Electron microscopy has allowed scientists to see individual atoms, but even at that resolution not everything is clear.
A team of researchers from both Cornell and the University of Chicago have developed a method for achieving ultra-high resolution without the need for “corrective lenses” for their microscope. The result is a world record for image resolution: In this case, of a layer of molybdenum disulfide just three atoms thick.
The lenses of electron microscopes have intrinsic imperfections known as aberrations, and special aberration correctors—“like eye glasses for your microscope,” said David Muller, the Samuel B. Eckert Professor of Engineering at Cornell University—have been developed over the years to correct these defects.
Aberration correctors only go so far, however, and to correct multiple aberrations, you need an ever-expanding collector of corrector elements. It’s like putting glasses on glasses on glasses—it becomes a bit unwieldy.
“It is like watching a high-def TV,” said Jiwoong Park, professor of chemistry and molecular engineering at the University of Chicago and coauthor on the paper. “You get to see so much more detail, including the tiniest blemishes—like a single missing atom!”
The microscopes you use in science class use light waves, but scientists who want to see down even further like to use electrons instead, because the wavelength of an electron is many times smaller than those of visible light. The trouble is that electron microscope lenses are not commensurately precise.
Typically, Muller said, the resolution of an electron microscope depends in large part on the numerical aperture of the lens. In a basic camera, this is notated with the “f-number”—the smaller the number, the better the resolution.
In a good camera, the lowest f-number or “f-stop” might be a little under two, but “an electron microscope has an f-number of about 100,” Muller said. Aberration correctors can bring that number down to about 40, he said, which is still not great.
Image resolution in electron microscopy has traditionally been improved by increasing both the numerical aperture of the lens and the energy of the electron beam, which does for the microscope what light does for a camera or an optical microscope: illuminates the subject.
But increasing the energy of the electron beam also means you might damage the sample you’re trying to look at. And by the time you get down to the sub-ångström level—where you can easily see individual atoms—the balance is very delicate.
Using a special device and a technique known as ptychography, the group was able to reach a resolution of 0.39 ångströms—a new world record.
They also tried using the aberration-corrected lenses at a lower, less damaging beam energy, and still achieved 0.98 ångströms.
Despite the low beam energy, the resolution is so good that the microscope is able to detect with startling clarity a missing sulfur atom—“a defect in the lattice,” Gruner said—in a 2-D material. “That’s astounding to me,” he said.
The lead authors were Cornell’s Yi Jiang and Zhen Chen. Other co-authors were UChicago researchers Saien Xie and Hui Gao, as well as Cornell scientists Sol Gruner, the John L. Wetherill Professor of Physics, and Veit Elser, professor of physics.
Citation: “Electron Ptychography of 2D Materials to Deep Sub-Ångström Resolution,” Jiang et al, Nature, July 19.
Funding: Kavli Institute at Cornell; the U.S. Department of Energy; National Science Foundation; Air Force Office of Scientific Research.