Molekulární nanostruktury na površích

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    Correlative Imaging of Individual CsPbBr3 Nanocrystals: Role of Isolated Grains in Photoluminescence of Perovskite Polycrystalline Thin Films
    (AMER CHEMICAL SOC, 2023-06-20) Liška, Petr; Musálek, Tomáš; Šamořil, Tomáš; Kratochvíl, Matouš; Matula, Radovan; Horák, Michal; Nedvěd, Matěj; Urban, Jakub; Planer, Jakub; Rovenská, Katarína; Dvořák, Petr; Kolíbal, Miroslav; Křápek, Vlastimil; Kalousek, Radek; Šikola, Tomáš
    We report on the optical properties of a CsPbBr3 polycrystallinethin film on a single grain level. A sample composed of isolated nanocrystals(NCs) mimicking the properties of the polycrystalline thin film grainsthat can be individually probed by photoluminescence spectroscopywas prepared. These NCs were analyzed using correlative microscopyallowing the examination of structural, chemical, and optical propertiesfrom identical sites. Our results show that the stoichiometry of theCsPbBr(3) NCs is uniform and independent of the NCs'morphology. The photoluminescence (PL) peak emission wavelength isslightly dependent on the dimensions of NCs, with a blue shift upto 9 nm for the smallest analyzed NCs. The magnitude of theblueshift is smaller than the emission line width, thus detectableonly by high-resolution PL mapping. By comparing the emission energiesobtained from the experiment and a rigorous effective mass model,we can fully attribute the observed variations to the size-dependentquantum confinement effect.
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    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.
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    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.
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    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.
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    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.