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 • 243 Software-in-the-loop optimization of actuator and sensor placement for a smart piezoelectric funnel-shaped inlet of a magnetic resonance imaging tomograph
    Nestorović, T. and Hassw, K. and Oveisi, A.
    Mechanical Systems and Signal Processing 147 (2021)
    Performance of smart piezoelectric structures strongly depends on placement of integrated piezoelectric actuators and sensors, which may be implemented in the form of thin film layers on the structure surface or embedded within the structure. In both cases actuator and sensor placement plays an important role, since after applying they remain permanently integrated with structure. In this paper the optimization procedure for piezoelectric structures with curved surfaces is proposed based on the Software-in-the-Loop (SiL) methodology and balanced modal order reduction in combination with H2 and H∞ norms used in placement indices. The optimization procedure is a global one, since it seeks for optima across the entire domain of the structure. A special challenge is tackling the problem of curved surfaces. This problem is solved in this work for a funnel shaped structure – inlet of the magnetic resonance imaging tomopraph. A thorough mesh convergence study with respect to the eigenfrequency analysis is performed in order to obtain a reliable numeric finite element model for further optimization purposes. Material parameter optimization is performed as well. Based on placement indices optimal placement study is performed under consideration of several eigenmodes of interest. The optimization is performed for individual modes as well as for simultaneous consideration of multiple modes. The SiL approach with recurrent communication in each iteration of the optimization between the numerical simulation FE software and optimization tool designed in Python is implemented through the evaluation of the placement indices for candidate locations over the entire curved surface of the structure. Depending on support conditions, optimal locations of piezoelectric actuators and sensors are proposed. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.ymssp.2020.107097
  • 2020 • 242 Preparation of antibiofouling nanocomposite PVDF/Ag-SiO2 membrane and long-term performance evaluation in the MBR system fed by real pharmaceutical wastewater
    Ahsani, M. and Hazrati, H. and Javadi, M. and Ulbricht, M. and Yegani, R.
    Separation and Purification Technology 249 (2020)
    In this work, the Ag-SiO2 nanoparticles were successfully synthesized and their antibacterial property was confirmed using the plate colony counting method. The SiO2 and Ag-SiO2 nanoparticles were used to prepare the PVDF/SiO2 and PVDF/Ag-SiO2 nanocomposite membranes, respectively. Pure water flux, contact angle and mechanical strength measurement analyses were conducted to characterize and compare the performance of the neat and nanocomposite membranes. Moreover, in order to investigate the structure of the prepared membranes scanning electron microscope (SEM) was used to obtain surface and cross-section images. A long-term filtration test was carried out in a bench scale submerged membrane bioreactor (MBR) system, fed by real pharmaceutical wastewater, to evaluate the antibiofouling performance of the prepared neat and nanocomposite membranes. In comparison to the neat PVDF, the pure water flux of the nanocomposite PVDF membrane (PVDF/Ag-SiO2; 0.6 wt%) increased about 60% and the water contact angle decreased from about 99° to 89°. The obtained results showed that the nanocomposite PVDF/Ag-SiO2 membrane exhibits considerable antibiofouling properties such that the accumulated dried biofilm as well as the extra cellular polymeric substances (EPSs) collected from the cake layer decreased considerably for the nanocomposite membrane. Moreover, the flux recovery ratio increased from 58% for the neat PVDF membrane to 76% for the nanocomposite PVDF/Ag-SiO2 membrane. The excitation and emission matrix (EEM) fluorescence spectroscopy analysis revealed that the accumulated protein on the surface of nanocomposite membranes decreased considerably in a way that the peak corresponding to tryptophan protein-like substances diminished completely, indicating the high antibiofouling potential of nanocomposite membranes. The chemical oxygen demand (COD) and ammonium removal efficiencies of the neat and nanocomposite membranes were higher than 90% and 95%, respectively, indicating negligible impact of the membrane modification on the effluents’ quality. © 2020
    view abstractdoi: 10.1016/j.seppur.2020.116938
  • 2020 • 241 Improvement of Fatigue Strength in Lightweight Selective Laser Melted Alloys by In-Situ and Ex-Situ Composition and Heat Treatment
    Awd, M. and Johannsen, J. and Chan, T. and Merghany, M. and Emmelmann, C. and Walther, F.
    Minerals, Metals and Materials Series 115-126 (2020)
    Selective laser melting is a powder-bed-fusion process that is applied to different alloys. Thus, it is essential to study what are the different process variables that affect the static, quasi-static, and cyclic mechanical properties. In this contribution, two examples of alloys are introduced: AlSi (AlSi12, AlSi10Mg) and Ti-6Al-4V. The influence of controlled cooling and degassing mechanisms of residual gases is investigated by structural analysis in electron microscopy and X-ray computed tomography. Controlled cooling through platform heating or multi-exposure treatments increased the dendritic width in AlSi alloys and decomposed alpha prime in Ti-6Al-4V. The alteration was a cause for enhanced ductility and slowing of crack propagation. The cyclic deformation is tracked during mechanical testing and is simulated in FE software using a high-throughput methodology to calculate Woehler curves based on Fatemi-Socie damage parameters. The cyclic deformation simulation is in agreement with the experimental data and quantified cyclic damage using Fatemi-Socie parameters. © 2020, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-36296-6_11
  • 2020 • 240 An investigation on burner rig testing of environmental barrier coatings for aerospace applications
    Bakan, E. and Mack, D.E. and Lobe, S. and Koch, D. and Vaßen, R.
    Journal of the European Ceramic Society (2020)
    In this study, burner rig testing of Si/Yb2Si2O7 environmental barrier coating protected SiC-based ceramic matrix composites was conducted. Tests were performed at standard conditions as well as with liquid water injection to the flame. Furthermore, the influence of the impingement angle of the flame (45° vs. 90°) on water vapor corrosion was explored. Gas flow rates were adapted in each test to adjust 1250 °C at the sample surface. The comparison of test results showed that water injection advances the corrosion of the Yb2Si2O7 topcoat and the impingement angle affects the size and shape of the corroded area on the sample surface. © 2020 Elsevier Ltd
    view abstractdoi: 10.1016/j.jeurceramsoc.2020.06.016
  • 2020 • 239 Spatially controlled VLS epitaxy of gallium arsenide nanowires on gallium nitride layers
    Blumberg, C. and Liborius, L. and Ackermann, J. and Tegude, F.-J. and Poloczek, A. and Prost, W. and Weimann, N.
    CrystEngComm 22 1239-1250 (2020)
    We present Au catalyzed p-GaAs nanowire growth on n-GaN layers as a possible method to grow an arsenide on a nitride compound semiconductor by metal organic vapor phase epitaxy. The GaAs growth position, the nanowire density and the nanowire growth direction are controlled by a combination of vapor-liquid-solid growth and selective area epitaxy. Thus, a spatially controlled nanowire growth is attained, which is mandatory for device fabrication. The growth position is defined by lithographically positioned Au discs on n-GaN. By adapting the growth conditions (QTBAs, presaturation) the nanowire density is optimized. Lateral and vertical anisotropic nanowire growth is attained through VLS growth in structured SiOx openings. Critical technological parameters for successful control of the growth direction are the positioning of the Au catalyst in relation to the SiOx mask, the size of the eutectic in relation to the opening dimensions, and the SiOx thickness. These results lead to distinct pn-junction positions and adjustable nanowire growth dimensions and directions. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ce01926j
  • 2020 • 238 Plasma-assisted gas-phase synthesis and in-line coating of silicon nanoparticles
    Dasgupta, M. and Fortugno, P. and Wiggers, H.
    Plasma Processes and Polymers 17 (2020)
    This study investigates the feasibility of plasma-supported in-line functionalization of silicon nanoparticles (NPs) in an atmospheric pressure gas-phase reactor. The approach utilizes the synthesis of core silicon NPs and their subsequent coating downstream of the particle formation zone. In-line coating is accomplished with a cylindrical coating nozzle to achieve homogenous mixing of coating precursor vapors with in-coming NPs. Multiple siloxanes were tested for their coating suitability and their ability towards coating homogeneity. It was found that tetraethyl orthosilicate is favored for thin layers consisting of almost pure silica while hexamethyldisiloxane and octamethylcyclotetrasiloxane (OMCTS) coatings contained reasonable amounts of hydrocarbons. Moreover, OMCTS showed a pronounced tendency towards homogeneous nucleation, thus leading to the additional formation of silica NPs due to homogeneous nucleation. © 2020 The Authors. Plasma Processes and Polymers published by WILEY-VCH Verlag GmbH & Co. KGaA
    view abstractdoi: 10.1002/ppap.201900245
  • 2020 • 237 Thermally Induced Crossover from 2D to 1D Behavior in an Array of Atomic Wires: Silicon Dangling-Bond Solitons in Si(553)-Au
    Hafke, B. and Brand, C. and Witte, T. and Sothmann, B. and Horn-Von Hoegen, M. and Erwin, S.C.
    Physical Review Letters 124 (2020)
    The self-assembly of submonolayer amounts of Au on the densely stepped Si(553) surface creates an array of closely spaced "atomic wires" separated by 1.5 nm. At low temperature, charge transfer between the terraces and the row of silicon dangling bonds at the step edges leads to a charge-ordered state within the row of dangling bonds with ×3 periodicity. Interactions between the dangling bonds lead to their ordering into a fully two-dimensional (2D) array with centered registry between adjacent steps. We show that as the temperature is raised, soliton defects are created within each step edge. The concentration of solitons rises with increasing temperature and eventually destroys the 2D order by decoupling the step edges, reducing the effective dimensionality of the system to 1D. This crossover from higher to lower dimensionality is unexpected and, indeed, opposite to the behavior in other systems. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.124.016102
  • 2020 • 236 Optimization of Transparent Passivating Contact for Crystalline Silicon Solar Cells
    Kohler, M. and Finger, F. and Rau, U. and Ding, K. and Pomaska, M. and Zamchiy, A. and Lambertz, A. and Duan, W. and Lentz, F. and Li, S. and Smirnov, V. and Kirchartz, T.
    IEEE Journal of Photovoltaics 10 46-53 (2020)
    A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO2/μc-SiC:H(n)] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the μc-SiC:H(n) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of μc-SiC:H(n) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO2/μc-SiC:H(n) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm2. All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization. © 2011-2012 IEEE.
    view abstractdoi: 10.1109/JPHOTOV.2019.2947131
  • 2020 • 235 Vibrational Raman spectroscopy on adsorbate-induced low-dimensional surface structures
    Speiser, E. and Esser, N. and Halbig, B. and Geurts, J. and Schmidt, W.G. and Sanna, S.
    Surface Science Reports 75 (2020)
    Low-dimensional self-organized surface structures, induced by (sub)monolayer metal adsorbates on semiconductor surfaces may give rise not only to a variety of emergent electronic properties, but also to a multitude of specific localized vibronic features. The focus of this review is on the analysis of these novel surface vibration eigenmodes. The application of in situ surface Raman spectroscopy under UHV conditions on clean semiconductor surfaces and those with self-ordered adsorbates, in close conjunction with the calculations of Raman spectra, based on the first-principles determination of the structural, electronic and vibronic properties, allows a consistent determination of the vibration eigenfrequencies, symmetry properties, and elongation patterns of the systems of interest. The localized nature of the surface eigenmodes determines the surface sensitivity, independent of the large penetration depth of light. The surface contribution can be selectively enhanced by employing resonance conditions to surface electronic transitions. Moreover, surface and bulk contributions can be separated by taking difference spectra between various stages of surface preparation. The relevant surfaces are Ge and especially Si with different orientations ((111) and vicinal (hhk)), on which the adsorption of various metals (Au, Sn, Pb, or In) gives rise to two- and quasi-one-dimensional structures (e.g. Au-(5 × 2)/Si(111)) with a variety of vibration modes. The Raman analysis of these modes not only enables the distinction between different proposed structural models (e.g. for Au-(3×3)/Si(111)), but also gives access to the role of electron-phonon coupling in structural phase transitions (e.g. for In-(8 × 2)–(4 × 1)/Si(111)). © 2020 The Authors
    view abstractdoi: 10.1016/j.surfrep.2020.100480
  • 2020 • 234 Surface Properties of Battery Materials Elucidated Using Scanning Electrochemical Microscopy: The Case of Type I Silicon Clathrate
    Tarnev, T. and Wilde, P. and Dopilka, A. and Schuhmann, W. and Chan, C.K. and Ventosa, E.
    ChemElectroChem 7 665-671 (2020)
    Silicon clathrates have attracted interest as potential anodes for lithium-ion batteries with unique framework structures. However, very little is known about the surface reactivity and solid electrolyte interphase (SEI) properties of clathrates. In this study, operando scanning electrochemical microscopy (SECM) is used to investigate the effect of pre-treatment on the formation dynamics and intrinsic properties of the SEI in electrodes prepared from type I Ba8Al16Si30 silicon clathrates. Although X-ray photoelectron spectroscopy (XPS) analysis does not reveal large changes in SEI composition, it is found through SECM measurements that ball-milling combined with chemical acid/base etching of the clathrates lead to a more stable and rapidly formed SEI as compared to purely ball-milled samples, resulting in enhanced coulombic efficiency. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201901688
  • 2020 • 233 Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe11.4Si1.6 H1.6
    Terwey, A. and Gruner, M.E. and Keune, W. and Landers, J. and Salamon, S. and Eggert, B. and Ollefs, K. and Brabänder, V. and Radulov, I. and Skokov, K. and Faske, T. and Hu, M.Y. and Zhao, J. and Alp, E.E. and Giacobbe, C. and G...
    Physical Review B 101 (2020)
    We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe11.4Si1.6H1.6 in terms of the vibrational (phonon) density of states (VDOS), which we determined with Fe57 nuclear resonant inelastic X-ray scattering (NRIXS) measurements and with density functional theory (DFT) based first-principles calculations in the ferromagnetic (FM) low-temperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between nonhydrogenated and hydrogenated samples. This shows that hydrogen not only shifts the temperature of the first-order phase transition, but also affects the elastic response of the Fe subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe-specific vibrational entropy ΔSlat across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and the hydrogen-free compounds. Although reduced by 50% as compared to the hydrogen-free system, the measured change ΔSlat of (3.2±1.9)JkgK across the FM-to-PM transition contributes with ∼35% significantly and cooperatively to the total isothermal entropy change ΔSiso. Hydrogenation is observed to induce an overall blueshift of the Fe VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of LaFe11.4Si1.6H1.6 was determined from the NRIXS and the DFT data. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.064415
  • 2020 • 232 Self-assembled nano-silicon/graphite hybrid embedded in a conductive polyaniline matrix for the performance enhancement of industrial applicable lithium-ion battery anodes
    Wiggers, H. and Sehlleier, Y.H. and Kunze, F. and Xiao, L. and Schnurre, S.M. and Schulz, C.
    Solid State Ionics 344 (2020)
    Nano-structured silicon-based composite materials have generated significant excitement for use as anode materials in high-performance Li-ion batteries. For making these materials commercially applicable, a high Coulombic efficiency at the first cycle must be achieved. Additionally, scalable synthesis routes need to be developed to provide access to practically-relevant material quantities. In this work, we propose a strategy for the production of Si/graphite/polyaniline (Si/graphite/PANI) composites that addresses both above mentioned challenges. Si nanoparticles were produced in a pilot-plant-scale microwave-plasma reactor using monosilane (SiH4) as precursor. This process enables the formation of high-purity Si nanoparticles with controllable crystal sizes at a production rate of 45 g/h. Si/graphite hybrids are fabricated through self-assembly by electrostatic attraction. The Si/graphite/PANI nanocomposite is then prepared by in situ polymerization of aniline monomer in the presence of the Si/graphite hybrid. With this approach, ~40 g of Si/graphite/PANI composite per batch can be produced at lab scale. The scalability of the underlying processes enables the use for commercial products. The nanocomposite shows favorable characteristics inherited from its three components: Si nanoparticles provide high capacity, graphite acts as an electrical conductor and gives a high Coulombic efficiency, and the polyaniline coating further enhances the electrical conductivity and protects the entire structure. A very good Coulombic efficiency of 86.2% at the initial cycle is recorded for this nanocomposite material. Galvanostatic charge/discharge tests demonstrate that this material can deliver a discharge capacity of 2000 mAh/g with a very good capacity retention of 76% after 500 cycles at a discharge rate of 0.5C (1.25 A/g). The capacity is 870 mAh/g measured at 5C (12.5 A/g). © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.ssi.2019.115117
  • 2020 • 231 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 Saénz, J.J. and Maier, S.A. and Stefani, F.D.
    Proceedings of SPIE - The International Society for Optical Engineering 11297 (2020)
    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 by tuning the light wavelength to their size-dependent magnetic dipolar resonance. © 2020 SPIE.
    view abstractdoi: 10.1117/12.2539265
  • 2019 • 230 Effect of mg and si content in aluminum alloys on friction surfacing processing behavior
    Ehrich, J. and Roos, A. and Hanke, S.
    Minerals, Metals and Materials Series 357-363 (2019)
    Friction surfacing (FS) coating layers are generated through severe plastic deformation (SPD) at elevated temperatures (≈0.8 Tmelt). Alloying elements in metals affect heat generation and dynamic recrystallization kinetics during SPD, and therefore require significant adjustments of FS processing conditions. In this study, custom made Aluminum alloys (AA 6060 with additions of 2 and 3.5 wt% Mg, and 6.6, 10.4 and 14.6 wt% Si) were processed by FS. It was found that for the high-Mg Aluminum alloys especially the rotational speeds require a downward adaption to achieve a steady state process. A higher content of Mg results in a reduced rate of thermal softening and more efficient heat generation. With regard to the plasticization behavior during FS, the high amount of hard phases in the high-Si alloys was expected to cause additional friction and increase heat generation. However, as the Si content increases, the process temperatures decrease. Influences of Mg and Si content on material efficiency and coating dimensions were evaluated and discussed. © 2019, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-05864-7_45
  • 2019 • 229 A numerical analysis of a microwave induced coaxial surface wave discharge fed with a mixture of oxygen and hexamethyldisiloxane for the purpose of deposition
    Kemaneci, E. and Mitschker, F. and Benedikt, J. and Eremin, D. and Awakowicz, P. and Brinkmann, R.P.
    Plasma Sources Science and Technology 28 (2019)
    A microwave induced coaxial surface wave discharge with a feeding gas mixture of oxygen and hexamethyldisiloxane used for the deposition of polymer coatings is numerically analysed by a volume-averaged zero-dimensional modelling formalism. A set of edge-to-centre ratios are analytically estimated for a self-consistent description of the positive ion and reactive neutral flux at the radial walls (Kemaneci et al 2017 J. Phys. D: Appl. Phys. 50 245203). The simulation results are compared with the measurements of a wide variety of distinct particle concentrations as well as of the electron temperature and an agreement is obtained with respect to the input power, the pressure and the oxygen to hexamethyldisiloxane flow ratios. The net charge density is dominated by Si2OC5H15 + with a negligible degree of electronegativity. Hexamethyldisiloxane is fragmented into methyl radical via the electron impact dissociation and the dissociative ionization. Large amounts of hydrocarbons, water, carbon monoxide, carbon dioxide and hydrogen molecules are produced. A significant portion of the net hydrocarbon and carbon monoxide production rates is formed by the emission from the solid surfaces due to the hydrogen and oxygen atom flux. The essential roles of C3H9SiO molecules and Si2OC5H15 + ions on the deposition process are verified. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab3f8a
  • 2019 • 228 Synthesis of silicon nanoparticles in a pilot-plant-scale microwave plasma reactor: Impact of flow rates and precursor concentration on the nanoparticle size and aggregation
    Kunze, F. and Kuns, S. and Spree, M. and Hülser, T. and Schulz, C. and Wiggers, H. and Schnurre, S.M.
    Powder Technology 342 880-886 (2019)
    This work is devoted to scale-up the microwave plasma synthesis of silicon nanoparticles from gaseous precursor monosilane (SiH4), previously investigated in lab-scale processes, to the pilot-plant-scale with production rates up to 200 g/h. The aim is to ensure reproducible, long-term operation of the reactor through gas-dynamic stabilization of the reacting flow and to control particle size and morphology via the gas flow velocity and the precursor concentration. Based on a newly designed nozzle, the lab-scale approach of stabilizing the plasma flow via a tangential sheath gas flow and an axial precursor gas flow was successfully transferred to the pilot-plant scale. At precursor concentrations up to 16 vol% of SiH4 diluted in argon and hydrogen, the as-synthesized particles have similar characteristics compared to those from lab-scale reactors. They are spherical, crystalline, mostly soft-agglomerated, and show a log-normal size distribution with a geometric standard deviation around 1.45 as expected for self-preserving aerosol size-distributions. In contrast to lab-scale experiments, an increase in SiH4 concentration up to 48 vol% does not lead to further growth of isolated primary particles but promotes aggregate formation from smaller primary particles. This is attributed to massive initial nucleation of very small particles due to strong supersaturation and their subsequent strong aggregation while suppressing complete coalescence due to the limited residence time at high temperature. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.powtec.2018.10.042
  • 2019 • 227 Transferability of interatomic potentials for molybdenum and silicon
    Lysogorskiy, Y. and Hammerschmidt, T. and Janssen, J. and Neugebauer, J. and Drautz, R.
    Modelling and Simulation in Materials Science and Engineering 27 (2019)
    Interatomic potentials are widely used in computational materials science, in particular for simulations that are too computationally expensive for density functional theory (DFT). Most interatomic potentials have a limited application range and often there is very limited information available regarding their performance for specific simulations. We carried out high-throughput calculations for molybdenum and silicon with DFT and a number of interatomic potentials. We compare the DFT reference calculations and experimental data to the predictions of the interatomic potentials. We focus on a large number of basic materials properties, including the cohesive energy, atomic volume, elastic coefficients, vibrational properties, thermodynamic properties, surface energies and vacancy formation energies, which enables a detailed discussion of the performance of the different potentials. We further analyze correlations between properties as obtained from DFT calculations and how interatomic potentials reproduce these correlations, and suggest a general measure for quantifying the accuracy and transferability of an interatomic potential. From our analysis we do not establish a clearcut ranking of the potentials as each potential has its strengths and weaknesses. It is therefore essential to assess the properties of a potential carefully before application of the potential in a specific simulation. The data presented here will be useful for selecting a potential for simulations of Mo or Si. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-651X/aafd13
  • 2019 • 226 Microstructural and tribological properties of sputtered AlCrSiWN films deposited with segmented powder metallurgic target materials
    Tillmann, W. and Fehr, A. and Stangier, D.
    Thin Solid Films 687 (2019)
    When synthesizing magnetron sputtered films with a complex stoichiometry, integrating the desired coating constituents into one target material is favorable in order to avoid nanolaminar film depositions and to enable a homogenous film growth. In contrast to alloyed targets, segmented plug targets allow to merge elements with different physical properties in one target material. Two targets, amalgamating 20 and 48 hot-pressed 85.6Cr9.9Si4.5W (at. %) plugs, respectively, into a monolithic aluminum target were fabricated and employed in a direct current magnetron sputtering process to deposit AlCrSiWN films on high-speed steel (AISI M3:2, 1.3344). The cathode powers for the Al(CrSiW)20 and Al(CrSiW)48 targets were varied between 7.5 and 17.5 W/cm2 to analyze how differently composed targets and various cathode powers affect the microstructure and tribological properties of the sputtered films. The results revealed that the chemical composition as well as the thickness of the films strongly depend on the target setup. All AlCrSiWN films exhibited a Cr/Si/W ratio of approximately 84/11/6. The Cr and Al contents were dominant (19–29 at. %), while the Si and W contents varied between 2 and 3 at. %. Especially the Al/Cr ratio of the films is affected by the varying Al/CrSiW surface area ratio of the manufactured plug targets. Furthermore, the mechanical properties are significantly influenced by the Al/Cr ratio, which is responsible for a dense coating growth and the crystalline structure of the films. All AlCrSiWN films were (111) textured indicating a B1 (Al, Cr, W)N structure, which exhibited a finer crystalline growth with an increasing cathode power on the Al(CrSiW)20 target. Tribological analyses of the films against Al2O3 balls further revealed that thinner films resulted in a decreased wear coefficient. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2019.137465
  • 2019 • 225 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 • 224 Intra- and inter-nanocrystal charge transport in nanocrystal films
    Aigner, W. and Bienek, O. and Falcão, B.P. and Ahmed, S.U. and Wiggers, H. and Stutzmann, M. and Pereira, R.N.
    Nanoscale 10 8042-8057 (2018)
    The exploitation of semiconductor nanocrystal (NC) films in novel electronic and optoelectronic applications requires a better understanding of charge transport in these systems. Here, we develop a model of charge transport in NC films, based on a generalization of the concept of transport energy level ET to nanocrystal assemblies, which considers both intra- and inter-NC charge transfer processes. We conclude that the role played by each of these processes can be probed from temperature-dependent measurements of charge carrier density n and mobility μ in the same films. The model also enables the determination of the position of the Fermi energy level EF with respect to ET, an important parameter of charge transport in semiconductor materials, from the temperature dependence of n. Moreover, we provide support to an essentially temperature-independent intra-NC charge carrier mobility, considered in the transport level concept, and consequently the frequently observed temperature dependence of the overall mobility μ in NC films results from a temperature variation of the inter-NC charge transport processes. Importantly, we also conclude that the temperature dependence of conductivity in NC films should result in general from a combination of temperature variations of both n and μ. By applying the model to solution-processed Si NC films, we conclude that transport within each NC is similar to that in amorphous Si (a-Si), with charges hopping along band tail states located below the conduction band edge. For Si NCs, we obtain values of ET - EF of ∼0.25 eV. The overall mobility μ in Si NC films is significantly further reduced with respect to that typically found in a-Si due to the additional transport constraints imposed by inter-NC transfer processes inherent to a nanoparticulate film. Our model accounting for inter- and intra-NC charge transport processes provides a simple and more general description of charge transport that can be broadly applied to films of semiconductor NCs. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8nr00250a
  • 2018 • 223 Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel
    Benzing, J.T. and Poling, W.A. and Pierce, D.T. and Bentley, J. and Findley, K.O. and Raabe, D. and Wittig, J.E.
    Materials Science and Engineering A 711 78-92 (2018)
    The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3 mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2017.11.017
  • 2018 • 222 Dynamic quadrupole interactions in semiconductors
    Dang, T.T. and Schell, J. and Lupascu, D.C. and Vianden, R.
    Journal of Applied Physics 123 (2018)
    The time differential perturbed angular correlation, TDPAC, technique has been used for several decades to study electric quadrupole hyperfine interactions in semiconductors such as dynamic quadrupole interactions (DQI) resulting from after-effects of the nuclear decay as well as static quadrupole interactions originating from static defects around the probe nuclei such as interstitial ions, stresses in the crystalline structure, and impurities. Nowadays, the quality of the available semiconductor materials is much better, allowing us to study purely dynamic interactions. We present TDPAC measurements on pure Si, Ge, GaAs, and InP as a function of temperature between 12 K and 110 K. The probe 111In (111Cd) was used. Implantation damage was recovered by thermal annealing. Si experienced the strongest DQI with lifetime, τg, increasing with rising temperature, followed by Ge. In contrast, InP and GaAs, which have larger band gaps and less electron concentration than Si and Ge in the same temperature range, presented no DQI. The results obtained also allow us to conclude that indirect band gap semiconductors showed the dynamic interaction, whereas the direct band gap semiconductors, restricted to GaAs and InP, did not. © 2018 Author(s).
    view abstractdoi: 10.1063/1.4993714
  • 2018 • 221 Si micro-cantilever sensor chips for space-resolved stress measurements in physical and plasma-enhanced chemical vapour deposition
    Grochla, D. and Banko, L. and Pfetzing-Micklich, J. and Behm, H. and Dahlmann, R. and Ludwig, Al.
    Sensors and Actuators, A: Physical 270 271-277 (2018)
    The control of extrinsic and intrinsic mechanical stresses in thin films is crucial. Stresses can limit the film performance e.g. by stress-induced delamination or undesired bending of film/substrate combinations; however, stresses can also be used to obtain functionality. Thus, understanding of stress-inducing mechanisms, correlations of stress with film synthesis parameters and controlling the sign and amplitude of stresses in thin films is important and a facile and reliable stress measurement method is necessary. Here, a stress measurement chip is presented which is based on the measurement of the residual overall film stress by a film-substrate combination curvature-based measurement technique. The novel Si-based cantilever sensor chip can measure residual stress in films from a few nanometers thickness up to several microns. Moreover, the sensor chips are applicable for determining the coefficient of thermal expansion, and for examining the film thickness homogeneity over large areas in a deposition system. They can be applied in physical vapor deposition and plasma-enhanced chemical vapor deposition processes with different geometrical and process-related boundary conditions. Exemplary results which were obtained with the sensor chips are discussed to demonstrate their easy applicability, accuracy, versatility, reliability, the thickness dependence of the residual stress and the homogeneity of SiOx films as well as the residual stress and the thermal expansion values of Al-Cr-N films. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.sna.2017.12.050
  • 2018 • 220 Bacterial cell division is involved in the damage of gram-negative bacteria on a nano-pillar titanium surface
    Köller, M. and Ziegler, N. and Sengstock, C. and Schildhauer, T.A. and Ludwig, Al.
    Biomedical Physics and Engineering Express 4 (2018)
    The role of bacterial cell division on the damage of adherent bacteria to titanium (Ti) nano-pillar cicada wing like surface was analyzed. Therefore nano-pillar Ti thin films were fabricated by glancing angle sputter deposition (GLAD) on silicon substrates. Gram-negative E. coli bacteria were allowed to adhere and to proliferate on these nanostructured samples for 3 h at 37 °C either under optimal cell growth conditions (brain heart infusion medium, BHI) or limited growth conditions (RPMI1640 medium). The bacteria adhered to the samples in both media. Compared to BHI medium the growth of E. coli in RPMI1640 medium was significantly inhibited. Concomitantly, the ratio of dead/living adherent bacteria on the nano-pillar surface was significantly decreased after the incubation period in RPMI1640. In addition, when the bacterial proliferation was biochemically halted using DL-serine-hydroxamate a comparable decrease in the ratio of dead/living adherent bacteria was also obtained in BHI medium. These results indicate that cell growth of adherent E. coli which is accompanied by cell elongations of the rod structure is involved in the damage induced by the titanium nano-pillar surface. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/2057-1976/aad2c1
  • 2018 • 219 Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries
    Liebscher, C.H. and Stoffers, A. and Alam, M. and Lymperakis, L. and Cojocaru-Mirédin, O. and Gault, B. and Neugebauer, J. and Dehm, G. and Scheu, C. and Raabe, D.
    Physical Review Letters 121 (2018)
    The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning. © 2018 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.121.015702
  • 2018 • 218 All gas-phase synthesis of graphene: Characterization and its utilization for silicon-based lithium-ion batteries
    Münzer, A. and Xiao, L. and Sehlleier, Y.H. and Schulz, C. and Wiggers, H.
    Electrochimica Acta 272 52-59 (2018)
    We report on a gas-phase synthesis method for the preparation of free-standing few-layer graphene in a microwave plasma reactor using pure ethanol as precursor. This scalable synthesis route produces gas-phase graphene (GPG) with lab-scale production rates up to a few hundred mg/h. The physico-chemical properties of the resulting GPG were characterized by XRD, FTIR-, and Raman spectroscopy, electrical conductivity measurements, XPS, and HRTEM in combination with EELS. The materials’ properties were compared with those of reduced graphene oxide (rGO) made by the established Hummers’ method. The results indicate that the gas-phase synthesis method provides highly-ordered few-layer graphene with extraordinary high purity, very low oxygen content of less than 1 at.%, and high specific conductivity. Both graphene materials were processed in combination with gas-phase synthesized silicon nanoparticles towards silicon-graphene nanocomposites for Li-ion battery anodes. Subsequent electrochemical testing revealed that the gas-phase graphene significantly enhances the long-term stability and Coulomb efficiency of the composite compared to pristine silicon and outperforms the composite fabricated from reduced graphene oxide. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2018.03.137
  • 2018 • 217 High-Temperature Rate Constants for H + Tetramethylsilane and H + Silane and Implications about Structure–Activity Relationships for Silanes
    Peukert, S. and Herzler, J. and Fikri, M. and Schulz, C.
    International Journal of Chemical Kinetics 50 57-72 (2018)
    The shock-tube technique has been used to investigate the reactions H + SiH4 → H2 + SiH3 (R1) and H + Si(CH3)4 → Si(CH3)3CH2 + H2 (R2) behind reflected shock waves. C2H5I was used as a thermal in situ source for H atoms. For reaction (R1), the experiments covered a temperature range of 1170–1251 K and for (R2) 1227–1320 K. In both cases, the pressures were near 1.5 bar. In these experiments, H atoms were monitored with atomic resonance absorption spectrometry. Fits to the H-atom temporal concentration profiles applying postulated chemical kinetic reaction mechanisms were used for determining the rate constants k1 and k2. Experimental rate constants were well represented by the Arrhenius equations k1(T) = 2.75 × 10−9 exp(−37.78 kJ mol−1/RT) cm3 s−1 and k2(T) = 1.17 × 10−7 exp(−86.82 kJ mol−1/RT) cm3 s−1. Transition state theory (TST) calculations based on CBS-QB3 and G4 levels of theory show good agreement with experimentally obtained rate constants; the experimental values for k1 and k2 are ∼40% lower and ∼50% larger than theoretical predictions, respectively. For the development of a mechanism describing the thermal decomposition of tetramethylsilane (Si(CH3)4; TMS), also TST-based rate constants for reaction CH3 + Si(CH3)4 → Si(CH3)3CH2 + CH4 (R3) were calculated. A comparison between experimental and theoretical rate constants k2 and k3 with available rate constants from the literature indicates that Si(CH3)4 has very similar reactivity toward H abstractions like neopentane (C(CH3)4), which is the analog hydrocarbon to TMS. Based on these results, the possibility of drawing reactivity analogies between hydrocarbons and structurally similar silicon-organic compounds for H-atom abstractions is discussed. © 2017 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/kin.21140
  • 2018 • 216 Influence of preparation method and doping of zirconium oxide onto the material characteristics and catalytic activity for the HDO reaction in nickel on zirconium oxide catalysts
    Pichler, C.M. and Gu, D. and Joshi, H. and Schüth, F.
    Journal of Catalysis 365 367-375 (2018)
    Different Ni/ZrO2 catalysts were prepared and tested for the hydrodeoxygenation (HDO) reaction of guaiacol. It was shown that the preparation mode of the ZrO2 support has a significant influence on the catalytic results, as remaining elements like Si and Na from the preparation can change the material properties. The influence of Si and Na onto these material properties, which were especially surface acidity and oxygen vacancy concentration, could be clarified. It could be also rationalized, how the change of these properties affects the results of the HDO reaction. Furthermore, it was demonstrated that the oxygen vacancy concentration is an important factor for the catalytic performance, although this property has hardly been considered in the design of HDO catalysts so far. La doping was found to be an efficient strategy to tune the oxygen vacancy concentration, and by using this approach the catalytic performance of the catalyst could be improved remarkably. © 2018
    view abstractdoi: 10.1016/j.jcat.2018.07.021
  • 2018 • 215 An examination of interactions between temperature, pressure, sintering time, and Si/W ratio on the sintering behavior of CrSiW
    Tillmann, W. and Fehr, A. and Ferreira, M. and Stangier, D.
    International Journal of Refractory Metals and Hard Materials 73 146-156 (2018)
    A Statistic Design of Experiments (DoE) was implemented with the aim to systematically investigate sintering parameter interactions of hot pressed 80Cr10Si10W. By varying the temperature between 999 and 1200 °C at pressures ranging from 2 to 8 MPa for 6.6 to 23.4 min, an initial examination of the ternary system is realized. The overall objective of this study is to minimize the porosity and to foster a diffusion of the elements. The investigations revealed that high temperatures (&gt;1000 °C) and pressures (&gt;6 MPa) support the diffusion between chromium and silicon while tungsten particles accumulate at the grain boundaries of silicon. XRD analyses confirmed the existence of a c-CrSi3 phase. The setup of the DoE, which promises the highest densification, was subsequently used to examine the influence of the silicon and tungsten content on the porosity according to the pattern 80Cr(20-x)Si(x)W for 0 ≥ x ≥ 20 (in wt%). This approach generated the lowest porosity (3.2 ± 0.93 area%) for 80Cr20W and led to homogenous particle distributions. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.ijrmhm.2018.02.012
  • 2017 • 214 Yb2Si2O7 Environmental Barrier Coatings Deposited by Various Thermal Spray Techniques: A Preliminary Comparative Study
    Bakan, E. and Marcano, D. and Zhou, D. and Sohn, Y.J. and Mauer, G. and Vaßen, R.
    Journal of Thermal Spray Technology 26 1011-1024 (2017)
    Dense, crack-free, uniform, and well-adhered environmental barrier coatings (EBCs) are required to enhance the environmental durability of silicon (Si)-based ceramic matrix composites in high pressure, high gas velocity combustion atmospheres. This paper represents an assessment of different thermal spray techniques for the deposition of Yb2Si2O7 EBCs. The Yb2Si2O7 coatings were deposited by means of atmospheric plasma spraying (APS), high-velocity oxygen fuel spraying (HVOF), suspension plasma spraying (SPS), and very low-pressure plasma spraying (VLPPS) techniques. The initial feedstock, as well as the deposited coatings, were characterized and compared in terms of their phase composition. The as-sprayed amorphous content, microstructure, and porosity of the coatings were further analyzed. Based on this preliminary investigation, the HVOF process stood out from the other techniques as it enabled the production of vertical crack-free coatings with higher crystallinity in comparison with the APS and SPS techniques in atmospheric conditions. Nevertheless, VLPPS was found to be the preferred process for the deposition of Yb2Si2O7 coatings with desired characteristics in a controlled-atmosphere chamber. © 2017, ASM International.
    view abstractdoi: 10.1007/s11666-017-0574-1
  • 2017 • 213 Thermal resistance of twist boundaries in silicon nanowires by nonequilibrium molecular dynamics
    Bohrer, J.K. and Schröer, K. and Brendel, L. and Wolf, D.E.
    AIP Advances 7 (2017)
    The thermal boundary resistance (Kapitza resistance) of (001) twist boundaries in silicon is investigated by nonequilibrium molecular dynamics simulations. In order to enable continuous adjustment of the mismatch angle, a cylindrical geometry with fixed atomic positions at the boundaries is devised. The influence of the boundary conditions on the Kapitza resistance is removed by means of a finite size analysis. Due to the diamond structure of silicon, twist boundaries with mismatch angles ϕ and 90°−ϕ are not equivalent, whereas those with ±ϕ or with 90°±ϕ are. The Kapitza resistance increases with mismatch angle up to 45°, where it reaches a plateau around 1.56±0.05Km2/GW. Between 80° and the 90°Σ1 grain boundary it drops by about 30%. Surprisingly, lattice coincidence at other angles (Σ5,Σ13,Σ27,Σ25) has no noticable effect on the Kapitza resistance. However, there is a clear correlation between the Kapitza resistance and the width of a non-crystalline layer at the twist boundaries. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4979982
  • 2017 • 212 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 • 211 Optical anisotropy of quasi-1D rare-earth silicide nanostructures on Si(001)
    Chandola, S. and Speiser, E. and Esser, N. and Appelfeller, S. and Franz, M. and Dähne, M.
    Applied Surface Science 399 648-653 (2017)
    Rare earth metals are known to interact strongly with Si(001) surfaces to form different types of silicide nanostructures. Using STM to structurally characterize Dy and Tb silicide nanostructures on vicinal Si(001), it will be shown that reflectance anisotropy spectroscopy (RAS) can be used as an optical fingerprint technique to clearly distinguish between the formation of a semiconducting two-dimensional wetting layer and the metallic one-dimensional nanowires. Moreover, the distinctive spectral features can be related to structural units of the nanostructures. RAS spectra of Tb and Dy nanostructures are found to show similar features. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2016.12.044
  • 2017 • 210 Spectroscopic models for laser-heated silicon and copper nanoparticles
    Daun, K. and Menser, J. and Mansmann, R. and Moghaddam, S.T. and Dreier, T. and Schulz, C.
    Journal of Quantitative Spectroscopy and Radiative Transfer 197 3-11 (2017)
    Interpreting laser-induced incandescence (LII) measurements on aerosolized nanoparticles requires a spectroscopic model that relates the measured spectral incandescence to the temperature of the nanoparticles. We present spectroscopic models for molten silicon and copper nanoparticles, which are evaluated through extinction and incandescence measurements on nanoaerosols. Measurements on molten silicon nanoparticles are consistent with the Drude theory in the Rayleigh limit of Mie theory. The copper nanoparticles were initially assumed to coalesce into spheres, but the observed spectral incandescence does not show a surface plasmon polariton (SPP) peak in the vicinity of 600. nm expected of spheres. A simulation based on the discrete dipole approximation (DDA) suggests that this effect could be explained by the structure of the copper aggregates. © 2016.
    view abstractdoi: 10.1016/j.jqsrt.2016.10.006
  • 2017 • 209 Silicon surface modifications produced by non-equilibrium He, Ne and Kr plasma jets
    Engelhardt, M. and Kartaschew, K. and Bibinov, N. and Havenith, M. and Awakowicz, P.
    Journal of Physics D-applied Physics 50 015206 (2017)
    In this publication the interaction of non-equilibrium plasma jets (N-APPJs) with silicon surfaces is studied. The N-APPJs are operated with He, Ne and Kr gas flows under atmospheric pressure conditions. Plasma bullets are produced by the He and Ne N-APPJs, while a filamentary discharge is ignited in the Kr flow. All these N-APPJs produce remarkable traces on silicon wafer surfaces treated in their effluents. Different types of etching tracks, blisters and crystals are observed on the treated surfaces. The observed traces and surface modifications of silicon wafers are analyzed with optical, atomic-force, scanning electron and Raman microscopes. Based on the material composition within the etching tracks and the position and dimension of blisters and crystals, the traces observed on the silicon wafer surfaces are interpreted as traces of micro-plasmoids. Amorphous silicon is found in the etching tracks. Blisters are produced through the formation of cracks inside the silicon crystal by the interaction with micro-plasmoids. The reason for these modifications is not clear now. The density of micro-plasmoids traces on the treated silicon surface and the depth and length of the etching tracks depends strongly on the type of the used carrier gas of the N-APPJ.
    view abstractdoi: 10.1088/1361-6463/50/1/015206
  • 2017 • 208 Silicon surface modifications produced by non-equilibrium He, Ne and Kr plasma jets
    Engelhardt, M. and Kartaschew, K. and Bibinov, N. and Havenith, M. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    In this publication the interaction of non-equilibrium plasma jets (N-APPJs) with silicon surfaces is studied. The N-APPJs are operated with He, Ne and Kr gas flows under atmospheric pressure conditions. Plasma bullets are produced by the He and Ne N-APPJs, while a filamentary discharge is ignited in the Kr flow. All these N-APPJs produce remarkable traces on silicon wafer surfaces treated in their effluents. Different types of etching tracks, blisters and crystals are observed on the treated surfaces. The observed traces and surface modifications of silicon wafers are analyzed with optical, atomic-force, scanning electron and Raman microscopes. Based on the material composition within the etching tracks and the position and dimension of blisters and crystals, the traces observed on the silicon wafer surfaces are interpreted as traces of micro-plasmoids. Amorphous silicon is found in the etching tracks. Blisters are produced through the formation of cracks inside the silicon crystal by the interaction with micro-plasmoids. The reason for these modifications is not clear now. The density of micro-plasmoids traces on the treated silicon surface and the depth and length of the etching tracks depends strongly on the type of the used carrier gas of the N-APPJ. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/50/1/015206
  • 2017 • 207 Parasitic Reactions in Nanosized Silicon Anodes for Lithium-Ion Batteries
    Gao, H. and Xiao, L. and Plümel, I. and Xu, G.-L. and Ren, Y. and Zuo, X. and Liu, Y. and Schulz, C. and Wiggers, H. and Amine, K. and Chen, Z.
    Nano Letters 17 1512-1519 (2017)
    When designing nano-Si electrodes for lithium-ion batteries, the detrimental effect of the c-Li15Si4 phase formed upon full lithiation is often a concern. In this study, Si nanoparticles with controlled particle sizes and morphology were synthesized, and parasitic reactions of the metastable c-Li15Si4 phase with the nonaqueous electrolyte was investigated. The use of smaller Si nanoparticles (∼60 nm) and the addition of fluoroethylene carbonate additive played decisive roles in the parasitic reactions such that the c-Li15Si4 phase could disappear at the end of lithiation. This suppression of c-Li15Si4 improved the cycle life of the nano-Si electrodes but with a little loss of specific capacity. In addition, the characteristic c-Li15Si4 peak in the differential capacity (dQ/dV) plots can be used as an early-stage indicator of cell capacity fade during cycling. Our findings can contribute to the design guidelines of Si electrodes and allow us to quantify another factor to the performance of the Si electrodes. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b04551
  • 2017 • 206 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 • 205 Laser-induced atomic emission of silicon nanoparticles during laser-induced heating
    Menser, J. and Daun, K. and Dreier, T. and Schulz, C.
    Applied Optics 56 E50-E57 (2017)
    The temporal luminescence behavior of silicon atoms during and after laser-heating of gas-borne silicon nano-particles was investigated. Silicon nanoparticles were formed in the exhaust stream of a microwave plasma reactor at 100 mbar. The observed prompt atomic line intensities correspond with thermal excitation of the evaporated species. A prompt signal at 251.61 and 288.15 nm originating from the 3s23p2 → 3s23p4s transitions showed a lifetime of 16 ns that matches the documented excited-state lifetime for the respective transitions. A secondary delayed signal contribution with similar peak intensities was observed commencing approximately 100-300 ns after the laser pulse and persisting for hundreds of nanoseconds. This signal contribution is attributed to electron impact excitation or recombination after electron impact ionization of the silicon evaporated as a consequence of the laser heating of the plasma leading to non-thermal population of electronically excited silicon. The observations support a nanoparticle evaporation model that can be used to recover nanoparticle sizes from time-resolved LII data. © 2017 Optical Society of America.
    view abstractdoi: 10.1364/AO.56.000E50
  • 2017 • 204 On measurement of the thermal diffusivity of moderate and heavily doped semiconductor samples using modulated photothermal infrared radiometry
    Pawlak, M. and Panas, A. and Ludwig, Ar. and Wieck, A.D.
    Thermochimica Acta 650 33-38 (2017)
    In this work, the accuracy of the thermal diffusivity estimation in moderately and heavily doped semiconductor samples using the modulated photothermal infrared radiometry is investigated. The studies were carried out on heavily doped Si and GaAs wafers, and on moderately doped Si and recently studied GaAs and CdSe samples. It is shown, that depending on the infrared properties of the semiconductor sample, the modulated photothermal infrared radiometry signal can yield information about thermal diffusivity, (effective) infrared absorption coefficient and electronic transport parameters (recombination lifetime, carrier diffusivity and surface recombination velocities). For the heavily doped samples, the modulated photothermal infrared radiometry signal consists only of the thermal response yielding information about the (effective) infrared absorption coefficient and thermal diffusivity. The relative expanded uncertainty with 0.95 level of confidence Ur of estimating the thermal diffusivity in this case is about Ur = 0.05. For moderately doped samples the modulated photothermal infrared radiometry signal consists of the thermal and of the photocarrier response. The relative expanded uncertainty with 0.95 level of confidence Ur of estimating the thermal diffusivity in this case varies between about Ur = 0.10 and about Ur = 0.30, depending on the existence of the maximum in the signal phase, but information about the electronic transport properties is derived. It is shown that not only infrared properties have the influence on the accuracy in estimating the thermal diffusivity of moderate doped semiconductor samples, but also the thermal, geometrical (thickness) and carrier recombination properties can play an important role. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.tca.2017.02.003
  • 2017 • 203 Perovskite Nanopillar Array Based Tandem Solar Cell
    Raja, W. and Schmid, M. and Toma, A. and Wang, H. and Alabastri, A. and Proietti Zaccaria, R.
    ACS Photonics 4 2025-2035 (2017)
    One of the promising approaches to improve the efficiency of conventional single-crystalline silicon (c-Si) solar cells is their integration in a tandem arrangement. In this perspective, inorganic-organic perovskites are an ideal blend of materials to combine with c-Si owing to their complementary light absorption characteristics. Even though interesting and promising combinations of perovskite/c-Si-based solar cells have been presented, their overall efficiency has been limited by the photocurrent reduction occurring in both perovskite and silicon due mostly to reflection and parasitic losses. Here, we envision and model a new design strategy for an efficient light-to-current conversion through the use of a nanopillar array based perovskite/c-Si tandem solar cell. The optical-electrical performance of the proposed architecture is analyzed by a 3D finite-element numerical model. In particular, we have searched for the best optical enhancement conditions through the tuning of the cell geometrical parameters, demonstrating the importance of optical resonances. Afterward, we have evaluated the electrical response of the optimized structures in a four-terminal (4-T) configuration by studying the current-voltage characteristics and power conversion efficiency. In particular, the introduced solar cell yields a conversion efficiency of 27%, with contributions of 18.5% and 8.51% from perovskite and c-Si, respectively. We have compared our proposed nanopatterned design with its planar counterpart characterized by the same quantity of active material, obtaining a relative efficiency enhancement of 21%. Importantly, the conversion efficiency of our proposed design surpasses the efficiency of single-junction perovskite and c-Si solar cells, and, similarly, it represents a new achievement for 4-T perovskite/c-Si tandem solar cells. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsphotonics.7b00406
  • 2017 • 202 Bismuth amides as promising ALD precursors for Bi2Te3 films
    Rusek, M. and Komossa, T. and Bendt, G. and Schulz, S.
    Journal of Crystal Growth 470 128-134 (2017)
    The thermal properties of five homoleptic bismuth amides of the general type (R1R2N)3Bi (R1, R2 = Me 1, R1, R2 = Et 2, R1, R2 = n-Pr 3, R1 = Me, R2 = Et 4, R1 = n-Bu, R2 = Et 5) and the cyclo-dibismadiazane [(Me3Si)2NBi-μ-NSiMe3]2 6 were studied by DSC and TGA/DTA (1, 6) and their reactions with (Et3Si)2Te were investigated by in situ NMR spectroscopy. Based on these results, the potential application of 1, 4 and 6 to serve as ALD precursors for the deposition of Bi2Te3 films in reactions with (Et3Si)2Te were investigated. The resulting films were characterized by XRD, EDX, AFM and SEM and the crucial role of the substrate material and substrate temperature on the film growth rate and the morphology and chemical composition of the Bi2Te3 films was determined. © 2017
    view abstractdoi: 10.1016/j.jcrysgro.2017.04.019
  • 2017 • 201 Microstructure and thermoelectric properties of Si-WSi2 nanocomposites
    Stoetzel, J. and Schneider, T. and Mueller, M.M. and Kleebe, H.-J. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Acta Materialia 125 321-326 (2017)
    Nanocomposites of n-doped Si/WSi2 were prepared and morphologically and thermoelectrically investigated. The composites were densified by spark-plasma-sintering of doped Si nanoparticles with WSi2 nanoinclusions. The nanoparticles were synthesized in a gas-phase process. The microstructure of the bulk nanocomposite shows an inhomogeneous distribution of the WSi2 nanoinclusions in form of WSi2-rich and -depleted regions. This inhomogeneity is not present in the starting material and is assigned to a self-organizing process during sintering. The inhomogeneities are in the micrometer range and may act as scattering centers for long-wavelength phonons. The WSi2 nanoinclusions grow during sintering from originally 3–7 nm up to 30–143 nm depending on the total W content and might act as scattering centers for the medium wavelength range of phonons. Further, the growth of Si grains is suppressed by the WSi2 inclusions, which leads to an enhanced grain boundary density. Adding 1 at% W reduces lattice thermal conductivity by almost 35% within the temperature range from 300 K to 1250 K compared to pure, nanocrystalline silicon (doped). By addition of 6 at% W a reduction of 54% in lattice thermal conductivity is achieved. Although little amounts of W slightly reduce the power factor an enhancement of the thermoelectric figure of merit of 50% at 1250 K compared to a tungsten-free reference was realized. © 2016
    view abstractdoi: 10.1016/j.actamat.2016.11.069
  • 2017 • 200 Correlating Atom Probe Tomography with Atomic-Resolved Scanning Transmission Electron Microscopy: Example of Segregation at Silicon Grain Boundaries
    Stoffers, A. and Barthel, J. and Liebscher, C.H. and Gault, B. and Cojocaru-Mirédin, O. and Scheu, C. and Raabe, D.
    Microscopy and Microanalysis 1-9 (2017)
    In the course of a thorough investigation of the performance-structure-chemistry interdependency at silicon grain boundaries, we successfully developed a method to systematically correlate aberration-corrected scanning transmission electron microscopy and atom probe tomography. The correlative approach is conducted on individual APT and TEM specimens, with the option to perform both investigations on the same specimen in the future. In the present case of a Σ9 grain boundary, joint mapping of the atomistic details of the grain boundary topology, in conjunction with chemical decoration, enables a deeper understanding of the segregation of impurities observed at such grain boundaries. © Microscopy Society of America 2017
    view abstractdoi: 10.1017/S1431927617000034
  • 2017 • 199 Effects of Post Annealing Treatments on the Interfacial Chemical Properties and Band Alignment of AlN/Si Structure Prepared by Atomic Layer Deposition
    Sun, Long and Lu, Hong-Liang and Chen, Hong-Yan and Wang, Tao and Ji, Xin-Ming and Liu, Wen-Jun and Zhao, Dongxu and Devi, Anjana and Ding, Shi-Jin and Zhang, David Wei
    Nanoscale Research Letters 12 102 (2017)
    The influences of annealing temperature in N-2 atmosphere on interfacial chemical properties and band alignment of AlN/Si structure deposited by atomic layer deposition have been investigated based on x-ray photoelectron spectroscopy and spectroscopic ellipsometry. It is found that more oxygen incorporated into AlN film with the increasing annealing temperature, resulting from a little residual H2O in N-2 atmosphere reacting with AlN film during the annealing treatment. Accordingly, the Si-N bonding at the interface gradually transforms to Si-O bonding with the increasing temperature due to the diffusion of oxygen from AlN film to the Si substrate. Specially, the Si-O-AI bonding state can be detected in the 900 degrees C-annealed sample. Furthermore, it is determined that the band gap and valence band offset increase with increasing annealing temperature.
    view abstractdoi: 10.1186/s11671-016-1822-x
  • 2017 • 198 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
  • 2017 • 197 Influence of bias voltage and sputter mode on the coating properties of TiAlSiN: Einfluss der Biasspannung und des Sputtermodus auf die Schichteigenschaften von TiAlSiN
    Tillmann, W. and Dildrop, M.
    Materialwissenschaft und Werkstofftechnik 48 855-861 (2017)
    Silicon offers promising opportunities to improve the characteristics of thin coatings. By adding silicon to TiAlN, the oxidation resistance as well as the tribological properties can be increased and improved. To analyze the influence of the silicon content on the coating properties of TiAlSiN, it is necessary to keep the ratio of the other coating elements constant by using the right target configuration. Within this study, TiAlSiN coatings were deposited on hot work steel AISI H11 by using magnetron sputtering (Cemecon CC800/9 sinox ML). This steel was previously plasma nitrided to increase the hardness and hence the carrying load of the substrate, avoiding shell egg effect during the analysis. Different sputter modes were used to analyze the possibility to produce TiAlSiN by utilizing a pure low conductive silicon target. The bias voltages were systematically varied to see their influence on the structure and chemical compositions of the coating which were investigated by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (EDX). Furthermore, the roughness of the surface of the coatings was measured by an optical three-dimensional surface analyzer. The results of this study serve as a basis for further investigations regarding the variation of the silicon content of TiAlSiN coatings. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201600731
  • 2017 • 196 Identification and separation of rectifier mechanisms in Si/SiGe ballistic cross junctions
    Von Pock, J.F. and Salloch, D. and Wieser, U. and Hackbarth, T. and Kunze, U.
    Journal of Applied Physics 121 (2017)
    Depending on the detailed geometry, gate voltage, and circuitry, nanoscale Si/SiGe cross junctions at low temperatures exhibit full-wave rectification arising from different mechanisms like change in the number of current-carrying modes, stationary ballistic charging of a current-free voltage lead, and hot-electron thermopower. We study the rectifier structures on high-mobility Si/SiGe heterostructures consisting of a straight voltage stem and oblique current-injecting leads. Local gate electrodes are used to control the electron density in the voltage or current channel. Compared to three-terminal Y-branch junctions, the four-terminal cross junction eliminates the mode effect. A gradual increase of output voltage as gate-voltage is reduced until threshold voltage is identified as contribution of hot-electron thermopower. Heating the initially cold reservoir from a second orthogonal cross junction eliminates the electron temperature gradient and suppresses the thermopower. Even if the operation as six-terminal device re-induces a mode-controlled contribution, we demonstrate that it is negligible. As expected, the ballistic signal can be reliably separated from other mechanisms by measurements under positive gate voltage. The ballistic voltage can be described by a parabolic function of the injected current and is proportional to the cosine of the injection angle. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4973279
  • 2017 • 195 Interaction between phase transformations and dislocations at incipient plasticity of monocrystalline silicon under nanoindentation
    Zhang, J. and Zhang, J. and Wang, Z. and Hartmaier, A. and Yan, Y. and Sun, T.
    Computational Materials Science 131 55-61 (2017)
    Structural phase transformation and dislocation slip are two important deformation modes of monocrystalline silicon. In the present work, we elucidate mechanisms of inhomogeneous elastic-plastic transition in spherical nanoindentation of monocrystalline silicon by means of molecular dynamics simulations. The Stillinger-Weber potential is utilized to present simultaneous phase transformations and dislocation activities in the silicon nanoindentation. And a bond angle analysis-based method is proposed to quantitatively clarify silicon phases. The influence of crystallographic orientation on the silicon nanoindentation is further addressed. Our simulation results indicate that prior to the “Pop-In” event, Si(0 1 0) undergoes inelastic deformation accompanied by the phase transformation from the Si-I to the Si-III/Si-XII, which is not occurred in Si(1 1 0) and Si(1 1 1). While the phase transformation from the Si-I to the bct-5 is the dominant mechanism of incipient plasticity for each crystallographic orientation, dislocation nucleation is also an operating deformation mode in the elastic-plastic transition of Si(0 1 0). Furthermore, interactions between phase transformations and dislocations are more pronounced in Si(0 1 0) than the other two crystallographic orientations. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.commatsci.2017.01.043
  • 2016 • 194 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 • 193 Interaction of an argon plasma jet with a silicon wafer
    Engelhardt, M. and Pothiraja, R. and Kartaschew, K. and Bibinov, N. and Havenith, M. and Awakowicz, P.
    Journal of Physics D: Applied Physics 49 (2016)
    A filamentary discharge is ignited in an argon plasma jet under atmospheric pressure conditions. The gas discharge is characterized with voltage-current measurements, optical emission spectroscopy and an ICCD-camera with a high temporal resolution down to 10 ns. In the effluent of the plasma jet, filaments come into contact with the surface of a silicon wafer and modify it, namely etching traces are produced and microcrystals are deposited. These traces are studied with optical and electron microscopes. The material of the deposited microcrystals and the surface modifications of the silicon wafer are analyzed with Raman microspectroscopy. Amorphous silicon is found within the etching traces. The largest part of the deposited microcrystals are composed of nitratine (NaNO3) and some of them are calcite (CaCO3). Analyzing the possible reasons for the silicon wafer modifications we come to the conclusion that plasmoids, which are produced near the substrate surface by interaction with ionization waves, are a plausible explanation for the observed surface modifications of the silicon wafer. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/14/145201
  • 2016 • 192 Molecular dynamics simulations of thermal transport in isotopically modulated semiconductor nanostructures
    Frieling, R. and Eon, S. and Wolf, D. and Bracht, H.
    Physica Status Solidi (A) Applications and Materials Science 213 549-556 (2016)
    In this paper, we investigate the effect of isotopic modulation on the thermal conductivity of semiconductor nanostructures. The isotope doping is of particular interest for the application of semiconductors as thermoelectric materials as it leaves the electronic properties practically unaffected while the phononic transport is retarded. This approach could increase the figure of merit of thermoelectric generators by decreasing the thermal conductivity of semiconductors. We use non-equilibrium molecular dynamics simulations to examine thermal transport in isotopically engineered semiconductors. The temperature profiles along the sample region deduced from the simulations allow the extraction of thermal conductivities. The reliability of the MD-predicted thermal conductivities is studied by analyzing the influence of the input parameters on the results. The first set of samples are isotopically modified silicon samples. The influence of temperature, isotopic composition, and ordering of isotopic defects on the thermal conductivity of silicon is studied. The second material system under investigation is silicon germanium alloys. The influence of isotopic modulation on the thermal conductivity of Si-Ge alloys is examined for varying chemical composition. The thermal conductivities predicted by MD are compared to results derived from the solution of the Boltzmann transport equation in the relaxation time approach. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532469
  • 2016 • 191 Microscale Fracture Behavior of Single Crystal Silicon Beams at Elevated Temperatures
    Jaya, B.N. and Wheeler, J.M. and Wehrs, J. and Best, J.P. and Soler, R. and Michler, J. and Kirchlechner, C. and Dehm, G.
    Nano Letters 16 7597-7603 (2016)
    The micromechanical fracture behavior of Si [100] was investigated as a function of temperature in the scanning electron microscope with a nanoindenter. A gradual increase in KC was observed with temperature, in contrast to sharp transitions reported earlier for macro-Si. A transition in cracking mechanism via crack branching occurs at ∼300 °C accompanied by multiple load drops. This reveals that onset of small-scale plasticity plays an important role in the brittle-to-ductile transition of miniaturized Si. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b03461
  • 2016 • 190 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 • 189 Preparing hydroxyapatite-silicon composite suspensions with homogeneous distribution of multi-walled carbon nano-tubes for electrophoretic coating of NiTi bone implant and their effect on the surface morphology
    Khalili, V. and Khalil-Allafi, J. and Xia, W. and Parsa, A.B. and Frenzel, J. and Somsen, C. and Eggeler, G.
    Applied Surface Science 366 158-165 (2016)
    Preparing a stable suspension is a main step towards the electrophoretically depositing of homogeneous and dense composite coatings on NiTi for its biomedical application. In the present study, different composite suspensions of hydroxyapatite, silicon and multi-walled carbon nano-tubes were prepared using n-butanol and triethanolamine as media and dispersing agent, respectively. Multi-walled carbon nanotubes were first functionalized in the nitric acid vapor for 15 h at 175 °C, and then mixed into suspensions. Thermal desorption spectroscopy profiles indicate the formation of functional groups on multi-walled carbon nano-tubes. An excellent suspension stability can be achieved for different amounts of triethanolamine. The amount of triethanolamine can be increased by adding a second component to a stable hydroxyapatite suspension due to an electrostatic interaction between components in suspension. The stability of composite suspension is less than that of the hydroxyapatite suspension, due to density differences, which under the gravitational force promote the demixing. The scanning electron microscopy images of the coatings surface show that more dense coatings are developed on NiTi substrate using electrophoretic deposition and sintering at 850 °C in the simultaneous presence of silicon and multi-walled carbon nanotubes in the hydroxyapatite coatings. The atomic force microscopy results of the coatings surface represent that composite coatings of hydroxyapatite-20 wt.% silicon and hydroxyapatite-20 wt.% silicon-1 wt.% multi-walled carbon nano-tubes with low zeta potential have rougher surfaces. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2016.01.053
  • 2016 • 188 The influence of Si as reactive bonding agent in the electrophoretic coatings of HA–Si–MWCNTs on NiTi alloys
    Khalili, V. and Khalil-Allafi, J. and Maleki-Ghaleh, H. and Paulsen, A. and Frenzel, J. and Eggeler, G.
    Journal of Materials Engineering and Performance 25 390-400 (2016)
    In this study, different composite coatings with 20 wt.% silicon and 1 wt.% multi-walled carbon nanotubes of hydroxyapatite were developed on NiTi substrate using a combination of electrophoretic deposition and reactive bonding during the sintering. Silicon was used as reactive bonding agent. During electrophoretic deposition, the constant voltage of 30 V was applied for 60 s. After deposition, samples were dried and then sintered at 850 °C for 1 h in a vacuum furnace. SEM, XRD and EDX were used to characterize the microstructure, phase and elemental identification of coatings, respectively. The SEM images of the coatings reveal a uniform and compact structure for HA–Si and HA–Si–MWCNTs. The presence of silicon as a reactive bonding agent as well as formation of new phases such as SiO2, CaSiO3 and Ca3SiO5 during the sintering process results in compact coatings and consumes produced phases from HA decomposition. Formation of the mentioned phases was confirmed using XRD analysis. The EDX elemental maps show a homogeneous distribution of silicon all over the composite coatings. Also, the bonding strength of HA–Si–MWCNTs coating is found to be 27.47 ± 1 MPa. © 2015, ASM International.
    view abstractdoi: 10.1007/s11665-015-1824-3
  • 2016 • 187 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 • 186 Magnetic microstructure in a stress-annealed Fe73.5Si15.5B7Nb3Cu1 soft magnetic alloy observed using off-axis electron holography and Lorentz microscopy
    Kovács, A. and Pradeep, K.G. and Herzer, G. and Raabe, D. and Dunin-Borkowski, R.E.
    AIP Advances 6 (2016)
    Fe-Si-B-Nb-Cu alloys are attractive for high frequency applications due to their low coercivity and high saturation magnetization. Here, we study the effect of stress annealing on magnetic microstructure in Fe73.5Si15.5B7Nb3Cu1 using off-axis electron holography and the Fresnel mode of Lorentz transmission electron microscopy. A stress of 50 MPa was applied to selected samples during rapid annealing for 4 s, resulting in uniaxial anisotropy perpendicular to the stress direction. The examination of focused ion beam milled lamellae prepared from each sample revealed a random magnetic domain pattern in the sample that had been rapidly annealed in the absence of stress, whereas a highly regular domain pattern was observed in the stress-annealed sample. We also measured a decrease in domain wall width from ∼ 94 nm in the sample annealed without stress to ∼ 80 nm in the stress-annealed sample. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4942954
  • 2016 • 185 Atom probe tomography of intermetallic phases and interfaces formed in dissimilar joining between Al alloys and steel
    Lemmens, B. and Springer, H. and Duarte, M.J. and De Graeve, I. and De Strycker, J. and Raabe, D. and Verbeken, K.
    Materials Characterization 120 268-272 (2016)
    While Si additions to Al are widely used to reduce the thickness of the brittle intermetallic seam formed at the interface during joining of Al alloys to steel, the underlying mechanisms are not clarified yet. The developed approach for the site specific atom probe tomography analysis revealed Si enrichments at grain and phase boundaries between the θ (Fe4Al13) and η (Fe2Al5) phase, up to about ten times that of the concentration in Al. The increase in Si concentration could play an important role for the growth kinetics of the intermetallic phases formed for example in hot-dip aluminizing of steel. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.matchar.2016.09.008
  • 2016 • 184 Nano fabricated silicon nanorod array with titanium nitride coating for on-chip supercapacitors
    Lu, P. and Ohlckers, P. and Müller, L. and Leopold, S. and Hoffmann, M. and Grigoras, K. and Ahopelto, J. and Prunnila, M. and Chen, X.
    Electrochemistry Communications 70 51-55 (2016)
    We demonstrate high aspect ratio silicon nanorod arrays by cyclic deep reactive ion etching (DRIE) process as a scaffold to enhance the energy density of a Si-based supercapacitor. By unique atomic layer deposition (ALD) technology, a conformal nanolayer of TiN was deposited on the silicon nanorod arrays as the active material. The TiN coated silicon nanorods as a supercapacitor electrode lead to a 6 times improvement in capacitance compared to flat TiN film electrode. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2016.07.002
  • 2016 • 183 Process flow to integrate nanostructures on silicon grass in surface micromachined systems
    Mehner, H. and Müller, L. and Biermann, S. and Hänschke, F. and Hoffmann, M.
    Journal of Physics: Conference Series 757 (2016)
    The process flow to integrate metallic nanostructures in surface micromachining processes is presented. The nanostructures are generated by evaporation of microstructured silicon grass with metal. The process flow is based on the lift-off of a thin amorphous silicon layer deposited using a CVD process. All steps feature a low temperature load beneath 120 °C and high compatibility with many materials as only well-established chemicals are used. As a result metallic nanostructures usable for optical applications can be generated as part of multilayered microsystems fabricated in surface micromachining. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/757/1/012022
  • 2016 • 182 Laser-induced incandescence from laser-heated silicon nanoparticles
    Menser, J. and Daun, K. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 122 (2016)
    This work describes the application of temporally and spectrally resolved laser-induced incandescence to silicon nanoparticles synthesized in a microwave plasma reactor. Optical properties for bulk silicon presented in the literature were extended for nanostructured particles analyzed in this paper. Uncertainties of parameters in the evaporation submodel, as well as measurement noise, are incorporated into the inference process by Bayesian statistics. The inferred nanoparticle sizes agree with results from transmission electron microscopy, and the determined accommodation coefficient matches the values of the preceding study. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-016-6551-4
  • 2016 • 181 Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing
    Morsdorf, L. and Pradeep, K.G. and Herzer, G. and Kovács, A. and Dunin-Borkowski, R.E. and Povstugar, I. and Konygin, G. and Choi, P. and Raabe, D.
    Journal of Applied Physics 119 (2016)
    Nucleation of soft magnetic Fe3Si nanocrystals in Cu-free Fe74.5Si15.5Nb3B7 alloy, upon rapid (10 s) and conventional (30 min) annealing, was investigated using x-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy, and atom probe tomography. By employing rapid annealing, preferential nucleation of Fe3Si nanocrystals was achieved, whereas otherwise there is simultaneous nucleation of both Fe3Si and undesired Fe-B compound phases. Analysis revealed that the enhanced Nb diffusivity, achieved during rapid annealing, facilitates homogeneous nucleation of Fe3Si nanocrystals while shifting the secondary Fe-B crystallization to higher temperatures resulting in pure soft magnetic nanocrystallization with very low coercivities of ∼10 A/m. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4944595
  • 2016 • 180 Solid-Supported Lipid Multilayers under High Hydrostatic Pressure
    Nowak, B. and Paulus, M. and Nase, J. and Salmen, P. and Degen, P. and Wirkert, F.J. and Honkimäki, V. and Tolan, M.
    Langmuir 32 2638-2643 (2016)
    In this work, the structure of solid-supported lipid multilayers exposed to increased hydrostatic pressure was studied in situ by X-ray reflectometry at the solid-liquid interface between silicon and an aqueous buffer solution. The layers' vertical structure was analyzed up to a maximum pressure of 4500 bar. The multilayers showed phase transitions from the fluid into different gel phases. With increasing pressure, a gradual filling of the sublayers between the hydrophilic head groups with water was observed. This process was inverted when the pressure was decreased, yielding finally smaller water layers than those in the initial state. As is commonly known, water has an abrasive effect on lipid multilayers by the formation of vesicles. We show that increasing pressure can reverse this process so that a controlled switching between multi- and bilayers is possible. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b04554
  • 2016 • 179 Development of a post-synthetic method for tuning the Al content of OSDA-free Beta as a catalyst for conversion of methanol to olefins
    Otomo, R. and Müller, U. and Feyen, M. and Yilmaz, B. and Meng, X. and Xiao, F.-S. and Gies, H. and Bao, X. and Zhang, W. and De Vos, D. and Yokoi, T.
    Catalysis Science and Technology 6 713-721 (2016)
    Zeolites synthesized without any organic structure-directing agent (OSDA) have several advantages over conventional zeolites synthesized with OSDAs. Their Al-rich compositions, however, are sometimes not suitable for applications as catalysts. In the present study, post-synthetic modification was performed using an Al-rich Beta zeolite synthesized without any OSDAs (designated as "Beta(OF)") to obtain high-silica Beta zeolites. We have successfully developed a facile post-synthetic method for tuning the Al content of Beta(OF) with the ∗BEA-type structure retained by calcination at &gt;750 °C followed by acid treatment. Solid-state 29Si and 27Al MAS NMR analyses revealed that during calcination, framework Al atoms were isomorphously substituted with Si atoms to form high-silica frameworks and concomitant extra-framework Al species. The stability of the obtained frameworks against acid treatment was evaluated in terms of the framework Al content, finding that the framework with the Si/Al ratio higher than 12 is well stabilized enough for acid treatment. Thus, dealuminated Beta(OF) zeolites with high-silica compositions were found to be effective catalysts for the methanol-to-olefin (MTO) reaction; particularly, the Beta zeolite with the Si/Al ratio of 112 maintained the conversion of methanol over 90% with selectivity to C2-C4 olefins over 70% even at 40 hours on stream at WHSV = 3.2 h-1. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c5cy00944h
  • 2016 • 178 Considerations on the symmetry of pure silica ITQ-7 zeolite (ISV) derived from 29Si MAS NMR and Rietveld analysis
    Paillaud, J.-L. and Tzanis, L. and Marler, B. and Rigolet, S. and Patarin, J. and Gies, H.
    Microporous and Mesoporous Materials 219 306-310 (2016)
    Pure silica ITQ-7 zeolite of topology ISV synthesized in fluoride media possesses a 3D 12-membered-rings pore system. Originally the structure of ITQ-7 zeolite was solved and refined from a calcined sample in the highest possible space group symmetry P42/mmc. This space group leads to only 5 independent silicon T sites for this structure. However, this high symmetry was not supported by solid state 29Si MAS NMR spectroscopy. In this short communication, we show from a revised 29Si solid state MAS NMR and Rietveld analyses, that lowering of the space group symmetry from P42/mmc to P42 consolidates structure model and experimental data, in particular, the number of non equivalent crystallographic T sites passing from 5 to 16, is in agreement with 29Si solid state NMR spectroscopy. © 2015 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2015.07.002
  • 2016 • 177 Thermoelectric properties of Ge/Si heterostructures: A combined theoretical and experimental study
    Reith, H. and Nielsch, K. and Fiedler, G. and Nausner, L. and Hu, Y. and Chen, P. and Rastelli, A. and Kratzer, P.
    Physica Status Solidi (A) Applications and Materials Science 213 524-532 (2016)
    We present a combined experimental and theoretical investigation of the thermoelectric properties of p-doped Ge/Si superlattices grown on Si(001) substrates by molecular beam epitaxy. Electrical conductivity is measured both in the direction parallel and perpendicular to the interfaces by means of a modified transfer length method. Electronic transport is strongly anisotropic, with the cross-plane conductivity being about five times lower than in plane. This result is in very good agreement with the theoretical predictions based on the tight-binding method combined with the Boltzmann equation applied to the experimentally investigated structure. The cross-plane thermal conductivity of doped superlattices is measured with the differential 3ω method and compared with that of undoped superlattices and alloys with similar average Ge content. The comparison reveals that superlattices have strongly reduced thermal conduction compared to alloys, and that doping increases their thermal conductivity by about 50%. Considering the used doping level, this increase appears surprising. The Seebeck coefficient of the structures is addressed theoretically and displays a less pronounced anisotropy compared to the electric conductivity. Combined with the knowledge of the other thermoelectric parameters, we conclude that, while p-doped Si/Ge superlattices may be used as model systems for the investigation of thermoelectric transport in nanostructured materials, their relevance for application is limited. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532486
  • 2016 • 176 Partitioning of Cr and Si between cementite and ferrite derived from first-principles thermodynamics
    Sawada, H. and Kawakami, K. and Körmann, F. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Acta Materialia 102 241-250 (2016)
    Partitioning of Cr and Si between cementite and ferrite was investigated by first-principles thermodynamics taking into account vibrational, electronic, and magnetic Gibbs energy contributions. At finite temperatures, these contributions lower the partitioning Gibbs energy and compete with the configurational entropy, which favors impurity segregation to ferrite due to its larger volume fraction compared to cementite. Due to the large positive partitioning enthalpy contribution of Si at T = 0 K, partitioning of Si to cementite is virtually absent in agreement with experiment. The situation is drastically different for Cr impurities. Incorporation of finite-temperature effects resolves the discrepancy between experimental observations and previous T = 0 K first-principles calculations. Cr strongly segregates to cementite due to the enhanced magnetic entropy of cementite above 400 K, i.e., near the Curie temperature of cementite. The increasing magnetic fluctuations in ferrite cause a strong reduction of the partitioning coefficient in the temperature range from 773 to 973 K in qualitative agreement with experiment. Quantitative agreement with calphad data and experimental data for equilibrium Cr concentrations in a wide range of alloy compositions is achieved by renormalizing the theoretical magnetic partitioning Gibbs energy by a constant scaling factor. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.09.010
  • 2016 • 175 Silicon-based nanocomposites for thermoelectric application
    Schierning, G. and Stoetzel, J. and Chavez, R. and Kessler, V. and Hall, J. and Schmechel, R. and Schneider, T. and Petermann, N. and Wiggers, H. and Angst, S. and Wolf, D.E. and Stoib, B. and Greppmair, A. and Stutzmann, M. and B...
    Physica Status Solidi (A) Applications and Materials Science 213 497-514 (2016)
    Here we present the realization of efficient and sustainable silicon-based thermoelectric materials from nanoparticles. We employ a gas phase synthesis for the nanoparticles which is capable of producing doped silicon (Si) nanoparticles, doped alloy nanoparticles of silicon and germanium (Ge), SixGe1-x, and doped composites of Si nanoparticles with embedded metal silicide precipitation phases. Hence, the so-called "nanoparticle in alloy" approach, theoretically proposed in the literature, forms a guideline for the material development. For bulk samples, a current-activated pressure-assisted densification process of the nanoparticles was optimized in order to obtain the desired microstructure. For thin films, a laser annealing process was developed. Thermoelectric transport properties were characterized on nanocrystalline bulk samples and laser-sintered-thin films. Devices were produced from nanocrystalline bulk silicon in the form of p-n junction thermoelectric generators, and their electrical output data were measured up to hot side temperatures of 750°C. In order to get a deeper insight into thermoelectric properties and structure forming processes, a 3D-Onsager network model was developed. This model was extended further to study the p-n junction thermoelectric generator and understand the fundamental working principle of this novel device architecture. Gas phase synthesis of composite nanoparticles; nanocrystalline bulk with optimized composite microstructure; laser-annealed thin film. The authors fabricated thermoelectric nanomaterials from doped silicon and silicon and germanium alloy nanoparticles, as well as composites of Si nanoparticles with embedded metal silicide nanoparticles. Processing was performed applying a current-activated pressure-assisted densification process for bulk samples and a laser annealing process for thin film samples. Devices were produced in the form of pn junction thermoelectric generators. A 3D-Onsager network model was used to understand the fundamental working principle of this novel device architecture. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201532602
  • 2016 • 174 High-yield and scalable synthesis of a Silicon/Aminosilane-functionalized Carbon NanoTubes/Carbon (Si/A-CNT/C) composite as a high-capacity anode for lithium-ion batteries
    Sehlleier, Y.H. and Dobrowolny, S. and Plümel, I. and Xiao, L. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Applied Electrochemistry 46 229-239 (2016)
    In this study, we present a novel anode architecture for high-performance lithium-ion batteries based on a Silicon/3-aminosilane-functionalized CNT/Carbon (Si/A-CNT/C) composite. A high-yield, low-cost approach has been developed to stabilize and support silicon as an active anode material. Silicon (Si) nanoparticles synthesized in a hot-wall reactor and aminosilane-functionalized carbon nanotubes (A-CNT) were dispersed in styrene and divinylbenzene (DVB) and subsequently polymerized forming a porous Si/A-CNT/C composite. Transmission electron microscopy showed that this method enables the interconnection and a uniform encapsulation of Si nanoparticles within a porous carbon matrix especially using aminosilane-functionalized CNT (A-CNT). Electrochemical characterization shows that this material can deliver a delithiation capacity of 2293 mAh g−1 with a capacity retention of more than 90 % after 200 cycles at lithiation and delithiation rate of 0.5 C. We conclude that the porous Si/A-CNT/C composite material can accommodate sufficient space for Si volume expansion and extraction and improve the electronic and ionic conduction. Excellent electrochemical performance during repeated cycling can thus be achieved. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10800-015-0897-x
  • 2016 • 173 New materials for the light-induced hydrogen evolution reaction from the Cu-Si-Ti-O system
    Stein, H.S. and Gutkowski, R. and Siegel, A. and Schuhmann, W. and Ludwig, Al.
    Journal of Materials Chemistry A 4 3148-3152 (2016)
    Cu-containing photocathodes are generally limited by fast photocorrosion under working conditions. Hence stabilization of these materials is a key factor in their potential application for the light-induced hydrogen evolution reaction (HER). In order to identify new materials, oxidized Cu-Si-Ti metallic thin film precursor materials libraries were evaluated using a combinatorial approach. High-throughput photoelectrochemical characterization using an automated optical scanning droplet cell was performed on a material library to analyze doping and alloying effects on the light-induced HER. The results revealed that compositions near Ti-doped CuSiO3 (dioptase and copper-polysilicate) and Si-doped Cu3TiOx act as comparatively stable and highly active materials for HER. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ta10186g
  • 2016 • 172 Effect of Si on the acceleration of bainite transformation by pre-existing martensite
    Toji, Y. and Matsuda, H. and Raabe, D.
    Acta Materialia 116 250-262 (2016)
    Bainite transformation was investigated focusing on the influence of pre-existing martensite on the transformation kinetics, morphology and crystallographic orientation of subsequently formed bainite using EBSD and atom probe tomography. Two 1.1 wt% C-3wt.%Mn steels with and without 2 wt% Si were used to clarify the effect of Si. Steels were rapidly cooled from 900 °C to 300 °C and held at this temperature, or quenched from 900 °C once in water to generate approximately 30 vol% martensite followed by holding at 300 °C. Bainite transformation was clearly accelerated by pre-existing martensite in both Si-containing and Si-free steels. Bainite surrounds the pre-existing martensite in the Si-free steel, whereas it grows to the interior of the austenite grains in the steel containing 2 wt% Si. The major orientation relationship between bainite and adjacent austenite was changed by the presence of martensite from Nishiyama-Wassermann (N-W) to Greninger-Troiano (G-T) regardless of Si content. Clear carbon partitioning from martensite into austenite was observed prior to the bainite transformation in the 2 wt% Si steel, which was not observed in the Si-free steel. We suggest that the dislocations introduced by the martensite transformation act as a primary factor accelerating the bainite transformation when martensite is introduced prior to the bainite transformation. © 2016 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2016.06.044
  • 2016 • 171 Antifouling and Antibacterial Multifunctional Polyzwitterion/Enzyme Coating on Silicone Catheter Material Prepared by Electrostatic Layer-by-Layer Assembly
    Vaterrodt, A. and Thallinger, B. and Daumann, K. and Koch, D. and Guebitz, G.M. and Ulbricht, M.
    Langmuir 32 1347-1359 (2016)
    The formation of bacterial biofilms on indwelling medical devices generally causes high risks for adverse complications such as catheter-associated urinary tract infections. In this work, a strategy for synthesizing innovative coatings of poly(dimethylsiloxane) (PDMS) catheter material, using layer-by-layer assembly with three novel functional polymeric building blocks, is reported, i.e., an antifouling copolymer with zwitterionic and quaternary ammonium side groups, a contact biocidal derivative of that polymer with octyl groups, and the antibacterial hydrogen peroxide (H2O2) producing enzyme cellobiose dehydrogenase (CDH). CDH oxidizes oligosaccharides by transferring electrons to oxygen, resulting in the production of H2O2. The design and synthesis of random copolymers which combine segments that have antifouling properties by zwitterionic groups and can be used for electrostatically driven layer-by-layer (LbL) assembly at the same time were based on the atom-transfer radical polymerization of dimethylaminoethyl methacrylate and subsequent partial sulfobetainization with 1,3-propane sultone followed by quaternization with methyl iodide only or octyl bromide and thereafter methyl iodide. The alternating multilayer systems were formed by consecutive adsorption of the novel polycations with up to 50% zwitterionic groups and of poly(styrenesulfonate) as the polyanion. Due to its negative charge, enzyme CDH was also firmly embedded as a polyanionic layer in the multilayer system. This LbL coating procedure was first performed on prefunctionalized silicon wafers and studied in detail with ellipsometry as well as contact angle (CA) and zetapotential (ZP) measurements before it was transferred to prefunctionalized PDMS and analyzed by CA and ZP measurements as well as atomic force microscopy. The coatings comprising six layers were stable and yielded a more neutral and hydrophilic surface than did PDMS, the polycation with 50% zwitterionic groups having the largest effect. Enzyme activity was found to be dependent on the depth of embedment in the multilayer coating. Depending on the used polymeric building block, up to a 60% reduction in the amount of adhering bacteria and clear evidence for killed bacteria due to the antimicrobial functionality of the coating could be confirmed. Overall, this work demonstrates the feasibility of an easy to perform and shape-independent method for preparing an antifouling and antimicrobial coating for the significant reduction of biofilm formation and thus reducing the risk of acquiring infections by using urinary catheters. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b04303
  • 2016 • 170 Understanding surface reactivity of Si electrodes in Li-ion batteries by: In operando scanning electrochemical microscopy
    Ventosa, E. and Wilde, P. and Zinn, A.-H. and Trautmann, M. and Ludwig, Al. and Schuhmann, W.
    Chemical Communications 52 6825-6828 (2016)
    In operando SECM is employed to monitor the evolution of the electrically insulating character of a Si electrode surface during (de-)lithiation. The solid-electrolyte interface (SEI) formed on Si electrodes is shown to be intrinsically electrically insulating. However, volume changes upon (de-)lithiation lead to the loss of the protecting character of the initially formed SEI. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6cc02493a
  • 2016 • 169 Phosphine-ligated dinitrosyl iron complexes for redox-controlled NO release
    Wittkamp, F. and Nagel, C. and Lauterjung, P. and Mallick, B. and Schatzschneider, U. and Apfel, U.-P.
    Dalton Transactions 45 10271-10279 (2016)
    Here we present the syntheses and structural, spectroscopic, as well as electrochemical properties of four dinitrosyl iron complexes (DNICs) based on silicon- and carbon-derived di- and tripodal phosphines. Whereas CH3C(CH2PPh2)3 and Ph2Si(CH2PPh2)2 coordinate iron in a η2-binding mode, CH3Si(CH2PPh2)3 undergoes cleavage of one Si-C bond to afford [Fe(NO)2(P(CH3)Ph2)2] at elevated temperatures. The complexes were characterized by IR spectroelectrochemistry as well as UV-vis measurements. The oxidized {Fe(NO)2}9 compounds were obtained by oxidation with (NH4)2[Ce(NO3)6] and their properties evaluated with Mössbauer and IR spectroscopy. Stability experiments on the complexes suggest that they are capable of releasing their NO-ligands in the oxidized {Fe(NO)2}9 but not in the reduced {Fe(NO)2}10 form. A detailed DFT analysis is provided in order to understand the electronic configurations and the complexes' ability to release NO. © The Royal Society of Chemistry 2016.
    view abstractdoi: 10.1039/c6dt01209d
  • 2016 • 168 Fe-doped Beta zeolite from organotemplate-free synthesis for NH3-SCR of NOX
    Zhu, Y. and Chen, B. and Zhao, R. and Zhao, Q. and Gies, H. and Xiao, F.-S. and De Vos, D. and Yokoi, T. and Bao, X. and Kolb, U. and Feyen, M. and Maurer, S. and Moini, A. and Müller, U. and Shi, C. and Zhang, W.
    Catalysis Science and Technology 6 6581-6592 (2016)
    Two types of Beta zeolites, one from organotemplate-free synthesis with a Si/Al ratio of 9 and the other from a commercial one with a Si/Al ratio of 19, were employed here to dope Fe for NH3-SCR of NOx. Fe-Beta (Si/Al = 9) exhibits much higher activity than Fe-Beta (Si/Al = 19), especially at low-temperature regions (< 250 °C). In addition, it also exhibits better hydrothermal stability as compared with Fe-Beta (Si/Al = 19), which demonstrates that it is a promising SCR catalyst with good activity as well as hydrothermal stability. The correlation between the quantitative calculation of the content of isolated Fe3+ in Beta zeolites and the NO conversion rate at 150 °C shows a linear relationship, suggesting that the isolated Fe3+ species affect the SCR activity directly. The higher activity of the Fe-Beta-9 catalyst is supposed to be related not only to the isolated Fe3+ but also to the acidity. Furthermore, the template-free synthesized Beta zeolite shows less dealumination during hydrothermal aging and therefore better hydrothermal stability during the SCR reaction. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6cy00231e
  • 2015 • 167 Film Stress of Amorphous Hydrogenated Carbon on Biaxially Oriented Polyethylene Terephthalate
    Bahre, H. and Behm, H. and Grochla, D. and Böke, M., and Dahlmann, R., and Hopmann, C., and Ludwig, Al., and Winter, J.
    Plasma Processes and Polymers 12 896-904 (2015)
    Amorphous hydrogenated carbon (a-C:H) deposited on steel with plasma enhanced chemical vapor deposition can be used as elongation tolerant oxygen barrier. However, the elongation tolerance of the a-C:H film is lost if deposited on a poly(ethylene terephthalate) (PET) for reasons unknown. To assess this phenomenon, a-C:H was deposited on PET, silicon substrates, and silicon micro-cantilevers, and the stress was determined by measuring the radius of curvature. a-C:H deposited on PET showed lower compressive stress than on silicon. This difference is not due to the formation of a gradient layer or plastic deformation of PET. Instead, the most probable explanation is that energetic ions cause a partial release of biaxial orientation within the PET, thereby reducing the compressive stress. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.201500045
  • 2015 • 166 Hybrid glasses from strong and fragile metal-organic framework liquids
    Bennett, T.D. and Tan, J.-C. and Yue, Y. and Baxter, E. and Ducati, C. and Terrill, N.J. and Yeung, H.H.-M. and Zhou, Z. and Chen, W. and Henke, S. and Cheetham, A.K. and Greaves, G.N.
    Nature Communications 6 (2015)
    Hybrid glasses connect the emerging field of metal-organic frameworks (MOFs) with the glass formation, amorphization and melting processes of these chemically versatile systems. Though inorganic zeolites collapse around the glass transition and melt at higher temperatures, the relationship between amorphization and melting has so far not been investigated. Here we show how heating MOFs of zeolitic topology first results in a low density 'perfect' glass, similar to those formed in ice, silicon and disaccharides. This order-order transition leads to a super-strong liquid of low fragility that dynamically controls collapse, before a subsequent order-disorder transition, which creates a more fragile high-density liquid. After crystallization to a dense phase, which can be remelted, subsequent quenching results in a bulk glass, virtually identical to the high-density phase. We provide evidence that the wide-ranging melting temperatures of zeolitic MOFs are related to their network topologies and opens up the possibility of 'melt-casting' MOF glasses. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms9079
  • 2015 • 165 A new zeolite formed from interlayer expansion of the precursor COK-5
    Bian, C. and Wu, Q. and Zhang, J. and Chen, F. and Pan, S. and Wang, L. and Meng, X. and Müller, U. and Feyen, M. and Yilmaz, B. and Gies, H. and Zhang, W. and Bao, X. and De Vos, D. and Yokoi, T. and Tatsumi, T. and Xiao, F.-S.
    Microporous and Mesoporous Materials 214 204-209 (2015)
    The layered silicate COK-5 has been used for an interlayer expansion reaction with dichlorodimethylsilane (DCDMS) at 180 °C to interconnect neighboring layers, yielding a new and crystalline microporous framework. The samples containing the methyl functional groups in the as-made form and having OH groups in the calcined form were designed as COE-5 and calcined COE-5. These samples were characterized with X-ray diffraction (XRD), N<inf>2</inf> sorption isotherms, inductively coupled plasma optical emission spectrometry (ICP-OES), infrared spectroscopy (IR), high-resolution transmission electron micrograph (HRTEM), thermogravimetry-differential thermal analysis (TG-DTA), and 29Si and 13C solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), as well as the contact angle techniques. XRD patterns and HRTEM images suggest that the sample interlayer spacing has been expanded by nearly 0.5 Å. The N<inf>2</inf> sorption isotherms of the materials show the BET surface areas are 165 m2/g for COE-5 and 340 m2/g for calcined COE-5. 29Si and 13C MAS NMR as well as IR spectroscopy confirm the insertion of the linker group -Si(CH<inf>3</inf>)<inf>2</inf>- connecting neighboring layers. Interestingly, calcined COE-5 shows enhanced catalytic performance in the acetalisation of glycerol with acetone to produce solketal, compared with COK-5. © 2015 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2015.04.017
  • 2015 • 164 Propanoate grafting on (H,OH)-Si(0 0 1)-2×1
    Bournel, F. and Gallet, J.-J. and Köhler, U. and Ellakhmissi, B.B. and Kubsky, S. and Carniato, S. and Rochet, F.
    Journal of Physics Condensed Matter 27 (2015)
    We have examined the reactivity of water-covered Si(0 0 1)-2?×1, (H,OH)-Si(0 0 1)-2?×1, with propanoic (C2H5COOH) acid at room temperature. Using a combination of spectroscopic techniques probing the electronic structure (XPS, NEXAFS) and the vibrational spectrum (HREELS), we have proved that the acid is chemisorbed on the surface as a propanoate. Once the molecule is chemisorbed, the strong perturbation of the electronic structure of the hydroxyls, and of their vibrational spectrum, suggests that the molecule makes hydrogen bonds with the surrounding hydroxyls. As we find evidence that surface hydroxyls are involved in the adsorption reaction, we discuss how a concerted or a radical-mediated reaction (involving the surface silicon dangling bonds) could lead to water elimination and formation of the ester. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/5/054005
  • 2015 • 163 Sample temperature profile during the excimer laser annealing of silicon nanoparticles
    Caninenberg, M. and Verheyen, E. and Kiesler, D. and Stoib, B. and Brandt, M.S. and Benson, N. and Schmechel, R.
    Optics and Laser Technology 74 132-137 (2015)
    Based on the heat diffusion equation we describe the temperature profile of a silicon nanoparticle thin film on silicon during excimer laser annealing using COMSOL Multiphysics. For this purpose system specific material parameters are determined such as the silicon nanoparticle melting point at 1683 K, the surface reflectivity at 248 nm of 20% and the nanoparticle thermal conductivity between 0.3 and 1.2 W/m K. To validate our model, the simulation results are compared to experimental data obtained by Raman spectroscopy, SEM microscopy and electrochemical capacitance-voltage measurements (ECV). The experimental data are in good agreement with our theoretical findings and support the validity of the model. © 2015, Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.optlastec.2015.05.020
  • 2015 • 162 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 • 161 Nanoscale heat transport from Ge hut, dome, and relaxed clusters on Si(001) measured by ultrafast electron diffraction
    Frigge, T. and Hafke, B. and Tinnemann, V. and Krenzer, B. and Horn-Von Hoegen, M.
    Applied Physics Letters 106 (2015)
    The thermal transport properties of crystalline nanostructures on Si were studied by ultra-fast surface sensitive time-resolved electron diffraction. Self-organized growth of epitaxial Ge hut, dome, and relaxed clusters was achieved by in-situ deposition of 8 monolayers of Ge on Si(001) at 550 °C under UHV conditions. The thermal response of the three different cluster types subsequent to impulsive heating by fs laser pulses was determined through the Debye-Waller effect. Time resolved spot profile analysis and life-time mapping was employed to distinguish between the thermal response of the different cluster types. While dome clusters are cooling with a time constant of τ = 150 ps, which agrees well with numerical simulations, the smaller hut clusters with a height of 2.3 nm exhibit a cooling time constant of τ = 50 ps, which is a factor of 1.4 slower than expected. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4907636
  • 2015 • 160 Metal-to-Insulator Transition in Au Chains on Si(111)-5×2-Au by Band Filling: Infrared Plasmonic Signal and Ab Initio Band Structure Calculation
    Hötzel, F. and Seino, K. and Chandola, S. and Speiser, E. and Esser, N. and Bechstedt, F. and Pucci, A.
    Journal of Physical Chemistry Letters 6 3615-3620 (2015)
    The Si(111)-5×2-Au surface is increasingly of interest because it is one of the rare atomic chain systems with quasi-one-dimensional properties. For the deposition of 0.7 monolayers of Au, these chains are metallic. Upon the evaporation of an additional submonolayer amount of gold, the surface becomes insulating but keeps the 5×2 symmetry. This metal-to-insulator transition was in situ monitored based on the infrared plasmonic signal change with coverage. The phase transition is theoretically explained by total-energy and band-structure calculations. Accordingly, it can be understood in terms of the occupation of the originally half-filled one-dimensional band at the Fermi level. By annealing the system, the additional gold is removed from the surface and the plasmonic signal is recovered, which underlines the stability of the metallic structure. So, recent results on the infrared plasmonic signals of the Si(111)-5 × 2-Au surface are supported. The understanding of potential one-dimensional electrical interconnects is improved. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpclett.5b01530
  • 2015 • 159 The nucleation of Mo-rich Laves phase particles adjacent to M23C6 micrograin boundary carbides in 12% Cr tempered martensite ferritic steels
    Isik, M.I. and Kostka, A. and Yardley, V.A. and Pradeep, K.G. and Duarte, M.J. and Choi, P.P. and Raabe, D. and Eggeler, G.
    Acta Materialia 90 94-104 (2015)
    We study the nucleation of Mo-rich Laves phase particles during aging and creep of 12 wt.% Cr tempered martensite ferritic steels (TMFS). Recently, in Isik et al. (2014) we reported that Laves phase particles tend to form at micrograin boundaries of TMFSs after Mo and Si had segregated from the ferritic matrix to these internal interfaces. In the present work, we employ transmission electron microscopy (TEM) and atom probe tomography (APT) to study the formation of Laves phase particles. We investigate the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides. Our results suggest that this joint precipitation effect is due to the combined segregation of Mo and Si from the matrix to the micrograin boundaries and Si and P enrichment around the growing carbides.
    view abstractdoi: 10.1016/j.actamat.2015.01.027
  • 2015 • 158 High-Speed GaN/GaInN Nanowire Array Light-Emitting Diode on Silicon(111)
    Koester, R. and Sager, D. and Quitsch, W.-A. and Pfingsten, O. and Poloczek, A. and Blumenthal, S. and Keller, G. and Prost, W. and Bacher, G. and Tegude, F.-J.
    Nano Letters 15 2318-2323 (2015)
    (Graph Presented). The high speed on-off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core-shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90-10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/nl504447j
  • 2015 • 157 Wet Nanoindentation of the Solid Electrolyte Interphase on Thin Film Si Electrodes
    Kuznetsov, V. and Zinn, A.-H. and Zampardi, G. and Borhani-Haghighi, S. and La Mantia, F. and Ludwig, Al. and Schuhmann, W. and Ventosa, E.
    ACS Applied Materials and Interfaces 7 23554-23563 (2015)
    The solid electrolyte interphase (SEI) film formed at the surface of negative electrodes strongly affects the performance of a Li-ion battery. The mechanical properties of the SEI are of special importance for Si electrodes due to the large volumetric changes of Si upon (de)insertion of Li ions. This manuscript reports the careful determination of the Young's modulus of the SEI formed on a sputtered Si electrode using wet atomic force microscopy (AFM)-nanoindentation. Several key parameters in the determination of the Young's modulus are considered and discussed, e.g., wetness and roughness-thickness ratio of the film and the shape of a nanoindenter. The values of the Young's modulus were determined to be 0.5-10 MPa under the investigated conditions which are in the lower range of those previously reported, i.e., 1 MPa to 10 GPa, pointing out the importance of the conditions of its determination. After multiple electrochemical cycles, the polymeric deposits formed on the surface of the SEI are revealed, by force-volume mapping in liquid using colloidal probes, to extend up to 300 nm into bulk solution. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b06700
  • 2015 • 156 Towards solar cell emitters based on colloidal Si nanocrystals
    Leendertz, C. and Chirvony, V.S. and García-Calzada, R. and Görög, L. and Töfflinger, J.A. and Korte, L. and Agouram, S. and Martínez-Pastor, J. and Petermann, N. and Wiggers, H. and Ulyashin, A.G.
    Physica Status Solidi (A) Applications and Materials Science 212 156-161 (2015)
    The application of layers of doped colloidal silicon nanocrystals sandwiched between hydrogenated amorphous silicon layers as emitters in silicon heterojunction solar cells is explored. It is shown that such emitters provide excellent interface passivation and reasonable conductivity. Final solar cells with such nanoparticle emitters reach conversion efficiencies on the same level as solar cells with conventional hetero emitters. Quantum efficiency measurements indicate that the light absorbed in the nanocrystals contributes to the current extracted from the solar cell. The remaining challenges that need to be addressed before the application of such colloidal silicon nanocrystals for the processing of low-cost and potentially printable emitter layers becomes feasible are discussed. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/pssa.201431264
  • 2015 • 155 Local electronic and magnetic properties of pure and Mn-containing magnetocaloric LaFe13-xSix compounds inferred from Mössbauer spectroscopy and magnetometry
    Makarov, S.I. and Krautz, M. and Salamon, S. and Skokov, K. and Teixeira, C.S. and Gutfleisch, O. and Wende, H. and Keune, W.
    Journal of Physics D: Applied Physics 48 (2015)
    Manganese containing La-Fe-Si alloys are important magnetocaloric compounds, since Mn atoms prevent segregation of hydrogen in partially hydrogenated La-Fe-Mn-Si alloys when their Curie temperature is tuned to room temperature by hydrogen. The effect of Mn alloying on the Fe atomic magnetic moment μ<inf>Fe</inf> is still rather unexplored. Therefore, we investigated the (local) magnetic and electric hyperfine interactions in the strongly magnetocaloric compound LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf> and, for comparison, LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 57Fe Mössbauer spectroscopy, and the global magnetic properties by vibrating sample magnetometry. The NaZn<inf>13</inf> structure was confirmed by x-ray diffraction. Two non-equivalent Fe lattice sites are known to exist in this material: the (96i) sites (Fe<inf>II</inf>) of low local symmetry, and the highly symmetrical (8b) sites (Fe<inf>I</inf>). At room temperature in the paramagnetic state, the electric hyperfine parameters of Fe atoms on both sites were obtained. At low temperatures (4.8 K), the observed magnetically split nuclear Zeeman sextets with broad apparent lines were analyzed in terms of a distribution P(B<inf>hf</inf>) of hyperfine magnetic fields B<inf>hf</inf>. The average hyperfine field 〈B<inf>hf</inf>〉, originating predominantly from Fe<inf>II</inf> sites, was found to be rather high (30.7(1) T at 4.8 K) for LaFe<inf>11.6</inf>Si<inf>1.4</inf>, and the approximate relation 〈B<inf>hf</inf>〉 = Aμ<inf>Fe</inf> is confirmed for Fe<inf>II</inf> sites, with A = 14.2 T/μ<inf>B</inf>. 〈B<inf>hf</inf>〉 is significantly reduced (to 27.7(1) T at 4.8 K) for the Mn-containing sample LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf>, providing evidence for a reduction by 9.7% of the average Fe atomic moment μ<inf>Fe</inf> from ∼2.16 μ<inf>B</inf> to a value of ∼1.95 μ<inf>B</inf> by Mn substitution of Fe. Our Mössbauer results are in good agreement with magnetometry, which reveals a reduction of the saturation magnetization of M<inf>s</inf> = 163.1(1) Am2 kg-1 of LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 10.5% due to Mn substitution. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/30/305006
  • 2015 • 154 Controlling the polarity of metalorganic vapor phase epitaxy-grown GaP on Si(111) for subsequent III-V nanowire growth
    Paszuk, A. and Brückner, S. and Steidl, M. and Zhao, W. and Dobrich, A. and Supplie, O. and Kleinschmidt, P. and Prost, W. and Hannappel, T.
    Applied Physics Letters 106 (2015)
    Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111). © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4922275
  • 2015 • 153 The influence of stacking fault energy on the microstructural and strain-hardening evolution of Fe-Mn-Al-Si steels during tensile deformation
    Pierce, D.T. and Jiménez, J.A. and Bentley, J. and Raabe, D. and Wittig, J.E.
    Acta Materialia 100 178-190 (2015)
    Understanding the relationship between the stacking-fault energy (SFE), deformation mechanisms, and strain-hardening behavior is important for alloying and design of high-Mn austenitic transformation- and twinning-induced plasticity (TRIP/TWIP) steels. The present study investigates the influence of SFE on the microstructural and strain-hardening evolution of three TRIP/TWIP alloys (Fe-22/25/28Mn-3Al-3Si wt.%). The SFE is increased by systemically increasing the Mn content from 22 to 28 wt.%. The Fe-22Mn-3Al-3Si alloy, with a SFE of 15 mJ m-2, deforms by planar dislocation glide and strain-induced ε<inf>hcp</inf>-/α<inf>bcc</inf>-martensite formation which occurs from the onset of plastic deformation, resulting in improved work-hardening at low strains but lower total elongation. With an increased SFE of 21 mJ m-2 in the Fe-25Mn-3Al-3Si alloy, both mechanical twinning and ε<inf>hcp</inf>-martensite formation are activated during deformation, and result in the largest elongation of the three alloys. A SFE of 39 mJ m-2 enables significant dislocation cross slip and suppresses ε<inf>hcp</inf>-martensite formation, causing reduced work-hardening during the early stages of deformation in the Fe-28Mn-3Al-3Si alloy while mechanical twinning begins to enhance the strain-hardening after approximately 10% strain. The increase in SFE from 15 to 39 mJ m-2 results in significant changes in the deformation mechanisms and, at low strains, decreased work-hardening, but has a relatively small influence on strength and ductility. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.08.030
  • 2015 • 152 Atomic scale study of CU clustering and pseudo-homogeneous Fe-Si nanocrystallization in soft magnetic FeSiNbB(CU) alloys
    Pradeep, K.G. and Herzer, G. and Raabe, D.
    Ultramicroscopy 159 285-291 (2015)
    A local electrode atom probe has been employed to trace the onset of Cu clustering followed by their coarsening and subsequent growth upon rapid (10s) annealing of an amorphous Fe73.5Si15.5Cu1Nb3B7 alloy. It has been found that the clustering of Cu atoms introduces heterogeneities in the amorphous matrix, leading to the formation of Fe rich regions which crystallizes pseudo-homogeneously into Fe-Si nanocrystals upon annealing. In this paper, we present the data treatment method that allows for the visualization of these different phases and to understand their morphology while still quantifying them in terms of their size, number density and volume fraction. The crystallite size of Fe-Si nanocrystals as estimated from the atom probe data are found to be in good agreement with other complementary techniques like XRD and TEM, emphasizing the importance of this approach towards accurate structural analysis. In addition, a composition driven data segmentation approach has been attempted to determine and distinguish nanocrystalline regions from the remaining amorphous matrix. Such an analysis introduces the possibility of retrieving crystallographic information from extremely fine (2-4nm sized) nanocrystalline regions of very low volume fraction (< 5Vol%) thereby providing crucial in-sights into the chemical heterogeneity induced crystallization process of amorphous materials. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.04.006
  • 2015 • 151 Energy transfer in argon atom - Surface interactions studied by Pt-SiO2-Si thin film chemoelectronic devices
    Scheele, M. and Nedrygailov, I.I. and Hasselbrink, E. and Diesing, D.
    Vacuum 111 137-141 (2015)
    The energy transferred from a heated platinum surface to an adjacent argon gas of several mbar pressure is studied. The cooling effects during accommodation of the argon atoms when colliding with the surface can be monitored by current changes in a chemoelectronic device, in this case consisting of silicon, silicon-oxide and platinum. A numerical heat flow model and the experimentally observed temperature dependence of the conduction in the device show that the observed signals are due to a cooling of the whole device. Temperature gradients and accompanying thermoelectric effects play only a minor role. © 2014 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.vacuum.2014.09.017
  • 2015 • 150 Complex Nanotwin Substructure of an Asymmetric Σ9 Tilt Grain Boundary in a Silicon Polycrystal
    Stoffers, A. and Ziebarth, B. and Barthel, J. and Cojocaru-Mirédin, O. and Elsässer, C. and Raabe, D.
    Physical Review Letters 115 (2015)
    Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.115.235502
  • 2015 • 149 Grain boundary segregation in multicrystalline silicon: Correlative characterization by EBSD, EBIC, and atom probe tomography
    Stoffers, A. and Cojocaru-Mirédin, O. and Seifert, W. and Zaefferer, S. and Riepe, S. and Raabe, D.
    Progress in Photovoltaics: Research and Applications 23 1742-1753 (2015)
    This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron-beam-induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a non-coherent (random) high-angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high-angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface-property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon. Copyright © 2015 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/pip.2614
  • 2015 • 148 Laser-Assisted Wet-Chemical Doping of Sintered Si and Ge Nanoparticle Films
    Stoib, B. and Greppmair, A. and Petermann, N. and Wiggers, H. and Stutzmann, M. and Brandt, M.S.
    Advanced Electronic Materials 1 (2015)
    Doped thin films of group-IV semiconductors can be fabricated using the adsorption of dopant species from a liquid source to a precursor nanoparticle film, followed by laser-sintering to incorporate and activate the dopants in the sintered thin film. A detailed study of the doping of germanium films with arsenic reveals diffusion of dopants into the film and their adsorption to the nanoparticle surface as kinetically governing steps, benefiting from the large internal surface area of the nanoparticle film. The resulting charge carrier concentration can be adjusted by the internal surface area via the nano­particle diameter, by controlling the dopant concentration in the liquid, and by the immersion time and temperature. It is shown that the method can be successfully transferred to silicon and silicon–germanium alloy films using group-III and -V elements, which lead to p- and n-type conductivity, respectively. Atomic dopant concentrations above 1020 cm−3 can be realized by laser-sintering, which are electrically active to a high extent and lead to effective conductivities well above 10 S cm–1 in the mesoporous films is investigated here. The method allows flexible printing of devices using inks for the nanoparticles and the dopant and avoids toxic substances for the doping of nanoparticles in the gas phase. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/aelm.201400029
  • 2015 • 147 Chemical vapor deposition of Si/SiC nano-multilayer thin films
    Weber, A. and Remfort, R. and Wöhrl, N. and Assenmacher, W. and Schulz, S.
    Thin Solid Films 593 44-52 (2015)
    Stoichiometric SiC films were deposited with the commercially available single source precursor Et3SiH by classical thermal chemical vapor deposition (CVD) as well as plasma-enhanced CVD at low temperatures in the absence of any other reactive gases. Temperature-variable deposition studies revealed that polycrystalline films containing different SiC polytypes with a Si to carbon ratio of close to 1:1 are formed at 1000°C in thermal CVD process and below 100°C in the plasma-enhanced CVD process. The plasma enhanced CVD process enables the reduction of residual stress in the deposited films and offers the deposition on temperature sensitive substrates in the future. In both deposition processes the film thickness can be controlled by variation of the process parameters such as the substrate temperature and the deposition time. The resulting material films were characterized with respect to their chemical composition and their crystallinity using scanning electron microscope, energy dispersive X-ray spectroscopy (XRD), atomic force microscopy, X-ray diffraction, grazing incidence X-ray diffraction, secondary ion mass spectrometry and Raman spectroscopy. Finally, Si/SiC multilayers of up to 10 individual layers of equal thickness (about 450 nm) were deposited at 1000°C using Et3SiH and SiH4. The resulting multilayers features amorphous SiC films alternating with Si films, which feature larger crystals up to 300 nm size as measured by transmission electron microscopy as well as by XRD. XRD features three distinct peaks for Si(111), Si(220) and Si(311). © 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.08.042
  • 2015 • 146 Si-CNT/rGO Nanoheterostructures as High-Performance Lithium-Ion-Battery Anodes
    Xiao, L. and Sehlleier, Y.H. and Dobrowolny, S. and Orthner, H. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    ChemElectroChem 2 1983-1990 (2015)
    A robust and electrochemically stable 3D nanoheterostructure consisting of Si nanoparticles (NPs), carbon nanotubes (CNTs) and reduced graphene oxide (rGO) is developed as an anode material (Si-CNT/rGO) for lithium-ion batteries (LIBs). It integrates the benefits from its three building blocks of Si NPs, CNTs, and rGO; Si NPs offer high capacity, CNTs act as a mechanical, electrically conductive support to connect Si NPs, and highly electrically conductive and flexible rGO provides a robust matrix with enough void space to accommodate the volume changes of Si NPs upon lithiation/delithiation and to simultaneously assure good electric contact. The composite material shows a high reversible capacity of 1665mAhg-1 with good capacity retention of 88.6% over 500 cycles when cycled at 0.5C, that is, a 0.02% capacity decay per cycle. The high-power capability is demonstrated at 10C (16.2Ag-1) where 755mAhg-1 are delivered, thus indicating promising characteristics of this material for high-performance LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500323
  • 2014 • 145 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 • 144 Thermal conductivity of isotopically controlled silicon nanostructures
    Bracht, H. and Eon, S. and Frieling, R. and Plech, A. and Issenmann, D. and Wolf, D. and Lundsgaard Hansen, J. and Nylandsted Larsen, A. and Ager Iii, J.W. and Haller, E.E.
    New Journal of Physics 16 (2014)
    Nanostructured semiconductors open the opportunity to independently tailor electric and thermal conductivity by manipulation of the phonon transport. Nanostructuring of materials is a highly promising strategy for engineering thermoelectric devices with improved efficiency. The concept of reducing the thermal conductivity without degrading the electrical conductivity is most ideally realized by controlled isotope doping. This work reports on experimental and theoretical investigations on the thermal conductivity of isotopically modulated silicon nanostructures. State-of-the-art pump-and-probe experiments are conducted to determine the thermal conductivity of the different nanostructures of isotopically enriched silicon layers epitaxially grown on natural silicon substrates. Concomitant molecular dynamics calculations are performed to study the impact of the silicon isotope mass, isotope interfaces, and of the isotope layer ordering and thickness on the thermal conductivity. Engineering the isotope distribution is a striking concept to reduce the thermal conductivity of silicon without affecting its electronic properties. This approach, using isotopically engineered silicon, might pave the way for future commercial thermoelectric devices. © 2014 IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/16/1/015021
  • 2014 • 143 On the mechanism that leads to vanishing thermal hysteresis of the B2-R phase transformation in multilayered (TiNi)/(W) shape memory alloy thin films
    Buenconsejo, P.J.S. and Zarnetta, R. and Young, M. and Brunken, H. and Mehta, A. and Ludwig, Al.
    Thin Solid Films 564 79-85 (2014)
    The film stresses in two-phase (TiNi)/(W) shape memory alloy (SMA) multilayer thin films were evaluated using synchrotron diffraction analysis. The phase transforming B2-TiNi phase is under tensile stress due to the mismatch of the coefficient-of-thermal-expansion (αB2-TiNi &gt; αW &gt; αSi-substrate) and the elastic modulus (EW &gt; ESi &gt; EB2-TiNi) with respect to the bcc-W layers and the Si-substrate. The amount of stress on the B2-TiNi phase increases with increasing W amount in the film, which is proportional to the W layer thickness. This led to important changes in the behavior of the B2-R transformation. On cooling, a B2-R transformation proceeds under increasing tensile stress which increases the transformation start temperature (R s). Upon transformation to the R phase, the TiNi layers undergo stress-relaxation by reorientation of R phase variants to accommodate the mismatch. During heating the film always starts from a relaxed stress-state, so the reverse transformation proceeds without adversely affecting the reverse transformation temperature (Af). With increasing amount of W in the film Rs increases more on cooling, while Af is not significantly affected on heating, and this leads to vanishing thermal hysteresis (ΔTB2-R = Af - Rs). © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.05.010
  • 2014 • 142 Multidiagnostic analysis of silicate speciation in clear solutions/sols for zeolite synthesis
    Castro, M. and Haouas, M. and Taulelle, F. and Lim, I. and Breynaert, E. and Brabants, G. and Kirschhock, C.E.A. and Schmidt, W.
    Microporous and Mesoporous Materials 189 158-162 (2014)
    The formation of zeolites in presence of tetraalkylammonium cations from so-called clear solutions using silicon alkoxides is a highly complex process which challenges experimental chemistry. Most clear solutions are better described as clear sols as they contain nanosized silicate particles, which are formed during hydrolysis of the Si source before self-assembly into the zeolite framework. This process spans multiple time- and length-scales and only a combination of different analysis methods allows revelation of molecular level zeolite formation mechanisms. On the example of the early stages of the formation of zeolite beta from clear solutions/sols the different windows of observation of liquid-state 29Si and 27Al nuclear magnetic resonance (NMR) spectroscopy, small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and mass spectrometry (MS) are demonstrated. Each diagnostic means by itself needs to be carefully assessed for its window of temporal and spatial resolution which can be achieved by exploiting the overlapping information available from their combination. © 2013 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2013.08.027
  • 2014 • 141 High temperature thermoelectric device concept using large area PN junctions
    Chavez, R. and Angst, S. and Hall, J. and Stoetzel, J. and Kessler, V. and Bitzer, L. and Maculewicz, F. and Benson, N. and Wiggers, H. and Wolf, D. and Schierning, G. and Schmechel, R.
    Journal of Electronic Materials 43 2376-2383 (2014)
    A new high temperature thermoelectric device concept using large area nanostructured silicon p-type and n-type (PN) junctions is presented. In contrast to conventional thermoelectric generators, where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we experimentally demonstrate a device concept in which a large area PN junction made from highly doped densified silicon nanoparticles is subject to a temperature gradient parallel to the PN interface. In the proposed device concept, the electrical contacts are made at the cold side eliminating the hot side substrate and difficulties that go along with high temperature electrical contacts. This concept allows temperature gradients greater than 300 K to be experimentally applied with hot side temperatures larger than 800 K. Electronic properties of the PN junctions and power output characterizations are presented. A fundamental working principle is discussed using a particle network model with temperature and electric fields as variables, and which considers electrical conductivity and thermal conductivity according to Fourier's law, as well as Peltier and Seebeck effects. © 2014 TMS.
    view abstractdoi: 10.1007/s11664-014-3073-x
  • 2014 • 140 Shape-selective organic-inorganic zeolitic catalysts prepared via interlayer expansion
    De Baerdemaeker, T. and Vandebroeck, W. and Gies, H. and Yilmaz, B. and Müller, U. and Feyen, M. and De Vos, D.
    Catalysis Today 235 169-175 (2014)
    Interlayer expansion of layered zeolite precursors is achieved via the insertion of an additional T-atom in between the layers, typically by means of a silylating agent as source of the T-atom. (3-Mercaptopropyl) methyldimethoxysilane was used as Si-source in the interlayer expansion of the layered zeolite precursors RUB-36 and RUB-39. The structure expansion was confirmed with PXRD. The incorporation of the silylating agent was followed with 29Si MAS NMR, 13C CP MAS NMR and thermogravimetric analysis. The incorporated thiol groups were oxidized with H2O 2 to obtain sulfonic acid groups in between the layers. 13C CP MAS NMR was used to characterize the organic species and monitor the conversion of thiol to propylsulfonic groups. The shape-selective properties of the obtained materials were investigated in acid-catalyzed tetrahydropyranylation reactions. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.cattod.2014.02.035
  • 2014 • 139 Elemental composition of particulate matter and the association with lung function
    Eeftens, M. and Hoek, G. and Gruzieva, O. and Mölter, A. and Agius, R. and Beelen, R. and Brunekreef, B. and Custovic, A. and Cyrys, J. and Fuertes, E. and Heinrich, J. and Hoffmann, B. and De Hoogh, K. and Jedynska, A. and Keuke...
    Epidemiology 25 648-657 (2014)
    BACKGROUND: Negative effects of long-term exposure to particulate matter (PM) on lung function have been shown repeatedly. Spatial differences in the composition and toxicity of PM may explain differences in observed effect sizes between studies. METHODS: We conducted a multicenter study in 5 European birth cohorts - BAMSE (Sweden), GINIplus and LISAplus (Germany), MAAS (United Kingdom), and PIAMA (The Netherlands) - for which lung function measurements were available for study subjects at the age of 6 or 8 years. Individual annual average residential exposure to copper, iron, potassium, nickel, sulfur, silicon, vanadium, and zinc within PM smaller than 2.5 μm (PM2.5) and smaller than 10 μm (PM10) was estimated using land-use regression models. Associations between air pollution and lung function were analyzed by linear regression within cohorts, adjusting for potential confounders, and then combined by random effects meta-analysis. RESULTS: We observed small reductions in forced expiratory volume in the first second, forced vital capacity, and peak expiratory flow related to exposure to most elemental pollutants, with the most substantial negative associations found for nickel and sulfur. PM10 nickel and PM10 sulfur were associated with decreases in forced expiratory volume in the first second of 1.6% (95% confidence interval = 0.4% to 2.7%) and 2.3% (-0.1% to 4.6%) per increase in exposure of 2 and 200 ng/m, respectively. Associations remained after adjusting for PM mass. However, associations with these elements were not evident in all cohorts, and heterogeneity of associations with exposure to various components was larger than for exposure to PM mass. CONCLUSIONS: Although we detected small adverse effects on lung function associated with annual average levels of some of the evaluated elements (particularly nickel and sulfur), lower lung function was more consistently associated with increased PM mass. Copyright © 2014 by Lippincott Williams & Wilkins.
    view abstractdoi: 10.1097/EDE.0000000000000136
  • 2014 • 138 Ab initio structure determination of interlayer expanded zeolites by single crystal rotation electron diffraction
    Guo, P. and Liu, L. and Yun, Y. and Su, J. and Wan, W. and Gies, H. and Zhang, H. and Xiao, F.-S. and Zou, X.
    Dalton Transactions 43 10593-10601 (2014)
    Layered solids often form thin plate-like crystals that are too small to be studied by single-crystal X-ray diffraction. Although powder X-ray diffraction (PXRD) is the conventional method for studying such solids, it has limitations because of peak broadening and peak overlapping. We have recently developed a software-based rotation electron diffraction (RED) method for automated collection and processing of 3D electron diffraction data. Here we demonstrate the ab initio structure determination of two interlayer expanded zeolites, the microporous silicates COE-3 and COE-4 (COE-n stands for International Network of Centers of Excellence-n), from submicron-sized crystals by the RED method. COE-3 and COE-4 are built of ferrierite-type layers pillared by (-O-Si(CH 3)2-O-) and (-O-Si(OH)2-O-) linker groups, respectively. The structures contain 2D intersecting 10-ring channels running parallel to the ferrierite layers. Because both COE-3 and COE-4 are electron-beam sensitive, a combination of RED datasets from 2 to 3 different crystals was needed for the structure solution and subsequent structure refinement. The structures were further refined by Rietveld refinement against the PXRD data. The structure models obtained from RED and PXRD were compared. This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4dt00458b
  • 2014 • 137 In-situ high-resolution low energy electron diffraction study of strain relaxation in heteroepitaxy of Bi(111) on Si(001): Interplay of strain state, misfit dislocation array and lattice parameter
    Hattab, H. and Jnawali, G. and Horn-Von Hoegen, M.
    Thin Solid Films 159-163 (2014)
    The relief of lattice mismatch-induced strain in Bi(111) on Si(001) heteroepitaxial system was investigated in real time as the Bi film relaxes, by means of high resolution low-energy electron diffraction (LEED). The inherent lattice mismatch of 2.5% at room temperature is accommodated through the formation of an ordered misfit dislocation array confined to the interface. The strain fields of the dislocations cause a periodic height undulation of the surface in the sub-Ångström regime, which is observed through spot splitting in LEED. From a simultaneous measurement of the position of the first-order LEED spots, which corresponds to the lattice parameter of the film, and of the separation of satellite spots, which corresponds to the ordering of the dislocation array, the evolution of the strain state during annealing of a 6 nm Bi film was determined. The strain is solely relaxed by full edge-type dislocations arranged in the ordered array at the interface. From the remaining strain of ε = 0.6% the critical thickness for generation of misfit dislocations under equilibrium conditions can be derived. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2014.08.013
  • 2014 • 136 Tailoring the morphology of mesoporous titania thin films through biotemplating with nanocrystalline cellulose
    Ivanova, A. and Fattakhova-Rohlfing, D. and Kayaalp, B.E. and Rathouský, J. and Bein, T.
    Journal of the American Chemical Society 136 5930-5937 (2014)
    The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing, and photocatalysis. Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distributions. By varying the titania-to-template ratio, it is possible to tune the surface area, pore size, pore anisotropy, and dimensions of titania crystallites in the films. Moreover, a post-treatment at high humidity and subsequent slow template removal can be used to achieve pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The resulting homogeneous transparent films can be directly spin- or dip- coated on glass, silicon, and transparent conducting oxide (TCO) substrates. The mesoporous titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol. Furthermore, the films can be successfully applied as anodes in dye-sensitized solar cells. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja411292u
  • 2014 • 135 Strain state, film and surface morphology of epitaxial topological insulator Bi2Se3 films on Si(111)
    Klein, C. and Vyshnepolsky, M. and Kompch, A. and Klasing, F. and Hanisch-Blicharski, A. and Winterer, M. and Horn-Von Hoegen, M.
    Thin Solid Films 564 241-245 (2014)
    Epitaxial Bi2Se3 films were grown by molecular beam epitaxy on Si(111)-Bi(3×3)R30° at temperatures between 200 and 250 °C. The surface and bulk morphology was characterized by high resolution low energy electron diffraction, X-ray diffraction, and atomic force microscopy for various film thicknesses between 6 and 90 nm. The films are atomically smooth without small angle mosaics or small angle rotational domains. The precise determination of lattice parameter reveals that films grown at higher temperature exhibit a smaller value for the vertical lattice parameter. The presence of random stacking faults in the film is reflected by a parabolic increase of the width of the diffraction peaks in X-ray diffraction. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.04.024
  • 2014 • 134 Silicon/Polyaniline nanocomposites as anode material for Lithium ion batteries
    Kummer, M. and Badillo, J.P. and Schmitz, A. and Bremes, H.-G. and Winter, M. and Schulz, C. and Wiggers, H.
    Journal of the Electrochemical Society 161 A40-A45 (2014)
    Due to of its high Li storage capacity, silicon is a promising anode material for lithium ion batteries. Unfortunately, this high specific capacity leads to extreme volume expansion of about 300% during lithiation and delithiation, that may lead to mechanical disintegration of the electrode and poor cycle life. To improve the cycling behavior, we combined nano-silicon (n-Si) active material with an inactive material that acts as a binder and buffering matrix. Stability, flexibility and conductivity are the main requirements for such matrix material. Polyaniline (PANi), a conducting polymer, meets all these requirements. With a theoretical capacity of 643 mAh g -1, the prepared n-Si/PANi sample showed a higher capacity in respect to the commonly used anode material, graphite. The electrochemical performance of the n-Si/PANi composite is stable compared to the performance of nano-silicon without PANi. After 300 cycles the composite still retains more than 60% of its theoretical capacity. © 2013 The Electrochemical Society.
    view abstractdoi: 10.1149/2.020401jes
  • 2014 • 133 Combining SiGe BiCMOS and InP processing in an on-top of chip integration approach
    Lisker, M. and Trusch, A. and Krüger, A. and Fraschke, M. and Kulse, P. and Marschmeyer, S. and Schmidt, J. and Meliani, C. and Tillack, B. and Weimann, N. and Kraemer, T. and Ostermay, I. and Krüger, O. and Jensen, T. and Al-Sa...
    ECS Transactions 64 177-194 (2014)
    Applications such as radar imaging and wideband communications are driving the research on millimeter-wave circuits. For some applications SiGe hetero junction bipolar transistors (HBTs) are limited in output power. III-V technologies (like InP) can realize devices showing a high product of peak transit frequency multiplied with the open base breakdown voltage. Therefore, merging the qualities of both III-V and Si technology will enable a new class of high-performance ICs. Our approach combines an InP-DHBT transferred-substrate process with a Si-BiCMOS process. The key method is an aligned face-to-face wafer bonding with a subsequent removal of the InP substrate. Different integrated signal sources with an output frequency up to 246 GHz were designed and produced using different combinations of BiCMOS and InP circuit building blocks to demonstrate the capabilities of the heterointegration routine. In this paper the influences of the wafer bonding and the finalization of the InP-DHBT process on SiGe devices were investigated. It was found that the influences on the BiCMOS devices were rather small. © The Electrochemical Society.
    view abstractdoi: 10.1149/06406.0177ecst
  • 2014 • 132 Monitoring of amorfization of the oxygen implanted layers in silicon wafers using photothermal radiometry and modulated free carrier absorption methods
    Maliński, M. and Pawlak, M. and Chrobak, Ł. and Pal, S. and Ludwig, Ar.
    Applied Physics A: Materials Science and Processing 118 1009-1014 (2014)
    This paper presents experimental results that characterize implanted layers in silicon being the result of a high energy implantation of O+6 ions. We propose a simple relation between attenuation of photothermal radiometry and/or modulated free carrier absorption amplitudes, the implanted layer thickness and its optical absorption coefficient. The thickness of the implanted layers was determined from capacitance–voltage characteristics and computations with the TRIM program. The obtained results allowed to estimate changes of the optical absorption coefficient of the oxygen implanted layers indicating the amorfization of the layers. © 2014, The Author(s).
    view abstractdoi: 10.1007/s00339-014-8859-4
  • 2014 • 131 Topotactic condensation of layer silicates with ferrierite-type layers forming porous tectosilicates
    Marler, B. and Wang, Y. and Song, J. and Gies, H.
    Dalton Transactions 43 10396-10416 (2014)
    Five different hydrous layer silicates (HLSs) containing fer layers (ferrierite-type layers) were obtained by hydrothermal syntheses from mixtures of silicic acid, water and tetraalkylammonium/tetraalkylphosphonium hydroxides. The organic cations had been added as structure directing agents (SDA). A characteristic feature of the structures is the presence of strong to medium strong hydrogen bonds between the terminal silanol/siloxy groups of neighbouring layers. The five-layered silicates differ chemically only with respect to the organic cations. Structurally, they differ with respect to the arrangement of the fer layers relative to each other, which is distinct for every SDA-fer-layer system. RUB-20 (containing tetramethylammonium) and RUB-40 (tetramethylphosphonium) are monoclinic with stacking sequence AAA and shift vectors between successive layers 1a0 + 0b0 + 0.19c 0 and 1a0 + 0b0 + 0.24c0, respectively. RUB-36 (diethyldimethylammonium), RUB-38 (methyltriethylammonium) and RUB-48 (trimethylisopropylammonium) are orthorhombic with stacking sequence ABAB and shift vectors 0.5a0 + 0b0 ± 0.36c 0, 0.5a0 + 0b0 + 0.5c0 and 0.5a 0 + 0b0 ± 0.39c0, respectively. Unprecedented among the HLSs, two monoclinic materials are made up of fer layers which possess a significant amount of ordered defects within the layer. The ordered defects involve one particular Si-O-Si bridge which is, to a fraction of ca. 50%, hydrolyzed to form nests of two Si-OH groups. When heated to 500-600 °C in air, the HLSs condense to form framework silicates. Although all layered precursors were moderately to well ordered, the resulting framework structures were of quite different crystallinity. The orthorhombic materials RUB-36, -38 and -48, general formula SDA4Si36O 72(OH)4, which possess very strong hydrogen bonds (d[O⋯O] ≈ 2.4 Å), transform into a fairly or well ordered CDO-type silica zeolite RUB-37. The monoclinic materials RUB-20 and -40, general formula SDA2Si18O36(OH)2OH, possessing medium strong hydrogen bonds (d[O⋯O] ≈ 2.65 Å) are transformed into poorly ordered framework silicates. Some rules of thumb can be established concerning the successful zeolite synthesis via a topotactic condensation of layered precursors. Favourably, the precursor (i) possesses already a well ordered structure without defects, (ii) contains strong inter-layer hydrogen bonds and does not contain strong intra-layer hydrogen bonds and (iii) contains a suitable cation. The nature of the organic cation (size, geometry, flexibility, thermal stability) plays a key role in the formation of a microporous tectosilicate with well ordered structure. RUB-36 which meets these criteria yields a well ordered condensation product (RUB-37). This journal is © the Partner Organisations 2014.
    view abstractdoi: 10.1039/c4dt00262h
  • 2014 • 130 Effect of localized states on the reciprocity between quantum efficiency and electroluminescence in Cu(In,Ga)Se2 and Si thin-film solar cells
    Müller, T.C.M. and Pieters, B.E. and Kirchartz, T. and Carius, R. and Rau, U.
    Solar Energy Materials and Solar Cells 129 95-103 (2014)
    We present the electroluminescence and quantum efficiency of three different types of thin-film solar cells based on absorbers made from Cu(In,Ga)Se2 as well as from μc-Si:H and a-Si:H. Simulations of our experimental results show that the main contribution to the electroluminescence spectrum originates from band-to-tail transitions in the case of Cu(In,Ga)Se2, from tail-to-tail transitions in μc-Si:H, and from tail-to-tail as well as from band-to-midgap-defect transitions in a-Si:H. By comparing the electroluminescence with the quantum efficiency, we analyze the effect of localized states on the optoelectronic reciprocity relation in the three material systems. The relatively steep band-tail density of states in Cu(In,Ga)Se2 is compatible with the reciprocity relation at room temperature while the shallower band-tail density of states in μc-Si:H and the deep mid-gap defect states in a-Si:H lead to substantial deviations from the reciprocity. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.solmat.2014.04.018
  • 2014 • 129 Graphitic nanostripes in silicon carbide surfaces created by swift heavy ion irradiation
    Ochedowski, O. and Osmani, O. and Schade, M. and Bussmann, B.K. and Ban-Detat, B. and Lebius, H. and Schleberger, M.
    Nature Communications 5 (2014)
    The controlled creation of defects in silicon carbide represents a major challenge. A well-known and efficient tool for defect creation in dielectric materials is the irradiation with swift (E kin ‰ 500 ‰keV/amu) heavy ions, which deposit a significant amount of their kinetic energy into the electronic system. However, in the case of silicon carbide, a significant defect creation by individual ions could hitherto not be achieved. Here we present experimental evidence that silicon carbide surfaces can be modified by individual swift heavy ions with an energy well below the proposed threshold if the irradiation takes place under oblique angles. Depending on the angle of incidence, these grooves can span several hundreds of nanometres. We show that our experimental data are fully compatible with the assumption that each ion induces the sublimation of silicon atoms along its trajectory, resulting in narrow graphitic grooves in the silicon carbide matrix.
    view abstractdoi: 10.1038/ncomms4913
  • 2014 • 128 Magnesiothermic conversion of the silicamineralizing golden algae Mallomonas caudata and Synura petersenii to elemental silicon with high geometric precision
    Petrack, J. and Jost, S. and Boenigk, J. and Epple, M.
    Beilstein Journal of Nanotechnology 5 554-560 (2014)
    Chrysophyceae, also known as golden algae, contain characteristic, three-dimensional biomineralized silica structures. Their chemical composition and microscopic structure was studied. By high-temperature conversion of the skeleton of Mallomonas caudata and Synura petersenii into elementary silicon by magnesium vapour, nanostructured defined replicates were produced which were clearly seen after removal of the formed magnesium oxide with acid. © 2014 Petrack et al; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.5.65
  • 2014 • 127 Chemical vapor infiltration of activated carbon with tetramethylsilane
    Pflitsch, C. and Curdts, B. and Helmich, M. and Pasel, C. and Notthoff, C. and Bathen, D. and Atakan, B.
    Carbon 79 28-35 (2014)
    Chemical vapor infiltration of activated carbon with tetramethylsilane (TMS) at 200 hPa total pressure and a gas phase concentration of 15 (mol-)% TMS in nitrogen is studied. The influence of temperature on the infiltration process is discussed in detail. Up to 873 K, the infiltration is performed in the kinetically controlled regime resulting in high loadings up to around 42 (wt.-)%. The modified materials show high values for BET-surface and pore volume indicating a sufficient adoption of the infiltrated silicon layer to the surface morphology of the carbon substrates. Low oxidation resistance of the infiltrated material and EDX measurements give rise to the assumption that the infiltrated material is silicon. At higher infiltration temperatures above 873 K, particles are formed which have the shape of cylindrical nanostructures. EDX measurements reveal that silicon carbide is produced at these temperatures. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2014.07.018
  • 2014 • 126 Benzaldehyde on water-saturated Si(001): Reaction with isolated silicon dangling bonds versus concerted hydrosilylation
    Pierucci, D. and Naitabdi, A. and Bournel, F. and Gallet, J.-J. and Tissot, H. and Carniato, S. and Rochet, F. and Köhler, U. and Laumann, D. and Kubsky, S. and Silly, M.G. and Sirotti, F.
    Journal of Physical Chemistry C 118 10005-10016 (2014)
    Despite strong similarities due to the common presence of silicon monohydrides and isolated silicon dangling bonds (silicon radicals), the water-saturated Si(001)-2 × 1 surface and the hydrogen-terminated Si(001)-2 × 1 surface show very different reactivities with respect to benzaldehyde. By using real-time scanning tunneling microscopy, synchrotron radiation photoemission, X-ray absorption, and high-resolution electron energy loss spectroscopies in combination, we demonstrated that benzaldehyde reacts with the silicon dangling bonds of water-saturated Si (001). As we found no evidence for the abstraction of a nearby H leading to the formation of a new dangling bond, the formation of a stable radical adduct is a plausible explanation. This observation contrasts with the H-terminated case for which benzaldehyde grafting occurs via a radical chain reaction that can propagate after abstraction of a nearby H by the radical adduct. Also at odds with the H-terminated case, a second chemisorption channel is observed [i.e., a concerted hydrosilylation reaction between a surface monohydride (SiH) and the carbonyl moiety] without any participation of the silicon dangling bond. We discuss how the presence of hydroxyls on water-saturated Si(001)-2 × 1 could make its reactivity markedly different from that of H-terminated Si(001)-2 × 1. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp4077678
  • 2014 • 125 Atom probe tomography study of ultrahigh nanocrystallization rates in FeSiNbBCu soft magnetic amorphous alloys on rapid annealing
    Pradeep, K.G. and Herzer, G. and Choi, P. and Raabe, D.
    Acta Materialia 68 295-309 (2014)
    Rapid annealing (4-10 s) induced primary crystallization of soft magnetic Fe-Si nanocrystals in a Fe73.5Si15.5Cu1Nb 3B7 amorphous alloy has been systematically studied by atom probe tomography in comparison with conventional annealing (30-60 min). It was found that the nanostructure obtained after rapid annealing is basically the same, irrespective of the different time scales of annealing. This underlines the crucial role of Cu during structure formation. Accordingly, the clustering of Cu atoms starts at least 50 C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization, resulting in a reduction of available nucleation sites during Fe-Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Nevertheless, rapid annealing in comparison with conventional annealing results in the formation of ∼30% smaller Fe-Si nanocrystals, but of identical structure, volume fraction and chemical composition, indicating the limited influence of thermal treatment on nanocrystallization, owing to the effect of Cu. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2014.01.031
  • 2014 • 124 The Ba 4d-4f giant dipole resonance in complex Ba/Si compounds
    Sahle, C.H.J. and Sternemann, C. and Sternemann, H. and Tse, J.S. and Gordon, R.A. and Desgreniers, S. and Maekawa, S. and Yamanaka, S. and Lehmkühler, F. and Wieland, D.C.F. and Mende, K. and Huotari, S. and Tolan, M.
    Journal of Physics B: Atomic, Molecular and Optical Physics 47 (2014)
    The shape of the Ba 4d-4f giant dipole resonance is studied for Ba atoms embedded inside complex Si networks covering structures consisting of Si nanocages and nanotubes, i.e. the clathrate Ba8Si46, the complex compound BaSi6, and the semiconducting BaSi2. Here, non-resonant x-ray Raman scattering is used to investigate confinement effects on the shape of the giant resonance in the vicinity of the Ba N IV, V-edge. The distinct momentum transfer dependence of the spectra is analyzed and discussed. The measurements are compared to calculations of the giant resonance within time-dependent local density approximation in the dipole limit. No modulation of the giant resonance's shape for Ba atoms confined in different local environments was observed, in contrast to the calculations. The absence of such shape modulation for complex Ba/Si compounds is discussed providing important implications for further studies of giant resonance phenomena utilizing both theory and experiment. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-4075/47/4/045102
  • 2014 • 123 Unoccupied electronic structure and relaxation dynamics of Pb/Si(1 1 1)
    Sandhofer, M. and Sklyadneva, I.Yu. and Sharma, V. and Trontl, V.M. and Zhou, P. and Ligges, M. and Heid, R. and Bohnen, K.-P. and Chulkov, E.V. and Bovensiepen, U.
    Journal of Electron Spectroscopy and Related Phenomena 195 278-284 (2014)
    The unoccupied electronic structure of epitaxial Pb films on Si(1 1 1) is analyzed by angle-resolved two-photon photoemission in the over(Γ, -) → over(M, -) direction close to the Brillouin zone center. The experimental results are compared to density functional theory calculations and we focus on the nature of the interaction of the 6pz states with the Si substrate. The experimentally obtained dispersion E(k||) of the unoccupied quantum well states is weaker than expected for freestanding films, in good agreement with their occupied counterparts. Following E(k||) of quantum well states as a function of momentum at different energies, which are degenerate and non-degenerate with the Si conduction band, we observe no influence of the Si bulk band and conclude a vanishing direct interaction of the Pb 6pz states with the Si band. However, the momentum range at which mixing of 6pz and 6px,y derived subbands is found to occur in the presence of the Si substrate is closer to over(Γ, -) than in the corresponding freestanding film, which indicates a substrate-mediated enhancement of the mixing of these states. Additional femtosecond time-resolved measurements show a constant relaxation time of hot electrons in unoccupied quantum well states as a function of parallel electron momentum which supports our conclusion of a px,y mediated interaction of the pz states with the Si conduction band. © 2014.
    view abstractdoi: 10.1016/j.elspec.2014.04.006
  • 2014 • 122 High-temperature creep and oxidation behavior of Mo-Si-B alloys with high Ti contents
    Schliephake, D. and Azim, M. and Von Klinski-Wetzel, K. and Gorr, B. and Christ, H.-J. and Bei, H. and George, E.P. and Heilmaier, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 45 1102-1111 (2014)
    Multiphase alloys in the Mo-Si-B system are potential high-temperature structural materials due to their good oxidation and creep resistance. Since they suffer from relatively high densities, the current study focuses on the influence of density-reducing Ti additions on creep and oxidation behavior at temperatures above 1273 K (1000 C). Two alloys with compositions of Mo-12.5Si-8.5B-27.5Ti and Mo-9Si-8B-29Ti (in at. pct) were synthesized by arc melting and then homogenized by annealing in vacuum for 150 hours at 1873 K (1600 C). Both alloys show similar creep behavior at stresses of 100 to 300 MPa and temperatures of 1473 K and 1573 K (1200 C and 1300 C), although they possess different intermetallic volume fractions. They exhibit superior creep resistance and lower density than a state-of-the-art Ni-base superalloy (single-crystalline CMSX-4) as well as other Mo-Si-B alloys. Solid solution strengthening due to Ti was confirmed by Vickers hardness measurements and is believed to be the reason for the significant increase in creep resistance compared to Mo-Si-B alloys without Ti, but with comparable microstructural length scales. The addition of Ti degrades oxidation resistance relative to a Mo-9Si-8B reference alloy due to the formation of a relatively porous duplex layer with titania matrix enabling easy inward diffusion of oxygen. © 2013 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-013-1944-z
  • 2014 • 121 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 • 120 In situ nanoparticle size measurements of gas-borne silicon nanoparticles by time-resolved laser-induced incandescence
    Sipkens, T.A. and Mansmann, R. and Daun, K.J. and Petermann, N. and Titantah, J.T. and Karttunen, M. and Wiggers, H. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 116 623-636 (2014)
    This paper describes the application of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used mainly for measuring soot primary particles, to size silicon nanoparticles formed within a plasma reactor. Inferring nanoparticle sizes from TiRe-LII data requires knowledge of the heat transfer through which the laser-heated nanoparticles equilibrate with their surroundings. Models of the free molecular conduction and evaporation are derived, including a thermal accommodation coefficient found through molecular dynamics. The model is used to analyze TiRe-LII measurements made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon and hydrogen. Nanoparticle sizes inferred from the TiRe-LII data agree with the results of a Brunauer-Emmett-Teller analysis. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-013-5745-2
  • 2014 • 119 In Situ Particle Size Measurements of Gas-borne Silicon Nanoparticles by Time-resolved Laser-induced Incandescence
    Sipkens, T. A. and Petermann, N. and Daun, K. J. and Titantah, J. and Karttunen, M. and Wiggers, H. and Dreier, T. and Schulz, C.
    Proceedings of the Asme Summer Heat Transfer Conference - 2013, Vol 1 V001T03A001 (2014)
    The functionality of silicon nanoparticles is strongly size-dependent, so there is a pressing need for laser diagnostics that can characterize aerosolized silicon nanoparticles. The present work is the first attempt to extend time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used for sizing soot, to size silicon nanoparticles. TiRe-LII measurements are made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon. Molecular dynamics (MD) is used to predict the accommodation coefficient between silicon nanoparticles and argon and helium, which is needed to interpret the TiRe-LII data. The MD-derived thermal accommodation coefficients will be validated by comparing them to experimentally-derived values found using transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis.
    view abstractdoi: 10.1115/HT2013-17246
  • 2014 • 118 Raman spectroscopy study of silicon nanoribbons on Ag(110)
    Speiser, E. and Buick, B. and Esser, N. and Richter, W. and Colonna, S. and Cricenti, A. and Ronci, F.
    Applied Physics Letters 104 (2014)
    The grating of self-assembled Si nanoribbons at the Ag(110) surface has been studied by Raman spectroscopy. The study, conducted in situ with uncapped samples, resulted in phonon frequencies in disagreement with the results of theoretical calculations reported in literature for freestanding silicene sheets and nanoribbons. These results suggest that the structure of these silicon nanoribbons is very different from the planar and lightly buckled structural models and that the influence of the substrate might be underestimated in the discussed structural models. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4872460
  • 2014 • 117 Spatially resolved determination of thermal conductivity by Raman spectroscopy
    Stoib, B. and Filser, S. and Stötzel, J. and Greppmair, A. and Petermann, N. and Wiggers, H. and Schierning, G. and Stutzmann, M. and Brandt, M.S.
    Semiconductor Science and Technology 29 (2014)
    We review the Raman shift method as a non-destructive optical tool to investigate the thermal conductivity and demonstrate the possibility to map this quantity with a micrometer resolution by studying thin film and bulk materials for thermoelectric applications. In this method, a focused laser beam both thermally excites a sample and undergoes Raman scattering at the excitation spot. The temperature dependence of the phonon energies measured is used as a local thermometer. We discuss that the temperature measured is an effective one and describe how the thermal conductivity is deduced from single temperature measurements to full temperature maps, with the help of analytical or numerical treatments of heat diffusion. We validate the method and its analysis on three- and two-dimensional single crystalline samples before applying it to more complex Si-based materials. A suspended thin mesoporous film of phosphorus-doped lasersintered Si 78 Ge 22 nanoparticles is investigated to extract the in-plane thermal conductivity from the effective temperatures, measured as a function of the distance to the heat sink. Using an iterative multigrid Gauss-Seidel algorithm the experimental data can be modelled yielding a thermal conductivity of 0.1 W/m K after normalizing by the porosity. As a second application we map the surface of a phosphorus-doped three-dimensional bulk-nanocrystalline Si sample which exhibits anisotropic and oxygen-rich precipitates. Thermal conductivities as low as 11 W/m K are found in the regions of the precipitates, signi ficantly lower than the 17 W/m K in the surrounding matrix. The present work serves as a basis to more routinely use the Raman shift method as a versatile tool for thermal conductivity investigations, both for samples with high and low thermal conductivity and in a variety of geometries. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0268-1242/29/12/124005
  • 2014 • 116 Pressure-induced changes on the electronic structure and electron topology in the direct FCC → SH transformation of silicon
    Tse, J.S. and Hanfland, M. and Flacau, R. and Desgreniers, S. and Li, Z. and Mende, K. and Gilmore, K. and Nyrow, A. and Moretti Sala, M. and Sternemann, C.
    Journal of Physical Chemistry C 118 1161-1166 (2014)
    X-ray diffraction experiments at 80 K show that when silicon is compressed under hydrostatic conditions the intermediate high-pressure phases are bypassed leading to a direct transformation to the simple hexagonal structure at 17 GPa. A maximum entropy analysis of the diffraction patterns reveals dramatic alterations in the valence electron distribution from tetrahedral covalent bonding to localization in the interstitial sites and along the one-dimensional silicon atom chain running along adjacent hexagonal layers. Changes in the orbital character of the unoccupied states are confirmed using X-ray Raman scattering spectroscopy and theoretical Bethe-Salpeter equation calculations. This is the first direct observation indicating that the silicon valence electrons in 3s and 3p orbitals are transferred to the 3d orbitals at high density which proves that electrons of compressed elemental solids migrate from their native bonding configuration to interstitial regions. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp408666q
  • 2014 • 115 Atomistic study of the influence of lattice defects on the thermal conductivity of silicon
    Wang, T. and Madsen, G.K.H. and Hartmaier, A.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Lattice defects such as vacancies, voids and dislocations are inevitably present in any material of technological interest. In this work, non-equilibrium molecular dynamics simulations are conducted to investigate how the monatomic vacancies and nanovoids influence the lattice thermal conductivity of silicon. The results show a clear non-linear decrease of the thermal conductivity with increasing defect volume fraction. Furthermore, it is found that for a given volume fraction of defects, a random distribution shows a lower lattice thermal conductivity. To develop a fundamental understanding of these observations, the spectral energy densities for all phonon branches obtained from 2D Fourier transformations of the atomic trajectories are analyzed. This yields the mean phonon group velocities and relaxation times, which are the main physical quantities contributing to the lattice thermal conductivity. Our analysis reveals that the phonon relaxation time is the most important parameter for describing the degrading of the thermal transport behavior in the defected structures. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/035011
  • 2014 • 114 Large recovery strain in Fe-Mn-Si-based shape memory steels obtained by engineering annealing twin boundaries
    Wen, Y.H. and Peng, H.B. and Raabe, D. and Gutierrez-Urrutia, I. and Chen, J. and Du, Y.Y.
    Nature Communications 5 (2014)
    Shape memory alloys are a unique class of materials that can recover their original shape upon heating after a large deformation. Ti-Ni alloys with a large recovery strain are expensive, while low-cost conventional processed Fe-Mn-Si-based steels suffer from a low recovery strain (<3%). Here we show that the low recovery strain results from interactions between stress-induced martensite and a high density of annealing twin boundaries. Reducing the density of twin boundaries is thus a critical factor for obtaining a large recovery strain in these steels. By significantly suppressing the formation of twin boundaries, we attain a tensile recovery strain of 7.6% in an annealed cast polycrystalline Fe-20.2Mn-5.6Si-8.9Cr-5.0Ni steel (weight%). Further attractiveness of this material lies in its low-cost alloying components and simple synthesis-processing cycle consisting only of casting plus annealing. This enables these steels to be used at a large scale as structural materials with advanced functional properties © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms5964
  • 2014 • 113 Formation of intermetallic phases in Al-coated hot-stamped 22MnB5 sheets in terms of coating thickness and Si content
    Windmann, M. and Röttger, A. and Theisen, W.
    Surface and Coatings Technology 246 17-25 (2014)
    AlSiFe coatings with differing thicknesses and Si contents were applied to steel sheets by hot dipping. The steel sheets were austenitized at TAUS=920°C for different dwell times and then quenched in water. Phase formation as a function of coating thickness and Si content at the steel substrate/coating interface was investigated by ex-situ phase analysis with synchrotron radiation and by electron backscatter diffraction (EBSD). X-ray diffraction (XRD) and EBSD investigations confirmed the formation of AlFe-rich intermetallics at the steel/coating interface as a result of a strong diffusion of the elements Al and Fe. Within the first minute, Fe diffusion into the partially melted Al-base coatings promotes the formation of intermetallics of type Al8Fe2Si, Al13Fe4, and Al5Fe2. After the coating has transformed completely into Al-Fe intermetallics, Al diffusion into the steel substrate becomes more pronounced, thus reducing the Al content in the Al-Fe intermetallics and promoting formation of the phases of type Al2Fe and AlFe in the coating and formation of an Al-rich bcc layer in the steel substrate. The transformation kinetics of the resulting Al-, Fe-rich intermetallics are influenced by the coating thickness and the chemical composition of the Al-base coating. On the one hand, faster saturation of Fe in the Al-base coating is promoted by a shorter diffusion path and therefore by a thinner coating thickness. Otherwise, Si influences the diffusivity of the elements Al and Fe in the Al-, Fe-rich intermetallics and promotes the formation of Si-richer intermetallics, which then act as nuclei for Fe-richer intermetallics. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2014.02.056
  • 2014 • 112 Superplastic Mn-Si-Cr-C duplex and triplex steels: Interaction of microstructure and void formation
    Zhang, H. and Ponge, D. and Raabe, D.
    Materials Science and Engineering A 610 355-369 (2014)
    Duplex and triplex microstructures consisting initially of ferrite plus carbide or of martensite, ferrite plus carbide, respectively, can undergo strain induced austenite formation during superplastic deformation at 30K below Ae1 (Ae1: equilibrium pearlite-austenite transformation temperature) and low strain rate (e.g. 2×10-3s-1). The effect leads to excellent superplasticity of the materials (elongation ~500%, flow stress < 50MPa) through fine austenite grains (~10μm). Using a deformation temperature just below Ae1 leads to a weak driving force for both, carbide dissolution and austenite formation. Thereby a sufficient volume fraction of carbides (1-2μm, 15vol%) is located at austenite grain boundaries suppressing austenite grain growth during superplastic deformation. Also, void nucleation and growth in the superplastic regime are slowed down within the newly transformed austenite plus carbide microstructure. In contrast, austenite grains and voids grow fast at a high deformation temperature (120K above Ae1). At a low deformation temperature (130K below Ae1), strain induced austenite formation does not occur and the nucleation of multiple voids at the ferrite-carbide interfaces becomes relevant. The fast growth of grains and voids as well as the formation of multiple voids can trigger premature failure during tensile testing in the superplastic regime. EBSD is used to analyze the microstructure evolution and void formation during superplastic deformation, revealing optimum microstructural and forming conditions for superplasticity of Mn-Si-Cr-C steels. The study reveals that excellent superplasticity can be maintained even at 120K above Ae1 by designing an appropriate initial duplex ferrite plus carbide microstructure. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2014.05.061
  • 2014 • 111 Potential-induced degradation in solar cells: Electronic structure and diffusion mechanism of sodium in stacking faults of silicon
    Ziebarth, B. and Mrovec, M. and Elsässer, C. and Gumbsch, P.
    Journal of Applied Physics 116 (2014)
    Sodium decorated stacking faults (SFs) were recently identified as the primary cause of potential-induced degradation in silicon (Si) solar-cells due to local electrical short-circuiting of the p-n junctions. In the present study, we investigate these defects by first principles calculations based on density functional theory in order to elucidate their structural, thermodynamic, and electronic properties. Our calculations show that the presence of sodium (Na) atoms leads to a substantial elongation of the Si-Si bonds across the SF, and the coverage and continuity of the Na layer strongly affect the diffusion behavior of Na within the SF. An analysis of the electronic structure reveals that the presence of Na in the SF gives rise to partially occupied defect levels within the Si band gap that participate in electrical conduction along the SF. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4894007
  • 2014 • 110 Mechanical properties of SiLix thin films at different stages of electrochemical Li insertion
    Zinn, A.-H. and Borhani-Haghighi, S. and Ventosa, E. and Pfetzing-Micklich, J. and Wieczorek, N. and Schuhmann, W. and Ludwig, Al.
    Physica Status Solidi (A) Applications and Materials Science 211 2650-2656 (2014)
    The mechanical properties of amorphous Si thin films, lithiated electrochemically to different Si£Li compositions are studied by ex situ nanoindentation. The compositions of the films are adjusted using an electrochemical routine that corrects for the Li consumed by SEI layer growth during initial lithiation. The mechanical properties such as Young's modulus and hardness are derived from nanoindentation. For compositions between Si and SiLi<inf>2.5</inf> the Young's modulus decreases with increasing Li content from ∼160 GPa to ∼8 GPa and the hardness decreases from ∼14 GPa to ∼0.1 GPa. The yield strength values, as deduced from hardness measurements, decrease from ∼5 GPa to 0.05 GPa. AFM imaging is used on the electrochemically cycled films to assess the SEIs impact on the nanomechanical measurements. XPS depth-profiling of the electrochemically cycled sample indicated a Li concentration gradient across the film thickness. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.201431303
  • 2013 • 109 Design and experimental evaluation of a new nanoparticle thermophoretic personal sampler
    Azong-Wara, N. and Asbach, C. and Stahlmecke, B. and Fissan, H. and Kaminski, H. and Plitzko, S. and Bathen, D. and Kuhlbusch, T.A.J.
    Journal of Nanoparticle Research 15 (2013)
    A personal sampler that thermophoretically samples particles between a few nanometers and approximately 300 nm has been designed and first prototypes built. The thermal precipitator (TP) is designed to take samples in the breathing zone of a worker in order to determine the personal exposure to airborne nanomaterials. In the sampler, particles are deposited onto silicon substrates that can be used for consecutive electron microscopic (EM) analysis of the particle size distribution and chemical composition of the sampled particles. Due to very homogeneous size-independent particle deposition on a large portion of the substrate, representative samples can be taken for offline analysis. The experimental evaluation revealed a good general agreement with numerical simulations concerning homogeneity of the deposit and a very high correlation (R^2 = 0.98) of the deposition rate per unit area with number concentrations simultaneously measured with an SMPS for particle sizes between 14 and 305 nm. The samplers' small size of only 45 x 32 × 97 mm3 and low weight of only 140 g make it perfectly suitable as a personal sampler. The power consumption for temperature control and pump is around 1.5 W and can be easily provided by batteries. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-013-1530-8
  • 2013 • 108 Internal detection of surface plasmon coupled chemiluminescence during chlorination of potassium thin films
    Becker, F. and Krix, D. and Hagemann, U. and Nienhaus, H.
    Journal of Chemical Physics 138 (2013)
    The interaction of chlorine with potassium surfaces is a prototype reaction with a strong non-adiabatic energy transfer leading to exoemission and chemiluminescence. Thin film K/Ag/p-Si(111) Schottky diodes with 8 nm potassium on a 5-200 nm thick Ag layer are used as 2π-photodetectors for the chemiluminescence during chlorination of the K film at 110 K. The observed photocurrent shows a sharp maximum for small exposures and decreases gradually with the increasing chloride layer. The time dependence can be explained by the reaction kinetics, which is governed initially by second-order adsorption processes followed by an electric field-assisted diffusion. The detector current corresponds to a yield of a few percent of elementary charge per reacting chlorine molecule and is orders of magnitude larger than for external detection. The photoyield can be enhanced by increasing the Ag film thickness. For Ag films of 30 and 50 nm, the yield exhibits a maximum indicating surface plasmon coupled chemiluminescence. Surface plasmon polaritons in the Ag layer are excited by the reaction and decay radiatively into Si leading to the observed currents. A model calculation for the reverse process in attenuated total reflection is applied to explain the observed current yield maxima. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4776156
  • 2013 • 107 Fabrication of periodic surface topographies via sequential photothermal laser microsintering of silicon nanoparticle films
    Behrenberg, D. and Franzka, S. and Petermann, N. and Wiggers, H. and Hartmann, N.
    Applied Surface Science 278 278-283 (2013)
    Photothermal laser microprocessing is exploited in order to induce sintering and compaction of thin silicon nanoparticle (Si NP) films. Ethanolic dispersions of Si NPs with an average diameter of 45 nm are spin-coated on silicon substrates yielding films with a thickness of about 400-500 nm. Scanning electron and atomic force microscopy are used for characterization of the resulting surface morphologies. Sequential processing of the coated layer with a microfocused cw-laser beam at a wavelength of 532 nm generates periodic surface structures. The periodicity of these structures is determined by the distance between adjacent laser-written lines. Despite a 1/e laser spot size of 1.4 μm, fabrication of topographic surface structures with submicrometer periodicities is feasible. In particular, surface topographies with periodicities of 600 nm and a topographic amplitude of 80 nm are fabricated. These results point to a high nonlinearity, which is attributed to the strongly activated, temperature-dependent laser sintering process. These experimental observations are reproduced qualitatively considering a simple photothermal model and an activated sintering process. Prospects of photothermal laser microsintering of nanoparticle films to fabricate biomimetic surface structures are discussed. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2013.01.205
  • 2013 • 106 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 • 105 Ultrathin magnetic oxide EuO films on Si(001) using SiOx passivation-Controlled by hard x-ray photoemission spectroscopy
    Caspers, C. and Flade, S. and Gorgoi, M. and Gloskovskii, A. and Drube, W. and Schneider, C.M. and Müller, M.
    Journal of Applied Physics 113 (2013)
    We present the chemical and structural optimization of ultrathin magnetic oxide EuO films on silicon. By applying a controlled in situ passivation of the Si(001) surface with SiOx in the monolayer regime, metallic silicide contaminations at the interface can be effectively reduced down to a sub-monolayer coverage, as was carefully quantified by interface-sensitive hard x-ray photoemission spectroscopy. Heteroepitaxial growth of EuO on Si(001) is sustained for this ultrathin SiOx-passivation, and bulk-near magnetic properties are observed for the 4 nm-thin EuO films. Our successful combination of chemically and structurally optimized EuO/Si(001) heterostructures by ultrathin in situ SiOx passivation makes this system promising for an application as alternative spin functional tunnel contacts in spin-FETs. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4795010
  • 2013 • 104 Solid state NMR structural studies of the lithiation of nano-silicon:: Effects of charging capacities, host-doping, and thermal treatment
    Cattaneo, A.S. and Dupke, S. and Schmitz, A. and Badillo, J.P. and Winter, M. and Wiggers, H. and Eckert, H.
    Solid State Ionics 249-250 41-48 (2013)
    The electrochemical lithiation of undoped, P-doped and B-doped nano-silicon particles (100-200 nm diameter) has been studied during the first cycle by ex-situ 6Li and 7Li magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Samples were charged within pouch cells up to capacities of 4000 mAh/g, at C/40 followed by NMR analysis. The spectra reveal important quantitative information on the local lithium environments during the various stages of the intercalation process. Approximate Li/Si ratios of the lithium silicides present in the nanoparticles can be deduced, based on the initial formation of the SEI layer, which accounts for an irreversible capacity of up to 500 mAh/g. Surface lithium silicide environments with high Li concentrations (corresponding to the composition Li15Si4) are preferentially formed at charging capacities near 1000 mAh/g. At higher charging capacities, irreversible capacity losses are lower and a wide distribution of lithium silicide environments is found, resembling those present in the crystalline phases Li12Si7, Li7Si 3, and Li13Si4. At a charging capacity of and above 2000 mAh/g the large majority of silicon is converted to lithiated silicide particles. Boron-doped nano-Si materials behave generally similar, while phosphorus-doping reveals clear beneficial effects, in particular concerning the initial lithiation stages. Both irreversible capacity losses and surface "over-lithiation" are significantly diminished in these samples. Exposure of lithiated nano-Si samples to elevated temperatures (400-440 K) results in the crystallization of Li7Si3 in all of those nano-Si samples charged with at least 1500 mAh/g. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.ssi.2013.07.013
  • 2013 • 103 A new thermoelectric concept using large area PN junctions
    Chavez, R. and Becker, A. and Kessler, V. and Engenhorst, M. and Petermann, N. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1543 3-8 (2013)
    A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines thermally induced charge generation and separation in a space charge region with the conventional Seebeck effect by applying a temperature gradient parallel to the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. The investigated PN junction devices are produced by stacking n-type and p-type nanopowder prior to a densification process. The nanoparticulate nature of the densified PN junction lowers thermal conductivity and increases the intraband traps density which we propose is beneficial for transport across the PN junction thus enhancing the thermoelectric properties. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator. © 2013 Materials Research Society.
    view abstractdoi: 10.1557/opl.2013.954
  • 2013 • 102 Morphology control of thin P3HT-Si-NCs composite films for hybrid photovoltaic cells
    Cunha, M. and Bernardo, G. and Hilliou, L. and Wiggers, H. and Pereira, R.N.
    Materials Science Forum 730-732 227-231 (2013)
    This work deals with an experimental investigation of the microstructure/morphology of spin-casted composite thin films of poly(3-hexylthiophene) (P3HT) and silicon nanocrystals (Si- NCs), in the weight proportion 1:1, which develop under different deposition conditions. The experimental parameters considered were the following: i) solvent quality; ii) spinning rate; iii) spinning time and iv) solution concentration. The developed morphologies were characterized by means of optical microscopy and X-ray diffraction (XRD) measurements. The present work aims at a) establishing the relationship between processing conditions and resultant morphology and b) defining the most relevant processing parameters that govern and are of significance for the induced morphology. © (2013) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/MSF.730-732.227
  • 2013 • 101 Theoretical prediction of improved figure-of-merit in Si/Ge quantum dot superlattices
    Fiedler, G. and Kratzer, P.
    New Journal of Physics 15 (2013)
    A detailed theoretical model for thermoelectric transport perpendicular to the multilayers of a Si-Ge heterostructure is presented. The electronic structure of a three-dimensional superlattice, consisting of a regular array of Ge quantum dots in each layer, capped by Si layers, is calculated using an atomistic tight-binding approach. The Seebeck coefficient, the electric conductivity and the contribution of the electrons to the thermal conductivity for n-doped samples are worked out within Boltzmann transport theory. Using experimental literature data for the lattice thermal conductivity, we determine the temperature dependence of the figure of merit ZT. A nonlinear increase of ZT with temperature is found, with ZT > 2 at T = 1000 K in highly doped samples. Moreover, we find an enhanced thermoelectric power factor already at room temperature and below, which is due to highly mobile electrons in strain-induced conductive channels. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/12/125010
  • 2013 • 100 Ultra-fast electron diffraction at surfaces: From nanoscale heat transport to driven phase transitions
    Hanisch-Blicharski, A. and Janzen, A. and Krenzer, B. and Wall, S. and Klasing, F. and Kalus, A. and Frigge, T. and Kammler, M. and Horn-von Hoegen, M.
    Ultramicroscopy 127 2-8 (2013)
    Many fundamental processes of structural changes at surfaces occur on a pico- or femtosecond time scale. In order to study such ultra-fast processes, we have combined modern surface science techniques with fs-laser pulses in a pump-probe scheme. Reflection high energy electron diffraction (RHEED) with grazing incident electrons ensures surface sensitivity for the probing electron pulses. Utilizing the Debye-Waller effect, we studied the cooling of vibrational excitations in monolayer adsorbate systems or the nanoscale heat transport from an ultra-thin film through a hetero-interface on the lower ps-time scale. The relaxation dynamics of a driven phase transition far away from thermal equilibrium is demonstrated with the In-induced (8×2) reconstruction on Si(111). This surface exhibits a Peierls-like phase transition at 100. K from a (8×2) ground state to (4×1) excited state. Upon excitation by a fs-laser pulse, this structural phase transition is driven into an excited (4×1) state at a sample temperature of 20. K. Relaxation into the (8×2) ground state occurs after more than 150. ps. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2012.07.017
  • 2013 • 99 Modelling the lattice dynamics in SixGe1-x alloys
    Katre, A. and Drautz, R. and Madsen, G.K.H.
    Journal of Physics Condensed Matter 25 (2013)
    The development of simplified models for the simulation of thermodynamic and thermal transport properties in random alloys is of great importance. In this paper we show how a simple second nearest neighbour model can reliably capture the lattice dynamics of SixGe1-x alloys. The model parameters are extracted from DFT-calculated force constant matrices for pure Si, pure Ge and the Si0.5Ge0.5 ordered alloy. We extract the nearest neighbour contributions directly from density functional theory, whereas effective interactions are obtained for the second nearest neighbour contributions. We demonstrate how the thermal properties, including the expansion coefficient, can be reliably reproduced and that the model is transferable to random SixGe1-x alloys. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/25/36/365403
  • 2013 • 98 Thermoelectric properties of nanocrystalline silicon from a scaled-up synthesis plant
    Kessler, V. and Gautam, D. and Hülser, T. and Spree, M. and Theissmann, R. and Winterer, M. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Advanced Engineering Materials 15 379-385 (2013)
    Silicon based thermoelectrics are promising candidates for high temperature energy scavenging applications. We present the properties of thermoelectrics made from highly boron doped silicon nanoparticles. The particles were produced by a continuous gas phase process in a scaled-up synthesis plant enabling production rates in the kg h-1 regime. The silicon nanoparticles were compacted by direct current assisted sintering to yield nanocrystalline bulk silicon with average crystallite size between 40 and 80 nm and relative densities above 97% of the density of single crystalline silicon. The influence of the sintering temperature on the thermoelectric properties is investigated. It was found that high sintering temperatures are beneficial for an enhancement of the power factor, while the thermal conductivity was only moderately affected. The optimization of the compaction procedure with respect to the transport properties leads to zT values of the p-type nanosilicon of 0.32 at 700 °C, demonstrating the potential of our method. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adem.201200233
  • 2013 • 97 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 • 96 Al-induced faceting of Si(113)
    Klein, C. and Heidmann, I. and Nabbefeld, T. and Speckmann, M. and Schmidt, T. and Meyer zu Heringdorf, F.-J. and Falta, J. and Horn-Von Hoegen, M.
    Surface Science 618 109-114 (2013)
    Adsorption of Al on a Si(113) substrate at elevated temperatures causes a faceting transition of the initially flat surface. The (113) surface decomposes into a quasi-periodic sequence of Al terminated (115)- and (112)-facets. The resulting surface morphology is characterized in-situ by reciprocal space maps obtained with in-situ spot profile analyzing low-energy electron diffraction and ex-situ atomic force microscopy. The periodicity length of the faceted surface increases with adsorption temperature from 7 nm at 650 C to 80 nm at 800 C. The stability of the Al terminated Si(112) surface is the driving force for the faceting transition. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.007
  • 2013 • 95 Low-temperature oxidation of alkali overlayers: Ionic species and reaction kinetics
    Krix, D. and Nienhaus, H.
    Applied Surface Science 270 231-237 (2013)
    Clean and oxidized alkali metal films have been studied using X-ray photoelectron spectroscopy (XPS). Thin films, typically 10 nm thick, of lithium, sodium, potassium, rubidium and cesium have been deposited on silicon substrates and oxidized at 120 K. Plasmon losses were found to dress the primary photo emission structures of the metals' core lines which confirms the metallic, bulk like nature of the films. The emission from the O 1s core levels was used to determine the chemical composition and the reaction kinetics during the exposure to molecular oxygen at low pressures. Molecular oxide ions O2- and O22- as well as atomic oxygen ions O2- were detected in varying amounts depending on the alkali metal used. Diffusive transport of material in the film is shown to greatly determine the composition of the oxides. Especially, the growth of potassium superoxide is explained by the diffusion of potassium atoms to the surface and growth at the surface in a Deal-Grove like model.
    view abstractdoi: 10.1016/j.apsusc.2013.01.008
  • 2013 • 94 Atomic scale investigation of redistribution of alloying elements in pearlitic steel wires upon cold-drawing and annealing
    Li, Y.J. and Choi, P. and Goto, S. and Borchers, C. and Raabe, D. and Kirchheim, R.
    Ultramicroscopy 132 233-238 (2013)
    A local electrode atom probe has been employed to analyze the redistribution of alloying elements including Si, Mn, and Cr in pearlitic steel wires upon cold-drawing and subsequent annealing. It has been found that the three elements undergo mechanical mixing upon cold-drawing at large strains, where Mn and Cr exhibit a nearly homogeneous distribution throughout both ferrite and cementite, whereas Si only dissolves slightly in cementite. Annealing at elevated temperatures leads to a reversion of the mechanical alloying. Si atoms mainly segregate at well-defined ferrite (sub)grain boundaries formed during annealing. Cr and Mn are strongly concentrated in cementite adjacent to the ferrite/cementite interface due to their lower diffusivities in cementite than in ferrite. © 2012.
    view abstractdoi: 10.1016/j.ultramic.2012.10.010
  • 2013 • 93 Blocking growth by an electrically active subsurface layer: The effect of si as an antisurfactant in the growth of GaN
    Markurt, T. and Lymperakis, L. and Neugebauer, J. and Drechsel, P. and Stauss, P. and Schulz, T. and Remmele, T. and Grillo, V. and Rotunno, E. and Albrecht, M.
    Physical Review Letters 110 (2013)
    Combining aberration corrected high resolution transmission electron microscopy and density functional theory calculations we propose an explanation of the antisurfactant effect of Si in GaN growth. We identify the atomic structure of a Si delta-doped layer (commonly called SiNx mask) as a SiGaN3 monolayer that resembles a √3×√3 R30 surface reconstruction containing one Si atom, one Ga atom, and a Ga vacancy (V Ga) in its unit cell. Our density functional theory calculations show that GaN growth on top of this SiGaN3 layer is inhibited by forming an energetically unfavorable electrical dipole moment that increases with layer thickness and that is caused by charge transfer between cation dangling bonds at the surface to VGa bound at subsurface sites. © 2013 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.110.036103
  • 2013 • 92 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 • 91 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 • 90 Strong enhancement of Eu+3 luminescence in europium-implanted GaN by Si and Mg codoping
    Mishra, J.K. and Langer, T. and Rossow, U. and Shvarkov, S. and Wieck, A. and Hangleiter, A.
    Applied Physics Letters 102 (2013)
    A strong enhancement of Eu3+ luminescence in europium-implanted GaN samples is obtained by codoping with silicon (Si) and magnesium (Mg), simultaneously. The Eu3+ intensity in the 5D0 to 7F2 transition region is found to be 30 times higher compared to europium-implanted undoped GaN. The major contribution to this overall enhancement is due a weak peak present only in europium-implanted Mg-doped GaN at 2.0031 eV (618.9 nm) which is strongly enhanced by codoping both Mg and Si. The excitation process of europium ions is proposed to take place through a donor-acceptor pair related energy transfer mechanism. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4793207
  • 2013 • 89 Low temperature diffusion of Li atoms into Si nanoparticles and surfaces
    Nienhaus, H. and Karacuban, H. and Krix, D. and Becker, F. and Hagemann, U. and Steeger, D. and Bywalez, R. and Schulz, C. and Wiggers, H.
    Journal of Applied Physics 114 (2013)
    The diffusion of Li atoms deposited on hydrogen-passivated Si(001) surfaces, chemically oxidized Si(001) surfaces, Si nanoparticle films, and thick SiO2 layers is investigated with electron-beam induced Auger electron spectroscopy. The nanoparticles exhibit an average diameter of 24 nm. The Li metal film is evaporated at a sample temperature below 120 K. The reappearance of the Si substrate Auger signal as a function of time and temperature can be measured to study the Li diffusion into the bulk material. Values for the diffusion barrier of 0.5 eV for H:Si(001) and 0.3 eV for the ox-Si(001) and Si nanoparticle films are obtained. The diffusion of the Li atoms results in the disruption of the crystalline Si surfaces observed with atomic force microscopy. Contrasting to that, the Si nanoparticle films show less disruption by Li diffusion due to filling of the porous films detected with cross section electron microscopy. Silicon dioxide acts as a diffusion barrier for temperatures up to 300 K. However, the electron beam induces a reaction between Li and SiO2, leading to LiOx and elemental Si floating on the surface. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4813872
  • 2013 • 88 Surface morphology and atomic structure of thin layers of Fe3Si on GaAs(001) and their magnetic properties
    Noor, S. and Barsukov, I. and Özkan, M.S. and Elbers, L. and Melnichak, N. and Lindner, J. and Farle, M. and Köhler, U.
    Journal of Applied Physics 113 (2013)
    The structural and magnetic properties of ultrathin near-stoichiometric Fe3Si layers on GaAs(001) are investigated after using scanning tunneling microscopy (STM) analysis to optimize the deposition process. This includes atomic resolution imaging of the surface as measured by STM revealing the atomic ordering and characteristic defects in the topmost layers. Emphasis is laid on connections between the layer morphology and its magnetic properties, which are analysed by in situ MOKE, FMR, and SQUID magnetometry. Upon nucleation, the Fe3Si islands behave like superparamagnetic nanoparticles where we find a quantitative agreement between the size of the nanoparticles and their superspin. At higher coverage, the Fe3Si layers show ferromagnetic behaviour. Here, we investigate the superposition of the magnetocrystalline and the uniaxial anisotropies where the latter can be excluded to be caused by shape anisotropy. Furthermore, an unexpected increase of the magnetic moment towards low coverage can be observed which apart from an increased orbital moment can be attributed to an increased step density. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4795163
  • 2013 • 87 Structural changes in amorphous GexSiOy on the way to nanocrystal formation
    Nyrow, A. and Sternemann, C. and Sahle, Ch.J. and Hohl, A. and Zschintzsch-Dias, M. and Schwamberger, A. and Mende, K. and Brinkmann, I. and Moretti Sala, M. and Wagner, R. and Meier, A. and Völklein, F. and Tolan, M.
    Nanotechnology 24 (2013)
    Temperature induced changes of the local chemical structure of bulk amorphous GexSiOy are studied by Ge K-edge x-ray absorption near-edge spectroscopy and Si L2/3-edge x-ray Raman scattering spectroscopy. Different processes are revealed which lead to formation of Ge regions embedded in a Si oxide matrix due to different initial structures of as-prepared samples, depending on their Ge/Si/O ratio and temperature treatment, eventually resulting in the occurrence of nanocrystals. Here, disproportionation of GeOx and SiOx regions and/or reduction of Ge oxides by pure Si or by a surrounding Si sub-oxide matrix can be employed to tune the size of Ge nanocrystals along with the chemical composition of the embedding matrix. This is important for the optimization of the electronic and luminescent properties of the material. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/24/16/165701
  • 2013 • 86 Nano-gold diggers: Au-Assisted SiO2-decomposition and desorption in supported nanocatalysts
    Ono, L.K. and Behafarid, F. and Cuenya, B.R.
    ACS Nano 7 10327-10334 (2013)
    An investigation of the thermal stability of size-selected Au nanoparticles (NPs) synthesized via inverse micelle encapsulation and deposited on SiO 2(4 nm)/Si(100) is presented. The size and mobility of individual Au NPs after annealing at elevated temperatures in ultrahigh vacuum (UHV) was monitored via atomic force microscopy (AFM). An enhanced thermal stability against coarsening and lack of NP mobility was observed up to 1343 K. In addition, a drastic decrease in the average NP height was detected with increasing annealing temperature, which was not accompanied by the sublimation of Au atoms/clusters in UHV. The apparent decrease in the Au NP height observed is assigned to their ability to dig vertical channels in the underlying SiO 2 support. More specifically, a progressive reduction in the thickness of the SiO2 support underneath and in the immediate vicinity of the NPs was evidenced, leading to NPs partially sinking into the SiO2 substrate. The complete removal of silicon oxide in small patches was observed to take place around the Au NPs after annealing at 1343 K in UHV. These results reveal a Au-assisted oxygen desorption from the support via reverse oxygen spillover to the NPs. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn404744b
  • 2013 • 85 High-throughput study of the structural stability and thermoelectric properties of transition metal silicides
    Opahle, I. and Parma, A. and McEniry, E.J. and Drautz, R. and Madsen, G.K.H.
    New Journal of Physics 15 (2013)
    The phase stability, electronic structure and transport properties of binary 3d, 4d and 5d transition metal silicides are investigated using high-throughput density functional calculations. An overall good agreement is found between the calculated 0 K phase diagrams and experiment. We introduce descriptors for the phase-stability and thermoelectric properties and hereby identify several candidates with potential for thermoelectric applications. This includes known thermoelectrics like Mn4Si7, β-FeSi2, Ru2Si3 and CrSi2 as well as new potentially meta-stable materials like Rh3Si5, Fe2Si3 and an orthorhombic CrSi2 phase. Analysis of the electronic structure shows that the gap formation in most of the semiconducting transition metal silicides can be understood with simple hybridization models. The transport properties of the Mn4Si 7, Ru2Ge3 and Ir3Si5 structure types and the orthorhombic CrSi2 phase are discussed. The calculated transport properties are in good agreement with available experimental data. It is shown that a better thermoelectric performance may be achieved upon optimal doping. Finally, the high-throughput data are analysed and rationalized using a simple tight-binding model. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/15/10/105010
  • 2013 • 84 Extension of the PC-SAFT based group contribution method for polymers to aromatic, oxygen- and silicon-based polymers
    Peters, F.T. and Herhut, M. and Sadowski, G.
    Fluid Phase Equilibria 339 89-104 (2013)
    A PC-SAFT group contribution method (GCM) for polymers developed earlier [10] is extended to aromatic, oxygen- and silicon-based (co-)polymers. Polymer parameters are determined using group contributions and applying simple arithmetic and geometric combination rules. Group contributions for six new groups are identified and parameterized: &gt;CHAr, &gt;CAr, O, &gt;CO, OH and &gt;Si< . The parameterization method is applied to liquid density and binary liquid-liquid equilibria and vapor-liquid equilibria as well as to excess enthalpies of polymers containing aromatic, oxygen- and silicon-containing monomer units in an extended spectrum of nonpolar, polar and associating solvents. Modeling results using both, GCM and fitted polymer parameters, show equally-good agreement with experimental data. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2012.11.031
  • 2013 • 83 Electronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111)
    Plucinski, L. and Herdt, A. and Fahrendorf, S. and Bihlmayer, G. and Mussler, G. and Döring, S. and Kampmeier, J. and Matthes, F. and Bürgler, D.E. and Grützmacher, D. and Blügel, S. and Schneider, C.M.
    Journal of Applied Physics 113 (2013)
    We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼ 170 meV centered ∼ 65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of the time-reversal symmetry protected topological surface states in Sb2 Te3 in the presence of impurities and defects. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4789353
  • 2013 • 82 Single-shot pulse duration monitor for extreme ultraviolet and X-ray free-electron lasers
    Riedel, R. and Al-Shemmary, A. and Gensch, M. and Golz, T. and Harmand, M. and Medvedev, N. and Prandolini, M.J. and Sokolowski-Tinten, K. and Toleikis, S. and Wegner, U. and Ziaja, B. and Stojanovic, N. and Tavella, F.
    Nature Communications 4 (2013)
    The resolution of ultrafast studies performed at extreme ultraviolet and X-ray free-electron lasers is still limited by shot-to-shot variations of the temporal pulse characteristics. Here we show a versatile single-shot temporal diagnostic tool that allows the determination of the extreme ultraviolet pulse duration and the relative arrival time with respect to an external pump-probe laser pulse. This method is based on time-resolved optical probing of the transient reflectivity change due to linear absorption of the extreme ultraviolet pulse within a solid material. In this work, we present measurements performed at the FLASH free-electron laser. We determine the pulse duration at two distinct wavelengths, yielding (184±14) fs at 41.5 nm and (21±19) fs at 5.5 nm. Furthermore, we demonstrate the feasibility to operate the tool as an online diagnostic by using a 20-nm-thin Si 3N4 membrane as target. Our results are supported by detailed numerical and analytical investigations. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms2754
  • 2013 • 81 Impact of composition and morphology on the optical properties of Si-NC/P3HT thin films processed from solution
    Rodrigues, F.D. and Cunha, M. and Hilliou, L. and Rino, L. and Correia, M.R. and Busani, T. and Bernardo, G. and Wiggers, H. and Filonovich, S.A. and Pereira, R.N.
    Applied Physics A: Materials Science and Processing 113 439-446 (2013)
    Blends of Si nanocrystals (Si-NCs) and organic semiconductors are promising materials for new optical and electronic devices processed from solutions. Here, we study how the optical properties of composite films containing Si- NCs and the organic semiconductor poly(3-hexylthiophene) (P3HT) are influenced by the composition and morphology resulting from different solution-processing parameters and different solvents used dichlorobenzene vs. chloroform). The optical spectra of the hybrid films are described using a simple phenomenological model, with which we can discern the contribution of each material in the films to the optical properties. From this analysis, we obtain quantitativeinformation about the composition and morphology of the hybrid nanostructured films, which otherwise would be obtained from more demanding microscopy and spectroscopy techniques. For the case of the Si-NC/P3HT blend, we find that in films deposited from dichlorobenzene solutions the Si-NCs contribute sizably to light absorption. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-012-7540-z
  • 2013 • 80 Insight into the Reaction Scheme of SiO2 Film Deposition at Atmospheric Pressure
    Rugner, K. and Reuter, R. and Ellerweg, D. and de los Arcos, T. and von Keudell, A. and Benedikt, J.
    Plasma Processes and Polymers 10 1061--1073 (2013)
    Characterisation of an atmospheric pressure microplasma jet in combination with simulations have been used to determine reaction mechanism of SiO2-like film formation and reaction rate constants for several gas phase reactions in the He/hexamethyldisiloxane (HMDSO)(/O-2) plasma chemistry. Using a variable-length quartz tube, the gas residence time in the plasma effluent could be well controlled without changing plasma properties. A possible reaction scheme has been developed. Deposition rates, deposited profiles, carbon content of the films and the depletion of HMDSO could be reproduced by the simulation. The simulation indicates that HMDSO in He(/O-2) plasma dissociates preferentially into (CH3)(3)SiO and Si(CH3)(3), where the former radical serves as a main growth precursor.
    view abstractdoi: 10.1002/ppap.201300059
  • 2013 • 79 Spatial Distributions of Alloying Elements Obtained from Atom Probe Tomography of the Amorphous Ribbon Fe75C11Si2B8Cr4
    Shin, J. and Yi, S. and Pradeep, K.G. and Choi, P.P. and Raabe, D.
    Korean Journal of Materials Research 23 190-193 (2013)
    Spatial distributions of alloying elements of an Fe-based amorphous ribbon with a nominal composition of Fe75C11Si2B8Cr4we re analyzed through the atom probe tomography method. The amorphous ribbon was prepared through the melt spinning method. The macroscopic amorphous natures were confirmed using an X-ray diffractometer (XRD) and a differential scanning calorimeter (DSC). Atom Probe (Cameca LEAP 3000X HR) analyses were carried out in pulsed voltage mode at a specimen base temperature of about 60 K, a pulse to base voltage ratio of 15 %, and a pulse frequency of 200 kHz. The target detection rate was set to 5 ions per 1000 pulses. Based on a statistical analyses of the data obtained from the volume of 59 x 59 x 33 nm3, homogeneous distributions of alloying elements in nano-scales were concluded. Even with high carbon and strong carbide forming element contents, nano-scale segregation zones of alloying elements were not detected within the Fe-based amorphous ribbon. However, the existence of small sub-nanometer scale clusters due to short range ordering cannot be completely excluded. © Materials Research Society of Korea, All rights reserved.
    view abstractdoi: 10.3740/MRSK.2013.23.3.190
  • 2013 • 78 Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion
    Springer, H. and Belde, M. and Raabe, D.
    Materials Science and Engineering A 582 235-244 (2013)
    The effect of local martensite-to-austenite reversion on microstructure and mechanical properties was studied with the aim of designing ductile martensitic steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0-5. wt% Mn, 0-2. wt% Si, constant 13.5. wt% Cr and 0.45. wt% C) and seven martensite tempering conditions (300-500. °C, 0-30. min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35. vol%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5. GPa ultimate tensile strength and 18% elongation), Si led to brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the austenite, as a result of the composition effects on austenite retention and reversion. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2013.06.036
  • 2013 • 77 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 • 76 Phase and elemental composition of silicon-containing hydroxyapatite-based coatings fabricated by RF-magnetron sputtering for medical implants
    Surmeneva, M.A. and Surmenev, R.A. and Chaikina, M.V. and Kachaev, A.A. and Pichugin, V.F. and Epple, M.
    Inorganic Materials: Applied Research 4 227-235 (2013)
    We studied by X-ray diffraction analysis, IR spectroscopy, and scanning electron microscopy (SEM) the phase composition and the structure of coatings based on silicon-containing hydroxyapatite (Si-HA) deposited by RF-magnetron sputtering. The sputtering target contained two phases (apatite and tricalcium phosphate) and was produced by the ceramic technology from a single-phase mechanically activated powder precursor. The structure of the coating deposited by sputtering from the two-phase target was single phase (hydroxyapatite) and textured in the (002) direction. During deposition, silicate anions partially replaced phosphate ions in the apatite lattice. © Pleiades Publishing, Ltd., 2013.
    view abstractdoi: 10.1134/S2075113313030131
  • 2013 • 75 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 • 74 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 • 73 Local junction voltages and radiative ideality factors of a-Si:H solar modules determined by electroluminescence imaging
    Tran, T.M.H. and Pieters, B.E. and Schneemann, M. and Müller, T.C.M. and Gerber, A. and Kirchartz, T. and Rau, U.
    Materials Research Society Symposium Proceedings 1536 105-111 (2013)
    In this contribution, we show that the dominant electroluminescent emission of hydrogenated amorphous silicon (a-Si:H) thin-film solar cells follows a diode law, whose radiative ideality factor nr is larger than one. This is in contrast to crystalline silicon and Cu(In, Ga)Se2 solar cells for which nr equals one. As a consequence, the existing quantitative analysis for the extraction of the local junction voltage V j(r) from luminescence images fails for a-Si:H solar cells. We expand the existing analysis method, and include the radiative ideality factor n r into the model. With this modification, we are able to determine the local junction voltage Vj(r) for a-Si:H solar cells and modules. We investigated the local junction voltage Vj(r) and the radiative ideality factor nr for both initial and stabilized a-Si:H solar modules. Furthermore, we show that the apparent radiative ideality factor is affected by the spectral sensitivity of the used camera system. © 2013 Materials Research Society.
    view abstractdoi: 10.1557/opl.2013.817
  • 2013 • 72 Quantitative evaluation method for electroluminescence images of a-Si: H thin-film solar modules
    Tran, T.M.H. and Pieters, B.E. and Schneemann, M. and Müller, T.C.M. and Gerber, A. and Kirchartz, T. and Rau, U.
    Physica Status Solidi - Rapid Research Letters 7 627-630 (2013)
    This work presents a method for extracting the absolute local junction voltage of a-Si:H thin-film solar cells and modules from electroluminescence (EL) images. It is shown that the electroluminescent emission of a-Si:H devices follows a diode law with a radiative ideality factor nr larger than one. We introduce an evaluation method that allows us to determine the absolute local junction voltage in cases of nr &gt; 1, while existing approaches rely on the assumption of nr = 1. Furthermore, we find that the experimentally determined values of nr vary from sample to sample. It is also explained why the derived radiative ideality factor is influenced by the spectral sensitivity of the camera system used in the experiment. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201308039
  • 2013 • 71 High-pressure water intrusion investigation of pure silica ITQ-7 zeolite
    Tzanis, L. and Marler, B. and Gies, H. and Patarin, J.
    Journal of Physical Chemistry C 117 4098-4103 (2013)
    Experimental water intrusion-extrusion isotherms were recorded at room temperature on pure silica ISV-type zeolite (ITQ-7 zeosil) possessing a 3D channel system. The water intrusion is obtained by applying a high hydraulic pressure corresponding to the intrusion step. When the pressure is released, the water extrusion occurs but at a lower pressure to that of the intrusion one. The ITQ-7 zeosil-water system behaves like a shock-absorber, but the phenomenon is nonreproducible. Several characterizations have been realized before and after water intrusion-extrusion experiments in order to reveal the presence or the absence of defects after such experiments. Structural modifications at the long-range order can be observed by XRD analysis after intrusion. At the short-range order, solid-state NMR spectroscopy shows evidence of the presence of Q2 ((HO-)2Si(-OSi)2) and Q3 ((HO-Si(-OSi)3) or (Si(-OSi)3O-)) groups revealing the breaking of some siloxane bridges after the intrusion step. The amount of defects increases (< 2% before and 15% after intrusion step). However, after a regeneration step (calcination of the intruded-extruded sample), the ITQ-7 zeosil-water system displays again a shock-absorber behavior with a similar intrusion pressure. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp311921d
  • 2013 • 70 Epitaxial growth of the topological insulator Bi2Se3 on Si(111): Growth mode, lattice parameter, and strain state
    Vyshnepolsky, M. and Klein, C. and Klasing, F. and Hanisch-Blicharski, A. and Horn-Von Hoegen, M.
    Applied Physics Letters 103 (2013)
    Using spot profile analysis low energy electron diffraction, we studied the growth mode and strain state of ultra-thin epitaxial Bi2Se 3(111) films grown by molecular beam epitaxy on Si(111). The first layer grows as complete quintuple layer and covers the Si substrate before the next layer nucleates. Its lateral lattice parameter is increased by 1% compared with the value of a- = 4.136 Å for a 6-nm-thick film. With increasing film thickness, a continuous change of the lattice parameter is observed to an asymptotic value, which is explained by a van der Waals-like bonding between the Bi2Se3 film and the Si substrate. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4821181
  • 2013 • 69 Re effects on phase stability and mechanical properties of Mo SS+Mo3Si+Mo5SiB2 alloys
    Yang, Y. and Bei, H. and Tiley, J. and George, E.P.
    Journal of Alloys and Compounds 556 32-38 (2013)
    In this paper, we investigate the effects of Re additions on the microstructure and mechanical properties of a ternary alloy with the composition Mo-12.5Si-8.5B (at.%). This alloy has a three-phase microstructure consisting of Mo solid-solution (MoSS), Mo3Si, and Mo 5SiB2 and our results show that up to 8.4 at.% Re can be added to it without changing its microstructure or forming any brittle σ phase at 1600 °C. Three-point bend tests using chevron-notched specimens showed that Re did not improve fracture toughness of the three-phase alloy. Nanoindentation performed on the MoSS phase in the three-phase alloy showed that Re increases Young's modulus, but does not lower hardness as in some Mo solid solution alloys. Based on our thermodynamic calculations and microstructural analyses, the lack of a Re softening effect is attributed to the increased Si levels in the Re-containing MoSS phase since Si is known to increase its hardness. This lack of softening is possibly why there is no Re-induced improvement in fracture toughness. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2012.12.047
  • 2012 • 68 Photothermal laser processing of thin silicon nanoparticle films: On the impact of oxide formation on film morphology
    Behrenberg, D. and Franzka, S. and Petermann, N. and Wiggers, H. and Hartmann, N.
    Applied Physics A: Materials Science and Processing 106 853-861 (2012)
    Photothermal laser processing of thin films of H-terminated silicon nanoparticles (Si NPs) is investigated. Ethanolic dispersions of Si NPs with an average diameter of 45 nm are spin-coated on silicon substrates yielding films with thicknesses ≤500 nm. Small-area laser processing is carried out using a microfocused scanning cw-laser setup operating at a wavelength of 532 nm and a 1/e laser spot size of 1.4 μm. In conjunction with microscopic techniques, this provides a highly reproducible and convenient approach in order to study the dependence of the resulting film morphology and composition on the experimental parameters. Processing in air results in strongly oxidized granular structures with sizes between 100 and 200 nm. The formation of these structures is dominated by surface oxidation. In particular, changing the processing parameters (i.e., laser power, writing speed, and/or the background air pressure) has little effect on the morphology. Only in vacuum at pressures <1 mbar, oxygen adsorption, and hence oxide formation, is largely suppressed. Under these conditions, irradiation at low laser powers results in mesoporous surface layers, whereas compact silicon films are formed at high laser powers. In agreement with these results, comparative experiments with films of H-terminated and surface-oxidized Si NPs reveal a strong impact of the surface oxide layer on the film morphology. Mechanistic aspects and implications for photothermal processing techniques, e.g., targeting photovoltaic and thermoelectric applications, are discussed. © 2012 Springer-Verlag.
    view abstractdoi: 10.1007/s00339-012-6779-8
  • 2012 • 67 Electron transport at surfaces and interfaces
    Bobisch, C.A. and Möller, R.
    Chimia 66 23-30 (2012)
    Here we present two techniques which give insight on transport phenomena with atomic resolution. Ballistic electron emission microscopy is used to study the ballistic transport through layered heterogeneous systems. The measured ballistic fraction of the tunneling current provides information about lossless transport channels through metallic layers and organic adsorbates. The transport characteristics of Bi(111)/Si Schottky devices and the influence of the organic adsorbates perylene tetracaboxylic dianhydride acid and C 60 on the ballistic current are discussed. Scanning tunneling potentiometry gives access to the lateral transport along a surface, thus scattering processes within two-dimensional electron systems for the Bi(111) surface and the Si(111)(√3×√3)-Ag surface could be visualized. © Schweizerische Chemische Gesellschaft.
    view abstractdoi: 10.2533/chimia.2012.23
  • 2012 • 66 The origin of gate hysteresis in p-type Si-doped AlGaAs/GaAs heterostructures
    Carrad, D. and Burke, A.M. and Waddington, D. and Lyttleton, R. and Tan, H.H. and Reece, P.J. and Klochan, O. and Hamilton, A.R. and Rai, A. and Reuter, D. and Wieck, A.D. and Micolich, A.P.
    Conference on Optoelectronic and Microelectronic Materials and Devices, Proceedings, COMMAD 9-10 (2012)
    Gate instability and hysteresis in Si-doped p-type AlGaAs/GaAs heterostructures impedes the development of nanoscale hole devices, which are of interest for topics from quantum computing to novel spin physics. We report an extended study conducted using matched n-type and p-type heterostructures, with and without insulated gates, aimed at understanding the origin of the hysteresis. We show the hysteresis is not due to the inherent 'leakiness' of gates on p-type heterostructures, as commonly believed. Instead, hysteresis arises from a combination of GaAs surface-state trapping and charge migration in the doping layer. © 2012 IEEE.
    view abstractdoi: 10.1109/COMMAD.2012.6472334
  • 2012 • 65 The influence of particle size and spacing on the fragmentation of nanocomposite anodes for Li batteries
    Dimitrijevic, B.J. and Aifantis, K.E. and Hackl, K.
    Journal of Power Sources 206 343-348 (2012)
    Experimental evidence has shown that composites comprised Si and Sn nanoparticles embedded inside a matrix are the most promising next generation anodes for Li-ion batteries. This is due to the ability of the matrix material to constrain/buffer the up to 300 volume expansion that Sn and Si undergo upon the formation of lithium rich alloys. Damage still occurs at the nanoparticle/matrix interface, and hence further materials design is required in order to commercialize such anodes. Initial theoretical works have predicted that low volume fractions and high aspect ratios of the nanoparticles result in a greater mechanical stability and hence better capacity retention. The most important design parameters, however, such as particle size and spacing have not been considered theoretically. In the present study, therefore, a gradient enhanced damage model will be employed to predict that damage during Li-insertion, is negligible when the particle size is 20 nm, and the interparticle half-spacing greater then 1.5 times the particle diameter. Furthermore, from the matrix materials considered herein graphene is predicted to be the most promising matrix, which is consistent with recent experimental data. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2012.01.065
  • 2012 • 64 The dangling-bond defect in amorphous silicon: Statistical random versus kinetically driven defect geometries
    Freysoldt, C. and Pfanner, G. and Neugebauer, J.
    Journal of Non-Crystalline Solids 358 2063-2066 (2012)
    Amorphous and micro-crystalline silicon (a-Si:H, μc-Si) are key materials for resource-saving thin-film solar cells. However, the efficiency of such devices is severely limited by light-induced Si dangling-bond defects, which can be detected by electron paramagnetic resonance (EPR). We report density-functional theory calculations on a set of random dangling bonds created in supercell models of a-Si:H and compare calculated hyperfine and g-tensor distributions to the ones obtained from a recent multi-frequency EPR spectral analysis. Our results show that the g-tensor does not exhibit axial symmetry as has been previously assumed, but is clearly rhombic. The hyperfine coupling to the undercoordinated Si atom, on the other hand, is almost perfectly axial. This apparent discrepancy in the symmetry properties is shown to be a consequence of the underlying coupling mechanisms and how these are influenced by structural disorder. However, the hyperfine distribution calculated from our random models underestimates the experimentally observed 30% red-shift when going from c-Si to a-Si:H. We suggest that only a subset of possible dangling-bond configurations is observed in experiment. We discuss plausible mechanisms that would give rise to such a selection, and new experiments to test these hypotheses. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jnoncrysol.2011.12.090
  • 2012 • 63 Nanoscale heat transport in self-organized Ge clusters on Si(001)
    Frigge, T. and Kalus, A. and Klasing, F. and Kammler, M. and Hanisch-Blicharski, A. and Horn-von Hoegen, M.
    Materials Research Society Symposium Proceedings 1456 51-56 (2012)
    Ultrafast time resolved transmission electron diffraction (TED) in a reflection geometry was used to study the cooling behavior of self-organized, well defined nanoscale germanium hut and dome clusters on Si(001). The clusters were heated in a pump-probe scheme by fs-laser pulses. The resulting transient temperature rise was then determined from the drop in diffraction intensity caused by the Debye-Waller effect. From a cooling time of τ=177 ps we estimated a strongly reduced heat transfer compared with homogeneous films of equivalent thickness. © 2013 Materials Research Society.
    view abstractdoi: 10.1557/opl.2013.148
  • 2012 • 62 Antiferromagnetic coupling in combined Fe/Si/MgO/Fe structures with controlled interface diffusion
    Gareev, R. and Stromberg, F. and Buchmeier, M. and Keune, W. and Back, C. and Wende, H.
    Applied Physics Express 5 (2012)
    We study antiferromagnetic coupling and interface diffusion in Fe/Si/MgO/Fe structures grown by molecular beam epitaxy. The Fe/Si/Fe samples with a 1.2-nm-thick Si spacer demonstrate antiferromagnetic coupling J 1 ∼- 1:5 mJ/m 2 and prevailing interdiffusion at the top Si/Fe interface, as revealed by conversion electron Mössbauer spectroscopy. For combined Si/MgO spacers with 0.9-nm-thick Si, interdiffusion continuously reduces upon changing the MgO thickness from 0.3 to 0.5nm accompanied by a decrease of antiferromagnetic coupling from |J 1| ∼ 1 mJ/m2 to |J 1| ∼ 0:002mJ/m 2. We emphasize that monolayer-scaled engineering of insulating spacers is a promising tool for the precise control of antiferromagnetic coupling and interface diffusion. © 2012 The Japan Society of Applied Physics.
    view abstractdoi: 10.1143/APEX.5.033003
  • 2012 • 61 Interlayer expansion of the hydrous layer silicate rub-36 to a functionalized, microporous framework silicate: Crystal structure analysis and physical and chemical characterization
    Gies, H. and Müller, U. and Yilmaz, B. and Feyen, M. and Tatsumi, T. and Imai, H. and Zhang, H. and Xie, B. and Xiao, F.-S. and Bao, X. and Zhang, W. and Baerdemaeker, T.D. and De Vos, D.
    Chemistry of Materials 24 1536-1545 (2012)
    The hydrous layer silicate RUB-36, (C 6H 16N) 4 [H 4Si 36O 76], has been used for an interlayer expansion reaction with dichlorodimethylsilane to interconnect neighboring ferrierite-type layers to a three-dimensional framework silicate. The linker group (-O-Si(CH 3) 2-O-) still has the two methyl groups in the as-synthesized form (material name COE-3 [Si 20O 38(CH 3) 4] for the silicate framework) rendering hydrophobic properties. The interlayer expanded zeolite, IEZ, is thermally stable and can be calcined at 550 °C to yield a hydrophilic material COE-4 [Si 20O 38(OH) 4]. 29Si solid state MAS NMR experiments confirm the insertion of the linker group and the methyl and hydroxyl substitution in the as-made and calcined form, respectively. The BET surface area is 238 m 2/g for COE-3 and 350 m 2/g for COE-4. COE-3 and COE-4 crystallize in space group Pm with a = 12.2503(3) Å b = 13.9752(2) Å c = 7.3850(1) Å and β = 107.33(1)° and a = 12.16985(4) Å, b = 13.95066(3) Å c = 7.37058(2) Å, and β = 107.30(1)°, respectively. Rietveld crystal structure refinement of the PXRD pattern of COE-3 and COE-4 reveal the expanded, two-dimensional 10-ring pore system including the linker group as homogeneous structural property of the materials. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm300525u
  • 2012 • 60 Effect of p-type doping on the oxidation of H-Si(111) studied by second-harmonic generation
    Gökce, B. and Dougherty, D.B. and Gundogdu, K.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 30 (2012)
    Atomic force microscopy and second-harmonic generation data show that boron doping enhances the rate of oxidation of H-terminated silicon. Holes cause a greater increase in the reactivity of the Si-H up bonds than that of the Si-Si back bonds. © 2012 American Vacuum Society.
    view abstractdoi: 10.1116/1.4721329
  • 2012 • 59 Control of the oxidation kinetics of H-terminated (111)Si by using the carrier concentration and the strain: A second-harmonic-generation investigation
    Gökce, B. and Gundogdu, K. and Aspnes, D.E.
    Journal of the Korean Physical Society 60 1685-1689 (2012)
    We discuss recent results regarding the effects of strain, carrier type and concentration on the oxidation of H-terminated (111)Si. Second-harmonic-generation data show that this is a two-stage process where the H of the "up" bonds of the outermost Si layer is replaced by OH, followed by O insertion into the "back" bonds. These data provide additional detailed information about both stages. In particular, directional control of the in-plane surface chemistry by using the applied uniaxial stress provides new opportunities for interface control. © 2012 The Korean Physical Society.
    view abstractdoi: 10.3938/jkps.60.1685
  • 2012 • 58 Effect of Si addition on the oxidation resistance of Co-Re-Cr-alloys: Recent attainments in the development of novel alloys
    Gorr, B. and Burk, S. and Depka, T. and Somsen, C. and Abu-Samra, H. and Christ, H.-J. and Eggeler, G.
    International Journal of Materials Research 103 24-30 (2012)
    The influence of silicon on the oxidation behaviour of Co- Re- Cr-alloys has been studied at 1 000 °C and 1 100 °C. Consideration was given to the synergetic effects between chromium and silicon with respect to the development of a protective Cr 2O 3 layer. The Si addition to the Co- Re-alloys produces a significant decrease in the evaporation rate of Re oxides. Moreover, the beneficial influence in the transient oxidation period results in a rapid formation of Cr2O3 scale. While the addition of 1 and 2 at.% Si to the ternary Co-17Re-23Cr alloy was insufficient to form a continuous Cr2O3 scale, the addition of 3 at.% silicon caused a change in the oxidation mode resulting in the formation of a nearly continuous Cr 2O 3 scale. On the oxide/alloy interface of the alloy Co-17Re-30Cr-2Si, a continuous and dense Cr 2O 3 scale was observed, which remained stable after 100 h exposure protecting the metallic substrate. © 2012 Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110626
  • 2012 • 57 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 • 56 Sacrificial ion beam etching process for seed layer removal of 6 μm pitch CuSn micro bumps
    Hess, J. and Vogt, H.
    IOP Conference Series: Materials Science and Engineering 41 (2012)
    Copper pillar bumps show a wide-ranging application for assembly and packaging according to the «More than Moore» roadmap. For the demand of higher input/output (I/O) densities and consequently smaller bump pitches the requirements on each process step in producing 6 μm pitch Cu-Sn bumps increase. In this case the removal of seed layer with wet etchants is no longer practicable due to high undercut. A sacrifical Ion Beam Etching (IBE) process was developed for removing the TiW/Cu seed layer without any undercut. Due to the high etching rate of the rough Sn surface a sacrificial layer of Ni was used to protect the solder layer. To optimize the layer thicknesses etch rates were characterized. Special attention was directed to the etched material which covered the bumps on the sidewalls after the etching process step. Energy-dispersive X-ray spectroscopy (EDX) measurements and reflow processes revealed the influence of the redepositioned material on the melting behavior and hence on the following bonding process. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/41/1/012005
  • 2012 • 55 Bioconjugated silicon quantum dots from one-step green synthesis
    Intartaglia, R. and Barchanski, A. and Bagga, K. and Genovese, A. and Das, G. and Wagener, P. and Di Fabrizio, E. and Diaspro, A. and Brandi, F. and Barcikowski, S.
    Nanoscale 4 1271-1274 (2012)
    Biofunctionalized silicon quantum dots were prepared through a one step strategy avoiding the use of chemical precursors. UV-Vis spectroscopy, Raman spectroscopy and HAADF-STEM prove oligonucleotide conjugation to the surface of silicon nanoparticle with an average size of 4 nm. The nanoparticle size results from the size-quenching effect during in situ conjugation. Photoemissive properties, conjugation efficiency and stability of these pure colloids were studied and demonstrate the bio-application potential, e.g. for nucleic acid vector delivery with semiconducting, biocompatible nanoparticles. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2nr11763k
  • 2012 • 54 Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al-Si-based alloys
    Li, J.H. and Zarif, M.Z. and Dehm, G. and Schumacher, P.
    Philosophical Magazine 92 3789-3805 (2012)
    The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al-5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al-5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al5 FeSi intermetallic was also observed, depending on subsequent controlled cooling. © 2012 Copyright Taylor and Francis Group, LLC.
    view abstractdoi: 10.1080/14786435.2012.687840
  • 2012 • 53 The realization of a pn-diode using only silicon nanoparticles
    Meseth, M. and Ziolkowski, P. and Schierning, G. and Theissmann, R. and Petermann, N. and Wiggers, H. and Benson, N. and Schmechel, R.
    Scripta Materialia 67 265-268 (2012)
    Si nanoparticles (Si-NPs) are a non-toxic and low-cost material resource that can be processed from dispersion for electrical thin film or from powder for bulk application using various sintering techniques. So far research on electronic applications using Si-NPs is limited. Few reports exist on thermoelectric research, or hybrid photovoltaic applications. In the following we demonstrate the realization of the first Si pn-diode using only Si-NPs in combination with field-assisted sintering. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.scriptamat.2012.04.039
  • 2012 • 52 Modelling of photo- and electroluminescence of hydrogenated microcrystalline silicon solar cells
    Müller, T.C.M. and Pieters, B.E. and Kirchartz, T. and Carius, R. and Rau, U.
    Physica Status Solidi (C) Current Topics in Solid State Physics 9 1963-1967 (2012)
    Photoluminescence (PL) and electroluminescence (EL) have received much attention as characterization techniques for photovoltaic devices. The methods are applied to study e.g. optical band-gap, defect states, or quasi-Fermi level splitting. Spatially resolved EL imaging is used to derive local junction voltage differences making it a fast inline characterization method for solar modules. However, the interpretation of EL and PL experiments on hydrogenated microcrystalline silicon (μc-Si:H) solar cells is more complex hampering the direct determination of local voltage differences. In this work we integrated an existing model for PL of μc-Si:H silicon with a commercial device simulator for thin-film silicon devices. This way we extended the existing model from a spatially zero dimensional model to a one dimensional model which can also model EL. Furthermore the connection with an electrical device simulator enables the consistent modeling of EL, PL, and the electrical properties of the device. We compared experimental and simulation results for EL, PL and dark-, and illuminated- current/voltage characteristics over a wide temperature range (80 - 300 K). The simulations and experiments are in good agreement in the temperature range from 170 K up to room temperature. In experiments we observed several effects which cannot be explained in the previous zero dimensional model. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.201200428
  • 2012 • 51 Noninvasive measurement and control of the temperature of Pt nanofilms on Si supports
    Nedrygailov, I.I. and Hasselbrink, E. and Diesing, D. and Dasari, S.K. and Hashemian, M.A. and Karpov, E.G.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 30 (2012)
    A direct, noninvasive thermometry method based on the temperature dependence of the in-plane electrical resistance R(T) of 20 nm Pt films on Si-based semiconducting substrates is presented. At the calibration stage, the entire sample is slowly heated or cooled by external means. For moderately doped semiconductors, the R(T) dependence is closely linear at low temperatures (T 300 K), when the nanofilm metal conductance dominates, followed by a maximum and a subsequent decrease attributed to an increasing substrate conductance at higher temperatures. The position of the maximum depends on the Pt-substrate interface conditions, in particular, on the presence of an oxide layer, and may vary from 350 to 500 K. With the R(T) dependence measured in equilibrium one can derive T(R), which provides a highly accurate method for noninvasive measuring and controlling the temperature of the nanofilm with a direct resistive heating technique. Accuracy of the present method for dynamical measurement is shown to be significantly better than that of a standard approach using a Pt resistance temperature detector attached to the nanofilm for heating rates of 1-7 K/s. © 2012 American Vacuum Society.
    view abstractdoi: 10.1116/1.3696973
  • 2012 • 50 Low-cost post-growth treatments of crystalline silicon nanoparticles improving surface and electronic properties
    Niesar, S. and Pereira, R.N. and Stegner, A.R. and Erhard, N. and Hoeb, M. and Baumer, A. and Wiggers, H. and Brandt, M.S. and Stutzmann, M.
    Advanced Functional Materials 22 1190-1198 (2012)
    Freestanding silicon nanocrystals (Si-ncs) offer unique optical and electronic properties for new photovoltaic, thermoelectric, and other electronic devices. A method to fabricate Si-ncs which is scalable to industrial usage has been developed in recent years. However, barriers to the widespread utilization of these nanocrystals are the presence of charge-trapping defects and an oxide shell formed upon ambient atmosphere exposure hindering the charge transport. Here, we exploit low-cost post-growth treatment routes based on wet-etching in hydrofluoric acid plus surface hydrosilylation or annealing enabling a complete native oxide removal and a reduction of the defect density by up to two orders of magnitude. Moreover, when compared with only H-terminated Si-ncs we report an enhancement of the conductivity by up to a factor of 400 for films of HF etched and annealed Si-ncs, which retain a defect density below that of untreated Si-ncs even after several months of air exposure. Further, we demonstrate that HF etched and hydrosilylated Si-ncs are extremely stable against oxidation and maintain a very low defect density after a long-term storage in air, opening the possibility of device processing in ambient atmosphere. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201101811
  • 2012 • 49 Damage in crystalline silicon by swift heavy ion irradiation
    Osmani, O. and Alzaher, I. and Peters, T. and Ban D'Etat, B. and Cassimi, A. and Lebius, H. and Monnet, I. and Medvedev, N. and Rethfeld, B. and Schleberger, M.
    Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms 282 43-47 (2012)
    We have studied damage of crystalline Si surfaces induced by electronic energy loss of swift heavy ions with an electronic stopping power of up to S e = 12 keV/nm. Scanning tunneling microscope images of the surface after irradiation under perpendicular as well as glancing angles of incidence showed no surface damage. We have performed theoretical calculations for the damage threshold within the two temperature model, resulting in Seth=8 keV/nm as the minimum stopping power to create a molten zone. We investigate the respective influence of the electron-phonon coupling, of the criterion at which the damage occurs and a possible effect of ballistic electrons. We show that the latter has the strongest effect on the calculated damage threshold. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.nimb.2011.08.036
  • 2012 • 48 High-quality epitaxial Bi(111) films on Si(111) by isochronal annealing
    Payer, T. and Klein, C. and Acet, M. and Ney, V. and Kammler, M. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Thin Solid Films 520 6905-6908 (2012)
    Bi(111) films grown on Si(111) at room temperature show a significantly higher roughness compared to Bi films grown on Si(100) utilizing a kinetic pathway based on a low-temperature process. Isochronal annealing steps of 3 min duration each with temperatures up to 200 °C cause a relaxation of the Bi films' lattice parameter toward the Bi bulk value and yield an atomically flat Bi surface. Driving force for the relaxation and surface reordering is the magic mismatch of 11 Bi atoms to 13 Si atoms that emerges at annealing temperatures above 150 °C and reduces the remaining strain to less than 0.2%. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2012.06.004
  • 2012 • 47 Surface damage of silicon after swift heavy ion irradiation
    Peters, T. and Alzaher, I. and Ban D'Etat, B. and Cassimi, A. and Monnet, I. and Lebius, H. and Schleberger, M.
    Journal of Physics: Conference Series 388 (2012)
    In order to answer the long-standing question, if silicon surfaces can be damaged by swift heavy ions, a set-up to study ion-irradiation damage of reactive surfaces is presented. This set-up allows for the first time to avoid oxidization of the silicone surface during the experimental study. Scanning tunneling microscopy as well as low-energy electron diffraction was used to study the surfaces before and after irradiation. Silicon surfaces were prepared by flash-heating before irradiation with swift heavy ions (Xenon at 0.9 MeV/u). The targets stayed in ultra-high vacuum during preparation, irradiation and surface imaging. No surface damage was detected, at normal as well as at grazing incidence angle. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/388/13/132035
  • 2012 • 46 Layer-by-layer fabrication of an anatase titania multilayer with gradual sponge-like morphology
    Perlich, J. and Memesa, M. and Diethert, A. and Metwalli, E. and Wang, W. and Roth, S.V. and Gutmann, J.S. and Müller-Buschbaum, P.
    Colloid and Polymer Science 290 119-126 (2012)
    The fabrication of a functional multilayer system with a gradually hierarchical order formed by individual titania thin films of different porosity is investigated. The porous or sponge-like nanostructures are fabricated using a diblock copolymer assisted sol-gel process. The successive spin-coating of the sol-gel solution onto the silicon substrate deposits a thin polymer nanocomposite film which is transformed to purely anatase titania nanostructures via calcination. In total, this procedure is repeated layer by layer for three times. This layer-by-layer approach is monitored with grazing incidence small-angle X-ray scattering (GISAXS) after each fabrication step. The GISAXS investigation is complemented in real space with a scanning electron microscopy characterization of the respective preparation stages. From the characterization, a porous titania multilayer system with gradually structured levels is clearly identified. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00396-011-2529-2
  • 2012 • 45 Stability of platinum nanoparticles supported on SiO2/Si(111): A high-pressure X-ray photoelectron spectroscopy study
    Porsgaard, S. and Merte, L.R. and Ono, L.K. and Behafarid, F. and Matos, J. and Helveg, S. and Salmeron, M. and Roldan Cuenya, B. and Besenbacher, F.
    ACS Nano 6 10743-10749 (2012)
    The stability of Pt nanoparticles (NPs) supported on ultrathin SiO 2 films on Si(111) was investigated in situ under H2 and O2 (0.5 Torr) by high-pressure X-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtOx desorption from the Pt NPs (∼2 and ∼4 nm NP sizes) at temperatures above 450 C in the presence of 0.5 Torr of O2. Under oxidizing conditions, the entire NPs were oxidized, although with no indication of a PtO2 phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e., minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nn3040167
  • 2012 • 44 Thermally induced reactions between lithiated nano-silicon electrode and electrolyte for lithium-ion batteries
    Profatilova, I.A. and Langer, T. and Badillo, J.P. and Schmitz, A. and Orthner, H. and Wiggers, H. and Passerini, S. and Winter, M.
    Journal of the Electrochemical Society 159 A657-A663 (2012)
    The thermal stability of nano-silicon electrodes before and after lithiation was studied by means of differential scanning calorimetry (DSC). It was found that pristine Si electrodes heated in presence of ECDEC 1M LiPF 6 electrolyte show exothermic reactions between sodium carboxymethylcellulose (Na CMC binder) and LiPF 6. The products of thermal decomposition of a lithiated nano-Si electrode with electrolyte at different temperatures were identified using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). SEI layer was found to be responsible for the thermal reactions in the range between 77 and 107°C. Exothermic events between 107 and 140°C were caused by partial decomposition of LiPF 6 salt, which products initiated further transformations of SEI layer compounds and esterification of Na CMC. Interaction between nano-Li xSi and ECDEC 1M LiPF 6 was the reason for the main exothermic peaks at temperatures between 150 and 300°C. Nano-Li xSi heated with ECDEC solvent mixture without LiPF 6 resulted in electrolyte decomposition at much lower temperatures (&gt;105°C). Therefore, the important role of LiPF 6 in the thermal stabilization of nano-Li xSi with electrolyte at temperatures < 140°C was confirmed while LiTFSI salt added to ECDEC was ineffective in the prevention of the main exothermic reaction starting at 105°C. © 2012 The Electrochemical Society.
    view abstractdoi: 10.1149/2.095205jes
  • 2012 • 43 Combined ab initio, experimental, and CALPHAD approach for an improved thermodynamic evaluation of the Mg-Si system
    Schick, M. and Hallstedt, B. and Glensk, A. and Grabowski, B. and Hickel, T. and Hampl, M. and Gröbner, J. and Neugebauer, J. and Schmid-Fetzer, R.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 37 77-86 (2012)
    A new thermodynamic evaluation of the well-known Mg-Si system is presented with the aim to resolve persistent uncertainties in the Gibbs energy of its only compound, Mg 2Si. For this purpose the heat capacity and enthalpy of melting of Mg 2Si were measured by differential scanning calorimetry. Using finite temperature density functional theory and the quasiharmonic approximation, thermodynamic properties of Mg 2Si were additionally calculated up to and above its melting temperature. Using these new data, in particular the heat capacity, the Mg-Si system was evaluated thermodynamically with the CALPHAD method leading to a thermodynamic description of the system within narrow bounds. In contrast to several previous evaluations there is no problem with an inverted miscibility gap in the liquid. Although present enthalpy of melting data turned out to be inconsistent with other data in this system, the new evaluation accurately describes all other available data in this system. In particular the Gibbs energy of Mg 2Si can now be considered reliably described.© 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.calphad.2012.02.001
  • 2012 • 42 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 • 41 Atomic scale effects of alloying, partitioning, solute drag and austempering on the mechanical properties of high-carbon bainitic-austenitic TRIP steels
    Seol, J.-B. and Raabe, D. and Choi, P.-P. and Im, Y.-R. and Park, C.-G.
    Acta Materialia 60 6183-6199 (2012)
    Understanding alloying and thermal processing at an atomic scale is essential for the optimal design of high-carbon (0.71 wt.%) bainitic-austenitic transformation-induced plasticity (TRIP) steels. We investigate the influence of the austempering temperature, chemical composition (especially the Si:Al ratio) and partitioning on the nanostructure and mechanical behavior of these steels by atom probe tomography. The effects of the austempering temperature and of Si and Al on the compositional gradients across the phase boundaries between retained austenite and bainitic ferrite are studied. We observe that controlling these parameters (i.e. Si, Al content and austempering temperature) can be used to tune the stability of the retained austenite and hence the mechanical behavior of these steels. We also study the atomic scale redistribution of Mn and Si at the bainitic ferrite/austenite interface. The observations suggest that either para-equilibrium or local equilibrium-negligible partitioning conditions prevail depending on the Si:Al ratio during bainite transformation. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2012.07.064
  • 2012 • 40 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 • 39 The interplay of topography and energy dissipation in pentacene thin films
    Wall, S. and Thien, D. and Meyer zu Heringdorf, F.-J.
    Journal of Electron Spectroscopy and Related Phenomena 185 436-440 (2012)
    Nonlinear photoemission electron microscopy was used to study the morphology-dependent lifetime of electronic excitations in pentacene islands on Si(0 0 1) and (√3 × √3)R30°-Ag/Si(1 1 1). After an optical excitation of electrons by a λ = 400 nm femtosecond laser pulse the characteristic decay times were measured with spatial resolution in a pump-probe setup. For pentacene on Si(0 0 1), the observed lifetimes vary by a factor of two between the wetting layer and the fractal-shaped pentacene islands. The measured lifetime difference is explained by a difference in the electronic coupling of the pentacene islands and the wetting layer to the substrate. For pentacene on (√3 × √3)R30°-Ag/Si(1 1 1), similar lifetimes are found, although the orientation of the pentacene molecules in the compact islands is rotated. Our findings suggest that electronic excitations in higher layers of the pentacene islands do not diffuse to the interface before they decay. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.elspec.2012.09.007
  • 2012 • 38 Cellular reactions toward nanostructured silicon surfaces created by laser ablation
    Wallat, K. and Dörr, D. and Le Harzic, R. and Stracke, F. and Sauer, D. and Neumeier, M. and Kovtun, A. and Zimmermann, H. and Epple, M.
    Journal of Laser Applications 24 (2012)
    Silicon wafers were structured with a femtosecond laser on the cm 2 scale with high spatial frequency laser-induced periodic surface structures. These areas are characterized by regular parallel ripples with a period of the order of 100 nm. The particular ripple spacing is determined by the illumination wavelength of the tunable femtosecond laser. The cellular reaction to the structured silicon wafers and to the same materials, coated with calcium phosphate nanoparticles by electrophoretic deposition, was studied using L929 fibroblasts, human mesenchymal stem cells, and epithelial cells. The cells adhered uniformly to structured and unprocessed areas after seeding but significantly preferred the unstructured silicon after 48 h. This behavior disappeared after coating the structured surface with calcium phosphate nanoparticles. © 2012 Laser Institute of America.
    view abstractdoi: 10.2351/1.4732594
  • 2012 • 37 Emission measurement and safety assessment for the production process of silicon nanoparticles in a pilot-scale facility
    Wang, J. and Asbach, C. and Fissan, H. and Hülser, T. and Kaminski, H. and Kuhlbusch, T.A.J. and Pui, D.Y.H.
    Journal of Nanoparticle Research 14 (2012)
    Emission into the workplace was measured for the production process of silicon nanoparticles in a pilot-scale facility at the Institute of Energy and Environmental Technology e.V. (IUTA). The silicon nanoparticles were produced in a hot-wall reactor and consisted of primary particles around 60 nm in diameter. We employed real-time aerosol instruments to measure particle number and lungdeposited surface area concentrations and size distribution; airborne particles were also collected for offline electron microscopic analysis. Emission of silicon nanoparticles was not detected during the processes of synthesis, collection, and bagging. This was attributed to the completely closed production system and other safety measures against particle release which will be discussed briefly. Emission of silicon nanoparticles significantly above the detection limit was only observed during the cleaning process when the production system was open and manually cleaned. The majority of the detected particles was in the size range of 100-400 nm and were silicon nanoparticle agglomerates first deposited in the tubing then resuspended during the cleaning process. Appropriate personal protection equipment is recommended for safety protection of the workers during cleaning. © 2012 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s11051-012-0759-y
  • 2011 • 36 Electronic structure of EuO spin filter tunnel contacts directly on silicon
    Caspers, C. and Müller, M. and Gray, A.X. and Kaiser, A.M. and Gloskovskii, A. and Fadley, C.S. and Drube, W. and Schneider, C.M.
    Physica Status Solidi - Rapid Research Letters 5 441-443 (2011)
    We present an electronic structure study of a magnetic oxide/ semiconductor model system, EuO on silicon, which is dedicated for efficient spin injection and spin detection in silicon-based spintronics devices. A combined electronic structure analysis of Eu core levels and valence bands using hard X-ray photoemission spectroscopy was performed to quantify the nearly ideal stoichiometry of EuO "spin filter" tunnel barriers directly on silicon, and the absence of silicon oxide at the EuO/Si interface. These results provide evidence for the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices. Hard X-ray photoemission spectroscopy of an Al/EuO/Si heterostructure probing the buried EuO and EuO/Si interface. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Hard X-ray photoemission spectroscopy reveals the nearly ideal stoichiometry of EuO spin filter tunnel barriers grown directly on silicon, and the absence of silicon oxide formation at the EuO/Si interface. These results demonstrate the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssr.201105403
  • 2011 • 35 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 • 34 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 • 33 Isolated silicon dangling bonds on a water-saturated n+-doped Si(001)-2 × 1 surface: An XPS and STM study
    Gallet, J.-J. and Bournel, F. and Rochet, F. and Köhler, U. and Kubsky, S. and Silly, M.G. and Sirotti, F. and Pierucci, D.
    Journal of Physical Chemistry C 115 7686-7693 (2011)
    Using Si 2p core-level X-ray photoelectron spectroscopy we have measured the upward band bending at the surface of n+-doped water-saturated Si(001)-2 × 1 and inferred the macroscopic negative surface charge density of the surface. These macroscopic results are in excellent accord with the microscopic view provided by dual-bias scanning tunneling microscopy showing that the isolated silicon dangling bonds (∼1.2 × 10-2 defects per Si atom) bear indeed a negative charge. Noting the structural analogy between isolated dangling bonds on water-saturated Si(001) and H-terminated Si(001), in the final, prospective section of the paper, we raise the question of the possible role that these defects could play in radical chain reactions with π-bonded molecules, in relationship with the hydride/hydroxyl patterns that are resolved in the scanning tunneling images. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp201262x
  • 2011 • 32 The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Gebhardt, T. and Music, D. and Ekholm, M. and Abrikosov, I.A. and Vitos, L. and Dick, A. and Hickel, T. and Neugebauer, J. and Schneider, J.M.
    Journal of Physics Condensed Matter 23 (2011)
    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Néel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/24/246003
  • 2011 • 31 Measurement and control of in-plane surface chemistry during oxidation of H-terminated (111)Si
    Gökce, B. and Adles, E.J. and Aspnes, D.E. and Gundogdu, K.
    AIP Conference Proceedings 1399 193-194 (2011)
    We demonstrate both directional control and measurement of the oxidation of H-terminated (111)Si. Control is achieved through externally applied strain, with strained back bonds oxidizing faster than unstrained ones. Real-time measurement is achieved by second-harmonic generation (SHG), with SHG anisotropy data analyzed with the anisotropic bond-charge model of nonlinear optics. Anisotropic oxidation also results in structural changes, which appear as rotations of the average orientations of the back bonds from their unperturbed directions. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3666321
  • 2011 • 30 Effect of strain on bond-specific reaction kinetics during the oxidation of H-terminated (111) Si
    Gökce, B. and Aspnes, D.E. and Gundogdu, K.
    Applied Physics Letters 98 (2011)
    Although strain is used in semiconductor technology for manipulating optical, electronic, and chemical properties of semiconductors, the understanding of the microscopic phenomena that are affected or influenced by strain is still incomplete. Second-harmonic generation data obtained during the air oxidation of H-terminated (111) Si reveal the effect of compressive strain on this chemical reaction. Even small amounts of strain manipulate the reaction kinetics of surface bonds significantly, with tensile strain enhancing oxidation and compressive strain retarding it. This dramatic change suggests a strain-driven charge transfer mechanism between Si-H up bonds and Si-Si back bonds in the outer layer of Si atoms. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3567528
  • 2011 • 29 Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping
    Gökce, B. and Aspnes, D.E. and Lucovsky, G. and Gundogdu, K.
    Applied Physics Letters 98 (2011)
    It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3537809
  • 2011 • 28 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 • 27 Freestanding silicon quantum dots: Origin of red and blue luminescence
    Gupta, A. and Wiggers, H.
    Nanotechnology 22 (2011)
    In this paper, we studied the behavior of silicon quantum dots (Si-QDs) after etching and surface oxidation by means of photoluminescence (PL) measurements, Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy (EPR). We observed that etching of red luminescing Si-QDs with HF acid drastically reduces the concentration of defects and significantly enhances their PL intensity together with a small shift in the emission spectrum. Additionally, we observed the emergence of blue luminescence from Si-QDs during the re-oxidation of freshly etched particles. Our results indicate that the red emission is related to the quantum confinement effect, while the blue emission from Si-QDs is related to defect states at the newly formed silicon oxide surface. © 2011 IOP Publishing Ltd Printed in the UK & the USA.
    view abstractdoi: 10.1088/0957-4484/22/5/055707
  • 2011 • 26 Nanoscale heat transport through epitaxial ultrathin hetero films: Bi(111)/Si(001) and Bi(111)/Si(111)
    Hanisch-Blicharski, A. and Wall, S. and Kalus, A. and Frigge, T. and Horn-Von Hoegen, M.
    Materials Research Society Symposium Proceedings 1404 80-85 (2011)
    The cooling process of ultrathin hetero films upon excitation with short laser pulses was studied for epitaxial Bi(111) films on Si(001) and Si(111) substrates by means of the Debye-Waller effect with ultrafast electron diffraction. From the exponential decay of the temperature, a cooling time constant was determined as a function of thickness for both substrates. For Bi/Si(111), a linear dependence between the decay constant and thickness was observed, even for 2.8 run thin films , as predicted from the diffuse mismatch model (DMM) and the acoustic mismatch model (AMM). However, with Bi/Si(001), a significant deviation from this linear dependence was observed for film thicknesses below 5 nm. © 2012 Materials Research Society.
    view abstractdoi: 10.1557/opl.2012.472
  • 2011 • 25 Improved interfacial local structural ordering of epitaxial Fe 3Si(001) thin films on GaAs(001) by a MgO(001) tunneling barrier
    Makarov, S.I. and Krumme, B. and Stromberg, F. and Weis, C. and Keune, W. and Wende, H.
    Applied Physics Letters 99 (2011)
    Although the quasi-Heusler compound Fe3Si is a promising candidate for spintronics applications, its combination with the reactive GaAs surface is problematic, since it deteriorates its beneficial attributes due to a large amount of interdiffusion at the Fe3Si/GaAs interface. Here, we show the epitaxial growth of Fe3Si with low evaporation rates on GaAs(001) and report on improved local structural D03 ordering in epitaxial Fe3Si(001) films grown on GaAs(001) by inserting a MgO buffer layer. Conversion-electron Mssbauer spectroscopy with 57Fe3Si tracer layers reveals that the effect of thermally induced interdiffusion at the Fe3Si/GaAs(001) interface is dramatically reduced by inserting a 30 MgO tunneling barrier between the film and the substrate. The chemical order of Fe3Si is comparable to that of Fe3Si films which are grown directly on MgO(001) single crystals. It is proposed that this preparation method can be useful to achieve high-efficiency spin-polarized electron currents from ferromagnetic Fe 3Si into semiconducting GaAs(001). © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3646390
  • 2011 • 24 Effect of alloying elements on hydrogen environment embrittlement of AISI type 304 austenitic stainless steel
    Martin, M. and Weber, S. and Theisen, W. and Michler, T. and Naumann, J.
    International Journal of Hydrogen Energy 36 15888-15898 (2011)
    The chemical composition of an AISI type 304 austenitic stainless was systematically modified in order to evaluate the influence of the elements Mo, Ni, Si, S, Cr and Mn on the material's susceptibility to hydrogen environment embrittlement (HEE). Mechanical properties were evaluated by tensile testing at room temperature in air at ambient pressure and in a 40 MPa hydrogen gas atmosphere. For every chemical composition, the corresponding austenite stability was evaluated by magnetic response measurements and thermodynamic calculations based on the Calphad method. Tensile test results show that yield and tensile strength are negligibly affected by the presence of hydrogen, whereas measurements of elongation to rupture and reduction of area indicate an increasing ductility loss with decreasing austenite stability. Concerning modifications of alloy composition, an increase in Si, Mn and Cr content showed a significant improvement of material's ductility compared to other alloying elements. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijhydene.2011.09.013
  • 2011 • 23 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 • 22 Solution-processed networks of silicon nanocrystals: The role of internanocrystal medium on semiconducting behavior
    Pereira, R.N. and Niesar, S. and You, W.B. and Da Cunha, A.F. and Erhard, N. and Stegner, A.R. and Wiggers, H. and Willinger, M.-G. and Stutzmann, M. and Brandt, M.S.
    Journal of Physical Chemistry C 115 20120-20127 (2011)
    We have produced networks of surface-oxidized and hydrogen-terminated silicon nanocrystals (Si-NCs), both intrinsic and n-type doped, on flexible plastic foil from nanoparticle inks. The charge transport in these networks was comprehensively studied by means of time-dependent conductivity, steady-state current versus voltage characteristics, and steady-state photocurrent measurements as a function of incident light intensity. These measurements were complemented by surface chemistry and structural/morphological analysis from Fourier transform infrared spectroscopy and electron microscopy. Whereas H-terminated Si-NC networks function as semiconductors (both in air and in vacuum), where conductivity enhancement upon impurity doping and photoconductivity were observed, these characteristics are not present in networks of surface-oxidized Si-NCs. For both network types, the observation of a power law behavior for steady-state current versus voltage and a current decaying with time at constant bias indicate that charge transport is controlled by space-charge-limited current (involving trap states) via percolation paths through the networks. We have also monitored the evolution of the networks (photo)conductivity when the internanocrystal separating medium formed by Si-H bonds is progressively replaced by a native oxide upon exposure to air. Although a decrease in the (photo)conductivity is observed, the networks still behave as semiconductors even after a long-term air exposure. From an analysis of all (photo)current data, we deduce that in networks of oxidized Si-NCs inter-NC charge transfer requires the participation of oxide-related electronic states, whereas in H-terminated Si-NC networks direct inter-NC charge transfer plays a major role in the overall long-range conduction process. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp205984m
  • 2011 • 21 Liquid-crystalline elastomer microvalve for microfluidics
    Sánchez-Ferrer, A. and Fischl, T. and Stubenrauch, M. and Albrecht, A. and Wurmus, H. and Hoffmann, M. and Finkelmann, H.
    Advanced Materials 23 4526-4530 (2011)
    Microactuators are an essential component in microsystems or microdevices, and in applications that include artificial muscles, pumps, valves, or switchers. Liquid-crystalline elastomers are a new class of actuator material in the field of microsystem technologies, which can be used in standard processes. This newly developed actuator provides new possibilities in microfluidics because of its dimensional changes activated by the increase in temperature. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adma.201102277
  • 2011 • 20 In-vitro investigation of magnetron-sputtered coatings based on silicon-substituted hydroxyapatite
    Surmeneva, M.A. and Surmenev, R.A. and Pichugin, V.F. and Chernousova, S.S. and Epple, M.
    Journal of Surface Investigation 5 1202-1207 (2011)
    Silicon-containing calcium phosphate (Si-CaP) coatings on titanium and austenite steel substrates have been prepared by method of high-frequency magnetron sputtering. The powder of silicon-containing hydroxyapatite Ca 10(PO 4) 6 - x(SiO 4) x(HO) 2 - x (Si-HA), where x = 0. 5 obtained using a mechanochemical technique, was used as a target material. The obtained coatings were X-ray amorphous; the elemental composition of the coatings depended on the composition of the target to be sputtered. The coatings were heated in air for 3 hours to the temperature 700°C with the aim of changing their structure. The bioactivity of the coatings was studied using in-vitro tests. The solution of the simulated body fluid (SBF) oversaturated with respect to HA was used as a model medium. The phase elemental composition and morphology of the deposited and annealed Si-CaP coatings before and after submersion into the solution were controlled using the methods of X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), and scanning electron microscopy (SEM). According to the XFA and IR-spectroscopy data, heat treatment in the air yields the formation of an apatite-like phase in the coating. Thermostating of "metal + coating" specimens in the solution of simulated body fluid revealed that all obtained coatings were biologically active, and a calcium phosphate layer was formed on the coating surface during mineralization. The annealed coatings show a higher chemical stability under physiological conditions as compared to amorphous coatings. © 2011 Pleiades Publishing, Ltd.
    view abstractdoi: 10.1134/S1027451011120135
  • 2011 • 19 Amphiphilic gold nanoparticles: Synthesis, characterization and adsorption to PEGylated polymer surfaces
    Tarnawski, R. and Ulbricht, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 374 13-21 (2011)
    The direct synthesis of water-soluble gold nanoparticles with a mixed shell of two different thiols, 1-mercaptoundec-11-yl-hexa(ethylene glycol) (EG6) and dodecanethiol (C12), and their characterization are reported. Data from IR spectroscopy and contact angle (CA) measurements as well as the solubility of the nanoparticles in water support that the composition of the shell is in the range of the thiol ratio used for synthesis (EG6:C12 = 72:28). Results of transmission electron microscopy and atomic force microscopy (AFM) for deposited particles as well as the UV-vis spectrum in solution are in line with a size of ≤10. nm. Self-assembled monolayers (SAMs) as model surfaces were prepared from mixtures of EG6 and C12 on planar gold films. Polystyrene (PSt) spin-coated films on silicon wafers and on gold-coated surface plasmon resonance (SPR) sensor disks were used as substrates for surface functionalization via adsorption/self-assembly of a polystyrene poly(ethylene glycol) diblock copolymer (PSt- b-PEG) from aqueous solutions. CA and AFM results revealed pronounced differences of the hydrophilicity/hydrophobicity and topography of the surface as a function of PSt- b-PEG concentration used for the modification. The adsorption of myoglobin and the novel gold nanoparticles to the PSt- b-PEGylated surfaces was analyzed by SPR. A control of adsorbed amounts by the degree of surface PEGylation, i.e. a reduction by up to 55% for the highest degree of modification, could be confirmed for both kinds of colloids. Adsorption of the novel gold nanoparticles to the mixed SAM surfaces as analyzed by SPR showed an even stronger dependency of surface composition. All experiments demonstrate that amphiphilic, water-soluble gold-based nanoparticles can be used as model colloids for the investigation of interactions with polymer surfaces of varied structure and architecture, and that they could be further developed for analytical or biological applications. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2010.10.027
  • 2011 • 18 Silicon and magnesium diffusion in a single crystal of MgSiO3 perovskite
    Xu, J. and Yamazaki, D. and Katsura, T. and Wu, X. and Remmert, P. and Yurimoto, H. and Chakraborty, S.
    Journal of Geophysical Research: Solid Earth 116 (2011)
    Si and Mg self-diffusion coefficients were measured simultaneously in single crystals of MgSiO<inf>3</inf> perovskite under lower mantle conditions. There is little difference in Si volume diffusivity measured directly using single crystals (this study) and those retrieved from experiments with polycrystals (earlier studies). This agreement between studies establishes the reliability of Si diffusion coefficients measured in perovskite. Within the uncertainties of our measurements, no anisotropy in the diffusion of either Si or Mg could be resolved. Diffusion of Si and Mg in perovskite are described by an Arrhenius equation, D=D<inf>0</inf> exp (-H/RT) at 25GPa, with D <inf>0</inf>=5.10×10-11m2/s for Si and 4.99×10-11m2/s for Mg, H=308kJ/mol for Si, and 305kJ/mol for Mg. Mg diffusivity in MgSiO<inf>3</inf> perovskite is distinctly lower than those measured in olivine, wadsleyite, and ringwoodite. We find that Mg has very similar diffusivity to Si in perovskite. As a consequence, the rheological properties of the lower mantle may be controlled by the coupled motion of Si and Mg. A point defect-based model is discussed that may account for the diffusion behavior of Si and Mg in MgSiO<inf>3</inf> perovskite. Our data indicate that, within realistic ranges of temperature, grain size, and state of stress, both diffusion creep as well as dislocation creep may be observed in the lower mantle. Copyright 2011 by the American Geophysical Union.
    view abstractdoi: 10.1029/2011JB008444
  • 2011 • 17 Superplastic martensitic Mn-Si-Cr-C steel with 900% elongation
    Zhang, H. and Bai, B. and Raabe, D.
    Acta Materialia 59 5787-5802 (2011)
    High-strength (1.2-1.5)C-(2-2.5)Mn-(1.5-2)Si-(0.8-1.5)Cr steels (mass%) consisting of martensite and carbides exhibit excellent superplastic properties (e.g. strain rate sensitivity m ≈ 0.5, elongation ≈900% at 1023 K). A homogeneous martensitic starting microstructure is obtained through thermomechanical processing (austenitization plus 1.2 true strain, followed by quenching). Superplastic forming leads to a duplex structure consisting of ferrite and spherical micro-carbides. Through 1.5-2% Si alloying, carbides precipitate at hetero-phase interfaces and martensite blocks at the beginning of superplastic forming. Via Ostwald ripening, these interface carbides grow at the expense of carbides precipitating at martensite laths, thereby promoting ferrite dynamic recrystallization. Simultaneously, carbides at ferrite grain boundaries retard the growth of recrystallized ferrite grains. Due to 2-2.5% Mn and 0.8-1.5% Cr alloying, carbide coarsening is suppressed owing to the slow diffusion of these elements. As a result, fine and homogeneous ferrite plus spherical carbide duplex microstructures with a ferrite grain size of ∼1.5 μm are obtained after superplastic forming. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2011.05.055
  • 2011 • 16 Ge-Si-O phase separation and Ge nanocrystal growth in Ge:SiO x/SiO2 multilayers - A new dc magnetron approach
    Zschintzsch, M. and Sahle, C.J. and Borany, J.V. and Sternemann, C. and Mücklich, A. and Nyrow, A. and Schwamberger, A. and Tolan, M.
    Nanotechnology 22 (2011)
    Ge:SiOx/SiO2 multilayers are fabricated using a new reactive dc magnetron sputtering approach. The influence of the multilayer stoichiometry on the ternary Ge-Si-O phase separation and the subsequent size-controlled Ge nanocrystal formation is explored by means of x-ray absorption spectroscopy, x-ray diffraction, electron microscopy and Raman spectroscopy. The ternary system Ge-Si-O reveals complete Ge-O phase separation at 400 °C which does not differ significantly to the binary Ge-O system. Ge nanocrystals of < 5nm size are generated after subsequent annealing below 700°C. It is shown that Ge oxides contained in the as-deposited multilayers are reduced by a surrounding unsaturated silica matrix. A stoichiometric regime was found where almost no GeO2 is present after annealing. Thus, the Ge nanocrystals become completely embedded in a stoichiometric silica matrix favouring the use for photovoltaic applications. © IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/48/485303
  • 2010 • 15 Formation, stability and crystal structure of the r phase in Mo-Re-Si alloys
    Bei, H. and Yang, Y. and Viswanathan, G.B. and Rawn, C.J. and George, E.P. and Tiley, J. and Chang, Y.A.
    Acta Materialia 58 6027-6034 (2010)
    The formation, stability and crystal structure of the σ phase in Mo-Re-Si alloys were investigated. Guided by thermodynamic calculations, six critically selected alloys were arc melted and annealed at 1600 °C for 150 h. Their as-cast and annealed microstructures, including phase fractions and distributions, the compositions of the constituent phases and the crystal structure of the r phase were analyzed by thermodynamic modeling coupled with experimental characterization by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. Two key findings resulted from this work. One is the large homogeneity range of the r phase region, extending from binary Mo-Re to ternary Mo-Re-Si. The other is the formation of a r phase in Mo-rich alloys either through the peritectic reaction of liquid + Moss → σ or primary solidification. These findings are important in understanding the effects of Re on the microstructure and providing guidance on the design of Mo-Re-Si alloys. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2010.07.020
  • 2010 • 14 Influence of the substrate lattice structure on the formation of quantum well states in thin in and Pb films on silicon
    Dil, J.H. and Hülsen, B. and Kampen, T.U. and Kratzer, P. and Horn, K.
    Journal of Physics Condensed Matter 22 (2010)
    The substrate lattice structure may have a considerable influence on the formation of quantum well states in a metal overlayer material. Here we study three model systems using angle resolved photoemission and low energy electron diffraction: indium films on Si(111) and indium and lead on Si(100). Data are compared with theoretical predictions based on density functional theory. We find that the interaction between the substrate and the overlayer strongly influences the formation of quantum well states; indium layers only exhibit well defined quantum well states when the layer relaxes from an initial face-centred cubic to the bulk body-centred tetragonal lattice structure. For Pb layers on Si(100) a change in growth orientation inhibits the formation of quantum well states in films thicker than 2ML. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/13/135008
  • 2010 • 13 Measurement and control of in-plane surface chemistry during the oxidation of H-terminated (111) Si
    Gökce, B. and Adles, E.J. and Aspnes, D.E. and Gundogdu, K.
    Proceedings of the National Academy of Sciences of the United States of America 107 17503-17508 (2010)
    In-plane directional control of surface chemistry during interface formation can lead to new opportunities regarding device structures and applications. Control of this type requires techniques that can probe and hence provide feedback on the chemical reactivity of bonds not only in specific directions but also in real time. Here, we demonstrate both control and measurement of the oxidation of H-terminated (111) Si. Control is achieved by externally applying uniaxial strain, and measurement by second-harmonic generation (SHG) together with the anisotropic-bond model of nonlinear optics. In this system anisotropy results because bonds in the strain direction oxidize faster than those perpendicular to it, leading in addition to transient structural changes that can also be detected at the bond level by SHG.
    view abstractdoi: 10.1073/pnas.1011295107
  • 2010 • 12 Stable colloidal dispersions of silicon nanoparticles for the fabrication of films using inkjet printing technology
    Gupta, A. and Khalil, A.S.G. and Winterer, M. and Wiggers, H.
    INEC 2010 - 2010 3rd International Nanoelectronics Conference, Proceedings 1018-1019 (2010)
    The formation of stable colloidal dispersions of nanoparticles is essential for the manufacture of electronic and optoelectronic devices using cost-effective printing technologies, In this study, we examined the stability of silicon nanoparticles (Si-NPs) in aqueous medium at different pH. The Si-NPs show high zeta potential values within pH = 6.5 - 8.5. In addition, the Si-NPs do not show any isoelectric point in the pH range studied, It IS observed that the stability of Si-NPs in aqueous medium increases after the addition of ethanol. In order to stabilize Si-NPs in organic solvents, their surface is functionalized with alkyl groups via a thermally induced alkylation process. The functionalized Si-NPs form nice, transparent dispersions in a variety of organic solvents and no sedimentation of functionalized samples was observed over any period of time. Fabricating films of Si-NPs using inkjet printing is currently under investigation. ©2010 IEEE.
    view abstractdoi: 10.1109/INEC.2010.5425068
  • 2010 • 11 Optical and electrical properties of silicon nanoparticles
    Gupta, A. and Hartner, S. and Wiggers, H.
    INEC 2010 - 2010 3rd International Nanoelectronics Conference, Proceedings 616-617 (2010)
    For the fabrication of optoelectronic devices based on silicon nanoparticles (Si-NPs), it is very important to understand their optical and electrical behavior. In this paper, we present the optical and electrical properties of Si-NPs. We demonstrate that the optical properties of Si-NPs depend on their size as well as their surface chemistry. The size of Si-NPs was finely tuned by etching them in a mixture of hydrofluoric acid (HF) and nitric acid (HNO 3) for different times. The resulting Si-NPs exhibit bright luminescence across the visible spectrum. In order to stabilize the optical emission, the surface of freshly etched Si-NPs was successfully functionalized with organic molecules. As the surface chemistry is also expected to strongly influence the electrical transport between Si-NPs and therefore the electrical properties of Si-NP ensembles, the conductivity of pellets consisting of Si-NPs was measured using impedance spectroscopy. The surface oxide of Si-NPs was removed by etching them with HF acid. The freshly etched Si-NPs showed much higher conductivity compared to as-prepared samples. The surface functionalization of freshly etched Si-NPs slightly decreases their conductivity. However, it was observed that the conductivity was still much higher compared to as-prepared samples. ©2010 IEEE.
    view abstractdoi: 10.1109/INEC.2010.5424734
  • 2010 • 10 Single shot damage mechanism of Mo/Si multilayer optics under intense pulsed XUV-exposure
    Khorsand, A.R. and Sobierajski, R. and Louis, E. and Bruijn, S. and Van Hattum, E.D. and Vande Kruijs, R.W.E. and Jurek, M. and Klinger, D. and Pelka, J.B. and Juha, L. and Burian, T. and Chalupsky, J. and Cihelka, J. and Hajkova,...
    Optics Express 18 700-712 (2010)
    We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference- polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources. © 2010 Optical Society of America.
    view abstractdoi: 10.1364/OE.18.000700
  • 2010 • 9 Growth of crystalline Gd2O3 thin films with a high-quality interface on Si(100) by low-temperature H2O-assisted atomic layer deposition
    Milanov, A.P. and Xu, K. and Laha, A. and Bugiel, E. and Ranjith, R. and Schwendt, D. and Osten, H.J. and Parala, H. and Fischer, R.A. and Devi, A.
    Journal of the American Chemical Society 132 36-37 (2010)
    (Figure Presented) This work documents the first example of deposition of high-quality Gd2O3 thin films in a surface-controlled, self-limiting manner by a water-based atomic layer deposition (ALD) process using the engineered homoleptic gadolinium guanidinate precursor [Gd(DPDMG) 3]. The potential of this class of compound is demonstrated in terms of a true ALD process, exhibiting pronounced growth rates, a high-quality interface between the film and the substrate without the need for any additional surface treatment prior to the film deposition, and most importantly, encouraging electrical properties. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja909102j
  • 2010 • 8 Silver induced faceting of Si(112)
    Nabbefeld, T. and Wiethoff, C. and Meyer zu Heringdorf, F.-J. and Horn-Von Hoegen, M.
    Applied Physics Letters 97 (2010)
    Si nanowires grown in ultrahigh vacuum by metal-catalyzed vapor-liquid-solid epitaxy are known to exhibit sidewalls with {112}-type orientation. For some metals the sidewalls show pronounced faceting. Ag induced faceting on Si(112) surfaces was studied in situ by spot-profile-analyzing low energy electron diffraction and ex situ atomic force microscopy. The (112) surface decomposes into (115)- and (111)- (√3×√3) -facets, both of which are Ag terminated. The width of the facets is kinetically limited and varies between 6 nm at T<550 °C and 30 nm at T=690 °C. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3464555
  • 2010 • 7 Defect reduction in silicon nanoparticles by low-temperature vacuum annealing
    Niesar, S. and Stegner, A.R. and Pereira, R.N. and Hoeb, M. and Wiggers, H. and Brandt, M.S. and Stutzmann, M.
    Applied Physics Letters 96 (2010)
    Using electron paramagnetic resonance, we find that vacuum annealing at 200 °C leads to a significant reduction in the silicon dangling bond (Si-db) defect density in silicon nanoparticles (Si-NPs). The best improvement of the Si-db density by a factor of 10 is obtained when the vacuum annealing is combined with an etching step in hydrofluoric acid (HF), whereas HF etching alone only removes the Si-dbs at the Si/ SiO2 interface. The reduction in the Si-db defect density is confirmed by photothermal deflection spectroscopy and photoconductivity measurements on thin Si-NPs films. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3428359
  • 2010 • 6 Passivation of Si(111) surfaces with electrochemically grafted thin organic films
    Roodenko, K. and Yang, F. and Hunger, R. and Esser, N. and Hinrichs, K. and Rappich, J.
    Surface Science 604 1623-1627 (2010)
    Ultra thin organic films (about 5 nm thick) of nitrobenzene and 4-methoxydiphenylamine were deposited electrochemically on p-Si(111) surfaces from benzene diazonium compounds. Studies based on atomic force microscopy, infrared spectroscopic ellipsometry and x-ray photoelectron spectroscopy showed that upon exposure to atmospheric conditions the oxidation of the silicon interface proceed slower on organically modified surfaces than on unmodified hydrogen passivated p-Si(111) surfaces. Effects of HF treatment on the oxidized organic/Si interface and on the organic layer itself are discussed. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2010.06.005
  • 2010 • 5 Micro- and nanopatterning of functional organic monolayers on oxide-free silicon by laser-induced photothermal desorption
    Scheres, L. and Klingebiel, B. and Ter Maat, J. and Giesbers, M. and De Jong, H. and Hartmann, N. and Zuilhof, H.
    Small 6 1918-1926 (2010)
    The photothermal laser patterning of functional organic monolayers, prepared on oxide-free hydrogen-terminated silicon, and subsequent backfi lling of the laser-written lines with a second organic monolayer that differs in its terminal functionality, is described. Since the thermal monolayer decomposition process is highly nonlinear in the applied laser power density, subwavelength patterning of the organic monolayers is feasible. After photothermal laser patterning of hexadecenyl monolayers, the lines freed up by the laser are backfi lled with functional acid fl uoride monolayers. Coupling of cysteamine to the acid fl uoride groups and subsequent attachment of Au nanoparticles allows easy characterization of the functional lines by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Depending on the laser power and writing speed, functional lines with widths between 1.1 μm and 250 nm can be created. In addition, trifl uoroethyl-terminated (TFE) monolayers are also patterned. Subsequently, the decomposed lines are backfi lled with a nonfunctional hexadecenyl monolayer, the TFE stripes are converted into thiol stripes, and then finally covered with Au nanoparticles. By reducing the lateral distance between the laser lines, Au-nanoparticle stripes with widths close to 100 nm are obtained. Finally, in view of the great potential of this type of monolayer in the fi eld of biosensing, the ease of fabricating biofunctional patterns is demonstrated by covalent binding of fl uorescently labeled oligo-DNA to acidfl uoride-backfi lled laser lines, which-as shown by fl uorescence microscopy-is accessible for hybridization.Copyright © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201000189
  • 2010 • 4 Anisotropy of Ag diffusion on vicinal Si surfaces
    Sindermann, S. and Wall, D. and Roos, K.R. and Horn-Von Hoegen, M. and Meyer zu Heringdorf, F.-J.
    e-Journal of Surface Science and Nanotechnology 8 372-376 (2010)
    Photoemission electron microscopy (PEEM) is used to study Ag surface diffusion on vicinal Si surfaces. The diffusion field is represented by Iso-Coverage Zones around Ag islands during desorption. By analyzing the shape and radius of the Iso-Coverage Zone we can determine diffusion parameters. For anisotropic diffusion the zone has an elliptical shape and the aspect ratio gives a measure for the anisotropy. Using this technique, we study the degree of anisotropy of Ag diffusion on vicinal Si(001) and Si(111). With increasing miscut angles, starting from Si(001) as well as from Si(111), we find a gradually increasing anisotropy, caused by the higher step density. On higher index surfaces, like Si(119), Si(115) and Si(113), we find isotropic diffusion for surfaces with comparable dimer and (double) step structure as on Si(001)-4°, where diffusion is strongly anisotropic. © 2010 The Surface Science Society of Japan.
    view abstractdoi: 10.1380/ejssnt.2010.372
  • 2010 • 3 Liquid injection MOCVD grown binary oxides and ternary rare-earth oxide as alternate gate-oxides for logic devices
    Thomas, R. and Ehrhart, P. and Waser, R. and Schubert, J. and Devi, A. and Katiyar, R.S.
    ECS Transactions 33 211-219 (2010)
    ZrO2, HfO2 and DyScO3 thin films having thickness in the range 2-20 nm were grown on SiOx/Si(100) substrates in a multi-wafer planetary MOCVD reactor combined with a liquid delivery system using engineered precursors. Growth rate, surface morphology, crystal structure, crystal density of the as-deposited films were analysed as a function of deposition temperature. The influence of post deposition annealing on the densification and crystallization was studied. Electrical properties of MIS capacitor structures are also discussed. Results on the optimised gate stack of Pt/ZrO2/SiOx/Si, Pt/HfO2/SiOx/Si, Pt/DyScO3/SiOx/Si are finally compared; and DyScO 3 seems to be promising high-k material candidate compared to Group-IVB oxides for the coming technology nodes. ©The Electrochemical Society.
    view abstractdoi: 10.1149/1.3481608
  • 2010 • 2 Magnetic properties of ultrathin Fe3Si films on GaAs(001)
    Weis, C. and Krumme, B. and Herper, H.C. and Stromberg, F. and Antoniak, C. and Warland, A. and Entel, P. and Keune, W. and Wende, H.
    Journal of Physics: Conference Series 200 (2010)
    For a detailed understanding of the interface between Fe3Si and GaAs, we investigate Fe3Si films in the ultrathin limit down to a few monolayers and compare the results to Fe3Si/MgO(001) which serves as a reference in the present study. From X-ray magnetic circular dichroism measurements we determine averaged spin and orbital magnetic Fe moments. Further insight follows from SPR-KKR calculations. Conversion electron Mössbauer spectroscopy (CEMS) yields information on the chemical ordering and is able to distinguish inequivalent Fe lattice sites. The CEMS results indicate structural disorder which we attribute to an interdiffusion at the Fe3Si/GaAs interface. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/200/7/072105
  • 2010 • 1 Effects of Ti, Zr, and Hf on the phase stability of Mo_ss + Mo3Si + Mo5SiB2 alloys at 1600 °C
    Yang, Y. and Bei, H. and Chen, S. and George, E.P. and Tiley, J. and Chang, Y.A.
    Acta Materialia 58 541-548 (2010)
    Understanding the stability of the three-phase Mo_ss + Mo3Si + Mo5SiB2 region is important for alloy design of Mo-Si-B-based refractory metal intermetallic composites. In this work, thermodynamic modeling is coupled with guided experiments to study phase stability in this three-phase region of the Mo-Si-B-X (X = Ti, Zr, Hf) system. Both the calculated and experimental results show that additions of Zr and Hf limit significantly the stability of the three-phase region because of the formation of the ternary phases MoSiZr and MoSiHf, while Ti addition leads to a much larger region of stability for the three-phase equilibrium. © 2009 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2009.09.032