Since their discovery by Wilhelm Conrad Roentgen in 1895, X-rays have found extensive application in various scientific, industrial, and technological fields. This widespread utilization stems from their exceptionally short wavelengths, ranging from 10 nanometers to 10 picometers, which allows X-rays to easily penetrate numerous materials that visible light, with a wavelength of hundreds of nanometers, cannot pass through.

We’re all familiar with X-rays used as a diagnostic tool in medicine, but they also play an important role in materials analysis, where X-rays can be used to determine what a specific material is made of or detect various defects in a sample.

Over the years, scientists have made great advancements in X-ray diagnostics, significantly enhancing its accuracy. However, until recently there was a limit to the amount of material that could be scanned using X-rays — about one attogram, which corresponds to roughly 10,000 atoms or more. This is because the X-ray signal produced by a smaller amount of matter is extremely weak, making it difficult for conventional X-ray detectors to pick it up.

Excitingly, this limitation seems to have been overcome. In a recent study published in Nature, an international team of researchers led by Saw Wai Hla of Ohio University and Argonne National Laboratory in the US, has unveiled a technique that can detect the X-ray signature of an individual atom, even determining the structure of its electron orbits.

“Atoms can be routinely imaged with scanning probe microscopes, but without X-rays, one cannot tell what they are made of. We can now detect exactly the type of a particular atom, one atom-at-a-time, and can simultaneously measure its chemical state,” said Hla in a press release. “Once we are able to do that, we can trace the materials down to ultimate limit of just one atom. This will have a great impact on environmental and medical sciences and maybe even find a cure that can have a huge impact for humankind. This discovery will transform the world.”

Enhancing scanning probe microscopy

Scanning probe microscopy is a method of analyzing the surface of a material using quantum tunneling — the process of passing a particle through a barrier, which, according to classical physics, it should not be able to penetrate.

This technique consists in placing a conducting probe above the surface of the material under study. The probe acts like a “sensor” for the electrons bound to atoms found in the material’s surface, as these electrons tunnel through the tiny gaps between the probe and the surface.

Different atoms have electron orbits with distinct shapes and sizes, so as these electrons tunnel into the probe, the intensity of their tunneling varies depending on the specific electron orbit of each atom, giving rise to different currents within the probe. By scanning the material’s surface with the microscope and measuring these currents, scientists can unravel the composition of the material.

The authors of the current study took this a step further and combined scanning probe microscopy with X-rays of a defined wavelength generated using a device called a synchotron. When these X-rays hit the surface of the material, they are absorbed by the atoms’ electrons, which, now with increased energies, can jump to higher orbits, making it easier for them to tunnel into the microscope’s probe.

This allowed the physicists to increase the accuracy of current measurements such that they could detect individual atoms and even determine their chemical states, making this method of analysis much more powerful.

“This achievement connects synchrotron X-rays with quantum tunneling process to detect X-ray signature of an individual atom and opens many exciting research directions including the research on quantum and spin (magnetic) properties of just one atom using synchrotron X-rays,” said Hla.

An initial probe

To demonstrate their method, the scientists placed one atom of iron and one atom of terbium in a specially prepared organic substance, and were able to accurately determine their positions and chemical properties. They found that the electron orbits of the iron atom interacted with the atoms in its surroundings while the electron orbits of the terbium atom remained unchanged.

While these were just preliminary probes, terbium was selected because it belongs to a group of elements called the rare earth metals, which are used in many electronic devices, such as smartphones, computers and televisions, to name a few. A deeper understanding of how these elements interact with other atoms could be very useful and could potentially lead to breakthroughs in areas such as quantum information technology and communications, where precise control over the states of individual atoms and molecules is of paramount importance.

The ability to accurately analyze and understand the composition of materials at such a fundamental level opens up unprecedented opportunities. The scientists envision that this method holds the potential to revolutionize many other fields, including medicine, environmental science, and countless others that rely on precise material analysis and the ability to manipulate the chemical properties of individual atoms.

“The technique used, and concept proven in this study, broke new ground in X-ray science and nanoscale studies,” concluded Tolulope Michael Ajayi of Ohio University and Argonne National Laboratory, who is the first author of the study.

“More so, using X-rays to detect and characterize individual atoms could revolutionize research and give birth to new technologies in areas such as quantum information and the detection of trace elements in environmental and medical research, to name a few,” he continued. “This achievement also opens the road for advanced materials science instrumentation.”

Reference: Tolulope Michael Ajayi et al., Characterization of just one atom using synchrotron X-rays, Nature (2023), DOI: 10.1038/s41586-023-06011-w

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