Electron Diffraction

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Student authors: Mint Pungsrisai 2019

Electron diffraction technique utilizes the wave nature of electron in studying the crystal structure of the sample of interest in terms of chemical positions and nanoscale’s atomic positions with high precision. This technique studies the phenomenon of the diffraction pattern resulting from the interference of a beam of electrons and the crystalline materials. The technique is usually performed in a transmitting electron microscope (TEM), and scanning electron microscope (SEM) as electron backscatter diffraction.

How electron diffraction works

The sample requires to be really thin so that it is transparent to electrons. In the instrument, electrons are accelerated in order to create an electron beam consisting of high-speed electrons with a short and known wavelength that is comparable to the spacing in the crystal structure. The beam is shined through a thin layer of a sample whose crystalline structure acts as a diffraction grating. Then, the electron beam is scattered into a diffraction pattern. The result diffraction pattern can be observed on a screen or film.

How to interpret the data

Figure \(\PageIndex{1}\): An example of an electron microscopy image of inorganic tantalum oxide.

The circle indicates the thin region that was further analyzed.

Figure \(\PageIndex{2}\): Electron diffraction pattern of tantalum oxide shown above.

The brightest spot in the middle is the beam that passed through the sample without diffracting.

The area that is usually chosen for the crystal structure analysis is the very thin area close to the edge of the sample, shown in Figure 1. The observed pattern in Figure 2 shows the interference of the diffracted electron. The images are actually very sensitive to defocus of the objective lens, bright spots could turn into dark spots on the other objective lens. The thickness of the sample also influences the contrast of the images making the analysis challenging. There are two methods to solve this problem, which are exit-wave function reconstruction method, and crystallographic image processing. The exit-wave function reconstruction requires several TEM images with different defocus for the computation of the exit-wave function, while crystallographic imaging processing only needs one image, which is suitable for the samples that could be damaged by the electron beam. Further improvement can be performed using crystal tilt compensation and searching for the most likely projected symmetry.

Good literature examples

1. Bendersky, L. A.; Gayle, F. W. Electron Diffraction Using Transmission Electron Microscopy. J Res Natl Inst Stand Technol. 2001. 106(6), 997–1012.

Works cited

https://en.Wikipedia.org/wiki/Electron_diffraction

https://en.Wikipedia.org/wiki/Electron_crystallography

https://en.Wikipedia.org/wiki/Crystallographic_image_processing

https://www.britannica.com/science/electron-diffraction

Kohli, Rajiv. Developments in Imaging and Analysis Techniques for Micro- and Nanosize Particles and Surface Features. In Developments in Surface Contamination and Cleaning: Detection, Characterization, and Analysis of Contaminants; Kohli, R., Mittal, K. L., Eds.; Elsevier: Cambridge, MA, 2012; pp 215-306.

Useful resources for in-depth reading

Sarney, W. L. Understanding Transmission Electron Microscopy Diffraction Patterns Obtained From Infrared Semiconductor Materials. Adelphi, MD: Army Research Laboratory, 2013.