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- ItemFibroblast Response to Nanocolumnar TiO2 Structures Grown by Oblique Angle Sputter Deposition(Weinheim : Wiley-VCH, 2021) Kapprell, Uta; Friebe, Sabrina; Grüner, Susann; Grüner, Christoph; Kupferer, Astrid; Rauschenbach, Bernd; Mayr, Stefan G.Cells are established to sense and respond to the properties, including nano- and microscale morphology, of the substrate they adhere to, which opens up the possibility to tailor bioactivity. With this background, the potential of tilted TiO2 nanostructures grown by oblique angle sputtering to affect fibroblasts with particular focus on inducing anisotropy in cell behavior is explored. By depositing TiO2 at different oblique angles relative to the substrate normal, morphologies, columnar tilt angle, roughness, and distances between neighbored nanocolumns can be adjusted. To assess bioactivity of the resulting structures, L929-mouse fibroblasts are seeded in vitro on TiO2 nanostructured substrates. Angle-dependent movement and velocity distributions of the cells on differently tilted columns and a smooth reference sample are studied. Cell proliferation rates and cell areas are additional factors which provide information about viability and the well-being of cells. It could be shown that the local topography of the surface has an influence on the directed movement of the cells. © 2021 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH
- ItemNanoporous Morphogenesis in Amorphous Carbon Layers: Experiments and Modeling on Energetic Ion Induced Self‐Organization(Weinheim : Wiley-VCH Verlag, 2021) Hoffmann, Daniel T.; Dietrich, Johannes; Mändl, Stephan; Zink, Mareike; Mayr, Stefan G.Nanoporous amorphous carbon constitutes a highly relevant material for a multitude of applications ranging from energy to environmental and biomedical systems. In the present work, it is demonstrated experimentally how energetic ions can be utilized to tailor porosity of thin sputter deposited amorphous carbon films. The physical mechanisms underlying self-organized nanoporous morphogenesis are unraveled by employing extensive molecular dynamics and phase field models across different length scales. It is demonstrated that pore formation is a defect induced phenomenon, in which vacancies cluster in a spinodal decomposition type of self-organization process, while interstitials are absorbed by the amorphous matrix, leading to additional volume increase and radiation induced viscous flow. The proposed modeling framework is capable to reproduce and predict the experimental observations from first principles and thus opens the venue for computer assisted design of nanoporous frameworks.
- ItemStructural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses(Basel : MDPI, 2020) Behrens, Mario; Lotnyk, Andriy; Bryja, Hagen; Gerlach, Jürgen W.; Rauschenbach, BerndGe-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge2Sb2Te5 (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts.
- ItemPhase and grain size engineering in Ge-Sb-Te-O by alloying with La-Sr-Mn-O towards improved material properties(Oxford : Elsevier Science, 2020) Kraft, Nikolas; Wang, Guoxiang; Bryja, Hagen; Prager, Andrea; Griebel, Jan; Lotnyk, AndriyGe-Sb-Te alloys are promising materials for non-volatile memory applications. Alloying of the materials with various elements is considered as prospective approach to enhance material properties. This work reports on the preparation and characterization of pure Ge-Sb-Te-O (GSTO) and alloyed with La-Sr-Mn-O (LSMO) thin films. Thermal heating of amorphous thin films to different temperatures show distinct crystallization behavior. A general trend is the decrease in the size of GSTO crystallites and the suppression in the formation of stable trigonal GSTO phase with increasing content of LSMO. Microstructural studies by transmission electron microscopy show the formation of metastable GSTO nanocrystallites dispersed in the amorphous matrix. Analysis of local chemical bonding by X-ray spectroscopy reveal the presence of different oxides in the GSTO-LSMO composites. Moreover, the composites with a high LSMO content exhibit higher crystallization temperature and significant larger sheet resistance in amorphous and crystalline phase, while a memory device made of GSTO-LSMO alloy reveals bipolar switching and synaptic behavior. In addition, the amount of LSMO in GSTO-LSMO thin films influences their optical properties and band gap. Overall, the results of this work reveal the highly promising potential of GSTO-LSMO nanocomposites for data storage and reconfigurable photonic applications as well as neuro-inspired computing.
- ItemEffects of methyl terminal and carbon bridging groups ratio on critical properties of porous organosilicate-glass films(Basel : MDPI, 2020) Vishnevskiy, Alexey S.; Naumov, Sergej; Seregin, Dmitry S.; Wu, Yu-Hsuan; Chuang, Wei-Tsung; Rasadujjaman, Md.; Zhang, Jing; Leu, Jihperng; Vorotilov, Konstantin A.; Baklanov, Mikhail R.Organosilicate glass-based porous low dielectic constant films with different ratios of terminal methyl to bridging organic (methylene, ethylene and 1,4-phenylene) groups are spin-on deposited by using a mixture of alkylenesiloxane with organic bridges and methyltrimethoxysilane, followed by soft baking at 120–200◦ C and curing at 430◦ C. The films’ porosity was controlled by using sacrificial template Brij® L4. Changes of the films’ refractive indices, mechanical properties, k-values, porosity and pore structure versus chemical composition of the film’s matrix are evaluated and compared with methyl-terminated low-k materials. The chemical resistance of the films to annealing in oxygen-containing atmosphere is evaluated by using density functional theory (DFT). It is found that the introduction of bridging groups changes their porosity and pore structure, increases Young’s modulus, but the improvement of mechanical properties happens simultaneously with the increase in the refractive index and k-value. The 1,4-phenylene bridging groups have the strongest impact on the films’ properties. Mechanisms of oxidative degradation of carbon bridges are studied and it is shown that 1,4-phenylene-bridged films have the highest stability. Methylene-and ethylene-bridged films are less stable but methylene-bridged films show slightly higher stability than ethylene-bridged films. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
- ItemTailoring morphology in titania nanotube arrays by implantation: experiments and modelling on designed pore size—and beyond(London [u.a.] : Taylor & Francis, 2021) Kupferer, Astrid; Mändl, Stephan; Mayr, Stefan G.Titania nanotube arrays are an exceptionally adaptable material for various applications ranging from energy conversion to biomedicine. Besides electronic properties, structural morphology on nanometre scale is essential. It is demonstrated that ion implantation constitutes a versatile method for the synthesis of tailored nanotube morphologies. Experimental-phenomenological observations reveal a successive closing behaviour of nanotubes upon ion implantation. Employing molecular dynamics calculations in combination with analytical continuum models, the physical origins of this scenario are unravelled by identifying ion bombardment induced viscous flow driven by capillarity as its underlying mechanism besides minor contributions from sputtering and redeposition. These findings enable the tailoring of nanotube arrays suitable for manifold applications.
- ItemHelium transmission rate as a rapid and reliable method for assessing the water vapour transmission rate of transparent PET-SiOx barrier foils(New York, NY [u.a.] : Wiley, 2021) Herbst, Florian; Großer, Stephan; With, Patrick C.; Prager, Lutz; Pander, MatthiasA single quadrupole mass spectrometer coupled measuring setup was developed for the investigation of the helium transmission rate (HeTR) of SiOx-coated polyethylene terephthalate (PET) barrier films. The setup allows the pressure-less and time-resolved measurements of the helium permeation at transient and steady-state conditions. Whereas standard water vapour transmission rate (WVTR) experiments took extended test times (in the range of several days), HeTR measurements were finished after 1 h. For the material system investigated here, an excellent linear correlation of WVTR and HeTR was proven over two orders of magnitude (regarding WVTR). Experiments with application of different strain loads on the coated films revealed a significant increase of both, HeTR and WVTR. Scanning electron microscope (SEM) measurements evidenced multiple ruptures of the SiOx coating depending on the applied strain and initial thickness of the SiOx layer. Considering virgin barrier films and strain-ruptured barrier films, a good correlation of WVTR and HeTR was shown.
- ItemSecondary electron yield engineering of copper surfaces by 532 nm ultrashort laser pulses(Amsterdam [u.a.] : Elsevier, 2022) Lorenz, Pierre; Bez, Elena; Himmerlich, Marcel; Ehrhardt, Martin; Taborelli, Mauro; Zimmer, KlausNanostructured surfaces exhibit outstanding properties and enable manifold industrial applications. In this study the laser surface processing of polycrystalline, flat copper surfaces by 532 nm picosecond laser irradiation for secondary electron yield (SEY) reduction is reported. The laser beam was scanned in parallel lines across the sample surface in order to modify large surface areas. Morphology and SEY are characterized in dependence of the process parameters to derive correlations and mechanisms of the laser-based SEY engineering process. The nano- and microstructure morphology of the laser-modified surface was characterized by scanning electron microscopy and the secondary electron yield was measured. In general, an SEY reduction with increasing accumulated laser fluence was found. In particular, at low scanning speed (1 mm/s - 10 mm/s) and “high” laser power (~ 1 W) compact nanostructures with a very low SEY maximum of 0.7 are formed.
- ItemSelf-cleaning stainless steel surfaces induced by laser processing and chemical engineering(Amsterdam [u.a.] : Elsevier, 2022) Lorenz, Pierre; Zajadacz, Joachim; Marquardt, Franka; Ehrhardt, Martin; Hommes, Gregor; Peter, Sebastian; Zimmer, KlausNanostructured surfaces show a variety of beneficial macroscopic effects. The combination of hierarchic nanostructures with a suitable chemical surface composition allows for the fabrication of surfaces with interesting fluidic properties beyond such effects. This approach enables the specification of nano/microstructure and chemical composition independent of each other. Various hierarchical micro- and nanostructures can be realized by laser texturing of stainless steel surfaces with infrared picosecond laser. Simultaneously, the surface is activated for chemical processing. The surface can now be tuned by bonding of a self-assembled monolayer on the laser-treated surface by chemical treatment. This two-step functionalization process allows the for separated adjusting of the surface topography and chemical composition and thus for the well-defined setting of the surface properties. The fabrication of superhydrophobic surfaces with self-cleaning properties are performed that can be functionalized further by subsequent laser-irradiation. Furthermore, the long-time stability of the surface functionalization in relation to the impact chemicals or radiation was investigated.