Scientific Output

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

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  • 2021 • 187 Synthesis and Characterization of Dendritic and Linear Glycol Methacrylates and Their Performance as Marine Antifouling Coatings
    Wanka, R. and Koschitzki, F. and Puzovic, V. and Pahl, T. and Manderfeld, E. and Hunsucker, K.Z. and Swain, G.W. and Rosenhahn, A.
    ACS Applied Materials and Interfaces (2021)
    Dendritic polyglycerol (PG) was covalently coupled to 2-hydroxyethyl methacrylate (HEMA) by an anionically catalyzed ring-opening polymerization generating a dendritic PG-HEMA with four PG repetition units (PG4MA). Coatings of the methacrylate monomer were prepared by grafting-through and compared against commercially available hydrophilic monomers of HEMA, poly(ethylene) glycol methacrylate (PEGMA), and poly(propylene) glycol methacrylate (PPGMA). The obtained coatings were characterized by modern surface analytical techniques, including water contact angle goniometry (sessile and captive bubble), attenuated total internal reflection Fourier transform infrared spectroscopy, and atomic force microscopy. The antifouling (AF) and fouling-release (FR) properties of the coatings were tested against the model organisms Cobetia marina and Navicula perminuta in laboratory-scale dynamic accumulation assays as well as in a dynamic short-term field exposure (DSFE) in the marine environment. In addition, the hydration of the coatings and their susceptibility toward silt uptake were evaluated, revealing a strong correlation between water uptake, silt incorporation, and field assay performance. While all glycol derivatives showed good resistance in laboratory settlement experiments, PPGMA turned out to be less susceptible to silt incorporation and outperformed PEGMA and PG4MA in the DSFE assay. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acsami.0c21212
  • 2020 • 186 Time-dependent plasticity in silicon microbeams mediated by dislocation nucleation
    Elhebeary, M. and Harzer, T. and Dehm, G. and Saif, M.T.A.
    Proceedings of the National Academy of Sciences of the United States of America 117 16864-16871 (2020)
    Understanding deformation mechanisms in silicon is critical for reliable design of miniaturized devices operating at high temperatures. Bulk silicon is brittle, but it becomes ductile at about 540 °C. It creeps (deforms plastically with time) at high temperatures (∼800 °C). However, the effect of small size on ductility and creep of silicon remains elusive. Here, we report that silicon at small scales may deform plastically with time at lower temperatures (400 °C) above a threshold stress. We achieve this stress by bending single-crystal silicon microbeams using an in situ thermomechanical testing stage. Small size, together with bending, localize high stress near the surface of the beam close to the anchor. This localization offers flaw tolerance, allowing ductility to win over fracture. Our combined scanning, transmission electron microscopy, and atomic force microscopy analysis reveals that as the threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high-stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time, lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon. © 2020 National Academy of Sciences. All rights reserved.
    view abstractdoi: 10.1073/pnas.2002681117
  • 2020 • 185 Characterisation of micropores in plasma deposited SiO xfilms by means of positron annihilation lifetime spectroscopy
    Hoppe, C. and Mitschker, F. and Butterling, M. and Liedke, M.O. and De Los Arcos, T. and Awakowicz, P. and Wagner, A. and Grundmeier, G.
    Journal of Physics D: Applied Physics 53 (2020)
    The effect of average incorporated ion energy and impinging atomic oxygen flux on the structure and permeability of SiO x thin films by a microwave driven low-pressure discharge with additional radio frequency bias is studied by means of positron annihilation lifetime spectroscopy (PALS) and complementary analytical approaches. The film growth and structure were controlled by the particle fluxes. A correlation between the pore sizes and pore size distribution as measured by PALS and the adjusted plasma parameters was established. The corresponding barrier performance was measured by oxygen transmission rate and could be explained by the pore size distribution. The dominant pore size characteristic for dangling bonds within the SiO x-network was found to be in the range of 0.8 nm. The chemical composition and morphology were analysed by means of X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy diffuse reflectance measurements and atomic force microscopy. It was observed that a combination of both an increase in incorporated energy per deposited Si atom and low oxygen to silicon ratio resulted in an enhanced cross-linking of the SiO x network and thereby led to a decrease in micropore density and to a shift of the pore size distribution function to lower values. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aba8ba
  • 2020 • 184 TETA-anchored graphene oxide enhanced polyamide thin film nanofiltration membrane for water purification; performance and antifouling properties
    Izadmehr, N. and Mansourpanah, Y. and Ulbricht, M. and Rahimpour, A. and Omidkhah, M.R.
    Journal of Environmental Management 276 (2020)
    This work investigates the performance and structure of polyamide thin film nanocomposite (PA-TFN) membrane incorporated with triethylenetetramine-modified graphene oxide (GO-TETA). The embedment of GO-TETA nanosheets within the structure of PA-TFN membrane was evaluated at different concentrations (0.005, 0.01, 0.03 wt%; in aqueous piperazine (PIP)) through interfacial polymerization (IP). The physicochemical properties of the prepared membrane were investigated by SEM, AFM, water contact angle, and zeta potential as well as ATR-IR spectroscopy. The presence of longer chains of amino groups (in comparison with the directly linked amino ones) among the stacked GO nanosheets was assumed to increase interlayer spacing, resulting in remarkable changes in water permeance and separation behavior of modified polyamide (PA) membrane. It is seen that GO-TETA nanosheets were uniformly distributed in the matrix of PA layer. With increasing the concentration of GO-TETA, the flux of TFN membranes under 6 bar was increased from 49.8 l/m2 h (no additive) to 73.2 l/m2 h (TFN comprising 0.03 wt% GO-TETA. In addition, more loading GO-TETA resulted in a significant decrease in the average thickness of the polyamide layer from ~380 to ~150 nm. Furthermore, addition of GO-TETA improved the hydrophilicity of nanocomposite membranes, resulting in superb water flux recovery (antifouling indicator) as high as 95% after filtration of bovine serum albumin solution. Also, the retention capability of the TFN membranes towards some textile dyes increased as high as 99.6%. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.jenvman.2020.111299
  • 2020 • 183 Modification of polysulfone ultrafiltration membranes via addition of anionic polyelectrolyte based on acrylamide and sodium acrylate to the coagulation bath to improve antifouling performance in water treatment
    Plisko, T.V. and Bildyukevich, A.V. and Burts, K.S. and Hliavitskaya, T.A. and Penkova, A.V. and Ermakov, S.S. and Ulbricht, M.
    Membranes 10 1-25 (2020)
    Surface modification of polysulfone ultrafiltration membranes was performed via addition of an anionic polymer flocculant based on acrylamide and sodium acrylate (PASA) to the coagulation bath upon membrane preparation by non-solvent induced phase separation (NIPS). The effect of PASA concentration in the coagulant at different coagulation bath temperatures on membrane formation time, membrane structure, surface roughness, hydrophilic-hydrophobic balance of the skin layer, surface charge, as well as separation and antifouling performance was studied. Scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, contact angle and zeta potential measurements were utilized for membrane characterization. Membrane barrier and antifouling properties were evaluated in ultrafiltration of model solutions containing human serum albumin and humic acids as well as with real surface water. PASA addition was found to affect the kinetics of phase separation leading to delayed demixing mechanism of phase separation due to the substantial increase of coagulant viscosity, which is proved by a large increase of membrane formation time. Denser and thicker skin layer is formed and formation of macrovoids in membrane matrix is suppressed. FTIR analysis confirms the immobilization of PASA macromolecules into the membrane skin layer, which yields improvement of hydrophilicity and change of zeta potential. Modified membrane demonstrated better separation and antifouling performance in the ultrafiltration of humic acid solution and surface water compared to the reference membrane. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/membranes10100264
  • 2020 • 182 Apparent differences between single layer molybdenum disulphide fabricated via chemical vapour deposition and exfoliation
    Pollmann, E. and Madauß, L. and Schumacher, S. and Kumar, U. and Heuvel, F. and vom Ende, C. and Yilmaz, S. and Güngörmüs, S. and Schleberger, M.
    Nanotechnology 31 (2020)
    Innovative applications based on two-dimensional solids require cost-effective fabrication processes resulting in large areas of high quality materials. Chemical vapour deposition is among the most promising methods to fulfill these requirements. However, for 2D materials prepared in this way it is generally assumed that they are of inferior quality in comparison to the exfoliated 2D materials commonly used in basic research. In this work we challenge this assumption and aim to quantify the differences in quality for the prototypical transition metal dichalcogenide MoS2. To this end single layers of MoS2 prepared by different techniques (exfoliation, grown by different chemical vapour deposition methods, transfer techniques and as vertical heterostructure with graphene) are studied by Raman and photoluminescence spectroscopy, complemented by atomic force microscopy. We demonstrate that as-prepared MoS2, directly grown on SiO2, differs from exfoliated MoS2 in terms of higher photoluminescence, lower electron concentration and increased strain. As soon as a water film is intercalated (e.g. by transfer) underneath the grown MoS2, in particular the (opto)electronic properties become practically identical to those of exfoliated MoS2. A comparison of the two most common precursors shows that the growth with MoO3 causes greater strain and/or defect density deviations than growth with ammonium heptamolybdate. As part of a heterostructure directly grown MoS2 interacts much stronger with the substrate and in this case an intercalated water film does not lead to the complete decoupling, which is typical for exfoliation or transfer. Our work shows that the supposedly poorer quality of grown 2D transition metal dichalcogenides is indeed a misconception. © 2020 The Author(s).
    view abstractdoi: 10.1088/1361-6528/abb5d2
  • 2020 • 181 Silicon Oxide Barrier Films Deposited on Polycarbonate Substrates in Pulsed Plasmas
    Shafaei, S. and Yang, L. and Rudolph, M. and Awakowicz, P.
    Plasma Chemistry and Plasma Processing 40 607-623 (2020)
    For many applications of polycarbonate (PC) from packaging to micro-electronics improved barrier properties are necessary. In this contribution, silica thin films were deposited from hexamethyldisiloxane/oxygen (HMDSO/O2) on polycarbonate substrate in three step plasma processes by combining a microwave (MW) surface wave discharge of 2.45 GHz with an optional radio-frequency (RF) bias of 13.56 MHz. The influence of interlayer thickness, HMDSO flow and oxygen to HMDSO ratio on barrier performance for three step-coating processes was investigated. The morphology and surface properties of the coated surface of PC were studied by atomic force microscopy (AFM). The surface topography showed a silica particles distribution on the PC substrate with relatively smooth surface roughness. AFM-QNM provides more insight into the surface morphology and stiffness. The results identify the coating structure for PC film coated with and without bias. High barrier improvement of the deposited films on PC substrates was obtained after plasma silicon coating process with a barrier improvement factor up to 337. It was found that the deposition process is optimal for food packaging applications by using combined MW-RF PECVD technology. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.
    view abstractdoi: 10.1007/s11090-019-10049-y
  • 2019 • 180 Electrical conductivity of silver nanoparticle doped carbon nanofibres measured by CS-AFM
    Ali, W. and Shabani, V. and Linke, M. and Sayin, S. and Gebert, B. and Altinpinar, S. and Hildebrandt, M. and Gutmann, J.S. and Mayer-Gall, T.
    RSC Advances 9 4553-4562 (2019)
    In this work, a pioneering study on the electrical properties of composite carbon nanofibres (CNFs) using current-sensitive atomic force microscopy (CS-AFM) has been demonstrated. CNFs are highly interesting materials which are usable in a wide array of applications e.g. electrode materials for biosensors, lithium ion batteries, fuel cells and supercapacitors. CNFs offer a high specific surface area and thus have a high contact area for charge transfer. CNFs can be produced using spinnable polyacrylonitrile (PAN) as a precursor for carbonisation. For the purpose of developing efficient CNFs with high conductivity and power density, silver nanoparticle (AgNPs)-containing PAN solutions were electrospun to form composite nanofibres which was followed by heat treatment. The applied voltage of the spinning setup and the content of both PAN and the silver nanoparticles in the spinning solution were varied in order to study their influence on the morphology and the electrical properties of the nanofibres. The resultant morphologies and fibre diameters were determined by scanning electron microscopy (SEM). The formation of silver nanoparticles was characterised in solution by UV-visible absorption spectroscopy and dynamic light scattering (DLS), while energy-dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM) were carried out to investigate the presence as well as the average diameter of the AgNPs. The electrical properties of the CNFs were investigated using CS-AFM. This technique gives us the possibility to explore the electrical properties of single fibers and hence derive relationships between the structural features and the electrical properties. Our results show that the composite CNFs have a higher electrical conductivity than the neat CNFs and both the average diameter of the fibers and the electrical conductivity increase with an increasing AgNP content. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/C8RA04594A
  • 2019 • 179 Tuning the charge flow between Marcus regimes in an organic thin-film device
    Atxabal, A. and Arnold, T. and Parui, S. and Hutsch, S. and Zuccatti, E. and Llopis, R. and Cinchetti, M. and Casanova, F. and Ortmann, F. and Hueso, L.E.
    Nature Communications 10 (2019)
    Marcus’s theory of electron transfer, initially formulated six decades ago for redox reactions in solution, is now of great importance for very diverse scientific communities. The molecular scale tunability of electronic properties renders organic semiconductor materials in principle an ideal platform to test this theory. However, the demonstration of charge transfer in different Marcus regions requires a precise control over the driving force acting on the charge carriers. Here, we make use of a three-terminal hot-electron molecular transistor, which lets us access unconventional transport regimes. Thanks to the control of the injection energy of hot carriers in the molecular thin film we induce an effective negative differential resistance state that is a direct consequence of the Marcus Inverted Region. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-10114-2
  • 2019 • 178 Monitoring of photochemically induced changes in phase-modulating samples with digital holographic microscopy
    Besaga, V.R. and Saetchnikov, A.V. and Gerhardt, N.C. and Ostendorf, A. and Hofmann, M.R.
    Applied Optics 58 G41-G47 (2019)
    This paper analyzes the performance of single-shot digital holographic microscopy for rapid characterization of static step-index structures in transparent polymer materials and for online monitoring of the photoinduced polymerization dynamics. The experiments are performed with a modified Mach–Zehnder transmission digital holographic microscope of high stability (phase accuracy of 0.69°) and of high magnification (of ≈90×). Use of near-infrared illumination allows both nondestructive examination of the manufactured samples and monitoring of optically induced processes in a photosensitive material concurrently with its excitation. The accuracy of the method for a precise sample’s topography evaluation is studied on an example of microchannel sets fabricated via two-photon polymerization and is supported by reference measurements with an atomic force microscope. The applicability of the approach for dynamic measurements is proved via online monitoring of the refractive index evolution in a photoresin layer illuminated with a focused laser beam at 405 nm. High correlation between the experimental results and a kinetics model for the photopolymerization process is achieved. © 2019 Optical Society of America
    view abstractdoi: 10.1364/AO.58.000G41
  • 2019 • 177 Bisphosphonate ligand mediated ultrasensitive capacitive protein sensor: complementary match of supramolecular and dynamic chemistry
    Ertürk, G. and Akhoundian, M. and Lueg-Althoff, K. and Shinde, S. and Yeung, S.Y. and Hedström, M. and Schrader, T. and Mattiasson, B. and Sellergren, B.
    New Journal of Chemistry 43 847-852 (2019)
    Modern healthcare demands rapid and accurate detection of proteins/enzymes at the ultratrace level. Herein we present a molecularly imprinted capacitive sensor for trypsin, developed by microcontact imprinting. High affinity and selectivity was achieved by doping the prepolymerization mixture with a stoichiometric amount of methacrylamide-based bisphosphonate (BP) monomer. Taking advantage of the specific interaction between bisphosphonate binding monomers and lysine/arginine residues on the surface of trypsin, we have constructed a powerful polymeric sensor. The BP based sensor has the ability to recognize trypsin over other arginine-rich proteins, even in high ionic strength buffers with a sub-picomolar detection limit (pM). We believe that the combination of supramolecular chemistry, molecular imprinting and advanced instrumentation has a potential for future drug development and diagnostics that extends beyond biomolecular recognition. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
    view abstractdoi: 10.1039/c8nj05238g
  • 2019 • 176 Thin plasma polymerised coatings for corrosion protection against strong alkaline solutions
    Jaritz, M. and Hopmann, C. and Wilski, S. and Kleines, L. and Rudolph, M. and Awakowicz, P. and Dahlmann, R.
    Surface and Coatings Technology 374 232-241 (2019)
    Thin plasma polymers were applied on gold- and aluminium substrates using low pressure microwave- and radiofrequency-excited hexamethyldisilazane (HMDSN) plasma. The corrosion resistance properties of these coatings against sodium hydroxide solution (NaOH) was characterised by means of time resolved electrochemical impedance spectroscopy (EIS) and light microscopy. The evaluated resistance values were correlated with coating topography, chemical composition, wetting properties, and morphology with particular focus on porosity. Coating porosity was determined by using cyclic voltammetry (CV) and light microscopy. The topography and chemistry of the coatings were characterised by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The wetting properties were determined by optical contact angle (OCA) measurements. It is shown that the plasma polymer's resistance against NaOH can be greatly increased by lowering the energy input during the deposition process. This can be attributed to the strong correlation between porosity and resistivity: low energy input during plasma deposition leads to the formation of smaller and more uniform particles in the plasma bulk and possibly a Stranski–Krastanov growth of the layers, resulting in a smoother coating topography and lower nano-porosity. A more dense and compact coating morphology leads to a better corrosion protection performance. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.05.069
  • 2019 • 175 A Branched Tripeptide with an Anion-Binding Motif as a New Delivery Carrier for Efficient Gene Transfection
    Jiang, H. and Hu, X.-Y. and Mosel, S. and Knauer, S.K. and Hirschhäuser, C. and Schmuck, C.
    ChemBioChem 20 1410-1416 (2019)
    Branched and dendrimeric cationic peptides have shown better transfection efficiency than linear peptides, owing to their superior capacity for inducing DNA condensation. We have designed and synthesized two analogously guanidinocarbonylpyrrole-substituted (GCP-substituted) branched cationic tripeptides that provide extremely strong electrostatic attraction towards DNA. Both ligands 1 and 2 can bind to DNA and form condensed complexes, owing to the branched structure and high positive charges, as demonstrated by isothermal titration calorimetry (ITC), ζ potential and atomic force microscopy (AFM). After the replacement of the carboxylate group by an amide group, binding of ligand 2 to DNA shows exothermic enthalpy and positive entropy changes relative to ligand 1. Rational interpretation would suggest that ligand 2 might aid the translocation of plasmid pF143 to HEK 293T cells, showing high gene transfection efficiency. This work therefore provides a facile way, by modifying a branched cationic tripeptide with GCP, to turn a peptide even a tripeptide into an efficient gene transfection vector. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cbic.201800728
  • 2019 • 174 Femtosecond laser patterning of graphene electrodes for thin-film transistors
    Kasischke, M. and Subaşı, E. and Bock, C. and Pham, D.-V. and Gurevich, E.L. and Kunze, U. and Ostendorf, A.
    Applied Surface Science 478 299-303 (2019)
    The aim of this study is to assess femtosecond laser patterning of graphene in air and in vacuum for the application as source and drain electrodes in thin-film transistors (TFTs). The analysis of the laser-patterned graphene with scanning electron microscopy, atomic force microscopy and Raman spectroscopy showed that processing in vacuum leads to less debris formation and thus re-deposited carbonaceous material on the sample compared to laser processing in air. It was found that the debris reduction due to patterning in vacuum improves the TFT characteristics significantly. Hysteresis disappears, the mobility is enhanced by an order of magnitude and the subthreshold swing is reduced from S sub = 2.5 V/dec to S sub = 1.5 V/dec. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2019.01.198
  • 2019 • 173 Sites of high local frustration in DNA origami
    Kosinski, R. and Mukhortava, A. and Pfeifer, W. and Candelli, A. and Rauch, P. and Saccà, B.
    Nature Communications 10 (2019)
    The self-assembly of a DNA origami structure, although mostly feasible, represents indeed a rather complex folding problem. Entropy-driven folding and nucleation seeds formation may provide possible solutions; however, until now, a unified view of the energetic factors in play is missing. Here, by analyzing the self-assembly of origami domains with identical structure but different nucleobase composition, in function of variable design and experimental parameters, we identify the role played by sequence-dependent forces at the edges of the structure, where topological constraint is higher. Our data show that the degree of mechanical stress experienced by these regions during initial folding reshapes the energy landscape profile, defining the ratio between two possible global conformations. We thus propose a dynamic model of DNA origami assembly that relies on the capability of the system to escape high structural frustration at nucleation sites, eventually resulting in the emergence of a more favorable but previously hidden state. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-09002-6
  • 2019 • 172 Dissociation of the Signaling Protein K-Ras4B from Lipid Membranes Induced by a Molecular Tweezer
    Li, L. and Erwin, N. and Möbitz, S. and Niemeyer, F. and Schrader, T. and Winter, R.H.A.
    Chemistry - A European Journal 25 9827-9833 (2019)
    Oncogenic Ras mutations occur in more than 30 % of human cancers. K-Ras4B is the most frequently mutated isoform of Ras proteins. Development of effective K-Ras4B inhibitors has been challenging, hence new approaches to inhibit this oncogenic protein are urgently required. The polybasic domain of K-Ras4B with its stretch of lysine residues is essential for its plasma membrane targeting and localization. Employing CD and fluorescence spectroscopy, confocal fluorescence, and atomic force microscopy we show that the molecular tweezer CLR01 is able to efficiently bind to the lysine stretch in the polybasic domain of K-Ras4B, resulting in dissociation of the K-Ras4B protein from the lipid membrane and disintegration of K-Ras4B nanoclusters in the lipid bilayer. These results suggest that targeting of the polybasic domain of K-Ras4B by properly designed tweezers might represent an effective strategy for inactivation of K-Ras4B signaling. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201901861
  • 2019 • 171 Highly Compact TiO 2 Films by Spray Pyrolysis and Application in Perovskite Solar Cells
    Möllmann, A. and Gedamu, D. and Vivo, P. and Frohnhoven, R. and Stadler, D. and Fischer, T. and Ka, I. and Steinhorst, M. and Nechache, R. and Rosei, F. and Cloutier, S.G. and Kirchartz, T. and Mathur, S.
    Advanced Engineering Materials 21 (2019)
    Transparent and pinhole free hole-blocking layers such as TiO 2 grown at low temperatures and by scalable processes are necessary to reduce production costs and thus enabling commercialization of perovskite solar cells. Here, the authors compare the transport properties of TiO 2 compact layers grown by spray pyrolysis from commonly used titanium diisopropoxide bisacetylacetonate ([Ti(OPr i ) 2 (acac) 2 ]) precursor to films grown by spray pyrolysis of TiCl 4 . Spray pyrolysis provides insights into the interdependence of precursor chemistry and electron transport properties of TiO 2 films and their influence on the performance of the perovskite solar cells. X-ray diffraction and X-ray photoelectron spectroscopy data confirm the chemical and structural composition of the obtained films. Thin film deposition at lower temperature (150 °C) are conducted using TiCl 4 to evaluate the influence of crystal growth and topography by scanning electron microscopy and atomic force microscopy as well as thickness (profilometry) and transmittance (UV/Vis spectroscopy) on the power conversion efficiency of perovskite solar cells. TiO 2 compact layers grown from TiCl 4 enhance the power conversion efficiency by acting as superior electron transfer medium and by reducing hysteresis behavior, when compared to films grown using titanium diisopropoxide bisacetylacetonate. UV/Vis spectroscopy and external quantum efficiency studies reveal the correlation of transmittance on the power conversion efficiency. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201801196
  • 2019 • 170 Engineering atomic-level complexity in high-entropy and complex concentrated alloys
    Oh, H.S. and Kim, S.J. and Odbadrakh, K. and Ryu, W.H. and Yoon, K.N. and Mu, S. and Körmann, F. and Ikeda, Y. and Tasan, C.C. and Raabe, D. and Egami, T. and Park, E.S.
    Nature Communications 10 (2019)
    Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-10012-7
  • 2019 • 169 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 32-43 (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 abstractdoi: 10.1016/j.surfcoat.2019.05.054
  • 2019 • 168 Antifouling Properties of Dendritic Polyglycerols against Marine Macrofouling Organisms
    Wanka, R. and Aldred, N. and Finlay, J.A. and Amuthalingam, A. and Clarke, J.L. and Clare, A.S. and Rosenhahn, A.
    Langmuir 35 16568-16575 (2019)
    Dendritic polyglycerols (PGs) were synthesized and postmodified by grafting of poly(ethylene glycol) (PEG) and polypropylene glycol (PPG) diglycidyl ether groups, and their antifouling and fouling-release properties were tested. Coating characterization by spectroscopic ellipsometry, contact angle goniometry, attenuated total internal reflection-Fourier transform infrared spectroscopy (ATR-FTIR), and atomic force microscopy showed brushlike morphologies with a high degree of microscale roughness and the ability to absorb large amounts of water within seconds. PGs with three different thicknesses were tested in laboratory assays against settlement of larvae of the barnacle Balanus improvisus and against the settlement and removal of zoospores of the alga Ulva linza. Very low coating thicknesses, e.g., 11 nm, reduced the settlement of barnacles, under static conditions, to 2% compared with 55% for an octadecyltrichlorosilane reference surface. In contrast, zoospores of U. linza settled readily but the vast majority were removed by exposure to a shear force of 52 Pa. Both PEG and PPG modification increased the antifouling properties of the PG films, providing a direct comparison of the ultralow fouling properties of all three polymers. Both, the modified and the nonmodified PGs are promising components for incorporation into amphiphilic fouling-resistant coatings. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.9b02720
  • 2019 • 167 Tail state limited photocurrent collection of thick photoactive layers in organic solar cells
    Wu, J. and Luke, J. and Lee, H.K.H. and Shakya Tuladhar, P. and Cha, H. and Jang, S.-Y. and Tsoi, W.C. and Heeney, M. and Kang, H. and Lee, K. and Kirchartz, T. and Kim, J.-S. and Durrant, J.R.
    Nature Communications 10 (2019)
    We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells. © 2019, The Author(s).
    view abstractdoi: 10.1038/s41467-019-12951-7
  • 2018 • 166 Near-Model Amphiphilic Polymer Conetworks Based on Four-Arm Stars of Poly(vinylidene fluoride) and Poly(ethylene glycol): Synthesis and Characterization
    Apostolides, D.E. and Patrickios, C.S. and Sakai, T. and Guerre, M. and Lopez, G. and Améduri, B. and Ladmiral, V. and Simon, M. and Gradzielski, M. and Clemens, D. and Krumm, C. and Tiller, J.C. and Ernould, B. and Gohy, J.-F.
    Macromolecules 51 2476-2488 (2018)
    Amphiphilic polymer conetworks (APCN) were prepared in N,N-dimethylformamide (DMF) by the interconnection of four-arm star poly(vinylidene fluoride) (PVDF, Mn = 8800 Da) end-functionalized with benzaldehyde groups and four-arm star poly(ethylene glycol) (PEG, Mn = 10 kDa) end-functionalized with benzaacylhydrazide groups. The PVDF stars were prepared via the reversible addition-fragmentation chain transfer polymerization of vinylidene fluoride using a tetraxanthate chain transfer agent. Equilibrium swelling of the APCNs in various solvents was dependent on the compatibility of the APCN components with the solvent, with the degrees of swelling (DS) varying from 22 in DMF (a good solvent for both PEG and PVDF), down to 8 in water (a good and selective solvent for PEG), and even down to 3 in diethyl ether (a nonsolvent for both polymers). Characterization of the conetworks in D2O using small-angle neutron scattering (SANS) indicated phase separation at the nanoscale, as evidenced by a (broad) correlation peak, consistent with a 19 nm spacing between the formed PVDF-based hydrophobic clusters of ∼10 nm diameter and an aggregation number of ca. 50 (growing in size with PVDF content). This behavior was independent of temperature from 25 to 70 °C and slightly dependent on deviations (±ca. 50 mol %) from the PVDF: PEG stoichiometry. Conetwork characterization in the bulk using atomic force microscopy (AFM) revealed a domain spacing of 14 ± 6 nm, in good agreement with the spacing of 11 nm calculated from the SANS results above (19 nm) but also taking into account the DS in D2O (5.5). Annealing the conetworks at 200 °C, a temperature above the melting point of PVDF, did not improve the morphological order in the AFM images. Finally, APCNs prepared in the room temperature ionic liquid binary mixture lithium bis(trifluoromethanesulfonyl)imide:1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (1:9 molar ratio) exhibited an electrochemical stability up to 4.3 V and a good room temperature ion conductivity of 0.6 mS cm-1. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.macromol.7b02475
  • 2018 • 165 Calorimetric studies of PEO-b-PMMA and PEO-b-PiPMA diblock copolymers synthesized via atom transfer radical polymerization
    Chaudhury, S. and Gaalken, J. and Meyer, J. and Ulbricht, M.
    Polymer (United Kingdom) 139 11-19 (2018)
    Poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA) and poly(ethylene oxide)-b-poly(isopropyl methacrylate) (PEO-b-PiPMA) diblock copolymers of different block ratios have been synthesized using atom transfer radical polymerization with functionalized PEO monomethylether as macroinitiator. The phase separation between the constituent blocks of the copolymers has been investigated by differential scanning calorimetry (DSC). In PEO-b-PMMA, the constituent blocks are completely miscible irrespective of their molar mass and block ratios. This behaviour remains the same for PEO-b-PiPMA with ≤15% PEO fraction; phase separation could only be observed for PEO-b-PiPMA with higher PEO content. These observations are supported by results of atomic force microscopy studies of films of two copolymers with comparable molecular weight and PEO fraction (≥24% PEO); a very well developed lamellar morphology was only observed for PEO-b-PiPMA, while the block domains were randomly dispersed in PEO-b-PMMA. Interestingly, the phase separation behaviour in PEO-b-PMMA with >30% PEO fraction has been found to be strongly dependent on its processing and thermal history. On the contrary, phase separation in PEO-b-PiPMA BCPs with ≥24% PEO fraction has not been affected by its processing or thermal history. Results indicate that the use of a block-selective solvent for the precipitation of the diblock copolymer promotes the formation of microphase separated structures even for copolymers with miscible blocks. The findings are relevant for ongoing attempts to utilize the microphase separation of such polymers to obtain well-defined nanoporous membranes and other materials. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.polymer.2018.01.082
  • 2018 • 164 Enhanced selective cellular proliferation by multi-biofunctionalization of medical implant surfaces with heterodimeric BMP-2/6, fibronectin, and FGF-2
    Ettelt, V. and Belitsky, A. and Lehnert, M. and Loidl-Stahlhofen, A. and Epple, M. and Veith, M.
    Journal of Biomedical Materials Research - Part A 106 2910-2922 (2018)
    Increasing cell adhesion on implant surfaces is an issue of high biomedical importance. Early colonization with endogenous cells reduces the risk of bacterial contamination and enhances the integration of an implant into the diverse cellular tissues surrounding it. In vivo integration of implants is controlled by a complex spatial and temporal interplay of cytokines and adhesive molecules. The concept of a multi-biofunctionalized TiO2 surface for stimulating bone and soft tissue growth is presented here. All supramolecular architectures were built with a biotin–streptavidin coupling system. Biofunctionalization of TiO2 with immobilized FGF-2 and heparin could be shown to selectively increase the proliferation of fibroblasts while immobilized BMP-2 only stimulated the growth of osteoblasts. Furthermore, TiO2 surfaces biofunctionalized with either the BMP-2 or BMP-2/6 growth factor and the cell adhesion-enhancing protein fibronectin showed higher osteoblast adhesion than a TiO2 surface functionalized with only one of these proteins. In conclusion, the presented immobilization strategy is applicable in vivo for a selective surface coating of implants in both hard and connective tissue. The combined immobilization of different extracellular proteins on implants has the potential to further influence cell-specific reactions. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2910–2922, 2018. © 2018 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jbm.a.36480
  • 2018 • 163 Robust nonlinear control of atomic force microscope via immersion and invariance
    Keighobadi, J. and Hosseini-Pishrobat, M. and Faraji, J. and Oveisi, A. and Nestorović, T.
    International Journal of Robust and Nonlinear Control (2018)
    This paper reports an immersion and invariance (I&I)–based robust nonlinear controller for atomic force microscope (AFM) applications. The AFM dynamics is prone to chaos, which, in practice, leads to performance degradation and inaccurate measurements. Therefore, we design a nonlinear tracking controller that stabilizes the AFM dynamics around a desired periodic orbit. To this end, in the tracking error state space, we define a target invariant manifold, on which the system dynamics fulfills the control objective. First, considering a nominal case with full state measurement and no modeling uncertainty, we design an I&I controller to render the target manifold exponentially attractive. Next, we consider an uncertain AFM dynamics, in which only the displacement of the probe cantilever is measured. In the framework of the I&I method, we recast the robust output feedback control problem as the immersion of the output feedback closed-loop system into the nominal full state one. For this purpose, we define another target invariant manifold that recovers the performance of the nominal control system. Moreover, to handle large uncertainty/disturbances, we incorporate the method of active disturbance rejection into the I&I output feedback control. Through Lyapunov-based analysis of the closed-loop stability and robustness, we show the semiglobal practical stability and convergence of the tracking error dynamics. Finally, we present a set of detailed, comparative software simulations to assess the effectiveness of the control method. © 2018 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/rnc.4421
  • 2018 • 162 Multifunctional Stimuli-Responsive Cellulose Nanocrystals via Dual Surface Modification with Genetically Engineered Elastin-Like Polypeptides and Poly(acrylic acid)
    Malho, J.-M. and Brand, J. and Pecastaings, G. and Ruokolainen, J. and Gröschel, A. and Sèbe, G. and Garanger, E. and Lecommandoux, S.
    ACS Macro Letters 7 646-650 (2018)
    Cellulose nanocrystals (CNCs) are promising candidates for a myriad of applications; however, successful utilization of CNCs requires balanced and multifunctional properties, which require ever more applied concepts for supramolecular tailoring. We present here a facile and straightforward route to generate dual functional CNCs using poly(acrylic acid) (PAA) and biosynthetic elastin-like polypeptides (ELPs). We utilize thiol-maleimide chemistry and SI-ATRP to harvest the temperature responsiveness of ELPs and pH sensitivity of PAA to confer multifunctionality to CNCs. Cryo-TEM and light microscopy are used to exhibit reversible temperature response, while atomic force microscopy (AFM) provides detailed information on the particle morphology. The approach is tunable and allows variation of the modifying molecules, inspiring supramolecular engineering beyond the currently presented motifs. The surge of genetically engineered peptides adds further possibilities for future exploitation of the potential of cellulose nanomaterials. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acsmacrolett.8b00321
  • 2018 • 161 Metalorganic Vapor-Phase Epitaxy Growth Parameters for Two-Dimensional MoS2
    Marx, M. and Grundmann, A. and Lin, Y.-R. and Andrzejewski, D. and Kümmell, T. and Bacher, G. and Heuken, M. and Kalisch, H. and Vescan, A.
    Journal of Electronic Materials 47 910-916 (2018)
    The influence of the main growth parameters on the growth mechanism and film formation processes during metalorganic vapor-phase epitaxy (MOVPE) of two-dimensional MoS2 on sapphire (0001) have been investigated. Deposition was performed using molybdenum hexacarbonyl and di-tert-butyl sulfide as metalorganic precursors in a horizontal hot-wall MOVPE reactor from AIXTRON. The structural properties of the MoS2 films were analyzed by atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. It was found that a substrate prebake step prior to growth reduced the nucleation density of the polycrystalline film. Simultaneously, the size of the MoS2 domains increased and the formation of parasitic carbonaceous film was suppressed. Additionally, the influence of growth parameters such as reactor pressure and surface temperature is discussed. An upper limit for these parameters was found, beyond which strong parasitic deposition or incorporation of carbon into MoS2 took place. This carbon contamination became significant at reactor pressure above 100 hPa and temperature above 900°C. © 2017, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/s11664-017-5937-3
  • 2018 • 160 Segregation Phenomena in Size-Selected Bimetallic CuNi Nanoparticle Catalysts
    Pielsticker, L. and Zegkinoglou, I. and Divins, N.J. and Mistry, H. and Chen, Y.-T. and Kostka, A. and Boscoboinik, J.A. and Cuenya, B.R.
    Journal of Physical Chemistry B 122 919-926 (2018)
    Surface segregation, restructuring, and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO2 hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.7b06984
  • 2018 • 159 Overcoming Ehrlich-Schwöbel barrier in (1 1 1)A GaAs molecular beam epitaxy
    Ritzmann, J. and Schott, R. and Gross, K. and Reuter, D. and Ludwig, Ar. and Wieck, A.D.
    Journal of Crystal Growth 481 7-10 (2018)
    In this work, we first study the effect of different growth parameters on the molecular beam epitaxy (MBE) growth of GaAs layers on (1 1 1)A oriented substrates. After that we present a method for the MBE growth of atomically smooth layers by sequences of growth and annealing phases. The samples exhibit low surface roughness and good electrical properties shown by atomic force microscopy (AFM), scanning electron microscopy (SEM) and van-der-Pauw Hall measurements. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2017.10.029
  • 2018 • 158 Stress induced magnetic-domain evolution in magnetoelectric composites
    Trivedi, H. and Shvartsman, V.V. and Lupascu, D.C. and Medeiros, M.S.A. and Pullar, R.C.
    Nanotechnology 29 (2018)
    Local observation of the stress mediated magnetoelectric (ME) effect in composites has gained a great deal of interest over the last decades. However, there is an apparent lack of rigorous methods for a quantitative characterization of the ME effect at the local scale, especially in polycrystalline microstructures. In the present work, we address this issue by locally probing the surface magnetic state of barium titante-hexagonal barium ferrite (BaTiO3-BaFe12O19) ceramic composites using magnetic force microscopy (MFM). The effect of the piezoelectrically induced local stress on the magnetostrictive component (BaFe12O19, BaM) was observed in the form of the evolution of the magnetic domains. The local piezoelectric stress was induced by applying a voltage to the neighboring BaTiO3 grains, using a conductive atomic force microscopy tip. The resulting stochastic evolution of magnetic domains was studied in the context of the induced magnetoelastic anisotropy. In order to overcome the ambiguity in the domain changes observed by MFM, certain generalizations about the observed MFM contrast are put forward, followed by application of an algorithm for extracting the average micromagnetic changes. An average change in domain wall thickness of 50 nm was extracted, giving a lower limit on the corresponding induced magnetoelastic anisotropy energy. Furthermore, we demonstrate that this induced magnetomechanical energy is approximately equal to the K1 magnetocrystalline anisotropy constant of BaM, and compare it with a modeled value of applied elastic energy density. The comparison allowed us to judge the quality of the interfaces in the composite system, by roughly gauging the energy conversion ratio. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aab181
  • 2017 • 157 Fabrication of modified polyethersulfone membranes for wastewater treatment by submerged membrane bioreactor
    Abdel-Karim, A. and Gad-Allah, T.A. and El-Kalliny, A.S. and Ahmed, S.I.A. and Souaya, E.R. and Badawy, M.I. and Ulbricht, M.
    Separation and Purification Technology 175 36-46 (2017)
    Polyethersulfone (PES) flat sheet membranes were fabricated using different additives, i.e. polyvinylpyrrolidone (PVP), linear Pluronic 31R1, and star-like Tetronic 904, with different contents, by nonsolvent-induced phase separation method at similar preparation conditions. Their effects on characteristics and performance of the flat sheet PES membranes were investigated in order to select the best membrane to be applied in submerged membrane bioreactor (SMBR). The characteristics of the fabricated membranes were investigated by using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), mechanical stability measurements, contact angle (CA) measurement, shrinkage measurement, and membrane porosity analysis. The membranes performance was examined by investigation of pure water permeability (PWP) and bovine serum albumin (BSA) rejection analyses. The characteristics and the performance of the neat PES membrane have been improved by the action of these additives. For instance, there is an increase in the PWP from 2.2 L m−2 h−1 bar−1 for neat PES membrane to more than 40, 116, and 64 L m−2 h−1 bar−1 for PES-PVP (3 wt.%), PES-P31R1 (5 wt.%), and PES-T904 (5 wt.%) modified membranes, respectively. Also, these membranes showed 60–70% BSA rejection compared to ∼90% for neat PES membrane. To sum up, these mentioned three membranes are considered as promising membranes for SMBRs. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2016.10.060
  • 2017 • 156 Silicon surface modifications produced by non-equilibrium He, Ne and Kr plasma jets
    Engelhardt, M. and Kartaschew, K. and Bibinov, N. and Havenith, M. and Awakowicz, P.
    Journal of Physics D-applied Physics 50 015206 (2017)
    In this publication the interaction of non-equilibrium plasma jets (N-APPJs) with silicon surfaces is studied. The N-APPJs are operated with He, Ne and Kr gas flows under atmospheric pressure conditions. Plasma bullets are produced by the He and Ne N-APPJs, while a filamentary discharge is ignited in the Kr flow. All these N-APPJs produce remarkable traces on silicon wafer surfaces treated in their effluents. Different types of etching tracks, blisters and crystals are observed on the treated surfaces. The observed traces and surface modifications of silicon wafers are analyzed with optical, atomic-force, scanning electron and Raman microscopes. Based on the material composition within the etching tracks and the position and dimension of blisters and crystals, the traces observed on the silicon wafer surfaces are interpreted as traces of micro-plasmoids. Amorphous silicon is found in the etching tracks. Blisters are produced through the formation of cracks inside the silicon crystal by the interaction with micro-plasmoids. The reason for these modifications is not clear now. The density of micro-plasmoids traces on the treated silicon surface and the depth and length of the etching tracks depends strongly on the type of the used carrier gas of the N-APPJ.
    view abstractdoi: 10.1088/1361-6463/50/1/015206
  • 2017 • 155 Silicon surface modifications produced by non-equilibrium He, Ne and Kr plasma jets
    Engelhardt, M. and Kartaschew, K. and Bibinov, N. and Havenith, M. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    In this publication the interaction of non-equilibrium plasma jets (N-APPJs) with silicon surfaces is studied. The N-APPJs are operated with He, Ne and Kr gas flows under atmospheric pressure conditions. Plasma bullets are produced by the He and Ne N-APPJs, while a filamentary discharge is ignited in the Kr flow. All these N-APPJs produce remarkable traces on silicon wafer surfaces treated in their effluents. Different types of etching tracks, blisters and crystals are observed on the treated surfaces. The observed traces and surface modifications of silicon wafers are analyzed with optical, atomic-force, scanning electron and Raman microscopes. Based on the material composition within the etching tracks and the position and dimension of blisters and crystals, the traces observed on the silicon wafer surfaces are interpreted as traces of micro-plasmoids. Amorphous silicon is found in the etching tracks. Blisters are produced through the formation of cracks inside the silicon crystal by the interaction with micro-plasmoids. The reason for these modifications is not clear now. The density of micro-plasmoids traces on the treated silicon surface and the depth and length of the etching tracks depends strongly on the type of the used carrier gas of the N-APPJ. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/50/1/015206
  • 2017 • 154 Complete Prevention of Dendrite Formation in Zn Metal Anodes by Means of Pulsed Charging Protocols
    Garcia, G. and Ventosa, E. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 18691-18698 (2017)
    Zn metal as anode in rechargeable batteries, such as Zn/air or Zn/Ni, suffers from poor cyclability. The formation of Zn dendrites upon cycling is the key limiting step. We report a systematic study of the influence of pulsed electroplating protocols on the formation of Zn dendrites and in turn on strategies to completely prevent Zn dendrite formation. Because of the large number of variables in electroplating protocols, a scanning droplet cell technique was adapted as a high-throughput methodology in which a descriptor of the surface roughness can be in situ derived by means of electrochemical impedance spectroscopy. Upon optimizing the electroplating protocol by controlling nucleation, zincate ion depletion, and zincate ion diffusion, scanning electron microscopy and atomic force microscopy confirmed the growth of uniform and homogenous Zn deposits with a complete prevention of dendrite growth. The implementation of pulsed electroplating as the charging protocol for commercially available Ni-Zn batteries leads to substantially prolonged cyclability demonstrating the benefits of pulsed charging in Zn metal-based batteries. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b01705
  • 2017 • 153 Transparent, mediator- and membrane-free enzymatic fuel cell based on nanostructured chemically modified indium tin oxide electrodes
    González-Arribas, E. and Bobrowski, T. and Di Bari, C. and Sliozberg, K. and Ludwig, R. and Toscano, M.D. and De Lacey, A.L. and Pita, M. and Schuhmann, W. and Shleev, S.
    Biosensors and Bioelectronics 97 46-52 (2017)
    We detail a mediator- and membrane-free enzymatic glucose/oxygen biofuel cell based on transparent and nanostructured conducting supports. Chemically modified indium tin oxide nanoparticle modified electrodes were used to substantially increase the active surface area without significantly compromising transparency. Two different procedures for surface nanostructuring were employed, viz. spray-coating and drop-coating. The spray-coated biodevice showed superior characteristics as compared to the drop-coated enzymatic fuel cell, as a result of the higher nanostructured surface area as confirmed by electrochemical characterisation, as well as scanning electron and atomic force microscopy. Subsequent chemical modification with silanes, followed by the immobilisation of either cellobiose dehydrogenase from Corynascus thermophiles or bilirubin oxidase from Myrothecium verrucaria, were performed to obtain the bioanodes and biocathodes, respectively. The optimised biodevice exhibited an OCV of 0.67 V and power output of up to 1.4 µW/cm2 at an operating voltage of 0.35 V. This is considered a significant step forward in the field of glucose/oxygen membrane- and mediator-free, transparent enzymatic fuel cells. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2017.05.040
  • 2017 • 152 Monitoring ion track formation using in situ RBS/c, ToF-ERDA, and HR-PIXE
    Karlušić, M. and Fazinić, S. and Siketić, Z. and Tadić, T. and Cosic, D.D. and Božičević-Mihalić, I. and Zamboni, I. and Jakšić, M. and Schleberger, M.
    Materials 10 (2017)
    The aim of this work is to investigate the feasibility of ion beam analysis techniques for monitoring swift heavy ion track formation. First, the use of the in situ Rutherford backscattering spectrometry in channeling mode to observe damage build-up in quartz SiO2 after MeV heavy ion irradiation is demonstrated. Second, new results of the in situ grazing incidence time-of-flight elastic recoil detection analysis used for monitoring the surface elemental composition during ion tracks formation in various materials are presented. Ion tracks were found on SrTiO3, quartz SiO2, a-SiO2, and muscovite mica surfaces by atomic force microscopy, but in contrast to our previous studies on GaN and TiO2, surface stoichiometry remained unchanged. Third, the usability of high resolution particle induced X-ray spectroscopy for observation of electronic dynamics during early stages of ion track formation is shown. © 2017 by the authors.
    view abstractdoi: 10.3390/ma10091041
  • 2017 • 151 Swift heavy ion track formation in SrTiO3 and TiO2 under random, channeling and near-channeling conditions
    Karlusic, M. and Jaksic, M. and Lebius, H. and Ban-d'Etat, B. and Wilhelm, R. A. and Heller, R. and Schleberger, M.
    Journal of Physics D-applied Physics 50 205302 (2017)
    Conditions for ion track formation in single crystal SrTiO3 and TiO2 (rutile) after irradiations using swift heavy ion beams with specific energies below 1 MeV/amu were investigated in this work. Rutherford backscattering spectroscopy in channeling was used to measure ion tracks in the bulk, while atomic force microscopy was used for observation of ion tracks on the surfaces. Variations in the ion track sizes and respective thresholds were observed after irradiations under random, channeling and near-channeling conditions close to normal incidence. These variations are attributed to the specifics of the electronic stopping power of swift heavy ions under the investigated conditions. In the case of ion channeling, electronic stopping power is reduced and observed ion tracks are smaller. The opposite was found under the near-channeling conditions when lowering of the ion track formation threshold was observed. We attribute this finding to the oscillating electronic stopping power with large peak values. For both materials, thresholds for bulk and surface ion track formation were found to be surprisingly close, around 10 keV nm(-1). Obtained results are compared with predictions of the analytical thermal spike model.
    view abstractdoi: 10.1088/1361-6463/aa678c
  • 2017 • 150 Swift heavy ion track formation in SrTiO3 and TiO2 under random, channeling and near-channeling conditions
    Karlušić, M. and Jakšić, M. and Lebius, H. and Ban-D'Etat, B. and Wilhelm, R.A. and Heller, R. and Schleberger, M.
    Journal of Physics D: Applied Physics 50 (2017)
    Conditions for ion track formation in single crystal SrTiO3 and TiO2 (rutile) after irradiations using swift heavy ion beams with specific energies below 1 MeV/amu were investigated in this work. Rutherford backscattering spectroscopy in channeling was used to measure ion tracks in the bulk, while atomic force microscopy was used for observation of ion tracks on the surfaces. Variations in the ion track sizes and respective thresholds were observed after irradiations under random, channeling and near-channeling conditions close to normal incidence. These variations are attributed to the specifics of the electronic stopping power of swift heavy ions under the investigated conditions. In the case of ion channeling, electronic stopping power is reduced and observed ion tracks are smaller. The opposite was found under the near-channeling conditions when lowering of the ion track formation threshold was observed. We attribute this finding to the oscillating electronic stopping power with large peak values. For both materials, thresholds for bulk and surface ion track formation were found to be surprisingly close, around 10 keV nm-1. Obtained results are compared with predictions of the analytical thermal spike model. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa678c
  • 2017 • 149 Low-Temperature Atomic Layer Deposition of Cobalt Oxide as an Effective Catalyst for Photoelectrochemical Water-Splitting Devices
    Kim, J. and Iivonen, T. and Hämäläinen, J. and Kemell, M. and Meinander, K. and Mizohata, K. and Wang, L. and Räisänen, J. and Beranek, R. and Leskelä, M. and Devi, A.
    Chemistry of Materials 29 5796-5805 (2017)
    We have developed a low-temperature atomic layer deposition (ALD) process for depositing crystalline and phase pure spinel cobalt oxide (Co3O4) films at 120 °C using [Co(tBu2DAD)2] and ozone as coreagent. X-ray diffraction, UV-vis spectroscopy, atomic force microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis were performed to characterize the structure and properties of the films. The as-deposited Co3O4 films are crystalline with a low amount of impurities (<2% C and <5% H) despite low deposition temperatures. Deposition of Co3O4 onto thin TiO2 photoanodes (100 nm) for water oxidation resulted in 30% improvement of photocurrent (after 10 ALD cycles yielding small Co3O4 particles) as compared to pristine TiO2 films), and exhibited no detrimental effects on photocurrent response up to 300 deposition cycles (approximately 35 nm thick films), demonstrating the applicability of the developed ALD process for deposition of effective catalyst particles and layers in photoelectrochemical water-splitting devices. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.6b05346
  • 2017 • 148 Influence of bidisperse self-assembled monolayer structure on the slip boundary condition of thin polymer films
    McGraw, J.D. and Klos, M. and Bridet, A. and Hähl, H. and Paulus, M. and Castillo, J.M. and Horsch, M. and Jacobs, K.
    Journal of Chemical Physics 146 (2017)
    Alkylsilane self-assembled monolayers (SAMs) are often used as model substrates for their ease of preparation and hydrophobic properties. We have observed that these atomically smooth monolayers also provide a slip boundary condition for dewetting films composed of unentangled polymers. This slip length, an indirect measure of the friction between a given liquid and different solids, is switchable and can be increased [R. Fetzer et al., Phys. Rev. Lett. 95, 127801 (2005); O. Bäumchen et al., J. Phys.: Condens. Matter 24, 325102 (2012)] if the alkyl chain length is changed from 18 to 12 backbone carbons, for example. Typically, this change in boundary condition is affected in a quantized way, using one or the other alkyl chain length, thus obtaining one or the other slip length. Here, we present results in which this SAM structure is changed in a continuous way. We prepare bidisperse mixed SAMs of alkyl silanes, with the composition as a control parameter. We find that all the mixed SAMs investigated show an enhanced slip boundary condition as compared to the single-component SAMs. The slip boundary condition is accessed using optical and atomic force microscopy, and we describe these observations in the context of X-ray reflectivity measurements. The slip length, varying over nearly two orders of magnitude, of identical polymer melts on chemically similar SAMs is found to correlate with the density of exposed alkyl chains. Our results demonstrate the importance of a well characterized solid/liquid pair, down to the angstrom level, when discussing the friction between a liquid and a solid. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4978676
  • 2017 • 147 Ion-induced interdiffusion of surface GaN quantum dots
    Rothfuchs, C. and Semond, F. and Portail, M. and Tottereau, O. and Courville, A. and Wieck, A.D. and Ludwig, Ar.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 409 107-110 (2017)
    In the flourishing fields of quantum technology gallium nitride (GaN) quantum dots (QDs) have great appeal by providing high stability and room-temperature operation. Here, we report on the ion implantation of surface GaN QDs grown in the hexagonal crystal structure. An uncapped sample (S1) and two samples capped by 8 ML (S2) and 16 ML (S3) of AlN are subjected to a 100 keV gallium (S1, S2) and a 210 keV erbium (S3) ion beam. The fluence ranged from 5×1010 cm−2 to 1×1015 cm−2 (S1, S2) and from 5×1010 cm−2 to 5×1013 cm−2 (S3). QD characterization is performed by cathodoluminescence measurements at 77 K and atomic force microscopy and scanning electron microscopy. Strong interdiffusion processes upon ion impact at the interfaces are evidenced leading besides other effects to a quenching of the quantum confined Stark effect. Moreover, a model for the QD morphology based on a fluence-dependent diffusion coefficient is developed. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.nimb.2017.04.036
  • 2017 • 146 An inverted supramolecular amphiphile and its step-wise self-assembly into vesicular networks
    Samanta, K. and Zellermann, E. and Zähres, M. and Mayer, C. and Schmuck, C.
    Soft Matter 13 8108-8112 (2017)
    A host-guest interaction between a multi-cationic dendrimer 1 functionalized with 16 guanidiniocarbonyl pyrrole (GCP) groups on its surface and naphthalene diimide dicarboxylic acid (NDIDC) in a 1:8 ratio leads to the formation of a new type of inverted amphiphile. This amphiphile further self-assembles in a step-wise manner first into reverse micelles and then into reverse vesicles, which adhere to form an extensive 3D network several micrometers in length. Self-assembly is based on the aromatic stacking interactions of the surface-bound NDIDC. Furthermore, these aggregates only form at neutral pH but not in acidic or basic solutions in which no ion pairing between 1 and NDIDC is possible. The step-wise self-assembly process of the inverted amphiphile which follows a theoretical prediction recently proposed for hyperbranched polymers was studied and visualized in detail using atomic force microscopy (AFM) and transmission electron microscopy (TEM). © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7sm01641g
  • 2017 • 145 The combination of micro-resonators with spatially resolved ferromagnetic resonance
    Schaffers, T. and Meckenstock, R. and Spoddig, D. and Feggeler, T. and Ollefs, K. and Schöppner, C. and Bonetti, S. and Ohldag, H. and Farle, M. and Ney, A.
    Review of Scientific Instruments 88 (2017)
    We present two new and complementary approaches to realize spatial resolution for ferromagnetic resonance (FMR) on the 100 nm-scale. Both experimental setups utilize lithographically fabricated micro-resonators. They offer a detection sensitivity that is increased by four orders of magnitude compared with resonator-based FMR. In the first setup, the magnetic properties are thermally modulated via the thermal near-field effect generated by the thermal probe of an atomic force microscope. In combination with lock-in detection of the absorbed microwave power in the micro-resonator, a spatial resolution of less than 100 nm is achieved. The second setup is a combination of a micro-resonator with a scanning transmission x-ray microscope (STXM). Here a conventional FMR is excited by the micro-resonator while focused x-rays are used for a time-resolved snap-shot detection of the FMR excitations via the x-ray magnetic circular dichroism effect. This technique allows a lateral resolution of nominally 35 nm given by the STXM. Both experimental setups combine the advantage of low-power FMR excitation in the linear regime with high spatial resolution to study single and coupled nanomagnets. As proof-of-principle experiments, two perpendicular magnetic micro-stripes (5 μm × 1 μm) were grown and their FMR excitations were investigated using both setups. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4996780
  • 2017 • 144 Investigations on "near perfect" poly(2-oxazoline) based amphiphilic polymer conetworks with a crystallizable block
    Schmidt, M. and Raidt, T. and Ring, S. and Gielke, S. and Gramse, C. and Wilhelm, S. and Katzenberg, F. and Krumm, C. and Tiller, J.C.
    European Polymer Journal 88 562-574 (2017)
    Amphiphilic polymer conetworks (APCNs) of near perfect structure were prepared by cross-linking defined poly(2-oxazoline)-based block copolymers. To this end, ABA triblock copolymers with poly(2-methyloxazoline) (PMOx) as A block and poly(2-heptyloxazoline) (PHepOx) as B block in different compositions and with different cross-linkable end groups were prepared and crosslinked. The resulting amphiphilic polymer conetworks were characterized with atomic force microscopy (AFM) and small as well as wide angle X-ray scattering (SAXS/WAXS). The results show that the PHepOx block crystallizes in the APCNs if the content is larger than 19. vol.%, but shows no thermal signature in the differential scanning calorimetry if the polymer content is below 68. vol.%. Further, the size of the polymer phase is the same in a composition range of 38-58. vol.% PHepOx. The phase size was confirmed by AFM. The found very regular interconnected structure over a wide range of compositions seems to be the generic structural motive in APCNs and is also formed when inserting a crystallizable polymer block. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.eurpolymj.2016.09.046
  • 2017 • 143 Development of electrically conductive microstructures based on polymer/CNT nanocomposites via two-photon polymerization
    Staudinger, U. and Zyla, G. and Krause, B. and Janke, A. and Fischer, D. and Esen, C. and Voit, B. and Ostendorf, A.
    Microelectronic Engineering 179 48-55 (2017)
    Femtosecond laser-induced two-photon polymerization (2PP) of carbon nanofiller doped polymers was utilized to produce electrically conductive microstructures, which are expected to be applicable as microelectronic components or micro-electromechanical systems in sensors. The nanocomposites were processed by compounding an inorganic-organic hybrid material with two different types (short and long) of single walled carbon nanotubes (SWCNTs). Different SWCNT contents were dispersed in the polymer by sonication to adjust the electrical conductivity of the nanocomposites. Low surface resistivity values of ~ 4.6 × 105 Ω/sq. could be measured for coated reference films with a thickness of 30 μm having an exceptionally low SWCNT content of 0.01 wt% of the long type of SWCNTs. In contrast, a higher minimum resistivity of 1.5 × 106 Ω/sq. was exhibited for composites with a higher content, 2 wt%, of short SWCNTs. The structural quality of the microstructures processed by 2PP was mainly influenced by the dispersion quality of the SWCNTs. To characterize the electrical conductivity, conductive atomic force microscopy was applied for the first time. In microstructures with 0.05 wt% of the long type of SWCNTs, a contact current could be detected over a wide range of the measured area visualizing the electrical conductive CNT network, which has not been reported before. © 2017
    view abstractdoi: 10.1016/j.mee.2017.04.024
  • 2017 • 142 Cross-Linking of a Hydrophilic, Antimicrobial Polycation toward a Fast-Swelling, Antimicrobial Superabsorber and Interpenetrating Hydrogel Networks with Long Lasting Antimicrobial Properties
    Strassburg, A. and Petranowitsch, J. and Paetzold, F. and Krumm, C. and Peter, E. and Meuris, M. and Köller, M. and Tiller, J.C.
    ACS Applied Materials and Interfaces 9 36573-36582 (2017)
    A hemocompatible, antimicrobial 3,4en-ionene (PBI) derived by polyaddition of trans-1,4-dibromo-2-butene and N,N,N′,N′-tetramethyl-1,3-propanediamine was cross-linked via its bromine end groups using tris(2-aminoethyl)amine (TREN) to form a fast-swelling, antimicrobial superabsorber. This superabsorber is taking up the 30-fold of its weight in 60 s and the granulated material is taking up 96-fold of its weight forming a hydrogel. It fully prevents growth of the bacterium Staphylococcus aureus. The PBI network was swollen with 2-hydroxyethyl acrylate and glycerol dimethacrylate followed by photopolymerization to form an interpenetrating hydrogel (IPH) with varying PBI content in the range of 2.0 to 7.8 wt %. The nanophasic structure of the IPH was confirmed by atomic force microscopy and transmission electron microscopy. The bacterial cells of the nosocomial strains Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa are killed on the IPH even at the lowest PBI concentration. The antimicrobial activity was retained after washing the hydrogels for up to 4 weeks. The IPHs show minor leaching of PBI far below its antimicrobial active concentration using a new quantitative test for PBI detection in solution. This leaching was shown to be insufficient to form an inhibition zone and killing bacterial cells in the surroundings of the IPH. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b10049
  • 2016 • 141 On the origin of contact resistances in graphene devices fabricated by optical lithography
    Chavarin, C.A. and Sagade, A.A. and Neumaier, D. and Bacher, G. and Mertin, W.
    Applied Physics A: Materials Science and Processing 122 1-5 (2016)
    The contact resistance is a key bottleneck limiting the performance of graphene-based electronic and optoelectronic devices. Using a combined approach of atomic force microscopy patterning, Kelvin probe force microscopy and micro-Raman mapping, we study the influence of optical lithography resists on the contact resistance in graphene devices. We find that devices fabricated by optical lithography show a significantly larger contact resistance compared to devices produced by electron beam lithography using polymethylmethacrylate as resist. This difference is attributed to a 3–4-nm-thick residual layer remaining in between the contact metal and the graphene after optical lithography. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-015-9582-5
  • 2016 • 140 Improved Antifouling Properties of Polydimethylsiloxane Films via Formation of Polysiloxane/Polyzwitterion Interpenetrating Networks
    Dundua, A. and Franzka, S. and Ulbricht, M.
    Macromolecular Rapid Communications 37 2030-2036 (2016)
    Nonspecific adsorption of proteins is a challenging problem for the development of biocompatible materials, as well as for antifouling and fouling-release coatings, for instance for the marine industry. The concept of preparing amphiphilic systems based on low surface energy hydrophobic materials via their hydrophilic modification is being widely pursued. This work describes a novel two-step route for the preparation of interpenetrating polymer networks of otherwise incompatible poly(dimethylsiloxane) and zwitterionic polymers. Changes in surface hydrophilicity as well as surface charge at different pH values are investigated. Characterization using atomic force microscopy provides thorough insight into surface changes upon hydrophilic modification. Protein fouling of the materials is assessed using fibrinogen as a model protein. (Figure presented.). © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/marc.201600473
  • 2016 • 139 Bound rubber morphology and loss tangent properties of carbon-black-filled rubber compounds
    Gabriel, D. and Karbach, A. and Drechsler, D. and Gutmann, J. and Graf, K. and Kheirandish, S.
    Colloid and Polymer Science 294 501-511 (2016)
    The bound rubber phenomenon of carbon-black-filled rubber compounds, which is still an intensively discussed subject, is visualized in this research as a stable nanoscale interphase. Using the novel amplitude and phase-modified atomic force microscope technique, a viscoelastic mapping mode, it becomes possible to quantify mechanical loss tangent properties that are defined as the ratio of loss modulus G″ to storage modulus G′. Imaging loss tangent enables the observation of separated energy dissipation of single constituents within a blend system as well as bound rubber dimensions. Determined with the conventional quantification of insoluble rubber, the amount of bound rubber is correlated with values from the analytical evaluation of loss tangent images. Comparing the loss tangent images and histograms to dynamic mechanical analyses allows the characterization of each single component. On the base of the time-temperature superposition principle, bound rubber dimensions and mechanical properties of filled compounds can be optimized. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00396-015-3802-6
  • 2016 • 138 Benchmarking the Performance of Thin-Film Oxide Electrocatalysts for Gas Evolution Reactions at High Current Densities
    Ganassin, A. and Maljusch, A. and Colic, V. and Spanier, L. and Brandl, K. and Schuhmann, W. and Bandarenka, A.
    ACS Catalysis 6 3017-3024 (2016)
    Oxide materials are among the state-of-the-art heterogeneous electrocatalysts for many important large-scale industrial processes, including O2 and Cl2 evolution reactions. However, benchmarking their performance is challenging in many cases, especially at high current densities, which are relevant for industrial applications. Serious complications arise particularly due to (i) the formation of a nonconducting gas phase which blocks the surface during the reactions, (ii) problems in determination of the real electroactive electrode area, and (iii) the large influence of surface morphology alterations (stability issues) under reaction conditions, among others. In this work, an approach overcoming many of these challenges is presented, with a focus on electrochemically formed thin-film oxide electrocatalysts. The approach is based on benefits provided by the use of microelectrodes, and it gives comprehensive information about the surface roughness, catalyst activity, and stability. The key advantages of the proposed method are the possibility of characterization of the whole microelectrode surface by means of atomic force microscopy and an accurate assessment of the specific activity (and subsequently stability) of the catalyst, even at very high current densities. Electrochemically deposited CoOx thin films have been used in this study as model catalysts. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00455
  • 2016 • 137 Swift heavy ion irradiation of CaF2 - From grooves to hillocks in a single ion track
    Gruber, E. and Salou, P. and Bergen, L. and El Kharrazi, M. and Lattouf, E. and Grygiel, C. and Wang, Y. and Benyagoub, A. and Levavasseur, D. and Rangama, J. and Lebius, H. and Ban-D'Etat, B. and Schleberger, M. and Aumayr, F.
    Journal of Physics Condensed Matter 28 (2016)
    A novel form of ion-tracks, namely nanogrooves and hillocks, are observed on CaF2 after irradiation with xenon and lead ions of about 100 MeV kinetic energy. The irradiation is performed under grazing incidence (0.3°-3°) which forces the track to a region in close vicinity to the surface. Atomic force microscopy imaging of the impact sites with high spatial resolution reveals that the surface track consists in fact of three distinct parts: each swift heavy ion impacting on the CaF2 surface first opens a several 100 nm long groove bordered by a series of nanohillocks on both sides. The end of the groove is marked by a huge single hillock and the further penetration of the swift projectile into deeper layers of the target is accompanied by a single protrusion of several 100 nm in length slowly fading until the track vanishes. By comparing experimental data for various impact angles with results of a simulation, based on a three-dimensional version of the two-temperature-model (TTM), we are able to link the crater and hillock formation to sublimation and melting processes of CaF2 due to the local energy deposition by swift heavy ions. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/40/405001
  • 2016 • 136 Silver nanoparticles with different size and shape: Equal cytotoxicity, but different antibacterial effects
    Helmlinger, J. and Sengstock, C. and Groß-Heitfeld, C. and Mayer, C. and Schildhauer, T.A. and Köller, M. and Epple, M.
    RSC Advances 6 18490-18501 (2016)
    The influence of silver nanoparticle morphology on the dissolution kinetics in ultrapure water as well as the biological effect on eukaryotic and prokaryotic cells was examined. Silver nanoparticles with different shapes but comparable size and identical surface functionalisation were prepared, i.e. spheres (diameter 40-80 and 120-180 nm; two different samples), platelets (20-60 nm), cubes (140-180 nm), and rods (diameter 80-120 nm, length > 1000 nm). All particles were purified by ultracentrifugation and colloidally stabilized with poly(N-vinyl pyrrolidone) (PVP). Their colloidal dispersion in ultrapure water and cell culture medium was demonstrated by dynamic light scattering. Size, shape, and colloidal stability were analysed by scanning electron microscopy, atomic force microscopy, dynamic light scattering, and differential centrifugal sedimentation. The dissolution in ultrapure water was proportional to the specific surface area of the silver nanoparticles. The averaged release rate for all particle morphologies was 30 ± 13 ng s-1 m-2 in ultrapure water (T = 25 ± 1°C; pH 4.8; oxygen saturation 93%), i.e. about 10-20 times larger than the release of silver from a macroscopic silver bar (1 oz), possibly due to the presence of surface defects in the nanoparticulate state. All particles were taken up by human mesenchymal stem cells and were cytotoxic in concentrations of >12.5 μg mL-1, but there was no significant influence of the particle shape on the cytotoxicity towards the cells. Contrary to that, the toxicity towards bacteria increased with a higher dissolution rate, suggesting that the toxic species against bacteria are dissolved silver ions. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5ra27836h
  • 2016 • 135 Formation of swift heavy ion tracks on a rutile TiO2 (001) surface
    Karlušic, M. and Bernstorff, S. and Siketic, Z. and Šantic, B. and Bogdanovic-Radovic, I. and Jakšic, M. and Schleberger, M. and Buljan, M.
    Journal of Applied Crystallography 49 1704-1712 (2016)
    Nanostructuring of surfaces and two-dimensional materials using swift heavy ions offers some unique possibilities owing to the deposition of a large amount of energy localized within a nanoscale volume surrounding the ion trajectory. To fully exploit this feature, the morphology of nanostructures formed after ion impact has to be known in detail. In the present work the response of a rutile TiO2 (001) surface to grazing-incidence swift heavy ion irradiation is investigated. Surface ion tracks with the well known intermittent inner structure were successfully produced using 23MeV I ions. Samples irradiated with different ion fluences were investigated using atomic force microscopy and grazing-incidence small-angle X-ray scattering. With these two complementary approaches, a detailed description of the swift heavy ion impact sites, i.e. the ion tracks on the surface, can be obtained even for the case of multiple ion track overlap. In addition to the structural investigation of surface ion tracks, the change in stoichiometry of the rutile TiO2 (001) surface during swift heavy ion irradiation was monitored using in situ time-of-flight elastic recoil detection analysis, and a preferential loss of oxygen was found.Formation of ion tracks on a rutile TiO2 (001) surface after exposure to swift heavy ions under grazing incidence is studied using atomic force microscopy, grazing-incidence small-angle X-ray scattering and in situ time-of-flight elastic recoil detection analysis and a preferential loss of oxygen was found. © Marko Karlušic et al. 2016.
    view abstractdoi: 10.1107/S1600576716013704
  • 2016 • 134 Preparing hydroxyapatite-silicon composite suspensions with homogeneous distribution of multi-walled carbon nano-tubes for electrophoretic coating of NiTi bone implant and their effect on the surface morphology
    Khalili, V. and Khalil-Allafi, J. and Xia, W. and Parsa, A.B. and Frenzel, J. and Somsen, C. and Eggeler, G.
    Applied Surface Science 366 158-165 (2016)
    Preparing a stable suspension is a main step towards the electrophoretically depositing of homogeneous and dense composite coatings on NiTi for its biomedical application. In the present study, different composite suspensions of hydroxyapatite, silicon and multi-walled carbon nano-tubes were prepared using n-butanol and triethanolamine as media and dispersing agent, respectively. Multi-walled carbon nanotubes were first functionalized in the nitric acid vapor for 15 h at 175 °C, and then mixed into suspensions. Thermal desorption spectroscopy profiles indicate the formation of functional groups on multi-walled carbon nano-tubes. An excellent suspension stability can be achieved for different amounts of triethanolamine. The amount of triethanolamine can be increased by adding a second component to a stable hydroxyapatite suspension due to an electrostatic interaction between components in suspension. The stability of composite suspension is less than that of the hydroxyapatite suspension, due to density differences, which under the gravitational force promote the demixing. The scanning electron microscopy images of the coatings surface show that more dense coatings are developed on NiTi substrate using electrophoretic deposition and sintering at 850 °C in the simultaneous presence of silicon and multi-walled carbon nanotubes in the hydroxyapatite coatings. The atomic force microscopy results of the coatings surface represent that composite coatings of hydroxyapatite-20 wt.% silicon and hydroxyapatite-20 wt.% silicon-1 wt.% multi-walled carbon nano-tubes with low zeta potential have rougher surfaces. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2016.01.053
  • 2016 • 133 Coupled molecular and cantilever dynamics model for frequency-modulated atomic force microscopy
    Klocke, M. and Wolf, D.E.
    Beilstein Journal of Nanotechnology 7 708-720 (2016)
    A molecular dynamics model is presented, which adds harmonic potentials to the atomic interactions to mimic the elastic properties of an AFM cantilever. It gives new insight into the correlation between the experimentally monitored frequency shift and cantilever damping due to the interaction between tip atoms and scanned surface. Applying the model to ionic crystals with rock salt structure two damping mechanisms are investigated, which occur separately or simultaneously depending on the tip position. These mechanisms are adhesion hysteresis on the one hand and lateral excitations of the cantilever on the other. We find that the short range Lennard-Jones part of the atomic interaction alone is sufficient for changing the predominant mechanism. When the long range ionic interaction is switched off, the two damping mechanisms occur with a completely different pattern, which is explained by the energy landscape for the apex atom of the tip. In this case the adhesion hysteresis is always associated with a distinct lateral displacement of the tip. It is shown how this may lead to a systematic shift between the periodic patterns obtained from the frequency and from the damping signal, respectively. © 2016 Klocke and Wolf.
    view abstractdoi: 10.3762/bjnano.7.63
  • 2016 • 132 Preparation and characterization of low fouling novel hybrid ultrafiltration membranes based on the blends of GO-TiO2 nanocomposite and polysulfone for humic acid removal
    Kumar, M. and Gholamvand, Z. and Morrissey, A. and Nolan, K. and Ulbricht, M. and Lawler, J.
    Journal of Membrane Science 506 38-49 (2016)
    In this work, graphene oxide (GO)-TiO2 nanocomposite was synthesized by in situ sol-gel reaction at pH=2 using GO nanosheets suspension and titanium isopropoxide precursor. The synthesized GO-TiO2 nanocomposite was explored as a filler to fabricate improved antifouling novel hybrid ultrafiltration membranes for removal of humic acid from aqueous solution. Membranes were fabricated from polymer blend solutions containing polysulfone and GO-TiO2 with varied loading amount (0-5 wt%) by the non-solvent induced phase separation (NIPS) method. Contact angle, atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and outer surface zeta potential studies were conducted in order to characterise the membranes in terms of roughness, structure, surface properties and charge. The porous hydrophilic hybrid membranes were shown to have an asymmetric structure with improved surface roughness. The water permeability and antifouling capacity of hybrid membranes with 10 ppm HA solution were dependent on the loading amount of GO-TiO2. Incorporation of GO-TiO2 nanocomposite was found to improve the antifouling characteristics of the membranes when challenged with HA solutions. Irreversible HA fouling was substantially reduced with increased loading of GO-TiO2 nanocomposite (wt%). The lowest irreversible fouling ratio (3.2%) was obtained for the membrane containing 5 wt% nanocomposite (to total wt% of PSf, MG-5). Ultrafiltration of HA solutions of varied concentrations using hybrid membranes was studied at pH=7 and 1 bar feed pressure. The removal efficiency of hybrid membranes for HA was controlled by the membrane surface charge concentration, porosity and HA exclusion. The membrane MG-5 had the highest HA removal efficiency for 10 ppm HA solution at pH=7. © 2016 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2016.02.005
  • 2016 • 131 Incorporation of a Non-Natural Arginine Analogue into a Cyclic Peptide Leads to Formation of Positively Charged Nanofibers Capable of Gene Transfection
    Li, M. and Ehlers, M. and Schlesiger, S. and Zellermann, E. and Knauer, S.K. and Schmuck, C.
    Angewandte Chemie - International Edition 55 598-601 (2016)
    Functionalization of the tetracationic cyclic peptide (Ka)4 with a single guanidiniocarbonyl pyrrole (GCP) moiety, a weakly basic but highly efficient arginine analogue, completely alters the self-assembly properties of the peptide. In contrast to the nonfunctionalized peptide 2, which does not self-assemble, GCP-containing peptide 1 forms cationic nanofibers of micrometer length. These aggregates are efficient gene transfection vectors. DNA binds to their cationic surface and is efficiently delivered into cells. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201508714
  • 2016 • 130 Understanding of the importance of the spore coat structure and pigmentation in the Bacillus subtilis spore resistance to low-pressure plasma sterilization
    Raguse, M. and Fiebrandt, M. and Denis, B. and Stapelmann, K. and Eichenberger, P. and Driks, A. and Eaton, P. and Awakowicz, P. and Moeller, R.
    Journal of Physics D: Applied Physics 49 (2016)
    Low-pressure plasmas have been evaluated for their potential in biomedical and defense purposes. The sterilizing effect of plasma can be attributed to several active agents, including (V)UV radiation, charged particles, radical species, neutral and excited atoms and molecules, and the electric field. Spores of Bacillus subtilis were used as a bioindicator and a genetic model system to study the sporicidal effects of low-pressure plasma decontamination. Wild-type spores, spores lacking the major protective coat layers (inner, outer, and crust), pigmentation-deficient spores or spore impaired in encasement (a late step in coat assembly) were systematically tested for their resistance to low-pressure argon, hydrogen, and oxygen plasmas with and without admixtures. We demonstrate that low-pressure plasma discharges of argon and oxygen discharges cause significant physical damage to spore surface structures as visualized by atomic force microscopy. Spore resistance to low-pressure plasma was primarily dependent on the presence of the inner, and outer spore coat layers as well as spore encasement, with minor or less importance of the crust and spore pigmentation, whereas spore inactivation itself was strongly influenced by the gas composition and operational settings. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/28/285401
  • 2016 • 129 Antifouling and Antibacterial Multifunctional Polyzwitterion/Enzyme Coating on Silicone Catheter Material Prepared by Electrostatic Layer-by-Layer Assembly
    Vaterrodt, A. and Thallinger, B. and Daumann, K. and Koch, D. and Guebitz, G.M. and Ulbricht, M.
    Langmuir 32 1347-1359 (2016)
    The formation of bacterial biofilms on indwelling medical devices generally causes high risks for adverse complications such as catheter-associated urinary tract infections. In this work, a strategy for synthesizing innovative coatings of poly(dimethylsiloxane) (PDMS) catheter material, using layer-by-layer assembly with three novel functional polymeric building blocks, is reported, i.e., an antifouling copolymer with zwitterionic and quaternary ammonium side groups, a contact biocidal derivative of that polymer with octyl groups, and the antibacterial hydrogen peroxide (H2O2) producing enzyme cellobiose dehydrogenase (CDH). CDH oxidizes oligosaccharides by transferring electrons to oxygen, resulting in the production of H2O2. The design and synthesis of random copolymers which combine segments that have antifouling properties by zwitterionic groups and can be used for electrostatically driven layer-by-layer (LbL) assembly at the same time were based on the atom-transfer radical polymerization of dimethylaminoethyl methacrylate and subsequent partial sulfobetainization with 1,3-propane sultone followed by quaternization with methyl iodide only or octyl bromide and thereafter methyl iodide. The alternating multilayer systems were formed by consecutive adsorption of the novel polycations with up to 50% zwitterionic groups and of poly(styrenesulfonate) as the polyanion. Due to its negative charge, enzyme CDH was also firmly embedded as a polyanionic layer in the multilayer system. This LbL coating procedure was first performed on prefunctionalized silicon wafers and studied in detail with ellipsometry as well as contact angle (CA) and zetapotential (ZP) measurements before it was transferred to prefunctionalized PDMS and analyzed by CA and ZP measurements as well as atomic force microscopy. The coatings comprising six layers were stable and yielded a more neutral and hydrophilic surface than did PDMS, the polycation with 50% zwitterionic groups having the largest effect. Enzyme activity was found to be dependent on the depth of embedment in the multilayer coating. Depending on the used polymeric building block, up to a 60% reduction in the amount of adhering bacteria and clear evidence for killed bacteria due to the antimicrobial functionality of the coating could be confirmed. Overall, this work demonstrates the feasibility of an easy to perform and shape-independent method for preparing an antifouling and antimicrobial coating for the significant reduction of biofilm formation and thus reducing the risk of acquiring infections by using urinary catheters. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b04303
  • 2016 • 128 Understanding surface reactivity of Si electrodes in Li-ion batteries by: In operando scanning electrochemical microscopy
    Ventosa, E. and Wilde, P. and Zinn, A.-H. and Trautmann, M. and Ludwig, Al. and Schuhmann, W.
    Chemical Communications 52 6825-6828 (2016)
    In operando SECM is employed to monitor the evolution of the electrically insulating character of a Si electrode surface during (de-)lithiation. The solid-electrolyte interface (SEI) formed on Si electrodes is shown to be intrinsically electrically insulating. However, volume changes upon (de-)lithiation lead to the loss of the protecting character of the initially formed SEI. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6cc02493a
  • 2015 • 127 Carbon Monoxide-Induced Stability and Atomic Segregation Phenomena in Shape-Selected Octahedral PtNi Nanoparticles
    Ahmadi, M. and Cui, C. and Mistry, H. and Strasser, P. and Roldan Cuenya, B.
    ACS Nano 9 10686-10694 (2015)
    The chemical and morphological stability of size- and shape-selected octahedral PtNi nanoparticles (NP) were investigated after different annealing treatments up to a maximum temperature of 700 °C in a vacuum and under 1 bar of CO. Atomic force microscopy was used to examine the mobility of the NPs and their stability against coarsening, and X-ray photoelectron spectroscopy to study the surface composition, chemical state of Pt and Ni in the NPs, and thermally and CO-induced atomic segregation trends. Exposing the samples to 1 bar of CO at room temperature before annealing in a vacuum was found to be effective at enhancing the stability of the NPs against coarsening. In contrast, significant coarsening was observed when the sample was annealed in 1 bar of CO, most likely as a result of Ni(CO)4 formation and their enhanced mobility on the support surface. Sample exposure to CO at room temperature prior to annealing led to the segregation of Pt to the NP surface. Nevertheless, oxidic PtOx and NiOx species still remained at the NP surface, and, irrespective of the initial sample pretreatment, Ni surface segregation was observed upon annealing in a vacuum at moderate temperature (T < 300 °C). Interestingly, a distinct atomic segregation trend was detected between 300 and 500 °C for the sample pre-exposed to CO; namely, Ni surface segregation was partially hindered. This might be attributed to the higher bonding energy of CO to Pt as compared to Ni. Annealing in the presence of 1 bar CO also resulted in the initial surface segregation of Ni (T < 400 °C) as long as PtOx and NiOx species were available on the surface as a result of the higher affinity of Ni for oxygen. Above 500 °C, and regardless of the sample pretreatment, the diffusion of Pt atoms to the NP surface and the formation of a Ni-Pt alloy are observed. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b01807
  • 2015 • 126 Van der Waals epitaxial MOCVD-growth of (BixSb1-x)2Te3 (0 < x < 1) films
    Bendt, G. and Sonntag, J. and Lorke, A. and Assenmacher, W. and Hagemann, U. and Schulz, S.
    Semiconductor Science and Technology 30 (2015)
    Epitaxial (Bi<inf>x</inf>Sb<inf>1-x</inf>)<inf>2</inf>Te<inf>3</inf> films with (0 < x < 1) were grown by the metal-organic chemical vapour deposition (MOCVD) process at 400 °C using the tailor-made precursors Et<inf>2</inf>Te<inf>2</inf>, i-Pr<inf>3</inf>Sb and Et<inf>3</inf>Bi. The films grown on Al<inf>2</inf>O<inf>3</inf>(0001) substrates show a very smooth surface morphology as shown by a scanning electron microscope (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM), while those grown on Si(100) are rather polycrystalline. The chemical composition of the crystalline films (x-ray powder diffraction (XRD)) was investigated by energy-dispersive x-ray (EDX) and x-ray photoelectron spectroscopy (XPS), and the in-plane transport properties were measured, and a strong dependency from the bismuth content was found, which allows the tuning of the carrier concentration and mobility in a wide range. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0268-1242/30/8/085021
  • 2015 • 125 Shear-Induced Detachment of Polystyrene Beads from SAM-Coated Surfaces
    Cho, K.L. and Rosenhahn, A. and Thelen, R. and Grunze, M. and Lobban, M. and Karahka, M.L. and Kreuzer, H.J.
    Langmuir 31 11105-11112 (2015)
    In this work we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow in order to develop a mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated as a thermally activated process applying an Arrhenius Ansatz to determine the activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data obtained in phosphate-buffered saline buffer results in an activation energy around 20 kJ/mol, which is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We conclude that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction) and not physical detachment, i.e., lifting the particle off the surface. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b02321
  • 2015 • 124 Isotropic macroporous polyethersulfone membranes as competitive supports for high performance polyamide desalination membranes
    ElSherbiny, I.M.A. and Ghannam, R. and Khalil, A.S.G. and Ulbricht, M.
    Journal of Membrane Science 493 782-793 (2015)
    Novel macroporous isotropic polyethersulfone (PES) base membranes were developed using combined processes of vapor- and non-solvent-induced phase separation. Optimization of different preparation parameters was carried out. The newly prepared PES membranes exhibited well-defined isotropic porous structures with optimized average barrier pore diameter of 100. nm as well as hydrophilic surface and high water permeability. These isotropic membranes together with two more supports (i.e. commercial PES microfiltration and anisotropic hydrophobic polysulfone membranes), were utilized for the fabrication of polyamide (PA) thin-film composite (TFC) desalination membranes. The resulted PA TFC membranes showed significantly different film morphologies, surface characteristics as well as separation performance. The PA TFC membranes based on the hydrophilic PES supports with isotropic and optimized pore size, developed in this study, showed superior water permeability compared to the composite membranes based on the other supports, without compromising the salt rejection and providing high stability for the PA selective layer. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2015.05.064
  • 2015 • 123 Correlation between surface properties and wettability of multi-scale structured biocompatible surfaces
    Gorodzha, S.N. and Surmeneva, M.A. and Prymak, O. and Wittmar, A. and Ulbricht, M. and Epple, M. and Teresov, A. and Koval, N. and Surmenev, R.A.
    IOP Conference Series: Materials Science and Engineering 98 (2015)
    The influence of surface properties of radio-frequency (RF) magnetron deposited hydroxyapatite (HA) and Si-containing HA coatings on wettability was studied. The composition and morphology of the coatings fabricated on titanium (Ti) were characterized using atomic force microscopy (AFM) and X-ray diffraction (XRD). The surface wettability was studied using contact angle analysis. Different geometric parameters of acid-etched (AE) and pulse electron beam (PEB)-treated Ti substrates and silicate content in the HA films resulted in the different morphology of the coatings at micro- and nano- length scales. Water contact angles for the HA coated Ti samples were evaluated as a combined effect of micro roughness of the substrate and nano-roughness of the HA films resulting in higher water contact angles compared with acid-etched (AE) or pulse electron beam (PEB) treated Ti substrates. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/98/1/012026
  • 2015 • 122 Response of GaN to energetic ion irradiation: Conditions for ion track formation
    Karlušić, M. and Kozubek, R. and Lebius, H. and Ban-d'Etat, B. and Wilhelm, R.A. and Buljan, M. and Siketić, Z. and Scholz, F. and Meisch, T. and Jakšić, M. and Bernstorff, S. and Schleberger, M. and Šantić, B.
    Journal of Physics D: Applied Physics 48 (2015)
    We investigated the response of wurzite GaN thin films to energetic ion irradiation. Both swift heavy ions (92 MeV Xe23+, 23 MeV I6+) and highly charged ions (100 keV Xe40+) were used. After irradiation, the samples were investigated using atomic force microscopy, grazing incidence small angle x-ray scattering, Rutherford backscattering spectroscopy in channelling orientation and time of flight elastic recoil detection analysis. Only grazing incidence swift heavy ion irradiation induced changes on the surface of the GaN, when the appearance of nanoholes is accompanied by a notable loss of nitrogen. The results are discussed in the framework of the thermal spike model. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/32/325304
  • 2015 • 121 MOCVD of TiO2 thin films from a modified titanium alkoxide precursor
    Kim, S.J. and Dang, V.-S. and Xu, K. and Barreca, D. and Maccato, C. and Carraro, G. and Bhakta, R.K. and Winter, M. and Becker, H.-W. and Rogalla, D. and Sada, C. and Fischer, R.A. and Devi, A.
    Physica Status Solidi (A) Applications and Materials Science 212 1563-1570 (2015)
    A new titanium precursor, [Ti(OPri)<inf>2</inf>(deacam)<inf>2</inf>] (deacam = N,N-diethylacetoacetamide), was developed by the reaction of the parent Ti alkoxide with the β-ketoamide. The compound, obtained as a monomeric six-coordinated complex, was used in metal organic chemical vapor deposition (MOCVD) of TiO<inf>2</inf> both as a single source precursor (SSP) and in the presence of oxygen. The high thermal stability of [Ti(OPri)<inf>2</inf>(deacam)<inf>2</inf>] enabled the fabrication of TiO<inf>2</inf> films over a wide temperature range, with steady growth rates between 500 and 800 °C. The microstructure of the obtained systems was analyzed by X-ray diffraction (XRD) and Raman spectroscopy, whereas atomic force microscopy (AFM) and field emission-scanning electron microscopy (FE-SEM) measurements were performed to investigate the surface morphology and nanoorganization. Film composition was investigated by complementary techniques like Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). The electrical properties of the layers were investigated by performing capacitance voltage (C-V) and leakage current measurements. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/pssa.201532271
  • 2015 • 120 Wet Nanoindentation of the Solid Electrolyte Interphase on Thin Film Si Electrodes
    Kuznetsov, V. and Zinn, A.-H. and Zampardi, G. and Borhani-Haghighi, S. and La Mantia, F. and Ludwig, Al. and Schuhmann, W. and Ventosa, E.
    ACS Applied Materials and Interfaces 7 23554-23563 (2015)
    The solid electrolyte interphase (SEI) film formed at the surface of negative electrodes strongly affects the performance of a Li-ion battery. The mechanical properties of the SEI are of special importance for Si electrodes due to the large volumetric changes of Si upon (de)insertion of Li ions. This manuscript reports the careful determination of the Young's modulus of the SEI formed on a sputtered Si electrode using wet atomic force microscopy (AFM)-nanoindentation. Several key parameters in the determination of the Young's modulus are considered and discussed, e.g., wetness and roughness-thickness ratio of the film and the shape of a nanoindenter. The values of the Young's modulus were determined to be 0.5-10 MPa under the investigated conditions which are in the lower range of those previously reported, i.e., 1 MPa to 10 GPa, pointing out the importance of the conditions of its determination. After multiple electrochemical cycles, the polymeric deposits formed on the surface of the SEI are revealed, by force-volume mapping in liquid using colloidal probes, to extend up to 300 nm into bulk solution. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b06700
  • 2015 • 119 Temperature dependence of surface reorganization characteristics of amphiphilic block copolymer in air and in water studied by scanning force microscopy
    Lee, S. and Flores, S.M. and Berger, R. and Gutmann, J.S. and Brehmer, M. and Conrad, L. and Funk, L. and Theato, P. and Yoon, D.Y.
    Journal of Plastic Film and Sheeting 31 434-448 (2015)
    We have investigated the surface reorganization characteristics of a novel amphiphilic diblock copolymer, poly(acetic acid-2-(2-(4-vinyl-phenoxy)-ethoxy)-ethylester)-block-polystyrene (PAEES-b-PS), in response to varying interfaces from air to water and vice-versa at various temperatures. The surface reorganization characteristics of the block copolymer films was monitored by scanning force microscopy, in order to delineate the kinetically controlled morphological process of surface reorganization of a diblock copolymer, with a particular emphasis on the phase contrast signal which allowed the determination of local composition patterns of PAEES-b-PS at the surface. Upon heating a water-annealed sample in air, the initially hydrophilic liquid-like surface exhibited a typical dewetting pattern comprising holes and elevations of different copolymer components. In contrast, air-annealed samples with glassy polystyrene surfaces exhibited a distinctly different reorganization pattern upon heating in water, possibly due to the swelling of the underlying liquid-like hydrophilic block by penetrated water. In both air and water environments, the major surface reorganization occurred around 70, well below the glass transition temperature (100) of the higher T<inf>g</inf> block, polystyrene, in the copolymer. ©The Author(s) 2015.
    view abstractdoi: 10.1177/8756087915595261
  • 2015 • 118 A molecular tweezer antagonizes seminal amyloids and HIV infection
    Lump, E. and Castellano, L.M. and Meier, C. and Seeliger, J. and Erwin, N. and Sperlich, B. and Stürzel, C.M. and Usmani, S. and Hammond, R.M. and Von Einem, J. and Gerold, G. and Kreppel, F. and Bravo-Rodriguez, K. and Pietschma...
    eLife 4 (2015)
    Semen is the main vector for HIV transmission and contains amyloid fibrils that enhance viral infection. Available microbicides that target viral components have proven largely ineffective in preventing sexual virus transmission. In this study, we establish that CLR01, a ‘molecular tweezer’ specific for lysine and arginine residues, inhibits the formation of infectivity-enhancing seminal amyloids and remodels preformed fibrils. Moreover, CLR01 abrogates semen-mediated enhancement of viral infection by preventing the formation of virion–amyloid complexes and by directly disrupting the membrane integrity of HIV and other enveloped viruses.We establish that CLR01 acts by binding to the target lysine and arginine residues rather than by a non-specific, colloidal mechanism. CLR01 counteracts both host factors that may be important for HIV transmission and the pathogen itself. These combined anti-amyloid and antiviral activities make CLR01 a promising topical microbicide for blocking infection by HIV and other sexually transmitted viruses. © 2015, Lump et al.
    view abstractdoi: 10.7554/eLife.05397
  • 2015 • 117 Adhesion measurement of a buried Cr interlayer on polyimide
    Marx, V.M. and Kirchlechner, C. and Zizak, I. and Cordill, M.J. and Dehm, G.
    Philosophical Magazine 95 1982-1991 (2015)
    A fundamental knowledge and understanding of the adhesion behaviour of metal-polymer systems is important as interface failure leads to a complete breakdown of flexible devices. A combination of in situ atomic force microscopy for studying topological changes and in situ synchrotron based stress measurements both during film tensile testing were used to estimate the adhesion energy of a thin bilayer film. The film systems consisted of 50-200 nm Cu with a 10 nm Cr adhesion layer on 50 μm thick polyimide. If the Cu film thickness is decreased to 50 nm the Cr interlayer starts dominating the system behaviour. An apparent transition from plastic to predominantly brittle deformation behaviour of the Cu can be observed. Then, compressive stresses in the transverse direction are high enough to cause delamination and buckling of the Cr interlayer from the substrate. This opens a new route to induce buckling of a brittle interlayer between a ductile film and a compliant substrate which is used to determine the interfacial adhesion energy. © 2015 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2014.920543
  • 2015 • 116 The influence of a brittle Cr interlayer on the deformation behavior of thin Cu films on flexible substrates: Experiment and model
    Marx, V.M. and Toth, F. and Wiesinger, A. and Berger, J. and Kirchlechner, C. and Cordill, M.J. and Fischer, F.D. and Rammerstorfer, F.G. and Dehm, G.
    Acta Materialia 89 278-289 (2015)
    Thin metal films deposited on polymer substrates are used in flexible electronic devices such as flexible displays or printed memories. They are often fabricated as complicated multilayer structures. Understanding the mechanical behavior of the interface between the metal film and the substrate as well as the process of crack formation under global tension is important for producing reliable devices. In the present work, the deformation behavior of copper films (50-200 nm thick), bonded to polyimide directly or via a 10 nm chromium interlayer, is investigated by experimental analysis and computational simulations. The influence of the various copper film thicknesses and the usage of a brittle interlayer on the crack density as well as on the stress magnitude in the copper after saturation of the cracking process are studied with in situ tensile tests in a synchrotron and under an atomic force microscope. From the computational point of view, the evolution of the crack pattern is modeled as a stochastic process via finite element based cohesive zone simulations. Both, experiments and simulations show that the chromium interlayer dominates the deformation behavior. The interlayer forms cracks that induce a stress concentration in the overlying copper film. This behavior is more pronounced in the 50 nm than in the 200 nm copper films. © Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2015.01.047
  • 2015 • 115 Impact of bacterial endotoxin on the structure of DMPC membranes
    Nagel, M. and Brauckmann, S. and Moegle-Hofacker, F. and Effenberger-Neidnicht, K. and Hartmann, M. and De Groot, H. and Mayer, C.
    Biochimica et Biophysica Acta - Biomembranes 1848 2271-2276 (2015)
    Abstract Bacterial lipopolysaccharides are believed to have a toxic effect on human cell membranes. In this study, the influence of a lipopolysaccharide (LPS) from Escherichia coli on the structure, the dynamics and the mechanical strength of phospholipid membranes are monitored by nuclear magnetic resonance spectroscopy (NMR) and by atomic force microscopy (AFM). Model membranes are formed from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and are either prepared as multilamellar bulk samples or multilamellar vesicles. Field gradient NMR data directly prove the rapid integration of LPS into DMPC membranes. Solid state NMR experiments primarily detect decreasing molecular order parameters with increasing LPS content. This is accompanied by a mechanical softening of the membrane bilayers as is shown by AFM indentation measurements. Altogether, the data prove that lipopolysaccharide molecules quickly insert into phospholipid bilayers, increase membrane fluctuation amplitudes and significantly weaken their mechanical stiffness. © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.bbamem.2015.06.008
  • 2015 • 114 Soft Nanocomposites-From Interface Control to Interphase Formation
    Pihan, S.A. and Emmerling, S.G.J. and Butt, H.-J. and Berger, R. and Gutmann, J.S.
    ACS Applied Materials and Interfaces 7 12380-12386 (2015)
    We report measurements of structure, mechanical properties, glass transition temperature, and contact angle of a novel nanocomposite material consisting of swellable silsesquioxane nanoparticles with grafted poly(ethyl methacrylate) (PEMA) brushes and PEMA matrices with varying molecular weight. We measured the interparticle distance at the surface of the composites using scanning probe microscopy (SPM) and in the bulk of ∼0.5 μm thick films by grazing incidence small angle X-ray scattering (GISAXS). For a given molecular weight of the brush unstable dispersions at high molecular weight of the matrix indicate an intrinsic incompatibility between polymer-grafted-nanoparticles and homopolymer matrices. This incompatibility is affirmed by a high contact angle between the polymer-grafted-nanoparticles and the high molecular weight matrix as measured by SPM. For unstable dispersions, we measured a decreased glass transition temperature along with a decreased plateau modulus by dynamic mechanical thermal analysis (DMTA) which indicates the formation of a liquid-like layer at the brush-matrix interface. This proves the ability to decouple the structural and mechanical properties from the potential to be swollen with small molecules. It opens a new area of use of these soft nanocomposites as slow release materials with tailored mechanical properties. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/am507572q
  • 2015 • 113 Determinants of amyloid fibril degradation by the PDZ protease HTRA1
    Poepsel, S. and Sprengel, A. and Saccà, B. and Kaschani, F. and Kaiser, M. and Gatsogiannis, C. and Raunser, S. and Clausen, T. and Ehrmann, M.
    Nature Chemical Biology 11 862-869 (2015)
    Excessive aggregation of proteins has a major impact on cell fate and is a hallmark of amyloid diseases in humans. To resolve insoluble deposits and to maintain protein homeostasis, all cells use dedicated protein disaggregation, protein folding and protein degradation factors. Despite intense recent research, the underlying mechanisms controlling this key metabolic event are not well understood. Here, we analyzed how a single factor, the highly conserved serine protease HTRA1, degrades amyloid fibrils in an ATP-independent manner. This PDZ protease solubilizes protein fibrils and disintegrates the fibrillar core structure, allowing productive interaction of aggregated polypeptides with the active site for rapid degradation. The aggregate burden in a cellular model of cytoplasmic tau aggregation is thus reduced. Mechanistic aspects of ATP-independent proteolysis and its implications in amyloid diseases are discussed. © 2015 Nature America, Inc. All rights reserved.
    view abstractdoi: 10.1038/nchembio.1931
  • 2015 • 112 Reversible reconfiguration of DNA origami nanochambers monitored by single-molecule FRET
    Saccà, B. and Ishitsuka, Y. and Meyer, R. and Sprengel, A. and Schöneweiß, E.-C. and Nienhaus, G.U. and Niemeyer, C.M.
    Angewandte Chemie - International Edition 54 3592-3597 (2015)
    Today, DNA nanotechnology is one of the methods of choice to achieve spatiotemporal control of matter at the nanoscale. By combining the peculiar spatial addressability of DNA origami structures with the switchable mechanical movement of small DNA motifs, we constructed reconfigurable DNA nanochambers as dynamic compartmentalization systems. The reversible extension and contraction of the inner cavity of the structures was used to control the distance-dependent energy transfer between two preloaded fluorophores. Interestingly, single-molecule FRET studies revealed that the kinetics of the process are strongly affected by the choice of the switchable motifs and/or actuator sequences, thus offering a valid method for fine-tuning the dynamic properties of large DNA nanostructures. We envisage that the proposed DNA nanochambers may function as model structures for artificial biomimetic compartments and transport systems. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201408941
  • 2015 • 111 Two-component self-assembly of a tetra-guanidiniocarbonyl pyrrole cation and Na4EDTA: Formation of pH switchable supramolecular networks
    Samanta, K. and Schmuck, C.
    Chemical Communications 51 16065-16067 (2015)
    A guanidiniocarbonyl pyrrole (GCP) cation forms stable H-bond assisted ion pairs with carboxylates even in aqueous solutions. A tetra GCP cation 1 undergoes efficient two-component self-assembly with Na4EDTA, a tetra-carboxylate, leading to 3D supramolecular networks. These networks show dual pH responsiveness and reversibly dissociate back into monomers upon addition of either acid or base. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5cc06392b
  • 2015 • 110 Resonant photothermal laser processing of hybrid gold/titania nanoparticle films
    Schade, L. and Franzka, S. and Dzialkowski, K. and Hardt, S. and Wiggers, H. and Reichenberger, S. and Wagener, P. and Hartmann, N.
    Applied Surface Science 336 48-52 (2015)
    Photothermal processing of thin anatase TiO2 and hybrid Au/anatase TiO2 nanoparticle films on glass supports is investigated using continuous-wave microfocused lasers at λ = 355 nm and λ = 532 nm. UV/Vis spectroscopy, Raman spectroscopy, optical microscopy, atomic force microscopy and scanning electron microscopy are used for characterization. Processing of TiO2 nanoparticle films is feasible at λ = 355 nm only. In contrast, the addition of Au nanoparticles enhances the overall absorbance of the material in the visible range and enables processing at both wavelengths, i.e. at λ = 355 nm and λ = 532 nm. Generally, laser heating induces a transition from anatase to rutile. The modification degree increases with increasing laser power and laser irradiation time. Resonant laser processing of hybrid Au/TiO2-mesoporous films provide promising perspectives in various applications, e.g. in photovoltaics, where embedded nanoparticulate Au could be exploited to enhance light trapping. © 2014 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2014.09.118
  • 2015 • 109 Avalanche-Discharge-Induced Electrical Forming in Tantalum Oxide-Based Metal-Insulator-Metal Structures
    Skaja, K. and Baumer, C. and Peters, O. and Menzel, S. and Moors, M. and Du, H. C. and Bornhofft, M. and Schmitz, C. and Feyer, V. and Jia, C. L. and Schneider, C. M. and Mayer, J. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 7154--7162 (2015)
    Oxide-based metal-insulator-metal structures are of special interest for future resistive random-access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite-like structure, which is induced by an avalanche discharge between the top electrode and the Ta2O5- x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite-like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta4+ and Ta5+, which takes place over the entire former Pt/Ta2O5- x interface of the dendrite-like structure.
    view abstractdoi: 10.1002/adfm.201502767
  • 2015 • 108 Conductivity Mechanisms in Sb-Doped SnO2 Nanoparticle Assemblies: DC and Terahertz Regime
    Skoromets, V. and Němec, H. and Kopeček, J. and Kužel, P. and Peters, K. and Fattakhova-Rohlfing, D. and Vetushka, A. and Müller, M. and Ganzerová, K. and Fejfar, A.
    Journal of Physical Chemistry C 119 19485-19495 (2015)
    Assemblies of undoped and antimony-doped tin oxide nanoparticles synthesized via a nonaqueous sol-gel procedure, pressed into pellets, and annealed under various conditions were investigated using time-domain terahertz spectroscopy, scanning electron microscopy, atomic force microscopy, and dc conductivity measurements. Combination of these methods made it possible to resolve the conductivity limitations imposed by intrinsic properties of the material and by the morphology of the samples. Percolation of the nanoparticles was confirmed in all samples. The undoped samples exhibit a weak hopping conductivity, whereas bandlike conduction of charges partially confined in the nanoparticles dominates in the doped samples. The conductivity of nanoparticles and their connectivity can be greatly controlled during the sample preparation, namely by the calcination temperature and by the order of technological steps. A substantial increase of the conductivity inside nanoparticles and of the charge transport between them is achieved upon calcination at 500 °C. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b05091
  • 2015 • 107 Non-destructive Patterning of Carbon Electrodes by Using the Direct Mode of Scanning Electrochemical Microscopy
    Stratmann, L. and Clausmeyer, J. and Schuhmann, W.
    ChemPhysChem 16 3477-3482 (2015)
    Patterning of glassy carbon surfaces grafted with a layer of nitrophenyl moieties was achieved by using the direct mode of scanning electrochemical microscopy (SECM) to locally reduce the nitro groups to hydroxylamine and amino functionalities. SECM and atomic force microscopy (AFM) revealed that potentiostatic pulses applied to the working electrode lead to local destruction of the glassy carbon surface, most likely caused by etchants generated at the positioned SECM tip used as the counter electrode. By applying galvanostatic pulses, and thus, limiting the current during structuring, corrosion of the carbon surface was substantially suppressed. After galvanostatic patterning, unambiguous proof of the formation of the anticipated amino moieties was possible by modulation of the pH value during the feedback mode of SECM imaging. This patterning strategy is suitable for the further bio-modification of microstructured surfaces. Alkaline phosphatase, as a model enzyme, was locally bound to the modified areas, thus showing that the technique can be used for the development of protein microarrays. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201500585
  • 2015 • 106 Poly(N,N-dimethylaminoethyl methacrylate) Brushes: PH-Dependent Switching Kinetics of a Surface-Grafted Thermoresponsive Polyelectrolyte
    Thomas, M. and Gajda, M. and Amiri Naini, C. and Franzka, S. and Ulbricht, M. and Hartmann, N.
    Langmuir 31 13426-13432 (2015)
    The temperature-dependent switching behavior of poly(N,N-dimethylaminoethyl methacrylate) brushes in alkaline, neutral, and acidic solutions is examined. A novel microscopic laser temperature-jump technique is employed in order to study characteristic thermodynamic and kinetic parameters. Static laser micromanipulation experiments allow one to determine the temperature-dependent variation of the swelling ratio. The data reveal a strong shift of the volume phase transition of the polymer brushes to higher temperatures when going from pH = 10 to pH = 4. Dynamic laser micromanipulation experiments offer a temporal resolution on a submillisecond time scale and provide a means to determine the intrinsic rate constants. Both the swelling and the deswelling rates strongly decrease in acidic solutions. Complementary experiments using in situ atomic force microscopy show an increased polymer layer thickness at these conditions. The data are discussed on the basis of pH-dependent structural changes of the polymer brushes including protonation of the amine groups and conformational rearrangements. Generally, repulsive electrostatic interactions and steric effects are assumed to hamper and slow down temperature-induced switching in acidic solutions. This imposes significant restrictions for smart polymer surfaces, sensors, and devices requiring fast response times. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b03448
  • 2015 • 105 Chemical vapor deposition of Si/SiC nano-multilayer thin films
    Weber, A. and Remfort, R. and Wöhrl, N. and Assenmacher, W. and Schulz, S.
    Thin Solid Films 593 44-52 (2015)
    Stoichiometric SiC films were deposited with the commercially available single source precursor Et3SiH by classical thermal chemical vapor deposition (CVD) as well as plasma-enhanced CVD at low temperatures in the absence of any other reactive gases. Temperature-variable deposition studies revealed that polycrystalline films containing different SiC polytypes with a Si to carbon ratio of close to 1:1 are formed at 1000°C in thermal CVD process and below 100°C in the plasma-enhanced CVD process. The plasma enhanced CVD process enables the reduction of residual stress in the deposited films and offers the deposition on temperature sensitive substrates in the future. In both deposition processes the film thickness can be controlled by variation of the process parameters such as the substrate temperature and the deposition time. The resulting material films were characterized with respect to their chemical composition and their crystallinity using scanning electron microscope, energy dispersive X-ray spectroscopy (XRD), atomic force microscopy, X-ray diffraction, grazing incidence X-ray diffraction, secondary ion mass spectrometry and Raman spectroscopy. Finally, Si/SiC multilayers of up to 10 individual layers of equal thickness (about 450 nm) were deposited at 1000°C using Et3SiH and SiH4. The resulting multilayers features amorphous SiC films alternating with Si films, which feature larger crystals up to 300 nm size as measured by transmission electron microscopy as well as by XRD. XRD features three distinct peaks for Si(111), Si(220) and Si(311). © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.08.042
  • 2015 • 104 Combined AFM/SECM Investigation of the Solid Electrolyte Interphase in Li-Ion Batteries
    Zampardi, G. and Klink, S. and Kuznetsov, V. and Erichsen, T. and Maljusch, A. and LaMantia, F. and Schuhmann, W. and Ventosa, E.
    ChemElectroChem 2 1607-1611 (2015)
    The solid electrolyte interphase (SEI) is an electronically insulating film formed from the decomposition of the organic electrolyte at the surface of the negative electrodes in Li-ion batteries (LIBs). This film is of vital importance in the performance and safety of LIBs. Atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) are combined in one platform for the consecutive insitu investigation of surface reactions in LIBs inside an Ar-filled glovebox. As proof of concept, the formation and the electrochemical properties of the SEI formed on glassy carbon electrodes are investigated. Changes in topography during film formation of the SEI are studied via AFM. The AFM tip is then used to partially remove a small area (50×50μm2) of the SEI, which is subsequently probed using SECM in feedback mode. The AFM-scratched spot is clearly visualized in the SECM image, demonstrating the strength of the AFM/SECM combination for the investigation in the field of LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500085
  • 2014 • 103 A Shape-adaptive, Antibacterial-coating of immobilized Quaternary-ammonium compounds tethered on hyperbranched polyurea and its mechanism of action
    Asri, L.A.T.W. and Crismaru, M. and Roest, S. and Chen, Y. and Ivashenko, O. and Rudolf, P. and Tiller, J.C. and Van Der Mei, H.C. and Loontjens, T.J.A. and Busscher, H.J.
    Advanced Functional Materials 24 346-355 (2014)
    Quaternary-ammonium-compounds are potent cationic antimicrobials used in everyday consumer products. Surface-immobilized, quaternary-ammonium-compounds create an antimicrobial contact-killing coating. We describe the preparation of a shape-adaptive, contact-killing coating by tethering quaternary-ammonium- compounds onto hyperbranched polyurea coatings, able to kill adhering bacteria by partially enveloping them. Even after extensive washing, coatings caused high contact-killing of Staphylococcus epidermidis, both in culture-based assays and through confocal-laser-scanning-microscopic examination of the membrane-damage of adhering bacteria. In culture-based assays, at a challenge of 1600 CFU/cm2, contact-killing was &gt;99.99%. The working-mechanism of dissolved quaternary-ammonium-compounds is based on their interdigitation in bacterial membranes, but it is difficult to envisage how immobilized quaternary-ammonium-molecules can exert such a mechanism of action. Staphylococcal adhesion forces to hyperbranched quaternary-ammonium coatings were extremely high, indicating that quaternary-ammonium-molecules on hyperbranched polyurea partially envelope adhering bacteria upon contact. These lethally strong adhesion forces upon adhering bacteria then cause removal of membrane lipids and eventually lead to bacterial death. Shape-adaptive, hyperbranched polyurea with quaternary ammonium compounds. The preparation of AB2 monomers and the covalently attached hyperbranched polyurea coatings with polyethyleneimine covalently coupled to hyperbranched polyurea coatings. The coatings demonstrate high contact-killing efficacies toward adhering staphylococci, without any demonstrable leaching of antibacterial compounds. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201301686
  • 2014 • 102 Structural and electronic properties of micellar Au nanoparticles: Size and ligand effects
    Behafarid, F. and Matos, J. and Hong, S. and Zhang, L. and Rahman, T.S. and Roldan Cuenya, B.
    ACS Nano 8 6671-6681 (2014)
    Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (∼1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method. Metallic Au NPs encapsulated by polystyrene 2-vinylpiridine (PS-P2VP) were studied in the solution phase (dispersed in toluene) as well as after deposition on γ-Al2O3. Our experimental data revealed a size-dependent contraction of the interatomic distances of the ligand-protected NPs with decreasing NP size. These findings are in good agreement with the results from DFT calculations of unsupported Au NPs surrounded by P2VP, as well as those obtained for pure (ligand-free) Au clusters of analogous sizes. A comparison of the experimental and theoretical results supports the conclusion that the P2VP ligands employed to stabilize the gold NPs do not lead to strong distortions in the average interatomic spacing. The changes in the electronic structure of the Au-P2VP NPs were found to originate mainly from finite size effects and not from charge transfer between the NPs and their environment (e.g., Au-ligand interactions). In addition, the isolated ligand-protected experimental NPs only display a weak interaction with the support, making them an ideal model system for the investigation of size-dependent physical and chemical properties of structurally well-defined nanomaterials. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn406568b
  • 2014 • 101 Transforming polyethylenimine into a pH-switchable hydrogel by additional supramolecular interactions
    Fleischer, M. and Schmuck, C.
    Chemical Communications 50 10464--10467 (2014)
    Attaching a self-complementary zwitterionic supramolecular binding site to the amino groups of polyethyleneimine leads to a pH switchable hydrogel as dimerization of the zwitterions introduces additional crosslinks between the polymer chains. This gel responds to both the addition of either acid or base, as only the zwitterion can self-assemble but neither the protonated or deprotonated form.
    view abstractdoi: 10.1039/c4cc03281k
  • 2014 • 100 Epitaxial Cu(001) films grown on a Cr/Ag/Fe/GaAs(001) buffer system
    Gottlob, D.M. and Jansen, T. and Hoppe, M. and Bürgler, D.E. and Schneider, C.M.
    Thin Solid Films 562 250-253 (2014)
    We present a procedure to prepare single-crystalline, high-purity Cu(001) films (templates) suitable as substrates for subsequent epitaxial thin-film growth. The template films were grown in a dedicated molecular-beam epitaxy system on a Cr/Ag/Fe/GaAs(001) buffer layer system. Low-energy electron diffraction and X-ray diffraction were applied to determine the surface orientation and the epitaxial relationship between all layers of the stack. Post-annealing at moderate temperatures enhances the quality of the film as shown by low-energy electron diffraction and atomic force microscopy. X-ray photoemission and Auger electron spectroscopy confirm that no atoms of the buffer layers diffuse into the Cu film during the initial preparation and the post-annealing treatment. The completed Cu(001) template system can be exposed to air and afterwards refurbished by Ar+-ion bombardment and annealing, enabling the transfer between vacuum systems. The procedure provides suitable conductive thin film templates for studies of epitaxial thin films, e.g. on the magnetic and magnetotransport properties of Co and Ni based films and multilayers. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.04.078
  • 2014 • 99 Atomic layer deposition of TiO2 and ZrO2 thin films using heteroleptic guanidinate precursors
    Kaipio, M. and Blanquart, T. and Banerjee, M. and Xu, K. and Niinistö, J. and Longo, V. and Mizohata, K. and Devi, A. and Ritala, M. and Leskelä, M.
    Chemical Vapor Deposition 20 209-216 (2014)
    In this study the atomic layer deposition (ALD) of TiO2 and ZrO2 using two heteroleptic amido-guanidinate precursors, [Ti(NEtMe)3(guan-NEtMe)] and [Zr(NEtMe)3(guan-NEtMe)], together with water or ozone as oxygen sources, are investigated. All processes exhibit self-limiting growth at a deposition temperature of 275°C. The zirconium precursor especially gives high growth rates (0.8/1.0Å per cycle with H2O/O3). The films are also relatively smooth, as determined by atomic force microscopy (AFM). The composition of the films is examined using X-ray photoelectron spectroscopy (XPS) and time of flight elastic recoil detection analysis (TOF-ERDA). When using ozone as the oxygen source the films present very high purity. The results are compared and discussed with respect to earlier studies on guanidinate, as well as homoleptic amido precursors. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201407115
  • 2014 • 98 Strain state, film and surface morphology of epitaxial topological insulator Bi2Se3 films on Si(111)
    Klein, C. and Vyshnepolsky, M. and Kompch, A. and Klasing, F. and Hanisch-Blicharski, A. and Winterer, M. and Horn-Von Hoegen, M.
    Thin Solid Films 564 241-245 (2014)
    Epitaxial Bi2Se3 films were grown by molecular beam epitaxy on Si(111)-Bi(3×3)R30° at temperatures between 200 and 250 °C. The surface and bulk morphology was characterized by high resolution low energy electron diffraction, X-ray diffraction, and atomic force microscopy for various film thicknesses between 6 and 90 nm. The films are atomically smooth without small angle mosaics or small angle rotational domains. The precise determination of lattice parameter reveals that films grown at higher temperature exhibit a smaller value for the vertical lattice parameter. The presence of random stacking faults in the film is reflected by a parabolic increase of the width of the diffraction peaks in X-ray diffraction. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.04.024
  • 2014 • 97 Dissipation signals due to lateral tip oscillations in FM-AFM
    Klocke, M. and Wolf, D.E.
    Beilstein Journal of Nanotechnology 5 2048-2057 (2014)
    We study the coupling of lateral and normal tip oscillations and its effect on the imaging process of frequency-modulated dynamic atomic force microscopy. The coupling is induced by the interaction between tip and surface. Energy is transferred from the normal to the lateral excitation, which can be detected as damping of the cantilever oscillation. However, energy can be transferred back into the normal oscillation, if not dissipated by the usually uncontrolled mechanical damping of the lateral excitation. For certain cantilevers, this dissipation mechanism can lead to dissipation rates larger than 0.01 eV per period. The mechanism produces an atomic contrast for ionic crystals with two maxima per unit cell in a line scan. © 2014 Klocke and Wolf.
    view abstractdoi: 10.3762/bjnano.5.213
  • 2014 • 96 Sudden stress-induced transformation events during nanoindentation of NiTi shape memory alloys
    Laplanche, G. and Pfetzing-Micklich, J. and Eggeler, G.
    Acta Materialia 78 144-160 (2014)
    This study investigates the stress-induced formation of martensite during nanoindentation of an austenitic NiTi shape memory alloy, where stress-induced martensite is stable at room temperature. An individual grain with a [1 1 1] surface normal was selected for spherical ex situ and in situ nanoindentation in a scanning electron microscope. The in situ load-displacement curves show several pop-ins which occur concomitantly with the formation of traces around the contact zone between the indenter tip and the sample. These traces exhibit a threefold symmetry around the remnant indent. A detailed study of the indentation-induced surface relief by atomic force microscopy before and after shape recovery allows to identify the formation of six twinned martensite plates. Post-mortem microstructural characterization shows that these twinned martensite plates are growing as the applied load is increasing. The activation of the experimentally observed twinned martensite plates is rationalized by analytical calculations of resolved shear stress and mechanical interaction energy density. Finally, the in situ nanoindentation results in combination with the post-mortem microstructural characterization show that the most likely deformation mechanism responsible for pop-in events corresponds to sudden increases of the thicknesses of twinned martensite plates. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.05.061
  • 2014 • 95 Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization
    Schröter, A. and Franzka, S. and Hartmann, N.
    Langmuir 30 14841-14848 (2014)
    Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e2 spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates. (Graph Presented). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la503814n
  • 2014 • 94 Hydration layers trapped between graphene and a hydrophilic substrate
    Temmen, M. and Ochedowski, O. and Schleberger, M. and Reichling, M. and Bollmann, T.R.J.
    New Journal of Physics 16 (2014)
    Graphene is mechanically exfoliated on CaF2(111) under ambient conditions. We demonstrate the formation of a several monolayer thick hydration layer on the hydrophilic substrate and its response to annealing at temperatures up to 750 K in an ultra-high vacuum environment. Upon heating, water is released, however, it is impossible to remove the first layer. The initially homogeneous film separates into water-containing and water-free domains by two-dimensional Ostwald ripening. Upon severe heating, thick graphene multilayers undergo rupture, while nanoblisters confining sealed water appear on thinner sheets, capable of the storage and release of material. From modeling the dimensions of the nanoblisters, we estimate the graphene/CaF2(111) interfacial adhesion energy to be 0.33 ± 0.13 J m-2, thereby viable for polymer-assisted transfer printing. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/16/5/053039
  • 2014 • 93 Single core-shell nanoparticle probes for non-invasive magnetic force microscopy
    Uhlig, T. and Wiedwald, U. and Seidenstücker, A. and Ziemann, P. and Eng, L.M.
    Nanotechnology 25 (2014)
    We present an easy, fast and reliable method for the preparation of magnetic force microscopy (MFM) probes based on single Co nanoparticles (NPs). Due to their dipolar character, these magnetic probes open up a new approach for quantitative and non-invasive MFM measurements on the nanometer length scale. To guarantee long-term stability of these tips under ambient conditions, an ultrathin protecting Au shell was grown around the Co NPs through photochemical deposition. Single magnetic particles were firmly attached to standard silicon AFM tips using bifunctional self-assembling molecules. Such probes were tested on longitudinal magnetic recording media and compared to the results as recorded with conventional thin-film MFM tips. Easy data interpretation of the magnetic nanoparticle probes in a point dipole model is shown. Our nanoparticle tips provide excellent endurance for MFM recording, enable non-invasive probing while maintaining a high sensitivity, resolution, and reproducibility. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/25/255501
  • 2014 • 92 Biofuel-Cell Cathodes Based on Bilirubin Oxidase Immobilized through Organic Linkers on 3D Hierarchically Structured Carbon Electrodes
    Vivekananthan, J. and Rincón, R.A. and Kuznetsov, V. and Pöller, S. and Schuhmann, W.
    ChemElectroChem 1 1901-1908 (2014)
    Different modification procedures to stabilize and control the orientation of Myrothecium verrucaria bilirubin oxidase (MvBOD) on 3D carbon nanotube/carbon microfiber-modified graphite electrode surfaces were evaluated for the development of biofuel-cell cathodes. The surface properties of different linkers for covalent binding of BOD were investigated by using atomic force microscopy-based techniques. For all immobilization strategies, the maximal current response was obtained at a pH value of 6.5 with temperatures between 20 and 35°C. The biocathode based on MvBOD immobilized through an imino bond to the electrode showed the highest current density (1600μAcm-2) and was resistant to the presence of chloride ions. A biofuel cell was constructed, and it exhibited a maximal power of 54μWcm-2 at 350mV with an open-circuit voltage of about 600mV by using a cellobiose dehydrogenase based bioanode and glucose as the fuel. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201402099
  • 2014 • 91 A facile solution-doping method to improve a low-temperature zinc oxide precursor: Towards low-cost electronics on plastic foil
    Weber, D. and Botnaraş, S. and Pham, D.V. and Merkulov, A. and Steiger, J. and Schmechel, R. and De Cola, L.
    Advanced Functional Materials 24 2537-2543 (2014)
    Optimization of thin-film transistors performance is usually accompanied by an increase of the process temperature. This work presents a method to raise the field effect mobility by a factor of 3 without a change of the process parameters. The modification involves a solution doping process where an ammine zinc complex is formed in the presence of metal ions of the 13th group, namely gallium and indium. Morphological studies, including scanning electron microscopy and atomic force microscopy, reveal the difference among the resulting films. Moreover, X-ray diffraction results show that the doping affects the preferred orientation of the zinc oxide crystals in the resulting film. The electrical properties vary distinctly and are best for a solution doped with both gallium and indium. With a double-layer system the performance of this new precursor exceeds field effect mobility values of 1 cm2 V-1 s-1 after a maximum process temperature of 160 °C. The performance of ZnO-based field-effect transistors is improved by a simple solution-doping procedure using ions of the 13th group. The method has a strong influence on the film morphology and orientation of the crystallites. This leads to field effect mobility values comparable to amorphous silicon. The low conversion temperature allows the fabrication on flexible substrates. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201303461
  • 2014 • 90 Plasma-enhanced chemical vapor deposition of graphene on copper substrates
    Wöhrl, N. and Ochedowski, O. and Gottlieb, S. and Shibasaki, K. and Schulz, S.
    AIP Advances 4 (2014)
    A plasma enhanced vapor deposition process is used to synthesize graphene from a hydrogen/methane gas mixture on copper samples. The graphene samples were transferred onto SiO2 substrates and characterized by Raman spectroscopic mapping and atomic force microscope topographical mapping. Analysis of the Raman bands shows that the deposited graphene is clearly SLG and that the sheets are deposited on large areas of several mm2. The defect density in the graphene sheets is calculated using Raman measurements and the influence of the process pressure on the defect density is measured. Furthermore the origin of these defects is discussed with respect to the process parameters and hence the plasma environment. © 2014 Author(s).
    view abstractdoi: 10.1063/1.4873157
  • 2014 • 89 Interface reactions of Ag@TiO2 nanocomposite films
    Zuo, J. and Rao, J. and Eggeler, G.
    Materials Chemistry and Physics 145 90-98 (2014)
    TiO2 films were sputtered on 100-nm-thick Ag layers at various O2 partial pressures to study forming processes at the interface. The interfacial reactions during the deposition process were investigated by means of transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and UV-vis reflection spectra. The sputtering process led to formation of Ag nanoparticles surrounded by Ag 2O and TiO2 in the TiO2 film matrix as well as on the surface. The presence of oxygen in the plasma resulted in enrichment of silver oxides on the surface and an intermixing of Ag in the TiO2 matrix. The film structures could be explained based on the interplay among the formation of silver oxide, the nucleation and growth of TiO2, as well as the mobility of silver and silver oxides within the growing TiO2 films. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.matchemphys.2014.01.041
  • 2013 • 88 Long-range segregation phenomena in shape-selected bimetallic nanoparticles: Chemical state effects
    Ahmadi, M. and Behafarid, F. and Cui, C. and Strasser, P. and Cuenya, B.R.
    ACS Nano 7 9195-9204 (2013)
    A study of the morphological and chemical stability of shape-selected octahedral Pt0.5Ni0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt0.5Ni0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H2, and O2 environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtOx species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (< 200-270 C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn403793a
  • 2013 • 87 Towards the understanding of sintering phenomena at the nanoscale: Geometric and environmental effects
    Behafarid, F. and Roldan Cuenya, B.
    Topics in Catalysis 56 1542-1559 (2013)
    One of the technologically most important requirements for the application of supported metal nanoparticles (NPs) to the field of heterogeneous catalysis is the achievement of thermally and chemically stable systems under reaction conditions. For this purpose, a thorough understanding of the different pathways underlying coarsening phenomena is needed. In particular, in depth knowledge must be achieved on the role of the NP synthesis method, geometrical features of the NPs (size and shape), initial NP dispersion on the support (interparticle distance), support pre-treatment (affecting its morphology and chemical state), and reaction environment (gaseous or liquid medium, pressure, temperature). This study provides examples of the stability and sintering behavior of nanoscale systems monitored ex situ, in situ, and under operando conditions via transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray absorption fine-structure spectroscopy. Experimental data corresponding to physical-vapor-deposited and micelle-synthesized metal (Pt, Au) NPs supported on TiO<inf>2</inf>, SiO<inf>2</inf> and Al<inf>2</inf>O <inf>3</inf> will be used to illustrate Ostwald-ripening and diffusion coalescence processes. In addition, the role of the annealing environment (H<inf>2</inf>, O<inf>2</inf>, water vapor) on the stability of NPs will be discussed. © 2013 Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s11244-013-0149-4
  • 2013 • 86 Fabrication of periodic surface topographies via sequential photothermal laser microsintering of silicon nanoparticle films
    Behrenberg, D. and Franzka, S. and Petermann, N. and Wiggers, H. and Hartmann, N.
    Applied Surface Science 278 278-283 (2013)
    Photothermal laser microprocessing is exploited in order to induce sintering and compaction of thin silicon nanoparticle (Si NP) films. Ethanolic dispersions of Si NPs with an average diameter of 45 nm are spin-coated on silicon substrates yielding films with a thickness of about 400-500 nm. Scanning electron and atomic force microscopy are used for characterization of the resulting surface morphologies. Sequential processing of the coated layer with a microfocused cw-laser beam at a wavelength of 532 nm generates periodic surface structures. The periodicity of these structures is determined by the distance between adjacent laser-written lines. Despite a 1/e laser spot size of 1.4 μm, fabrication of topographic surface structures with submicrometer periodicities is feasible. In particular, surface topographies with periodicities of 600 nm and a topographic amplitude of 80 nm are fabricated. These results point to a high nonlinearity, which is attributed to the strongly activated, temperature-dependent laser sintering process. These experimental observations are reproduced qualitatively considering a simple photothermal model and an activated sintering process. Prospects of photothermal laser microsintering of nanoparticle films to fabricate biomimetic surface structures are discussed. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2013.01.205
  • 2013 • 85 Cooperative self-assembly of discoid dimers: Hierarchical formation of nanostructures with a pH switch
    Fenske, M.T. and Meyer-Zaika, W. and Korth, H.-G. and Vieker, H. and Turchanin, A. and Schmuck, C.
    Journal of the American Chemical Society 135 8342-8349 (2013)
    Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to dimerize to 1•1 with an aggregation constant of ca. &gt;1010 M-l in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of sec-amide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif. A butyl amide substituent as in 2b enables the discoid dimers to further aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-assisted π-π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja4025148
  • 2013 • 84 Ultra-thin MoS2 irradiated with highly charged ions
    Hopster, J. and Kozubek, R. and Krämer, J. and Sokolovsky, V. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 317 165-169 (2013)
    Single MoS2 layers exfoliated on KBr have been irradiated with highly charged Xe ions, i.e. with Xe35+ and Xe40+. By atomic force microscopy (AFM) we identified pits and hillocks induced by single ion impacts. The latter ones appear on single layer and bulk-like MoS 2 after both irradiations, whereas their diameter and height apparently depend on the charge state q and layer number. By comparison of contact mode and tapping mode AFM measurements we deduce that these ion induced defects are topographical hillocks accompanied by an enhanced friction. In contrast to this, pit-like structures were only observed on single layer MoS2 irradiated with q = 40. Taking into account the well known ion induced pit formation on KBr due to defect mediated sputtering, we deduce that pit formation takes place in the substrate and not in the MoS2 layer. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.nimb.2013.02.038
  • 2013 • 83 Electrochemical formation and surface characterisation of Cu 2-xTe thin films with adjustable content of Cu
    Huang, M. and Maljusch, A. and Calle-Vallejo, F. and Henry, J.B. and Koper, M.T.M. and Schuhmann, W. and Bandarenka, A.S.
    RSC Advances 3 21648-21654 (2013)
    Electrochemically driven "intercalation" of Cu into Te was used to prepare Cu<inf>2-x</inf>Te (0.2 < x ≤ 2) thin films and accurately control the composition of the resulting samples. A thorough theoretical analysis of the system using density functional theory (DFT) calculations showed that in the absence of external electric fields the driving forces for Cu atoms to move into the subsurface layers of the Te electrodes depend on the surface coverage of copper atoms. The Cu atoms tend to preferentially occupy the subsurface layers in the telluride films. The effective electric charge on Cu atoms inside the Te-electrodes is positive. These effective charge differences with respect to pure Cu and pure Te are only 0.2 e-. Scanning Kelvin probe (SKP), atomic force microscopy (AFM) and electrochemical techniques were used to characterise the surface status of the obtained samples. Both, DFT-calculated work function differences and the SKP-measured contact potential differences (CPD) change non-linearly with the variation of the film composition. Interfacial (solid/liquid) properties evaluated using electrochemical impedance spectroscopy depend on the nominal composition of the samples and display an abrupt change that correlates with a large change in the work function and CPD. While the proposed electrochemical synthetic route can efficiently and accurately control the composition of the Cu<inf>2-x</inf>Te thin films, SKP-measurements performed under close to ambient conditions in combination with DFT calculations can provide a promising tool to link fundamental surface properties and parameters which define the interface between solids and liquids. © The Royal Society of Chemistry 2013.
    view abstractdoi: 10.1039/c3ra42504e
  • 2013 • 82 Nanoscale thermomechanics of wear-resilient polymeric bilayer systems
    Kaule, T. and Zhang, Y. and Emmerling, S. and Pihan, S. and Foerch, R. and Gutmann, J. and Butt, H.-J. and Berger, R. and Duerig, U. and Knoll, A.W.
    ACS Nano 7 748-759 (2013)
    We explore the effect of an ultrathin elastic coating to optimize the mechanical stability of an underlying polymer film for nanoscale applications. The coating consists of a several nanometer thin plasma-polymerized norbornene layer. Scanning probes are used to characterize the system in terms of shear-force-induced wear and thermally assisted indentation. The layer transforms a weakly performing polystyrene film into a highly wear-resistive system, ideal for high-density and low-power data storage applications. The result can be understood from the indentation characteristics with a hot and sharp indenter tip. The latter gives rise to a deformation mode in the fully plastic regime, enabling a simple interpretation of the results. The softening transition and the yield stress of the system on a microsecond time scale and a nanometer size scale were obtained. We show that the plastic deformation is governed by yielding in the polystyrene sublayer, which renders the overall system soft for plastic deformation. The ultrathin protection layer contributes as an elastic skin, which shields part of the temperature and pressure and enables the high wear resistance against lateral forces. Moreover, the method of probing polymers at microsecond and nanometer size scales opens up new opportunities for studying polymer physics in a largely unexplored regime. Thus, we find softening temperatures of more than 100 °C above the polystyrene glass transition, which implies that for the short interaction time scales the glassy state of the polymer is preserved up to this temperature. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nn305047m
  • 2013 • 81 Intrinsic nitrogen-doped CVD-grown TiO2 thin films from all-N-coordinated Ti precursors for photoelectrochemical applications
    Kim, S.J. and Xu, K. and Parala, H. and Beranek, R. and Bledowski, M. and Sliozberg, K. and Becker, H.-W. and Rogalla, D. and Barreca, D. and Maccato, C. and Sada, C. and Schuhmann, W. and Fischer, R.A. and Devi, A.
    Chemical Vapor Deposition 19 45-52 (2013)
    N-doped titanium dioxide (TiO2) thin films are grown on Si(100) and indium tin oxide (ITO)-coated borosilicate glass substrates by metal-organic (MO)CVD. The intrinsic doping of TiO2 thin films is achieved using all-nitrogen-coordinated Ti precursors in the presence of oxygen. The titanium amide-guanidinate complex, [Ti(NMe2)3(guan)] (guan = N,N′-diisopropyl-2-dimethylamidoguanidinato) has been developed to compensate for the thermal instability of the parent alkylamide [Ti(NMe 2)4]. Both of these amide-based compounds are tested and compared as precursors for intrinsically N-doped TiO2 at various deposition temperatures in the absence of additional N sources. The structure and morphology of TiO2 thin films are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Rutherford back scattering (RBS), nuclear reaction analysis (NRA), and secondary ion mass spectrometry (SIMS) analyses are performed to determine N content and distribution in the films. The optical and photoelectrochemical properties of TiO2 thin films on ITO substrates are also examined. N-doped TiO2 thin films, grown from [Ti(NMe 2)3(guan)] at 600 °C, exhibit the lowest optical absorption edge (3.0 eV) and the highest visible light photocurrent response. When compared to undoped TiO2, while in UV light photoconversion efficiency decreases significantly, the intrinsically N-doped TiO2 shows enhanced photocurrents under visible light irradiation. The intrinsic doping of TiO2 thin films with nitrogen by MOCVD and the investigation of the photo-electrochemical properties of the films are reported. N-doped anatase phase TiO2 thin films are grown on Si(100) and ITO substrates under specific processing conditions, using [Ti(NMe2) 4] (1) and [Ti(NMe2)3(guan)] (2) (guan = N,N′-diisopropyl-2-dimethylamidoguanidinato) as precursors. The films grown from [Ti(NMe2)3(guan)] at 600 °C show relatively large surface roughness and lower bandgap related with high N content. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201206996
  • 2013 • 80 Al-induced faceting of Si(113)
    Klein, C. and Heidmann, I. and Nabbefeld, T. and Speckmann, M. and Schmidt, T. and Meyer zu Heringdorf, F.-J. and Falta, J. and Horn-Von Hoegen, M.
    Surface Science 618 109-114 (2013)
    Adsorption of Al on a Si(113) substrate at elevated temperatures causes a faceting transition of the initially flat surface. The (113) surface decomposes into a quasi-periodic sequence of Al terminated (115)- and (112)-facets. The resulting surface morphology is characterized in-situ by reciprocal space maps obtained with in-situ spot profile analyzing low-energy electron diffraction and ex-situ atomic force microscopy. The periodicity length of the faceted surface increases with adsorption temperature from 7 nm at 650 C to 80 nm at 800 C. The stability of the Al terminated Si(112) surface is the driving force for the faceting transition. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.007
  • 2013 • 79 Combined in situ FTIR-spectroscopic and electrochemical analysis of nanopores in ultra-thin SiOx-like plasma polymer barrier films
    Liu, C.-N. and Ozkaya, B. and Steves, S. and Awakowicz, P. and Grundmeier, G.
    Journal of Physics D: Applied Physics 46 (2013)
    Plasma polymerized SiOx barrier films were investigated by means of in situ spectroscopic and electrochemical methods to correlate the process parameters such as applied substrate bias with the resulting barrier properties. SiOx layers with various hexamethyldisiloxane/oxygen ratio were deposited with and without applied substrate bias. The resulting film morphologies were characterized by means of atomic force microscopy, and the presence of nanopores was analysed by cyclic voltammetry. In order to compare the film density and the presence of nanopore structure, evaluation of interfacial hydroxyl groups was performed by means of discrete polarization modulation Fourier transform infrared reflection-absorption spectroscopy in atmospheres with controlled partial pressures of H2O or D 2O. It could be shown that the electrochemical and in situ spectroscopic approach allows for the analysis of nanopores and that a clear correlation of process parameters and film structure can be established. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/8/084015
  • 2013 • 78 Low temperature diffusion of Li atoms into Si nanoparticles and surfaces
    Nienhaus, H. and Karacuban, H. and Krix, D. and Becker, F. and Hagemann, U. and Steeger, D. and Bywalez, R. and Schulz, C. and Wiggers, H.
    Journal of Applied Physics 114 (2013)
    The diffusion of Li atoms deposited on hydrogen-passivated Si(001) surfaces, chemically oxidized Si(001) surfaces, Si nanoparticle films, and thick SiO2 layers is investigated with electron-beam induced Auger electron spectroscopy. The nanoparticles exhibit an average diameter of 24 nm. The Li metal film is evaporated at a sample temperature below 120 K. The reappearance of the Si substrate Auger signal as a function of time and temperature can be measured to study the Li diffusion into the bulk material. Values for the diffusion barrier of 0.5 eV for H:Si(001) and 0.3 eV for the ox-Si(001) and Si nanoparticle films are obtained. The diffusion of the Li atoms results in the disruption of the crystalline Si surfaces observed with atomic force microscopy. Contrasting to that, the Si nanoparticle films show less disruption by Li diffusion due to filling of the porous films detected with cross section electron microscopy. Silicon dioxide acts as a diffusion barrier for temperatures up to 300 K. However, the electron beam induces a reaction between Li and SiO2, leading to LiOx and elemental Si floating on the surface. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4813872
  • 2013 • 77 Detecting swift heavy ion irradiation effects with graphene
    Ochedowski, O. and Akcöltekin, S. and Ban-D'Etat, B. and Lebius, H. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 314 18-20 (2013)
    In this paper we show how single layer graphene can be utilized to study swift heavy ion (SHI) modifications on various substrates. The samples were prepared by mechanical exfoliation of bulk graphite onto SrTiO3, NaCl and Si(1 1 1), respectively. SHI irradiations were performed under glancing angles of incidence and the samples were analysed by means of atomic force microscopy in ambient conditions. We show that graphene can be used to check whether the irradiation was successful or not, to determine the nominal ion fluence and to locally mark SHI impacts. In case of samples prepared in situ, graphene is shown to be able to catch material which would otherwise escape from the surface. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.nimb.2013.03.063
  • 2013 • 76 Radiation hardness of graphene and MoS2 field effect devices against swift heavy ion irradiation
    Ochedowski, O. and Marinov, K. and Wilbs, G. and Keller, G. and Scheuschner, N. and Severin, D. and Bender, M. and Maultzsch, J. and Tegude, F.J. and Schleberger, M.
    Journal of Applied Physics 113 (2013)
    We have investigated the deterioration of field effect transistors based on two-dimensional materials due to irradiation with swift heavy ions. Devices were prepared with exfoliated single layers of MoS2 and graphene, respectively. They were characterized before and after irradiation with 1.14 GeV U28+ ions using three different fluences. By electrical characterization, atomic force microscopy, and Raman spectroscopy, we show that the irradiation leads to significant changes of structural and electrical properties. At the highest fluence of 4 × 1011 ions/cm 2, the MoS2 transistor is destroyed, while the graphene based device remains operational, albeit with an inferior performance. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4808460
  • 2013 • 75 Nano-gold diggers: Au-Assisted SiO2-decomposition and desorption in supported nanocatalysts
    Ono, L.K. and Behafarid, F. and Cuenya, B.R.
    ACS Nano 7 10327-10334 (2013)
    An investigation of the thermal stability of size-selected Au nanoparticles (NPs) synthesized via inverse micelle encapsulation and deposited on SiO 2(4 nm)/Si(100) is presented. The size and mobility of individual Au NPs after annealing at elevated temperatures in ultrahigh vacuum (UHV) was monitored via atomic force microscopy (AFM). An enhanced thermal stability against coarsening and lack of NP mobility was observed up to 1343 K. In addition, a drastic decrease in the average NP height was detected with increasing annealing temperature, which was not accompanied by the sublimation of Au atoms/clusters in UHV. The apparent decrease in the Au NP height observed is assigned to their ability to dig vertical channels in the underlying SiO 2 support. More specifically, a progressive reduction in the thickness of the SiO2 support underneath and in the immediate vicinity of the NPs was evidenced, leading to NPs partially sinking into the SiO2 substrate. The complete removal of silicon oxide in small patches was observed to take place around the Au NPs after annealing at 1343 K in UHV. These results reveal a Au-assisted oxygen desorption from the support via reverse oxygen spillover to the NPs. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn404744b
  • 2013 • 74 Surface grafting of Corchorus olitorius fibre: A green approach for the development of activated bioadsorbent
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Majumder, S.B. and Adhikari, B.
    Carbohydrate Polymers 92 2118-2127 (2013)
    The present work is an endeavor to prepare lignocellulosic biomass based adsorbent, suitable for removal of organic and inorganic pollutants from industrial effluents. Lignocellulosic Corchorus olitorius fibre (jute fibre) surface was grafted with naturally available polyphenol, tannin, preceded by the epoxy-activation of fibre surface with epichlorohydrin under mild condition in an aqueous suspension. The reaction parameters for the modification, viz., concentration of epichlorohydrin and tannin, time, and temperature were optimized. The successful occurrence of surface modification of jute fibre (JF) was characterized and estimated from weight gain percent, elemental analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron and atomic force microscopy, and thermogravimetric analysis. An extensive analysis of deconvoluted FTIR spectra using the Voigt model was utilized to ensure the surface grafting. The microbiological susceptibility study revealed high persistency of JF towards biodegradation after efficient grafting with tannin. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.carbpol.2012.11.039
  • 2013 • 73 Formation of carbon nanofilms on single crystal quartz
    Samsonau, S.V. and Dzedzits, E. and Shvarkov, S.D. and Meinerzhagen, F. and Wieck, A.D. and Zaitsev, A.M.
    Sensors and Actuators, B: Chemical 186 610-613 (2013)
    In this work formation of the very first layers of carbon nanofilms on single crystal quartz substrates is studied. Films where grown by molecular beam growth, and have been characterized by Raman spectroscopy and atomic force microscopy. Formation of a non-conductive carbon layer of low crystallinity on the initial stage of the growth process is reported. Ab-initio calculations with an atom-by-atom approach have been performed to explain the experimental data. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2013.06.023
  • 2013 • 72 Growth of graphene-like films for NO2 detection
    Samsonau, S.V. and Shvarkov, S.D. and Meinerzhagen, F. and Wieck, A.D. and Zaitsev, A.M.
    Sensors and Actuators, B: Chemical 182 66-70 (2013)
    Chemical vapor deposition of graphene-like (GL) films (nano-crystalline graphite films) from methane on single crystal quartz substrates is reported. The GL films have been characterized by means of Raman spectroscopy, atomic force microscopy and electrical measurements. The thinnest GL films obtained so far have a thickness of 1.5 nm, a relatively rough surface structure and an electrical conductivity in the range of 20 kΩ. The high chemical sensitivity of GL films has been demonstrated by measuring the change of their conductance during exposure to a NO2-containing atmosphere. The optimal electrical conductance, high chemical sensitivity as well as the simple growth method make the GL films promising for practical applications as a chemically sensitive material. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.snb.2013.02.067
  • 2013 • 71 Electric in-plane polarization in multiferroic CoFe2O 4/BaTiO3 nanocomposite tuned by magnetic fields
    Schmitz-Antoniak, C. and Schmitz, D. and Borisov, P. and De Groot, F.M.F. and Stienen, S. and Warland, A. and Krumme, B. and Feyerherm, R. and Dudzik, E. and Kleemann, W. and Wende, H.
    Nature Communications 4 (2013)
    Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix. The competing mechanisms of clamping and relaxation effects are discussed in detail. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms3051
  • 2013 • 70 Fabrication of chemical templates via selective laser-induced desorption of hexadecanethiol self-assembled monolayers
    Schröter, A. and Mathieu, M. and Franzka, S. and Feydt, J. and Irsen, S. and Hartmann, N.
    Applied Surface Science 278 57-61 (2013)
    A nonlinear photothermal laser patterning technique for rapid fabrication of chemical templates is demonstrated. Hexadecanethiol monolayers on Au-coated Si substrates are processed at λ = 532 nm, a 1/e2 spot diameter of d1/e2=2.8Tμm and ambient conditions. Local laser irradiation at high laser powers and short irradiation times in the micro-/millisecond range induces desorption of thiol molecules. The laser-depleted areas are backfilled with mercaptohexadecanoic acid in order to build up chemical templates. Atomic force microscopy, scanning electron microscopy and scanning Auger electron spectroscopy are used for characterization of these templates. In agreement with a selective laser process, the results indicate the formation of flat chemical patterns with well-defined boundaries. Complementary condensation experiments demonstrate the functionality of the patterns as hydrophilic/hydrophobic templates. In particular, upon decreasing the temperature below the dew point, selective formation of water droplets on the backfilled areas is observed. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2012.12.135
  • 2013 • 69 Star-shaped poly(styrene)-block-Poly(4-vinyl-N-methylpyridiniumiodide) for semipermanent antimicrobial coatings
    Siedenbiedel, F. and Fuchs, A. and Moll, T. and Weide, M. and Breves, R. and Tiller, J.C.
    Macromolecular Bioscience 13 1447-1455 (2013)
    Goal of the present work is to develop an antimicrobial coating that can be applied from an aqueous solution and resists short washing cycles, but can be rinsed off by thorough washing. To this end, a series of star-shaped polystyrene-block-poly(4-vinyl-N-methylpyridinium iodide) polymers are synthesized by anionic polymerization using a core-first approach. The optimal resulting polymers are applied as coatings on glass slides, showing high antimicrobial efficiency against Staphylococcus aureus as well as Escherichia coli. The coatings, characterized by atomic force microscopy and transmission electron microscopy, stay at the surface even after at least 20 flush-like washings with water, and retain their antimicrobial activity. Semipermanent antimicrobial coatings can be achieved by synthesizing 3-arm star-blockcopolymer with an inner poly(styrene)-block and an outer poly(4-vinyl-N-methylpyridiniumiodide)-block. Solubility, antimicrobial activity, and coating stability strongly depend on the block ratios. The coatings can be applied from aqueous solution, withstand flush-like washings and prevent growth of S. aureus and E. coli. Nonetheless, the coatings are removable of by extended rinsing. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mabi.201300219
  • 2013 • 68 Silicon oxide barrier films deposited on PET foils in pulsed plasmas: Influence of substrate bias on deposition process and film properties
    Steves, S. and Ozkaya, B. and Liu, C.-N. and Ozcan, O. and Bibinov, N. and Grundmeier, G. and Awakowicz, P.
    Journal of Physics D: Applied Physics 46 (2013)
    A widely used plastic for packaging, polyethylene terephtalate (PET) offers limited barrier properties against gas permeation. For many applications of PET (from food packaging to micro electronics) improved barrier properties are essential. A silicon oxide barrier coating of PET foils is applied by means of a pulsed microwave driven low-pressure plasma. While the adjustment of the microwave power allows for a control of the ion production during the plasma pulse, a substrate bias controls the energy of ions impinging on the substrate. Detailed analysis of deposited films applying oxygen permeation measurements, x-ray photoelectron spectroscopy and atomic force microscopy are correlated with results from plasma diagnostics describing the deposition process. The influence of a change in process parameters such as gas mixture and substrate bias on the gas temperature, electron density, mean electron energy, ion energy and the atomic oxygen density is studied. An additional substrate bias results in an increase in atomic oxygen density up to a factor of 6, although plasma parameter such as electron density of ne = 3.8 ± 0.8 x 1017 m-3 and electron temperature of kBT e = 1.7 ± 0.1 eV are unmodified. It is shown that atomic oxygen densities measured during deposition process higher than nO = 1.8 x 1021 m-3 yield in barrier films with a barrier improvement factor up to 150. Good barrier films are highly cross-linked and show a smooth morphology. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/8/084013
  • 2013 • 67 Routes to rupture and folding of graphene on rough 6H-SiC(0001) and their identification
    Temmen, M. and Ochedowski, O. and Kleine Bussmann, B. and Schleberger, M. and Reichling, M. and Bollmann, T.R.J.
    Beilstein Journal of Nanotechnology 4 625-631 (2013)
    Twisted few layer graphene (FLG) is highly attractive from an application point of view, due to its extraordinary electronic properties. In order to study its properties, we demonstrate and discuss three different routes to in situ create and identify (twisted) FLG. Single layer graphene (SLG) sheets mechanically exfoliated under ambient conditions on 6H-SiC(0001) are modified by (i) swift heavy ion (SHI) irradiation, (ii) by a force microscope tip and (iii) by severe heating. The resulting surface topography and the surface potential are investigated with non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). SHI irradiation results in rupture of the SLG sheets, thereby creating foldings and bilayer graphene (BLG). Applying the other modification methods creates enlarged (twisted) graphene foldings that show rupture along preferential edges of zigzag and armchair type. Peeling at a folding over an edge different from a low index crystallographic direction can result in twisted BLG, showing a similar height as Bernal (or AA-stacked) BLG in NC-AFM images. The rotational stacking can be identified by a significant contrast in the local contact potential difference (LCPD) measured by KPFM. © 2013 Komkova et al; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.4.69
  • 2013 • 66 Hot embossed microtopographic gradients reveal morphological cues that guide the settlement of zoospores
    Xiao, L. and Thompson, S.E.M. and Röhrig, M. and Callow, M.E. and Callow, J.A. and Grunze, M. and Rosenhahn, A.
    Langmuir 29 1093-1099 (2013)
    Among different surface cues, the settlement of cells and larvae of marine macrofouling organisms has been found to be strongly influenced by surface microtopographies. In this article, the settlement of zoospores of the green alga Ulva linza on a surface topographic gradient has been investigated. "Honeycomb" gradient structures with feature sizes ranging from 1 to 10 μm were prepared by hot embossing, and the effect on the density of spores that attached in settlement assays was quantified. The highest density of spores was found when the size of the microstructures was similar to or larger than the size of the spores. With decreasing size of the structures, spore settlement density decreased. Interestingly, spore settlement density correlated with the Wenzel roughness of the surfaces. "Kink sites" on the surface played an important role and resembled preferred attachment positions. Furthermore, the gradients allowed the minimum pit size that the spores were able to squeeze into to be determined. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la303832u
  • 2013 • 65 "grafting-from" Polymerization of PMMA from stainless steel surfaces by a raft-mediated polymerization process
    Zammarelli, N. and Luksin, M. and Raschke, H. and Hergenröder, R. and Weberskirch, R.
    Langmuir 29 12834-12843 (2013)
    The synthesis of grafted PMMA homopolymer films is reported using a surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization from a RAFT-agent immobilized on a silanized stainless steel surface. Therefore, stainless steel surfaces were hydroxylated with piranha solution followed by silanization with 3-aminopropylsilane (APS). The pendant primary amino groups of the cross-linked polysiloxane layer were reacted with 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid N-hydroxysuccinimide ester to produce a surface with covalently immobilized RAFT agents. PMMA homopolymers of different molecular weights between 13 060 and 45 000 g/mol were then prepared by a surface-initiated RAFT polymerization. Molecular weight (MW) and polydispersity index (PDI) were determined from sacrificial polymerization in solution. The different steps of stainless steel surface modification and the ultrathin films were investigated using atomic force microscopy (AFM), static, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), and ellipsometry. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/la402870p
  • 2013 • 64 Cation induced conformation changes in hyaluronate solution
    Zellermann, A.-M. and Bergmann, D. and Mayer, C.
    European Polymer Journal 49 70-79 (2013)
    The influence of barium as a bivalent metal cation on the molecular conformation of partially deprotonated hyaluronic acid is studied by viscosimetry and atomic force microscopy (AFM). The resulting data are compared with corresponding observations on dextran as an uncharged representative of the group of polysaccharides. Titrations monitored by rotational viscosimetry and conductometry are carried out using diluted aqueous solutions of hyaluronate and dextran, stepwise adding small amounts of a Ba2+ solution. The AFM measurements are performed on single molecules of hyaluronate and dextran in absence and presence of Ba2+ ions. The cation induced conformational changes lead to a decrease of the solution viscosity in case of the hyaluronate. The corresponding AFM micrographs reveal that Ba2+ ions induce a transformation from extended and slightly curved chains to internally linked molecules exhibiting sharp bends and significantly smaller hydrodynamic radii. In contrast, the solution viscosity and the conformation of dextran remain largely unchanged in presence of barium.© 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.eurpolymj.2012.09.025
  • 2012 • 63 Folding graphene with swift heavy ions
    Akcöltekin, S. and Bukowska, H. and Akcol̈tekin, E. and Lebius, H. and Schleberger, M.
    Materials Research Society Symposium Proceedings 1354 67-72 (2012)
    Swift heavy ion induced modifications on graphene were investigated by means of atomic force microscopy and Raman spectroscopy. For the experiment graphene was exfoliated onto different substrates (SrTiO3 (100), TiO2(100), Al2O3(1102) and 90 nm SiO 2/Si) by the standard technique. After irradiation with heavy ions of 93 MeV kinetic energy and under glancing angles of incidence, characteristic folding structures are observed. The folding patterns on crystalline substrates are generally larger and are created with a higher efficiency than on the amorphous SiO2. This difference is attributed to the relatively large distance between graphene and SiO2 of d ≈ 1 nm. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/opl.2011.1278
  • 2012 • 62 Scanning Probe Methods: From Microscopy to Sensing
    Berger, R. and Gutmann, J. and Schäfer, R.
    Methods in Physical Chemistry 1 737-761 (2012)
    doi: 10.1002/9783527636839.ch24
  • 2012 • 61 A facile route to reassemble titania nanoparticles into ordered chain-like networks on substrate
    Cheng, Y.-J. and Wolkenhauer, M. and Bumbu, G.-G. and Gutmann, J.S.
    Macromolecular Rapid Communications 33 218-224 (2012)
    A facile route to reassemble titania nanoparticles within the titania-block copolymer composite films has been developed. The titania nanoparticles templated by the amphiphilic block copolymer of poly(styrene)-block-poly (ethylene oxide) (PS-b-PEO) were frozen in the continuous PS matrix. Upon UV exposure, the PS matrix was partially degraded, allowing the titania nanoparticles to rearrange into chain-like networks exhibiting a closer packing. The local structures of the Titania chain-like networks were investigated by both AFM and SEM; the lateral structures and vertical structures of the films were studied by GISAXS and X-ray reflectivity respectively. Both the image analysis and X-ray scattering characterization prove the reassembly of the titania nanoparticles after UV exposure. The mechanism of the nanoparticle assembly is discussed. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/marc.201100638
  • 2012 • 60 Influence of process parameters on the crystallinity, morphology and composition of tungsten oxide-based thin films grown by metalorganic chemical vapor deposition
    De Los Arcos, T. and Cwik, S. and Milanov, A.P. and Gwildies, V. and Parala, H. and Wagner, T. and Birkner, A. and Rogalla, D. and Becker, H.-W. and Winter, J. and Ludwig, Al. and Fischer, R.A. and Devi, A.
    Thin Solid Films 522 11-16 (2012)
    The growth of tungsten oxide (WO 3) based thin films was achieved via metalorganic chemical vapor deposition using an all-nitrogen coordinated tungsten precursor in combination with oxygen. Film growth was performed on Si(100) substrates in the temperature range of 400-800 °C. Employing multi-technique approaches like X-ray diffraction, scanning electron microscopy, atomic force microscopy, Rutherford back scattering, nuclear reaction analysis and X-ray photoelectron spectroscopy, the variation of the growth characteristics and film properties with deposition temperature were studied in terms of crystallinity, structure, surface roughness and composition. Special attention was devoted to the investigation of variations in the film composition for the as-deposited and annealed films. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2011.12.007
  • 2012 • 59 Scaling potential of local redox processes in memristive SrTiO 3 thin-film devices
    Dittmann, R. and Muenstermann, R. and Krug, I. and Park, D. and Menke, T. and Mayer, J. and Besmehn, A. and Kronast, F. and Schneider, C. M. and Waser, R.
    Proceedings of the IEEE 100 1979-1990 (2012)
    In this work, we address the following question: Where do the resistive switching processes take place in memristive thin-film devices of the single crystalline model material Fe-doped SrTiO 3? We compare resistive switching induced by the tip of the atomic force microscope on the bare thin-film surface with the switching properties observed in memristive devices with Pt top electrode. In order to close the gap between these two approaches, we combine conductive-tip atomic force microscopy with a delamination technique to remove the top electrode of Fe-doped SrTiO 3 metal-insulator-metal (MIM) structures to gain insight into the active switching interface with nanoscale lateral resolution. This enables us to prove the coexistence of a filamentary and area-dependent switching process with opposite switching polarities in the same sample. The spatially resolved analysis by transmission electron microscopy and photoelectron spectromicroscopy gives us some hints that the two switching types take place in device regions with different defect density and significant stoichiometry difference. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/JPROC.2012.2188771
  • 2012 • 58 Detection of lipid raft domains in neutral and anionic Langmuir monolayers and bilayers of complex lipid composition
    Evers, F. and Jeworrek, C. and Weise, K. and Tolan, M. and Winter, R.
    Soft Matter 8 2170-2175 (2012)
    In reference to the complexity and heterogeneity of cellular membranes, the structure and lateral ordering of lipid monolayers and bilayers composed of multi-component lipid mixtures have been investigated and compared in the present study. These complex model biomembrane systems represent valuable model systems, e.g. for studies of lipid-peptide interactions, where an integrated in situ multi-technique approach using both monolayer and bilayer techniques is required. A zwitterionic (3 components) and an anionic (5 components) heterogeneous model membrane system have been characterized that consist of saturated and unsaturated phospholipids as well as cholesterol. Lipid monolayers were analyzed by surface X-ray scattering techniques, and both the vertical structure (electron density profile) and the degree of in-plane ordering were determined as a function of surface pressure and temperature. The corresponding structure and lateral organisation of the bilayer membranes were characterized by atomic force microscopy. Both monolayers and bilayers reveal ordered domain formation, the monolayer ones being of much smaller size and different temperature stability, however. Furthermore, the charge density of the lipid monolayer has a drastic influence on the size of ordered domains as well as the intermolecular distances of the lipid molecules. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2sm06835d
  • 2012 • 57 Effect of p-type doping on the oxidation of H-Si(111) studied by second-harmonic generation
    Gökce, B. and Dougherty, D.B. and Gundogdu, K.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 30 (2012)
    Atomic force microscopy and second-harmonic generation data show that boron doping enhances the rate of oxidation of H-terminated silicon. Holes cause a greater increase in the reactivity of the Si-H up bonds than that of the Si-Si back bonds. © 2012 American Vacuum Society.
    view abstractdoi: 10.1116/1.4721329
  • 2012 • 56 Direct experimental observation of the aggregation of α-amino acids into 100-200 nm clusters in aqueous solution
    Hagmeyer, D. and Ruesing, J. and Fenske, T. and Klein, H.-W. and Schmuck, C. and Schrader, W. and Piedade, M.E.M.D. and Epple, M.
    RSC Advances 2 4690-4696 (2012)
    Spherical supramolecular aggregates of α-amino acids with a typical diameter of 100-200 nm are formed spontaneously after dissolution in water at a concentration of a few mM, i.e. well below the solubility limit. Their presence was shown by nanoparticle tracking analysis (NTA), atomic force microscopy (AFM), and ESI mass spectrometry (ESI-MS). There is a dynamic equilibrium between the aggregates and dissolved individual molecules which allows them to penetrate through dialysis membranes and filters. The same phenomenon was observed for para-amino salicylic acid and two dipeptides. Thermodynamic considerations suggest an entropy-controlled process. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2ra01352e
  • 2012 • 55 Growth optimization and characterization of lattice-matched Al 0.82In 0.18N optical confinement layer for edge emitting nitride laser diodes
    Kim-Chauveau, H. and Frayssinet, E. and Damilano, B. and De Mierry, P. and Bodiou, L. and Nguyen, L. and Vennéguès, P. and Chauveau, J.-M. and Cordier, Y. and Duboz, J.Y. and Charash, R. and Vajpeyi, A. and Lamy, J.-M. and Akhte...
    Journal of Crystal Growth 338 20-29 (2012)
    We present the growth optimization and the doping by the metal organic chemical vapor deposition of lattice-matched Al 0.82In 0.18N bottom optical confinement layers for edge emitting laser diodes. Due to the increasing size and density of V-shaped defects in Al 1-xIn xN with increasing thickness, we have designed an Al 1-xIn xN/GaN multilayer structure by optimizing the growth and thickness of the GaN interlayer. The Al 1-xIn xN and GaN interlayers in the multilayer structure were both doped using the same SiH 4 flow, while the Si levels in both layers were found to be significantly different by SIMS. The optimized 8×(Al 0.82In 0.18N/GaN=54/6 nm) multilayer structures grown on free-standing GaN substrates were characterized by high resolution X-ray diffraction, atomic force microscopy and transmission electron microscopy, along with the in-situ measurements of stress evolution during growth. Finally, lasing was obtained from the UV (394 nm) to blue (436 nm) wavelengths, in electrically injected, edge-emitting, cleaved-facet laser diodes with 480 nm thick Si-doped Al 1-xIn xN/GaN multilayers as bottom waveguide claddings. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jcrysgro.2011.10.016
  • 2012 • 54 Interactions of multicationic bis(guanidiniocarbonylpyrrole) receptors with double-stranded nucleic acids: Syntheses, binding studies, and atomic force microscopy imaging
    Klemm, K. and Stojković, M.R. and Horvat, G. and Tomišić, V. and Piantanida, I. and Schmuck, C.
    Chemistry - A European Journal 18 1352-1363 (2012)
    Compounds 1-3, composed of two guanidiniocarbonylpyrrole moieties linked by oligoamide bridges and differing in number and type of basic groups, were prepared. The sites and degree of protonation of 1-3 depend strongly on the pH value. The interactions of these compounds with several double-stranded (ds) DNA and dsRNA were investigated by means of UV/Vis and CD spectroscopy as well as isothermal titration microcalorimetry (ITC). These studies revealed that the binding of 1-3 to the polynucleotides is driven by three factors, the presence of aliphatic amino groups, the protonation state of the compounds, and the steric properties of the polynucleotide binding site, that is, the shape and structure of their grooves. The results obtained by all applied methods consistently indicated that receptors 1-3 bind to the minor groove of DNA, but, by contrast, to the major groove of RNA. Additionally, it was shown by atomic force microscopy (AFM) imaging that upon interaction of compound 2 with calf thymus (ct) DNA induced aggregation of the DNA occurs, leading to pronounced changes in its secondary structure. © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201101544
  • 2012 • 53 Protective ability of hybrid nano-composite coatings with cerium sulphate as inhibitor against corrosion of AA2024 aluminium alloy
    Kozhukharov, S. and Kozhukharov, V. and Schem, M. and Aslan, M. and Wittmar, M. and Wittmar, A. and Veith, M.
    Progress in Organic Coatings 73 95-103 (2012)
    The corrosion protective ability of hybrid oxy silane nano-composite coatings deposited on AA2024 by sol-gel technique was studied. The coatings are developed as an environmentally friendly alternative of the toxic chromium containing coatings on aluminium. A cerium salt, Ce2(SO 4)3, was used as inhibitor of the corrosion process. Two methods were applied to introduce the salt in the hybrid matrix: directly in the matrix, or by porous Al2O3 nano-particles preliminary loaded by the salt. Atomic force microscopy (AFM) was used to evaluate the superficial morphology of the coatings, while their layer structure was studied by means of scanning electron microscopy (SEM). Linear voltammetry (LVA) and electrochemical impedance spectroscopy (EIS) were used for assessment of the barrier ability. The hybrid matrix was found to possess remarkable barrier ability which was preserved even after prolonged exposure of the coatings to a model corrosive medium of 0.05 M NaCl. In all cases, the cerium salt involved either directly or by Al2O3 nano-particles proved to deteriorate the protective properties of the coatings and to accelerate pitting nucleation. The experimental results have shown that cerium sulphate, introduced in the by the both manners in the hybrid matrix did not efficiently inhibit the corrosion of AA2024, unlike the reported inhibiting properties of other cerium salts. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.porgcoat.2011.09.005
  • 2012 • 52 New insights into hard phases of CoCrMo metal-on-metal hip replacements
    Liao, Y. and Pourzal, R. and Stemmer, P. and Wimmer, M.A. and Jacobs, J.J. and Fischer, A. and Marks, L.D.
    Journal of the Mechanical Behavior of Biomedical Materials 12 39-49 (2012)
    The microstructural and mechanical properties of the hard phases in CoCrMo prosthetic alloys in both cast and wrought conditions were examined using transmission electron microscopy and nanoindentation. Besides the known carbides of M23C6-type (M=Cr, Mo, Co) and M6C-type which are formed by either eutectic solidification or precipitation, a new mixed-phase hard constituent has been found in the cast alloys, which is composed of ~100nm fine grains. The nanosized grains were identified to be mostly of M23C6 type using nano-beam precession electron diffraction, and the chemical composition varied from grain to grain being either Cr- or Co-rich. In contrast, the carbides within the wrought alloy having the same M23C6 structure were homogeneous, which can be attributed to the repeated heating and deformation steps. Nanoindentation measurements showed that the hardness of the hard phase mixture in the cast specimen was ~15.7GPa, while the M23C6 carbides in the wrought alloy were twice as hard (~30.7GPa). The origin of the nanostructured hard phase mixture was found to be related to slow cooling during casting. Mixed hard phases were produced at a cooling rate of 0.2°C/s, whereas single phase carbides were formed at a cooling rate of 50°C/s. This is consistent with sluggish kinetics and rationalizes different and partly conflicting microstructural results in the literature, and could be a source of variations in the performance of prosthetic devices in-vivo. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2012.03.013
  • 2012 • 51 Template-directed mild synthesis of anatase hybrid nanotubes within cylindrical core-shell-corona polymer brushes
    Müllner, M. and Lunkenbein, T. and Schieder, M. and Gröschel, A.H. and Miyajima, N. and Förtsch, M. and Breu, J. and Caruso, F. and Müller, A.H.E.
    Macromolecules 45 6981-6988 (2012)
    We demonstrate the synthesis of uniform one-dimensional (1D) titania hybrid nanotubes using core-shell-corona cylindrical polymer brushes (CPBs) as soft templates. The CPBs consist of a polymethacrylate backbone with densely grafted poly(ε-caprolactone) (PCL) as the core, poly(2-(dimethlamino)ethyl methacrylate) (PDMAEMA) as the cationic shell, and poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) as the corona. The weak polyelectrolyte shell complexed an oppositely charged titania precursor, namely titanium(IV) bis(ammonium lactate) dihydroxide (TALH), and then acted as a nanoreactor for the hydrolysis and condensation of TALH, resulting in crystalline TiO 2. The POEGMA shell provides solubility in aqueous and organic solvents. The hybrid titania nanotubes containing anatase nanoparticles were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electrion microscopy (SEM). The phase purity of the crystalline nanostructures was verified by powder X-ray diffractometry (PXRD). © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ma301232m
  • 2012 • 50 Stability of platinum nanoparticles supported on SiO2/Si(111): A high-pressure X-ray photoelectron spectroscopy study
    Porsgaard, S. and Merte, L.R. and Ono, L.K. and Behafarid, F. and Matos, J. and Helveg, S. and Salmeron, M. and Roldan Cuenya, B. and Besenbacher, F.
    ACS Nano 6 10743-10749 (2012)
    The stability of Pt nanoparticles (NPs) supported on ultrathin SiO 2 films on Si(111) was investigated in situ under H2 and O2 (0.5 Torr) by high-pressure X-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtOx desorption from the Pt NPs (∼2 and ∼4 nm NP sizes) at temperatures above 450 C in the presence of 0.5 Torr of O2. Under oxidizing conditions, the entire NPs were oxidized, although with no indication of a PtO2 phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e., minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nn3040167
  • 2012 • 49 Self-assembly of a triple-zwitterion in polar solutions: Hierarchical formation of nanostructures
    Rehm, T.H. and Gröhn, F. and Schmuck, C.
    Soft Matter 8 3154-3162 (2012)
    The self-assembly of triple-zwitterion 1 in dimethyl sulfoxide is described. 1 forms dimers which polymerize upon increasing concentration into flexible twisted ribbons. These ribbons finally coil into soft nanospheres. The step-wise aggregation process is primarily based on the very strong tendency of the self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion to form H-bond assisted ion paired dimers (K &gt; 10 10 M -1 in DMSO). Using a variety of analytical techniques such as diffusion ordered NMR spectroscopy (DOSY NMR), small angle neutron scattering (SANS), as well as atomic force microscopy (AFM) and molecular modelling for visualization, a consistent picture of the hierarchical self-assembly process of triple-zwitterion 1 in solution is obtained. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2sm07153c
  • 2012 • 48 Nanolattices of switchable DNA-based motors
    Saccà, B. and Siebers, B. and Meyer, R. and Bayer, M. and Niemeyer, C.M.
    Small 8 3000-3008 (2012)
    Miniaturization is an important aspect of device fabrication. Despite the advancements of modern top-down approaches, scaling-down to the sub-nanometer size is still a challenge. As an alternative, bottom-up approaches, such as the use of DNA as an engineering material, are therefore emerging, allowing control of matter at the single-molecule level. A DNA-based self-assembly method for the construction of switchable DNA devices is descrbied here based on G-quadruplex moieties, which are patterned on quasi-planar DNA arrays with nanoscale precision. The reversible switching of the devices is triggered by addition of DNA sequences ('fuels') and translated into linear extension/contractile movements. The conformational change of the devices was visualized by atomic force microscopy and FRET spectroscopy. Steady state fluorescence spectroscopy indicated that scaffolding of the G4 motors to either individual tiles or extended superlattices had no significant impact on the switching and optical performance of the system. However, time-resolved spectroscopy revealed that ordering in the microstructural environment enhances the fraction of molecules subject to FRET. Altogether, our study confirms that DNA superstructures are well-suited scaffolds for accommodation of mechanically switchable units and thus opens the door to the development of more sophisticated nanomechanical devices. Ordered planar nanoarrays bearing DNA-responsive devices are produced by DNA self-assembly procedures. The nanomotors display extension and contraction movements in response to addition of DNA fuels with efficient cycling operation even after repetitive cycles. This DNA-based assembly of nanomechanical units offers full control over the spatial arrangement of each single molecule and opens the door to the development of more sophisticated nanomechanical devices. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201200703
  • 2012 • 47 Human high temperature requirement serine protease A1 (HTRA1) degrades tau protein aggregates
    Tennstaedt, A. and Pöpsel, S. and Truebestein, L. and Hauske, P. and Brockmann, A. and Schmidt, N. and Irle, I. and Saccà, B. and Niemeyer, C.M. and Brandt, R. and Ksiezak-Reding, H. and Tirniceriu, A.L. and Egensperger, R. and ...
    Journal of Biological Chemistry 287 20931-20941 (2012)
    Protective proteases are key elements of protein quality control pathways that are up-regulated, for example, under various protein folding stresses. These proteases are employed to prevent the accumulation and aggregation of misfolded proteins that can impose severe damage to cells. The high temperature requirement A (HtrA) family of serine proteases has evolved to perform important aspects of ATP-independent protein quality control. So far, however, no HtrA protease is known that degrades protein aggregates. We show here that human HTRA1 degrades aggregated and fibrillar tau, a protein that is critically involved in various neurological disorders. Neuronal cells and patient brains accumulate less tau, neurofibrillary tangles, and neuritic plaques, respectively, when HTRA1 is expressed at elevated levels. Furthermore, HTRA1 mRNA and HTRA1 activity are up-regulated in response to elevated tau concentrations. These data suggest that HTRA1 is performing regulated proteolysis during protein quality control, the implications of which are discussed. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc.
    view abstractdoi: 10.1074/jbc.M111.316232
  • 2012 • 46 Atomic layer deposition of HfO 2 thin films employing a heteroleptic hafnium precursor
    Xu, K. and Milanov, A.P. and Parala, H. and Wenger, C. and Baristiran-Kaynak, C. and Lakribssi, K. and Toader, T. and Bock, C. and Rogalla, D. and Becker, H.-W. and Kunze, U. and Devi, A.
    Chemical Vapor Deposition 18 27-35 (2012)
    The application of a heteroleptic hafnium amide-guanidinate precursor for the deposition of HfO 2 thin films via a water-assisted atomic layer deposition (ALD) process is demonstrated for the first time. HfO 2 films are grown in the temperature range 100-300 °C using the compound [Hf(NMe 2) 2(NMe 2-Guan) 2] (1). This compound shows self-limiting ALD-type growth characteristics with growth rates of the order of 1.0-1.2 Å per cycle in the temperature range 100-225 °C. The saturation behavior and a linear dependence on film thickness as a function of number of cycles are verified at various temperatures within the ALD window. The as-deposited HfO 2 films are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS), and electrical measurements. For a direct comparison of the precursor performance with that of the parent alkyl amide [Hf(NMe 2) 4] (2), ALD experiments are also performed employing compound 2 under similar process conditions, and in this case no typical ALD characteristics are observed. The application of a heteroleptic hafnium amide-guanidinate precursor [Hf(NMe 2) 2(NMe 2-Guan) 2] for the deposition of HfO 2 thin films via a water assisted ALD process has been demonstrated for the first time. This compound showed self-limiting ALD type growth characteristics with the growth rates as high as 1.0-1.2 Å per cycle in the temperature range 100-225 °C. Typical ALD characteristics such as saturation behavior and linear dependence on the film thickness as a function of number of cycles were verified at different temperatures within the ALD window. The as-deposited HfO 2 films were characterized by AFM, SEM, RBS, XPS and electrical measurements. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201106934
  • 2012 • 45 Atomic layer deposition of Gd 2O 3 and Dy 2O 3: A study of the ALD characteristics and structural and electrical properties
    Xu, K. and Ranjith, R. and Laha, A. and Parala, H. and Milanov, A.P. and Fischer, R.A. and Bugiel, E. and Feydt, J. and Irsen, S. and Toader, T. and Bock, C. and Rogalla, D. and Osten, H.-J. and Kunze, U. and Devi, A.
    Chemistry of Materials 24 651-658 (2012)
    Gd 2O 3 and Dy 2O 3 thin films were grown by atomic layer deposition (ALD) on Si(100) substrates using the homoleptic rare earth guanidinate based precursors, namely, tris(N,N′- diisopropyl-2-dimethylamido-guanidinato)gadolinium(III) [Gd(DPDMG) 3] (1) and tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)dysprosium(III) [Dy(DPDMG) 3] (2), respectively. Both complexes are volatile and exhibit high reactivity and good thermal stability, which are ideal characteristics of a good ALD precursor. Thin Gd 2O 3 and Dy 2O 3 layers were grown by ALD, where the precursors were used in combination with water as a reactant at reduced pressure at the substrate temperature ranging from 150 °C to 350 °C. A constant growth per cycle (GPC) of 1.1 Å was obtained at deposition temperatures between 175 and 275 °C for Gd 2O 3, and in the case of Dy 2O 3, a GPC of 1.0 Å was obtained at 200-275 °C. The self-limiting ALD growth characteristics and the saturation behavior of the precursors were confirmed at substrate temperatures of 225 and 250 °C within the ALD window for both Gd 2O 3 and Dy 2O 3. Thin films were structurally characterized by grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analyses for crystallinity and morphology. The chemical composition of the layer was examined by Rutherford backscattering (RBS) analysis and Auger electron spectroscopy (AES) depth profile measurements. The electrical properties of the ALD grown layers were analyzed by capacitance-voltage (C-V) and current-voltage (I-V) measurements. Upon subjection to a forming gas treatment, the ALD grown layers show promising dielectric behavior, with no hysteresis and reduced interface trap densities, thus revealing the potential of these layers as high-k oxide for application in complementary metal oxide semiconductor based devices. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm2020862
  • 2012 • 44 Hybrid sol-gel silica films with (TiO 2-CeO 2) binary nanopowders
    Zaharescu, M. and Nicolescu, M. and Gartner, M. and Barau, A. and Predoana, L. and Anastasescu, M. and Stoica, M. and Szekeres, A.
    Journal of Physics: Conference Series 356 (2012)
    In the present work the preparation of hybrid sol-gel silica coatings doped with binary TiO 2-CeO 2 nanopowders was studied. The oxide powder was embedded in the hybrid matrix either by in-situ generation or by previously prepared powder dispersion. The main objective of the work was to establish a correlation between the method of generation of the dopant particles in the system and the properties of the films. The films were deposited on silicon wafer and glass substrates by the 'dip-coating' method and characterized in the as-prepared stage and after annealing at 120 °C. The optical and morphological properties of the films deposited on glass and silicon wafer were determined by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/356/1/012018
  • 2012 • 43 Orientation informed nanoindentation of α-titanium: Indentation pileup in hexagonal metals deforming by prismatic slip
    Zambaldi, C. and Yang, Y. and Bieler, T.R. and Raabe, D.
    Journal of Materials Research 27 356-367 (2012)
    This study reports on the anisotropic indentation response of α-titanium. Coarse-grained titanium was characterized by electron backscatter diffraction. Sphero-conical nanoindentation was performed for a number of different crystallographic orientations. The grain size was much larger than the size of the indents to ensure quasi-single-crystal indentation. The hexagonal c-axis was determined to be the hardest direction. Surface topographies of several indents were measured by atomic force microscopy. Analysis of the indent surfaces, following Zambaldi and Raabe (Acta Mater. 58(9), 3516-3530), revealed the orientation-dependent pileup behavior of α-titanium during axisymmetric indentation. Corresponding crystal plasticity finite element (CPFE) simulations predicted the pileup patterns with good accuracy. The constitutive parameters of the CPFE model were identified by a nonlinear optimization procedure, and reproducibly converged toward easy activation of prismatic glide systems. The calculated critical resolved shear stresses were 150 ± 4, 349 ± 10, and 1107 ± 39 MPa for prismatic and basal «a»-glide and pyramidal «c + a»-glide, respectively. © 2011 Materials Research Society.
    view abstractdoi: 10.1557/jmr.2011.334
  • 2011 • 42 Plasma enhanced-CVD of undoped and fluorine-doped Co3O 4 nanosystems for novel gas sensors
    Barreca, D. and Bekermann, D. and Comini, E. and Devi, A. and Fischer, R.A. and Gasparotto, A. and Gavagnin, M. and MacCato, C. and Sada, C. and Sberveglieri, G. and Tondello, E.
    Sensors and Actuators, B: Chemical 160 79-86 (2011)
    Co3O4-based nanosystems were prepared on polycrystalline Al2O3 by plasma enhanced-chemical vapor deposition (PE-CVD), at temperatures ranging between 200 and 400 °C. The use of two different precursors, Co(dpm)2 (dpm = 2,2,6,6-tetramethyl-3, 5-heptanedionate) and Co(hfa)2·TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′- tetramethylethylenediamine) enabled the synthesis of undoped and fluorine-doped Co3O4 specimens, respectively. A thorough characterization of their properties was performed by glancing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), field emission-scanning electron microscopy (FE-SEM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). For the first time, the gas sensing properties of such PE-CVD nanosystems were investigated in the detection of ethanol and acetone. The results show an appreciable response improvement upon doping and functional performances directly dependent on the fluorine content in the Co3O4 system. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.snb.2011.07.016
  • 2011 • 41 MOCVD of ZnO films from bis(ketoiminato)Zn(II) precursors: Structure, morphology and optical properties
    Bekermann, D. and Ludwig, Ar. and Toader, T. and MacCato, C. and Barreca, D. and Gasparotto, A. and Bock, C. and Wieck, A.D. and Kunze, U. and Tondello, E. and Fischer, R.A. and Devi, A.
    Chemical Vapor Deposition 17 155-161 (2011)
    Two closely related bis(ketoiminato) zinc precursors, which are air stable and possess favorable properties for metal-organic (MO)CVD, are successfully employed for the growth of ZnO films on silicon and borosilicate glass substrates at temperatures between 400 and 700 °C. The as-deposited films are investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), nuclear reaction analysis (NRA), as well as by UV-vis absorption spectroscopy and photoluminescence (PL) measurements. The structure, morphology, and composition of the as-grown films show a strong dependence on the substrate temperature. The formation of pure and (001)-oriented wurtzite-type stoichiometric ZnO is observed. PL measurements are performed both at room temperature and 77 K, revealing a defect-free emission of ZnO films. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201006898
  • 2011 • 40 Doping of graphene exfoliated on SrTiO3
    Bußmann, B.K. and Ochedowski, O. and Schleberger, M.
    Nanotechnology 22 (2011)
    We present atomic force microscopy and scanning Kelvin probe data obtained under ultra-high vacuum conditions from graphene exfoliated on crystalline SrTiO3 substrates. The contact potential difference shows a monotonic increase with the number of graphene layers until after five layers of saturation is reached. By identifying the saturation value with the work function of graphite we determine the work function of single and bilayer graphene to be ΦSLG = 4.409 0.039eV and ΦBLG = 4.516 0.035eV, respectively. In agreement with the higher work function of single-layer graphene with respect to free-standing graphene, our measurements indicate an accumulation of charge carriers corresponding to a doping of the exfoliated graphene layer with electrons. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/26/265703
  • 2011 • 39 Interface failure and adhesion measured by focused ion beam cutting of metal-polymer interfaces
    Cordill, M.J. and Schmidegg, K. and Dehm, G.
    Philosophical Magazine Letters 91 530-536 (2011)
    New developments in flexible electronics require metal films to adhere to polymer substrates. Measuring the interfacial adhesion of these systems is challenging, requiring the formulation of new techniques and models. A strategy to measure the adhesion of Cr-polyethylene terephthalate (PET) interfaces using tensile straining and buckle formation is presented in this article. Focused ion beam cross-sectioning of the buckles reveals that the polymer substrate can locally fail, which may lead to an overestimate of adhesion. Cr-PET adhesion energy of 9.4±1.6 J/m2 is determined with the present approach. © 2011 Taylor &Francis.
    view abstractdoi: 10.1080/09500839.2011.593575
  • 2011 • 38 Basic investigation of HfO2 based metal-insulator-metal diodes
    Dudek, P. and Schmidt, R. and Lukosius, M. and Lupina, G. and Wenger, C. and Abrutis, A. and Albert, M. and Xu, K. and Devi, A.
    Thin Solid Films 519 5796-5799 (2011)
    Very fast frequency response of metal-insulator-metal (MIM) diodes extends into the terahertz regime making them attractive as key elements as alternative to photovoltaic solar energy harvesting and ultrahigh speed wireless communication systems. The tunnelling phenomena, which is crucial for achieving high performance in these devices is extremely sensitive to the nanoscale structural and chemical quality of interface regions. Modern chemical deposition techniques like Pulsed Injected Metal-Organic Chemical Vapour Deposition (PICVD), Atomic Layer Deposition (ALD) and Atomic Vapour Deposition (AVD®) will be used for the extremely precise growth of thin HfO2 films on TiN bottom electrodes. However, different deposition techniques may give unpredictably different results in terms of film density, surface and interface property and consequently in physical properties of the device. In this work, the influence of deposition techniques on the charge transport characteristics of HfO2 MIM diodes was investigated by Conducting Atomic Force Microscopy (C-AFM) and X-ray Photoelectron Spectroscopy (XPS). © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.12.195
  • 2011 • 37 Self-assembly of calcium phosphate nanoparticles into hollow spheres induced by dissolved amino acids
    Hagmeyer, D. and Ganesan, K. and Ruesing, J. and Schunk, D. and Mayer, C. and Dey, A. and Sommerdijk, N.A.J.M. and Epple, M.
    Journal of Materials Chemistry 21 9219-9223 (2011)
    Nanoparticles of calcium phosphate assemble spontaneously within a few seconds into hollow spheres with a diameter around 200-300 nm in the presence of dissolved amino acids and dipeptides. The process of formation was followed by cryo-transmission electron microscopy (cryoTEM), proving their hollow nature which was also confirmed by nano-indentation by atomic force microscopy (AFM). The hollow spheres were analyzed by scanning electron microscopy (SEM), dynamic light scattering (DLS), nanoparticle tracking analysis (NTA) and elemental analysis. The hollow spheres were moderately stable against heating and ultrasonication. A self-assembly of the primarily formed calcium phosphate nanoparticles around amino acid-rich domains in water is proposed. As this process was observed with different amino acids, it appears to be a more general phenomenon. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1jm11316j
  • 2011 • 36 Atomic vapor deposition approach to In 2O 3 thin films
    Hellwig, M. and Parala, H. and Cybinksa, J. and Barreca, D. and Gasparotto, A. and Niermann, B. and Becker, H.-W. and Rogalla, D. and Feydt, J. and Irsen, S. and Mudring, A.-V. and Winter, J. and Fischer, R.A. and Devi, A.
    Journal of Nanoscience and Nanotechnology 11 8094-8100 (2011)
    In 2O 3 thin films were grown by atomic vapor deposition (AVD) on Si(100) and glass substrates from a tris-guanidinate complex of indium [In(NiPr 2guanid) 3] under an oxygen atmosphere. The effects of the growth temperature on the structure, morphology and composition of In 2O 3 films were investigated. X-ray diffraction (XRD) measurements revealed that In 2O 3 films deposited in the temperature range 450-700°C crystallised in the cubic phase. The film morphology, studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), was strongly dependent on the substrate temperature. Stoichiometric In 2O 3 films were formed under optimised processing conditions as was confirmed by X-ray photoelectron and X-ray excited Auger electron spectroscopies (XPS, XE-AES), as well as by Rutherford backscattering spectrometry (RBS). Finally, optical properties were investigated by photoluminescence (PL) measurements, spectroscopic ellipsometry (SE) and optical absorption. In 2O 3 films grown on glass exhibited excellent transparency (≈90%) in the Visible (Vis) spectral region. Copyright © 2011 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5024
  • 2011 • 35 Methylated [(arene)(1,3-cyclohexadiene)Ru(0)] complexes as low-melting MOCVD precursor complexes with a controlled follow-up chemistry of the ligands
    Jipa, I. and Danova, K. and Popovska, N. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Cremer, T. and Maier, F. and Marbach, H. and Steinrück, H.-P. and Heinemann, F.W. and Zenneck, U.
    Journal of Materials Chemistry 21 3014-3024 (2011)
    [(Benzene)(2-methyl-1,3-cyclohexadiene)Ru(0)] (1), [(1,3-cyclohexadiene) (toluene)Ru(0)] (2), and [(methyl-cyclohexadiene)(toluene)Ru(0)] (3, mixture of isomers) have been prepared and tested as new metal organic ruthenium precursor complexes for chemical vapor deposition (MOCVD) with favorable properties. 1 is a low-melting precursor complex (mp = 29 °C) and the isomeric mixture 3 forms a liquid at room temperature. X-ray diffraction studies of single crystals of complexes 1 and 2 are characteristic for true Ru(0) π-complexes without molecular structure peculiarities or significant intermolecular interactions in the solid state, which could hinder undecomposed evaporation. Differential thermal analysis (DTA), differential scanning calorimetry (DSC) and vapor pressure data qualify the compounds as almost ideal MOCVD precursors. Thin ruthenium films have been deposited successfully on silicon wafers and substrate temperatures between 200 and 450 °C in inert gas atmospheres. Film growth and properties were evaluated by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and four-point probe conductivity measurements. All films consist of polycrystalline metallic ruthenium with a low surface roughness. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c0jm02652b
  • 2011 • 34 Methylated [(benzene)(1,3-butadiene)Ru0] derivatives as novel MOCVD precursors with favorable properties
    Jipa, I. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Marbach, H. and Cremer, T. and Maier, F. and Steinrück, H.-P. and Danova, K. and Popovska, N. and Heinemann, F.W. and Zenneck, U.
    Chemical Vapor Deposition 17 15-21 (2011)
    [(Benzene)(2-methyl-1,3-butadiene)Ru0] (1), [(benzene)(2,3- dimethyl-1,3-butadiene)Ru0] (2), and [(2,3-dimethyl-1,3-butadiene) (toluene)Ru0] (3) are prepared and tested as new metal-organic (MO) ruthenium precursor complexes with favorable deposition properties for the CVD of thin ruthenium films. X-ray diffraction (XRD) studies of single crystals of the complexes are characteristic for true Ru0 π-complexes without molecular structure peculiarities or significant intermolecular interactions in the solid state, which can hinder undecomposed evaporation. Differential thermal analysis (DTA) and vapor pressure data qualify the compounds as almost ideal MOCVD precursors. Thin ruthenium films are deposited successfully on silicon wafers at substrate temperatures between 200 and 400°C in a nitrogen gas atmosphere. X-ray photoelectron spectroscopy (XPS), four-point probe conductivity measurements, and atomic force microscopy (AFM) are used to characterize the films. All films consist of polycrystalline metallic ruthenium with a low surface roughness. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201006853
  • 2011 • 33 MBE growth optimization of topological insulator Bi2Te 3 films
    Krumrain, J. and Mussler, G. and Borisova, S. and Stoica, T. and Plucinski, L. and Schneider, C.M. and Grützmacher, D.
    Journal of Crystal Growth 324 115-118 (2011)
    We investigated the growth of the topological insulator Bi 2Te3 on Si(1 1 1) substrates by means of molecular-beam epitaxy (MBE). The substrate temperature as well as the Bi and Te beam-equivalent pressure (BEP) was varied in a large range. The structure and morphology of the layers were studied using X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM). The layer-by-layer growth mode with quintuple layer (QL) as an unit is accomplished on large plateaus if the MBE growth takes place in a Te overpressure. At carefully optimized MBE growth parameters, we obtained atomically smooth, single-crystal Bi 2Te3 with large area single QL covering about 75% of the layer surface. Angular-resolved photoelectron spectroscopy reveals a linear energy dispersion of charge carriers at the surface, evidencing topologically insulating properties of the Bi2Te3 epilayers. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2011.03.008
  • 2011 • 32 Influence of dislocation density on the pop-in behavior and indentation size effect in CaF2 single crystals: Experiments and molecular dynamics simulations
    Lodes, M.A. and Hartmaier, A. and Göken, M. and Durst, K.
    Acta Materialia 59 4264-4273 (2011)
    In this work, the indentation size effect and pop-in behavior are studied for indentations in undeformed and locally pre-deformed CaF2 single crystals, using both nanoindentation experiments and molecular dynamics simulations. To study the influence of dislocation density on the indentation behavior, small-scale indentations are carried out inside the plastic zone of larger indentations. This experiment is mimicked in the simulations by indenting a small sphere into the center of the residual impression of a larger sphere. The undeformed material shows the well-known pop-in behavior followed by the indentation size effect. Pre-deforming the material leads to a reduction in the indentation size effect both for experiments and simulations, which is in accordance with the Nix-Gao theory. Furthermore, the pop-in load is reduced in the experiments, whereas a smooth transition from elastic to plastic deformation is found in the simulations. There, plasticity is initiated by the movement of pre-existing dislocation loops in the vicinity of the plastic zone. The simulations thus give a detailed insight into the deformation mechanism during indentation and highlight the importance of the dislocation microstructure for the indentation size effect and dislocation nucleation. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.03.050
  • 2011 • 31 Growth process, characterization, and modeling of electronic properties of coupled InAsSbP nanostructures
    Marquardt, O. and Hickel, T. and Neugebauer, J. and Gambaryan, K.M. and Aroutiounian, V.M.
    Journal of Applied Physics 110 (2011)
    Quaternary III-V InAsSbP quantum dots (QDs) have been grown in the form of cooperative InAsSb/InAsP structures using a modified version of the liquid phase epitaxy. High resolution scanning electron microscopy, atomic force microscopy, and Fourier-transform infrared spectrometry were used to investigate these so-called nano-camomiles, mainly consisting of a central InAsSb QD surrounded by six InAsP-QDs, that shall be referred to as leaves in the following. The observed QDs average density ranges from 0.8 to 2 × 109 cm -2, with heights and widths dimensions from 2 to 20 nm and 5 to 45 nm, respectively. The average density of the leaves is equal to (6-10) × 109 cm-2 with dimensions of approx. 5 to 40 nm in width and depth. To achieve a first basic understanding of the electronic properties, we have modeled these novel nanostructures using second-order continuum elasticity theory and an eight-band k p model to calculate the electronic structure. Our calculations found a clear localization of hole states in the central InAsSb dot. The localization of electron states, however, was found to be weak and might thus be easily influenced by external electric fields or strain. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3624621
  • 2011 • 30 Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots
    Marquardt, B. and Geller, M. and Baxevanis, B. and Pfannkuche, D. and Wieck, A.D. and Reuter, D. and Lorke, A.
    Nature Communications 2 (2011)
    Self-assembled quantum dots (QDs) are prominent candidates for solid-state quantum information processing. For these systems, great progress has been made in addressing spin states by optical means. In this study, we introduce an all-electrical measurement technique to prepare and detect non-equilibrium many-particle spin states in an ensemble of self-assembled QDs at liquid helium temperature. The excitation spectra of the one- (QD hydrogen), two- (QD helium) and three- (QD lithium) electron configuration are shown and compared with calculations using the exact diagonalization method. An exchange splitting of 10 meV between the excited triplet and singlet spin states is observed in the QD helium spectrum. These experiments are a starting point for an all-electrical control of electron spin states in self-assembled QDs above liquid helium temperature. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms1205
  • 2011 • 29 Oxygen chemisorption, formation, and thermal stability of Pt oxides on Pt nanoparticles supported on SiO2/Si(001): Size effects
    Ono, L.K. and Croy, J.R. and Heinrich, H. and Roldan Cuenya, B.
    Journal of Physical Chemistry C 115 16856-16866 (2011)
    The changes induced in the structure and chemical state of size-selected Pt nanoparticles (NPs) supported on ultrathin SiO2 films upon exposure to oxygen have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), in situ X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For low atomic oxygen exposures, chemisorbed oxygen species were detected on all samples. Exposure to higher atomic oxygen coverages at room temperature leads to the formation and stabilization of PtOx species (PtO2 and PtO). On all samples, a two-step thermal decomposition process was observed upon annealing in ultrahigh vacuum: PtO2 → PtO → Pt. For NPs in the 2-6 nm range, the NP size was found to affect the strength of the O binding. Contrary to the case of Pt(111), where no oxides were detected above 700 K, 10-20% PtO was detected on the NP samples via XPS at the same temperature, suggesting the presence of strongly bound oxygen species. In addition, for identical atomic oxygen exposures, decreasing the NP size was found to favor their ability to form oxides. Interestingly, regardless of whether the desorption of chemisorbed oxygen species or that of oxygen in PtOx species was considered, our TPD data revealed higher O2-desorption temperatures for the Pt NPs as compared with the Pt(111) surface. Furthermore, a clear size-dependent trend was observed, with an increase in the strength of the oxygen bonding with decreasing NP size. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp204743q
  • 2011 • 28 Covalent tethering of protruding arms for addressable DNA nanostructures
    Saccà, B. and Niemeyer, C.M.
    Small 7 2887-2898 (2011)
    Functionalization of self-assembled DNA nanostructures is of fundamental importance for the realization of their application in nanotechnology and biosensing. Approaches reported so far suffer from lack of general applicability and usually require careful system design to avoid poor yields in the assembly of target structures. A novel approach well suited for fabrication of addressable DNA superstructures is reported here to generate DNA tile motifs. The method is based on the covalent linkage of a single-stranded protruding arm (covPA) to one of the oligomers forming the tile. Subsequent to assembly of tile motifs and superlattices, the covPA can be addressed by hybridization with complementary oligonucleotides or DNA-protein conjugates. The covPA can be located at arbitrary positions in a given tile motif without changing the general design and without compromising the structural integrity of the tile. The covPA strategy can also be readily extended to different PA sequences and multiple covPA arms can be linked to a tile. Superlattices obtained by self-assembly of covPA tiles reveal partial folding into double layers which possess an intrinsic order at the ultrastructural level. This phenomenon is likely associated with the increased flexibility of the covPA and might open up novel ways for DNA-based functionalization of solid surfaces and other applications of structural DNA nanotechnology. A novel class of DNA tile motifs bears a covalently linked, single-stranded protruding arm. The increased flexibility of this arm enables efficient modification of tiles and superlattices with complementary DNA-linked objects and induces formation of partially folded bilayers which posses an intrinsic order at the ultrastructural level. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201101010
  • 2011 • 27 Wetting morphologies and their transitions in grooved substrates
    Seemann, R. and Brinkmann, M. and Herminghaus, S. and Khare, K. and Law, B.M. and McBride, S. and Kostourou, K. and Gurevich, E. and Bommer, S. and Herrmann, C. and Michler, D.
    Journal of Physics Condensed Matter 23 (2011)
    When exposed to a partially wetting liquid, many natural and artificial surfaces equipped with complex topographies display a rich variety of liquid interfacial morphologies. In the present article, we focus on a few simple paradigmatic surface topographies and elaborate on the statics and dynamics of the resulting wetting morphologies. It is demonstrated that the spectrum of wetting morphologies increases with increasing complexity of the groove structure. On elastically deformable substrates, additional structures in the liquid morphologies can be observed, which are caused by deformations of the groove geometry in the presence of capillary forces. The emergence of certain liquid morphologies in grooves can be actively controlled by changes in wettability and geometry. For electrically conducting solid substrates, the apparent contact angle can be varied by electrowetting. This allows, depending on groove geometry, a reversible or irreversible transport of liquid along surface grooves. In the case of irreversible liquid transport in triangular grooves, the dynamics of the emerging instability is sensitive to the apparent hydrodynamic slip at the substrate. On elastic substrates, the geometry can be varied in a straightforward manner by stretching or relaxing the sample. The imbibition velocity in deformable grooves is significantly reduced compared to solid grooves, which is a result of the microscopic deformation of the elastic groove material close to the three phase contact line. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184108
  • 2011 • 26 Amphiphilic gold nanoparticles: Synthesis, characterization and adsorption to PEGylated polymer surfaces
    Tarnawski, R. and Ulbricht, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 374 13-21 (2011)
    The direct synthesis of water-soluble gold nanoparticles with a mixed shell of two different thiols, 1-mercaptoundec-11-yl-hexa(ethylene glycol) (EG6) and dodecanethiol (C12), and their characterization are reported. Data from IR spectroscopy and contact angle (CA) measurements as well as the solubility of the nanoparticles in water support that the composition of the shell is in the range of the thiol ratio used for synthesis (EG6:C12 = 72:28). Results of transmission electron microscopy and atomic force microscopy (AFM) for deposited particles as well as the UV-vis spectrum in solution are in line with a size of ≤10. nm. Self-assembled monolayers (SAMs) as model surfaces were prepared from mixtures of EG6 and C12 on planar gold films. Polystyrene (PSt) spin-coated films on silicon wafers and on gold-coated surface plasmon resonance (SPR) sensor disks were used as substrates for surface functionalization via adsorption/self-assembly of a polystyrene poly(ethylene glycol) diblock copolymer (PSt- b-PEG) from aqueous solutions. CA and AFM results revealed pronounced differences of the hydrophilicity/hydrophobicity and topography of the surface as a function of PSt- b-PEG concentration used for the modification. The adsorption of myoglobin and the novel gold nanoparticles to the PSt- b-PEGylated surfaces was analyzed by SPR. A control of adsorbed amounts by the degree of surface PEGylation, i.e. a reduction by up to 55% for the highest degree of modification, could be confirmed for both kinds of colloids. Adsorption of the novel gold nanoparticles to the mixed SAM surfaces as analyzed by SPR showed an even stronger dependency of surface composition. All experiments demonstrate that amphiphilic, water-soluble gold-based nanoparticles can be used as model colloids for the investigation of interactions with polymer surfaces of varied structure and architecture, and that they could be further developed for analytical or biological applications. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2010.10.027
  • 2011 • 25 Thermally driven solid-phase epitaxy of laser-ablated amorphous AlFe films on (0001)-oriented sapphire single crystals
    Trautvetter, M. and Wiedwald, U. and Paul, H. and Minkow, A. and Ziemann, P.
    Applied Physics A: Materials Science and Processing 102 725-730 (2011)
    Solid-phase epitaxy is demonstrated for the metallic binary alloy AlFe. Stoichiometric thin films are deposited at ambient temperature onto c-cut sapphire by pulsed laser deposition (PLD), resulting in smooth amorphous films as revealed by X-ray diffraction (XRD) and atomic force microscopy (AFM). By annealing at 600°C, still smooth epitaxial AlFe films are obtained exhibiting the B2 phase with the (110) direction parallel to the substrate normal and an in-plane orientation as given by AlFe[001]||Al2O 3[112̄0]. While ferromagnetism is observed for the amorphous phase, the formation of the B2 structure is accompanied by paramagnetic behavior, confirming the high structural quality. © Springer-Verlag 2010.
    view abstractdoi: 10.1007/s00339-010-5972-x
  • 2010 • 24 Perfluorocarbon-filled poly(lactide-co-gylcolide) nano- and microcapsules as artificial oxygen carriers for blood substitutes: A physico-chemical assessment
    Bauer, J. and Zähres, M. and Zellermann, A. and Kirsch, M. and Petrat, F. and De Groot, H. and Mayer, C.
    Journal of Microencapsulation 27 122-132 (2010)
    The physico-chemical suitability of perfluorocarbon-filled capsules as artificial oxygen carriers for blood substitutes is assessed on the example of biodegradable poly(lactide-co-gylcolide) micro- and nanocapsules with a liquid content of perfluorodecalin. The morphology of the capsules is studied by confocal laser scanning microscopy using Nile red as a fluorescent marker. The mechanical stability and the wall flexibility of the capsules are examined by atomic force microscopy. The permeability of the capsule walls in connection with the oxygen uptake is detected by nuclear magnetic resonance. It is shown that the preparation in fact leads to nanocapsules with a mechanical stability which compares well with the one of red blood cells. The capsule walls exhibit sufficient permeability to allow for the exchange of oxygen in aqueous environment. In the fully saturated state, the amount of oxygen dissolved within the encapsulated perfluorodecalin in aqueous dispersion is as large as for bulk perfluorodecalin. Simple kinetic studies are presently restricted to the time scale of minutes, but so far indicate that the permeability of the capsule walls could be sufficient to allow for rapid gas exchange.
    view abstractdoi: 10.3109/02652040903052002
  • 2010 • 23 Amphiphilic polymer conetworks as matrices for phase transfer reactions
    Bruns, N. and Hanko, M. and Dech, S. and Ladisch, R. and Tobis, J. and Tiller, J.C.
    Macromolecular Symposia 291-292 293-301 (2010)
    Amphiphilic polymer conetworks were prepared, characterized and used as highly activating matrices for phase transfer reactions. Several applications such as biocatalysis in organic solvents, metathesis reactions in water, biosensor designs for detecting metabolites in organic solvents, as well as gas sensor designs are discussed. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/masy.201050534
  • 2010 • 22 γ-Fe2O3 nanoparticle adsorption at an OTS Langmuir monolayer
    Degen, P. and Paulus, M. and Leick, S. and Tolan, M. and Rehage, H.
    Colloid and Polymer Science 288 643-651 (2010)
    The assembling of magnetic nanoparticles in ordered structures as well as the preparation of very thin magnetic switchable polymer membranes is an important aim in many technical fields. We studied the influence of γ-Fe2O3 nanoparticles on the polymerization process and on the properties of the poly(organosiloxane)/nanoparticle-composite layer by surface rheological measurements, surface pressure/area (π/A) isotherm measurements, and Brewster angle microscopy. The adsorption process dynamics were studied by X-ray reflectivity and surface potential measurements. The results confirm the presence of attractive electrostatic interactions between the partial negatively charged monolayer and the positively charged nanoparticles. For further investigations, we prepared Langmuir-Blodgett layers of these polymer-nanoparticle composite and investigated them by atomic force microscopy and UV-Vis spectroscopy. We found that the concentration of nanoparticles was very low and the particles were mainly arranged below the polymer layer. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s00396-010-2191-0
  • 2010 • 21 Bending of single crystal microcantilever beams of cube orientation: Finite element model and experiments
    Demir, E. and Roters, F. and Raabe, D.
    Journal of the Mechanics and Physics of Solids 58 1599-1612 (2010)
    The aim of this work is to investigate the microstructure evolution, stressstrain response and strain hardening behavior of microscale beams. For that purpose, two single crystal cantilever beams in the size dependent regime were manufactured by ion beam milling and beams were bent with an indenter device. A crystal plasticity material model for large deformations was implemented in a finite element framework to further investigate the effect of boundary constraints. Simulations were performed using bulk material properties of single crystal copper without any special treatment for the strain gradients. The difference between the slopes of the experimental and the simulated force displacement curves suggested negligible amount of strain gradient hardening compared to the statistical hardening mechanisms. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.jmps.2010.07.007
  • 2010 • 20 Growth and characterization of ti-ta-o thin films on si substrates by liquid injection MOCVD for high-k applications from modified titanium and tantalum precursors
    Devi, A. and Hellwig, M. and Barreca, D. and Parala, H. and Thomas, R. and Becker, H.-W. and Katiyar, R.S. and Fischer, R.A. and Tondello, E.
    Chemical Vapor Deposition 16 157-165 (2010)
    Titanium oxide (TiO2) and titanium-tantalum oxide (Ti-Ta-O) thin films are deposited by liquid injection (LI) metal-organic (MO) CVD using metal amide-malonate complexes, [Ti(NR2)2 (dbml) 2], and tantalum, [Ta(NMe2)2 (dbml)] (R Me, Et; dbml di-tert-butylmalonato). TiO2 and Ti-Ta-O films are deposited on Si(100) in the temperature ranges 350-650°C and 500-700°C, respectively. The structure, morphology, and chemical composition of the films are evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), and X-ray photoelectron spectroscopy (XPS). The electrical properties of the films, namely the dielectric properties, are assessed by carrying out capacitance-voltage (C-V) measurements on metal-oxide-semiconductor (MOS) capacitor structures.
    view abstractdoi: 10.1002/cvde.200906813
  • 2010 • 19 Modification of GaAs surface by low-current Townsend discharge
    Gurevich, E.L. and Kittel, S. and Hergenröder, R. and Astrov, Y.A. and Portsel, L.M. and Lodygin, A.N. and Tolmachev, V.A. and Ankudinov, A.V.
    Journal of Physics D: Applied Physics 43 (2010)
    The influence of stationary spatially homogeneous Townsend discharge on the (1 0 0) surface of semi-insulating GaAs samples is studied. Samples exposed to both electrons and ions in a nitrogen discharge at a current density j = 60 μA cm-2 are studied by means of x-ray photoelectron spectroscopy, ellipsometry and atomic force microscopy. It is shown that an exposure to low-energy ions (< 1 eV) changes the crystal structure of the semiconductor for a depth of up to 10-20 nm, although the stoichiometric composition does not change. The exposure to low-energy electrons (< 10 eV) forms an oxide layer, which is 5-10 nm thick. Atomic force microscopy demonstrates that the change in the surface potential of the samples may exceed 100 mV, for both discharge polarities, while the surface roughness does not increase. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/43/27/275302
  • 2010 • 18 [cis-(1,3-diene) 2W(CO) 2] complexes as MOCVD precursors for the deposition of thin tungsten - Tungsten carbide films
    Jipa, I. and Heinemann, F.W. and Schneider, A. and Popovska, N. and Siddiqi, M.A. and Siddiqui, R.A. and Atakan, B. and Marbach, H. and Papp, C. and Steinrück, H.-P. and Zenneck, U.
    Chemical Vapor Deposition 16 239-247 (2010)
    Tungsten - tungsten carbide thin films are deposited by metal-organic (MO)CVD on silica-coated silicon wafers using [cis-(1,3-butadiene) 2W(CO) 2] and [cis-(1,3-cyclohexadiene) 2W(CO) 2], respectively, as tunable precursor complexes. The compounds are prepared through photochemical ligand exchange reactions from [W(CO) 6] and fully characterized, including X-ray structure determination and detailed differential thermal analysis (DTA)/thermogravimetry (TG) investigations. Gas-phase diffusion coefficients and the vapor pressure of the compounds are calculated. The MOCVD experiments are performed in a vertical cold-wall reactor and the exhaust gas is analyzed by gas chromatography (GC). X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) measurements are utilized for film characterization. Consequences of the high oxophilicity of freshly formed tungsten surfaces, consecutive surface reactions of the complex ligands, film growth, and film properties are discussed. Inside the layers, tungsten carbide is identified as the main component. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201006852
  • 2010 • 17 Energy threshold for the creation of nanodots on SrTiO3 by swift heavy ions
    Karlušić, M. and Akcöltekin, S. and Osmani, O. and Monnet, I. and Lebius, H. and Jakšić, M. and Schleberger, M.
    New Journal of Physics 12 (2010)
    We present experimental and theoretical data on the threshold behaviour of nanodot creation with swift heavy ions. A model calculation based on a two-temperature model that takes into account the spatially resolved electron density gives a threshold of 12keVnm-1 below which the energy density at the end of the track is no longer high enough to melt the material. In the corresponding experiments, we irradiated SrTiO3 surfaces under grazing incidence with swift heavy ions. The resulting chains of nanodots were analysed by atomic force microscopy (AFM). In addition, some samples of SrTiO3 irradiated under normal incidence were analysed by transmission electron microscopy (TEM). Both experiments showed two thresholds, which were connected with the appearance of tracks and the creation of fully developed tracks. The threshold values were similar for surface and bulk tracks, suggesting that the same processes occur at both glancing and normal incidence. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/12/4/043009
  • 2010 • 16 Single shot damage mechanism of Mo/Si multilayer optics under intense pulsed XUV-exposure
    Khorsand, A.R. and Sobierajski, R. and Louis, E. and Bruijn, S. and Van Hattum, E.D. and Vande Kruijs, R.W.E. and Jurek, M. and Klinger, D. and Pelka, J.B. and Juha, L. and Burian, T. and Chalupsky, J. and Cihelka, J. and Hajkova,...
    Optics Express 18 700-712 (2010)
    We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference- polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources. © 2010 Optical Society of America.
    view abstractdoi: 10.1364/OE.18.000700
  • 2010 • 15 Photothermally induced bromination and decomposition of alkylsiloxane monolayers on surface-oxidized silicon substrates
    Klingebiel, B. and Schröter, A. and Franzka, S. and Hartmann, N.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 28 834-837 (2010)
    Photothermal laser processing of alkylsiloxane monolayers in gaseous bromine is investigated. Surface-oxidized silicon samples are coated with octadecylsiloxane monolayers and locally irradiated with a focused beam of an Ar+ -laser at λ=514 nm and a 1/ e2 spot diameter of 3 μm. For characterization, atomic force microscopy, scanning electron microscopy, and optical microscopy in conjunction with labeling techniques and condensation experiments are used. At low laser powers, monolayer bromination in micron-sized areas is observed. Additionally, at high laser powers, decomposition of the monolayer takes place at the center of the brominated areas. Prospects and limitations of this procedure in fabrication of multifunctional templates are discussed. © 2010 American Vacuum Society.
    view abstractdoi: 10.1116/1.3386590
  • 2010 • 14 Biomimetic formation of thin, coherent iron oxide films under Langmuir monolayers
    Maas, M. and Degen, P. and Rehage, H. and Nebel, H. and Epple, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 354 149-155 (2010)
    This study focuses on the biomimetic formation and growth of thin iron oxide films under Langmuir monolayers. These coherent film structures were formed in the presence of different iron chloride solutions during the addition of an ammonia atmosphere. Stearic acid, stearyl amine and stearyl alcohol were used as film forming surfactants while the subphase contained FeCl2, FeCl3 or a mixture of both salts. The thin, coherent films consisted of X-ray amorphous iron oxide, hydroxide or oxyhydroxide. The films were studied by scanning electron microscopy, atomic force microscopy, X-ray diffraction, dynamic light scattering and surface potential measurements. Based on the experimental results we propose a growth mechanism that is guided by the formation of nanoparticles in the subphase and their assembly and aggregation underneath the Langmuir films. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.colsurfa.2009.04.049
  • 2010 • 13 Micron-sized fracture experiments on amorphous SiOx films and SiOx/SiNx multi-layers
    Matoy, K. and Schönherr, H. and Detzel, T. and Dehm, G.
    Thin Solid Films 518 5796-5801 (2010)
    In this study miniaturized monolithic cantilevers of thermally grown silicon oxide and multi-layer cantilevers of plasma enhanced chemical vapor deposited silicon oxide and nitride were mechanically characterized. In order to determine the fracture stress as well as the fracture toughness, un-notched and focused ion beam pre-notched cantilevers were tested. While the thickness of the monolithic cantilevers was varied from 280 nm to 2380 nm, the individual sub-layer thickness of the multi-layer cantilevers was adjusted to 50 nm. Bending experiments reveal a small increase of the fracture stresses with decreasing cantilever thicknesses. For the multi-layer stacks the tensile stress at fracture slightly exceeds the strength values of the corresponding monolithic materials. Furthermore, it is demonstrated that the specimens pre-notched by focused ion beam do not show significant changes in fracture toughness with varying pre-notch size. This makes the applied test a reproducible technique to determine fracture toughness of brittle films. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.05.114
  • 2010 • 12 Growth of crystalline Gd2O3 thin films with a high-quality interface on Si(100) by low-temperature H2O-assisted atomic layer deposition
    Milanov, A.P. and Xu, K. and Laha, A. and Bugiel, E. and Ranjith, R. and Schwendt, D. and Osten, H.J. and Parala, H. and Fischer, R.A. and Devi, A.
    Journal of the American Chemical Society 132 36-37 (2010)
    (Figure Presented) This work documents the first example of deposition of high-quality Gd2O3 thin films in a surface-controlled, self-limiting manner by a water-based atomic layer deposition (ALD) process using the engineered homoleptic gadolinium guanidinate precursor [Gd(DPDMG) 3]. The potential of this class of compound is demonstrated in terms of a true ALD process, exhibiting pronounced growth rates, a high-quality interface between the film and the substrate without the need for any additional surface treatment prior to the film deposition, and most importantly, encouraging electrical properties. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja909102j
  • 2010 • 11 Silver induced faceting of Si(112)
    Nabbefeld, T. and Wiethoff, C. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Applied Physics Letters 97 (2010)
    Si nanowires grown in ultrahigh vacuum by metal-catalyzed vapor-liquid-solid epitaxy are known to exhibit sidewalls with {112}-type orientation. For some metals the sidewalls show pronounced faceting. Ag induced faceting on Si(112) surfaces was studied in situ by spot-profile-analyzing low energy electron diffraction and ex situ atomic force microscopy. The (112) surface decomposes into (115)- and (111)- (√3×√3) -facets, both of which are Ag terminated. The width of the facets is kinetically limited and varies between 6 nm at T<550 °C and 30 nm at T=690 °C. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3464555
  • 2010 • 10 Formation and thermal stability of platinum oxides on size-selected platinum nanoparticles: Support effects
    Ono, L.K. and Yuan, B. and Heinrich, H. and Roldan Cuenya, B.
    Journal of Physical Chemistry C 114 22119-22133 (2010)
    This article presents a systematic study of the formation and thermal stability of Pt oxide species on sizeselected Pt nanoparticles (NPs) supported on SiO2, ZrO2, and TiO2 thin films. The studies were carried out in ultrahigh vacuum (UHV) by temperature-dependent X-ray photoelectron spectroscopy (XPS) measurements and ex situ transmission electron microscopy and atomic force microscopy. The NPs were synthesized by inverse micelle encapsulation and oxidized in UHV at room temperature by an oxygen plasma treatment. For a given particle size distribution, the role played by the NP support on the stability of Pt oxides was analyzed. PtO2 species are formed on all supports investigated after O2-plasma exposure. A two-step thermal decomposition (PtO2 → PtO → Pt) is observed from 300 to 600 K upon annealing in UHV. The stability of oxidized Pt species was found to be enhanced on ZrO2 under annealing treatments in O2. Strong NP/support interactions and the formation of Pt-Ti-O alloys are detected for Pt/TiO2 upon annealing in UHV above 550 K but not under an identical treatment in O2. Furthermore, thermal treatments in both environments above 700 K lead to the encapsulation of Pt by TiOx. The final shape of the micellar Pt NPs is influenced by the type of underlying support as well as by the post-deposition treatment. Spherical Pt NPs are stable on SiO2, ZrO2, and TiO 2 after in situ ligand removal with atomic oxygen at RT. However, annealing in UHV at 1000 K leads to NP flattening on ZrO2 and to the diffusion of Pt NPs into TiO2. The stronger the nature of the NP/support interaction, the more dramatic is the change in the NP shape (TiO2 &gt; ZrO2 &gt; SiO2). © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp1086703
  • 2010 • 9 Nanoscale engineering and optical addressing of single spins in diamond
    Pezzagna, S. and Wildanger, D. and Mazarov, P. and Wieck, A.D. and Sarov, Y. and Rangelow, I. and Naydenov, B. and Jelezko, F. and Hell, S.W. and Meijer, J.
    Small 6 2117-2121 (2010)
    The artificial creation of shallow nitrogen-vacancy (NV) centres in diamond with 25 nm lateral resolution is performed by collimated implantation of low-energy nitrogen ions. The electron spin associated to this defect is the most promising qubit for a scalable quantum computer working at room temperature. Individual optical addressing of two single centres separated by only 16 nm is demonstrated with stimulated emission depletion (STED) microscopy.
    view abstractdoi: 10.1002/smll.201000902
  • 2010 • 8 Passivation of Si(111) surfaces with electrochemically grafted thin organic films
    Roodenko, K. and Yang, F. and Hunger, R. and Esser, N. and Hinrichs, K. and Rappich, J.
    Surface Science 604 1623-1627 (2010)
    Ultra thin organic films (about 5 nm thick) of nitrobenzene and 4-methoxydiphenylamine were deposited electrochemically on p-Si(111) surfaces from benzene diazonium compounds. Studies based on atomic force microscopy, infrared spectroscopic ellipsometry and x-ray photoelectron spectroscopy showed that upon exposure to atmospheric conditions the oxidation of the silicon interface proceed slower on organically modified surfaces than on unmodified hydrogen passivated p-Si(111) surfaces. Effects of HF treatment on the oxidized organic/Si interface and on the organic layer itself are discussed. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2010.06.005
  • 2010 • 7 Solving the structure of size-selected Pt nanocatalysts synthesized by inverse micelle encapsulation
    Roldan Cuenya, B. and Croy, J.R. and Mostafa, S. and Behafarid, F. and Li, L. and Zhang, Z. and Yang, J.C. and Wang, Q. and Frenkel, A.I.
    Journal of the American Chemical Society 132 8747-8756 (2010)
    The structure, size, and shape of γ-Al2O 3-supported Pt nanoparticles (NPs) synthesized by inverse micelle encapsulation have been resolved via a synergistic combination of imaging and spectroscopic tools. It is shown that this synthesis method leads to 3D NP shapes even for subnanometer clusters, in contrast to the raft-like structures obtained for the same systems via traditional deposition-precipitation methods. Furthermore, a high degree of atomic ordering is observed for the micellar NPs in H2 atmosphere at all sizes studied, possibly due to H-induced surface reconstruction in these high surface area clusters. Our findings demonstrate that the influence of NP/support interactions on NP structure can be diminished in favor of NP/adsorbate interactions when NP catalysts are prepared by micelle encapsulation methods. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja101997z
  • 2010 • 6 Micro- and nanopatterning of functional organic monolayers on oxide-free silicon by laser-induced photothermal desorption
    Scheres, L. and Klingebiel, B. and Ter Maat, J. and Giesbers, M. and De Jong, H. and Hartmann, N. and Zuilhof, H.
    Small 6 1918-1926 (2010)
    The photothermal laser patterning of functional organic monolayers, prepared on oxide-free hydrogen-terminated silicon, and subsequent backfi lling of the laser-written lines with a second organic monolayer that differs in its terminal functionality, is described. Since the thermal monolayer decomposition process is highly nonlinear in the applied laser power density, subwavelength patterning of the organic monolayers is feasible. After photothermal laser patterning of hexadecenyl monolayers, the lines freed up by the laser are backfi lled with functional acid fl uoride monolayers. Coupling of cysteamine to the acid fl uoride groups and subsequent attachment of Au nanoparticles allows easy characterization of the functional lines by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Depending on the laser power and writing speed, functional lines with widths between 1.1 μm and 250 nm can be created. In addition, trifl uoroethyl-terminated (TFE) monolayers are also patterned. Subsequently, the decomposed lines are backfi lled with a nonfunctional hexadecenyl monolayer, the TFE stripes are converted into thiol stripes, and then finally covered with Au nanoparticles. By reducing the lateral distance between the laser lines, Au-nanoparticle stripes with widths close to 100 nm are obtained. Finally, in view of the great potential of this type of monolayer in the fi eld of biosensing, the ease of fabricating biofunctional patterns is demonstrated by covalent binding of fl uorescently labeled oligo-DNA to acidfl uoride-backfi lled laser lines, which-as shown by fl uorescence microscopy-is accessible for hybridization.Copyright © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201000189
  • 2010 • 5 MBE growth of cubic AlN on 3C-SiC substrate
    Schupp, T. and Rossbach, G. and Schley, P. and Goldhahn, R. and Rp̈ppischer, M. and Esser, N. and Cobet, C. and Lischka, K. and As, D.J.
    Physica Status Solidi (A) Applications and Materials Science 207 1365-1368 (2010)
    We present our recent results on the growth of cubic AlN (001) layers by plasma assisted molecular beam epitaxy (PAMBE) using freestanding 3C-SiC (001) substrate. For high-quality c-AlN layers reflection high-electron energy diffraction (RHEED) patterns in all azimuths show RHEED patterns of the cubic lattice, hexagonal reflections are absent. Highresolution X-ray diffraction (HRXRD) measurements confirm the cubic structure of the c-AlN layers with a lattice parameter of 4.373Å. Atomic force microscopy (AFM) scans show an atomically smooth surface with a roughness of 0.2nm RMS. Ellipsometry studies yield the dielectric function (DF) of c-AlN from 1 to 10eV. The direct gap is determined with 5.93eV at room temperature, while the indirect one is below 5.3 eV (onset of adsorption). The high-energy part of the DF is dominated by two transitions at 7.20 and 7.95 eV attributed to critical points of the band structure. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.200983437
  • 2010 • 4 Inactivation of bacteria and biomolecules by low-pressure plasma discharges
    Von Keudell, A. and Awakowicz, P. and Benedikt, J. and Raballand, V. and Yanguas-Gil, A. and Opretzka, J. and Flötgen, C. and Reuter, R. and Byelykh, L. and Halfmann, H. and Stapelmann, K. and Denis, B. and Wunderlich, J. and Mur...
    Plasma Processes and Polymers 7 327-352 (2010)
    (Figure Presented) The inactivation of bacteria and biomolecules using plasma discharges were investigated within the European project BIODECON. The goal of the project was to identify and isolate inactivation mechanisms by combining dedicated beam experiments with especially designed plasma reactors. The plasma reactors are based on a fully computer-controlled, low-pressure inductively-coupled plasma (ICP). Four of these reactors were built and distributed among the consortium, thereby ensuring comparability of the results between the teams. Based on this combined effort, the role of UV light, of chemical sputtering (i.e. the combined impact of neutrals and ions), and of thermal effects on bacteria such as Bacillus atrophaeus, Aspergillus niger, as well as on biomolecules such as LPS, Lipid A, BSA and prions have been evaluated. The particle fluxes emerging from the plasmas are quantified by using mass spectrometry, Langmuir probe measurements, retarding field measurements and optical emission spectroscopy. The effects of the plasma on the biological systems are evaluated using atomic force microscopy, ellipsometry, electrophoresis, specially-designed western blot tests, and animal models. A quantitative analysis of the plasma discharges and the thorough study of their effect on biological systems led to the identification of the different mechanisms operating during the decontamination process. Our results confirm the role of UV in the 200-2 50 nm range for the inactivation of microorganisms and a large variability of results observed between different strains of the same species. Moreover, we also demonstrate the role of chemical sputtering corresponding to the synergism between ion bombardment of a surface with the simultaneous reaction of active species such as O, O2 or H. Finally, we show that plasma processes can be efficient against different micro-organisms, bacteria and fungi, pyrogens, model proteins and prions. The effect of matrices is described, and consequences for any future industrial implementation are discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/ppap.200900121
  • 2010 • 3 Molecular recognition of vesicles: Host-guest interactions combined with specific dimerization of zwitterions
    Voskuhl, J. and Fenske, T. and Stuart, M.C.A. and Wibbeling, B. and Schmuck, C. and Ravoo, B.J.
    Chemistry - A European Journal 16 8300-8306 (2010)
    The aggregation of β-cyclodextrin vesicles can be induced by an adamantyl-substituted zwitterionic guanidiniocarbonylpyrrole carboxylate guest molecule (1). Upon addition of 1 to the cyclodextrin vesicles at neutral pH, the vesicles aggregate (but do not fuse), as shown by using UV/Vis and fluorescence spectroscopy, dynamic light scattering, ζ-potential measurements, cryogenic transmission electron microscopy, and atomic force microscopy. Aggregation of the vesicles is in-duced by a twofold supramolecular interaction. First, the adamantyl group of 1 forms an inclusion complex with ßcyclodextrin. Second, at neutral pH the guanidiniocarbonylpyrrole carboxylate zwitterion dimerizes through the formation of hydrogen-bonded ion pairs. Because the dimerization of 1 depends on the zwitterionic protonation state of 1, the aggregation of the cyclodextrin vesicles is also pH dependent; the cyclodextrin vesicles do not interact at pH 5 or 9, at which 1 is either cationic or anionic and, therefore, not self-complementary. These observations are consistent with molecular recognition of the vesicles through a combination of two different supramolecular interactions, that is, host-guest inclusion and dimerization of zwitterions, at the bilayer membrane surface. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201000623
  • 2010 • 2 Plastic anisotropy of γ-TiAl revealed by axisymmetric indentation
    Zambaldi, C. and Raabe, D.
    Acta Materialia 58 3516-3530 (2010)
    Single crystals of γ-TiAl cannot be grown in the near-stoichiometric compositions that are present inside two-phase γ / α2-microstructures with attractive mechanical properties. Therefore, the single-crystal constitutive behavior of γ-TiAl was studied by nanoindentation experiments in single-phase regions of these γ / α2-microstructures. The experiments were characterized by orientation microscopy and atomic force microscopy to quantify the orientation-dependent mechanical response during nanoindentation. Further, they were analyzed by a three-dimensional crystal plasticity finite element model that incorporated the deformation behavior of γ-TiAl. The spatially resolved activation of competing deformation mechanisms during indentation was used to assess their relative strengths. A convention was defined to unambiguously relate any indentation axis to a crystallographic orientation. Experiments and simulations were combined to study the orientation-dependent surface pile-up. The characteristic pile-up topographies were simulated throughout the unit triangle of γ-TiAl and represented graphically in the newly introduced inverse pole figure of pile-up patterns. Through this approach, easy activation of ordinary dislocation glide in stoichiometric γ-TiAl was confirmed independently from dislocation observation by transmission electron microscopy. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.02.025
  • 2010 • 1 SiRNA-loaded multi-shell nanoparticles incorporated into a multilayered film as a reservoir for gene silencing
    Zhang, X. and Kovtun, A. and Mendoza-Palomares, C. and Oulad-Abdelghani, M. and Fioretti, F. and Rinckenbach, S. and Mainard, D. and Epple, M. and Benkirane-Jessel, N.
    Biomaterials 31 6013-6018 (2010)
    In this study, we presented a new type of coating based on polyelectrolyte multilayers containing sequentially adsorbed active shRNA calcium phosphate nanoparticles for locally defined and temporarily variable gene silencing. Therefore, we investigated multi-shell calcium phosphate-shRNA nanoparticles embedded into a polyelectrolyte multilayer for gene silencing. As model system, we synthesized triple-shell calcium phosphate-shRNA nanoparticles (NP) and prepared polyelectrolyte multilayers films made of nanoparticles and poly-(l-lysine) (PLL). The biological activities of these polyelectrolyte multilayers films were tested by the production of osteopontin and osteocalcin in the human osteoblasts (HOb) which were cultivated on the PEM films. This new strategy can be used to efficiently control the bone formation and could be applicable in tissue engineering. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.biomaterials.2010.04.024