Beam-sensitive materials at atomic resolution


Rebecca Pool

Monday, January 22, 2018 - 16:30
High-resolution TEM image from a MOF UiO-66 crystal; arrows point to the benzene rings. Overlays are a simulated projected potential map and structural model, [KAUST].
Researchers from  King Abdullah University of Science and Technology (KAUST), Saudi Arabia, have developed new TEM methods to acquire atomic-resolution images of electron beam-sensitive metal organic frameworks.
Using their high resolution TEM imaging, Professor Yu Han from Chemical Science, Dr Kun Li, Director of the Imaging and Characterization Core Lab, and colleagues, could identify individual metal atomic chains, different surface terminations and benzene rings in the organic linkers of the MOFs.
"[Our methods] have significantly broadened the applications of high resolution TEM, and also provided beam-sensitive materials researchers with a powerful tool to investigate the structure of beam-sensitive materials in much more localized detail than traditional X-ray diffraction techniques permit," highlights Li.
High-resolution TEM images of MOF UiO-66 acquired from different crystallographic zone axes, [KAUST].
High-resolution imaging of electron beam-sensitive materials is one of the most difficult TEM applications with key challenges including the acquisition of images with extremely low beam doses, the search for crystal zone axes, and precise image alignment. 
Given this, Han, Li and colleagues first developed an algorithm to align the zone axis of the MOF crystal while keeping the specimen intact, in a single step.
But as Han points out: "Due to inherent issues when dealing with beam-sensitive materials, high resolution TEM would still produce blurred images mainly due to sample drift during exposure."
A series of successive short exposures was then taken with an amplitude filter method also developed to minimize noise and accurately align all the frames.
To reconstruct the structure of the MOFs, the researchers also devised a process that made use of the instability of beam-sensitive materials to determine the absolute defocus value from a purposely amorphized region.
The KAUST processes are not just limited to beam-sensitive materials.
For example, the method for zone axis alignment can be used to align nanosized crystals while the image alignment method also applies to noisy images with periodic features.
"[Our methods] will undoubtedly facilitate beam-sensitive materials researchers in the designing of new structures with enhanced performance," says Li.
Research is published in Science.
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