Dr. Christian Sternemann

DELTA
TU Dortmund University

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  • Frustrated flexibility in metal-organic frameworks
    Pallach, R. and Keupp, J. and Terlinden, K. and Frentzel-Beyme, L. and Kloß, M. and Machalica, A. and Kotschy, J. and Vasa, S.K. and Chater, P.A. and Sternemann, C. and Wharmby, M.T. and Linser, R. and Schmid, R. and Henke, S.
    Nature Communications 12 (2021)
    Stimuli-responsive flexible metal-organic frameworks (MOFs) remain at the forefront of porous materials research due to their enormous potential for various technological applications. Here, we introduce the concept of frustrated flexibility in MOFs, which arises from an incompatibility of intra-framework dispersion forces with the geometrical constraints of the inorganic building units. Controlled by appropriate linker functionalization with dispersion energy donating alkoxy groups, this approach results in a series of MOFs exhibiting a new type of guest- and temperature-responsive structural flexibility characterized by reversible loss and recovery of crystalline order under full retention of framework connectivity and topology. The stimuli-dependent phase change of the frustrated MOFs involves non-correlated deformations of their inorganic building unit, as probed by a combination of global and local structure techniques together with computer simulations. Frustrated flexibility may be a common phenomenon in MOF structures, which are commonly regarded as rigid, and thus may be of crucial importance for the performance of these materials in various applications. © 2021, The Author(s).
    view abstract10.1038/s41467-021-24188-4
  • Ion association in hydrothermal aqueous NaCl solutions: implications for the microscopic structure of supercritical water
    Elbers, M. and Schmidt, C. and Sternemann, C. and Sahle, C.J. and Jahn, S. and Albers, C. and Sakrowski, R. and Gretarsson, H. and Sundermann, M. and Tolan, M. and Wilke, M.
    Physical Chemistry Chemical Physics 23 (2021)
    Knowledge of the microscopic structure of fluids and changes thereof with pressure and temperature is important for the understanding of chemistry and geochemical processes. In this work we investigate the influence of sodium chloride on the hydrogen-bond network in aqueous solution up to supercritical conditions. A combination ofin situX-ray Raman scattering andab initiomolecular dynamics simulations is used to probe the oxygen K-edge of the alkali halide aqueous solution in order to obtain unique information about the oxygen's local coordination around the ions,e.g.solvation-shell structure and the influence of ion pairing. The measured spectra exhibit systematic temperature dependent changes, which are entirely reproduced by calculations on the basis of structural snapshots obtainedvia ab initiomolecular dynamics simulations. Analysis of the simulated trajectories allowed us to extract detailed structural information. This combined analysis reveals a net destabilizing effect of the dissolved ions which is reduced with rising temperature. The observed increased formation of contact ion pairs and occurrence of larger polyatomic clusters at higher temperatures can be identified as a driving force behind the increasing structural similarity between the salt solution and pure water at elevated temperatures and pressures with drawback on the role of hydrogen bonding in the hot fluid. We discuss our findings in view of recent results on hot NaOH and HCl aqueous fluids and emphasize the importance of ion pairing in the interpretation of the microscopic structure of water. © the Owner Societies 2021.
    view abstract10.1039/d1cp01490k
  • Isomeric effects in structure formation and dielectric dynamics of different octanols
    Bolle, J. and Bierwirth, S.P. and Požar, M. and Perera, A. and Paulus, M. and Münzner, P. and Albers, C. and Dogan, S. and Elbers, M. and Sakrowski, R. and Surmeier, G. and Böhmer, R. and Tolan, M. and Sternemann, C.
    Physical Chemistry Chemical Physics 23 (2021)
    The understanding of the microstructure of associated liquids promoted by hydrogen-bonding and constrained by steric hindrance is highly relevant in chemistry, physics, biology and for many aspects of daily life. In this study we use a combination of X-ray diffraction, dielectric spectroscopy and molecular dynamics simulations to reveal temperature induced changes in the microstructure of different octanol isomers,i.e., linear 1-octanol and branched 2-, 3- and 4-octanol. In all octanols, the hydroxyl groups form the basis of chain-, cyclic- or loop-like bonded structures that are separated by outwardly directed alkyl chains. This clustering is analyzed through the scattering pre-peaks observed from X-ray scattering and simulations. The charge ordering which pilots OH aggregation can be linked to the strength of the Debye process observed in dielectric spectroscopy. Interestingly, all methods used here converge to the same interpretation: as one moves from 1-octanol to the branched octanols, the cluster structure evolves from loose large aggregates to a larger number of smaller, tighter aggregates. All alcohols exhibit a peculiar temperature dependence of both the pre-peak and Debye process, which can be understood as a change in microstructure promoted by chain association with increased chain length possibly assisted by ring-opening effects. All these results tend to support the intuitive picture of the entropic constraint provided by branching through the alkyl tails and highlight its capital entropic role in supramolecular assembly. © the Owner Societies 2021.
    view abstract10.1039/d1cp02468j
  • Process characteristics, particle behavior and coating properties during HVOF spraying of conventional, fine and nanostructured WC-12Co powders
    Baumann, I. and Hagen, L. and Tillmann, W. and Hollingsworth, P. and Stangier, D. and Schmidtmann, G. and Tolan, M. and Paulus, M. and Sternemann, C.
    Surface and Coatings Technology 405 (2021)
    In recent years, great effort has been taken in science and industry to find novel material-related solutions, which provide improved properties for future technological applications. One of these approaches is the use of fine structured and nanostructured materials. Within the field of wear protection, the use of fine or nanostructured WC-Co powder feedstock in the thermal spray process enables the application of highly wear resistant, thin near net-shape coatings on parts with complex geometries. In this study, the processing of WC-12Co powders by means of High Velocity Oxy-Fuel (HVOF) flame spraying is fundamentally investigated and the results are compared to those obtained with conventional powders. The influence of process parameter and scaling effects on the spray process and the thermo-kinetic particle behavior in the flame, the heating of the substrate as well as on the coating properties, the microstructure, the behavior of elements and phases and the residual stress is discussed comprehensively. The investigations of this work have shown that HVOF spraying of fine and nanostructured WC-12Co powders instead of conventional ones leads to a significant alteration of the thermo-kinetic spray conditions. Under optimized spray conditions, achieved by the use of special spray equipment and statistical design of experiments (DoE), improvements in terms of the economy of the spray process (higher deposition efficiencies) and the mechanical properties (higher microhardness and fracture toughness, lower porosity and roughness) can be achieved. © 2020
    view abstract10.1016/j.surfcoat.2020.126716
  • Reflective imaging, on-axis laser heating and radiospectrometry of samples in diamond anvil cells with a parabolic mirror
    Spiekermann, G. and Libon, L. and Albers, C. and Sakrowski, R. and Petitgirard, S. and Sahle, C.J. and Sundermann, M. and Gretarsson, H. and Sergueev, I. and Sternemann, C. and Wilke, M. and Murakami, M.
    High Pressure Research 41 (2021)
    We describe the use of a silver-coated 90 (Formula presented.) parabolic mirror of 33 mm focal length as objective for imaging, on-axis laser heating and radiospectrometric temperature measurements of a sample compressed in a diamond anvil cell in a laser heating system. There, spatial resolution and imaging quality of the parabolic mirror are similar to the one of a 10× objective. The temperature measurements between 500 and 900 nm are essentially free from chromatic aberration. The parabolic mirror was also perforated with a 220-μm hole, allowing for on-axis imaging, laser heating and incidence of X-rays simultaneously at synchrotron facilities. The parabolic mirror is thus a well-suited alternative to existing refractive and reflective objectives in laboratory and synchrotron laser heating systems. © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/08957959.2021.1921173
  • Transition from elastic to plastic strain release in core−shell nanowires revealed by in-plane x-ray diffraction
    Hassan, A.A. and Salehi, W.A. and Lewis, R.B. and Anjum, T. and Sternemann, C. and Geelhaar, L. and Pietsch, U.
    Nanotechnology 32 (2021)
    We investigate the strain evolution and relaxation process as function of increasing lattice mismatch between the GaAs core and surrounding InxGa1−xAs shell in core-shell nanowire heterostructures grown on Si(111) substrates. The dimensions of the core and shell are kept constant whereas the indium concentration inside the shell is varied. Measuring the 224¯ and 220 ¯ in-plane Bragg reflections normal to the nanowire side edges and side facets, we observe a transition from elastic to plastic strain release for a shell indium content x > 0.5. Above the onset of plastic strain relaxation, indium rich mounds and an indium poor coherent shell grow simultaneously around the GaAs core. Mound formation was observed for indium contents x = 0.5 and 0.6 by scanning electron microscopy. Considering both the measured radial reflections and the axial 111 Bragg reflection, the 3D strain variation was extracted separately for the core and the InxGa1−xAs shell. © 2021 The Author(s). Published by IOP Publishing Ltd Printed in the UK
    view abstract10.1088/1361-6528/abe5db
  • A portable on-Axis laser-heating system for near-90° X-ray spectroscopy: Application to ferropericlase and iron silicide
    Spiekermann, G. and Kupenko, I. and Petitgirard, S. and Harder, M. and Nyrow, A. and Weis, C. and Albers, C. and Biedermann, N. and Libon, L. and Sahle, C.J. and Cerantola, V. and Glazyrin, K. and Konopkova, Z. and Sinmyo, R. and Morgenroth, W. and Sergueev, I. and Yavas, H. and Dubrovinsky, L. and Tolan, M. and Sternemann, C. and Wilke, M.
    Journal of Synchrotron Radiation 27 (2020)
    A portable IR fiber laser-heating system, optimized for X-ray emission spectroscopy (XES) and nuclear inelastic scattering (NIS) spectroscopy with signal collection through the radial opening of diamond anvil cells near 90°with respect to the incident X-ray beam, is presented. The system offers double-sided on-Axis heating by a single laser source and zero attenuation of incoming X-rays other than by the high-pressure environment. A description of the system, which has been tested for pressures above 100GPa and temperatures up to 3000K, is given. The XES spectra of laser-heated Mg0.67Fe0.33O demonstrate the potential to map the iron spin state in the pressure-Temperature range of the Earth's lower mantle, and the NIS spectra of laser-heated FeSi give access to the sound velocity of this candidate of a phase inside the Earth's core. This portable system represents one of the few bridges across the gap between laser heating and high-resolution X-ray spectroscopies with signal collection near 90°. © 2020 Journal of Synchrotron Radiation.
    view abstract10.1107/S1600577519017041
  • Hydration in aqueous osmolyte solutions: The case of TMAO and urea
    Sahle, C.J. and Schroer, M.A. and Niskanen, J. and Elbers, M. and Jeffries, C.M. and Sternemann, C.
    Physical Chemistry Chemical Physics 22 (2020)
    The hydration and hydrogen-bond topology of small water solvated molecules such as the naturally occurring organic osmolytes trimethylamine N-oxide (TMAO) and urea are under intense investigation. We aim at furthering the understanding of this complex hydration by combining experimental oxygen K-edge excitation spectra with results from spectra calculated via the Bethe-Salpeter equation based on structures obtained from ab initio molecular dynamics simulations. Comparison of experimental and calculated spectra allows us to extract detailed information about the immediate surrounding of the solute molecules in the solvated state. We quantify and localize the influence of the solute on the hydrogen bond network of the water solvent and find spectroscopic fingerprints of a clear directional asymmetry around TMAO with strong and local kosmotropic influence around TMAO's NO head group and slight chaotropic influence around the hydrophobic methyl groups. The influence of urea on the local water network is qualitatively similar to that of TMAO but weaker in magnitude. The strongest influence of both molecules on the shape of the oxygen K-edge spectra is found in the first hydration shells. This journal is © the Owner Societies.
    view abstract10.1039/c9cp06785j
  • 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 (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 abstract10.1021/acs.jpcb.0c05932
  • Pressure stability of the first hydration shell of yttrium in aqueous YCl3 solution
    Elbers, M. and Sternemann, C. and Julius, K. and Paulus, M. and Surmeier, G. and König, N. and Nase, J. and Bolle, J. and Wagner, R. and Irifune, T. and Tolan, M.
    High Pressure Research (2020)
    The pressure stability of the first hydration shell of Y3+ ions in aqueous solution has been investigated by means of extended x-ray absorption fine-structure spectroscopy at the yttrium K-edge for hydrostatic pressures up to 4.5 kbar and concentrations between 1 M and 2.5 M. We find approximately 8.4 water molecules surrounding the yttrium cation at a mean distance of 2.370 Å at ambient conditions, independent on the concentration. The yttrium hydration shell has a low compressibility in the order of (-3.8 ± 0.7)×10-4 Å kbar-1 emphasizing its pressure stability in the kbar range. At the studied conditions, no indication for chloride complexation was observed. © 2020, © 2020 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/08957959.2020.1724998
  • Reorientational dynamics of trimethoxyboroxine: A molecular glass former studied by dielectric spectroscopy and 11B nuclear magnetic resonance
    Hoffmann, L. and Beerwerth, J. and Greim, D. and Senker, J. and Sternemann, C. and Hiller, W. and Böhmer, R.
    Journal of Chemical Physics 152 (2020)
    In this work, trimethoxyboroxine (TMB) is identified as a small-molecule glass former. In its viscous liquid as well as glassy states, static and dynamic properties of TMB are explored using various techniques. It is found that, on average, the structure of the condensed TMB molecules deviates from threefold symmetry so that TMB's electric dipole moment is nonzero, thus rendering broadband dielectric spectroscopy applicable. This method reveals the super-Arrhenius dynamics that characterizes TMB above its glass transition, which occurs at about 204 K. To extend the temperature range in which the molecular dynamics can be studied, 11B nuclear magnetic resonance experiments are additionally carried out on rotating and stationary samples: Exploiting dynamic second-order shifts, spin-relaxation times, line shape effects, as well as stimulated-echo and two-dimensional exchange spectroscopy, a coherent picture regarding the dynamics of this glass former is gained. © 2020 Author(s).
    view abstract10.1063/1.5129769
  • Cholesterol modulates the pressure response of DMPC membranes
    Surmeier, G. and Paulus, M. and Salmen, P. and Dogan, S. and Sternemann, C. and Nase, J.
    Biophysical Chemistry 252 (2019)
    In this work, the effect of cholesterol on the pressure response of solid-supported phospholipid multilayers is analyzed. It is shown that DMPC multilayers become highly pressure-responsive by the incorporation of low amounts of cholesterol, resulting in a strong pressure-induced expansion of the bilayer spacing. This is accompanied by a high tendency of the multilayer system to detach from the substrate. Increasing the cholesterol concentration reduces the pressure-induced expansion and the membrane structure remains largely unchanged upon pressurization, consequently the stability of the multilayers improves. For a determination of the influence of the substrate, the pressure-dependent behavior of multilayers is compared to that of solid-supported bilayers and multi-lamellar vesicles in bulk solution. While single-supported bilayers remain largely unaffected by external pressure independent of their cholesterol content, multi-lamellar vesicles and multilayers behave similarly. © 2019 Elsevier B.V.
    view abstract10.1016/j.bpc.2019.106210
  • Combining X-ray Kβ 1,3 , valence-to-core, and X-ray Raman spectroscopy for studying Earth materials at high pressure and temperature: The case of siderite
    Weis, C. and Spiekermann, G. and Sternemann, C. and Harder, M. and Vankó, G. and Cerantola, V. and Sahle, C.J. and Forov, Y. and Sakrowski, R. and Kupenko, I. and Petitgirard, S. and Yavaş, H. and Bressler, C. and Gawelda, W. and Tolan, M. and Wilke, M.
    Journal of Analytical Atomic Spectrometry 34 (2019)
    X-ray emission and X-ray Raman scattering spectroscopy are powerful tools to investigate the local electronic and atomic structure of high and low Z elements in situ. Notably, these methods can be applied for in situ spectroscopy at high pressure and high temperature using resistively or laser-heated diamond anvil cells in order to achieve thermodynamic conditions which appear in the Earth's interior. We present a setup for combined X-ray emission and X-ray Raman scattering studies at beamline P01 of PETRA III using a portable wavelength-dispersive von Hamos spectrometer together with the permanently installed multiple-analyzer Johann-type spectrometer. The capabilities of this setup are exemplified by investigating the iron spin crossover of siderite FeCO 3 up to 49.3 GPa by measuring the Fe M 2,3 -edge and the Fe Kβ 1,3 emission line simultaneously. With this setup, the Fe valence-to-core emission can be detected together with the Kβ 1,3 emission line providing complementary information on the sample's electronic structure. By implementing a laser-heating device, we demonstrate the strength of using a von Hamos type spectrometer for spin state mapping at extreme conditions. Finally, we give different examples of low Z elements' absorption edges relevant for application in geoscience that are accessible with the Johann-type XRS spectrometer. With this setup new insights into the spin transition and compression mechanisms of Earth's mantle materials can be obtained of importance for comprehension of the macroscopic physical and chemical properties of the Earth's interior. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8ja00247a
  • Control of structural flexibility of layered-pillared metal-organic frameworks anchored at surfaces
    Wannapaiboon, S. and Schneemann, A. and Hante, I. and Tu, M. and Epp, K. and Semrau, A.L. and Sternemann, C. and Paulus, M. and Baxter, S.J. and Kieslich, G. and Fischer, R.A.
    Nature Communications 10 (2019)
    Flexible metal-organic frameworks (MOFs) are structurally flexible, porous, crystalline solids that show a structural transition in response to a stimulus. If MOF-based solid-state and microelectronic devices are to be capable of leveraging such structural flexibility, then the integration of MOF thin films into a device configuration is crucial. Here we report the targeted and precise anchoring of Cu-based alkylether-functionalised layered-pillared MOF crystallites onto substrates via stepwise liquid-phase epitaxy. The structural transformation during methanol sorption is monitored by in-situ grazing incidence X-ray diffraction. Interestingly, spatially-controlled anchoring of the flexible MOFs on the surface induces a distinct structural responsiveness which is different from the bulk powder and can be systematically controlled by varying the crystallite characteristics, for instance dimensions and orientation. This fundamental understanding of thin-film flexibility is of paramount importance for the rational design of MOF-based devices utilising the structural flexibility in specific applications such as selective sensors. © 2019, The Author(s).
    view abstract10.1038/s41467-018-08285-5
  • Density variations of TMAO solutions in the kilobar range: Experiments, PC-SAFT predictions, and molecular dynamics simulations
    Knierbein, M. and Held, C. and Hölzl, C. and Horinek, D. and Paulus, M. and Sadowski, G. and Sternemann, C. and Nase, J.
    Biophysical Chemistry 253 (2019)
    We present measurements, molecular dynamics (MD) simulations, and predictions using Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) of the density of aqueous solutions in a pressure range from 1 bar to 5000 bar, a pressure regime that is highly relevant for both biochemical applications and the fundamental understanding of solvation. The accurate determination of density data of pressurized solutions remains challenging. We determined relative density changes from the variations in X-ray absorption through the sample and developed a new water parameter set for PC-SAFT modeling that is appropriate for high pressure conditions in the kilobar regime. As a showcase, we studied trimethylamine N-oxide (TMAO) solutions and demonstrated that their compressibility decreases with the TMAO content. This result is linked to the stabilizing effect of TMAO on the local H-bond network of water. Experiments and calculations, which represent two independent methods, are in very good agreement and are in accordance with results of force field molecular dynamics simulations of the same systems. © 2019 Elsevier B.V.
    view abstract10.1016/j.bpc.2019.106222
  • Influence of etching-pretreatment on nano-grained WC-Co surfaces and properties of PVD/HVOF duplex coatings
    Tillmann, W. and Hagen, L. and Stangier, D. and Krabiell, M. and Schröder, P. and Tiller, J. and Krumm, C. and Sternemann, C. and Paulus, M. and Elbers, M.
    Surface and Coatings Technology 374 (2019)
    The deposition of coatings by means of Physical Vapor Deposition (PVD)is an established process to enhance the lifetime and performance of carbide bulk tools. Although the effect of surface pretreatments on conventional WC-Co surfaces is well known, this investigation examines for the first time, how different surface pretreatments affect the surface integrity of thermally sprayed WC-Co substrates prior a subsequent PVD layer deposition and its resulting properties. Therefore, a WC-12Co feedstock with an average WC particle size of 100 nm was thermally sprayed on AISI M3 steel substrates using High Velocity Oxy-Fuel (HVOF)technique. Hereinafter, the HVOF sprayed WC-Co coatings were grounded and polished, thus serving as substrates for further surface pretreatments and the deposition of a CrAlN PVD hard coating by means of magnetron sputtering. To evaluate the influence of various surface pretreatments on the HVOF sprayed WC-Co coatings, several sequences such as heating, inert gas ion etching, metal ion etching, and High Power Impulse Magnetron Sputtering (HIPIMS)-etching were carried out. With respect to the subsequent PVD layer deposition, the results show that the pretreatment does neither affect the hardness nor Young's modulus of the CrAlN top layer. Yet, different effects on the WC-Co surface and PVD coating adhesion are observed. Inert gas ion etching leads to a faster removal of the carbides than of the Co-binder matrix. In contrast, metal ion etching provides a “micro-blasting” effect and removes the binder matrix as verified by Atomic Force Microscope (AFM)measurements. As a result, a decrease of the compressive residual stress state and an increase of the surface free energy are observed. With respect to HIPIMS-etching, a Cr-nanolayer was applied onto the WC-Co surface, which enhances the adhesion of the CrAlN top layer. Nevertheless, HRC Rockwell adhesion and scratch tests reveal a superior adhesion for samples pretreated with the metal ion etching. © 2019 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2019.05.054
  • Persistent Octahedral Coordination in Amorphous GeO2 Up to 100 GPa by Kβ′′ X-Ray Emission Spectroscopy PERSISTENT OCTAHEDRAL COORDINATION in ... G. SPIEKERMANN et al.
    Spiekermann, G. and Harder, M. and Gilmore, K. and Zalden, P. and Sahle, C.J. and Petitgirard, S. and Wilke, M. and Biedermann, N. and Weis, C. and Morgenroth, W. and Tse, J.S. and Kulik, E. and Nishiyama, N. and Yavaş, H. and Sternemann, C.
    Physical Review X 9 (2019)
    We measure valence-to-core x-ray emission spectra of compressed crystalline GeO2 up to 56 GPa and of amorphous GeO2 up to 100 GPa. In a novel approach, we extract the Ge coordination number and mean Ge-O distances from the emission energy and the intensity of the Kβ′′ emission line. The spectra of high-pressure polymorphs are calculated using the Bethe-Salpeter equation. Trends observed in the experimental and calculated spectra are found to match only when utilizing an octahedral model. The results reveal persistent octahedral Ge coordination with increasing distortion, similar to the compaction mechanism in the sequence of octahedrally coordinated crystalline GeO2 high-pressure polymorphs. © 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» 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.
    view abstract10.1103/PhysRevX.9.011025
  • Structure and dynamics of short-chain polymerized ionic liquids
    Wieland, F. and Bocharova, V. and Münzner, P. and Hiller, W. and Sakrowski, R. and Sternemann, C. and Böhmer, R. and Sokolov, A.P. and Gainaru, C.
    Journal of Chemical Physics 151 (2019)
    Combining experimental results obtained with X-ray scattering and field-gradient nuclear magnetic resonance (NMR) and an assessment of new and previous dielectric and rheology data, our study focuses on the molecular weight (Mw) evolution of local structure and dynamics in a homologous series of covalently bonded ionic liquids. Performed on a family of electrolytes with a tailored degree of ionic decoupling, this study reveals the differences between monomeric and oligomeric melts with respect to their structural organization, mass and charge transport, and molecular diffusion. Our study demonstrates that for the monomeric compound, the broadband conductivity and mechanical spectra reflect the same underlying distribution of activation barriers and that the Random Barrier Model describes fairly well both the ionic and structural relaxation processes in these materials. Moreover, the oligomers with chains comprising ten segments only exhibit both structural and dynamical fingerprints of a genuine polymer. A comparison of conductivity levels estimated using the self-diffusion coefficients probed via NMR and those probed directly with dielectric spectroscopy reveals the emerging of ion correlations which are affecting the macroscopic charge transport in these materials in a chain-length dependent manner. © 2019 Author(s).
    view abstract10.1063/1.5109228
  • Ultrathin Films of 2D Hofmann-Type Coordination Polymers: Influence of Pillaring Linkers on Structural Flexibility and Vertical Charge Transport
    Rubio-Giménez, V. and Escorcia-Ariza, G. and Bartual-Murgui, C. and Sternemann, C. and Galbiati, M. and Castells-Gil, J. and Real, J.A. and Tatay, S. and Martí-Gastaldo, C.
    Chemistry of Materials 31 (2019)
    Searching for novel materials and controlling their nanostructuration into electronic devices is a challenging task ahead of chemists and chemical engineers. Even more so when this new application requires an exquisite control over the morphology, crystallinity, roughness, and orientation of the films produced. In this context, it is of critical importance to analyze the influence of the chemical composition of perspective materials on their properties at the nanoscale. We report the fabrication of ultrathin films (thickness &lt; 30 nm) of a family of FeII Hofmann-like coordination polymers (CPs) by using an optimized liquid-phase epitaxy (LPE) set up. The series [Fe(L)2{Pt(CN)4}] (L = pyridine, pyrimidine, and isoquinoline) conform an ideal platform for correlating the effect of the axial nitrogenated ligand with changes to their structural response to guests or electrical resistance. All film properties relevant to device integration have been thoroughly analyzed with complementary surface techniques for a meaningful comparison. Our results reveal that changes to this ligand can hinder the structural transformation triggered by the absorption of guest molecules previously reported for the pyridine phase. Also important, it can substantially hinder vertical charge transport across the layers, even at the ultrathin film limit. © 2019 American Chemical Society.
    view abstract10.1021/acs.chemmater.9b01634
  • A high pressure study of calmodulin-ligand interactions using small-angle X-ray and elastic incoherent neutron scattering
    Cinar, S. and Al-Ayoubi, S. and Sternemann, C. and Peters, J. and Winter, R. and Czeslik, C.
    Physical Chemistry Chemical Physics 20 (2018)
    Calmodulin (CaM) is a Ca2+ sensor and mediates Ca2+ signaling through binding of numerous target ligands. The binding of ligands by Ca2+-saturated CaM (holo-CaM) is governed by attractive hydrophobic and electrostatic interactions that are weakened under high pressure in aqueous solutions. Moreover, the potential formation of void volumes upon ligand binding creates a further source of pressure sensitivity. Hence, high pressure is a suitable thermodynamic variable to probe protein-ligand interactions. In this study, we compare the binding of two different ligands to holo-CaM as a function of pressure by using X-ray and neutron scattering techniques. The two ligands are the farnesylated hypervariable region (HVR) of the K-Ras4B protein, which is a natural binding partner of holo-CaM, and the antagonist trifluoperazine (TFP), which is known to inhibit holo-CaM activity. From small-angle X-ray scattering experiments performed up to 3000 bar, we observe a pressure-induced partial unfolding of the free holo-CaM in the absence of ligands, where the two lobes of the dumbbell-shaped protein are slightly swelled. In contrast, upon binding TFP, holo-CaM forms a closed globular conformation, which is pressure stable at least up to 3000 bar. The HVR of K-Ras4B shows a different binding behavior, and the data suggest the dissociation of the holo-CaM/HVR complex under high pressure, probably due to a less dense protein contact of the HVR as compared to TFP. The elastic incoherent neutron scattering experiments corroborate these findings. Below 2000 bar, pressure induces enhanced atomic fluctuations in both holo-CaM/ligand complexes, but those of the holo-CaM/HVR complex seem to be larger. Thus, the inhibition of holo-CaM by TFP is supported by a low-volume ligand binding, albeit this is not associated with a rigidification of the complex structure on the sub-ns Å-scale. © 2018 the Owner Societies.
    view abstract10.1039/c7cp07399b
  • Completing the Series: New Coordination Networks of Composition 3 ∞[RE2(ADC)3(H2O)6]·2H2O with RE = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Y and ADC2– = Acetylenedicarboxylate (–O2C–C≡C–CO2 –)
    Gramm, V.K. and Schuy, A. and Suta, M. and Wickleder, C. and Sternemann, C. and Ruschewitz, U.
    Zeitschrift fur Anorganische und Allgemeine Chemie 644 (2018)
    The crystal structures of 3 ∞[RE2(ADC)3(H2O)6]·2H2O (RE = Pr, Nd, Sm, Eu, Tb, Dy) were solved and refined from X-ray single crystal data. They crystallize in a structure type already known for RE = La, Ce and Gd (P1, no. 2, Z = 2), which is characterized by REO9 polyhedra forming dimeric units being the nodes of a 3D framework structure linked by ADC2– anions (–O2C–C≡C–CO2 – = acetylenedicarboxylate). From synchrotron powder diffraction data it was shown that isostructural coordination networks are formed for RE = Ho, Er, Y, whereas for RE = Tm, Yb, Lu a new structure type crystallizing in a highly complex crystal structure with a large orthorhombic unit cell is found. All compounds are obtained by slow evaporation of an aqueous solution containing RE(OAc)3·xH2O and acetylenedicarboxylic acid (H2ADC). The coordination networks of composition 3 ∞[RE2(ADC)3(H2O)6]·2H2O were thoroughly investigated by thermal analysis and for RE = Eu, Tb, a strong red and green photoluminescence was observed and investigated by means of UV/Vis spectroscopy. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/zaac.201700361
  • Different Breathing Mechanisms in Flexible Pillared-Layered Metal-Organic Frameworks: Impact of the Metal Center
    Schneemann, A. and Vervoorts, P. and Hante, I. and Tu, M. and Wannapaiboon, S. and Sternemann, C. and Paulus, M. and Wieland, D.C.F. and Henke, S. and Fischer, R.A.
    Chemistry of Materials 30 (2018)
    The pillared-layered metal-organic framework compounds M2(BME-bdc)2(dabco) (M2+ = Zn2+, Co2+, Ni2+, Cu2+; BME-bdc2- = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M2+. This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO2 pressure reveal different mechanisms for the np to lp phase transition during CO2 adsorption. © 2018 American Chemical Society.
    view abstract10.1021/acs.chemmater.7b05052
  • Investigation on the oxidation behavior of AlCrVxN thin films by means of synchrotron radiation and influence on the high temperature friction
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Applied Surface Science 427 (2018)
    Friction minimization is an important topic which is pursued in research and industry. In addition to the use of lubricants, friction-reducing oxide phases can be utilized which occur during. These oxides are called Magnéli phases and especially vanadium oxides exhibit good friction reducing properties. Thereby, the lubrication effect can be traced back to oxygen deficiencies. AlCrN thin films are being used as coatings for tools which have to withstand high temperatures. A further improvement of AlCrN thin films concerning their friction properties is possible by incorporation of vanadium. This study analyzes the temperature dependent oxidation behavior of magnetron sputtered AlCrVN thin films with different vanadium contents up to 13.5 at.-% by means of X-ray diffraction and X-ray absorption near-edge spectroscopy. Up to 400 °C the coatings show no oxidation. A higher temperature of 700 °C leads to an oxidation and formation of Magnéli phases of the coatings with vanadium contents above 10.7 at.-%. Friction coefficients, measured by ball-on-disk test are correlated with the oxide formation in order to figure out the effect of vanadium oxides. At 700 °C a decrease of the friction coefficient with increasing vanadium content can be observed, due to the formation of VO2, V2O3 and the Magnéli phase V4O7. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.09.029
  • On the Origin of Microtubules’ High-Pressure Sensitivity
    Gao, M. and Berghaus, M. and Möbitz, S. and Schuabb, V. and Erwin, N. and Herzog, M. and Julius, K. and Sternemann, C. and Winter, R.
    Biophysical Journal 114 (2018)
    For over 50 years, it has been known that the mitosis of eukaryotic cells is inhibited already at high hydrostatic pressure conditions of 30 MPa. This effect has been attributed to the disorganization of microtubules, the main component of the spindle apparatus. However, the structural details of the depolymerization and the origin of the pressure sensitivity have remained elusive. It has also been a puzzle how complex organisms could still successfully inhabit extreme high-pressure environments such as those encountered in the depth of oceans. We studied the pressure stability of microtubules at different structural levels and for distinct dynamic states using high-pressure Fourier-transform infrared spectroscopy and Synchrotron small-angle x-ray scattering. We show that microtubules are hardly stable under abyssal conditions, where pressures up to 100 MPa are reached. This high-pressure sensitivity can be mainly attributed to the internal voids and packing defects in the microtubules. In particular, we show that lateral and longitudinal contacts feature different pressure stabilities, and they define also the pressure stability of tubulin bundles. The intactness of both contact types is necessary for the functionality of microtubules in vivo. Despite being known to dynamically stabilize microtubules and prevent their depolymerization, we found that the anti-cancer drug taxol and the accessory protein MAP2c decrease the pressure stability of microtubule protofilaments. Moreover, we demonstrate that the cellular environment itself is a crowded place and accessory proteins can increase the pressure stability of microtubules and accelerate their otherwise highly pressure-sensitive de novo formation. © 2018 Biophysical Society
    view abstract10.1016/j.bpj.2018.01.021
  • Connecting structurally and dynamically detected signatures of supramolecular Debye liquids
    Büning, T. and Lueg, J. and Bolle, J. and Sternemann, C. and Gainaru, C. and Tolan, M. and Böhmer, R.
    Journal of Chemical Physics 147 (2017)
    The monohydroxy alcohol 2-ethyl-1-hexanol mixed with the halogen-substituted alkyl halides 2-ethyl-1-hexyl chloride and 2-ethyl-1-hexyl bromide was studied using synchrotron-based x-ray scattering. In the diffraction patterns, an oxygen-related prepeak appears. The concentration dependence of its intensity, shape, and position indicates that the formation of the hydrogen-bonded associates of monohydroxy alcohols is largely hindered by the halogen alkane admixture. Using dielectric spectroscopy and high-resolution rheology on the same liquid mixtures, it is shown that these structural features are correlated with the relaxation mechanisms giving rise to supramolecular low-frequency dynamics. © 2017 Author(s).
    view abstract10.1063/1.4986866
  • Investigation of the influence of the vanadium content on the high temperature tribo-mechanical properties of DC magnetron sputtered AlCrVN thin films
    Tillmann, W. and Kokalj, D. and Stangier, D. and Paulus, M. and Sternemann, C. and Tolan, M.
    Surface and Coatings Technology 328 (2017)
    The forming of high-strength steels or new aluminum alloys leads to a steady increase of the load of tools and coatings. One approach is to positively influence the manufacturing process by using thin solid films with self-lubricating features, provided by oxides at high temperatures with low decohesion energies. For the purpose of this study, AlCrN provides the matrix, while vanadium oxides are used to enhance the frictional and wear properties. However, it is not yet clear which minimum amount of vanadium has to be incorporated in DC magnetron sputtered AlCrN coatings to improve the tribological behavior. Therefore, in this study, AlCrVN coatings are synthesized with an increasing vanadium content by means of reactive DC magnetron sputtering. Additionally, a vanadium-free AlCrN coating is used as reference for the tribo-mechanical investigations. The coatings were synthesized up to a vanadium content of 13.5 at.-% and no phase change could be detected by means of x-ray diffraction. Moreover, no hexagonal AlN phase, which reduces the mechanical properties and the oxidation resistance, was formed. In contrast to the vanadium-free coating, the hardness of the coatings containing vanadium is slightly reduced. The coating with the smallest vanadium content shows the highest hardness of all analyzed coatings. A heat treatment at 400 °C does not lead to any significant changes with respect to mechanical properties, but at 700 °C hardness, modulus of elasticity and critical load decreased for all coatings, indicating a significant change in mechanical properties. The ball-on-disc test at room-temperature, 400 °C, and 700 °C shows the highest wear coefficient for the coating with the lowest vanadium content, due to the poor adhesion of the coating, although this coating shows the highest H/E-ratio. © 2017 Elsevier B.V.
    view abstract10.1016/j.surfcoat.2017.08.046
  • Miniature diamond anvils for X-ray Raman scattering spectroscopy experiments at high pressure
    Petitgirard, S. and Spiekermann, G. and Weis, C. and Sahle, C. and Sternemann, C. and Wilke, M.
    Journal of Synchrotron Radiation 24 (2017)
    X-ray Raman scattering (XRS) spectroscopy is an inelastic scattering method that uses hard X-rays of the order of 10 keV to measure energy-loss spectra at absorption edges of light elements (Si, Mg, O etc.), with an energy resolution below 1 eV. The high-energy X-rays employed with this technique can penetrate thick or dense sample containers such as the diamond anvils employed in high-pressure cells. Here, we describe the use of custom-made conical miniature diamond anvils of less than 500 μm thickness which allow pressure generation of up to 70 GPa. This set-up overcomes the limitations of the XRS technique in very high-pressure measurements (&gt;10 GPa) by drastically improving the signal-to-noise ratio. The conical shape of the base of the diamonds gives a 70° opening angle, enabling measurements in both low- and high-angle scattering geometry. This reduction of the diamond thickness to one-third of the classical diamond anvils considerably lowers the attenuation of the incoming and the scattered beams and thus enhances the signal-to-noise ratio significantly. A further improvement of the signal-to-background ratio is obtained by a recess of ∼20 μm that is milled in the culet of the miniature anvils. This recess increases the sample scattering volume by a factor of three at a pressure of 60 GPa. Examples of X-ray Raman spectra collected at the O K-edge and Si L-edge in SiO2 glass at high pressures up to 47 GPa demonstrate the significant improvement and potential for spectroscopic studies of low-Z elements at high pressure. © 2017 International Union of Crystallography.
    view abstract10.1107/S1600577516017112
  • Pressure driven spin transition in siderite and magnesiosiderite single crystals
    Weis, C. and Sternemann, C. and Cerantola, V. and Sahle, C.J. and Spiekermann, G. and Harder, M. and Forov, Y. and Kononov, A. and Sakrowski, R. and Yavaş, H. and Tolan, M. and Wilke, M.
    Scientific Reports 7 (2017)
    Iron-bearing carbonates are candidate phases for carbon storage in the deep Earth and may play an important role for the Earth's carbon cycle. To elucidate the properties of carbonates at conditions of the deep Earth, we investigated the pressure driven magnetic high spin to low spin transition of synthetic siderite FeCO3 and magnesiosiderite (Mg0.74Fe0.26)CO3 single crystals for pressures up to 57 GPa using diamond anvil cells and x-ray Raman scattering spectroscopy to directly probe the iron 3d electron configuration. An extremely sharp transition for siderite single crystal occurs at a notably low pressure of 40.4 ± 0.1 GPa with a transition width of 0.7 GPa when using the very soft pressure medium helium. In contrast, we observe a broadening of the transition width to 4.4 GPa for siderite with a surprising additional shift of the transition pressure to 44.3 ± 0.4 GPa when argon is used as pressure medium. The difference is assigned to larger pressure gradients in case of argon. For magnesiosiderite loaded with argon, the transition occurs at 44.8 ± 0.8 GPa showing similar width as siderite. Hence, no compositional effect on the spin transition pressure is observed. The spectra measured within the spin crossover regime indicate coexistence of regions of pure high- and low-spin configuration within the single crystal. © 2017 The Author(s).
    view abstract10.1038/s41598-017-16733-3
  • Temperature dependence of the hydrogen bond network in trimethylamine: N -oxide and guanidine hydrochloride-water solutions
    Lehmkühler, F. and Forov, Y. and Elbers, M. and Steinke, I. and Sahle, C.J. and Weis, C. and Tsuji, N. and Itou, M. and Sakurai, Y. and Poulain, A. and Sternemann, C.
    Physical Chemistry Chemical Physics 19 (2017)
    We present an X-ray Compton scattering study on aqueous trimethylamine N-oxide (TMAO) and guanidine hydrochloride solutions (GdnHCl) as a function of temperature. Independent from the concentration of the solvent, Compton profiles almost resemble results for liquid water as a function of temperature. However, the number of hydrogen bonds per water molecule extracted from the Compton profiles suggests a decrease of hydrogen bonds with rising temperature for all studied samples, and the differences between water and the solutions are weak. Nevertheless, the data indicate a reduced bond weakening with rising TMAO concentration up to 5 M of 7.2% compared to 8% for pure water. In contrast, the addition of GdnHCl appears to behave differently for concentrations up to 3.1 M with a weaker impact on the temperature response of the hydrogen bond structure. © This journal is the Owner Societies.
    view abstract10.1039/c7cp04958g
  • Bulk sensitive determination of the Fe3+/FeTot-ratio in minerals by Fe L2/3-edge X-ray Raman scattering
    Nyrow, A. and Sternemann, C. and Tse, J.S. and Weis, C. and Sahle, C.J. and Mende, K. and Wieland, D.C.F. and Cerantola, V. and Gordon, R.A. and Spiekermann, G. and Regier, T. and Wilke, M. and Tolan, M.
    Journal of Analytical Atomic Spectrometry 31 (2016)
    We present the first measurements of the iron L2/3-edge of the compounds FeO, Fe2O3, and Fe3O4 at ambient pressure and of FeCO3 at high pressures of 2.4 and 40 GPa using a diamond anvil cell by X-ray Raman scattering spectroscopy, a bulk sensitive probe of soft X-ray absorption edges making use of hard X-rays. We show that the spectral shape of the Fe L2/3-edge can be analyzed quantitatively to reveal the oxidation state of iron in matter. Consequently, in situ X-ray Raman scattering spectroscopy at the iron L-edge at high pressure and temperature opens exciting perspectives to characterize the local coordination, oxidation, and spin state of iron at high pressure and temperature, conditions that are of relevance for e.g. geological sciences or chemical processing. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c5ja00261c
  • Formation of CaB6 in the thermal decomposition of the hydrogen storage material Ca(BH4)2
    Sahle, C.J. and Sternemann, C. and Giacobbe, C. and Yan, Y. and Weis, C. and Harder, M. and Forov, Y. and Spiekermann, G. and Tolan, M. and Krisch, M. and Remhof, A.
    Physical Chemistry Chemical Physics 18 (2016)
    Using a combination of high resolution X-ray powder diffraction and X-ray Raman scattering spectroscopy at the B K- and Ca L2,3-edges, we analyzed the reaction products of Ca(BH4)2 after annealing at 350 °C and 400 °C under vacuum conditions. We observed the formation of nanocrystalline/amorphous CaB6 mainly and found only small contributions from amorphous B for annealing times larger than 2 h. For short annealing times of 0.5 h at 400 °C we observed neither CaB12H12 nor CaB6. The results indicate a reaction pathway in which Ca(BH4)2 decomposes to B and CaH2 and finally reacts to form CaB6. These findings confirm the potential of using Ca(BH4)2 as a hydrogen storage medium and imply the desired cycling capabilities for achieving high-density hydrogen storage materials. © the Owner Societies 2016.
    view abstract10.1039/c6cp02495e
  • In situ characterization of the decomposition behavior of Mg(BH4)2 by X-ray Raman scattering spectroscopy
    Sahle, C.J. and Kujawski, S. and Remhof, A. and Yan, Y. and Stadie, N.P. and Al-Zein, A. and Tolan, M. and Huotari, S. and Krisch, M. and Sternemann, C.
    Physical Chemistry Chemical Physics 18 (2016)
    We present an in situ study of the thermal decomposition of Mg(BH4)2 in a hydrogen atmosphere of up to 4 bar and up to 500 °C using X-ray Raman scattering spectroscopy at the boron K-edge and the magnesium L2,3-edges. The combination of the fingerprinting analysis of both edges yields detailed quantitative information on the reaction products during decomposition, an issue of crucial importance in determining whether Mg(BH4)2 can be used as a next-generation hydrogen storage material. This work reveals the formation of reaction intermediate(s) at 300 °C, accompanied by a significant hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB12H12. At temperatures between 300 °C and 400 °C, further hydrogen release proceeds via the formation of higher boranes and crystalline MgH2. Above 400 °C, decomposition into the constituting elements takes place. Therefore, at moderate temperatures, Mg(BH4)2 is shown to be a promising high-density hydrogen storage material with great potential for reversible energy storage applications. © 2016 the Owner Societies.
    view abstract10.1039/c5cp06571b
  • Intramolecular structure and energetics in supercooled water down to 255 K
    Lehmkühler, F. and Forov, Y. and Büning, T. and Sahle, C.J. and Steinke, I. and Julius, K. and Buslaps, T. and Tolan, M. and Hakala, M. and Sternemann, C.
    Physical Chemistry Chemical Physics 18 (2016)
    We studied the structure and energetics of supercooled water by means of X-ray Raman and Compton scattering. Under supercooled conditions down to 255 K, the oxygen K-edge measured by X-ray Raman scattering suggests an increase of tetrahedral order similar to the conventional temperature effect observed in non-supercooled water. Compton profile differences indicate contributions beyond the theoretically predicted temperature effect and provide a deeper insight into local structural changes. These contributions suggest a decrease of the electron mean kinetic energy by 3.3 ± 0.7 kJ (mol K)-1 that cannot be modeled within established water models. Our surprising results emphasize the need for water models that capture in detail the intramolecular structural changes and quantum effects to explain this complex liquid. © the Owner Societies 2016.
    view abstract10.1039/c5cp07721d
  • PDMS embedded Ag clusters: Coalescence and cluster-matrix interaction
    Roese, S. and Engemann, D. and Hoffmann, S. and Latussek, K. and Sternemann, C. and Hövel, H.
    Journal of Physics: Conference Series 712 (2016)
    Polydimethylsiloxane (PDMS) has proven to be a suitable embedding medium for silver clusters to prevent aggregation. In order to investigate the influence of the PDMS on the electronic and local atomic structure of the clusters the measurement of x-ray absorption near edge structure (XANES) spectra for different coverages of silver clusters in PDMS and calculations of corresponding XANES spectra have been performed. The coalescence process and the cluster-PDMS interaction were investigated with XANES.
    view abstract10.1088/1742-6596/712/1/012068
  • Polaron-induced lattice distortion of (In,Ga)As/GaAs quantum dots by optically excited carriers
    Tiemeyer, S. and Bombeck, M. and Göhring, H. and Paulus, M. and Sternemann, C. and Nase, J. and Wirkert, F.J. and Möller, J. and Büning, T. and Seeck, O.H. and Reuter, D. and Wieck, A.D. and Bayer, M. and Tolan, M.
    Nanotechnology 27 (2016)
    We report on a high resolution x-ray diffraction study unveiling the effect of carriers optically injected into (In,Ga)As quantum dots on the surrounding GaAs crystal matrix. We find a tetragonal lattice expansion with enhanced elongation along the [001] crystal axis that is superimposed on an isotropic lattice extension. The isotropic contribution arises from excitation induced lattice heating as confirmed by temperature dependent reference studies. The tetragonal expansion on the femtometer scale is tentatively attributed to polaron formation by carriers trapped in the quantum dots. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/42/425702
  • Spectroscopy of low and intermediate Z elements at extreme conditions: in situ studies of Earth materials at pressure and temperature via X-ray Raman scattering
    Sternemann, C. and Wilke, M.
    High Pressure Research 36 (2016)
    X-ray Raman scattering spectroscopy is an emerging method in the study of low and intermediate Z elements' core-electron excitations at extreme conditions in order to reveal information on local structure and electronic state of matter in situ. We discuss the capabilities of this method to address questions in Earth materials' science and demonstrate its sensitivity to detect changes in the oxidation state, electronic structure, coordination, and spin state. Examples are presented for the study of the oxygen K-, silicon L- and iron M-edges. We assess the application of both temperature and pressure in such investigations exploiting diamond anvil cells in combination with resistive or laser heating which is required to achieve realistic conditions of the Earth's crust, mantle, and core. © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstract10.1080/08957959.2016.1198903
  • The detachment behavior of polycarbonate on thin films above the glass transition temperature
    Tillmann, W. and Hagen, L. and Hoffmann, F. and Dildrop, M. and Wibbeke, A. and Schöppner, V. and Resonnek, V. and Pohl, M. and Krumm, C. and Tiller, J.C. and Paulus, M. and Sternemann, C.
    Polymer Engineering and Science 56 (2016)
    When producing mono-axially stretched films made of amorphous polycarbonate, a self-reinforcement is generated due to the stretching process. This leads to an increase of the strength and stiffness. The mono-axial stretching process is conducted at temperatures above the glass transition temperature, whereas better mechanical properties are obtained at higher stretching temperatures. However, the film tends to adhere to the rolls, especially at temperatures from 10°C above the glass transition temperature. The rolls of the mono-axial stretching unit are made of an induction hardened and polished quenched and tempered steel 1.7225 – 42CrMo4. This work reports on the investigation of the detachment behavior of polycarbonate on different coatings as a function of the temperature and contact time. The main intention is to find a suitable coating on which the polycarbonate film adheres only slightly at temperatures clearly exceeding the glass transition temperature. POLYM. ENG. SCI., 56:786–797, 2016. © 2016 Society of Plastics Engineers. © 2016 Society of Plastics Engineers
    view abstract10.1002/pen.24307
  • Iron speciation in minerals and glasses probed by M2/3-edge X-ray Raman scattering spectroscopy
    Nyrow, A. and Sternemann, C. and Wilke, M. and Gordon, R.A. and Mende, K. and Yavaş, H. and Simonelli, L. and Hiraoka, N. and Sahle, C.J. and Huotari, S. and Andreozzi, G.B. and Woodland, A.B. and Tolan, M. and Tse, J.S.
    Contributions to Mineralogy and Petrology 167 (2014)
    We present a spectroscopic study of the iron M2/3-edge for several minerals and compounds to reveal information about the oxidation state and the local coordination of iron. We describe a novel approach to probe the iron M2/3-edge bulk sensitively using X-ray Raman scattering. Significant changes in the onset and shape of the Fe M2/3-edge were observed on ferrous and ferric model compounds with Fe in octahedral and tetrahedral coordination. Simulation of the spectra is possible using an atomic multiplet code, which potentially allows determination of, e.g., crystal-field parameters in a quantitative manner. A protocol is discussed for determination of the Fe oxidation state in compounds by linear combination of spectra of ferric and ferrous end members. The presented results demonstrate the capabilities of Fe M2/3-edge spectroscopy by X-ray Raman scattering to extract information on the ratio of trivalent to total iron Fe3+/∑Fe and local coordination. As X-ray Raman scattering is performed with hard X-rays, this approach is suitable for in situ experiments at high pressure and temperature. It thus may provide indispensable information on oxidation state, electronic structure and local structure of materials that are important for physical and chemical processes of the deep Earth. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00410-014-1012-8
  • Pressure induced spin transition revealed by iron M2,3-edge spectroscopy
    Nyrow, A. and Tse, J.S. and Hiraoka, N. and Desgreniers, S. and Büning, T. and Mende, K. and Tolan, M. and Wilke, M. and Sternemann, C.
    Applied Physics Letters 104 (2014)
    We present a method to characterize pressure induced magnetic high to low spin transition in iron sulphide using x-ray Raman scattering spectroscopy at the iron M2,3-edge. The advantage of this method is that the observed spectral changes between pressures of 1.7 GPa and 10.1 GPa can be used with the help of atomic multiplet calculations to determine the crystal field splitting parameters associated with the spin transition. We discuss the potential of this M2,3-edge spectroscopy to investigate the irons electronic spin state in-situ at the conditions of the inner Earth, i.e., at high temperature and high pressure, providing exciting opportunities for geophysical and materials science applications. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4886971
  • Pressure-induced changes on the electronic structure and electron topology in the direct FCC → SH transformation of silicon
    Tse, J.S. and Hanfland, M. and Flacau, R. and Desgreniers, S. and Li, Z. and Mende, K. and Gilmore, K. and Nyrow, A. and Moretti Sala, M. and Sternemann, C.
    Journal of Physical Chemistry C 118 (2014)
    X-ray diffraction experiments at 80 K show that when silicon is compressed under hydrostatic conditions the intermediate high-pressure phases are bypassed leading to a direct transformation to the simple hexagonal structure at 17 GPa. A maximum entropy analysis of the diffraction patterns reveals dramatic alterations in the valence electron distribution from tetrahedral covalent bonding to localization in the interstitial sites and along the one-dimensional silicon atom chain running along adjacent hexagonal layers. Changes in the orbital character of the unoccupied states are confirmed using X-ray Raman scattering spectroscopy and theoretical Bethe-Salpeter equation calculations. This is the first direct observation indicating that the silicon valence electrons in 3s and 3p orbitals are transferred to the 3d orbitals at high density which proves that electrons of compressed elemental solids migrate from their native bonding configuration to interstitial regions. © 2013 American Chemical Society.
    view abstract10.1021/jp408666q
  • Supramolecular x-ray signature of susceptibility amplification in hydrogen-bonded liquids
    Bierwirth, S.P. and Büning, T. and Gainaru, C. and Sternemann, C. and Tolan, M. and Böhmer, R.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 90 (2014)
    Mixing two nonconducting hydrogen-bonded liquids, each exhibiting a low dielectric relaxation strength, can result in a highly electrically absorbing fluid. This susceptibility amplification effect is demonstrated for mixtures of monohydroxy alcohols. Whereas in the pure liquids a tendency to form ringlike low-dipole moment clusters prevails, in the mixtures such supramolecular structures are disfavored leading to an up to tenfold enhancement of the dielectric loss. The compositional evolution of density and mean cluster-cluster separation is traced using x-ray scattering and indicates deviations from ideal mixing with decreased C-C but simultaneously increased O-O correlation lengths. Thus, the variation in the supramolecular absorption strength could be tracked using a static scattering technique. These observations are in harmony with volume exclusion and ring open effects that predict an optimized susceptibility amplification for mixtures in which the two components occupy equal volume fractions as experimentally observed. © 2014 American Physical Society.
    view abstract10.1103/PhysRevE.90.052807
  • Temperature-driven adsorption and desorption of proteins at solid-liquid interfaces
    Kiesel, I. and Paulus, M. and Nase, J. and Tiemeyer, S. and Sternemann, C. and Rüster, K. and Wirkert, F.J. and Mende, K. and Büning, T. and Tolan, M.
    Langmuir 30 (2014)
    The heat-induced desorption and adsorption of the proteins lysozyme, ribonuclease A, bovine serum albumin, and fibronectin at protein layers was investigated in two different environments: pure buffer and protein solution. Using two different environments allows us to distinguish between thermodynamic and kinetic mechanisms in the adsorption process. We observed a desorption in buffer and an adsorption in protein solution, depending upon protein properties, such as size, stability, and charge. We conclude that the desorption in buffer is mainly influenced by the mobility of the proteins at the interface, while the adsorption in protein solution is driven by conformational changes and, thereby, a gain in entropy. These results are relevant for controlling biofilm formation at solid-liquid interfaces. © 2014 American Chemical Society.
    view abstract10.1021/la404884a
  • The Ba 4d-4f giant dipole resonance in complex Ba/Si compounds
    Sahle, C.H.J. and Sternemann, C. and Sternemann, H. and Tse, J.S. and Gordon, R.A. and Desgreniers, S. and Maekawa, S. and Yamanaka, S. and Lehmkühler, F. and Wieland, D.C.F. and Mende, K. and Huotari, S. and Tolan, M.
    Journal of Physics B: Atomic, Molecular and Optical Physics 47 (2014)
    The shape of the Ba 4d-4f giant dipole resonance is studied for Ba atoms embedded inside complex Si networks covering structures consisting of Si nanocages and nanotubes, i.e. the clathrate Ba8Si46, the complex compound BaSi6, and the semiconducting BaSi2. Here, non-resonant x-ray Raman scattering is used to investigate confinement effects on the shape of the giant resonance in the vicinity of the Ba N IV, V-edge. The distinct momentum transfer dependence of the spectra is analyzed and discussed. The measurements are compared to calculations of the giant resonance within time-dependent local density approximation in the dipole limit. No modulation of the giant resonance's shape for Ba atoms confined in different local environments was observed, in contrast to the calculations. The absence of such shape modulation for complex Ba/Si compounds is discussed providing important implications for further studies of giant resonance phenomena utilizing both theory and experiment. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0953-4075/47/4/045102
  • X-ray reflectivity measurements of liquid/solid interfaces under high hydrostatic pressure conditions
    Wirkert, F.J. and Paulus, M. and Nase, J. and Möller, J. and Kujawski, S. and Sternemann, C. and Tolan, M.
    Journal of Synchrotron Radiation 21 (2014)
    A high-pressure cell for in situ X-ray reflectivity measurements of liquid/solid interfaces at hydrostatic pressures up to 500 MPa (5 kbar), a pressure regime that is particularly important for the study of protein unfolding, is presented. The original set-up of this hydrostatic high-pressure cell is discussed and its unique properties are demonstrated by the investigation of pressure-induced adsorption of the protein lysozyme onto hydrophobic silicon wafers. The presented results emphasize the enormous potential of X-ray reflectivity studies under high hydrostatic pressure conditions for the in situ investigation of adsorption phenomena in biological systems.© 2014 International Union of Crystallography.
    view abstract10.1107/S1600577513021516
  • A solid-solution approach to mixed-metal metal-organic frameworks - Detailed characterization of local structures, defects and breathing behaviour of Al/V frameworks
    Kozachuk, O. and Meilikhov, M. and Yusenko, K. and Schneemann, A. and Jee, B. and Kuttatheyil, A.V. and Bertmer, M. and Sternemann, C. and Pöppl, A. and Fischer, R.A.
    European Journal of Inorganic Chemistry (2013)
    The doping of [Al(OH)L]n [L = 1,4-benzenedicarboxylate (bdc) or 1,4-naphthalenedicarboxylate (ndc)] with vanadium ions yields crystalline porous mixed-metal solid-solution metal-organic frameworks (MOFs) of general formula [(AlOH)1-x(VO)xL]n (x can be varied in the whole range from 0 to 1). Several characterization methods, including powder X-ray diffraction (PXRD), electron paramagnetic resonance (EPR), solid-state NMR and FTIR spectroscopy, strongly support the effective incorporation of vanadium cations. The Al/V-doped MOFs are isostructural to the parent monometallic MOFs and show a characteristic uniform dependence of the cell parameters on the metal ratios. Detailed spectroscopic investigation provided evidence that the introduced species are fairly well ordered. Interestingly, for low amounts of doped vanadium for both activated and as-synthesized Al/V phases, the EPR results revealed the presence of vanadyl units as local defects in pseudo-octahedral or square-pyramidal environments, which are different from those in the parent MIL-47(V). This observation matches the nonlinear response of the adsorption properties on variation of the composition. Remarkably, the presence of such mixed Al/V chains strongly affects the breathing behaviour of the materials. Both CO2 sorption and in situ PXRD studies validated a gradual change from highly flexible (with easily induced phase transitions) to totally rigid structures upon increasing vanadium content. Systematic studies on the doping of [Al(OH)L]n [L = 1,4-benzenedicarboxylate (bdc) or 1,4-naphthalenedicarboxylate (ndc)] metal-organic frameworks (MOFs) with V ions to obtain [(AlOH)1-x(VO)xL]n (0 < x < 1) solid-solution materials are reported. Detailed characterization allowed us to examine the correlations between compositions and the structures and properties of the final mixed-metal materials. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ejic.201300591
  • CsNaC2, CsKC2, CsRbC2 -syntheses and crystal structures of three new acetylides
    Liebig, S. and Paulus, M. and Sternemann, C. and Ruschewitz, U.
    Zeitschrift fur Anorganische und Allgemeine Chemie 639 (2013)
    Three new alkali metal acetylides CsNaC2, CsKC2, and CsRbC2 have been synthesized and characterized by means of synchrotron powder diffraction studies. As a new synthetic approach, the binary alkali metal acetylides were reacted at relatively low temperatures (200 °C). DSC measurements were performed to prove the general usability of this reaction. CsKC2 and CsRbC2 crystallize in a variant of the anti-PbCl2-type structure (Pnma, Z = 4), while for CsNaC2 a new structure type (Pbcm, Z = 4) is found. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/zaac.201300147
  • Microscopic structure of water at elevated pressures and temperatures
    Sahle, C.J. and Sternemann, C. and Schmidt, C. and Lehtola, S. and Jahn, S. and Simonelli, L. and Huotari, S. and Hakala, M. and Pylkkänen, T. and Nyrow, A. and Mende, K. and Tolan, M. and Hämäläinen, K. and Wilke, M.
    Proceedings of the National Academy of Sciences of the United States of America 110 (2013)
    We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley's K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6at600 °C and p = 134 MPa.
    view abstract10.1073/pnas.1220301110
  • Stress-induced stabilization of crystals in shape memory natural rubber
    Heuwers, B. and Quitmann, D. and Hoeher, R. and Reinders, F.M. and Tiemeyer, S. and Sternemann, C. and Tolan, M. and Katzenberg, F. and Tiller, J.C.
    Macromolecular Rapid Communications 34 (2013)
    In contrast to all known shape memory polymers, the melting temperature of crystals in shape memory natural rubber (SMNR) can be greatly manipulated by the application of external mechanical stress. As shown previously, stress perpendicular to the prior programming direction decreases the melting temperature by up to 40 K. In this study, we investigated the influence of mechanical stress parallel to prior stretching direction during programming on the stability of the elongation-stabilizing crystals. It was found that parallel stress stabilizes the crystals, which is indicated by linear increase of the trigger temperature by up to 17 K. The crystal melting temperature can be increased up to 126.5 °C under constrained conditions as shown by X-ray diffraction measurements. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/marc.201200594
  • Structural changes in amorphous GexSiOy on the way to nanocrystal formation
    Nyrow, A. and Sternemann, C. and Sahle, Ch.J. and Hohl, A. and Zschintzsch-Dias, M. and Schwamberger, A. and Mende, K. and Brinkmann, I. and Moretti Sala, M. and Wagner, R. and Meier, A. and Völklein, F. and Tolan, M.
    Nanotechnology 24 (2013)
    Temperature induced changes of the local chemical structure of bulk amorphous GexSiOy are studied by Ge K-edge x-ray absorption near-edge spectroscopy and Si L2/3-edge x-ray Raman scattering spectroscopy. Different processes are revealed which lead to formation of Ge regions embedded in a Si oxide matrix due to different initial structures of as-prepared samples, depending on their Ge/Si/O ratio and temperature treatment, eventually resulting in the occurrence of nanocrystals. Here, disproportionation of GeOx and SiOx regions and/or reduction of Ge oxides by pure Si or by a surrounding Si sub-oxide matrix can be employed to tune the size of Ge nanocrystals along with the chemical composition of the embedding matrix. This is important for the optimization of the electronic and luminescent properties of the material. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/24/16/165701
  • Study of time and pressure dependent phenomena at the hard X-ray beamline BL9 of DELTA
    Wirkert, F.J. and Paulus, M. and Sternemann, C. and Nase, J. and Schroer, M.A. and Wieland, D.C.F. and Bieder, S. and Degen, P. and Rehage, H. and Tolan, M.
    Journal of Physics: Conference Series 425 (2013)
    The beamline BL9 of DELTA (Dortmund ELecTron Accelerator) is a multi-purpose beamline operating in an energy range between 4 and 27 keV. A short overview of the beamline and the experimental endstation is given. Exemplarily three typical applications, namely X-ray diffraction from interfaces, small angle X-ray scattering under high hydrostatic pressure and fast X-ray reflectivity measurements, are discussed in some detail in order to demonstrate the capabilities of the beamline.
    view abstract10.1088/1742-6596/425/20/202006
  • Study of time and pressure dependent phenomena at the hard X-ray beamline BL9 of DELTA
    Wirkert, F.J. and Paulus, M. and Sternemann, C. and Nase, J. and Schroer, M.A. and Wieland, D.C.F. and Bieder, S. and Degen, P. and Rehage, H. and Tolan, M.
    Journal of Physics: Conference Series 425 (2013)
    The beamline BL9 of DELTA (Dortmund ELecTron Accelerator) is a multi-purpose beamline operating in an energy range between 4 and 27 keV. A short overview of the beamline and the experimental endstation is given. Exemplarily three typical applications, namely X-ray diffraction from interfaces, small angle X-ray scattering under high hydrostatic pressure and fast X-ray reflectivity measurements, are discussed in some detail in order to demonstrate the capabilities of the beamline.
    view abstract10.1088/1742-6596/425/20/202006
  • X-ray Raman scattering: An exciting tool for the study of matter at conditions of the Earth's interior
    Sternemann, C. and Sahle, C.J. and Mende, K. and Schmidt, C. and Nyrow, A. and Simonelli, L. and Sala, M.M. and Tolan, M. and Wilke, M.
    Journal of Physics: Conference Series 425 (2013)
    The study of minerals and melts at in situ conditions is highly relevant to understand the physical and chemical properties of the Earth's crust and mantle. Here, X-ray Raman scattering provides a valuable tool to investigate the local atomic and electronic structure of Earth materials consisting predominantly of low Z elements at high pressures and temperatures. The capabilities of X-ray Raman scattering to investigate silicate minerals, glasses, and melts are discussed and the application of the method to in situ studies of silicate melts using a hydrothermal diamond anvil cell is demonstrated.
    view abstract10.1088/1742-6596/425/20/202011
  • Adsorption of nanoparticles at the solid-liquid interface
    Brenner, T. and Paulus, M. and Schroer, M.A. and Tiemeyer, S. and Sternemann, C. and Möller, J. and Tolan, M. and Degen, P. and Rehage, H.
    Journal of Colloid and Interface Science 374 (2012)
    The adsorption of differently charged nanoparticles at liquid-solid interfaces was investigated by in situ X-ray reflectivity measurements. The layer formation of positively charged maghemite (γ-Fe 2O 3) nanoparticles at the aqueous solution-SiO 2 interface was observed while negatively charged gold nanoparticles show no adsorption at this interface. Thus, the electrostatic interaction between the particles and the charged surface was determined as the driving force for the adsorption process. The data analysis shows that a logarithmic particle size distribution describes the density profile of the thin adsorbed maghemite layer. The size distribution in the nanoparticle solution determined by small angle X-ray scattering shows an average particle size which is similar to that found for the adsorbed film. The formed magehemite film exhibits a rather high stability. © 2012 Elsevier Inc.
    view abstract10.1016/j.jcis.2012.02.010
  • Performance of fluorene and terthiophene copolymer in bilayer photovoltaic devices: The role of the polymer conformations
    Marchiori, C.F.N. and Yamamoto, N.A.D. and Grova, I.R. and MacEdo, A.G. and Paulus, M. and Sternemann, C. and Huotari, S. and Akcelrud, L. and Roman, L.S. and Koehler, M.
    Organic Electronics: physics, materials, applications 13 (2012)
    We report experiments using fluorene and terthiophene copolymer as the active layer in bilayer devices with C 60. The highest short circuit current, open circuit voltage and power conversion efficiency upon AM1.5 illumination were 6.8 mA/cm 2, 0.68 V and 2.33%, respectively. Density functional theory analysis was used to identify the most stable configurations of the terthiophene moieties in the polymer: the most stable form has the thiophene rings in the alternate configuration (anti) and the second conformation has the thiophene rings pointing to the same direction (syn). Comparing theoretical results with measurements of absorbance, X-ray diffraction, and X-ray reflectometry experiments, we conclude that the annealing treatment produces conformational anti to syn transition along the backbone of poly[9,9′-n-dihexyl-2,7-fluorene-alt-2,5-terthiophene] (LaPPS45). The syn segments of the chain condensed then in a lamellar ordered structure which increases the degree of crystallinity of the annealed samples and improve the light harvest at long wavelengths. From absorption measurements of films submitted to different annealing temperatures and with the help of theoretical calculations we propose a "wave-like" aggregation pattern to the syn segments in those lamellas. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.orgel.2012.08.002
  • Self-assembled iron oxide nanoparticle multilayer: X-ray and polarized neutron reflectivity
    Mishra, D. and Benitez, M.J. and Petracic, O. and Badini Confalonieri, G.A. and Szary, P. and Brüssing, F. and Theis-Bröhl, K. and Devishvili, A. and Vorobiev, A. and Konovalov, O. and Paulus, M. and Sternemann, C. and Toperverg, B.P. and Zabel, H.
    Nanotechnology 23 (2012)
    We have investigated the structure and magnetism of self-assembled, 20nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipoledipole interaction is rather strong, dominating the collective magnetic properties at room temperature. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/23/5/055707
  • Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity
    Hähl, H. and Evers, F. and Grandthyll, S. and Paulus, M. and Sternemann, C. and Loskill, P. and Lessel, M. and Hüsecken, A.K. and Brenner, T. and Tolan, M. and Jacobs, K.
    Langmuir 28 (2012)
    The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading. © 2012 American Chemical Society.
    view abstract10.1021/la300850g
  • Charge excitations in stripe-ordered La5/3Sr 1/3NiO4 and La15/8Ba1/8CuO 4: Interpretation of the anomalous momentum transfer dependence via fluorescence interferometry
    Schülke, W. and Sternemann, C.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Fluorescence interferometry (FI) makes use of the temporal and spatial coherence of absorption and re-emission process in resonant inelastic x-ray scattering (RIXS), which leads to interference terms in the RIXS cross section of a group of identical, physically equivalent, and interacting atoms. These interferences contain structural information and make RIXS site selective. The basics of FI are derived within the third-order perturbation treatment of indirect RIXS. The final relations for the FI related RIXS cross section were used to interpret the unique momentum-transfer dependence of the in-gap charge excitations observed for 214-type nickelates (La5/3Sr 1/3NiO4) and cuprates (La15/8Ba 1/8CuO4). By using well-known stripe models, the momentum-transfer dependence of the corresponding indirect RIXS features is traced back to interference terms in the RIXS cross section based on the principles of FI. Thus analyzing the momentum-transfer dependence of the re-emitted radiation yields information about details of the charge stripe structure. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.085143
  • Ge-Si-O phase separation and Ge nanocrystal growth in Ge:SiO x/SiO2 multilayers - A new dc magnetron approach
    Zschintzsch, M. and Sahle, C.J. and Borany, J.V. and Sternemann, C. and Mücklich, A. and Nyrow, A. and Schwamberger, A. and Tolan, M.
    Nanotechnology 22 (2011)
    Ge:SiOx/SiO2 multilayers are fabricated using a new reactive dc magnetron sputtering approach. The influence of the multilayer stoichiometry on the ternary Ge-Si-O phase separation and the subsequent size-controlled Ge nanocrystal formation is explored by means of x-ray absorption spectroscopy, x-ray diffraction, electron microscopy and Raman spectroscopy. The ternary system Ge-Si-O reveals complete Ge-O phase separation at 400 °C which does not differ significantly to the binary Ge-O system. Ge nanocrystals of < 5nm size are generated after subsequent annealing below 700°C. It is shown that Ge oxides contained in the as-deposited multilayers are reduced by a surrounding unsaturated silica matrix. A stoichiometric regime was found where almost no GeO2 is present after annealing. Thus, the Ge nanocrystals become completely embedded in a stoichiometric silica matrix favouring the use for photovoltaic applications. © IOP Publishing Ltd.
    view abstract10.1088/0957-4484/22/48/485303
  • Influence of hydrogen on thermally induced phase separation in GeO/SiO 2 multilayers
    Sahle, C.J. and Zschintzsch, M. and Sternemann, C. and Von Borany, J. and Mücklich, A. and Nyrow, A. and Jeutter, N.M. and Wagner, R. and Frahm, R. and Tolan, M.
    Nanotechnology 22 (2011)
    The influence of the annealing atmosphere on the temperature induced phase separation of Ge oxide in GeOx/SiO2 multilayers (x ≈ 1), leading to size controlled growth of Ge nanocrystals, is explored by means of x-ray absorption spectroscopy at the Ge K-edge. Ge sub-oxides contained in the as-deposited multilayers diminish with increasing annealing temperature, showing complete phase separation at approximately 450 °C using inert N 2 ambient. The use of reducing H2 in the annealing atmosphere influences the phase separation even at an early stage of the disproportionation. In particular, the temperature regime where the phase separation occurs is lowered by at least 50 °C. At temperatures above 400 °C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO2 by H 2. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/22/12/125709
  • Intercalation in layered metal-organic frameworks: Reversible inclusion of an extended π-system
    Arslan, H.K. and Shekhah, O. and Wieland, D.C.F. and Paulus, M. and Sternemann, C. and Schroer, M.A. and Tiemeyer, S. and Tolan, M. and Fischer, R.A. and Wöll, C.
    Journal of the American Chemical Society 133 (2011)
    We report the synthesis of layered [Zn 2(bdc) 2(H 2O) 2] and [Cu 2(bdc) 2(H 2O) 2] (bdc = benzdicarboxylate) metal-organic frameworks (MOF) carried out using the liquid-phase epitaxy approach employing self-assembled monolayer (SAM) modified Au-substrates. We obtain Cu and Zn MOF-2 structures, which have not yet been obtained using conventional, solvothermal synthesis methods. The 2D Cu 2+ dimer paddle wheel planes characteristic for the MOF are found to be strictly planar, with the planes oriented perpendicular to the substrate. Intercalation of an organic dye, DXP, leads to a reversible tilting of the planes, demonstrating the huge potential of these surface-anchored MOFs for the intercalation of large, planar molecules. © 2011 American Chemical Society.
    view abstract10.1021/ja2037996
  • Multiple phase-transitions upon selective CO 2 adsorption in an alkyl ether functionalized metal-organic framework - An in situ X-ray diffraction study
    Henke, S. and Florian Wieland, D.C. and Meilikhov, M. and Paulus, M. and Sternemann, C. and Yusenko, K. and Fischer, R.A.
    CrystEngComm 13 (2011)
    The flexible alkyl ether functionalized metal-organic framework [Zn 2(BME-bdc) 2(dabco)] n (BME-bdc = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2. 2]octane) shows remarkable structural changes upon selective adsorption of CO 2 as determined by in situ X-ray diffraction at 195 K. Upon accommodation of carbon dioxide [Zn 2(BME-bdc) 2(dabco)] n transfers from a narrow pore form to an open pore form, which exhibits a much higher unit cell volume. Due to the slow adsorption kinetics an unexpected metastable intermediate form could be identified. © 2011 The Royal Society of Chemistry.
    view abstract10.1039/c1ce05446e
  • Pressure-induced electron topological transitions in Ba-doped Si clathrate
    Tse, J.S. and Yang, L. and Zhang, S.J. and Jin, C.Q. and Sahle, C.J. and Sternemann, C. and Nyrow, A. and Giordano, V. and Jiang, J.Z. and Yamanaka, S. and Desgreniers, S. and Tulk, C.A.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Ba8Si46 is the archetype of the Si clathrates family. X-ray diffractions have revealed an unusual homothetic isostructural transition at ∼14-16 GPa. Raman experiments, however, suggested even more transitions at lower pressure. We present evidence showing that successive electronic topological transitions are responsible for the transformations. It is shown that the electronic structure of Ba8Si46 is easily perturbed by the environment. Reverse Monte Carlo calculations and in-situ resistivity measurements revealed continual changes in the structure and electrical properties upon compression. This finding is corroborated by results of x-ray Raman scattering study in the vicinity of the Ba N4,5 and Si L2,3 absorption edges. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.184105
  • Temperature-induced structural changes of tetrahydrofuran clathrate and of the liquid water/tetrahydrofuran mixture
    Lehmkühler, F. and Sakko, A. and Steinke, I. and Sternemann, C. and Hakala, M. and Sahle, C.J. and Buslaps, T. and Simonelli, L. and Galambosi, S. and Paulus, M. and Pylkkänen, T. and Tolan, M. and Hämäläinen, K.
    Journal of Physical Chemistry C 115 (2011)
    We present two complementary inelastic X-ray scattering studies on the structure of tetrahydrofuran (THF) clathrate hydrate and the supercooled stoichiometric liquid mixture of water and THF. Compton scattering experiments of the liquid mixture show that formation of hydrate precursors is unlikely. By comparing experimental spectra of THF hydrate and water/THF mixtures at temperatures above 250 K with density functional theory calculations, structural changes that manifest in OH bond length changes are observed. X-ray Raman scattering measurements of the oxygen K-edge in the same temperature range corroborate these results. The experimental results of THF hydrate at temperatures between 20 and 244 K can be modeled best by assuming thermal expansion only. Therefore, dependency on the system's temperature different structural behavior of THF hydrate is reported. © 2011 American Chemical Society.
    view abstract10.1021/jp207027p
  • Analysis of the ion distribution at a charged solid-liquid interface using X-ray standing waves
    Brücher, M. and Jacob, P. and Von Bohlen, A. and Franzke, J. and Sternemann, C. and Paulus, M. and Hergenröder, R.
    Langmuir 26 (2010)
    Functionalized solid-liquid interfaces were analyzed by X-ray standing waves (XSW) combined with, streaming current measurements to study surface charges, interfacial potential, and ion distributions. Thin films of aqueous solution containing Br- anions and Fe3+ cations at a concentration of 10 mg/L were prepared on functionalized silicon wafers. Functionalization of Si surfaces was accomplished by aminosilane groups shifting the interfacial potential, toward, positive values. The ion distribution was measured with nanometer resolution, which allows distinguishing between absorbed and mobile ions at the surface and in the diffusive layer, respectively. For Br-, different degrees of ion attraction were measured for the pH values 5.7 and 2.8. The ion Debye length values of the diffuse layer were 4 and 2 nm, respectively. © 2009 American Chemical Society.
    view abstract10.1021/la902385d
  • Anomalous energetics in tetrahydrofuran clathrate hydrate revealed by X-ray compton scattering
    Lehmkühler, F. and Sakko, A. and Sternemann, C. and Hakala, M. and Nygård, K. and Sahle, C.J. and Galambosi, S. and Steinke, I. and Tiemeyer, S. and Nyrow, A. and Buslaps, T. and Pontoni, D. and Tolan, M. and Hämääinen, K.
    Journal of Physical Chemistry Letters 1 (2010)
    Changes in the ground-state electron momentum density of tetrahydrofuran clathrate hydrate are studied in a temperature range between 93 and 275 K by means of X-ray Compton scattering. At temperatures above 253 K, large rather unexpected differences from the Compton profiles of ice are observed. Configurational enthalpies are extracted and exhibit a rapid rise above 253 K, whereas a constant configurational heat capacity of 0.23 ± 0.07 J g -1 K-1 is found below 253 K. Density functional theory calculations suggest that this anomalous behavior originates from the structural change of the hydrate, however, no indication was found for the formation of hydrogen bonds between tetrahydrofuran and water molecules. © 2010 American Chemical Society.
    view abstract10.1021/jz1010362
  • EuxSr1-xC2 (0 ≤ x ≤ 1): A dicarbide solid solution [1] with perfect vegard behavior
    Link, P. and Wandner, D. and Schellenberg, I. and Pöttgen, R. and Paulus, M. and Sahle, C.J. and Sternemann, C. and Ruschewitz, U.
    Zeitschrift fur Anorganische und Allgemeine Chemie 636 (2010)
    A solid solution EuxSr1-xC2 (0 ≤ x ≤ 1) was synthesized by direct reaction of the elements at 1123 K. The crystal structures of these compounds, investigated by synchrotron powder diffraction, depend upon x. For x &gt; 0.5 the monoclinic ThC2 type structure (C2/c, Z = 4) is observed and for x = 0.5 the ThC2 type structure coexists with the tetragonal CaC2 type structure (I4/mmm, Z = 2). The unit cell volumes per formula unit of all EuxSr1-xC2 compounds show perfect Vegard behavior, which is due to the almost identical ionic radii of Eu2+ and Sr2+. Mössbauer spectroscopic investigations indeed reveal that europium is in the divalent state over the whole composition range. EuxSr 1-xC2 exhibits several temperature dependent phase transitions that were studied by synchrotron powder diffraction and differential thermal analysis. The transition to a cubic high-temperature modification (Fm3̄ m, Z = 4) is of special interest, as it contains information about strain effects appearing inside the modifications with ordered C2 dumbbells (ThC2 and CaC2 type structures). The linear temperature dependence of the obtained transition temperatures TPh shows that no observable strain exists in EuxSr1-xC2, which is again due to the almost identical radii of Eu2+ and Sr 2+. EuxSr1-xC2 may therefore be described as a strain free dicarbide solid solution with perfect Vegard behavior. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/zaac.201000206
  • Phase separation and Si nanocrystal formation in bulk SiO studied by x-ray scattering
    Feroughi, O.M. and Sternemann, C. and Sahle, Ch.J. and Schroer, M.A. and Sternemann, H. and Conrad, H. and Hohl, A. and Seidler, G.T. and Bradley, J. and Fister, T.T. and Balasubramanian, M. and Sakko, A. and Pirkkalainen, K. and Hämäläinen, K. and Tolan, M.
    Applied Physics Letters 96 (2010)
    We present an x-ray scattering study of the temperature-induced phase separation and Si nanocrystal formation in bulk amorphous SiOx with x≈1. X-ray Raman scattering at the Si LII,III -edge reveals a significant contribution of suboxides present in native amorphous SiO. The suboxide contribution decreases with increasing annealing temperature between 800-1200 °C pointing toward a phase separation of SiO into Si and SiO2 domains. In combination with x-ray diffraction and small angle x-ray scattering the SiO microstructure is found to be dominated by internal suboxide interfaces in the native state. For higher annealing temperatures above 900 °C growth of Si nanocrystals with rough surfaces embedded in a silicon oxide matrix can be observed. © 2010 American Institute of Physics.
    view abstract10.1063/1.3323106
  • Step-by-step growth of highly oriented and continuous seeding layers of [Cu2(ndc)2(dabco)] on bare oxide and nitride substrates
    Yusenko, K. and Meilikhov, M. and Zacher, D. and Wieland, F. and Sternemann, C. and Stammer, X. and Ladnorg, T. and Wöll, C. and Fischer, R.A.
    CrystEngComm 12 (2010)
    The step-by-step growth of highly oriented and continuous thin films of [Cu2(ndc)2(dabco)] (1) at 50 °C was studied and compared with growth directly from solvothermal mother solution at 120 °C. The substrates were bare unmodified SiO2, Al2O3 grown by atomic layer deposition (ALD), Ta2O5 and Si 3N4. The deposited layers of 1 were characterized via in-plane and out-of-plane X-ray powder diffraction (PXRD) and Scanning Electron Microscopy (SEM). The stepwise film formation process was studied by the variation of the reaction conditions and washing procedures indicating an island growth mode and the importance of storage effects. The highly oriented layers obtained by the step-by-step method were used as seeds for the deposition of thicker films of 500-700 nm with the same orientation directly from solvothermal mother solution. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b927212g
  • Suboxide interface in disproportionating a -SiO studied by x-ray Raman scattering
    Sakko, A. and Sternemann, C. and Sahle, C.J. and Sternemann, H. and Feroughi, O.M. and Conrad, H. and Djurabekova, F. and Hohl, A. and Seidler, G.T. and Tolan, M. and Hämäläinen, K.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    The microscopic structure of disproportionating amorphous silicon monoxide is studied by inelastic x-ray scattering at the silicon LII,III edge. This material arranges into nanocrystalline regions of Si embedded in amorphous SiO2 at proper annealing temperatures and in this work we demonstrate how the contribution of the suboxide interfaces between these regions can be extracted from the experimental data. The resulting near-edge spectra are analyzed in detail using a computational framework that combines molecular-dynamics simulations and density-functional theory calculations. The results indicate that the amount of silicon atoms with oxidation states between +1 and +3 is significant and depends strongly on the annealing temperature. Furthermore, the presented s, p, and d -type local densities of states (DOS) demonstrate that the most significant differences are found in the p -type DOS. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.205317
  • The new x-ray lithography beamline BL1 at DELTA
    Lietz, D. and Paulus, M. and Sternemann, C. and Berges, U. and Hippert, B. and Tolan, M.
    AIP Conference Proceedings 1234 (2010)
    Lithography using synchrotron radiation in the x-ray regime provides a powerful method to produce mechanical components of sub-millimeter size with a very good quality for microtechnological applications. In recent years the demand for x-ray lithography beamtime for industrial production of microparts increased rapidly resulting in the development of new experimental endstations at synchrotron radiation sources dedicated for the production of micromechanical devices. We present in this work the layout of the new x-ray lithography beamline BL1 at the synchrotron radiation source DELTA in Dortmund and discuss first results of exposure tests. © 2010 American Institute of Physics.
    view abstract10.1063/1.3463213
  • adsorption

  • density functional theory

  • nanoscience

  • silicon

  • x ray scattering

  • x-ray diffraction

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