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Start date: 2020 × DDC: 530 × DDC: 670 ×

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Now showing 1 - 10 of 21
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    Tailoring 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.
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    Photoluminescence Mapping over Laser Pulse Fluence and Repetition Rate as a Fingerprint of Charge and Defect Dynamics in Perovskites
    (Weinheim : Wiley-VCH, 2023) Rao, Shraddha M.; Kiligaridis, Alexander; Yangui, Aymen; An, Qingzhi; Vaynzof, Yana; Scheblykin, Ivan G.
    Defects in metal halide perovskites (MHP) are photosensitive, making the observer effect unavoidable when laser spectroscopy methods are applied. Photoluminescence (PL) bleaching and enhancement under light soaking and recovery in dark are examples of the transient phenomena that are consequent to the creation and healing of defects. Depending on the initial sample composition, environment, and other factors, the defect nature and evolution can strongly vary, making spectroscopic data analysis prone to misinterpretations. Herein, the use of an automatically acquired dependence of PL quantum yield (PLQY) on the laser pulse repetition rate and pulse fluence as a unique fingerprint of both charge carrier dynamics and defect evolution is demonstrated. A simple visual comparison of such fingerprints allows for assessment of similarities and differences between MHP samples. The study illustrates this by examining methylammonium lead triiodide (MAPbI3) films with altered stoichiometry that just after preparation showed very pronounced defect dynamics at time scale from milliseconds to seconds, clearly distorting the PLQY fingerprint. Upon weeks of storage, the sample fingerprints evolve toward the standard stoichiometric MAPbI3 in terms of both charge carrier dynamics and defect stability. Automatic PLQY mapping can be used as a universal method for assessment of perovskite sample quality.
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    Self-organized formation of unidirectional and quasi-one-dimensional metallic Tb silicide nanowires on Si(110)
    (Amsterdam [u.a.] : Elsevier, 2022) Appelfeller, Stephan; Franz, Martin; Karadag, Murat; Kubicki, Milan; Zielinski, Robert; Krivenkov, Maxim; Varykhalov, Andrei; Preobrajenski, Alexei; Dähne, Mario
    Terbium induced nanostructures on Si(110) and their growth are thoroughly characterized by low energy electron diffraction, scanning tunneling microscopy and spectroscopy, core-level and valence band photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. For low Tb coverage, a wetting layer forms with its surface fraction continuously decreasing with increasing Tb coverage in favor of the formation of unidirectional Tb silicide nanowires. These nanowires show high aspect ratios for high annealing temperatures or on substrates already containing Tb in the bulk. Both wetting layer and nanowires are stable for temperatures up to 750°C. In contrast to the nanowires, the wetting layer is characterized by a band gap. Thus, the metallic nanowires, which show a quasi-one-dimensional electronic band structure, are embedded in a semiconducting surrounding of wetting layer and substrate, insulating the nanowires from each other.
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    Tunable Circular Dichroism by Photoluminescent Moiré Gratings
    (Weinheim : Wiley-VCH, 2020) Aftenieva, Olha; Schnepf, Max; Mehlhorn, Börge; König, Tobias A.F.
    In nanophotonics, there is a current demand for ultrathin, flexible nanostructures that are simultaneously easily tunable, demonstrate a high contrast, and have a strong response in photoluminescent polarization. In this work, the template-assisted self-assembly of water-dispersed colloidal core–shell quantum dots into 1D light-emitting sub-micrometer gratings on a flexible substrate is demonstrated. Combining such structures with a light-absorbing metallic counterpart by simple stacking at various angles results in a tunable Moiré pattern with strong lateral contrast. Furthermore, a combination with an identical emitter-based grating leads to a chiroptical effect with a remarkably high degree of polarization of 0.72. Such a structure demonstrates direct circular polarized photoluminescence, for the first time, without a need for an additional chiral template as an intermediary. The suggested approach allows for reproducible, large-area manufacturing at reasonable costs and is of potential use for chiroptical sensors, photonic circuit applications, or preventing counterfeit. © 2020 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH
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    Spectrometer‐free Optical Hydrogen Sensing Based on Fano‐like Spatial Distribution of Transmission in a Metal−Insulator−Metal Plasmonic Doppler Grating
    (Weinheim : Wiley-VCH, 2021) Chen, Yi‐Ju; Lin, Fan‐Cheng; Singh, Ankit Kumar; Ouyang, Lei; Huang, Jer‐Shing
    Optical nanosensors are promising for hydrogen sensing because they are small, free from spark generation, and feasible for remote optical readout. Conventional optical nanosensors require broadband excitation and spectrometers, rendering the devices bulky and complex. An alternative is spatial intensity-based optical sensing, which only requires an imaging system and a smartly designed platform to report the spatial distribution of analytical optical signals. Here, a spatial intensity-based hydrogen sensing platform is presented based on Fano-like spatial distribution of the transmission in a Pd-Al2O3-Au metal-insulator-metal plasmonic Doppler grating (MIM-PDG). The MIM-PDG manifests the Fano resonance as an asymmetric spatial transmission intensity profile. The absorption of hydrogen changes the spatial Fano-like transmission profiles, which can be analyzed with a “spatial” Fano resonance model and the extracted Fano resonance parameters can be used to establish analytical calibration lines. While gratings sensitive to hydrogen absorption are suitable for hydrogen sensing, hydrogen insensitive gratings are also found, which provide an unperturbed reference signal and may find applications in nanophotonic devices that require a stable optical response under fluctuating hydrogen atmosphere. The MIM-PDG platform is a spectrometer-free and intensity-based optical sensor that requires only an imaging system, making it promising for cellphone-based optical sensing applications. © 2021 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH.
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    Waveguide-Integrated Broadband Spectrometer Based on Tailored Disorder
    (Weinheim : Wiley-VCH Verlag, 2020) Hartmann, Wladick; Varytis, Paris; Gehring, Helge; Walter, Nicolai; Beutel, Fabian; Busch, Kurt; Pernice, Wolfram
    Compact, on-chip spectrometers exploiting tailored disorder for broadband light scattering enable high-resolution signal analysis while maintaining a small device footprint. Due to multiple scattering events of light in the disordered medium, the effective path length of the device is significantly enhanced. Here, on-chip spectrometers are realized for visible and near-infrared wavelengths by combining an efficient broadband fiber-to-chip coupling approach with a scattering area in a broadband transparent silicon nitride waveguiding structure. Air holes etched into a structured silicon nitride slab terminated with multiple waveguides enable multipath light scattering in a diffusive regime. Spectral-to-spatial mapping is performed by determining the transmission matrix at the waveguide outputs, which is then used to reconstruct the probe signals. Direct comparison with theoretical analyses shows that such devices can be used for high-resolution spectroscopy from the visible up to the telecom wavelength regime. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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    Tailoring Plasmonics of Au@Ag Nanoparticles by Silica Encapsulation
    (Weinheim : Wiley-VCH, 2021) Schultz, Johannes; Kirner, Felizitas; Potapov, Pavel; Büchner, Bernd; Lubk, Axel; Sturm, Elena V.
    Hybrid metallic nanoparticles (NPs) encapsulated in oxide shells are currently intensely studied for plasmonic applications in sensing, medicine, catalysis, and photovoltaics. Here, a method for the synthesis of Au@Ag@SiO2 cubes with a uniform silica shell of variable and adjustable thickness in the nanometer range is introduced and their excellent, highly reproducible, and tunable optical response is demonstrated. Varying the silica shell thickness, the excitation energies of the single NP plasmon modes can be tuned in a broad spectral range between 2.55 and 3.25 eV. Most importantly, a strong coherent coupling of the surface plasmons is revealed at the silver–silica interface with Mie resonances at the silica–vacuum interface leading to a significant field enhancement at the encapsulated NP surface in the range of 100% at shell thicknesses t ≃ 20 nm. Consequently, the synthesis method and the field enhancement open pathways to a widespread use of silver NPs in plasmonic applications including photonic crystals and may be transferred to other non-precious metals. © 2021 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH
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    Self-stabilization of the equilibrium state in ferroelectric thin films
    (Amsterdam : Elsevier, 2022) Gaal, Peter; Schmidt, Daniel; Khosla, Mallika; Richter, Carsten; Boesecke, Peter; Novikov, Dmitri; Schmidbauer, Martin; Schwarzkopf, Jutta
    (K,Na)NbO3 is a lead-free and sustainable ferroelectric material with electromechanical parameters comparable to Pb(Zr,Ti)O3 (PZT) and other lead-based solid solutions. It is therefore a promising candidate for caloric cooling and energy harvesting applications. Specifically, the structural transition from the low-temperature Mc- to the high-temperature c-phase displays a rich hierarchical order of domains and superdomains, that forms at specific strain conditions. The relevant length scales are few tens of nanometers for the domain and few micrometers for the superdomain size, respectively. Phase-field calculations show that this hierarchical order adds to the total free energy of the solid. Thus, domains and their formation has a strong impact on the functional properties relevant for electrocaloric cooling or energy harvesting applications. However, monitoring the formation of domains and superdomains is difficult and requires both, high spatial and high temporal resolution of the experiment. Synchrotron-based time-resolved X-ray diffraction methods in combination with scanning imaging X-ray microscopy is applied to resolve the local dynamics of the domain morphology with sub-micrometer spatial and nanosecond temporal resolution. In this regime, the material displays a novel self-stabilization mechanism of the domain morphology, which may be a general property of first-order phase transitions.
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    Fabrication of four-level hierarchical topographies through the combination of LIPSS and direct laser interference pattering on near-beta titanium alloy
    (New York, NY [u.a.] : Elsevier, 2022) Schell, Frederic; Alamri, Sabri; Hariharan, Avinash; Gebert, Annett; Lasagni, Andrés Fabián; Kunze, Tim
    Complex repetitive periodic surface patterns were produced on a near-beta Ti-13Nb-13Zr alloy, using two-beam Direct Laser Interference Patterning (DLIP) employing a picosecond-pulsed laser source with wavelengths of 355 nm, 532 nm and 1064 nm. Different types of Laser-induced periodic surface structures (LIPSS) are produced, including low and high spatial frequency LIPSS, which are observed frequently on top of the line-like DLIP microstructures, as well as quasi-periodic microstructures with periods greater than the laser wavelength. The feature size of the fabricated LIPSS features could be tuned as function of the utilized laser process parameters.
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    The Potential of Combining Thermal Scanning Probes and Phase-Change Materials for Tunable Metasurfaces
    (Weinheim : Wiley-VCH, 2020) Michel, Ann-Katrin U.; Meyer, Sebastian; Essing, Nicolas; Lassaline, Nolan; Lightner, Carin R.; Bisig, Samuel; Norris, David J.; Chigrin, Dmitry N.
    Metasurfaces allow for the spatiotemporal variation of amplitude, phase, and polarization of optical wavefronts. Implementation of active tunability of metasurfaces promises compact flat optics capable of reconfigurable wavefront shaping. Phase-change materials (PCMs) are a prominent material class enabling reconfigurable metasurfaces due to their large refractive index change upon structural transition. However, commonly employed laser-induced switching of PCMs limits the achievable feature sizes and restricts device miniaturization. Thermal scanning-probe-induced local switching of the PCM germanium telluride is proposed to realize near-infrared metasurfaces with feature sizes far below what is achievable with diffraction-limited optical switching. The design is based on a planar multilayer and does not require fabrication of protruding resonators as commonly applied in the literature. Instead, it is numerically demonstrated that a broad-band tuning of perfect absorption can be realized by the localized tip-induced crystallization of the PCM. The spectral response of the metasurface is explained using resonance mode analysis and numerical simulations. To facilitate experimental realization, a theoretical description of the tip-induced crystallization employing multiphysics simulations is provided to demonstrate the great potential for fabricating compact reconfigurable metasurfaces. The concept can be applied not only for plasmonic sensing and spatial frequency filtering, but also be transferred to all-dielectric metasurfaces. © 2020 Wiley-VCH GmbH