X-ray spectroscopy of a solitary atom

Physics 16, 96

A technique combining X-ray spectroscopy with tunneling microscopy yielded X-ray spectra of single atoms.

Sketch of an SX-STM setup. The setup involves using an STM tip to measure a tunnel current (red beam) induced by X-ray (blue beam) illumination of a sample.

X-ray spectroscopy offers exquisite tools for studying the composition and structure of samples relevant to biology, chemistry, environmental studies, and material sciences. It can not only probe the “fingerprints” of specific chemical elements in a sample, but also reveal a wealth of information about the chemical states and structural arrangement of the atoms in the sample. Until now, however, experiments have needed thousands of atoms, or oneattogram of sample, to get a clean enough signal. Now a team led by Saw-Wai Hla of Ohio University and Argonne National Laboratory in Illinois has been able, for the first time, to measure the X-ray spectra of single atoms [1]. The breakthrough was achieved with a technique that combines X-ray spectroscopy with the best single-atom resolution technique: scanning tunneling microscopy (STM).

Developed in 1981, STM has accustomed us to extraordinary visions of the microscopic world. A decade ago, IBM researchers even made the world’s “smallest” movie, A boy and his atom, whose frames were made of molecules moved with an STM tip. STM, however, does not provide information on atomic species. “I regularly image and manipulate atoms with STM. But with STM alone I couldn’t identify what types of atoms are present in an unknown sample,” says Hla.

The STM’s lack of elemental specificity arises from the fact that the tunneling current measured with the STM tip involves the electrons that are easiest to strip away from an atom: its outermost electrons, whose characteristics are not element-specific . X-ray spectroscopy, on the other hand, targets electrons in an atom’s nucleus, which can offer unique fingerprints in the form of element-specific absorption lines. Furthermore, the details of these absorption lines can reveal information about the chemical state of the atom, such as how it might share electrons with its neighbors. The X-ray absorption of a single atom, however, is so weak that many atoms are typically required to produce a measurable signal on a detector.

For years, Hla and his team have worked to combine the best of both worlds: the atomic resolution of STM and the chemical sensitivity of X-ray spectroscopy. Their approach makes use of a previously demonstrated technique called X-ray STM of synchrotron (SX-STM). In the technique, a sample is scanned by an STM tip while being exposed to the X-ray beam emitted by a synchrotron. The tip registers a large tunneling current when the X-ray energy resonates with transitions centrally in the sample atoms. By measuring the current as the X-ray energy varies, researchers can recover an X-ray absorption spectrum of the sample under the tip.

Structure of part of the supramolecular complex used to measure the X-ray absorption spectrum of a single iron atom. The iron atom (red) is contained within several ring-shaped structures. (The specific complex was named one of the “coolest molecules of 2020” by Chemistry and Engineering News).Structure of part of the supramolecular complex used to measure the X-ray absorption spectrum of a single iron atom. The iron atom (red) is contained within several ring-shaped structures. (The specific complex has been named among the “coldest molecules of … Show more

While SX-STM has been available and applied to nanoscale imaging since 2009 [2], reducing resolution to single atoms was far from simple, says Hla. A key challenge was preparing a suitable sample in which a specific atom could be singled out from its environment to be reliably measured. To do this, the team synthesized special supramolecular complexes, which made it possible to place a single atom in a controlled and reproducible position.

In the new work, the team studied two different elements, iron and terbium, each of which was inserted into a supramolecular complex that isolated the target atoms from other atoms of the same species. This design ensured that the measured peak current could be unambiguously attributed to a single atom. As they scanned, for example, their tip along the length of a complex containing two terbium atoms, the researchers recorded X-ray spectra consistent with terbium”m edge” (involving core electron transitions from 3d at 4f States). In a second experiment on another supramolecular complex, they found the “L edge” (involving 2P-to-3d transitions) at points within an iron-transporting complex.

Because the tip was scanned at ten locations in a sample containing two terbium atoms, it picked up a signal only from the locations (2 and 9) where the terbium was located (left: STM image; right: molecular structure sketch corresponding).

As well as identifying iron and terbium atoms in the samples, the results also showed the potential to reveal subtle chemical details. Spectral analysis has confirmed, for example, that iron is in an oxidation state of +2 and that it interacts strongly with its surroundings (technically speaking, the atom d the orbitals mix, or hybridize, with the P orbitals of six neighboring nitrogen atoms). For terbium, the team confirmed an expected property: isolation from its surroundings due to lack of terbium hybridization f orbitals. This loose coupling is the key to the many technological applications of terbium and other rare earths.

“The idea of ​​performing X-ray spectroscopy on a single atom is quite extraordinary, and successfully performing such a measurement is an incredible tour de forcesays condensed matter physicist Michael Crommie of the University of California, Berkeley, who was not involved in the study. Paul Scherrer Institute in Switzerland Such single-atom resolving power could help to understand and design so-called metal-organic coordination networks, materials that hold promise for applications from surface catalysis to quantum technologies, he says.

Hla says his team is working to further improve the setup. Specifically, the researchers aim to make measurements using polarized X-rays. This approach would make the experiment sensitive to the spin state of a single atom, which would be invaluable for studying rare-earth-based materials that have tantalizing applications in spintronics and magnetic memory technology.

Matthew Rini

Matteo Rini is the Director of Physics magazine.

References

  1. TM Ajayi et al.Single-atom characterization using synchrotron X-rays, Nature 61869 (2023).
  2. T. Okuda et al.Nanoscale chemical imaging using synchrotron radiation-assisted scanning tunneling microscopy, Phys. Rev. Lett. 102105503 (2009).

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