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    Volumetrical characterization of sheet molding compounds
    (Basel : MDPI, 2010) Calvimontes, A.; Grund, K.; Müller, A.
    For a comprehensive study of Sheet Molding Compound (SMC) surfaces, topographical data obtained by chromatic confocal imaging were submitted systematically for the development of a profile model to understand the formation of cavities on the surface. In order to qualify SMC surfaces and to predict their coatability, a characterization of cavities is applied. To quantify the effect of surface modification treatments, a new parameter (Surface Relative Smooth) is presented, applied and probed. The parameter proposed can be used for any surface modification of any solid material. © 2010 by the authors.
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    Polyester textile functionalization through incorporation of pH/thermo-responsive microgels. Part II: Polyester functionalization and characterization
    (Heidelberg : Springer, 2012) Glampedaki, P.; Calvimontes, A.; Dutschk, V.; Warmoeskerken, M.M.C.G.
    A new approach to functionalize the surface of polyester textiles is described in this study. Functionalization was achieved by incorporating pH/temperature-responsive polyelectrolyte microgels into the textile surface layer using UV irradiation. The aim of functionalization was to regulate polyester wettability according to ambient conditions by imparting stimuli-responsiveness from the microgel to the textile itself. Microgels consisted of pH/thermo-responsive microparticles of poly(N-isopropylacrylamide- co-acrylic acid) either alone or complexed with the pH-responsive natural polysaccharide chitosan. Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy, ζ-potential measurements, and topographical analysis were used for surface characterization. Wettability of polyester textiles was assessed by dynamic wetting, water vapor transfer, and moisture regain measurements. One of the main findings showed that the polyester surface was rendered pH-responsive, both in acidic and alkaline pH region, owing to the microgel incorporation. With a marked relaxation in their structure and an increase in their microporosity, the functionalized textiles exhibited higher water vapor transfer rates both at 20 and 40 °C, and 65% relative humidity compared with the reference polyester. Also, at 40 °C, i.e., above the microgel Lower Critical Solution Temperature, the functionalized polyester textiles had lower moisture regains than the reference. Finally, the type of the incorporated microgel affected significantly the polyester total absorption times, with an up to 300% increase in one case and an up to 80% decrease in another case. These findings are promising for the development of functional textile materials with possible applications in biotechnology, technical, and protective clothing.
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    Direct imaging of delayed magneto-dynamic modes induced by surface acoustic waves
    (London : Nature Publishing Group, 2017) Foerster, Michael; Macià, Ferran; Statuto, Nahuel; Finizio, Simone; Hernández-Mínguez, Alberto; Lendínez, Sergi; Santos, Paulo V.; Fontcuberta, Josep; Hernàndez, Joan Manel; Kläui, Mathias; Aballe, Lucia
    The magnetoelastic effect—the change of magnetic properties caused by the elastic deformation of a magnetic material—has been proposed as an alternative approach to magnetic fields for the low-power control of magnetization states of nanoelements since it avoids charge currents, which entail ohmic losses. Here, we have studied the effect of dynamic strain accompanying a surface acoustic wave on magnetic nanostructures in thermal equilibrium. We have developed an experimental technique based on stroboscopic X-ray microscopy that provides a pathway to the quantitative study of strain waves and magnetization at the nanoscale. We have simultaneously imaged the evolution of both strain and magnetization dynamics of nanostructures at the picosecond time scale and found that magnetization modes have a delayed response to the strain modes, adjustable by the magnetic domain configuration. Our results provide fundamental insight into magnetoelastic coupling in nanostructures and have implications for the design of strain-controlled magnetostrictive nano-devices.
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    Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films
    ([London] : Nature Publishing Group UK, 2022) Li, Zhan Hua; He, Jia Xing; Lv, Xiao Hu; Chi, Ling Fei; Egbo, Kingsley O.; Li, Ming-De; Tanaka, Tooru; Guo, Qi Xin; Yu, Kin Man; Liu, Chao Ping
    As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for copper iodide thin films by a synergistic approach employing a combination of analytical techniques. Steady-state photoluminescence spectra reveal that the emission at ~420 nm arises from the recombination of electrons with neutral copper vacancies. The photogenerated carrier density dependent ultrafast physical processes are elucidated with using the femtosecond transient absorption spectroscopy. Both the effects of hot-phonon bottleneck and the Auger heating significantly slow down the cooling rate of hot-carriers in the case of high excitation density. The effect of defects on the carrier recombination and the two-photon induced ultrafast carrier dynamics are also investigated. These findings are crucial to the optoelectronic applications of copper iodide.
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    Effect of chemical solvents on the wetting behavior over time of femtosecond laser structured ti6al4v surfaces
    (Basel : MDPI, 2020) Schnell, Georg; Polley, Christian; Bartling, Stephan; Seitz, Hermann
    The effect of chemical solvents on the wetting state of laser-structured surfaces over time is systematically examined in this paper. By using a 300-fs laser, nanostructures were generated on Ti6Al4V, subsequently cleaned in an ultrasonic bath with different solvents and stored in ambient air. The static contact angle showed significant differences for cleaning with various solvents, which, depending on the applied cleaning and time, amounted up to 100°. X-ray photoelectron spectroscopy analyses reveal that the cleaning of the laser-structured surfaces affects the surface chemistry and the aging behavior of the surfaces, even with highly volatile solvents. The effect of the chemical surface modification is particularly noticeable when using alcohols for cleaning, which, due to their OH groups, cause highly hydrophilic behavior of the surface after one day of storage. Over the course of 14 days, enrichment with organic groups from the atmosphere occurs on the surface, which leads to poorer wetting on almost every structured surface. In contrast, the cleaning in hexane leads to a fast saturation of the surface with long-chain carbon groups and thus to a time-independent hydrophobic behavior.
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    Mapping the band structure of GeSbTe phase change alloys around the Fermi level
    (London : Nature Publishing Group, 2018) Kellner, J.; Bihlmayer, G.; Liebmann, M.; Otto, S.; Pauly, C.; Boschker, J.E.; Bragaglia, V.; Cecchi, S.; Wang, R.N.; Deringer, V.L.; Küppers, P.; Bhaskar, P.; Golias, E.; Sánchez-Barriga, J.; Dronskowski, R.; Fauster, T.; Rader, O.; Calarco, R.; Morgenstern, M.
    Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin.
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    Fermi surface nesting in several transition metal dichalcogenides
    (Milton Park : Taylor & Francis, 2008) Inosov, D.S.; Zabolotnyy, V.B.; Evtushinsky, D.V.; Kordyuk, A.A.; Büchner, B.; Follath, R.; Berger, H.; Borisenko, S.V.
    By means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), we have studied the fermiology of 2H transition metal dichalcogenide polytypes TaSe2, NbSe2 and Cu0.2NbS 2. The tight-binding model of the electronic structure, extracted from ARPES spectra for all three compounds, was used to calculate the Lindhard function (bare spin susceptibility), which reflects the propensity to charge density wave (CDW) instabilities observed in TaSe2 and NbSe 2. We show that though the Fermi surfaces of all three compounds possess an incommensurate nesting vector in the close vicinity of the CDW wave vector, the nesting and ordering wave vectors do not exactly coincide, and there is no direct relationship between the magnitude of the susceptibility at the nesting vector and the CDW transition temperature. The nesting vector persists across the incommensurate CDW transition in TaSe2 as a function of temperature despite the observable variations of the Fermi surface geometry in this temperature range. In Cu0.2NbS2, the nesting vector is present despite different doping levels, which leads us to expect a possible enhancement of the CDW instability with Cu intercalation in the Cu xNbS2 family of materials.
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    Formation of resonant bonding during growth of ultrathin GeTe films
    (London : Nature Publishing Group, 2017) Wang, Ruining; Zhang, Wei; Momand, Jamo; Ronneberger, Ider; Boschker, Jos E.; Mazzarello, Riccardo; Kooi, Bart J.; Riechert, Henning; Wuttig, Matthias; Calarco, Raffaella
    A highly unconventional growth scenario is reported upon deposition of GeTe films on the hydrogen passivated Si(111) surface. Initially, an amorphous film forms for growth parameters that should yield a crystalline material. The entire amorphous film then crystallizes once a critical thickness of four GeTe bilayers is reached, subsequently following the GeTe(111) 
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    Ordered Peierls distortion prevented at growth onset of GeTe ultra-thin films
    (London : Nature Publishing Group, 2016) Wang, Ruining; Campi, Davide; Bernasconi, Marco; Momand, Jamo; Kooi, Bart J.; Verheijen, Marcel A.; Wuttig, Matthias; Calarco, Raffaella
    Using reflection high-energy electron diffraction (RHEED), the growth onset of molecular beam epitaxy (MBE) deposited germanium telluride (GeTe) film on Si(111)-(√3 × √3)R30°-Sb surfaces is investigated, and a larger than expected in-plane lattice spacing is observed during the deposition of the first two molecular layers. High-resolution transmission electron microscopy (HRTEM) confirms that the growth proceeds via closed layers, and that those are stable after growth. The comparison of the experimental Raman spectra with theoretical calculated ones allows assessing the shift of the phonon modes for a quasi-free-standing ultra-thin GeTe layer with larger in-plane lattice spacing. The manifestation of the latter phenomenon is ascribed to the influence of the interface and the confinement of GeTe within the limited volume of material available at growth onset, either preventing the occurrence of Peierls dimerization or their ordered arrangement to occur normally.
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    Far-infrared and Raman spectroscopy investigation of phonon modes in amorphous and crystalline epitaxial GeTe-Sb2Te3 alloys
    (London : Nature Publishing Group, 2016) Bragaglia, V.; Holldack, K.; Boschker, J.E.; Arciprete, F.; Zallo, E.; Flissikowski, T.; Calarco, R.
    A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the Fm3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.