Prof. Dr.-Ing. Alfred Ludwig

Institute for Materials
Ruhr-Universität Bochum

Contact

Hub
  • Complex-Solid-Solution Electrocatalyst Discovery by Computational Prediction and High-Throughput Experimentation**
    Batchelor, T.A.A. and Löffler, T. and Xiao, B. and Krysiak, O.A. and Strotkötter, V. and Pedersen, J.K. and Clausen, C.M. and Savan, A. and Li, Y. and Schuhmann, W. and Rossmeisl, J. and Ludwig, Al.
    Angewandte Chemie - International Edition (2021)
    Complex solid solutions (“high entropy alloys”), comprising five or more principal elements, promise a paradigm change in electrocatalysis due to the availability of millions of different active sites with unique arrangements of multiple elements directly neighbouring a binding site. Thus, strong electronic and geometric effects are induced, which are known as effective tools to tune activity. With the example of the oxygen reduction reaction, we show that by utilising a data-driven discovery cycle, the multidimensionality challenge raised by this catalyst class can be mastered. Iteratively refined computational models predict activity trends around which continuous composition-spread thin-film libraries are synthesised. High-throughput characterisation datasets are then used as input for refinement of the model. The refined model correctly predicts activity maxima of the exemplary model system Ag-Ir-Pd-Pt-Ru. The method can identify optimal complex-solid-solution materials for electrocatalytic reactions in an unprecedented manner. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202014374
  • Influence of low Bi contents on phase transformation properties of VO2studied in a VO2:Bi thin film library
    Wang, X. and Rogalla, D. and Kostka, A. and Ludwig, Al.
    RSC Advances 11 (2021)
    A thin-film materials library in the system V-Bi-O was fabricated by reactive co-sputtering. The composition of Bi relative to V was determined by Rutherford backscattering spectroscopy, ranging from 0.06 to 0.84 at% along the library. The VO2phase M1 was detected by X-ray diffraction over the whole library, however a second phase was observed in the microstructure of films with Bi contents > 0.29 at%. The second phase was determined by electron diffraction to be BiVO4, which suggests that the solubility limit of Bi in VO2is only ∼0.29 at%. For Bi contents from 0.08 to 0.29 at%, the phase transformation temperatures of VO2:Bi increase from 74.7 to 76.4 °C by 8 K per at% Bi. With X-ray photoemission spectroscopy, the oxidation state of Bi was determined to be 3+. The V5+/V4+ratio increases with increasing Bi content from 0.10 to 0.84 at%. The similarly increasing tendency of the V5+/V4+ratio andTcwith Bi content suggests that although the ionic radius of Bi3+is much larger than that of V4+, the charge doping effect and the resulting V5+are more prominent in regulating the phase transformation behavior of Bi-doped VO2 © The Royal Society of Chemistry 2021.
    view abstract10.1039/d0ra09654g
  • Nanocrystalline equiatomic CoCrFeNi alloy thin films: Are they single phase fcc?
    Kini, M.K. and Lee, S. and Savan, A. and Breitbach, B. and Addab, Y. and Lu, W. and Ghidelli, M. and Ludwig, Al. and Bozzolo, N. and Scheu, C. and Chatain, D. and Dehm, G.
    Surface and Coatings Technology 410 (2021)
    The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO2 and c-sapphire substrates by magnetron co-sputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites. © 2021 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2021.126945
  • Combinatorial Exploration and Mapping of Phase Transformation in a Ni-Ti-Co Thin Film Library
    Al Hasan, N.M. and Hou, H. and Gao, T. and Counsell, J. and Sarker, S. and Thienhaus, S. and Walton, E. and Decker, P. and Mehta, A. and Ludwig, Al. and Takeuchi, I.
    ACS Combinatorial Science 22 (2020)
    Combinatorial synthesis and high-throughput characterization of a Ni-Ti-Co thin film materials library are reported for exploration of reversible martensitic transformation. The library was prepared by magnetron co-sputtering, annealed in vacuum at 500 °C without atmospheric exposure, and evaluated for shape memory behavior as an indicator of transformation. Composition, structure, and transformation behavior of the 177 pads in the library were characterized using high-throughput wavelength dispersive spectroscopy (WDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and four-point probe temperature-dependent resistance (R(T)) measurements. A new, expanded composition space having phase transformation with low thermal hysteresis and Co > 10 at. % is found. Unsupervised machine learning methods of hierarchical clustering were employed to streamline data processing of the large XRD and XPS data sets. Through cluster analysis of XRD data, we identified and mapped the constituent structural phases. Composition-structure-property maps for the ternary system are made to correlate the functional properties to the local microstructure and composition of the Ni-Ti-Co thin film library. © 2020 American Chemical Society. All rights reserved.
    view abstract10.1021/acscombsci.0c00097
  • Combinatorial Search for New Solar Water Splitting Photoanode Materials in the Thin-Film System Fe-Ti-W-O
    Kumari, S. and Khare, C. and Xi, F. and Nowak, M. and Sliozberg, K. and Gutkowski, R. and Bassi, P.S. and Fiechter, S. and Schuhmann, W. and Ludwig, Al .
    Zeitschrift fur Physikalische Chemie 234 (2020)
    In order to identify new solar water splitting photoanodes, Fe-Ti-W-O materials libraries were fabricated by combinatorial reactive co-sputtering and investigated by high-throughput characterization methods to elucidate compositional, thickness, and structural properties. In addition, photoelectrochemical measurements such as potentiodynamic photocurrent determination and open circuit potential measurements were performed using an automated scanning droplet cell. In the thin-film library, a quaternary photoactive region Fe30-49Ti29-55W13-22Ox was identified as a hit composition region, comprising binary and ternary phases. The identified region shows a distinct surface morphology with larger grains (∼200 nm) being embedded into a matrix of smaller grains (∼80-100 nm). A maximum photocurrent density of 117 μA/cm2 at a bias potential of 1.45 V vs. RHE in NaClO4 as an electrolyte under standard solar simulating conditions was recorded. Additional samples with compositions from the hit region were fabricated by reactive co-sputtering and spin coating followed by annealing. Synchrotron X-ray diffraction of sputtered Fe32Ti52W16Ox thin-films, annealed in air (600 °C, 700 °C, 800 °C) revealed the presence of the phases FeTiO3 and Ti0.54W0.46O2. The composition Fe48Ti30W22Ox from the hit region was fabricated by spin coating and subsequent annealing for a detailed investigation of its structure and photoactivity. After annealing the spin-coated sample at 650 °C for 6 h, X-ray diffraction results showed a dominant pattern with narrow diffraction lines belonging to a distorted FeWO4 (ferberite) phase along with broad diffraction lines addressed as Fe2TiO5 and in a small fraction also, Fe1.7Ti0.23O3. In hematite, Fe can be substituted by Ti, therefore we suggest that in the newfound ferberite-type phase, Ti partially substitutes for Fe leading to a small lattice distortion and a doubling of the monoclinic unit cell. In addition, Na from the substrate stabilizes the new phase: its tentative chemical formula is NaxFe0.33Ti0.67W2O8. A maximum photocurrent density of around 0.43 mA/cm2 at 1.45 V vs. RHE in 1M NaOH (pH ∼13.6) as an electrolyte was measured. Different aspects of the dependence of annealing and precursor solution concentration on phase transformation and photoactivity are discussed. © 2020 Alfred Ludwig et al., published by De Gruyter, Berlin/Boston 2020.
    view abstract10.1515/zpch-2019-1462
  • Combinatorial Synthesis and High-Throughput Characterization of Microstructure and Phase Transformation in Ni–Ti–Cu–V Quaternary Thin-Film Library
    Al Hasan, N.M. and Hou, H. and Sarkar, S. and Thienhaus, S. and Mehta, A. and Ludwig, Al. and Takeuchi, I.
    Engineering 6 (2020)
    Ni–Ti–based shape memory alloys (SMAs) have found widespread use in the last 70 years, but improving their functional stability remains a key quest for more robust and advanced applications. Named for their ability to retain their processed shape as a result of a reversible martensitic transformation, SMAs are highly sensitive to compositional variations. Alloying with ternary and quaternary elements to fine-tune the lattice parameters and the thermal hysteresis of an SMA, therefore, becomes a challenge in materials exploration. Combinatorial materials science allows streamlining of the synthesis process and data management from multiple characterization techniques. In this study, a composition spread of Ni–Ti–Cu–V thin-film library was synthesized by magnetron co-sputtering on a thermally oxidized Si wafer. Composition-dependent phase transformation temperature and microstructure were investigated and determined using high-throughput wavelength dispersive spectroscopy, synchrotron X-ray diffraction, and temperature-dependent resistance measurements. Of the 177 compositions in the materials library, 32 were observed to have shape memory effect, of which five had zero or near-zero thermal hysteresis. These compositions provide flexibility in the operating temperature regimes that they can be used in. A phase map for the quaternary system and correlations of functional properties are discussed with respect to the local microstructure and composition of the thin-film library. © 2020 THE AUTHORS
    view abstract10.1016/j.eng.2020.05.003
  • Combinatorial synthesis of Ni–Mn–Ga-(Fe,Co,Cu) high temperature ferromagnetic shape memory alloys thin films
    Alexandrakis, V. and Barandiaran, J.M. and Pérez-Checa, A. and Lázpita, P. and Decker, P. and Salomon, S. and Feuchtwanger, J. and Ludwig, Al. and Chernenko, V.
    Scripta Materialia 178 (2020)
    High temperature ferromagnetic shape memory alloys (HT-FSMAs) in form of a thin film materials library of Ni–Mn–Ga, alloyed with Fe, Co and Cu has been fabricated by co-sputtering and characterized by high-throughput screening techniques. The weak dependence of martensitic transformation temperature and tetragonal ratio of the martensitic lattice on the doping composition and their irregular variation are ascribed to the competitive influence of the alloying elements Fe, Co and Cu. © 2019 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2019.10.043
  • Comparative study of the residual stress development in HMDSN-based organosilicon and silicon oxide coatings
    Jaritz, M. and Hopmann, C. and Wilski, S. and Kleines, L. and Banko, L. and Grochla, D. and Ludwig, Al. and Dahlmann, R.
    Journal of Physics D: Applied Physics 53 (2020)
    To investigate the stress formation mechanisms in thin plasma polymers, a comparative study of organosilicon (SiNOCH) and silicon oxide (SiOx) coatings in dependence of power input for deposition was conducted. Both coating types were produced in a low-pressure (15 Pa) microwave excited hexamethyldilisazane (HMDSN) plasma. Residual stress values were obtained using a high-throughput, time resolved and in-situ measurement method, including a CCD-camera, a line laser and micro-machined cantilever sensor chips. Both plasma polymer types were shown to form residual stresses with opposite signs. The stress evolution in the coatings revealed a strong dependency on the variation of power input for deposition. The SiOx coating exhibits mostly compressive stresses. Higher power inputs constitute higher ion momentums as well as a higher degree of fragmentation of the monomer. The SiOx coatings were deposited with a high oxygen flow and with a higher average energy of the plasma for all investigated parameter sets than the SiNOCH coating. Therefore, it is conceivable that ion peening is mostly responsible for the compressive stress formation in the SiOx coatings. In contrast to the SiOx coating, the SiNOCH coating can be applied without residual stress. For higher excitation powers, tensile stresses are predominant, most likely due to attractive forces between island or column boundaries and crosslinking. © 2020 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab8ceb
  • Correlative chemical and structural investigations of accelerated phase evolution in a nanocrystalline high entropy alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 183 (2020)
    Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.03.016
  • Design of Complex Solid-Solution Electrocatalysts by Correlating Configuration, Adsorption Energy Distribution Patterns, and Activity Curves
    Löffler, T. and Savan, A. and Meyer, H. and Meischein, M. and Strotkötter, V. and Ludwig, Al. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 (2020)
    Complex solid-solution electrocatalysts (also referred to as high-entropy alloy) are gaining increasing interest owing to their promising properties which were only recently discovered. With the capability of forming complex single-phase solid solutions from five or more constituents, they offer unique capabilities of fine-tuning adsorption energies. However, the elemental complexity within the crystal structure and its effect on electrocatalytic properties is poorly understood. We discuss how addition or replacement of elements affect the adsorption energy distribution pattern and how this impacts the shape and activity of catalytic response curves. We highlight the implications of these conceptual findings on improved screening of new catalyst configurations and illustrate this strategy based on the discovery and experimental evaluation of several highly active complex solid solution nanoparticle catalysts for the oxygen reduction reaction in alkaline media. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201914666
  • Enhanced antibacterial performance of ultrathin silver/platinum nanopatches by a sacrificial anode mechanism
    Abuayyash, A. and Ziegler, N. and Meyer, H. and Meischein, M. and Sengstock, C. and Moellenhoff, J. and Rurainsky, C. and Heggen, M. and Garzón-Manjón, A. and Scheu, C. and Tschulik, K. and Ludwig, Al. and Köller, M.
    Nanomedicine: Nanotechnology, Biology, and Medicine 24 (2020)
    The development of antibacterial implant surfaces is a challenging task in biomaterial research. We fabricated a highly antibacterial bimetallic platinum (Pt)/silver(Ag) nanopatch surface by short time sputtering of Pt and Ag on titanium. The sputter process led to a patch-like distribution with crystalline areas in the nanometer-size range (1.3–3.9 nm thickness, 3–60 nm extension). Structural analyses of Pt/Ag samples showed Ag- and Pt-rich areas containing nanoparticle-like Pt deposits of 1–2 nm. The adhesion and proliferation properties of S. aureus on the nanopatch samples were analyzed. Consecutively sputtered Ag/Pt nanopatches (Pt followed by Ag) induced enhanced antimicrobial activity compared to co-sputtered Pt/Ag samples or pure Ag patches of similar Ag amounts. The underlying sacrificial anode mechanism was proved by linear sweep voltammetry. The advantages of this nanopatch coating are the enhanced antimicrobial activity despite a reduced total amount of Ag/Pt and a self-limited effect due the rapid Ag dissolution. © 2019 Elsevier Inc.
    view abstract10.1016/j.nano.2019.102126
  • Experimental and Theoretical Investigation on Phase Formation and Mechanical Properties in Cr-Co-Ni Alloys Processed Using a Novel Thin-Film Quenching Technique
    Naujoks, D. and Schneider, M. and Salomon, S. and Pfetzing-Micklich, J. and Subramanyam, A.P.A. and Hammerschmidt, T. and Drautz, R. and Frenzel, J. and Kostka, A. and Eggeler, G. and Laplanche, G. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    The Cr-Co-Ni system was studied by combining experimental and computational methods to investigate phase stability and mechanical properties. Thin-film materials libraries were prepared and quenched from high temperatures up to 700 °C using a novel quenching technique. It could be shown that a wide A1 solid solution region exists in the Cr-Co-Ni system. To validate the results obtained using thin-film materials libraries, bulk samples of selected compositions were prepared by arc melting, and the experimental data were additionally compared to results from DFT calculations. The computational results are in good agreement with the measured lattice parameters and elastic moduli. The lattice parameters increase with the addition of Co and Cr, with a more pronounced effect for the latter. The addition of ∼20 atom % Cr results in a similar hardening effect to that of the addition of ∼40 atom % Co. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acscombsci.9b00170
  • Fast-Track to Research Data Management in Experimental Material Science-Setting the Ground for Research Group Level Materials Digitalization
    Banko, L. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    Research data management is a major necessity for the digital transformation in material science. Material science is multifaceted and experimental data, especially, is highly diverse. We demonstrate an adjustable approach to a group level data management based on a customizable document management software. Our solution is to continuously transform data management workflows from generalized to specialized data management. We start up fast with a relatively unregulated base setting and adapt continuously over the period of use to transform more and more data procedures into specialized data management workflows. By continuous adaptation and integration of analysis workflows and metadata schemes, the amount and the quality of the data improves. As an example of this process, in a period of 36 months, data on over 1800 samples, mainly materials libraries with hundreds of individual samples, were collected. The research data management system now contains over 1700 deposition processes and more than 4000 characterization documents. From initially mainly user-defined data input, an increased number of specialized data processing workflows was developed allowing the collection of more specialized, quality-assured data sets. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acscombsci.0c00057
  • High-Throughput Characterization of (FexCo1- x)3O4Thin-Film Composition Spreads
    Piotrowiak, T.H. and Wang, X. and Banko, L. and Kumari, S. and Sarker, S. and Mehta, A. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    Thin-film continuous composition spreads of Fe-Co-O were fabricated by reactive cosputtering from elemental Fe and Co targets in reactive Ar/O2 atmosphere using deposition temperatures ranging from 300 to 700 °C. Fused silica and platinized Si/SiO2 strips were used as substrates. Ti and Ta were investigated as adhesion layer for Pt and the fabrication of the Fe-Co-O films. The thin-film composition spreads were characterized by high-throughput electron-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and optical transmission spectroscopy. The Fe-content ranged from 28 to 72 at. %. The spinel phases Fe2CoO4 and FeCo2O4 could be synthesized and stabilized at all deposition temperatures with a continuous variation in spinel composition in between. The dependence of the film surface microstructure on the deposition temperature and the composition was mapped. Moreover, the band gap values, ranging from 2.41 eV for FeCo2O4 to 2.74 eV for Fe2CoO4, show a continuous variation with the composition. ©
    view abstract10.1021/acscombsci.0c00126
  • High-throughput characterization of Ag–V–O nanostructured thin-film materials libraries for photoelectrochemical solar water splitting
    Kumari, S. and Helt, L. and Junqueira, J.R.C. and Kostka, A. and Zhang, S. and Sarker, S. and Mehta, A. and Scheu, C. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 45 (2020)
    Ag–V–O thin-film materials libraries, with both composition (Ag22-77V23-78Ox) and thickness (123–714 nm) gradients were fabricated using combinatorial reactive magnetron co-sputtering aiming on establishing relations between composition, structure, and functional properties. As-deposited libraries were annealed in air at 300 °C for 10 h. High-throughput characterization methods of composition, structure and functional properties were used to identify photoelectrochemically active regions. The phases AgV6O15, Ag2V4O11, AgVO3, and Ag4V2O7 were observed throughout the composition gradient. The photoelectrochemical properties of Ag–V–O films are dependent on composition and morphology. An enhanced photocurrent density (~300–554 μA/cm2) was obtained at 30 to 45 at.% Ag along the thickness gradient. Thin films of these compositions show a nanowire morphology, which is an important factor for the enhancement of photoelectrochemical performance. The photoelectrochemically active regions were further investigated by high-throughput synchrotron-X-ray diffraction and transmission electron microscopy (Ag32V68Ox) which confirmed the presence of Ag2V4O11 as the dominating phase along with the minor phases AgV6O15 and AgVO3. This enhanced photoactive region shows bandgap values of ~2.30 eV for the direct and ~1.87 eV for the indirect bandgap energies. The porous nanostructured films improve charge transport and are hence of interest for photoelectrochemical water splitting. © 2020 Hydrogen Energy Publications LLC
    view abstract10.1016/j.ijhydene.2020.02.154
  • High-Throughput Characterization of Structural and Photoelectrochemical Properties of a Bi-Mo-W-O Thin-Film Materials Library
    Nowak, M. and Gutkowski, R. and Junqueira, J. and Schuhmann, W. and Ludwig, Al
    Zeitschrift fur Physikalische Chemie 234 (2020)
    A Bi-W-Mo-O thin-film materials library was fabricated by combinatorial reactive magnetron sputtering. The composition spread was investigated using high-throughput methods to determine crystalline phases, composition, morphology, optical properties, and photoelectrochemical performance. The aurivillius phase (Bi2O2)2+ (BiM(W1-NMoN)M-1O3M+1)2- is the predominantly observed crystal structure, indicating that the thin films in the library are solid solutions. With increasing amounts of Mo ≙ 7-22% the diffraction peak at 2θ = 28° ≙ [131] shifts due to lattice distortion, the photoelectrochemical activity is increasing up to a wavelength of 460 nm with an incident photon to current efficiency (IPCE) of 4.5%, and the bandgap decreases. A maximum photocurrent density of 31 μA/cm2 was measured for Bi31W62Mo7Oz at a bias potential of 1.23 V vs. RHE (0.1 M Na2SO4). © 2020 Wolfgang Schuhmann, Alfred Ludwig et al., published by De Gruyter, Berlin/Boston 2020.
    view abstract10.1515/zpch-2019-1439
  • High-Throughput Exploration of Metal Vanadate Thin-Film Systems (M-V-O, M = Cu, Ag, W, Cr, Co, Fe) for Solar Water Splitting: Composition, Structure, Stability, and Photoelectrochemical Properties
    Kumari, S. and Junqueira, J.R.C. and Schuhmann, W. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    Combinatorial synthesis and high-throughput characterization of thin-film materials libraries enable to efficiently identify both photoelectrochemically active and inactive, as well as stable and instable systems for solar water splitting. This is shown on six ternary metal vanadate (M-V-O, M = Cu, Ag, W, Cr, Co, Fe) thin-film materials libraries, fabricated using combinatorial reactive magnetron cosputtering with subsequent annealing in air. By means of high-throughput characterization of these libraries correlations between composition, crystal structure, photocurrent density, and stability of the M-V-O systems in different electrolytes such as acidic, neutral and alkaline media were identified. The systems Cu-V-O and Ag-V-O are stable in alkaline electrolyte and exhibited photocurrents of 170 and 554 μA/cm2, respectively, whereas the systems W-V-O, Cr-V-O, and Co-V-O are not stable in alkaline electrolyte. However, the Cr-V-O and Co-V-O systems showed an enlarged photoactive region in acidic electrolyte, albeit with very low photocurrents (<10 μA/cm2). Complete data sets obtained from these different screening sets, including information on nonpromising systems, lays groundwork for their use to predict new systems for solar water splitting, for example, by machine learning. ©
    view abstract10.1021/acscombsci.0c00150
  • Microstructure evolution and thermal stability of equiatomic CoCrFeNi films on (0001) α-Al2O3
    Addab, Y. and Kini, M.K. and Courtois, B. and Savan, A. and Ludwig, Al. and Bozzolo, N. and Scheu, C. and Dehm, G. and Chatain, D.
    Acta Materialia 200 (2020)
    Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001) α-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick films were investigated between 973 K and 1423 K. The fcc-phase persists while the original &lt;111&gt; texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 µm wide grains upon annealing. Only the metallic M grains having two specific orientation relationships (ORs) to the c-sapphire grow. These ORs are OR1 (M(111)[11¯0]//α-Al2O3(0001)[11¯00]) and OR2 (M(111)[11¯0]//α-Al2O3(0001)[112¯0])and their twin-related variants (OR1t and OR2t). They are identical to those reported for several pure fcc metal (M) films. Thus, the ORs in these fcc/c-sapphire systems appear not to be controlled by the fcc phase chemistry or its lattice parameter as usually assumed in literature. Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. Below 1000 K, the film microstructure stabilizes into 10 µm wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.09.064
  • On the effects of diluted and mixed ionic liquids as liquid substrates for the sputter synthesis of nanoparticles
    Meischein, M. and Fork, M. and Ludwig, Al.
    Nanomaterials 10 (2020)
    The synthesis of nanoparticles by combinatorial sputtering in ionic liquids is a versatile approach for discovering new materials. Whereas the influence on nanoparticle formation of different pure ionic liquids has been addressed, the influence of (I) dilution of ionic liquid with solvents and (II) different mixtures of ionic liquids is less known. Therefore, mixtures of the ionic liquid [Bmim][(Tf)2N] with the organic solvent anisole and other ionic liquids ([Bmim][(Pf)2N], [BmPyr][(Tf)2N]) were used as liquid substrates for the sputter synthesis of nanoparticles, in order to investigate the influence of these mixtures on the size of the nanoparticles. First, mixtures of anisole with a suspension of sputtered Ag nanoparticles in [Bmim][(Tf)2N] were prepared in different volumetric steps to investigate if the stabilization of the NPs by the ionic liquid could be reduced by the solvent. However, a continuous reduction in nanoparticle size and amount with increasing anisole volume was observed. Second, Ag, Au and Cu were sputtered on ionic liquid mixtures. Ag nanoparticles in [Bmim][(Tf)2N]/[Bmim][(Pf)2N] mixtures showed a decrease in size with the increasing volumetric fraction of [Bmim][(Tf)2N], whereas all nanoparticles obtained from [Bmim][(Tf)2N]/[BmPyr][(Tf)2N] mixtures showed increasing size and broadening of the size distribution. Maximum sizes of sputtered Ag and Au NPs were reached in mixtures of [Bmim][(Tf)2N] with 20 vol.% and 40 vol.% [BmPyr][(Tf)2N]. The results indicate that ionic liquid mixtures with different portions of cations and anions have the capability of influencing the ionic liquid stabilization characteristics with respect to, e.g., nanoparticle size and size distribution. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10030525
  • Phase decomposition in a nanocrystalline CrCoNi alloy
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Scripta Materialia 188 (2020)
    Phase stability of a nanocrystalline CrCoNi alloy is investigated using the combinatorial processing platform approach, which enables synthesis, processing and direct atomic-scale characterizations of alloys by atom probe tomography and transmission electron microscopy. Phase decomposition with formation of CoNi-rich phase occurs faster in the smaller (10 nm) grain-sized region than the larger one (20 nm), both being present in the same sample. Chemical analyses indicate that diffusion of Co and Cr plays an important role in phase decomposition. Comparison of phase stability between CrMnFeCoNi and CrCoNi implies that elemental segregation may promote phase decomposition by providing an additional chemical driving force for it. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2020.07.054
  • Recent Developments in Small-Scale Shape Memory Oxides
    Wang, X. and Ludwig, Al.
    Shape Memory and Superelasticity 6 (2020)
    This review presents an overview of the developments in small-scale shape memory materials: from alloys to oxides and ceramics. Shape memory oxides such as zirconia, different ferroelectric perovskites and VO2-based materials have favorable characteristics of high strength, high operating temperature and chemical resistance, which make this class of shape memory materials interesting for special applications, e.g., in harsh environments or at the nanoscale. Because of the constraint and mismatch stress from neighboring grains in polycrystalline/bulk oxides, the transformation strain of shape memory oxides is relatively small, and micro-cracks can appear after some cycles. However, recent progress in shape memory oxide research related to small-scale approaches such as decreasing the amounts of grain boundaries, strain-engineering, and application in the form of nanoscale thin films shows that some oxides are capable to exhibit excellent shape memory effects and superelasticity at nano/micro-scales. The materials systems ZrO2, BiFO3, and VO2 are discussed with respect to their shape memory performance in bulk and small-scale. © 2020, The Author(s).
    view abstract10.1007/s40830-020-00299-7
  • Sputter deposition of highly active complex solid solution electrocatalysts into an ionic liquid library: Effect of structure and composition on oxygen reduction activity
    Manjón, A.G. and Löffler, T. and Meischein, M. and Meyer, H. and Lim, J. and Strotkötter, V. and Schuhmann, W. and Ludwig, Al. and Scheu, C.
    Nanoscale 12 (2020)
    Complex solid solution electrocatalysts (often called high-entropy alloys) present a new catalyst class with highly promising features due to the interplay of multi-element active sites. One hurdle is the limited knowledge about structure-activity correlations needed for targeted catalyst design. We prepared Cr-Mn-Fe-Co-Ni nanoparticles by magnetron sputtering a high entropy Cantor alloy target simultaneously into an ionic liquid library. The synthesized nanoparticles have a narrow size distribution but different sizes (from 1.3 ± 0.1 nm up to 2.6 ± 0.3 nm), different crystallinity (amorphous, face-centered cubic or body-centered cubic) and composition (i.e. high Mn versus low Mn content). The Cr-Mn-Fe-Co-Ni complex solid solution nanoparticles possess an unprecedented intrinsic electrocatalytic activity for the oxygen reduction reaction in alkaline media, some of them even surpassing that of Pt. The highest intrinsic activity was obtained for body-centered cubic nanoparticles with a low Mn and Fe content which were synthesized using the ionic liquid 1-etyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Emimi][(Tf)2N]. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0nr07632e
  • Structural and photoelectrochemical properties in the thin film system Cu-Fe-V-O and its ternary subsystems Fe-V-O and Cu-V-O
    Kumari, S. and Junqueira, J.R.C. and Sarker, S. and Mehta, A. and Schuhmann, W. and Ludwig, Al.
    Journal of Chemical Physics 153 (2020)
    Thin-film material libraries in the ternary and quaternary metal oxide systems Fe-V-O, Cu-V-O, and Cu-Fe-V-O were synthesized using combinatorial reactive co-sputtering with subsequent annealing in air. Their compositional, structural, and functional properties were assessed using high-throughput characterization methods. Prior to the investigation of the quaternary system Cu-Fe-V-O, the compositions (Fe61V39)Ox and (Cu52V48)Ox with promising photoactivity were identified from their ternary subsystems Fe-V-O and Cu-V-O, respectively. Two Cu-Fe-V-O material libraries with (Cu29-72Fe4-27V22-57)Ox and (Cu11-55Fe27-73V12-34)Ox composition spread were investigated. Seven mixed ternary and quaternary phase regions were identified: I (α-Cu3FeV6O26/FeVO4), II (Cu5V2O10/FeVO4/α-Cu3Fe4V6O26), III (Cu5V2O10), IV (Cu5V2O10/FeVO4, V (FeVO4/γ-Cu2V2O7/α-Cu3Fe4V6O26), VI (β-Cu2V2O7/α-Cu3Fe4V6O26/FeVO4), and VII (β-Cu3Fe4V6O26/FeVO4). In the investigated composition range, two photoactive regions, (Cu53Fe7V40)Ox and (Cu45Fe21V34)Ox, were identified, exhibiting 103 μA/cm2 and 108 μA/cm2 photocurrent density for the oxygen evolution reaction at 1.63 V vs reversible hydrogen electrode, respectively. The highest photoactive region (Cu45Fe21V34)Ox comprises the dominant α-Cu3Fe4V6O24 phase and minor FeVO4 phase. This photoactive region corresponds to having an indirect bandgap of 1.87 eV and a direct bandgap of 2.58 eV with an incident photon-to-current efficiency of 30% at a wavelength of 310 nm. © 2020 Author(s).
    view abstract10.1063/5.0009512
  • Structure zone investigation of multiple principle element alloy thin films as optimization for nanoindentation measurements
    Savan, A. and Allermann, T. and Wang, X. and Grochla, D. and Banko, L. and Kalchev, Y. and Kostka, A. and Pfetzing-Micklich, J. and Ludwig, Al.
    Materials 13 (2020)
    Multiple principal element alloys, also often referred to as compositionally complex alloys or high entropy alloys, present extreme challenges to characterize. They show a vast, multidimensional composition space that merits detailed investigation and optimization to identify compositions and to map the composition ranges where useful properties are maintained. Combinatorial thin film material libraries are a cost-effective and efficient way to create directly comparable, controlled composition variations. Characterizing them comes with its own challenges, including the need for high-speed, automated measurements of dozens to hundreds or more compositions to be screened. By selecting an appropriate thin film morphology through predictable control of critical deposition parameters, representative measured values can be obtained with less scatter, i.e., requiring fewer measurement repetitions for each particular composition. In the present study, equiatomic CoCrFeNi was grown by magnetron sputtering in different locations in the structure zone diagram applied to multinary element alloys, followed by microstructural and morphological characterizations. Increasing the energy input to the deposition process by increased temperature and adding high-power impulse magnetron sputtering (HiPIMS) plasma generators led to denser, more homogeneous morphologies with smoother surfaces until recrystallization and grain boundary grooving began. Growth at 300 ffiC, even without the extra particle energy input of HiPIMS generators, led to consistently repeatable nanoindentation load-displacement curves and the resulting hardness and Young's modulus values. © 2020 by the authors.
    view abstract10.3390/ma13092113
  • Subtoxic cell responses to silica particles with different size and shape
    Kersting, M. and Olejnik, M. and Rosenkranz, N. and Loza, K. and Breisch, M. and Rostek, A. and Westphal, G. and Bünger, J. and Ziegler, N. and Ludwig, Al. and Köller, M. and Sengstock, C. and Epple, M.
    Scientific Reports 10 (2020)
    Health risks from particles are a priority challenge to health protection at work. Despite the ubiquitous exposure to a wide range of particles and the many years of research in this field, there are fundamental unresolved questions regarding the prevention of particle-related respiratory diseases. Here, the highly relevant particulate material silicon dioxide was analyzed with emphasis on defined size and shape. Silica particles were prepared with different size and shape: Spheres (NS nanospheres 60 nm; SMS submicrospheres 230 nm; MS microspheres 430 nm) and rods (SMR submicrorods with d = 125 nm, L = 230 nm; aspect ratio 1:1.8; MR microrods with d = 100 nm, L = 600 nm; aspect ratio 1:6). After an in-depth physicochemical characterization, their effects on NR8383 alveolar macrophages were investigated. The particles were X-ray amorphous, well dispersed, and not agglomerated. Toxic effects were only observed at high concentrations, i.e. ≥ 200 µg mL−1, with the microparticles showing a stronger significant effect on toxicity (MS≈MR &gt; SMR≈SMS≈NS) than the nanoparticles. Special attention was directed to effects in the subtoxic range (less than 50% cell death compared to untreated cells), i.e. below 100 µg mL−1 where chronic health effects may be expected. All particles were readily taken up by NR8383 cells within a few hours and mainly found associated with endolysosomes. At subtoxic levels, neither particle type induced strongly adverse effects, as probed by viability tests, detection of reactive oxygen species (ROS), protein microarrays, and cytokine release (IL-1β, GDF-15, TNF-α, CXCL1). In the particle-induced cell migration assay (PICMA) with leukocytes (dHL-60 cells) and in cytokine release assays, only small effects were seen. In conclusion, at subtoxic concentrations, where chronic health effects may be expected, neither size and nor shape of the synthesized chemically identical silica particles showed harmful cell-biological effects. © 2020, The Author(s).
    view abstract10.1038/s41598-020-78550-5
  • Synthesis of plasmonic Fe/Al nanoparticles in ionic liquids
    Schmitz, A. and Meyer, H. and Meischein, M. and Garzón Manjón, A. and Schmolke, L. and Giesen, B. and Schlüsener, C. and Simon, P. and Grin, Y. and Fischer, R.A. and Scheu, C. and Ludwig, Al. and Janiak, C.
    RSC Advances 10 (2020)
    Bottom-up and top-down approaches are described for the challenging synthesis of Fe/Al nanoparticles (NPs) in ionic liquids (ILs) under mild conditions. The crystalline phase and morphology of the metal nanoparticles synthesized in three different ionic liquids were identified by powder X-ray diffractometry (PXRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and fast Fourier transform (FFT) of high-resolution TEM images. Characterization was completed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) for the analysis of the element composition of the whole sample consisting of the NPs and the amorphous background. The bottom-up approaches resulted in crystalline FeAl NPs on an amorphous background. The top-down approach revealed small NPs and could be identified as Fe4Al13 NPs which in the IL [OPy][NTf2] yield two absorption bands in the green-blue to green spectral region at 475 and 520 nm which give rise to a complementary red color, akin to appropriate Au NPs. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0ra01111h
  • Thin-Film Microtensile-Test Structures for High-Throughput Characterization of Mechanical Properties
    Oellers, T. and Arigela, V.G. and Kirchlechner, C. and Dehm, G. and Ludwig, Al.
    ACS Combinatorial Science 22 (2020)
    A photolithographic process for the rapid fabrication of thin-film tensile-test structures is presented. The process is applicable to various physical vapor deposition techniques and can be used for the combinatorial fabrication of thin-film tensile-test structure materials libraries for the high-throughput characterization of mechanical properties. The functionality of the fabrication process and the feasibility of performing high-quality measurements with these structures are demonstrated with Cu tensile-test structures. In addition, the scalability from unary structures to libraries with compositional variations is demonstrated. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acscombsci.9b00182
  • Bimetallic silver-platinum nanoparticles with combined osteo-promotive and antimicrobial activity
    Breisch, M. and Grasmik, V. and Loza, K. and Pappert, K. and Rostek, A. and Ziegler, N. and Ludwig, Al. and Heggen, M. and Epple, M. and Tiller, J.C. and Schildhauer, T.A. and Köller, M. and Sengstock, C.
    Nanotechnology 30 (2019)
    Bimetallic alloyed silver-platinum nanoparticles (AgPt NP) with different metal composition from Ag10Pt90 to Ag90Pt10 in steps of 20 mol% were synthesized. The biological effects of AgPt NP, including cellular uptake, cell viability, osteogenic differentiation and osteoclastogenesis as well as the antimicrobial activity towards Staphylococcus aureus and Escherichia coli were analyzed in comparison to pure Ag NP and pure Pt NP. The uptake of NP into human mesenchymal stem cells was confirmed by cross-sectional focused-ion beam preparation and observation by scanning and transmission electron microscopy in combination with energy-dispersive x-ray analysis. Lower cytotoxicity and antimicrobial activity were observed for AgPt NP compared to pure Ag NP. Thus, an enhanced Ag ion release due to a possible sacrificial anode effect was not achieved. Nevertheless, a Ag content of at least 50 mol% was sufficient to induce bactericidal effects against both Staphylococcus aureus and Escherichia coli. In addition, a Pt-related (≥50 mol% Pt) osteo-promotive activity on human mesenchymal stem cells was observed by enhanced cell calcification and alkaline phosphatase activity. In contrast, the osteoclastogenesis of rat primary precursor osteoclasts was inhibited. In summary, these results demonstrate a combinatory osteo-promotive and antimicrobial activity of bimetallic Ag50Pt50 NP. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/ab172b
  • Combinatorial Synthesis of Binary Nanoparticles in Ionic Liquids by Cosputtering and Mixing of Elemental Nanoparticles
    Meischein, M. and Garzón-Manjón, A. and Frohn, T. and Meyer, H. and Salomon, S. and Scheu, C. and Ludwig, Al.
    ACS Combinatorial Science (2019)
    Binary alloy nanoparticles were fabricated by two combinatorial methods: (I) cosputtering from elemental targets into the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][(Tf)2N] and (II) by mixing elemental nanoparticles after sputtering them separately into [Bmim][(Tf)2N]. Both methods lead to the formation of Au-Cu nanoparticles (2.3 nm for cosputtered, 3.6 nm for mixed), however with different resulting compositions: cosputtered nanoparticles show a composition range of Au80-90Cu20-10; mixing of Au- and Cu-loaded ionic liquids leads to the formation of Au75Cu25 nanoparticles. Annealing the binary nanoparticles at 100 °C shows that the mixed nanoparticles grow to sizes of 4.1 nm, whereas the cosputtered nanoparticles grow only to 3 nm. © 2019 American Chemical Society.
    view abstract10.1021/acscombsci.9b00140
  • Development of a high-temperature micromechanics stage with a novel temperature measurement approach
    Arigela, V.G. and Oellers, T. and Ludwig, Al. and Kirchlechner, C. and Dehm, G.
    Review of Scientific Instruments 90 (2019)
    The study of mechanical properties of materials at high temperatures at the microstructural length regime requires dedicated setups for testing. Despite the advances in the instrumentation in these setups over the last decade, further optimization is required in order to extend the temperature range well-beyond 600 °C. Particularly, an improvement of the contact temperature measurement is required. A design with a novel approach of temperature measurement with independent tip and sample heating is developed to characterize materials at high temperatures. This design is realized by modifying a displacement controlled room temperature microstraining rig with the addition of two miniature hot stages, one each carrying the sample and indenter tip. The sample reaches temperatures of &gt;600 °C with a 50 W diode laser system. The stages have slots for the working sample as well as a reference sample on both ends for precise temperature measurements, relying on the symmetry of the stage toward the ends. The whole setup is placed inside a custom-made steel chamber, capable of attaining a vacuum of 10-4 Pa. Alternatively, the apparatus can be operated under environmental conditions by applying various gases. Here, the unique design and its high temperature capabilities will be presented together with the first results of microtension experiments on freestanding copper thin films at 400 °C. © 2019 Author(s).
    view abstract10.1063/1.5086261
  • Discovery of new materials using combinatorial synthesis and high-throughput characterization of thin-film materials libraries combined with computational methods
    Ludwig, Al.
    npj Computational Materials 5 (2019)
    This perspective provides an experimentalist’s view on materials discovery in multinary materials systems—from nanoparticles over thin films to bulk—based on combinatorial thin-film synthesis and high-throughput characterization in connection with high-throughput calculations and materials informatics. Complete multinary materials systems as well as composition gradients which cover all materials compositions necessary for verification/falsification of hypotheses and predictions are efficiently fabricated by combinatorial synthesis of thin-film materials libraries. Automated high-quality high-throughput characterization methods enable comprehensive determination of compositional, structural and (multi)functional properties of the materials contained in the libraries. The created multidimensional datasets enable data-driven materials discoveries and support efficient optimization of newly identified materials, using combinatorial processing. Furthermore, these datasets are the basis for multifunctional existence diagrams, comprising correlations between composition, processing, structure and properties, which can be used for the design of future materials. © 2019, The Author(s).
    view abstract10.1038/s41524-019-0205-0
  • Effects of the Ion to Growth Flux Ratio on the Constitution and Mechanical Properties of Cr1- x-Alx-N Thin Films
    Banko, L. and Ries, S. and Grochla, D. and Arghavani, M. and Salomon, S. and Pfetzing-Micklich, J. and Kostka, A. and Rogalla, D. and Schulze, J. and Awakowicz, P. and Ludwig, Al.
    ACS Combinatorial Science 21 (2019)
    Cr-Al-N thin film materials libraries were synthesized by combinatorial reactive high power impulse magnetron sputtering (HiPIMS). Different HiPIMS repetition frequencies and peak power densities were applied altering the ion to growth flux ratio. Moreover, time-resolved ion energy distribution functions were measured with a retarding field energy analyzer (RFEA). The plasma properties were measured during the growth of films with different compositions within the materials library and correlated to the resulting film properties such as phase, grain size, texture, indentation modulus, indentation hardness, and residual stress. The influence of the ion to growth flux ratio on the film properties was most significant for films with high Al-content (xAl = 50 at. %). X-ray diffraction with a 2D detector revealed hcp-AlN precipitation starting from Al-concentration xAl ≥ 50 at. %. This precipitation might be related to the kinetically enhanced adatom mobility for a high ratio of ions per deposited atoms, leading to strong intermixing of the deposited species. A structure zone transition, induced by composition and flux ratio JI/JG, from zone T to zone Ic structure was observed which hints toward the conclusion that the combination of increasing flux ratio and Al-concentration lead to opposing trends regarding the increase in homologous temperature. © 2019 American Chemical Society.
    view abstract10.1021/acscombsci.9b00123
  • Electrical and Structural Properties of the Partial Ternary Thin-Film System Ni-Si-B
    Wambach, M. and Nguyen, N. and Hamann, S. and Nishio, M. and Yagyu, S. and Chikyow, T. and Ludwig, Al.
    ACS Combinatorial Science 21 (2019)
    High-throughput and combinatorial materials science methods were used to investigate the dependence of the work function in the Ni-Si system on the B content (0-30 at. %). Alloying of NiSi is used to adapt its properties to suit the needs as a gate electrode material. Thin-film materials libraries were fabricated and investigated with respect to their structural and electrical properties. Further the work function values of selected samples in the region of interest were analyzed. The results show that the work function can be adjusted between 4.86 eV (B = 4.2 at. %) and 5.16 eV (B = 29.2 at. %) for (NiSi)Bx. © 2019 American Chemical Society.
    view abstract10.1021/acscombsci.8b00175
  • Glancing-angle deposition of nanostructures on an implant material surface
    Ziegler, N. and Sengstock, C. and Mai, V. and Schildhauer, T.A. and Köller, M. and Ludwig, Al.
    Nanomaterials 9 (2019)
    Cell-compatible and antibacterial surfaces are needed for implants, which frequently have complex and rough surfaces. Bio-inspired columnar nanostructures can be grown on flat substrates; however, the application of these nanostructures on clinically relevant, complex, and rough surfaces was pending. Therefore, a titanium plasma spray (TPS) implant surface was coated with titanium nano-spikes via glancing angle magnetron sputter deposition (GLAD) at room temperature. Using GLAD, it was possible to cover the three-dimensional, highly structured macroscopic surface (including cavities, niches, clefts, and curved areas) of the TPS homogeneously with nano-spikes (TPS+), creating a cell-compatible and antibacterial surface. The adherence and spreading of mesenchymal stem cells (MSC) were similar for TPS and TPS+ surfaces. However, MSC adherent to TPS+ expressed less and shorter pseudopodia. The induced osteogenic response of MSC was significantly increased in cells cultivated on TPS+ compared with TPS. In addition, Gram-negative bacteria (E. coli) adherent to the nano-spikes were partly destructed by a physico-mechanical mechanism; however, Gram-positive bacteria (S. aureus) were not significantly damaged. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano9010060
  • Influence of Cr Alloying (1.5 to 5 at.%) on Martensitic Phase Transformation Temperatures in Co-Ni-Ga-Cr Thin Films
    Decker, P. and Fortmann, J. and Salomon, S. and Krooß, P. and Niendorf, T. and Ludwig, Al.
    Shape Memory and Superelasticity 5 (2019)
    A Co-Ni-Ga-Cr thin film materials library with a Cr concentration gradient ranging from 1.5 at.% to 5 at.% was magnetron-sputtered to determine the influence of Cr additions on the martensite transformation temperature of non-stoichiometric Co2NiGa-based Heusler alloys. An increase of the phase transformation temperature from 30 °C for 5 at.% Cr up to 100 °C for 1.5 at.% Cr was determined by a peak fitting analysis of temperature-dependent X-ray diffraction data. © 2019, ASM International.
    view abstract10.1007/s40830-019-00209-6
  • Influences of Si Substitution on Existence, Structural and Magnetic Properties of the CoMnGe Phase Investigated in a Co-Mn-Ge-Si Thin-Film Materials Library
    Salomon, S. and Wöhrle, F. and Hübner, P. and Decker, P. and Ludwig, Al.
    ACS Combinatorial Science 21 (2019)
    Materials exhibiting a giant magnetocaloric effect (GMCE) need to be finely tuned with respect to their magnetic and structural properties, specifically their respective transition temperatures. Co-Mn-Ge and Co-Mn-Ge-Si thin-film materials libraries (MLs) were fabricated and characterized to determine the composition range in which the CoMnGe phase is stable and analyze selected single-phase measurement regions. Phase analysis was performed on these MLs and refined in regard to the CoMnGe single-phase region by synchrotron diffraction experiments. A comparison of the MLs revealed that the CoMnGe (HT) single-phase region gets smaller with the addition of Si and exists for 26.5 at. % &lt; Co &lt; 29.5 at. %, 34 at. % &lt; Mn &lt; 37 at. %, Ge ∼35 at. %, and Si ∼1.5 at. % in the quaternary ML. The investigation of magnetic and structural transition properties in this region revealed hard-magnetic behavior with Curie temperatures (Tc) between 261 and 274 K and large hysteresis widths with values &gt;100 K for the structural transition. A low Co content was necessary to achieve an overlap of Tc and structural transition temperatures, leading to the most favorable properties for a GMCE to be found in Co26.5Mn37Ge35Si1.5. © 2019 American Chemical Society.
    view abstract10.1021/acscombsci.9b00107
  • Ion energy control via the electrical asymmetry effect to tune coating properties in reactive radio frequency sputtering
    Ries, S. and Banko, L. and Hans, M. and Primetzhofer, D. and Schneider, J.M. and Ludwig, Al. and Awakowicz, P. and Schulze, J.
    Plasma Sources Science and Technology 28 (2019)
    A knowledge-based understanding of the plasma-surface-interaction with the aim to precisely control (reactive) sputtering processes for the deposition of thin films with tailored and reproducible properties is highly desired for industrial applications. In order to understand the effect of plasma parameter variations on the film properties, a single plasma parameter needs to be varied, while all other process and plasma parameters should remain constant. In this work, we use the Electrical Asymmetry Effect in a multi-frequency capacitively coupled plasma to control the ion energy at the substrate without affecting the ion-to-growth flux ratio by adjusting the relative phase between two consecutive driving harmonics and their voltage amplitudes. Measurements of the ion energy distribution function and ion flux at the substrate by a retarding field energy analyzer combined with the determined deposition rate R d for a reactive Ar/N2 (8:1) plasma at 0.5 Pa show a possible variation of the mean ion energy at the substrate E m ig within a range of 38 and 81 eV that allows the modification of the film characteristics at the grounded electrode, when changing the relative phase shift θ between the applied voltage frequencies, while the ion-to-growth flux ratio Γig/Γgr can be kept constant. AlN thin films are deposited and exhibit an increase in compressive film stress from -5.8 to -8.4 GPa as well as an increase in elastic modulus from 175 to 224 GPa as a function of the mean ion energy. Moreover, a transition from the preferential orientation (002) at low ion energies to the (100), (101) and (110) orientations at higher ion energies is observed. In this way, the effects of the ion energy on the growing film are identified, while other process relevant parameters remain unchanged. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6595/ab504b
  • Photocurrent Recombination through Surface Segregation in Al-Cr-Fe-O Photocathodes
    Stein, H.S. and Zhang, S. and Li, Y. and Scheu, C. and Ludwig, Al.
    Zeitschrift fur Physikalische Chemie (2019)
    Chemical surface segregation is a design variable in the optimization of phocathodes but has largely been investigated through surface passivation or decoration. In this study a long charge carrier lifetime material, Al-Cr-Fe-O, exhibiting strong photocurrent recombination is investigated for its atomic scale crystallographic and chemical inhomogeneity. Combined scanning transmission electron microscopy and atom probe tomography unveils that insulating Al- and Cr-rich surface layers form during processing. These are discussed to be the primary reason for experimentally observed charge carrier recombination. This study highlights the importance of processing in the design, discovery and optimization of new light absorber materials for photoelectrochemical water splitting. ©2019 Walter de Gruyter GmbH, Berlin/Boston.
    view abstract10.1515/zpch-2019-1459
  • Reconciling Experimental and Theoretical Data in the Structural Analysis of Ti–Ta Shape-Memory Alloys
    Ferrari, A. and Kadletz, P.M. and Chakraborty, T. and Liao, K. and Langenkämper, D. and Motemani, Y. and Paulsen, A. and Lysogorskiy, Y. and Frenzel, J. and Rogal, J. and Ludwig, Al. and Somsen, C. and Drautz, R. and Schmahl, W.W.
    Shape Memory and Superelasticity 5 (2019)
    The structural characterization of the various phases that occur in Ti–Ta-based high-temperature shape-memory alloys is complicated by the presence of many competing phases as a function of composition. In this study, we resolve apparent inconsistencies between experimental data and theoretical calculations by suggesting that phase separation and segregation of undesired phases are not negligible in these alloys, and that finite temperature effects should be taken into account in the modeling of these materials. Specifically, we propose that the formation of the ω phase at low Ta content and of the σ phase at high Ta content implies a difference between the nominal alloy composition and the actual composition of the martensitic and austenitic phases. In addition, we show that temperature affects strongly the calculated values of the order parameters of the martensitic transformation occurring in Ti–Ta. © 2018, ASM International.
    view abstract10.1007/s40830-018-00201-6
  • Structural and Functional Properties of the Thin Film System Ti-Ni-Si
    Wambach, M. and Ziolkowski, P. and Müller, E. and Ludwig, Al.
    ACS Combinatorial Science 21 (2019)
    The thin film system Ti-Ni-Si was investigated using methods of combinatorial materials science. A thin film composition spread library of the system was fabricated using combinatorial magnetron sputtering. The functional properties Seebeck coefficient, electrical resistivity, and luminance were determined using high-throughput characterization techniques. A thin-film phase diagram was established by the assessment of high-throughput X-ray diffraction results. Correlations between composition, phase constitution, and functional properties with focus on the binary composition space are discussed. © Copyright 2019 American Chemical Society.
    view abstract10.1021/acscombsci.8b00181
  • The 2019 materials by design roadmap
    Alberi, K. and Nardelli, M.B. and Zakutayev, A. and Mitas, L. and Curtarolo, S. and Jain, A. and Fornari, M. and Marzari, N. and Takeuchi, I. and Green, M.L. and Kanatzidis, M. and Toney, M.F. and Butenko, S. and Meredig, B. and Lany, S. and Kattner, U. and Davydov, A. and Toberer, E.S. and Stevanovic, V. and Walsh, A. and Park, N.-G. and Aspuru-Guzik, A. and Tabor, D.P. and Nelson, J. and Murphy, J. and Setlur, A. and Gregoire, J. and Li, H. and Xiao, R. and Ludwig, Al. and Martin, L.W. and Rappe, A.M. and Wei, S.-H. and Perkins, J.
    Journal of Physics D: Applied Physics 52 (2019)
    Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aad926
  • Toward a Paradigm Shift in Electrocatalysis Using Complex Solid Solution Nanoparticles
    Löffler, T. and Savan, A. and Garzón-Manjón, A. and Meischein, M. and Scheu, C. and Ludwig, Al. and Schuhmann, W.
    ACS Energy Letters 4 (2019)
    Complex solid solution (CSS) nanoparticles were recently discovered as efficient electrocatalysts for a variety of reactions. As one of many advantages, they exhibit the potential to replace noble-metal catalysts with multinary combinations of transition metals because they offer formation of new unique and tailorable active sites of multiple elements located next to each other. This Perspective reports on the current state and on challenges of the (combinatorial) synthesis of multinary nanoparticles and advanced electron microscopy characterization techniques for revealing structure-activity correlations on an atomic scale. We discuss what distinguishes this material class from common catalysts to highlight their potential to act as electrocatalysts and rationalize their nontypical electrochemical behavior. We provide an overview about challenges in synthesis, characterization, and electrochemical evaluation and propose guidelines for future design of CSS catalysts to achieve further progress in this research field, which is still in its infancy. © 2019 American Chemical Society.
    view abstract10.1021/acsenergylett.9b00531
  • Accelerated atomic-scale exploration of phase evolution in compositionally complex materials
    Li, Y.J. and Savan, A. and Kostka, A. and Stein, H.S. and Ludwig, Al.
    Materials Horizons 5 (2018)
    Combining nanoscale-tip arrays with combinatorial thin film deposition and processing as well as direct atomic-scale characterization (APT and TEM) enables accelerated exploration of the temperature- and environment-dependent phase evolution in multinary materials systems. Results from nanocrystalline CrMnFeCoNi show that this alloy is unstable and already decomposes after 1 h at low temperatures of around 300 °C. The combinatorial processing platform approach is extendible to explore oxidation and corrosion in complex structural and functional materials on the atomic scale. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c7mh00486a
  • Antibacterial Efficacy of Sacrifical Anode Thin Films Combining Silver with Platinum Group Elements within a Bacteria-Containing Human Plasma Clot
    Abuayyash, A. and Ziegler, N. and Gessmann, J. and Sengstock, C. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Advanced Engineering Materials 20 (2018)
    Silver (Ag) dots arrays (64 and 400 dots per mm2) are fabricated on a continuous platinum (Pt), palladium (Pd), or iridium (Ir) thin film (sacrifical anode systems for Ag) and for comparison on titanium (Ti) film (non-sacrifical anode system for Ag) by sputter deposition and photolithographic patterning. The samples are embedded within a tissue-like plasma clot matrix containing Staphylococcus aureus (S. aureus), cultivated for 24 h. Bacterial growth is analyzed by fluorescence microscopy. Among platinum group sacrifical anode elements and a dense Ag sample, only the high Ag ion releasing Ag–Ir system is able to inhibit the bacterial growth within the adjacent plasma clot matrix. This study demonstrates that the antibacterial efficiency of Ag coatings is reduced under tissue-like conditions. However, the new sacrificial anode based Ag–Ir system can overcome this limitation. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201700493
  • Application of High-Throughput Seebeck Microprobe Measurements on Thermoelectric Half-Heusler Thin Film Combinatorial Material Libraries
    Ziolkowski, P. and Wambach, M. and Ludwig, Al. and Mueller, E.
    ACS Combinatorial Science 20 (2018)
    In view of the variety and complexity of thermoelectric (TE) material systems, combinatorial approaches to materials development come to the fore for identifying new promising compounds. The success of this approach is related to the availability and reliability of high-throughput characterization methods for identifying interrelations between materials structures and properties within the composition spread libraries. A meaningful characterization starts with determination of the Seebeck coefficient as a major feature of TE materials. Its measurement, and hence the accuracy and detectability of promising material compositions, may be strongly affected by thermal and electrical measurement conditions. This work illustrates the interrelated effects of the substrate material, the layer thickness, and spatial property distributions of thin film composition spread libraries, which are studied experimentally by local thermopower scans by means of the Potential and Seebeck Microprobe (PSM). The study is complemented by numerical evaluation. Material libraries of the half-Heusler compound system Ti-Ni-Sn were deposited on selected substrates (Si, AlN, Al2O3) by magnetron sputtering. Assuming homogeneous properties of a film, significant decrease of the detected thermopower Sm can be expected on substrates with higher thermal conductivity, yielding an underestimation of materials thermopower between 15% and 50%, according to FEM (finite element methods) simulations. Thermally poor conducting substrates provide a better accuracy with thermopower underestimates lower than 8%, but suffer from a lower spatial resolution. According to FEM simulations, local scanning of sharp thermopower peaks on lowly conductive substrates is linked to an additional deviation of the measured thermopower of up to 70% compared to homogeneous films, which is 66% higher than for corresponding cases on substrates with higher thermal conductivity of this study. © 2017 American Chemical Society.
    view abstract10.1021/acscombsci.7b00019
  • Atomic-scale investigation of fast oxidation kinetics of nanocrystalline CrMnFeCoNi thin films
    Li, Y.J. and Kostka, A. and Savan, A. and Ludwig, Al.
    Journal of Alloys and Compounds 766 (2018)
    Atom probe tomography was combined with transmission electron microscopy to characterize in atomic detail the structure of nanocrystalline high entropy alloy CrMnFeCoNi thin films before and after exposure to air at 500 °C for 5 min. Mn and Cr oxide scales were observed on the surface of the sample. These results on the nanoscale for short experimental time agree with literature reports on bulk counterparts after oxidation at 900 °C for 100 h, which means that the oxidation performance of complex materials can be tested in an accelerated way. Moreover, oxidation-related sub-surface depletion of Mn and Cr together with partial decomposition of the initial Cantor phase yield a FeCo-B2 sub-surface phase. In comparison to coarse-grained bulk material, the faster oxidation of the nanocrystalline material was attributed to an enhanced outwards diffusion of Cr and Mn along grain boundaries and within the newly formed B2 phase. © 2018 Elsevier B.V.
    view abstract10.1016/j.jallcom.2018.07.048
  • Bacterial cell division is involved in the damage of gram-negative bacteria on a nano-pillar titanium surface
    Köller, M. and Ziegler, N. and Sengstock, C. and Schildhauer, T.A. and Ludwig, Al.
    Biomedical Physics and Engineering Express 4 (2018)
    The role of bacterial cell division on the damage of adherent bacteria to titanium (Ti) nano-pillar cicada wing like surface was analyzed. Therefore nano-pillar Ti thin films were fabricated by glancing angle sputter deposition (GLAD) on silicon substrates. Gram-negative E. coli bacteria were allowed to adhere and to proliferate on these nanostructured samples for 3 h at 37 °C either under optimal cell growth conditions (brain heart infusion medium, BHI) or limited growth conditions (RPMI1640 medium). The bacteria adhered to the samples in both media. Compared to BHI medium the growth of E. coli in RPMI1640 medium was significantly inhibited. Concomitantly, the ratio of dead/living adherent bacteria on the nano-pillar surface was significantly decreased after the incubation period in RPMI1640. In addition, when the bacterial proliferation was biochemically halted using DL-serine-hydroxamate a comparable decrease in the ratio of dead/living adherent bacteria was also obtained in BHI medium. These results indicate that cell growth of adherent E. coli which is accompanied by cell elongations of the rod structure is involved in the damage induced by the titanium nano-pillar surface. © 2018 IOP Publishing Ltd.
    view abstract10.1088/2057-1976/aad2c1
  • Charge Carrier Lifetimes in Cr-Fe-Al-O Thin Films
    Stein, H.S. and Müller, S. and Schwarzburg, K. and Friedrich, D. and Ludwig, Al. and Eichberger, R.
    ACS Applied Materials and Interfaces 10 (2018)
    The effect of compositional variation on charge carrier lifetimes of Cr1Fe0.84Al0.16O3, a promising material for solar water splitting recently identified using combinatorial materials science, is explored using ultrafast time-resolved optical reflectance. The transient signal can be described by a biexponential decay, where the shorter time constant varies over 1 order of magnitude with changing Cr content while the longer one stays constant. Intrinsic performance limitations such as a low charge carrier mobility on the order of 10-3 cm2/(Vs) are identified. Charge carrier lifetime and mobility are discussed as screening criteria for solar water splitting materials. © 2018 American Chemical Society.
    view abstract10.1021/acsami.8b04900
  • Combinatorial metallurgical synthesis and processing of high-entropy alloys
    Li, Z. and Ludwig, Al. and Savan, A. and Springer, H. and Raabe, D.
    Journal of Materials Research (2018)
    High-entropy alloys (HEAs) with multiple principal elements open up a practically infinite space for designing novel materials. Probing this huge material universe requires the use of combinatorial and high-throughput synthesis and processing methods. Here, we present and discuss four different combinatorial experimental methods that have been used to accelerate the development of novel HEAs, namely, rapid alloy prototyping, diffusion-multiples, laser additive manufacturing, and combinatorial co-deposition of thin-film materials libraries. While the first three approaches are bulk methods which allow for downstream processing and microstructure adaptation, the latter technique is a thin-film method capable of efficiently synthesizing wider ranges of composition and using high-throughput measurement techniques to characterize their structure and properties. Additional coupling of these high-throughput experimental methodologies with theoretical guidance regarding specific target features such as phase (meta)stability allows for effective screening of novel HEAs with beneficial property profiles. Copyright © Materials Research Society 2018
    view abstract10.1557/jmr.2018.214
  • Combinatorial Study on Phase Formation and Oxidation in the Thin Film Superalloy Subsystems Co-Al-Cr and Co-Al-Cr-W
    Naujoks, D. and Weiser, M. and Salomon, S. and Stein, H. and Virtanen, S. and Ludwig, Al.
    ACS Combinatorial Science 20 (2018)
    Two Co-based superalloy subsystems, the ternary system Co-Al-Cr and the quasi-ternary system Co-Al-Cr-W with a constant amount of 10 at. % W, were deposited as thin-film materials libraries and analyzed in terms of phase formation and oxidation behavior at 500 °C in air. By combining energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy high-throughput composition measurements, a detailed evaluation of the dependence between the initial multinary metal composition and the oxide scale composition which is forming upon oxidation on the surface of the thin film is established. Phase maps for both materials libraries are provided by high-throughput X-ray diffraction. In addition, the oxidation of a Co-Al-Cr-W bulk sample was analyzed and compared to a corresponding film in the library. © 2018 American Chemical Society.
    view abstract10.1021/acscombsci.8b00078
  • Combinatorial Synthesis and High-Throughput Characterization of Fe-V-O Thin-Film Materials Libraries for Solar Water Splitting
    Kumari, S. and Gutkowski, R. and Junqueira, J.R.C. and Kostka, A. and Hengge, K. and Scheu, C. and Schuhmann, W. and Ludwig, Al.
    ACS Combinatorial Science 20 (2018)
    The search for suitable materials for solar water splitting is addressed with combinatorial material science methods. Thin film Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron cosputtering and subsequent annealing in air. The design of the libraries comprises a combination of large compositional gradients (from Fe10V90Ox to Fe79V21Ox) and thickness gradients (from 140 to 425 nm). These material libraries were investigated by high-throughput characterization techniques in terms of composition, structure, optical, and photoelectrochemical properties to establish correlations between composition, thickness, crystallinity, microstructure, and photocurrent density. Results show the presence of the Fe2V4O13 phase from ∼11 to 42 at. % Fe (toward low-Fe region) and the FeVO4 phase from ∼37 to 79 at. % Fe (toward Fe-rich region). However, as a third phase, Fe2O3 is present throughout the compositional gradients (from low-Fe to Fe-rich region). Material compositions with increasing crystallinity of the FeVO4 phase show enhanced photocurrent densities (∼160 to 190 μA/cm2) throughout the thickness gradients whereas compositions with the Fe2V4O13 phase show comparatively low photocurrent densities (∼28 μA/cm2). The band gap energies of Fe-V-O films were inferred from Tauc plots. The highest photocurrent density of ∼190 μA/cm2 was obtained for films with ∼54 to 66 at. % Fe for the FeVO4 phase with ∼2.04 eV for the indirect and ∼2.80 eV for the direct band gap energies. © 2018 American Chemical Society.
    view abstract10.1021/acscombsci.8b00030
  • Controlling the amorphous and crystalline state of multinary alloy nanoparticles in an ionic liquid
    Garzón-Manjón, A. and Meyer, H. and Grochla, D. and Löffler, T. and Schuhmann, W. and Ludwig, Al. and Scheu, C.
    Nanomaterials 8 (2018)
    Controlling the amorphous or crystalline state of multinary Cr-Mn-Fe-Co-Ni alloy nanoparticles with sizes in the range between ~1.7 and ~4.8 nm is achieved using three processing routes. Direct current sputtering from an alloy target in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide leads to amorphous nanoparticles as observed by high-resolution transmission electron microscopy. Crystalline nanoparticles can be achieved in situ in a transmission electron microscope by exposure to an electron beam, ex situ by heating in vacuum, or directly during synthesis by using a high-power impulse magnetron sputtering process. Growth of the nanoparticles with respect to the amorphous particles was observed. Furthermore, the crystal structure can be manipulated by the processing conditions. For example, a body-centered cubic structure is formed during in situ electron beam crystallization while longer ex situ annealing induces a face-centered cubic structure. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano8110903
  • Crystallographic Structure Analysis of a Ti-Ta Thin Film Materials Library Fabricated by Combinatorial Magnetron Sputtering
    Kadletz, P.M. and Motemani, Y. and Iannotta, J. and Salomon, S. and Khare, C. and Grossmann, L. and Maier, H.J. and Ludwig, Al. and Schmahl, W.W.
    ACS Combinatorial Science 20 (2018)
    Ti-Ta thin films exhibit properties that are of interest for applications as microactuators and as biomedical implants. A Ti-Ta thin film materials library was deposited at T = 25 °C by magnetron sputtering employing the combinatorial approach, which led to a compositional range of Ti87Ta13 to Ti14Ta86. Subsequent high-throughput characterization methods permitted a quick and comprehensive study of the crystallographic, microstructural, and morphological properties, which strongly depend on the chemical composition. SEM investigation revealed a columnar morphology having pyramidal, sharp tips with coarser columns in the Ti-rich and finer columns in the Ta-rich region. By grazing incidence X-ray diffraction four phases were identified, from Ta-lean to Ta-rich: ω phase, α″ martensite, β phase, and a tetragonal Ta-rich phase (Ta(tetr)). The crystal structure and microstructure were analyzed by Rietveld refinement and clear trends could be determined as a function of Ta-content. The lattice correspondences between β as the parent phase and α″ and ω as derivative phases were expressed in matrix form. The β α″ phase transition shows a discontinuity at the composition where the martensitic transformation temperatures fall below room temperature (between 34 and 38 at. % Ta) rendering it first order and confirming its martensitic nature. A short study of the α″ martensite employing the Landau theory is included for a mathematical quantification of the spontaneous lattice strain at room temperature (max = 22.4(6) % for pure Ti). Martensitic properties of Ti-Ta are beneficial for the development of high-temperature actuators with actuation response at transformation temperatures higher than 100 °C. © 2018 American Chemical Society.
    view abstract10.1021/acscombsci.7b00135
  • Development of Single-Crystal Ni-Base Superalloys Based on Multi-criteria Numerical Optimization and Efficient Use of Refractory Elements
    Markl, M. and Müller, A. and Ritter, N. and Hofmeister, M. and Naujoks, D. and Schaar, H. and Abrahams, K. and Frenzel, J. and Subramanyam, A.P.A. and Ludwig, Al. and Pfetzing-Micklich, J. and Hammerschmidt, T. and Drautz, R. and Steinbach, I. and Rettig, R. and Singer, R.F. and Körner, C.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 (2018)
    The development of new Ni-base superalloys with a complex composition consisting of eight or more alloying elements is a challenging task. The experimental state-of-the-art development cycle is based on the adaption of already existing compositions. Although new alloy compositions with potentially improved material properties are expected to be similar to already known superalloys, this procedure impedes efficiently finding these compositions in the large multi-dimensional design-space of all alloying elements. Modern alloy development combines numerical optimization methods with experimental validation to guide the development towards promising compositions. In this work, an improved numerical multi-criteria optimization tool using CALPHAD calculations and semi-empirical models for alloy development is presented. The model improvements to its predecessor are described and the successful application for the development of rhenium-free single-crystal Ni-base superalloys ERBO/13 and ERBO/15 is revisited. The optimization tool is described and the designed alloys are discussed regarding phase stability. Finally, a possible phase stability model extending the optimization tool and improving the alloy composition predictions is presented. © 2018, The Author(s).
    view abstract10.1007/s11661-018-4759-0
  • Discovery of a Multinary Noble Metal–Free Oxygen Reduction Catalyst
    Löffler, T. and Meyer, H. and Savan, A. and Wilde, P. and Garzón Manjón, A. and Chen, Y.-T. and Ventosa, E. and Scheu, C. and Ludwig, Al. and Schuhmann, W.
    Advanced Energy Materials 8 (2018)
    In the endeavor of discovering new noble metal–free electrocatalysts for the oxygen reduction reaction, noble metal–free multinary transition metal nanoparticle libraries are investigated. The complexity of such multiple principal element alloys provides access to a large variety of different elemental compositions, each with potentially unique properties. The strategy for efficient identification of novel electrocatalytically active systems comprises combinatorial co-sputtering into an ionic liquid followed by potential-assisted immobilization of the formed nanoparticles at a microelectrode which allows the evaluation of their intrinsic electrocatalytic activity in alkaline media. A surprisingly high intrinsic activity is found for the system Cr–Mn–Fe–Co–Ni, which is at least comparable to Pt under the same conditions, an unexpected result based on the typical properties of its constituents. Systematic removal of each element from the quinary alloy system yields a significant drop in activity for all quaternary alloys, indicating the importance of the synergistic combination of all five elements, likely due to formation of a single solid solution phase with altered properties which enables the limitations of the single elements to be overcome. Multinary transition metal alloys as a novel material class in electrocatalysis with basically unlimited possibilities for catalyst design, targeting the replacement of noble metal–based materials, are suggested. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201802269
  • Effect of Pt and Au current collector in LiMn2O4 thin film for micro-batteries
    Trócoli, R. and Dushina, A. and Borhani-Haghighi, S. and Ludwig, Al. and La Mantia, F.
    Nanotechnology 29 (2018)
    The crystal orientation and morphology of sputtered LiMn2O4 thin films is strongly affected by the current collector. By substituting Pt with Au, it is possible to observe in the x-ray diffraction pattern of LiMn2O4 a change in the preferential orientation of the grains from (111) to (400). In addition, LiMn2O4 thin films deposited on Au show a higher porosity than films deposited on Pt. These structural differences cause an improvement in the electrochemical performances of the thin films deposited on Au, with up to 50% more specific charge. Aqueous cells using thin film based on LiMn2O4 sputtered on Au or Pt as the cathode electrode present a similar retention of specific charge, delivering 85% and 100%, respectively, of the initial values after 100 cycles. The critical role of the nature of the substrate used in the morphology and electrochemical behaviour observed could permit the exploration of similar effects for other lithium intercalation electrodes. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa9e33
  • Improved homogeneity of plasma and coating properties using a lance matrix gas distribution in MW-PECVD
    Kirchheim, D. and Wilski, S. and Jaritz, M. and Mitschker, F. and Oberberg, M. and Trieschmann, J. and Banko, L. and Brochhagen, M. and Schreckenberg, R. and Hopmann, C. and Böke, M. and Benedikt, J. and de los Arcos, T. and Grundmeier, G. and Grochla, D. and Ludwig, Al. and Mussenbrock, T. and Brinkmann, R.P. and Awakowicz, P. and Dahlmann, R.
    Journal of Coatings Technology and Research (2018)
    Plasma reactors for the application of silicon oxide coatings (SiOx) are often customized to optimize the processes regarding substrate properties and targeted functionalities. The design of these reactors is often based on qualitative considerations. This paper evaluates the use of a numerical, free simulation software for continuous mechanical problems (OpenFOAM) as a tool to evaluate reactor design options. As demonstrator for this purpose serves a given reactor for large-area pulsed microwave plasmas with a precursor inlet in the form of a shower ring. Previous results indicate that the shower ring may lead to an inhomogeneity in plasma and coatings properties along the substrate surface. Thus, a new precursor inlet design shall be developed. For this, the distribution of the process gases in the reactor for a variety of gas inlet designs and gas flows was simulated and a design chosen based on the results. The reactor was modified accordingly, and the simulations correlated with experimental results of plasma and coating properties. The results show that, despite many simplifications, a simulation of the neutral gas distribution using an open-access software can be a viable tool to support reactor and process design development. © 2018, American Coatings Association.
    view abstract10.1007/s11998-018-0138-4
  • Influences of W Content on the Phase Transformation Properties and the Associated Stress Change in Thin Film Substrate Combinations Studied by Fabrication and Characterization of Thin Film V1- xWxO2 Materials Libraries
    Wang, X. and Rogalla, D. and Ludwig, Al.
    ACS Combinatorial Science 20 (2018)
    The mechanical stress change of VO2 film substrate combinations during their reversible phase transformation makes them promising for applications in micro/nanoactuators. V1-xWxO2 thin film libraries were fabricated by reactive combinatorial cosputtering to investigate the effects of the addition of W on mechanical and other transformation properties. High-throughput characterization methods were used to systematically determine the composition spread, crystalline structure, surface topography, as well as the temperature-dependent phase transformation properties, that is, the hysteresis curves of the resistance and stress change. The study indicates that as x in V1-xWxO2 increases from 0.007 to 0.044 the crystalline structure gradually shifts from the VO2 (M) phase to the VO2 (R) phase. The transformation temperature decreases by 15 K/at. % and the resistance change is reduced to 1 order of magnitude, accompanied by a wider transition range and a narrower hysteresis with a minimal value of 1.8 K. A V1-xWxO2 library deposited on a Si3N4/SiO2-coated Si cantilever array wafer was used to study simultaneously the temperature-dependent stress change σ(T) of films with different W content through the phase transformation. Compared with σ(T) of ∼700 MPa of a VO2 film, σ(T) in V1-xWxO2 films decreases to ∼250 MPa. Meanwhile, σ(T) becomes less abrupt and occurs over a wider temperature range with decreased transformation temperatures. © 2018 American Chemical Society.
    view abstract10.1021/acscombsci.7b00192
  • Martensitic transformation hysteresis in Ni(Co)-Mn-Sn/MgO metamagnetic shape memory thin films
    Alexandrakis, V. and Aseguinolaza, I.R. and Decker, P. and Salomon, S. and Barandiarán, J.M. and Ludwig, Al. and Chernenko, V.A.
    Scripta Materialia 156 (2018)
    Hysteresis of martensitic transformations (MTs) is a crucial factor which affects the efficiency and lifetime of actuators and magnetocaloric devices. In the present work we found a tenfold reduction of the thermal hysteresis of MT caused by increasing the MT temperature in a series of Ni(Co)-Mn-Sn thin films deposited onto MgO (001). We have observed that this evolution is accompanied by a drastic change of the temperature dependence of the lattice parameter in the temperature interval of MT. Such a behaviour can be interpreted using the concept of the critical state, where an anhysteretic behaviour is expected. © 2018
    view abstract10.1016/j.scriptamat.2018.07.015
  • 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 (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 abstract10.1016/j.tsf.2017.10.030
  • PEALD of SiO2 and Al2O3 Thin Films on Polypropylene: Investigations of the Film Growth at the Interface, Stress, and Gas Barrier Properties of Dyads
    Gebhard, M. and Mai, L. and Banko, L. and Mitschker, F. and Hoppe, C. and Jaritz, M. and Kirchheim, D. and Zekorn, C. and De Los Arcos, T. and Grochla, D. and Dahlmann, R. and Grundmeier, G. and Awakowicz, P. and Ludwig, Al. and Devi, A.
    ACS Applied Materials and Interfaces 10 (2018)
    A study on the plasma-enhanced atomic layer deposition of amorphous inorganic oxides SiO2 and Al2O3 on polypropylene (PP) was carried out with respect to growth taking place at the interface of the polymer substrate and the thin film employing in situ quartz-crystal microbalance (QCM) experiments. A model layer of spin-coated PP (scPP) was deposited on QCM crystals prior to depositions to allow a transfer of findings from QCM studies to industrially applied PP foil. The influence of precursor choice (trimethylaluminum (TMA) vs [3-(dimethylamino)propyl]-dimethyl aluminum (DMAD)) and of plasma pretreatment on the monitored QCM response was investigated. Furthermore, dyads of SiO2/Al2O3, using different Al precursors for the Al2O3 thin-film deposition, were investigated regarding their barrier performance. Although the growth of SiO2 and Al2O3 from TMA on scPP is significantly hindered if no oxygen plasma pretreatment is applied to the scPP prior to depositions, the DMAD process was found to yield comparable Al2O3 growth directly on scPP similar to that found on a bare QCM crystal. From this, the interface formed between the Al2O3 and the PP substrate is suggested to be different for the two precursors TMA and DMAD due to different growth modes. Furthermore, the residual stress of the thin films influences the barrier properties of SiO2/Al2O3 dyads. Dyads composed of 5 nm Al2O3 (DMAD) + 5 nm SiO2 exhibit an oxygen transmission rate (OTR) of 57.4 cm3 m-2 day-1, which correlates with a barrier improvement factor of 24 against 5 when Al2O3 from TMA is applied. © 2018 American Chemical Society.
    view abstract10.1021/acsami.7b14916
  • Rapid Assessment of Sputtered Nanoparticle Ionic Liquid Combinations
    Meyer, H. and Meischein, M. and Ludwig, Al.
    ACS Combinatorial Science 20 (2018)
    A high-throughput method is presented for the efficient assessment of the formation and stability of nanoparticle suspensions in ionic liquids which differ in their cations and anions. As a proof of principle, Ag was sputtered on a cavity array filled with 9 different ionic liquids. Not all nanoparticle ionic liquid combinations form a stable suspension with separated nanoparticles. Directly after synthesis, the formation of nonagglomerated nanoparticle suspensions with sizes from 4 to 9 nm is observed by transmission electron microscopy as well as different time dependencies of the suspension stabilities. Only 3 out of the tested 9 nanoparticle ionic liquid suspensions show long-term stability: Stable suspension of spherical nanoparticles are formed in the ionic liquids 1-butyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide [Bmim][(Pf)2N], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][(Tf)2N], and 1-butyl-1-methylpyrrolidinum bis(trifluoromethylsulfonyl)imide [BmPyr][(Tf)2N]. © 2018 American Chemical Society.
    view abstract10.1021/acscombsci.8b00017
  • Si micro-cantilever sensor chips for space-resolved stress measurements in physical and plasma-enhanced chemical vapour deposition
    Grochla, D. and Banko, L. and Pfetzing-Micklich, J. and Behm, H. and Dahlmann, R. and Ludwig, Al.
    Sensors and Actuators, A: Physical 270 (2018)
    The control of extrinsic and intrinsic mechanical stresses in thin films is crucial. Stresses can limit the film performance e.g. by stress-induced delamination or undesired bending of film/substrate combinations; however, stresses can also be used to obtain functionality. Thus, understanding of stress-inducing mechanisms, correlations of stress with film synthesis parameters and controlling the sign and amplitude of stresses in thin films is important and a facile and reliable stress measurement method is necessary. Here, a stress measurement chip is presented which is based on the measurement of the residual overall film stress by a film-substrate combination curvature-based measurement technique. The novel Si-based cantilever sensor chip can measure residual stress in films from a few nanometers thickness up to several microns. Moreover, the sensor chips are applicable for determining the coefficient of thermal expansion, and for examining the film thickness homogeneity over large areas in a deposition system. They can be applied in physical vapor deposition and plasma-enhanced chemical vapor deposition processes with different geometrical and process-related boundary conditions. Exemplary results which were obtained with the sensor chips are discussed to demonstrate their easy applicability, accuracy, versatility, reliability, the thickness dependence of the residual stress and the homogeneity of SiOx films as well as the residual stress and the thermal expansion values of Al-Cr-N films. © 2017 Elsevier B.V.
    view abstract10.1016/j.sna.2017.12.050
  • The stability number as a metric for electrocatalyst stability benchmarking
    Geiger, S. and Kasian, O. and Ledendecker, M. and Pizzutilo, E. and Mingers, A.M. and Fu, W.T. and Diaz-Morales, O. and Li, Z. and Oellers, T. and Fruchter, L. and Ludwig, Al. and Mayrhofer, K.J.J. and Koper, M.T.M. and Cherevko, S.
    Nature Catalysis 1 (2018)
    Reducing the noble metal loading and increasing the specific activity of the oxygen evolution catalysts are omnipresent challenges in proton-exchange-membrane water electrolysis, which have recently been tackled by utilizing mixed oxides of noble and non-noble elements. However, proper verification of the stability of these materials is still pending. Here we introduce a metric to explore the dissolution processes of various iridium-based oxides, defined as the ratio between the amounts of evolved oxygen and dissolved iridium. The so-called stability number is independent of loading, surface area or involved active sites and provides a reasonable comparison of diverse materials with respect to stability. The case study on iridium-based perovskites shows that leaching of the non-noble elements in mixed oxides leads to the formation of highly active amorphous iridium oxide, the instability of which is explained by the generation of short-lived vacancies that favour dissolution. These insights are meant to guide further research, which should be devoted to increasing the utilization of highly durable pure crystalline iridium oxide and finding solutions to stabilize amorphous iridium oxides. © 2018 The Author(s).
    view abstract10.1038/s41929-018-0085-6
  • Using Instability of a Non-stoichiometric Mixed Oxide Oxygen Evolution Catalyst As a Tool to Improve Its Electrocatalytic Performance
    Kasian, O. and Geiger, S. and Schalenbach, M. and Mingers, A.M. and Savan, A. and Ludwig, Al. and Cherevko, S. and Mayrhofer, K.J.J.
    Electrocatalysis 9 (2018)
    Owing to their superior electrocatalytic performance, non-stoichiometric mixed oxides are often considered as promising electrocatalysts for the acidic oxygen evolution reaction (OER). Their activity and stability can be superior to those of the state-of-the-art IrO2 catalysts, although the exact nature of this phenomenon is not yet understood. In the current work, a Ir0.7Sn0.3O2-x thin-film electrode is taken as a representative example for a thorough evaluation of OER activity of the non-stoichiometric oxides. Complementary activity and stability analysis of Ir0.7Sn0.3O2-x electrodes is achieved using a setup based on an electrochemical scanning flow cell and ICP-MS. The obtained ICP-MS data presents an unambiguous proof of the preferential dissolution of the less noble Sn from the mixed oxide during OER. While less than a monolayer of Ir is dissolved after a prolonged electrolysis of 1400 min during which its dissolution rate drops to near zero, the amount of Sn lost is ten monolayers. The latter finding is confirmed by XPS analysis, which besides showing Ir surface enrichment also indicates a gradual transformation of Ir0 to IrIII species. This transition is beneficial for electrode activity, as the overpotential for OER at j = 5 mA cm−2 was decreasing up to 300 mV. The increase in electrode activity is attributed to several mechanisms including generation of IrIII active sites and overall surface area increase. A generalized description of OER catalysis by Ir-based materials is given, including data from the current work as well as from other Ir-based mixed oxides, such as Ir-Ru-O and Ir-Ni-O. [Figure not available: see fulltext.]. © 2017, The Author(s).
    view abstract10.1007/s12678-017-0394-6
  • A Unified Interdisciplinary Approach to Design Antibacterial Coatings for Fast Silver Release
    El Arrassi, A. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Köller, M. and Ludwig, Al. and Tschulik, K.
    ChemElectroChem (2017)
    The increasing number of surgical treatments performed per year requires novel approaches to inhibit implant-associated infections, caused by multi-antibiotic resistant bacteria. Silver ions (Ag+) are known for their effective antimicrobial activity. Therefore, a system that efficiently and locally releases the minimum required amount of Ag+ directly after the surgical treatment is in high demand. Herein we study electrochemically, microbiologically, microscopically and spectroscopically sacrificial Ag anode coatings for antibacterial implant applications. It is found that Ag dot arrays deposited on noble metals (Pd, Ir) release Ag+ much faster than continuous Ag thin films. The Ag+ release qualitatively scales with the difference of standard potentials between Ag and the noble metal. Furthermore, with higher numbers of Ag dots, the total amount of released Ag+ increases, while the release efficiency declines. Notably, an efficient killing of Staphylococcus aureus bacteria was seen for coatings containing as little as 23ng of Ag per mm2. Thus, the use of sacrificial Ag anodes as highly efficient antibacterial coating materials is evaluated. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201700247
  • Antibacterial activity of microstructured sacrificial anode thin films by combination of silver with platinum group elements (platinum, palladium, iridium)
    Köller, M. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Tschulik, K. and Schildhauer, T.A. and Ludwig, Al.
    Materials Science and Engineering C 74 (2017)
    Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards . S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd. © 2016 Elsevier B.V.
    view abstract10.1016/j.msec.2016.12.075
  • Combinatorial screening of Pd-based quaternary electrocatalysts for oxygen reduction reaction in alkaline media
    Li, J. and Stein, H.S. and Sliozberg, K. and Liu, J. and Liu, Y. and Sertic, G. and Scanley, E. and Ludwig, Al. and Schroers, J. and Schuhmann, W. and Taylor, A.D.
    Journal of Materials Chemistry A 5 (2017)
    The implementation of electrochemical systems such as fuel cells has been hindered by the slow development of low cost high activity catalysts. Here we examine the oxygen reduction reaction performance of a combinatorial Pd-Au-Ag-Ti thin film library using high-throughput screening and correlate the electrochemical behavior to the crystallographic properties. We find compositions of ca. 40-60 at% Pd and 30-35 at% Au exhibit both a low overpotential of close to the value of pure Pt as well as high current density. We also observe a volcano-like relationship between the overpotential and the solid formation strain. This study provides compositional guidance towards the future synthesis of nanostructured quaternary Pd-Au-Ag-Ti alloys and suggests the potential for broader application of high-throughput electrochemical characterization by means of an automatic scanning droplet cell. © The Royal Society of Chemistry.
    view abstract10.1039/C6TA08088J
  • Combinatorial study of Fe-Co-V hard magnetic thin films
    Fackler, S.W. and Alexandrakis, V. and König, D. and Kusne, A.G. and Gao, T. and Kramer, M.J. and Stasak, D. and Lopez, K. and Zayac, B. and Mehta, A. and Ludwig, Al. and Takeuchi, I.
    Science and Technology of Advanced Materials 18 (2017)
    Thin film libraries of Fe-Co-V were fabricated by combinatorial sputtering to study magnetic and structural properties over wide ranges of composition and thickness by high-throughput methods: synchrotron X-ray diffraction, magnetometry, composition, and thickness were measured across the Fe-Co-V libraries. In-plane magnetic hysteresis loops were shown to have a coercive field of 23.9 kA m–1 (300 G) and magnetization of 1000 kA m–1. The out-of-plane direction revealed enhanced coercive fields of 207 kA m–1 (2.6 kG) which was attributed to the shape anisotropy of column grains observed with electron microscopy. Angular dependence of the switching field showed that the magnetization reversal mechanism is governed by 180° domain wall pinning. In the thickness-dependent combinatorial study, co-sputtered composition spreads had a thickness ranging from 50 to 500 nm and (Fe70Co30)100-xVx compositions of x = 2–80. Comparison of high-throughput magneto-optical Kerr effect and traditional vibrating sample magnetometer measurements show agreement of trends in coercive fields across large composition and thickness regions. © 2017 The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis.
    view abstract10.1080/14686996.2017.1287520
  • Combinatorial synthesis and high-throughput characterization of structural and photoelectrochemical properties of Fe:WO3 nanostructured libraries
    Khare, C. and Sliozberg, K. and Stepanovich, A. and Schuhmann, W. and Ludwig, Al.
    Nanotechnology 28 (2017)
    Porous and photoelectrochemically active Fe-doped WO3 nanostructures were obtained by a combinatorial dealloying method. Two types of precursor materials libraries, exhibiting dense and nano-columnar morphology were fabricated by using two distinct magnetron sputter deposition geometries. Both libraries were subjected to combinatorial dealloying enabling preparation and screening of a large quantity of compositions having different nanostructures. This approach allows identifying materials with interesting photoelectrochemical characteristics. The dealloying process selectively dissolved Fe from the composition gradient precursor W-Fe materials library, resulting in formation of monoclinic single crystalline nanoblade-like structures over the entire surface. Photoelectrochemical properties of nanostructured Fe:WO3 films were found to be composition-dependent. The measurement region doped with ∼1.7 at % Fe and a film thickness of ∼ 900-1100 nm displayed highly porous WO3 nanostructures and exhibited the highest photocurrent density of ∼ 72 μA cm-2. This enhanced photocurrent density is attributed to the decreased bandgap values, suppressed recombination of electron-hole pairs, improved light absorption as well as efficient charge transport in the highly porous Fe-doped film with single crystalline WO3 nanoblades. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa6964
  • Correlating Oxygen Evolution Catalysts Activity and Electronic Structure by a High-Throughput Investigation of Ni 1-y-z Fe y Cr z O x
    Schwanke, C. and Stein, H.S. and Xi, L. and Sliozberg, K. and Schuhmann, W. and Ludwig, Al. and Lange, K.M.
    Scientific Reports 7 (2017)
    High-throughput characterization by soft X-ray absorption spectroscopy (XAS) and electrochemical characterization is used to establish a correlation between electronic structure and catalytic activity of oxygen evolution reaction (OER) catalysts. As a model system a quasi-ternary materials library of Ni 1-y-zFe y Cr z O x was synthesized by combinatorial reactive magnetron sputtering, characterized by XAS, and an automated scanning droplet cell. The presence of Cr was found to increase the OER activity in the investigated compositional range. The electronic structure of Ni II and Cr III remains unchanged over the investigated composition spread. At the Fe L-edge a linear combination of two spectra was observed. These spectra were assigned to Fe III in O h symmetry and Fe III in T d symmetry. The ratio of Fe III O h to Fe III T d increases with the amount of Cr and a correlation between the presence of the Fe III O h and a high OER activity is found.
    view abstract10.1038/srep44192
  • Correlative plasma-surface model for metastable Cr-Al-N: Frenkel pair formation and influence of the stress state on the elastic properties
    Music, D. and Banko, L. and Ruess, H. and Engels, M. and Hecimovic, A. and Grochla, D. and Rogalla, D. and Brögelmann, T. and Ludwig, Al. and Von Keudell, A. and Bobzin, K. and Schneider, J.M.
    Journal of Applied Physics 121 (2017)
    Correlatively employing density functional theory and experiments congregated around high power pulsed magnetron sputtering, a plasma-surface model for metastable Cr0.8Al0.2N (space group Fm 3 m) is developed. This plasma-surface model relates plasma energetics with film composition, crystal structure, mass density, stress state, and elastic properties. It is predicted that N Frenkel pairs form during Cr0.8Al0.2N growth due to high-energy ion irradiation, yielding a mass density of 5.69 g cm-3 at room temperature and Young's modulus of 358-130 GPa in the temperature range of 50-700 K for the stress-free state and about 150 GPa larger values for the compressive stress of 4 GPa. Our measurements are consistent with the quantum mechanical predictions within 5% for the mass density and 3% for Young's modulus. The hypothesis of a stress-induced Young's modulus change may at least in part explain the spread in the reported elasticity data ranging from 250 to 420 GPa. © 2017 Author(s).
    view abstract10.1063/1.4985172
  • Expediting combinatorial data set analysis by combining human and algorithmic analysis
    Stein, H.S. and Jiao, S. and Ludwig, Al.
    ACS Combinatorial Science 19 (2017)
    A challenge in combinatorial materials science remains the efficient analysis of X-ray diffraction (XRD) data and its correlation to functional properties. Rapid identification of phase-regions and proper assignment of corresponding crystal structures is necessary to keep pace with the improved methods for synthesizing and characterizing materials libraries. Therefore, a new modular software called htAx (highthroughput analysis of X-ray and functional properties data) is presented that couples human intelligence tasks used for "ground-truth" phase-region identification with subsequent unbiased verification by an algorithm to efficiently analyze which phases are present in a materials library. Identified phases and phase-regions may then be correlated to functional properties in an expedited manner. For the functionality of htAx to be proven, two previously published XRD benchmark data sets of the materials systems Al-Cr-Fe-O and Ni-Ti-Cu are analyzed by htAx. The analysis of ∼1000 XRD patterns takes less than 1 day with htAx. The proposed method reliably identifies phase-region boundaries and robustly identifies multiphase structures. The method also addresses the problem of identifying regions with previously unpublished crystal structures using a special daisy ternary plot. © 2016 American Chemical Society.
    view abstract10.1021/acscombsci.6b00151
  • Fundamental study of an industrial reactive HPPMS (Cr,Al)N process
    Bobzin, K. and Brögelmann, T. and Kruppe, N.C. and Engels, M. and Von Keudell, A. and Hecimovic, A. and Ludwig, Al. and Grochla, D. and Banko, L.
    Journal of Applied Physics 122 (2017)
    In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the plasma is nearly constant for the frequency variation. In contrast, the metal to gas ion flux ratio is changed from JM/JG = 0.51 to JM/JG = 0.10 for increasing frequency. Regarding the coating properties, a structure refinement as well as lower residual stresses, higher universal hardness, and a changing crystal orientation from (111) to (200) were observed at higher frequencies. By correlating the plasma and coating properties, it can be concluded that the change in the gas ion to metal ion flux ratio results in a competitive crystal growth of the film, which results in changing coating properties. © 2017 Author(s).
    view abstract10.1063/1.4990997
  • High-Density Droplet Microarray of Individually Addressable Electrochemical Cells
    Zhang, H. and Oellers, T. and Feng, W. and Abdulazim, T. and Saw, E.N. and Ludwig, Al. and Levkin, P.A. and Plumeré, N.
    Analytical Chemistry 89 (2017)
    Microarray technology has shown great potential for various types of high-throughput screening applications. The main read-out methods of most microarray platforms, however, are based on optical techniques, limiting the scope of potential applications of such powerful screening technology. Electrochemical methods possess numerous complementary advantages over optical detection methods, including its label-free nature, capability of quantitative monitoring of various reporter molecules, and the ability to not only detect but also address compositions of individual compartments. However, application of electrochemical methods for the purpose of high-throughput screening remains very limited. In this work, we develop a high-density individually addressable electrochemical droplet microarray (eDMA). The eDMA allows for the detection of redox-active reporter molecules irrespective of their electrochemical reversibility in individual nanoliter-sized droplets. Orthogonal band microelectrodes are arranged to form at their intersections an array of three-electrode systems for precise control of the applied potential, which enables direct read-out of the current related to analyte detection. The band microelectrode array is covered with a layer of permeable porous polymethacrylate functionalized with a highly hydrophobic-hydrophilic pattern, forming spatially separated nanoliter-sized droplets on top of each electrochemical cell. Electrochemical characterization of single droplets demonstrates that the underlying electrode system is accessible to redox-active molecules through the hydrophilic polymeric pattern and that the nonwettable hydrophobic boundaries can spatially separate neighboring cells effectively. The eDMA technology opens the possibility to combine the high-throughput biochemical or living cell screenings using the droplet microarray platform with the sequential electrochemical read-out of individual droplets. © 2017 American Chemical Society.
    view abstract10.1021/acs.analchem.7b00008
  • High-throughput heterodyne thermoreflectance: Application to thermal conductivity measurements of a Fe-Si-Ge thin film alloy library
    D'Acremont, Q. and Pernot, G. and Rampnoux, J.-M. and Furlan, A. and Lacroix, D. and Ludwig, Al. and Dilhaire, S.
    Review of Scientific Instruments 88 (2017)
    A High-Throughput Time-Domain ThermoReflectance (HT-TDTR) technique was developed to perform fast thermal conductivity measurements with minimum user actions required. This new setup is based on a heterodyne picosecond thermoreflectance system. The use of two different laser oscillators has been proven to reduce the acquisition time by two orders of magnitude and avoid the experimental artefacts usually induced by moving the elements present in TDTR systems. An amplitude modulation associated to a lock-in detection scheme is included to maintain a high sensitivity to thermal properties. We demonstrate the capabilities of the HT-TDTR setup to perform high-throughput thermal analysis by mapping thermal conductivity and interface resistances of a ternary thin film silicide library FexSiyGe100-x-y (20&lt;x,y&lt;80) deposited by wedge-type multi-layer method on a 100 mm diameter sapphire wafer offering more than 300 analysis areas of different ternary alloy compositions. © 2017 Author(s).
    view abstract10.1063/1.4986469
  • High-Throughput Structural and Functional Characterization of the Thin Film Materials System Ni-Co-Al
    Decker, P. and Naujoks, D. and Langenkämper, D. and Somsen, C. and Ludwig, Al.
    ACS Combinatorial Science 19 (2017)
    High-throughput methods were used to investigate a Ni-Co-Al thin film materials library, which is of interest for structural and functional applications (superalloys, shape memory alloys). X-ray diffraction (XRD) measurements were performed to identify the phase regions of the Ni-Co-Al system in its state after annealing at 600 °C. Optical, electrical, and magneto-optical measurements were performed to map functional properties and confirm XRD results. All results and literature data were used to propose a ternary thin film phase diagram of the Ni-Co-Al thin film system. © 2017 American Chemical Society.
    view abstract10.1021/acscombsci.6b00176
  • High-throughput study of binary thin film tungsten alloys
    Nikolić, V. and Wurster, S. and Savan, A. and Ludwig, Al. and Pippan, R.
    International Journal of Refractory Metals and Hard Materials 69 (2017)
    Combinatorial magnetron co-sputtering from elemental sources was applied to produce W-alloy thin film composition spread materials libraries with well-defined, continuous composition gradients (film thicknesses between 1 and 2.5 μm). Three systems were studied: W-Fe (0–7 at.%), W-Ti (0–15 at.%) and W-Ir (0–12 at.%). High-throughput characterization of the materials libraries comprised of chemical, morphological and microstructural analyses. Scanning electron microscope investigations revealed that the films have a columnar structure of inverted cone-like units separated by voided boundaries, with a strong correlation to the alloying element content. Significant morphological changes occurred with an increase in the amount of the added element; W films with lower at.% of the alloying element had higher density and tighter grain boundaries, altering towards an increased amount of voids as the concentration of the alloying element increased. Electron backscatter diffraction scanning was used to determine microstructural components (grain size, grain shape, texture evolution), in dependence on the concentration of the alloying element. © 2017 Elsevier Ltd
    view abstract10.1016/j.ijrmhm.2017.07.017
  • Identification of a ternary μ-phase in the Co-Ti-W system – An advanced correlative thin-film and bulk combinatorial materials investigation
    Naujoks, D. and Eggeler, Y.M. and Hallensleben, P. and Frenzel, J. and Fries, S.G. and Palumbo, M. and Koßmann, J. and Hammerschmidt, T. and Pfetzing-Micklich, J. and Eggeler, G. and Spiecker, E. and Drautz, R. and Ludwig, Al.
    Acta Materialia 138 (2017)
    The formation of a ternary μ-phase is documented for the system Co-Ti-W. The relevant compositional stability range is identified by high-throughput energy dispersive X-ray spectroscopy, electrical resistance and X-ray diffraction maps from a thin-film materials library (1 μm thickness). Bulk samples of the identified compositions were fabricated to allow for correlative film and bulk studies. Using analytical scanning and transmission electron microscopy, we demonstrate that in both, thin film and bulk samples, the D85 phase (μ-phase) coexists with the C36-phase and the A2-phase at comparable average chemical compositions. Young's moduli and hardness values of the μ-phase and the C36-phase were determined by nanoindentation. The trends of experimentally obtained elastic moduli are consistent with density functional theory (DFT) calculations. DFT analysis also supports the experimental findings, that the μ-phase can solve up to 18 at.% Ti. Based on the experimental and DFT results it is shown that CALPHAD modeling can be modified to account for the new findings. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.07.037
  • Influence of residual stress on the adhesion and surface morphology of PECVD-coated polypropylene
    Jaritz, M. and Hopmann, C. and Behm, H. and Kirchheim, D. and Wilski, S. and Grochla, D. and Banko, L. and Ludwig, Al. and Böke, M. and Winter, J. and Bahre, H. and Dahlmann, R.
    Journal of Physics D: Applied Physics 50 (2017)
    The properties of plasma-enhanced chemical vapour deposition (PECVD) coatings on polymer materials depend to some extent on the surface and material properties of the substrate. Here, isotactic polypropylene (PP) substrates are coated with silicon oxide (SiOx) films. Plasmas for the deposition of SiOx are energetic and oxidative due to the high amount of oxygen in the gas mixture. Residual stress measurements using single Si cantilever stress sensors showed that these coatings contain high compressive stress. To investigate the influence of the plasma and the coatings, residual stress, silicon organic (SiOCH) coatings with different thicknesses between the PP and the SiOx coating are used as a means to protect the substrate from the oxidative SiOx coating process. Pull-off tests are performed to analyse differences in the adhesion of these coating systems. It could be shown that the adhesion of the PECVD coatings on PP depends on the coatings' residual stress. In a PP/SiOCH/SiOx-multilayer system the residual stress can be significantly reduced by increasing the thickness of the SiOCH coating, resulting in enhanced adhesion. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa8798
  • Influence of Substrate Temperature and Film Thickness on Thermal, Electrical, and Structural Properties of HPPMS and DC Magnetron Sputtered Ge Thin Films
    Furlan, A. and Grochla, D. and D'Acremont, Q. and Pernot, G. and Dilhaire, S. and Ludwig, Al.
    Advanced Engineering Materials (2017)
    Ge was deposited as thickness gradient films at temperatures up to 800°C by direct current (DC) and high power pulsed magnetron sputtering (HPPMS). Structural characterization shows increased crystallization with increasing substrate temperature and film thickness. Thermal conductivity was measured by a novel high-throughput time-domain thermo-reflectance method. Thermo-electrical properties correlate to the degree of crystallization. Conductivities increase with increasing substrate temperature up to 500°C. For higher temperatures the trend reverses. A room temperature deposited/annealed film displays smaller crystallites (10nm) and lower thermal conductivity (5Wm-1K-1) compared to 25Wm-1K-1 for hot DC deposition. Compared to DC, HPPMS films show higher thermal conductivities up to 45Wm-1K-1. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201600854
  • Microstructural evolution and functional fatigue of a Ti–25Ta high-temperature shape memory alloy
    Maier, H.J. and Karsten, E. and Paulsen, A. and Langenkämper, D. and Decker, P. and Frenzel, J. and Somsen, C. and Ludwig, Al. and Eggeler, G. and Niendorf, T.
    Journal of Materials Research (2017)
    Titanium–tantalum based alloys can demonstrate a martensitic transformation well above 100 °C, which makes them attractive for shape memory applications at elevated temperatures. In addition, they provide for good workability and contain only reasonably priced constituents. The current study presents results from functional fatigue experiments on a binary Ti–25Ta high-temperature shape memory alloy. This material shows a martensitic transformation at about 350 °C along with a transformation strain of 2 pct at a bias stress of 100 MPa. The success of most of the envisaged applications will, however, hinge on the microstructural stability under thermomechanical loading. Thus, light and electron optical microscopy as well X-ray diffraction were used to uncover the mechanisms that dominate functional degradation in different temperature regimes. It is demonstrated the maximum test temperature is the key parameter that governs functional degradation in the thermomechanical fatigue tests. Specifically, ω-phase formation and local decomposition in Ti-rich and Ta-rich areas dominate when T max does not exceed ≈430 °C. As T max is increased, the detrimental phases start to dissolve and functional fatigue can be suppressed. However, when T max reaches ≈620 °C, structural fatigue sets in, and fatigue life is again deteriorated by oxygen-induced crack formation. Copyright © Materials Research Society 2017
    view abstract10.1557/jmr.2017.319
  • 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 (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 abstract10.1021/acscombsci.7b00065
  • Unraveling compositional effects on the light-induced oxygen evolution in Bi(V-Mo-X)O4 material libraries
    Gutkowski, R. and Khare, C. and Conzuelo, F. and Kayran, Y.U. and Ludwig, Al. and Schuhmann, W.
    Energy and Environmental Science 10 (2017)
    The influence of co-deposited transition metals X (X = Ta, W, Nb) with various relative concentrations on the photoelectrochemical performance of BiVO4 is investigated. Thin film material libraries with well-defined composition gradients of Bi, V and two transition metals are fabricated by combinatorial sputter co-deposition. Materials with the highest photoelectrochemical performance are identified by high-throughput characterization of the Bi(V-Mo-X)O4 material libraries using an optical scanning droplet cell. Bi(V-Mo-W)O4 and Bi(V-Mo-Nb)O4 material libraries show the highest improvement in the photocurrent, with ten times higher photocurrents of up to 1 mA cm-2 compared to a BiVO4 reference material library. Deviations from the V:Bi equiatomic ratio lead to a decrease in the photocurrent for pristine monoclinic BiVO4. By the addition of transition metals this effect is minimized and no significant decrease in the photocurrent occurs up to 10 at% variation from the equiatomic V:Bi ratio. Excellent photoelectrochemical performance is reached under these conditions in regions with a V:Bi atomic ratio of 70:30 and co-deposited Nb concentrations of &gt;10 at%. Scanning photoelectrochemical microscopy allows the evaluation of the correlation between the generated oxygen at a photoanode and the measured photocurrent. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ee00287d
  • Combining sensor and protective functionalities in ferromagnetic nanocomposite thin films for applications in harsh environments
    Seemann, K. and Leiste, H. and Stüber, M. and Krüger, K. and Brunken, H. and Ludwig, Al. and Thede, C. and Quandt, E.
    Advanced Engineering Materials 18 (2016)
    The present paper discusses the development of Fe48Co52/TiN, Fe32Co44Hf12N12/TiN, Fe-Co/Al-N multilayer, and Fe2B single layer films produced by magnetron sputtering processes. These films combine appropriate magnetic and wear resistant multifunctional properties for the employment as sensor components for monitoring the actual state of stress and/or temperature of cutting tools used in the metal processing industry. At this point, the ferromagnetic layers enable the sensor function, whereas the TiN or Al-N layers support mechanical features. Being exposed to stress and/or temperature, the change of polarization or permeability can be utilized as a response for reading-out the sensor component. The paper introduces the development of Fe48Co52/TiN, Fe32Co44Hf12N12/TiN, Fe-Co/Al-N multilayer, and Fe2B single layer films as sensor components for monitoring, e.g., the actual state of stress and/or temperature of cutting tools which are used in metal processing. The films are produced by magnetron sputtering processes. They ought to combine appropriate magnetic property changes and wear resistance. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201500474
  • Nanostructured Ti-Ta thin films synthesized by combinatorial glancing angle sputter deposition
    Motemani, Y. and Khare, C. and Savan, A. and Hans, M. and Paulsen, A. and Frenzel, J. and Somsen, C. and Mücklich, F. and Eggeler, G. and Ludwig, Al.
    Nanotechnology 27 (2016)
    Ti-Ta alloys are attractive materials for applications in actuators as well as biomedical implants. When fabricated as thin films, these alloys can potentially be employed as microactuators, components for micro-implantable devices and coatings on surgical implants. In this study, Ti100-xTa x (x = 21, 30) nanocolumnar thin films are fabricated by glancing angle deposition (GLAD) at room temperature using Ti73Ta27 and Ta sputter targets. Crystal structure, morphology and microstructure of the nanostructured thin films are systematically investigated by XRD, SEM and TEM, respectively. Nanocolumns of ∼150-160 nm in width are oriented perpendicular to the substrate for both Ti79Ta21 and Ti70Ta30 compositions. The disordered α″ martensite phase with orthorhombic structure is formed in room temperature as-deposited thin films. The columns are found to be elongated small single crystals which are aligned perpendicular to the and planes of α″ martensite, indicating that the films' growth orientation is mainly dominated by these crystallographic planes. Laser pre-patterned substrates are utilized to obtain periodic nanocolumnar arrays. The differences in seed pattern, and inter-seed distances lead to growth of multi-level porous nanostructures. Using a unique sputter deposition geometry consisting of Ti73Ta27 and Ta sputter sources, a nanocolumnar Ti-Ta materials library was fabricated on a static substrate by a co-deposition process (combinatorial-GLAD approach). In this library, a composition spread developed between Ti72.8Ta27.2 and Ti64.4Ta35.6, as confirmed by high-throughput EDX analysis. The morphology over the materials library varies from well-isolated nanocolumns to fan-like nanocolumnar structures. The influence of two sputter sources is investigated by studying the resulting column angle on the materials library. The presented nanostructuring methods including the use of the GLAD technique along with pre-patterning and a combinatorial materials library fabrication strategy offer a promising technological approach for investigating Ti-Ta thin films for a range of applications. The proposed approaches can be similarly implemented for other materials systems which can benefit from the formation of a nanocolumnar morphology. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/49/495604
  • 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 (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 abstract10.1039/c5ta10186g
  • On the origin of the improved ruthenium stability in RuO2-IrO2 mixed oxides
    Kasian, O. and Geiger, S. and Stock, P. and Polymeros, G. and Breitbach, B. and Savan, A. and Ludwig, Al. and Cherevko, S. and Mayrhofer, K.J.J.
    Journal of the Electrochemical Society 163 (2016)
    High oxygen evolution reaction activity of ruthenium and long term stability of iridium in acidic electrolytes make their mixed oxides attractive candidates for utilization as anodes in water electrolyzers. Indeed, such materials were addressed in numerous previous studies. The application of a scanning flow cell connected to an inductively coupled plasma mass spectrometer allowed us now to examine the stability and activity toward oxygen evolution reaction of such mixed oxides in parallel. The whole composition range of Ir-Ru mixtures has been covered in a thin film material library. In the whole composition range the rate of Ru dissolution is observed to be much higher than that of Ir. Eventually, due to the loss of Ru, the activity of the mixed oxides approaches the value corresponding to pure IrO2. Interestingly, the loss of only a few percent of a monolayer in Ru surface concentration results in a significant drop in activity. Several explanations of this phenomenon are discussed. It is concluded that the herein observed stability of mixed Ir-Ru oxide systems is most likely a result of high corrosion resistance of the iridium component, but not due to an alteration of the material's electronic structure. © 2016 by the Authors.
    view abstract10.1149/2.0131611jes
  • Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability
    Cherevko, S. and Geiger, S. and Kasian, O. and Kulyk, N. and Grote, J.-P. and Savan, A. and Shrestha, B.R. and Merzlikin, S. and Breitbach, B. and Ludwig, Al. and Mayrhofer, K.J.J.
    Catalysis Today 262 (2016)
    Metallic iridium and ruthenium as well as their oxides are among the most active oxygen evolution (OER) electrocatalysts in acidic media, and are also of interest for the catalysis of the hydrogen evolution (HER). The stability of these materials under different operating conditions is, however, still not fully understood. In the current work, activity and stability of well-defined Ru, RuO2, Ir, and IrO2 thin film electrodes are evaluated in acidic and alkaline electrolytes using an electrochemical scanning flow cell (SFC) connected to an inductively coupled plasma mass spectrometer (ICP-MS). Identical experimental protocols are intentionally employed for all electrodes and electrolytes, to obtain unambiguous and comparable information on intrinsic activity and stability of the electrodes. It is found that independent of the electrolyte, OER activity decreases as Ru &gt; Ir ≈ RuO2 &gt; IrO2, while dissolution increases as IrO2 « RuO2 < Ir « Ru. Moreover, dissolution of these metals in both solutions is 2-3 orders of magnitude higher compared to their respective oxides, and dissolution is generally more intense in alkaline solutions. Similarly to the OER, metallic electrodes are more active catalysts for HER. They, however, suffer from dissolution during native oxide reduction, while IrO2 and RuO2 do not exhibit significant dissolution. The obtained results on activity and stability of the electrodes are discussed in light of their potential applications, i.e. water electrolysers or fuel cells. © 2015 Elsevier B.V.
    view abstract10.1016/j.cattod.2015.08.014
  • Phase Formation and Oxidation Behavior at 500 °c in a Ni-Co-Al Thin-Film Materials Library
    Naujoks, D. and Richert, J. and Decker, P. and Weiser, M. and Virtanen, S. and Ludwig, Al.
    ACS Combinatorial Science 18 (2016)
    The complete ternary system Ni-Co-Al was fabricated as a thin film materials library by combinatorial magnetron sputtering and was annealed subsequently in several steps in Ar and under atmospheric conditions at 500 °C. Ni-Co-Al is the base system for both Ni- and Co-based superalloys. Therefore, the phases occurring in this system and their oxidation behavior is of high interest. The Ni-Co-Al materials library was investigated using high-throughput characterization methods such as optical measurements, resistance screening, automated EDX, automated XRD, and XPS. From the obtained data a thin film phase diagram for the Ni-Co-Al system in its state after annealing at 500 °C in air was established. Furthermore, a surface oxide composition map of the full Ni-Co-Al system for oxidation at 500 °C was concluded. As a result, it could be shown that at 500 °C an amount of 10 at. % Al is necessary for a Ni-Co-Al thin film to produce a protective Al-oxide scale. © 2016 American Chemical Society.
    view abstract10.1021/acscombsci.6b00052
  • 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 (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 abstract10.1016/j.jcat.2016.02.026
  • Structural and multifunctional properties of magnetron-sputtered Fe-P(-Mn) thin films
    Decker, P. and Stein, H.S. and Salomon, S. and Brüssing, F. and Savan, A. and Hamann, S. and Ludwig, Al.
    Thin Solid Films 603 (2016)
    Structural and magnetic properties of magnetron-sputtered Fe-P(-Mn) thin films with compositions around the Fe2P single phase region are reported, revealing the compositional range of the Fe2P-type structure and the change of the magnetic properties within this composition spread. The structural analysis shows that in order to obtain crystalline Fe-P phases the P content must be higher than (Fe0.97Mn0.03)2.33P. A maximum phase fraction of the Fe2P-type structure is obtained in the examined (Fe0.97Mn0.03)1.78P sample. The hysteresis loops for the Fe2P(-Mn) thin films show a two-step magnetic reversal with one part belonging to an amorphous phase fraction and the other to the Fe2P(-Mn) phase. A maximum coercivity of 0.36 T was measured for the Fe2P(-Mn) phase fraction also at the composition of (Fe0.97Mn0.03)1.78P. Furthermore, electrochemical properties of FeP2(-Mn) thin films as hydrogen evolution catalysts (HER) are studied. FeP2(-Mn) shows a HER onset potential about 200 mV lower than that of Pt. Chronoamperometric testing at - 11.5 mA/cm2 for over 3500 s revealed no obvious decay in current density, suggesting good stability under typical working conditions in a photoelectrochemical device. © 2016 Elsevier B.V.
    view abstract10.1016/j.tsf.2016.02.023
  • Synthesis of nanostructured LiMn2O4 thin films by glancing angle deposition for Li-ion battery applications
    Borhani-Haghighi, S. and Khare, C. and Trócoli, R. and Dushina, A. and Kieschnick, M. and Lamantia, F. and Ludwig, Al.
    Nanotechnology 27 (2016)
    The development of electric vehicles and portable electronic devices demands lighter and thinner batteries with improved specific charge and rate capabilities. In this work, thin films of LiMn2O4 were fabricated by rf magnetron sputtering. Glancing angle deposition is introduced as a promising approach for fabrication of porous cathode thin films with 2.6 times the capacity in comparison with conventionally sputtered films of the same thickness. Surface morphology and crystallinity of the films are studied along with their electrochemical performance in an aqueous electrolyte. The glancing angle deposited films can reach a rate capability of up to 4 mA cm-2 with minimal energy loss, and a life cycle longer than 100 charge/discharge cycles. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/45/455402
  • Understanding surface reactivity of Si electrodes in Li-ion batteries by: In operando scanning electrochemical microscopy
    Ventosa, E. and Wilde, P. and Zinn, A.-H. and Trautmann, M. and Ludwig, Al. and Schuhmann, W.
    Chemical Communications 52 (2016)
    In operando SECM is employed to monitor the evolution of the electrically insulating character of a Si electrode surface during (de-)lithiation. The solid-electrolyte interface (SEI) formed on Si electrodes is shown to be intrinsically electrically insulating. However, volume changes upon (de-)lithiation lead to the loss of the protecting character of the initially formed SEI. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c6cc02493a
  • Unraveling Self-Doping Effects in Thermoelectric TiNiSn Half-Heusler Compounds by Combined Theory and High-Throughput Experiments
    Wambach, M. and Stern, R. and Bhattacharya, S. and Ziolkowski, P. and Muller, E. and Madsen, G. K. H. and Ludwig, Al.
    Advanced Electronic Materials 2 (2016)
    The control of the carrier concentration is a key topic in the optimization of the thermoelectric power factor. It depends intricately on the defect chemistry of a host phase (here: TiNiSn) and the boundary conditions set by competing phases. The large impact of a slight off-stoichiometry in the intermetallic half-Heusler phase TiNiSn makes combinatorial techniques ideally suited for systematic optimization of its thermoelectric performance. In this work, computational thermochemistry, combinatorial synthesis, and high-throughput characterization are combined to obtain a complete map of the thermoelectric power factor for the Ti-Ni-Sn system. The role of the chemical potential of the constituents in determining the detailed nonstoichiometric composition of the intermetallic half-Heusler phase TiNiSn is elucidated. This work not only confirms the assumption of a large phase-width in terms of Ni surplus but also demonstrates that TiNiSn phases with a relatively large Ti surplus can be produced. This can serve as a new route for achieving high carrier concentrations by self-doping in the ternary system Ti-Ni-Sn. The defect thermochemistry calculations for the carrier concentration are in excellent agreement with the experimental results. The findings of this work suggest new ways of improving the thermoelectric performance of half-Heusler phases such as TiNiSn.
    view abstract10.1002/aelm.201500208
  • A combinatorial study of photoelectrochemical properties of Fe-W-O thin films
    Sliozberg, K. and Schäfer, D. and Meyer, R. and Ludwig, Al. and Schuhmann, W.
    ChemPlusChem 80 (2015)
    A continuous mixed Fe-W-O thin-films materials library was fabricated by means of reactive co-sputtering from elemental iron and tungsten targets in argon/oxygen. The materials library was screened for its local photoelectrochemical properties by using an automated optical scanning droplet cell. Local scanning electron microscopy (SEM) and electron-dispersive X-ray spectroscopy (EDX) measurements of the materials library were performed to correlate the composition, morphology, and photocurrents. The iron content was varied in the range from Fe32W68Ox to Fe81W19Ox. A strong dependence of the film morphology and the measured photocurrents on the composition was observed with a maximal photocurrent from a measurement area containing 55 at% iron. The most photoactive area showed a porous structure with a high surface area. The bandgap values of these materials were assessed by photocurrent spectroscopy and showed a systematic variation of the bandgap values with the composition. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201402277
  • 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 (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 abstract10.1002/pssa.201532384
  • Antibacterial activity of microstructured Ag/Au sacrificial anode thin films
    Köller, M. and Sengstock, C. and Motemani, Y. and Khare, C. and Buenconsejo, P.J.S. and Geukes, J. and Schildhauer, T.A. and Ludwig, Al.
    Materials Science and Engineering C 46 (2015)
    Ten different Ag dot arrays (16 to 625 microstructured dots per square mm) were fabricated on a continuous Au thin film and for comparison also on Ti film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Escherichia coli and Staphylococcus aureus were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards both bacterial strains was induced by Ag dot arrays on fabricated Au thin film (sacrificial anode system for Ag), due to the release of Ag ions from dissolution of Ag dots in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag) which remained intact to the original dot shape. The required number of Ag dots on gold film to achieve complete bactericidal effects for both bacterial strains was seven times lower than that observed with Ag dot arrays on Ti film. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.msec.2014.10.058
  • Combinatorial Development of Fe-Co-Nb Thin Film Magnetic Nanocomposites
    Alexandrakis, V. and Wallisch, W. and Hamann, S. and Varvaro, G. and Fidler, J. and Ludwig, Al.
    ACS Combinatorial Science 17 (2015)
    A Fe-Co-Nb thin film materials library was deposited by combinatorial magnetron sputtering and investigated by high-throughput methods to identify new noncubic ferromagnetic phases, indicating that combinatorial experimentation is an efficient method to discover new ferromagnetic phases adequate for permanent magnet applications. Structural analysis indicated the formation of a new magnetic ternary compound (Fe,Co)3Nb with a hexagonal crystal structure (C36) embedded in an FeCo-based matrix. This nanocomposite exhibits characteristics of a two-phase ferromagnetic system, the so-called hard-soft nanocomposites, indicating that the new phase (Fe,Co)3Nb is ferromagnetic. Magnetic hysteresis loops at various angles revealed that the magnetization reversal process is governed by a domain wall pinning mechanism. © 2015 American Chemical Society.
    view abstract10.1021/acscombsci.5b00116
  • Combinatorial synthesis and high-throughput characterization of the thin film materials system Co-Mn-Ge: Composition, structure, and magnetic properties
    Salomon, S. and Hamann, S. and Decker, P. and Savan, A. and Meshi, L. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 212 (2015)
    Co-Mn-Ge is a system of interest for magnetocaloric applications as a solid state magnetic refrigerant. A thin film materials library covering a large fraction of the Co-Mn-Ge ternary composition space was fabricated by sputter deposition. After deposition, it was annealed at 600°C for 3 h and quenched subsequently. An energy-dispersive X-ray spectroscopy and X-ray diffraction-based cluster analysis revealed the regions of existence for the CoMnGe and the Co<inf>2</inf>MnGe single phase areas. Furthermore, high intensity diffraction peaks revealed the presence of the hexagonal (Co, Mn)<inf>7</inf>Ge<inf>6</inf> phase in a region that also featured the CoMnGe phase. In this region, a non-linear, symmetric, and hysteretic shift of the (200) diffraction peak of the (Co, Mn)<inf>7</inf>Ge<inf>6</inf> phase was observed by temperature-dependent X-ray diffraction for Co<inf>23</inf>Mn<inf>33</inf>Ge<inf>44</inf>, indicating a structural phase transition taking place between 350 and 375 K upon heating and 325 and 300 K upon cooling. This coincides with a magnetic transition near 325 K from the ferromagnetic to the paramagnetic state. However, no magnetostructural coupling was identified in the temperature range from 330 to 300 K upon cooling. Magnetostriction and an undetected structural transition of the CoMnGe phase were ruled out as probable causes for the non-linear shifts. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201532075
  • Exploration of ternary subsystems of superalloys by high-throughput thin film experimentation: Optical and electrical data of the Co-Al-W system
    Naujoks, D. and Koenig, D. and Ludwig, Al.
    Materials Research Society Symposium Proceedings 1760 (2015)
    The complete ternary system Co-Al-W was fabricated as a thin film materials library by combinatorial magnetron sputtering. The materials library was investigated using high-throughput characterization methods such as optical measurements as well as automated resistance screening. The obtained data indicate possible phase regions and compositional regions which show early surface oxidation. The demonstrated approach illustrates that using high-throughput measurement methods provides a fast access to data of relatively unexplored materials systems. The gained data provides a valuable basis for further in-depth studies of the investigated materials systems. © 2015 Materials Research Society.
    view abstract10.1557/opl.2015.56
  • Fe-cr-al containing oxide semiconductors as potential solar water-splitting materials
    Sliozberg, K. and Stein, H.S. and Khare, C. and Parkinson, B.A. and Ludwig, Al. and Schuhmann, W.
    ACS Applied Materials and Interfaces 7 (2015)
    A high-throughput thin film materials library for Fe-Cr-Al-O was obtained by reactive magnetron cosputtering and analyzed with automated EDX and XRD to elucidate compositional and structural properties. An automated optical scanning droplet cell was then used to perform photoelectrochemical measurements of 289 compositions on the library, including electrochemical stability, potentiodynamic photocurrents and photocurrent spectroscopy. The photocurrent onset and open circuit potentials of two semiconductor compositions (n-type semiconducting: Fe51Cr47Al2Ox, p-type semiconducting Fe36.5Cr55.5Al8Ox) are favorable for water splitting. Cathodic photocurrents are observed at 1.0 V vs RHE for the p-type material exhibiting an open circuit potential of 0.85 V vs RHE. The n-type material shows an onset of photocurrents at 0.75 V and an open circuit potential of 0.6 V. The p-type material showed a bandgap of 1.55 eV, while the n-type material showed a bandgap of 1.97 eV. © 2015 American Chemical Society.
    view abstract10.1021/am508946e
  • Film Stress of Amorphous Hydrogenated Carbon on Biaxially Oriented Polyethylene Terephthalate
    Bahre, H. and Behm, H. and Grochla, D. and Böke, M., and Dahlmann, R., and Hopmann, C., and Ludwig, Al., and Winter, J.
    Plasma Processes and Polymers 12 (2015)
    Amorphous hydrogenated carbon (a-C:H) deposited on steel with plasma enhanced chemical vapor deposition can be used as elongation tolerant oxygen barrier. However, the elongation tolerance of the a-C:H film is lost if deposited on a poly(ethylene terephthalate) (PET) for reasons unknown. To assess this phenomenon, a-C:H was deposited on PET, silicon substrates, and silicon micro-cantilevers, and the stress was determined by measuring the radius of curvature. a-C:H deposited on PET showed lower compressive stress than on silicon. This difference is not due to the formation of a gradient layer or plastic deformation of PET. Instead, the most probable explanation is that energetic ions cause a partial release of biaxial orientation within the PET, thereby reducing the compressive stress. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ppap.201500045
  • High-Throughput Investigation of the Oxidation and Phase Constitution of Thin-Film Ni-Al-Cr Materials Libraries
    König, D. and Eberling, C. and Kieschnick, M. and Virtanen, S. and Ludwig, Al.
    Advanced Engineering Materials 17 (2015)
    Thin-film materials libraries of the intermetallic model system Ni-Al-Cr were fabricated and their oxidation behavior was studied by compositional, optical, electrical, and structural high-throughput characterization methods. The study reveals the compositional regions of the binary and ternary compositions which withstand longest to annealing in air (up to 700 C), and are, therefore, resistant to oxidation and delamination under these conditions. A complete ternary thin-film phase diagram for the Ni-Al-Cr system in its state after 9 h annealing in air at 500 C was determined. Optical high-throughput characterization is shown to be valid for rapid identification of oxidizing phases. Generally, the initially metallic phases show different oxidation behavior in air. We find that the ternary compositions are more resistant to oxidation than the binary phases. Compositions around Ni<inf>25</inf>Al<inf>12.5</inf>Cr<inf>62.5</inf> were found to show very good oxidation resistance. These results were supported by additional information from corresponding electrical and optical property investigations. The presented high-throughput approach is generic for the efficient study of multinary thin-film materials in harsh environments. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201500023
  • High-Throughput Screening of Thin-Film Semiconductor Material Libraries I: System Development and Case Study for Ti-W-O
    Sliozberg, K. and Schäfer, D. and Erichsen, T. and Meyer, R. and Khare, C. and Ludwig, Al. and Schuhmann, W.
    ChemSusChem 8 (2015)
    An automated optical scanning droplet cell (OSDC) enables high-throughput quantitative characterization of thin-film semiconductor material libraries. Photoelectrochemical data on small selected measurement areas are recorded including intensity-dependent photopotentials and -currents, potentiodynamic and potentiostatic photocurrents, as well as photocurrent (action) spectra. The OSDC contains integrated counter and double-junction reference electrodes and is fixed on a precise positioning system. A Xe lamp with a monochromator is coupled to the cell through a thin poly(methyl methacrylate) (PMMA) optical fiber. A specifically designed polytetrafluoroethylene (PTFE) capillary tip is pressed on the sample surface and defines through its diameter the homogeneously illuminated measurement area. The overall and wavelength-resolved irradiation intensities and the cell surface area are precisely determined and calibrated. System development and its performance are demonstrated by means of screening of a Ti-W-O thin film. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cssc.201402917
  • High-throughput screening of thin-film semiconductor material libraries II: Characterization of Fe-W-O libraries
    Meyer, R. and Sliozberg, K. and Khare, C. and Schuhmann, W. and Ludwig, Al.
    ChemSusChem 8 (2015)
    Metal oxides are promising materials for solar water splitting. To identify suitable materials within the ternary system Fe-W-O, thin-film material libraries with combined thickness and compositional gradients were synthesized by combinatorial reactive magnetron sputtering. These libraries (>1000 different samples) were investigated by means of structural and functional high-throughput characterization techniques to establish correlations between composition, crystallinity, morphology, thickness, and photocurrent density in the compositional range between (Fe<inf>6</inf>W<inf>94</inf>)O<inf>x</inf> and (Fe<inf>61</inf>W<inf>39</inf>)O<inf>x</inf>. In addition to the well-known phase WO<inf>3</inf>, the binary phase W<inf>5</inf>O<inf>14</inf> and the ternary phase Fe<inf>2</inf>O<inf>6</inf>W show enhanced photoelectrochemical activity. The highest photocurrent density of 65 μA cm-2 was achieved for the composition (Fe<inf>15</inf>W<inf>85</inf>)O<inf>x</inf>, which contains the W<inf>5</inf>O<inf>14</inf> phase and has a thickness of 1060 nm. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201402918
  • Microstructure, Shape Memory Effect and Functional Stability of Ti67Ta33 Thin Films
    Motemani, Y. and Kadletz, P.M. and Maier, B. and Rynko, R. and Somsen, C. and Paulsen, A. and Frenzel, J. and Schmahl, W.W. and Eggeler, G. and Ludwig, Al.
    Advanced Engineering Materials 17 (2015)
    Ti-Ta based alloys are an interesting class of high-temperature shape memory materials. When fabricated as thin films, they can be used as high-temperature micro-actuators with operation temperatures exceeding 100 °C. In this study, microstructure, shape memory effect and thermal cycling stability of room-temperature sputter deposited Ti<inf>67</inf>Ta<inf>33</inf> thin films are investigated. A disordered α martensite (orthorhombic) phase is formed in the as-deposited Ti<inf>67</inf>Ta<inf>33</inf> films. The films show a columnar morphology with the columns being oriented perpendicular to the substrate surface. They are approximately 200 nm in width. XRD texture analysis reveals a martensite fiber texture with {120} and {102} fiber axes. The XRD results are confirmed by TEM analysis, which also shows columnar grains with long axes perpendicular to the {120} and {102} planes of α martensite. The shape memory effect is analyzed in the temperature range of -10 to 240 °C using the cantilever deflection method, with special emphasis placed on cyclic stability. Ti<inf>67</inf>Ta<inf>33</inf> thin films undergo a forward martensitic transformation at M<inf>s</inf> ≈ 165 °C, with a stress relaxation of approximately 33 MPa during the transformation. The actuation response of the film actuators degrades significantly during thermal cycling. TEM analysis shows that this degradation is related to the formation of nanoscale ω precipitates (5-13 nm) which form above the austenite finish temperature. These precipitates suppress the martensitic transformation, as they act as obstacles for the growth of martensite variants. Ti-Ta thin films can be used as high-temperature micro-actuators. In this study, microstructure, shape memory effect, and functional stability of room-temperature sputter deposited Ti<inf>67</inf>Ta<inf>33</inf> thin films are systematically investigated. The actuation response of the film actuators degrades significantly during thermal cycling. This degradation is related to the formation of nanoscale ω precipitates (5-13 nm) which form above the austenite finish temperature. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201400576
  • 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 (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 abstract10.1016/j.actamat.2014.11.035
  • Temperature dependent low-field measurements of the magnetocaloric Δ T with sub-mK resolution in small volume and thin film samples
    Döntgen, J. and Rudolph, J. and Gottschall, T. and Gutfleisch, O. and Salomon, S. and Ludwig, Al. and Hägele, D.
    Applied Physics Letters 106 (2015)
    We present temperature dependent ΔT measurements of the magnetocaloric effect in a thin film sample of Gd, employing magnetomodulation and detection of thermal radiation. A bulk sample of the metamagnetic material LaFe11.05Co0.91Si1.04 shows a strong broadening of the ΔT peak for increasing field amplitudes between 4 and 45mT. Bulk Gd in comparison shows only a weak broadening. All investigated samples exhibit a clear quadratic dependence of ΔT on the external field Hext at the ΔT peak maximum, contrary to earlier predictions. An analytic expression is derived that interpolates between the Hext2-behavior at low and the well-known Hext2/3-behavior at high fields. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4906426
  • The effect of cast microstructure and crystallography on rafting, dislocation plasticity and creep anisotropy of single crystal Ni-base superalloys
    Nörtershäuser, P. and Frenzel, J. and Ludwig, Al. and Neuking, K. and Eggeler, G.
    Materials Science and Engineering A 626 (2015)
    In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.12.030
  • Wet Nanoindentation of the Solid Electrolyte Interphase on Thin Film Si Electrodes
    Kuznetsov, V. and Zinn, A.-H. and Zampardi, G. and Borhani-Haghighi, S. and La Mantia, F. and Ludwig, Al. and Schuhmann, W. and Ventosa, E.
    ACS Applied Materials and Interfaces 7 (2015)
    The solid electrolyte interphase (SEI) film formed at the surface of negative electrodes strongly affects the performance of a Li-ion battery. The mechanical properties of the SEI are of special importance for Si electrodes due to the large volumetric changes of Si upon (de)insertion of Li ions. This manuscript reports the careful determination of the Young's modulus of the SEI formed on a sputtered Si electrode using wet atomic force microscopy (AFM)-nanoindentation. Several key parameters in the determination of the Young's modulus are considered and discussed, e.g., wetness and roughness-thickness ratio of the film and the shape of a nanoindenter. The values of the Young's modulus were determined to be 0.5-10 MPa under the investigated conditions which are in the lower range of those previously reported, i.e., 1 MPa to 10 GPa, pointing out the importance of the conditions of its determination. After multiple electrochemical cycles, the polymeric deposits formed on the surface of the SEI are revealed, by force-volume mapping in liquid using colloidal probes, to extend up to 300 nm into bulk solution. © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b06700
  • 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 (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 abstract10.1002/adem.201400317
  • Adherence of human mesenchymal stem cells on Ti and TiO2 nano-columnar surfaces fabricated by glancing angle sputter deposition
    Motemani, Y. and Greulich, C. and Khare, C. and Lopian, M. and Buenconsejo, P.J.S. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Applied Surface Science 292 (2014)
    The interaction of human mesenchymal stem cells (hMSCs) with Ti and TiO2 nano-columnar surfaces fabricated using glancing angle sputter deposition was investigated. The adherence and proliferation of hMSCs on different nano-columnar surfaces, including vertical columns, slanted columns and chevrons, were examined with calcein-acetoxymethyl ester fluorescence staining and scanning electron microscopy. For comparison, adherence of hMSCs on compact, dense films was also studied. After 24 h and 7 days, adherent and viable cells were observed on both, Ti nano-columns as well as dense Ti films, which confirms the biocompatibility of these nanostructures. Very small pseudopodia with width of approximately 20-35 nm and length varying from 20 to 200 nm were observed between the nano-columns, independent of the type of the nano-columnar morphology. Large inter-column spacing and effectively increased surface area make these nanostructures promising candidates for bio-functionalization or drug loading on the surface of Ti-based implants. © 2013 Elsevier B.V.
    view abstract10.1016/j.apsusc.2013.12.022
  • Characterization of Ta-Ti Thin Films by using a Scanning Droplet Cell in Combination with AC Linear Sweep Voltammetry
    Fan, M. and Sliozberg, K. and La Mantia, F. and Miyashita, N. and Hagymási, M. and Schnitter, C. and Ludwig, Al. and Schuhmann, W.
    ChemElectroChem 1 (2014)
    A binary Ta-Ti thin film composition-spread materials library is prepared through magnetron sputter co-deposition. An automated microelectrochemical investigation on selected surface areas, corresponding to a concentration gradient of Ti varying from 0.5 to 36at%, is achieved by using a scanning droplet cell. Simultaneously, during the anodic oxide growth, a small alternating current (AC) voltage is superimposed on the increasing direct current (DC) potential in order to record the capacitance of the mixed-metal oxide by using alternating current linear sweep voltammetry (AC-LSV). Valve metal behavior, with the current stabilizing after an initial rapid increase, is found for all investigated compositions. AC-LSV allows the ratio of the formation factor to the relative permittivity for different compositions to be calculated. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201300153
  • 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 (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 abstract10.1002/celc.201300078
  • Composition-structure-function diagrams of Ti-Ni-Au thin film shape memory alloys
    Buenconsejo, P.J.S. and Ludwig, Al.
    ACS Combinatorial Science 16 (2014)
    Ti-Ni-Au thin film materials libraries were prepared from multilayer precursors by combinatorial sputtering. The materials libraries were annealed at 500, 600, and 700 °C for 1 h and then characterized by high-throughput methods to investigate the relations between composition, structure and functional properties. The identified relations were visualized in functional phase diagrams. The goal is to identify composition regions that are suitable as high temperature shape memory alloys. Phase transforming compositions were identified by electrical resistance measured during thermal cycles in the range of -20 and 250 °C. Three phase transformation paths were confirmed: (1) B2-R, (2) B2-R-B19', and (3) B2-B19. For the materials library annealed at 500 °C only the B2-R transformation was observed. For the materials libraries annealed at 600 and 700 °C, all transformation paths were observed. High transformation temperatures (Ms ≈100 °C) were only obtained by annealing at 600 or 700 °C, and with compositions of Ti ≈ 50 at. % and Au &gt; 20 at. %. This is the composition range that undergoes B2-B19 transformation. The phase transformation behaviors were explained according to the compositional and annealing temperature dependence of phase/structure formation, as revealed by X-ray diffraction analysis of the materials libraries. © 2014 American Chemical Society.
    view abstract10.1021/co5000745
  • CrN/AlN nanolaminate coatings deposited via high power pulsed and middle frequency pulsed magnetron sputtering
    Bagcivan, N. and Bobzin, K. and Ludwig, Al. and Grochla, D. and Brugnara, R.H.
    Thin Solid Films 572 (2014)
    Nanolaminate coatings based on transition metal nitrides such as CrN, AlN and TiN deposited via physical vapor deposition (PVD) have shown great advantage as protective coatings on tools and components subject to high loads in tribological applications. By varying the individual layer materials and their thicknesses it is possible to optimize the coating properties, e.g. hardness, Young's modulus and thermal stability. One way for further improvement of coating properties is the use of advanced PVD technologies. High power pulsed magnetron sputtering (HPPMS) is an advancement of pulsed magnetron sputtering (MS). The use of HPPMS allows a better control of the energetic bombardment of the substrate due to the higher ionization degree of metallic species. It provides an opportunity to influence chemical and mechanical properties by varying the process parameters. The present work deals with the development of CrN/AlN nanolaminate coatings in an industrial scale unit by using two different PVD technologies. Therefore, HPPMS and mfMS (middle frequency magnetron sputtering) technologies were used. The bilayer period Λ, i.e. the thickness of a CrN/AlN double layer, was varied between 6.2nm and 47.8 nm by varying the rotational speed of the substrate holders. In a second step the highest rotational speed was chosen and further HPPMS CrN/AlN coatings were deposited applying different HPPMS pulse lengths (40, 80, 200 μs) at the same mean cathode power and frequency. Thickness, morphology, roughness and phase composition of the coatings were analyzed by means of scanning electron microscopy (SEM), confocal laser microscopy, and X-ray diffraction (XRD), respectively. The chemical composition was determined using glow discharge optical emission spectroscopy (GDOES). Detailed characterization of the nanolaminate was conducted by transmission electron microscopy (TEM). The hardness and the Young's modulus were analyzed by nanoindentation measurements. The residual stress was determined via Si microcantilever curvature measurements. The phase analysis revealed the formation of h-Cr2N, c-CrN and c-AlN mixed phases for the mfMS CrN/AlN coatings, whereas the HPPMS coatings exhibited only cubic phases (c-CrN, c-AlN). A hardness of 31.0 GPa was measured for the HPPMS coating with a bilayer period of 6.2 nm. The decrease of the HPPMS pulse length at constant mean power leads to a considerable increase of the cathode current on the Cr and Al target associated with an increased ion flux towards the substrate. Furthermore, it was observed that the deposition rate of HPPMS CrN/AlN decreases with shorter pulse lengths, so that a CrN/AlN coating with a bilayer period of 2.9 nm, a high hardness of 40.8 GPa and a high compressive stress (- 4.37 GPa) was achieved using a short pulse length of 40 μs. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2014.06.058
  • Electrochemistry on binary valve metal combinatorial libraries: Niobium-tantalum thin films
    Mardare, A.I. and Ludwig, Al. and Savan, A. and Hassel, A.W.
    Electrochimica Acta 140 (2014)
    A Nb-Ta thin film compositional spread obtained from a co-sputtering process was analysed. The microstructure and crystallographic investigations revealed the presence of a compositional threshold at Nb-60 at.%Ta where the change from tetragonal to cubic symmetry was evidenced by a mixed tetragonal-cubic phase. The electrochemical properties of the anodic oxides were studied via cyclic voltammetry and the oxide formation factors were mapped along the entire compositional spread. Values ranging from 1.8 nm·V -1 at the Ta-rich side to 2.6 nm·V-1 at the Nb-rich side of the library were measured. All Nb-Ta mixed anodic oxides were found to exhibit a type-n semiconducting behaviour as evidenced by Mott-Schottky analysis. The chemical composition of the surface anodic oxides differed from the composition of the parent metal alloys and no clear trend could be identified regarding their mismatch. © 2014 Elsevier Ltd.
    view abstract10.1016/j.electacta.2014.02.138
  • Fabrication of a Ni-Cu thin film material library using pulsed electrodeposition
    Srinivas, P. and Hamann, S. and Wambach, M. and Ludwig, Al. and Dey, S.R.
    Journal of the Electrochemical Society 161 (2014)
    A thin film composition gradient library of the Ni-Cu alloy system is generated through an electrodeposition technique using a complexing citrate electrolyte bath in a modified Hull cell. Energy dispersive X-ray spectroscopy, scanning electron microscopy and automated X-ray diffraction are performed to assess composition, surface morphology, and crystallographic structure of the deposited film as a function of the lateral position on the materials library. The results confirmed deposition of single phase polycrystalline f.c.c. Ni-Cu alloy system with varied lateral composition and lattice parameter, afcc as well. © 2014 The Electrochemical Society. All rights reserved.
    view abstract10.1149/2.0451410jes
  • Functional and structural fatigue of titanium tantalum high temperature shape memory alloys (HT SMAs)
    Niendorf, T. and Krooß, P. and Batyrsina, E. and Paulsen, A. and Motemani, Y. and Ludwig, Al. and Buenconsejo, P. and Frenzel, J. and Eggeler, G. and Maier, H.J.
    Materials Science and Engineering A 620 (2014)
    Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel-titanium (Ni-Ti) can only be employed at temperatures up to about 100°C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni-Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium-tantalum (Ti-Ta) system has been developed to overcome these issues. Binary Ti-Ta provides relatively high MS temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti-Ta-Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and structural fatigue of this alloy. Functional fatigue is dominated by ω-phase evolution, while structural fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for fatigue life extension proposed very recently for binary Ti-Ta, is demonstrated to be also applicable for the ternary Ti-Ta-Al. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.10.038
  • High-Temperature Shape Memory Effect in Ti-Ta Thin Films Sputter Deposited at Room Temperature
    Motemani, Y. and Buenconsejo, P.J.S. and Craciunescu, C. and Ludwig, Al.
    Advanced Materials Interfaces 1 (2014)
    Ti-Ta based alloys are potential high-temperature shape memory materials with operation temperatures above 100°C. In this study, the room temperature fabrication of Ti-Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200°C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α martensite (orthorhombic) phase is formed in the as-deposited co-sputtered Ti<inf>70</inf>Ta<inf>30</inf>, Ti<inf>68</inf>Ta<inf>32</inf> and Ti<inf>67</inf>Ta<inf>33</inf> films, independent of the substrate. A Ti<inf>70</inf>Ta<inf>30</inf> free-standing film shows a reversible martensitic transformation, as confirmed by temperature-dependent XRD measurements during thermal cycling between 125°C to 275°C. Furthermore, a one-way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti-Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies. The room temperature fabrication of Ti-Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200°C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/admi.201400019
  • High-throughput fabrication of Au-Cu nanoparticle libraries by combinatorial sputtering in ionic liquids
    König, D. and Richter, K. and Siegel, A. and Mudring, A.-V. and Ludwig, Al.
    Advanced Functional Materials 24 (2014)
    Materials libraries of binary alloy nanoparticles (NPs) are synthesized by combinatorial co-sputter deposition of Cu and Au into the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 1C4im][Tf2N]), which is contained in a micromachined cavity array substrate. The resulting NPs and NP-suspensions are investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis measurements (UV-Vis), and attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy. Whereas the NPs can be directly observed in the IL using TEM, for XRD measurements the NP concentration is too low to lead to satisfactory results. Thus, a new NP isolation process involving capping agents is developed which enables separation of NPs from the IL without changing their size, morphology, composition, and state of aggregation. The results of the NP characterization show that next to the unary Cu and Au NPs, both stoichiometric and non-stoichiometric Cu-Au NPs smaller than 7 nm can be readily obtained. Whereas the size and shape of the alloy NPs change with alloy composition, for a fixed composition the NPs have a small size distribution. The measured lattice constants of all capped NPs show unexpected increased values, which could be related to the NP/surfactant interactions. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201303140
  • Investigation of ternary subsystems of superalloys by thin-film combinatorial synthesis and high-throughput analysis
    König, D. and Pfetzing-Micklich, J. and Frenzel, J. and Ludwig, Al.
    MATEC Web of Conferences 14 (2014)
    A Co-Ti-W thin film materials library was fabricated by magnetron sputtering. By using automated high-throughput measurement techniques (resistance mapping, automated XRD measurements) and cluster analysis a yet unknown phase region was revealed. The existence region of the new ternary phase is close to the composition Co60Ti15W25. In order to transfer the results from thin film to bulk material, a bulk sample was prepared by arc melting and subsequent heat treatment. Scanning electron microscopy and chemical micro-analysis data support that a yet unknown ternary phase exists in the system Co-Ti-W. © 2014 Owned by the authors, published by EDP Sciences.
    view abstract10.1051/matecconf/20141418002
  • Investigation of the thin-film phase diagram of the Cr-Ni-Re system by high-throughput experimentation
    Janghorban, A. and Pfetzing-Micklich, J. and Frenzel, J. and Ludwig, Al.
    Advanced Engineering Materials 16 (2014)
    The Cr-Ni-Re system was investigated over the whole composition range using combinatorial fabrication methods combined with high-throughput characterization techniques in order to establish its thin film phase diagram. After annealing at 940 and 1100°C, the phase equilibrium was reached in the Ni-rich part of the ternary in agreement with the published bulk phase diagram. Annealing the materials library at 940°C is not sufficient to achieve the equilibrium state in the Re-rich part of the system, however by annealing the materials library at 1100°C the formation of expected phases (three solid-solutions and a topologically close packed compound) could be observed. As a result of this study, a thin film phase diagram of the complete Cr-Ni-Re at 1100°C was established, which is well comparable to the bulk phase diagram. This shows that the combinatorial thin film phase diagram approach is feasible and especially promising for materials systems with expensive and/or high melting point constituents. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201300430
  • Mechanical properties of SiLix thin films at different stages of electrochemical Li insertion
    Zinn, A.-H. and Borhani-Haghighi, S. and Ventosa, E. and Pfetzing-Micklich, J. and Wieczorek, N. and Schuhmann, W. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 211 (2014)
    The mechanical properties of amorphous Si thin films, lithiated electrochemically to different Si£Li compositions are studied by ex situ nanoindentation. The compositions of the films are adjusted using an electrochemical routine that corrects for the Li consumed by SEI layer growth during initial lithiation. The mechanical properties such as Young's modulus and hardness are derived from nanoindentation. For compositions between Si and SiLi<inf>2.5</inf> the Young's modulus decreases with increasing Li content from ∼160 GPa to ∼8 GPa and the hardness decreases from ∼14 GPa to ∼0.1 GPa. The yield strength values, as deduced from hardness measurements, decrease from ∼5 GPa to 0.05 GPa. AFM imaging is used on the electrochemically cycled films to assess the SEIs impact on the nanomechanical measurements. XPS depth-profiling of the electrochemically cycled sample indicated a Li concentration gradient across the film thickness. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201431303
  • On the mechanism that leads to vanishing thermal hysteresis of the B2-R phase transformation in multilayered (TiNi)/(W) shape memory alloy thin films
    Buenconsejo, P.J.S. and Zarnetta, R. and Young, M. and Brunken, H. and Mehta, A. and Ludwig, Al.
    Thin Solid Films 564 (2014)
    The film stresses in two-phase (TiNi)/(W) shape memory alloy (SMA) multilayer thin films were evaluated using synchrotron diffraction analysis. The phase transforming B2-TiNi phase is under tensile stress due to the mismatch of the coefficient-of-thermal-expansion (αB2-TiNi &gt; αW &gt; αSi-substrate) and the elastic modulus (EW &gt; ESi &gt; EB2-TiNi) with respect to the bcc-W layers and the Si-substrate. The amount of stress on the B2-TiNi phase increases with increasing W amount in the film, which is proportional to the W layer thickness. This led to important changes in the behavior of the B2-R transformation. On cooling, a B2-R transformation proceeds under increasing tensile stress which increases the transformation start temperature (R s). Upon transformation to the R phase, the TiNi layers undergo stress-relaxation by reorientation of R phase variants to accommodate the mismatch. During heating the film always starts from a relaxed stress-state, so the reverse transformation proceeds without adversely affecting the reverse transformation temperature (Af). With increasing amount of W in the film Rs increases more on cooling, while Af is not significantly affected on heating, and this leads to vanishing thermal hysteresis (ΔTB2-R = Af - Rs). © 2014 Elsevier B.V.
    view abstract10.1016/j.tsf.2014.05.010
  • Potential-resolved dissolution of Pt-Cu: A thin-film material library study
    Schuppert, A.K. and Savan, A. and Ludwig, Al. and Mayrhofer, K.J.J.
    Electrochimica Acta 144 (2014)
    Within the search for new catalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells, alloys of Pt with other transition metals are of great interest due to their increased specific and especially mass activity. However, the drawback of these catalysts is their reduced stability due to the dissolution of the less-noble metal from the alloy. To resolve the potential dependence of these dissolution processes, we investigate a material library of Pt-Cu thin-film alloys with compositions ranging from 0 at% Cu up to 60 at% Cu. Utilizing our combinatorial scanning flow cell technique coupled to a mass spectrometer several aspects of dissolution are revealed. The onset of relevant Cu dissolution was found to be around 0.9 VRHE, independent of the composition. Although this is well below the onset potential of the Pt dissolution (1.15 VRHE), the two dissolution processes are clearly correlated, especially when the surface is already depleted of Cu. In contrast to Pt, however, Cu preferentially dissolves during anodic polarization rather than during the cathodic sweep. Additionally, at Cu compositions above the parting limit between 50 and 57 at% Cu a breakdown of passivity and massive Cu dissolution leads to porosity formation. The critical potential for the alloy with 57 at% Cu was detected around 1.3 VRHE, which is above the stability potential of Pt. While the absolute activity during the porosity formation increases due to the formation of more active sites in the pores, the specific activity decays to a value close to pure Pt. © 2014 The Authors.
    view abstract10.1016/j.electacta.2014.07.113
  • Properties of anodic oxides grown on a hafnium-tantalum-titanium thin film library
    Mardare, A.I. and Ludwig, Al. and Savan, A. and Hassel, A.W.
    Science and Technology of Advanced Materials 15 (2014)
    A ternary thin film combinatorial materials library of the valve metal system Hf-Ta-Ti obtained by co-sputtering was studied. The microstructural and crystallographic analysis of the obtained compositions revealed a crystalline and textured surface, with the exception of compositions with Ta concentration above 48 at.% which are amorphous and show a flat surface. Electrochemical anodization of the composition spread thin films was used for analysing the growth of the mixed surface oxides. Oxide formation factors, obtained from the potentiodynamic anodization curves, as well as the dielectric constants and electrical resistances, obtained from electrochemical impedance spectroscopy, were mapped along two dimensions of the library using a scanning droplet cell microscope. The semiconducting properties of the anodic oxides were mapped using Mott-Schottky analysis. The degree of oxide mixing was analysed qualitatively using x-ray photoelectron spectroscopy depth profiling. A quantitative analysis of the surface oxides was performed and correlated to the as-deposited metal thin film compositions. In the concurrent transport of the three metal cations during oxide growth a clear speed order of Ti > Hf > Ta was proven. © 2014 National Institute for Materials Science.
    view abstract10.1088/1468-6996/15/1/015006
  • Rapid identification of areas of interest in thin film materials libraries by combining electrical, optical, X-ray diffraction, and mechanical high-throughput measurements: A case study for the system ni-al
    Thienhaus, S. and Naujoks, D. and Pfetzing-Micklich, J. and König, D. and Ludwig, Al.
    ACS Combinatorial Science 16 (2014)
    The efficient identification of compositional areas of interest in thin film materials systems fabricated by combinatorial deposition methods is essential in combinatorial materials science. We use a combination of compositional screening by EDX together with high-throughput measurements of electrical and optical properties of thin film libraries to determine efficiently the areas of interest in a materials system. Areas of interest are compositions which show distinctive properties. The crystallinity of the thus determined areas is identified by X-ray diffraction. Additionally, by using automated nanoindentation across the materials library, mechanical data of the thin films can be obtained which complements the identification of areas of interest. The feasibility of this approach is demonstrated by using a Ni-Al thin film library as a reference system. The obtained results promise that this approach can be used for the case of ternary and higher order systems. © 2014 American Chemical Society.
    view abstract10.1021/co5000757
  • Self-directed localization of ZIF-8 thin film formation by conversion of ZnO nanolayers
    Khaletskaya, K. and Turner, S. and Tu, M. and Wannapaiboon, S. and Schneemann, A. and Meyer, R. and Ludwig, Al. and Van Tendeloo, G. and Fischer, R.A.
    Advanced Functional Materials 24 (2014)
    Control of localized metal-organic framework (MOF) thin film formation is a challenge. Zeolitic imidazolate frameworks (ZIFs) are an important sub-class of MOFs based on transition metals and imidazolate linkers. Continuous coatings of intergrown ZIF crystals require high rates of heterogeneous nucleation. In this work, substrates coated with zinc oxide layers are used, obtained by atomic layer deposition (ALD) or by magnetron sputtering, to provide the Zn 2+ ions required for nucleation and localized growth of ZIF-8 films ([Zn(mim)2]; Hmim = 2-methylimidazolate). The obtained ZIF-8 films reveal the expected microporosity, as deduced from methanol adsorption studies using an environmentally controlled quartz crystal microbalance (QCM) and comparison with bulk ZIF-8 reference data. The concept is transferable to other MOFs, and is applied to the formation of [Al(OH)(1,4-ndc)]n (ndc = naphtalenedicarboxylate) thin films derived from Al2O3 nanolayers. The self-template route for the manufacturing of ZIF-8 films on silicon (Si) and quartz crystal microbalance (QCM) substrates involves the pre-deposition of ZnO films prepared by sputtering or atomic layer deposition methods and the subsequent conversion of the immobilized ZnO phase into crystalline and homogeneously dense ZIF-8 films via microwave-assisted synthesis. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201400559
  • Structure-related antibacterial activity of a titanium nanostructured surface fabricated by glancing angle sputter deposition
    Sengstock, C. and Lopian, M. and Motemani, Y. and Borgmann, A. and Khare, C. and Buenconsejo, P.J.S. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Nanotechnology 25 (2014)
    The aim of this study was to reproduce the physico-mechanical antibacterial effect of the nanocolumnar cicada wing surface for metallic biomaterials by fabrication of titanium (Ti) nanocolumnar surfaces using glancing angle sputter deposition (GLAD). Nanocolumnar Ti thin films were fabricated by GLAD on silicon substrates. S. aureus as well as E. coli were incubated with nanostructured or reference dense Ti thin film test samples for one or three hours at 37 °C. Bacterial adherence, morphology, and viability were analyzed by fluorescence staining and scanning electron microscopy and compared to human mesenchymal stem cells (hMSCs). Bacterial adherence was not significantly different after short (1 h) incubation on the dense or the nanostructured Ti surface. In contrast to S. aureus the viability of E. coli was significantly decreased after 3 h on the nanostructured film compared to the dense film and was accompanied by an irregular morphology and a cell wall deformation. Cell adherence, spreading and viability of hMSCs were not altered on the nanostructured surface. The results show that the selective antibacterial effect of the cicada wing could be transferred to a nanostructured metallic biomaterial by mimicking the natural nanocolumnar topography. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/25/19/195101
  • Synthesis of WO3 nanoblades by the dealloying of glancing angle deposited W-Fe nanocolumnar thin films
    Khare, C. and Stepanovich, A. and Buenconsejo, P.J.S. and Ludwig, Al.
    Nanotechnology 25 (2014)
    Glancing angle co-deposition of well-separated W-Fe nanocolumns was carried out using a W oblique angle sputter source and a Fe confocal incidence source. As-deposited nanocolumns with an overall composition of W64.6Fe35.4 (at.%) exhibited an average column width w nc of 77 ± 15 nm with predominant growth in the β-W phase. With the aim of synthesizing highly porous nanostructures, the as-deposited precursor W-Fe nanocolumnar thin films were immersed in aqueous HNO3 solution for various dealloying durations (t d ). Formation of nanoflake-, nanocactus-, and nanoblade-like structures were observed during the dealloying treatment, as a result of selective dissolution of Fe from the W-Fe precursor films and simultaneous oxidation of W adatoms. By increasing the dealloying duration, the Fe concentration within the film reduced drastically and the film thickness increased by about three times in comparison to the as-deposited film. The dealloyed film exhibited an overall composition of W95.6Fe4.4, where the effective surface area of the film increased substantially. It was found that W adatom diffusion and subsequent rearrangement are crucially important in determining the resultant thin film morphology. The morphological development, corresponding compositions and crystallographic properties of different nanostructures were found to be significantly dependent on the dealloying duration. For optimized processing parameters, the selective dissolution process led to formation of single crystal monoclinic WO3 nanoblades, with growth along [002] and [020] axes. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/25/20/205606
  • The Bain library: A Cu-Au buffer template for a continuous variation of lattice parameters in epitaxial films
    Kauffmann-Weiss, S. and Hamann, S. and Reichel, L. and Siegel, A. and Alexandrakis, V. and Heller, R. and Schultz, L. and Ludwig, Al. and Fähler, S.
    APL Materials 2 (2014)
    Smallest variations of the lattice parameter result in significant changes in material properties. Whereas in bulk, lattice parameters can only be changed by composition or temperature, coherent epitaxial growth of thin films on single crystals allows adjusting the lattice parameters independently. Up to now only discrete values were accessible by using different buffer or substrate materials. We realize a lateral variation of in-plane lattice parameters using combinatorial film deposition of epitaxial Cu-Au on a 4-in. Si wafer. This template gives the possibility to adjust the in-plane lattice parameter over a wide range from 0.365 nm up to 0.382 nm. © 2014 Author(s).
    view abstract10.1063/1.4870759
  • Ag-stabilized few-layer graphene dispersions in low boiling point solvents for versatile nonlinear optical applications
    Sun, Z. and Dong, N. and Wang, K. and König, D. and Nagaiah, T.C. and Sánchez, M.D. and Ludwig, Al. and Cheng, X. and Schuhmann, W. and Wang, J. and Muhler, M.
    Carbon 62 (2013)
    A solution stabilization strategy that uses an easily removable media is critical to graphene (G) applications. Here, we demonstrate that highly stable graphene dispersions in low boiling point solvents such as isopropanol can be readily achieved by the uniform deposition of Ag nanoparticles (NPs) on the surface of graphene. Optimizing the synthesis parameters such as ultrasonic intensity, feeding strategy, loading content and precursor concentration allowed us to tune the particle size and, in this way, the stabilizing effects of the NPs on the dispersions. The as-obtained Ag/G/i-PrOH dispersions exhibit versatile nonlinear optical properties suggesting a great potential in nanophotonic applications such as absorber for ultrafast lasers and eye protection. © 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2013.06.010
  • Compositional trends and magnetic excitations in binary and ternary Fe-Pd-X magnetic shape memory alloys
    Gruner, M.E. and Hamann, S. and Brunken, H. and Ludwig, Al. and Entel, P.
    Journal of Alloys and Compounds 577 (2013)
    High throughput thin film experiments and first-principles calculations are combined in order to get insight into the relation between finite temperature transformation behavior and structural ground state properties of ternary Fe-Pd-X alloys. In particular, we consider the binding surface, i.e., the energy of the disordered alloy calculated along the Bain path between bcc and fcc which we model by a 108 atom supercell. We compare stoichiometric Fe 75Pd25 with ternary systems, where 4.6% of the Fe atoms were substituted by Cu and Mn, respectively. The computational trends are related to combinatorial experiments on thin film libraries for the systems Fe-Pd-Mn and Fe-Pd-Cu which reveal a systematic evolution of the martensitic start temperature with composition within the relevant concentration range for magnetic shape memory (MSM) applications. Our calculations include atomic relaxations, which were shown to be relevant for a correct description of the structural properties. Furthermore, we find that magnetic excitations can substantially alter the binding surface. The comparison of experimental and theoretical trends indicates that, both, compositional changes and magnetic excitations contribute significantly to the structural stability which may thus be tailored by specifically adding antiferromagnetic components. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2012.02.033
  • Development of Ni-Cu materials library by using combinatorial pulsed electrodeposition
    Srinivas, P. and Hamann, S. and Wambach, M. and Kieschnick, M. and Ludwig, Al. and Dey, S.R.
    Transactions of the Indian Institute of Metals 66 (2013)
    Combinatorial materials fabrication and high-throughput characterization methods offer a new experimental paradigm to accelerate the enhancement of known and the discovery of new materials in materials science and engineering. In this study, the composition spread library of binary Ni-Cu alloy system is synthesized combinatorially onto a copper substrate using pulsed electrodeposition (PED) from a single sulfate bath with a complexing agent trisodium citrate in a modified Hull cell. Different current densities are expected on the tilted cathode which varied the composition along lateral direction and generated Ni-Cu binary spread. Crystallographic structure as well as atomic concentration of the constituent elements of the deposited binary Ni-Cu alloy film along the lateral direction is determined as a function of fabrication parameters. The presented results indicate the successful development of Ni-Cu binary spread (10-90 at.%) via PED. © 2013 Indian Institute of Metals.
    view abstract10.1007/s12666-013-0249-7
  • Erratum: Dynamics of photogenerated holes in TiO2-polyheptazine hybrid photoanodes for visible light-driven water splitting (Journal of the Electrochemical Society (2012) 159 (H616))
    Wang, L. and Bledowski, M. and Ramakrishnan, A. and König, D. and Ludwig, Al. and Beranek, R.
    Journal of the Electrochemical Society 160 (2013)
    view abstract10.1149/2.087311jes
  • High-throughput compositional and structural evaluation of a Li a(NixMnyCoz)Or thin film battery materials library
    Borhani-Haghighi, S. and Kieschnick, M. and Motemani, Y. and Savan, A. and Rogalla, D. and Becker, H.-W. and Meijer, J. and Ludwig, Al.
    ACS Combinatorial Science 15 (2013)
    A Lia(NixMnyCoz)Or cathode materials library was fabricated by combinatorial magnetron sputtering. The compositional analysis of the library was performed by a new high-throughput approach for Li-content measurement in thin films, which combines automated energy-dispersive X-ray spectroscopy, Deuteron-induced gamma emission, and Rutherford backscattering measurements. Furthermore, combining this approach with thickness measurements allows the mapping of density values of samples from the materials library. By correlating the obtained compositional data with structural data from high-throughput X-ray diffraction measurements, those compositions which show a layered (R3Ì...m) structure and are therefore most interesting for Li-battery applications (for cathode (positive) electrodes) can be rapidly identified. This structure was identified as being most pronounced in the compositions Li0.6(Ni0.16Mn 0.35Co0.48)O2, Li0.7(Ni 0.10Mn0.37Co0.51)O2, Li 0.6(Ni0.23Mn0.33Co0.43)O 2, Li0.3(Ni0.65Mn0.08Co 0.26)O2, Li0.3(Ni0.63Mn 0.08Co0.29)O2, Li0.4(Ni 0.56Mn0.09Co0.34)O2, Li 0.5(Ni0.45Mn0.13Co0.42)O 2, and Li0.6(Ni0.34Mn0.14Co 0.52)O2. © 2013 American Chemical Society.
    view abstract10.1021/co4000166
  • Layered WO3/TiO2 nanostructures with enhanced photocurrent densities
    Khare, C. and Sliozberg, K. and Meyer, R. and Savan, A. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 38 (2013)
    Layered WO3/TiO2 nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO2 and WO3 layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO3 microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO2 nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO3 and TiO2 films show thickness and microstructure dependence. Among individual WO3 layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO2 nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO3-wedge type layer with a layer of TiO2 nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO3 layers. The highest photocurrent that is measured in the combinatorial library of porous WO3/TiO2 films is as high as 0.11 mA/cm2. Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.
    view abstract10.1016/j.ijhydene.2013.09.142
  • Nanostructured few-layer graphene with superior optical limiting properties fabricated by a catalytic steam etching process
    Sun, Z. and Dong, N. and Xie, K. and Xia, W. and König, D. and Nagaiah, T.C. and Sánchez, M.D. and Ebbinghaus, P. and Erbe, A. and Zhang, X. and Ludwig, Al. and Schuhmann, W. and Wang, J. and Muhler, M.
    Journal of Physical Chemistry C 117 (2013)
    Tailoring the morphology and structure of graphene can result in novel properties for advanced applications. Here, we demonstrate the fabrication of nanostructured few-layer graphene through a mild etching process via catalytic steam gasification of carbon by Fe nanoparticles (NPs). Controlling the reaction temperature, steam concentration, and the loading density of the NPs enables the fine-tuning of the etching level of graphene. Well-defined nanotrenches with a width of less than 25 nm were formed by channeling of the catalytic NPs. Etching caves and quasi-semicircular etched edges were observed as well. The nonlinear optical properties of the resulting nanostructured graphene were studied under a 532 nm nanosecond pulse laser through an open-aperture apparatus. At the same level of the linear extinction coefficient, it exhibits superior optical limiting performance in comparison with pristine graphene and C60, showing a large potential in nanophotonic devices. This enhancement is ascribed to the defects formed by etching resulting in a finite band gap in nanostructured graphene. © 2013 American Chemical Society.
    view abstract10.1021/jp401736n
  • 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 (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 abstract10.1149/05801.0587ecst
  • Scanning droplet cell microscopy on a wide range hafnium-niobium thin film combinatorial library
    Mardare, A.I. and Ludwig, Al. and Savan, A. and Hassel, A.W.
    Electrochimica Acta 110 (2013)
    A wide-range thin film Hf-Nb combinatorial library deposited by co-sputtering is studied. The microstructure and crystallographic properties of the thin film alloys locally investigated by SEM and GIXRD are mapped along the entire compositional spread from 14 to 94 at.% Nb. Scanning droplet cell microscopy (SDCM) is used for mapping the electrochemical properties of the naturally oxidised metallic surfaces. Anodisation of the Hf-Nb thin films alloys is achieved with a high throughput due to computer-controlled scanning, made with a composition resolution of 1 at.%. The electrical properties of the anodic oxides are mapped by EIS and a maximum electrical permittivity close to 75 was found for Hf-33 at.% Nb. Semiconducting properties of the mixed anodic oxides are studied using Mott-Schottky analysis and their composition and mixing is investigated by XPS depth profiling.© 2013 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2013.03.065
  • Synthesis of Au microwires by selective oxidation of Au-W thin-film composition spreads
    Hamann, S. and Brunken, H. and Salomon, S. and Meyer, R. and Savan, A. and Ludwig, Al.
    Science and Technology of Advanced Materials 14 (2013)
    We report on the stress-induced growth of Au microwires out of a surrounding Au-W matrix by selective oxidation, in view of a possible application as 'micro-Velcro'. The Au wires are extruded due to the high compressive stress in the tungsten oxide formed by oxidation of elemental W. The samples were fabricated as a thin-film materials library using combinatorial sputter deposition followed by thermal oxidation. Sizes and shapes of the Au microwires were investigated as a function of the W to Au ratio. The coherence length and stress state of the Au microwires were related to their shape and plastic deformation. Depending on the composition of the Au-W precursor, the oxidized samples showed regions with differently shaped Au microwires. The Au48W52 composition yielded wires with the maximum length to diameter ratio due to the high compressive stress in the tungsten oxide matrix. The values of wire length (35 μm) and diameter (2 μm) achieved at the Au48W52 composition are suitable for micro-Velcro applications. © 2013 National Institute for Materials Science.
    view abstract10.1088/1468-6996/14/1/015003
  • Time- and space-resolved high-throughput characterization of stresses during sputtering and thermal processing of Al-Cr-N thin films
    Grochla, D. and Siegel, A. and Hamann, S. and Buenconsejo, P.J.S. and Kieschnick, M. and Brunken, H. and König, D. and Ludwig, Al.
    Journal of Physics D: Applied Physics 46 (2013)
    (Al100-xCrx)N thin-film materials libraries (x = 31-79 at%) were fabricated on micro-machined cantilever arrays, in order to simultaneously investigate the evolution of stresses during film growth as well as during thermal processing by analysing the changes in cantilever curvature. The issue of the dependence of stress in the growing films on composition, at comparable film thicknesses, was investigated. Among the various experimental parameters studied, it was found that the applied substrate bias has the strongest influence on stress evolution and microstructure formation. The compositions of the films, as well as the applied substrate bias, have a pronounced effect on the lattice parameter and the coherence length. For example, applying a substrate bias in general leads to compressive residual stress, increases the lattice parameter and decreases the coherence length. Moreover, bias can change the film texture from [1 1 1] orientation to [2 0 0]. Further detailed analysis using x-ray diffraction and transmission electron microscopy clearly revealed the presence of a [1 1 1] highly textured face centred cubic (B1 type) Al-Cr-N phase in the as-deposited state as well as the coexistence of the hexagonal [1 1 0] textured Cr2N phase, which forms in the Cr-rich region. These results show that the combinatorial approach provides insight into how stresses and compositions are related to phases and microstructures of different Al-Cr-N compositions fabricated in the form of materials libraries. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/46/8/084011
  • Combinatorial development of nanoporous WO 3 thin film photoelectrodes for solar water splitting by dealloying of binary alloys
    Stepanovich, A. and Sliozberg, K. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 37 (2012)
    A combinatorial materials approach is suggested for the development of nanoporous thin film oxides for photoelectrochemical solar water splitting. As a precursor for nanoporous WO 3 films, metallic nanoporous W films were synthesized by dealloying sputtered W 1-xAl x and W 1-xFe x (0.06 < x < 0.67) thin film materials libraries in aqueous HNO 3 solutions with different concentrations for 24 h under open circuit conditions. The variation of the etchant concentration provided different film nanostructures. The films were then transformed into nanoporous WO 3 by controlled thermal oxidation at 500 °C in air. Screening of the photoelectrochemical properties of nanoporous WO 3 films shows a strong porosity- and thickness-dependence of the photocurrent. At the same time the photocurrent density does not depend on precursor composition, because dealloying in acid solutions of certain concentration leads to formation of identical nanostructures in a broad range of precursor compositions. ©, 2012 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2012.05.039
  • Dual frequency capacitive plasmas in Fe and Ni sputter applications: Correlation of discharge properties on thin film properties
    Bienholz, S. and Semmler, E. and Awakowicz, P. and Brunken, H. and Ludwig, Al.
    Plasma Sources Science and Technology 21 (2012)
    Dual frequency capacitively coupled plasmas (CCPs) are widely used in (large area) etching and plasma enhanced chemical vapor deposition processes. However, applications in physical vapor deposition (PVD) are still sparse due to the well-established dc magnetron cathode discharges. Nevertheless, there exist critical applications such as ferromagnetic or ceramic thin film deposition which are difficult to handle even for dc magnetron systems. For these materials systems dual frequency CCPs pose a good alternative, because for insulators charging can be avoided and for ferromagnetic materials the target thickness becomes independent of the magnetron configuration at comparable deposition rates. In this work we investigate two separate subjects. First, in dual frequency capacitive discharges a complex coupling of the applied excitation frequencies can be observed, which from a plasma parameter point of view limits the separability of ion flux (usually controlled by frequencies >60MHz) and ion bombarding energy (usually controlled by frequency <15MHz) onto the sputter target. By performing deposition experiments it was found that by following simple tuning guidelines a very good degree of separability is achievable. Additionally, the deposition homogeneity is not affected. Second, we correlate the growth conditions with crystalline and magnetic properties as well as the degree of O content for Fe and Ni films. Therefore, we applied different signals as a substrate bias to influence thin film growth. It was found that the crystalline and magnetic properties can be influenced for both Fe and Ni films but is more pronounced for Ni. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0963-0252/21/1/015010
  • Dynamics of photogenerated holes in TiO2-polyheptazine hybrid photoanodes for visible light-driven water splitting
    Wang, L. and Bledowski, M. and Ramakrishnan, A. and König, D. and Ludwig, Al. and Beranek, R.
    Journal of the Electrochemical Society 159 (2012)
    The dynamics of visible-light photogenerated holes in nanocrystalline TiO2-polyheptazine (TiO2-PH) hybrid photoelectrodes for water photooxidation was investigated by polychromatic and wavelength-resolved photocurrent measurements. The evaluation of the hole reactivity was addressed by direct comparison to photoelectrodes based on pristine TiO2. The visible-light generated holes in TiO2-PH are located in the thin polyheptazine ("graphitic carbon nitride") layer at the surface of TiO2 and possess a lower oxidation potential (by ∼0.9 V) as compared to UV light-photogenerated holes in pristine TiO2. Due to their slow water oxidation kinetics, the photoholes accumulate at the surface, which leads to negligible oxygen evolution and increased recombination. This problem can be overcome by introducing a suitable co-catalyst (IrO2 nanoparticles), as evidenced by dioxygen evolution under visible light (λ &gt; 420 nm) irradiation. © 2012 The Electrochemical Society.
    view abstract10.1149/2.010207jes
  • Enhancing magnetocrystalline anisotropy of the Fe 70Pd 30 magnetic shape memory alloy by adding Cu
    Kauffmann-Weiss, S. and Hamann, S. and Gruner, M.E. and Schultz, L. and Ludwig, Al. and Fähler, S.
    Acta Materialia 60 (2012)
    Fe-Pd-Cu thin films are of great interest for applications in magnetic shape memory microsystems due to their increased martensitic transformation temperature. Here we analyse the consequences of Cu addition to Fe-Pd on the binding energy and magnetic properties by a combination of thin film experiments and first-principles calculations. Strained epitaxial growth of Fe 70Pd 30-xCu x with x = 0, 3, 7 is used to freeze intermediate stages during the martensitic transformation. This makes a large range of tetragonal distortion susceptible for analysis, ranging from body-centred cubic to beyond face-centred cubic (1.07 < c/a bct < 1.57). We find that Cu enhances the quality of epitaxial growth, while spontaneous polarization and Curie temperature are reduced only moderately, in agreement with our calculations. Beyond c/a bct &gt; 1.41 the samples undergo structural relaxations through adaptive nanotwinning. Cu enhances the magnetocrystalline anisotropy constant K 1 at room temperature, which reaches a maximum of -2.4 × 10 5 J m -3 around c/a bct = 1.33. This value exceeds those of binary Fe 70Pd 30 and the prototype Ni-Mn-Ga magnetic shape memory system. Since K 1 represents the maximum driving energy for variant reorientation in magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a promising route towards microactuator applications with significantly improved work output. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.08.001
  • 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 (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 abstract10.1016/j.intermet.2012.03.044
  • Influence of process parameters on the crystallinity, morphology and composition of tungsten oxide-based thin films grown by metalorganic chemical vapor deposition
    De Los Arcos, T. and Cwik, S. and Milanov, A.P. and Gwildies, V. and Parala, H. and Wagner, T. and Birkner, A. and Rogalla, D. and Becker, H.-W. and Winter, J. and Ludwig, Al. and Fischer, R.A. and Devi, A.
    Thin Solid Films 522 (2012)
    The growth of tungsten oxide (WO 3) based thin films was achieved via metalorganic chemical vapor deposition using an all-nitrogen coordinated tungsten precursor in combination with oxygen. Film growth was performed on Si(100) substrates in the temperature range of 400-800 °C. Employing multi-technique approaches like X-ray diffraction, scanning electron microscopy, atomic force microscopy, Rutherford back scattering, nuclear reaction analysis and X-ray photoelectron spectroscopy, the variation of the growth characteristics and film properties with deposition temperature were studied in terms of crystallinity, structure, surface roughness and composition. Special attention was devoted to the investigation of variations in the film composition for the as-deposited and annealed films. © 2011 Elsevier B.V.
    view abstract10.1016/j.tsf.2011.12.007
  • Microgradient-heaters as tools for high-throughput experimentation
    Meyer, R. and Hamann, S. and Ehmann, M. and Thienhaus, S. and Jaeger, S. and Thiede, T. and Devi, A. and Fischer, R.A. and Ludwig, Al.
    ACS Combinatorial Science 14 (2012)
    A microgradient-heater (MGH) was developed, and its feasibility as a tool for high-throughput materials science experimentation was tested. The MGH is derived from microhot plate (MHP) systems and allows combinatorial thermal processing on the micronano scale. The temperature gradient is adjustable by the substrate material. For an Au-coated MGH membrane a temperature drop from 605 to 100 °C was measured over a distance of 965 μm, resulting in an average temperature change of 0.52 K/μm. As a proof of principle, we demonstrate the feasibility of MGHs on the example of a chemical vapor deposition (CVD) process. The achieved results show discontinuous changes in surface morphology within a continuous TiO 2 film. Furthermore the MGH can be used to get insights into the energetic relations of film growth processes, giving it the potential for microcalorimetry measurements. © 2012 American Chemical Society.
    view abstract10.1021/co3000488
  • Preparation of 24 ternary thin film materials libraries on a single substrate in one experiment for irreversible high-throughput studies
    Buenconsejo, P.J.S. and Siegel, A. and Savan, A. and Thienhaus, S. and Ludwig, Al.
    ACS Combinatorial Science 14 (2012)
    For different areas of combinatorial materials science, it is desirable to have multiple materials libraries: especially for irreversible high-throughput studies, like, for example, corrosion resistance testing in different media or annealing of complete materials libraries at different temperatures. Therefore a new combinatorial sputter-deposition process was developed which yields 24 materials libraries in one experiment on a single substrate. It is discussed with the example of 24 Ti-Ni-Ag materials libraries. They are divided based on the composition coverage and orientation of composition gradient into two sets of 12 nearly identical materials libraries. Each materials library covers at least 30-40% of the complete ternary composition range. An acid etch test in buffered-HF solution was performed, illustrating the feasibility of our approach for destructive materials characterization. The results revealed that within the composition range of Ni < 30 at.%, the films were severely etched. The composition range which shows reversible martensitic transformations was confirmed to be outside this region. The high output of the present method makes it attractive for combinatorial studies requiring multiple materials libraries. © 2011 American Chemical Society.
    view abstract10.1021/co2001263
  • Rapid and surfactant-free synthesis of bimetallic Pt-Cu nanoparticles simply via ultrasound-assisted redox replacement
    Sun, Z. and Masa, J. and Xia, W. and König, D. and Ludwig, Al. and Li, Z.-A. and Farle, M. and Schuhmann, W. and Muhler, M.
    ACS Catalysis 2 (2012)
    The synthesis of bimetallic nanoparticles (NPs) with well-defined morphology and a size of <5 nm remains an ongoing challenge. Here, we developed a facile and efficient approach to the design of bimetallic nanostructures by the galvanic replacement reaction facilitated by high-intensity ultrasound (100 W, 20 kHz) at low temperatures. As a model system, Pt-Cu NPs deposited on nitrogen-doped carbon nanotubes (NCNTs) were synthesized and characterized by spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) inspection shows that the mean diameter of Pt-Cu NPs can be as low as ≈2.8 nm, regardless of the much larger initial Cu particle size, and that a significant increase in particle number density by a factor of 35 had occurred during the replacement process. The concentration of the Pt precursor solution as well as of the size of the seed particles were found to control the size of the bimetallic NPs. Energy dispersive X-ray spectroscopy performed in the scanning TEM mode confirmed the alloyed nature of the Pt-Cu NPs. Electrochemical oxygen reduction measurements demonstrated that the resulting Pt-Cu/NCNT catalysts exhibit an approximately 2-fold enhancement in both mass- and area-related activities compared with a commercial Pt/C catalyst. © 2012 American Chemical Society.
    view abstract10.1021/cs300187z
  • 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 (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 abstract10.1016/j.actamat.2011.09.037
  • Understanding the magnetic shape memory system Fe-Pd-X by thin film experiments and first principle calculations
    Kauffmann-Weiss, S. and Hamann, S. and Gruner, M.E. and Buschbeck, J. and Ludwig, Al. and Schultz, L. and Fähler, S.
    Advanced Engineering Materials 14 (2012)
    The magnetic shape memory (MSM) alloy Fe 70Pd 30 is of particular interest for novel microactuator and sensor applications. This review summarizes the underlying physical and material science concepts for this MSM alloy system. First-principles calculations of the electronic and crystallographic structure together with combinatorial and epitaxial film studies are presented. By these complementary methods we can address the open key questions of MSM alloys and microsystems: Which are the driving forces for a martensitic transformation and how does this transformation proceed? How is it possible to improve the MSM properties by adding third elements? What is the role of external interfaces and which routes allow the preparation of freestanding epitaxial films? Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200052
  • A new prototype two-phase (TiNi)-(β-W) SMA system with tailorable thermal hysteresis
    Buenconsejo, P.J.S. and Zarnetta, R. and König, D. and Savan, A. and Thienhaus, S. and Ludwig, Al.
    Advanced Functional Materials 21 (2011)
    The Ti-Ni-W two-phase shape memory alloy (SMA) thin film system is presented as a prototype for new SMAs with tailorable thermal transformation hysteresis (ΔT). The concept is to combine the SMA TiNi with almost insoluble W to create the two-phase system (TiNi)-(β-W). This system behaves like a pseudobinary TiNi system. Phase transformation behavior for compositions above the solubility limit of W in TiNi exhibit a B2-R phase transformation with characteristically small ΔT. Moreover, ΔT is dependent on the amount of W and it can be tailored to zero and even negative. This phenomenon is rationalized as being due to the mechanical interaction between the phases B2-TiNi and β-W. The presented results are very promising for the development of high-speed Ti-Ni-based SMA actuators. We present an approach to inkjet print high-performance organic transistors by printing the organic semiconductor ink on a thin, continuous, and solvent-absorbing layer of insulating material. The ink spreading is effectively controlled by local dissolution of the layer, and during drying the characteristic circular morphology with high rims and inner plateau forms. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201001697
  • A novel high-throughput fatigue testing method for metallic thin films
    Burger, S. and Eberl, C. and Siegel, A. and Ludwig, Al. and Kraft, O.
    Science and Technology of Advanced Materials 12 (2011)
    Thin films are used in a wide variety of computing and communication applications although their fatigue behavior and its dependence on alloying elements are not very well known. In this paper, we present an experimental implementation of a novel high-throughput fatigue testing method for metallic thin films. The methodology uses the fact that the surface strain amplitude of a vibrating cantilever decreases linearly from the fixed end to the free end. Therefore, a thin film attached to a vibrating cantilever will experience a gradient of strain and corresponding stress amplitudes along the cantilever. Each cantilever can be used to extract a lifetime diagram by measuring the fatigue-induced damage front that progresses along the cantilever during up to 10 8 load cycles. © 2011 National Institute for Materials Science.
    view abstract10.1088/1468-6996/12/5/054202
  • Applications of an energy-dispersive pnCCD for X-ray reflectivity: Investigation of interdiffusion in Fe-Pt multilayers
    Abboud, A. and Send, S. and Hartmann, R. and Strüder, L. and Savan, A. and Ludwig, Al. and Zotov, N. and Pietsch, U.
    Physica Status Solidi (A) Applications and Materials Science 208 (2011)
    A frame store pn-junction CCD (pnCCD) detector was applied to study thermally induced interdiffusion in Fe/Pt thin film multilayers (MLs) in a temperature range between 300 and 585K. Based on the energy resolution of the detector the reflectivity was measured simultaneously in a spectral range between 8keV< E< 20keV including the Pt L-edge energies close to 11.5keV. Above T=533K we find a strong drop of intensities at 1st and 2nd order ML Bragg peak interpreted by mutual interdiffusion. Considering a simulated model of interdiffusion it has been found that the concentration of iron that diffuses into the platinum sub layers is higher than that of platinum into iron. The time dependence of inter diffusion was also calculated in the range of 533-568K and was described by the Arrhenius equation D(T)=D 0exp(-H a/k BT). The activation energy for the MLs used [Fe 1.7nm/Pt 2nm] 50 was found to be 0.94±0.22eV. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201184268
  • Combinatorial investigation of Fe-B thin-film nanocomposites
    Brunken, H. and Grochla, D. and Savan, A. and Kieschnick, M. and Meijer, J. D. and Ludwig, Al.
    Science and Technology of Advanced Materials 12 (2011)
    Combinatorial magnetron sputter deposition from elemental targets was used to create Fe-B composition spread type thin film materials libraries on thermally oxidized 4-in. Si wafers. The materials libraries consisting of wedge-type multilayer thin films were annealed at 500 or 700 degrees C to transform the multilayers into multiphase alloys. The libraries were characterized by nuclear reaction analysis, Rutherford backscattering, nanoindentation, vibrating sample magnetometry, x-ray diffraction (XRD) and transmission electron microscopy (TEM). Young's modulus and hardness values were related to the annealing parameters, structure and composition of the films. The magnetic properties of the films were improved by annealing in a H-2 atmosphere, showing a more than tenfold decrease in the coercive field values in comparison to those of the vacuum-annealed films. The hardness values increased from 8 to 18 GPa when the annealing temperature was increased from 500 to 700 degrees C. The appearance of Fe2B phases, as revealed by XRD and TEM, had a significant effect on the mechanical properties of the films.
    view abstract10.1088/1468-6996/12/5/054208
  • Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering
    Vidyarthi, V.S. and Hofmann, M. and Savan, A. and Sliozberg, K. and König, D. and Beranek, R. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 36 (2011)
    Polycrystalline WO3 thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O2 gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO3, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm-2 at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO3 films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2011.01.087
  • Fatigue testing of thin films
    Burger, S. and Rupp, B. and Ludwig, Al. and Kraft, O. and Eberl, C.
    Key Engineering Materials 465 (2011)
    Thin film processing has been a driving technology in microelectronics and mechanics for years. The reliability of such devices is often limited by the failure of thin films. Therefore a deeper understanding of fatigue mechanisms of thin films through experiments is necessary to develop physical based lifetime models. Thus, this paper focuses on a novel setup for micro beam bending of thin metal films on Si cantilever substrate and first results will be presented. © (2011) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/KEM.465.552
  • High-throughput characterization of film thickness in thin film materials libraries by digital holographic microscopy
    Lai, Y.W. and Krause, M. and Savan, A. and Thienhaus, S. and Koukourakis, N. and Hofmann, M.R. and Ludwig, Al.
    Science and Technology of Advanced Materials 12 (2011)
    A high-throughput characterization technique based on digital holography for mapping film thickness in thin-film materials libraries was developed. Digital holographic microscopy is used for fully automatic measurements of the thickness of patterned films with nanometer resolution. The method has several significant advantages over conventional stylus profilometry: it is contactless and fast, substrate bending is compensated, and the experimental setup is simple. Patterned films prepared by different combinatorial thin-film approaches were characterized to investigate and demonstrate this method. The results show that this technique is valuable for the quick, reliable and high-throughput determination of the film thickness distribution in combinatorial materials research. Importantly, it can also be applied to thin films that have been structured by shadow masking. © 2011 National Institute for Materials Science.
    view abstract10.1088/1468-6996/12/5/054201
  • 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 (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 abstract10.1016/j.msea.2011.05.006
  • High-throughput characterization of Pt supported on thin film oxide material libraries applied in the oxygen reduction reaction
    Schäfer, D. and Mardare, C. and Savan, A. and Sanchez, M.D. and Mei, B. and Xia, W. and Muhler, M. and Ludwig, Al. and Schuhmann, W.
    Analytical Chemistry 83 (2011)
    Thin film metal oxide material libraries were prepared by sputter deposition of nanoscale Ti/Nb precursor multilayers followed by ex situ oxidation. The metal composition was varied from 6 at.% Nb to 27 at.% Nb. Additionally, thin wedge-type layers of Pt with a nominal thickness gradient from 0 to 5 nm were sputter-deposited on top of the oxides. The materials libraries were characterized with respect to metallic film composition, oxide thickness, phases, electrical conductivity, Pt thickness, and electrochemical activity for the oxygen reduction reaction (ORR). Electrochemical investigations were carried out by cyclic voltammetry using an automated scanning droplet cell. For a nominal Pt thickness >1 nm, no significant dependence of the ORR activity on the Pt thickness or the substrate composition was observed. However, below that critical thickness, a strong decrease of the surface-normalized activity in terms of reduction currents and potentials was observed. For such thin Pt layers, the conductivity of the substrate seems to have a substantial impact on the catalytic activity. Results from X-ray photoelectron spectroscopy (XPS) measurements suggest that the critical Pt thickness coincides with the transition from a continuous Pt film into isolated particles at decreasing nominal Pt thickness. In the case of isolated Pt particles, the activity of Pt decisively depends on its ability to exchange electrons with the oxide layer, and hence, a dependence on the substrate conductivity is rationalized. © 2011 American Chemical Society.
    view abstract10.1021/ac102303u
  • High-throughput characterization of stresses in thin film materials libraries using Si cantilever array wafers and digital holographic microscopy
    Lai, Y.W. and Hamann, S. and Ehmann, M. and Ludwig, Al.
    Review of Scientific Instruments 82 (2011)
    We report the development of an advanced high-throughput stress characterization method for thin film materials libraries sputter-deposited on micro-machined cantilever arrays consisting of around 1500 cantilevers on 4-inch silicon-on-insulator wafers. A low-cost custom-designed digital holographic microscope (DHM) is employed to simultaneously monitor the thin film thickness, the surface topography and the curvature of each of the cantilevers before and after deposition. The variation in stress state across the thin film materials library is then calculated by Stoneys equation based on the obtained radii of curvature of the cantilevers and film thicknesses. DHM with nanometer-scale out-of-plane resolution allows stress measurements in a wide range, at least from several MPa to several GPa. By using an automatic x-y translation stage, the local stresses within a 4-inch materials library are mapped with high accuracy within 10 min. The speed of measurement is greatly improved compared with the prior laser scanning approach that needs more than an hour of measuring time. A high-throughput stress measurement of an as-deposited Fe-Pd-W materials library was evaluated for demonstration. The fast characterization method is expected to accelerate the development of (functional) thin films, e.g., (magnetic) shape memory materials, whose functionality is greatly stress dependent. © 2011 American Institute of Physics.
    view abstract10.1063/1.3600594
  • High-throughput characterization of the seebeck coefficient of a-(Cr 1 - XSix)1 - yOy thin film materials libraries as verification of the extended thermopower formula
    Sonntag, J. and Ziolkowski, P. and Savan, A. and Kieschnick, M. and Ludwig, Al.
    Journal of Physics Condensed Matter 23 (2011)
    In a previous paper (Sonntag 2010 J. Phys.: Condens. Matter 22 235501) the classical thermopower formula has been argued to be incomplete, because it only takes into account the scattering properties of the carriers, but not the temperature dependence of the electrochemical potential μ caused by variation of the carrier density and/or band edge shift with temperature T. This argument is now checked experimentally by high-throughput measurements of the thermopower (Seebeck coefficient) S of a-(Cr1 - xSix) 1 - yOy thin film materials libraries. The concentration dependences of S differ depending on whether the measurements are done with the complete film (where x ranges continuously from x≈0.3 to 0.8; y≈0.1-0.2) or with the separated pieces (each piece with another average value of x). These differences are especially large if, in addition, an oxygen gradient is present. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/23/26/265501
  • Interdiffusion in Fe/Pt multilayers: In situ high temperature synchrotron radiation reflectivity study
    Zotov, N. and Feydt, J. and Savan, A. and Ludwig, Al. and Von Borany, J.
    Advanced Engineering Materials 13 (2011)
    Thermal annealing of Fe/Pt multilayers (ML) is reported to reduce significantly the formation temperature of FePt hard magnetic thin films. The transformation mechanisms of [Fe 1.38 nm/Pt 2.24 nm]50 ML, prepared by magnetron sputtering, is investigated in the present communication by high temperature X-ray reflectivity using synchrotron radiation. Complete degradation of the ML periodic structure is observed at about 610 K. The variation with annealing temperature of the intensity of the first Bragg peak, the correlated vertical roughness, and the lateral correlation length of the ML show that the ML transform in two stages with a cross-over temperature of about 515 ± 15 K. This behavior cannot be simply explained by the change in the measured interdiffusion coefficient below and above the cross-over temperature, suggesting the formation of FePt nanograins along the interfaces. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201000358
  • Modular high-throughput test stand for versatile screening of thin-film materials libraries
    Thienhaus, S. and Hamann, S. and Ludwig, Al.
    Science and Technology of Advanced Materials 12 (2011)
    Versatile high-throughput characterization tools are required for the development of new materials using combinatorial techniques. Here, we describe a modular, high-throughput test stand for the screening of thin-film materials libraries, which can carry out automated electrical, magnetic and magnetoresistance measurements in the temperature range of -40 to 300 degrees C. As a proof of concept, we measured the temperature-dependent resistance of Fe-Pd-Mn ferromagnetic shape-memory alloy materials libraries, revealing reversible martensitic transformations and the associated transformation temperatures. Magneto-optical screening measurements of a materials library identify ferromagnetic samples, whereas resistivity maps support the discovery of new phases. A distance sensor in the same setup allows stress measurements in materials libraries deposited on cantilever arrays. A combination of these methods offers a fast and reliable high-throughput characterization technology for searching for new materials. Using this approach, a composition region has been identified in the Fe-Pd-Mn system that combines ferromagnetism and martensitic transformation.
    view abstract10.1088/1468-6996/12/5/054206
  • Phase transformation, structural and functional fatigue properties of Ti-Ni-Hf shape memory thin films
    König, D. and Zarnetta, R. and Savan, A. and Brunken, H. and Ludwig, Al.
    Acta Materialia 59 (2011)
    The shape memory thin film system Ti-Ni-Hf was investigated with regard to its structural, phase transformation and functional fatigue properties by means of combinatorial and high-throughput methods. Temperature-dependent resistance measurements revealed a broad compositional region showing a reversible phase transformation. A ternary Laves phase was identified using X-ray diffraction as a precipitate phase within the transforming composition region. With increasing Ti content, the amount of the Laves phase increases, which results in an increase in the thermal hysteresis and a simultaneous decrease in the transformation temperatures. Shape memory properties were characterized by temperature-dependent stress change measurements using micromachined Si cantilever array wafers coated with Ti-Ni-Hf. The recovery stress was found to increase for small amounts of Laves phase precipitates. Strengthening of the matrix due to the Laves phase precipitates is concluded to be responsible for the observed increase in recovery stress and improved functional fatigue properties for (Ti,Hf)-rich alloy compositions (Ti 40.0Ni 47.5Hf 12.5). © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2011.01.066
  • Reversible fcc ↔ bcc transformation in freestanding epitaxially grown Fe-Pd ferromagnetic shape memory films
    Edler, T. and Hamann, S. and Ludwig, Al. and Mayr, S.G.
    Scripta Materialia 64 (2011)
    Using temperature-dependent X-ray diffraction and magnetization measurements, a reversible face-centered cubic (fcc) to body-centered cubic (bcc) structural transformation was confirmed in freestanding epitaxially grown Fe70Pd30 films after lift-off from their MgO (1 0 0) substrates - a transformation generally considered irreversible in bulk samples. The latter is accompanied by a distinct change of the sample magnetization. In contrast, substrate constraints were found to suppress the thermoelastic fcc to bcc transformation in substrate-attached films. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2010.09.013
  • Small-scale deposition of thin films and nanoparticles by microevaporation sources
    Meyer, R. and Hamann, S. and Ehmann, M. and König, D. and Thienhaus, S. and Savan, A. and Ludwig, Al.
    Journal of Microelectromechanical Systems 20 (2011)
    This paper reports on a novel miniaturized deposition technique based on micro-hotplates which are used as microevaporation sources (MES) for a localized deposition of thin films and nanoparticles. The feasibility of this small-scale deposition technique and its general properties are shown for depositions of Ag on unpatterned and microstructured substrates. The deposited films are rotationally symmetric and show a distinct lateral thickness change. We take advantage of this latter effect, as, e.g., all stages of film condensation can be observed within one experiment on one sample, in a size suitable for transmission electron microscopy investigations. For realizing the most laterally confined depositions, a micro-Knudsen cell was used. It is shown that the use of MES is also very suitable for the fabrication and deposition of nanoparticles. © 2011 IEEE.
    view abstract10.1109/JMEMS.2010.2090506
  • The effects of grain size on the phase transformation properties of annealed (Ti/Ni/W) shape memory alloy multilayers
    Buenconsejo, P.J.S. and Zarnetta, R. and Ludwig, Al.
    Scripta Materialia 64 (2011)
    (Ti/Ni/W) n multilayer films were annealed to form a two-phase (B2-TiNi and β-W) system. Grain sizes extracted from X-ray diffraction profiles of annealed films revealed that B2-TiNi decreases with increasing W, due to the immiscible W layers obstructing its grain growth. With decreasing B2-TiNi grain size the R s (B2-R) transformation temperature is not affected but the M s (R-B19′) transformation temperature decreases significantly. Thus the addition of W to Ti-Ni is effective to induce the B2-R single-step transformation due to grain size effects. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2011.02.021
  • Artificial single variant martensite in freestanding Fe70Pd 30 films obtained by coherent epitaxial growth
    Bechtold, C. and Buschbeck, J. and Lotnyk, A. and Erkartal, B. and Hamann, S. and Zamponi, C. and Schultz, L. and Ludwig, Al. and Kienle, L. and Fähler, S. and Quandt, E.
    Advanced Materials 22 (2010)
    (Figure Presented) The mechanically soft behavior of the magnetic shape-memory material Fe70Pd30 allows huge tetragonal distortions to be stabilized in sputtered thin films by coherent epitaxial growth on various metallic buffers. Furthermore, it is demonstrated that epitaxial films more than 1 μm thick can be grown, which makes possible freestanding films in an artificial single variant state suitable for microactuators and sensors. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/adma.201000599
  • Combinatorial investigation of Hf-Ta thin films and their anodic oxides
    Mardare, A.I. and Ludwig, Al. and Savan, A. and Wieck, A.D. and Hassel, A.W.
    Electrochimica Acta 55 (2010)
    A co-sputtering technique was used for the fabrication of a thin film combinatorial library (Hf-21 at.% Ta to 91 at.% Ta) based on alloying of Hf and Ta. The microstructure and crystallography of individual metallic alloy compositions were analyzed using SEM and XRD mapping, respectively. Three different zones of microstructure were identified within the range of alloys, going from hexagonal to tetragonal through an intermediate amorphous region. The local oxidation of Hf-Ta parent metal alloys at different compositions was investigated in steps of 1 at.% using an automated scanning droplet cell in the confined droplet mode. Potentiodynamic anodisation cycles combined with in situ impedance spectroscopy provide basic knowledge regarding the oxide formation and corresponding electrical properties. Dielectric constants were mapped for the entire composition range and XPS depth profiles allowed investigation of the oxide compositions. © 2010 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.electacta.2010.03.066
  • Correlation of phase transformations and magnetic properties in annealed epitaxial Fe-Pd magnetic shape memory alloy films
    Buschbeck, J. and Hamann, S. and Ludwig, Al. and Holzapfel, B. and Schultz, L. and Fähler, S.
    Journal of Applied Physics 107 (2010)
    Single-crystal-like films are promising candidates for magnetic shape memory (MSM) applications on the microscale. For defect reduction and stress relaxation, we apply a heat treatment to pulsed laser deposited, partial epitaxial Fe-Pd films with different compositions. By recrystallization starting from the epitaxial interface, single-crystal-like films are obtained. Deformation twins being present in the as-deposited state are completely eliminated. The epitaxial (100) orientation allows clear monitoring of the transformation from face centered cubic (fcc) austenite to face centered tetragonal (fct) martensite by x-ray diffraction experiments. Transformation from fcc austenite to fct martensite is hindered by constraints from the substrate. At temperatures down to 125 K residual fcc austenite is present. Magnetic measurements performed down to 50 K indicate that during further cooling the phase transformation to body centered tetragonal martensite occurs. The results show that annealing of laser deposited films is a promising route to obtain epitaxial Fe-Pd MSM films that are suitable for applications. © 2010 American Institute of Physics.
    view abstract10.1063/1.3383055
  • Development and characterization of Fe70Pd30 ferromagnetic shape memory splats
    Kock, I. and Hamann, S. and Brunken, H. and Edler, T. and Mayr, S.G. and Ludwig, Al.
    Intermetallics 18 (2010)
    Freestanding Fe70Pd30 foils with a thickness of about 60 μm were fabricated using the splat-quenching technique. A shift of the martensitic transformation temperatures as a function of different annealing treatments (600 °C, 700 °C, 800 °C, 900 °C, 1000 °C for 15 min) was observed by temperature-dependent X-ray diffraction (XRD), resistance and magnetization measurements. The sample annealed at 800 °C showed the highest degree of crystallinity for the (200) fcc austenite peak and no secondary phases. Samples annealed below 800 °C kept austenite remainders even at -25 °C. The transformation temperatures, determined by all three-measurement methods, showed an increase with increasing annealing temperature. © 2009 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.intermet.2009.12.019
  • Effects of annealing time on the structural and magnetic properties of L10 FePt thin films
    Zotov, N. and Hiergeist, R. and Savan, A. and Ludwig, Al.
    Thin Solid Films 518 (2010)
    Thermal annealing of [Fe 1.65 nm/Pt 1.84 nm]50 multilayers at 673 K for various annealing times between 60 and 12000 s leads to the direct formation of the fully ordered L10 FePt phase with (111) texture. The average grain sizes, determined from X-ray diffraction size-strain analysis, are smaller than the critical size for multi-domain FePt particles, suggesting the presence of single-domain (SD) grains. The coercivity increases with annealing time and increasing grain size and reaches values of about 955 kA/m. The remanence values are typical for randomly oriented weakly-interacting particles. A decrease of the remanence with annealing time suggests a decrease of the intergrain exchange interactions with annealing time. Analysis of minor loops and the initial magnetization curves shows the presence of a broad distribution of critical fields, which the individual SD particles have to overcome for the magnetization reversal. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2010.03.076
  • Engineered Tungsten Oxy-Nitride Thin Film Materials for Photocatalytical Water Splitting Fabricated by MOCVD
    Cwik, S. and Milanov, A. P. and Gwildies, V. and Thiede, T. B. and Vidyarthi, V. S. and Savan, A. and Meyer, R. and Becker, H. W. and Rogalla, D. and Ludwig, Al. and Fischer, R. A. and Devi, A.
    Advanced Organic and Inorganic Materials for Electrochemical Power Sources 28 (2010)
    The growth of tungsten oxynitride thin films by metalorganic chemical vapor deposition (MOCVD) was achieved using an all nitrogen coordinated tungsten imido-amidinato precursor in the presence of oxygen as the reactive gas. In particular the influence of CVD process parameters on the structure, morphology and composition of the films was studied by scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA) and X-ray diffraction XRD. It was possible to tune the content of the nitrogen of the films which could be an advantage in terms of functional properties of the WO3 based materials.
    view abstract10.1149/1.3490695
  • Epitaxially stabilized TiN/(Ti,Fe,Co)N multilayer thin films in (pseudo-)fcc crystal structure by sequential magnetron sputter deposition
    Klever, C. and Seemann, K. and Stüber, M. and Ulrich, S. and Brunken, H. and Ludwig, Al. and Leiste, H.
    Journal of Physics D: Applied Physics 43 (2010)
    Multilayer thin films were grown by non-reactive sequential magnetron sputter deposition from ceramic TiN and metallic FeCo targets addressing a combination of wear resistance and sensoric functionality. Coatings with bilayer period values ranging from 449 nm down to 2.6 nm were grown with the total amount of either material maintained constant. The multilayer thin films were post-annealed ex situ at 600 °C for 60 min in vacuum. X-ray diffraction results imply the multilayer thin films undergo significant changes in their crystalline structure when the bilayer period is decreased. Using high-resolution transmission electron microscopy as well as selected-area electron diffraction it is shown that in the case of multilayer thin films with bilayer periods of several tens of nanometres and higher, FeCo layers and TiN layers in their respective common CsCl-and NaCl-type crystal structures alternate. In contrast, in the multilayer thin films with bilayer periods of only a few nanometres, grain growth across the interfaces between the individual layers takes place and a strongly textured microstructure is formed which features columns in (pseudo-)fcc crystal structure grown in heteroepitaxial growth mode. It is suggested that the experimental findings imply the latter multilayer thin films to be alternately composed of TiN layers and (Ti,Fe,Co)N solid solution layers which have been formed by a solid-state reaction during the deposition process. As a consequence, heteroepitaxially stabilized columnar grains in strongly textured (pseudo-)fcc crystal structure are formed. This crystal structure is preserved after the annealing procedure which qualifies these coatings for use in applications where temperatures of up to 600 °C are reached. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/43/39/395406
  • 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 abstract10.1088/0964-1726/19/6/065032
  • 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 Eggeler, G. and Ludwig, Al.
    Advanced Functional Materials 20 (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 abstract10.1002/adfm.200902336
  • Integrity of micro-hotplates during high-temperature operation monitored by digital holographic microscopy
    Lai, Y.W. and Koukourakis, N. and Gerhardt, N.C. and Hofmann, M.R. and Meyer, R. and Hamann, S. and Ehmann, M. and Hackl, K. and Darakis, E. and Ludwig, Al.
    Journal of Microelectromechanical Systems 19 (2010)
    An investigation on the integrity of micro-hotplates using in situ digital holographic microscopy is reported. The surface topography and surface evolution of the devices during high-temperature operation (heating/cooling cycles) is measured with nanometer-scale resolution. A localized permanent out-of-plane surface deformation of 40% of the membrane thickness caused by the top measurement electrodes occurring after the first cycle is observed. The integrity-related issues caused by such a permanent deformation are discussed. © 2006 IEEE.
    view abstract10.1109/JMEMS.2010.2067442
  • Micro- to nanostructured devices for the characterization of scaling effects in shape-memory thin films
    König, D. and Ehmann, M. and Thienhaus, S. and Ludwig, Al.
    Journal of Microelectromechanical Systems 19 (2010)
    Two microfabricated devices designed as test platforms for the investigation of scaling effects in micro- to nanosized substrate-attached shape-memory alloy (SMA) thin films as well as freestanding (suspended) thin-film microbridges are presented. These micromachined test platforms allow for simultaneous nanomechanical, electrical, and thermal tests on thin-film microbridges and can be seen as a basis for nanoscale SMA thin-film applications. The functionality of these devices is demonstrated for Ti 52 Ni32 Cu16 thin films as active material. The martensitic phase-transition temperatures for the thin films as substrate-attached or suspended microstructures as well as the dependence on the lateral dimensions were examined. It was found that decreasing the bridge width from 4 to 1 μm leads to a substantial and asymmetrical decrease of the phase-transition temperatures: 20% [austenite finish temperature (Af) and martensite start temperature (Ms)] and 80% [austenite start temperature (As
    view abstract10.1109/JMEMS.2010.2067441
  • Microstructure and magnetic properties of FeCo/Ti thin film multilayers annealed in nitrogen
    Brunken, H. and Somsen, C. and Savan, A. and Ludwig, Al.
    Thin Solid Films 519 (2010)
    Multifunctional nanocomposites consisting of at least one ferromagnetic phase (e.g. FeCo) and one protective, wear resistant phase (e.g. TiN) are of interest for applications as sensors or actuators in harsh environments. This paper reports on the fabrication and characterization of nanocomposite thin films, prepared from FeCo/Ti metallic precursor multilayer composition spreads using a combinatorial sputter-deposition system. After deposition, the composition spread was annealed in nitrogen (5 × 10 5 Pa pressure) at 850 °C for 1.5 h, leading to preferential nitriding of Ti to TiN, thus forming the protective phase. Automated energy dispersive X-ray analysis, Auger electron spectroscopy, X-ray diffraction measurements, transmission electron microscopy (TEM) and vibrating sample magnetometry were used for the characterization of the as deposited and nitrided composition spreads. As an unexpected result, the appearance of a Heusler phase (Co 2FeSi) in the nanocomposite was observed by TEM. After N 2 annealing, the nanocomposites show reduced saturation magnetization values μ 0M S between 0.5 and 0.95 T and improved coercive field values μ 0H c between 4 and 13.8 mT, dependent on the TiN content. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2010.09.008
  • 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 (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 abstract10.1016/j.actamat.2010.07.011
  • alloys

  • array

  • energy conversion

  • high-throughput methods

  • martensite

  • materials libraries

  • microstructure

  • nanostructures

  • shape-memory alloys

« back