arXiv subjects

Michal Horák

Publications and source records attributed to Michal Horák.

At least 19 recordsLinked to original sources

arXiv2026

Time-resolved study of carbonization and growth of ultrathin 3C-SiC on Si(111) under ultra-high vacuum

The early-stage formation of silicon carbide (SiC) on Si(111) by ethylene exposure under ultra-high vacuum (UHV) was investigated to resolve the time-dependent chemical and morphological evolution of an ultrathin layer. Clean Si(111) substrates were exposed to C$_2$H$_4$ at 800 °C for 2 min to 4 h, and the surfaces followed by in-situ X-ray photoelectron spectroscopy (XPS) and ex-situ AFM, SEM, AES, SIMS and TEM. Si 2p and C 1s peak analysis shows the progressive conversion of elemental silicon into a carbidic Si-C phase, the SiC fraction overtaking the elemental component between 120 and 160 min and saturating near 80-81% beyond 180 min, leaving about 19-20% residual elemental silicon. Correlative SEM and AFM reveal a parallel morphological progression, from sparse isolated islands to a coalesced, near-continuous layer. AES depth profiling confirms carbon incorporated into the near-surface region rather than weakly adsorbed as contamination, assigning the islands to early SiC nuclei. TEM confirms the zinc-blende lattice and the presence of cubic silicon carbide (3C-SiC). Together, these results provide a time-resolved picture of SiC nucleation, coalescence and layer growth on Si(111), relevant to 3C-SiC heteroepitaxy, and can be utilized in the optimization of SiC/Si(111) templates for growth of III-nitride and other carbide systems on silicon (e.g. Mo$_2$C).

cond-mat.mtrl-sci↗

arXiv2026

Lead nanoparticles, the deep-ultraviolet to near-infrared plasmonic platform

Among the other non-noble metals, lead (Pb) is a material of particular interest for plasmonic applications in the deep ultraviolet spectral region. However, experimental studies on its plasmonic performance have not yet been conducted. In this work, the dependence of the optical properties of spherical lead nanoparticles on their diameter is demonstrated. The plasmonic performance of chemically synthesized lead nanoparticles is evaluated at the single-particle level by means of a combination of scanning transmission electron microscopy and electron energy loss spectroscopy. Our findings demonstrate that these nanoparticles support localized surface plasmon resonances across the entire spectrum from near-infrared to deep-ultraviolet. This range was identified as the most extensive among all plasmonic elemental metals, extending to wavelengths below 200 nanometers. Consequently, lead nanoparticles exhibit stable plasmonic performance over a remarkably broad wavelength range, thereby substantiating their potential as a multispectral plasmonic platform.

physics.optics↗

arXiv2026

Plasmonics of non-noble metals

Localized surface plasmon resonances are self-sustained, collective oscillations of free electrons in metallic nanostructures. They have a wide range of applications. The most common plasmonic metals are noble metals, such as gold and silver. However, there are applications, such as surface-enhanced Raman spectroscopy, in which using non-noble metals is advantageous. This review summarizes the investigation of localized surface plasmons in non-noble metal nanoparticles, providing an overview of the plasmonic properties of non-noble metals. We cover the following metals: aluminium (Al), antimony (Sb), bismuth (Bi), chromium (Cr), copper (Cu), gallium (Ga), indium (In), lead (Pb), magnesium (Mg), molybdenum (Mo), nickel (Ni), potassium (K), selenium (Se), sodium (Na), tellurium (Te), tin (Sn), titanium (Ti), tungsten (W), and zinc (Zn). Our summary therefore compares the plasmonic properties of non-noble metals and briefly introduces their potential to the readers.

physics.optics↗

arXiv2026

Tuning of Localized Surface Plasmons in Vanadium Dioxide Nanoparticles via Size and Insulator-Metal Transition

Vanadium dioxide has been identified as a promising phase-changing material for use in tunable plasmonic devices. In this study, we present a comprehensive modal analysis of single-phase and multi-phase vanadium dioxide nanoparticles. In-situ high-resolution electron energy loss spectroscopy was utilized to experimentally resolve the dipole plasmon peak, higher-order and breathing plasmonic modes, and bulk losses as a function of nanoparticle size. Furthermore, the focus is directed toward capturing the dynamic nanoscale optical response throughout the metal-insulator transition in a vanadium dioxide nanoparticle. This system possesses the ability to be gradually switched on and off in terms of the emergence of near-infrared plasmonic absorption. The switching is accompanied by a gradual spectral shift of the absorption peak, amounting to 0.18 eV for a 120 nm nanoparticle. It is envisioned that this phenomenon can be generalized to larger nanostructures with a higher aspect ratio, thereby introducing a wider tunability of the system, which is essential for functional nanodevices based on vanadium dioxide.

physics.optics↗

arXiv2025

Charge reservoir as a design concept for plasmonic antennas

Plasmonic antennas exploit localized surface plasmons to shape, confine, and enhance electromagnetic fields with subwavelength resolution. The field enhancement is contributed to by various effects, such as the inherent surface localization of plasmons or the plasmonic lightning-rod effect. Inspired by nanofocusing observed for propagating plasmons, we test the hypothesis that plasmonic antennas with a large cross-section represent a large charge reservoir, enabling large induced charge and field enhancement. Our study reveals that a large charge reservoir is accompanied by large radiative losses, which are the dominant factor, resulting in a low field enhancement.

physics.optics↗

arXiv2025

Plasmonic properties of individual bismuth nanoparticles

Bismuth nanoparticles are being investigated due to their reported photothermal and photocatalytic properties. In this study, we synthesised spherical bismuth nanoparticles (50-600 nm) and investigated their structural and optical properties at the single particle level using analytical transmission electron microscopy. Our experimental results, supported by numerical simulations, demonstrate that bismuth nanoparticles support localised surface plasmon resonances, which can be tuned from the near-infrared to the ultraviolet spectral region by changing the nanoparticle size. Furthermore, plasmonic resonances demonstrate stability across the entire spectral bandwidth, enhancing the attractiveness of bismuth nanoparticles for applications over a wide spectral range. Bismuth's lower cost, biocompatibility, and oxidation resistance make it a suitable candidate for utilisation, particularly in industrial and large-scale plasmonic applications.

physics.optics↗

arXiv2025

Bismuth plasmonic antennas

Bismuth is a particularly promising alternative plasmonic metal because of its theoretically predicted wide spectral bandwidth. In this study, we experimentally demonstrated the correlation between the shape and size of individual bismuth plasmonic antennas and their optical properties. To this end, we employed a combination of scanning transmission electron microscopy and electron energy loss spectroscopy. Bar-shaped and bowtie bismuth plasmonic antennas of various sizes were fabricated by focused ion beam lithography of a polycrystalline bismuth thin film. Our experimental findings demonstrate that these antennas support localised surface plasmon resonances and their dipole modes can be tuned through their size from the near-infrared to the entire visible spectral region. Furthermore, our findings demonstrate that bismuth exhibits a plasmon dispersion relation that is nearly identical to that of gold while maintaining its plasmonic performance even at higher plasmon energies, thus rendering it a promising low-cost alternative to gold.

physics.optics↗

arXiv2025

Ab initio study of angle-resolved electron reflection spectroscopy of few-layer graphene

We present ab initio theory for electron reflection spectroscopy of few-layer graphene for arbitrary angles of incidence. The inelastic effects are included in a consistent way using the optical potential retrieved from ab initio simulations of electron energy-loss spectra. We demonstrate a significant impact of inelastic effects even for single-layer graphene. Next, we address the ability of the electron reflection spectroscopy to determine specific parameters of graphene including not only the number of layers in the few-layer graphene but also the stacking type in the graphene multilayers, and to resolve moiré patterns in twisted graphene bilayers. We show that the predicted contrast, although significantly reduced by inelastic effects, is sufficient for the experimental detection of all considered parameters. Our findings are corroborated by a fair correspondence of our theoretical predictions with experimental data, both our own and recently published by other authors.

cond-mat.mes-hall↗

arXiv2024

Coexisting phases of individual VO$_2$ nanoparticles for multilevel nanoscale memory

Vanadium dioxide (VO$_2$) has received significant interest in the context of nanophotonic metamaterials and memories owing to its reversible insulator-metal transition associated with significant changes in its optical and electronic properties. While the VO$_2$ transition has been extensively studied for several decades, the hysteresis dynamics of individual single-crystal VO$_2$ nanoparticles (NPs) remains largely unexplored. Here, employing transmission electron microscopy techniques, we investigate phase transitions of single VO$_2$ NPs in real time. Our analysis reveals the statistical distribution of the transition temperature and steepness and how they differ during forward (heating) and backward (cooling) transitions. We assess the stability of coexisting phases in individual NPs and prove the persistent multilevel memory at near-room temperatures using only a few VO$_2$ NPs. Our findings shed new light on the underlying physical mechanisms governing the hysteresis of VO$_2$ and establish VO$_2$ NPs as a promising component of optoelectronic and memory devices with enhanced functionalities.

cond-mat.mes-hall↗

arXiv2024

Efficient nanoscale imaging of solid-state phase transitions by transmission electron microscopy demonstrated on vanadium dioxide nanoparticles

We present annular dark field scanning transmission electron microscopy (ADF-STEM) as an efficient, fast, and non-destructive nanoscale tool for monitoring solid-state phase transition. Using metal-insulator transition in vanadium dioxide nanoparticles as an example, we characterize lattice and electronic signatures of the phase transition using analytical transmission electron microscopy including diffraction and electron energy-loss spectroscopy. We demonstrate that ADF-STEM shows a clear contrast across the transition, interpreted with the help of convergent electron beam diffraction as stemming from the crystal-lattice modification accompanying the transition. In addition, ADF-STEM utilizes 3--6 orders of magnitude lower electron dose when compared to electron microscopy techniques able to reveal the phase transition with the same spatial resolution and universality. The benefits of ADF-STEM are emphasized by recording a full hysteresis loop for the metal-insulator transition of a single vanadium dioxide nanoparticle. Our study opens the prospect for fast, non-destructive, large-area and nanoscale characterization of solid-state phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

Plasmonic lightning-rod effect

The plasmonic lightning-rod effect refers to the formation of a strong electric near field of localized surface plasmons at the sharp features of plasmonic antennas. While this effect is intuitively utilized in the design and optimization of plasmonic antennas, the relation between the magnitude of the electric field and the local curvature of the plasmonic antenna has not been yet rigorously established. Here, we provide such a study. We design sets of plasmonic antennas that allow to isolate the role of the local curvature from other effects influencing the field. The near electric field is inspected by electron energy loss spectroscopy and electrodynamic simulations. We demonstrate the existence of the plasmonic lightning-rod effect and establish its quantitative description, showing that its strength is comparable to the electrostatic lightning-rod effect. We also provide a simple phenomenological formula for the spatial dependence of the field. Finally, we introduce the effective radius of curvature related to the spatial distribution of induced charge in plasmonic antennas, significantly smaller than their geometrical radius.

physics.optics↗

arXiv2024

Influence of deposition parameters on the plasmonic properties of gold nanoantennas fabricated by focused ion beam lithography

The behavior of plasmonic antennas is influenced by a variety of factors, including their size, shape, and material. Even minor changes in the deposition parameters during the thin film preparation process may have a significant impact on the dielectric function of the film, and thus on the plasmonic properties of the resulting antenna. In this work, we deposited gold thin films with thicknesses of 20 nm, 30 nm, and 40 nm at various deposition rates using an ion-beam-assisted deposition. We evaluate their morphology and crystallography by atomic force microscopy, X ray diffraction, and transmission electron microscopy. Next, we examined the ease of fabricating plasmonic antennas using focused-ion-beam lithography. Finally, we evaluate their plasmonic properties by electron energy loss spectroscopy measurements of individual antennas. Our results show that the optimal gold thin film for plasmonic antenna fabrication of a thickness of 20 and 30 nm should be deposited at the deposition rate of around 0.1 nm/s. The thicker 40 nm film should be deposited at a higher deposition rate like 0.3 nm/s.

cond-mat.mtrl-sci↗

arXiv2024

Analytical electron microscopy analysis of insulating and metallic phases in nanostructured vanadium dioxide

Vanadium dioxide (VO$_2$) is a strongly-correlated material that exhibits insulator-to-metal transition (IMT) near room temperature, which makes it a promising candidate for applications in nanophotonics or optoelectronics. However, creating VO$_2$ nanostructures with the desired functionality can be challenging due to microscopic inhomogeneities that can significantly impact the local optical and electronic properties. Thin lamellas, produced by focused ion beam milling from a homogeneous layer, provide a useful prototype for studying VO$_2$ at the truly microscopic level using a scanning transmission electron microscope (STEM). High-resolution imaging is used to identify structural inhomogeneities while electron energy-loss spectroscopy (EELS) supported by statistical analysis helps to detect V$_x$O$_y$ stoichiometries with a reduced oxidation number of vanadium at the areas of thickness below 70 nm. On the other hand, the thicker areas are dominated by vanadium dioxide, where the signatures of IMT are detected in both core-loss and low-loss EELS experiments with in-situ heating. The experimental results are interpreted with ab-initio and semi-classical calculations. This work shows that structural inhomogeneities such as pores and cracks present no harm to the desired optical properties of VO$_2$ samples.

cond-mat.mtrl-sci↗

arXiv2023

Plasmonic sensing using Babinet's principle

Developing methods to sense local variations in nearby materials, such as their refractive index and thickness, is important in different fields including chemistry and biomedical applications, among others. Localized surface plasmons (LSPs) excited in plasmonic nanostructures have demonstrated to be useful in this context due to the spectral location of their associated resonances being sensitive to changes near the plasmonic structures. In this manuscript, Babinet's principle is explored by exploiting LSP resonances excited in complementary metal-dielectric cylindrical plasmonic structures (plasmonic particle-dimers and aperture-dimers in our case). Both plasmonic structures are evaluated numerically and experimentally using Electron Energy Loss Spectroscopy (EELS), providing a full physical understanding of the complementary nature of the excited LSP resonances. The studied plasmonic structures are then exploited for dielectric sensing under two configurations: when a thin dielectric film is positioned atop the plasmonic structures and when the analyte surrounds/fills the plasmonic particles/apertures. The complementary sensing performance of both proposed structures is also evaluated, showing the approximate validity of the Babinet principle with sensitivities values of up to 700 nm/RIU for thin dielectric sensing.

physics.optics↗

arXiv2023

Correlative Imaging of Individual CsPbBr3 Nanocrystals: Role of Isolated Grains in Photoluminescence of Perovskite Polycrystalline Thin Films

We report on the optical properties of CsPbBr3 polycrystalline thin film on a single grain level. A sample comprised of isolated nanocrystals (NCs) mimicking the properties of the polycrystalline thin film grains that can be individually probed by photoluminescence spectroscopy was prepared. These NCs were analyzed using correlative microscopy allowing the examination of structural, chemical, and optical properties from identical sites. Our results show that the stoichiometry of the CsPbBr3 NCs is uniform and independent of the NCs' morphology. The photoluminescence (PL) peak emission wavelength is slightly dependent on the dimensions of NCs, with the blue shift up to 9 nm for the smallest analyzed NCs. The magnitude of the blueshift is smaller than the emission linewidth, thus detectable only by high-resolution PL mapping. By comparing the emission wavelengths obtained from the experiment and a rigorous effective mass model we can fully attribute the observed variations to the size-dependent quantum confinement effect.

physics.chem-ph↗

arXiv2023

Spatio-spectral metrics in electron energy loss spectroscopy as a tool to resolve nearly degenerate plasmon modes in dimer plasmonic antennas

Electron energy loss spectroscopy (EELS) is often utilized to characterize localized surface plasmon modes supported by plasmonic antennas. However, the spectral resolution of this technique is only mediocre, and it can be rather difficult to resolve modes close in the energy, such as coupled modes of dimer antennas. Here we address this issue for a case study of the dimer plasmonic antenna composed of two gold discs. We analyze four nearly degenerate coupled plasmon modes of the dimer: longitudinal and transverse bonding and antibonding dipole modes. With a traditional approach, which takes into account the spectral response of the antennas recorded at specific points, the modes cannot be experimentally identified with EELS. Therefore, we employ the spectral and spatial sensitivity of EELS simultaneously. We propose several metrics that can be utilized to resolve the modes. First, we utilize electrodynamic simulations to verify that the metrics indeed represent the spectral positions of the plasmon modes. Next, we apply the metrics to experimental data, demonstrating their ability to resolve three of the above-mentioned modes (with transverse bonding and antibonding modes still unresolved), identify them unequivocally, and determine their energies. In this respect, the spatio-spectral metrics increase the information extracted from electron energy loss spectroscopy applied to plasmonic antennas.

physics.optics↗

arXiv2022

Plasmonic properties of individual gallium nanoparticles

Gallium is a plasmonic material offering ultraviolet to near-infrared tunability, facile and scalable preparation, and good stability of nanoparticles. In our contribution, we experimentally demonstrate the link between the shape and size of individual gallium nanoparticles and their optical properties. To this end, we utilize scanning transmission electron microscopy combined with electron energy loss spectroscopy. Lens-shaped gallium nanoparticles with a diameter between 10 nm and 200 nm were grown directly on a silicon nitride membrane using an in-house developed effusion cell operated at ultra-high vacuum conditions. We have experimentally proved that they support localized surface plasmon resonances and their dipole mode can be tuned through their size from ultraviolet to near-infrared spectral region. The measurements are supported by numerical simulations using realistic particle shapes and sizes. Our results open the way for future applications of gallium nanoparticles such as hyperspectral absorption of sunlight in energy harvesting or plasmon-enhanced luminescence of ultraviolet emitters.

physics.optics↗

arXiv2020

Structural and optical properties of monocrystalline and polycrystalline gold plasmonic nanorods

Plasmonic structures are often fabricated by lithographic patterning of a thin metallic film. Properties of the thin film are intimately related to the quality of the resulting structures. Here we compare two kinds of thin gold films on silicon nitride membrane: a conventional polycrystalline thin film deposited by magnetron sputtering, and monocrystalline gold microplates which were chemically synthesised directly on the membrane's surface for the first time. Both pristine metals were used to fabricate plasmonic nanorods using focused ion beam lithography. Structural and optical properties of the nanorods were characterized by analytical transmission electron microscopy. The dimensions of the nanorods in both substrates reproduced well the designed size of $240 \times 80\ \mathrm{nm^2}$ with the deviations up to 20 nm in both length and width. The shape reproducibility was considerably improved among monocrystalline nanorods fabricated from the same microplate. Interestingly, monocrystal nanorods featured inclined boundaries while the boundaries of the polycrystal nanorods were upright. Q factors and peak loss probabilities of the modes in both structures are within the experimental uncertainty identical. We conclude that the optical response of the plasmonic antennas is not deteriorated when the polycrystalline metal is used instead of the monocrystalline metal.

physics.app-ph↗