Nano domino with molecules – How a tilting molecule helps nanotechnology –

Three years ago, physicists succeeded for the first time in setting up a nano domino in a controlled manner – a flat molecule stood upright on a silver surface. Now, after months of experimentation, they have knocked it over again. What sounds more like a game than relevant research, has an important purpose: Because erect molecules could be important components of nanomachines and sensors, their stability has to be known – and also which forces influence them.

Whether it’s a molecular machine, nano-robot or quantum component: in nanotechnology, the goal is to construct tiny sensors, machines and other components from individual molecules or atomic building blocks – be it by self-organization or with the help of programmable nano-assemblers. Usually, a substrate serves as the background for this nano-Lego.

How stable are three-dimensional nano-components?

The problem, however, is that the molecular components can often only show their full functionality if they do not lie flat on this surface, but are erect and only weakly coupled to their base. In 2018, researchers succeeded for the first time in placing a domino-shaped organic molecule in a controlled manner on a silver surface under ultra-cold conditions and then erecting it so that it stood on edge.

However, if you want to connect such “dominoes” to larger constructions or to use them practically, they have to remain reasonably stable in this position: “Beyond the flat land of the surface there is an enormous wealth of structures and functionalities, but stability is crucial for them”, explain Marvin Knol from Forschungszentrum Jülich and his colleagues.

In the scanning tunneling microscope it can be seen whether the nano domino is lying or standing. © Knol et al./ Science Advances, CC-by-sa 4.0

Tipped over by heat

Therefore, the team has now subjected the domino molecule to a stress test and examined when it falls over again and why. “In our case, we increased the temperature step by step, shaking the molecule more and more, until it finally fell over,” explains senior author Christian Wagner from FZ Jülich. The increase in temperature supplies the molecule and its base with energy and this leads to ever stronger vibrations.

Using a scanning tunneling microscope, the team was able to observe when the critical point had been reached. The result: A temperature increase of around ten degrees – from five to around 14 Kelvin – was enough to make the nano domino fall over. This confirms that the molecule is only very weakly coupled to the surface, the researchers report.

Two opposing forces

Even more important, however: the physicists were also able to determine which forces are decisive for the stability of the domino molecule. According to this, its precarious balance is based on the interaction of two opposing effects – the short-distance covalent bonds between the atoms and the van der Waals force that also acts between the nanodomino and the substrate. The latter strives to pull the molecule back flat onto the surface.

“The balance of interactions that keep the molecule from tipping over is extremely subtle and a real challenge for our methods of quantum chemical simulation,” explains co-author Reinhard Maurer from the University of Warwick. “The project not only revealed to us which fundamental mechanisms stabilize such unusual nanostructures, it also helps us to refine our methods.”

Helpful for future nanoconstructs

In the opinion of the research team, the insights gained represent an important step towards molecular production in three dimensions. Because based on the new findings, more stable molecular constructs can now be developed that remain upright even at not-so-low temperatures. (Science Advances, 2021; doi: 10.1126/sciadv.abj9751)

Source: University of Warwick, Forschungszentrum Jülich

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