Researchers in Ben McMorran’s University of Oregon physics lab had a great 2018, publishing four papers about their efforts to bring new life to scanning transmission electron microscopes for medical and materials research.
They’ve created a technique, STEM holography, that sends electrons along two separate paths, one going through a sample and one not. This allows them to measure the delay between them to create a high-resolution image. It provides improved atomic resolution of a sample’s outer structure and unveils previously unseen interfaces between a sample and underlying material.
The researchers have tested their technique on gold nanoparticles, carbon substrates and electrical fields. Eventually, it could be tweaked for use on live biological samples, said McMorran, an associate professor in the Department of Physics.
“This technique allows us to study materials at high resolution, measure them accurately and understand them better than was possible before,” said doctoral student Fehmi Yasin. “Can we image biomolecular materials at atomic resolution without destroying them? No yet, but our technique is a good first step.”
Researchers in Germany, Japan and the United States theorized 30 years ago that such an approach was possible, but available technology did not allow them to demonstrate it as a practical imaging technique, Yasin said. UO researchers have now shown — using microscopes at the UO, Lawrence Berkeley National Laboratory and Hitachi Ltd. Research and Development Group in Japan — that STEM holography works.
The technique builds on electron holography, another recent advance that requires state-of-the-art, cost-prohibitive electron guns, specially built apertures and highly stable power supplies to deliver atomic-scale resolution.
“Using flexible STEM holography, an offshoot we developed in collaboration with Toshiaki Tanigaki at Hitachi, we now can capture with more precision the interesting geometries of materials,” Yasin said, “Previously, the field of view of STEM holography was limited to maybe 30 nanometers. Using flexible STEM holography expands the field of view.”
The first transmission electron microscope was made in Germany by Max Knoll, an electrical engineer, and Ernst Ruska, a physicist, in 1931. The first commercial version emerged in 1939. Ruska won the Nobel Prize in physics for his efforts in 1986.
The multimillion-dollar microscopes create micrographs as a beam of electrons passes through a thin slice of a sample. Traditionally in scanning transmission electron microscopes, magnetic fields are used to focus the beam to an atom-sized spot of a sample. That beam then is scanned across a sample, but large numbers of electrons are required to see anything because most of them go through a sample without getting deflected.
The UO approach places a diffraction grating above a sample, creating additional beams hitting the sample and a hologram below it. That captures signals from electrons that are not scattered and details about how others are slowed as they pass through a sample.
The recent series of papers confirmed that STEM holography matches computer simulations.
“We put the electron microscope in conditions where we could isolate the signal that we care about, and we looked at several different kinds of samples,” said former UO doctoral student Tyler Harvey, now a postdoctoral researcher at the University of Gottingen. “We also simulated images of one sample and found that the simulations matched the experiment very well.”