Macro lenses are awesome photographic tools because they let you see the world around you with greater detail and in fresh, new ways. But what if you had a way to see even closer, so close that you could see actual atoms? This is what a research team led by David Muller has been doing at Cornell University. In 2018, researchers at Cornell built a high-powered detector that, when combined with an algorithm-driven process called ptychography, captured an image of atoms at triple the resolution of a state-of-the-art electron microscope. Three years later, Muller, the Samuel B. Eckert Professor of Engineering at Cornell University, is leading research with an even more impressive detector.
The team’s paper, ‘Electron Ptychography Achieves Atomic-Resolution Limits Set by Lattice Vibrations,’ outlines a new electron microscope pixel array detector (EMPAD) that includes more sophisticated 3D reconstruction algorithms. The combination of the EMPAD and algorithms is so finely tuned that the only blurring of atoms in the image is due to the atoms’ ‘thermal jiggling.’
‘This image shows an electron ptychographic reconstruction of a praseodymium orthoscandate (PrScO3) crystal, zoomed in 100 million times.’ Image and caption credit: Cornell University, 2021.
‘This doesn’t just set a new record,’ said Muller, ‘It’s reached a regime which is effectively going to be an ultimate limit for resolution. We basically can now figure out where the atoms are in a very easy way. This opens up a whole lot of new measurement possibilities of things we’ve wanted to do for a very long time. It also solves a long-standing problem – undoing the multiple scattering of the beam in the sample, which Hans Bethe laid out in 1928 – that has blocked us from doing this in the past.’
This scattering problem impacted Muller’s team back in 2018, image seen below. At that time, given the detector and algorithm they were using, the ptychography process was limited by the thickness of the sample. They could only image samples that were only a few atoms thicker. Thicker samples would cause electrons in the sample to scatter in ways that were then impossible to disentangle.
‘A ptychographic image of two sheets of molybdenum disulfide, with one rotated by 6.8 degrees with respect to the other. The distances between individual atoms range from a full atomic bond length down to complete overlap.’ Image and caption credit: Cornell University, 2018
‘Ptychography works by scanning overlapping scattering patterns from a material sample and looking for changes in the overlapping region,’ said David Nutt of Cornell University. ‘We’re chasing speckle patterns that look a lot like those laser-pointer patterns that cats are equally fascinated by,’ Muller said. ‘By seeing how the pattern changes, we are able to compute the shape of the object that caused the pattern.’
Whereas traditional photographers typically aim to achieve perfect focus, when capturing images to atoms, it’s better to have the EMPAD slightly defocused. This allows the team to capture a wider range of data, which can then be reconstructed via complex algorithms to create a final, precise and sharp image. The image is precise down to one-trillionth of a meter, or a picometer.
‘With these new algorithms, we’re now able to correct for all the blurring of our microscope to the point that the largest blurring factor we have left is the fact that the atoms themselves are wobbling, because that’s what happens to atoms at finite temperature,’ said Muller.
‘This schematic shows how an electron probe is defocused to capture a wide range of data that is reconstructed into an ultraprecise image. The bottom three images are the diffraction patterns simulated when the probe is illuminated at the positions circled above.’ Image and caption credit: Cornell University, 2020
While Muller believes the team has reached a plateau, it’s possible the team could outdo itself yet again by using a material with heavier atoms, which would wobble less, or by reducing the temperature of the sample. Although, even so, atoms still experience quantum fluctuations, so the improvement may not be large, certainly not as large as the improvement from 2018’s image to the image published this month.
The team’s imaging method has possible applications outside of academia. Being able to locate individual atoms in three dimensions could prove useful when searching for impurities in semiconductors, catalysts and quantum materials, such as those used in quantum computing. The imaging method could be used on biological cells or tissues, perhaps even synapse connections inside the brain.
Muller, who co-directs the Kavli Institute at Cornell for Nanoscale Science and co-chairs the Nanoscale Science and Microsystems Engineering (NEXT Nano) Task Force, says, ‘We want to apply this to everything we do. Until now, we’ve all been wearing really bad glasses. And now we actually have a really good pair. Why wouldn’t you want to take off the old glasses, put on the new ones, and use them all the time?’
The paper’s lead author is postdoctoral researcher Zhen Chen. Co-authors include Darrel Schlom, Yi Jiang, Yu-Tsun Shao, Megan Holtz and researchers from the Paul Scherrer Institute and the Leibniz Institute for Crystal Growth. The research was supported by the National Science Foundation through Cornell’s Platform for the Accelerated Realization, Analysis and Discovery of Interface Materials (PARADIM). The researchers also used the Cornell Center for Materials Research, which is supported by the NSF’s Materials Research Science and Engineering Center program.
Image credits: Photos courtesy of Cornell University