Since the discovery of X-rays by Wilhelm Röntgen in 1895, its use has been ubiquitous, from medical and environmental applications to materials sciences. X-ray characterization requires a large number of atoms and reducing the material quantity is a long-standing goal. Argonne National Laboratory researcher Saw Wai Hla and colleagues now show that X-rays can be used to characterize the elemental and chemical state of just one atom.
When X-rays (blue color) illuminate onto an iron (Fe) atom (red ball at the center of the molecule), core level electrons are excited. X-ray excited electrons are then tunnel to the detector tip (gray) via overlapping atomic/molecular orbitals, which provide elemental and chemical information of the iron atom. Image credit: Ajayi et al., doi: 10.1038/s41586-023-06011-w.
“Atoms can be routinely imaged with scanning probe microscopes, but without X-rays one cannot tell what they are made of,” said Dr. Hla, who is also the director of the Nanoscale and Quantum Phenomena Institute at Ohio University.
“We can now detect exactly the type of a particular atom, one atom-at-a-time, and can simultaneously measure its chemical state.”
Dr. Hla and co-authors conducted their experiment at the XTIP beamline at the Advanced Photon Source and the Center for Nanoscale Materials at Argonne National Laboratory.
For demonstration, they chose an iron atom and a terbium atom, both inserted in respective molecular hosts.
To detect X-ray signal of one atom, they used a technique known as synchrotron X-ray scanning tunneling microscopy.
“X-ray characterization of materials has been revolutionized after the invention of synchrotron X-rays in the mid-twentieth century,” they said.
“The capabilities of synchrotron light sources have been continuously upgraded to improve resolution and minimum sample quantity required for measurements.”
“So far, an attogram amount of sample can be detected by X-rays. However, it is still in the range of over 10,000 atoms and gaining access to a much smaller sample is becoming extremely arduous.”
“If X-rays could be used to detect just one atom, it would further revolutionize their applications to an unprecedented level, from quantum information technology to environmental and medical research.”
“One way to overcome these challenges is to supplant conventional detectors with a specialized detector made of a sharp metal tip positioned at extreme proximity to the sample to collect X-ray-excited electrons, a technique known as synchrotron X-ray scanning tunneling microscopy (SX-STM).
An image of a ring shaped supramolecule (left) where only one iron atom is present in the entire ring; and X-ray signature of just one iron atom (right). Image credit: Ajayi et al., doi: 10.1038/s41586-023-06011-w.
X-ray spectroscopy in SX-STM is triggered by photoabsorption of core level electrons, which constitutes elemental fingerprints and is effective in identifying the elemental type of the materials directly.
“The spectrums are like fingerprints, each one being unique and able to detect exactly what it is,” Dr. Hla said.
“The technique used, and concept proven in this study, broke new ground in X-ray science and nanoscale studies,” added Tolulope Michael Ajayi, also from Argonne National Laboratory and Ohio University.
“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. This achievement also opens the road for advanced materials science instrumentation.”
In addition to achieving X-ray signature of one atom, the team’s key goal was to use this technique to investigate the environmental effect on a single rare-earth atom.
“We have detected the chemical states of individual atoms as well,” Dr. Hla said.
“By comparing the chemical states of an iron atom and a terbium atom inside respective molecular hosts, we find that the terbium atom, a rare-earth metal, is rather isolated and does not change its chemical state while the iron atom strongly interacts with its surrounding.”
The team’s work appears in the journal Nature.
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T.M. Ajayi et al. 2023. Characterization of just one atom using synchrotron X-rays. Nature 618, 69-73; doi: 10.1038/s41586-023-06011-w
