Energy-filtered transmission electron microscopy

The energy-filtered transmission electron microscopy (engl. Energy filtered transmission electron microscopy ; EFTEM) is a development of the transmission electron microscopy (TEM, electron energy loss spectroscopy). With it, only inelastically scattered electrons with certain characteristic energies are used for the image display , whereby only these contribute to the generation of energy-filtered electron diffraction images. This enables the distribution of chemical elements in the image field to be measured quickly and effectively.

principle

In the EFTEM (energy filtering TEM), as in the normal TEM, the image is created by the electron scattering in the sample. A large part of the electrons penetrate the sample unhindered, but some interact with the sample. They are scattered elastically with a change in direction and inelastically with a change in direction and speed. The energy loss of inelastically scattered electrons is characteristic of certain interactions, such as the ionization of an atom.

While with conventional TEM the electrons scattered at certain angles are selected for image generation , with EFTEM the electrons are also selected according to their energy. For this purpose, only the electrons with a certain energy loss are selected from the spectrum of the transmitted electrons with the help of a spectrometer . The parts that reduce the contrast are masked out. Only electrons with a specific energy loss contribute to the image, which improves all types of contrast. Since the method has no effect on the spatial resolution, both thin and non-contrasted, as well as frozen or unconventionally thick specimens can be displayed with excellent contrast. Small, diffusible molecules such as ions can be most reliably detected in cryopreparations in the EFTEM. In addition, electrons with a very special scattering behavior can be selected in this way, which lead to structure- or element-specific images.

The following methods and techniques are used for automatic recording and analysis of spectra and energy-filtered images.

The processing and creation of ESI (electron spectroscopic imaging) distribution images is mainly used to examine radiation-sensitive preparations.
The editing and creation of parallel and serial electron energy loss spectra (EELS short of English: electron energy loss spectroscopy ) of image series (image EELS) of different energy losses of the analysis and imaging is used lowest element concentrations in the frame and allows an improvement in the local spectral sensitivity. Only after the creation are individual images selected and it is thus determined which energy losses are used to create the resulting element distribution image for further analysis. The parallel recording of energy loss spectra with a CCD camera for the rapid identification of elements and their distribution in the sample is known as parallel EELS. Elements can be identified using typical intensity profiles in an EELS spectrum (such as an ionization edge). In the simplest case, two images are recorded for this purpose: an image with electrons whose energy loss corresponds exactly to the ionization energy of a certain element, and an image with an energy loss immediately in front of the ionization edge. A division of these two images ("edge: front edge") shows those areas of the image in which the element is intensified.
as well as a quantification and the determination of the layer thickness.

technology

To select electrons of a certain kinetic energy, they are first sorted according to their energy. This happens because the electrons pass through a magnetic field in which they are deflected due to the Lorentz force . The faster an electron is, the smaller its deflection angle. Finally, several magnetic lenses map the electron spectrum. The desired energy range is selected from this spectrum with the aid of a mechanical slot. This can for example correspond to the characteristic absorption range of a chemical element. With further lenses a distribution image of this element is created. The combination of a magnetic field and imaging lenses is called an energy filter.

Individual evidence

^ ^A ^b C. Barry Carter, David B. Williams: Transmission Electron Microscopy Diffraction, Imaging, and Spectrometry . Springer, 2016, ISBN 978-3-319-26651-0 , pp. 378 ( limited preview in Google Book search).
↑ ^a ^b ^c ^d ^e ^f ^g Microscopic technique: Romeis - microscopic technique . Springer-Verlag, 2015, ISBN 978-3-642-55190-1 , p. 33 ( limited preview in Google Book search).

[C._Barry_Carter,_David_B._Williams-1] A ^b C. Barry Carter, David B. Williams: Transmission Electron Microscopy Diffraction, Imaging, and Spectrometry . Springer, 2016, ISBN 978-3-319-26651-0 , pp. 378 ( limited preview in Google Book search).

[Mikroskopische_Technik-2] ↑ ^a ^b ^c ^d ^e ^f ^g Microscopic technique: Romeis - microscopic technique . Springer-Verlag, 2015, ISBN 978-3-642-55190-1 , p. 33 ( limited preview in Google Book search).