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Thermal desorption spectroscopy in prototype furnace for chemical vapor deposition
Průcha, Lukáš ; Daniel, Benjamin ; Piňos, Jakub ; Mikmeková, Eliška
Cleaning of the sample surfaces is crucial for scanning electron microscopy, especially for\nlow energy electron microscopy or for the deposition of thin layers, such as graphene,\nwhere surface has to be well prepared. In the best case, every unwanted particle should be\ncleaned from the sample surface for best low energy electron microscopy observation or thin\nfilm deposition. Unfortunately, the standard cleaning procedures can leave residues on the\nsample surface. This work is focused on thermal desorption spectroscopy (TDS). TDS is a method of observing desorbed molecules from a sample surface during the increase of\ntemperature of the sample. The aim of this study was to determine optimum conditions:\ntemperature and time, to achieve clean surfaces in the shortest time.
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Stable Ce4+ centres - a tool to optimize cathodoluminescence performance in garnet scintillators
Lalinský, Ondřej ; Schauer, Petr ; Rathaiah, M. ; Kučera, M.
Garnet single crystals are widely used as scintillators in electron detectors. Cerium activated lutetium aluminum garnet Cex:Lu3-xAl5O12 (LuAG:Ce) is a promising example of such material for these applications. This is mainly due to its high light yield (LY) of 25 kph/MeV, short decay time of 60–80 ns, high atomic density (6.7 g/cm3), and high radiation stability with no hygroscopicity. The cathodoluminescence (CL) performance can be improved by Ga and Gd doping the garnet matrix. Proper admixture of these elements can increase the LY to 50–60 kph/MeV in addition to eliminating unwanted slower decay components. There was an idea that further decay acceleration can be achieved by doping the garnet with monovalent (Li+) or divalent ions (Mg2+, Ca2+). This should increase the valency of some Ce3+ centres to Ce4+ which should better compete with electron traps, and thus accelerate the decay. Our previous work proved the same decay trend, however, at a price of the LY. Such LY loss may induce the idea, if the stable Ce4+ centres are really participating in Ce3+ emission.
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STEM modes in SEM
Konvalina, Ivo ; Paták, Aleš ; Mikmeková, Eliška ; Mika, Filip ; Müllerová, Ilona
The segmented semiconductor STEM detector in the Magellan 400 FEG SEM microscope\n(https://www.fei.com/) is used to detect transmitted electrons (TEs) and allows observing\nsamples in four imaging modes. Two modes of objective lens, namely high resolution (HR)\nand ultra-high resolution (UHR), differ by their resolution and by the presence or absence of\na magnetic field around the sample. If the beam deceleration (BD) mode is chosen, then\nan electrostatic field around the sample is added and two further microscope modes HR + BD\nand UHR + BD, become available. Trajectories of TEs are studied with regard to their angular\nand energy distribution in each mode in this work.\n
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Field emission from W5O14 nanowires
Saqib, M. ; Knápek, Alexandr ; Jelenc, J. ; Pirker, L.
The W5O14 (O/W=2.8) nanowires are metallic oxides with specific resistivity of 25 microOhm/cm and\ndiameters bellow 100 nm [1]. They were synthesized by iodine transport method using nickel\nas a growth promoter and WO3 as source of tungsten and oxygen. The field emission\ncharacteristics of single nanowires [2] and the films composed of these nanowires have been\nreported [3]. The emitting current densities up to 6.4 mA/cm2 have been obtained at relatively\nlow average electric field of about 3 V/Ohm*m. The samples were allowed to emit for more than\n100 hours without showing significant decays of the emitting current and without substantial\ncurrent oscillations. Here, we present field emission properties of single W5O14 nanowires\nexposed to two ranges of average electric fields (0.7–0.85 V/Ohm*m and up to 37–39 V/Ohm*m.
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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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Difraction in a scanning electron microscopie
Řiháček, Tomáš ; Mika, Filip ; Matějka, Milan ; Krátký, Stanislav ; Müllerová, Ilona
Manipulation with the primary beam phase in a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) has drawn significant attention in the microscopy community in the recent years. Although a few applications were found long before, some are still subjects of a future research. One of them is the use of electron vortex beams, which has very promising potential. It ranges from probing magnetic materials and manipulating with nanoparticles to spin polarization of a beam in an electron microscope.\nThe methods for producing electron vortex beams have undergone a lot of development in recent years as well. The most versatile way is holographic reconstruction using computer-generated holograms modifying either phase or amplitude. As the method is\nbased on diffraction, beam coherence is a very important parameter here. It is usually performed in TEM at energies of about 100 – 300 keV which are well suited for diffraction on artificial structures for two reasons. The coherence of the primary beam is often reasonable, and the diffraction pattern is easily observed. This is however not the case for a standard scanning electron microscope (SEM) with typical energy up to 30 keV.
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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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Bandpass filter for secondary electrons in SEM - experiments
Mika, Filip ; Konvalina, Ivo ; Krátký, Stanislav ; Müllerová, Ilona
Bandpass energy filtering using a through-the-lens secondary electron (TLD) detector in a field emission gun SEM (FEG-SEM) has been known over a decade. During energy filtering, image contrast is changed and new information about the material can be observed. Our motivation for this study was to compare theoretical calculations with the experimental data\nof the SE bandpass energy filter in Magellan 400 FEG SEM. The TLD detector works as a bandpass energy filter for the special setup of electrode potentials inside the objective lens, with the positive potential on the specimen regulating the energy window.
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New detectors for low-energy BSE
Lalinský, Ondřej ; Schauer, Petr ; Kučera, M. ; Hanuš, M. ; Lučeničová, Z.
Backscattered electrons (BSE) are mostly used to study the specimen’s topography. Nowadays, low energy (units of keV) electron beam imaging is often necessary for example for the research of nanomaterials, biomaterials or semiconductors. Because BSE detectors are mostly non-accelerating or low-accelerating, electrons with approximately the same energy as primary beam (PB) have to be detected. Therefore, BSE detectors need to become optimized for such low-energy electrons. For the scintillation detectors, the biggest problem probably lies in the scintillator. Semiconductor detectors aren’t studied in this work. Cerium activated bulk single crystals of yttrium aluminium garnet (YAG:Ce)Ce(X):Y(3-X)Al(5)O(12) are widely used as scintillators for the detection of high-energy backscattered electrons (BSE). However, commonly used YAG:Ce single crystal strongly loses its light yield (LY) with the decrease of the PB energy. As possible available alternatives for this application, bulk single crystals of yttrium aluminium perovskite (YAP:Ce) Ce(x)Y(1-X)AlO(3) and CRY018 can be predicted. However, similar LY drop can be expected also with these scintillators.
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