Analytical techniques in electron microscopes
There are many ways to extract information from an electron microscope. The following section aims at giving you a short overview over the most important techniques.
For an elaborate description please consult the book "Transmission Electron Microscopy" by Reimer, Kohl (Springer Verlag).
The periodic structure of a crystalline solid acts as a diffraction grating, scattering the electrons in a predictable manner. The diffraction pattern is typical for the structure of the crystal. A combination of diffraction and imaging mode of a TEM allows to identify lattice defects.
EELS and ESI
Both are techniques to analyze the energy distribution of the transmitted electrons, whose energy loss is element specific.
EELS was developed in the 1940s but became widely used not before the 1990s since it necessitates a good vacuum technology and powerful microscopes.
The energy loss of the electrons is analyzed to determine the underlying interaction and the kind of atoms involved. The amount of lost energy is measured with a spectrometer.
Electron energy-loss spectroscopy (EELS) records the intensity as a function of the energy loss from selected regions. Elemental mapping with electron spectroscopic imaging (ESI) uses energy-filtered images at an element-specific energy loss from which a background image has to be subtracted.
Ionization of an inner shell results in an unstable state. The inner orbit of the electron is taken by an electron of a higher orbit. The energy difference between these two states can be emitted in form of x-ray radiation, whose wavelength is typical for the atom. The strength of this typical radiation depends on the atom concentration in the sample. This type of element analysis works well for elements with an atomic number beyond 10, for atomic numbers less than 10 the yield of Auger electrons is higher than that of x-rays.
Thin specimens must be used to prevent multiple scattering. In bulk specimens the resolution is limited by the diameter of the electron-diffusion cloud to 0.1-1 µm.
Comparison of EDX and EELS
EELS shows a high detection efficiency for elements with low atomic numbers, whereas EDX is preferable for the detection of high atomic numbers. EELS is a very fast method and has a very good energy resolution (0.3 - 2 eV), but requires complex processing. EDX is slow and has a rather low energy resolution (>100 eV), but yields results which are easy to interpret.
For further information you may consult the webpages of the University of Zurich.
Ionization of an inner shell results in an unstable state. The inner orbit of the electron is taken by an electron of a higher orbit. The energy thus released can not only be emitted as x-rays but can also cause the emission of an electron. This electron is called an Auger electron after Pierre Auger who discovered this relaxation process. The energy of the Auger electron is characteristic of the element that emitted it, and can thus be used to identify the element. Neither hydrogen nor helium can be detected (because they are not able to produce Auger electrons) but all other elements, best those with low atomic numbers. The information delivered by Auger electrons is restricted to a depth of 0,5 - 3 nm, therefore this technique is best suited to surface analysis.
The detection of secondary or backscattered electrons allows a direct image of the specimen given by topographic and material contrast. The signal results from interactions at or at least near the surface. The intensity of BSE is related to the atomic number of the elements involved.
|diffraction||EELS (electron energy loss spectroscopy)||ESI (electron spectroscopic imaging)||EDXS (x-ray microanalysis)||Auger microscopy||surface imaging|
|interaction||elastic scattering at the nuclei||inelastic scattering||inelastic scattering||excitation of an electron from an inner shell, the recombination results in characteristic x-ray emission||excitation of an electron from an inner shell, the recombination results in emission of an Auger electron||excitation of a valence/conduction electron (secondary electron)|
|detection of||primary and scattered electrons||energy loss of the primary electrons||energy loss of the primary electrons||x-ray radiation||Auger electrons||secondary (SE) or backscattered electrons (BSE)|
|detection system||CCD, photographic emulsion, fluorescent screen||spectrometer behind the final image or imaging energy filter||filter + CCD, filter + emulsion, filter + fluorescent screen||energy dispersive Si or Ge detector||Auger-detector + spectrometer||SE-detector + screen|
|application||diffraction image||micro-analysis||element-specific image||micro-analysis||micro-analysis||surface image|
|used in||TEM||TEM, STEM||TEM, STEM||TEM, STEM, SEM||SEM||SEM|
|information about||crystal symmetry and orientation||element composition, chemical bonding||element distribution, chemical bonding||element composition||element composition||topography, potential, material contrast|
Except of ESI all of these techniques are applied by operating at a small electron probe, the spatial resolution is 0.2-100 nm.