Joachim Mayer

RWTH Aachen University, Aachen, North Rhine-Westphalia, Germany

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Publications (87)356.27 Total impact

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    Hongchu Du · Chun-Lin Jia · Joachim Mayer
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    ABSTRACT: The highly sensitive and selective properties of monodisperse faceted nanocrystals inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is merely determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, atomic details on the faceted surfaces need to be studied on the atomic level. Here, we demonstrate that the surface atomic structure of faceted nanocrystals of complex oxides, {1 0 0}-faceted strontium titanate zirconate nanocubes, can be unambiguously resolved by aberration-corrected scanning transmission electron microscopy. The resolved surface atomic details reveal a layerwise growth process of the nanocubes, thereby allowing an in-depth understanding of the growth mechanism.
    Full-text · Article · Jan 2016 · Chemistry of Materials
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    ABSTRACT: Marchewka et al. investigate the reset kinetics of TaOx-based nano-crossbars using time-resolved voltage-pulse measurements in the sub-μs regime and numerical simulations in article number 1500233. The advanced simulation model of nonisothermal electronic-ionic drift-diffusion allows for studying donor migration and related filament evolution in the oxide layer, thus identifying the balance between donor drift and diffusion as the root cause of the gradual reset behavior.
    No preview · Article · Jan 2016
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    ABSTRACT: The investigation aims at obtaining a life time extension of FeCrAlY heating elements by increasing the emissivity of the alumina surface scales. This approach will allow the use of a lower heating element temperature without decreasing the usable heat output. For this purpose, the oxidation mechanisms of Zr-doped FeCrAlY model alloys were investigated. For thick specimens a high Zr addition is accompanied by an undesired rapid increase of the oxidation rate adversely affecting the life time of the component. However, for thin foil heating elements (typically 50-200μm thickness) an optimized high Zr content might be a suitable concept for obtaining a high emissivity, because apparently, a dark appearing, high emissivity oxide may be obtained without occurrence of dramatic internal oxidation.
    No preview · Article · Dec 2015 · Advanced Engineering Materials
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    ABSTRACT: Resistive switching memories based on the valence change mechanism have attracted great attention due to their potential use in future nanoelectronics. The working principle relies on ion migration in an oxide matrix and subsequent nanoscale redox processes leading to a resistance change. While switching from a low resistive to a high resistive state, different intermediate resistance levels can be programmed by changing the maximum applied voltage, making resistive switches highly interesting for multibit data storage and neuromorphic applications. To date, this phenomenon, which is known as gradual reset, has been reported in various experimental studies, but a comprehensive physical understanding of this key phenomenon is missing. Here, a combined experimental and numerical modeling approach is presented to address these issues. Time-resolved pulse measurements are performed to study the reset kinetics in TaOx-based nano-crossbar structures. The results are analyzed using a 2D dynamic model of nonisothermal drift–diffusion transport in the mixed electronic–ionic conducting oxide including the effect of contact potential barriers. The model accurately describes the experimental data and provides physical insights into the processes determining the gradual reset. The gradual nature can be attributed to the temperature-accelerated oxygen-vacancy motion being governed by drift and diffusion processes approaching an equilibrium situation.
    Full-text · Article · Nov 2015
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    ABSTRACT: The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably.
    Full-text · Article · Oct 2015 · Nature Communications
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    ABSTRACT: Oxide-based metal-insulator-metal structures are of special interest for future resistive random-access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite-like structure, which is induced by an avalanche discharge between the top electrode and the Ta2O5-x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite-like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta4+ and Ta5+, which takes place over the entire former Pt/Ta2O5-x interface of the dendrite-like structure.
    Full-text · Article · Oct 2015 · Advanced Functional Materials
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    Full-text · Article · Aug 2015 · Acta Crystallographica Section A: Foundations and Advances
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    ABSTRACT: The atomic structure of antiphase nanodomains in Fe-doped SrTiO3 films is revealed directly by aberration-corrected scanning transmission electron microscopy. In particular, the crystallographic translation vector between the antiphase nanodomains and the matrix is determined to be a/2 [0 1 1], distinct from a/2 [0 1 1] for the well-known Ruddlesden–Popper (RP) planar faults in perovskite structure. The antiphase boundaries lie mostly in the {1 0 0} planes and partially in the {1 1 0} planes. The atomic structure of the antiphase boundaries is found to consist of chains of edge-sharing TiO6 octahedra implying a local Ti-enrichment, which is in strong contrast to local Sr-enrichment at the RP type antiphase boundaries.
    Full-text · Article · May 2015 · Advanced Functional Materials
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    ABSTRACT: Dislocations in perovskite oxides have been found to have important impacts on their electronic and ionic transportation properties, which are believed to be related to the structure and chemistry of dislocation cores. For dislocation cores at a 6° low-angle [0 0 1] tilt grain boundary in SrTiO3, an embedded TiOx rocksalt-like structure has been suggested, consistent with a deficiency of Sr. However, direct evidence supporting these suggestions has not been obtained up to now. In this work, we reveal the atomic structure and chemistry of edge dislocation cores at a low-angle [0 0 1] symmetric tilt-boundary in SrTiO3 bicrystals by imaging techniques of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS) with an FEI Titan cube3 60–300 (PICO) microscope operated at 80 kV. The experimental results demonstrate direct evidence for a local coordination of edge-sharing TiO6 octahedra at the dislocation cores, which can be understood as the result of strain. The local coordination of edge-sharing TiO6 octahedra is associated with the face-centered cubic (FCC) NaCl-type TiO phase. The present study therefore provides a solid structural and chemical basis for understanding the properties of dislocations.
    Full-text · Article · May 2015 · Acta Materialia
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    ABSTRACT: We introduce a novel solvothermal synthesis of individual single crystalline Sb2 Te3 micro- and nanocrystals as a model material for phase-change switching. We identified different intermediates along the reaction path to the final Sb2 Te3 hexagonal platelets (HPs) and discuss their forming mechanism. By means of nanodiffraction (ND) in a scanning transmission electron microscope we demonstrate that the intermediates follow a hexagonal shape evolution in the amorphous state. In situ nanomanipulator measurements reveal electrical phase-change switching properties of the individual Sb2 Te3 hexagonal platelets. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    Full-text · Article · Apr 2015 · Angewandte Chemie International Edition
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    ABSTRACT: Wir stellen eine neue solvothermale Synthese des Modellmaterials Sb2Te3 für Phasenwechselschalten in Form von einzelnen einkristallinen Mikro- und Nanokristallen vor. Im Verlauf des Reaktionsprozesses zum finalen Produkt, bestehend aus hexagonalen Sb2Te3-Plättchen (HPs), haben wir vier verschiedene Intermediate identifiziert. Durch Untersuchungen mittels Nanodiffraktion (ND) im Rastertransmissionselektronenmikroskop konnten wir eine ungewöhnliche Formentwicklung der Intermediate im amorphen Zustand nachweisen. In In-situ-Messungen mit einem Nanomanipulatoraufbau konnten wir die Phasenwechseleigenschaften der Sb2Te3-HPs zeigen.
    No preview · Article · Apr 2015 · Angewandte Chemie
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    ABSTRACT: Adhesive organs like arolia of insects allow these animals to climb on different substrates by creating high adhesion forces. According to the Dahlquist criterion, adhesive organs must be very soft exhibiting an effective Young's modulus of below 100 kPa to adhere well to substrates. Such a low effective Young's modulus allows the adhesive organs to make almost direct contact to the substrate and results in van der Waals forces beside capillary forces. In previous studies the effective Young's moduli of adhesive organs were determined using indentation tests yielding their structure to be very soft indeed. However, adhesive organs show a layered structure, thus the measured values comprise the effective Young's moduli of several layers of the adhesive organs. In this study, a new approach is illustrated to measure the Young's modulus of the outermost layer of the arolium, i.e. of the epicuticle, of the stick insect Carausius morosus. As a result of the epicuticle being supported by upright fibres tensile tests allow the determination of the Young's modulus of the epicuticle hardly affected by subjacent layers. In our tensile tests arolia of stick insects adhering on a latex membrane were stretched by stretching the membrane while the elongation of the contact area between an arolium and the membrane was recorded. For analysis mathematical models of the mechanical system were developed. When fed with the observed elongations, these models yield estimates for the Young's modulus of the epicuticle of about a hundred megapascal. Thus in arolia a very thin layer (~225 nm) of a rather stiff material, which is less susceptible to abrasion, makes contact to the substrates while the inner fibrous structure of arolia is responsible for their softness.
    Full-text · Article · Sep 2014 · Journal of Experimental Biology

  • No preview · Article · Aug 2014 · Microscopy and Microanalysis
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    ABSTRACT: The effect of thermal annealing treatments on the morphology and structure of amorphous unalloyed and amorphous Si alloyed alumina thin films has been investigated. All amorphous thin films were deposited by filtered cathodic arc deposition at room temperature onto Si3N4 coated Si substrates and subsequently annealed in argon atmosphere at temperatures in the range of 610 °C–1100 °C with a heating rate of 20 °C/min. After each heating sequence the thin film samples were investigated by means of transmission electron microscopy. Upon alloying 2 at.% of Si to alumina, the amorphous to crystalline transition is shifted by ≥ 290 °C to higher temperatures. For the unalloyed thin film crystallization of γ-Al2O3 in an amorphous matrix is observed at 630 °C. Fully crystalline γ-Al2O3 is formed at 750 °C. Evidence for the transition of γ-Al2O3 to α-Al2O3 is obtained after annealing at 900 °C concomitant with substantial crack formation and coating-failure. In contrast, the Si alloyed alumina thin film remains amorphous until 900 °C. At 950 °C first traces of γ-Al2O3 in an amorphous matrix are observed and further annealing at 1100 °C results in the formation of a mullite phase in addition to the γ-Al2O3-phase. The thermal stability range of amorphous alumina thin films is hence significantly enhanced by alloying with 2 at.% of Si.
    No preview · Article · Aug 2014 · Surface and Coatings Technology
  • Sarah Haigh · Joachim Mayer

    No preview · Article · Aug 2014 · Micron
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    ABSTRACT: Ultrathin ferroelectric heterostructures (SrTiO3/BaTiO3/BaRuO3/SrRuO3) were studied by scanning transmission electron microscopy (STEM) in terms of structural distortions and atomic displacements. The TiO2-termination at the top interface of the BaTiO3 layer was changed into a BaO-termination by adding an additional BaRuO3 layer. High-angle annular dark-field (HAADF) imaging by aberration-corrected STEM revealed that an artificially introduced BaO-termination can be achieved by this interface engineering. By using fast sequential imaging and frame-by-frame drift correction, the effect of the specimen drift was significantly reduced and the signal-to-noise ratio of the HAADF images was improved. Thus, a quantitative analysis of the HAADF images was feasible, and an in-plane and out-of-plane lattice spacing of the BaTiO3 layer of 3.90 and 4.22 Å were determined. A 25 pm shift of the Ti columns from the center of the unit cell of BaTiO3 along the c-axis was observed. By spatially resolved electron energy-loss spectroscopy studies, a reduction of the crystal field splitting (CFS, ΔL3=1.93 eV) and an asymmetric broadening of the e g peak were observed in the BaTiO3 film. These results verify the presence of a ferroelectric polarization in the ultrathin BaTiO3 film.
    No preview · Article · Jun 2014 · Microscopy and Microanalysis
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    ABSTRACT: Spatially defined networks of 15 nm-sized DNA-functionalized gold nanoparticles (DNA–AuNPs) were studied using dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), as well as optical extinction spectroscopy (OES). We use a combination of these techniques with Monte Carlo simulations of pair-distance distribution function (PDDF) curves and generalized Mie theory simulations as well as in situ-transmission electron microscopy (in situ-TEM) to analyze the internal structure of the finite-size assemblies. The DLS data show that monodisperse, spherical networks with hydrodynamic radii of ca. 30 nm are found for reaction mixtures of complementarily functionalized DNA–AuNPs between 1:15 and 1:20. Different interparticle distances within these assemblies are identified and quantified. By controlling the network morphology through selection of the reaction mixture, center-shell geometries are obtained. The number of shell-AuNPs surrounding each center-AuNP is determined from the SAXS data and Monte Carlo simulations. This number is quantified to be ca. 10, with the exact number depending on the linking DNA double strand. The optical spectra of the networks are found to be consistent with the structural properties. The structural information gained here enables a quantitative description of optical and other physical properties, which is expected to prove useful for the construction and application of such systems, for example, in drug release, gene regulation, or external-stimuli-responsive materials.
    No preview · Article · Mar 2014 · The Journal of Physical Chemistry C
  • Sarah Haigh · Joachim Mayer

    No preview · Article · Jan 2014 · Micron
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    ABSTRACT: We present a theoretical and experimental study on the influence of the Ba/Sr and Co/Fe ratios as well as the oxygen-non-stoichiometry on the stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF). Thin-layer depositions are analysed by looking at TEM images and EDX spectra. Bond-analytical calculations are performed to explain the stability difference between hexagonal and cubic BSCF. Finally, annealing experiments analysed using XRD give an insight into the differences of phase-fraction growth with respect to the Ba/Sr ratio.
    No preview · Article · Dec 2013 · Physical Chemistry Chemical Physics
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    ABSTRACT: Cr-Ni-Fe alloy deposits containing different Cr, Ni and Fe contents were created by varying the electroplating current density in an eco-friendly trivalent Cr-based bath with Ni2 + and Fe2 + ions. The Cr content of the Cr-Ni-Fe alloy deposits increased from 14 to 75 wt.% when the electroplating current density changed from 15 to 25 A/dm2. The mapping result obtained with a scanning electron microscope showed that a uniform ternary Cr-Ni-Fe alloy deposit could be obtained from the proposed bath, and X-ray diffraction results indicated the Fe-based alloy electroplated at 15 A/dm2 was crystalline, whereas the Cr-based alloy deposits electroplated at a high current density had an amorphous structure. The electrochemical corrosion test results showed that the corrosion resistance of the Cr-Ni-Fe alloy deposit electroplated at 20 A/dm2 was better than that of the Cr-Ni-Fe alloy deposit obtained at 25 A/dm2, despite having almost identical chemical compositions. These results were obtained by performing an electrochemical test and microstructure observations.
    No preview · Article · Oct 2013 · Thin Solid Films