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 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 of these electrons will interact with the specimen and result 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, the energy loss is characteristic of the element the electron interacted with.
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 you position just before the ionization edge (pre-edge) and one you position just after the edge (post-edge). The resultant images are divided pixel-by-pixel to yield a qualitative map that is bright when 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 that is proportional to the element concentration in the sample.
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.
Midgley, P.A., Weyland, M., “3D electron microscopy in the physical sciences: the development of Z-contrast and EFTEM tomography”, Ultramicroscopy 96 (2003) 413-431
Egerton, R. F. Electron Energy Loss Spectroscopy in the Electron Microscope. Springer. 3rd ed. New York: 2011.