Molekulární nanostruktury na površích

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CO-Induced Dimer Decay Responsible for Gem-Dicarbonyl Formation on a Model Single-Atom Catalyst
(WILEY-V C H VERLAG GMBH, 2024-04-15) Wang, Chunlei; Sombut, Panukorn; Puntscher, Lena; Jakub, Zdeněk; Meier, Matthias; Pavelec, Jiří; Bliem, Roland; Schmid, Michael; Diebold, Ulrike; Franchini, Cesare; Parkinson, Gareth S.
The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh1 adatoms and minority Rh2 dimers on Fe3O4(001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh1 sites results exclusively in stable Rh1CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh1(CO)2 gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh2 dimers via an unstable Rh2(CO)3 intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis. Time-lapse scanning tunneling microscopy movies are combined with theoretical computations to study CO adsorption on a model Rh1/Fe3O4(001) catalyst under ultrahigh vacuum conditions. Direct CO adsorption at Rh1 sites results in monocarbonyl species. Rh1-(CO)2 gem dicarbonyl species are observed, but from only via the CO-induced break-up of Rh2 dimer species.+ image
Reversible Intercalation of Organic Solvents in Graphite and Its Hindrance by a Strongly Adsorbing Supramolecular Monolayer
(WILEY, 2024-12-01) Badami-Behjat, Arash; Rinkovec, Tamara; Procházka, Pavel; Bazylevska, Anastasiia; RodríguezGonzález, Miriam C.; Cao, Hai; Čechal, Jan; De Feyter, Steven; Lackinger, Markus
At elevated temperatures, the prototypical organic solvents used to study the self-assembly of supramolecular monolayers at liquid-solid interfaces alter a graphite substrate by intercalation. As a consequence, less strongly bound supramolecular monolayers become thermodynamically unstable, as probed by scanning tunneling microscopy. Complementary characterization by atomic force microscopy, confocal Raman spectroscopy and low energy electron microscopy consistently points to subsurface changes in the top few layers of the graphite substrate due to solvent intercalation. High-temperature annealing at 900 degrees C in the vacuum restores the adsorption properties of the graphite substrates, indicating a high activation energy for deintercalation. However, strongly adsorbing hydrogen-bonded monolayers of trimesic acid inhibit solvent intercalation and thus protect the graphite substrate. Mildly solvent-intercalated graphite may prove useful as an easily prepared graphitic material with further weakened adsorption properties. The solvents commonly used for self-assembly studies at liquid-solid interfaces alter graphite substrates at elevated temperatures by intercalation, rendering weakly bound supramolecular monolayers thermodynamically unstable. This solvent intercalation can be reversed by high-temperature vacuum annealing at 900 degrees C or prevented by strong and persistent adsorption of supramolecular monolayers of trimesic acid. image
Rapid oxygen exchange between hematite and water vapor
(Nature Portfolio, 2021-11-10) Jakub, Zdeněk; Meier, Matthias; Kraushofer, Florian; Balajka, Jan; Pavelec, Jiří; Schmid, Michael; Franchini, Cesare; Diebold, Ulrike; Parkinson, Gareth S.
Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline "r-cut" (1 (1) over bar 02) surface of hematite (alpha-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 degrees C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid-water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.
Single-layer graphene on epitaxial FeRh thin films
(ELSEVIER, 2020-06-01) Uhlíř, Vojtěch; Pressacco, Frederico; Arregi Uribeetxebarria, Jon Ander; Procházka, Pavel; Průša, Stanislav; Potoček, Michal; Šikola, Tomáš; Čechal, Jan; Bendounan, Azzedine; Sirotti, F.
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 degrees, 15 degrees, 11 degrees, and 19 degrees 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.
Step-edge assisted large scale FeSe monolayer growth on epitaxial Bi(2)Se(3)thin films
(IOP Publishing, 2020-07-01) Fikáček, Jan; Procházka, Pavel; Stetsovych, Vitalii; Průša, Stanislav; Vondráček, Martin; Kormoš, Lukáš; Skála, Tomáš; Vlaic, Petru; Caha, Ondřej; Carva, Karel; Čechal, Jan; Springholz, Gunther; Honolka, Jan
Enhanced superconductivity of FeSe in the 2D limit on oxide surfaces as well as the prediction oftopological superconductivityat the interface to topological insulators makes the fabrication of Fe-chalcogenide monolayers a topic of current interest. So far superconductive properties of the latter are mostly studied by scanning tunneling spectroscopy, which can detect gaps in the local density of states as an indicator for Cooper pairing. Direct macroscopic transport properties, which can prove or falsify a true superconducting phase, are yet widely unexplored due to the difficulty to grow monolayer films with homogeneous material properties on a larger scale. Here we report on a promising route to fabricate micron-scale continuous carpets of monolayer thick FeSe on Bi(2)Se(3)topological insulators. In contrast to previous procedures based on ultraflat bulk Bi(2)Se(3)surfaces, we use molecular beam epitaxy grown Bi(2)Se(3)films with high step-edge densities (terrace widths 10-100 nm). We observe that step edges promote the almost strainless growth of coalescing FeSe domains without compromising the underlying Bi(2)Se(3)crystal structure.

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