electron microscopy



Comparison of Electron (ED) and X-ray Diffraction (XRD)

Both, ED and XRD, are caused by constructive interference of scattered waves, and the same fundamental laws (e.g., Bragg law, extinction rules) can be applied for the interpretation of the resulting diffraction patterns. In both cases, diffraction patterns of powders and of single crystals appear.

However, ED and XRD show some distinct differences:

1. The wavelength of electrons (e.g., 1.97 pm for 300 keV electrons) is much shorter than that of X-rays (about 100 pm). Therefore, the radius of the Ewald sphere is much larger and more reflections are observed by ED than by XRD.

2. The diffraction angles are very small in ED: 0 < Θ < 2° (cf., XRD: 0 < Θ < 180º)

3. Electrons are scattered by the positive potential inside the electron cloud (Coulomb interaction), while X-rays interact with the electron cloud. As the result, the interaction of electrons with matter is much stronger (106-107×) than that of X-rays.
This has the advantage that the diffracted electron beams have a high intensity and exposure times are in the order of a few seconds. ED patterns can directly be observed on the viewing screen of the electron microscope. Thus, orienting a crystal along a direction can be easily achieved by tilting while observing changes of the ED pattern simultaneously. Furthermore, diffraction patterns can be obtained from very small crystals selected with a diffracted aperture (Selected Area Electron Diffraction SAED) and by a focused electron beam even from nm-sized regions (Convergent Beam Electron Diffraction CBED).
The disadvantage of the strong interaction is that multiple scattering plays an important role, and the intensities of the reflections are much influenced by this dynamical effect. This makes structure determination from ED more difficult and less reliable than that from XRD data. A method for getting more reliable quantitative data is precession diffraction.


ED:Basics | Bragg law | Ewald sphere | Examples


ETH Zürich | ETH chemistry department | ETH inorganic chemistry

modified: 6 February, 2015 by F. Krumeich | © ETH Zürich and the authors