Ohio University physicist Saw Wai Hla and his colleagues were able to scan a single iron atom hidden in a complex molecule, something that had never been done with an X-ray before.
Extremely powerful microscopes can take pictures of individual atoms, and indeed they almost always do, because we live in the future. But without X-rays and spectroscopy (a way of taking an object’s chemical fingerprint based on the wavelengths of light it absorbs or emits), images alone can’t tell scientists which element they’re looking at.
In recent experiments, Hla and his colleagues have identified individual atoms and measured some of their key properties. To do this, the researchers combined powerful, focused X-rays from a particle accelerator called a synchrotron with a technique called scanning tunneling microscopy, which uses a conductive tip to scan the surface of a sample. Their goal, says Hla Reverse, was to use X-ray spectroscopy at the ultimate limit of the atomic scale. X-rays were discovered in 1895, nearly 130 years ago, but have never been able to detect a single atom.
They published their findings in the journal Nature.
Fast electrons and bright lights
A synchrotron, like the one at Argonne National Laboratory in Illinois, accelerates electrons to nearly the speed of light, then sends them hurtling along a curved track. As the racing electrons round each bend in the track, they flash a bright light that is a tiny version of it Tron. Equipment connected to the synchrotron splits light into different wavelengths, sending infrared light along one beamline and X-ray light along another, for example.
The X-rays produced this way are brighter and more concentrated than what an ordinary X-ray machine can offer. Physicists can observe how light interacts with molecules in a material to learn tiny details about its structure and composition. But how small is small? If you want to know if there is iron in a particular material, for example, you should hope that your sample contains at least a few thousand iron atoms, otherwise synchrotron radiography probably won’t notice it.
But there’s a good reason materials scientists want to detect individual atoms in much smaller samples.
If the elemental and chemical state of an atom can be detected, then there will be a huge impact in many areas of research, Hla says.
The Quantum Realm
This is where quantum tunneling comes in handy. When synchrotron X-rays hit a sample, they pump energy into the atoms. That new burst of energy angers some of the electrons orbiting closer to the center of the atom, and they manage to break free.
Hla and his colleagues held the sharp metal tip of a scanning tunneling microscope instrument just half a nanometer away from the sample close enough to form a quantum tunnel, letting the newly freed electrons drift from the sample to the instrument.
The electrons arriving at the detector tip carry information about the atom they came from, such as which wavelengths of X-ray light the atom has absorbed. Since each chemical element absorbs, reflects, and emits a specific set of wavelengths of light, knowing which wavelengths the sample absorbed can reveal exactly which element an electron came from.
And in this case, Hla and his colleagues had just scanned the single iron atom in a large, complicated ring-shaped molecule. The scan also revealed how many electrons the iron atom was missing: two, in this case. This affects how an atom can react with other atoms to form new chemical bonds, so it’s an important thing to know.
Hla and his colleagues repeated the experiment with a different, large and complicated molecule, this time with a single atom of a rare-earth element called terbium hidden within it. And once again, the detector identified the terbium atom and its chemical state.
Being able to combine detailed images with X-ray scans of a single atom of such a precious element could be of great help to engineers and materials scientists in the future.
It could also be useful in medical research, Hla says. It will also impact quantum information science, just to name a few. Next, Hla says he and his colleagues hope to measure the magnetic properties of a single atom, which will be useful for solid-state electronics.
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