In a phenomenal feat, scientists have for the first time identified an element by X-raying a single atom.
Discovered in 1895 by German physicist Wilhelm Conrad Rontgen, X-rays have become a staple in our world, from medical diagnostics to security screenings in airports.
In materials science, X-rays are an important way to identify the type of a material. As scientists improved this technology through the years, the quantity of a sample required for X-rays to identify it has plummeted. Until the current experiment was conducted at Argonne National Laboratory, Illinois, the minimum amount required for X-ray detection was an attogram (which is 1-quintillionth of a gram). This is about the weight of a small virus.
The reason there’s a minimum amount of material required is that an atom’s response to being hit by X-rays can be very weak. The more atoms there are, the better detectors can pick up on them.
While scientists have previously imaged atoms using scanning microscopes, X-raying an individual atom could help them identify which element it is as well, which was not possible before.
So how did the scientists X-ray a lone atom?
The team, led by Saw Wai Hla from Ohio University, used a synchrotron X-ray instrument developed specifically for the experiment. As samples, they used a single atom of iron and terbium, a rare earth metal, each.
The study paper, published in Nature on May 31, said that the scientists modified a conventional X-ray detector to add a sharp metal tip that would be moved to be extremely close to a sample. This is to improve the detector’s ability to record any signals from the atom.
They used a method called synchrotron X-ray scanning tunnelling microscopy or SX-STM. The atom was hit with X-ray photons. As expected, the electrons in the atom absorbed only photons of certain frequencies. Photons of the other frequencies passed through. Using a spectroscope, the team determined which frequencies had been absorbed.
This absorption spectrum is unique to each element and can be used to identify it.
Second, when electrons absorb the X-ray photons, they become excited and tunnel their way to the metal tip of the detector. There, a small voltage allows the electron to create an electric current, which is recorded and measured. In one version of the experiment, the scientists placed the metal tip as close as 0.5 nanometres from the atom.
Being able to identify a material using only one atom could, according to the group, revolutionise research in material science, quantum mechanics, and other areas.
The study also characterised the chemical states of the atoms. They found that as a rare earth metal, the terbium atom maintains its chemical state and is isolated while the iron atom interacts heavily with its surroundings.
This is useful because rare earth metals find widespread application in electronic devices, and understanding their properties better could help researchers manipulate their atoms to greater precision.
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