Scientific Output

Link to project open research data repository: ZENODO

P. Kepič: Arrays of Plasmonic Nanostructures Made of Phase-Change Materials (BUT, 2019) (bachelor’s thesis)

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Fundamentals of cathodoluminescence in a STEM: The impact of sample geometry and electron beam energy on light emission of semiconductors by Michael Stöger-Pollach, Kristýna Bukvišová, Sabine Schwarz, Michal Kvapil, Tomáš Šamořil and Michal Horák

Ultramicroscopy Volume 200, May 2019, Pages 111-124
DOI: 10.1016/j.ultramic.2019.03.001 

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Link to ORDP experimental dataset 

Abstract:

Cathodoluminescence has attracted interest in scanning transmission electron microscopy since the advent of commercial available detection systems with high efficiency, like the Gatan Vulcan or the Attolight Mönch system. In this work we discuss light emission caused by high-energy electron beams when traversing a semiconducting specimen. We nd that it is impossible to directly interpret the spectrum of the emitted light to the inter-band transitions excited by the electron beam, because the Čerenkov effect and the related light guiding modes as well as transition radiation is altering the spectra. Total inner re ection and subsequent interference effects are changing the spectral shape dependent on the sample shape and geometry, sample thickness, and beam energy, respectively. A detailed study on these parameters is given using silicon and GaAs as test materials.

Formation of Tungsten Oxide Nanowires by Electron-Beam-Enhanced Oxidation of WS2 Nanotubes and Platelets by Miroslav Kolíbal, Kristýna Bukvišová, Lukáš Kachtík, Alla Zak, Libor Novák, and Tomáš Šikola  

J. Phys. Chem. C, DOI: 10.1021/acs.jpcc.9b00592, March 9 2019  

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Abstract:

Oxidation of van der Waals-bonded layered semiconductors plays a key role in deterioration of their superior optical and electronic properties. The oxidation mechanism of these materials is, however, different from non-layered semiconductors in many aspects. Here, we show a rather unusual oxidation of tungsten disulfide (WS2) nanotubes and platelets in a high vacuum chamber at a presence of water vapor and at elevated temperatures. The process results in formation of small tungsten oxide nanowires on the surface of WS2. Utilizing real-time scanning electron microscopy we are able to unravel the oxidation mechanism, which proceeds via reduction of initially formed WO3 phase into W18O49 nanowires. Moreover, we show that the oxidation reaction can be localized and enhanced by an electron beam irradiation, which allows for on-demand growth of tungsten oxide nanowires. 

Geometric-Phase Microscopy for Quantitative Phase Imaging of Isotropic, Birefringent and SpaceVariant Polarization Samples by Petr Bouchal; Lenka Štrbková; Zbyněk Dostál; Radim Chmelík, and Zdeněk Bouchal

Scientific Reports 9, Article number: 3608 (2019), March 5 2019 (Open Access)

Link to ORDP experimental dataset

Abstract:

We present geometric-phase microscopy allowing a multipurpose quantitative phase imaging in which the ground-truth phase is restored by quantifying the phase retardance. The method uses broadband spatially incoherent light that is polarization sensitively controlled through the geometric (Pancharatnam-Berry) phase. The assessed retardance possibly originates either in dynamic or geometric phase and measurements are customized for quantitative mapping of isotropic and birefringent samples or multi-functional geometric-phase elements. The phase restoration is based on the self-interference of polarization distinguished waves carrying sample information and providing pure reference phase, while passing through an inherently stable common-path setup. The experimental configuration allows an instantaneous (single-shot) phase restoration with guaranteed subnanometer precision and excellent ground-truth accuracy (well below 5 nm). The optical performance is demonstrated in advanced yet routinely feasible noninvasive biophotonic imaging executed in the automated manner and predestined for supervised machine learning. The experiments demonstrate measurement of cell dry mass density, cell classification based on the morphological parameters and visualization of dynamic dry mass changes. The multipurpose use of the method was demonstrated by restoring variations in the dynamic phase originating from the electrically induced birefringence of liquid crystals and by mapping the geometric phase of a space-variant polarization directed lens.

High-Resolution Quantitative Phase Imaging of Plasmonic Metasurfaces with Sensitivity down to a Single Nanoantenna by Petr Bouchal, Petr Dvořák, Jiří Babocký, Zdeněk Bouchal, Filip Ligmajer, Martin Hrtoň, Vlastimil Křápek, Alexander Fassbender, Stefan Linden, Radim Chmelík, and Tomáš Šikola 

Nano Lett. 2019, 19 (2), 1242-1250;  DOI: 10.1021/acs.nanolett.8b04776, January 2 2019  

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Link to ORDP experimental dataset 

Abstract:

Optical metasurfaces have emerged as a new generation of building blocks for multi-functional optics. Design and realization of metasurface elements place ever-increasing demands on accurate assessment of phase alterations introduced by complex nanoantenna arrays, a process referred to as quantitative phase imaging. Despite considerable effort, the widefield (non-scanning) phase imaging that would approach resolution limits of optical microscopy and indicate the response of a single nanoantenna still remains a challenge. Here, we report on a new strategy in incoherent holographic imaging of metasurfaces, in which unprecedented spatial resolution and light sensitivity are achieved by taking full advantage of the polarization selective control of light through the geometric (Pancharatnam-Berry) phase. The measurement is carried out in an inherently stable common-path setup composed of a standard optical microscope and an add-on imaging module. Phase information is acquired from the mutual coherence function attainable in records created in broadband spatially incoherent light by the self-interference of scattered and leakage light coming from the metasurface. In calibration measurements, the phase was mapped with the precision and spatial background noise better than 0.01 rad and 0.05 rad, respectively. The imaging excels at the high spatial resolution that was demonstrated experimentally by the precise amplitude and phase restoration of vortex metalenses and a metasurface grating with 833 lines/mm. Thanks to superior light sensitivity of the method, we demonstrated, for the first time to our knowledge, the widefield measurement of the phase altered by a single nanoantenna, while maintaining the precision well below 0.15 rad.

Preparation of ultrafine fibrous uranium dioxide by electrospinning by Vojtech Kundrat, Ales Patak, and Jiri Pinkas 

Journal of Nuclear Materials; available online 1 Nov 2019  DOI: 10.1016/j.jnucmat.2019.151877 

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Link to ORDP experimental dataset 

Abstract:

We are introducing for the first time a robust method for the preparation of uranium oxide nanofibers with extraordinary thin diameters. We have studied the preparation of U3O8 and UO2 nanofibers by the electrospinning method from uranyl acetylacetonate in mixed organic solvents. Polyvinylpyrrolidone (PVP) was used as a supporting polymer. Besides studying and optimizing the solution systems, we have also examined the effects of collector material on the properties of the fiber mat. We have found that calcination of electrospun green composite fibers deposited on aluminum foil produces U3O8 nanofibers with the lowest average diameter of 20 ± 5 nm as their adhesion prevents fiber shrinking. We have also studied calcination of fibers on ashless paper and freestanding composite mats. Reduction of U3O8 fibers in H2/N2 atmosphere at various temperatures provided uranium dioxide nanofibers with an average diameter of 90 nm.


Complex k-uniform tilings by a simple bitopic precursor self-assembled on Ag(001) surface by Lukáš Kormoš; Pavel Procházka; Anton O. Makoveev; and Jan Čechal

Nature Communications 11, 1856 (2020), April 20, 2020 (Open Access)

Link to ORDP experimental dataset

Abstract:

The realization of complex long-range ordered structures in a Euclidean plane presents a significant challenge en route to the utilization of their unique physical and chemical properties. Recent progress in on-surface supramolecular chemistry has enabled the engineering of regular and semi-regular tilings, expressing translation symmetric, quasicrystalline, and fractal geometries. However, the k-uniform tilings possessing several distinct vertices remain largely unexplored. Here, we show that these complex geometries can be prepared from a simple bitopic molecular precursor – 4,4’-biphenyl dicarboxylic acid (BDA) – by its controlled chemical transformation on the Ag(001) surface. The realization of 2- and 3-uniform tilings is enabled by partially carboxylated BDA mediating the seamless connection of two distinct binding motifs in a single long-range ordered molecular phase. These results define the basic self-assembly criteria, opening way to the utilization of complex supramolecular tilings.

Multi-Electrode Array with a Planar Surface for Cell Patterning by Microprinting by Jan Slavík; Josef Skopalík; Ivo Provazník; and Jaromír Hubálek

Sensors 2019, 19(24), 5379, December 5, 2019 (Open Access)

Link to ORDP experimental dataset

Abstract:

Multielectrode arrays (MEAs) are devices for non-invasive electrophysiological measurements of cell populations. This paper describes a novel fabrication method of MEAs with a fully planar surface. The surface of the insulation layer and the surface of the electrodes were on one plane; we named this device the planar MEA (pMEA). The main advantage of the pMEA is that it allows uniform contact between the pMEA surface and a substrate for positioning of microfluidic channels or microprinting of a cell adhesive layer. The fabrication of the pMEA is based on a low adhesive Au sacrificial peel-off layer. In divergence from conventional MEAs with recessed electrodes, the electrodes of the pMEA lead across the sloped edge of the insulation layer. To make this, the profile of the edge of the insulation layer was measured and the impedance of the planar electrodes was characterized. The impedance of the pMEA was comparable with the impedance of conventional MEA electrodes. The pMEA was tested for patterning HL-1 cells with a combination of imprinting fibronectin and coating by antifouling poly (l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG). The HL-1 cells remained patterned even at full confluency and presented spontaneous and synchronous beating activity.

Independent engineering of individual plasmon modes in plasmonic dimers with conductive and capacitive coupling by Vlastimil Křápek; Andrea Konečná; Michal Horák; Filip Ligmajer;   Michael Stöger-Pollach; Martin Hrtoň; Jiří Babocký; and Tomáš Šikola

Nanophotonics, Volume 9, Issue 3, Pages 623–632, eISSN 2192-8614, December 9, 2019 (Open Access)

Link to ORDP experimental dataset (coming soon)

Abstract:

We revisit plasmon modes in nanoparticle dimers with conductive or insulating junction resulting in conductive or capacitive coupling. In our study, which combines electron energy loss spectroscopy, optical spectroscopy, and numerical simulations, we show the coexistence of strongly and weakly hybridised modes. While the properties of the former ones strongly depend on the nature of the junction, the properties of the latter ones are nearly unaffected. This opens up a prospect for independent engineering of individual plasmon modes in a single plasmonic antenna. In addition, we show that Babinet’s principle allows to engineer the near field of plasmon modes independent of their energy. Finally, we demonstrate that combined electron energy loss imaging of a plasmonic antenna and its Babinet-complementary counterpart allows to reconstruct the distribution of both electric and magnetic near fields of localised plasmon resonances supported by the antenna, as well as charge and current antinodes of related charge oscillations.

Vacuum Rabi splitting of a dark plasmonic cavity mode revealed by fast electrons by Ora Bitton; Satyendra Nath Gupta; Lothar Houben; Michal Kvapil; Vlastimil Křápek; Tomáš Šikola; and Gilad Haran

Nature Communications 11, 487 (2020), January 24, 2020 (Open Access)

Link to ORDP experimental dataset 

Abstract:

Recent years have seen a growing interest in strong coupling between plasmons and excitons, as a way to generate new quantum optical testbeds and influence chemical dynamics and reactivity. Strong coupling to bright plasmonic modes has been achieved even with single quantum emitters. Dark plasmonic modes fare better in some applications due to longer lifetimes, but are difficult to probe as they are subradiant. Here, we apply electron energy loss (EEL) spectroscopy to demonstrate that a dark mode of an individual plasmonic bowtie can interact with a small number of quantum emitters, as evidenced by Rabi-split spectra. Coupling strengths of up to 85 meV place the bowtie-emitter devices at the onset of the strong coupling regime. Remarkably, the coupling occurs at the periphery of the bowtie gaps, even while the electron beam probes their center. Our findings pave the way for using EEL spectroscopy to study exciton-plasmon interactions involving non-emissive photonic modes.

Single-layer graphene on epitaxial FeRh thin films by Vojtěch Uhlíř; Federico Pressacco; Jon Ander Arregi; Pavel Procházka; Stanislav Průša; Michal Potoček; Tomáš Šikola; Jan Čechal; Azzedine Bendounan; and Fausto Sirotti

Apllied Surface Science Volume 514, 1 June 2020, 145923, February 27, 2020 (Open Access)

Link to ORDP experimental dataset (coming soon)

Abstract:

Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by X-ray Photoemission and X-ray Absorption Spectroscopy, Low-Energy Ion Scattering, Scanning Tunneling Microscopy, and Low-Energy Electron Microscopy shows that graphene is single-layer, polycrystalline and covers more than 97% of the substrate. Graphene displays several preferential orientations on the FeRh(0 0 1) surface with unit vectors of graphene rotated by 30°, 15°, 11°, and 19° with respect to FeRh substrate unit vectors. In addition, the graphene layer is capable to protect the films from oxidation when exposed to air for several months. Therefore, it can be also used as a protective layer during fabrication of magnetic elements or as an atomically thin spacer, which enables incorporation of switchable magnetic layers within stacks of 2D materials in advanced devices.

Preparation of thorium dioxide nanofibers by electrospinning by Vojtěch Kundrát; Zdeněk Moravec and  Jiří Pinkas

Journal of Nuclear Materials; available online 14 April 2020  DOI: 10.1016/j.jnucmat.2020.152153

Link to Open access version 

Link to ORDP experimental dataset 

Abstract:

For the first time, we report the preparation of thorium dioxide with nanofibrous morphology by the electrospinning method. Two approaches were employed with different electrospun solution compositions and inorganic precursors. Thorium nitrate with polyvinyl alcohol (PVA) as a supporting polymer was successfully electrospun from an aqueous solution resulting in green composite fibers that were calcined at 773 K to pure ThO2 nanofibers of diameter 76 ± 25 nm. Another precursor solution was based on organic solvents and thorium acetylacetonate complex with polyvinylpyrrolidone (PVP). Green fibers were calcined at 673 K to porous nanofibers of ThO2 with an average diameter of 34 ± 18 nm and surface area of 85 m2 g-1 . The prepared materials were characterized by TGA-DSC, SEM, TEM, PXRD, and BET methods.