ILs and their catalytic applications is given. The basic principles and the supports that are being used in various concepts emerging from supported ionic liquids (SILs) including supported ionic liquid films (SILFs), supported ionic liquid catalysis/catalysts (SILC/SILCAs), solid catalysts with ILs (SCILs), solid catalysts with IL layer (SCILL), supported IL nanoparticles (SILnPs) and supported ionic liquid phase (SILP) catalysts (SILPC) are first discussed. In addition, we provide a comprehensive review of the SILs concept dealing with inorganic supports, such as layered silicates, porous glass, zeolites, mesoporous (MCM) and PMO (periodic mesoporous organosilica) materials; and their applications. (C) 2012 Elsevier B.V. All rights reserved.
Keywords: Ionic liquids; Supported ionic liquids; Inorganic supports; Non-porous and porous materials

Support nanostructure boosts oxygen transfer to catalytically active platinum nanoparticles - Vayssilov, Georgi N.; Lykhach, Yaroslava; Migani, Annapaola; Staudt, Thorsten; Petrova, Galina P.; Tsud, Nataliya; Skala, Tomas; Bruix, Albert; Illas, Francesc; Prince, Kevin C.; Matolin, Vladimir; Neyman, Konstantin M.; et al. (Nature Materials, 2011)
Interactions of metal particles with oxide supports can radically enhance the performance of supported catalysts. At the microscopic level, the details of such metal-oxide interactions usually remain obscure. This study identifies two types of oxidative metal-oxide interaction on well-defined models of technologically important Pt-ceria catalysts: (1) electron transfer from the Pt nanoparticle to the support, and (2) oxygen transfer from ceria to Pt. The electron transfer is favourable on ceria supports, irrespective of their morphology. Remarkably, the oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt and, thus, is inherently a nanoscale effect. Our findings enable us to detail the formation mechanism of the catalytically indispensable Pt-O species on ceria and to elucidate the extraordinary structure-activity dependence of ceria-based catalysts in general.

Suppression of Hysteresis Effects in Organohalide Perovskite Solar Cells - Hou, Yi; Scheiner, Simon; Tang, Xiaofeng; Gasparini, Nicola; Richter, Moses; Li, Ning; Schweizer, Peter; Chen, Shi; Chen, Haiwei; Ramírez Quiroz, César Omar; Du, Xiaoyan; Matt, Gebhard; et al. (Advanced Materials Interfaces, 2017)
Thin-film solar cell based on hybrid perovskites shows excellent light-to-power conversion efficiencies exceeding 22%. However, the mixed ionic-electronic semiconductor hybrid perovskite exhibits many unusual properties such as slow photocurrent instabilities, hysteresis behavior, and low-frequency giant capacitance, which still question us so far. This study presents a direct surface functionalization of transparent conductive oxide electrode with an ultrathin ≈2 nm thick phosphonic acid based mixed C60/organic self-assembled monolayer (SAM) that significantly reduces hysteresis. Moreover, due to the strong phosphonates bonds with indium tin oxide (ITO) substrates, the SAM/ITO substrates also exhibit an excellent recyclability merit from the perspective of cost effectiveness. Impedance studies find the fingerprint of an ion-based diffusion process in the millisecond to second regime for TiO2-based devices, which, however, is not observed for SAM-based devices at these low frequencies. It is experimentally demonstrated that ion migration can be considerably suppressed by carefully engineering SAM interfaces, which allows effectively suppressing hysteresis and unstable diode behavior in the frequency regime between ≈1 and 100 Hz. It is suggested that a reduced density of ionic defects in combination with the absence of charge carrier accumulation at the interface is the main physical origin for the reduced hysteresis.

Suppression of the critical angle of diffraction in thin-film colloidal photonic crystals - Romanov, Sergei G.; Peschel, Ulf; Bardosova, Maria; Essig, Sabine; Busch, Kurt (Physical Review B, 2010)
We present an experimental and numerical investigation of the polarization anisotropy of the zero-order backdiffracted light from three-dimensional thin-film photonic crystals assembled from colloidal spheres. In particular, we compare simulations of reflectance spectra from perfectly ordered fcc lattice of spheres with measured reflectance data from self-organized opal films and forced-assembled Langmuir-Blodgett crystal films. We identify cross-polarization couplings and interactions between photonic crystal eigenmodes as the major physical mechanisms for resonance depolarization effects. Based on this, we find that a necessary condition for the observation of critical angles of diffraction in three-dimensional lattices is that the orientation of the light's plane of incidence coincides with a high-symmetry plane of the crystal lattice. We further show that for currently achievable colloidal photonic crystals with moderate refractive index contrast, this resonance depolarization mechanisms together with the destructive influence of lattice disorder effectively renders the meaning of a critical angle of diffraction obsolete.
EAM Import::2019-03-13

Suppression of Thermally Induced Fullerene Aggregation in Polyfullerene-Based Multiacceptor Organic Solar Cells - Dowland, Simon A.; Salvador, Michael Filipe; Perea Ospina, Jose Dario; Gasparini, Nicola; Langner, Stefan; Rajoelson, Sambatra; Ramanitra, Hasina H.; Lindner, Benjamin; Osvet, Andres; Brabec, Christoph; Hiorns, Roger C.; Egelhaaf, Hans-Joachim (ACS Applied Materials and Interfaces, 2017)
A novel main-chain polyfullerene, poly[fullerene-alt-2,5-bis(octyloxy)terephthalaldehyde] (PPC4), is investigated for its hypothesized superior morphological stability as an electron-accepting material in organic photovoltaics relative to the widely used fullerene phenyl-C61-butyric acid methyl ester (PCBM). When mixed with poly(3-hexylthiophene-2,5-diyl) (P3HT), PPC4 affords low-charge-generation yields because of poor intermixing within the blend. The adoption of a multiacceptor system, by introducing PCBM into the P3HT:polyfullerene blend, was found to lead to a 3-fold enhancement in charge generation, affording power conversion efficiencies very close to that of the prototypical P3HT:PCBM binary control. Upon thermal stressing and in contrast to the P3HT:PCBM binary, photovoltaic devices based on the multiacceptor system demonstrated significantly improved stability, outperforming the control because of suppression of the PCBM migration and aggregation processes responsible for rapid device failure. We rationalize the influence of the fullerene miscibility and its implications on the device performance in terms of a thermodynamic model based on Flory-Huggins solution theory. Finally, the potential universal applicability of this approach for thermal stabilization of organic solar cells is demonstrated, utilizing an alternative low-band-gap polymer-donor system.
Keywords: fullerene aggregation; main-chain polyfullerenes; multi acceptor composite blend; organic photovoltaics; thermal stability

Supramolecular assembly of self-labeled amphicalixarenes - Becherer, Miriam; Schade, Boris; Boettcher, Christoph; Hirsch, Andreas (Chemistry - A European Journal, 2009)
The synthesis and precise supramolecular organization of new amphicalixarene 4 bearing four rodlike aligned fluorescent terephthalic benzamide moieties in the upper rim is reported. The aggregation of 4 was monitored by a combination of fluorescence, conductometry, and cryo-TEM measurements at different pH values and revealed significant structural differences. Most interestingly, we found that exactly 12 molecules of 4 assemble to form a spherical and structurally persistent micelle at pH 7, which coexists with rodlike micelles. In contrast to other examples of structurally defined micelles reported previously, this new type of self-labeled amphiphile serves as a fluorescence reporter with the terephthalic units additionally providing an extended cavity structure in the hydrophilic part, which facilitates the inclusion of guest molecules. In addition to the observed formation of well-defined micelles, the other highly important finding from this study is the fact that guest molecules directly influence the micellar organization because they can interact with both the free amphiphiles below the critical micelle concentration (cmc) or the micellar aggregate itself. These two types of interactions are especially pronounced in the case of the cationic pyrene derivative 10, which binds electrostatically with the amphiphile 4. In addition, a very unique membrane structure exhibiting a regular hexagonal pattern of 5 nm pores is formed by 4 at pH 4. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

Supramolecular order and structural dynamics: A STM study of 2H-tetraphenylporphycene on Cu(111) - Stark, Michael Werner; Träg, Johannes; Ditze, Stefanie; Brenner, Wolfgang; Jux, Norbert; Steinrück, Hans-Peter; Marbach, Hubertus (Journal of Chemical Physics, 2015)
The adsorption of 2H-tetraphenylporphycene (2HTPPc) on Cu(111) was investigated by scanning tunneling microscopy (STM). At medium coverages, supramolecular ordered islands are observed. The individual 2HTPPc molecules appear as two pairs of intense protrusions which are separated by an elongated depression. In the islands, the molecules are organized in rows oriented along one of the close packed Cu(111) substrate rows; the structure is stabilized by T-type interactions of the phenyl substituents of neighboring molecules. Two types of rows are observed, namely, highly ordered rows in which all molecules exhibit the same orientation, and less ordered rows in which the molecules exhibit two perpendicular orientations. Altogether, three different azimuthal orientations of 2HTPPc are observed within one domain, all of them rotated by 15° ± 1° relative to one closed packed Cu direction. The highly ordered rows are always separated by either one or two less ordered rows, with the latter structure being the thermodynamically more stable one. The situation in the islands is highly dynamic, such that molecules in the less ordered rows occasionally change orientation, also complete highly ordered rows can move. The supramolecular order and structural dynamics are discussed on the basis of the specific molecule-substrate and molecule-molecule interactions.

Surface analysis of Pd/ZnO catalysts dispersed on micro-channeled Al-foils by XPS - Dumbuya, K.; Denecke, Reinhard; Steinrück, Hans-Peter (Applied Catalysis A-General, 2008)
We have investigated the surface composition of Pd/ZnO catalysts dispersed on micro-channeled Al-foils in a non-activated and in the activated state, between 300 and 700 K using X-ray photoelectron spectroscopy (XPS). While the surface of the non-activated catalyst, as obtained after calcination, is composed of PdO, the surface of the activated (reduced) catalyst consists of predominantly PdZn and ∼30% metallic Pd. When increasing the temperature up to 700 K in UHV significant structural surface changes were observed for both samples. For the non-activated sample, PdO was reduced mainly to the Pd metal with some fraction of PdZn alloy. For the activated catalyst, the fraction of metallic Pd was also converted to PdZn, yielding a pure alloy, which is stable to at least 600 K. At 700 K both samples show changes due to segregation of Mg from the micro-channeled Al-foils onto which the powdered catalysts were deposited, which contains 3% Mg. © 2008 Elsevier B.V. All rights reserved.

Surface-Assisted Organic Synthesis of Hyperbenzene Nanotroughs - Fan, Qitang; Wang, Cici; Han, Yong; Zhu, Junfa; Hieringer, Wolfgang; Kuttner, Julian; Hilt, Gerhard; Gottfried, J. Michael (Angewandte Chemie International Edition, 2013)
A hexagonal macrocycle consisting of 18 phenylene units (hyperbenzene) was synthesized on a Cu(111) surface in ultrahigh vacuum by Ullmann coupling of six 4,4′′-dibromo-m-terphenyl molecules. The large diameter of 21.3Å and the ability to assemble in arrays makes hyperbenzene an interesting candidate for a nanotrough that could enclose metallic, semiconducting, or molecular quantum dots.
EAM Import::2019-03-12

Surface characterization of functionalized imidazolium-based ionic liquids - Kolbeck, Claudia; Killian, Manuela; Maier, Florian; Paape, Natalia; Wasserscheid, Peter; Steinrück, Hans-Peter (Langmuir, 2008)
The surface composition of oligo(ethylene glycol) ether functionalized bis(trifluoromethylsulfonyl)imide ionic liquids has been studied by means of X-ray photoelectron spectroscopy (XPS). For [Me(EG)MIM] [TfN], [Et(EG)MIM]-[TfN], and [Me(EG)MIM][Tf N], which vary by the number of ethylene glycol (EG) units (from 1 to 3), we have shown that the stoichiometry of the surface near region is in excellent agreement with the bulk stoichiometry, which confirms the high purity of the ionic liquid samples investigated and rules out pronounced surface orientation effects. This has been deduced from the experimental observation that the angle-resolved XP spectra of all elements present in the IL anions and cations (C, N, O, F, S) show identical signals in the bulk and surfaces sensitive geometry, i.e., at 0° and 70° emission angle, respectively. The relative intensity ratios of all elements were found to be in nearly perfect agreement with the nominal values for the individual ILs. In contrast to these findings, we identified surfaceactive impurities in [Me(EG)MIM]I, which is the starting material for the final anion exchange step to synthesize [Me(EG)MIM][Tf2N]. Sputtering of the surface led to a depletion of this layer, which however recovered with time. The buildup of this contamination is attributed to a surface enrichment of a minor bulk contamination that shows surface activity in the iodide melt. © 2008 American Chemical Society.

Surface Charging and Interfacial Water Structure of Amphoteric Colloidal Particles - Peukert, Wolfgang; Braunschweig, Björn; Sauerbeck, Christian (Journal of Physical Chemistry C, 2014)
Colloidal stability and adsorption processes at particle surfaces are controlled by the generation of surface charge and the molecular structure of the resulting electrical double layers. We have applied second-harmonic light scattering (SHS) to address doublelayer charging and the orientation of water molecules at surfaces of amphoteric particles in unprecedented detail. For that purpose we have performed the first SHS experiments of neat polystyrene particles with a mixture of sulfate (R−O−SO3-) and amino (R-NH2) functional groups. Changing the pH has a dramatic effect on the surface potential which can be tuned from positive to negative values and has allowed us to record changes in SHS signal while the isoelectric point of the particles is being crossed. The SHS signal is a function of two contributions which are directly related to the first molecular layer, its orientation, and to a second electric field induced contribution of the unidirectional field within the interfacial electric double layer. Through additional charge screening experiments we measured the surface charge density for positively as well as for negatively charged particle surfaces and confirmed the isoelectric point, where an increase in ionic strength had little effect on the SHS intensity. Furthermore, we have determined the net orientation of water molecules directly adsorbed to the particle surface from pH-dependent changes in the relative phase of the two SHS contributions.
Keywords: colloid; adsorption; particle; surface; charge; electric; ionic; strength; double; layer

Surface enrichment of Pt in Ga2O3 films grown on liquid Pt/Ga alloys - Grabau, Mathias; Krick Calderon, Sandra; Rietzler, Florian; Niedermaier, Inga; Taccardi, Nicola; Wasserscheid, Peter; Maier, Florian; Steinrück, Hans-Peter; Papp, Christian (Surface Science, 2016)
The formation of surface GaO films on liquid samples of Ga, and Pt-Ga alloys with 0.7 and 1.8 at.% Pt was examined using near-ambient pressure (NAP) X-ray photoelectron spectroscopy (XPS). Thickness, composition and growth of the oxide films were deduced as a function of temperature and Pt content of the alloys, in ultra-high vacuum and at oxygen pressures of 3 × 10, 3 × 10 and 1 mbar. We examined oxide layers up to a thickness of 37 Å. Different growth modes were found for oxidation at low and high pressures. The formed GaO oxide films showed an increased Pt content, while the pristine GaPt alloy showed a surface depletion of Pt at the examined temperatures. Upon growth of GaO on Pt/Ga alloys a linear increase of Pt content was observed, due to the incorporation of 3.6 at.% Pt in the GaO. The Pt content in GaO, at the examined temperatures and bulk Pt concentrations is found to be independent of pressure, temperature and the nominal Pt content of the metallic alloy.
Keywords: Bimetallic catalysts; Gallium oxide; NAP XPS; Pt/Ga alloys

Surface exciton separation in photoexcited MgO nanocube powders - Sternig, Andreas; Stankic, Slavica; Mueller, Markus; Siedl, Nicolas; Diwald, Oliver (Nanoscale, 2012)
In MgO nanocube powders surface excitons can separate and the resulting charge carriers provide reactive adsorption sites at well-defined surface elements. We employed photoluminescence (PL) emission bands originating from the photoexcitation of nanocube corners and edges as quantitative probes to explore their chemical reactivity towards molecular hydrogen. Surface excitons which form at corners and edges exhibit similar cross-sections for separation in vacuum. The separation of edge excitons, however, is significantly enhanced in hydrogen atmosphere when hydrogen adsorption occurs as a simultaneous surface process. The electronic structure of MgO nanocube edges which split hydrogen heterolytically upon generation of surface hydroxyls and hydrides is unaffected by the photoexcitation of corners. Respective edges, however, are efficient absorption sites for UV photons. Transfer of exciton energy to oxygen ions in corners is followed by exciton separation which transforms corner ions into surface radicals leading to a well-defined starting point for the site selective functionalization of metal oxide nanostructures.
Keywords: nanoparticle

Surface functionalization and electronic interactions of ZnO nanorods with a porphyrin derivative - Klaumünzer, Martin; Kahnt, Axel; Burger, Alexandra; Mackovic, Mirza; Muenzel, Corinna; Srikantharajah, Rubitha; Spiecker, Erdmann; Hirsch, Andreas; Peukert, Wolfgang; Guldi, Dirk Michael (ACS Applied Materials and Interfaces, 2014)
To optimize electron transfer and optoelectronic properties in nanoparticulate thin films for electronics we show the surface functionalization of ZnO nanorods by means of replacing surface active 2-[2-(2-methoxyethoxy) ethoxy]acetic acid (TODA) by a redoxactive organic component, that is, 5,10,15,20-(phenoxyacetat)-porphyrin bearing four carboxylic acids as possible ZnO anchors. Microscopy-transmission electron microscopy-And spectroscopy-optical spectroscopy-verifies the successful and homogenous integration of the porphyrin onto the surface of ZnO nanorods, a process that is facilitated by the four anchoring groups. Photophysical investigations based on emission and absorption spectroscopy prompt to distinct electronic interactions between ZnO nanorods and the porphyrins. Consequently, we performed further photophysical studies flanked by pulse radiolysis assays to corroborate the nature of the electronic interactions. © 2014 American Chemical Society.

Surface functionalization of ZnO nanorods with C60 derivatives carrying phosphonic acid functionalities - Voigt, Michael; Klaumünzer, Martin; Ebel, Alexander; Werner, Fabian; Yang, Guang; Marczak, Renata; Spiecker, Erdmann; Guldi, Dirk Michael; Hirsch, Andreas; Peukert, Wolfgang (Journal of Physical Chemistry C, 2011)
ZnO with its wide band gap (3.37 eV) emerged as a promising semiconductor material for applications in the fields of printable and flexible electronics such as field effect transistors, etc. The fact that contacts at particle/particle interfaces exert a limiting impact on device performances motivates research on aligning the electronic levels at particle/particle interfaces. To this end, we focus in the present study on the surface functionalization of ZnO nanorods by a series of C derivatives. The latter share in common (i) a phosphonic acid as an anchor to ZnO, (ii) an alkyl chain as a flexible spacer, and (iii) a C as the headgroup. To shed light onto the electronic interactions between ZnO and C, we vary the alkyl chain length from n = 3 to n = 18. HRTEM and EELS prove the successful surface functionalization. Fundamental to this work are steady state absorption and photoluminescence measurements, which confirm the presence of appreciable electronic couplings between ZnO nanorods and C. Especially the photoluminescence measurements are important since they provide strong support for the dependence between spacer length and strength of electronic communication. In particular, a blue-shift of the ZnO free exciton emission band amounting to as much as 50 meV is observed with a decrease in spacer length. Furthermore, time-resolved absorption measurements corroborate ultrafast charge injection - facilitated by strong coupling - from the photoexcited ZnO nanorods to C. © 2011 American Chemical Society.
Keywords: Functionalization; Oxides; Reaction products; Organophosphorus compounds; Nanorods

Surface-functionalized ionic liquid crystal-supported ionic liquid phase materials: Ionic liquid crystals in mesopores - Kohler, Florian; Morain, Bruno; Weiß, Alexander; Laurin, Mathias; Libuda, Jörg; Wagner, Valentin; Melcher, Berthold Ulrich; Wang, Xinjiao; Meyer, Karsten; Wasserscheid, Peter (ChemPhysChem, 2011)
The influence of confinement on the ionic liquid crystal (ILC) [C C Im][OTf] is studied using differential scanning calorimetry (DSC), polarized optical microscopy (POM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The ILC studied is supported on Si-based powders and glasses with pore sizes ranging from 11 to 50 nm. The temperature of the solid-to-liquid-crystalline phase transition seems mostly unaffected by the confinement, whereas the temperature of the liquid-crystalline-to-liquid phase transition is depressed for smaller pore sizes. A contact layer with a thickness in the order of 2 nm is identified. The contact layer exhibits a phase transition at a temperature 30 K lower than the solid-to-liquid-crystalline phase transition observed for the neat ILC. For applications within the "supported ionic liquid phase (SILP)" concept, the experiments show that in pores of diameter 50 nm a pore filling of α>0.4 is sufficient to reproduce the phase transitions of the neat ILC. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Surface-Mediated in Situ Metalation of Porphyrins at the Solid-Vacuum Interface - Marbach, Hubertus (Accounts of Chemical Research, 2015)
The investigation of porphyrin derivatives at the solid-vacuum interface has become a vivid research field with the prospect to tailor functional molecular architectures and as prototype examples to study the fundamental properties of porphyrin derivatives in regard to their vital role in many natural processes. The functional properties of the porphyrin derivatives are mainly determined by the central metal atom. Thus, the recent exploration of the surface-confined in situ metalation of porphyrins is an important step toward the realization of molecule-based functional devices. The corresponding metalation reaction of free base porphyrin derivatives can be conveniently realized in situ in ultrahigh vacuum by post- or predeposition of metal atoms or directly with substrate atoms in the so-called self-metalation. Moderate heating above room temperature (RT) might be necessary either to realize the transport of the metal to the porphyrin via diffusion or to overcome an activation barrier determined by the redox reaction itself. Surface science techniques like scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) are powerful tools to scrutinize the reaction and give valuable insights into the metalation process. For example, the completed metalation can be reflected in an enhanced apparent height of the corresponding porphyrin in STM or can be evidenced by characteristic changes in the N 1s region in XPS. These signatures allow monitoring of the progress of the metalation, and it was found that the reaction generally proceeds with very high yield. Surface diffusion of the coadsorbed metal atoms mediates the reaction and is crucial for the high yields of the corresponding reactions with pre- and postadsorbed metals. It was also demonstrated that the completed metalation can indeed significantly alter the adsorption behavior and the electronic properties and thus the functionality of the porphyrin molecules. These alterations can be used to monitor the kinetics of a particular porphyrin self-metalation reaction by STM and to estimate the activation barrier for that reaction based on isothermal measurements at different temperatures. Also TPD measurements of the H2 and D2 signals allow for the determination of corresponding activation energies for the metalation of free base porphyrins and their deuterized analogues. Gas phase DFT calculations of the metalation of the free base porphyrin macrocycle identify intermediate reaction steps with the transfer of the first hydrogen atom to the metal center being the main barrier to overcome. The values from these calculations are in fair agreement with experimentally determined ones. However, TPD based results indicate that exchanges of deuterium and hydrogen between the central nitrogen and the surface occur, which indicate an active role of the surface and challenge the findings from gas phase DFT. The in situ metalation of porphyrins at the solid-vacuum interface is established as a novel and convenient route to tailor functional molecular architectures. With different surface science techniques detailed insights into the surface-mediated metalation reaction were achieved for this class of prototype functional molecules.
EAM Import::2019-03-12

Surface modification of an alumina-based bioceramic for cement application - Pedimonte, J.; Travitzky, Nahum; Korn, Matthias; Kriegelstein, S.; Greil, Peter (Advanced Engineering Materials, 2011)
Biograde zirconia toughened alumina (ZTA) has found wide application in load bearing endoprosthetic implants due to high strength, fracture toughness, and wear resistance. In order to enhance bonding to acrylic bone cement (BC) for implants, fixation modification of ZTA with a thin layer of porous anodic alumina (PAA) was investigated. An Al-layer of approximately 500 nm was sputtered on the ZTA substrate which subsequently was electrochemically oxidized by anodic polarization in H C O or H PO solution. PAA layers with a total porosity ranging from 11 to 30%, mean pore spacing of 90-200 nm and pore diameters of 30-110 nm were prepared. Compared to unmodified ZTA/BC interface (≈ 30 MPa), the PAA modified specimens (ZTA/PAA/BC) achieved a significantly higher interface bonding strength (≈ 60 MPa) measured by four point bending on composite beam specimens. While crack propagation in the unmodified ZTA/BC specimen was found to proceed along the interface, fracture analysis on the ZTA/PAA/BC specimens showed a mixed mode fracture with part of the fracture propagation localized along the PAA/BC interface and part through BC. Thus, pore structure controlled mechanical interlocking is expected to offer a high potential for applying PAA surface modification to improve biomaterial to BC bonding. Bonding of ZrO toughened Al O (ZTA) to acrylic bone cement (BC) was improved by a PAA film, formed in H C O or H PO by electrochemical anodization of a sputtered Al-layer on the ZTA substrate. Compared to unmodified interface (≈ 30 MPa), the PAA modified specimens achieved a significantly higher interface bonding strength (≈ 60 MPa), due to mixed mode fracture with part of the crack propagation along the PAA/BC interface and part through BC. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Surface Modification of ZnO Nanorods with Hamilton Receptors - Hirsch, Andreas; Peukert, Wolfgang; Zeininger, Lukas; Klaumünzer, Martin (International Journal of Molecular Sciences, 2015)
A new prototype of a Hamilton receptor suitable for the functionalization of inorganic nanoparticles was synthesized and characterized. The hydrogen bonding receptor was coupled to a catechol moiety, which served as anchor group for the functionalization of metal oxides, in particular zinc oxide. Synthesized zinc oxide nanorods [ZnO] were used for surface functionalization. The wet-chemical functionalization procedure towards monolayer-grafted particles [ZnO-HR] is described and a detailed characterization study is presented. In addition, the detection of specific cyanurate molecules is demonstrated. The hybrid structures [ZnO-HR-CA] were stable towards agglomeration and exhibited enhanced dispersability in apolar solvents. This observation, in combination with several spectroscopic experiments gave evidence of the highly directional supramolecular recognition at the surface of nanoparticles.
Keywords: nanorod; functional; particle; ZnO; zinc; oxide; monolayer; stabilization

Surface polymerization of poly(p -phenylene-terephthalamide) on Ag(111) investigated by X-ray photoelectron spectroscopy and scanning tunneling microscopy - Schmitz, Christoph H.; Schmid, Martin; Gärtner, Stefan; Steinrück, Hans-Peter; Gottfried, Michael; Sokolowski, Moritz (Journal of Physical Chemistry C, 2011)
The formation of poly(p-phenylene-terephthalamide) (PPTA) by surface-assisted polymerization of terephthaloyl chloride (TPC) and p-phenylenediamine (PPD) on Ag(111) has been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Analysis of the XPS data reveals that the polymer is formed in the (sub-) monolayer regime already at room temperature. Upon vacuum deposition of TPC onto the single-crystal surface, the carbon-chlorine bond is cleaved, and the polymerization proceeds via a p-phenylene dicarbonyl species, which is presumably surface stabilized. This contrasts the reaction mechanism in solution, where the chlorine atom is expelled after a nucleophilic attack by the amine group. Annealing of the sample leads to desorption of residual PPD monomers, lateral ordering of the polymer chains, and further progress of the polymerization that can be followed by XPS. The chain length distribution of the polyamide, as extracted from the XP spectra, is consistent with the typical chain lengths found in STM images. © 2011 American Chemical Society.

Surface Reaction of CO on Carbide-Modified Mo(110) - Gleichweit, Christoph; Neiß, Christian; Maisel, Sven; Bauer, Udo; Späth, Florian; Höfert, Oliver; Görling, Andreas; Steinrück, Hans-Peter; Papp, Christian (Journal of Physical Chemistry C, 2017)
The adsorption and reaction of CO on a monolayer carbide and a bulk carbide, prepared on Mo(110), was studied with synchrotron-based XPS, TPD, and density-functional calculations using slab models. In the experiments on the monolayer carbide, we find two CO species at 140 K, with a saturation coverage of similar to 0.7 ML, while on the bulk carbide, Mo2C, three molecular adsorption states are found, showing a similar total coverage of similar to 0.7 ML at saturation. In addition, CO partly dissociates on both surfaces (monolayer carbide: 7%, bulk carbide: 15%). The calculations on the monolayer carbide show that the adsorption of CO on Mo sites is most stable. At increased coverages, several different adsorption sites on the monolayer carbide become possible. From the core level shifts, an assignment to the experimentally found species becomes available. Upon heating, we find on both carbides the competing processes of desorption, interconversion of different CO species, and dissociation of CO. The detailed quantitative analysis of these processes shows that desorption and dissociation to atomic oxygen and carbon is completed at similar to 400 K on the monolayer carbide and similar to 450 K on the bulk carbide; in both cases, about 35% (0.25 ML) of the initially adsorbed CO decomposes upon heating. Above 800 K, atomic carbon and oxygen desorb associatively, and at 1200 K the carbide surfaces are restore

Surface Reactions of Dicyclohexylmethane on Pt(111) - Gleichweit, Christoph; Amende, Maximilian; Höfert, Oliver; Xu, Tao; Späth, Florian; Brückner, Nicole; Wasserscheid, Peter; Libuda, Jörg; Steinrück, Hans-Peter; Papp, Christian (Journal of Physical Chemistry C, 2015)
We investigated the surface reaction of the liquid organic hydrogen carrier dicydohexylmethane (DCHM) on Pt(111) in ultrahigh vacuum by high-resolution X-ray photoelectron spectroscopy, temperature-programmed desorption, near-edge X-ray absorption fine structure, and infrared reflection absorption spectroscopy. Additionally, the hydrogen-lean molecule diphenylmethane and the relevant molecular fragments of DCHM, methylcyclohexane, and toluene were studied to elucidate the reaction steps of DCHM. We find dehydrogenation of DCHM in the range of 200-260 K, to form a double-sided pi-allylic species coadsorbed with hydrogen. Subsequently, similar to 30% of the molecules desorb, and for similar to 70%, one of the pi-allyls reacts to a phenyl group between 260 and 330 K, accompanied by associative hydrogen desorption. Above 360 K, the second pi-allylic species is dehydrogenated to a phenyl ring. This is accompanied by C-H bond scission at the methylene group, which is an unwanted decomposition step in the hydrogen storage cycle, as it alters the original hydrogen carrier DCHM. Above 450 K, we find further decomposition steps which we assign to C-H abstraction at the phenyl rings.

Surface science and model catalysis with ionic liquid-modified materials - Steinrück, Hans-Peter; Libuda, Jörg; Wasserscheid, Peter; Cremer, Till; Kolbeck, Claudia; Laurin, Mathias; Maier, Florian; Sobota, Marek; Schulz, Peter; Stark, Michael Werner (Advanced Materials, 2011)
Materials making use of thin ionic liquid (IL) films as support-modifying functional layer open up a variety of new possibilities in heterogeneous catalysis, which range from the tailoring of gas-surface interactions to the immobilization of molecularly defined reactive sites. The present report reviews recent progress towards an understanding of "supported ionic liquid phase (SILP)" and "solid catalysts with ionic liquid layer (SCILL)" materials at the microscopic level, using a surface science and model catalysis type of approach. Thin film IL systems can be prepared not only ex-situ, but also in-situ under ultrahigh vacuum (UHV) conditions using atomically well-defined surfaces as substrates, for example by physical vapor deposition (PVD). Due to their low vapor pressure, these systems can be studied in UHV using the full spectrum of surface science techniques. We discuss general strategies and considerations of this approach and exemplify the information available from complementary methods, specifically photoelectron spectroscopy and surface vibrational spectroscopy. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Surface science goes liquid ! - Steinrück, Hans-Peter (Surface Science, 2010)
Ionic liquids are a new class of materials with most interesting properties. They are liquid at room temperature, but have a negligible vapour pressure. Consequently they can, in contrast to normal liquids, be investigated by all UHV-based methods of surface science. This allows to determine their properties with the same atomic level accuracy that is presently common for solid surfaces and conventional adsorbate systems. Apart from the investigation of the specific properties of ionic liquids, which are relevant for many applications, this also opens the possibility to obtain more detailed insight in the general physical and chemical properties of liquids. In that sense it opens the door to a new chapter of surface science - "Ionic Liquid Surface Science". © 2010 Elsevier B.V. All rights reserved.

Surface sites on Pt-CeO2 mixed oxide catalysts probed by CO adsorption: a synchrotron radiation photoelectron spectroscopy study - Neitzel, Armin; Lykhach, Yaroslava; Skala, Tomas; Tsud, Nataliya; Vorokhta, Mykhailo; Mazur, Daniel; Prince, Kevin C.; Matolin, Vladimir; Libuda, Jörg (Physical Chemistry Chemical Physics, 2014)
By means of synchrotron radiation photoemission spectroscopy, we have investigated Pt-CeO2 mixed oxide films prepared on CeO2(111)/Cu(111). Using CO molecules as a probe, we associate the corresponding surface species with specific surface sites. This allows us to identify the changes in the composition and morphology of Pt-CeO2 mixed oxide films caused by annealing in an ultrahigh vacuum. Specifically, two peaks in C 1s spectra at 289.4 and 291.2 eV, associated with tridentate and bidentate carbonate species, are formed on the nanostructured stoichiometric CeO2 film. The peak at 290.5-291.0 eV in the C 1s spectra indicates the onset of restructuring, i.e. coarsening, of the Pt-CeO2 film. This peak is associated with a carbonate species formed near an oxygen vacancy. The onset of cerium oxide reduction is indicated by the peak at 287.8-288.0 eV associated with carbonite species formed near Ce3+ cations. The development of surface species on the Pt-CeO2 mixed oxides suggests that restructuring of the films occurs above 300 K irrespective of Pt loadings. We do not find any adsorbed CO species associated with Pt4+ or Pt2+. The onset of Pt2+ reduction is indicated by the peak at 286.9 eV in the C 1s spectra due to CO adsorption on metallic Pt particles. The thermal stability of Pt2+ in Pt-CeO2 mixed oxide depends on Pt loading. We find excellent stability of Pt2+ for 12% Pt content in the CeO2 film, whereas at a Pt concentration of 25% in the CeO2 film, a large fraction of the Pt2+ is converted into metallic Pt particles above 300 K.

Surface strain evolution of ultrafine-grained aluminum alloy laminates under tension-Microscale plastic instabilities and the Portevin-Le Chatelier effect - Kurtovic, A.; Niendorf, Thomas; Hausöl, Tina; Höppel, Heinz Werner; Göken, Mathias; Maier, H. J. (Scripta Materialia, 2013)
The surface strain evolution in accumulative roll bonded aluminum alloys was studied in situ using digital image correlation. Laminated multicomponent materials, consisting of two ultrafine-grained aluminum alloys with different layer architectures (laminate and sandwich), were analyzed and compared to the pure alloys after the same number of accumulative roll bonding cycles. The results obtained show differences in the surface strain evolution of the accumulative roll bonded and differently stacked materials, which is attributed to microscale plastic instabilities.
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Surface strengthening of extrusion-formed polymer/filler-derived ceramic composites - Schlier, Lorenz; Travitzky, Nahum; Gegner, J.; Greil, Peter (Journal of Ceramic Science and Technology, 2012)
Surface nitridation of extrusion-formed Fe-Si-Cr-filler-loaded polysiloxane polymer filaments was investigated. After the filaments were exposed to a nitrogen atmosphere at temperatures above 1000 °C, a gas-solid reaction caused the formation of a nitridation reaction layer covering the filament surface. Thermo-chemical calculations of equilibrium phase compositions at different nitrogen activity suggest the formation of SiN O and SiN near the filament surface (high nitrogen content) whereas SiC and unreacted CrSi and FeSi dominate in the core region (low nitrogen content), which was confirmed by means ofXRDanalyses. Compared to filaments annealed in an inert Ar atmosphere (no nitride reaction layer), specimens covered with a nitride surface layer of only 20 μm in thickness obtained a bending strength increment of + 35%(mean fracture stress 400 MPa). The improved mechanical properties were attributed to a pronounced volume increase triggered by the nitride reaction, which gives rise to pore filling and crack healing. Since post-fabrication treatment in a reactive atmosphere is independent of the component shape and size, formation of a surface reaction zone with densified microstructure (reduced porosity and flaws) may offer a versatile route for improving the properties of bulk polymer-filler-derived ceramic components. © 2012 Göller Verlag.
Keywords: Polymer derived ceramics; Reaction surface zone; Surface strengthening

Surface tension of isotropic-nematic interfaces: Fundamental measure theory for hard spherocylinders - Wittmann, Rene; Mecke, Klaus (Journal of Chemical Physics, 2014)
A fluid constituted of hard spherocylinders is studied using a density functional theory for nonspherical hard particles, which can be written as a function of weighted densities. This is based on an extended deconvolution of the Mayer f-function for arbitrarily shaped convex hard bodies in tensorial weight functions, which depend each only on the shape and orientation of a single particle. In the course of an examination of the isotropic-nematic interface at coexistence the functional is applied to anisotropic and inhomogeneous problems for the first time. We find good qualitative agreement with other theoretical predictions and also with Monte Carlo simulations. (c) 2014 AIP Publishing LLC.

Surface thermography using dual channel imaging based on the blue and red emission of Ba3MgSi2O8:Eu2+, Mn2+ - Hashemi, Amir; Gast, Jessica; Ali, Amjad; Osvet, Andres; Batentschuk, Miroslaw; Brabec, Christoph (Measurement Science and Technology, 2017)
In this investigation, we present a surface temperature determination method, based on the luminescence characteristics of Ba3MgSi2O8:Eu+2,Mn+2. Simple optics utilizing only one RGB scientific CMOS camera makes this measurement technique easily applicable. Four different imaging methods were developed and are presented together with the corresponding calibration curves. We successfully utilized the methods to take surface temperature gradient images of glass plates with a point heat source between them.
Keywords: Phosphor spectroscopy; Dual channel lock in method; Phase sensitive averaging detection method; Imaging; Temperature measurement; Photoluminescence decay; Intensity ratio

Surface waves at an interface of two metamaterial structures with interelement coupling - Radkovskaya, A.; Tatartschuk, E.; Sydoruk, O.; Shamonina, E.; Stevens, C. J.; Edwards, D. J.; Solymar, L. (Physical Review B, 2010)
A configuration of two strongly coupled homogeneous two-dimensional metamaterial lattices of resonant elements is shown to be able to propagate surface magnetoinductive waves along the interface by virtue of coupling between the elements at the boundary. A study of the dispersion equations reveals the existence of two separate pass bands for surface waves which may partly overlap with pass bands supporting bulk waves. Experiments are reported on a structure consisting of 90 magnetically coupled capacitively loaded resonant rings designed to operate around 55 MHz. The measured current distributions and dispersion curves extracted from the experimental data are compared both with numerical simulations, using the generalized Kirchhoff's equation and with analytical expressions derived on the assumption of nearest-neighbor interaction. Excellent agreement between the three approaches is found. Considering that surface waves of various kinds have found a wide range of applications in the past, it is envisaged that this surface wave will open up fresh possibilities. A number of examples are presented. It is conjectured that other existing metamaterial structures might also be suitable candidates for propagating analogous surface waves.
EAM Import::2019-03-12

'Suspended' Pt nanoparticles over TiO2 nanotubes for enhanced photocatalytic H2 evolution - Nguyen, Nhat Truong; Yoo, Jeong Eun; Altomarea, Marco; Schmuki, Patrik (Chemical Communications, 2014)
In the present work we introduce a technique to form a photocatalyst based on Pt nanoparticles suspended over the mouth of anodic TiO2 nanotubes. These structures are obtained by decorating the top end of highly ordered TiO2 nanotubes with a web of TiO2 nanofibrils, followed by sputter deposition of a minimum amount of Pt. A subsequent thermal dewetting step forms 3–6 nm-sized Pt nanoparticles along the nanofibrils. These structures, when compared to conventional Pt decoration techniques of TiO2 nanotubes, show strongly enhanced photocatalytic H2 evolution efficiency.
EAM Import::2019-03-12

Switching adsorption and growth behavior of ultrathin [C(2)C(1)lm][OTf] films on Au(111) by Pd deposition - Rietzler, Florian; May, Benjamin; Steinrück, Hans-Peter; Maier, Florian (Physical Chemistry Chemical Physics, 2016)
Combining in vacuo deposition of ultrathin ionic liquid (UTIL) films with angle-resolved X-ray photoelectron spectroscopy (ARXPS), we demonstrate that by deposition of submonolayer amounts of Pd onto Au(111) the initial growth mode of the ionic liquid (IL) 1-ethyl-3-methylimidazolium trifluoro-methanesulfonate ([C(2)C(1)lm][OTf]) can be switched from three-dimensional (3D) to two-dimensional (2D) growth, that is, from non-wetting to wetting. On clean Au(111), pronounced 3D growth occurs on top of an initially formed 2D wetting layer with cations and anions next to each other in a checkerboard arrangement. After pre- or postdeposition of only 0.7 ML Pd, two-dimensional layer-by-layer growth is found, which is attributed to strong attractive interactions between [C(2)C(1)lm][OTf] and surface Pd. For Pd post deposition onto the IL, the ARXPS data revealed particularly strong interactions between the dialkylimidazolium cation and Pd atoms, which considerably reduce the regular surface alloying of Pd with the Au substrate stabilizing Pd at the metal surface. In the context of heterogeneous catalysis using the SCILL (solid catalyst coated with ionic liquid layer) concept, these results directly provide a possible explanation on the molecular level for the beneficial influence of the IL layer in case of heterogeneous metal alloy catalysts.

Switching behaviour of individual Ag-TCNQ nanowires: an in situ transmission electron microscopy study - Ran, Ke; Rösner, Benedikt; Butz, Benjamin; Fink, Rainer; Spiecker, Erdmann (Nanotechnology, 2016)
The organic semiconductor silver-tetracyanoquinodimethane (Ag-TCNQ) exhibits electrical switching and memory characteristics. Employing a scanning tunnelling microscopy setup inside a transmission electron microscope, the switching behaviour of individual Ag-TCNQ nanowires (NWs) is investigated in detail. For a large number of NWs, the switching between a high (OFF) and a low (ON) resistance state was successfully stimulated by negative bias sweeps. Fitting the experimental I-V curves with a Schottky emission function makes the switching features prominent and thus enables a direct evaluation of the switching process. A memory cycle including writing, reading and erasing features is demonstrated at an individual NW. Moreover, electronic failure mechanisms due to Joule heating are discussed. These findings have a significant impact on our understanding of the switching behaviour of Ag-TCNQ.
Keywords: Ag-TCNQ nanowires; in situ transmission electron microscopy; electrical switching; organic memory device; organic electronics

Switching from Ultraweak to Strong Adhesion - Fenz, Susanne F.; Bihr, Timo; Merkel, Rudolf; Seifert, Udo; Sengupta, Kheya; Smith, Ana-Suncana (Advanced Materials, 2011)
Intrinsically strong avidin-neutravidin bonds can exhibit ultraweak adhesion mediated by transiently bound domains and can undergo a transition to a stable strong adhesion by locally increasing bond density.
Keywords: INTERFERENCE CONTRAST MICROSCOPY; CELL-ADHESION; SUPPORTED MEMBRANES; DIFFUSION; DYNAMICS; FLUCTUATIONS; MOLECULES; VESICLES; SURFACE; BONDS

Switching of a coupled spin pair in a single-molecule junction - Wagner, Stefan; Kißlinger, Ferdinand; Ballmann, Stefan; Schramm, Frank; Chandrasekar, Rajadurai; Bodenstein, Tilmann; Fuhr, Olaf; Secker, Daniel; Fink, Karin; Ruben, Mario; Weber, Heiko B. (Nature Nanotechnology, 2013)
Single-molecule spintronics investigates electron transport through magnetic molecules that have an internal spin degree of freedom. To understand and control these individual molecules it is important to read their spin state. For unpaired spins, the Kondo effect has been observed as a low-temperature anomaly at small voltages. Here, we show that a coupled spin pair in a single magnetic molecule can be detected and that a bias voltage can be used to switch between two states of the molecule. In particular, we use the mechanically controlled break-junction technique to measure electronic transport through a single-molecule junction containing two coupled spin centres that are confined on two Co 2+ ions. Spin-orbit configuration interaction methods are used to calculate the combined spin system, where the ground state is found to be a pseudo-singlet and the first excitations behave as a pseudo-triplet. Experimentally, these states can be assigned to the absence and occurrence of a Kondo-like zero-bias anomaly in the low-temperature conductance data, respectively. By applying finite bias, we can repeatedly switch between the pseudo-singlet state and the pseudo-triplet state. © 2013 Macmillan Publishers Limited. All rights reserved.
UnivIS-Import:2015-04-14:Pub.2013.nat.dphy.IAP.LAP.switch

Switching the conductance of a molecular junction using a proton transfer reaction - Hofmeister, Chriszandro; Härtle, Rainer; Rubio-Pons, Oscar; Brana Coto, Pedro; Sobolewski, Andrzej L.; Thoss, Michael (Journal of Molecular Modeling, 2014)
A novel mechanism for switching a molecular junction based on a proton transfer reaction triggered by an external electrostatic field is proposed. As a specific example to demonstrate the feasibility of the mechanism, the tautomers [2,5-(4-hydroxypyridine)] and 2,5-[4(1H)-pyridone] are considered. Employing a combination of first-principles electronic structure calculations and Landauer transport theory, we show that both tautomers exhibit very different conductance properties and realize the "on" and "off" states of a molecular switch. Moreover, we provide a proof of principle that both forms can be reversibly converted into each other using an external electrostatic field. © Springer-Verlag 2014.
UnivIS-Import:2015-04-14:Pub.2014.nat.dphy.ITP.ppem.switch
Keywords: Charge transport; Density functional theory; Molecular electronics; Molecular switch; Single molecule junction

Symmetry-Adapted Perturbation Theory Applied to Endohedral Fullerene Complexes: A Stability Study of H2@C60 and 2H2@C60 - Korona, Tatiana; Heßelmann, Andreas; Dodziuk, Helena (Journal of Chemical Theory and Computation, 2009)
Because of difficulties in a description of host-guest interactions, various theoretical methods predict different numbers of hydrogen molecules which can be inserted into the C60 cavity, ranging from one to more than 20. On the other hand, only one H_2 molecule inside the C60 fullerene has been detected experimentally. Moreover, a recently synthesized H_2@C70 complex prevails in the mixture formed with 2H_2@C70. To get a deeper insight into the stability of the complexes created from C60 and hydrogen molecules, we carried out highly accurate calculations for complexes of one or two hydrogen molecules with fullerene applying symmetry-adapted perturbation theory (SAPT) and a large TZVPP basis set for selected points on the potential energy surfaces of H_2@C60 and 2H_2@C60. The electron correlation in the host and guests has been treated by density functional theory. Our calculations yield the stability of the recently synthesized H_2@C60 complex. In addition, for all tried positions of the H_2 dimer inside the C60 cage, the 2H_2@C60 complex has been characterized by a positive interaction energy corresponding to the instability of this species. Contrary to the conclusions of several theoretical studies, this finding, as well as model considerations and literature experimental data, indicates that only one hydrogen molecule can reside inside the C60 cage. The calculated energy components have been analyzed to identify the most important contributions to the interaction energy. Supermolecular interaction energies obtained with MP2, SCS-MP2, and DFT+Disp methods are also reported and compared to those of DFT-SAPT. The DFT-SAPT interaction energy has also been calculated for several points on the potential energy surface for a larger 2H_2@C70 complex, confirming, in agreement with recent experimental findings, that this species is stable. The DFT-SAPT approach has been used for the first time to obtain interaction energies for van der Waals endohedral complexes, demonstrating that the method is capable of handling such difficult cases.
EAM Import::2019-03-12

Symmetry detection of auxetic behaviour in 2D frameworks - Mitschke, Holger; Schröder-Turk, Gerd; Mecke, Klaus; Fowler, P. W.; Guest, S. D. (EPL - Europhysics Letters, 2013)
A symmetry-extended Maxwell treatment of the net mobility of periodic bar-and-joint frameworks is used to derive a sufficient condition for auxetic behaviour of a 2D material. The type of auxetic behaviour that can be detected by symmetry has Poisson's ratio -1, with equal expansion/contraction in all directions, and is here termed equiauxetic. A framework may have a symmetry-detectable equiauxetic mechanism if it belongs to a plane group that includes rotational axes of order n = 6, 4, or 3. If the reducible representation for the net mobility contains mechanisms that preserve full rotational symmetry (A modes), these are equiauxetic. In addition, for n = 6, mechanisms that halve rotational symmetry (B modes) are also equiauxetic. Copyright (c) EPLA, 2013

Synchronization properties of two mutually delay-coupled semiconductor lasers - Junges, Leandro; Gallas, Jason; Gavrielides, Athanasios (Journal of the Optical Society of America B-Optical Physics, 2016)
This paper reports a detailed numerical study of the synchronization properties of two mutually delay-coupled semiconductor lasers in the framework of the Lang-Kobayashi model. By computing high-definition stability diagrams we predict the complex distribution of periodic and chaotic laser oscillations on the coupling versus detuning control parameter plane. Such diagrams provide details concerning the behavior of the laser intensities, quantify objectively the synchronization between their electric fields, and display in-phase and out-of-phase laser behavior. In addition, we also describe the presence of a conspicuous abrupt change in the optimal shift for the greatest value of the cross-correlation function when varying the detuning between the optical angular frequencies of the laser

Synergy of Catechol-Functionalized Zinc Oxide Nanorods and Porphyrins in Layer-by-Layer Assemblies - Burger, Alexandra; Kunzmann, Andreas; Costa, Ruben Dario; Srikantharajah, Rubitha; Peukert, Wolfgang; Guldi, Dirk Michael; Hirsch, Andreas (Chemistry - A European Journal, 2018)
Catechol-functionalized, positively charged ZnO nanorods (NRs) and anionic porphyrins were integrated into layer-by-layer (LbL) assemblies. In general, this study focuses on the impact that different porphyrins, varying in size and number of negative charges, exert on the LbL architecture in terms of morphology and spectroscopy. In particular, through a combination of analytical methods, including UV/Vis spectroscopy, SEM, and profilometry, valuable insights into LbL assembly formation were gathered. A key feature was the surface coverage in the resulting films. Denser films and surface coverages were realized when highly negatively charged and sterically demanding porphyrins were employed. As a complement to basic characterization, the LbL assembled films were used to fabricate proof-of-concept solar cells.
Keywords: porphyrins; solar energy conversion; layer-by-layer assemblies; layered compounds; thin films; ZnO nanorods

Synthese mikroporöser Kohlenstoffschäume für katalytische Anwendungen - Glenk, Friedrich; Schirmer, Matthias; Gütlein, Stefan; Etzold, Bastian (Chemie Ingenieur Technik, 2010)
Mikroporöse Kohlenstoffschäume wurden über die Chlorierung von Carbiden zur Verwendung als strukturierte Katalysatorträger hergestellt. Die selbsttragenden Strukturen zeigten eine enge Mikroporenverteilung, welche Formselektivität induzieren können. Die Eignung dieser Materialien als Katalysatorträger wurde durch Nassimprägnierung von Pulver mit Platin und die Verwendung in einer Modellhydrierung von Olefinen getestet. Es ist anzunehmen, dass die Ergebnisse der Nassimprägnierung und Hydrierung auf Kohlenstoffschaumstrukturen übertragbar sind.
EAM Import::2019-03-12

Synthesis and Characterization of a Uranium(II) Monoarene Complex Supported by delta Backbonding - la Pierre, Henry Storms; Scheurer, Andreas; Heinemann, Frank Wilhelm; Hieringer, Wolfgang; Meyer, Karsten (Angewandte Chemie International Edition, 2014)
The low-temperature (<−35 °C) reduction of the trivalent uranium monoarene complex [(Ad,MeArO)3mesU] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)][((Ad,MeArO)3mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature 1H NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f 4 electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at −2.495 V vs. Fc/Fc+ is principally metal-centere
Keywords: arenes; electronic structure; magnetic properties; uranium; X-ray diffraction

Synthesis and Characterization of Gelatin-Based Magnetic Hydrogels - Helminger, Maria; Wu, Baohu; Kollmann, Tina; Benke, Dominik; Schwahn, Dietmar; Pipich, Vitaliy; Faivre, Damien; Zahn, Dirk; Coelfen, Helmut (Advanced Functional Materials, 2014)
A simple preparation of thermoreversible gelatin-based ferrogels in water provides a constant structure defined by the crosslinking degree for gelatin contents between 6 and 18 wt%. The possibility of varying magnetite nanoparticle concentration between 20 and 70 wt% is also reported. Simulation studies hint at the suitability of collagen to bind iron and hydroxide ions, suggesting that collagen acts as a nucleation seed to iron hydroxide aggregation, and thus the intergrowth of collagen and magnetite nanoparticles already at the precursor stage. The detailed structure of the individual ferrogel components is characterized by small-angle neutron scattering (SANS) using contrast matching. The magnetite structure characterization is supplemented by small-angle X-ray scattering and microscopy only visualizing magnetite. SANS shows an unchanged gelatin structure of average mesh size larger than the nanoparticles with respect to gel concentration while the magnetite nanoparticles size of around 10 nm seems to be limited by the gel mesh size. Swelling measurements underline that magnetite acts as additional crosslinker and therefore varying the magnetic and mechanical properties of the ferrogels. Overall, the simple and variable synthesis protocol, the cheap and easy accessibility of the components as well as the biocompatibility of the gelatin-based materials suggest them for a number of applications including actuators.
EAM Import::2019-03-12

Synthesis and derivatization of expanded [n]radialenes (n=3, 4) - Gholami, Mojtaba; Ramsaywack, Sharwatie; Chaur, Manuel N.; Murray, Adrian H.; Mcdonald, Robert; Ferguson, Michael J.; Echegoyen, Luis; Tykwinski, Rik (Chemistry - A European Journal, 2013)
Versatile, iterative synthetic protocols to form expanded [n]radialenes have been developed (n=3 and 4), which allow for a variety of groups to be placed around the periphery of the macrocyclic framework. The successful use of the Sonogashira cross-coupling reaction to complete the final ring closure demonstrates the ability of this reaction to tolerate significant ring strain while producing moderate to excellent product yields. The resulting radialenes show good stability under normal laboratory conditions in spite of their strained, cyclic structures. The physical and electronic characteristics of the macrocycles have been documented by UV-visible spectroscopy, electrochemical methods, and X-ray crystallography (four derivatives), and these studies provide insight into the properties of these compounds as a function of pendent substitution in terms of conjugation and donor/acceptor functionalization. Feeling the strain: By using a handful of versatile building blocks, expanded radialenes are easily assembled through Sonogashira cross-coupling reactions. X-ray crystallography shows expanded [3]radialenes have a rather significantly strained enyne framework, whereas those of the expanded [4]radialenes show much less strain (see figure). © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis and magnetic properties of a nitrogen-containing fullerene dimer - Hörmann, Frank; Hirsch, Andreas; Porfyrakis, Kyriakos; Briggs, G. Andrew D. (European Journal of Organic Chemistry, 2011)
We report here on the two-step synthesis of fullerene dimer 3 from monomer 2. The structures were confirmed by MALDI mass spectrometry, UV/Vis, FTIR, H NMR, and C NMR spectroscopy. The magnetic properties of the analogous species of dimer 3, N@C H O , were studied by continuous-wave electron-spin resonance (CW-EPR), indicating that the functionalization introduces a permanent zero-field splitting (ZFS) D = 8.6 MHz and E = 0.45 MHz. The same synthetic procedure could be used to produce a N- N dimer enabling spin resonance experiments that could demonstrate entanglement between electron spins. N@C -containing dimers have been proposed as quantum bits (qubits) for a quantum information processing device. Herein, we demonstrate the feasibility of synthesizing such dimeric systems, while at the same time retaining the extraordinary paramagnetic character of N@C . © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis and orthogonal functionalization of [60]fullerene e,e,e-trisadducts with two spherically defined addend zones - Beuerle, Florian; Hirsch, Andreas (Chemistry - A European Journal, 2009)
e,e,e-Trisadducts 13 and 15 have been prepared by a highly regioselective threefold cyclopropanation of tripodal malonates 10 and 12 with C. The yield and regioselectivity depend on the length and structure of the tethers that connect the malonate units to the focal benzene core of 13-15. As a consequence of the template-directed synthesis, all ce.e-trisadducts were formed as in/out isomers exclusively and contain two spherically well-defined addend zones with equatorial and polar orientation, respectively. By variation of the outer malonate termini of the tethers, selective functionalization of the equatorial addend zone could be achieved, thus leading to fine-tuning of intermolecular interactions, such as solubility or aggregation phenomena. After removal of the focal benzene moiety in 14 and 15, selective functionalization of the polar addend zone could be achieved. Strong intramolecular hydrogen-bonding networks of the polar substituents in the polar addend zone could be observed by H NMR spectroscopic analysis. By orthogonal functionalization of both addend zones, fullerene derivatives 44-48 could be synthesized as one single in/out isomer, thus greatly enhancing the potential of e,e,e-trisadducts as building blocks in supramolecular architectures. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

Synthesis and photoinduced electron-transfer properties of phthalocyanine-[60]fullerene conjugates. - Quintiliani, M.; Kahnt, Axel; Wölfle, Thorsten; Hieringer, Wolfgang; Vázquez, P.; Görling, Andreas; Guldi, Dirk Michael; Torres, T. (Chemistry - A European Journal, 2008)
A series of three novel ZnPc-C conjugates (Pc = phthalocyanine) 1a-c bearing different spacers (single, double, and triple bond) between the two electroactive moieties was synthesized and compared to that of ZnPc-C conjugate 2, in which the two electroactive moieties are linked directly. The synthetic strategy - towards the preparation of 1a-c - involved palladium-catalyzed cross-coupling reactions over a monoiodophthalocyanine precursor 4 to introduce the corresponding spacer, and subsequent dipolar cycloaddition reaction to C . Detailed photophysical investigations of 1a-c and 2 prompted an intramolecular electron transfer that evolves from the photoexcited ZnPc to the electron-accepting C . In particular, with the help of femtosecond laser photolysis charge separation was indeed confirmed as the major deactivation channel. Complementary time-dependent density functional calculations supported the spectral assignment, namely, the spectral identity of the ZnPc radical cation and the C radical anion as seen in the differential absorption spectra. The lifetimes of the correspondingly formed radical ion-pair states depend markedly on the solvent polarity: they increase as polarity decreases. Similarly, although to a lesser extent, the nature of the linker impacts the lifetime of the radical ion-pair states. In general, the lifetimes of these states tend to be shortest in the system that lacks any spacer at all (2), whereas the longest lifetimes were found in the system that carries the triple-bond spacer (1a). © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Synthesis and Photophysical Properties of Multichromophoric Carbonyl-Bridged Triarylamines - Haedler, Andreas T.; Beyer, Sebastian R.; Hammer, Natalie; Hildner, Richard; Kivala, Milan; Koehler, Juergen; Schmidt, Hans-Werner (Chemistry - A European Journal, 2014)
The synthesis and photophysical properties of two novel multichromophoric compounds is presented. Their molecular design comprises a carbonyl-bridged triarylamine core and either naphthalimides or 4-(5-hexyl-2,2'-bithiophene)naphthalimides as second chromophore in the periphery. The lateral chromophores are attached to the core via an amide linkage and a short alkyl spacer. The synthetic approach demonstrates a straightforward functionalization strategy for carbonyl-bridged triarylamines. Steady-state and time-resolved spectroscopic investigations of these compounds, in combination with three reference compounds, provide clear evidence for energy transfer in both multichromophoric compounds. The direction of the energy transfer depends on the lateral chromophore used. Furthermore, the compound bearing the lateral 4-(bithiophene)naphthaimides is capable of forming fluorescent gels at very low concentrations in the sub-millimolar regime whilst retaining its energy transfer properties.
EAM Import::2019-03-12

Synthesis and Properties of Arylvinylidene-Bridged Triphenylamines - Hammer, Natalie; Shubina, Tatyana; Gisselbrecht, Jean-Paul; Hampel, Frank; Kivala, Milan (Journal of Organic Chemistry, 2015)
A series of arylvinylidene-bridged triphenylamines were efficiently synthesized through the thionation/Barton-Kellogg olefination sequence from their corresponding carbonyl precursors. The electrochemical investigations identified these highly distorted scaffolds as fairly strong electron donors capable of several reversible oxidation steps with the first oxidation occurring at a potential comparable to that of ferrocene for the n-hexyl-substituted diphenylvinylidene-bridged compound.
EAM Import::2019-03-12

Synthesis and properties of long [n]cumulenes (n ≥ 5) - Januszewski, Johanna; Tykwinski, Rik (Chemical Society Reviews, 2014)
Molecules composed of a contiguous sequence of double bonds, the [n]cumulenes, share structural similarities to both of their conjugated relatives, the polyenes and polyynes. The synthesis and properties of [n]cumulenes are, however, quite different from those of either polyenes or polyynes. At an infinite length, [n]cumulenes would provide one structural form of the hypothetical sp-hybridized carbon allotrope carbyne, while shorter derivatives offer model compounds to help to predict the properties of carbyne. Finally, derivatization of the π-electron framework of [n]cumulenes provides a number of different synthetic transformations, with cycloaddition reactions being the most common. In this review, both historical and recent synthetic achievements toward long [n]cumulenes (n ≥ 5) are discussed. This is followed by a description of our current understanding of the physical and electronic structure of [n]cumulenes based on UV/vis spectroscopy and X-ray crystallography. Finally, the reactivity of long [n]cumulenes is described. This journal is © the Partner Organisations 2014.

Synthesis of amphiphilic [60]fullerene [3:3]hexakisadducts with four spherically defined addend zones - Beuerle, Florian; Hirsch, Andreas (Chemistry - A European Journal, 2009)
New [3:3]hexakisadducts 3, 6, and 7 have been synthesized by a stepwise addition of two tripodal malonate tethers to pristine [60]fullerene. For the first time, [3:3]hexakisadducts of two sets of asymmetrically substituted malonate addends could be prepared as single in/out isomers by following this synthetic protocol. Thereby, four spherically defined addend zones I-IV were created with various similar degrees of latitude on the fullerene sphere. Four amphiphilic [3:3]hexakisadducts 12, 14, 18, and 19 were prepared with different relative arrangements of both the hydrophilic and lipophilic moieties in the addend zones I/II and III/IV, respectively. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

Synthesis of carbon core–shell pore structures and their performance as supercapacitors - Ariyanto, Ariyanto; Dyatkin, Boris; Zhang, Guirong; Kern, Andreas; Gogotsi, Yury; Etzold, Bastian (Microporous and Mesoporous Materials, 2015)
High-power supercapacitors require excellent electrolyte mobility within the pore network and high electrical conductivity for maximum capacitance and efficiency. Achieving high power typically requires sacrificing energy densities, as the latter demands a high specific surface area and narrow porosity that impedes ion transport. We present a novel solution for this optimization problem: a nanostructured core-shell carbonaceous material that exhibits a microporous carbon core surrounded by a mesoporous, graphitic shell. Our tunable synthesis parameters yielded a structure that features either a sharp or a gradual transition between the core and shell sections. Electrochemical supercapacitor testing using organic electrolyte revealed that these novel core-shell materials outperform carbons with homogeneous pore structures. The hybrid core-shell materials showed a combination of good capacitance retention, typical for the carbon present in the shell and high specific capacitance, typical for the core material. These materials achieved power densities in excess of 40 kW kg-1 at energy densities reaching 27 Wh kg-1.
EAM Import::2019-03-12

Synthesis of Goethite α-FeOOH Particles by Air Oxidation of Ferrous Hydroxide Fe(OH)2 Suspensions: Insight on the Formation Mechanism - Klupp Taylor, Robin; Encina, Ezequiel Roberto; Distaso, Monica; Taylor, Robin N. Klupp; Peukert, Wolfgang (Crystal Growth & Design, 2015)
Iron oxide and iron oxyhydroxide particles, particularly, the goethite extgreeka-FeOOH phase, are environmentally friendly materials and are used in various technological applications as adsorbents, precursors of Fe powders for magnetic recording media, and pigments. In this work, the formation process of extgreeka-FeOOH by air oxidation of Fe(OH)2 suspensions has been studied. The effects of the air flow rate, as well as of the reactant concentration ratio, R (=[(OH)-]/[Fe(II)]), on the reaction product were analyzed. It has been found that the morphology and the size of the extgreeka-FeOOH particles can be modified by means of the air flow rate. Furthermore, by performing a detailed microscopic analysis of the morphology of the initial, intermediate, and final reaction products, we have obtained evidence of epitaxial growth of extgreeka-FeOOH on the Fe(OH)2 substrate. It is suggested that the similarity between the anion arrangements in both phases facilitates this process. Based on these results, a pathway for the formation of extgreeka-FeOOH in highly alkaline medium is proposed in which the size and shape of the initial Fe(OH)2 particles plays a significant role in the formation the extgreeka-FeOOH particles obtained upon completion of the oxidation process.
EAM Import::2019-03-12
Keywords: Oxidation reactions; Atmospheric chemistry; Suspensions; Anions; Oxidation

Synthesis of Microporous Carbon Foams as Catalyst Supports - Glenk, Friedrich; Knorr, Tilman; Schirmer, Matthias; Gütlein, Stefan; Etzold, Bastian (Chemical Engineering & Technology, 2010)
Microporous carbon foams were synthesized as structured catalyst supports by the carbide-derived carbon (CDC) method. The self-supporting foams showed a narrow pore size distribution which seems to induce shape-selective effects during reactions. The applicability of these materials as supports for active metals was tested in a hydrogenation reaction of different olefins with a CDC powder wet impregnated with platinum
EAM Import::2019-03-12
Keywords: Carbide-derived carbon; CDC method; Foams; Microporous carbon; CARBIDE-DERIVED CARBONS; HYDROGEN STORAGE; ACTIVATED CARBON; MONOLITHS; NITROBENZENE; POROSITY; REACTOR

Synthesis of multilamellar MFI-type zeolites under static conditions: The role of gel composition on their properties - Machoke, Albert; Knoke, I. Y.; Lopez-Orozco, S.; Schmiele, Martin; Selvam, Thangaraj; Marthala, V. R. R.; Spiecker, E.; Unruh, Tobias; Hartmann, Martin; Schwieger, Wilhelm; Hartmann, Martin; Spiecker, Erdmann (Microporous and Mesoporous Materials, 2014)
The formation of multilamellar MFI-type zeolite has been studied systematically by investigating the crystallization kinetics and the influence of different synthesis parameters under static conditions. Depending on the template amount and alkalinity, multilamellar MFI-type zeolites with different degrees of purity were obtained. In accordance with X-ray diffraction, SAXS and SEM results, highly crystalline multilamellar MFI-type zeolites were obtained within 4 days of crystallization time. Depending on the Si/Al ratio of the synthesis gel, different crystallization rates, textural properties and product morphologies were observed. (C) 2014 Elsevier Inc. All rights reserved.
Keywords: Nanosheets of MFI; Multilamellar MFI; Hierarchical zeolites; ZSM-5; Static conditions

Synthesis of silicon nanoparticles with a narrow size distribution: A theoretical study - Peukert, Wolfgang; Körmer, Richard; Menz, William; Shekar, Shraddha; Brownbridge, George; Mosbach, Sebastian; Kraft, Markus (Journal of Aerosol Science, 2012)
This work presents a study of the processes involved in synthesis of narrowly distributed silicon nanoparticles from the thermal decomposition of silane. Two models are proposed, one which simultaneously solves the kinetic mechanism of Swihart & Girshick (1999, Journal of Physical Chemistry B 103, 64–76) while adjusting the sintering parameters; and another which adjusts the kinetic and surface growth mechanisms while neglecting coagulation and sintering. The models are applied to simulate the centreline of the hot-wall reactor and process conditions of Körmer et al. (2010, Journal of Aerosol Science 41, 998–1007). Both models are shown to give good agreement with experimental PSDs at a range of process conditions. However, it is reported that an unphysical sintering process is obtained when attempting to use Swihart & Girshick’s kinetic mechanism, while solving for the sintering parameters. The model with adjusted gas-phase and surface growth processes gives better quantitative and qualitative agreement with experimental results. It is therefore recommended that further study into the kinetic and heterogeneous growth mechanisms be conducted in order to better understand the fundamental processes occurring in this hot-wall reactor.
Keywords: synthesis; nanoparticle; thermal; decomposition; sintering; coagulation

Synthesis of silver nanoparticle necklaces without explicit addition of reducing or templating agents - Hanisch, Mathias; Mackovic, Mirza; Taccardi, Nicola; Spiecker, Erdmann; Klupp Taylor, Robin (Chemical Communications, 2012)
Here we report that silver nanoparticle necklaces can be readily formed by treatment of colloidal silica with ammoniacal silver complex solution followed by washing, deposition and ageing. We investigate the morphology of the produced materials and elucidate the key variables that influence this promising new approach to one-dimensional nanostructuring. © The Royal Society of Chemistry 201
Keywords: templating; agent; nano; particle

Synthesis of silver nanoparticles in melts of amphiphilic polyesters - Vasylyev, Serhiy; Damm, Cornelia; Segets, Doris; Hanisch, Mathias; Taccardi, Nicola; Wasserscheid, Peter; Peukert, Wolfgang (Nanotechnology, 2013)
The current work presents a one-step procedure for the synthesis of amphiphilic silver nanoparticles suitable for production of silver-filled polymeric materials. This solvent free synthesis via reduction of Tollens' reagent as silver precursor in melts of amphiphilic polyesters consisting of hydrophilic poly(ethylene glycol) blocks and hydrophobic alkyl chains allows the production of silver nanoparticles without any by-product formation. This makes them especially interesting for the production of medical devices with antimicrobial properties. In this article the influences of the chain length of the hydrophobic block in the amphiphilic polyesters and the process temperature on the particle size distribution (PSD) and the stability of the particles against agglomeration are discussed. According to the results of spectroscopic and viscosimetric investigations the silver precursor is reduced to elemental silver nanoparticles by a single electron transfer process from the poly(ethylene glycol) chain to the silver ion.

Synthesis of ZIF-11-Effect of water residues in the solvent onto the phase transition from ZIF-11 to ZIF-7-III - Reif, Benjamin; Fabisch, Florian; Hovestadt, Maximilian; Hartmann, Martin; Schwieger, Wilhelm (Microporous and Mesoporous Materials, 2017)
The syntheses of MOFs and ZIFs are often accompanied by the formation of unwanted byproducts. Also for the ZIF-11 synthesis in N,N-Diethylformamide (DEF), phase impurities have been reported. In this systematic study, water residues in the reaction mixture could be identified as one major reason for the formation of the phase impurity ZIF-7-Ill within the ZIF-11 synthesis. The parameters 'crystallinity' and 'phase index' were introduced to quantify the phase transition. (C) 2017 Elsevier Inc. All rights reserved.
Keywords: ZIF-11 synthesis; Phase transition; ZIF-7-III

Synthesis, Radiosynthesis and Biological Evaluation of Buprenorphine-Derived Phenylazocarboxamides as Novel mu-Opioid Receptor Ligands - Krüll, Jasmin; Fehler, Stefanie; Hofmann, Laura; Nebel, Natascha; Maschauer, Simone; Prante, Olaf; Gmeiner, Peter; Lanig, Harald; Hübner, Harald; Heinrich, Markus (ChemMedChem, 2020)
Targeted structural modifications have led to a novel type of buprenorphine-derived opioid receptor ligand displaying an improved selectivity profile for the mu-OR subtype. On this basis, it is shown that phenylazocarboxamides may serve as useful bioisosteric replacements for the widely occurring cinnamide units, without loss of OR binding affinity or subtype selectivity. This study further includes functional experiments pointing to weak partial agonist properties of the novel mu-OR ligands, as well as docking and metabolism experiments. Finally, the unique bifunctional character of phenylazocarboxylates, herein serving as precursors for the azocarboxamide subunit, was exploited to demonstrate the accessibility of an F-18-fluorinated analogue.
Keywords: azo compounds; bioisosters; buprenorphine; opioid receptors; radiosynthesis

Systematic increase of electrocatalytic turnover at nanoporous platinum surfaces prepared by atomic layer deposition - Assaud, Loic; Schumacher, Johannes; Tafel, Alexander; Bochmann, Sebastian; Christiansen, Silke; Bachmann, Julien (Journal of Materials Chemistry A, 2015)
We establish a procedure for the fabrication of electrocatalytically active, nanoporous surfaces coated with Pt and exhibiting a high geometric area. Firstly, the mechanism of the surface reactions between platinum(II) acetylacetonate and ozone is investigated by piezoelectric microbalance measurements. The data reveal that ozone oxidizes the metallic Pt surface to an extent which can exceed one monolayer depending on the reaction conditions. Proper reaction parameters yield a self-limited growth in atomic layer deposition (ALD) mode. Secondly, the ALD procedure is applied to porous anodic oxide substrates. The morphology and the crystal structure of the deposits are characterized. The ALD coating results in a continuous layer of Pt nanocrystallites along deep pore walls (aspect ratio 70). Thirdly, the Pt/TiO2 surfaces are shown to be electrochemically active in both acidic and alkaline media, in a way that qualitatively conforms to literature precedents based on Pt. Finally, we apply the anodization and ALD procedure to commercial Ti felts and demonstrate systematically how the electrochemical current density is increased by the large specific surface area and by the presence of the catalyst. Thereby, the catalyst loading, as well as its efficient utilization, can be optimized accurately. The preparative approach demonstrated here can be generalized and applied to the various electrocatalytic reactions of energy conversion device

Systematic Onset of Periodic Patterns in Random Disk Packings - Topic, Nikola; Pöschel, Thorsten; Gallas, Jason (Physical Review Letters, 2018)
We report startling evidence of a systematic onset of periodic structures in large piles of disks deposited between rigid walls, independently of the pile width, observed in numerically constructed monodisperse packings with up to 107 disks. Disordered transient phases preceding the periodicity onset are found to obey power-laws as a function of the channel height. Independently of the width, we invariably find packings to be asymptotically periodic after disordered transients which, however, may become very large as the channel width grows without bound. The probability density of finding periodic structures of a given period displays a series of discrete peaks which, however, are washed out when the channel width grows indefinitely.

Systematic Surface Phase Transition of Ag Thin Films by Iodine Functionalization at Room Temperature: Evolution of Optoelectronic and Texture Properties - Bashouti, Muhammad Y.; Talebi, Razieh; Kassar, Thaer; Nahal, Arashmid; Ristein, Jürgen; Unruh, Tobias; Christiansen, Silke H. (Scientific Reports, 2016)
We show a simple room temperature surface functionalization approach using iodine vapour to control a surface phase transition from cubic silver (Ag) of thin films into wurtzite silver-iodid (beta-AgI) films. A combination of surface characterization techniques (optical, electronical and structural characterization) reveal distinct physical properties of the new surface phase. We discuss the AgI thin film formation dynamics and related transformation of physical properties by determining the work-function, dielectric constant and pyroelectric behavior together with morphological and structural thin film properties such as layer thickness, grain structure and texture formation. Notable results are: (i) a remarkable increase of the work-function (by 0.9 eV) of the Ag thin layer after short a iodine exposure time (<= 60 s), with simultaneous increase of the thin film transparency (by two orders of magnitude), (ii) pinning of the Fermi level at the valance band maximum upon iodine functionalization, (iii) 84% of all crystallites grain were aligned as a result of the evolution of an internal electric field. Realizing a nanoscale layer stack composed of a dielectric AgI layer on top of a metallic thin Ag layer with such a simple method has some technological implications e.g. to realize optical elements such as planar optical waveguides.

Systematic tuning of segmented magnetic nanowires into three-dimensional arrays of ‘bits’ - Bochmann, Sebastian; Fernández-Pacheco, Amalio; Mackovic, Mirza; Neff, Andreas; Siefermann, Katrin Rita; Spiecker, Erdmann; Cowburn, Russell P.; Bachmann, Julien (RSC Advances, 2017)
A method is presented for the preparation of a three-dimensional magnetic data storage material system. The major ingredients are an inert nanoporous matrix prepared by anodization and galvanic plating of magnetic and non-magnetic metals in wire shape inside the cylindrical pores. The individual nanomagnets consist of a nickel–cobalt alloy, the composition of which is tuned systematically by adjusting the electrolytic bath composition at one optimal applied potential. The lowest magnetocrystalline anisotropy is obtained at the composition Ni60Co40, as quantified by superconducting quantum interference device magnetometry. Wires of this composition experience a pinning-free propagation of magnetic domain walls, as determined by single-wire magneto-optical Kerr effect magnetometry. Adding copper into the electrolyte allows one to generate segments of Ni60Co40 separated by non-magnetic copper. The segment structure is apparent in individual nanowires imaged by scanning electron microscopy, UV-photoelectron emission microscopy, and transmission electron microscopy. The single-domain structure of the wire segments is evidenced by magnetic force microscopy.

Tackling time-reversibility in transition path sampling molecular dynamics simulations - Zahn, Dirk (Molecular Simulation, 2012)
We discuss various implementations of the transition path sampling (TPS) method to investigate rare-events in complex systems. The key focus is dedicated to the different demands posed by molecular dynamics simulations of barrier crossing events. Thus, technical issues and tricks of trade are collected from earlier simulations of reactions, conformational changes, phase transition and phase separation processes. Along this line, the improvement of time-reversible molecular dynamics algorithms and the sampling of pathways with limited time-reversibility represent the central obstacles. The latter aspect not only extends the perspectives of the conventional TPS approach, but also leads over to the transition interface sampling concept.
EAM Import::2019-03-12

Ta-Doped TiO2 Nanotubes for Enhanced Solar-Light Photoelectrochemical Water Splitting - Altomare, Marco; Lee, Kiyoung; Killian, Manuela; Selli, Elena; Schmuki, Patrik (Chemistry - A European Journal, 2013)
A little dopey: Ta-doped titania (TiO2) nanotube (NT) arrays can be grown by electrochemical anodization onto low-Ta-concentration (0.03–0.4at % Ta) Ti–Ta alloys. Under optimized conditions (0.1at % Ta, annealing at 650 °C and 7 extgreekmm thickness), Ta-doped TiO2 NT arrays show a significantly enhanced activity in photoelectrochemical water splitting under simulated sunlight conditions.
EAM Import::2019-03-12

Ta doping for an enhanced efficiency of TiO2 nanotube based dye-sensitized solar cells - Lee, Kiyoung; Schmuki, Patrik (Electrochemistry Communications, 2012)
Ta doped TiO2 nanotube layers were fabricated by anodization of different Ti–Ta alloys with Ta contents of 0.03, 0.1 and 0.4 at.%. For all layers well defined nanotubular structures could be formed. The layers were then characterized in dye-sensitized solar cells. Under optimized conditions, that is for a 0.1 at.% Ta alloy, the solar cell efficiency could be enhanced by 25% compared to non-doped TiO2 nanotubes. Intensity modulated photocurrent spectra (IMPS) and intensity modulated photovoltage spectra (IMVS) measurements performed with the solar cells indicate that the beneficial effect is due to suppression of recombination effects.
EAM Import::2019-03-12

Tagged-particle motion in a dense confined liquid - Lang, Simon; Franosch, Thomas (Physical Review E, 2014)
We investigate the dynamics of a tagged particle embedded in a strongly interacting confined liquid enclosed between two opposing flat walls. Using the Zwanzig-Mori projection operator formalism we obtain an equation of motion for the incoherent scattering function suitably generalized to account for the lack of translational symmetry. We close the equations of motion by a self-consistent mode-coupling ansatz. The interaction of the tracer with the surrounding liquid is encoded in generalized direct correlation functions. We extract the in-plane dynamics and provide a microscopic expression for the diffusion coefficient parallel to the walls. The solute particle may differ in size or interaction from the surrounding host-liquid constituents offering the possibility of a systematic analysis of dynamic effects on the tagged-particle motion in confinement.
EAM Import::2019-03-12

Tailored catalyst pellet specification for improved fixed-bed transport characteristics: A shortcut method for the model-based reactor design - Pietschak, Alexander; Kaiser, Markus; Freund, Hannsjörg (Chemical Engineering Research & Design, 2018)
tTo improve the transport characteristics of a catalytic fixed-bed via optimal design of catalystpellet specifications such as the pellet diameter it is necessary to account for all physico-chemical phenomena influenced by the pellet. For the approximative description of thetransport phenomena on the catalyst pellet scale a new shortcut method is developed inthis work. It enables a generalized and system independent treatment of the aforementionedprocesses and can be applied to arbitrary reaction networks and reaction kinetic models.Based on linearization and decoupling of the pellet balance equations the method yields ananalytical solution. This allows for model-based design of the reactor-catalyst system viadynamic optimization at reduced computational costs as the use of complex heterogeneousmodels is avoided. In order to ensure accurate predictions of the method, regions with highcatalyst utilization are targeted. To indicate the potential of improved bed transport characteristics, the shortcut method isapplied to the reactor-catalyst system for ethylene oxide synthesis. The system is optimizedin terms of reducing the pressure drop while meeting other reactor performance constraints.The pressure drop could be reduced by more than 60%. The shortcut method is validatedusing a rigorous stand-alone model of the catalyst pellet.
Keywords: Reactor optimization; Thiele modulus; Fixed-bed reactors; Process intensification; Catalyst pellet design

Tailored heat treated accumulative roll bonded aluminum blanks: failure under bending stresses - Nguyen, H.; Boehm, W.; Tsoupis, Ioannis; Göken, Mathias; Höppel, Heinz Werner; Schmidt, Christian; Hausöl, Tina; Ruppert, Mathis; Merklein, Marion (Production Engineering, 2016)
Ultrafine-grained accumulative roll bonded (ARB) sheet metals of aluminum alloys have a high potential for lightweight construction. The mechanical properties can be enhanced regarding strength and ductility by the combination of ARB and a local heat treatment according to the Tailor Heat Treated Blanks technology. The present investigation focuses on the failure behavior of ultrafine-grained ARB blanks. Due to the low formability of these high-strength ARB metals, a detailed understanding of the failure mechanisms is essential. For this purpose, an established approach to determine the different stages of damage of the material for conventional materials is now applied to multilayered aluminum in the as-received and heat-treated condition. In this context, air bending tests are used to qualify and quantify the bending and forming behavior of ARB sheets of AA1050A and AA6016 aluminum alloys.
Keywords: Tailor Heat Treated Blanks (THTB); Ultrafine-grained microstructure (UFG); Failure; Formability; Bending; Accumulative roll bonding (ARB)

Tailored heat treated accumulative roll bonded aluminum blanks: Microstructure and mechanical behavior - Maier, Verena; Hausöl, Tina; Schmidt, Christian; Böhm, Wolfgang; Hung Nguyen,; Merklein, Marion; Höppel, Heinz Werner; Göken, Mathias (Metallurgical and Materials Transactions A-Physical Metallurgy and Materials Science, 2012)
Aluminum alloy AA6016 was accumulative roll bonded up to eight cycles and investigated regarding formability by bending tests. Due to the limited bendability of accumulative roll bonding (ARB) processed materials, a tailored laser heat treatment was performed along the bending edge before forming. This tailored laser heat treatment causes a local recrystallization and recovery of the bending samples at the deformation zone, which locally increases ductility and allows higher bending angles achievable with lower forming forces. Between the recrystallized heat treated zone and the unaffected ultrafine-grained (UFG) base material, a gradient in grain size with a bimodal region is formed. This observed microstructural profile is confirmed by local mechanical testing measuring the hardness and strain rate sensitivity by nanoindentation techniques.
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Tailoring Material Properties of Aluminum by Local Laser Heat Treatment - Merklein, Marion; Böhm, Wolfgang; Lechner, Michael (Physics Procedia, 2012)
The local laser heat treatment of precipitation hardenable aluminum alloys enables the tailoring of the materials properties and the manufacturing of lightweight and crash-proof components for the body in white. For conventional aluminum sheet metals and profiles the formability is enhanced by an indirect approach. Thereby specific parts of the products are softened in order to improve the material flow towards crack critical areas during the forming operation. For innovative high-strength aluminum sheet metal with nanocrystalline grain structure produced in the Accumulative Roll Bonding process in addition to the indirect approach a direct enhancement of the formability by recrystallization effects is possible.
EAM Import::2019-03-12

Tailoring materials properties by accumulative roll bonding - Hausöl, Tina; Maier, Verena; Schmidt, Christian; Winkler, Michael; Höppel, Heinz Werner; Göken, Mathias (Advanced Engineering Materials, 2010)
Accumulative roll bonding (ARB) as a method of severe plastic deformation (SPD) is an interesting established process to produce ultrafine-grained (UFG) sheet materials with high potential for light weight constructions. The ARB process offers a high flexibility for tailored material design. Al_2O_3 particles, carbon fibers and titanium foils are used as reinforcement of aluminum sheets introduced during accumulative roll bonding. Furthermore multicomponent materials are produced by cladding of different aluminum alloys. These sandwich-like structures allow to combine desired properties of the materials involved. Post-ARB heat treatment offers another possibility for tailoring materials properties of graded structures as shown by formation of TiAl_3 in Al/Ti laminates. The tailored materials are investigated by means of SEM, EDX, nanoindentation experiments and tensile testing.
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Tailoring materials properties of UFG aluminium alloys by accumulative roll bonded sandwich-like sheets - Hausöl, Tina; Höppel, Heinz Werner; Göken, Mathias (Journal of Materials Science, 2010)
Accumulative roll bonding (ARB) as a method of severe plastic deformation is a well-established process to produce ultrafine-grained (UFG) sheet materials with extraordinary mechanical properties. In this work ARB is applied to combine different sheet materials in order to tailor the materials properties by producing sandwich-like structures. The high strength aluminium alloy AA5754, after 4 ARB cycles (N4), is used as a core material. To achieve high corrosion resistance and good visual properties, it is cladded with commercially pure aluminium AA1050A (N4) at room temperature and alternatively with AA6014 (N4) at 230 °C. All materials are UFG and satisfactory bonding between the different layers of aluminium alloys is achieved. Nanoindentation measurements reveal that there is a sharp transition in hardness at the interface. The yield and tensile strength of the core material are fully retained in the case of the AA6014/AA5754 sandwich. The strength of the AA1050A/AA5754 sandwich is slightly lower compared to the core material but still twice as high as the clad material. The serrated yielding effect which is strongly visible in tensile tests on the pure AA5754 alloy completely disappears in the sandwich sheets, which means the surface quality is strongly enhanced.
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Tailoring nanostructured, graded, and particle-reinforced Al laminates by accumulative roll bonding - Göken, Mathias; Höppel, Heinz Werner (Advanced Materials, 2011)
Accumulative roll bonding (ARB) is a very attractive process for processing large sheets to achieve ultrafine-grained microstructure and high strength. Commercial purity Al and many Al alloys from the 5xxx and the precipitation strengthened 6xxx alloy series have been successfully processed by the ARB process into an ultrafine-grained state and superior ductility have been achieved for some materials like technical purity Al. It has also been shown that the ARB process can be successfully used to produce multi-component materials with tailored properties by reinforcement or grading, respectively. This allows optimizing the properties based on two or more materials/alloys. For example, to achieve high corrosion resistance and good visual surface properties it is interesting to produce a composite of two different Al alloys, where for example a high strength alloy of the 5xxx series is used as the core material and a 6xxx series alloy as the clad material. It has been shown that such a composite achieves more or less the same strength as the core material although 50% of the composite consists of the significant softer clad alloy. Furthermore, it has been found, that the serrated yielding which typically appears in 5xxx series alloys and limits applications as outer skin materials completely disappears. Moreover, the ARB process allows many other attractive ways to design new composites and graded material structures with unique properties by the introduction of particles, fibres and sheets. Strengthening with nanoparticles for example is a very attractive way to improve the properties and accelerate the grain refining used in the severe plastic deformation process. With an addition of only 0.1 vol.-% Al_2O_3 nanoparticles a significantly accelerated grain refinement has been found which reduces the number of ARB passes necessary to achieve the maximum in strength. The paper provides a short review on recent developments in the field of ARB processing for producing multicomponent ultrafine-grained sheet materials with tailored properties.
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Tailoring re-entrant geometry in inverse colloidal monolayers to control surface wettability - Utech, Stefanie; Bley, Karina; Aizenberg, Joanna; Vogel, Nicolas (Journal of Materials Chemistry A, 2016)
Controlling the microscopic wetting state of a liquid in contact with a structured surface is the basis for the design of liquid repellent as well as anti-fogging coatings by preventing or enabling a given liquid to infiltrate the surface structures. Similarly, a liquid can be confined to designated surface areas by locally controlling the wetting state, with applications ranging from liquid transport on a surface to creating tailored microenvironments for cell culture or chemical synthesis. The control of the wetting of a low-surface-tension liquid is substantially more difficult compared to water and requires surface structures with overhanging features, known as re-entrant geometries. Here, we use colloidal self-assembly and templating to create two-dimensional nanopore arrays with tailored re-entrant geometry. These pore arrays, termed inverse monolayers, are prepared by backfilling a sacrificial colloidal monolayer with a silica sol-gel precursor material. Varying the precursor concentration enables us to control the degree to which the colloids are embedded into the silica matrix. Upon calcination, nanopores with different opening angles result. The pore opening angle directly correlates with the re-entrant curvature of the surface nanostructures and can be used to control the macroscopic wetting behavior of a liquid sitting on the surface structures. We characterize the wetting of various liquids by static and dynamic contact angles and find correlation between the experimental results and theoretical predictions of the wetting state based on simple geometric considerations. We demonstrate the creation of omniphobic surface coatings that support Cassie–Baxter wetting states for liquids with low surface tensions, including octane ( extgreekg = 21.7 mN m-1). We further use photolithography to spatially confine such low-surface-tension liquids to desired areas of the substrate with high accuracy.

Tailoring the graphene/silicon carbide interface for monolithic wafer-scale electronics - Hertel, Stefan; Waldmann, Daniel; Jobst, Johannes; Albert, Andreas; Albrecht, Matthäus; Krieger, Michael; Reshanov, Sergey; Schöner, Adolf; Weber, Heiko B. (Nature Communications, 2012)
Graphene is an outstanding electronic material, predicted to have a role in post-silicon electronics. However, owing to the absence of an electronic bandgap, graphene switching devices with high on/off ratio are still lacking. Here in the search for a comprehensive concept for wafer-scale graphene electronics, we present a monolithic transistor that uses the entire material system epitaxial graphene on silicon carbide (0001). This system consists of the graphene layer with its vanishing energy gap, the underlying semiconductor and their common interface. The graphene/semiconductor interfaces are tailor-made for ohmic as well as for Schottky contacts side-by-side on the same chip. We demonstrate normally on and normally off operation of a single transistor with on/off ratios exceeding 10(4) and no damping at megahertz frequencies. In its simplest realization, the fabrication process requires only one lithography step to build transistors, diodes, resistors and eventually integrated circuits without the need of metallic interconnects.

Tantalum Nitride Nanorod Arrays: Introducing Ni–Fe Layered Double Hydroxides as a Cocatalyst Strongly Stabilizing Photoanodes in Water Splitting - Wang, Lei; Dionigi, Fabio; Nguyen, Nhat Truong; Kirchgeorg, Robin; Gliech, Manuel; Grigorescu, Sabina Georgiana; Strasser, Peter; Schmuki, Patrik (Chemistry of Materials, 2015)
Ta3N5 nanostructures are widely explored as anodes for photoelectrochemical (PEC) water splitting. Although the material shows excellent semiconductive properties for this purpose, the key challenge is its severe photocorrosion when used in typical aqueous environments. In the present work we introduce a NiFe layered double hydroxide (LDH) cocatalyst that dramatically reduces photocorrosion effects. To fabricate the Ta3N5 electrode, we use through-template anodization of Ta and obtain oxide nanorod arrays that then are converted to Ta3N5 by high temperature nitridation. After modification with our cocatalyst system, we obtained solar photocurrents of 6.3 mA cm–2 at 1.23 VRHE in 1 M KOH, and an electrode maintains about 80% of the initial activity for extended irradiation times.
EAM Import::2019-03-12

TEM and ToF-SIMS studies on the corrosion behavior of vanadium and chromium containing WC-Co hard metals in alkaline solutions - Kellner, Florian; Killian, Manuela; Yang, Guang; Spiecker, Erdmann; Virtanen, Sannakaisa (International Journal of Refractory Metals and Hard Materials, 2011)
The corrosion behavior of hard metals with VC and Cr_3C_2 grain growth inhibitors was investigated in alkaline solutions by electrochemical methods. The two inhibitors have opposite effects on the corrosion behavior: Cr_3C_2 significantly improves the corrosion behavior, whereas VC-containing alloys show a poor resistance. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and analytical transmission electron microscopy (TEM) analyses of the distributions of Cr and V in the composite material, as well as in the surface layers formed during corrosion were employed to clarify the influence of these elements on the corrosion behavior. The measurements showed that VC is precipitated mostly along the WC/binder interface after the liquid-phase sintering process, while Cr_3C_2 is almost homogeneously dissolved in the binder. As VC is chemically instable in alkaline solutions, it completely dissolves out of the binder. In accordance with this observation no V was found in the corrosion product layer on the surface. As WC is more noble than Co, galvanic coupling between the two phases reinforces the Co dissolution, while the WC-phase is protected cathodically. Contrary to VC, chromium is stable in alkaline environments and forms a passivating Cr_2O_3 layer. Enrichment of Cr in the corrosion product layers was detected by TEM and ToF-SIMS. Due to surface passivation by Cr_2O_3, galvanic coupling effects between Co and WC play a much less important role in the corrosion process of the composite material.

Temperature Dependence of the Non-Stokesian Charge Transport in Binary Blends of Ionic Liquids - Wachter, Philipp; Zistler, Markus; Schreiner, Christian; Fleischmann, Matthias; Gerhard, Dirk; Wasserscheid, Peter; Barthel, Joseph; Gores, Heiner J. (Journal of Chemical and Engineering Data, 2009)
A comprehensive characterization of an ionic liquid based electrolyte for dye-sensitized solar cells (DSSC) was performed by determination of triiodide diffusion coefficients, viscosities, specific conductivities, and densities. The observed non-Stokesian transport behavior was ensured by determination of triiodide diffusion coefficients with two independent methods, steady-state cyclic voltammetry at ultramicroelectrodes, and polarization measurements at thin layer cells. The electrolyte, consisting of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF_4]), 1-methyl-3-propylimidazolium iodide ([MPIM][I]), and iodine, was examined at fixed iodine concentration over a broad mixing range with varying ionic liquid molar ratio and over a broad temperature range as well. The triiodide diffusion coefficients and the specific conductivity increase with decreasing [MPIM][I] mole fraction or increasing temperature, caused by decreasing electrolyte viscosity. The Einstein-Stokes ratios strongly increase with increasing [MPIM][I] mole fraction and viscosity and thus do not obey the Einstein-Stokes equation. The magnitude of this strong non-Stokesian behavior decreases with increasing temperature. Additional non-Stokesian behavior was found for [MPIM][I]-rich blends since for these blends the Einstein-Stokes ratios strongly decrease at increasing temperatures and simultaneously decreasing viscosity.

Temperature-dependent chemical and structural transformations from 2H-tetraphenylporphyrin to copper(II)-tetraphenylporphyrin on Cu(111) - Xiao, Jie; Ditze, Stefanie; Chen, Min; Buchner, Florian; Stark, Michael Werner; Drost, Martin; Steinrück, Hans-Peter; Gottfried, J. Michael; Marbach, Hubertus (Journal of Physical Chemistry C, 2012)
Temperature-dependent chemical and structural changes of a submonolayer of 2H-tetraphenylporphyrin (2HTPP) on Cu(111) were studied with photoelectron spectroscopy (XPS/UPS) and scanning tunneling microscopy (STM). 2HTPP reacts with Cu atoms from the substrate to form copper(II)-tetraphenylporphyrin (CuTPP). This metalation reaction starts at about 400 K and was investigated at various temperatures up to 500 K. At room temperature, adsorbed 2HTPP adopts an orientation with the molecular plane parallel to the substrate; the same holds for its reaction product CuTPP after annealing to 400 K. In contrast, annealing at 450 K yields a tilted orientation of CuTPP, as indicated by STM and supported by C 1s XPS shifts and changes in the Cu(111) surface state. Subsequent annealing at 500 K restores a flat-lying orientation; however, the appearance of the complex in STM images differs from the original appearance of CuTPP. In summary, 2HTPP undergoes three irreversible transformations upon annealing on Cu(111), a metalation reaction to CuTPP followed by two intramolecular structural changes. © 2012 American Chemical Society.

Temperature-dependent optical spectra of single-crystal (CH3NH3)PbBr3 cleaved in ultrahigh vacuum - Niesner, Daniel; Schuster, Oskar; Wilhelm, Max; Levchuk, Ievgen; Osvet, Andres; Shrestha, Shreetu; Batentschuk, Miroslaw; Brabec, Christoph; Fauster, Thomas (Physical Review B - Condensed Matter and Materials Physics, 2017)
We measure temperature-dependent one-photon and two-photon induced photoluminescence from (CH3NH3)PbBr3 single crystals cleaved in ultrahigh vacuum. An approach is presented to extract absorption spectra from a comparison of both measurements. Cleaved crystals exhibit broad photoluminescence spectra. We identify the direct optical band gap of 2.31 eV. Below 200 K, the band gap increases with temperature, and it decreases at elevated temperature, as described by the Bose-Einstein model. An excitonic transition is found 22 meV below the band gap at temperatures <200 K. Defect emission occurs at photon energies <2.16 eV. In addition, we observe a transition at 2.25 eV (2.22 eV) in the orthorhombic (tetragonal and cubic) phase. Below 200 K, the associated exciton binding energy is also 22 meV, and the transition redshifts at higher temperature. The binding energy of the exciton related to the direct band gap, in contrast, decreases in the cubic phase. High-energy emission from free carriers is observed with higher intensity than reported in earlier studies. It disappears after exposing the crystals to ai

Temperature-dependent surface-enrichment effects of imidazolium-based ionic liquids - Kolbeck, Claudia; Deyko, Alexey; Matsuda, Takashi; Kohler, Florian; Wasserscheid, Peter; Maier, Florian; Steinrück, Hans-Peter (ChemPhysChem, 2013)
We present the first systematic study of the influence of temperature on the degree of surface enrichment of 1-alkyl-3-methylimidazolium-based ionic liquids (ILs). Using angle-resolved X-ray photoelectron spectroscopy, we demonstrate that the degree of surface enrichment strongly decreases with increasing temperature for all the studied ILs. For ILs with the same cation, but different anions, [CCIm]Br, [CC Im][TfO] and [CCIm][TfN], no significant differences of the temperature-induced partial loss of surface enrichment are found. Measurements for [CCIm][TfO], [CCIm][TfO] and [CCIm][TfO] indicate a small effect of the chain length. For [CC Im][TfO], a continuous decrease of alkyl surface enrichment is found with increasing temperature, with no abrupt changes at the phase-transition temperature from the smectic A to the isotropic phase, indicating that the surface enrichment is not affected by this phase transition. Loss of order! The surface enrichment of the alkyl chains of five imidazolium-based ionic liquids (ILs) at the liquid/vacuum interface is studied as a function of temperature by angle-resolved X-ray photoelectron spectroscopy; both the anion and the alkyl chain length at the cation are varied. At room temperature, the alkyl chains are enriched at the outer surface. Upon increasing the temperature, the degree of surface enrichment decreases. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Temperature of a granular gas with regard to the stochastic nature of particle interactions - Gunkelmann, Nina; Serero, Dan; Pöschel, Thorsten (New Journal of Physics, 2013)
The decay of temperature of a force-free granular gas in the homogeneous cooling state depends on the specific model for particle interaction. For the case of rough spheres, in recent experimental and theoretical work, the coefficient of restitution was characterized as a fluctuating quantity. We show that for such particles, the decay of temperature with time follows the law which deviates from Haff's law, T ∼ t, obtained for gases of particles interacting via a constant coefficient of restitution also from obtained for gases of viscoelastic particles. Our results are obtained from kinetic theory and are in very good agreement with Monte Carlo simulations. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Template-assisted formation of fullerenes from short-chain hydrocarbons by supported platinum nanoparticles - Vines Solana, Francesc; Görling, Andreas (Angewandte Chemie International Edition, 2011)
A C bubble-maker: Induced by platinum nanoparticle templates, carbon atoms formed during the dehydrogenation of small hydrocarbon molecules gather and, reaching a certain coverage on the nanoparticle templates, form fullerenes (see picture). The tailor-made Pt nanoparticle templates exhibit defined shapes and sizes and are supported on inert or basic oxide supports. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Template-Dependent Photochemical Reactivity of Molecular Metal Oxides - Tucher, Johannes; Peuntinger, Katrin; Margraf, Johannes; Clark, Timothy; Guldi, Dirk Michael; Streb, Carsten (Chemistry - A European Journal, 2015)
A combined experimental and theoretical study shows that the photooxidative activity of two isostructural metal oxide clusters depends on their internal templates. To this end, two halide-templated bismuth vanadium oxide clusters [X(Bi(dmso)(3))(2)V12O33](-) (X=Cl-, Br-) are reported and fully characterized. The two clusters show similar absorption features and illustrate that bismuth incorporation results in increased visible-light absorption. Significantly higher photooxidative activity is observed for the bromide-templated cluster compared with the chloride-templated one. Detailed photophysical assays and complementary DFT calculations suggest that the more efficient triplet excited state formation in the Br–containing cluster is the decisive step in the photocatalysis and is due to the heavy-atom effect of the bromide. This concept can therefore open new pathways towards the optimization of photocatalytic activity in metal oxide clusters.
Keywords: density functional calculations; metal oxide; polyoxometalate; self-assembly; template

Templating Approaches Using Natural Cellular Plant Tissue - Greil, Peter (Mrs Bulletin, 2010)
Biological preforms such as plant tissue offer a novel approach for manufacturing biomorphous ceramics with an anisotropic cellular micro- and macrostructure pseudomorphous to the natural template structure. Mimicking the hierarchical microstructure of the native template at different length scales from large vessels (mm) down to a cell wall microstructure ( extgreekmm to nm) offers the possibility to tailor the local strut microstructure in biomorphous ceramics in order to improve mechanical properties at low density. Mineralization may be achieved by intercalation of the cell walls with an inorganic, metal organic, or organometallic sol. Heating above the pyrolysis temperature of the hydrocarbons forming the cell wall material in an inert atmosphere finally results in a positive replica of the cellular structure with a metal oxide/carbon composite forming the cell walls. Amorphous, nano- or microcrystalline C/Si-O-C(-N) composite materials are obtained by infiltration with a low viscosity preceramic polymeric precursor, such as polycarbosilane, -silazane, -siloxane, or a copolymer or mixture thereof. Pyrolysis into a biocarbon template and subsequent metal alloy melt or vapor infiltration and reaction at high temperatures above 1000°C is an alternate way to produce single and multiphase carbides and composites.
EAM Import::2019-03-12

TEM preparation method for site- and orientation-specific sectioning of individual anisotropic nanoparticles based on shadow-FIB geometry - Vieweg, Benito; Butz, Benjamin; Peukert, Wolfgang; Klupp Taylor, Robin; Spiecker, Erdmann (Ultramicroscopy, 2012)
For the comprehensive characterization of nanoparticles cross-sectional investigation on the atomic scale by analytical and high-resolution transmission electron microscopy (TEM) is indispensable. Cross-sectioning is especially important for anisotropic nanoparticles to gain information on structure and chemistry along all important projections. We present a focused ion beam (FIB) method for site- and orientation-specific cross-sectioning of arbitrary nanoparticles that are dispersed on a substrate. By adopting a shadow geometry originally developed for thin sensitive films' protection of the specimen by a platinum layer is avoided. This enables simultaneous observation (from the front side) by the electron beam and ion-beam sectioning (from the back side of the supporting substrate) of individually selected particles with excellent accuracy on the nanometer scale. The feasibility and general applicability of the method is demonstrated by site-specific sectioning and cross-section HRTEM investigation of two types of anisotropic nanostructures: silver nanorods with five-fold twin structure and Janus-type silver patchy particles. © 2011 Elsevier B.V.
Keywords: FIB; High-resolution transmission electron microscopy; Nanoparticles; Nanorods; Patchy particles; TEM preparation

Tensorial density functional theory for non-spherical hard-body fluids - Hansen-Goos, Hendrik; Mecke, Klaus (Journal of Physics: Condensed Matter, 2010)
In a recent publication (Hansen-Goos and Mecke 2009 Phys. Rev. Lett. 102 018302) we constructed a free energy functional for the inhomogeneous hard-body fluid, which reduces to Rosenfeld's fundamental measure theory (Rosenfeld 1989 Phys. Rev. Lett. 63 980) when applied to hard spheres. The new functional is able to yield the isotropic-nematic transition for the hard-spherocylinder fluid in contrast to Rosenfeld's fundamental measure theory for non-spherical particles (Rosenfeld 1994 Phys. Rev. E 50 R3318). The description of inhomogeneous isotropic fluids is also improved when compared with data from Monte Carlo simulations for hard spherocylinders in contact with a planar hard wall. However, the new functional for the inhomogeneous fluid in general does not comply with the exact second order virial expansion. We introduced the. correction in order to minimize the deviation from Onsager's exact result in the isotropic bulk fluid. In this article we give a detailed account of the construction of the new functional. An extension of the. correction makes the latter better suited for non-isotropic particle distributions. The extended. correction is shown to improve the description of the isotropic-nematic bulk phase diagram while it has little effect on the results for the isotropic but inhomogeneous hard-spherocylinder fluid. We argue that the gain from using higher order tensorial weighted densities in the theory is likely to be inferior to the associated increase in complexity.

Tensorial Minkowski functionals and anisotropy measures for planar patterns - Schröder-Turk, Gerd; Kapfer, Sebastian; Breidenbach, B.; Beisbart, C.; Mecke, Klaus (Journal of Microscopy, 2010)
Quantitative measures for anisotropic characteristics of spatial structure are needed when relating the morphology of microstructured heterogeneous materials to tensorial physical properties such as elasticity, permeability and conductance. Tensor-valued Minkowski functionals, defined in the framework of integral geometry, provide a concise set of descriptors of anisotropic morphology. In this article, we describe the robust computation of these measures for microscopy images and polygonal shapes. We demonstrate their relevance for shape description, their versatility and their robustness by applying them to experimental data sets, specifically microscopy data sets of non-equilibrium stationary Turing patterns and the shapes of ice grains from Antarctic cores.
Keywords: Anisotropy; integral geometry; microstructured and cellular; materials; Minkowski functionals; morphology; strain and deformation; Turing patterns

Tensorial Minkowski functionals of triply periodic minimal surfaces - Mickel, Walter; Schröder-Turk, Gerd; Mecke, Klaus (Interface Focus, 2012)
A fundamental understanding of the formation and properties of a complex spatial structure relies on robust quantitative tools to characterize morphology. Asystematic approach to the characterization of average properties of anisotropic complex interfacial geometries is provided by integral geometry which furnishes a family of morphological descriptors known as tensorial Minkowski functionals. These functionals are curvature-weighted integrals of tensor products of position vectors and surface normal vectors over the interfacial surface. We here demonstrate their use by application to non-cubic triply periodic minimal surface model geometries, whose Weierstrass parametrizations allow for accurate numerical computation of the Minkowski tensors.
Keywords: morphology; anisotropy; lipid self-assembly; minimal surface; porous material; integral geometry

Texture evolution and microstructural changes during solid-state dewetting: A correlative study by complementary in situ TEM techniques - Niekiel, Florian; Kraschewski, Simon; Schweizer, Peter; Butz, Benjamin; Spiecker, Erdmann (Acta Materialia, 2016)
The transition of a thin film into an energetically favorable set of particles at temperatures below the melting point of the bulk material is known as solid-state dewetting. In this work the dewetting behavior of 16 nm thick discontinuous Au thin films on amorphous silicon nitride membranes is quantitatively studied by complementary in situ transmission electron microscopy techniques taking advantage of the unique capabilities of a chip-based heating system. The combination of dedicated imaging and diffraction techniques is used to investigate the interplay of grain growth and texture evolution with the process of dewetting. The results show an initial coarsening of the microstructure preceding the other processes. Texture evolution is highly correlated to material retraction and agglomeration during the following dewetting process. In-plane grain rotation has been observed, acting as an additional mechanism for orientation changes. From a methodological perspective this work demonstrates the capabilities of today's transmission electron microscopy in combination with state-of-the-art in situ instrumentation. In particular the combination of complementary information from different dedicated techniques in one and the same setup is demonstrated to be highly beneficial. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Solid-state dewetting; Thin films; Texture; Grain growth; Grain rotation

Texture, microstructure and mechanical properties of ultrafine grained aluminum produced by accumulative roll bonding - Scharnweber, Juliane; Skrotzki, Werner; Oertel, Carl-Georg; Brokmeier, Heinz-Guenter; Höppel, Heinz Werner; Topic, Irena; Jaschinski, Joern (Advanced Engineering Materials, 2010)
Texture, microstructure, and tensile behavior of technically pure aluminum AA1050 and of the age-hardening alloy AA6016 produced by accumulative roll bonding (ARB) were studied for different numbers of ARB cycles. After eight cycles an ultrafine grained microstructure with grain sizes of the order of 0.5 µm is reached. The grain size decreases with increasing alloying content. The texture consists of a major copper component and a minor brass component. The latter strengthens with alloying. Due to shear deformation in the surface region also a rotated cube component is found. It is stronger in the pure material and is partly added up in the bulk during ARB. Due to dislocation and grain boundary hardening the tensile strength increases with increasing ARB cycles following the Hall-Petch behavior while a moderate ductility is kept. Within the sheet plane no significant influence of the tensile direction on the observed mechanical properties was found.
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The Direct Writing of Plasmonic Gold Nanostructures by Electron-Beam-Induced Deposition - Hoeflich, Katja; Yang, Ren Bin; Berger, Andreas; Leuchs, Gerd; Christiansen, Silke (Advanced Materials, 2011)
Various nanostructures are directly written by electron-beam-induced deposition using dimethyl-gold(III)-acetylacetonate as the precursor gas. After purification, their potential applications include plasmonic devices and metamaterials. Carbon contamination of the as-written structures can be completely removed by low-temperature ozone treatment, leaving polycrystalline pure gold structures (see figure). This treatment reduces the size of the nanostructures but does not substantially alter their functional shape.
EAM Import::2019-03-12

The effect of a negative Poisson's ratio on thermal stresses in cellular metallic structures - Adler, Lucas; Warmuth, Franziska; Lodes, Matthias; Osmanlic, Fuad; Körner, Carolin (Smart Materials and Structures, 2016)
To investigate the influence of auxetic behavior on thermally induced stresses, finite element analysis simulations of the displacement and stress fields in locally heated three dimensional auxetic and cubic structures are compared. The simulations were carried out in Comsol for a cubic as well as a chiral array of 30 mm length in each spatial dimension (3 × 3 × 3 unit cells). The center cells of these arrays were heated for 20 s. For two boundary conditions (free and clamped), deformation mechanisms are analyzed. It is found, that the auxetic behavior can effectively reduce thermal stresses by internal node rotation and strut bending, especially for constrained (clamped) boundary conditions. A stress reduction of a factor of 3.3 in comparisons to a simple cubic cell array could be evaluated.
Keywords: additive manufacturing; auxetic effect; cellular structures; FEA simulation; inhomogeneous heating; negative Poisson's ratio; selective electron beam melting (SEBM)

The effects of post-processing on the surface and the optical properties of copper indium sulfide quantum dots - Akdas, Tugce; Distaso, Monica; Kuhri, Susanne; Winter, Benjamin; Birajdar, Balaji; Spiecker, Erdmann; Guldi, Dirk Michael; Peukert, Wolfgang (Advances in Colloid and Interface Science, 2015)
In the current contribution we report on investigations regarding the surface of CuInS2 quantum dots and on different strategies to control the amount of surface ligands in a post-processing step. In particular, the reactivity of the organic components, that is, 1-dodecanthiol and 1-octadecene as ligand and solvent, respectively, during nanocrystal formation was studied. A new method to remove residuals from the reaction mixture and to detach excess organics from the surface of the nanocrystals is reported. Our new method, which is based on the utilization of acids, is compared with standard purification procedures by means of thermogravimetric analysis (TGA) with particular focus on its efficiency to remove organics. As a complement, the surface chemistry is analyzed by nuclear magnetic resonance spectroscopy (NMR) to shed light on the nature of the organic components still present after purification. Further analysis of the product by inductively coupled plasma optical emission spectroscopy (ICP-OES) is performed to verify the influence of the new purification method on surface composition and properties. Moreover, steady state and time resolved spectroscopies give insights into excitonic behavior as well as recombination processes. Finally, the new method is optimized for the purification of CuInS2-ZnS nanocrystals, which show enhanced optical properties.

The Electronic Structure of Amorphous Carbon Nanodots - Margraf, Johannes; Strauß, Volker; Guldi, Dirk Michael; Clark, Timothy (Journal of Physical Chemistry B, 2015)
We have studied hydrogen-passivated amorphous carbon nanostructures with semiempirical molecular orbital theory in order to provide an understanding of the factors that affect their electronic properties. Amorphous structures were first constructed using periodic calculations in a melt/quench protocol. Pure periodic amorphous carbon structures and their counterparts doped with nitrogen and/or oxygen feature large electronic band gaps. Surprisingly, descriptors such as the elemental composition and the number of sp3-atoms only influence the electronic structure weakly. Instead, the exact topology of the sp2-network in terms of effective conjugation defines the band gap. Amorphous carbon nanodots of different structures and sizes were cut out of the periodic structures. Our calculations predict the occurrence of localized electronic surface states, which give rise to interesting effects such as amphoteric reactivity and predicted optical band gaps in the near-UV/visible range. Optical and electronic gaps display a dependence on particle size similar to that of inorganic colloidal quantum dots.

The electronic structure of cobalt(II) phthalocyanine adsorbed on Ag(111) - Schmid, Martin; Kaftan, Andre; Steinrück, Hans-Peter; Gottfried, Michael (Surface Science, 2012)
The electronic states of submonolayers and multilayers of cobalt(II) phthalocyanine (CoPc) adsorbed on Ag(111) were examined with photoelectron spectroscopy to obtain insight into the details of the substrate-adsorbate interaction. UV photoelectron spectroscopy (UPS) reveals the presence of two interaction-related valence states in the direct vicinity of the Fermi edge, in agreement with previous DFT calculations. X-ray photoelectron spectra indicate that the substrate-adsorbate interaction results in transfer of electron density from the substrate to the Co(II) ion. Substantial changes in the Co 2p multiplet structure, a spectral pattern induced by the open-shell character of the central Co(II) ion, indicate a complete quenching of the molecular spin. While pristine CoPc molecules are paramagnetic with S = 1/2, molecules in direct contact to the Ag(111) substrate appear to be in a diamagnetic state. © 2012 Elsevier B.V. All rights reserved.

The era of carbon allotropes - Hirsch, Andreas (Nature Materials, 2010)
Carbon is the element in the periodic table that provides the basis for life on Earth. It is also important for many technological applications, ranging from drugs to synthetic materials. This role is a consequence of carbon's ability to bind to itself and to nearly all elements in almost limitless variety. The resulting structural diversity of organic compounds and molecules is accompanied by a broad range of chemical and physical properties. The tools of modern synthetic chemistry allow the tailored design of these properties by the controlled combination of structural and functional building blocks in new target systems. Elemental carbon exists in two natural allotropes, diamond and graphite, which consist of extended networks of sp3- and sp2-hybridized carbon atoms, respectively. Both forms show unique physical properties such as hardness, thermal conductivity, lubrication behaviour or electrical conductivity. Conceptually, many other ways to construct carbon allotropes are possible by altering the periodic binding motif in networks consisting of sp3-, sp2- and sp-hybridized carbon atoms1, 2. As a consequence of the expected remarkable physical properties of these elusive carbon allotropes, it has been appealing, for some time, to develop concepts for their preparation in macroscopic quantities. However, diamond and graphite represented the only known allotropes of carbon for a long time. This situation changed in 1985, with the advent of fullerenes (Fig. 1), which were observed for the first time by Kroto et al.3. This serendipitous discovery marked the beginning of an era of synthetic carbon allotropes. Now, as we celebrate buckminsterfullerene's 25th birthday, it is also the time to reflect on a growing family of synthetic carbon allotropes, which includes the synthesis of carbon nanotubes in 19914 and the rediscovery of graphene in 20045. Keeping in mind the numerous possible carbon modifications and the number of scientists investigating this challenge, these revelations have certainly not come to an end. In this family, fullerenes represent the most intensively investigated class. Fullerene chemistry is a mature field and many well-defined derivatives with outstanding properties have been synthesized. The first fullerene-based products such as organic solar cells have already entered the market. The materials properties of the carbon nanotubes and especially those of graphene are considered to be even more promising. However, it is still difficult to control their chemistry and also the bulk production of uniform monodisperse samples, for example, mass production of tubes with single helicities remains a challenge. Graphene chemistry is in its early infancy. Thinking to the future, there are a huge number of elusive carbon allotropes whose predicted properties are unprecedented. Synthetic chemists are developing concepts at present for their preparation and have already synthesized partial structures.

The Eshelby Theorem and Application to the optimization of an Elastic Patch - Leugering, Günter; Nazarov, Sergei; Schury, Fabian; Stingl, Michael (SIAM Journal on Applied Mathematics, 2012)
shWwWe present the analysis for finding optimal locations and rotations of anisotropic material inclusions in a matrix material by using the polarization matrix. We compare different types of cost functionals, in particular local ones, and show their respective differences. We use the Eshelby theorem and the representation of stresses based on the link matrix. As an analytical model reduction technique, this allows for efficent numerical computation which is demonstrated for two selected examples. W
Keywords: asymptotic analysis; Eshelby theorem; optimization; elastic patch; pointwise error estimates; integral and local cost functionals

The Eshelby Theorem and its Variants for Piezoelectric Media - Leugering, Günter; Nazarov, S. A. (Archive for Rational Mechanics and Analysis, 2014)
We prove the Eshelby theorem for an ellipsoidal piezoelectric inclusion in an infinite piezoelectric material. Explicit formulas for the link and polarization matrices are derived. Passing to the limits with respect to parameters in the corresponding equations, the result is extended to cases when either the inclusion or the surrounding material is purely elastic.

The evolution of crystalline ordering for ligand-ornamented zinc oxide nanoparticles - Zobel, Mirijam; Windmueller, Anna; Schmidt, Ella; Götz, Klaus; Milek, Theodor; Zahn, Dirk; Kimber, Simon A. J.; Hudspeth, Jessica M.; Neder, Reinhard (Crystengcomm, 2016)
Recent total scattering experiments have opened up the possibility to study nanoparticle formation in situ and to observe the structural transformation from precursor clusters to adult particles. Organic ligand molecules interact with precursors of metal oxide nanoparticles, yet their influence onto the evolution of crystallinity during particle formation has not been addressed in detail; nor have in situ total scattering experiments ventured into the field of low-concentration, room-temperature syntheses in organic solvents to date. In this report, we follow the crystallization of ZnO nanoparticles in ethanol in the presence of different organic ligands. Low coordinated zinc precursor clusters rapidly polymerize upon base addition to particles of ca. 1 nm in diameter. In situ SAXS experiments reveal that the overall particle size increases to 2 to 4 nm with advancing reaction time. Complementary in situ PDF experiments show smaller crystalline domain sizes, which are only one third to half as large as the particle diameter. The ZnO particles thus feature a crystalline core surrounded by a disordered shell. Both, the core and the shell diameter are influenced by the different surface-bound organic ligands, which prevent an immediate relaxation to fully crystalline particles. A slow crystallization takes place in solution. We assume a dynamic equilibrium of the ligand and solvent molecules at the particle surface, which enables gradual bond restructuring. With suitably adjusted synthesis conditions, in our case by a continuous base addition, we show how to bypass the disordered intermediates, allowing the spontaneous nucleation of fully crystalline nanoparticles.

The fabrication of color-tunable organic light-emitting diode displays via solution processing - Guo, Fei; Karl, André; Xue, Qifan; Tam, Kai C.; Forberich, Karen; Brabec, Christoph (Light: Science & Applications, 2017)
Electroluminescent devices based on organic semiconductors have attracted significant attention owing to their promising applications in flat-panel displays. The conventional display pixel consisting of side-by-side arrayed red, green and blue subpixels represents the mature technology but bears an intrinsic deficiency of a low pixel density. Constructing an individual color-tunable pixel that comprises vertically stacked subpixels is considered an advanced technology. Although color-tunable organic light-emitting diodes (OLEDs) have been fabricated using the vacuum deposition of small molecules, the solution processing of conjugated polymers would enable a much simpler and inexpensive manufacturing process. Here we present the all-solution processing of color-tunable OLEDs comprising two vertically stacked polymer emitters. A thin layer of highly conducting and transparent silver nanowires is introduced as the intermediate charge injection contact, which allows the emission spectrum and intensity of the tandem devices to be seamlessly manipulated. To demonstrate a viable application of this technology, a 4-by-4 pixelated matrix color-tunable display was fabricated.
Keywords: color tunable; oLED display; organic light-emitting diode; silver nanowires; solution processing; tandem

The formation of CdS quantum dots and Au nanoparticles - Schiener, Andreas; Schmidt, Ella; Bergmann, Christoph; Seifert, Soenke; Zahn, Dirk; Krach, Alexander; Weihrich, Richard; Magerl, Andreas (Zeitschrift fur Kristallographie - Crystalline Materials, 2017)
We report on microsecond-resolved in-situ SAXS experiments of the early nucleation and growth behavior of both cadmium sulfide (CdS) quantum dots in aqueous solution including the temperature dependence and of gold (Au) nanoparticles. A novel free-jet setup was developped to access reaction times as early as 20 μs. As the signal in particular in the beginning of the reaction is weak the containment-free nature of this sample environment prooved crucial. The SAXS data reveal a two-step pathway with a surprising stability of a structurally relaxed cluster with a diameter of about 2 nm. While these develop rapidly by ionic assembly, a further slower growth is attributed to cluster attachment. WAXS diffraction confirms, that the particles at this early stage are not yet crystalline. This growth mode is confirmed for a temperature range from 25°C to 45°C. An energy barrier for the diffusion of primary clusters in water of 0.60 eV was experimentally observed in agreement with molecular simulations. To access reaction times beyond 100 ms, a stopped-drop setup -again contaiment- free is introduced. SAXS experiments on the growth of Au nanoparticles on an extended time scale provide a much slower growth with one population only. Further, the influence of ionizing X-ray radiation on the Au particle fromation and growth is discussed.
Keywords: Au nanoparticles; CdS quantum dots; Containment-free; In-situ SAXS; Nucleation and growth

The holy grail of microfluidics: sub-laminar drag by layout of periodically embedded microgrooves - Daschiel, Gerti; Peric, Milovan; Jovanovic, Jovan; Delgado, Antonio (Microfluidics and Nanofluidics, 2013)
The sub-laminar drag effect of microgroove surfaces was studied numerically in a steady two-dimensional channel flow at subcritical Reynolds numbers. Considerations are restricted to grooves of a few viscous length scales in depth, which are assumed not to promote the laminar to turbulent transition process. It was found that the drag reduction effect is due to the layout of grooves with respect to the flow direction and contour geometry. Results of computations show that for grooves of curved contour placed normal to the flow direction, drag arising from viscous and pressure forces is modulated due to the functional dependence of forces on the surface area projected in the flow direction. Such a groove layout leads to a large skin-friction reduction, but a comparable increase in pressure drag results in sub-laminar drag if drag over flat surface is considered as a reference. For a curved groove contour, the drag reduction increases with increasing Reynolds number and reaches about 5 % at Reynolds numbers approaching critical. © 2013 Springer-Verlag Berlin Heidelberg.
UnivIS-Import:2015-03-09:Pub.2013.tech.ITC.stmmec.thehol
Keywords: Drag reduction surfaces; Flow control; Viscous drag reduction; Viscous flow; Wall-bounded flows

The Impact of Helium Co-Implantation on Hydrogen Induced Donor Profiles in Float Zone Silicon - Laven, Johannes; Job, R.; Schulze, H. J.; Niedernostheide, F. J.; Häublein, V.; Schulze, H.; Schustereder, W.; Ryssel, Heiner; Frey, Lothar (ECS Transactions, 2010)
Doping profiles in proton and helium co-implanted and annealed n-type float zone silicon wafers are analyzed by means of spreading resistance measurements. After annealing at sufficiently high temperatures and/or long-enough duration, the hydrogen-related donor profiles known from proton implantations are significantly enhanced by the helium irradiation. The resulting profile shape exhibits a strong resemblance to the radiation damage distribution of the co-implantation. By increasing the ultimately introduced num-ber of hydrogen related donors without varying the available amount of hydrogen, it is shown that the donor profile resulting from proton implantation and annealing is limited by the defect species and the implanted hydrogen is supplied in surplus. The maximum of the additionally induced donor distribution by the he-lium implantation shows a different dependency on the fluence compared to the donor maximum induced by mere proton irradia-tion. The diffusion of protons through the irradiated layer during annealing is impacted by the He-irradiation.
EAM Import::2019-03-12

The Impact of Self-Assembled Monolayer Thickness in Hybrid Gate Dielectrics for Organic Thin-Film Transistors - Jedaa, Abdesselam; Burkhardt, Martin; Zschieschang, Ute; Klauk, Hagen; Habich, Dana; Schmid, Günter; Halik, Marcus (Organic Electronics, 2009)
We have investigated the electrical characteristics of hybrid dielectrics with a thickness of 6 nm or less that are composed of a plasma-grown aluminum oxide (AlOx) layer and a self-assembled monolayer (SAM) of an aliphatic phosphonic acid. The impact of the quality of the AlOx layer on the insulating properties of the double-layer dielectrics was assessed by comparing two different oxidation procedures, and the influence of the thickness of the organic SAM was evaluated by employing molecules with five different chain lengths. In order to decouple the relative contributions of the oxide and the SAM to the performance of the double-layer dielectrics we have also performed cyclic voltammetry measurements on indium tin oxide (ITO)/SAM devices without AlOx layer. Finally, we have evaluated how the quality of the AlOx layer and the thickness of the SAM affect the performance of low-voltage organic thin-film transistors (TFTs) that employ the thin AlOx/SAM dielectrics as the gate dielectric. The results confirm the important role of the SAM in determining the breakdown voltage, in limiting the current density, and in compensating the somewhat lower quality of AlOx layers produced under mild plasma conditions.

The influence of hydrogenated amorphous carbon coatings (a-C:H) on the fatigue life of coated steel specimens - Schaufler, Jens; Durst, Karsten; Haas, Thomas; Nolte, Roland; Höppel, Heinz Werner; Göken, Mathias (International Journal of Fatigue, 2012)
The influence of a hydrogenated amorphous carbon coating on the fatigue life of an austenitic steel was evaluated in this work. The fatigue damage mechanisms have been compared for both coated and uncoated specimens. A strong increase in fatigue life of more than 300% for low total strain amplitudes was found for the coated specimens. The influence of the coating on the fatigue life increases with decreasing total strain amplitude. The damage mechanisms were analyzed using atomic force microscopy and focus ion beam cross sectioning. The coated specimens show a significantly reduced surface roughening. Slip bands in the steel sample could not penetrate the coating and thus slip step formation on the surface was avoided, leading to the enhanced fatigue life. The compressive residual stress in the coating is discussed being the main reason for the enhanced fatigue life of the specimens.
WW1::948

The Interaction of Cobalt with CeO2(111) Prepared on Cu(111) - Vari, Gabor; Ovari, Laszlo; Papp, Christian; Steinrück, Hans-Peter; Kiss, Janos; Konya, Zoltan (Journal of Physical Chemistry C, 2015)
The interaction of cobalt with an ultrathiri CeO2(111) film on Cu(111) was studied by low energy ion scattering spectroscopy and X-ray photoelectron spectroscopy. At room temperature, Co grows three-dimensionally on the ceria film. For small Co coverages (0.2 ML), oxidation to Co2+ occurs together with the formation Of Ce3+ in the ceria. At higher Co coverages (07 and 2 ML), an increasing fraction of Co remained Metallic after deposition. Annealing to higher temperatures induced diffusion of Co2+ ions into the ceria film, and for metallic Co, presumably agglomeration occurs. At T >= 800 K, oxidation of significant amounts of Co-0 to Co2+ is found, along with diffusion, into the ceria. film, and partly also in the copper crystal underneath. On a reduced CeOx surface, the redox reaction between cobalt and cerium was hindered, which, in turn, slowed down cobalt migration into the ceria film.

The local electron affinity for non-minimal basis sets - Clark, Timothy (Journal of Molecular Modeling, 2010)
A technique known as intensity filtering is introduced to select valence-like virtual orbitals for calculating the local electron affinity, EAL. Intensity filtering allows EAL to be calculated using semiempirical molecular orbital techniques that include polarisation functions. Without intensity filtering, such techniques yield spurious EAL values that are dominated by the polarisation functions. As intensity filtering should also be applicable for ab initio or density functional theory calculations with large basis sets, it also makes EAL available for these techniques.

The mechanical and optical properties of injection-moulded PMMA, filled with glass particles of a matching refractive index - Wildner, Wolfgang; Drummer, Dietmar (Polymers and Polymer Composites, 2017)
In this paper, the optical and mechanical properties of injection-molded plates (thickness: 1 mm) made of poly(methyl methacrylate) (PMMA) and glass particles as filler with a matching refractive index are investigated. Tests show that the orientation resulting from the injection molding process greatly affects the tensile strength and elongation at break, whereas the elastic modulus merely depends on the filling degree, which is investigated up to 13 vol.%. In terms of the optical properties, the specimens' haze depends on the difference in the refractive index between a glass and polymer. However, the absolute value of haze increases with an increasing filling degree and a decreasing particle size. Surface defects resulting from the injection molding process were found to be the primary reason for this haze even with identical RI between PMMA and glass.

The Mechanism of Hydrocarbon Oxygenate Reforming: C-C Bond Scission, Carbon Formation, and Noble-Metal-Free Oxide Catalysts - Lykhach, Yaroslava; Neitzel, Armin; Sevcikova, Klara; Johanek, Viktor; Tsud, Nataliya; Skala, Tomas; Prince, Kevin C.; Matolin, Vladimir; Libuda, Jörg (Chemsuschem, 2014)
Towards a molecular understanding of the mechanism behind catalytic reforming of bioderived hydrocarbon oxygenates, we explore the C-C bond scission of C-2 model compounds (acetic acid, ethanol, ethylene glycol) on ceria model catalysts of different complexity, with and without platinum. Synchrotron photoelectron spectroscopy reveals that the reaction pathway depends very specifically on both the reactant molecule and the catalyst surface. Whereas C-C bond scission on Pt sites and on oxygen vacancies involves intermittent surface carbon species, the reaction occurs without any carbon formation and deposition for ethylene glycol on CeO2(111).
Keywords: catalytic reforming; ceria; heterogeneous catalysis; photoelectron spectroscopy; platinum

The microscopic structure of mono-disperse granular heaps and sediments of particles on inclined surfaces - Topic, Nikola; Schaller, Fabian; Schröder-Turk, Gerd; Pöschel, Thorsten (Soft Matter, 2016)
Granular heaps of particles created by deposition of mono-disperse particles raining from an extended source of finite size are characterized by a non-homogeneous field of density. It was speculated that this inhomogeneity is due to the transient shape of the sediment during the process of construction of the heap, thus reflecting the history of the creation of the heap. By comparison of structural characteristics of the heap with sediments created on top of inclined planes exploiting the method of Minkowski tensors, we provide further evidence to support this hypothesis. Moreover, for the case of sediments generated by homogeneous rain on surfaces, we provide relationships between the inclination of the surface and the Minkowski measures characterizing the isotropy of local particle environments.

The Molecular Mechanism of α-Resorcinol's Asymmetric Crystal Growth from the Melt - Ectors, Philipp; Sae-Tang, Wang; Chatchawalsaisin, Jittima; Zahn, Dirk; Anwar, Jamshed (Crystal Growth & Design, 2015)
Polar crystals are characterized by an axis that has a nonzero dipole due to the nature of the molecular packing. For these crystals, the growth rates of the faces delineating the polar axis are generally expected to be equal. Recent experiments, however, have revealed a few exceptions where the growth of these faces from the vapor phase is asymmetric, a notable case being crystals of resorcinol. Here, we present the mechanics of resorcinol crystal growth from the melt for the hemihedral faces (011) and (01̅1̅) delineating the polar axis as revealed by molecular dynamics simulations. The simulations reveal asymmetric growth consistent with experiment. The asymmetry is attributed to the slow-growing (011) face being less able to direct the correct alignment of the oncoming molecules and the presence of an alternate resorcinol conformation that readily incorporates into the lattice at this surface, serving to poison and retard subsequent growth. Putting the issue of the rogue conformation aside, the identified factors that influence molecular recognition are considered to be applicable to other polar crystals, which suggest asymmetric growth along the polar axis to be a common feature.
EAM Import::2019-03-12

The morphology of integrated self-assembled monolayers and their impact on devices – A computational and experimental approach - Novak, Michael; Jäger, Christof; Rumpel, Armin; Kropp, Henning; Peukert, Wolfgang; Clark, Timothy; Halik, Marcus (Organic Electronics, 2010)
We report on a combined experimental and computational study on self-assembled monolayers (SAMs). The dielectric properties of SAMs based on n-alkyl phosphonic acids in large area thin film devices (organic field-effect transistors with good hole mobilities up to 0.3 cm² V−1 s−1 at −1 V and capacitors) depend on their chain length, but not consistently to theoretical pictures of tunneling through saturated n-alkanes. An unexpected saturation of current density with increasing chain length was obtained in devices, what impact on the leakage current in transistors and current density in capacitors. This is attributed to different self-assembled monolayer morphology ranging from an amorphous state for short alkyl chains to a quasi-crystalline state for longer alkyl chains. Molecular dynamics (MD) simulations provide a deeper insight into the nature of the three-dimensional intermolecular interactions and support the proposed SAM morphology. The change in morphology leads to a reduced effective SAM thickness in devices, which could be described by a Simmons approach quantitatively. The morphological aspect of self-assembled molecules is of enormous importance beyond the applications of SAMs in low-voltage, high mobility organic transistors because of it’s relevance for the hole field of molecular scale electronics. It demonstrates that even small changes in the molecular design can change the molecular interactions and monolayer assembly.

The mutual influence of surface energy and substrate temperature on the saturation mobility in organic semiconductors - Sarcletti, Marco; Schmaltz, Thomas; Halik, Marcus (Organic Electronics, 2014)
We report on a mutual correlation between the substrate temperature during semiconductor deposition and the surface energy of the gate dielectric on the charge carrier mobility in bottom gate top contact organic field effect transistors (OFETs) with N,N′-diphenyl-3,4,9,10-perylene tetracarboxylic diimide (DP-PDI) as organic semiconductor. The gate dielectric consists of a thin gate aluminum oxide and a self-assembled monolayer (SAM) with which the surface energy of the hybrid dielectric is tuned. The substrate temperature during evaporation of the DP-PDI semiconductor was systematically varied to obtain the best charge carrier mobility for each SAM. We found that the optimum substrate temperature correlates with the surface energy. Devices with low surface energy reach best mobility at higher substrate temperatures than devices with a higher surface energy.

The Nature of One-Dimensional Carbon: Polyynic versus Cumulenic - Neiß, Christian; Trushin, Egor; Görling, Andreas (ChemPhysChem, 2014)
A question of both fundamental as well as practical importance is the nature of one-dimensional carbon, in particular whether a one-dimensional carbon allotrope is polyynic or cumulenic, that is, whether bond-length alternation occurs or not. By combining the concept of aromaticity and antiaromaticity with the rule of Peierls distortion, the occurrence and magnitude of bond-length alternation in carbon chains with periodic boundary conditions and corresponding carbon rings as a function of the chain or ring length can be explained. The electronic properties of one-dimensional carbon depend crucially on the bond-length alternation. Whereas it is generally accepted that carbon chains in the limit of infinite length have a polyynic structure at the minimum of the potential energy surface with bond-length alternation, we show here that zero-point vibrations lead to an effective equalization of all carboncarbon bond lengths and thus to a cumulenic structure.
Keywords: bond-length alternation; carbynes; cumulenes; density functional calculations; polyynes

The Optical Janus Effect: Asymmetric Structural Color Reflection Materials - England, Grant T.; Russell, Calvin; Shirman, Elijah; Kay, Theresa; Vogel, Nicolas; Aizenberg, Joanna (Advanced Materials, 2017)
Structurally colored materials are often used for their resistance to photobleaching and their complex viewing-direction-dependent optical properties. Frequently, absorption has been added to these types of materials in order to improve the color saturation by mitigating the effects of nonspecific scattering that is present in most samples due to imperfect manufacturing procedures. The combination of absorbing elements and structural coloration often yields emergent optical properties. Here, a new hybrid architecture is introduced that leads to an interesting, highly directional optical effect. By localizing absorption in a thin layer within a transparent, structurally colored multilayer material, an optical Janus effect is created, wherein the observed reflected color is different on one side of the sample than on the other. A systematic characterization of the optical properties of these structures as a function of their geometry and composition is performed. The experimental studies are coupled with a theoretical analysis that enables a precise, rational design of various optical Janus structures with highly controlled color, pattern, and fabrication approaches. These asymmetrically colored materials will open applications in art, architecture, semitransparent solar cells, and security features in anticounterfeiting materials.
Keywords: absorption; multilayers; photonic crystals; structural color; thin films

Theoretical and experimental insights into the surface chemistry of semiconductor quantum dots - Margraf, Johannes; Ruland Palaia, Andres; Sgobba, Vito; Guldi, Dirk Michael; Clark, Timothy (Langmuir, 2013)
We present a series of non-stoichiometric cadmium sulfide quantum-dot (QD) models. Using density functional theory (DFT) and semi-empirical molecular orbital (MO) calculations, we explore the ligand binding and exchange chemistry of these models. Their surface morphology allows for these processes to be rationalized on the atomic scale. This is corroborated by ultraviolet-visible (UV-vis), infrared (IR), and inductively coupled plasma-optical emission spectroscopy (ICP-OES). © 2013 American Chemical Society.

The physics of wind-blown sand and dust - Ribeiro Parteli, Eric Josef; Kok, Jasper F.; Michaels, Timothy I.; Karam, Diana Bou (Reports on Progress in Physics, 2012)
The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This paper presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.

The potential of molecular self-assembled monolayers in organic electronic devices - Halik, Marcus; Hirsch, Andreas (Advanced Materials, 2011)
Functionalized molecules that organize to self-assembled monolayers (SAMs) are gaining importance in organic electronic devices. They are fully compatible with flexible substrates, are amenable to low-cost processing, and show reliable film-forming behavior. Highly integrated devices, such as sensor arrays or memories, have also been demonstrated. Starting from auxiliary layers, which improve and modify surfaces and interfaces in traditional thin-film devices, the applications of SAMs develop towards molecular scale electronics, including active molecular device layers and multifunctional SAMs, which fulfill several layer functions of a device within one monolayer. Mixed SAMs make new and tunable device features possible, by stoichiometric control of the composition of different SAM-forming molecules. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The process of solid-state dewetting of Au thin films studied by in situ scanning transmission electron microscopy - Niekiel, Florian; Schweizer, Peter; Kraschewski, Simon; Butz, Benjamin; Spiecker, Erdmann (Acta Materialia, 2015)
Solid-state dewetting describes the transformation of thin films into a set of particles or droplets at temperatures well below the melting temperature of the bulk. In this work in situ scanning transmission electron microscopy has been used to study the dewetting behavior of discontinuous Au thin films (15 nm and 22 nm thick) on amorphous silicon nitride membranes at temperatures ranging from 300 °C to 600 °C. The combination of microscopic and statistical information enabled not only the qualitative discussion of the observed processes but also the quantification of the kinetics as well as the development of a model of the underlying morphological mechanism. A model-free master curve approach to the temporal evolution of the covered area at different temperatures is used to determine the activation energy of dewetting (1.04±0.14eV for the 15 nm thick film). A closer inspection reveals a multiple power law behavior, which is discussed in the frame of depercolation. Retraction of finger-like structures is found to be the dominant morphological mechanism based on the observed linear relationship between covered area and boundary length.
Keywords: Image analysis; Morphology; Solid-state dewetting; Temporal evolution; Thin films

The Protonation States of the Active-Site Histidines in (6-4) Photolyase - Condic-Jurkic, Karmen; Smith, Ana-Suncana; Zipse, Hendrik; Smith, David M. (Journal of Chemical Theory and Computation, 2012)
The active sites of the (6-4) photolyases contain two conserved histidine residues, which, in the Drosophila melanogaster enzyme, correspond to His365 and His369. While there are nine combinations in which the three possible protonation states of the two histidines (with protons on N delta (HID), N epsilon (HIE), or both N delta and N epsilon (HIP)) can be paired, there is presently no consensus as to which of these states is present, let alone mechanistically relevant. EPR hyperfine couplings for selected protons of the FADH* radical have previously been used to address this issue. Our QM/MM calculations show, however, that the experimental couplings are equally well reproduced by each of the nine combinations. Since the EPR results seemingly cannot be used to unequivocally assign the protonation states, the pK(a) values of the two histidines were calculated using the popular PROPKA, H++, and APBS approaches in various environments and for several lesions. These techniques consistently indicate that, at pH = 7, both His365 and His369 should be neutral, although His369 is found to be more prone to becoming protonated. In a comparative approach, a series of molecular dynamics simulations was performed with all nine combinations, employing various reference crystal structures and different oxidation states of the FAD cofactor. The overall result of this approach is in agreement with our pKa results. Consequently, although the introduction of the reduced cofactor results in an increased stability for selected protonated states, particularly the His365=HID and His369=HIP combination, the neutral combination His365=HID and His365=HIE stands out as the most relevant state for the activity of the enzyme.
Keywords: DIMER RADICAL-ANION; AMBER FORCE-FIELD; DNA PHOTOLYASE; MECHANICAL CALCULATIONS; MOLECULAR-DYNAMICS; CRYSTAL-STRUCTURE; ELECTRON-TRANSFER; NUCLEIC-ACIDS; PK(A) VALUES; RESP MODEL

The quasi-free-standing nature of graphene on H-saturated SiC(0001) - Speck, Florian; Jobst, Johannes; Fromm, Felix; Ostler, Markus; Waldmann, Daniel; Hundhausen, Martin; Weber, Heiko B.; Seyller, Thomas (Applied Physics Letters, 2011)
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001) which was prepared by intercalation of hydrogen under the buffer layer. Using infrared absorption spectroscopy, we prove that the SiC(0001) surface is saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer layer into graphene which exhibits a slight tensile strain and short range defects. The layers are hole doped (p 5.0 - 6.5 1012 cm -2) with a carrier mobility of 3100 cm2/Vs at room temperature. Compared to graphene on the buffer layer, a strongly reduced temperature dependence of the mobility is observed for graphene on H-terminated SiC(0001) which justifies the term quasi-free-standing. © 2011 American Institute of Physics.
UnivIS-Import:2015-03-09:Pub.2011.nat.dphy.IAP.exphys.thequa

The relationship between threshold voltage and dipolar character of self-assembled monolayers in organic thin-film transistors - Salinas Batallas, Michael; Jäger, Christof; Yousefi Amin, Atefeh; Dral, Pavlo; Meyer-Friedrichsen, Timo; Hirsch, Andreas; Clark, Timothy; Halik, Marcus (Journal of the American Chemical Society, 2012)
We report a quantitative study that describes and correlates the threshold voltage of low-voltage organic field-effect transistors with the molecular structure of self-assembled monolayer dielectrics. We have observed that the component of the dipole moment of such self-assembled molecules perpendicular to the surface correlates linearly with the threshold voltage shift in devices. The model was validated using three different organic semiconductors (pentacene, α,α′-dihexylsexithiophene, and fullerene-C ) on six different self-assembled monolayers. The correlation found can help optimize future devices, by tuning the dipole moments of the molecules that constitute the self-assembled monolayer. © 2012 American Chemical Society.

Thermal air oxidation of Fe: rapid hematite nanowire growth and photoelectrochemical water splitting performance - Grigorescu, Sabina Georgiana; Lee, Chong-Yong; Lee, Kiyoung; Albu, Sergiu; Paramasivam, Indhumati; Demetrescu, Ioana; Schmuki, Patrik (Electrochemistry Communications, 2012)
Different iron oxide structures were formed by annealing of iron foils in air at temperatures between 500 °C to 800 °C. Depending on temperature, a significant variation in the hematite/magnetite ratio and a strongly temperature dependent morphology is obtained. While over a wide range of conditions more or less compact Fe3O4/Fe2O3 layers are obtained, at 600 °C rapid growth (several micrometer per hour) of highly crystalline hematite nanowires can be observed. Visible light photocurrent measurements in 1 M NaOH under AM 1.5 100 mW/cm2 conditions show that photocurrent density and the onset potential for water oxidation strongly shifted in the cathodic direction for the nanowire morphology. The results indicate that a simple air oxidation of iron can provide a rapid path to form hematite nanowires. Obtained layers are considerably active as photoanodes for solar water splitting.
EAM Import::2019-03-12

Thermal Conductivity of Ionic Liquids: Measurement and Prediction - Fröba, Andreas Paul; Rausch, Michael Heinrich; Krzeminski, Kamil; Assenbaum, Daniel; Wasserscheid, Peter; Leipertz, Alfred (International Journal of Thermophysics, 2010)
This study reports thermal-conductivity data for a series of [EMIM] (1-ethyl-3-methylimidazolium)-based ionic liquids (ILs) having the anions [NTf2] (bis(trifluoromethylsulfonyl)imide), [OAc] (acetate), [N(CN)2] (dicyanimide), [C(CN)3] (tricyanomethide), [MeOHPO2] (methylphosphonate), [EtSO4] (ethylsulfate), or [OcSO4] (octylsulfate), and in addition for ILs with the [NTf2]-anion having the cations [HMIM] (1-hexyl-3-methylimidazolium), [OMA] (methyltrioctylammonium), or [BBIM] (1,3-dibutylimidazolium). Measurements were performed in the temperature range between (273.15 and 333.15) K by a stationary guarded parallelplate instrument with a total measurement uncertainty of 3%(k = 2). For all ILs, the temperature dependence of the thermal conductivity can well be represented by a linear temperature dependence of the thermal conductivity can well be represented by a linear equation. While for the [NTf2]-based ILs, a slight increase of the thermal conductivity with increasingmolar mass of the cation is found at a given temperature, the [EMIM]based ILs show a pronounced, approximately linear decrease with increasing molar mass of the different probed anions. Based on the experimental data obtained in this study, a simple relationship between the thermal conductivity,molar mass, and density is proposed for the prediction of the thermal-conductivity data of ILs. For this, also densities were measured for [EMIM][OAc], [EMIM][C(CN)3], and [HMIM][NTf2]. Themean absolute percentage deviation of all thermal-conductivity data for ILs found in the literature from the proposed prediction is about 7 %. This result represents a convenient simplification in the acquisition of thermal conductivity information for the enormous amount of structurally different IL cation/anion combinations available.

Thermal decomposition, gas phase hydration and liquid phase reconstruction in the system Mg/Al hydrotalcite/mixed oxide: A comparative study - Mokhtar, M.; Inayat, Alexandra; Ofili, Ifeanyi Jimmy; Schwieger, Wilhelm (Applied Clay Science, 2010)
The thermal decomposition of a synthetic Mg/Al hydrotalcite and the subsequent hydration of the calcined products in the presence of water vapor at 30 degrees C were studied by in situ XRD. A comparative study between the gas phase hydration and reconstruction of calcined hydrotalcite in Na(2)CO(3) solution at 25 and 70 degrees C was investigated. TG coupled to a mass spectrometer was used to study the thermal behaviour of synthetic hydrotalcite. SEM images were taken to show the layering and size of crystals. Thermal decomposition of hydrotalcite led to the formation of a dehydrated intermediate phase at 200 degrees C. followed by crystalline MgO formation at 450 degrees C. Hydration of calcined hydrotalcite in H(2)O/N(2) resulted in the formation of broad pattems of the meixnerite phase after 2 h. This phase retained its structure even after 52 h exposure to water vapor. Reconstruction of the calcined products in carbonated water at 25 and 70 degrees C resulted in intensified patterns of the hydrotalcite phase. Due to the different structural and compositional properties of the layered products obtained by different chemical reactions of the mixed oxide phase a clarification of nomenclature for all phase transforming steps is proposed. (c) 2010 Elsevier B.V. All rights reserved.
Keywords: Hydrotalcites; Hydration; Rehydration; Reconstruction; In-situ XRD

Thermal evolution of cobalt deposits on Co3O4(111): Atomically dispersed cobalt, two-dimensional CoO islands, and metallic Co nanoparticles - Mehl, Sascha; Ferstl, Pascal; Schuler, Marina; Toghan, A.; Brummel, Olaf; Hammer, Lutz; Schneider, M. Alexander; Libuda, Jörg (Physical Chemistry Chemical Physics, 2015)
Cobalt oxide nanomaterials show high activity in several catalytic reactions thereby offering the potential to replace noble metals in some applications. We have developed a well-defined model system for partially reduced cobalt oxide materials aiming at a molecular level understanding of cobalt-oxide-based catalysis. Starting from a well-ordered Co3O4(111) film on Ir(100), we modified the surface by deposition of metallic cobalt. Growth, structure, and adsorption properties of the cobalt-modified surface were investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and infrared reflection absorption spectroscopy (IRAS) using CO as a probe molecule. The deposition of a submonolayer of cobalt at 300 K leads to the formation of atomically dispersed cobalt ions distorting the surface layer of the Co3O4 film. Upon annealing to 500 K the Co ions are incorporated into the surface layer forming ordered two-dimensional CoO islands on the Co3O4 grains. At 700 K, Co ions diffuse from the CoO islands into the bulk and the ordered Co3O4(111) surface is restored. Deposition of larger amounts of Co at 300 K leads to formation of metallic Co aggregates on the dispersed cobalt phase. The metallic particles sinter at 500 K and diffuse into the bulk at 700 K. Depending on the degree of bulk reduction, extended Co3O4 grains switch to the CoO(111) structure. All above structures show characteristic CO adsorption behavior and can therefore be identified by IR spectroscopy of adsorbed CO.

Thermally Activated Self-metalation of Carboxy-functionalized Porphyrin Films on MgO Nanocubes - Kollhoff, Fabian; Schneider, Johannes; Berger, Thomas; Diwald, Oliver; Libuda, Jörg (ChemPhysChem, 2018)
We investigated the adsorption of different free-base carboxyl-functionalized porphyrins, 5,10,15,20-tetrakis(4-carboxyphenyl)-21,23H-porphyrin (2H-TCPP) and 5(4-carboxyphenyl)-10,15,20-triphenyl-21,23H-porphyrin (2H-MCTPP), on MgO nanocubes combining IR, UV/Vis and photoluminescence emission spectroscopy. The thermal behavior of the films was monitored in-situ during annealing. Both porphyrins bind to the nanocubes via one and two acid groups respectively, yielding monolayer films consisting of tilted molecules. For 2H-TCPP, two acid groups remain free and give rise to a characteristic IR band. Self-assembly in a tilted adsorbate layer suppresses metalation at room temperature, in contrast to non-functionalized 2H-TPP, which adsorbs flat-lying. Upon heating, 2H-MCTPP undergoes full metalation at temperatures below 280 degrees C, whereas 2H-TCPP does not metalate at all. The hindered metalation reaction is attributed to the rigidity of the adsorbate film preventing complexation. Our results show that the properties of porphyrin films on oxides can be tuned in a wide range via the position and arrangement of carboxyl anchoring groups.
Keywords: porphyrin; oxide nanoparticles; infrared spectroscopy; UV; Vis spectroscopy; in-situ metalation

Thermally Induced Growth of ZnO Nanocrystals on Mixed Metal Oxide Surfaces - Inayat, Alexandra; Makky, Ayman; Giraldo, Jose; Kuhnt, Andreas; Busse, Corinna; Schwieger, Wilhelm (Chemistry - A European Journal, 2014)
An in situ method for the growth of ZnO nanocrystals on Zn/Al mixed metal oxide (MMO) surfaces is presented. The key to this method is the thermal treatment of Zn/Al layered double hydroxides (Zn/Al LDHs) in the presence of nitrate anions, which results in partial demixing of the LDH/MMO structure and the subsequent crystallization of ZnO crystals on the surface of the forming MMO layers. In a first experimental series, thermal treatment of Zn/Al LDHs with different fractions of nitrate and carbonate in the interlayer space was examined by thermogravimetry coupled with mass spectrometry (TG-MS) and in situ XRD. In a second experimental series, Zn/Al LDHs with only carbonate in the interlayer space were thermally treated in the presence of different amounts of an external nitrate source (NH4NO3). All obtained Zn/Al MMO samples were analysed by electron microscopy, nitrogen physisorption and powder X-ray diffraction. The gas phase formed during nitrate decomposition turned out to be responsible for the formation of crystalline ZnO nanoparticles. Accordingly, both interlayer nitrate and the presence of ammonium nitrate led to the formation of supported ZnO nanocrystals with mean diameters between 100 and 400 nm, and both methods offer the possibility to tailor the amount and size of the ZnO crystals by means of the amount of nitrate.
Keywords: layered compounds; mixed metal oxides; nanoparticles; nitrogen oxides; zinc

Thermally stable bis(trifluoromethylsulfonyl)imide salts and their mixtures - Scheuermeyer, Marlene; Kusche, Matthias Manfred; Agel, Friederike; Schreiber, Patrick; Maier, Florian; Steinrück, Hans-Peter; Davis, James H.; Heym, Florian; Jess, Andreas; Wasserscheid, Peter (New Journal of Chemistry, 2016)
We show that both tetraphenylphosphonium bis(trifluoromethylsulfonyl)imide ([PPh4][NTf2]) and Cs[NTf2] are low melting salts of exceptionally high and also very similar thermal stability. This similarity indicates that the thermal stability is dominated by the anion. Moreover, eutectic mixtures of [PPh4][NTf2] and Cs[NTf2] with melting points below 100 degrees C are presented. Surface analysis of the latter in the liquid state reveals a surprising depletion of (PPh4](+) ions from the surface.

Thermoanalytical Evidence of Metastable Molecular Defects in Form I of Benzamide - Butterhof, Christian; Martin, Thomas; Ectors, Philipp; Zahn, Dirk; Niemietz, Paul; Senker, Juergen; Nather, Christian; Breu, Josef (Crystal Growth & Design, 2012)
Friedrich Wöhler and Justus von Liebig discovered the first case of molecular polymorphism when crystallizing benzamide in 1832.(1) Thus far only microcrystalline mixtures of metastable phases with the thermodynamically stable form I were available. Optimizing the conditions for crystal growth now gave access to single crystals of the metastable form III which allowed for a detailed comparison of form I and form III by Raman and IR spectroscopy and thermoanalysis. Surprisingly, in the course of detailed differential scanning calorimetry (DSC) measurements for form I an endothermic peak prior to melting was observed. As proven by temperature dependent powder diffraction, thermal microscopy, molecular dynamic (MD) simulations, and solid-state NMR, this premelting thermosignal is not related to an enantiotropic phase transition of form I into form III with a transition temperature close to the melting point. Rather the thermosignal is related to a significant population of metastable defects in form I. While form III melts via a conventional disorder nucleation and growth mechanism, form I accumulates thermally activated metastable point defects involving locally more mobile defect molecules well below Tm. The spin–lattice relaxation experiments suggest an activation barrier larger than 55 kJ/mol. From a combination of DSC, solid state NMR, and MD simulations we provide conclusive evidence for such defects that could play a significant role in selective polymorph transformations of molecular crystals.
EAM Import::2019-03-12

Thermodynamics and Kinetics of Prenucleation Clusters, Classical and Non-Classical Nucleation - Zahn, Dirk (ChemPhysChem, 2015)
Recent observations of prenucleation species and multi-stage crystal nucleation processes challenge the long-established view on the thermodynamics of crystal formation. Here, we review and generalize extensions to classical nucleation theory. Going beyond the conventional implementation as has been used for more than a century now, nucleation inhibitors, precursor clusters and non-classical nucleation processes are rationalized as well by analogous concepts based on competing interface and bulk energy terms. This is illustrated by recent examples of species formed prior to/instead of crystal nucleation and multi-step nucleation processes. Much of the discussed insights were obtained from molecular simulation using advanced sampling techniques, briefly summarized herein for both nucleation-controlled and diffusion-controlled aggregate formation.
EAM Import::2019-03-12

The role of orbital transformations in coupled-pair functionals - Kollmar, Christian; Heßelmann, Andreas (Theoretical Chemistry Accounts, 2010)
The replacement of single excitations by orbital transformations in coupled-pair functionals derived from a single double configuration interaction approach is discussed. It is demonstrated that this modification leads to considerably improved density matrices and better agreement with results from coupled cluster singles doubles calculations taken as a reference. A comparison between the variationally optimized orbitals and the Brueckner orbitals shows that these two sets of orbitals are different.
EAM Import::2019-03-12

The Role of Si during the Growth of GaN Micro- and Nanorods - Tessarek, C.; Heilmann, M.; Butzen, E.; Haab, A.; Hardtdegen, Hilde; Dieker, Christel; Spiecker, Erdmann; Christiansen, S. (Crystal Growth & Design, 2014)
The role of Si during the metal-organic vapor phase epitaxy of GaN rods is investigated. Already a small amount of Si strongly enhances the vertical growth of GaN. Reactive ion etching experiments show that the inner volume of the rod is much more strongly etched than the m-plane surface layer. Transmission electron microscopy and energy dispersive X-ray spectroscopy measurements reveal that Si is predominiantly incorporated in the surface layer of the m-plane sidewall facets of the rods. The formation of a SiN layer prevents growth on and etching of the m-planes and enhances the mobility of atoms promoting vertical growth. Annealing experiments demonstrate the extraordinary thermal resistivity in comparison to undoped GaN rod structures and GaN layers. The subsequent InGaN quantum well growth on the GaN rods reveals the antisurfactant effect of the SiN layer. A model based on the vapor-liquid-solid growth mode is proposed. The results help to understand the role of Si during growth of GaN rod structures to improve the performance of rod based light emitting and electronic devices.

The role of the C2 position in interionic interactions of imidazolium based ionic liquids: a vibrational and NMR spectroscopic study - Noack, Kristina; Schulz, Peter; Paape, Natalia; Kiefer, Johannes; Wasserscheid, Peter; Leipertz, Alfred (Physical Chemistry Chemical Physics, 2010)
Methylation of the C2 position of 1,3-dialkylimidazolium based ionic liquids disrupts the predominant hydrogen-bonding interaction between cation and anion leading to unexpected changes of the physicochemical properties. We found the viscosity of 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C2C1C1Im][Tf2N], for example, to be about three times higher than that of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C2C1Im][Tf2N]. In order to explain these macroscopic changes upon methylation we investigated the vibrational as well as the magnetic resonance structure of [Tf2N]- salts involving the cations 1-ethyl-3-methylimidazolium [C2C1Im]+, 1-ethyl-2,3-dimethylimidazolium [C2C1C1Im]+, 1-butyl-3-methylimidazolium [C4C1Im]+, and 1-butyl-2,3-dimethylimidazolium [C4C1C1Im]+ by means of Fourier-transform infrared (FTIR), Raman and 13C NMR as well as 1H NMR spectroscopy aiming a better microscopic understanding of the cation–anion interaction. To reveal the impact of methylating the C2 position and changing the alkyl side chain length of the imidazolium a detailed assignment of the individual peaks is followed by a comparative discussion of the spectral features also considering already published work. Our spectroscopic findings deduce electron density changes leading to changes in the position and strength of interionic interactions and reduced configurational variations. Both facts are represented on a macroscopic level by the viscosity and melting point. Therefore changes on a macroscopic level clearly express molecular alterations which in turn can be observed using spectroscopic methods as Raman, IR and NMR.