Method to control ‘hot’ electrons comes a step closer

London: In a promising step towards being able to manipulate and control the behaviour of high energy, or ‘hot’, electrons, scientists have, for the first time, identified a method of visualising the quantum behaviour of electrons on a surface.

Hot electrons are necessary for a number of processes and the implications of being able to manipulate their behaviour are far-reaching — from enhancing the efficiency of solar energy, to improving the targetting of radiotherapy for cancer treatment.

“Hot electrons are essential for a number of processes — certain technologies are entirely reliant on them. But they’re notoriously difficult to observe due to their short lifespan, about a millionth of a billionth of a second,” said one of the researchers Peter Sloan from University of Bath in England.

“This visualisation technique gives us a really new level of understanding,” Sloan noted.

In the experiment, a Scanning Tunnelling Microscope was used to inject electrons into a silicon surface, decorated with toluene molecules. As the injected charge propagated from the tip, it induced the molecules to react and ‘lift off’ from the surface.

By measuring the precise atomic positions from which molecules departed on injection, the team were able to identify that electrons were governed by quantum mechanics close to the tip, and then by more classical behaviour further away.

The team found that the molecular lift-off was “suppressed” near the point of charge injection, because the classical behaviour was inhibited.

The number of reactions close to the tip increased rapidly until reaching a radius, up to 15 nanometres away, before seeing relatively slow decay of reactions beyond that point more in keeping with classical behaviour.

This radius, at which the behaviour changes from quantum to classical, could be altered by varying the energy of the electrons injected, said the study published in the journal Nature Communications.

“When an electron is captured by a molecule of toluene, we see the molecule lift off from the surface — imagine the Apollo lander leaving the moon’s surface. By comparing before and after images of the surface we measure the pattern of these molecular launch sites and reveal the behaviour of electrons in a manner not possible before,” Professor Richard Palmer from the University of Birmingham explained.