Scientific Output

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

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)

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

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

APL Photonics 4, 126102, 2019 (Open Access)

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.

Non-diffracting light in nature: Anomalously reflected self-healing Bessel beams from jewel scarabs by Petr Bouchal, Josef Kapitán, Martin Konečný, Marek Zbončák a Zdeněk Bouchal

Nat Commun 11, 1856 (2020) - (Open Access)

Abstract:

We report a study of spatial light modulation in the photonic structure of jewel scarabs, revealing the interplay of the polarization and phase control of light, which is not possible with the current optical technology. Phase measurements performed on jewel scarabs demonstrate that the polarization anomalous (helicity-preserving) reflection of light occurs together with alteration of the dynamic phase associated with the optical path length. This control of light differs from the operation of artificially prepared polarization-sensitive structures, shaping light through the geometric phase altered by the polarization transformation. Challenging three-dimensional imaging of the cuticle, requiring high-resolution quantitative mapping of steep phase changes, has been achieved owing to the optical performance of recently developed geometric-phase microscopy. We find that the cuticle of jewel scarabs is formed of micrometer-sized axicon cells, generating thousands of Bessel beams with subwavelength spot size. The nondiffracting features and the self-healing ability of the Bessel beams originating from the beetle Chrysina gloriosa are demonstrated experimentally. Considering Bragg reflection and shaping of RGB components of white light Bessel beams, we explain the spatial structuring of colors in microscopic images of jewel scarabs and reveal the conversion of colors when changing the distance from the cuticle. The functionality and performance of the cuticle axicon cells are discussed in comparison with high-aperture dielectric meta-axicons, and potential applications in colorimetric refractive index sensing are outlined.

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

Link to the 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)

Applied Surface Science, Volume 497, 16 August 2019, 143705

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.

Low temperature selective growth of GaN single crystals on pre-patterned Si substrates by Jindřich Mach, Jakub Piastek, Jaroslav Maniš, Vojtěch Čalkovský, Tomáš Šamořil, Jana Damková, Miroslav Bartošík, Stanislav Voborný, Martin Konečný and Tomáš Šikola

(link to open access version)

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

Abstract:

We report on a hybrid method for fabrication of arrays of GaN nanocrystals by low-temperature UHV selective growth on pre-patterned silicon substrates covered by native oxide. Patterning of the substrates was performed by using a gallium focused ion beam (FIB). To get GaN nanocrystals at specific positions, Ga droplets were created at FIB patterned sites by evaporation of Ga atoms at 280 °C substrate temperature first, and then modified by their post-nitridation using an ultra-low energy (50 eV) nitrogen ion–beam at a sample temperature of 200 °C. To get larger arrays of GaN nanocrystals (≈150 nm and 200 nm in diameter), such a sequential process was repeated in several cycles at slightly modified operation conditions. The quality of the nanocrystals was checked by measurement of their photoluminescence properties which proved the occurrence of the peak of a band edge emission at around 367 nm (3.38 eV).

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

Link to Open Access Version

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.

Effect of deposition angle on fabrication of plasmonic gold nanocones and nanodiscs by Jiří Liška, Filip Ligmajer, Pedro V. Pinho N., Lukáš Kejík, Michal Kvapil, Petr Dvořák, Michal Horký, Nikolaus S. Leitner, Erik Reimhult, and Tomáš Šikola;

Microelectronic Engineering, Volume 228, 1 May 2020, 111326.

Link to ORPD experimental dataset

Abstract:

Metal nanocones can exhibit several strong plasmonic resonances, which are associated with intense and accessible electromagnetic hot spots. They can thus be used to enhance light–matter interactions or to facilitate location-specific sensing while enabling separation of some non-specific contributions towards the sensing signal. Nanocones and similar 3D structures are often fabricated with the use of the so-called self-shading effect, which occurs during the evaporation of a metal film into circular nanowells. Unfortunately, a full description of a successful deposition process with all the essential details is currently missing in literature. Here we present a detailed view of the fabrication of ordered arrays of conical gold nanostructures using electron beam lithography and gold electron beam evaporation. We show that the symmetry of the fabricated nanostructures is influenced by the lateral position of the substrate on the sample holder during the deposition. Off-axis deposition or tilt of the sample leads to asymmetric nanostructures. When the deposited film is thick enough, or the nanowells narrow enough, the entrance aperture is clogged, and nanocones with sharp tips are formed. In contrast, flat-top truncated cones are produced for thinner films or wider nanowells. All these findings help to identify inherent limits for the production of wafer-scale arrays of such non-planar nanostructures. On the other hand, they also suggest new fabrication possibilities for more complicated structures such as mutually connected nanocones for electrically addressable chips.

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)

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

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

Calphad, Volume 69, June 2020, 101741 (link to Open Access Version)

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.

Thermal properties of Ag@Ni core-shell nanoparticles by Vít Vykoukal, František Zelenka, Jiří Bursík, Tomáš Kaňa, Aleš Kroupa, and Jiří Pinkas

Link to ORPD experimental dataset

Abstract:

We synthesized Ag@Ni core-shell nanoparticles by the solvothermal hot injection method and characterized them as for their shape and size by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). We previously demonstrated their core-shell structure by scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS). The silver/nickel phase diagram was calculated by the CALPHAD method, and the melting points of 10, 15, and 20 nm silver nanoparticles were predicted at 930.2, 940.7, and 946.0 °C, respectively. We took advantage of the nickel shell to avoid silver sintering and to confirm the calculated melting point depression (MPD). The results obtained from the differential scanning calorimetry (DSC) experiments revealed the melting points of 11–15 nm nanoparticles at 944–949 °C in agreement with calculated values.

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

Ultramicroscopy, Volume 214, July 2020, 113011 (Open Access)

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.

Using Čerenkov radiation for measuring the refractive index in thick samples by interferometric cathodoluminescence by Michael Stöger-Pollach, Stefan Löffler, Niklas Maurer, and Kristýna Bukvišová

Link to download full-text

Abstract:

Cathodoluminescence (CL) has evolved into a standard analytical technique in (scanning) transmission electron microscopy. CL utilizes light excited due to the interactions between the electron-beam and the sample. In the present study we focus on Cˇerenkov radiation. We make use of the fact that the electron transparent specimen acts as a Fabry-Pérot interferometer for coherently emitted radiation. From the wavelength dependent interference pattern of thickness dependent measurements we calculate the refractive index of the studied material. We describe the limits of this approach and compare it with the determination of the refractive index by using valence electron energy loss spectrometry (VEELS).

Magnetic Near Field Imaging with Electron Energy Loss Spectroscopy Based on Babinet's Principle by Michal Horák, Vlastimil Křápek, Martin Hrtoň, Andrea Konečná, Filip Ligmajer, Michael Stöger-Pollach and Tomáš Šikola

Conference paper in the proceedings of Microscopy & Microanalysis 2020 (4th–7th August, Milwaukee, USA, held in a virtual form) published as a supplement of impact journal Microscopy and Microanalysis.

Optical topography of rough surfaces using vortex localization of fluorescent markers by Petr Schovánek, Petr Bouchal, and Zdeněk Bouchal

Optics Letters Vol. 45, Issue 16, pp. 4468-4471 (2020) (Link to open access version)

J. Phys. Chem. Lett. 2020, 11, XXX, 6498–6504 (27 July 2020) - (Link to open access version)

Abstract:

Measuring rough surfaces is challenging because the proven topographic methods are impaired by the adverse effects of diffuse light. In our method, the measured surface is marked by fluorescent nanobeads allowing a complete suppression of diffuse light by bandpass filtering. Light emitted by each fluorescent bead is shaped to a double-helix point spread function used for three-dimensional bead localization on the surface. This non-interferometric measurement of rough surface topography is implemented in a vibration resistant setup. The comparison of our method with vertical scanning interferometry shows that a commercial profiler is surpassed when ground glass surfaces with steep slopes are measured.

Collector Droplet Behavior during Formation of Nanowire Junctions by Yanming Wang, Tomáš Šikola, and
Miroslav Kolíbal

Abstract:

Formation of nanowire networks is an appealing strategy for demonstrating novel phenomena at the nanoscale, e.g., detection of Majorana Fermions, as well as an essential step in realizing complex nanowire-based architectures. However, a detailed description of mechanisms taking place during growth of such complex structures is lacking. Here, the experimental observations of gold-catalyzed germanium nanowire junction formation are explained utilizing phase field modeling corroborated with real-time in situ scanning electron microscopy. When the two nanowires collide head on during the growth, we observe two scenarios. (i) Two catalytic droplets merge into one, and the growth continues as a single nanowire. (ii) The droplets merge and subsequently split again, giving rise to the growth of two daughter nanowires. Both the experiments and modeling indicate the critical importance of the liquid–solid growth interface anisotropy and the growth kinetics in facilitating the structural transition during the nanowire merging process.

Mechanism and Suppression of Physisorbed-Water-Caused Hysteresis in Graphene FET Sensors by Miroslav Bartošík, Jindřich Mach, Jakub Piastek, David Nezval, Martin Konečný, Vojtěch Švarc, Klaus Ensslin, and Tomáš Šikola

JACS Sensors 2020 5 (9), 2940-2949 - open access version

Nano Lett. 2021, 21, 1, 590–596 (December 18, 2020) - (open access)

Abstract:

Hysteresis is a problem in field-effect transistors (FETs) often caused by defects and charge traps inside a gate isolating (e.g., SiO2) layer. This work shows that graphene-based FETs also exhibit hysteresis due to water physisorbed on top of graphene determined by the relative humidity level, which naturally happens in biosensors and ambient operating sensors. The hysteresis effect is explained by trapping of electrons by physisorbed water, and it is shown that this hysteresis can be suppressed using short pulses of alternating gate voltages.

Continuous Flow Synthesis of Iron Oxide Nanoparticles Using Water-in-Oil Microemulsion by Sopoušek J., Pinkas J., Buršík J., Svoboda M., Krásenský P.

Colloid Journal volume 82, pages 727–734 (2020) (Green Open Access)

Link to ORDP dataset

Abstract:

A continuous laminar flow reactor for the synthesis of nanopowder in microemulsion is described. The reactor is suitable for separated handling with nucleation, growth, and stabilization processes. The synthesis of iron oxide nanoparticles was selected as a model case. A water−sodium dodecyl sulphate−cyclohexene system was used as the microemulsion system for dissolving reactive aqueous solution, precursor, and a particle stabilizer. The product was purified and transferred to the aqueous phase. The result was a colloid solution of iron oxide nanoparticles in water of 50–200 nm in size with a zeta potential ranging from –25 to –57 mV. The product was characterized by UV-VIS spectroscopy, powder XRD, dynamic light scattering, electron microscopy, and electron diffraction. The results showed that water-in-oil microemulsion method is useful for the synthesis of nanopowders to obtain large amounts of stable product.

Fundamental Limit of Plasmonic Cathodoluminescence by Franz-Philipp Schmidt, Arthur Losquin,
Michal Horák, Ulrich Hohenester, Michael Stöger-Pollach, and Joachim R. Krenn

Journal of Materials Science volume 56, pages 10834–10846 (2021)

Abstract:

We use cathodoluminescence (CL) spectroscopy in a transmission electron microscope to probe the radial breathing mode of plasmonic silver nanodisks. A two-mirror detection system sandwiching the sample collects the CL emission in both directions, that is, backward and forward with respect to the electron beam trajectory. We unambiguously identify a spectral shift of about 8 nm in the CL spectra acquired from both sides and show that this asymmetry is induced by the electron beam itself. By numerical simulations, we confirm the observations and identify the underlying physical effect due to the interference of the CL emission patterns of an electron-beam-induced dipole and the breathing mode. This effect can ultimately limit the achievable fidelity in CL measurements on any system involving multiple excitations and should therefore be considered with care in high-precision experiments.

Synthesis and characterization of WS2/SiO2 microfibers by Vojtěch Kundrát; Rita Rosentsveig; Olga Brontvein; Reshef Tenne; and Jiří Pinkas

Link to open access version

Nature Communications volume 12, Article number: 1310 (2021) (Open Access)

Abstract:

Tungsten disulfide polycrystalline microfibers were successfully synthesized by a process involving electrospinning, calcination, and sulfidation steps. We used an aqueous solution of silicotungstic acid (H₄SiW₁₂O₄₀) and polyvinyl alcohol as precursors for the synthesis of composite fibers by the needle-less electrospinning technique. The obtained green composite fibers (av. diam. 460 nm) were converted by calcination in air to tungsten oxide WO₃ fibers with traces of SiO₂ and a smaller diameter (av. diam. 335 nm). The heat treatment of the WO₃ fibers under flowing H₂/H2_S/N₂ stream led to conversion to tungsten disulfide WS₂ with retention of the fibrous morphology (av. diam. 196 nm). Characterization of the intermediate and final fibers was performed by the XRD, SEM, TEM, HAADF STEM EDS, elemental analyses ICP-OES, and IR spectroscopy methods.

Complex plasmon-exciton dynamics revealed through quantum dot light emission in a nanocavity by Satyendra Nath Gupta, Ora Bitton, Tomas Neuman, Ruben Esteban, Lev Chuntonov, Javier Aizpurua & Gilad Haran

Ultramicroscopy Volume 224, May 2021, 113260 (Open access)

Abstract:

Plasmonic cavities can confine electromagnetic radiation to deep sub-wavelength regimes. This facilitates strong coupling phenomena to be observed at the limit of individual quantum emitters. Here, we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots. Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss interferometry, we observe non-classical emission, allowing us to directly determine the number of emitters in each device. Surprising features in photoluminescence spectra point to the contribution of multiple excited states. Using model simulations based on an extended Jaynes-Cummings Hamiltonian, we find that the involvement of a dark state of the quantum dots explains the experimental findings. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states.

Coherent light emission in cathodoluminescence when using GaAs in a scanning (transmission) electron microscope by Michael Stöger-Pollach, Cornelia F. Pichler, Topa Dan, Gregor A. Zickler, Kristýna Bukvišová, Oliver Eibl, Franz Brandstätter

Nature volume 592, pages722–725 (2021) (link to the Green OA)

Abstract:

For most materials science oriented applications incoherent cathodoluminescence (CL) is of main interest, for which the recombination of electron–hole pairs yields the emission of light. However, the incoherent signal is superimposed by coherently excited photons, similar to the situation for X-rays in Energy-Dispersive X-ray spectra (EDX). In EDX two very different processes superimpose in each spectrum: Bremsstrahlung and characteristic X-ray radiation. Both processes yield X-rays, however, their origin is substantially different. Therefore, in the present CL study we focus on the coherent emission of light, in particular Čerenkov radiation. We use a 200μm thick GaAs sample, not electron transparent and therefore not acting as a light guide, and investigate the radiation emitted from the top surface of the sample generated by back-scattered electrons on their way out of the specimen. The CL spectra revealed a pronounced peak corresponding to the expected interband transition. This peak was at 892 nm at room temperature and shifted to 845 nm at 80 K. The coherent light emission significantly modifies the shape of CL spectra at elevated beam energies. For the first time, by the systematic variation of current and energy of primary electrons we could distinguish the coherent and incoherent light superimposed in CL spectra. These findings are essential for the correct interpretation of CL spectra in STEM. The Čerenkov intensity as well as the total intensity in a spectrum scales linearly with the beam current. Additionally, we investigate the influence of asymmetric mirrors on the spectral shapes, collecting roughly only half of the whole solid angle. Different emission behaviour of different physical causes thus lead to changes in the overall spectral shape.

Direct assessment of the acidity of individual surface hydroxyls by Margareta Wagner, Bernd Meyer, Martin Setvin, Michael Schmid, and Ulrike Diebold

Abstract:

The state of deprotonation/protonation of surfaces has far-ranging implications in chemistry, from acid–base catalysis1 and the electrocatalytic and photocatalytic splitting of water2, to the behaviour of minerals3 and biochemistry4. An entity’s acidity is described by its proton affinity and its acid dissociation constant pKa (the negative logarithm of the equilibrium constant of the proton transfer reaction in solution). The acidity of individual sites is difficult to assess for solids, compared with molecules. For mineral surfaces, the acidity is estimated by semi-empirical concepts, such as bond-order valence sums5, and increasingly modelled with first-principles molecular dynamics simulations6,7. At present, such predictions cannot be tested—experimental measures, such as the point of zero charge8, integrate over the whole surface or, in some cases, individual crystal facets9. Here we assess the acidity of individual hydroxyl groups on In2O3(111)—a model oxide with four different types of surface oxygen atom. We probe the strength of their hydrogen bonds with the tip of a non-contact atomic force microscope and find quantitative agreement with density functional theory calculations. By relating the results to known proton affinities of gas-phase molecules, we determine the proton affinity of the different surface sites of In2O3 with atomic precision. Measurements on hydroxylated titanium dioxide and zirconium oxide extend our method to other oxides.

Kinetics of guided growth of horizontal gan nanowires on flat and faceted sapphire surfaces by Amnon Rothman, Jaroslav Maniš, Vladimir G. Dubrovskii, Tomáš Šikola, Jindřich Mach and Ernesto Joslevich

Nanomaterials 2021, 11, 624. (2021) (open access)

Abstract:

The bottom-up assembly of nanowires facilitates the control of their dimensions, structure, orientation and physical properties. Surface-guided growth of planar nanowires has been shown to enable their assembly and alignment on substrates during growth, thus eliminating the need for additional post-growth processes. However, accurate control and understanding of the growth of the planar nanowires were achieved only recently, and only for ZnSe and ZnS nanowires. Here, we study the growth kinetics of surface-guided planar GaN nanowires on flat and faceted sapphire surfaces, based on the previous growth model. The data are fully consistent with the same model, presenting two limiting regimes—either the Gibbs–Thomson effect controlling the growth of the thinner nanowires or surface diffusion controlling the growth of thicker ones. The results are qualitatively compared with other semiconductors surface-guided planar nanowires materials, demonstrating the generality of the growth mechanism. The rational approach enabled by this general model provides better control of the nanowire (NW) dimensions and expands the range of materials systems and possible application of NW-based devices in nanotechnology.

Single-Shot Three-Dimensional Orientation Imaging of Nanorods Using Spin to Orbital Angular Momentum Conversion by Fordey T., Bouchal P., Schovánek P., Baránek M., Bouchal Z., Dvořák P., Hrtoň M., Rovenská K., Ligmajer F., Chmelík R., and Šikola T.

Nano Letters Volume 21, Issue 17, Pages 7244 - 7251, 2021, (Open Access)

Abstract:

The key information about any nanoscale system relates to the orientations and conformations of its parts. Unfortunately, these details are often hidden below the diffraction limit, and elaborate techniques must be used to optically probe them. Here we present imaging of the 3D rotation motion of metal nanorods, restoring the distinct nanorod orientations in the full extent of azimuthal and polar angles. The nanorods imprint their 3D orientation onto the geometric phase and space-variant polarization of the light they scatter. We manipulate the light angular momentum and generate optical vortices that create self-interference images providing the nanorods’ angles via digital processing. After calibration by scanning electron microscopy, we demonstrated time-resolved 3D orientation imaging of sub-100 nm nanorods under Brownian motion (frame rate up to 500 fps). We also succeeded in imaging nanorods as nanoprobes in live-cell imaging and reconstructed their 3D rotational movement during interaction with the cell membrane (100 fps).

Ga interaction with ZnO surfaces: diffusion and melt-back etching by Tomáš Pejchal, Kristýna Bukvišová, Stella Vallejos, Daniel Citterberg, Tomáš Šikola and Miroslav Kolíbal

Applied Surface Science, Volume 583, 1 May 2022, 152475 (link to Open Access version)

Abstract:

Despite being technologically very attractive, highly-doped zinc oxide whiskers with precisely defined morphology and doping level are difficult to prepare. Here, as an advancing step towards this goal, we show that pre-annealing of ZnO in oxygen-poor conditions (e.g. high vacuum) encourages a deeper diffusion of Ga into the ZnO crystal lattice in contrast to ZnO pre-annealed in oxygen-rich conditions. We also demonstrate that gallium acts as a reactant causing ZnO etching at diffusion temperatures, contrary to Al-based doping of ZnO systems. This behaviour, being similar to gallium melt-back etching during GaN epitaxy on silicon, has not been observed for ZnO so far and can represent a significant hurdle for the post-growth diffusion doping of ZnO nanostructures. The paper suggests possible ways how to diminish this effect.

Preparation of polycrystalline tungsten nanofibers by needleless electrospinning by Vojtech Kundrat, Vit Vykoukal, Zdenek Moravec, Lucie Simonikova, Karel Novotny, and Jiri Pinkas

Journal of Alloys and Compounds, Volume 900, 15 April 2022, 163542 (link to Open Access version)

Abstract:

One-dimensional metal nanostructures are of great interest for applications in electronic and micromechanical devices, solar cells, sensors, and heterogeneous (photo)catalysts. We describe the multigram preparation method for nanoscopic tungsten fibers (diameter 107 ± 49 nm) for the first time. The material was prepared by needleless electrospinning from an aqueous polyvinyl alcohol and silicotungstic acid solution. A green fibrous composite mat was calcined in air to WO3/SiO2 fibers and further reduced in forming gas at various temperatures (500–1000 °C). The reduction process proceeded from nanofibrous blue tungsten oxide to mixtures of reduced tungsten oxides with metallic tungsten and finally at 800 °C to pure metallic W in the form of polycrystalline fibers. These nanofibers consist of individual tungsten nanoparticles covered and interconnected by amorphous silica. All prepared materials were characterized by the TG-DSC, TG-DTA, SEM, TEM, STEM-EDS, and XRD methods. This optimized fabrication method could be scaled up to supply ample amounts of metallic tungsten nanofibers for future applications.

Nanotubes from the Misfit Layered Compound (SmS)1.19TaS2: Atomic Structure, Charge Transfer, and Electrical Properties by M. B. Sreedhara; Kristýna Bukvišová; Azat Khadiev; Daniel Citterberg; Hagai Cohen; Viktor Balema; Arjun K. Pathak; Dmitri Novikov; Gregory Leitus; Ifat Kaplan-Ashiri; Miroslav Kolíbal; Andrey N. Enyashin; Lothar Houben and Reshef Tenne

Chem. Mater. 2022, 34, 4, 1838–1853, February 2022 (link to Open Access version)

Abstract:

Misfit layered compounds (MLCs) MX-TX2, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX2 (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX2 sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS2 nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS2 nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS2 sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5dz2 level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 10–4 Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K.

Correlative Raman Imaging and Scanning Electron Microscopy: The Role of Single Ga Islands in Surface-Enhanced Raman Spectroscopy of Graphene by Jakub Piastek, Jindřich Mach*, Stanislav Bardy, Zoltán Édes, Miroslav Bartošík, Jaroslav Maniš, Vojtěch Čalkovský, Martin Konečný, Jiří Spousta and Tomáš Šikola

J. Phys. Chem. C 2022, 126, 9, 4508–4514, February 2022 (link to Open Access version)

Abstract:

Surface-enhanced Raman spectroscopy (SERS) is a perspective nondestructive analytic technique enabling the detection of individual nanoobjects, even single molecules. In this paper, we have studied the morphology of Ga islands deposited on chemical vapor deposition graphene by ultrahigh vacuum evaporation and local optical response of this system by the correlative Raman imaging and scanning electron microscopy (RISE). Contrary to the previous papers, where only an integral Raman response from the whole ununiformed Ga nanoparticles (NPs) ensembles on graphene was investigated, the RISE technique has enabled us to detect graphene Raman peaks enhanced by single Ga islands and particularly to correlate the Raman signal with the shape and size of these single particles. In this way and by a support of numerical simulations, we have proved a plasmonic nature of the Raman signal enhancement related to localized surface plasmon resonances. It has been found that this enhancement is island-size-dependent and shows a maximum for medium-sized Ga islands. A reasonable agreement between the simulations of the plasmon enhancement of electric fields in the vicinity of Ga islands and the experimental intensities of corresponding Raman peaks proved the plasmonic origin of the observed effect known as SERS.

Nanofibers of solid-solution thorium(IV)-uranium(IV) oxides by electrospinning by Vojtech Kundrat, Vit Vykoukal, Zdenek Moravec, Jiri Pinkas∗

Journal of Nuclear Materials 566 (2022) 153731, April 2022 (link to Open Access version)

Abstract:

Thorium-uranium dioxide nanofibers were prepared in a three-step process. Green composite microfibers were electrospun from solutions composed of Th(IV) and U(VI) acetylacetonate complexes in different molar ratios, polyvinylpyrrolidone, and acetone/ethanol mixed solvents. The second step converted the green composite fibers by calcination in the air to mixed Th/U oxide fibers. In the final step, the heat treatment under forming gas provided the nanofibers with diameters of 28−46 nm composed of solid solutions ThxU1−xO2 (x = 1, 0.75, 0.5, 0.25, 0). The XRD and STEM-EDS analyses of the prepared Th/U mixed oxide nanofibers attested to their atomic homogeneity.

Low temperature 2D GaN growth on Si(111) 7 × 7 assisted by hyperthermal nitrogen ions by Jaroslav Maniš, Jindřch Mach, Miroslav Bartošík, Tomáš Šamořil, Michal Horák, Vojtěch Čalkovský, David Nezval, Lukáš Kachtík, Martin Konečný a Tomáš Šikola

Nanoscale Advances 4 (2022) Issue 17, 153731, 3549 - 355619 July 2022 (Open Access)