Scientific Output

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

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

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  • 2022 • 207 A review of the metastable omega phase in beta titanium alloys: the phase transformation mechanisms and its effect on mechanical properties
    Ballor, J. and Li, T. and Prima, F. and Boehlert, C.J. and Devaraj, A.
    International Materials Reviews (2022)
    Since its discovery in 1954, the omega (ω) phase in titanium and its alloys has attracted substantial attention from researchers. The β-to-ω and ω-to-α phase transformations are central to β-titanium alloy design, but the transformation mechanisms have been a subject of debate. With new generations of aberration-corrected transmission electron microscopy and atom probe tomography, both the spatial resolution and compositional sensitivity of phase transformation analysis have been rapidly improving. This review provides a detailed assessment of the new understanding gained and related debates in this field enabled by advanced characterization methods. Specifically, new insights into the possibility of a coupled diffusional-displacive component in the β-to-ω transformation and key nucleation driving forces for the ω-assisted α phase formation are discussed. Additionally, the influence of ω phase on the mechanical properties of β-titanium alloys is also reviewed. Finally, a perspective on open questions and future direction for research is discussed. © This material is authored by Battelle Memorial Institute with the US Department of Energy under Contract No. DE-AC05-76RL01830. The US Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, and irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
    view abstractdoi: 10.1080/09506608.2022.2036401
  • 2022 • 206 Origins of the hydrogen signal in atom probe tomography: Case studies of alkali and noble metals
    Yoo, S.-H. and Kim, S.-H. and Woods, E. and Gault, B. and Todorova, M. and Neugebauer, J.
    New Journal of Physics 24 (2022)
    Atom probe tomography (APT) analysis is being actively used to provide near-atomic-scale information on the composition of complex materials in three-dimensions. In recent years, there has been a surge of interest in the technique to investigate the distribution of hydrogen in metals. However, the presence of hydrogen in the analysis of almost all specimens from nearly all material systems has caused numerous debates as to its origins and impact on the quantitativeness of the measurement. It is often perceived that most H arises from residual gas ionization, therefore affecting primarily materials with a relatively low evaporation field. In this work, we perform systematic investigations to identify the origin of H residuals in APT experiments by combining density-functional theory (DFT) calculations and APT measurements on an alkali and a noble metal, namely Na and Pt, respectively. We report that no H residual is found in Na metal samples, but in Pt, which has a higher evaporation field, a relatively high signal of H is detected. These results contradict the hypothesis of the H signal being due to direct ionization of residual H2 without much interaction with the specimen's surface. Based on DFT, we demonstrate that alkali metals are thermodynamically less likely to be subject to H contamination under APT-operating conditions compared to transition or noble metals. These insights indicate that the detected H-signal is not only from ionization of residual gaseous H2 alone, but is strongly influenced by material-specific physical properties. The origin of H residuals is elucidated by considering different conditions encountered during APT experiments, specifically, specimen-preparation, transportation, and APT-operating conditions by taking thermodynamic and kinetic aspects into account. © 2022 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ac40cd
  • 2022 • 205 Out-of-plane longitudinal sound velocity in SnS2determined via broadband time-domain Brillouin scattering
    Cheng, M. and Pichugin, K. and Maas, A. and Schleberger, M. and Sciaini, G.
    Journal of Applied Physics 132 (2022)
    Here, we report time-resolved broadband transient reflectivity measurements performed in a single crystal of SnS2. We made use of time-domain Brillouin scattering and a broadband probe to measure the out-of-plane longitudinal sound velocity, υ L = (2950 ± 100) m s - 1, in this semiconducting two-dimensional metal dichalcogenide. Our study illustrates the potential of this non-invasive all-optical pump-probe technique for the study of the elastic properties of transparent brittle materials and provides the value of the elastic constant c 33 = (39 ± 3) GPa. © 2022 Author(s).
    view abstractdoi: 10.1063/5.0103224
  • 2022 • 204 Quantitative analysis of grain boundary diffusion, segregation and precipitation at a sub-nanometer scale
    Peng, Z. and Meiners, T. and Lu, Y. and Liebscher, C.H. and Kostka, A. and Raabe, D. and Gault, B.
    Acta Materialia 225 (2022)
    Grain boundaries are intrinsic and omnipresent microstructural imperfections in polycrystalline and nanocrystalline materials. They are short-circuit diffusion paths and preferential locations for alloying elements, dopants, and impurities segregation. They also facilitate heterogeneous nucleation and the growth of secondary phases. Therefore, grain boundaries strongly influence many materials' properties and their stabilities during application. Here, we propose an approach to measure diffusion, segregation, and segregation-induced precipitation at grain boundaries at a sub-nanometer scale by combining atom probe tomography and scanning transmission electron microscopy. Nanocrystalline multilayer thin films with columnar grain structure were used as a model system as they offer a large area of random high-angle grain boundaries and inherent short diffusion distance. Our results show that the fast diffusion flux proceeds primarily through the core region of the grain boundary, which is around 1 nm. While the spatial range that the segregated solute atoms occupied is larger: below the saturation level, it is 1,2 nm; as the segregation saturates, it is 2–3.4 nm in most grain boundary areas. Above 3.4 nm, secondary phase nuclei seem to form. The observed distributions of the solutes at the matrix grain boundaries evidence that even at a single grain boundary, different regions accommodate different amounts of solute atoms and promote secondary phase nuclei with different compositions, which is caused by its complex three-dimensional topology. © 2021 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2021.117522
  • 2022 • 203 Structural and electron spin state changes in an x-ray heated iron carbonate system at the Earth's lower mantle pressures
    Kaa, J.M. and Sternemann, C. and Appel, K. and Cerantola, V. and Preston, T.R. and Albers, C. and Elbers, M. and Libon, L. and Makita, M. and Pelka, A. and Petitgirard, S. and Plückthun, C. and Roddatis, V. and Sahle, C.J. and Sp...
    Physical Review Research 4 (2022)
    The determination of the spin state of iron-bearing compounds at high pressure and temperature is crucial for our understanding of chemical and physical properties of the deep Earth. Studies on the relationship between the coordination of iron and its electronic spin structure in iron-bearing oxides, silicates, carbonates, iron alloys, and other minerals found in the Earth's mantle and core are scarce because of the technical challenges to simultaneously probe the sample at high pressures and temperatures. We used the unique properties of a pulsed and highly brilliant x-ray free electron laser (XFEL) beam at the High Energy Density (HED) instrument of the European XFEL to x-ray heat and probe samples contained in a diamond anvil cell. We heated and probed with the same x-ray pulse train and simultaneously measured x-ray emission and x-ray diffraction of an FeCO3 sample at a pressure of 51 GPa with up to melting temperatures. We collected spin state sensitive Fe Kβ1,3 fluorescence spectra and detected the sample's structural changes via diffraction, observing the inverse volume collapse across the spin transition. During x-ray heating, the carbonate transforms into orthorhombic Fe4C3O12 and iron oxides. Incipient melting was also observed. This approach to collect information about the electronic state and structural changes from samples contained in a diamond anvil cell at melting temperatures and above will considerably improve our understanding of the structure and dynamics of planetary and exoplanetary interiors. © 2022 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevResearch.4.033042
  • 2021 • 202 Coherent Spin Dynamics of Electrons and Holes in CsPbBr3Colloidal Nanocrystals
    Grigoryev, P.S. and Belykh, V.V. and Yakovlev, D.R. and Lhuillier, E. and Bayer, M.
    Nano Letters 21 8481-8487 (2021)
    The spin dynamics in CsPbBr3 lead halide perovskite nanocrystals are studied by picosecond pump-probe Faraday rotation in an external magnetic field. Coherent Larmor precession of electrons and holes with spin dephasing times of ∼600 ps is detected in a transversal magnetic field. The longitudinal spin relaxation time in weak magnetic fields reaches 80 ns at a temperature of 5 K. In this regime, the carrier spin dynamics is governed by nuclear spin fluctuations characterized by an effective hyperfine field strength of 25 mT. The Landé factors determining the carrier Zeeman splittings are ge = +1.73 for electrons and gh = +0.83 for holes. A comparison with a CsPbBr3 polycrystalline film and bulk single crystals evidences that the spatial confinement of electrons and holes in the nanocrystals only slightly affects their g factors and spin dynamics. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.1c03292
  • 2021 • 201 Control of residual oxygen of the process atmosphere during laser-powder bed fusion processing of Ti-6Al-4V
    Pauzon, C. and Dietrich, K. and Forêt, P. and Dubiez-Le Goff, S. and Hryha, E. and Witt, G.
    Additive Manufacturing 38 (2021)
    The effect of the residual oxygen concentration in the process atmosphere during laser-powder bed fusion (L-PBF) of Ti-6Al-4V was investigated, using an external oxygen monitoring system equipped with two types of oxygen sensors typically used in L-PBF hardware: a lambda probe and an electrochemical oxygen sensor. The recordings of the oxygen variations during L-PBF highlighted that the electrochemical sensor is more reliable than the lambda probe, whose signal showed a maximum deviation of about 700 ppm O2 after 7 h, attributed to its sensitivity to hydrogen present in the system. The study revealed that proper monitoring of the oxygen in the laboratory scale L-PBF system used is necessary to limit oxygen and nitrogen pick-ups by the built material. Concentrations as high as 2200 ppm O2 and 500 ppm N2 in the Ti-6Al-4V part built under standard conditions were measured, compared to maximum levels of 1800 ppm O2 and 250 ppm N2 with the external oxygen control. In addition, the findings underline the critical effect of the component design, such as the high aspect ratio columns or the lattice structures, on the heat accumulation in case of Ti-6Al-4V, leading to enhanced oxygen and nitrogen pick-up, as high as 600 ppm O2 and 150 ppm N2 difference between the bottom and top of the cylindrical samples of 70 mm height used in this study. The determination of tensile properties of samples built at different heights put in evidence the detrimental effect of the oxygen increase with build height on the ductility, which decreased from 12% to below 6% between the bottom and top positions. This work highlights that the possible presence of impurities in the L-PBF atmosphere can have harmful impact on the properties of Ti-6Al-4V components, which can be mitigated adjusting the oxygen control system. © 2020 The Authors
    view abstractdoi: 10.1016/j.addma.2020.101765
  • 2021 • 200 Dopant-segregation to grain boundaries controls electrical conductivity of n-type NbCo(Pt)Sn half-Heusler alloy mediating thermoelectric performance
    Luo, T. and Serrano-Sánchez, F. and Bishara, H. and Zhang, S. and Villoro, B. and Kuo, J.J. and Felser, C. and Scheu, C. and Snyder, G.J. and Best, J.P. and Dehm, G. and Yu, Y. and Raabe, D. and Fu, C. and Gault, B.
    Acta Materialia 217 (2021)
    Science-driven design of future thermoelectric materials requires a deep understanding of the fundamental relationships between microstructure and transport properties. Grain boundaries in polycrystalline materials influence the thermoelectric performance through the scattering of phonons or the trapping of electrons due to space-charge effects. Yet, the current lack of careful investigations on grain boundary-associated features hinders further optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler alloys, which were synthesized by ball milling and spark plasma sintering (SPS). Post-SPS annealing was performed on one sample, leading to improved low-temperature electrical conductivity. The microstructure of both samples was examined by electron microscopy and atom probe tomography. The grain size increases from ~230 nm to ~2.38 μm upon annealing. Pt is found within grains and at grain boundaries, where it locally reduces the resistivity, as assessed by in situ four-point-probe electrical conductivity measurement. Our work showcases the correlation between microstructure and electrical conductivity, providing opportunities for future microstructural optimization by tuning the chemical composition at grain boundaries. © 2021 The Authors
    view abstractdoi: 10.1016/j.actamat.2021.117147
  • 2021 • 199 Influence of the sampling probe on flame temperature, species, residence times and on the interpretation of ion signals of methane/oxygen flames in molecular beam mass spectrometry measurements
    Karakaya, Y. and Sellmann, J. and Wlokas, I. and Kasper, T.
    Combustion and Flame 229 (2021)
    Laminar flames are widely used to analyze the fundamentals of combustion processes using molecular beam mass spectrometry. The extraction of a representative sample from a flame by an intrusive sampling technique is challenging because of two main issues. First, the sampling probe itself perturbs the flow and temperature field, affecting the species profiles. These effects need to be characterized by 2-D fluid dynamic simulations to reveal sources of perturbations that are in particular suction and flame cooling. Second, some intermediate species interact with the sampling probe and are removed from the gas sample before analysis. The concentrations of these intermediates in the flames are often low and close to the detection limit. Naturally occurring ions can also be extracted from the flame by molecular beam sampling. Coupled with modern ion optical devices for ion transfer to the mass analyzer very high sensitivity can be reached in the detection of ionic species in flames. Similarities in the shape of measured relative concentration profiles indicate a connection between neutrals and the corresponding protonated molecules by proton transfer reactions. A quantification method of neutral flame species based on signals of the flame-sampled ions is presented and evaluated for the intermediate methanol in methane/oxygen/argon flames. The proposed method is based on equilibrium calculations that depend on temperature. To characterize the sampling process and demonstrate the validity of the quantification approach for ion measurements, the influence of the sampling probe on flame temperature and mole fraction profiles of the main species and the intermediate methanol are investigated by a combined experimental and simulation study. A comparison of the methanol profiles measured by conventional molecular beam sampling and the novel ion sampling technique reveal acceptable agreement. This work shows that if all aspects of sampling are considered as well as possible, the ion sampling technique allows access to kinetic data of neutral intermediates. © 2021
    view abstractdoi: 10.1016/j.combustflame.2021.02.034
  • 2021 • 198 Investigation of an atomic-layer-deposited Al2O3 diffusion barrier between Pt and Si for the use in atomic scale atom probe tomography studies on a combinatorial processing platform
    Li, Y. and Zanders, D. and Meischein, M. and Devi, A. and Ludwig, A.
    Surface and Interface Analysis 53 727-733 (2021)
    In order to enable the application of atomic probe tomography combinatorial processing platforms for atomic-scale investigations of phase evolution at elevated temperatures, the pre-sharpened Si tip of 10–20 nm in diameter must be protected against interdiffusion and reaction of the reactive Si with a film of interest by a conformal coating on the Si tip. It is shown that unwanted reactions can be suppressed by introducing a 20-nm-thick intermediate Al2O3 layer grown by atomic layer deposition (ALD). As a representative case, Pt is chosen as a film of interest, as it easily forms silicides. Whereas without the ALD coating diffusion/reactions occur, with the protective film, this is prevented for temperatures up to at least 600°C. The effectiveness of the Al2O3 layer serving as a diffusion barrier is not limited to a sharpened Si tip but works generally for all cases where a Si substrate is used. © 2021 The Authors. Surface and Interface Analysis published by John Wiley & Sons Ltd.
    view abstractdoi: 10.1002/sia.6955
  • 2021 • 197 On the Multipole Resonance Probe: Current Status of Research and Development
    Oberrath, J. and Friedrichs, M. and Gong, J. and Oberberg, M. and Pohle, D. and Schulz, C. and Wang, C. and Awakowicz, P. and Brinkmann, R.P. and Lapke, M. and Mussenbrock, T. and Musch, T. and Rolfes, I.
    IEEE Transactions on Plasma Science (2021)
    During the last decade a new probe design for active plasma resonance spectroscopy, the multipole resonance probe (MRP), was proposed, analyzed, developed, and characterized in two different designs: the spherical MRP (sMRP) and the planar MRP (pMRP). The advantage of the latter is that it can be integrated into the chamber wall and can minimize the perturbation of the plasma. Both designs can be applied for monitoring and control purposes of plasma processes for industrial applications. As usual for this measurement technique, a mathematical model is required to determine plasma parameter (electron density, electron temperature, and collision frequency of electrons with neutral atoms) from the measured resonances. Based on the cold plasma model a simple relationship between the resonance frequency and the electron density can be derived and leads to excellent measurement results. However, a simultaneous measurement of the electron temperature in low-pressure plasmas requires a kinetic model, because the half-width of the resonance peak is broadened by kinetic effects. Such a model has been derived and first results show the broadening of the spectra as expected. Deriving a relation between the half-width and the electron temperature will allow the simultaneous measurement and an improvement of monitoring and control concepts. IEEE
    view abstractdoi: 10.1109/TPS.2021.3113832
  • 2020 • 196 Characterization of structural and chemical modifications during the steam activation of activated carbons
    Muthmann, J. and Bläker, C. and Pasel, C. and Luckas, M. and Schledorn, C. and Bathen, D.
    Microporous and Mesoporous Materials 309 (2020)
    An anthracite-based material of low porosity was activated in a nitrogen stream enriched with water vapor to produce four activated carbons, which differ systematically in their structural and chemical properties. Characterization of the structural properties of the base and the activated materials was carried out by volumetric measurements and mercury porosimetry. A general classification of chemical composition was done by ultimate analysis. The surface chemistry was analyzed by Boehm titration. The adsorption performance was investigated by measuring excess isotherms and load-dependent adsorption enthalpies of selected probe molecules: acetone (polar), n-heptane (non-polar) and toluene (aromatic). Assuming distribution rules for selective adsorption, the molar ratios of the surface groups were estimated from the excess isotherms. The combination of these methods enables the observation and differentiation of structural and surface chemical changes of the carbons during activation. For example, it could be shown that the broader pore width distribution with an increasing proportion of mesopores occurring in the course of activation leads to a more pronounced energetic heterogeneity. It was also found that an increase in surface area with increasing activation time is accompanied by an increase in aromatic surface groups and a reduction of polar and non-polar surface groups. Especially the adsorption of the polar probe molecule acetone revealed the heterogeneity of the surface chemistry, which together with the heterogeneous structural composition resulted in a distinct decrease of the adsorption enthalpy with increasing loading. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2020.110549
  • 2020 • 195 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 183 122-126 (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 abstractdoi: 10.1016/j.scriptamat.2020.03.016
  • 2020 • 194 Could face-centered cubic titanium in cold-rolled commercially-pure titanium only be a Ti-hydride?
    Chang, Y. and Zhang, S. and Liebscher, C.H. and Dye, D. and Ponge, D. and Scheu, C. and Dehm, G. and Raabe, D. and Gault, B. and Lu, W.
    Scripta Materialia 178 39-43 (2020)
    A face-centered cubic (FCC) phase in electro-polished specimens for transmission electron microscopy of commercially pure titanium has sometimes been reported. Here, a combination of atom-probe tomography, scanning transmission electron microscopy and low-loss electron energy loss spectroscopy is employed to study both the crystal structural and chemical composition of this FCC phase. Our results prove that the FCC phase is actually a TiHx (x ≥ 1) hydride, and not a new allotrope of Ti, in agreement with previous reports. The formation of the hydride is discussed. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2019.11.010
  • 2020 • 193 Determination of atomic oxygen state densities in a double inductively coupled plasma using optical emission and absorption spectroscopy and probe measurements
    Fiebrandt, M. and Bibinov, N. and Awakowicz, P.
    Plasma Sources Science and Technology 29 (2020)
    A collisional radiative model for fast estimation and monitoring of atomic oxygen ground and excited state densities and fluxes in varying Ar:O2 mixtures is developed and applied in a double inductively coupled plasma source at a pressure of 5 Pa and incident power of 500 W. The model takes into account measured line intensities of 130.4 nm, 135.6 nm, 557.7 nm, and 777.5 nm, the electron densities and electron energy distribution functions determined using a Langmuir probe and multipole resonance probe as well as the state densities of the first four excited states of argon measured with the branching fraction method and compared to tunable diode laser absorption spectroscopy. The influence of cascading and self absorption is included and the validity of the used cross sections and reaction rates is discussed in detail. The determined atomic oxygen state densities are discussed for their plausibility, sources of error, and compared to other measurements. Furthermore, the results of the model are analyzed to identify the application regimes of much simpler models, which could be used more easily for process control, e.g. actinometry. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab7cbe
  • 2020 • 192 Multiferroic bismuth ferrite: Perturbed angular correlation studies on its ferroic α-β Phase transition
    Marschick, G. and Schell, J. and Stöger, B. and Gonçalves, J.N. and Karabasov, M.O. and Zyabkin, D. and Welker, A. and Escobar C., M. and Gärtner, D. and Efe, I. and Santos, R.A. and Laulainen, J.E.M. and Lupascu, D.C.
    Physical Review B 102 (2020)
    Work of numerous research groups has shown different outcomes of studies of the transition from the ferroelectric α-phase to the high temperature β-phase of the multiferroic, magnetoelectric perovskite Bismuth Ferrite (BiFeO3 or BFO). Using the perturbed angular correlation (PAC) method with Cd111m as the probe nucleus, the α to β phase transition was characterized. The phase transition temperature, the change of the crystal structure, and its parameters were supervised with measurements at different temperatures using a six detector PAC setup to observe the γ-γ decay of the Cd111m probe nucleus. The temperature dependence of the hyperfine parameters shows a change in coordination of the probe ion, which substitutes for the bismuth site, forecasting the phase transition to β-BFO by either increasing disorder or formation of a polytype transition structure. A visible drop of the quadrupole frequency ω0 at a temperature of about Tc≈820∘C indicates the α-β phase transition. For a given crystal symmetry, the DFT-calculations yield a specific local symmetry and electric field gradient value of the probe ion. The Pbnm (β-BFO) crystal symmetry yields calculated local electric field gradients, which very well match our experimental results. The assumption of other crystal symmetries results in significantly different computed local environments not corresponding to the experiment. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.224110
  • 2020 • 191 On the atomic solute diffusional mechanisms during compressive creep deformation of a Co-Al-W-Ta single crystal superalloy
    He, J. and Zenk, C.H. and Zhou, X. and Neumeier, S. and Raabe, D. and Gault, B. and Makineni, S.K.
    Acta Materialia 184 86-99 (2020)
    We investigated the solute diffusional behavior active during compressive creep deformation at 150 MPa / 975 °C of a Co-Al-W-Ta single crystal superalloy in the [001] orientation. We report the formation of shear-bands that involves re-orientation of γ/γʹ rafts to {111} from {001} planes, referring to as γ/γ′ raft-rotation. In the shear-band regions, we observed abundant micro-twins, stacking faults (SFs), disordered zones within the γʹ termed as ‘γ pockets’ and also few geometrically-close-packed (GCP) phases. We used a correlative approach blending electron microscopy and atom probe tomography to characterize the structure and composition of these features. The SFs were identified as intrinsic and exhibit a W enrichment up to 14.5 at.% and an Al deficiency down to 5.1 at.%, with respect to the surrounding γʹ phase. The micro-twin boundaries show a solute enrichment similar to the SFs with a distinct W compositional profile gradients perpendicular from the boundaries into the twin interior, indicating solute diffusion within the micro-twins. The γ-pockets have a composition close to that of γ but richer in W/Ta. Based on these observations, we propose (i) a solute diffusion mechanism taking place during micro-twinning, (ii) a mechanism for the γ/γʹ raft-rotation process and evaluate their influence on the overall creep deformation of the present Co-based superalloy. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.035
  • 2020 • 190 Probing catalytic surfaces by correlative scanning photoemission electron microscopy and atom probe tomography
    Schweinar, K. and Nicholls, R.L. and Rajamathi, C.R. and Zeller, P. and Amati, M. and Gregoratti, L. and Raabe, D. and Greiner, M. and Gault, B. and Kasian, O.
    Journal of Materials Chemistry A 8 388-400 (2020)
    The chemical composition and the electronic state of the surface of alloys or mixed oxides with enhanced electrocatalytic properties are usually heterogeneous at the nanoscale. The non-uniform distribution of the potential across their surface affects both activity and stability. Studying such heterogeneities at the relevant length scale is crucial for understanding the relationships between structure and catalytic behaviour. Here, we demonstrate an experimental approach combining scanning photoemission electron microscopy and atom probe tomography performed at identical locations to characterise the surface's structure and oxidation states, and the chemical composition of the surface and sub-surface regions. Showcased on an Ir-Ru thermally grown oxide, an efficient catalyst for the anodic oxygen evolution reaction, the complementary techniques yield consistent results in terms of the determined surface oxidation states and local oxide stoichiometry. Significant chemical heterogeneities in the sputter-deposited Ir-Ru alloy thin films govern the oxide's chemistry, observed after thermal oxidation both laterally and vertically. While the oxide grains have a composition of Ir0.94Ru0.06O2, the composition in the grain boundary region varies from Ir0.70Ru0.30O2 to Ir0.40Ru0.60O2 and eventually to Ir0.75Ru0.25O2 from the top surface into the depth. The influence of such compositional non-uniformities on the catalytic performance of the material is discussed, along with possible engineering levers for the synthesis of more stable and reactive mixed oxides. The proposed method provides a framework for investigating materials of interest in the field of electrocatalysis and beyond. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta10818a
  • 2020 • 189 Revealing nano-chemistry at lattice defects in thermoelectric materials using atom probe tomography
    Yu, Y. and Zhou, C. and Zhang, S. and Zhu, M. and Wuttig, M. and Scheu, C. and Raabe, D. and Snyder, G.J. and Gault, B. and Cojocaru-Mirédin, O.
    Materials Today 32 260-274 (2020)
    The population of all non-equilibrium lattice defects in materials is referred to as microstructure. Examples are point defects such as substitutional and interstitial atoms, and vacancies; line defects such as dislocations; planar defects such as interfaces and stacking faults; or mesoscopic defects such as second-phase precipitates. These types of lattice imperfections are usually described in terms of their structural features, breaking the periodicity of the otherwise regular crystalline structure. Recent analytical probing at the nanoscale has revealed that their chemical features are likewise important and characteristic. The structure of the defects as well as their individual chemical composition, that is their chemical decoration state, which results from elemental partitioning with the adjacent matrix, can significantly influence the electrical and thermal transport properties of thermoelectric materials. The emergence of atom probe tomography (APT) has now made routinely accessible the mapping of three-dimensional chemical composition with sub-nanometer spatial accuracy and elemental sensitivity in the range of tens of ppm. Here, we review APT-based investigations and results related to the local chemical decoration states of various types of lattice defects in thermoelectric materials. APT allows to better understand the interplay between thermoelectric properties and microstructural features, extending the concept of defect engineering to the field of segregation engineering so as to guide the rational design of high-performance thermoelectric materials. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.mattod.2019.11.010
  • 2020 • 188 Spinodal decomposition versus classical γ′ nucleation in a nickel-base superalloy powder: An in-situ neutron diffraction and atomic-scale analysis
    Collins, D.M. and D'Souza, N. and Panwisawas, C. and Papadaki, C. and West, G.D. and Kostka, A. and Kontis, P.
    Acta Materialia 200 959-970 (2020)
    Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by their thermo-mechanical treatments during processing. Here, the influence of a thermal exposure to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ′ volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ′ phase transformations. Subsolvus heat treatments yielded a unimodal γ′ distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ′ population during heating, but dissolves as the temperature increases further. The γ′ then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ′ formed during heating or cooling. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.09.055
  • 2020 • 187 The Planar Multipole Resonance Probe: A Minimally Invasive Monitoring Concept for Plasma-Assisted Dielectric Deposition Processes
    Pohle, D. and Schulz, C. and Oberberg, M. and Awakowicz, P. and Rolfes, I.
    IEEE Transactions on Microwave Theory and Techniques 68 2067-2079 (2020)
    In this article, a novel minimally invasive approach to plasma monitoring in the challenging environment of dielectric deposition processes based on the planar multipole resonance probe (pMRP) is presented. By placing the sensor on the plasma-remote side of a dielectric substrate to be coated, perturbations of the process due to its presence can be significantly reduced. Since the electric field of the sensor is able to penetrate dielectric layers, a plasma supervision through the substrate is enabled. To investigate the effect of increasing coating thicknesses on the measurement performance for a broad spectrum of materials and plasma conditions, the results of extensive 3-D full-wave simulations performed with CST Microwave Studio are evaluated. Finally, real-time monitoring results of an argon-oxygen plasma during a sputter deposition with aluminum oxide on a polyethylene terephthalate (PET) film substrate together with a comparison to external process parameters are presented. The results demonstrate both the applicability of the proposed concept and its insensitivity to additional dielectric coatings. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/TMTT.2020.2974835
  • 2019 • 186 A concept to generate ultrashort ion pulses for pump-probe experiments in the keV energy range
    Breuers, A. and Herder, M. and Kucharczyk, P. and Schleberger, M. and Sokolowski-Tinten, K. and Wucher, A.
    New Journal of Physics 21 (2019)
    The impact of an energetic particle onto a solid surface generates a strongly perturbed and extremely localized non-equilibrium state, which relaxes on extremely fast time scales. In order to facilitate a time-resolved observation of the relaxation dynamics using established ultrafast pump-probe techniques, it is necessary to pinpoint the projectile impact in time with sufficient accuracy. In this paper, we propose a concept to generate ultrashort ion pulses via femtosecond photoionization of rare gas atoms entrained in a supersonic jet, combined with ion optical bunching of the resulting ion package. We calculate the photoion cloud generated by an intense focused laser pulse and show that Arq+ ions with q = 1-5 can be generated with a standard table-top laser system, which are then accelerated to energies in the keV range over a very short distance and bunched to impinge onto the target surface in a time-focused manner. Detailed ion trajectory simulations show that single ion pulses of sub-picosecond duration can be generated this way. The influence of space charge broadening is included in the simulations, which reveal that flight time broadening is insignificant for pulses containing up to 10-20 ions and starts to increase the pulse width above ∼50 ions/pulse. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab1775
  • 2019 • 185 Atomic-scale investigation of hydrogen distribution in a Ti–Mo alloy
    Yan, F. and Mouton, I. and Stephenson, L.T. and Breen, A.J. and Chang, Y. and Ponge, D. and Raabe, D. and Gault, B.
    Scripta Materialia 162 321-325 (2019)
    Ingress of hydrogen is often linked to catastrophic failure of Ti-alloys. Here, we quantify the hydrogen distribution in fully β and α + β Ti–Mo alloys by using atom probe tomography. Hydrogen does not segregate at grain boundaries in the fully β sample but segregates at some α/β phase boundaries with a composition exceeding 20 at.% in the α + β sample. No stable hydrides were observed in either sample. The hydrogen concentration in β phases linearly decreases from ~13 at. % to ~4 at. % with increasing Mo-content, which is ascribed to the suppression of hydrogen uptake by Mo addition. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.11.040
  • 2019 • 184 Carbon and strain partitioning in a quenched and partitioned steel containing ferrite
    Tan, X. and Ponge, D. and Lu, W. and Xu, Y. and Yang, X. and Rao, X. and Wu, D. and Raabe, D.
    Acta Materialia 165 561-576 (2019)
    We applied a hot rolling direct quenching and partitioning (HDQ&P) process to a low-C low-Si Al-added steel and obtained a Q&P steel containing 40 vol % of ferrite. Microstructure characterization was performed by means of SEM, EBSD, TEM and XRD. Atomic-scale characterization of carbon partitioning among the phases was carried out by atom probe tomography (APT). The carbon distribution in the retained austenite and near the martensite/retained austenite interfaces was quantitatively analyzed to study its partitioning behavior. The macroscopic strain distribution evolution across the tensile sample surface was investigated using macro digital image correlation (DIC) analysis. Combining these results with joint micro-DIC and EBSD analysis during quasi in-situ tensile testing, we investigated the strain partitioning among the different phases and the TRIP effect. Coupling of these results enabled us to reveal the relation among carbon partitioning, strain partitioning and the TRIP effect. The large blocky retained austenite with a side length of about 300–600 nm located near the ferrite/martensite (F/M) interfaces has low stability and transforms to martensite during the early deformation stages, i.e. at average strain below 21%. The retained austenite films in the centers of the martensite regions are more stable. The carbon distribution in both, the martensite and the retained austenite are inhomogeneous, with 0.5–2.0 at. % in the martensite and 4.0–7.5 at. % in the retained austenite. Strong carbon concentration gradients of up to 1.1 at. %/nm were observed near the martensite/retained austenite interfaces. The large blocky retained austenite (300–600 nm in side length) near the F/M interfaces has 1.5–2.0 at. % lower carbon content than that in the narrow retained austenite films (20–150 nm in thickness). The ferrite is soft and deforms prior to the martensite. The strain distribution in ferrite and martensite is inhomogeneous, varying by up to 20% within the same phase at an average strain of about 20%. Ferrite deformation is the main origin of ductility of the material. The balance between ferrite fraction and martensite morphology controls the TRIP effect and its efficiency in reaching a suited combination of strength and ductility. Reducing the ferrite volume fraction and softening the martensite by coarsening and polygonization can enhance the strain carried by the martensite, thus promoting more retained austenite in the martensite regions enabling a TRIP effect. The enhancement of the TRIP effect and the decrease of the strain contrast between ferrite and martensite jointly optimize the micromechanical deformation compatibility of the adjacent phases, thus improving the material's ductility. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.019
  • 2019 • 183 Carbon partitioning and microstructure evolution during tempering of an Fe-Ni-C steel
    Harding, I. and Mouton, I. and Gault, B. and Raabe, D. and Kumar, K.S.
    Scripta Materialia 172 38-42 (2019)
    Partitioning of C during tempering of quenched Fe-9.6Ni-0.5C-0.6Mn-0.6Mo-0.7Cr-0.1V (at.%) steel is determined by atom probe tomography and the resulting microstructure is described. The precipitated austenite size, together with its C and Ni content control its thermal stability and these can vary differently with tempering time and temperature. Thus, both austenite and strong carbide formers compete for the available C early in the process. Due to widely different transport kinetics, C likely plays a dominant role early but is either fully consumed or its role diminishes by dilution, and Ni partitioning eventually takes over as the austenite stability-controlling species. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.06.036
  • 2019 • 182 Composition of the nanosized orthorhombic O′ phase and its direct transformation to fine α during ageing in metastable β-Ti alloys
    Li, T. and Lai, M. and Kostka, A. and Salomon, S. and Zhang, S. and Somsen, C. and Dargusch, M.S. and Kent, D.
    Scripta Materialia 170 183-188 (2019)
    The structure and chemistry of the orthorhombic O′ phase after quenching of a Ti-23 at.%Nb-2 at.%O was measured using aberration-corrected transmission electron microscopy and atom probe tomography. The nanosized O′ phase, formed in the vicinity of ω, is enriched with oxygen and slightly depleted in Nb. Upon annealing, ω dissolves and the O′ phase develops in β up to 350 °C, above which temperature it transforms to colonies of fine α phase. Another needle-like form of α with lower Nb content is thought to nucleate from Nb-lean regions related to spinodal decomposition of β. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2019.06.008
  • 2019 • 181 Crystal plasticity finite element simulation and experiment investigation of nanoscratching of single crystalline copper
    Wang, Z. and Zhang, H. and Li, Z. and Li, G. and Zhang, J. and Zhang, J. and Hassan, H.U. and Yan, Y. and Hartmaier, A. and Sun, T.
    Wear 430-431 100-107 (2019)
    Mechanical properties of crystalline materials strongly correlate with deformation behaviour at the grain level. In the present work, we establish a 3D crystal plasticity finite element model of nanoscratching of single crystalline copper using a Berkovich probe, which is capable of addressing the crystallography influence. In particular, nanoindentation experiments and high resolution electron back-scatter diffraction characterization are jointly carried out to precisely calibrate parameters used in the crystal plasticity finite element model. Subsequent finite element simulations of nanoscratching are performed to reveal fundamental deformation behaviour of single crystalline copper in terms of mechanical response and surface pile-up topography, as well as their dependence on crystallographic orientation. Furthermore, nanoscratching experiments with the same parameters used in the finite element simulations are carried out, the results of which are further compared with predication results by the finite element simulations. Simulation data and experimental results jointly demonstrate the strong anisotropic characteristics of single crystalline copper under nanoscratching, due to the crystallographic orientation dependent coupled effects of intrinsic dislocation slip and extrinsic discrete stress distribution by probe geometry. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2019.04.024
  • 2019 • 180 Current-induced domain wall oscillations in a nanowire imaged by time-resolved photoemission electron microscopy
    Khan, M.I. and Cramm, S. and Bürgler, D.E. and Parlak, U. and Nemšák, S. and Gottlob, D.M. and Hackl, J. and Doğanay, H. and Schneider, C.M.
    Journal of Magnetism and Magnetic Materials 476 538-545 (2019)
    We study reversible domain wall motion in half-ring Ni 80 Fe 20 nanowires on a nanosecond (ns) timescale in a truly current-induced pump-probe experiment using an energy filtered, aberration-corrected photoemission electron microscope. The X-ray magnetic circular dichroism signal is probed at different time delays before, during and after the current pulse in a stroboscopic mode with circularly polarized synchrotron radiation in the energy range of the Fe L 3 -edge (707 eV). We observe lateral domain wall oscillations with a frequency of ∼0.4 GHz. Comparing the results to a proposed string model, we find that the domain wall oscillations can be described as string-like asymmetric oscillations. © 2019
    view abstractdoi: 10.1016/j.jmmm.2019.01.003
  • 2019 • 179 Elemental re-distribution inside shear bands revealed by correlative atom-probe tomography and electron microscopy in a deformed metallic glass
    Balachandran, S. and Orava, J. and Köhler, M. and Breen, A.J. and Kaban, I. and Raabe, D. and Herbig, M.
    Scripta Materialia 168 14-18 (2019)
    A density variation in shear bands visible by electron microscopy is correlated with compositionally altered locations measured by atom-probe tomography in plastically-deformed Al 85.6 Y 7.5 Fe 5.8 metallic-glass ribbons. Two compositionally distinct regions are identified along shear bands, one is Al-rich (~92 at.%), the other is Al-depleted (~82.5 at.%) and both regions show marginal concentration fluctuations of Y and Fe. The elemental re-distribution is observed within shear bands only, and no chemical exchange with the surrounding glassy matrix is observed. © 2019
    view abstractdoi: 10.1016/j.scriptamat.2019.04.014
  • 2019 • 178 Experimental and numerical investigation on the flow in a return channel of multistage centrifugal compressors
    Dolle, B. and Brillert, D. and Dohmen, H.J. and Benra, F.-K.
    Proceedings of the ASME Turbo Expo 2B-2019 (2019)
    Multistage radial compressors are major components in plenty of industrial applications. Today, compressor downsizing for CAPEX reduction is of utmost importance. Accordingly, the pressure ratio must be increased, accommodated in a most compact design, accepting nearly no penalties in the performance level achieved so far. In order to investigate the complex flow in stator parts of multistage centrifugal compressors and to increase the confidence level of numerical methods a new test rig is developed and taken into operation. This test rig allows to separate stationary flow effects from time variant effects leading to an in depth insight into the physical flow features. The aim is to investigate the flow in different stator designs in detail for varying flow coefficients. Additive manufacturing techniques are applied to achieve low costs simultaneously with short production time for the tested parts. In this publucation, the measured flow field in the stator parts is discussed. The experimental results comprise velocity and pressure data from five-hole-probes and unsteady velocity data from single-film hot-film-probes. Using unsteady velocity data, turbulent statistics such as turbulence intensity and the turbulent kinetic energy will be determined. Subsequently, the experimental results are compared to numerical results. Therefore, (U)RANS simulations are performed using a commercial CFD-code. The simulation results are evaluated at planes appropriate to the measuring planes in the test rig to guarantee a directly comparable data base. Copyright © 2019 ASME.
    view abstractdoi: 10.1115/GT2019-90455
  • 2019 • 177 Influence of composition and precipitation evolution on damage at grain boundaries in a crept polycrystalline Ni-based superalloy
    Kontis, P. and Kostka, A. and Raabe, D. and Gault, B.
    Acta Materialia 166 158-167 (2019)
    The microstructural and compositional evolution of intergranular carbides and borides prior to and after creep deformation at 850 °C in a polycrystalline nickel-based superalloy was studied. Primary MC carbides, enveloped within intergranular γ′ layers, decomposed resulting in the formation of layers of the undesirable η phase. These layers have a composition corresponding to Ni3Ta as measured by atom probe tomography and their structure is consistent with the D024 hexagonal structure as revealed by transmission electron microscopy. Electron backscattered diffraction reveals that they assume various misorientations with regard to the adjacent grains. As a consequence, these layers act as brittle recrystallized zones and crack initiation sites. The composition of the MC carbides after creep was altered substantially, with the Ta content decreasing and the Hf and Zr contents increasing, suggesting a beneficial effect of Hf and Zr additions on the stability of MC carbides. By contrast, M5B3 borides were found to be microstructurally stable after creep and without substantial compositional changes. Borides at 850 °C were found to coarsen, resulting in some cases into γ′- depleted zones, where, however, no cracks were observed. The major consequences of secondary phases on the microstructural stability of superalloys during the design of new polycrystalline superalloys are discussed. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.039
  • 2019 • 176 Intensity-dependent degenerate and non-degenerate nonlinear optical absorption of direct-gap semiconductors
    Hannes, W.-R. and Krauß-Kodytek, L. and Ruppert, C. and Betz, M. and Meier, T.
    Proceedings of SPIE - The International Society for Optical Engineering 10916 (2019)
    The nonlinear optical response of direct-gap semiconductors is investigated with a focus on non-degenerate multiphoton absorption processes. The theoretical approach is based on the semiconductor Bloch equations and yields the absorption rate either perturbatively or non-perturbatively in the incident light intensities. We describe the semiconductor by a two-band model and consider a pump-probe scheme where the weak probe pulse provides one of the simultaneously absorbed photons. The perturbative response can be described analytically within some approximations and we give simple expressions for two, three, and four-photon absorption coefficients. These are compared with numerical results for the absorption of pulses with a finite duration, where the influence of dephasing and relaxation as well as higher-order corrections are also investigated. For strong pump fields that are treated non-perturbatively we demonstrate non-trivial dependencies of the absorption on the time delay between the pulses. In the non-perturbative response of a single light pulse characteristic modulations appear in the absorption dependence on the field strength that may be interpreted as multi-photon Rabi oscillations. Finally, we present measurements of the non-degenerate two-photon absorption coefficient of bulk GaAs via time-delay and polarization-dependent transmissivity changes in a pump probe setup. The observed strong increase of the absorption coefficient with frequency ratios deviating from unity qualitatively agrees with theoretical expectations. Copyright © 2019 SPIE.
    view abstractdoi: 10.1117/12.2503539
  • 2019 • 175 Martensite to austenite reversion in a high-Mn steel: Partitioning-dependent two-stage kinetics revealed by atom probe tomography, in-situ magnetic measurements and simulation
    Souza Filho, I.R. and Kwiatkowski da Silva, A. and Sandim, M.J.R. and Ponge, D. and Gault, B. and Sandim, H.R.Z. and Raabe, D.
    Acta Materialia 166 178-191 (2019)
    Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (TCurie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the TCurie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.046
  • 2019 • 174 Microscopic dynamics of electron hopping in a semiconductor quantum well probed by spin-dependent photon echoes
    Kosarev, A.N. and Poltavtsev, S.V. and Golub, L.E. and Glazov, M.M. and Salewski, M. and Kozyrev, N.V. and Zhukov, E.A. and Yakovlev, D.R. and Karczewski, G. and Chusnutdinow, S. and Wojtowicz, T. and Akimov, I.A. and Bayer, M.
    Physical Review B 100 (2019)
    Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that, independent of the particular spin relaxation mechanism, the hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.100.121401
  • 2019 • 173 Multi-scale characterization of austenite reversion and martensite recovery in a cold-rolled medium-Mn steel
    Benzing, J.T. and Kwiatkowski da Silva, A. and Morsdorf, L. and Bentley, J. and Ponge, D. and Dutta, A. and Han, J. and McBride, J.R. and Van Leer, B. and Gault, B. and Raabe, D. and Wittig, J.E.
    Acta Materialia 166 512-530 (2019)
    A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc ® simulations to balance the fraction and stacking fault energy of reverted austenite. Intercritical annealing for 0.5, 8 and 48 h was carried out at 585 °C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning-transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered α′-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered α′-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered α′-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C-enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium (LE) model such that the driving force between two equilibrium phases moves the mobile interface, as confirmed by DICTRA simulations (a Thermo-Calc ® diffusion module). The sequence of austenite reversion is: (i) formation of Mn- and C-enriched face-centered-cubic nuclei from decorated dislocations and/or particles; (ii) co-partitioning of Mn and C and (iii) growth of austenite controlled by the LE mode. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.01.003
  • 2019 • 172 On the compositional partitioning during phase transformation in a binary ferromagnetic MnAl alloy
    Palanisamy, D. and Raabe, D. and Gault, B.
    Acta Materialia 174 227-236 (2019)
    We introduce a new perspective on the classical massive mode of solid-state phase transformation enabled by the correlative use of atomic-scale electron microscopy and atom probe tomography. This is demonstrated in a binary MnAl alloy which has Heusler-like characteristics. In this system, the τ phase formed by a massive transformation from the high-temperature ε phase is metastable and ferromagnetic. The transformation results in a high density of micro-twins inside the newly grown τ phase. Atomic-scale compositional analysis across the interface boundaries and atomic structure of the micro-twins reveals the involvement of both structural modification and also the compositional partitioning during the growth of the τ phase. This is assisted by the migrating τ/ε interface boundary during transformation. Finally, the role of micro-twins on nucleating the equilibrium phases and the influence of the defects and phase formation on the magnetic properties are discussed. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.05.037
  • 2019 • 171 On the influence of the heat treatment on microstructure formation and mechanical properties of near-α Ti-Fe alloys
    Sandlöbes, S. and Korte-Kerzel, S. and Raabe, D.
    Materials Science and Engineering A 748 301-312 (2019)
    We study the microstructure formation and mechanical properties of Ti-1Fe (wt%) and Ti-3Fe (wt%) alloys for different heat treatments in the β-phase and α + β-phase regions. By applying different heat treatment routes, we observe different microstructure formation mechanisms causing a wide range of mechanical properties from high strength (1.3 GPa) and low ductility (2%) to intermediate strength (700 MPa) and high ductility (30%) in these simple binary alloys. We performed microstructure characterizsation using scanning electron microscopy, transmission electron microscopy and atom probe tomography to show that the alloying content and heat treatment significantly affect the local martensitic and / or diffusional phase transformations causing the substantial changes in the mechanical behavior. © 2018
    view abstractdoi: 10.1016/j.msea.2018.12.071
  • 2019 • 170 Phase-Tunable Thermal Rectification in the Topological SQUIPT
    Bours, L. and Sothmann, B. and Carrega, M. and Strambini, E. and Braggio, A. and Hankiewicz, E.M. and Molenkamp, L.W. and Giazotto, F.
    Physical Review Applied 11 (2019)
    We theoretically explore the behavior of thermal transport in the topological SQUIPT, in the linear and nonlinear regime. The device consists of a topological Josephson junction based on a two-dimensional topological insulator in contact with two superconducting leads, and a probe tunnel coupled to the topological edge states of the junction. We compare the performance of a normal metal and a graphene probe, showing that the topological SQUIPT behaves as a passive thermal rectifier and that it can reach a rectification coefficient of up to 145% with the normal metal probe. Moreover, the interplay between the superconducting leads and the helical edge states leads to a unique behavior due to a Doppler-shift-like effect that allows one to influence quasiparticle transport through the edge channels via the magnetic flux that penetrates the junction. Exploiting this effect, we can greatly enhance the rectification coefficient for temperatures below the critical temperature TC in an active rectification scheme. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevApplied.11.044073
  • 2019 • 169 Pump-probe microscopy of femtosecond laser ablation in air and liquids
    Kanitz, A. and Förster, D.J. and Hoppius, J.S. and Weber, R. and Ostendorf, A. and Gurevich, E.L.
    Applied Surface Science 475 204-210 (2019)
    The ablation process of femtosecond laser pulses of iron in air and different liquids was investigated for fluences of 0.5 J/cm2 and 2 J/cm2 by means of femtosecond pump-probe microscopy. Measurements of the relative change in reflectivity suggest that the surrounding liquid has a significant impact on the ablation process. During the heating phase of the metal within the first picoseconds after laser beam impact, the change in reflectivity in air and liquids is similar. Afterwards, the vapor and melt expulsion in air leads to a strong decrease in reflectivity, while the change in reflectivity in the liquids shows a more complex fluence and time-dependent behavior. This behavior is suggested to be triggered by the expansion of the molten surface and chemical reactions on the picosecond timescale. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.12.184
  • 2019 • 168 Quantification of solute deuterium in titanium deuteride by atom probe tomography with both laser pulsing and high-voltage pulsing: Influence of the surface electric field
    Chang, Y.H. and Mouton, I. and Stephenson, L. and Ashton, M. and Zhang, G.K. and Szczpaniak, A. and Lu, W.J. and Ponge, D. and Raabe, D. and Gault, B.
    New Journal of Physics 21 (2019)
    Atom probe tomography (APT) has been increasingly used to investigate hydrogen embrittlement in metals due to its unique capacity for direct imaging of H atoms interacting with microstructural features. The quantitativeness of hydrogen measurements by APT is yet to be established in views of erroneous compositional measurements of bulk hydrides and the influence of spurious hydrogen, e.g. residual gas inside the analysis chamber. Here, we analyzed titanium deuteride (approx. 65.0 at%-66.6 at% D) in lieu of hydride to minimize the overlap with residual gas, both with laser pulsing and high-voltage (HV) pulsing. Strategies were deployed to prevent H pick-up during specimen fabrication, including preparing specimens at cryogenic temperature. The measured composition of deuterium by APT with laser pulsing decreases significantly with the applied laser pulse energy, which is interpreted with regards to the strength of the corresponding surface electrostatic field, as assessed by the evolution of charge-state ratio. In contrast, compositional analyses with HV pulsing are roughly independent of the applied experimental parameters, although approx. 15 at%-20 at% off the nominal composition. Aided by plotting paired mass-to-charge correlations, the mechanisms of composition bias in both pulsing modes are discussed. A special emphasis is placed on the local variations of the measured composition as a function of the local electric field across the specimen's surface, which is not uniform due to asymmetric heat distribution related to the localized laser absorption and the faceted nature of surface caused by the crystallographic structure. Our investigations demonstrate the challenges of quantitative analysis of solute deuterium by APT but nevertheless provide insight to achieving the best possible experimental protocol. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/ab1c3b
  • 2019 • 167 Radiofrequency driving of coherent electron spin dynamics in n -GaAs detected by Faraday rotation
    Belykh, V.V. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 99 (2019)
    We suggest a pump-probe method for studying semiconductor spin dynamics based on pumping of carrier spins by a pulse of oscillating radiofrequency (rf) magnetic field and probing by measuring the Faraday rotation of a short laser pulse. We demonstrate this technique on n-GaAs and observe the onset and decay of coherent spin precession during and after the course of rf pulse excitation. We show that the rf field resonantly addresses the electron spins with Larmor frequencies close to that of the rf field. This opens the opportunity to determine the homogeneous spin coherence time T2, that is inaccessible directly in standard all-optical pump-probe experiments. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.161205
  • 2019 • 166 Role of elemental intermixing at the In 2 S 3 /CIGSe heterojunction deposited using reactive RF magnetron sputtering
    Soni, P. and Raghuwanshi, M. and Wuerz, R. and Berghoff, B. and Knoch, J. and Raabe, D. and Cojocaru-Mirédin, O.
    Solar Energy Materials and Solar Cells 195 367-375 (2019)
    In the present work, sputtered In 2 S 3 buffer layers are deposited on Cu(In,Ga)Se 2 absorbers with no or minimal sputter damage. Buffer deposition at slower sputter rates (0.22 Å/s) with H 2 S as a reactive gas improved the interface quality and uniformity. We obtained crystalline In 2 S 3 films at room temperature with the deposition parameters used in this work. Elemental intermixing effect at the In 2 S 3 /CIGSe heterointerface at different annealing temperatures was mapped in 3-dimensions using atom probe tomography (APT). APT results revealed the induced sputter damage during buffer layer deposition, and the effect of diffusion and segregation of elements at the heterointerface. Knowledge of elemental redistribution at the buffer-absorber heterointerface can help better understand the relation of the chemical intermixing with electrical performance of the cell. X-ray photoelectron spectroscopy (XPS) reveal accelerated Cu + doping of the In 2 S 3 buffer at 225 °C. Cu-depletion on CIGSe surface up to a few nanometers improves the cell performance. However, at higher annealing temperature of 275 °C, CuIn 5 S 8 and Cu(In x Ga 1-x ) 5 Se 8 phases are formed which is detrimental for cell performance. Na diffusing from the CIGSe absorber segregates at the In 2 S 3 /CIGSe heterojunction. This actively suppresses the formation of charged antisites defects, lowering the recombination in the space charged region. Cu-depletion at CIGSe surface, Cu and Na diffusion in In 2 S 3 and passivation of interfacial defect states by Na, giving an efficiency of 13.84% (14.83% with CdS-reference) for sputtered In 2 S 3 buffer layers. Consequently, this work elucidates the chemistry of buried hetero-interfaces and their significance to improve the electrical performance of solar cells. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.solmat.2019.03.026
  • 2019 • 165 Segregation-driven grain boundary spinodal decomposition as a pathway for phase nucleation in a high-entropy alloy
    Li, L. and Li, Z. and Kwiatkowski da Silva, A. and Peng, Z. and Zhao, H. and Gault, B. and Raabe, D.
    Acta Materialia 178 1-9 (2019)
    Elemental segregation to grain boundaries (GBs) can induce structural and chemical transitions at GBs along with significant changes in material properties. The presence of multiple principal elements interacting in high-entropy alloys (HEAs) makes the GB segregation and interfacial phase transformation a rather challenging subject to investigate. Here, we explored the temporal evolution of the chemistry for general high-angle GBs in a typical equiatomic FeMnNiCoCr HEA during aging heat treatment through detailed atom probe tomography (APT) analysis. We found that the five principal elements segregate heterogeneously at the GBs. More specifically, Ni and Mn co-segregate to some regions of the GBs along with the depletion of Fe, Co and Cr, while Cr is enriched in other regions of the GBs where Ni and Mn are depleted. The redistribution of these elements on the GBs follow a periodic characteristic, spinodal-like compositional modulation. The accumulation of elements at the GBs can create local compositions by shifting their state from a solid solution (like in the adjacent bulk region) into a spinodal regime to promote interfacial phase-like transitions as segregation proceeds. These results not only shed light on phase precursor states and the associated nucleation mechanism at GBs in alloy systems with multiple principal elements but also help to guide the microstructure design of advanced HEAs in which formation of embrittling phases at interfaces must be avoided. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.07.052
  • 2019 • 164 The role of lattice defects, element partitioning and intrinsic heat effects on the microstructure in selective laser melted Ti-6Al-4V
    Haubrich, J. and Gussone, J. and Barriobero-Vila, P. and Kürnsteiner, P. and Jägle, E.A. and Raabe, D. and Schell, N. and Requena, G.
    Acta Materialia 167 136-148 (2019)
    The microstructure and phase composition in selective laser melted (SLM) Ti-6Al-4V plays a key role for its mechanical performance. The microstructure evolution in SLM Ti-6Al-4V was studied in the as-built condition and after sub-transus heat treatments between 400 °C and 800 °C focusing on elemental partitioning and the role of lattice defects on precipitation of the β phase. With SLM parameters corresponding to low volume energy density (E V = 77 J/mm 3 ) the as-built microstructure consisted of acicular martensite and showed a higher density of lattice defects than that synthesized under high E V = 145 J/mm 3 condition. High energy X-ray synchrotron diffraction indicated the presence of ∼2 wt.% β-phase at this high E V . Moreover, atom-probe tomography revealed enrichments in β-stabilizers at one- and two-dimensional lattice defects. These fine enriched one-dimensional columnar and two-dimensional features are identified as precursors of β-phase, revealing the role of lattice defects for β-precipitation. Upon annealing at 400 °C and 530 °C, β-films began to fragment into β−platelets and nanoparticles, whereas annealing at 800 °C led to a coarse-lamellar α/β-microstructure. Moreover, α 2 -Ti 3 Al was found in the 400 °C annealed condition. In line with the microstructure changes, Vickers hardness increased upon annealing at temperatures up to 530 °C and dropped when coarsening occurred at higher temperatures. Substantial element partitioning occurred during thermally driven martensite decomposition, which was significantly stronger for Fe than for V. © 2019 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2019.01.039
  • 2019 • 163 Thermal annealing effects in polycrystalline EuTiO3 and Eu2Ti2O7
    Schell, J. and Kamba, S. and Kachlik, M. and Maca, K. and Drahokoupil, J. and Rano, B.R. and Nuno Gonçalves, J. and Thanh Dang, T. and Costa, A. and Noll, C. and Vianden, R. and Lupascu, D.C.
    AIP Advances 9 (2019)
    Using time-differential perturbed angular correlation of γrays, we investigated the electric-field gradient of polycrystalline EuTiO3 and Eu2Ti2O7, with 181Hf(181Ta) as a probe, following different thermal treatments. The measurements were performed at ISOLDE-CERN following 80 keV implantation at the Bonn Radioisotope Separator. The experimental results indicated successful induction of different phases in the implantation recovery process at 1273 and 1373 K. These observations were combined with ab initio calculations and X-ray diffraction measurements. A comparison of ab initio calculated electric-field gradients with the measured values discriminates between different structures and defects and rules out many possible cases. The Ta probe at the Ti site in the Eu2Ti2O7 phase is found to be the most probable case of site occupation after annealing at 1373 K, while annealing at 1273 K keeps EuTiO3 in the vicinity of the Ta probe. A discussion of the hyperfine interactions that promote variation in the interaction strength at the 181Ta site is presented. © 2019 Author(s).
    view abstractdoi: 10.1063/1.5115466
  • 2018 • 162 Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials
    Yu, Y. and Zhang, S. and Mio, A.M. and Gault, B. and Sheskin, A. and Scheu, C. and Raabe, D. and Zu, F. and Wuttig, M. and Amouyal, Y. and Cojocaru-Mirédin, O.
    ACS Applied Materials and Interfaces 10 3609-3615 (2018)
    Dislocations have been considered to be an efficient source for scattering midfrequency phonons, contributing to the enhancement of thermoelectric performance. The structure of dislocations can be resolved by electron microscopy whereas their chemical composition and decoration state are scarcely known. Here, we correlate transmission Kikuchi diffraction and (scanning) transmission electron microscopy in conjunction with atom probe tomography to investigate the local structure and chemical composition of dislocations in a thermoelectric Ag-doped PbTe compound. Our investigations indicate that Ag atoms segregate to dislocations with a 10-fold excess of Ag compared with its average concentration in the matrix. Yet the Ag concentration along the dislocation line is not constant but fluctuates from ∼0.8 to ∼10 atom % with a period of about 5 nm. Thermal conductivity is evaluated applying laser flash analysis, and is correlated with theoretical calculations based on the Debye-Callaway model, demonstrating that these Ag-decorated dislocations yield stronger phonon scatterings. These findings reduce the knowledge gap regarding the composition of dislocations needed for theoretical calculations of phonon scattering and pave the way for extending the concept of defect engineering to thermoelectric materials. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b17142
  • 2018 • 161 Atomic scale analysis of grain boundary deuteride growth front in Zircaloy-4
    Breen, A.J. and Mouton, I. and Lu, W. and Wang, S. and Szczepaniak, A. and Kontis, P. and Stephenson, L.T. and Chang, Y. and da Silva, A.K. and Liebscher, C.H. and Raabe, D. and Britton, T.B. and Herbig, M. and Gault, B.
    Scripta Materialia 156 42-46 (2018)
    Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%) was electrochemically charged with deuterium to create deuterides and subsequently analysed with atom probe tomography and scanning transmission electron microscopy to understand zirconium hydride formation and embrittlement. At the interface between the hexagonal close packed (HCP) α-Zr matrix and a face centred cubic (FCC) δ deuteride (ZrD1.5–1.65), a HCP ζ phase deuteride (ZrD0.25–0.5) has been observed. Furthermore, Sn is rejected from the deuterides and segregates to the deuteride/α-Zr reaction front. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.06.044
  • 2018 • 160 Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale
    Chang, Y. and Breen, A.J. and Tarzimoghadam, Z. and Kürnsteiner, P. and Gardner, H. and Ackerman, A. and Radecka, A. and Bagot, P.A.J. and Lu, W. and Li, T. and Jägle, E.A. and Herbig, M. and Stephenson, L.T. and Moody, M.P. and...
    Acta Materialia 150 273-280 (2018)
    Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.02.064
  • 2018 • 159 Coherent transmission of superconducting carriers through a ∼2 μm polar semiconductor
    Chakraborti, H. and Deb, S. and Schott, R. and Thakur, V. and Chatterjee, A. and Yadav, S. and Saroj, R.K. and Wieck, A. and Shivaprasad, S.M. and Das Gupta, K. and Dhar, S.
    Superconductor Science and Technology 31 (2018)
    Coherent transmission of Cooper pairs through a non-superconducting medium is a key challenge for hybrid electronics with superconductors, normal metals and semiconductors. While superconductor-normal metal-superconductor (SNS) junctions have been known for quite sometime, including a low carrier density region or a two-dimensional electron gas (2DEG) in the path of superconducting electrons is relatively less explored. Indeed, this is due to the limited choice of materials that would make ohmic contacts to such systems, while simultaneously supporting a superconducting phase. In this paper we show a coherent transmission of supercurrent through a degenerate semiconductor over a length ≈2 μm with a critical magnetic field B c ≈ 8 T at 1.6 K and T c ≈ 5 K at zero magnetic field. This length scale is much larger than the typical thickness of a Josephson junction. Our system is a fragment of a GaN nanowall network that has been shown to support a high mobility 2DEG (μ n > 104 cm2 V-1 s-1 ). The current and voltage probes were superconducting tungsten-gallium composite electrodes and the measurements could be done in four-probe geometry. We demonstrate ballistic type carrier transport with a near ideal transparency of 1 and a critical current (I c) large enough such that the Josephson coupling parameter . Some features in magneto-transport data suggest that there is possibly a small magnetic moment forming in the semiconductor fragment. In addition the combination of a T c typical of elemental metallic superconductors, but a critical field that appears to be higher than the Clogston-Chandrasekhar limit, may be indicative of the emergence of a triplet pairing mechanism in these structures. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6668/aacd89
  • 2018 • 158 Competition between formation of carbides and reversed austenite during tempering of a medium-manganese steel studied by thermodynamic-kinetic simulations and atom probe tomography
    Kwiatkowski da Silva, A. and Inden, G. and Kumar, A. and Ponge, D. and Gault, B. and Raabe, D.
    Acta Materialia 147 165-175 (2018)
    We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 °C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations, we predicted the growth of M23C6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 °C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable γ-M23C6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.01.022
  • 2018 • 157 Compositional evolution of long-period stacking ordered structures in magnesium studied by atom probe tomography
    Kim, J.-K. and Guo, W. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 156 55-59 (2018)
    Mg alloys containing long-period stacking ordered (LPSO) structures are strong and ductile compared to conventional Mg alloys. We study here the compositional evolution of LPSO structures in a Mg97Y2Zn1 (at.%) alloy upon annealing at 500 °C using atom probe tomography. In the material annealed for 2.5 h, the Zn/Y ratio of the building blocks in the interdendritic LPSO phase (0.73) is close to the stoichiometric composition of Y8Zn6 L12 clusters while that in plate-type LPSO structures (0.66) slightly deviates from the ideal value. The Y/Zn enrichment in LPSO structures in the α-Mg matrix slightly decreases with increasing annealing time. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.07.017
  • 2018 • 156 Correlative transmission Kikuchi diffraction and atom probe tomography study of Cu(In,Ga)Se2 grain boundaries
    Schwarz, T. and Stechmann, G. and Gault, B. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 26 196-204 (2018)
    We combined transmission Kikuchi diffraction and atom probe tomography techniques to investigate the relationship between the structure and chemistry of grain boundaries in Cu(In,Ga)Se2 thin films. Kikuchi patterns with the tetragonal structure of Cu(In,Ga)Se2 were simulated to emphasize the pseudosymmetry issue in this material system and, hence, the orientation determination ambiguity in case of indexing with a cubic zinc-blende structure. We compared these patterns with experimental data. We detect an elemental redistribution at random high-angle grain boundaries but no chemical fluctuations at Σ3 twin boundaries. The atom probe tomography analyses reveal Cu depletion as well as In and Se enrichment at random grain boundaries and, at some random grain boundaries, a slight Ga depletion. This In on Cu scenario is accompanied by cosegregation of Na and K originating from the soda-lime glass substrate. The amount of impurity segregation does vary not only from one grain boundary to another but also along an individual grain boundary. Hence, our results suggest that the degree of passivation of detrimental, nonradiative recombination centers does differ not only between Σ3 twin boundaries and random grain boundaries but also within the same random grain boundary. Copyright © 2017 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2966
  • 2018 • 155 Dynamic quadrupole interactions in semiconductors
    Dang, T.T. and Schell, J. and Lupascu, D.C. and Vianden, R.
    Journal of Applied Physics 123 (2018)
    The time differential perturbed angular correlation, TDPAC, technique has been used for several decades to study electric quadrupole hyperfine interactions in semiconductors such as dynamic quadrupole interactions (DQI) resulting from after-effects of the nuclear decay as well as static quadrupole interactions originating from static defects around the probe nuclei such as interstitial ions, stresses in the crystalline structure, and impurities. Nowadays, the quality of the available semiconductor materials is much better, allowing us to study purely dynamic interactions. We present TDPAC measurements on pure Si, Ge, GaAs, and InP as a function of temperature between 12 K and 110 K. The probe 111In (111Cd) was used. Implantation damage was recovered by thermal annealing. Si experienced the strongest DQI with lifetime, τg, increasing with rising temperature, followed by Ge. In contrast, InP and GaAs, which have larger band gaps and less electron concentration than Si and Ge in the same temperature range, presented no DQI. The results obtained also allow us to conclude that indirect band gap semiconductors showed the dynamic interaction, whereas the direct band gap semiconductors, restricted to GaAs and InP, did not. © 2018 Author(s).
    view abstractdoi: 10.1063/1.4993714
  • 2018 • 154 Elemental partitioning and site-occupancy in γ/γ′ forming Co-Ti-Mo and Co-Ti-Cr alloys
    Im, H.J. and Makineni, S.K. and Gault, B. and Stein, F. and Raabe, D. and Choi, P.-P.
    Scripta Materialia 154 159-162 (2018)
    We report on the sub-nanometer scale characterization of Co-12Ti-4Mo and Co-12Ti-4Cr (at.%) model alloys. Atom probe tomography reveals that Co and Cr partition to γ, whereas Ti and Mo to γ′. Additions of Mo and Cr to the reference Co-12Ti system lead to strong increases in γ′ volume fraction by about 25% and 12%, respectively. Element-specific spatial distribution maps along the [001] direction of the L12-ordered γ′ phase reveal that both Mo and Cr preferentially replace Ti on its sublattice. The remaining excess Ti is available for formation of additional γ′, resulting in enhanced γ′ volume fractions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.05.041
  • 2018 • 153 Elemental segregation to twin boundaries in a MnAl ferromagnetic Heusler alloy
    Palanisamy, D. and Raabe, D. and Gault, B.
    Scripta Materialia 155 144-148 (2018)
    Electron microscopy and atom probe tomography were combined to investigate the crystallography and chemistry of a single twin boundary (TB) in a rare-earth-free ferromagnetic MnAl Heusler alloy. The results establish a significant segregation of Mn along the twin boundaries. An enrichment of approx. ~8 at.% Mn was measured along the twin boundary with a confined depletion outside the twin boundary, suggesting short range solute diffusion occurring during massive transformation. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.06.037
  • 2018 • 152 Kelvin probe force microscopy studies of the charge effects upon adsorption of carbon nanotubes and C60 fullerenes on hydrogen-terminated diamond
    Kölsch, S. and Fritz, F. and Fenner, M.A. and Kurch, S. and Wöhrl, N. and Mayne, A.J. and Dujardin, G. and Meyer, C.
    Journal of Applied Physics 123 (2018)
    Hydrogen-terminated diamond is known for its unusually high surface conductivity that is ascribed to its negative electron affinity. In the presence of acceptor molecules, electrons are expected to transfer from the surface to the acceptor, resulting in p-type surface conductivity. Here, we present Kelvin probe force microscopy (KPFM) measurements on carbon nanotubes and C60 adsorbed onto a hydrogen-terminated diamond(001) surface. A clear reduction in the Kelvin signal is observed at the position of the carbon nanotubes and C60 molecules as compared with the bare, air-exposed surface. This result can be explained by the high positive electron affinity of carbon nanotubes and C60, resulting in electron transfer from the surface to the adsorbates. When an oxygen-terminated diamond(001) is used instead, no reduction in the Kelvin signal is obtained. While the presence of a charged adsorbate or a difference in work function could induce a change in the KPFM signal, a charge transfer effect of the hydrogen-terminated diamond surface, by the adsorption of the carbon nanotubes and the C60 fullerenes, is consistent with previous theoretical studies. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5019486
  • 2018 • 151 Nano-laminated thin film metallic glass design for outstanding mechanical properties
    Kontis, P. and Köhler, M. and Evertz, S. and Chen, Y.-T. and Schnabel, V. and Soler, R. and Bednarick, J. and Kirchlechner, C. and Dehm, G. and Raabe, D. and Schneider, J.M. and Gault, B.
    Scripta Materialia 155 73-77 (2018)
    We report the enhancement of fracture toughness and strength of a cobalt‑tantalum-based metallic glass thin film with increasing boron content. The improvement of the mechanical performance is attributed to the formation of a compositionally lamellar compared to uniform glass microstructure, which becomes thinner with increasing boron content as revealed by transmission electron microscopy. Compositional variations across the lamellar structure are revealed by atom probe tomography. Cobalt- and boron-rich regions alternate sequentially, whereas tantalum exhibits slight variations across the lamellae. Our results can be utilized in future design efforts for metallic glass thin films with outstanding mechanical performance. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2018.06.015
  • 2018 • 150 Nonequilibrium electron and lattice dynamics of strongly correlated Bi2Sr2CaCu2O8+d single crystals
    Konstantinova, T. and Rameau, J.D. and Reid, A.H. and Abdurazakov, O. and Wu, L. and Li, R. and Shen, X. and Gu, G. and Huang, Y. and Rettig, L. and Avigo, I. and Ligges, M. and Freericks, J.K. and Kemper, A.F. and Dürr, H.A. and...
    Science Advances 4 (2018)
    The interplay between the electronic and lattice degrees of freedom in nonequilibrium states of strongly correlated systems has been debated for decades. Although progress has been made in establishing a hierarchy of electronic interactions with the use of time-resolved techniques, the role of the phonons often remains in dispute, a situation highlighting the need for tools that directly probe the lattice. We present the first combined megaelectron volt ultrafast electron diffraction and time- and angle-resolved photoemission spectroscopy study of optimally doped Bi2Sr2CaCu2O8+d. Quantitative analysis of the lattice and electron subsystems’ dynamics provides a unified picture of nonequilibrium electron-phonon interactions in the cuprates beyond the N-temperature model. The work provides new insights on the specific phonon branches involved in the nonequilibrium heat dissipation from the high-energy Cu–O bond stretching “hot” phonons to the lowest-energy acoustic phonons with correlated atomic motion along the <110> crystal directions and their characteristic time scales. It reveals a highly nonthermal phonon population during the first several picoseconds after the photoexcitation. The approach, taking advantage of the distinct nature of electrons and photons as probes, is applicable for studying energy relaxation in other strongly correlated electron systems. Copyright © 2018 The Authors.
    view abstractdoi: 10.1126/sciadv.aap7427
  • 2018 • 149 Nucleation driving force for ω-assisted formation of α and associated ω morphology in β-Ti alloys
    Li, T. and Kent, D. and Sha, G. and Liu, H. and Fries, S.G. and Ceguerra, A.V. and Dargusch, M.S. and Cairney, J.M.
    Scripta Materialia 155 149-154 (2018)
    The structural and chemical changes at ω/β interfaces and the evolution of the morphology of ω in a near-β alloy during isothermal ageing at 573 K were investigated by atom probe tomography and aberration-corrected high-resolution transmission electron microscopy. Ledges and local O enrichment at semi-coherent isothermal ω interfaces are proposed to provide the key driving force for nucleation of ω-assisted α. Following nucleation of α, the morphology of ω evolves from ellipsoidal to rod-like, induced by rapid consumption of ω by α. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2018.06.039
  • 2018 • 148 On the diffusive phase transformation mechanism assisted by extended dislocations during creep of a single crystal CoNi-based superalloy
    Makineni, S.K. and Kumar, A. and Lenz, M. and Kontis, P. and Meiners, T. and Zenk, C. and Zaefferer, S. and Eggeler, G. and Neumeier, S. and Spiecker, E. and Raabe, D. and Gault, B.
    Acta Materialia 155 362-371 (2018)
    We propose here a deformation-induced diffusive phase transformation mechanism occurring during shearing of γ′ ordered phase in a γ/γ′ single crystalline CoNi-based superalloy. Shearing involved the creation and motion of a high density of planar imperfections. Through correlative electron microscopy and atom probe tomography, we captured a superlattice intrinsic stacking fault (SISF) and its associated moving leading partial dislocation (LPD). The structure and composition of these imperfections reveal characteristic chemical – structural contrast. The SISF locally exhibits a D019 ordered structure coherently embedded in the L12 γ′ and enriched in W and Co. Interestingly, the LPD is enriched with Cr and Co, while the adjoining planes ahead of the LPD are enriched with Al. Quantitative analysis of the three-dimensional compositional field in the vicinity of imperfections sheds light onto a new in-plane diffusion mechanism as the LPD moves on specific {111} planes upon application of stress at high temperature. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.05.074
  • 2018 • 147 On the grain boundary strengthening effect of boron in γ/γ′ Cobalt-base superalloys
    Kolb, M. and Freund, L.P. and Fischer, F. and Povstugar, I. and Makineni, S.K. and Gault, B. and Raabe, D. and Müller, J. and Spiecker, E. and Neumeier, S. and Göken, M.
    Acta Materialia 145 247-254 (2018)
    Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.12.020
  • 2018 • 146 Parameter free quantitative analysis of atom probe data by correlation functions: Application to the precipitation in Al-Zn-Mg-Cu
    Zhao, H. and Gault, B. and Ponge, D. and Raabe, D. and De Geuser, F.
    Scripta Materialia 154 106-110 (2018)
    Atom probe tomography enables precise quantification of the composition of second phase particles from their early stages, leading to improved understanding of the thermodynamic and kinetic mechanisms of phase formation and quantify structure-property relationships. Here we demonstrate how approaches developed for small-angle scattering can be adapted to atom probe tomography. By exploiting nearest-neighbor distributions and radial distribution function, we introduce a parameter free methodology to efficiently extract information such as particle size, composition, volume fraction, number density and inter-particle distance. We demonstrate the strength of this approach in the analysis of a precipitation-hardened model Al-Zn-Mg-Cu high-strength lightweight alloy. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.scriptamat.2018.05.024
  • 2017 • 145 A novel type of Co–Ti–Cr-base γ/γ′ superalloys with low mass density
    Zenk, C.H. and Povstugar, I. and Li, R. and Rinaldi, F. and Neumeier, S. and Raabe, D. and Göken, M.
    Acta Materialia 135 244-251 (2017)
    A γ′ strengthened Co–Ti–Cr superalloy is presented with a mass density ∼14 % below that of typical Co–Al–W-based alloys. The lattice misfit is sufficiently low to form coherent cuboidal γ′ precipitates. Atom probe tomography shows that Cr partitions to the γ phase, but increases the γ′ volume fraction compared to a binary Co-Ti alloy to more than 60 %. The solubility of Cr in the γ′ phase is significantly higher than expected from previously published values. The γ′ solvus temperature is above 1100 °C. The yield strength shows a distinct increase above 600 °C surpassing that of Co–9Al–8W (at.%) and conventional Co-base superalloys, even more so when it is normalized by the mass density. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.06.024
  • 2017 • 144 Cd and Impurity Redistribution at the CdS/CIGS Interface after Annealing of CIGS-Based Solar Cells Resolved by Atom Probe Tomography
    Koprek, A. and Cojocaru-Miredin, O. and Wuerz, R. and Freysoldt, C. and Gault, B. and Raabe, D.
    IEEE Journal of Photovoltaics 7 313-321 (2017)
    Cd and impurity redistribution in the vicinity of the CdS/C(In,Ga)Se2 (CIGS) interface is studied by means of atom probe tomography. We find an increase of the Cd content in the CIGS layer and redistribution of O (in form of O+ and OH+) at the CdS/CIGS interface after annealing the samples at 200, 250, and 300 °C. About 0.2 at% of Na impurity is observed at the interface, across the range of heat treatments performed here. Simultaneously, the J-V measurements of the treated samples show a drop in the open-circuit voltage and fill factor, and thus of the cell efficiency, compared with the untreated sample. © 2011-2012 IEEE.
    view abstractdoi: 10.1109/JPHOTOV.2016.2629841
  • 2017 • 143 Cd and In-doping in thin film SnO2
    Schell, J. and Lupascu, D.C. and Wilson Carbonari, A. and Domingues Mansano, R. and Freitas, R.S. and Gonçalves, J.N. and Dang, T.T. and Vianden, R.
    Journal of Applied Physics 121 (2017)
    In this paper, we investigate the effects of doping in the local structure of SnO2 by measuring the hyperfine interactions at impurity nuclei using the Time Differential Perturbed Gamma-Gamma Angular Correlation (TDPAC) method in addition to density functional theory simulations. The hyperfine field parameters have been probed as a function of the temperature in thin film samples. The experimental results reveal that 117Cd/In and 111In/Cd are incorporated and stabilized in the SnO2 lattice replacing the cationic site. Significant differences in the electric field gradient were observed from TDPAC measurements with both the probe nuclei. Furthermore, the absence of strongly damped spectra further indicates that implanted Cd atoms (for 117Cd/In probe nuclei measurements) easily occupy regular substitutional Sn sites with good stability. The simulated value for the electric field gradient obtained with the first oxygen neighbor removed is closer to the experimental value observed for 117Cd, which also indicates this configuration as stable and present in the sample. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4983669
  • 2017 • 142 Collision frequency determination of low-pressure plasmas based on RF-reflectometry
    Schulz, C. and Rolfes, I. and Oberberg, M. and Awakowicz, P.
    European Microwave Week 2016: "Microwaves Everywhere", EuMW 2016 - Conference Proceedings; 46th European Microwave Conference, EuMC 2016 807-810 (2017)
    This paper introduces a robust collision frequency determination of low-pressure plasma processes. Based on the multipole resonance probe and in-situ reflection measurements, the input admittance of the system probe-plasma can be calculated. It can be used to determine the collision frequency via its width. The proposed evaluation is investigated by numerous parameter variations within 3D electromagnetic field simulations. For plasma electron frequencies, which can be determined simultaneously, over 4 GHz or collision frequencies below 200 MHz, a two-step compensation is necessary for reliable results. Based on the proposed compensation, a maximum error of ± 5% can be reached within the simulations. The final measurements in an argon-oxygen plasma confirm the suitability of the presented evaluation. © 2016 EuMA.
    view abstractdoi: 10.1109/EuMC.2016.7824466
  • 2017 • 141 Comparison of maraging steel micro- and nanostructure produced conventionally and by laser additive manufacturing
    Jägle, E.A. and Sheng, Z. and Kürnsteiner, P. and Ocylok, S. and Weisheit, A. and Raabe, D.
    Materials 10 (2017)
    Maraging steels are used to produce tools by Additive Manufacturing (AM) methods such as Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Although it is well established that dense parts can be produced by AM, the influence of the AM process on the microstructure-in particular the content of retained and reversed austenite as well as the nanostructure, especially the precipitate density and chemistry, are not yet explored. Here, we study these features using microhardness measurements, Optical Microscopy, Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Atom Probe Tomography (APT) in the as-produced state and during ageing heat treatment. We find that due to microsegregation, retained austenite exists in the as-LMD- and as-SLM-produced states but not in the conventionally-produced material. The hardness in the as-LMD-produced state is higher than in the conventionally and SLM-produced materials, however, not in the uppermost layers. By APT, it is confirmed that this is due to early stages of precipitation induced by the cyclic re-heating upon further deposition-i.e., the intrinsic heat treatment associated with LMD. In the peak-aged state, which is reached after a similar time in all materials, the hardness of SLM- and LMD-produced material is slightly lower than in conventionally-produced material due to the presence of retained austenite and reversed austenite formed during ageing. © 2017 by the authors.
    view abstractdoi: 10.3390/ma10010008
  • 2017 • 140 Comparison of the quantitative analysis performance between pulsed voltage atom probe and pulsed laser atom probe
    Takahashi, J. and Kawakami, K. and Raabe, D.
    Ultramicroscopy 175 105-110 (2017)
    The difference in quantitative analysis performance between the voltage-mode and laser-mode of a local electrode atom probe (LEAP3000X HR) was investigated using a Fe-Cu binary model alloy. Solute copper atoms in ferritic iron preferentially field evaporate because of their significantly lower evaporation field than the matrix iron, and thus, the apparent concentration of solute copper tends to be lower than the actual concentration. However, in voltage-mode, the apparent concentration was higher than the actual concentration at 40 K or less due to a detection loss of matrix iron, and the concentration decreased with increasing specimen temperature due to the preferential evaporation of solute copper. On the other hand, in laser-mode, the apparent concentration never exceeded the actual concentration, even at lower temperatures (20 K), and this mode showed better quantitative performance over a wide range of specimen temperatures. These results indicate that the pulsed laser atom probe prevents both detection loss and preferential evaporation under a wide range of measurement conditions. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2017.01.015
  • 2017 • 139 Confined chemical and structural states at dislocations in Fe-9wt%Mn steels: A correlative TEM-atom probe study combined with multiscale modelling
    Kwiatkowski da Silva, A. and Leyson, G. and Kuzmina, M. and Ponge, D. and Herbig, M. and Sandlöbes, S. and Gault, B. and Neugebauer, J. and Raabe, D.
    Acta Materialia 124 305-315 (2017)
    We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.11.013
  • 2017 • 138 Electron transport in stepped Bi2Se3 thin films
    Bauer, S. and Bobisch, C.A.
    Journal of Physics Condensed Matter 29 (2017)
    We analyse the electron transport in a 16 quintuple layer thick stepped Bi2Se3 film grown on Si(1 1 1) by means of scanning tunnelling potentiometry (STP) and multi-point probe measurements. Scanning tunnelling microscopy images reveal that the local structure of the Bi2Se3 film is dominated by terrace steps and domain boundaries. From a microscopic study on the nm scale by STP, we find a mostly linear gradient of the voltage on the Bi2Se3 terraces which is interrupted by voltage drops at the position of the domain boundaries. The voltage drops indicate that the domain boundaries are scatterers for the electron transport. Macroscopic resistance measurements (2PP and in-line 4PP measurement) on the μm scale support the microscopic results. An additional rotational square 4PP measurement shows an electrical anisotropy of the sheet conductance parallel and perpendicular to the Bi2Se3 steps of about 10%. This is a result of the anisotropic step distribution at the stepped Bi2Se3 surface while domain boundaries are distributed isotropically. The determined value of the conductivity of the Bi2Se3 steps of about 1000 S cm-1 verifies the value of an earlier STP study. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7a3c
  • 2017 • 137 Fluorination of supramolecular liquid crystals-tuning tool and analytical probe
    Spengler, M. and Dong, R.Y. and Michal, C.A. and Pfletscher, M. and Giese, M.
    Journal of Materials Chemistry C 5 2235-2239 (2017)
    A study on the impact of fluorination on the properties of photo-responsive hydrogen-bonded liquid crystals is reported. The chosen modular approach allows for efficient screening of a variety of hydrogen-bonded assemblies. Tremendous effects on the mesophase stabilization and morphology are described. In addition, the fluorine substituents are used as analytical probes for 19F solid-state NMR studies, providing insight into the molecular alignment of the assemblies. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6tc05472b
  • 2017 • 136 Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency
    Schwarz, T. and Cojocaru-Mirédin, O. and Mousel, M. and Redinger, A. and Raabe, D. and Choi, P.-P.
    Acta Materialia 132 276-284 (2017)
    Atom probe tomography and transmission electron microscopy are used to study the formation of nano-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films. For a Cu-rich precursor, which was deposited at 320 °C under Cu- and Zn-rich growth conditions, Cu2-xSe grains at the surface are detected. During annealing the precursor at 500 °C in a SnSe + Se atmosphere most of the Cu2-xSe is transformed to Cu2ZnSnSe4 via the consumption of excessive ZnSe and incorporation of Sn. However, atom probe tomography studies also reveal the formation of various nanometer-sized Cu-Sn-Se particles close to the CdS/Cu2ZnSnSe4 interface. One of those particles has a composition close to the Cu2SnSe3 compound. This phase has a smaller band gap than Cu2ZnSnSe4 and is proposed to lead to a significant drop in the open-circuit voltage and could be the main cause for a detrimental p-n junction and the zero efficiency of the final device. Possible effects of the other phases on solar cell performance and formation mechanisms are discussed as well. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.04.056
  • 2017 • 135 High-resolution analysis of photoanodes for water splitting by means of scanning photoelectrochemical microscopy
    Conzuelo, F. and Sliozberg, K. and Gutkowski, R. and Grutzke, S. and Nebe, M. and Schuhmann, W.
    Analytical Chemistry 89 1222-1228 (2017)
    In pursuance of efficient tools for the local analysis and characterization of novel photoelectrocatalytic materials, several SECM-based techniques have been developed, aiming on the combined benefit of a local irradiation of the analyzed sample and a microelectrode probe for the localized electrochemical analysis of the surface. We present the development and application of scanning photoelectrochemical microscopy (SPECM) for the laterally resolved characterization of photoelectrocatalytic materials. Particularly, the system was developed for the photoelectrochemical characterization of n-type semiconductor- based photoanodes for water splitting. By using the tip microelectrode simultaneously for local irradiation and as an electrochemical probe, SPECM was capable to simultaneously provide information about the local photocurrent generated at the sample under irradiation and to detect the photoelectrocatalytically evolved oxygen at the microelectrode. In combination with a novel means of irradiation of the interrogated sample, local analysis of semiconductor materials for light-induced water splitting with improved lateral resolution is achieved. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b03706
  • 2017 • 134 Interfacial hydrogen localization in austenite/martensite dual-phase steel visualized through optimized silver decoration and scanning Kelvin probe force microscopy
    Nagashima, T. and Koyama, M. and Bashir, A. and Rohwerder, M. and Tasan, C.C. and Akiyama, E. and Raabe, D. and Tsuzaki, K.
    Materials and Corrosion 68 306-310 (2017)
    The hydrogen distribution in an austenite-martensite dual-phase steel was investigated using silver decoration and scanning Kelvin probe force microscopy. The silver decoration technique optimized for spacial resolution reveals interfacial segregation of hydrogen along the plate-type martensite-martensite grain boundaries. In addition, the scanning Kelvin probe force microscopy kinetically elucidates that hydrogen preferentially diffused out from the martensite-martensite grain boundaries. These preferential sites of hydrogen desorption correspond to the regions of hydrogen-assisted damage. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201609104
  • 2017 • 133 Long-term thermal stability of nanoclusters in ODS-Eurofer steel: An atom probe tomography study
    Zilnyk, K.D. and Pradeep, K.G. and Choi, P. and Sandim, H.R.Z. and Raabe, D.
    Journal of Nuclear Materials 492 142-147 (2017)
    Oxide-dispersion strengthened materials are important candidates for several high-temperature structural applications in advanced nuclear power plants. Most of the desirable mechanical properties presented by these materials are due to the dispersion of stable nanoparticles in the matrix. Samples of ODS-Eurofer steel were annealed for 4320 h (6 months) at 800 °C. The material was characterized using atom probe tomography in both conditions (prior and after heat treatment). The particles number density, size distribution, and chemical compositions were determined. No significant changes were observed between the two conditions indicating a high thermal stability of the Y-rich nanoparticles at 800 °C. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jnucmat.2017.05.027
  • 2017 • 132 Low Resistive Edge Contacts to CVD-Grown Graphene Using a CMOS Compatible Metal
    Shaygan, M. and Otto, M. and Sagade, A.A. and Chavarin, C.A. and Bacher, G. and Mertin, W. and Neumaier, D.
    Annalen der Physik (2017)
    doi: 10.1002/andp.201600410
  • 2017 • 131 Massive nanoprecipitation in an Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition
    Kürnsteiner, P. and Wilms, M.B. and Weisheit, A. and Barriobero-Vila, P. and Jägle, E.A. and Raabe, D.
    Acta Materialia 129 52-60 (2017)
    Due to the layer-by-layer build-up of additively manufactured parts, the deposited material experiences a cyclic re-heating in the form of a sequence of temperature pulses. In the current work, this “intrinsic heat treatment (IHT)” was exploited to induce the precipitation of NiAl nanoparticles in an Fe-19Ni-xAl (at%) model maraging steel, a system known for rapid clustering. We used Laser Metal Deposition (LMD) to synthesize compositionally graded specimens. This allowed for the efficient screening of effects associated with varying Al contents ranging from 0 to 25 at% and for identifying promising concentrations for further studies. Based on the existence of the desired martensitic matrix, an upper bound for the Al concentration of 15 at% was defined. Owing to the presence of NiAl precipitates as observed by Atom Probe Tomography (APT), a lower bound of 3–5 at% Al was established. Within this concentration window, increasing the Al concentration gave rise to an increase in hardness by 225 HV due to an exceptionally high number density of 1025 NiAl precipitates per m3, as measured by APT. This work demonstrates the possibility of exploiting the IHT of the LMD process for the production of samples that are precipitation strengthened during the additive manufacturing process without need for any further heat treatment. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.02.069
  • 2017 • 130 Nanostructure of and structural defects in a Mo2BC hard coating investigated by transmission electron microscopy and atom probe tomography
    Gleich, S. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Soler, R. and Pradeep, K.G. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Journal of Applied Physics 122 (2017)
    In this work, the nanostructure of a Mo2BC hard coating was determined by several transmission electron microscopy methods and correlated with the mechanical properties. The coating was deposited on a Si (100) wafer by bipolar pulsed direct current magnetron sputtering from a Mo2BC compound target in Ar at a substrate temperature of 630 °C. Transmission electron microscopy investigations revealed structural features at various length scales: bundles (30 nm to networks of several micrometers) consisting of columnar grains (∼10 nm in diameter), grain boundary regions with a less ordered atomic arrangement, and defects including disordered clusters (∼1.5 nm in diameter) as well as stacking faults within the grains. The most prominent defect with a volume fraction of ∼0.5% is the disordered clusters, which were investigated in detail by electron energy loss spectroscopy and atom probe tomography. The results provide conclusive evidence that Ar is incorporated into the Mo2BC film as disordered Ar-rich Mo-B-C clusters of approximately 1.5 nm in diameter. Hardness values of 28 ± 1 GPa were obtained by nanoindentation tests. The Young's modulus of the Mo2BC coating exhibits a value of 462 ± 9 GPa, which is consistent with ab initio calculations for crystalline and defect free Mo2BC and measurements of combinatorically deposited Mo2BC thin films at a substrate temperature of 900 °C. We conclude that a reduction of the substrate temperature of 270 °C has no significant influence on hardness and Young's modulus of the Mo2BC hard coating, even if its nanostructure exhibits defects. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4999304
  • 2017 • 129 Plasma diagnostics in dielectric deposition processes
    Schulz, C. and Rolfes, I.
    Proceedings of IEEE Sensors (2017)
    This contribution presents an in-situ plasma probe, which is capable to measure precisely in the challenging environment of deposition processes. The probe is inserted into the plasma in order to determine critical parameters, which are required for a process control. Therefore, the effects of deposited dielectric materials, which adsorb onto the probe, are investigated within numerous pseudo deposition processes by 3D electromagnetic field simulations. Here, the adsorbed material is varied in its relative permittivity and layer thickness for two different loss tangents. The corresponding evaluations demonstrate the suitability and the prospects of the probe within these simulations. The final measurements in an argon-oxygen plasma, depositing TiO2, confirm the insensitivity of the probe. © 2016 IEEE.
    view abstractdoi: 10.1109/ICSENS.2016.7808810
  • 2017 • 128 Recent progress in microstructural hydrogen mapping in steels: quantification, kinetic analysis, and multi-scale characterisation
    Koyama, M. and Rohwerder, M. and Tasan, C.C. and Bashir, A. and Akiyama, E. and Takai, K. and Raabe, D. and Tsuzaki, K.
    Materials Science and Technology (United Kingdom) 1-16 (2017)
    This paper gives an overview of recent progress in microstructure-specific hydrogen mapping techniques. The challenging nature of mapping hydrogen with high spatial resolution, i.e. at the scale of finest microstructural features, led to the development of various methodologies: thermal desorption spectrometry, silver decoration, the hydrogen microprint technique, secondary ion mass spectroscopy, atom probe tomography, neutron radiography, and the scanning Kelvin probe. These techniques have different characteristics regarding spatial and temporal resolution associated with microstructure-sensitive hydrogen detection. Employing these techniques in a site-specific manner together with other microstructure probing methods enables multi-scale, quantitative, three-dimensional, high spatial, and kinetic resolution hydrogen mapping, depending on the specific multi-probe approaches used. Here, we present a brief overview of the specific characteristics of each method and the progress resulting from their combined application to the field of hydrogen embrittlement. © 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/02670836.2017.1299276
  • 2017 • 127 Role of Nanostructuring and Microstructuring in Silver Antimony Telluride Compounds for Thermoelectric Applications
    Cojocaru-Mirédin, O. and Abdellaoui, L. and Nagli, M. and Zhang, S. and Yu, Y. and Scheu, C. and Raabe, D. and Wuttig, M. and Amouyal, Y.
    ACS Applied Materials and Interfaces 9 14779-14790 (2017)
    Thermoelectric (TE) materials are of utmost significance for conversion of heat flux into electrical power in the low-power regime. Their conversion efficiency depends strongly on the microstructure. AgSbTe2-based compounds are high-efficiency TE materials suitable for the mid-temperature range. Herein, we explore an Ag16.7Sb30Te53.3 alloy (at %) subjected to heat treatments at 380 °C for different durations aimed at nucleation and coarsening of Sb2Te3-precipitates. To characterize the Sb2Te3-precipitation, we use a set of methods combining thermal and electrical measurements in concert with transmission electron microscopy and atom probe tomography. We find correlations between the measured TE transport coefficients and the applied heat treatments. Specifically, the lowest electrical and thermal conductivity values are obtained for the as-quenched state, whereas the highest values are observed for alloys aged for 8 h. In turn, long-term heat treatments result in intermediate values of transport coefficients. We explain these findings in terms of interplay between precipitate formation and variations in the matrix composition, highlighting the importance of thermal stability of the material under service conditions. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b00689
  • 2017 • 126 TDPAC study of Fe-implanted titanium dioxide thin films
    Schell, J. and Schaaf, P. and Vetter, U. and Lupascu, D.C.
    AIP Advances 7 (2017)
    Fe-doping in TiO2 has been proven to improve several of its properties, including the photocatalytic activity. Time-differential perturbed angular correlation (TDPAC) as the applied spectroscopy method is particularly interesting because it can probe the electric and magnetic interactions on a local atomic scale. In this work the hyperfine interactions on 111Cd atoms substituting Ti atoms in TiO2 due to nearby Fe atoms also diluted within the TiO2 lattice were measured as a function of temperature. The results review two fractions with distinct quadrupole interaction parameters. One site, occupied by the 111Cd probes, presents the smaller quadrupole interaction frequency, namely υq1 = 45 MHz, and can be ascribed to sites that are more distant from the Fe substitutional site whereas the second site characterized with υq2 = 62 MHz is related to Cd probe atoms that are closer to the Fe defect. Additionally, the system has been characterized using electron dispersive spectroscopy. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4994247
  • 2017 • 125 Upgrade of a commercial four-probe scanning tunneling microscopy system
    Ma, R. and Huan, Q. and Wu, L. and Yan, J. and Zou, Q. and Wang, A. and Bobisch, C.A. and Bao, L. and Gao, H.-J.
    Review of Scientific Instruments 88 (2017)
    Upgrade of a commercial ultra-high vacuum four-probe scanning tunneling microscopy system for atomic resolution capability and thermal stability is reported. To improve the mechanical and thermal performance of the system, we introduced extra vibration isolation, magnetic damping, and double thermal shielding, and we redesigned the scanning structure and thermal links. The success of the upgrade is characterized by its atomically resolved imaging, steady cooling down cycles with high efficiency, and standard transport measurement capability. Our design may provide a feasible way for the upgrade of similar commercial systems. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4986466
  • 2016 • 124 Analysis of the contrast in normal-incidence surface plasmon photoemission microscopy in a pump–probe experiment with adjustable polarization
    Podbiel, D. and Kahl, P. and Meyer zu Heringdorf, F.-J.
    Applied Physics B: Lasers and Optics 122 (2016)
    We investigate the fringe contrast in surface plasmon polariton-based two-photon photoemission microscopy in a normal-incidence geometry. In a pump–probe experiment with freely adjustable polarization of the probe pulse, we find a maximum contrast whenever the probe pulse polarization is parallel (or anti-parallel) to the propagation direction of the surface plasmon polariton wave packet. The experimental observation is compared to a wave simulation based on the known TM solution for surface plasmon polaritons. We estimate that at the Au/vacuum interface the in-plane component of the electric field of the surface plasmon polariton inside the metal is about five times larger than its out-of-plane component. We conclude that the locations of maximum plasmon-related nonlinear photoemission yield in a pump–probe experiment are the ones where the in-plane component of the electric field of the surface plasmon polariton is maximal. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-016-6363-6
  • 2016 • 123 Atom probe tomography of intermetallic phases and interfaces formed in dissimilar joining between Al alloys and steel
    Lemmens, B. and Springer, H. and Duarte, M.J. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Materials Characterization 120 268-272 (2016)
    While Si additions to Al are widely used to reduce the thickness of the brittle intermetallic seam formed at the interface during joining of Al alloys to steel, the underlying mechanisms are not clarified yet. The developed approach for the site specific atom probe tomography analysis revealed Si enrichments at grain and phase boundaries between the θ (Fe4Al13) and η (Fe2Al5) phase, up to about ten times that of the concentration in Al. The increase in Si concentration could play an important role for the growth kinetics of the intermetallic phases formed for example in hot-dip aluminizing of steel. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2016.09.008
  • 2016 • 122 Atom probe tomography of metallic nanostructures
    Hono, K. and Raabe, D. and Ringer, S.P. and Seidman, D.N.
    MRS Bulletin 41 23-29 (2016)
    This article focuses on four topics that demonstrate the importance of atom probe tomography for obtaining nanostructural information that provides deep insights into the structures of metallic alloys, leading to a better understanding of their properties. First, we discuss the microstructure-coercivity relationship of Nd-Fe-B permanent magnets, essential for developing a higher coercivity magnet. Second, we address equilibrium segregation at grain boundaries with the aim of manipulating their interfacial structure, energies, compositions, and properties, thereby enabling beneficial material behavior. Third, recent progress in the search to extend the performance and practicality of the next generation of advanced high-strength steels is discussed. Finally, a study of the temporal evolution of a Ni-Al-Cr alloy through the stages of nucleation, growth, and coarsening (Ostwald ripening) and its relationship with the predictions of a model for quasi-stationary coarsening is described. This information is critical for understanding high-Temperature mechanical properties of the material. © Copyright Materials Research Society 2016.
    view abstractdoi: 10.1557/mrs.2015.314
  • 2016 • 121 Characterization of azimuthal and radial velocity fields induced by rotors in flows with a low Reynolds number
    Köhler, J. and Friedrich, J. and Ostendorf, A. and Gurevich, E.L.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 93 (2016)
    We theoretically and experimentally investigate the flow field that emerges from a rodlike microrotor rotating about its center in a nonaxisymmetric manner. A simple theoretical model is proposed that uses a superposition of two rotlets as a fundamental solution to the Stokes equation. The predictions of this model are compared to measurements of the azimuthal and radial microfluidic velocity field components that are induced by a rotor composed of fused microscopic spheres. The rotor is driven magnetically and the fluid flow is measured with the help of a probe particle fixed by an optical tweezer. We find considerable deviations of the mere azimuthal flow pattern induced by a single rotating sphere as it has been reported by Di Leonardo et al. [Phys. Rev. Lett. 96, 134502 (2006)PRLTAO0031-900710.1103/PhysRevLett.96.134502]. Notably, the presence of a radial velocity component that manifests itself by an oscillation of the probe particle with twice the rotor frequency is observed. These findings open up a way to discuss possible radial transport in microfluidic devices. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.93.023108
  • 2016 • 120 Chemical Surface Characterization of Activated Carbons by Adsorption Excess of Probe Molecules
    Treese, J. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemical Engineering and Technology 39 1144-1150 (2016)
    Activated carbons are one of the most common industrial adsorbents in liquid-phase applications. It is known that the surface groups of the activated carbon can have a significant influence on the adsorption process from the liquid phase. Therefore, it is desirable to measure surface groups on activated carbons. This opens up the possibility to use group-contribution methods to model and predict adsorption isotherms. An idea is presented to characterize the inner surface of activated carbons by three types of surface groups: aromatic, polar, and nonpolar surface groups. The amounts of these surface groups were calculated from excess adsorption isotherms of probe molecules on ten activated carbons. This lays the groundwork for further simulation studies of liquid-phase adsorption using group-contribution methods. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ceat.201500571
  • 2016 • 119 Combined atom probe tomography and density functional theory investigation of the Al off-stoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low density steel
    Yao, M.J. and Dey, P. and Seol, J.-B. and Choi, P. and Herbig, M. and Marceau, R.K.W. and Hickel, T. and Neugebauer, J. and Raabe, D.
    Acta Materialia 106 229-238 (2016)
    We report on the investigation of the off-stoichiometry and site-occupancy of κ-carbide precipitates within an austenitic (γ), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the κ-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L′12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the κ/γ mismatch, which facilitates occupation of Al sites in κ-carbide by Mn atoms (Mnγ Al anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.01.007
  • 2016 • 118 Diagnostics of plasma processes based on parallelized spatially resolved in-situ reflection measurements
    Schulz, C. and Runkel, J. and Oberberg, M. and Awakowicz, P. and Rolfes, I.
    IEEE Transactions on Microwave Theory and Techniques 64 616-623 (2016)
    A parallelized in-situ plasma measurement setup, consisting of two multipole resonance probes (MRP), a passive signal divider, and two coaxial cables with different lengths is presented in this contribution. The combined reflection coefficient of the applied probes is measured, separated in the time domain, and evaluated. Here, each MRP is able to measure the spatially resolved plasma electron density via its resonance behavior precisely and quasi-simultaneously. Furthermore, the return loss (RL) changes with the collision frequency, which can be detected for each probe. The parallelization and the applied signal processing are confirmed by simulations and combined measurements in CST Schematic as well as by in-situ measurements in an argon plasma. The resulting error is below 1% for the resonance frequency and below 8% for the corresponding RL. Hence, the input power and gas pressure of a plasma process can be controlled effectively. © 2015 IEEE.
    view abstractdoi: 10.1109/TMTT.2015.2510653
  • 2016 • 117 Effect of Si on the acceleration of bainite transformation by pre-existing martensite
    Toji, Y. and Matsuda, H. and Raabe, D.
    Acta Materialia 116 250-262 (2016)
    Bainite transformation was investigated focusing on the influence of pre-existing martensite on the transformation kinetics, morphology and crystallographic orientation of subsequently formed bainite using EBSD and atom probe tomography. Two 1.1 wt% C-3wt.%Mn steels with and without 2 wt% Si were used to clarify the effect of Si. Steels were rapidly cooled from 900 °C to 300 °C and held at this temperature, or quenched from 900 °C once in water to generate approximately 30 vol% martensite followed by holding at 300 °C. Bainite transformation was clearly accelerated by pre-existing martensite in both Si-containing and Si-free steels. Bainite surrounds the pre-existing martensite in the Si-free steel, whereas it grows to the interior of the austenite grains in the steel containing 2 wt% Si. The major orientation relationship between bainite and adjacent austenite was changed by the presence of martensite from Nishiyama-Wassermann (N-W) to Greninger-Troiano (G-T) regardless of Si content. Clear carbon partitioning from martensite into austenite was observed prior to the bainite transformation in the 2 wt% Si steel, which was not observed in the Si-free steel. We suggest that the dislocations introduced by the martensite transformation act as a primary factor accelerating the bainite transformation when martensite is introduced prior to the bainite transformation. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.06.044
  • 2016 • 116 Elemental distributions within multiphase quaternary Pb chalcogenide thermoelectric materials determined through three-dimensional atom probe tomography
    Yamini, A. and Li, T. and Mitchell, D.R.G. and Cairney, J.M.
    Nano Energy 26 157-163 (2016)
    Nanostructured multiphase p-type lead chalcogenides have shown the highest efficiencies amongst thermoelectric materials. However, their electronic transport properties have been described assuming homogenous distribution of dopants between phases. Here, we have analyzed elemental distributions in precipitates and matrices of nanostructured multiphase quaternary Pb chalcogenides doped to levels below and above the solubility limit of the matrix, using three-dimensional atom probe tomography. We demonstrate that partitioning of sodium and selenium occur between the matrix and secondary phase in both lightly- and heavily-doped compounds and that the concentrations of sodium and selenium in precipitates are higher than those in the matrices. This can contribute to the transport properties of such multiphase compounds The sodium concentration reached ~3 at% in sulfur-rich (PbS) precipitates and no nano precipitates of Na-rich phases were observed within either phase, a result that is supported by high resolution TEM analysis, indicating that the solubility limit of sodium in PbS is much higher than previously thought. However, non-equilibrium segregation of sodium is identified at the precipitates/matrix interfaces. These findings can lead to further advances in designing and characterizing multiphase thermoelectric materials. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.nanoen.2016.05.019
  • 2016 • 115 Elemental partitioning, lattice misfit and creep behaviour of Cr containing gammaprime strengthened Co base superalloys
    Povstugar, I. and Zenk, C.H. and Li, R. and Choi, P.-P. and Neumeier, S. and Dolotko, O. and Hoelzel, M. and Göken, M. and Raabe, D.
    Materials Science and Technology (United Kingdom) 32 220-225 (2016)
    Novel Cr containing Co-Al-W base superalloys were studied by atom probe tomography and neutron diffraction. Cr is found to predominantly partition to the gamma matrix and decrease partitioning of W to gammaprime. Furthermore, Cr significantly enhances the gammaprime volume fraction, decreases the gamma/gammaprime lattice misfit and deteriorates the creep resistance. Addition of Ni to the Cr containing alloys affects partitioning of W and Al, further decreases the lattice misfit and results in the formation of irregularly shaped precipitates. Al, W and Cr tend to occupy the ‘B'sublattice in the gammaprime-A3B phase (L12 type), while Co and Ni reside in the ‘A' sublattice. © 2016 Institute of Materials.
    view abstractdoi: 10.1179/1743284715Y.0000000112
  • 2016 • 114 Formation of nanosized grain structure in martensitic 100Cr6 bearing steels upon rolling contact loading studied by atom probe tomography
    Li, Y.J. and Herbig, M. and Goto, S. and Raabe, D.
    Materials Science and Technology (United Kingdom) 32 1100-1105 (2016)
    To understand the origin of white etching cracks (WECs), a systematic microstructural characterisation in the regions affected from the near-surface region down to the subsurface layers where WECs occur is necessary. As a starting point, we focus on the near-surface region of an axial thrust bearing, made of martensitic 100Cr6 steel, to study the influence of rolling contact loading on the microstructure and the resulting distributions of the major alloying elements C and Cr using atom probe tomography. We find that upon rolling contact loading the original plate-like martensitic structure evolves into a nanosized equiaxed grain structure with C segregation up to 5 at.-% at the grain boundaries. Cementite particles, located at grain boundaries and triple junctions, undergo spheroidisation. The originally homogeneously distributed Cr becomes enriched in spheroidised cementite particles. The microstructural changes give strong hints that rolling contact loading induces plastic deformation and an increased temperature on the near-surface region. This paper is part of a Themed Issue on Recent developments in bearing steels. © 2016 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1080/02670836.2015.1120458
  • 2016 • 113 Growth of bainitic ferrite and carbon partitioning during the early stages of bainite transformation in a 2 mass% silicon steel studied by in situ neutron diffraction, TEM and APT
    Timokhina, I.B. and Liss, K.D. and Raabe, D. and Rakha, K. and Beladi, H. and Xiong, X.Y. and Hodgson, P.D.
    Journal of Applied Crystallography 49 399-414 (2016)
    In situ neutron diffraction, transmission electron microscopy (TEM) and atom probe tomography (APT) have been used to study the early stages of bainite transformation in a 2 mass% Si nano-bainitic steel. It was observed that carbon redistribution between the bainitic ferrite and retained austenite at the early stages of the bainite transformation at low isothermal holding occurred in the following sequence: (i) formation of bainitic ferrite nuclei within carbon-depleted regions immediately after the beginning of isothermal treatment; (ii) carbon partitioning immediately after the formation of bainitic ferrite nuclei but substantial carbon diffusion only after 33 min of bainite isothermal holding; (iii) formation of the carbon-enriched remaining austenite in the vicinity of bainitic laths at the beginning of the transformation; (iv) segregation of carbon to the dislocations near the austenite/ferrite interface; and (v) homogeneous redistribution of carbon within the remaining austenite with the progress of the transformation and with the formation of bainitic ferrite colonies. Bainitic ferrite nucleated at internal defects or bainite/austenite interfaces as well as at the prior austenite grain boundary. Bainitic ferrite has been observed in the form of an individual layer, a colony of layers and a layer with sideplates at the early stages of transformation. © 2016 International Union of Crystallography.
    view abstractdoi: 10.1107/S1600576716000418
  • 2016 • 112 Labeling and Selective Inactivation of Gram-Positive Bacteria Employing Bimodal Photoprobes with Dual Readouts
    Galstyan, A. and Block, D. and Niemann, S. and Grüner, M.C. and Abbruzzetti, S. and Oneto, M. and Daniliuc, C.G. and Hermann, S. and Viappiani, C. and Schäfers, M. and Löffler, B. and Strassert, C.A. and Faust, A.
    Chemistry - A European Journal 22 5243-5252 (2016)
    Carbohydrate-conjugated silicon(IV) phthalocyanines with bimodal photoactivity were developed as probes with both fluorescent labeling and photosensitizing capabilities, and the concomitant fluorescent labeling and photoinduced inactivation of Gram-positive and Gram-negative models was explored. The maltohexaose-conjugated photoprobe provides a dual readout to distinguish between both groups of pathogens, as only the Gram-positive species was inactivated, even though both appeared labeled with near-infrared luminescence. Antibiotic resistance did not hinder the phototoxic effect, as even the methicillin-resistant pathogen Staphylococcus aureus (MRSA) was completely photoinactivated. Time-resolved confocal fluorescence microscopy analysis suggests that the photoprobe sticks onto the outer rim of the microorganisms, explaining the resistance of Gram-negative species on the basis of their membrane constitution. The mannose-conjugated photoprobe yields a different readout because it is able to label and to inactivate only the Gram-positive strain. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201504935
  • 2016 • 111 Monitoring of low pressure plasmas with a calibrated probe
    Runkel, J. and Schulz, C. and Rolfes, I. and Oberberg, M. and Awakowicz, P.
    GeMiC 2016 - 2016 German Microwave Conference 43-46 (2016)
    In this contribution the use of the planar multipole resonance probe (pMRP) as a monitoring tool for low pressure plasmas is presented. By 3D electromagnetic simulations, the probe's ability to monitor two important plasma parameters is investigated and a full one-port calibration is applied to ensure maximum monitoring precision. Measurements in a double inductively coupled argon plasma confirm the simulation results and prove the suitability of the calibrated pMRP for precise plasma monitoring. © 2016 Institut fur Mikrowellen und Antennentechnik-IMA.
    view abstractdoi: 10.1109/GEMIC.2016.7461551
  • 2016 • 110 New insights into the phase transformations to isothermal ω and ω-assisted α in near β-Ti alloys
    Li, T. and Kent, D. and Sha, G. and Stephenson, L.T. and Ceguerra, A.V. and Ringer, S.P. and Dargusch, M.S. and Cairney, J.M.
    Acta Materialia 106 353-366 (2016)
    For multicomponent near-β alloys, we have investigated the mechanisms responsible for the β-to-ω and ω-to-α phase transformations upon isothermal ageing at 573 K. Experimental evidence from atom probe tomography and aberration-corrected high-resolution transmission electron microscopy indicates that the formation of isothermal ω involves a structural reconstruction assisted by nanoscale spinodal decomposition of the β matrix, prior to the specific chemistry change required to form ω, rather than a mixed-mode process with structure and chemistry changes occurring simultaneously as has been previously suggested. First, incommensurate embryonic ω evolve via a displacive mechanism within Mo-lean regions created by second-order coherent spinodal decomposition of the β matrix. The subtle spinodal decomposition in β and chemistry of embryonic ω are carefully analysed by an advanced atom probe data analysis algorithm. When the size of embryonic É·exceeds a critical value, commensurate isothermal É·forms through the exit of the other alloying solutes. O-rich regions present at the isothermal ω/β interface provide potent sites for the formation of α. The concurrent compositional partitioning of solutes in É·and α indicates the transformation from ω to α involves both a rapid lattice reconstruction at the ω/α interface and a slow Al diffusion at the α/β, therefore a mixed-mode displacive-diffusive process. This study provides novel experimental evidence to understand the much-disputed transformation processes and elucidate the mechanisms responsible for these important phase transformations. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.12.046
  • 2016 • 109 Spatio-temporal imaging of surface plasmon polaritons in two photon photoemission microscopy
    Meyer zu Heringdorf, F.-J. and Podbiel, D. and Raß, N. and Makris, A. and Buckanie, N.M. and Kahl, P.A.
    Proceedings of SPIE - The International Society for Optical Engineering 9921 (2016)
    A two-photon photoemission microscopy experiment with femtosecond time-resolution for imaging of propagating surface plasmon polaritons is discussed. The experimental setup of an actively Pancharatnam's phase stabilized interferometer is described, and a temporal stability in time-resolved two-photon photoemission microscopy of less than 20 attoseconds is demonstrated. The time-resolved setup is applied to investigate the interaction of a surface plasmon polariton wave packet with a plasmonic beam-splitter. Pump-probe data recorded at times before and after the interaction of the surface plasmon polariton wave packet with the beam-splitter indicate transmission and reflection coefficients of T ≈ 0.3 and R ≈ 0.4, respectively. © 2016 SPIE.
    view abstractdoi: 10.1117/12.2239878
  • 2016 • 108 Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation
    Plötzing, M. and Adam, R. and Weier, C. and Plucinski, L. and Eich, S. and Emmerich, S. and Rollinger, M. and Aeschlimann, M. and Mathias, S. and Schneider, C.M.
    Review of Scientific Instruments 87 (2016)
    The fundamental mechanism responsible for optically induced magnetization dynamics in ferromagnetic thin films has been under intense debate since almost two decades. Currently, numerous competing theoretical models are in strong need for a decisive experimental confirmation such as monitoring the triggered changes in the spin-dependent band structure on ultrashort time scales. Our approach explores the possibility of observing femtosecond band structure dynamics by giving access to extended parts of the Brillouin zone in a simultaneously time-, energy- and spin-resolved photoemission experiment. For this purpose, our setup uses a state-of-the-art, highly efficient spin detector and ultrashort, extreme ultraviolet light pulses created by laser-based high-order harmonic generation. In this paper, we present the setup and first spin-resolved spectra obtained with our experiment within an acquisition time short enough to allow pump-probe studies. Further, we characterize the influence of the excitation with femtosecond extreme ultraviolet pulses by comparing the results with data acquired using a continuous wave light source with similar photon energy. In addition, changes in the spectra induced by vacuum space-charge effects due to both the extreme ultraviolet probe- and near-infrared pump-pulses are studied by analyzing the resulting spectral distortions. The combination of energy resolution and electron count rate achieved in our setup confirms its suitability for spin-resolved studies of the band structure on ultrashort time scales. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4946782
  • 2016 • 107 Structural and optical properties of (1122) InGaN quantum wells compared to (0001) and (1120)
    Pristovsek, M. and Han, Y. and Zhu, T. and Oehler, F. and Tang, F. and Oliver, R.A. and Humphreys, C.J. and Tytko, D. and Choi, P.-P. and Raabe, D. and Brunner, F. and Weyers, M.
    Semiconductor Science and Technology 31 (2016)
    We benchmarked growth, microstructure and photo luminescence (PL) of (112-2) InGaN quantum wells (QWs) against (0001) and (112-0). In incorporation, growth rate and the critical thickness of (112-2) QWs are slightly lower than (0001) QWs, while the In incorporation on (112-0) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20% In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (112-0). The slight deviation of (112-2) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (112-2) and (0001) while (112-0) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4 lower on (112-2) and more than 10 lower on (112-0). © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0268-1242/31/8/085007
  • 2016 • 106 Structure and dynamics of shear bands in amorphous-crystalline nanolaminates
    Guo, W. and Gan, B. and Molina-Aldareguia, J.M. and Poplawsky, J.D. and Raabe, D.
    Scripta Materialia 110 28-32 (2016)
    The velocities of shear bands in amorphous CuZr/crystalline Cu nanolaminates were quantified as a function of strain rate and crystalline volume fraction. A rate-dependent transition in flow response was found in a 100 nm CuZr/10 nm Cu nanolaminates. When increasing the Cu layer thickness from 10 nm to 100 nm, the instantaneous velocity of the shear band in these nanolaminates decreases from 11.2 μm/s to <∼500 nm/s. Atom probe tomography and transmission election microcopy observation revealed that in post-deformed pillars both grain rotation in the crystalline portion and non-diffusive crystallization in the amorphous layer affect the viscosity of shear bands. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2015.07.038
  • 2016 • 105 The role of ω in the precipitation of α in near-β Ti alloys
    Li, T. and Kent, D. and Sha, G. and Cairney, J.M. and Dargusch, M.S.
    Scripta Materialia 117 92-95 (2016)
    To identify the conditions under which ω assists α formation in a near-β Ti alloy, we employed transmission electron microscopy and atom probe tomography to study α precipitation in alloys designed to contain two different types of ω. Coherent incommensurate embryonic ω formed upon isothermal ageing, does not directly assist α precipitation. When this incommensurate embryonic ω grows to a critical size, it transforms into commensurate isothermal ω, during which stress is thought to be the dominant factor. Regions of O enrichment at the semi-coherent isothermal ω/β interfaces are observed, which is thought to promote α formation. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2016.02.026
  • 2015 • 104 3D structural and atomic-scale analysis of lath martensite: Effect of the transformation sequence
    Morsdorf, L. and Tasan, C.C. and Ponge, D. and Raabe, D.
    Acta Materialia 95 366-377 (2015)
    To improve the fundamental understanding of the multi-scale characteristics of martensitic microstructures and their micro-mechanical properties, a multi-probe methodology is developed and applied to low-carbon lath martensitic model alloys. The approach is based on the joint employment of electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), atom probe tomography (APT) and nanoindentation, in conjunction with high precision and large field-of-view 3D serial sectioning. This methodology enabled us to resolve (i) size variations of martensite sub-units, (ii) associated dislocation sub-structures, (iii) chemical heterogeneities, and (iv) the resulting local mechanical properties. The identified interrelated microstructure heterogeneity is discussed and related to the martensitic transformation sequence, which is proposed to intrinsically lead to formation of a nano-composite structure in low-carbon martensitic steels. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.05.023
  • 2015 • 103 Advanced scale bridging microstructure analysis of single crystal Ni-base superalloys
    Parsa, A.B. and Wollgramm, P. and Buck, H. and Somsen, C. and Kostka, A. and Povstugar, I. and Choi, P.-P. and Raabe, D. and Dlouhy, A. and Müller, J. and Spiecker, E. and Demtroder, K. and Schreuer, J. and Neuking, K. and Eggeler, G.
    Advanced Engineering Materials 17 216-230 (2015)
    In the present work, we show how conventional and advanced mechanical, chemical, and microstructural methods can be used to characterize cast single crystal Ni-base superalloy (SX) plates across multiple length scales. Two types of microstructural heterogeneities are important, associated with the castmicrostructure (dendrites (D) and interdendritic (ID) regions - large scale heterogeneity) and with the well-known γ/γ′ microstructure (small scale heterogeneity). Using electron probe microanalysis (EPMA), we can showthat elements such as Re, Co, andCr partition to the dendrites while ID regions contain more Al, Ta, and Ti. Analytical transmission electron microscopy and atom probe tomography (APT) show that Al, Ta, and Ti partition to the γ′ cubes while g channels show higher concentrations of Co, Cr, Re, andW.We can combine large scale (EPMA) and small-scale analytical methods (APT) to obtain reasonable estimates for γ′ volume fractions in the dendrites and in the ID regions. The chemical and mechanical properties of the SX plates studied in the present work are homogeneous, when they are determined from volumes with dimensions, which are significantly larger than the dendrite spacing. For the SX plates (140mm x 100mm x 20mm) studied in the present work this holds for the average chemical composition as well as for elastic behavior and local creep properties. We highlight the potential of HRTEM and APT to contribute to a better understanding of the role of dislocations during coarsening of the γ′ phase and the effect of cooling rates after high temperature exposure on the microstructure. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/adem.201400136
  • 2015 • 102 Atom probe informed simulations of dislocation-precipitate interactions reveal the importance of local interface curvature
    Prakash, A. and Guénolé, J. and Wang, J. and Müller, J. and Spiecker, E. and Mills, M.J. and Povstugar, I. and Choi, P. and Raabe, D. and Bitzek, E.
    Acta Materialia 92 33-45 (2015)
    The interaction of dislocations with precipitates is an essential strengthening mechanism in metals, as exemplified by the superior high-temperature strength of Ni-base superalloys. Here we use atomistic simulation samples generated from atom probe tomography data of a single crystal superalloy to study the interactions of matrix dislocations with a γ′ precipitate in molecular dynamics simulations. It is shown that the precipitate morphology, in particular its local curvature, and the local chemical composition significantly alter both, the misfit dislocation network which forms at the precipitate interface, and the core structure of the misfit dislocations. Simulated tensile tests reveal the atomic scale details of many experimentally observed dislocation-precipitate interaction mechanisms, which cannot be reproduced by idealized simulation setups with planar interfaces. We thus demonstrate the need to include interface curvature in the study of semicoherent precipitates and introduce as an enabling method atom probe tomography-informed atomistic simulations. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.03.050
  • 2015 • 101 Atom probe tomography investigation of heterogeneous short-range ordering in the 'komplex' phase state (K-state) of Fe-18Al (at.%)
    Marceau, R.K.W. and Ceguerra, A.V. and Breen, A.J. and Palm, M. and Stein, F. and Ringer, S.P. and Raabe, D.
    Intermetallics 64 23-31 (2015)
    We study an Fe-18Al (at.%) alloy after various thermal treatments at different times (24-336 h) and temperatures (250-1100 °C) to determine the nature of the so-called 'komplex' phase state (or "K-state"), which is common to other alloy systems having compositions at the boundaries of known order-disorder transitions and is characterised by heterogeneous short-range-ordering (SRO). This has been done by direct observation using atom probe tomography (APT), which reveals that nano-sized, ordered regions/particles do not exist. Also, by employing shell-based analysis of the three-dimensional atomic positions, we have determined chemically sensitive, generalised multicomponent short-range order (GM-SRO) parameters, which are compared with published pairwise SRO parameters derived from bulk, volume-averaged measurement techniques (e.g. X-ray and neutron scattering, Mössbauer spectroscopy) and combined ab-initio and Monte Carlo simulations. This analysis procedure has general relevance for other alloy systems where quantitative chemical-structure evaluation of local atomic environments is required to understand ordering and partial ordering phenomena that affect physical and mechanical properties. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2015.04.005
  • 2015 • 100 Atom probe tomography study of internal interfaces in Cu2ZnSnSe4 thin-films
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P. and Mousel, M. and Redinger, A. and Siebentritt, S. and Raabe, D.
    Journal of Applied Physics 118 (2015)
    We report on atom probe tomography studies of the composition at internal interfaces in Cu<inf>2</inf>ZnSnSe<inf>4</inf> thin-films. For Cu<inf>2</inf>ZnSnSe<inf>4</inf> precursors, which are deposited at 320 °C under Zn-rich conditions, grain boundaries are found to be enriched with Cu irrespective of whether Cu-poor or Cu-rich growth conditions are chosen. Cu<inf>2</inf>ZnSnSe<inf>4</inf> grains are found to be Cu-poor and excess Cu atoms are found to be accumulated at grain boundaries. In addition, nanometer-sized ZnSe grains are detected at or near grain boundaries. The compositions at grain boundaries show different trends after annealing at 500 °C. Grain boundaries in the annealed absorber films, which are free of impurities, are Cu-, Sn-, and Se-depleted and Zn-enriched. This is attributed to dissolution of ZnSe at the Cu-enriched grain boundaries during annealing. Furthermore, some of the grain boundaries of the absorbers are enriched with Na and K atoms, stemming from the soda-lime glass substrate. Such grain boundaries show no or only small changes in composition of the matrix elements. Na and K impurities are also partly segregated at some of the Cu<inf>2</inf>ZnSnSe<inf>4</inf>/ZnSe interfaces in the absorber, whereas for the precursors, only Na was detected at such phase boundaries possibly due to a higher diffusivity of Na compared to K. Possible effects of the detected compositional fluctuations on cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4929874
  • 2015 • 99 Atomic scale investigation of non-equilibrium segregation of boron in a quenched Mo-free martensitic steel
    Li, Y.J. and Ponge, D. and Choi, P. and Raabe, D.
    Ultramicroscopy 159 240-247 (2015)
    B-added low carbon steels exhibit excellent hardenability. The reason has been frequently attributed to B segregation at prior austenite grain boundaries, which prevents the austenite to ferrite transformation and favors the formation of martensite. The segregation behavior of B at prior austenite grain boundaries is strongly influenced by processing conditions such as austenitization temperatures and cooling rates and by alloying elements such as Mo, Cr, and Nb. Here an local electrode atom probe was employed to investigate the segregation behavior of B and other alloying elements (C, Mn, Si, and Cr) in a Cr-added Mo-free martensitic steel. Similar to our previous results on a Mo-added steel, we found that in both steels B is segregated at prior austenite grain boundaries with similar excess values, whereas B is neither detected in the martensitic matrix nor at martensite-martensite boundaries at the given cooling rate of 30 K/s. These results are in agreement with the literature reporting that Cr has the same effect on hardenability of steels as Mo in the case of high cooling rates. The absence of B at martensite-martensite boundaries suggests that B segregates to prior austenite grain boundaries via a non-equilibrium mechanism. Segregation of C at all boundaries such as prior austenite grain boundaries and martensite-martensite boundaries may occur by an equilibrium mechanism. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.03.009
  • 2015 • 98 Atomic scale study of CU clustering and pseudo-homogeneous Fe-Si nanocrystallization in soft magnetic FeSiNbB(CU) alloys
    Pradeep, K.G. and Herzer, G. and Raabe, D.
    Ultramicroscopy 159 285-291 (2015)
    A local electrode atom probe has been employed to trace the onset of Cu clustering followed by their coarsening and subsequent growth upon rapid (10s) annealing of an amorphous Fe73.5Si15.5Cu1Nb3B7 alloy. It has been found that the clustering of Cu atoms introduces heterogeneities in the amorphous matrix, leading to the formation of Fe rich regions which crystallizes pseudo-homogeneously into Fe-Si nanocrystals upon annealing. In this paper, we present the data treatment method that allows for the visualization of these different phases and to understand their morphology while still quantifying them in terms of their size, number density and volume fraction. The crystallite size of Fe-Si nanocrystals as estimated from the atom probe data are found to be in good agreement with other complementary techniques like XRD and TEM, emphasizing the importance of this approach towards accurate structural analysis. In addition, a composition driven data segmentation approach has been attempted to determine and distinguish nanocrystalline regions from the remaining amorphous matrix. Such an analysis introduces the possibility of retrieving crystallographic information from extremely fine (2-4nm sized) nanocrystalline regions of very low volume fraction (< 5Vol%) thereby providing crucial in-sights into the chemical heterogeneity induced crystallization process of amorphous materials. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.04.006
  • 2015 • 97 Carbon partitioning during quenching and partitioning heat treatment accompanied by carbide precipitation
    Toji, Y. and Miyamoto, G. and Raabe, D.
    Acta Materialia 86 137-147 (2015)
    Carbon partitioning from martensite into austenite in the quenching and partitioning (Q&P) process has been suggested to be controlled by the constrained carbon equilibrium (CCE) criterion. It defines an approach for predicting the carbon concentration in austenite under the condition that competing reactions such as carbide formation and bainite transformation are suppressed. Carbide precipitation in martensite is, however, often observed during the partitioning step, even in low-carbon steels as well as in high-carbon steels, even when containing a high amount of Si. Therefore, carbon partitioning from martensite into austenite is studied here, considering carbide precipitation in martensite. Carbon partitioning was investigated by means of a field-emission electron probe micro analysis (FE-EPMA) and atom probe tomography (APT), using 1.07 wt.% and 0.59 wt.% carbon steels with various martensite volume fractions. Carbon partitioning from martensite to austenite was clearly observed in all specimens, even though a considerable amount of carbide precipitated inside the martensite. The austenite carbon concentration after the partitioning step was not influenced by either the martensite volume fraction or the bulk carbon content. A modified model for predicting the austenite carbon concentration after the partitioning step was proposed to explain the experimental results by assuming carbon equilibria between austenite, ferrite and cementite under a constrained condition. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.11.049
  • 2015 • 96 Cd and impurity redistribution at the p-n junction of CIGS based solar cells resolved by atom-probe tomography
    Koprek, A. and Cojocaru-Miredin, O. and Wuerz, R. and Freysoldt, C. and Raabe, D.
    2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 (2015)
    Cd and impurity redistribution in the vicinity of CdS/CIGS interface is studied by means of atom probe tomography (APT). We find an increase of the Cd content in the CIGS layer and redistribution of O at the CdS/CIGS interface after annealing the samples at 200 °C, 250 °C, or 300 °C. Very small amounts (∼0.1 at. %) of Na impurity where observed at the p-n junction independent on the heat treatment. Simultaneously, the I-V measurements of the treated samples show a drop in the open circuit voltage and thus of the efficiency compared to the untreated sample. The effect of Cd diffusion in CIGS and of O and Na segregation at the CdS/CIGS interface on the cell performance is discussed. © 2015 IEEE.
    view abstractdoi: 10.1109/PVSC.2015.7355651
  • 2015 • 95 Combining structural and chemical information at the nanometer scale by correlative transmission electron microscopy and atom probe tomography
    Herbig, M. and Choi, P. and Raabe, D.
    Ultramicroscopy 153 32-39 (2015)
    In many cases, the three-dimensional reconstructions from atom probe tomography (APT) are not sufficiently accurate to resolve crystallographic features such as lattice planes, shear bands, stacking faults, dislocations or grain boundaries. Hence, correlative crystallographic characterization is required in addition to APT at the exact same location of the specimen. Also, for the site-specific preparation of APT tips containing regions of interest (e.g. grain boundaries) correlative electron microscopy is often inevitable. Here we present a versatile experimental setup that enables performing correlative focused ion beam milling, transmission electron microscopy (TEM), and APT under optimized characterization conditions. The setup was designed for high throughput, robustness and practicability. We demonstrate that atom probe tips can be characterized by TEM in the same way as a standard TEM sample. In particular, the use of scanning nanobeam diffraction provides valuable complementary crystallographic information when being performed on atom probe tips. This technique enables the measurement of orientation and phase maps as known from electron backscattering diffraction with a spatial resolution down to one nanometer. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.02.003
  • 2015 • 94 Deformation induced alloying in crystalline - metallic glass nano-composites
    Guo, W. and Yao, J. and Jägle, E.A. and Choi, P.-P. and Herbig, M. and Schneider, J.M. and Raabe, D.
    Materials Science and Engineering A 628 269-280 (2015)
    We study the mechanisms of deformation driven chemical mixing in a metallic nanocomposite model system. More specific, we investigate shear banding at the atomic scale in an amorphous CuZr/ crystalline Cu nanolaminate, deformed by microindentation. Three CuZr/Cu multilayer systems (100 nm Cu/100 nm CuZr, 50 nm Cu/100 nm CuZr, and 10 nm Cu/100 nm CuZr) are fabricated to study the effect of layer thickness on shear band formation and deformation induced alloying. The chemical and structural evolution at different strain levels are traced by atom probe tomography and transmission electron microscopy combined with nano-beam diffraction mapping. The initially pure crystalline Cu and amorphous CuZr layers chemically mix by cross-phase shear banding after reaching a critical layer thickness. The Cu inside the shear bands develops a high dislocation density and can locally undergo transition to an amorphous state when sheared and mixed. We conclude that the severe deformation in the shear bands in the amorphous layer squeeze Zr atoms into the Cu dislocation cores in the Cu layers (thickness <5 nm), resulting in local chemical mixing. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2015.01.062
  • 2015 • 93 Detection of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases in co-evaporated Cu2ZnSnSe4 thin-films
    Schwarz, T. and Marques, M.A.L. and Botti, S. and Mousel, M. and Redinger, A. and Siebentritt, S. and Cojocaru-Mirédin, O. and Raabe, D. and Choi, P.-P.
    Applied Physics Letters 107 (2015)
    Cu2ZnSnSe4 thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu2Zn5SnSe8 and Cu2Zn6SnSe9 composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu2ZnSnSe4 and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meV below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu2Zn5SnSe8 and Cu2Zn6SnSe9 phases. Possible effects of these phases on solar cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4934847
  • 2015 • 92 Dicke superradiance as a nondestructive probe for quantum quenches in optical lattices
    Ten Brinke, N. and Schützhold, R.
    Physical Review A - Atomic, Molecular, and Optical Physics 92 (2015)
    We study Dicke superradiance as collective and coherent absorption and (time-delayed) emission of photons from an ensemble of ultracold atoms in an optical lattice. Since this process depends on the coherence properties of the atoms (e.g., superfluidity), it can be used as a probe for their quantum state. In analogy to pump-probe spectroscopy in solid-state physics, this detection method facilitates the investigation of nonequilibrium phenomena and is less invasive than time-of-flight experiments or direct (projective) measurements of the atom number (or parity) per lattice site, which both destroy properties of the quantum state such as phase coherence. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevA.92.013617
  • 2015 • 91 Doping Level of Boron-Doped Diamond Electrodes Controls the Grafting Density of Functional Groups for DNA Assays
    Švorc, L. and Jambrec, D. and Vojs, M. and Barwe, S. and Clausmeyer, J. and Michniak, P. and Marton, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 7 18949-18956 (2015)
    The impact of different doping levels of boron-doped diamond on the surface functionalization was investigated by means of electrochemical reduction of aryldiazonium salts. The grafting efficiency of 4-nitrophenyl groups increased with the boron levels (B/C ratio from 0 to 20 000 ppm). Controlled grafting of nitrophenyldiazonium was used to adjust the amount of immobilized single-stranded DNA strands at the surface and further on the hybridization yield in dependence on the boron doping level. The grafted nitro functions were electrochemically reduced to the amine moieties. Subsequent functionalization with a succinic acid introduced carboxyl groups for subsequent binding of an amino-terminated DNA probe. DNA hybridization significantly depends on the probe density which is in turn dependent on the boron doping level. The proposed approach opens new insights for the design and control of doped diamond surface functionalization for the construction of DNA hybridization assays. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b06394
  • 2015 • 90 Dynamic strain aging studied at the atomic scale
    Aboulfadl, H. and Deges, J. and Choi, P. and Raabe, D.
    Acta Materialia 86 34-42 (2015)
    Dynamic strain aging arises from the interaction between solute atoms and matrix dislocations in strained metallic alloy. It initiates jerky dislocation motion and abrupt softening, causing negative strain rate sensitivity. This effect leads to instable flow phenomena at the macroscopic scale, appearing as a serrated stress-strain response and deformation banding. These macroscopic features are referred to as the Portevin-Le Chatelier effect (PLC). Here we study the atomistic origin of dynamic strain aging in an Al-4.8 at.% Mg alloy using atom probe tomography (APT) and transmission electron microscopy (TEM). Samples were prepared from as-cold rolled (90% thickness reduction), stabilized (120 °C, 20 h) and recrystallized sheets (400°C, 10 min), respectively. In the stabilized state, Mg was found to decorate <1 1 0> aligned dislocations with up to ∼12.5 at.%. Tensile tests in combination with thermographic and laser speckle observations were used to map the deformation bands for the site-specific extraction of APT samples from regions inside the PLC bands. We observed an asymmetrical Mg distribution along some of the dislocations, matching model predictions for high dislocation speeds at peak drag stress by Zhang and Curtin. In this case, the Mg distribution is characterized by depletion in the compressive regime above the dislocation slip plane and enrichment in the dilatation region below the slip plane. Mg also depletes in a tail-like form behind fast-moving dislocations, further promoting slip localization. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.028
  • 2015 • 89 Dynamic strain-induced transformation: An atomic scale investigation
    Zhang, H. and Pradeep, K.G. and Mandal, S. and Ponge, D. and Springer, H. and Raabe, D.
    Scripta Materialia 109 23-27 (2015)
    Phase transformations provide the most versatile access to the design of complex nanostructured alloys in terms of grain size, morphology, local chemical constitution etc. Here we study a special case of deformation induced phase transformation. More specifically, we investigate the atomistic mechanisms associated with dynamic strain-induced transformation (DSIT) in a dual-phased multicomponent iron-based alloy at high temperatures. DSIT phenomena and the associated secondary phase nucleation were observed at atomic scale using atom probe tomography. The obtained local chemical composition was used for simulating the nucleation process which revealed that DSIT, occurring during load exertion, proceeds by a diffusion-controlled nucleation process. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2015.07.010
  • 2015 • 88 Effects of Ru on elemental partitioning and precipitation of topologically close-packed phases in Ni-based superalloys
    Peng, Z. and Povstugar, I. and Matuszewski, K. and Rettig, R. and Singer, R. and Kostka, A. and Choi, P.-P. and Raabe, D.
    Scripta Materialia 101 44-47 (2015)
    Two Ni-based superalloys (one Ru-free and one containing 1 at.% Ru) were comparatively studied by Atom Probe Tomography and complimentary techniques. Ru addition impedes precipitation of the σ phase at 950 °C. Ru partitions nearly equally to the γ and σ phase. Neither reverse elemental partitioning nor destabilization of the γ' phase is detected when adding Ru. We propose an increase in the γ/σ lattice misfit caused by Ru as a major reason for impeded nucleation of the σ phase. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2015.01.014
  • 2015 • 87 Electron density determination for plasma assisted sterilization processes
    Schulz, C. and Runkel, J. and Rolfes, I.
    2015 IEEE MTT-S International Microwave Workshop Series on RF and Wireless Technologies for Biomedical and Healthcare Applications, IMWS-BIO 2015 - Proceedings 112-113 (2015)
    An effective measurement concept for the precise determination of the electron density in plasma assisted sterilization processes is presented in this paper. Here, plasmas offer a promising approach for heat or chemical sensitive materials, like surgery instruments. Therefore, the Multipole Resonance Probe (MRP) is investigated over a wide frequency range. Based on the evaluation of an unambiguous resonance behaviour, the resonance frequency can be determined from 0.5 to 6 GHz. Proven in 3D electromagnetic field simulations and measurements in an excited Argon plasma, the additional center of gravity method increases the performance and the MRP can be used for an evaluation of a wide density range. Hence, the density can be controlled effectively and used in a feedback control loop. © 2015 IEEE.
    view abstractdoi: 10.1109/IMWS-BIO.2015.7303803
  • 2015 • 86 Experimental characterization of the combustion of single lithium particles with CO2
    Fischer, P. and Schiemann, M. and Scherer, V. and Maas, P. and Schmid, G. and Taroata, D.
    Fuel 153 90-101 (2015)
    Combustion and temperature measurement of single lithium particles (dp &lt; 250 μm) with CO2 was carried out in a laminar flow reactor. An imaging two-color pyrometer system was used to measure particle and flame size as well as combustion temperatures. The results indicate two different combustion phenomena, which have been identified in literature before: Gas-phase reaction at temperatures above 2500 K and surface reaction of lithium with CO2 at temperatures between 1500 and 1800 K. In addition, a sampling probe was utilized to extract burning particles from the reactor. The extracted probes were analyzed concerning their constituents using X-ray diffraction analysis and their shape and surface with scanning electron microscopy. The results showed lithium carbonate as main reaction product and a relatively smooth surface of the particles after burn-out. Combining the experimental findings, a single particle combustion model was suggested and apparent reaction kinetics was determined. © 2015 Elsevier Ltd.All rights reserved.
    view abstractdoi: 10.1016/j.fuel.2015.02.098
  • 2015 • 85 Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel
    Kuzmina, M. and Ponge, D. and Raabe, D.
    Acta Materialia 86 182-192 (2015)
    We study grain boundary embrittlement in a quenched and tempered Fe-Mn high-purity model martensite alloy using Charpy impact tests and grain boundary characterization by atom probe tomography. We observe that solute Mn directly embrittles martensite grain boundaries while reversion of martensite to austenite at high-angle grain boundaries cleans the interfaces from solute Mn by partitioning the Mn into the newly formed austenite, hence restoring impact toughness. Microalloying with B improves the impact toughness in the quenched state and delays temper embrittlement at 450 °C. Tempering at 600 °C for 1 min significantly improves the impact toughness and further tempering at lower temperature does not cause the embrittlement to return. At higher temperatures, regular austenite nucleation and growth takes place, whereas at lower temperature, Mn directly promotes its growth. ©2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.12.021
  • 2015 • 84 Grain boundary segregation in multicrystalline silicon: Correlative characterization by EBSD, EBIC, and atom probe tomography
    Stoffers, A. and Cojocaru-Mirédin, O. and Seifert, W. and Zaefferer, S. and Riepe, S. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 1742-1753 (2015)
    This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron-beam-induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non-coherent (random) high-angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high-angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface-property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2614
  • 2015 • 83 Guided mass spectrum labelling in atom probe tomography
    Haley, D. and Choi, P. and Raabe, D.
    Ultramicroscopy 159 338-345 (2015)
    Atom probe tomography (APT) is a valuable near-atomic scale imaging technique, which yields mass spectrographic data. Experimental correctness can often pivot on the identification of peaks within a dataset, this is a manual process where subjectivity and errors can arise. The limitations of manual procedures complicate APT experiments for the operator and furthermore are a barrier to technique standardisation. In this work we explore the capabilities of computer-guided ranging to aid identification and analysis of mass spectra. We propose a fully robust algorithm for enumeration of the possible identities of detected peak positions, which assists labelling. Furthermore, a simple ranking scheme is developed to allow for evaluation of the likelihood of each possible identity being the likely assignment from the enumerated set. We demonstrate a simple, yet complete work-chain that allows for the conversion of mass-spectra to fully identified APT spectra, with the goal of minimising identification errors, and the inter-operator variance within APT experiments. This work chain is compared to current procedures via experimental trials with different APT operators, to determine the relative effectiveness and precision of the two approaches. It is found that there is little loss of precision (and occasionally gain) when participants are given computer assistance. We find that in either case, inter-operator precision for ranging varies between 0 and 2 "significant figures" (2 σ confidence in the first n digits of the reported value) when reporting compositions. Intra-operator precision is weakly tested and found to vary between 1 and 3 significant figures, depending upon species composition levels. Finally it is suggested that inconsistencies in inter-operator peak labelling may be the largest source of scatter when reporting composition data in APT. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.03.005
  • 2015 • 82 Implications of electron heating and non-uniformities in a VHF-CCP for sterilization of medical instruments
    Stapelmann, K. and Fiebrandt, M. and Styrnoll, T. and Baldus, S. and Bibinov, N. and Awakowicz, P.
    Plasma Sources Science and Technology 24 (2015)
    A capacitively coupled plasma driven at a frequency of 81.36 MHz from the VHF-band is investigated by means of optical emission spectroscopy (OES) and multipole resonance probe (MRP). The discharge is operated with hydrogen, yielding an electropositive discharge, as well as oxygen, yielding an electronegative discharge, and mixtures of both. Pressure is varied from p = 5 Pa to p = 25 Pa. Homogeneity of the discharge is investigated by CCD camera recordings as well as spatially resolved multipole resonance probe measurements. The results indicate the presence of electromagnetic edge effects as well as standing wave effects. Furthermore, a largely homogeneous discharge can be achieved with hydrogen as process gas at a pressure of p = 5-10 Pa. With increasing pressure as well as with increasing oxygen content, the discharge appears less homogeneously. The transition from an electropositive to an electronegative discharge leads to a change in electron heating mechanisms, with pronounced local maxima of electron density at the sheath edges. A comparison of OES and MRP results reveal a significant difference in electron density, which can be explained by a non-Maxwellian distribution function of electrons. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/24/3/034014
  • 2015 • 81 Interface engineering and characterization at the atomic-scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin-film solar cells
    Cojocaru-Mirédin, O. and Fu, Y. and Kostka, A. and Sáez-Araoz, R. and Beyer, A. and Knaub, N. and Volz, K. and Fischer, C.-H. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 705-716 (2015)
    In this work, we investigate the p-n junction region for two different buffer/Cu(In,Ga)(Se,S)<inf>2</inf> (CIGSSe) samples having different conversion efficiencies (the cell with pure In<inf>2</inf>S<inf>3</inf> buffer shows a lower efficiency than the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered one). To explain the better efficiency of the sample with nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffer layer, combined transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies were performed. In the pure In<inf>2</inf>S<inf>3</inf> buffered sample, a CuIn<inf>3</inf>Se<inf>5</inf> ordered-defect compound is observed at the CIGSSe surface, whereas in the nano-ZnS/In<inf>2</inf>S<inf>3</inf> buffered sample no such compound is detected. The absence of an ordered-defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn<inf>3</inf>Se<inf>5</inf>, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered-defect compound. In the nano-ZnS/In<inf>2</inf>S<inf>3</inf> sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se<inf>2</inf>) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p-n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2484
  • 2015 • 80 Interface engineering and nanoscale characterization of Zn(S,O) alternative buffer layer for CIGS thin film solar cells
    Soni, P. and Cojocaru-Miredin, O. and Raabe, D.
    2015 IEEE 42nd Photovoltaic Specialist Conference, PVSC 2015 (2015)
    The buffer layers in Cu(In,Ga)Se2 solar cells play a crucial role in device performance, although their thickness is only a few tens of nanometers. Moreover, often Zn(S,O) alternative buffer layers have been studied in view of their structure, band alignment, and optical properties, but not much work exists on their nanoscale chemical properties. This work focuses on the chemical characterization of Zn(S,O) using x-ray photoelectron spectroscopy for determination of the Zn(S,O) and Cu(In,Ga)Se2 depth composition, and atom probe tomography for probing the nano-scale chemical fluctuations at the Zn(S,O)/Cu(In,Ga)Se2 interface. The Zn(O,S) buffer layer was deposited by RF magnetron sputtering. The aim is to study the nanoscale concentration changes and atomic interdiffusion between CIGS and Zn(S,O) after sputter deposition at room temperature and after post-deposition heat treatment at 200°C. © 2015 IEEE.
    view abstractdoi: 10.1109/PVSC.2015.7355889
  • 2015 • 79 Investigation of the sampling nozzle effect on laminar flat flames
    Deng, L. and Kempf, A. and Hasemann, O. and Korobeinichev, O.P. and Wlokas, I.
    Combustion and Flame 162 1737-1747 (2015)
    Sampling probes used for the mass spectrometric sampling of a flame can affect the flame's flow field. Although this effect is already compensated for by heuristic correction functions, state of the art 3-D simulations may permit an even better consideration of this effect. This work has investigated the perturbations induced by sampling probes in burner-stabilized, laminar, flat flames using numerical simulations. Any deviations in the flow and temperature fields from the ideal, one-dimensional flat flame were generated here by a perforated burner plate; they are also examined. Corresponding mass spectrometric measurements were performed in flames of CH4/O2/Ar and H2/O2/N2, burning under atmospheric conditions. In the present study, heat transfer from the flame to the sampling nozzle was studied with a conjugate heat transfer model. Combustion was described using a finite rate chemistry model, employing a detailed reaction mechanism for a H2/O2/N2 flame and a reduced mechanism for a CH4/O2/Ar flame. Compared to the ideal, one-dimensional, and unperturbed flame, the probe was found to affect the measurements of the concentrations of some species by up to 50%. The results highlight the value of supporting numerical simulations of both the flow and combustion for such measurements with invasive probing. © 2014 The Combustion Institute.
    view abstractdoi: 10.1016/j.combustflame.2014.11.035
  • 2015 • 78 Mapping near-field plasmonic interactions of silver particles with scanning near-field optical microscopy measurements
    Andrae, P. and Song, M. and Haggui, M. and Fumagalli, P. and Schmid, M.
    Proceedings of SPIE - The International Society for Optical Engineering 9547 (2015)
    A scanning near-field optical microscope (SNOM) is a powerful tool to investigate optical effects that are smaller than Abbe's limit. Its greatest strength is the simultaneous measurement of high-resolution topography and optical nearfield data that can be correlated to each other. However, the resolution of an aperture SNOM is always limited by the probe. It is a technical challenge to fabricate small illumination tips with a well-defined aperture and high transmission. The aperture size and the coating homogeneity will define the optical resolution and the optical image whereas the tip size and shape influence the topographic accuracy. Although the technique has been developing for many years, the correlation between simulated near-field data and measurement is still not convincing. To overcome this challenge, the mapping of near-field plasmonic interactions of silver nanoparticles is investigated. Different nanocluster samples with diverse distributions of silver particles are characterized via SNOM in illumination and collection mode. This will lead to topographical and optical images that can be used as an input for SNOM simulations with the aim of estimating optical artifacts. Including tip, particles, and substrate, our finite-elementmethod (FEM) simulations are based on the realistic geometry. Correlating the high-precision SNOM measurement and the detailed simulation of a full image scan will enable us to draw conclusions regarding near-field enhancements caused by interacting particles. © 2015 SPIE.
    view abstractdoi: 10.1117/12.2187358
  • 2015 • 77 Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation
    Schuh, B. and Mendez-Martin, F. and Völker, B. and George, E.P. and Clemens, H. and Pippan, R. and Hohenwarter, A.
    Acta Materialia 96 258-268 (2015)
    An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to ∼520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of ∼630 HV at about 450 °C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450 °C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites. © 2015 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2015.06.025
  • 2015 • 76 Microstructure design and mechanical properties in a near-α Ti-4Mo alloy
    Tarzimoghadam, Z. and Sandlöbes, S. and Pradeep, K.G. and Raabe, D.
    Acta Materialia 97 291-304 (2015)
    Abstract We study the effects of different heat treatment routes on microstructure engineering and the resulting mechanical response in a plain binary Ti-4Mo (wt%) model alloy. We observe a broad variety of microstructure formation mechanisms including diffusion driven allotropic phase transformations as well as shear and/or diffusion dominated modes of martensitic transformations, enabling a wealth of effective microstructure design options even in such a simple binary Ti alloy. This wide variety of microstructures allows tailoring the mechanical properties ranging from low yield strength (350 MPa) and high ductility (30-35% tensile elongation) to very high yield strength (1100 MPa) and medium ductility (10-15% tensile elongation) as well as a variety of intermediate states. Mechanical testing and microstructure characterization using optical microscopy, scanning electron microscopy based techniques, transmission electron microscopy and atom probe tomography were performed revealing that minor variations in the heat treatment cause significant changes in the resulting microstructures (e.g. structural refinement, transition between diffusive and martensitic transformations). The experimental results on microstructure evolution during the applied different heat treatment routes are discussed with respect to the mechanical properties. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.06.043
  • 2015 • 75 Model-independent measurement of the charge density distribution along an Fe atom probe needle using off-axis electron holography without mean inner potential effects
    Migunov, V. and London, A. and Farle, M. and Dunin-Borkowski, R.E.
    Journal of Applied Physics 117 134301 (2015)
    The one-dimensional charge density distribution along an electrically biased Fe atom probe needle is measured using a model-independent approach based on off-axis electron holography in the transmission electron microscope. Both the mean inner potential and the magnetic contribution to the phase shift are subtracted by taking differences between electron-optical phase images recorded with different voltages applied to the needle. The measured one-dimensional charge density distribution along the needle is compared with a similar result obtained using model-based fitting of the phase shift surrounding the needle. On the assumption of cylindrical symmetry, it is then used to infer the three-dimensional electric field and electrostatic potential around the needle with ∼10 nm spatial resolution, without needing to consider either the influence of the perturbed reference wave or the extension of the projected potential outside the field of view of the electron hologram. The present study illustrates how a model-independent approach can be used to measure local variations in charge density in a material using electron holography in the presence of additional contributions to the phase, such as those arising from changes in mean inner potential and specimen thickness. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4916609
  • 2015 • 74 New approaches to nanoparticle sample fabrication for atom probe tomography
    Felfer, P. and Li, T. and Eder, K. and Galinski, H. and Magyar, A.P. and Bell, D.C. and Smith, G.D.W. and Kruse, N. and Ringer, S.P. and Cairney, J.M.
    Ultramicroscopy 159 413-419 (2015)
    Due to their unique properties, nano-sized materials such as nanoparticles and nanowires are receiving considerable attention. However, little data is available about their chemical makeup at the atomic scale, especially in three dimensions (3D). Atom probe tomography is able to answer many important questions about these materials if the challenge of producing a suitable sample can be overcome. In order to achieve this, the nanomaterial needs to be positioned within the end of a tip and fixed there so the sample possesses sufficient structural integrity for analysis. Here we provide a detailed description of various techniques that have been used to position nanoparticles on substrates for atom probe analysis. In some of the approaches, this is combined with deposition techniques to incorporate the particles into a solid matrix, and focused ion beam processing is then used to fabricate atom probe samples from this composite. Using these approaches, data has been achieved from 10-20 nm core-shell nanoparticles that were extracted directly from suspension (i.e. with no chemical modification) with a resolution of better than ±1 nm. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.ultramic.2015.04.014
  • 2015 • 73 Parallelization concept for spatially resolved in-situ plasma measurements
    Schulz, C. and Runkel, J. and Rolfes, I.
    2015 IEEE MTT-S International Microwave Symposium, IMS 2015 (2015)
    A novel, simple, and passive parallelization concept for the simultaneous determination of plasma density by spatially resolved plasma probes is presented in this paper. Based on the so-called Multipole Resonance Probe (MRP), a Wilkinson divider, and two coaxial cables with different length, the resulting reflection coefficients of each probe can be separated in time domain. Hence, the corresponding plasma density at different positions in a reactor can be determined without limitations. The basic concept is confirmed by simulative investigations in CST Schematic. Therefore, 3D electromagnetic simulations are combined with numerical models and the signal processing is both, investigated and evaluated. Compared to a measurement based setup, an excellent agreement is achieved and the concept is proven. © 2015 IEEE.
    view abstractdoi: 10.1109/MWSYM.2015.7167113
  • 2015 • 72 Quantitative chemical-structure evaluation using atom probe tomography: Short-range order analysis of Fe-Al
    Marceau, R.K.W. and Ceguerra, A.V. and Breen, A.J. and Raabe, D. and Ringer, S.P.
    Ultramicroscopy 157 12-20 (2015)
    Short-range-order (SRO) has been quantitatively evaluated in an Fe-18Al (at%) alloy using atom probe tomography (APT) data and by calculation of the generalised multicomponent short-range order (GM-SRO) parameters, which have been determined by shell-based analysis of the three-dimensional atomic positions. The accuracy of this method with respect to limited detector efficiency and spatial resolution is tested against simulated D0<inf>3</inf> ordered data. Whilst there is minimal adverse effect from limited atom probe instrument detector efficiency, the combination of this with imperfect spatial resolution has the effect of making the data appear more randomised. The value of lattice rectification of the experimental APT data prior to GM-SRO analysis is demonstrated through improved information sensitivity. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.05.001
  • 2015 • 71 Segregation of boron at prior austenite grain boundaries in a quenched martensitic steel studied by atom probe tomography
    Li, Y.J. and Ponge, D. and Choi, P. and Raabe, D.
    Scripta Materialia 96 13-16 (2015)
    The distribution of B and other alloying elements (C, Cr, Mo) at prior austenite grain boundaries (PAGBs) and in the matrix was quantified by atom probe tomography in a quenched martensitic steel. B and Mo were observed to be segregated only at PAGBs and to be absent at martensite-martensite boundaries. C is segregated both at PAGBs and at martensite-martensite boundaries, whereas Cr is homogeneously distributed in the probed volume. Our results indicate that B undergoes a non-equilibrium segregation. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2014.09.031
  • 2015 • 70 Spatially and kinetically resolved mapping of hydrogen in a twinning-induced plasticity steel by use of Scanning Kelvin Probe Force Microscopy
    Koyama, M. and Bashir, A. and Rohwerder, M. and Merzlikin, S.V. and Akiyama, E. and Tsuzaki, K. and Raabe, D.
    Journal of the Electrochemical Society 162 C638-C647 (2015)
    The hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1-2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption. © 2015 The Electrochemical Society.
    view abstractdoi: 10.1149/2.0131512jes
  • 2015 • 69 Swift Heavy Ion Induced Optical and Electronic Modifications of Graphene-TiO2 Nanocomposites
    Mishra, M. and Meinerzhagen, F. and Schleberger, M. and Kanjilal, D. and Mohanty, T.
    Journal of Physical Chemistry C 119 21270-21277 (2015)
    The effect of swift heavy ions irradiation on optical and electronic properties of chemically synthesized graphene-TiO<inf>2</inf> nanocomposites is presented. Modification of surface properties of these nanocomposites by irradiation with three different ions and with varying fluence was analyzed by Raman spectroscopy, transmission electron microscopy, and scanning Kelvin probe microscopy techniques. Raman spectra of irradiated samples exhibit systematic changes in the characteristic peaks of both graphene and TiO<inf>2</inf>. The nanocrystallite dimension calculated from Raman peak intensity decreases with fluence, indicating the occurrence of peripheral fragmentation. Furthermore, measurement of the surface contact potential difference using scanning Kelvin probe reveals that the work function of graphene-titanium dioxide nanocomposites can be effectively increased by more than 1 eV. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b07297
  • 2015 • 68 The evolution of microstructure and mechanical properties of Ti-5Al-5Mo-5V-2Cr-1Fe during ageing
    Ahmed, M. and Li, T. and Casillas, G. and Cairney, J.M. and Wexler, D. and Pereloma, E.V.
    Journal of Alloys and Compounds 629 260-273 (2015)
    The phase transformations and compositional changes occurring during thermo-mechanical processing and subsequent high temperature ageing of Ti-5Al-5Mo-5V-2Cr-1Fe (wt.%) were investigated using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). High resolution STEM revealed nano-sized α (< 10 nm) and athermal ω (∼1-3 nm) formed during accelerated cooling from 800°C and slow heating to an ageing temperature of 650°C. Nuclei of α were found to form heterogeneously in the β matrix as well as at the ω phase. APT revealed pronounced Mo compositional fluctuations in the β matrix. No direct connection was established between Mo-rich or Mo-lean regions and α or ω nuclei. APT also failed to detect the ω phase, which supports theories that it forms by a shuffle mechanism, without any compositional difference from the β phase. Very small α particles, after initial ageing, showed only a minute change in composition with respect to the β matrix, indicative of a displacive-diffusional transformation. With further ageing, growth of the α lamellae was accompanied by compositional changes according to the diffusion rates of β-stabilising elements. Pile-up of the slowest diffusing solutes (Mo, V) at the α/β interface were pronounced in the initial stages of ageing. The best combination of mechanical properties (1200 MPa ultimate tensile strength with 15% total elongation) was recorded after 3.6 ks of ageing. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.01.005
  • 2015 • 67 The influence of partitioning on the growth of intragranular α in near-β Ti alloys
    Li, T. and Ahmed, M. and Sha, G. and Shi, R. and Casillas, G. and Yen, H.-W. and Wang, Y. and Pereloma, E.V. and Cairney, J.M.
    Journal of Alloys and Compounds 643 212-222 (2015)
    Abstract We report on partitioning of alloying elements during the formation of fine intragranular α plates in a Ti-55521 alloy after thermo-mechanical processing (TMP) and isothermal ageing at 923 K. The microstructures were characterised using atom probe tomography and high-resolution transmission electron microscopy. The partitioning of Mo, V and Al are strongly affected by their diffusivities and their mutual interaction. This leads to a deviation of the measured contents of alloying elements in the two phases from the predicted equilibrium values. The alloying elements at the broad faces and tips of α plates were found to exhibit different pile-up and segregation behaviours, which is thought to affect the lengthening and thickening kinetics of the α plates. As a result, the aspect ratio of α plates decreased rapidly with increasing ageing time. This study suggests that careful selection of alloying elements could be an effective way in controlling the growth anisotropy of α plates and thus α + β microstructures in near-β Ti alloys. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.04.143
  • 2015 • 66 The mechanism of ω-assisted α phase formation in near β-Ti alloys
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Scripta Materialia 104 75-78 (2015)
    Partitioning of alloying elements during the ω-to-α phase transformation in a near-β alloy after isothermal ageing at 573 K was measured using atom probe tomography and high-resolution transmission electron microscopy. O-rich regions associated with ω precipitates were observed for the first time, and likely serve as nucleation sites for the α phase. The partitioning behaviours of Al and O, unlike other elements, are different for α and ω, suggesting a mixed-mode mechanism for the ω-to-α phase transformation. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2015.04.007
  • 2015 • 65 The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Yardley, V.A. and Pradeep, K.G. and Duarte, M.J. and Choi, P.P. and Raabe, D. and Eggeler, G.
    Acta Materialia 90 94-104 (2015)
    We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr tempered martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
    view abstractdoi: 10.1016/j.actamat.2015.01.027
  • 2015 • 64 The Planar Multipole Resonance Probe: Challenges and Prospects of a Planar Plasma Sensor
    Schulz, C. and Styrnoll, T. and Awakowicz, P. and Rolfes, I.
    IEEE Transactions on Instrumentation and Measurement 64 857-864 (2015)
    A novel compact plasma sensor applicable for the supervision and control of industrial plasma processes is presented in this contribution. Based on the multipole resonance probe (MRP), the new planar MRP (pMRP) is introduced as a powerful and economical monitoring tool, flush-mounted into the reactor wall. Hence, it can be used for an effective suppression of disturbances of the plasma process itself. Using 3D electromagnetic field simulations with CST Microwave Studio, the pMRP is investigated and challenges as well as prospects of the new sensor design are discussed in detail. Three different sensor versions are presented and compared with the resonance behavior of the MRP. Furthermore, limitations concerning position tolerances are shown and the suitability of the pMRP is proven. Measurements in a double inductive coupled plasma, with argon as process gas and varying excitation powers, demonstrate the suitability of the pMRP for monitoring purposes. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/TIM.2014.2358111
  • 2015 • 63 Thermal dissolution mechanisms of AlN/CrN hard coating superlattices studied by atom probe tomography and transmission electron microscopy
    Tytko, D. and Choi, P.-P. and Raabe, D.
    Acta Materialia 85 32-41 (2015)
    AlN/CrN superlattices with a B1 cubic crystal structure and a bilayer period of 4 nm were deposited by reactive radiofrequency magnetron sputtering. The coatings were investigated with respect to their thermal stability and changes in microstructure and chemical composition at 900 °C. The AlN layers show high chemical stability but undergo dissolution by pinching off at grain boundaries. A transformation from cubic to hexagonal AlN with subsequent coarsening at grain boundary triple junctions is observed. In contrast to AlN, the CrN layers show poor chemical stability and their compositions are shifted towards Cr2N upon annealing in a protective argon atmosphere due to nitrogen loss. However, even after establishing Cr2N stoichiometry the crystal structure of the layers remains cubic. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.11.004
  • 2015 • 62 Ultrafast laser-excited spin transport in Au/Fe/MgO(001): Relevance of the Fe layer thickness
    Alekhin, A. and Bürstel, D. and Melnikov, A. and Diesing, D. and Bovensiepen, U.
    Springer Proceedings in Physics 159 241-243 (2015)
    Propagation dynamics of spin-dependent optical excitations is investigated by back-pump front-probe experiments in Au/Fe/Mg0(001). We observe a decrease for all pump-probe signals detected at the Au surface, if the Fe thickness in increased. Relaxation processes within Fe limit the emission region of ballistic spins at the Fe/Au interface to ∼1 nm. © Springer International Publishing Switzerland 2015.
    view abstractdoi: 10.1007/978-3-319-07743-7_75
  • 2015 • 61 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 23554-23563 (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 abstractdoi: 10.1021/acsami.5b06700
  • 2014 • 60 A new approach on advanced compact plasma sensors for industrial plasma applications
    Schulz, C. and Rolfes, I.
    2014 IEEE Sensors Applications Symposium, SAS 2014 - Proceedings 263-266 (2014)
    A novel compact plasma sensor applicable for the supervision and control of industrial plasma processes is presented in this contribution. Based on the multipole resonance probe (MRP), the new planar multipole resonance probe (pMRP) flush-mounted into the reactor wall can be used for an effective suppression of disruptions on the plasma process itself. Using 3D-electromagnetic field simulations, the MRP and the pMRP are investigated and compared. Furthermore, limitations concerning position tolerances are shown and the suitability is demonstrated. © 2014 IEEE.
    view abstractdoi: 10.1109/SAS.2014.6798958
  • 2014 • 59 A stacked sensor concept for industry compatible plasma diagnostics
    Schulz, C. and Will, B. and Rolfes, I.
    Proceedings - 2014 International Conference on Electromagnetics in Advanced Applications, ICEAA 2014 683-686 (2014)
    This paper presents a stacked sensor concept, which is producible entirely on standard printed circuit boards and applicable for the control of industrial plasma processes. Starting with the so-called multipole resonance probe (MRP) the prospects and the development towards the stacked MRP (sMRP) are discussed. The conversion of the MRP into a discretized spherical setup, using printed half discs of varying radii, is investigated. 3D electromagnetic field simulations yield an optimized assembly of 25 PCBs for a replication of the probe head. Measurements of a first prototype in a double inductive coupled plasma with an excited argon-hydrogen plasma have proven the expected resonance behaviour. In comparison to the MRP and the simulations, the applicability of the stacked sensor concept is demonstrated. © 2014 IEEE.
    view abstractdoi: 10.1109/ICEAA.2014.6903945
  • 2014 • 58 Characterization of microporous activated carbons using molecular probe method
    Helmich, M. and Luckas, M. and Pasel, C. and Bathen, D.
    Carbon 74 22-31 (2014)
    Accurate knowledge of an adsorbent's porosity is fundamental for scientific and industrial applications of adsorption technology. Over the last decades many approaches have been established to assess porosity of adsorbent materials by analyzing their nitrogen uptake at 77 K with volumetric measurement devices. Despite using highly sophisticated physical models, all approaches make assumptions on pore shape as well as on the interactions between adsorbent and adsorptive molecules. Subsequently, significant differences in pore size distributions are observed depending on which modeling parameters were used. The molecular probe method presented in this paper therefore restrains to a minimum of approximations by measuring isotherms of chemically similar substances of increasing molecular size. Differences in pore volume can be reduced to sterical limitations in micropores below the size of adsorptive, leading to a high-resolution pore size distribution below 0.7 nm where only few comparable methods exist. The analytical procedure was customized to take account of the amorphous and heterogeneous pore structure of activated carbon. By measuring adsorption isotherms of N2, n-hexane, iso-octane and cyclohexane on various activated carbons, it is shown that differences in pore accessibility of tested adsorptives are specific for each adsorbent. Using molecular probe molecules hence appears to be a promising method for a complementary porosity analysis of activated carbons. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2014.02.077
  • 2014 • 57 Comparative scanning near-field optical microscopy studies of plasmonic nanoparticle concepts
    Andrae, P. and Fumagalli, P. and Schmid, M.
    Proceedings of SPIE - The International Society for Optical Engineering 9132 (2014)
    We use scanning near-field optical microscopy (SNOM) to characterize different plasmonic-nanoparticle situations with high spatial and spectral resolution in this comparative study. The near-field enhancement is measured with an aperture probe (Al coated glass fiber) and two CCD spectrometers for simultaneous detection of reflection and transmission. The images of transmission and reflection show a correlation to the topography. We present a new way to access the relative absorption and discuss the results with consideration of artifact influences. Near-field enhancements are deeper understood by imaging isolated particles. This near field will be compared to measurements of random-particle distributions. Therefore, we will show normalized reflection and transmission images of random structures that lay the foundation for an absolute interpretation of near-field images. The normalization considers both the far-field UV/VIS results and a reference image of the substrate. The near-field reflection of nanoparticle arrays shows an enhancement of 25 %. In view of specific applications, particle distributions implemented in two ways: as far-field scatters and as near field enhancing objects. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2051760
  • 2014 • 56 Counting of oxygen defects versus metal surface sites in methanol synthesis catalysts by different probe molecules
    Fichtl, M.B. and Schumann, J. and Kasatkin, I. and Jacobsen, N. and Behrens, M. and Schlögl, R. and Muhler, M. and Hinrichsen, O.
    Angewandte Chemie - International Edition 53 7043-7047 (2014)
    Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2. A combination of N2O reactive frontal chromatography and H2 temperature-programmed desorption is used to analyze the interplay of copper and zinc oxide in methanol synthesis catalysts. This method provides an easy in situ approach to quantify the direct copper-zinc interaction (SMSI effect) and offers an important possibility to rational catalyst design also for other supported metal catalysts. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201400575
  • 2014 • 55 Electrical potential-assisted DNA hybridization. How to mitigate electrostatics for surface DNA hybridization
    Tymoczko, J. and Schuhmann, W. and Gebala, M.
    ACS Applied Materials and Interfaces 6 21851-21858 (2014)
    Surface-confined DNA hybridization reactions are sensitive to the number and identity of DNA capture probes and experimental conditions such as the nature and the ionic strength of the electrolyte solution. When the surface probe density is high or the concentration of bulk ions is much lower than the concentration of ions within the DNA layer, hybridization is significantly slowed down or does not proceed at all. However, high-density DNA monolayers are attractive for designing high-sensitivity DNA sensors. Thus, circumventing sluggish DNA hybridization on such interfaces allows a high surface concentration of target DNA and improved signal/noise ratio. We present potential-assisted hybridization as a strategy in which an external voltage is applied to the ssDNA-modified interface during the hybridization process. Results show that a significant enhancement of hybridization can be achieved using this approach. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/am5027902
  • 2014 • 54 Femtospex: A versatile optical pump-soft x-ray probe facility with 100fs x-ray pulses of variable polarization
    Holldack, K. and Bahrdt, J. and Balzer, A. and Bovensiepen, U. and Brzhezinskaya, M. and Erko, A. and Eschenlohr, A. and Follath, R. and Firsov, A. and Frentrup, W. and Le Guyader, L. and Kachel, T. and Kuske, P. and Mitzner, R. a...
    Journal of Synchrotron Radiation 21 1090-1104 (2014)
    Here the major upgrades of the femtoslicing facility at BESSYII (Khan et al., 2006) are reviewed, giving a tutorial on how elliptical-polarized ultrashort soft X-ray pulses from electron storage rings are generated at high repetition rates. Employing a 6kHz femtosecond-laser system consisting of two amplifiers that are seeded by one Ti:Sa oscillator, the total average flux of photons of 100fs duration (FWHM) has been increased by a factor of 120 to up to 10 6 photons s -1 (0.1% bandwidth) -1 on the sample in the range from 250 to 1400eV. Thanks to a new beamline design, a factor of 20 enhanced flux and improvements of the stability together with the top-up mode of the accelerator have been achieved. The previously unavoidable problem of increased picosecond-background at higher repetition rates, caused by 'halo' photons, has also been solved by hopping between different 'camshaft' bunches in a dedicated fill pattern ('3+1 camshaft fill') of the storage ring. In addition to an increased X-ray performance at variable (linear and elliptical) polarization, the sample excitation in pump-probe experiments has been considerably extended using an optical parametric amplifier that supports the range from the near-UV to the far-IR regime. Dedicated endstations covering ultrafast magnetism experiments based on time-resolved X-ray circular dichroism have been either upgraded or, in the case of time-resolved resonant soft X-ray diffraction and reflection, newly constructed and adapted to femtoslicing requirements. Experiments at low temperatures down to 6K and magnetic fields up to 0.5T are supported. The FemtoSpeX facility is now operated as a 24h user facility enabling a new class of experiments in ultrafast magnetism and in the field of transient phenomena and phase transitions in solids. © 2014 International Union of Crystallography.
    view abstractdoi: 10.1107/S1600577514012247
  • 2014 • 53 Grain boundary characterization in multicrystalline silicon using joint EBSD, EBIC, and atom probe tomography
    Stoffers, A. and Cojocaru-Miredin, O. and Breitenstein, O. and Seifert, W. and Zaefferer, S. and Raabe, D.
    2014 Ieee 40th Photovoltaic Specialist Conference (pvsc) 42--46 (2014)
    The efficiency of multicrystalline silicon solar cells suffers from the presence of extended defects like dislocations and grain boundaries. In fact, the defects themselves do not implicitly have to be harmful, but their interaction with impurities makes them detrimental for the cell efficiencies. Here, we present a systematic method to correlate the grain boundary charge recombination activity with local grain boundary properties and the site specific segregation information. For that, electron beam induced current is used to characterize the recombination activity at the grain boundaries, while electron backscatter diffraction is used to map the grain boundary crystallography. Atom probe tips containing the desired grain boundary are cut by using a novel site-specific sample preparation. Finally, atom probe tomography is used to reveal the 3D distribution of the impurities at the selected grain boundary. In conclusion, this work is one of the first studies based on understanding the correlation between the charge recombination activity and structural as well as chemical properties at grain boundaries in multicrystalline silicon solar cells.
    view abstractdoi: 10.1109/PVSC.2014.6925089
  • 2014 • 52 Nanojunction-mediated photocatalytic enhancement in heterostructured CdS/ZnO, CdSe/ZnO, and CdTe/ZnO nanocrystals
    Eley, C. and Li, T. and Liao, F. and Fairclough, S.M. and Smith, J.M. and Smith, G. and Tsang, S.C.E.
    Angewandte Chemie - International Edition 53 7838-7842 (2014)
    A series of highly efficient semiconductor nanocrystal (NC) photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots (QDs). The resulting contact between two small but high-quality crystals creates novel CdX/ZnO heterostructured semiconductor nanocrystals (HSNCs) with extensive type-II nanojunctions that exhibit more efficient photocatalytic decomposition of aqueous organic molecules under UV irradiation. Catalytic testing and characterization indicate that catalytic activity increases as a result of a combination of both the intrinsic chemistry of the chalcogenide anions and the heterojunction structure. Atomic probe tomography (APT) is employed for the first time to probe the spatial characteristics of the nanojunction between cadmium chalcogenide and ZnO crystalline phases, which reveals various degrees of ion exchange between the two crystals to relax large lattice mismatches. In the most extreme case, total encapsulation of CdTe by ZnO as a result of interfacial alloying is observed, with the expected advantage of facilitating hole transport for enhanced exciton separation during catalysis. That's a (quantum dot) wrap! A series of highly active semiconductor photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots. The resulting heterostructured CdX/ZnO nanocrystals with extensive type-II nanojunctions exhibit rapid photocatalytic decomposition of organic molecules in aqueous media. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201404481
  • 2014 • 51 On the electrical asymmetry effect in large area multiple frequency capacitively coupled plasmas
    Bienholz, S. and Styrnoll, T. and Awakowicz, P.
    Journal of Physics D: Applied Physics 47 (2014)
    Recently, many publications have dealt with the electrical asymmetry effect in capacitively coupled radio frequency-discharges. The idea of this concept is the possibility of controlling the self-bias voltage by tuning the relative phase of harmonics in relation to the fundamental wave. In this work, we apply the electrical asymmetry effect on a large-area multiple frequency capacitively coupled plasma used for reactive sputtering by varying the relative phase of the 13.56 and 27.12 MHz excitation. The resulting voltage waveforms at the electrode are recorded using a high-voltage probe. The shape of the waveform is then analysed by Fourier analysis to study the influence of higher harmonics excited at the non-linearity of the plasma boundary sheath. To investigate the influence of the relative phase on the plasma itself, radially resolved multipole resonance probe measurements are performed. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/47/6/065201
  • 2014 • 50 On the spheroidized carbide dissolution and elemental partitioning in high carbon bearing steel 100Cr6
    Song, W. and Choi, P.-P. and Inden, G. and Prahl, U. and Raabe, D. and Bleck, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 595-606 (2014)
    We report on the characterization of high carbon bearing steel 100Cr6 using electron microscopy and atom probe tomography in combination with multi-component diffusion simulations. Scanning electron micrographs show that around 14 vol pct spheroidized carbides are formed during soft annealing and only 3 vol pct remain after dissolution into the austenitic matrix through austenitization at 1123 K (850 °C) for 300 seconds. The spheroidized particles are identified as (Fe, Cr)3C by transmission electron microscopy. Atom probe analysis reveals the redistribution and partitioning of the elements involved, i.e., C, Si, Mn, Cr, Fe, in both, the spheroidized carbides and the bainitic matrix in the sample isothermally heat-treated at 773 K (500 °C) after austenitization. Homogeneous distribution of C and a Cr gradient were detected within the spheroidized carbides. Due to its limited diffusivity in (Fe, Cr) 3C, Cr exhibits a maximum concentration at the surface of spheroidized carbides (16 at. pct) and decreases gradually from the surface towards the core down to about 2 at. pct. The atom probe results also indicate that the partially dissolved spheroidized carbides during austenitization may serve as nucleation sites for intermediate temperature cementite within bainite, which results in a relatively softer surface and harder core in spheroidized particles. This microstructure may contribute to the good wear resistance and fatigue properties of the steel. Good agreement between DICTRA simulations and experimental composition profiles is obtained by an increase of mobility of the substitutional elements in cementite by a factor of five, compared to the mobility in the database MOBFE2. © The Minerals, Metals & Materials Society and ASM International 2013.
    view abstractdoi: 10.1007/s11661-013-2048-5
  • 2014 • 49 Optical tweezers as manufacturing and characterization tool in microfluidics
    Köhler, J. and Ghadiri, R. and Ksouri, S.I. and Gurevich, E.L. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 9164 (2014)
    Pumping and mixing of small volumes of liquid samples are basic processes in microfluidic applications. Among the number of different principles for active transportation of the fluids microrotors have been investigated from the beginning. The main challenge in microrotors, however, has been the driving principle. In this work a new approach for a very simple magnetic driving principle has been realized. More precisely, we take advantage of optical grippers to fabricate various microrotors and introduce an optical force method to characterize the fluid flow generated by rotating the structures through magnetic actuation. The microrotors are built of silica and magnetic microspheres which are initially coated with Streptavidin or Biotin molecules. Holographic optical tweezers (HOT) are used to trap, to position, and to assemble the microspheres with the chemical interaction of the biomolecules leading to a stable binding. Using this technique, complex designs of microrotors can be realized. The magnetic response of the magnetic microspheres enables the rotation and control of the structures through an external magnetic field. The generated fluid flow around the microrotor is measured optically by inserting a probe particle next to the rotor. While the probe particle is trapped by optical forces the flow force leads to a displacement of the particle from the trapping position. This displacement is directly related to the flow velocity and can be measured and calibrated. Variations of the microrotor design and rotating speed lead to characteristic flow fields. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2063080
  • 2014 • 48 Precipitation of the α-phase in an ultrafine grained beta-titanium alloy processed by severe plastic deformation
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Materials Science and Engineering A 605 144-150 (2014)
    A fine and uniform distribution of α phase at grain boundaries is expected to improve the mechanical properties and thermal stability of beta Ti alloys. To design high strength alloys, a key factor is the volume fraction of α, which is related to the concentration of the α phase. In this study, α-phase precipitates were characterized in an ultrafine-grained Ti-15Nb-2Mo-2Zr-1Sn (at%) alloy processed by severe plastic deformation in two different ways (hot drawing and cold rolling in conjunction with annealing). A combination of transmission Kikuchi diffraction, transmission electron microscopy and atom-probe tomography revealed that ultra-fine α precipitates precipitate at grain boundaries in hot-drawn material or at sub-grain boundaries in the cold-rolled samples. The Nb concentrations of α phases formed were not those expected for an equilibrium state, which highlights the importance of understanding the chemistry of the α precipitates for engineering microstructures in advanced Ti alloys. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.03.044
  • 2014 • 47 Single core-shell nanoparticle probes for non-invasive magnetic force microscopy
    Uhlig, T. and Wiedwald, U. and Seidenstücker, A. and Ziemann, P. and Eng, L.M.
    Nanotechnology 25 (2014)
    We present an easy, fast and reliable method for the preparation of magnetic force microscopy (MFM) probes based on single Co nanoparticles (NPs). Due to their dipolar character, these magnetic probes open up a new approach for quantitative and non-invasive MFM measurements on the nanometer length scale. To guarantee long-term stability of these tips under ambient conditions, an ultrathin protecting Au shell was grown around the Co NPs through photochemical deposition. Single magnetic particles were firmly attached to standard silicon AFM tips using bifunctional self-assembling molecules. Such probes were tested on longitudinal magnetic recording media and compared to the results as recorded with conventional thin-film MFM tips. Easy data interpretation of the magnetic nanoparticle probes in a point dipole model is shown. Our nanoparticle tips provide excellent endurance for MFM recording, enable non-invasive probing while maintaining a high sensitivity, resolution, and reproducibility. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/25/255501
  • 2014 • 46 Study on electrostatic and electromagnetic probes operated in ceramic and metallic depositing plasmas
    Styrnoll, T. and Bienholz, S. and Lapke, M. and Awakowicz, P.
    Plasma Sources Science and Technology 23 (2014)
    This paper discusses plasma probe diagnostics, namely the multipole resonance probe (MRP) and Langmuir probe (LP), operated in depositing plasmas. The aim of this work is to show that the combination of both probes provides stable and robust measurements and clear determination of plasma parameters for metallic and ceramic coating processes. The probes use different approaches to determine plasma parameters, e.g. electron density ne and electron temperature Te. The LP is a well-established plasma diagnostic, and its applicability in technological plasmas is well documented. The LP is a dc probe that performs a voltage sweep and analyses the measured current, which makes it insensitive against conductive metallic coating. However, once the LP is dielectrically coated with a ceramic film, its functionality is constricted. In contrast, the MRP was recently presented as a monitoring tool, which is insensitive to coating with dielectric ceramics. It is a new plasma diagnostic based on the concept of active plasma resonance spectroscopy, which uses the universal characteristic of all plasmas to resonate on or near the electron plasma frequency. The MRP emits a frequency sweep and the absorption of the signal, the |S11| parameter, is analysed. Since the MRP concept is based on electromagnetic waves, which are able to transmit dielectrics, it is insensitive to dielectric coatings. But once the MRP is metallized with a thin conductive film, no undisturbed RF-signal can be emitted into the plasma, which leads to falsified plasma parameter. In order to compare both systems, during metallic or dielectric coating, the probes are operated in a magnetron CCP, which is equipped with a titanium target. We present measurements in metallic and dielectric coating processes with both probes and elaborate advantages and problems of each probe operated in each coating environment. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/23/2/025013
  • 2014 • 45 The multipole resonance probe: Progression and evaluation of a process compatible plasma sensor
    Schulz, C. and Styrnoll, T. and Storch, R. and Awakowicz, P. and Musch, T. and Rolfes, I.
    IEEE Sensors Journal 14 3408-3417 (2014)
    A robust and sensitive plasma sensor, the multipole resonance probe (MRP), and its process compatibility are presented and discussed in this paper. Based on its innovative concept and simple model describing the system 'probe-plasma', three steps of development are introduced. 3D electromagnetic field simulations are applied as an indispensable tool for an economical and efficient investigation and optimization of different sensor layouts. Independent of the chosen sensor design, a developed pulse-based measurement device yields an economical signal generation and evaluation. Electron density profiles, determined with the MRP and the pulse-based system utilized in a capacitive coupled plasma, confirm and demonstrate the simulation results and the measurement concept, respectively. © 2014 IEEE.
    view abstractdoi: 10.1109/JSEN.2014.2333659
  • 2013 • 44 Characterization of low-pressure microwave and radio frequency discharges in oxygen applying optical emission spectroscopy and multipole resonance probe
    Steves, S. and Styrnoll, T. and Mitschker, F. and Bienholz, S. and Nikita, B. and Awakowicz, P.
    Journal of Physics D: Applied Physics 46 (2013)
    Optical emission spectroscopy (OES) and multipole resonance probe (MRP) are adopted to characterize low-pressure microwave (MW) and radio frequency (RF) discharges in oxygen. In this context, both discharges are usually applied for the deposition of permeation barrier SiOx films on plastic foils or the inner surface of plastic bottles. For technological reasons the MW excitation is modulated and a continuous wave (cw) RF bias is used. The RF voltage produces a stationary low-density plasma, whereas the high-density MW discharge is pulsed. For the optimization of deposition process and the quality of the deposited barrier films, plasma conditions are characterized using OES and MRP. To simplify the comparison of applied diagnostics, both MW and RF discharges are studied separately in cw mode. The OES and MRP diagnostic methods complement each other and provide reliable information about electron density and electron temperature. In the MW case, electron density amounts to n e = (1.25 ± 0.26) x 10^17 m-3, and kTe to 1.93 ± 0.20 eV, in the RF case ne = (6.8 ± 1.8) x 10^15 m-3 and kTe = 2.6 ± 0.35 eV. The corresponding gas temperatures are 760±40 K and 440±20 K. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/44/445201
  • 2013 • 43 Picosecond opto-acoustic interferometry and polarimetry in high-index GaAs
    Scherbakov, A.V. and Bombeck, M. and Jäger, J.V. and Salasyuk, A.S. and Linnik, T.L. and Gusev, V.E. and Yakovlev, D.R. and Akimov, A.V. and Bayer, M.
    Optics Express 21 16473-16485 (2013)
    By means of a metal opto-acoustic transducer we generate quasilongitudinal and quasi-transverse picosecond strain pulses in a (311)-GaAs substrate and monitor their propagation by picosecond acoustic interferometry. By probing at the sample side opposite to the transducer the signals related to the compressive and shear strain pulses can be separated in time. In addition to conventional monitoring of the reflected probe light intensity we monitor also the polarization rotation of the optical probe beam. This polarimetric technique results in improved sensitivity of detection and provides comprehensive information about the elasto-optical anisotropy. The experimental observations are in a good agreement with a theoretical analysis. © 2013 Optical Society of America.
    view abstractdoi: 10.1364/OE.21.016473
  • 2013 • 42 Probing ultrafast carrier tunneling dynamics in individual quantum dots and molecules
    Müller, K. and Bechtold, A. and Ruppert, C. and Kaldewey, T. and Zecherle, M. and Wildmann, J.S. and Bichler, M. and Krenner, H.J. and Villas-Bôas, J.M. and Abstreiter, G. and Betz, M. and Finley, J.J.
    Annalen der Physik 525 49-58 (2013)
    Ultrafast pump-probe spectroscopy is employed to directly monitor the tunneling of charge carriers from single and vertically coupled quantum dots and probe intra-molecular dynamics. Immediately after resonant optical excitation, several peaks are observed in the pump-probe spectrum arising from Coulomb interactions between the photogenerated charge carriers. The influence of few-Fermion interactions in the photoexcited system and the temporal evolution of the optical response is directly probed in the time domain. In addition, the tunneling times for electrons and holes from the QD nanostructure are independently determined. In polarization resolved measurements, near perfect Pauli-spin blockade is observed in the spin-selective absorption spectrum as well as stimulated emission. While electron and hole tunneling from single quantum dots is shown to be well explained by the WKB formalism, for coupled quantum dots pronounced resonances in the electron tunneling rate are observed arising from elastic and inelastic electron tunneling between the different dots. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/andp.201200195
  • 2013 • 41 Process diagnostics and monitoring using the multipole resonance probe in an inhomogeneous plasma for ion-assisted deposition of optical coatings
    Styrnoll, T. and Harhausen, J. and Lapke, M. and Storch, R. and Brinkmann, R.P. and Foest, R. and Ohl, A. and Awakowicz, P.
    Plasma Sources Science and Technology 22 (2013)
    The application of a multipole resonance probe (MRP) for diagnostic and monitoring purposes in a plasma ion-assisted deposition (PIAD) process is reported. Recently, the MRP was proposed as an economical and industry compatible plasma diagnostic device (Lapke et al 2011 Plasma Sources Sci. Technol. 20 042001). The major advantages of the MRP are its robustness against dielectric coating and its high sensitivity to measure the electron density. The PIAD process investigated is driven by the advanced plasma source (APS), which generates an ion beam in the deposition chamber for the production of high performance optical coatings. With a background neutral pressure of p 0 ~ 20 mPa the plasma expands from the source region into the recipient, leading to an inhomogeneous spatial distribution. Electron density and electron temperature vary over the distance from substrate (ne ~ 109 cm-3 and Te,eff ~ 2 eV) to the APS (ne >~ 1012 cm-3 and Te,eff ~ 20 eV) (Harhausen et al 2012 Plasma Sources Sci. Technol. 21 035012). This huge variation of the plasma parameters represents a big challenge for plasma diagnostics to operate precisely for all plasma conditions. The results obtained by the MRP are compared to those from a Langmuir probe chosen as reference diagnostics. It is demonstrated that the MRP is suited for the characterization of the PIAD plasma as well as for electron density monitoring. The latter aspect offers the possibility to develop new control schemes for complex industrial plasma environments. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/22/4/045008
  • 2013 • 40 Routes to rupture and folding of graphene on rough 6H-SiC(0001) and their identification
    Temmen, M. and Ochedowski, O. and Kleine Bussmann, B. and Schleberger, M. and Reichling, M. and Bollmann, T.R.J.
    Beilstein Journal of Nanotechnology 4 625-631 (2013)
    Twisted few layer graphene (FLG) is highly attractive from an application point of view, due to its extraordinary electronic properties. In order to study its properties, we demonstrate and discuss three different routes to in situ create and identify (twisted) FLG. Single layer graphene (SLG) sheets mechanically exfoliated under ambient conditions on 6H-SiC(0001) are modified by (i) swift heavy ion (SHI) irradiation, (ii) by a force microscope tip and (iii) by severe heating. The resulting surface topography and the surface potential are investigated with non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). SHI irradiation results in rupture of the SLG sheets, thereby creating foldings and bilayer graphene (BLG). Applying the other modification methods creates enlarged (twisted) graphene foldings that show rupture along preferential edges of zigzag and armchair type. Peeling at a folding over an edge different from a low index crystallographic direction can result in twisted BLG, showing a similar height as Bernal (or AA-stacked) BLG in NC-AFM images. The rotational stacking can be identified by a significant contrast in the local contact potential difference (LCPD) measured by KPFM. © 2013 Komkova et al; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.4.69
  • 2013 • 39 Supervision and control of medical sterilization processes utilizing the multipole resonance probe
    Schulz, C. and Styrnoll, T. and Awakowicz, P. and Rolfes, I.
    2013 IEEE MTT-S International Microwave Workshop Series on RF and Wireless Technologies for Biomedical and Healthcare Applications, IMWS-BIO 2013 - Proceedings (2013)
    An innovative and sensitive plasma probe suitable for the supervision and control of low-temperature plasma sterilization processes is presented in this contribution. For heat or chemical sensitive materials, plasmas are an indispensable tool regarding the sterilization of surgery instruments, for example. The presented Multipole Resonance Probe (MRP) allows for the simultaneous determination of plasma density, plasma temperature, and collision frequency by a simple and fast evaluation of its frequency response. Fed by an rf-signal, the MRP yields sensitive and local measurements for the determination of lowest fluctuations and for the application of a sensor network, respectively. With a minimal distance of 3 cm between two probes, the MRP can be deployed effectively as sensor network inside the plasma for the supervision of its stability and homogeneity. Based on 3D-electromagnetic field simulations the advantages of the MRP are discussed in detail. Compared to a Langmuir probe, measurements in a Double Inductive Coupled Plasma (DICP) show the suitability of the MRP inside an argon plasma. © 2013 IEEE.
    view abstractdoi: 10.1109/IMWS-BIO.2013.6756158
  • 2013 • 38 The multipole resonance probe: Evolution of a plasma sensor
    Schulz, C. and Rolfes, I. and Styrnoll, T. and Awakowicz, P. and Oberrath, J. and Mussenbrock, T. and Brinkmann, R.P. and Storch, R. and Musch, T.
    Proceedings of IEEE Sensors (2013)
    A robust and sensitive plasma probe, the multipole resonance probe (MRP), and its importance for industrial purposes is presented and discussed in this paper. Based on its innovative concept and its simple model of the system 'probe-plasma', a novel wall-mounted sensor is introduced. This sensor represents an optimized design of one sector of the MRP's assembly and is investigated within 3D-electromagnetic field simulations and compared to measurements of the MRP in an argon plasma. The resulting wall-mounted sensor can be designed for a desired application, which operates within a limited frequency range. The presented sensor covers a density range of approximately ne = 1016 m-3.. 1017 m-3, which is sufficient for the considered process. © 2013 IEEE.
    view abstractdoi: 10.1109/ICSENS.2013.6688324
  • 2013 • 37 Understanding the detection of carbon in austenitic high-Mn steel using atom probe tomography
    Marceau, R.K.W. and Choi, P. and Raabe, D.
    Ultramicroscopy 132 239-247 (2013)
    A high-Mn TWIP steel having composition Fe-22Mn-0.6C (wt%) is considered in this study, where the need for accurate and quantitative analysis of clustering and short-range ordering by atom probe analysis requires a better understanding of the detection of carbon in this system. Experimental measurements reveal that a high percentage of carbon atoms are detected as molecular ion species and on multiple hit events, which is discussed with respect to issues such as optimal experimental parameters, correlated field evaporation and directional walk/migration of carbon atoms at the surface of the specimen tip during analysis. These phenomena impact the compositional and spatial accuracy of the atom probe measurement and thus require careful consideration for further cluster-finding analysis. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2013.01.010
  • 2012 • 36 Application of endoscopic OH*-chemiluminescence measurements at a full-scale high-pressure gas turbine combustion test rig
    Witzel, B. and Heinze, J. and Kock, B.F. and Stockhausen, G. and Streb, H. and Van Kampen, J. and Schulz, C. and Willert, C. and Fleing, C.
    Proceedings of the ASME Turbo Expo 1 817-825 (2012)
    Single burner combustion tests play a key role in the Siemens gas turbine combustion system development process. The main scope of these tests is to assess the performance of combustor design variants in terms emissions or combustion stability at gas turbine relevant operation conditions. Both emissions and combustion stability strongly depend on the flame front and flame position. A pragmatic approach to investigate the flame is to detect the chemiluminescence signal of the combustion intermediate species OH*. Thus, the OH*- chemiluminescence signal was recorded at high-pressure combustion tests to get more insight in the complex interactions between combustor design, operation conditions and combustion performance. To minimize the impact of the measurement system on the combustion behavior, the optical access to the test rig was realized by using a water-cooled probe with an UV-transparent endoscope. The probe was located in the test rig side-wall, downstream of the burner outlet, viewing towards the burner with a 90° angle relative to the endoscope orientation. The experimental setup was completed by a combination of bandpass filters and an ICCD camera. During the experiments acoustic pressure oscillations inside the combustion chamber were recorded simultaneously to the chemiluminescence images to allow for phase-sorting of the recorded images during the image post-processing. The postprocessed images then were correlated with the pressure oscillations to investigate the relationship of the heat release to the pressure oscillations. The measurements were carried out during single burner gas turbine combustion tests at realistic gas turbine operation conditions at a scaled pressure of 9 bar. This paper presents selected test results and discusses how they give new insight in the complex combustion processes at full-scale high-pressure gas turbine combustion tests. Copyright © 2012 by ASME.
    view abstractdoi: 10.1115/GT2012-68965
  • 2012 • 35 Characterization of oxidation and reduction of a Palladium-Rhodium alloy by atom-probe tomography
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
    Journal of Physical Chemistry C 116 4760-4766 (2012)
    Platinum group metals (PGMs) are used in numerous catalyst applications, including conversion of engine exhaust gases and hydrocarbon reforming. Reducing the loading of PGMs without diminishing the overall catalyst activity is a major challenge. Fundamental studies of PGMs under reactive conditions can assist the design/synthesis of "nanoengineered" catalysts, tunable and optimized for cost, stability, and performance. In the present study, the oxidation and reduction behavior of a Pd-6.4 at. % Rh alloy is investigated following treatment at 873 K for various exposure times using atom-probe tomography. For short oxidation times (10 min), an oxide layer with PdO stoichiometry grows on the surface. As the oxidation time increases, two phases with stoichiometries of (Rh 1Pd 1)O 2 and (Pd 2O) evolve. When the alloy is subsequently reduced in hydrogen, a nanoscale dispersion of Rh-rich metallic regions remains. This provides a route for the synthesis of multifunctional catalysts with different nanosurface regions in close proximity to one another. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp211687m
  • 2012 • 34 Combined high resolution Scanning Kelvin probe - Scanning electrochemical microscopy investigations for the visualization of local corrosion processes
    Maljusch, A. and Senöz, C. and Rohwerder, M. and Schuhmann, W.
    Electrochimica Acta 82 339-348 (2012)
    An integrated SKP-SECM system was successfully optimised with respect to improved lateral resolution. An aluminum alloy was synthesised by solidification of a liquid melt of pure Al, Cu and Mg metal powders in order to visualize single S-phase intermetallic particles (IMPs) using a newly proposed "glass free" SKP-SECM tip. The obtained IMPs were randomly distributed in the solid solution matrix of the alloy and their average chemical composition was in agreement with that of S-phase IMPs in commercially available AA2024-T351 alloys. The S-phase IMPs were localized in the SKP mode of the SKP-SECM system. The increased electrochemical activity of the S-phase IMPs was visualized using the feedback mode of SECM and the in situ consumption of O 2 on the surface of a single S-phase IMP was visualized in the redox-competition mode of the SECM using the same tip. Thus, the local Volta potential difference obtained in the SKP mode could be overlaid with the local electrochemical activity for O 2 reduction. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2012.05.134
  • 2012 • 33 Computer simulation of two-step atomization in graphite furnaces for analytical atomic spectrometry
    Zakharov, Yu.A. and Kokorina, O.B. and Lysogorskii, Yu.V. and Staroverov, A.E.
    Journal of Analytical Chemistry 67 714-721 (2012)
    The processes of sample fractionation by two-step atomization with the intermediate condensation of the analyte on a cold surface in graphite furnaces were theoretically studied. The transfer equation was solved for the atoms, molecules, and condensed particles of the sample from a flow of argon directed along this surface. The spatial distributions of vapor and the condensate formed were calculated depending on the composition and flow rate. It was found that a cold surface section with a length of 6 mm is sufficient for the complete trapping of atomic analyte vapor from an argon layer having a velocity of about 1 m/sec and a thickness of 5 mm. In this case, the molecules and clusters condensation coefficients smaller than unity were deposited insignificantly; that is, they were fractionally separated. The results of the shadow spectral visualization of the process of sample fractionation on a cold probe surface of in commercial HGA and THGA atomizers were interpreted. The advantages of analytical signals upon the evaporation of a sample condensate from the probe in these atomizers and inductively coupled plasma were demonstrated. © 2012 Pleiades Publishing, Inc.
    view abstractdoi: 10.1134/S1061934812060214
  • 2012 • 32 Electrical control of interdot electron tunneling in a double InGaAs quantum-dot nanostructure
    Müller, K. and Bechtold, A. and Ruppert, C. and Zecherle, M. and Reithmaier, G. and Bichler, M. and Krenner, H.J. and Abstreiter, G. and Holleitner, A.W. and Villas-Boas, J.M. and Betz, M. and Finley, J.J.
    Physical Review Letters 108 (2012)
    We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photogenerated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5ps) interdot tunneling is shown to occur over a surprisingly wide bandwidth, up to ∼8meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.108.197402
  • 2012 • 31 Electronic characterization of single-layer MoS2 sheets exfoliated on SrTiO3
    Bußmann, B.K. and Marinov, K. and Ochedowski, O. and Scheuschner, N. and Maultzsch, J. and Schleberger, M.
    Materials Research Society Symposium Proceedings 1474 30-34 (2012)
    Single layer regions of MoS2 on SiO2 and SrTiO 3 were identified by Raman spectroscopy and μ-photoluminescence before Kelvin probe force microscopy was performed. For the already known system MoS2/SiO2 we find 1.839 eV for the direct bandgap, in good agreement with earlier results. On MoS2/SrTiO3 the direct bandgap was determined to be 1.829 eV. From our Kelvin probe data we infer that the SrTiO3 substrate leads to a dipole layer at the interface of the MoS2 single layer. The corresponding μ-PL measurements however show no significant decrease of the bandgap. This shows, that in the case of MoS2 the carrier type as well as concentration is not significantly influenced by the choice of SrTiO3 as the substrate compared to SiO2. © 2012 Materials Research Society.
    view abstractdoi: 10.1557/opl.2012.1463
  • 2012 • 30 Probing ultrafast charge and spin dynamics in a quantum dot molecule
    Müller, K. and Bechtold, A. and Ruppert, C. and Krenner, H.J. and Bichler, M. and Villas-Bôas, J.M. and Abstreiter, G. and Betz, M. and Finley, J.J.
    Proceedings of SPIE - The International Society for Optical Engineering 8260 (2012)
    We apply ultrafast pump-probe photocurrent spectroscopy to directly probe few Fermion charge and spin dynamics in an artificial molecule formed by vertically stacking a pair of InGaAs self-assembled quantum dots. As the relative energy of the orbital states in the two dots are energetically tuned by applying static electric fields, pronounced anticrossings are observed arising from electron tunnel couplings. Time resolved photocurrent measurements performed in the vicinity of these anticrossings provide direct information on the comparative roles of elastic and inelastic resonant tunneling processes between the two quantum dots forming the molecule. Resonant pumping of the neutral exciton in the upper dot with circularly polarized light facilitates ultrafast initialization of hole spin qubits over timescales limited only by the laser pulse duration (<5ps) and a near perfect Pauli spin-blockade with a near unity suppression of absorption (>96%) for spin forbidden transitions. Such a spin selective photocurrent response opens the way to probe spin dynamics in the system over ultrafast timescales. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstractdoi: 10.1117/12.907795
  • 2012 • 29 Quantitative label-free monitoring of peptide recognition by artificial receptors: A comparative FT-IR and UV resonance Raman spectroscopic study
    Niebling, S. and Kuchelmeister, H.Y. and Schmuck, C. and Schlücker, S.
    Chemical Science 3 3371-3377 (2012)
    Vibrational spectroscopic investigations on molecular recognition processes are surprisingly rare, even at the qualitative level. In this first comparative study, we employ Fourier-transform infrared (FT-IR) and UV resonance Raman (UVRR) spectroscopy for quantitative label-free monitoring of molecular recognition processes. Specifically, the complexation of two different tetrapeptide ligands by an artificial receptor is investigated. The central advantage of UVRR is its capability to selectively probe the binding site of the receptor in the free/unbound and complexed form. In contrast, FT-IR probes the entire receptor-ligand complex without spectral selectivity, thereby providing complementary vibrational information. Multivariate analysis of the experimental IR/UVRR binding studies is required for determining association constants and the vibrational spectrum of the complex, which is not directly accessible. Both FT-IR and UVRR spectroscopy provide similar association constants for the two different tetrapeptide ligands. Complementary DFT calculations support the interpretation of the observed spectral changes upon complexation, which is a prerequisite for extracting structural information from vibrational binding studies. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2sc20811c
  • 2012 • 28 The interplay of topography and energy dissipation in pentacene thin films
    Wall, S. and Thien, D. and Meyer zu Heringdorf, F.-J.
    Journal of Electron Spectroscopy and Related Phenomena 185 436-440 (2012)
    Nonlinear photoemission electron microscopy was used to study the morphology-dependent lifetime of electronic excitations in pentacene islands on Si(0 0 1) and (√3 × √3)R30°-Ag/Si(1 1 1). After an optical excitation of electrons by a λ = 400 nm femtosecond laser pulse the characteristic decay times were measured with spatial resolution in a pump-probe setup. For pentacene on Si(0 0 1), the observed lifetimes vary by a factor of two between the wetting layer and the fractal-shaped pentacene islands. The measured lifetime difference is explained by a difference in the electronic coupling of the pentacene islands and the wetting layer to the substrate. For pentacene on (√3 × √3)R30°-Ag/Si(1 1 1), similar lifetimes are found, although the orientation of the pentacene molecules in the compact islands is rotated. Our findings suggest that electronic excitations in higher layers of the pentacene islands do not diffuse to the interface before they decay. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.elspec.2012.09.007
  • 2012 • 27 Ultrafast demagnetization measurements using extreme ultraviolet light: Comparison of electronic and magnetic contributions
    La-O-Vorakiat, C. and Turgut, E. and Teale, C.A. and Kapteyn, H.C. and Murnane, M.M. and Mathias, S. and Aeschlimann, M. and Schneider, C. M. and Shaw, J.M. and Nembach, H.T. and Silva, T.J.
    Physical Review X 2 (2012)
    Ultrashort pulses of extreme ultraviolet light from high-harmonic generation are a new tool for probing coupled charge, spin, and phonon dynamics with element specificity, attosecond pump-probe synchronization, and time resolution of a few femtoseconds in a tabletop apparatus. In this paper, we address an important question in magneto-optics that has implications for understanding magnetism on the fastest time scales: Is the signal from the transverse magneto-optical Kerr effect at the M 2,3 edges of a magnetic material purely magnetic or is it perturbed by nonmagnetic artifacts? Our measurements demonstrate conclusively that transverse magneto-optical Kerr measurements at the M 2,3 edges sensitively probe the magnetic state, with almost negligible contributions from the transient variation of the refractive index by the nonequilibrium hot-electron distribution. In addition, we compare pump-probe demagnetization dynamics measured by both high harmonics and conventional visible-wavelength magneto-optics and find that the measured demagnetization times are in agreement.
    view abstractdoi: 10.1103/PhysRevX.2.011005
  • 2012 • 26 Ultrafast field-resolved semiconductor spectroscopy utilizing quantum interference control of currents
    Ruppert, C. and Lohrenz, J. and Thunich, S. and Betz, M.
    Optics Letters 37 3879-3881 (2012)
    We implement a versatile concept to time-resolve optical nonlinearities of semiconductors in amplitude and phase. A probe pulse transmitted through the optically pumped sample is superimposed with first subharmonic spectral components derived from the same laser source. This effective ω/2ω pulse pair induces a coherently controlled current in a time-integrating semiconductor detector. Current interferograms obtained by scanning the ω/2ω time delay then reveal the electric field of the 2ω part as well as its pump-induced modifications. As a paradigm we analyze the excitonic optical nonlinearity of a CdTe thin film at frequencies around 385 THz. We then move on to resolve the pump-induced amplitude-and phase-distortions of a probe pulse related to two-photon absorption and cross-phase modulation in ZnSe. © 2012 Optical Society of America.
    view abstractdoi: 10.1364/OL.37.003879
  • 2011 • 25 A (S)TEM and atom probe tomography study of InGaN
    Mehrtens, T. and Bley, S. and Schowalter, M. and Sebald, K. and Seyfried, M. and Gutowski, J. and Gerstl, S.S.A. and Choi, P.-P. and Raabe, D. and Rosenauer, A.
    Journal of Physics: Conference Series 326 (2011)
    In this work we show how the indium concentration in high indium content InxGa1-xN quantum wells, as they are commonly used in blue and green light emitting diodes, can be deduced from high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. This method bases on introducing normalized intensities which can be compared with multislice simulations to determine the specimen thickness or the indium concentration. The evaluated concentrations are compared with atom probe tomography measurements. It is also demonstrated how the quality of focused ion beam prepared TEM-lamellas can be improved by an additional etching with low energy ions.
    view abstractdoi: 10.1088/1742-6596/326/1/012029
  • 2011 • 24 Application of AC-SECM in corrosion science: Local visualisation of inhibitor films on active metals for corrosion protection
    Pähler, M. and Santana, J.J. and Schuhmann, W. and Souto, R.M.
    Chemistry - A European Journal 17 905-911 (2011)
    The suitability of frequency-dependent alternating-current scanning electrochemical microscopy (4D AC-SECM) for investigation of thin passivating layers covering the surface of corrosion-inhibited metals has been demonstrated. Inhibition of copper corrosion by benzotriazole (BTAH) and methylbenzotriazole (MBTAH), which are effective inhibitors for this metal under many environmental conditions, was investigated. Strong dependencies were found for the AC z-approach curves with both the duration of the inhibitor treatment and the frequency of the AC excitation signal applied in AC-SECM. Both negative and positive feedback behaviours were observed in the AC approach curves for untreated copper and for Cu/BTAH and Cu/MBTAH samples. Negative feedback behaviour occurred in the low-frequency range, whereas a positive feedback effect was observed at higher frequencies. A threshold frequency related to the passage from negative to positive regimes could be determined in each case. The threshold frequency for inhibitor-modified samples was found always to be significantly higher than for the untreated metal, because the inhibitor film provides electrical insulation for the surface. Moreover, the threshold frequency increased with increasing surface coverage by the inhibitor. 4D AC-SECM was successfully applied to visualizing spatially resolved differences in local electrochemical activity between inhibitor-free and inhibitor-covered areas of the sample. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201000689
  • 2011 • 23 Atom probe tomography characterization of heavily cold drawn pearlitic steel wire
    Lia, Y.J. and Choi, P. and Borchers, C. and Chen, Y.Z. and Goto, S. and Raabe, D. and Kirchheim, R.
    Ultramicroscopy 111 628-632 (2011)
    Atom Probe Tomography (APT) was used to analyze the carbon distribution in a heavily cold drawn pearlitic steel wire with a true strain of 6.02. The carbon concentrations in cementite and ferrite were separately measured by a sub-volume method and compared with the literature data. It is found that the carbon concentration in ferrite saturates with strain. The carbon concentration in cementite decreases with the lamellar thickness, while the carbon atoms segregate at dislocations or cell/grain boundaries in ferrite. The mechanism of cementite decomposition is discussed in terms of the evolution of dislocation structure during severe plastic deformation. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.11.010
  • 2011 • 22 Atomic-scale distribution of impurities in cuinse2-based thin-film solar cells
    Cojocaru-Miredin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Ultramicroscopy 111 552-556 (2011)
    Atom Probe Tomography was employed to investigate the distribution of impurities, in particular sodium and oxygen, in a cuinse2-based thin-film solar cell. It could be shown that sodium, oxygen, and silicon diffuse from the soda lime glass substrate into the cuinse2 film and accumulate at the grain boundaries. Highly dilute concentrations of sodium and oxygen were measured in the bulk. Selenium was found to be depleted at the grain boundaries. These observations could be confirmed by complementary energy dispersive X-ray spectroscopy studies. Our results support the model proposed by Kronik et al. (1998) [1], which explains the enhanced photovoltaic efficiency of sodium containing cuinse2 solar cells by the passivation of selenium vacancies at grain boundaries. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.034
  • 2011 • 21 Atomic-scale mechanisms of deformation-induced cementite decomposition in pearlite
    Li, Y.J. and Choi, P. and Borchers, C. and Westerkamp, S. and Goto, S. and Raabe, D. and Kirchheim, R.
    Acta Materialia 59 3965-3977 (2011)
    Pearlitic steel can exhibit tensile strengths higher than 5 GPa after severe plastic deformation, where the deformation promotes a refinement of the lamellar structure and cementite decomposition. However, a convincing correlation between deformation and cementite decomposition in pearlite is still absent. In the present work, a local electrode atom probe was used to characterize the microstructural evolution of pearlitic steel, cold-drawn with progressive strains up to 5.4. Transmission electron microscopy was also employed to perform complementary analyses of the microstructure. Both methods yielded consistent results. The overall carbon content in the detected volumes as well as the carbon concentrations in ferrite and cementite were measured by atom probe. In addition, the thickness of the cementite filaments was determined. In ferrite, we found a correlation of carbon concentration with the strain, and in cementite, we found a correlation of carbon concentration with the lamella thickness. Direct evidence for the formation of cell/subgrain boundaries in ferrite and segregation of carbon atoms at these defects was found. Based on these findings, the mechanisms of cementite decomposition are discussed in terms of carbon-dislocation interaction. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.022
  • 2011 • 20 Characterization of grain boundaries in Cu(In,Ga)Se 2 films using atom-probe tomography
    Cojocaru-Mirédin, O. and Choi, P.-P. and Abou-Ras, D. and Schmidt, S.S. and Caballero, R. and Raabe, D.
    IEEE Journal of Photovoltaics 1 207-212 (2011)
    This paper discusses the advantages of pulsed laser atom-probe tomography (APT) to analyze Cu(In,Ga)Se 2-based solar cells. Electron backscatter diffraction (EBSD) was exploited for site-specific preparation of APT samples at selected Cu(In,Ga)Se 2 grain boundaries. This approach is very helpful not only to determine the location of grain boundaries but also to classify them as well. We demonstrate that correlative transmission electron microscopy (TEM) analyses on atom-probe specimens enable the atom-probe datasets to be reconstructed with high accuracy. Moreover, EBSD and TEM can be very useful to obtain complementary information about the crystal structure in addition to the compositional analyses. The local chemical compositions at grain boundaries of a solar grade Cu(In,Ga)Se 2 film are presented here. Na, K, and O impurities are found to be segregated at grain boundaries. These impurities most likely diffuse from the soda lime glass substrate into the absorber layer during cell fabrication and processing. Based on the experimental results, we propose that Na, K, and O play an important role in the electrical properties of grain boundaries in Cu(In,Ga)Se 2 thin films for solar cells. © 2011 IEEE.
    view abstractdoi: 10.1109/JPHOTOV.2011.2170447
  • 2011 • 19 Characterization of nano-sized precipitates in a Mn-based lean maraging steel by atom probe tomography
    Millán, J. and Ponge, D. and Raabe, D. and Choi, P. and Dmitrieva, O.
    Steel Research International 82 137-145 (2011)
    We present atom probe tomography results of a precipitation-hardened Mn-based maraging steel (9 Mn, 1.9 Ni, 0.6 Mo, 1.1 Ti, 0.33 Al; in at.%). The alloy is characterized by the surprising effect that both, strength and total elongation increase upon aging. The material reveals a high ultimate tensile strength (UTS) up to 1GPa and good ductility (total elongation (TE) of up to 15% in a tensile test) depending on aging conditions. We map the evolution of the precipitates after 450°C aging treatment using atom probe tomography in terms of chemical composition and size distribution. Copyright © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/srin.201000274
  • 2011 • 18 Characterization of oxidation and reduction of a platinum-rhodium alloy by atom-probe tomography
    Li, T. and Marquis, E.A. and Bagot, P.A.J. and Tsang, S.C. and Smith, G.D.W.
    Catalysis Today 175 552-557 (2011)
    An active challenge in heterogeneous catalysis is to minimize the quantities of the expensive platinum group metals used without causing degradation of the overall catalytic efficiency in a chemical reaction. To achieve this goal, a thorough atomic-scale understanding of these materials under reactive conditions is required. This will enable the design and production of "nano-engineered" catalysts, optimised for cost, stability and performance. In this study, the oxidation and reduction behaviour of a Pt-Rh alloy between 873 and 1073K was investigated by atom-probe tomography (APT). Detailed observations of the concentration profiles at the oxide/metal interfaces show that the growth of Rh2O3 oxide is limited by diffusion of Rh in the alloy. By varying the oxidation conditions, it was possible to calculate the activation energy for Rh diffusion in Pt-Rh as 236 ± 41 kJ/mol, together with diffusion coefficients for Rh for a range of temperatures. Reduction of the oxide phase left a thin, almost pure, layer of the most reactive (and expensive) element, Rh, on the surface of the specimen, suggesting a simple route for engineering the formation of the core-shell structure Pt-Rh nanoparticles. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2011.03.046
  • 2011 • 17 Chemical gradients across phase boundaries between martensite and austenite in steel studied by atom probe tomography and simulation
    Dmitrieva, O. and Ponge, D. and Inden, G. and Millán, J. and Choi, P. and Sietsma, J. and Raabe, D.
    Acta Materialia 59 364-374 (2011)
    Partitioning at phase boundaries of complex steels is important for their properties. We present atom probe tomography results across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculations (using a finite interface mobility). © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.09.042
  • 2011 • 16 Comparative atom probe study of Cu(In,Ga)Se 2 thin-film solar cells deposited on soda-lime glass and mild steel substrates
    Choi, P.-P. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Journal of Applied Physics 110 (2011)
    We report on a comparative study of Cu(In,Ga)Se 2 solar cells deposited on soda-lime glass and mild steel substrates, using atom probe tomography in conjunction with secondary ion mass spectrometry, x-ray fluorescence, current density-voltage, and external quantum efficiency measurements. Cu(In,Ga)Se 2 films deposited on soda-lime glass substrates and on steel substrates with a NaF precursor layer on top of the Mo back contact contain a significant amount of Na impurities and yield an enhanced open circuit voltage and fill factor. Using atom probe tomography, Na atoms are found to be segregated at grain boundaries and clustered in both bulk and grain boundaries. The atom probe data indicate that Na Cu point defects are most likely formed at grain boundaries, reducing the number of compensating In Cu point defects and thus contributing to an enhanced cell efficiency. However, for steel substrates the positive effect of Na on the cell performance is counterbalanced by the incorporation of Fe impurities into the Cu(In,Ga)Se 2 film. Fe atoms are homogeneously distributed inside the grains suggesting that Fe introduces point defects in the bulk © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3665723
  • 2011 • 15 In situ scanning tunneling microscopy study of selective dissolution of Au3Cu and Cu3Au (0 0 1)
    Renner, F.U. and Eckstein, G.A. and Lymperakis, L. and Dakkouri-Baldauf, A. and Rohwerder, M. and Neugebauer, J. and Stratmann, M.
    Electrochimica Acta 56 1694-1700 (2011)
    We present an electrochemical study of Au3Cu (0 0 1) single crystal surfaces in 0.1 mol dm-3 H2SO4 and 0.1 mol dm-3 H2SO4 + 0.1 mmol dm-3 HCl, and of Cu3Au (0 0 1) in 0.1 mol dm-3 H2SO 4. The focus is on in situ scanning tunneling microscopy experiments. The changes of the surface morphology, which are time- and potential-dependent, have been observed, clearly resolving single atomic steps and mono-atomic islands and pits. Chloride additives enhance the surface diffusion and respective morphologies are observed earlier. All surfaces have shown considerable roughening already in the passive region far below the critical potential. © 2010 Elsevier Ltd All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2010.09.061
  • 2011 • 14 Integrated scanning kelvin probe-scanning electrochemical microscope system: Development and first applications
    Maljusch, A. and Schönberger, B. and Lindner, A. and Stratmann, M. and Rohwerder, M. and Schuhmann, W.
    Analytical Chemistry 83 6114-6120 (2011)
    The integration of a scanning Kelvin probe (SKP) and a scanning electrochemical microscope (SECM) into a single SKP-SECM setup, the concept of the proposed system, its technical realization, and first applications are presented and discussed in detail. A preloaded piezo actuator placed in a grounded stainless steel case was used as the driving mechanism for oscillation of a Pt disk electrode as conventionally used in SECM when the system was operated in the SKP mode. Thus, the same tip is recording the contact potential difference (CPD) during SKP scanning and is used as a working electrode for SECM imaging in the redox-competition mode (RC-SECM). The detection of the local CPD is established by amplification of the displacement current at an ultralow noise operational amplifier and its compensation by application of a variable backing potential (V b) in the external circuit. The control of the tip-to-sample distance is performed by applying an additional alternating voltage with a much lower frequency than the oscillation frequency of the Kelvin probe. The main advantage of the SKP-SECM system is that it allows constant distance measurements of the CPD in air under ambient conditions and in the redox-competition mode of the SECM in the electrolyte of choice over the same sample area without replacement of the sample or exchange of the working electrode. The performance of the system was evaluated using a test sample made by sputtering thin Pt and W films on an oxidized silicon wafer. The obtained values of the CPD correlate well with known data, and the electrochemical activity for oxygen reduction is as expected higher over Pt than W. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ac200953b
  • 2011 • 13 Local voltage drop in a single functionalized graphene sheet characterized by Kelvin probe force microscopy
    Yan, L. and Punckt, C. and Aksay, I.A. and Mertin, W. and Bacher, G.
    Nano Letters 11 3543-3549 (2011)
    We studied the local voltage drop in functionalized graphene sheets of subμm size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current-voltage characteristics and an intrinsic conductivity of about 3.7 × 10 5 S/m corresponding to a sheet resistance of 2.7 kΩ/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found to be < 6.3 × 10-7 Ωcm2. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl201070c
  • 2011 • 12 Plasma and optical thin film technologies
    Stenzel, O. and Wilbrandt, S. and Kaiser, N. and Schmitz, C. and Turowski, M. and Ristau, D. and Awakowicz, P. and Brinkmann, R.P. and Musch, T. and Rolfes, I. and Steffen, H. and Foest, R. and Ohl, A. and Köhler, T. and Dolgonos...
    Proceedings of SPIE - The International Society for Optical Engineering 8168 (2011)
    The PluTO project is aimed at combining thin-film and plasma technologies. Accordingly, the consortium comprises experts in optical coating (Laser Zentrum Hannover, Fraunhofer IOF) and such in plasma technology (INP Greifswald, Ruhr University of Bochum RUB). The process plasmas available, especially the sheath layers, will be thoroughly characterized by means of special probes, so that the types, numbers and energies of the particles participating in the coating formation processes can be determined comprehensively in every detail for the first time. The data thus obtained will provide a basis for a numerical modelling of layer growth at atomic scale (Bremen Center for Computational Materials Science BCCMS). The results are expected to deepen the understanding of the physical mechanisms responsible for the influence of plasma action on the layer properties. In parallel, suitable tools for process monitoring will be identified and made available. Some first results have already been achieved which prove the viability of the approach. © 2011 SPIE.
    view abstractdoi: 10.1117/12.895323
  • 2011 • 11 Plasma and particle temperature measurements in thermal spray: Approaches and applications
    Mauer, G. and Vaßen, R. and Stöver, D.
    Journal of Thermal Spray Technology 20 391-406 (2011)
    Growing demands on the quality of thermally sprayed coatings require reliable methods to monitor and optimize the spraying processes. Thus, the importance of diagnostic methods is increasing. A critical requirement of diagnostic methods in thermal spray is the accurate measurement of temperatures. This refers to the hot working gases as well as to the in-flight temperature of the particles. This article gives a review of plasma and particle temperature measurements in thermal spray. The enthalpy probe, optical emission spectroscopy, and computer tomography are introduced for plasma measurements. To determine the in-flight particle temperatures mainly multicolor pyrometry is applied and is hence described in detail. The theoretical background, operation principles and setups are given for each technique. Special interest is attached to calibration methods, application limits, and sources of errors. Furthermore, examples of fields of application are given in the form of results of current research work. © 2010 ASM International.
    view abstractdoi: 10.1007/s11666-010-9603-z
  • 2011 • 10 Pulsed-laser atom probe studies of a precipitation hardened maraging TRIP steel
    Dmitrieva, O. and Choi, P. and Gerstl, S.S.A. and Ponge, D. and Raabe, D.
    Ultramicroscopy 111 623-627 (2011)
    A precipitation hardened maraging TRIP steel was analyzed using a pulsed laser atom probe. The laser pulse energy was varied from 0.3 to 1.9 nJ to study its effect on the measured chemical compositions and spatial resolution. Compositional analyses using proximity histograms did not show any significant variations in the average matrix and precipitate compositions. The only remarkable change in the atom probe data was a decrease in the++/+ charge state ratios of the elements. The values of the evaporation field used for the reconstructions exhibit a linear dependence on the laser pulse energy. The adjustment of the evaporation fields used in the reconstructions for different laser pulse energies was based on the correlation of the obtained cluster shapes to the TEM observations. No influence of laser pulse energy on chemical composition of the precipitates and on the chemical sharpness of their interfaces was detected. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.007
  • 2011 • 9 The multipole resonance probe: Characterization of a prototype
    Lapke, M. and Oberrath, J. and Schulz, C. and Storch, R. and Styrnoll, T. and Zietz, C. and Awakowicz, P. and Brinkmann, R.P. and Musch, T. and Mussenbrock, T. and Rolfes, I.
    Plasma Sources Science and Technology 20 (2011)
    The multipole resonance probe (MRP) was recently proposed as an economical and industry compatible plasma diagnostic device (Lapke et al 2008 Appl. Phys. Lett. 93 051502). This communication reports the experimental characterization of a first MRP prototype in an inductively coupled argon/nitrogen plasma at 10 Pa. The behavior of the device follows the predictions of both an analytical model and a numerical simulation. The obtained electron densities are in excellent agreement with the results of Langmuir probe measurements. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0963-0252/20/4/042001
  • 2011 • 8 Thermal stability of TiAIN/CrN multilayer coatings studied by atom probe tomography
    Choi, P.-P. and Povstugar, I. and Ahn, J.-P. and Kostka, A. and Raabe, D.
    Ultramicroscopy 111 518-523 (2011)
    This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 °C. At 700 °C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 °C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.11.012
  • 2011 • 7 Tracer percentage prediction of dive reflex samplers
    Bensmann, S. and Lockow, E. and Walzel, P. and Weihs, C.
    Powder Technology 208 63-71 (2011)
    Instead of the frequently applied monochromatic light probes a whie light fibre optic system was employed at the Laboratory of Mechanical Process Design, TU Dortmund University, in order to exploit the color in formations for concentration measurements within bulk solid. The system is applied to obtain local particle concentrations of blue- and red-colored quartz sand within the bed of a rotary drum. 16 solid mixtures with one or two particle sizes from 100 μm to 2000 μm and different species concentration were analyzed and the relationship between probe measurement values and red sand content was determined by statistical regression methods. After transformation of the data, linear models were found to derive the red sand content from given measurement values. Based thereupon, an all-purpose scheme for mono- and bi-disperse solid mixtures was developed and verified in an example with a mean error of 5%. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2010.12.004
  • 2010 • 6 Electrical scanning probe microscopy of an integrated blocking layer
    Weber, S.A.L. and Memesa, M. and Berger, R. and Butt, H.-J. and Gutmann, J.S.
    Journal of Nanoscience and Nanotechnology 10 6840-6844 (2010)
    Scanning probe microscopy was performed on an integrated blocking layer system developed for hybrid organic solar cells. A nanocomposite consisting of titania and an amphiphilic triblock copolymer ((PEO)MA-PDMS-MA(PEO)) was prepared by sol-gel chemistry. After plasma treatment and annealing of a spin casted film of 30-100 nm thickness a granular structure with a typical titania grain diameter of 20 nm was found. Conductive scanning force microscopy revealed that on top of almost every grain on the surface there is an increased conductivity compared to the average value. The correlation of grains and conductivity indicated that titania particles formed interconnecting paths through the film. For the resistivity of these pathways we found that effects of tip-sample and sample-electrode resistivity dominate. Additionally, conductive scanning force microscopy revealed non-conducting structures attributed to the thermal treatment. Kelvin probe microscopy of pristine samples on one side and plasma treated plus annealed samples on the other side showed that there is a shift in work function (0.8±0.2 eV) as expected for the transition of amorphous to anatase titania. Copyright © 2010 American Scientific Publishers.
    view abstractdoi: 10.1166/jnn.2010.2974
  • 2010 • 5 Process diagnostics in suspension plasma spraying
    Mauer, G. and Guignard, A. and Vaßen, R. and Stöver, D.
    Surface and Coatings Technology 205 961-966 (2010)
    Benefits and limitations of process diagnostics are investigated for the suspension plasma spraying of yttria-stabilized zirconia thermal barrier coatings. The methods applied were enthalpy probe measurements, optical emission spectroscopy, and in-flight particle diagnostic.It was proved that the plasma characteristics are not affected negatively by the injection of the ethanol based suspension since the combustion of species resulting from ethanol decomposition achieves a gain of plasma enthalpy. Furthermore, the conditions of the suspension injection into the plasma were found to be optimum as a significant content of evaporated powder material could be detected. Regarding the void content and segmentation crack density of the coatings, the in-flight particle diagnostic showed that the spray distance should be dimensioned in a way that the molten particles reach the substrates just before solidification starts. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2010.03.007
  • 2010 • 4 SERS microscopy: Plasmonic nanoparticle probes and biomedical applications
    Gellner, M. and Schütz, M. and Salehi, M. and Packeisen, J. and Ströbel, P. and Marx, A. and Schmuck, C. and Schlücker, S.
    Proceedings of SPIE - The International Society for Optical Engineering 7757 (2010)
    Nanoparticle probes for use in targeted detection schemes and readout by surface-enhanced Raman scattering (SERS) comprise a metal core, Raman reporter molecules and a protective shell. One design of SERS labels specifically optimized for biomedical applications in conjunction with red laser excitation is based on tunable gold/silver nanoshells, which are completely covered by a self-assembled monolayer (SAM) of Raman reporters. A shell around the SAMcoated metal core stabilizes the colloid and prevents particle aggregation. The optical properties and SERS efficiencies of these plasmonic nanostructures are characterized both experimentally and theoretically. Subsequent bioconjugation of SERS probes to ligands such as antibodies is a prerequisite for the selective detection of the corresponding target molecule via the characteristic Raman signature of the label. Biomedical imaging applications of SERS-labeled antibodies for tumor diagnostics by SERS microscopy are presented, using the localization of the tumor suppressor p63 in prostate tissue sections as an example. © 2010 SPIE.
    view abstractdoi: 10.1117/12.859253
  • 2010 • 3 Suspension plasma spraying: Process characteristics and applications
    Vaßen, R. and Kaner, H. and Mauer, G. and Stöver, D.
    Journal of Thermal Spray Technology 19 219-225 (2010)
    Suspension plasma spraying (SPS) offers the manufacture of unique microstructures which are not possible with conventional powdery feedstock. Due to the considerably smaller size of the droplets and also the further fragmentation of these in the plasma jet, the attainable microstructural features like splat and pore sizes can be downsized to the nanometer range. Our present understanding of the deposition process including injection, suspension plasma plume interaction, and deposition will be outlined. The drawn conclusions are based on analysis of the coating microstructures in combination with particle temperature and velocity measurements as well as enthalpy probe investigations. The last measurements with the water cooled stagnation probe gives valuable information on the interaction of the carrier fluid with the plasma plume. Meanwhile, different areas of application of SPS coatings are known. In this paper, the focus will be on coatings for energy systems. Thermal barrier coatings (TBCs) for modern gas turbines are one important application field. SPS coatings offer the manufacture of strain-tolerant, segmented TBCs with low thermal conductivity. In addition, highly reflective coatings, which reduce the thermal load of the parts from radiation, can be produced. Further applications of SPS coatings as cathode layers in solid oxide fuel cells (SOFC) and for photovoltaic (PV) applications will be presented. © 2009 ASM International.
    view abstractdoi: 10.1007/s11666-009-9451-x
  • 2010 • 2 Synthesis of glass-coated SERS nanoparticle probes via SAMs with terminal SiO2 precursors
    Schütz, M. and Küstner, B. and Bauer, M. and Schmuck, C. and Schlücker, S.
    Small 6 733-737 (2010)
    A short synthesis route to silica-encapsulated nanoparticles coated with a self-assembled monolayer (SAM) is presented. The organic molecules within the SAM contain a SiO2 precursor to render the surface vitreophilic. Due to the high mechanical and chemical stability of a glass shell, such particles can be used as probes in targeted research with surface-enhanced Raman scattering as the read-out method. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.200902065
  • 2010 • 1 Two-beam high-order harmonics from solids: Coupling mechanisms
    Tarasevitch, A. and Wieczorek, J. and Kohn, R. and Bovensiepen, U. and Von Der Linde, D.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 82 (2010)
    The polarization of the two beam (driver-probe) high-order harmonic generation from solids is measured. The experiments, together with computer simulations, allow us to distinguish two different coupling mechanisms of the driver and the probe, resulting in different harmonic efficiencies and spectral slopes. We find that in the nonrelativistic regime the coupling is mostly due to the nonlinear plasma density modulation. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.82.056410