Energy Filtered Transmission Electron Microscopy (EFTEM)

Energy Filtered Transmission Electron Microscopy (EFTEM)

Image formation using energy filtered transmission electron microscopy (EFTEM) is the same process that is found in cTEM and HRTEM imaging: electron lenses are used to magnify and focus the electron beam after it has passed through the specimen. The difference between EFTEM and these other TEM imaging techniques is that for EFTEM imaging, an "energy filter" is placed between the specimen and the plane where the final image is recorded. Because some of the electrons in the electron beam will lose energy when they interact with the specimen, the energy filter is used to select for image formation only the electrons that have a defined energy spread. Energy filters can be used to form images using those electrons that have not lost any energy, that have lost only a small amount of energy or that have lost discrete amounts of energy defined by the elements in the specimen.

Although the same fundamental physics of electron scattering gives rise to energy filtered transmission electron microscopy (EFTEM) and to electron energy loss spectroscopy (EELS), the two techniques differ in how the scattered electrons are used. EFTEM forms images (and electron diffraction patterns) using the scattered electrons while EELS is the analysis of the energy loss spectrum itself. These two are obviously related and most modern transmission electron microscopes (TEMs) that are equipped for one are capable of both. It can also be enormously informative to combine both EFTEM and EELS during the analysis of a single sample, though this is not always necessary (or done).

The various types of electron scattering and the signals produced, giving rise to auger electrons, second electrons (SE), characteristic X-rays, electron-hole pairs, Bremstrahlung, and inelstically scattered electrons.

Electrons interact with matter elastically (where no energy is lost to the interaction) and inelastically (where some amount of energy is lost), with significantly more inelastic events than elastic events. This implies that even if the electrons in the electron beam had exactly the same energy before interacting with the specimen, inelastic events as the beam passes through the specimen would produce an energy spread in the electrons after the specimen. An equivalent way to think about this energy spread is to think in terms of the wavelength, λ, of the electrons: even if all the electrons before the specimen have exactly the same wavelength, electrons after the specimen will have different wavelengths due to inelastic scattering. Various types of electron scattering, producing signals, and the inelastic scattering events (resulting in energy loss) give rise to the following signals:

  • Auger electrons
  • Secondary electrons (SE)
  • Characteristic x-rays
  • Electron-hle pairs
  • Bremstrahlung
  • Inelasticlaly scattered electrons

Energy Loss Spectrum

A diagram of the typical energy loss spectrum associated with any given element.

The energy loss spectrum associated with any given element and obtained using EELS has several regions where the different types of scattering events occur. For the discussion here, we will categorize these events into three regions:

The Zero Loss Region

This region of the spectrum contains both electrons that are unscattered (the vast majority of all electrons that pass through a thin specimen) and those that are scattered elastically.

The Plasmon Region

This region contains electrons that have lost a small amount of energy (usually defined to be less than 50 or 100 eV). This energy loss is due to many different processes that include interactions of the electrons in the beam with outer shell electrons, with the crystalline lattice if present (generally referred to as phonon effects) and with a compound's electronic band gap structure. There are also additional effects such as Čerenkov radiation that contribute to this energy loss region. For all atoms, there are significantly more inelastically scattered electrons in the plasmon region than in all other regions of the energy loss spectrum. The study of this low energy loss region is often referred to as valence electron energy loss spectroscopy (VEELS) and is a very active area both in terms of the theory behind it and in terms of using this region for analytical purposes.

Inner Shell Ionization

Energy losses beyond the plasmon region correspond to scattering events that involve the ionization of inner shell electrons. Since these energy losses are tied directly to specific inner shell ionization events, they are specific for not only the type of atom involved but also for the electronic state of that atom. This is the property of the energy loss spectrum that makes EELS such a powerful tool: not only can types of atoms be identified by their energy loss signature, but it is also possible to determine properties such as the oxidation state(s) of the identified atoms.

An ionization threshold energy is associated with any specific ionization event. This threshold causes the energy loss spectrum for such an event to exhibit a very sharp rise followed by a slow decay. This feature in the energy loss spectrum is normally called an "edge" because of this shape (and not a "peak" which implies a more Gaussian feature). These edges are classified as belonging to the K, L, M, N and O electron shells, which correspond to primary quantum numbers 1, 2, 3, 4 and 5 from atomic orbital descriptions. In addition, EELS edges beyond K have an associated number that can be related to the shape (subshell) of of the atomic orbital. For example, L1 refers to 2s electrons, M4,5 refers to 3d electrons and N2,3 refers to 4p electrons. In principle, there are separate edges at (for example) the L2 and L3 energies, but in practice these can only be resolved for very high ionization energies.