In 2018, Cornell researchers built a high-powered detector that, in
combination with an algorithm-driven process called ptychography, set a world
record by tripling the resolution of a state-of-the-art electron microscope.
As successful as it was, that approach had a weakness. It only worked with
ultrathin samples that were a few atoms thick. Anything thicker would cause
the electrons to scatter in ways that could not be disentangled.
Now a team, again led by David Muller, the Samuel B. Eckert Professor of
Engineering, has bested its own record by a factor of two with an electron
microscope pixel array detector (EMPAD) that incorporates even more
sophisticated 3D reconstruction algorithms.
The resolution is so fine-tuned, the only blurring that remains is the
thermal jiggling of the atoms themselves.
The group's paper, "Electron Ptychography Achieves Atomic-Resolution Limits
Set by Lattice Vibrations," published May 20 in Science. The paper's lead
author is postdoctoral researcher Zhen Chen.
"This doesn't just set a new record," Muller said. "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."
Ptychography works by scanning overlapping scattering patterns from a
material sample and looking for changes in the overlapping region.
"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."
The detector is slightly defocused, blurring the beam, in order to capture
the widest range of data possible. This data is then reconstructed via
complex algorithms, resulting in an ultraprecise image with picometer
(one-trillionth of a meter) precision.
"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," Muller said. "When we talk about
temperature, what we're actually measuring is the average speed of how much
the atoms are jiggling."
The researchers could possibly top their record again by using a material
that consists of heavier atoms, which wobble less, or by cooling down the
sample. But even at zero temperature, atoms still have quantum fluctuations,
so the improvement would not be very large.
This latest form of electron ptychography will enable scientists to locate
individual atoms in all three dimensions when they might be otherwise hidden
using other imaging methods. Researchers will also be able to find impurity
atoms in unusual configurations and image them and their vibrations, one at
a time. This could be particularly helpful in imaging semiconductors,
catalysts and quantum materials—including those used in quantum computing—as
well as for analyzing atoms at the boundaries where materials are joined
together.
The imaging method could also be applied to thick biological cells or
tissues, or even the synapse connections in the brain—what Muller refers to
as "connectomics on demand."
While the method is time-consuming and computationally demanding, it could
be made more efficient with more powerful computers in conjunction with
machine learning and faster detectors.
"We want to apply this to everything we do," said 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, part of
Cornell's Radical Collaboration initiative. "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?"
Reference:
Electron ptychography achieves atomic-resolution limits set by lattice
vibrations. Science, 21 May 2021: DOI:
10.1126/science.abg2533