National Repository of Grey Literature 33 records found  1 - 10nextend  jump to record: Search took 0.01 seconds. 
Role of hydrogen during growth of epitaxial graphene on SiC
Sýkora, Petr ; Kunc, Jan (advisor) ; Čechal, Jan (referee)
This work is focused on preparation of graphene by epitaxial growth on silicon face of silicon carbide. In the introduction, the interesting properties of graphene are briefly described, and its possible applications are mentioned. Subsequently, the methods of preparation are discussed with an emphasis on epitaxial growth and the role of hydrogen on the process itself. Further, the possible ways of measuring graphene samples are explained and three of them are analyzed in more detail - Raman spectroscopy, atomic force microscopy, and Hall effect measurement. The experiment itself is focused on the effect of argon-hydrogen atmosphere on the growth of graphene. 1
Deposition of large organic molecules under UHV
Krajňák, Tomáš ; Novák,, Jiří (referee) ; Čechal, Jan (advisor)
In this thesis, large organic molecules (DM15N, DM18N, Cu(dbm)2) were deposited. These molecules are cannot be deposited by thermal sublimation due the fact that they decompose at lower temperature than they sublime. The employed molecules to single molecular magnets, which can be potentially used as quantum bites (qubit). The new method of deposition atomic layer injection made by Bihur Crystal company was introduced and tested. The method uses liquid solution with molecules which is driven by argon gas through pulse valve to the sample placed in ultra-high vacuum chamber. During the deposition, droplets of solution are formed on the sample surface. The solvent can be removed by light annealing or by keeping the sample in the vacuum for couple of days. The molecules were investigated by x-ray photoelectron spectroscopy and by scanning electron microscopy to determine fragmentation of the molecules, to study topography of the resultant surface and homogeneity of the deposited layer. We found conditions at which the intact molecules are deposited on the sample surfaces and form molecular nano- and micro- crystals.
Interaction of low-energy electrons with graphene field effect transistors
Vysocký, Filip ; Kunc, Jan (referee) ; Čechal, Jan (advisor)
This diploma thesis is focused on fabrication of graphene field-effect transistors, characterisation of their transport properties and investigation of low-energy electron beam influence on the devices' properties under UHV conditions. The theoretical part of this work describes graphene fabrication methods, options of graphene transfer onto the substrates for graphene field-effect transistor manufacture. Furthermore, model of graphene doping via electrostatic interaction or photon, resp. electron beam exposition is explained. The experimental part of this work consist of manufacture of the graphene field-effect transistor in order to examine the change of its transport properties induced by doping of the graphene via low-energy electron beam exposition.
Hydrogen quantification by electron spectroscopy
Endstrasser, Zdeněk ; Polčák, Josef (referee) ; Čechal, Jan (advisor)
The absence of hydrogen in the photoelectron spectrum remains one of the greatest limitations of X-ray photoelectron spectroscopy. The main aim of this work is therefore to determine the concentration of hydrogen atoms in the near surface region of a sample. The REELS electron spectroscopic method based on quasi-elastic electron scattering is used for this purpose. The choice of this method was given mainly by the fact that the photoelectron spectrometers has an electron source in the basic equipment, so there is no requirement to purchase additional equipment. In our case, measurements were realized using the AXIS Supra photoelectron spectrometer in CEITEC laboratories. We were primarily focused on the study of polymer samples in this work. The gus cluster ion source was used for sputter cleaning of the sample surface. Then the XPS and REELS measurements were performed on such cleaned samples. The obtained data were subsequently evaluated, and the results discussed. The most important output of this work was to develop the methodology of detection and quantification of hydrogen atoms. However, the theoretical description of the methods used, and the appreciation of critical aspects associated with the use of the REELS method are also embraced. In particular, the effect of sample charging and electron dose on measurement results is discussed.
Controllable synthesis, treatment and characterization of anodes for Direct Formic Acid Fuell Cell
Bieloshapka, Igor ; Jiříček, Petr (advisor) ; Khalakhan, Ivan (referee) ; Čechal, Jan (referee)
Title: Controllable synthesis, treatment and characterization of anodes for Direct Formic Acid Fuell Cell Author: Mgr. Igor Bieloshapka Department/Institute: Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University Supervisor of the doctoral thesis: Ing. Petr Jiříček, CSc., Institute of Physics of the Czech Academy of Sciences, Division of Solid State Physics, Department of Optical Materials Abstract: At this doctoral thesis, anodes were prepared by novel DC magnetron sputtering technique for direct formic acid fuel cells (DFAFCs). Anode part consisted of support material and catalyst. Carbon cloth and polyaniline were used as a support. Palladium and palladium-copper bimetallic catalysts were deposited on the top of the support. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used for investigation of the morphology of the anodes. Composition and chemical states on the anode part were studied by x-ray photoelectron spectroscopy (XPS). Transmission electron microscope (TEM) together with the XPS technique were used for characterizing graphene oxide (GO) and reduced graphene oxide (RGO) as a promising support for the polymer membrane fuel cells. For decreasing the role of substoichiometric PdCx phase and other contaminations on the surface of...
Role of surface defects in ceria-based catalysis
Tovt, Andrii ; Mysliveček, Josef (advisor) ; Čechal, Jan (referee) ; Švec, Martin (referee)
Title: Role of surface defects in ceria-based catalysis Author: Andrii Tovt Department: Department of Surface and Plasma Science Supervisor of the doctoral thesis: doc. Mgr. Josef Mysliveček Ph.D., Department of Surface and Plasma Science Abstract: This work concentrates on the analysis of fundamental physicochemical properties of Pt-CeOx, single-atom Pt1 /CeOx, and inverse CeOx/Cu(111) catalysts. Preparation method for stabilized atomically-dispersed Pt2+ ions on ceria was developed and adsorption sites for Pt ions were thoroughly studied using advanced surface science techniques supported by theoretical methods. The mechanism of Pt2+ stabilization on ceria steps was revealed and the step capacity towards Pt2+ ions was estimated. Also, the preparation method for well-ordered cerium oxide ultrathin films with different stoichiometry and ordering of surface oxygen vacancies was developed, and the Ceria/Cu(111) interaction was investigated. Key words: heterogeneous catalysis, model systems, single-atom catalysis, platinum ions, cerium oxide.
Atomic structure of FeRh(001) surface
Ondračka, Václav ; Bábor, Petr (referee) ; Čechal, Jan (advisor)
Alloy of iron-rhodium, Fe50Rh50, shows a low-temperature metamagnetic phase transition between its antiferromagnetic and ferromagnetic state. A detailed description of this transition using the tools of surface analysis has not yet been done. This bachelor’s thesis focuses on preparation and characterisation of the alloy’s clean surface using X-ray photoemission spectroscopy and low-energy electron diffraction. A flat field correction and a diffraction measurement callibration is done using a sample with known crystal lattice in order to quantify the diffraction results.
Graphene doping by low-energy electrons
Stará, Veronika ; Kunc, Jan (referee) ; Čechal, Jan (advisor)
Tato diplomová práce se zabývá dotováním grafenu nízkoenergiovými elektrony. Na křemíkový substrát pokrytý vrstvou SiO2 jsou pomocí litograficky vyrobené masky nadeponované kovové kontakty z titanu a zlata. Grafen vyrobený pomocí metody depozice z plynné fáze je přenesen na substrát a slouží jako vodivé spojení kovových elektrod, které vytvářejí kolektor a emitor. Na křemík je ze spodu přivedeno napětí, které tak vytváří spodní hradlo. Takto vytvořený grafenový tranzistor je ozařován nízkoenergiovými elektrony, které mění dotování grafenu. Z polohy maxima v závislosti odporu grafenu na hradlovém napětí lze vyčíst typ dotování. Toto maximum udává napětí, při kterém Fermiho meze grafenu prochází Diracovým bodem v pásové struktuře grafenu. Velikost hradlového napětí, primární energie elektronového svazku a proud svazku jsou tři parametry, které mají velký vliv na změny dotování. Při ozařování transistoru dochází ke změně typu dotování právě tehdy, když odpor grafenu v závislosti na hradlovém napětí dosáhne maxima. Vývoj této změny je zkoumán pro různé energie a proudy primárního svazku v závislosti na hradlovém napětí i v čase. Typ dotování je také prozkoumán při zastavení ozařování v různých fázích smyčky hradlového napětí. Dopování grafenu nízkoenergiovými elektrony je popsáno v teoretickém modelu.
Self-assembled molecular layers on epitaxial graphene
Kovařík, Štěpán ; Švec, Martin (referee) ; Čechal, Jan (advisor)
Samouspořádání organických molekul je spontánní proces tvorby nanostruktur, při kterém je výsledná struktura určena mezimolekulárními and molekulárně-substrátovými interakcemi. Pochopení principů samouspořádávání je klíčem k přípravě funkčních nanostruktur s atomární přesností. Tato diplomová práce se zaměřuje na přípravu a studium samouspořádaných molekulárních struktur 4,4’-diphenyl dikarboxylové kyseliny na povrchu grafenu připraveného na Ir(111). Pro studium vlastností molekulárních struktur je využito rastrovací tunelovací mikroskopie a nízkoenergiové elektronové mikroskopie. Tato kombinace umožňuje získat informace z oblastí o velikosti v řádu nanometrů až milimetrů. V práci je popsána molekulární strukuktura stabilní při pokojové teplotě. Vazebný motiv této struktury je dán interakcí karboxylových skupin sousedních molekul.
Environment-Dependent Surface Structure of Fe2O3(012)
Komora, Mojmír ; Dubroka, Adam (referee) ; Čechal, Jan (advisor)
Táto diplomová práca sa zaoberá štúdiom alfa-Fe2O3(012) vystaveného vodnému prostrediu. Súčasný stav poznania ohľadom oxidov železa s pozornosťou vkladanou do popisu alfa-Fe2O3 a jeho (012) povrchu je stručne zhrnutá. Experimentálná časť tejto práce začína s popisom unikátného zariadenia na depozíciu kvapalnej vody na povrchy monokryštalických vzoriek compatibilného s podmienkami ultra vysokého vákuua. Jednotlivé konštrukčné časti a detaily sú diskutované. Navrhnutý systém bol testovaný na vzorkách striebornej fólie a monokryštálu Fe3O4 s povrchovom v rovine [100]. Chemické zloženie a morfológia povrchu po experimente s kvapalnou vodou je diskutovaná. Navrhnuté zariadenie bolo použité na štúdium interakcie vody so známymi (1x1) a (2x1) povrchovými alfa-Fe2O3(012). Experimentálne dáta ukazujú, že voda sa viaže na obe reconštrukcie disociatívne s určitým množstvom molekulárnej vody naviazanej na adsorbované hydroxydi. (1x1) sa zdá javý stabilná po expozícii rôznym tlakom vodnej pary, zatial čo (2x1) vykazuje zmenu na (1x1) pri expozíciach vyšším tlakom vodnej pary alebo po niekolo minútovej expozíci electrónovému zväzku. Ďalej sú v tejto práci prezentované prvé výsledky z štúdie prechodu (1x1) rekonštrukcie na (2x1) pomocou mikroskpie nízkoenergiových elektrónov. Výsledky ukazujú, že táto premena na nižších teplotách je inciovaná na hranách atomárnych vrstiev a defektov na povrchu, ktorá potom postupuje smerom k stredu atomárnych terás. Meranie na vyšších teplotách vykazuje postupnú premenu povrchu naznačúju viaceru nukleačných centrech z ktorých sa (2x1) povrch širi ďalej.

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