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Zobrazování deformace kovových materiálů pomalými elektrony
Piňos, Jakub ; Kolíbal, Miroslav (oponent) ; Kasl, Josef (oponent) ; Frank, Luděk (vedoucí práce)
Rastrovací elektronová mikroskopie patří k běžným technikám analýzy moderních strojírenských materiálů. Rozvoj různých zobrazovacích technik umožňuje volit vhodný způsob pozorování pro získání nových informací o vzorku. Tato práce se zabývá přímým zobrazením deformace v kovových vzorcích pomocí rastrovací mikroskopie pomalými elektrony v průběhu in-situ tahové zkoušky. Experimenty byly provedeny na vzorcích z čisté mědi. V průběhu zkoušek byly získány snímky umožňující sledovat vývoj vlivu deformace na mikrostrukturu materiálu od jeho prvních projevů ve struktuře při intenzitách plastické deformace 3-4% až po extrémní plastickou deformaci na čele trhliny.
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Role sekundární emise v nabíjení prachových zrn
Richterová, Ivana ; Němeček, Zdeněk (vedoucí práce) ; Stöckel, Jan (oponent) ; Tomková, Eva (oponent) ; Frank, Luděk (oponent)
Název práce: Role sekundární emise v nabíjení prachových zrn Autor: Ivana Richterová Katedra: Katedra fyziky povrchů a plazmatu Školitel: Prof. RNDr. Zdeněk Němeček, DrSc. Abstrakt: Tato práce představuje model sekundární emise zaměřený na mikronová a submikronová prachová zrnka. Výsledky napomohly objasnit průběh rovnovážného potenciálu zrnka nabíjeného svazkem elektronů o energii mezi 50 eV a 15 keV, kdy s klesající velikostí zrn dochází k jejich průstřelu primárními elektrony. Chování zrn je rovněž ovlivněno jejich tvarem. Předpovědi modelu byly experimentálně ověřeny na skleněných, uhlíkových, zlatých kulových zrnkách a na krystalických zrnkách napodobující měsíční prach. Model nachází uplatnění při výpočtu náboje zrn v prachovém plazmatu s horkými elektrony, tedy například v magnetosférách planet a v okrajovém plazmatu tokamaků. Náboj zrn spolu s parametry plazmatu určují dynamiku celého systému. Klíčová slova: Sekundární emise, prachová zrnka, nabíjení prach
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Transmission of very slow electrons as a diagnostic tool
Frank, Luděk ; Nebesářová, Jana ; Vancová, Marie ; Paták, Aleš ; Mikmeková, Eliška ; Müllerová, Ilona
The penetration of electrons through solids is retarded by sequences of their interactions with the matter in which the electron changes its direction of motion and loses its energy. Inelastic collisions, the intensity of which reaches a maximum at around 50 electronvolts (eV) and drops steeply on both sides of this fuzzy threshold, are decisive for the penetration of electrons. Transmission microscopy (TEM or STEM) observes thin samples of tens to hundreds of nanometres in thickness by passing electrons of energies of tens to hundreds of kiloelectronvolts through them. The range below 50 eV has recently been utilized in the examination of surfaces with reflected electrons, where high image resolution is achieved thanks to the retardation of electrons close to the sample surface in the ´cathode lens´ . In this lens, the role of the cathode is played by the sample itself, biased to a high negative potential. This principle can also be utilized in the transmission mode with samples of a thickness at and below 10 nm. This method has recently been implemented and verified on graphene samples prepared by various methods. The results have made it possible to diagnose the continuity and quality of the graphene flakes. Furthermore, series of experiments have been performed involving the observation of ultrathin tissue sections with electrons decelerated to about 500 eV and less, where they provide an image contrast of the cell ultrastructure much higher than that provided by traditional microscopic modes.
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Real time observation of strain in the SEM sample
Piňos, Jakub ; Frank, Luděk
The SEM with various detector arrangements and analytical attachments represents an\nirreplaceable tool in material research. One of the techniques available in most contemporary\nmicroscopes is the scanning low energy electron microscopy (SLEEM) with biased specimen, marketed as the beam deceleration mode, gentle beam and others. The SLEEM allows\ncontrolling the information depth of the backscatter electron (BSE) imaging within a wide\nrange by altering the landing energy of electrons.
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Very low energy electron transmission spectromicroscopy
Daniel, Benjamin ; Radlička, Tomáš ; Piňos, Jakub ; Mikmeková, Šárka ; Konvalina, Ivo ; Frank, Luděk ; Müllerová, Ilona
For more than 25 years, Scanning Low Energy Electron Microscopy (SLEEM) has been\ndeveloped at the Institute of Scientific Instruments (ISI), with several commercially available SEMs adapted with a cathode lens for SLEEM use, as well as a dedicated self-built UHVSLEEM setup.\nFor a better understanding of contrast formation at low energies, especially at very low energies below 50 eV, where the local density of states plays an important role, more general knowledge about the interaction of (very) low energy electrons with solids is required. This will be achieved using a newly developed ultra-high vacuum (UHV SLEEM) setup which includes several enhancements compared to other available machines. Data processing is presented in, and processed data will be further used and tested with the Monte Carlosimulation package BRUCE, which is being developed by Werner et al. at TU Vienna.
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Examination of 2D crystals in a low voltage SEM/STEM
Mikmeková, Eliška ; Frank, Luděk ; Polčák, J. ; Paták, Aleš ; Lejeune, M.
Development of new types of materials such as 2D crystals (graphene, MoS2, WS2, h-BN, etc.) requires emergence of new surface-sensitive techniques for their characterization. As regards the “surface” sensitivity, the (ultra) low energy electron microscopy can become a very powerful tool for true examination of these atom-thick materials, capable of confirming physical phenomena predicted to occur on their surfaces. Modern commercial scanning electron microscopes enable imaging and analyses by low energy electrons even at very high magnification. In the case of the SEM, resolution even below 1 nm can be achieved at low landing energy of electrons. Since specimen contamination increases with increasing electron dose and decreasing landing energy, specimen cleanness is a critical factor in obtaining meaningful data. A range of various specimen cleaning methods can be applied to selected samples. Typical cleaning methods, such as solvent rinsing, heating, bombarding with ions and plasma etching have their limitations. Electron-induced in situ cleaning procedure can be gentle, experimentally convenient and very effective for wide range of specimens. Even a small amount of hydrocarbon contamination can severely impact on the results obtained with low energy electrons, as illustrated in Figure 1A. During the scanning of surfaces by electrons, the image usually darkens because of a carbonaceous layer gradually deposited on the top from adsorbed hydrocarbon precursors.
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Role sekundární emise v nabíjení prachových zrn
Richterová, Ivana ; Němeček, Zdeněk (vedoucí práce) ; Stöckel, Jan (oponent) ; Tomková, Eva (oponent) ; Frank, Luděk (oponent)
Název práce: Role sekundární emise v nabíjení prachových zrn Autor: Ivana Richterová Katedra: Katedra fyziky povrchů a plazmatu Školitel: Prof. RNDr. Zdeněk Němeček, DrSc. Abstrakt: Tato práce představuje model sekundární emise zaměřený na mikronová a submikronová prachová zrnka. Výsledky napomohly objasnit průběh rovnovážného potenciálu zrnka nabíjeného svazkem elektronů o energii mezi 50 eV a 15 keV, kdy s klesající velikostí zrn dochází k jejich průstřelu primárními elektrony. Chování zrn je rovněž ovlivněno jejich tvarem. Předpovědi modelu byly experimentálně ověřeny na skleněných, uhlíkových, zlatých kulových zrnkách a na krystalických zrnkách napodobující měsíční prach. Model nachází uplatnění při výpočtu náboje zrn v prachovém plazmatu s horkými elektrony, tedy například v magnetosférách planet a v okrajovém plazmatu tokamaků. Náboj zrn spolu s parametry plazmatu určují dynamiku celého systému. Klíčová slova: Sekundární emise, prachová zrnka, nabíjení prach
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The information depth of backscattered electron imaging
Piňos, Jakub ; Mikmeková, Šárka ; Frank, Luděk
Of the conventional imaging signals in the scanning electron microscope (SEM), the secondary electrons generally reflect surface properties of the sample, while the backscattered electrons (BSE) are capable of providing information about complex properties of the target down to a certain subsurface depth. Contrast mechanisms are combined according to the energy of incident electrons and energy and angular acceptance of BSE detection. In all cases, a question arises concerning the information depth of this mode. No applicable answer provides a definition declaring this depth as that from which we still obtain useful information about the object. We can employ software simulating the electron scattering in solids,\nwhile experimental approaches are also possible. Moreover, two analytic formulas can be found in the literature.
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Scanning transmission microscopy at very low energies
Müllerová, Ilona ; Mikmeková, Eliška ; Konvalina, Ivo ; Frank, Luděk
To operate down to units of eV with a small primary spot size, a cathode lens with a biased specimen was introduced into the SEM. The reflected signal, accelerated secondary and backscattered electrons, is collected by detectors situated above the specimen.\nWhen we insert a detector below the specimen, the transmitted electron signal can also be used for imaging down to zero energy. Fig. 1 also shows an example of the simulated signal trajectories of electrons that impact on the detector of reflected electrons, based on an Yttrium Aluminium Garnet (YAG) crystal, and trajectories of electrons transmitted through the specimen and incident on a semiconductor detector based on the PIN structure.
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Treatment of surfaces with slow electrons
Frank, Luděk ; Mikmeková, Eliška
Historically, the most annoying obstacle to acquiring SEM micrographs, in particular higher magnification micrographs taken with the ambition of resolving the finest observable details, may be said to be carbonaceous contamination “highlighting” the previous field of view with a black rectangle contoured by an even darker frame. This contamination is generated by decomposition of adsorbed hydrocarbon molecules with incident electrons leaving a crosslinked\nlayer of carbon atoms as a surface coating. The darker contours come from high surface mobility of hydrocarbon molecules from outside the field. The situation has been improved in recent decades by a lower pressure and dryer vacuum in specimen chambers, but even under ultrahigh vacuum (UHV) conditions the phenomenon occurs due to hydrocarbon molecules deposited on the specimen when loaded. Therefore, only in-situ cleaning with an\nattachment producing an ion beam solves this problem in UHV, while some plasma cleaners have also started appearing in standard-vacuum SEM chambers. The goal of complete removal of hydrocarbons is motivated by the supposed unavoidability of their decomposition with primary electrons. However, we have found hydrocarbon molecules being released, rather than their decomposition, when the energy of the impinging electrons drops beneath 50 eV or so.
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