Researchers discover why single molecules fall over

Nanoscale machinery has many uses, including in drug delivery, single-atom transistor technology and memory storage. But nanoscale machinery obviously needs to be assembled at the nanoscale, which is a considerable challenge.

For nanotechnology engineers, the ultimate goal is to be able to assemble functional machinery part-by-part at the nanoscale. In the macroscopic world, we can simply grab items to assemble them. While 'grabbing' single molecules is now possible, their quantum nature makes their response to manipulation unpredictable, limiting the ability to assemble molecules one-by-one.

But this prospect is now a step closer to reality thanks to an international effort led by Christian Wagner at the Research Centre Jülich in Germany, which also involved researchers from the Department of Chemistry at the University of Warwick in the UK. In a paper in Science Advances, this research team reports developing a generic stabilization mechanism for a single standing molecule, which can be used for the rational design and construction of three-dimensional molecular devices at surfaces.

The scanning probe microscope (SPM) has brought the promise of molecular-scale fabrication closer to reality, because it offers the capability to rearrange atoms and molecules on surfaces, thereby allowing the creation of metastable structures that do not form spontaneously. Using SPM, Wagner and his team were able to interact with a single standing molecule – perylene-tetracarboxylic dianhydride (PTCDA) – on a surface. This allowed them to study the thermal stability and temperature at which the molecule would cease to be stable and would drop back into its natural state where it lies flat on the surface. They found that this temperature was -259.15°C, only 14°C above absolute zero.

Quantum chemical calculations performed in collaboration with Reinhard Maurer at the University of Warwick revealed that the subtle stability of the molecule stems from the competition of two strong counteracting quantum forces. These are the long-range attraction from the surface and the short-range restoring force arising from the anchor point between the molecule and the surface.

“The balance of interactions that keeps the molecule from falling over is very subtle and a true challenge for our quantum chemical simulation methods,” said Maurer. “In addition to teaching us about the fundamental mechanisms that stabilize such unusual nanostructures, the project also helped us to assess and improve the capabilities of our methods.”

“To make technological use of the fascinating quantum properties of individual molecules, we need to find the right balance: they must be immobilized on a surface, but without fixing them too strongly, otherwise they would lose these properties,” said Wagner. “Standing molecules are ideal in that respect. To measure how stable they actually are, we had to stand them up over and over again with a sharp metal needle and time how long they survived at different temperatures.”

Now that the interactions that give rise to a stable standing molecule are known, future research can work towards designing better molecules and molecule-surface links to tune those quantum interactions. This can help to increase the stability and the temperature at which molecules can be switched into standing arrays under workable conditions, raising the prospect of nanofabrication of machinery at the nanoscale.

This story is adapted from material from the University of Warwick, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.