The world’s first x-ray of a single atom reveals chemistry at the smallest level

Atoms may not have bones, but we still want to know how they are put together. These tiny particles are the foundation upon which all normal matter (including our bones) is built, and understanding them helps us understand the larger Universe.

We currently use high-energy X-ray light to help us understand atoms and molecules and how they are arranged, capturing diffracted beams to reconstruct their configurations in crystal form.

Now, scientists have used X-rays to characterize the properties of a single atom, showing that this technique can be used to understand matter at the level of its smallest building blocks.

“Here,” writes an international team led by physicist Tolulope Ajayi of Ohio University and Argonne National Laboratory in the United States, “we demonstrate that X-rays can be used to characterize the elemental and chemical state of a single atom.” .

Diagram of the supramolecular assembly of iron, with the iron atom in red and the rubidium in cyan. (Ajayi et al., Nature2023)

X-rays are considered a suitable probe for the characterization of materials at the atomic level because their wavelength distribution is comparable to the size of an atom.

And there are different techniques for shooting X-rays at things to see how they’re put together on really tiny scales.

One of these is synchrotron X-rays, in which electrons are accelerated along a circular track to the point where they shine brightly with high-energy light.

To try to resolve really fine scales, Ajayi and his colleagues used a technique that combines synchrotron X-rays with a microscopy technique for imaging the atomic scale called scanning tunneling microscopy. This employs an excellent sharp-tipped conductive probe that interacts with electrons in the test material in what is known as ‘quantum tunnelling’.

At very close distances (like half a nanometer), the precise location of an electron is uncertain, smearing it across the gap between the material and the probe; the state of the atom can then be measured in the resulting current.

Together, the two techniques are known as synchrotron X-ray scanning tunneling microscopy (SX-STM). The amplified X-ray excites the sample and the needle detector collects the resulting photoelectrons. And it’s an exciting technique that opens up some pretty incredible possibilities: Last year, the team published a paper on using SX-STM to rotate a single molecule.

This time they’ve gotten even smaller, attempting to measure the properties of a single iron atom. They created supramolecular assemblies separately, including iron and terbium ions within a ring of atoms in what’s called a ligand. One iron and six rubidium atoms were bonded with terpyridine ligands; terbium, oxygen and bromine were linked using pyridine-2,6-dicarboxamide ligands.

Left: Diagram of terbium supramolecular assembly, with terbium in cyan, bromine in blue, and oxygen in red. Left: SX-STM image of terbium supramolecular assemblies. (Ajayi et al., Nature2023)

These samples were then subjected to SX-STM.

The light that the detector receives is not the same as the light radiated on the sample. Some wavelengths are absorbed by electrons in the atomic nucleus, which means that there are some darker lines on the received X-ray spectrum.

These darker lines, the team found, are consistent with the absorbed wavelengths of iron and terbium, respectively. Absorption spectra could also be analyzed to determine the chemical states of these atoms.

Something interesting happened to the iron atom. The X-ray signal could only be detected when the probe tip was exactly above the iron atom in its supramolecular structure and in extremely close proximity.

This, the researchers say, confirms the detection in the tunneling regime. Since tunneling is a quantum phenomenon, this has implications for the study of quantum mechanics.

‘Our work,’ the researchers write, ‘links synchrotron X-rays with a quantum tunneling process and opens up future X-ray experiments for simultaneous characterizations of the elementary and chemical properties of materials at the ultimate limit of the single atom.’

It’s probably at least as good as the bones.

The research was published in Nature.

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