Příprava a charakterizace nanostruktur

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Direct Observation of Structural Phase Transformations during Phosphorene Formation on Cu(111)
(American Chemical Society, 2025-01-22) David, Jiří; Jeřábek, František; Procházka, Pavel; Černý, Miroslav; Ciobanu, Cristian V.; Průša, Stanislav; Šikola, Tomáš; Kodambaka, Suneel; Kolíbal, Miroslav
Blue phosphorene, a two-dimensional, hexagonal-structured, semiconducting phosphorus, has gained attention as it is considered easier to synthesize on metal surfaces than its allotrope, black phosphorene. Recent studies report different structures of phosphorene, for example, on Cu(111), but the underlying mechanisms of their formation are not known. Here, using a combination of in situ ultrahigh vacuum low-energy electron microscopy and in vacuo scanning tunneling microscopy, we determine the time evolution of the surface structure and morphology during the deposition of phosphorus on single-crystalline Cu(111). We find that during the early stages of deposition phosphorus intermixes with Cu, resulting in copper phosphide structures. With the increasing surface concentration of phosphorus, the phosphide phase disappears, and a blue phosphorene layer forms, followed by the self-assembly of highly ordered phosphorus clusters that eventually grow into multilayer islands. We attribute the unexpected transformation of stable phosphide to a phosphorene layer to the presence of a large concentration of P2 dimers on the surface. Our results constitute direct evidence for a growth mode leading to a flat phosphorene layer via an intermediary phase, which could underpin the growth of other 2D materials on strongly interacting substrates.
Coexisting Phases of Individual VO₂ Nanoparticles for Multilevel Nanoscale Memory
(AMER CHEMICAL SOC, 2025-01-02) Kepič, Peter; Horák, Michal; Kabát, Jiří; Hájek, Martin; Konečná, Andrea; Šikola, Tomáš; Ligmajer, Filip
Vanadium dioxide (VO2) 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. The phase transition of VO2 has been extensively studied for several decades, and the ways how to control its hysteresis characteristics relevant for memory applications have significantly improved. However, the hysteresis dynamics and stability of coexisting phases during the transition have not been studied on the level of individual single-crystal VO2 nanoparticles (NPs), although they represent the fundamental component of ordinary polycrystalline films and can also act like nanoscale memory units on their own. Here, employing transmission electron microscopy techniques, we investigate phase transitions of single VO2 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 evaluate the stability of coexisting phases in individual NPs and prove the persistent multilevel memory at near room temperatures using only a few VO2 NPs. Our findings unveil the physical mechanisms that govern the hysteresis of VO2 at the nanoscale and establish VO2 NPs as a promising component of optoelectronic and memory devices with enhanced functionalities.
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.
Next-generation photocatalytic system: Ga2O3-modified Ga Nanoislands on graphene for H2 production
(Elsevier, 2024-05-24) Bartošík, Miroslav; Edelmannová, Miroslava Filip; Mach, Jindřich; Kočí, Kamila
Study investigates Ga2O3/Ga on Graphene/SiO2/Si, prepared via temperature-controlled growth of Ga, for photocatalytic hydrogen generation, marking the first exploration of this system for such purpose. Thin surface ultra-wide band gap Ga2O3 layer effectively photoemits electron-hole pairs under UVC and the metallic Ga cores enhance electric field separating charge carriers by formation of localized surface plasmon (LSP) resonances. These effects increase hydrogen yields. Computational analysis of LSP-induced electric field enhancement quantitatively supports proposed fundamental mechanism underlying sample's photoactivity. The most active photocatalyst (Ga-M) with medium-size Ga particles (radius 14 nm) exhibited ten thousand times higher activity per gram than commercial TiO2.
Observation of Mermin-Wagner behavior in LaFeO₃/SrTiO₃ superlattices
(NATURE PORTFOLIO, 2024-06-22) Kiaba, Michal; Suter, Andreas; Salman, Zaher; Prokscha, Thomas; Chen, B.; Koster, Gertjan; Dubroka, Adam
Two-dimensional magnetic materials can exhibit new magnetic properties due to the enhanced spin fluctuations that arise in reduced dimension. However, the suppression of the long-range magnetic order in two dimensions due to long-wavelength spin fluctuations, as suggested by the Mermin-Wagner theorem, has been questioned for finite-size laboratory samples. Here we study the magnetic properties of a dimensional crossover in superlattices composed of the antiferromagnetic LaFeO3 and SrTiO3 that, thanks to their large lateral size, allowed examination using a sensitive magnetic probe - muon spin rotation spectroscopy. We show that the iron electronic moments in superlattices with 3 and 2 monolayers of LaFeO3 exhibit a static antiferromagnetic order. In contrast, in the superlattices with single LaFeO3 monolayer, the moments do not order and fluctuate to the lowest measured temperature as expected from the Mermin-Wagner theorem. Our work shows how dimensionality can be used to tune the magnetic properties of ultrathin films.

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