Using transmission electron microscopy (TEM) to both image and energize a bunch of atoms, scientists have for the very first time recorded footage of atoms forming and breaking bonds in real time.
“To our knowledge, this is the first time when bond evolution, breaking and formation was recorded on film at the atomic scale,” said Andrei Khlobystov, professor at the University of Nottingham and one of the lead authors of the new study.
TEM resembles traditional film photography, in the sense that it blasts a beam of electrons onto a very thin sample to produce images of such a high resolution that individual atoms can be discerned.
Chemical bonding takes place a microscopic scale of about half a million times smaller than the width of a human hair.
Imaging the ultra-fast processes that occur in such a tiny world was a huge challenge but the international team of researchers rose to the occasion.
The researchers, which included scientists from the University of Nottingham and the SALVE Project at the University of Ulm, introduced pairs of rhenium atoms into a nano-test tube made of single-walled carbon nanotubes. An individual rhenium atom is just 205 picometers across, or 205 trillionths of a meter, whereas a carbon nanotube’s diameter ranges from one nanometer to a few nanometers.
The rhenium atoms bonded right to the side of the carbon nanotubes, forming a quadruple bond between them — and this confinement proved critical for focusing and recording footage of the atomic bonding. Writing in the journal Science Advances, the authors explain that the curvature of the nanotubes influences the bonding modes available to the rhenium atoms to form rhenium molecules.
“It was surprisingly clear how the two atoms move in pairs, clearly indicating a bond between them,” said Kecheng Cao, an author of the study. “Importantly, as Re2 moves down the nanotube, the bond length changes, indicating that the bond becomes stronger or weaker depending on the environment around the atoms.”
Eventually, the researchers managed to record videos of the atoms forming pure metal-metal bonds in real-time. What’s more, the microscope’s electron beam not only recorded the experiment but also facilitated it by adding electron energy.
In the video, the atom pairs (black dots) travel along the narrow gap between the carbon nanotubes. When the length of the bond exceeded the size of the atoms, the bond broke. After traveling independently for a while, the two atoms reformed once more into a molecule because this more stable configuration lowers their energy.
The findings are exciting not just because they signify an unprecedented achievement, but also because they add to our knowledge of metallic bonds, which are still quite mysterious. Metallic bonds are notoriously difficult to study because other atoms and molecules are drawn to interact with them, distorting bond length and other factors that scientists would like to measure.