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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  • 2023 • 244 Machine learning transferable atomic forces for large systems from underconverged molecular fragments
    Herbold, M. and Behler, J.
    Physical Chemistry Chemical Physics 25 12979-12989 (2023)
    Machine learning potentials (MLP) enable atomistic simulations with first-principles accuracy at a small fraction of the costs of electronic structure calculations. Most modern MLPs rely on constructing the potential energy, or a major part of it, as a sum of atomic energies, which are given as a function of the local chemical environments up to a cutoff radius. Since analytic forces are readily available, nowadays it is common practice to make use of both, reference energies and forces, for training these MLPs. This can be computationally demanding since often large systems are required to obtain structurally converged reference forces experienced by atoms in realistic condensed phase environments. In this work we show how density-functional theory calculations of molecular fragments, which are too small to provide such structurally converged forces, can be used to learn forces exhibiting excellent transferability to extended systems. The general procedure and the accuracy of the method are illustrated for metal-organic frameworks using second-generation high-dimensional neural network potentials. © 2023 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d2cp05976b
  • 2022 • 243 A chiral one-dimensional atom using a quantum dot in an open microcavity
    Antoniadis, N.O. and Tomm, N. and Jakubczyk, T. and Schott, R. and Valentin, S.R. and Wieck, A.D. and Ludwig, A. and Warburton, R.J. and Javadi, A.
    npj Quantum Information 8 (2022)
    In a chiral one-dimensional atom, a photon propagating in one direction interacts with the atom; a photon propagating in the other direction does not. Chiral quantum optics has applications in creating nanoscopic single-photon routers, circulators, phase-shifters, and two-photon gates. Here, we implement chiral quantum optics using a low-noise quantum dot in an open microcavity. We demonstrate the non-reciprocal absorption of single photons, a single-photon diode. The non-reciprocity, the ratio of the transmission in the forward-direction to the transmission in the reverse direction, is as high as 10.7 dB. This is achieved by tuning the photon-emitter coupling in situ to the optimal operating condition (β = 0.5). Proof that the non-reciprocity arises from a single quantum emitter lies in the photon statistics—ultralow-power laser light propagating in the diode’s reverse direction results in a highly bunched output (g(2)(0) = 101), showing that the single-photon component is largely removed. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41534-022-00545-z
  • 2022 • 242 A Hessian-based assessment of atomic forces for training machine learning interatomic potentials
    Herbold, M. and Behler, J.
    Journal of Chemical Physics 156 (2022)
    In recent years, many types of machine learning potentials (MLPs) have been introduced, which are able to represent high-dimensional potential-energy surfaces (PESs) with close to first-principles accuracy. Most current MLPs rely on atomic energy contributions given as a function of the local chemical environments. Frequently, in addition to total energies, atomic forces are also used to construct the potentials, as they provide detailed local information about the PES. Since many systems are too large for electronic structure calculations, obtaining reliable reference forces from smaller subsystems, such as molecular fragments or clusters, can substantially simplify the construction of the training sets. Here, we propose a method to determine structurally converged molecular fragments, providing reliable atomic forces based on an analysis of the Hessian. The method, which serves as a locality test and allows us to estimate the importance of long-range interactions, is illustrated for a series of molecular model systems and the metal-organic framework MOF-5 as an example for a complex organic-inorganic hybrid material. © 2022 Author(s).
    view abstractdoi: 10.1063/5.0082952
  • 2022 • 241 A single-Pt-atom-on-Ru-nanoparticle electrocatalyst for CO-resilient methanol oxidation
    Poerwoprajitno, A.R. and Gloag, L. and Watt, J. and Cheong, S. and Tan, X. and Lei, H. and Tahini, H.A. and Henson, A. and Subhash, B. and Bedford, N.M. and Miller, B.K. and O’Mara, P.B. and Benedetti, T.M. and Huber, D.L. and Z...
    Nature Catalysis 5 231-237 (2022)
    Single Pt atom catalysts are key targets because a high exposure of Pt substantially enhances electrocatalytic activity. In addition, PtRu alloy nanoparticles are the most active catalysts for the methanol oxidation reaction. To combine the exceptional activity of single Pt atom catalysts with an active Ru support we must overcome the synthetic challenge of forming single Pt atoms on noble metal nanoparticles. Here we demonstrate a process that grows and spreads Pt islands on Ru branched nanoparticles to create single-Pt-atom-on-Ru catalysts. By following the spreading process by in situ TEM, we found that the formation of a stable single atom structure is thermodynamically driven by the formation of strong Pt–Ru bonds and the lowering of the surface energy of the Pt islands. The stability of the single-Pt-atom-on-Ru structure and its resilience to CO poisoning result in a high current density and mass activity for the methanol oxidation reaction over time. [Figure not available: see fulltext.] © 2022, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41929-022-00756-9
  • 2022 • 240 Bistable H2Pc Molecular Conductance Switch on Ag(100)
    Kamiński, W. and Antczak, G. and Morgenstern, K.
    Journal of Physical Chemistry C 126 16767-16776 (2022)
    Scanning tunneling microscopy (STM) and density functional theory (DFT) were used to study the tautomerization reaction of an H2Pc molecule adsorbed on a Ag(100) surface. The presence of two hydrogen atoms in the cavity of the H2Pc molecule enforces the existence of two molecular tautomers. It causes a reduction from 4- to 2-fold symmetry in STM images that can be recorded as two current states over the H2Pc molecule with a high-to-low current state ratio of ∼1.2. These findings are confirmed by the spatial distributions of the all-atom electron charge density calculated by using DFT and transmission maps together with tunneling current ratios (∼1.2) determined from the nonequilibrium Green's function transport calculations. Therefore, we demonstrate that an H2Pc molecule adsorbed on a Ag(100) surface is a good candidate for a bistable molecular conductance switch since neither the presence of the Ag(100) surface nor that of the STM tip alters the tautomerization. © 2022 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.2c03485
  • 2022 • 239 Efficient Nitrate Conversion to Ammonia on f-Block Single-Atom/Metal Oxide Heterostructure via Local Electron-Deficiency Modulation
    Kumar, A. and Lee, J. and Kim, M.G. and Debnath, B. and Liu, X. and Hwang, Y. and Wang, Y. and Shao, X. and Jadhav, A.R. and Liu, Y. and Tüysüz, H. and Lee, H.
    ACS Nano 16 15297-15309 (2022)
    Exploring single-atom catalysts (SACs) for the nitrate reduction reaction (NO3-NitRR) to value-added ammonia (NH3) offers a sustainable alternative to both the Haber-Bosch process and NO3--rich wastewater treatment. However, due to the insufficient electron deficiency and unfavorable electronic structure of SACs, resulting in poor NO3--adsorption, sluggish proton (H*) transfer kinetics, and preferred hydrogen evolution, their NO3--to-NH3selectivity and yield rate are far from satisfactory. Herein, a systematic theoretical prediction reveals that the local electron deficiency of an f-block Gd single atom (GdSA) can be significantly regulated upon coordination with oxygen-defect-rich NiO (GdSA-D-NiO400) support. Thus, facilitating stronger NO3-adsorption via strong Gd5d-O2porbital coupling and further improving the protonation kinetics of adsorption intermediates by rapid H∗ capture from water dissociation catalyzed by the adjacent oxygen vacancy site along with suppressed H∗ dimerization synergistically boosts the NH3selectivity/yield rate. Motivated by DFT prediction, we delicately stabilized electron-deficient (strongly electrophilic) GdSAon D-NiO400(?84% strong electrophilic sites), which exhibited excellent alkaline NitRR activity (NH3Faradaic efficiency ?97% and yield rate ?628 μg/(mgcath)) along with superior structural stability, as revealed by in situ Raman spectroscopy, significantly outperforming weakly electrophilic Gd nanoparticles, defect-free GdSA-P-NiO400, and reported state-of-the-art catalysts. © 2022 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/acsnano.2c06747
  • 2022 • 238 Enhancement of Proton Therapy Efficiency by Noble Metal Nanoparticles Is Driven by the Number and Chemical Activity of Surface Atoms
    Zwiehoff, S. and Johny, J. and Behrends, C. and Landmann, A. and Mentzel, F. and Bäumer, C. and Kröninger, K. and Rehbock, C. and Timmermann, B. and Barcikowski, S.
    Small 18 (2022)
    Proton-based radiotherapy is a modern technique for the treatment of solid tumors with significantly reduced side effects to adjacent tissues. Biocompatible nanoparticles (NPs) with high atomic numbers are known to serve as sensitizers and to enhance treatment efficacy, which is commonly believed to be attributed to the generation of reactive oxygen species (ROS). However, little systematic knowledge is available on how either physical effects due to secondary electron generation or the particle surface chemistry affect ROS production. Thereto, ligand-free colloidal platinum (Pt) and gold (Au) NPs with well-controlled particle size distributions and defined total surface area are proton-irradiated. A fluorescence-based assay is developed to monitor the formation of ROS using terephthalic acid as a cross-effect-free dye. The findings indicate that proton irradiation (PI)-induced ROS formation sensitized by noble metal NPs is driven by the total available particle surface area rather than particle size or mass. Furthermore, a distinctive material effect with Pt being more active than Au is observed which clearly indicates that the chemical reactivity of the NP surface is a main contributor to ROS generation upon PI. These results pave the way towards an in-depth understanding of the NP-induced sensitizing effects upon PI and hence a well-controlled enhanced therapy. © 2021 The Authors. Small published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/smll.202106383
  • 2022 • 237 Inner relaxations in equiatomic single-phase high-entropy cantor alloy
    Smekhova, A. and Kuzmin, A. and Siemensmeyer, K. and Abrudan, R. and Reinholz, U. and Buzanich, A.G. and Schneider, M. and Laplanche, G. and Yusenko, K.V.
    Journal of Alloys and Compounds 920 (2022)
    The superior properties of high-entropy multi-functional materials are strongly connected with their atomic heterogeneity through many different local atomic interactions. The detailed element-specific studies on a local scale can provide insight into the primary arrangements of atoms in multicomponent systems and benefit to unravel the role of individual components in certain macroscopic properties of complex compounds. Herein, multi-edge X-ray absorption spectroscopy combined with reverse Monte Carlo simulations was used to explore a homogeneity of the local crystallographic ordering and specific structure relaxations of each constituent in the equiatomic single-phase face-centered cubic CrMnFeCoNi high-entropy alloy at room temperature. Within the considered fitting approach, all five elements of the alloy were found to be distributed at the nodes of the fcc lattice without any signatures of the additional phases at the atomic scale and exhibit very close statistically averaged interatomic distances (2.54 – 2.55 Å) with their nearest-neighbors. Enlarged structural displacements were found solely for Cr atoms. The macroscopic magnetic properties probed by conventional magnetometry demonstrate no opening of the hysteresis loops at 5 K and illustrate a complex character of the long-range magnetic order after field-assisted cooling in± 5 T. The observed magnetic behavior is assigned to effects related to structural relaxations of Cr. Besides, the advantages and limitations of the reverse Monte Carlo approach to studies of multicomponent systems like high-entropy alloys are highlighted. © 2022 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2022.165999
  • 2022 • 236 Molecular dynamics study on the role of Ar ions in the sputter deposition of Al thin films
    Gergs, T. and Mussenbrock, T. and Trieschmann, J.
    Journal of Applied Physics 132 (2022)
    Compressive stresses in sputter deposited thin films are generally assumed to be caused by forward sputtered (peened) built-in particles and entrapped working gas atoms. While the former are assumed to be predominant, the effect of the latter on interaction dynamics and thin film properties is scarcely clarified (concurrent or causative). The overlay of the ion bombardment induced processes renders an isolation of their contribution impracticable. This issue is addressed by two molecular dynamics case studies considering the sputter deposition of Al thin films in Ar working gas. First, Ar atoms are fully retained. Second, they are artificially neglected, as implanted Ar atoms are assumed to outgas anyhow and not alter the ongoing dynamics significantly. Both case studies share common particle dose impinging Al(001) surfaces. Ion energies from 3 to 300 eV and Al / Ar + flux ratios from 0 to 1 are considered. The surface interactions are simulated by hybrid reactive molecular dynamics/force-biased Monte Carlo simulations and characterized in terms of mass density, Ar concentration, biaxial stress, shear stress, ring statistical connectivity profile, Ar gas porosity, Al vacancy density, and root-mean-squared roughness. Implanted Ar atoms are found to form subnanometer sized eventually outgassing clusters for ion energies exceeding 100 eV. They fundamentally govern a variety of surface processes (e.g., forward sputtering/peening) and surface properties (e.g., compressive stresses) in the considered operating regime. © 2022 Author(s).
    view abstractdoi: 10.1063/5.0098040
  • 2022 • 235 Molecular Emissions from Stretched Excitation Pulse in Nanosecond Phase-Selective Laser-Induced Breakdown Spectroscopy of TiO2 Nanoaerosols
    Xiong, G. and Zhang, Y. and Schulz, C. and Tse, S.D.
    Applied Spectroscopy (2022)
    In phase-selective laser-induced breakdown spectroscopy (PS-LIBS), gas-borne nanoparticles are irradiated with laser pulses (∼2.4 GW/cm2) resulting in breakdown of the nanoparticle phase but not the surrounding gas phase. In this work, the effect of excitation laser-pulse duration and energy on the intensity and duration of TiO2–nanoparticle PS-LIBS emission signal is investigated. Laser pulses from a frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (532 nm) are stretched from 8 ns (full width at half maximum, FWHM) up to ∼30 ns at fixed pulse energy using combinations of two optical cavities. The intensity of the titanium atomic emissions at around 500 nm wavelength increases by ∼60%, with the stretched pulse and emissions at around 482 nm, attributed to TiO, enhanced over 10 times. While the atomic emissions rise with the stretched laser pulse and decay around 20 ns after the end of the laser pulse, the TiO emissions reach their peak intensity at about 20 ns later and last longer. At low laser energy (i.e., 1 mJ/pulse, or 80 MW/cm2), the TiO emissions dominate, but their increase with laser energy is lower compared to the atomic emissions. The origin of the 482 nm emission is explored by examining several different aerosol setups, including Ti–O, Ti–N, and Ti–O–N from a spark particle generator and Ti–O–N–C–H aerosol from flame synthesis. The 482 nm emissions are attributed to electronically excited TiO, likely resulting from the reaction of excited titanium atoms with surrounding oxidizing (carbonaceous and/or radical) species. The effects of pulse length are attributed to the shift of absorption from the initial interaction with the particle to the prolonged interaction with the plasma through inverse bremsstrahlung. © The Author(s) 2022.
    view abstractdoi: 10.1177/00037028211072583
  • 2022 • 234 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 • 233 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 • 232 Selecting the Reaction Path in On-Surface Synthesis through the Electron Chemical Potential in Graphene
    Kraus, S. and Herman, A. and Huttmann, F. and Krämer, C. and Amsharov, K. and Tsukamoto, S. and Wende, H. and Atodiresei, N. and Michely, T.
    Journal of the American Chemical Society 144 11003-11009 (2022)
    The organometallic on-surface synthesis of the eight-membered sp2 carbon-based ring cyclooctatetraene (C8H8, Cot) with the neighboring rare-earth elements ytterbium and thulium yields fundamentally different products for the two lanthanides, when conducted on graphene (Gr) close to the charge neutrality point. Sandwich-molecular YbCot wires of more than 500 Å length being composed of an alternating sequence of Yb atoms and upright-standing Cot molecules result from the on-surface synthesis with Yb. In contrast, repulsively interacting TmCot dots consisting of a single Cot molecule and a single Tm atom result from the on-surface synthesis with Tm. While the YbCot wires are bound through van der Waals interactions to the substrate, the dots are chemisorbed to Gr via the Tm atoms being more electropositive compared to Yb atoms. When the electron chemical potential in Gr is substantially raised (n-doping) through backside doping from an intercalation layer, the reaction product in the synthesis with Tm can be tuned to TmCot sandwich-molecular wires rather than TmCot dots. By use of density functional theory, it is found that the reduced electronegativity of Gr upon n-doping weakens the binding as well as the charge transfer between the reaction intermediate TmCot dot and Gr. Thus, the assembly of the TmCot dots to long TmCot sandwich-molecular wires becomes energetically favorable. It is thereby demonstrated that the electron chemical potential in Gr can be used as a control parameter in an organometallic on-surface synthesis to tune the outcome of a reaction. © 2022 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/jacs.2c04359
  • 2022 • 231 Tailoring magnetic anisotropy by graphene-induced selective skyhook effect on 4f-metals
    Herman, A. and Kraus, S. and Tsukamoto, S. and Spieker, L. and Caciuc, V. and Lojewski, T. and Günzing, D. and Dreiser, J. and Delley, B. and Ollefs, K. and Michely, T. and Atodiresei, N. and Wende, H.
    Nanoscale 14 7682-7691 (2022)
    From macroscopic heavy-duty permanent magnets to nanodevices, the precise control of the magnetic properties in rare-earth metals is crucial for many applications used in our daily life. Therefore, a detailed understanding and manipulation of the 4f-metals’ magnetic properties are key to further boosting the functionalization and efficiency of future applications. We present a proof-of-concept approach consisting of a dysprosium-iridium surface alloy in which graphene adsorption allows us to tailor its magnetic properties. By adsorbing graphene onto a long-range ordered two-dimensional dysprosium-iridium surface alloy, the magnetic 4f-metal atoms are selectively lifted from the surface alloy. This selective skyhook effect introduces a giant magnetic anisotropy in dysprosium atoms as a result of manipulating its geometrical structure within the surface alloy. Introducing and proving this concept by our combined theoretical and experimental approach provides an easy and unambiguous understanding of its underlying mechanism. Our study sets the ground for an alternative path on how to modify the crystal field around 4f-atoms and therefore their magnetic anisotropies. © 2022 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d2nr01458k
  • 2022 • 230 Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution
    Schröter, M. and Herper, H.C. and Grünebohm, A.
    Journal of Physics D: Applied Physics 55 (2022)
    Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties. © 2021 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/ac2a66
  • 2021 • 229 A bin and hash method for analyzing reference data and descriptors in machine learning potentials
    Paleico, M.L. and Behler, J.
    Machine Learning: Science and Technology 2 (2021)
    In recent years the development of machine learning potentials (MLPs) has become a very active field of research. Numerous approaches have been proposed, which allow one to perform extended simulations of large systems at a small fraction of the computational costs of electronic structure calculations. The key to the success of modern MLPs is the close-To first principles quality description of the atomic interactions. This accuracy is reached by using very flexible functional forms in combination with high-level reference data from electronic structure calculations. These data sets can include up to hundreds of thousands of structures covering millions of atomic environments to ensure that all relevant features of the potential energy surface are well represented. The handling of such large data sets is nowadays becoming one of the main challenges in the construction of MLPs. In this paper we present a method, the bin-And-hash (BAH) algorithm, to overcome this problem by enabling the efficient identification and comparison of large numbers of multidimensional vectors. Such vectors emerge in multiple contexts in the construction of MLPs. Examples are the comparison of local atomic environments to identify and avoid unnecessary redundant information in the reference data sets that is costly in terms of both the electronic structure calculations as well as the training process, the assessment of the quality of the descriptors used as structural fingerprints in many types of MLPs, and the detection of possibly unreliable data points. The BAH algorithm is illustrated for the example of high-dimensional neural network potentials using atom-centered symmetry functions for the geometrical description of the atomic environments, but the method is general and can be combined with any current type of MLP. © 2021 Machine Learning: Science and Technology. All rights reserved.
    view abstractdoi: 10.1088/2632-2153/abe663
  • 2021 • 228 Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys
    Schneider, A. and Fu, C.-C. and Waseda, O. and Barreteau, C. and Hickel, T.
    Physical Review B 103 (2021)
    Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.024421
  • 2021 • 227 Absolute concentration imaging using self-calibrating laser-induced fluorescence: application to atomic iron in a nanoparticle flame-synthesis reactor
    Lalanne, M.R. and Pilipodi-Best, A. and Blumer, O. and Wollny, P. and Nanjaiah, M. and Wlokas, I. and Cheskis, S. and Rahinov, I.
    Applied Physics B: Lasers and Optics 127 (2021)
    Quantitative, spatially-resolved measurements of intermediates in flame synthesis reactors are required for the validation of precursor decomposition and oxidation mechanisms, preceding the nanoparticle formation. In this work we demonstrate how the laser-induced fluorescence (LIF) can be used in a self-calibrating fashion to image absolute concentrations of the strong absorber, such as atomic iron generated during the thermal decomposition of iron pentacarbonyl precursor in the preheat zone of synthesis flame. Flame symmetry facilitates deduction of the absolute concentrations. A comparison of LIF fluorescence patterns on both sides of axisymmetric flow configuration cancels out symmetric factors such as fluorescence quantum yield, fluorescence trapping and optical aberrations. This approach, utilizing one laser beam and one spectral transition provides a refinement of previous methods that have used either two spectral transitions or two collinear laser beams in counter-propagating geometry. Its spectral resolution and the detection sensitivity of que are not compromised when the spectral width of the laser exceeds that of the absorber. The measured Fe-atom concentration field is qualitatively consistent with the predictions of nucleation theory approach and suggest that flame synthesis model should be expanded beyond the formation of small incipient iron clusters to several nm-sized iron particles. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00340-021-07672-z
  • 2021 • 226 Atomic oxygen generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and O2
    Korolov, I. and Steuer, D. and Bischoff, L. and Hübner, G. and Liu, Y. and Schulz-Von der Gathen, V. and Böke, M. and Mussenbrock, T. and Schulze, J.
    Journal of Physics D: Applied Physics 54 (2021)
    Absolute atomic oxygen densities measured space resolved in the active plasma volume of a COST microplasma reference jet operated in He/O2 and driven by tailored voltage waveforms are presented. The measurements are performed for different shapes of the driving voltage waveform, oxygen admixture concentrations, and peak-to-peak voltages. Peaks- and valleys-waveforms constructed based on different numbers of consecutive harmonics, N, of the fundamental frequency f 0 =13.56 MHz, different relative phases and amplitudes are used. The results show that the density of atomic oxygen can be controlled and optimized by voltage waveform tailoring (VWT). It is significantly enhanced by increasing the number of consecutive driving harmonics at fixed peak-to-peak voltage. The shape of the measured density profiles in the direction perpendicular to the electrodes can be controlled by VWT as well. For N >1 and peaks-/valleys-waveforms, it exhibits a strong spatial asymmetry with a maximum at one of the electrodes due to the spatially asymmetric electron power absorption dynamics. Thus, the atomic oxygen flux can be directed primarily towards one of the electrodes. The generation of atomic oxygen can be further optimized by changing the reactive gas admixture and by tuning the peak-to-peak voltage amplitude. The obtained results are understood based on a detailed analysis of the spatio-temporal dynamics of energetic electrons revealed by phase resolved optical emission spectroscopy. © 2021 Institute of Physics Publishing. All rights reserved.
    view abstractdoi: 10.1088/1361-6463/abd20e
  • 2021 • 225 Atomic scale understanding of phase stability and decomposition of a nanocrystalline CrMnFeCoNi Cantor alloy
    Li, Y.J. and Savan, A. and Ludwig, A.
    Applied Physics Letters 119 (2021)
    High entropy alloys (HEAs) provide superior mechanical and functional properties. However, these advantages may disappear when a metastable single-phase solid solution decomposes at low temperatures upon long-term annealing. Therefore, understanding the underlying phase separation mechanisms is important for the design of new HEAs with controlled properties. In the current work, the thermal stability of a nanocrystalline CrMnFeCoNi HEA was investigated at different annealing conditions using a combinatorial processing platform, involving fast and parallel synthesis of nanocrystalline thin films, short annealing time for a rapid phase evolution, and direct characterization by atom probe tomography. The microstructural features of the decomposed CrMnFeCoNi alloy as well as its decomposition process were analyzed in terms of elemental distributions at the near-atomic scale. The results show that the segregation of Ni and Mn to grain boundaries in the initial single-phase alloy is a prerequisite and is observed to be the only occurring physical process at the early stage of phase decomposition. When the concentrations of Ni and Mn reach a certain value, phase decomposition starts and a MnNi-rich phase forms at grain boundaries. Next, two Cr-rich phases form at the interface between the MnNi-rich phase and the matrix. Meanwhile, a FeCo-rich phase forms in the grain interior. Based on these observations, the underlying mechanisms involving nucleation, diffusivity as well as thermodynamic considerations were discussed. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0069107
  • 2021 • 224 Decay of quantum sensitivity due to three-body loss in Bose-Einstein condensates
    Rätzel, D. and Schützhold, R.
    Physical Review A 103 (2021)
    In view of the coherent properties of a large number of atoms, Bose-Einstein condensates (BECs) have a high potential for sensing applications. Several proposals have been put forward to use collective excitations such as phonons in BECs for quantum-enhanced sensing in quantum metrology. However, the associated highly nonclassical states tend to be very vulnerable to decoherence. In this article, we investigate the effect of decoherence due to the omnipresent process of three-body loss in BECs. We find strong restrictions for a wide range of parameters, and we discuss possibilities to limit these restrictions. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevA.103.063321
  • 2021 • 223 Efficient electronic passivation scheme for computing low-symmetry compound semiconductor surfaces in density-functional theory slab calculations
    Yoo, S.-H. and Lymperakis, L. and Neugebauer, J.
    Physical Review Materials 5 (2021)
    Removing artificial bands from the back side of surface slabs with pseudohydrogen atoms has become the method of choice to boost the convergence of density-functional theory (DFT) surface calculation with respect to slab thickness. In this paper we apply this approach to semipolar compound semiconductor surfaces, which have recently become attractive for device applications. We show that approaches employing saturation of dangling bonds by pseudohydrogen atoms alone are inadequate to properly passivate the surfaces, remove spurious surface states from the fundamental band gap, and achieve flat band conditions in the slab. We propose and successfully apply to technologically interesting semipolar wurtzite surfaces of III-N, III-V, and II-VI semiconductors a reconstruction-inspired passivation scheme that utilizes native anions to passivate cation dangling bonds and pseudohydrogen atoms to obey the electron counting rule and compensate for polarization-induced surface-bound charges. This scheme is generic and robust and can be straightforwardly implemented in DFT investigations of low-symmetry surfaces as well as in high-throughput and machine learning studies. © 2021 authors.
    view abstractdoi: 10.1103/PhysRevMaterials.5.044605
  • 2021 • 222 Fabrication of Gd: XFeyOzfilms using an atomic layer deposition-type approach
    Yu, P. and Beer, S.M.J. and Devi, A. and Coll, M.
    CrystEngComm 23 730-740 (2021)
    The growth of complex oxide thin films with atomic precision offers bright prospects to study improved properties and novel functionalities. Here we tackle the fabrication of gadolinium iron oxide thin films by an atomic layer deposition-type approach in which iron and gadolinium tailor-made metalorganic precursors (bis(N-isopropyl ketoiminate)iron(ii), [Fe(ipki)2] and tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)gadolinium(iii), [Gd(DPDMG)3]) are alternately reacted with ozone and deposited on silicon substrates at 250 °C. The structure, chemical composition and magnetic properties of the resulting films are compared with those obtained from a commercially available ferrocene precursor [Fe(Cp)2] and [Gd(DPDMG)3]. All films resulted in cation ratio close to nominal stoichiometry with negligible amount of organic species. The tailor-made metalorganic precursors, designed to provide similar thermal behavior, result in the formation of polycrystalline Gd3Fe5O12 films coexisting with GdFeO3, Gd2O3 and Fe2O3 whereas the combination of [Fe(Cp)2] and [Gd(DPDMG)3] mainly favors the formation of Gd3Fe5O12 films coexisting with traces of Gd2O3. This study demonstrates that this is a viable route to prepare complex GdxFeyOz films and could be used for the design of complex oxide films with improved properties upon rigorous study of the compatibility of metalorganic precursors. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ce01252a
  • 2021 • 221 First-principles computational exploration of ferromagnetism in monolayer GaS via substitutional doping
    Khan, R. and Rahman, A.U. and Zhang, Q. and Kratzer, P. and Ramay, S.M.
    Journal of Physics Condensed Matter 33 (2021)
    Using first-principles calculations, functionalization of the monolayer-GaS crystal structure through N or Cr-doping at all possible lattice sites has been investigated. Our results show that pristine monolayer-GaS is an indirect-bandgap, non-magnetic semiconductor. The bandgap can be tuned and a magnetic moment (MM) can be induced by the introduction of N or Cr atomic anion/cation doping in monolayer GaS. For instance, the intrinsic character of monolayer GaS can be changed by substitution of N for the S-site to p-type, while substitution of Cr at the S-site or Ga-site induces half-metallicity at sufficiently high concentrations. The defect states are located in the electronic bandgap region of the GaS monolayer. These findings help to extend the application of monolayer-GaS structures in nano-electronics and spintronics. Since the S-sites at the surface are more easily accessible to doping in experiment, we chose the S-site for further investigations. Finally, we perform calculations with ferromagnetic (FM) and antiferromagnetic (AFM) alignment of the MMs at the dopants. For pairs of impurities of the same species at low concentrations we find Cr atoms to prefer the FM state, while N atoms prefer the AFM state, both for impurities on opposite surfaces of the GaS monolayer and for impurities sharing a common Ga neighbor sitting at the same surface. Extending our study to higher concentrations of Cr atoms, we find that clusters of four Cr atoms prefer AFM coupling, whereas the FM coupling is retained for Cr atoms at larger distance arranged on a honeycomb lattice. For the latter arrangement, we estimate the FM Curie temperature T C to be 241 K. We conclude that the Cr-doped monolayer-GaS crystal structure offers enhanced electronic and magnetic properties and is an appealing candidate for spintronic devices operating close to room temperature. © 2021 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/ac04ce
  • 2021 • 220 Four Generations of High-Dimensional Neural Network Potentials
    Behler, J.
    Chemical Reviews 121 10037-10072 (2021)
    Since their introduction about 25 years ago, machine learning (ML) potentials have become an important tool in the field of atomistic simulations. After the initial decade, in which neural networks were successfully used to construct potentials for rather small molecular systems, the development of high-dimensional neural network potentials (HDNNPs) in 2007 opened the way for the application of ML potentials in simulations of large systems containing thousands of atoms. To date, many other types of ML potentials have been proposed continuously increasing the range of problems that can be studied. In this review, the methodology of the family of HDNNPs including new recent developments will be discussed using a classification scheme into four generations of potentials, which is also applicable to many other types of ML potentials. The first generation is formed by early neural network potentials designed for low-dimensional systems. High-dimensional neural network potentials established the second generation and are based on three key steps: First, the expression of the total energy as a sum of environment-dependent atomic energy contributions; second, the description of the atomic environments by atom-centered symmetry functions as descriptors fulfilling the requirements of rotational, translational, and permutation invariance; and third, the iterative construction of the reference electronic structure data sets by active learning. In third-generation HDNNPs, in addition, long-range interactions are included employing environment-dependent partial charges expressed by atomic neural networks. In fourth-generation HDNNPs, which are just emerging, in addition, nonlocal phenomena such as long-range charge transfer can be included. The applicability and remaining limitations of HDNNPs are discussed along with an outlook at possible future developments. © 2021 The Author. Published by American Chemical Society.
    view abstractdoi: 10.1021/acs.chemrev.0c00868
  • 2021 • 219 Grain boundary energy landscape from the shape analysis of synthetically stabilized embedded grains
    Schratt, A.A. and Steinbach, I. and Mohles, V.
    Computational Materials Science 193 (2021)
    The Gibbs free energy of grain boundaries (GBs) in Al bicrystals has been investigated by Molecular Dynamics (MD) simulations. In our novel approach, one grain is fully embedded in a large matrix grain with fixed misorientation. Hence all inclinations are considered simultaneously since the boundary covers the full orientation subspace. A synthetical driving force is employed to counteract the shrinkage of the embedded grain by the capillary forces. Hence, the number of atoms of the embedded grain is kept constant, but its shape adjusts itself at elevated temperatures in order to minimize the total GB energy. The quasi-equilibrium shapes are used to derive the GB energy γ(n) as functions of the GB plane normal n. For GBs with the misorientations Σ5〈001〉 and Σ7〈111〉, analytical functions were derived and validated in a mesoscopic front-tracking simulation: the latter simulations recovered the grain shapes observed in MD simulations. For the Σ5〈001〉 misorientation it is shown that the anisotropy of γ(n) varies quite strongly with temperature. For a Σ9〈110〉 misorientation, the derived numerical energy function was found to be rather complex, showing pronounced energy minima for mixed tilt/twist GBs parallel to 111 crystal planes. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2021.110384
  • 2021 • 218 How Hydrogen Admixture Changes Plasma Jet Characteristics in Spray Processes at Low Pressure
    Mauer, G.
    Plasma Chemistry and Plasma Processing 41 109-132 (2021)
    In plasma spraying, hydrogen is widely used as a secondary working gas besides argon. In particular under low pressure, there are strong effects on the plasma jet characteristics even by small hydrogen percentages. Under such conditions, fundamental mechanisms like diffusion and recombination are affected while this is less relevant under atmospheric conditions. This was investigated for argon–hydrogen mixtures by optical emission spectroscopy (OES). The small electron densities under the investigated low pressure conditions implied specific difficulties in the application of several OES-based methods which are discussed in detail. Adding hydrogen to the plasma gas effected an increased plasma enthalpy. Moreover, the jet expanded radially as the reactive part of the thermal conductivity was enhanced by recombination of atomic hydrogen so that the shock waves were less reflected at the cold jet rims. In the jet cores, the lowest temperatures were found for the highest hydrogen admixture because the energy consumption due to the dissociation of molecular hydrogen outbalanced the increase of the plasma enthalpy. Variations in the radial temperature profiles were related to the jet structure and radial thermal conductivity. The local hydrogen–argon concentration ratios revealed an accumulation of hydrogen atoms at the jet rims. Clear indications were found, that higher hydrogen contents promoted the fast recombination of electrons and ions. However, it is assumed that the transport properties of the plasma were hardly affected by this, since the electron densities and thus the ionization degrees were generally small due to the low pressure conditions. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11090-020-10143-6
  • 2021 • 217 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 • 216 Parallel hybrid Monte Carlo / Molecular Statics for Simulation of Solute Segregation in Solids
    Ganesan, H. and Longsworth, M. and Sutmann, G.
    Journal of Physics: Conference Series 1740 (2021)
    A parallel hybrid Monte Carlo/molecular statics method is presented for studying segregation of interstitial atoms in the solid state. The method is based on the efficient use of virtual atoms as placeholders to find energetically favorable sites for interstitials in a distorted environment. MC trial moves perform an exchange between a randomly chosen virtual atom with a carbon atom followed by a short energy minimization via MS to relax the lattice distortion. The proposed hybrid method is capable of modeling solute segregation in deformed crystalline metallic materials with a moderate MC efficiency. To improve sampling efficiency, the scheme is extended towards a biased MC approach, which takes into account the history of successful trial moves in the system. Parallelization of the hybrid MC/MS method is achieved by a Manager-Worker model which applies a speculative execution of trial moves, which are asynchronously executed on the cores. The technique is applied to an Fe-C system including a dislocation as a symmetry breaking perturbation in the system. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1740/1/012001
  • 2021 • 215 Polymorphism of dimethylaminoborane N(CH3)2-BH2
    Bodach, A. and Bernert, T. and Fischer, M. and Leya, M.B. and Weidenthaler, C.
    Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials 77 299-306 (2021)
    Dehydrocoupling of the adduct of dimethylamine and borane, NH(CH3)2-BH3 leads to dimethylaminoborane with formal composition N(CH3)2-BH2. The structure of this product depends on the conditions of the synthesis; it may crystallize either as a dimer in a triclinic space group forming a four-membered ring [N(CH3)2-BH2]2 or as a trimer forming a six-membered ring [N(CH3)2-BH2]3 in an orthorhombic space group. Due to the denser packing, the six-membered ring in the trimer structure should be energetically more stable than the four-membered ring. The triclinic structure is stable at low temperatures. Heating the triclinic phase above 290K leads to a second-order phase transition to a new monoclinic polymorph. While the crystal structures of the triclinic and orthorhombic phases were already known in the literature, the monoclinic crystal structure was determined from powder diffraction data in this study. Monoclinic dimethylaminoborane crystallizes in space group C2/m with the boron and nitrogen atoms located on the mirror plane, Wyckoff position 4i, while the carbon and hydrogen atoms are on the general position 8j. © 2021.
    view abstractdoi: 10.1107/S2052520621001979
  • 2021 • 214 Properties of α-Brass Nanoparticles II: Structure and Composition
    Weinreich, J. and Paleico, M.L. and Behler, J.
    Journal of Physical Chemistry C 125 14897-14909 (2021)
    Nanoparticles have become increasingly interesting for a wide range of applications because in principle it is possible to tailor their properties by controlling size, shape, and composition. One of these applications is heterogeneous catalysis, and a fundamental understanding of the structural details of the nanoparticles is essential for any knowledge-based improvement of reactivity and selectivity. In this work, we investigate the atomic structure of brass nanoparticles containing up to 5000 atoms as a typical example for a binary alloy consisting of Cu and Zn. As systems of this size are too large for electronic structure calculations, in our simulations, we use a recently parameterized machine learning potential providing close to density functional theory accuracy. This potential is employed for a structural characterization as a function of chemical composition by various types of simulations such as Monte Carlo in the semigrand canonical ensemble and simulated annealing molecular dynamics. Our analysis reveals that the distribution of both elements in the nanoparticles is inhomogeneous, and zinc accumulates in the outermost layer, while the first subsurface layer shows an enrichment of copper. Only for high zinc concentrations, alloying can be found in the interior of the nanoparticles, and regular patterns corresponding to crystalline bulk phases of α-brass can then be observed. The surfaces of the investigated clusters exhibit well-ordered single-crystal facets, which can give rise to grain boundaries inside the clusters. The melting temperature of the nanoparticles is found to decrease with increasing zinc-atom fraction, a trend which is well known also for the bulk phase diagram of brass. © 2021 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.1c02314
  • 2021 • 213 Rapid Interchangeable Hydrogen, Hydride, and Proton Species at the Interface of Transition Metal Atom on Oxide Surface
    Wu, S. and Tseng, K.-Y. and Kato, R. and Wu, T.-S. and Large, A. and Peng, Y.-K. and Xiang, W. and Fang, H. and Mo, J. and Wilkinson, I. and Soo, Y.-L. and Held, G. and Suenaga, K. and Li, T. and Chen, H.-Y.T. and Tsang, S.C.E.
    Journal of the American Chemical Society 143 9105-9112 (2021)
    Hydrogen spillover is the phenomenon where a hydrogen atom, generated from the dissociative chemisorption of dihydrogen on the surface of a metal species, migrates from the metal to the catalytic support. This phenomenon is regarded as a promising avenue for hydrogen storage, yet the atomic mechanism for how the hydrogen atom can be transferred to the support has remained controversial for decades. As a result, the development of catalytic support for such a purpose is only limited to typical reducible oxide materials. Herein, by using a combination of in situ spectroscopic and imaging technique, we are able to visualize and observe the atomic pathway for which hydrogen travels via a frustrated Lewis pair that has been constructed on a nonreducible metal oxide. The interchangeable status between the hydrogen, proton, and hydride is carefully characterized and demonstrated. It is envisaged that this study has opened up new design criteria for hydrogen storage material. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/jacs.1c02859
  • 2021 • 212 Revealing atomic-scale vacancy-solute interaction in nickel
    Morgado, F.F. and Katnagallu, S. and Freysoldt, C. and Klaes, B. and Vurpillot, F. and Neugebauer, J. and Raabe, D. and Neumeier, S. and Gault, B. and Stephenson, L.T.
    Scripta Materialia 203 (2021)
    It is widely accepted that the different types of crystalline imperfections, such as vacancies or dislocations, greatly influence a material's physical and mechanical properties. However, imaging individual vacancies in solids and revealing their atomic neighborhood remains one of the frontiers of microscopy and microanalysis. Here, we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of Ta with vacancies, supporting positive solute-vacancy interactions previously predicted by atomistic simulations. © 2021
    view abstractdoi: 10.1016/j.scriptamat.2021.114036
  • 2021 • 211 Synthesis of Cu Single Atoms Supported on Mesoporous Graphitic Carbon Nitride and Their Application in Liquid-Phase Aerobic Oxidation of Cyclohexene
    Büker, J. and Huang, X. and Bitzer, J. and Kleist, W. and Muhler, M. and Peng, B.
    ACS Catalysis 11 7863-7875 (2021)
    Different loadings of Cu single atoms were anchored on a graphitic carbon nitride (g-C3N4) matrix using a two-step thermal synthesis method and applied in liquid-phase cyclohexene oxidation under mild conditions using molecular O2 as the oxidizing agent. The oxidation state of Cu was determined to be Cu+, which is in linear coordination with two neighboring nitrogen atoms at a distance of 1.9 Å. The catalyst with 0.9 wt % Cu pyrolyzed at 380 °C was found to exhibit the best catalytic performance with the highest conversion up to 82% with an allylic selectivity of 55%. It also showed high reusability over four catalytic runs without any detectable Cu leaching. Cyclohexene oxidation followed first-order kinetics with an apparent activation energy of 66.2 kJ mol-1. The addition of hydroquinone as a radical scavenger confirmed that cyclohexene oxidation proceeds via a radical mechanism. Time-resolved in situ attenuated total reflection infrared (ATR-IR) spectroscopy was carried out to qualitatively monitor the cyclohexene oxidation pathways. The comparison with the homogeneous analogue Cu(I) iodide indirectly verified the linearly N-coordinated single Cu(I) species to be the active sites for cyclohexene oxidation. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.1c01468
  • 2020 • 210 A Mechanistic Study on Reactions of Group 13 Diyls LM with Cp*SbX2: From Stibanyl Radicals to Antimony Hydrides
    Helling, C. and Wölper, C. and Cutsail, G.E., III and Haberhauer, G. and Schulz, S.
    Chemistry - A European Journal 26 13390-13399 (2020)
    Oxidative addition of Cp*SbX2 (X=Cl, Br, I; Cp*=C5Me5) to group 13 diyls LM (M=Al, Ga, In; L=HC[C(Me)N (Dip)]2, Dip=2,6-iPr2C6H3) yields elemental antimony (M=Al) or the corresponding stibanylgallanes [L(X)Ga]Sb(X)Cp* (X=Br 1, I 2) and -indanes [L(X)In]Sb(X)Cp* (X=Cl 5, Br 6, I 7). 1 and 2 react with a second equivalent of LGa to eliminate decamethyl-1,1’-dihydrofulvalene (Cp*2) and form stibanyl radicals [L(X)Ga]2Sb. (X=Br 3, I 4), whereas analogous reactions of 5 and 6 with LIn selectively yield stibanes [L(X)In]2SbH (X=Cl 8, Br 9) by elimination of 1,2,3,4-tetramethylfulvene. The reactions are proposed to proceed via formation of [L(X)M]2SbCp* as reaction intermediate, which is supported by the isolation of [L(Cl)Ga]2SbCp (11, Cp=C5H5). The reaction mechanism was further studied by computational calculations using two different models. The energy values for the Ga- and the In-substituted model systems showing methyl groups instead of the very bulky Dip units are very similar, and in both cases the same products are expected. Homolytic Sb−C bond cleavage yields van der Waals complexes from the as-formed radicals ([L(Cl)M]2Sb. and Cp*.), which can be stabilized by hydrogen atom abstraction to give the corresponding hydrides, whereas the direct formation of Sb hydrides starting from [L(Cl)M]2SbCp* via concerted β-H elimination is unlikely. The consideration of the bulky Dip units reveals that the amount of the steric overload in the intermediate I determines the product formation (radical vs. hydride). © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/chem.202001739
  • 2020 • 209 A model for grain boundary thermodynamics
    Darvishi Kamachali, R.
    RSC Advances 10 26728-26741 (2020)
    Systematic microstructure design requires reliable thermodynamic descriptions of each and all microstructure elements. While such descriptions are well established for most bulk phases, thermodynamic assessment of microstructure defects is challenging because of their individualistic nature. In this paper, a model is devised for assessing grain boundary thermodynamics based on available bulk thermodynamic data. We propose a continuous relative atomic density field and its spatial gradients to describe the grain boundary region with reference to the homogeneous bulk and derive the grain boundary Gibbs free energy functional. The grain boundary segregation isotherm and phase diagram are computed for a regular binary solid solution, and qualitatively benchmarked for the Pt-Au system. The relationships between the grain boundary's atomic density, excess free volume, and misorientation angle are discussed. Combining the current density-based model with available bulk thermodynamic databases enables constructing databases, phase diagrams, and segregation isotherms for grain boundaries, opening possibilities for studying and designing heterogeneous microstructures. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra04682e
  • 2020 • 208 An experimentally validated neural-network potential energy surface for H-atom on free-standing graphene in full dimensionality
    Wille, S. and Jiang, H. and Bünermann, O. and Wodtke, A.M. and Behler, J. and Kandratsenka, A.
    Physical Chemistry Chemical Physics 22 26113-26120 (2020)
    We present a first principles-quality potential energy surface (PES) describing the inter-atomic forces for hydrogen atoms interacting with free-standing graphene. The PES is a high-dimensional neural network potential that has been parameterized to 75 945 data points computed with density-functional theory employing the PBE-D2 functional. Improving over a previously published PES [Jiang et al., Science, 2019, 364, 379], this neural network exhibits a realistic physisorption well and achieves a 10-fold reduction in the RMS fitting error, which is 0.6 meV per atom. The chemisorption barrier is 172 meV, which is lower than that of the REBO-EMFT PES (260 meV). We used this PES to calculate about 1.5 million classical trajectories with carefully selected initial conditions to allow for direct comparison to results of H- and D-atom scattering experiments performed at incidence translational energy of 1.9 eV and a surface temperature of 300 K. The theoretically predicted scattering angular and energy loss distributions are in good agreement with experiment, despite the fact that the experiments employed graphene grown on Pt(111). Compared to previous calculations, the agreement with experiments is improved. The remaining discrepancies between experiment and theory are likely due to the influence of the Pt substrate only present in the experiment. This journal is © the Owner Societies.
    view abstractdoi: 10.1039/d0cp03462b
  • 2020 • 207 Atomic scale configuration of planar defects in the Nb-rich C14 Laves phase NbFe2
    Šlapáková, M. and Zendegani, A. and Liebscher, C.H. and Hickel, T. and Neugebauer, J. and Hammerschmidt, T. and Ormeci, A. and Grin, J. and Dehm, G. and Kumar, K.S. and Stein, F.
    Acta Materialia 183 362-376 (2020)
    Laves phases belong to the group of tetrahedrally close-packed intermetallic phases, and their crystal structure can be described by discrete layer arrangements. They often possess extended homogeneity ranges and the general notion is that deviations from stoichiometry are accommodated by anti-site atoms or vacancies. The present work shows that excess Nb atoms in a Nb-rich NbFe2 C14 Laves phase can also be incorporated in various types of planar defects. Aberration-corrected scanning transmission electron microscopy and density functional theory calculations are employed to characterize the atomic configuration of these defects and to establish stability criteria for them. The planar defects can be categorized as extended or confined ones. The extended defects lie parallel to the basal plane of the surrounding C14 Laves phase and are fully coherent. They contain the characteristic Zr4Al3-type (O) units found in the neighboring Nb6Fe7 µ phase. An analysis of the chemical bonding reveals that the local reduction of the charge transfer is a possible reason for the preference of this atomic arrangement. However, the overall layer stacking deviates from that of the perfect µ phase. The ab initio calculations establish why these exceptionally layered defects can be more stable configurations than coherent nano-precipitates of the perfect µ phase. The confined defects are observed with pyramidal and basal habit planes. The pyramidal defect is only ~1 nm thick and resembles the perfect µ phase. In contrast, the confined basal defect can be regarded as only one single O unit and it appears as if the stacking sequence is disrupted. This configuration is confirmed by ab initio calculations to be metastable. © 2019
    view abstractdoi: 10.1016/j.actamat.2019.11.004
  • 2020 • 206 Atomic Scale Origin of Metal Ion Release from Hip Implant Taper Junctions
    Balachandran, S. and Zachariah, Z. and Fischer, A. and Mayweg, D. and Wimmer, M.A. and Raabe, D. and Herbig, M.
    Advanced Science 7 (2020)
    Millions worldwide suffer from arthritis of the hips, and total hip replacement is a clinically successful treatment for end-stage arthritis patients. Typical hip implants incorporate a cobalt alloy (Co–Cr–Mo) femoral head fixed on a titanium alloy (Ti-6Al-4V) femoral stem via a Morse taper junction. However, fretting and corrosion at this junction can cause release of wear particles and metal ions from the metallic implant, leading to local and systemic toxicity in patients. This study is a multiscale structural-chemical investigation, ranging from the micrometer down to the atomic scale, of the underlying mechanisms leading to metal ion release from such taper junctions. Correlative transmission electron microscopy and atom probe tomography reveals microstructural and compositional alterations in the subsurface of the titanium alloy subjected to in vitro gross-slip fretting against the cobalt alloy. Even though the cobalt alloy is comparatively more wear-resistant, changes in the titanium alloy promote tribocorrosion and subsequent degradation of the cobalt alloy. These observations regarding the concurrent occurrence of electrochemical and tribological phenomena are vital to further improve the design and performance of taper junctions in similar environments. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/advs.201903008
  • 2020 • 205 Correlation analysis of strongly fluctuating atomic volumes, charges, and stresses in body-centered cubic refractory high-entropy alloys
    Ishibashi, S. and Ikeda, Y. and Körmann, F. and Grabowski, B. and Neugebauer, J.
    Physical Review Materials 4 (2020)
    Local lattice distortions in a series of body-centered cubic alloys, including refractory high-entropy alloys, are investigated by means of atomic volumes, atomic charges, and atomic stresses defined by the Bader charge analysis based on first-principles calculations. Analyzing the extensive data sets, we find large distributions of these atomic properties for each element in each alloy, indicating a large impact of the varying local chemical environments. We show that these local-environment effects can be well understood and captured already by the first and the second nearest neighbor shells. Based on this insight, we employ linear regression models up to the second nearest neighbor shell to accurately predict these atomic properties. Finally, we find that the elementwise-averaged values of the atomic properties correlate linearly with the averaged valence-electron concentration of the considered alloys. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.4.023608
  • 2020 • 204 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 • 203 Direct Atomic-Level Imaging of Zeolites: Oxygen, Sodium in Na-LTA and Iron in Fe-MFI
    Mayoral, A. and Zhang, Q. and Zhou, Y. and Chen, P. and Ma, Y. and Monji, T. and Losch, P. and Schmidt, W. and Schüth, F. and Hirao, H. and Yu, J. and Terasaki, O.
    Angewandte Chemie - International Edition 59 19510-19517 (2020)
    Zeolites are becoming more versatile in their chemical functions through rational design of their frameworks. Therefore, direct imaging of all atoms at the atomic scale, basic units (Si, Al, and O), heteroatoms in the framework, and extra-framework cations, is needed. TEM provides local information at the atomic level, but the serious problem of electron-beam damage needs to be overcome. Herein, all framework atoms, including oxygen and most of the extra-framework Na cations, are successfully observed in one of the most electron-beam-sensitive and lowest framework density zeolites, Na-LTA. Zeolite performance, for instance in catalysis, is highly dependent on the location of incorporated heteroatoms. Fe single atomic sites in the MFI framework have been imaged for the first time. The approach presented here, combining image analysis, electron diffraction, and DFT calculations, can provide essential structural keys for tuning catalytically active sites at the atomic level. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstractdoi: 10.1002/anie.202006122
  • 2020 • 202 Freestanding and Supported MoS2Monolayers under Cluster Irradiation: Insights from Molecular Dynamics Simulations
    Ghaderzadeh, S. and Ladygin, V. and Ghorbani-Asl, M. and Hlawacek, G. and Schleberger, M. and Krasheninnikov, A.V.
    ACS Applied Materials and Interfaces 12 37454-37463 (2020)
    Two-dimensional (2D) materials with nanometer-size holes are promising systems for DNA sequencing, water purification, and molecule selection/separation. However, controllable creation of holes with uniform sizes and shapes is still a challenge, especially when the 2D material consists of several atomic layers as, e.g., MoS2, the archetypical transition metal dichalcogenide. We use analytical potential molecular dynamics simulations to study the response of 2D MoS2 to cluster irradiation. We model both freestanding and supported sheets and assess the amount of damage created in MoS2 by the impacts of noble gas clusters in a wide range of cluster energies and incident angles. We show that cluster irradiation can be used to produce uniform holes in 2D MoS2 with the diameter being dependent on cluster size and energy. Energetic clusters can also be used to displace sulfur atoms preferentially from either top or bottom layers of S atoms in MoS2 and also clean the surface of MoS2 sheets from adsorbents. Our results for MoS2, which should be relevant to other 2D transition metal dichalcogenides, suggest new routes toward cluster beam engineering of devices based on 2D inorganic materials. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c09255
  • 2020 • 201 Grain boundary energy effect on grain boundary segregation in an equiatomic high-entropy alloy
    Li, L. and Kamachali, R.D. and Li, Z. and Zhang, Z.
    Physical Review Materials 4 (2020)
    Grain boundary (GB) segregation has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The mechanism of nanoscale segregation at the various GBs in multicomponent alloys is of great challenge to reveal and remains elusive so far. To address this issue, we studied the GB segregation in a representative equiatomic FeMnNiCoCr high-entropy alloy (HEA) aged at 450 °C. By combining transmission Kikuchi diffraction, atom probe tomography analysis and a density-based thermodynamics modeling, we uncover the nanoscale segregation behavior at a series of well-characterized GBs of different characters. No segregation occurs at coherent twin boundaries; only slight nanoscale segregation of Ni takes place at the low-angle GBs and vicinal ς29b coincidence site lattice GBs. Ni and Mn show cosegregation of high levels at the general high-angle GBs with a strong depletion in Fe, Cr, and Co. Our density-based thermodynamic model reveals that the highly negative energy of mixing Ni and Mn is the main driving force for nanoscale cosegregation to the GBs. This is further assisted by the opposite segregation of Ni and Cr atoms with a positive enthalpy of mixing. It is also found that GBs of higher interfacial energy, possessing lower atomic densities (higher disorder and free volume), show higher segregation levels. By clarifying the origins of GB segregations in the FeMnNiCoCr HEA, the current work provides fundamental ideas on nanoscale segregation at crystal defects in multicomponent alloys. © 2020 authors.
    view abstractdoi: 10.1103/PhysRevMaterials.4.053603
  • 2020 • 200 Highly charged ion impact on graphene leading to the emission of low energy electrons
    Schwestka, J. and Niggas, A. and Creutzburg, S. and Kozubek, R. and Madauß, L. and Heller, R. and Schleberger, M. and Facsko, S. and Wilhelm, R.A. and Aumayr, F.
    Journal of Physics: Conference Series 1412 (2020)
    Recent experiments found that the neutralisation of highly charged ions interacting with a freestanding single layer of graphene proceeds on a femtosecond time scale. This ultra-fast deexcitation was attributed to Interatomic Coulombic Decay (ICD), a process in which core holes in the projectile are filled by previously captured outer electrons and the energy is transferred to electrons of the surrounding carbon atoms. ICD therefore predicts the emission of many low energy electrons. We now present experimental evidence that e.g. Xe40+ indeed emits up to 85 electrons with energies below 20 eV. © 2019 Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1412/20/202012
  • 2020 • 199 Insights into the mechanochemical synthesis of Sn-β: Solid-state metal incorporation in beta zeolite
    Joshi, H. and Ochoa-Hernández, C. and Nürenberg, E. and Kang, L. and Wang, F.R. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
    Microporous and Mesoporous Materials 309 (2020)
    Sn-β zeolite is an active material for the isomerization of glucose to fructose, which is one of the critical reactions for the valorization of biomass. The material is synthesized either by a top-down or bottom-up approach. In this work, we use a top-down approach for the synthesis of Sn-β to incorporate the tin atoms into the *BEA framework. As compared to the literature, we replace the process of manual grinding with the use of ball milling to make the process reproducible, flexible, and scalable. The primary focus of this work is to investigate the processes occurring during the synthesis by a variety of characterization tools. These techniques include thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), physisorption, X-ray diffraction (XRD), and chemisorption monitored by Fourier-transform infrared spectroscopy (FTIR). The synthesis is followed by characterizing the material at various stages of synthesis. Finally, the materials are tested for the isomerization of glucose to fructose to assess the chemical nature of Sn-β zeolites. The results of this investigation provide several insights into the mechanochemical process for the incorporation of atoms in a zeolite framework. For instance, the importance of the size of precursors, distribution of Sn atoms during synthesis, and chemical changes occurring during milling are highlighted. These insights could produce a blueprint for the synthesis of a variety of solid catalysts. © 2020
    view abstractdoi: 10.1016/j.micromeso.2020.110566
  • 2020 • 198 Magnetic response of FeRh to static and dynamic disorder
    Eggert, B. and Schmeink, A. and Lill, J. and Liedke, M.O. and Kentsch, U. and Butterling, M. and Wagner, A. and Pascarelli, S. and Potzger, K. and Lindner, J. and Thomson, T. and Fassbender, J. and Ollefs, K. and Keune, W. and Bal...
    RSC Advances 10 14386-14395 (2020)
    Atomic scale defects generated using focused ion as well as laser beams can activate ferromagnetism in initially non-ferromagnetic B2 ordered alloy thin film templates. Such defects can be induced locally, confining the ferromagnetic objects within well-defined nanoscale regions. The characterization of these atomic scale defects is challenging, and the mechanism for the emergence of ferromagnetism due to sensitive lattice disordering is unclear. Here we directly probe a variety of microscopic defects in systematically disordered B2 FeRh thin films that are initially antiferromagnetic and undergo a thermally-driven isostructural phase transition to a volatile ferromagnetic state. We show that the presence of static disorder i.e., the slight deviations of atoms from their equilibrium sites is sufficient to induce a non-volatile ferromagnetic state at room temperature. A static mean square relative displacement of 9 × 10-4 Å-2 is associated with the occurrence of non-volatile ferromagnetism and replicates a snapshot of the dynamic disorder observed in the thermally-driven ferromagnetic state. The equivalence of static and dynamic disorder with respect to the ferromagnetic behavior can provide insights into the emergence of ferromagnetic coupling as well as achieving tunable magnetic properties through defect manipulations in alloys. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0ra01410a
  • 2020 • 197 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 • 196 On the X-ray Scattering Pre-peak of Linear Mono-ols and the Related Microstructure from Computer Simulations
    Požar, M. and Bolle, J. and Sternemann, C. and Perera, A.
    Journal of Physical Chemistry B 124 8358-8371 (2020)
    The X-ray scattering intensities (I(k)) of linear alkanols OH(CH2)n-1CH3 obtained from experiments (methanol to 1-undecanol) and computer simulations (methanol to 1-nonanol) of different force field models are comparatively studied particularly in order to explain the origin and the properties of the scattering pre-peak in the k-vector range 0.3-1 Å-1. The experimental I(k) values show two apparent features: the pre-peak position kP decreases with increasing n, and more intriguingly, the amplitude AP goes through a maximum at 1-butanol (n = 4). The first feature is well reproduced by all force-field models, while the second shows strong model dependence. The simulations reveal various shapes of clusters of the hydroxyl head-group from n>2. kP is directly related to the size of the meta-objects corresponding to such clusters surrounded by their alkyl tails. The explanation of the AP turnover at n = 4 is more involved in terms of cancellations of atom-atom structure factor S(k) contributions related to domain ordering. The flexibility of the alkyl tails tends to reduce the cross contributions, thus revealing the crucial importance of this parameter in the models. Force fields with all-atom representation are less successful in reproducing the pre-peak features for smaller alkanols, n<6, possibly because they blur the charge ordering process since all atoms bear partial charges. The analysis clearly shows that it is not possible to obtain a model-free explanation of the features of I(k). Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.0c05932
  • 2020 • 195 One-Pot Cooperation of Single-Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization-Hydrosilylation Process
    Sarma, B.B. and Kim, J. and Amsler, J. and Agostini, G. and Weidenthaler, C. and Pfänder, N. and Arenal, R. and Concepción, P. and Plessow, P. and Studt, F. and Prieto, G.
    Angewandte Chemie - International Edition 59 5806-5815 (2020)
    Realizing the full potential of oxide-supported single-atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one-pot combination of Ru1/CeO2 and Rh1/CeO2 SACs enables a highly selective olefin isomerization-hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double-bond migration and anti-Markovnikov α-olefin hydrosilylation, respectively. First-principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single-pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio-selectivity (&gt;95 %) even from industrially-relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide-supported single-atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201915255
  • 2020 • 194 Particle emission from two-dimensional MoS2 induced by highly charged ion impact
    Skopinski, L. and Ernst, P. and Herder, M. and Schleberger, M.
    Journal of Physics: Conference Series 1412 (2020)
    For many attractive applications of single layer MoS2 such as in optoelectronics e.g., the sample is supported by a substrate. Its importance for the modification through ion irradiation is here experimentally investigated by the analysis of sputtered particle of MoS2 on SiO2 and Au substrates under highly charged ion irradiation. The velocity distribution of the sputtered atoms is less affected by the substrate using highly charged projectiles than using slightly charged ones. Furthermore, we can show that potential sputtering causes additional emission of particles with lower kinetic energy. © 2019 Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/1412/20/202007
  • 2020 • 193 Performance and Cost Assessment of Machine Learning Interatomic Potentials
    Zuo, Y. and Chen, C. and Li, X. and Deng, Z. and Chen, Y. and Behler, J. and Csányi, G. and Shapeev, A.V. and Thompson, A.P. and Wood, M.A. and Ong, S.P.
    Journal of Physical Chemistry A 124 731-745 (2020)
    Machine learning of the quantitative relationship between local environment descriptors and the potential energy surface of a system of atoms has emerged as a new frontier in the development of interatomic potentials (IAPs). Here, we present a comprehensive evaluation of machine learning IAPs (ML-IAPs) based on four local environment descriptors - atom-centered symmetry functions (ACSF), smooth overlap of atomic positions (SOAP), the spectral neighbor analysis potential (SNAP) bispectrum components, and moment tensors - using a diverse data set generated using high-throughput density functional theory (DFT) calculations. The data set comprising bcc (Li, Mo) and fcc (Cu, Ni) metals and diamond group IV semiconductors (Si, Ge) is chosen to span a range of crystal structures and bonding. All descriptors studied show excellent performance in predicting energies and forces far surpassing that of classical IAPs, as well as predicting properties such as elastic constants and phonon dispersion curves. We observe a general trade-off between accuracy and the degrees of freedom of each model and, consequently, computational cost. We will discuss these trade-offs in the context of model selection for molecular dynamics and other applications. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.9b08723
  • 2020 • 192 Phonons in magnetically disordered materials: Magnetic versus phononic time scales
    Dutta, B. and Körmann, F. and Ghosh, S. and Sanyal, B. and Neugebauer, J. and Hickel, T.
    Physical Review B 101 (2020)
    The lattice dynamics in magnetic materials, such as Fe depends on the degree of disorder of the atomic magnetic moments and the time scale of spin fluctuations. Using first-principles methods, we have studied this effect by determining the force constant matrix in two limits: (i) When spin fluctuations are much faster than the atom vibrations, their combined impact is captured by a spin-space averaged force constant matrix, (ii) when individual spin fluctuations are sufficiently slow to scatter the phonon modes, the itinerant coherent potential approximation with spin-pair resolved force constants (i.e., φ↑↑,φ↓↓, and φ↑↓) is employed in this paper. The physical consequences for the vibrational spectral functions are analyzed by systematically modifying the input parameters (magnetization and ratio of force constants betweens atoms with equal and opposite spin directions) and by deriving them for the prototype material system bcc Fe from first-principles calculations. In the paramagnetic regime, the two limits yield identical phonon spectra. Below the Curie temperature, however, there are regions in the parametric study that show qualitative differences, including a broadening of the phonon peaks. For bcc Fe, however, the quantitative modifications of phonon frequencies turn out to be small. © 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.
    view abstractdoi: 10.1103/PhysRevB.101.094201
  • 2020 • 191 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 • 190 Revealing the two-step nucleation and growth mechanism of vanadium carbonitrides in microalloyed steels
    Wang, H. and Li, Y. and Detemple, E. and Eggeler, G.
    Scripta Materialia 187 350-354 (2020)
    Combining high-resolution transmission electron microscopy (HR-TEM) and 3-dimensional atom probe tomography (3D-APT), the early stages of nucleation and growth of vanadium carbonitrides (VCN) were revealed. VCN nucleation starts with locally distorted body-centered cubic (bcc) lattices due to a substitution of Fe atoms by V atoms, which results in the formation of an intermediate coherent crystal structure within the ferrite matrix. Misfit strain self-accommodation leads to twining within the VCN particles. As the particles grow, the precipitates gradually lose coherency and grow into discs or plates. Simultaneously, the intermediate crystal structure of the nucleus transforms into the equilibrium VCN-structure. © 2020
    view abstractdoi: 10.1016/j.scriptamat.2020.06.041
  • 2020 • 189 Single-layer Janus black arsenic-phosphorus (b-AsP): Optical dichroism, anisotropic vibrational, thermal, and elastic properties
    Li, L.L. and Bacaksiz, C. and Nakhaee, M. and Pentcheva, R. and Peeters, F.M. and Yagmurcukardes, M.
    Physical Review B 101 (2020)
    By using density functional theory (DFT) calculations, we predict a puckered, dynamically stable Janus single-layer black arsenic-phosphorus (b-AsP), which is composed of two different atomic sublayers, arsenic and phosphorus atoms. The calculated phonon spectrum reveals that Janus single-layer b-AsP is dynamically stable with either pure or coupled optical phonon branches arising from As and P atoms. The calculated Raman spectrum indicates that due to the relatively strong P-P bonds, As atoms have no contribution to the high-frequency optical vibrations. In addition, the orientation-dependent isovolume heat capacity reveals anisotropic contributions of LA and TA phonon branches to the low-temperature thermal properties. Unlike pristine single layers of b-As and b-P, Janus single-layer b-AsP exhibits additional out-of-plane asymmetry which leads to important consequences for its electronic, optical, and elastic properties. In contrast to single-layer b-As, Janus single-layer b-AsP is found to possess a direct band gap dominated by the P atoms. Moreover, real and imaginary parts of the dynamical dielectric function, including excitonic effects, reveal the highly anisotropic optical feature of the Janus single-layer. A tight-binding (TB) model is also presented for Janus single-layer b-AsP, and it is shown that, with up to seven nearest hoppings, the TB model reproduces well the DFT band structure in the low-energy region around the band gap. This TB model can be used in combination with the Green's function approach to study, e.g., quantum transport in finite systems based on Janus single-layer b-AsP. Furthermore, the linear-elastic properties of Janus single-layer b-AsP are investigated, and the orientation-dependent in-plane stiffness and Poisson ratio are calculated. It is found that the Janus single layer exhibits strong in-plane anisotropy in its Poisson ratio much larger than that of single-layer b-P. This Janus single layer is relevant for promising applications in optical dichroism and anisotropic nanoelasticity. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.134102
  • 2020 • 188 Solute hydrogen and deuterium observed at the near atomic scale in high-strength steel
    Breen, A.J. and Stephenson, L.T. and Sun, B. and Li, Y. and Kasian, O. and Raabe, D. and Herbig, M. and Gault, B.
    Acta Materialia 188 108-120 (2020)
    Observing solute hydrogen (H) in matter is a formidable challenge, yet, enabling quantitative imaging of H at the atomic-scale is critical to understand its deleterious influence on the mechanical strength of many metallic alloys that has resulted in many catastrophic failures of engineering parts and structures. Here, we report on the APT analysis of hydrogen (H) and deuterium (D) within the nanostructure of an ultra-high strength steel with high resistance to hydrogen embrittlement. Cold drawn, severely deformed pearlitic steel wires (Fe–0.98C–0.31Mn–0.20Si–0.20Cr–0.01Cu–0.006P–0.007S wt%, ε=3.1) contains cementite decomposed during the pre-deformation of the alloy and ferrite. We find H and D within the decomposed cementite, and at some interfaces with the surrounding ferrite. To ascertain the origin of the H/D signal obtained in APT, we explored a series of experimental workflows including cryogenic specimen preparation and cryogenic-vacuum transfer from the preparation into a state-of-the-art atom probe. Our study points to the critical role of the preparation, i.e. the possible saturation of H-trapping sites during electrochemical polishing, how these can be alleviated by the use of an outgassing treatment, cryogenic preparation and transfer prior to charging. Accommodation of large amounts of H in the under-stoichiometric carbide likely explains the resistance of pearlite against hydrogen embrittlement. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.004
  • 2020 • 187 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 • 186 Study of grain boundary self-diffusion in iron with different atomistic models
    Starikov, S. and Mrovec, M. and Drautz, R.
    Acta Materialia 188 560-569 (2020)
    We studied grain boundary (GB) self-diffusion in body-centered cubic iron using ab initio calculations and molecular dynamics simulations with various interatomic potentials. A combination of different models allowed us to determine the principal characteristics of self-diffusion along different types of GBs. In particular, we found that atomic self-diffusion in symmetric tilt GBs is mostly driven by self-interstitial atoms. In contrast, in general GBs atoms diffuse predominantly via an exchange mechanism that does not involve a particular defect but is similar to diffusion in a liquid. Most observed mechanisms lead to a significant enhancement of self-diffusion along GBs as compared to diffusion in the bulk. The results of simulations are verified by comparison with available experimental data. © 2020 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2020.02.027
  • 2020 • 185 The role of molecular interactions on Michaelis constants of α-chymotrypsin catalyzed peptide hydrolyses
    Knierbein, M. and Held, C. and Sadowski, G.
    Journal of Chemical Thermodynamics 148 (2020)
    In this work, the effects of co-solvent and pressure on Michaelis constants at ambient temperature were analyzed for the enzymatic peptide hydrolyses of L-phenylalanine-p-nitroanilide (HPNA) and of N-succinyl-L-phenylalanine-p-nitroanilide (SPNA). These two substrates resemble each other in their molecular structure. That is, at the position of SPNA's succinyl-group (S), HPNA possesses a hydrogen atom (H). Two co-solvents were considered: trimethylamine N-oxide and dimethyl sulfoxide. The thermodynamic model Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) was used to predict the activity of HPNA and SPNA under different reaction conditions regarding solvent composition and pressure. The PC-SAFT parameters (pure-component parameters and one binary parameter between substrate and solvent) were fitted to solubility data of HPNA in different solvents (water, ethanol, ethyl acetate, dimethyl sulfoxide), which were measured in this work at 30 °C and 1 bar. The resulting PC-SAFT predicted Michaelis constants were validated by experimental literature data. Results show that pressure decreased the Michaelis constants of both reactions, HPNA hydrolysis and SPNA hydrolysis. In spite of that, co-solvent effects on the Michaelis constants were predicted to be contrary for the two hydrolysis reactions. For the hydrolysis of HPNA, the co-solvents under investigation decreased the Michaelis constant while the co-solvents increased the Michaelis constant for the hydrolysis of SPNA. These PC-SAFT predictions were in qualitative agreement with the experimental literature data. This shows that molecular interactions are the key to understand the effects of co-solvents on Michaelis constants for the considered reactions. Applying the thermodynamic model PC-SAFT allowed predicting the observed combined effects of co-solvent and pressure on enzymatic reaction kinetics, which opens the door for solvent design of enzymatic reactions in the future. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.jct.2020.106142
  • 2020 • 184 Ultrafast Construction of Oxygen-Containing Scaffold over Graphite for Trapping Ni2+into Single Atom Catalysts
    Liu, Z. and Li, S. and Yang, J. and Tan, X. and Yu, C. and Zhao, C. and Han, X. and Huang, H. and Wan, G. and Liu, Y. and Tschulik, K. and Qiu, J.
    ACS Nano 14 11662-11669 (2020)
    Ultrafast construction of oxygen-containing scaffold over graphite for trapping Ni2+ into single atom catalysts (SACs) was developed and presented by a one-step electrochemical activation technique. The present method for Ni SACs starts with graphite foil and is capable of achieving ultrafast preparation (1.5 min) and mass production. The defective oxygen featuring the strong electronegativity enables primarily attracting Ni2+ ions and stabilizing Ni atoms via Ni-O6 coordination instead of conventional metal-C or metal-N. In addition, the oxygen defects for trapping are tunable through altering the applied voltage or electrolyte, further altering the loading of Ni atoms, indicative of enhanced oxygen evolution activity. This simple and ultrafast electrochemical synthesis is promising for the mass and controllable production of oxygen-coordinated Ni SACs, which exhibit good performance for oxygen evolution reaction. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.0c04210
  • 2020 • 183 Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy
    Knippertz, J. and Kelly, L.L. and Franke, M. and Kumpf, C. and Cinchetti, M. and Aeschlimann, M. and Stadtmüller, B.
    Physical Review B 102 (2020)
    Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter organic molecules or metallic substrates offers the opportunity to optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control metal-organic interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence X-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg2 surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg2 surface alloy [B. Stadtmüller, Phys. Rev. Lett. 117, 096805 (2016)PRLTAO0031-900710.1103/PhysRevLett.117.096805]. In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local σ-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.075447
  • 2019 • 182 A critical comparison of neural network potentials for molecular reaction dynamics with exact permutation symmetry
    Li, J. and Song, K. and Behler, J.
    Physical Chemistry Chemical Physics 21 9672-9682 (2019)
    The availability of accurate full-dimensional potential energy surfaces (PESs) is a mandatory condition for efficient computer simulations of molecular systems. Much effort has been devoted to developing reliable PESs with physically sound properties, such as the invariance of the energy with respect to the permutation of chemically identical atoms. In this work, we compare the performance of four neural network (NN)-based approaches with a rigorous permutation symmetry for fitting five typical reaction systems: OH + CO, H + H2S, H + NH3, H + CH4 and OH + CH4. The methods can be grouped into two categories, invariant polynomial based NNs and high-dimensional NN potentials (HD-NNPs). For the invariant polynomial based NNs, three types of polynomials, permutation invariant polynomials (PIPs), non-redundant PIPs (NRPIPs) and fundamental invariants (FIs), are used in the input layer of the NN. In HD-NNPs, the total energy is the sum of atomic contributions, each of which is given by an individual atomic NN with input vectors consisting of sets of atom-centered symmetry functions. Our results show that all methods exhibit a similar level of accuracy for the energies with respect to ab initio data obtained at the explicitly correlated coupled cluster level of theory (CCSD(T)-F12a). The HD-NNP method allows study of systems with larger numbers of atoms, making it more generally applicable than invariant polynomial based approaches, which in turn are computationally more efficient for smaller systems. To illustrate the applicability of the obtained potentials, quasi-classical trajectory calculations have been performed for the OH + CH4 → H2O + CH3 reaction to reveal its complicated mode specificity. © the Owner Societies 2019.
    view abstractdoi: 10.1039/c8cp06919k
  • 2019 • 181 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 • 180 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 • 179 Degradation of iridium oxides via oxygen evolution from the lattice: Correlating atomic scale structure with reaction mechanisms
    Kasian, O. and Geiger, S. and Li, T. and Grote, J.-P. and Schweinar, K. and Zhang, S. and Scheu, C. and Raabe, D. and Cherevko, S. and Gault, B. and Mayrhofer, K.J.J.
    Energy and Environmental Science 12 3548-3555 (2019)
    Understanding the fundamentals of iridium degradation during the oxygen evolution reaction is of importance for the development of efficient and durable water electrolysis systems. The degradation mechanism is complex and it is under intense discussion whether the oxygen molecule can be directly released from the oxide lattice. Here, we define the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces. We combine isotope labelling with atom probe tomography, online electrochemical and inductively coupled plasma mass spectrometry. Our data reveal that, unlike rutile IrO2, Ir hydrous oxide contains -IrIIIOOH species which directly contribute to the oxygen evolution from the lattice. This oxygen evolution mechanism results in faster degradation and dissolution of Ir. In addition, near surface bulk regions of hydrous oxide are involved in the oxygen catalysis and dissolution, while only the topmost atomic layers of rutile IrO2 participate in both reactions. Overall our data provide a contribution to the fundamental understanding of the exceptional stability of Ir-oxides towards the oxygen evolution reaction. The proposed approach to a quantitative assessment of the degree of lattice oxygen participation in the oxygen evolution reaction can be further applied to other oxide catalyst systems. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ee01872g
  • 2019 • 178 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 • 177 Elemental site occupancy in the L12 A3B ordered intermetallic phase in Co-based superalloys and its influence on the microstructure
    Pandey, P. and Makineni, S.K. and Samanta, A. and Sharma, A. and Das, S.M. and Nithin, B. and Srivastava, C. and Singh, A.K. and Raabe, D. and Gault, B. and Chattopadhyay, K.
    Acta Materialia 163 140-153 (2019)
    We explore the effects of the elemental site occupancy in γ′-A3B (L12) intermetallic phases and their partitioning across the γ/γ′ interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ′ phase and its effect on the γ/γ′ partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-XCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ′ phase (KMo&gt;1) into the γ matrix (KMo&lt;1). At 5 at.% Cr, the Cr also has a preference to replace the excess anti-site Co atoms from the B-sites. At 8 at.% Cr, the Cr develops an additional preference to replace Co atoms from the A-sites. These compositional changes in the phases and the site partitioning behavior in the γ′ phase are accompanied by an overall decrease in the lattice misfit (δ) across the γ/γ′ interfaces as measured by high-resolution X-ray diffraction at room temperature. The reduction in misfit triggers a change in morphology of the γ′ phase from cuboidal (δ ∼ +0.48% at 0 at.% Cr) to round-cornered (δ ∼ +0.34% at 5 at.% Cr) to spheroidal shaped (δ ∼ +0.19% at 8 at.% Cr) precipitates. We also observed an increase in the solvus temperature from 1066 °C to 1105 °C when adding 5 at.% Cr to the alloy. These results on the effects of Cr in Co-base superalloys enable tuning the microstructure of these alloys and widening the alloy spectrum for designing improved high temperature alloys. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.09.049
  • 2019 • 176 High-Dimensional Neural Network Potentials for Atomistic Simulations
    Hellström, M. and Behler, J.
    ACS Symposium Series 1326 49-59 (2019)
    Machine-learning methods have become increasingly popular for describing potential energy surfaces for molecular and materials simulations, and they are even beginning to challenge the present-day dominance of force fields for this task. This chapter reviews high-dimensional neural network potentials (HDNNPs), which are a general-purpose reactive potential method that can be used for simulations of an arbitrary number of atoms, can describe all types of chemical interactions (e.g., covalent, metallic, and dispersion), and includes the breaking and forming of chemical bonds. Before an HDNNP can be applied, it must be parameterized using electronic structure data, and great care must be taken at the parameterization stage to ensure that all pertinent parts of the potential energy surface are adequately covered. Typically, this is done iteratively through the addition of more training data and refitting of parameters. This chapter illustrates these points through the use of two case studies from our recent work for aqueous NaOH solutions and the ZnO/water interface. © 2019 American Chemical Society. All rights reserved.
    view abstractdoi: 10.1021/bk-2019-1326.ch003
  • 2019 • 175 Imaging individual solute atoms at crystalline imperfections in metals
    Katnagallu, S. and Stephenson, L.T. and Mouton, I. and Freysoldt, C. and Subramanyam, A.P.A. and Jenke, J. and Ladines, A.N. and Neumeier, S. and Hammerschmidt, T. and Drautz, R. and Neugebauer, J. and Vurpillot, F. and Raabe, D. ...
    New Journal of Physics 21 (2019)
    Directly imaging all atoms constituting a material and, maybe more importantly, crystalline defects that dictate materials' properties, remains a formidable challenge. Here, we propose a new approach to chemistry-sensitive field-ion microscopy (FIM) combining FIM with time-of-flight mass-spectrometry (tof-ms). Elemental identification and correlation to FIM images enabled by data mining of combined tof-ms delivers a truly analytical-FIM (A-FIM). Contrast variations due to different chemistries is also interpreted from density-functional theory (DFT). A-FIM has true atomic resolution and we demonstrate how the technique can reveal the presence of individual solute atoms at specific positions in the microstructure. The performance of this new technique is showcased in revealing individual Re atoms at crystalline defects formed in Ni-Re binary alloy during creep deformation. The atomistic details offered by A-FIM allowed us to directly compare our results with simulations, and to tackle a long-standing question of how Re extends lifetime of Ni-based superalloys in service at high-temperature. © 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/ab5cc4
  • 2019 • 174 Impact of interstitial C on phase stability and stacking-fault energy of the CrMnFeCoNi high-entropy alloy
    Ikeda, Y. and Tanaka, I. and Neugebauer, J. and Körmann, F.
    Physical Review Materials 3 (2019)
    Interstitial alloying in CrMnFeCoNi-based high-entropy alloys is known to modify their mechanical properties. Specifically, strength can be increased due to interstitial solid-solution hardening, while simultaneously affecting ductility. In this paper, first-principles calculations are carried out to analyze the impact of interstitial C atoms on CrMnFeCoNi in the fcc and the hcp phases. Our results show that C solution energies are widely spread and sensitively depend on the specific local environments. Using the computed solution-energy distributions together with statistical mechanics concepts, we determine the impact of C on the phase stability. C atoms are found to stabilize the fcc phase as compared to the hcp phase, indicating that the stacking-fault energy of CrMnFeCoNi increases due to C alloying. Using our extensive set of first-principles computed solution energies, correlations between them and local environments around the C atoms are investigated. This analysis reveals, e.g., that the local valence-electron concentration around a C atom is well correlated with its solution energy. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.113603
  • 2019 • 173 Loss of Specific Active-Site Iron Atoms in Oxygen-Exposed [FeFe]-Hydrogenase Determined by Detailed X-ray Structure Analyses
    Esselborn, J. and Kertess, L. and Apfel, U.-P. and Hofmann, E. and Happe, T.
    Journal of the American Chemical Society 141 17721-17728 (2019)
    The [FeFe]-hydrogenases catalyze the uptake and evolution of hydrogen with unmatched speed at low overpotential. However, oxygen induces the degradation of the unique [6Fe-6S] cofactor within the active site, termed the H-cluster. We used X-ray structural analyses to determine possible modes of irreversible oxygen-driven inactivation. To this end, we exposed crystals of the [FeFe]-hydrogenase CpI from Clostridium pasteurianum to oxygen and quantitatively investigated the effects on the H-cluster structure over several time points using multiple data sets, while correlating it to decreases in enzyme activity. Our results reveal the loss of specific Fe atoms from both the diiron (2FeH) and the [4Fe-4S] subcluster (4FeH) of the H-cluster. Within the 2FeH, the Fe atom more distal to the 4FeH is strikingly more affected than the more proximal Fe atom. The 4FeH interconverts to a [2Fe-2S] cluster in parts of the population of active CpIADT, but not in crystals of the inactive apoCpI initially lacking the 2FeH. We thus propose two parallel processes: dissociation of the distal Fe atom and 4FeH interconversion. Both pathways appear to play major roles in the oxidative damage of [FeFe]-hydrogenases under electron-donor deprived conditions probed by our experimental setup. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/jacs.9b07808
  • 2019 • 172 Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Tin(IV) Oxide from a Functionalized Alkyl Precursor: Fabrication and Evaluation of SnO2-Based Thin-Film Transistor Devices
    Mai, L. and Zanders, D. and Subaşl, E. and Ciftyurek, E. and Hoppe, C. and Rogalla, D. and Gilbert, W. and Arcos, T.D.L. and Schierbaum, K. and Grundmeier, G. and Bock, C. and Devi, A.
    ACS Applied Materials and Interfaces (2019)
    A bottom-up process from precursor development for tin to plasma-enhanced atomic layer deposition (PEALD) for tin(IV) oxide and its successful implementation in a working thin-film transistor device is reported. PEALD of tin(IV) oxide thin films at low temperatures down to 60 °C employing tetrakis-(dimethylamino)propyl tin(IV) [Sn(DMP)4] and oxygen plasma is demonstrated. The liquid precursor has been synthesized and thoroughly characterized with thermogravimetric analyses, revealing sufficient volatility and long-term thermal stability. [Sn(DMP)4] demonstrates typical saturation behavior and constant growth rates of 0.27 or 0.42 Å cycle-1 at 150 and 60 °C, respectively, in PEALD experiments. Within the ALD regime, the films are smooth, uniform, and of high purity. On the basis of these promising features, the PEALD process was optimized wherein a 6 nm thick tin oxide channel material layer deposited at 60 °C was applied in bottom-contact bottom-gate thin-film transistors, showing a remarkable on/off ratio of 107 and field-effect mobility of μFE ≈ 12 cm2 V-1 s-1 for the as-deposited thin films deposited at such low temperatures. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsami.8b16443
  • 2019 • 171 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 • 170 Oxygen vacancies and hydrogen doping in LaAlO3/SrTiO3 heterostructures: Electronic properties and impact on surface and interface reconstruction
    Piyanzina, I.I. and Eyert, V. and Lysogorskiy, Yu.V. and Tayurskii, D.A. and Kopp, T.
    Journal of Physics Condensed Matter 31 (2019)
    We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in a LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 heterostructure slabs with three LaAlO3 overlayers lead to the emergence of interface conductivity. Calculated defect formation energies reveal strong preference of hydrogen dopant atoms for surface sites for all structures and concentrations considered. Strong decrease of the defect formation energy of hydrogen adatoms with increasing thickness of the LaAlO3 overlayer and crossover from positive to negative values, taken together with the metallic conductivity induced by hydrogen adatoms, seamlessly explains the semiconductor-metal transition observed for these heterostructures as a function of the overlayer thickness. Moreover, we show that the potential drop and concomitant shift of (layer resolved) band edges is suppressed for the metallic configuration. Finally, magnetism with stable local moments, which form atomically thin magnetic layers at the interface, is generated by oxygen vacancies either at the surface or the interface, or by hydrogen atoms buried at the interface. In particular, oxygen vacancies in the TiO2 interface layer cause drastic downshift of the 3d eg states of the Ti atoms neighboring the vacancies, giving rise to strongly localized magnetic moments, which add to the two-dimensional background magnetization. © 2019 IOP Publishing Ltd Printed in the UK.
    view abstractdoi: 10.1088/1361-648X/ab1831
  • 2019 • 169 Partitioning of interstitial segregants during decohesion: A DFT case study of the ∑3 symmetric tilt grain boundary in ferritic steel
    Huang, X. and Janisch, R.
    Materials 12 (2019)
    The effect of hydrogen atoms at grain boundaries in metals is usually detrimental to the cohesion of the interface. This effect can be quantified in terms of the strengthening energy, which is obtained following the thermodynamic model of Rice and Wang. A critical component of this model is the bonding or solution energy of the atoms to the free surfaces that are created during decohesion. At a grain boundary in a multicomponent system, it is not immediately clear how the different species would partition and distribute on the cleaved free surfaces. In this work, it is demonstrated that the choice of partitioning pattern has a significant effect on the predicted influence of H and C on grain boundary cohesion. To this end, the ∑3(112)[110] symmetric tilt grain boundary in bcc Fe with different contents of interstitial C and H was studied, taking into account all possible distributions of the elements, as well as surface diffusion effects. H as a single element has a negative influence on grain boundary cohesion, independent of the details of the H distribution. C, on the other hand, can act both ways, enhancing or reducing the cohesion of the interface. The effect of mixed H and C compositions depends on the partition pattern. However, the general trend is that the number of detrimental cases increases with increasing H content. A decomposition of the strengthening energy into chemical and mechanical contributions shows that the elastic contribution dominates at high C contents, while the chemical contribution sets the trend for high H contents. © 2019 by the authors.
    view abstractdoi: 10.3390/ma12182971
  • 2019 • 168 Potential Precursor Alternatives to the Pyrophoric Trimethylaluminium for the Atomic Layer Deposition of Aluminium Oxide
    Mai, L. and Boysen, N. and Zanders, D. and de los Arcos, T. and Mitschker, F. and Mallick, B. and Grundmeier, G. and Awakowicz, P. and Devi, A.
    Chemistry - A European Journal 25 7489-7500 (2019)
    New precursor chemistries for the atomic layer deposition (ALD) of aluminium oxide are reported as potential alternatives to the pyrophoric trimethylaluminium (TMA) which is to date a widely used Al precursor. Combining the high reactivity of aluminium alkyls employing the 3-(dimethylamino)propyl (DMP) ligand with thermally stable amide ligands yielded three new heteroleptic, non-pyrophoric compounds [Al(NMe2)2(DMP)] (2), [Al(NEt2)2(DMP)] (3, BDEADA) and [Al(NiPr2)2(DMP)] (4), which combine the properties of both ligand systems. The compounds were synthesized and thoroughly chemically characterized, showing the intramolecular stabilization of the DMP ligand as well as only reactive Al−C and Al−N bonds, which are the key factors for the thermal stability accompanied by a sufficient reactivity, both being crucial for ALD precursors. Upon rational variation of the amide alkyl chains, tunable and high evaporation rates accompanied by thermal stability were found, as revealed by thermal evaluation. In addition, a new and promising plasma enhanced (PE)ALD process using BDEADA and oxygen plasma in a wide temperature range from 60 to 220 °C is reported and compared to that of a modified variation of the TMA, namely [AlMe2(DMP)] (DMAD). The resulting Al2O3 layers are of high density, smooth, uniform, and of high purity. The applicability of the Al2O3 films as effective gas barrier layers (GBLs) was successfully demonstrated, considering that coating on polyethylene terephthalate (PET) substrates yielded very good oxygen transmission rates (OTR) with an improvement factor of 86 for a 15 nm film by using DMAD and a factor of 25 for a film thickness of just 5 nm by using BDEDA compared to bare PET substrates. All these film attributes are of the same quality as those obtained for the industrial precursor TMA, rendering the new precursors safe and potential alternatives to TMA. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201900475
  • 2019 • 167 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 • 166 Realizing facile regeneration of spent NaBH4 with Mg-Al alloy
    Zhong, H. and Ouyang, L. and Zeng, M. and Liu, J. and Wang, H. and Shao, H. and Felderhoff, M. and Zhu, M.
    Journal of Materials Chemistry A 7 10723-10728 (2019)
    The regeneration of sodium borohydride (NaBH4) is crucial to form a closed cycle after it either supplies hydrogen energy via a hydrolysis process or provides energy through electron transfer at the anode of direct borohydride fuel cells (DBFCs). In both of these cases, the spent fuels are NaB(OH)4 from NaBO2 aqueous solution. However, the current regeneration process from (NaB(OH)4)·xH2O to form NaBH4 by reduction reaction and calcination at high temperature with metal hydrides as reducing agents is very expensive. In this work, we developed a simple regeneration process via ball milling with Mg-Al alloys as the reducing agent for NaB(OH)4 under an argon atmosphere. Under optimized conditions, a high yield of about 72% of NaBH4 could be obtained. Mechanistic study showed that all the hydrogen atoms from NaB(OH)4 remain in NaBH4 and no additional hydrogen sources are needed for the reduction process. The inexpensive Mg-Al alloy works as a reducing agent transforming the H+ to H- in NaBH4. This approach demonstrates a ∼20-fold cost reduction compared with the method using metal hydrides. This opens the door to the commercial implementation of simple ball milling processes for the regeneration of spent NaBH4 from NaB(OH)4 with cheap reducing agents. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta00769e
  • 2019 • 165 Roadmap on photonic, electronic and atomic collision physics: III. Heavy particles: With zero to relativistic speeds
    Aumayr, F. and Ueda, K. and Sokell, E. and Schippers, S. and Sadeghpour, H. and Merkt, F. and Gallagher, T.F. and Dunning, F.B. and Scheier, P. and Echt, O. and Kirchner, T. and Fritzsche, S. and Surzhykov, A. and Ma, X. and Rivar...
    Journal of Physics B: Atomic, Molecular and Optical Physics 52 (2019)
    We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6455/ab26ea
  • 2019 • 164 Surface structural phase transition induced by the formation of metal-organic networks on the Si(111) - In surface
    Suzuki, T. and Lawrence, J. and Morbec, J.M. and Kratzer, P. and Costantini, G.
    Nanoscale 11 21790-21798 (2019)
    We studied the adsorption of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Si(111)- √7 × √3-In surface, a known surface superconductor. Scanning tunneling microscopy shows the development of a surface-confined metal-organic network (SMON) where TCNQ molecules coordinate with indium atoms from the underlying √7 × √3 reconstruction. The formation of the SMON causes a surface structural phase transition from the √7 × √3 reconstruction to a previously unknown 5 × 5 reconstruction of the Si(111)-In surface. Scanning tunneling spectroscopy measurements indicate that the 5 × 5 reconstruction has a stronger insulating character than the √7 × √3 reconstruction. Density-functional-theory calculations are used to evaluate the atomic arrangement and stability of the 5 × 5 and √7 × √3 reconstructions as a function of In coverage, and suggest that the structural phase transition is driven by a slight reduction of the In coverage, caused by the incorporation of indium atoms into the SMON. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9nr07074e
  • 2018 • 163 Advanced data mining in field ion microscopy
    Katnagallu, S. and Gault, B. and Grabowski, B. and Neugebauer, J. and Raabe, D. and Nematollahi, A.
    Materials Characterization (2018)
    Field ion microscopy (FIM) allows to image individual surface atoms by exploiting the effect of an intense electric field. Widespread use of atomic resolution imaging by FIM has been hampered by a lack of efficient image processing/data extraction tools. Recent advances in imaging and data mining techniques have renewed the interest in using FIM in conjunction with automated detection of atoms and lattice defects for materials characterization. After a brief overview of existing routines, we review the use of machine learning (ML) approaches for data extraction with the aim to catalyze new data-driven insights into high electrical field physics. Apart from exploring various supervised and unsupervised ML algorithms in this context, we also employ advanced image processing routines for data extraction from large sets of FIM images. The outcomes and limitations of such routines are discussed, and we conclude with the possible application of energy minimization schemes to the extracted point clouds as a way of improving the spatial resolution of FIM. © 2018 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2018.02.040
  • 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 An N-Heterocyclic Carbene Based Silver Precursor for Plasma-Enhanced Spatial Atomic Layer Deposition of Silver Thin Films at Atmospheric Pressure
    Boysen, N. and Hasselmann, T. and Karle, S. and Rogalla, D. and Theirich, D. and Winter, M. and Riedl, T. and Devi, A.
    Angewandte Chemie - International Edition 57 16224-16227 (2018)
    A new N-heterocyclic carbene (NHC)-based silver amide compound, 1,3-di-tert-butyl-imidazolin-2-ylidene silver(I) 1,1,1-trimethyl-N-(trimethylsilyl)silanaminide [(NHC)Ag(hmds)] was synthesized and analyzed by single-crystal X-ray diffraction, 1H and 13C NMR spectroscopy, as well as EI mass spectrometry, and subsequently evaluated for its thermal characteristics. This new halogen- and phosphine-free Ag atomic layer deposition (ALD) precursor was tested successfully for silver thin film growth in atmospheric pressure plasma enhanced spatial (APP-ALD). High-purity conductive Ag thin films with a low sheet resistance of 0.9 Ω/sq (resistivity: 10−5 Ωcm) were deposited at 100 °C and characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, optical transmittance, and Rutherford back-scattering techniques. The carbene-based Ag precursor and the new APP-ALD process are significant developments in the field of precursor chemistry as well as metal ALD processing. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201808586
  • 2018 • 160 Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors
    Ren, Q.-H. and Zhang, Y. and Lu, H.-L. and Wang, Y.-P. and Liu, W.-J. and Ji, X.-M. and Devi, A. and Jiang, A.-Q. and Zhang, D.W.
    ACS Applied Materials and Interfaces 10 468-476 (2018)
    A novel hybrid core-shell structure of ZnO nanowires (NWs)/Ni as a pseudocapacitor electrode was successfully fabricated by atomic layer deposition of a nickel shell, and its capacitive performance was systemically investigated. Transmission electron microscopy and X-ray photoelectron spectroscopy results indicated that the NiO was formed at the interface between ZnO and Ni where the Ni was oxidized by ZnO during the ALD of the Ni layer. Electrochemical measurement results revealed that the Ti/ZnO NWs/Ni (1500 cycles) electrode with a 30 nm thick Ni-NiO shell layer had the best supercapacitor properties including ultrahigh specific capacitance (∼2440 F g-1), good rate capability (80.5%) under high current charge-discharge conditions, and a relatively better cycling stability (86.7% of the initial value remained after 750 cycles at 10 A g-1). These attractive capacitive behaviors are mainly attributed to the unique core-shell structure and the combined effect of ZnO NW arrays as short charge transfer pathways for ion diffusion and electron transfer as well as conductive Ni serving as channel for the fast electron transport to Ti substrate. This high-performance Ti/ZnO NWs/Ni hybrid structure is expected to be one of a promising electrodes for high-performance supercapacitor applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b13392
  • 2018 • 159 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 • 158 Atomic-scale insights into surface species of electrocatalysts in three dimensions
    Li, T. and Kasian, O. and Cherevko, S. and Zhang, S. and Geiger, S. and Scheu, C. and Felfer, P. and Raabe, D. and Gault, B. and Mayrhofer, K.J.J.
    Nature Catalysis 1 300-305 (2018)
    The topmost atomic layers of electrocatalysts determine the mechanism and kinetics of reactions in many important industrial processes, such as water splitting, chlor-electrolysis or fuel cells. Optimizing the performance of electrocatalysts requires a detailed understanding of surface-state changes during the catalytic process, ideally at the atomic scale. Here, we use atom probe tomography to reveal the three-dimensional structure of the first few atomic layers of electrochemically grown iridium oxide, an efficient electrocatalyst for the oxygen evolution reaction. We unveil the formation of confined, non-stoichiometric Ir-O species during oxygen evolution. These species gradually transform to IrO2, providing improved stability but also a decrease in activity. Additionally, electrochemical growth of oxide in deuterated solutions allowed us to trace hydroxy-groups and water molecules present in the regions of the oxide layer that are favourable for the oxygen evolution and iridium dissolution reactions. Overall, we demonstrate how tomography with near-atomic resolution advances the understanding of complex relationships between surface structure, surface state and function in electrocatalysis. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41929-018-0043-3
  • 2018 • 157 Automatic selection of atomic fingerprints and reference configurations for machine-learning potentials
    Imbalzano, G. and Anelli, A. and Giofré, D. and Klees, S. and Behler, J. and Ceriotti, M.
    Journal of Chemical Physics 148 (2018)
    Machine learning of atomic-scale properties is revolutionizing molecular modeling, making it possible to evaluate inter-atomic potentials with first-principles accuracy, at a fraction of the costs. The accuracy, speed, and reliability of machine learning potentials, however, depend strongly on the way atomic configurations are represented, i.e., the choice of descriptors used as input for the machine learning method. The raw Cartesian coordinates are typically transformed in "fingerprints," or "symmetry functions," that are designed to encode, in addition to the structure, important properties of the potential energy surface like its invariances with respect to rotation, translation, and permutation of like atoms. Here we discuss automatic protocols to select a number of fingerprints out of a large pool of candidates, based on the correlations that are intrinsic to the training data. This procedure can greatly simplify the construction of neural network potentials that strike the best balance between accuracy and computational efficiency and has the potential to accelerate by orders of magnitude the evaluation of Gaussian approximation potentials based on the smooth overlap of atomic positions kernel. We present applications to the construction of neural network potentials for water and for an Al-Mg-Si alloy and to the prediction of the formation energies of small organic molecules using Gaussian process regression. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5024611
  • 2018 • 156 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 • 155 Co2+-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions
    Yang, J. and Muckel, F. and Choi, B.K. and Lorenz, S. and Kim, I.Y. and Ackermann, J. and Chang, H. and Czerney, T. and Kale, V.S. and Hwang, S.-J. and Bacher, G. and Hyeon, T.
    Nano Letters 18 7350-7357 (2018)
    Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (&gt;21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)13 clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co-Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small-sized clusters. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.8b03627
  • 2018 • 154 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 • 153 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 • 152 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 • 151 Dispersion interactions between neighboring Bi atoms in (BiH3)2 and Te(BiR2)2
    Haack, R. and Schulz, S. and Jansen, G.
    Journal of Computational Chemistry 39 1413-1423 (2018)
    Triggered by the observation of a short Bi⋯Bi distance and a BiTeBi bond angle of only 86.6° in the crystal structure of bis(diethylbismuthanyl)tellurane quantum chemical computations on interactions between neighboring Bi atoms in Te(BiR2)2 molecules (R = H, Me, Et) and in (BiH3)2 were undertaken. Bi⋯Bi distances atoms were found to significantly shorten upon inclusion of the d shells of the heavy metal atoms into the electron correlation treatment, and it was confirmed that interaction energies from spin component-scaled second-order Møller–Plesset theory (SCS-MP2) agree well with coupled-cluster singles and doubles theory including perturbative triples (CCSD(T)). Density functional theory-based symmetry-adapted perturbation theory (DFT-SAPT) was used to study the anisotropy of the interplay of dispersion attraction and steric repulsion between the Bi atoms. Finally, geometries and relative stabilities of syn–syn and syn–anti conformers of Te(BiR2)2 (R = H, Me, Et) and interconversion barriers between them were computed. © 2018 Wiley Periodicals, Inc. © 2018 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jcc.25209
  • 2018 • 150 Electron-Blocking and Oxygen Evolution Catalyst Layers by Plasma-Enhanced Atomic Layer Deposition of Nickel Oxide
    Hufnagel, A.G. and Henß, A.-K. and Hoffmann, R. and Zeman, O.E.O. and Häringer, S. and Fattakhova-Rohlfing, D. and Bein, T.
    Advanced Materials Interfaces 5 (2018)
    A plasma-enhanced atomic layer deposition (ALD) process is presented, capable of producing thin conformal films of nickel(II) oxide (NiO) on various substrates. Nickelocene (NiCp2) is used as an inexpensive metal precursor with oxygen plasma as the oxidant. The film growth rate saturates with both nickel precursor and plasma exposure. An ALD window is observed between 225 and 275 °C. Linear growth is achieved at 250 °C with a growth rate of 0.042 nm per cycle. The thickness is highly uniform and the surface roughness is below 1 nm rms for 52 nm thick films on Si(100). Substrates with aspect ratios up to 1:10 can be processed. As-deposited, the films consist of polycrystalline, cubic NiO, and are transparent over the entire visible range with an optical bandgap of 3.7 eV. The films consist of stoichiometric NiO and contain ≈1% of carbon impurities. Two promising applications of these films are showcased in renewable energy conversion and storage devices: The films are pinhole-free and exhibit excellent electron blocking capabilities, making them potential hole-selective contact layers in solar cells. Also, high electrocatalytic activity of ultrathin NiO films is demonstrated for the alkaline oxygen evolution reaction, especially in electrolytes containing Fe3+. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/admi.201701531
  • 2018 • 149 Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption
    Ur Rahman, A. and Rahman, G. and Kratzer, P.
    Journal of Physics Condensed Matter 30 (2018)
    The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aab8b8
  • 2018 • 148 Enhancement of hardness, modulus and fracture toughness of the tetragonal (Fe,Cr)2B and orthorhombic (Cr,Fe)2B phases with addition of Cr
    Lentz, J. and Röttger, A. and Großwendt, F. and Theisen, W.
    Materials and Design 156 113-124 (2018)
    This study analyzes the influence of Cr content on hardness H, elastic modulus E and fracture toughness KIC of the M2B boride by means of nanoindentation experiments. Additionally, properties of the Fe3(C,B) phase are determined. Samples of the M2B phase are casted and microstructurally characterized by means of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. At a Cr content higher than 14.7 atom% the M2B phase transforms from tetragonal into orthorhombic structure. The tetragonal M2B type possesses an optimum of H (21 ± 1 GPa), E (373 ± 6) GPa and KIC (3.5 ± 0.7 MPam) at 4–5 atom% Cr. The hardness, modulus and toughness of the orthorhombic M2B phase increase with Cr content and reach values of H = 27 ± 0.7 GPa, E = 473 ± 9 of and KIC = 3.26 ± 0.8 MPam at maximal investigated Cr content of 55 atom%. The hardness of the M2B phases decreases around 2.3–3.2 GPa as a function of indentation depth, which is known as the indentation size effect. Hardness and fracture toughness of M2B phase outperform conventionally used M7C3 carbides and are similar to MC-carbides. Findings can be used in novel alloying approaches in order to optimize the performance and reduce cost of tool steels. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2018.06.040
  • 2018 • 147 Formation of eta carbide in ferrous martensite by room temperature aging
    Lu, W. and Herbig, M. and Liebscher, C.H. and Morsdorf, L. and Marceau, R.K.W. and Dehm, G. and Raabe, D.
    Acta Materialia 158 297-312 (2018)
    For several decades, the formation of carbon(C)-rich domains upon room temperature aging of supersaturated martensite has been a matter of debate. C-rich tweed-like patterns are observed to form after short aging times at room temperature and coarsen upon further aging. Here, we present a systematic atomic-scale investigation of carbide formation in Fe-15Ni-1C (wt.%) martensite after two to three years of isothermal room temperature aging by a combination of atom probe tomography and transmission electron microscopy. Owing to the sub-zero martensite start temperature of −25 °C, a fully austenitic microstructure is maintained at room temperature and the martensitic phase transformation is initiated during quenching in liquid nitrogen. In this way, any diffusion and redistribution of C in martensite is suppressed until heating up the specimen and holding it at room temperature. The microstructural changes that accompany the rearrangement of C atoms have been systematically investigated under controlled isothermal conditions. Our results show that after prolonged room temperature aging nanometer-sized, plate-shaped η-Fe2C carbides form with a macroscopic martensite habit plane close to {521}. The orientation relationship between the η-Fe2C carbides and the parent martensite grain (α′) follows [001]α’//[001]η, (1¯10) α’//(020)η. The observation of η-Fe2C–carbide formation at room temperature is particularly interesting, as transition carbides have so far only been reported to form above 100 °C. After three years of room temperature aging a depletion of Fe is observed in the η carbide while Ni remains distributed homogenously. This implies that the substitutional element Fe can diffuse several nanometers in martensite at room temperature within three years. © 2018
    view abstractdoi: 10.1016/j.actamat.2018.07.071
  • 2018 • 146 Fracture ab initio: A force-based scaling law for atomistically informed continuum models
    Möller, J.J. and Bitzek, E. and Janisch, R. and Ul Hassan, H. and Hartmaier, A.
    Journal of Materials Research 33 3750-3761 (2018)
    In fracture mechanics, established methods exist to model the stability of a crack tip or the kinetics of crack growth on both the atomic and the macroscopic scale. However, approaches to bridge the two scales still face the challenge in terms of directly converting the atomic forces at which bonds break into meaningful continuum mechanical failure stresses. Here we use two atomistic methods to investigate cleavage fracture of brittle materials: (i) we analyze the forces in front of a sharp crack and (ii) we study the bond breaking process during rigid body separation of half crystals without elastic relaxation. The comparison demonstrates the ability of the latter scheme, which is often used in ab initio density functional theory calculations, to model the bonding situation at a crack tip. Furthermore, we confirm the applicability of linear elastic fracture mechanics in the nanometer range close to crack tips in brittle materials. Based on these observations, a fracture mechanics model is developed to scale the critical atomic forces for bond breaking into relevant continuum mechanical quantities in the form of an atomistically informed scale-sensitive traction separation law. Such failure criteria can then be applied to describe fracture processes on larger length scales, e.g., in cohesive zone models or extended finite element models. Copyright © Materials Research Society 2018 This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (.
    view abstractdoi: 10.1557/jmr.2018.384
  • 2018 • 145 High-Temperature Rate Constants for H + Tetramethylsilane and H + Silane and Implications about Structure–Activity Relationships for Silanes
    Peukert, S. and Herzler, J. and Fikri, M. and Schulz, C.
    International Journal of Chemical Kinetics 50 57-72 (2018)
    The shock-tube technique has been used to investigate the reactions H + SiH4 → H2 + SiH3 (R1) and H + Si(CH3)4 → Si(CH3)3CH2 + H2 (R2) behind reflected shock waves. C2H5I was used as a thermal in situ source for H atoms. For reaction (R1), the experiments covered a temperature range of 1170–1251 K and for (R2) 1227–1320 K. In both cases, the pressures were near 1.5 bar. In these experiments, H atoms were monitored with atomic resonance absorption spectrometry. Fits to the H-atom temporal concentration profiles applying postulated chemical kinetic reaction mechanisms were used for determining the rate constants k1 and k2. Experimental rate constants were well represented by the Arrhenius equations k1(T) = 2.75 × 10−9 exp(−37.78 kJ mol−1/RT) cm3 s−1 and k2(T) = 1.17 × 10−7 exp(−86.82 kJ mol−1/RT) cm3 s−1. Transition state theory (TST) calculations based on CBS-QB3 and G4 levels of theory show good agreement with experimentally obtained rate constants; the experimental values for k1 and k2 are ∼40% lower and ∼50% larger than theoretical predictions, respectively. For the development of a mechanism describing the thermal decomposition of tetramethylsilane (Si(CH3)4; TMS), also TST-based rate constants for reaction CH3 + Si(CH3)4 → Si(CH3)3CH2 + CH4 (R3) were calculated. A comparison between experimental and theoretical rate constants k2 and k3 with available rate constants from the literature indicates that Si(CH3)4 has very similar reactivity toward H abstractions like neopentane (C(CH3)4), which is the analog hydrocarbon to TMS. Based on these results, the possibility of drawing reactivity analogies between hydrocarbons and structurally similar silicon-organic compounds for H-atom abstractions is discussed. © 2017 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/kin.21140
  • 2018 • 144 High-Temperature Rate Constants for the Reaction of Hydrogen Atoms with Tetramethoxysilane and Reactivity Analogies between Silanes and Oxygenated Hydrocarbons
    Peukert, S. and Yatsenko, P. and Fikri, M. and Schulz, C.
    Journal of Physical Chemistry A 122 5289-5298 (2018)
    The shock-tube technique has been used to investigate the H-abstraction reaction H + Si(OCH3)4 → H2 + Si(OCH2)(OCH3)3 behind reflected shock waves. C2H5I was used as a thermal in situ source for H atoms. The experiments covered a temperature range of 1111-1238 K, and pressures of 1.3-1.4 bar. H atom concentrations were monitored with atomic resonance absorption spectrometry (ARAS). Fits to the temporal H atom concentration profiles based on a developed chemical kinetics reaction mechanism were used for determining bimolecular rate constants. Experimental total H-abstraction rate constants were well represented by the Arrhenius equation ktotal(T) = 10-9.16±0.24 exp(-25.5 ± 5.6 kJ mol-1/RT) cm3 s-1. Transition state theory (TST) calculations based on the G4 level of theory show excellent agreement with experimentally obtained rate constants, i.e., the theory values of k(T) deviate by less than 25% from the experimental results. Regarding H abstractions, we have compared the reactivity of C-H bonds in Si(OCH3)4 with the reactivity of C-H bonds in dimethyl ether (CH3OCH3). Present experimental and theoretical results indicate that at high temperatures, i.e., T &gt; 500 K, CH3OCH3 is a good reactivity analog to Si(OCH3)4, i.e., kH+Si(OCH3)4(T) ∼ 1.5 × kH+CH3OCH3(T). On the basis of these results, we discuss the possibility of drawing reactivity analogies between oxygenated silanes and oxygenated hydrocarbons. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.8b03160
  • 2018 • 143 Impact of local electrostatic field rearrangement on field ionization
    Katnagallu, S. and Dagan, M. and Parviainen, S. and Nematollahi, A. and Grabowski, B. and Bagot, P.A.J. and Rolland, N. and Neugebauer, J. and Raabe, D. and Vurpillot, F. and Moody, M.P. and Gault, B.
    Journal of Physics D: Applied Physics 51 (2018)
    Field ion microscopy allows for direct imaging of surfaces with true atomic resolution. The high charge density distribution on the surface generates an intense electric field that can induce ionization of gas atoms. We investigate the dynamic nature of the charge and the consequent electrostatic field redistribution following the departure of atoms initially constituting the surface in the form of an ion, a process known as field evaporation. We report on a new algorithm for image processing and tracking of individual atoms on the specimen surface enabling quantitative assessment of shifts in the imaged atomic positions. By combining experimental investigations with molecular dynamics simulations, which include the full electric charge, we confirm that change is directly associated with the rearrangement of the electrostatic field that modifies the imaging gas ionization zone. We derive important considerations for future developments of data reconstruction in 3D field ion microscopy, in particular for precise quantification of lattice strains and characterization of crystalline defects at the atomic scale. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aaaba6
  • 2018 • 142 Improved Models for Metallic Nanoparticle Cores from Atomic Pair Distribution Function (PDF) Analysis
    Banerjee, S. and Liu, C.-H. and Lee, J.D. and Kovyakh, A. and Grasmik, V. and Prymak, O. and Koenigsmann, C. and Liu, H. and Wang, L. and Abeykoon, A.M.M. and Wong, S.S. and Epple, M. and Murray, C.B. and Billinge, S.J.L.
    Journal of Physical Chemistry C 122 29498-29506 (2018)
    X-ray atomic pair distribution functions (PDFs) were collected from a range of canonical metallic nanomaterials, both elemental and alloyed, prepared using different synthesis methods and exhibiting drastically different morphological properties. Widely applied shape-tuned attenuated crystal (AC) fcc models proved inadequate, yielding structured, coherent, and correlated fit residuals. However, equally simple discrete cluster models could account for the largest amplitude features in these difference signals. A hypothesis testing based approach to nanoparticle structure modeling systematically ruled out effects from crystallite size, composition, shape, and surface faceting as primary factors contributing to the AC misfit. On the other hand, decahedrally twinned cluster cores were found to be the origin of the AC structure misfits for a majority of the nanomaterials reported here. It is further motivated that the PDF can readily differentiate between the arrangement of domains in these multiply twinned motifs. Most of the nanomaterials surveyed also fall within the sub-5 nm size regime where traditional electron microscopy cannot easily detect and quantify domain structures, with sampling representative of the average nanocrystal synthesized. The results demonstrate that PDF analysis is a powerful method for understanding internal atomic interfaces in small noble metallic nanomaterials. Such core cluster models, easily built algorithmically, should serve as starting structures for more advanced models able to capture atomic positional disorder, ligand induced or otherwise, near nanocrystal surfaces. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b05897
  • 2018 • 141 Influence of average ion energy and atomic oxygen flux per Si atom on the formation of silicon oxide permeation barrier coatings on PET
    Mitschker, F. and Wißing, J. and Hoppe, C. and De Los Arcos, T. and Grundmeier, G. and Awakowicz, P.
    Journal of Physics D: Applied Physics 51 (2018)
    The respective effect of average incorporated ion energy and impinging atomic oxygen flux on the deposition of silicon oxide (SiOx) barrier coatings for polymers is studied in a microwave driven low pressure discharge with additional variable RF bias. Under consideration of plasma parameters, bias voltage, film density, chemical composition and particle fluxes, both are determined relative to the effective flux of Si atoms contributing to film growth. Subsequently, a correlation with barrier performance and chemical structure is achieved by measuring the oxygen transmission rate (OTR) and by performing x-ray photoelectron spectroscopy. It is observed that an increase in incorporated energy to 160 eV per deposited Si atom result in an enhanced cross-linking of the SiOx network and, therefore, an improved barrier performance by almost two orders of magnitude. Furthermore, independently increasing the number of oxygen atoms to 10 500 per deposited Si atom also lead to a comparable barrier improvement by an enhanced cross-linking. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aab1dd
  • 2018 • 140 Influence of Chain Length and Branching on the Structure of Functionalized Gold Nanoparticles
    Giri, A.K. and Spohr, E.
    Journal of Physical Chemistry C 122 26739-26747 (2018)
    Functionalized gold nanoparticles (GNPs) in aqueous NaCl solutions have been studied using molecular dynamics simulations to assess the suitability of various functionalization chemistries to effectively shield the metallic core. Alkane thiol chains of various chain length (Cl) containing 6, 12, 18, and 24 carbon atoms are grafted onto the surface of the gold core. We compare the properties of GNPs functionalized with nonpolar CH3-terminated and polar COO-- and NH3 +-terminated chains, where the nanoparticle charge is compensated by appropriate numbers of excess Na+ or Cl- counterions. In addition to linear chains, we also investigate branched Y-shaped chains with the branching sites at the 4th, 8th, or 12th carbon atom from the sulfur atom that connects the chain to the gold core. The penetration depth of water and ions into the diffuse hydrocarbon shell region and its dependence on chain length, branching, and terminating group is found to increase with decreasing chain length irrespective of termination. Long linear chains, however, tend to form bundles independent of terminal group and can thus leave fractions of the nanoparticle surface exposed to small molecules, whereas shorter and branched chains do not form bundles and can cover the GNPs more homogeneously. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.8b08590
  • 2018 • 139 Investigation of the local austenite stability related to hydrogen environment embrittlement of austenitic stainless steels
    Fussik, R. and Egels, G. and Theisen, W. and Weber, S.
    Materials Science Forum 941 MSF 263-268 (2018)
    Hydrogen is increasingly considered as fuel for future mobility or for stationary applications. However, the safe distribution and storage of pure hydrogen is only possible with suitable materials. Interstitially dissolved hydrogen atoms in the lattice of numerous metals are responsible for hydrogen embrittlement (HE). If hydrogen is introduced by an external source, it is called hydrogen environment embrittlement (HEE). Commonly, steels like AISI 316L with a high resistance to HEE include a large number of alloying elements and in high amount. High alloying levels result in a decrease of cost-efficiency. Therefore, the systematic investigation of lean-alloyed austenitic stainless steels is necessary in order to understand the mechanism of HEE. For that purpose, the steel grades AISI 304L and AISI 316L are selected in this work. Tensile tests in air and 400 bar hydrogen gas atmospheres are performed. After tensile testing in H, AISI 304L revealed secondary cracks at the specimen surface, which are related to the local austenite stability, which in turn is affected by the level of micro-segregation. The microstructural investigations of the crack environment directly contribute to the understanding of the micro-mechanisms of HEE. Property-maps generated from experimentally measured distributions of alloying elements allow to correlate the impact of micro-segregations on the local austenite stability. It is shown, that local segregation-bands affect the initiation and propagation of secondary cracks. In this context, the local austenite stability which is significantly affected by the Ni distribution will be discussed in detail by comparison of the metastable austenitic steel grades AISI 304L and AISI 316L. © 2018 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2018 • 138 Machine-learning-based atom probe crystallographic analysis
    Wei, Y. and Gault, B. and Varanasi, R.S. and Raabe, D. and Herbig, M. and Breen, A.J.
    Ultramicroscopy 194 15-24 (2018)
    Atom probe tomography is known for its accurate compositional analysis at the nanoscale. However, the patterns created by successive hits on the single particle detector during experiments often contain complementary information about the specimen's crystallography, including structure and orientation. This information remains in most cases unexploited because it is, up to now, retrieved predominantly manually. Here, we propose an approach combining image analysis techniques for feature selection and deep-learning to automatically interpret the patterns. Application of unsupervised machine learning techniques allows to build and train a deep neural network, based on a library generated from theoretically known crystallographic angular relationships. This approach enables direct interpretation of the detector hit maps, as shown here on the example of numerous pure-Al, and is robust enough to function under various conditions of base temperature, pulsing mode and pulse fraction. We benchmark our approach against recent attempts to automate the pattern identification via Hough-transform and discuss the current limitations of our approach. This new automated approach renders crystallographic atom probe tomography analysis more efficient, feature-sensitive, robust, user-independent and reliable. With that, deep-learning algorithms show a great potential to give access to combined atom probe crystallographic and compositional analysis to a large community of users. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2018.06.017
  • 2018 • 137 MC/MD coupling for scale bridging simulations of solute segregation in solids: An application study
    Ganesan, H. and Begau, C. and Sutmann, G.
    Communications in Computer and Information Science 889 112-127 (2018)
    A parallel hybrid Monte Carlo/Molecular Dynamics coupled framework has been developed to overcome the time scale limitation in simulations of segregation of interstitial atoms in solids. Simulations were performed using the proposed coupling approach to demonstrate its potential to model carbon segregation in ferritic steels with a single dislocation. Many simulations were carried out for different background carbon concentrations. This paper is a first step towards understanding the effect of segregation of interstitial atoms in solids and its influence on dislocation mobility in external fields. To this end, we carried out MD simulations, where shear forces were applied to mechanically drive screw dislocation on configurations with segregated carbon atoms. The results are compared with a reference system containing homogeneously distributed carbon atoms where the influence of segregated carbon on dislocation mobility could be observed. Simulation results gave qualitative evidence that the local concentration of interstitial solutes like carbon provides a significant pinning effect for the dislocation. © Springer Nature Switzerland AG 2018.
    view abstractdoi: 10.1007/978-3-319-96271-9_7
  • 2018 • 136 Measuring displacement currents during fabrication of Mg/Si Schottky diodes due to band-bending evolution
    Hagemann, U. and Huba, K. and Nienhaus, H.
    Journal of Applied Physics 124 (2018)
    The generation of a rectifying metal-semiconductor contact forms a charge depletion layer in the semiconductor surface. The resulting space charge leads to a surface band bending and the formation of a Schottky barrier. The present study introduces an unconventional method to measure and monitor the surface band bending during metal atom deposition by recording the displacement current between the metal and the semiconductor. Magnesium atoms are evaporated at 130 K onto hydrogen-passivated p-Si(001) surfaces. During deposition, the time-dependent reverse current in the diode is detected. A sharp current peak of a few nA can be attributed to the displaced charge when the first monolayers of the Mg film are formed. The currents are proportional to the number of Mg atoms impinging onto the surface. Integrating the observed displacement currents over time yields the total space charge densities at the interface between 8 and 23 nC/cm 2. This is in excellent agreement with the calculated value for a Schottky barrier of 0.5 eV and assuming flatband condition for hydrogen-passivated Si(001) surfaces. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5055206
  • 2018 • 135 On the detection of multiple events in atom probe tomography
    Peng, Z. and Vurpillot, F. and Choi, P.-P. and Li, Y. and Raabe, D. and Gault, B.
    Ultramicroscopy 189 54-60 (2018)
    In atom probe tomography (APT), multiple events can arise as a consequence of e.g. correlated field evaporation and molecular ion dissociation. They represent challenging cases for single-particle detectors and can cause compositional as well as spatial inaccuracies. Here, two state-of-the-art atom probe microscopes (Cameca LEAP 5000 XS and 5000 XR) were used to investigate cemented tungsten carbide, which exhibits high amounts of multiple events. By advanced data analysis methods, the natural character of the multiple events, as well as the performance of the APT detectors, are assessed. Accordingly, possible signal loss mechanisms are discussed. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2018.03.018
  • 2018 • 134 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 • 133 Optically excited structural transition in atomic wires on surfaces at the quantum limit: A femtosecond ultrafast surface electron diffraction study
    Horn-von Hoegen, M
    Proceedings of SPIE - The International Society for Optical Engineering 10673 (2018)
    Ultrafast electron diffraction is employed for study of structural dynamics at surfaces in the time domain. Experiments are performed in a pump probe setup with fs-laser excitation and subsequent probing through diffraction of a fs electron pulse at a temporal resolution of 350 fs. The system of interest is one atomic layer of indium atoms on a Si(111) surface: Through self-assembly In atomic wires and form which exhibits a Peierls-like, insulator to metal phase transition which can be driven non-thermally through a femtosecond-laser pulse. Through the transient intensity of the diffraction spots we observe the lifting of the Peierls transition and melting of a charge density wave in only 700 fs, heating of the surface in 6 ps, and formation of a metastable and supercooled phase which exists for nanoseconds. © 2018 SPIE.
    view abstractdoi: 10.1117/12.2312239
  • 2018 • 132 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
  • 2018 • 131 Single-atom vacancy in monolayer phosphorene: A comprehensive study of stability and magnetism under applied strain
    Morbec, J.M. and Rahman, G. and Kratzer, P.
    Journal of Magnetism and Magnetic Materials 465 546-553 (2018)
    Using first-principles calculations based on density-functional theory we systematically investigate the effect of applied strain on the stability and on the electronic and magnetic properties of monolayer phosphorene with single-atom vacancy. We consider two types of single vacancies: the symmetric SV-55|66, which has a metallic and non-magnetic ground state, and the asymmetric SV-5|9, which is energetically more favorable and exhibits a semiconducting and magnetic character. Our results show that compressive strain up to 10%, both biaxial and uniaxial along the zigzag direction, reduces the formation energy of both single-atom vacancies with respect to the pristine configuration and can stabilize these defects in phosphorene. We found that the magnetic moment of the SV-5|9 system is robust under uniaxial strain in the range of −10 to +10%, and it is only destroyed under biaxial compressive strain larger than 8%, when the system also suffers a semiconductor-to-metal transition. Additionally, we found that magnetism can be induced in the SV-55|66 system under uniaxial compressive strain larger than 4% along the zigzag direction and under biaxial tensile strain larger than 6%. Our findings of small formation energies and non-zero magnetic moments for both SV-5|9 and SV-55|66 systems under zigzag uniaxial compressive strain larger than 4% strongly suggest that a magnetic configuration in monolayer phosphorene can be easily realized by single-vacancy formation under uniaxial compressive strain. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2018.06.016
  • 2018 • 130 Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries
    Liebscher, C.H. and Stoffers, A. and Alam, M. and Lymperakis, L. and Cojocaru-Mirédin, O. and Gault, B. and Neugebauer, J. and Dehm, G. and Scheu, C. and Raabe, D.
    Physical Review Letters 121 (2018)
    The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.121.015702
  • 2017 • 129 A quantum chemical and kinetics modeling study on the autoignition mechanism of diethyl ether
    Sakai, Y. and Herzler, J. and Werler, M. and Schulz, C. and Fikri, M.
    Proceedings of the Combustion Institute 36 195-202 (2017)
    A detailed chemical kinetics model has been developed to elucidate the auto-ignition behavior of diethyl ether (DEE) under conditions relevant for internal combustion engines. The present model is composed of a C0-C4 base module from literature and a DEE module. For the low-temperature oxidation mechanism, the reactions of ROO and QOOH radicals were studied previously with a quantum-chemical and transition state theory approach by Sakai et al. (2015). In the present study, the potential energy surfaces for the unimolecular reactions of OOQOOH isomers and 1- and 2-ethoxyethyl radicals were determined with a CBSQB3 composite method. In the presence of an OOH group, the reaction barrier of the hydrogen shift from the β site (terminal carbon atom) decreases as it does in alkane oxidation but there is no effect on the hydrogen shift from the α site (next to the ether oxygen atom). Therefore, the reaction barriers of OOQOOH isomers have the same trend as the corresponding ROO radical and rate constants for the reactions of OOQOOH isomers were determined. The constructed model was validated against the recent data of ignition delay times provided in literature by Werler et al. (2015). The agreement is good over the temperature range 500-1300K and pressure range 1-40bar, although, open questions remain regarding the non-consensus at 900-1150K and 40bar. Reaction-path and sensitivity analyses attribute the importance of the reactivity at the α site to the decrease of the C H bond dissociation energy due to the ether oxygen atom. © 2016.
    view abstractdoi: 10.1016/j.proci.2016.06.037
  • 2017 • 128 Absolutely calibrated mass spectrometry measurement of reactive and stable plasma chemistry products in the effluent of a He/H2O atmospheric plasma
    Willems, G. and Benedikt, J. and Von Keudell, A.
    Journal of Physics D: Applied Physics 50 (2017)
    Mass spectrometry has been used to analyse the effluent of a micro-scaled atmospheric plasma jet operated in helium with a controlled concentration of water vapour. Absolute densities of H2O2 and OH have been measured as function of water vapour concentration and distance from the jet nozzle. The trend for both species densities are correlated and after an initial increase, the densities of H2O2 and OH saturate around 5000 ppm to 6000 ppm of water admixture. The largest densities for H2O2 (2.37 × 1014 cm-3) and OH (1.96 × 1014 cm-3) were measured at 7980 ppm water admixture and 2 mm distance from the jet. Densities of HO2 (1 × 1014 cm-3) and O2 (4 × 1014 cm-3) have been measured as well, although no trend could be observed. The direct electron impact ionisation cross-section of H2O2 at 70 eV electron energy was experimentally determined to be 1.02 × 1016 cm2. The measured densities and profiles have been compared to a 2D axially symmetric fluid model of species transport and recombination reactions. The effluent reaction chemistry is dominated by the hydroxyl radical, where the hydrogen atoms seem to play an important role as well. The analysis of neutral plasma chemistry products have been complemented by measurements of qualitative ion signals. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa77ca
  • 2017 • 127 Characterization of DC magnetron plasma in Ar/Kr/N2 mixture during deposition of (Cr,Al)N coating
    Bobzin, K. and Bagcivan, N. and Theiß, S. and Brugnara, R. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    doi: 10.1088/1361-6463/aa4ea2
  • 2017 • 126 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 • 125 Crystal structures of Fe4C vs. Fe4N analysed by DFT calculations: Fcc-based interstitial superstructures explored
    Leineweber, A. and Hickel, T. and Azimi-Manavi, B. and Maisel, S.B.
    Acta Materialia 140 433-442 (2017)
    Knowledge of the thermodynamic and structural properties of iron carbide and nitride phases is crucial for understanding phase transformations and related microstructure formation in steels. While the existence and crystal structure of the primitive cubic fcc-based γ′-Fe4N1-z phase is experimentally well-established, there is no consensus in contemporary literature about an analogous γ′-Fe4C compound. Here, we present DFT calculations for all fcc-like Fe4C and Fe4N superstructures with up to two formula units per primitive unit cell, providing energy values and the relaxed atomic structures, which were analysed mathematically and by visual inspection of the atomic arrangement. Notably, all considered Fe4C and Fe4N superstructures are metastable with respect to α-Fe and cementite-Fe3C/ε-Fe3N. Unsurprisingly, we find the well-known γ′ compound's crystal structure to be most favourable among these metastable Fe4N superstructures. However, we find the equivalent superstructure to be quite unfavourable in Fe4C. The most favourable among these metastable Fe4C structures are stabilised by a partial Bain-like distortion into the direction of a body-centred cubic arrangement of Fe atoms. This makes the particular C-ordering interesting for comparison with the short-range order in Fe-C martensites. However, even the lowest-energy Fe4C structure releases about 0.056 eV/atom upon decomposition into α + Fe3C, much more than it is the case for Fe4N (0.019 eV/atom). That energy difference is difficult to overcome even at T &gt; 0 K, in agreement with the lack of clear experimental evidence for existence of a Fe4C phase. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2017.08.059
  • 2017 • 124 Detection of adsorbed transition-metal porphyrins by spin-dependent conductance of graphene nanoribbon
    Kratzer, P. and Tawfik, S.A. and Cui, X.Y. and Stampfl, C.
    RSC Advances 7 29112-29121 (2017)
    Electronic transport in a zig-zag-edge graphene nanoribbon (GNR) and its modification by adsorbed transition metal porphyrins is studied by means of density functional theory calculations. The detachment reaction of the metal centre of the porphyrin is investigated both in the gas phase and for molecules adsorbed on the GNR. As most metal porphyrins are very stable against this reaction, it is found that these molecules bind only weakly to a perfect nanoribbon. However, interaction with a single-atom vacancy in the GNR results in chemical bonding by the transition metal centre being shared between nitrogen atoms in the porphyrin ring and the carbon atoms next to the vacancy in the GNR. For both the physisorbed and the chemisorbed geometry, the inclusion of van der Waals interaction results in a significant enlargement of the binding energy and reduction of the adsorption height. Electronic transport calculations using non-equilibrium Greens functions show that the conductivity of the GNR is altered by the chemisorbed porphyrin molecules. Since the metal centers of porphyrins carry an element-specific magnetic moment, not only the net conductance, but also the spin-dependent conductance of the GNR is affected. In particular, the adsorption of Ru-porphyrin on the single-atom vacancy results in a very large spin polarization of the current of 88% at small applied source-drain voltages. Based on our results, we suggest that a spin valve constructed from a GNR with ferromagnetic contacts could be used as a sensitive detector that could discriminate between various metal porphyrins. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ra04594h
  • 2017 • 123 First principles modeling of 3d-metal doped three-layer fluorite-structured TiO2 (4,4) nanotube to be used for photocatalytic hydrogen production
    Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Spohr, E. and D'yachkov, P.N.
    Vacuum 146 562-569 (2017)
    We have estimated theoretically the photocatalytic suitability of thinnest single-wall fluorite-structured titania (4,4) nanotube (NT) possessing three layers each (O-Ti-O) and doped by Sc, V, Cr, Mn, Fe, Co, Ni, Cu and Zn atoms substituted for host Ti atoms. For this goal, we have performed large-scale ab initio calculations on TiO2 NTs with three-layer morphology doped by 3d transition metals, using (i) the method of linear combination of atom-centered Gaussian-type orbitals (LCAO) based on the hybrid density functional theory (DFT) incorporating the Hartree-Fock (HF) exchange contribution (DFT+HF) and (ii) the method of linearized augmented cylindrical waves (LACW) with the muffin-tin approximation based on the local density functional approach (LDA). We have compared the ground state electronic structure, particularly the one-electron densities of states (DOSs) from the LCAO and LACW calculations for periodic arrangements of the 3d-metal dopant atoms. The results show clear evidence for a potential photocatalytic application for water splitting in the case of the Sc-doped titania nanotubes only. These NTs show both a reduced band gap of 2.0 eV relative to the pristine NT and an absence of defect-induced levels between the redox potentials of hydrogen and oxygen, so that electron-hole recombination becomes unlikely. Other 3d dopants with higher atomic number, although their band gap also covers the favorable green to orange region of the solar spectrum, are unsuitable because their defect-induced levels are positioned between the redox potential of oxygen and hydrogen, which can be expected to lead to rapid electron-hole recombination. © 2017 Elsevier Ltd
    view abstractdoi: 10.1016/j.vacuum.2017.05.002
  • 2017 • 122 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 • 121 Influence of PE-CVD and PE-ALD on defect formation in permeation barrier films on PET and correlation to atomic oxygen fluence
    Mitschker, F. and Steves, S. and Gebhard, M. and Rudolph, M. and Schücke, L. and Kirchheim, D. and Jaritz, M. and Brochhagen, M. and Hoppe, C. and Dahlmann, R. and Böke, M. and Benedikt, J. and Giner, I. and De los Arcos, T. and...
    Journal of Physics D: Applied Physics 50 (2017)
    doi: 10.1088/1361-6463/aa6e28
  • 2017 • 120 Laser-induced atomic emission of silicon nanoparticles during laser-induced heating
    Menser, J. and Daun, K. and Dreier, T. and Schulz, C.
    Applied Optics 56 E50-E57 (2017)
    The temporal luminescence behavior of silicon atoms during and after laser-heating of gas-borne silicon nano-particles was investigated. Silicon nanoparticles were formed in the exhaust stream of a microwave plasma reactor at 100 mbar. The observed prompt atomic line intensities correspond with thermal excitation of the evaporated species. A prompt signal at 251.61 and 288.15 nm originating from the 3s23p2 → 3s23p4s transitions showed a lifetime of 16 ns that matches the documented excited-state lifetime for the respective transitions. A secondary delayed signal contribution with similar peak intensities was observed commencing approximately 100-300 ns after the laser pulse and persisting for hundreds of nanoseconds. This signal contribution is attributed to electron impact excitation or recombination after electron impact ionization of the silicon evaporated as a consequence of the laser heating of the plasma leading to non-thermal population of electronically excited silicon. The observations support a nanoparticle evaporation model that can be used to recover nanoparticle sizes from time-resolved LII data. © 2017 Optical Society of America.
    view abstractdoi: 10.1364/AO.56.000E50
  • 2017 • 119 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 • 118 Mössbauer spectroscopy evidence of intrinsic non-stoichiometry in iron telluride single crystals
    Kiiamov, A.G. and Lysogorskiy, Y.V. and Vagizov, F.G. and Tagirov, L.R. and Tayurskii, D.A. and Croitori, D. and Tsurkan, V. and Loidl, A.
    Annalen der Physik 529 (2017)
    The FeTe parent compound for iron-superconductor chalcogenides was studied applying Mössbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room-temperature (RT) Mössbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of over-stoichiometric iron or impurity phases. Low-temperature Mössbauer spectra of the magnetically ordered compound could be well described by four hyperfine-split sextets, although no other foreign phases different from Fe1.05Te were detected by XRD and microanalysis within the sensitivity limits of the equipment. Density functional ab initio calculations have shown that over-stoichiometric iron atoms significantly affect electron charge and spin density up to the second coordination sphere of the iron sub-lattice, and, as a result, four non-equivalent groups of iron atoms are formed by their local environment. The resulting four-group model consistently describes the angular dependence of the single crystals Mössbauer spectra as well as intensity asymmetry of the doublet absorption lines in powdered samples at RT. We suppose that our approach could be extended to the entire class of Fe1+ySe1-xTex compounds, which contain excess iron atoms. (Figure presented.). © 2016 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/andp.201600241
  • 2017 • 117 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 • 116 Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts
    Zegkinoglou, I. and Zendegani, A. and Sinev, I. and Kunze, S. and Mistry, H. and Jeon, H.S. and Zhao, J. and Hu, M.Y. and Alp, E.E. and Piontek, S. and Smialkowski, M. and Apfel, U.-P. and Körmann, F. and Neugebauer, J. and Hicke...
    Journal of the American Chemical Society 139 14360-14363 (2017)
    Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b07902
  • 2017 • 115 Recommended reading list of early publications on atomic layer deposition-Outcome of the "Virtual Project on the History of ALD"
    Ahvenniemi, Esko and Akbashev, Andrew R. and Ali, Saima and Bechelany, Mikhael and Berdova, Maria and Boyadjiev, Stefan and Cameron, David C. and Chen, Rong and Chubarov, Mikhail and Cremers, Veronique and Devi, Anjana and Drozd, ...
    Journal of Vacuum Science & Technology A 35 010801 (2017)
    Atomic layer deposition (ALD), a gas-phase thin film deposition technique based on repeated, self-terminating gas-solid reactions, has become the method of choice in semiconductor manufacturing and many other technological areas for depositing thin conformal inorganic material layers for various applications. ALD has been discovered and developed independently, at least twice, under different names: atomic layer epitaxy (ALE) and molecular layering. ALE, dating back to 1974 in Finland, has been commonly known as the origin of ALD, while work done since the 1960s in the Soviet Union under the name "molecular layering" (and sometimes other names) has remained much less known. The virtual project on the history of ALD (VPHA) is a volunteer-based effort with open participation, set up to make the early days of ALD more transparent. In VPHA, started in July 2013, the target is to list, read and comment on all early ALD academic and patent literature up to 1986. VPHA has resulted in two essays and several presentations at international conferences. This paper, based on a poster presentation at the 16th International Conference on Atomic Layer Deposition in Dublin, Ireland, 2016, presents a recommended reading list of early ALD publications, created collectively by the VPHA participants through voting. The list contains 22 publications from Finland, Japan, Soviet Union, United Kingdom, and United States. Up to now, a balanced overview regarding the early history of ALD has been missing; the current list is an attempt to remedy this deficiency. (C) 2016 Author(s).
    view abstractdoi: 10.1116/1.4971389
  • 2017 • 114 Review Article: Recommended reading list of early publications on atomic layer deposition - Outcome of the "virtual Project on the History of ALD"
    Ahvenniemi, E. and Akbashev, A.R. and Ali, S. and Bechelany, M. and Berdova, M. and Boyadjiev, S. and Cameron, D.C. and Chen, R. and Chubarov, M. and Cremers, V. and Devi, A. and Drozd, V. and Elnikova, L. and Gottardi, G. and Gri...
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 35 (2017)
    Atomic layer deposition (ALD), a gas-phase thin film deposition technique based on repeated, self-terminating gas-solid reactions, has become the method of choice in semiconductor manufacturing and many other technological areas for depositing thin conformal inorganic material layers for various applications. ALD has been discovered and developed independently, at least twice, under different names: atomic layer epitaxy (ALE) and molecular layering. ALE, dating back to 1974 in Finland, has been commonly known as the origin of ALD, while work done since the 1960s in the Soviet Union under the name "molecular layering" (and sometimes other names) has remained much less known. The virtual project on the history of ALD (VPHA) is a volunteer-based effort with open participation, set up to make the early days of ALD more transparent. In VPHA, started in July 2013, the target is to list, read and comment on all early ALD academic and patent literature up to 1986. VPHA has resulted in two essays and several presentations at international conferences. This paper, based on a poster presentation at the 16th International Conference on Atomic Layer Deposition in Dublin, Ireland, 2016, presents a recommended reading list of early ALD publications, created collectively by the VPHA participants through voting. The list contains 22 publications from Finland, Japan, Soviet Union, United Kingdom, and United States. Up to now, a balanced overview regarding the early history of ALD has been missing; the current list is an attempt to remedy this deficiency. © 2016 Author(s).
    view abstractdoi: 10.1116/1.4971389
  • 2017 • 113 Review Article: Unraveling synergistic effects in plasma-surface processes by means of beam experiments
    Von Keudell, A. and Corbella, C.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 35 (2017)
    The interaction of plasmas with surfaces is dominated by synergistic effects between incident ions and radicals. Film growth is accelerated by the ions, providing adsorption sites for incoming radicals. Chemical etching is accelerated by incident ions when chemical etching products are removed from the surface by ion sputtering. The latter is the essence of anisotropic etching in microelectronics, as elucidated by the seminal paper of Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)]. However, ion-radical-synergisms play also an important role in a multitude of other systems, which are described in this article: (1) hydrocarbon thin film growth from methyl radicals and hydrogen atoms; (2) hydrocarbon thin film etching by ions and reactive neutrals; (3) plasma inactivation of bacteria; (4) plasma treatment of polymers; and (5) oxidation mechanisms during reactive magnetron sputtering of metal targets. All these mechanisms are unraveled by using a particle beam experiment to mimic the plasma-surface interface with the advantage of being able to control the species fluxes independently. It clearly shows that the mechanisms in action that had been described by Coburn and Winters [J. Appl. Phys. 50, 3189 (1979)] are ubiquitous. © 2017 Author(s).
    view abstractdoi: 10.1116/1.4983275
  • 2017 • 112 Tracing the coupled atomic shear and shuffle for a cubic to a hexagonal crystal transition
    Wang, H.L. and Hao, Y.L. and He, S.Y. and Du, K. and Li, T. and Obbard, E.G. and Hudspeth, J. and Wang, J.G. and Wang, Y.D. and Wang, Y. and Prima, F. and Lu, N. and Kim, M.J. and Cairney, J.M. and Li, S.J. and Yang, R.
    Scripta Materialia 133 70-74 (2017)
    Tracing the rearranged atoms in the first-order phase transformation is unrealistic due to the discrete structure change. Here we report that, by tuning a nano-scale decomposition in a titanium alloy, the bcc crystal distorts successively toward the hcp crystal by keeping an orthorhombic symmetry. Thus, the shear-shuffle relationship is traced experimentally to enrich the well-known Burgers mechanism. Our results reveal also that the successive tuning on crystal structure at the atomic level leads to some novel properties which are unexpected from the discrete phase transformations. © 2017 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.scriptamat.2017.02.024
  • 2017 • 111 Unearthing [3-(Dimethylamino)propyl]aluminium(III) Complexes as Novel Atomic Layer Deposition (ALD) Precursors for Al2O3: Synthesis, Characterization and ALD Process Development
    Mai, L. and Gebhard, M. and de los Arcos, T. and Giner, I. and Mitschker, F. and Winter, M. and Parala, H. and Awakowicz, P. and Grundmeier, G. and Devi, A.
    Chemistry - A European Journal 23 10768-10772 (2017)
    Identification and synthesis of intramolecularly donor-stabilized aluminium(III) complexes, which contain a 3-(dimethylamino)propyl (DMP) ligand, as novel atomic layer deposition (ALD) precursors has enabled the development of new and promising ALD processes for Al2O3 thin films at low temperatures. Key for this promising outcome is the nature of the ligand combination that leads to heteroleptic Al complexes encompassing optimal volatility, thermal stability and reactivity. The first ever example of the application of this family of Al precursors for ALD is reported here. The process shows typical ALD like growth characteristics yielding homogeneous, smooth and high purity Al2O3 thin films that are comparable to Al2O3 layers grown by well-established, but highly pyrophoric, trimethylaluminium (TMA)-based ALD processes. This is a significant development based on the fact that these compounds are non-pyrophoric in nature and therefore should be considered as an alternative to the industrial TMA-based Al2O3 ALD process used in many technological fields of application. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201702939
  • 2016 • 110 Ab Initio Molecular Dynamics Simulations of Nitrogen/VN(001) Surface Reactions: Vacancy-Catalyzed N2 Dissociative Chemisorption, N Adatom Migration, and N2 Desorption
    Sangiovanni, D.G. and Mei, A.B. and Hultman, L. and Chirita, V. and Petrov, I. and Greene, J.E.
    Journal of Physical Chemistry C 120 12503-12516 (2016)
    We use density-functional ab initio molecular dynamics to investigate the kinetics of N/VN(001) surface reactions at temperatures ranging from 1600 to 2300 K. N adatoms (Nad) on VN(001) favor epitaxial atop-V positions and diffuse among them by transiting through 4-fold hollow (FFH) sites, at which they are surrounded by two V and two N surface atoms. After several atop-V → FFH → atop-V jumps, isolated N adatoms bond strongly with an underlying N surface (Nsurf) atom. Frequent Nad/Nsurf pair exchange reactions lead to N2 desorption, which results in the formation of an anion surface vacancy. N vacancies rapidly migrate via in-plane 〈110〉 jumps and act as efficient catalysts for the dissociative chemisorption of incident N2 molecules. During exposure of VN(001) to incident atomic N gas atoms, Nad/Nad recombination and desorption is never observed, despite a continuously high N monomer surface coverage. Instead, N2 desorption is always initiated by a N adatom removing a N surface atom or by energetic N gas atoms colliding with Nad or Nsurf atoms. Similarities and differences between N/VN(001) vs. previous N/TiN(001) results, discussed on the basis of temperature-dependent ab initio electronic structures and chemical bonding, provide insights for controlling the reactivity of NaCl-structure transition-metal nitride (001) surfaces via electron-concentration tuning. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.6b02652
  • 2016 • 109 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 • 108 Atomic displacement in the CrMnFeCoNi high-entropy alloy - A scaling factor to predict solid solution strengthening
    Okamoto, N.L. and Yuge, K. and Tanaka, K. and Inui, H. and George, E.P.
    AIP Advances 6 (2016)
    Although metals strengthened by alloying have been used for millennia, models to quantify solid solution strengthening (SSS) were first proposed scarcely seventy years ago. Early models could predict the strengths of only simple alloys such as dilute binaries and not those of compositionally complex alloys because of the difficulty of calculating dislocation-solute interaction energies. Recently, models and theories of SSS have been proposed to tackle complex high-entropy alloys (HEAs). Here we show that the strength at 0 K of a prototypical HEA, CrMnFeCoNi, can be scaled and predicted using the root-mean-square atomic displacement, which can be deduced from X-ray diffraction and first-principles calculations as the isotropic atomic displacement parameter, that is, the average displacements of the constituent atoms from regular lattice positions. We show that our approach can be applied successfully to rationalize SSS in FeCoNi, MnFeCoNi, MnCoNi, MnFeNi, CrCoNi, CrFeCoNi, and CrMnCoNi, which are all medium-entropy subsets of the CrMnFeCoNi HEA. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4971371
  • 2016 • 107 Atomic layer deposition and high-resolution electron microscopy characterization of nickel nanoparticles for catalyst applications
    Dashjav, E. and Lipińska-Chwałek, M. and Grüner, D. and Mauer, G. and Luysberg, M. and Tietz, F.
    Surface and Coatings Technology 307 428-435 (2016)
    Ni nanoparticles (diameter <  10 nm) are deposited on Si and ceramic substrates of porous lanthanum-substituted strontium titanate/yttrium-stabilized zirconia (LST/YSZ) composites by a two-step process. First, NiO films are produced by atomic layer deposition at 200 °C using bis(methylcyclopentadienyl)nickel(II) (Ni(MeCp)2) and H2O as precursors. In the second step, the NiO films are reduced in H2 atmosphere at 400–800 °C. The size of the resulting Ni nanoparticles is controlled by the temperature. The largest particles with a diameter of about 7 nm are obtained at 800 °C. NiO film and Ni nanoparticles deposited on Si substrates are characterized by high-resolution electron microscopy. It was found that the Ni(MeCp)2 precursor reacts with the substrate, leading to the formation of NiSi2 precipitates beneath the surface of the Si wafer and amorphization of the surrounding area, resulting in a 10 nm thick top layer of the Si wafer. After reductive annealing, NiSi2 precipitates are preserved but Si recrystallizes and the amorphous NiO film transforms into crystalline Ni nanoparticles well distributed on the wafer surface. Process parameters were optimized for Si substrates and transfer of the process to ceramic LST/YSZ substrates is possible in principle. However, a much higher number of ALD cycles (1200 compared to 100 for Si) are necessary to obtain Ni nanoparticles of similar size and the number density of particles is lower than observed for Si substrates. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.074
  • 2016 • 106 Atomic mobility in the overheated amorphous GeTe compound for phase change memories
    Sosso, G.C. and Behler, J. and Bernasconi, M.
    Physica Status Solidi (A) Applications and Materials Science 213 329-334 (2016)
    Phase change memories rest on the ability of some chalcogenide alloys to undergo a fast and reversible transition between the crystalline and amorphous phases upon Joule heating. The fast crystallization is due to a high nucleation rate and a large crystal growth velocity which are actually possible thanks to the fragility of the supercooled liquid that allows for the persistence of a high atomic mobility at high supercooling where the thermodynamical driving force for crystallization is also high. Since crystallization in the devices occurs by rapidly heating the amorphous phase, hysteretic effects might arise with a different diffusion coefficient and viscosity on heating than on cooling. In this work, we have quantified these hysteretic effects in the phase change compound GeTe by means of molecular dynamics simulations. The atomic mobility in the overheated amorphous phase is lower than in supercooled liquid at the same temperature and the viscosity is consequently higher. Still, the simulations of the overheated amorphous phase reveal a breakdown of the Stokes-Einstein relation between the diffusion coefficient and the viscosity, similarly to what we found previously in the supercooled liquid. Evidences are provided that the breakdown is due to the emergence of dynamical heterogeneities at high supercooling. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532378
  • 2016 • 105 Atomic-layer-controlled deposition of TEMAZ/O2-ZrO2 oxidation resistance inner surface coatings for solid oxide fuel cells
    Keuter, T. and Mauer, G. and Vondahlen, F. and Iskandar, R. and Menzler, N.H. and Vaßen, R.
    Surface and Coatings Technology 288 211-220 (2016)
    Solid oxide fuel cells (SOFCs) directly convert the chemical energy of fuels into electrical energy with high efficiency. Under certain conditions oxygen can diffuse to the Ni/8 mol% Y2O3-doped ZrO2 substrate of anode-supported SOFCs, then the nickel re-oxidizes, leading to cracks in the electrolyte and cell failure thus limiting the durability of SOFCs. In order to improve the stability of SOFCs with respect to oxidation, the inner surface of the porous substrate is coated with a ZrO2 oxidation resistance layer using atomic layer deposition (ALD) with the precursors tetrakis(ethylmethylamino)zirconium (TEMAZ) and molecular oxygen. This TEMAZ/O2-ZrO2 ALD process has not yet been reported in the literature and hence, the development of the process is described in this paper. The inner surface of the porous substrate is coated with ZrO2 and the film thickness is compared with theoretical predictions, verifying the ALD model. Furthermore, the coating depth can be estimated using a simple analytical equation. The ALD ZrO2 film protects the nickel in the substrate against oxidation for at least 17 re-oxidation/re-reduction cycles. The ZrO2 inner surface coating is a highly promising candidate for enhancing the resistance of SOFCs to re-oxidation because of the excellent oxidation resistance and good cycling stability of the film. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.01.026
  • 2016 • 104 Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters - A kinetic in situ study
    Jendrzej, S. and Gökce, B. and Amendola, V. and Barcikowski, S.
    Journal of Colloid and Interface Science 463 299-307 (2016)
    Unintended post-synthesis growth of noble metal colloids caused by excess amounts of reactants or highly reactive atom clusters represents a fundamental problem in colloidal chemistry, affecting product stability or purity. Hence, quantified kinetics could allow defining nanoparticle size determination in dependence of the time. Here, we investigate in situ the growth kinetics of ps pulsed laser-fragmented platinum nanoparticles in presence of naked atom clusters in water without any influence of reducing agents or surfactants. The nanoparticle growth is investigated for platinum covering a time scale of minutes to 50 days after nanoparticle generation, it is also supplemented by results obtained from gold and palladium. Since a minimum atom cluster concentration is exceeded, a significant growth is determined by time resolved UV/Vis spectroscopy, analytical disc centrifugation, zeta potential measurement and transmission electron microscopy. We suggest a decrease of atom cluster concentration over time, since nanoparticles grow at the expense of atom clusters. The growth mechanism during early phase (<1. day) of laser-synthesized colloid is kinetically modeled by rapid barrierless coalescence. The prolonged slow nanoparticle growth is kinetically modeled by a combination of coalescence and Lifshitz-Slyozov-Wagner kinetic for Ostwald ripening, validated experimentally by the temperature dependence of Pt nanoparticle size and growth quenching by Iodide anions. © 2015.
    view abstractdoi: 10.1016/j.jcis.2015.10.032
  • 2016 • 103 Beam-induced atomic migration at Ag-containing nanofacets at an asymmetric Cu grain boundary
    Peter, N.J. and Liebscher, C.H. and Kirchlechner, C. and Dehm, G.
    Journal of Materials Research 32 968-982 (2016)
    Besides the high spatial resolution achieved in aberration-corrected scanning transmission microscopy, beam-induced dynamic effects have to be considered for quantitative chemical characterization on the level of single atomic columns. The present study investigates the influence of imaging conditions in an aberration-corrected scanning transmission electron microscope on the beam-induced atomic migration at a complex Ag-segregated, nanofaceted Cu grain boundary. Three distinct imaging conditions including static single image and serial image acquisition have been utilized. Chemical information on the Ag column occupation of single atomic columns at the grain boundary was extracted by the evolution of peak intensity ratios and compared to idealized scanning transmission electron microscopy image simulations. The atomic column occupation is underestimated when using conventional single frame acquisition due to an averaging of Ag atomic migration events during acquisition. Possible migration paths for the beam-induced atomic motion at a complex Cu grain boundary are presented. Copyright © Materials Research Society 2016
    view abstractdoi: 10.1557/jmr.2016.398
  • 2016 • 102 Bi atoms mobility-driven circular domains at the Bi/InAs(111) interface
    Richter, M.C. and Mariot, J.-M. and Gafoor, M.A. and Nicolaï, L. and Heckmann, O. and Djukic, U. and Ndiaye, W. and Vobornik, I. and Fujii, J. and Barrett, N. and Feyer, V. and Schneider, C.M. and Hricovini, K.
    Surface Science 651 147-153 (2016)
    Bi films deposited on InAs(111) A and B sides have been studied by photoemission electron microscopy. A series of snapshots acquired during sequential annealing of the interfaces at temperatures below and above the melting temperature of Bi allowed obtaining a comprehensive image of the topographic and chemical evolutions of the Bi films that are found to be InAs side dependent. On the A side, a morphology of circular patterns controlled by Bi atoms mobility is observed. The patterns are formed on the pristine In-terminated InAs(111) surface covered by a weakly bonded Bi bilayer. On the B side, no particular morphology is observed due to a stronger chemical interaction between Bi and As atoms as evidenced by the spatially-resolved core-level photoelectron spectra. © 2016 Published by Elsevier Inc.
    view abstractdoi: 10.1016/j.susc.2016.03.032
  • 2016 • 101 Collective Atomic Displacements during Complex Phase Boundary Migration in Solid-Solid Phase Transformations
    Duncan, J. and Harjunmaa, A. and Terrell, R. and Drautz, R. and Henkelman, G. and Rogal, J.
    Physical Review Letters 116 (2016)
    The A15 to bcc phase transition is simulated at the atomic scale based on an interatomic potential for molybdenum. The migration of the phase boundary proceeds via long-range collective displacements of entire groups of atoms across the interface. To capture the kinetics of these complex atomic rearrangements over extended time scales we use the adaptive kinetic Monte Carlo approach. An effective barrier of 0.5 eV is determined for the formation of each new bcc layer. This barrier is not associated with any particular atomistic process that governs the dynamics of the phase boundary migration. Instead, the effective layer transformation barrier represents a collective property of the complex potential energy surface. © 2016 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.116.035701
  • 2016 • 100 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 • 99 Diffusion of solutes in fcc Cobalt investigated by diffusion couples and first principles kinetic Monte Carlo
    Neumeier, S. and Rehman, H.U. and Neuner, J. and Zenk, C.H. and Michel, S. and Schuwalow, S. and Rogal, J. and Drautz, R. and Göken, M.
    Acta Materialia 106 304-312 (2016)
    The interdiffusivity of Al and the transition metal solutes Ti, V, Cr, Mn, Fe, Nb, Mo, Ru, Ta, W, and Re in fcc Co is characterized at 1373 K, 1473 K and 1573 K by binary diffusion couples. The experimental results are complemented by first-principles calculations in combination with kinetic Monte Carlo simulations to investigate the diffusion of Re, W, Mo and Ta in fcc Co. The interdiffusion coefficients of alloying elements in fcc Co are generally smaller than in fcc Ni, but the correlation between interdiffusion coefficients and the atomic number of metal solutes is comparable in Co and Ni. With increasing atomic number and decreasing atomic radii the interdiffusion coefficients of the investigated elements, except for Mn and Fe, decrease strongly. The trends in the diffusivity determined by experiment and simulation are in excellent agreement. Re is the slowest diffusing element in fcc Co among the investigated elements. The electronic structure calculations indicate that this is caused by strong directional bonds between Re and neighboring Co atoms. The overall lower diffusivity of solute atoms in Co as compared to Ni suggests that diffusion controlled processes could be slower in Co-base superalloys. © 2016 Acta Materialia Inc. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2016.01.028
  • 2016 • 98 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 • 97 Estimation of energy of cubic iron-carbon nanoclusters by molecular mechanic method: Berechnung der Energie von kubischen Eisen-Kohlenstoff-Nanoclustern durch molekularmechanische Methoden
    Epple, M. and Prylutskyy, Y. and Nedolya, A.V. and Shapar, D.Y.
    Materialwissenschaft und Werkstofftechnik 47 128-132 (2016)
    The energy of cubic iron-carbon nanoclusters was evaluated using the method of molecular mechanics. The focus was on two types of interstitial sites: octahedral and tetrahedral, in which the carbon atoms can be located. The calculation results showed that in the surface layer of the face-centered cubic nanocluster, all of the tetrahedral interstitial sites were energetically equivalent. If a carbon atom changes position between two tetrahedral interstices in the direction of the 111, it can occupy an energetically preferable position in octahedral interstitial space. The comparison of the nanoclusters energy between the cases of surface and subsurface location of the carbon atoms in the octahedral interstice showed that the system has lower energy in the former case. For body-centered cubic nanocluster, octahedral interstitial sites are more energetically favorable for carbon atoms than the tetrahedral interstice, excluding the surface. However, the octahedral interstitial sites on the surface are more preferable than tetrahedral interstice. Based on the calculations it was found that body-centered cubic and face-centered cubic nanoclusters could be unstable by the volumetric concentration of carbon. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201600481
  • 2016 • 96 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 • 95 Large-scale molecular dynamics simulations of TiN/TiN(001) epitaxial film growth
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 34 (2016)
    Large-scale classical molecular dynamics simulations of epitaxial TiN/TiN(001) thin film growth at 1200 K are carried out using incident flux ratios N/Ti = 1, 2, and 4. The films are analyzed as a function of composition, island size distribution, island edge orientation, and vacancy formation. Results show that N/Ti = 1 films are globally understoichiometric with dispersed Ti-rich surface regions which serve as traps to nucleate 111-oriented islands, leading to local epitaxial breakdown. Films grown with N/Ti = 2 are approximately stoichiometric and the growth mode is closer to layer-by-layer, while N/Ti = 4 films are stoichiometric with N-rich surfaces. As N/Ti is increased from 1 to 4, island edges are increasingly polar, i.e., 110-oriented, and N-terminated to accommodate the excess N flux, some of which is lost by reflection of incident N atoms. N vacancies are produced in the surface layer during film deposition with N/Ti = 1 due to the formation and subsequent desorption of N2 molecules composed of a N adatom and a N surface atom, as well as itinerant Ti adatoms pulling up N surface atoms. The N vacancy concentration is significantly reduced as N/Ti is increased to 2; with N/Ti = 4, Ti vacancies dominate. Overall, our results show that an insufficient N/Ti ratio leads to surface roughening via nucleation of small dispersed 111 islands, whereas high N/Ti ratios result in surface roughening due to more rapid upper-layer nucleation and mound formation. The growth mode of N/Ti = 2 films, which have smoother surfaces, is closer to layer-by-layer. © 2016 American Vacuum Society.
    view abstractdoi: 10.1116/1.4953404
  • 2016 • 94 Local Platinum Environments in a Solid Analogue of the Molecular Periana Catalyst
    Soorholtz, M. and Jones, L.C. and Samuelis, D. and Weidenthaler, C. and White, R.J. and Titirici, M.-M. and Cullen, D.A. and Zimmermann, T. and Antonietti, M. and Maier, J. and Palkovits, R. and Chmelka, B.F. and Schüth, F.
    ACS Catalysis 6 2332-2340 (2016)
    Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl2, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl2. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl2. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl2, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl2, along with differences that account for their respective properties. (Figure Presented). © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b02305
  • 2016 • 93 Multiscale description of carbon-supersaturated ferrite in severely drawn pearlitic wires
    Nematollahi, Gh.A. and Grabowski, B. and Raabe, D. and Neugebauer, J.
    Acta Materialia 111 321-334 (2016)
    A multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.03.052
  • 2016 • 92 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 • 91 Reversible 2D Phase Transition Driven by an Electric Field: Visualization and Control on the Atomic Scale
    Wortmann, B. and Vörden, D.V. and Graf, P. and Robles, R. and Abufager, P. and Lorente, N. and Bobisch, C.A. and Möller, R.
    Nano Letters 16 528-533 (2016)
    We report on a reversible structural phase transition of a two-dimensional system that can be locally induced by an external electric field. Two different structural configurations may coexist within a CO monolayer on Cu(111). The balance between the two phases can be shifted by an external electric field, causing the domain boundaries to move, increasing the area of the favored phase controllable both in location and size. If the field is further enhanced new domains nucleate. The arrangement of the CO molecules on the Cu surface is observed in real time and real space with atomic resolution while the electric field driving the phase transition is easily varied over a broad range. Together with the well-known molecular manipulation of CO adlayers, our findings open exciting prospects for combining spontaneous long-range order with man-made CO structures such as "molecule cascades" or "molecular graphene". Our new manipulation mode permits us to bridge the gap between fundamental concepts and the fabrication of arbitrary atomic patterns in large scale, by providing unprecedented insight into the physics of structural phase transitions on the atomic scale. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.5b04174
  • 2015 • 90 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 • 89 Ab initio simulations on N and S co-doped titania nanotubes for photocatalytic applications
    Chesnokov, A. and Lisovski, O. and Bocharov, D. and Piskunov, S. and Zhukovskii, Y.F. and Wessel, M. and Spohr, E.
    Physica Scripta 90 (2015)
    In this paper we present the results of quantum chemical modeling for energetically stable anatase (001) TiO<inf>2</inf> nanotubes, undoped, doped, and codoped with N<inf>O</inf> and S<inf>O</inf> atoms. We calculate the electronic structure of one-dimensional (1D) nanotubes and zero-dimensional (0D) atomic fragments cut out from these nanotubes, employing hybrid density functional theory with a partial incorporation of an exact, nonlocal Hartree-Fock exchange within the formalism of the linear combination of atomic orbitals, as implemented in both CRYSTAL and NWChem total energy codes. Structural optimization of 1D nanotubes has been performed using CRYSTAL09 code, while the cut-out 0D fragments have been modelled using the NWChem code. The electronic properties of the studied systems prove that the band structure of the pristine TiO<inf>2</inf> nanotube can be substantially modified by introducing substitutional impurity defects. The N-doped nanotube creates a midgap state that largely has a nitrogen 2p character. The S-doped nanotube has a defect state that almost coincides with the top of the valence bond for the pristine material. For nanotubes codoped with both S and N, we observe a downward shift of the gap state of nitrogen relative to the purely N-doped state by about 0.3 eV. This results in a system with a filled gap state about 0.3 eV below the O<inf>2</inf>/H<inf>2</inf>O oxidation level, making it a very promising candidate for photocatalytic hydrogen generation under visible light, because due to the presence of sulfur, the bottom of the conduction band is only about 2.2 eV above the occupied midgap state, and also, clearly above the standard hydrogen electrode level. © 2015 The Royal Swedish Academy of Sciences.
    view abstractdoi: 10.1088/0031-8949/90/9/094013
  • 2015 • 88 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 • 87 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 • 86 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 • 85 Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction
    Yan, X. and Xiong, H. and Bai, Q. and Frenzel, J. and Si, C. and Chen, X. and Eggeler, G. and Zhang, Z.
    RSC Advances 5 19409-19417 (2015)
    Atomic layer-by-layer construction of Pd on nanoporous gold (NPG) has been investigated through the combination of underpotential deposition (UPD) with displacement reaction. It has been found that the UPD of Cu on NPG is sensitive to the applied potential and the deposition time. The optimum deposition potential and time were determined through potential- and time-sensitive stripping experiments. The NPG-Pd electrode shows a different voltammetric behavior in comparison to the bare NPG electrode, and the deposition potential was determined through the integrated charge control for the monolayer UPD of Cu on the NPG-Pd electrode. Five layers of Pd were constructed on NPG through the layer-by-layer deposition. In addition, the microstructure of the NPG-Pdx (x = 1, 2, 3, 4 and 5) films was probed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The microstructural observation demonstrates that the atomic layers of Pd form on the ligament surface of NPG through epitaxial growth, and have no effect on the nanoporous structure of NPG. In addition, the hydrogen storage properties of the NPG-Pdx electrodes have also been addressed. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra17014h
  • 2015 • 84 Atomic oxygen dynamics in an air dielectric barrier discharge: A combined diagnostic and modeling approach
    Baldus, S. and Schröder, D. and Bibinov, N. and Schulz-Von Der Gathen, V. and Awakowicz, P.
    Journal of Physics D: Applied Physics 48 (2015)
    Cold atmospheric pressure plasmas are a promising alternative therapy for treatment of chronic wounds, as they have already shown in clinical trials. In this study an air dielectric barrier discharge (DBD) developed for therapeutic use in dermatology is characterized with respect to the plasma produced reactive oxygen species, namely atomic oxygen and ozone, which are known to be of great importance to wound healing. To understand the plasma chemistry of the applied DBD, xenon-calibrated two-photon laser-induced fluorescence spectroscopy and optical absorption spectroscopy are applied. The measured spatial distributions are shown and compared to each other. A model of the afterglow chemistry based on optical emission spectroscopy is developed to cross-check the measurement results and obtain insight into the dynamics of the considered reactive oxygen species. The atomic oxygen density is found to be located mostly between the electrodes with a maximum density of n<inf>O<inf>3</inf></inf> = 6 x 10^16 cm. Time resolved measurements reveal a constant atomic oxygen density between two high voltage pulses. The ozone is measured up to 3 mm outside the active plasma volume, reaching a maximum value of nO = 3 x 1016 cm-3 between the electrodes.
    view abstractdoi: 10.1088/0022-3727/48/27/275203
  • 2015 • 83 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 • 82 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 • 81 Autonomous Repair Mechanism of Creep Damage in Fe-Au and Fe-Au-B-N Alloys
    Zhang, S. and Kwakernaak, C. and Tichelaar, F.D. and Sloof, W.G. and Kuzmina, M. and Herbig, M. and Raabe, D. and Brück, E. and van der Zwaag, S. and van Dijk, N.H.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 5656-5670 (2015)
    The autonomous repair mechanism of creep cavitation during high-temperature deformation has been investigated in Fe-Au and Fe-Au-B-N alloys. Combined electron-microscopy techniques and atom probe tomography reveal how the improved creep properties result from Au precipitation within the creep cavities, preferentially formed on grain boundaries oriented perpendicular to the applied stress. The selective precipitation of Au atoms at the free creep cavity surface results in pore filling, and thereby, autonomous repair of the creep damage. The large difference in atomic size between the Au and Fe strongly hampers the nucleation of precipitates in the matrix. As a result, the matrix acts as a reservoir for the supersaturated solute until damage occurs. Grain boundaries and dislocations are found to act as fast transport routes for solute gold from the matrix to the creep cavities. The mechanism responsible for the self-healing can be characterized by a simple model for cavity growth and cavity filling. © 2015, The Author(s).
    view abstractdoi: 10.1007/s11661-015-3169-9
  • 2015 • 80 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 • 79 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 • 78 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 • 77 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 • 76 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 • 75 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 • 74 Evaluation of the Electrochemical Stability of Model Cu-Pt(111) Near-Surface Alloy Catalysts
    Tymoczko, J. and Calle-Vallejo, F. and Čolić, V. and Schuhmann, W. and Bandarenka, A.S.
    Electrochimica Acta 179 469-474 (2015)
    Better understanding of the factors responsible for the long-term stability of electrocatalysts is of increasing importance for the development of new generations of efficient electrode materials relevant for sustainable energy provision. Therefore, experiments with model, often single-crystal catalytic surfaces are of significance for fundamental electrochemistry and technological applications. Among model electrocatalysts, near-surface alloys (NSAs) of Pt with Cu, Ni and other metals formed via electrochemical deposition and thermal annealing have shown remarkable properties, demonstrating high activity towards a number of important reactions, including the oxygen reduction reaction (ORR) and CO oxidation. However, relatively little is known about the electrochemical stability and mechanisms of degradation of model NSAs. In this work, we employ a simple electrochemical approach, supported by density functional theory calculations, to evaluate the stability of Cu-Pt(111) NSAs in 0.1 M HClO4. Our results show that ∼30% of the Cu atoms initially incorporated into the second atomic layer of Pt are lost within the first 2000 cycles performed between 0.05 V and 1.0 V (RHE). After 5000 cycles, ca. half of the Cu atoms initially placed in the second atomic layer still remained in the subsurface region. The dissolution of Cu has a substantial impact on the measured shift in the average OH-binding energy for the catalyst surface and, consequently, on the ORR activity. Interestingly, after dissolution of Cu from NSAs, voltammetric features, which are characteristic to the Pt(111) facets, are partially restored suggesting the formation of NSA and Pt(111) domains in the resulting surface. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2015.02.110
  • 2015 • 73 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 • 72 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 • 71 Homogeneity and composition of AlInGaN: A multiprobe nanostructure study
    Krause, F.F. and Ahl, J.-P. and Tytko, D. and Choi, P.-P. and Egoavil, R. and Schowalter, M. and Mehrtens, T. and Müller-Caspary, K. and Verbeeck, J. and Raabe, D. and Hertkorn, J. and Engl, K. and Rosenauer, A.
    Ultramicroscopy 156 29-36 (2015)
    The electronic properties of quaternary AlInGaN devices significantly depend on the homogeneity of the alloy. The identification of compositional fluctuations or verification of random-alloy distribution is hence of grave importance. Here, a comprehensive multiprobe study of composition and compositional homogeneity is presented, investigating AlInGaN layers with indium concentrations ranging from 0 to 17. at% and aluminium concentrations between 0 and 39 at% employing high-angle annular dark field scanning electron microscopy (HAADF STEM), energy dispersive X-ray spectroscopy (EDX) and atom probe tomography (APT). EDX mappings reveal distributions of local concentrations which are in good agreement with random alloy atomic distributions. This was hence investigated with HAADF STEM by comparison with theoretical random alloy expectations using statistical tests. To validate the performance of these tests, HAADF STEM image simulations were carried out for the case of a random-alloy distribution of atoms and for the case of In-rich clusters with nanometer dimensions. The investigated samples, which were grown by metal-organic vapor phase epitaxy (MOVPE), were thereby found to be homogeneous on this nanometer scale. Analysis of reconstructions obtained from APT measurements yielded matching results. Though HAADF STEM only allows for the reduction of possible combinations of indium and aluminium concentrations to the proximity of isolines in the two-dimensional composition space. The observed ranges of composition are in good agreement with the EDX and APT results within the respective precisions. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.04.012
  • 2015 • 70 How Atomic Steps Modify Diffusion and Inter-adsorbate Forces: Empirical Evidence from Hopping Dynamics in Na/Cu(115)
    Godsi, O. and Corem, G. and Kravchuk, T. and Bertram, C. and Morgenstern, K. and Hedgeland, H. and Jardine, A.P. and Allison, W. and Ellis, J. and Alexandrowicz, G.
    Journal of Physical Chemistry Letters 6 4165-4170 (2015)
    We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico- to nanoseconds using helium spin echo spectroscopy. The well-defined stepped structure of Cu(115) allows us to study the effect that atomic steps have on the adsorption properties, the rate for motion parallel and perpendicular to the step edge, and the interaction between the Na atoms. With the support of a molecular dynamics simulation we show that the Na atoms perform strongly anisotropic 1D hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms that lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.5b01939
  • 2015 • 69 Hydrogen diffusion and segregation in α iron ∑ 3 (111) grain boundaries
    Hamza, M. and Hatem, T.M. and Raabe, D. and El-Awady, J.A.
    ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) 9-2015 (2015)
    Polycrystalline material generally exhibits degradation in its mechanical properties and shows more tendency for intergranular fracture due to segregation and diffusion of hydrogen on the grain boundaries (GBs). Understanding the parameters affecting the diffusion and binding of hydrogen within GBs will allow enhancing the mechanical properties of the commercial engineering materials and developing interface dominant materials. In practice during forming processes, the coincidence site lattice (CSL) GBs are experiencing deviations from their ideal configurations. Consequently, this will change the atomic structural integrity by superposition of sub-boundary dislocation networks on the ideal CSL interfaces. For this study, the ideal ∑ 3 111 [11 0] GB structure and its angular deviations in BCC iron within the range of Brandon criterion will be studied comprehensively using molecular statics (MS) simulations. The clean GB energy will be quantified, followed by the GB and free surface segregation energies calculations for hydrogen atoms. Rice-Wang model will be used to assess the embrittlement impact variation over the deviation angles. The results showed that the ideal GB structure is having the greatest resistance to embrittlement prior GB hydrogen saturation, while the 3° deviated GB is showing the highest susceptibility to embrittlement. Upon saturation, the 5° deviated GB appears to have the highest resistance instead due to the lowest stability of hydrogen atoms observed in the free surfaces of its simulation cell. Molecular dynamics (MD) simulations are then applied to calculate hydrogen diffusivity within the ideal and deviated GB structure. It is shown that hydrogen diffusivity decreases significantly in the deviated GB models. In addition, the 5° deviated GB is representing the local minimum for diffusivity results suggesting the existence of the highest atomic disorder and excessive secondary dislocation accommodation within this interface. Copyright © 2015 by ASME.
    view abstractdoi: 10.1115/IMECE2015-53118
  • 2015 • 68 Influence of Adsorption Kinetics upon the Electrochemically Reversible Hydrogen Oxidation Reaction
    Lin, C. and Jiao, X. and Tschulik, K. and Batchelor-Mcauley, C. and Compton, R.G.
    Journal of Physical Chemistry C 119 16121-16130 (2015)
    The hydrogen oxidation reaction was studied at bright polycrystalline platinum microelectrodes. A smaller steady-state current was observed in experiment as compared to that anticipated for a diffusion limited process. To facilitate physical insight into this system, a simulation model based on the Tafel-Volmer mechanism for the hydrogen oxidation reaction was developed. Under conditions of reversible electron transfer, the adsorption kinetics k<inf>a</inf> and k<inf>d</inf> are found to have distinctly different influences upon the voltammetry responses. Correspondence between the simulated and the experimental voltammograms is found, confirming the decrease of the steady-state current is caused by the slow adsorption process. The combined adsorption parameter k<inf>a</inf>γmax2 on the Tafel-Volmer mechanism was approximately 5.0 × 10-4 m s-1, where γ <inf>max</inf> (mol m-2) is the maximum surface coverage of adsorption hydrogen atoms. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b04293
  • 2015 • 67 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 • 66 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 • 65 Local electronic and magnetic properties of pure and Mn-containing magnetocaloric LaFe13-xSix compounds inferred from Mössbauer spectroscopy and magnetometry
    Makarov, S.I. and Krautz, M. and Salamon, S. and Skokov, K. and Teixeira, C.S. and Gutfleisch, O. and Wende, H. and Keune, W.
    Journal of Physics D: Applied Physics 48 (2015)
    Manganese containing La-Fe-Si alloys are important magnetocaloric compounds, since Mn atoms prevent segregation of hydrogen in partially hydrogenated La-Fe-Mn-Si alloys when their Curie temperature is tuned to room temperature by hydrogen. The effect of Mn alloying on the Fe atomic magnetic moment μ<inf>Fe</inf> is still rather unexplored. Therefore, we investigated the (local) magnetic and electric hyperfine interactions in the strongly magnetocaloric compound LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf> and, for comparison, LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 57Fe Mössbauer spectroscopy, and the global magnetic properties by vibrating sample magnetometry. The NaZn<inf>13</inf> structure was confirmed by x-ray diffraction. Two non-equivalent Fe lattice sites are known to exist in this material: the (96i) sites (Fe<inf>II</inf>) of low local symmetry, and the highly symmetrical (8b) sites (Fe<inf>I</inf>). At room temperature in the paramagnetic state, the electric hyperfine parameters of Fe atoms on both sites were obtained. At low temperatures (4.8 K), the observed magnetically split nuclear Zeeman sextets with broad apparent lines were analyzed in terms of a distribution P(B<inf>hf</inf>) of hyperfine magnetic fields B<inf>hf</inf>. The average hyperfine field 〈B<inf>hf</inf>〉, originating predominantly from Fe<inf>II</inf> sites, was found to be rather high (30.7(1) T at 4.8 K) for LaFe<inf>11.6</inf>Si<inf>1.4</inf>, and the approximate relation 〈B<inf>hf</inf>〉 = Aμ<inf>Fe</inf> is confirmed for Fe<inf>II</inf> sites, with A = 14.2 T/μ<inf>B</inf>. 〈B<inf>hf</inf>〉 is significantly reduced (to 27.7(1) T at 4.8 K) for the Mn-containing sample LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf>, providing evidence for a reduction by 9.7% of the average Fe atomic moment μ<inf>Fe</inf> from ∼2.16 μ<inf>B</inf> to a value of ∼1.95 μ<inf>B</inf> by Mn substitution of Fe. Our Mössbauer results are in good agreement with magnetometry, which reveals a reduction of the saturation magnetization of M<inf>s</inf> = 163.1(1) Am2 kg-1 of LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 10.5% due to Mn substitution. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/30/305006
  • 2015 • 64 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 • 63 Modelling and evaluation of hydrogen desorption kinetics controlled by surface reaction and bulk diffusion for magnesium hydride
    Drozdov, I.V. and Vaßen, R. and Stöver, D.
    RSC Advances 5 5363-5371 (2015)
    The 'shrinking core' model has been applied for the evaluation of hydrogen desorption kinetics during decomposition of magnesium hydride. According to our estimation, the full desorption time is expected to have a quadratic dependence on the size of powder particles, if the bulk diffusion of hydrogen atoms in magnesium is a rate controlling step. However, for the actual diffusion rate for hydrogen in magnesium bulk the diffusion cannot significantly influence the overall desorption kinetics for microand nano-powders. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4ra08089k
  • 2015 • 62 Nanocatalysis: Size- and shape-dependent chemisorption and catalytic reactivity
    Roldan Cuenya, B. and Behafarid, F.
    Surface Science Reports 70 135-187 (2015)
    In recent years, the field of catalysis has experienced an astonishing transformation, driven in part by more demanding environmental standards and critical societal and industrial needs such as the search for alternative energy sources. Thanks to the advent of nanotechnology, major steps have been made towards the rational design of novel catalysts. Striking new catalytic properties, including greatly enhanced reactivities and selectivities, have been reported for nanoparticle (NP) catalysts as compared to their bulk counterparts. However, in order to harness the power of these nanocatalysts, a detailed understanding of the origin of their enhanced performance is needed. The present review focuses on the role of the NP size and shape on chemisorption and catalytic performance. Since homogeneity in NP size and shape is a prerequisite for the understanding of structure-reactivity correlations, we first review different synthesis methods that result in narrow NP size distributions and shape controlled NPs. Next, size-dependent phenomena which influence the chemical reactivity of NPs, including quantum size-effects and the presence of under-coordinated surface atoms are examined. The effect of the NP shape on catalytic performance is discussed and explained based on the existence of different atomic structures on the NP surface with distinct chemisorption properties. The influence of additional factors, such as the oxidation state of the NPs and NP-support interactions, is also considered in the frame of the size- and shape-dependency that these phenomena present. Ultimately, our review highlights the importance of achieving a systematic understanding of the factors that control the activity and selectivity of a catalyst in order to avoid trial and error methods in the rational design of the new generation of nanocatalysts with properties tunable at the atomic level. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.surfrep.2015.01.001
  • 2015 • 61 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 • 60 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 • 59 Relationship Between Damping Capacity and Variations of Vacancies Concentration and Segregation of Carbon Atom in an Fe-Mn Alloy
    Wen, Y. and Xiao, H. and Peng, H. and Li, N. and Raabe, D.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 4828-4833 (2015)
    We investigated effects of quenching temperature and ageing on variations of vacancies concentration and segregation of solute atoms and their relationship with damping capacity in an Fe-Mn alloy. The damping capacity can be remarkably improved by lowering vacancies concentration but deteriorated by segregation of carbon atoms. A higher damping capacity can be obtained by furnace cooling or quenching and then ageing in Fe-Mn alloy with lower carbon content or addition of Ti or Nb. © 2015, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-015-3111-1
  • 2015 • 58 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 • 57 Size matters: An experimental and computational study of the influence of particle size on the lattice energy of NaCl
    Range, S. and Bernardes, C.E.S. and Simões, R.G. and Epple, M. and Da Piedade, M.E.M.
    Journal of Physical Chemistry C 119 4387-4396 (2015)
    One of the most interesting features of nanomaterials is the change in properties that normally accompanies a decrease in particle size. Enthalpy of solution measurements in water, at 298 K, carried out with sodium chloride samples spanning a 500-fold particle size range (120 nm to 60 μm) evidenced the effect of the increase in surface area to volume ratio in the enthalpy of solution and cohesive energy of NaCl. The nanoscopic samples were prepared by a new malonic ester synthesis, which allowed the production of well-formed and approximately cubic crystals. It was found that a very small change in lattice energy (∼0.01%) can be translated into a comparatively much larger change in enthalpy of solution (∼4%) and that the largest changes in properties are expected to occur for particle sizes below ∼100 nm where a steep decrease in lattice energy (spanning a ∼230 kJ·mol-1 range) down to the limit of monomeric NaCl is expected to occur. The experimental findings were corroborated by the results of atom-atom pair potential calculations, which further suggested that the lattice energy within each crystal layer varies from site to site, with the energy differences between adjacent sites decreasing on moving from the periphery to the center of the crystal. The atoms at the outmost surface layer exhibit the lowest lattice energies. Finally the most stable atoms in terms of lattice energy are located in the second layer possibly because repulsive interactions with ions of similar type beyond the crystal surface are absent. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jp5124772
  • 2015 • 56 Solubility and ordering of Ti, Ta, Mo and W on the Al sublattice in L12-Co3Al
    Koßmann, J. and Hammerschmidt, T. and Maisel, S. and Müller, S. and Drautz, R.
    Intermetallics 64 44-50 (2015)
    Co-Al-W-based alloys are promising new materials for high-temperature applications. They owe their high-temperature strength to hardening by ternary L1<inf>2</inf>-Co<inf>3</inf>(Al<inf>1-x</inf>W<inf>x</inf>) precipitates, which may form even though binary Co<inf>3</inf>Al is not stable. In the current work, density functional theory calculations are performed to study the solubility and ordering of the transition metals W, Mo, Ti, and Ta at the Al sublattice in L1<inf>2</inf>-Co<inf>3</inf>Al. The sublattice disorder is modelled with a newly parametrised cluster expansion and compared to results using special quasi-random structures. Our results for W and Mo show that the mixing energy exhibits a minimum at approximately x = 0.7. However, the computed small values of the mixing energies indicate that W and Mo atoms are fully disordered with the Al atoms already at low temperatures. For Ti and Ta we find no sizeable driving force for ordering with the Al atoms. The computed solubilities on the Al sublattice obtained are in the range of 40-80 meV/atom for W and Mo and less than 25 meV/atom for Ti and Ta. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2015.04.009
  • 2015 • 55 Spectroscopic and Microscopic Investigations of Degradation Processes in Polymer Surface-Near Regions during the Deposition of SiOx Films
    Mitschker, F. and Dietrich, J. and Ozkaya, B., Dr. and De los Arcos, T., Dr. and Giner, I., Dr. and Awakowicz, P., Prof. and Grundmeier, G., Prof.
    Plasma Processes and Polymers 12 1002-1009 (2015)
    Atomic oxygen densities and fluences in a microwave plasma are determined by means of optical emission spectroscopy for different oxygen to hexamethyldisiloxane (HMDSO) ratios during deposition of SiO<inf>x</inf> and SiO<inf>x</inf>C<inf>y</inf>H<inf>z</inf> like coatings on molecularly defined organic surfaces. The plasma coatings are deposited on octadecanethiol self-assembled monolayers that serve as a sensor layer. They are used for tracing the interfacial changes induced during plasma deposition as a function of the O<inf>2</inf> to HMDSO ratio and absolutely quantified atomic oxygen fluence. The interfacial chemical changes are monitored by means of polarization modulation IR reflection-absorption spectroscopy. The data reveal that significant oxidative degradation of the sensor layer is reached for exposure to an atomic oxygen fluence of 1.0 · 1022 m-2. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.201500085
  • 2015 • 54 Structural stability of Fe-based topologically close-packed phases
    Ladines, A.N. and Hammerschmidt, T. and Drautz, R.
    Intermetallics 59 59-67 (2015)
    Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe-Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.12.009
  • 2015 • 53 The dynamics of TiNx (x = 1-3) admolecule interlayer and intralayer transport on TiN/TiN(001) islands
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Petrov, I. and Greene, J.E. and Chirita, V.
    Thin Solid Films 589 133-144 (2015)
    It has been shown both experimentally and by density functional theory calculations that the primary diffusing species during the epitaxial growth of TiN/TiN(001) are Ti and N adatoms together with TiN<inf>x</inf> complexes (x = 1, 2, 3), in which the dominant N-containing admolecule species depends upon the incident N/Ti flux ratio. Here, we employ classical molecular dynamics (CMD) simulations to probe the dynamics of TiN<inf>x</inf> (x = 1-3) admolecules on 8 × 8 atom square, single-atom-high TiN islands on TiN(001), as well as pathways for descent over island edges. The simulations are carried out at 1000 K, a reasonable epitaxial growth temperature. We find that despite their lower mobility on infinite TiN(001) terraces, both TiN and TiN<inf>2</inf> admolecules funnel toward descending steps and are incorporated into island edges more rapidly than Ti adatoms. On islands, TiN diffuses primarily via concerted translations, but rotation is the preferred diffusion mechanism on infinite terraces. TiN<inf>2</inf> migration is initiated primarily by rotation about one of the N admolecule atoms anchored at an epitaxial site. TiN admolecules descend from islands by direct hopping over edges and by edge exchange reactions, while TiN<inf>2</inf> trimers descend exclusively by hopping. In contrast, TiN<inf>3</inf> admolecules are essentially stationary and serve as initiators for local island growth. Ti adatoms are the fastest diffusing species on infinite TiN(001) terraces, but on small TiN/TiN(001) islands, TiN dimers provide more efficient mass transport. The overall results reveal the effect of the N/Ti precursor flux ratio on TiN(001) surface morphological evolution and growth modes. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.05.013
  • 2014 • 52 Analytic bond-order potentials for the bcc refractory metals Nb, Ta, Mo and W
    Čák, M. and Hammerschmidt, T. and Rogal, J. and Vitek, V. and Drautz, R.
    Journal of Physics Condensed Matter 26 (2014)
    Bond-order potentials (BOPs) are based on the tight-binding approximation for determining the energy of a system of interacting atoms. The bond energy and forces are computed analytically within the formalism of the analytic BOPs. Here we present parametrizations of the analytic BOPs for the bcc refractory metals Nb, Ta, Mo and W. The parametrizations are optimized for the equilibrium bcc structure and tested for atomic environments far from equilibrium that had not been included in the fitting procedure. These tests include structural energy differences for competing crystal structures; tetragonal, trigonal, hexagonal and orthorhombic deformation paths; formation energies of point defects as well as phonon dispersion relations. Our tests show good agreement with available experimental and theoretical data. In practice, we obtain the energetic ordering of vacancy, [1 1 1], [1 1 0], and [1 0 0] self-interstitial atom in agreement with density functional theory calculations. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/19/195501
  • 2014 • 51 Atomic imaging of carbon-supported Pt, Pt/Co, and Ir@Pt nanocatalysts by atom-probe tomography
    Li, T. and Bagot, P.A.J. and Christian, E. and Theobald, B.R.C. and Sharman, J.D.B. and Ozkaya, D. and Moody, M.P. and Tsang, S.C.E. and Smith, G.D.W.
    ACS Catalysis 4 695-702 (2014)
    Atom probe tomography (APT) has been used to characterize commercially prepared Pt, Pt/Co alloy, and Ir@Pt core-shell nanoparticles supported on high-surface-area carbon black. Concentration profiles and 3D atom maps revealing the detailed internal structures and compositions of Pt, Pt/Co alloy, and Ir@Pt core-shell particles have been generated, and the distribution of trace impurity elements, including Na and Cl, has been examined. The observation of retained Na on the support, especially in the Pt nanoparticle system, indicates a more rigorous washing procedure is required. In the Pt/Co alloyed carbon-supported nanoparticle system, a marked variation in both compositions and particle sizes is observed. In the case of Ir@Pt, significant intermixing of the Ir core and Pt shell atoms takes place, which would be very difficult to measure by other techniques. All such observations will likely impact the catalytic performance of these materials. We envisage that the single nanoparticle analysis capability of APT, providing atomic-scale structures and chemical mapping, can also act as a means of quality control, identifying differences in the final product compared with the intended specification. Although the catalytic activity of these nanoparticles was not part of current study, the detailed information offered by such studies will permit knowledge-based improvements in nanoscale catalyst preparation methods and will also provide new ways of investigating structure and activity relationships at the nanometer scale. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cs401117e
  • 2014 • 50 Atomic-Layer-Deposited Aluminum and Zirconium Oxides for Surface Passivation of TiO2 in High-Efficiency Organic Photovoltaics
    Vasilopoulou, M. and Georgiadou, D.G. and Soultati, A. and Boukos, N. and Gardelis, S. and Palilis, L.C. and Fakis, M. and Skoulatakis, G. and Kennou, S. and Botzakaki, M. and Georga, S. and Krontiras, C.A. and Auras, F. and Fatta...
    Advanced Energy Materials 4 (2014)
    The reduction in electronic recombination losses by the passivation of surfaces is a key factor enabling high-efficiency solar cells. Here a strategy to passivate surface trap states of TiO<inf>2</inf> films used as cathode interlayers in organic photovoltaics (OPVs) through applying alumina (Al<inf>2</inf>O<inf>3</inf>) or zirconia (ZrO<inf>2</inf>) insulating nanolayers by thermal atomic layer deposition (ALD) is investigated. The results suggest that the surface traps in TiO<inf>2</inf> are oxygen vacancies, which cause undesirable recombination and high electron extraction barrier, reducing the open-circuit voltage and the short-circuit current of the complete OPV device. It is found that the ALD metal oxides enable excellent passivation of the TiO<inf>2</inf> surface followed by a downward shift of the conduction band minimum. OPV devices based on different photoactive layers and using the passivated TiO<inf>2</inf> electron extraction layers exhibit a significant enhancement of more than 30% in their power conversion efficiencies compared to their reference devices without the insulating metal oxide nanolayers. This is a result of significant suppression of charge recombination and enhanced electron extraction rates at the TiO<inf>2</inf>/ALD metal oxide/organic interface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201400214
  • 2014 • 49 Control of solid catalysts down to the atomic scale: Where is the limit?
    Schüth, F.
    Angewandte Chemie - International Edition 53 8599-8604 (2014)
    Down to the last detail: Nanostructured solid catalysts were already known in the early 20th century, but their exact structure was unclear. Nowadays, the arrangement of atoms and particles in solids can be manipulated and analyzed down to the atomic scale (see image). The use of specific highly active catalysts enables industrially relevant reactions to be performed at room temperature. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201402251
  • 2014 • 48 Crystallization, phase evolution and corrosion of Fe-based metallic glasses: An atomic-scale structural and chemical characterization study
    Duarte, M.J. and Kostka, A. and Jimenez, J.A. and Choi, P. and Klemm, J. and Crespo, D. and Raabe, D. and Renner, F.U.
    Acta Materialia 71 20-30 (2014)
    Understanding phase changes, including their formation and evolution, is critical for the performance of functional as well as structural materials. We analyze in detail microstructural and chemical transformations of the amorphous steel Fe50Cr15Mo14C15B6 during isothermal treatments at temperatures ranging from 550 to 800 °C. By combining high-resolution transmission electron microscopy and Rietveld analyses of X-ray diffraction patterns together with the local chemical data obtained by atom probe tomography, this research provides relevant information at the atomic scale about the mechanisms of crystallization and the subsequent phases evolution. During the initial stages of crystallization a stable (Fe,Cr) 23(C,B)6 precipitates as well as two metastable intermediates of M3(C,B) and the intermetallic χ-phase. When full crystallization is reached, only a percolated nano-scale Cr-rich (Fe,Cr) 23(C,B)6 and Mo-rich η-Fe3Mo3C structure is detected, with no evidence to suggest that other phases appear at any subsequent time. Finally, the corrosion behavior of the developed phases is discussed from considerations of the obtained atomic information. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.02.027
  • 2014 • 47 Impact of Mn on the solution enthalpy of hydrogen in austenitic Fe-Mn alloys: A first-principles study
    Von Appen, J. and Dronskowski, R. and Chakrabarty, A. and Hickel, T. and Spatschek, R. and Neugebauer, J.
    Journal of Computational Chemistry 35 2239-2244 (2014)
    Hydrogen interstitials in austenitic Fe-Mn alloys were studied using density-functional theory to gain insights into the mechanisms of hydrogen embrittlement in high-strength Mn steels. The investigations reveal that H atoms at octahedral interstitial sites prefer a local environment containing Mn atoms rather than Fe atoms. This phenomenon is closely examined combining total energy calculations and crystal orbital Hamilton population analysis. Contributions from various electronic phenomena such as elastic, chemical, and magnetic effects are characterized. The primary reason for the environmental preference is a volumetric effect, which causes a linear dependence on the number of nearest-neighbour Mn atoms. A secondary electronic/magnetic effect explains the deviations from this linearity. © 2014 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jcc.23742
  • 2014 • 46 Position of Cu atoms at the Pt(111) electrode surfaces controls electrosorption of (H)SO4 (2)- from H2SO4 electrolytes
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    ChemElectroChem 1 (2014)
    Selective positioning of monolayer amounts of foreign atoms at the surface and subsurface regions of metal electrodes is a promising way to fine-tune the properties of the electrode/ electrolyte interface. The latter is critical as it largely governs the adsorption of electrolyte components and reaction intermediates and, therefore, controls many key electrocatalytic processes. Using model Pt(111) single-crystal electrodes, we demonstrate how the relative position of Cu atoms at the surface drastically changes the adsorption energies for (bi)sulfate anions. Our measurements involve pseudomorphic overlayers of Cu on Pt(111) as well as Cu-Pt(111) surface and sub-surface alloys, where Cu atoms were located either in the first or in the second atomic layers of Pt, respectively. In the case of Cu- Pt(111) surface alloys, specific adsorption of the anions starts earlier compared to the unmodified Pt(111) surface. In contrast, placing Cu atoms into the second atomic layer weakens the binding between the surface and the anions. Surprisingly, Cu pseudomorphic overlayers do not reveal any specific adsorption of (bi)sulfates (within the region of the overlayer stability). Taking into account that electrified interfaces between Pt(111) electrodes and sulfate-containing electrolytes often play the role of benchmark systems in fundamental physico-chemical and, particularly, electrocatalytic studies, our findings demonstrate a promising and relatively easy route of tuning the properties of these interfaces. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300107
  • 2014 • 45 Quantum-mechanical study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Friák, M. and Zhu, L.-F. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Key Engineering Materials 592-593 335-341 (2014)
    We use quantum-mechanical calculations to study single-crystalline elastic properties of (Ca,Mg)CO3 crystals with concentrations ranging from calcite CaCO3 to magnesite MgCO3. By analyzing results for a dense set of distributions of Ca and Mg atoms within 30-atom supercells, our theoretical study shows that those atomic configurations, that minimize the total energy for a given concentration, are characterized by elastic constants that either increase with the Mg content or remain nearly constants. Employing these ab initio calculated single-crystalline elastic parameters, the polycrystalline elastic properties of (Ca,Mg)CO3 aggregates are determined using a mean-field self-consistent homogenization method. The computed integral elastic moduli (bulk and shear) show a significant stiffening impact of Mg atoms on calcite crystals. Our analysis also demonstrates that it is not advantageous to use a granular two-phase composite of stoichiometric calcite and magnesite instead of substituting individual Ca and Mg atoms. Such two-phase aggregates are not significantly thermodynamically favorable and do not offer any strong additional stiffening effect. © (2014) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2014 • 44 Reactivity of metal catalysts in glucose-fructose conversion
    Loerbroks, C. and vanRijn, J. and Ruby, M.-P. and Tong, Q. and Schüth, F. and Thiel, W.
    Chemistry - A European Journal 20 12298–12309 (2014)
    A joint experimental and computational study on the glucose-fructose conversion in water is reported. The reactivity of different metal catalysts (CrCl3, AlCl3, CuCl2, FeCl3, and MgCl2) was analyzed. Experimentally, CrCl3 and AlCl3 achieved the best glucose conversion rates, CuCl2 and FeCl3 were only mediocre catalysts, and MgCl2 was inactive. To explain these differences in reactivity, DFT calculations were performed for various metal complexes. The computed mechanism consists of two proton transfers and a hydrogen-atom transfer; the latter was the rate-determining step for all catalysts. The computational results were consistent with the experimental findings and rationalized the observed differences in the behavior of the metal catalysts. To be an efficient catalyst, a metal complex should satisfy the following criteria: moderate Brønsted and Lewis acidity (pKa=4-6), coordination with either water or weaker σ donors, energetically low-lying unoccupied orbitals, compact transition-state structures, and the ability for complexation of glucose. Thus, the reactivity of the metal catalysts in water is governed by many factors, not just the Lewis acidity. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201402437
  • 2014 • 43 Representing potential energy surfaces by high-dimensional neural network potentials
    Behler, J.
    Journal of Physics Condensed Matter 26 (2014)
    The development of interatomic potentials employing artificial neural networks has seen tremendous progress in recent years. While until recently the applicability of neural network potentials (NNPs) has been restricted to low-dimensional systems, this limitation has now been overcome and high-dimensional NNPs can be used in large-scale molecular dynamics simulations of thousands of atoms. NNPs are constructed by adjusting a set of parameters using data from electronic structure calculations, and in many cases energies and forces can be obtained with very high accuracy. Therefore, NNP-based simulation results are often very close to those gained by a direct application of first-principles methods. In this review, the basic methodology of high-dimensional NNPs will be presented with a special focus on the scope and the remaining limitations of this approach. The development of NNPs requires substantial computational effort as typically thousands of reference calculations are required. Still, if the problem to be studied involves very large systems or long simulation times this overhead is regained quickly. Further, the method is still limited to systems containing about three or four chemical elements due to the rapidly increasing complexity of the configuration space, although many atoms of each species can be present. Due to the ability of NNPs to describe even extremely complex atomic configurations with excellent accuracy irrespective of the nature of the atomic interactions, they represent a general and therefore widely applicable technique, e.g. for addressing problems in materials science, for investigating properties of interfaces, and for studying solvation processes. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/26/18/183001
  • 2014 • 42 Shape-dependent catalytic oxidation of 2-butanol over Pt nanoparticles supported on γ-Al2O3
    Mistry, H. and Behafarid, F. and Zhou, E. and Ono, L.K. and Zhang, L. and Roldan Cuenya, B.
    ACS Catalysis 4 109-115 (2014)
    This study illustrates the effect of nanoparticle (NP) shape on the reactivity of size-selected Pt/γ-Al2O3 nanocatalysts for 2-butanol oxidation. Nanoparticles similar in size [transmission electron microscopy (TEM) diameter of ∼1 nm] but with different shapes were prepared via encapsulation in inverse micelles. The NP shape was resolved by combining information extracted from extended X-ray absorption fine structure spectroscopy (EXAFS) data, TEM, and modeling. A correlation was observed between the average first nearest neighbor coordination number of atoms at the NP surface and their catalytic activity. In particular, the NPs with the largest number of weakly coordinated surface atoms (i.e., edges and corners) were found to be the least active for the total oxidation of 2-butanol. This result highlights that not only size but also shape control must be achieved to tailor the catalytic properties of nanoscale materials. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400888n
  • 2014 • 41 Site occupation of Nb atoms in ternary Ni-Ti-Nb shape memory alloys
    Shi, H. and Frenzel, J. and Martinez, G.T. and Van Rompaey, S. and Bakulin, A. and Kulkova, S. and Van Aert, S. and Schryvers, D.
    Acta Materialia 74 85-95 (2014)
    Nb occupancy in the austenite B2-NiTi matrix and Ti2Ni phase in Ni-Ti-Nb shape memory alloys was investigated by aberration-corrected scanning transmission electron microscopy and precession electron diffraction. In both cases, Nb atoms were found to prefer to occupy the Ti rather than Ni sites. A projector augmented wave method within density functional theory was used to calculate the atomic and electronic structures of the austenitic B2-NiTi matrix phase and the Ti2Ni precipitates both with and without addition of Nb. The obtained formation energies and analysis of structural and electronic characteristics explain the preference for Ti sites for Nb over Ni sites. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.03.062
  • 2014 • 40 Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands
    Edström, D. and Sangiovanni, D.G. and Hultman, L. and Chirita, V. and Petrov, I. and Greene, J.E.
    Thin Solid Films 558 37-46 (2014)
    We use classical molecular dynamics and the modified embedded atom method to determine residence times and descent pathways of Ti and N adatoms on square, single-atom-high, TiN islands on TiN(001). Simulations are carried out at 1000 K, which is within the optimal range for TiN(001) epitaxial growth. Results show that the frequency of descent events, and overall adatom residence times, depend strongly on both the TiN(001) diffusion barrier for each species as well as the adatom island-edge location immediately prior to descent. Ti adatoms, with a low diffusion barrier, rapidly move toward the island periphery, via funneling, where they diffuse along upper island edges. The primary descent mechanism for Ti adatoms is via push-out/exchange with Ti island-edge atoms, a process in which the adatom replaces an island edge atom by moving down while pushing the edge atom out onto the terrace to occupy an epitaxial position along the island edge. Double push-out events are also observed for Ti adatoms descending at N corner positions. N adatoms, with a considerably higher diffusion barrier on TiN(001), require much longer times to reach island edges and, consequently, have significantly longer residence times. N adatoms are found to descend onto the terrace by direct hopping over island edges and corner atoms, as well as by concerted push-out/exchange with N atoms adjacent to Ti corners. For both adspecies, we also observe several complex adatom/island interactions, before and after descent onto the terrace, including two instances of Ti island-atom ascent onto the island surface. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2014.02.053
  • 2013 • 39 Ab initio and atomistic study of generalized stacking fault energies in Mg and Mg-Y alloys
    Pei, Z. and Zhu, L.-F. and Friák, M. and Sandlöbes, S. and Von Pezold, J. and Sheng, H.W. and Race, C.P. and Zaefferer, S. and Svendsen, B. and Raabe, D. and Neugebauer, J.
    New Journal of Physics 15 (2013)
    Magnesium-yttrium alloys show significantly improved room temperature ductility when compared with pure Mg. We study this interesting phenomenon theoretically at the atomic scale employing quantum-mechanical (so-called ab initio) and atomistic modeling methods. Specifically, we have calculated generalized stacking fault energies for five slip systems in both elemental magnesium (Mg) and Mg-Y alloys using (i) density functional theory and (ii) a set of embedded-atom-method (EAM) potentials. These calculations predict that the addition of yttrium results in a reduction in the unstable stacking fault energy of basal slip systems. Specifically in the case of an I2 stacking fault, the predicted reduction of the stacking fault energy due to Y atoms was verified by experimental measurements. We find a similar reduction for the stable stacking fault energy of the non-basal slip system. On the other hand, other energies along this particular γ-surface profile increase with the addition of Y. In parallel to our quantum-mechanical calculations, we have also developed a new EAM Mg-Y potential and thoroughly tested its performance. The comparison of quantum-mechanical and atomistic results indicates that the new potential is suitable for future large-scale atomistic simulations. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/4/043020
  • 2013 • 38 Ab initio study of single-crystalline and polycrystalline elastic properties of Mg-substituted calcite crystals
    Zhu, L.-F. and Friák, M. and Lymperakis, L. and Titrian, H. and Aydin, U. and Janus, A.M. and Fabritius, H.-O. and Ziegler, A. and Nikolov, S. and Hemzalová, P. and Raabe, D. and Neugebauer, J.
    Journal of the Mechanical Behavior of Biomedical Materials 20 296-304 (2013)
    We employ ab initio calculations and investigate the single-crystalline elastic properties of (Ca,Mg)CO3 crystals covering the whole range of concentrations from pure calcite CaCO3 to pure magnesite MgCO3. Studying different distributions of Ca and Mg atoms within 30-atom supercells, our theoretical results show that the energetically most favorable configurations are characterized by elastic constants that nearly monotonously increase with the Mg content. Based on the first principles-derived single-crystalline elastic anisotropy, the integral elastic response of (Ca,Mg)CO3 polycrystals is determined employing a mean-field self-consistent homogenization method. As in case of single-crystalline elastic properties, the computed polycrystalline elastic parameters sensitively depend on the chemical composition and show a significant stiffening impact of Mg atoms on calcite crystals in agreement with the experimental findings. Our analysis also shows that it is not advantageous to use a higher-scale two-phase mix of stoichiometric calcite and magnesite instead of substituting Ca atoms by Mg ones on the atomic scale. Such two-phase composites are not significantly thermodynamically favorable and do not provide any strong additional stiffening effect. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2013.01.030
  • 2013 • 37 Combining emission and absorption spectroscopy at rare earth spectral lines: Plasma temperature measurements in ceramic metal halide lamps
    Ruhrmann, C. and Westermeier, M. and Höbing, T. and Bergner, A. and Denissen, C. and Suijker, J. and Awakowicz, P. and Mentel, J.
    Journal of Physics D: Applied Physics 46 (2013)
    Presently, most high intensity discharge (HID) lamps contain mercury to generate a high pressure buffer gas and thereby an appropriate power input into the arc. Due to its toxicity, the replacement of Hg is of particular interest in recent research on HID lamps. Up to now, the emission coefficient of an atomic Hg double line is widely used to determine the plasma temperature Tpl in HID lamps. Tpl is needed to calculate the total density of atoms and ions of elements inside these lamps. A combination of optical emission and broadband absorption spectroscopy allows us to evaluate Tpl independently of Hg emission lines. The method is required for a determination of Tpl if the Hg line intensity within the investigated lamp is too low, is superimposed by other lines or if environmental-friendly Hg-free lamps are developed. Within this work, phase-resolved plasma temperatures are determined in front of the electrode of Hg-containing MH lamps by emission spectroscopy at atomic Hg lines. Above all, temperatures are measured by a combination of emission and absorption spectroscopy at atomic rare earth lines, namely Dy and Tm. A comparison of Tpl determined by both methods agree within an error margin of < 10%. Total phase-resolved rare earth atom densities are obtained by means of the measured ground state densities and Tpl. The combination of emission and absorption spectroscopy is also applied to the bulk plasma of lamps where the intensity of the Hg emission lines is too low for plasma temperature measurements or Hg is absent. It provides the partial rare earth pressure and by comparison with thermodynamic data cold spot temperatures within the lamps. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/29/295202
  • 2013 • 36 Effect of adsorbed magnetic and non-magnetic atoms on electronic transport through surfaces with strong spin-orbit coupling
    Lükermann, D. and Sologub, S. and Pfnür, H. and Klein, C. and Horn-von Hoegen, M. and Tegenkamp, C.
    Materialwissenschaft und Werkstofftechnik 44 210-217 (2013)
    Adsorption-induced reduction of the surface state conductivity in epitaxial Bi(111) films, a prototype system with a large Rashba-induced surface state splitting by adsorbed atoms of Bi, Fe and Co was investigated by macroscopic surface magneto-transport measurements at a temperature of 10 K. A detailed analysis of magneto-transport, DC-transport and Hall data reveals that the scattering efficiencies for Co and Fe are by a factor of two larger than for Bi. While for the latter, charge transfer and change of band filling near the Fermi level is negligible, we found an increase of hole concentration upon Co and Fe adsorption. These atoms act as acceptors and retract roughly 0.5 electrons from the surface per adsorbed atom. Besides the dominant classical magneto-conductance signal the films show signatures of weak anti-localization (WAL) reflecting the strong spin-orbit coupling in Bi(111) surface states. Our measurements show that the control of hybridization is important in order to make use of local spin-moments and to increase the backscattering rate in strongly spin-orbit coupled systems, e. g., topological insulators. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300114
  • 2013 • 35 Electronically nonadiabatic processes in the interaction of H with a Au surface revealed using MIM junctions: The temperature dependence
    Schindler, B. and Diesing, D. and Hasselbrink, E.
    Journal of Physical Chemistry C 117 6337-6345 (2013)
    The chemicurrent response of Au-TaOx-Ta metal-insulator-metal junctions exposed to a flux of atomic hydrogen has been studied in detail for device temperatures between 110 and 300 K. Currents of some 100 pA are observed at an H atom flux of 4 × 1014 atoms cm-2 s-1. The steady-state current closely tracks the rate expected for the Langmuir-Hinschelwood recombination reaction as a function of temperature. The current trace reflects the reaction kinetics when the H-atom flux is modulated. The rate constant is directly determined from the individual trace at each temperature. Moreover, it is observed that the Fermi level of the 20 nm thick top metal film shifts with varying H coverage, giving rise to an additional charging/discharging current. For device temperatures below 200 K, subsurface hydrogen becomes significant. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp4009459
  • 2013 • 34 Generation of AuGe nanocomposites by co-sparking technique and their photoluminescence properties
    Kala, S. and Theissmann, R. and Kruis, F.E.
    Journal of Nanoparticle Research 15 (2013)
    The feasibility of spark discharge technique for preparing metal-semiconductor nanocomposites is demonstrated. In the AuGe system, Au shows only 10-3 atomic percent solid solubility in Ge, whereas 3.1 at.% Ge is soluble in Au. During the co-sparking, Au is used as anode material; the cathode is composed of Ge. The relative atomic percent of Au and Ge in the initially generated mixture can be changed by changing the charging current to the capacitor used to trigger the sparking. Depending upon the atomic ratio of Au and Ge in the initial mixture, AuGe agglomerates form AuGe composite nanoparticles on subsequent sintering, in which AuGe alloy nanoparticles are found dispersed in a Ge matrix. The size of the dispersed AuGe alloy nanoparticles depend on the relative atomic concentration of Au and Ge in the initial mixture as well as on the sintering temperature. AuGe alloy nanoparticles dispersed in the Ge matrix are observed to exhibit an intense photoluminescence between 550 and 600 nm. © 2013 Springer Science+Business Media.
    view abstractdoi: 10.1007/s11051-013-1963-0
  • 2013 • 33 Interfacial structure and chemistry of GaN on Ge(111)
    Zhang, S. and Zhang, Y. and Cui, Y. and Freysoldt, C. and Neugebauer, J. and Lieten, R.R. and Barnard, J.S. and Humphreys, C.J.
    Physical Review Letters 111 (2013)
    The interface of GaN grown on Ge(111) by plasma-assisted molecular beam epitaxy is resolved by aberration corrected scanning transmission electron microscopy. A novel interfacial structure with a 5:4 closely spaced atomic bilayer is observed that explains why the interface is flat, crystalline, and free of GeNx. Density functional theory based total energy calculations show that the interface bilayer contains Ge and Ga atoms, with no N atoms. The 5:4 bilayer at the interface has a lower energy than a direct stacking of GaN on Ge(111) and enables the 5:4 lattice-matching growth of GaN. © 2013 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.111.256101
  • 2013 • 32 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 • 31 A DFT study of formation energies of Fe-Zn-Al intermetallics and solutes
    Klaver, T.P.C. and Madsen, G.K.H. and Drautz, R.
    Intermetallics 31 137-144 (2012)
    We report Density Functional Theory results on FeAl and FeZn intermetallics and Fe, Zn and Al solute atoms. The formation energies of fully relaxed intermetallic geometries were determined, as well as solution energies of the three elements in host lattices of the other two elements. Since it is know that the outcome of the magnetic states of some FeAl intermetallics and Fe solutes in Al depends on subtle details of how the calculations are carried out, we have determined many of our results with two different parameterisations, PBE and PBEsol, so see how the parameterisation influences the results. The relaxed intermetallic geometries are in good agreement with experimental results, with PBEsol calculations resulting in slightly smaller geometries than PBE calculations (0.7-2.1%). Intermetallic formation energies fall within ranges of experimental results where available, and are in excellent or reasonable agreement with other DFT results, except for the FeAl 2 phase. For this phase a structure revision was recently suggested and the heat of formation of the newly suggested structure is 0.1 eV/atom lower than for the long-accepted structure. The formation energies of Fe aluminides are an order of magnitude more negative than those of FeZn intermetallics. Most of the calculated magnetic states of the intermetallics are at odds with experimental results. However, the intermetallic formation energies are often not strongly affected by this. Fe/Al solute systems have the most negative solution energies. All other solution energies are positive and smaller in absolute value than the Fe/Al solution energies. Solution energies were all some tenths of eV. Where comparisons could be made, calculated and experimental results differed by some hundredths of eV. The magnetic moment found on an Fe solute in Al is at odds with experimental results. As with FeAl, the outcome of the magnetic state subtly depends on the details of how calculations were performed and has little energetic effect. Lattice relaxation around solute atoms is mostly in agreement with simple atomic size considerations. The slight relaxation of Al neighbours away from a Zn solute is at odds with this pattern, and also with experimental results. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2012.06.017
  • 2012 • 30 A scheme to combine molecular dynamics and dislocation dynamics
    Brinckmann, S. and Mahajan, D.K. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 20 (2012)
    Many engineering challenges occur on multiple interacting length scales, e.g. during fracture atoms separate on the atomic scale while plasticity develops on the micrometer scale. To investigate the details of these events, a concurrent multiscale model is required which studies the problem at appropriate length- and time-scales: the atomistic scale and the dislocation dynamics scale. The AtoDis multiscale model is introduced, which combines atomistics and dislocation dynamicsinto a fully dynamic model that is able to simulate deformation mechanisms at finite temperature. The model uses point forces to ensure mechanical equilibrium and kinematic continuity at the interface. By resolving each interface atom analytically, and not numerically, the framework uses a coarse FEM mesh and intrinsically filters out atomistic vibrations. This multiscale model allows bi-directional dislocation transition at the interface of both models with no remnant atomic disorder. Thereby, the model is able to simulate a larger plastic zone than conventional molecular dynamics while reducing the need for constitutive dislocation dynamics equations. This contribution studies dislocation nucleation at finite temperature and investigates the absorption of dislocations into the crack wake. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/20/4/045001
  • 2012 • 29 Atomic ordering effect in Ni 50Mn 37Sn 13 magnetocaloric ribbons
    Wu, D. and Xue, S. and Frenzel, J. and Eggeler, G. and Zhai, Q. and Zheng, H.
    Materials Science and Engineering A 534 568-572 (2012)
    High-performance Ni 50Mn 37Sn 13 magnetocaloric materials are produced using melt spinning technique in the present work and the atomic order dependence of phase transition behaviors and magnetic properties is established. The effective refrigeration capacity of the melt-spun ribbon annealed at 1273K for 15min reaches 95.27J/kg for a magnetic field change of 18kOe, demonstrating great potential for magnetic refrigeration applications near ambient temperature. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.12.009
  • 2012 • 28 Atomic scale effects of alloying, partitioning, solute drag and austempering on the mechanical properties of high-carbon bainitic-austenitic TRIP steels
    Seol, J.-B. and Raabe, D. and Choi, P.-P. and Im, Y.-R. and Park, C.-G.
    Acta Materialia 60 6183-6199 (2012)
    Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic-austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.064
  • 2012 • 27 Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28)
    Graetsch, H.A. and Pandey, C.S. and Schreuer, J. and Burianek, M. and Mühlberg, M.
    Acta Crystallographica Section B: Structural Science 68 101-106 (2012)
    The incommensurately modulated crystal structures of Ca 0.28Ba 0.72Nb 2O 6 (CBN28) and Ce 0.02Ca 0.25Ba 0.72Nb 2O 6 (Ce:CBN28) were refined in the supercentred setting X4bm(AA0,-AA0) of the 3 + 2-dimensional superspace group P4bm(aa1/2,-aa). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO 6 octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium. © 2012 International Union of Crystallography Printed in Singapore-all rights reserved.
    view abstractdoi: 10.1107/S0108768111054863
  • 2012 • 26 Thermally induced structural changes in incommensurate calcium barium niobate Ca 0.28Ba 0.72Nb 2O 6 (CBN28)
    Graetsch, H.A. and Schreuer, J. and Burianek, M. and Mühlberg, M.
    Journal of Solid State Chemistry 196 255-266 (2012)
    The incommensurately modulated crystal structure of relaxor ferroelectric CBN28 was refined at elevated temperatures up to 330 °C in the 32 dimensional superspace group P4bm(αα1/2,-αα1/2). The structural modulations mainly consisting of cooperative tilting of NbO 6 octahedra and an occupational modulation of the large cation site Me2 persist beyond the diffuse ferroelectric transition with slightly reduced amplitudes. A change of symmetry was not observed. Both symmetrically non-equivalent NbO 6 octahedra are distorted by off center shifts of the Nb atoms in the same direction along the tetragonal c-axis. The displacements of the Nb atoms are gradually reduced with increasing temperatures until one of the Nb atoms crosses the center of its coordination polyhedron near the transition temperature T M adopting an uncompensated anti-ferroelectric configuration. This change is accompanied by enhanced thermal motions of the Nb atoms along the c-axis. Structural distortions and electric polarization do not completely vanish at T M but may persist in fluctuating polar nanodomains. © 2012 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jssc.2012.06.028
  • 2011 • 25 Anodic repassivation of low energy Au-implanted ultra-thin anodic Al 2O 3
    Mardare, A.I. and Melnikov, A. and Wieck, A.D. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 208 1270-1274 (2011)
    Ultrathin anodic alumina with a film thickness of 11nm was implanted by Au atoms with low energy of 2, 5 or 10keV. Stopping range simulations yielded three essentially different geometries ranging from surface near implantation over well penetrated oxide to near oxide metal interface implantation, covering the entire range of possible implantation modifications. This work aims at demonstrating how to perform band gap engineering in alumina not only on an energetic level but also targeting a certain geometrical position of the doping atoms by means of the implantation parameters. Beside the intended implantation the oxide destruction in the implantation path and its possible repair was of interest. The repassivation behaviour was considerably different showing a significant redox contribution of the gold nanoclusters on top of the simple oxide repassivation. Near surface implanted Au remained electrochemically active for low repassivation potentials. Higher repassivation potentials always buried the implanted Au atoms under anodic alumina. The repassivation charge determined allowed determining the volume destructed by the implantation. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201001212
  • 2011 • 24 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 • 23 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 • 22 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 • 21 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 • 20 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 • 19 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 • 18 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 • 17 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 • 16 First-principles investigation of the effect of carbon on the stacking fault energy of Fe-C alloys
    Abbasi, A. and Dick, A. and Hickel, T. and Neugebauer, J.
    Acta Materialia 59 3041-3048 (2011)
    The intrinsic stacking fault energy (SFE) is a critical parameter that defines the type of plasticity mechanisms in austenitic high-Mn steels. We have performed ab initio investigations to study the effect of interstitial carbon atoms on the SFE of face-centred cubic (fcc) Fe-C alloys. Simulating the stacking fault explicitly, we observe a strong dependence of the SFE on the position of carbon atoms with respect to the stacking-fault layer and the carbon concentration. To determine the SFE for realistic carbon distributions we consider two scenarios, assuming (i) an equilibration of the carbon atoms in response to the stacking fault formation and (ii) a homogeneous distribution of the C atoms when creating the stacking fault (i.e. diffusion is suppressed). This distinction allows us to interpret two sets of apparently contradicting experimental data sets, where some find an almost negligible dependence on the carbon concentration while others report a large carbon dependence. In particular, our results for the second scenario show a significant increase in the SFE as a function of carbon concentration. These trends are consistently found for the explicit calculations as well as for the computationally much more efficient axial next-nearest-neighbour Ising approach. They will be decisive for the selection of specific plasticity mechanisms in steels (such as twin formation, martensitic transformations and dislocation gliding). © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.01.044
  • 2011 • 15 Induced magnetic Cu moments and magnetic ordering in Cu2MnAl thin films on MgO(0 0 1) observed by XMCD
    Krumme, B. and Herper, H.C. and Erb, D. and Weis, C. and Antoniak, C. and Warland, A. and Westerholt, K. and Entel, P. and Wende, H.
    Journal of Physics D: Applied Physics 44 (2011)
    The disorder-order transition of a highly defective A2-ordered Cu 2MnAl film on MgO(0 0 1) upon annealing at 600 K was monitored by means of x-ray absorption spectroscopy (XAS) at the Cu and Mn L2,3 edges. Additionally, x-ray magnetic circular dichroism (XMCD) was employed to determine element-specific orbital and spin resolved magnetic moments of the Cu and Mn atoms. A small induced total magnetic moment of ≈0.04 0.01μB per atom was detected at the Cu sites, whereas a total magnetic moment of 3.57 0.52μB is carried by the Mn atoms. The experimental XAS and XMCD spectra of Cu agree reasonably with the results from ab initio calculations, magnetic moments derived by the sum rules are in accordance with the calculations. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/41/415004
  • 2011 • 14 Investigating the influence of the operating frequency on the gas phase emitter effect of dysprosium in ceramic metal halide lamps
    Reinelt, J. and Westermeier, M. and Ruhrmann, C. and Bergner, A. and Luijks, G.M.J.F. and Awakowicz, P. and Mentel, J.
    Journal of Physics D: Applied Physics 44 (2011)
    The dependence of the gas phase emitter effect of Dy on a variation of the operating frequency between a few Hz and 2 kHz is investigated in a high intensity discharge lamp. The buffer gas of the lamp consisting of Ar, Kr and predominantly Hg is seeded with DyI3, its burner vessel is formed from transparent yttrium-alumina-garnet material. Phase and spatial resolved emission spectroscopy in front of the lamp electrode and pyrometric temperature measurements along the tungsten electrode are performed with a spectroscopic setup. Dy atom and ion densities in front of the electrode are deduced from absolute intensities of optically thin Dy lines and a plasma temperature, derived from the absolute intensity of mercury lines. Phase resolved values of the electrode tip temperature Ttip and input power Pin are obtained from temperature distributions along the electrode. Distinctly higher Dy ion and atom densities are measured in front of the electrode within the cathodic phase. With increasing operating frequency a reduction in both atoms and ions is observed in front of the cathode. In contrast, an increase in the ion density in front of the anode is seen. Moreover, the Dy ion density is drastically reduced by an additional seeding of the lamp with TlI. It is found that an up rating of the Dy ion density is correlated with a decline of T tip and Pin. At higher frequencies this effect takes place not only within the cathodic phase but also within the anodic phase. The reduction of the average electrode tip temperature of the order of several hundred kelvin compared with a YAG lamp with a pure mercury filling is explained by a Dy monolayer on the electrode surface which is sustained by a Dy ion current. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/22/224006
  • 2011 • 13 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 • 12 Segregation and stability in surface alloys: PdxRu 1-x/Ru(0001) and PtxRu1-x/Ru(0001)
    Bergbreiter, A. and Hoster, H.E. and Behm, R.J.
    ChemPhysChem 12 1148-1154 (2011)
    The stability of PdRu/Ru(0001) and PtRu/Ru(0001) surface alloys and the tendency for surface segregation of Pd and Pt subsurface guest metals in these surface alloys is studied by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Atomic resolution STM imaging and AES measurements reveal that upon overgrowing the surface alloys with a 1-2 monolayer Ru film and subsequent annealing to the temperatures required for initial surface alloy formation, the Ru-covered Pd (Pt) atoms float back to the outermost layer. The lateral distribution of these species is also essentially identical to that of the initial surface alloys, before overgrowth by Ru. In combination, this clearly demonstrates that the surface alloys represent stable surface configurations, metastable only towards entropically favored bulk dissolution, and that there is a distinct driving force for surface segregation of these species. Consequences of these data on the mechanism for surface alloy formation are discussed. Floating in PtRu (PdRu) surface alloys on Ru(0001): The PtRu (PdRu)monolayer surface alloy layer is covered with the substrate metal Ru by means of physical vapour depositon. Subsequent annealing to temperatures necessary for surface alloy formation reconstitutes the original Pt (Pd) amount as well as the original atom distribution of the initial equilibrated alloy layer (see picture). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201001087
  • 2011 • 11 Spatial dynamics of helium metastables in sheath or bulk dominated rf micro-plasma jets
    Niermann, B. and Hemke, T. and Babaeva, N.Y. and Böke, M. and Kushner, M.J. and Mussenbrock, T. and Winter, J.
    Journal of Physics D: Applied Physics 44 (2011)
    Space resolved concentrations of helium He (3S1) metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. The spatial profile of metastable atoms in the volume between the electrodes was deduced for various electrode gap distances. Density profiles reveal the sheath structure and reflect the plasma excitation distribution, as well as the dominance of the α-mode discharge. Gap width variations show the transition from a normal glow plasma to a pure sheath discharge. In order to analyse and verify the experimentally observed profiles of the metastable atoms, a two-dimensional simulation model was set up. Applying an appropriate He/N2/O 2 chemistry model, the correlation between the metastable profiles and the underlying excitation mechanisms was obtained. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/48/485204
  • 2011 • 10 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
  • 2010 • 9 Ab initio study of the anomalous volume-composition dependence in Fe-Al alloys
    Friák, M. and Neugebauer, J.
    Intermetallics 18 1316-1321 (2010)
    The experimentally observed anomalous compositional dependence of the lattice constant of Fe-Al crystals has been theoretically investigated employing density functional theory (DFT) within the generalized gradient approximation (GGA). The formation energies, equilibrium volumes and magnetic states have been determined for a dense set of different aluminium concentrations and a large variety of atomic configurations. The spin-polarized calculations for Fe-rich compounds reproduce very well the anomalous lattice-constant behavior in contrast to both the nonmagnetic and fixed-spin-moment calculations that result in nearly linear trends without any anomaly. We thus identify the change in magnetism of iron atoms as caused by an increasing number of Al atoms in the first coordination spheres to be the decisive driving force of the anomalous behavior. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.03.014
  • 2010 • 8 Atomic mobilities and diffusivities in the fcc, L12 and B2 phases of the Ni-Al system
    Zhang, L. and Du, Y. and Chen, Q. and Steinbach, I. and Huang, B.
    International Journal of Materials Research 101 1461-1475 (2010)
    A phenomenological model was utilized to describe diffusivities in the γ (fcc)/γ′ (L12) and A2/B2 phases of the Ni-Al system. An effective strategy, which takes the homogeneity range and defect concentration into account, was developed in the present work to optimize the atomic mobilities of γ′ phase. Such a strategy results in a dramatic decrease in the number of atomic mobility parameters to be evaluated for the L12 phase. The measured composition-and temperature-dependent diffusivities in the Ni-Al system have been well replicated by the present mobility descriptions. For the L12 phase, comprehensive comparisons show that with fewer model parameters the presently obtained mobilities yield a better fit to experimental diffusivities, compared with previous assessments. The mobility descriptions are further validated by comparing calculated and measured concentration profiles for various diffusion couples. The time-dependent Al composition profile for the annealed vapor Al/γ couple is accurately described for the first time. © Carl Hanser Verlag GmbH & Co. KG ISSN 1862-5282.
    view abstractdoi: 10.3139/146.110428
  • 2010 • 7 Characterization of the effluent of a He/O-2 microscale atmospheric pressure plasma jet by quantitative molecular beam mass spectrometry
    Ellerweg, D. and Benedikt, J. and von Keudell, A. and Knake, N. and Schulz-von der Gathen, V.
    New Journal of Physics 12 013021 (2010)
    The effluent of a microscale atmospheric pressure plasma jet (mu-APPJ) operated in helium with a small admixture of molecular oxygen (< 1.6%) has been analyzed by means of two independent diagnostics, quantitative molecular beam mass spectrometry (MBMS) and two-photon absorption laser-induced fluorescence spectroscopy (TALIF). The atomic oxygen density, the ozone density and the depletion of molecular oxygen have been measured by MBMS and the atomic oxygen density has been validated by TALIF. Absolute atomic oxygen densities in the effluent up to 4.7x10(15) cm(-3) could be measured with a very good agreement between both diagnostics. In addition, ozone densities in the effluent up to 1.4x10(15) cm(-3) and an O-2 depletion up to 10% could be measured by MBMS. The atomic oxygen density shows a maximum value at an O-2 admixture of 0.6%, whereas the ozone density continues to increase toward higher O-2 admixtures. With increasing distance from the jet, the atomic oxygen density decreases but is still detectable at a distance of 30 mm. The ozone density increases with distance, saturating at a distance of 40 mm. By applying higher powers to the mu-APPJ, the atomic oxygen density increases linearly whereas the ozone density exhibits a maximum.
    view abstractdoi: 10.1088/1367-2630/12/1/013021
  • 2010 • 6 Development of a method to determine Burgers vectors from atomistic data
    Hua, J. and Hartmaier, A.
    Journal of Physics: Conference Series 240 (2010)
    Large-scale molecular dynamics simulations have been widely used to investigate the mechanical behaviour of materials. But complex datasets, involving the positions of millions of atoms, generated during the simulations make quantitative data analysis quite a challenge. This paper presents a novel method to determine not only dislocations in the crystal, but also to quantify their Burgers vectors. This is achieved by combining geometrical methods to determine the atoms lying in the dislocations cores, like for example the common neighbour analysis or the bond angle analysis, with the slip vector analysis. The first methods are used to filter out the atoms lying in undisturbed regions of the crystal; the latter method yields the relative slip of the remaining atoms and thus indicates the Burgers vector of those atoms lying in the dislocation cores. The validity of the method is demonstrated here on a single edge dislocation in a relatively small sample. Furthermore a way will be sketched how this analysis can be used to determine densities of statistically stored and geometrically necessary dislocations, respectively. Hence, this method can be expected to provide valuable input for strain gradient plasticity models. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/240/1/012010
  • 2010 • 5 Diffusivities of an Al-Fe-Ni melt and their effects on the microstructure during solidification
    Zhang, L. and Du, Y. and Steinbach, I. and Chen, Q. and Huang, B.
    Acta Materialia 58 3664-3675 (2010)
    A systematical investigation of the diffusivities in an Al-Fe-Ni melt was presented. Based on the experimental and theoretical data about diffusivities, the temperature- and composition-dependent atomic mobilities were evaluated for the elements in Al-Ni, Al-Fe, Fe-Ni and Al-Fe-Ni melts via an effective approach. Most of the reported diffusivities can be reproduced well by the obtained atomic mobilities. In particular, for the first time the ternary diffusivity of the liquid in a ternary system is described in conjunction with the established atomic mobilities. The effect of the atomic mobilities in a liquid on microstructure and microsegregation during solidification was demonstrated with one Al-Ni binary alloy. The simulation results indicate that accurate databases of mobilities in the liquid phase are much needed for the quantitative simulation of microstructural evolution during solidification by using various approaches, including DICTRA and the phase-field method. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.03.002
  • 2010 • 4 Effect of dehydration on the local structure of framework aluminum atoms in mixed linker MIL-53(Al) materials studied by solid-state NMR spectroscopy
    Jiang, Y. and Huang, J. and Marx, S. and Kleist, W. and Hunger, M. and Baiker, A.
    Journal of Physical Chemistry Letters 1 2886-2890 (2010)
    The present study features 1H and 27Al MAS NMR spectroscopic investigations on mixed ligand metal-organic frameworks (MOFs) of MIL-53(Al) type with benzene-1,4-dicarboxylate (BDC) and 2-aminobenzene-1,4- dicarboxylate (ABDC) linkers. The excellent resolution of the 1H spectra allowed an elegant and facile quantitative analysis of the organic linkers using solid-state NMR. The actual molar fraction of ABDC in the dehydrated mixed linker MOFs was determined by evaluating the intensity of the -NH2 signal at 5.6 ppm. The incorporation of amine groups led to higher field shifts of the corner-sharing AlOH signals and a more homogeneous charge distribution in the local structure of framework aluminum atoms corresponding to a decrease of the quadrupole coupling constant by ∼1 MHz compared to that of aluminum coordinated to BDC. Upon rehydration, the local structures of the framework aluminum atoms exhibited a much lower symmetry, as indicated by an increase of the 27Al quadrupole coupling constant by up to 3 MHz. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jz1010835
  • 2010 • 3 Electronic excitations generated by the deposition of Mg on Mg films
    Hagemann, U. and Krix, D. and Nienhaus, H.
    Physical Review Letters 104 (2010)
    Nonadiabatic processes are observed during growth of Mg atoms from the gas phase on Mg films. Chemicurrents are measured in thin film Mg/p-Si(001) Schottky diodes which are exposed to thermally evaporated Mg atoms. The photonic and chemical contributions to the observed reverse currents in the devices can be distinguished by varying the Mg atom flux and by independent measurements using an empty evaporator as a source of heat radiation. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.104.028301
  • 2010 • 2 Nonequilibrium current and noise in inelastic tunneling through a magnetic atom
    Sothmann, B. and König, J.
    New Journal of Physics 12 (2010)
    In a recent experiment, Hirjibehedin et al (2007 Science 317 1199) performed inelastic tunneling spectroscopy of a single iron atom absorbed on a nonmagnetic substrate. The observed steps in the differential conductance marked the spin excitation energies. In this paper, we explain the observed nonmonotonicities in the differential conductance by a nonequilibrium population of the atom spin states. Furthermore, we predict super-Poissonian current noise due to this nonequilibrium situation. We argue that the remarkable absence of nonequilibrium features at certain conductance steps indicates the presence of an anisotropic relaxation channel. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/8/083028
  • 2010 • 1 Shape-dependent catalytic properties of Pt nanoparticles
    Mostafa, S. and Behafarid, F. and Croy, J.R. and Ono, L.K. and Li, L. and Yang, J.C. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 132 15714-15719 (2010)
    Tailoring the chemical reactivity of nanomaterials at the atomic level is one of the most important challenges in catalysis research. In order to achieve this elusive goal, fundamental understanding of the geometric and electronic structure of these complex systems at the atomic level must be obtained. This article reports the influence of the nanoparticle shape on the reactivity of Pt nanocatalysts supported on γ-Al2O3. Nanoparticles with analogous average size distributions (∼0.8-1 nm), but with different shapes, synthesized by inverse micelle encapsulation, were found to display distinct reactivities for the oxidation of 2-propanol. A correlation between the number of undercoordinated atoms at the nanoparticle surface and the onset temperature for 2-propanol oxidation was observed, demonstrating that catalytic properties can be controlled through shape-selective synthesis. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja106679z