Properties of SiO2 and Si3 N4 as gate dielectrics for printed ZnO transistors - Walther, Sabine; Polster, Sebastian; Meyer, Benedikt; Jank, M. P. M.; Ryssel, Heiner; Frey, Lothar (Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics, 2011)
In this article, the authors report on thin film transistors based on gas phase synthesized ZnO nanoparticles using low temperature deposited silicon dioxide and silicon nitride as gate dielectrics. For bottom gate transistors, the devices using silicon nitride as gate insulator show the lowest off-current for a given induced charge and the steepest subthreshold slope. The charge carrier mobility of around 3×10-3 cm2/V s and an Ion/Ioff ratio of around 105 are almost independent of the insulator material. In a double gated thin film transistor using low stress silicon nitride as a top gate insulator, transistor parameters are extracted for the identical semiconducting layer when the bottom and top gates are used. It is shown that the extracted charge carrier mobility is not inherent for the ZnO nanoparticle layer but rather an effective value for the device under investigation.

Protective Layer of Al2O3 Nanocomposites and Surface Composition after Electrical Stress - Meichsner, Christian; Clark, Timothy; Groeppel, Peter; Winter, Benjamin; Butz, Benjamin; Spiecker, Erdmann (IEEE Transactions on Dielectrics and Electrical Insulation, 2015)
The performance of nanocomposites for improved resistance to electrical discharges has resulted in increased interest in nano-loaded thermosetting resins. First assessments of the performance of new systems can be obtained by a plate-electrode construction according to IEC(b). Epoxy resins loaded with different amounts of spherical alumina particles were tested following this procedure and parameters such as erosion depth and standardized volume analyzed. Transmission electron microscopy (TEM) and a three-dimensional (3D) reconstruction using electron tomography were used to determine the particle size distribution. TEM analysis after preparation of the eroded samples indicates the advantages and disadvantages of using alumina nanoparticles to improve the erosion resistance and showed a novel effect of the surface-grafted polysiloxanes. A comparison with conventional nano-silica-loaded resins reveals that the lower sintering tendency of Al2O3 is important.
Keywords: Alumina; epoxy resin; erosion resistance; nanoparticles; transmission electron microscopy; electron tomography

Protein adsorption on magnesium and its alloys: A review - Höhn, Sarah; Virtanen, Sannakaisa; Boccaccini, Aldo R. (Applied Surface Science, 2019)
Keywords: Corrosion; Adsorption; Magnesium; Interface; Proteins

Proteomic Analysis of Human Bladder Epithelial Cells by 2D Blue Native SDS-PAGE Reveals TCDD-Induced Alterations of Calcium and Iron Homeostasis Possibly Mediated by Nitric Oxide - Verma, Nisha; Pink, Mario; Petrat, Frank; Rettenmeier, Albert W.; Schmitz-Spanke, Simone (Journal of Proteome Research, 2015)
A proteomic analysis of the interaction among multiprotein complexes involved in 2,3,7,8-dibenzo-p-dioxin (TCDD)-mediated toxicity in urinary bladder epithelial RT4 cells was performed using two-dimensional blue native SDS-PAGE (2D BN/SDS-PAGE). To enrich the protein complexes, unexposed and TCDD-exposed cells were fractionated. BN/SDS-PAGE of the resulting fractions led to an effective separation of proteins and protein complexes of various origins, including cell membrane, mitochondria, and other intracellular compartments. Major differences between the proteome of control and exposed cells involved the alteration of many calcium-regulated proteins (calmodulin, protein S100-A2, annexin A5, annexin A10, gelsolin isoform b) and iron-regulated proteins (ferritin, heme-binding protein 2, transferrin). On the basis of these findings, the intracellular calcium concentration was determined, revealing a significant increase after 24 h of exposure to TCDD. Moreover, the concentration of the labile iron pool (LIP) was also significantly elevated in TCDD-exposed cells. This increase was strongly inhibited by the calmodulin (CaM) antagonist W-7, which pointed toward a possible interaction between iron and calcium signaling. Because nitric oxide (NO) production was significantly enhanced in TCDD-exposed cells and was also inhibited by W-7, we hypothesize that alterations in calcium and iron homeostasis upon exposure to TCDD may be linked through NO generated by CaM-activated nitric oxide synthase. In our model, we propose that NO produced upon TCDD exposure interacts with the iron centers of iron-regulatory proteins (IRPs) that modulate the alteration of ferritin and transferrin, resulting in an augmented cellular LIP and, hence, increased toxicity.
EVALuna2:9715

Prototyping of catalyst pore-systems by a combined synthetic, analytical and computational approach: Application to mesoporous TiO2 - Novak, Vladimir; Ortel, Erik; Winter, Benjamin; Butz, Benjamin; Paul, Benjamin; Koci, Petr; Marek, Milos; Spiecker, Erdmann; Kraehnert, Ralph (Chemical Engineering Journal, 2014)
We propose a strategy that enables virtual prototyping and computational optimization of the pore system of a solid catalyst. The method combines template-assisted synthesis of porous metal oxides with tunable pore-space morphology, 3D imaging of the catalysts nanostructure by the state-of-the-art electron tomography in HAADF STEM mode, and multi-scale mathematical modeling that provides computational evaluation of the pore size distribution, effective diffusivity and tortuosity in the reconstructed system as well as macroscopic performance of the catalytic layer in terms of reactant conversion. This general approach is demonstrated on mesoporous TiO2 layers. (c) 2014 Elsevier B.V. All rights reserved.
Keywords: Catalyst; Nanocasting; Synthesis; Electron tomography; Multi-scale mathematical modeling; Diffusion

Pseudomorphic transformation of amorphous calcium carbonate films follows spherulitic growth mechanisms and can give rise to crystal lattice tilting - Harris, Joseph; Mey, Ingo; Hajir, M; Mondeshki, Mihail; Wolf, Stephan (Crystengcomm, 2015)
Amorphous calcium carbonate films synthesized by the polymer-induced liquid-precursor (PILP) process convert into crystallographically complex calcite spherulites. Tuning the experimental parameters allows for the generation of crystal lattice tilting similar to that found in calcareous biominerals. This contribution evidences the role of spherulitic growth mechanisms in pseudomorphic transformations of calcium carbonate.

Pseudotachylyte in muscovite-bearing quartzite: Coseismic friction-induced melting and plastic deformation of quartz - Bestmann, Michel; Pennacchioni, Giorgio; Frank, Gerhard; Göken, Mathias; de Wall, Helga (Journal of Structural Geology, 2011)
Thin (0.5–2 mm thick) pseudotachylyte veins occur within muscovite-bearing ( 10% volume), amphibolite-facies quartzites of the Schneeberg Normal Fault Zone (Austroalpine, Southern Tyrol, Italy). Pseudotachylytes are associated with precursor localized plastic microshear zones (50–150 extgreekmm thick) developed sub-parallel to the host-rock foliation and with conjugate sets oriented at a high angle to the foliation. Such microshear zones are characterized by recrystallization to ultrafine-grained (1–2 extgreekmm grain size) mosaic aggregates of quartz showing a transition from a host-controlled to a random crystallographic preferred orientation towards the shear zone interior. Subsequent coseismic slip mainly exploited these microshear zones. Microstructural analysis provides evidence of extensive friction-induced melting of the muscovite-bearing quartzite, producing a bimodal melt composition. First, the host-rock muscovite was completely melted and subsequently crystallized, mainly as K-feldspar. Then, about 60% volume of the ultrafine-grained quartz underwent melting and crystallized as spherulitic rims (mostly consisting of quartz ± Ti ± Fe) around melt-corroded quartz clasts. The two melts show immiscibility structures in the major injection veins exploiting microshear zones at high angles to the quartzite foliation. In contrast, they were mechanically mixed during flow along the main fault veins.
WW1::896

Pt‒CeO2 catalysts for fuel cell applications: From surface science to electrochemistry - Lykhach, Yaroslava; Brummel, Olaf; Bruix, Albert; Fabris, Stefano; Matolinova, Iva; Matolin, Vladimir; Neyman, Konstantin M.; Libuda, Jörg (2017)
EAM Import::2019-03-08

Pulsed direct flame deposition and thermal annealing of transparent amorphous indium zinc oxide films as active layers in field effect transistors - Kilian, Daniel; Polster, Sebastian; Vogeler, Isabell; Frey, Lothar; Jank, Michael; Peukert, Wolfgang (ACS Applied Materials and Interfaces, 2014)
Indium-zinc oxide (IZO) films were deposited via flame spray pyrolysis (FSP) by pulsewise shooting a Si/SiO2 substrate directly into the combustion area of the flame. Based on UV-vis measurements of thin-films deposited on glass substrates, the optimal deposition parameters with respect to low haze values and film thicknesses of around 100 nm were determined. Thermal annealing of the deposited films at temperatures between 300 and 700 °C was carried out and staggered bottom gate thin-film transistors (TFT) were fabricated. The thin films were investigated by scanning electron microscopy, atomic force microscopy, X-ray diffraction, Fourier transformed infrared spectroscopy, and room-temperature photoluminescence measurements. The outcome of these investigations lead to two major requirements in order to implement a working TFT: (i) organic residues from the deposition process need to be removed and (ii) the net free charge carrier concentration has to be minimized by controlling the trap states in the semiconductor. The optimal annealing temperature was 300 °C as both requirements are fulfilled best in this case. This leads to field effect transistors with a low hysteresis, a saturation mobility of μSat = 0.1 cm2/(V s), a threshold voltage of Vth = -18.9 V, and an Ion/Ioff ratio on the order of 107. Depending on thermal treatment, the defect density changes significantly strongly influencing the transfer characteristics of the device. © 2014 American Chemical Society.
UnivIS-Import:2015-03-09:Pub.2014.tech.IE.LEB.pulsed
Keywords: amorphous metal oxides; direct deposition; field-effect transistor; flame spray pyrolysis; indium-zinc oxide; thermal annealing

Purely entropic self-assembly of the bicontinuous Ia3d gyroid phase in equilibrium hard-pear systems - Schoenhoefer, Philipp W. A.; Ellison, Laurence J.; Marechal, Mattheus; Cleaver, Douglas J.; Schroder-Turk, Gerd E. (Interface Focus, 2017)
We investigate a model of hard pear-shaped particles which forms the bicontinuous Ia[Formula: see text]d structure by entropic self-assembly, extending the previous observations of Barmes et al. (2003 Phys. Rev. E 68, 021708. (doi:10.1103/PhysRevE.68.021708)) and Ellison et al. (2006 Phys. Rev. Lett. 97, 237801. (doi:10.1103/PhysRevLett.97.237801)). We specifically provide the complete phase diagram of this system, with global density and particle shape as the two variable parameters, incorporating the gyroid phase as well as disordered isotropic, smectic and nematic phases. The phase diagram is obtained by two methods, one being a compression-decompression study and the other being a continuous change of the particle shape parameter at constant density. Additionally, we probe the mechanism by which interdigitating sheets of pears in these systems create surfaces with negative Gauss curvature, which is needed to form the gyroid minimal surface. This is achieved by the use of Voronoi tessellation, whereby both the shape and volume of Voronoi cells can be assessed in regard to the local Gauss curvature of the gyroid minimal surface. Through this, we show that the mechanisms prevalent in this entropy-driven system differ from those found in systems which form gyroid structures in nature (lipid bilayers) and from synthesized materials (di-block copolymers) and where the formation of the gyroid is enthalpically driven. We further argue that the gyroid phase formed in these systems is a realization of a modulated splay-bend phase in which the conventional nematic has been predicted to be destabilized at the mesoscale due to molecular-scale coupling of polar and orientational degrees of freedom.
EAM Import::2019-03-08

Pushing efficiency limits for semitransparent perovskite solar cells - Ramírez Quiroz, César Omar; Levchuk, Ievgen; Bronnbauer, Carina; Salvador, Michael Filipe; Forberich, Karen; Heumüller, Thomas; Hou, Yi; Schweizer, Peter; Spiecker, Erdmann; Brabec, Christoph (Journal of Materials Chemistry A, 2015)
While perovskite-based semitransparent solar cells deliver competitive levels of transparency and efficiency to be envisioned for urban infrastructures, the complexity and sensitivity of their processing conditions remain challenging. Here, we introduce two robust protocols for the processing of sub-100 nm perovskite films, allowing fine-tuning of the active layer without compromising the crystallinity and quality of the semiconductor. Specifically, we demonstrate that a method based on solvent-induced crystallization with a rapid drying step affords perovskite solar cells with 37% average visible transmittance (AVT) and 7.8% PCE. This process enhances crystallization with a preferential phase orientation presumably at the interface, yielding a high fill factor of 72.3%. The second method is based on a solvent-solvent extraction protocol, enabling active layer films as thin as 40 nm and featuring room-temperature crystallization in an ambient environment on a few second time span. As a result, we demonstrate a maximum AVT of 46% with an efficiency of 3.6%, which is the highest combination of efficiency and transparency for a full device stack to date. By combining the two methods presented here we cover a broad range of thicknesses vs. transparency values and confirm that solvent-induced crystallization represents a powerful processing strategy toward high-efficiency semitransparent solar cells. Optical simulations support our experimental findings and provide a global perspective of the opportunities and limitations of semitransparent perovskite photovoltaic devices.

Putting a positive spin on molecular bridges - Guldi, Dirk Michael (Angewandte Chemie International Edition, 2010)
Getting the message across: Insight into electronic coupling beween electron donors and acceptors connected by molecular bridges (see example) has been achieved by measuring the magnitude of the spin-spin exchange interactions. A kinetic analysis of the magnetic field effects showed that charge transfer in highly π-conjugated bridges is a function of the spin-selective charge-recombination pathways. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Pyridine on flat Pt(111) and stepped Pt(355) - An in situ HRXPS investigation of adsorption and thermal evolution - Steinrück, Hans-Peter; Welke, Martin; Eilert, Andre; Denecke, Reinhard; Schoeppke, Matthias; Wöckel, Claudia (Journal of Chemical Physics, 2016)
We investigated the adsorption and reaction of pyridine on flat Pt(111) and stepped Pt(355) surfaces via high-resolution in situ x-ray photoelectron spectroscopy. The surfaces were exposed to pyridine at temperatures between 112 and 300 K while simultaneously recording XP spectra. Subsequently, the crystals were annealed and the temperature dependencies of the N 1s and C 1s core levels were studied again in a continuous and quantitative way. Various surface species were found, namely, physisorbed, flat-lying and end-on pyridine, α-pyridyl species on the terraces and on the steps and several unidentified high temperature species. We were able to show an influence of the steps of Pt(355) by pre-adsorbing silver next to the step, which selectively suppresses the step adsorption.

Qualitative analysis of bulk-heterojunction solar cells without device fabrication: An elegant and contactless method - Baran, Derya; Li, Ning; Breton, Anne-Catherine; Osvet, Andres; Ameri, Tayebeh; Leclerc, Mario; Brabec, Christoph (Journal of the American Chemical Society, 2014)
The enormous synthetic efforts on novel solar cell materials require a reliable and fast technique for the rapid screening of novel donor/acceptor combinations in order to quickly and reliably estimate their optimized parameters. Here, we report the applicability of such a versatile and fast evaluation technique for bulk heterojunction (BHJ) organic photovoltaics (OPV) by utilizing a steady-state photoluminescence (PL) method confirmed by electroluminescence (EL) measurements. A strong relation has been observed between the residual singlet emission and the charge transfer state emission in the blend. Using this relation, a figure of merit (FOM) is defined from photoluminescence and also electroluminescence measurements for qualitative analysis and shown to precisely anticipate the optimized blend parameters of bulk heterojunction films. Photoluminescence allows contactless evaluation of the photoactive layer and can be used to predict the optimized conditions for the best polymer-fullerene combination. Most interestingly, the contactless, PL-based FOM method has the potential to be integrated as a fast and reliable inline tool for quality control and material optimization. © 2014 American Chemical Society.
Keywords: Engineering controlled terms: ElectroluminescencePhotoluminescenceSolar cells Compendex keywords Bulk heterojunction (BHJ)Bulk-heterojunction solar cellsCharge transfer stateFigure of merit (FOM)Material optimizationOptimized conditionsOrganic photovoltaicsSolar cell materials Engineering main heading: Heterojunctions

Qualitatively correct charge-transfer excitation energies in HeH + by time-dependent density-functional theory due to exact exchange Kohn-Sham eigenvalue differences - Gimon, Tino; Ipatov, Andrey; Heßelmann, Andreas; Görling, Andreas (Journal of Chemical Theory and Computation, 2009)
Time-dependent density-functional theory in the response regime is shown to yield qualitatively correct charge-transfer excitation energies in the system HeH+ if the exact Kohn-Sham exchange potential is employed to determine the Kohn-Sham orbitals and eigenvalues entering the time-dependent density-functional calculation. The employed exactexchange kernel is frequency-independent and, like conventional kernels in the local density approximation or in generalized gradient approximations, does not contribute to the chargetransfer excitation energy. This shows that it can be that not the exchange-correlation kernel, as generally believed, but the exchange-correlation potential plays the crucial role in the description of charge-transfer excitations. © 2009 American Chemical Society.

Quality control of polymer solar modules by lock-in thermography - Hoppe, Harald; Bachmann, Jonas; Muhsin, Burhan; Druee, Karl-Heinz; Riedel, Ingo; Gobsch, Gerhard; Buerhop-Lutz, Claudia; Brabec, Christoph; Dyakonov, Vladimir (Journal of Applied Physics, 2010)
We have characterized lateral imperfections of photovoltaic modules based on solution processed polymer-fullerene semiconductor blends by means of lock-in thermography (LIT). The active layer of the solar cell modules is based on the heterogeneous organic semiconductor system poly(3-hexylthiophene):phenyl-C -butyric acid methyl ester and the power conversion efficiency of the modules reached nearly 2% under irradiation of an AM 1.5 solar simulator. Applying highly sensitive LIT allowed us to detect several kinds of laterally distributed defects originating from imperfections in the respective functional layers as well as in the quality of encapsulation. We show that LIT is a powerful method for the quality control of large area polymer solar cells and modules, enabling fast feedback for optimization of production parameters. © 2010 American Institute of Physics.

Quantitative analysis of a wind energy conversion model - Zucker, Florian; Strunz, Andreas; Gräbner, Anna; Meyn, Jan-Peter (European Journal of Physics, 2015)
A rotor of 12 cm diameter is attached to a precision electric motor, used as a generator, to make a model wind turbine. Output power of the generator is measured in a wind tunnel with up to 15 m s-1 air velocity. The maximum power is 3.4 W, the power conversion factor from kinetic to electric energy is cp = 0.15. The v3 power law is confirmed. The model illustrates several technically important features of industrial wind turbines quantitatively.
UnivIS-Import:2015-04-14:Pub.2015.nat.dphy.PI.PDP.quanti
Keywords: Betz factor; demonstration experiment; model; renewable energy; wind energy; wind turbine

Quantitative broadband chemical sensing in air-suspended solid-core fibers - Euser, Tijmen; Chen, J. S. Y.; Scharrer, Michael; Russell, Philip St. John; Farrer, N. J.; Sadler, P. J. (Journal of Applied Physics, 2008)
We demonstrate a quantitative broadband fiber sensor based on evanescent-field sensing in the cladding holes of an air-suspended solid-core photonic crystal fiber. We discuss the fabrication process, together with the structural and optical characterization of a range of different fibers. Measured mode profiles are in good agreement with finite element method calculations made without free parameters. The fraction of the light in the hollow cladding can be tuned via the core diameter of the fiber. Dispersion measurements are in excellent agreement with theory and demonstrate tuning of the zero dispersion wavelength via the core diameter. Optimum design parameters for absorption sensors are discussed using a general parameter diagram. From our analysis, we estimate that a sensitivity increase of three orders of magnitude is feasible compared to standard cuvette measurements. Our study applies to both liquid and gas fiber sensors. We demonstrate the applicability of our results to liquid chemical sensing by measuring the broad absorption peak of an aqueous NiCl2 solution. We find excellent agreement with the reference spectrum measured in a standard cuvette, even though the sample volume has decreased by three orders of magnitude. Our results demonstrate that air-suspended solid-core photonic crystal fibers can be used in quantitative broadband chemical-sensing measurements.
EAM Import::2019-03-12

Quantitative Determination and Comparison of the Surface Binding of Phosphonic Acid, Carboxylic Acid, and Catechol Ligands on TiO2 Nanoparticles - Zeininger, Lukas; Portilla Berlanga, Luis; Halik, Marcus; Hirsch, Andreas (Chemistry - A European Journal, 2016)
The adsorption, desorption, co-adsorption, and exchange behavior of phosphonic acid, carboxylic acid, and catechol derivatives on the surface of titanium oxide (anatase) nanoparticles are investigated. Thermogravimetric analysis provides a facile and fast-track quantitative determination of the wet-chemical monolayer adsorption constants and grafting densities of ten adsorbates, all under neutral pH conditions. This characterization protocol allows straightforward quantification of the relevant thermodynamic data of ligand adsorption and a comparison of ligand adsorption strengths. The reported procedure is proposed as a universal tool and it should be applicable to many other colloidal metal oxide materials. Moreover, the determined values for the adsorption constants and the monolayer grafting densities provide a toolbox for the assessment of the adsorbates' behavior in desorption, exchange, and co-adsorption equilibria. This versatile evaluation procedure will help to identify optimal monolayer-surface combinations and to evaluate critical parameters, such as monolayer robustness, ligand exchange rates, or targeted mixed assembly of functionalities.

Quantitative evaluation of nanoparticle classification by size-exclusion chromatography - Süß, Sebastian; Metzger, Christoph; Damm, Cornelia; Segets, Doris; Peukert, Wolfgang (Powder Technology, 2018)
Although few attempts for classification of nanoparticles (NPs) in labscale do exist, the transfer to industrial applications is still challenging. One promising separation method, which is already established for biological molecules, is chromatography. Herein, we study the classification of differently sized gold NPs (AuNPs) by size-exclusion chromatography (SEC). First, we investigated the interactions of AuNPs with potential stationary phases in order to identify a suitable material for the chromatographic process where irreversible NP adhesion is excluded. Then, we demonstrate the high reproducibility of our SEC experiments by multiple sample injections that lead to constant peak areas. In particular, we show the size-dependent elution behavior of AuNP mixtures resulting in bimodal elution peaks, where size separation was confirmed by inline measured UV/Vis spectra. Finally, NP classification results by using a fraction collector are characterized by retention time, mass balances and size-dependent separation efficiencies. The adjustment of the particle size distributions (PSDs) is demonstrated by changing the switching time of the fraction collector. Our study evidences the high potential of SEC for preparative and continuous separation of NPs.
Keywords: (Gold) particles; Separation efficiency; Chromatography; Separation

Quantitative evaluation of size selective precipitation of Mn-doped ZnS quantum dots by size distributions calculated from UV/Vis absorbance spectra - Segets, Doris; Komada, So; Butz, Benjamin; Spiecker, Erdmann; Mori, Yasushige; Peukert, Wolfgang (Journal of Nanoparticle Research, 2013)
We demonstrate the quantitative evaluation of the sharp classification of manganese-doped zinc sulfide (ZnS:Mn) quantum dots by size selective precipitation. The particles were characterized by the direct conversion of absorbance spectra to particle size distributions (PSDs) and high-resolution transmission electron micrographs (HRTEM). Gradual addition of a poor solvent (2-propanol) to the aqueous colloid led to the flocculation of larger particles. Though the starting suspension after synthesis had an already narrow PSD between 1.5 and 3.2 nm, different particle size fractions were subsequently isolated by the careful adjustment of the good solvent/poor solvent ratio. Moreover, due to the fact that for the analysis of the classification results the size distributions were available, an in-depth understanding of the quality of the distinct classification steps could be achieved. From the PSDs of the feed, as well as the coarse and the fine fractions with their corresponding yields determined after each classification step, an optimum after the first addition of poor solvent was identified with a maximal separation sharpness κ as high as 0.75. Only by the quantitative evaluation of classification results leading to an in-depth understanding of the relevant driving forces, a future transfer of this lab scale post-processing to larger quantities will be possible.

Quantitative investigation of the fragmentation process and defect density evolution of oxo-functionalized graphene due to ultrasonication and milling - Halbig, Christian Eberhard; Nacken, Thomas; Walter, Johannes; Damm, Cornelia; Eigler, Siegfried; Peukert, Wolfgang (Carbon, 2016)
In the view of technical and medical applications we examined the effect of ultrasonication and ball milling of oxo-functionalized graphene bearing an almost intact carbon lattice (oxo-G) with respect to the lateral flake size reduction and defect formation in the carbon lattice for the first time. The energy input was screened for both methods and quantitative changes in lateral flake size distribution were characterized in solution by analytical ultracentrifugation (AUC) supported by atomic force microscopy (AFM). For the first time, we applied statistical Raman spectroscopy to reliably evaluate the formation of lattice defects with processing time. Surprisingly, the density of lattice defects remains unaffected with energy inputs below a limiting threshold. Furthermore, for both processing techniques, the size reduction scales with the energy input according to a power law function, which is well known for emulsification and mechanical comminution processes. With the results of this study it is possible to precisely predetermine the lateral size of flakes of oxo-G upon sonication or milling processing, preserving the carbon lattice. Understanding this controlled formation of oxo-G flakes enables target oriented technical, medical and electronic applications with tailored properties.
EAM Import::2019-03-13

Quantitative material analysis by dual-energy computed tomography for industrial NDT applications - Nachtrab, Frank; Weis, S.; Kessling, P.; Sukowski, F.; Hassler, U.; Fuchs, T.; Uhlmann, N.; Hanke, R. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2011)
Dual-energy computed tomography (DECT) is an established method in the field of medical CT to obtain quantitative information on a material of interest instead of mean attenuation coefficients only. In the field of industrial X-ray imaging dual-energy techniques have been used to solve special problems on a case-by-case basis rather than as a standard tool. Our goal is to develop an easy-to-use dual-energy solution that can be handled by the average industrial operator without the need for a specialist. We are aiming at providing dual-energy CT as a measurement tool for those cases where qualitative images are not enough and one needs additional quantitative information (e.g. mass density extgreekr and atomic number Z) about the sample at hand. Our solution is based on an algorithm proposed by Heismann et al. (2003) [1] for application in medical CT . As input data this algorithm needs two CT data sets, one with low (LE) and one with high effective energy (HE). A first order linearization is applied to the raw data, and two volumes are reconstructed thereafter. The dual-energy analysis is done voxel by voxel, using a pre-calculated function F(Z) that implies the parameters of the low and high energy measurement (such as tube voltage, filtration and detector sensitivity). As a result, two volume data sets are obtained, one providing information about the mass density extgreekr in each voxel, the other providing the effective atomic number Z of the material therein. One main difference between medical and industrial CT is that the range of materials that can be contained in a sample is much wider and can cover the whole range of elements, from hydrogen to uranium. Heismann's algorithm is limited to the range of elements Z=1–30, because for Z>30 the function F(Z) as given by Heismann is not a bijective function anymore. While this still seems very suitable for medical application, it is not enough to cover the complete range of industrial applications. We therefore investigated the possibilities of extending the afore mentioned approach to dual-energy imaging to a wider range of materials.
EAM Import::2019-03-13

Quantum-dot-sensitized solar cells: Understanding linker molecules through theory and experiment - Margraf, Johannes; Ruland, Andres; Sgobba, Vito; Guldi, Dirk Michael; Clark, Timothy (Langmuir, 2013)
We have investigated the role of linker molecules in quantum-dot-sensitized solar cells (QDSSCs) using density-functional theory (DFT) and experiments. Linkers not only govern the number of attached QDs but also influence charge separation, recombination, and transport. Understanding their behavior is therefore not straightforward. DFT calculations show that mercaptopropionic acid (MPA) and cysteine (Cys) exhibit characteristic binding configurations on TiO2 surfaces. This information is used to optimize the cell assembly process, yielding Cys-based cells that significantly outperform MPA cells, and reach power conversion efficiencies (PCE) as high as 2.7% under AM 1.5 illumination. Importantly, the structural information from theory also helps understand the cause for this improved performance.

Quantum Interference and Decoherence in Single-Molecule Junctions: How Vibrations Induce Electrical Current - Härtle, Rainer; Butzin, M.; Rubio-Pons, O.; Thoss, Michael (Physical Review Letters, 2011)
Quantum interference and decoherence in single-molecule junctions is analyzed employing a nonequilibrium Green's function approach. Electrons tunneling through quasidegenerate states of a molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. This decoherence mechanism may cause significantly larger electrical currents and is particularly pronounced if the junction is vibrationally highly excited, e.g., due to inelastic processes in the resonant transport regime. © 2011 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2011.nat.dphy.ITP.ppem.quantu

Quantum oscillations and quantum Hall effect in epitaxial graphene - Jobst, Johannes; Waldmann, Daniel; Speck, Florian; Hirner, R.; Maude, Duncan; Seyller, Thomas; Weber, Heiko B. (Physical Review B, 2010)
We investigate the transport properties of high-quality single-layer graphene, epitaxially grown on a 6H-SiC(0001) substrate. We have measured transport properties, in particular charge-carrier density, mobility, conductivity, and magnetoconductance of large samples as well as submicrometer-sized Hall bars which are entirely lying on atomically flat substrate terraces. The results display high mobilities, independent of sample size. The temperature dependence of the conductance indicates a rather strong coupling to the SiC substrate. An analysis of the Shubnikov-de Haas effect yields the Landau-level spectrum of single-layer graphene. When gated close to the Dirac point, the mobility increases substantially and the graphenelike quantum Hall effect occurs.

Quantum yield of Eu2+ emission in (Ca1-xSr x)S:Eu light emitting diode converter at 20-420 K - Xia, Qi; Batentschuk, Miroslaw; Osvet, Andres; Winnacker, Albrecht; Schneider, Juergen (Radiation Measurements, 2010)
A series of red-emitting light converters Ca1-xSrxS:Eu2+, with tunable composition-dependent emission maxima were synthesized and characterized concerning their photoluminescent (PL) properties. X-ray diffraction patterns, photoluminescence spectra, luminosities and quantum yields were compared for phosphors with strontium concentrations varying from 0 to 100%. The maxima wavelength of emission shifts from 663 down to 619 nm, originating from the dependence of Eu2+ 5d state energy on the surrounding crystal field. Upon increasing the temperature from 20 to 420 K, a broadening of emission spectra along with thermal quenching of emission intensity and quantum yield occurs. Satisfying PL properties and their thermal stability demonstrate that the phosphors could be used as light converters in light emitting diodes (LEDs).
Keywords: Light emitting diodes; Quantum yield; Phosphors; Temperature stability; Sulfides

Quasicrystalline nanocrystal superlattice with partial matching rules - Ye, Xingchen; Chen, Jun; Irrgang, M. Eric; Engel, Michael; Dong, Angang; Glotzer, Sharon C.; Murray, Christopher B. (Nature Materials, 2017)
Expanding the library of self-assembled superstructures provides insight into the behaviour of atomic crystals and supports the development of materials with mesoscale order. Here we build on recent findings of soft matter quasicrystals and report a quasicrystalline binary nanocrystal superlattice that exhibits correlations in the form of partial matching rules reducing tiling disorder. We determine a three-dimensional structure model through electron tomography and direct imaging of surface topography. The 12-fold rotational symmetry of the quasicrystal is broken in sublayers, forming a random tiling of rectangles, large triangles and small triangles with 6-fold symmetry. We analyse the geometry of the experimental tiling and discuss factors relevant for the stabilization of the quasicrystal. Our joint experimental-computational study demonstrates the power of nanocrystal superlattice engineering and further narrows the gap between the richness of crystal structures found with atoms and in soft matter assemblies.
EAM Import::2019-03-13

Quasicrystalline Nanocrystal Superlattice with Partial Matching Rules Xingchen Ye, Jun Chen, M. Eric Irrgang, Michael Engel, Angang Dong, Sharon C. Glotzer & Christopher B. Murray - Ye, Xingchen; Chen, Jun; Irrgang, M. Eric; Engel, Michael; Dong, Angang; Glotzer, Sharon C.; Murray, Christopher B. (Nature Materials, 2016)
Expanding the library of self-assembled superstructures provides insight into the behaviour of atomic crystals and supports the development of materials with mesoscale order1, <a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4759.html#ref2" id="ref-link-2" title="Tschierske, C. Liquid crystal engineering-new complex mesophase structures and their relations to polymer morphologies, nanoscale patterning and crystal engineering. Chem. Soc. Rev. 36, 1930-1970 (2007).">2. Here we build on recent findings of soft matter quasicrystals<a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4759.html#ref3" id="ref-link-3" title="Wasio, N. A. et al. Self-assembly of hydrogen-bonded two-dimensional quasicrystals. Nature 507, 86-89 (2014).">3, 4, <a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4759.html#ref5" id="ref-link-5" title="Takano, A. et al. A mesoscopic Archimedean tiling having a new complexity in an ABC star polymer. J. Polym. Sci. B 43, 2427-2432 (2005).">5, <a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4759.html#ref6" id="ref-link-6" title="Talapin, D. V. et al. Quasicrystalline order in self-assembled binary nanoparticle superlattices. Nature 461, 964-967 (2009).">6 and report a quasicrystalline binary nanocrystal superlattice that exhibits correlations in the form of partial matching rules reducing tiling disorder. We determine a three-dimensional structure model through electron tomography<a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4759.html#ref7" id="ref-link-7" title="Midgley, P. A. & Weyland, M. 3D electron microscopy in the physical sciences: the development of Z-contrast and EFTEM tomography. Ultramicroscopy 96, 413-431 (2003).">7, <a href="http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4759.html#ref8" id="ref-link-8" title="Friedrich, H. et al. Quantitative structural analysis of binary nanocrystal superlattices by electron tomography. Nano Lett. 9, 2719-2724 (2009).">8 and direct imaging of surface topography. The 12-fold rotational symmetry of the quasicrystal is broken in sublayers, forming a random tiling of rectangles, large triangles and small triangles with 6-fold symmetry. We analyse the geometry of the experimental tiling and discuss factors relevant for the stabilization of the quasicrystal. Our joint experimental–computational study demonstrates the power of nanocrystal superlattice engineering and further narrows the gap between the richness of crystal structures found with atoms and in soft matter assemblies.

Quasi-freestanding Graphene on SiC(0001) - Speck, Florian; Ostler, Markus; Röhrl, Jonas; Jobst, Johannes; Waldmann, Daniel; Hundhausen, Martin; Ley, Lothar; Weber, Heiko B.; Seyller, Thomas (Materials Science Forum, 2010)
We report on a comprehensive study of the properties of quasi-freestanding monolayer and bilayer graphene produced by conversion of the (6√3×6√3)R30° reconstruction into graphene via intercalation of hydrogen. The conversion is confirmed by photoelectron spectroscopy and Raman spectroscopy. By using infrared absorption spectroscopy we show that the underlying SiC(0001) surface is terminated by hydrogen in the form of Si-H bonds. Using Hall effect measurements we have determined the carrier concentration and type as well as the mobility which lies well above 1000 cm2/Vs despite a significant amount of short range scatterers detected by Raman spectroscopy. © (2010) Trans Tech Publications.
UnivIS-Import:2015-04-14:Pub.2010.nat.dphy.IAP.LAP.quasif
Keywords: ARPES; Graphene; Hydrogen; Infrared absorption spectroscopy; Intercalation; Raman spectroscopy; XPS

Quaternized Pyridyloxy Phthalocyanines Render Aqueous Electron-Donor Carbon Nanotubes as Unprecedented Supramolecular Materials for Energy Conversion - Anaya-Plaza, Eduardo; Moreno-Oliva, Maria; Kunzmann, Andreas; Romero-Nieto, Carlos; Costa, Ruben Dario; De La Escosura, Andres; Guldi, Dirk Michael; Torres, Tomas (Advanced Functional Materials, 2015)
Exploring new properties in known materials, sometimes even achieving behaviors opposite to those traditionally encountered, is a fundamental aspect of innovation in materials science. In the ﬁeld of energy conversion, for example, the development of water-processed organic solar cells provides environmentally friendlier materials, which contribute to reduce health risks. Herein, a novel approach is described to produce water-soluble electron-donor single wall carbon nanotube (SWCNT) hybrids based on the noncovalent immobilization of quaternized pyridyloxy zinc phthalocyanines (ZnPc) with a varying number of pyridyl substituents. Moreover, the excellent electron-accepting ability of the latter ZnPcs is reported. The introduction of tert-butylphenyl groups at the pyridines enables for the ﬁrst time a complete characterization. The electron-acceptor nature of the ZnPcs enables switching the role of SWCNTs within the resulting supramolecular hybrids. Finally, a proof-of-concept demonstration of the SWCNT/ZnPc hybrids' capacity for energy conversion is presented, paving their way to possible use as active layer material in solar cells processed entirely from aqueous solutions.
EAM Import::2019-03-13

QVibeplot: A Program To Visualize Molecular Vibrations in Two Dimensions - Laurin, Mathias (Journal of Chemical Education, 2013)
QVibeplot is a software program that automatically generates twodimensional visualizations of molecular vibrations. The representations show the changes of bond lengths, angles, and torsions occurring upon a vibration. This is consistent with the experimentalist's understanding of molecular vibrations that makes a distinction between stretching and deformation modes. Twodimensionality is achieved by basing the representation on a skeletal formula of the molecule. The program also displays the spectrum and the list of frequencies. The phase and amplitude are indicated as well, providing a comprehensive visualization of molecular vibrations. The software is available online as a free and open-source software.
EAM Import::2019-03-13

Radial Growth in 2D Revisited: The Effect of Finite Density, Binding Affinity, Reaction Rates, and Diffusion - Bihr, Timo; Sadafi, Fabrizio-Zagros; Smith, Ana-Suncana; Klupp Taylor, Robin; Seifert, Udo (Advanced Materials Interfaces, 2017)
Dendrite growth of metal patches on colloidal particles shows a variety of structures depending on the preparation conditions. The morphology of these patches suggests a cross-over from a reaction to a diffusion limited growth, implicating diffusion on the particle surface. Interestingly, the morphological and optical characteristics of the patches continuously change between two limiting behaviors. To understand this growth process, extensive simulations are performed, studying the fractal dimension and the dynamics of growth of a patch on a particle of a finite size, as a function of the initial density and the binding affinity of diffusing tracers. Several important growth regimes are characterized that enable to optimize the pathway for the synthesis of optically active, patchy particles.
Keywords: LIMITED AGGREGATION; CELL-ADHESION; FRACTAL AGGREGATION; COLLOIDAL SYNTHESIS; NANOSPHERES; DIMENSIONS; SIMULATION; NUCLEATION; PARTICLES; STABILITY

Radiation stability of CsBr:Eu needle image plates - Batentschuk, Miroslaw; Neudert, S.; Weidner, Martina; Osvet, Andres; Struye, L.; Tahon, J. -P.; Leblans, P. (Journal of Applied Physics, 2009)
Needle image plates (NIPs) based on CsBr:Eu are a good alternative to the BaFBr:Eu powder image plates due to their higher sensitivity and improved spatial resolution. The x-ray radiation stability of the NIPs produced by Agfa Gevaert was investigated and it was found that the radiation stability of the optimized NIPs is as high as that of the commercial CsI:Tl single crystals and much higher than reported in literature for the CsBr:Eu NIPs. The dependence of the sensitivity of the NIPs on the accumulated dose was determined for three different types of x-ray irradiation. It is shown that degradation of the sensitivity starts at about 10 Gy and it is the strongest for the most hard x-ray beam. If the energy absorbed by a NIP is taken into account, the degradation does not depend on the hardness of the applied x-ray beam. It is suggested that the main reason for the observed high radiation stability of the CsBr:Eu NIPs is the use of the oxygen-free Eu2+-containing precursor CsEuBr3 for the doping in the manufacturing process.

Raman spectra of epitaxial graphene on SiC(0001) - Röhrl, Jonas; Hundhausen, Martin; Emtsev, Konstantin; Seyller, Thomas; Graupner, Ralf; Ley, Lothar (Applied Physics Letters, 2008)
We present Raman spectra of epitaxial graphene layers grown on 63×63 reconstructed silicon carbide surfaces during annealing at elevated temperature. In contrast to exfoliated graphene a significant phonon hardening is observed. We ascribe that phonon hardening to a minor part to the known electron transfer from the substrate to the epitaxial layer, and mainly to mechanical strain that builds up when the sample is cooled down after annealing. Due to the larger thermal expansion coefficient of silicon carbide compared to the in-plane expansion coefficient of graphite this strain is compressive at room temperature. © 2008 American Institute of Physics.
UnivIS-Import:2015-03-09:Pub.2008.nat.dphy.IAP.exphys.ramans

Raman spectroscopy and electrical transport studies of free-standing epitaxial graphene: Evidence of an AB-stacked bilayer - Shivaraman, Shriram; Jobst, Johannes; Waldmann, Daniel; Weber, Heiko B.; Spencer, Michael (Physical Review B, 2013)
We fabricate free-standing graphene structures from epitaxial graphene on silicon carbide using a photoelectrochemical (PEC) etching process. A combination of Raman spectroscopy and magnetotransport measurements was used to investigate multiterminal devices in various geometries. From the analysis of Raman data and Shubnikov-de Haas oscillations, we conclude that the buffer layer is converted into a graphene layer and, thus, monolayer graphene on SiC gets converted to a free-standing AB-stacked bilayer. The bilayer exhibits inversion-symmetry breaking because of differential doping between the layers. Additionally, lateral inhomogeneities exist in the form of domains with nonuniform mobility. The same PEC process on a pure buffer layer, however, does not yield monolayer graphene.

Random phase approximation correlation energies with exact Kohn-Sham exchange - Heßelmann, Andreas; Görling, Andreas (Molecular Physics, 2010)
The random phase approximation (RPA) correlation energy is expressed in terms of the exact local Kohn-Sham (KS) exchange potential and corresponding adiabatic and nonadiabatic exchange kernels for density-functional reference determinants. The approach naturally extends the RPA method in which, conventionally, only the Coulomb kernel is included. By comparison with the coupled cluster singles doubles with perturbative triples method it is shown for a set of small molecules that the new RPA method based on KS exchange yields correlation energies more accurate than RPA on the basis of Hartree-Fock exchange. © 2010 Taylor & Francis.

Random-phase approximation correlation methods for molecules and solids - Heßelmann, Andreas; Görling, Andreas (Molecular Physics, 2011)
Random-phase approximation (RPA) correlation methods based on Kohn-Sham density-functional theory and Hartree-Fock are derived using the adiabatic-connection fluctuation dissipation theorem. It is shown that the correlation energy within the adiabatic-connection fluctuation-dissipation theorem is exact in a Kohn-Sham framework while for Hartree-Fock reference states this is not the case. This shows that Kohn-Sham reference states are probably better suited to describe electron correlation for use in RPA methods than Hartree-Fock reference states. Both, Kohn-Sham and Hartree-Fock RPA methods are related to each other both by comparing the underlying correlation functionals and numerically through the comparison of total energies and reaction energies for a set of small organic molecules. Copyright © 2011 Taylor and Francis Group, LLC.

Rapid Anodic Formation of High Aspect Ratio WO 3 Layers with Self-Ordered Nanochannel Geometry and Use in Photocatalysis - Wei, Wei; Shaw, Santosh; Lee, Kiyoung; Schmuki, Patrik (Chemistry - A European Journal, 2012)
The formation of WO3 layers with an aligned nanochannel morphology by using self-organizing anodization of a tungsten metal substrate is demonstrated. The nanochannel layers with diameters of approximately 9nm can be grown to about 10 extgreekmm thickness. Layers optimized for length and structure are promising for visible-light photocatalytic applications, such as water-splitting photoanodes.
EAM Import::2019-03-13

Rare-earth ion doped up-conversion materials for photovoltaic applications - Wang, Hao; Batentschuk, Miroslaw; Osvet, Andres; Pinna, Luigi; Brabec, Christoph (Advanced Materials, 2011)
With the aim of utilizing the infrared region of solar radiation to improve solar cell performance, significant progress, including theoretical analysis and experimental achievement, has been made in the field of up-conversion for photovoltaic applications. This Research News article reviews recent progress in the development of rear-earth (RE) ion doped up-conversion materials for solar cell applications. In addition, new trends for RE-ion-doped phosphors are briefly discussed, among them trivalent RE-ion-doped up-conversion materials for organic solar cell applications. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: nonlinear optical materials; phosphors; photovoltaic devices; rare earth ions

Ratcheting and tumbling motion of Vibrots - Scholz, Christian; Dsilva, Sean; Pöschel, Thorsten (New Journal of Physics, 2016)
Systems of granular rotors (Vibrots), that is, small devices that convert linear vibrational motion into rotation by frictional impact, are of scientific interest since they could reveal various types of collective behavior. Looking at an isolated Vibrot, we note at least two dierent dynamical modes, depending on the parameters of the vibrational driving. By means of Finite Element simulations, we reveal the driving mechanism for both cases which may be correspondingly identified as ratcheting and tumbling. The transition between both modes resembles period doubling in certain bouncing ball problems leading eventually to chaotic motion in such systems.

Reactivity of atomically dispersed Pt2+ species towards H-2: model Pt-CeO2 fuel cell catalyst - Lykhach, Yaroslava; Figueroba, Alberto; Camellone, Matteo Farnesi; Neitzel, Armin; Skala, Tomas; Negreiros, Fabio R.; Vorokhta, Mykhailo; Tsud, Nataliya; Prince, Kevin C.; Fabris, Stefano; Neyman, Konstantin M.; Matolin, Vladimir; et al. (Physical Chemistry Chemical Physics, 2016)
The reactivity of atomically dispersed Pt2+ species on the surface of nanostructured CeO2 films and the mechanism of H-2 activation on these sites have been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. Isolated Pt2+ sites are found to be inactive towards H-2 dissociation due to high activation energy required for H-H bond scission. Trace amounts of metallic Pt are necessary to initiate H-2 dissociation on Pt-CeO2 films. H-2 dissociation triggers the reduction of Ce4+ cations which, in turn, is coupled with the reduction of Pt2+ species. The mechanism of Pt2+ reduction involves reverse oxygen spillover and formation of oxygen vacancies on Pt-CeO2 films. Our calculations suggest the existence of a threshold concentration of oxygen vacancies associated with the onset of Pt2+ reduction.

Reactivity of CO and C2H4 on Bimetallic PtxAg1-x/Pt(111) Surface Alloys Investigated by High-Resolution X-ray Photoelectron Spectroscopy - Bauer, Udo; Späth, Florian; Düll, Fabian; Bachmann, Phillip; Steinhauer, Johann; Steinrück, Hans-Peter; Papp, Christian (ChemPhysChem, 2018)
We have investigated the adsorption and thermal reactivity of CO and C2H4 on well-defined bimetallic PtxAg1-x/Pt(111) surface alloys (with x=0.67, 0.50, 0.38 and 0.32) using insitu synchrotron-based high-resolution X-ray photoelectron spectroscopy. During low-temperature (140K) adsorption of CO, we find population of the energetically more stable Pt on-top site followed by the Pt bridge site, as known from clean Pt(111); CO does not adsorb on Ag facets under the applied conditions. The total CO coverage linearly decreases with Ag surface content. Upon heating, COon-top and CObridge desorb at higher temperatures than on bare Pt(111), for example, by approximate to 65K for COon-top for an Ag content of 33%. This shift is due to the well-known electronic ligand-effect of Ag atoms surrounding Pt atoms/clusters. For C2H4 adsorption on the AgPt alloys at low temperature (120K), we find two species in the C1s XP spectrum, each displaying a vibrational fine structure; this contrasts the situation on clean Pt(111), where only one species is found. Upon heating, ethylene reacts with ethylidyne being the dominant intermediate; in contrast to clean Pt(111), on the alloy surfaces the reaction to CCH3 occurs at much lower temperatures, e.g. by approximate to 60K for the alloy with 68% Ag, and no ethylene desorption is found.
Keywords: carbon monoxide; ethylene; platinum; silver; X-ray photoelectron spectroscopy

Reactivity of CO on Sulfur-Passivated Graphene-Supported Palladium Nanocluster Arrays - Düll, Fabian; Späth, Florian; Bachmann, Philipp; Bauer, Udo; Steinrück, Hans-Peter; Papp, Christian (Journal of Physical Chemistry C, 2017)
Nanocluster arrays on graphene are suitable model systems for catalysis. We use this model system to study sulfur poisoning, which is a major deactivation process of heterogeneous catalysts. Using high-resolution X-ray photoelectron spectroscopy, we investigated the adsorption and desorption of CO on sulfur-poisoned graphene-supported palladium nanoparticles. We find that sulfur blocks CO adsorption sites, with hollow sites being most affected. From the unchanged desorption temperatures, we conclude that the Pd-CO bond strength is not altered by co-adsorbed sulfur. The degree of site blocking compared to the amount of sulfur indicates that the palladium nanoparticle surface is dominated by (111) and (100) facets.

Reactivity of Graphene-Supported Pt Nanocluster Arrays - Gotterbarm, Karin; Späth, Florian; Bauer, Udo; Bronnbauer, Carina; Steinrück, Hans-Peter; Papp, Christian (ACS Catalysis, 2015)
Nanocluster arrays on graphene (Gr) are intriguing model systems for catalysis. We studied the adsorption and oxidation of CO on Pt/Gr/Rh(111) with synchrotron-based high-resolution X-ray photoelectron spectroscopy. On the nanoclusters, CO is found to adsorb at three different sites: namely, on-top, bridge, and step. The C 1s spectra exhibit remarkable similarities to those on the stepped Pt(355) surface. Similar to the case for stepped Pt surfaces, a clear preference for the adsorption on the step sites is found, while the preference for the adsorption on the on-top site over the bridge site on the terraces is much less pronounced in comparison to that on Pt single crystals. Temperature-programmed X-ray photoelectron spectroscopy revealed an enhanced binding energy for the cluster step sites, similar to the situation on stepped Pt surfaces. The oxidation of CO on graphene-supported Pt nanoclusters follows a pseudo-first-order rate law. Applying an Arrhenius analysis, we found an activation energy of 13 ± 4 kJ/mol, which is much smaller than that on Pt(111), due to the more reactive step and kink sites on the nanoclusters.

Reactivity of TiO2 Nanotube-Supported Platinum Particles in the CO Oxidation Reaction - Krick Calderon, Sandra; Grabau, Mathias; Yoo, Jeong Eun; Killian, Manuela; Schmuki, Patrik; Steinrück, Hans-Peter; Papp, Christian (ChemCatChem, 2017)
The CO oxidation on different platinum model catalysts, in particular differently sized Pt particles on vertically aligned TiO nanotubes (d=80, l=200nm) was investigated by near-ambient-pressure X-ray photoelectron spectroscopy and simultaneous online gas analysis. The results are compared to the behavior on a TiO(110) rutile crystal with a similar Pt coverage and a Pt(111) single crystal. Temperature-programmed reaction experiments at total pressures of 10 to 1.0mbar, and a CO/O ratio of 1:4 were performed to evaluate the reactivity of the samples. The onset temperature of the reaction on the Pt/TiO nanotube samples was found to be lower than on Pt(111) and Pt/TiO(110) and to decrease with decreasing particle size. For all particle samples, a steep increase in reaction rate at a defined temperature was observed, and at high temperatures the reaction became mass transfer-limited in all cases. Isothermal measurements showed that no CO is adsorbed on the particles when CO oxidation has a high activity and metallic platinum is the active catalyst. Ageing of the TiO-supported samples is observed resulting in changes of the reaction onset and CO conversion.
Keywords: Nanotubes; Oxidation; Platinum; Supported catalysts; X-ray absorption spectroscopy

Realization of a computed tomography setup to achieve resolutions below 1μm - Salamon, M.; Hanke, R.; Krueger, P.; Uhlmann, Norman; Voland, V. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2008)
The amelioration of present X-ray imaging systems especially in computed tomography to resolutions below 1 extgreekmm necessitates very precise equipment details such as the manipulators, focus size and stability of the tube and the detector adjustment. In this paper, we present an experimental setup of a computed tomography system with sub-micrometer resolution. The requirements on and characteristics of the components and their influence on the measurements are discussed and exemplary results of different test objects in the field of non-destructive testing with sub-micrometer resolution are presented.
EAM Import::2019-03-13

Realization of Vertical and Zigzag Single Crystalline Silicon Nanowire Architectures - Sivakov, V. A.; Broenstrup, G.; Pecz, Bela; Berger, A.; Radnoczi, G. Z.; Krause, Michael; Christiansen, S. H. (Journal of Physical Chemistry C, 2010)
Silicon nanowire (SiNW) ensembles with vertical and zigzag architectures have been realized using wet chemical etching of bulk silicon wafers (p-Si(111) and p-Si(100)) with a mask of silver nanoparticles that are deposited by wet electroless deposition. The etching of SiNWs is based on subsequent treatments in chemical solutions such as 0.02 M aqueous solutions of silver nitrate (AgNO_3) followed by 5 M hydrofluoric acid and 30% hydrogen peroxide (H_2O_2). The etching of the Si wafers is mediated by the reduction of silver on the silicon surface and in parallel by the oxidation of Si thereby forming SiO_2 which is dissolved in the HF surroundings. The morphology of the starting silver (Ag) layer/Ag nanoparticles that form during processing on the Si wafer surfaces strongly influences the morphology of the SiNW ensembles and homogeneity of the etch profile. Our observations suggest that the Ag layer/Ag nanoparticles not only catalyze the wet chemical etching of silicon but also strongly catalyze the decomposition of H_2O_2 so that the temperature of the etching solution substantially increases (strong exothermic reaction) and thus the etching velocity of bulk material. The morphology and microstructure of single crystalline SiNWs with respect to their crystallographic orientation was investigated by scanning (SEM) and transmission electron (TEM) microscopies and by electron backscatter diffraction (EBSD) in an SEM. Straight SiNWs as well as zigzag SiNWs can be realized depending on processing peculiarities. The optical characteristics such as absorption, transmission, and reflectance of the different silicon 1D architectures were investigated in an integrating sphere. Strong absorption and less reflection of visible and near-infrared light by the SiNW ensembles suggest that such material can be applied in the fields of opto-electronics, photonics and photovoltaics.
EAM Import::2019-03-13

Real-time evaluation of thin film drying kinetics using an advanced, multi-probe optical setup - Güldal, Nusret Sena; Kassar, Thaer; Berlinghof, Marvin; Ameri, Tayebeh; Osvet, Andres; Pacios, Roberto; Li Destri, Giovanni; Unruh, Tobias; Brabec, Christoph (Journal of Materials Chemistry C, 2016)
Solution-processed organic photovoltaic devices are advantageous due to their low-cost large area manufacturing techniques, such as slot-die coating, gravure printing and roll-to-roll coating. The final microstructure of a polymer:fullerene bulk-heterojunction (BHJ) film is a fine interplay between solution thermodynamics (e.g. solubility, miscibility...) and kinetics (e.g. solvent evaporation, polymer ordering, phase separation...) during the drying process. In order to design better performing organic photovoltaic devices, gaining knowledge over the drying properties of polymer:fullerene thin films is essential. A novel in situ thin film drying characterization chamber, equipped with white-light reflectometry, laser light scattering and photoluminescence, is presented in combination with grazing-incidence X-ray diffraction on two different polymer:fullerene bulk heterojunctions based on poly-(3-hexylthiophene-2,5-diyl) (P3HT) and polythieno[3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T) polymers. With photoluminescence applied for the first time as an in situ method for such drying studies, these single-chamber measurements track the fine interplay between thermodynamics and kinetics of thin film drying and provide invaluable information on solution behavior and microstructure formation.
Keywords: Engineering controlled terms: Coatings; Drying; Fullerenes; Heterojunctions; Kinetics; Light scattering; Microstructure; Phase separation; Photoluminescence; Plastic coatings; Polymer films; Polymers; Rolls (machine components); Solubility; Thermodynamics; Thiophene; X ray diffraction Bulk heterojunction (BHJ); Grazing incidence X-ray diffraction; Manufacturing techniques; Microstructure formation; Organic photovoltaic devices; Poly(3-hexylthiophene-2; 5-diyl); Solution thermodynamics; Thermodynamics and kinetics Engineering main heading: Thin films

Real-time in-situ observation of morphological changes in organic bulk-heterojunction solar cells by means of capacitance measurements - Lungenschmied, Christoph; Bauer, Siegfried; Schwoediauer, Reinhard; Rodman, Sheila; Fournier, Darcy; Dennler, Gilles; Brabec, Christoph (Journal of Applied Physics, 2011)
We present a method to directly study temperature induced structural and morphological changes in the active layer of organic solar cells by capacitance determination. At sufficiently high frequencies the change in device capacitance with temperature is dominated by the expansion of the organic layer. Variations in the expansion coefficient or the permittivity are visible in the capacitive response. We have studied thin film devices based on poly(3- hexyl)thiophene (P3HT) as well as organic solar cells made from blends of P3HT with a fullerene derivative. Temperature induced transitions are correlated to structural changes in the polymer (e.g., glass transition) as well as morphological changes of the blend; the morphology transition driven by phase separation sets in around 120 C. The advantage of our capacitance method is substantiated as we have demonstrated that confinement due to the substrate and the top electrode alters the effect of temperature on thin films as compared to the bulk material. © 2011 American Institute of Physics.
Keywords: Active Layer; Bulk materials; Capacitance method; Capacitive response; Device capacitance; Effect of temperature; Expansion coefficients; Fullerene derivative; High frequency; In-situ observations; Morphological changes; Morphology transitions; Organic bulk-heterojunction solar cells; Organic layers; Organic solar cell; Structural change; Temperature-induced Engineering controlled terms: Capacitance; Capacitance measurement; Glass transition; Heterojunctions; Morphology; Phase separation; Polymer films; Thin film devices; Thin films; Thiophene Engineering main heading: Solar cells

Real-Time Investigation of Intercalation and Structure Evolution in Printed Polymer:Fullerene Bulk Heterojunction Thin Films - Kassar, Thaer; Güldal, Nusret Sena; Berlinghof, Marvin; Ameri, Tayebeh; Kratzer, Andreas; Schroeder, Bob C.; Li Destri, Giovanni; Hirsch, Andreas; Heeney, Martin; Mcculloch, Iain; Brabec, Christoph; Unruh, Tobias (Advanced Energy Materials, 2016)
The complex intermixing morphology is critical for the performance of the nanostructured polymer:fullerene bulk heterojunction (BHJ) solar cells. Here, time resolved in situ grazing incidence X-ray diffraction and grazing incidence small angle X-ray scattering are used to track the structure formation of BHJ thin films formed from the donor polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) with different fullerene derivative acceptors. The formation of stable bimolecular crystals through the intercalation of fullerene molecules between the side chains of polymer crystallites is investigated. Such systems exhibit more efficient exciton dissociation but lower photo-conductance and faster decay of charges. On the basis of the experimental observations, intercalation obviously takes place before or with the formation of the crystalline polymer domains. It results in more stable structures whose volume remains constant upon further drying. Three distinct periods of drying are observed and the formation of unidimensional fullerene channels along the π-stacking direction of polymer crystallites is confirmed.
Keywords: fullerenes; grazing incidence diffraction; intercalation dynamics; organic solar cells; thin films

Real-Time Monitoring of the Nucleation and Growth of ZnO Nanoparticles Using an Optical Hyper-Rayleigh Scattering Method - Segets, Doris; Martinez Tomalino, Lars-Owe; Gradl, Johannes; Peukert, Wolfgang (Journal of Physical Chemistry C, 2009)
Incoherent nonlinear hyper-Rayleigh scattering (HRS) has been applied to monitor the different stages in the synthesis of sub-10 nm ZnO nanoparticles. During time-resolved investigation of the ripening process, we measured an increase in the particle hyperpolarizability, βZnO, with increasing particle size. Millisecond time resolution measurements revealed regions of prevalent nucleation, growth, and ripening. Using the size-dependent hyperpolarizabilities, a new general approach is introduced for the quantitative determination of nucleation, growth, and ripening rates via in situ HRS measurements. The derived solubility also allowed the determination of reasonable values of the ZnO surface energy.

Recent advances in multifunctional nanocarbons used in dye-sensitized solar cells - Costa Riquelme, Rubén Dario; Lodermeyer, Fabian; Casillas Pacheco, Ruben; Guldi, Dirk Michael (Energy and Environmental Science, 2014)
Throughout recent years the implementation of nanocarbons into dye-sensitized solar cells (DSSC) has resulted in important breakthroughs. The most relevant of them in this context are (i) the enhancement of charge transport and charge collection in nanocarbon-doped electrodes, (ii) the introduction of nanocarbon interlayers that simultaneously reduce the charge recombination and increase the charge collection efficiency, (iii) the use of nanocarbon-based, iodine-free, solid-state electrolytes featuring excellent diffusion coefficients and catalytic efficiencies, (iv) the use of novel nanocarbon-based hybrid dyes, and (v) the use of nanocarbons towards platinum-free counter electrodes. The first four aforementioned aspects are thoroughly described in this review. This journal is © the Partner Organisations 2014.

Recent developments in the study of ionic liquid interfaces using X-ray photoelectron spectroscopy and potential future directions - Steinrück, Hans-Peter (Physical Chemistry Chemical Physics, 2012)
A perspective is given on the potential of X-ray photoelectron spectroscopy to investigate ionic liquid (IL) interfaces and related bulk properties. To allow for cross-correlation of different IL properties, the study is restricted to imidazolium-based systems. By systematic variation of alkyl chain lengths, by investigation of different anions, and by introduction of functional groups detailed information on surface orientation, enrichment effects, cation-anion interactions, and the correlation to macroscopic properties such as density and surface tension of ILs are derived. Furthermore, the molecular arrangement and growth behaviour of ultrathin ionic liquid layers on various substrates, in situ monitoring of chemical reactions, and the dissolution of transition metal complexes are addressed. © 2012 the Owner Societies.

Recommendations for the Production of Silicon Carbide-derived Carbon Based on Intrinsic Kinetic Data - Knorr, Tilman; Strobl, Florian; Glenk, Friedrich; Etzold, Bastian (Chemical Engineering & Technology, 2012)
Recommendations for mode of operation and critical dimensions regarding inner mass transfer limitation were deduced for the technical extraction of silicon from silicon carbide to produce nanoporous carbon (carbide-derived carbon, CDC). The recommendations are based on intrinsic kinetics derived from experimental data and reaction engineering simulation of the transient process (shrinking-core like mechanism). As the reaction takes place at 1000 °C under chlorine atmosphere, kinetic data as well as diffusion coefficients are extracted from integral conversion data in a lab scale reactor. The reaction rate dependency for varying chlorine concentrations is represented best by a Hougen-Watson type rate approach with dissociative adsorption of a reactant. From the experimental data the effective diffusion coefficient of chlorine in the resulting nanoporous carbon could also be derived to 4 · 10–8 m2s–1 at 1000 °C.
EAM Import::2019-03-13

Recurrent inflation and collapse in horizontally shaken granular materials - Pöschel, Thorsten; Rosenkranz, Dirk E.; Gallas, Jason (Physical Review E, 2012)
We report a striking effect observed experimentally in several granular materials when shaken horizontally: The material displays a recurrent alternation between a slow inflation phase, characterized by an increase in its volume, and a fast collapse phase, when the volume abruptly returns to its original value. The frequency of such phase alternations is totally decoupled from the frequency of the external drive. We argue that the inflation and collapse alternation arises from an interplay between the mechanical stability of the material and Reynolds dilatancy due to convective motion. © 2012 American Physical Society.

Recycling 4.0 - Heinzerling, Peter (Chemie Konkret - Forum für Unterricht und Didaktik, 2019)
CRIS-Team WoS Importer:2019-04-16

Redox chemistry, solubility, and surface distribution of Pt(II) and Pt(IV) complexes dissolved in ionic liquids - Kolbeck, Claudia; Taccardi, Nicola; Paape, Natalia; Schulz, Peter; Wasserscheid, Peter; Steinrück, Hans-Peter; Maier, Florian (Journal of Molecular Liquids, 2014)
An X-ray photoelectron spectroscopy (XPS) study of Pt(II) and Pt(IV) complexes dissolved in ionic liquids (ILs) revealed the occurrence of a rich redox chemistry, which is partly induced by X-ray radiation. Depending on the IL anion either a reduction of Pt(IV) to Pt(II) in the case of ethylsulfate ([EtOSO])-based ILs, or an oxidation of Pt(II) to Pt(IV) in the case of bis[(trifluoromethyl)sulfonyl]imide ([TfN] )-based ILs was observed upon X-ray irradiation. Moreover, surface enrichment or depletion effects of the Pt complexes were deduced from angle-resolved XPS (ARXPS) data; while the Pt complexes dissolved in imidazolium-based ILs tended to deplete from the surface, in pyrrolidinium-based solutions a surface enrichment of the Pt complexes was observed. © 2013 Elsevier B.V. All rights reserved.

Redox-mediated conversion of atomically dispersed platinum to sub-nanometer particles - Lykhach, Yaroslava; Figueroba, Alberto; Skala, Tomas; Duchon, Tomas; Tsud, Nataliya; Aulicka, Marie; Neitzel, Armin; Veltruska, Katerina; Prince, Kevin C.; Matolin, Vladimir; Neyman, Konstantin M.; Libuda, Jörg (Journal of Materials Chemistry A, 2017)
The stability and the conversion of atomically dispersed Pt2+ species to sub-nanometer Pt particles have been investigated as a function of the Sn concentration in Pt-CeO2 films by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and angle-resolved X-ray photoelectron spectroscopy in combination with density functional calculations. The deposition of Sn onto the Pt-CeO2 films triggers the reduction of Ce4+ cations to Ce3+ yielding Sn2+ cations. Consecutively, the redox coupling between the Ce3+ and Pt2+ species triggers the reduction of Pt2+ species yielding sub-nanometer Pt particles. The onset of reduction of Pt2+ species is directly related to the concentration of Ce3+ centers which, in turn, is controlled by the concentration of Sn2+ cations in the Pt-CeO2 film. On average, the formation of 6Ce(3+) centers corresponding to the adsorption of 3Sn atoms gives rise to the reduction of one Pt2+ species. The analysis of the depth distribution of Sn atoms in the Pt-CeO2 films revealed preferential adsorption of Sn2+ at the surface followed by diffusion of Sn2+ ions into the bulk at higher Sn coverages. Density functional modeling suggested that the adsorption of three Sn atoms in the vicinity of the Pt2+ species results in a rearrangement of the local coordination accompanied by substantial destabilization of the Pt2+ species followed by its conversion to Pt-0 atoms. The formation of sub-nanometer Pt particles is coupled with re-oxidation of two Ce3+ centers per one Pt2+ species reduced. Annealing of the Pt-CeO2 films in the presence of metallic Sn also leads to the reduction of the Pt2+ species due to thermally triggered oxidation of metallic Sn residues followed by diffusion of Sn2+ into the bulk. Annealing of the Pt-CeO2 films to temperatures above 600 K results in a loss of Sn yielding sub-nanometer Pt particles supported on nearly stoichiometric and Sn-free CeO2 films.

Reducing burn-in voltage loss in polymer solar cells by increasing the polymer crystallinity - Heumueller, Thomas; Mateker, William R.; Sachs-Quintana, I. T.; Vandewal, Koen; Bartelt, Jonathan A.; Burke, Timothy M.; Ameri, Tayebeh; Brabec, Christoph; Mcgehee, Michael D. (Energy and Environmental Science, 2014)
In order to commercialize polymer solar cells, the fast initial performance losses present in many high efficiency materials will have to be managed. This burn-in degradation is caused by light-induced traps and its characteristics depend on which polymer is used. We show that the light-induced traps are in the bulk of the active layer and we find a direct correlation between their presence and the open-circuit voltage loss in devices made with amorphous polymers. Solar cells made with crystalline polymers do not show characteristic open circuit voltage losses, even though light-induced traps are also present in these devices. This indicates that crystalline materials are more resistant against the influence of traps on device performance. Recent work on crystalline materials has shown there is an energetic driving force for charge carriers to leave amorphous, mixed regions of bulk heterojunctions, and charges are dominantly transported in pure, ordered phases. This energetic landscape allows efficient charge generation as well as extraction and also may benefit the stability against light-induced traps. This journal is © the Partner Organisations 2014.
Keywords: Engineering controlled terms: Crystalline materials; Heterojunctions; Open circuit voltage; Solar cells Amorphous polymers; Bulk heterojunction; Charge generation; Crystalline polymers; Device performance; Open circuit voltage loss; Polymer crystallinity; Polymer Solar Cells Engineering main heading: Polymers GEOBASE Subject Index: crystallinity; energy efficiency; photovoltaic system; polymer; solar power

Reduction of microkinetic reaction models for reactor optimization exemplified for hydrogen production from methane - Karst, Florian; Maestri, Matteo; Freund, Hannsjörg; Sundmacher, Kai (Chemical Engineering Journal, 2015)
Sustainable and efficient processes require optimal design and operating conditions. The determination of optimal process routes, however, is a challenging task. Either the models and underlying chemical reaction rate equations are not able to describe the process in a wide ranges of reaction conditions and thus limit the optimization space, or the models are too complex and numerically challenging to be used in dynamic optimization. To address this problem, in this contribution, a reduction technique for chemical reaction networks is proposed. It focuses on the sensitivity of the reaction kinetic model with respect to the removal of selected reaction steps and evaluates their significance for the prediction of the overall system behavior. The method is demonstrated for a C-1 microkinetic model describing methane conversion to syngas on Rh/Al2O3 as catalyst. The original and the reduced microkinetic model show excellent qualitative and quantitative agreement. Subsequently, the reduced kinetic model is used for the optimization of a methane reformer to produce a hydrogen rich gas mixture as feed for polymer electrolyte membrane (PEM) fuel cell applications. (C) 2015 The Authors. Published by Elsevier B.V.
Keywords: Microkinetic modeling; Reactor optimization; Reaction networks; Methane conversion; Process design

Reduction of Pt2+ species in model Pt-CeO2 fuel cell catalysts upon reaction with methanol - Neitzel, Armin; Johanek, Viktor; Lykhach, Yaroslava; Skala, Tomas; Tsud, Nataliya; Vorokhta, Mykhailo; Matolin, Vladimir; Libuda, Jörg (Applied Surface Science, 2016)
The stability of atomically dispersed Pt2+ species on the surface of nanostructured CeO2 films during the reaction with methanol has been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The isolated Pt2+ species were prepared at low Pt concentration in Pt-CeO2 film. Additionally, Pt2+ species coexisting with metallic Pt particles were prepared at high Pt concentration. We found that adsorption of methanol yields similar decomposition products regardless of Pt concentration in Pt-CeO2 films. A small number of oxygen vacancies formed during the methanol decomposition can be replenished in the Pt-CeO2 film with low Pt concentration by diffusion of oxygen from the bulk. In the presence of supported Pt particles, a higher number of oxygen vacancies leads to a partial reduction of the Pt2+ species. The isolated Pt2+ species are reduced under rather strongly reducing conditions only, i.e. during annealing under continuous exposure to methanol. Reduction of isolated Pt2+ species results in the formation of ultra-small Pt particles containing around 25 atoms per particle or less. Such ultra-small Pt particles demonstrate excellent stability against sintering during annealing of Pt-CeO2 film with low Pt concentration under reducing conditions. (C) 2016 Elsevier B.V. All rights reserved.
Keywords: platinum; Ceria Model catalyst; Fuel cell; Methanol; Synchrotron radiation photoelectron spectroscopy; Resonant photoemission spectroscopy

Reductive retrofunctionalization of single-walled carbon nanotubes - Syrgiannis, Zois; Gebhardt, Benjamin; Dotzer, Christoph; Hauke, Frank; Graupner, Ralf; Hirsch, Andreas; Hauke, Frank (Angewandte Chemie International Edition, 2010)
(Figure Presented) As if nothing had happened: A detailed mechanistic investigation of the nucleophilic addition of organolithium reagents onto single-walled carbon nanotubes (SWCNTs) revealed that the initial functionalization step is part of an equilibrium. Consequently, the functionalization of the sidewalls of SWCNTs is reversible, and the introduced substituents can be detached easily by reduction. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

Regeneration of LOHC dehydrogenation catalysts: In-situ IR spectroscopy on single crystals, model catalysts, and real catalysts from UHV to near ambient pressure - Amende, Maximilian; Kaftan, Andre; Bachmann, Philipp; Brehmer, Richard; Preuster, Patrick; Koch, Marcus; Wasserscheid, Peter; Libuda, Jörg (Applied Surface Science, 2016)
The Liquid Organic Hydrogen Carrier (LOHC) concept offers an efficient route to store hydrogen using organic compounds that are reversibly hydrogenated and dehydrogenated. One important challenge towards application of the LOHC technology at a larger scale is to minimize degradation of Pt-based dehydrogenation catalysts during long-term operation. Herein, we investigate the regeneration of Pt/alumina catalysts poisoned by LOHC degradation. We combine ultrahigh vacuum (UHV) studies on Pt(111), investigations on well-defined Pt/AlO model catalysts, and near-ambient pressure (NAP) measurements on real core-shell Pt/AlO catalyst pellets. The catalysts were purposely poisoned by reaction with the LOHC perhydro-dibenzyltoluene (H18-MSH) and with dicyclohexylmethane (DCHM) as a simpler model compound. We focus on oxidative regeneration under conditions that may be applied in real dehydrogenation reactors. The degree of poisoning and regeneration under oxidative reaction conditions was quantified using CO as a probe molecule and measured by infrared reflection-absorption spectroscopy (IRAS) and diffuse reflectance Fourier transform IR spectroscopy (DRIFTS) for planar model systems and real catalysts, respectively. We find that regeneration strongly depends on the composition of the catalyst surface. While the clean surface of a poisoned Pt(111) single crystal is fully restored upon thermal treatment in oxygen up to 700 K, contaminated Pt/AlO model catalyst and core-shell pellet were only partially restored under the applied reaction conditions. Whereas partial regeneration on facet-like sites on supported catalysts is more facile than on Pt(111), carbonaceous deposits adsorbed at low-coordinated defect sites impede full regeneration of the Pt/AlO catalysts.
Keywords: Infrared spectroscopy; Liquid Organic Hydrogen Carrier; Materials gap; Model catalysis; Pressure gap; Real catalysis

Region-selective Deposition of Core–Shell Nanoparticles for 3 D Hierarchical Assemblies by the Huisgen 1,3-Dipolar Cycloaddition - Etschel, Sebastian; Portilla Berlanga, Luis; Kirschner, Johannes; Drost, Martin; Marbach, Hubertus; Tykwinski, Rik; Halik, Marcus (Angewandte Chemie International Edition, 2015)
A method for the region-selective deposition of nanoparticles (NPs) by the Huisgen 1,3-dipolar cycloaddition is presented. The approach enables defined stacking of various oxide NPs in any order with control over layer thickness. Thereby the reaction is performed between a substrate, functionalized with a self-assembled monolayer of an azide-bearing phosphonic acid (PA) and aluminum oxide (AlOx) NPs functionalized with an alkyne bearing PA. The layer of alkyne functionalized AlOx NPs is then used as substrate for the deposition of azide-functionalized indium tin oxide (ITO) NPs to provide a binary stack. This progression is then conducted with alkyne-functionalized CeO2 NPs, yielding a ternary stack of NPs with three different NP cores. The stacks are characterized by AFM and SEM, defining the region-selectivity of the deposition technique. Finally, these assemblies have been tested in devices as a dielectric to form a capacitor resulting in a dramatic increase in the measured capacitance.

Regularity Theory and Adjoint-Based Optimality Conditions for a Nonlinear Transport Equation with Nonlocal Velocity - Gröschel, Michael; Keimer, Alexander; Leugering, Günter; Wang, Zhiqiang (SIAM Journal on Control and Optimization, 2014)
In this contribution the optimal boundary control problem for a first order nonlinear, nonlocal hyperbolic PDE is studied. Motivated by various applications ranging from re-entrant manufacturing systems to particle synthesis processes, we establish the regularity of solutions for W1,p-data. Based on a general L2 tracking type cost functional, the existence, uniqueness, and regularity of the adjoint system in W1,p is derived using the special structure induced from the nonlocal flux function of the state equation. The assumption of W1,p - and not Lp-regularity comes thereby due to the fact that the adjoint equation asks for more regularity to be well defined. This problem is discussed in detail, and we give a solution by defining a special type of cost functional, such that the corresponding optimality system is well defined.

Relaminarization of wall turbulence by high-pressure ramps at low reynolds numbers - Song, Kwonyul; Jovanovic, Jovan; Al-Salaymeh, Ahmed; Rauh, Cornelia; Delgado, Antonio (Thermal Science, 2016)
Reverse transition from the turbulent towards the laminar flow regime was investigated experimentally by progressively increasing the pressure up to 400 MPa in a fully developed pipe flow operated with silicone oil as the working fluid. Using hot-wire anemometry, it is shown indirectly that at low Reynolds numbers a rapid increase in pressure modifies the turbulence dynamics owing to the processes which induce the effects caused by fluid compressibility in the region very close to the wall. The experimental results confirm that under such circumstances, the traditional mechanism responsible for self-maintenance of turbulence in wall-bounded flows is altered in such a way as to lead towards a state in which turbulence cannot persist any longe
UnivIS-Import:2018-02-22:Pub.2016.tech.ITC.stmmec.relami
Keywords: High pressure ramp; Relaminarization; Wall turbulence

Relation between exchange-only optimized potential and Kohn-Sham methods with finite basis sets, and effect of linearly dependent products of orbital basis functions - Görling, Andreas; Heßelmann, Andreas; Jones, M.; Levy, Mel (Journal of Chemical Physics, 2008)
Recently, Staroverov, Scuseria, and Davidson [J. Chem. Phys. 124, 141103 (2006)] presented examples of exchange-only optimized effective potential (xOEP) calculations that yield exactly the Hartree-Fock (HF) total energy. Here, building on their work, arguments showing under which conditions xOEP methods, with finite basis sets, do or do not yield the HF ground state energy but a higher one, are given. While the orbital products of a complete basis are linearly dependent, the HF ground state energy can only be obtained via a finite basis set xOEP scheme in the case that all products of occupied and unoccupied orbitals emerging from the employed orbital basis set are linearly independent of each other. Further, exchange potentials leading to the HF ground state energy likely exhibit unphysical oscillations and do not represent a Kohn-Sham (KS) exchange potential as a functional derivative of the exchange energy. These findings appear to explain the seemingly paradoxical results of Staroverov that certain finite basis set xOEP calculations lead to the HF ground state energy despite the fact that within a real space (or complete basis) representation, the xOEP ground state energy is always higher than the HF energy. Moreover, independent of whether or not the occupied and unoccupied orbital products are linearly dependent, it is shown that finite basis set xOEP methods only represent exact exchange-only (EXX) KS methods, i.e., proper density-functional methods, if the orbital basis set and the auxiliary basis set representing the exchange potential are balanced to each other, i.e., if the orbital basis is comprehensive enough for a given auxiliary basis. Otherwise xOEP methods do not represent EXX KS methods and yield unphysical exchange potentials. The question whether a xOEP method properly represents a KS method with an exchange potential that is a functional derivative of the exchange energy is related to the problem of the definition of local multiplicative operators in finite basis representations. Plane wave calculations for bulk silicon illustrate the findings of this work. © 2008 American Institute of Physic

Relation of Nanostructure and Recombination Dynamics in a Low-Temperature Solution-Processed CuInS2 Nanocrystalline Solar Cell - Azimi, Seyed Hamed; Heumüller, Thomas; Gerl, Andreas; Matt, Gebhard; Kubis, Peter; Distaso, Monica; Ahmad, Rameez; Akdas, Tugce; Richter, Moses; Peukert, Wolfgang; Brabec, Christoph (Advanced Energy Materials, 2013)
The understanding and control of nanostructures with regard to transport and recombination mechanisms is of key importance in the optimization of the power conversion efficiency (PCE) of solar cells based on inorganic nanocrystals. Here, the transport properties of solution-processed solar cells are investigated using photo-CELIV (photogenerated charge carrier extraction by linearly increasing voltage) and transient photovoltage techniques; the solar cells are prepared by an in-situ formation of CuInS2 nanocrystals (CIS NCs) at the low temperature of 270 °C. Structural and morphological analyses reveal the presence of a metastable CuIn5S8 phase and a disordered morphology in the CuInS2 nanocrytalline films consisting of polycrystalline grains at the nanoscale range. Consistent with the disordered morphology of the CIS NC thin films, the CIS NC devices are characterized by a low carrier mobility. The carrier density dynamic indicates that the recombination kinetics in these devices follows the dispersive bimolecular recombination model and does not fully behave in a diffusion-controlled manner, as expected by Langevin-type recombination. The mobility–lifetime product of the charge carriers properly explains the performance of the thin (200 nm) CIS NC solar cell with a high fill-factor of 64% and a PCE of over 3.5\
Keywords: nanostructures; power; conversion; nanocrystal

Relaxation of a spring with an attached granular damper - Kollmer, Jonathan; Sack, Achim; Heckel, Michael; Pöschel, Thorsten (New Journal of Physics, 2013)
The oscillation of a spring may be attenuated by means of a granular damper. In difference to viscous dampers, the amplitude decays nearly linearly in time up to a finite value, from there on it decays much slower. We quantitatively explain the linear decay, which was a long-standing question. © IOP Publishing and Deutsche Physikalische Gesellschaft.

Reliable Metal Deposition into TiO2 Nanotubes for Leakage-Free Interdigitated Electrode Structures and Use as a Memristive Electrode - Liu, Ning; Lee, Kiyoung; Schmuki, Patrik (Angewandte Chemie International Edition, 2013)
Nearly 100 % filling of TiO2 nanotubes with metals, including Ag, Cu, Au, and Pt, was achieved by defect-sealing treatment at the bottom of the nanotubes, followed by metal deposition using nuclei formation/coalescence. The resulting short-circuit-free interdigitated electrode configurations can, for example, be used to fabricate memristive electrodes.
EAM Import::2019-03-13

Residual Defect Density in Random Disks Deposits - Topic, Nikola; Pöschel, Thorsten; Gallas, Jason (Scientific Reports, 2015)
We investigate the residual distribution of structural defects in very tall packings of disks deposited randomly in large channels. By performing simulations involving the sedimentation of up to 50 × 10 9 particles we find all deposits to consistently show a non-zero residual density of defects obeying a characteristic power-law as a function of the channel width. This remarkable finding corrects the widespread belief that the density of defects should vanish algebraically with growing height. A non-zero residual density of defects implies a type of long-range spatial order in the packing, as opposed to only local ordering. In addition, we find deposits of particles to involve considerably less randomness than generally presumed.

Resolution of identity approach for the Kohn-Sham correlation energy within the exact-exchange random-phase approximation - Bleiziffer, Patrick; Heßelmann, Andreas; Görling, Andreas (Journal of Chemical Physics, 2012)
Two related methods to calculate the Kohn-Sham correlation energy within the framework of the adiabatic-connection fluctuation-dissipation theorem are presented. The required coupling-strength-dependent density-density response functions are calculated within exact-exchange time-dependent density-functional theory, i.e., within time-dependent density-functional response theory using the full frequency-dependent exchange kernel in addition to the Coulomb kernel. The resulting resolution-of-identity exact-exchange random-phase approximation (RI-EXXRPA) methods in contrast to previous EXXRPA methods employ an auxiliary basis set (RI basis set) to improve the computational efficiency, in particular, to reduce the formal scaling of the computational effort with respect to the system size N from N to N . Moreover, the presented RI-EXXRPA methods, in contrast to previous ones, do not treat products of occupied times unoccupied orbitals as if they were linearly independent. Finally, terms neglected in previous EXXRPA methods can be included, which leads to a method designated RI-EXXRPA, while the method without these extra terms is simply referred to as RI-EXXRPA. Both EXXRPA methods are shown to yield total energies, reaction energies of small molecules, and binding energies of noncovalently bonded dimers of a quality that is similar and in some cases even better than that obtained with quantum chemistry methods such as Moller-Plesset perturbation theory of second order (MP2) or with the coupled cluster singles doubles method. In contrast to MP2 and to conventional density-functional methods, the presented RI-EXXRPA methods are able to treat static correlation. © 2012 American Institute of Physics.

Resolving diffusion in clay minerals at different time scales: Combination of experimental and modeling approaches - Churakov, Sergey V.; Gimmi, Thomas; Unruh, Tobias; Van Loon, Luc R.; Juranyi, Fanni (Applied Clay Science, 2014)
The size of the interlayer and the size of Mt particles are two characteristic dimensions which determine the time dependent behavior of water diffusion in Mt. MD simulations show that at very short time scales water dynamics has the characteristic features of an oscillatory motion in the cage formed by neighbors in the first coordination shell. At longer time scales, the interaction of water with the surface determines the water dynamics, and the effect of confinement on the overall water mobility within the interlayer becomes evident. At time scales corresponding to an average water displacement equivalent to the average size of Mt particles, the effects of tortuosity are observed in the meso- to macroscopic pore scale simulations. Consistent with the picture obtained in the simulations, the QENS data can be described using a (local) 3D diffusion at short observation times, whereas at sufficiently long observation times a 2D diffusive motion is clearly observed. The effects of tortuosity measured in macroscopic tracer diffusion experiments are in qualitative agreement with RW simulations. By using experimental data to calibrate molecular and mesoscopic theoretical models, a consistent description of water mobility in clay minerals from the molecular to the macroscopic scale can be achieved. In turn, simulations help in choosing optimal conditions for the experimental measurements and the data interpretation. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: Restricted water diffusion; QENS; Molecular dynamics; 2D confinement; Montmorillonite

Resonance amplification of defect emission in ZnO-inverted opal - Khunsin, Worawut; Scharrer, Michael; Aagesen, Larry K.; Anderson, Mark A.; Chang, Robert P. H.; Sotomayor Torres, Clivia M.; Romanov, Sergei G. (Optics Letters, 2009)
Transformation of broadband emission of oxygen defects in the carcass of ZnO-inverted opal into a multiple-mode amplified spontaneous emission band has been observed in the spectral interval of a photonic bandgap upon increasing excitation intensity. The mode structure has been assigned to amplification of emission coupled to resonance modes of the self-selected distributed Bragg resonator. The surprisingly low 2 W/cm^2 onset of amplification has been explained by the long radiative decay time of defect states populated according to the three-level excitation scheme. The decrease of emission intensity between amplified peaks has been associated with the saturation of the ZnO defect emission.
EAM Import::2019-03-13

Resonant electron transport in single-molecule junctions: Vibrational excitation, rectification, negative differential resistance, and local cooling - Härtle, Rainer; Thoss, Michael (Physical Review B, 2011)
Vibronic effects in resonant electron transport through single-molecule junctions are analyzed. The study is based on generic models for molecular junctions, which include electronic states on the molecular bridge that are vibrationally coupled and exhibit Coulomb interaction. The transport calculations employ a master equation approach. The results, obtained for a series of models with increasing complexity, show a multitude of interesting transport phenomena, including vibrational excitation, rectification, negative differential resistance, as well as local cooling. While some of these phenomena have been observed or proposed before, the present analysis extends previous studies and allows a more detailed understanding of the underlying transport mechanisms. In particular, it is shown that many of the observed phenomena can only be explained if electron-hole pair creation processes at the molecule-lead interface are taken into account. Furthermore, vibronic effects in systems with multiple electronic states and their role for the stability of molecular junctions are analyzed. © 2011 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2011.nat.dphy.ITP.ppem.resona

Resonant metamaterials for contrast enhancement in optical lithography - Dobmann, Sabine; Shyroki, Dzmitry; Banzer, Peter; Erdmann, Andreas; Peschel, Ulf (Optics Express, 2012)
The transmission through ultra-thin metal films is noticeable and thus limits their potential for the formation of lithographic masks. By sub-wavelength patterning of a metal film with a post structure, a resonant metamaterial is formed, which can effectively suppress the transmission. Measurements as well as calculations identify the width of the metal islands as a critical geometrical feature. Hence, the extraordinarily low transmission effect can be explained by the resonant response of single scatterers known as Localized Surface Plasmon Resonances (LSPR). A potential application of this suppressed transmission effect to thin metal masks in optical lithography is experimentally investigated.
EAM Import::2019-03-13

Resonant Vibrations, Peak Broadening, and Noise in Single Molecule Contacts: The Nature of the First Conductance Peak - Secker, Daniel; Wagner, Stefan; Ballmann, Stefan Dieter; Härtle, Rainer; Thoss, Michael; Weber, Heiko B. (Physical Review Letters, 2011)
We carry out experiments on single-molecule junctions at low temperatures, using the mechanically controlled break junction technique. Analyzing the results obtained with various molecules, the nature of the first peak in the differential conductance spectra is elucidated. We observe an electronic transition with a vibronic fine structure, if the first peak occurs at small voltages. This regime can accurately be described by the resonant tunneling model. At higher voltages, additional smearing is observed and no fine structure can be resolved. A detailed analysis of the noise signal indicates that the onset of current is associated with strong fluctuations as a precursor of current flow. The data indicate that a complex fluctuation-driven transport mechanism takes over in this regime. © 2011 American Physical Society.
UnivIS-Import:2015-04-14:Pub.2011.nat.dphy.IAP.LAP.resona

Revealing Minor Electrical Losses in the Interconnecting Layers of Organic Tandem Solar Cells - Prosa, Mario; Li, Ning; Gasparini, Nicola; Bolognesi, Margherita; Seri, Mirko; Muccini, Michele; Brabec, Christoph (Advanced Materials Interfaces, 2017)
The interconnection layer (ICL) of organic multijunction solar cells represents one of the most delicate parts to ensure an efficient device operation. In view of pushing the efficiencies toward the theoretical limit, the individuation of minor losses affecting the ICL operation is of crucial importance. However, the difficulties arising from its position within the complex device structure typically hamper an accurate and selective investigation of the ICL. Here, a method based on the analysis of the photo-generated current density-voltage (Jph-V) response of solar cells, in the region of bias over the open-circuit voltage, is proved to individuate minor electrical losses within the ICL. Interestingly, the proposed method is demonstrated to effectively operate on tandem substructures, where different ICLs are investigated through the combination of materials having diverse characteristics. Furthermore, the use of a complementary investigation technique based on electroluminescence (EL) analysis allows to distinguish the specific nature of the electrical losses. The combination of Jph-V and EL analyses represents an elegant and advanced approach to shed light on the quality of ICLs in tandem substructures by avoiding the fabrication of the more complex tandem architecture. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Electrical losses; Electroluminescence; Interconnecting layers; Organic tandem solar cells; Photo-generated current

Reversible Hydrogenation of Graphene on Ni(111)-Synthesis of ’Graphone’ - Zhao, Wei; Gebhardt, Julian; Späth, Florian; Gotterbarm, Karin; Gleichweit, Christoph; Steinrück, Hans-Peter; Görling, Andreas; Papp, Christian (Chemistry - A European Journal, 2015)
Understanding the adsorption and reaction between hydrogen and graphene is of fundamental importance for developing graphene-based concepts for hydrogen storage and for the chemical functionalization of graphene by hydrogenation. Recently, theoretical studies of single-sided hydrogenated graphene, so called graphone, predicted it to be a promising semiconductor for applications in graphene-based electronics. Here, we report on the synthesis of graphone bound to a Ni(111) surface. We investigate the formation process by X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density-functional theory calculations, showing that the hydrogenation of graphene with atomic hydrogen indeed leads to graphone, that is, a hydrogen coverage of 1ML (4.2 wt %). The dehydrogenation of graphone reveals complex desorption processes that are attributed to coverage-dependent changes in the activation energies for the associative desorption of hydrogen as molecular H2 .
Keywords: dehydrogenation; density functional calculations; graphene; graphone; hydrogenation; X-ray photoelectron spectroscopy

Review - Single-Walled Carbon Nanohorn-Based Dye-Sensitized Solar Cells - Lodermeyer, Fabian; Costa, Ruben Dario; Guldi, Dirk Michael (ECS Journal of Solid State Science and Technology, 2017)
Efforts in the broad field of dye-sensitized solar cells (DSSC) are governed by four key challenges: i) the development of photoelectrodes with high charge injection yields and charge collection efficiencies, ii) the development of low-cost buffer layers, which effectively reduce the back reaction of electrons from the transparent conductive oxide (TCO) to the electrolyte, iii) the design of platinum-free counter electrodes (CE), which feature good electrolyte regeneration yields, and iv) the implementation of (quasi) solid-state and iodine-free electrolytes featuring excellent ionic diffusion and conductivity. The introduction of nanocarbons into each component of DSSCs has emerged as a promising strategy to tackle all the aforementioned challenges. Leading examples of nanocarbons are graphene, carbon black (CB), multi-walled carbon nanotubes (MWCNT), and single-walled carbon nanotubes (SWCNT). Here, we focus on single-walled carbon nanohorns (SWCNH), which were successfully integrated into each of the aforementioned parts of DSSCs.
EAM Import::2019-03-13

Rhodium-Phosphite SILP Catalysis for the Highly Selective Hydroformylation of Mixed C4 Feedstocks - Jakuttis, Michael; Schönweiz, Andreas; Werner, Sebastian; Franke, Robert; Wiese, Klaus-Diether; Haumann, Marco; Wasserscheid, Peter (Angewandte Chemie International Edition, 2011)
The cat. that got the butene: Highly active and selective diphosphite ligands are applied for rhodium-catalyzed supported ionic liquid phase (SILP) hydroformylation and make it possible to form linear n-pentanal with exceptional selectivity from a mixed butene feedstock by combined isomerization and hydroformylation (see picture, vials 1–4 show typical SILP catalysts employed in this study.). The SILP catalyst is stable for more than 800h on stream.

Rhodomyrtals A-D, four unusual phloroglucinol-sesquiterpene adducts from Rhodomyrtus psidioides - Shou, Qingyao; Smith, Joshua E.; Mon, Htwe; Brkljaca, Zlatko; Smith, Ana-Suncana; Smith, David M.; Griesser, Hans J.; Wohlmuth, Hans (RSC Advances, 2014)
Four novel compounds, rhodomyrtals A-D (1-4), with two unprecendented carbon frameworks of phloroglucinol coupled eudesmane, together with the known compound eucalyptin A (5) have been isolated from the leaves of the Australian plant Rhodomyrtus psidioides. The structures of compounds 1-4 were elucidated by spectroscopic analysis and ECD calculations. Some of the compounds showed good antibacterial activity against selected Gram-positive strains.
Keywords: TOMENTOSA AITON HASSK.; STAPHYLOCOCCUS-AUREUS; EUCALYPTUS-GLOBULUS; MYRTACEAE; CANDIDATE; LEAVES

Rigorous design of multiphase reactors: Identification of optimal conditions for mass transfer limited reactions - Xie, Mingquan; Freund, Hannsjörg (Chemical Engineering and Processing, 2018)
Multiphase reaction systems are often complex due to the interaction of reaction and transport processes. To design optimal reactors for such systems, a rigorous optimization is necessary. In this regard, this work proposes extensions of a reactor design methodology based on rigorous optimization. Each phase is represented by one fluid element, subject to different mass and energy fluxes. While the unlimited external fluxes are optimally provided along the reaction route, sensitivity analysis of the parameters related to the limited external fluxes is performed. The extended methodology is applied in the reactor design for chemical absorption of CO2 as an example for complex multiphase reaction systems with severe mass transfer limitation. An accurate model which is also numerically suitable for optimization has been set up and validated with pilot plant data from literature. Optimal temperature profiles which result from balancing the different temperature-dependent phenomena are identified for cases with different inlet conditions, and tailor-made heat control strategies are shown for these cases. The potential of the developments in improving mass transfer rates by, e.g., structural design or solvent design has been quantified. This model-based design methodology is generally applicable to different multiphase reaction systems with severe mass transfer limitation.
Keywords: Reactor design; Multiphase system; Mass transfer; Carbon dioxide; Chemical absorption; Process intensification

Robocasting of alumina lattice truss structures - Schlordt, Tobias; Keppner, Felix; Travitzky, Nahum; Greil, Peter (Journal of Ceramic Science and Technology, 2012)
Robocasting of aqueous colloidal α-AlO gels for manufacturing cellular ceramics with periodical lattice truss structures was investigated. Coagulation of gels loaded with 48 vol% α-Al O was induced by addingCHCOONH. The gels exhibit shear-thinning behavior, shear elastic moduli ranging from 6.7 to 390 kPa and yield-stresses from 25 to 570 Pa. Continuous filaments with a diameter of 0.5mmwere extruded with a deposition speed of up to 35 mm/s on a high-precision six-axis robotic system equipped with a single-screw micro-extruder. The lattice structures consist of alternating layers formed by a linear array of circular rods aligned parallel with a distance of 1mmand an angle of 90° between alternating layers. After being dried for 12 h, the robocast grids were sintered in air at 1650 °C for 2 hresulting in a fractional strut density > 0.95, amean filament diameter of 400 μm, a volume filling fraction of 0.49 (sealed walls) and 0.35 (meshed walls), and macro-cells in the deposition plane of quadratic shape with a mean area of 0.136mm ± 0.017mm Based on gravitation-driven viscous flow, model conditions for attaining free spanning ligaments were discussed. © 2012 Göller Verlag.
Keywords: Alumina gel; Lattice truss structures; Robocasting

Robust graphene membranes in a silicon carbide frame - Waldmann, Daniel; Butz, Benjamin; Bauer, Sebastian; Englert, Jan; Jobst, Johannes; Ullmann, Konrad; Fromm, Felix; Ammon, Maximilian Michael; Enzelberger-Heim, Michael; Hirsch, Andreas; Maier, Sabine; Schmuki, Patrik; et al. (ACS nano, 2013)
We present a fabrication process for freely suspended membranes consisting of bi- and trilayer graphene grown on silicon carbide. The procedure, involving photoelectrochemical etching, enables the simultaneous fabrication of hundreds of arbitrarily shaped membranes with an area up to 500 μm(2) and a yield of around 90%. Micro-Raman and atomic force microscopy measurements confirm that the graphene layer withstands the electrochemical etching and show that the membranes are virtually unstrained. The process delivers membranes with a cleanliness suited for high-resolution transmission electron microscopy (HRTEM) at atomic scale. The membrane, and its frame, is very robust with respect to thermal cycling above 1000 °C as well as harsh acidic or alkaline treatment.

Robust Pore Size Analysis of Filamentous Networks from Three-Dimensional Confocal Microscopy - Mickel, Walter; Münster, Stefan; Jawerth, Louise M.; Vader, David A.; Weitz, David A.; Sheppard, Adrian P.; Mecke, Klaus; Fabry, Ben; Schröder-Turk, Gerd (Biophysical Journal, 2008)
We describe a robust method for determining morphological properties of. lamentous biopolymer networks, such as collagen or other connective tissue matrices, from confocal microscopy image stacks. Morphological properties including pore size distributions and percolation thresholds are important for transport processes, e. g., particle diffusion or cell migration through the extracellular matrix. The method is applied to fluorescently labeled fiber networks prepared from rat-tail tendon and calf-skin collagen, at concentrations of 1.2, 1.6, and 2.4 mg/ml. The collagen fibers form an entangled and branched network. The medial axes, or skeletons, representing the collagen fibers are extracted from the image stack by threshold intensity segmentation and distance-ordered homotopic thinning. The size of the fiuid pores as defined by the radii of largest spheres that fit into the cavities between the collagen fibers is derived from Euclidean distance maps and maximal covering radius transforms of the fluid phase. The size of the largest sphere that can traverse the fluid phase between the collagen fibers across the entire probe, called the percolation threshold, was computed for both horizontal and vertical directions. We demonstrate that by representing the fibers as the medial axis the derived morphological network properties are both robust against changes of the value of the segmentation threshold intensity and robust to problems associated with the point-spread function of the imaging system. We also provide empirical support for a recent claim that the percolation threshold of a fiber network is close to the fiber diameter for which the Euler index of the networks becomes zero.

Role of Flagella in Adhesion of Escherichia coli to Abiotic Surfaces - Friedlander, Ronn S.; Vogel, Nicolas; Aizenberg, Joanna (Langmuir, 2015)
Understanding the interfacial activity of bacteria is of critical importance due to the huge economic and public health implications associated with surface fouling and biofilm formation. The complexity of the process and difficulties of predicting microbial adhesion to novel materials demand study of the properties of specific bacterial surface features and their potential contribution to surface attachment. Here, we examine flagella, cell appendages primarily studied for their cell motility function, to elucidate their potential role in the surface adhesion of Escherichia coli-a model organism and potential pathogen. We use self-assembled monolayers (SAMs) of thiol-bearing molecules on gold films to generate surfaces of varying hydrophobicity, and measure adhesion of purified flagella using quartz crystal microbalance. We show that flagella adhere more extensively and bind more tightly to hydrophobic SAMs than to hydrophilic ones, and we propose a two-step vs a single-step adhesion mechanism that accounts for the observed dissipation and frequency changes for the two types of surfaces, respectively. Subsequently, study of the adhesion of wild-type and flagella knockout cells confirms that flagella improve adhesion to hydrophobic substrates, whereas cells lacking flagella do not show preferred affinity to hydrophobic substrates. Together, these properties bring about an interesting ability of cells with flagella to stabilize emulsions of aqueous culture and dodecane, not observed for cells lacking flagella. This work contributes to our overall understanding of nonspecific bacterial adhesion and confirms that flagella, beyond motility, may play an important role in surface adhesion.

Role of Oxygen in Acetic Acid Decomposition on Pt(111) - Neitzel, Armin; Lykhach, Yaroslava; Johanek, Viktor; Tsud, Nataliya; Skala, Tomas; Prince, Kevin C.; Matolin, Vladimir; Libuda, Jörg (Journal of Physical Chemistry C, 2014)
We have investigated the role of coadsorbed atomic oxygen during the decomposition of acetic acid on Pt(111) by means of temperature-programmed desorption (TPD) and synchrotron radiation photoelectron spectroscopy (SRPES). Reaction mechanisms have been established through identification of desorbing products and surface species formed during decomposition of acetic acid, both on Pt(111) and oxygen pre-exposed p(2 X 2)-O/Pt(111). Acetate and molecularly adsorbed acetic acid are formed on both samples during the adsorption of acetic acid at 150 K. On p(2 X 2)-O/Pt(111), however, surface acetyl is identified as the principal species. The major decomposition channel for acetate and acetic acid involves formation of ketene and acetaldehyde at 222 K, and this reaction is not affected by coadsorbed oxygen. In the following reactions, partial decomposition of acetaldehyde yielded ethylene, ethylidene, ethylidyne, and small amounts of CO and methoxy on both samples. Above 222 K, decomposition of acetate on Pt(111) yields acetic acid, hydrogen, methane, and CO. In contrast, the species desorbing from p(2 X 2)-O/Pt(111) are the products of acetyl decomposition. In particular, the reaction of acetyl with atomic oxygen and surface hydroxyl groups yields methanol and acetic anhydride at 300 and 450 K, and methane and CO2 at 390 K. Decomposition of acetic acid on both Pt(111) and p(2 x 2)-O/Pt(111) results in surface carbon from decomposition of ethylidyne and partial C-C bond cleavage in the acetyl species.

Role of Short-Range Order and Hyperuniformity in the Formation of Band Gaps in Disordered Photonic Materials - Luis S., Froufe-Pérez; Engel, Michael; Damasceno, Pablo F.; Muller, Nicolas; Haberko, Jakub; Glotzer, Sharon C.; Scheffold, Frank (Physical Review Letters, 2016)
We study photonic band gap formation in two-dimensional high-refractive-index disordered materials where the dielectric structure is derived from packing disks in real and reciprocal space. Numerical calculations of the photonic density of states demonstrate the presence of a band gap for all polarizations in both cases. We find that the band gap width is controlled by the increase in positional correlation inducing short-range order and hyperuniformity concurrently. Our findings suggest that the optimization of short-range order, in particular the tailoring of Bragg scattering at the isotropic Brillouin zone, are of key importance for designing disordered PBG materials.

Role of Specific Intermolecular Interactions for the Arrangement of Ni(II)-5, 10, 15, 20-Tetraphenyltetrabenzoporphyrin on Cu(111) - Lepper, Michael; Zhang, Liang; Stark, Michael Werner; Ditze, Stefanie; Lungerich, Dominik; Jux, Norbert; Hieringer, Wolfgang; Steinrück, Hans-Peter; Marbach, Hubertus (Journal of Physical Chemistry C, 2015)
We have studied the coverage-dependent adsorption behavior of Ni(II)-5,10,15,20-tetraphenyltetra-benzoporphyrin on Cu(111) by scanning tunneling microscopy (STM) at room temperature. At medium coverages, the molecules self-assemble into two-dimensional islands, due to mutual stabilization through intermolecular interactions. Altogether, three different supramolecular arrangements coexist at low-to-medium coverages. On the basis of high-resolution STM images and density functional theory calculations, models for the three arrangements and the corresponding intramolecular conformations of the individual molecules are proposed. The observed polymorphism is attributed to a complex interplay of specific T-type and pi-pi stacking interactions between the phenyl groups. For Ni(II)-meso-tetrakis (4-tert-butylphenyl) benzoporphyrin, in which the aromatic periphery is modified by the attachment of tert-butyl groups, only one supramolecular arrangement on Cu(111) is found. This difference highlights the fact that the choice of peripheral ligands of the porphyrin derivatives plays an important role in the fabrication and tailoring of functional molecular architectures.

Role of Transparent Electrodes for High Efficiency TiO2 Nanotube Based Dye-Sensitized Solar Cells - Lee, Kiyoung; Kirchgeorg, Robin; Schmuki, Patrik (Journal of Physical Chemistry C, 2014)
In the present work, we investigate the performance of anodic TiO2 nanotube layers in dye-sensitized solar cells under front- and back-side illumination configurations. To fabricate transparent TiO2 nanotube electrodes, we evaporated 5 extgreekmm thick metallic Ti layers on FTO glass-then the metal layers are completely anodized to form aligned nanotube layers. We compare different types of FTO glass (conductivity and transparency) and use them for working electrodes as well as transparent platinized counter electrodes. The results show that for TiO2 nanotube electrodes the resulting light conversion efficiency in DSSCs is highly affected by the type of glass used (efficiency range-depending on configuration from 4.62% to 7.58%).
EAM Import::2019-03-13

Roll to roll compatible fabrication of inverted organic solar cells with a self-organized charge selective cathode interfacial layer - Zhang, Hong; Tan, Wan-Yi; Fladischer, Stefanie; Ke, Lili; Ameri, Tayebeh; Li, Ning; Turbiez, Mathieu; Spiecker, Erdmann; Zhu, Xu-Hui; Cao, Yong; Brabec, Christoph (Journal of Materials Chemistry A, 2016)
We successfully demonstrate a simple approach to printing efficient, inverted organic solar cells (OSCs) with a self-organized charge selective cathode interface layer based on the small-molecule Phen-NaDPO. Different from previous studies, Phen-NaDPO molecules were blended into a polymer/fullerene blend, comprising a low bandgap diketopyrrolopyrrole-quinquethiophene alternating copolymer pDPP5T-2 and phenyl-C-butyric acid methyl ester (PCBM), and processed by doctor blading in air. We observed a spontaneous, surface energy driven migration of Phen-NaDPO towards the ZnO interface and a subsequent formation of electron selective and barrier free extraction contacts. In the presence of 0.5 wt% Phen-NaDPO, a PCE of 5.4% was achieved for the inverted device based on an ITO/ZnO cathode. Notably, the photovoltaic performances remained at the same level with increasing the Phen-NaDPO concentration in the active layer from 0.25 to 1 wt%. Furthermore, this approach could be proven to effectively work with other cathodes such as bare ITO and ITO/AZO. The self-organization of Phen-NaDPO through spontaneous vertical phase separation is mainly attributed to its high surface energy and strong interaction with the cathode material. The present results highlight that a self-organized cathode interfacial material processed from a "ternary" active layer is fully compatible with the requirements for roll-to-roll fabrication of inverted organic solar cells.
Keywords: Engineering controlled terms: Butyric acid; Cathodes; Electrodes; Interfacial energy; Molecules; Organic solar cells; Phase separation Alternating copolymer; Cathode interfacial layers; Diketopyrrolopyrroles; High surface energy; Inverted organic solar cells; Photovoltaic performance; Roll-to-roll fabrication; Vertical phase separations Engineering main heading: Solar cells

Rotating robots move collectively and self-organize. - Scholz, Christian; Engel, Michael; Pöschel, Thorsten (Nature Communications, 2018)
Biological organisms and artificial active particles self-organize into swarms and patterns. Open questions concern the design of emergent phenomena by choosing appropriate forms of activity and particle interactions. A particularly simple and versatile system are 3D-printed robots on a vibrating table that can perform self-propelled and self-spinning motion. Here we study a mixture of minimalistic clockwise and counter-clockwise rotating robots, called rotors. Our experiments show that rotors move collectively and exhibit super-diffusive interfacial motion and phase separate via spinodal decomposition. On long time scales, confinement favors symmetric demixing patterns. By mapping rotor motion on a Langevin equation with a constant driving torque and by comparison with computer simulations, we demonstrate that our macroscopic system is a form of active soft matter.

Roughness of silicon nanowire sidewalls and room temperature photoluminescence - Sivakov, Vladimir A.; Voigt, Felix; Berger, Andreas; Bauer, Gottfried; Christiansen, Silke H. (Physical Review B, 2010)
Strong room temperature visible (red-orange) photoluminescence (PL) has been observed in silicon nanowires (SiNWs) that were realized by wet chemical etching of heavily (arsenic, As: 1020 cm-3) and lowly doped (boron, B: 1015 cm-3) single crystalline silicon (Si) wafers. Optical characterization of these SiNWs by PL combined with structural characterization by transmission and scanning electron microscopy strongly suggest that the visible PL at room temperature results from the rough SiNW sidewall structure that is composed of nanoscale features in which quantum confinement effects may occur.
EAM Import::2019-03-13

Round-Robin Studies on Roll-Processed ITO-free Organic Tandem Solar Cells Combined with Inter-Laboratory Stability Studies - Livi, Francesco; Sondergaard, Roar R.; Andersen, Thomas R.; Roth, Berenger; Gevorgyan, Suren; Dam, Henrik F.; Carle, Jon E.; Helgesen, Martin; Spyropoulos, Georgios; Adams, Jens; Ameri, Tayebeh; Brabec, Christoph; et al. (Energy Technology, 2015)
Roll-processed, indium tin oxide (ITO)-free, flexible, organic tandem solar cells and modules have been realized and used in round-robin studies as well as in parallel inter-laboratory stability studies. The tandem cells/modules show no significant difference in comparison to their single-junction counterparts and the use of round-robin studies as a consensus tool for evaluation of organic solar cell parameters is judged just as viable for the tandem solar cells as for single-junction devices. The inter-laboratory stability studies were conducted according to testing protocols ISOS-D-2, ISOS-D-3, and ISOS-L-2, and in spite of a much more complicated architecture the organic tandem solar cells show no significant difference in stability in comparison to their single-junction counterparts. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: ITO-free; Photovoltaics; Roll processing; Stability; Tandem solar cells

Ru-doped TiO2 nanotubes: Improved performance in dye-sensitized solar cells - So, Seulgi; Lee, Kiyoung; Schmuki, Patrik (Physica Status Solidi-Rapid Research Letters, 2012)
In the present work we use a series of Ti–Ru alloys, with minor amounts of Ru (0.01, 0.02, 0.05 and 0.2 at%) to grow anodic self-organized Ru-doped TiO2 nanotube layers. When used in dye-sensitized solar cells (DSSCs), the nanotube layers with an optimum amount of Ru (0.02 at% Ru in the alloy) show a considerable increase in solar cell efficiency ( extgreekh = 5.2%) under AM1.5 (100 mW/cm2) conditions compared with non-doped TiO2 nanotubes ( extgreekh = 4.3%).
EAM Import::2019-03-13

Scalable, ambient atmosphere roll-to-roll manufacture of encapsulated large area, flexible organic tandem solar cell modules - Andersen, Thomas R.; Dam, Henrik F.; Hosel, Markus; Helgesen, Martin; Carle, Jon E.; Larsen-Olsen, Thue T.; Gevorgyan, Suren A.; Andreasen, Jens W.; Adams, Jens; Li, Ning; Machui, Florian; Spyropoulos, Georgios; et al. (Energy and Environmental Science, 2014)
Inline printing and coating methods have been demonstrated to enable a high technical yield of fully roll-to-roll processed polymer tandem solar cell modules. We demonstrate generality by employing different material sets and also describe how the ink systems must be carefully co-developed in order to reach the ambitious objective of a fully printed and coated 14-layer flexible tandem solar cell stack. The roll-to-roll methodologies involved are flexographic printing, rotary screen printing, slot-die coating, X-ray scattering, electrical testing and UV-lamination. Their combination enables the manufacture of completely functional devices in exceptionally high yields. Critical to the ink and process development is a carefully chosen technology transfer to industry method where first a roll coater is employed enabling contactless stack build up, followed by a small roll-to-roll coater fitted to an X-ray machine enabling in situ studies of wet ink deposition and drying mechanisms, ultimately elucidating how a robust inline processed recombination layer is key to a high technical yield. Finally, the transfer to full roll-to-roll processing is demonstrated. This journal is © the Partner Organisations 2014.
Keywords: Engineering controlled terms: Coatings; Manufacture; Technology transfer; Testing Ambient atmosphere; Flexographic printing; Functional devices; Polymer tandem solar cells; Process development; Recombination layers; Roll-to-roll processing; Rotary screen printing Engineering main heading: Solar cells GEOBASE Subject Index: coating; photovoltaic system; polymer; solar power; technological development

Scalable production of graphene sheets by mechanical delamination - Knieke, Catharina; Berger, Angela; Voigt, Michael; Klupp Taylor, Robin; Röhrl, Jonas; Peukert, Wolfgang (Carbon, 2010)
Mono- and multilayer graphene sheets have been successfully produced from commercial graphite powder in a wet grinding process under mild milling conditions. The shear forces in the milling chamber lead to a continuous delamination of ultrathin graphene flakes which are dispersed in a liquid medium. To avoid agglomeration of the exfoliated flakes the anionic surfactant sodium dodecyl sulfate was used. By adjusting the process parameters in a way to overcome the weak interlayer forces between the sheets without breaking them leads to the fabrication of thin flakes with high aspect ratios. The presented scalable process allows a high-yield and low-cost production of free-standing graphene sheets for various application
Keywords: monolayer; graphene; graphite; wet; grinding; interlayer; forces; flake; scaleable; sheet

Scale adaptive simulation of bubble column flows - Masood, Rao Muhammad Atif; Khalid, Yasir Noman; Delgado, Antonio (Chemical Engineering Journal, 2015)
Numerical investigations using scale adaptive simulation (SAS) in 3D transient Euler-Euler framework for bubble columns have been carried out and all the simulation results are compared with available experimental data. The performance of SAS model was also compared with re-normalization group (RNG), large eddy simulation (LES) and k-. ε models. Numerical results are in good agreement with experiments in predicting time averaged turbulent kinetic energy and axial velocity profiles. The superior flow predicting ability of SAS model is highlighted and it has shown that SAS turbulence model can capture large scale turbulence behavior and can be used efficiently for simulating flow field and turbulent quantities in case of bubble column flows.
UnivIS-Import:2015-03-09:Pub.2015.tech.ITC.stmmec.scalea
Keywords: Bubble column; Computational fluid dynamics; Scale adaptive simulation; Turbulence models

Scanning-Raman-microscopy for the statistical analysis of covalently functionalized graphene - Englert, Jan; Vecera, Philipp; Knirsch, Kathrin; Schäfer, Ricarda; Hauke, Frank; Hirsch, Andreas; Hauke, Frank (Acs Nano, 2013)
We report on the introduction of a systematic method for the quantitative and reliable characterization of covalently functionalized graphene based on Scanning-Raman-Microscopy (SRM). This allows for recording and analyzing several thousands of Raman spectra per sample and straightforward display of various Raman properties and their correlations with each other in histograms or coded 2D-plots. In this way, information about the functionalization efficiency of a given reaction, the reproducibility of the statistical analysis, and the sample homogeneity can be easily deduced. Based on geometric considerations, we were also able to provide, for the first time, a correlation between the mean defect distance of densely packed point defects and the Raman I/I ratio directly obtained from the statistical analysis. This proved to be the prerequisite for determining the degree of functionalization, termed θ. As model compounds, we have studied a series of arylated graphenes (GPh) for which we have developed new synthetic procedures. Both graphite and graphene grown by chemical vapor deposition (CVD) were used as starting materials. The best route toward GPh consisted of the initial reduction of graphite with a Na/K alloy in 1,2-dimethoxyethane (DME) as it yields the highest overall homogeneity of products reflected in the widths of the Raman I /I histograms. The Raman results correlate nicely with parallel thermogravimetric analysis (TGA) coupled with mass spectrometry (MS) studies. © 2013 American Chemical Society.

Science meets Fiction. Nano-Modellierungen: Poesie und Mikroskopie - Heydenreich, Aura; Mecke, Klaus (homunculus verlag, 2018)
Bilder von der Natur im 21. Jahrhundert – das müssen nicht mehr nur Bilder von Wäldern, Tieren oder dem Mond sein, sondern können auch Dinge und Lebewesen des Nanokosmos zeigen, ins Auge gefasst durch ein Mikroskop. Was aber würde ein literarisches Mikroskop zum Vorschein bringen? Diese Frage stellte das Exzellenzcluster Engineering of Advanced Materials (EAM), ein interdisziplinärer Forschungsverbund an der Friedrich- Alexander-Universität Erlangen-Nürnberg (FAU), der neuartige Hochleistungsmaterialien erforscht und entwickelt. Mit Texten von Maria Anne Anders, Victoria Bauer, Thora Engel, Tobias Falberg, Suzanne Fischer, Axel Görlach, Birgit Honikel, Ricarda Howe, Mathias Jeschke, Philipp Kampa, Michael Kanofsky, Christine Kappe, Michael Köhler, Kristiane Kondrat, Simone Kornappel, Petra Ina Lang, Friederike Langer, Ute Paul, Andreas Peukert, Stephan Roiss, Wolfgang Roth, Veronica Rummel-Damian, Tanja Rupel Tera, Franziska Ruprecht, Kirsten Sanders, Barbara Schibli, Claus-Peter Schmidt, Rolf Schönlau, Anna Stern, Marlies Strübbe-Tewes, Gerhard Stübner, Cornelia Travnicek, Dorothea Weinfurtner, Bernhard Winter, Melanie Wolf und Franziska Wotzinge

Screening of Supported Ionic Liquid Phase (SILP) catalysts for the very low temperature water–gas-shift reaction - Werner, Sebastian; Szesni, Normen; Bittermann, Agnes; Schneider, Martin; Härter, Peter; Haumann, Marco; Wasserscheid, Peter (Applied Catalysis A-General, 2010)
Screening of transition metal complexes typically involves elaborative experiments in batch reactors in organic solvents under non-steady-state conditions. In contrast, screening of immobilized catalysts in a fixed-bed continuous gas-phase reactor enables highly efficient and precise investigations on activity and stability under steady-state. In this work we present a technique to immobilize numerous homogeneous transition metal complexes (Ru, Ir, Os, Fe, Cu, Mo, Pd Re, Rh) in the ionic liquid [BMMIM][OTf] on the highly porous support Silica 100 using the Supported Ionic Liquid Phase (SILP) technology. All prepared SILP catalysts were investigated in the water–gas shift reaction (WGSR) in a continuously operated fixed-bed reactor at temperatures between 120 and 160 °C and ambient pressure. Best activities and stabilities were found for ruthenium-based systems.

Second Harmonic Light Scattering from Spherical Polyelectrolyte Brushes - Peukert, Wolfgang; Schürer, Benedikt; Hoffmann, Martin; Wunderlich, Sarina; Peschel, Ulf; Harnau, Ludger; Ballauf, Matthias (Journal of Physical Chemistry C, 2011)
Optical second harmonic generation (SHG) was applied to study the shell layer of spherical polyelectrolyte brushes (SPB) as a function of ionic strength. The particles consist of a polystyrene core from which linear chains of a quenched polyelectrolyte are grafted. Compared to all systems studied by SHG previously, the shell layer in this study has a remarkable thickness of ca. 100 nm. Angle-resolved second harmonic (SH) scattering profiles were recorded and compared to simulations with different model geometries using a nonlinear Mie scattering model. The analysis reveals that the second harmonic signal is not generated by the polyelectrolyte molecules, but rather by oriented and polarized water molecules at the inner (polystyrene / polyelectrolyte) and at the outer (polyelectrolyte / solvent) interface of the polyelectrolyte shell. The results demonstrate that angle-resolved SH light scattering provides direct access to the structure of water and the screening of charges at the inner and outer interface of the polyelectrolyte shell. Correspondingly, this work sets the experimental and theoretical basis for the study of the extended interfacial layer of electrosterically stabilized particles by SH light scattering.
Keywords: optical; second; harmonic; generation; particle; polyelectrolyte

Selection of potential iron oxide nanoparticles for breast cancer treatment based on in vitro cytotoxicity and cellular uptake - Poller, Johanna; Zaloga, Jan; Schreiber, Eveline; Unterweger, Harald; Janko, Christina; Radon, Patricia; Eberbeck, Dietmar; Trahms, Lutz; Alexiou, Christoph; Friedrich, Ralf P. (International Journal of Nanomedicine, 2017)
Superparamagnetic iron oxide nanoparticles (SPIONs) are promising tools for the treatment of different diseases. Their magnetic properties enable therapies involving magnetic drug targeting (MDT), hyperthermia or imaging. Depending on the intended treatment, specific characteristics of SPIONs are required. While particles used for imaging should circulate for extended periods of time in the vascular system, SPIONs intended for MDT or hyperthermia should be accumulated in the target area to come into close proximity of, or to be incorporated into, specific tumor cells. In this study, we determined the impact of several accurately characterized SPION types varying in size, zeta potential and surface coating on various human breast cancer cell lines and endothelial cells to identify the most suitable particle for future breast cancer therapy. We analyzed cellular SPION uptake, magnetic properties, cell proliferation and toxicity using atomic emission spectroscopy, magnetic susceptometry, flow cytometry and microscopy. The results demonstrated that treatment with dextran-coated SPIONs (SPIONDex) and lauric acid-coated SPIONs (SPIONLA) with an additional protein corona formed by human serum albumin (SPIONLA-HSA) resulted in very moderate particle uptake and low cytotoxicity, whereas SPIONLAhad in part much stronger effects on cellular uptake and cellular toxicity. In summary, our data show significant dose-dependent and particle type-related response differences between various breast cancer and endothelial cells, indicating the utility of these particle types for distinct medical applications.
EVALuna2:33205

Selective catalytic conversion of biobased carbohydrates to formic acid using molecular oxygen - Woelfel, Rene; Taccardi, Nicola; Bösmann, Andreas; Wasserscheid, Peter (Green Chemistry, 2011)
A new and straightforward method to transform carbohydrate-based biomass to formic acid (FA) by oxidation with molecular oxygen in aqueous solution using a Keggin-type H5PV2Mo10O40 polyoxometalate as catalyst is presented. Several water-soluble carbohydrates were fully and selectively converted to formic acid and CO2 under very mild conditions. It is worth noting, even complex biomass mixtures, such as poplar wood sawdust, were transformed to formic acid, giving 19 wt% yield (11% based on the carbon atoms in the feedstock) under non-optimized conditions.

Selective catalytic oxidation of humins to low-chain carboxylic acids with tailor-made polyoxometalate catalysts - Maerten, Stephanie; Voß, Dorothea; Liauw, Marcel; Albert, Jakob (ChemistrySelect, 2017)
For the first time, a selective catalytic conversion of thermally induced side products (humins) from biomass valorization to short-chain carboxylic acids like formic acid and acetic acid is reported. Very interestingly, we found that homogeneous polyoxometalate catalysts could selectively oxidize humins from sugar-derived levulinic acid production to formic acid and acetic acid under very mild conditions in a simple monophasic aqueous approach. This green and sustainable process offers a very cost-effective and sustainable way for the conversion of undesired humins and benefits from its low reaction temperature (90-120 °C) and its tunable product distribution. By variation of process parameters and chemical composition of the POM structures, we could control both product selectivity and humin conversion. Furthermore, catalyst recycling over three batches has been successfully carried out.

Selective electron beam melting of cellular titanium: Mechanical properties - Heinl, Peter; Körner, Carolin; Singer, Robert (Advanced Engineering Materials, 2008)
The mechanical properties of cellular titanium with non-stochastic structure fabricated through Selective Electron Beam Melting was investigated. The mechanical behavior depending on different unit cell sizes and process settings to provide a starting basis for optimizing the manufacturing process and predicting the mechanical properties was also investigated. Cellular titanium structures were fabricated with an electron beam melting facility and constant layer thickness of 70 μm. The mechanical properties of the fabricated cellular structures were evaluated in case of the dynamic elastic modulus by the impulse excitation technique. The determination of the dynamic elastic Modulus was conducted with an impulse excitation apparatus conforming to ASTM E 1876 standard. It was observed that a change in the quality of the structure could not be observed with different process settings.

Selective oxidation of lignocellulosic biomass to formic acid and high-grade cellulose with tailor-made polyoxometalate catalysts - Albert, Jakob (Faraday Discussions, 2017)
The main goal of this project was to identify and to optimize tailor-made polyoxometalate catalysts for a fractionated oxidation of lignocellulosic biomass (i.e. wood and residues from sugar or paper industries) to produce formic acid (FA) and high-grade cellulose for further processing i.e. in white biotechnology to bio-ethanol. Homogeneous vanadium precursor like sodium metavanadate and vanadyl sulfate as well as Keggin-type polyoxometalates (POMs) and more exotic structures like Anderson-, Wells-Dawson and Lindqvist type POMs have been screened for the desired catalytic performance. The most promising behaviour was found using the Lindqvist-type POM K5V3W3O19 showing for the first time in literature a selective oxidation of solely hemicellulose and lignin to formic acid while the cellulose fraction was untrapped. However, this could be only a first step for the project goal as little product yields could be achieved.
EAM Import::2019-03-13

Selectivity enhancement in the catalytic hydrogenation of propionitrile using ionic liquid multiphase reaction systems - Obert, Katharina; Roth, Daniel; Ehrig, Martin; Schönweiz, Andreas; Assenbaum, Daniel; Lange, Harald; Wasserscheid, Peter (Applied Catalysis A-General, 2009)
A new ionic liquid based, multiphase reaction system was investigated for the catalytic hydrogenation of propionitrile. The reaction system under investigation consisted of a solid heterogeneous Ru-catalyst (Ru on carbon), two liquid phases and the hydrogen gas-phase. By using two liquid phases in the reactor – of which one was an ionic liquid – it was possible to improve significantly the selectivity for the formation of propylamine by suppressing consecutive reactions to dipropylamine and tripropylamine. This enhanced selectivity resulted either from the protonation of the primary formed amine (in case of the Brønsted acidic ionic liquids dimethylcyclohexylammonium hydrogensulfate and 1-butylimidazolium hydrogensulfate) or from the extraction of the primary amine into the organic phase (in case of a 1-ethyl-3-methylimidazolium ethylsulfate/1,2,4-trichlorobenzene biphasic system).

Self-alignment of zinc oxide nanorods into a 3D-smectic phase - Schäfer, Stefan; Srikantharajah, Rubitha; Klaumünzer, Martin; Lobaz, Volodymyr; Voigt, Michael; Peukert, Wolfgang (Thin Solid Films, 2014)
We focus on thin films from ZnO nanorod dispersions in order to generate a high lateral order and a dense space filling structure applicable e. g. in printed optoelectronic devices. Highly stable dispersions with a solid content of up to 60 wt.% in ethanol were prepared using trioxadecanoic acid as stabilizer. With the dispersions we prepared thin films on substrates with a 3D-smectic phase in lateral dimension of cm2 and thicknesses from below 100 nm up to 1 μm. We show how the concentration of the initial solid content is influencing the formation of domains in our films and discuss the self-assembly process into ordered structures.

Self-Assembled Monolayer Exchange Reactions as a Tool for Channel Interface Engineering in Low-Voltage Organic Thin-Film Transistors - Lenz, Thomas; Schmaltz, Thomas; Novak, Michael; Halik, Marcus (Langmuir, 2012)
In this work, we compared the kinetics of monolayer self-assembly long-chained carboxylic acids and phosphonic acids on thin aluminum oxide surfaces and investigated their dielectric properties in capacitors and low-voltage organic thin-film transistors. Phosphonic acid anchor groups tend to substitute carboxylic acid molecules on aluminum oxide surfaces and thus allow the formation of mixed or fully exchanged monolayers. With different alkyl chain substituents (n-alkyl or fluorinated alkyl chains), the exchange reaction can be monitored as a function of time by static contact angle measurements. The threshold voltage in α,α′-dihexyl-sexithiophene thin-film transistors composed of such mixed layer dielectrics correlates with the exchange progress and can be tuned from negative to positive values or vice versa depending on the dipole moment of the alkyl chain substituents. The change in the dipole moment with increasing exchange time also shifts the capacitance of these devices. The rate constants for exchange reactions determined by the time-dependent shift of static contact angle, threshold voltage, and capacitance exhibit virtually the same value thus proving the exchange kinetics to be highly controllable. In general, the exchange approach is a powerful tool in interface engineering, displaying a great potential for tailoring of device characteristics.

Self-Assembled Monolayer Field-Effect Transistors based on Oligo-9,9’-dioctylfluorene Phosphonic Acids - Gothe, Bastian; de Roo, Tjaard; Will, Johannes; Unruh, Tobias; Mecking, Stefan; Halik, Marcus (Nanoscale, 2017)
The use of functional oligomers of pi-conjugated oligofluorenes led to a region-selective assembly of amorphous monolayers which exhibit robust lateral charge transport pathways in self-assembled monolayer field-effect transistors over long distances and even in mixed monolayers of semiconducting and insulating molecules. This oligomer concept might stimulate a new molecular design of self-assembling semiconducting materials.

Self-Assembled Monolayers Get Their Final Finish via a Quasi-Langmuir-Blodgett Transfer - Meltzer, Christian; Dietrich, Hanno; Zahn, Dirk; Peukert, Wolfgang; Braunschweig, Björn (Langmuir, 2015)
The growth of self-assembled monolayers (SAMs) of octadecylphosphonic acid (ODPA) molecules on α-Al2O3(0001) and subsequent dewetting of the SAMs were studied with a combination of in situ sum-frequency generation (SFG) and molecular dynamics (MD) simulations. Although SAM growth after deposition times >8 h reduces to nearly negligible values, the resultant ODPA SAMs in solution are still not in a well-ordered state with the alkyl chains in all-trans configurations. In fact, in situ SFG spectroscopy revealed a comparatively high concentration of gauche defects of the SAM in the ODPA 2-propanol solution even after a growth time of 16 h. Here, results of the MD simulations strongly suggest that defects can be caused by ODPA molecules which are not attached to the substrate but are incorporated into the SAM layer with the polar headgroup oriented into the 2-propanol solvent. This inverted adsorption geometry of additional ODPA molecules blocks adsorption sites and thus stabilizes the SAM without improving ordering to an extent that all molecules are in the all-trans configuration. While persistent in solution, the observed defects can be healed out when the SAMs are transferred from the solvent to a gas phase. During this process, a quasi-Langmuir-Blodgett transfer of molecules takes place which drives the SAM into a higher conformational state and significantly improves its quality.

Self-assembling depsipeptide dendrimers and dendritic fullerenes with new cis- and trans-symmetric Hamilton receptor functionalized zn-porphyrins: Synthesis, photophysical properties and cooperativity phenomena - Maurer, Katja; Grimm, Bruno; Wessendorf, Florian; Hartnagel, Kristine; Guldi, Dirk Michael; Hirsch, Andreas (European Journal of Organic Chemistry, 2010)
Two novel supramolecular building blocks, namely cis- and trans-symmetric zinc-porphyrins bearing Hamilton receptors as their focal points (10 and 15), were self-assembled with (i) first-, second- and third-generation depsipeptide cyanurates (21-23); (ii) second-generation dendrofullerene (24), and (iii) cyanuric acid substituted fullerene (20), and probed in a series of photophysical investigations. Importantly, an overall 1:2 stoichiometry was confirmed with binding con-stants Kn, and Hill coefficients nH that indicate remarkable cooperativities. As a general feature, stronger binding evolves from the trans-symmetric porphyrin isomer, although the increased steric demand of the depsipeptide ligands means that the binding constants decrease with each generation. The self-assembly of 10 or 15 with 20 affords novel electron donor-acceptor hybrids that reveal, upon excitation, intra-hybrid charge-transfer events. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Self-assembly and coverage dependent thermally induced conformational changes of Ni(II)-meso-tetrakis (4-tert-butylphenyl) benzoporphyrin on Cu(111) - Zhang, Liang; Lepper, Michael; Stark, Michael Werner; Lungerich, Dominik; Jux, Norbert; Hieringer, Wolfgang; Steinrück, Hans-Peter; Marbach, Hubertus (Physical Chemistry Chemical Physics, 2015)
A systematic scanning tunnelling microscopy investigation of the self-assembly and of thermally induced conformational changes of Ni(II)-meso-tetrakis (4-tert-butylphenyl) benzoporphyrin (Ni-TTBPBP) on Cu(111) is presented. At room temperature, Ni-TTBPBPs diffuse on the surface and self-assemble into ordered islands with well-defined registry to the substrate, with two different azimuthal orientations. The formation of the characteristic supramolecular structure is attributed to van der Waals interactions between the tert-butyl groups. Upon moderate heating, the intramolecular conformation changes irreversibly due to a dehydrogenative intramolecular aryl-aryl coupling reaction. This reaction is coverage dependent, with a lower rate at higher initial coverage; this behaviour is attributed to a stabilization of Ni-TTBPBP in the ordered islands at higher coverage.

Self-Assembly and Stability of Hydrogen-Bonded Networks of Bridged Triphenylamines on Au(111) and Cu(111) - Steiner, Christian; Gliemann, Bettina; Meinhardt, Ute; Gurrath, Martin; Meyer, Bernd; Kivala, Milan; Maier, Sabine (Journal of Physical Chemistry C, 2015)
The adsorption, chemical nature, and self-assembly of diaminotriazinyl- and carboxyl-substituted triphenylamines with dimethylmethylene bridges were studied on Au(111) and Cu(111) at submonolayer coverage by low-temperature scanning tunneling microscopy and density functional theory. On Au(111), both molecules form extended porous honeycomb networks. The geometry of the networks agrees well with density functional theory optimized hydrogen-bonded gas phase structures. Therefore, the self-assemblies on Au(111) are strongly directed by intermolecular hydrogen bond interactions. In contrast, on Cu(111) both molecules aggregate in dense islands owing to the stronger moleculesurface interaction. While the carboxyl substituents partially deprotonate at room temperature on Cu(111), the diaminotriazinyl-substituted triphenylamines adsorb mainly intact. The diaminotriazinyl groups deprotonate gradually at increased adsorption temperatures.

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials - Engel, Michael; Talapin, Dmitri V.; Boles, Michael A. (Chemical Reviews, 2016)
Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micrometer colloids and block copolymer assembly. We outline the extensive catalog of superlattices prepared to date using hydrocarbon-capped nanocrystals with spherical, polyhedral, rod, plate, and branched inorganic core shapes, as well as those obtained by mixing combinations thereof. We also provide an overview of structural defects in nanocrystal superlattices. We then explore the unique possibilities offered by leveraging nontraditional surface chemistries and assembly environments to control superlattice structure and produce nonbulk assemblies. We end with a discussion of the unique optical, magnetic, electronic, and catalytic properties of ordered nanocrystal superlattices, and the coming advances required to make use of this new class of solids.

Self-assembly of structurally persistent micelles is controlled by specific-ion effects and hydrophobic guests - Jäger, Christof; Hirsch, Andreas; Schade, Boris; Ludwig, Kai; Böttcher, Christoph; Clark, Timothy (Langmuir, 2010)
A combined study using cryo-TEM experiments and molecular dynamics simulations reveals remarkable details of the factors that affect the self-organization of specifically designed T-shaped amphiphilic dendrimers upon treatment of an aqueous solution with ultrasound under a layer of hexane. This treatment leads to dodecameric, structured micelles rather than the heptameric ones observed without hexane. Three-dimensional reconstruction of the cryo-TEM images provides very detailed structures of the micelles, and molecular dynamics simulations suggest that approximately 36 hexane molecules are needed to stabilize the dodecameric micelles. Sodium counterions are found to exert a significant stabilizing effect that results in an apparent attraction between the highly negatively charged polycarboxylate headgroups. DFT calculations support the observation that the formation of ion multiplets is especially crucial for this stabilizing counterion effect, which reduces headgroup repulsion. This and the increased hydrophobic stabilization that results from the hexane-enlarged core of the micelle lead to stable dodecameric micelles. The specific effects found for sodium counterions are largely absent for potassium. © 2009 American Chemical Society.

Self-assembly of supramolecular architectures and polymers by orthogonal metal complexation and hydrogen-bonding motifs - Grimm, Felix; Ulm, Nadine; Gröhn, Franziska; Düring, Jasmin; Hirsch, Andreas (Chemistry - A European Journal, 2011)
A modular construction kit with two orthogonal noncovalent binding sites for self-assembly of supramolecular architectures is presented. The heteroditopic building blocks contain a terpyridine (tpy) unit for coordination of metal ions and a Hamilton receptor for multiple H-bonding of cyanuric acid derivatives. The association constants of ligand binding of M complexes (M=Ru, Zn, Fe, and Pt) with a dendritic end cap were determined to be in the range of 10 and 10 L mol in chloroform. The capabilities for binding of metal ions were investigated by H NMR and UV/Vis spectroscopy. The Fe complexes are most appropriate for the generation of discrete and high-ordered architectures due to their strong tendency to form FeL complexes. Superstructures are readily formed in a one-pot procedure at room temperature. No mutual interactions between the orthogonal binding motifs were observed, and this demonstrates the highly specific nature of each binding process. Decomplexation experiments were carried out to examine the reversibility of Fe-tpy coordination. Substitution of the terminal end cap with a homoditopic bis-cyanurate linkage leads to formation of an iron-containing supramolecular strand. Formation of coordination polymers was confirmed by viscosity measurements. The supramolecular polymer strands can be reversibly cleaved by addition of a terminating cyanuric acid building block, and this proves the dynamic nature of this noncovalent polymerization process. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Self-assembly of supramolecular oligo-phenylene-ethynylene wires consisting of double Hamilton receptor modified OPE rods and a tetraphenylporphyrin cyanurate - Wessendorf, Florian; Hirsch, Andreas (Tetrahedron, 2008)
The syntheses and characterizations of new oligo-phenylene-ethynylene (OPE) bridged bis-Hamilton receptors 8-9 and their linear H-bonding behaviour with a new cyanuric acid modified tetraphenylporphyrin (TPP) 13 are reported. The resulting rigid complexes consisting of an oligo-phenylene-acetylene wire and two terminating TPPs were characterized by H NMR, UV/vis and fluorescence spectroscopies. The 1:2 compositions of the supramolecular complexes, the association constants K and the cooperativity of binding expressed by Scatchard plots and Hill coefficients η were determined by H NMR titration experiments. The strength of the association constants K in CDCl at rt was found to be in a range of 10 and 10 mol dm for the first complexation and 10-10 mol dm for the second complexation, which is rather strong. © 2008 Elsevier Ltd. All rights reserved.

Self-association and electron transfer in donor-acceptor dyads connected by meta-substituted oligomers - Molina-Ontoria, Agustin; Fernandez, Gustavo; Wielopolski, Mateusz; Atienza, Carmen; Sanchez, Luis; Gouloumis, Andreas; Clark, Timothy; Martin, Nazario; Guldi, Dirk Michael (Journal of the American Chemical Society, 2009)
The synthesis of a new series of electron donor-acceptor conjugates (5, 10, 13, and 16) in which the electron acceptor - C - and the electron donor - π-extended tetrathiafulvalene (exTTF) - are bridged by means of m-phenyleneethynylene spacers of variable length is reported. The unexpected self-association of these hybrids was first detected to occur in the gas phase by means of MALDI-TOF spectrometry and subsequently corroborated in solution by utilizing concentration-dependent and variable-temperature H NMR experiments. Furthermore, the ability of these new conjugates to form wirelike structures upon deposition onto a mica surface has been demonstrated by AFM spectroscopy. In light of their photoactivity and redoxactivity, 5, 10, 13, and 16 were probed in concentration-dependent photophysical experiments. Importantly, absorption and fluorescence revealed subtle dissimilarities for the association constants, that is, a dependence on the length of the m-phenylene spacers. The binding strength is in 5 greatly reduced when compared with those in 10, 13, and 16. Not only that, the spacer length also plays a decisive role in governing excited-state interactions in the corresponding electron donor-acceptor conjugates (5, 10, 13, and 16). To this end, 5, in which the photo- and electroactive constituents are bridged by just one aromatic ring, displayssexclusively and independent of the concentration (10 to 10 M) - efficient intramolecular electron transfer events on the basis of a "through-bond" mechanism. On the contrary, the lack of conjugation throughout the bridges in 10 (two m-phenyleneethynylene rings), 13 (three m-phenyleneethynylene rings), and 16 (four m-phenyleneethynylene rings) favors at low concentration (10 M) "through space" intramolecular electron transfer events. These are, however, quite ineffective and, in turn, lead to excited-state deactivations that are at high concentrations (10 M) dominated by intracomplex electron transfer events, namely, between exTTF of one molecule and C of another molecule, and that stabilize the resulting radical ion pair state with lifetimes reaching 4.0 μs. © 2009 American Chemical Society.

Self-Catalyzed Growth of Vertically Aligned InN Nanorods by Metal-Organic Vapor Phase Epitaxy - Tessarek, C.; Fladischer, Stefanie; Dieker, Christel; Sarau, G.; Hoffmann, Björn; Bashouti, M.; Goebelt, M.; Heilmann, M.; Latzel, Michael; Butzen, E.; Figge, S.; Gust, A.; et al. (Nano Letters, 2016)
Vertically aligned hexagonal InN nanorods were grown mask-free by conventional metal organic vapor phase epitaxy without any foreign catalyst. The In droplets on top of the nanorod's indicate a self-catalytic vapor liquid solid growth mode. A systematic study on important growth parameters has been carried out for the optimization of nanorod morphology. The nanorod N-polarity, induced by high temperature nitridation of the sapphire substrate, IS necessary to achieve vertical growth. Hydrogen, usually inapplicable during InN growth due to formation of metallic indium, and silane are needed to enhance the aspect ratio and to reduce parasitic deposition beside the nanorods on the sapphire surface. The results reveal many similarities between InN and GaN nanorod growth showing that the process despite the large difference in growth temperature is similar. Transmission electron microscopy, spatially resolved energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy have been performed to analyze the structural properties. Spatially resolved cathodoluminescence investigations are carried out to verify the optical activity of the InN nanorods. The InN nanorods are expected to be the material of choice for high-efficiency hot carrier solar cells.
Keywords: InN; nanorods; MOVPE; growth; vapor-liquid-solid; cathodoluminescence

Self-consistent field convergence for proteins: a comparison of full and localized-molecular-orbital schemes - Wick, Christian; Hennemann, Matthias; Stewart, James J. P.; Clark, Timothy (Journal of Molecular Modeling, 2014)
Proteins in the gas phase present an extreme (and unrealistic) challenge for self-consistent-field iteration schemes because their ionized groups are very strong electron donors or acceptors, depending on their formal charge. This means that gas-phase proteins have a very small band gap but that their frontier orbitals are localized compared to “normal” conjugated semiconductors. The frontier orbitals are thus likely to be separated in space so that they are close to, but not quite, orthogonal during the SCF iterations. We report full SCF calculations using the massively parallel EMPIRE code and linear scaling localized-molecular-orbital (LMO) calculations using Mopac2009. The LMO procedure can lead to artificially over-polarized wavefunctions in gas-phase proteins. The full SCF iteration procedure can be very slow to converge because many cycles are needed to overcome the over-polarization by inductive charge shifts. Example molecules have been constructed to demonstrate this behavior. The two approaches give identical results if solvent effects are included.

Self-consistent Kohn-Sham method based on the adiabatic-connection fluctuation-dissipation theorem and the exact-exchange kernel - Görling, Andreas; Bleiziffer, Patrick; Krug, Marcel (Journal of Chemical Physics, 2015)
A self-consistent Kohn-Sham method based on the adiabatic-connection fluctuation-dissipation (ACFD) theorem, employing the frequency-dependent exact exchange kernel f(x) is presented. The resulting SC-exact-exchange-only (EXX)-ACFD method leads to even more accurate correlation potentials than those obtained within the direct random phase approximation (dRPA). In contrast to dRPA methods, not only the Coulomb kernel but also the exact exchange kernel f(x) is taken into account in the EXX-ACFD correlation which results in a method that, unlike dRPA methods, is free of self-correlations, i.e., a method that treats exactly all one-electron systems, like, e.g., the hydrogen atom. The self-consistent evaluation of EXX-ACFD total energies improves the accuracy compared to EXX-ACFD total energies evaluated non-self-consistently with EXX or dRPA orbitals and eigenvalues. Reaction energies of a set of small molecules, for which highly accurate experimental reference data are available, are calculated and compared to quantum chemistry methods like Moller-Plesset perturbation theory of second order (MP2) or coupled cluster methods [CCSD, coupled cluster singles, doubles, and perturbative triples (CCSD(T))]. Moreover, we compare our methods to other ACFD variants like dRPA combined with perturbative corrections such as the second order screened exchange corrections or a renormalized singles correction. Similarly, the performance of our EXX-ACFD methods is investigated for the non-covalently bonded dimers of the S22 reference set and for potential energy curves of noble gas, water, and benzene dimers. The computational effort of the SC-EXX-ACFD method exhibits the same scaling of N5 with respect to the system size N as the non-self-consistent evaluation of only the EXX-ACFD correlation energy; however, the prefactor increases significantly. Reaction energies from the SC-EXX-ACFD method deviate quite little from EXX-ACFD energies obtained non-self-consistently with dRPA orbitals and eigenvalues, and the deviation reduces even further if the Coulomb kernel is scaled by a factor of 0.75 in the dRPA to reduce self-correlations in the dRPA correlation potential. For larger systems, such a non-self-consistent EXX-ACFD method is a competitive alternative to high-level wave-function-based methods, yielding higher accuracy than MP2 and CCSD methods while exhibiting a better scaling of the computational effort than CCSD or CCSD(T) methods. Moreover, EXX-ACFD methods were shown to be applicable in situation characterized by static correlation. (C) 2015 AIP Publishing LLC.

Self-decoration of Pt metal particles on TiO2 nanotubes used for highly efficient photocatalytic H2 production - Basahel, Sulaiman N.; Lee, Kiyoung; Hahn, Robert; Schmuki, Patrik; Bawaked, Salem M.; Al-Thabaiti, Shaeel A. (Chemical Communications, 2014)
Pt decorated TiO2 has, over the past decades, been a key material for photocatalytic hydrogen production. The present work shows that growing anodic self-organized TiO2 nanotubes from Ti-Pt alloy with a low Pt content of 0.2 at% leads to oxide nanotube layers that are self-decorated with Pt nanoparticles of 4-5 nm in diameter. The average particle spacing is in the range of 50 nm and is partially adjustable by the anodization conditions. This intrinsic decoration of TiO2 nanotubes with Pt leads to a highly active photocatalyst for the production of H2 under UV or visible light conditions.
EAM Import::2019-03-13

Self-Healing Materials - Hager, Martin D.; Greil, Peter; Leyens, Christoph; Van Der Zwaag, Sybrand; Schubert, Ulrich S. (Advanced Materials, 2010)
Self-healing materials are able to partially or completely heal damage inflicted on them, e.g., crack formation; it is anticipated that the original functionality can be restored. This article covers the design and generic principles of self-healing materials through a wide range of different material classes including metals, ceramics, concrete, and polymers. Recent key developments and future challenges in the field of self-healing materials are summarised, and generic, fundamental material-independent principles and mechanism are discussed and evaluated.
EAM Import::2019-03-13

Self-induced current oscillations during anodization of Ti in LA containing DMSO electrolyte - Özkan, Selda; Mazare, Anca Valentina; Schmuki, Patrik (Electrochemistry Communications, 2016)
During self-organized nanotubular oxide growth by electrochemical anodization of Ti in lactic acid (LA) containing dimethyl sulfoxide (DMSO) electrolytes, we observe the occurrence of regular current oscillations (COs). Key factors that influence COs are the LA concentration, the surface roughness and diffusion limitation. The oscillation characteristics (amplitude and period) depend on both LA concentration and applied voltage. Under optimized oscillation conditions a sequential growth/detachment of the TiO2 layers is observed that leads to a self-stacking of TiO2 nanostructured multilayers.
EAM Import::2019-03-13

Self-organization of antiperiodic oscillations - Freire, J. G.; Cabeza, C.; Marti, A. C.; Pöschel, Thorsten; Gallas, Jason (European Physical Journal - Special Topics, 2014)
Antiperiodic oscillations forming infinite cascades of spirals were recently found experimentally and numerically in the control parameter space of an autonomous electronic circuit. They were discovered while recording one specific voltage of the circuit. Here, we show that such regular self-organization may be measured in any of the four variables of the circuit. Although the relative size of individual phases, their boundaries and the number of peaks of each characteristic oscillation depends on the physical quantity used to record them, the global structural organization of the complex phase diagrams is an invariant of the circuit. Tunable families of antiperiodic oscillations cast fresh light on new intricate behavior of nonlinear systems and open the possibility of studying hitherto unobserved phenomena.

Self-organization of pulsing and bursting in a CO2 laser with opto-electronic feedback - Gallas, Jason A.; Gallas, Jason; Freire, J. G.; Meucci, Riccardo; Areccchi, Fortunato (Chaos, 2015)
We report a detailed investigation of the stability of a CO 2 laser with feedback as described by a six-dimensional rate-equations model which provides satisfactory agreement between numerical and experimental results. We focus on experimentally accessible parameters, like bias voltage, feedback gain, and the bandwidth of the feedback loop. The impact of decay rates and parameters controlling cavity losses are also investigated as well as control planes which imply changes of the laser physical medium. For several parameter combinations, we report stability diagrams detailing how laser spiking and bursting is organized over extended intervals. Laser pulsations are shown to emerge organized in several hitherto unseen regular and irregular phases and to exhibit a much richer and complex range of behaviors than described thus far. A significant observation is that qualitatively similar organization of laser spiking and bursting can be obtained by tuning rather distinct control parameters, suggesting the existence of unexpected symmetries in the laser control space.

Self-Organized Arrays of Single-Metal Catalyst Particles in TiO 2 Cavities: A Highly Efficient Photocatalytic System - Yoo, Jeong Eun; Lee, Kiyoung; Altomare, Marco; Selli, Elena; Schmuki, Patrik (Angewandte Chemie International Edition, 2013)
Peas in a pod: A highly aligned Aunp@TiO2 photocatalyst was formed by self-organizing anodization of a Ti substrate followed by dewetting of a gold thin film. This leads to exactly one Au nanoparticle (np) per TiO2 nanocavity. Such arrays are highly efficient photocatalysts for hydrogen generation from ethanol.
EAM Import::2019-03-13

Self-organized cobalt fluoride nanochannel layers used as a pseudocapacitor material - Lee, Chong-Yong; Su, Zixue; Lee, Kiyoung; Tsuchiya, Hiroaki; Schmuki, Patrik (Chemical Communications, 2014)
Aligned CoF2 nanochannel layers have been formed by self-ordering electrochemical anodization. In voltammograms these layers provide multiple oxidation states, an almost ideal rectangular pseudocapacitive behavior, a high specific capacitance and good capacitance retention. These layers may thus be promising for supercapacitor applications.
EAM Import::2019-03-13

Self-organized nano-tubes of TiO2-MoO3 with enhanced electrochromic properties - Shrestha, Nabeen Kumar; Nah, Yoon-Chae; Tsuchiya, Hiroaki; Schmuki, Patrik (Chemical Communications, 2009)
Self-organized TiO_2–MoO_3 composite oxide nano-tubes with tunable dimensions are successfully fabricated by anodization of a Ti–Mo alloy and these nano-tube layers exhibit a significantly enhanced electrochromic color contrast compared with plain TiO_2 nano-tubes.
EAM Import::2019-03-13

Self-organized shocks in the sedimentation of a granular gas - Almazan Torres, Lidia; Serero, Dan; Salueña, Clara; Pöschel, Thorsten (Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 2015)
A granular gas in gravity heated from below develops a certain stationary density profile. When the heating is switched off, the granular gas collapses. We investigate the process of sedimentation using computational hydrodynamics, based on the Jenkins-Richman theory, and find that the process is significantly more complex than generally acknowledged. In particular, during its evolution, the system passes several stages which reveal distinct spatial regions of inertial (supersonic) and diffusive (subsonic) dynamics. During the supersonic stages, characterized by Mach>1, the system develops supersonic shocks which are followed by a steep front of the hydrodynamic fields of temperature and density, traveling upward.
Keywords: granular gas

Self-organized TiO2 nanotubes: Visible light activation by Ni oxide nanoparticle decoration - Shrestha, Nabeen Kumar; Yang, Mingjun; Nah, Yoon-Chae; Paramasivam, Indhumati; Schmuki, Patrik (Electrochemistry Communications, 2010)
Self-organized TiO_2 nanotubes grown by anodization of Ti-substrate in a fluoride based electrolyte were loaded with Ni oxide nanoparticles by a simple chemical bath precipitation technique. The photoelectrochemical and photocatalytic activities of these nanoparticle decorated nanotubes were investigated. In comparison with the classical approaches to enhance the visible light response of TiO_2 nanotubes (N-doping), the Ni oxide loaded nanotube layers show clearly a higher visible light photoresponse and photocatalytic activity.
EAM Import::2019-03-13

Semiempirical UNO–CAS and UNO–CI: Method and Applications in Nanoelectronics - Dral, Pavlo O.; Clark, Timothy (Journal of Physical Chemistry A, 2011)
Unrestricted Natural Orbital–Complete Active Space Configuration Interaction, abbreviated as UNO–CAS, has been implemented for NDDO-based semiempirical molecular-orbital (MO) theory. A computationally more economic technique, UNO–CIS, in which we use a configuration interaction (CI) calculation with only single excitations (CIS) to calculate excited states, has also been implemented and tested. The class of techniques in which unrestricted natural orbitals (UNOs) are used as the reference for CI calculations is denoted UNO–CI. Semiempirical UNO–CI gives good results for the optical band gaps of organic semiconductors such as polyynes and polyacenes, which are promising materials for nanoelectronics. The results of these semiempirical UNO–CI techniques are generally in better agreement with experiment than those obtained with the corresponding conventional semiempirical CI methods and comparable to or better than those obtained with far more computationally expensive methods such as time-dependent density-functional theory. We also show that symmetry breaking in semiempirical UHF calculations is very useful for predicting the diradical character of organic compounds in the singlet spin state.

Semimetallic TiO2 Nanotubes - Hahn, Robert; Schmidt-Stein, Felix; Salonen, Jarno; Thiemann, Stefan; Song, Yanyan; Kunze, Julia; Lehto, Vesa-Pekka; Schmuki, Patrik (Angewandte Chemie International Edition, 2009)
Orchestrating conductors: A carbo-thermal reduction treatment of a TiO_2 nanotube layer in acetylene converts the tube walls into highly conductive and stable oxy carbide compounds. The resulting semimetallic TiO_2 nanotube layers (TiOxCy) are electrode materials with properties that differ from other TiO_2 nanotubes (see picture). They have a high oxygen overpotential and can be used as a catalyst support, for example, for methanol oxidation.
EAM Import::2019-03-13

Semitransparent Organic Light Emitting Diodes with Bidirectionally Controlled Emission - Bronnbauer, Carina; Osvet, Andres; Brabec, Christoph; Forberich, Karen (ACS Photonics, 2016)
Semitransparent OLEDs are a candidate for large-area eco-friendly light sources that can be integrated into building facades, suggesting colorful windows that become luminescent if the OLED is switched on. However, since the light is emitted in two directions, smart light engineering has to be implemented to direct the light into a preferred direction and to prevent for instance huge energetic losses to the outside of a building. We introduce an unprecedented device architecture, composed of a dielectric mirror attached to a semitransparent OLED. Such a system features a dual functionality that depends on the viewing direction: changing the color perception and/or enhancing the light directionality while still preserving a high overall device transparency. First, we motivate the potential of this concept with a theoretical study, showing that broad modifications in the color range can be realized via device optimization and that the maximum possible emission enhancement of the OLED is limited only by the transparency of the interfacial layers and the electrodes. Then, experimental investigations with a semitransparent yellow OLED (transparency = 58.2%) in combination with six different dielectric mirrors validate the theoretical results. Retaining the same color perception, up to 80% of the total emitted light can be directed toward one side while the color is modified at the other side of the device stack. Here, modifications from yellow to purple to dark or light blue can be realized.
Keywords: color modification; dielectric mirror; fully printed; light management; OLED; semitransparent

Sensitivity of CO oxidation toward metal oxidation state in ceria-supported catalysts: an operando DRIFTS-MS study - Kaftan, Andre; Kollhoff, Fabian; Thanh-Son Nguyen,; Piccolo, Laurent; Laurina, Mathias; Libuda, Jörg (Catalysis: Science and Technology, 2016)
The oxidation of carbon monoxide has been studied on pristine CeO2, Rh-CeO2, and Pt-CeO2 powders prepared in one step by solution combustion synthesis (SCS). The reaction was cycled between an oxygenrich and a CO-rich feed with regard to the stoichiometric conditions. CO2 production was monitored by mass spectrometry, while the surface species were probed by operando DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy). Whereas the reaction starts above 150 degrees C on CeO2 and Rh-CeO2 and does not depend on the state of the surface (oxidized or reduced), the reaction on Pt-CeO2 shows strong dependency on the initial state and substantial activity is achieved at much lower temperatures with the CO-rich feed. We relate this result to the change in the oxidation state of Pt via strong interaction with ceria.

Setting the pace of microswimmers: when increasing viscosity speeds up self-propulsion - Pande, Jayant; Merchant, Laura; Kruger, Timm; Harting, Jens; Smith, Ana-Suncana (New Journal of Physics, 2017)
It has long been known that some microswimmers seem to swim counter-intuitively faster when the viscosity of the surrounding fluid is increased, whereas others slow down. This conflicting dependence of the swimming velocity on the viscosity is poorly understood theoretically. Here we explain that any mechanical microswimmer with an elastic degree of freedom in a simple Newtonian fluid can exhibit both kinds of response to an increase in the fluid viscosity for different viscosity ranges, if the driving is weak. The velocity response is controlled by a single parameter Gamma, the ratio of the relaxation time of the elastic component of the swimmer in the viscous fluid and the swimming stroke period. This defines two velocity-viscosity regimes, which we characterize using the bead-spring microswimmer model and analyzing the different forces acting on the parts of this swimmer. The analytical calculations are supported by lattice-Boltzmann simulations, which accurately reproduce the two velocity regimes for the predicted values of Gamma.
Keywords: microswimming; anomalous increase in velocity; analytical modeling; lattice-Boltzmann simulations

Setup of an electron probe micro analyzer for highest resolution radioscopy - Hanke, R.; Nachtrab, Frank; Burtzlaff, S.; Voland, V.; Uhlmann, N.; Porsch, F.; Johansson, W. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2009)
Structural investigation and characterization of objects below the micrometer-scale level often require either big efforts in experimental setup and critical time schedules as is the case with synchrotron sources or a big effort in test object preparation including destructive methods (e.g. grinding) for the investigation within an electron microscope. The limiting factor for high-resolution imaging with conventional X-ray sources is the focal spot size of the X-ray tube used. Focal spot sizes down to 500 nm can be achieved with double-focusing X-ray tubes and transmission targets. To be able to use X-ray transmission imaging for structural investigation with highest resolution in everybody's laboratory, we modified an electron probe micro analyzer (EPMA) to be used as an X-ray source. With this it is possible to obtain electron beam diameters below 200 nm and to produce X-rays using a special transmission target. Due to the low X-ray intensity and in order to profit from the high magnification, a high-resolution, low-noise detector is to be used. We chose a Medipix2 detector for that purpose. We will present the setup and first results of radioscopic images with a resolution of about 240 nm.
EAM Import::2019-03-13

Shape and Symmetry Determine Two-Dimensional Melting Transitions of Hard Regular Polygons - Anderson, Joshua A.; Antonaglia, James; Millan, Jaime A.; Engel, Michael; Glotzer, Sharon C. (Physical Review X, 2017)
The melting transition of two-dimensional systems is a fundamental problem in condensed matter and statistical physics that has advanced significantly through the application of computational resources and algorithms. Two-dimensional systems present the opportunity for novel phases and phase transition scenarios not observed in 3D systems, but these phases depend sensitively on the system and, thus, predicting how any given 2D system will behave remains a challenge. Here, we report a comprehensive simulation study of the phase behavior near the melting transition of all hard regular polygons with 3≤n≤14 vertices using massively parallel Monte Carlo simulations of up to 1×106 particles. By investigating this family of shapes, we show that the melting transition depends upon both particle shape and symmetry considerations, which together can predict which of three different melting scenarios will occur for a given n. We show that systems of polygons with as few as seven edges behave like hard disks; they melt continuously from a solid to a hexatic fluid and then undergo a first-order transition from the hexatic phase to the isotropic fluid phase. We show that this behavior, which holds for all 7≤n≤14, arises from weak entropic forces among the particles. Strong directional entropic forces align polygons with fewer than seven edges and impose local order in the fluid. These forces can enhance or suppress the discontinuous character of the transition depending on whether the local order in the fluid is compatible with the local order in the solid. As a result, systems of triangles, squares, and hexagons exhibit a Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) predicted continuous transition between isotropic fluid and triatic, tetratic, and hexatic phases, respectively, and a continuous transition from the appropriate x-atic to the solid. In particular, we find that systems of hexagons display continuous two-step KTHNY melting. In contrast, due to symmetry incompatibility between the ordered fluid and solid, systems of pentagons and plane-filling fourfold pentilles display a one-step first-order melting of the solid to the isotropic fluid with no intermediate phase.

Shape-dependent ordering of gold nanocrystals into large-scale superlattices - Gong, Jianxiao; Newman, Richmond S.; Engel, Michael; Zhao, Man; Bian, Fenggang; Glotzer, Sharon C.; Tang, Zhiyong (Nature Communications, 2017)
Self-assembly of individual building blocks into highly ordered structures, analogous to spontaneous growth of crystals from atoms, is a promising approach to realize the collective properties of nanocrystals. Yet the ability to reliably produce macroscopic assemblies is unavailable and key factors determining assembly quality/yield are not understood. Here we report the formation of highly ordered superlattice films, with single crystalline domains of up to half a millimetre in two dimensions and thickness of up to several microns from nanocrystals with tens of nanometres in diameter. Combining experimental and computational results for gold nanocrystals in the shapes of spheres, cubes, octahedra and rhombic dodecahedra, we investigate the entire self-assembly process from disordered suspensions to large-scale ordered superlattices induced by nanocrystal sedimentation and eventual solvent evaporation. Our findings reveal that the ultimate coherence length of superlattices strongly depends on nanocrystal shape. Factors inhibiting the formation of high-quality large-scale superlattices are explored in detail.
EAM Import::2019-03-13

Shape optimization for the Helmholtz equation - Seifrt, Frantiek; Leugering, Günter; Rohan, Eduard (Proceedings in Applied Mathematics and Mechanics, 2008)
The aim of our work is to develop optimal dielectric composite structures with specific qualities. The task is to design interfaces of given material components such that the originated structure attains certain optical properties. Propagation of the electromagnetic waves in the composite is described by the Helmholtz equation. Success of the structure is enumerated by the objective function which is to be minimized. Interfaces of the given materials are parametrized by the cubic B–spline curves. The design variables are afterwards the positions of B–spline control points. For objective function evaluation one forward computation of the Helmholtz equation is needed. To get the sensitivity of the objective function we solve the backward (adjoint) equation.

Shape sensitivity analysis of a quasi-electrostatic piezoelectric system in multilayered media - Leugering, Günter; Novotny, A. A.; Perla Menzala, G.; Sokolowski, J. (Mathematical Methods in the Applied Sciences, 2010)
The optimization of shape and topology of piezo-patches or layered piezo-electrical material attached to structural parts, such as elastic bodies, plates and shells, plays a major role in the design of smart structures, as piezo-mechanic-acoustic devices in loudspeakers or energy harvesters. While the design for time-harmonic motions is genuinely frequency-dependent, as has been reported in the literature in the context of density optimization with the SIMP-method, time-varying piezoelectric material has not been investigated with respect to the optimal design so far. Therefore, shape sensitivities for layered piezoelectric material and time-varying loads and charges are derived in this paper. In particular, we provide the shape-derivatives for nested piezo-layers associated with a class of shape functional. More general layers can be dealt with similar approach.

Shape Transformation Mechanism of Silver Nanorods in Aqueous Solution - Damm, Cornelia; Segets, Doris; Yang, Guang; Vieweg, Benito; Spiecker, Erdmann; Peukert, Wolfgang (Small, 2011)
The spontaneous shape transformation of silver nanorods with an initial length of several hundred nanometers towards spherical particle shapes in aqueous solution is investigated by means of scanning electron microscopy, UV–vis absorption spectroscopy, anodic stripping voltammetry, and high-resolution transmission electron microscopy (HRTEM). The consolidation of the results reveals an increase in the particle number density with time. Moreover, HRTEM image analysis along the cross section of the rods evidences the presence of fivefold twinning defects which extend along the whole rod length. According to the analytical model of Monk et al. this kind of rod structure is only thermodynamically stable if the rod length is below a critical value at a given diameter. The rods investigated in the present work do not fulfill the stability criterion as they exceed the critical length. Thus, the rods decay into smaller “nanobuns” and defective as well as defect-free spheres. A mechanism based on findings from the literature, HRTEM image analysis of former rods, transition states, and the final particle structures is proposed. The defects along the surface are seen as starting points for the dissolution of material, which is reintegrated into the solid phase by homogeneous as well as heterogeneous nucleation and growth.

Shearing in a biomimetic apatite-protein composite: Molecular dynamics of slip zone formation, plastic flow and backcreep mechanisms - Zahn, Dirk; Bitzek, Erik (PLoS ONE, 2014)
We report molecular dynamics simulations of shear in a biomimetic hydroxyapatite-collagen composite. Our model exhibits elastic properties fully dominated by the inorganic component. However, beyond the elastic regime the biomolecules along with the hierarchical nature of the composite account for the formation of structure-inherent slip zones. These accommodate shear without compromising the overall structure and lead to the sliding of intrinsically defined rods at roughly constant restoring force. Upon releasing load, rod displacement is reversible and backcreep is observed as gradual ionic rearrangement in the slip zone, subjected to an activation barrier. © 2014 Zahn, Bitze

Shedding light on the effective fluorophore structure of high fluorescence quantum yield carbon nanodots - Wang, Wenshuo; Wang, Bingzhe; Embrechts, Heidemarie; Damm, Cornelia; Cadranel, Alejandro; Strauß, Volker; Distaso, Monica; Hinterberger, Vanessa; Guldi, Dirk Michael; Peukert, Wolfgang (RSC Advances, 2017)
Carbon nanodots (CD) have great potential for imaging and sensing applications, due to their unique luminescence properties in combination with their low toxicity. Although CDs are currently the subject of intensive research activity, the exact structure of their effective fluorophore species has to be clarified because it can depend on the starting materials and reaction conditions. Most importantly, the limited photostability of some CDs presents an impediment to the much-needed breakthroughs required in the field. In this work, the photobleaching of CDs synthesized hydrothermally from citric acid and urea undervarious conditions is studied in detail. By visualizing changes in the absorption and fluorescence properties of CDs over time upon exposure to UV-light by means of in situ spectroscopic probes, we demonstrate a trade-off between high initial quantum yield and photostability. We found that the CDs consist of low-molecular weight fluorophores bound to extgreekp-conjugated domains (carbon core). In the case of CDs synthesized at higher reaction temperatures and/or for longer reaction times, lower initial quantum yields and higher photostability originate from partial conversion of the low-molecular weight fluorophores to extgreekp-conjugated domains during synthesis. Thus, the remaining surface bound fluorophores are bound to larger extgreekp-conjugated domains, which, in turn, enable dissipation of absorbed UV-energy and protection against photochemical damage. Reference experiments were performed with citrazinic acid – a pyridone-based structure – using ultrafast time-resolved spectroscopy, steady-state spectroscopy, and theoretical DFT simulations. We conclude that pyridone-like structures are most likely responsible for both the high quantum yields and the photobleaching of CDs.

Shedding light on the growth of gold nanoshells - Sauerbeck, Christian; Haderlein, Michael; Schürer, Benedikt; Braunschweig, Björn; Peukert, Wolfgang; Klupp Taylor, Robin (ACS nano, 2014)
Nanostructured particles containing noble metals can have highly tunable localized surface plasmon resonances and are therefore of particular interest for numerous applications. Nanoshells comprising a dielectric core and gold or silver shell are a widely researched systems because of the strong dependence of their optical properties on the ratio of core diameter to shell thickness. Although seeded-growth procedures have been developed to produce these particles, the many reported studies show significant variation in the nanoshell morphologies and hence optical properties. In order to establish processes that reproducibly synthesize nanoshells with high optical quality, it is necessary to develop techniques that monitor changes at the core particle surface during shell growth. For that purpose, we have carried out in situ nonlinear second-harmonic scattering (SHS) and linear vis-NIR extinction spectroscopy simultaneously during the seeded growth of gold nanoshells on silica core particles. Our SHS measurements show a striking variation in the nonlinear optical properties of the growing gold nanoshells. In comparison with linear optical measurements and with scanning electron microscopy (SEM) images made of gold nanoshells produced with varying shell completenesses, the SHS signal was observed to reach a peak intensity at a stage prior to shell closure. We attribute this high sensitivity of the SHS signal to the incomplete nanoshell surface morphology to the generation and subsequent degeneration of regions of electric field enhancement at gaps between isolated gold islands, which grow and coalesce. This conclusion is corroborated by finite-difference time-domain simulations of incomplete nanoshells. We suggest that the in situ analytical approach demonstrated here offers significant promise for future activities regarding the in-process optimization of the morphology and optical properties of metal nanoshells and other nanostructured plasmonic particles. © 2014 American Chemical Society.
Keywords: electric field enhancement; in situ vis-NIR spectroscopy; nanostructured particles; plasmonic particles; second-harmonic generation

Shortcomings of the bond orientational order parameters for the analysis of disordered particulate matter - Mickel, Walter; Kapfer, Sebastian; Schröder-Turk, Gerd; Mecke, Klaus (Journal of Chemical Physics, 2013)
Local structure characterization with the bond-orientational order parameters q(4), q(6), ... introduced by Steinhardt et al. [Phys. Rev. B 28, 784 (1983)] has become a standard tool in condensed matter physics, with applications including glass, jamming, melting or crystallization transitions, and cluster formation. Here, we discuss two fundamental flaws in the definition of these parameters that significantly affect their interpretation for studies of disordered systems, and offer a remedy. First, the definition of the bond-orientational order parameters considers the geometrical arrangement of a set of nearest neighboring (NN) spheres, NN(p), around a given central particle p; we show that the choice of neighborhood definition can have a bigger influence on both the numerical values and qualitative trend of q(l) than a change of the physical parameters, such as packing fraction. Second, the discrete nature of neighborhood implies that NN(p) is not a continuous function of the particle coordinates; this discontinuity, inherited by q(l), leads to a lack of robustness of the q(l) as structure metrics. Both issues can be avoided by a morphometric approach leading to the robust Minkowski structure metrics q(l)'. These q(l)' are of a similar mathematical form as the conventional bond-orientational order parameters and are mathematically equivalent to the recently introduced Minkowski tensors [G. E. Schroder-Turk et al., Europhys. Lett. 90, 34001 (2010); S. Kapfer et al., Phys. Rev. E 85, 030301(R) (2012)]. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4774084]

Shrinking core like fluid solid reactions-A dispersion model accounting for fluid phase volume change and solid phase particle size distributions - Knorr, Tilman; Kaiser, Markus; Glenk, Friedrich; Etzold, Bastian (Chemical Engineering Science, 2012)
A mathematical model is derived for the shrinking core like conversion of solids in a reacting fluid. As a technical example for this type of reaction the model is used to evaluate kinetic experiments on the production of carbide-derived carbon from silicon carbide. Therefore the simultaneous influence of reaction as well as diffusion is combined with the residence time behaviour of the reactor (tubular reactor), the non-constant gas phase volume over the extent of reaction and the particle size distribution of the solid substrate. The derived model is validated for accuracy against known solutions of boarder cases (e.g. shrinking core model of a single sphere, fluid phase volume change without dispersion or shrinking core and limitation by reactant feed).
EAM Import::2019-03-13

SiC ceramic micropatterns from polycarbosilanes - Jang, Yeon-Suk; Jank, Michael; Maier, Verena; Durst, Karsten; Travitzky, Nahum; Zollfrank, Cordt (Journal of the European Ceramic Society, 2010)
Micropatterned SiC ceramics were fabricated from polycarbosilanes applying a softlithographic replication technique. A polydimethylsiloxane mould replicated from a photolithographic microstructured silicon wafer was used as master structure. The polydimethylsiloxane mould was coated with a solution containing a mixture of two different polycarbosilanes in n-octane. After treatment at 200–400 °C the cross-linked polycarbosilane films were debonded and pyrolysed at 900 °C in nitrogen and subsequently crystallised at temperatures up to 1500 °C in argon. The cross-linking and thermal degradation behaviour of the polycarbosilanes was investigated by Fourier-transform infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. X-ray diffractrometry showed the expected development of a nanocrystalline extgreekb-SiC (3 nm) as the main phase with increasing temperature. However, traces of extgreeka-SiO2 derived from the polycarbosilane precursors were also detected by X-ray analysis. Removal of the extgreeka-SiO2 dioxide with hydrofluoric acid in the pyrolysed samples and subsequent increased the crystallite size to 7 nm. The Young's modulus determined by nanoindentation was increased from 3 GPa after cross-linking to 110 GPa after crystallisation. Scanning electron microscopy revealed, that the initial micropatterns were fully retained in the pyrolysed and crystallised SiC ceramics. The micropatterned cross-linked and crystallised extgreekb-SiC based substrates exhibited light scattering characteristics, which qualify them as promising candidates for diffractive optical elements in microoptical applications.
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Side-Chain Engineering for Enhancing the Properties of Small Molecule Solar Cells: A Trade-off Beyond Efficiency - Min, Jie; Cui, Chaohua; Heumüller, Thomas; Fladischer, Stefanie; Cheng, Xiao; Spiecker, Erdmann; Li, Yongfang; Brabec, Christoph (Advanced Energy Materials, 2016)
Three small molecules with different substituents on bithienyl-benzo[1,2-b:4,5-b′]dithiophene (BDTT) units, BDTT-TR (meta-alkyl side chain), BDTT-O-TR (meta-alkoxy), and BDTT-S-TR (meta-alkylthio), are designed and synthesized for systematically elucidating their structure–property relationship in solution-processed bulk heterojunction organic solar cells. Although all three molecules show similar molecular structures, thermal properties and optical band gaps, the introduction of meta-alkylthio-BDTT as the central unit in the molecular backbone substantially results in a higher absorption coefficient, slightly lower highest occupied molecular orbital level and significantly more efficient and balanced charge transport property. The bridging atom in the meta-position to the side chain is found to impact the microstructure formation which is a subtle but decisive way: carrier recombination is suppressed due to a more balanced carrier mobility and BDTT based devices with the meta-alkylthio side chain (BDTT-S-TR) show a higher power conversion efficiency (PCE of 9.20%) as compared to the meta-alkoxy (PCE of 7.44% for BDTT-TR) and meta-alkyl spacer (PCE of 6.50% for BDTT-O-TR). Density functional density calculations suggest only small variations in the torsion angle of the side chains, but the nature of the side chain linkage is further found to impact the thermal as well as the photostability of corresponding devices. The aim is to provide comprehensive insight into fine-tuning the structure–property interrelationship of the BDTT material class as a function of side chain engineering.
Keywords: absorption coefficient; microstructure formation; photostability; side chain engineering; structure-property relationships

Si-doped Fe2O3 nanotubular/nanoporous layers for enhanced photoelectrochemical water splitting - Lee, Chong-Yong; Wang, Lei; Kado, Yuya; Kirchgeorg, Robin; Schmuki, Patrik (Electrochemistry Communications, 2013)
The present work reports the enhancement of the photoelectrochemical water splitting performance of in-situ silicon (Si)-doped nanotubular/nanoporous (NT/NP) layers. These layers were grown by self-organizing anodization on Fe-Si alloys of various Si content. The incorporation of Si is found to retard the layer growth rates, leads to a more pronounced nanotubular morphology, and most importantly, an improved photoelectrochemical behavior. By increasing Si content from 1, 2 to 5 at.% in the iron oxide NT/NP photoanodes, the photocurrent onset potential shifts favorably to lower values. At 1.3 V vs. RHE, hematite layer with 5 at.% Si shows a 5-fold increase of the photocurrent, i.e. 0.5 mA cm- 2 in comparison to 0.1 mA cm- 2 for the undoped samples. The study also reveals that a suitable layer thickness is essential to achieve a beneficial effect of the Si doping.
EAM Import::2019-03-13

Signal Amplification Strategy Based on TiO 2 -Nanotube Layers and Nanobeads Carrying Quantum Dots for Electrochemiluminescent Immunosensors - Gao, Zhi-Da; Zhuang, Qian-Lan; Song, Yan-Yan; Lee, Kiyoung; Schmuki, Patrik (ChemistryOpen, 2013)
Self-organized TiO2-nanotube layers can be used for immunoassay-type sensing in combination with amplifying CdTe labels in a direct and very sensitive electrochemiluminescent (ECL) configuration. Key properties for this method are the conductivity of the TiO2-nanotubes, and their transparency for light emitted from the CdTe labels at approximately 2.4 eV. To demonstrate the potential of this platform, we constructed a sandwich-type immunoassay onto the TiO2-nanotube wall with a layer of (3-aminopropyl)triethoxysilane as the cross-linker for antibody immobilization. For the counter part of the sandwich, we created an amplification system consisting of TiO2 nano-beads carrying the secondary antibody and multiple CdTe quantum dots (multiQD). For antigen (IgG) detection, we find that this combination of 3D transparent electrode with multi-QD labels allows for an ECL detection limit of 0.05 pgmL}1 a linearity of the signal in the range of 0.1–108 pg mL-1
EAM Import::2019-03-13

Signature of a Nonharmonic Potential as Revealed from a Consistent Shape and Fluctuation Analysis of an Adherent Membrane - Schmidt, Daniel; Monzel, Cornelia; Bihr, Timo; Merkel, Rudolf; Seifert, Udo; Sengupta, Kheya; Smith, Ana-Suncana (Physical Review X, 2014)
The interaction of fluid membranes with a scaffold, which can be a planar surface or a more complex structure, is intrinsic to a number of systems from artificial supported bilayers and vesicles to cellular membranes. In principle, these interactions can be either discrete and protein mediated, or continuous. In the latter case, they emerge from ubiquitous intrinsic surface interaction potentials as well as nature-designed steric contributions of the fluctuating membrane or from the polymers of the glycocalyx. Despite the fact that these nonspecific potentials are omnipresent, their description has been a major challenge from experimental and theoretical points of view. Here, we show that a full understanding of the implications of the continuous interactions can be achieved only by expanding the standard superposition models commonly used to treat these types of systems, beyond the usual harmonic level of description. Supported by this expanded theoretical framework, we present three independent, yet mutually consistent, experimental approaches to measure the interaction potential strength and the membrane tension. Upon explicitly taking into account the nature of shot noise as well as the nature of finite experimental resolution, excellent agreement with the augmented theory is obtained, which finally provides a coherent view of the behavior of the membrane in the vicinity of a scaffold.
Keywords: INTERFERENCE CONTRAST MICROSCOPY; FLUID MEMBRANES; BENDING ELASTICITY; LIPID-BILAYERS; CELL-ADHESION; BROWNIAN-MOTION; DYNAMICS; VESICLES; BIOMEMBRANES; MECHANICS

Significant performance enhancement of InGaN/GaN nanorod LEDs with multi-layer graphene transparent electrodes by alumina surface passivation - Latzel, Michael; Buettner, P.; Sarau, George; Hoeflich, K.; Heilmann, M.; Chen, Weijian; Wen, X.; Conibeer, Gavin; Christiansen, S. H. (Nanotechnology, 2017)
Nanotextured surfaces provide an ideal platform for efficiently capturing and emitting light. However, the increased surface area in combination with surface defects induced by nanostructuring e.g. using reactive ion etching (RIE) negatively affects the device's active region and, thus, drastically decreases device performance. In this work, the influence of structural defects and surface states on the optical and electrical performance of InGaN/GaN nanorod (NR) light emitting diodes (LEDs) fabricated by top-down RIE of c-plane GaN with InGaN quantum wells was investigated. After proper surface treatment a significantly improved device performance could be shown. Therefore, wet chemical removal of damaged material in KOH solution followed by atomic layer deposition of only 10 nm alumina as wide bandgap oxide for passivation were successfully applied. Raman spectroscopy revealed that the initially compressively strained InGaN/GaN LED layer stack turned into a virtually completely relaxed GaN and partially relaxed InGaN combination after RIE etching of NRs. Time-correlated single photon counting provides evidence that both treatments—chemical etching and alumina deposition—reduce the number of pathways for non-radiative recombination. Steady-state photoluminescence revealed that the luminescent performance of the NR LEDs is increased by about 50% after KOH and 80% after additional alumina passivation. Finally, complete NR LED devices with a suspended graphene contact were fabricated, for which the effectiveness of the alumina passivation was successfully demonstrated by electroluminescence measurements.
EAM Import::2019-03-13

SILICONBORONOXYCARBIDE (SIBOC) FOAM FROM METHYL BOROSILOXANE - Krishnan, Sreejith; Fey, Tobias; Greil, Peter (Ceramic Transactions, 2014)
Siliconboronoxycarbide (SiBOC) foams were prepared from borosiloxane precursors by sacrificial pu foam template method. Methyl borosiloxane oligomer was synthesized from boric acid and methyltriethoxysilane. This SiBOC precursor (methylborosiloxane) was pyrolyzed at 1000 degrees C in argon with a ceramic yield of amorphous SiBOC of similar to 78%. Compressive strength of the foam sample achieved values of 410kPa at a total porosity of 85% and a mean pore cell diameter of approx. 700 mu p.. Annealing at 1400 degrees C in argon resulted in beta-SiC crystallization and a SiC/SiBOC nano composite foam was formed.

Silicon Nanowire-Based Solar Cells on Glass: Synthesis, Optical Properties, and Cell Parameters - Sivakov, V.; Andrae, G.; Gawlik, A.; Berger, A.; Plentz, J.; Falk, F.; Christiansen, S. H. (Nano Letters, 2009)
Silicon nanowire (SiNW)-based solar cells on glass substrates have been fabricated by wet electroless chemical etching (using silver nitrate and hydrofluoric acid) of 2.7 extgreekmm multicrystalline p+nn+ doped silicon layers thereby creating the nanowire structure. Low reflectance (<10%, at 300-800 nm) and a strong broadband optical absorption (>90% at 500 nm) have been measured. The highest open-circuit voltage (Voc) and short-circuit current density (Jsc) for AM1.5 illumination were 450 mV and 40 mA/cm2, respectively at a maximum power conversion efficiency of 4.4%.
EAM Import::2019-03-13

Silicon nitride and intrinsic amorphous silicon double antireflection coatings for thin-film solar cells on foreign substrates - Li, Da; Kunz, Thomas; Wolf, Nadine; Liebig, Jan Philipp; Wittmann, Stephan; Ahmad, Taimoor; Hessmann, Maik T.; Auer, Richard; Göken, Mathias; Brabec, Christoph (Thin Solid Films, 2015)
Hydrogenated intrinsic amorphous silicon (a-Si:H) was investigated as a surface passivationmethod for crystalline silicon thin film solar cells on graphite substrates. The results of the experiments, including quantumefficiency and current density-voltage measurements, show improvements in cell performance. This improvement is due to surface passivation by an a-Si:H(i) layer, which increases the open circuit voltage and the fill factor. In comparison with our previous work, we have achieved an increase of 0.6% absolute cell efficiency for a 40 μm thick 4 cm2 aperture area on the graphite substrate. The optical properties of the SiNx/a-Si:H(i) stack were studied using spectroscopic ellipsometer techniques. Scanning transmission electron microscopy inside a scanning electron microscope was applied to characterize the cross section of the SiNx/a-Si:H(i) stack using focus ion beam preparatio
Keywords: Amorphous silicon; Crystalline silicon thin film; Double layer antireflection coating; Focus ion beam; Graphite substrate; Scanning transmission electron microscopy; Surface passivation

Silver-assisted colloidal synthesis of gold patchy particles with tunable patchiness - Bao, Huixin; Butz, Benjamin; Zhou, Zhou; Spiecker, Erdmann; Hartmann, Martin; Klupp Taylor, Robin (Langmuir, 2012)
Patchy particles possessing heterogeneous surface composition show great promise as self-organizing building blocks for new classes of hierarchical functional structures. A major hurdle is the scalable synthesis of stable patches on nanosized core particles with arbitrarily defined patch number and coverage. So far, few methods have been reported which could be expected to meet these challenges. Recently we described the heterogeneous nucleation and growth of silver patches on silica nanospheres via a template free colloidal route. The patches produced, although tunable in size and number and showing interesting plasmon resonant properties, were rather unstable and degraded rapidly during attempts to process them further. In the present work, therefore, we set out to explore if related approaches can be employed to produce patchy particles involving gold, which is known to be more stable. The differences between typical patch precursors Ag+ and [AuClx (OH)4-x]- and their respective interactions with amorphous silica make this a significant challenge. We show that preformed small silver patches in addition to the presence of a reducing agent are necessary for the formation of gold patches conformal to the silica nanosphere surface. Systematic study of the process parameters and their influence on the patchiness as well as in-depth analytical TEM investigation of the patch composition reveal that patches spread over the silica surface via a cycle of galvanic dissolution and redeposition of silver. The resulting gold patchy particles remain stable during subsequent storage or washing and display tunable plasmon resonances within the visible and near-IR spectrum.
EAM Import::2019-03-13

Simple and efficient method to fabricate nano cone arrays by FIB milling demonstrated on planar substrates and on protruded structures - Rommel, Mathias; Bauer, A. J.; Frey, Lothar (Microelectronic Engineering, 2012)
A simple, fast and efficient method is demonstrated for the fabrication of very homogeneous periodic nano-cone arrays on planar Si substrates by focused ion beam milling. The approach makes use of the actual shape of the ion beam by simply milling lines in vertical and horizontal direction with the lines separated by the targeted periodicity. If beam currents and, thus, beam diameters are chosen appropriately, this patterning approach results in nano-cone arrays with apex radii of 25 nm and below. Arrays with periodicities of 600, 300, and 150 nm are realized with minimum processing times of the order of 30 s for an area of 30 extgreekmm2 without a change in the quality of the cones by area scaling. To a certain extent, height, aspect ratio, apex radius and shape of the individual cones can be varied using different beam diameters. While constant dwell times are used for planar samples, it is shown that homogeneous nano-cone arrays can only be realized on the whole surface of protruded rectangular mesa structures if the patterning strategy is adapted appropriately. In the successfully modified approach the dwell time is gradually decreased along the milled lines from the center of the array towards the edges and a frame like pattern is added at the edges of the protruded structures.
Keywords: FIB; FIB patterning; Focused ion beam; Nano cone array; Patterning strategy; Tip array

Simple and recyclable ionic liquid based system for the selective decomposition of formic acid to hydrogen and carbon dioxide - Berger, Markus; Assenbaum, Daniel; Taccardi, Nicola; Spiecker, Erdmann; Wasserscheid, Peter (Green Chemistry, 2011)
Exploitation of hydrogen as an energy carrier requires the development of systems for its storage and delivery. Formic acid has been proposed as valuable hydrogen carrier compound, due to its relatively high hydrogen content (53 g L-1), the latter being easily and cleanly released in catalytic reactions under mild conditions (HCOOH → H_2 + CO_2). Ionic liquids are interesting solvents for homogeneous catalyzed formic acid decomposition systems as their extremely low volatility avoids solvent contamination of the produced hydrogen stream. In this paper an outstandingly simple, robust and active catalyst system is presented, namely RuCl_3 dissolved in 1-ethyl-2,3-dimethylimidazolium acetate (RuCl_3/[EMMIM][OAc]). This system proved to be fully recyclable over 10 times. Turnover frequencies (TOF) of 150 h-1 and 850 h-1 were obtained at 80 °C and 120 °C, respectively.

Simple solutions for spectroscopic, photon counting X-ray imaging detectors - Nachtrab, Frank; Hofmann, T.; Firsching, M.; Uhlmann, N.; Hanke, R. (Nuclear Science Symposium Conference Record (NSS/MIC), 2009)
The central idea of our approach is to use standard imaging sensors for industrial optical imaging as X-ray sensors. We use them as a simple solution for a spectroscopic, single photon counting X-ray detector with a reduction of the pixel size by a factor of almost 10 compared to commercially available photon counting X-ray detectors. In principle, each of the sensors photodiodes can act as a direct converting X-ray sensor pixel. We compare the acquisition of images in integrating and photon-counting mode and notice a much better spatial resolution in photon-counting mode compared to integrating- mode. Using the benefits of direct detection we gather spectroscopic information of the incident photons. Gray value and energy deposition are correlated linearly. Energy resolutions down to 700 eV are by limitation to single event clusters. Furthermore detection efficiency, multiplicity, DQE(0), MTF and the radiation hardness are investigated.
EAM Import::2019-03-13

Simulating charge transport in flexible systems - Clark, Timothy (Perspectives in Science, 2015)
Systems in which movements occur on two significantly different time domains, such as organic electronic components with flexible molecules, require different simulation techniques for the two time scales. In the case of molecular electronics, charge transport is complicated by the several different mechanisms (and theoretical models) that apply in different cases. We cannot yet combine time scales of molecular and electronic movement in simulations of real systems. This review describes our progress towards this goal.

Simulating ’Soft’ Electronic Devices - Clark, Timothy; Halik, Marcus; Hennemann, Matthias; Jäger, Christof (2013)
A combination of classical molecular dynamics simulations and very large scale semiempirical molecular orbital calculations has been used to simulate field-effect transistors in which both the dielectric layer and the semiconductor are incorporated in a self-assembled monolayer of suitable functionalized alkylphosphonic acids. In such simulations, both the dynamics of the flexible organic molecules and the electronic properties of the molecular aggregates must be taken into account. First steps towards realistic simulations of such devices are described.

Simulation of a hard-spherocylinder liquid crystal with the pe - Fischermeier, Ellen; Bartuschat, Dominik; Preclik, Tobias; Marechal, Mattheus; Mecke, Klaus (Computer Physics Communications, 2014)
The pe physics engine is validated through the simulation of a liquid crystal model system consisting of hard spherocylinders. For this purpose we evaluate several characteristic parameters of this system, namely the nematic order parameter, the pressure, and the Frank elastic constants. We compare these to the values reported in literature and find a very good agreement, which demonstrates that the pe physics engine can accurately treat such densely packed particle systems. Simultaneously we are able to examine the influence of finite size effects, especially on the evaluation of the Frank elastic constants, as we are far less restricted in system size than earlier simulations.
UnivIS-Import:2015-03-09:Pub.2014.tech.IMMD.lsinfs.simula_0
Keywords: Frank elastic constants; Hard rod; Liquid crystal; Parallel framework; Rigid body dynamics

Simulation of charge transport in organic semiconductors: A time-dependent multiscale method based on nonequilibrium Green's functions - Leitherer, Susanne; Jager, C. M.; Krause, Andreas; Halik, Marcus; Clark, Timothy; Thoss, Michael (Physical Review Materials, 2017)
In weakly interacting organic semiconductors, static disorder and dynamic disorder often have an important impact on transport properties. Describing charge transport in these systems requires an approach that correctly takes structural and electronic fluctuations into account. Here, we present a multiscale method based on a combination of molecular-dynamics simulations, electronic-structure calculations, and a transport theory that uses time-dependent nonequilibrium Green's functions. We apply the methodology to investigate charge transport in C-60-containing self-assembled monolayers, which are used in organic field-effect transistors.

Simulation of fracture in heterogeneous elastic materials with cohesive zone models - Prechtel, Marina; Ronda, Pavel Leiva; Janisch, Rebecca; Hartmaier, Alexander; Leugering, Günter; Steinmann, Paul; Stingl, Michael (International Journal of Fracture, 2010)
In brittle composite materials, failure mechanisms like debonding of the matrix-fiber interface or fiber breakage can result in crack deflection and hence in the improvement of the damage tolerance. More generally it is known that high values of fracture energy dissipation lead to toughening of the material.Ouraimis toinvestigate the influence of material parameters and geometrical aspects of fibers on the fracture energy as well as the crack growth for given load scenarios. Concerning simulations of crack growth the cohesive element method in combination with the Discontinuous Galerkin method provides a framework to model the fracture considering strength, stiffness and failure energy in an integrated manner. Cohesive parameters are directly determined by DFT supercell calculations.We perform studies with prescribed crack paths as well as free crack path simulations. In both cases computational results reveal that fracture energy depends on both the material parameters but also the geometry of the fibers. In particular it is shown that the dissipated energy can be increased by appropriate choices of cohesive parametersof the interface and geometrical aspects of the fiber. In conclusion, our results can help to guide themanufacturing process ofmaterials with a high fracture toughness.
UnivIS-Import:2015-03-09:Pub.2010.tech.FT.FT-TM.simula

Simulations of bubble growth and interaction in high viscous fluids using the lattice Boltzmann method - Cubeddu, Alessandro; Rauh, C.; Ulrich, V. (International Journal of Multiphase Flow, 2017)
Phenomena of growth, coalescence and breakdown of bubbles within high viscous fluids are of great interest in the fluid dynamics of multiphase fluids because of their industrial relevance, e.g. in polymer, metal alloy and food processing fields. The dynamics of multiple bubble growth in hot viscous fluids is a complex issue governed by pressure forces, vapour diffusion, surface tension and viscous forces. Effects of water evaporation from the mixture surface are responsible for phenomena like glass transition, viscous increase and dough solidification. This article presents Lattice Boltzmann simulations of nucleating bubbles with large density ratio, that grow and interact in a hot high-viscous fluid. The work focuses on the first phases of the bubble expansion, neglecting the effects of evaporation. The simulations are performed using the Lattice Boltzmann Method (LBM). The Free Surface method is used to reduce a liquid/gas two-phase flow to a single-phase flow. The interface layer between gas and fluid is tracked using the volume of fluid (VOF) method. To avoid numerical instabilities due to the high viscosity (n = 100 Pa s), the problem is scaled from physical to LB-units through non-dimensional quantities. The bubbles are initially punched randomly into the domain with a dimension comparable with the dimension of nucleation and are allowed to grow under an internal over-pressure. The simulated final structure of the bubbles is compared with images of a pure starch fluid, extruded under same conditions. It is shown as the final bubble distribution, matrix dimension and bubble diameters in the simulation are in good agreement with the real final conformation.
EAM Import::2019-03-13

Simultaneous 3D observation of different kinetic subprocesses for precipitation in a T-mixer - Gradl, Johannes; Peukert, Wolfgang (Chemical Engineering Science, 2009)
A coupled direct numerical simulation (DNS)—population balance equation (PBE)–DLVO approach is presented to calculate the product properties of particles originating from precipitation in a T-mixer. A special focus is given to the coupling of different subprocesses such as mixing, nucleation, growth, stabilization and aggregation for a rapid precipitation process. Based on the predictive computation of the resulting particle size distributions (PSD) full 3D-field information of all transient parameters are accessible, e.g., supersaturations, nucleation rates, particle sizes and stabilities. By interpolating the kinetic data, relevant subprocesses can be visualized and studied separately in the mixer. The resulting PSDs are predicted for different conditions where aggregation can be neglected. For unstable conditions aggregation and transient stabilization effects are determined by the DLVO theory. The size of the particles, their surface potential and the ionic strength of the solution vary extensively during the solid formation process. The dynamic evolution of the particle stability leads to a better description of the PSD based on the DNS–PBE–DLVO approach for unstable conditions and allows to develop strategies how to better stabilize particles. This approach is verified for barium sulphate precipitation but can be transferred to any other ionic system for which the relevant material specific data are known.

Simultaneous Analysis of Hydrodynamic and Optical Properties Using Analytical Ultracentrifugation Equipped with Multiwavelength Detection - Peukert, Wolfgang; Walter, Johannes; Lin, Wei; Segets, Doris; Sherwood, Peter; Stafford, Walter (Analytical Chemistry, 2015)
Analytical ultracentrifugation (AUC) has proven to be a powerful tool for the study of particle size distributions, particle shapes, and interactions with high accuracy and unrevealed resolution. In this work we show how the analysis of sedimentation velocity data from the AUC equipped with a multiwavelength detector (MWL) can be used to gain an even deeper understanding of colloidal and macromolecular mixtures. New data evaluation routines have been integrated in the software SEDANAL to allow for the handling of MWL data. This opens up a variety of new possibilities because spectroscopic information becomes available for individual components in mixtures at the same time using MWL-AUC. For systems of known optical properties information on the hydrodynamic properties of the individual components in a mixture becomes accessible. For the first time, the determination of individual extinction spectra of components in mixtures is demonstrated via MWL evaluation of sedimentation velocity data. In our paper we first provide the informational background for the data analysis and expose the accessible parameters of our methodology. We further demonstrate the data evaluation by means of simulated data. Finally, we give two examples which are highly relevant in the field of nanotechnology using colored silica and gold nanoparticles of different size and extinction properties.
Keywords: analytical; ultracentrifugation; sedimentation; colloid; macromolecular; simulation

Simultaneous design of the optimal reaction and process concept for multiphase systems - Hentschel, Benjamin; Peschel, Andreas; Freund, Hannsjörg; Sundmacher, Kai (Chemical Engineering Science, 2014)
In complex chemical processes optimizing the reactor with respect to a stand-alone reactor performance criterium such as product selectivity does not necessarily yield the best process-wide reaction concept. Especially in case of multiphase reaction systems, which often involve complex reaction networks, nonideal phase behavior and multiple recycles, reactor and process have to be optimized simultaneously since the reactor affects the separation units and vice versa. In order to derive the process-wide optimal reaction concept in combination with optimal process parameters, a systematic optimization procedure based on a large scale optimization problem constrained by a system of differential algebraic equations (DAE) is presented. The optimization problem contains a detailed model of the reaction section, a process model describing the other units of the process, and models for operational and fixed costs. The solution of the optimization is a set of process parameters and optimal profiles of heat and material fluxes over the reaction coordinate which minimize the production costs. The method is exemplified on the rhodium catalyzed hydroformylation of 1-dodecene in a thermomorphic solvent system. (C) 2013 Elsevier Ltd. All rights reserved.
Keywords: Reactor design; Process design; Simultaneous optimization; Multiphase systems; Hydroformylation; Process economics

Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution - Peukert, Wolfgang; Karabudak, Engin; Brookes, Emre; Lesnyak, Vladimir; Gaponik, Nikolai; Eychmüller, Alexander; Walter, Johannes; Segets, Doris; Wohlleben, Wendel; Demeler, Borries; Cölfen, Helmut (Angewandte Chemie-International Edition, 2016)
We report an unsurpassed solution characterization technique based on analytical ultracentrifugation, which demonstrates exceptional potential for resolving particle sizes in solution with sub-nm resolution. We achieve this improvement in resolution by simultaneously measuring UV/Vis spectra while hydrodynamically separating individual components in the mixture. By equipping an analytical ultracentrifuge with a novel multi-wavelength detector, we are adding a new spectral discovery dimension to traditional hydrodynamic characterization, and amplify the information obtained by orders of magnitude. We demonstrate the power of this technique by characterizing unpurified CdTe nanoparticle samples, avoiding tedious and often impossible purification and fractionation of nanoparticles into apparently monodisperse fractions. With this approach, we have for the first time identified the pure spectral properties and band-gap positions of discrete species present in the CdTe mixture.
Keywords: analytical methods; colloids; nanoparticles

Single-mode hollow-core photonic crystal fiber made from soft glass - Jiang, Xin; Euser, T. G.; Abdolvand, Amir; Babic, F.; Tani, Francesco; Joly, Nicolas; Travers, J. C.; Russell, Philip St. John (Optics Express, 2011)
We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagomé-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter 30 µm) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices.
EAM Import::2019-03-13

Single phase 3D phononic band gap material - Warmuth, Franziska; Wormser, Maximilian; Körner, Carolin (Scientific Reports, 2017)
Phononic band gap materials are capable of prohibiting the propagation of mechanical waves in certain frequency ranges. Band gaps are produced by combining different phases with different properties within one material. In this paper, we present a novel cellular material consisting of only one phase with a phononic band gap. Different phases are modelled by lattice structure design based on eigenmode analysis. Test samples are built from a titanium alloy using selective electron beam melting. For the first time, the predicted phononic band gaps via FEM simulation are experimentally verified. In addition, it is shown how the position and extension of the band gaps can be tuned by utilizing knowledge-based design.

Single-speed molecular dynamics of hard parallel squares and cubes - Bannerman, Marcus; Hoover, Wm. G.; Hoover, Carol G. (Journal of Statistical Physics, 2009)
The fluid and solid equations of state for hard parallel squares and cubes are reinvestigated here over a wide range of densities.We use a novel single-speed version of molecular dynamics. Our results are compared with those from earlier simulations, as well as with the predictions of the virial series, the cell model, and Kirkwood's many-body singleoccupancy model. The single-occupancy model is applied to give the absolute entropy of the solid phases just as was done earlier for hard disks and hard spheres. As we should expect, the excellent agreement found here with all relevant previous work shows very clearly that configurational properties, such as the equation of state, do not require the maximumentropy Maxwell-Boltzmann velocity distribution. For both hard squares and hard cubes the free-volume theory provides a good description of the high-density solid-phase pressure. Hard parallel squares appear to exhibit a second-order melting transition at a density of 0.79 relative to close-packing. Hard parallel cubes have a more complicated equation of state, with several relatively-gentle curvature changes, but nothing so abrupt as to indicate a firstorder melting transition. Because the number-dependence for the cubes is relatively large the exact nature of the cube transition remains unknown. © The Author(s) 2009.
Keywords: Computational methods; Melting transition; Molecular dynamics

Singlet Fission for Photovoltaics with 130 % Injection Efficiency - Kunzmann, Andreas; Gruber, Marco; Casillas Pacheco, Ruben; Zirzlmeier, Johannes; Stanzel, Melanie; Peukert, Wolfgang; Tykwinski, Rik; Guldi, Dirk Michael (Angewandte Chemie International Edition, 2018)
A novel pentacene dimer (P2) and a structurally analogous monomer (P1) were synthesized for use in n-type dye-sensitized solar cells. In P2, the triplet excited states formed by the rapid, spin-allowed process singlet fission were expected to enable carrier multiplication in comparison to the slow, spin-forbidden intersystem crossing seen in P1. A meta-positioning of the two pentacenes and the carboxylate anchor were chosen in P2 to balance the intramolecular dynamics of singlet fission and electron injection. Electron injection from energetically low-lying triplet excited states of pentacene units necessitated the intrinsic and extrinsic lowering of the Fermi level of the semiconductor. Indium-zinc oxide in the presence of Li+ was found to be the optimum choice for the photoelectrodes. Efficient electron injection from the triplet excited states of P1 and P2 was found, with a carrier multiplication of nearly 130 %.
Keywords: pentacene sensitizers; singlet fission; dye-sensitized solar cells; transient absorption spectroscopy

Singlet fission in pentacene dimers - Zirzlmeier, Johannes; Lehnherr, Dan; Brana Coto, Pedro; Chernick, Erin; Casillas, Ruben; Basel, Bettina; Thoss, Michael; Tykwinski, Rik; Guldi, Dirk Michael (Proceedings of the National Academy of Sciences of the United States of America, 2015)
Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley-Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.

Single-Walled TiO2 Nanotubes: Enhanced Carrier-Transport Properties by TiCl4 Treatment - Hwang, Imgon; So, Seulgi; Mokhtar, Mohamed; Alshehri, Abdelmohsen; Al-Thabaiti, Shaeel A.; Mazare, Anca Valentina; Schmuki, Patrik (Chemistry - A European Journal, 2015)
In the present work we report significant enhancement of the photoelectrochemical properties of self- organized TiO2 nanotubes by a combined -coringof classic nanotubes followed by an appropiate TiCl4 treatment. We show that, except for the expected particle decoration, a key effect of the TiCl4 treatment is that the electron transport characteristics in TiO2 nanotubes can be drastically improved, for example, we observe an enhancement of up to 70 % in electron-transport times.
EAM Import::2019-03-13

Site blocking and CO/sulfur site exchange processes on stepped Pt surfaces - Streber, Regine; Papp, Christian; Lorenz, Michael Peter Andreas; Bayer, Andreas; Wickert, Sandra; Schöppke, Matthias; Denecke, Reinhard; Steinrück, Hans-Peter (Journal of Physics: Condensed Matter, 2009)
The influence of preadsorbed sulfur on the adsorption of CO on Pt(355) and Pt(322) is investigated systematically for sulfur precoverages between 0.02 and 0.30ML by insitu x-ray photoelectron spectroscopy of the C 1s and S 2p core levels. The two surfaces have the same nominal terrace width of 5 atomic rows, but different step orientation. For both, at low temperatures (130 and 150K), S preferentially adsorbs at the steps and passivates them for COadsorption. The required S precoverage is significantly smaller for Pt(322), because of the lower number of steps as a result of the S-induced double-step formation. Upon heating, population of step sites with CO occurs due to irreversible CO/S site exchange. At low S precoverages, the characteristic transition temperatures of 165K for Pt(355) and 245K for Pt(322) are significantly different, indicating a higher activation barrier for the CO/S site exchange process for Pt(322). For medium to high S precoverages the transition temperature stays unchanged for Pt(322) but increases for Pt(355). The observed behavior is attributed to a kinetic passivation of the steps by sulfur at low temperatures, which is lifted upon heating. © 2009 IOP Publishing Lt

Size and structure effects controlling the stability of the liquid organic hydrogen carrier dodecahydro- N -ethylcarbazole during dehydrogenation over pt model catalysts - Amende, Maximilian; Gleichweit, Christoph; Schernich, Stefan; Höfert, Oliver; Lorenz, Michael Peter Andreas; Zhao, Wei; Koch, Marcus; Obesser, Katharina; Papp, Christian; Wasserscheid, Peter; Steinrück, Hans-Peter; Libuda, Jörg (Journal of Physical Chemistry Letters, 2014)
Hydrogen can be stored conveniently using so-called liquid organic hydrogen carriers (LOHCs), for example, N-ethylcarbazole (NEC), which can be reversibly hydrogenated to dodecahydro-N-ethylcarbazole (H-NEC). In this study, we focus on the dealkylation of H-NEC, an undesired side reaction, which competes with dehydrogenation. The structural sensivity of dealkylation was studied by high-resolution X-ray photoelectron spectroscopy (HR-XPS) on AlO-supported Pt model catalysts and Pt(111) single crystals. We show that the morphology of the Pt deposit strongly influences LOHC degradation via C-N bond breakage. On smaller, defect-rich Pt particles, the onset of dealkylation is shifted by 90 K to lower temperatures as compared to large, well-shaped particles and well-ordered Pt(111). We attribute these effects to a reduced activation barrier for C-N bond breakage at low-coordinated Pt sites, which are abundant on small Pt aggregates but are rare on large particles and single crystal surfaces. © 2014 American Chemical Society.

Size-controlled PtNi nanoparticles as highly efficient catalyst for hydrodechlorination reactions - Munoz, Macarena; Ponce, Sebastian; Zhang, Guirong; Etzold, Bastian (Applied Catalysis B-Environmental, 2016)
The application of size-controlled PtNi nanoparticles (NPs) as catalyst on the aqueous-phase hydrodechlorination (HDC) of 4-chlorophenol (4-CP) under ambient conditions (25 °C, 1 atm) has been investigated. NPs were synthesized as a co-reduction of both metals following a solvothermal method which allowed a proper control of the PtNi alloyed NP size in the range of 4.4–12.0 nm. To make the active sites of the nanoalloy accessible, the stabilizer poly(N-vinyl-2-pyrrolidone) was removed by washing of the NPs with H2O2/H2SO4. Evaluating the NP structure/composition influence on the activity, a clear synergic effect between Pt and Ni was found. In this sense, complete conversion of 4-CP was achieved in 1 h reaction time with the bimetallic NPs whereas only 70% was reached with the monometallic Pt counterpart (NP size = 12 nm, [Pt] = 80 mg L-1, [4-CP]0 = 78 extgreekmmol L-1). That effect could be related to the higher resistance of PtNi nanoalloy against chlorine poisoning compared to the monometallic Pt. In fact, PtNi NPs showed a remarkable stability and negligible deactivation even after storing the catalyst one month in the reaction mixture, whereas for the monometallic Pt fully deactivation resulted. The structure-sensitivity of HDC with PtNi nanocatalysts was finally confirmed as the catalytic performance was clearly dependent on the size. A volcano plot like behaviour was obtained, being 5.5 nm the optimum size. PtNi-5.5 led to the complete conversion of 4-CP in 15 min at a HDC rate of 3.12 L min-1 gPt-1.
EAM Import::2019-03-13

Size-Dependent Phase Stability of a Molecular Nanocrystal: a Proxy for Investigating the Early Stages of Crystallization - Zahn, Dirk; Anwar, Jamshed (Chemistry - A European Journal, 2011)
We make the link between the size-dependent phase stability of a nanocrystal and the phase-transition behavior of emerging crystallites during the earliest stages of crystallization, by using the former as a proxy for the latter. We outline an extension of the classical nucleation theory to describe crystal nucleation and subsequent transformations of competing polymorphic phases that characterize Ostwald's rule of stages. The theoretical framework reveals that the relative stability of the competing phases is a function of cluster size, which in turn varies with time, and therefore explains the complex transformation behavior observed for some systems. We investigated the stability of a nanocrystal of dl-norleucine by means of molecular simulation as a proxy for post-nucleation phase-transformation behavior in emerging crystallites. The simulations reveal that, for nanocrystals, the surface energy of the transition state of a transformation can dominate the barrier to phase change, thus causing metastable phases to be stabilized, not because they are thermodynamically stable, but rather due to kinetic hindering. Therefore, in the context of the earliest stages of crystal growth, not only does phase stability vary as a function of cluster size, and hence time, but thermodynamically feasible transformations are also prone to kinetic hindering.
EAM Import::2019-03-13

Sizing aerosolized fractal nanoparticle aggregates through Bayesian analysis of wide-angle light scattering (WALS) data - Huber, Franz; Daun, Kyle James; Will, Stefan (Journal of Quantitative Spectroscopy & Radiative Transfer, 2016)
Inferring the size distribution of aerosolized fractal aggregates from the angular distribution of elastically scattered light is a mathematically ill-posed problem. This paper presents a procedure for analyzing Wide-Angle Light Scattering (WALS) data using Bayesian inference. The outcome is probability densities for the recovered size distribution and aggregate morphology parameters. This technique is applied to both synthetic data and experimental data collected on soot-laden aerosols, using a measurement equation derived from Rayleigh-Debye-Gans fractal aggregate (RDG-FA) theory. In the case of experimental data, the recovered aggregate size distribution parameters are generally consistent with TEM-derived values, but the accuracy is impaired by the well-known limited accuracy of RDG-FA theory. Finally, we show how this bias could potentially be avoided using the approximation error technique. (C) 2016 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles; Fractal aggregates; Elastic light scattering; Bayesian analysis; Size distribution; Credibility intervals

Slope constrained material design - Schury, Fabian; Stingl, Michael; Wein, Fabian (Structural and Multidisciplinary Optimization, 2012)
We investigate a class of constrained inverse homogenization problems. The complexity of the topological solution is restricted using slope constraint regularization. We show existence of the solution for the inverse optimization problem in function space and outline a converging approximation scheme. We demonstrate how a proper numerical implementation can lead to a stable material design approach. We finally describe results for a comprehensive set of numerical test cases.
Keywords: Slope constraints; Inverse homogenization; Topology optimization; SIMP

Slow waves in magnetic metamaterials: history, fundamentals and applications - Shamonina, E. (physica status solidi (b), 2008)
Magnetic metamaterials consist of small metallic resonators of the split ring type. Microscopic properties of these structures are governed by near field coupling between individual elements. This strongly anisotropic coupling leads to propagation of slow magnetoinductive waves with the wavelength much shorter than that of the electromagnetic radiation. This opens up novel possibilities of designing miniaturised subwavelength waveguide components and near-field lenses in a wide range of frequencies, from radio frequencies, with a potential for medical applications in MRI, to the IR and visible range, aiming at fast signal processing. This paper provides an overview of contributions that led to the birth of the subject of magnetic metamaterials. The properties of slow waves on magnetic metamaterials including their dispersion characteristics, excitation and applications are discussed. The emphasis is laid on phenomena such as backward and forward slow waves, polaritonic hybridization of the interaction of a metamaterial with an incident electromagnetic wave yielding negative values of magnetic permeability, the possibility of positive and negative refraction of slow waves as well as mechanisms for near field subwavelength imaging.
EAM Import::2019-03-13

Slow waves on magnetic metamaterials and on chains of plasmonic nanoparticles: Driven solutions in the presence of retardation - Zhuromskyy, Oleksandr; Sydoruk, Oleksiy; Shamonina, E.; Solymar, L. (Journal of Applied Physics, 2009)
Slow waves on chains or lattices of resonant elements offer a unique tool for guiding and manipulating the electromagnetic radiation on a subwavelength scale. Applications range from radio waves to optics with two major classes of structures being used: (i) metamaterials made of coupled ring resonators supporting magnetoinductive waves and (ii) plasmonic crystals made of nanoparticles supporting waves of near-field coupling. We derive dispersion equations of both types of slow waves for the case when the interelement coupling is governed by retardation effects, and show how closely they are related. The current distribution is found from Kirchhoff's equation by inverting the impedance matrix. In contrast to previous treatments power conservation is demonstrated in a form relevant to a finite structure: the input power is shown to be equal to the radiated power plus the powers absorbed in the Ohmic resistance of the elements and the terminal impedance. The relations between frequency and wave number are determined for a 500-element line for two excitations using three different methods. Our approach of retrieval of the dispersion from driven solutions of finite lines is relevant for practical applications and may be used in the design of metamaterials and plasmonic crystals with desired properties.
EAM Import::2019-03-13

Small diameter TiO2 nanotubes vs. nanopores in dye sensitized solar cells - Liu, Ning; Lee, Kiyoung; Schmuki, Patrik (Electrochemistry Communications, 2012)
Highly ordered nanoporous and nanotubular TiO2 geometries can be anodically grown on Ti substrates in fluoride containing ethylene glycol electrolytes using different water contents. Here we fabricate 1 extgreekmm thick layers consisting of arrayed tubes or pores with an open diameter of 15 nm. We compare these small diameter structures to classical 100 nm diameter TiO2 nanotubes for their application in Grätzel-type solar cells. The results show clearly that a small diameter nanotube geometry significantly enhances the solar cell conversion efficiency.
EAM Import::2019-03-13

Smart Optical Composite Materials: Dispersions of Metal-Organic Framework@Superparamagnetic Microrods for Switchable Isotropic-Anisotropic Optical Properties - Mandel, Karl; Granath, Tim; Wehner, Tobias; Rey, Marcel; Stracke, Werner; Vogel, Nicolas; Sextl, Gerhard; Mueller-Buschbaum, Klaus (Acs Nano, 2017)
A smart optical composite material with dynamic isotropic and anisotropic optical properties by combination of luminescence and high reflectivity was developed. This combination enables switching between luminescence and angle-dependent reflectivity by changing the applied wavelength of light. The composite is formed as anisotropic core/shell particles by coating superparamagnetic iron oxide-silica microrods with a layer of the luminescent metal-organic framework (MOF) 3ınfty[Eu2(BDC)3]·2DMF·2H2O (BDC2- = 1,4-benzenedicarboxylate). The composite particles can be rotated by an external magnet. Their anisotropic shape causes changes in the reflectivity and diffraction of light depending on the orientation of the composite particle. These rotation-dependent optical properties are complemented by an isotropic luminescence resulting from the MOF shell. If illuminated by UV light, the particles exhibit isotropic luminescence while the same sample shows anisotropic optical properties when illuminated with visible light. In addition to direct switching, the optical properties can be tailored continuously between isotropic red emission and anisotropic reflection of light if the illuminating light is tuned through fractions of both UV and visible light. The integration and control of light emission modes within a homogeneous particle dispersion marks a smart optical material, addressing fundamental directions for research on switchable multifunctional materials. The material can function as an optic compass or could be used as an optic shutter that can be switched by a magnetic field, e.g., for an intensity control for waveguides in the visible range.
Keywords: anisotropy; composites; isotropy; magnetic actuation; metal organic frameworks; optical properties; smart materials; superparamagnetism; switching

Smooth conformal alpha-NEM for Gradient Elasticity - Rajagopal, Amirtham; Scherer, Michael; Steinmann, Paul; Sukumar, Natarajan (The International journal of structural changes in solids: mechanics and applications, 2009)
Strain gradient theory for continuum analysis has kinematic relations, which include terms from second gradients of the deformation map. This results in balance equations that have fourth order spatial derivatives, supplemented with higher order boundary conditions. The weak formulation of fourth order operators stipulate that the basis functions must be globally C 1 continuous; very few finite elements in two dimensions meet this requirement. In this paper, we propose a meshfree methodology for the analysis of gradient continua using a conformal alpha-shape based natural element method (NEM). The conformal alpha-NEM allows the construction of models entirely in terms of nodes, and ensures quadratic precision of the interpolant over convex and non-convex boundaries. Smooth natural neighbor interpolants are achieved by a transformation of Farin's C 1 interpolant, which are obtained by embedding Sibson's natural neighbor coordinates in the Bernstein-Bezier surface representation of a cubic simplex. Numerical examples are presented to demonstrate the accuracy and efficiency of the proposed method.<br
UnivIS-Import:2015-03-09:Pub.2009.tech.FT.FT-TM.smooth

Smoothly Tunable Surface Properties of Aluminum Oxide Core-Shell Nanoparticles By A Mixed-Ligand Approach - Portilla Berlanga, Luis; Halik, Marcus (ACS Applied Materials and Interfaces, 2014)
We present a facile solution-based procedure for tailoring the surface properties of aluminum oxide nanoparticles (AlOx-NPs) by the formation of tunable core-shell systems with self-assembled monolayers. By employing chained molecules with a phosphonic acid anchor group and either hydrophobic or hydrophilic chains the surface properties of the nanoparticles change dramatically. So, the solubility can be tuned orthogonal from trifluorotoluene (CF3-C6H5) for hydrophobic shell to water (H2O) for hydrophilic functionalization respectively. Spray coated films of those functionalized nanoparticles exhibited superhydrophobic or superhydrophilic properties. The surface properties can be tuned smoothly by the formation of a mixed ligand monolayer from corresponding stoichiometric mixtures of the ligands. The core-shell nanoparticles were investigated by means of thermogravimetric analysis, TGA; Fourier transform infrared spectroscopy, FTIR; and static contact angle goniometry, SCA. The effect of different dipole moments of the SAM molecules in mixed shell nanoparticles to their stability in dispersions was studied by zeta potential measurements.

SO2 adsorption and thermal evolution on clean and oxygen precovered Pt(111) - Streber, Regine; Papp, Christian; Lorenz, Michael Peter Andreas; Höfert, Oliver; Darlatt, Erik; Bayer, Andreas; Denecke, Reinhard; Steinrück, Hans-Peter (Chemical Physics Letters, 2010)
We studied the adsorption and reaction of sulfur dioxide on clean and oxygen precovered platinum surfaces by in situ high resolution X-ray photoelectron spectroscopy and discuss earlier, contradicting results. On the clean surface we find flat lying and upright standing SO, adsorbing in similar amounts at low temperature. Upon heating, the ratio changes strongly favoring the standing SO; subsequently almost all SO desorbs. On the oxygen precovered surface the standing SO is preferably populated and even at low temperatures a direct reaction of SO to SO occurs. Upon heating, SO oxidation to SO and subsequently to SO is found. © 2010 Elsevier B.V. All rights reserve

SO2 adsorption on Pt(111) and oxygen precovered Pt(111): A combined infrared reflection absorption spectroscopy and density functional study - Happel, Markus; Luckas, Nicola; Vines Solana, Francesc; Sobota, Marek; Laurin, Mathias; Görling, Andreas; Libuda, Jörg (Journal of Physical Chemistry C, 2011)
We have performed a combined molecular beam (MB)/infrared reflection absorption spectroscopy (IRAS) and density functional (DF) study on the adsorption of sulfur dioxide (SO) on clean and oxygen precovered Pt(111). The adsorbate species formed as a function of coverage and their structural and chemical transformations upon subsequent annealing were followed systematically by IRAS. It is shown that, upon adsorption on clean Pt(111) at 100 K, SO adsorbs molecularly in two geometries. Calculations reveal that these geometries correspond to molecules oriented parallel or perpendicular to the surface. Upon heating up to 250 K, conversion between the two species occurs with the fraction of upright standing SO increasing with increasing temperature. Between 300 and 350 K, all surface species desorb without decomposition. This is in sharp contrast to oxygen precovered Pt(111), on which, in addition to the formation of molecular SO in flat and perpendicular geometry, partial formation of SO is observed even at low temperature. Upon annealing, part of the surface SO species are converted to SO and, subsequently, to SO, which remains stable up to 450 K. DF calculations on the stable adsorption structures of SO species provide characteristic IR frequencies, which can be correlated with the present IRAS data and photoelectron spectroscopy (PES) data previously published. © 2010 American Chemical Society.

SO2 Decomposition on Pt/CeO2(111) Model Catalysts: On the Reaction Mechanism and the Influence of H-2 and CO - Happel, Markus; Lykhach, Yaroslava; Tsud, Nataliya; Skala, Tomas; Johanek, Viktor; Prince, Kevin C.; Matolin, Vladimir; Libuda, Jörg (Journal of Physical Chemistry C, 2012)
We have performed a synchrotron radiation photoelectron spectroscopy (SR-PES) study on the mechanism of SO2 decomposition on Pt/CeO2(111) model catalysts prepared on Cu(111). Resonant photoemission spectroscopy (RPES) was applied to monitor changes of the cerium oxidation state. Adsorption of SO2 at 150 K on Pt/CeO2(111)/Cu(111) yields surface sulfites (SO32-) on ceria and upright standing molecular SO2 (SO2st) on Pt along with traces of atomic sulfur. Partial decomposition and desorption of the adsorbed species in the form of SO2 occur between 150 and 300 K. In this temperature region, slow accumulation of atomic sulfur is accompanied by reduction of the ceria support. Above 300 K, all surface species completely transform into atomic sulfur; this process is accompanied by a strong reoxidation of ceria. As probed by CO adsorption, the majority of the atomic sulfur species is located on the surface of the Pt particles. Possible reaction mechanisms involving spillover of numerous sulfur-derived species are considered. Finally, reverse spillover of the sulfur from Pt to the ceria support and formation of a cerium oxysulfide phase occurs above 550 K. The spillover of atomic sulfur is readily promoted under reducing conditions, e.g., in the presence of coadsorbed hydrogen or CO.

Sodium effect on self-organization of amphiphilic carboxylates: Formation of structured micelles and superlattices - Rosenlehner, Karin; Schade, Boris; Böttcher, Christoph; Jäger, Christof; Clark, Timothy; Heinemann, Frank Wilhelm; Hirsch, Andreas (Chemistry - A European Journal, 2010)
Not only the self-aggregation of dendritic polycarboxylates intostructurally persistent micelles, but also that of the micelles themselves into superlatticesis controlled by alkali-metalcounterions and shows a pronounced sodium effect. Our combined experimental and computational work has revealed the formation of superlattices for the first time. The behavior of a variety of amphiphilic carboxylates and the different effects of the alkali cations Li, Na, and K have been investigated by conductivity measurements, cryogenic transmission electron microscopy (cryo-TEM), and molecular-dynamics (MD) simulations. Together, these show that sodium salts of the amphiphiles give the most stable micelles, followed by lithium and potassium. Our results suggest that ion multiplets in bridging positions, rather than contact ion pairs, are responsible for the enhanced stability and the formation of hexagonally ordered superlattices with sodium counter ions. Potassium ions do not form such ion multiplets and cannot therefore induce aggregation of the micelles. This sodium effect has far-reaching consequences for a large number of biological and technical systems and sheds new light on the origin of specific-ion effects. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Solar Light and Dopant-Induced Recombination Effects: Photoactive Nitrogen in TiO2 as a Case Study - D'Arienzo, Massimiliano; Siedl, Nicolas; Sternig, Andreas; Scotti, Roberto; Morazzoni, Franca; Bernardi, Johannes; Diwald, Oliver (Journal of Physical Chemistry C, 2010)
The spectral response of newly synthesized and doped TiO_2 nanostructures, which are designed for the operation under solar light, is key to their potential performance as photocatalyst materials. In the case of nitrogen-doped TiO_2 particles, the presence of the photoactive defect can be verified via characteristic spectroscopic fingerprints, such as (1) an optical absorption band above extgreekl > 400 nm, (2) paramagnetic defect states related to substitutional and interstitial nitrogen atoms, and (3) a photoluminescence emission feature that can be induced by band gap excitation. We report these spectroscopic fingerprints for hydrothermally derived and N-doped TiO_2 powder samples. On the basis of defined annealing procedures, we also found a way to eliminate the photoactive nitrogen defects and performed light-induced charge separation experiments on samples before and after dopant elimination in O_2 atmosphere. The concentrations of adsorbed O_2 - ions, which stand for scavenged photogenerated electrons, were determined by electron paramagnetic resonance (EPR) spectroscopy. On the basis of these data, we calculated the number of photons, which (a) were additionally utilized because of the beneficial effect of photoactive nitrogen and (b) were lost because of the dopant-induced recombination of UV light generated charge carriers. As a major result, we found for the doped materials that both effects actually compensate each other. With regard to the synthesis and characterization of doped TiO_2 systems, these findings underline the importance to include dopant-induced charge-carrier recombination effects in the evaluation of the photocatalyst's spectral response in order to reach a fair evaluation of its properties.
EAM Import::2019-03-13

Solar water splitting: preserving the beneficial small feature size in porous α-Fe2O3 photoelectrodes during annealing - Wang, Lei; Lee, Chong-Yong; Schmuki, Patrik (Journal of Materials Chemistry A, 2012)
We prepared mesoporous extgreeka-Fe2O3/FTO electrodes by a simple anodic precipitation and annealing method. The classic problem is that when annealed in air at >500 °C, the individual particle size increases from ≈20 nm to 40–60 nm. This grain growth is highly detrimental for the water splitting efficiency. In the present work we show that grain coarsening can efficiently be prevented by a two-step annealing method in Ar, leading to photocurrent densities for such electrodes (obtained in 1 M KOH solution) of up to 2.6 mA cm-2 at 1.6 V vs. RHE under AM 1.5 100 mW cm-2 illumination, which is significantly higher than 1.5 mA cm-2 for conventionally annealed samples.
EAM Import::2019-03-13

Solid-Solid Interface Formation in TiO2 Nanoparticle Networks - Baumann, Stefan O.; Elser, Michael J.; Auer, Michael; Bernardi, Johannes; Huesing, Nicola; Diwald, Oliver (Langmuir, 2011)
Aiming at a comparison of microstructure and paramagnetic properties of mesoporous TiO_2 nanoparticle networks, we subjected entirely different TiO2-x precursor structures to vacuum annealing. The transformation of an amorphous TiO2-x gel—obtained by sol-gel processing of an ethylene glycol-modified titanium precursor—into a network of interconnected anatase nanocrystals was explored by means of X-ray diffraction, nitrogen sorption, and electron microscopy. Crystalline junctions between the particles emerge from temperature treatment. This process of particle network formation is different from that related to the vapor phase grown anatase nanocrystals where particle-particle interface formation is induced by contact with water. It was found that, after annealing up to 873 K and controlled sample purification in oxygen atmosphere, both types of samples exhibit high concentrations of particle-particle interfaces and comparable properties in terms of surface area, porosity, and microstructure. With electron paramagnetic resonance (EPR) we observed on nonstoichiometric TiO2-x networks an identical type of subsurface defect which is related to the presence of solid-solid interfaces.
EAM Import::2019-03-13

Solid-state structures of double-long-chain imidazolium ionic liquids: Influence of anion shape on cation geometry and crystal packing - Wang, Xinjiao; Vogel, Carola; Heinemann, Frank Wilhelm; Wasserscheid, Peter; Meyer, Karsten (Crystal Growth & Design, 2011)
The syntheses and solid-state structures of a series of imidazolium (IM) salt-based, double C alkyl chain functionalized ionic liquids, namely, [CCIM] [A], where the anion A is I , I, I, N(CN) , C(CN), B(CN), or SbF, are reported. All compounds were fully characterized by CHN elemental analysis, H and C NMR spectroscopy, and X-ray diffraction studies on single crystals. The molecular structure of the IM [CCIM] cation, as found in the individual crystal packing arrangements, is discussed in relation to the different anions used for crystallization. Depending on the geometry of the counteranions used (linear, bent, planar, and spherical), different molecular structures of the IM cations (rod-, V-, and U-shaped) resulted. The crystal packing in the solid-state structure is examined on the basis of a Hirshfeld surface analysis and is discussed in terms of polar and nonpolar regions. © 2011 American Chemical Society.