Molekulární nanostruktury na površích

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ProLEED Studio: software for modeling low-energy electron diffraction patterns
(INT UNION CRYSTALLOGRAPHY, 2024-02-01) Procházka, Pavel; Čechal, Jan
Low-energy electron diffraction patterns contain precise information about the structure of the surface studied. However, retrieving the real space lattice periodicity from complex diffraction patterns is challenging, especially when the modeled patterns originate from superlattices with large unit cells composed of several symmetry-equivalent domains without a simple relation to the substrate. This work presents ProLEED Studio software, built to provide simple, intuitive and precise modeling of low-energy electron diffraction patterns. The interactive graphical user interface allows real-time modeling of experimental diffraction patterns, change of depicted diffraction spot intensities, visualization of different diffraction domains, and manipulation of any lattice points or diffraction spots. The visualization of unit cells, lattice vectors, grids and scale bars as well as the possibility of exporting ready-to-publish models in bitmap and vector formats significantly simplifies the modeling process and publishing of results.
Remarkably stable metal-organic frameworks on an inert substrate: M-TCNQ on graphene (M = Ni, Fe, Mn)
(Royal Society of Chemistry, 2022-07-07) Jakub, Zdeněk; Kurowská, Anna; Herich, Ondrej; Černá, Lenka; Kormoš, Lukáš; Shahsavar, Azin; Procházka, Pavel; Čechal, Jan
Potential applications of 2D metal-organic frameworks (MOF) require the frameworks to be monophase and well-defined at the atomic scale, to be decoupled from the supporting substrate, and to remain stable at the application conditions. Here,we present three systems meeting this elusive set of requirements: M-TCNQ (M = Ni, Fe, Mn) on epitaxial graphene/Ir(111). We study the systems experimentally by scanning tunneling microscopy, low energy electron microscopy and x-ray photoelectron spectroscopy. When synthesized on graphene, the 2D M-TCNQ MOFs are monophase with M1(TCNQ)1 stoichiometry, no alternative structure was observed with slight variation of the preparation protocol. We further demonstrate a remarkable chemical and thermal stability of TCNQ-based 2D MOFs: All the studied systems survive exposure to ambient conditions, with Ni-TCNQ doing so without any significant changes to its atomic-scale structure or chemical state. Thermally, the most stable system is Fe-TCNQ which remains stable above 500 °C, while all the tested MOFs survive heating to 250 °C. Overall, the modular M-TCNQ/graphene system combines the atomic-scale definition required for fundamental studies with the robustness and stability needed for applications, thus we consider it an ideal model for research in single atom catalysis, spintronics or high-density storage media.
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.
Polymer pencil leads as a porous nanocomposite graphite material for electrochemical applications: The impact of chemical and thermal treatments
(Elsevier, 2021-05-01) Trnková, Libuše; Třísková, Iveta; Čechal, Jan; Farka, Zdeněk
Pencil graphite electrodes are a simple, disposable, and low-cost alternative to screen-printed graphite electrodes. In terms of stability and sensitivity, pencil electrodes often outperform conventional carbon ones. This paper discusses and emphasizes the superior properties of polymer pencil graphite electrodes (pPeGEs), which can be exploited in the electrochemical analysis of molecules, such as chlorides, whose signals are missing on common graphite electrodes. The chemical and structural behaviour of pencil leads after exposure to acids (HF, HNO3, HClO4) or organic solvents (CH3CN, CH3Cl) was monitored via X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The electrochemical activity of pristine and treated pPeGEs was studied by the cyclic voltammetry (CV) responses of reversible redox probes [Fe(CN)6]3/4- and [Ru(NH3)6]3+/ 2+. XPS proved the presence of siloxanes in the surface matrix of the pencil leads; this finding relates to the hydrophobic surface character of the electrodes. SEM then provided images of the pencil surfaces with microplates and flakes and revealed the removal of siloxanes upon chemical treatment. The CVs of non-dried and dried pPeGEs displayed surface changes in the polymer matrix, accompanied by water loss. Our study shows that the pPeGE retains the character of a stable graphite sensor when exposed to acids and organic solvents, except for HF and chloroform. The discovered effects explain the electrochemical processes occurring on pPeGEs and can contribute to their application in electrochemical sensing and energy storage.
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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