The real-space observation of chemistry using scanning tunneling microscopy (STM) or synchrotron-based x-ray microscopy (XM) continues to have a tremendous impact on our understanding of nanoscale materials. However, although STM provides high spatial resolution, it lacks direct chemical contrast. On the other hand, XM can provide chemical sensitivity, but the spatial resolution is inferior. In order to overcome these limitations, we have developed a technique that combines the chemical contrast of synchrotron x-rays with the locality of STM.
In this presentation, 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 x-ray signal can be sensed only when the tip detector 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 chemical properties of materials at the ultimate single atom limit.
This work was performed at the Advanced Photon Source and the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility under Contract No. DE-AC02-06CH11357.  Nature 618, 69-73 (2023).