Energy filtered transmission electron microscopy
Energy-filtered transmission electron microscopy (EFTEM) is a technique used in Transmission electron microscopy
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

, in which only electrons of particular kinetic energies are used to form the image or diffraction pattern. The technique can be used to aid chemical analysis of the sample in conjunction with complementary techniques such as electron crystallography.

The Principle

If a very thin sample is illuminated with a beam of high-energy electrons, then a majority of the electrons will pass unhindered through the sample but some will interact with the sample, being scattered elastically or inelastically (phonon
In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

 scattering, plasmon
In physics, a plasmon is a quantum of plasma oscillation. The plasmon is a quasiparticle resulting from the quantization of plasma oscillations just as photons and phonons are quantizations of light and mechanical vibrations, respectively...

 scattering or inner shell ionisation
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...

). Inelastic scattering results in both a loss of energy and a change in momentum, which in the case of inner shell ionisation is characteristic of the element in the sample.

If the electron beam emerging from the sample is passed through a magnetic prism, then the flight path of the electrons will vary depending on their energy. This technique is used to form spectra in Electron energy loss spectroscopy
Electron energy loss spectroscopy
In electron energy loss spectroscopy a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected...

(EELS), but it is also possible to place an adjustable slit to allow only electrons with a certain range of energies through, and reform an image using these electrons on a detector.

The energy slit can be adjusted so as to only allow electrons which have not lost energy to pass through to form the image. This prevents inelastic scattering from contributing to the image, and hence produces an enhanced contrast image.

Adjusting the slit to only allow electrons which have lost a specific amount of energy can be used to obtain elementally sensitive images. As the ionisation signal is often significantly smaller than the background signal, it is normally necessary to obtain more than one image at varying energies to remove the background effect. The simplest method is known as the jump ratio technique, where an image recorded using electrons at the energy of the maximum of the absorption peak caused by a particular inner shell ionisation is divided by an image recorded just before the ionisation energy. It is often necessary to cross-correlate the images to compensate for relative drift of the sample between the two images.

Improved elemental maps can be obtained by taking a series of images, allowing quantitative analysis and improved accuracy of mapping where more than one element is involved. By taking a series of images, it is also possible to extract the EELS profile from particular features.

External links

Further reading

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