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Studium vlastností nanočástic oxidu titaničitého
Procházka, Jan ; Kavan, Ladislav (vedoucí práce) ; Mička, Zdeněk (oponent) ; Šubrt, Jan (oponent)
Univerzita Karlova v Praze Přírodovědecká fakulta Katedra anorganické chemie STUDIUM VLASTNOSTÍ NANOČÁSTIC OXIDU TITANIČITÉHO Disertační práce Souhrn Ing. Jan Procházka Praha 2009 Tato doktorská disertační práce je koncipována jako soubor publikovaných článků a podaných patentů, na nichž se autor během studia spolupodílel. Jejich seznam je zařazen spolu s životopisem v doktorské práci. Studium vlastností nanočástic TiO2 přineslo řadu poznatků umožňujících zdokonalit syntézu nanočástic oxidu titaničitého ve formě tzv. templátovaných struktur, jejichž solární účinnost, při použití jako anody v solárních článcích, výrazně převyšuje tradiční TiO2 filmy. Systematický výzkum TiO2 nanostruktur vedl k vyladění jejich strukturních parametrů. Během této práce byly získány zásadní poznatky, a různé TiO2 struktury byly optimalizovány pro použití jako anody v DSC. Nejprve byly určeny všechny faktory, 1. ovlivňující správnou funkci TiO2 vícevrstvého filmu, vytvořeného postupným nanášením jednotlivých vrstev, a posléze proběhlo zdokonalení procesu jejich přípravy. Zásadním kvalitativním přínosem bylo dopování TiO2 fosforem během syntézy mesoporéznéch templátovaných vrstev. Přítomnost fosforu stabilizovala plochu povrchu TiO2 při vyšších teplotách a umožnila přípravu fázově čistého anatasu. Fázová čistota anatasu...
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Dynamika živých organismů
Vondrák, Marek ; Pelikán, Josef (vedoucí práce) ; Kavan, Ladislav (oponent)
Počítačová animace artikulovaných postav je jedna z nejzajzajímavějších a nejvíce se rozvíjejících oblastí moderní počítačové grafiky. Cílem této práce je seznámit čtenáře s teorií simulace tuhých těles s omezeními, která je následně použita ke konstrukci obecného simulátoru tuhých těles s omezeními a třením a animační knihovny pro artikulované postavy. Postavy jsou reprezentovány soustavami tuhých těles (segmentů) propojených klouby a jejich pohyb určen dynamikou příslušných segmentů. Dodatečná omezení, předepisující např. požadované úhly v kloubech nebo pozice vybraných bodů na povrchu segmentů, umožňují řídit pohyb postav. Bohaté interaktivní demonstrační příklady předvádí vlastnosti samotného simulátoru a možnosti animační knihovny pro zpracování motion capture dat (přehrávání animací, přizpůsobování animací externím vlivům, mapování "syrových" motion capture dat na pohyb segmentů, atd.).
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Experimental and Theoretical Comparative Study of Monolayer and Bulk MoS2 under Compression
del Corro, Elena ; Morales-García, A. ; Peňa-Alvarez, M. ; Kavan, Ladislav ; Kalbáč, Martin ; Frank, Otakar
Recently, a new family of 2D materials with exceptional optoelectronic properties has stormed into the scene of nanotechnology, the transition metal dichalcogenides (e.g., MoS2). In contrast with graphene, which is a zero band gap semiconductor, many of the single layered materials from this family show a direct band-gap in the visible range. This band-gap can be tuned by several factors, including the thickness of the sample; the transition from a direct to indirect semiconductor state takes place in MoS2 when increasing the number of layers from 1 towards the bulk. Applying strain/stress has been revealed as another tool for promoting changes in the electronic structure of these materials; however, only a few experimental works exist for MoS2. In this work we present a comparative study of single layered and bulk MoS2 subjected to direct out-of-plane compression, using high pressure anvil cells and monitoring with non-resonant Raman spectroscopy; accompanying the results with theoretical DFT studies. In the case of monolayer MoS2 we observe transitions from direct to indirect band-gap semiconductor and to semimetal, analogous to the transitions observed under hydrostatic pressure, but promoted at more accessible pressure ranges (similar to 25 times lower pressure). For bulk MoS2, both regimes, hydrostatic and uniaxial, lead to the semimetallization at similar pressure values, around 30 GPa. Our calculations reveal different driving forces for the metallization in bulk and monolayer samples.
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EXPERIMENTAL STUDY OF PIB-BASED CVD GRAPHENE TRANSFER EFFICIENCY
Bouša, Milan ; Kalbáč, Martin ; Jirka, Ivan ; Kavan, Ladislav ; Frank, Otakar
The transfer of graphene prepared by Chemical Vapor Deposition (CVD) from metal catalyst to target substrate is an important step in preparing desirable nanoscale structures in various fields of science, and thus searching for fast, cheap and clean method attracts great interest. Investigation of mechanical properties of graphene, which are crucial for applications in flexible electronics, performed on bendable synthetic materials, requires a transfer technique using polymers soluble in aliphatic solvents harmless for target polymer substrates. In this study we explore a dry technique using polydimethylsiloxane (PDMS) as stamping polymer and polyisobutylene (PIB) layer as graphene-support polymer. After the transfer PDMS is peeled off and PIB is dissolved in hexane, hence this method fulfils the above mentioned prerequisite. The effectiveness of this transfer was examined by scanning electron microscopy, optical microscopy and Raman microspectroscopy including micro-mapping, and finally by X-ray photoelectron spectroscopy. With all methods carried out, it was found that this sort of stamp-technique is suitable for a high precision transfer of small grains of CVD graphene onto polymer substrates with large yields and similar purity compared to poly(methylmethacrylate) (PMMA)based transfer methods. However, it introduces substantial quantity of surface discontinuities, and therefore this is not a proper method for large scale applications.
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STRAIN ENGINEERING OF THE ELECTRONIC STRUCTURE OF 2D MATERIALS
del Corro, Elena ; Peňa-Alvarez, M. ; Morales-García, A. ; Bouša, Milan ; Řáhová, Jaroslava ; Kavan, Ladislav ; Kalbáč, Martin ; Frank, Otakar
The research on graphene has attracted much attention since its first successful preparation in 2004. It possesses many unique properties, such as an extreme stiffness and strength, high electron mobility, ballistic transport even at room temperature, superior thermal conductivity and many others. The affection for graphene was followed swiftly by a keen interest in other two dimensional materials like transition metal dichalcogenides. As has been predicted and in part proven experimentally, the electronic properties of these materials can be modified by various means. The most common ones include covalent or non-covalent chemistry, electrochemical, gate or atomic doping, or quantum confinement. None of these methods has proven universal enough in terms of the devices' characteristics or scalability. However, another approach is known mechanical strain/stress, but experiments in that direction are scarce, in spite of their high promises.\nThe primary challenge consists in the understanding of the mechanical properties of 2D materials and in the ability to quantify the lattice deformation. Several techniques can be then used to apply strain to the specimens and thus to induce changes in their electronic structure. We will review their basic concepts and some of the examples so far documented experimentally and/or theoretically.
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Electrochemical Characterization of TiO2 Blocking Layers for Dye-Sensitized Solar Cells
Kavan, Ladislav
Thin compact layers of TiO2 are grown by thermal oxidation of Ti, by spray pyrolysis, by electrochemical deposition, and by atomic layer deposition. These layers are used in dye-sensitized solar cells to prevent recombination of electrons from the substrate (FTO or Ti) with the hole-conducting medium at this interface. The quality of blocking is evaluated electrochemically by methylviologen, ferro/ferricyanide, and spiro-OMeTAD as the model redox probes. Two types of pinholes in the blocking layers are classified, and their effective area is quantified. Frequency-independent Mott–Schottky plots are fitted from electrochemical impedance spectroscopy. Certain films of the thicknesses of several nanometers allow distinguishing the depletion layer formation both in the TiO2 film and in the FTO substrate underneath the titania film. The excellent blocking function of thermally oxidized Ti, electrodeposited film (60 nm), and atomic-layer-deposited films (>6 nm) is documented by the relative pinhole area of less than 1%. However, the blocking behavior of electrodeposited and atomic-layer-deposited films is strongly reduced upon calcination at 500 °C. The blocking function of spray-pyrolyzed films is less good but also less sensitive to calcination. The thermally oxidized Ti is well blocking and insensitive to calcination.
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DYE SENSITISED SOLAR CELLS: NEW CHALLENGES FROM GRAPHENE CATHODE AND IODINE-FREE ELECTROLYTE
Kavan, Ladislav
Graphene nanoplatelets (GNP) in the form of thin semitransparent film on F-doped SnO2 (FTO) exhibit high electrocatalytic activity for Co(L)2; where L is 6-(lH-pyrazol-l-yl)- 2,2‘-bipyridine. GNP films with optical transmission below 88% are outperforming the activity of platinized FTO for the Co2+/3+(L)2 redox reaction. Dye-sensitized solar cells with Y123 dye adsorbed on TiO2 photoanode achieved energy conversion efficiencies between 8 to 10 % for both GNP and Pt-based cathodes. However, the cell with GNP cathode is superior to that with Pt-FTO cathode in fill factors and in the efficiency at higher illumination intensities.
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Electrochemical and spectroelectrochemical study of nanocrystalline TiO2 anatase with exposed (001) faces
Lásková, Barbora ; Zukalová, Markéta ; Bouša, Milan ; Kavan, Ladislav ; Liska, P. ; Grätzel, M.
The electrochemical and spectroelectrochemical behavior ot TiO2 anatase with a predominant (001) face was studied and compared to a reference anatase material with dominating (101) face. The electrochemical measurements indicate that Li-ion insertion/extraction is facilitated for TiO2 anatase with (001) faces as compared to (101) one. The performance of both different crystal morphologies as a photoanode material in DSC (dye sensitized solar cell) was tested too. The (001) face adsorbed smaller amount of the used dye sensitizer (C101) but provides larger open circuit voltage (Uoc) of the solar cell. The different band energetics of both particular morphologies was suggested to be the reason for the voltage enhancement of the device employing (001) anatase. To prove our hypothesis and determine their flatband potential, a spectroelectrochemical study of transparent films of nanocrystalline (001) and (101) anatase on coaducting glass was carried out. UV/Vis spectra were measured at potentials between 0 and -1.4V vs SCE and confirmed the negative shift of flatband potential for (001) anatase as compared to (101) one.
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