Communications of the ACM
Scientists Visualize Quantum Behavior of Hot Electrons
"These findings suggest future atomic-scale quantum devices could work without the need for a tank of liquid helium coolant," says Professor Richard Palmer from the University of Birmingham, shown here with a Scanning Tunneling Microscope.
Credit: University of Birmingham
Scientists have, for the first time, identified a method of visualizing the quantum behavior of electrons on a surface. The findings present a promising step forward towards being able to manipulate and control the behavior of high energy, or 'hot,' electrons.
A Scanning Tunneling 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 was able to identify that electrons were governed by quantum mechanics close to the tip, and then by more classical behavior further away.
The team found that the molecular lift-off was "suppressed" near the point of charge injection, because the classical behavior was inhibited. The number of reactions close to the tip increased rapidly until reaching a radius, up to 15 nanometers away, before seeing relatively slow decay of reactions beyond that point more in keeping with classical behavior. This radius, at which the behavior changes from quantum to classical, could be altered by varying the energy of the electrons injected.
The research is described in "Initiating and Imaging the Coherent Surface Dynamics of Charge Carriers in Real Space," published in the journal Nature Communications, and is the result of ongoing collaboration between the University of Birmingham and the University of Bath.
"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," says Professor Richard Palmer, from the University of Birmingham. "By comparing before and after images of the surface we measure the pattern of these molecular launch sites and reveal the behavior of electrons in a manner not possible before.
"These findings are, crucially, undertaken at room temperature" Palmer says. "They show that the quantum behavior of electrons which is easily accessible at close to absolute zero temperature [-273°C] persist under the more balmy conditions of room temperature and over a 'large' 15-nanometer scale. These findings suggest future atomic-scale quantum devices could work without the need for a tank of liquid helium coolant."
Peter Sloan, from the University of Bath, adds: "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. This visualization technique gives us a really new level of understanding."
Now that the team has developed the method of visualizing quantum transport, the goal is to understand how to control and manipulate the wave function of the electron. This could be by injecting electrons through a cluster of metal atoms, or by manipulating the surfaces themselves to harness the quantum effects of electrons.
The implications of being able to manipulate the behavior of hot electrons are far-reaching; from improving the efficiency of solar energy, to improving the targeting of radiotherapy for cancer treatment.
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