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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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Very low energy STEM for biology
Frank, Luděk ; Nebesářová, Jana ; Vancová, Marie ; Paták, Aleš ; Müllerová, Ilona
Examination of tissue sections with transmitted electrons has been performed at energies of hundreds and tens of eV with thicknesses of sections of 10 nm or less. This was possible by employing the cathode lens principle working without lowest energy limitations with the help of biasing the sample to a high negative potential. The reflected and transmitted electrons were attracted with the same electric field to earthed detectors situated above and below the sample. Very high image contrasts have been obtained even for samples free of any heavy metal salts for contrast enhancement.
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Surface crystallinity at the sight of electrons
Frank, Luděk ; Mikmeková, Šárka ; Mika, Filip ; Müllerová, Ilona
Scattering of electrons, injected into solids in order to produce an electron optical image of their surfaces, is governed by inner potentional of the sample with its spatial distribution inherent to the target structure. Except truly amorphous materials of the spatial arrangement range shorter than the interaction volume of electrons, we meet anisotropic scattering with the resulting image signal responding to the local crystalline structure. Incident electrons undergo scattering events the number of wich depends on their energy and on the scattering cross section of the material and the final emitted current results from statistics of these events.
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Scanning Very Low Energy Electron Microscopy
Müllerová, Ilona ; Hovorka, Miloš ; Mikmeková, Šárka ; Pokorná, Zuzana ; Mikmeková, Eliška ; Frank, Luděk
Recent developments in applications of the scanning very low energy electron microscopy in selected branches of materials science are reviewed. The examples include visualization of grains in conductive polycrystals including ultrafine grained metals, identification of the local crystal orientation upon reflectance of very slow electrons, transmission mode with ultrathin free-standing films including graphene, acquisition of a quantitative dopant contrast in semiconductors, and examination of thin surface coverages.
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