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New scanning electron microscopes for medical and material research

Credit: University of Oregon

Researchers Ben McMorran at the Oregon University Physics Laboratory had a great 2018 with four articles about their efforts to get a new life for scanning electron microscopes for medical and material research.

They have created the technique, STEM holography, which sends electrons through two separate paths, one passes through the sample and one does not. This allows them to measure the delay between them to create a high resolution image. This provides improved sample resolution of the outer structure of the sample and reveals previously unseen interfaces between the sample and the underlying material.

Researchers have tested their technique with gold nanoparticles, carbon substrates and electric fields. Finally, it could be inherited for use in living biological samples, said McMorran, Associate Professor of Physics.

"This method allows us to study high-resolution materials, accurately measure and better understand them than was possible before," said PhD student Fehmi Yasin. "Can we display biomolecular materials in atomic resolution without destroying them? Not yet, but our technique is a good first step."

Researchers in Germany, Japan and the United States 30 years ago theoretically claimed that such an approach was possible, but the available technology did not allow it to be demonstrated as a practical imaging technique, said Yasin. UO researchers have now demonstrated – using microscopes UO, Lawrence Berkeley National Laboratory and Hitachi Ltd. research and development group in Japan – STEM holography.

This method is based on electron holography, another recent achievement that requires state-of-the-art, inaccessible electron cannons, specially designed openings, and highly stable power supplies to deliver atomic scale resolution.

"Using flexible STEM holography, we developed collaboration with Toshiaki Tanigaki at Hitachi, now we can get more accurate material with interesting geometry," Yasin said, "In the past, the field of STEM holography was limited to up to 30.

The first transmission electron microscope in Germany was made by electrical engineer Max Knoll and physicist Ernst Ruska in 1931. The first commercial version appeared in 1939. Ruska won the Nobel Prize in Physics for his efforts in 1986.

The multi-million dollar microscopes create micrographs because the electron beam crosses the plan sample. Traditionally, magnetic fields have been used in scanning transmission electron microscopes to focus light on the atomic size of the sample. The plaque is then scanned through the sample, but to see something, a large number of electrons are needed, as most of them pass through the sample without departing.

The UO approach places the diffraction grating over the sample, creating additional beams that get into the sample and a hologram underneath it. This includes signals from non-scattered electrons and information about how others are slowed down when they pass through the sample.

Recent series of publications confirmed that STEM holography meets computer simulations.

Credit: University of Oregon

"We placed an electron microscope in conditions where we could isolate the signal we care about and we looked at several different types of samples," said former UO doctoral student Tyler Harvey, who is now a postdoctoral student at the University of Gotingen. "We also modeled the images of one sample and found that the simulation was very good for the experiment."

The December book, led by Harvey magazine Suitable physical examination, The UO team described in detail the technique and how it works theoretically.

In a separate document Nano lettersThe Yasin-led team showed that this method provides subnometer resolution images of carbon materials. Color is a thickness that complements the third dimension and improves the measurement.

The researchers noted that the images were as clear as expected with a small number of electrons.

"We think STEM holography will be a great tool for material science and biology," said Harvey. "This technique really stands out with electric and magnetic field imaging and can be done by making the majority of material scientists look after: see where the atoms are."

The ability to use this method in biological samples is far from that, but being able to do it safely could be a huge cost, Yasin said.

"We now have a lot of drugs that attack cancer," said Yasin. "But this composition is similar to our entire body, so these cancer drugs attack both diseased cells and other body cells at the same time. If we knew the state of each atom in the cancer cell, we could develop much better, more effective drugs without the deadly side effects."

For the first time, McMorran wrote about the idea of ​​using a hologram approach in the January 2011 Science document when he was with the National Institute for Standards and Technology in Maryland.

In their UO laboratory, supported by the National Science Foundation and the US Department of Energy, researchers have implemented four areas where everyone is looking for parts of image materials that are difficult to detect.

Four areas focus on transparent materials, including biomaterials or other organic molecules; electric fields such as charge and its distribution in separate transistors; magnetic fields, such as materials that are now on the hard disk and may be useful for spintronics; and electrons and qubitus to be used on quantum computers.

"Any of these four things may not work," said McMorran, who is also a member of the Material Science Institute and Oregon Center for Optical, Molecular and Quantum Science. "There may be a better technique that ends with what is best for some. We can develop a useful tool to get all four options or maybe just one of them. At the moment, all the arrows point to all four."

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More information:
Fehmi S Yasin et al. Electron interferometry by electron microscopy detached by road, t Physics Magazine D: Applied Physics (2018). DOI: 10.1088 / 1361-6463 / aabc47

Fehmi S. Yasin et al. Projection light atoms at subnanometer resolution: Holographic realization of scanning electron microscope Nano letters (2018). DOI: 10.1021 / acs.nanolett.8b03166

Fehmi S. Yasin et al. Adjustable path separated electron interferometer with amplitude grid plate Business Physics Letters (2018). DOI: 10.1063 / 1.5051380

Tyler R. Harvey et al. Interpretometer contrast scan electron microscope with diffraction grating splitter, Suitable physical examination (2018). DOI: 10.1103 / PhysRevApplied.10.061001

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