Abstract
Since the discovery of X-rays by Roentgen in 1895, its use has been ubiquitous, from medical and environmental applications to materials sciences1,2,3,4,5. X-ray characterization requires a large number of atoms and reducing the material quantity is a long-standing goal. Here we show that X-rays can be used to characterize the elemental and chemical state of just one atom. Using a specialized tip as a detector, X-ray-excited currents generated from an iron and a terbium atom coordinated to organic ligands are detected. The fingerprints of a single atom, the L2,3 and M4,5 absorption edge signals for iron and terbium, respectively, are clearly observed in the X-ray absorption spectra. The chemical states of these atoms are characterized by means of near-edge X-ray absorption signals, in which X-ray-excited resonance tunnelling (X-ERT) is dominant for the iron atom. The X-ray signal can be sensed only when the tip is located directly above the atom in extreme proximity, which confirms atomically localized detection in the tunnelling regime. Our work connects synchrotron X-rays with a quantum tunnelling process and opens future X-rays experiments for simultaneous characterizations of elemental and chemical properties of materials at the ultimate single-atom limit.
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Data availability
All data are available in the main text, extended data and supplementary materials. The source data for the manuscript, for the extended data figures and Supplementary Figs. 13–16 are provided with the manuscript. All the theory data are deposited at https://doi.org/10.19061/iochem-bd-6-165. Source data are provided with this paper.
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Acknowledgements
We acknowledge financial support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Science and Engineering Division. Work performed at the Center for Nanoscale Materials and Advanced Photon Source, both U.S. Department of Energy Office of Science User Facilities, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We gratefully acknowledge the computing resources provided on Bebop, a high-performance computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory.
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S.-W.H. conceived and designed the experiments. T.M.A., S.Wieghold, N.S., S.-W.H., V.R. and S.P. performed the SX-STM experiments. D.J.T., K.Z.L., S.S., S.Wang and Y.Li prepared the samples and performed STM imaging. T.M.A., S.Wieghold, N.S. and S.-W.H analysed the SX-STM data. Y.Li and X.L. synthesized the Fe and Ru assemblies. E.M. designed the Tb complex, X.C. synthesized it and N.K. obtained its X-ray crystal structure. D.R. and Y.Liu prepared the coaxial tips for SX-STM. T.R., N.K.D. and A.T.N. performed the calculations. All authors discussed the results and commented on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 XTIP beamline and SX-STM technique.
a, Schematic presentation of XTIP. b, Mechanistic model of the SX-STM process. (1) Photoabsorption process. (2) X-ray-excited electrons and Auger electrons with energies between the Fermi level EF and the work function Φ fill up unoccupied states of the sample, whereas those having energies exceeding the work function escape (‘X-ray-ejected electrons’). (3) X-ray-ejected electrons captured by the tip produce a photocurrent. (4) The electrons that fill up states between EF and Φ tunnel to the tip, generating an X-ray-excited tunnelling current. Both the X-ray-ejected electron current and X-ray-excited tunnelling current are produced by the same initial photoabsorption process (1). c, Left, photoabsorption process excites core-level electrons either to unoccupied orbitals below the work function or above the vacuum level, depending on the energy of the photon. This process leaves a hole in a core level. Right, in the de-excitation process, an electron from a higher level fills up the hole. The excess energy can cause the release of Auger electrons. d, Measured photon flux of the XTIP beamline.
Extended Data Fig. 2 Coaxial detector tip.
a, Transmission X-ray microscopy image of a detector tip shows several materials composing the tip. Scanning electron microscopy image of a detector tip before (b) and after (c) incision. Here the PtIr conducting core is used to collect electrons, whereas the SiO2 insulating layer prevents collection of ejected electrons from the sample by the side wall of the tip. The outer Au conducting layer is to protect from the charging effect34,35.
Extended Data Fig. 3 Formation of dimer complexes through Ullmann coupling.
a, STM images Tb(pcam-Br)3 on Au(111) acquired at 5 K substrate temperature. A single Tb- (pcam-Br)3 complex is indicated with a circle. Tb(pcam-Br)3 is mobile (indicated with arrows) during scanning with the STM tip even at 5 K. b, STM image of Tb(pcam-Br)3 clusters on Au(111). c, A model depicting debromination and covalent linking of two monomers (upper part). Heating the sample to 473 K results in the formation of a dimer, [Tb(pcam)3]2 (lower part). d, A model of [Tb(pcam)3]2. The yellow balls in c and d are Tb ions. e, An STM image of a dimer. f, A profile measured along the white line shown in e gives the length of the dimer as roughly 1.67 nm, which agrees well with the expected length of about 1.61 nm shown in d. Image parameters for a, b and e: It = 100 pA, Vt = 0.5 V.
Extended Data Fig. 4 Simultaneously measured X-ray-excited currents in the tunnelling regime before background subtraction.
Sample channel (a) and tip channel (b). Background subtraction is performed by setting the photocurrent at 702.5 eV to zero and applying a linear slope correction.
Extended Data Fig. 5 STM-XAS on Ru ions.
The supramolecular ring (see Fig. 1a and Extended Data Fig. 6b) includes only one Fe ion and the rest of the tpy bridges are formed by Ru ions. a–f, Simultaneously measured STM-XAS spectra in the sample and tip channels focusing on the Fe L3 edge region on the six <tpy-Ru-tpy> bridges. The tip channel in the tunnelling regime detects atomically localized signal and, thus, it does not show any Fe L3 edge signature when measured on the <tpy-Ru-tpy> bridges, because the Fe ions are not present there. However, the sample channel gives a strong Fe L3 edge signal because it is produced by the entire X-ray-illuminated surface area in which many supramolecular rings, each having one Fe ion, are present.
Extended Data Fig. 6 STM-XAS on different locations of a <tpy-Fe-tpy> bridge.
a, An STM image of a single supramolecular ring measured with the SX-STM setup. Image parameters: It = 100 pA, Vt = −1.0 V. b, Atomic positions of Fe and Ru ions in a. c–e, Simultaneously measured STM-XAS spectra in sample and tip channels in the tunnelling regime focusing on the L3 absorption edge of Fe. Cogent L3 edge signals of Fe ion are observed in both the sample and tip channels in c measured at the centre of the <tpy-Fe-tpy> bridge in which the Fe ion is located (position A in a). Weak Fe edge signals are observed when measured on the <tpy-Fe-tpy> bridge but the tip is not positioned on top of the Fe ion, such as in position B (d and e).
Extended Data Fig. 7 STM-XAS spectra of Tb M4,5 absorption edges in the far-field regime.
At the sample channel (a) and at the tip channel (b).
Extended Data Fig. 8 STM-XAS signals in the tunnelling regime when the tip is not on top of the Tb ion.
The sample channel (a) and the tip channel (b).
Extended Data Fig. 9 STM-NEXAFS spectra of Fe ion in the far field.
The sample channel (a) and the tip channel (b). The satellite peaks are labelled as i, ii, iii, iv, v and vi.
Extended Data Fig. 10 Theory calculations.
a, Spherically averaged potential of the <tpy-Fe-tpy> bridge on Au(111). The vacuum level is indicated. PDOS of combined Fe ‘d’ and N ‘p’ orbitals calculated by B3LYP/def2TZVP-based theory (b) and energy differences between the peaks in b with the experimentally measured data (c).
Supplementary information
Supplementary Information
This file contains details about the synthesis and characterization of Tb(pcam-Br)3, STM-XAS data before background subtraction, Supplementary Figs. 1–16, Supplementary Tables 1 and 2 and Supplementary References.
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Ajayi, T.M., Shirato, N., Rojas, T. et al. Characterization of just one atom using synchrotron X-rays. Nature 618, 69–73 (2023). https://doi.org/10.1038/s41586-023-06011-w
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DOI: https://doi.org/10.1038/s41586-023-06011-w
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