Příprava a charakterizace nanostruktur

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Zero-field spin wave turns
(AIP Publishing, 2024-03-11) Klíma, Jan; Wojewoda, Ondřej; Roučka, Václav; Molnár, Tomáš; Holobrádek, Jakub; Urbánek, Michal
Spin-wave computing, a potential successor to CMOS-based technologies, relies on the efficient manipulation of spin waves for information processing. While basic logic devices such as magnon transistors, gates, and adders have been experimentally demonstrated, the challenge for complex magnonic circuits lies in steering spin waves through sharp turns. In this study, we demonstrate with micromagnetic simulations and Brillouin light scattering microscopy experiments, that dipolar spin waves can propagate through 90 degrees turns without distortion. The key lies in carefully designed in-plane magnetization landscapes, addressing challenges posed by anisotropic dispersion. The experimental realization of the required magnetization landscape is enabled by spatial manipulation of the uniaxial anisotropy using corrugated magnonic waveguides. The findings presented in this work should be considered in any magnonic circuit design dealing with anisotropic dispersion and spin wave turns.
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.
Singular and Nonsingular Transitions in the Infrared Plasmons of Nearly Touching Nanocube Dimers
(AMER CHEMICAL SOC, 2024-05-28) Wu, Yina; Konečná, Andrea; Cho, Shin Hum; Milliron, Delia J.; Hachtel, Jordan A.; de Abajo, F. Javier García
Narrow gaps between plasmon-supporting materials can confine infrared electromagnetic energy at the nanoscale, thus enabling applications in areas such as optical sensing. However, in nanoparticle dimers, the nature of the transition between touching (zero gap) and nearly nontouching (nonzero gap less than or similar to 15 nm) regimes is still a subject of debate. Here, we observe both singular and nonsingular transitions in infrared plasmons confined to dimers of fluorine-doped indium oxide nanocubes when moving from touching to nontouching configurations depending on the dimensionality of the contact region. Through spatially resolved electron energy-loss spectroscopy, we find a continuous spectral evolution of the lowest-order plasmon mode across the transition for finite touching areas, in excellent agreement with the simulations. This behavior challenges the widely accepted idea that a singular transition always emerges in the near-touching regime of plasmonic particle dimers. The apparent contradiction is resolved by theoretically examining different types of gap morphologies, revealing that the presence of a finite touching area renders the transition nonsingular, while one-dimensional and point-like contacts produce a singular behavior in which the lowest-order dipolar mode in the touching configuration, characterized by a net induced charge in each of the particles, becomes unphysical as soon as they are separated. Our results provide valuable insights into the nature of dimer plasmons in highly doped semiconductors.
Magneto-optical investigations of molecular nanomagnet monolayers
(Royal Society of Chemistry, 2016-04-11) Rozbořil, Jakub; Rechkemmer, Y.; Bloos, Dominik; Münz, Filip; Wang, C. Nan; Neugebauer, Petr; Čechal, Jan; Novák, Jiří; van Slageren, Joris
We report field-dependent magnetization measurements on monolayers of [Dy(Pc)2] on quartz, prepared by the Langmuir–Blodgett technique. The films are thoroughly characterized by means of X-ray reflectivity and atomic force microscopy. The magnetisation of the sample is measured through the magnetic circular dichroism of a ligand-based electronic transition.
Superflux of an organic adlayer towards its local reactive immobilization
(NATURE PORTFOLIO, 2023-10-18) Salamon, David; Bukvišová, Kristýna; Jan, Vít; Potoček, Michal; Čechal, Jan
On-surface mass transport is the key process determining the kinetics and dynamics of on-surface reactions, including the formation of nanostructures, catalysis, or surface cleaning. Volatile organic compounds (VOC) localized on a majority of surfaces dramatically change their properties and act as reactants in many surface reactions. However, the fundamental question "How far and how fast can the molecules travel on the surface to react?" remains open. Here we show that isoprene, the natural VOC, can travel similar to 1 mu m s(-1), i.e., centimeters per day, quickly filling low-concentration areas if they become locally depleted. We show that VOC have high surface adhesion on ceramic surfaces and simultaneously high mobility providing a steady flow of resource material for focused electron beam synthesis, which is applicable also on rough or porous surfaces. Our work established the mass transport of reactants on solid surfaces and explored a route for nanofabrication using the natural VOC layer.

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