Atomic-scale imaging emerged in the mid-1950s and has been advancing rapidly ever since—so much so, that back in 2008, physicists successfully used an electron microscope to image a single hydrogen atom. Five years later, scientists were able to peer inside a hydrogen atom using a "quantum microscope," resulting in the first direct observation of electron orbitals. And now we have the first X-ray taken of a single atom, courtesy of scientists from Ohio University, Argonne National Laboratory, and the University of Illinois-Chicago, according to a new paper published in the journal Nature.
“Atoms can be routinely imaged with scanning probe microscopes, but without X-rays one cannot tell what they are made of," said co-author Saw-Wai Hla, a physicist at Ohio University and Argonne National Laboratory. "We can now detect exactly the type of a particular atom, one atom at a time, and can simultaneously measure its chemical state. Once we are able to do that, we can trace the materials down to [the] ultimate limit of just one atom. This will have a great impact on environmental and medical sciences.”
When the average non-scientist thinks of an atom, chances are they envision some popularized version of the classic, much-maligned Bohr model of the atom. That's the one where electrons move about the atomic nucleus in circular orbits, like planets orbiting the Sun in our Solar System. The orbits have set discrete energies, and those energies are related to an orbit’s size: The lowest energy, or “ground state,” is associated with the smallest orbit. Whenever an electron changes speed or direction (according to the Bohr model), it emits radiation in the specific frequencies associated with particular orbitals.
The model has been superseded since Niels Bohr first proposed it in 1913, as our understanding of the quantum world advanced. Erwin Schroedinger proposed a new atomic model that dispensed with orbits in favor of energy levels. It still shares some similar concepts with the Bohr model. For instance, if an atom heats up (i.e., is energized), its electrons move to higher levels. As they cool and fall back to their normal ground state, the excess energy has to go somewhere, so it’s emitted as photons. And those photons possess frequencies that match the change in energy levels.
Technically, the electrons don’t really “move” around the nucleus in orbits. Electrons are really waves—they show up as particles when you perform an experiment to determine position—and those waves are stationary. You can check to see where an electron is, but each time you do, it will show up in a different position, not because it’s moving but because of the superposition of states. The electron doesn’t have a fixed position until you look at it, and the wave function collapses. That said, if you make a lot of individual measurements and plot the positions of the electron for each one, eventually you’ll get a ghostly orbit-like cloud pattern that is much closer to what an individual atom "looks" like.
As Hla notes, physicists can now routinely image atoms with scanning-probe microscopes. These work by running a very sharp tip over a surface and forming the image of the surface from a signal read from the tip—akin to a record player reading the grooves on a record to play sound. The first of these techniques, scanning tunneling microscopy (STM), was developed by IBM researchers in 1981. STM relies on quantum mechanical tunneling effects. As the microscope's tip is scanned over a surface, electrons tunnel from the tip into the surface. The tunneling current is measured and can be transformed into an image. (Fun fact: In 1989, IBM researchers used STM to spell out "IBM" using 35 xenon atoms on a nickel substrate.)
Hla has been working for the last 12 years to develop an X-ray version of STM: synchrotron X-ray-scanning tunneling microscopy, or SX-STM, which would enable scientists to identify the type of atom and its chemical state. X-ray imaging methods like synchrotron radiation are widely used across myriad disciplines, including art and archaeology. But the smallest amount to date that can be X-rayed is an attogram, or roughly 10,000 atoms. That's because the X-ray emission of a single atom is just too weak to be detected—until now.
SX-STM combines conventional synchrotron radiation with quantum tunneling. It replaces the conventional X-ray detector used in most synchrotron radiation experiments with a different kind of detector: a sharp metal tip placed extremely close to the sample, the better to collect electrons pushed into an excited state by the X-rays.
With Hla et al.'s method, X-rays hit the sample and excite the core electrons, which then tunnel to the detector tip. The photoabsorption of the core electrons serves as a kind of elemental fingerprint for identifying the type of atoms in a material. The team tested their method at the XTIP beam line at Argonne's Advanced Photon Source, using an iron atom and a terbium atom (inserted into supramolecules, which served as hosts).
And that's not all. “We have detected the chemical states of individual atoms as well,” said Hla. “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.” Also, Hla's team has developed another technique called X-ray-excited resonance tunneling (X-ERT), which will allow them to detect the orientation of the orbital of a single molecule on a material surface.
DOI: Nature, 2023. 10.1038/s41586-023-06011-w (About DOIs).
