Joachim Mayer

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

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Publications (65)262.28 Total impact

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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.
    Journal of Experimental Biology 09/2014; · 3.00 Impact Factor
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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.
    Surface and Coatings Technology 08/2014; 257:338–347. · 2.20 Impact Factor
  • Sarah Haigh, Joachim Mayer
    Micron 08/2014; 63C:1. · 2.06 Impact Factor
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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 eg peak were observed in the BaTiO3 film. These results verify the presence of a ferroelectric polarization in the ultrathin BaTiO3 film.
    Microscopy and Microanalysis 06/2014; FirstView:1-8. · 1.76 Impact Factor
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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.
    The Journal of Physical Chemistry C 03/2014; 118(13):7174–7184. · 4.84 Impact Factor
  • Sarah Haigh, Joachim Mayer
    Micron 01/2014; 63:1. · 2.06 Impact Factor
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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.
    Physical Chemistry Chemical Physics 12/2013; · 4.20 Impact Factor
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    ABSTRACT: Phase-change materials are the basis for next-generation memory devices and reconfigurable electronics, but fundamental understanding of the unconventional kinetics of their phase transitions has been hindered by challenges in the experimental quantification. Here we obtain deeper understanding based on the temperature dependence of the crystal growth velocity of the phase-change material AgInSbTe, as derived from laser-based time-resolved reflectivity measurements. We observe a strict Arrhenius behaviour for the growth velocity over eight orders of magnitude (from ~10 nm s(-1) to ~1 m s(-1)). This can be attributed to the formation of a glass at elevated temperatures because of rapid quenching of the melt. Further, the temperature dependence of the viscosity is derived, which reveals that the supercooled liquid phase must have an extremely high fragility (>100). Finally, the new experimental evidence leads to an interpretation, which comprehensively explains existing data from various different experiments reported in literature.
    Nature Communications 08/2013; 4:2371. · 10.74 Impact Factor
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    ABSTRACT: Mixing a bis-hydrophilic, cationic miktoarm star polymer with a linear polyanion leads to the formation of unilamellar polymersomes, which consist of an interpolyelectrolyte complex (IPEC) wall sandwiched between poly(ethylene oxide) brushes. The experimental finding of this rare IPEC morphology is rationalized theoretically: the star architecture forces the assembly into a vesicular shape due to the high entropic penalty for stretching of the insoluble arms in non-planar morphologies. The transmission electron microscopy of vitrified samples (cryogenic TEM) is compared with the samples at ambient conditions (in situ TEM), giving one of the first TEM reports on soft matter in its pristine environment.
    Macromolecular Rapid Communications 03/2013; 34(10). · 4.61 Impact Factor
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    ABSTRACT: A laminar structure with nanosized segregation was observed in Cr–Ni–Fe alloy deposits electroplated in a Cr3+-based bath with Ni2+ and Fe2+ ions. The formation of laminar structure could be attributed to the depletion of preferentially reduced ions, or cathodic polarization, adjacent to the cathodic surface. This nanosized segregation could be clearly observed with scanning-transmission-electron microscope in high-angle angular dark-field (HAADF) mode. A crystalline Fe-rich alloy deposit with a grain size of 4 nm can be achieved with an electroplating current density of 15 A dm−2, whereas an amorphous Cr-rich alloy deposit was produced with 25 A dm−2. Owing to the approach toward thermal equilibrium in the two-phase field, uphill diffusion took place in the Cr-rich alloy deposit during annealing at 500 °C, forming a Cr- and Fe-rich multilayer.
    Materials Letters 02/2013; 93:107–110. · 2.27 Impact Factor
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    ABSTRACT: Purpose – The purpose of this paper is to show how controlled exposure of electromagnetic fields toward bearing steel vulnerates the microstructure. The ability of Barkhausen Noise signal processing is used for detecting phenomena such as dislocation and subgrain formation processes as the beginning of later failures. Design/methodology/approach – A Barkhausen noise signal measurement equipment is used for detecting subsurface distress of 100Cr6 as a function of the applied electromagnetic and mechanical stress. Barkhausen noise signal is mathematically processed by use of fractal dimension analysis. Findings – The paper cleary reveals significant impact of electromagnetic field in junction with mechanical loading. Electromagnetic impact depends on the magnitude of the field. Research limitations/implications – Research limitations are given by the fact that in real field applications, e.g. wind power plants, bearings are exposed by multiple influences and the methodology is not applicable to those conditions. Practical implications – The methodology can be applied to real field applications in condition monitoring systems. Up to now, no reasonable on-line measurement is in use determining sub surface fatigue phenomena. The paper hence, reveals the possibility to raise condition monitoring into a new perspective. Originality/value – The use of Barkhausen noise signal processing, as presented here, is original with respect to real field applications, such as wind power plants with a high demand in condition monitoring, especially off-shore plants.
    Industrial Lubrication and Tribology 08/2012; 64(5):247-252. · 0.32 Impact Factor
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    ABSTRACT: Ein difunktionelles DNA‐Templat wird vorgestellt, das durch eine Klickreaktion kovalent an funktionalisierte Substrate gebunden und anschließend kontrolliert metallisiert werden kann. Diese Immobilisierungs‐/Metallisierungsmethode ermöglicht die Herstellung metallisch leitfähiger Nanodrähte mit regulierbarem Durchmesser.
    Angewandte Chemie 07/2012; 124(30).
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    ABSTRACT: Investment in copper, silver, and gold: A difunctional DNA template is first immobilized on a functionalized substrate by a copper-catalyzed reaction, modified with silver nucleation centers, and then metalized by gold deposition. The diameter of the resulting metallically conductive nanowires can be adjusted.
    Angewandte Chemie International Edition 06/2012; 51(30):7586-8. · 11.34 Impact Factor
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    ABSTRACT: Corrugated SiCN ceramic substrates fabricated by a facile replication process using nonlithographic PDMS masters were employed for the directed assembly of block copolymer microdomains. During thermal annealing of polystyrene-b-polybutadiene diblock copolymer, the material transport was guided by a wrinkled substrate to form regular modulations in the film thickness. As a consequence of the thickness-dependent morphological behavior of cylinder forming block copolymer, the film surface appears as sequenced patterns of alternative microphase-separated structures. The ordering process is attributed to the formation of inverted terraces which match the substrate topography, so that the resulting surface patterns are free from the surface relief structures within macroscopically large areas. The issues of the film thickness, the substrate surface energy, and the pattern geometry are addressed. Our approach demonstrates an effective synergism of external confinement and internal polymorphism of block copolymers toward complex hierarchically structured patterned surfaces.
    Macromolecules 03/2012; 45(5):2494-2501. · 5.93 Impact Factor
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    ABSTRACT: Let there be light: A heterogeneous photocatalytic system based on easily recyclable TiO(2) or ZnO allows cross dehydrogenative coupling reactions of tertiary amines. The newly developed protocols have successfully been applied to various C-C and C-P bond-forming reactions to provide nitro amines as well as amino ketones, nitriles and phosphonates.
    Chemistry - A European Journal 02/2012; 18(12):3478-81. · 5.93 Impact Factor
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    ABSTRACT: For the guided block copolymer assembly we used corrugated SiCN ceramic substrates which were fabricated by a facile replication process using non-lithographic PDMS masters. During thermal annealing of polystyrene-b-polybutadiene diblock copolymer, the material transport was guided by the wrinkled substrate to form regular modulations in the film thickness. As a consequence of the thickness-dependent morphological behavior of block copolymers, the film surface appears as sequenced patterns of alternative microphase separated structures. The ordering process is attributed to the formation of reverse terraces which match the substrate topography, so that the resulting surface patterns are free from the surface relief structures within macroscopically-large areas. The issues of the film thickness, the substrate surface energy and the pattern geometry are addressed. Our approach demonstrates an effective synergism of external confinement and internal polymorphism of block copolymers towards complex hierarchically-structured patterned surfaces.
    02/2012;
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    ABSTRACT: In this work, we address the following question: Where do the resistive switching processes take place in memristive thin-film devices of the single crystalline model material Fe-doped SrTiO$_{3}$? We compare resistive switching induced by the tip of the atomic force microscope on the bare thin-film surface with the switching properties observed in memristive devices with Pt top electrode. In order to close the gap between these two approaches, we combine conductive-tip atomic force microscopy with a delamination technique to remove the top electrode of Fe-doped SrTiO$_{3}$ metal–insulator–metal (MIM) structures to gain insight into the active switching interface with nanoscale lateral resolution. This enables us to prove the coexistence of a filamentary and area-dependent switching process with opposite switching polarities in the same sample. The spatially resolved analysis by transmission electron microscopy and photoelectron spectromicroscopy gives us some hints that the two switching types take place in device regions with different defect density and significant stoichiometry difference.
    Proceedings of the IEEE 01/2012; PP(99):1. · 5.47 Impact Factor
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    ABSTRACT: For directed block copolymer assembly we used corrugated SiCN ceramic substrates which were fabricated by a facile replication process using non-lithog. PDMS masters. During thermal annealing of polystyrene-b-polybutadiene (SB) diblock copolymer, the material transport was channeled by the wrinkled substrate to form regular modulations in the film thickness. As a consequence of the thickness-dependent morphol. behavior of block copolymers, the film surface appears as sequenced patterns of alternative microphase sepd. structures. The ordering process is attributed to the formation of reverse terraces which match the substrate topog., so that the resulting surface patterns are free from the surface relief structures within macroscopically-large areas. The issues of the film thickness, the substrate surface energy and the pattern geometry are addressed. Our approach demonstrates an effective synergism of external confinement and internal polymorphism of block copolymers towards complex hierarchically-structured patterned surfaces. [on SciFinder(R)]
    PMSE Prepr. 01/2012;
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    ABSTRACT: Dynamic core–shell nanoparticles have received increasing attention in recent years. This paper presents a detailed study of Au–Hg nanoalloys, whose composing elements show a large difference in cohesive energy. A simple method to prepare Au@Hg particles with precise control over the composition up to 15 atom% mercury is introduced, based on reacting a citrate stabilized gold sol with elemental mercury. Transmission electron microscopy shows an increase of particle size with increasing mercury content and, together with X-ray powder diffraction, points towards the presence of a core–shell structure with a gold core surrounded by an Au–Hg solid solution layer. The amalgamation process is described by pseudo-zero-order reaction kinetics, which indicates slow dissolution of mercury in water as the rate determining step, followed by fast scavenging by nanoparticles in solution. Once adsorbed at the surface, slow diffusion of Hg into the particle lattice occurs, to a depth of ca. 3 nm, independent of Hg concentration. Discrete dipole approximation calculations relate the UV–vis spectra to the microscopic details of the nanoalloy structure. Segregation energies and metal distribution in the nanoalloys were modeled by density functional theory calculations. The results indicate slow metal interdiffusion at the nanoscale, which has important implications for synthetic methods aimed at core–shell particles.
    Advanced Functional Materials 09/2011; 21(17). · 10.44 Impact Factor
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    ABSTRACT: Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tunable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfill this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this process, the SAM is converted into a nanocrystalline graphene sheet with well-defined thickness and arbitrary dimensions. Electric transport data demonstrate that this transformation is accompanied by an insulator to metal transition that can be utilized to control electrical properties such as conductivity, electron mobility, and ambipolar electric field effect of the fabricated graphene sheets. The suggested route opens broad prospects toward the engineering of free-standing 2D carbon materials with tunable properties on various solid substrates and on holey substrates as suspended membranes.
    ACS Nano 05/2011; 5(5):3896-904. · 12.03 Impact Factor

Publication Stats

684 Citations
262.28 Total Impact Points

Institutions

  • 2006–2014
    • RWTH Aachen University
      • Central Facility for Electron Microscopy
      Aachen, North Rhine-Westphalia, Germany
  • 2010
    • Forschungszentrum Jülich
      Jülich, North Rhine-Westphalia, Germany