In a quiet lab at Argonne National Laboratory, Saw-Wai Hla and his team were huddled around their instruments late one night when they detected the spectral signature they had been searching for. The excitement in the room was electric. After more than a decade of research, they had done it. They had captured the X-ray fingerprint of a single atom.
For Hla, a professor at Ohio University and physicist at Argonne, the discovery was a career-defining moment. “I could not sleep for probably two, three days,” Hla recalls. “It was one of the best moments of my life.”
When Wilhelm Roentgen first discovered X-rays in 1895, he couldn’t have imagined how far this technology would advance. From revolutionizing medicine to exploring the surface of Mars, X-rays have become indispensable across various fields. Yet, despite these leaps, one goal remained elusive for over a century: detecting the X-ray signature of a single atom. That is, until now.
The implications are nothing short of transformative. Scientists can now detect exactly the type of a particular atom, one atom at a time, and simultaneously measure its chemical state. This could have a major impact on environmental and medical sciences, and may even lead to cures to currently untreateable diseases.
A New Frontier in Atomic Physics
Imaging individual atoms is an extraordinary achievement, but it’s not exactly novel. Everything changed in 1955 when a German physicist named Erwin Müller, with an unwavering obsession for pushing the boundaries of what could be observed, unveiled an extraordinary invention: the field ion microscope (FIM). With this new tool, Müller achieved the unthinkable — he produced the first-ever image of individual atoms on the surface of a metal. The resulting images, grainy and almost ghostly by today’s standards, were breathtaking. For the first time in human history, the very building blocks of matter were laid bare, captured as faint points of light dancing on a screen.
The real breakthrough, however, came in 1981, with a technology that would forever change our ability to observe the atomic world. That year, two physicists at IBM in Zurich, Gerd Binnig and Heinrich Rohrer, introduced the world to the scanning tunneling microscope (STM). The STM didn’t just image atoms — it felt them, like a blind person reading Braille.
By running a sharp metal tip just a few angstroms above the surface of a material, the microscope measured tiny quantum tunneling currents that varied depending on the distance between the tip and the atoms beneath it. These variations could then be translated into topographical maps at an atomic scale.
The achievement earned Binnig and Rohrer the Nobel Prize in Physics in 1986, shared with Ernst Ruska, who had developed the electron microscope decades earlier.
From STM to X-rays
The STM’s success sparked a flurry of innovations, leading to the development of the atomic force microscope (AFM) and other scanning probe technologies that could not only image atoms but also manipulate them. In a now-legendary experiment in 1989, IBM researchers used an STM to precisely move 35 xenon atoms on a nickel surface to spell out their company’s name — “IBM” — on an atomic scale.
Yet, as powerful as scanning tunneling microscopy (STM) was, it had a major blind spot. Yes, scientists could now visualize atoms, could even move them into neat little rows like an atomic-scale game of checkers. But there was a lingering question that STM simply couldn’t answer: What exactly were those atoms?
STM is inherently blind to the chemical composition of the atoms it images. To fill in the blanks, you need X-rays. In the early 20th century, scientists discovered that when atoms are bombarded with high-energy X-rays, they emit a kind of fingerprint — a unique set of spectral lines that reveals their identity. This was yet another major breakthrough in science. Here was a way to peer into the heart of matter and understand not just its structure, but its composition.
The X-Ray ‘Fingerprint’ of One Atom
When Wilhelm Roentgen first discovered X-rays in 1895, he couldn’t have imagined how far this technology would advance. They peer inside bodies, inspect luggage at airports, and analyze paintings for hidden masterpieces. But the story of X-ray technology is far from over. Scientists are pushing it beyond Röntgen’s wildest dreams, probing the tiniest structures in biology, chemistry, and materials science.
Traditionally, X-ray methods require at least 10,000 atoms to generate a detectable signal. This is because the X-ray signal produced by a single atom is incredibly weak. But today, the boundary between seeing and not seeing has shrunk to the size of an atom.
“Each element in the periodic table has its own unique fingerprint,” Hla explained to me during an interview in Berlin, at the 2024 Falling Walls conference. “We can easily identify gold or silver with X-rays, but if you gave me a single atom, I wouldn’t be able to tell you what it was — until now.”
Hla’s team shattered the thousands-of-atom barrier using a new technique called synchrotron X-ray scanning tunneling microscopy (SX-STM). In other words, this method combines the best of both worlds.
The key to their success is a highly sensitive detector made from a sharp metal tip positioned just nanometers away from the atom. This setup allows the team to collect X-ray excited electrons with unprecedented precision.
The team chose two elements to demonstrate their technique: iron and the rare earth metal terbium. Both atoms were carefully embedded within molecular hosts, allowing the researchers to study their properties in detail. For instance, these measurements not only confirmed the presence of these atoms but also revealed their chemical states.
Fundamental Research With Far-Reaching Implications
The journey was anything but straightforward. Hla joined the Argonne National Laboratory over a decade ago with this goal in mind. The early stages of the project required overcoming significant technical hurdles, such as reducing background noise and building specialized equipment. “We built our beam line from scratch,” he said, crediting the efforts of his colleagues and the support of the Department of Energy for enabling this groundbreaking research.
The project was truly a collaborative effort, involving scientists and students from multiple institutions. Tolulope Michael Ajayi, a Ph.D. candidate at Ohio University, served as the first author of the Nature paper. His work focused on refining the SX-STM technique to achieve the level of sensitivity required for single-atom detection. Ajayi wasn’t alone. Over the course of 12 years, Hla supervised four Ohio University graduate students who completed their Ph.D. theses on this project.
This breakthrough, Hla believes, will open new doors in fields as diverse as medicine, quantum computing, and environmental science. Rare earth elements like terbium are critical components in everyday devices such as smartphones, computers, and televisions. Understanding how these elements behave at the atomic level could lead to more efficient designs and new technological advancements. But there’s much more it.
For instance, the ability to pinpoint and analyze the chemical state of a single atom could lead to innovations in catalysis, where controlling atomic interactions is crucial.
“If we can identify one atom, there are many scientific phenomena that start with just one,” Hla said. “Proteins, for instance, rely on specific atoms to function properly. A single misplaced atom could potentially cause diseases.”
“If we can find that particular atom, then we will have a cure,” he added.
The implications for medicine are tantalizing. By isolating the atoms involved in biological processes, researchers may gain insights into the mechanisms behind protein misfolding and plaque formation — processes associated with neurodegenerative diseases like Alzheimer’s. “We might even find a way to prevent these plaques from forming,” Hla suggested, though he acknowledged that much work remains to be done.
Beyond biology, the technique has potential applications in quantum technology. Hla is particularly interested in how this could advance spintronics, a field focused on using electron spins for data storage and processing. “X-rays can also manipulate atomic spins,” he noted, hinting at the possibility of more efficient quantum computing technologies.
“That’s just one little piece I’m telling you, but there are so many different things, because X-rays are so useful,” Hla told ZME Science.
“I am not saying that this will happen tomorrow. As usual, every discovery takes some time to get to the point of application.”
Making the Technology Accessible
While the discovery is revolutionary, translating it into practical applications will take time. Currently, Hla’s setup requires a synchrotron — a type of particle accelerator available only at select research facilities. These machines are powerful but cumbersome, limiting the immediate accessibility of this technology.
“In order for science to grow, we need instrumentation that is accessible to many,” Hla emphasized. “The synchrotron facilities are open to many higher institutions free of charge, but applying for beam time and waiting for access takes time.”
To address this challenge, Hla is exploring ways to develop more compact X-ray sources as a turnkey solution that could replicate his team’s results. This way, other labs across the world could image single atoms with X-rays. It’s an ambitious goal, but one that he believes is achievable. “Think of computers,” he said. “They were once the size of rooms, but now we have them in our pockets. I’m optimistic that this technology won’t take 50 years to become accessible.”
Despite the challenges, Hla’s optimism is infectious. “We’ve already proved that it’s possible,” he said with a smile. “Now it’s just a matter of time before we see where this breakthrough can take us.”
For now, Hla and his team are celebrating their hard-earned achievement. But they’re already planning the next steps, driven by the possibilities that lie ahead. As Hla puts it, “The potential applications you can imagine are limitless.”
