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Deterministicky aperiodické obrazové zařízení
Horáček, Miroslav ; Meluzín, Petr ; Krátký, Stanislav ; Knápek, Alexandr ; Mika, Filip ; Chlumská, Jana ; Matějka, František ; Král, Stanislav ; Brunn, Ondřej ; Giričová, D. ; Kopal, Jaroslav ; Kolařík, Vladimír
Příspěvek se týká analýzy, návrhu a přípravy difrakčního opticky variabilního obrazového zařízení tvořeného deterministicky aperiodickou sítí základních optických prvků. Teoretický základ je doplněn numerickou analýzou a prezentací realizované matrice a jejích replik.
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Scanning very low energy electron microscopy for the characterization of polycrystalline metal samples
Pokorná, Zuzana ; Knápek, Alexandr
We explored the possibility of a Scanning Electron Microscopy technique for the determination of crystallographic orientation, based on the measurement of the reflectivity of very low energy electrons. Our experiments are based on the concept that in the incident electron energy range 0–30 eV, electron reflectivity can be correlated with the electronic structure of the material, which varies with the local crystallographic orientation of the specimen.\nThe motivation for the development of this technique was to achieve a quick and highresolution means for determining the crystallographic orientation of very small grains in a polycrystalline material. The key limiting factor was the cleanliness of the sample surface and also the geometrical setup of the experiment.
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Field emission from the surface of highly ordered pyrolytic graphite
Knápek, Alexandr ; Pokorná, Zuzana
This paper deals with the electrical characterization of highly ordered pyrolytic graphite (HOPG) surface based on field emission of electrons. The effect of field emission, occurs only at disrupted surface, i.e. surface containing ripped and warped shreds of the uppermost layers of graphite. These deformations provide the necessary field gradients which are required for measuring tunneling current caused by field electron emission. Results of the field emission measurements are correlated with other surface\ncharacterization methods such as scanning near-field optical microscopy (SNOM) or atomic force microscopy. A simple method utilizing the field emission of electrons has been devised to characterize the sample surface. Electron and probe microscopies were used to determine the structure of both the bulk sample and the partially exfoliated shreds of the uppermost layers of graphite in locations where field emission is observed.
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Metody zobrazování a analýzy biologických vzorků
Novotná, Veronika ; Tománek, Pavel (oponent) ; Knápek, Alexandr (vedoucí práce)
Bakalářská práce se věnuje vlastnostem lidských tkání a bakterií z mikroskopického hlediska. Dále popisuje fyzikální principy mikroskopovacích technik SNOM a elektronové mikroskopie (TEM, SEM, ESEM) a jejich vhodnost pro pozorování biologických preparátů. Následující část se zabývá transformací dat z elektronového mikroskopu z časové do obrazové roviny a práci se vzniklými obrazy. Další kapitola je věnována experimentální přípravě preparátů pro SEM/ESEM a SNOM. Protože jsou zde v současné době tendence využít mikroskopovací techniku SNOM v biologické výzkumu je cílem práce srovnání možností zmíněných mikroskopovacích technik pro pozorování biologických vzorků a zhodnocení, zda je možné pro tyto účely nahradit obecně drahý EM levnějším SNOM. EM i SNOM totiž umožňují získat rozdílnou nicméně srovnatelnou a komplementární informaci o zkoumaném vzorku.
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Methods of Preparation and Characterization of Experimental Field-Emission Cathodes
Knápek, Alexandr ; Luňáček, Jiří (oponent) ; Lazar, Josef (oponent) ; Grmela, Lubomír (vedoucí práce)
This PhD thesis describes, and covers the preparation of field-emission cathodes that represent a high-quality, inexpensive source of electrons for devices that work with a focused electron beam. In this work, an improved method of electrochemical fabrication is used for the preparation of a composite field-emission cathode. The composite structure improves current stability in comparison to tungsten-based pure field-emission cathodes. Improvements to the technology used were implemented on the basis of findings obtained using spectrum noise diagnostics, which are based on measuring and evaluating the emission current in the time and in frequency domains. Additionally, noise spectroscopy provides important information about charge transport, electron mobility, and lifespan, which is essential for future research. Results obtained using the experimental method have been further supplemented with theoretical data provided by a computer simulation in order to establish characterization procedures for improved field-emission cathodes.
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Methodics of characterisation for the cold field-emission sources intended for electron microscopy
Štrbková, Lenka ; Grmela, Lubomír (oponent) ; Knápek, Alexandr (vedoucí práce)
The master’s thesis deals with the characterization method of the electron emitter for an electron microscope – Fowler-Nordheim analysis. The method is based on the presence of quantum tunnelling, which occurs during the field emission of electrons. The work contains the theoretical background to the topic, as well as the practical part. The theoretical introduction describes the electron emission, quantum tunnelling phenomenon and the parameters of the electron emitters. In the second part, the thesis focuses on the experimental study of the field-emission cathode. The essential characteristics of the emitter are computed. These characteristics determine the overall quality of electron emission, as well as provide information about the actual condition of the emitter. The results of the F-N analysis are subsequently compared with the results of the computer simulation of the field-emission. The simulation is performed in COMSOL Multiphysics software using the finite element method (FEM) and enables to assess the relevance and accuracy of the F-N analysis.
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Pyroelectric detector signal measurement and processing
Knápek, Alexandr ; Sedláková, Vlasta (oponent) ; Majzner, Jiří (vedoucí práce)
The thesis analyzes the physical properties of the pyroelectric sensors and its practical use. Essential part of the work is the design and realization of the measuring set-up, which is used for the measurements of the sensors physical properties. With this workbench, main parameters of the pyroelectric sensors have been obtained. The second part of the work deals with a low noise preamplifier designing. This device was designed for the pyroelectric sensor signal measurements. The amplifier is designed to be used for a low noise, wide band measuring. During the process of amplifier designing, all the noise components have been investigated separately, using operational amplifiers models. The objective of the last part of this work is to develop the system, which would be able to localize an infrared (IR) emitting source located somewhere in the space between the installed pyroelectric sensors. For this purpose, classical localization methods could be used as well as the artificial neural networks (ANN), which are becoming still more popular these days. The system is able to detect the exact placement of the IR radiation source.
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Imaging of carbon nanostructures by low energy STEM below 5 keV
Pokorná, Zuzana ; Knápek, Alexandr ; Jašek, O. ; Prášek, J. ; Majzlíková, P.
Our work deals with the imaging of nanostructures composed of light biogenic elements, such as carbon nanotubes, by low energy scanning transmission electron microscopy (STEM). Compared to imaging at the voltages commonly used for TEM and STEM, low energy electrons seem very promising in terms of specimen damage that is caused by a number of elastic andn inelastic collisions. In carbonaceous materials, the most problematic is probably the knock-on damage, where the structure can be impaired by carbon atom displacement. To avoid this problem with structures composed of light elements, a reduction in beam voltage going down to 5 keV has recently been proposed. The range below 5 keV has not been explored yet for this purpose, although electron scattering in matter is lower for these energies, which allows achieving a higher spatial resolution. We aim to demonstrate that additional reduction of incident electron energy may yield interesting contrast features.
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Surface examination of ultra-sharp cold field-emission cathode
Knápek, A. ; Brüstlová, J. ; Trčka, T. ; Klampár, M. ; Mikmeková, Šárka
An experimental cold field-emission cathode, based on metal-oxide-insulator structure has been analyzed by several non-destructive evaluation methods in order to describe the material properties and electrical behaviour of the sample. Owing to the fact that the tip of the cathode reaches nanoscopic-scale, several different evaluation methods have been incorporated. Firstly, the scanning electron microscopy (SEM), along with the electron dispersive spectroscopy (EDS) was performed to describe general proportions of the shape and the abundance of elements present in the surface layer. Secondly, the scanning low-energy electron microscopy (SLEEM) was applied in order to visualize the crystalline microstructure of the cathode surface. Finally, the epoxy coating covering the cathode tip was characterised using the dielectric relaxation spectroscopy (DRS) method.
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