How one photon can excite multiple electrons

By a conservative estimate, the researchers report that within their superlattice one photon could "kick" as many as five electrons to flow as current. (Credit: iStockphoto)

New work with graphene exploits some surprising quantum-level interactions to give one photon multiple potential electron partners.

In the quest to harvest light for electronics, the focal point is the moment when photons—light particles—encounter electrons, those negatively-charged subatomic particles that form the basis of our modern electronic lives. If conditions are right when electrons and photons meet, an exchange of energy can occur. Maximizing that transfer of energy is the key to making efficient light-captured energetics possible.

“This is the ideal, but finding high efficiency is very difficult,” says Sanfeng Wu, a doctoral student in physics at the University of Washington. “Researchers have been looking for materials that will let them do this—one way is to make each absorbed photon transfer all of its energy to many electrons, instead of just one electron in traditional devices.”

The discovery could lead to highly efficient devices that could harvest light with a large energy profit.

superlattice gif
The Moiré superlattice the researchers created by aligning graphene and boron-nitride. (Credit: Sanfeng Wu)

In traditional light-harvesting methods, energy from one photon only excites one electron or none depending on the absorber’s energy gap, transferring just a small portion of light energy into electricity. The remaining energy is lost as heat. But in the paper in Science Advances, Wu, associate professor Xiaodong Xu, and colleagues describe one promising approach to coax photons into stimulating multiple electrons.

“Graphene is a substance with many exciting properties,” says Wu, the paper’s lead author. “For our purposes, it shows a very efficient interaction with light.”

Graphene sandwiches

Graphene is a two-dimensional hexagonal lattice of carbon atoms bonded to one another, and electrons are able to move easily within graphene. The researchers took a single layer of graphene—just one sheet of carbon atoms thick—and sandwiched it between two thin layers of a material called boron-nitride.

“Boron-nitride has a lattice structure that is very similar to graphene, but has very different chemical properties,” says Wu. “Electrons do not flow easily within boron-nitride; it essentially acts as an insulator.”

Xu and Wu discovered that when the graphene layer’s lattice is aligned with the layers of boron-nitride, a type of “superlattice” is created with properties allowing efficient optoelectronics that researchers had sought. These properties rely on quantum mechanics, the occasionally baffling rules that govern interactions between all known particles of matter. Wu and Xu detected unique quantum regions within the superlattice known as Van Hove singularities.

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“These are regions of huge electron density of states, and they were not accessed in either the graphene or boron-nitride alone,” says Wu. “We only created these high electron density regions in an accessible way when both layers were aligned together.”

When Xu and Wu directed energetic photons toward the superlattice, they discovered that those Van Hove singularities were sites where one energized photon could transfer its energy to multiple electrons that are subsequently collected by electrodes—not just one electron or none with the remaining energy lost as heat. By a conservative estimate, Xu and Wu report that within this superlattice one photon could “kick” as many as five electrons to flow as current.

‘Graphene is a tiger’

Future work would need to uncover how to organize the excited electrons into electrical current for optimizing the energy-converting efficiency and remove some of the more cumbersome properties of their superlattice, such as the need for a magnetic field. But they believe this efficient process between photons and electrons represents major progress.

“Graphene is a tiger with great potential for optoelectronics, but locked in a cage,” says Wu. “The singularities in this superlattice are a key to unlocking that cage and releasing graphene’s potential for light harvesting application.”

Coauthors are from Columbia University; the National High Magnetic Field Laboratory in Florida; Carnegie Mellon University; the National Institute for Materials Science in Japan. The National Science Foundation and the US Air Force Office of Scientific Research supported the work at the University of Washington. Xu, who has appointments in the department of materials science & engineering and the department of physics, acknowledges the support from the Boeing Distinguished Professorship and Washington’s state-funded Clean Energy Institute.

Source: University of Washington