A comprehensive analysis of electron diffraction limitations

Nanocrystalline materials are common in different fields of both materials and life science nowadays. The minimal crystal size can either be obtained on purpose, since it is the nano-crystallinity that is responsible for the peculiar properties of the material, or it can be an undesired effect of the crystallization process. Solid state syntheses very often deliver powder polycrystalline samples in which the grain size is of the order of 1 micron. Organic molecules that are of pharmaceutical interest cannot be crystallized into a large single crystal. Proteins sometimes cannot be crystallized at all, or the only output of a crystallization attempt are precipitates.

Limitations of powder diffraction mode (PWD)

When the size of the crystal falls below a few tens of microns, the standard diffraction techniques (X-ray or neutrons) are unable to deliver acceptable signals in single crystal diffraction (SCD) mode, and the powder diffraction mode (PWD) is the only available option. In powder diffraction, the signal is enhanced by the large number of crystals simultaneously diffracting, but the information is only one-dimensional. While in single crystal diffraction we have access to the entire three-dimensional reciprocal space with an unambiguous integration of every reflection, in powder diffraction we can measure only the scattering intensity vs. scattering angle. This causes overlapping of those reflections which, although far in reciprocal space, are indexed by reciprocal vectors with similar lengths, but also the overlapping of reflections belonging to different crystalline phases if the sample is not pure. The overlapping prevents proper integration and a reliable intensity integration with the risk of failure of the structure analysis.

Creating a strong case for electron diffraction

The stronger interaction of the electron with matter with respect to x-ray and neutrons and the possibility to focus the electrons in bright beams of a few nanometers make electron diffraction the technique of choice in the case of nanocrystals. Crystals of a few tens of nanometers diffract as single crystals once they are illuminated with an electron beam, and a single crystal diffraction experiment can be performed on them. While originally the application of electron diffraction to structure solution was limited due to the dynamical character of the electron scattering that spoils the simple linear relation between reflection intensities and the square modulus of the structure factor, since 2009 the invention of 3D electron diffraction (3D ED) methods has been opening up a completely new perspective. In 3D ED, a sequence of patterns is collected while the sample is rotated around the goniometer axis and the reciprocal space of the crystal is fully sampled. Thanks to geometry and the reciprocal space integration, the intensities collected are not severely affected by dynamical scattering and can be successfully used for structure solution.

The method has already been successfully applied to a wide class of crystals. Initially, it was extensively applied to inorganic crystals that are moderately beam sensitive. Successively, the advent of single electrons detectors for diffraction and fast diffraction procedures has enabled data collection in very low doses, opening up the possibility of investigating organic and macromolecular crystals.

The need for a dedicated electron diffractometer

3D ED is now a well-established technique, with a solid theoretical background and a great deal of successful results. However, it is not widespread – still only few laboratories have mastered it. The reason is simple: 3D ED needs a transmission electron microscope (TEM) to be performed, and transmission electron microscopes are designed and optimized mainly for imaging. To perform a 3D ED experiment, it takes a fully committed scientist who tries to use a microscope as an electron diffractometer. For a fully automatic and fast experiment on a crystal of a few hundreds of nanometers, the TEM goniometer should have a mechanical stability of the same order of magnitude as the crystal size for a rotation of more than 100°, and there are no TEM goniometers specifically designed for that. For this reason, a large number of very beam-sensitive and extremely interesting organic crystals cannot be studied even if the methodology is basically already available.

If a dedicated electron diffractometer – where the electron optics, the sample stage, the detectors, the imaging system and the control software are completely designed and optimized for diffraction – were to be introduced on the market, it would be a breakthrough in the field, removing the problematic step of growing a large single crystal to determine the crystal structure.

More Scientific Content from ELDICO:

What are Electron Diffraction and Nanocrystallography and why are they important? (White Paper) — Rapid Structure Determination of Microcrystalline Molecular Compounds Using Electron Diffraction (Peer-Reviewed Paper) — Can Electron Diffraction distinguish between carbon and nitrogen atoms? (Application Note).