Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2022 • 238 Carbon-nitrogen bond formation on Cu electrodes during CO2 reduction in NO3- solution
    Krzywda, P.M. and Paradelo Rodríguez, A. and Benes, N.E. and Mei, B.T. and Mul, G.
    Applied Catalysis B: Environmental 316 (2022)
    We demonstrate by Raman Spectroscopy that simultaneous reduction of NO3- and CO2 on Cu surfaces leads to formation of Cu-C[tbnd]N–like species, showing Raman bands at 2080 and 2150 cm−1 when associated with reduced or oxidized Cu surfaces, respectively. Furthermore Cu-C[tbnd]N–like species are soluble, explaining vast restructuring of the Cu surface observed after co-electrolysis of CO2 and nitrate. Oxidation of deposited Cu-C[tbnd]N–like species results in the formation of NO. Cu-C[tbnd]N–like species do not form in electrolytes containing i) NH4+ and CO2, or ii) NO3- and HCOO-, suggesting these likely originate from Cu-CO, the commonly accepted intermediate in electrochemical reduction of CO2, and Cu-NHx species, previously identified in the literature as intermediate towards C-N bond formation. The implications of the previously unresolved formation of Cu-C[tbnd]N–like species for the development of electrodes and processes for electrochemical formation of carbon-nitrogen bonds, including urea, amines or amides, are briefly discussed. © 2022 The Authors
    view abstractdoi: 10.1016/j.apcatb.2022.121512
  • 2022 • 237 Cementite decomposition in 100Cr6 bearing steel during high-pressure torsion: Influence of precipitate composition, size, morphology and matrix hardness
    Kiranbabu, S. and Tung, P.-Y. and Sreekala, L. and Prithiv, T.S. and Hickel, T. and Pippan, R. and Morsdorf, L. and Herbig, M.
    Materials Science and Engineering A 833 (2022)
    Premature failure of rail and bearing steels by White-Etching-Cracks leads to severe economic losses. This failure mechanism is associated with microstructure decomposition via local severe plastic deformation. The decomposition of cementite plays a key role. Due to the high hardness of this phase, it is the most difficult obstacle to overcome in the decaying microstructure. Understanding the mechanisms of carbide decomposition is essential for designing damage-resistant steels for industrial applications. We investigate cementite decomposition in the bearing steel 100Cr6 (AISI 52100) upon exposure to high-pressure torsion (maximum shear strain, Ƴmax = 50.2). Following-up on our earlier work on cementite decomposition in hardened 100Cr6 steel (Qin et al., Act. Mater. 2020 [1]), we now apply a modified heat treatment to generate a soft-annealed microstructure where spherical and lamellar cementite precipitates are embedded in a ferritic matrix. These two precipitate types differ in morphology (spherical vs. lamellar), size (spherical: 100–1000 nm diameter, lamellar: 40–100 nm thickness) and composition (Cr and Mn partitioning). We unravel the correlation between cementite type and its resistance to decomposition using multi-scale chemical and structural characterization techniques. Upon high-pressure torsion, the spherical cementite precipitates did not decompose, but the larger spherical precipitates (≥ 1 μm) deformed. In contrast, the lamellar cementite precipitates underwent thinning followed by decomposition and dissolution. Moreover, the decomposition behavior of cementite precipitates is affected by the type of matrix microstructure. We conclude that the cementite size and morphology, as well as the matrix mechanical properties are the predominating factors influencing the decomposition behavior of cementite. The compositional effects of Cr and Mn on cementite stability calculated by complementary density functional theory (DFT) calculations are minor in the current scenario. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2021.142372
  • 2022 • 236 Halogen and structure sensitivity of halobenzene adsorption on copper surfaces
    Schunke, C. and Miller, D.P. and Zurek, E. and Morgenstern, K.
    Physical Chemistry Chemical Physics 24 4485-4492 (2022)
    The adsorption orientation of molecules on surfaces influences their reactivity, but it is still challenging to tailor the interactions that govern their orientation. Here, we investigate how the substituent and the surface structure alter the adsorption orientation of halogenated benzene molecules from parallel to tilted relative to the surface plane. The deviation of the parallel orientation of bromo-, chloro-, and fluorobenzene molecules adsorbed on Cu(111) and Cu(110) surfaces is determined, utilising the surface selection rule in reflection-absorption infrared spectroscopy. On Cu(111), all three halogenated molecules are adsorbed with their molecular plane almost parallel to the surface at low coverages. However, they are tilted at higher coverages; yet, the threshold coverages differ. On Cu(110), merely bromo- and chlorobenzene follow this trend, albeit with a lower threshold for both. In contrast, fluorobenzene molecules are tilted already at low coverages. The substantial influence of the halogen atom and the surface structure on the adsorption orientation, resulting from an interplay of molecule-molecule and molecule-surface interactions, is highly relevant for reactivity confined to two dimensions. © 2022 the Owner Societies.
    view abstractdoi: 10.1039/d1cp05660c
  • 2022 • 235 Microstructural evolution of Cu/W nano-multilayers filler metal during thermal treatment
    Li, H. and Xing, Z.-C. and Li, B.-J. and Liu, X.-S. and Lehmert, B. and Matthias, M. and Li, Z.-X. and Tillmann, W.
    Vacuum 200 (2022)
    Copper/Tungsten (Cu/W) nano-multilayers show potential for application as novel low-temperature brazing filler metals. Therefore, researchers are interested in understanding phase stability and microstructural evolution of the nano-multilayers during thermal treatment. A repetition of 50 alternating nanolayers of Cu and W with individual thicknesses of 10 nm were prepared by magnetron-sputtering on silicon substrates. The structural evolution of Cu/W nano-multilayers (NMLs) within the temperature range 400 °C–800 °C was monitored using real-time in-situ XRD, SEM, TEM, SAXS, DSC and in-house XRD system. The results showed that the melting point of Cu/W nano-multilayers determined using DSC was 793.694 °C was remarkably lower than the melting point of bulk Cu(1083 °C) and W (3140 °C). After annealing at 400 °C for 30 min, the surface of the NMLs exhibited more copper grains, with significant coarsening of the copper grains. The layered structure of the Cu/W NMLs was unaffected after annealed at 400 °C. When annealed at 600 °C for 30 min, some Cu particles migrated into the W layers along the internal interface leading to cracks which partially collapsed the original stratified structure. The nano-multilayered structure was completely destroyed when annealed at 800 °C. Further, the in-situ XRD results showed that the copper grains grew substantially, while the tungsten size remained unchanged with increasing temperature. © 2022
    view abstractdoi: 10.1016/j.vacuum.2022.111007
  • 2022 • 234 Nanoscale copper and silver thin film systems display differences in antiviral and antibacterial properties
    Meister, T.L. and Fortmann, J. and Breisch, M. and Sengstock, C. and Steinmann, E. and Köller, M. and Pfaender, S. and Ludwig, Al.
    Scientific Reports 12 (2022)
    The current Coronavirus Disease 19 (COVID-19) pandemic has exemplified the need for simple and efficient prevention strategies that can be rapidly implemented to mitigate infection risks. Various surfaces have a long history of antimicrobial properties and are well described for the prevention of bacterial infections. However, their effect on many viruses has not been studied in depth. In the context of COVID-19, several surfaces, including copper (Cu) and silver (Ag) coatings have been described as efficient antiviral measures that can easily be implemented to slow viral transmission. In this study, we detected antiviral properties against Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) on surfaces, which were coated with Cu by magnetron sputtering as thin Cu films or as Cu/Ag ultrathin bimetallic nanopatches. However, no effect of Ag on viral titers was observed, in clear contrast to its well-known antibacterial properties. Further enhancement of Ag ion release kinetics based on an electrochemical sacrificial anode mechanism did not increase antiviral activity. These results clearly demonstrate that Cu and Ag thin film systems display significant differences in antiviral and antibacterial properties which need to be considered upon implementation. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41598-022-11212-w
  • 2021 • 233 Combining Nanoconfinement in Ag Core/Porous Cu Shell Nanoparticles with Gas Diffusion Electrodes for Improved Electrocatalytic Carbon Dioxide Reduction
    Junqueira, J.R.C. and O'Mara, P.B. and Wilde, P. and Dieckhöfer, S. and Benedetti, T.M. and Andronescu, C. and Tilley, R.D. and Gooding, J.J. and Schuhmann, W.
    ChemElectroChem 8 4848-4853 (2021)
    Bimetallic silver-copper electrocatalysts are promising materials for electrochemical CO2 reduction reaction (CO2RR) to fuels and multi-carbon molecules. Here, we combine Ag core/porous Cu shell particles, which entrap reaction intermediates and thus facilitate the formation of C2+ products at low overpotentials, with gas diffusion electrodes (GDE). Mass transport plays a crucial role in the product selectivity in CO2RR. Conventional H-cell configurations suffer from limited CO2 diffusion to the reaction zone, thus decreasing the rate of the CO2RR. In contrast, in the case of GDE-based cells, the CO2RR takes place under enhanced mass transport conditions. Hence, investigation of the Ag core/porous Cu shell particles at the same potentials under different mass transport regimes reveals: (i) a variation of product distribution including C3 products, and (ii) a significant change in the local OH- activity under operation. © 2021 The Authors. ChemElectroChem published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/celc.202100906
  • 2021 • 232 Faceting diagram for Ag segregation induced nanofaceting at an asymmetric Cu tilt grain boundary
    Peter, N.J. and Duarte, M.J. and Kirchlechner, C. and Liebscher, C.H. and Dehm, G.
    Acta Materialia 214 (2021)
    In this work, we experimentally establish the isothermal nanofacet evolution at an asymmetric ∑5 tilt grain boundary in the Cu-Ag system using a diffusion couple approach. We investigate the nanofacet formation along the grain boundary in dependence of the Ag solute excess concentration. The initial grain boundary dissociates into asymmetric Ag-lean segments and Ag-rich symmetric (210) segments. Increasing Ag excess leads to an increase in Ag-rich facet segment length, while the length of the asymmetric facets remains constant. From this, we construct a grain boundary nanofaceting diagram deduced from our experiments relating local atomic structure, overall inclination and Ag solute excess. © 2021 The Author(s)
    view abstractdoi: 10.1016/j.actamat.2021.116960
  • 2021 • 231 Ferrous to Ferric Transition in Fe-Phthalocyanine Driven by NO2 Exposure
    Cojocariu, I. and Carlotto, S. and Sturmeit, H.M. and Zamborlini, G. and Cinchetti, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Jugovac, M. and Puschnig, P. and Piamonteze, C. and Casarin, M. and Feyer, V. and Schneider, C.M.
    Chemistry - A European Journal 27 3526-3535 (2021)
    Due to its unique magnetic properties offered by the open-shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d-states of FePc and the sp-band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature. © 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202004932
  • 2021 • 230 In situ correlation between metastable phase-transformation mechanism and kinetics in a metallic glass
    Orava, J. and Balachandran, S. and Han, X. and Shuleshova, O. and Nurouzi, E. and Soldatov, I. and Oswald, S. and Gutowski, O. and Ivashko, O. and Dippel, A.-C. and Zimmermann, M. and Ivanov, Y.P. and Greer, A.L. and Raabe, D. and...
    Nature Communications 12 (2021)
    A combination of complementary high-energy X-ray diffraction, containerless solidification during electromagnetic levitation and transmission electron microscopy is used to map in situ the phase evolution in a prototype Cu-Zr-Al glass during flash-annealing imposed at a rate ranging from 102 to 103 K s−1 and during cooling from the liquid state. Such a combination of experimental techniques provides hitherto inaccessible insight into the phase-transformation mechanism and its kinetics with high temporal resolution over the entire temperature range of the existence of the supercooled liquid. On flash-annealing, most of the formed phases represent transient (metastable) states – they crystallographically conform to their equilibrium phases but the compositions, revealed by atom probe tomography, are different. It is only the B2 CuZr phase which is represented by its equilibrium composition, and its growth is facilitated by a kinetic mechanism of Al partitioning; Al-rich precipitates of less than 10 nm in a diameter are revealed. In this work, the kinetic and chemical conditions of the high propensity of the glass for the B2 phase formation are formulated, and the multi-technique approach can be applied to map phase transformations in other metallic-glass-forming systems. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41467-021-23028-9
  • 2021 • 229 Influence of the Lattice Structure of Copper Surfaces on Ammonia Dimer Formation
    Srivastava, P. and Miller, D.P. and Morgenstern, K.
    Journal of Physical Chemistry C 125 24363-24370 (2021)
    The restriction imposed by the lattice structure of different surfaces is used to investigate the influence of the distance between two monomers on their ability to bind to each other. We compare the interaction of ammonia monomers at two distinct distances imposed by the surface structure of a Cu(511) high-index surface to that of a Cu(110) low-index surface using low-temperature scanning tunneling microscopy, inelastic tunneling spectroscopy, and density functional theory. Frustrated translational and rotational modes, the Mulliken and Bader charge analyses, and electrostatic potential mapping indicate chemisorption of ammonia monomers on both surfaces, with their dipoles oriented perpendicular to the surface plane. At a larger intermolecular distance of around 0.51 nm on step edges of Cu(511), the monomers slightly repel each other due to electrostatic repulsion. At a shorter distance of around 0.36 nm perpendicular to the close-packed rows on Cu(110), a noticeable charge transfer between adjacent monomers indicates binding, that is, dimer formation in parallel orientation. This binding energy of the molecules compensates for the electrostatic repulsion. Our results outline how the choice of the surface structure may be utilized to alter the intermolecular interaction of solvent molecules and to enforce or suppress dimer formation. ©
    view abstractdoi: 10.1021/acs.jpcc.1c06275
  • 2021 • 228 Limited Elemental Mixing in Nanoparticles Generated by Ultrashort Pulse Laser Ablation of AgCu Bilayer Thin Films in a Liquid Environment: Atomistic Modeling and Experiments
    Shih, C.-Y. and Chen, C. and Rehbock, C. and Tymoczko, A. and Wiedwald, U. and Kamp, M. and Schuermann, U. and Kienle, L. and Barcikowski, S. and Zhigilei, L.V.
    Journal of Physical Chemistry C (2021)
    Pulsed laser ablation in liquids (PLAL) is a promising technique for the generation of colloidal alloy nanoparticles that are of high demand in a broad range of fields, including catalysis, additive manufacturing, and biomedicine. Many of the applications have stringent requirements on the nanoparticle composition and size distributions, which can only be met through innovations in the PLAL technique guided by a clear understanding of the nanoparticle formation mechanisms. In this work, we undertake a combined computational and experimental study of the nanoparticle formation mechanisms in ultrashort PLAL of Ag/Cu and Cu/Ag bilayer thin films. Experimental probing of the composition of individual nanoparticles and predictions from large-scale atomistic simulations provide consistent evidence of limited mixing between the two components from bilayer films by PLAL. The simulated and experimental distributions of nanoparticle compositions exhibit an enhanced abundance of Ag-rich and Cu-rich nanoparticles, as well as a strongly depressed population of well-mixed alloy nanoparticles. The surprising observation that the nanoscale phase separation of the two components in the bilayer films manifests itself in the sharp departure from the complete quantitative mixing in the colloidal nanoparticles is explained by the complex dynamic interaction between the ablation plume and liquid environment revealed in the simulations of the initial stage of the ablation process. The simulations predict that rapid deceleration of the ablation plume by the liquid environment results in the formation of a transient hot and dense metal region at the front of the plume, which hampers the mixing of the two components and, at the same time, contributes to the stratification of the plume in the emerging cavitation bubble. As a result, nanoparticles of different sizes and compositions are produced in different parts of the emerging cavitation bubble during the first nanoseconds of the ablation process. Notably, the diameters of the largest nanoparticles generated in the simulations of the initial stage of the ablation process are more than twice larger than the thickness of the original bilayer films. This observation provides a plausible scenario for the formation of large nanoparticles observed in the experiments. The conclusion on limited elemental mixing in the nanoparticles is validated in simulations of bilayers with different spatial order of Cu and Ag layers, even though the two systems exhibit some notable quantitative differences mainly related to the different strength of electron-phonon coupling in Cu and Ag. Overall, the results of this study provide new insights into the formation mechanism of bimetallic nanoparticles in ultrashort PLAL from thin bilayer targets and suggest that the formation of alloy nanoparticles from immiscible elements may be hampered for targets featuring distinctive elemental segregation. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.0c09970
  • 2021 • 227 Performant implementation of the atomic cluster expansion (PACE) and application to copper and silicon
    Lysogorskiy, Y. and Oord, C. and Bochkarev, A. and Menon, S. and Rinaldi, M. and Hammerschmidt, T. and Mrovec, M. and Thompson, A. and Csányi, G. and Ortner, C. and Drautz, R.
    npj Computational Materials 7 (2021)
    The atomic cluster expansion is a general polynomial expansion of the atomic energy in multi-atom basis functions. Here we implement the atomic cluster expansion in the performant C++ code PACE that is suitable for use in large-scale atomistic simulations. We briefly review the atomic cluster expansion and give detailed expressions for energies and forces as well as efficient algorithms for their evaluation. We demonstrate that the atomic cluster expansion as implemented in PACE shifts a previously established Pareto front for machine learning interatomic potentials toward faster and more accurate calculations. Moreover, general purpose parameterizations are presented for copper and silicon and evaluated in detail. We show that the Cu and Si potentials significantly improve on the best available potentials for highly accurate large-scale atomistic simulations. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41524-021-00559-9
  • 2021 • 226 Phase decomposition in nanocrystalline Cr0.8Cu0.2 thin films
    Chakraborty, J. and Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 888 (2021)
    Metastable Cr0.8Cu0.2 alloy thin films with nominal thickness of 360 nm have been deposited on Si(100) substrate by co-evaporation of Cu and Cr using molecular beam epitaxy (MBE). Phase evolution, microstructure, stress development, and crystallographic texture in Cr0.8Cu0.2 thin films have been investigated by X-ray diffraction (XRD), atom probe tomography (APT) and transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) during annealing of the films in the temperature range 200–450 °C. X-ray diffraction of the as-deposited thin film shows single phase bcc crystal structure of the film whereas APT observation of fine precipitates in the film matrix due to inherent compositional fluctuation indicates onset of phase separation via spinodal decomposition regime. XRD (in-situ) and APT investigation of 300 °C annealed film reveals that the early stage of phase separation involves localized formation of metastable intermediate bcc precipitate phase having 60 at% Cr and 40 at% Cu approximately (~Cr0.6Cu0.4). For longer duration of annealing at temperature ≥350 °C, such metastable bcc precipitates act as heterogeneous nucleation sites for the onset of precipitation of Cu rich fcc Cu(Cr) phase which indicates a change of phase separation mechanism from ‘spinodal decomposition’ to ‘nucleation and growth’. Annealing of the film at temperature ≥400 °C for longer duration leads to the formation of a two phase structure with Cu rich fcc precipitate phase in a Cr rich bcc matrix. Observed phase decomposition is accompanied by significant changes in the microstructure, residual stress and crystallographic texture in the Cr rich bcc film matrix which leads to the minimization of both surface and strain energies and thereby a reduction of total Gibbs free energy of the thin film. Thermodynamic model calculation has been presented in order to understand the nucleation pathway of Cu rich stable fcc Cu(Cr) precipitates via non-classical nucleation of metastable intermediate bcc Cr0.6Cu0.4 phase. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2021.161391
  • 2021 • 225 Understanding Grain Boundary Electrical Resistivity in Cu: The Effect of Boundary Structure
    Bishara, H. and Lee, S. and Brink, T. and Ghidelli, M. and Dehm, G.
    ACS Nano 15 16607-16615 (2021)
    Grain boundaries (GBs) in metals usually increase electrical resistivity due to their distinct atomic arrangement compared to the grain interior. While the GB structure has a crucial influence on the electrical properties, its relationship with resistivity is poorly understood. Here, we perform a systematic study on the resistivity-structure relationship in Cu tilt GBs, employing high-resolution in situ electrical measurements coupled with atomic structure analysis of the GBs. Excess volume and energies of selected GBs are calculated using molecular dynamics simulations. We find a consistent relation between the coincidence site lattice (CSL) type of the GB and its resistivity. The most resistive GBs are in the high range of low-angle GBs (14°-18°) with twice the resistivity of high angle tilt GBs, due to the high dislocation density and corresponding strain fields. Regarding the atomistic structure, GB resistivity approximately correlates with the GB excess volume. Moreover, we show that GB curvature increases resistivity by ∼80%, while phase variations and defects within the same CSL type do not considerably change it. © 2021 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/acsnano.1c06367
  • 2020 • 224 A carbene stabilized precursor for the spatial atomic layer deposition of copper thin films
    Boysen, N. and Misimi, B. and Muriqi, A. and Wree, J.-L. and Hasselmann, T. and Rogalla, D. and Haeger, T. and Theirich, D. and Nolan, M. and Riedl, T. and Devi, A.
    Chemical Communications 56 13752-13755 (2020)
    This paper demonstrates a carbene stabilized precursor [Cu(tBuNHC)(hmds)] with suitable volatility, reactivity and thermal stability, that enables the spatial plasma-enhanced atomic layer deposition (APP-ALD) of copper thin films at atmospheric pressure. The resulting conductive and pure copper layers were thoroughly analysed and a comparison of precursor and process with the previously reported silver analogue [Ag(tBuNHC)(hmds)] revealed interesting similarities and notable differences in precursor chemistry and growth characteristics. This first report of APP-ALD grown copper layers is an important starting point for high throughput, low-cost manufacturing of copper films for nano- and optoelectronic devices. © 2020 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0cc05781a
  • 2020 • 223 A swift technique to hydrophobize graphene and increase its mechanical stability and charge carrier density
    Madauß, L. and Pollmann, E. and Foller, T. and Schumacher, J. and Hagemann, U. and Heckhoff, T. and Herder, M. and Skopinski, L. and Breuer, L. and Hierzenberger, A. and Wittmar, A. and Lebius, H. and Benyagoub, A. and Ulbricht, ...
    npj 2D Materials and Applications 4 (2020)
    Despite the improvement of the quality of CVD grown single-layer graphene on copper substrates, transferring the two-dimensional layer without introducing any unintentional defects still poses a challenge. While many approaches focus on optimizing the transfer itself or on necessary post-transfer cleaning steps, we have focused on developing a pre-treatment of the monolayer graphene on copper to improve the quality and reproducibility of the transfer process. By pressing an ethylene-vinyl acetate copolymer foil onto the monolayer graphene on copper using a commercially available vacuum bag sealer graphene is stabilized by the attachment of functional carbon groups. As a result, we are able to transfer graphene without the need of any supporting layer in an all-H2O wet-chemical transfer step. Despite the general belief that the crumbling of graphene without a support layer in a H2O environment is caused due to differences in surface energy, we will show that this assumption is false and that this behavior is caused rather by the polar interactions between graphene and water. Suppressing these interactions protects graphene from ripping and results in extremely clean, highly crystalline graphene with a coverage close to 100%. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41699-020-0148-9
  • 2020 • 222 Conductive films prepared from inks based on copper nanoparticles synthesized by transferred arc discharge
    Fu, Q. and Stein, M. and Li, W. and Zheng, J. and Kruis, F.E.
    Nanotechnology 31 (2020)
    Copper nanoparticles (NPs) are considered as a promising alternative for silver and gold NPs in conductive inks for the application of printing electronics, since copper shows a high electrical conductivity but is significantly cheaper than silver and gold. In this study, copper NPs were synthesized in the gas phase by transferred arc discharge, which has demonstrated scale-up potential. Depending on the production parameters, copper NPs can be continuously synthesized at a production rate of 1.2-5.5 g h-1, while their Brunauer-Emmett-Teller sizes were maintained below 100 nm. To investigate the suitability in electronic printing, we use ball milling technique to produce copper conductive inks. The effect of ball milling parameters on ink stability was discussed. In addition, the electrical resistivity of copper films sintered at 300 °C in reducing atmosphere was measured to be 5.4 ± 0.6 μΩ cm which is about three times higher than that of bulk copper (1.7 μΩ cm). This indicates that conductive inks prepared from gas-phase synthesized copper NPs are competitive to the conductive inks prepared from chemically synthesized copper NPs. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/ab4524
  • 2020 • 221 Crystal anisotropy-dependent shear angle variation in orthogonal cutting of single crystalline copper
    Wang, Z. and Zhang, J. and Xu, Z. and Zhang, J. and Li, G. and Zhang, H. and Li, Z. and Hassan, H.U. and Fang, F. and Hartmaier, A. and Yan, Y. and Sun, T.
    Precision Engineering 63 41-48 (2020)
    Shear deformation that dominates elementary chip formation in metal cutting greatly relies on crystal anisotropy. In the present work we investigate the influence of crystallographic orientation on shear angle in ultra-precision orthogonal diamond cutting of single crystalline copper by joint crystal plasticity finite element simulations and in-situ experiments integrated in scanning electron microscope. In particular, the experimental cutting conditions including a straight cutting edge are the same with that used in the 2D finite element simulations. Both simulations and experiments demonstrate a well agreement in chip profile and shear angle, as well as their dependence on crystallography. A series of finite element simulations of orthogonal cutting along different cutting directions for a specific crystallographic orientation are further performed, and predicated values of shear angle are used to calibrate an extended analytical model of shear angle based on the Ernst–Merchant relationship. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.precisioneng.2020.01.006
  • 2020 • 220 Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal–Organic Frameworks
    Bitzer, J. and Otterbach, S. and Thangavel, K. and Kultaeva, A. and Schmid, R. and Pöppl, A. and Kleist, W.
    Chemistry - A European Journal 26 5667-5675 (2020)
    Metal–organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed-metal metal–organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed-metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed-metal FeCuBTC material with HKUST-1 structure, which contained bimetallic Fe−Cu paddlewheels as well as monometallic Cu−Cu and Fe−Fe units under optimized synthesis conditions. The in-depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed-metal metal–organic framework materials and the thorough characterization thereof is particularly important to derive structure–property or structure–activity correlations. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201905596
  • 2020 • 219 Global optimization of copper clusters at the ZnO(10 1 ¯ 0) surface using a DFT-based neural network potential and genetic algorithms
    Paleico, M.L. and Behler, J.
    Journal of Chemical Physics 153 (2020)
    The determination of the most stable structures of metal clusters supported at solid surfaces by computer simulations represents a formidable challenge due to the complexity of the potential-energy surface. Here, we combine a high-dimensional neural network potential, which allows us to predict the energies and forces of a large number of structures with first-principles accuracy, with a global optimization scheme employing genetic algorithms. This very efficient setup is used to identify the global minima and low-energy local minima for a series of copper clusters containing between four and ten atoms adsorbed at the ZnO(101¯0) surface. A series of structures with common structural features resembling the Cu(111) and Cu(110) surfaces at the metal-oxide interface has been identified, and the geometries of the emerging clusters are characterized in detail. We demonstrate that the frequently employed approximation of a frozen substrate surface in global optimization can result in missing the most relevant structures. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0014876
  • 2020 • 218 Investigation of wetting behaviour of hardenable copper based-alloys for brazing applications
    Tillmann, W. and Lehmert, B. and Manka, M. and Wojarski, L. and Holewa, M.
    Science and Technology of Welding and Joining 25 290-296 (2020)
    Tools for machining are exposed to high loads, wear, and elevated temperatures. Commonly, such tools consist of cemented carbides and tool steel. To combine the advantages of both materials, high-quality-joints with high strengths are desired. When brazing these materials, the main challenge is the mismatch of the Coefficient of Thermal Expansion (CTE) and the poor wettability of cemented carbides by molten filler alloys. In this regard, the feasibility of two custom-made alloys (CuNi12Si5, CuNi12Si5B0.4) was analysed. Besides being a cost-efficient alternative, these alloys offer the possibility to modify their mechanical properties by precipitation hardening to reduce stresses within the final joints. Thus, this paper shows that a temperature of 1060°C is suitable for wetting and brazing tests on different substrates. © 2019, © 2019 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute.
    view abstractdoi: 10.1080/13621718.2019.1688458
  • 2020 • 217 Irreversible Structural Changes of Copper Hexacyanoferrate Used as a Cathode in Zn-Ion Batteries
    Lim, J. and Kasiri, G. and Sahu, R. and Schweinar, K. and Hengge, K. and Raabe, D. and La Mantia, F. and Scheu, C.
    Chemistry - A European Journal 26 4917-4922 (2020)
    The structural changes of copper hexacyanoferrate (CuHCF), a Prussian blue analogue, which occur when used as a cathode in an aqueous Zn-ion battery, are investigated using electron microscopy techniques. The evolution of ZnxCu1−xHCF phases possessing wire and cubic morphologies from initial CuHCF nanoparticles are monitored after hundreds of cycles. Irreversible introduction of Zn ions to CuHCF is revealed locally using scanning transmission electron microscopy. A substitution mechanism is proposed to explain the increasing Zn content within the cathode material while simultaneously the Cu content is lowered during Zn-ion battery cycling. The present study demonstrates that the irreversible introduction of Zn ions is responsible for the decreasing Zn ion capacity of the CuHCF cathode in high electrolyte concentration. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201905384
  • 2020 • 216 Molecular anchoring stabilizes low valence Ni(i)TPP on copper against thermally induced chemical changes
    Sturmeit, H.M. and Cojocariu, I. and Jugovac, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Sala, A. and Comelli, G. and Moro, S. and Stredansky, M. and Corva, M. and Vesselli, E. and Puschnig, P. and Schneider, C.M. and...
    Journal of Materials Chemistry C 8 8876-8886 (2020)
    Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. Here, we study nickel porphyrin molecules on a copper surface as an archetypal system containing a metal center whose oxidation state can be controlled through the interaction with the metal substrate. We demonstrate that the strong molecule-surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin to the substrate. Upon thermal treatment, the molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0tc00946f
  • 2020 • 215 Observations of grain-boundary phase transformations in an elemental metal
    Meiners, T. and Frolov, T. and Rudd, R.E. and Dehm, G. and Liebscher, C.H.
    Nature 579 375-378 (2020)
    The theory of grain boundary (the interface between crystallites, GB) structure has a long history1 and the concept of GBs undergoing phase transformations was proposed 50 years ago2,3. The underlying assumption was that multiple stable and metastable states exist for different GB orientations4–6. The terminology ‘complexion’ was recently proposed to distinguish between interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexion and transitions between complexions have been characterized, mostly in binary or multicomponent systems8–19. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems20–24. However, the direct experimental observation and transformation kinetics of GBs in an elemental metal have remained elusive. Here we demonstrate atomic-scale GB phase coexistence and transformations at symmetric and asymmetric [11 1 ¯] tilt GBs in elemental copper. Atomic-resolution imaging reveals the coexistence of two different structures at Σ19b GBs (where Σ19 is the density of coincident sites and b is a GB variant), in agreement with evolutionary GB structure search and clustering analysis21,25,26. We also use finite-temperature molecular dynamics simulations to explore the coexistence and transformation kinetics of these GB phases. Our results demonstrate how GB phases can be kinetically trapped, enabling atomic-scale room-temperature observations. Our work paves the way for atomic-scale in situ studies of metallic GB phase transformations, which were previously detected only indirectly9,15,27–29, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41586-020-2082-6
  • 2020 • 214 On the crystallographic anisotropy of plastic zone size in single crystalline copper under Berkovich nanoindentation
    Wang, Z. and Zhang, J. and Ma, A. and Hartmaier, A. and Yan, Y. and Sun, T.
    Materials Today Communications 25 (2020)
    Aiming at revealing plastic deformation mechanisms of nanoindentation tests, we investigate the crystallographic orientation-influenced indentation size effect in the Berkovich nanoindentation tests of single crystalline copper, by using the nonlocal crystal plasticity finite element approach and specifically designed experiments. In our simulation model of nanoindentation, a new geometrically necessary dislocation density-based crystal plasticity model is proposed, and the utilized model parameters are calibrated by fitting the measured load-displacement curves of indentation tests. Then the size of plastic zone of indentation tests is defined by the surface pile-up profile, i.e. the diameter of a circle consisting of material points with half of maximum pile-up height. It is found that the modified plastic zone model incorporated with the newly developed scaling factor provides good predication of the indentation depth-dependent hardness of single crystalline copper. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.mtcomm.2020.101314
  • 2020 • 213 Stochastic modeling of classifying aerodynamic lenses for separation of airborne particles by material and size
    Furat, O. and Masuhr, M. and Kruis, F.E. and Schmidt, V.
    Advanced Powder Technology 31 2215-2226 (2020)
    A flexible stochastic approach is described to model separation processes, in which air-borne particles are separated via a setup of one or more aerodynamic lens orifices. Varying the size of the orifices, the focusing pressure and the mass flow rate through the lens, leads to focusing of particles with distinct aerodynamic properties on the central axis. In this modeling approach a bivariate transfer function is used to describe the passage probability of particles depending on their size and mass. The distribution of feed particles and the changes of the distribution due to the separation process is described via probability densities. The modeling procedure is applicable to various kinds of separation methods and allows optimization of geometric and operation parameters. To this end, the model utilizes flexibly defined separation performance measures which are illustrated in a case study that considers the separation of Cu particles from SiO2 particles. The spherical particles in the considered virtual mixtures are described by their log-normally distributed diameters and their normally distributed mass densities. Furthermore, the cases are selected in such a manner that the mean aerodynamic diameters of both Cu and SiO2 particles are equal. © 2020 The Society of Powder Technology Japan
    view abstractdoi: 10.1016/j.apt.2020.03.014
  • 2020 • 212 Tantalum and zirconium induced structural transitions at complex [111] tilt grain boundaries in copper
    Meiners, T. and Duarte, J.M. and Richter, G. and Dehm, G. and Liebscher, C.H.
    Acta Materialia 190 93-104 (2020)
    Alloying nanocrystalline copper (Cu) with immiscible elements, such as tantalum (Ta) and zirconium (Zr), is a promising technique to manipulate grain boundary properties and by this suppress grain growth at elevated temperatures. However, insights on the atomistic origins on the influence of impurity elements on grain boundaries are lacking. In this study, the atomistic effects of Ta and Zr on [111] tilt grain boundaries in Cu are investigated by high resolution scanning transmission electron microscopy techniques. In case of Ta, the formation of spherical, nano-scale precipitates in close vicinity to the grain boundaries is observed, but no sign of segregation. The particles induce a repelling force to migrating boundaries and act as local pinning points. The segregation of Zr is observed to occur either at confined grain boundary steps or homogeneously along the boundaries without steps. In both cases a strong disordering of the defect or grain boundary structure is revealed. Furthermore, at low Zr concentrations it induces structural grain boundary transitions and partial atomic reordering of the grain boundary structural units. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.064
  • 2020 • 211 Towards an understanding of grain boundary step in diamond cutting of polycrystalline copper
    Wang, Z. and Zhang, J. and Zhang, J. and Li, G. and Zhang, H. and ul Hassan, H. and Hartmaier, A. and Yan, Y. and Sun, T.
    Journal of Materials Processing Technology 276 (2020)
    Microstructural deformation at the grain level has an inherent impact on the achievable ultimate machining accuracy of polycrystalline materials. In the present work, numerical simulations and experiments of diamond cutting of polycrystalline copper are carried out to investigate the formation of surface step at grain boundaries on machined surface. Single crystal diamond cutting tool with straight cutting edge is chosen for experiments to mimic the tool geometry utilized in 2D crystal plasticity finite element simulations. Moreover, the same crystallography configuration of bi-crystal Cu is employed between experiments and simulations. Formation mechanisms of surface steps at grain boundaries are revealed by finite element simulations and corresponding experimental validation, as well as cross-sectional transmission electron microscope characterization. Finally, finite element simulations of orthogonal cutting of bi-crystal Cu are carried out to examine effects of both extrinsic cutting edge radius of diamond cutting tool and intrinsic misorientation angle of grain boundary on the propensity of grain boundary surface step formation. The present work provides theoretical guidelines on the strategy of suppressing grain boundary surface step formation for achieving superior surface finish of polycrystalline materials by diamond cutting. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116400
  • 2019 • 210 Corrosion and material alterations of a CuZn38Pb3 brass under acoustic cavitation
    Abedini, M. and Reuter, F. and Hanke, S.
    Ultrasonics Sonochemistry 58 (2019)
    An alloy that is exposed to cavitation may experience mechanical cavitation damages as well as accelerated corrosion. In the present paper, the evolution of corrosion erosion behavior of brass samples (CuZn38Pb3) during continuous exposure to ultrasonic cavitation in a salt solution (NaCl) was investigated. Various samples were sonicated for times between 0 min and 5 h. The average surface roughness and the effective surface area of the samples were measured by confocal microscopy, and the surfaces were inspected by scanning electron microscopy. Different erosion behavior of the phases present on the surface is discussed. Complementary to the surface inspection, the corrosion behavior of the samples before, during and after sonication was investigated through open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The results show that at the initial times of sonication preferably the lead islets were removed from the brass surface, resulting in a change in the open circuit potential. α and β′ phases showed ductile and brittle behavior under sonication, respectively. The corrosion rate of the alloy under cavitation increased as the sonication time increased, mainly related to the increase in effective surface area and the rise of plastic deformation of the surface material. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultsonch.2019.104628
  • 2019 • 209 Crystal plasticity finite element modeling and simulation of diamond cutting of polycrystalline copper
    Wang, Z. and Zhang, J. and Xu, Z. and Zhang, J. and Hassan, H.U. and Li, G. and Zhang, H. and Hartmaier, A. and Fang, F. and Yan, Y. and Sun, T.
    Journal of Manufacturing Processes 38 187-195 (2019)
    Microstructural-related deformation behavior leads to anisotropic machining characteristics of polycrystalline materials. In the present work, we develop a crystal plasticity finite element model of ultra-precision diamond cutting of polycrystalline copper, aiming to evaluate the influence of grain boundaries on the correlation between microscopic deformation behavior of the material and macroscopic machining results. The crystal plasticity dealing with the anisotropy of polycrystalline copper is implemented in a user subroutine (UMAT), and an efficient element deletion technique based on the Johnson-Cook damage model is adopted to describe material removal and chip formation. The effectiveness of as-established crystal plasticity finite element model is verified by experiments of nanoindentation, nanoscratching and in-situ diamond microcutting. Subsequent crystal plasticity finite element simulation of diamond cutting across a high angle grain boundary demonstrates significant anisotropic machining characteristics in terms of machined surface quality, chip profile and cutting force, due to heterogeneous plastic deformation behavior in the grain level. © 2019
    view abstractdoi: 10.1016/j.jmapro.2019.01.007
  • 2019 • 208 Crystal plasticity finite element simulation and experiment investigation of nanoscratching of single crystalline copper
    Wang, Z. and Zhang, H. and Li, Z. and Li, G. and Zhang, J. and Zhang, J. and Hassan, H.U. and Yan, Y. and Hartmaier, A. and Sun, T.
    Wear 430-431 100-107 (2019)
    Mechanical properties of crystalline materials strongly correlate with deformation behaviour at the grain level. In the present work, we establish a 3D crystal plasticity finite element model of nanoscratching of single crystalline copper using a Berkovich probe, which is capable of addressing the crystallography influence. In particular, nanoindentation experiments and high resolution electron back-scatter diffraction characterization are jointly carried out to precisely calibrate parameters used in the crystal plasticity finite element model. Subsequent finite element simulations of nanoscratching are performed to reveal fundamental deformation behaviour of single crystalline copper in terms of mechanical response and surface pile-up topography, as well as their dependence on crystallographic orientation. Furthermore, nanoscratching experiments with the same parameters used in the finite element simulations are carried out, the results of which are further compared with predication results by the finite element simulations. Simulation data and experimental results jointly demonstrate the strong anisotropic characteristics of single crystalline copper under nanoscratching, due to the crystallographic orientation dependent coupled effects of intrinsic dislocation slip and extrinsic discrete stress distribution by probe geometry. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2019.04.024
  • 2019 • 207 Dislocation mechanism based size-dependent crystal plasticity modeling and simulation of gradient nano-grained copper
    Lu, X. and Zhang, X. and Shi, M. and Roters, F. and Kang, G. and Raabe, D.
    International Journal of Plasticity 113 52-73 (2019)
    Overcoming the trade-off between strength and ductility in metallic materials is a grand challenge. Recently, materials with a gradient nano-grained (GNG) surface layer adhering to a ductile coarse-grained (CG) substrate have been proposed to overcome this long-standing dilemma. Constitutive modeling and simulation are crucial to understand the deformation mechanisms controlling the strength and ductility in GNG/CG materials, and to enable theory to guide microstructure optimization for upscaling. Here, we develop a dislocation mechanism based size-dependent crystal plasticity model, where multiple dislocation evolution mechanisms are considered. Furthermore, damage evolution and mechanically driven grain growth during the deformation of GNG/CG materials are incorporated into the constitutive model to study the role of microstructure gradient in the overall plastic response. The developed size-dependent constitutive model was implemented within a finite-strain crystal plasticity finite element framework, and used to predict the tensile mechanical behavior of GNG/CG copper, including yield stress, strain-hardening and ductility with a highly simplified geometrical representation of the microstructure. The simulations reveal some of the underlying deformation mechanisms controlling ductility and strengthening in terms of the spatial distribution and temporal evolution of microstructure and damage. The model was also used to demonstrate optimization of strength and ductility of GNG/CG copper. By manipulating the thickness of the GNG layer and the grain size of the CG substrate, the strength increase is associated with a loss of ductility showing the same linear inverse relationship observed experimentally for GNG/CG copper, which indicates the improvement over the typical nonlinear trade-off between strength and ductility. © 2018 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ijplas.2018.09.007
  • 2019 • 206 Quasi-Fermi-Level Splitting of Cu -Poor and Cu -Rich CuIn S2 Absorber Layers
    Lomuscio, A. and Rödel, T. and Schwarz, T. and Gault, B. and Melchiorre, M. and Raabe, D. and Siebentritt, S.
    Physical Review Applied 11 (2019)
    Cu(In,Ga)S2-based solar cells are interesting tandem partners for Si or chalcopyrite solar cells, but suffer from a low open-circuit voltage. Recently, record efficiencies have been achieved by using higher growth temperatures for the absorber. To understand the effect of higher growth temperatures, we investigate the structural and electronic properties of CuInS2 absorbers. By investigating the absorber alone as opposed to complete solar cells, we can separate changes in the absorber from effects of the interface properties. We show that the quasi-Fermi-level splitting, which indicates the maximum open-circuit voltage an absorber is capable of, increases with higher growth temperature. The quasi-Fermi-level splitting is limited by a deep defect, the concentration of which decreases with higher growth temperature and is less prominent in Cu-rich films. Thus, we demonstrate that the open-circuit voltage of CuInS2-based solar cells is limited to below 850 mV by the absorber itself, independent of the interface. In contrast to the changes in the electronic properties, the structural properties are rather independent of temperature within the range investigated but are significantly influenced by the composition. © 2019 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.11.054052
  • 2019 • 205 Recent advances in the preparation of zeolites for the selective catalytic reduction of NOx in diesel engines
    Zhang, L. and Wu, Q. and Meng, X. and Müller, U. and Feyen, M. and Dai, D. and Maurer, S. and McGuire, R. and Moini, A. and Parvulescu, A.-N. and Zhang, W. and Shi, C. and Yokoi, T. and Pan, X. and Bao, X. and Gies, H. and Marler...
    Reaction Chemistry and Engineering 4 975-985 (2019)
    Metal-exchanged zeolites with small pore sizes have attracted much attention in recent years due to their application in the selective catalytic reduction (SCR) of NOx in diesel engines. Typically, copper-chabazite (e.g. Cu-SSZ-13) has been gradually used as an SCR catalyst in heavy-duty diesel vehicles over the last decade due to its relatively excellent catalytic performance and stability. However, most SSZ-13 zeolites are still prepared via the traditional hydrothermal process in the presence of organic templates, requiring consecutive solid separation and thermal treatment steps to achieve the final zeolite products. In recent years, several strategies for the environmentally friendly preparation of zeolites have been reported, which are also applicable for the synthesis of zeolites for emission control applications. These concepts include copper-amine templating, organotemplate-free synthesis, and solvent-free synthesis. In this review, we briefly summarize the potential advantages of the environmentally friendly synthesis of zeolites for SCR. © The Royal Society of Chemistry 2019.
    view abstractdoi: 10.1039/c8re00214b
  • 2019 • 204 Sensitive and selective detection of Cu2+ ions based on fluorescent Ag nanoparticles synthesized by R-phycoerythrin from marine algae Porphyra yezoensis
    Xu, Y. and Hou, Y. and Wang, Y. and Wang, Y. and Li, T. and Song, C. and Wei, N. and Wang, Q.
    Ecotoxicology and Environmental Safety 168 356-362 (2019)
    In this study, using a natural and green protein R-phycoerythrin (R-PE) extracted from marine Porphyra yezoensis as the stabilizer and reducer, silver nanoparticles (AgNPs) were synthesized. Based on this, a highly sensitive and selective method for the detection of Cu2+ ions was developed using R-PE-AgNPs as fluorescent probe. The interactions between R-PE-AgNPs and Cu2+ ions were systematically characterized by fluorescence spectroscopy, transmission electron microscopy (TEM), elemental mapping and Fourier transform infrared (FTIR). It was found that Cu2+ ions could cause aggregation of the R-PE-AgNPs, accompanied by the greatly increased particle size. Importantly, the method offered a wide linear detection range from 0 μM to 100.0 μM with a detection limit of 0.0190 μM. Moreover, the proposed method was successfully applied to analyze Cu2+ ions in tap water and lake water samples, acquiring satisfactory recovery between 91.6% and 102.2%. Such a green, fast and cost-effective fluorimetric method of the R-PE-AgNPs probe has great potential for tracing Cu2+ ions in diverse aqueous media. © 2018 Elsevier Inc.
    view abstractdoi: 10.1016/j.ecoenv.2018.10.102
  • 2019 • 203 Tailoring the Surface Structure of Silicon Carbide Support for Copper Catalyzed Ethanol Dehydrogenation
    Li, M.-Y. and Lu, W.-D. and He, L. and Schüth, F. and Lu, A.-H.
    ChemCatChem 11 481-487 (2019)
    The production of acetaldehyde through biomass-derived ethanol dehydrogenation is a sustainable alternative compared to the fossil-feedstock based process, for which Cu-based catalysts are considered to be the most efficient. Herein, we modified the surface of silicon carbide (SiC) to alter the properties of the interface from SiO2-rich to C-rich, and we prepared a series of Cu-supported catalysts (Cu/SiC, Cu/SiO2/SiC, and Cu/C/SiC) with the aim of insight into the effect of the interface structure and composition on catalytic dehydrogenation of ethanol. At 280 °C, the Cu/SiO2/SiC catalyst exhibits high ethanol conversion due to the excellent dispersion of Cu nanoparticles promoted by SiO2-rich interface. In contrast, Cu nanoparticles dispersed on C/SiC shows somewhat lower activity but excellent acetaldehyde selectivity with trace amounts of by-products under identical reaction conditions. This difference is attributed to the fast removal of acetaldehyde because of its low affinity for the relatively inert C-rich interface (C/SiC). This work provides an in-depth understanding of Cu−Si−C multi-interfacial structure and the ethanol dehydrogenation behavior, which may shed light on the design of novel catalysts with tailored interfacial structures. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201801742
  • 2019 • 202 The interaction between grain boundary and tool geometry in nanocutting of a bi-crystal copper
    Wang, Z. and Sun, T. and Zhang, H. and Li, G. and Li, Z. and Zhang, J. and Yan, Y. and Hartmaier, A.
    International Journal of Extreme Manufacturing 1 (2019)
    Anisotropy is one central influencing factor on achievable ultimate machined surface integrity of metallic materials. Specifically, grain boundary has a strong impact on the deformation behaviour of polycrystalline materials and correlated material removal at the microscale. In the present work, we perform molecular dynamics simulations and experiments to elucidate the underlying grain boundary-associated mechanisms and their correlations with machining results of a bi-crystal Cu under nanocutting using a Berkovich tool. Specifically, crystallographic orientations of simulated bi-crystal Cu with a misorientation angle of 44.1° are derived from electron backscatter diffraction characterization of utilized polycrystalline copper specimen. Simulation results reveal that blocking of dislocation motion at grain boundaries, absorption of dislocations by grain boundaries and dislocation nucleation from grain boundaries are operating deformation modes in nanocutting of the bi-crystal Cu. Furthermore, heterogeneous grain boundary-associated mechanisms in neighbouring grains lead to strong anisotropic machining behaviour in the vicinity of the grain boundary. Simulated machined surface morphology and machining force evolution in the vicinity of grain boundary qualitatively agree well with experimental results. It is also found that the geometry of Berkovich tool has a strong impact on grain boundary-associated mechanisms and resultant ploughing-induced surface pile-up phenomenon. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the IMMT
    view abstractdoi: 10.1088/2631-7990/ab4b68
  • 2019 • 201 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO 2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO 2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H 2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO 2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol -1 for Cu/ZrO 2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2019 • 200 The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol over Cu/ZrO2 in the aqueous phase
    Gabrysch, T. and Muhler, M. and Peng, B.
    Applied Catalysis A: General 576 47-53 (2019)
    The kinetics of glycerol hydrodeoxygenation to 1,2-propanediol via the selective cleavage of the primary C-O bond was systematically studied in the aqueous phase over a co-precipitated Cu/ZrO2 catalyst. Unsupported pure metallic Cu was used as reference catalyst. Batch experiments were performed in an autoclave by varying the reaction temperature (175–225 °C), H2 partial pressure (25–35 bar) and initial glycerol concentration (2–8 wt%). The Cu/ZrO2 catalyst was found to be highly selective to 1,2propanediol (up to 95%), and ethylene glycol was obtained as major by-product from parallel C–]C bond hydrogenolysis. The apparent activation energies amounting to 106 and 105 kJ mol-1 for Cu/ZrO2 and pure metallic Cu, respectively, of the hydrodeoxygenation pathway provide further evidence for metallic Cu acting as the active site. Kinetic analysis of the rate of glycerol consumption yielded a zero-order dependence on the concentration of glycerol suggesting an essentially almost full coverage of adsorbed glycerol as most strongly bound organic adsorbate. In contrast, a first-order dependence on hydrogen concentration was observed. Hydrogen is assumed to be not only required for the fast hydrogenation of the intermediate acetol, but also for the removal of adsorbed atomic oxygen originating from water dissociation to create empty sites for dissociative glycerol adsorption. Thus, the active Cu sites are assumed to be fully adsorbate-covered under reaction conditions. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcata.2019.03.001
  • 2018 • 199 An in situ XPS study of L-cysteine co-adsorbed with water on polycrystalline copper and gold
    Jürgensen, A. and Raschke, H. and Esser, N. and Hergenröder, R.
    Applied Surface Science 435 870-879 (2018)
    The interactions of biomolecules with metal surfaces are important because an adsorbed layer of such molecules introduces complex reactive functionality to the substrate. However, studying these interactions is challenging: they usually take place in an aqueous environment, and the structure of the first few monolayers on the surface is of particular interest, as these layers determine most interfacial properties. Ideally, this requires surface sensitive analysis methods that are operated under ambient conditions, for example ambient pressure x-ray photoelectron spectroscopy (AP-XPS). This paper focuses on an AP-XPS study of the interaction of water vapour and l-Cysteine on polycrystalline copper and gold surfaces. Thin films of l-Cysteine were characterized with XPS in UHV and in a water vapour atmosphere (P ≤ 1 mbar): the structure of the adsorbed l-Cysteine layer depended on substrate material and deposition method, and exposure of the surface to water vapour led to the formation of hydrogen bonds between H2O molecules and the COO− and NH2 groups of adsorbed l-Cysteine zwitterions and neutral molecules, respectively. This study also proved that it is possible to investigate monolayers of biomolecules in a gas atmosphere with AP-XPS using a conventional laboratory Al-Kα x-ray source. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.11.150
  • 2018 • 198 Coherently strained [Fe-Co(C)/Au-Cu]n multilayers: A path to induce magnetic anisotropy in Fe-Co films over large thicknesses
    Giannopoulos, G. and Salikhov, R. and Varvaro, G. and Psycharis, V. and Testa, A.M. and Farle, M. and Niarchos, D.
    Journal of Physics D: Applied Physics 51 (2018)
    Among novel critical-element-free materials for permanent magnets, the nearly equiatomic Fe-Co alloy has recently attracted a great deal of attention as a large magneto-crystalline anisotropy can be induced by straining the Fe-Co unit cell. In thin film systems, the use of a suitable underlayer allows a tetragonal reconstruction of the Fe-Co to be triggered up to a critical thickness of few nanometers, above which the crystal structure relaxes to the magnetically soft cubic phase. Scaling-up the thickness of the metastable tetragonal Fe-Co phase is of crucial significance for different nanoscale applications, such as magnetic micro- and nano-electromechanical systems. To suppress the strain relaxation occurring at high thicknesses, we explored a novel approach based on Fe-Co(C)/Au-Cu multilayer films, where both Au-Cu interlayers and carbon (C) doping were used to stabilize the strained Fe-Co tetragonal phase over large thicknesses. Both doped and un-doped multilayer structures show a coherently strained regime, persisting up to a thickness of 60 nm, which leads, possibly in combination with the surface anisotropy induced at the Au-Cu interfaces, to the appearance of a large out-of-plane anisotropy (up to 0.4 MJ m-3), thus suggesting the potential of such an approach to develop critical-element-free thin film permanent magnets for a variety of nanoscale applications. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aaa41c
  • 2018 • 197 Computational Structure Prediction of (4,4)-Connected Copper Paddle-wheel-based MOFs: Influence of Ligand Functionalization on the Topological Preference
    Impeng, S. and Cedeno, R. and Dürholt, J.P. and Schmid, R. and Bureekaew, S.
    Crystal Growth and Design 18 2699-2706 (2018)
    The effect of linkers with extended π-system on the topological preference of (4,4)-connected copper paddle-wheel-based metal-organic frameworks (MOFs) was investigated using the reverse topological approach (RTA) in which a genetic algorithm (GA) and the DFT-derived force field MOF-FF were used for ranking and predicting the most stable phase. Three tetracarboxylate linkers bearing different functionality, namely, phenylene (L1), naphthalene (L2), and anthracene (L3) groups, were studied. All potential topologies including nbo-b, ssa, ssb, pts, and lvt-b were considered. The computational results reveal that nbo-b is the most stable topology for all three investigated linkers. However, L2 is also formed in ssb according to experimental findings. Our simulation results show that the CH-π interactions with a Y-shaped configuration between naphthalene moieties of L2 stabilize the ssb framework. Unlike L2, CH-π interactions are not favorable for L1 and L3 because of unsuitable size of the π-system. The results of the RTA predictions are in agreement with experimentally reported data, suggesting the capability of RTA for accurate structural predictions of MOFs. More importantly, this work shows the exemption of reticular chemistry in which linker functionalization can result in alteration of the resulting topology, as found in the case of linker L2. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.cgd.8b00238
  • 2018 • 196 Copper Supported on Hybrid C@SiO2 Hollow Submicron Spheres as Active Ethanol Dehydrogenation Catalyst
    Lu, W.-D. and Wang, Q.-N. and He, L. and Li, W.-C. and Schüth, F. and Lu, A.-H.
    ChemNanoMat 4 505-509 (2018)
    The dehydrogenation of ethanol to acetaldehyde (DHEA) is an environmentally benign alternative for synthetic chemistry and for the fine chemical industry. The key is to design Cu-based catalysts with certain structures to obtain high acetaldehyde selectivity. Herein, hybrid C@SiO2 hollow submicron spheres were designed and synthesized using a confined pyrolysis method. This hybrid structure processes a layer of carbon-silica hybrid shell. After loading the Cu, the Cu/C@SiO2 catalyst exhibited 36.1% conversion of ethanol and ∼99% acetaldehyde selectivity at 260 °C. The hybrid support combined the two favorable properties of carbon and silica and thus improving both selectivity and stability. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cnma.201800021
  • 2018 • 195 Correlative transmission Kikuchi diffraction and atom probe tomography study of Cu(In,Ga)Se2 grain boundaries
    Schwarz, T. and Stechmann, G. and Gault, B. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 26 196-204 (2018)
    We combined transmission Kikuchi diffraction and atom probe tomography techniques to investigate the relationship between the structure and chemistry of grain boundaries in Cu(In,Ga)Se2 thin films. Kikuchi patterns with the tetragonal structure of Cu(In,Ga)Se2 were simulated to emphasize the pseudosymmetry issue in this material system and, hence, the orientation determination ambiguity in case of indexing with a cubic zinc-blende structure. We compared these patterns with experimental data. We detect an elemental redistribution at random high-angle grain boundaries but no chemical fluctuations at Σ3 twin boundaries. The atom probe tomography analyses reveal Cu depletion as well as In and Se enrichment at random grain boundaries and, at some random grain boundaries, a slight Ga depletion. This In on Cu scenario is accompanied by cosegregation of Na and K originating from the soda-lime glass substrate. The amount of impurity segregation does vary not only from one grain boundary to another but also along an individual grain boundary. Hence, our results suggest that the degree of passivation of detrimental, nonradiative recombination centers does differ not only between Σ3 twin boundaries and random grain boundaries but also within the same random grain boundary. Copyright © 2017 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2966
  • 2018 • 194 Coupled effect of crystallographic orientation and indenter geometry on nanoindentation of single crystalline copper
    Wang, Z. and Zhang, J. and Hassan, H.U. and Zhang, J. and Yan, Y. and Hartmaier, A. and Sun, T.
    International Journal of Mechanical Sciences 148 531-539 (2018)
    Surface pile-up topography is very significant for property extraction in nanoindentation tests. In the present work, we perform crystal plasticity finite element simulations of Berkovich nanoindentation of single crystalline copper with different crystallographic orientations, which derive quantitatively comparable mechanical properties and surface pile-up topographies with experimental data. Simulation results demonstrate that there is a coupled effect of crystallographic orientation of indented material and indenter geometry on surface pile-up behavior, due to the interaction between intrinsic dislocation slip events and extrinsic discrete stress distribution patterns. Based on the relative spatial orientation between crystallographic orientation of indented material and indenter geometry, a surface pile-up density factor mp is proposed to qualitatively characterize the propensity of surface pile-up behavior in nanoindentation tests of single crystalline copper. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijmecsci.2018.09.007
  • 2018 • 193 Development of forming and product properties of copper wire in a linear coil winding process
    Komodromos, A. and Lobbe, C. and Tekkaya, A.E.
    2017 7th International Electric Drives Production Conference, EDPC 2017 - Proceedings 2017-December 1-7 (2018)
    Since electric motors are becoming more important in many application fields, e. g. hybrid electric vehicles, the optimization of the linear coil winding process is an important contribution to a higher productivity and flexibility. For the investigation of the forming behavior of the winding wire the material behavior is characterized in different experimental setups using wire diameters of 0.63 mm-3.35 mm. By comparing the results of tensile tests and compression tests, a tension-compression anisotropy of the wire behavior can be noticed. The Young's Modulus, measured in cyclic tensile tests from 99-116 GPa (dependent on the amount of strain), is used for the characterization of the elastic behavior of the copper wire. Subsequently, numerical investigations of the linear winding process in a case study for a rectangular bobbin are carried out in order to analyze the influence forming parameters have on the resulting properties of the wound coil. The wire tensile force represents a key parameter concerning the geometrical properties of the wound wire and the clearance between wire and bobbin. Numerical results show that the wire cross-section decreases through bending already at standard wire tensile forces. Additionally, the clearance between wire and bobbin increases with larger wire diameters. The aforementioned results serve as fundamentals for a comprehensive modeling of the linear winding process of noncircular coil bobbins with copper wire. © 2017 IEEE.
    view abstractdoi: 10.1109/EDPC.2017.8328143
  • 2018 • 192 Different Breathing Mechanisms in Flexible Pillared-Layered Metal-Organic Frameworks: Impact of the Metal Center
    Schneemann, A. and Vervoorts, P. and Hante, I. and Tu, M. and Wannapaiboon, S. and Sternemann, C. and Paulus, M. and Wieland, D.C.F. and Henke, S. and Fischer, R.A.
    Chemistry of Materials 30 1667-1676 (2018)
    The pillared-layered metal-organic framework compounds M2(BME-bdc)2(dabco) (M2+ = Zn2+, Co2+, Ni2+, Cu2+; BME-bdc2- = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M2+. This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO2 pressure reveal different mechanisms for the np to lp phase transition during CO2 adsorption. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b05052
  • 2018 • 191 Dislocation slip transmission through a coherent Σ3{111} copper twin boundary: Strain rate sensitivity, activation volume and strength distribution function
    Malyar, N.V. and Grabowski, B. and Dehm, G. and Kirchlechner, C.
    Acta Materialia 161 412-419 (2018)
    We present the first measurement of the strain rate sensitivity of the ideal dislocation slip transmission through a coherent Σ3{111} copper twin boundary. For this purpose we have deformed 129 geometrically identical samples at different strain rates. The micron-sized samples are either single crystalline (87 pillars) or contain one vertical Σ3{111} twin boundary (42 pillars). The strain rate sensitivity of the ideal slip transmission event is 0.015 ± 0.009. This value is considerably lower than the strain rate sensitivity observed for nano-twinned bulk materials, which is addressed to multiple simultaneously activated deformation processes present in the latter case. The activation volume of the ideal slip transmission points towards a cross-slip like transmission process of dislocations through the twin boundary. Furthermore, the high number of geometrically identical samples is used to discuss the ability to identify the strength distribution function of micropillars. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.09.045
  • 2018 • 190 Highly Efficient Photocatalytic Degradation of Dyes by a Copper–Triazolate Metal–Organic Framework
    Liu, C.-X. and Zhang, W.-H. and Wang, N. and Guo, P. and Muhler, M. and Wang, Y. and Lin, S. and Chen, Z. and Yang, G.
    Chemistry - A European Journal 24 16804-16813 (2018)
    A copper(I) 3,5-diphenyltriazolate metal–organic framework (CuTz-1) was synthesized and extensively characterized by using a multi-technique approach. The combined results provided solid evidence that CuTz-1 features an unprecedented Cu5tz6 cluster as the secondary building unit (SBU) with channels approximately 8.3 Å in diameter. This metal–organic framework (MOF) material, which is both thermally and chemically (basic and acidic) stable, exhibited semiconductivity and high photocatalytic activity towards the degradation of dyes in the presence of H2O2. Its catalytic performance was superior to that of reported MOFs and comparable to some composites, which has been attributed to its high efficiency in generating .OH, the most active species for the degradation of dyes. It is suggested that the photogenerated holes are trapped by CuI, which yields CuII, the latter of which behaves as a catalyst for a Fenton-like reaction to produce an excess amount of .OH in addition to that formed through the scavenging of photogenerated electrons by H2O2. Furthermore, it was shown that a dye mixture (methyl orange, methyl blue, methylene blue, and rhodamine B) could be totally decolorized by using CuTz-1 as a photocatalyst in the presence of H2O2 under the irradiation of a Xe lamp or natural sunlight. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201803306
  • 2018 • 189 In situ atomic-scale observation of oxidation and decomposition processes in nanocrystalline alloys
    Guo, J. and Haberfehlner, G. and Rosalie, J. and Li, L. and Duarte, M.J. and Kothleitner, G. and Dehm, G. and He, Y. and Pippan, R. and Zhang, Z.
    Nature Communications 9 (2018)
    Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu-Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu-Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03288-8
  • 2018 • 188 Local dynamics of copper active sites in zeolite catalysts for selective catalytic reduction of NOx with NH3
    Chen, P. and Khetan, A. and Jabłońska, M. and Simböck, J. and Muhler, M. and Palkovits, R. and Pitsch, H. and Simon, U.
    Applied Catalysis B: Environmental 237 263-272 (2018)
    In Cu-zeolite based selective catalytic reduction of NOx with NH3 (NH3-SCR), Cu species (in particular CuI) solvated by NH3 molecules are predicted theoretically to be highly mobile with their mobility being decisive for the NH3-SCR reactivity at low temperatures (<250 °C). Direct experimental observation of the Cu mobility after NH3 solvation, however, has not been achieved yet. Here we show that complex impedance-based modulus spectroscopy, performed by following the corresponding dielectric relaxation processes at high frequencies (104 to 106 Hz), can be applied to monitor directly the dynamic local movement of Cu ions in zeolite catalysts under NH3-SCR related reaction conditions. Simultaneous in situ impedance and infrared spectroscopy studies, assisted by periodic DFT calculations with reliable van der Waals dispersion corrections, allowed us to identify the key factors determining the local dynamics of Cu ions in two representative Cu-zeolites, i.e. Cu-ZSM-5 and Cu-SAPO-34. The co-adsorption and interaction of NO and NH3 on CuII sites led to the formation of highly mobile CuI species and NH4+ intermediates, and, consequently, significantly enhanced local dynamics of Cu ions in both zeolite catalysts. The re-oxidation of CuI, which is the rate-determining step of NH3-SCR reaction, was more favorable in Cu-SAPO-34 than in Cu-ZSM-5, which can be attributed to the close coupling of NH4+ intermediate and Cu site promoting the formation of CuII-NO2/NH4+. As a result, the overall local dynamics of Cu, largely determined by CuI species, is less dependent on the NH4+ intermediate in Cu-SAPO-34 than in Cu-ZSM-5. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2018.05.091
  • 2018 • 187 Microstructure and mechanical properties in the thin film system Cu-Zr
    Oellers, T. and Raghavan, R. and Chakraborty, J. and Kirchlechner, C. and Kostka, A. and Liebscher, C.H. and Dehm, G. and Ludwig, Al.
    Thin Solid Films 645 193-202 (2018)
    A composition-spread Cu-Zr thin film library with Zr contents from 2.5 up to 6.5 at.% was synthesized by magnetron sputtering on a thermally oxidized Si wafer. The compositional and microstructural variations of the Cu-Zr thin film across the composition gradient were examined using energy dispersive X-ray spectroscopy, X-ray diffraction, and high-resolution scanning and transmission electron microscopy of cross-sections fabricated by focused ion beam milling. Composition-dependent hardness and elastic modulus values were obtained by nanoindentation for measurement areas with discrete Zr contents along the composition gradient. Similarly, the electrical resistivity was investigated by 4-point resistivity measurements to study the influence of Zr composition and microstructural changes in the thin film. Both, the mechanical and electrical properties reveal a significant increase in hardness and resistivity with increasing Zr content. The trends of the mechanical and functional properties are discussed with respect to the local microstructure and composition of the thin film library. © 2017
    view abstractdoi: 10.1016/j.tsf.2017.10.030
  • 2018 • 186 On the growth mechanisms of polar (100) surfaces of ceria on copper (100)
    Hackl, J. and Duchoň, T. and Gottlob, D.M. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Surface Science 671 1-5 (2018)
    We present a study of temperature dependent growth of nano-sized ceria islands on a Cu (100) substrate. Low-energy electron microscopy, micro-electron diffraction, X-ray absorption spectroscopy, and photoemission electron microscopy are used to determine the morphology, shape, chemical state, and crystal structure of the grown islands. Utilizing real-time observation capabilities, we reveal a three-way interaction between the ceria, substrate, and local oxygen chemical potential. The interaction manifests in the reorientation of terrace boundaries on the Cu (100) substrate, characteristic of the transition between oxidized and metallic surface. The reorientation is initiated at nucleation sites of ceria islands, whose growth direction is influenced by the proximity of the terrace boundaries. The grown ceria islands were identified as fully stoichiometric CeO2 (100) surfaces with a (2 × 2) reconstruction. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2018.01.008
  • 2018 • 185 On-surface nickel porphyrin mimics the reactive center of an enzyme cofactor
    Zamborlini, G. and Jugovac, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Lüftner, D. and Puschnig, P. and Feyer, V. and Schneider, C.M.
    Chemical Communications 54 13423-13426 (2018)
    Metal-containing enzyme cofactors achieve their unusual reactivity by stabilizing uncommon metal oxidation states with structurally complex ligands. In particular, the specific cofactor promoting both methanogenesis and anaerobic methane oxidation is a porphyrinoid chelated to a nickel(i) atom via a multi-step biosynthetic path, where nickel reduction is achieved through extensive molecular hydrogenation. Here, we demonstrate an alternative route to porphyrin reduction by charge transfer from a selected copper substrate to commercially available 5,10,15,20-tetraphenyl-porphyrin nickel(ii). X-ray absorption measurements at the Ni L3-edge unequivocally show that NiTPP species adsorbed on Cu(100) are stabilized in the highly reactive Ni(i) oxidation state by electron transfer to the molecular orbitals. Our approach highlights how some fundamental properties of synthetically inaccessible biological cofactors may be reproduced by hybridization of simple metalloporphyrins with metal surfaces, with implications towards novel approaches to heterogenous catalysis. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8cc06739b
  • 2018 • 184 Segregation-Induced Nanofaceting Transition at an Asymmetric Tilt Grain Boundary in Copper
    Peter, N.J. and Frolov, T. and Duarte, M.J. and Hadian, R. and Ophus, C. and Kirchlechner, C. and Liebscher, C.H. and Dehm, G.
    Physical Review Letters 121 (2018)
    We show that chemistry can be used to trigger a nanofaceting transition. In particular, the segregation of Ag to an asymmetric tilt grain boundary in Cu is investigated. Aberration-corrected electron microscopy reveals that annealing the bicrystal results in the formation of nanometer-sized facets composed of preferentially Ag-segregated symmetric Σ5{210} segments and Ag-depleted {230}/{100} asymmetric segments. Our observations oppose an anticipated trend to form coarse facets. Atomistic simulations confirm the nanofacet formation observed in the experiment and demonstrate a concurrent grain boundary phase transition induced by the anisotropic segregation of Ag. © 2018 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.121.255502
  • 2018 • 183 Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers
    Colombara, D. and Werner, F. and Schwarz, T. and Cañero Infante, I. and Fleming, Y. and Valle, N. and Spindler, C. and Vacchieri, E. and Rey, G. and Guennou, M. and Bouttemy, M. and Manjón, A.G. and Peral Alonso, I. and Melchior...
    Nature Communications 9 (2018)
    Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-03115-0
  • 2018 • 182 The Role of Metallic Copper in the Selective Hydrodeoxygenation of Glycerol to 1,2-Propanediol over Cu/ZrO2
    Gabrysch, T. and Peng, B. and Bunea, S. and Dyker, G. and Muhler, M.
    ChemCatChem 10 1344-1350 (2018)
    A series of Cu/ZrO2 catalysts with nominal CuO loadings of 5, 10, 18 and 31 wt.% was synthesized by co-precipitation, characterized and applied in the hydrodeoxygenation of glycerol under mild reaction conditions (200 °C, 25 bar H2). These catalysts were highly selective for the cleavage of C−O bonds while preserving C−C bonds leading to 95 % selectivity to 1,2-propanediol. The conversion of glycerol was observed to be linearly correlated with the specific copper surface area derived from N2O frontal chromatography. The reaction was found to occur through the dehydration of glycerol to acetol followed by its hydrogenation to 1,2-propanediol. Metallic copper was identified as the active site for both reactions suggesting the acid ZrO2 sites to be blocked by water. Reusability studies showed that the catalyst was relatively stable and the conversion decreased by only 18 % after three cycles. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cctc.201701748
  • 2017 • 181 Bipyridine copper functionalized polymer resins as support materials for the aerobic oxidation of alcohols
    Sand, H. and Weberskirch, R.
    Polymer International 66 428-435 (2017)
    Here, we report the first polymer resin supported Cu(I)/bipyridine/N-oxyl catalyst systems for the aerobic oxidation of alcohols at room temperature with ambient air. We chose polystyrene-poly(ethylene glycol) copolymer (TentaGel®) and Merrifield resin as support materials because of their different swelling properties in polar and nonpolar solvents. The bromo functionalized TentaGel resin TG1 or Merrifield resin MR1 were functionalized with 4,4′-dimethoxy-2,2′-bipyridine (MeObpy) to give the ligand modified polymer resin TG2/MR2 that was loaded with CuI(Br) to give the final CuI(Br)/bipyridine support TG3/MR3. These resins were characterized by Fourier transform infrared, SEM, SEM energy dispersive X-ray spectroscopy and elemental analysis. Catalytic activity and recyclability of TG3 was investigated in acetonitrile and cyclohexane and displayed high activities in acetonitrile but also high metal leaching. In cyclohexane as solvent leaching was reduced to 1% − 2%, and catalytic activity was still at 75% after the fifth run. MR3 was consequently tested in cyclohexane and toluene. In both solvents low metal leaching was observed with higher activity in toluene as solvent, showing still over 90% conversion after the seventh run with 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO) and 80% with 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO). © 2016 Society of Chemical Industry. © 2016 Society of Chemical Industry
    view abstractdoi: 10.1002/pi.5277
  • 2017 • 180 Comparison of the quantitative analysis performance between pulsed voltage atom probe and pulsed laser atom probe
    Takahashi, J. and Kawakami, K. and Raabe, D.
    Ultramicroscopy 175 105-110 (2017)
    The difference in quantitative analysis performance between the voltage-mode and laser-mode of a local electrode atom probe (LEAP3000X HR) was investigated using a Fe-Cu binary model alloy. Solute copper atoms in ferritic iron preferentially field evaporate because of their significantly lower evaporation field than the matrix iron, and thus, the apparent concentration of solute copper tends to be lower than the actual concentration. However, in voltage-mode, the apparent concentration was higher than the actual concentration at 40 K or less due to a detection loss of matrix iron, and the concentration decreased with increasing specimen temperature due to the preferential evaporation of solute copper. On the other hand, in laser-mode, the apparent concentration never exceeded the actual concentration, even at lower temperatures (20 K), and this mode showed better quantitative performance over a wide range of specimen temperatures. These results indicate that the pulsed laser atom probe prevents both detection loss and preferential evaporation under a wide range of measurement conditions. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2017.01.015
  • 2017 • 179 Effect of Post-Treatment on Structure and Catalytic Activity of CuCo-based Materials for Glycerol Oxidation
    Dodekatos, G. and Tüysüz, H.
    ChemCatChem 9 610-619 (2017)
    A series of CuCo-based materials prepared by co-precipitation with varied Co/Cu ratios and different post-treatments were applied in the selective oxidation of glycerol in the aqueous phase under basic conditions. The influence of the post-treatment on the structure of the materials and the catalytic performance was investigated in detail. As-prepared materials without calcination and materials calcined under air with subsequent reduction under ethanol/N2 gas stream showed higher conversion of glycerol compared to samples solely calcined under air or to samples calcined under air with subsequent reduction under H2/Ar gas stream. The main products identified in the liquid phase were glyceric, glycolic, and formic acids. Systematic catalytic studies for differently prepared samples with varied Cu content and subsequent characterization of the materials by N2 physisorption, XRD, TEM, and EDX allowed for the identification of CoO(OH) in contact with CuO as the potentially active phases. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201601219
  • 2017 • 178 Electronic structure of metastable bcc Cu-Cr alloy thin films: Comparison of electron energy-loss spectroscopy and first-principles calculations
    Liebscher, C.H. and Freysoldt, C. and Dennenwaldt, T. and Harzer, T.P. and Dehm, G.
    Ultramicroscopy 178 96-104 (2017)
    Metastable Cu-Cr alloy thin films with nominal thickness of 300nm and composition of Cu67Cr33 (at%) are obtained by co-evaporation using molecular beam epitaxy. The microstructure, chemical phase separation and electronic structure are investigated by transmission electron microscopy (TEM). The thin film adopts the body-centered cubic crystal structure and consists of columnar grains with ~50nm diameter. Aberration-corrected scanning TEM in combination with energy dispersive X-ray spectroscopy confirms compositional fluctuations within the grains. Cu- and Cr-rich domains with composition of Cu85Cr15 (at%) and Cu42Cr58 (at%) and domain size of 1-5nm are observed. The alignment of the interface between the Cu- and Cr-rich domains shows a preference for (110)-type habit plane. The electronic structure of the Cu-Cr thin films is investigated by electron energy loss spectroscopy (EELS) and is contrasted to an fcc-Cu reference sample. The experimental EEL spectra are compared to spectra computed by density functional theory. The main differences between bcc-and fcc-Cu are related to differences in van Hove singularities in the electron density of states. In Cu-Cr solid solutions with bcc crystal structure a single peak after the L3-edge, corresponding to a van Hove singularity at the N-point of the first Brillouin zone is observed. Spectra computed for pure bcc-Cu and random Cu-Cr solid solutions with 10at% Cr confirm the experimental observations. The calculated spectrum for a perfect Cu50Cr50 (at%) random structure shows a shift in the van Hove singularity towards higher energy by developing a Cu-Cr d-band that lies between the delocalized d-bands of Cu and Cr. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2016.07.011
  • 2017 • 177 Enhanced spin-orbit coupling in tetragonally strained Fe-Co-B films
    Salikhov, R. and Reichel, L. and Zingsem, B. and Abrudan, R. and Edström, A. and Thonig, D. and Rusz, J. and Eriksson, O. and Schultz, L. and Fähler, S. and Farle, M. and Wiedwald, U.
    Journal of Physics Condensed Matter 29 (2017)
    Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on a 20 nm thick Au0.55Cu0.45 buffer layer. Different ratios of the perpendicular to in-plane lattice constant c/a = 1.013, 1.034 and 1.02 were stabilized by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment mainly originates from B impurities in octahedral position and the reduced symmetry around B atoms. These findings offer the possibility to enhance SOC phenomena - namely the magnetocrystalline anisotropy and the orbital moment - by stabilizing anisotropic strain by doping 4 at.% B. Results on the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation are also presented. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7498
  • 2017 • 176 Exploiting micro-scale structural and chemical observations in real time for understanding chemical conversion: LEEM/PEEM studies over CeOx–Cu(111)
    Duchoň, T. and Hackl, J. and Höcker, J. and Veltruská, K. and Matolín, V. and Falta, J. and Cramm, S. and Nemšák, S. and Schneider, C.M. and Flege, J.I. and Senanayake, S.D.
    Ultramicroscopy 183 1339-1351 (2017)
    Proper consideration of length-scales is critical for elucidating active sites/phases in heterogeneous catalysis, revealing chemical function of surfaces and identifying fundamental steps of chemical reactions. Using the example of ceria thin films deposited on the Cu(111) surface, we demonstrate the benefits of multi length-scale experimental framework for understanding chemical conversion. Specifically, exploiting the tunable sampling and spatial resolution of photoemission electron microscopy, we reveal crystal defect mediated structures of inhomogeneous copper–ceria mixed phase that grow during preparation of ceria/Cu(111) model systems. The density of the microsized structures is such that they are relevant to the chemistry, but unlikely to be found during investigation at the nanoscale or with atomic level investigations. Our findings highlight the importance of accessing micro-scale when considering chemical pathways over heteroepitaxially grown model systems. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2017.05.003
  • 2017 • 175 Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency
    Schwarz, T. and Cojocaru-Mirédin, O. and Mousel, M. and Redinger, A. and Raabe, D. and Choi, P.-P.
    Acta Materialia 132 276-284 (2017)
    Atom probe tomography and transmission electron microscopy are used to study the formation of nano-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films. For a Cu-rich precursor, which was deposited at 320 °C under Cu- and Zn-rich growth conditions, Cu2-xSe grains at the surface are detected. During annealing the precursor at 500 °C in a SnSe + Se atmosphere most of the Cu2-xSe is transformed to Cu2ZnSnSe4 via the consumption of excessive ZnSe and incorporation of Sn. However, atom probe tomography studies also reveal the formation of various nanometer-sized Cu-Sn-Se particles close to the CdS/Cu2ZnSnSe4 interface. One of those particles has a composition close to the Cu2SnSe3 compound. This phase has a smaller band gap than Cu2ZnSnSe4 and is proposed to lead to a significant drop in the open-circuit voltage and could be the main cause for a detrimental p-n junction and the zero efficiency of the final device. Possible effects of the other phases on solar cell performance and formation mechanisms are discussed as well. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.056
  • 2017 • 174 From the Precursor to the Active State: Monitoring Metamorphosis of Electrocatalysts During Water Oxidation by In Situ Spectroscopy
    Hollmann, D. and Rockstroh, N. and Grabow, K. and Bentrup, U. and Rabeah, J. and Polyakov, M. and Surkus, A.-E. and Schuhmann, W. and Hoch, S. and Brückner, A.
    ChemElectroChem 4 2117-2122 (2017)
    In situ Raman and in situ EPR spectroscopy in combination with electrochemistry have been used to investigate the behavior of mixed cobalt nickel and cobalt copper oxides in the oxygen evolution reaction (OER). All experiments were carried out in homemade electrochemical cells using 0.1 M KOH as the electrolyte. The OER activities vary depending on the annealing conditions of the catalyst precursors, also reflected by different behaviours during the in situ spectroscopic experiments. The different activity of the Co/Ni oxides is most likely related to the formation of either γ- or β-NiO(OH), characterized by distinct features in the Raman spectra. Thus, a higher percentage of β-NiO(OH) is present in the more active catalyst. A different behaviour of Co/Cu catalysts has been shown by in situ Raman spectroscopy too, but the active phase could not be identified because of missing spectral features. However, in situ EPR spectroscopy revealed the partial dissolution of Cu(II), suggesting the formation of a Co-enriched oxide/hydroxide surface. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201700142
  • 2017 • 173 Grain boundary character distribution in electroplated nanotwinned copper
    Ratanaphan, S. and Raabe, D. and Sarochawikasit, R. and Olmsted, D.L. and Rohrer, G.S. and Tu, K.N.
    Journal of Materials Science 52 4070-4085 (2017)
    The grain boundary character distribution (GBCD) of nanotwinned copper, fabricated by electroplating inside small-scale through-wafer vias, was characterized using a stereological interpretation of electron backscatter diffraction maps. The GBCD of electroplated nanotwinned copper, specified by five macroscopic parameters (three for the lattice misorientation and two for the grain boundary plane inclination), is similar to the GBCD of coarse-grained polycrystalline copper used here as a reference material. The GBCD was compared to calculated grain boundary energies determined from atomistic simulations. We find that the grain boundary population in the electroplated nanotwinned and coarse-grained reference copper is both on average inversely correlated to the grain boundary energies. The slopes of the relationships between grain boundary population and energy for the most highly populated misorientations (Σ3, Σ9, and Σ11) are different. The relationships are strongly influenced by the geometric constraints at the triple junctions and multiple twinning, which enhanced the observed frequencies of Σ9 boundaries. The results suggest that the grain boundary network and the GBCD in the polycrystalline specimens are strongly influenced by the microstructure, grain boundary energy, and multiple twining. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-016-0670-5
  • 2017 • 172 Impact of Synthesis Parameters on the Formation of Defects in HKUST-1
    Zhang, W. and Kauer, M. and Guo, P. and Kunze, S. and Cwik, S. and Muhler, M. and Wang, Y. and Epp, K. and Kieslich, G. and Fischer, R.A.
    European Journal of Inorganic Chemistry 2017 925-931 (2017)
    By employing various synthetic conditions, such as Cu(NO3)2·3H2O and Cu(BF4)2·6H2O as copper ion sources and different solvents, defect-engineered analogues of metal–organic framework (MOF) [Cu3(BTC)2] (HKUST-1; BTC = 1,3,5-benzenetricarboxylate) with isophthalate (IP) incorporation (DEMOFs) were synthesised and characterised by powder XRD, SEM, IR spectroscopy, thermogravimetric analysis, NMR spectroscopy and N2 sorption. The results show that the choice of counter ions impacts the properties of the samples especially at high concentrations of IP. The combination of DMF and Cu(BF4)2·6H2O turns out to be superior for DEMOFs with IP incorporation up to 25 %. Ultrahigh-vacuum IR spectra recorded upon CO adsorption together with the results of X-ray photoelectron spectroscopic studies show the generation of coordinatively unsaturated Cu+ sites. The results suggest the presence of two different defect types, that is, missing-linker defects and missing paddlewheels for high concentrations of IP. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ejic.201601239
  • 2017 • 171 In situ and operando observation of surface oxides during oxygen evolution reaction on copper
    Toparli, C. and Sarfraz, A. and Wieck, A.D. and Rohwerder, M. and Erbe, A.
    Electrochimica Acta 236 104-115 (2017)
    Formation and dissolution of oxide on copper under transpassive conditions, i.e. during OER and transpassive dissolution, in alkaline electrolyte was investigated by a combination of electrochemical techniques and in situ and operando Raman and photoluminescence (PL) spectroscopy, as well as spectropscopic ellipsometry. Experiments were conducted under potentiodynamic and potentiostatic polarisation in 0.1M NaOH. In chronoamperometry experiments with steps between potentials, oxide thickness continued increasing beyond the onset of OER. The thickness dropped significantly from >10 nm to <5 nm ≈400 mV above the OER onset. The presence of CuO, Cu2O and Cu4O3 was observed by Raman spectroscopy after the onset of OER. Correlating with the thickness drop, strong PL was observed at 1.55 eV, indicating the formation of singly charged oxygen vacancies VO+, following the classical PL spectrum interpretation from the literature. PL observation speaks against vacancy pair coalescence as mechanism of dissolution. After electrochemical experiments, the films were n-type semiconductors, not p-type conductors as expected for copper oxides. Results indicate that transpassive dissolution may be triggered by the instability of the oxide with respect to defect formation. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2017.03.137
  • 2017 • 170 In–situ TEM study of diffusion kinetics and electron irradiation effects on the Cr phase separation of a nanocrystalline Cu–4 at.% Cr thin film alloy
    Harzer, T.P. and Duarte, M.J. and Dehm, G.
    Journal of Alloys and Compounds 695 1583-1590 (2017)
    The Cr phase separation process and kinetics of a metastable Cu96Cr4 alloy film were investigated by isothermal annealing at different temperatures of up to 500 °C using transmission electron microscopy. It is shown that the Cr phase separation proceeds predominantly via enrichment of Cr at grain boundaries and grain boundary diffusion. Temperature dependent diffusion coefficients and the activation energy for grain boundary diffusion of Cr in face–centered cubic Cu are determined from analytical in–situ transmission electron microscopy experiments. In addition, the influence of electron beam irradiation on the diffusion kinetics is considered. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2016.10.302
  • 2017 • 169 Laser-induced surface activation of biocomposites for electroless metallization
    Rytlewski, P. and Bahners, T. and Polewski, F. and Gebert, B. and Gutmann, J.S. and Hartmann, N. and Hagemann, U. and Moraczewski, K.
    Surface and Coatings Technology 311 104-112 (2017)
    In this work biocomposites containing polylactide (PLA), polycaprolactone (PCL), copper(II) oxide and copper acetylacetonate were manufactured by an extrusion process. The extruded composites differed with respect to the PLA/PCL ratio whereas the content of mixed copper(II) oxide and copper acetylacetonate powders was held constant at 20 wt%. The main aims for the addition of PCL was to increase impact strength resistance, improve surface catalytic properties and reduce the temperature of extrusion, thus limiting degradation effects initiated by copper acetylacetonate. The composite samples were irradiated with an ArF excimer laser varying the number of laser pulses and then metalized by electroless plating. Based on optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements, it was found that (i) PCL was dispersed in the form of droplets in all volume of PLA, (ii) the copper compounds were preferably located in the dispersed PCL phase, and (iii) composites with higher PCL content were more effectively metalized. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.12.048
  • 2017 • 168 Local photocurrent mapping and cell performance behaviour on a nanometre scale for monolithically interconnected Cu(In,Ga)Se2 solar cells
    Haggui, M. and Reinhold, B. and Andrae, P. and Greiner, D. and Schmid, M. and Fumagalli, P.
    Journal of Microscopy 268 66-72 (2017)
    The local efficiency of lamellar shaped Cu(In,Ga)Se2 solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm). © 2017 The Authors Journal of Microscopy © 2017 Royal Microscopical Society
    view abstractdoi: 10.1111/jmi.12587
  • 2017 • 167 Localized Synthesis of Conductive Copper-Tetracyanoquinodimethane Nanostructures in Ultrasmall Microchambers for Nanoelectronics
    Xing, Y. and Sun, G. and Speiser, E. and Esser, N. and Dittrich, P.S.
    ACS Applied Materials and Interfaces 9 17271-17278 (2017)
    In this work, the microfluidic-assisted synthesis of copper-tetracyanoquinodimethane (Cu-TCNQ) nanostructures in an ambient environment is reported for the first time. A two-layer microfluidic device comprising parallel actuated microchambers was used for the synthesis and enabled excellent fluid handling for the continuous and multiple chemical reactions in confined ultrasmall chambers. Different precautions were applied to ensure the reduction state of copper (Cu) for the synthesis of Cu-TCNQ charge-transfer compounds. The localized synthesis of Cu and in situ transformation to Cu-TCNQ complexes in solution were achieved by applying different gas pressures in the control layer. Additionally, various diameters of the Cu-TCNQ nano/microstructures were obtained by adjusting the concentration of the precursors and reaction time. After the synthesis, platinum (Pt) microelectrode arrays, which were aligned at the microchambers, could enable the in situ measurements of the electronic properties of the synthesized nanostructures without further manipulation. The as-prepared Cu-TCNQ wire bundles showed good conductivity and a reversible hysteretic switching effect, which proved the possibility in using them to build advanced nanoelectronics. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b01664
  • 2017 • 166 Maintaining strength in supersaturated copper–chromium thin films annealed at 0.5 of the melting temperature of Cu
    Raghavan, R. and Harzer, T.P. and Djaziri, S. and Hieke, S.W. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Science 52 913-920 (2017)
    The thermal stability of evaporated copper–chromium alloy films was studied by correlating hardness trends from nanoindentation to nanostructural–compositional changes from transmission electron microscopy. In particular, the hardness evolution with ageing time at ambient and elevated temperatures of two compositions, dilute (Cu96Cr4) and chromium-rich (Cu67Cr33) solutions, was studied. Due to the negligible mutual miscibility of copper and chromium, the chosen solid solutions are trapped in metastable states as supersaturated solid solutions with face-centred cubic and body-centred cubic phases. Nano-mechanical probing of the nanostructural evolution as a function of temperature provided interesting insights into the phase separation of these systems. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-016-0386-6
  • 2017 • 165 Orientation of ripples induced by ultrafast laser pulses on copper in different liquids
    Maragkaki, S. and Elkalash, A. and Gurevich, E.L.
    Applied Physics A: Materials Science and Processing 123 (2017)
    Formation of laser-induced periodic surface structures (LIPSS or ripples) was studied on a metallic surface of polished copper using irradiation with multiple femtosecond laser pulses in different environmental conditions (air, water, ethanol and methanol). Uniform LIPSS have been achieved by controlling the peak fluence and the overlapping rate. Ripples in both orientations, perpendicular and parallel to laser polarization, were observed in all liquids simultaneously. The orientation of these ripples in the center of the ablated line was changing with the incident light intensity. For low intensities the orientation of the ripples is perpendicular to the laser polarization, whereas for high intensities it turns parallel to it without considerable changes in the period. Multi-directional LIPSS formation was also observed for moderate peak fluence in liquid environments. © 2017, Springer-Verlag GmbH Germany.
    view abstractdoi: 10.1007/s00339-017-1336-0
  • 2017 • 164 Origin of Structural Modulations in Ultrathin Fe Films on Cu(001)
    Zhang, X. and Hickel, T. and Rogal, J. and Neugebauer, J.
    Physical Review Letters 118 (2017)
    Employing ab initio calculations we demonstrate that the complex structural modulations experimentally observed in ultrathin Fe films on Cu(001) originate from Fe bulk phases that arise under extreme deformations. Specifically, we show that the structural modulations correspond to the motifs observed when transforming fcc Fe to bcc Fe in the Pitsch orientation relationship [(001)fcc||(110)bcc]. The observed structural equivalence between surface and unstable bulk structures naturally explains the experimentally reported magnetic and structural transitions when going from low (two to four MLs) to intermediate (four to ten MLs) film coverages. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.118.236101
  • 2017 • 163 Plasma-Activated Copper Nanocube Catalysts for Efficient Carbon Dioxide Electroreduction to Hydrocarbons and Alcohols
    Gao, D. and Zegkinoglou, I. and Divins, N.J. and Scholten, F. and Sinev, I. and Grosse, P. and Roldan Cuenya, B.
    ACS Nano 11 4825-4831 (2017)
    Carbon dioxide electroreduction to chemicals and fuels powered by renewable energy sources is considered a promising path to address climate change and energy storage needs. We have developed highly active and selective copper (Cu) nanocube catalysts with tunable Cu(100) facet and oxygen/chlorine ion content by low-pressure plasma pretreatments. These catalysts display lower overpotentials and higher ethylene, ethanol, and n-propanol selectivity, resulting in a maximum Faradaic efficiency (FE) of ∼73% for C2 and C3 products. Scanning electron microscopy and energy-dispersive X-ray spectroscopy in combination with quasi-in situ X-ray photoelectron spectroscopy revealed that the catalyst shape, ion content, and ion stability under electrochemical reaction conditions can be systematically tuned through plasma treatments. Our results demonstrate that the presence of oxygen species in surface and subsurface regions of the nanocube catalysts is key for achieving high activity and hydrocarbon/alcohol selectivity, even more important than the presence of Cu(100) facets. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.7b01257
  • 2017 • 162 Pre- and post-buckling behavior of bi-crystalline micropillars: Origin and consequences
    Kirchlechner, C. and Toth, F. and Rammerstorfer, F.G. and Fischer, F.D. and Dehm, G.
    Acta Materialia 124 195-203 (2017)
    Compression of micropillars is routinely used to measure the material response under uniaxial load. In bi-crystalline pillars an S-shaped grain-boundary together with an S-shaped pillar is often observed after deformation raising the question of its origin and consequences for stress-strain materials data. In addition to dislocation and grain-boundary based mechanisms, this observation can be caused by buckling and subsequent post-buckling deformation. Deviations from the classical pre- and post-buckling deformation behavior are assigned to imperfections, which are categorized in extrinsic and intrinsic imperfections in this work. In the present paper, the S-shaped actual deformation state is particularly promoted by an intrinsic imperfection, caused by a material heterogeneity (due to the bi-crystal arrangement). This kind of deformation behavior is investigated by micro-compression experiments on 7 × 7 × 21 μm3 sized bi-crystal copper pillars with nearly elastic (axial Young's modulus) homogeneity and identical Schmid factors for both grain orientations. Complementary finite element simulations are performed, in which also the role of friction and of an extrinsic imperfection in the form of initial misalignment of the loading on the S-shape are considered. There, a material model describing the flow stress distribution caused by a dislocation pile-up at the grain-boundary is applied. Finally, suggestions to prevent buckling and, thus, transversal post-buckling displacements during micropillar compression tests are given with the goal to extract engineering stress-strain curves. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.10.075
  • 2017 • 161 Preferential Carbon Monoxide Oxidation over Copper-Based Catalysts under In Situ Ball Milling
    Eckert, R. and Felderhoff, M. and Schüth, F.
    Angewandte Chemie - International Edition 56 2445-2448 (2017)
    In situ ball milling of solid catalysts is a promising yet almost unexplored concept for boosting catalytic performance. The continuous preferential oxidation of CO (CO-PROX) under in situ ball milling of Cu-based catalysts such as Cu/Cr2O3 is presented. At temperatures as low as −40 °C, considerable activity and more than 95 % selectivity were achieved. A negative apparent activation energy was observed, which is attributed to the mechanically induced generation and subsequent thermal healing of short-lived surface defects. In situ ball milling at sub-zero temperatures resulted in an increase of the CO oxidation rate by roughly 4 orders of magnitude. This drastic and highly selective enhancement of CO oxidation showcases the potential of in situ ball milling in heterogeneous catalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201610501
  • 2017 • 160 Production of precursors for micro-concentrator solar cells by femtosecond laser-induced forward transfer
    Andree, S. and Heidmann, B. and Ringleb, F. and Eylers, K. and Bonse, J. and Boeck, T. and Schmid, M. and Krüger, J.
    Applied Physics A: Materials Science and Processing 123 (2017)
    Single-pulse femtosecond laser-induced forward transfer (LIFT, 30 fs, 790 nm) is used to deposit micron-sized dots of copper and/or indium onto a molybdenum layer on glass. Such systems can serve as precursors for the bottom–up manufacturing of micro-concentrator solar cells based on copper–indium–gallium–diselenide. The influence of the thickness of the copper, indium, and combined copper–indium donor layers on the quality of the transferred dots was qualified by scanning electron microscopy, energy-dispersive X-ray analysis, and optical microscopy. The potential for manufacturing of a spatial arrangement adapted to the geometry of micro-lens arrays needed for micro-concentrator solar cells is demonstrated. © 2017, Springer-Verlag GmbH Germany.
    view abstractdoi: 10.1007/s00339-017-1282-x
  • 2017 • 159 Role of Dissolved and Molecular Oxygen on Cu and PtCu Alloy Particle Structure during Laser Ablation Synthesis in Liquids
    Marzun, G. and Bönnemann, H. and Lehmann, C. and Spliethoff, B. and Weidenthaler, C. and Barcikowski, S.
    ChemPhysChem 18 1175-1184 (2017)
    The role of molecular oxygen dissolved in the solvent is often discussed as being an influential factor on particle oxidation during pulsed laser ablation in liquids. However, the formation of the particles during laser synthesis takes place under extreme conditions that enable the decomposition of the liquid medium. Reactive species of the solvent may then affect particle formation due to a chemical reaction in the reactive plasma. Experimental results show a difference between the role of dissolved molecular oxygen and the contribution from the oxygen in water molecules. Using a metallic Cu target in air-saturated water, laser ablation led to 20.5 wt % Cu, 11.5 wt % Cu2O, and 68 wt % CuO nanoparticles, according to X-ray diffraction results. In contrast to particles obtained in air-saturated water, no CuO was observed in the colloid synthesized in a Schlenk ablation chamber in completely oxygen-free water. Under these conditions, less-oxidized nanoparticles (25 wt % Cu and 75 wt % Cu2O) were synthesized. The results show that nanoparticle oxidation during laser synthesis is mainly caused by reactive oxygen species from the decomposition of water molecules. However, the addition of molecular oxygen promotes particle oxidation. Storage of the Cu colloid in the presence of dissolved oxygen leads, due to aging, to nanostructures with a higher oxidation state than the freshly prepared colloid. The XRD pattern of the sample prepared in air-saturated acetone showed no crystalline phases, which is possibly due to small crystallites or low particle concentration. Concentration of the particles by centrifugation showed that in the large fraction (>20 nm), even less oxidized nanoparticles (46 wt % Cu and 54 wt % Cu2O) were present, although the solubility of molecular oxygen is higher in acetone than in water. The nanoparticles in acetone were stable due to a Cu-catalyzed graphite layer formed on their surfaces. The influence of the solvent on alloy synthesis is also crucial. Laser ablation of PtCu3 in air-saturated water led to separated large CuO and Pt-rich spherical nanoparticles, whereas homogeneous PtCu3 alloy nanoparticles were formed in acetone. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cphc.201601315
  • 2017 • 158 Self-Diffusion of Surface Defects at Copper-Water Interfaces
    Kondati Natarajan, S. and Behler, J.
    Journal of Physical Chemistry C 121 4368-4383 (2017)
    Solid-liquid interfaces play an important role in many fields like electrochemistry, corrosion, and heterogeneous catalysis. For understanding the related processes, detailed insights into the elementary steps at the atomic level are mandatory. Here we unravel the properties of prototypical surface-defects like adatoms and vacancies at a number of copper-water interfaces including the low-index Cu(111), Cu(100), and Cu(110), as well as the stepped Cu(211) and Cu(311) surfaces. Using a first-principles quality neural network potential constructed from density functional theory reference data in combination with molecular dynamics and metadynamics simulations, we investigate the defect diffusion mechanisms and the associated free energy barriers. Further, the solvent structure and the mobility of the interfacial water molecules close to the defects are analyzed and compared to the defect-free surfaces. We find that, like at the copper-vacuum interface, hopping mechanisms are preferred compared to exchange mechanisms, while the associated barriers for hopping are reduced in the presence of liquid water. The water structure close to adatoms and vacancies exhibits pronounced local features and differs strongly from the structure at the ideal low-index surfaces. Moreover, in particular at Cu(111) the adatoms are very mobile and hopping events along the surface are more frequent than the exchange of coordinating water molecules in their local environment. Consequently, adatom self-diffusion processes at Cu(111) involve entities of adatoms and their associated solvation shells. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.6b12657
  • 2017 • 157 Shape memory micro-and nanowire libraries for the high-throughput investigation of scaling effects
    Oellers, T. and König, D. and Kostka, A. and Xie, S. and Brugger, J. and Ludwig, Al.
    ACS Combinatorial Science 19 574-584 (2017)
    The scaling behavior of Ti-Ni-Cu shape memory thin-film micro- and nanowires of different geometry is investigated with respect to its influence on the martensitic transformation properties. Two processes for the highthroughput fabrication of Ti-Ni-Cu micro- to nanoscale thin film wire libraries and the subsequent investigation of the transformation properties are reported. The libraries are fabricated with compositional and geometrical (wire width) variations to investigate the influence of these parameters on the transformation properties. Interesting behaviors were observed: Phase transformation temperatures change in the range from 1 to 72 °C (austenite finish, (Af), 13 to 66 °C (martensite start, Ms) and the thermal hysteresis from -3.5 to 20 K. It is shown that a vanishing hysteresis can be achieved for special combinations of sample geometry and composition. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscombsci.7b00065
  • 2017 • 156 Size-dependent reactivity of gold-copper bimetallic nanoparticles during CO2 electroreduction
    Mistry, H. and Reske, R. and Strasser, P. and Roldan Cuenya, B.
    Catalysis Today 288 30-36 (2017)
    New catalysts are needed to achieve lower overpotentials and higher faradaic efficiency for desirable products during the electroreduction of CO2. In this study, we explore the size-dependence of monodisperse gold-copper alloy nanoparticles (NPs) synthesized by inverse micelle encapsulation as catalysts for CO2 electroreduction. X-ray spectroscopy revealed that gold-copper alloys were formed and were heavily oxidized in their initial as prepared state. Current density was found to increase significantly for smaller NPs due to the increasing population of strongly binding low coordinated sites on NPs below 5nm. Product analysis showed formation of H2, CO, and CH4, with faradaic selectivity showing a minor dependence on size. The selectivity trends observed are assigned to reaction-induced segregation of gold atoms to the particle surface and altered electronic or geometric properties due to alloying. © 2016.
    view abstractdoi: 10.1016/j.cattod.2016.09.017
  • 2017 • 155 Spatially resolved measurements of the physical plasma parameters and the chemical modifications in a twin surface dielectric barrier discharge for gas flow purification
    Offerhaus, B. and Lackmann, J.-W. and Kogelheide, F. and Bracht, V. and Smith, R. and Bibinov, N. and Stapelmann, K. and Awakowicz, P.
    Plasma Processes and Polymers 14 (2017)
    A twin surface dielectric barrier discharge consisting of an aluminium oxide plate with grid-structured copper traces on both sides is presented. Due to the size of the electrode configuration spatially resolved optical emission spectroscopy for characterisation of the discharge is performed on two different length scales in order to show its homogeneous behaviour. A broadband echelle spectrometer is employed for a comparison of the plasma parameters at different sites along the copper traces with a spatial resolution on a scale of millimetres. In addition, an ICCD camera with bandpass filters yields homogeneity of the plasma parameters on a scale of micrometres at a given node of the grid-structured copper traces. The discharge is shown to be homogeneous all along the electrode. However, due to the changing composition of the gas stream, it cannot be concluded that the gas phase chemistry follows the same trend. Therefore, FTIR spectroscopy of cysteine is used to monitor the spatial dependence of the gas phase chemistry, showing a transition from purely oxygen-related modifications at the front of the electrode to a mixture of oxygen-related and nitrogen-related modifications at the rear. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ppap.201600255
  • 2017 • 154 Spectroscopic models for laser-heated silicon and copper nanoparticles
    Daun, K. and Menser, J. and Mansmann, R. and Moghaddam, S.T. and Dreier, T. and Schulz, C.
    Journal of Quantitative Spectroscopy and Radiative Transfer 197 3-11 (2017)
    Interpreting laser-induced incandescence (LII) measurements on aerosolized nanoparticles requires a spectroscopic model that relates the measured spectral incandescence to the temperature of the nanoparticles. We present spectroscopic models for molten silicon and copper nanoparticles, which are evaluated through extinction and incandescence measurements on nanoaerosols. Measurements on molten silicon nanoparticles are consistent with the Drude theory in the Rayleigh limit of Mie theory. The copper nanoparticles were initially assumed to coalesce into spheres, but the observed spectral incandescence does not show a surface plasmon polariton (SPP) peak in the vicinity of 600. nm expected of spheres. A simulation based on the discrete dipole approximation (DDA) suggests that this effect could be explained by the structure of the copper aggregates. © 2016.
    view abstractdoi: 10.1016/j.jqsrt.2016.10.006
  • 2017 • 153 Stability, phase separation and oxidation of a supersaturated nanocrystalline Cu-33 at.% Cr thin film alloy
    Harzer, T.P. and Dehm, G.
    Thin Solid Films 623 48-58 (2017)
    A binary nanocrystalline Cu67Cr33 thin film alloy consisting of columnar grains was synthesized via co-evaporation of the constituent elements under non-equilibrium ultra-high vacuum conditions using molecular beam epitaxy. In the as-deposited state, the alloy film is a supersaturated solid solution with a single-phase body-centered cubic structure. In order to study the thermal stability of the microstructure and phase separation behavior towards the two phase equilibrium structure, isothermal annealing experiments in a temperature range of 150 °C – 500 °C were conducted inside a transmission electron microscope and compared to data obtained by X-ray diffraction under protective N2 atmosphere. It is shown that the single-phase nature of the alloy film is maintained for annealing temperatures of ≤ 300 °C, whereas heat treatment at temperatures of ≥ 400 °C results in the formation of a second phase, i.e. the equilibrium face-centered cubic phase of Cu. Phase separation proceeds predominantly by a spinodal-type decomposition process but a simultaneous diffusion of Cr along the columnar grain boundaries to the surface of the alloy film is observed as well. Temperature dependent diffusion coefficients for volume and grain boundary diffusion along with the activation energy for volume diffusion of Cr within the crystal lattice of the alloy film in a temperature range between 400 °C – 500 °C are determined from analytical in situ transmission electron microscopy experiments. Moreover, grain boundary diffusion of Cr leads to the growth of an external Cr-rich oxide scale. It is found that the growth kinetics of this oxide scale exhibits a transition from a linear to a nearly parabolic growth rate. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2016.12.048
  • 2017 • 152 Strain rate dependence of the slip transfer through a penetrable high angle grain boundary in copper
    Malyar, N.V. and Dehm, G. and Kirchlechner, C.
    Scripta Materialia 138 88-91 (2017)
    Micro pillar compression is used to analyze the strain rate dependence of copper pillars containing a penetrable high-angle grain boundary via in situ compression tests at strain rates ranging from  10− 1 to 10− 4 s− 1. While the grain-boundary containing pillars exhibit a clear strain-rate dependence of m = 0.04 ± 0.02, their single crystal counterparts seem to have a weak strain rate dependence of m = 0.01 ± 0.01. The results strongly suggest that the movement of the dislocation line in the grain boundary, required to change its orientation from the incoming to the outgoing slip plane, is the critical process in deforming this kind of grain-boundary containing pillars. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.05.042
  • 2017 • 151 Strengthening Fe – TiB2 based high modulus steels by precipitations
    Szczepaniak, A. and Springer, H. and Aparicio-Fernández, R. and Baron, C. and Raabe, D.
    Materials and Design 124 183-193 (2017)
    We systematically studied the microstructure, mechanical and physical properties of hyper-eutectic Fe – TiB2 high modulus steels (20 vol% TiB2) with (Si, Mn, Ni) and Cu additions for the formation of G-phase and Cu precipitates during ageing treatments. Alloying with Si, Mn and Ni led predominantly to pronounced solid solution strengthening, reaching tensile strength (UTS) values up to 1100 MPa after quenching. While G-phase formation could be observed in aged materials, its preferential formation on grain boundaries significantly deteriorated ductility. Its effects on strength were partially balanced by a decrease of grain boundary density. Additions of 1 and 2 wt% Cu, respectively, led to lower strength in the as quenched state, but also to significant strengthening via ageing. The peak ageing conditions as well as the Cu particle structure and size are comparable to values reported for Cu strengthened interstitial free steels and Fe-Cu alloys. Both alloying additions slightly lowered the specific elastic modulus of the HMS, most pronounced for Cu addition with a drop of about 3 GPa cm3 g− 1 per wt% and also promoted embrittlement. Microstructure-property relationships and consequences for future alloy design, especially towards more ductile hypo-eutectic HMS, are outlined and discussed. © 2017
    view abstractdoi: 10.1016/j.matdes.2017.03.042
  • 2017 • 150 Synthesis and evaluation of new copper ketoiminate precursors for a facile and additive-free solution-based approach to nanoscale copper oxide thin films
    Karle, Sarah and Rogalla, Detlef and Ludwig, Arne and Becker, Hans-Werner and Wieck, Andreas Dirk and Grafen, Markus and Ostendorf, Andreas and Devi, Anjana
    Dalton Transactions 46 2670--2679 (2017)
    Novel copper ketoiminate compounds were synthesized and for the first time applied for additive-free solution-based deposition of nanoscale copper oxide thin films. The two closely related compounds, namely the bis[4-(2-ethoxyethyl-imino)-3-pentanonato] copper, [Cu(EEKI)(2)], and bis[4-(3-methoxypropylimino)- 3-pentanonato] copper, [Cu(MPKI)(2)], were characterized by means of elemental and thermogravimetric analysis (TGA), as well as electron impact mass spectrometry (EI-MS). The advantages of these compounds are that they are liquid and possess excellent solubility in common organic solvents in addition to an optimum reactivity towards ambient moisture that enables a facile solution-based approach to nanoscale copper oxide thin films. Moreover, no additives or aging is needed to stabilize the solution processing of the copper oxide layers. [Cu(MPKI)(2)] was tested in detail for the deposition of copper oxide thin films by spin coating. Upon one-step annealing, high-quality, uniform, crystalline copper oxide thin films were deposited on Si, SiO2, as well as on quartz substrates. Structural, morphological and compositional characteristics of the copper oxide nanostructures were investigated in detail by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and a combined analysis using Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA). It was possible to control the copper oxide phases (CuO and Cu2O) by systematic tuning of the post-deposition annealing conditions. The functional properties in terms of optical band gap were investigated using UV/Vis spectroscopy, while the transport properties, such as resistivity, mobility and carrier concentration were analyzed employing Hall measurements, which confirmed the p-type conductivity of the copper oxide layers.
    view abstractdoi: 10.1039/c6dt04399b
  • 2017 • 149 Ultrafast inhomogeneous magnetization dynamics analyzed by interface-sensitive nonlinear magneto-optics
    Chen, J. and Wieczorek, J. and Eschenlohr, A. and Xiao, S. and Tarasevitch, A. and Bovensiepen, U.
    Applied Physics Letters 110 (2017)
    We analyze laser-induced ultrafast, spatially inhomogeneous magnetization dynamics of epitaxial Co/Cu(001) films in a 0.4-10 nm thickness range with time-resolved magnetization-induced second harmonic generation, which probes femtosecond spin dynamics at the vacuum/Co and Co/Cu interfaces. The interference of these two contributions makes the overall signal particularly sensitive to differences in the transient magnetization redistribution between the two interfaces, i.e., ultrafast magnetization profiles in the ferromagnetic film. We conclude that the magnetization dynamics within the first several hundred femtoseconds is characteristically dependent on the Co film thickness. In films up to 3 nm thickness, we find a stronger demagnetization at the film surface compared to the Cu/Co interface, which we explain by a spin current from Co into the Cu substrate with an effective mean free path of about 3 nm. For film thicknesses larger than 3 nm, the transient magnetization profile over the Co film reverses its sign since spins can be transferred into the substrate only from the interface near region. Our work emphasizes that spatial inhomogeneities in the dynamic magnetic response to femtosecond laser excitation allow conclusions on the underlying microscopic processes. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4977767
  • 2016 • 148 Assessment of strain hardening in copper single crystals using in situ SEM microshear experiments
    Wieczorek, N. and Laplanche, G. and Heyer, J.-K. and Parsa, A.B. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 113 320-334 (2016)
    The effect of a pre-strain on the plasticity of copper single crystals subjected to in situ microshear deformation in a scanning electron microscope (SEM) is investigated. Pre-strains of 6.5 and 20% are imposed using [1 0 0] tensile testing. During tensile pre-deformation, several slip systems are activated and irregularly spaced slip bands form. A trace analysis revealed the presence of several slip bands on the tensile specimen near the grips while one family of slip bands parallel to the (1 1 1) crystallographic plane were detected in the middle of the tensile specimen. From the middle of the pre-deformed tensile specimens double microshear samples were prepared using focused ion beam (FIB) machining such that the [0 -1 -1] (1 -1 1) slip system could be directly activated. The results show how microshear behavior reacts to different levels of tensile pre-deformation. Sudden deformation events (SDEs) are observed during microshear testing. The critical stress associated with the first SDE is shown to increase with increasing pre-deformation as a result of an increasing number of slip bands introduced during pre-deformation per shear zone. The results allow also to obtain information on the interaction between dislocations activated during microshearing ([0 -1 -1] (1 -1 1)) and those which were introduced during tensile pre-deformation ([1 0 -1] (1 1 1) and [1 -1 0] (1 1 1)). When these slip systems interact glissile junctions and Lomer-Cottrell locks are likely to form. In the light of this analysis, we rationalize the occurrence of sudden deformation events based on piled up dislocation assemblies which overcome Lomer-Cottrell lock barriers. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.04.055
  • 2016 • 147 Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells - a review
    Abou-Ras, D. and Schmidt, S.S. and Schäfer, N. and Kavalakkatt, J. and Rissom, T. and Unold, T. and Mainz, R. and Weber, A. and Kirchartz, T. and Simsek Sanli, E. and van Aken, P.A. and Ramasse, Q.M. and Kleebe, H.-J. and Azulay,...
    Physica Status Solidi - Rapid Research Letters 10 363-375 (2016)
    The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high-efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep-defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley-Read-Hall recombination is still enhanced with respect to defect-free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two-dimensional device simulations suggest that these defects are one origin of the reduced open-circuit voltage of the photovoltaic devices. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) This review gives an overview on the current understanding of line and planar defects in Cu(In,Ga)Se2 thin films and their impacts on the corresponding solar-cell devices. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201510440
  • 2016 • 146 Concentrating light in Cu(In,Ga)Se2 solar cells
    Schmid, M. and Yin, G. and Song, M. and Duan, S. and Heidmann, B. and Sancho-Martinez, D. and Kämmer, S. and Köhler, T. and Manley, P. and Lux-Steiner, M.C.
    Proceedings of SPIE - The International Society for Optical Engineering 9937 (2016)
    Light concentration has proven beneficial for solar cells, most notably for highly efficient but expensive absorber materials using high concentrations and large scale optics. Here we investigate light concentration for cost efficient thinfilm solar cells which show nano-or microtextured absorbers. Our absorber material of choice is Cu(In,Ga)Se2 (CIGSe) which has a proven stabilized record efficiency of 22.6% and which-despite being a polycrystalline thin-film material-is very tolerant to environmental influences. Taking a nanoscale approach, we concentrate light in the CIGSe absorber layer by integrating photonic nanostructures made from dielectric materials. The dielectric nanostructures give rise to resonant modes and field localization in their vicinity. Thus when inserted inside or adjacent to the absorber layer, absorption and efficiency enhancement are observed. In contrast to this internal absorption enhancement, external enhancement is exploited in the microscale approach: mm-sized lenses can be used to concentrate light onto CIGSe solar cells with lateral dimensions reduced down to the micrometer range. These micro solar cells come with the benefit of improved heat dissipation compared to the large scale concentrators and promise compact high efficiency devices. Both approaches of light concentration allow for reduction in material consumption by restricting the absorber dimension either vertically (ultra-thin absorbers for dielectric nanostructures) or horizontally (micro absorbers for concentrating lenses) and have significant potential for efficiency enhancement. © 2016 SPIE.
    view abstractdoi: 10.1117/12.2238056
  • 2016 • 145 Copper(II), zinc(II) and copper(II)/zinc(II)-containing carbonate-substituted hydroxyapatite: Synthesis, characterization and thermal behaviour
    Livitska, O. and Strutynska, N. and Zatovsky, I. and Nikolenko, I. and Slobodyanik, N. and Prylutskyy, Y. and Epple, M. and Prymak, O. and Byeda, A.
    Materialwissenschaft und Werkstofftechnik 47 85-91 (2016)
    A new approach for the preparation of nanoscale copper- and zinc-containing sodium- and carbonate-substituted apatites is presented. The thermal transformations of the samples in the temperature range 80-1000 °C were determined by temperature-programmed desorption mass spectroscopy and thermogravimetry. The chemical and phase compositions of the copper- and zinc-containing sodium- and carbonate-substituted apatites were studied by atomic absorption spectroscopy and X-ray diffraction, respectively. The degree and nature of the carbonate substitution were determined by elemental analysis (C, H, N) and infrared spectroscopy, respectively. In addition, scanning electron microscopy (SEM) showed nanoparticles (about 10-20 nm in diameter) with a stability to aggregation under processes by microwave radiation. Samples annealed at 700 °C were crystalline and had an apatite structure. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201600460
  • 2016 • 144 Crystal plasticity modeling of size effects in rolled multilayered Cu-Nb composites
    Jia, N. and Raabe, D. and Zhao, X.
    Acta Materialia 111 116-128 (2016)
    We present size-dependent crystal plasticity finite element simulations of the deformation microstructure, plastic flow and texture evolution in multilayered Cu-Nb composites during cold rolling. The model is based on a constitutive framework incorporating thermally activated dislocation slip, mechanical twinning and non-crystallographic shear banding. It also accounts for the dislocation density evolution and its dependence on initial grain size. By performing a series of quadricrystal simulations considering characteristic heterophase microstructures, the underlying micromechanics and texture of the composites are explored. Significant shear banding occurs in both phases, primarily determined by their initial orientations. For each phase, the activation of shear banding is also affected by the mechanical properties and orientations of the adjacent phase. For composites with an initial single layer thickness of 35 μm or 4 μm, the layer thickness reduction after rolling is non-uniform and the typical rolling textures for bulk pure metals develop in the respective phases. For the 75 nm initial single layer thickness composite, both phases are reduced uniformly in thickness and the initial orientations prevail. The predictions agree well with experimental observations in cold-rolled Cu-Nb thin films. The simulations reveal that for the composites with initial single layer thickness of micrometer scale, dislocation slip is the dominant deformation mechanism although shear banding increasingly carries the deformation at larger strains. For the samples with initial single layer thickness of a few tens of nanometers, shear banding and dislocation slip are the dominant mechanisms. This transition in deformation characteristics leads to different textures in micrometer- and nanometer-scaled multilayers. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.03.055
  • 2016 • 143 Effect of Ni Incorporation into Malachite Precursors on the Catalytic Properties of the Resulting Nanostructured CuO/NiO Catalysts
    Garcia, Y. and Su, B.-L. and Ortega, K.F. and Hüttner, A. and Heese, J. and Behrens, M.
    European Journal of Inorganic Chemistry 2016 2063-2071 (2016)
    Synthetic nickelian malachite nanopowders (Cu1-xNix)2(OH)2CO3 with x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1 were prepared by constant-pH coprecipitation. N2 sorption isotherms confirmed a steady increase of the BET surface area with increasing Ni content for the as-synthesized and calcined mesoporous materials. Powder XRD patterns for x ≤ 0.1 indicate the formation of single-phase materials with an anisotropic contraction of the unit cell. This is related to the gradual decrease of the Jahn-Teller distortion in the malachite structure. An XRD-amorphous hydroxide-rich phase is formed for x > 0.1, which appears as spongelike regions in SEM images. Thermogravimetric analysis showed that nickel lowers the onset of thermal decomposition. Powder XRD patterns of the calcined samples evidence the formation of a tenorite structure despite the presence of Ni. Heterogeneous Fenton-like decomposition of Bismarck Brown Y with H2O2 showed that a Cu/Ni ratio of 92:8 in the nanostructured oxide leads to the highest reaction rate constant derived from a pseudo-first-order kinetic rate law expression. Temperature-programmed CO oxidation experiments revealed that pure CuO achieved the highest activity. Similar performance was observed for the binary system obtained through calcination of the precursor prepared with a Cu/Ni ratio of 96:4. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ejic.201501425
  • 2016 • 142 Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene
    Mistry, H. and Varela, A.S. and Bonifacio, C.S. and Zegkinoglou, I. and Sinev, I. and Choi, Y.-W. and Kisslinger, K. and Stach, E.A. and Yang, J.C. and Strasser, P. and Cuenya, B.R.
    Nature Communications 7 (2016)
    There is an urgent need to develop technologies that use renewable energy to convert waste products such as carbon dioxide into hydrocarbon fuels. Carbon dioxide can be electrochemically reduced to hydrocarbons over copper catalysts, although higher efficiency is required. We have developed oxidized copper catalysts displaying lower overpotentials for carbon dioxide electroreduction and record selectivity towards ethylene (60%) through facile and tunable plasma treatments. Herein we provide insight into the improved performance of these catalysts by combining electrochemical measurements with microscopic and spectroscopic characterization techniques. Operando X-ray absorption spectroscopy and cross-sectional scanning transmission electron microscopy show that copper oxides are surprisingly resistant to reduction and copper+ species remain on the surface during the reaction. Our results demonstrate that the roughness of oxide-derived copper catalysts plays only a partial role in determining the catalytic performance, while the presence of copper+ is key for lowering the onset potential and enhancing ethylene selectivity.
    view abstractdoi: 10.1038/ncomms12123
  • 2016 • 141 Improvement of catalytic activity over Cu–Fe modified Al-rich Beta catalyst for the selective catalytic reduction of NOx with NH3
    Xu, L. and Shi, C. and Chen, B. and Zhao, Q. and Zhu, Y. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and Feyen, M. and Maurer, S. and Moini, A. and Müller, U. and Zhang, W.
    Microporous and Mesoporous Materials 236 211-217 (2016)
    Copper and iron bimetal modified Al-rich Beta zeolites from template-free synthesis were prepared for selective catalytic reduction (SCR) of NOx with NH3 in exhaust gas streams. Comparing to the Cu-based and Fe-based mono-component Beta catalysts, Cu(3.0)-Fe(1.3)-Beta bi-component catalyst shows better low-temperature activity and wider reaction-temperature window. Over 80% of NO conversion can be achieved at the temperature region of 125–500 °C. Due to the synergistic effect of copper and iron evidenced by XRD, UV–Vis–NIR, EPR and XPS measurements, the dispersion state of active components as well as the ratio of Cu2+/Cu+ and Fe3+/Fe2+ were improved over Cu(3.0)-Fe(1.3)-Beta. Isolated Cu2+ and Fe3+ ions which located at the exchange sites could be the active species at the low-temperature region, while FeOx cluster species may be more important to the high-temperature activity. During the test of sulfur resistance, Fe-containing samples including Cu(3.0)-Fe(1.3)-Beta and Fe(2.7)-Beta-4 present better performance compared to Cu(4.1)-Beta-4. Deactivation of Cu-based catalyst is attributed to the easier deposition of sulfates over the surface according to the results of TGA coupled with TPD experiments. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2016.08.042
  • 2016 • 140 Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster
    Tymoczko, J. and Calle-Vallejo, F. and Schuhmann, W. and Bandarenka, A.S.
    Nature Communications 7 (2016)
    Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5-1.0 mg cm-2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm-2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.
    view abstractdoi: 10.1038/ncomms10990
  • 2016 • 139 New materials for the light-induced hydrogen evolution reaction from the Cu-Si-Ti-O system
    Stein, H.S. and Gutkowski, R. and Siegel, A. and Schuhmann, W. and Ludwig, Al.
    Journal of Materials Chemistry A 4 3148-3152 (2016)
    Cu-containing photocathodes are generally limited by fast photocorrosion under working conditions. Hence stabilization of these materials is a key factor in their potential application for the light-induced hydrogen evolution reaction (HER). In order to identify new materials, oxidized Cu-Si-Ti metallic thin film precursor materials libraries were evaluated using a combinatorial approach. High-throughput photoelectrochemical characterization using an automated optical scanning droplet cell was performed on a material library to analyze doping and alloying effects on the light-induced HER. The results revealed that compositions near Ti-doped CuSiO3 (dioptase and copper-polysilicate) and Si-doped Cu3TiOx act as comparatively stable and highly active materials for HER. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ta10186g
  • 2016 • 138 Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing
    Morsdorf, L. and Pradeep, K.G. and Herzer, G. and Kovács, A. and Dunin-Borkowski, R.E. and Povstugar, I. and Konygin, G. and Choi, P. and Raabe, D.
    Journal of Applied Physics 119 (2016)
    Nucleation of soft magnetic Fe3Si nanocrystals in Cu-free Fe74.5Si15.5Nb3B7 alloy, upon rapid (10 s) and conventional (30 min) annealing, was investigated using x-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy, and atom probe tomography. By employing rapid annealing, preferential nucleation of Fe3Si nanocrystals was achieved, whereas otherwise there is simultaneous nucleation of both Fe3Si and undesired Fe-B compound phases. Analysis revealed that the enhanced Nb diffusivity, achieved during rapid annealing, facilitates homogeneous nucleation of Fe3Si nanocrystals while shifting the secondary Fe-B crystallization to higher temperatures resulting in pure soft magnetic nanocrystallization with very low coercivities of ∼10 A/m. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4944595
  • 2016 • 137 Promoting the Synthesis of Methanol: Understanding the Requirements for an Industrial Catalyst for the Conversion of CO2
    Behrens, M.
    Angewandte Chemie - International Edition 55 14906-14908 (2016)
    doi: 10.1002/anie.201607600
  • 2016 • 136 Screening of material libraries for electrochemical CO2 reduction catalysts – Improving selectivity of Cu by mixing with Co
    Grote, J.-P. and Zeradjanin, A.R. and Cherevko, S. and Savan, A. and Breitbach, B. and Ludwig, Al. and Mayrhofer, K.J.J.
    Journal of Catalysis 343 248-256 (2016)
    The efficiency of the direct electrochemical CO2 reduction can be improved by the development of new alloy catalysts, but to do so a highly resolved composition screening remains to be connected to complex sample preparation and time consuming analysis. We have developed a technique that allows a fast and easy initial catalyst composition screening by analyzing thin film composition spread samples, utilizing a scanning flow cell coupled to an online electrochemical mass spectrometer (SFC-OLEMS). As a first case example, the investigation of a Cu–Co thin film material library demonstrates the benefits and high potential of this approach. In particular, a shift in selectivity toward C2 species for low Co content (5–15 at.%) has been found and is discussed as being related to changed adsorption energies of intermediate products and the consequent modification of reaction pathways. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcat.2016.02.026
  • 2016 • 135 The Temperature-Programmed Desorption of H2 from Cu/ZrO2
    Schittkowski, J. and Buesen, D. and Toelle, K. and Muhler, M.
    Catalysis Letters 146 1011-1017 (2016)
    The desorption kinetics of hydrogen from a polycrystalline Cu/ZrO2 catalyst was investigated under atmospheric pressure using temperature-programmed desorption (TPD) experiments in a microreactor set-up. Different heating rates were applied under equal conditions with a carefully reduced catalyst. The hydrogen TPD peaks were symmetric and centered slightly above 300 K indicating associative desorption of H2 from metallic Cu. Using heating rate variation, the kinetic parameters Ades and Edes were determined to be 1.24 × 109 s-1 and 68 kJ mol-1, respectively. As the modeling with constant values of Ades and Edes yielded signals which were too narrow, dependence of Edes on coverage was introduced applying Edes - K (ΘH)n. By application of the "full-analysis" method an optimal fit to the experimental data was found. Setting n = 1 resulted in the best fit and a value of 61 kJ mol-1 - (6.25 kJ mol-1 × ΘH) for Edes was determined. © Springer Science+Business Media New York 2016.
    view abstractdoi: 10.1007/s10562-016-1712-y
  • 2016 • 134 Tuning Catalytic Selectivity at the Mesoscale via Interparticle Interactions
    Mistry, H. and Behafarid, F. and Reske, R. and Varela, A.S. and Strasser, P. and Roldan Cuenya, B.
    ACS Catalysis 6 1075-1080 (2016)
    The selectivity of heterogeneously catalyzed chemical reactions is well-known to be dependent on nanoscale determinants, such as surface atomic geometry and composition. However, principles to control the selectivity of nanoparticle (NP) catalysts by means of mesoscopic descriptors, such as the interparticle distance, have remained largely unexplored. We used well-defined copper catalysts to deconvolute the effect of NP size and distance on product selectivity during CO2 electroreduction. Corroborated by reaction-diffusion modeling, our results reveal that mesoscale phenomena such as interparticle reactant diffusion and readsorption of intermediates play a defining role in product selectivity. More importantly, this study uncovers general principles of tailoring NP activity and selectivity by carefully engineering size and distance. These principles provide guidance for the rational design of mesoscopic catalyst architectures in order to enhance the production of desired reaction products. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b02202
  • 2015 • 133 A structure zone diagram obtained by simultaneous deposition on a novel step heater: A case study for Cu2O thin films
    Stein, H. and Naujoks, D. and Grochla, D. and Khare, C. and Gutkowski, R. and Grützke, S. and Schuhmann, W. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 212 2798-2804 (2015)
    In thin film deposition processes, the deposition temperature is one of the crucial process parameters for obtaining films with desired properties. Usually the optimum deposition temperature is found by conducting several depositions sequentially in a time consuming process. This paper demonstrates a facile and rapid route of the simultaneous thin film deposition at six different deposition temperatures ranging from 100 to 1000 °C. Cuprite (Cu2O) was chosen for the study as this material is of interest for energy applications. The thin films are assessed for their crystallographic, microstructural, Raman scattering, and photoelectrochemical properties. The results show that the utilization of a step heater leads to the rapid optimization of thin film microstructures of an absorber material used in photoelectrochemistry. This results in a structure zone diagram for Cu2O. For a substrate temperature of 600 °C, an optimum between crystallinity and morphology occurs. © 2015 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532384
  • 2015 • 132 Adhesion measurement of a buried Cr interlayer on polyimide
    Marx, V.M. and Kirchlechner, C. and Zizak, I. and Cordill, M.J. and Dehm, G.
    Philosophical Magazine 95 1982-1991 (2015)
    A fundamental knowledge and understanding of the adhesion behaviour of metal-polymer systems is important as interface failure leads to a complete breakdown of flexible devices. A combination of in situ atomic force microscopy for studying topological changes and in situ synchrotron based stress measurements both during film tensile testing were used to estimate the adhesion energy of a thin bilayer film. The film systems consisted of 50-200 nm Cu with a 10 nm Cr adhesion layer on 50 μm thick polyimide. If the Cu film thickness is decreased to 50 nm the Cr interlayer starts dominating the system behaviour. An apparent transition from plastic to predominantly brittle deformation behaviour of the Cu can be observed. Then, compressive stresses in the transverse direction are high enough to cause delamination and buckling of the Cr interlayer from the substrate. This opens a new route to induce buckling of a brittle interlayer between a ductile film and a compliant substrate which is used to determine the interfacial adhesion energy. © 2015 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2014.920543
  • 2015 • 131 Atom probe tomography study of internal interfaces in Cu2ZnSnSe4 thin-films
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P. and Mousel, M. and Redinger, A. and Siebentritt, S. and Raabe, D.
    Journal of Applied Physics 118 (2015)
    We report on atom probe tomography studies of the composition at internal interfaces in Cu<inf>2</inf>ZnSnSe<inf>4</inf> thin-films. For Cu<inf>2</inf>ZnSnSe<inf>4</inf> precursors, which are deposited at 320 °C under Zn-rich conditions, grain boundaries are found to be enriched with Cu irrespective of whether Cu-poor or Cu-rich growth conditions are chosen. Cu<inf>2</inf>ZnSnSe<inf>4</inf> grains are found to be Cu-poor and excess Cu atoms are found to be accumulated at grain boundaries. In addition, nanometer-sized ZnSe grains are detected at or near grain boundaries. The compositions at grain boundaries show different trends after annealing at 500 °C. Grain boundaries in the annealed absorber films, which are free of impurities, are Cu-, Sn-, and Se-depleted and Zn-enriched. This is attributed to dissolution of ZnSe at the Cu-enriched grain boundaries during annealing. Furthermore, some of the grain boundaries of the absorbers are enriched with Na and K atoms, stemming from the soda-lime glass substrate. Such grain boundaries show no or only small changes in composition of the matrix elements. Na and K impurities are also partly segregated at some of the Cu<inf>2</inf>ZnSnSe<inf>4</inf>/ZnSe interfaces in the absorber, whereas for the precursors, only Na was detected at such phase boundaries possibly due to a higher diffusivity of Na compared to K. Possible effects of the detected compositional fluctuations on cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4929874
  • 2015 • 130 Atomistic investigation of wear mechanisms of a copper bi-crystal
    Zhang, J. and Begau, C. and Geng, L. and Hartmaier, A.
    Wear 332-333 941-948 (2015)
    In the present work, we investigate the wear mechanisms of a Cu bi-crystal containing a random high angle grain boundary by means of molecular dynamics simulations. The underlying deformation behavior of the material is analyzed and further related to the observed characteristics of mechanical response and resulting morphology of the worn surface. In particular, the grain boundary-associated mechanisms are characterized by advanced analysis techniques for lattice defects. Our simulation results indicate that in addition to dislocation slip and dislocation-grain boundary interactions, grain boundary migration plays an important role in the plastic deformation of Cu bi-crystal. It is found that the wear behavior of Cu depends on the crystallographic orientation of the worn surface and can be altered quite significantly by the presence of a grain boundary. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2015.02.023
  • 2015 • 129 Cd and impurity redistribution at the p-n junction of CIGS based solar cells resolved by atom-probe tomography
    Koprek, A. and Cojocaru-Miredin, O. and Wuerz, R. and Freysoldt, C. and Raabe, D.
    2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 (2015)
    Cd and impurity redistribution in the vicinity of CdS/CIGS interface is studied by means of atom probe tomography (APT). We find an increase of the Cd content in the CIGS layer and redistribution of O at the CdS/CIGS interface after annealing the samples at 200 °C, 250 °C, or 300 °C. Very small amounts (∼0.1 at. %) of Na impurity where observed at the p-n junction independent on the heat treatment. Simultaneously, the I-V measurements of the treated samples show a drop in the open circuit voltage and thus of the efficiency compared to the untreated sample. The effect of Cd diffusion in CIGS and of O and Na segregation at the CdS/CIGS interface on the cell performance is discussed. © 2015 IEEE.
    view abstractdoi: 10.1109/PVSC.2015.7355651
  • 2015 • 128 Comparing small scale plasticity of copper-chromium nanolayered and alloyed thin films at elevated temperatures
    Raghavan, R. and Harzer, T.P. and Chawla, V. and Djaziri, S. and Phillipi, B. and Wehrs, J. and Wheeler, J.M. and Michler, J. and Dehm, G.
    Acta Materialia 93 175-186 (2015)
    Abstract The yield strengths and deformation mechanisms of Cu-Cr nanolayered and alloyed thin films were studied by microcompression testing at elevated temperatures. The mechanical response of the films with alternating layers of Cu and Cr with sub-100 nm interlayer thicknesses and alloyed films of the same average composition was compared to determine the role of the interfaces on deformation. Higher resistance to plastic flow at elevated temperatures was exhibited by the nanolayered films with smaller interlayer thickness among the layered films, while the alloyed film revealed an anomalous increase in strength with temperature exhibiting a deformation mechanism similar to the pure Cr film. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.04.008
  • 2015 • 127 Copper-based nanostructured coatings for low-temperature brazing applications
    Lehmert, B. and Janczak-Rusch, J. and Pigozzi, G. and Zuraw, P. and La Mattina, F. and Wojarski, L. and Tillmann, W. and Jeurgens, L.P.H.
    Materials Transactions 56 1015-1018 (2015)
    This feasibility study demonstrates the possibility to apply nanostructured filler materials for novel low-temperature brazing applications by exploiting the size-dependent melting behavior of metals and alloys when confined to the nano-scale regime. As an example, a copper-based nanostructured brazing filler is presented, which allows metal brazing of coated Ti-6Al-4V components at 750°C, much below the bulk melting point of copper (1083°C). The copper-based nanostructured brazing fillers can be produced in the form of coatings and free-standing brazing foils. The nano-confinement of Cu is abrogated after brazing and, consequently, the brazed joints can be operated well above their reduced brazing temperatures. © 2015 The Japan Institute of Metals and Materials.
    view abstractdoi: 10.2320/matertrans.MI201419
  • 2015 • 126 Detection of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases in co-evaporated Cu2ZnSnSe4 thin-films
    Schwarz, T. and Marques, M.A.L. and Botti, S. and Mousel, M. and Redinger, A. and Siebentritt, S. and Cojocaru-Mirédin, O. and Raabe, D. and Choi, P.-P.
    Applied Physics Letters 107 (2015)
    Cu2ZnSnSe4 thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu2ZnSnSe4 and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meV below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases. Possible effects of these phases on solar cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4934847
  • 2015 • 125 Effect of heat treatment on phase structure and thermal conductivity of a copper-infiltrated steel
    Klein, S. and Weber, S. and Theisen, W.
    Journal of Materials Science 50 3586-3596 (2015)
    Infiltration of tool steels with copper is a suitable and cheap method to create dense parts using powder metallurgy. In this work, it is shown that the copper network that forms inside the steel skeleton during infiltration enhances the thermal conductivity of the resulting composite. The level of enhancement is dependent on the thermal conductivity of the copper phase and the volume fraction of copper. Multiple heat treatments of this composite revealed a strong dependency between the thermal conductivity of the composite and the solution state of Fe in the copper network. The latter is highly dependent on the heat-treated condition of the multi-phase material. Using infiltration, the thermal and electrical conductivity was increased from (Formula presented.) respectively, for aged steel-copper composite in comparison with original X245VCrMo9-4-4 steel. In addition, a model alloy that represents the copper-phase network in the composite was manufactured. By measuring both, the thermal conductivity of this model alloy and the bulk steel, and comparing it to the data for the composite, different models for calculating the overall conductivity of the composite are discussed. © 2015, The Author(s).
    view abstractdoi: 10.1007/s10853-015-8919-y
  • 2015 • 124 Formation of dislocation networks in a coherent Cu Σ3(1 1 1) twin boundary
    Jeon, J.B. and Dehm, G.
    Scripta Materialia 102 71-74 (2015)
    Molecular dynamics simulations were performed to investigate dislocation network formations in a coherent twin boundary in Cu. Depending on the activated glide system, the initial flawless twin boundary can be heavily or sparsely decorated by a dislocation network. The dislocation mechanism leading to a heavy dislocation network at the twin boundary and its consequence on mechanical properties will be discussed. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2015.02.016
  • 2015 • 123 Grain boundary segregation in multicrystalline silicon: Correlative characterization by EBSD, EBIC, and atom probe tomography
    Stoffers, A. and Cojocaru-Mirédin, O. and Seifert, W. and Zaefferer, S. and Riepe, S. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 1742-1753 (2015)
    This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron-beam-induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non-coherent (random) high-angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high-angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface-property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2614
  • 2015 • 122 Hydrogenation of CO2 to methanol and CO on Cu/ZnO/Al2O3: Is there a common intermediate or not? This work is dedicated to the memory and achievements of Dr. Haldor Topsøe.
    Kunkes, E.L. and Studt, F. and Abild-Pedersen, F. and Schlögl, R. and Behrens, M.
    Journal of Catalysis 328 43-48 (2015)
    H/D exchange experiments on a Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalyst have shown that methanol synthesis and RWGS display a strong thermodynamic isotope effect, which is attributed to differences in the zero-point energy of hydrogenated vs. deuterated species. The effect is larger for methanol synthesis and substantially increases the equilibrium yield in deuterated syngas. In the kinetic regime of CO<inf>2</inf> hydrogenation, an inverse kinetic isotope effect of H/D substitution was observed, which is stronger for methanol synthesis than for CO formation suggesting that the two reactions do not share a common intermediate. Similar observations were also made on other catalysts such as Cu/MgO, Cu/SiO<inf>2</inf>, and Pd/SiO<inf>2</inf>. In contrast to CO<inf>2</inf> hydrogenation, the CO hydrogenation on Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> did not show such a strong kinetic isotope effect indicating that methanol formation from CO<inf>2</inf> does not proceed via consecutive reverse water gas shift and CO hydrogenation steps. The inverse KIE is consistent with formate hydrogenation being the rate-determining step of methanol synthesis from CO<inf>2</inf>. Differences in the extent of product inhibition by water, observed for methanol synthesis and reverse water gas shift indicate that the two reactions proceed on different surface sites in a parallel manner. The consequences for catalyst design for effective methanol synthesis from CO<inf>2</inf> are discussed. © 2014 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2014.12.016
  • 2015 • 121 Importance of dislocation pile-ups on the mechanical properties and the Bauschinger effect in microcantilevers
    Kapp, M.W. and Kirchlechner, C. and Pippan, R. and Dehm, G.
    Journal of Materials Research 30 791-797 (2015)
    Copper microcantilevers were produced by focused ion beam milling and tested in situ using a scanning electron microscope. To provide different interfaces for piling up dislocations, cantilevers were fabricated to be single crystalline, bicrystalline, or single crystalline with a slit in the region of the neutral axis. The aim of the experiment was to study the influence of dislocation pile-ups on (i) strength and (ii) Bauschinger effects in micrometer-sized, focused ion beam milled bending cantilevers. The samples were loaded monotonically for several times under displacement control. Even though the cantilevers exhibited the same nominal strain gradient the strength varied by 34% within the three cantilever geometries. The Bauschinger effect can be promoted and prohibited by the insertion of different interfaces. © 2015 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2015.49
  • 2015 • 120 Influence of inclined twin boundaries on the deformation behavior of Cu micropillars
    Imrich, P.J. and Kirchlechner, C. and Dehm, G.
    Materials Science and Engineering A 642 65-70 (2015)
    In situ micromechanical compression tests on Cu pillars were performed to evaluate the influence of twin boundaries on the mechanical behavior. The 1. μm sized Cu samples on a Si substrate prepared by focused ion beam milling were either single crystalline or contained 2-5 twin boundaries that were inclined to the compression direction. The strengths of the pillars vary, depending on the crystal orientation, associated twin boundary inclination and orientation of slip systems. Results show, that multiple slip systems are activated in each pillar. However, slip parallel to the twin boundaries prevails due to the long mean free path for dislocation movement. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.06.064
  • 2015 • 119 Influence of initial microstructure on thermomechanical fatigue behavior of Cu films on substrates
    Heinz, W. and Robl, W. and Dehm, G.
    Microelectronic Engineering 137 5-10 (2015)
    During a switch event in a power semiconductor device temperature changes of up to 300 K can occur in the Cu layer. Repeated switching operations causes cyclic thermal cycling which may finally lead to thermomechanical fatigue with severe microstructural changes. In this study, the influence of the starting microstructure and film thickness (600 nm and 5000 nm) on thermomechanical fatigue was investigated for epitaxial and polycrystalline Cu films for up to 1000 thermal cycles. Severe surface roughening and a texture change (crystal rotation) are detected during thermal cycling for the polycrystalline Cu films, while the epitaxial films maintain their microstructure. Controlling the initial microstructure of a Cu layer in a device exposed to cyclic thermomechanical straining is a route to delay surface damage. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.mee.2014.10.024
  • 2015 • 118 Integration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se2 solar cells
    Yin, G. and Steigert, A. and Andrae, P. and Goebelt, M. and Latzel, M. and Manley, P. and Lauermann, I. and Christiansen, S. and Schmid, M.
    Applied Surface Science 355 800-804 (2015)
    Integration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se2 (CIGSe) solar cells is investigated. X-ray photoelectron spectroscopy results show that Ag nanoparticles underneath a Sn:In2O3 back contact could not be thermally passivated even at a low substrate temperature of 440 °C during CIGSe deposition. It is shown that a 50 nm thick Al2O3 film prepared by atomic layer deposition is able to block the diffusion of Ag, clearing the thermal obstacle in utilizing Ag nanoparticles as a back reflector in ultra-thin CIGSe solar cells. Via 3-D finite element optical simulation, it is proved that the Ag nanoparticles show the potential to contribute the effective absorption in CIGSe solar cells. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.07.195
  • 2015 • 117 Internal and external stresses: In situ TEM compression of Cu bicrystals containing a twin boundary
    Imrich, P.J. and Kirchlechner, C. and Kiener, D. and Dehm, G.
    Scripta Materialia 100 94-97 (2015)
    Uniaxial compression experiments on single- and twinned bicrystalline Cu samples using conventional and scanning in situ transmission electron microscopy reveal no increase in flow stress for the bicrystals. Dislocation curvature and dislocation source size analysis combined with indenter force measurements show agreement between local internal stresses acting on the dislocations and external stresses imposed by the indenter, indicating no stress concentrations due to the twin boundary. Furthermore, the dislocation density evolution shows stochastic variations but never a complete dislocation starvation. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2014.12.023
  • 2015 • 116 Kinetics of deactivation on Cu/ZnO/Al2O3 methanol synthesis catalysts
    Fichtl, M.B. and Schlereth, D. and Jacobsen, N. and Kasatkin, I. and Schumann, J. and Behrens, M. and Schlögl, R. and Hinrichsen, O.
    Applied Catalysis A: General 502 262-270 (2015)
    Deactivation behavior is an important topic in catalyst development. In case of methanol synthesis the conventional Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> system is commonly known to be prone to sintering, however, information about the structural development during deactivation or the sintering mechanism(s) are scarce. We present a systematic deactivation study on three different Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> catalysts which are aged under constant conditions and periodically analyzed using kinetic measurements and N<inf>2</inf>O chemisorption. A power law model for the catalyst activity with time on stream is derived. Furthermore it is found, that the presence of water provokes a steep loss in active surface area and specific activity. Also, the TEM particle size distributions generated during the aging treatment are evaluated and discussed. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2015.06.014
  • 2015 • 115 Large scale Molecular Dynamics simulation of microstructure formation during thermal spraying of pure copper
    Wang, T. and Begau, C. and Sutmann, G. and Hartmaier, A.
    Surface and Coatings Technology 280 72-80 (2015)
    Thermal spray processes are widely used for the manufacture of advanced coating systems, e.g. metallic coatings for wear and corrosion protection. The desired coating properties are closely related to the microstructure, which is highly influenced by the processing parameters, such as temperature, size and velocity of the sprayed particles. In this paper, large scale Molecular Dynamics simulations are conducted to investigate the microstructure formation mechanisms during the spraying process of hot nano-particles onto a substrate at room temperature using pure copper as a benchmark material representing for a wider class of face-centered-cubic metals. To evaluate the influence of processing parameters on the coating morphology, a number of simulations are performed in which the initial temperature, size and velocity of copper particles are systematically varied in order to investigate the thermal and microstructural evolution during impaction. Two distinct types of microstructural formation mechanisms, resulting in different coating morphologies, are observed in the present investigation, which are either governed by plastic deformation or by the process of melting and subsequent solidification. Furthermore, a thermodynamically motivated model as a function of the particle temperature and velocity is developed, which predicts the microstructural mechanisms observed in the simulations. The results provide an elementary insight into the microstructure formation mechanisms on an atomistic scale, which can serve as basic input for continuum modeling of thermal spray process. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2015.08.034
  • 2015 • 114 Microscopic analysis of the composition driven spin-reorientation transition in NixPd1-x/Cu(001)
    Gottlob, D.M. and Doğanay, H. and Nickel, F. and Cramm, S. and Krug, I.P. and Nemšák, S. and Schneider, C.M.
    Ultramicroscopy 159 503-507 (2015)
    The spin-reorientation transition (SRT) in epitaxial NixPd1-x/Cu(001) is studied by photoemission microscopy utilizing the X-ray magnetic circular dichroism effect at the Ni L2,3 edge. In a composition/thickness wedged geometry, a composition driven SRT could be observed between 37ML and 60ML, and 0 and 38% of Pd. Microspectroscopy in combination with azimuthal sample rotation confirms a magnetization preference changing from the [001] to an in-plane easy axis. At this increased thickness, the domain patterns arrange comparable to SRTs in ultrathin films. The images document domains equivalent to a canted state SRT, at which an additional effect of in-plane anisotropies could be identified. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.05.022
  • 2015 • 113 Monolithically interconnected lamellar Cu(In,Ga)Se2 micro solar cells under full white light concentration
    Reinhold, B. and Schmid, M. and Greiner, D. and Schüle, M. and Kieven, D. and Ennaoui, A. and Lux-Steiner, M.Ch.
    Progress in Photovoltaics: Research and Applications 23 1929-1939 (2015)
    Thin film solar cells already benefit from significant material and energy savings. By using photon management, the conversion efficiency and the power density can be enhanced further, including a reduction of material costs. In this work, micrometer-sized Cu(In,Ga)Se2 (CIGS) thin film solar cells were investigated under concentrated white light illumination (1-50×). The cell design is based on industrially standardized, lamellar shaped solar cells with monolithic interconnects (P-scribe). In order to characterize the shunt and serial resistance profiles and their impact on the device performance the cell width was reduced stepwise from 1900 to 200 μm and the P1-scribe thickness was varied between 45 and 320 μm. The results are compared to macroscopic solar cells in standard geometry and dot-shaped microcells with ring contacts. Under concentrated white light, the maximal conversion efficiency could be increased by more than 3.8% absolute for the lamellar microcells and more than 4.8% absolute in case of dot-shaped microcells compared to their initial values at 1 sun illumination. The power density could be raised by a factor of 51 and 70, respectively. But apparently, the optimum concentration level and the improvement in performance strongly depend on the chosen cell geometry, the used contact method and the electrical material properties. It turns out, that the widely used industrial thin film solar cell design pattern cannot simply be adapted to prepare micro-concentrator CIGS solar modules, without significant optimization. Based on the experimental and simulated results, modifications for the cell design are proposed. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2611
  • 2015 • 112 New Au-Cu-Al thin film shape memory alloys with tunable functional properties and high thermal stability
    Buenconsejo, P.J.S. and Ludwig, Al.
    Acta Materialia 85 378-386 (2015)
    An Au-Cu-Al thin film materials library prepared by combinatorial sputter-deposition was characterized by high-throughput experimentation in order to identify and assess new shape memory alloys (SMAs) in this alloy system. Automated resistance measurements during thermal cycling between -20 and 250 °C revealed a wide composition range that undergoes reversible phase transformations with martensite transformation start temperatures, reverse transformation finish temperatures and transformation hysteresis ranging from -15 to 149 °C, 5 to 185 °C and 8 to 60 K, respectively. High-throughput X-ray diffraction analysis of the materials library confirmed that the phase-transforming compositions can be attributed to the existence of the β-AuCuAl parent phase and its martensite product. The formation of large amount of phases based on face-centered cubic (Au-Cu), Al-Cu and Al-Au is responsible for limiting the range of phase-transforming compositions. Selected alloys in this system show excellent thermal cyclic stability of the phase transformation. The functional properties of these alloys, combined with the inherent properties of Au-based alloys, i.e. aesthetic value, oxidation and corrosion resistance, makes them attractive as smart materials for a wide range of applications, including applications as SMAs for elevated temperatures in harsh environment. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.11.035
  • 2015 • 111 Oxidative potential of particulate matter at a German motorway
    Hellack, B. and Quass, U. and Nickel, C. and Wick, G. and Schins, R.P.F. and Kuhlbusch, T.A.J.
    Environmental Sciences: Processes and Impacts 17 868-875 (2015)
    Ambient particulate matter (PM10) was sampled alongside a motorway in North-Rhine Westphalia, Germany, during a one-year period. In sum, 120 PM10 samples on quartz fibre filters, 60 samples at each side of the motorway, were taken during clear cross-wind direction situations, i.e. upwind (local background situation) and downwind (traffic influenced). To quantify the traffic-related oxidative potential (OP), or more precisely the hydroxyl radical (OH) generation potency, these samples were analysed to study their hydrogen peroxide dependent oxidant generation by Electron Paramagnetic Resonance (EPR) spectroscopy using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide. In addition the PM10 mass, the chemical composition and the NO<inf>x</inf> concentrations were determined. For PM10 mass and traffic tracers like Sb, Ba, elemental and organic carbon as well as for NO<inf>x</inf>, an additional contribution to the background concentration caused by the traffic was observed (factor: 1.3-6.0). The downwind measurements showed in 72% of cases higher OH generation potencies with an average factor of 1.4. Significant correlations to OH were detected for Fe (r > 0.58) and Cu (r > 0.57) for the upwind and overall (upwind + downwind, r > 0.44) dataset. At the downwind side these correlations were absent and are assumed to be covered by the interferences with additional soot particles leading to a quenching of OH. Accordingly, no significant overall correlation of the OH generation potency with the traffic intensity was detected. The suggested quenching effect was confirmed via standard diesel soot (SRM 2975) measurements using the EPR approach. In summary, the traffic related PM causes an intrinsic OH generation via Fenton-like reaction but obviously also leads to interferences and scavenging by traffic related carbonaceous compounds. In consequence, for future studies that would link the intrinsic OP and adverse health effects we suggest to analyse the relationship to EC/OC and to use in parallel also a further OP detection method. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4em00605d
  • 2015 • 110 Promoting strong metal support interaction: Doping ZnO for enhanced activity of Cu/ZnO:M (M = Al, Ga, Mg) catalysts
    Schumann, J. and Eichelbaum, M. and Lunkenbein, T. and Thomas, N. and Álvarez Galván, M.C. and Schlögl, R. and Behrens, M.
    ACS Catalysis 5 3260-3270 (2015)
    The promoting effect of Al, Ga, and Mg on the support in Cu/ZnO catalysts for methanol synthesis has been investigated. Different unpromoted and promoted ZnO supports were synthesized and impregnated with Cu metal in a subsequent step. All materials, supports, and calcined and activated catalysts were characterized by various methods, including contactless (microwave) conductivity measurements under different gas atmospheres. Small amounts of promoters were found to exhibit a significant influence on the properties of the oxide support, concerning textural as well as electronic properties. We found correlations between the conductivity of the ZnO support and the activity of the catalyst in the reverse water-gas shift reaction (rWGS) as well as in methanol synthesis. In rWGS the activation energy and reaction order in H<inf>2</inf> are decreased upon promotion of the ZnO support with the trivalent promoters Al3+ and Ga3+, indicating an electronic promotion. In methanol synthesis, results point to a structural promotion by Al3+ and Ga3+. A detrimental effect of Mg2+ doping was observed in both reactions. This effect is discussed in the context of the reducibility of ZnO under reaction conditions, which can be tuned by the promoter in different ways. The reducibility is seen as a critical property for the dynamic metal support interaction of the Cu/ZnO system. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b00188
  • 2015 • 109 Pseudomorphic Generation of Supported Catalysts for Glycerol Oxidation
    Deng, X. and Dodekatos, G. and Pupovac, K. and Weidenthaler, C. and Schmidt, W. and Schüth, F. and Tüysüz, H.
    ChemCatChem 7 3832-3837 (2015)
    A catalyst consisting of copper nanoparticles (15-20 nm in size) supported on ordered mesoporous cobalt monoxide was synthesized by the one-step reduction of ethanol from nanocast copper cobalt spinel oxides. The small-angle X-ray scattering patterns showed that the ordered mesostructure was maintained after post-treatment, and the cross-section scanning electron microscopy images showed that the Cu nanoparticles were distributed homogeneously throughout the mesoporous CoO framework. The materials were tested as noble-metal-free catalysts for the oxidation of glycerol under alkaline conditions. The catalytic data showed that the presence of Cu nanoparticles greatly enhanced the catalytic performance. Nothing noble: A catalyst consisting of copper nanoparticles (NPs, 15-20 nm in size) supported on ordered mesoporous cobalt monoxide is synthesized by the one-step reduction with ethanol from nanocast copper cobalt spinel oxides. As a noble-metal-free catalyst for the oxidation of glycerol, the presence of Cu NPs greatly enhances the catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201500703
  • 2015 • 108 The influence of a brittle Cr interlayer on the deformation behavior of thin Cu films on flexible substrates: Experiment and model
    Marx, V.M. and Toth, F. and Wiesinger, A. and Berger, J. and Kirchlechner, C. and Cordill, M.J. and Fischer, F.D. and Rammerstorfer, F.G. and Dehm, G.
    Acta Materialia 89 278-289 (2015)
    Thin metal films deposited on polymer substrates are used in flexible electronic devices such as flexible displays or printed memories. They are often fabricated as complicated multilayer structures. Understanding the mechanical behavior of the interface between the metal film and the substrate as well as the process of crack formation under global tension is important for producing reliable devices. In the present work, the deformation behavior of copper films (50-200 nm thick), bonded to polyimide directly or via a 10 nm chromium interlayer, is investigated by experimental analysis and computational simulations. The influence of the various copper film thicknesses and the usage of a brittle interlayer on the crack density as well as on the stress magnitude in the copper after saturation of the cracking process are studied with in situ tensile tests in a synchrotron and under an atomic force microscope. From the computational point of view, the evolution of the crack pattern is modeled as a stochastic process via finite element based cohesive zone simulations. Both, experiments and simulations show that the chromium interlayer dominates the deformation behavior. The interlayer forms cracks that induce a stress concentration in the overlying copper film. This behavior is more pronounced in the 50 nm than in the 200 nm copper films. © Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2015.01.047
  • 2015 • 107 The influence of different pre-treatments of current collectors and variation of the binders on the performance of Li4Ti5O12 anodes for lithium ion batteries
    Wennig, S. and Langklotz, U. and Prinz, G.M. and Schmidt, A. and Oberschachtsiek, B. and Lorke, A. and Heinzel, A.
    Journal of Applied Electrochemistry 45 1043-1055 (2015)
    In order to optimize the electron transfer between the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf>-based active mass and the current collector, the surface of aluminum foil was modified either by alkaline etching or by a carbon coating. The as-modified aluminum foils were coated with an active mass of Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> mixed with polyvinylidene fluoride, sodium carboxymethyl cellulose, or polyacrylic acid as binders. Untreated aluminum and copper foils served as reference current collectors. The corrosion reactions of aluminum foil with the applied binder solutions were studied and the electrode structure has been analyzed, depending on the binder. Finally, the electrochemical performance of the prepared electrodes was investigated. Based on these measurements, conclusions concerning the electrical contact between the different current collectors and the active masses were drawn. The energy density of the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> electrodes cast on carbon-coated aluminum foils was significantly increased, compared to the corresponding electrodes with a copper current collector. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10800-015-0878-0
  • 2015 • 106 The Mechanism of CO and CO2 Hydrogenation to Methanol over Cu-Based Catalysts
    Studt, F. and Behrens, M. and Kunkes, E.L. and Thomas, N. and Zander, S. and Tarasov, A. and Schumann, J. and Frei, E. and Varley, J.B. and Abild-Pedersen, F. and Nørskov, J.K. and Schlögl, R.
    ChemCatChem 7 1105-1111 (2015)
    Methanol, an important chemical, fuel additive, and precursor for clean fuels, is produced by hydrogenation of carbon oxides over Cu-based catalysts. Despite the technological maturity of this process, the understanding of this apparently simple reaction is still incomplete with regard to the reaction mechanism and the active sites. Regarding the latter, recent progress has shown that stepped and ZnO<inf>x</inf>-decorated Cu surfaces are crucial for the performance of industrial catalysts. Herein, we integrate this insight with additional experiments into a full microkinetic description of methanol synthesis. In particular, we show how the presence or absence of the Zn promoter dramatically changes not only the activity, but unexpectedly the reaction mechanism itself. The Janus-faced character of Cu with two different sites for methanol synthesis, Zn-promoted and unpromoted, resolves the long-standing controversy regarding the Cu/Zn synergy and adds methanol synthesis to the few major industrial catalytic processes that are described on an atomic level. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cctc.201500123
  • 2015 • 105 Three-dimensional Cu foam-supported single crystalline mesoporous Cu2O nanothorn arrays for ultra-highly sensitive and efficient nonenzymatic detection of glucose
    Dong, C. and Zhong, H. and Kou, T. and Frenzel, J. and Eggeler, G. and Zhang, Z.
    ACS Applied Materials and Interfaces 7 20215-20223 (2015)
    Highly sensitive and efficient biosensors play a crucial role in clinical, environmental, industrial, and agricultural applications, and tremendous efforts have been dedicated to advanced electrode materials with superior electrochemical activities and low cost. Here, we report a three-dimensional binder-free Cu foam-supported Cu<inf>2</inf>O nanothorn array electrode developed via facile electrochemistry. The nanothorns growing in situ along the specific direction of <011> have single crystalline features and a mesoporous surface. When being used as a potential biosensor for nonenzyme glucose detection, the hybrid electrode exhibits multistage linear detection ranges with ultrahigh sensitivities (maximum of 97.9 mA mM-1 cm-2) and an ultralow detection limit of 5 nM. Furthermore, the electrode presents outstanding selectivity and stability toward glucose detection. The distinguished performances endow this novel electrode with powerful reliability for analyzing human serum samples. These unprecedented sensing characteristics could be ascribed to the synergistic action of superior electrochemical catalytic activity of nanothorn arrays with dramatically enhanced surface area and intimate contact between the active material (Cu<inf>2</inf>O) and current collector (Cu foam), concurrently supplying good conductivity for electron/ion transport during glucose biosensing. Significantly, our findings could guide the fabrication of new metal oxide nanostructures with well-organized morphologies and unique properties as well as low materials cost. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b05738
  • 2015 • 104 Transition from shear to stress-assisted diffusion of copper-chromium nanolayered thin films at elevated temperatures
    Raghavan, R. and Wheeler, J.M. and Harzer, T.P. and Chawla, V. and Djaziri, S. and Thomas, K. and Philippi, B. and Kirchlechner, C. and Jaya, B.N. and Wehrs, J. and Michler, J. and Dehm, G.
    Acta Materialia 100 73-80 (2015)
    The mechanical behavior of Cu-Cr nanolayered films and an alloy film of nominal composition Cu<inf>20</inf>Cr<inf>80</inf> at.% was studied by microcompression testing at temperatures from 25 °C to 300 °C. Comparing nanolayered films, plastic deformation and failure occurred at consistently higher stress levels in the film with the smaller layer thicknesses. Plasticity in the nanolayered films always initiated in the softer Cu layers followed by a finite strain-hardening response in the stress-strain curves. Failure indicated by a strain-softening response following the higher peak strength due to shearing and tearing at columnar boundaries of Cr was observed in the nanolayered films at 25 °C and 100 °C. A transition from shearing and crack formation across the Cu-Cr interfaces leading to anomalous grain growth or beading of the nanocrystalline Cu layers was observed at elevated temperatures of 200 °C and 300 °C. On the other hand, the Cu<inf>20</inf>Cr<inf>80</inf> at.% alloy film exhibited failure by columnar buckling consistently at elevated temperatures, but shearing promoted by buckling at the highest strengths among the films at ambient temperature. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.08.016
  • 2015 • 103 X-ray photoelectron spectroscopy investigations of the surface reaction layer and its effects on the transformation properties of nanoscale Ti51Ni38Cu11 shape memory thin films
    König, D. and Naujoks, D. and De Los Arcos, T. and Grosse-Kreul, S. and Ludwig, Al.
    Advanced Engineering Materials 17 669-673 (2015)
    The depth-dependent chemical constitution of Ti<inf>51</inf>Ni<inf>38</inf>Cu<inf>11</inf> thin films of different total film thickness from 400 to 50-nm was characterized using X-ray photoelectron spectroscopy (XPS). It was analyzed how reaction layers, which form on the surface of the film significantly change the chemical composition of the transforming phase, which leads in turn to altered phase transformation properties. For thinner films, the deviation from the nominal chemical composition increases. For a film thickness of 50-nm, a Ti loss of ≈9-at% is observed. The Ni content is increased by ≈5-at%, whereas the Cu content stays relatively constant for films of different thickness. The results are summarized in a layer model, which supports designing nanoscale shape memory thin films. Ti<inf>51</inf>Ni<inf>38</inf>Cu<inf>11</inf> thin films of different film thickness are investigated regarding the influence of the reaction layers on the chemical composition of the transforming phase and the corresponding functional properties. A model is proposed describing the different reaction layers on the surface of the thin film and at the substrate/thin film interface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201400317
  • 2014 • 102 Composition-Dependent Oxygen Reduction Activity and Stability of Pt-Cu Thin Films
    Schuppert, A.K. and Topalov, A.A. and Savan, A. and Ludwig, Al. and Mayrhofer, K.J.J.
    ChemElectroChem 1 358-361 (2014)
    Catalyst considerations: Pt-Cu alloys are prepared as a thin-film material library with a composition gradient. By using a scanning flow cell coupled to on-line mass spectrometry, this library can be screened over to measure the activity towards the oxygen reduction reaction as well as the time-resolved dissolution of both alloy components in parallel. This results in comprehensive insights into the composition-dependent performance of the Pt-Cu system. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300078
  • 2014 • 101 Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules
    Fichtl, M.B. and Schumann, J. and Kasatkin, I. and Jacobsen, N. and Behrens, M. and Schlögl, R. and Muhler, M. and Hinrichsen, O.
    Angewandte Chemie - International Edition 53 7043-7047 (2014)
    Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2. A combination of N2O reactive frontal chromatography and H2 temperature-programmed desorption is used to analyze the interplay of copper and zinc oxide in methanol synthesis catalysts. This method provides an easy in situ approach to quantify the direct copper-zinc interaction (SMSI effect) and offers an important possibility to rational catalyst design also for other supported metal catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201400575
  • 2014 • 100 Cu-based catalyst resulting from a Cu,Zn,Al hydrotalcite-like compound: A microstructural, thermoanalytical, and in situ XAS study
    Kühl, S. and Tarasov, A. and Zander, S. and Kasatkin, I. and Behrens, M.
    Chemistry - A European Journal 20 3782-3792 (2014)
    A Cu-based methanol synthesis catalyst was obtained from a phase pure Cu,Zn,Al hydrotalcite-like precursor, which was prepared by co-precipitation. This sample was intrinsically more active than a conventionally prepared Cu/ZnO/Al2O3 catalyst. Upon thermal decomposition in air, the [(Cu0.5Zn0.17Al0.33)(OH) 2(CO3)0.17]×mH2O precursor is transferred into a carbonate-modified, amorphous mixed oxide. The calcined catalyst can be described as well-dispersed "CuO" within ZnAl 2O4 still containing stabilizing carbonate with a strong interaction of Cu2+ ions with the Zn-Al matrix. The reduction of this material was carefully analyzed by complementary temperature-programmed reduction (TPR) and near-edge X-ray absorption fine structure (NEXAFS) measurements. The results fully describe the reduction mechanism with a kinetic model that can be used to predict the oxidation state of Cu at given reduction conditions. The reaction proceeds in two steps through a kinetically stabilized CuI intermediate. With reduction, a nanostructured catalyst evolves with metallic Cu particles dispersed in a ZnAl2O4 spinel-like matrix. Due to the strong interaction of Cu and the oxide matrix, the small Cu particles (7 nm) of this catalyst are partially embedded leading to lower absolute activity in comparison with a catalyst comprised of less-embedded particles. Interestingly, the exposed Cu surface area exhibits a superior intrinsic activity, which is related to a positive effect of the interface contact of Cu and its surroundings. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201302599
  • 2014 • 99 Damage evolution during cyclic tension-tension loading of micron-sized Cu lines
    Wimmer, A. and Leitner, A. and Detzel, T. and Robl, W. and Heinz, W. and Pippan, R. and Dehm, G.
    Acta Materialia 67 297-307 (2014)
    In this study, the low-cycle fatigue properties (1-15,000 cycles) of electrodeposited Cu, which is frequently used as metallization in the semiconductor industry, is analyzed with respect to its microstructure. Freestanding Cu tensile samples 20 μm × 20 μm × 130 μm were fabricated by a lithographic process. The grain size of the samples was modified by using three different process conditions for electrochemical Cu deposition. All samples were subjected to cyclic tension-tension testing performed with a miniaturized stress-controlled stage in situ in a scanning electron microscope until failure occurred. The number of cycles sustained prior to failure depends on the accumulated creep strain and can be related to the failure strain in a tensile test. It will be shown that the microstructure influences the number of cycles to failure and the failure mode. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2013.12.006
  • 2014 • 98 Differences in deformation behavior of bicrystalline Cu micropillars containing a twin boundary or a large-angle grain boundary
    Imrich, P.J. and Kirchlechner, C. and Motz, C. and Dehm, G.
    Acta Materialia 73 240-250 (2014)
    Micrometer-sized compression pillars containing a grain boundary are investigated to better understand under which conditions grain boundaries have a strengthening effect. The compression experiments were performed on focused ion beam fabricated micrometer-sized bicrystalline Cu pillars including either a large-angle grain boundary (LAGB) or a coherent twin boundary (CTB) parallel to the compression axis and additionally on single-crystalline reference samples. Pillars containing a LAGB show increased strength, stronger hardening and smaller load drops compared to single crystals and exhibit a bent boundary and pillar shape. Samples with a CTB show no major difference in stress-strain data compared to the corresponding single-crystalline samples. This is due to the special orientation and symmetry of the twin boundary and is reflected in a characteristic pillar shape after deformation. The experimental findings can be related to the dislocation-boundary interactions at the different grain boundaries and are compared with three-dimensional discrete dislocation dynamics simulations. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.04.022
  • 2014 • 97 Effect of constant-rate reduction on the performance of a ternary Cu/ZnO/Al2O3 catalyst in methanol synthesis
    Ruland, H. and Busser, W. and Otto, H. and Muhler, M.
    Chemie-Ingenieur-Technik 86 1890-1893 (2014)
    A multi-functional flow set-up was developed for the rate- and temperature-controlled reduction of copper catalysts, their application in high-pressure methanol synthesis and the determination of the copper surface area by N2O frontal chromatography. The influence of constant-rate reduction on the catalytic properties of a ternary Cu/ZnO/Al2O3 catalyst was investigated. The temperature during the constant-rate reduction was found to decrease, indicating autocatalytic kinetics, but no significant catalytic effect of the milder reduction conditions was observed compared with a slow linear heating ramp. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cite.201400065
  • 2014 • 96 Effect of localized states on the reciprocity between quantum efficiency and electroluminescence in Cu(In,Ga)Se2 and Si thin-film solar cells
    Müller, T.C.M. and Pieters, B.E. and Kirchartz, T. and Carius, R. and Rau, U.
    Solar Energy Materials and Solar Cells 129 95-103 (2014)
    We present the electroluminescence and quantum efficiency of three different types of thin-film solar cells based on absorbers made from Cu(In,Ga)Se2 as well as from μc-Si:H and a-Si:H. Simulations of our experimental results show that the main contribution to the electroluminescence spectrum originates from band-to-tail transitions in the case of Cu(In,Ga)Se2, from tail-to-tail transitions in μc-Si:H, and from tail-to-tail as well as from band-to-midgap-defect transitions in a-Si:H. By comparing the electroluminescence with the quantum efficiency, we analyze the effect of localized states on the optoelectronic reciprocity relation in the three material systems. The relatively steep band-tail density of states in Cu(In,Ga)Se2 is compatible with the reciprocity relation at room temperature while the shallower band-tail density of states in μc-Si:H and the deep mid-gap defect states in a-Si:H lead to substantial deviations from the reciprocity. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.solmat.2014.04.018
  • 2014 • 95 Elemental composition of particulate matter and the association with lung function
    Eeftens, M. and Hoek, G. and Gruzieva, O. and Mölter, A. and Agius, R. and Beelen, R. and Brunekreef, B. and Custovic, A. and Cyrys, J. and Fuertes, E. and Heinrich, J. and Hoffmann, B. and De Hoogh, K. and Jedynska, A. and Keuke...
    Epidemiology 25 648-657 (2014)
    BACKGROUND: Negative effects of long-term exposure to particulate matter (PM) on lung function have been shown repeatedly. Spatial differences in the composition and toxicity of PM may explain differences in observed effect sizes between studies. METHODS: We conducted a multicenter study in 5 European birth cohorts - BAMSE (Sweden), GINIplus and LISAplus (Germany), MAAS (United Kingdom), and PIAMA (The Netherlands) - for which lung function measurements were available for study subjects at the age of 6 or 8 years. Individual annual average residential exposure to copper, iron, potassium, nickel, sulfur, silicon, vanadium, and zinc within PM smaller than 2.5 μm (PM2.5) and smaller than 10 μm (PM10) was estimated using land-use regression models. Associations between air pollution and lung function were analyzed by linear regression within cohorts, adjusting for potential confounders, and then combined by random effects meta-analysis. RESULTS: We observed small reductions in forced expiratory volume in the first second, forced vital capacity, and peak expiratory flow related to exposure to most elemental pollutants, with the most substantial negative associations found for nickel and sulfur. PM10 nickel and PM10 sulfur were associated with decreases in forced expiratory volume in the first second of 1.6% (95% confidence interval = 0.4% to 2.7%) and 2.3% (-0.1% to 4.6%) per increase in exposure of 2 and 200 ng/m, respectively. Associations remained after adjusting for PM mass. However, associations with these elements were not evident in all cohorts, and heterogeneity of associations with exposure to various components was larger than for exposure to PM mass. CONCLUSIONS: Although we detected small adverse effects on lung function associated with annual average levels of some of the evaluated elements (particularly nickel and sulfur), lower lung function was more consistently associated with increased PM mass. Copyright © 2014 by Lippincott Williams & Wilkins.
    view abstractdoi: 10.1097/EDE.0000000000000136
  • 2014 • 94 Energetics of the water-gas-shift reaction on the active sites of the industrially used Cu/ZnO/Al2O3 catalyst
    Studt, F. and Behrens, M. and Abild-Pedersen, F.
    Catalysis Letters 144 1973-1977 (2014)
    The energy profile for the water-gas-shift reaction has been calculated on the active sites of the industrially used Cu/ZnO/Al2O3 catalyst using the BEEF-vdW functional. Our theoretical results suggest that both active site motifs, a copper (211) step as well as a zinc decorated step, are equally active for the water-gas-shift reaction. We find that the splitting of water into surface OH∗and H∗constitutes the rate-limiting step and that the reaction proceeds through the carboxyl mechanism. Our findings also suggest that mixed copper-zinc step sites are most likely to exhibit superior activity. © Springer Science+Business Media New York 2014.
    view abstractdoi: 10.1007/s10562-014-1363-9
  • 2014 • 93 Epitaxial Cu(001) films grown on a Cr/Ag/Fe/GaAs(001) buffer system
    Gottlob, D.M. and Jansen, T. and Hoppe, M. and Bürgler, D.E. and Schneider, C.M.
    Thin Solid Films 562 250-253 (2014)
    We present a procedure to prepare single-crystalline, high-purity Cu(001) films (templates) suitable as substrates for subsequent epitaxial thin-film growth. The template films were grown in a dedicated molecular-beam epitaxy system on a Cr/Ag/Fe/GaAs(001) buffer layer system. Low-energy electron diffraction and X-ray diffraction were applied to determine the surface orientation and the epitaxial relationship between all layers of the stack. Post-annealing at moderate temperatures enhances the quality of the film as shown by low-energy electron diffraction and atomic force microscopy. X-ray photoemission and Auger electron spectroscopy confirm that no atoms of the buffer layers diffuse into the Cu film during the initial preparation and the post-annealing treatment. The completed Cu(001) template system can be exposed to air and afterwards refurbished by Ar+-ion bombardment and annealing, enabling the transfer between vacuum systems. The procedure provides suitable conductive thin film templates for studies of epitaxial thin films, e.g. on the magnetic and magnetotransport properties of Co and Ni based films and multilayers. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.04.078
  • 2014 • 92 Heterogeneous Catalysis of CO2 Conversion to Methanol on Copper Surfaces
    Behrens, M.
    Angewandte Chemie - International Edition 53 12022-12024 (2014)
    Combined experimental and theoretical approaches resulted in a better understanding of the hydrogenation of CO2 to methanol on copper-based catalysts. These results highlight the important role of the reducible oxide promoter for CO2 activation. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201409282
  • 2014 • 91 Micromechanical investigations and modelling of a Copper-Antimony-Alloy under creep conditions
    Vöse, M. and Otto, F. and Fedelich, B. and Eggeler, G.
    Mechanics of Materials 69 41-62 (2014)
    In many practical applications, creep damage is the limiting factor of a component's lifetime. A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal within the framework of finite element analysis. The model considers grain boundary cavitation and sliding according to a micromechanically motivated cohesive zone model while creep deformation of the grains is described following the slip system theory. The model can be applied to idealised polycrystalline structures, such as a Voronoi tessellation or, like demonstrated here, to real grain structures of miniature creep specimens. Creep tests with pure Cu single crystals and with a coarse-grained polycrystalline Cu-1 wt.% Sb alloy at 823 K have been performed and used to calibrate the polycrystal model. The grain structure of the polycrystalline Cu-Sb specimens has been revealed by the EBSD method. Extensive grain boundary sliding and cavitation has been observed in the crept specimens. Grain boundary sliding has been found to promote wedge-type damage at grain boundary triple junctions and to contribute significantly to the total creep strain. Furthermore, the assumed stress sensitivity of the models grain boundary cavity nucleation rate strongly influences the development of wedge-type damage. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.mechmat.2013.09.013
  • 2014 • 90 Microstructural and defect analysis of metal nanoparticles in functional catalysts by diffraction and electron microscopy: The Cu/ZnO catalyst for methanol synthesis
    Kandemir, T. and Kasatkin, I. and Girgsdies, F. and Zander, S. and Kühl, S. and Tovar, M. and Schlögl, R. and Behrens, M.
    Topics in Catalysis 57 188-206 (2014)
    The application of different methods for a microstructural analysis of functional catalysts is reported for the example of different Cu/ZnO-based methanol synthesis catalysts. Transmission electron microscopy and diffraction were used as complementary techniques to extract information on the size and the defect concentration of the Cu nano-crystallites. The results, strengths and limitations of the two techniques and of different evaluation methods for line profile analysis of diffraction data including Rietveld-refinement, Scherrer- and (modified) Williamson-Hall-analyses, single peak deconvolution and whole powder pattern modeling are compared and critically discussed. It was found that in comparison with a macrocrystalline pure Cu sample, the catalysts were not only characterized by a smaller crystallite size, but also by a high concentration of lattice defects, in particular stacking faults. Neutron diffraction was introduced as a valuable tool for such analysis, because of the larger number of higher-order diffraction peaks that can be detected with this method. An attempt is reported to quantify the different types of defects for a selected catalyst. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-013-0175-2
  • 2014 • 89 Multifunctional, defect-engineered metal-organic frameworks with ruthenium centers: Sorption and catalytic properties
    Kozachuk, O. and Luz, I. and Llabrés I Xamena, F.X. and Noei, H. and Kauer, M. and Albada, H.B. and Bloch, E.D. and Marler, B. and Wang, Y. and Muhler, M. and Fischer, R.A.
    Angewandte Chemie - International Edition 53 7058-7062 (2014)
    A mixed-linker solid-solution approach was employed to modify the metal sites and introduce structural defects into the mixed-valence Ru II/III structural analogue of the well-known MOF family [M 3 II,II(btc)2] (M=Cu, Mo, Cr, Ni, Zn; btc=benzene-1,3,5-tricarboxylate), with partly missing carboxylate ligators at the Ru2 paddle-wheels. Incorporation of pyridine-3,5-dicarboxylate (pydc), which is the same size as btc but carries lower charge, as a second, defective linker has led to the mixed-linker isoreticular derivatives of Ru-MOF, which display characteristics unlike those of the defect-free framework. Along with the creation of additional coordinatively unsaturated sites, the incorporation of pydc induces the partial reduction of ruthenium. Accordingly, the modified Ru sites are responsible for the activity of the "defective" variants in the dissociative chemisorption of CO 2, the enhanced performance in CO sorption, the formation of hydride species, and the catalytic hydrogenation of olefins. The defect engineering in Ru-based metal-organic frameworks (MOFs) at coordinatively unsaturated metal centers (CUS) induces partial reduction of the metal nodes and leads to properties that are absent for the parent MOF, such as dissociative chemisorption of CO2 and enhanced sorption capacity of CO. The modified MOFs offer new perspectives as multifunctional materials whose performance is controlled by design of the defects. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201311128
  • 2014 • 88 Ordered mesoporous Cu-Ce-O catalysts for CO preferential oxidation in H2-rich gases: Influence of copper content and pretreatment conditions
    Gu, D. and Jia, C.-J. and Bongard, H. and Spliethoff, B. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
    Applied Catalysis B: Environmental 152-153 11-18 (2014)
    Highly ordered mesoporous Cu-Ce-O catalysts with different Cu contents have been synthesized by using ordered mesoporous silica KIT-6 as a hard template. The mesostructural order of the negative replica is influenced by the ratio of Cu to Ce. Using XRD, HR-SEM, TEM and EDX analysis, it was found that the ordered mesostructures of the nanocomposites degenerate with increasing Cu concentration, due to CuO leaching during the template removal process and a phase separation at high Cu concentration. Cu ions can replace Ce-ion in the structure of CeO2 at Cu concentrations below 40mol%. However, the Cu concentration in the final materials is lower than expected from the ratio used in the synthesis. The activity in preferential oxidation of CO in H2-rich gases (PROX) was tested at a space velocity of 60,000mLh-1gcat -1. The activity of the mesoporous catalysts increases with the concentration of Cu and becomes stable for Cu concentrations higher than 20mol%. A CO conversion around 100 % can be attained with Cu0.20Ce0.80O2 as catalyst at 160°C. The exit CO concentration can be as low as 70ppm under these conditions. The CO2 selectivity can reach 100 % at low temperature (60- 80°C). Direct loading of CuO on the surface of mesoporous CeO2 leads to large CuO crystals and correspondingly low activity. The influence of the pretreatment atmosphere on activity was also studied. Oxidation-reduction-reoxidation cycling can improve the catalytic activity of the catalysts. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.apcatb.2014.01.011
  • 2014 • 87 Photo-enhanced antinodal conductivity in the pseudogap state of high-T c cuprates
    Cilento, F. and Dal Conte, S. and Coslovich, G. and Peli, S. and Nembrini, N. and Mor, S. and Banfi, F. and Ferrini, G. and Eisaki, H. and Chan, M.K. and Dorow, C.J. and Veit, M.J. and Greven, M. and Van Der Marel, D. and Comin, R...
    Nature Communications 5 (2014)
    A major challenge in understanding the cuprate superconductors is to clarify the nature of the fundamental electronic correlations that lead to the pseudogap phenomenon. Here we use ultrashort light pulses to prepare a non-thermal distribution of excitations and capture novel properties that are hidden at equilibrium. Using a broadband (0.5-2 eV) probe, we are able to track the dynamics of the dielectric function and unveil an anomalous decrease in the scattering rate of the charge carriers in a pseudogap-like region of the temperature (T) and hole-doping (p) phase diagram. In this region, delimited by a well-defined T* neq (p) line, the photoexcitation process triggers the evolution of antinodal excitations from gapped (localized) to delocalized quasiparticles characterized by a longer lifetime. The novel concept of photo-enhanced antinodal conductivity is naturally explained within the single-band Hubbard model, in which the short-range Coulomb repulsion leads to a k-space differentiation between nodal quasiparticles and antinodal excitations. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5353
  • 2014 • 86 Photodeposition of copper and chromia on gallium oxide: The role of co-catalysts in photocatalytic water splitting
    Busser, G.W. and Mei, B. and Pougin, A. and Strunk, J. and Gutkowski, R. and Schuhmann, W. and Willinger, M.-G. and Schlögl, R. and Muhler, M.
    ChemSusChem 7 1030-1034 (2014)
    Split second: The photocatalytic activity of gallium oxide (β-Ga 2O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu2+ and CrO42-. The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx: it is essential for the oxidation of water. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201301065
  • 2014 • 85 Plasma-enhanced chemical vapor deposition of graphene on copper substrates
    Wöhrl, N. and Ochedowski, O. and Gottlieb, S. and Shibasaki, K. and Schulz, S.
    AIP Advances 4 (2014)
    A plasma enhanced vapor deposition process is used to synthesize graphene from a hydrogen/methane gas mixture on copper samples. The graphene samples were transferred onto SiO2 substrates and characterized by Raman spectroscopic mapping and atomic force microscope topographical mapping. Analysis of the Raman bands shows that the deposited graphene is clearly SLG and that the sheets are deposited on large areas of several mm2. The defect density in the graphene sheets is calculated using Raman measurements and the influence of the process pressure on the defect density is measured. Furthermore the origin of these defects is discussed with respect to the process parameters and hence the plasma environment. © 2014 Author(s).
    view abstractdoi: 10.1063/1.4873157
  • 2014 • 84 Reflectance difference spectroscopy of water on Cu(110)
    Denk, M. and Hohage, M. and Suna, L.D. and Zeppenfeld, P. and Esser, N. and Cobet, C.
    Surface Science 627 16-22 (2014)
    The adsorption of H2O on Cu(110)was probed bymeans of reflectance difference spectroscopy (RDS) in the energy range between 1.5 and 9.3 eV and by scanning tunnelingmicroscopy (STM). The adsorption ofwater on the pristine Cu(110) substrate mainly induces changes in the Cu surface state related optical transitions. Furthermore, the H2O adsorbate modifies the Cu bulk transitions. In particular, our investigations demonstrate that the coverage-dependent phase transition from 1D pentamer chains to a (7 × 8) superstructure can bemonitored by means of RDS. In the vacuum-UV range, new RD features assigned to Cu bulk transitions were detected. Adsorption on themetal surface stronglymodifies or quenches theH2OHOMO-LUMOtransition, whereby a distinct RD feature of the water molecules themselves in the vacuum-UV range is absent. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2014.04.009
  • 2014 • 83 Support and challenges to the melanosomal casing model based on nanoscale distribution of metals within iris melanosomes detected by X-ray fluorescence analysis
    Gorniak, T. and Haraszti, T. and Suhonen, H. and Yang, Y. and Hedberg-Buenz, A. and Koehn, D. and Heine, R. and Grunze, M. and Rosenhahn, A. and Anderson, M.G.
    Pigment Cell and Melanoma Research 27 831-834 (2014)
    Melanin within melanosomes exists as eumelanin or pheomelanin. Distributions of these melanins have been studied extensively within tissues, but less often within individual melanosomes. Here, we apply X-ray fluorescence analysis with synchrotron radiation to survey the nanoscale distribution of metals within purified melanosomes of mice. The study allows a discovery-based characterization of melanosomal metals, and, because Cu is specifically associated with eumelanin, a hypothesis-based test of the 'casing model' predicting that melanosomes contain a pheomelanin core surrounded by a eumelanin shell. Analysis of Cu, Ca, and Zn shows variable concentrations and distributions, with Ca/Zn highly correlated, and at least three discrete patterns for the distribution of Cu vs. Ca/Zn in different melanosomes - including one with a Cu-rich shell surrounding a Ca/Zn-rich core. Thus, the results support predictions of the casing model, but also suggest that in at least some tissues and genetic contexts, other arrangements of melanin may co-exist. © 2014 John Wiley & Sons A/S.
    view abstractdoi: 10.1111/pcmr.12278
  • 2014 • 82 Synthesis and characterisation of a highly active Cu/ZnO: Al catalyst
    Schumann, J. and Lunkenbein, T. and Tarasov, A. and Thomas, N. and Schlögl, R. and Behrens, M.
    ChemCatChem 6 2889–2897 (2014)
    We report the application of an optimised synthesis protocol of a Cu/ZnO:Al catalyst. The different stages of synthesis are all well-characterised by using various methods with regard to the (micro-)structural, textural, solid-state kinetic, defect and surface properties. The low amount of the Al promoter (3%) influences but does not generally change the phase evolution known for binary Cu/ZnO catalysts. Its main function seems to be the introduction of defect sites in ZnO by doping. These sites as well as the large Cu surface area are responsible for the large N2O chemisorption capacity. Under reducing conditions, the Al promoter, just as Zn, is found enriched at the surface suggesting an active role in the strong metal-support interaction between Cu and ZnO:Al. The different stages of the synthesis are comprehensively analysed and found to be highly reproducible in the 100g scale. The resulting catalyst is characterised by a uniform elemental distribution, small Cu particles (8nm), a porous texture (pore size of approximately 25nm), high specific surface area (approximately 120m2g-1), a high amount of defects in the Cu phase and synergetic Cu-ZnO interaction. A high and stable performance was found in methanol synthesis. We wish to establish this complex but well-studied material as a benchmark system for Cu-based catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.201402278
  • 2014 • 81 Synthesis of 7-pentafluorophenyl-1 H -indole: An anion receptor for anion-π interactions
    Sun, Z.-H. and Albrecht, M. and Giese, M. and Pan, F. and Rissanen, K.
    Synlett 25 2075-2077 (2014)
    7-Pentafluorophenyl-1H-indole has the potential to be a key compound for the investigation of anion-π interactions in solution. Unfortunately, it was not possible to obtain it by aryl-aryl coupling reaction. Finally, it has been prepared by Bartoli indole synthesis. The key compound as well as analogues were submitted to preliminary studies of anion binding. Single crystals of two key receptors were obtained. © Georg Thieme Verlag Stuttgart New York.
    view abstractdoi: 10.1055/s-0034-1378449
  • 2014 • 80 Texture and microstructure evolution during non-crystallographic shear banding in a plane strain compressed Cu-Ag metal matrix composite
    Jia, N. and Raabe, D. and Zhao, X.
    Acta Materialia 76 238-251 (2014)
    We studied the texture and microstructure evolution in a plane strain compressed Cu-Ag metal matrix composite (MMC) with a heterophase microstructure using crystal plasticity finite element simulations. Lattice reorientations induced by both crystallographic (dislocation slip and twinning) and non-crystallographic (shear banding) mechanisms are addressed. First, simulations on a polycrystalline composite are made. Quite similar texture trends are observed for the composites and for the individual single-phase materials, namely, copper-type texture components in the Cu phase and brass-type texture components in the Ag phase. This result differs from experimental data that show less copper-type and more brass-type textures in both phases for the composite materials. To explore co-deformation mechanisms that lead to the specific crystallographic textures in the MMC, bicrystal simulations for the composite with specific initial orientation combinations are performed. The bicrystal simulations reproduce the experimentally observed trends of texture evolution in the respective phases of the composite, indicating that the localized stress and strain fields as well as the co-deformation mechanisms within the actual heterophase microstructure are well captured. The modeling shows that to accommodate plastic deformation between adjacent phases in the bicrystals, pronounced shear bands are triggered by stress concentration at the hetero-interfaces. With further deformation the bands penetrate through the phase boundaries and lead to larger lattice rotations. The simulations confirm that the shear banding behavior in heterophase composites is different from that in single-phase metals and the texture evolution in composite materials is strongly influenced by the starting texture, the local constraints exerted from the phase boundaries and the constitutive properties of the abutting phases. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.05.036
  • 2013 • 79 Accessing 4f-states in single-molecule spintronics
    Fahrendorf, S. and Atodiresei, N. and Besson, C. and Caciuc, V. and Matthes, F. and Blügel, S. and Kögerler, P. and Bürgler, D.E. and Schneider, C.M.
    Nature Communications 4 (2013)
    Magnetic molecules are potential functional units for molecular and supramolecular spintronic devices. However, their magnetic and electronic properties depend critically on their interaction with metallic electrodes. Charge transfer and hybridization modify the electronic structure and thereby influence or even quench the molecular magnetic moment. Yet, detection and manipulation of the molecular spin state by means of charge transport, that is, spintronic functionality, mandates a certain level of hybridization of the magnetic orbitals with electrode states. Here we show how a judicious choice of the molecular spin centres determines these critical molecule-electrode contact characteristics. In contrast to late lanthanide analogues, the 4f-orbitals of single bis(phthalocyaninato)-neodymium(III) molecules adsorbed on Cu(100) can be directly accessed by scanning tunnelling microscopy. Hence, they contribute to charge transport, whereas their magnetic moment is sustained as evident from comparing spectroscopic data with ab initio calculations. Our results showcase how tailoring molecular orbitals can yield all-electrically controlled spintronic device concepts. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms3425
  • 2013 • 78 Adsorption of methanethiolate and atomic sulfur at the Cu(111) surface: A computational study
    Seema, P. and Behler, J. and Marx, D.
    Journal of Physical Chemistry C 117 337-348 (2013)
    Density-functional theory calculations have been carried out to study the adsorption of methanethiolate and atomic sulfur as a nonmolecular reference at the Cu(111) surface. A large number of surface models have been investigated considering a variety of binding sites and coverages at the ideal and reconstructed surface. For methanethiolate, we find that the proposed [5013] supercell commonly used to approximate the experimentally observed noncommensurate pseudo(100) reconstruction yields the lowest surface energy, but several similar local minima exist differing in the positions of the copper atoms. None of these structures show the regular nearly square coordination of the thiolate species observed in scanning tunneling microscopy (STM). Modifying the chemical composition of the relaxed layer, e.g., by adding another copper atom, yields structures of comparable stability. It is thus very likely that the proposed supercell is not a good approximation to the true pseudo(100) phase and that larger unit cells are needed to allow for a realistic relaxation of the reconstructed layer. For atomic sulfur, it is well established that the most stable phase at Cu(111) is a (√7 × √7)R19.1 reconstruction. Its structure, however, has been discussed controversially in the literature for many years. While there is a consensus that the unit cell contains three sulfur atoms, there are still several competing models differing in the number of copper adatoms in the reconstructed layer. We find that three models have a very similar stability, and a three-copper adatom model is only marginally preferred. These results will be of importance for many fields from heterogeneous catalysis to covalent mechanochemistry and molecular nanomechanics. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp309728w
  • 2013 • 77 Atom probe study of Cu2ZnSnSe4 thin-films prepared by co-evaporation and post-deposition annealing
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P. and Mousel, M. and Redinger, A. and Siebentritt, S. and Raabe, D.
    Applied Physics Letters 102 (2013)
    We use atom probe tomography (APT) for resolving nanometer scale compositional fluctuations in Cu2ZnSnSe4 (CZTSe) thin-films prepared by co-evaporation and post-deposition annealing. We detect a complex, nanometer-sized network of CZTSe and ZnSe domains in these films. Some of the ZnSe domains contain precipitates having a Cu- and Sn-rich composition, where the composition cannot be assigned to any of the known equilibrium phases. Furthermore, Na impurities are found to be segregated at the CZTSe/ZnSe interface. The insights given by APT are essential for understanding the growth of CZTSe absorber layers for thin-film solar cells and for optimizing their optoelectronic properties. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4788815
  • 2013 • 76 Atom Probe Tomography Studies on the Cu(In,Ga)Se-2 Grain Boundaries
    Cojocaru-Miredin, O. and Schwarz, T. and Choi, P. P. and Herbig, M. and Wuerz, R. and Raabe, D.
    Jove-journal of Visualized Experiments UNSP e50376 (2013)
    Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the nanoscale and in three dimensions. Indeed, APT possesses high sensitivity (in the order of ppm) and high spatial resolution (sub nm). Considerable efforts were done here to prepare an APT tip which contains the desired grain boundary with a known structure. Indeed, sitespecific sample preparation using combined focused-ion-beam, electron backscatter diffraction, and transmission electron microscopy is presented in this work. This method allows selected grain boundaries with a known structure and location in Cu(In, Ga)Se-2 thin-films to be studied by atom probe tomography. Finally, we discuss the advantages and drawbacks of using the atom probe tomography technique to study the grain boundaries in Cu(In,Ga)Se-2 thin-film solar cells.
    view abstractdoi: 10.3791/50376
  • 2013 • 75 Chemical activity of thin oxide layers: Strong interactions with the support yield a new thin-film phase of zno
    Schott, V. and Oberhofer, H. and Birkner, A. and Xu, M. and Wang, Y. and Muhler, M. and Reuter, K. and Wöll, C.
    Angewandte Chemie - International Edition 52 11925-11929 (2013)
    Influential support: Metal substrates affect the chemical properties of ZnO layers, which are important catalyst materials for the industrial production of methanol through the oxidation of CO. Interactions with the substrate lead to the formation of a new, planar ZnO thin-film phase, in which less highly oxidized Zn atoms bind CO more strongly than the Zn atoms in the normal wurtzite modification. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201302315
  • 2013 • 74 Cu doped V2O5 flowers as cathode material for high-performance lithium ion batteries
    Yu, H. and Rui, X. and Tan, H. and Chen, J. and Huang, X. and Xu, C. and Liu, W. and Yu, D.Y.W. and Hng, H.H. and Hoster, H.E. and Yan, Q.
    Nanoscale 5 4937-4943 (2013)
    Hierarchical Cu doped vanadium pentoxide (V2O5) flowers were prepared via a simple hydrothermal approach followed by an annealing process. The flower precursors are self-assembled with 1D nanobelts surrounding a central core. The morphological evolution is investigated and a plausible mechanism is proposed. As the cathode material for lithium ion batteries, the Cu doped V2O5 samples exhibit improved electrochemical performance compared to the un-doped ones. Among them Cu 0.02V1.98O5 delivered higher reversible specific capacities, better cycling stabilities and excellent rate capabilities, e.g. 97 mA h g-1 at 20.0 C. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3nr00548h
  • 2013 • 73 Cu2MnAl thin films grown onto sapphire and MgO substrates: Exchange stiffness and magnetic anisotropy
    Belmeguenai, M. and Tuzcuoglu, H. and Chérif, S.M. and Westerholt, K. and Chauveau, T. and Mazaleyrat, F. and Moch, P.
    Physica Status Solidi (A) Applications and Materials Science 210 553-558 (2013)
    Cu2MnAl films of different thicknesses (50, 70, and 100 nm) were grown by UHV RF-sputtering on a-plane sapphire or on MgO (100) substrates. Their structural and static magnetic properties have been studied by X-rays diffraction (XRD) and by vibrating sample magnetometry (VSM), respectively. The Cu2MnAl films exhibit a (100) and (110)-texture when grown on MgO and sapphire substrates, respectively. The best growth quality and the higher magnetization at saturation were obtained for the films grown on MgO. Dynamic magnetic properties were investigated using micro-strip line ferromagnetic resonance (MS-FMR). From the resonance measurements varying the direction and the amplitude of the in-plane and out-of-plane applied magnetic fields we derive the effective magnetization, the Landé g-factor (g = 2.11), the exchange constant (Aex = 0.34 μerg cm-1) and the magnetic anisotropy terms. The in-plane anisotropy can be described as a superposition of two terms showing a small twofold and a dominant fourfold symmetry. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201228187
  • 2013 • 72 Effect of carrier gas composition on transferred arc metal nanoparticle synthesis
    Stein, M. and Kiesler, D. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    Metal nanoparticles are used in a great number of applications; an effective and economical production scaling-up is hence desirable. A simple and cost-effective transferred arc process is developed, which produces pure metal (Zn, Cu, and Ag) nanoparticles with high production rates, while allowing fast optimization based on energy efficiency. Different carrier gas compositions, as well as the electrode arrangements and the power input are investigated to improve the production and its efficiency and to understand the arc production behavior. The production rates are determined by a novel process monitoring method, which combines an online microbalance method with a scanning mobility particle sizer for fast production rate and size distribution measurement. Particle characterization is performed via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction measurements. It is found that the carrier gas composition has the largest impact on the particle production rate and can increase it with orders of magnitude. This appears to be not only a result of the increased heat flux and melt temperature but also of the formation of tiny nitrogen (hydrogen) bubbles in the molten feedstock, which impacts feedstock evaporation significantly in bi-atomic gases. A production rate of sub 200 nm particles from 20 up to 2,500 mg/h has been realized for the different metals. In this production range, specific power consumptions as low as 0.08 kWh/g have been reached. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-012-1400-9
  • 2013 • 71 Electrochemical formation and surface characterisation of Cu 2-xTe thin films with adjustable content of Cu
    Huang, M. and Maljusch, A. and Calle-Vallejo, F. and Henry, J.B. and Koper, M.T.M. and Schuhmann, W. and Bandarenka, A.S.
    RSC Advances 3 21648-21654 (2013)
    Electrochemically driven "intercalation" of Cu into Te was used to prepare Cu<inf>2-x</inf>Te (0.2 < x ≤ 2) thin films and accurately control the composition of the resulting samples. A thorough theoretical analysis of the system using density functional theory (DFT) calculations showed that in the absence of external electric fields the driving forces for Cu atoms to move into the subsurface layers of the Te electrodes depend on the surface coverage of copper atoms. The Cu atoms tend to preferentially occupy the subsurface layers in the telluride films. The effective electric charge on Cu atoms inside the Te-electrodes is positive. These effective charge differences with respect to pure Cu and pure Te are only 0.2 e-. Scanning Kelvin probe (SKP), atomic force microscopy (AFM) and electrochemical techniques were used to characterise the surface status of the obtained samples. Both, DFT-calculated work function differences and the SKP-measured contact potential differences (CPD) change non-linearly with the variation of the film composition. Interfacial (solid/liquid) properties evaluated using electrochemical impedance spectroscopy depend on the nominal composition of the samples and display an abrupt change that correlates with a large change in the work function and CPD. While the proposed electrochemical synthetic route can efficiently and accurately control the composition of the Cu<inf>2-x</inf>Te thin films, SKP-measurements performed under close to ambient conditions in combination with DFT calculations can provide a promising tool to link fundamental surface properties and parameters which define the interface between solids and liquids. © The Royal Society of Chemistry 2013.
    view abstractdoi: 10.1039/c3ra42504e
  • 2013 • 70 Formation and characterization of Fe3+-/Cu2+-modified zirconium oxide conversion layers on zinc alloy coated steel sheets
    Lostak, T. and Krebs, S. and Maljusch, A. and Gothe, T. and Giza, M. and Kimpel, M. and Flock, J. and Schulz, S.
    Electrochimica Acta 112 14-23 (2013)
    Zirconium oxide conversion layers are considered as environmentally friendly alternatives replacing chromate-based passivation layers in the coil-coating industry. Based on excellent electronic barrier properties they provide an effective corrosion protection of the metallic substrate. In this work, thin layers were grown on HDG-steel-substrates by increasing the local pH at the surface and were characterized using potentiodynamic polarization technique. The influence of Cu(NO3)2·3H 2O or Fe(NO3)3·9H2O on morphology and thickness of deposited protective layers were investigated by XPS, ToF-SIMS and FE-SEM. A significant film thickness increase was found by adding Cu2+ or Fe3+ ions to the conversion solution. In addition, growth kinetics was studied by in situ measurements of corrosion potential using potentiodynamic polarization technique. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2013.08.161
  • 2013 • 69 How to prepare a good Cu/ZnO catalyst or the role of solid state chemistry for the synthesis of nanostructured catalysts
    Behrens, M. and Schlögl, R.
    Zeitschrift fur Anorganische und Allgemeine Chemie 639 2683-2695 (2013)
    In this research report we summarize recent progress that has been made in the field of Cu/ZnO catalyst synthesis. We briefly introduce the fields of application of this catalyst: methanol synthesis, the water gas shift reaction, and methanol steam reforming. The review is focused on the well-documented industrial synthesis protocol and on the early stages of catalyst synthesis. The setting of the most critical synthesis parameters during co-precipitation and ageing, like pH and temperature, is discussed in detail. We show how these parameters effect the phase formation and identify zincian malachite, (Cu, Zn)2(OH)2CO3, as the relevant precursor phase for high-performance catalysts. A special emphasis is placed on the solid state chemistry of this precursor phase, in particular on the structural effects of Cu, Zn substitution. Based on the structural analysis, it is shown that the industrial synthesis recipe was empirically optimized to maximize the zinc incorporation into zincian malachite. From this insight a simple and generic geometric concept for the synthesis of nanostructured composite catalysts based on de-mixing of solid solution precursors is derived. With this concept, the complex multi-step industrial synthesis can be rationalized and the so-called "chemical memory" of this catalyst synthesis can be understood. We also demonstrate how application of this concept can lead to new interesting catalytic materials, which help to address fundamental questions of this catalyst system like to role of the Al2O3 promoter or the so-called Cu-Zn synergy. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/zaac.201300356
  • 2013 • 68 Interaction of antitumor flavonoids with dsDNA in the absence and presence of Cu(II)
    Temerk, Y.M. and Ibrahim, M.S. and Kotb, M. and Schuhmann, W.
    Analytical and Bioanalytical Chemistry 405 3839-3846 (2013)
    The binding of antitumor flavonoids, namely 3-hydroxyflavone (3HF) and hesperidin (Hesp) with dsDNA was investigated in the absence and presence of Cu(II) using cyclic voltammetry and square wave voltammetry at the hanging mercury drop electrode. The reduction currents of 3HF, 3HF-Cu complex, and the 3HF-β-cyclodextrin inclusion complex decreased after intercalation into dsDNA. The intercalation of Hesp into dsDNA is weak. dsDNA is reduced at a potential of -1.48 V overlaying the reduction of Hesp. In contrast, in the presence of Cu(II), the interaction of Hesp with dsDNA leads to a much stronger intercalation. The binding constants of the flavonoid-Cu complex with dsDNA were evaluated and calibration graphs for the determination of dsDNA were obtained from the decrease in the peak current in the cyclic voltammograms of 3HF in the presence of dsDNA. The proposed method exhibited good recovery and reproducibility for indirect determination of dsDNA. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00216-012-6675-2
  • 2013 • 67 Neural network potentials for metals and oxides - First applications to copper clusters at zinc oxide
    Artrith, N. and Hiller, B. and Behler, J.
    Physica Status Solidi (B) Basic Research 250 1191-1203 (2013)
    The development of reliable interatomic potentials for large-scale molecular dynamics (MD) simulations of chemical processes at surfaces and interfaces is a formidable challenge because a wide range of atomic environments and very different types of bonding can be present. In recent years interatomic potentials based on artificial neural networks (NNs) have emerged offering an unbiased approach to the construction of potential energy surfaces (PESs) for systems that are difficult to describe by conventional potentials. Here, we review the basic properties of NN potentials and describe their construction for materials like metals and oxides. The accuracy and efficiency are demonstrated using copper and zinc oxide as benchmark systems. First results for a potential of the combined ternary CuZnO system aiming at the description of oxide-supported copper clusters are reported. Model of a copper cluster at the ZnO($10\overline {1} 0$) surface. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201248370
  • 2013 • 66 Performance improvement of nanocatalysts by promoter-induced defects in the support material: Methanol synthesis over Cu/ZnO:Al
    Behrens, M. and Zander, S. and Kurr, P. and Jacobsen, N. and Senker, J. and Koch, G. and Ressler, T. and Fischer, R.W. and Schlögl, R.
    Journal of the American Chemical Society 135 6061-6068 (2013)
    Addition of small amounts of promoters to solid catalysts can cause pronounced improvement in the catalytic properties. For the complex catalysts employed in industrial processes, the fate and mode of operation of promoters is often not well understood, which hinders a more rational optimization of these important materials. Herein we show for the example of the industrial Cu/ZnO/Al2O3 catalyst for methanol synthesis how structure-performance relationships can deliver such insights and shed light on the role of the Al promoter in this system. We were able to discriminate a structural effect and an electronic promoting effect, identify the relevant Al species as a dopant in ZnO, and determine the optimal Al content of improved Cu/ZnO:Al catalysts. By analogy to Ga- and Cr-promoted samples, we conclude that there is a general effect of promoter-induced defects in ZnO on the metal-support interactions and propose the relevance of this promotion mechanism for other metal/oxide catalysts also. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja310456f
  • 2013 • 65 Preparation of thin film Cu-Pt(1 1 1) near-surface alloys: One small step towards up-scaling model single crystal surfaces
    Henry, J.B. and Maljusch, A. and Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 112 887-893 (2013)
    A method for the preparation of Pt(1 1 1) like thin films and thin film Cu-Pt(1 1 1) near-surface alloys (where Cu is preferentially located in the subsurface region) is reported in detail. Cyclic voltammograms of the resultant Pt(1 1 1)-like thin films in 0.1 M HClO4 demonstrate characteristic "butterfly" peaks attributed to the disorder/order phase transition in the adsorbed*OH layer, typical for a large Pt(1 1 1) crystals. Modification of the film surface with a monolayer of Cu and subsequent annealing in a reducing Ar/H2(5%) atmosphere, the voltammograms resemble those obtained for Cu-Pt(1 1 1) near-surface alloys prepared on commercial bulk single crystals. This method shows promise for the fabrication of extended (1 1 1)-type alloy surfaces of Pt and its alloys and can additionally be used to up-scale model objects for wider industrial and laboratory applications. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2012.11.139
  • 2013 • 64 Pt-Cu alloys as catalysts for the oxygen reduction reaction - A thin-film study of activity and stability
    Schuppert, A.K. and Topalov, A.A. and Savan, A. and Ludwig, Al. and Mayrhofer, K.J.J.
    ECS Transactions 58 587-592 (2013)
    Critical factors for the commercial application of fuel cells are the high costs and the limited stability of Pt catalysts. In order to improve the activity and material efficiency, Pt-alloys with nonnoble metals play an essential role. However, stability remains a critical factor for this type of catalysts. In order to understand the dissolution of Pt-alloys and eventually improve their performance, we therefore analyze a Pt-Cu thin-film alloy with varying composition using a combinatorial screening approach coupled to online analytics. © The Electrochemical Society.
    view abstractdoi: 10.1149/05801.0587ecst
  • 2013 • 63 Surface-induced dechlorination of FeOEP-Cl** on Cu(111)
    Van Vörden, D. and Lange, M. and Schaffert, J. and Cottin, M.C. and Schmuck, M. and Robles, R. and Wende, H. and Bobisch, C.A. and Möller, R.
    ChemPhysChem 14 3472-3475 (2013)
    To be or not to be chlorinated: When octaethylporphyrin iron(III) chloride (FeOEP-Cl) molecules are sublimated onto Cu(111) surfaces, two different molecular species are observed through scanning tunneling microscopy, showing either a protrusion or a depression at the center. In combination with van der Waals-corrected density functional calculations, our experiments reveal that one species corresponds to FeOEP-Cl molecules with the chlorine atom pointing away from the surface, whereas the other species has been dechlorinated. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201300497
  • 2013 • 62 Synthesis and characterization of Cu-doped ZnO one-dimensional structures for miniaturized sensor applications with faster response
    Chow, L. and Lupan, O. and Chai, G. and Khallaf, H. and Ono, L.K. and Roldan Cuenya, B. and Tiginyanu, I.M. and Ursaki, V.V. and Sontea, V. and Schulte, A.
    Sensors and Actuators, A: Physical 189 399-408 (2013)
    Detection of chemicals and biological species is an important issue to human health and safety. In this paper, we report the hydrothermal synthesis at 95 °C of Cu-doped ZnO low-dimensional rods for room-temperature (RT) sensing applications and enhanced sensor performances. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman and photoluminescence are used to characterize the material properties. To demonstrate the suitability of the Cu-doped ZnO rods for gas sensor applications and for comparison with pure ZnO, we fabricated a double rod device using Focused Ion Beam. The responses of pure-ZnO and Cu-doped ZnO rods studied in exactly the same condition are reported. We found that Cu-ZnO sensors have enhanced RT sensitivity, faster response time, and good selectivity. Miniaturized Cu-ZnO rod-based sensors can serve as a good candidate for effective H2 detectors with low power consumption. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.sna.2012.09.006
  • 2013 • 61 The role of the oxide component in the development of copper composite catalysts for methanol synthesis
    Zander, S. and Kunkes, E.L. and Schuster, M.E. and Schumann, J. and Weinberg, G. and Teschner, D. and Jacobsen, N. and Schlögl, R. and Behrens, M.
    Angewandte Chemie - International Edition 52 6536-6540 (2013)
    The design of solid catalysts for industrial processes remains a major challenge in synthetic materials chemistry. Based on the investigation of the industrial Cu/ZnO/Al2O3 catalyst, a modular concept is introduced that helps to develop novel methanol synthesis catalysts that operate in different feed gas mixtures. SA=surface area, SMSI=strong metal-support interaction. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201301419
  • 2013 • 60 The transition from spark to arc discharge and its implications with respect to nanoparticle production
    Hontañón, E. and Palomares, J.M. and Stein, M. and Guo, X. and Engeln, R. and Nirschl, H. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    The synthesis of nanoparticles by means of electrical discharges between two electrodes in an inert gas at atmospheric pressure, as driven by a constant current ranging from a few milliamps to tens of amps, is investigated in this work. An extensive series of experiments are conducted with copper as a consumable electrode and pure nitrogen as the inert gas. Three different DC power supplies are used to drive electrical discharges for the entire operating current range. Then, three electrical discharge regimes (spark, glow, and arc) with distinct voltage-current characteristics and plasma emission spectra are recognized. For the first time, nanoparticles are synthesized by evaporation of an electrode by atmospheric pressure inert gas DC glow discharge of a few millimeters in size. The discharge regimes are characterized in terms of the mass output rate and the particle size distribution of the copper aerosols by means of online (tapered element oscillating microbalance, TEOM; and scanning mobility particle sizer, SPMS) and offline (gravimetric analysis; small and wide angle X-ray scattering, SWAXS; and transmission electron microscopy, TEM) techniques. The electrical power delivered to the electrode gap and the gas flow rate are two major parameters determining the aerosol mass output rate and the aerosol particle size distribution. The mass output rate of copper aerosols raises from 2 mg h-1 to 2 g h-1 when increasing the electrical power from 9 to 900 W. The particle mean size (SMPS dg) varies between 20 and 100 nm depending upon the electrical power and the gas flow rate, whereas the particle size dispersion (SMPS σg) ranges from 1.4 to 1.7 and is only weakly dependent on the gas flow rate. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1957-y
  • 2012 • 59 Annealing effect on structural and magnetic properties of Cu 2MnAL heusler alloy films
    Topkaya, R. and Yilgin, R. and Kazan, S. and Akdoǧan, N. and Obaida., M. and Inam, H. and Westerholt, K.
    Journal of Superconductivity and Novel Magnetism 25 2605-2609 (2012)
    Cu2MnAl Heusler alloy films were grown on MgO (001) substrates by using the ion beam sputtering technique. The films were post-annealed at varying temperatures in order to investigate the influence of annealing on crystal structure and magnetic properties. The structural properties of Cu 2MnAl films have been investigated by using x-ray diffraction (XRD) and magnetic properties have been investigated by both vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) techniques. The experimental data indicates that the crystal structure of the films strongly depends on the annealing temperature. When the films were annealed at 200 °C, the saturation magnetization (Ms = 250 emu/cm3) achieved its maximum and the coercive field (Hc 7 Oe) reached its minimum with B2 ordered structure. In addition, FMR results have revealed that the Cu2MnAl film annealed 200 °C has the highest effective magnetization. The combination of structural and magnetic characterization indicates that the optimum growth temperature is 200 °C for the Cu2MnAl Heusler alloy films on MgO substrates. © Springer Science+Business Media, LLC 2011.
    view abstractdoi: 10.1007/s10948-011-1228-z
  • 2012 • 58 Charge transfer dynamics in molecular solids and adsorbates driven by local and non-local excitations
    Föhlisch, A. and Vijayalakshmi, S. and Pietzsch, A. and Nagasono, M. and Wurth, W. and Kirchmann, P.S. and Loukakos, P.A. and Bovensiepen, U. and Wolf, M. and Tchaplyguine, M. and Hennies, F.
    Surface Science 606 881-885 (2012)
    Charge transfer pathways and charge transfer times in molecular films and in adsorbate layers depend both on the details of the electronic structure as well as on the degree of the initial localization of the propagating excited electronic state. For C 6F 6 molecular adsorbate films on the Cu(111) surface we determined the interplay between excited state localization and charge transfer pathways. In particular we selectively prepared a free-particle-like LUMO band excitation and compared it to a molecularly localized core-excited C1s → π * C 6F 6 LUMO state using time-resolved two-photon photoemission (tr-2PPE) and core-hole-clock (CHC) spectroscopy, respectively. For the molecularly localized core-excited C1s → π * C 6F 6 LUMO state, we separate the intramolecular dynamics from the charge transfer dynamics to the metal substrate by taking the intramolecular dynamics of the free C 6F 6 molecule into account. Our analysis yields a generally applicable description of charge transfer within molecular adsorbates and to the substrate. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2011.12.014
  • 2012 • 57 Exploring the p-n junction region in Cu(In,Ga)Se 2 thin-film solar cells at the nanometer-scale
    Cojocaru-Mirédin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Applied Physics Letters 101 (2012)
    In this work we study the CdS/Cu(In,Ga)Se 2 p-n junction region in Cu(In,Ga)Se 2 thin-film solar cells using atom probe tomography. A Cu-, Ga-depleted, and Cd-doped region of about 1 nm thickness is detected at the Cu(In,Ga)Se 2 side of the CdS/Cu(In,Ga)Se 2 interface. Furthermore, Cd is also found to be enriched at Cu(In,Ga)Se 2 grain boundaries connected to the CdS layer. Na and O impurities decorate the CdS/CIGS interface, where Na-rich clusters are preferentially located in CdS regions abutting to Cu(In,Ga)Se 2 grain boundaries. The experimental findings of this work demonstrate the capability of atom probe tomography in studying buried interfaces and yield vital information for understanding and modeling the p-n junction band structure in Cu(In,Ga)Se 2 solar cells. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4764527
  • 2012 • 56 Facile remediation method of copper sulfide by nitrogen pre-treatment
    Yap, P.L. and Yoong, Y.L.A. and Kutty, M.G. and Timpe, O. and Behrens, M. and Abd Hamid, S.B.
    Advanced Materials Research 361-363 1445-1450 (2012)
    The deactivation and destabilization of copper sulfide when exposed to an oxidizing environment has led to the economical concerns as this sulfidic material can be easily destroyed by a series of oxidation processes. A promising and effective remediation technique in limiting the contact between covellite (CuS) and oxygen has been developed using a simple, hassle-free, non-corrosive, and eco-friendly pre-treatment of nitrogen approach. This remediation technique is remarkably effective as various techniques such as powder XRD, EDX, elemental mapping, and TGA-MS analyses have confirmed that covellite prepared with the pre-treatment of nitrogen does not oxidize to any mixed phase compound. Meanwhile, the study also shows that covellite stored without the pre-treatment of nitrogen has transformed to a mixed phase of pentahydrate copper sulfate and covellite. Hence, this method can be practically exercised not only on covellite, but possibly on other metal sulfides which are prone to be attacked by oxygen and water molecules in oxidizing environment. © (2012) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 55 High-throughput study of martensitic transformations in the complete Ti-Ni-Cu system
    Zarnetta, R. and Buenconsejo, P.J.S. and Savan, A. and Thienhaus, S. and Ludwig, Al.
    Intermetallics 26 98-109 (2012)
    The shape memory properties of the complete Ti-Ni-Cu thin film system were investigated using combinatorial methods, i.e. fabrication and high-throughput characterization of thin film materials libraries. Thin film composition spreads were deposited using a wedge-type multilayer technique and annealed at 500°C, 600°C and 700°C for 1 h for alloy formation. The complete composition regions showing reversible phase transformations were identified for each annealing temperature. These regions are well extended in comparison to prior knowledge. Furthermore, the composition-structure-property relations governing the phase transformation characteristics in the thin film samples were determined. For films annealed at 500°C and 600°C the transformation temperatures are highest for compositions close to Ti 50Ni 50-xCu x and decrease as the thin film compositions deviate. Similarly, the thermal transformation hysteresis is found to be smallest for "stoichiometric" (Ti 50Ni 50-xCu x) compositions. Precipitation of Ti-rich and (Ni,Cu)-rich phases is found to be responsible. With increasing annealing temperature the transformation temperatures increase and the thermal hysteresis values decrease for compositions showing B2→B19 phase transformation paths, due to coarsening of the precipitate phases. The alloying process of the multilayer thin films leads to the formation of the equilibrium phases. The formation of Guinier-Preston zones is suppressed. For thin films annealed at 700°C the transforming composition region is comparatively smaller and the phase transformation properties are influenced by Ti 2Ni precipitates. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2012.03.044
  • 2012 • 54 Investigation of copper corrosion inhibition with frequency-dependent alternating-current scanning electrochemical microscopy
    Santana, J.J. and Pähler, M. and Schuhmann, W. and Souto, R.M.
    ChemPlusChem 77 707-712 (2012)
    Alternating current scanning electrochemical microscopy (ACSECM) is used to investigate the inhibition of copper corrosion by four potential inhibitors, namely benzotriazole (BTAH), 5-methyl-benzotriazole (MBTAH), 2-mercaptobenzimidazole (MBI), and ethyl xanthate (EX). It is shown that the formation of inhibitor films on the metal can be followed from the decrease of the substrate's local electrochemical activity associated with the formation of inhibitor-containing surface layers. Sensitive imaging can be performed that accounts for changes in electrochemical activity of the modified surfaces, as well as for their corrosive attack from the environment. AC-SECM is shown to be a powerful technique for the investigation of corrosion processes. © 2012 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cplu.201200091
  • 2012 • 53 Investigation of reversible plasticity in a micron-sized, single crystalline copper bending beam by X-ray μlaue diffraction
    Kirchlechner, C. and Grosinger, W. and Kapp, M.W. and Imrich, P.J. and Micha, J.-S. and Ulrich, O. and Keckes, J. and Dehm, G. and Motz, C.
    Philosophical Magazine 92 3231-3242 (2012)
    The observed mechanical behaviour of micron-sized samples raises fundamental questions about the influence of size on the underlying dislocation plasticity. In situ μLaue diffraction on a single crystalline copper bending beam was performed to study the feasibility of bending tests and their contribution to our understanding of size-dependent dislocation plasticity. Theoretical considerations lead to a minimum sample size where in situ Laue experiments are useable. A critical size is evidenced below which, depending on Young's modulus and maximum stress, the elastic and plastic contributions to the lattice curvature cannot be separated. The experiment shows the increase in geometrically necessary dislocations during plastic deformation followed by a decrease during unloading. This can be explained by the formation and dissolution of a dislocation pile-up at the neutral axis of the bending cantilever. The dissolution of the dislocation pile-up is caused by the back stress of the pile-up and a direct observation of the Bauschinger effect, which is consistent with the non-purely elastic mechanical behaviour when unloading the sample. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.669067
  • 2012 • 52 Microtomographic analysis of splat formation and layer build-up of a thermally sprayed coating
    Tillmann, W. and Nellesen, J. and Abdulgader, M.
    Journal of Thermal Spray Technology 21 514-521 (2012)
    Thermal spraying is a material processing technique, which is based on the combination of thermal and kinetic energy. The used feedstock is melted in a hot flame. The melt is atomized and accelerated by means of atomization or process gases. As the formed particles hit a pre-treated substrate they are rapidly solidified and consolidate to form splats. The splats pile one-on-top-of-other forming lamellas creating the final coating. In the work presented here a combination of cored wire (WC as filling powder) and massive wire (copper) were simultaneously sprayed using the twin wire arc spraying process. 3D micro tomography was used in order to gain knowledge about splat formation and layer build-up. Due to the high attenuation coefficient of tungsten in comparison with copper and carbon, tungsten-rich particles and splats can easily be spotted in the tomogram of the coating layer. It turns out that besides irregular formed flat splats also ball-shaped particles exist in the coating layer which suggests that the spherical particles impacted on the substrate in an un-molten state. By 3D data processing tungsten-rich particles were visualized to analyze their spatial distributions and to quantify their geometric parameters. This work aims at contributing to the understanding of spraying processes. © ASM International.
    view abstractdoi: 10.1007/s11666-012-9737-2
  • 2012 • 51 Microwave-hydrothermal synthesis and characterization of nanostructured copper substituted ZnM2O4 (M = Al, Ga) spinels as precursors for thermally stable Cu catalysts
    Conrad, F. and Massue, C. and Kühl, S. and Kunkes, E. and Girgsdies, F. and Kasatkin, I. and Zhang, B. and Friedrich, M. and Luo, Y. and Armbrüster, M. and Patzke, G.R. and Behrens, M.
    Nanoscale 4 2018-2028 (2012)
    Nanostructured Cu<inf>x</inf>Zn<inf>1-x</inf>Al<inf>2</inf>O<inf>4</inf> with a Cu:Zn ratio of: has been prepared by a microwave-assisted hydrothermal synthesis at 150°C and used as a precursor for Cu/ZnO/Al<inf>2</inf>O <inf>3</inf>-based catalysts. The spinel nanoparticles exhibit an average size of approximately 5 nm and a high specific surface area (above 250 m2 g-1). Cu nanoparticles of an average size of 3.3 nm can be formed by reduction of the spinel precursor in hydrogen and the accessible metallic Cu(0) surface area of the reduced catalyst was 8 m2 g-1. The catalytic performance of the material in CO<inf>2</inf> hydrogenation and methanol steam reforming was compared with conventionally prepared Cu/ZnO/Al<inf>2</inf>O<inf>3</inf> reference catalysts. The observed lower performance of the spinel-based samples is attributed to a lack of synergetic interaction of the Cu nanoparticles with ZnO due to the incorporation of Zn 2+ in the stable spinel lattice. Despite its lower performance, however, the nanostructured nature of the spinel catalyst was stable after thermal treatment up to 500°C in contrast to other Cu-based catalysts. Furthermore, a large fraction of the re-oxidized copper migrates back into the spinel upon calcination of the reduced catalyst, thereby enabling a regeneration of sintered catalysts after prolonged usage at high temperatures. Similarly prepared samples with Ga instead of Al exhibit a more crystalline catalyst with a spinel particle size around 20 nm. The slightly decreased Cu(0) surface area of 3.2 m2 g-1 due to less copper incorporation is not a significant drawback for the methanol steam reforming. © The Royal Society of Chemistry 2012.
    view abstractdoi: 10.1039/c2nr11804a
  • 2012 • 50 Non-crystallographic shear banding in crystal plasticity FEM simulations: Example of texture evolution in α-brass
    Jia, N. and Roters, F. and Eisenlohr, P. and Kords, C. and Raabe, D.
    Acta Materialia 60 1099-1115 (2012)
    We present crystal plasticity finite element simulations of the texture evolution in α-brass polycrystals under plane strain compression. The novelty is a non-crystallographic shear band mechanism [Anand L, Su C. J Mech Phys Solids 2005;53:1362] that is incorporated into the constitutive model in addition to dislocation and twinning. Non-crystallographic deformation associated with shear banding leads to weaker copper and S texture components and to a stronger brass texture compared to simulations enabling slip and twinning only. The lattice rotation rates are reduced when shear banding occurs. This effect leads to a weaker copper component. Also, the initiation of shear banding promotes brass-type components. In summary the occurrence of non-crystallographic deformation through shear bands shifts face-centered-cubic deformation textures from the copper type to the brass type. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.10.047
  • 2012 • 49 Non-syngas direct steam reforming of methanol to hydrogen and carbon dioxide at low temperature
    Yu, K.M.K. and Tong, W. and West, A. and Cheung, K. and Li, T. and Smith, G. and Guo, Y. and Tsang, S.C.E.
    Nature Communications 3 (2012)
    A non-syngas direct steam reforming route is investigated for the conversion of methanol to hydrogen and carbon dioxide over a CuZnGaOx catalyst at 150-200 °C. This route is in marked contrast with the conventional complex route involving steam reformation to syngas (CO/H 2) at high temperature, followed by water gas shift and CO cleanup stages for hydrogen production. Here we report that high quality hydrogen and carbon dioxide can be produced in a single-step reaction over the catalyst, with no detectable CO (below detection limit of 1 ppm). This can be used to supply proton exchange membrane fuel cells for mobile applications without invoking any CO shift and cleanup stages. The working catalyst contains, on average, 3-4 nm copper particles, alongside extremely small size of copper clusters stabilized on a defective ZnGa2O4 spinel oxide surface, providing hydrogen productivity of 393.6 ml g-1-cat h-1 at 150 °C. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms2242
  • 2012 • 48 Novel temperature dependent tensile test of freestanding copper thin film structures
    Smolka, M. and Motz, C. and Detzel, T. and Robl, W. and Griesser, T. and Wimmer, A. and Dehm, G.
    Review of Scientific Instruments 83 (2012)
    The temperature dependent mechanical properties of the metallization of electronic power devices are studied in tensile tests on micron-sized freestanding copper beams at temperatures up to 400 °C. The experiments are performed in situ in a scanning electron microscope. This allows studying the micromechanical processes during the deformation and failure of the sample at different temperatures. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4725529
  • 2012 • 47 On the effect of grain boundary segregation on creep and creep rupture
    Otto, F. and Viswanathan, G.B. and Payton, E.J. and Frenzel, J. and Eggeler, G.
    Acta Materialia 60 2982-2998 (2012)
    The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu-0.008 wt.% Bi and Cu-0.92 wt.% Sb at stresses ranging from 10 to 20 MPa and temperatures between 773 and 873 K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu-Bi) and high apparent activation energies for creep (Cu-Bi and Cu-Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.02.004
  • 2012 • 46 On the limits of the interfacial yield model for fragmentation testing of brittle films on polymer substrates
    Taylor, A.A. and Cordill, M.J. and Dehm, G.
    Philosophical Magazine 92 3363-3380 (2012)
    Fragmentation testing is frequently used to probe film fracture strain and the interfacial properties of thin brittle films on compliant substrates. A model based upon complete yield of the film/substrate interface is frequently used to analyse data after cracking has saturated. Additionally, the film is either assumed to have a single-valued failure stress or a distribution of strengths described by Weibull statistics. Recent work by the authors showed that consideration of film thickness variations and the application of neighbour ratio analysis brought 96% of the data for an Al x O y /Cu film/substrate system into compliance with the predictions for a film with a single-valued failure stress. In the present work Cr/PI (polyimide) and Cr/PET (polyethylene teraphthalate) systems are analysed according to the same methodology. The Cr films on polymer substrates crack such that the neighbour ratios considerably exceed the predicted limit of 2. The influence of the relative thickness of the film and substrate and the strain rate of the test is investigated. A deviation from the idealised mechanical model due to the large difference in elastic moduli of film and substrate is put forward as a possible cause of the observed behaviour. The importance of these results to the application of the interfacial yield model is discussed. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.723145
  • 2012 • 45 Orientation dependence of shear banding in face-centered-cubic single crystals
    Jia, N. and Eisenlohr, P. and Roters, F. and Raabe, D. and Zhao, X.
    Acta Materialia 60 3415-3434 (2012)
    We present crystal plasticity finite element simulations of plane strain compression of α-Brass single crystals with different initial orientations. The aim is to study the fundamentals of mesoscale structure and texture development in face-centered-cubic (fcc) metals with low stacking fault energy (SFE). Shear banding depends on the initial orientation of the crystals. In Copper and Brass-R-oriented crystals which show the largest tendency to form shear bands, an inhomogeneous texture distribution induced by shear banding is observed. To also understand the influence of the micromechanical boundary conditions on shear band formation, simulations on Copper-oriented single crystals with varying sample geometry and loading conditions are performed. We find that shear banding can be understood in terms of a mesoscopic softening mechanism. The predicted local textures and the shear banding patterns agree well with experimental observations in low SFE fcc crystals. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.03.005
  • 2012 • 44 Sorption profile of Hg(II) onto mixed phase of copper sulphide and copper sulphate
    Yoong, Y.L.A. and Yap, P.L. and Kutty, M.G. and Timpe, O. and Behrens, M. and Hamid, S.B.A. and Schlögl, R.
    Advanced Materials Research 356-360 537-546 (2012)
    The use of surface oxidized covellite (CuS), namely mixed phase copper sulphide (CuS and CuSO4) was studied for the removal of mercury from aqueous solution under the effect of various reaction parameters (pH, time, Hg(II) concentration). From batch sorption studies, the equilibrium data revealed that the sorption behaviour of Hg(II) onto mixed phase copper sulphide follows well with Langmuir isotherm and the maximum sorption capacity (Q max) determined π 400mg Hg(II) /g of sorbent. Meanwhile, all the unreacted and reacted mixed phase copper sulphides were also characterized by Powder XRD, SEM and XPS techniques. The results indicated that the sorption of Hg(II) onto mixed phase copper sulphide occurs initially through the dissolution of surface oxidized CuSO4 layer. After that, the surface complexation product formed and sorbed onto the surface of CuS. These outcomes suggest the potential ability of CuS in removing Hg(II) even if the CuS layer is being surrounded by oxidized layer of CuSO4. © (2012) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2012 • 43 The active site of methanol synthesis over Cu/ZnO/Al2O 3 industrial catalysts
    Behrens, M. and Studt, F. and Kasatkin, I. and Kühl, S. and Hävecker, M. and Abild-Pedersen, F. and Zander, S. and Girgsdies, F. and Kurr, P. and Kniep, B.-L. and Tovar, M. and Fischer, R.W. and Nørskov, J.K. and Schlögl, R.
    Science 336 893-897 (2012)
    One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al2O 3methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.
    view abstractdoi: 10.1126/science.1219831
  • 2012 • 42 The biocompatibility of metal-organic framework coatings: An investigation on the stability of SURMOFs with regard to water and selected cell culture media
    Hanke, M. and Arslan, H.K. and Bauer, S. and Zybaylo, O. and Christophis, C. and Gliemann, H. and Rosenhahn, A. and Wöll, C.
    Langmuir 28 6877-6884 (2012)
    Highly porous thin films based on a [Cu(bdc) 2] n (bdc = benzene-1,4-dicarboxylic acid) metal-organic framework, MOF, grown using liquid-phase epitaxy (LPE) show remarkable stability in pure water as well as in artificial seawater. This opens the possibility to use these highly porous coatings for environmental and life science applications. Here we characterize in detail the stability of these SURMOF 2 thin films under aqueous and cell culture conditions. We find that the material degrades only very slowly in water and artificial seawater (ASW) whereas in typical cell culture media (PBS and DMEM) a rapid dissolution is observed. The release of Cu 2+ ions resulting from the dissolution of the SURMOF 2 in the liquids exhibits no adverse effect on the adhesion of fibroblasts, prototype eukaryotic cells, to the substrate and their subsequent proliferation, thus demonstrating the biocompatibility of SURMOF 2 surface coatings. Thus, the results are an important step toward application of these porous materials as a slow release matrix, for example, for pharmaceuticals and growth factors. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la300457z
  • 2012 • 41 Thickness-dependence of the B2-B19 martensitic transformation in nanoscale shape memory alloy thin films: Zero-hysteresis in 75 nm thick Ti 51Ni38Cu11 thin films
    König, D. and Buenconsejo, P.J.S. and Grochla, D. and Hamann, S. and Pfetzing-Micklich, J. and Ludwig, Al.
    Acta Materialia 60 306-313 (2012)
    The influence of film thickness on the B2-B19 martensitic transformation properties of nanoscale Ti51Ni38Cu11 thin films with thicknesses ranging from 750 to 50 nm is reported. For these films an unexpected behavior of the phase transformation temperatures was observed: Af and Os initially decrease with decreasing film thickness but increase sharply again for thicknesses < 100 nm. The phase transformation temperatures and thermal hysteresis width range from 58 to 35 °C (Af) and 14 to ∼0 K, respectively. For the first time we can show that substrate-attached Ti-Ni-Cu thin films as thin as 50 nm show reversible B2-B19 phase transformations. Furthermore, it is shown that with decreasing film thickness a change in the tetragonality of the B19 martensite phase occurs. This leads to fulfilling the so-called λ2 criterion, causing a vanishing hysteresis for a film thickness of 75 nm. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.09.037
  • 2012 • 40 Thin-film Cu-Pt(111) near-surface alloys: Active electrocatalysts for the oxygen reduction reaction
    Henry, J.B. and Maljusch, A. and Huang, M. and Schuhmann, W. and Bondarenko, A.S.
    ACS Catalysis 2 1457-1460 (2012)
    A simple method is presented for the formation of thin films of Cu-Pt(111) near-surface alloys (NSA). In these thin films, the solute metal (Cu) is preferentially located in the second platinum layer and protected by a Pt surface layer. The NSA-films act as active and fairly stable electrocatalysts for the reduction of oxygen with the activity and stability which approach those for bulk single crystalline Pt-alloy surfaces and ∼5 times more active than state-of-the-art Pt thin films. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs300165t
  • 2012 • 39 Yield stress influenced by the ratio of wire diameter to grain size - A competition between the effects of specimen microstructure and dimension in micro-sized polycrystalline copper wires
    Yang, B. and Motz, C. and Rester, M. and Dehm, G.
    Philosophical Magazine 92 3243-3256 (2012)
    Polycrystalline copper wires with diameters of 25, 30 and 50m were annealed at temperatures between 200°C and 900°C, resulting in different microstructures with ratios of wire diameter to grain size between 1.1 and 15.6. The microstructure evolution and tensile behavior were studied systematically. In comparison with experimental data available in the literature, the results revealed that the tensile yield stresses of these micro-sized wires are influenced not only by the grain size but also by the ratio of wire diameter to grain size. This is clearly seen when comparing identical grain sizes but different wire diameters where thinner wires reveal smaller flow stress values. A model is proposed to explain the smaller is softer phenomenon, taking into account the higher strengthening effect of grain boundaries compared to the free surface. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.693215
  • 2011 • 38 An electron energy loss spectrometer designed for studies of electronic energy losses and spin waves in the large momentum regime
    Ibach, H. and Rajeswari, J. and Schneider, C.M.
    Review of Scientific Instruments 82 (2011)
    Based on 143° electrostatic deflectors we have realized a new spectrometer for electron energy loss spectroscopy which is particularly suitable for studies on surface spin waves and other low energy electronic energy losses. Contrary to previous designs high resolution is maintained even for diffuse inelastic scattering due to a specific management of the angular aberrations in combination with an angle aperture. The performance of the instrument is demonstrated with high resolution energy loss spectra of surface spin waves on a cobalt film deposited on the Cu(100) surface. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3670731
  • 2011 • 37 Application of AC-SECM in corrosion science: Local visualisation of inhibitor films on active metals for corrosion protection
    Pähler, M. and Santana, J.J. and Schuhmann, W. and Souto, R.M.
    Chemistry - A European Journal 17 905-911 (2011)
    The suitability of frequency-dependent alternating-current scanning electrochemical microscopy (4D AC-SECM) for investigation of thin passivating layers covering the surface of corrosion-inhibited metals has been demonstrated. Inhibition of copper corrosion by benzotriazole (BTAH) and methylbenzotriazole (MBTAH), which are effective inhibitors for this metal under many environmental conditions, was investigated. Strong dependencies were found for the AC z-approach curves with both the duration of the inhibitor treatment and the frequency of the AC excitation signal applied in AC-SECM. Both negative and positive feedback behaviours were observed in the AC approach curves for untreated copper and for Cu/BTAH and Cu/MBTAH samples. Negative feedback behaviour occurred in the low-frequency range, whereas a positive feedback effect was observed at higher frequencies. A threshold frequency related to the passage from negative to positive regimes could be determined in each case. The threshold frequency for inhibitor-modified samples was found always to be significantly higher than for the untreated metal, because the inhibitor film provides electrical insulation for the surface. Moreover, the threshold frequency increased with increasing surface coverage by the inhibitor. 4D AC-SECM was successfully applied to visualizing spatially resolved differences in local electrochemical activity between inhibitor-free and inhibitor-covered areas of the sample. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201000689
  • 2011 • 36 Atomic-scale distribution of impurities in cuinse2-based thin-film solar cells
    Cojocaru-Miredin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Ultramicroscopy 111 552-556 (2011)
    Atom Probe Tomography was employed to investigate the distribution of impurities, in particular sodium and oxygen, in a cuinse2-based thin-film solar cell. It could be shown that sodium, oxygen, and silicon diffuse from the soda lime glass substrate into the cuinse2 film and accumulate at the grain boundaries. Highly dilute concentrations of sodium and oxygen were measured in the bulk. Selenium was found to be depleted at the grain boundaries. These observations could be confirmed by complementary energy dispersive X-ray spectroscopy studies. Our results support the model proposed by Kronik et al. (1998) [1], which explains the enhanced photovoltaic efficiency of sodium containing cuinse2 solar cells by the passivation of selenium vacancies at grain boundaries. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.034
  • 2011 • 35 Characterization of grain boundaries in Cu(In,Ga)Se 2 films using atom-probe tomography
    Cojocaru-Mirédin, O. and Choi, P.-P. and Abou-Ras, D. and Schmidt, S.S. and Caballero, R. and Raabe, D.
    IEEE Journal of Photovoltaics 1 207-212 (2011)
    This paper discusses the advantages of pulsed laser atom-probe tomography (APT) to analyze Cu(In,Ga)Se 2-based solar cells. Electron backscatter diffraction (EBSD) was exploited for site-specific preparation of APT samples at selected Cu(In,Ga)Se 2 grain boundaries. This approach is very helpful not only to determine the location of grain boundaries but also to classify them as well. We demonstrate that correlative transmission electron microscopy (TEM) analyses on atom-probe specimens enable the atom-probe datasets to be reconstructed with high accuracy. Moreover, EBSD and TEM can be very useful to obtain complementary information about the crystal structure in addition to the compositional analyses. The local chemical compositions at grain boundaries of a solar grade Cu(In,Ga)Se 2 film are presented here. Na, K, and O impurities are found to be segregated at grain boundaries. These impurities most likely diffuse from the soda lime glass substrate into the absorber layer during cell fabrication and processing. Based on the experimental results, we propose that Na, K, and O play an important role in the electrical properties of grain boundaries in Cu(In,Ga)Se 2 thin films for solar cells. © 2011 IEEE.
    view abstractdoi: 10.1109/JPHOTOV.2011.2170447
  • 2011 • 34 Comparative atom probe study of Cu(In,Ga)Se 2 thin-film solar cells deposited on soda-lime glass and mild steel substrates
    Choi, P.-P. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Journal of Applied Physics 110 (2011)
    We report on a comparative study of Cu(In,Ga)Se 2 solar cells deposited on soda-lime glass and mild steel substrates, using atom probe tomography in conjunction with secondary ion mass spectrometry, x-ray fluorescence, current density-voltage, and external quantum efficiency measurements. Cu(In,Ga)Se 2 films deposited on soda-lime glass substrates and on steel substrates with a NaF precursor layer on top of the Mo back contact contain a significant amount of Na impurities and yield an enhanced open circuit voltage and fill factor. Using atom probe tomography, Na atoms are found to be segregated at grain boundaries and clustered in both bulk and grain boundaries. The atom probe data indicate that Na Cu point defects are most likely formed at grain boundaries, reducing the number of compensating In Cu point defects and thus contributing to an enhanced cell efficiency. However, for steel substrates the positive effect of Na on the cell performance is counterbalanced by the incorporation of Fe impurities into the Cu(In,Ga)Se 2 film. Fe atoms are homogeneously distributed inside the grains suggesting that Fe introduces point defects in the bulk © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3665723
  • 2011 • 33 Deformation resistance in the transition from coarse-grained to ultrafine-grained Cu by severe plastic deformation up to 24 passes of ECAP
    Blum, W. and Li, Y.J. and Zhang, Y. and Wang, J.T.
    Materials Science and Engineering A 528 8621-8627 (2011)
    Pure Cu was subjected to severe plastic predeformation by p=1, 2, 4, 8, 16 and 24 passes of equal channel angular pressing (ECAP) on route BC at ambient temperature and subsequently tested in uniaxial compression parallel to the extrusion direction at constant rate or constant stress and temperatures from ambient temperature up to 418K. The maximum compressive strength of the ECAPed Cu varies in a systematic fashion with p, until a steady state is finally reached between p=8 and 16 where the rate sensitivity of flow stress is maximal. The results are quantitatively interpreted in terms of the boundary structure, considering the superposition of hardening due to refinement of low-angle boundaries and softening due to enhanced thermal recovery at high-angle boundaries. Beyond the maximum the compressive strength declines with strain for relatively low rate and/or elevated temperature of compression. This is explained by dynamic grain coarsening towards the new steady state developing in compression. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.08.010
  • 2011 • 32 High-throughput characterization of mechanical properties of Ti-Ni-Cu shape memory thin films at elevated temperature
    Zarnetta, R. and Kneip, S. and Somsen, C. and Ludwig, Al.
    Materials Science and Engineering A 528 6552-6557 (2011)
    Hardness and Young's moduli values for TixNi90-xCu10 (37at.%< x< 67at.%) thin films from a continuous composition spread type materials library, annealed at 500°C for 1h, were determined at room temperature (martensitic state) and 80°C (austenitic state) using high-throughput nanoindentation experiments. These values are found to increase as the compositions deviate from Ti contents close to 50at.%. The increases in hardness is correlated to the presence of Ti-rich and (Ni,Cu)-rich precipitates resulting in precipitate hardening and grain size refinement (Hall-Petch effect). The increase of the Young's moduli is rationalized by considering the significantly higher Young's moduli of the different precipitate phases and applying the rule of mixtures. The contributions of the precipitate phases and the matrix to the combined Young's modulus were estimated by evaluating the load-displacement curves in detail. The obtained results are in good agreement with the Young's moduli determined from thin film curvature measurements [R. Zarnetta et al., Smart Mater. Struct. 19 (2010) 65032]. Thus, the experimental restrictions for nanoindentation experiments at elevated temperatures are concluded to not adversely affect the validity of the results. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.05.006
  • 2011 • 31 Indirect magnetic coupling of manganese porphyrin to a ferromagnetic cobalt substrate
    Chylarecka, D. and Kim, T.K. and Tarafder, K. and Müller, K. and Gödel, K. and Czekaj, I. and Wäckerlin, C. and Cinchetti, M. and Ali, Md.E. and Piamonteze, C. and Schmitt, F. and Wüstenberg, J.-P. and Ziegler, C. and Nolting,...
    115 1295-1301 (2011)
    The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106822s
  • 2011 • 30 Induced magnetic Cu moments and magnetic ordering in Cu2MnAl thin films on MgO(0 0 1) observed by XMCD
    Krumme, B. and Herper, H.C. and Erb, D. and Weis, C. and Antoniak, C. and Warland, A. and Westerholt, K. and Entel, P. and Wende, H.
    Journal of Physics D: Applied Physics 44 (2011)
    The disorder-order transition of a highly defective A2-ordered Cu 2MnAl film on MgO(0 0 1) upon annealing at 600 K was monitored by means of x-ray absorption spectroscopy (XAS) at the Cu and Mn L2,3 edges. Additionally, x-ray magnetic circular dichroism (XMCD) was employed to determine element-specific orbital and spin resolved magnetic moments of the Cu and Mn atoms. A small induced total magnetic moment of ≈0.04 0.01μB per atom was detected at the Cu sites, whereas a total magnetic moment of 3.57 0.52μB is carried by the Mn atoms. The experimental XAS and XMCD spectra of Cu agree reasonably with the results from ab initio calculations, magnetic moments derived by the sum rules are in accordance with the calculations. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/41/415004
  • 2011 • 29 Intercalation in layered metal-organic frameworks: Reversible inclusion of an extended π-system
    Arslan, H.K. and Shekhah, O. and Wieland, D.C.F. and Paulus, M. and Sternemann, C. and Schroer, M.A. and Tiemeyer, S. and Tolan, M. and Fischer, R.A. and Wöll, C.
    Journal of the American Chemical Society 133 8158-8161 (2011)
    We report the synthesis of layered [Zn 2(bdc) 2(H 2O) 2] and [Cu 2(bdc) 2(H 2O) 2] (bdc = benzdicarboxylate) metal-organic frameworks (MOF) carried out using the liquid-phase epitaxy approach employing self-assembled monolayer (SAM) modified Au-substrates. We obtain Cu and Zn MOF-2 structures, which have not yet been obtained using conventional, solvothermal synthesis methods. The 2D Cu 2+ dimer paddle wheel planes characteristic for the MOF are found to be strictly planar, with the planes oriented perpendicular to the substrate. Intercalation of an organic dye, DXP, leads to a reversible tilting of the planes, demonstrating the huge potential of these surface-anchored MOFs for the intercalation of large, planar molecules. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja2037996
  • 2011 • 28 Investigation of the spin-dependent properties of electron doped cobalt-CuPc interfaces
    Steil, S. and Goedel, K. and Ruffing, A. and Sarkar, I. and Cinchetti, M. and Aeschlimann, M.
    161 570-574 (2011)
    We have grown metal-organic interfaces by in-situ deposition of ultrathin copper phthalocyanine (CuPc) films on a thin cobalt film on Cu(0 0 1). Evidence for layer-by-layer growth is found. The spin-dependent electronic properties of the Co-CuPc interface and their modification under caesium doping are investigated by spin-resolved photoemission spectroscopy. We observe a doping-induced shift of the highest occupied molecular orbital (HOMO) of CuPc away from the Fermi level (EF), accompanied by the formation of an unpolarised gap-state at 0.7 eV below EF in the high doping regime. Such features are reflected in the behaviour of the detected interfacial spin-polarisation. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.synthmet.2010.11.031
  • 2011 • 27 Light driven reactions of single physisorbed azobenzenes
    Bazarnik, M. and Henzl, J. and Czajka, R. and Morgenstern, K.
    Chemical Communications 47 7764-7766 (2011)
    We present a successful attempt of decoupling a dye molecule from a metallic surface via physisorption for enabling direct photoisomerization. Effective switching between the isomers is possible by exposure to UV light via the rotation pathway. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1cc11578b
  • 2011 • 26 Metal ion release kinetics from nanoparticle silicone composites
    Hahn, A. and Brandes, G. and Wagener, P. and Barcikowski, S.
    Journal of Controlled Release 154 164-170 (2011)
    Metal ion release kinetics from silver and copper nanoparticle silicone composites generated by laser ablation in liquids are investigated. The metal ion transport mechanism is studied by using different model equations and their fit to experimental data. Results indicate that during the first 30 days of immersion, Fickian diffusion is the dominant transport mechanism. After this time period, the oxidation and dissolution of nanoparticles from the bulk determine the ion release. This second mechanism is very slow since the dissolution of the nanoparticle is found to be anisotropic. Silver ion release profile is best described by pseudo-first order exponential equation. Copper ion release profile is best described by a second order exponential equation. For practical purposes, the in vitro release characteristics of the bioactive metal ions are evaluated as a function of nanoparticle loading density, the chemistry and the texture of the silicone. Based on the proposed two-step release model, a prediction of the release characteristics over a time course of 84 days is possible and a long-term ion release could be demonstrated. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jconrel.2011.05.023
  • 2011 • 25 Microstructure and adhesion of as-deposited and annealed Cu/Ti films on polyimide
    Cordill, M.J. and Taylor, A. and Schalko, J. and Dehm, G.
    International Journal of Materials Research 102 729-734 (2011)
    The ability to measure the adhesion energy of metal thin films on polymer substrates is important for the design of reliable flexible electronic devices. One technique is to create well-defined areas of delamination (buckles) as a consequence of lateral compressive stresses induced by tensile straining of the film-substrate system. The adhesion energy is calculated from the buckle dimensions. In order to improve the adhesion between the metal film and polymer substrate, thin adhesion layers can be incorporated. However, interdiffusion and reactions can occur between the adhesion layer and the metal film when subjected to elevated temperatures. This is detrimental for the interfacial adhesion, as will be discussed for Cu films on polyimide with a Ti interlayer subjected to annealing at 350°C. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110513
  • 2011 • 24 Non-invasive nano-imaging of ion implanted and activated copper in silicon
    Ballout, F. and Samson, J.-S. and Schmidt, D.A. and Bründermann, E. and Mathis, Y.-L. and Gasharova, B. and Wieck, A. and Havenith, M.
    Journal of Applied Physics 110 (2011)
    Using vibrational imaging techniques including Fourier-transform infrared (FTIR) synchrotron microscopy, Raman microscopy, and scattering scanning near-field infrared microcscopy (s-SNIM), we mapped a sample of phosphor and copper ions implanted in a high-purity silicon wafer. While Raman microscopy monitors the structural disorder within the implantation fields, the aforementionedinfrared techniques provide a detailed picture of the distribution of the free carriers. On a large scale (tens of micrometers), we visualized the channeling effects of phosphor dopants in silicon using FTIR microscopy. In comparison, using s-SNIM we were able to image, on a nanometer scale, local variations of the dielectric properties of the silicon substrate due to the activation of copper dopants. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3606415
  • 2011 • 23 On the evolution of microstructure in oxygen-free high conductivity copper during thermomechanical processing using rotary swaging
    Otto, F. and Frenzel, J. and Eggeler, G.
    International Journal of Materials Research 102 363-370 (2011)
    In the present work, the processing parameters which govern the evolution of microstructure during rotary swaging and intermediate/subsequent heat treatments in copper rods were studied. Copper ingots with an initial diameter of 40 mm were reduced to a final diameter of 11.7 mm by rotary swaging. Processing sequences were applied with different intermediate anneals and various final heat treatments. The resulting microstructures were characterized using orientation imaging microscopy, optical microscopy and hardness measurements. Special emphasis was placed on the evolution of microstructure with respect to the radial and longitudinal position in the rod. Most importantly, microstructural evidence for torsional loading during swaging was found, and a spiral grain morphology was observed. Moreover, localized deformation events were identified and evidence for abnormal grain growth was found. Finally, a combination of swaging and heat treatment parameters was identified which allowed a homogeneous grain structure to be produced. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110501
  • 2011 • 22 On the influence of small quantities of Bi and Sb on the evolution of microstructure during swaging and heat treatments in copper
    Otto, F. and Frenzel, J. and Eggeler, G.
    Journal of Alloys and Compounds 509 4073-4080 (2011)
    In the present work, the influence of small amounts of Bi and Sb on the microstructural evolution of Cu during an ingot metallurgy processing route is investigated. Both elements are known to segregate to grain boundaries in Cu. Cu ingots with an outer diameter of 40 mm containing 0.008 wt.% Bi and 0.92 wt.% Sb, respectively, were vacuum induction melted, cast, and gradually swaged down to a final diameter of 11.7 mm with several intermediate annealing steps. Subsequent annealing treatments were conducted to investigate the microstructural evolution of the swaged bars. Optical microscopy, hardness testing and orientation imaging microscopy were used to characterize the deformation and recrystallization behavior, as well as the evolution of texture in the alloys. The results are then compared to those obtained for pure Cu. It is shown that even small amounts of alloying elements significantly alter the hardening behavior and suppress recrystallization at low temperatures. At higher temperatures, recrystallization in Cu, Cu-Bi and Cu-Sb leads to different textures. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2010.12.178
  • 2011 • 21 Stepwise deposition of metal organic frameworks on flexible synthetic polymer surfaces
    Meilikhov, M. and Yusenko, K. and Schollmeyer, E. and Mayer, C. and Buschmann, H.-J. and Fischer, R.A.
    Dalton Transactions 40 4838-4841 (2011)
    Thin films of [Cu3(btc)2]n (btc = 1,3,5-benzenetricarboxylate) metal organic framework were deposited in a stepwise manner on surfaces of flexible organic polymers. The thickness of films can be precisely controlled. The deposition of the first cycles was monitored by UV-vis spectroscopy. The porosity was proven by the adsorption of pyrazine, which was monitored by FT-IR and thermogravimetric analysis. The deposition of MOF thin films on flexible polymer surfaces might be a new path for the fabrication of functional materials for different applications, such as protection layers for working clothes and gas separation materials in the textile industry. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0dt01820a
  • 2011 • 20 The effect of film thickness variations in periodic cracking: Analysis and experiments
    Taylor, A.A. and Edlmayr, V. and Cordill, M.J. and Dehm, G.
    Surface and Coatings Technology 206 1830-1836 (2011)
    Periodic cracking experiments are frequently used in the assessment of interface quality in brittle film/compliant substrate systems. Through these techniques it is possible to extract a quantitative measure of interface shear strength and therefore assess the mechanical suitability of the films for application. The influence of film thickness inhomogeneities on the crack spacing is assessed in this study. While film thickness inhomogeneities are always present in thin film systems, only nominal thickness values have been considered up to this point. By defining two separate regimes of film thickness variation, roughness and unevenness, in relation to the crack spacing, the influence of such variation on the data is analysed. The results of this analysis are then considered in reference to a model system of an amorphous alumina film on a copper substrate (AlxOy/Cu), the limits of this analysis are then discussed. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2011.07.047
  • 2011 • 19 The effect of temperature and strain rate on the periodic cracking of amorphous AlxOy films on Cu
    Taylor, A.A. and Edlmayr, V. and Cordill, M.J. and Dehm, G.
    Surface and Coatings Technology 206 1855-1859 (2011)
    The high temperature properties of metal/ceramic interfaces play an important role in the operation of microprocessors and coated products. Determination of the interface properties over a range of testing conditions is critical in understanding and improving the performance of such systems. Periodic cracking of ceramic films on metal substrates provides a direct measure of the interface strength. A model AlxOy/Cu system is investigated over temperatures of 25-650°C and at strain rates of 3.5{dot operator}10-2s-1 and 1.7{dot operator}10-5s-1. For this system it is found that temperature does not significantly affect the spacing of film cracks in the steady state, at a given strain rate. However, the high strain rate tests broaden the measured crack spacing distributions compared to the low strain rate tests. This broadening causes an increase in the average crack spacing at high strain rate. The minimum crack spacing was increased 15-20% by both increasing strain rate and increasing temperature. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2011.08.011
  • 2011 • 18 Transient (000)-order attenuation effects in ultrafast transmission electron diffraction
    Ligges, M. and Rajkovi, I. and Streubhr, C. and Brazda, T. and Zhou, P. and Posth, O. and Hassel, C. and Dumpich, G. and Von Der Linde, D.
    Journal of Applied Physics 109 (2011)
    We discuss the observation of a transient (000)-order attenuation in time-resolved transmission electron diffraction experiments. It is shown that this effect causes a decrease of the diffraction intensity of all higher diffraction orders. This effect is not unique to specific materials as it was observed in thin Au, Ag and Cu films. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3554405
  • 2010 • 17 An in situ tensile tester for studying electrochemical repassivation behavior: Fabrication and challenges
    Neelakantan, L. and Schönberger, B. and Eggeler, G. and Hassel, A.W.
    Review of Scientific Instruments 81 (2010)
    An in situ tensile rig is proposed, which allows performing electrochemical (repassivation) experiments during dynamic mechanical testing of wires. Utilizing the basic components of a conventional tensile tester, a custom-made minitensile rig was designed and fabricated. The maximal force that can be measured by the force sensor is 80 N, with a sensitivity of 0.5 mV/V. The maximum travel range of the crosshead induced by the motor is 10 mm with a minimum step size of 0.5 nm. The functionality of the tensile test rig was validated by investigating Cu and shape memory NiTi wires. Wires of lengths between 40 and 50 mm with varying gauge lengths can be tested. An interface between wire and electrochemical setup (noncontact) with a smart arrangement of electrodes facilitated the electrochemical measurements during tensile loading. Preliminary results on the repassivation behavior of Al wire are reported. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3292685
  • 2010 • 16 Cyclic loading behavior of micro-sized polycrystalline copper wires
    Yang, B. and Motz, C. and Grosinger, W. and Dehm, G.
    Procedia Engineering 2 925-930 (2010)
    Micro-sized polycrystalline copper wires of diameters ranging from 5 μm to 36 μm were cyclically loaded under stress-control in many steps with increasing the applied stress amplitude after a saturation state in plastic strain is reached. It is observed that the thicker wires show smaller plastic strain at saturation as well as smaller creep strain compared to the thinner wires. The results were discussed combined with the monotonic tensile results of these micro-sized wires. © 2010 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2010.03.100
  • 2010 • 15 Fracture and delamination of chromium thin films on polymer substrates
    Cordill, M.J. and Taylor, A. and Schalko, J. and Dehm, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 870-875 (2010)
    New emerging technologies in the field of flexible electronic devices require that metal films adhere well and flex with polymer substrates. Common thin film materials used for these applications include copper (Cu) with an adhesion interlayer of chromium (Cr). Copper can be quite ductile and easily move with the polymer substrate. However, Cr is more brittle and fractures at lower strains than Cu. This study aims to examine the fracture and subsequent buckling and delamination of strained Cr films on polyimide (PI). In-situ scanning electron microscope (SEM) straining is used to systematically study the influence of film thickness on fracture and buckling strains. Film fracture and delamination depend on film thickness, and increases in crack and buckle density with decreasing thickness are explored by a shear lag model. © 2009 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-009-9988-9
  • 2010 • 14 Identification of optimized Ti-Ni-Cu shape memory alloy compositions for high-frequency thin film microactuator applications
    Zarnetta, R. and Ehmann, M. and Savan, A. and Ludwig, Al.
    Smart Materials and Structures 19 (2010)
    Ti-Ni-Cu shape memory thin films within a broad composition range were investigated by the cantilever deflection method using combinatorial methods. Optimal compositions with improved functional properties, i.e.large recovery stress, high transformation temperatures, low thermal hysteresis width and small temperature interval of transformation, were identified using a newly defined figure of merit. Of the investigated alloys, Ti50Ni 41Cu9 and Ti45Ni46Cu9 exhibit the best shape memory properties for compositions showing a B2 → B19 and a B2 → R-phase transformation, respectively. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0964-1726/19/6/065032
  • 2010 • 13 Identification of quaternary shape memory alloys with near-zero thermal hysteresis and unprecedented functional stability
    Zarnetta, R. and Takahashi, R. and Young, M.L. and Savan, A. and Furuya, Y. and Thienhaus, S. and Maaß, B. and Rahim, M. and Frenzel, J. and Brunken, H. and Chu, Y.S. and Srivastava, V. and James, R.D. and Takeuchi, I. and Eggele...
    Advanced Functional Materials 20 1917-1923 (2010)
    Improving the functional stability of shape memory alloys (SMAs), which undergo a reversible martensitic transformation, is critical for their applications and remains a central research theme driving advances in shape memory technology. By using a thin-film composition-spread technique and high-throughput characterization methods, the lattice parameters of quaternary Ti-Ni-Cu-Pd SMAs and the thermal hysteresis are tailored. Novel alloys with near-zero thermal hysteresis, as predicted by the geometric nonlinear theory of martensite, are identified. The thin-film results are successfully transferred to bulk materials and near-zero thermal hysteresis is observed for the phase transformation in bulk alloys using the temperaturedependent alternating current potential drop method. A universal behavior of hysteresis versus the middle eigenvalue of the transformation stretch matrix is observed for different alloy systems. Furthermore, significantly improved functional stability, investigated by thermal cycling using differential scanning calorimetry, is found for the quaternary bulk alloy Ti50.2Ni34.4Cu12.3Pd3.1 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.200902336
  • 2010 • 12 Mechanical and microstructural single-crystal Bauschinger effects: Observation of reversible plasticity in copper during bending
    Demir, E. and Raabe, D.
    Acta Materialia 58 6055-6063 (2010)
    We study the Bauschinger effect on a bent and straightened micro-sized single-crystal copper beam (width: 8.64 lm; thickness: 7.05 lm) over three consecutive cycles. The reverse yield strengths (straightening step) are much smaller than those in forward loading (bending step). An upper bound estimate shows a load drop of 73% (1st cycle), 76% (2nd cycle) and 83% (3rd cycle) relative to the forward yield stress. Electron backscatter diffraction reveals a dramatic reduction in the bending-induced misorientation gradients upon load reversal (straightening), documenting an unexpected form of microstructure reversibility. The observed Bauschinger softening is interpreted in terms of two effects. The first consists of internal backstresses that support load reversal. They are created by polarized dislocation arrays that are accumulated during forward bending. The second effect is the reduced requirement to activate dislocation sources during reverse loading as the dislocations that were stored during bending did not participate much in cross-hardening and, hence, serve as mobile dislocations upon reverse loading. After straightening the misorientation gradients are largely removed but the non-polarized dislocations remain. We therefore introduce a revised terminology, namely the "mechanical Bauschinger effect" and the "microstructural Bauschinger effect". The former term describes a yield stress drop and the latter one the degree of microstructure reversibility upon load path changes. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.023
  • 2010 • 11 Stearate-Based Cu Colloids in Methanol Synthesis: Structural Changes Driven by Strong Metal-Support Interactions
    Schimpf, S. and Rittermeier, A. and Zhang, X. and Li, Z.-A. and Spasova, M. and van den Berg, M.W.E. and Farle, M. and Wang, Y. and Fischer, R.A. and Muhler, M.
    ChemCatChem 2 214-222 (2010)
    Metal stearate-stabilized Cu nanoparticles, synthesized by an efficient one-step process, were applied in the continuous liquid-phase synthesis of methanol. After optimizing the reduction procedure, twofold higher rates of methanol formation were found for Cu-Zn colloids, compared to the conventional ternary Cu/ZnO/Al2O3 catalyst applied as fine powder in the liquid phase. Structural changes were investigated as a function of time on stream; after reduction in H2, spherical, well-separated 5-10 nm Cu particles stabilized by a Zn stearate shell were found. Under catalytic high-pressure conditions Zn stearate was hydrolyzed forming ZnO. High-resolution transmission electron microscopy revealed the presence of triangular ZnO prisms with truncated edges. Applying optimized synthesis conditions these triangularly shaped ZnO particles were found to be mostly attached to the spherical Cu particles. The catalytic results and the structural and spectroscopic characterization suggest that these ZnO particles act as a reservoir, releasing ZnOx species, which diffuse onto the Cu particles and promote the catalytic activity. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cctc.200900252
  • 2010 • 10 Stress, sheet resistance, and microstructure evolution of electroplated cu films during self-annealing
    Huang, R. and Robl, W. and Ceric, H. and Detzel, T. and Dehm, G.
    IEEE Transactions on Device and Materials Reliability 10 47-54 (2010)
    Electroplated copper films are known to change their microstructure due to the self-annealing effect. The self-annealing effect of electroplated copper films was investigated by measuring the time dependence of the film stress and sheet resistance for different layer thicknesses between 1.5 and 20 μm. While the sheet resistance was found to decrease as time elapsed, a size-dependent change in film stress was observed. Films with the thickness of 5 μm; and below decrease in stress, while thicker films initially reveal an increase in film stress followed by a stress relaxation at a later stage. This behavior is explained by the superposition of grain growth and grain-size-dependent yielding. © 2010 IEEE.
    view abstractdoi: 10.1109/TDMR.2009.2032768
  • 2010 • 9 Structural effects of Cu/Zn substitution in the malachite-rosasite system
    Behrens, M. and Girgsdies, F.
    Zeitschrift fur Anorganische und Allgemeine Chemie 636 919-927 (2010)
    Synthetic zincian malachite samples (Cu1-xZnx) 2(OH)2CO3 with x = 0, 0.1, 0.2 and 0.3 were characterized by powder X-ray diffraction and optical spectroscopy. The XRD patterns of the samples up to x = 0.2 indicate single phase materials with an approximately linear dependence of the refined lattice parameters on the zinc content. In contrast, the sample with a nominal zinc content x = 0.3 shows the formation of a small amount of aurichalcite (Zn,Cu)5(OH) 6(CO3)2 as an additional phase. Based on the lattice parameter variations, the zinc content of the zincian malachite component in this sample is estimated to be x ≈ 0.27, which seems to represent the maximum possible substitution in zincian malachite under the synthesis conditions applied. The results are discussed in relation to preparation of Cu/ZnO catalysts and the crystal structures of the minerals malachite and rosasite. One striking difference between these two structurally closely related phases is the orientation of the Jahn-Teller elongated axes of the CuO6 octahedra in the unit cell, which seems to be correlated with the placement of the monoclinic β angle. The structural and chemical relationship between these crystallographically distinct phases is discussed using a hypothetical intermediate Zn2(OH)2CO3 phase of higher orthorhombic symmetry. In addition to the crystallographic analysis, optical spectroscopy proves to be a useful tool for estimation of the Cu:Zn ratio in (Cu1-xZnx)2(OH) 2CO3 samples. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/zaac.201000028
  • 2010 • 8 The ferromagnetic shape memory system Fe-Pd-Cu
    Hamann, S. and Gruner, M.E. and Irsen, S. and Buschbeck, J. and Bechtold, C. and Kock, I. and Mayr, S.G. and Savan, A. and Thienhaus, S. and Quandt, E. and Fähler, S. and Entel, P. and Ludwig, Al.
    Acta Materialia 58 5949-5961 (2010)
    A new ferromagnetic shape memory thin film system, Fe-Pd-Cu, was developed using ab initio calculations, combinatorial fabrication and high-throughput experimentation methods. Reversible martensitic transformations are found in extended compositional regions, which have increased fcc-fct transformation temperatures in comparison to previously published results. High resolution transmission electron microscopy verified the existence of a homogeneous ternary phase without precipitates. Curie temperature, saturation polarization and orbital magnetism are only moderately decreased by alloying with nonmagnetic Cu. Compared to the binary system; enhanced Invar-type thermal expansion anomalies in terms of an increased volume magnetostriction are predicted. Complementary experiments on splat-fabricated bulk Fe-Pd-Cu samples showed an enhanced stability of the disordered transforming Fe70Pd30 phase against decomposition. From the comparison of bulk and thin film results, it can be inferred that, for ternary systems, the Fe content, rather than the valence electron concentration, should be regarded as the decisive factor determining the fcc-fct transformation temperature. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.011
  • 2010 • 7 The influence of the surface state onto the distance distribution of single molecules and small molecular clusters
    Mehlhorn, M. and Simic-Milosevic, V. and Jaksch, S. and Scheier, P. and Morgenstern, K.
    Surface Science 604 1698-1704 (2010)
    We investigate the distance distribution of two anorganic molecules (CO, H2O), one organic radical (parabenzyne), and one strongly dipolar molecule (ortho-dinitrobenzene) on the (111) faces of copper and silver. Above the onset of diffusion, their distribution is influenced by the surface state and oscillates. While CO, H2O, and para-benzyne show the expected oscillation period of ≈λF / 2, ortho-dinitrobenzene oscillates with λF / 4. The position of the first maximum in these oscillations is consistent with a perfect scatterer for the anorganic molecules, but inconsistent for the radical and the dipolar molecule. This observation is utilized to explain the double periodicity observed in the distance distribution of ortho-dinitrobenzene. © 2010 Elsevier B.V. All rights reserved. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2010.06.018
  • 2010 • 6 The mechanical size effect as a mean-field breakdown phenomenon: Example of microscale single crystal beam bending
    Demir, E. and Raabe, D. and Roters, F.
    Acta Materialia 58 1876-1886 (2010)
    Single crystalline copper beams with thicknesses between 0.7 and 5 μm are manufactured with a focused ion beam technique and bent in a nanoindenter. The yield strengths of the beams show a mechanical size effect (smaller-is-stronger). The geometrically necessary dislocation (GND) densities estimated from misorientation maps do not explain the observed size effect. Also, accumulation of GNDs principally requires pre-straining. We hence introduce a mean-field breakdown theory and generalize it to small-scale mechanical tests other than bending. The mean-field breakdown limit is defined in terms of a microstructural correlation measure (characteristic dislocation bow-out length) below which the local availability of dislocation sources and not the density of GNDs dominates the mechanical size effect. This explains why a size dependence can occur for samples that are not pre-strained (by using a very small critical strain to define the yield strength). After pre-straining, when GNDs build up, they can contribute to the flow stress. The mean-field breakdown theory can also explain the large scatter typically observed in small-scale mechanical tests as the availability of sufficiently soft sources at scales around or below the correlation length does not follow statistical laws but highly depends on the position where the probe is taken. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2009.11.031
  • 2010 • 5 The Potential of Microstructural Optimization in Metal/Oxide Catalysts: Higher Intrinsic Activity of Copper by Partial Embedding of Copper Nanoparticles
    Behrens, M. and Furche, A. and Kasatkin, I. and Trunschke, A. and Busser, W. and Muhler, M. and Kniep, B. and Fischer, R. and Schlögl, R.
    ChemCatChem 2 816-818 (2010)
    doi: 10.1002/cctc.201000017
  • 2010 • 4 The synthesis of highly loaded Cu/Al2O3 and Cu/Zno/Al2O3 catalysts by the two-step CVD of Cu IIdiethylamino-2-propoxide in a fluidized-bed reactor
    Becker, M. and D'Alnoncourt, R.N. and Kähler, K. and Sekulic, J. and Fischer, R.A. and Muhler, M.
    Chemical Vapor Deposition 16 85-92 (2010)
    Highly loaded copper catalysts supported on alumina are synthesized applying the cyclic two-step CVD of the precursor copper(II)diethylamino-2- propoxide in a fluidized-bed reactor. Copper/zinc oxide/alumina composites are synthesized by either the CVD of the precursor bis[bis (trimethylsilyl) amido]zinc on Cu/Al2O3, or the CVD of the Cu precursor on Zn-pretreated alumina, impregnating with diethyl zinc in addition. The composites are extensively characterized by atomic absorption spectroscopy (AAS), elemental analysis (EA), mass spectrometry (MS), N2 physisorption, N2O reactive frontal chromatography (RFC), and X-ray diffraction (XRD). The Cu and ZnO nanoparticles originating from the efficient two-step procedure, consisting of adsorption and subsequent decomposition of the adsorbed species in two separated steps, are highly dispersed, X-ray amorphous, and, in the case of the Cu-containing catalysts, have high specific Cu surface areas. The catalytic activities are determined both in methanol synthesis, to judge the contact between the deposited Cu and ZnO nanoparticles, and in the steam reforming of methanol (SRM) to probe the stability of the Cu particles. The turn-over frequencies (TOF) in methanol synthesis of these Cu/ZnO/Al 2O3 catalysts are higher than that of a commercial ternary catalyst. The varied sequence of the CVD of Cu and ZnO on alumina leads to catalysts with similar activities in the case of similar specific Cu areas. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.200906808
  • 2010 • 3 Therapeutic window for bioactive nanocomposites fabricated by laser ablation in polymer-doped organic liquids
    Hahn, A. and Stöver, T. and Paasche, G. and Löbler, M. and Sternberg, K. and Rohm, H. and Barcikowski, S.
    Advanced Engineering Materials 12 B156-B162 (2010)
    Polymeric nanomaterials are gaining increased interest in medical applications due to the sustained release ofbioactive agents. Within this study nanomaterials are fabricated using laser ablation of silver and copper in polymer-doped organic liquids thus allowing to produce customized drug release systems. A strategy is shown to determine the therapeutic window for cells relevant for cochlear implant electrodes, defined by the viability of L929 fibroblasts, PC12 neuronal cells, and spiral ganglion cells on different concentrations ofsilver and copper ions. The distribution ofnanoparticles within the silicone polymer matrix is determined using transmission electron microscopy. Hexane doped with 1% silicone resin is found to be an appropriate liquid matrix to fabricate a nanocomposite with a constant ion release rate. Silver ions of 10 μmol L-1 or copper ions of 100 μmol L -1 cause a suppression of tissue growth without inhibiting neuronal cell growth. The copper nanoparticle content of 0.1 wt% of the silicone composite releases ion concentrations which fit the therapeutic window. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.200980071
  • 2010 • 2 Thin films of the Heusler alloys Cu2MnAl and Co2MnSi: Recovery of ferromagnetism via solid-state crystallization from the x-ray amorphous state
    Erb, D. and Nowak, G. and Westerholt, K. and Zabel, H.
    Journal of Physics D: Applied Physics 43 (2010)
    X-ray amorphous thin films of the Heusler alloys Cu2MnAl and Co2MnSi have been prepared by magnetron sputter deposition at room temperature. In the amorphous state the Cu2MnAl phase is non-ferromagnetic; Co2MnSi is weakly ferromagnetic with a ferromagnetic Curie temperature of 170 K. By solid-state crystallization at high temperatures strong ferromagnetic order and high Curie temperatures are established in both alloys. The saturation magnetization of the Co 2MnSi alloy reaches 5.1μB/f.u. at 4 K, corresponding to 100% of the theoretical value; for Cu2MnAl we obtain 2.8μB/f.u. at 4 K, which corresponds to 87.5% of the theoretical value. In samples of the Co2MnSi phase with optimum saturation magnetization Bragg reflections as indicators of a long-range chemical order are missing, whereas for the Cu2MnAl phase Bragg reflections confirm epitaxial quality and long-range L21 order. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/43/28/285001
  • 2010 • 1 Using Ab initio calculations in designing bcc MgLi-X alloys for ultra-lightweight applications
    Counts, W.A. and Friák, M. and Raabe, D. and Neugebauer, J.
    Advanced Engineering Materials 12 1198-1205 (2010)
    Body centered cubic (bcc) Mg-Li-based alloys are a promising light-weight structural material. In order to tailor the Mg-Li composition with respect to specific industrial requirements, systematic materials-design concepts need to be developed and applied. Quantum-mechanical calculations are increasingly employed when designing new alloys as they accurately predict basic thermodynamic, structural, and functional properties using only the atomic composition as input. We have therefore performed a quantum-mechanical study using density functional theory (DFT) to systematically explore fundamental physical properties of a broad set of bcc MgLi-based compounds. These DFT-determined properties are used to calculate engineering parameters such as (i) the specific Young's modulus (Y/ρ) or (ii) the bulk over shear modulus ratio (B/G) which allow differentiating between brittle and ductile behavior. As we have recently shown, it is not possible to increase both specific Young's modulus, as a measure of strength, and B/G ratio, as a proxy for ductility, by changing only the composition in the binary bcc Mg-Li system. In an attempt to bypass such fundamental materials-design limitations, a large set of MgLi-X substitutional ternaries derived from stoichiometric MgLi with CsCl structure are studied. Motivated by the fact that for Mg-Li alloys (i) 3rd row Si and Al and (ii) 4th row Zn are industrially used as alloying elements, we probe the alloying performance of the 3rd (Na, Al, Si, P, S, Cl) and 4th row transition metal (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) elements. The studied solutes offer a variety of properties but none is able to simultaneously improve both specific Young's modulus and ductility. Therefore, in order to explore the alloying performance of yet a broader set of solutes, we predict the bulk modulus of MgX and LiX B2-compounds running over 40 different elements. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201000225