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What is Diffraction Imaging and SPED?
Electron diffraction is one of the most useful techniques in TEM. Crystallographic information about the structure, composition of the sample and defects within it can be obtained with a few orders of magnitude better resolution than other techniques. The diffraction patterns from thin crystalline materials consist of very high-intensity peaks due to Brag scattering, however, with a very sensitive detector, scientists can recover signals from defects, inelastic scattering, diffuse scattering and others. If the detector is not only sensitive but also fast, diffraction tomography and precession methods become more practical.
Maximising information from diffraction experiments
One of the key advantages of electron diffraction with a parallel probe is that it can be done with a very small electron dose. In this case, a relatively larger area defined by aperture is illuminated and diffraction image collected. One of the disadvantages of the technique was that microscopists had to manually select a small area of the sample from which to collect the diffraction patterns. However, the development of fast framing detectors such as MerlinEM in recent years made collections of scanning diffraction datasets with parallel illumination highly attractive. The illustration image is an example of such a dataset (it can be argued that this is also an example of a 4D-STEM technique). In this case, diffraction patterns from a larger area of the sample were averaged and used to determine what crystallographic structure was responsible for a loss of efficiency of halide perovskite materials solar cell materials now published in Nature (link below).
[Obtained under CC BY 4.0 licence from Supporting Data repository, Doherty, TAS. et al, Nature 580.7803 (2020): 360-366.]
Precession for easier interpretation
Interpretation of electron diffraction experiments can be difficult for samples with strong dynamic diffraction effects – specialist multistep simulations are usually required. Because the dynamic diffraction is very angle and orientation sensitive, beam precessing techniques were developed to help with overcoming the problem. By precessing the electron beam over a particular spot of the sample, the dynamic contribution to the diffraction can be averaged out and the data will mostly correspond to a kinematical pattern which is interpretable with direct reconstruction methods. As mentioned before, the characteristics of the MerlinEM detector are very applicable in diffraction imaging and precession diffraction experiments are not an exception. NanoMEGAS, the provider of precession diffraction solutions, has integrated MerlinEM detector to their toolkit which allows users to collect faster and more detailed precession and orientation maps of their sample. In the image on the right, we show a comparison of the conventional camera mapping and mapping using the MerlinEM direct electron counting detector demonstrating the enhancement available to our users.
[Data courtesy Dr Ian MacLaren, University of Glasgow]
The Quantum C100 detector, has been optimised for 100 keV and has been developed as a key component to facilitate economic access of the emerging field of imaging around the world.Read more