Effects of oligothiophene π-bridge length on physical and photovoltaic properties of star-shaped molecules for bulk heterojunction solar cells - Min, Jie; Luponosov, Yuriy N.; Baran, Derya; Chvalun, Sergei N.; Shcherbina, Maxim A.; Bakirov, Artem V.; Dmitryakov, Petr V.; Peregudova, Svetlana M.; Kausch-Busies, Nina; Ponomarenko, Sergei A.; Ameri, Tayebeh; Brabec, Christoph (Journal of Materials Chemistry A, 2014)
The preparation of four different star-shaped donor (D)-π-acceptor (A) small molecules (N(Ph-1T-DCN-Me)3, N(Ph-2T-DCN-Me)3, N(Ph-2T-DCN-Hex)3 and N(Ph-3T-DCN-Hex)3) possessing various oligothiophene π-bridge lengths and their use in solution-processed bulk heterojunction small molecule solar cells is reported. Optical and electrochemical data show that increasing oligothiophene π-bridge length leads to a decrease of the optical band gap due to a parallel increase of the highest occupied molecular orbital (HOMO) level. Furthermore, subtle modifications of a molecular π-bridge length strongly affect the thermal behavior, solubility, crystallization, film morphology and charge carrier mobility, which in turn significantly change the device performance. Although the moderately increasing oligothiophene π-bridge length uplifts the HOMO level, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the short circuit current (Jsc) and fill factor (FF). The study demonstrates that such an approach can represent an interesting tool for the effective modulation of the photovoltaic properties of the organic solar cells (OSCs) at a moderate cost.
EAM Import::2019-03-14
Keywords: Bulk heterojunction solar cells; Oligothiophenes; Photovoltaic property; Star-shaped molecules

Effects of support and Rh additive on co-based catalysts in the ethanol steam reforming reaction - Ferencz, Zs.; Erdohelyi, A.; Baan, K.; Oszko, A.; Ovari, L.; Konya, Zoltan; Papp, Christian; Steinrück, Hans-Peter; Kiss, J. (ACS Catalysis, 2014)
The effect of the nature of the support and the promotion achieved by a Rh additive on Co-based catalysts in the ethanol steam reforming reaction were studied. The catalysts with 2% Co loading were characterized by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). In situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS) identified the surface intermediates formed during the reaction, whereas gas phase products were detected by gas chromatography (GC). Upon heating in hydrogen to 773 K, cobalt could not be reduced to Co on alumina, but on silica the reduction was almost complete. On ceria, half of the Co could be reduced to the metallic state. By the presence of a small amount (0.1%) of Rh promoter, the reduction of both cobalt and ceria was greatly enhanced. For Co on the acidic AlO support, the dehydration mechanism was dominant, although on the basic CeO support, a significant amount of hydrogen was also formed. Addition of a small amount of Rh as promoter to the Co/CeO catalyst resulted in a significant further increase in the hydrogen selectivity. © 2014 American Chemical Society.

Efficient calculation of constraint back-offs for optimization under uncertainty: A case study on maleic anhydride synthesis - Maußner, Johannes; Freund, Hannsjörg (Chemical Engineering Science, 2018)
In the present work we propose an efficient and general algorithm for optimization under uncertainty based on the work of Srinivasan et al. (2003). We use specialized cubature rules to speed up the uncertainty propagation step which results in a significant reduction of the overall computational effort. The approach is illustrated by studying the optimal design of a fixed bed reactor for the synthesis of maleic anhydride from raffinate II feedstock, where the amount of n-butane and n-butenes in the feed is assumed to be uncertain. Applying the algorithm results in a robustified reactor design which shows significantly less temperature constraint violations and runaway conditions while still satisfying reactor performance criteria such as minimally required conversion and maximum allowable pressure drop.
Keywords: Reaction engineering; Reactor optimization; Maleic anhydride; Optimization under uncertainty; Cubature rules

Efficient exact-exchange time-dependent density-functional theory methods and their relation to time-dependent Hartree-Fock - Heßelmann, Andreas; Görling, Andreas (Journal of Chemical Physics, 2011)
A recently introduced time-dependent exact-exchange (TDEXX) method, i.e., a response method based on time-dependent density-functional theory that treats the frequency-dependent exchange kernel exactly, is reformulated. In the reformulated version of the TDEXX method electronic excitation energies can be calculated by solving a linear generalized eigenvalue problem while in the original version of the TDEXX method a laborious frequency iteration is required in the calculation of each excitation energy. The lowest eigenvalues of the new TDEXX eigenvalue equation corresponding to the lowest excitation energies can be efficiently obtained by, e.g., a version of the Davidson algorithm appropriate for generalized eigenvalue problems. Alternatively, with the help of a series expansion of the new TDEXX eigenvalue equation, standard eigensolvers for large regular eigenvalue problems, e.g., the standard Davidson algorithm, can be used to efficiently calculate the lowest excitation energies. With the help of the series expansion as well, the relation between the TDEXX method and time-dependent Hartree-Fock is analyzed. Several ways to take into account correlation in addition to the exact treatment of exchange in the TDEXX method are discussed, e.g., a scaling of the Kohn-Sham eigenvalues, the inclusion of (semi)local approximate correlation potentials, or hybrids of the exact-exchange kernel with kernels within the adiabatic local density approximation. The lowest lying excitations of the molecules ethylene, acetaldehyde, and pyridine are considered as examples. © 2011 American Institute of Physics.

Efficient hydrogen release from perhydro-N-ethylcarbazole using catalyst-coated metallic structures produced by selective electron beam melting - Peters, Willi; Eypasch, Martin; Frank, T.; Schwerdtfeger, Jan; Körner, Carolin; Bösmann, Andreas; Wasserscheid, Peter (Energy and Environmental Science, 2015)
A particularly suitable reactor concept for the continuous dehydrogenation of perhydro-N-ethylcarbazole in the context of hydrogen and energy storage applications is described. The concept addresses the fact that dehydrogenation is a highly endothermic gas evolution reaction. Thus, for efficient dehydrogenation a significant amount of reaction heat has to be provided to a reactor that is essentially full of gas. This particular challenge is addressed in our study by the use of a catalyst-coated (Pt on alumina), structured metal reactor obtained by selective electron beam melting. The so-obtained reactor was tested both as a single tube set-up and as a Hydrogen Release Unit (HRU) with ten parallel reactors. In stationary operation, the HRU realized a hydrogen release capacity of 1.75 kW(therm) (960 W-el in a subsequent fuel cell) with up to 1.12 g(H2) min(-1) g(Pt)(-1) and a power density of 4.32 kW(el) L-1 of HRU reactor.

Efficient laser induced consolidation of nanoparticulate ZnO thin films with reduced thermal budget - Baum, Marcus; Polster, Sebastian; Jank, Michael; Alexeev, Ilya; Frey, Lothar; Schmidt, Michael (Applied Physics A-Materials Science & Processing, 2012)
Layers of ZnO nanoparticles with thicknesses of about 40 nm were prepared on Si substrates. It was shown that UV laser irradiation is suitable for consolidation and significant densification of the ZnO particle layers under ambient conditions. Both experiments and simulations show that an underlying SiO 2 particle layer has a beneficial effect in inhibiting heat transfer towards the substrate and thus enables the application of temperature-sensitive carrier substrates like polymer foils despite the extremely high melting temperature of ZnO. © 2012 Springer-Verlag.
UnivIS-Import:2015-03-09:Pub.2012.tech.IE.LEB.effici
Keywords: ZnO; nanoparticle; laser; simulation; thin film; layer

Efficient light harvesting anionic heptamethine cyanine-[60] and [70]fullerene hybrids - Villegas, Carmen; Krokos, Evangelos; Bouit, Pierre-Antoine; Delgado, Juan Luis; Guldi, Dirk Michael; Martin, Nazario (Energy and Environmental Science, 2011)
New anionic fullerene-heptamethine conjugates absorbing as far as 900 nm have been synthesized and fully characterized by physicochemical means. In terms of electron transfer, we have succeeded in realizing the rarely occurring scenario of a charge transfer commencing with a higher excited state. Kinetic studies reveal that charge separation and charge recombination dynamics occur in the normal and inverted Marcus region, respectively. © 2011 The Royal Society of Chemistry.

Efficient Nitrogen Doping of Single-Layer Graphene Accompanied by Negligible Defect Generation for Integration into Hybrid Semiconductor Heterostructures - Sarau, George; Heilmann, Martin; Bashouti, Muhammad; Latzel, Michael; Tessarek, Christian; Christiansen, Silke (ACS Applied Materials and Interfaces, 2017)
While doping enables application-specific tailoring of graphene properties, it can also produce high defect densities that degrade the beneficial features. In this work, we report efficient nitrogen doping of ∼11 atom % without virtually inducing new structural defects in the initial, large-area, low defect, and transferred single-layer graphene. To shed light on this remarkable high-doping–low-disorder relationship, a unique experimental strategy consisting of analyzing the changes in doping, strain, and defect density after each important step during the doping procedure was employed. Complementary micro-Raman mapping, X-ray photoelectron spectroscopy, and optical microscopy revealed that effective cleaning of the graphene surface assists efficient nitrogen incorporation accompanied by mild compressive strain resulting in negligible defect formation in the doped graphene lattice. These original results are achieved by separating the growth of graphene from its doping. Moreover, the high doping level occurred simultaneously with the epitaxial growth of n-GaN micro- and nanorods on top of graphene, leading to the flow of higher currents through the graphene/n-GaN rod interface. Our approach can be extended toward integrating graphene into other technologically relevant hybrid semiconductor heterostructures and obtaining an ohmic contact at their interfaces by adjusting the doping level in graphene.
EAM Import::2019-03-14

Efficient Photocatalytic H2 Evolution: Controlled Dewetting-Dealloying to Fabricate Site-Selective High-Activity Nanoporous Au Particles on Highly Ordered TiO2 Nanotube Arrays - Nguyen, Nhat Truong; Altomare, Marco; Yoo, Jeong Eun; Schmuki, Patrik (Advanced Materials, 2015)
Anodic self-organized TiO2 nanostumps are formed and exploited for self-ordering dewetting of Au–Ag sputtered films. This forms ordered particle configurations at the tube top (crown position) or bottom (ground position). By dealloying from a minimal amount of noble metal, porous Au nanoparticles are then formed, which, when in the crown position, allow for a drastically improved photocatalytic H2 production compared with nanoparticles produced by conventional dewetting processes.
EAM Import::2019-03-14

Efficient self-consistent treatment of electron correlation within the random phase approximation - Bleiziffer, Patrick; Heßelmann, Andreas; Görling, Andreas (Journal of Chemical Physics, 2013)
A self-consistent Kohn-Sham (KS) method is presented that treats correlation on the basis of the adiabatic-connection dissipation-fluctuation theorem employing the direct random phase approximation (dRPA), i.e., taking into account only the Coulomb kernel while neglecting the exchange-correlation kernel in the calculation of the Kohn-Sham correlation energy and potential. The method, denoted self-consistent dRPA method, furthermore treats exactly the exchange energy and the local multiplicative KS exchange potential. It uses Gaussian basis sets, is reasonably efficient, exhibiting a scaling of the computational effort with the forth power of the system size, and thus is generally applicable to molecules. The resulting dRPA correlation potentials in contrast to common approximate correlation potentials are in good agreement with exact reference potentials. The negatives of the eigenvalues of the highest occupied molecular orbitals are found to be in good agreement with experimental ionization potentials. Total energies from self-consistent dRPA calculations, as expected, are even poorer than non-self-consistent dRPA total energies and dRPA reaction and non-covalent binding energies do not significantly benefit from self-consistency. On the other hand, energies obtained with a recently introduced adiabatic-connection dissipation-fluctuation approach (EXXRPA+, exact-exchange random phase approximation) that takes into account, besides the Coulomb kernel, also the exact frequency-dependent exchange kernel are significantly improved if evaluated with orbitals obtained from a self-consistent dRPA calculation instead of an exact exchange-only calculation. Total energies, reaction energies, and noncovalent binding energies obtained in this way are of the same quality as those of high-level quantum chemistry methods, like the coupled cluster singles doubles method which is computationally more demanding. © 2013 AIP Publishing LLC.

Efficient synthetic access to cationic dendrons and their application for ZnO nanoparticles surface functionalization: New building blocks for dye-sensitized solar cells - Gnichwitz, Jan-Frederik; Marczak, Renata; Werner, Fabian; Lang, Nina; Jux, Norbert; Guldi, Dirk Michael; Peukert, Wolfgang; Hirsch, Andreas (Journal of the American Chemical Society, 2010)
A new concept for the efficient synthesis of cationic dendrons, 4-tert-butyl-1-(3-(3,4-dihydroxybenzamido)benzyl)pyridinium bromide (17), 1,1′-(5-(3,4-dihydroxybenzamido)-1,3-phenylene)bis(methylene) bis(4-tert-butylpyridinium) bromide (18), N1,N7-bis(3-(4-tert-butyl-pyridium- methyl)phenyl)-4-(3-(3-(4-tert-butyl-pyridinium-methyl)phenyl-amino) -3-oxopropyl)-4-(3,4-dihydroxybenzamido)heptanediamide tribromide (19), and N1,N7-bis(3,5-bis(4-tert-butyl-pyridium-methyl)phenyl)-4-(3-(3, 5-bis(4-tert-butyl-pyridinium-methyl)phenylamino)-3-oxopropyl)-4-(3, 4-dihydroxybenzamido)heptanediamide hexabromide (20), and their facile binding to zinc oxide (ZnO) nanostructures is introduced. Dendrons containing highly reactive benzylic bromides reacted readily with 4-tert-butyl-pyridine and resulted in cationic dendrons. Furthermore, these permanently positively charged dendrons were equipped with a catechol anchor group. This enabled ZnO surface functionalization by simple immersion. The adsorption of 17, 18, 19, and 20 on the colloidal nanoparticles was monitored by Langmuir isotherms. The highest obtained experimental loadings correspond to 99.5%, 98.6%, 99.1%, and 42.5% of the particle surface for 17, 18, 19, and 20, respectively. These results indicate insufficient adsorption of the largest molecule 20 leading to reduced colloidal stability of the nanoparticles, while an enhanced stability after grafting with 17, 18, and 19 was observed. Mesoporous films suitable for the use as electrodes in dye-sensitized solar cells (DSSCs) were prepared. Subsequently, the films were functionalized with 18, 19, or 20 and sensitized with zinc-5,15-bis-[2′,6′-bis-2″,2″-bis-(carboxy)- ethyl-methyl-4′-tert-butyl-pheny]-10,20-bis-(4′-tert-butylphenyl) porphyrin-octasodium-salt. UV-vis absorption spectra confirmed that 18, 19, and 20 are suitable for the stable electrostatic attachment of the dye. Current-voltage characteristics of complete cells demonstrated that increasing positive functionalization of the ZnO surface leads to decreased open circuit voltages (V ). All V values were around 0.4 V with a maximum for the 18 functionalized ZnO film of 0.45 V. The maximum cell efficiency obtained (0.31%) is rather high, considering the narrow spectral absorption of the dye and the rather thin ZnO films used. Finally, incident photon to current efficiency (IPCE) measurements confirmed photoinduced electron injection from the dye. These features are important assets for applications in particle technology and even facilitated advanced devices like a supramolecular DSSC complete with a subsequent layer of negatively charged porphyrins. © 2010 American Chemical Society.

Efficient two-scale optimization of manufacturable graded structures - Schury, Fabian; Stingl, Michael; Wein, Fabian (SIAM Journal on Scientific Computing, 2012)
A new method for the optimal design of inhomogeneous meso-structured components is presented. The method is based on a variant of the free material optimization (FMO) method. Instead of coupling a macroscopic model directly with the meso-scale in the framework of a two-scale approach, information from the meso-scale is incorporated into the FMO model by additional constraints. The FMO result, given in terms of optimized tensors, is interpreted by the inverse homogenization approach. Rather than only computing the meso-structure for each "FMO cell" independently, approximate continuity of the meso-structure is enforced. Thus, the final result is a manufacturable structurally graded material. The feasibility of the whole method is demonstrated by means of an academic example.
Keywords: free material optimization; inverse homogenization; structurally graded materials; topology optimization; numerical validation

Efficient utilization of higher-lying excited states to trigger charge-transfer events - Bouit, Pierre-Antoine; Spänig, Fabian; Kuzmanich, Gregory; Krokos, Evangelos; Oelsner, Christian; Garcia-Garibay, Miguel A.; Delgado, Juan Luis; Martin, Nazario; Guldi, Dirk Michael (Chemistry - A European Journal, 2010)
Several new fullerene-heptamethine conjugates, which absorb as far as into the infrared spectrum as 800 nm, have been synthesized and fully characterized by physicochemical means. In terms of optical and electrochemical characteristics, appreciable electronic coupling between both electroactive species is deduced. The latter also reflect the excited-state features. To this end, time-resolved, transient absorption measurements revealed that photoexcitation is followed by a sequence of charge-transfer events which evolve from higher singlet excited states (i.e., S-fast charge transfer) and the lowest singlet excited state of the heptamethine cyanine (i.e., S - slow charge transfer), as the electron donor, to either a covalently linked C or C as the electron acceptor. Finally, charge transfer from photoexcited C/ C completes the charge-transfer sequence. The slow internal conversion within the light-harvesting heptamethine cyanine and the strong electronic coupling between the individual constituents are particularly beneficial to this process. © 2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.

Elasticity of nematic phases with fundamental measure theory - Wittmann, Rene; Marechal, Mattheus; Mecke, Klaus (Physical Review E, 2015)
In a previous publication [R. Wittmann, M. Marechal, and K. Mecke, Europhys. Lett. 109, 26003 (2015)], we introduced fundamental mixed measure theory (FMMT) for mixtures of anisotropic hard bodies, which shows that earlier results with an empirical parameter are inaccurate. Now we provide a deeper insight into the background of this theory in integral geometry. We study the Frank elastic coefficients in the nematic phase of the hard spherocylinder fluid. The framework of FMMT provides us with the required direct correlation function without additional input of an equation of state. A series representation of the mixed measure gives rise to closed analytical formulas for the elastic constants that only depend on the density, order parameters, and the particle geometry, pointing out a significant advantage of our geometry-based approach compared to other density functionals. Our elastic coefficients are in good agreement with computer simulations and increase with the density and the nematic order parameter. We confirm earlier mean-field predictions in the limits of low orientational order and infinitely long rods.

Electrical circuit model of arrays of resonant elements - Lomanets, Vadym; Zhuromskyy, Oleksandr; Onishchukov, G.; Peschel, Ulf (Physical Review B, 2012)
We present an effective electrical circuit model that can be used for a quasianalytic analysis of electromagnetic oscillations in arrays of coupled elements, resonant in the microwave domain. The model accounts for electric and magnetic interactions between charges and currents excited in individual resonators. Respective coupling coefficients can be calculated from the field and current distributions in a subsystem of just one or two elements, provided by a finite-difference electromagnetic solver. The model was used to investigate current distributions and dispersion relations of wave propagation on chains of coupled split-ring resonators. The change of the dispersion characteristics from forward to backward propagating wave type observed experimentally is readily reproduced by the model.
EAM Import::2019-03-14

Electrical properties of solution processed layers based on Ge-Si alloy nanoparticles - Meric, Zeynep; Mehringer, Christian; Jank, Michael. P. M.; Peukert, Wolfgang; Frey, Lothar (MRS Advances, 2016)
Ge-Si alloy nanoparticles (NPs) covering the full range of compositions were studied in regard to their suitability as semiconducting channel layer in thin-film transistors (TFTs). Special focus is given to the influence of annealing and encapsulation techniques on the contact and channel properties. Therefore, electrical characterization methods separating contact from channel characteristics are highlighted and applied. It is demonstrated that appropriate passivation of the nanoparticle surfaces can improve the Ion/Ioff ratios by modulation of the density of free charge carriers and also can suppress hysteresis effects. Ge-rich NP alloys can generally be passivated more effectively regardless if passivation is done with solution-processed poly(methyl methacrylate) (PMMA) or by aluminum oxide (Al2O3) from Atomic Layer Deposition (ALD). Sufficient annealing improves the contact formation between aluminum electrodes and Ge-Si particles by modification of charge injection. The presented analysis leads to a better understanding interface and surface effects in porous nanoparticle semiconductors for application in TFT devices.
Keywords: thin film; passivation; nanostructure

Electrifying model catalysts for understanding electrocatalytic reactions in liquid electrolytes - Faisal, Firas; Stumm, Corinna; Bertram, Manon; Waidhas, Fabian; Lykhach, Yaroslava; Cherevko, Serhiy; Xiang, Feifei; Ammon, Maximilian Michael; Vorokhta, Mykhailo; Smid, Bretislav; Skala, Tomas; Tsud, Nataliya; et al. (Nature Materials, 2018)
Electrocatalysis is at the heart of our future transition to a renewable energy system. Most energy storage and conversion technologies for renewables rely on electrocatalytic processes and, with increasing availability of cheap electrical energy from renewables, chemical production will witness electrification in the near future(1-3). However, our fundamental understanding of electrocatalysis lags behind the field of classical heterogeneous catalysis that has been the dominating chemical technology for a long time. Here, we describe a new strategy to advance fundamental studies on electrocatalytic materials. We propose to 'electrify' complex oxide-based model catalysts made by surface science methods to explore electrocatalytic reactions in liquid electrolytes. We demonstrate the feasibility of this concept by transferring an atomically defined platinum/cobalt oxide model catalyst into the electrochemical environment while preserving its atomic surface structure. Using this approach, we explore particle size effects and identify hitherto unknown metal-support interactions that stabilize oxidized platinum at the nanoparticle interface. The metal-support interactions open a new synergistic reaction pathway that involves both metallic and oxidized platinum. Our results illustrate the potential of the concept, which makes available a systematic approach to build atomically defined model electrodes for fundamental electrocatalytic studies.

Electrocatalysis with Atomically Defined Model Systems: Metal-Support Interactions between Pt Nanoparticles and Co3O4(111) under Ultrahigh Vacuum and in Liquid Electrolytes - Faisal, Firas; Bertram, Manon; Stumm, Corinna; Wähler, Tobias; Schuster, Ralf; Lykhach, Yaroslava; Neitzel, Armin; Skala, Tomas; Tsud, Nataliya; Beranova, Klara; Prince, Kevin C.; Matolin, Vladimir; et al. (Journal of Physical Chemistry C, 2018)
Electronic metal-support interactions play a key role in the design of heterogeneous catalysts, as they provide a tool for tuning catalytic properties and enhancing catalyst stability. In this work, we explore the role of metal- support interactions in electrocatalysis using a model approach. We investigate the adsorption and reaction behavior of atomically defined Pt/Co3O4 model catalysts under ultrahigh vacuum (UHV) and under electrochemical conditions. The model systems were prepared by physical vapor deposition (PVD) of Pt onto well-ordered Co3O4(111) films on Ir(100), varying the average Pt nanoparticle (NP) size between 10 and 500 atoms per NP. In UHV, the model catalysts were characterized by synchrotron radiation photoelectron spectroscopy (SRPES), temperature-programmed desorption (TPD), and infrared reflection-absorption spectroscopy (IRAS). By SRPES, we show that partially oxidized Pt delta+ species are formed at the interface with the Co3O4 support. CO adsorbs weakly on these Pt delta+ sites and can be identified by IRAS at 115 K. Upon heating, CO adsorbed on metallic Pt-0 reacts with oxygen released from Co3O4 and gives rise to CO2 between 450 and 500 K. As a result of oxygen depletion, the Pt delta+ at the NP interface is reduced to Pt-0. Subsequently, we investigated the adsorption and oxidation of CO under electrochemical conditions on the same Pt/Co3O4 model catalysts. After preparation and characterization in UHV, the model systems were transferred into the electrochemical environment without exposure to ambient conditions. CO adsorption and electrooxidation were performed under conditions where the model system is stable (pH 10, 0.33-1.03 V-RHE, phosphate buffer). By electrochemical infrared reflection-absorption spectroscopy (EC-IRRAS), we show that CO does not adsorb at the partially oxidized Pt delta+ sites in the electrolyte at 300 K. Nevertheless, the Pet(delta+) species at the NP/oxide interface is reduced to Pt-0 upon repeated experimental cycles. This effect increases with decreasing NP size, in line with the behavior observed under UHV conditions. Our findings suggest that electronic metal-support interactions in metal/oxide catalysts play a very similar role in reactions with gaseous reactants and at the electrified interface.

Electrochemical synthesis of self-organized TiO2 nanotubular structures using an ionic liquid (BMIM-BF4) - Paramasivam, Indhumati; Macák, Jan; Selvam, T.; Schmuki, Patrik (Electrochimica Acta, 2008)
We show that an ionic liquid consisting of imidazolium salt with a BF_4 counter ion (BMIM-BF_4) can directly be used to grow well-defined layers of self-organized TiO_2 nanotubes. For this a Ti metal substrate is anodized in this electrolyte for potential range between 3 VAg/AgCl and 10 VAg/AgCl without addition of free fluoride species (fluorides are used in all previous tube growth procedures). Key factors that influence the morphology and geometry of the resulting nanotubular layer are the anodic potential, the anodization time and particularly the water content in the ionic liquid. The resulting nanotubes layers have thickness in the range of approximately 300–650 nm; with individual tubes that have diameters between 27 nm and 43 nm.
EAM Import::2019-03-14

Electrochemical wettability control on conductive TiO2 nanotube surfaces modified with a ferrocene redox system - Schmidt-Stein, F.; Gnichwitz, Jan-Frederik; Salonen, J.; Hahn, Robert; Hirsch, Andreas; Schmuki, Patrik (Electrochemistry Communications, 2009)
This work reports on an electrochemical system which allows the control of surface wettability properties by voltage induced changes in contact angle (Θ) of ΔΘ ≥ 50°. For this we used conductive TiO nanotubular layers that were modified with ferrocene coupled to the TiO surface via triethoxysilane. To enhance the hydrophobic character of the nanotubular TiO surface, also mixed organic monolayers namely perfluorotriethoxysilane, were explored. Formation of the ferrocene and mixed organic monolayer was confirmed by X-ray-photoelectron-spectroscopy (XPS). Contact angle combined with electrochemical measurements show that ferrocene in these monolayers can successfully be switched from Fe to Fe and that this change in the redox state considerably alters the wetting properties. Using a conductive nanotube substrate allows us to amplify this change by a factor of more than 10, and thus this surface can be used to trigger significant wetting alterations. © 2009 Elsevier B.V. All rights reserved.

Electroluminescence imaging of organic photovoltaic modules - Hoyer, U.; Wagner, M.; Swonke, Th.; Bachmann, J.; Auer, R.; Osvet, Andres; Brabec, Christoph (Applied Physics Letters, 2010)
We report on electroluminescence (EL) imaging of organic photovoltaic cells and modules with poly(3-hexylthiophene)/[6,6]-phenyl C61 butyric acid methyl ester as semiconductor layer. The dominant EL emission is found in a spectral regime between 1200 and 1400 nm and is identified as the radiative decay of the charge transfer complex formed between the polymer and the fullerene. Electroluminescence emission from the pristine compounds is either much weaker or completely absent. Overall, electroluminescence imaging is shown to give valuable information on the defects but also on the performance of organic solar modules. © 2010 American Institute of Physics.

Electron-beam induced deposition and autocatalytic decomposition of Co(CO)(3)NO - Vollnhals, Florian; Drost, Martin; TU, Fan; Carrasco Burgos, Esther; Späth, Andreas; Fink, Rainer; Steinrück, Hans-Peter; Marbach, Hubertus (Beilstein Journal of Nanotechnology, 2014)
The autocatalytic growth of arbitrarily shaped nanostructures fabricated by electron beam-induced deposition (EBID) and electron beam-induced surface activation (EBISA) is studied for two precursors: iron pentacarbonyl, Fe(CO)(5), and cobalt tricarbonyl nitrosyl, Co(CO)(3)NO. Different deposits are prepared on silicon nitride membranes and silicon wafers under ultrahigh vacuum conditions, and are studied by scanning electron microscopy (SEM) and scanning transmission X-ray microscopy (STXM), including near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It has previously been shown that Fe(CO)(5) decomposes autocatalytically on Fe seed layers (EBID) and on certain electron beam-activated surfaces, yielding high purity, polycrystalline Fe nanostructures. In this contribution, we investigate the growth of structures from Co(CO)(3)NO and compare it to results obtained from Fe(CO)(5). Co(CO)(3)NO exhibits autocatalytic growth on Co-containing seed layers prepared by EBID using the same precursor. The growth yields granular, oxygen-, carbon- and nitrogen-containing deposits. In contrast to Fe(CO)(5) no decomposition on electron beam-activated surfaces is observed. In addition, we show that the autocatalytic growth of nanostructures from Co(CO)(3)NO can also be initiated by an Fe seed layer, which presents a novel approach to the fabrication of layered nanostructures.
Keywords: autocatalytic growth; cobalt tricarbonyl nitrosyl; electron-beam induced deposition; nanofabrication; scanning transmission X-ray microscopy

Electron-beam-induced deposition and post-treatment processes to locally generate clean titanium oxide nanostructures on Si(100) - Schirmer, Michael; Walz, Marie-Madeleine; Vollnhals, Florian; Lukasczyk, Thomas; Sandmann, Achim; Chen, C.; Steinrück, Hans-Peter; Marbach, Hubertus (Nanotechnology, 2011)
We have investigated the lithographic generation of TiO nanostructures on Si(100) via electron-beam-induced deposition (EBID) of titanium tetraisopropoxide (TTIP) in ultra-high vacuum (UHV) by scanning electron microscopy (SEM) and local Auger electron spectroscopy (AES). In addition, the fabricated nanostructures were also characterized ex situ via atomic force microscopy (AFM) under ambient conditions. In EBID, a highly focused electron beam is used to locally decompose precursor molecules and thereby to generate a deposit. A drawback of this nanofabrication technique is the unintended deposition of material in the vicinity of the impact position of the primary electron beam due to so-called proximity effects. Herein, we present a post-treatment procedure to deplete the unintended deposits by moderate sputtering after the deposition process. Moreover, we were able to observe the formation of pure titanium oxide nanocrystals (>100 nm) in situ upon heating the sample in a well-defined oxygen atmosphere. While the nanocrystal growth for the as-deposited structures also occurs in the surroundings of the irradiated area due to proximity effects, it is limited to the pre-defined regions, if the sample was sputtered before heating the sample under oxygen atmosphere. The described two-step post-treatment procedure after EBID presents a new pathway for the fabrication of clean localized nanostructures. © 2011 IOP Publishing Ltd.

Electron-beam-induced deposition in ultrahigh vacuum: Lithographic fabrication of clean iron nanostructures - Lukasczyk, Thomas; Schirmer, Michael; Steinrück, Hans-Peter; Marbach, Hubertus (Small, 2008)
The generation of nanostructures with arbitrary shapes and well-defined chemical composition is still a challenge and targets the core of the fast-growing field of nanotechnology. One approach is the maskless nanofabrication technique of electron-beam-induced deposition (EBID). Up to now, the purity of these EBID structures has been rather poor. Here we demonstrate that by performing the EBID process solely under ultrahigh vacuum conditions, the lithographic generation of iron nanostructures on Si(100) with an unprecedented purity of higher than 95% is possible. One particular new aspect is the formation of EBID deposits with reduced size in a strain-induced diffusive process, resulting in deposits significantly smaller than 10 nm. © 2008 Wiley-VCH Verlag GmbH & Co.

Electron beam induced surface activation: a method for the lithographic fabrication of nanostructures via catalytic processes - Marbach, Hubertus (Applied Physics A-Materials Science & Processing, 2014)
In focused electron beam induced processing (FEBIP), the very narrow electron beam of a scanning electron microscope or transmission electron microscope is used to locally modify matter on the nanometer scale. Recently, the family of FEBIP could be considerably expanded by the technique of focused electron beam induced surface activation (EBISA). In EBISA, the surface itself gets chemically activated by the impact of the electron beam without the presence of precursor molecules. In the second EBISA processing step, the surface is exposed to a precursor molecule which is then catalytically decomposed at the pre-irradiated/activated areas and eventually continues to grow autocatalytically upon prolonged precursor dosage. In this way, electron irradiation and precursor dosage are effectively separated. One of the advantages is that, due to the autocatalytic growth, the size of the corresponding nanostructures can be controlled by the precursor dosage and corresponding electron proximity effects can be omitted. Another advantage is the parallel processing of the pre-irradiated regions during precursor dosage. This bears the potential to significantly reduce the fabrication times for larger deposits compared to the classical electron beam induced deposition approach, in which precursor molecules are sequentially dissociated by the impact of the electron. The fundamentals and apparent further developments as well as the potential and challenges of the comparably new EBISA technique, and more general of catalytic effects in FEBIP are presented and discussed.
EAM Import::2019-03-14

Electron beam induced surface activation of ultrathin porphyrin layers on Ag(111) - Vollnhals, Florian; Wintrich, Patrick; Walz, Marie-Madeleine; Steinrück, Hans-Peter; Marbach, Hubertus (Langmuir, 2013)
We demonstrate how a focused electron beam can be used to chemically activate porphyrin layers on Ag(111) such that they become locally reactive toward the decomposition of iron pentacarbonyl, Fe(CO). This finding considerably expands the scope of electron beam induced surface activation (EBISA) and also has implications for electron beam induced deposition (EBID). The influence of the porphyrin layer thickness on both processes is studied in detail using scanning tunneling microscopy (STM) and scanning electron microscopy (SEM) as well as Auger electron spectroscopy (AES) and scanning Auger microscopy (SAM). While a closed monolayer of porphyrin molecules does exhibit some activity toward Fe(CO) decomposition after electron irradiation, a growth enhancement is found for bi- and multilayer films. This is attributed to a partial quenching of activated centers in the first layer due to the close proximity of the silver substrate. In addition, we demonstrate that the catalytic decomposition of gaseous Fe(CO) on Ag(111) can be effectively inhibited by introducing a densely packed monolayer of 2H-tetraphenylporphyrin (2HTPP) molecules. © 2013 American Chemical Society.

Electron beam-induced writing of nanoscale iron wires on a functional metal oxide - Vollnhals, Florian; Woolcot, Tom; Walz, Marie-Madeleine; Seiler, Steffen; Steinrück, Hans-Peter; Thornton, Geoff; Marbach, Hubertus (Journal of Physical Chemistry C, 2013)
Electron beam-induced surface activation (EBISA) has been used to grow wires of iron on rutile TiO(110)-(1 × 1) in ultrahigh vacuum. The wires have a width down to ∼20 nm and hence have potential utility as interconnects on this dielectric substrate. Wire formation was achieved using an electron beam from a scanning electron microscope to activate the surface, which was subsequently exposed to Fe(CO). On the basis of scanning tunneling microscopy and Auger electron spectroscopy measurements, the activation mechanism involves electron beam-induced surface reduction and restructuring. © 2013 American Chemical Society.

Electron-donating behavior of few-layer graphene in covalent ensembles with electron-accepting phthalocyanines - Ragoussi, Maria-Eleni; Katsukis, Georgios; Roth, Alexandra Marina; Malig, Jenny; De La Torre, Gema; Guldi, Dirk Michael; Torres, Tomas (Journal of the American Chemical Society, 2014)
We describe herein the first example of highly exfoliated graphene covalently linked to electron accepting phthalocyanines. The functionalization of the nanocarbon surface with alkylsulfonyl phthalocyanines was attained by means of a "click" chemistry protocol. The new ensemble was fully characterized (thermogravimetric analysis, atomic force microscopy, transmission electron microscopy and Raman, as well as ground-state absorption) and was studied in terms of electron donor-acceptor interactions in the ground and in the excited state. In particular, a series of steady-state and time-resolved spectroscopy experiments demonstrated photoinduced electron transfer from the graphene to the electron-accepting phthalocyanines. This is the first example of an electron donor-acceptor nanoconjugate, that is, few-layer graphene/phthalocyanine, pinpointing the uncommon electron donating character of graphene. © 2014 American Chemical Society.

Electron-Electron Interaction in the Magnetoresistance of Graphene - Jobst, Johannes; Waldmann, Daniel; Gornyi, Igor; Mirlin, Alexander; Weber, Heiko B. (Physical Review Letters, 2012)
We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive ( magneto) transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.

Electronic Properties of Si-Hx Vibrational Modes at Si Waveguide Interface. - Bashouti, Muhammad Y.; Yousefi, Peyman; Ristein, Jürgen; Christiansen, Silke H. (Journal of Physical Chemistry Letters, 2015)
Attenuated total reflectance (ATR) and X-ray photoelectron spectroscopy in suite with Kelvin probe were conjugated to explore the electronic properties of Si-Hx vibrational modes by developing Si waveguide with large dynamic detection range compared with conventional IR. The Si 2p emission and work-function related to the formation and elimination of Si-Hx bonds at Si surfaces are monitored based on the detection of vibrational mode frequencies. A transition between various Si-Hx bonds and thus related vibrational modes is monitored for which effective momentum transfer could be demonstrated. The combination of the aforementioned methods provides for results that permit a model for the kinetics of hydrogen termination of Si surfaces with time and advanced surface characterizing of hybrid-terminated semiconducting solids.

Electronic Structure of Magnesia-Ceria Model Catalysts, CO2 Adsorption, and CO2 Activation: A Synchrotron Radiation Photoelectron Spectroscopy Study - Staudt, Thorsten; Lykhach, Yaroslava; Tsud, Nataliya; Skala, Tomas; Prince, Kevin C.; Matolin, Vladimir; Libuda, Jörg (Journal of Physical Chemistry C, 2011)
We have studied the electronic properties of single crystal based ceria and magnesia-ceria model catalysts, the CO2, adsorption, and the CO2-induced reoxidation of these systems by synchrotron radiation photoelectron spectroscopy (SR-PES). All model systems were prepared starting from a fully stoichiometric and well-ordered CeO2(111) film grown on Cu(111). Different magnesia-ceria mixed oxide films were prepared by physical vapor deposition (PVD) of magnesium, oxygen treatment, and subsequent annealing. The preparation procedure was varied to obtain samples with different oxidation state, structure, and surface composition. Different carbon-containing species were identified, including surface carbonates formed in the vicinity of Mg2+ and Ce3+/4+ and surface carboxylates. The presence of Mg2+ was observed to strongly enhance carbonate formation but suppress the formation of carboxylates. Changes in the oxidation state of ceria upon CO2 exposure were monitored with highest sensitivity by resonant photoelectron spectroscopy (RPES). Reoxidation of Ce3+ was observed to be suppressed on magnesia-containing samples, even in the presence of a high surface concentration of Ce3+. The findings suggest that carboxylates are an intermediate step of reoxidation, whereas the generation of stable surface carbonates inhibits formation of this intermediate and, therefore, CO2-induced reoxidation

Electrons as "Invisible Ink": Fabrication of nanostructures by local electron beam induced activation of SiOx - Walz, Marie-Madeleine; Schirmer, Michael; Vollnhals, Florian; Lukasczyk, Thomas; Steinrück, Hans-Peter; Marbach, Hubertus (Angewandte Chemie International Edition, 2010)
Figure Presented Beam me up: A novel two-step process allows iron nanostructures to be generated locally on SiO/Si at 300 K. The surface is first locally activated by an electron beam. Then the activated structures are exposed to [Fe(CO)], which decomposes and grows autocatalytically to give pure Fe nanocrystals. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electron transfer through rigid organic molecular wires enhanced by electronic and electron–vibration coupling - Sukegawa, Junpei; Schubert, Christina; Zhu, Xiaozhang; Tsuji, Hayato; Guldi, Dirk Michael; Nakamura, Eiichi (Nature Chemistry, 2014)
Electron transfer (ET) is a fundamental process in a wide range of biological systems, photovoltaics and molecular electronics. Therefore to understand the relationship between molecular structure and ET properties is of prime importance. For this purpose, photoinduced ET has been studied extensively using donor–bridge–acceptor molecules, in which π-conjugated molecular wires are employed as bridges. Here, we demonstrate that carbon-bridged oligo-p-phenylenevinylene (COPV), which is both rigid and flat, shows an 840-fold increase in the ET rate compared with the equivalent flexible molecular bridges. A 120-fold rate enhancement is explained in terms of enhanced electronic coupling between the electron donor and the electron acceptor because of effective conjugation through the COPVs. The remainder of the rate enhancement is explained by inelastic electron tunnelling through COPV caused by electron–vibration coupling, unprecedented for organic molecular wires in solution at room temperature. This type of nonlinear effect demonstrates the versatility and potential practical utility of COPVs in molecular device applications.

Electron-vibron coupling in halogenated acenaphthenequinone upon O K-edge soft x-ray absorption - Schmidt, Norman Anja; Clark, Timothy; Urquhart, Stephen G.; Fink, Rainer (Journal of Chemical Physics, 2011)
We present high resolution oxygen K near-edge x-ray absorption spectra of the acenaphthenequinone (ANQ) derivative 3,8-dibromo-5,6-dichloro-ANQ (Br Cl-ANQ). The spectral features exhibit an almost identical vibronic fine structure compared to that shown by ANQ. The unequal distances of the vibronic levels as derived from the least-squares fit analysis of the vibronic progressions allows us to determine the anharmonicity of the excited state potentials involved. We conclude that a single vibrational progression couples to the resonant excitation of O 1s core electrons preferentially. Comparison of the two ANQ derivatives gives a clear indication that the vibronic mode corresponds to a C=O stretching mode, rather than coupling to a C-H mode as suggested previously. These conclusions are supported by density functional theory calculations. © 2011 American Institute of Physics.

Electrophoretic Deposition of alpha-Fe2O3/Chitosan Nanocomposite Coatings for Functional and Biomedical Applications - Cabanas Polo, Sandra; Distaso, Monica; Peukert, Wolfgang; Boccaccini, Aldo R. (Journal of Nanoscience and Nanotechnology, 2015)
Promising composite coatings based on hematite (alpha-Fe2O3) mesocrystals of size 110 nm and chitosan (CHT) molecules for different biotechnological applications have been successfully obtained by electrophoretic deposition (EPD). Homogeneous and reproducible coatings have been obtained by studying and controlling the chemical interactions between both phases (alpha-Fe2O3 and CHT). A voltage of 25 V and a deposition time of 5 min were chosen as best deposition conditions, which resulted in highly homogeneous coatings with well-distributed alpha-Fe2O3 particles. According to TGA measurements, the content of alpha-Fe2O3 and chitosan in the final composite coating were found to be 74 and 26 wt%, respectively. The presence of both phases in the composite coating was determined by XRD analysis and the coatings microstructure was observed by SEM.
Keywords: Iron Oxide Nanoparticles; Chitosan; Composite Coating; Electrophoretic Deposition

Electrophoretic deposition of anisotropic α-Fe2O3/PVP/chitosan nanocomposites for biomedical applications - Ramos Rivera, Laura; Distaso, Monica; Peukert, Wolfgang; Boccaccini, Aldo R. (Materials Letters, 2017)
Electrophoretic deposition (EPD) has been used to fabricate composite coatings based on anisotropic hematite particles ( extgreeka-Fe2O3) comprising residual poly-N-vinyl-pyrrolidone (PVP) and chitosan (CHT) on stainless steel substrates. The experimental conditions towards homogeneous surface, non-delamination and good distribution of extgreeka-Fe2O3/PVP particles are described. The coatings were characterized by SEM, XRD and TGA. A molecular interaction between PVP adsorbed on the surface of the particles and CHT is identified.
EAM Import::2019-03-14
Keywords: Electrophoretic deposition; Hierarchical structures; Porous layers; PVP/Chitosan interaction

Electrophoretic Deposition of Boehmite on Additively Manufactured, Interpenetrating Periodic Open Cellular Structures for Catalytic Applications - Do, Giang; Stiegler, Thomas; Fiegl, Markus; Adler, Lucas; Körner, Carolin; Bösmann, Andreas; Freund, Hannsjörg Dieter; Schwieger, Wilhelm; Wasserscheid, Peter (Industrial & Engineering Chemistry Research, 2017)
Our contribution demonstrates that the combination of additive manufacturing and electrophoretic deposition offers great potential for the future manufacturing of tailor-made catalytic structures for continuous flow applications. A new protocol for the controlled and homogeneous coating of both electrodes of interpenetrating porous open cell structures (interPOCS) with layers of boehmite is presented. Moreover, it has been found that by applying different coating voltages in an electrophoretic deposition (EPD) process, the properties of the obtained coating can be fine-tuned with respect to layer thickness, density, and porosity. This offers very interesting options for optimizing the catalysis-relevant properties from two sides by the use of the special interPOCS support design and by adjusting the coating through the parameters of the EPD coating process.

Electrospray ionization deposition of ultrathin ionic liquid films: [C 8C1Im]Cl and [C8C1Im][Tf 2N] on Au(111) - Rietzler, Florian; Piermaier, Marius; Deyko, Alexey; Steinrück, Hans-Peter; Maier, Florian (Langmuir, 2014)
We introduce a new method for preparing ultrathin ionic liquid (IL) films on surfaces by means of electrospray ionization deposition (ESID) under ultraclean and well-defined ultra-high-vacuum (UHV) conditions. In contrast to physical vapor deposition (PVD) of ILs under UHV, ESID even allows deposition of ILs, which are prone to thermal decomposition. As proof of concept, we first investigated ultrathin [CCIm][TfN] (=1-methyl-3-octyl imidazolium bis(trifluoromethyl)imide) films on Au(111) by angle-resolved X-ray photoelectron spectroscopy (ARXPS). Films obtained by ESID are found to be virtually identical to films grown by standard PVD. Thereafter, ESID of [CCIm]Cl on Au(111) was studied as a first example of an IL that cannot be prepared as ultrathin film otherwise. [C CIm]Cl forms a wetting layer with a checkerboard arrangement with the cationic imidazolium ring and the chloride anion adsorbed next to each other on the substrate and the alkyl chain pointing toward vacuum. This arrangement within the wetting layer is similar to that observed for [CCIm][TfN], albeit with a higher degree of order of the alkyl chains. Further deposition of [CC Im]Cl leads to a pronounced island growth on top of the wetting layer, which is independently confirmed by ARXPS and atomic force microscopy. This behavior contrasts the growth behavior found for [CC Im][TfN], where layer-by-layer growth on top of the wetting layer is observed. The dramatic difference between both ILs is attributed to differences in the cation-anion interactions and in the degree of order in the wetting layer of the two ILs. © 2014 American Chemical Society.

Electrostatic layer-by-layer construction and characterization of photoelectrochemical solar cells based on water soluble polythiophenes and carbon nanotubes - Sgobba, Vito; Troeger, Anna; Cagnoli, Rita; Mateo-Alonso, Aurelio; Prato, Maurizio; Parenti, Francesca; Mucci, Adele; Schenetti, Luisa; Guldi, Dirk Michael (Journal of Materials Chemistry, 2009)
Positively and negatively charged functionalized polythiophenes and single walled carbon nanotubes were sequentially adsorbed on indium tin oxide electrodes by electrostatic and van der Waals interactions using the layer-by-layer deposition method. The resulting donor/acceptor photoelectrodes were tested as photoelectrochemical cells. Upon illumination, monochromatic incident photoconversion efficiencies of 8.2 and 4.3% were determined for eight stacks of single walled carbon nanotubes with ammonium and sulfonate polythiophene derivatives, respectively. © The Royal Society of Chemistry 2009.

Elucidating the Excited-State Properties of CuInS2 Nanocrystals upon Phase Transformation: Quasi-Quantum Dots Versus Bulk Behavior - Azimi, Seyed Hamed; Kuhri, Susanne; Stahl, Melissa S.; Hou, Yi; Guldi, Dirk Michael; Brabec, Christoph (Advanced Electronic Materials, 2015)
New insights into the complex relation between the thermal phase transformation and the excited state properties of chalcopyrite CuInS nanocrystals (CIS NCs) are presented. An in situ solution processing method via depositing molecular precursors is applied, offering advantages in terms of simplicity, low-temperature processability, and control over crystallite size, and optical properties. At low annealing temperatures, strong carrier quantum confinement and small crystallite sizes are realized for CIS NC films. CIS crystal growth is found to set in at higher annealing temperatures inducing a complete transformation from quasi-quantum-dot (QD) via bulk-like to bulk behavior. The transition of a near-infrared localized surface plasmon resonance (LSPR) towards a bulk-like plasmon resonance documents the crystal growth and further acts as a valuable probe to relate crystallite size with copper-deficient stoichiometries. In addition, time-resolved photophysical investigations help to shed light on the dynamics and mechanisms of exciton and charge carrier generation of CIS NC films as a function of annealing temperature. The in situ-prepared CIS NC films are further passivated by a thin CdS layer, leading to the formation of long-lived excitons and an effective CIS ground-state depletion.
Keywords: chalcopyrite; CuInS2 nanocrystals; exciton generation; localized surface plasmon resonance; transient absorption spectroscopy

Elucidating water dynamics in MgCl2 hydrates from molecular dynamics simulation - Huinink, Hendrik Pieter; Zahn, Dirk (Solid State Sciences, 2017)
The water mobility in single crystals of MgCl2 hydrates has been investigated with molecular dynamics. Standard force fields have been benchmarked for molecular dynamics simulations of MgCl2 hydrates. To provide a reliable molecular mechanics model, force fields are selected on their ability to reproduce the structure of MgCl2·6H2O at 300 K. The selected force fields are then tested on their ability to also reproduce the structures of the different hydrates (n = 12, 8, 6, 4, 2, 1) and available thermodynamic data. For the currently best force-field available, constant-temperature, constant-pressure molecular dynamics simulations are preformed to elucidate the mechanisms of hydrate water mobility in perfect single crystals of the tetra- and hexahydrate. Long range water diffusion was not observed; each water molecule remained in the coordination sphere of its original Mg2+ host. However, collective ring-like motions of four water molecules at once within the coordination shell of a Magnesium ion were observed.
EAM Import::2019-03-14

Embedded Palladium Activation as a Facile Method for TiO2 - Mazare, Anca Valentina; Liu, Ning; Lee, Kiyoung; Killian, Manuela; Schmuki, Patrik (ChemistryOpen, 2013)
A direct and efficient way to decorate TiO2 nanotubes with copper, Cu2O and CuO particles is shown. Active palladium species embedded in the TiO2 nanotube walls allow specific initiation sites to be triggered for electroless copper deposition. These Cu2O-decorated TiO2 nanotubes could be used for water splitting.
EAM Import::2019-03-14

Emission of semiconductor nanocrystals in photonic crystal environment - Romanov, Sergey; Peschel, Ulf (Nanocrystals: Synthesis, Properties and Applications, 2010)
EAM Import::2019-03-14

EMPIRE: a highly parallel semiempirical molecular orbital program: 1: self-consistent field calculations - Hennemann, Matthias; Clark, Timothy (Journal of Molecular Modeling, 2014)
No abstract available

EMPIRE: a highly parallel semiempirical molecular orbital program: 2: periodic boundary conditions - Margraf, Johannes; Hennemann, Matthias; Meyer, Bernd; Clark, Timothy (Journal of Molecular Modeling, 2015)
No abstract available for this article

Encapsulation of silver nanowire networks by atomic layer deposition for indium-free transparent electrodes - Göbelt, Manuela; Keding, Ralf; Schmitt, Sebastian W.; Hoffmann, Bjoern; Jaeckle, Sara; Latzel, Michael; Radmilovic, Vuk V.; Radmilovic, Velimir R.; Spiecker, Erdmann; Christiansen, Silke; Hoffmann, Björn (Nano Energy, 2015)
We report on the development of a novel nano-composite transparent electrode material to be used in various energy applications e.g. as contacts for solar cells, composed of a wet-chemically synthesized silver nanowire (AgNW) network encapsulated in a transparent conductive oxide (TCO) which was deposited with nano-scale precision by atomic layer deposition (ALD). The AgNWs form a random network on a substrate of choice when being drop casted. ALD encapsulation of AgNWs guarantees a conformal and thickness controlled coating of the wires e.g. by the selected aluminum doped zinc oxide (AZO). Annealing of the AgNWs prior to ALD coating, yield a local sintering of AgNWs at their points of intersection, which improves the conductivity of the composite electrodes by reducing their sheet resistance. To demonstrate the performance of these AgNW/AZO composite transparent electrodes, they were used as a top electrode on wafer-based silicon (Si) - solar cells. A novel combination of scanning electron microscopy and image processing is used to determine the degree of percolation of the AgNWs on large areas of the nano-composite AgNW/AZO electrodes. Our results show that the solar cell with percolated AgNW/AZO electrode show the highest short circuit current density (28 mA/cm(2)) and a series resistance in the same order of magnitude compared to reference solar cells with a thermally evaporated silver grid electrode. The electrode example we chose reveals that the developed AgNW/AZO electrode is a technologically relevant and cheap alternative to conventional solar cell screen printed grid electrodes, which contain similar to 95% more Ag per device area, with a high potential to be further systematically optimized by the presented image processing method. (C) 2015 Elsevier Ltd. All rights reserved.
Keywords: Atomic layer deposition; Silver nano wires; Transparent electrode; Encapsulation; Aluminum doped zinc oxide (AZO); Solar cell

Endohedral fullerenes for organic photovoltaic devices - Ross, Russel B.; Cardona, Claudia M.; Guldi, Dirk Michael; Sankaranarayanan, Shankara Gayathri; Reese, Matthew O.; Kopidakis, Nikos; Peet, Jeff; Walker, Bright; Bazan, Guillermo C.; Van Keuren, Edward; Holloway, Brian C.; Drees, Martin (Nature Materials, 2009)
So far, one of the fundamental limitations of organic photovoltaic (OPV) device power conversion efficiencies (PCEs) has been the low voltage output caused by a molecular orbital mismatch between the donor polymer and acceptor molecules. Here, we present a means of addressing the low voltage output by introducing novel trimetallic nitride endohedral fullerenes (TNEFs) as acceptor materials for use in photovoltaic devices. TNEFs were discovered in 1999 by Stevenson et al.; for the first time derivatives of the TNEF acceptor, Lu 3 N@C 80, are synthesized and integrated into OPV devices. The reduced energy offset of the molecular orbitals of Lu 3 N@C 80 to the donor, poly(3-hexyl)thiophene (P3HT), reduces energy losses in the charge transfer process and increases the open circuit voltage (Voc) to 260 mV above reference devices made with [6,6]-phenyl-C 61 -butyric methyl ester (C 60 -PCBM) acceptor. PCEs >4% have been observed using P3HT as the donor material. This work clears a path towards higher PCEs in OPV devices by demonstrating that high-yield charge separation can occur with OPV systems that have a reduced donor/acceptor lowest unoccupied molecular orbital energy offset.

Energy decay in a granular gas collapse - Almazan Torres, Lidia; Serero, Dan; Salueña, Clara; Pöschel, Thorsten (New Journal of Physics, 2017)
An inelastic hard ball bouncing repeatedly off the ground comes to rest in finite time by performing an infinite number of collisions. Similarly, a granular gas under the influence of external gravity, condenses at the bottom of the confinement due to inelastic collisions. By means of hydrodynamical simulations, we find that the condensation process of a granular gas reveals a similar dynamics as the bouncing ball. Our result is in agreement with both experiments and particle simulations, but disagrees with earlier simplified hydrodynamical description. Analyzing the result in detail, we find that the adequate modeling of pressure plays a key role in continuum modeling of granular matter.

Energy dissipation in driven granular matter in the absence of gravity - Sack, Achim; Heckel, Michael; Kollmer, Jonathan; Zimber, Fabian; Pöschel, Thorsten (Physical Review Letters, 2013)
We experimentally investigate the energy dissipation rate in sinusoidally driven boxes which are partly filled by granular material under conditions of weightlessness. We identify two different modes of granular dynamics, depending on the amplitude of driving, A. For intense forcing, A>A, the material is found in the collect-and-collide regime where the center of mass of the granulate moves synchronously with the driven container, while for weak forcing, A

Energy Storage in Strained Organic Molecules: (Spectro)Electrochemical Characterization of Norbornadiene and Quadricyclane - Brummel, Olaf; Besold, Daniel; Döpper, Tibor; Wu, Yanlin; Bochmann, Sebastian; Lazzari, Federica; Waidhas, Fabian; Bauer, Udo; Bachmann, Philipp; Papp, Christian; Steinrück, Hans-Peter; Görling, Andreas; et al. (Chemsuschem, 2016)
We have investigated the electrochemically triggered cycloreversion of quadricyclane (QC) to norbornadiene (NBD), a system that holds the potential to combine both energy storage and conversion in a single molecule. Unambiguous voltammetric traces are obtained for pure NBD and pure QC, the latter a strained polycyclic isomer of the former. The difference in redox potentials is smaller than the energy difference between the neutral molecules. This is owing to a significant energy difference between the corresponding radical cations, as demonstrated by density functional theory (DFT) calculations. The vibrational modes of each pure compound are characterized experimentally in the fingerprint region and identified by DFT methods. Thermal and electrochemical transformations of NBD and QC are monitored insitu by IR spectroelectrochemical methods. The kinetics of the cycloreversion of QC to NBD, which is catalyzed by oxidizing equivalents, can be controlled by an applied electrode potential, which implies the ability to adjust in real time the release of thermal power stored in QC.
Keywords: Infrared spectroscopy; Norbornadiene; Quadricyclane; Spectroelectrochemistry; Voltammetry

Engineered disorder and light propagation in a planar photonic glass - Romanov, Sergey; Orlov, Sergej; Ploss, Daniel; Weiss, Clemens K.; Vogel, Nicolas; Peschel, Ulf (Scientific Reports, 2016)
The interaction of light with matter strongly depends on the structure of the latter at wavelength scale. Ordered systems interact with light via collective modes, giving rise to diffraction. In contrast, completely disordered systems are dominated by Mie resonances of individual particles and random scattering. However, less clear is the transition regime in between these two extremes, where diffraction, Mie resonances and near-field interaction between individual scatterers interplay. Here, we probe this transitional regime by creating colloidal crystals with controlled disorder from two-dimensional self-assembly of bidisperse spheres. Choosing the particle size in a way that the small particles are transparent in the spectral region of interest enables us to probe in detail the effect of increasing positional disorder on the optical properties of the large spheres. With increasing disorder a transition from a collective optical response characterized by diffractive resonances to single particles scattering represented by Mie resonances occurs. In between these extremes, we identify an intermediate, hopping-like light transport regime mediated by resonant interactions between individual spheres. These results suggest that different levels of disorder, characterized not only by absence of long range order but also by differences in short-range correlation and interparticle distance, exist in colloidal glasses.

Engineered Light Scattering in Colloidal Photonic Heterocrystals - Ding, Boyang; Bardosova, Maria; Povey, Ian; Pemble, Martyn E.; Romanov, Sergei G. (Advanced Functional Materials, 2010)
Photonic heterocrystals are prepared by sandwiching films of self-assembled opal and force-assembled Langmuir–Blodgett colloidal crystals. Anomalously strong light scattering in conjunction with low reflectivity is observed with increasing angle of incidence in the spectral range of photonic bandgaps. The occurrence of light scattering at the interface has been assigned to the optical mode mismatch between the two types of photonic crystals. Photonic bandgap-related mechanisms of trapping the decaying photonic crystal modes at the interface are suggested.
EAM Import::2019-03-14

Engineering a three-dimensional, photoelectrochemically active p-NiO/i-Sb2S3 junction by atomic layer deposition - Barr, Maissa K. S.; Assaud, Loic; Wu, Yanlin; Laffon, Carine; Parent, Philippe; Bachmann, Julien; Santinacci, Lionel (Electrochimica Acta, 2015)
A p-NiO/i-Sb2S3 semiconductor junction is created as an array of parallel, coaxially structured, hollow nanocylinders. The preparation bases on two consecutive steps of atomic layer deposition (ALD) onto the pore walls of anodic alumina, used as an inert template. ALD allows for the conformal coating of the deep pores. Characterization by x-ray photoelectron spectroscopy, x-ray diffraction, and transmission electron microscopy demonstrates the high purity, perfect stoichiometry, and nanocrystalline structure of both layers. Annealing the samples increases the crystallite size but disrupts the continuity of the Sb2S3 film. Electrochemical and photoelectrochemical curves evidence the injection of holes from the light absorber Sb2S3 into the p-type NiO, but no significant photoinduced cathodic electron transfer to the electrolyte. (C) 2015 Elsevier Ltd. All rights reserved.
Keywords: Atomic layer deposition; Photoelectrochemical cell; Sb2S3; NiO; Anodic aluminum oxide

Engineering Nanoporous Iron(III) Oxide into an Effective Water Oxidation Electrode - Haschke, Sandra; Wu, Yanlin; Bashouti, Muhammad; Christiansen, Silke; Bachmann, Julien (ChemCatChem, 2015)
The geometric effects of nanostructuring a pure Fe2O3 surface on its electrochemical water oxidation performance at neutral pH were systematically explored. Atomic layer deposition was used to coat the inner walls of cylindrical "anodic" nanopores ordered in parallel arrays with a homogeneous Fe2O3 layer. Annealing and electrochemical treatments generated a roughened surface, as demonstrated by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy, the larger geometric area of which increases current densities. Combining these treatments with the "anodic" pore geometry delivered an effective increase in turnover by almost three orders of magnitude with respect to a smooth, planar Fe2O3 surface. However, the current density depended on the pore length in a non-monotonic manner. An optimal length was found that maximized turnover by equating the rate of transport in the electrolyte with that of charge transfer across the interface.
Keywords: atomic layer deposition; electrochemistry; iron oxide; mesoporous materials; oxygen evolution reaction

Enhanced Activity and Selectivity in Catalytic Methanol Steam Reforming by Basic Alkali Metal Salt Coatings - Kusche, Matthias Manfred; Enzenberger, Florian; Bajus, Stephanie; Niedermeyer, Heiko Jannes; Bösmann, Andreas; Kaftan, Andre; Laurin, Mathias; Libuda, Jörg; Wasserscheid, Peter (Angewandte Chemie International Edition, 2013)
Pt on alumina catalysts can be used for converting methanol and water into hydrogen and carbon dioxide, for applications such as hydrogen storage. Both the activity and selectivity could be enhanced by coating these materials with a thin layer of a molten salt mixture of Li/K/Cs acetate. Potassium doping was indentified by DRIFTS measurements to be an important factor for the boost in catalyst performance.

Enhanced activity and selectivity in n-octane isomerization using a bifunctional SCILL catalyst - Meyer, Carolin; Hager, Veit; Schwieger, Wilhelm; Wasserscheid, Peter (Journal of Catalysis, 2012)
Bifunctional Solid Catalyst with Ionic Liquid Layer (SCILL) systems are presented and applied in the skeleton isomerization of n-octane in a slurry-phase reaction mode. It is demonstrated that Pt on silica, coated with a thin film of acidic chloroaluminate ionic liquid exhibits remarkable catalytic performance under mild conditions in the presence of hydrogen. Interestingly, both selectivity and activity of n-octane isomerization increase in these systems as a function of hydrogen pressure. This does not only suggest a hydrogenation activity of the catalytic Pt-centers embedded in the strongly Lewis acidic ionic liquid but also a significant increase in the proton acidity in this system as a function of the hydrogen pressure.

Enhanced Ambipolar Charge Injection with Semiconducting Polymer/Carbon Nanotube Thin Films for Light-Emitting Transistors - Gwinner, Michael C.; Jakubka, Florian; Gannott, Florentina; Sirringhaus, Henning; Zaumseil, Jana (ACS nano, 2012)
We investigate the influence of small amounts of semiconducting single-walled carbon nanotubes (SWNTs) dispersed in polyfluorenes such as poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT) and poly(9,9-dioctylfluorene) (F8) on device characteristics of bottom contact/top gate ambipolar light-emitting field-effect transistors (LEFETs) based on these conjugated polymers. We find that the presence of SWNTs within the semiconducting layer at concentrations below the percolation limit significantly increases both hole and electron injection, even for a large band gap semiconductor like F8, without leading to significant luminescence quenching of the conjugated polymer. As a result of the reduced contact resistance and lower threshold voltages, larger ambipolar currents and thus brighter light emission are observed. We examine possible mechanisms of this effect such as energy level alignment, reduced bulk resistance above the contacts, and field-enhanced injection at the nanotube tips. The observed ambipolar injection improvement is applicable to most conjugated polymers in staggered transistor configurations or similar organic electronic devices where injection barriers are an issue.
EAM Import::2019-03-14

Enhanced Crystallization of Lysozyme Mediated by the Aggregation of Inorganic Seed Particles - Weichsel, Ulrike; Segets, Doris; Thajudeen, Thaseem; Maier, Eva-Maria; Peukert, Wolfgang (Crystal Growth and Design, 2017)
We show that aggregation plays a major role in seeded growth of protein crystals. The seeded batch approach provides the opportunity to set the starting conditions for protein crystallization by adding a defined amount of well-characterized seed particles. The experimental observations for tetragonal hen egg-white lysozyme (LSZ) confirm the concept of the oriented aggregation of larger building blocks to form a protein crystal. It was shown that the aggregation of the seed particles/bioconjugates is advantageous for the product quality in terms of larger and more defined LSZ crystals and in terms of accelerated reaction kinetics. We present a population balance (PB) model for the seeded batch crystallization of LSZ considering the aggregation of growth units to form protein crystals. For the modeling of crystal growth, evolving particle size distributions (PSDs) of agglomerating LSZ molecules were measured by dynamic light scattering (DLS). Moreover, the aggregation of seed particles in LSZ solutions under crystallization conditions was investigated by DLS. In line with our expectations, the number of seeds was found to be important as it strongly affects the collision frequency in the aggregation term of our PB model. Finally, the applied model gives trends of the supersaturation depletion curves and orders of magnitude of the measured CSDs in particle size correctly, ranging from only a few nanometers up to micrometer-sized particles/crystals. Thus, by the combination of PB modeling and experimentally determined crystallization parameters, insights into the crystal formation mechanism were obtained. To the best of our knowledge, this is the first attempt to model growth within a crystal population by an aggregation mechanism induced by seeding with foreign particles.

Enhanced dissociation of charge-transfer states in narrow band gap polymer:fullerene solar cells processed with 1,8-octanedithiol - Moet, D. J. D.; Lenes, M.; Morana, M.; Azimi, H.; Brabec, Christoph; Blom, P. W. M. (Applied Physics Letters, 2010)
The improved photovoltaic performance of narrow band gap polymer:fullerene solar cells processed from solutions containing small amounts of 1,8-octanedithiol is analyzed by modeling of the experimental photocurrent. In contrast to devices that are spin coated from pristine chlorobenzene, these cells do not produce a recombination-limited photocurrent. Modeling of the experimental data reveals that a sixfold reduction in the decay rate of photogenerated bound electron-hole pairs can account for the marked increase in short-circuit current density and fill factor. At short-circuit conditions, the dissociation probability of bound pairs is found to increase from 48% to 70%. © 2010 American Institute of Physics.

Enhanced fatigue lives in AA1050A/AA5005 laminated metal composites produced by accumulative roll bonding - Kümmel, Frank; Hausöl, Tina; Höppel, Heinz Werner; Göken, Mathias (Acta Materialia, 2016)
In this work laminated metal composites with alternating layers of commercial pure aluminum AA1050A and aluminum alloy AA5005 were produced by ARB. In order to vary the layer thickness and the number of interfaces different numbers of ARB cycles (4, 8 and 12) were applied. Subsequently, fatigue tests were performed and the related deformation and damage mechanisms were investigated. At high amplitudes crack growth occurs very straight through the whole sheet and is more or less unaffected by the interfaces of the individual layers in the composite. At lower amplitudes, where pronounced grain coarsening of the AA1050A layers occurs, the cracks deflect in front of the AA5005 layers and propagate in the coarsened AA1050A layers parallel to the bonding plane and the loading direction. Contrary to the composite, in both conventionally ARB processed AA1050A and AA5005 mono-material sheets grains are only locally coarsened around the crack tip and the fatigue cracks propagate very straight through the sheet width. Compared to AA1050A mono-material ARB sheets fatigue life and the cyclic stability are clearly enhanced in all composites. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Accumulative roll bonding (ARB); Ultrafine-grained (UFG); Laminated metal composites (LMCs); Fatigue behavior; Dynamic grain coarsening

Enhanced Formability of Ultrafine-Grained Aluminum Blanks by Local Heat Treatments - Merklein, Marion; Vogt, Uwe (Key Engineering Materials, 2009)
High strength aluminum blanks can be obtained by grain-refinement due to an Accumulativ Roll Bonding (ARB) process, in which two sheets are iteratively brushed, stacked on top of each other and subsequently rolled together. The high shear stresses during the rolling cycles result in an ultrafine-grained microstructure with an average grain size ranging between 200 to 1000 nm. Whereas the grain-refinement causes a drastically increased strength of the aluminum material, the formability of the ARB-blanks made of industrially used aluminum alloys such as the AA6016 deteriorates to the same degree as the strength rises. In this context, a local heat treatment of the ARB-blank reducing the material's strength and increasing its ductility in specific zones will allow to recover the blank's formability again. The research work presented in this paper studies the microstructural effects of a short-term heat treatment on the mechanical properties of ARB-blanks made of AA6016. Experimental investigations including hardness measurements, tensile tests as well as microscopic analyses show that heat treatments of only several seconds already result in significant increases of the material's ductility and decreases of the material's strength. By applying these microstructural mechanisms in terms of a specific heat treatment layout, functional gradients of strength and ductility adapted to the succeeding forming operation can be setup significantly enhancing the ARB-blank's formability.
EAM Import::2019-03-14

Enhanced performance of dye-sensitized solar cells based on TiO2 nanotube membranes using an optimized annealing profile - Mohammadpour, F.; Moradi, M.; Lee, Kiyoung; Cha, Gi; So, Seulgi; Kahnt, Axel; Guldi, Dirk Michael; Altomare, Marco; Schmuki, Patrik (Chemical Communications, 2015)
We use free-standing TiO2 nanotube membranes that are transferred onto FTO slides in front-side illuminated dye-sensitized solar cells (DSSCs). We investigate the key parameters for solar cell arrangement of self-ordered anodic TiO2 nanotube layers on the FTO substrate, namely the influence of the annealing procedure on the DSSC light conversion efficiency. The results show that using an optimal temperature annealing profile can significantly enhance the DSSC efficiency (in our case extgreekh = 9.8%), as it leads to a markedly lower density of trapping states in the tube oxide, and thus to strongly improved electron transport properties.
EAM Import::2019-03-14

Enhanced Photoelectrochemical Water Splitting Efficiency of a Hematite-Ordered Sb:SnO2 Host-Guest System - Wang, Lei; Palacios-Padros, Anna; Kirchgeorg, Robin; Tighineanu, Alexei; Schmuki, Patrik (ChemSusChem, 2014)
Host–guest systems such as hematite/SnO2 have attracted a great deal of interest as photoanodes for photoelectrochemical water splitting. In the present work we form an ordered porous tin oxide layer formed by self-organizing anodization of Sn films on a FTO substrate. Subsequently the anodic tin oxide nanostructure is doped with antimony (ATO) by a simple impregnation and annealing treatment, and then decorated with hematite using anodic deposition. Photoelectrochemical water splitting experiments show that compared to conventional SnO2 nanostructures, using a Sb doped nanochannel SnO2 as a host leads to a drastic increase of the water splitting photocurrent response up to 1.5mA cm-2 at 1.6V (vs. RHE) in 1 M KOH under AM 1.5 (100mW cm-2) conditions compared to 0.04mA cm-2 for the non-Sb doped SnO2 scaffold.
EAM Import::2019-03-14

Enhanced photoluminescence from mixed-valence Eu-doped nanocrystalline silicate glass ceramics - Gao, Guojun; Da, Ning; Reibstein, Sindy; Wondraczek, Lothar (Optics Express, 2010)
Intense photoluminescence was observed from mixed-valence Eu-doped nanocrystalline BaAl2Si2O8/LaBO3 glass ceramics. For preparation in air, the ratio between Eu3+ and Eu2+ luminescence depends on synthesis temperature. XRD, TEM and IR absorption spectra were employed to characterize the crystallization process and structural properties of the precursor glass and corresponding glass ceramics. When annealed at 950 °C, the material exhibited photoluminescence more than ten times stronger than was found in its glassy state. Spectroscopic data indicate that during such a heat treatment, even in air, a significant part of the Eu3+ ions is reduced to Eu2+. Lifetime of the 5D0 state of Eu3+ increases with increasing heat treatment temperature. Eu3+ species are largely incorporated on La3+ sites in LaBO3 crystallites whereas Eu2+ locates on Ba2+ sites in the hexacelsian phase. A mechanism for the internal reduction of Eu3+ to Eu2+ is proposed. Spectroscopic properties of the material suggest application in additive luminescent light generation.
EAM Import::2019-03-14

Enhanced photosynthetic activity in Spinacia oleracea by spectral modification with a photoluminescent light converting material - Xia, Qi; Batentschuk, Miroslaw; Osvet, Andres; Richter, Peter; Häder, Donat-Peter; Schneider, Juergen; Brabec, Christoph; Wondraczek, Lothar; Winnacker, Albrecht (Optics Express, 2013)
The spectral conversion of incident sunlight by appropriate photoluminescent materials has been a widely studied issue for improving the efficiency of photovoltaic solar energy harvesting. By using phosphors with suitable excitation/emission properties, also the light conditions for plants can be adjusted to match the absorption spectra of chlorophyll dyes, in this way increasing the photosynthetic activity of the plant. Here, we report on the application of this principle to a high plant, Spinacia oleracea. We employ a calcium strontium sulfide phosphor doped with divalent europium (Ca0.4Sr0.6S:Eu2+, CSSE) on a backlight conversion foil in photosynthesis experiments. We show that this phosphor can be used to effectively convert green to red light, centering at a wavelength of 650 nm which overlaps the absorption peaks of chlorophyll a/b pigments. A measurement system was developed to monitor the photosynthetic activity, expressed as the CO2 assimilation rate of spinach leaves under various controlled light conditions. Results show that under identical external light supply which is rich in green photons, the CO2 assimilation rate can be enhanced by more than 25% when the actinic light is modified by the CSSE conversion foil as compared to a purely reflecting reference foil. These results show that the phosphor could be potentially applied to modify the solar spectrum by converting the green photons into photosynthetically active red photons for improved photosynthetic activity. © 2013 Optical Society of Americ
Keywords: Engineering controlled terms: Carbon dioxide; Light emission; Phosphors; Photons; Plants (botany); Solar energy; Calcium Assimilation rate; Measurement system; Photoluminescent materials; Photosynthetic activity; Photovoltaic solar energy; Spectral conversion; Spectral modifications; Strontium sulfide phosphor; Divalent europium Engineering main heading: Light EMTREE drug terms: carbon dioxide; strontium EMTREE medical terms: article; chemistry; luminescence; metabolism; photon; photosynthesis; physiology; spectrofluorometry; spinach; time MeSH: Carbon Dioxide; Luminescence; Photons; Photosynthesis; Spectrometry; Fluorescence; Spinacia oleracea; Strontium; Time Factors Medline is the source for the MeSH terms of this document.

Enhanced reactivity of Pt nanoparticles supported on ceria thin films during ethylene dehydrogenation - Lykhach, Yaroslava; Staudt, Thorsten; Tsud, Nataliya; Skala, Tomas; Prince, Kevin Charles; Matolin, Vladimir; Libuda, Jörg (Physical Chemistry Chemical Physics, 2011)
The adsorption and reaction of ethylene on Pt/CeO(2-x)/Cu(111) model catalysts were studied by means of high resolution photoelectron spectroscopy (HR-PES) in conjunction with resonant photoemission spectroscopy (RPES). The dehydrogenation mechanism is compared to the HR-PES data obtained on a Pt(111) single crystal under identical conditions. It was found that the Pt nanoparticle system shows a substantially enhanced reactivity and several additional reaction pathways. In sharp contrast to Pt(111), partial dehydrogenation of ethylene on the supported Pt nanoparticles already starts at temperatures as low as 100 K. Similar to the single crystal surface, dehydrogenation occurs via the isomer ethylidene (CHCH(3)) and then mainly via ethylidyne (CCH(3)). In the temperature region between 100 and 250 K there is strong evidence for spillover of hydrocarbon fragments to the ceria support. In addition, splitting of ethylene to C(1) fragments is more facile than on Pt(111), giving rise to the formation of CH species and CO in the temperature region between 250 and 400 K. Upon further annealing, carbonaceous deposits are formed at 450 K. By heating to 700 K, these carbon deposits are completely removed from the surface by reaction with oxygen, provided by reverse spillover of oxygen from the ceria support.

Enhanced visible light photocurrent generation at surface-modified TiO2 nanotubes - Beranek, Radim; Macák, Jan; Gärtner, Marc; Meyer, Karsten; Schmuki, Patrik (Electrochimica Acta, 2009)
Photoelectrodes consisting of TiO nanotube layers with different thicknesses (0.5 μm, 1.7 μm, 3 μm, 6 μm, 9 μm, and 18 μm) were prepared by anodization of titanium substrates and subsequent surface modification by a heat treatment at 400 °C in the presence of urea pyrolysis products. In contrast to unmodified TiO nanotubes, the modified photoelectrodes exhibit photocurrents under visible light irradiation down to 750 nm. Photocurrent transients indicate enhanced recombination unless a suitable hole-scavenger, like iodide, is present since the photogenerated holes do not oxidize water efficiently. In the visible light the photoconversion efficiency increases significantly with nanotube length. The maximum incident photon-to-current efficiency (IPCE) was observed for tubes with the length of 6-9 μm (IPCE ∼4.5% and 1.4% at 450 nm and 550 nm, respectively) and the photocurrent enhancement with increasing tube length is found to be stronger at longer irradiations wavelengths. © 2008 Elsevier Ltd. All rights reserved.

Enhanced water splitting activity of M-doped Ta3N5 (M = Na, K, Rb, Cs) - Kado, Yuya; Lee, Chong-Yong; Lee, Kiyoung; Müller, Julian; Moll, Matthias; Spiecker, Erdmann; Schmuki, Patrik (Chemical Communications, 2012)
We use a highly aligned Ta2O5 nanochannel structure to fabricate alkali metal ion (Na, K, Rb or Cs) doped Ta3N5 via solution seeding and thermal conversion in NH3. Under optimized conditions the resulting doped structures show a strongly enhanced visible light water splitting performance in comparison to undoped Ta3N5.

Enhancement of the Carbothermal Reduction of Hafnium Oxide by Silicon - Weisenseel, Bastian; Harris, Joseph; Stumpf, Martin; Wolf, Stephan; Fey, Tobias; Greil, Peter (Advanced Engineering Materials, 2017)
Mixtures of hafnia and carbon powders, containing 0.0-10mol% of silicon, are heated and analyzed by XRD, SEM, TEM, and DTA. Furthermore, hafnia and carbon pellets, with varying degrees of silicon content, are investigated to determine the nature of the silicon influence on the reaction. The addition of up to 5.0mol% silicon increases the yield of carbide by 400%. TEM reveal that silicon causes the formation of an amorphous layer on the hafnia particles, on which nucleation of the carbide phase take place. Taking into account that the carbide formation in Si-free samples is only observed on the carbon surfaces, the authors conclude that the silicon-induced layer on the hafnia surfaces increases the accessible area for carbide formation and consequently the reaction rat

Enhancement of up-conversion luminescence in Er,Ce doped Y3-xYbxAG single crystalline films - Zorenko, Yu.; Gorbenko, V.; Zorenko, T.; Paprocki, Kazimierz; Osvet, Andres; Batentschuk, Miroslaw; Brabec, Christoph; Fedorov, A. (Journal of Luminescence, 2016)
The paper deals with investigation of the luminescent properties of Er and Er/Ce doped solid solutions of YYbAG garnets with x values ranging from 0 to 3.0, prepared by the LPE method in the form of single crystalline films (SCF). We have found that the effective Ce→Yb energy transfer is observed in the YYbAlO:Er,Ce garnets (YYbAG:Er,Ce). Apart from that, strong enhancement of the Er upconvesion luminescence in YYbAG:Er,Ce garnets was found due to the Ce codoping. Among all the investigated samples of solid solutions of mixed YYbAG:Er,Ce garnets, the highest intensity of the up-conversion luminescence is observed for the YYbAG:Er,Ce SCF.
Keywords: Er and Ce dopant; Liquid phase luminescence; Mixed Y3-xYbxAG garnet; Single crystalline films; Up conversion luminescence

Enhancing In Vitro Bioactivity of Melt-Derived 45S5 Bioglass® by Comminution in a Stirred Media Mill - Romeis, Stefan; Hoppe, Alexander; Eisermann, Claudia; Schneider, Nadine; Boccaccini, Aldo R.; Schmidt, Jochen; Peukert, Wolfgang (Journal of the American Ceramic Society, 2014)
45S5 Bioglassâ (45S5 BG) is a frequently applied Type A bioactive material, capable of forming an inherent bond to bone and soft tissue. Currently, applied melt-derived bioactive glass powders (BG) exhibit particle sizes between a few to several hundred micrometers. Recent studies on nanometer-sized bioactive glasses (nBGs), produced by bottom-up methods like sol– gel processing or flame spray pyrolysis, have indicated their great potential for several biomedical applications. In this study, the feasibility of top-down processing starting from bulk 45S5 BG by wet comminution in a stirred media mill was investigated. The products were assessed by in vitro hydroxycarbonate apatite (HCAp) formation in simulated body fluid, which is a marker for bioactive behavior. The study reveals the paramount influence of the used solvent for a successful topdown processing: In comparison with the as-received material bioactivity is lost for powders processed in water, preserved for comminution in ethanol and increased for powders processed using the alcohols n-butanol, n-pentanol, and n-hexanol. It was also found that only for the latter solvents, the chemical composition of the glass is maintained during comminution. Flakelike, slightly porous particles with specific surface areas of  25–30 m2/g are obtained. Thus, the presented comminution approach offers a convenient technique to process 45S5 BG with enhanced bioactivity.
UnivIS-Import:2015-03-09:Pub.2013.tech.IW.LM.enhanc

Enhancing photocurrent efficiencies by resonance energy transfer in CdTe quantum dot multilayers: Towards rainbow solar cells - Ruland Palaia, Andres; Schulz-Drost, Christian; Sgobba, Vito; Guldi, Dirk Michael (Advanced Materials, 2011)
The impact of Förster resonant energy transfer (FRET) in CdTe quantum dot (QD) based photoelectrochemical cells is investigated. By deposition of different CdTe QD sizes onto indium on oxide electrodes, FRET across the photoactive film could be obtained, resulting in a 25% enhancement of the photon-to-current efficiency when compared to reference systems that lack FRET. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Enhancing the dispersability of TiO2 nanorods and gaining control over region-selective layer formation - Etschel, Sebastian; Tykwinski, Rik; Halik, Marcus (Langmuir, 2016)
We demonstrate that the dispersibility and reactivity of core-shell TiO2 nanorods (NRs) can be controlled significantly through functionalization with a combination of ligands based on phosphonic acid derivatives (PAs). Specifically, a glycol based PA allows dispersion of the NRs in methanol (MeOH). On the other hand, incorporating an alkyne terminated PA in the ligand shell of the NRs allows for a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction with an azide-patterned aluminum oxide (AlOx) substrate and forms a region-selectively deposited film of NRs. We clearly demonstrate that the quality of the NR films correlates strongly with the stability of the NR dispersions in the reaction medium. In particular, tuning the concentration of alkyne PA in the ligand shell inhibits aggregation of the NRs on the substrate, while reactivity for the CuAAC reaction is maintained. The surface coverage with NRs fits the Langmuir model. This study illustrates that surface functionalization of AlOx substrates can be effectively and conveniently controlled through enhancing the dispersibility of the NRs using mixed ligand shells.

Enhancing the Water Splitting Efficiency of Sn-Doped Hematite Nanoflakes by Flame Annealing - Wang, Lei; Lee, Chong-Yong; Mazare, Anca Valentina; Lee, Kiyoung; Müller, Julian; Spiecker, Erdmann; Schmuki, Patrik (Chemistry - A European Journal, 2014)
The effect of flame annealing on the water-splitting properties of Sn decorated hematite (-Fe2O3) nanoflakes has been investigated. It is shown that flame annealing can yield a considerable enhancement in the maximum photocurrent under AM 1.5 (100mWcm(-2)) conditions compared to classic furnace annealing treatments. Optimizing the annealing time (10s at 1000 degrees C) leads to a photocurrent of 1.1mAcm(-2) at 1.23V (vs. RHE) with a maximum value 1.6mAcm(-2) at 1.6V (vs. RHE) in 1M KOH. The improvement in photocurrent can be attributed to the fast direct heating that maintains the nanoscale morphology, leads to optimized Sn decoration, and minimizes detrimental substrate effects.
Keywords: doping; hematite nanoflakes; water splitting

Environmental and health impact assessment of Liquid Organic Hydrogen Carrier (LOHC) systems - challenges and preliminary results - Markiewicz, M.; Zhang, Y. Q.; Bösmann, Andreas; Brückner, Nicole; Thöming, J.; Wasserscheid, Peter; Stolte, S. (Energy and Environmental Science, 2015)
Liquid Organic Hydrogen Carrier (LOHC) systems offer a very attractive way to store and transport hydrogen, a technical feature that is highly desirable to link unsteady energy production from renewables with the vision of a sustainable, CO2-free, hydrogen-based energy system. LOHCs can be charged and discharged with considerable amounts of hydrogen in cyclic, catalytic hydrogenation and dehydrogenation processes. As their physico-chemical properties are very similar to diesel, today's infrastructure for liquid fuels can be used for their handling thus greatly facilitating the step-wise transition from today's fossil system to a CO2 emission free energy supply for both, stationary and mobile applications. However, for a broader application of these liquids it is mandatory to study in addition to their technical performance also their potential impact on the environment and human health. This paper presents the first account on the toxicological profile of some potential LOHC structures. Moreover, it documents the importance of an early integration of hazard assessment in technology development and reveals for the specific case of LOHC structures the need for additional research in order to overcome some challenges in the hazard assessment for these liquids.

Environmentally Printing Efficient Organic Tandem Solar Cells with High Fill Factors: A Guideline Towards 20% Power Conversion Efficiency - Li, Ning; Baran, Derya; Spyropoulos, George D.; Zhang, Hong; Berny, Stephane; Turbiez, Mathieu; Ameri, Tayebeh; Krebs, Frederik C.; Brabec, Christoph (Advanced Energy Materials, 2014)
The tandem concept involves stacking two or more cells with complementary absorption spectra in series or parallel connection, harvesting photons at the highest possible potential. It is strongly suggested that the roll-to-roll production of organic solar cells will employ the tandem concept to enhance the power conversion efficiency (PCE). However, due to the undeveloped deposition techniques, the challenges in ink formulation as well as the lack of commercially available high performance active materials, roll-to-roll fabrication of highly efficient organic tandem solar cells currently presents a major challenge. The reported high PCE values from lab-scale spin-coated devices are, of course, representative, but not helpful for commercialization. Here, organic tandem solar cells with exceptionally high fill factors and PCE values of 7.66% (on glass) and 5.56% (on flexible substrate), which are the highest values for the solution-processed tandem solar cells fabricated by a mass-production compatible coating technique under ambient conditions, are demonstrated. To predict the highest possible performance of tandem solar cells, optical simulation based on experimentally feasible values is performed. A maximum PCE of 21% is theoretically achievable for an organic tandem solar cell based on the optimized bandgaps and achieved fill factors.
Keywords: flexible electronics; large-scale production; organic photovoltaics; organic solar cells; solution processing

Epitaxial Graphenes on Silicon Carbide - Seyller, Thomas; et al. (Mrs Bulletin, 2010)
The materials science of graphene grown epitaxially on the hexagonal basal planes of SiC crystals is reviewed. We show that the growth of epitaxial graphene on Si-terminated SiC(0001) is much different than growth on the C-terminated SiC(000 -1) surface, and discuss the physical structure of these graphenes. The unique electronic structure and transport properties of each type of epitaxial graphene is described, as well as progress toward the development of epitaxial graphene devices. This materials system is rich in subtleties, and graphene grown on the two polar faces differs in important ways, but all of the salient features of ideal graphene are found in these epitaxial graphenes, and wafer-scale fabrication of multi-GHz devices already has been achieved.
EAM Import::2019-03-14

Epitaxial Growth of PbSe Quantum Dots on MoS2 Nanosheets and their Near-Infrared Photoresponse - Schornbaum, Julia; Winter, Benjamin; Schießl, Stefan; Gannott, Florentina; Katsukis, Georgios; Guldi, Dirk Michael; Spiecker, Erdmann; Zaumseil, Jana (Advanced Functional Materials, 2014)
A facile one-pot synthesis of hybrid materials consisting of PbSe quantum dots (QDs) that grow epitaxially on MoS2 nanoflakes resulting in three equivalent orientation variants of the PbSe QDs with respect to the MoS2 lattice is demonstrated. The epitaxial growth and cross-sectional high-resolution transmission electron microscopy (HRTEM) investigations verify a direct and linker-free contact between the quantum dots and the transition metal dichalcogenide (TMD) nanoflakes, while maintaining surface passivation of the PbSe with oleic acid ligands on the outside. Solution-processed photodetectors based on PbSe-MoS2 hybrids exhibit stable photoconduction when illuminated with near-IR light (wavelength > 1200 nm) without any laborious ligand-exchange steps. Flexible devices fabricated on polyethylene terephthalate (PET) substrates show excellent stability upon repeated bending. These hybrid materials are air-stable and solution-processable at low temperatures and thus promising for low-cost flexible near-IR photodetectors.

Epitaxial upward transport of Al at the beginning of the Al-induced layer exchange process - Birajdar, Balaji; Antesberger, Tobias; Stutzmann, Martin; Spiecker, Erdmann (Physica Status Solidi-Rapid Research Letters, 2011)
Al-induced layer exchange (ALILE) and crystallization in a stack of amorphous Si (a-Si, 100 nm)/Al (50 nm)/quartz substrate, annealed at 450 degrees C, have been investigated at different length scales by optical microscopy as well as analytical scanning and transmission electron microscopy. Significant lateral and vertical redistribution of Al was observed, yielding Al deficient dendritic cell centers surrounded by an about 10 mu m wide Al excess zone containing epitaxial islands of "pushed-up" Al whose number density and size decreases with increasing distance from the cell boundary. This new finding is likely to be a key step forward in the understanding of the complex redistribution of Al and Si during the ALILE process. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: thin films; solar cells; crystallization; electron microscopy

EPR investigations of non-oxidized silicon nanoparticles from thermal pyrolysis of silane - Jank, Michael; Frey, Lothar; Peukert, Wolfgang; Körmer, Richard; Wu, Jia; Otto, Michael (Physica Status Solidi, 2011)
Electron paramagnetic resonance (EPR) investigations of non-oxidized crystalline silicon nanoparticles (SiNP) from thermal silane pyrolysis revealed that the dangling bond (db) concentration strongly depends on the synthesis temperature. For SiNP synthesized at 1000-1200 °C and processed entirely in inert atmosphere, approximately 1013 cm-2 dbs were found which is at least one order of magnitude higher than the values previously reported for oxidized SiNP. After the synthesis, the dbs can be quenched by the addition of bromine (Br2) that proves their location at the particle surface. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
UnivIS-Import:2015-03-09:Pub.2011.tech.IE.LEB.eprinv
Keywords: Electron paramagnetic resonance; Nanoparticles; Pyrolysis; Silicon

Erlangen pipe flow: the concept and DNS results for microflow control of near-wall turbulence - Lammers, Peter; Jovanovic, Jovan; Frohnapfel, Bettina; Delgado, Antonio (Microfluidics and Nanofluidics, 2012)
The concept of a micropatterned surface morphology capable of producing self-stabilization of turbulence in wall-bounded flows is considered in pipes of non-circular cross-sections which act to restructure fluctuations towards the limiting state where these must be entirely suppressed. Direct numerical simulations of turbulence in pipes of polygon-shaped cross-sections with straight and profiled sides were performed at a Reynolds number Res ' 300 based on the wall shear velocity and the hydraulic diameter. Using the lattice Boltzmann numerical algorithm, turbulence was resolved with up to 540 * 10 6 grid points (8;192 * 257 * 256 in the x 1, x 2 and x 3 directions). The DNS results show a decrease in the viscous drag around corners, resulting in a reduction of the skin-friction coefficient compared with expectations based on the wellestablished concept of hydraulic diameter and the use of the Blasius correlation. These findings support the conjecture that turbulence might be completely suppressed if the pipe cross-section is a polygon consisting of a sufficient number of profiled sides of the same length which intersect at right angles at the corners. © Springer-Verlag 2012.
UnivIS-Import:2015-03-09:Pub.2012.tech.ITC.stmmec.erlang

Erratum to: Stable algorithm for event detection in event-driven particle dynamics: logical states - Strobl, Severin; Bannerman, Marcus; Pöschel, Thorsten (Computational Particle Mechanics, 2016)
The original publication of the article contains an error in line number 8 of Algorithm 2. The correct version of the Algorithm 2 is provided in this erratum.

Etching of Crystalline ZnO Surfaces upon Phosphonic Acid Adsorption: Guidelines for the Realization of Well-Engineered Functional Self-Assembled Monolayers - Ostapenko, Alexandra; Klöffel, Tobias; Eussner, Jens; Harms, Klaus; Dehnen, Stefanie; Meyer, Bernd; Witte, Gregor (ACS Applied Materials and Interfaces, 2016)
Functionalization of metal oxides by means of covalently bound self-assembled monolayers (SAMs) offers a tailoring of surface electronic properties such as their work function and, in combination with its large charge carrier mobility, renders ZnO a promising conductive oxide for use as transparent electrode material in optoelectronic devices. In this study, we show that the formation of phosphonic acid-anchored SAMs on ZnO competes with an unwanted chemical side reaction, leading to the formation of surface precipitates and severe surface damage at prolonged immersion times of several days. Combining atomic force microscopy (AFM), X-ray diffraction (XRD), and thermal desorption spectroscopy (TDS), the stability and structure of the aggregates formed upon immersion of ZnO single crystal surfaces of different orientations [(0001̅), (0001), and (101̅0)] in phenylphosphonic acid (PPA) solution were studied. By intentionally increasing the immersion time to more than 1 week, large crystalline precipitates are formed, which are identified as zinc phosphonate. Moreover, the energetics and the reaction pathway of this transformation have been evaluated using density functional theory (DFT), showing that zinc phosphonate is thermodynamically more favorable than phosphonic acid SAMs on ZnO. Precipitation is also found for phosphonic acids with fluorinated aromatic backbones, while less precipitation occurs upon formation of SAMs with phenylphosphinic anchoring units. By contrast, no precipitates are formed when PPA monolayer films are prepared by sublimation under vacuum conditions, yielding smooth surfaces without noticeable etching.
EAM Import::2019-03-14

Ethene adsorption and dehydrogenation on clean and oxygen precovered Ni(111) studied by high resolution x-ray photoelectron spectroscopy - Lorenz, Michael Peter Andreas; Fuhrmann, Thomas; Streber, R.; Bayer, Andreas; Bebensee, Fabian; Gotterbarm, Karin; Kinne, Martin; Tränkenschuh, Barbara; Zhu, J. F.; Papp, Christian; Denecke, R.; Steinrück, Hans-Peter (Journal of Chemical Physics, 2010)
The adsorption and thermal evolution of ethene (ethylene) on clean and oxygen precovered Ni(111) was investigated with high resolution x-ray photoelectron spectroscopy using synchrotron radiation at BESSY II. The high resolution spectra allow to unequivocally identify the local environment of individual carbon atoms. Upon adsorption at 110 K, ethene adsorbs in a geometry, where the two carbon atoms within the intact ethene molecule occupy nonequivalent sites, most likely hollow and on top; this new result unambiguously solves an old puzzle concerning the adsorption geometry of ethene on Ni(111). On the oxygen precovered surface a different adsorption geometry is found with both carbon atoms occupying equivalent hollow sites. Upon heating ethene on the clean surface, we can confirm the dehydrogenation to ethine (acetylene), which adsorbs in a geometry, where both carbon atoms occupy equivalent sites. On the oxygen precovered surface dehydrogenation of ethene is completely suppressed. For the identification of the adsorbed species and the quantitative analysis the vibrational fine structure of the x-ray photoelectron spectra was analyzed in detail. © 2010 American Institute of Physic

Ethene-induced temporary inhibition of grubbs metathesis catalysts - Scholz, Judith; Loekman, Soebiakto; Szesni, Normen; Hieringer, Wolfgang; Görling, Andreas; Haumann, Marco; Wasserscheid, Peter (dvanced Synthesis & Catalysis, 2011)
Grubbs-type catalysts were immobilized in form of supported ionic liquid phase (SILP) materials and applied in the gas-phase cross-metathesis of various substrates in order to study the effect of ethene on catalyst activity and stability in detail. From our theoretical and experimental findings we suggest that ethene causes temporary inhibition effects due to a reversible shift of ruthenium complexes from the productive catalytic cycle towards unproductive, dormant ruthenium species. By avoiding the presence of ethene, constant catalytic activity and selectivity for more than 20 days of continuous operation could be realized. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ethylene to 2-butene in a continuous gas phase reaction using silp-type cationic nickel catalysts - Scholz, Judith; Hager, Veit; Wang, Xinjiao; Kohler, Florian; Sternberg, Michelle; Haumann, Marco; Szesni, Normen; Meyer, Karsten; Wasserscheid, Peter (ChemCatChem, 2014)
Owing to shifting market demands, it is important to convert ethylene to propylene. One attractive way to achieve this conversion is the dimerization of ethylene to 1-butene, followed by isomerization to 2-butene and subsequent metathesis of 2-butene/ethylene olefin. Our contribution focuses on combining the first two steps. Herein, we report a highly selective tandem dimerization/isomerization of ethylene to 2-butene catalyzed by homogeneously dissolved cationic nickel complexes. These catalysts can be efficiently immobilized by using the supported ionic liquid phase technology. Such supported ionic liquid phase materials have been tested under continuous gas phase conditions and demonstrated attractive catalytic performance with respect to both catalyst stability and productivity after the optimization of support, ionic liquid, ligand, and process parameters. The limited thermal stability of the nickel complexes and olefin condensation at too low temperatures require a careful thermal management of the fixed-bed reactor. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Evaluation of Electron Donor Materials for Solution-Processed Organic Solar Cells via a Novel Figure of Merit - Min, Jie; Luponosov, Yuriy N.; Cui, Chaohua; Kan, Bin; Chen, Haiwei; Wan, Xiangjian; Chen, Yongsheng; Ponomarenko, Sergei A.; Li, Yongfang; Brabec, Christoph (Advanced Energy Materials, 2017)
Organic photovoltaic (OPV) technology offers many advantages, although no commercial applications have been achieved after more than a decade of intensive research and development. Several challenges have yet to be overcome including high power conversion efficiency (PCE), good processability, low cost, and excellent long-term stability, and so on. In this article, these fundamental challenges are significantly addressed by surveying and analyzing a new merit factor for material applied accessibility containing three parameters: synthetic complexity, device efficiency, and photostability. Thirty-five donor small molecules are introduced to assess their synthetic accessibility. Furthermore, the PCEs and device photostability of these molecules are carried out, and further measured under one sun illumination within 200 h, respectively. Combining with the characteristics of these three factors, investigated molecules are ranked according to an industrial figure of merit (i-FOM), while some guidelines for the material design and synthesis are given. It is suggested that a PCE of >14% and an i-FOM of >20% via active material engineering are realistic for possible industry future of OPV. Along with the systematic study, it is believed that this i-FOM can be taken into consideration at an early stage of molecular design and provides valuable insight for efficient evaluation of photovoltaic materials for possible commercial applications. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Conversion efficiency; Efficiency; Molecules; Solar cells; Synthesis (chemical); Transportation; Figure of merits; Long term stability; Low costs; Power conversion efficiencies; Processability; Organic solar cells

Evaluation of Industrially Applied Heat-Transfer Fluids as Liquid Organic Hydrogen Carrier Systems - Brückner, Nicole; Obesser, Katharina; Bösmann, Andreas; Teichmann, Daniel; Arlt, Wolfgang; Dungs, Jennifer; Wasserscheid, Peter (Chemsuschem, 2014)
Liquid organic hydrogen carrier (LOHC) systems offer a very attractive method for the decentralized storage of renewable excess energy. In this contribution, industrially well-established heat-transfer oils (typically sold under trade names, e.g., Marlotherm) are proposed as a new class of LOHC systems. It is demonstrated that the liquid mixture of isomeric dibenzyltoluenes (m.p. -39° to -34°C, b.p. 390°C) can be readily hydrogenated to the corresponding mixture of perhydrogenated angenated to the corresponding mixture of perhydrogenated an-alogues by binding 6.2 wt% of H2. The liquid H2 -rich form can be stored and transported similarly to diesel fuel. It readily undergoes catalytic dehydrogenation at temperatures above 260°C, which proves its applicability as a reversible H2 carrier. The presented LOHC systems are further characterized by their excellent technical availability at comparably low prices, full registration of the H2-lean forms, and excellent thermal stabilities.

Evaluation of NbN thin films grown by MOCVD and plasma-enhanced ALD for gate electrode application in high-k/SiO2 gate stacks - Hinz, J.; Bauer, A. J.; Thiede, T.; Fischer, R. A.; Frey, Lothar (Semiconductor Science and Technology, 2010)
NbN was deposited by plasma-enhanced ALD (atomic layer deposition) as well as MOCVD (metal organic chemical vapour deposition) as gate electrode for MOS capacitors with SiO2 and HfO2 as dielectric. Finally, different contact materials were deposited on the NbN electrodes and the gate stacks were annealed. Dependent on the applied dielectric and the composition of the contact materials, variation in work function, increase in EOT, and even oxide degradation occurs after thermal treatment. This can be related to chemical reactions due to interdiffusion between the NbN gate electrode, the contact material and the gate dielectric. Employing PEALD (plasma-enhanced atomic layer deposition) for NbN electrode deposition on HfO2 and poly-Si as contact material, however, degradation can be significantly reduced and the work function remains stable at 4.8 eV. This makes NbN deposited by PEALD an attractive gate electrode material for future low power consuming PMOS-transistors.

Evaluation of the film formation and the charge transport mechanism of indium tin oxide nanoparticle films - Mahajeri, Mahdi; Voigt, Michael; Klupp Taylor, Robin; Reindl, Armin; Peukert, Wolfgang (Thin Solid Films, 2010)
The structure formation and charge transfer of thin nanoparticulate indium tin oxide (ITO) films prepared by dip-coating was studied as a function of stabilizer before and after annealing at different temperatures. The analysis of the film structure by optical methods revealed that it is a function of the stability. Suspensions containing an optimum stabilizer concentration of 0.1 mol/l resulted in densely packed films with a peak specific conductivity of 8.3 S cm− 1 after annealing at 550 °C for 1 h in air and 121 S cm− 1 after annealing in forming gas at 250 °C for 1 h, respectively. Furthermore, for the densely packed films fluctuation-induced tunnelling was found to be the dominant charge transport mechanism, whereas for the low density films a thermally activated charge transport was observed. That the films of maximum density showed a metallic charge transport behaviour at temperatures above 300 K indicated the optimal contact between ITO particles had been achieve
Keywords: Indium tin oxide; Nanoparticles; Dispersion; Stability; Transmittance; Conductance; Charge transport

Evaluation of X-ray phase-contrast imaging with the Medipix - Bartl, Peter; Michel, Thilo; Nachtrab, F.; Uhlmann, N.; Anton, Gisela (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2011)
In the recent decade phase-contrast imaging in the X-ray regime has been developed as a competitive alternative to conventional absorption radiography and computed tomography Wilkins et al. (1996) [1]. As phase-contrast is an energy dependent phenomenon, the use of a spectroscopic detector like the Medipix is a key tool to characterize and optimize imaging methods such as in-line or interferometric phase-contrast imaging. In the following the energy dependency of interference patterns caused by phase shifts in a coherent wavefront will be shown. Moreover, the spatial coherence of X-rays, considered as wavefronts, is crucial for the formation of detectable interference effects. The newest generations of X-ray sources produce small but finite focal spot sizes, so that partial spatial coherence of the wave-field has to be taken into account. The contribution of partial coherence to the image recorded in the detector plane has been investigated and the outcome will be presented. For a fuller understanding of a complete phase-contrast imaging setup the incoherent scattering effects were additionally simulated with Monte-Carlo methods. Furthermore, to develop a realistic simulation-tool for X-ray phase-contrast imaging, a good comprehension of the detection system is needed. This was also achieved by Monte-Carlo simulations. The comparison with measurements of the simulations of a complete setup shows a very good agreement.
EAM Import::2019-03-14

Event-driven DEM of soft spheres - Pöschel, Thorsten; Müller, Patric (7th International Conference on Micromechanics of Granular Media: Powders and Grains 2013, 2013)
In this paper an algorithm is described which combines the efficiency of event-driven Molecular-Dynamics (eMD) and the physical correctness of force-based Molecular-Dynamics (MD) for dilute granular systems of frictionless spheres. © 2013 AIP Publishing LL
Keywords: event-driven Molecular-Dynamics; granular Gases; hard-sphere model; Molecular-Dynamics

Event-driven molecular dynamics of soft particles - Müller, Patric; Pöschel, Thorsten (Physical Review E, 2013)
The dynamics of dissipative soft-sphere gases obeys Newton's equations of motion, which are commonly solved numerically by (force-based) Molecular Dynamics (MD) schemes. With the assumption of instantaneous, pairwise collisions, the simulation can be accelerated considerably using event-driven MD, where the coefficient of restitution is derived from the interaction force between particles. Recently it was shown, however, that this approach may fail dramatically, that is, the obtained trajectories deviate significantly from the ones predicted by Newton's equations. In this paper, we generalize the concept of the coefficient of restitution and derive a numerical scheme which, in the case of dilute systems and frictionless interaction, allows us to perform highly efficient event-driven MD simulations even for noninstantaneous collisions. We show that the particle trajectories predicted by our scheme agree perfectly with the corresponding (force-based) MD, except for a short transient period whose duration corresponds to the duration of the contact. Thus, the new algorithm solves Newton's equations of motion like force-based MD while preserving the advantages of event-driven simulations. © 2013 American Physical Society.

Evidence for an active oxygen species on Au/TiO 2(1 1 0) model catalysts during investigation with in situ X-ray photoelectron spectroscopy - Dumbuya, K.; Cabailh, G.; Lazzari, R.; Jupille, J.; Ringel, Lorenz; Pistor, Martina; Lytken, Ole; Steinrück, Hans-Peter; Gottfried, Michael (Catalysis Today, 2012)
The influence of oxygen (O ) and carbon monoxide (CO) on Au nanoparticles supported on TiO (1 1 0) in the size range of 2-3 nm has been studied using X-ray photoelectron spectroscopy (XPS) and in situ (high pressure) XPS at 300 K for O and/or CO pressures of 0.1-1 mbar. These experiments were aimed at revisiting Au 4f core level shifts as reported in the literature and most importantly, to establish the dependence of the core-level shifts on the knowledge that there exists a maximum in reactivity for CO oxidation. Two samples were prepared with a coverage corresponding to that maximum (Au coverage 0.14-0.2 ML, particle size estimated to ∼2-2.5 nm) while a third sample was expected to be less reactive (Au coverage 0.4 ML, particle size estimated to ∼3.3 nm). At elevated O pressures, a new Au 4f component at higher binding energy (2.4-2.6 eV relative to the Au(0) bulk signal) evolved at all particle sizes. Its appearance was attributed to a radiation-induced activation of oxygen and simultaneous oxidation of gold. The activation was much more efficient on the ∼2-2.5 nm particles. The relative intensity of the oxide component depended strongly on O pressure and, thus, on the equilibrium coverage of O . While not present in 0.1 mbar O regardless of exposure time and particle size, it dominated the Au 4f spectrum of particles ∼2-2.5 nm in size at 1 mbar oxygen pressure. This pressure-dependent formation reconciles previously conflicting XPS data. Finally, the activated oxygen species were very reactive toward CO as manifested by the rapid disappearance of the new Au 4f component in a 1:1 mixture of CO and O . The rates of evolution and consumption of this component were found to depend on gold coverage (and thus, particle size) and were highest for the smaller particles. © 2011 Elsevier B.V. All rights reserved.

Evidence for a precursor adcomplex during the metalation of 2HTPP with iron on Ag(100) - Röckert, Michael; Franke, Matthias; Tariq, Quratulain; Steinrück, Hans-Peter; Lytken, Ole (Chemical Physics Letters, 2015)
Using X-ray photoelectron spectroscopy (XPS), we demonstrate that free-base tetraphenylporphyrin (2HTPP) on Ag(1 0 0) forms a theoretically predicted adcomplex with post-deposited iron at 100 K. The formation of the adcomplex, as a precursor for metalation, can be deduced from the N Is spectra showing clearly not-metalated 2HTPP molecules, but with strongly shifted iminic nitrogen peaks, caused by the interaction of the iminic nitrogen atoms with the Fe adatoms. (C) 2015 Elsevier B.V. All rights reserved.

Evolution of full phononic band gaps in periodic cellular structures - Wormser, Maximilian; Warmuth, Franziska; Körner, Carolin (Applied Physics A: Solids and Surfaces, 2017)
Cellular materials not only show interesting static properties, but can also be used to manipulate dynamic mechanical waves. In this contribution, the existence of phononic band gaps in periodic cellular structures is experimentally shown via sonic transmission experiment. Cellular structures with varying numbers of cells are excited by piezoceramic actuators and the transmitted waves are measured by piezoceramic sensors. The minimum number of cells necessary to form a clear band gap is determined. A rotation of the cells does not have an influence on the formation of the gap, indicating a complete phononic band gap. The experimental results are in good agreement with the numerically obtained dispersion relation.

Exact calculation of the overlap volume of spheres and mesh elements - Strobl, Severin; Formella, Arno; Pöschel, Thorsten (Journal of Computational Physics, 2016)
An algorithm for the exact calculation of the overlap volume of a sphere and a tetrahedron, wedge, or hexahedron is described. The method can be used to determine the exact local solid fractions for a system of spherical, non-overlapping particles contained in a complex mesh, a question of significant relevance for the numerical solution of many fluid-solid interaction problems. While challenging due to the limited machine precision, a numerically robust version of the calculation maintaining high computational efficiency is devised. The method is evaluated with respect to the numerical precision and computational cost. It is shown that the exact calculation is only limited by the machine precision and can be applied to a wide range of size ratios, contrary to previously published methods. Eliminating this constraint enables the usage of meshes with higher resolution near the system boundaries for coupled CFD-DEM simulations. The numerical robustness is further illustrated by applying the method to highly deformed mesh elements. The full source code of the reference implementation is made available under an open-source license.
Keywords: Local solid fraction; Numerical robustness; Overlap volume; Sphere-mesh overlap

Exact on-event expressions for discrete potential systems - Bannerman, Marcus; Lue, Leo (Journal of Chemical Physics, 2010)
The properties of systems composed of atoms interacting though discrete potentials are dictated by a series of events which occur between pairs of atoms. There are only four basic event types for pairwise discrete potentials and the square-well/shoulder systems studied here exhibit them all. Closed analytical expressions are derived for the on-event kinetic energy distribution functions for an atom, which are distinct from the Maxwell-Boltzmann distribution function. Exact expressions are derived that directly relate the pressure and temperature of equilibrium discrete potential systems to the rates of each type of event. The pressure can be determined from knowledge of only the rate of core and bounce events. The temperature is given by the ratio of the number of bounce events to the number of disassociation/association events. All these expressions are validated with event-driven molecular dynamics simulations and agree with the data within the statistical precision of the simulations. © 2010 American Institute of Physics.

Exchange-correlation potentials with proper discontinuities for physically meaningful Kohn-Sham eigenvalues and band structures - Görling, Andreas (Physical Review B, 2015)
It is shown how to properly construct exchange-correlation (xc) potentials in the Kohn-Sham (KS) formalism of density-functional theory such that physically meaningful KS eigenvalues result. A potential adjustor Delta(N)(Hxc)- is derived which enables for any approximate functional for the xc energy the construction of a consistent xc potential leading to an eigenvalue of the energetically highest occupied KS orbital equal to the negative of the ionization potential. Together with a second potential adjustor Delta(N+)(Hxc) KS band structures can be converted in approximate quasiparticle band structures exhibiting the exact physical instead of the KS band gap. This represents an alternative route to the fundamental quasiparticle band gap, completely within the KS formalism without the need to resort to many-body perturbation theory approaches like the GW method. It is shown that, for any finite system, approximate xc potentials including those in the local density and generalized gradient approximations, in contrast to common belief, always exhibit consistent nonzero discontinuities at integer electron numbers, if constructed properly. Thus the discontinuity of xc potentials is identified as a derived quantity which emerges automatically by properly constructing xc potentials without the need to be specifically incorporated in approximate xc functionals. It is demonstrated that all relevant objects of the ensembleKS formalism, in particular functional derivatives and their discontinuities, can be expressed in terms of quantities readily available in the KS formalism of integer electron numbers within an approach named integer electron ensemble approach (IEEA). Attempts to specifically construct ensemble density functionals are shown to be needless. Taking the known asymptotic behavior of the electron density into account an internal consistency condition for xc potentials is presented which justifies tuning procedures of xc functionals and, furthermore, indicates how asymptotic corrections for xc potentials have to be properly employed.

Excitation of plasmonic gap waveguides by nanoantennas - Romanov, Sergey; et al. (Optics Express, 2009)
We model and optimize the excitation of a plasmonic gap waveguide by a dipole antenna. The coupling efficiency strongly depends on antenna and waveguide properties where impedanec matching plays a critical role. The optimization of antenna lengths and gap widths shows that concepts of circuit networks can likewise be applied to optical frequencies. Using classical optimization schemes known from electrical engineering we manage to increase the coupling efficiency by a factor of 129 compared with the situation without antennas. © 2009 Optical Society of America.
UnivIS-Import:2015-03-09:Pub.2009.nat.dphy.optik.1optik.excita

Excited state properties of anodic TiO2 nanotubes - Kahnt, Axel; Oelsner, Christian; Werner, Fabian; Guldi, Dirk Michael; Albu, Sergiu; Kirchgeorg, Robin; Lee, Kiyoung; Schmuki, Patrik (Applied Physics Letters, 2013)
Charge carriers, that is, holes as well as trapped and "free" electrons were investigated by means of time resolved spectroscopy in anodic TiO nanotubes that were heat treated at different temperatures. The lifetimes of the charge carrier were compared with those generated in reference layers of Solaronix Ti-Nanoxide D and Degussa P25 nanoparticles. Remarkably long lived "free" electrons were only noted in the TiO nanotubes. These findings have significance in view of any photoelectrochemical applications of TiO nanotubes. © 2013 2013 Author(s).

Exciton Formation at Solid-Solid Interfaces: a systematic experimental and ab initio Study on compressed MgO Nanopowders - Sternig, Andreas; Koller, David; Siedl, Nicolas; Diwald, Oliver; Mckenna, Keith (Journal of Physical Chemistry C, 2012)
An important and so far neglected class of structural elements affecting the overall properties of metal oxide nanopowders are interfaces between individual nanocrystals. In this work we show experimentally that these defects inside a powder of compressed MgO nanocubes are subject to photoexcitation in the UV light range. In particular, we identify a so far unobserved photoluminescence emission process at 2.5 eV. First principles calculations of the optical properties of nanocrystal interfaces provide plausible candidates for both light absorbing and emitting sites which involve different types of interface features. It was found that edge dislocations that arise from interfaces between nanocube edges and terraces induce a significant electrostatic perturbation of the interfacial electronic states. This leads to exciton generation and luminescence at even lower energies than those related to corners and edges of MgO nanocubes
EAM Import::2019-03-14

Exfoliation of hexa-peri-hexabenzocoronene in water - Englert, Jan; Hauke, Frank; Hirsch, Andreas; Feng, Xingliang; Müllen, Klaus (Chemical Communications, 2010)
Unsubstituted hexa-peri-hexabenzocoronene (HBC) - a small and well defined model system for graphite/graphene - was exfoliated and dissolved in water and the resulting individualized nano graphene sheets were characterized spectroscopically for the first time. © 2010 The Royal Society of Chemistry.

Experimental and Theoretical Assessment of Ni-Based Binary Compounds for the Hydrogen Evolution Reaction - Ledendecker, Marc; Schlott, Hannah; Antonietti, Markus; Meyer, Bernd; Shalom, Menny (Advanced Energy Materials, 2017)
Metallic binary compounds have emerged in recent years as highly active and stable electrocatalysts toward the hydrogen evolution reaction. In this work, the origin of their high activity from a theoretical and experimental point of view is elucidated. Here, different metallic ceramics as Ni3S2, Ni3N, or Ni5P4 are grown directly on Ni support in order to avoid any contaminations. The correlation of theoretical calculations with detailed material characterization and electrochemical testing paves the way to a deeper understanding of possible active adsorption sites for each material and the observed catalytic activity. It is shown that heteroatoms as P, S, and N actively take part in the reaction and do not act as simple spectator. Due to the anisotropic nature of the materials, a variety of adsorption sites with highly coverage-dependent properties exists, leading to a general shift in hydrogen adsorption free energies extgreekDGH close to zero. Extending the knowledge gained about the here described materials, a new catalyst is prepared by modifying a high surface Ni foam, for which current densities up to 100 mA cm-2 at around 0.15 V (for Ni3N) are obtained.
EAM Import::2019-03-14

Experimental and theoretical confirmation of the scaling exponent 2 in pyramidal load displacement data for depth sensing indentation - Merle, Benoit; Maier, Verena; Durst, Karsten (Scanning, 2014)
A series of articles by Kaupp et al. have recently been published in ,containing erroneous claims about the curvature of pyramidal nanoindentation loading curves. The present paper recalls the theoretical reasons why, for self-similar indenter shapes like pyramidal or conical indentations, the load scales with the indentation depth squared. Furthermore, experimental evidence for that behavior is provided for a wide variety of materials, ranging from ceramics to metals and polymers.
WW1::996

Experimental and Theoretical Studies of the Colloidal Stability of Nanoparticles−A General Interpretation Based on Stability Maps - Segets, Doris; Marczak, Renata; Schäfer, Stefan; Paula, Carolin; Gnichwitz, Jan-Frederik; Hirsch, Andreas; Peukert, Wolfgang (Acs Nano, 2011)
The current work addresses the understanding of the stabilization of nanoparticles in suspension. Specifically, we study ZnO in ethanol for which the Influence of particle size and reactant ratio as well as surface coverage on colloidal stability in dependence of the purification progress was investigated. The results revealed that the well-known zeta-potential determines not only the colloidal stability but also the surface coverage of acetate groups bound to the particle surface. The acetate groups act as molecular spacers between the nanoparticles and prevent agglomeration. Next to DLVO calculations based on the theory of Derjaguin, Landau, Verwey and Overbeek using a core shell model we find that the stability is better understood in terms of dimensionless numbers which represent attractive forces as well as electrostatic repulsion, steric effects, transport properties, and particle concentration. Evaluating the colloidal stability in dependence of time by means of UV-vis absorption measurements a stability map for ZnO is derived. From this map it becomes clear that the dimensionless steric contribution to colloidal stability scales with a stability parameter including dimensionless repulsion and attraction as well as particle concentration and diffusivity of the particles according to a power law with an exponent of -0.5. Finally, we show that our approach is valid for other stabilizing molecules like cationic dendrons and is generally applicable for a wide range of other material systems within the limitations of vanishing van der Weals forces in refractive index matched situations, vanishing zeta-potential and systems without a stabilizing shell around the particle surface.
Keywords: nanoparticles; stabilization; particle size; electrostatic; repulsion

Experimental cross-polarization detection of coupling far-field light to highly confined plasmonic gap modes via nanoantennas - Wen, J.; Banzer, Peter; Kriesch, Arian; Ploss, Daniel; Schmauß, Bernhard; Peschel, Ulf (Applied Physics Letters, 2011)
We experimentally demonstrate the coupling of far-field light to highly confined plasmonic gap modes via connected nanoantennas. The excitation of plasmonic gap modes is shown to depend on the polarization, position, and wavelength of the incident beam. Far-field measurements performed in crossed polarization allow for the detection of extremely weak signals re-emitted from gap waveguides and can increase the signal-to-noise ratio dramatically.
lhft_intern.bib::Wen11

Experimental determination of the effective indenter shape and ε-factor for nanoindentation by continuously measuring the unloading stiffness - Merle, Benoit; Maier, Verena; Göken, Mathias; Durst, Karsten (Journal of Materials Research, 2012)
The Oliver and Pharr method for evaluating nanoindentation load–displacement data is based on the measurement of the contact stiffness, which is usually determined at the very beginning of the unloading sequence, or, using dynamic nanoindentation, continuously during the whole loading segment. A new experimental method has been developed to continuously monitor the contact stiffness throughout the unloading sequence. It provides supplementary information about the shape and area of the residual impression, as well as a direct measurement of the shape of the effective indenter previously introduced by Pharr and Bolshakov. The new method was applied to indentations on fused silica, sapphire, nanocrystalline nickel, and ultrafine-grained aluminum. Lastly, the new procedure was adapted to directly measure the epsilon factor used in the Oliver and Pharr method. A value of 0.76 was found from indentation into fused silica, in close agreement with literature values.
EAM Import::2019-03-14

Experimental Evidence for Quantum Interference and Vibrationally Induced Decoherence in Single-Molecule Junctions - Ballmann, Stefan; Härtle, Rainer; Brana Coto, Pedro; Elbing, Mark; Mayor, Marcel; Bryce, Martin; Thoss, Michael; Weber, Heiko B. (Physical Review Letters, 2012)
We analyze quantum interference and decoherence effects in single-molecule junctions both experimentally and theoretically by means of the mechanically controlled break junction technique and density-functional theory. We consider the case where interference is provided by overlapping quasidegenerate states. Decoherence mechanisms arising from electronic-vibrational coupling strongly affect the electrical current flowing through a single-molecule contact and can be controlled by temperature variation. Our findings underline the universal relevance of vibrations for understanding charge transport through molecular junctions. © 2012 American Physical Society.
UnivIS-Import:2015-04-14:Pub.2012.nat.dphy.IAP.LAP.experi

Experimental Investigation and Numerical Modeling of the Bond Shear Strength of Multi-layered 6000 Series Aluminum Alloys - Herrmann, Jürgen; Suttner, Sebastian; Merklein, Marion (Procedia Engineering, 2017)
The use of high-strength aluminum alloys is one key factor for the realization of lightweight design in industrial applications. Besides alloying, it is possible to produce high-strength sheet materials using the Accumulative Roll Bonding (ARB) process. Based on a repetitive stacking and roll bonding process, multi-layered sheets with a nanocrystalline grain structure are produced. The bond formation is created by a cold welding process due to the high pressure in the rolling gap and cracks in the oxide film of the aluminum sheet material caused by the thickness reduction. The experimental investigation of the bond strength is crucial for the assessment of the formability of the multi-layered sheet material. High bonding quality is necessary in order to prevent failure mechanisms like delamination in subsequent forming operations. Furthermore, a numerical model, which contains discrete layers over the sheet thickness as well as their bonding mechanisms will be implemented in finite element analysis. This enables a process layout for forming operations by the prediction of delamination effects. Within this investigation, an experimental test method is presented to characterize the bond shear strength of multi-layered sheet material. The use of an optical strain measurement system enables the identification of the material behavior over the sheet thickness during the shear test. For this investigation a 16-layered aluminum alloy AA6014 is used. However, the bond shear strength of the last bonded respectively middle layer of the sheet material is investigated. The 16-layered material has 15 bonds over the thickness but the one in the middle has the lowest bond strength as it was only roll bonded once compared to the others, which were rolled at least twice. The bonding mechanisms are modelled using a tiebreak contact formulation in the finite element software LS-DYNA. Concluding, the numerical model is validated by comparison with the experimental results.
EAM Import::2019-03-14

Experimental investigation of gas-liquid distribution in periodic open cellular structures as potential catalyst supports - Lämmermann, Markus; Schwieger, Wilhelm; Freund, Hannsjörg (Catalysis Today, 2016)
Radial and axial liquid distribution was measured in a packed column of 500 mm length and 100 mm inner diameter under various gas- and liquid superficial velocities to achieve trickle flow regime. Liquid maldistribution was determined using a liquid collector at the outlet consisting of 21 collection zones of equal area. In this work, as packings inside the column, periodic open cellular structures (POCS) were manufactured and applied. Such POCS can be used as structured catalyst support and/or liquid distributor. The hydrodynamic behavior of packings with Kelvin cell, Diamond cell and a hybrid combination of both unit cells was studied to identify unit cells with high potential for achieving homogeneous liquid distribution over the entire reactor cross-section. The liquid superficial velocity had a strong influence on the liquid distribution, while an increasing gas flow rate did not change the flow patterns. Kelvin cell packings tend to distribute the liquid towards the center whereas Diamond cell packings predominantly spread the fluid towards the rim. As a consequence of these observations, in this work a novel POCS structure is proposed: The combination of Kelvin and Diamond unit cells, called hybrid DiaKel unit cell. With POCS consisting of this unit cell type, a significantly improved liquid distribution can be achieved. Thus, this structure features high potential as liquid distributor and/or catalyst support in gas-liquid reaction systems.

Experimental investigation of the protein foam flow structure in horizontal channels:Flow regime and corresponding bubble size distribution - Zoheidi, Ladan; Panradl, Christian; Rauh, Cornelia; Delgado, Antonio (Journal of Food Process Engineering, 2017)
The present study offers the flow patterns and associated structure of protein foam flowing in a horizontal channel. An experimental equipment was developed to study the protein foam flow regime and the related bubble size distribution. The foam formation was performed with the help of a foam generator and transported in a horizontal channel. The optical measurements were performed by a high speed camera which enabled the foam flow visualization at the channel wall. The foam flow pattern and the corresponding bubble size distribution were studied as a function of the foam quality. As a consequence of the higher drainage and gravitational effect at higher gas flowrates, the foam structure resulted in different flow patterns. The bubble size distribution was studied as a function of the volumetric gas and liquid flowrate. The foam consisting of bubbles on the millimeter scale showed a close fit to the log‐normal distribution. Practical applications One of the most important attributes of the two‐phase flow is the bubble structure resulting in different foam flow regimes. Since the final appearance of foam food products and their textural properties such as their smoothness are associated with the foam uniform distribution of the bubbles, the main objective of the present work is given to provide the first experimental results of the milk protein foam flow regime and corresponding bubble sizes during mechanical transport through a horizontal duct.
UnivIS-Import:2018-02-22:Pub.2017.tech.ITC.stmmec.flowre

Experimental studies of the electronic structure of graphene - Bostwick, Aaron; Mcchesney, Jessica; Ohta, Taisuke; Rotenberg, Eli; Seyller, Thomas; Horn, Karsten (Progress in Surface Science, 2009)
Graphene, the single layer of hexagonally coordinated carbon, is a two-dimensional material with many unusual properties; its physical realization a few years ago has caused a storm of activities in the solid state physics and materials science communities. The intriguing ``massless Dirac Fermion'' character of its charge carriers renders graphene a unique study object in condensed matter physics, and we discuss how surface-related techniques such as photoemission, STM and LEED play a prominent role in these investigations. We report on experimental studies of the growth and electronic structure of epitaxial single and few layer graphene on silicon carbide. The unusual band structure of single layer graphene and its evolution as layers are added towards bulk graphite is studied. In the special case of the bilayer, the opening of a gap by inducing an asymmetry through the influence of doping is examined. Finally, the influence of many body processes on the spectral function is discussed on the basis of high resolution photoemission data. The discussion of these aspects gives a comprehensive overview of the electronic structure of graphene as examined by experiment.
EAM Import::2019-03-14

Experiments and simulation of the diffusion and activation of the n-type dopants P, As, and Sb implanted into germanium - Koffel, S.; Kaiser, R. J.; Bauer, A. J.; Amon, B.; Pichler, Peter; Lorenz, J.; Frey, Lothar; Scheiblin, P.; Mazzocchi, V.; Barnes, J. -P.; Claverie, A. (Microelectronic Engineering, 2011)
The effects of implant dose and annealing conditions on the diffusion, activation, and out-diffusion of the typical n-type dopants in germanium (phosphorus, arsenic, antimony) were studied. First, short annealing times were used to limit the diffusion of dopants and to match the conditions needed for the realization of shallow junctions. Sb is not well suited to achieve high activation levels because honeycomb voids can already form at doses of 3 × 1014 cm-2. Of the other two, P is a better candidate than As because it was possible to maximize the activation level to up to about 4.5 × 1019 cm-3 without noticeable diffusion. This absolute value has only a limited accuracy, though, since the mobility models available in literature lead to values which differ by more than one order of magnitude. Longer annealing times were then used to study the redistribution of the dopants. For P, a model based on migration predominantly via complexes with doubly negatively charged vacancies and dopant loss was implemented which allowed the simultaneous simulation of our experimental profiles with one set of parameters. The extracted diffusion coefficient with an activation energy of 2.2 eV is comparable to the results obtained in previous studies. No noticeable P clustering was observed in these experiments. The model was then adapted to simulate the redistribution of As and Sb. For Sb, clustering is apparent in the diffusion profiles and has to be taken into account in the simulations.
Keywords: Activation; Diffusion; Germanium; n-type doping; Simulation

Exploring the fabrication of Co and Mn nanostructures with focused soft x-ray beam induced deposition - TU, Fan; Späth, Andreas; Drost, Martin; Vollnhals, Florian; Krick Calderon, Sandra; Fink, Rainer; Marbach, Hubertus (Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics, 2017)
Focused soft X-ray beam induced deposition of metallic deposits from metal organic precursors is a promising novel technique for additive nanostructure fabrication. In the present work, the authors present a comparative study for deposition and in situ characterization of Co and Mn nanostructures in a scanning transmission x-ray microscope. The authors detect a significant selectivity of the deposition process with respect to the incident photon energy that arises from the enhanced x-ray absorption cross section of the precursor molecules for near-threshold excitation. This effect has been investigated for the L2,3-edges of the respective metal centers of two different precursor molecules as well as the N and O K-edges of the respective ligands. The authors find a photon-limited growth mode for deposition from cobalt tricarbonyl nitrosyl [Co(CO)3NO], while the process is precursor-limited for methylcyclopentadienyl manganese tricarbonyl [MeCpMn(CO)3] possibly due to a comparably low vapor pressure of the latter precursor.
EAM Import::2019-03-14

Exploring the Limiting Open-Circuit Voltage and the Voltage Loss Mechanism in Planar CH3NH3PbBr3 Perovskite Solar Cells - Chen, Shi; Hou, Yi; Chen, Haiwei; Richter, Moses; Guo, Fei; Kahmann, Simon; Tang, Xiaofeng; Stubhan, Tobias; Zhang, Hong; Li, Ning; Gasparini, Nicola; Ramírez Quiroz, César Omar; et al. (Advanced Energy Materials, 2016)
Perovskite solar cells based on CHNHPbBr with a band gap of 2.3 eV are attracting intense research interests due to their high open-circuit voltage (V) potential, which is specifically relevant for the use in tandem configuration or spectral splitting. Many efforts have been performed to optimize the V of CHNHPbBr solar cells; however, the limiting V (namely, radiative V:V) and the corresponding ΔV (the difference between V and V) mechanism are still unknown. Here, the average V of 1.50 V with the maximum value of 1.53 V at room temperature is achieved for a CHNHPbBr solar cell. External quantum efficiency measurements with electroluminescence spectroscopy determine the V of CHNHPbBr cells with 1.95 V and a ΔV of 0.45 V at 295 K. When the temperature declines from 295 to 200 K, the obtained V remains comparably stable in the vicinity of 1.5 V while the corresponding ΔV values show a more significant increase. Our findings suggest that the V of CHNHPbBr cells is primarily limited by the interface losses induced by the charge extraction layer rather than by bulk dominated recombination losses. These findings are important for developing strategies how to further enhance the V of CHNHPbBr-based solar cells.
Keywords: CH3NH3PbBr3 solar cells; interface materials based losses; nonradiative recombination losses; open-circuit voltage

Exploring the Stability of Novel Wide Bandgap Perovskites by a Robot Based High Throughput Approach - Chen, Shi; Hou, Yi; Chen, Haiwei; Tang, Xiaofeng; Langner, Stefan; Li, Ning; Stubhan, Tobias; Levchuk, Ievgen; Gu, Ening; Osvet, Andres; Brabec, Christoph (Advanced Energy Materials, 2017)
Currently, lead-based perovskites with mixed multiple cations and hybrid halides are attracting intense research interests due to their promising stability and high efficiency. A tremendous amount of 3D and 2D perovskite compositions and configurations are causing a strong demand for high throughput synthesis and characterization. Furthermore, wide bandgap (≈1.75 eV) perovskites as promising top-cell materials for perovskite–silicon tandem configurations require the screening of different compositions to overcome photoinduced halide segregation and still yielding a high opencircuit voltage (Voc). Herein, a home-made high throughput robot setup is introduced performing automatic perovskite synthesis and characterization. Subsequently, four kinds of compositions (i.e., cation mixtures of Cs–methylammonium (MA), Cs– formamidinium (FA), MA–FA, and FA–MA) with an optical bandgap of ≈1.75 eV are identified as promising device candidates. For Cs–MA and Cs–FA films it is found that the Br–I phase segregation indeed can be overcome. Moreover, Cs–MA, MA–FA, and Cs–FA based devices exhibit an average Voc of 1.17, 1.17, 1.12 V, and their maximum values approached 1.18, 1.19, 1.14 V, respectively, which are among the highest Voc (≈1.2 V) values for ≈40% Br perovskite. These findings highlight that the high throughput approach can effectively and efficiently accelerate the invention of novel perovskites for advanced applications.

Expression Templates Revisited: A Performance Analysis of the Current ET Methodology - Iglberger, Klaus; Hager, Georg; Eitzinger, Jan; Rüde, Ulrich (SIAM Journal on Scientific Computing, 2011)
In the last decade, Expression Templates (ET) have gained a reputation as an efficient performance optimization tool for C++ codes. This reputation builds on several ET-based linear algebra frameworks focused on combining both elegant and high-performance C++ code. However, on closer examination the assumption that ETs are a performance optimization technique cannot be maintained. In this paper we demonstrate and explain the inability of current ET-based frameworks to deliver high performance for dense and sparse linear algebra operations, and introduce a new ET implementation that truly allows the combination of high performance code with the elegance and maintainability of a domain-specific language.
UnivIS-Import:2015-03-09:Pub.2011.tech.IMMD.lsinfs.expres

Extended van Hove Singularity and Superconducting Instability in Doped Graphene - Mcchesney, J. L.; Bostwick, Aaron; Ohta, Taisuke; Seyller, Thomas; Horn, Karsten; Gonzalez, Jose; Rotenberg, Eli (Physical Review Letters, 2010)
We have investigated the effects of doping on a single layer of graphene using angle-resolved photoemission spectroscopy. We show that many-body interactions severely warp the Fermi surface, leading to an extended van Hove singularity (EVHS) at the graphene M point. The ground state properties of graphene with such an EVHS are calculated, analyzing the competition between a magnetic instability and the tendency towards superconductivity. We find that the latter plays the dominant role as it is enhanced by the strong modulation of the interaction along the Fermi line, leading to an energy scale for the onset of the pairing instability as large as 1 meV when the Fermi energy is sufficiently close to the EVHS.
EAM Import::2019-03-14

Extending the environmental lifetime of unpackaged perovskite solar cells through interfacial design - Chen, Haiwei; Hou, Yi; Halbig, Christian Eberhard; Chen, Shi; Zhang, Hong; Li, Ning; Guo, Fei; Tang, Xiaofeng; Gasparini, Nicola; Levchuk, Ievgen; Kahmann, Simon; Ramírez Quiroz, César Omar; et al. (Journal of Materials Chemistry A, 2016)
Solution-processed oxo-functionalized graphene (oxo-G) is employed to substitute hydrophilic PEDOT:PSS as an anode interfacial layer for perovskite solar cells. The resulting devices exhibit a reasonably high power conversion efficiency (PCE) of 15.2% in the planar inverted architecture with oxo-G as a hole transporting material (HTM), and most importantly, deploy the full open-circuit voltage (V) of up to 1.1 V. Moreover, oxo-G effectively slows down the ingress of water vapor into the device stack resulting in significantly enhanced environmental stability of unpackaged cells under illumination with 80% of the initial PCE being reached after 500 h. Without encapsulation, ∼60% of the initial PCE is retained after ∼1000 h of light soaking under 0.5 sun and ambient conditions maintaining the temperature beneath 30 °C. Moreover, the unsealed perovskite device retains 92% of its initial PCE after about 1900 h under ambient conditions and in the dark. Our results underpin that controlling water diffusion into perovskite cells through advanced interface engineering is a crucial step towards prolonged environmental stability.
Keywords: Engineering controlled terms: Cell engineering; Conducting polymers; Open circuit voltage; Perovskite; Slow light; Solar cells Environmental stability; Functionalized graphene; High power conversion; Hole-transporting materials; Interface engineering; Interfacial design; Inverted architectures; Solution-processed Engineering main heading: Perovskite solar cells

Extension of exact-exchange density functional theory of solids to finite temperatures - Greiner, Maximilian; Carrier, Pierre; Görling, Andreas (Physical Review B, 2010)
A general finite-temperature exact-exchange (EXX) formalism derived within the framework of finite-temperature density functional theory for grand canonical ensembles is presented. Based on this formalism a finite-temperature EXX method for solids using plane-wave basis sets is presented. The method is generally applicable, i.e., applicable to insulators, semiconductors, or metals and enables the investigation of temperature effects. More important, however, is that the finite-temperature EXX method enables an EXX treatment of metals by introducing a physically motivated Fermi broadening technique. We tested the method by applying it to sodium, magnesium, and aluminum and compare EXX and LDA (local density approximation) band structures as well as the density of states for the three metals. Differences between LDA and EXX band structures are negligible up to the Fermi level. Above the Fermi level, however, differences between LDA and EXX band structures of magnitudes of 1-2 eV start to build up for energetically higher bands. The magnitude of these differences is of the same order as that of the increases in EXX band gaps compared to LDA band gaps as they are reported for semiconductors and insulators. © 2010 The American Physical Society.

Extension of the Deep UV-Capabilities in Multiwavelength Spectrometry in Analytical Ultracentrifugation: The Role of Oil Deposits - Walter, Johannes; Segets, Doris; Peukert, Wolfgang (Particle & Particle Systems Characterization, 2016)
The optical performance of an analytical ultracentrifuge equipped with a fiber-coupled detector is investigated with special emphasis on the influence of oil leakage on the long-term deep ultraviolet stability. Two strategies for enhanced optical performance are presented. The first includes regeneration of fiber transmittance under atmospheric conditions, whereas the second utilizes an optimized vacuum system consisting of a turbomolecular pump.
EAM Import::2019-03-14

Extension of the LOPLS-AA Force Field for Alcohols, Esters, and Monoolein Bilayers and its Validation by Neutron Scattering Experiments. - Pluhackova, Kristyna; Morhenn, Humphrey; Lautner, Lisa; Lohstroh, Wiebke; Nemkovski, Kirill; Unruh, Tobias; Böckmann, Rainer (Journal of Physical Chemistry B, 2015)
The recently presented LOPLS-AA all-atom force field for long hydrocarbon chains, based on the OPLS-AA force field, was extended to alcohols, esters, and glyceryl monooleate (GMO) lipids as a model lipid. Dihedral angles were fitted against high level ab initio calculations, and ester charges were increased to improve their hydration properties. Additionally, the ester Lennard-Jones parameters were readjusted to reproduce experimental liquid bulk properties, densities, and heats of vaporization. This extension enabled the setup of LOPLS-AA parameters for GMO molecules. The properties of the lipid force field were tested for the liquid-crystalline phase of a GMO bilayer. The obtained area per lipid for GMO is in good agreement with experiment. Additionally, the lipid dynamics on the subpicosecond to the nanosecond time scale is in excellent agreement with results from time-of-flight (TOF) quasielastic neutron scattering (QENS) experiments on a multilamellar monoolein system, enabling here for the first time the critical evaluation of the short-time dynamics obtained from a molecular dynamics simulation of a membrane system.

Extracting the limiting factors in photocurrent measurements on TiO2 nanotubes and enhancing the photoelectrochemical properties by Nb doping - Özkan, Selda; Mazare, Anca Valentina; Schmuki, Patrik (Electrochimica Acta, 2015)
In the present work, we extract the key factors that influence and limit the photoresponse of anodic TiO2 nanotubular layers. For this we formed nanotube layers of different thicknesses, annealed the structures at different temperatures and evaluated the doping effect of Nb. Using various analytical characterization tools (SEM, XRD, XPS, ToF-SIMS), we found that a most crucial impact on limiting the photoresponse in nanotubular layers is the formation of a rutile layer at the metal/oxide interface during the thermal annealing process (by thermal oxidation of Ti metal underneath tubes). This rutile layer increases severely in thickness with annealing temperature. We show that this detrimental effect can be overcome by doping the TiO2 NTs with very low amounts of Nb (0.1 at% Nb), which can result in a 12-fold increase of the IPCE for high temperature annealed TiO2 NTs.
EAM Import::2019-03-14

Extraction coupled oxidative desulfurization of fuels to sulfate and water-soluble sulfur compounds using polyoxometalate catalysts and molecular oxygen - Albert, Jakob; Wasserscheid, Peter; Bertleff, Benjamin; Claußnitzer, Johannes; Korth, Wolfgang; Jess, Andreas (ACS Sustainable Chemistry & Engineering, 2017)
Our contribution demonstrates a new way of fuel desulfurization, namely selective oxidation of organic S-compounds present in fuels to water-soluble sulfur compounds followed by in situ extraction of the latter into an aqueous phase. Different from common oxidative desulfurization (ODS) processes, we demonstrate a technique that converts sulfur compounds in fuel to a large extent to sulfate (60–70%) using oxygen as the oxidant and an aqueous H8PV5Mo7O40 (HPA-5) solution as the catalyst phase. Other water-soluble desulfurization products are sulfoacetic acid (SAA) with a share of 10–20%, 2-sulfobenzoic acid (2-SBA), and 2-(sulfooxy)benzoic acid (2-SOBA), the latter two with a share of <10%. The new desulfurization method has been optimized for removing benzothiophene from isooctane, giving the best results with a degree of desulfurization of 99% applying 120 °C, 20 bar oxygen pressure, and 1000 rpm of 6 h reaction time using a volume water/oil ration of 10/1. Furthermore, we also successfully demonstrated the desulfurization of a domestic fuel oil with 973 ppmw sulfur content with a degree of desulfurization of 28% under nonoptimized conditions.

Extraordinary low transmission effects for ultra-thin patterned metal films - Reibold, David; Shao, F.; Erdmann, Andreas; Peschel, Ulf (Optics Express, 2009)
Thin metal films show a residual transmission for light in the visible and UV spectral range. This transmission can be strongly reduced by an appropriate sub-wavelength patterning of the metal film. Our investigation is focused on metal films with a thickness much below 100nm, where the transmission response is dominated by the individual posts acting like antennas and cannot be attributed to the excitation of surface plasmons. The almost complete suppression of transmission for ultra-thin metal films depends mainly on the absorber width, but not on the pitch of the pattern. The effect is robust with respect to imperfections of the geometry or larger features imprinted into the sub-wavelength pattern. © 2008 Optical Society of America.
UnivIS-Import:2017-07-10:Pub.2009.tech.IE.LEB.extrao

Extraordinary transmission through a single coaxial aperture in a thin metal film - Banzer, Peter; Kindler, Jochen; Quabis, Susanne; Peschel, Ulf; Leuchs, Gerd (Optics Express, 2010)
We investigate experimentally the transmission properties of single sub-wavelength coaxial apertures in thin metal films (t = 110 nm). Enhanced transmission through a single sub-wavelength coaxial aperture illuminated with a strongly focused radially polarized light beam is reported. In our experiments we achieved up to four times enhanced transmission through a single coaxial aperture as compared to a (hollow) circular aperture with the same outer diameter.We attribute this enhancement of transmission to the excitation of a TEM-mode for illumination with radially polarized light inside the single coaxial aperture. A strong polarization contrast is observed between the transmission for radially and azimuthally polarized illumination. Furthermore, the observed transmission through a single coaxial aperture can be strongly reduced if surface plasmons are excited. The experimental results are in good agreement with finite difference time domain (FDTD) simulations.
EAM Import::2019-03-14

Extrinsic corrugation-assisted mechanical exfoliation of monolayer graphene - Pang, Shuping; Englert, Jan; Tsao, Hoi Nok; Hernandez, Yenny; Hirsch, Andreas; Feng, Xinliang; Muellen, Klaus (Advanced Materials, 2010)
An extrinsic corrugation-assisted mechanical exfoliation process is introduced to fabricate ultra-large, patterned monolayer graphene on silicon substrates. This exfoliation method, without any contamination, offers not only an ideal alternative for the scalable deposition of monolayer graphene on silicon substrates, but also provides a promising basis for the construction of graphene-based integrated circuits. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Extrusion Foaming of a Preceramic Silicone Resin with a Variety of Profiles and Morphologies - Wolff, Friedrich; Ceron Nicolat, Bruno; Fey, Tobias; Greil, Peter; Münstedt, Helmut (Advanced Engineering Materials, 2012)
Preceramic foams with a variety of profiles and morphologies were fabricated by a continuous foam extrusion process based on a single-screw extruder. A methyl silicone resin was used as the preceramic polymer and carbon dioxide as the foaming agent. Variations of the foam morphology were achieved by changing the key process parameters carbon dioxide content, foaming pressure, and foaming temperature. Samples of different porosities, cell densities, pore sizes, and pore size gradients were obtained. From these results, conditions for the fabrication of particular foam structures can be derived. Various foam profiles as rods, tapes, and tubes were extruded and pyrolysed into ceramic foams.

Fabrication and characterisation of a fully auxetic 3D lattice structure via selective electron beam melting - Warmuth, Franziska; Adler, Lucas; Osmanlic, Fuad; Lodes, Matthias; Körner, Carolin (Smart Materials and Structures, 2017)
A three-dimensional fully auxetic cellular structure with negative Poisson's ratio is presented. Samples are fabricated from Ti6Al4V powder via selective electron beam melting. The influence of the strut thickness and the amplitude of the strut on the mechanical properties and the deformation behaviour of cellular structures is studied.
Keywords: additive manufacturing; auxetic; negative Poissons ratio; periodic cellular metals; selective electron beam melting (SEBM); Ti6Al4V

Fabrication and characterization of plasmonic nanocone antennas for strong spontaneous emission enhancement - Hoffmann, Björn; Vassant, Simon; Chen, Xue-Wen; Götzinger, Stephan; Sandoghdar, Vahid; Christiansen, Silke (Nanotechnology, 2015)
Plasmonic antennas are attractive nanostructures for a large variety of studies ranging from fundamental aspects of light-matter interactions at the nanoscale to industry-relevant applications such as ultrasensitive sensing, enhanced absorption in solar cells or solar fuel generation. A particularly interesting feature of these antennas is that they can enhance the fluorescence properties of emitters. Theoretical calculations have shown that nanocone antennas provide ideal results, but a high degree of manufacturing precision and control is needed to reach optimal performance. In this study, we report on the fabrication of nanocones with base diameters and heights in the range of 100 nm with variable aspect ratios using focused ion beam milling of sputtered nano-crystalline gold layers. The controlled fabrication process allows us to obtain cones with tailored plasmon resonances. The measured plasmon spectra show very good agreement with finite-difference time-domain calculations. Theoretical investigations predict that these nanocones can enhance the spontaneous emission rate of a quantum emitter by several hundred times while keeping its quantum efficiency above 60%.
Keywords: focused ion beam; nanocone antenna; plasmon spectroscopy; plasmonics; radiative rate enhancement

Fabrication and characterization of three-dimensional biomimetic chiral composites - Turner, Mark D.; Schröder-Turk, Gerd; Gu, Min (Optics Express, 2011)
Here we show the fabrication and characterization of a novel class of biomimetic photonic chiral composites inspired by a recent finding in butterfly wing-scales. These three-dimensional networks have cubic symmetry, are fully interconnected, have robust mechanical strength and possess chirality which can be controlled through the composition of multiple chiral networks, providing an excellent platform for developing novel chiral materials. Using direct laser writing we have fabricated different types of chiral composites that can be engineered to form novel photonic devices. We experimentally show strong circular dichroism and compare with numerical simulations to illustrate the high quality of these three-dimensional photonic structures.
EAM Import::2019-03-14

Fabrication and pressure drop behavior of novel monolithic structures with zeolitic architectures - Schwieger, Wilhelm; Thangaraj, Thangaraj; Warmuth, Franziska; Warnick, Karl-Georg; Klumpp, Michael; Lodes, Matthias; Körner, Carolin (Chemical Engineering Journal, 2016)
Novel monolithic structures with zeolitic [sodalite (SOD), Linde-Type A (LTA) and faujasite (FAU)-type] architectures having well-defined pores, cages and channels are successfully fabricated by selective electron beam melting (SEBM) using Ti-6Al-4V powder. The open porosities of the monoliths are 92.87% (SOD), 94.64% (LTA) and 95.67% (FAU), respectively. Although the porosity of the FAU monolith is relatively higher than the other two monoliths (SOD and LTA), but exhibits higher normalized pressure drop due to its inherent pore tortuosity (supercages interconnected by 12-ring channels in all three dimensions). This represents a significant step forward for the design and development of innovative structured supports and/or reactors with desired porosities and pressure drop characteristics.
Keywords: Monoliths; Pressure drop; SEBM; Structured supports/reactors; Zeolitic architecture

Fabrication, charge carrier transport, and application of printable nanocomposites based on indium tin oxide nanoparticles and conducting polymer 3,4-ethylenedioxythiophene/polystyrene sulfonic acid - Maksimenko, Ilja; Kilian, Daniel; Mehringer, Christian; Voigt, Michael; Peukert, Wolfgang; Wellmann, Peter (Journal of Applied Physics, 2011)
Printable transparent hybrid composites consisting of indium tin oxide (In2O3:Sn; ITO) nanoparticles and conducting polymer 3,4-polyethylenedioxythiophene (PEDOT) as matrix material were developed. The basic idea is to fill up the pores of the highly porous nanoparticulate ITO network to improve the interparticle contact and hence the conductivity of printed ITO thin films. Ready-to-use and stable aqueous dispersions were fabricated starting from ITO nanoparticles and aqueous formulation of conducting PEDOT and polystyrene sulfonic acid (PSS). This report presents and discusses key factors to obtain stable ITO-PEDOT dispersions with different mixing ratios and their application for printable devices as transparent electrode material. It was found that the f-potential value is crucial for preparation of stable dispersions. Electrical and optical properties of the hybrid ITO-PEDOT coatings were analyzed. Temperature dependent resistivity measurements reveal that conduction occurs by fluctuation induced tunneling. Transparent and conducting nanocomposite layers with a conductivity as high as 132 X1cm1 were fabricated by a low-temperature (T¼130 C) and entirely vacuum-free process. An all-printed electroluminescent lamp on a flexible substrate was realized as a demonstrator showing the applicability of such coatings for voltage-driven optoelectronic devices.<br

Fabrication of gold-nanoshell/polycaprolactonecomposite films with high electrical conductivity - Naseri, Shiva; Diba, Mani; Golkar, Saeedeh; Boccaccini, Aldo R.; Klupp Taylor, Robin (Materials Letters, 2014)
Gold nanoshells on 1.1 μm diameter silica cores were produced via a seeded growth approach and their quality evaluated using optical measurements. The particles were combined with polycaprolactone to produce a flexible biocompatible composite film with high conductivity (1.51 S cm-1) and relatively low effective gold loading (16.86 wt%) in comparison with other reports in the literature. The morphology of the composite was shown to comprise nanoshell-filled pores percolating a polymer-rich matrix. The obtained films have potential for biomedical applications such as in patches for cardiac tissue engineering, combining biocompatibility, flexibility and electrical conductivity. © 2014 Elsevier B.V.
Keywords: Conductivity; Metal nanoshells; Nanocomposite; Plasmon resonance; Polycaprolactone

Fabrication of layered nanostructures by successive electron beam induced deposition with two precursors: Protective capping of metallic iron structures - Schirmer, Michael; Walz, Marie-Madeleine; Papp, Christian; Kronast, F.; Gray, A.X.; Balke, B.; Cramm, S.; Fadley, C.S.; Steinrück, Hans-Peter; Marbach, Hubertus (Nanotechnology, 2011)
We report on the stepwise generation of layered nanostructures via electron beam induced deposition (EBID) using organometallic precursor molecules in ultra-high vacuum (UHV). In a first step a metallic iron line structure was produced using iron pentacarbonyl; in a second step this nanostructure was then locally capped with a 2-3nm thin titanium oxide-containing film fabricated from titanium tetraisopropoxide. The chemical composition of the deposited layers was analyzed by spatially resolved Auger electron spectroscopy. With spatially resolved x-ray absorption spectroscopy at the Fe L edge, it was demonstrated that the thin capping layer prevents the iron structure from oxidation upon exposure to air. © 2011 IOP Publishing Ltd.

Fabrication of metallic SPM tips by combining UV nanoimprint lithography and focused ion beam processing - Jambreck, J. D.; Schmitt, H.; Amon, B.; Rommel, Mathias; Bauer, A. J.; Frey, Lothar (Microelectronic Engineering, 2010)
UV nanoimprint lithography (UV-NIL) is applied for the fabrication of metallic scanning probe microscopy (SPM) tips. The NIL templates are generated by focused ion beam (FIB) processing. The tips are imprinted by UV-NIL using a resist which is a mixture of Ag particles and an UV-curing polymer. After imprinting, tips are further shaped by FIB to achieve a high aspect ratio and a small radius at the apex. The resulting tips are analyzed and methods for increasing the metal-to-polymer ratio are presented and discussed.
Keywords: FIB; Focused ion beam; Full metal tip; Nanoimprint lithography; NIL; Scanning probe microscopy; SPM

Fabrication, Optical Modeling, and Color Characterization of Semitransparent Bulk-Heterojunction Organic Solar Cells in an Inverted Structure - Ameri, Tayebeh; Dennler, Gilles; Waldauf, Christoph; Azimi, Hamed; Seemann, Andrea; Forberich, Karen; Hauch, Jens; Scharber, Markus; Hingerl, Kurt; Brabec, Christoph (Advanced Functional Materials, 2010)
Semitransparent inverted organic photodiodes are fabricated with a Baytron PH500 ethylene-glycol layer/silver grid as the top electrode. Reasonable performances are obtained under both rear- and front-side illumination and efficiencies up to 2% are achieved. Some light is shed on visual prospects through optical simulations for a semitransparent device of poly(3-hexylthiophene) (P3HT) and the C60 derivative 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C71 (PC70BM) in the inverted structure. These calculations allow the maximum efficiency achievable to be predicted for semitransparent cells based on P3HT:PC70BM versus the transparency perception for a human eye. The simulations suggest that low-bandgap materials such as poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) have a better potential for semitransparent devices. In addition, the color range recognized by the human eye is predicted by the optical simulation for some semitransparent devices including different active layers.

Facile and quick preparation of carbon nanohorn-based counter electrodes for efficient dye-sensitized solar cells - Lodermeyer, Fabian; Prato, Maurizio; Costa, Ruben Dario; Guldi, Dirk Michael (Nanoscale, 2016)
For the first time, Pt-free counter electrodes based on carbon nanohorns for highly efficient dye-sensitized solar cells were assembled by a facile and fast drop cast technique. These novel electrodes feature an effective catalytic behavior towards the reduction of I3- and, as such, afford even higher short-circuit current densities compared to Pt-based references. In a final device, solar cells with 7.7% efficiency were achieved.
EAM Import::2019-03-14

Facile colloidal coating of polystyrene nanospheres with tunable gold dendritic patches - Bao, Huixin; Bihr, Timo; Smith, Ana-Suncana; Klupp Taylor, Robin (Nanoscale, 2014)
Patchy particles comprise regions of differing material or chemical functionality on otherwise isotropic cores. To meet the great potential of these anisotropic structures in a wide range of application fields, completely new approaches are sought for the scalable and tunable production of patchy particles, particularly those with nanoscale dimensions. In this paper the synthesis of patchy particles via a simple colloidal route is investigated. Using surfactant-free cationic polystyrene nanospheres as core particles, gold patches are produced through the in situ reduction of chloroauric acid with ascorbic acid. The fact that such nanostructured metal patches can be heterogeneously nucleated on polymer nanospheres is related to the electrostatic interaction between core and metal precursor. Furthermore, the lateral expansion of the gold patches over the polystyrene surface is facilitated by an excess of ascorbic acid. The morphology of the patches is highly dendritic and process-induced variations in the structure are related to gold surface mobility using Monte Carlo simulations based on the diffusion limited aggregation principle. Considering the pH dependent behaviour of ascorbic acid it is possible to predict the moiety which most likely adsorbs to the polymer surface and promotes gold surface diffusion. This enables the judicious adjustment of the pH to also obtain non-dendritic patches. On account of the plasmonic behaviour of gold, the patchy particles have morphology-dependent optical properties. The systematic development of the synthetic approach described here is expected to lay a foundation for the development of functional materials based on the self- or directed-assembly of nanoscale building blocks with anisotropic interactions and properties. © the Partner Organisations 2014.
Keywords: patchy; particle; functionality; anisotropic; morphology; Monte-Carlo; simulation; surface

Facile decoration of functionalized single-wall carbon nanotubes with phthalocyanines via "click chemistry" - Campidelli, Stephane; Ballesteros, Beatriz; Filoramo, Arianna; Diaz Diaz, David; De La Torre, Gema; Torres, Tomas; Rahman, Aminur; Ehli, Christian; Kießling, Daniel; Werner, Fabian; Sgobba, Vito; Guldi, Dirk Michael; et al. (Journal of the American Chemical Society, 2008)
We describe the functionalization of single-wall carbon nanotubes (SWNTs) with 4-(2-trimethylsilyl)ethynylaniline and the subsequent attachment of a zinc-phthalocyanine (ZnPc) derivative using the reliable Huisgen 1,3-dipolar cycloaddition. The motivation of this study was the preparation of a nanotube-based platform which allows the facile fabrication of more complex functional nanometer-scale structures, such as a SWNT-ZnPc hybrid. The nanotube derivatives described here were fully characterized by a combination of analytical techniques such as Raman, absorption and emission spectroscopy, atomic force and scanning electron microscopy (AFM and SEM), and thermogravimetric analysis (TGA). The SWNT-ZnPc nanoconjugate was also investigated with a series of steady-state and time-resolved spectroscopy experiments, and a photoinduced communication between the two photoactive components (i.e., SWNT and ZnPc) was identified. Such beneficial features lead to monochromatic internal photoconversion efficiencies of 17.3% when the SWNT-ZnPc hybrid material was tested as photoactive material in an ITO photoanode. © 2008 American Chemical Society.

Facile route to morphologically tailored silver patches on colloidal particles - Klupp Taylor, Robin; Bao, Huixin; Vasylyev, Serhiy; Peukert, Wolfgang; Tian, Chenting (Langmuir, 2010)
Here we demonstrate, for the first time, the heterogeneous nucleation and growth of silver patches on submicrometer silica spheres. While patches can be grown directly onto native silica particles, it is shown that a higher patch yield can be obtained by first treating the silica with a mixture of an alkanolamine and silver nitrate. Variation of the pretreatment and subsequent coating reactions allowed the patch yield, number, size, thickness, and shape to be adjusted. The patchy particles were shown to possess plasmon modes extending from the visible into the near-IR region, making these structures highly interesting for both their asymmetric morphological and functional properties.
Keywords: tailored; particle; colloid; heterogeneous; nucleation

Facile synthesis and photovoltaic applications of a new alkylated bismethano fullerene as electron acceptor for high open circuit voltage solar cells - Baran, Derya; Erten-Ela, Sule; Kratzer, Andreas; Ameri, Tayebeh; Brabec, Christoph; Hirsch, Andreas (RSC Advances, 2015)
It is crucial to control the lowest unoccupied molecular orbital (LUMO) of electron accepting materials for producing efficient charge transfer in bulk heterojunction (BHJ) solar cells. Due to their high LUMO level, soluble bis-adducts of C60 are of high interest for improving the Voc in BHJ solar cells. In this work, we have developed a novel bis-4-propylpentyl[6,6]methanofullerene bis-adduct, NCBA, using a alkyl solubilizing group. The optoelectronic, electrochemical and photovoltaic properties of this bis-product are investigated. NCBA is successfully applied as the electron acceptor with poly(3-hexylthiophene) (P3HT) in a BHJ solar cell showing a high Voc of 0.73 V.
Keywords: Engineering controlled terms: Charge transfer; Heterojunctions; Molecular orbitals; Open circuit voltage; Solar power generation Bulk heterojunction (BHJ); Electron acceptor; Electron-accepting; Lowest unoccupied molecular orbital; Methanofullerenes; Photovoltaic applications; Photovoltaic property; Poly-3-hexylthiophene Engineering main heading: Solar cells

Facile synthesis and post-processing of eco-friendly, highly conductive copper zinc tin sulphide nanoparticles - Ahmad, Rameez; Distaso, Monica; Azimi, Seyed Hamed; Brabec, Christoph; Peukert, Wolfgang (Journal of Nanoparticle Research, 2013)
Cu2ZnSnS4 (CZTS) nanoparticles have shown promising properties to be used as an energy harvesting material. They are usually synthesised under inert atmosphere or vacuum, whereas the subsequent step of film formation is carried out under an atmosphere of sulphur and/or Sn in order to avoid the decomposition of CZTS nanoparticles into binary and ternary species as well as the formation of the corresponding oxides. In the present paper we show that both the synthesis of CZTS nanoparticles and the film formation from the corresponding suspension can be considerably simplified. Namely, the synthesis can be carried out without controlling the atmosphere, whereas during the film annealing a nitrogen atmosphere is sufficient to avoid the depletion of the CZTS kesterite phase. Furthermore, an integrated approach including in-depth Raman analysis is developed in order to deal with the challenges associated with the characterization of CZTS nanoparticles in comparison to bulk systems. The formation of competitive compounds during the synthesis such as binary and ternary sulphides as well as metal oxides nanoparticles is discussed in detail. Finally, the as-produced films have ten times higher conductivity than the state of the art.
Keywords: By-products; Defects; Kesterite; Raman spectroscopy; Stannite CZTS nanoparticles; Thermodynamic study

Facilitated Lattice Oxygen Depletion in Consolidated TiO2 Nanocrystal Ensembles: A Quantitative Spectroscopic O2 Adsorption Study - Elser, Michael J.; Diwald, Oliver (Journal of Physical Chemistry C, 2012)
On TiO2 nanocrystal powders, pressure-induced consolidation and subsequent annealing generates mesopores inside the pellet and leads to the depletion of specific surface area. This loss corresponds to the introduction of particle–particle interfaces and grain boundaries. In previous work, they were found to be susceptible to facilitated lattice oxygen depletion as compared to free particle surfaces during vacuum annealing. For the first time, we determined the adsorption of molecular oxygen on these consolidated and nonstoichiometric nanocrystal samples. As a function of repeated cycles of vacuum annealing and O2 exposure, the amount of paramagnetic and diamagnetic surface oxygen species was determined using mass spectroscopy in combination with FT-IR, electron paramagnetic resonance (EPR), and UV–vis–NIR diffuse reflectance.
EAM Import::2019-03-14

Failure mechanisms of a hydrogenated amorphous carbon coating in load-scanning tests - Hetzner, Harald; Schaufler, Jens; Tremmel, Stephan; Durst, Karsten; Wartzack, Sandro (Surface & Coatings Technology, 2012)
A diamond-like carbon (DLC) coating with a top layer of pure hydrogenated amorphous carbon (a-C:H) and an interlayer of tungsten-modified hydrogenated amorphous carbon (a-C:H:W) was deposited onto polished cylindrical specimens of a hardened and tempered cold work tool steel. On a load-scanning test rig, tribological-mechanical tests under dry conditions with DLC coated specimens sliding against identical, but uncoated specimens were performed. Additionally, comparative tests with DLC sliding against DLC and tool steel sliding against tool steel were carried out. During each test cycle, the normal load was gradually increased from 13 to 350. N, corresponding to a Hertzian contact pressure of 1.0 to 3.0. GPa. The coefficient of friction was monitored as a function of the normal load, with a significant increase in friction indicating failure of the coating. The tests were repeated and stopped at different total numbers of load cycles. After the tests, a FIB-assisted microscopical analysis in terms of wear and damage of the DLC coating was performed, revealing the (subsurface) failure mechanisms. For DLC sliding against steel, the coating fails within only few load cycles; first tribologically and after that mechanically. Failure is initiated by adherence and subsequent transfer of steel of the counter body. Below adhered steel flakes, tensile cracks form in the a-C:H top layer, with sharp crack edges removing even more steel from the counter body. In following load cycles, coating fragments are being pulled out at these spots, representing the final failure mode. In contrast, for DLC sliding against DLC, no coating failure and also no significant wear are observed, even after a considerably higher number of load cycles. © 2012 Elsevier B.
UnivIS-Import:2015-03-09:Pub.2012.tech.FT.FT-KLMEFK.failur
Keywords: a-C:H; Coating failure; Diamond-like carbon; Load-scanning test

Failure mechanisms of a tungsten-modified hydrogenated amorphous carbon coating in load-scanning tests - Hetzner, Harald; Schaufler, Jens; Tremmel, Stephan; Durst, Karsten; Wartzack, Sandro (Surface & Coatings Technology, 2012)
Polished cylindrical specimens of a hardened and tempered cold work steel were coated with a tungsten-modified hydrogenated amorphous carbon coating (a-C:H:W). On a load-scanning test rig, tribological-mechanical tests under dry conditions with coated specimens sliding against identical, but uncoated specimens were performed. Additionally, comparative tests with a-C:H:W coated specimens sliding against a-C:H:W coated specimens were carried out. During each test cycle, the normal load was gradually increased from 13 to 350. N (Hertzian contact pressure of 1.0 to 3.0. GPa). The coefficient of friction was monitored as a function of the normal load, with a significant increase in friction indicating failure of the coating. These tests were repeated and stopped at different total numbers of load cycles. After the tests, a FIB-assisted microscopical analysis in terms of wear and damage of the coating was performed, revealing the (subsurface) failure mechanisms. The results obtained show that at Hertzian pressures of more than 2. GPa and after about eight load cycles, failure of the a-C:H:W coating is initiated by locally accumulated adhesion of steel flakes, repeatedly occurring at random contact pressures in this load range. The adherence of steel goes along with a local increase in friction, inducing the formation of small shear cracks in the coating top layer. Continued crack formation finally results in partial delamination of coating material and disruption of the surface, in affected areas only. Reference tests with the pairing a-C:H:W/a-C:H:W underline that, very much unlike pure hydrogenated amorphous carbon (a-C:H), a-C:H:W is a rather ductile material with the ability to stick to itself. A discussion of this study's results against the background of a similarly conducted previous study of an a-C:H-based coating system reveals some similarities, but also significant differences in the tribological and mechanical behaviour of both types of amorphous carbon. © 2012 Elsevier B.
UnivIS-Import:2015-03-09:Pub.2012.tech.FT.FT-KLMEFK.failur_1
Keywords: a-C:H:W; Coating failure; Diamond-like carbon; Load-scanning test

Fast and Slow Ligand Exchange at the Surface of Colloidal Gold Nanoparticles - Dinkel, Rebecca; Braunschweig, Björn; Peukert, Wolfgang (Journal of Physical Chemistry C, 2016)
Applications of gold nanoparticles often demand that the particle's ligand shell is modified after particle formation. Obviously, there is a great need for a molecular understanding of this process which is often not accessible in situ. Here, we have applied second-harmonic light scattering (SHS) to investigate the ligand exchange at the surface of colloidal gold nanoparticles in situ and in real time. We demonstrate that the ligand exchange at the surface of citrate-covered Au nanoparticles with 3-mercapto-1-propanesulfonate (MPS) must be described by a fast (<100 s) and a slow reaction process (<23 min), which can be attributed to MPS adsorption on low- and high-coordinated Au surface sites. Using a modified Langmuir isotherm, the average Gibbs free energy of adsorption extgreekDG (-46 kJ/mol) and the surface coverage extgreekG ( 3.5 extgreekmmol/m2) for MPS on Au nanoparticles were determined. The latter was found to be much smaller compared to planar gold surfaces which points to coadsorption of MPS with citrate on high-coordinated sites, i.e., Au terraces. On more reactive low-coordinated Au sites, i.e., edge sites, citrate is easily replaced by MPS. In fact, we find that a substantial portion (49%) of the surface-adsorbed MPS is present on these low-coordinated sites.
EAM Import::2019-03-14

Fast formation of aligned high-aspect ratio TiO2 nanotube bundles that lead to increased open circuit voltage when used in dye sensitized solar cells - Jha, Himendra; Roy, Poulomi; Hahn, Robert; Paramasivam, Indhumati; Schmuki, Patrik (Electrochemistry Communications, 2011)
Ultrafast formation of high-aspect ratio titanium dioxide nanotube is carried out by the rapid-breakdown-anodizing technique using a two compartment electrochemical cell. The change in surface morphology of the titanium substrate during anodization, and physical parameters such as pH and temperature are investigated. Under optimized conditions, aligned TiO2 nanotube powder is obtained that beneficially can be used in dye-sensitized solar cells. Solar cell constructed using the nanotube powder can show a significantly higher open circuit voltage than when constructed with commercially available nanoparticles.
EAM Import::2019-03-14

Fast production of monolithic carbide-derived carbons with secondary porosity produced by chlorination of carbides containing a free metal phase - Schmirler, Martina; Knorr, Tilman; Fey, Tobias; Lynen, Arthur; Greil, Peter; Etzold, Bastian (Carbon, 2011)
Hierarchical structured carbide-derived carbons (CDC) are produced by high temperature chlorination of silicon carbides containing free silicon (Si/SiC). The influence of free silicon in the precursor carbide on the resulting pore and carbon structure and production rate is studied. The two phases - free silicon and silicon carbide - of Si/SiC gives the possibility to synthesize a monolithic carbon with the typical microporous character and narrow pore size distribution combined with larger voids in the micrometer range, while the carbon structure itself stays unchanged. The study revealed that using Si/SiC material increases the production rate for carbide-derived carbons dramatically, due to higher reactive surface area and lower mass transfer limitations, which allows for the time effective production of larger monoliths. (C) 2011 Elsevier Ltd. All rights reserved.

Fast synthesis of optimal chemical reactor networks based on a universal system representation - Xie, Mingquan; Freund, Hannsjörg (Chemical Engineering and Processing, 2018)
We propose a new approach for the synthesis of optimal chemical reactor networks based on the concept of elementary process functions [1] and dynamic optimization. This approach is capable of incorporating process intensification options at an early design stage. Only one single differential equation is required to describe the mass balance of the whole reactor system along the reaction route. This equation contains terms describing mass fluxes from outside the reactor system and mass fluxes which flow between different positions of the reaction route. We demonstrate that different reactor types and different reactor connections can be represented by such a formulation. After solving an optimization problem using such a model, optimal profiles of state variables and fluxes are obtained. Based on these profiles, the optimal reactor system is determined. After the presentation of the theoretical background and the derivation of the new approach, it is illustrated in case studies and validated by comparison with literature results. The advantages of this new approach presented in our contribution are its simplicity in description and implementation, universality in representing different reactor types and networks, and the potential to consider different intensification and integration options at an early design stage.
Keywords: Reactor design; Process design; Optimization; Reactor network; Process intensification

FE2AT - Finite element informed atomistic simulations - Möller, Johannes; Prakash, Aruna; Bitzek, Erik (Modelling and Simulation in Materials Science and Engineering, 2013)
Atomistic simulations play an important role in advancing our understanding of the mechanical properties of materials. Currently, most atomistic simulations are performed using relatively simple geometries under homogeneous loading conditions, and a significant part of the computer time is spent calculating the elastic response of the material, while the focus of the studies lies usually on the mechanisms of plastic deformation and failure. Here we present a simple but versatile approach called FE2AT to use finite element calculations to provide appropriate initial and boundary conditions for atomistic simulations. FE2AT allows us to forgo the simulation of large parts of the elastic loading process, even in the case of complex sample geometries and loading conditions. FE2AT is open source and can be used in combination with different atomistic simulation codes and methods. Its application is demonstrated using the bending of a nano-beam and the determination of the displacement field around a crack tip as examples.
WW1::963

Few-Layer Antimonene by Liquid-Phase Exfoliation - Gibaja, Carlos; Rodriguez-San-Miguel, David; Ares, Pablo; Gomez-Herrero, Julio; Varela, Maria; Gillen, Roland; Maultzsch, Janina; Hauke, Frank; Hirsch, Andreas; Abellan Saez, Gonzalo; Zamora, Felix (Angewandte Chemie International Edition, 2016)
We report on a fast and simple method to produce highly stable isopropanol/water (4:1) suspensions of few-layer antimonene by liquid-phase exfoliation of antimony crystals in a process that is assisted by sonication but does not require the addition of any surfactant. This straightforward method generates dispersions of few-layer antimonene suitable for on-surface isolation. Analysis by atomic force microscopy, scanning transmission electron microscopy, and electron energy loss spectroscopy confirmed the formation of high-quality few-layer antimonene nanosheets with large lateral dimensions. These nanolayers are extremely stable under ambient conditions. Their Raman signals are strongly thickness-dependent, which was rationalized by means of density functional theory calculations.
Keywords: Antimonene; Atomic force microscopy; Liquid-phase exfoliation; Raman spectroscopy; Two-dimensional materials

Field-driven pattern formation in nematic liquid crystals: mesoscopic simulations of electroconvection - Lee, Kuang-Wu; Pöschel, Thorsten (RSC Advances, 2017)
As an environment for rich pattern formation, the electroconvection (EC) of nematic liquid crystals (LCs) is studied via fully nonlinear simulations for the first time. Previously, EC was mostly studied by experiments or by linear/weakly nonlinear hydrodynamic theory for its instability criteria. While the negative dielectric LCs are used in most EC analytical and experimental investigations, EC with positive dielectric LCs is limited to experiments only, due to their more complex nonlinear behavior. In this work we take a step beyond the existing weakly nonlinear EC research by using a fully nonlinear particle-based simulation. To investigate the distinct dynamics of positive and negative dielectric LCs, we modified the molecular potential in the LC stochastic rotational model (LC SRD) [Lee et al., J. Chem. Phys., 2015, 142, 164110] to incorporate the dielectric characteristics and the field-particle interaction. As a result, different convection patterns known in the EC experiments were observed in our simulations, for which those patterns appeared orderly, as a function of external field strength. The simulated director and flow fields correspond to each other well, as found in our experiments. For the positive dielectric LC, we discovered a net directional flow accompanying the travelling EC rolls. This numerical model and its hydrodynamic analysis could be used for precise flow control at the micro-scale, such as nematic colloidal transportation in microfluidics.

FIMOR: An efficient simulation for ZnO quantum dot ripening applied to the optimization of nanoparticle synthesis - Haderlein, Michael; Segets, Doris; Gröschel, Michael; Pflug, Lukas; Leugering, Günter; Peukert, Wolfgang (Chemical Engineering Journal, 2015)
This work presents the application of a Fully Implicit Method for Ostwald Ripening (FIMOR) for simulating the ripening of ZnO quantum dots (QDs). Its stable numerics allow FIMOR to employ the full exponential term of the Gibbs–Thomson equation which significantly outperforms the common Taylor-approximation at typical QD sizes below 10 nm. The implementation is consistent with experimental data for temperatures between 10 and 50 °C and the computational effort is reduced by a factor of 100–1000 compared to previous approaches. This reduced the simulation time on a standard PC from several hours to a few minutes. In the second part, we demonstrate the high potential and accuracy of FIMOR by its application to several challenging studies. First, we compare numeric results obtained for ripening of ZnO QDs exposed to temperature ramps with experimental data. The deviation between simulation and experiment in the mean volume weighted particle size was as small as 5%. Second, a map for the process parameter space spanned by ripening time and temperature is created based on a large number (>50) of FIMOR runs. From this map appropriate process parameters to adjust a desired dispersity are easily deduced. Further data analysis reveals in agreement with literature findings that the particle size distribution converges towards a self-preserving stable shape. Equations describing the time dependent particle size distribution with high accuracy are presented. Finally, we realized the transfer from low volume batch experiments to continuous QD processing. We modeled the continuous ZnO synthesis in a fully automated microreaction plant and found an excellent agreement between the numeric prediction and the experimental results by considering the residence time distribution.

Finding Auxetic Frameworks in Periodic Tessellations - Mitschke, Holger; Schwerdtfeger, Jan; Schury, Fabian; Stingl, Michael; Körner, Carolin; Singer, Robert; Robins, Vanessa; Mecke, Klaus; Schröder-Turk, Gerd (Advanced Materials, 2011)
It appears that most models for micro-structured materials with auxetic deformations were found by clever intuition, possibly combined with optimization tools, rather than by systematic searches of existing structure archives. Here we review our recent approach of finding micro-structured materials with auxetic mechanisms within the vast repositories of planar tessellations. This approach has produced two previously unknown auxetic mechanisms, which have Poisson's ratio v(ss) = -1 when realized as a skeletal structure of stiff incompressible struts pivoting freely at common vertices. One of these, baptized Triangle-Square Wheels, has been produced as a linear-elastic cellular structure from Ti-6Al-4V alloy by selective electron beam melting. Its linear-elastic properties were measured by tensile experiments and yield an effective Poisson's ratio v(LE) approximate to -0.75, also in agreement with finite element modeling. The similarity between the Poisson's ratios v(SS) of the skeletal structure and v(LE) of the linear-elastic cellular structure emphasizes the fundamental role of geometry for deformation behavior, regardless of the mechanical details of the system. The approach of exploiting structure archives as candidate geometries for auxetic materials also applies to spatial networks and tessellations and can aid the quest for inherently three-dimensional auxetic mechanisms.

Finite auxetic deformations of plane tessellations - Mitschke, Holger; Robins, Vanessa; Mecke, Klaus; Schröder-Turk, Gerd (Proceedings of the Royal Society A-Mathematical Physical and Engineering Sciences, 2013)
We systematically analyse the mechanical deformation behaviour, in particular Poisson's ratio, of floppy bar-and-joint frameworks based on periodic tessellations of the plane. For frameworks with more than one deformation mode, crystallographic symmetry constraints or minimization of an angular vertex energy functional are used to lift this ambiguity. Our analysis allows for systematic searches for auxetic mechanisms in archives of tessellations; applied to the class of one- or two-uniform tessellations by regular or star polygons, we find two auxetic structures of hexagonal symmetry and demonstrate that several other tessellations become auxetic when retaining symmetries during the deformation, in some cases with large negative Poisson ratios nu < -1 for a specific lattice direction. We often find a transition to negative Poisson ratios at finite deformations for several tessellations, even if the undeformed tessellation is infinitesimally non-auxetic. Our numerical scheme is based on a solution of the quadratic equations enforcing constant edge lengths by a Newton method, with periodicity enforced by boundary conditions.
Keywords: Poisson's ratio; strain amplification; tilings and tessellations; isostaticity; skeletal frameworks; cellular structures

First results on laboratory nano-CT with a needle reflection target and an adapted toolchain - Stahlhut, P.; Dremel, K.; Dittmann, J.; Engel, J. M.; Zabler, S.; Hoelzing, A.; Hanke, R. (Proceedings of SPIE, 2016)
Recently, we introduced a nano Computed Tomography (nano-CT) system based on a customized JEOL scanning electron microscope applying the principle of shadow microscopy and yielding a spatial resolution of approximately 3000lp/mm. The system has been upgraded and now comprises a photon counting PIXIRAD-2 detector as well as a customized nano-positioning stage for object and electron target. The latter is a tungsten needle with a tip radius of 100nm produced by electrochemical etching. Here we present for the first time nano-CT volume images of microstructures within an AlCu29 sample recorded by the upgraded system (XRM-II). The quality of the iteratively reconstructed and regularized volumes is assessed by means of detail visibility and line spread. We found the spatial resolution to be at least 300nm. The image processing chain, in particular geometric misalignment correction is of critical importance for a successful nano-CT measurement with the XRM-II.
EAM Import::2019-03-14

Flame annealing effects on self-organized TiO2 nanotubes - Mazare, Anca Valentina; Paramasivam, Indhumati; Schmidt-Stein, Felix; Lee, Kiyoung; Demetrescu, Ioana; Schmuki, Patrik (Electrochimica Acta, 2012)
In the present work we study the modification of ordered TiO2 nanotube layers by flame annealing. We investigate different type of nanotubes (with variation in tube length, diameter, and grown in different electrolytes). We vary flame annealing parameters and analyze the morphology, phase composition and the resulting photoelectrochemical properties of the modified TiO2 nanotubes. The results show that flame annealing allows the crystallization of TiO2 nanotubes to anatase or rutile within seconds. The crystallization times strongly depend on the nanotube length and flame parameters. Samples from the outer cone show typically carbon burn off, while in the inner cone carbon accumulation occurs. The resulting photo electrochemical properties are to a large extent determined by the phase composition of the nanotubes (and not by the carbon content). Additionally, it may be noted that although the flame annealing process is fast and cheap, we could not achieve a maximum UV photoelectrochemical response comparable to classic furnace annealing.
EAM Import::2019-03-14

Flexible Copper-7,7,8,8 Tetracyanochinodimethane Memory Devices - Operation, Cross Talk and Bending - Novak, Michael; Burkhardt, Martin; Jedaa, Abdesselam; Halik, Marcus (Thin Solid Films, 2010)
Non-volatile memory devices based on the charge transfer complex copper-7,7,8,8 tetracyanochinodimethane were fabricated on ridged and flexible substrates with special emphasis on their general functionality and cross talk behaviour in 4 × 4 passive matrix arrays. Device characteristics have been investigated at elevated temperatures during operation (ranging from room temperature to 120 °C) under ambient conditions without encapsulation. To explore the influence of mechanical stress on device performance, the memory cells on flexible polyethylene terephthalate substrates were bended during operation, up to a convex and concave radius of 4 mm. The detected shift of the switching voltages and the decrease of reliability can be attributed to stress induced cracks in the active layer.

Flexible organic tandem solar modules with 6% efficiency: combining roll-to-roll compatible processing with high geometric fill factors - Spyropoulos, Georgios; Kubis, Peter; Li, Ning; Baran, Derya; Lucera, Luca; Salvador, Michael Filipe; Ameri, Tayebeh; Voigt, Monika; Krebs, F. C.; Brabec, Christoph (Energy and Environmental Science, 2014)
Organic solar cell technology bears the potential for high photovoltaic performance combined with truly low-cost, high-volume processing. Here we demonstrate organic tandem solar modules on flexible substrates fabricated by fully roll-to-roll compatible processing at temperatures <70 °C. By using ultrafast laser patterning we considerably reduced the “dead area” of the modules and achieved geometric fill factors beyond 90%. The modules revealed very low interconnection-resistance compared to the single tandem cells and exhibited a power conversion efficiency of up to 5.7%. Bending tests performed on the modules suggest high mechanical resilience for this type of device. Our findings inform concrete steps towards high efficiency photovoltaic applications on curved, foldable and moving surfaces.
Keywords: Fill factor; Roll to Roll; Solar module GEOBASE Subject Index: photovoltaic system; resilience; solar power; volume

Flow cytometry for intracellular SPION quantification: specificity and sensitivity in comparison with spectroscopic methods - Friedrich, Ralf P.; Janko, Christina; Poettler, Marina; Tripal, Philipp; Zaloga, Jan; Cicha, Iwona; Dürr, Stephan; Nowak, Johannes; Odenbach, Stefan; Slabu, Ioana; Liebl, Maik; Trahms, Lutz; et al. (International Journal of Nanomedicine, 2015)
Due to their special physicochemical properties, iron nanoparticles offer new promising possibilities for biomedical applications. For bench to bedside translation of super-paramagnetic iron oxide nanoparticles (SPIONs), safety issues have to be comprehensively clarified. To understand concentration-dependent nanoparticle-mediated toxicity, the exact quantification of intracellular SPIONs by reliable methods is of great importance. In the present study, we compared three different SPION quantification methods (ultraviolet spectrophotometry, magnetic particle spectroscopy, atomic adsorption spectroscopy) and discussed the shortcomings and advantages of each method. Moreover, we used those results to evaluate the possibility to use flow cytometric technique to determine the cellular SPION content. For this purpose, we correlated the side scatter data received from flow cytometry with the actual cellular SPION amount. We showed that flow cytometry provides a rapid and reliable method to assess the cellular SPION content. Our data also demonstrate that internalization of iron oxide nanoparticles in human umbilical vein endothelial cells is strongly dependent to the SPION type and results in a dose-dependent increase of toxicity. Thus, treatment with lauric acid-coated SPIONs (SEON(LA)) resulted in a significant increase in the intensity of side scatter and toxicity, whereas SEON(LA) with an additional protein corona formed by bovine serum albumin (SEON(LA-BSA)) and commercially available Rienso(®) particles showed only a minimal increase in both side scatter intensity and cellular toxicity. The increase in side scatter was in accordance with the measurements for SPION content by the atomic adsorption spectroscopy reference method. In summary, our data show that flow cytometry analysis can be used for estimation of uptake of SPIONs by mammalian cells and provides a fast tool for scientists to evaluate the safety of nanoparticle products.
EVALuna2:19955

Fluctuations in surface pH of maturing rat incisor enamel are a result of cycles of H(+)-secretion by ameloblasts and variations in enamel buffer characteristics - Damkier, Helle H.; Josephsen, Kaj; Takano, Yoshiro; Zahn, Dirk; Fejerskov, Ole; Frische, Sebastian (Bone, 2014)
It is disputed if ameloblasts in the maturation zone of the enamel organ mainly buffer protons released by hydroxyapatite (HA) crystal growth or if they periodically secrete protons to create alternating acidic and alkaline conditions. The latter hypothesis predicts alternating pH regimes in maturing enamel, which would be affected by pharmacological interference with ameloblast H+-secretion. This study tests these predictions. Colorimetric pH-indicators and ratiometric fluorometry were used to measure surface pH in maturation zone enamel of rat incisors. Alternating acidic (down to pH 6.24 ± 0.06) and alkaline zones (up to pH 7.34 ± 0.08) were found along the tooth coinciding with ameloblast morphological cycles. Underlying the cyclic pattern, a gradual decrease in pH towards the incisal edge was seen. Vinblastine or FR167356 (H+-ATPase-inhibitor) disturbed ameloblast acid-secretion, especially in the early parts of acidic zones. Enamel surface pH reflects the titration state of surface PO43–ions. At the pH-values observed, PO43- would be protonated (pKa > 12) and HA dissolved. However, by molecular dynamics simulations we estimate the pKa of HPO42- at an ideal HA surface to be 4.3. The acidic pH measured at the enamel surface may thus only dissolve non-perfect domains of HA crystals in which PO43- is less electrostatically shielded. During repeated alkaline/acidic cycles, near-perfect HA-domains may therefore gradually replace less perfect HA-domains resulting in near-perfect HA-crystals. In conclusion, cyclic changes in ameloblast H+-secretion and the degree of enamel maturation determine enamel surface pH. This is in accordance with a hypothesis implicating H+-ATPase mediated acid-secretion by ameloblasts.
EAM Import::2019-03-14

Fluid dynamic characterisation of porous solids in catalytic fixed-bed reactors - Horneber, Tobias; Rauh, Cornelia; Delgado, Antonio (Microporous and Mesoporous Materials, 2012)
3D numerical simulations of structured open-cell foams, more precisely the tetrakaidecahedron geometry (Kelvin cell geometry) are presented. The velocity and pressure field for a single cell with varied surface roughness and the whole reactor with the same pore size are calculated in order to show the influence of the surface roughness regarding the pressure drop and the influence of the length of the whole reactor regarding the velocity development, respectively. The numerical results are compared using dimensionless numbers such as Reynolds number, Euler number and friction factor. © 2011 Elsevier Inc. All rights reserved.
UnivIS-Import:2015-03-09:Pub.2012.tech.ITC.stmmec.fluidd
Keywords: CFD of metal foam; Kelvin cell geometry; Numerical simulations; Pressure drop; Surface roughness

Fluid flow simulation on the Cell Broadband Engine using the lattice Boltzmann method - Stürmer, Markus; Götz, Jan; Richter, G.; Dörfler, Arnd; Rüde, Ulrich (Computers & Mathematics with Applications, 2009)
In this paper we present a fast lattice Boltzmann fluid solver that has been performance optimized and tailored for the Cell Broadband Engine Architecture. Many design decisions were motivated by the long range objective to simulate blood flow in human blood vessels, especially in aneurysms, but have proven to be much more generally applicable. After explaining implementation details and how they were influenced by the target platform, the performance and memory requirements of this prototype solver are evaluated. © 2009 Elsevier Ltd. All rights reserved.
UnivIS-Import:2015-03-09:Pub.2009.tech.IMMD.lsinfs.fluidf
Keywords: Aneurysm; Blood flow; CBEA; Cell processor; Hemodynamics; High performance computing; Lattice Boltzmann

Fluidization of a horizontally driven granular monolayer - Heckel, Michael; Sack, Achim; Kollmer, Jonathan; Pöschel, Thorsten (Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2015)
We consider the transition of a horizontally vibrated monodisperse granular monolayer between its condensed state and its three-dimensional gaseous state as a function of the vibration parameters, amplitude, and frequency as well as particle number density. The transition is characterized by an abrupt change of the dynamical state which leaves its fingerprints in several measurable quantities including dissipation rate, sound emission, and a gap size which characterizes the sloshing motion of the material. The transition and its pronounced hysteresis is explained through the energy due to the collective motion of the particles relative to the container.

Fluids in Extreme Confinement - Franosch, Thomas; Lang, Simon; Schilling, Rolf (Physical Review Letters, 2012)
For extremely confined fluids with a two-dimensional density n in slit geometry of an accessible width L, we prove that in the limit Li̊ghtarrow0, the lateral and transversal degrees of freedom decouple, and the latter become ideal-gas-like. For a small wall separation, the transverse degrees of freedom can be integrated out and renormalize the interaction potential. We identify nL2 as the hidden smallness parameter of the confinement problem and evaluate the effective two-body potential analytically, which allows calculating the leading correction to the free energy exactly. Explicitly, we map a fluid of hard spheres in extreme confinement onto a 2D fluid of disks with an effective hard-core diameter and a soft boundary layer. Two-dimensional phase transitions are robust and the transition point experiences a shift O(nL2).
EAM Import::2019-03-14

Fluorescence-based remote irradiation sensor in liquid-filled hollow-core photonic crystal fiber - Zeltner, Richard; Bykov, D. S.; Xie, S.; Euser, T. G.; Russell, Philip St. John (Applied Physics Letters, 2016)
We report an irradiation sensor based on a fluorescent ``flying particle'' that is optically trapped and propelled inside the core of a water-filled hollow-core photonic crystal fiber. When the moving particle passes through an irradiated region, its emitted fluorescence is captured by guided modes of the fiber core and so can be monitored using a filtered photodiode placed at the fiber end. The particle speed and position can be precisely monitored using in-fiber Doppler velocimetry, allowing the irradiation profile to be measured to a spatial resolution of 10 extgreekmm. The spectral response can be readily adjusted by appropriate choice of particle material. Using dye-doped polystyrene particles, we demonstrate detection of green (532 nm) and ultraviolet (340 nm) light.
EAM Import::2019-03-14

Fluorine implantation for effective work function control in p-type metal-oxide-semiconductor high-k metal gate stacks - Fet, A.; Haeublein, V.; Bauer, A. J.; Ryssel, H.; Frey, Lothar (Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics, 2011)
The instability of the p-type metal effective work function of high-k/metal gate metal-oxide-semiconductor stacks after high temperature treatment results in device threshold voltage shifts and is one of the big challenges for the gate-first integration of high-k dielectrics in the future complementary metal-oxide semiconductor process flow. The exact cause of this instability is a subject of intense debate. In this article, it is shown that by implanting the gate stack with a fluorine dose of 1015 cm-2, it is possible to achieve an appropriate silicon valence band-edge effective work function of 5.1 eV for p-type metal-oxide-semiconductor devices. It is also shown that the fluorine doping can be accomplished not only with F+, but also with BF2+ ions. The influence of the implantation energy on the obtained effective work function is demonstrated and discussed. The origin of the induced shift is also discussed. Leakage current measurements show that the leakage properties of high-k stacks are not worsened by F implantation, while the implantation of BF2 slightly affects the leakage currents.

Flux saturation length of sediment transport - Ribeiro Parteli, Eric Josef; Pähtz, Thomas; Kok, Jasper F.; Herrmann, Hans J. (Physical Review Letters, 2013)
Sediment transport along the surface drives geophysical phenomena as diverse as wind erosion and dune formation. The main length scale controlling the dynamics of sediment erosion and deposition is the saturation length L , which characterizes the flux response to a change in transport conditions. Here we derive, for the first time, an expression predicting L as a function of the average sediment velocity under different physical environments. Our expression accounts for both the characteristics of sediment entrainment and the saturation of particle and fluid velocities, and has only two physical parameters which can be estimated directly from independent experiments. We show that our expression is consistent with measurements of L in both aeolian and subaqueous transport regimes over at least 5 orders of magnitude in the ratio of fluid and particle density, including on Mars. © 2013 American Physical Society.

Flying particle microlaser and temperature sensor in hollow-core photonic crystal fiber - Zeltner, Richard; Pennetta, Riccardo; Xie, Shangran; Russell, Philip St. John (Optics Letters, 2018)
Whispering-gallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dye-doped microparticle within a liquid-filled hollow-core photonic crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into core-guided modes that can be detected at the endfaces of the HC-PCF. Using radiation forces, the microlaser can be freely propelled along the HC-PCF over multi-centimeter distances-orders of magnitude farther than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperature-dependent frequency shift of the lasing modes, and may also permit precise delivery of light to remote locations.
EAM Import::2019-03-14

Foaming of thin films of a fluorinated ethylene propylene copolymer using supercritical carbon dioxide - Zirkel, Larissa; Jakob, Martin; Münstedt, Helmut (Journal of Supercritical Fluids, 2009)
The foaming of films of a fluorinated ethylene propylene copolymer (FEP) using supercritical carbon dioxide (scCO_2) was investigated. For this purpose, a one-step foaming process was applied to the films inside a pressure vessel at temperatures in excess of 200 °C and pressures up to 30 MPa. The films prepared show a homogeneous cellular structure with thin compact polymer layers on both sides. The foaming behaviour of the FEP films is determined in dependence on the exposure time of the polymer to the scCO_2, the processing temperature and pressure as well as on the pressure drop rate during the foaming process. The resulting density and the foam morphology can be related to the pressure level during the saturation process. The size of the cells generated mainly depends on the foaming temperature, whereas their number per volume is determined by the magnitude of the applied pressure. Furthermore, the pressure drop rate influences the size of the cells in a way that significantly fewer but larger bubbles are found for longer times of depressurizing. This finding can be explained by nucleation rates becoming the smaller the slower the rate of pressure decrease.
EAM Import::2019-03-14

Focused electron beam based direct-write fabrication of graphene and amorphous carbon from oxo-functionalized graphene on silicon dioxide - Schindler, Severin; Vollnhals, Florian; Halbig, Christian Eberhard; Marbach, Hubertus; Steinrück, Hans-Peter; Papp, Christian; Eigler, Siegfried (Physical Chemistry Chemical Physics, 2017)
Controlled patterning of graphene is an important task towards device fabrication and thus is the focus of current research activities. Graphene oxide (GO) is a solution-processible precursor of graphene. It can be patterned by thermal processing. However, thermal processing of GO leads to decomposition and CO2 formation. Alternatively, focused electron beam induced processing (FEBIP) techniques can be used to pattern graphene with high spatial resolution. Based on this approach, we explore FEBIP of GO deposited on SiO2. Using oxo-functionalized graphene (oxo-G) with an in-plane lattice defect density of 1% we are able to image the electron beam-induced effects by scanning Raman microscopy for the first time. Depending on electron energy (2-30 keV) and doses (50-800 mC m(-2)) either reduction of GO or formation of permanent lattice defects occurs. This result reflects a step towards controlled FEBIP processing of oxo-G.

Forces and shapes as determinants of micro-swimming: effect on synchronisation and the utilisation of drag - Pande, Jayant; Smith, Ana-Suncana (Soft Matter, 2015)
In this analytical study we demonstrate the richness of behaviour exhibited by bead-spring micro-swimmers, both in terms of known yet not fully explained effects such as synchronisation, and hitherto undiscovered phenomena such as the existence of two transport regimes where the swimmer shape has fundamentally different effects on the velocity. For this purpose we employ a micro-swimmer model composed of three arbitrarily-shaped rigid beads connected linearly by two springs. By analysing this swimmer in terms of the forces on the different beads, we determine the optimal kinematic parameters for sinusoidal driving, and also explain the pusher/puller nature of the swimmer. Moreover, we show that the phase difference between the swimmer's arms automatically attains values which maximise the swimming speed for a large region of the parameter space. Apart from this, we determine precisely the optimal bead shapes that maximise the velocity when the beads are constrained to be ellipsoids of a constant volume or surface area. On doing so, we discover the surprising existence of the aforementioned transport regimes in micro-swimming, where the motion is dominated by either a reduction of the drag force opposing the beads, or by the hydrodynamic interaction amongst them. Under some conditions, these regimes lead to counter-intuitive effects such as the most streamlined shapes forming locally the slowest swimmers.
Keywords: LOW-REYNOLDS-NUMBER; MOTORS; CILIA; DRIVEN; CARGO

Formability of accumulative roll bonded aluminum AA1050 and AA6016 investigated using bulge tests - Topic, Irena; Höppel, Heinz Werner; Staud, Detlev; Merklein, Marion; Geiger, Manfred; Göken, Mathias (Advanced Engineering Materials, 2008)
The paper reports one of the very first attempts to investigate the formability of ultrafine- grained aluminum sheets produced by a severe plastic deformation process known as accumulative roll bonding. During hydraulic bulge testing the samples showed a tendency to higher achievable burst pressures and/or von Mises equivalent strains with increasing number of accumulative roll bonding cycles, indicating promising deformation behaviour and good formability.
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Formability of ultrafine-grained AA6016 sheets processed by accumulative roll bonding - Hausöl, Tina; Schmidt, Christian; Maier, Verena; Böhm, Wolfgang; Nguyen, Manh Hung; Merklein, Marion; Höppel, Heinz Werner; Göken, Mathias (Key Engineering Materials, 2012)
Aluminium alloy AA6016 was accumulative roll bonded up to eight cycles in order to produce an ultrafine-grained microstructure. The formability of these sheets was investigated by means of bending tests. Furthermore the influence of a local laser heat treatment at the bending edge is observed. The strength of the UFG samples is increased by a factor of around two compared to the conventionally grained T4 condition which also results in up to 50 % higher punch forces needed for bending of ARB processed samples. An anisotropic bending behaviour is observed. By applying a tailored laser heat treatment along the bending edge prior to the bending tests a local recrystallization and recovery at the deformation zone of the samples is achieved. Thus, ductility is increased locally whereby bending to an angle of 80° is possible with lower forming forces compared to the non-heat treated specimens. The results are compared to previous studies on mechanical properties and formability investigations of ARB processed AA6016.
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Formation and Decomposition of CO2 Intercalated Graphene Oxide - Eigler, Siegfried; Dotzer, Christoph; Hirsch, Andreas; Enzelberger-Heim, Michael; Müller, Paul (Chemistry of Materials, 2012)
The formation, stability, and decomposition of CO intercalated graphene oxide was analyzed by FTIR, TGAMS, TGA-IR, AFM, and SEM for the first time. We found that the formation starts at 50 °C and develops up to 120 °C. The formation process can be best observed by FTIR spectroscopy, and the product is stable at ambient conditions. At higher temperatures, the decomposition of CO intercalated graphene oxide occurs due to the release of water, CO , and CO that can be monitored by TGA-MS and TGA-IR analysis. AFM and SEM images can visualize the formation of blisters in GO films that become instable at 210 °C. We further prepared graphene oxide with a low water-content and found that the formation of CO was significantly suppressed and CO became the major species responsible for the weight loss. In addition we prepared OH treated graphene oxide to elucidate the formation process of CO and found C O O by TGA-MS analysis that proves the crucial role of water during CO formation. From these experiments we propose that hydrate species are key-intermediates for the formation of CO . Hence, it seems likely that rearrangement reactions that can proceed via hydrate intermediates, known from organic chemistry, are probably responsible for the formation of carboxylic acids at the edges of graphene oxide sheets after sonication of graphite oxide. Further, our investigations prove that graphene oxide is less stable than shown by TGA measurements. This has a high impact on the electronic properties of reduced graphene oxide, especially for all those using it for electronic applications. © 2012 American Chemical Society.

Formation and Dissolution of Twin ZnO Nanostructures Promoted by Water and Control over Their Emitting Properties - Distaso, Monica; Mackovic, Mirza; Spiecker, Erdmann; Peukert, Wolfgang (Chemistry - A European Journal, 2014)
By using ZnO as a model system, the formation of twinned nanostructures has been investigated under microwave irradiation, exploiting experimental conditions ranging from purely solvothermal when N,N-dimethylformamide was used, to purely hydrothermal when water was the solvent. A progressive increase in size, elongation and roughness of the surface was observed with increasing water content in the solvent mixture. Particular attention was paid to the reactivity of the ZnO surfaces towards dissolution. Our results show that the formation of twinned nanorods is a dynamic process and that the coupling interphase itself is highly reactive. Consequently, the twinned rods undergo a number of complex dissolution processes that are responsible for the appearance of a wide distribution of defects either on the surface or inside the structure. Poly(N-vinyl pyrrolidone) influences the photoluminescent properties of the as-synthesised materials and allows control of the ratio of the intensity of the UV and visible emission.

Formation and Stability of Phenylphosphonic Acid Monolayers on ZnO: Comparison of In Situ and Ex Situ SAM Preparation - Ostapenko, Alexandra; Klöffel, Tobias; Meyer, Bernd; Witte, Gregor (Langmuir, 2016)
Self-assembled monolayers (SAMs) enable an electronic interface tailoring of conductive metal oxides and offer an alternative to common transparent electrodes in optoelectronic devices. Here, the influence of surface orientation and pretreatment on the formation and stability of SAMs has been studied for the case of phenylphosphonic acid (PPA) on ZnO single crystals. Using thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray adsorption fine structure spectroscopy (NEXAFS) and density-functional theory (DFT) calculations, the thermal stability and orientational ordering of PPA-SAMs on the polar and mixed-terminated ZnO surfaces were analyzed. On all surfaces, PPA-SAMs remain stable up to 550 K, while at higher temperatures a C–P bond cleavage and dissociative desorption takes place yielding two distinct desorption peaks. Based on DFT calculations, these desorption channels are attributed to protonated and deprotonated chemisorbed PPA molecules, which can be related to tri- and bidentate species, hence allowing to determine their relative abundance from the intensity ratio. Beside immersion, an alternative monolayer preparation based on vacuum deposition in combination with controlled desorption of excess multilayers is demonstrated. This enables a SAM preparation on bare ZnO surfaces without any precoating due to exposure to ambient air, which is further compared with SAM formation on intentionally hydroxylated substrates. Corresponding TDS data indicate that initial hydroxylation favors the formation of tridentate and deprotonated bidentate, while the OMBD preparation on bare surfaces yields a larger fraction of protonated bidentate species. The orientation of PPA molecules adopted in the SAMs was determined from the dichroism of K-edge NEXAFS measurements and reveals an almost upright orientation for the deprotonated species, while a slight tilting is obtained for monolayer films with a large fraction of protonated bidentate molecules.
EAM Import::2019-03-14

Formation and structure of ionic (Na, Zn) sulfophosphate glasses - Da, Ning; Grassmé, Oliver; Nielsen, Karsten Hansgaard; Peters, Gerhard; Wondraczek, Lothar (Journal of Non-Crystalline Solids, 2011)
Supercooled ortho- and pyrophosphate melts of the type P2O5-EO-A2O (E = Zn2+, Ca2+, Sr2+, Ba2+, Mg2+, Mn2+; A = Li+, Na+, K+) exhibit unusually high sulfate solubility. This enables facile fabrication of low-melting sulfophosphate glasses with SO3 content of up to 25 mol.%. Sulfur is incorporated into those glasses practically exclusively in the form of isolated SO42- groups. The Q-group distribution of phosphate species is dominated by Q1 and Q0 groups, whereby replacing P2O5 with SO3 results in increasing depolymerization. In this way, the glass forming region can readily be extended to compounds with an average number of bridging oxygen per phosphate anion of less than 0.5. In ionic (Na, Zn) sulfophosphate glasses, cations Zn2+ and Na2+ appear to cluster distinctively around PO43- and SO42- groups, respectively.
EAM Import::2019-03-14

Formation of aligned nanoporous/nanotubular layers of vanadium oxy-nitrides - Yang, Yang; Kirchgeorg, Robin; Hahn, Robert; Schmuki, Patrik (Electrochemistry Communications, 2014)
Self-organized vanadium oxy-fluoride nanotubular or nanoporous layers were grown by anodization of vanadium in a [BF4]- electrolyte. By a thermal conversion of this amorphous VOxFy precursor structure in ammonia gas, aligned vanadium oxy-nitride (VOxNy) nanoporous/nanotubular layers could be formed. We established conditions to reach conversion of precursor structures to a well crystallized VOxNy nanotube structure, without the formation of any secondary phases or the collapse of the ordered morphology. Such vanadium-nitride based structures have considerable potential in supercapacitor applications.
EAM Import::2019-03-14

Formation of anodic TiO2 nanotube or nanosponge morphology determined by the electrolyte hydrodynamic conditions - Sanchez-Tovar, Rita; Lee, Kiyoung; Garcia-Anton, Jose; Schmuki, Patrik (Electrochemistry Communications, 2013)
The present work studies the effect of hydrodynamic conditions on the growth of anodic TiO2 nanostructures on Ti in a glycerol/water/NH4F electrolyte. Parameter screening for fluoride content, anodization voltage and rotation rate (Reynolds number) of a rotating anode showed that two distinctly different TiO2 morphologies could be obtained: the classic ordered nanotubes or a nanoscale sponge layer. We present conditions for TiO2 sponge formation and growth to several micrometers of thickness, and show that in specific cases sponge layers can be superior to tube morphologies, as illustrated for photoelectrochemical water-splitting under standard AM 1.5 conditions.
EAM Import::2019-03-14

Formation of a Non-Thickness-Limited Titanium Dioxide Mesosponge and its Use in Dye-Sensitized Solar Cells - Kim, Dam-Hee; Lee, Kiyoung; Roy, Poulomi; Birajdar, Balaji; Spiecker, Erdmann; Schmuki, Patrik (Angewandte Chemie International Edition, 2009)
Etch a sketch: Robust TiO_2 mesoporous layers can be fabricated on Ti surfaces by a combination of non-thickness-limited anodization and selective etching. Layers more than 50 extgreekmm thick with highly regular feature and pore sizes in the range 5–10 nm can be produced (see picture, FTO=fluorine doped tin oxide). The layers are highly flexible, well-adhered, and can be used directly after appropriate annealing in efficient dye-sensitized solar cells.
Keywords: dyes/pigments; mesoporous materials; solar cells; titanates; titanium

Formation of a protective layer during IEC(b) test of epoxy resin loaded with silica nanoparticles - Meichsner, Christian; Clark, Timothy; Groeppel, Peter; Winter, Benjamin; Butz, Benjamin; Spiecker, Erdmann (IEEE Transactions on Dielectrics and Electrical Insulation, 2012)
The erosion resistance of materials against partial discharges (PD) is a useful first indicator of their suitability for use as high voltage insulators. Epoxy resin-based composites loaded with different concentrations of silica nanoparticles were exposed to high voltage stress via an IEC(b) electrode. Laser scanning analysis was used to investigate the eroded volume and the erosion depth. The aged samples were investigated by transmission electron microscopy (TEM) analysis and scanning transmission electron microscopy (STEM), which show the formation of a protective layer of partially fused nanoparticles. The results suggest that improved erosion resistance against PD using the IEC(b) electrode is caused by the in situ formation of a protective layer consisting of sintered nanoparticles.

Formation of CdS in Supramolecular Dendrimer-Dye Assemblies: Electrostatic and Electrostatic-Coordination Templating - Düring, Jasmin; Butz, Benjamin; Spiecker, Erdmann; Gröhn, Franziska (Macromolecules, 2015)
A novel concept for the formation of organic inorganic hybrid nanostructures based on a tunable supra-molecular templating mechanism involving several types of noncovalent interactions is presented. Different organic inorganic hybrid structures with narrow size distributions composed of a cationic dendrimer, an oppositely charged divalent diazo dye, and CdS can be created: First, larger supramolecular structures consisting of several 100 to 1000 dendrimers is built based on ionic and pi-pi interaction. Second, with this self-assembled template, the hybrid nanostructure forms based on electrostatics and coordination interaction. The influence on shape and internal structure of the supramolecular hybrid architectures was investigated. A size control of the assemblies is possible in a range of 60-420 nm. The prospect to direct self-assembly and templating through the controlled combination of different noncovalent interactions potentially allows to create tailor-made systems for various applications.

Formation of highly charged quasi-molecular ions of a polycationic [60]fullerene hexakis-adduct and their fragmentation behavior in the gas phase - Li, Jing; Nye, Leanne C.; Wasserthal, Lennard; Vinh, Chau; Kirschbaum, Rolf; Ivanovic-Burmazovic, Ivana; Hirsch, Andreas; Drewello, Thomas (European Journal of Organic Chemistry, 2015)
A novel polycationic [60]fullerene hexakis-adduct has been synthesized and investigated by electrospray ionization and tandem mass spectrometry. The polycationic ligand system comprises 12 pre-formed positive charges, compensated in the neutral molecule by bromide anions. Stable quasi-molecular cations were obtained through the release of the anionic counter ions covering a charge-state envelope from 3+ to 12+. Collision-induced dissociation experiments revealed three fragmentation pathways. A dominant neutral loss channel leads to daughter ions of the same charge state as the highly charged precursor ion and singly charged daughter ions can be expelled from the polycations through two distinct dissociation channels involving both the neutral and the charge-carrying part of their ligands.
Keywords: Electrospray ionization; Fullerenes; Mass spectrometry; Polycations

Formation of Highly Ordered Nanochannel Nb Oxide by Self-Organizing Anodization - Lee, Kiyoung; Yang, Yang; Yang, Mingjun; Schmuki, Patrik (Chemistry - A European Journal, 2012)
A highly aligned nanochannel Nb oxide structure can be formed by self organizing anodization of Nb in a K2HPO4-containing glycerol electrolyte at elevated temperatures (180 °C). Under optimized electrochemical conditions, a high degree of ordering can be achieved. These layers can be used as supercapacitors.
EAM Import::2019-03-14

Formation of Highly Ordered VO2 Nanotubular/Nanoporous Layers and Their Supercooling Effect in Phase Transitions - Yang, Yang; Lee, Kiyoung; Zobel, Mirijam; Mackovic, Mirza; Unruh, Tobias; Spiecker, Erdmann; Schmuki, Patrik (Advanced Materials, 2012)
The fabrication of self-organized VO2 nanotubular/nanoporous layers is demonstrated by self-organizing anodization, followed by a suitable heat treatment. These VO2 layers show a reversible metal to insulator transition (MIT) at 70 and 44 °C, when heating and cooling, respectively.
Keywords: vanadium dioxide; nanoporous layers; anodization; supercooling; metal-insulator transition

Formation of liquid crystalline phases in aqueous suspensions of platelet-like tripalmitin nanoparticles - Schmiele, Martin; Gehrer, Simone; Westermann, Martin; Steiniger, Frank; Unruh, Tobias (Journal of Chemical Physics, 2014)
Suspensions of platelet-like shaped tripalmitin nanocrystals stabilized by the pure lecithin DLPC and the lecithin blend S100, respectively, have been studied by small-angle x-ray scattering (SAXS) and optical observation of their birefringence at different tripalmitin (PPP) concentrations phi(PPP). It could be demonstrated that the platelets of these potential drug delivery systems start to form a liquid crystalline phase already at pharmaceutically relevant concentrations phi(PPP) of less than 10 wt. %. The details of this liquid crystalline phase are described here for the first time. As in a previous study [A. Illing et al., Pharm. Res. 21, 592 (2004)] some platelets are found to self-assemble into lamellar stacks above a critical tripalmitin concentration phi(st)(PPP) of 4 wt. %. In this study another critical concentration phi(lc)(PPP) approximate to 7 wt. % for DLPC and phi(lc)(PPP) approximate to 9 wt. % for S100 stabilized dispersions, respectively, has been observed phi(lc)(PPP) describes the transition from a phase of randomly oriented stacked lamellae and remaining non-assembled individual platelets to a phase in which the stacks and non-assembled platelets exhibit an overall preferred orientation. A careful analysis of the experimental data indicates that for concentrations above phi(lc)(PPP) the stacked lamellae start to coalesce to rather small liquid crystalline domains of nematically ordered stacks. These liquid crystalline domains can be individually very differently oriented but possess an overall preferred orientation over macroscopic length scales which becomes successively more expressed when further increasing phi(PPP). The lower critical concentration for the formation of liquid crystalline domains of the DLPC-stabilized suspension compared to phi(lc)(PPP) of the S100-stabilized suspension can be explained by a larger aspect ratio of the corresponding tripalmitin platelets. A geometrical model based on the excluded volumes of individual platelets and stacked lamellae has been developed and successfully applied to reproduce the critical volume fractions for both, the onset of stack formation and the appearance of the liquid crystalline phase. (C) 2014 AIP Publishing LLC.

Formation of Mefenamic Acid Nanocrystals with Improved Dissolution Characteristics - Konnerth, Christoph-Günther; Braig, Veronika; Schmidt, Jochen; Lee, Geoffrey; Peukert, Wolfgang; Ito, Atsutoshi (Chemie Ingenieur Technik, 2017)
An industrially feasible formulation approach combining media milling and spray-drying was applied to improve dissolu- tion characteristics of the poorly soluble drug mefenamic acid (MA). The approach was studied for two MA polymorphs at different stressing and pH conditions. It was found that final MA product particle sizes are rather determined by the solid-liquid equilibrium than by mechanical fracture. Obtained drug particles are only composed of the most stable poly- morph. Direct compressed tablets containing MA nanocrystals exhibit a significant improvement of in vitro dissolution kinetics as compared to tablets with micronized drug particles.
UnivIS-Import:2017-12-18:Pub.2017.nat.dchph.lphte.format
Keywords: Dissolution rate enhancement; Media milling; Nanocrystal; Polmorphism; Poorly water-soluble drug; Spray drying

Formation of Self-Organized Superlattice Nanotube Arrays - Embedding Heterojunctions into Nanotube Walls - Wei, Wei; Jha, Himendra; Yang, Guang; Hahn, Robert; Paramasivam, Indhumati; Berger, Steffen; Spiecker, Erdmann; Schmuki, Patrik (Advanced Materials, 2010)
In the present work we introduce the growth of 3D-superlattice nanotubes by a simple anodization technique. Highly defined self-organized, oxide tubes with modulated tube wall composition can be created that allow altering and tuning optical, electrical, and chemical properties of such nanometer-size architectures. The image shows a stacked nanotube array with built-in TiO2/Ta-2 O-5 heterojunctions.

Formation of `single walled' TiO2 nanotubes with significantly enhanced electronic properties for higher efficiency dye-sensitized solar cells - Mirabolghasemi, Hamed; Liu, Ning; Lee, Kiyoung; Schmuki, Patrik (Chemical Communications, 2013)
The present work establishes a process for the anodic growth of self-organized TiO2 nanotubes with a single-walled morphology. This yields not only a much more defined tube geometry but also provides a significant increase in properties and prospect for a wide range of TiO2 nanotube applications.
EAM Import::2019-03-14

Formation of the calcium/poly(3-hexylthiophene) interface: Structure and energetics - Zhu, Junfa; Bebensee, Fabian; Hieringer, Wolfgang; Zhao, Wei; Baricuatro, J. H.; Farmer, Jason A.; Bai, Yun; Steinrück, Hans-Peter; Gottfried, Michael; Campbell, C. T. (Journal of the American Chemical Society, 2009)
The adsorption of Ca on poly(3-hexylthiophene) (P3HT) has been studied by adsorption microcalorimetry, atomic beam/surface scattering, X-ray photoelectron spectroscopy (XPS), low-energy He ion scattering spectroscopy (LEIS), and first-principles calculations. The sticking probability of Ca on P3HT is initially 0.35 and increases to almost unity by 5 ML. A very high initial heat of adsorption in the first 0.02 ML (625-500 kJ/mol) is attributed to the reaction of Ca with defect sites or residual contamination. Between 0.1 and 0.5 ML, there is a high and nearly constant heat of adsorption of 405 kJ/mol, which we ascribe to Ca reacting with subsurface sulfur atoms from the thiophene rings of the polymer. This is supported by the absence of LEIS signal for Ca and the shift of the S 2p XPS binding energy by -2.8 eV for reacted S atoms. The heat of adsorption decreases above 0.6 ML coverage, reaching the sublimation enthalpy of Ca, 178 kJ/mol, by 4 ML. This is attributed to the formation of Ca nanoparticles and eventually a continuous solid Ca film, on top of the polymer. LEIS and XPS measurements, which show only a slow increase of the signals related to solid Ca, support this model. Incoming Ca atoms are subject to a kinetic competition between diffusing into the polymer to react with subsurface thiophene units versus forming or adding to three-dimensional Ca clusters on the surface. At ∼1.6 ML Ca coverage, Ca atoms have similar probabilities for either process, with the former dominating at lower coverage. Ultimately about 1.6 ML of Ca (1.2 x 10 atoms/cm) reacts with S atoms, corresponding to a reacted depth of ∼3 nm, or nearly five monomerunit layers. Density-functional theory calculations confirm that the heat of reaction and the shift of the S 2p signal are consistent with Ca abstracting S from the thiophene rings to form small CaS clusters. © 2009 American Chemical Society.

Formation of the Vilsmeier-Haack complex: the performance of different levels of theory - Ozpinar, Gul Altinbas; Kaufmann, Dieter E.; Clark, Timothy (Journal of Molecular Modeling, 2011)
Because of discrepancies in the available experimental data, an extensive theoretical investigation of the formation of the Vilsmeier-Haack (VH) complex has been carried out. The barriers to complex formation calculated using eight different density functional methods (BLYP, B2-PLYP, B3LYP, B3PW91, MPW1K, M06-2X, and PBE1PBE), MP2, and extrapolation techniques (CBS-QB3, G3B3) with several basis sets (6 − 31 + G**, 6 − 311++G**, 6 − 311 + (3df,2p), aug-cc-pVDZ, and aug-cc-pVTZ) were compared with experimental data. For the overall reaction, MP2/aug-cc-pVDZ and M06-2X/6−31 + G(d,p) perform best compared to the CBS techniques. The results help clarify some open mechanistic questions.

Formic Acid-Based Fischer-Tropsch Synthesis for Green Fuel Production from Wet Waste Biomass and Renewable Excess Energy - Albert, Jakob; Wasserscheid, Peter; Jess, Andreas; Kern, Christoph; Pöhlmann, Ferdinand; Glowienka, Kevin (ACS Sustainable Chemistry & Engineering, 2016)
While the production of hydrocarbons by Fischer-Tropsch synthesis (FTS) is a widely recognized, yet technically quite complex way to transform biomass via syngas (mostly from biomass gasification) into liquid fuels, we here present an alternative route transforming biomass first into formic acid (FA) followed by syngas formation by decomposition of FA and finally FTS using regenerative hydrogen (or if needed H2 from the stored FA) to balance the C:H ratio. The new method builds on the recently developed, selective oxidation of biomass to formic acid using Keggin-type polyoxometalates of the general formula (H3+x[PVxMo12-xO40]) as homogeneous catalysts, oxygen as the oxidant, and water as the solvent. This method is able to transform a wide range of complex and wet biomass mixtures into FA as the sole liquid product at mild reaction conditions (90 °C, 20-30 bar O2). We propose to convert FA with hydrogen from water electrolysis–the electrolysis step producing also the oxygen for the biomass oxidation to FA–to green hydrocarbon fuels using a typical Co-based FT catalyst.

Fractal substructure of a nanopowder - Schwager, Thomas; Wolf, Dietrich E.; Pöschel, Thorsten (Physical Review Letters, 2008)
The structural evolution of a nanopowder by repeated dispersion and settling can lead to characteristic fractal substructures. This is shown by numerical simulations of a two-dimensional model agglomerate of adhesive rigid particles. The agglomerate is cut into fragments of a characteristic size , which then are settling under gravity. Repeating this procedure converges to a loosely packed structure, the properties of which are investigated: (a)The final packing density is independent of the initialization, (b) the short-range correlation function is independent of the fragment size, (c) the structure is fractal up to the fragmentation scale with a fractal dimension close to 1.7, and (d) the relaxation time increases linearly with. © 2008 The American Physical Society.

Fractal Substructure of a nanopowder generated by repeated fragmentation and sedimentation: The role of the dust - Topic, Nikola; Wolf, Dietrich E.; Pöschel, Thorsten (Granular Matter, 2016)
Packings of cohesive nanoparticles, that is nano powders, may be obtained as the result of repeated fragmentation–reagglomeration cycles (Schwager et al. in Phys Rev Lett 100:218002, 2008) such that the resulting sediment reveals a fractal structure. The size distribution of the fragments after a fragmentation step is a superposition of a narrow distribution of large particles (chunks) whose size is determined by the cutting length and a power-law distribution for small particles, representing scale invariant dust. It was shown that the exponent of the power-law, τ , is in non-trivial relation to the fractal dimension, d f , via d f (2 − τ ) = 1. This poses the question for the structure of the sediment created by repeated fragmentation–reagglomeration cycles when the dust particles are excluded from the reagglomeration step. We found that even in this case, repeated fragmentation-reagglomeration cycles yield a sediment of fractal structure with slightly reduced fractal dimension while the dust exponent, τ , remains unchanged.

Fractal substructures due to fragmentation and reagglomeration - Wolf, D. E.; Pöschel, Thorsten; Schwager, T.; Weuster, A.; Brendel, L. (6th International Conference on Micromechanics of Granular Media, Powders and Grains 2009, 2009)
Cohesive powders form agglomerates that can be very porous. Hence they are also very fragile. Consider a process of complete fragmentation on a characteristic length scale I, where the fragments are subsequently allowed to settle under gravity. If this fragmentation-reagglomeration cycle is repeated sufficiently often, the powder develops a fractal substructure with robust statistical properties. The structural evolution is discussed for two different models: The first one is an off-lattice model, in which a fragment does not stick to the surface of other fragments that have already settled, but rolls down until it finds a locally stable position. The second one is a simpler lattice model, in which a fragment sticks at first contact with the agglomerate of fragments that have already settled. Results for the fragment size distribution are shown as well. One can distinguish scale invariant dust and fragments of a characteristic size. Their role in the process of structure formation will be addressed. © 2009 American Institute of Physic
Keywords: Fractals macroscopic aggregates; Granular systems; Pattern formation; Porous materials; Powders

Fracture toughness and bond trapping of grain boundary cracks - Möller, Johannes; Bitzek, Erik (Acta Materialia, 2014)
The fracture toughness of materials in which brittle cracks initiate and propagate along grain boundaries (GBs) depends not only on the energy of the GB, but also on its microscopic structure. Exactly how and to what degree brittle GB fracture is affected by the local atomic structure at the crack tip has not yet been studied in detail. Here, we use molecular static simulations to study the atomic-scale fracture behavior of six large-angle tilt GBs in tungsten bicrystals. The fracture toughness depends critically on the propagation direction and on the position of the crack tip within the structural units of the GB. Furthermore, the GB fracture toughness can be significantly larger than for single crystals in the same orientation. These results cannot be explained by the usual thermodynamic approach in continuum-scale fracture mechanics but can be understood by considering the effect of bond trapping of GB cracks. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserve
Keywords: Atomistic simulation; Bond trapping; Fracture; Grain boundary; Tungsten

Fracture toughness evaluation of NiAl single crystals by microcantilevers-a new continuous J-integral method - Ast, Johannes; Merle, Benoit; Durst, Karsten; Göken, Mathias (Journal of Materials Research, 2016)
The fracture toughness of NiAl single crystals is evaluated with a new method based on the J-integral concept. The new technique allows the measurement of continuous crack resistance curves at the microscale by continuously recording the stiffness of the microcantilevers with a nanoindenter. The experimental procedure allows the determination of the fracture toughness directly at the onset of stable crack growth. Experiments were performed on notched microcantilevers which were prepared by focused ion beam milling from NiAl single crystals. Stoichiometric NiAl crystals and NiAl crystals containing 0.14 wt% Fe were investigated in the so-called "hard" orientation. The fracture toughness was evaluated to be 6.4 +/- 0.5 MPa m(1/2) for the stoichiometric sample and 7.1 +/- 0.5 MPa m(1/2) for the iron containing sample, indicating that the addition of iron enhances the ductility. This effect is intensified with ongoing crack propagation where the Fe-containing sample exhibits a stronger crack resistance behavior than the stoichiometric NiAl single crystal. These findings are in good agreement with macroscopic fracture toughness measurements, and validate the new micromechanical testing approach.