Příprava a charakterizace nanostruktur

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Engineering of Active and Passive Loss in High-Quality-Factor Vanadium Dioxide-Based BIC Metasurfaces
(AMER CHEMICAL SOC, 2024-08-27) Aigner, Andreas; Ligmajer, Filip; Rovenská, Katarína; Holobrádek, Jakub; Idesová, Beáta; Maier, Stefan A.; Tittl, Andreas; Menezes, Leonardo de S.
Active functionalities of metasurfaces are of growing interest in nanophotonics. The main strategy employed to date is spectral resonance tuning affecting predominantly the far-field response. However, this barely influences other essential resonance properties like near-field enhancement, signal modulation, quality factor, and absorbance, which are all vital for numerous applications. Here we introduce an active metasurface approach that combines temperature-tunable losses in vanadium dioxide with far-field coupling tunable symmetry-protected bound states in the continuum. This method enables exceptional precision in independently controlling both radiative and nonradiative losses. Consequently, it allows for the adjustment of both the far-field response and, notably, the near-field characteristics like local field enhancement and absorbance. We experimentally demonstrate continuous tuning from under- through critical- to overcoupling, achieving quality factors of 200 and a relative switching contrast of 78%. Our research marks a significant step toward highly tunable metasurfaces, controlling both near- and far-field properties.
Influence of Deposition Parameters on the Plasmonic Properties of Gold Nanoantennas Fabricated by Focused Ion Beam Lithography
(AMER CHEMICAL SOC, 2024-08-20) Foltýn, Michael; Patočka, Marek; Řepa, Rostislav; Šikola, Tomáš; Horák, Michal
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, 30, 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.
Mechanism of WS₂ Nanotube Formation Revealed by in Situ/ex Situ Imaging
(AMER CHEMICAL SOC, 2024-05-03) Kundrát, Vojtěch; Novák, Libor; Bukvišová, Kristýna; Zálešák, Jakub; Kolíbalová, Eva; Rosentveig, Rita; Sreedhara, M.B.; Shalom, Hila; Yadgarov, Lena; Zak, Alla; Kolíbal, Miroslav; Tenne, Reshef
Multiwall WS2 nanotubes have been synthesized from W18O49 nanowhiskers in substantial amounts for more than a decade. The established growth model is based on the "surface-inward" mechanism, whereby the high-temperature reaction with H2S starts on the nanowhisker surface, and the oxide-to-sulfide conversion progresses inward until hollow-core multiwall WS2 nanotubes are obtained. In the present work, an upgraded in situ SEM mu Reactor with H-2 and H2S sources has been conceived to study the growth mechanism in detail. A hitherto undescribed growth mechanism, named "receding oxide core", which complements the "surface-inward" model, is observed and kinetically evaluated. Initially, the nanowhisker is passivated by several WS2 layers via the surface-inward reaction. At this point, the diffusion of H2S through the already existing outer layers becomes exceedingly sluggish, and the surface-inward reaction is slowed down appreciably. Subsequently, the tungsten suboxide core is anisotropically volatilized within the core close to its tips. The oxide vapors within the core lead to its partial out-diffusion, partially forming a cavity that expands with reaction time. Additionally, the oxide vapors react with the internalized H2S gas, forming fresh WS2 layers in the cavity of the nascent nanotube. The rate of the receding oxide core mode increases with temperatures above 900 degrees C. The growth of nanotubes in the atmospheric pressure flow reactor is carried out as well, showing that the proposed growth model (receding oxide core) is also relevant under regular reaction parameters. The current study comprehensively explains the WS2 nanotube growth mechanism, combining the known model with contemporary insight.
Near-field digital holography: a tool for plasmon phase imaging
(Royal Society of Chemistry, 2018-12-07) Dvořák, Petr; Kvapil, Michal; Bouchal, Petr; Édes, Zoltán; Šamořil, Tomáš; Hrtoň, Martin; Ligmajer, Filip; Křápek, Vlastimil; Šikola, Tomáš
The knowledge of the phase distribution of near electromagnetic field has become very important for many applications. However, its experimental observation is still technologically very demanding task. In this work, we propose a novel method for the measurement of the phase distribution of near electric field based on the principles of phase-shifting digital holography. In contrast with previous methods the holographic interference occurs already in the near field and the phase distribution can be determined purely from the scanning near-field optical microscopy measurements without need of additional far-field interferometric methods. This opens a way towards onchip phase imaging. We demonstrate the capabilities of the proposed method by reconstruction of the phase difference between interfering surface plasmon waves and by imaging the phase of single surface plasmon wave. We also demonstrate a selectivity of the method towards individual components of the field.
Synthesis Dynamics of Graphite Oxide
(Elsevier, 2018-05-10) Bannov, Alexander G.; Manakhov, Anton; Shibaev, Alexander A.; Ukhina, A.V.; Polčák, Josef; Maksimovskii, E. A.
Graphite oxide synthesis dynamics were investigated using a sampling technique. The synthesis of graphite oxide was carried out by a modified Hummers’ method. Small samples of the solid phase (30–50 mg) were collected from the reaction mixture and analyzed by thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, Raman spectroscopy, energy dissipative X-ray spectroscopy, and X-ray photoelectron spectroscopy. The strongest oxidation was detected 10 min after the start of the synthesis, i.e., after the addition of KMnO4, when the formation of the graphite oxide phase with intercalated guest molecules begins. The intercalation of graphite started after 30 min of synthesis when the temperature was increased to 35°C. The addition of ice into the reaction mixture leads to the increase in the COOH group concentration, whereas the concentration of C=O groups slightly changes, and the concentration of the C–O and C=O groups remains almost constant. It was found that the degree of oxidation of graphite oxide exhibited complex change, and H2O2 plays a significant role not only in the removal of impurities but also in the increase in the GO oxidation degree that is reflected by a higher concentration of oxygen-containing functional groups. Differential scanning calorimetry and thermogravimetric analysis data confirmed that the additions of ice and H2O2 induce the stronger formation of surface functional groups instead of intercalated guest species.

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