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 • 222 Novel approach to study diffusion of hydrogen bearing species in silicate glasses at low temperatures
    Bissbort, T. and Becker, H.-W. and Fanara, S. and Chakraborty, S.
    Chemical Geology 562 (2021)
    Diffusion of hydrogen bearing species in glasses plays a significant role in numerous applications in commercial as well as scientific domains. The investigation of diffusion of water in glasses at low temperatures led to experimental and analytical difficulties in the past. We present a new approach that lets us overcome these complications. Diffusion couples of An50Di50 glass (mol %, NBO/T = 0.67) were produced by coating anhydrous glass substrates with thin films of hydrated glass (~200 nm, ~2 wt% H2O) using pulsed laser deposition (PLD). Bonding the diffusant to the glass matrix of the thin film instead of using free water at the interface during experiments precludes other glass altering processes such as dissolution and precipitation. This allows us to confidently interpret the measured profiles to be a result of diffusion only. Nanoscale concentration profiles that result from diffusion at low temperatures on experimentally feasible time scales were measured with the Nuclear Resonance Reaction Analysis (NRRA, 1H(15N,αγ)12C). The non-destructive nature of NRRA enables us to observe and better understand the evolution of diffusion profiles with time within one sample. Evaluation of the sample quality by EPMA, SEM, optical microscopy, Rutherford backscattering spectroscopy (RBS), and NRRA was performed and confirmed the suitability of the samples for diffusion studies. Experiments at 1 atm in a box furnace and at 2 kbar in a CSPV (pressure medium = water) and an IHPV (pressure medium = Argon) prove that the diffusion couples can be used under various experimental conditions. We present diffusion profiles that were measured in experiments carried out in these devices and discuss the distinct features of each that result from different boundary conditions in the experiments. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.chemgeo.2020.120037
  • 2021 • 221 Investigation of the frequency dependent spatio-temporal dynamics and controllability of microdischarges in unipolar pulsed plasma electrolytic oxidation
    Hermanns, P. and Boeddeker, S. and Bracht, V. and Bibinov, N. and Grundmeier, G. and Awakowicz, P.
    Journal of Physics D: Applied Physics 54 (2021)
    The unipolar pulsed-plasma electrolytic oxidation (PEO) of aluminum has been replaced by bipolar pulsed methods that use a so-called 'soft-sparking'mode. This method results in an effective reduction of intense microdischarges, which are detrimental to the oxide layer. In a previous publication, we developed an in-situ multivariable microdischarge control scheme using unipolar pulsing. Using this method, it is possible to restrict the mean microdischarge size to well-defined limits, while at the same time influencing the mean microdischarge energy, number density or spectral emission behaviour. This method operates well inside a frequency range of f = 1-20 kHz. Although this method shows highly desirable plasma control properties, the mechanisms defining this frequency-dependent controllability are unclear. The aim of this study is to visualize the spatio-temporal behavior of microdischarges in higher frequency ranges. First, a wavelet transform was performed to estimate the temporal evolution of microdischarge lifetimes. Ceramic coatings were then deposited on aluminum alloy substrates in an aqueous solution using unipolar pulsed galvanostatic PEO. The aluminum samples were coated for 30 min at frequencies of f1 = 50 Hz, f2 = 5 kHz and f3 = 100 kHz. High-speed imaging was carried out utilizing four synchronized intensified charge-coupled device (ICCD) cameras, each with a 500 ns exposure time. At f2 = 5 kHz, the microdischarges were still able to follow the electrical pulses. In this regime, the process can be divided into two stages, an initial charging of the substrate surface without plasma emission and a subsequent slower evolution of microdischarges. Equivalent circuit model descriptions are given for both processes. At f3 = 100 kHz, microdischarges were not able to follow the pulse frequency, as the lifetimes and risetimes of the microdischarge characteristics were longer than the pulse length. Reignition at the same spatial location, clustering and permanent ignition through pulse periods were observed. © 2020 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/abbde4
  • 2021 • 220 Influence of spray gun parameters on inflight particle's characteristics, the splat-type distribution, and microstructure of plasma-sprayed YSZ coatings
    Tillmann, W. and Khalil, O. and Baumann, I.
    Surface and Coatings Technology 406 (2021)
    Ceramic coatings, fabricated with specific properties using the atmospheric plasma spray (APS) process, are widely used for many applications in which the porosity and splat interfaces are the main factors affecting the performance. Since the coating microstructure is composed of large numbers of molten and semi-molten particles impinged successfully at the substrate (known as splats), the produced coatings are characterized by the melting degree of these particles and their relative splat-type fractions. In the present work, the effect of process parameters settings has been studied systematically, relating the characteristics of impinging particles to splat formation and eventually to microstructure development and properties of the coating. Therefore, individual splats were collected on mirror-polished substrates and observed using image analysis (IA). These were evaluated and categorized into different splat types, based on their melting degree and morphology, under each combination of spray conditions. It was found that gun current and standoff distance have a profound effect on the characteristics of impinging particles. These, in turn, determine the relative fractions of splat types, layered structure, and final properties of the deposit. The effect of splat-type distribution on the bonding strength between layers, lamellar structure, and coating porosity was investigated. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.126705
  • 2020 • 219 Inspection of semiconductor-based planar wave-guiding structures with a near-infrared transmission digital holographic microscopy
    Besaga, V.R. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 11369 (2020)
    In this paper we report on practical investigations aimed at failure detection of the integrated optical circuits (IOC) on Silicon substrate during the control measurements of the items in use. Experiments are performed with a near-infrared (1064 nm) digital holographic microscope (≈90×magnification) in transmission mode. The instrument provides non-destructive and fast (<380 ms reconstruction time for 4112×3008 pixels images) data analysis at the diffraction-limited accuracy (lateral resolution of 760 nm). High quality of the instrument performance is shown on example of topography reconstruction of a standard glass-substrate test target. Practical applicability of the approach was proven on example of diffractive input elements of the IOCs designed for sensing purposes. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstractdoi: 10.1117/12.2553911
  • 2020 • 218 Improved Adhesion of Different Environmental Barrier Coatings on Al2O3/Al2O3-Ceramic Matrix Composites
    Gatzen, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Advanced Engineering Materials 22 (2020)
    In high-temperature combustion atmospheres, well-adhering environmental barrier coatings (EBCs) are required to protect the underlying ceramic matrix composites (CMCs) from corrosion. Herein the adhesion mechanisms of three different coatings produced by atmospheric plasma spraying (APS) on an Al2O3/Al2O3-CMC are investigated. In particular, the influence of surface structuring by laser ablation prior to coating production is investigated. Y2O3, yttria-stabilized zirconia (YSZ), and Gd2Zr2O7 are chosen as potential EBCs. The coating adhesion on CMC-substrates with and without surface structuring is analyzed by furnace cycling, pull-adhesion tests, and burner-rig tests with gradient. Special interest is paid to the interactions at the coating–substrate interface before and after heat treatment and their effect on the coating adhesion and lifetime. Two different adhesion mechanisms are found: adhesion promoted by chemical reaction and adhesion promoted by mechanical interlocking. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.202000087
  • 2020 • 217 High-performance positively charged hollow fiber nanofiltration membranes fabricated via green approach towards polyethyleneimine layer assembly
    Ghiasi, S. and Behboudi, A. and Mohammadi, T. and Ulbricht, M.
    Separation and Purification Technology 251 (2020)
    In this study, high performance positively charged hyper-branched polyethyleneimine (PEI) nanofiltration (NF) layer was assembled successfully on negatively charged polyethersulfone/polyimide (PES/PI) hollow fiber ultrafiltration (UF) membranes under different conditions. In accordance with principles of green chemistry, glutaraldehyde (GA) as cross-linker and purely aqueous solutions were used as a less hazardous alternative compared to, e.g. trimesoyl chloride in an organic solvent. The effects of the surface roughness and charge of the substrate UF membranes, due to the presence of PI, and various fabrication conditions, such as pH of PEI aqueous solution, GA/PEI ratio and crosslinking reaction time, were investigated and discussed. Electron microscopy images revealed the successful assembly of the PEI NF layer at uniform coverage of the PES/PI UF membranes. It was found that the varied preparation conditions drastically affect the membrane surface hydrophilicity, surface zeta potential, permeation flux, and salt rejection. The membrane fabricated at optimum conditions had a molecular weight cut-off of ≤ 400 Da; steric hindrance and Donnan exclusion resulted to achieve salt rejections of 94.2% and 87.4% for MgCl2 and MgSO4, respectively. Moreover, fabricated membranes were tested through three cycles of six-hour filtrations and over 95% flux recovery after the filtration of salts via the backwashing process was recorded. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2020.117313
  • 2020 • 216 Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing, testing under thermal gradient loading, and future challenges
    Gonzalez-Julian, J. and Mauer, G. and Sebold, D. and Mack, D.E. and Vassen, R.
    Journal of the American Ceramic Society 103 2362-2375 (2020)
    Cr2AlC layers with thickness up to 100 µm were deposited by high-velocity-atmospheric plasma spray (HV-APS) on Inconel 738 substrates to analyze the potential of MAX phases as bond coat in thermal barrier coating systems (TBCs). The deposited Cr2AlC layers showed high purity with theoretical densities up to 93%, although some secondary phases were detected after the deposition process. On top of this MAX phase layer, a porous yttria-stabilized zirconia (YSZ) was deposited by atmospheric plasma spraying. The system was tested under realistic thermal loading conditions using a burner rig facility, achieving surface and substrate temperatures of 1400°C and 1050°C, respectively. The system failed after 745 cycles mainly for three reasons: (i) open porosity of the bond coat layer, (ii) oxidation of secondary phases, and (iii) inter-diffusion. Nevertheless, these results show a high potential of Cr2AlC and other Al-based MAX phases as bond coat material for high-temperature applications. Furthermore, future challenges to transfer MAX phases as eventual bond coat or protective layer are discussed. © 2019 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals, Inc. on behalf of American Ceramic Society (ACERS)
    view abstractdoi: 10.1111/jace.16935
  • 2020 • 215 Ionization probability of sputtered indium atoms under impact of slow highly charged ions
    Herder, M. and Ernst, P. and Skopinski, L. and Weidtmann, B. and Schleberger, M. and Wucher, A.
    Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics 38 (2020)
    In order to investigate the different role of kinetic and potential projectile energy for secondary ion formation, the authors have measured the ionization probability of indium atoms sputtered from a clean indium surface under irradiation with rare gas (Xeq+) ions of different charge states q at the same kinetic impact energy of 20 keV. In this energy range, the kinetic energy of the projectile is predominantly deposited via nuclear stopping, leading to a collision-dominated sputtering process. The authors find that the ionization probability increases significantly if a highly charged ion is used as a projectile, where the ionization energy becomes comparable to or even exceeds the kinetic energy, indicating that a higher level of electronic substrate excitation induced by the potential energy stored in the projectile can boost the secondary ion formation process. This experimental result is discussed in terms of microscopic model calculations describing the secondary ion formation process. At the same time, the authors observe a significant change of the emission velocity distribution of the sputtered particles, leading to a pronounced low-energy contribution at higher projectile charge states. It is shown that this "potential sputtering"contribution strongly depends on surface chemistry even under conditions where the surface is dynamically cleaned by interleaved 5 keV Ar+ ion bombardment. © 2020 Author(s).
    view abstractdoi: 10.1116/6.0000171
  • 2020 • 214 Investigation of roughness correlation in polymer brushes via X-ray scattering
    Hildebrandt, M. and Shin, E. and Yang, S. and Ali, W. and Altinpinar, S. and Gutmann, J.S.
    Polymers 12 (2020)
    Thin polymer films and coatings are used to tailor the properties of surfaces in various applications such as protection against corrosion, biochemical functionalities or electronic resistors. Polymer brushes are a certain kind of thin polymer films, where polymer chains are covalently grafted to a substrate and straighten up to form a brush structure. Here we report on differences and similarities between polymer brushes and spin-coated polymer films from polystyrene and polymethyl methacrylate with special emphasis on surface roughness and roughness correlation. The phenomenon of roughness correlation or conformality describes the replication of the roughness profile from the substrate surface to the polymer surface. It is of high interest for polymer physics of brush layers as well as applications, in which a homogeneous polymer layer thickness is required. We demonstrate that spin-coated films as well as polymer brushes show roughness correlation, but in contrast to spin-coated films, the correlation in brushes is stable to solvent vapor annealing. Roughness correlation is therefore an intrinsic property of polymer brushes. © 2020 by the authors.
    view abstractdoi: 10.3390/POLYM12092101
  • 2020 • 213 Chemisorption and Physisorption at the Metal/Organic Interface: Bond Energies of Naphthalene and Azulene on Coinage Metal Surfaces
    Kachel, S.R. and Klein, B.P. and Morbec, J.M. and Schöniger, M. and Hutter, M. and Schmid, M. and Kratzer, P. and Meyer, B. and Tonner, R. and Gottfried, J.M.
    Journal of Physical Chemistry C 124 8257-8268 (2020)
    Organic/inorganic hybrid interfaces play a prominent role in organic (opto)electronics, heterogeneous catalysis, sensors, and other current fields of technology. The performance of the related devices and processes depends critically on the nature and strength of interfacial interaction. Here, we use the molecular isomers naphthalene (Nt) and azulene (Az) on the Ag(111) and Cu(111) surfaces as model systems that cover different bonding regimes from physisorption to chemisorption. Az also serves as a model for nonalternant molecular electronic materials and for topological 5-7 defects in graphene. The interaction energies are determined from the quantitative analysis of temperature-programmed desorption data. On both surfaces, Az binds more strongly than Nt, with zero-coverage desorption energies (in kJ/mol) of 120 for Az/Ag and 179 for Az/Cu, compared to 103 for Nt/Ag and 114 for Nt/Cu. The integrated experimental energies are compared with adsorption energies from density-functional theory (DFT) calculations, which include van der Waals contributions using four different correction schemes for the PBE functional: (1) the DFT-D3 scheme with Becke-Johnson damping, (2) the vdWsurf correction based on DFT-TS, (3) a many-body dispersion correction scheme, and (4) the D3surf scheme. Differences in the performance of these methods are discussed. Periodic energy decomposition analysis reveals details of the surface chemical bond and confirms that Az/Cu forms a chemisorptive bond, while the other systems are physisorbed. The variation of the adsorbate-substrate interaction with the topology of the Ï-electron system and the type of surface can be employed to modify the interface properties in graphene-based and organic electronic devices. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.0c00915
  • 2020 • 212 Dislocation plasticity and detwinning under thermal stresses in nanotwinned Ag thin films
    Kini, M.K. and Merola, C. and Breitbach, B. and Klapproth, D. and Philippi, B. and Molin, J.-B. and Kirchlechner, C. and Dehm, G.
    Acta Materialia 198 61-71 (2020)
    Wafer curvature measurements reported in literature for polycrystalline (often textured) and epitaxial fcc metal thin films on hard substrates show a characteristic “signature” in the stress-temperature evolution for either type of films. While epitaxial films reveal characteristic elastic – ideal plastic deformation with no dislocation storage and highly repeatable cycles, polycrystalline films show considerable hardening upon cooling in addition to the relaxation by diffusional creep at elevated temperatures. In the present study, we study the deformation characteristics of an electron beam deposited epitaxial nanotwinned Ag on Si (111) substrate. The twin spacing λ of the nanotwinned Ag is controlled by suitable heat treatment and the “signature” thermomechanical deformation curves by wafer curvature measurements are recorded for twin spacings varying from 20 nm to 1 μm. Further, deformation is compared to other small scale deformation studies on fcc metals such as epitaxial bicrystal films, bicrystal micropillars containing a coherent twin boundary and nanotwinned micropillars. © 2020
    view abstractdoi: 10.1016/j.actamat.2020.07.056
  • 2020 • 211 The role of molecular interactions on Michaelis constants of α-chymotrypsin catalyzed peptide hydrolyses
    Knierbein, M. and Held, C. and Sadowski, G.
    Journal of Chemical Thermodynamics 148 (2020)
    In this work, the effects of co-solvent and pressure on Michaelis constants at ambient temperature were analyzed for the enzymatic peptide hydrolyses of L-phenylalanine-p-nitroanilide (HPNA) and of N-succinyl-L-phenylalanine-p-nitroanilide (SPNA). These two substrates resemble each other in their molecular structure. That is, at the position of SPNA's succinyl-group (S), HPNA possesses a hydrogen atom (H). Two co-solvents were considered: trimethylamine N-oxide and dimethyl sulfoxide. The thermodynamic model Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) was used to predict the activity of HPNA and SPNA under different reaction conditions regarding solvent composition and pressure. The PC-SAFT parameters (pure-component parameters and one binary parameter between substrate and solvent) were fitted to solubility data of HPNA in different solvents (water, ethanol, ethyl acetate, dimethyl sulfoxide), which were measured in this work at 30 °C and 1 bar. The resulting PC-SAFT predicted Michaelis constants were validated by experimental literature data. Results show that pressure decreased the Michaelis constants of both reactions, HPNA hydrolysis and SPNA hydrolysis. In spite of that, co-solvent effects on the Michaelis constants were predicted to be contrary for the two hydrolysis reactions. For the hydrolysis of HPNA, the co-solvents under investigation decreased the Michaelis constant while the co-solvents increased the Michaelis constant for the hydrolysis of SPNA. These PC-SAFT predictions were in qualitative agreement with the experimental literature data. This shows that molecular interactions are the key to understand the effects of co-solvents on Michaelis constants for the considered reactions. Applying the thermodynamic model PC-SAFT allowed predicting the observed combined effects of co-solvent and pressure on enzymatic reaction kinetics, which opens the door for solvent design of enzymatic reactions in the future. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.jct.2020.106142
  • 2020 • 210 Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy
    Knippertz, J. and Kelly, L.L. and Franke, M. and Kumpf, C. and Cinchetti, M. and Aeschlimann, M. and Stadtmüller, B.
    Physical Review B 102 (2020)
    Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter organic molecules or metallic substrates offers the opportunity to optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control metal-organic interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence X-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg2 surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg2 surface alloy [B. Stadtmüller, Phys. Rev. Lett. 117, 096805 (2016)PRLTAO0031-900710.1103/PhysRevLett.117.096805]. In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local σ-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.075447
  • 2020 • 209 Picosecond ultrasonics with miniaturized semiconductor lasers
    Kobecki, M. and Tandoi, G. and Di Gaetano, E. and Sorel, M. and Scherbakov, A.V. and Czerniuk, T. and Schneider, C. and Kamp, M. and Höfling, S. and Akimov, A.V. and Bayer, M.
    Ultrasonics 106 (2020)
    There is a great desire to extend ultrasonic techniques to the imaging and characterization of nanoobjects. This can be achieved by picosecond ultrasonics, where by using ultrafast lasers it is possible to generate and detect acoustic waves with frequencies up to terahertz and wavelengths down to nanometers. In our work we present a picosecond ultrasonics setup based on miniaturized mode-locked semiconductor lasers, whose performance allows us to obtain the necessary power, pulse duration and repetition rate. Using such a laser, we measure the ultrasonic echo signal with picosecond resolution in a 112 nm thick Al film deposited on a semiconductor substrate. We show that the obtained signal is as good as the signal obtained with a standard bulky mode-locked Ti-Sa laser. The experiments pave the way for designing integrated portable picosecond ultrasonic setups on the basis of miniaturized semiconductor lasers. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultras.2020.106150
  • 2020 • 208 WS2 monolayer based light emitting devices fabricated by scalable deposition techniques
    Kümmell, T. and Andrzejewski, D. and Beckmann, Y. and Abdelbaky, M. and Yeow, T. and Grundmann, A. and Heuken, M. and Kalisch, H. and Vescan, A. and Musselman, K. and Bacher, G.
    Proceedings of SPIE - The International Society for Optical Engineering 11302 (2020)
    Transition metal dichalcogenides (TMDC) have become attractive candidates for 2D electronics and optoelectronics. While several concepts for light emitting devices have been reported, many of them realized using exfoliated TMDC flakes of micrometer size, only few approaches tackle the challenge of upscaling to relevant device sizes. We demonstrate a light emitting diode based on WS2 monolayers in a scalable design. The devices are fabricated by combining two industrially relevant deposition processes in a vertical p-n architecture: Metal organic CVD (MOCVD) is used to realize the optically active WS2 monolayers, while ZnO deposited by spatial atomic layer deposition (sALD) is employed as an electron injection layer on the cathode side. Organic layers spin-coated on an ITO covered glass substrate provide hole injection and transport. The resulting devices exhibit rectifying behavior and red electroluminescence from an area of 6 mm2. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstractdoi: 10.1117/12.2544926
  • 2020 • 207 InP-based THz Beam Steering Leaky-Wave Antenna
    Lu, P. and Haddad, T. and Sievert, B. and Khani, B. and Makhlouf, S. and Dulme, S. and Fernandez Estevez, J. and Rennings, A. and Erni, D. and Pfeiffer, U.R. and Stohr, A.
    IEEE Transactions on Terahertz Science and Technology (2020)
    For mobile THz applications, integrated beam steering THz transmitters are essential. Beam steering approaches using leaky-wave antennas (LWAs) are quite attractive in that regard since they do not require complex feeding control circuits because beam steering is simply accomplished by sweeping the operating frequency. To date, only a few THz LWAs have been reported. These LWAs are based on polymer or graphene substrates and thus it is quite impossible to monolithically integrate these antennas with state-of-the-art indium phosphide (InP) based photonic or electronic THz sources and receivers. Therefore, in this paper, we report on an InP-based THz LWA for the first time. The developed and fabricated THz LWA consists of a periodic leaking microstrip line integrated with a grounded coplanar waveguide to microstrip line (GCPW-MSL) transition for future integration with InP-based photodiodes. For fabrication, a substrate-transfer process using silicon as carrier substrate for a 50 m thin InP THz antenna chip has been established. By changing the operating frequency from 230 GHz to 330 GHz, the fabricated antenna allows to sweep the beam direction quasi-linearly from -46 to 42, i.e. the total scanning angle is 88. The measured average realized gain and 3 dB beam width of a 1.5 mm wide InP LWA are ~11 dBi and 10. The paper furthermore discusses the use of the fabricated LWA for THz interconnects. CCBY
    view abstractdoi: 10.1109/TTHZ.2020.3039460
  • 2020 • 206 Direct growth of graphene on GaN via plasma-enhanced chemical vapor deposition under N2 atmosphere
    Mischke, J. and Pennings, J. and Weisenseel, E. and Kerger, P. and Rohwerder, M. and Mertin, W. and Bacher, G.
    2D Materials 7 (2020)
    One of the bottlenecks in the implementation of graphene as a transparent electrode in modern opto-electronic devices is the need for complicated and damaging transfer processes of high-quality graphene sheets onto the desired target substrates. Here, we study the direct, plasma-enhanced chemical vapor deposition (PECVD) growth of graphene on GaN-based light-emitting diodes (LEDs). By replacing the commonly used hydrogen (H2) process gas with nitrogen (N2), we were able to suppress GaN surface decomposition while simultaneously enabling graphene deposition at &lt;800 °C in a single-step growth process. Optimizing the methane (CH4) flow and varying the growth time between 0.5 h and 8 h, the electro-optical properties of the graphene layers could be tuned to sheet resistances as low as ∼1 kΩ/D with a maximum transparency loss of ∼12%. The resulting high-quality graphene electrodes show an enhanced current spreading effect and an increase of the emission area by a factor of ∼8 in operating LEDs. © 2020 The Author(s).
    view abstractdoi: 10.1088/2053-1583/ab8969
  • 2020 • 205 The Planar Multipole Resonance Probe: A Minimally Invasive Monitoring Concept for Plasma-Assisted Dielectric Deposition Processes
    Pohle, D. and Schulz, C. and Oberberg, M. and Awakowicz, P. and Rolfes, I.
    IEEE Transactions on Microwave Theory and Techniques 68 2067-2079 (2020)
    In this article, a novel minimally invasive approach to plasma monitoring in the challenging environment of dielectric deposition processes based on the planar multipole resonance probe (pMRP) is presented. By placing the sensor on the plasma-remote side of a dielectric substrate to be coated, perturbations of the process due to its presence can be significantly reduced. Since the electric field of the sensor is able to penetrate dielectric layers, a plasma supervision through the substrate is enabled. To investigate the effect of increasing coating thicknesses on the measurement performance for a broad spectrum of materials and plasma conditions, the results of extensive 3-D full-wave simulations performed with CST Microwave Studio are evaluated. Finally, real-time monitoring results of an argon-oxygen plasma during a sputter deposition with aluminum oxide on a polyethylene terephthalate (PET) film substrate together with a comparison to external process parameters are presented. The results demonstrate both the applicability of the proposed concept and its insensitivity to additional dielectric coatings. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/TMTT.2020.2974835
  • 2020 • 204 Defect Segregation and Its Effect on the Photoelectrochemical Properties of Ti-Doped Hematite Photoanodes for Solar Water Splitting
    Scherrer, B. and Li, T. and Tsyganok, A. and Döbeli, M. and Gupta, B. and Malviya, K.D. and Kasian, O. and Maman, N. and Gault, B. and Grave, D.A. and Mehlman, A. and Visoly-Fisher, I. and Raabe, D. and Rothschild, A.
    Chemistry of Materials 32 1031-1040 (2020)
    Optimizing the photoelectrochemical performance of hematite photoanodes for solar water splitting requires better understanding of the relationships between dopant distribution, structural defects, and photoelectrochemical properties. Here, we use complementary characterization techniques including electron microscopy, conductive atomic force microscopy (CAFM), Rutherford backscattering spectroscopy (RBS), atom probe tomography (APT), and intensity-modulated photocurrent spectroscopy (IMPS) to study this correlation in Ti-doped (1 cat. %) hematite films deposited by pulsed laser deposition (PLD) on F:SnO2 (FTO)-coated glass substrates. The deposition was carried out at 300 °C followed by annealing at 500 °C for 2 h. Upon annealing, Ti was observed by APT to segregate to the hematite/FTO interface and into some hematite grains. Since no other pronounced changes in microstructure and chemical composition were observed by electron microscopy and RBS after annealing, a nonuniform Ti redistribution seems to be the reason for reduced interfacial recombination in the annealed films, as observed by IMPS. This results in a lower onset potential, higher photocurrent, and larger fill factor with respect to the as-deposited state. This work provides atomic-scale insights into the microscopic inhomogeneity in Ti-doped hematite thin films and the role of defect segregation in their electrical and photoelectrochemical properties. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.9b03704
  • 2020 • 203 Effects of heat treatments on the microstructure and the adhesion of Cr, Ti, Al, Zr HiPIMS films deposited on APS Al2O3 and ZrO2-8Y2O3 coatings
    Tillmann, W. and Fehr, A. and Stangier, D.
    Surface and Coatings Technology 393 (2020)
    While several studies deal with thin film depositions on bulk ceramics, little is known about film applications on thermally sprayed ceramics. Hence, atmospheric plasma sprayed Al2O3 and ZrO2-8Y2O3 coatings served as substrates for the deposition of metallic films (Cr, Ti, Al, and Zr) by means of high power impulse magnetron sputtering (HiPIMS). Subsequently, these duplex coatings were heat treated in a vacuum furnace at 300 °C and 600 °C to investigate influences on the phase formation, residual stress state, mechanical properties, and film adhesion. The Cr and Al films crystallized in a cubic lattice and the Zr films exhibited a hexagonal lattice. The crystallization of the Ti film was substrate dependent, i.e. on alumina substrate the Ti film was cubic, while it was hexagonal on zirconia. Furthermore, heat treating the Ti and Zr films at 600 °C in vacuum led to an oxygen exchange generating c-TiO and m-ZrO. Heat treating the Cr film on Al2O3 substrate provoked a significant stress relaxation leading to an increased adhesion. However, depositing Al films on alumina and zirconia substrates resulted in the highest adhesion in the as-deposited state due to the low Young's modulus. In general, it was shown that phase transformations, increased lattice mismatches as well as differences in the thermal expansion behavior of the substrate and the film had a negative influence on the film adhesion. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125766
  • 2020 • 202 LPBF-M manufactured Zr-based bulk metallic glasses coated with magnetron sputtered ZrN films
    Tillmann, W. and Fehr, A. and Wegner, J. and Stangier, D. and Kleszczynski, S. and Witt, G.
    Surface and Coatings Technology 386 (2020)
    The fabrication of Zr-based bulk metallic glasses (BMGs) by means of laser powder bed fusion of metals (LPBF-M) is recently emerging. This production route allows to widely overcome current geometrical restrictions of casting routes while maintaining the amorphous character, which is decisive for the unique mechanical properties, for instance. However, the roughness of the LPBF-M fabricated BMGs is still a challenging property, impeding the application of near-net shaped thin films that modify BMG surfaces, e.g. with respect to wear resistance. Zr59.3Cu28.8Al10.4Nb1.5 (at.%) substrates were manufactured by means of LPBFM, applying various exposure strategies, including laser remelting of the last solidified layer to influence the surface topography. Furthermore, BMG substrates were post-treated by grinding and polishing. Thus, varying degrees of crystallinity as well as surface roughness states were generated to analyze the effect of these characteristics on the microstructural properties of additionally applied magnetron sputtered ZrN films. Substrates that were fabricated with higher energy densities during LPBF-M exhibited (101)-Zr as well as (013)- and (110)-CuZr2 phases, which were accompanied by a decreased surface roughness. It was shown that all films had a crystalline structure on amorphous and partly crystalline BMG surfaces. A decreased surface roughness of the BMG substrates could be directly correlated with a higher hardness and a better adhesion of the ZrN film. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125463
  • 2020 • 201 Adhesion of HVOF-Sprayed WC-Co Coatings on 316L Substrates Processed by SLM
    Tillmann, W. and Hagen, L. and Schaak, C. and Liß, J. and Schaper, M. and Hoyer, K.-P. and Aydinöz, M.E. and Garthe, K.-U.
    Journal of Thermal Spray Technology 29 1396-1409 (2020)
    Different studies have been demonstrated that the surface integrity of substrate bulk materials to be coated has a significant impact on the adhesion of thermally sprayed coatings. It is known that the surface integrity of parts processed by selective laser melting (SLM) differs from those obtained from bulk materials. Although 316L stainless steel is among the most investigated material for SLM, the adhesion of thermally sprayed coatings on 316L stainless steel substrates processed by SLM has not been studied yet. This study aims at evaluating the effect of various mechanical pre-treatments onto 316L stainless steel substrates processed by SLM and their effect on the adhesion of high velocity oxy-fuel (HVOF)-sprayed WC-Co coatings. To differentiate between topographical effects and residual stress-related phenomena, a stress-relief heat treatment of the SLM substrates served as a reference throughout the investigations. The differently pre-treated SLM substrates were investigated with regard to the surface roughness and residual stresses. For the HVOF-sprayed SLM composites, Vickers interfacial indentation tests were conducted to assess the resulting coating adhesion. The findings demonstrated that the HVOF-sprayed WC-Co coatings predominantly exhibit good adhesion to the SLM 316L substrates. However, it was found that the stress state in the SLM 316L substrate surface is more likely to affect the adhesion of the WC-Co coating, while the substrate surface roughness showed a marginal effect. © 2020, The Author(s).
    view abstractdoi: 10.1007/s11666-020-01081-y
  • 2020 • 200 Tribo-mechanical properties and adhesion behavior of DLC coatings sputtered onto 36NiCrMo16 produced by selective laser melting
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Hagen, L. and Schaper, M. and Hengsbach, F. and Hoyer, K.-P.
    Surface and Coatings Technology 394 (2020)
    The combination of selective laser melted nickel-chromium-molybdenum alloyed steel and diamond-like carbon (DLC) coatings is an aspiring approach to produce light-weight components with improved tribological properties. In order to assure the tribo-mechanical properties of the DLC coatings, good adhesion on the additively manufactured substrates under high loads is essential. 36NiCrMo16, produced by Selective Laser Melting (SLM) and wrought material techniques, served as substrate material for hydrogen-free a-C and hydrogenated a-C:H coatings with a chromium carbide interlayer. The structural and mechanical properties of 36NiCrMo16 were examined in order to consider their effects on the tribo-mechanical properties and adhesion of the DLC coatings deposited by magnetron sputtering. In x-ray diffraction analyses, retained austenite was identified for the 36NiCrMo16 substrate processed by SLM. This increased austenite content in the SLM built steel is attributed to the pre-heating temperature of 200 °C during fabrication. Additionally, a relative density of 99.98% was detected for the SLM manufactured tempering steel by using an x-ray microscope. In contrast to the conventional 36NiCrMo16, the SLM substrates exhibited a higher surface roughness, which is ascribed to the different phase composition and microstructure. In nanoindentation tests, the a-C and a-C:H coatings deposited onto 36NiCrMo16 exhibited a hardness of ~22 and ~19–20 GPa, respectively. In general, the DLC coatings revealed a good adhesion to the conventional and SLM 36NiCrMo16 in Rockwell C indentation tests. Local, limited spalling failures were identified for a-C and a-C:H on the SLM substrate, while no delamination was observed for the coatings on conventional steel. The differences in microstructure, such as the retained austenite and higher surface roughness, do appear to affect the adhesion of the DLC coatings. In tribometer tests against 100Cr6 counterparts, the DLC coatings significantly reduced the friction and increased the wear resistance of 36NiCrMo16. Therefore, the DLC coatings are promising to enhance the tribo-mechanical properties of SLM 36NiCrMo16 substrates, but it is crucial to consider the surface integrity of SLM steel on the adhesion behavior of DLC coatings. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125748
  • 2020 • 199 Nitrogen doping of MoSx thin films sputtered by reactive High Power Impulse Magnetron Sputtering
    Tillmann, W. and Wittig, A. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
    Thin Solid Films 713 (2020)
    Incorporating nitrogen into non-stoichiometric molybdenum disulfide (MoSx) thin films is a promising approach in order to improve the mechanical properties. Nevertheless, the adhesion between the film and the substrate is still challenging and the interaction between the mechanical and the tribological properties is not fully understood yet. Subsequently, reactive High Power Impulse Magnetron Sputtering (HiPIMS) is used to deposit nitrogen doped MoSx thin films with different nitrogen amounts on 16MnCr5 steel. The interaction between the structural changes, the mechanical properties and the tribological behavior depending on the nitrogen amount is investigated. The results prove that an increasing amount of nitrogen significantly affects the structure and the tribo-mechanical properties of the thin films. X-ray diffraction analysis reveals a transformation from crystalline to amorphous with an increasing amount of nitrogen from (7.1 ± 0.3) at.-% to (19.5 ± 0.5) at.-%. This transformation is related to a suppression of the columnar microstructure as well as an increasing hardness and Young‘s modulus from (0.14 ± 0.02) GPa, and (5.28 ± 0.32) GPa for the undoped film, to (5.12 ± 0.32) GPa and (92.5 ± 6.2) GPa, for the film with the highest nitrogen amount. The results of the Rockwell indentation tests show that the films with a small amount of nitrogen exhibit an improved adhesion behavior. The wear coefficient can be reduced to a quarter of the value of the undoped MoSx film, whereas coefficients of friction are at similar level of 0.2 in ambient air. Reactive HiPIMS has proven to be promising to deposit nitrogen doped MoSx thin films on steel substrates, which reveal improved mechanical properties and an excellent transfer film built-up during the tribo-tests without failures. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2020.138267
  • 2019 • 198 Cold spray deposition of Cr2AlC MAX phase for coatings and bond-coat layers
    Go, T. and Sohn, Y.J. and Mauer, G. and Vaßen, R. and Gonzalez-Julian, J.
    Journal of the European Ceramic Society 39 860-867 (2019)
    Highly pure Cr2AlC powders were synthesized and deposited for the first time by cold spray technology on stainless steel substrates. The Cr2AlC coatings were relative dense, up to 91%, and present high purity (&gt; 98%) since only small traces of Cr2Al, Al2O3 and Cr2O3 were detected by XRD, SEM and EDX. The microstructure of the coatings is homogeneous, although some preferential orientation in the basal plane was observed by XRD pole figures. The adhesion between the coating and the substrate is strong, and compressive residual stresses up to 300 MPa in the coating were determined by XRD. Furthermore, a conventional YSZ Thermal Barrier Coating (TBCs) was deposited by Atmospheric Plasma Spray (APS) on top of the cold sprayed Cr2AlC coating in order to demonstrate the processing feasibility of Cr2AlC MAX phases as a bond-coat layer. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2018.11.035
  • 2019 • 197 H2S-free Metal-Organic Vapor Phase Epitaxy of Coalesced 2D WS2 Layers on Sapphire
    Grundmann, A. and Andrzejewski, D. and Kümmell, T. and Bacher, G. and Heuken, M. and Kalisch, H. and Vescan, A.
    MRS Advances 4 593-599 (2019)
    The 2D transition metal dichalcogenide (TMDC) tungsten disulfide (WS2) has attracted great interest due to its unique properties and prospects for future (opto)electronics. However, compared to molybdenum disulfide (MoS2), the development of a reproducible and scalable deposition process for 2D WS2 has not advanced very far yet. Here, we report on the systematic investigation of 2D WS2 growth on hydrogen (H2)-desorbed sapphire (0001) substrates using a hydrogen sulfide (H2S)-free metal-organic vapor phase epitaxy (MOVPE) process in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Tungsten hexacarbonyl (WCO, 99.9 %) and di-tert-butyl sulfide (DTBS, 99.9999 %) were used as MO sources, nitrogen (N2) was selected as carrier gas for the deposition processes (standard growth time 10 h). In an initial study, the impact of growth temperature on nucleation and growth was investigated and an optimal value of 820 °C was found. The influence of the WCO flow on lateral growth was investigated. The aim was to maximize the edge length of triangular crystals as well as the total surface coverage. Extending gradually the growth time up to 20 h at optimized WCO flow conditions yields fully coalesced WS2 samples without parasitic carbon-related Raman peaks and with only sparse bilayer nucleation. After substrate removal, a fully coalesced WS2 film was implemented into a light-emitting device showing intense red electroluminescence (EL). © Materials Research Society 2018.
    view abstractdoi: 10.1557/adv.2018.669
  • 2019 • 196 Via-Less Microstrip to Rectangular Waveguide Transition on InP
    Hussain, B. and Serafino, G. and Ghelfi, P. and Bogoni, A. and Stohr, A.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2019-September (2019)
    Indium-Phosphide (InP) is one of the most common materials used for realizing active devices working in the millimeter frequency range. The isotropic etching profile of InP substrates limits the realization of passive devices, thus requiring an expensive and lossy hybrid platform. This paper presents a via-less, cost-effective and efficient solution for InP substrate. By using the proposed planar solution, it is demonstrated that rectangular waveguides can be realized on InP by fabricating a bed of nails structure which acts as a reflecting boundary for an impinging millimeter wave. As a proof of concept, a transition from microstrip to rectangular waveguide structure is realized within H-band (220-320 GHz) with a return loss of-18dB over a bandwidth of 30 GHz. © 2019 IEEE.
    view abstractdoi: 10.1109/IRMMW-THz.2019.8874316
  • 2019 • 195 Lanthanum tungstate membranes for H 2 extraction and CO 2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology 100-112 (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H 2 extraction and CO 2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H 2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La 28−x W 4+x O 54+δ , LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H 2 flux of ca. 0.4 ml/min cm 2 measured at 825 °C in 50 vol% H 2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H 2 separation membranes and it is well comparable with the H 2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2019.03.015
  • 2019 • 194 Lanthanum tungstate membranes for H2 extraction and CO2 utilization: Fabrication strategies based on sequential tape casting and plasma-spray physical vapor deposition
    Ivanova, M.E. and Deibert, W. and Marcano, D. and Escolástico, S. and Mauer, G. and Meulenberg, W.A. and Bram, M. and Serra, J.M. and Vaßen, R. and Guillon, O.
    Separation and Purification Technology 219 100-112 (2019)
    In the context of energy conversion efficiency and decreasing greenhouse gas emissions from power generation and energy-intensive industries, membrane technologies for H2 extraction and CO2 capture and utilization become pronouncedly important. Mixed protonic-electronic conducting ceramic membranes are hence attractive for the pre-combustion integrated gasification combined cycle, specifically in the water gas shift and H2 separation process, and also for designing catalytic membrane reactors. This work presents the fabrication, microstructure and functional properties of Lanthanum tungstates (La28−xW4+xO54+δ, LaWO) asymmetric membranes supported on porous ceramic and porous metallic substrates fabricated by means of the sequential tape casting route and plasma spray-physical vapor deposition (PS-PVD). Pure LaWO and W site substituted LaWO were employed as membrane materials due to the promising combination of properties: appreciable mixed protonic-electronic conductivity at intermediate temperatures and reducing atmospheres, good sinterability and noticeable chemical stability under harsh operating conditions. As substrate materials porous LaWO (non-substituted), MgO and Crofer22APU stainless steel were used to support various LaWO membrane layers. The effect of fabrication parameters and material combinations on the assemblies’ microstructure, LaWO phase formation and gas tightness of the functional layers was explored along with the related fabrication challenges for shaping LaWO layers with sufficient quality for further practical application. The two different fabrication strategies used in the present work allow for preparing all-ceramic and ceramic-metallic assemblies with LaWO membrane layers with thicknesses between 25 and 60 μm and H2 flux of ca. 0.4 ml/min cm2 measured at 825 °C in 50 vol% H2 in He dry feed and humid Ar sweep configuration. Such a performance is an exceptional achievement for the LaWO based H2 separation membranes and it is well comparable with the H2 flux reported for other newly developed dual phase cer-cer and cer-met membranes. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2019.03.015
  • 2019 • 193 Repair of Ni-based single-crystal superalloys using vacuum plasma spray
    Kalfhaus, T. and Schneider, M. and Ruttert, B. and Sebold, D. and Hammerschmidt, T. and Frenzel, J. and Drautz, R. and Theisen, W. and Eggeler, G. and Guillon, O. and Vassen, R.
    Materials and Design 168 (2019)
    Turbine blades in aviation engines and land based gas-turbines are exposed to extreme environments. They suffer damage accumulation associated with creep, oxidation and fatigue loading. Therefore, advanced repair methods are of special interest for the gas-turbine industry. In this study, CMSX-4 powder is sprayed by Vacuum Plasma Spray (VPS) on single-crystalline substrates with similar compositions. The influence of the substrate temperature is investigated altering the temperature of the heating stage between 850 °C to 1000 °C. Different spray parameters were explored to identify their influence on the microstructure. Hot isostatic pressing (HIP) featuring fast quenching rates was used to minimize porosity and to allow for well-defined heat-treatments of the coatings. The microstructure was analysed by orientation imaging scanning electron microscopy (SEM), using electron backscatter diffraction (EBSD). The effects of different processing parameters were analysed regarding their influence on porosity and grain size. The results show that optimized HIP heat-treatments can lead to dense coatings with optimum γ/γ′ microstructure. The interface between the coating and the substrate is oxide free and shows good mechanical integrity. The formation of fine crystalline regions as a result of fast cooling was observed at the single-crystal surface, which resulted in grain growth during heat-treatment in orientations determined by the crystallography of the substrate. © 2019
    view abstractdoi: 10.1016/j.matdes.2019.107656
  • 2019 • 192 Redox-controlled epitaxy and magnetism of oxide heterointerfaces: EuO/SrTiO3
    Lömker, P. and Müller, M.
    Physical Review Materials 3 (2019)
    We demonstrate a route to prepare thin films of the ferromagnetic insulator europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate SrTiO3 as the supplier of oxygen. The process allows tuning the electronic, magnetic, and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.061401
  • 2019 • 191 A Selective Cucurbit[8]uril-Peptide Beacon Ensemble for the Ratiometric Fluorescence Detection of Peptides
    Maity, D. and Assaf, K.I. and Sicking, W. and Hirschhäuser, C. and Nau, W.M. and Schmuck, C.
    Chemistry - A European Journal 25 13088-13093 (2019)
    A convenient supramolecular strategy for constructing a ratiometric fluorescent chemosensing ensemble, consisting of a macrocyclic host (cucurbit[8]uril CB[8]), and a pyrene-tagged amphiphilic peptide beacon (AP 1), is reported. AP 1 unfolds upon encapsulation of the pyrene termini into the hydrophobic CB[8] cavity. This changes pyrene excimer to monomer emission. Substrates with higher affinity for the CB[8] cavity can displace AP 1 from the ensemble. The released AP 1 folds again to form a pyrene excimer, which allows for the ratiometric fluorescence monitoring of the substrate. In this report, the ensemble capacity for ratiometric fluorescence monitoring of biological substrates, such as amino acid derivatives, specific peptides, and proteins, in aqueous media is demonstrated. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.201901037
  • 2019 • 190 PS-PVD Processing of Single-Phase Lanthanum Tungstate Layers for Hydrogen-Related Applications
    Marcano, D. and Ivanova, M.E. and Mauer, G. and Sohn, Y.J. and Schwedt, A. and Bram, M. and Menzler, N.H. and Vaßen, R.
    Journal of Thermal Spray Technology 28 1554-1564 (2019)
    This work presents a systematic study of the lanthanum tungstate (LaWO) ceramic layers formation on porous metallic substrates as a function of the PS-PVD processing parameters including plasma characteristics, support type and temperature, as well as addition of O2 during the spraying. Through precise control of the PS-PVD parameters, a set of processing conditions were found that led to He gas-tight purely cubic LaWO layers with negligible secondary phase precipitations. Being dependent on process conditioning, the formation and evolution of the cubic La6−xWO12−δ (x = 0.3-0.6) as the main phase of functional importance and of the undesired secondary phases (La2O3 and La6W2O15) was strongly affected by the cation and oxygen stoichiometries. The rapid cooling of the feedstock at particle impact on the substrate led to the formation of highly La-saturated compositions which exhibited significant lattice expansion in comparison with conventionally processed LaWO and is considered beneficial in terms of material performance. And indeed, the H2 permeation performance of the PS-PVD processed LaWO ceramic layers shown earlier by our group was 0.4 ml/min∙cm2 at 825 °C for 60 µm thickness of the functional layer, the highest value reported for this type of proton conducting ceramics, so far. © 2019, ASM International.
    view abstractdoi: 10.1007/s11666-019-00935-4
  • 2019 • 189 Adsorption-controlled growth and properties of epitaxial SnO films
    Mei, A.B. and Miao, L. and Wahila, M.J. and Khalsa, G. and Wang, Z. and Barone, M. and Schreiber, N.J. and Noskin, L.E. and Paik, H. and Tiwald, T.E. and Zheng, Q. and Haasch, R.T. and Sangiovanni, D.G. and Piper, L.F.J. and Schlom, D.G.
    Physical Review Materials 3 (2019)
    When it comes to providing the unusual combination of optical transparency, p-type conductivity, and relatively high mobility, Sn2+-based oxides are promising candidates. Epitaxial films of the simplest Sn2+ oxide, SnO, are grown in an adsorption-controlled regime at 380°C on Al2O3 substrates by molecular-beam epitaxy, where the excess volatile SnOx desorbs from the film surface. A commensurately strained monolayer and an accompanying van der Waals gap is observed near the substrate interface, promoting layers with high structural perfection notwithstanding a large epitaxial lattice mismatch (-12%). The unintentionally doped films exhibit p-type conductivity with carrier concentration 2.5×1016cm-3 and mobility 2.4cm2V-1s-1 at room temperature. Additional physical properties are measured and linked to the Sn2+ valence state and corresponding lone-pair charge-density distribution. ©2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.105202
  • 2019 • 188 Variable chemical decoration of extended defects in Cu-poor C u2ZnSnS e4 thin films
    Schwarz, T. and Redinger, A. and Siebentritt, S. and Peng, Z. and Gault, B. and Raabe, D. and Choi, P.-P.
    Physical Review Materials 3 (2019)
    We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich Cu2ZnSnSe4 thin films. For a precursor film, which was co-evaporated at 320C, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at 500C, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed Cu2ZnSnSe4 film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.035402
  • 2019 • 187 Effects of selenization time and temperature on the growth of Cu2ZnSnSe4 thin films on a metal substrate for flexible solar cells
    Stanchik, A.V. and Gremenok, V.F. and Juskenas, R. and Tyukhov, I.I. and Tivanov, M.S. and Fettkenhauer, C. and Shvartsman, V.V. and Giraitis, R. and Hagemann, U. and Lupascu, D.C.
    Solar Energy 142-149 (2019)
    Thin film Cu2ZnSnSe4 (CZTSe) solar cells can be grown on flexible and lightweight metal substrates allowing their direct integration on bendable surfaces and where the weight of solar cell is an important criterion. Flexible substrates make it possible to use the roll-to-roll technology of solar cells, which leads to an additional reduction in the cost of production and final cost of solar cells. The CZTSe thin films were fabricated by selenization of electrodeposited metallic precursors onto tantalum (Ta) flexible substrates at different temperature and time. The results of the effect of selenization temperature and time on the morphology, structural, and optical property of the CZTSe films are presented in this work. It was found that the morphology of the CZTSe thin films depend on their elemental composition and time of selenization. Experimental data indicate that composition of the CZTSe films selenized within 10 and 20 min at 560 °C have the CZTSe basic phase and secondary phases (CuSe, SnSe and ZnSe). In contrast, the increase in selenization temperature and/or time leads to disappearing of the secondary phases (CuSe, SnSe) and better crystallization of the CZTSe films. It was found that films selenized at 560 and 580 °C within the same time have similar characteristics. Depending on selenization time and temperature of the CZTSe, thin films exhibited a shift in band gap from 1.16 to 1.19 and to 1.22 eV, respectively. The change of band gap of the CZTSe thin films is associated with changes of elemental and phase compositions, and thickness of the film. These results showed that the received CZTSe films on Ta foil can be used for fabrication of thin film solar cells. © 2018
    view abstractdoi: 10.1016/j.solener.2018.12.025
  • 2019 • 186 Effects of AISI H11 surface integrity on the residual stresses and adhesion of TiAlN/substrate compounds
    Tillmann, W. and Grisales, D. and Stangier, D.
    Surface and Coatings Technology 357 466-472 (2019)
    The material of the substrate prior to the deposition of PVD coatings as well as its surface roughness and microstructure, hardness, and Young's modulus both on the surface as well as in the material matrix, and their residual stresses on the surface and in depth, have a high influence on the adhesion of the coating to the substrate. During metal forming processes, tools are exposed to high loads, which promote plastic deformation in the material of the tool and thus reduce its lifespan. In this regard, different substrate pre-treatments, allow modifying the hardness and toughness of the substrate, positively affecting the adhesion of the coating/substrate compound as well as the overall performance of the PVD coated tool. In this investigation, four different pre-treatments were performed on the hot work tool steel AISI HI I (X37CrMoV5-1). Plasma nitriding, two different heat treatments, and polishing sequences (austenitizing, quenching and double tempering) as well as a combination of these two processes (heat treatment prior plasma nitriding). Subsequently, TiAlN coatings with a 3 μm thickness were deposited onto the four differently treated substrates by means of Direct Current Magnetron Sputtering. Residual stresses of the substrate surface, before and after coating deposition were determined by means of X-ray diffraction. Additionally, residual stresses depth profiles of the steel after each pre-treatment prior to the deposition of the coatings were measured utilizing selective electropolishing of the substrate surface. To evaluate the adhesion of the TiAlN monolayers to the coatings, scratch tests and Rockwell C adhesion tests were performed on the coating/substrate compounds. The adhesion of the coatings, deposited on steel with lower residual stresses prior to the deposition and flatter residual stress gradients, was improved. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2018.10.032
  • 2019 • 185 Influence of the etching processes on the adhesion of TiAlN coatings deposited by DCMS, HiPIMS and hybrid techniques on heat treated AISI H11
    Tillmann, W. and Grisales, D. and Stangier, D. and Ben Jebara, I. and Kang, H.
    Surface and Coatings Technology 378 (2019)
    Ensuring a good adhesion of the coatings to the substrate is one of the key points during the manufacturing of machining and forming tools. The nature of the substrate material and the way it is pre-treated and cleaned before the deposition plays an important role in the adhesion of the coatings. The in-situ cleaning processes as plasma etching and metal ion etching have demonstrated to have an influence on the adhesion of different coating/substrate systems. The introduction of HiPIMS technologies for the in-situ cleaning and the deposition of PVD coatings throughout this technique has opened varied opportunities to improve the performance of the coated parts. Systematic and comparative analysis of the influence of plasma etching itself (PE), plasma etching and metal ion etching (PE + MIE), and plasma etching and HiPIMS etching (PE + HiPE) etching processes on the adhesion of the TiAlN to the hot work tool steel AISI H11 has been performed. Additionally, subsequent to the etching processes, TiAlN coatings have been deposited using DCMS, HiPIMS and hybrid (DCMS/HiPIMS) technologies. Residual stresses of the heat treated AISI H11 were evaluated before and after the different etching process as well as after coating's deposition. It has been shown that the etching process affects the growth direction and microstructure of the TiAlN coatings, especially of those deposited by DCMS and hybrid. For instance, DCMS and hybrid TiAlN coatings deposited after PE have the presence of the TiAlN (200) reflection, not evidenced on the coatings deposited after PE + MIE and PE + HiPE. Moreover, hybrid coatings on PE + HiPE have a preferential (220) growth orientation and a (111) orientation when deposited on PE and PE + MIE cleaned substrate. Finally, in order to evaluate the adhesion of the coatings to the substrate, both scratch test and Rockwell C indentation test were used. The coatings deposited on the mere plasma etched (PE) substrate turn out to be the coatings with the highest critical load Lc3 and the lowest HF standards (HF1–HF3). This behaviour is attributed to the conservation of the substrate's surface integrity and the no promotion of surface tensions that can act in detriment of the adhesion of the coating. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.125075
  • 2019 • 184 Influence of plasma nitriding pretreatments on the tribo-mechanical properties of DLC coatings sputtered on AISI H11
    Tillmann, W. and Lopes Dias, N.F. and Stangier, D.
    Surface and Coatings Technology 357 1027-1036 (2019)
    The duplex treatment, consisting of plasma nitriding and the deposition of a DLC coating, was carried out on the hot-work tool steel AISI H11. The coating structure, composed of Cr-based interlayers and a hydrogenated carbon layer, was sputtered on non-nitrided, nitrided, as well as nitrided-repolished AISI H11 steel with an either annealed or quenched and tempered base condition to examine the influence of the pretreatment condition on the tribo-mechanical properties of the DLC coating. Besides the graded hardness profile, plasma nitriding leads to a roughness increase, which affects the microstructure as well as the mechanical properties of the DLC coating. The rougher surface favors a film growth of a carbon layer with larger cluster-like structures. As a result, these DLC coatings exhibit hardness values below 22 GPa, while the coating systems sputtered on substrates with smoother surfaces reach values of approximately 26 GPa and showed a good adherence. The heat treatment condition influences the load-bearing capacity of the nitrided substrate as the higher core hardness enhances the mechanical support of the coating and reaches the highest adhesion class HF1 in the Rockwell C tests. Due to the lower film adhesion and the low hardness of the DLC coatings sputtered on nitrided non-repolished AISI H11, high coefficients of frictions and wear coefficients of up to 0.59 and 3.19 ∗ 10−5 mm3/N∗m were determined in tribometer tests against WC/Co counterparts. In contrast, the nitrided repolished steel exhibits a low coefficient of friction of 0.12 as well as a low wear coefficient of 0.06 ∗ 10−5 mm3/N∗m. Therefore, a repolishing of the nitrided AISI H11 with quenched and tempered base condition ensures the highest load-bearing capability of the substrate as well as an improved friction and wear behavior of the DLC coating. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2018.11.002
  • 2019 • 183 Superior cyclic life of thermal barrier coatings with advanced bond coats on single-crystal superalloys
    Vorkötter, C. and Mack, D.E. and Guillon, O. and Vaßen, R.
    Surface and Coatings Technology 361 150-158 (2019)
    Advanced thermal barrier coatings are essential to further increase the efficiency of gas turbine engines. One limiting factor of the TBC lifetime is the temperature dependent formation of the thermally grown oxide (TGO) during thermal exposure resulting in critical stress levels at the top coat-bond coat interface. Oxide dispersion strengthened (ODS) bond coats demonstrated slower oxygen scale growth during thermal exposure in comparison to standard bond coats. In this study for the first time TBC samples on single-crystal substrates (comparable to CMSX4) with thin ODS Co-based flash coats on the same Co-based bond coat (Amdry 995) and a porous atmospherically plasma sprayed (APS) yttria stabilized zirconia (YSZ) topcoat were manufactured by thermal spray techniques and evaluated with respect to their thermal cyclic behavior. As the major performance test cyclic burner rigs, which can establish thermal conditions similar to those in gas turbines, were applied. TBC samples with the new material combination show superior performance compared to previous samples. Cross sections of the samples were analyzed by scanning electron and laser scanning microscopy. Lifetime data and failure mode of the samples are discussed with respect to material properties such as thermal expansion coefficients, microstructural changes and TGO growth. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2019.01.001
  • 2019 • 182 Dynamic-range enhancement of heterodyne THz imaging by the use of a soft paraffin-wax substrate lens on the detector
    Yuan, H. and Lisauskas, A. and Zhang, M. and Rennings, A. and Erni, D. and Roskos, H.G.
    2019 Photonics and Electromagnetics Research Symposium - Fall, PIERS - Fall 2019 - Proceedings 2607-2611 (2019)
    Imaging with terahertz (THz) radiation gains increasing interest, because it reaches better spatial resolution than imaging with microwaves, while it maintains the advantages of a non-ionizing character and the transparency of many materials. Coherent detection, which provides both intensity and phase information, plays a key role in many emerging THz imaging applications. However, in conventional heterodyne detection, usually a semi-transparent beam-combiner is employed to couple the object wave and the reference wave onto the same beam path in front of the detector chip. This results in a bulky detector system and considerable loss. In the measurement systems considered in our work, the waves are coupled to the detector element, a THz-sensitive field-effect transistor (TeraFET), through the detector chip's substrate to which a silicon substrate lens is attached. Coupling the reference radiation onto the TeraFET from the front side would allow to avoid the use of the beam-combiner. However, front-side coupling is inefficient because of the absence of a silicon substrate lens whose implementation to the chip's front side is prohibited by the presence of the detector's bonding wires. In this contribution, we introduce a novel lens-formation approach using paraffin wax with a low melting temperature. Dripping the liquid wax onto the detector allows to embed the bonding wires within the lens. The surface tension of the molten wax leads to a close-to-super-hemispherical shape of the droplet which is maintained upon solidification. Care has to be taken to correctly center the lens on the detector. Imaging results with a TeraFET without and with an integrated wax lens (of a diameter of 8 mm) in a Fourier imaging experiment show that the paraffin wax lens improves the coupling efficiency of the 300-GHz reference radiation by at least 6 dB. The dynamic range and the useful focal-plane area of the raw data are enhanced by the wax lens, with the consequence that the resolution of the reconstructed image is improved considerably. © 2019 IEEE.
    view abstractdoi: 10.1109/PIERS-Fall48861.2019.9021735
  • 2019 • 181 Size-Selective Optical Printing of Silicon Nanoparticles through Their Dipolar Magnetic Resonance
    Zaza, C. and Violi, I.L. and Gargiulo, J. and Chiarelli, G. and Schumacher, L. and Jakobi, J. and Olmos-Trigo, J. and Cortes, E. and König, M. and Barcikowski, S. and Schlücker, S. and Sáenz, J.J. and Maier, S.A. and Stefani, F.D.
    ACS Photonics 6 815-822 (2019)
    Silicon nanoparticles possess unique size-dependent optical properties due to their strong electric and magnetic resonances in the visible range. However, their widespread application has been limited, in comparison with other (e.g., metallic) nanoparticles, because their preparation on monodisperse colloids remains challenging. Exploiting the unique properties of Si nanoparticles in nano- and microdevices calls for methods able to sort and organize them from a colloidal suspension onto specific positions of solid substrates with nanometric precision. We demonstrate that surfactant-free silicon nanoparticles of a predefined and narrow (σ < 10 nm) size range can be selectively immobilized on a substrate by optical printing from a polydisperse colloidal suspension. The size selectivity is based on differential optical forces that can be applied on nanoparticles of different sizes by tuning the light wavelength to the size-dependent magnetic dipolar resonance of the nanoparticles. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsphotonics.8b01619
  • 2018 • 180 Low-temperature MOCVD deposition of Bi2Te3 thin films using Et2BiTeEt as single source precursor
    Bendt, G. and Gassa, S. and Rieger, F. and Jooss, C. and Schulz, S.
    Journal of Crystal Growth 490 77-83 (2018)
    Et2BiTeEt was used as single source precursor for the deposition of Bi2Te3 thin films on Si(1 0 0) substrates by metal organic chemical vapor deposition (MOCVD) at very low substrate temperatures. Stoichiometric and crystalline Bi2Te3 films were grown at 230 °C, which is approximately 100 °C lower compared to conventional MOCVD processes using one metal organic precursors for each element. The Bi2Te3 films were characterized using scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. The elemental composition of the films, which was determined by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, was found to be strongly dependent of the substrate temperature. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jcrysgro.2018.03.021
  • 2018 • 179 Structural and thermoelectrical characterization of epitaxial Sb2Te3 high quality thin films grown by thermal evaporation
    Bendt, G. and Kaiser, K. and Heckel, A. and Rieger, F. and Oing, D. and Lorke, A. and Rodriguez, N.P. and Schierning, G. and Jooss, C. and Schulz, S.
    Semiconductor Science and Technology 33 (2018)
    Thermal evaporation of Sb2Te3 powder was systematically studied under various pressure and temperature conditions. Low pressure experiments (5 •10-6 mbar) conducted inside a horizontal tube reactor at a temperature range of 500 °C-600 °C generated rough polycrystalline films on Si(100) substrates. Based on these experiments, the chemical composition of the resulting films were determined by the furnace temperature. Enhancing the reactor pressure to 20 mbar shifted the growth zone towards higher temperature ranges and yielded highly c-oriented Sb2Te3 films on Si(100) and Al2O3(0001) substrates. Additional experiments were conducted inside a special reactor containing two independent heaters to study the effects of the evaporator and substrate temperatures independently. In contrast to the samples generated in the previous reactor, a two-zone heating reactor allowed the growth of epitaxial Sb2Te3 films with a very smooth surface topology on Al2O3(0001) substrates, as shown by SEM, EDX, XPS, and HRTEM. The electrical in-plane conductivity of the Sb2Te3 films decreased with increasing temperature, ultimately reaching 3950 S •cm-1 at 300 K. The films showed a p-type carrier concentration of 4.3 •10-19 cm-3 at 300 K and a very high carrier mobility of 558 cm2 •V-1 •s-1. The Seebeck coefficient increased monotonically from 94 μV •K-1 at 270 K to 127 μV •K-1 at 420 K. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6641/aad7a3
  • 2018 • 178 Laser-Textured Metal Substrates as Photoanodes for Enhanced PEC Water Splitting Reactions
    Bialuschewski, D. and Hoppius, J.S. and Frohnhoven, R. and Deo, M. and Gönüllü, Y. and Fischer, T. and Gurevich, E.L. and Mathur, S.
    Advanced Engineering Materials 20 (2018)
    We demonstrate the effect of femtosecond laser structuring of titanium substrates to increase the absorption, photoconversion, and overall photoelectrochemical water splitting (PEC) performance compared to pristine metal substrates, independent of any additional top coat layers. The influence of ultra short laser pulse patterning on PEC efficiency is investigated toward spectroscopic (UV-Vis), microscopic (SEM), crystallographic (XRD), and compositional (XPS) properties. The beneficial effect of a periodically patterned substrate is attributed to enhanced specific surface area and improved in-plane light trapping when compared to flat surfaces. Photoanodes for water splitting experiments fabricated by titanium and iron oxide films on laser pre-patterned Ti substrates are also found to show enhanced PEC efficiency (0.057 mA cm−2) when compared to unpatterened substrates (0.028 mA cm−2). The lower absolute PEC efficiencies are due to extreme thin films. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201800167
  • 2018 • 177 Experimental and Numerical Investigations on Interdiffusion Profiles in Compounds Produced by Sinter-Cladding
    Blüm, M. and Theisen, W. and Weber, S.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 49 4991-5000 (2018)
    Tools used for mineral processing applications are affected by strong abrasive wear and high dynamic loads. This results in opposing demands on the mechanical properties of these tools. Therefore, modern concepts for the manufacturing of mineral processing tools include a composite tool concept consisting of a low-alloyed substrate and a high-alloyed, wear-resistant cladding material. These coatings can be applied using different production processes such as composite casting, deposit welding, and HIP cladding. During the deposition of the cladding, interdiffusion between the substrate and cladding material occurs. This interdiffusion may have a negative impact on the compound, since characteristics such as wear resistance, mechanical properties, and the local microstructure are influenced. This article is focused on the investigation and simulation of interdiffusion processes in supersolidus-sintered compounds using computational thermodynamics, diffusion calculations, optical emission spectrometry, hardness profiles, and microstructural investigations. It is shown that the interdiffusion processes between the solid substrate and the semisolid cladding can be simulated using a dispersed phase model to give results with a close concordance to optical emission spectrometry measurements. © 2018, The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-018-4750-9
  • 2018 • 176 Tailoring metal oxide nanoparticle dispersions for inkjet printing
    Gebauer, J.S. and Mackert, V. and Ognjanović, S. and Winterer, M.
    Journal of Colloid and Interface Science 526 400-409 (2018)
    There is a growing interest in science and industry for printed electronics. Printed electronics enable the production of large quantities of electronic components at low cost. Even though organic semiconductors are already widely used for printed components, inorganic materials may be advantageous due to their higher durability and superior device performance. Nevertheless, inorganic materials still remain difficult to print making the development of printable and functional inks a necessity. In this work we present the formulation, inkjet printing and processing of newly developed inks based on ethylene glycol as dispersion medium. Different metal oxide nanoparticles (ZnO, TiO2, CuO, SnO2 and In2O3) with high crystallinity and narrow size distribution were produced by chemical vapor synthesis. The particles were stabilized and the colloidal stability was evaluated by a combination of DLVO simulations and dynamic light scattering measurements. Measurements of rheological and interfacial properties, like viscosity and surface tension, are used to determine the printability on the basis of the inverse Ohnesorge number. Inks, developed in this work, have adjustable rheological properties as well as long-term stabilities without particle sedimentation over a period of several months. They are suitable for printing on different substrate materials like silicon and flexible polymeric substrates. © 2018 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2018.05.006
  • 2018 • 175 Synthesis of carbon nanowalls from a single-source metal-organic precursor
    Giese, A. and Schipporeit, S. and Buck, V. and Wöhrl, N.
    Beilstein Journal of Nanotechnology 9 1895-1905 (2018)
    In this work, the deposition of carbon nanowalls (CNWs) by inductively coupled plasma enhanced chemical vapor deposition (ICPPECVD) is investigated. The CNWs are electrically conducting and show a large specific surface area, which is a key characteristic to make them interesting for sensors, catalytic applications or energy-storage systems. It was recently discovered that CNW films can be deposited by the use of the single-source metal-organic precursor aluminium acetylacetonate. This precursor is relatively unknown in combination with the ICP-PECVD deposition method in literature and, thus, based on our previous publication is further investigated in this work to better understand the influence of the various deposition parameters on the growth. Silicon, stainless steel, nickel and copper are used as substrate materials. The CNWs deposited are characterized by scanning electron microscopy (SEM), Raman spectroscopy and Auger electron spectroscopy (AES). The combination of bias voltage, the temperature of the substrate and the substrate material had a strong influence on the morphology of the graphitic carbon nanowall structures. With regard to these results, a first growth model for the deposition of CNWs by ICP-PECVD and aluminium acetylacetonate is proposed. This model explains the formation of four different morphologies (nanorods as well as thorny, straight and curled CNWs) by taking the surface diffusion into account. The surface diffusion depends on the particle energies and the substrate material and thus explains the influence of these parameters. © 2018 Giese et al.
    view abstractdoi: 10.3762/bjnano.9.181
  • 2018 • 174 Thermal stability of nanocomposite Mo2BC hard coatings deposited by magnetron sputtering
    Gleich, S. and Breitbach, B. and Peter, N.J. and Soler, R. and Bolvardi, H. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Surface and Coatings Technology 349 378-383 (2018)
    The investigation of hard coatings under thermal load is crucial in order to obtain information on the thermal stability and possible changes of microstructure and mechanical properties. In addition, advanced heating studies may also provide feedback for the grain growth mechanism occurring during annealing and thus, help to predict optimum post-growth annealing conditions for producing high-performance hard coatings. Here, we investigate the thermal response of Mo2BC, deposited by bipolar pulsed direct current magnetron sputtering in an industrial chamber on a silicon substrate at a substrate temperature of 380 °C. Ex-situ and in-situ X-ray diffraction and transmission electron microscopy studies are performed at elevated temperatures to track changes in the structure. Whereas the as-deposited nanocomposite coating exhibits small spherical nanocrystals (1.2 nm in diameter) embedded in an amorphous matrix, a fully crystalline structure, mainly consisting of elongated and interconnected crystals with lengths of up to 1 μm, is obtained at elevated annealing temperatures. Hardness and Young's modulus increase by ~8% and ~47%, respectively, compared to the as-deposited coating. Delamination from the silicon substrate only occurs at temperatures above 840 °C. Thus, our detailed study of the micro- and nanostructure evolution upon thermal annealing suggests that heat treatments below 840 °C are a suitable method to improve the crystallinity and mechanical properties of nanocomposite Mo2BC coatings. © 2018
    view abstractdoi: 10.1016/j.surfcoat.2018.06.006
  • 2018 • 173 Modifying the nanostructure and the mechanical properties of Mo2BC hard coatings: Influence of substrate temperature during magnetron sputtering
    Gleich, S. and Soler, R. and Fager, H. and Bolvardi, H. and Achenbach, J.-O. and Hans, M. and Primetzhofer, D. and Schneider, J.M. and Dehm, G. and Scheu, C.
    Materials and Design 142 203-211 (2018)
    A reduction in synthesis temperature is favorable for hard coatings, which are designed for industrial applications, as manufacturing costs can be saved and technologically relevant substrate materials are often temperature-sensitive. In this study, we analyzed Mo2BC hard coatings deposited by direct current magnetron sputtering at different substrate temperatures, ranging from 380 °C to 630 °C. Transmission electron microscopy investigations revealed that a dense structure of columnar grains, which formed at a substrate temperature of 630 °C, continuously diminishes with decreasing substrate temperature. It almost vanishes in the coating deposited at 380 °C, which shows nanocrystals of ~1 nm in diameter embedded in an amorphous matrix. Moreover, Argon from the deposition process is incorporated in the film and its amount increases with decreasing substrate temperature. Nanoindentation experiments provided evidence that hardness and Young's modulus are modified by the nanostructure of the analyzed Mo2BC coatings. A substrate temperature rise from 380 °C to 630 °C resulted in an increase in hardness (21 GPa to 28 GPa) and Young's modulus (259 GPa to 462 GPa). We conclude that the substrate temperature determines the nanostructure and the associated changes in bond strength and stiffness and thus, influences hardness and Young's modulus of the coatings. © 2018 The Authors
    view abstractdoi: 10.1016/j.matdes.2018.01.029
  • 2018 • 172 Thermal cycling testing of TBCs on Cr2AlC MAX phase substrates
    Gonzalez-Julian, J. and Go, T. and Mack, D.E. and Vaßen, R.
    Surface and Coatings Technology 340 17-24 (2018)
    Thermal barrier coatings (TBCs) based on yttria-stabilized zirconia (YSZ) were deposited by Atmospheric Plasma Spray (APS) on highly dense and pure Cr2AlC substrates. The Cr2AlC/YSZ systems were tested under thermal cycling conditions at temperatures between 1100 and 1300 °C testing up to 500 h. The response of the system was excellent due to the strong adhesion between the substrate and the coating, and the formation of an outer and protective layer based on α-Al2O3. The oxide scale is formed due to the diffusion of Al atoms from the crystal structure of the Cr2AlC, followed by the reaction with oxygen in the air. The thickness of the oxide scale was 8.9, 17.6 and 39.7 μm at 1100, 1200 and 1300 °C, respectively, which is rather thick in comparison with the classical superalloy/TBC systems. Cr2AlC/YSZ systems survived without any damage under the severe cycling conditions at 1100 and 1200 °C due to the protective oxide scale layer and the sufficient thermal expansion match between the Cr2AlC, YSZ and α-Al2O3. At 1300 °C and after 268 h of cycling conditions, the system failed due to the formation of a porous carbide layer underneath of the oxide scale. The results are rather promising and confirm the potential of the MAX phases to operate under long term applications of high temperature and oxidizing environments. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2018.02.035
  • 2018 • 171 A Hetero-Integrated W-Band Transmitter Module in InP-on-BiCMOS Technology
    Hossain, M. and Eissa, M.H. and Hrobak, M. and Stoppel, D. and Weimann, N. and Malignaggi, A. and Mai, A. and Kissinger, D. and Heinrich, W. and Krozer, V.
    EuMIC 2018 - 2018 13th European Microwave Integrated Circuits Conference 97-100 (2018)
    This paper presents a W -band hetero-integrated transmitter module using InP-on-BiCMOS technology. It consists of a Phase Locked Loop (PLL) in 0.25 μm BiCMOS technology and a frequency multiplier followed by a double-balanced Gilbert mixer cell in 0.8 μm InP-HBT technology, which is integrated on top of the BiCMOS MMIC in a wafer-level BCB bonding process. The PLL operates from 45 GHz to 47 GHz and the module achieves a measured single sideband (SSB) power conversion loss of 20 dB and 22 dB at 88 GHz and 95 GHz, respectively, limited by the output power from the PLL source. The entire circuit consumes 434 mW DC power. The chip area of the module is 2.5×1.3 mm 2 , To the knowledge of the authors, this is the first complex hetero-Integrated module reported so far. © 2018 European Microwave Association - EuMA.
    view abstractdoi: 10.23919/EuMIC.2018.8539915
  • 2018 • 170 Systematic Investigation on the Influence of Spray Parameters on the Mechanical Properties of Atmospheric Plasma-Sprayed YSZ Coatings
    Mutter, M. and Mauer, G. and Mücke, R. and Guillon, O. and Vaßen, R.
    Journal of Thermal Spray Technology 1-15 (2018)
    In the atmospheric plasma spray (APS) process, micro-sized ceramic powder is injected into a thermal plasma where it is rapidly heated and propelled toward the substrate. The coating formation is characterized by the subsequent impingement of a large number of more or less molten particles forming the so-called splats and eventually the coating. In this study, a systematic investigation on the influence of selected spray parameters on the coating microstructure and the coating properties was conducted. The investigation thereby comprised the coating porosity, the elastic modulus, and the residual stress evolution within the coating. The melting status of the particles at the impingement on the substrate in combination with the substrate surface condition is crucial for the coating formation. Single splats were collected on mirror-polished substrates for selected spray conditions and evaluated by identifying different types of splats (ideal, distorted, weakly bonded, and partially molten) and their relative fractions. In a previous study, these splat types were evaluated in terms of their effect on the above-mentioned coating properties. The particle melting status, which serves as a measure for the particle spreading behavior, was determined by in-flight particle temperature measurements and correlated to the coating properties. It was found that the gun power and the spray distance have a strong effect on the investigated coating properties, whereas the feed rate and the cooling show minor influence. © 2018 ASM International
    view abstractdoi: 10.1007/s11666-018-0697-z
  • 2018 • 169 Effect of substrate orientation on local magnetoelectric coupling in bi-layered multiferroic thin films
    Naveed-Ul-Haq, M. and Webers, S. and Trivedi, H. and Salamon, S. and Wende, H. and Usman, M. and Mumtaz, A. and Shvartsman, V.V. and Lupascu, D.C.
    Nanoscale 10 20618-20627 (2018)
    In this study we explore the prospect of strain-mediated magnetoelectric coupling in CoFe2O4-BaTiO3 bi-layers as a function of different interfacial boundary conditions. Pulsed laser deposition fabricated thin films on Nb:SrTiO3(100) and Nb:SrTiO3(111) single crystal substrates were characterized in terms of their peculiarities related to the structure-property relationship. Despite the homogeneous phase formation in both films, transmission electron microscopy showed that the bi-layers on Nb:SrTiO3(100) exhibit a higher number of crystallographic defects when compared to the films on Nb:SrTiO3(111). This signifies an intrinsic relationship of the defects and the substrate orientation. To analyze the consequences of these defects on the overall magnetoelectric coupling of the bi-layered films, piezoresponse force microscopy was performed in situ with an applied magnetic field. The local magnetic field dependence of the piezoresponse was obtained using principal component analysis. A detailed analysis of this dependence led to a conclusion that the bi-layers on Nb:SrTiO3(111) exhibit better strain-transfer characteristics between the magnetic and the piezoelectric layer than those which were deposited on Nb:SrTiO3(100). These strain transfer characteristics correlate well with the interface quality and the defect concentration. This study suggests that in terms of overall magnetoelectric coupling, the Nb:SrTiO3(111) grown bi-layers are expected to outperform their Nb:SrTiO3(100) grown counterparts. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8nr06041j
  • 2018 • 168 Correlative transmission Kikuchi diffraction and atom probe tomography study of Cu(In,Ga)Se2 grain boundaries
    Schwarz, T. and Stechmann, G. and Gault, B. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 26 196-204 (2018)
    We combined transmission Kikuchi diffraction and atom probe tomography techniques to investigate the relationship between the structure and chemistry of grain boundaries in Cu(In,Ga)Se2 thin films. Kikuchi patterns with the tetragonal structure of Cu(In,Ga)Se2 were simulated to emphasize the pseudosymmetry issue in this material system and, hence, the orientation determination ambiguity in case of indexing with a cubic zinc-blende structure. We compared these patterns with experimental data. We detect an elemental redistribution at random high-angle grain boundaries but no chemical fluctuations at Σ3 twin boundaries. The atom probe tomography analyses reveal Cu depletion as well as In and Se enrichment at random grain boundaries and, at some random grain boundaries, a slight Ga depletion. This In on Cu scenario is accompanied by cosegregation of Na and K originating from the soda-lime glass substrate. The amount of impurity segregation does vary not only from one grain boundary to another but also along an individual grain boundary. Hence, our results suggest that the degree of passivation of detrimental, nonradiative recombination centers does differ not only between Σ3 twin boundaries and random grain boundaries but also within the same random grain boundary. Copyright © 2017 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2966
  • 2018 • 167 A Highly Efficient Ultrawideband Traveling-Wave Amplifier in InP DHBT Technology
    Shivan, T. and Weimann, N. and Hossain, M. and Stoppel, D. and Boppel, S. and Ostinelli, O. and Doerner, R. and Bolognesi, C.R. and Krozer, V. and Heinrich, W.
    IEEE Microwave and Wireless Components Letters 28 1029-1031 (2018)
    This letter presents a 1 to >110-GHz ultrawideband traveling-wave amplifier (TWA) based on 500-nm transferred-substrate InP double-heterojunction bipolar transistor technology. The HBT cells are realized with inductive peaking at the output and match the phase delay between individual stages. The collector bias is slightly below the value for the maximum current gain. This allows a frequency-invariant high-output power characteristic with a flat group delay. The amplifier exhibits a gain of 13 dB with a measured bandwidth of 1-110 GHz and a uniform 10-dBm 1-dB compression output power with an associated maximum PAE of 8% at 110 GHz. To the best of our knowledge, this is the highest PAE and the maximum flat output power reported for such a TWA covering frequencies up to 170 GHz. The amplifier consumes only 129 mW, which is among the lowest dc power dissipations reported for the given gain-bandwidth product. Moreover, the group delay is flat across the band, which makes the TWA very useful in high-speed optical communication systems. © 2001-2012 IEEE.
    view abstractdoi: 10.1109/LMWC.2018.2871336
  • 2018 • 166 Influence of coating thickness on residual stress and adhesion-strength of cold-sprayed Inconel 718 coatings
    Singh, R. and Schruefer, S. and Wilson, S. and Gibmeier, J. and Vassen, R.
    Surface and Coatings Technology 350 64-73 (2018)
    In the cold spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at the temperatures well below their melting point. The high particle impact velocity causes high local stresses which lead to deforming the particles and the substrate plastically in the proximity of the particle–substrate interface. As a result, high residual stresses are introduced in cold spray coatings due to the peening effect of the particles collisions with the substrate. In this study, a powder based on the chemical composition of IN 718 was cold-sprayed on IN 718 substrates by using nitrogen gas for an application as a repair tool for aero engine components. The magnitude of the residual stress and its distribution through the thickness were measured by using the hole-drilling and the bending methods. Residual stress was also estimated by using an approach based on the physical process parameters. Mainly compressive residual stresses were observed in cold-sprayed IN 718 coatings. Accumulation of residual stresses in the coatings is highly affected by peening during deposition and it decreases with increase in thickness. It has been observed that the adhesion-strengths of cold-sprayed IN 718 coatings are highly influenced by coating thickness and residual stress states of the coating/substrate system. In the presence of residual stresses in the coatings, adhesion-strength decreases with increasing coating thickness. The energy-release-rate criterion has been used to predict adhesion-strength with increasing coating thickness. Predicted bond-strength values are close to the measured adhesion-strength values and decrease with increase in coating thickness. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2018.06.080
  • 2018 • 165 Prediction and Experimental Validation of Co-Solvent Influence on Michaelis Constants: A Thermodynamic Activity-Based Approach
    Wangler, A. and Böttcher, D. and Hüser, A. and Sadowski, G. and Held, C.
    Chemistry - A European Journal 24 16418-16425 (2018)
    Co-solvents are known to influence the Michaelis constant KM of enzyme-catalyzed reactions. In the literature, co-solvent effects on KM are usually explained by interactions between enzyme and co-solvent. Very recent works replaced substrate concentrations with thermodynamic activities to separate enzyme–co-solvent from substrate–co-solvent interactions This yields the thermodynamic-activity-based Michalis constant Ka M. In this work, this approach was extended to alcohol dehydrogenase (ADH)-catalyzed reduction of acetophenone (ACP), a two-substrate reaction. It was experimentally found that polyethylene glycol (PEG) 6000 increased KM of ACP and decreased KM of nicotinamide adenine dinucleotide (NADH). To predict Ka M values, non-covalent interactions between substrates and reaction media were taken into account by electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) modelling. In contrast to experimental KM values, their activity-based pendants Ka M were independent of co-solvent. To further verify the approach, the reduction of 2-pentanone catalyzed by the same ADH was investigated. Interestingly, the addition of PEG caused a decrease of both KM of 2-pentanone and KM of NADH. Based on Ka M values obtained from in co-solvent-free conditions and activity coefficients from ePC-SAFT, the influence of the co-solvent on KM was quantitatively predicted. Thus, the approach known for pseudo one-substrate reactions was successfully transferred to two-substrate reactions. Furthermore, the advantage of thermodynamic activities over concentrations in the field of enzyme kinetics is highlighted. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201803573
  • 2018 • 164 Transferred-Substrate InP/GaAsSb Heterojunction Bipolar Transistor Technology with f max∼ 0.53 THz
    Weimann, N.G. and Johansen, T.K. and Stoppel, D. and Matalla, M. and Brahem, M. and Nosaeva, K. and Boppel, S. and Volkmer, N. and Ostermay, I. and Krozer, V. and Ostinelli, O. and Bolognesi, C.R.
    IEEE Transactions on Electron Devices 65 3704-3710 (2018)
    We report on the realization mymargin of transferred-substrate InP/GaAsSb double heterostructure bipolar transistors in a terahertz monolithic integrated circuit process. Transistors with 0.4- mu m -wide single emitters reached unilateral gain cutoff frequencies of around 530 GHz with simultaneous current gain cutoff frequencies above 350 GHz. Extrinsic collector capacitance is effectively reduced in the transfer-substrate process. In combination with the high collector breakdown voltage in the InP/GaAsSb heterobipolar transistor structure of 5 V, this process is amenable to analog power applications at millimeter (mm-wave) and sub-mm-wave frequencies. We demonstrate reliable extraction procedures for unilateral gain and current gain cutoff frequencies. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/TED.2018.2854546
  • 2018 • 163 Ideal Dimers of Gold Nanospheres for Precision Plasmonics: Synthesis and Characterization at the Single-Particle Level for Identification of Higher Order Modes
    Yoon, J.H. and Selbach, F. and Langolf, L. and Schlücker, S.
    Small 14 (2018)
    Ideal dimers comprising gold nanoparticles with a smooth surface and high sphericity are synthesized by a substrate-based assembly strategy with efficient cetyltrimethylammonium bromide removal. An unprecedented structural and plasmonic uniformity at the single-particle level is observed since inhomogeneities resulting from variations in gap morphology are eliminated. Single ideal dimers are analyzed by polarization-resolved dark-field scattering spectroscopy. Contributions from transverse as well as quadrupolar and octupolar longitudinal plasmon coupling modes can be discriminated because of their orthogonal polarization behavior. The assignment of these higher order coupling modes is supported by computer simulations. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201702754
  • 2017 • 162 Film forming properties of silicon nanoparticles on SixNy coated substrates during excimer laser annealing
    Caninenberg, M. and Kiesler, D. and Benson, N. and Schmechel, R.
    Optics and Laser Technology 90 33-39 (2017)
    In this article we investigate the film forming properties of excimer laser annealed silicon nanoparticles on non-silicon substrates. In contrast to their film forming properties on oxide free silicon substrates, the nanoparticle thin film tends to dewet and form a porous µ-structure on the silicon nitrite covered glass model substrates considered for our investigation. This is quantified using a SEM study in conjunction with image processing software, in order to evaluate the µ-structure size and inter µ-structure distance in dependence of the laser energy density. To generalize our results, the film forming process is described using a COMSOL Multiphysics ® fluid dynamics model, which solves the Navier Stokes equation for incompressible Newtonian fluids. To account for the porous nanoparticle thin film structure in the simulation, an effective medium approach is used by applying a conservative level set one phase method to our mesh. This effort allows us to predict the Si melt film formation ranging from a porous Si µ-structure to a compact 100% density Si thin film in dependence of the substrate / thin film interaction, as well as the laser energy used for the nanoparticle processing. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.optlastec.2016.11.010
  • 2017 • 161 Hybrid biocomposites based on titania nanotubes and a hydroxyapatite coating deposited by RF-magnetron sputtering: Surface topography, structure, and mechanical properties
    Chernozem, R.V. and Surmeneva, M.A. and Krause, B. and Baumbach, T. and Ignatov, V.P. and Tyurin, A.I. and Loza, K. and Epple, M. and Surmenev, R.A.
    Applied Surface Science 426 229-237 (2017)
    In this study, biocomposites based on porous titanium oxide structures and a calcium phosphate (CaP) or hydroxyapatite (HA) coating are described and prepared. Nanotubes (NTs) with different pore dimensions were processed using anodic oxidation of Ti substrates in a NH4F-containing electrolyte solution at anodization voltages of 30 and 60 V with a DC power supply. The external diameters of the nanotubes prepared at 30 V and 60 V were 53 ± 10 and 98 ± 16 nm, respectively. RF-magnetron sputtering of the HA target in a single deposition run was performed to prepare a coating on the surface of TiO2 NTs prepared at 30 and 60 V. The thickness of the CaP coating deposited on the mirror-polished Si substrate in the same deposition run with TiO2 NTs was determined by optical ellipsometry (SE) 95 ± 5 nm. Uncoated and CaP-coated NTs were annealed at 500 °C in air. Afterwards, the presence of TiO2 (anatase) was observed. The scanning electron microscopy (SEM), X-ray diffraction (XRD), photoelectron spectroscopy (XPS) and nanoindentation results revealed the influence that the NT dimensions had on the CaP coating deposition process. The tubular surfaces of the NTs were completely coated with the HA coating when prepared at 30 V, and no homogeneous CaP coating was observed when prepared at 60 V. The XRD patterns show peaks assigned to crystalline HA only for the coated TiO2 NTs prepared at 30 V. High-resolution XPS spectra show binding energies (BE) of Ca 2p, P 2p and O 1s core-levels corresponding to HA and amorphous calcium phosphate on TiO2 NTs prepared at 30 V and 60 V, respectively. Fabrication of TiO2 NTs results in a significant decrease to the elastic modulus and nanohardness compared to the Ti substrate. The porous structure of the NTs causes an increase in the elastic strain to failure of the coating (H/E) and the parameter used to describe the resistance of the material to plastic deformation (H3/E2) at the nanoscale level compared to the Ti substrate. Furthermore, only the HA coating on the NTs exhibits a significantly increased H/E ratio and H3/E2 factor compared to the NTs and Ti substrate. Increases in resistance to penetration for the indenter were also observed for HA-coated TiO2 NTs prepared at 30 V compared to uncoated and CaP-coated NTs prepared at 60 V. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.07.199
  • 2017 • 160 Water-free synthesis of ZnO quantum dots for application as an electron injection layer in light-emitting electrochemical cells
    Daumann, S. and Andrzejewski, D. and Di Marcantonio, M. and Hagemann, U. and Wepfer, S. and Vollkommer, F. and Bacher, G. and Epple, M. and Nannen, E.
    Journal of Materials Chemistry C 5 2344-2351 (2017)
    Large-area light emitters like organic (OLEDs) or quantum dot light-emitting devices (QLEDs) and light-emitting electrochemical cells (LECs) have gained increasing interest due to their cost-effective fabrication on various even flexible substrates. The implementation of ZnO nanoparticles as an electron injection layer in large-area emitters leads to efficient solution-based devices. However, ZnO support layers are frequently in direct contact with water-sensitive emitter materials, which requires ZnO nanoparticles with minimum water content. A water-free synthesis route (except for the small amount of water formed during the synthesis) of ligand-free ZnO nanoparticles is presented. The spherical ZnO nanoparticles have a diameter of 3.4 nm, possess a high crystallinity, and form stable dispersions in ethanol or 1-hexanol. Their application together with a transition metal complex (iTMC)-LEC as an additional electron injection layer resulted in an increase of the device efficiency from 1.6 to 2.4 lm W−1 as well as the reduction of the run-up time to one fifth, compared to the same system without ZnO nanoparticles. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6tc05571k
  • 2017 • 159 Modifications of aluminum film caused by micro-plasmoids and plasma spots in the effluent of an argon non-equilibrium plasma jet
    Engelhardt, M. and Ries, S. and Hermanns, P. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    A smooth layer of hard aluminium film is deposited onto a glass substrate with a multi-frequency CCP discharge and then treated in the effluent of a non-equilibrium atmospheric pressure plasma jet (N-APPJ) operated with Ar flow. A thin filament is formed in the argon N-APPJ through contraction of a diffuse feather-like discharge. The aluminium surface treated in the effluents of the N-APPJ is significantly modified. Erosion tracks of different forms and micro-balls composed of aluminium are observed on the treated surface. Based on CCD images of active plasma discharge channels, SEM images of the treated surface and current-voltage characteristics, these surface modifications are interpreted as traces of plasma spots and plasmoids. Plasma spots are focused plasma channels, which are characterized by an intense emission in CCD images at the contact point of a plasma channel with the treated metal surface and by deep short tracks on the aluminium surface, observed in SEM images. Plasmoids are plasma objects without contact to any power supply which can produce long, thin and shallow traces, as can be observed on the treated surface using electron microscopy. Based on observed traces and numerous transformations of plasma spots to plasmoids and vice versa, it is supposed that both types of plasma objects are formed by an extremely high axial magnetic field and differ from each other due to the existence or absence of contact to a power supply and the consequential transport of electric current. The reason for the magnetic field at the axis of these plasma objects is possibly a circular current of electron pairs in vortices, which are formed in plasma by the interaction of ionization waves with the substrate surface. The extremely high magnetic field of plasma spots and plasmoids leads to a local destruction of the metal film and top layer of the glass substrate and to an attraction of paramagnetic materials, namely aluminium and oxygen. The magnetic attraction of aluminium is a reason for the extraction of some pieces of metal and the formation of erosion tracks and holes in the metal film. In the absence of metal atomization, the extracted aluminium forms spherical micro-particles, which are distributed over the surface of the treated metal film by the gas flow. A thin (100 nm) gold (diamagnetic) layer on top of the aluminium film surface reduces the erosion rate of plasma spots and plasmoids drastically (more than three orders of magnitude). © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa802f
  • 2017 • 158 Dependence of the optical constants and the performance in the SPREE gas measurement on the thickness of doped tin oxide over coatings
    Fischer, D. and Hertwig, A. and Beck, U. and Negendank, D. and Lohse, V. and Kormunda, M. and Esser, N.
    Applied Surface Science 421 480-486 (2017)
    In this study, thickness related changes of the optical properties of doped tin oxide were studied. Two different sets of samples were prepared. The first set was doped with iron or nickel on silicon substrate with thicknesses of 29-56. nm, the second was iron doped on gold/glass substrate with 1.6-6.3. nm. The optical constants were determined by using spectral ellipsometry (SE) followed by modelling of the dielectric function with an oscillator model using Gaussian peaks. The analysis of the optical constants shows a dependence of the refraction and the absorption on the thickness of the doped tin oxide coating. In addition to the tin oxide absorption in the UV, one additional absorption peak was found in the near-IR/red which is related to plasmonic effects due to the doping. This peak shifts from the near-IR to the red part of the visible spectrum and becomes stronger by reducing the thickness, probably due to the formation of metal nanoparticles in this layer. These results were found for two different sets of samples by using the same optical model. Afterwards the second sample set was tested in the Surface Plasmon Resonance Enhanced Ellipsometric (SPREE) gas measurement with CO gas. It was found that the thickness has significant influence on the sensitivity and thus the adsorption of the CO gas. By increasing the thickness from 1.6. nm to 5.1. nm, the sensing ability is enhanced due to a higher coverage of the surface with the over coating. This is explained by the high affinity of CO molecules to the incorporated Fe-nanoparticles in the tin oxide coating. By increasing the thickness further to 6.3. nm, the sensing ability drops because the layer disturbs the SPR sensing effect too much. © 2016.
    view abstractdoi: 10.1016/j.apsusc.2016.11.188
  • 2017 • 157 An active balanced up-converter module in InP-on-BiCMOS technology
    Hossain, M. and Meliani, C. and Schukfeh, M.I. and Weimann, N. and Lisker, M. and Krozer, V. and Heinrich, W.
    IEEE MTT-S International Microwave Symposium Digest 953-956 (2017)
    This paper presents an active up-converter realized as hetero-integrated module in InP-on-BiCMOS technology. It consists of a fundamental Voltage Controlled Oscillator (VCO) in 0.25 μm BiCMOS technology and a frequency multiplier followed by double balanced Gilbert mixer cell in 0.8 μm transferred substrate (TS) InP-HBT technology, which is integrated on top of the BiCMOS MMIC. The fundamental VCO operates at 54 GHz. The module achieves a single-sideband (SSB) power up-conversion gain of 2.5 dB and -3.5 dB at 82 GHz and 106 GHz, respectively. It exhibits > 25 GHz IF bandwidth. To the knowledge of the authors, this is the first heterointegrated module reported so far. © 2017 IEEE.
    view abstractdoi: 10.1109/MWSYM.2017.8058745
  • 2017 • 156 The effect of UV radiation from oxygen and argon plasma on the adhesion of organosilicon coatings on polypropylene
    Jaritz, M. and Behm, H. and Hopmann, C. and Kirchheim, D. and Mitschker, F. and Awakowicz, P. and Dahlmann, R.
    Journal of Physics D-applied Physics 50 015201 (2017)
    The influence of ultraviolet (UV) radiation from oxygen and argon pretreatment plasmas on a plastic substrate has not been fully understood yet. In particular, its influence on the adhesion properties has not been sufficiently researched so far. This paper addresses this issue by comparing the bond strength of a plasmapolymerized silicon organic coating (SiOxCyHz) on polypropylene (PP) after oxygen and argon plasma pretreatment and pretreatment by UV radiation emitted by the same plasmas. The UV radiation is isolated from the other species from the plasma by means of a magnesium fluoride (MgF2) optical filter. It could be shown that UV radiation originating from an oxygen plasma has a significant impact on both substrate surface chemistry and coating adhesion. The same maximum bond strength enhancement can be reached by pretreating the polypropylene surface either with pulsed oxygen plasma, or with only the UV radiation from this oxygen plasma. Also, similar surface chemistry and topography modifications are induced. For argon plasma no significant influence of its UV radiation on the substrate could be observed in this study.
    view abstractdoi: 10.1088/1361-6463/50/1/015201
  • 2017 • 155 The effect of UV radiation from oxygen and argon plasma on the adhesion of organosilicon coatings on polypropylene
    Jaritz, M. and Behm, H. and Hopmann, C. and Kirchheim, D. and Mitschker, F. and Awakowicz, P. and Dahlmann, R.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of ultraviolet (UV) radiation from oxygen and argon pretreatment plasmas on a plastic substrate has not been fully understood yet. In particular, its influence on the adhesion properties has not been sufficiently researched so far. This paper addresses this issue by comparing the bond strength of a plasmapolymerized silicon organic coating (SiOxCyHz) on polypropylene (PP) after oxygen and argon plasma pretreatment and pretreatment by UV radiation emitted by the same plasmas. The UV radiation is isolated from the other species from the plasma by means of a magnesium fluoride (MgF2) optical filter. It could be shown that UV radiation originating from an oxygen plasma has a significant impact on both substrate surface chemistry and coating adhesion. The same maximum bond strength enhancement can be reached by pretreating the polypropylene surface either with pulsed oxygen plasma, or with only the UV radiation from this oxygen plasma. Also, similar surface chemistry and topography modifications are induced. For argon plasma no significant influence of its UV radiation on the substrate could be observed in this study. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/50/1/015201
  • 2017 • 154 Influence of residual stress on the adhesion and surface morphology of PECVD-coated polypropylene
    Jaritz, M. and Hopmann, C. and Behm, H. and Kirchheim, D. and Wilski, S. and Grochla, D. and Banko, L. and Ludwig, Al. and Böke, M. and Winter, J. and Bahre, H. and Dahlmann, R.
    Journal of Physics D: Applied Physics 50 (2017)
    The properties of plasma-enhanced chemical vapour deposition (PECVD) coatings on polymer materials depend to some extent on the surface and material properties of the substrate. Here, isotactic polypropylene (PP) substrates are coated with silicon oxide (SiOx) films. Plasmas for the deposition of SiOx are energetic and oxidative due to the high amount of oxygen in the gas mixture. Residual stress measurements using single Si cantilever stress sensors showed that these coatings contain high compressive stress. To investigate the influence of the plasma and the coatings, residual stress, silicon organic (SiOCH) coatings with different thicknesses between the PP and the SiOx coating are used as a means to protect the substrate from the oxidative SiOx coating process. Pull-off tests are performed to analyse differences in the adhesion of these coating systems. It could be shown that the adhesion of the PECVD coatings on PP depends on the coatings' residual stress. In a PP/SiOCH/SiOx-multilayer system the residual stress can be significantly reduced by increasing the thickness of the SiOCH coating, resulting in enhanced adhesion. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa8798
  • 2017 • 153 Bioactivity and electrochemical behavior of hydroxyapatite-silicon-multi walled carbon nano-tubes composite coatings synthesized by EPD on NiTi alloys in simulated body fluid
    Khalili, V. and Khalil-Allafi, J. and Frenzel, J. and Eggeler, G.
    Materials Science and Engineering C 71 473-482 (2017)
    In order to improve the surface bioactivity of NiTi bone implant and corrosion resistance, hydroxyapatite coating with addition of 20 wt% silicon, 1 wt% multi walled carbon nano-tubes and both of them were deposited on a NiTi substrate using a cathodic electrophoretic method. The apatite formation ability was estimated using immersion test in the simulated body fluid for 10 days. The SEM images of the surface of coatings after immersion in simulated body fluid show that the presence of silicon in the hydroxyapatite coatings accelerates in vitro growth of apatite layer on the coatings. The Open-circuit potential and electrochemical impedance spectroscopy were measured to evaluate the electrochemical behavior of the coatings in the simulated body fluid at 37 °C. The results indicate that the compact structure of hydroxyapatite-20 wt% silicon and hydroxyapatite-20 wt% silicon-1 wt% multi walled carbon nano-tubes coatings could efficiently increase the corrosion resistance of NiTi substrate. © 2016
    view abstractdoi: 10.1016/j.msec.2016.10.036
  • 2017 • 152 Hollow fiber membrane lumen modified by polyzwitterionic grafting
    Lieu Le, N. and Quilitzsch, M. and Cheng, H. and Hong, P.-Y. and Ulbricht, M. and Nunes, S.P. and Chung, T.-S.
    Journal of Membrane Science 522 1-11 (2017)
    In this study, we demonstrate an effective way to modify the lumen of polyetherimide hollow fibers by grafting zwitterionic poly(sulfobetaine) to increase the membrane resistance to fouling. Surface-selective grafting of the protective hydrogel layers has been achieved in a facile two-step process. The first step is the adsorption of a macromolecular redox co-initiator on the lumen-side surface of the membrane, which in the second step, after flushing the lumen of the membrane with a solution comprising monomers and a complementary redox initiator, triggers the in situ cross-linking copolymerization at room temperature. The success of grafting reaction has been verified by the surface elemental analyses using X-ray photoelectron spectroscopy (XPS) and the surface charge evaluation using zeta potential measurements. The hydrophilicity of the grafted porous substrate is improved as indicated by the change of contact angle value from 44° to 30°, due to the hydration layer on the surface produced by the zwitterionic poly(sulfobetaine). Compared to the pristine polyetherimide (PEI) substrate, the poly(sulfobetaine) grafted substrates exhibit high fouling resistance against bovine serum albumin (BSA) adsorption, E. coli attachment and cell growth on the surface. Fouling minimization in the lumen is important for the use of hollow fibers in different processes. For instance, it is needed to preserve power density of pressure-retarded osmosis (PRO). In high-pressure PRO tests, a control membrane based on PEI with an external polyamide selective layer was seriously fouled by BSA, leading to a high water flux drop of 37%. In comparison, the analogous membrane, whose lumen was modified with poly(sulfobetaine), not only had a less water flux decline but also had better flux recovery, up to 87% after cleaning and hydraulic pressure impulsion. Clearly, grafting PRO hollow fiber membranes with zwitterionic polymeric hydrogels as a protective layer potentially sustains PRO performance for power generation. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2016.08.038
  • 2017 • 151 Size- and density-controlled deposition of Ag nanoparticle films by a novel low-temperature spray chemical vapour deposition method—research into mechanism, particle growth and optical simulation
    Liu, Y. and Plate, P. and Hinrichs, V. and Köhler, T. and Song, M. and Manley, P. and Schmid, M. and Bartsch, P. and Fiechter, S. and Lux-Steiner, M.C. and Fischer, C.-H.
    Journal of Nanoparticle Research 19 (2017)
    Ag nanoparticles have attracted interest for plasmonic absorption enhancement of solar cells. For this purpose, well-defined particle sizes and densities as well as very low deposition temperatures are required. Thus, we report here a new spray chemical vapour deposition method for producing Ag NP films with independent size and density control at substrate temperatures even below 100 °C, which is much lower than for many other techniques. This method can be used on different substrates to deposit Ag NP films. It is a reproducible, low-cost process which uses trimethylphosphine (hexafluoroacetylacetonato) silver as a precursor in alcoholic solution. By systematic variation of deposition parameters and classic experiments, mechanisms of particle growth and of deposition processes as well as the low decomposition temperature of the precursor could be explained. Using the 3D finite element method, absorption spectra of selected samples were simulated, which fitted well with the measured results. Hence, further applications of such Ag NP films for generating plasmonic near field can be predicted by the simulation. © 2017, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-017-3834-6
  • 2017 • 150 Functional performance of Gd2Zr2O7/YSZ multi-layered thermal barrier coatings deposited by suspension plasma spray
    Mahade, S. and Curry, N. and Björklund, S. and Markocsan, N. and Nylén, P. and Vaßen, R.
    Surface and Coatings Technology 318 208-216 (2017)
    7-8. wt.% yttria stabilized zirconia (YSZ) is the standard ceramic top coat material used in gas turbines to insulate the underlying metallic substrate. However, at higher temperatures (>. 1200. C), phase stability and sintering becomes an issue for YSZ. At these temperatures, YSZ is also susceptible to CMAS (calcium magnesium alumino silicates) infiltration. New ceramic materials such as pyrochlores have thus been proposed due to their excellent properties such as lower thermal conductivity and better CMAS attack resistance compared to YSZ. However, pyrochlores have inferior thermo mechanical properties compared to YSZ. Therefore, double-layered TBCs with YSZ as the intermediate layer and pyrochlore as the top ceramic layer have been proposed. In this study, double layer TBC comprising gadolinium zirconate (GZ)/YSZ and triple layer TBC (GZdense/GZ/YSZ) comprising relatively denser GZ top layer on GZ/YSZ were deposited by suspension plasma spray. Also, single layer 8YSZ TBC was suspension plasma sprayed to compare its functional performance with the multi-layered TBCs. Cross sections and top surface morphology of as sprayed TBCs were analyzed by scanning electron microscopy (SEM). XRD analysis was done to identify phases formed in the top surface of as sprayed TBCs. Porosity measurements were made using water intrusion and image analysis methods. Thermal diffusivity of the as sprayed TBCs was measured using laser flash analysis and thermal conductivity of the TBCs was calculated. The multi-layered GZ/YSZ TBCs were shown to have lower thermal conductivity than the single layer YSZ. The as sprayed TBCs were also subjected to thermal cyclic testing at 1300. C. The double and triple layer TBCs had a longer thermal cyclic life compared to YSZ. The thermo cycled samples were analyzed by SEM. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.12.062
  • 2017 • 149 Recent developments in plasma spray processes for applications in energy technology
    Mauer, G. and Jarligo, M.O. and Marcano, D. and Rezanka, S. and Zhou, D. and Vaßen, R.
    IOP Conference Series: Materials Science and Engineering 181 (2017)
    This work focuses on recent developments of plasma spray processes with respect to specific demands in energy technology. High Velocity Atmospheric Plasma Spraying (HV-APS) is a novel variant of plasma spraying devoted to materials which are prone to oxidation or decomposition. It is shown how this process can be used for metallic bondcoats in thermal barrier coating systems. Furthermore, Suspension Plasma Spraying (SPS) is a new method to process submicron-sized feedstock powders which are not sufficiently flowable to feed them in dry state. SPS is presently promoted by the development of novel torch concepts with axial feedstock injection. An example for a columnar structured double layer thermal barrier coating is given. Finally, Plasma Spray-Physical Vapor Deposition (PS-PVD) is a novel technology operating in controlled atmosphere at low pressure and high plasma power. At such condition, vaporization even of high-melting oxide ceramics is possible enabling the formation of columnar structured, strain tolerant coatings with low thermal conductivity. Applying different conditions, the deposition is still dominated by liquid splats. Such process is termed Low Pressure Plasma Spraying-Thin Film (LPPS-TF). Two examples of applications are gas-tight and highly ionic and electronic conductive electrolyte and membrane layers which were deposited on porous metallic substrates. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/181/1/012001
  • 2017 • 148 Analysis of the time-resolved magneto-optical Kerr effect for ultrafast magnetization dynamics in ferromagnetic thin films
    Razdolski, I. and Alekhin, A. and Martens, U. and Bü Rstel, D. and Diesing, D. and Münzenberg, M. and Bovensiepen, U. and Melnikov, A.
    Journal of Physics Condensed Matter 29 (2017)
    We discuss fundamental aspects of laser-induced ultrafast demagnetization probed by the time-resolved magneto-optical Kerr effect (MOKE). Studying thin Fe films on MgO substrate in the absence of electronic transport, we demonstrate how to disentangle pump-induced variations of magnetization and magneto-optical coefficients. We provide a mathematical formalism for retrieving genuine laser-induced magnetization dynamics and discuss its applicability in real experimental situations. We further stress the importance of temporal resolution achieved in the experiments and argue that measurements of both time-resolved MOKE rotation and ellipticity are needed for the correct assessment of magnetization dynamics on sub-picosecond timescales. The framework developed here sheds light onto the details of the time-resolved MOKE technique and contributes to the understanding of the interplay between ultrafast laser-induced optical and magnetic effects. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa63c6
  • 2017 • 147 Growth and shape of indium islands on molybdenum at micro-roughened spots created by femtosecond laser pulses
    Ringleb, F. and Eylers, K. and Teubner, T. and Schramm, H.-P. and Symietz, C. and Bonse, J. and Andree, S. and Heidmann, B. and Schmid, M. and Krüger, J. and Boeck, T.
    Applied Surface Science 418 548-553 (2017)
    Indium islands on molybdenum coated glass can be grown in ordered arrays by surface structuring using a femtosecond laser. The effect of varying the molybdenum coated glass substrate temperature and the indium deposition rate on island areal density, volume and geometry is investigated and evaluated in a physical vapor deposition (PVD) process. The joined impact of growth conditions and spacing of the femtosecond laser structured spots on the arrangement and morphology of indium islands is demonstrated. The results yield a deeper understanding of the island growth and its precise adjustment to industrial requirements, which is indispensable for a technological application of such structures at a high throughput, for instance as precursors for the preparation of Cu(In,Ga)Se2 micro concentrator solar cells. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2016.11.135
  • 2017 • 146 Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films with High Anisotropy Grown from an Alloy Mn55Bi45 Target
    Sabet, S. and Hildebrandt, E. and Römer, F.M. and Radulov, I. and Zhang, H. and Farle, M. and Alff, L.
    IEEE Transactions on Magnetics 53 (2017)
    Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 eμcm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of \sim 1.86 {\cdot 10{7}} erg/cm3 (1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2016.2636817
  • 2017 • 145 Kinetic investigation of the ion angular distribution in capacitive radio-frequency plasmas
    Shihab, M. and Mussenbrock, T.
    Physics of Plasmas 24 (2017)
    One of the key parameters in the context of plasma assisted processing in semiconductor fabrication using capacitive radio-frequency plasmas is the ion flux distribution at the substrate. Whereas the ion energy distribution function determines the etching rate and selectivity, the ion angular distribution controls the etching profile. In this contribution, we reveal the effect of the ion flux and the sheath potential on the ion angular distribution and the direct ion heat flux at the bottom of etching profiles in geometrically symmetric plasma reactors. The ion angular distribution and the direct ion heat flux are calculated as a function of the sheath potential, the driving frequency, and the phase shift between the two distinct harmonics of the driving voltage of dual frequency discharges. For this task, self-consistent particle-in-cell simulations subject to Monte Carlo collision are carried out. The results from particle-in-cell simulations which are computationally very expensive are compared and verified with those from the novel ensemble-in-spacetime model. It is confirmed that increasing the voltage of the high-frequency component, the high-frequency component, and/or make a phase shift of π/2 between the dual frequency, narrow the ion angular distribution and increase the direct ion heat flux to the etching profile bottom. In all simulation cases, a correlation between the narrowing of the ion angular distribution and the increase of the sheath potential and the sheath ion flux is found. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4994754
  • 2017 • 144 Effects of substrate roughness and spray-angle on deposition behavior of cold-sprayed Inconel 718
    Singh, R. and Rauwald, K.-H. and Wessel, E. and Mauer, G. and Schruefer, S. and Barth, A. and Wilson, S. and Vassen, R.
    Surface and Coatings Technology 319 249-259 (2017)
    In this study, Inconel 718 powder particles were successfully cold-sprayed on Inconel 718 substrate by using nitrogen gas for a repair application of aero engine components. The effects of substrate roughness and spray-angle on the deposition behavior of Inconel 718 particles were investigated. It has been found that the deposition behavior of Inconel 718 powder on Inconel 718 substrates is highly influenced by substrate surface roughness. Single powder particle interaction with substrates of different roughness showed that plastic deformation and interfacial material mixing is higher if powder particles interact with substrates of higher roughness. Consequently, substrates with lower roughness demonstrated many abnormalities (e.g. cracks, peeling-off) during the deposition. Substrate roughness is limited to influence the deposition efficiency of the first few layers. Besides, it is also observed that particle deformation and coating quality are significantly affected by the spray-angle because of asymmetric deformation of the particle due to additional tangential momentum. Moreover, it is observed that deposition efficiency is reduced with a decrease in spray-angle, while the coating porosity and coating roughness show an inverse trend. Furthermore, the effects of three input parameters, namely substrate preparation, spray angle and stand-off distance, on four outputs (responses), such as thickness, roughness, porosity and Vickers-hardness of the coating were studied using non-linear statistical regression analysis. The above mentioned four outputs were found to be significantly dependent on substrate preparation and spray-angle. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.03.072
  • 2017 • 143 RF magnetron sputtering of a hydroxyapatite target: A comparison study on polytetrafluorethylene and titanium substrates
    Surmenev, R.A. and Surmeneva, M.A. and Grubova, I.Y. and Chernozem, R.V. and Krause, B. and Baumbach, T. and Loza, K. and Epple, M.
    Applied Surface Science 414 335-344 (2017)
    A pure hydroxyapatite (HA) target was used to prepare the biocompatible coating of HA on the surface of a polytetrafluorethylene (PTFE) substrate, which was placed on the same substrate holder with technically pure titanium (Ti) in the single deposition runs by radio-frequency (RF) magnetron sputtering. The XPS, XRD and FTIR analyses of the obtained surfaces showed that for all substrates, instead of the HA coating deposition, the coating of a mixture of calcium carbonate and calcium fluoride was grown. According to SEM investigations, the surface of PTFE was etched, and the surface topography of uncoated Ti was preserved after the depositions. The FTIR results reveal no phosphate bonds; only calcium tracks were observed in the EDX-spectra on the surface of the coated PTFE substrates. Phosphate oxide (V), which originated from the target, could be removed using a vacuum pump system, or no phosphate-containing bonds could be formed on the substrate surface because of the severe substrate bombardment process, which prevented the HA coating deposition. The observed results may be connected with the surface re-sputtering effect of the growing film by high-energy negatively charged ions (most probably oxygen or fluorine), which are accelerated in the cathode dark sheath. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.04.090
  • 2017 • 142 Influence of Si content on mechanical and tribological properties of TiAlSiN PVD coatings at elevated temperatures
    Tillmann, W. and Dildrop, M.
    Surface and Coatings Technology 321 448-454 (2017)
    TiN- and CrN-based binary or ternary coatings have been used for many years in order to extend the service life of machining tools. The increasing demands in the metalworking industry require more efficient coating systems. According to recent studies, silicon offers promising opportunities to positively influence the characteristics of thin titanium or chromium-based coatings. The nanocomposite TiAlSiN presents a high hardness and a fine grain structure. Furthermore, by adding silicon, the oxidation resistance as well as the tribological properties can be increased and improved. In this study, TiAlSiN coatings with different Si contents (0–10.9 at%) were produced by means of magnetron sputtering. In order to test the possibility to sputter pure, low conductive silicon targets, different sputter and bias modes were tested. The ratios of the other coating elements were kept constant while varying the silicon content inside the PVD coatings. Nitrided steel samples (AISI H11) were used as substrate materials. The influence of the Si content on the tribomechanical properties of TiAlSiN were analyzed. The analyses focused on the coatings with a silicon content of 5–10 at% due to the change of the coating morphology within this range. The coating morphology and different chemical compositions of the silicon-doped coatings were investigated by means of scanning electron microscopy and EDX analyses. Phase analyses were conducted and residual stresses were measured by means of X-ray diffraction. The hardness and Young's modulus of the PVD coatings were investigated using nanoindentation. Furthermore, scratch tests were performed in order to characterize the adhesion between the substrate and the coating. Finally, high temperature tribometer tests were executed to determine the wear resistance of the TiAlSiN coatings at room temperature as well as at elevated temperatures (500 °C, 800 °C). © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2017.05.014
  • 2016 • 141 High-temperature stability and interfacial reactions of Ti and TiN thin films on Al2O3 and ZrO2
    Brust, S. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 307 47-55 (2016)
    Metallic thin films are used in many applications to modify ceramic surfaces. However, during subsequent processing, chemical interactions may change the properties of the coating. In addition, differences in thermal expansion can lead to delamination of the coating. In this study, titanium and titanium nitride thin films were deposited via physical and chemical vapor deposition (PVD and CVD, respectively) on alumina- and yttria-stabilised zirconia substrates, before being heat-treated at 1200 °C or 1500 °C in static argon atmosphere and analysed via SEM, EDS and XRD to investigate the effect of temperature on the thin films. It was shown that the chemical interactions between TiN and both Al2O3 and ZrO2 are weak. However, partial delamination of the TiN coating on alumina was observed after both annealing temperatures. The TiN coating on zirconia remained adherent. In contrast, the Ti coatings underwent a transformation to cubic TiO on both oxide substrates. This was due to partial reduction of the ZrO2 to ZrO2 − x and dissolution of the Al2O3, which leads to a Ti3Al0.9O1.1 interlayer. The TiO coating which formed remained adherent on the alumina at both annealing temperatures, but delaminated from the ZrO2 substrate after annealing at 1500 °C. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.060
  • 2016 • 140 Adsorption of Helium Atoms on Two-Dimensional Substrates
    Burganova, R. and Lysogorskiy, Y. and Nedopekin, O. and Tayurskii, D.
    Journal of Low Temperature Physics 185 392-398 (2016)
    The study of the adsorption phenomenon of helium began many decades ago with the discovery of graphite as a homogeneous substrate for the investigation of physically adsorbed monolayer films. In particular, helium monoatomic layers on graphite were found to exhibit a very rich phase diagram. In the present work we have investigated the adsorption phenomenon of helium atoms on graphene and silicene substrates by means of density functional theory with Born–Oppenheimer approximation. Helium–substrate and helium–helium interactions were considered from first principles. Vibrational properties of adsorbed monolayers have been used to explore the stability of the system. This approach reproduces results describing the stability of a helium monolayer on graphene calculated by quantum Monte Carlo (QMC) simulations for low and high coverage cases. However, for the moderate coverage value there is a discrepancy with QMC results due to the lack of helium zero point motion. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10909-016-1473-4
  • 2016 • 139 Spin-hybrids: A single-molecule approach to spintronics
    Bürgler, D.E. and Heß, V. and Esat, T. and Fahrendorf, S. and Matthes, F. and Schneider, C. M. and Besson, C. and Monakhov, K.Y. and Kögerler, P. and Ghisolfi, A. and Braunstein, P. and Atodiresei, N. and Caciuc, V. and Blügel, S.
    e-Journal of Surface Science and Nanotechnology 14 17-22 (2016)
    Molecular spintronics aims at exploiting and controlling spin-dependent transport processes at the molecular level. Achieving this aim requires not only appropriate molecules, molecular structures and preparation procedures. Equally important is the understanding and engineering of the electronic and spin-dependent interactions between different molecular species, molecule and substrate, as well as molecule and electrodes. These interactions may not only determine the spin-dependent functionality of the molecular structures, but also their integrity on the substrate. Likewise, there may be also a modification of the surface properties below and in the vicinity of a molecule. We have investigated several molecules on different metallic surfaces, among them magnetic Nd doubledecker phthalocyanines, a cubane-type {Ni4} complex with single-molecule magnet properties, and a nonmagnetic triazine-based molecule. For NdPc2 molecules adsorbed on a Cu(100) surface, our scanning tunneling microscopy and spectroscopy studies show specific electronic states of the molecule-substrate complex. We find that the electric field between STM tip and sample must be taken into account to properly describe the electronic states associated with the upper Pc ligand. © 2016 The Surface Science Society of Japan.
    view abstractdoi: 10.1380/ejssnt.2016.17
  • 2016 • 138 Atomic layer deposition and high-resolution electron microscopy characterization of nickel nanoparticles for catalyst applications
    Dashjav, E. and Lipińska-Chwałek, M. and Grüner, D. and Mauer, G. and Luysberg, M. and Tietz, F.
    Surface and Coatings Technology 307 428-435 (2016)
    Ni nanoparticles (diameter <  10 nm) are deposited on Si and ceramic substrates of porous lanthanum-substituted strontium titanate/yttrium-stabilized zirconia (LST/YSZ) composites by a two-step process. First, NiO films are produced by atomic layer deposition at 200 °C using bis(methylcyclopentadienyl)nickel(II) (Ni(MeCp)2) and H2O as precursors. In the second step, the NiO films are reduced in H2 atmosphere at 400–800 °C. The size of the resulting Ni nanoparticles is controlled by the temperature. The largest particles with a diameter of about 7 nm are obtained at 800 °C. NiO film and Ni nanoparticles deposited on Si substrates are characterized by high-resolution electron microscopy. It was found that the Ni(MeCp)2 precursor reacts with the substrate, leading to the formation of NiSi2 precipitates beneath the surface of the Si wafer and amorphization of the surrounding area, resulting in a 10 nm thick top layer of the Si wafer. After reductive annealing, NiSi2 precipitates are preserved but Si recrystallizes and the amorphous NiO film transforms into crystalline Ni nanoparticles well distributed on the wafer surface. Process parameters were optimized for Si substrates and transfer of the process to ceramic LST/YSZ substrates is possible in principle. However, a much higher number of ALD cycles (1200 compared to 100 for Si) are necessary to obtain Ni nanoparticles of similar size and the number density of particles is lower than observed for Si substrates. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.08.074
  • 2016 • 137 Analyzing ultrafast laser-induced demagnetization in Co/Cu(001) via the depth sensitivity of the time-resolved transversal magneto-optical Kerr effect
    Eschenlohr, A. and Wieczorek, J. and Chen, J. and Weidtmann, B. and Rösner, M. and Bergeard, N. and Tarasevitch, A. and Wehling, T.O. and Bovensiepen, U.
    Proceedings of SPIE - The International Society for Optical Engineering 9746 (2016)
    Ultrafast demagnetization after femtosecond laser excitation of thin ferromagnetic films has been shown to occur due to a combination of spin-flip scattering in the film and spin transport to a conducting substrate or adjacent layer. Here we demonstrate that the inherent depth sensitivity of the transversal magneto-optical Kerr effect can be employed to derive conclusions on a transient spatial profile in the magnetization in the direction normal to the sample surface. This magnetization profile is qualitatively different for demagnetization caused by spin flips and spin transport. With the help of simulations based on simple phenomenological models we show that spin transport to the substrate in Co/Cu(001) films dominates the demagnetization before the thermalization of the electronic system, i.e. at times < 100 fs, while after approximately 200 fs mainly spin-flip scattering determines the magnetization profile, in agreement with our earlier findings employing the longitudinal magneto-optical Kerr effect. © 2016 SPIE.
    view abstractdoi: 10.1117/12.2211055
  • 2016 • 136 Thermodynamic stability and control of oxygen reactivity at functional oxide interfaces: EuO on ITO
    Gerber, T. and Lömker, P. and Zijlstra, B. and Besson, C. and Mueller, D.N. and Zander, W. and Schubert, J. and Gorgoi, M. and Müller, M.
    Journal of Materials Chemistry C 4 1813-1820 (2016)
    As a prototypical all-oxide heterostructure, the ferromagnetic insulator europium monoxide (EuO) is synthesized on transparent and conductive indium tin oxide (ITO) virtual substrates. Non-destructive hard X-ray photoelectron spectroscopy is employed to depth profile the chemical composition of the magnetic layer and the buried oxide-oxide interface. We find that thermally activated oxygen diffusion from ITO affects the EuO growth process. We present how to control the oxygen reactivity at the interface and discuss its origin in a thermodynamic analysis. Our complementary methodical strategy allows for a significant improvement of the EuO chemical quality with sizeable magnetic properties. Generally, our approach derives guidelines for the proper choice of oxide substrates and buffer layer materials for functional all-oxide heterostructures. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6tc00170j
  • 2016 • 135 Epitaxial growth of thermally stable cobalt films on Au(111)
    Haag, N. and Laux, M. and Stöckl, J. and Kollamana, J. and Seidel, J. and Großmann, N. and Fetzer, R. and Kelly, L.L. and Wei, Z. and Stadtmüller, B. and Cinchetti, M. and Aeschlimann, M.
    18 (2016)
    Ferromagnetic thin films play a fundamental role in spintronic applications as a source for spin polarized carriers and in fundamental studies as ferromagnetic substrates. However, it is challenging to produce such metallic films with high structural quality and chemical purity on single crystalline substrates since the diffusion barrier across the metal-metal interface is usually smaller than the thermal activation energy necessary for smooth surface morphologies. Here, we introduce epitaxial thin Co films grown on an Au(111) single crystal surface as a thermally stable ferromagnetic thin film. Our structural investigations reveal an identical growth of thin Co/Au(111) films compared to Co bulk single crystals with large monoatomic Co terraces with an average width of 500 Å, formed after thermal annealing at 575 K. Combining our results from photoemission and Auger electron spectroscopy, we provide evidence that no significant diffusion of Au into the near surface region of the Co film takes place for this temperature and that no Au capping layer is formed on top of Co films. Furthermore, we show that the electronic valence band is dominated by a strong spectral contribution from a Co 3d band and a Co derived surface resonance in the minority band. Both states lead to an overall negative spin polarization at the Fermi energy. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/18/10/103054
  • 2016 • 134 Selection of Highly SERS-Active Nanostructures from a Size Gradient of Au Nanovoids on a Single Bipolar Electrode
    Kayran, Y.U. and Eßmann, V. and Grützke, S. and Schuhmann, W.
    ChemElectroChem 3 399-403 (2016)
    As surface-enhanced Raman scattering (SERS) crucially depends on the morphology of nanostructured metal surfaces, we developed a convenient approach to produce a size gradient of truncated spherical Au nanovoids on a single bipolar electrode. The continuous potential drop in solution implies a linearly changing interfacial potential difference at the wireless electrode, leading to a linearly changing rate of Au electrodeposition. Such a structural gradient enables fast and reproducible screening for those structures, evoking high SERS intensity in a particular experiment. The optimal Au deposition potential with respect to the highest SERS amplification was determined and applied for the fabrication of highly active SERS substrates. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500423
  • 2016 • 133 Characterization of mechanical properties of hydroxyapatite-silicon-multi walled carbon nano tubes composite coatings synthesized by EPD on NiTi alloys for biomedical application
    Khalili, V. and Khalil-Allafi, J. and Sengstock, C. and Motemani, Y. and Paulsen, A. and Frenzel, J. and Eggeler, G. and Köller, M.
    Journal of the Mechanical Behavior of Biomedical Materials 59 337-352 (2016)
    Release of Ni1+ ions from NiTi alloy into tissue environment, biological response on the surface of NiTi and the allergic reaction of atopic people towards Ni are challengeable issues for biomedical application. In this study, composite coatings of hydroxyapatite-silicon multi walled carbon nano-tubes with 20 wt% Silicon and 1 wt% multi walled carbon nano-tubes of HA were deposited on a NiTi substrate using electrophoretic methods. The SEM images of coated samples exhibit a continuous and compact morphology for hydroxyapatite-silicon and hydroxyapatite-silicon-multi walled carbon nano-tubes coatings. Nano-indentation analysis on different locations of coatings represents the highest elastic modulus (45.8 GPa) for HA-Si-MWCNTs which is between the elastic modulus of NiTi substrate (66.5 GPa) and bone tissue (≈30 GPa). This results in decrease of stress gradient on coating-substrate-bone interfaces during performance. The results of nano-scratch analysis show the highest critical distance of delamination (2.5 mm) and normal load before failure (837 mN) as well as highest critical contact pressure for hydroxyapatite-silicon-multi walled carbon nano-tubes coating. The cell culture results show that human mesenchymal stem cells are able to adhere and proliferate on the pure hydroxyapatite and composite coatings. The presence of both silicon and multi walled carbon nano-tubes (CS3) in the hydroxyapatite coating induce more adherence of viable human mesenchymal stem cells in contrast to the HA coated samples with only silicon (CS2). These results make hydroxyapatite-silicon-multi walled carbon nano-tubes a promising composite coating for future bone implant application. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jmbbm.2016.02.007
  • 2016 • 132 The effect of silicon-substrate orientation on the local piezoelectric characteristics of LiNbO3 films
    Kiselev, D.A. and Zhukov, R.N. and Ksenich, S.V. and Kubasov, I.V. and Temirov, A.A. and Timushkin, N.G. and Bykov, A.S. and Malinkovich, M.D. and Shvartsman, V.V. and Lupascu, D.C. and Parkhomenko, Y.N.
    Journal of Surface Investigation 10 742-747 (2016)
    The domain structure of lithium-niobate thin films grown on Si(111) and Si(100) substrates coated with a native oxide layer with a thickness of no less than 2 nm is investigated by X-ray diffraction, scanning electron microscopy and piezoresponse force microscopy. The films are synthesized by the rf magnetron sputtering of a single-crystal lithium-niobate target. A high degree of grain orientation in the polycrystalline films is demonstrated. The piezoelectric coefficients dzz of the lithium-niobate films on Si(111) and Si(100) substrates are calculated from the measured dependences of the amplitude of the piezoresponse signal on the ac voltage applied between the cantilever tip and the substrate. Piezoelectric hysteresis loops are obtained in the remanent piezoelectric response regime © 2016, Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1027451016040091
  • 2016 • 131 Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates
    Marocchi, S. and Candini, A. and Klar, D. and Van Den Heuvel, W. and Huang, H. and Troiani, F. and Corradini, V. and Biagi, R. and De Renzi, V. and Klyatskaya, S. and Kummer, K. and Brookes, N.B. and Ruben, M. and Wende, H. and De...
    ACS Nano 10 9353-9360 (2016)
    We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.6b04107
  • 2016 • 130 Guanidiniocarbonyl-pyrrole-aryl conjugates as inhibitors of human dipeptidyl peptidase III: Combined experimental and computational study
    Matić, J. and Šupljika, F. and Tir, N. and Piotrowski, P. and Schmuck, C. and Abramić, M. and Piantanida, I. and Tomić, S.
    RSC Advances 6 83044-83052 (2016)
    Dipeptidyl peptidase III (DPP III) is a zinc dependant peptidase which catalyses hydrolysis of the second peptide bond from the N-termini of its substrates. DPP III is an enzyme of broad substrate specificity and it has been found in many organisms. It has been recognised in several processes of interest for drug development like pain modulation and defence against oxidative stress. However, its fundamental physiological role is unknown and specific inhibitors would be of significant help in identifying this role. In this work we combined experimental (UV/Vis, fluorimetry and microcalorimetry experiments) with molecular dynamic simulations to study the binding of several newly designed and synthesised guanidiniocarbonyl-pyrrole-aryl conjugates to human DPP III. We found that new compounds bind with micromolar affinity to the enzyme and with varied efficiency inhibit hydrolysis of Arg-Arg-2-naphthylamide, the standard synthetic substrate of DPP III. The molecular modelling study revealed multiple binding modes of the guanidiniocarbonyl-pyrrole-aryl conjugates into the active site of human DPP III. In order to elucidate which one is the most favourable we studied the molecular determinants for their binding to DPP III as well as their influence on protein structure. It seems that the main requirement for a good DPP III inhibitor is a bulky aryl-substituent and a linker of suitable length and flexibility between it and the guanidiniocarbonyl-pyrrole. The obtained results gave directions for future development and improvement of DPP III inhibitors. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6ra16966j
  • 2016 • 129 Thickness effect on the structure, grain size, and local piezoresponse of self-polarized lead lanthanum zirconate titanate thin films
    Melo, M. and Araújo, E.B. and Shvartsman, V.V. and Shur, V.Y. and Kholkin, A.L.
    Journal of Applied Physics 120 (2016)
    Polycrystalline lanthanum lead zirconate titanate (PLZT) thin films were deposited on Pt/TiO2/SiO2/Si substrates to study the effects of the thickness and grain size on their structural and piezoresponse properties at nanoscale. Thinner PLZT films show a slight (100)-orientation tendency that tends to random orientation for the thicker film, while microstrain and crystallite size increases almost linearly with increasing thickness. Piezoresponse force microscopy and autocorrelation function technique were used to demonstrate the existence of local self-polarization effect and to study the thickness dependence of correlation length. The obtained results ruled out the bulk mechanisms and suggest that Schottky barriers near the film-substrate are likely responsible for a build-in electric field in the films. Larger correlation length evidence that this build-in field increases the number of coexisting polarization directions in larger grains leading to an alignment of macrodomains in thinner films. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4960137
  • 2016 • 128 Nanostructured Ti-Ta thin films synthesized by combinatorial glancing angle sputter deposition
    Motemani, Y. and Khare, C. and Savan, A. and Hans, M. and Paulsen, A. and Frenzel, J. and Somsen, C. and Mücklich, F. and Eggeler, G. and Ludwig, Al.
    Nanotechnology 27 (2016)
    Ti-Ta alloys are attractive materials for applications in actuators as well as biomedical implants. When fabricated as thin films, these alloys can potentially be employed as microactuators, components for micro-implantable devices and coatings on surgical implants. In this study, Ti100-xTa x (x = 21, 30) nanocolumnar thin films are fabricated by glancing angle deposition (GLAD) at room temperature using Ti73Ta27 and Ta sputter targets. Crystal structure, morphology and microstructure of the nanostructured thin films are systematically investigated by XRD, SEM and TEM, respectively. Nanocolumns of ∼150-160 nm in width are oriented perpendicular to the substrate for both Ti79Ta21 and Ti70Ta30 compositions. The disordered α″ martensite phase with orthorhombic structure is formed in room temperature as-deposited thin films. The columns are found to be elongated small single crystals which are aligned perpendicular to the and planes of α″ martensite, indicating that the films' growth orientation is mainly dominated by these crystallographic planes. Laser pre-patterned substrates are utilized to obtain periodic nanocolumnar arrays. The differences in seed pattern, and inter-seed distances lead to growth of multi-level porous nanostructures. Using a unique sputter deposition geometry consisting of Ti73Ta27 and Ta sputter sources, a nanocolumnar Ti-Ta materials library was fabricated on a static substrate by a co-deposition process (combinatorial-GLAD approach). In this library, a composition spread developed between Ti72.8Ta27.2 and Ti64.4Ta35.6, as confirmed by high-throughput EDX analysis. The morphology over the materials library varies from well-isolated nanocolumns to fan-like nanocolumnar structures. The influence of two sputter sources is investigated by studying the resulting column angle on the materials library. The presented nanostructuring methods including the use of the GLAD technique along with pre-patterning and a combinatorial materials library fabrication strategy offer a promising technological approach for investigating Ti-Ta thin films for a range of applications. The proposed approaches can be similarly implemented for other materials systems which can benefit from the formation of a nanocolumnar morphology. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/49/495604
  • 2016 • 127 Surface-enhanced Raman spectroscopy on laser-engineered ruthenium dye-functionalized nanoporous gold
    Schade, L. and Franzka, S. and Biener, M. and Biener, J. and Hartmann, N.
    Applied Surface Science 374 19-22 (2016)
    Photothermal processing of nanoporous gold with a microfocused continuous-wave laser at λ = 532 nm provides a facile means in order engineer the pore and ligament size of nanoporous gold. In this report we take advantage of this approach in order to investigate the size-dependence of enhancement effects in surface-enhanced Raman spectroscopy (SERS). Surface structures with laterally varying pore sizes from 25 nm to ≥200 nm are characterized using scanning electron microscopy and then functionalized with N719, a commercial ruthenium complex, which is widely used in dye-sensitized solar cells. Raman spectroscopy reveals the characteristic spectral features of N719. Peak intensities strongly depend on the pore size. Highest intensities are observed on the native support, i.e. on nanoporous gold with pore sizes around 25 nm. These results demonstrate the particular perspectives of laser-fabricated nanoporous gold structures in fundamental SERS studies. In particular, it is emphasized that laser-engineered porous gold substrates represent a very well defined platform in order to study size-dependent effects with high reproducibility and precision and resolve conflicting results in previous studies. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.08.168
  • 2016 • 126 Orientation of FePt nanoparticles on top of a-SiO2/Si(001), MgO(001) and sapphire(0001): Effect of thermal treatments and influence of substrate and particle size
    Schilling, M. and Ziemann, P. and Zhang, Z. and Biskupek, J. and Kaiser, U. and Wiedwald, U.
    Beilstein Journal of Nanotechnology 7 591-604 (2016)
    Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: "small" NPs with diameters in the range of 2-3 nm and "large" ones in the range of 5-8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600-650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min. © 2016 Schilling et al.
    view abstractdoi: 10.3762/bjnano.7.52
  • 2016 • 125 Influence of PVD-coating technology and pretreatments on residual stresses for sheet-bulk metal forming tools
    Tillmann, W. and Stangier, D. and Denkena, B. and Grove, T. and Lucas, H.
    Production Engineering 10 17-24 (2016)
    Residual stresses in the substrate material are significantly influencing the performance of PVD-coated parts and tools which are exposed to high forces. Especially for forming operations, such as sheet-bulk metal forming, during which normal contact pressures of 1.4 GPa can occur, the reduction of friction and, at the same time, the wear protection by means of thin Cr-based coatings are essential. To ensure a long service life of forming tool and tool coating, each step of the substrate pre-treatment, as well as the magnetron-sputtering process, has to be coordinated and compatible. Therefore, polished as well as nitrided samples consisting of high-speed steel (AISI M3:2) are exposed to a sequence of plasma-based pre-treatments prior to depositing a CrAlN coating. Hardness and Young’s modulus of the substrate and the coating are analysed by means of nanoindentation. To determine the adhesion between coating and substrate, scratch tests are conducted and analysed using a scanning electron microscope. For each step, the residual stresses are determined using sin2ψ measurements, which are correlated to the mechanical properties. A plasma-nitriding process before the CrAlN coating induces high compressive residual stresses into the sample subsurface and at the same time increases the hardness of the surface. This results in higher critical loads during the scratch tests and therefore a better adhesion of the coating on the substrate. © 2015, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-015-0653-4
  • 2015 • 124 G-band frequency doubler based on InP transferred-substrate technology
    Al-Sawaf, T. and Hossain, M. and Weimann, N. and Kruger, O. and Krozer, V. and Heinrich, W.
    European Microwave Week 2015: 61-64 (2015)
    A G-band broadband frequency doubler based on InP transferred-substrate (TS) InP-DHBT technology is presented. The MMIC utilizes a two 2-finger HBTs with a total emitter size of 4 × 0.8 × 5 μm2. Total chip size is 0.9 × 0.78 mm2. The doubler delivers a maximum output power of 10 dBm at 160 GHz. At the same frequency, the circuit exhibits a conversion gain up to 4 dB and a maximum output efficiency of 14 % for a total DC consumption of 72 mW. Output power stays above 7 dBm from 140 to 180 GHz, which yields a 3-dB bandwidth of 40 GHz. The positive gain values demonstrate the inherent advantage of active multipliers against their passive counterparts. © 2015 EuMA.
    view abstractdoi: 10.1109/EuMIC.2015.7345068
  • 2015 • 123 Insights into the structural, electronic, and magnetic properties of Fe2−xTixO3/Fe2O3 thin films with x = 0.44 grown on Al2O3 (0001)
    Dennenwaldt, T. and Lübbe, M. and Winklhofer, M. and Müller, A. and Döblinger, M. and Nabi, H.S. and Gandman, M. and Cohen-Hyams, T. and Kaplan, W.D. and Moritz, W. and Pentcheva, R. and Scheu, C.
    Journal of Materials Science 50 122-137 (2015)
    The interface between hematite (α-Fe2 IIIO3) and ilmenite (FeIITiO3), a weak ferrimagnet and an antiferromagnet, respectively, has been suggested to be strongly ferrimagnetic due to the formation of a mixed valence layer of Fe2+/Fe3+ (1:1 ratio) caused by compensation of charge mismatch at the chemically abrupt boundary. Here, we report for the first time direct experimental evidence for a chemically distinct layer emerging at heterointerfaces in the hematite—Ti-doped-hematite system. Using molecular beam epitaxy, we have grown thin films (~25 nm thickness) of α-Fe2O3 on α-Al2O3 (0001) substrates, which were capped with a ~25 nm thick Fe2−xTixO3 layer (x = 0.44). An additional 3 nm cap of α-Fe2O3 was deposited on top. The films were structurally characterized in situ with surface X-ray diffraction, which showed a partial low index orientation relationship between film and substrate in terms of the [0001] axis and revealed two predominant domains with (Formula presented.) one with (Formula presented.) and a twin domain with (Formula presented.). Electron energy loss spectroscopy profiles across the Fe2−xTixO3/Fe2O3 interface show that Fe2+/Fe3+ ratios peak right at the interface. This strongly suggests the formation of a chemically distinct interface layer, which might also be magnetically distinct as indicated by the observed magnetic enhancement in the Fe2−xTixO3/α-Fe2O3/Al2O3 system compared to the pure α-Fe2O3/Al2O3 system. © 2014, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10853-014-8572-x
  • 2015 • 122 Radio frequency microelectromechanical system-platform based on silicon-ceramic composite substrates
    Fischer, M. and Gropp, S. and Nowak, J. and Capraro, B. and Sommer, R. and Hoffmann, M. and Mülle, J.
    Journal of Microelectronics and Electronic Packaging 12 37-42 (2015)
    In the last few years, several low-temperature coefficient of expansion of low temperature cofired ceramic (LTCC) materials have been developed for direct wafer bonding to silicon. BGK, a sodium-containing LTCC, was originally developed for anodic bonding of the sintered LTCC, whereas BCT (bondable ceramic tape) was tailored for direct silicon bonding of green LTCC tapes to fabricate a quasi-monolithic, silicon ceramic compound substrate. This so-called silicon-on-ceramic (SiCer) technique is based on homogeneous nanostructuring of a silicon substrate, a lamination step of BCT and Si, and a subsequent pressure-assisted sintering. We present a new approach for an integrated radio frequency (RF)-platform setup combining passive, active, and mechanical elements on one SiCer substrate. In this context, RF parameters of the Si-adapted LTCC tapes and the use of commercial metal pastes on BCT with respect to bondability and solderability are investigated. We show first technological results of creating cavities at the SiCer interface for SiCer-specific contacting options (e.g., exposed contact pads at the interface), as well as windows in the ceramic layer of the SiCer substrate for additional Si processing (e.g., Si backside thin-film wiring, plasma etching). A further investigated platform technology is deep reactive-ion etching of the SiCer composite substrate. The etching behavior of Si and BCT is demonstrated and discussed. With the SiCer technique, it is possible to reduce the Si content at the setup of RF microelectromechanical system to a minimum (low signal damping). © 2015 International Microelectronics Assembly and Packaging Society.
    view abstractdoi: 10.4071/imaps.442
  • 2015 • 121 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 • 120 The effect of patterned titanium substrates on the properties of silver-doped hydroxyapatite coatings
    Grubova, I.Y. and Surmeneva, M.A. and Ivanova, A.A. and Kravchuk, K. and Prymak, O. and Epple, M. and Buck, V. and Surmenev, R.A.
    Surface and Coatings Technology 276 595-601 (2015)
    This paper reports the effect of substrate nano/micro-structure design on the grain size, mechanical properties and surface wettability of nanostructured radio frequency (RF) magnetron sputter-deposited silver-containing hydroxyapatite (Ag-HA) coatings containing 0.13-0.36. wt.% silver. The results of this study revealed that the Ag-HA coating microstructure could be designed by controlling the pre-treated surface topography of titanium. The nano/micro-patterned surfaces of titanium were prepared by sand-blasting followed by acid-etching. The size of the nano-patterns on the surface of titanium was also affected by the sand-blasting procedure; namely, the lower the pressure was, the larger the size of the nano-structures and the distance between them. The effect of the coating grain size on the surface wettability and physico-mechanical properties of the biocomposites was revealed. The hydrophobic properties were imparted to the rough titanium by a nanostructured Ag-HA coating. Although according to the XRD patterns the coatings were mainly composed of HA, some differences in the morphology were observed. Therefore, the decreased wettability of the Ag-HA coatings could be explained by taking into account the different grain sizes of the films rather than the changes to the surface chemistry. Nanoindentation studies revealed that in the case of the Ag-HA-coated samples, smaller grains resulted in significantly higher nanohardness and Young's modulus. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.surfcoat.2015.06.010
  • 2015 • 119 Sliding wear behaviour of a Cr-base alloy after microstructure alterations induced by friction surfacing
    Hanke, S. and Fischer, A. and dos Santos, J.F.
    Wear 338-339 332-338 (2015)
    Friction surfacing is a method suitable to generate a wide variety of metallic coatings by means of frictional heating and severe shear deformation. It is a solid-state joining method, and therefore may be applied to non-fusion weldable as well as non-deformable brittle materials, as Cr-based alloys are. In the present study coatings of Cr60Ni40 alloy are generated onto Nimonic 80A substrates. Microstructural investigations of the coating material are carried out and compared to the usual cast state. The wear behaviour of the coatings as well as the cast material is examined under reciprocating sliding against 52100 ball bearing steel by means of a ball-on-flat test rig, lubricated with silicone oil to prevent oxidation. In this tribological system, wear takes place by abrasion with microploughing being the predominant submechanism, surface fatigue as well as adhesion by materials transfer of Cr60Ni40 from the flats to the steel balls. White etching layers form on Cr60Ni40 underneath the worn surfaces, which show cracks and delaminations. The amount of wear of all coatings is within the same magnitude compared to the cast state but slightly smaller. This can be explained by the distinctly finer microstructure (grain boundary strengthening) and a high degree of supersaturation of the solid solutions (solid solution strengthening) within the coatings. The results of this study show that it is possible to generate coatings of brittle alloys like Cr60Ni40 by friction surfacing, which show a slightly better wear behaviour under reciprocating sliding. Thus, in combination with a ductile substrate, these coatings are likely to extend the range of applicability of such high-temperature wear and corrosion resistant alloys. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2015.07.010
  • 2015 • 118 Hybrid biocomposite with a tunable antibacterial activity and bioactivity based on RF magnetron sputter deposited coating and silver nanoparticles
    Ivanova, A.A. and Surmenev, R.A. and Surmeneva, M.A. and Mukhametkaliyev, T. and Loza, K. and Prymak, O. and Epple, M.
    Applied Surface Science 329 212-218 (2015)
    In this work, we describe fabrication techniques used to prepare a multifunctional biocomposite based on a hydroxyapatite (HA) coating and silver nanoparticles (AgNPs). AgNPs synthesized by a wet chemical reduction method were deposited on Ti substrates using a dripping/drying method followed by deposition of calcium phosphate (CaP) coating via radio-frequency (RF) magnetron sputter-deposition. The negatively charged silver nanoparticles (zeta potential -21 mV) have a spherical shape with a metallic core diameter of 50 ± 20 nm. The HA coating was deposited as a dense nanocrystalline film over a surface of AgNPs. The RF-magnetron sputter deposition of HA films on the AgNPs layer did not affect the initial content of AgNPs on the substrate surface as well as NPs size and shape. SEM cross-sectional images taken using the backscattering mode revealed a homogeneous layer of AgNPs under the CaP layer. The diffraction patterns from the coatings revealed reflexes of crystalline HA and silver. The concentration of Ag ions released from the biocomposites after 7 days of immersion in phosphate and acetate buffers was estimated. The obtained results revealed that the amount of silver in the solutions was 0.27 ± 0.02 μg mL-1 and 0.54 ± 0.02 μg mL-1 for the phosphate and acetate buffers, respectively, which corresponded well with the minimum inhibitory concentration range known for silver ions in literature. Thus, this work establishes a new route to prepare a biocompatible layer using embedded AgNPs to achieve a local antibacterial effect. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2014.12.153
  • 2015 • 117 Deposition and characterization of single magnetron deposited Fe:SnOx coatings
    Kormunda, M. and Fischer, D. and Hertwig, A. and Beck, U. and Sebik, M. and Pavlik, J. and Esser, N.
    Thin Solid Films 595 200-208 (2015)
    Coatings deposited bymagnetron co-sputtering froma single RF magnetronwith a ceramic SnO2 target with iron inset in argon plasma were studied. Themass spectra of the process identified Sn+ and SnO+ species as the dominant species sputtered fromthe target, but no SnO2 + specieswere detected. The dominant positive ions in argon plasma are Ar+ species. The only detected negative ions were O-. Sputtered neutral tin related species were not detected. Iron related species were also not detected because their concentration is below the detection limit. The concentration of iron dopant in the tin oxide coatings was controlled by the RF bias applied on the substrate holder while the discharge pressure also has some influence. The iron concentration was in the range from 0.9 at.% up to 19 at.% increasing with the substrate bias while the sheet resistivity decreases. The stoichiometry ratio of O/(Sn+Fe) in the coatings increased from 1.7 up to 2 in dependence on the substrate bias from floating bias (-5 V) up to-120 V of RF self-bias, respectively. The tin in the coatings was mainly bonded in Sn4+ state and iron was mainly in Fe2+ state when other tin bonding states were detected only in a small amounts. Iron bonding states in contrary to elemental compositions of the coatings were not influenced by the RF bias applied on the substrate. The coatings showed high transparency in the visible spectral range. However, an increased metallic behavior could be detected by using a higher RF bias for the deposition. The X-ray diffraction patterns and electron microscopy pictures made on the coatings confirmed the presence of an amorphous phase. © 2015 Published by Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2015.11.009
  • 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 Visibility of two-dimensional layered materials on various substrates
    Müller, M.R. and Gumprich, A. and Ecik, E. and Kallis, K.T. and Winkler, F. and Kardynal, B. and Petrov, I. and Kunze, U. and Knoch, J.
    Journal of Applied Physics 118 (2015)
    For the investigation of 2D layered materials such as graphene, transition-metal dichalcogenides, boron nitride, and their heterostructures, dedicated substrates are required to enable unambiguous identification through optical microscopy. A systematic study is conducted, focusing on various 2D layered materials and substrates. The simulated colors are displayed and compared with microscopy images. Additionally, the issue of defining an appropriate index for measuring the degree of visibility is discussed. For a wide range of substrate stacks, layer thicknesses for optimum visibility are given along with the resulting sRGB colors. Further simulations of customized stacks can be conducted using our simulation tool, which is available for download and contains a database featuring a wide range of materials. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4930574
  • 2015 • 114 Improved thermal management of InP transistors in transferred-substrate technology with diamond heat-spreading layer
    Nosaeva, K. and Weimann, N. and Rudolph, M. and John, W. and Krueger, O. and Heinrich, W.
    Electronics Letters 51 1010-1012 (2015)
    A method to improve the thermal management of indium phosphide (InP) double-hetero bipolar transistors (DHBTs) fabricated in a transferred- substrate technology is presented. A vapour-phase deposited diamond layer acting as a heat spreader is heterogeneously integrated into the vertical layer stack. It is observed that the diamond layer reduces the thermal resistance of a 0.8 × 5 μm2 single emitter-finger HBT by roughly 75% down to 1.1 K/mW which is, to the authors' knowledge, the best value reported thus far for InP HBTs of comparable size. It is also the first demonstration of heterogeneous integration of diamond into an InP HBT monolithic microwave integrated circuit. © 2015 The Institution of Engineering and Technology.
    view abstractdoi: 10.1049/el.2015.1135
  • 2015 • 113 Atom probe tomography study of internal interfaces in Cu2ZnSnSe4 thin-films
    Schwarz, T. and Cojocaru-Mirédin, O. and Choi, P. and Mousel, M. and Redinger, A. and Siebentritt, S. and Raabe, D.
    Journal of Applied Physics 118 (2015)
    We report on atom probe tomography studies of the composition at internal interfaces in Cu<inf>2</inf>ZnSnSe<inf>4</inf> thin-films. For Cu<inf>2</inf>ZnSnSe<inf>4</inf> precursors, which are deposited at 320 °C under Zn-rich conditions, grain boundaries are found to be enriched with Cu irrespective of whether Cu-poor or Cu-rich growth conditions are chosen. Cu<inf>2</inf>ZnSnSe<inf>4</inf> grains are found to be Cu-poor and excess Cu atoms are found to be accumulated at grain boundaries. In addition, nanometer-sized ZnSe grains are detected at or near grain boundaries. The compositions at grain boundaries show different trends after annealing at 500 °C. Grain boundaries in the annealed absorber films, which are free of impurities, are Cu-, Sn-, and Se-depleted and Zn-enriched. This is attributed to dissolution of ZnSe at the Cu-enriched grain boundaries during annealing. Furthermore, some of the grain boundaries of the absorbers are enriched with Na and K atoms, stemming from the soda-lime glass substrate. Such grain boundaries show no or only small changes in composition of the matrix elements. Na and K impurities are also partly segregated at some of the Cu<inf>2</inf>ZnSnSe<inf>4</inf>/ZnSe interfaces in the absorber, whereas for the precursors, only Na was detected at such phase boundaries possibly due to a higher diffusivity of Na compared to K. Possible effects of the detected compositional fluctuations on cell performance are discussed. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4929874
  • 2015 • 112 Influence of the handling parameters on residual stresses of HVOF-sprayed WC-12Co coatings
    Selvadurai, U. and Hollingsworth, P. and Baumann, I. and Hussong, B. and Tillmann, W. and Rausch, S. and Biermann, D.
    Surface and Coatings Technology 268 30-35 (2015)
    In this research work HVOF WC-12Co coatings were deposited on C45 steel (1.0503) substrates. Unfavorable residual stresses can lead to delamination and spallation and thus have to be avoided by optimizing the handling parameters. This study investigates the influence of the following handling parameters: substrate temperature, number of overruns, stand-off distance, track pitch, and gun velocity on the residual stress and hardness. Additionally, the effect of post-treating the coating by means of surface grinding was determined. For the HVOF spraying experiments, fine agglomerated and sintered WC-12Co powders (2-10. μm) with WC particles in the submicron range (400. nm) were chosen. It was determined that alterations of these handling parameters had significant effects on the residual stress and the hardness. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.11.055
  • 2015 • 111 Influence of film thickness and composition on the martensitic transformation in epitaxial Ni-Mn-Sn thin films
    Teichert, N. and Auge, A. and Yüzüak, E. and Dincer, I. and Elerman, Y. and Krumme, B. and Wende, H. and Yildirim, O. and Potzger, K. and Hütten, A.
    Acta Materialia 86 279-285 (2015)
    Two series of epitaxial Ni-Mn-Sn thin films of different thickness are investigated for the thickness and composition dependence of the martensitic transformation. Thin films ranging in thickness from 20 to 200 nm (series A) and 10 to 100 nm (series B) were prepared by magnetron co-sputtering and deposited on heated MgO(0 0 1) substrates. The structural characterization was done by temperature-dependent X-ray diffraction measurements. Magnetization and resistivity measurements were performed to investigate the transformation characteristics. We find a strong influence of the film thickness on the relative amount of material undergoing the martensitic transformation, the temperature range of the transformation, and the transformation temperatures. The main contribution originates from the rigid substrate which delays the transformation of the Ni-Mn-Sn near the interface and even leads to a layer of residual austenite at low temperatures. Another issue are size effects which presumably broaden the martensitic transformation and decrease the transformation temperatures. By variation of the thin film composition we find changes of the substrate influence due to a different mismatch between the lattice of MgO and austenite. A better phase compatibility between martensite and austenite, denoted by λ2, not only results in a smaller hysteresis but is also beneficial for the transformation of material close to the substrate. © 2014 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2014.12.019
  • 2015 • 110 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
  • 2014 • 109 Selective laser patterning in organic solar cells
    Abreu Fernandes, S. and Maragkaki, S. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 9180 (2014)
    Selective laser patterning for integrative serious connection has been industrially established in inorganic thin film solar cells based on glass substrates since a few years. In organic solar cells (OSC) the used materials significantly differ in terms of their patterning behavior. Due to their processability by wet chemical methods inverted architectures are often preferred in organic solar cells which allow the patterning by ultrashort laser pulses in substrate and superstrate configuration. Starting with an introduction of the ablation mechanisms taking place in OSC thin films, an overview of the current state-of-the-art in laser patterning of organic solar cells is presented. Besides progress in research also current achievements in industrial applications are illustrated. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2061248
  • 2014 • 108 MOCVD of TiO2 thin films using a heteroleptic titanium complex: Precursor evaluation and investigation of optical, photoelectrochemical and electrical properties
    Banerjee, M. and Dang, V.-S. and Bledowski, M. and Beranek, R. and Becker, H.-W. and Rogalla, D. and Edengeiser, E. and Havenith, M. and Wieck, A.D. and Devi, A.
    Chemical Vapor Deposition 20 224-233 (2014)
    A new heteroleptic titanium precursor with a mixed oxygen/nitrogen coordination sphere [Ti(dmap)2(NMe2)2] (Hdmap=1-dimethylamino-2-propanol) is synthesized by a simple elimination reaction on tetrakis-dimethylaminotitanium(IV) [Ti(NMe2)4]. The compound shows encouraging results in terms of chemical and thermal stability compared to the parent alkyl amide [Ti(NMe2)4], and is therefore more suitable for MOCVD applications. TiO2 thin films are grown on Si(100) and ITO-coated borosilicate glass substrates via MOCVD in the temperature range 500-800°C. The deposition temperature has a significant effect on the phase and microstructure of the TiO2 films obtained, which influences the functional properties. The optical bandgaps of the films are in the range 2.92-3.36eV. The best photocurrent response (1.5mAcm-2 under AM 1.5G conditions) in aqueous electrolytes is observed for films grown at 700°C having improved crystallinity and porous columnar structure. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cvde.201407125
  • 2014 • 107 Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloys as magnetic electrodes
    Belmeguenai, M. and Genevois, C. and Zighem, F. and Roussigné, Y. and Chérif, S.M. and Westerholt, K. and El Bahoui, A. and Fnidiki, A. and Moch, P.
    Thin Solid Films 551 163-170 (2014)
    High resolution transmission electron microscopy, nano-beam electronic diffraction, energy dispersive X-rays scanning spectroscopy, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) techniques are used in view of comparing (static and dynamic) magnetic and structural properties of Co 2MnGe(13 nm)/Al2O3(3 nm)/Co(13 nm) tunnel magnetic junctions (TMJs), deposited on various single crystalline substrates (a-plane sapphire, MgO(100) and Si(111)). They allow for providing a correlation between these magnetic properties and the fine structure investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and contains a large concentration of Co atoms, which, however, is significantly reduced when using a sapphire substrate. The Co layer is polycrystalline and shows larger grains for films grown on a sapphire substrate. The VSM investigation reveals in-plane anisotropy only for samples grown on a sapphire substrate. The FMR spectra of the TMJs are compared to the obtained ones with a single Co and Co2MnGe films of identical thickness deposited on a sapphire substrate. As expected, two distinct modes are detected in the TMJs while only one mode is observed in each single film. For the TMJ grown on a sapphire substrate, the FMR behavior does not significantly differ from the superposition of the individual spectra of the single films, allowing for a conclusion that the exchange coupling between the two magnetic layers is too small to give rise to observable shifts. For TMJs grown on a Si or on a MgO substrate, the resonance spectra reveal one mode which is nearly identical to the obtained one in the single Co film, while the other observed resonance shows a considerably smaller intensity and cannot be described using the magnetic parameters appropriate to the single Co2MnGe film. The large Co concentration in the Al2O3 interlayer prevents for a simple interpretation of the observed spectra when using Si or MgO substrates. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.11.090
  • 2014 • 106 A chemically inert Rashba split interface electronic structure of C 60, FeOEP and PTCDA on BiAg2/Ag(111) substrates
    Cottin, M.C. and Lobo-Checa, J. and Schaffert, J. and Bobisch, C.A. and Möller, R. and Ortega, J.E. and Walter, A.L.
    New Journal of Physics 16 (2014)
    The fields of organic electronics and spintronics have the potential to revolutionize the electronics industry. Finding the right materials that can retain their electrical and spin properties when combined is a technological and fundamental challenge. We carry out the study of three archetypal organic molecules in intimate contact with the BiAg2 surface alloy. We show that the BiAg2 alloy is an especially suited substrate due to its inertness as support for molecular films, exhibiting an almost complete absence of substrate-molecular interactions. This is inferred from the persistence of a completely unaltered giant spin-orbit split surface state of the BiAg 2 substrate, and from the absence of significant metallic screening of charged molecular levels in the organic layer. Spin-orbit split states in BiAg2 turn out to be far more robust to organic overlayers than previously thought. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/16/4/045002
  • 2014 • 105 Adsorption geometry and electronic structure of iron phthalocyanine on Ag surfaces: A LEED and photoelectron momentum mapping study
    Feyer, V. and Graus, M. and Nigge, P. and Wießner, M. and Acres, R.G. and Wiemann, C. and Schneider, C.M. and Schöll, A. and Reinert, F.
    Surface Science 621 64-68 (2014)
    We present a comprehensive study of the adsorption behavior of iron phthalocyanine on the low-index crystal faces of silver. By combining measurements of the reciprocal space by means of photoelectron momentum mapping and low energy electron diffraction, the real space adsorption geometries are reconstructed. At monolayer coverage ordered superstructures exist on all studied surfaces containing one molecule in the unit cell in case of Ag(100) and Ag(111), and two molecules per unit cell for Ag(110). The azimuthal tilt angle of the molecules against the high symmetry directions of the substrate is derived from the photoelectron momentum maps. A comparative analysis of the momentum patterns on the substrates with different symmetry indicates that both constituents of the twofold degenerate FePc lowest unoccupied molecular orbital are occupied by charge transfer from the substrate at the interface. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.10.020
  • 2014 • 104 Self-directed localization of ZIF-8 thin film formation by conversion of ZnO nanolayers
    Khaletskaya, K. and Turner, S. and Tu, M. and Wannapaiboon, S. and Schneemann, A. and Meyer, R. and Ludwig, Al. and Van Tendeloo, G. and Fischer, R.A.
    Advanced Functional Materials 24 4804-4811 (2014)
    Control of localized metal-organic framework (MOF) thin film formation is a challenge. Zeolitic imidazolate frameworks (ZIFs) are an important sub-class of MOFs based on transition metals and imidazolate linkers. Continuous coatings of intergrown ZIF crystals require high rates of heterogeneous nucleation. In this work, substrates coated with zinc oxide layers are used, obtained by atomic layer deposition (ALD) or by magnetron sputtering, to provide the Zn 2+ ions required for nucleation and localized growth of ZIF-8 films ([Zn(mim)2]; Hmim = 2-methylimidazolate). The obtained ZIF-8 films reveal the expected microporosity, as deduced from methanol adsorption studies using an environmentally controlled quartz crystal microbalance (QCM) and comparison with bulk ZIF-8 reference data. The concept is transferable to other MOFs, and is applied to the formation of [Al(OH)(1,4-ndc)]n (ndc = naphtalenedicarboxylate) thin films derived from Al2O3 nanolayers. The self-template route for the manufacturing of ZIF-8 films on silicon (Si) and quartz crystal microbalance (QCM) substrates involves the pre-deposition of ZnO films prepared by sputtering or atomic layer deposition methods and the subsequent conversion of the immobilized ZnO phase into crystalline and homogeneously dense ZIF-8 films via microwave-assisted synthesis. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201400559
  • 2014 • 103 Micro-patterning of self-assembled organic monolayers by using tunable ultrafast laser pulses
    Maragkaki, S. and Aumann, A. and Schulz, F. and Schröter, A. and Schöps, B. and Franzka, S. and Hartmann, N. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 8972 (2014)
    We study the application of tunable ultrafast laser pulses in micropatterning self- assembled organic monolayer (SAMs) employing non collinear optical parametric amplification (NOPA). SAMs are ultrathin organic monolayers, which can be used in a variety of ways to assemble functionalized surface structures. In our study, we investigate the characteristics of SAMs as monomolecular resists during etching of gold. NOPA is a versatile method which provides the generation of ultrafast laser pulses, with a tunable wavelength in the visible and near infrared range. Due to the noncollinear geometry, a broadened spectral range can be amplified. The NOPA delivers wavelengths in the range of 480 nm to 950 nm at laser pulse lengths in the sub- 30 femtosecond range using a prism compressor after the nonlinear conversion. The ultrashort laser technology together with the advantages of the NOPA system guarantee high precision and allows us to determine the optimum conditions of sub-wavelength patterning by studying the effects of the fluence and the wavelength. At the same time, single-pulse processing allows us to selectively remove the ultrathin organic coating, while it ensures short processing time. In our study we used thiol-based SAMs as ultrathin layers on gold-coated glass substrates with a film thickness of 1-2 nm and 40 nm respectively. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2037716
  • 2014 • 102 MCrAlY bondcoats by high-velocity atmospheric plasma spraying
    Mauer, G. and Sebold, D. and Vaßen, R.
    Journal of Thermal Spray Technology 23 140-146 (2014)
    MCrAlY bondcoats (M = Co, Ni) are used to protect metallic substrates from oxidation and to improve adhesion of ceramic thermal barrier coatings for high temperature applications, such as in land-based and aviation turbines. Since MCrAlYs are prone to take up oxygen during thermal spraying, bondcoats often are manufactured under inert gas conditions at low pressure. Plasma spraying at atmospheric conditions is a cost-effective alternative if it would be possible to limit the oxygen uptake as well as to obtain sufficiently dense microstructures. In the present work, high-velocity spray parameters were developed for the TriplexPro 210 three-cathode plasma torch using MCrAlY powders of different particle size fractions to achieve these objectives. The aims are conflictive since the former requires cold conditions, whereas the latter is obtained by more elevated particle temperatures. High particle velocities can solve this divergence as they imply shorter time for oxidation during flight and contribute to coating densification by kinetic rather than thermal energy. Further aims of the experimental work were high deposition efficiencies as well as sufficient surface roughness. The oxidation behavior of the sprayed coatings was characterized by thermal gravimetric analyses and isothermal heat treatments. © 2013 ASM International.
    view abstractdoi: 10.1007/s11666-013-0026-5
  • 2014 • 101 Carbon-based micro-ball and micro-crystal deposition using filamentary pulsed atmospheric pressure plasma
    Pothiraja, R. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 47 (2014)
    Thin plasma filaments are produced by the propagation of ionization waves from a spiked driven electrode in a quartz tube in an argon/methane gas mixture (2400sccm/2sccm) at atmospheric pressure. The position of the touch point of filaments on the substrate surface is controlled in our experiment by applying various suitable substrate configurations and geometries of the grounded electrode. The gas conditions at the touch point are varied from argon to ambient air. Based on microphotography and discharge current waveforms, the duration of the filament touching the substrate is estimated to be about one microsecond. Carbon-based materials are deposited during this time at the touch points on the substrate surface. Micro-balls are produced if the filament touch points are saved from ambient air by the argon flow. Under an air admixture, micro-crystals are formed. The dimension of both materials is approximately one micrometre (0.5-2m) and corresponds to about 1010-1012 carbon atoms. Neither the diffusion of neutral species nor drift of ions can be reason for the formation of such a big micro-material during this short period of filament-substrate interaction. It is possible that charged carbon-based materials are formed in the plasma channel and transported to the surface of the substrate. The mechanism of this transport and characterization of micro-materials, which are formed under different gas conditions in our experiment, will be studied in the future. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/47/31/315203
  • 2014 • 100 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 • 99 The influence of multilayer design on residual stress gradients in Ti/TiAlN systems
    Selvadurai, U. and Tillmann, W. and Fischer, G. and Sprute, T.
    Materials Science Forum 768-769 264-271 (2014)
    In this research work, Ti/TiAlN multilayers of various designs were deposited on substrates pretreated by nitriding and etching procedures. The influence of the multilayer design on residual stress depth profiles was systematically analyzed for multilayers with different Titanium interlayer thicknesses. The depth-dependency of stress was measured by a modified sin2ψ method, using various defined gracing incident angles and measuring angles that ensure constant penetration depths. The residual stresses were investigated by synchrotron X-ray diffraction (SXRD) at the HASYLAB at DESY in Hamburg, Germany. SXRD allows a phase specific stress evaluation of the ceramic and metallic layers of the multilayer systems and the adjacent substrate region. This investigation reveals an influence of the Ti layer thickness on the values and the slope of the residual stress profile in ceramic TiAlN layers. © (2014) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/MSF.768-769.264
  • 2014 • 98 Influence of substrate pre-treatments on residual stresses and tribo-mechanical properties of TiAlN-based PVD coatings
    Sprute, T. and Tillmann, W. and Grisales, D. and Selvadurai, U. and Fischer, G.
    Surface and Coatings Technology 260 369-379 (2014)
    Residual stresses in the substrate and in the PVD coating have a significant influence on the coating adhesion and lifespan of machining as well as forming tools. Therefore, the understanding and control of the system's residual stresses will lead to a better performance of the coated components. Moreover, although investigations were conducted in the field of stress analysis of PVD coatings, they do not focus on interdependencies of residual stresses in the substrate and in the coating. In this investigation, three different metallographically prepared substrates were used. SiC grinding, diamond grinding, and SiC grinding and plasma nitriding preparations were selected, due to the substantial differences in their final residual stress states. Additionally, a Ti/TiAlN multilayer coating and a reference TiAlN monolayer were deposited on each pre-treated substrate. Their initial and final residual stress states were measured by means of X-ray diffraction. In addition to the residual stress analyses, tribo-mechanical tests, such as nano-indentation, ball-on-disc, and scratch tests were performed in order to correlate the results with these residual stress states. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.08.075
  • 2014 • 97 Nanostructured Er2O3 thin films grown by metalorganic chemical vapour deposition
    Xu, K. and Dang, V.-S. and Ney, A. and De Los Arcos, T. and Devi, A.
    Journal of Nanoscience and Nanotechnology 14 5095-5102 (2014)
    Metalorganic chemical vapor deposition (MOCVD) of nanostructured Er 2O3 thin films was performed using the Er-tris-guanidinate precursor [Er(DPDMG)3] (DPDMG = diisopropyl-2- dimethylamidoguanidinato) as the Er source and oxygen. Film deposition was carried out on Si(100) and quartz glass substrates and the process parameters namely temperature, pressure and oxygen flow rate were varied. The resulting thin films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) for investigating the crystallinity and morphology, respectively. The chemical composition of the film was investigated by X-ray photoelectron spectroscopy (XPS) measurements. Transmittance and absorption spectra of the 600 °C film grown on glass substrates were performed by UV-vis measurements revealing more than 80% transmittance. The potential of Er2O3 thin films as gate dielectrics was verified by carrying out capacitance-voltage (C-V ) and current-voltage (I-V ) measurements. Dielectric constants estimated from the accumulation capacitance were found to be in the range of 10-12 in AC frequencies of 1 MHz down to 10 kHz and the leakage current of the order of 2×10-8 A/cm2 at the applied field of 1 MV cm-1 was measured for films deposited under optimised process conditions. The low leakage current and high dielectric constant implies good quality of the Er2O3 layers relevant for high-k applications. These layers were found to be paramagnetic with a slightly reduced magnetic moment of the Er3+ ions. Copyright © 2014 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2014.8848
  • 2014 • 96 Growth modes and epitaxy of FeAl thin films on a-cut sapphire prepared by pulsed laser and ion beam assisted deposition
    Yao, X. and Wiedwald, U. and Trautvetter, M. and Ziemann, P.
    Journal of Applied Physics 115 (2014)
    FeAl films around equiatomic composition are grown on a-cut (11 2 0) sapphire substrates by ion beam assisted deposition (IBAD) and pulsed laser deposition (PLD) at ambient temperature. Subsequent successive annealing is used to establish chemical order and crystallographic orientation of the films with respect to the substrate. We find a strongly [110]-textured growth for both deposition techniques. Pole figures prove the successful preparation of high quality epitaxial films by PLD with a single in-plane orientation. IBAD-grown films, however, exhibit three in-plane orientations, all of them with broad angular distributions. The difference of the two growth modes is attributed to the existence of a metastable intermediate crystalline orientation as concluded from nonassisted sputter depositions at different substrate temperatures. The formation of the chemically ordered crystalline B2 phase is accompanied by the expected transition from ferromagnetic to paramagnetic behavior of the films. In accordance with the different thermally induced structural recovery, we find a step-like magnetic transition to paramagnetic behavior after annealing for 1 h at TA 300 °C for IBAD deposition, while PLD-grown films show a gradual decrease of ferromagnetic signals with rising annealing temperatures. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4861377
  • 2013 • 95 Preparation of graphene with graphane areas of controlled hydrogen isotope composition on opposite sides
    Balgar, T. and Kim, H. and Hasselbrink, E.
    Journal of Physical Chemistry Letters 4 2094-2098 (2013)
    Monolayer graphene was prepared on an Ir(111) substrate where it exhibits a 25 × 25 Å2 moiré pattern. Molecular hydrogen was dosed first, allowing it to dissociate on open areas of the Ir substrate. The generated H atoms formed an intercalated reservoir that can bind to the graphene subsequently. Next, atomic hydrogen was dosed, which binds to the graphene sheet and also initiates the transfer of H from the Ir substrate to the graphene sheet. The opposite sides of the sheet can be hydrogenated with isotope selectivity, as a sequence of difference isotopes, H or D, can be chosen at will in the preparation procedure. Sum-frequency generation spectra prove that as consequence of the dosing sequence, C-H bonds are predominantly pointing toward the Ir substrate side when H2 is dosed first and alternatively toward the vacuum side when D2 is dosed first. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jz400690w
  • 2013 • 94 Magnetization dynamics in Co2MnGe/Al2O3/Co tunnel junctions grown on different substrates
    Belmeguenai, M. and Tuzcuoglu, H. and Zighem, F. and Chérif, S.-M. and Roussigné, Y. and Westerholt, K. and Moch, P. and El Bahoui, A. and Genevois, C. and Fnidiki, A.
    Sensor Letters 11 2043-2048 (2013)
    We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3 nm)/Co (13 nm) tunnel magnetic junctions, deposited on various single crystalline substrates (a-plane sapphire, MgO(100), Si(111)). The results are compared to the magnetic properties of Co and of Co2MnGe single films lying on sapphire substrates. X-rays diffraction always shows (110) orientation of the Co2MnGe films. Structural observations obtained by high resolution transmission electron microscopy confirmed the high quality of the tunnel magnetic junction grown on sapphire. Our vibrating sample magnetometry measurements reveal in-plane anisotropy only in samples grown on a sapphire substrate. Depending on the substrate, the ferromagnetic resonance spectra of the tunnel magnetic junctions, studied by the microstrip technique, show one or two pseudo-uniform modes. In the case of MgO and of Si substrates only one mode is observed: it is described by magnetic parameters (g-factor, effective magnetization, in-plane magnetic anisotropy) derived in the frame of a simple expression of the magnetic energy density; these parameters are practically identical to those obtained for the Co single film. With a sapphire substrate two modes are present: one of them does not appreciably differ from the observed mode in the Co single film while the other one is similar to the mode appearing in the Co2MnGe single film: their magnetic parameters can thus be determined independently, using a classical model for the energy density in the absence of interlayer exchange coupling. Copyright © 2013 American Scientific Publishers.
    view abstractdoi: 10.1166/sl.2013.3064
  • 2013 • 93 Dynamic of the growth flux at the substrate during high-power pulsed magnetron sputtering (HiPIMS) of titanium
    Breilmann, W. and Maszl, C. and Benedikt, J. and von Keudell, A.
    Journal of Physics D-applied Physics 46 485204 (2013)
    The temporal distribution of the incident fluxes of argon and titanium ions on the substrate during an argon HiPIMS pulse to sputter titanium with pulse lengths between 50 to 400 mu s and peak powers up to 6 kW are measured by energy-resolved ion mass spectrometry with a temporal resolution of 2 mu s. The data are correlated with time-resolved growth rates and with phase-resolved optical emission spectra. Four ion contributions impinging on the substrate at different times and energies are identified: (i) an initial argon ion burst after ignition, (ii) a titanium and argon ion flux in phase with the plasma current due to ionized neutrals in front of the target, (iii) a small energetic burst of ions after plasma shut off, and (iv) cold ions impinging on the substrate in the late afterglow showing a pronounced maximum in current. The last contribution originates from ions generated during the plasma current maximum at 50 mu s after ignition in the magnetic trap in front of the target. They require long transport times of a few 100 mu s to reach the substrate. All energy distributions can be very well fitted with a shifted Maxwellian indicating an efficient thermalization of the energetic species on their travel from target to substrate. The energy of titanium is higher than that of argon, because they originate from energetic neutrals of the sputter process. The determination of the temporal sequence of species, energies and fluxes in HiPIMS may lead to design rules for the targeted generation of these discharges and for synchronized biasing concepts to further improve the capabilities of high-power impulse magnetron sputtering (HiPIMS) processes.
    view abstractdoi: 10.1088/0022-3727/46/48/485204
  • 2013 • 92 Structural phase transitions in ferroelectric crystals and thin films studied by VUV spectroscopic ellipsometry with synchrotron radiation
    Dorywalski, K. and Andriyevsky, B. and Piasecki, M. and Kityk, I. and Cobet, C. and Esser, N. and Patryn, A.
    Phase Transitions 86 932-940 (2013)
    Results of thermo-optical investigations for lead germanate (Pb5Ge3O11), potassium dihydrogen phosphate (KH2PO4) ferroelectric crystals, and lead titanate (PbTiO3) thin films expitaxially deposited on (001) SrTiO3 substrate are presented in this article. The measurements were performed using VUV spectroscopic ellipsometry with synchrotron radiation in the temperature range comprising structural phase transitions (PTs) in the materials under study. Distinct anomalies in the temperature dependencies of complex dielectric permittivity and the intensity of reflected synchrotron light are found in the vicinity of ferroelectric-to-paraelectric PT point and discussed. Different temperature points of the characteristic anomalies at the PT of the real and imaginary components of the pseudo-dielectric function are observed for Pb5Ge3O11 and KH2PO4. It is crucial that anomalies in the measured temperature dependences for PbTiO3 thin films on SrTiO3 substrates are observed at the temperature near 105 K, corresponding to the PT in the SrTiO3 single crystal. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1080/01411594.2012.745540
  • 2013 • 91 Cavitation erosion of Cr60Ni40 coatings generated by friction surfacing
    Hanke, S. and Beyer, M. and Silvonen, A. and dos Santos, J.F. and Fischer, A.
    Wear 301 415-423 (2013)
    CrNi-alloys with high Cr-content generally are quite brittle and, therefore, only available as castings and regarded as neither weldable nor deformable. The process of friction surfacing offers a possibility to generate Cr60Ni40 coatings e.g. on steel or Ni-base substrates. Cavitation tests were carried out using an ultrasonic vibratory test rig (~ASTM G32) with cast specimens and friction surfaced coatings. The coatings show less deformation and smaller disruptions, and wear rates in steady state were found to be three times higher for the cast and heat treated samples than for the coatings, caused by a highly wear resistant Cr-rich phase. The results of this study show that it is possible to generate defect free coatings of Cr60Ni40 with a thickness of about 250. μm by friction surfacing, which under cavitation show a better wear behavior than the cast material. Thus, in combination with a ductile substrate, these coatings are likely to extend the range of applicability of such high-temperature corrosion resistant alloys. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2012.11.016
  • 2013 • 90 Facile synthesis of polymer core@silver shell hybrid nanoparticles with super surface enhanced Raman scattering capability
    Huo, D. and He, J. and Yang, S. and Zhou, Z. and Hu, Y. and Epple, M.
    Journal of Colloid and Interface Science 393 119-125 (2013)
    Silver nano-shells (SNSs) were synthesized via a two-step seeds-mediated method. Polymer cores were composed of ultrafine gold nanoparticles (NPs) modified chitosan-poly(acrylic acid) nanoparticles (CS-PAA NPs). Then, deposition of silver upon gold nucleus leads to the seed enlargement and finally forms silver shell on the surface of CS-PAA NPs to get SNSs. Transmission electron microscope (TEM) showed SNSs had a discrete silver shell plus some pores and gaps, which could acted as "hot spots" and provided the great potential of these SNSs to be used as SERS substrates with wavelength ranging from visible to infrared region (700-1000. nm) by tuning shell coverage of silver. SERS experiments with dibenzyl disulphide (DBDS) as the indicator showed that the resulting SNSs allowed the production of highly consistent enhancement of the Raman signals down to nM concentrations of DBDS. Considering the excellent biocompatibility of polymer core and their small size, these SNSs are highly desirable candidates as the enhancers for high performance SERS analysis and as SERS optical labels in biomedical imaging. © 2012 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2012.11.003
  • 2013 • 89 Influence of the substrate bias on the stoichiometry and structure of RF-magnetron sputter-deposited silver-containing calcium phosphate coatings
    Ivanova, A.A. and Surmeneva, M.A. and Grubova, I.Y. and Sharonova, A.A. and Pichugin, V.F. and Chaikina, M.V. and Buck, V. and Prymak, O. and Epple, M. and Surmenev, R.A.
    Materialwissenschaft und Werkstofftechnik 44 218-225 (2013)
    A coating on the basis of silver-containing hydroxyapatite (silver-hydroxyapatite) was deposited by radio frequency (RF) magnetron sputtering. X-ray diffractometry, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and ellipsometry were used to analyse the change in structure and stoichiometry of the coatings upon the change of the negative electrical bias (-50 V and -100 V) on the substrate. The chemical composition of the sputter-deposited coating was identical to the target. However, an increase in the negative electrical bias on the substrate led to a decrease of the coating thickness. In addition, the average size of the grains decreased from 55 ± 15 nm (grounded substrate) up to 30 ± 10 nm when an electrical bias of -100 V was applied. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300101
  • 2013 • 88 Controlling adsorption of semiflexible polymers on planar and curved substrates
    Kampmann, T.A. and Boltz, H.-H. and Kierfeld, J.
    Journal of Chemical Physics 139 (2013)
    We study the adsorption of semiflexible polymers such as polyelectrolytes or DNA on planar and curved substrates, e.g., spheres or washboard substrates via short-range potentials using extensive Monte Carlo simulations, scaling arguments, and analytical transfer matrix techniques. We show that the adsorption threshold of stiff or semiflexible polymers on a planar substrate can be controlled by polymer stiffness: adsorption requires the highest potential strength if the persistence length of the polymer matches the range of the adsorption potential. On curved substrates, i.e., an adsorbing sphere or an adsorbing washboard surface, the adsorption can be additionally controlled by the curvature of the surface structure. The additional bending energy in the adsorbed state leads to an increase of the critical adsorption strength, which depends on the curvature radii of the substrate structure. For an adsorbing sphere, this gives rise to an optimal polymer stiffness for adsorption, i.e., a local minimum in the critical potential strength for adsorption, which can be controlled by curvature. For two- and three-dimensional washboard substrates, we identify the range of persistence lengths and the mechanisms for an effective control of the adsorption threshold by the substrate curvature. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4813021
  • 2013 • 87 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 • 86 Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films
    Klar, D. and Klyatskaya, S. and Candini, A. and Krumme, B. and Kummer, K. and Ohresser, P. and Corradini, V. and de Renzi, V. and Biagi, R. and Joly, L. and Kappler, J.-P. and Pennino, U.D. and Affronte, M. and Wende, H. and Ruben, M.
    Beilstein Journal of Nanotechnology 4 320-324 (2013)
    The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100) substrate, to an in situ deposited submonolayer of TbPc2 molecules. Because of the element specificity of the X-ray absorption spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film. On both substrates the TbPc2 molecules couple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface. © 2013 Klar et al.
    view abstractdoi: 10.3762/bjnano.4.36
  • 2013 • 85 Formation and characterization of Fe3+-/Cu2+-modified zirconium oxide conversion layers on zinc alloy coated steel sheets
    Lostak, T. and Krebs, S. and Maljusch, A. and Gothe, T. and Giza, M. and Kimpel, M. and Flock, J. and Schulz, S.
    Electrochimica Acta 112 14-23 (2013)
    Zirconium oxide conversion layers are considered as environmentally friendly alternatives replacing chromate-based passivation layers in the coil-coating industry. Based on excellent electronic barrier properties they provide an effective corrosion protection of the metallic substrate. In this work, thin layers were grown on HDG-steel-substrates by increasing the local pH at the surface and were characterized using potentiodynamic polarization technique. The influence of Cu(NO3)2·3H 2O or Fe(NO3)3·9H2O on morphology and thickness of deposited protective layers were investigated by XPS, ToF-SIMS and FE-SEM. A significant film thickness increase was found by adding Cu2+ or Fe3+ ions to the conversion solution. In addition, growth kinetics was studied by in situ measurements of corrosion potential using potentiodynamic polarization technique. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2013.08.161
  • 2013 • 84 Eu-doped ZnO nanowire arrays grown by electrodeposition
    Lupan, O. and Pauporté, T. and Viana, B. and Aschehoug, P. and Ahmadi, M. and Cuenya, B.R. and Rudzevich, Y. and Lin, Y. and Chow, L.
    Applied Surface Science 282 782-788 (2013)
    The preparation of efficient light emitting diodes requires active optical layers working at low voltage for light emission. Trivalent lanthanide doped wide-bandgap semiconducting oxide nanostructures are promising active materials in opto-electronic devices. In this work we report on the electrochemical deposition (ECD) of Eu-doped ZnO (ZnO:Eu) nanowire arrays on glass substrates coated with F-doped polycrystalline SnO2. The structural, chemical and optical properties of ZnO:Eu nanowires have been systematically characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and photoluminescence. XRD results suggest the substitution of Zn2+ by Eu ions in the crystalline lattice. High-resolution TEM and associated electron diffraction studies indicate an interplanar spacing of 0.52 nm which corresponds to the (0 0 0 1) crystal plane of the hexagonal ZnO, and a growth along the c-direction. The ZnO:Eu nanowires have a single crystal structure, without noticeable defects. According to EDX, SIMS and XPS studies, cationic Eu species are detected in these samples showing the incorporation of Eu into the ZnO matrix. The oxidation states of europium ions in the nanowires are determined as +3 (74%) and +2 (26%). Photoluminescence studies demonstrated red emission from the Eu-doped ZnO nanowire arrays. When Eu was incorporated during the nanowire growth, the sharp 5D0-7F 2 transition of the Eu3+ ion at around 612 nm was observed. These results suggest that Eu doped ZnO nanowires could pave the way for efficient, multispectral LEDs and optical devices. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2013.06.053
  • 2013 • 83 Excimer laser doping using highly doped silicon nanoparticles
    Meseth, M. and Kunert, B.C. and Bitzer, L. and Kunze, F. and Meyer, S. and Kiefer, F. and Dehnen, M. and Orthner, H. and Petermann, N. and Kummer, M. and Wiggers, H. and Harder, N.-P. and Benson, N. and Schmechel, R.
    Physica Status Solidi (A) Applications and Materials Science 210 2456-2462 (2013)
    Laser doping of crystalline Si (c-Si) using highly doped Si nanoparticles (NPs) as the dopant source is investigated. For this purpose Si NPs are deposited onto c-Si substrates from dispersion using a spin coater and subsequently laser annealed by scanning over the sample with a 248 nm line profile excimer laser. Scanning electron microscope (SEM) investigations demonstrate that the laser intensity as well as the oxide concentration in the NP thin film strongly influence the film forming properties of the annealed NPs. Substrate doping is substantiated using electrochemical capacitance voltage (ECV) measurements on realized pn-junctions. In dependence of the laser fluencies ranging from 0.81 to 2.54 J cm-2, the effective doping depth is determined to be in the range of 50 to 250 nm. The rectifying behaviour of the pn- or np-junctions is verified by current voltage measurements. A homogeneous in-plane doping distribution realized by the laser doping process is demonstrated on the μm scale by light beam induced current measurements. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201329012
  • 2013 • 82 Laser-doping of crystalline silicon substrates using doped silicon nanoparticles
    Meseth, M. and Lamine, K. and Dehnen, M. and Kayser, S. and Brock, W. and Behrenberg, D. and Orthner, H. and Elsukova, A. and Hartmann, N. and Wiggers, H. and Hülser, T. and Nienhaus, H. and Benson, N. and Schmechel, R.
    Thin Solid Films 548 437-442 (2013)
    Crystalline Si substrates are doped by laser annealing of solution processed Si. For this experiment, dispersions of highly B-doped Si nanoparticles are deposited onto intrinsic Si and laser processed using an 807.5 nm continuous wave laser. During laser processing the particles as well as a surface-near substrate layer are melted to subsequently crystallize in the same orientation as the substrate. The doping profile is investigated by secondary ion mass spectroscopy revealing a constant B concentration of 2 × 10 18 cm- 3 throughout the entire analyzed depth of 5 μm. Four-point probe measurements demonstrate that the effective conductivity of the doped sample is increased by almost two orders of magnitude. The absolute doping depth is estimated to be in between 8 μm and 100 μm. Further, a pn-diode is created by laser doping an n-type c-Si substrate using the Si NPs. © 2013 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.09.056
  • 2013 • 81 Bimodal substrate biasing to control gamma-Al2O3 deposition during reactive magnetron sputtering
    Prenzel, M. and Kortmann, A. and Stein, A. and von Keudell, A. and Nahif, F. and Schneider, J. M.
    Journal of Applied Physics 114 113301 (2013)
    Al2O3 thin films have been deposited at substrate temperatures between 500 degrees C and 600 degrees C by reactive magnetron sputtering using an additional arbitrary substrate bias to tailor the energy distribution of the incident ions. The films were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The film structure being amorphous, nanocrystalline, or crystalline was correlated with characteristic ion energy distributions. The evolving crystalline structure is connected with different levels of displacements per atom (dpa) in the growing film as being derived from TRIM simulations. The boundary between the formation of crystalline films and amorphous or nanocrystalline films was at 0.8 dpa for a substrate temperature of 500 degrees C. This threshold shifts to 0.6 dpa for films grown at 550 degrees C. (C) 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4819227
  • 2013 • 80 Chemical activity of thin oxide layers: Strong interactions with the support yield a new thin-film phase of zno
    Schott, V. and Oberhofer, H. and Birkner, A. and Xu, M. and Wang, Y. and Muhler, M. and Reuter, K. and Wöll, C.
    Angewandte Chemie - International Edition 52 11925-11929 (2013)
    Influential support: Metal substrates affect the chemical properties of ZnO layers, which are important catalyst materials for the industrial production of methanol through the oxidation of CO. Interactions with the substrate lead to the formation of a new, planar ZnO thin-film phase, in which less highly oxidized Zn atoms bind CO more strongly than the Zn atoms in the normal wurtzite modification. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201302315
  • 2013 • 79 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 • 78 The structure of an rf-magnetron sputter-deposited silicate-containinghydroxyapatite-based coating investigated by high-resolution techniques
    Surmeneva, M.A. and Chaikina, M.V. and Zaikovskiy, V.I. and Pichugin, V.F. and Buck, V. and Prymak, O. and Epple, M. and Surmenev, R.A.
    Surface and Coatings Technology 218 39-46 (2013)
    A biocompatible nanostructured silicate-containing hydroxyapatite-based (Si-HA) thin coatingwas deposited by radio-frequency (RF) magnetron sputtering on silicon and titanium substrates. The morphology of the Si-HA coating was pore-free, dense and followed the topography of the underlying substrates. Energy-dispersive X-ray spectroscopy (EDX) gave molar Ca/P and Ca/(P+Si) ratios of 1.78 and 1.45, respectively. According to XRD-analysis, the coatingwas nanocrystallinewith a crystallite size in the range of 10-50 nm. The ultrastructure of the coating was analyzed by high-resolution transmission electron spectroscopy (HRTEM) combinedwith fast Fourier transform (FFT) analysis. The average crystallite size calculated by the Rietveld method was in good agreement with the HRTEM results. Moreover, HRTEM-observations indicated the presence of atomic layer misorientations originating from imperfections between the nanocrystals in the coating. The average coating nanohardness (11.6±1.7 GPa) was significantly higher than that of the uncoated Ti substrate (4.0±0.3 GPa), whereas no significant difference between the Young'smodulus of the coating (125±20 GPa) and the substrate (115±10 GPa) was found. Immersion of the coated substrates in simulated body fluid (SBF) led to the deposition of an apatite layer. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.12.023
  • 2013 • 77 Preparation of a silicate-containing hydroxyapatite-based coating by magnetron sputtering: Structure and osteoblast-like MG63 cells in vitro study
    Surmeneva, M.A. and Kovtun, A. and Peetsch, A. and Goroja, S.N. and Sharonova, A.A. and Pichugin, V.F. and Grubova, I.Y. and Ivanova, A.A. and Teresov, A.D. and Koval, N.N. and Buck, V. and Wittmar, A. and Ulbricht, M. and Prymak,...
    RSC Advances 3 11240-11246 (2013)
    Silicate-containing hydroxyapatite-based coatings with different structure and calcium/phosphate ratios were prepared by radio-frequency magnetron sputtering on silicon and titanium substrates, respectively. Scanning electron microscopy, X-ray diffraction and IR spectroscopy were used to investigate the effect of the substrate bias on the properties of the silicate-containing hydroxyapatite-based coatings. The deposition rate, composition, and microstructure of the deposited coatings were all controlled by changing the bias voltage from grounded (0 V) to -50 and -100 V. The biocompatibility was assessed by cell culture with human osteoblast-like cells (MG-63 cell line), showing a good biocompatibility and cell growth on the substrates. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3ra40446c
  • 2013 • 76 Adhesion properties of a silicon-containing calcium phosphate coating deposited by RF magnetron sputtering on a heated substrate
    Surmeneva, M.A. and Surmenev, R.A. and Pichugin, V.F. and Koval', N.N. and Teresov, A.D. and Ivanova, A.A. and Grubova, I.Y. and Ignatov, V.P. and Primak, O. and Epple, M.
    Journal of Surface Investigation 7 944-951 (2013)
    Silicon-containing hydroxyapatite coatings 400-700 nm in thickness are prepared by means of radio-frequency (RF) magnetron sputtering on a heated (to 200°C) titanium substrate chemically etched and treated with a pulsed electron beam. The morphology and phase composition of the coating are studied. The morphology and roughness of the composite "calcium-phosphate coating-titanium substrate" differ depending on the treatment procedure of the substrate before deposition. The scratch test method is used to assess the adhesion strength of the coatings formed at different values of bias potential applied to the substrate. It is observed that the adhesion strength of the coating changes with decreasing crystallite size. © 2013 Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S102745101305039X
  • 2013 • 75 Surface morphology of MnSi thin films grown on Si(111)
    Suzuki, T. and Lutz, T. and Geisler, B. and Kratzer, P. and Kern, K. and Costantini, G.
    Surface Science 617 106-112 (2013)
    The surface morphology of MnSi thin films grown on Si(111)-7 × 7 substrates was investigated by systematically changing the amount of deposited Mn. A new 3 × 3 surface reconstruction was found at the very initial growth stages, whose atomic configuration was analyzed both experimentally and theoretically. At a coverage of 0.1 monolayers, the formation of nanometer-sized MnSi islands was observed in coexistence with Mn nanoclusters that fit within the 7 × 7 half unit cell. With increasing Mn deposition, the MnSi islands grow, develop extended flat tops and eventually coalesce into an atomically flat film with a high corrugated 3×3 reconstruction punctuated by several holes. The successive film growth mode is characterized by the formation of MnSi quadlayers with a low corrugated 3×3 reconstruction. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.005
  • 2013 • 74 Quality prediction of twin wire Arc sprayed coatings using acoustic Emission Analysis
    Tillmann, W. and Abdulgader, M. and Wang, G. and Zielke, R.
    Journal of Thermal Spray Technology 22 380-390 (2013)
    In this work, acoustic emission analysis is utilized in the twin wire arc spraying (TWAS) process to study the influence of the adjustable process parameters on the simultaneously obtained acoustic signals at the nozzle and at the substrate. The amplitude of recorded signals at the substrate was in general much higher than those recorded at the nozzle. At the substrate side, the amplitude of emitted acoustic signals is dependent on feedstock materials and is higher when using solid wires. The acoustic signals were recorded at the spraying gun for different gas pressures without arc ignition (as dry runs) in order to reveal the effect of the arc on the emitted acoustic signals. A correlation between controllable parameters, the acoustic signals, and the obtained in-flight particle characteristics was observed. This work contributes to the online control of TWAS processes and is one of many proposed publications in the research field of the conducted acoustic emission analysis. © 2012 ASM International.
    view abstractdoi: 10.1007/s11666-012-9861-z
  • 2013 • 73 Influence of bias voltage on residual stresses and tribological properties of TiAlVN-coatings at elevated temperatures
    Tillmann, W. and Sprute, T. and Hoffmann, F. and Chang, Y.-Y. and Tsai, C.-Y.
    Surface and Coatings Technology 231 122-125 (2013)
    The extension of tool life is a crucial goal for heat resistant forming tools. Therefore, the industry is interested to reduce the friction and wear for these tools. The employment of metals, polymeric composites, and ceramics as solid lubricants increases the production as well as maintenance costs. Thus, the thin film technology and especially new self-lubricating coatings will become increasingly important. Titanium aluminum vanadium nitride as a self-lubricating coating has a high potential to improve the tribological behavior of heat resisting tool surfaces and has good mechanical properties such as a high hardness (more than 40GPa). In this study, TiAlVN coatings were deposited on HS6-5-2C high speed steel substrates by using a magnetron sputtering system. After annealing at 650°C, a V2O5 (Magnéli phase) which adds self-lubricating qualities to the coatings could be detected in the TiAlVN layer. Due to the influence of adhesive and cohesive damage processes, resulting from the residual stress behavior in the layer close to the substrate area, it is critical to measure residual stresses in order to increase the wear resistance. In addition to the phase analyses, residual stress measurements were investigated by means of x-ray diffractometry as well. An experimental method, based on the traditional sin2ψ-method and utilizing a grazing-incidence diffraction geometry was used in order to enhance the irradiation volume of thin film samples. This resulted in a higher intensity for high-angle Bragg peaks than for the Bragg-Brentano geometry. Furthermore, the mechanical and tribological properties of the TiAlVN coatings were characterized at elevated temperatures. The required results were provided by a high temperature ball-on-disk device and a nanoindenter. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2012.03.012
  • 2013 • 72 Conformational disorder in alkylsiloxane monolayers at elevated temperatures
    Weber, J. and Balgar, T. and Hasselbrink, E.
    Journal of Chemical Physics 139 (2013)
    Vibrational sum frequency generation spectroscopy is used to characterize octadecylsiloxane monolayers on glass substrates at ambient conditions with a focus on thermally induced conformational disorder. Different modes of the C-H stretching vibrations of the terminal methyl groups and the methylene groups are therefore monitored in the frequency range of 2850-3000 cm-1. We observe a progressive increase of conformational disorder of the alkyl chains due to gauche defects over the temperature range from 300 to 510 K. The conformational disorder is reversible over a temperature range from 300 to about 410 K. But after heating to temperatures above 410 K, order is not reestablished on the time scale of the experiment. These results suggest that the assumption of an all-trans configuration of the alkyl chains is an over-simplification which increasingly misrepresents the situation for elevated temperatures which are still well below the one at which decomposition starts. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4846298
  • 2013 • 71 Phase formation at the interface between a boron alloyed steel substrate and an Al-rich coating
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 226 130-139 (2013)
    Al-base coating (AlSi10Fe3) was applied to a steel substrate (22MnB5) by hot dipping. The coated steel substrates were austenitized at 920. °C for several dwells, and phase formation at the steel/coating interface was investigated by means of ex-situ phase analysis with synchrotron radiation and EBSD. Phase identification by EBSD and XRD confirmed the formation of Al-rich intermetallics during austenitization. Increasing the dwell time led to Fe diffusion into the Al-base coating as well as Al diffusion into the substrate. As a result of the diffusion processes, Al-rich intermetallics in the coating transformed to more Fe-rich intermetallics. Simultaneously, Al diffusion into the substrate changed the microstructure of the steel substrate near the coating interface. Formation of FeAl intermetallics and thus the mechanical properties of the AlSi10Fe3 coating can be influenced by heat treatment. Higher austenitization temperatures and longer dwell times support the formation of more ductile FeAl intermetallics but also lead to grain growth; thus having a negative effect on the mechanical properties of the steel. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2013.03.045
  • 2013 • 70 Investigation of a conjugated polyelectrolyte interlayer for inverted polymer:fullerene solar cells
    Xia, R. and Leem, D.-S. and Kirchartz, T. and Spencer, S. and Murphy, C. and He, Z. and Wu, H. and Su, S. and Cao, Y. and Kim, J.S. and Demello, J.C. and Bradley, D.D.C. and Nelson, J.
    Advanced Energy Materials 3 718-723 (2013)
    Inverted bulk heterojunction solar cells are fabricated using a conjugated polyelectrolyte (PFN) as a cathode interlayer. Enhanced photovoltaic performance is achieved by adjusting the PFN thickness. Measurements of the optical transmittance, cathode work function (via UPS) and surface atomic composition (via XPS) provide insights into this optimization. Drift-diffusion simulations point to a reduction in recombination of holes at the cathode as the main cause for improving Voc. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/aenm.201200967
  • 2013 • 69 Improving the current density and the coulombic efficiency by a cascade reaction of glucose oxidizing enzymes
    Zafar, M.N. and Shao, M. and Ludwig, R. and Leech, D. and Schuhmann, W. and Gorton, L.
    ECS Transactions 53 131-143 (2013)
    Improvements in current density and coulombic efficiency of a glucose oxidizing electrode were realized by a combination of pyranose dehydrogenase from Agaricus meleagris (AmPDH) with glucose dehydrogenase from Glomerella cingulata (GcGDH). The mixed enzyme electrode oxidizes glucose in several combinations at the C-1, C-2 and C-3 positions of the pyranose ring. This concerted action of enzymes increases (i) the coulombic efficiency by extracting more than 2e- per substrate molecule and (ii) the current density of the electrode when the mass-transfer of substrates becomes rate limiting. The electrodes were investigated with flow injection analysis (FIA) using different substrates under physiological conditions (pH 7.4). These investigations showed that the product of one enzyme can be used as substrate for the other enzyme and maximally 6e- can be gained from the oxidation of one glucose molecule using mixed enzyme electrode AmPDH/GcGDH/Os-polymer. We propose a bioanode for use in biofuel cells with an increased current density and coulombic efficiency obtained by a cascade reaction catalyzed by redox enzymes with a different site-specificity for glucose. © The Electrochemical Society.
    view abstractdoi: 10.1149/05302.0131ecst
  • 2012 • 68 Photo-initiated lamination of polyethylene (PE) and poly(ethylene terephthalate) (PET)
    Bahners, T. and Gutmann, J.S.
    Journal of Adhesion Science and Technology 26 121-130 (2012)
    Besides plasma-based processes, photo-initiated surface modifications have an interesting potential for adhesion promotion. This is of special interest with applications ranging from classical finishing to composites. Photo-chemical processes using continuous UV sources monochromatic as well as broad band are based on radical activation and ensuing reaction with the atmosphere. Achievable effects are addition of atoms e.g., introduction of oxygen (photo-oxidation) resulting in increased surface energy or grafting of functional groups. Both have certain potentials for adhesion promotion in a physico-chemical way. Based on the fundamental scheme of these processes i.e., a photon-initiated radical reaction at the substrate-atmosphere interface a direct 'inter-linking' of coating polymer and substrate is presented in this paper. The principal idea is to apply a thin layer of coating polymer on the substrate and irradiate this composite system at certain UV wavelengths. Given a low absorption of the radiation by the thin coating and at the same time a high absorption by the substrate, the radiation will penetrate the coating layer and generate radicals at the interface, which will induce cross-linking between the coating polymer and substrate. It is shown that for the example of laminates of polyethylene (PE) film on fabrics made of poly(ethylene terephthalate) (PET), extremely high adhesion strenghths are achieved without any use of additional adhesion promoters. © 2012 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1163/016942411X569336
  • 2012 • 67 Coarsening phenomena of metal nanoparticles and the influence of the support pre-treatment: Pt/TiO 2(110)
    Behafarid, F. and Roldan Cuenya, B.
    Surface Science 606 908-918 (2012)
    One of the technologically most important requirements for the application of oxide-supported metal nanoparticles (NPs) in the fields of molecular electronics, plasmonics, and catalysis is the achievement of thermally stable systems. For this purpose, a thorough understanding of the different pathways underlying thermally-driven coarsening phenomena, and the effect of the nanoparticle synthesis method, support morphology, and degree of support reduction on NP sintering is needed. In this study, the sintering of supported metal NPs has been monitored via scanning tunneling microscopy combined with simulations following the Ostwald ripening and diffusion-coalescence models. Modifications were introduced to the diffusion-coalescence model to incorporate the correct temperature dependence and energetics. Such methods were applied to describe coarsening phenomena of physical-vapor deposited (PVD) and micellar Pt NPs supported on TiO 2(110). The TiO 2(110) substrates were exposed to different pre-treatments, leading to reduced, oxidized and polymer-modified TiO 2 surfaces. Such pre-treatments were found to affect the coarsening behavior of the NPs. No coarsening was observed for the micellar Pt NPs, maintaining their as-prepared size of ~ 3 nm after annealing in UHV at 1060 °C. Regardless of the initial substrate pre-treatment, the average size of the PVD-grown NPs was found to increase after identical thermal cycles, namely, from 0.5 ± 0.2 nm to 1.0 ± 0.3 nm for pristine TiO 2, and from 0.8 ± 0.3 nm to 1.3 ± 0.6 nm for polymer-coated TiO 2 after identical thermal treatments. Although no direct real-time in situ microscopic evidence is available to determine the dominant coarsening mechanism of the PVD NPs unequivocally, our simulations following the diffusion-coalescence coarsening route were in significantly better agreement with the experimental data as compared to those based on the Ostwald-ripening model. The enhanced thermal stability of the micellar NPs as compared to the PVD clusters might be related to their initial larger NP size, narrower size distribution, and larger interparticle distances. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2012.01.022
  • 2012 • 66 Structural and magnetic properties of Co 2MnSi thin films
    Belmeguenai, M. and Zighem, F. and Faurie, D. and Tuzcuoglu, H. and Chérif, S.-M. and Moch, P. and Westerholt, K. and Seiler, W.
    Physica Status Solidi (A) Applications and Materials Science 209 1328-1333 (2012)
    Co 2MnSi (CMS) films of different thicknesses (20, 50, and 100 nm) were grown by radio frequency (RF) sputtering on a-plane sapphire substrates. Our X-rays diffraction (XRD) study shows that, in all the samples, the cubic 〈110〉 CMS axis is normal to the substrate and that six well defined preferential in-plane orientations are present. Static and dynamic magnetic properties were investigated using vibrating sample magnetometry (VSM) and microstrip line ferromagnetic resonance (MS-FMR), respectively. From the resonance measurements versus the direction and the amplitude of an applied magnetic field, most of the magnetic parameters are derived, i.e.: the magnetization, the gyromagnetic factor, the exchange stiffness coefficient, and the magnetic anisotropy terms. The in-plane anisotropy results from the superposition of two terms showing a twofold and a fourfold symmetry, respectively. The observed behavior of the hysteresis loops is in agreement with this complex form of the in-plane anisotropy. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201228039
  • 2012 • 65 Inas nanowire circuits fabricated by field-assisted self-assembly on a host substrate
    Blekker, K. and Richter, R. and Oda, R. and Taniyama, S. and Benner, O. and Keller, G. and Münstermann, B. and Lysov, A. and Regolin, I. and Waho, T. and Prost, W.
    IEICE Transactions on Electronics E95-C 1369-1375 (2012)
    We report on the fabrication and analysis of basic digital circuits containing InAs nanowire transistors on a host substrate. The nanowires were assembled at predefined positions by means of electric field-assisted self-assembly within each run generating numerous circuits simultaneously. Inverter circuits composed of two separated nanowire transistors forming a driver and an active load have been fabricated. The inverter circuits exhibit a gain (>1) in the MHz regime and a time constant of about 0.9 ns. A sample & hold core element is fabricated based on an InAs nanowire transistor connected to a hold capacitor, both on a Silicon and an InP isolating substrate, respectively. The low leakage read-out of the hold capacitor is done by InP-based metal-insulator heterojunction FET grown on the same substrate prior to nanowire FET fabrication. Experimental operation of the circuit is demonstrated at 100 MHz sampling frequency. The presented approach enables III/V high-speed, low-voltage logic circuits on a wide variety of host substrates which may be up scaled to high volume circuits. Copyright © 2012 The Institute of Electronics, Information and Communication Engineers.
    view abstractdoi: 10.1587/transele.E95.C.1369
  • 2012 • 64 SintClad: A new approach for the production of wear-resistant tools
    Blüm, M. and Hill, H. and Moll, H. and Weber, S. and Theisen, W.
    Journal of Materials Engineering and Performance 21 756-763 (2012)
    Tools used in the mineral processing industry are required to feature high wear resistance to facilitate an adequate cost efficiency. These kinds of tools are made of composite materials based on a low-alloyed substrate material and a high-alloyed coating. The coatings can be applied in different ways using production processes like HIP cladding, deposit welding, and composite casting. The article is concerned with the problem of a novel and cost-effective coating alternative: sinter cladding, using the principle of supersolidus liquid-phase sintering (SLPS). Usually SLPS represents a sintering technique, which is used for the compaction of high-alloyed metal powders. However, no recognizable efforts were made to use the SLPSprocess for applying a PM-coating on a bulk substrate material. Sinter cladding for the first time uses SLPS to combine the process of powder compaction with the application of a coating to a solid steel substrate into one single step. Another advantage of the process is the possibility to produce massive bulk coatings with thicknesses exceeding 20 mm. This article is original in the scope of question and investigation methods in terms of microstructure, hardness profiles, EDX measurements, diffusion calculations, and computational thermodynamics. © ASM International.
    view abstractdoi: 10.1007/s11665-012-0199-y
  • 2012 • 63 Robust mechanical performance of chromium-coated polyethylene terephthalate over a broad range of conditions
    Cordill, M.J. and Taylor, A.A. and Berger, J. and Schmidegg, K. and Dehm, G.
    Philosophical Magazine 92 3346-3362 (2012)
    Mechanical properties of metal films on polymer substrates are normally studied in terms of the fracture and adhesion of the film, while the properties of the polymer substrate and testing conditions are overlooked. Substrate orientation and thickness, as well as strain rate and temperature effects, are examined using Cr films deposited onto polyethylene terephthalate substrates. A faster strain rate affects only the initial fracture strain of the Cr film and not the crack and buckle spacings in the high strain condition. The substrate orientation slightly changes the average crack spacing while the substrate thickness has little effect on the cracking and buckling behaviour. Straining experiments at high temperature increased the average crack spacing and led to a change in buckling mode. The lack of sizeable changes in the mechanical behaviour over the large range of testing procedures leads to a resilient material system for flexible applications. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.700418
  • 2012 • 62 Heat transport through interfaces with and without misfit dislocation arrays
    Hanisch-Blicharski, A. and Krenzer, B. and Wall, S. and Kalus, A. and Frigge, T. and Horn-Von Hoegen, M.
    Journal of Materials Research 27 2718-2723 (2012)
    In spite of its large lattice mismatch, Bi grows epitaxially in (111) orientation and almost free of defects on Si substrates. On Si(111), the Bi film is under compressive strain of less than 2% and shows a 6-7 registry to the Si(111)-(7 - 7) substrate. On Si(001), the compressive lattice strain of 2.3% results in the formation of an array of misfit dislocations with a periodicity of 20 nm. We studied the cooling process of ultrathin bismuth films deposited on Si(111) and Si(001) substrates upon excitation with short laser pulses. With ultrafast electron diffraction, we determined the thermal boundary conductance σ K from the exponential decay of the transient film temperature. Within the error bars of 7%, the experimentally determined thermal boundary conductances are the same for both substrates and thus independent of the presence of a periodic array of misfit dislocations and the different substrate orientation. © Materials Research Society 2012.
    view abstractdoi: 10.1557/jmr.2012.316
  • 2012 • 61 A quick method for the preparation of Pt(111)-like thin films
    Maljusch, A. and Henry, J.B. and Schuhmann, W. and Bondarenko, A.S.
    Electrochemistry Communications 16 88-91 (2012)
    A simple and quick method for forming Pt(111)-like thin films on Si/Ti substrates for electrochemical and/or electrocatalytic experiments is reported. This method involves physical vapour deposition followed by flame annealing, electrochemical cleaning and a short heat treatment under a controlled atmosphere. Careful selection of the substrate, surface preparation and cooling atmosphere allows production of Pt thin films which show voltammetry features typical of large Pt(111) single crystal electrodes in 0.1 M HClO 4. This technique promises a method for the production of Pt(111) type surfaces on a larger scale. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2011.12.004
  • 2012 • 60 Formation of highly ordered alloy nanoparticles based on precursor-filled latex spheres
    Manzke, A. and Plettl, A. and Wiedwald, U. and Han, L. and Ziemann, P. and Schreiber, E. and Ziener, U. and Vogel, N. and Weiss, C.K. and Landfester, K. and Fauth, K. and Biskupek, J. and Kaiser, U.
    Chemistry of Materials 24 1048-1054 (2012)
    An experimental approach is presented, allowing the preparation of substrate supported, hexagonally arranged metallic alloy nanoparticles with narrow size distributions, well-defined interparticle distances, and controlled chemical composition. The method is based on miniemulsion polymerization and isotropic plasma etching. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) colloids-in the present study containing Fe- and Pt-precursor complexes in a predefined ratio-are deposited onto hydrophilic Si/SiO 2 substrates by dip-coating, forming a highly ordered monolayer. Contrary to colloidal lithography, here, precursor-filled polystyrene colloids serve as carriers for the alloy forming elements. After reactive ion etching and annealing, hexagonally ordered arrays of crystalline FePt nanoparticles are formed exhibiting the desired 1:1 Fe-Pt ratio, as revealed by detailed analysis after each preparation step. Formation of stoichiometric binary alloy FePt nanoparticles is confirmed by determining magnetic hysteresis loops, as well as applying aberration-corrected high-resolution transmission electron microscopy. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm203241p
  • 2012 • 59 Solvent-Free catalytic depolymerization of cellulose to water-soluble oligosaccharides
    Meine, N. and Rinaldi, R. and Schüth, F.
    ChemSusChem 5 1449-1454 (2012)
    The use of cellulose is hampered by difficulties with breaking up the biopolymer into soluble products. Herein, we show that the impregnation of cellulosic substrates with catalytic amounts of a strong acid (e.g., H 2SO 4, HCl) is a highly effective strategy for minimizing the contact problem commonly experienced in mechanically assisted, solid-state reactions. Milling the acid-impregnated cellulose fully converts the substrate into water-soluble oligosaccharides within 2a H. In aqueous solution, soluble products are easily hydrolyzed at 130°C in 1a H, leading to 91% conversion of the glucan fraction of α-cellulose into glucose, and 96% of the xylans into xylose. Minor products are glucose dimers (8%), 5-hydroxymethylfurfural (1%) and furfural (4%). Milling practical feedstocks (e.g., wood, sugarcane bagasse, and switchgrass) also results to water-soluble products (oligosaccharides and lignin fragments). The integrated approach (solid-state depolymerization in combination with liquid-phase hydrolysis) could well hold the key to a highly efficient "entry process" in biorefinery schemes. Reactive milling: Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201100770
  • 2012 • 58 Effect of exchange bias on magnetic anisotropies in fe/coo bilayers
    Öztürk, M. and Demirci, E. and Topkaya, R. and Kazan, S. and Akdoǧan, N. and Obaida, M. and Westerholt, K.
    Journal of Superconductivity and Novel Magnetism 25 2597-2603 (2012)
    We report on the structural and magnetic properties of exchange-biased Fe/CoO bilayers grown on MgO (001) substrates by using rf-sputtering. For varying Fe thicknesses (4 nm, 10 nm, and 20 nm) the ferromagnetic resonance (FMR) spectra of bilayers have been studied as a function of temperature at X-band frequency. The resonance lines of FMR spectra have a relatively small linewidth indicating a high crystallinity of the Fe films. The roomtemperature FMR data also show that the easy axis of Fe is in the film plane and parallel to the [110] crystallographic direction of MgO substrate. In addition, M versus H loops were recorded at selected temperatures by using VSM magnetometry. The VSM measurements indicate that the Fe thickness and temperature dependence of exchangebias properties are in good agreement with the previous results on similar systems. However, the blocking temperature of the exchange-biased system is strongly reduced compared to the bulk values. This reduction in the blocking temperature is explained by both the thickness and superstoichiometric structure of antiferromagnetic CoO layer. © Springer Science+Business Media, LLC 2011.
    view abstractdoi: 10.1007/s10948-011-1227-0
  • 2012 • 57 Substrate-mediated effects in photothermal patterning of alkanethiol self-assembled monolayers with microfocused continuous-wave lasers
    Schröter, A. and Kalus, M. and Hartmann, N.
    Beilstein Journal of Nanotechnology 3 65-74 (2012)
    In recent years, self-assembled monolayers (SAMs) have been demonstrated to provide promising new approaches to nonlinear laser processing. Most notably, because of their ultrathin nature, indirect excitation mechanisms can be exploited in order to fabricate subwavelength structures. In photothermal processing, for example, microfocused lasers are used to locally heat the substrate surface and initiate desorption or decomposition of the coating. Because of the strongly temperature-dependent desorption kinetics, the overall process is highly nonlinear in the applied laser power. For this reason, subwavelength patterning is feasible employing ordinary continuous-wave lasers. The lateral resolution, generally, depends on both the type of the organic monolayer and the nature of the substrate. In previous studies we reported on photothermal patterning of distinct types of SAMs on Si supports. In this contribution, a systematic study on the impact of the substrate is presented. Alkanethiol SAMs on Au-coated glass and silicon substrates were patterned by using a microfocused laser beam at a wavelength of 532 nm. Temperature calculations and thermokinetic simulations were carried out in order to clarify the processes that determine the performance of the patterning technique. Because of the strongly temperature-dependent thermal conductivity of Si, surface-temperature profiles on Au/Si substrates are very narrow ensuring a particularly high lateral resolution. At a 1/e spot diameter of 2 μm, fabrication of subwavelength structures with diameters of 300-400 nm is feasible. Rapid heat dissipation, though, requires high laser powers. In contrast, patterning of SAMs on Au/glass substrates is strongly affected by the largely distinct heat conduction within the Au film and in the glass support. This results in broad surface temperature profiles. Hence, minimum structure sizes are larger when compared with respective values on Au/Si substrates. The required laser powers, though, are more than one order of magnitude lower. Also, the laser power needed for patterning decreases with decreasing Au layer thickness. These results demonstrate the impact of the substrate on the overall patterning process and provide new perspectives in photothermal laser patterning of ultrathin organic coatings. © 2012 Schröter et al.
    view abstractdoi: 10.3762/bjnano.3.8
  • 2012 • 56 Kinetics and driving forces of abnormal grain growth in thin Cu films
    Sonnweber-Ribic, P. and Gruber, P.A. and Dehm, G. and Strunk, H.P. and Arzt, E.
    Acta Materialia 60 2397-2406 (2012)
    The abnormal growth of individual (1 0 0) oriented grains is monitored by the in situ electron backscatter diffraction technique for more than 24 h at three different annealing temperatures (90 °C, 104 °C and 118 °C) in 1-5 μm thick Cu films on polyimide substrates. The (1 0 0) grain growth velocity increases with higher film thickness and annealing temperature, as suggested by an earlier model by Thompson and Carel. As a result, the final (1 0 0) texture fraction becomes more dominant for higher annealing temperatures and larger film thicknesses. The Thompson-Carel model, however, predicts that the (1 1 1) grains will preferably grow at temperatures up to 118 °C. Our calculations of the driving forces revealed that in addition to minimization of the strain energy (due to the thermal mismatch between film and substrate) and of the surface energy, the energy stored in the dislocations plays a decisive role in grain growth. Our observations can be understood by the notion that initially available (1 0 0) grain nuclei start to grow very rapidly, due to dislocation annihilation, and thus "overrun" the (1 1 1) grains in size. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.12.030
  • 2012 • 55 Biocompatible nanostructured coatings based on calcium phosphates prepared by means of rf-magnetron sputtering deposition
    Surmeneva, M. and Surmenev, R. and Pichugin, V. and Ivanova, A. and Grubova, I. and Chaikina, M. and Khlusov, I. and Kovtun, A. and Epple, M.
    Proceedings - 2012 7th International Forum on Strategic Technology, IFOST 2012 (2012)
    The aim of this study was to prevent the problems associated with implants failure. Biocompatible nanostructured thin films of either Si- or Ag-containing non-stoichiometric hydroxyapatite (HA) were deposited by method of radio-frequency (rf) magnetron sputtering. Plates of Ti, Ti6Al4V and 316 L SS were used as substrates. The thin coatings were characterized by EDX, ESEM, XRD, IR spectroscopy, HRTEM, nanoindentation and scratch-test. HRTEM observations of the coatings showed a nanocrystalline structure mixed with amorphous regions. It was found that the morphology, structure and the preferred orientation of the films are greatly affected by the parameters of deposition (rf-power, substrate temperature and voltage bias). The as-deposited modified CaP-based coatings are dense, pore-free and their composition resembles that of the precursor target composition. The Si- and Ag- containing HA coatings had a hardness of 10-12 GPa. A low rf-power (30 W) resulted in amorphous or low crystalline CaP coating structure. An increase in rf-power (> 200 W) induced the coating crystallization. The occurrence of the different structure types is described as function of the bias voltage and temperatures. The negative substrate bias allowed to vary the Ca/P ratio in the range of 1.53 to 4. In vitro biocompatibility assessments of the films using the MG63 osteoblast-like cells indicated excellent cell adherence and surface colonization. Si-containing rf-magnetron films promote osteogenic differentiation of human stromal stem cells in vitro. The coatings are prospective to be used in clinical practice: in stomatology or craniofacial medicine, where the leaching of toxic ions from the substrate is necessary or the initial material surface porosity for a further bone in growth should be preserved. © 2012 IEEE.
    view abstractdoi: 10.1109/IFOST.2012.6357526
  • 2012 • 54 On the limits of the interfacial yield model for fragmentation testing of brittle films on polymer substrates
    Taylor, A.A. and Cordill, M.J. and Dehm, G.
    Philosophical Magazine 92 3363-3380 (2012)
    Fragmentation testing is frequently used to probe film fracture strain and the interfacial properties of thin brittle films on compliant substrates. A model based upon complete yield of the film/substrate interface is frequently used to analyse data after cracking has saturated. Additionally, the film is either assumed to have a single-valued failure stress or a distribution of strengths described by Weibull statistics. Recent work by the authors showed that consideration of film thickness variations and the application of neighbour ratio analysis brought 96% of the data for an Al x O y /Cu film/substrate system into compliance with the predictions for a film with a single-valued failure stress. In the present work Cr/PI (polyimide) and Cr/PET (polyethylene teraphthalate) systems are analysed according to the same methodology. The Cr films on polymer substrates crack such that the neighbour ratios considerably exceed the predicted limit of 2. The influence of the relative thickness of the film and substrate and the strain rate of the test is investigated. A deviation from the idealised mechanical model due to the large difference in elastic moduli of film and substrate is put forward as a possible cause of the observed behaviour. The importance of these results to the application of the interfacial yield model is discussed. © 2012 Taylor & Francis.
    view abstractdoi: 10.1080/14786435.2012.723145
  • 2012 • 53 Deposition of hard and adherent TiBCN films for cutting tools applications
    Tillmann, W. and Bejarano, G. and Hoffmann, F.
    Physica Status Solidi (A) Applications and Materials Science 209 1520-1525 (2012)
    Metal cutting tools having wear resistant and chemically stable ceramic coatings are in many applications superior in performance to uncoated tools. Titanium boron carbon nitride (TiBCN) is a hard material particularly suitable as a protective coating for cutting tools due to its excellent properties, such as a high hardness and high wear and corrosion resistance, among other. TiBCN films were grown on Si (100) and high speed steel substrates by means of reactively pulsed DC magnetron sputtering technique. Two B 4C- and two Ti-targets, to which a pulsed DC voltage of middle frequency was applied, were used for the deposition of TiBCN. A chromium layer was first deposited to obtain a better adhesion of TiBCN to the substrates. The mechanical properties of these coatings deposited under different N 2 contents were investigated. The substrates were biased through a medium frequency power supply. The bias voltage value was -90 V for all coatings. The total film thickness was maintained at approximately 2 μm. The hardness of the coatings increased with reduced nitrogen content, while the adhesion decreased from 40.8 to 24.2 N, and the wear rate increased from 0.154 to 0.744 × 10 -16 m 3/N.m, the latter probably caused by the low content of the self-lubricating amorphous matrix of our coatings. However, the sample deposited by a nitrogen gas flow of 60 sccm presented a wear rate of four orders of magnitude smaller than the uncoated sample. The deposition method presented in this work seems very promising for the manufacture of TiBCN coatings. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201228130
  • 2012 • 52 An improved electrical and thermal model of a microbolometer for electronic circuit simulation
    Würfel, D. and Vogt, H.
    Advances in Radio Science 10 183-186 (2012)
    The need for uncooled infrared focal plane arrays (IRFPA) for imaging systems has increased since the beginning of the nineties. Examples for the application of IRFPAs are thermography, pedestrian detection for automotives, fire fighting, and infrared spectroscopy. It is very important to have a correct electro-optical model for the simulation of the microbolometer during the development of the readout integrated circuit (ROIC) used for IRFPAs. The microbolometer as the sensing element absorbs infrared radiation which leads to a change of its temperature due to a very good thermal insulation. In conjunction with a high temperature coefficient of resistance (TCR) of the sensing material (typical vanadium oxide or amorphous silicon) this temperature change results in a change of the electrical resistance. During readout, electrical power is dissipated in the microbolometer, which increases the temperature continuously. The standard model for the electro-optical simulation of a microbolometer includes the radiation emitted by an observed blackbody, radiation emitted by the substrate, radiation emitted by the microbolometer itself to the surrounding, a heat loss through the legs which connect the microbolometer electrically and mechanically to the substrate, and the electrical power dissipation during readout of the microbolometer (Wood, 1997). The improved model presented in this paper takes a closer look on additional radiation effects in a real IR camera system, for example the radiation emitted by the casing and the lens. The proposed model will consider that some parts of the radiation that is reflected from the casing and the substrate is also absorbed by the microbolometer. Finally, the proposed model will include that some fraction of the radiation is transmitted through the microbolometer at first and then absorbed after the reflection at the surface of the substrate. Compared to the standard model temperature and resistance of the microbolometer can be modelled more realistically when these higher order effects are taken into account. A Verilog-A model for electronic circuit simulations is developed based on the improved thermal model of the microbolometer. Finally, a simulation result of a simple circuit is presented. © 2012 Author(s).
    view abstractdoi: 10.5194/ars-10-183-2012
  • 2012 • 51 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 • 50 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
  • 2011 • 49 Intercalation in layered metal-organic frameworks: Reversible inclusion of an extended π-system
    Arslan, H.K. and Shekhah, O. and Wieland, D.C.F. and Paulus, M. and Sternemann, C. and Schroer, M.A. and Tiemeyer, S. and Tolan, M. and Fischer, R.A. and Wöll, C.
    Journal of the American Chemical Society 133 8158-8161 (2011)
    We report the synthesis of layered [Zn 2(bdc) 2(H 2O) 2] and [Cu 2(bdc) 2(H 2O) 2] (bdc = benzdicarboxylate) metal-organic frameworks (MOF) carried out using the liquid-phase epitaxy approach employing self-assembled monolayer (SAM) modified Au-substrates. We obtain Cu and Zn MOF-2 structures, which have not yet been obtained using conventional, solvothermal synthesis methods. The 2D Cu 2+ dimer paddle wheel planes characteristic for the MOF are found to be strictly planar, with the planes oriented perpendicular to the substrate. Intercalation of an organic dye, DXP, leads to a reversible tilting of the planes, demonstrating the huge potential of these surface-anchored MOFs for the intercalation of large, planar molecules. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja2037996
  • 2011 • 48 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 • 47 Raman spectra of graphene exfoliated on insulating crystalline substrates
    Bukowska, H. and Meinerzhagen, F. and Akcöltekin, S. and Ochedowski, O. and Neubert, M. and Buck, V. and Schleberger, M.
    New Journal of Physics 13 (2011)
    We have investigated single layer, bilayer and few-layer graphene exfoliated on SiO 2 and on single crystal surfaces of SrTiO 3, Al 2O 3 and TiO 2 using Raman spectroscopy. The typical 'fingerprint' 2D peak turns out to be indicative of the number of graphene layers independent of the substrate material. The morphological quality of the graphene is as good as on SiO 2 substrates for all the materials. We find evidence for substrate-induced changes due to doping. With most substrates, hole doping is observed, but with SrTiO 3 we have identified a dielectric substrate with which electron accumulation in graphene can be achieved. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/13/6/063018
  • 2011 • 46 Synthesis of active carbon-based catalysts by chemical vapor infiltration for nitrogen oxide conversion
    Busch, M. and Bergmann, U. and Sager, U. and Schmidt, W. and Schmidt, F. and Notthoff, C. and Atakan, B. and Winterer, M.
    Journal of Nanoscience and Nanotechnology 11 7956-7961 (2011)
    Direct reduction of nitrogen oxides is still a challenge. Strong efforts have been made in developing noble and transition metal catalysts on microporous support materials such as active carbons or zeolites. However, the required activation energy and low conversion rates still limit its break-through. Furthermore, infiltration of such microporous matrix materials is commonly performed by wet chemistry routes. Deep infiltration and homogeneous precursor distribution are often challenging due to precursor viscosity or electrostatic shielding and may be inhibited by pore clogging. Gas phase infiltration, as an alternative, can resolve viscosity issues and may contribute to homogeneous infiltration of precursors. In the present work new catalysts based on active carbon substrates were synthesized via chemical vapor infiltration. Iron oxide nano clusters were deposited in the microporous matrix material. Detailed investigation of produced catalysts included nitrogen oxide adsorption, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Catalytic activity was studied in a recycle flow reactor by time-resolved mass spectrometry at a temperature of 423 K. The infiltrated active carbons showed very homogeneous deposition of iron oxide nano clusters in the range of below 12 to 19 nm, depending on the amount of infiltrated precursor. The specific surface area was not excessively reduced, nor was the pore size distribution changed compared to the original substrate. Catalytic nitrogen oxides conversion was detected at temperatures as low as 423 K. Copyright © 2011 American Scientific Publishers. All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5074
  • 2011 • 45 Comparative atom probe study of Cu(In,Ga)Se 2 thin-film solar cells deposited on soda-lime glass and mild steel substrates
    Choi, P.-P. and Cojocaru-Mirédin, O. and Wuerz, R. and Raabe, D.
    Journal of Applied Physics 110 (2011)
    We report on a comparative study of Cu(In,Ga)Se 2 solar cells deposited on soda-lime glass and mild steel substrates, using atom probe tomography in conjunction with secondary ion mass spectrometry, x-ray fluorescence, current density-voltage, and external quantum efficiency measurements. Cu(In,Ga)Se 2 films deposited on soda-lime glass substrates and on steel substrates with a NaF precursor layer on top of the Mo back contact contain a significant amount of Na impurities and yield an enhanced open circuit voltage and fill factor. Using atom probe tomography, Na atoms are found to be segregated at grain boundaries and clustered in both bulk and grain boundaries. The atom probe data indicate that Na Cu point defects are most likely formed at grain boundaries, reducing the number of compensating In Cu point defects and thus contributing to an enhanced cell efficiency. However, for steel substrates the positive effect of Na on the cell performance is counterbalanced by the incorporation of Fe impurities into the Cu(In,Ga)Se 2 film. Fe atoms are homogeneously distributed inside the grains suggesting that Fe introduces point defects in the bulk © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3665723
  • 2011 • 44 Indirect magnetic coupling of manganese porphyrin to a ferromagnetic cobalt substrate
    Chylarecka, D. and Kim, T.K. and Tarafder, K. and Müller, K. and Gödel, K. and Czekaj, I. and Wäckerlin, C. and Cinchetti, M. and Ali, Md.E. and Piamonteze, C. and Schmitt, F. and Wüstenberg, J.-P. and Ziegler, C. and Nolting,...
    115 1295-1301 (2011)
    The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp106822s
  • 2011 • 43 Atomic-scale distribution of impurities in cuinse2-based thin-film solar cells
    Cojocaru-Miredin, O. and Choi, P. and Wuerz, R. and Raabe, D.
    Ultramicroscopy 111 552-556 (2011)
    Atom Probe Tomography was employed to investigate the distribution of impurities, in particular sodium and oxygen, in a cuinse2-based thin-film solar cell. It could be shown that sodium, oxygen, and silicon diffuse from the soda lime glass substrate into the cuinse2 film and accumulate at the grain boundaries. Highly dilute concentrations of sodium and oxygen were measured in the bulk. Selenium was found to be depleted at the grain boundaries. These observations could be confirmed by complementary energy dispersive X-ray spectroscopy studies. Our results support the model proposed by Kronik et al. (1998) [1], which explains the enhanced photovoltaic efficiency of sodium containing cuinse2 solar cells by the passivation of selenium vacancies at grain boundaries. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2010.12.034
  • 2011 • 42 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 • 41 Microstructure and adhesion of as-deposited and annealed Cu/Ti films on polyimide
    Cordill, M.J. and Taylor, A. and Schalko, J. and Dehm, G.
    International Journal of Materials Research 102 729-734 (2011)
    The ability to measure the adhesion energy of metal thin films on polymer substrates is important for the design of reliable flexible electronic devices. One technique is to create well-defined areas of delamination (buckles) as a consequence of lateral compressive stresses induced by tensile straining of the film-substrate system. The adhesion energy is calculated from the buckle dimensions. In order to improve the adhesion between the metal film and polymer substrate, thin adhesion layers can be incorporated. However, interdiffusion and reactions can occur between the adhesion layer and the metal film when subjected to elevated temperatures. This is detrimental for the interfacial adhesion, as will be discussed for Cu films on polyimide with a Ti interlayer subjected to annealing at 350°C. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110513
  • 2011 • 40 Characterization and simulation of a-Si:H/μc-Si:H tandem solar cells
    Ding, K. and Kirchartz, T. and Pieters, B.E. and Ulbrich, C. and Ermes, A.M. and Schicho, S. and Lambertz, A. and Carius, R. and Rau, U.
    Solar Energy Materials and Solar Cells 95 3318-3327 (2011)
    We simulated device characteristics of a-Si:H single junction, μc-Si:H single junction and a-Si:H/μc-Si:H tandem solar cells with the numerical device simulator Advanced Semiconductor Analysis (ASA). For this purpose we measured and adjusted electrical and optical input parameters by comparing measured and simulated external quantum efficiency, current-voltage characteristic and reflectivity spectra. Consistent reproducibility of experimental data by numerical simulation was achieved for all types of cells investigated in this work. We also show good correspondence between the experimental and simulated characteristics for a-Si:H/μc-Si:H tandem solar cells with various absorber thicknesses on both Asahi U-type SnO2:F and sputtered/etched (Jülich) ZnO:Al substrates. Based on this good correlation between experiment and theory, we provide insight into device properties that are not directly measurable like the spatially resolved absorptance and the voltage-dependent carrier collection. These data reveal that the difference between tandem solar cells grown on Asahi U-type and Jülich ZnO substrates primarily arises from their optical properties. In addition, we find out that the doped layers do not contribute to the photocurrent except for the front p-layer. We also calculated the initial efficiencies of a-Si:H/μc-Si:H tandem solar cells with different combinations of a-Si:H and μc-Si:H absorber layer thicknesses. The maximum efficiency is found at 260 nm/1500 nm for tandem solar cells on Asahi U-type substrates and at 360 nm/850 nm for tandem solar cells on Jülich ZnO substrates. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.solmat.2011.07.023
  • 2011 • 39 Reversible fcc ↔ bcc transformation in freestanding epitaxially grown Fe-Pd ferromagnetic shape memory films
    Edler, T. and Hamann, S. and Ludwig, Al. and Mayr, S.G.
    Scripta Materialia 64 89-92 (2011)
    Using temperature-dependent X-ray diffraction and magnetization measurements, a reversible face-centered cubic (fcc) to body-centered cubic (bcc) structural transformation was confirmed in freestanding epitaxially grown Fe70Pd30 films after lift-off from their MgO (1 0 0) substrates - a transformation generally considered irreversible in bulk samples. The latter is accompanied by a distinct change of the sample magnetization. In contrast, substrate constraints were found to suppress the thermoelastic fcc to bcc transformation in substrate-attached films. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2010.09.013
  • 2011 • 38 Time-resolved photoluminescence and optical gain of Ga(NAsP) heterostructures pseudomorphically grown on silicon (001) substrate
    Gerhardt, N.C. and Koukourakis, N. and Klimasch, M. and Funke, D.A. and Hofmann, M.R. and Kunert, B. and Liebich, S. and Trusheim, D. and Zimprich, M. and Volz, K. and Stolz, W.
    International Conference on Transparent Optical Networks (2011)
    The novel metastable dilute nitride material Ga(NAsP) is a very promising candidate for electrically pumped lasers on silicon because it can be pseudomorphically grown on silicon substrate. Here we investigate the optical properties of a series of multi-quantum well Ga(NAsP) samples grown lattice matched on GaP and Si substrates. Temperature and excitation resolved photoluminescence spectroscopy indicates a significant impact of disorder-induced carrier localization effects on the optical properties. On the other hand, optical gain measurements reveal high modal gain up to 80 cm -1 at room temperature and demonstrate the suitability of this new material as an active material for laser devices. A comparative analysis of optical gain and photoluminescence data demonstrates a strong impact of the barrier-growth conditions on the optical quality of the material. © 2011 IEEE.
    view abstractdoi: 10.1109/ICTON.2011.5970794
  • 2011 • 37 RioK1, a new interactor of protein arginine methyltransferase 5 (PRMT5), competes with pICln for binding and modulates PRMT5 complex composition and substrate specificity
    Guderian, G. and Peter, C. and Wiesner, J. and Sickmann, A. and Schulze-Osthoff, K. and Fischer, U. and Grimmler, M.
    Journal of Biological Chemistry 286 1976-1986 (2011)
    Protein arginine methylation plays a critical role in differential gene expression through modulating protein-protein and protein-DNA/RNA interactions. Although numerous proteins undergo arginine methylation, only limited information is available on how protein arginine methyltransferases (PRMTs) identify their substrates. The human PRMT5 complex consists of PRMT5, WD45/MEP50 (WD repeat domain 45/methylosome protein 50), and pICln and catalyzes the symmetrical arginine dimethylation of its substrate proteins. pICln recruits the spliceosomal Sm proteins to the PRMT5 complex for methylation, which allows their subsequent loading onto snRNA to form small nuclear ribonucleoproteins. To understand how the PRMT5 complex is regulated, we investigated its biochemical composition and identified RioK1 as a novel, stoichiometric component of the PRMT5 complex. We show that RioK1 and pICln bind to PRMT5 in a mutually exclusive fashion. This results in a PRMT5-WD45/MEP50 core structure that either associates with pICln or RioK1 in distinct complexes. Furthermore, we show that RioK1 functions in analogy to pICln as an adapter protein by recruiting the RNA-binding protein nucleolin to the PRMT5 complex for its symmetrical methylation. The exclusive interaction of PRMT5 with either pICln or RioK1 thus provides the first mechanistic insight into how a methyltransferase can distinguish between its substrate proteins. © 2011 by The American Society for Biochemistry and Molecular Biology, Inc.
    view abstractdoi: 10.1074/jbc.M110.148486
  • 2011 • 36 Synthesis and ink-jet printing of highly luminescing silicon nanoparticles for printable electronics
    Gupta, A. and Khalil, A.S.G. and Offer, M. and Geller, M. P. and Winterer, M. and Lorke, A. and Wiggers, H.
    Journal of Nanoscience and Nanotechnology 11 5028-5033 (2011)
    The formation of stable colloidal dispersions of silicon nanoparticles (Si-NPs) is essential for the manufacturing of silicon based electronic and optoelectronic devices using cost-effective printing technologies. However, the development of Si-NPs based printable electronics has so far been hampered by the lack of long-term stability, low production rate and poor optical properties of Si- NPs ink. In this paper, we synthesized Si-NPs in a gas phase microwave plasma reactor with very high production rate, which were later treated to form a stable colloidal dispersion. These particles can be readily dispersed in a variety of organic solvents and the dispersion is stable for months. The particles show excellent optical properties (quantum yields of about 15%) and long-term photoluminescence (PL) stability. The stable ink containing functionalized Si-NPs was successfully used to print structures on glass substrates by ink-jet printing. The homogeneity and uniformity of large-area printed film was investigated using photoluminescence (PL) mapping. Copyright © 2011 American Scientific Publishers.
    view abstractdoi: 10.1166/jnn.2011.4184
  • 2011 • 35 Cavitation erosion of NiAl-bronze layers generated by friction surfacing
    Hanke, S. and Fischer, A. and Beyer, M. and dos Santos, J.
    Wear 273 32-37 (2011)
    Friction surfacing is a solid-state process, which allows deposition welding at temperatures below the melting range. For this investigation coating layers of NiAl-bronze were deposited by friction surfacing on self-mating substrates, followed by microstructural characterisation. Further, cavitation tests were performed in order to investigate wear resistance. Cavitation erosion mechanisms were analysed by means of optical and electron microscopy. All coatings show incubation periods about twice as long as those of the substrate material, while their average rate of material loss is about one half of that of the substrate. The differences in cavitation erosion resistance are due to more ductile behaviour of the coatings, as well as corrosion increasing the wear of the as-cast material. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2011.06.002
  • 2011 • 34 Grain resolved orientation changes and texture evolution in a thermally strained Al film on Si substrate
    Heinz, W. and Dehm, G.
    Surface and Coatings Technology 206 1850-1854 (2011)
    Temperature changes induce thermal stresses in thin films on substrates due to differences in the thermal expansion coefficients. Repeated thermal cycling may finally lead to severe surface roughening and a change in film texture. In this study we investigate the orientation changes for a 600. nm thick Al film during subsequent thermal cycles between 25 °C and 450 °C by analyzing individual grains. The results reveal orientation changes by up to 3° after one thermal cycles and unexpected large orientation gradients within individual grains. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2011.07.046
  • 2011 • 33 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 • 32 Carbon nanowalls deposited by inductively coupled plasma enhanced chemical vapor deposition using aluminum acetylacetonate as precursor
    Jain, H.G. and Karacuban, H. and Krix, D. and Becker, H.-W. and Nienhaus, H. and Buck, V.
    Carbon 49 4987-4995 (2011)
    Well aligned carbon nanowalls, a few nanometers thick, were fabricated by continuous flow of aluminum acetylacetonate (Al (acac)3) without a catalyst, and independent of substrate material. The nanowalls were grown on Si, and steel substrates using inductively coupled plasma-enhanced chemical vapor deposition. Deposition parameters like flow of argon gas and substrate temperature were correlated with the growth of carbon nanowalls. For a high flow of argon carrier gas, an increased amount of aluminum in the film and a reduced lateral size of the carbon walls were found. The aluminum is present inside the carbon nanowall matrix in the form of well crystallized nanosized Al4C3 precipitates. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2011.07.002
  • 2011 • 31 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 • 30 Morphologies of small droplets on patterned hydrophobic substrates
    Moradi, N. and Gross, M. and Varnik, F. and Zikos, G. and Steinbach, I.
    Modelling and Simulation in Materials Science and Engineering 19 (2011)
    For situations, in which the size of a droplet is comparable to the roughness scale of the solid substrate, we explore possible wetting morphologies on patterned hydrophobic substrates and investigate their dependence on the initial droplet position, droplet volume and the surface geometry. For a regular array of cubical pillars, small perturbations of a symmetric droplet state are restored by capillary forces. Larger deviations, on the other hand, may lead to completely new morphologies. Our studies also suggest that the previously reported 'reentrant transition' upon quasi-static evaporation (a transition from the suspended state to partial penetration and then back to the suspended state) (Gross, et al. 2009 Europhys. Lett. 88 26002) is not restricted to a symmetric initial state but may occur for quite non-symmetric morphologies as well. In contrast, a change in the substrate geometry may lead to a completely different behavior, fully precluding the reentrant transition. This underlines the importance of substrate design for the use of reentrant transition as a self-cleaning mechanism. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/19/4/045005
  • 2011 • 29 Structure and flow of droplets on solid surfaces
    Müller-Buschbaum, P. and Magerl, D. and Hengstler, R. and Moulin, J.-F. and Körstgens, V. and Diethert, A. and Perlich, J. and Roth, S.V. and Burghammer, M. and Riekel, C. and Gross, M. and Varnik, F. and Uhlmann, P. and Stamm, ...
    Journal of Physics Condensed Matter 23 (2011)
    The structure and flow of droplets on solid surfaces is investigated with imaging and scattering techniques and compared to simulations. To access nanostructures at the liquid-solid interface advanced scattering techniques such as grazing incidence small-angle x-ray scattering (GISAXS) with micro-and nanometer-sized beams, GISAXS and insitu imaging ellipsometry and GISAXS tomography are used. Using gold nanoparticle suspensions, structures observed in the wetting area due to deposition are probed insitu during the drying of the droplets. After drying, nanostructures in the wetting area and inside the dried droplets are monitored. In addition to drying, a macroscopic movement of droplets is caused by body forces acting on an inclined substrate. The complexity of the solid surfaces is increased from simple silicon substrates to binary polymer brushes, which undergo a switching due to the liquid in the droplet. Nanostructures introduced in the polymer brush due to the movement of droplets are observed. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184111
  • 2011 • 28 Atmospheric pressure Chemical Vapor Infiltration (CVI) for the preparation of biomorphic SiC ceramics derived from paper
    Pflitsch, C. and Curdts, B. and Atakan, B.
    Journal of Nanoscience and Nanotechnology 11 8416-8419 (2011)
    Chemical Vapor Infiltration of biological structures such as paper is used here to produce biomorphic SiC ceramics with high temperature resistance. The biological substrate materials are infiltrated with tetramethylsilane (TMS) at atmospheric pressure and elevated temperatures of 790 °C. A simple tube furnace (hot-wall reactor) is used for the infiltration process. As result, porous SiC-ceramics are grown which are around 20% smaller and 70% lighter than the initial substrates. This can be explained by the pyrolytic reaction of the substrates while heating them up to 790 °C, which is necessary for the infiltration process. Nevertheless, besides the shrinking of the substrates the geometrical form remains nearly unchanged. The resulting materials were heated up to 1000 °C in oxygen atmosphere in order to analyze their resistance against oxidation. After this treatment, all of them were still mechanically stable and of unchanged shape while a further mass loss was observed. This confirms the high temperature stability of the prepared materials. Copyright © 2011 American Scientific Publishers All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5096
  • 2011 • 27 Mechanical properties of thermally sprayed Fe based coatings
    Röttger, A. and Weber, S. and Theisen, W. and Rajasekeran, B. and Vassen, R.
    Materials Science and Technology 27 973-982 (2011)
    An additional coating against wear or corrosion on component parts is required for many applications. These coatings protect the substrate material against external influences, thus increasing the economic lifetime of the component. Coating processes such as build-up welding and thermal spraying are well established and commonly used. The thermal spray process, in particular, permits deposition of metals, ceramics, or cermets materials to produce near net shape coatings on complex surface geometries. However, commonly used coating materials suffer from high raw material costs, thus decreasing the cost effectiveness of the coating process. Fe based materials are low priced and possess noteworthy mechanical properties; they thus provide the possibility of substituting the expensive Ni and Co based materials commonly used for thermal spray processes. In this work, 2 mm thick high velocity oxyfuel sprayed Fe based coatings in the as sprayed and thermally sprayed and hot isostatic pressed condition were investigated with respect to their mechanical and wear properties. Additionally, the fracture surface was investigated by scanning electron microscopy to characterise the fracture behaviour. It could be demonstrated that the substrate and the heat treatment have the greatest impact on the shear strength of thermally sprayed cold work tool steel. It is shown that the substrate materials as well as the heat treatment are promoting diffusion processes across the interface between the coating and the substrate. Hence, a material integrated bond is formed. The microstructures of the thermally sprayed coatings become more important regarding the mechanisms of failure of the four point bending tests. The material strength is influenced by quenching and tempering and the specimen deflection is influenced by diffusion reactions induced by hot isostatic pressing treatment. The thermally sprayed coatings in the as sprayed condition feature the highest wear resistance due to their hardness. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.
    view abstractdoi: 10.1179/1743284710Y.0000000002
  • 2011 • 26 Growth of epitaxial sodium-bismuth-titanate films by metal-organic chemical vapor phase deposition
    Schwarzkopf, J. and Schmidbauer, M. and Duk, A. and Kwasniewski, A. and Anooz, S.B. and Wagner, G. and Devi, A. and Fornari, R.
    Thin Solid Films 520 239-244 (2011)
    The liquid-delivery spin metal-organic chemical vapor phase deposition method was used to grow epitaxial sodium-bismuth-titanate films of the system Bi4Ti3O12 + xNa0.5Bi 0.5TiO3 on SrTiO3(001) substrates. Na(thd), Ti(OiPr)2(thd)2 and Bi(thd)3, solved in toluene, were applied as source materials. Depending on the substrate temperature and the Na/Bi ratio in the gas phase several structural phases of sodium-bismuth-titanate were detected. With increasing temperature and/or Na/Bi ratio, phase transitions from an Aurivillius phase with m = 3 to m = 4 via an interleaved state with m = 3.5, and, finally, to Na0.5Bi 0.5TiO3 with perovskite structure (m = ∞) were established. These phase transitions proceed at remarkably lower temperatures than in ceramics or bulk crystals for which they had been exclusively observed so far. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2011.07.050
  • 2011 • 25 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 • 24 Epitaxial Ag wires with a single grain boundary for electromigration
    Sindermann, S. and Witt, C. and Spoddig, D. and Horn-Von Hoegen, M. and Dumpich, G. and Meyer zu Heringdorf, F.-J.
    Review of Scientific Instruments 82 (2011)
    Test structures for electromigration with defined grain boundary configurations can be fabricated using focused ion beam (FIB). We present a novel approach of combining epitaxial growth of Ag islands with FIB milling. Depending on the growth parameters, bi-crystalline Ag islands can be grown on Si(111) surfaces and can be structured into wires by FIB. To avoid doping effects of the used Ga FIB, silicon on insulator (SOI) substrates are used. By cutting through the device layer of the SOI substrate with deep trenches, the Ag wire can be electrically separated from the rest of the substrate. In this way, Ag wires with one isolated grain boundary of arbitrary direction can be assembled. Using scanning electron microscopy we demonstrate the feasibility of our approach. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3671802
  • 2011 • 23 Thin tantalum films on crystalline silicon - a metallic glass
    Stella, K. and Bürstel, D. and Hasselbrink, E. and Diesing, D.
    Physica Status Solidi - Rapid Research Letters 5 68-70 (2011)
    Thin amorphous tantalum films are prepared on Si(111) substrates in a metallic glassy state. The amorphous monoatomic state of the film is characterized by X-ray diffraction studies. The glassy state leads to a negative temperature coefficient of the resistivity (TCR) for low sample temperatures < 200 K which is attributed to incipient localization. Above 200 K a positive TCR is observed as expected for a normal Boltzmann transport regime. Upon heating the Si substrate to 1200 K TaSi2 is formed out of the amorphous tantalum film and the silicon substrate. The TaSi2 layer is crystalline as evident from X-ray diffraction data. Schematic drawing of the evaporation setup on either glass or silicon samples. Scheme of annealing effects. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201004510
  • 2011 • 22 Infrared spectroscopic ellipsometry (IRSE) and X-ray photoelectron spectroscopy (XPS) monitoring the preparation of maleimide-functionalized surfaces: From Au towards Si (111)
    Sun, G. and Hovestädt, M. and Zhang, X. and Hinrichs, K. and Rosu, D.M. and Lauermann, I. and Zielke, C. and Vollmer, A. and Löchel, H. and Ay, B. and Holzhütter, H.-G. and Schade, U. and Esser, N. and Volkmer, R. and Rappich, J.
    Surface and Interface Analysis 43 1203-1210 (2011)
    The IR ellipsometric technique was used to identify the surface species and to control the preparation of maleimide-terminated surfaces. Because of higher s/n ratios for metallic substrates, the protocol was initially developed on Au surfaces, was later successfully transferred to technologically more relevant Si (111) substrates. The functionalized surfaces were achieved by electrochemical deposition of diazonium linker films and following chemical adsorption steps. Complementary XPS was also employed to detect the surface species in the process of preparation. The immobilization of different functional molecules was proven by interpreting the specific vibrational bands in IR spectra and additionally confirmed by XPS experiments. The surface homogeneity was investigated by FT-IR synchrotron mapping ellipsometry. This work shows that the proposed protocol is an effective pathway to achieve the desired functionalized surfaces. Copyright © 2010 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.3699
  • 2011 • 21 Stability and dynamics of droplets on patterned substrates: Insights from experiments and lattice Boltzmann simulations
    Varnik, F. and Gross, M. and Moradi, N. and Zikos, G. and Uhlmann, P. and Müller-Buschbaum, P. and Magerl, D. and Raabe, D. and Steinbach, I. and Stamm, M.
    Journal of Physics Condensed Matter 23 (2011)
    The stability and dynamics of droplets on solid substrates are studied both theoretically and via experiments. Focusing on our recent achievements within the DFG-priority program 1164 (Nano-and Microfluidics), we first consider the case of (large) droplets on the so-called gradient substrates. Here the term gradient refers to both a change of wettability (chemical gradient) or topography (roughness gradient). While the motion of a droplet on a perfectly flat substrate upon the action of a chemical gradient appears to be a natural consequence of the considered situation, we show that the behavior of a droplet on a gradient of topography is less obvious. Nevertheless, if care is taken in the choice of the topographic patterns (in order to reduce hysteresis effects), a motion may be observed. Interestingly, in this case, simple scaling arguments adequately account for the dependence of the droplet velocity on the roughness gradient (Moradi et al 2010 Europhys. Lett. 8926006). Another issue addressed in this paper is the behavior of droplets on hydrophobic substrates with a periodic arrangement of square shaped pillars. Here, it is possible to propose an analytically solvable model for the case where the droplet size becomes comparable to the roughness scale (Gross et al 2009 Europhys. Lett. 8826002). Two important predictions of the model are highlighted here. (i)There exists a state with a finite penetration depth, distinct from the full wetting (Wenzel) and suspended (Cassie-Baxter, CB) states. (ii)Upon quasi-static evaporation, a droplet initially on the top of the pillars (CB state) undergoes a transition to this new state with a finite penetration depth but then (upon further evaporation) climbs up the pillars and goes back to the CB state again. These predictions are confirmed via independent numerical simulations. Moreover, we also address the fundamental issue of the internal droplet dynamics and the terminal center of mass velocity on a flat substrate. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184112
  • 2011 • 20 Influence of Ti/TiAlN-multilayer designs on their residual stresses and mechanical properties
    Vogli, E. and Tillmann, W. and Selvadurai-Lassl, U. and Fischer, G. and Herper, J.
    Applied Surface Science 257 8550-8557 (2011)
    In this research work, Ti/TiAlN multilayers of various designs were deposited onto substrates pretreated by different etching procedures. The influence of multilayer design and substrate pretreatment on multilayers adhesion, hardness, wear and friction coefficients was systematically analyzed and correlated with residual stresses of these multilayers as well as with residual stresses on the coating-near substrate region, which were analyzed by synchrotron X-ray diffraction at HZB-BESSYII. These investigations show that the adhesion can be improved by a specific etching procedure, which cause increased compressive stress in the coating-near the substrate region. Additionally, it was found, that the multilayer with the thickest ceramic layers has the highest hardness and the lowest wear coefficients as well as the lowest compressive residual stress within studied multilayers. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2011.05.013
  • 2011 • 19 Picosecond laser direct patterning of poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate) (PEDOT:PSS) thin films
    Xiao, S. and Abreu Fernandes, S. and Esen, C. and Ostendorf, A.
    Journal of Laser Micro Nanoengineering 6 249-254 (2011)
    Laser selective micro patterning of thin film of Poly (3,4-ethylenedioxythiophene):Poly (styrene sulfonate) (PEDOT:PSS) blend is investigated. Picosecond pulsed laser with 355 nm and 1064 nm wavelengths are used to study the ablation behavior of PEDOT:PSS thin film coated on glass. We present and discuss ablation thresholds for different film thicknesses as well as ablation lines with different overlapping rates. The results observed by SEM and white-light interference microscopy reveal that PEDOT:PSS film on glass substrates can be selectively patterned by optimized laser parameters.
    view abstractdoi: 10.2961/jlmn.2011.03.0015
  • 2011 • 18 Selective patterning of ITO on flexible PET Substrate by 1064nm picosecond Laser
    Xiao, S. and Fernandes, S.A. and Ostendorf, A.
    Physics Procedia 12 125-132 (2011)
    Pulsed picosecond laser ablation of indium tin oxide (ITO) coated on flexible polyethylene terephthalate (PET) substrate has been investigated. Pulses with 355 nm, 532 nm and 1064 nm wavelengths are performed for comparison. Laser irradiation from the front side and the back side are analyzed. The single pulse ablation threshold is found at 0.56 J/cm2 from the study of the spot sizes varied according to pulse fluence. The overlap rate in multi pulses ablation of ITO layer is discussed during the scribing of lines. Optical microscopy, SEM, surface stylus and EDX analysis reveal that the ITO layer can be completely removed with little damage of the PET substrate using optimized parameters. © 2011 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.phpro.2011.03.114
  • 2010 • 17 Dissolution kinetics of Si into Ge (111) substrate on the nanoscale
    Balogh, Z. and Erdélyi, Z. and Beke, D.L. and Wiedwald, U. and Pfeiffer, H. and Tschetschetkin, A. and Ziemann, P.
    Thin Solid Films 519 952-955 (2010)
    In this paper we present experiments and simulations on the dissolution of Si into single crystalline Ge(111) substrates. The interface shift during the dissolution was tracked by X-ray Photoelectron Spectroscopy. It was obtained that the interface remained sharp and shifted according to anomalous kinetics similarly to our previous measurement in the Si/amorphous-Ge system. The interface shift, x, can be described by a power function of time x ∞ t kc with a kinetic exponent, kc, of 0.85 ± 0.1, larger than the one measured for the amorphous system (0.7 ± 0.1). Both exponents, however, are different from the kc = 0.5 Fickian (parabolic) value and it is interpreted as a nanoscale diffusional anomaly caused by the strong composition dependence of the diffusion coefficients. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.08.146
  • 2010 • 16 Structural, static and dynamic magnetic properties of Co2MnGe thin films on a sapphire a-plane substrate
    Belmeguenai, M. and Zighem, F. and Chauveau, T. and Faurie, D. and Roussigń, Y. and Ch́rif, S.M. and Moch, P. and Westerholt, K. and Monod, P.
    Journal of Applied Physics 108 (2010)
    Magnetic properties of Co2 MnGe thin films of different thicknesses (13, 34, 55, 83, 100, and 200 nm), grown by rf sputtering at 400 °C on single crystal sapphire substrates, were studied using vibrating sample magnetometry and conventional or microstrip line ferromagnetic resonance. Their behavior is described assuming a magnetic energy density showing twofold and fourfold in-plane anisotropies with some misalignment between their principal directions. For all the samples, the easy axis of the fourfold anisotropy is parallel to the c -axis of the substrate while the direction of the twofold anisotropy easy axis varies from sample to sample and seems to be strongly influenced by the growth conditions. Its direction is most probably monitored by the slight unavoidable miscut angle of the Al2 O 3 substrate. The twofold in-plane anisotropy field Hu is almost temperature independent, in contrast with the fourfold field H 4 which is a decreasing function of the temperature. Finally, we study the frequency dependence of the observed line-width of the resonant mode and we conclude to a typical Gilbert damping constant α value of 0.0065 for the 55-nm-thick film. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3475501
  • 2010 • 15 Adhesion energies of Cr thin films on polyimide determined from buckling: Experiment and model
    Cordill, M.J. and Fischer, F.D. and Rammerstorfer, F.G. and Dehm, G.
    Acta Materialia 58 5520-5531 (2010)
    For the realization of flexible electronic devices, the metal-polymer interfaces upon which they are based need to be optimized. These interfaces are prone to fracture in such systems and hence form a weak point. In order to quantify the interfacial adhesion, novel mechanical tests and modeling approaches are required. In this study, a tensile testing approach that induces buckling of films by lateral contraction of the substrate is employed to cause delamination of the film. Based on a newly developed energy balance model, the adhesion energy of Cr films on polyimide substrates is determined by measuring the buckle geometry induced by the tensile test. The obtained minimum values for the adhesion energy (about 4.5 J m-2) of 50-190 nm thick films compare well to those found in the literature for metal films on polymer substrates. © 2010 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.actamat.2010.06.032
  • 2010 • 14 Fracture and delamination of chromium thin films on polymer substrates
    Cordill, M.J. and Taylor, A. and Schalko, J. and Dehm, G.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 870-875 (2010)
    New emerging technologies in the field of flexible electronic devices require that metal films adhere well and flex with polymer substrates. Common thin film materials used for these applications include copper (Cu) with an adhesion interlayer of chromium (Cr). Copper can be quite ductile and easily move with the polymer substrate. However, Cr is more brittle and fractures at lower strains than Cu. This study aims to examine the fracture and subsequent buckling and delamination of strained Cr films on polyimide (PI). In-situ scanning electron microscope (SEM) straining is used to systematically study the influence of film thickness on fracture and buckling strains. Film fracture and delamination depend on film thickness, and increases in crack and buckle density with decreasing thickness are explored by a shear lag model. © 2009 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-009-9988-9
  • 2010 • 13 Study of Ni2 -Mn-Ga phase formation by magnetron sputtering film deposition at low temperature onto Si substrates and LaNi O3 /Pb (Ti,Zr) O3 buffer
    Figueiras, F. and Rauwel, E. and Amaral, V.S. and Vyshatko, N. and Kholkin, A.L. and Soyer, C. and Remiens, D. and Shvartsman, V.V. and Borisov, P. and Kleemann, W.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 28 6-10 (2010)
    Film deposition of Ni2 MnGa phaselike alloy by radio frequency (rf) magnetron sputtering was performed onto bare Si(100) substrates and LaNi O3 /Pb (Ti,Zr) O3 (LNO/PZT) ferroelectric buffer layer near room temperature. The prepared samples were characterized using conventional x-ray diffraction (XRD), superconducting quantum interference device, and electron dispersive x-ray spectroscopy from scanning electron microscope observations. The optimized films deposited under high rf power and low argon pressure present good surface quality and highly textured phase crystallization. The positioning distance between the substrate and the target-holder axis has some limited effect on the film's composition due to the specific diffusion behavior of each element in the sputtering plasma. Extended four pole high resolution XRD analysis allowed one to discriminate the intended Ni-Mn-Ga tetragonal martensitic phase induced by the (100) LNO/PZT oriented buffer. This low temperature process appears to be very promising, allowing separate control of the functional layer's properties, while trying to achieve high electromagnetoelastic coupling. © 2010 American Vacuum Society.
    view abstractdoi: 10.1116/1.3256200
  • 2010 • 12 Small droplets on superhydrophobic substrates
    Gross, M. and Varnik, F. and Raabe, D. and Steinbach, I.
    Physical Review E - Statistical, Nonlinear, and Soft Matter Physics 81 (2010)
    We investigate the wetting behavior of liquid droplets on rough hydrophobic substrates for the case of droplets that are of comparable size to the surface asperities. Using a simple three-dimensional analytical free-energy model, we have shown in a recent letter that, in addition to the well-known Cassie-Baxter and Wenzel states, there exists a further metastable wetting state where the droplet is immersed into the texture to a finite depth, yet not touching the bottom of the substrate. Due to this new state, a quasistatically evaporating droplet can be saved from going over to the Wenzel state and instead remains close to the top of the surface. In the present paper, we give an in-depth account of the droplet behavior based on the results of extensive computer simulations and an improved theoretical model. In particular, we show that releasing the assumption that the droplet is pinned at the outer edges of the pillars improves the analytical results for larger droplets. Interestingly, all qualitative aspects, such as the existence of an intermediate minimum and the "reentrant transition," remain unchanged. We also give a detailed description of the evaporation process for droplets of varying sizes. Our results point out the role of droplet size for superhydrophobicity and give hints for achieving the desired wetting properties of technically produced materials. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevE.81.051606
  • 2010 • 11 A comparative study of (Fe, Fe3Si)/GaAs and Heusler/MgO for spintronics applications
    Grünebohm, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Entel, P.
    Journal of Physics: Conference Series 200 (2010)
    Many Heusler-like compounds as well as elemental Fe show large spin polarization in their bulk phase, which makes them possible candidates for spintronic applications. However, it has turned out that the magnetic properties may change significantly if the ferromagnet is grown on a semiconductor. Here, we investigate from first principles the magnetic properties of thin Fe, Fe 3Si, and Heusler films on GaAs(110) and MgO(001) substrates, which show rather flat interfaces due to the absence of reconstruction. We observe high spin polarization for the Fe containing systems and both Heusler alloys considered here, even for ultra thin layers. These systems are mainly unaffected by the substrate. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/200/7/072038
  • 2010 • 10 Optical characterisation of BiFeO3 epitaxial thin films grown by pulsed-laser deposition
    Himcinschi, C. and Vrejoiu, I. and Friedrich, M. and Ding, L. and Cobet, C. and Esser, N. and Alexe, M. and Zahn, D.R.T.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 296-299 (2010)
    Epitaxial thin films of bismuth ferrite, BiFeO3, were deposited by pulsed laser deposition (PLD) on SrTiO3 (100), Nbdoped SrTiO 3 (100) and DyScO3 (110) substrates. Ellipsometric spectra are obtained in the energy range 0.73-9.5 eV by combining Variable Angle Spectroscopic Ellipsometry (VASE) and vacuum ultraviolet (VUV) ellipsometry with synchrotron radiation. The optical constants of BiFeO3 films were determined by analysing the ellipsometric spectra with a model that describes the optical response of a system consisting of air, film and substrate. The shift towards higher energies of the refractive index and extinction coefficient of the film deposited onto Nb-doped SrTiO3 as compared with that deposited onto DyScO3 was attributed to a more compressive in-plane epitaxial strain. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.200982414
  • 2010 • 9 Photothermal micro- and nanopatterning of organic/silicon interfaces
    Klingebiel, B. and Scheres, L. and Franzka, S. and Zuilhof, H. and Hartmann, N.
    Langmuir 26 6826-6831 (2010)
    Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at λ = 514 nm and a 1/e2 spot diameter of 2.6 μm results in local decomposition of the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for subwavelength patterning, with line widths between 0.4 and 1.4 μm. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 nm are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g., for hybrid microelectronic and sensor applications. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/la903926z
  • 2010 • 8 Fabrication and RF performance of InAs Nanowire FET
    Prost, W. and Tegude, F.J.
    Device Research Conference - Conference Digest, DRC 279-282 (2010)
    Nanowires can excellently be controlled during synthesis with respect to physical and chemical characteristics, including composition, size, electronic and optical properties. They may be used both as devices and interconnects, and thus can open doors for downscaled integration concepts not seen before. The non-lithographic bottom up synthesis approach on the nanoscale may be extremely cost-effective, especially when making use of the large material diversity stemming from decoupling of device from substrate material without loss of structural quality, e.g. growing metallic, Ge or III-V nanowires on Si substrates. Going down to very small dimensions one may make use of quantum confinement effects like reduced phonon scattering and related high carrier mobility, tunable electrical and optical properties, or implementing heterostructures for quantum dot and single electron devices. © 2010 IEEE.
    view abstractdoi: 10.1109/DRC.2010.5551958
  • 2010 • 7 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 • 6 X-ray diffraction residual stress analysis on PVD-multilayer coatings
    Tillmann, W. and Selvadurai-Laßl, U. and Vogli, E. and Fischer, G. and Hoffmann, F.
    Materialwissenschaft und Werkstofftechnik 41 482-488 (2010)
    Hard and wear resistant thin layers provides significant improvements in tools employed in manufacturing industry and are recently of great interest to increase as well as to enhance the tools? performance and lifetime. Ceramic PVD-layers already feature a high hardness combined with a high abrasive wear resistance. However, such layers possess only a limited lifetime due to their low toughness. To increase the toughness as well as the durability of such layers multilayer systems are steadily developed. This research work presents the first results of the influence of the steel substrate pretreatments, multilayer designs and coating process parameters on the properties of two innovative PVD metal-ceramic multilayers, Ti/TiAIN and Cr/CrAIN. It was obviously that the substrate pretreatment and the multilayer design have a large influence on the layer properties and residual stresses. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201000630
  • 2010 • 5 Nanostructured bionic PVD-coatings for forming tools
    Tillmann, W. and Vogli, E. and Herper, J. and Haase, M.
    Key Engineering Materials 438 41-48 (2010)
    It is very important to minimize wear and friction in forming processes in order to avoid adhesion between work piece and tool. For the realization of these requirements, the PVD-coating system CrAlN was deposited and tested on substrates made from high speed steel 1.3343 by means of a reactive sputter process. The coatings were deposited as single- and multilayers with a metallic Crinterlayer. Prior to realizing the appropriate coating design, the substrates were pre-structured. For this purpose natural surfaces were used as a pattern and tested in this research work. The skin of an insect serves as a model and its fine structures were reproduced on the substrate surface by milling. The generated specimens were analyzed with a scanning electron microscope, nanoindenter and ball-on-disc tester to compare the performance of the coating systems. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/KEM.438.41
  • 2010 • 4 Influence of substrate nitriding on adhesion, friction and wear resistance of DLC (diamond-like carbon)-coatings
    Tillmann, W. and Vogli, E. and Hoffmann, F. and Kemdem, P.
    Key Engineering Materials 438 211-218 (2010)
    Since diamond like carbon layers feature excellent mechanical and tribological behavior under defined environmental circumstances, they are well established in a wide field of industrial and automotive applications in the last decade. However, the pretreatment of the substrate plays also an important role in supporting and enforcing the excellent properties of the coatings. This work analyses the effect of the plasma nitrided cold working steel substrate (80CrV2) on the adhesion, friction and wear resistance of DLC-coatings and compares it to the performance of DLC-coatings applied on a non-hardened substrate material. Therefore the grinded and polished specimens were nitrogen-hardened in an Arc-PVD (Physical Vapor Deposition)-device before the DLC-coating was applied in a Magnetron Sputter-PVD-process. In order to measure the hardness of the thin film coating, a nanoindenter was used. The adhesion was tested with a scratch tester and the wear resistance was measured by using a Ball-on-disc-tester. A 3D-profilometer and a SEM (Scanning Electron Microscope) were utilized to analyze the scratches and wear tracks on the samples. With these results correlations between the substrate nitriding and the mechanical and tribological performance of the DLC-coating were made. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/KEM.438.211
  • 2010 • 3 Effect of plasma treatment on adhesion of DLC layers to steels
    Tillmann, W. and Vogli, E. and Momeni, S.
    Materials Science Forum 638-642 812-817 (2010)
    Diamond like carbon layers play a key role in industrial applications. However the layers quality deteriorates often due to insufficient interfacial adhesion. In this research work a prior plasma treatment of steel substrates was employed aimed to improve the interfacial adhesion of DLC-layers to steel substrates. Three different kinds of steels were employed and their microstructures as well as their compositions before and after plasma treatment were analyzed. The interfacial adhesion of DLC layers on the non-nitrided and nitrided steels was observed and the influence of the steel microstructure on the interfacial adhesion was studied. © (2010) Trans Tech Publications.
    view abstractdoi: 10.4028/www.scientific.net/MSF.638-642.812
  • 2010 • 2 Active transport of amino acids by a guanidiniocarbonyl-pyrrole receptor
    Urban, C. and Schmuck, C.
    Chemistry - A European Journal 16 9502-9510 (2010)
    Herein we report the synthesis and characterization of a transporter 9 for N-acetylated amino acids. Transporter 9 is a conjugate of a guanidiniocarbonyl pyrrole cation, one of the most efficient carboxylate binding motifs reported so far, and a hydrophobictris (dodecylbenzyl) group, which ensures solubility in organic solvents. In its protonated form, 9 binds N-acetylated amino acid carboxylates in wetorganic solvents with association constantsin the range of 104m-1 as estimated by extraction experiments. Aromatic amino acids are preferred due to additional cation-α-interactions of the amino acid side chain with the guanidiniocarbonyl pyrrole moiety. U-tube experiments established efficient transport across a bulk liquid chloroform phase with fluxes approaching 10-6 molm-2s-1. In experiments with single substrates, the release rate of the amino acid from the receptor-substrate complex at the interface with the receiving phase is rate determining. In contrast to this, in competition experiments with several substrates, the therm-odynamic affinity to 9 becomes decisive. As 9 can only transport anions in its protonated form and has a pKa value of approximately 7, pH-driven active transport of amino acids is also possible. Transport occurs as a symport of the amino acid carboxylate and a proton. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201000509
  • 2010 • 1 Gas-phase synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates
    Xia, W. and Mei, B. and Muhler, M.
    Nanoscience and Nanotechnology Letters 2 1-6 (2010)
    Despite the advances in high throughput experimentation in recent years the synthesis of realistic catalyst libraries especially gradient catalyst libraries remains as a challenge in material science. Recently, we have developed a method for the synthesis of gradient catalyst libraries consisting of nanoparticles supported on high surface area porous substrates. Chemical vapor deposition (CVD) was employed as a gas-phase method for the synthesis. The method made use of the lateral concentration profile of the precursor-loaded carrier gas stream during CVD, resulting in concentration profile of the deposits on porous substrates. In this report, high surface area materials of both powders (e.g., silica) and bulk composites (e.g., hierarchical carbon structures) were successfully employed as substrates for the deposition of single metal or bimetallic catalyst libraries. The synthesis was achieved by controlling the flow behavior of the effluent precursor stream. The resulting effusion cone led to a radial deposition gradient on the substrate. Different from thin film-type model catalyst libraries, the obtained catalysts can be tested under realistic reaction conditions. Methanol oxidation was studied as a test reaction using scanning mass spectrometry. Copyright © 2010 American Scientific Publishers.
    view abstractdoi: 10.1166/nnl.2010.1046