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Why use precession?
Why use precession electron diffraction
for nanocrystal structure analysis ?
X-ray crystallography is very well adapted to structure analysis of perfect single crystals larger than a few micrometers. X-ray crystal interactions are kinematical and thus the structure factors can be directly derived from the diffracted intensity data. For nanocrystals, powder X-ray diffraction may be used, but this technique presents severe limitations when the grain size is very small, when the powder is not well crystallized or when it contains several unknown phases. Then, it becomes very difficult to solve unknown structures from X-ray diffraction of nanocrystals ab-initio.
Due to its high lateral resolution, Transmission Electron Microscopy (TEM) is very well adapted to the imaging and the analysis of nanocrystals. By means of the electron micrograph of the studied specimen, it is possible to select (possibly in a defect free area) and probe a tiny area, smaller than the nanocrystal size, in order to obtain an electron diffraction pattern.
Despite these interesting features, electron diffraction was rarely used in the past as a standard tool for crystal identification mainly because the electron interactions with matter are about 10,000 times stronger than that of X‑rays. As a result, the scattering is not kinematic but dynamical, so that the diffracted intensities are so much altered that they cannot be trusted and used for crystal structure determination, unless the crystal thickness is very thin or very demanding dynamical calculations are undertaken.
The electron beam precession technique proposed by Vincent & Midgley offers a solution to this problem by decreasing the dynamical behaviour of electron diffraction. This technique is equivalent to the Buerger precession technique used in X-ray diffraction, where the specimen is precessed with respect to the incident X-ray beam. In the electron precession technique, the electron beam is tilted and precessed along a conical surface, having a common axis with the TEM optical axis.
As a result of this precession movement:
- only a few reflections are simultaneously excited,
- many more reflections are visible,
- the diffracted intensity of the beams is the integrated intensity.
- the resulting diffraction pattern can be considered less dynamical.
This means that kinematically forbidden reflections and multiple scatterings are greatly reduced, making space group identification easier. Several minerals, catalysts, and complex oxides have already been solved ab initio from quasi‑kinematical precession intensities [see references].
Every TEM (old or new) can be easily upgraded to perform precession electron diffraction by theSpinning Star precession device from NanoMEGAS ( see fig left).
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