Abstract: How do we identify a single atom using X-rays? This colloquium will explore the fundamental
physics of X-rays and their interaction with matter, leading to the ability to investigate individual
atoms through X-ray spectroscopy. Beginning with an introduction to X-ray generation,
synchrotron radiation, and conventional X-ray microscopy, we will explore how X-rays reveal the
structure, chemistry, and physical properties of materials.
I will then share the scientific journey of developing Synchrotron X-ray Scanning Tunneling
Microscopy (SX-STM), a technique that my team and I have pursued over much of my professional
career. By combining the elemental, chemical, and magnetic sensitivity of synchrotron X-rays with
the atomic-scale spatial resolution of scanning tunneling microscopy, SX-STM extends X-ray
spectroscopy beyond the spatial resolution limits of conventional X-ray microscopy. Through the
detection of X-ray-excited tunneling currents, we have achieved X-ray spectroscopic
characterization of individual atoms, enabling the determination of their elemental identities and
local chemical states. I will discuss the experimental challenges and breakthroughs that made these
measurements possible, followed by recent advances in studying chemically complex materials,
resolving magnetism at the atomic-layer level, and exploring the magnetic properties of individual
atoms.
The underlying concepts, light-matter interactions, quantum tunneling, and magnetism, connect to
fundamental questions across physics. The goal of this colloquium is to make these ideas accessible
to a broad audience while illustrating how scientific curiosity and experimental innovation can push
the boundaries of what we can measure.
This work was performed at the Advanced Photon Source and the Center for Nanoscale Materials, U.S. Department of
Energy Office of Science User Facilities, under Contract No. DE-AC02-06CH11357.
[1] Nature 618, 69–73 (2023).
[2] Advanced Materials 36, 2402442 (2024).
[3] Physical Review Materials 10, L091401 (2026).