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Detection of single atom using soft X-ray spectroscopy (May 31, 2023) A research group at Argonne National Laboratory's APS synchrotron radiation facility has reported the detection of single atoms using synchrotron radiation X-rays (Tolulope M. Ajayi, Nozomi Shirato, Tomas Rojas, Sarah Wieghold, Xinyue Cheng, Kyaw Zin Latt, Daniel J. Trainer, Naveen K. Dandu, Yiming Li, Sineth Premarathna, Sanjoy Sarkar, Daniel Rosenmann, Yuzi Liu, Nathalie Kyritsakas, Shaoze Wang, Eric Masson, Volker Rose, Xiaopeng Li, Anh T. Ngo and Saw-Wai Hla, “Characterization of just one atom using synchrotron X-rays,” Nature, 618, 69–73 (2023) https://doi.org/10.1038/s41586-023-06011-w). This research was performed at XTIP21, a beamline equipped with an STM probe in an ultrahigh vacuum chamber that can simultaneously perform STM imaging and soft X-ray spectroscopy experiments. The research sample is a supramolecular assembly with a ring structure consisting of seven terpyridine-metal-terpyridine bridges, with six ruthenium atoms and one iron atom on the metal. First, the sample was imaged by STM, and then, under the same conditions, the X-ray energy near the L2,3 absorption edge of iron was scanned with a monochromator, and the excitation current was measured by the probe. The electric current changed at the absorption edge, indicating that a single atom could be detected. Since the X-ray signal was detected only when the probe was placed extremely close to the atom, X-ray excitation resonance tunneling is dominant, confirming the detection of atom localization in the tunneling region. Another sample in this study, a terbium complex, was also measured. In this complex, the terbium is firmly anchored by three brominated pyridine-2,6-dicarboxamide ligands. As with the previous sample, STM imaging was performed and the synchrotron radiation energy was varied near the M4,5 absorption edge of terbium under the same conditions and a single atom was detected. Some readers may ask whether such a measurement can be achieved with X-ray fluorescence. It would depend on the sample conditions. The analysis of very small numbers of atoms by X-ray fluorescence spectroscopy is mainly limited by the background. If a sample has almost no background, in contrast to conventional X-ray analysis, it would theoretically be possible to detect a single atom. For example, if we have a sample in which only one atom is trapped in a fullerene, and the fullerene is placed in a carbon nanotube, it is possible to detect characteristic X-rays excited by electron beams with a semiconductor detector while observing the single atom with a transmission electron microscope. This was reported more than 10 years ago. For more details, see T. C. Lovejoy, Q. M. Ramasse, M. Falke, A. Kaeppel, R. Terborg, R. Zan, N. Dellby, and O. L. Krivanek, “Single atom identification by energy dispersive x- ray spectroscopy,” Appl. Phys. Lett. 100, 154101 (2012). https://doi.org/10.1063/1.3701598, and Kazu Suenaga, Toshiya Okazaki, Eiji Okunishi, and Syo Matsumura, “Detection of photons emitted from single erbium atoms in energy dispersive x-ray spectroscopy,” Nature Photonics 6, 545–548 (2012). https://doi.org/10.1038/nphoton.2012.148.