National Repository of Grey Literature 37 records found  beginprevious28 - 37  jump to record: Search took 0.01 seconds. 
Hyperfine interactions in maghemite and magnetite particles
Křišťan, Petr
Thesis is aimed at studying of magnetic iron oxide particles of submicron and nanoscale dimensions by means of nuclear magnetic resonance (NMR). 57 Fe NMR inves- tigations were carried out in composite bentonite/maghemite with respect to tempera- ture of calcination (Tcalc) during the sample preparation and in magnetite submicron powders with respect to various range of the particles size. One of the main findings is that increasing Tcalc improves resolution in the NMR spectra, which is most likely connected with higher degree of atomic ordering in the spinel structure. Evaluating the integral intensities of NMR spectra allowed us to determine the relative content of maghemite phase in particular samples of the series: the content rapidly grows for Tcalc up to ∼420 deg. An approach to distinguish signal from tetrahedral and octahedral irons was developed and tested on pure maghemite sample. Analysis based on vacancy- distribution models was performed in the spinel structure and the results were compared to the experiment. 57 Fe NMR spectra in submicron magnetite samples were found to differ markedly from spectrum of a single crystal. It was concluded that the investigated powders possess high amount of defects in the crystal structure or contain additional phase (probably closely related to the maghemite phase).
Studium optických nelinearit v polovodičích a polovodičových nanostrukturách
Kozák, Martin ; Trojánek, František (advisor) ; Herynková, Kateřina (referee) ; Kužel, Petr (referee)
This Ph.D. thesis is focused on the study of optical nonlinearities and dynamics of excited charge carriers in monocrystalline diamond, nanocrystalline diamond and silicon. The dynamics of high density carriers in bulk diamond is investigated in detail (the transition from excitons and free carriers to electron-hole liquid or plasma). We study the picosecond dynamics of electron-hole liquid condensation using several techniques of time-resolved optical spectroscopy and demonstrate its evaporation by femtosecond laser pulses. We also propose two new optical techniques for measurement of lifetime, diffusion coefficient and surface recombination velocity of excitons in diamond. The results obtained by these techniques are described theoretically using diffusion equation and compared with the results obtained by the transient grating diffraction measurement. Further we study two- and three- photon absorption and nonlinear refractive index in diamond. In nanocrystalline diamond we study the second and third harmonic generation and its physical origin. In superlattices of silicon nanocrystals in SiO2 matrix we investigate the nonlinear transient absorption dynamics and carrier diffusion.
Optical properties of silicon nanostructures for photovoltaics
Salava, Jan ; Trojánek, František (advisor) ; Žídek, Karel (referee)
Název práce: Optické vlastnosti křemíkových nanostruktur pro fotovoltaiku Autor: Bc. Jan Salava Katedra: Katedra chemické fyziky a optiky Vedoucí diplomové práce: doc. RNDr. František Trojánek, Ph.D., katedra chemické fyziky a optiky Abstrakt: V předložené práci jsou studovány křemíkové nanokrystaly umístěné v SiC matrici - jednotlivé vzorky se odlišují přidáním dopantu (boru) do příslušné vrstvy struktury během depozice metodou PECVD a pasivací vodíkem. Křemíkové nanokrystaly jsou významné zejména tím, že oproti své objemové verzi vykazují účinnou fotoluminiscenci a absorpci ve viditelné oblasti spektra. Změnami parametr· při přípravě lze ladit jejich vlastnosti s ohledem na konkrétní aplikaci. Základní myšlenka integrace křemíkových nanostruktur do solárních článk· spočívá ve zvýšení účinnosti konverze slunečního spektra kombinací několika tenkých vrstev s nanokrystaly a objemového Si článku tak, aby každá vrstva sluneční cely absorbo- vala určitou část spektra. Procesy, které se v těchto strukturách dějí krátce po excitaci nosič· náboje, však stále nejsou zcela popsány. Cílem práce je charakterizace těchto jev· metodami ča- sově rozlišené spektroskopie. Dalším úkolem je popsat vliv dopování jednotlivých částí materiálu a jeho pasivace ve vodíkové atmosféře na chování fotoexcitovaných nosič· a intenzitu...
Study of semiconductors by methods of laser spectroscopy
Dzurňák, Branislav ; Trojánek, František (advisor) ; Oswald, Jiří (referee) ; Herynková, Kateřina (referee)
Title: Study of semiconductors by methods of time resolved laser spectroscopy: Luminescence spectroscopy of nanocrystalline diamond Author: Branislav Dzurňák Department: Department of Chemical Physics and Optics Supervisor: doc. RNDr. František Trojánek, Ph.D. Abstract: The PhD thesis is focused on optical properties of nanocrystalline diamond prepared by chemical vapour deposition method. Photoluminescence of nanocrystalline diamond samples and effects of ambient temperature, pressure, pH and UV irradiation on it are studied by laser spectroscopy. Results suggest the keyrole of water and air adsorbates which affect the energy states in the sub-bandgap region of diamond. Photoluminescence decay of samples of different surface termination and structure and its dependency on ambient pressure and temperature is studied by methods of ultrafast (picosecond and nanosecond scale) laser spectroscopy. Results are analysed by power-law decay function which fits well the luminescence decay curves and also describes the dynamics of charge carriers in states localised within the bandgap. The model of interaction of nanocrystalline diamond with air adsorbates is proposed. Non-linear optical properties of nanocrystalline diamond are also studied, namely the generation of second and third harmonic frequency. The thesis...
Nonlinear optical properties of silicon nanostructures
Žídek, Karel ; Trojánek, František (advisor) ; Bryknar, Zdeněk (referee) ; Kuldová, Karla (referee)
Název práce: Nelineární optické vlastnosti křemíkových nanostruktur Autor: Karel Žídek Katedra (ústav): Katedra chemické fyziky a optiky Vedoucí disertační práce: Doc. RNDr. František Trojánek, Ph.D. E-mail vedoucího: trojanek@karlov.mff.cuni.cz Abstrakt: Disertační práce se zabývá nelineárními optickými jevy a ultrarychlým vývojem luminis- cence křemíkových nanokrystalů. Pomocí metody optického hradlování signálu (časové rozlišení až 250 fs) porovnáváme ultrarychlý vývoj luminiscence křemíkových nanokrystalů s různými ve- likostmi (v řádu jednotek nanometrů) a také s rozdílnými formami pasivace. Pro nanokrystaly, kde po excitaci dominuje vliv zachytávání nosičů do povrchových stavů nanokrystalu, navrhujeme teoretický popis závislosti rychlosti těchto procesů na vlastnostech nanokrystalů. Dále v práci podrobně zkoumáme působení Augerovy rekombinace, která se projevuje jak v časově rozlišené, tak i v časově integrované emisi vzorků. Experimentální data velmi dobře popisuje námi navržený model na bázi kinetických rovnic. Závěr práce se zaměřuje na zkoumání ultrarychle dohasínající stimulované emise. U stávajících metod měření optického zisku (VSL a SES) navrhujeme jejich rozšíření pro...
Hyperfine interactions in maghemite and magnetite particles
Křišťan, Petr ; Štěpánková, Helena (advisor) ; Procházka, Ivan (referee)
Thesis is aimed at studying of magnetic iron oxide particles of submicron and nanoscale dimensions by means of nuclear magnetic resonance (NMR). 57 Fe NMR inves- tigations were carried out in composite bentonite/maghemite with respect to tempera- ture of calcination (Tcalc) during the sample preparation and in magnetite submicron powders with respect to various range of the particles size. One of the main findings is that increasing Tcalc improves resolution in the NMR spectra, which is most likely connected with higher degree of atomic ordering in the spinel structure. Evaluating the integral intensities of NMR spectra allowed us to determine the relative content of maghemite phase in particular samples of the series: the content rapidly grows for Tcalc up to ∼420 deg. An approach to distinguish signal from tetrahedral and octahedral irons was developed and tested on pure maghemite sample. Analysis based on vacancy- distribution models was performed in the spinel structure and the results were compared to the experiment. 57 Fe NMR spectra in submicron magnetite samples were found to differ markedly from spectrum of a single crystal. It was concluded that the investigated powders possess high amount of defects in the crystal structure or contain additional phase (probably closely related to the maghemite phase).
Highly luminescent nanophosphors - new physics, technologies and applications
Nikl, Martin ; Čuba, V. ; Bárta, J. ; Jarý, Vítězslav
The application of radiation synthesis in the manufacturing nanopowders of binary and ternary oxide nanophosphors is reviewed. Technological routes are described: irradiation of specific solutions containing soluble metal salts and OH radical scavenger by UV or ionizing radiation results in the formation of finely dispersed solid phase which is separated from the solution and subsequently converted by further thermal treatment into crystalline nanopowders with typical dimension of grains of several tens of nanometers. Doped ZnO, Y2O3, Y3Al5O12, Lu3Al5O12 and Gd3(Ga,Al)5O12 nanophosphors were prepared and their luminescence and scintillation characteristics were measured. In the photoluminescence decay of doped garnets the distinct effect of nanosized grains was found consisting in the slowing-down of the decay due to the change of effective refractive index.
Porovnání difuzivit Fe a Mo ve sktrukturních složkách Fe91-yMo8Cu1By slitin
Čermák, Jiří ; Stloukal, Ivo
Iron and molybdenum tracer diffusion in three Fe91-yMo8Cu1By (y = 12, 15 and 17) alloys was studied by serial sectioning method at temperature interval 548 – 773 K. Three modes of diffusion were observed.
Preparation and properties of MgO-Ni(Fe) nanocrystalline composite
Schneeweiss, Oldřich ; Pizúrová, Naděžda ; Jirásková, Yvonna ; Žák, Tomáš ; Bezdička, Petr ; Večerníková, Eva
We have prepared MgO-Ni(Fe) nanocrystalline composites by controlled heat treatment of compacted spark synthesized nanocrystalline powders. Besides particle sizes and their chemical compositions, influence of interaction with surrounding atmosphere and adsorbed gases on phase composition and magnetic properties were investigated. Mössbauer spectroscopy, x-ray diffraction and TEM were used for phase analysis. Thermomagnetic curves and DTA/TG measurements were applied for detection of changes in structure and phase composition during heat treatments. The simultaneous gas analysis using mass spectroscopy by heating yielded information on adsorbed and dissolved gases and their reactions with the nanoparticles.

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