Behold the highest-resolution image of atoms ever seen. Cornell University researchers captured a sample from a crystal in three dimensions and magnified it 100 million times, doubling the resolution that earned the same scientists a Guinness World Record in 2018. Their work could help develop materials for designing more powerful and efficient phones, computers and other electronics, as well as longer-lasting batteries.
The researchers obtained the image using a technique called electron ptychography. It involves shooting a beam of electrons, about a billion of them per second, at a target material. The beam moves infinitesimally as the electrons are fired, so they hit the sample from slightly different angles each time—sometimes they pass through cleanly, and other times they hit atoms and bounce around inside the sample on their way out. Cornell physicist David Muller, whose team conducted the recent study, likens the technique to playing dodgeball against opponents who are standing in the dark. The dodgeballs are electrons, and the targets are individual atoms. Though Muller cannot see the targets, he can see where the “dodgeballs” end up, thanks to advanced detectors. Based on the speckle pattern generated by billions of electrons, machine-learning algorithms can calculate where the atoms were in the sample and what their shapes might be.
Previously, electron ptychography had only been used to image extremely flat samples: those merely one to a few atoms thick. The new study, published in Science, now allows it to capture multiple layers tens to hundreds of atoms thick. That makes the technique much more relevant to materials scientists, who typically study the properties of samples with a thickness of about 30 to 50 nanometers. (That range is smaller than the length your fingernails grow in a minute but many times thicker than what electron ptychography could image in the past.) “They can actually look at stacks of atoms now, so it’s amazing,” says Andrew Maiden, an engineer at the University of Sheffield in England, who helped develop ptychography but was not involved with the new study. “The resolution is just staggering.”
This marks an important advancement in the world of electron microscopy. Invented in the early 1930s, standard electron microscopes made it possible to see objects such as polioviruses, which are smaller than the wavelengths of visible light. But electron microscopes had a limit: increasing their resolution required raising the energy of the electron beam—and eventually the necessary energy would become so great that it would damage the sample. One way to avoid this problem was ptychography, which researchers developed in theory in the 1960s. But because of limitations in detectors and computational power, as well as the complex math required, it was decades before the technique was put into practice. Early versions only worked with visible light and x-rays, not the electron beams required to image atomic-size objects. Meanwhile scientists kept finding ways to improve electron microscopes, which worked so well that electron ptychography could not keep up. “You had to be a true believer in ptychography to be paying attention to it,” Muller says.
It was only in the past several years that Muller and his team developed a detector good enough for electron ptychography to work experimentally. By 2018, they had figured out how to reconstruct two-dimensional samples with the technique, producing “the highest-resolution image by any method in the world,” Muller says—which won that Guinness World Record. And the researchers did so with a lower-energy wavelength than other methods, allowing them to better preserve their samples.
Thicker samples, however, presented multiple challenges. Instead of bouncing just once before detection, an electron wave ricochets around atoms in a three-dimensional sample. “You know where it ended up, but you don’t know what path it took in the material,” Muller says. This pinballing is called the “multiple scattering problem,” and he and his team spent the past several years trying to solve it. With enough overlapping speckle patterns and computing power, they found they could work backward to determine which layout of atoms produced a given pattern. The researchers did so by fine-tuning a model until the speckle pattern it generated matched the experimentally produced one. Solving the multiple scattering problem is a major advancement, Muller says. Referring to the resolution his team’s technique can capture for samples 300 atoms thick or smaller, he contends that “we can do better than anyone else, and we can do better than anyone else by factors of two to four.”
Such high-resolution imaging techniques are essential for developing the next generation of electronic devices. For example, researchers are looking to move beyond silicon-based computer chips in search of more efficient semiconductors. To make this happen, engineers need to know what they are working with at an atomic level—which means taking advantage of technologies such as electron ptychography. “We have these tools sitting there, waiting to help us optimize what will become the next generation of devices,” says J. Murray Gibson, dean of the Florida A&M University–Florida State University College of Engineering, who was not involved in the new study. “Without these tools, we couldn’t do it.”
Batteries are a particularly promising area for applying imaging techniques such as electron ptychography, says Roger Falcone, a physicist at the University of California, Berkeley, who was also not involved with the research. “How do we make the structure of batteries,” he asks, “such that they can store a lot of energy and yet still be safe?” This is an essential question, especially for the transition from fossil fuels to renewable energies, including wind and solar. “Imaging technologies are very important to improving batteries because we can look at the chemical reactions in detail,” Falcone says.
But there is still a long way to go. In order for electron ptychography to lead to a new breakthrough for your cell phone or laptop, it must do more than take a picture—it has to be capable of precisely locating an individual atom in a material. Though the researchers demonstrated how the technique could do so theoretically, they have not yet performed an experimental demonstration. “With any new technique, it always takes a bit of time for your fellow researchers to try this out and see if it bears out into real, practical uses,” says Leslie Thompson, former manager of materials analysis and characterization at IBM Research–Almaden, who was not involved in the new study.
“To the extent that you invent a new tool like a high-resolution microscope, my sense is that you tend to be surprised [by] what problem it’s applied to solve,” Falcone adds. “People will look at things that we can’t even imagine now—and solve a problem that we’re not even sure we have yet.”