EFTEM

Energy-filtered transmission electron microscopy (EFTEM) is a family of imaging techniques that utilize properties of the energy loss spectrum to increase contrast, reduce the effects of chromatic aberration, and create unique contrast effects in the image. Key applications include:

• Contrast enhancement – Improves contrast and resolution in images and diffraction patterns by allowing only a narrow band of electron energies to form the image
  
  • Zero-loss filtering – Boosts contrast and resolution through the removal of inelastically scattered electrons
  • Most probable loss imaging – Improves signal-to-noise ratio (SNR) in thick samples and tomography applications
  • Contrast tuning – Highlights a particular material phase or property in the image
  • Pre-carbon imaging – Boosts contrast of non-carbon structures in unstained biological and polymer samples
• Mapping – Creates elemental/chemical maps at nanometer resolution by forming images with inelastically scattered electrons
  
  • 2-window jump-ratio imaging
  • 3-window elemental mapping
  • Chemical state mapping

The principle behind EFTEM is to illuminate a very thin specimen with a beam of high-energy electrons. Some electrons interact with the specimen, resulting in elastic or inelastic scattering. Inelastic scattering results in both a loss of energy and a change in momentum, which, in the case of inner shell ionization, is the energy loss characteristic of the element with which the electron interacted.

After the electron energy loss spectrum forms in the energy filter, an adjustable energy slit allows only electrons that have not lost energy to pass through to form the image. This is known as zero-loss filtering. The filtering prevents inelastically scattered electrons from contributing to the image plus enhances contrast image and resolution. In addition to zero-loss filtering, you can adjust the system to select electrons that have lost a specific amount of energy to obtain additional contrast effects and compositionally sensitive images.

Jump-ratio imaging is a technique that requires two energy-filtered images: one placed just before the ionization edge (pre-edge) and one placed just after the edge (post-edge). The resultant images are divided pixel-by-pixel to yield a qualitative map that is bright where the element is present and dark where it is not.

The 3-window technique requires two images before the ionization edge and one after. You can use the pre-edge images to compute the approximate background contained in the post-edge window. Once the background is determined and removed pixel-by-pixel, the resultant map shows a signal proportional to the sample's element concentration.

Alternatively, you can obtain a series of images over a broad range of energies. Results will then contain a continuous range of energies. This allows quantitative analysis and improves the accuracy of mapping where more than one element is involved. This EFTEM data stack is known as a spectrum image.