Energy filtered transmission electron microscopy

Energy Filtered Transmission Electron Microscopy ( engl. energy filtered transmission electron microscopy, EFTEM ) is a variant of the transmission electron microscopy. It utilizes the kinetic energy of the different electrons of the sample after passage of a chemical analysis of the sample.

Principle

Irradiation of a very thin sample by a beam of high-energy electrons then passes through a large portion of the electrons, the sample unhindered, but occurs with a part of the sample interact. They are elastically scattered and inelastically under direction change with a change of direction and speed. The energy loss of inelastically scattered electrons is characteristic of certain interactions, such as the ionization of an atom.

If only the number of electrons counted with a specific energy loss, so operates one electron energy loss spectroscopy ( EELS short, of English: electron energy loss spectroscopy). Using typical intensity gradients in an EELS spectrum ( about one ionization ), elements can be identified.

In EFTEM only certain electron kinetic energy can be added ( a section of the EELS spectrum) for imaging. The image intensity can then be associated with a specific interaction process and makes it possible to make the distribution of elements in the sample figuratively visible.

For this purpose, in the simplest case, two images are taken: An image with electrons whose energy loss corresponds exactly to the ionization energy of a particular element, and an image with an energy loss just before the ionization. A division of these two images ( " edge: fore side " ) shows that bright image areas where the element is particularly strong.

Technology

To select electrons of a given 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 the electron, the lower is its deflection angle. Several magnetic lenses eventually form from the electron spectrum. Using a mechanical slot of the desired energy is selected from this range. This may for example correspond to the characteristic absorption region of a chemical element. With other lenses creates a distribution image of this element. The combination of magnetic field and imaging lenses is referred to as an energy filter.

• Electron optics