Pssst… Annoying Graphene Tunneling Habit Tamed By Nanoscale “Whispering Gallery”

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Graphene has firmly entrenched itself as the nanomaterial of the next millennium, but its finicky, fussy behavior has challenged researchers who seek to tap its powerful electronic properties for next-generation solar technology and other devices. Part of the problem has been the difficulty involved in fabricating pure, defect-free samples of a material that is only one atom thick — which is what graphene is, a sheet of carbon atoms arranged in a distinctive chickenwire lattice.

However, now that pure graphene is more readily available for study, the game is afoot. The latest graphene mystery to unravel comes to us courtesy of a research team headed up by NIST, the US Commerce Department’s National Institute of Standards and Technology. It was inspired by the “whispering gallery” effect, so let’s take a quick peek at the whispering gallery phenomenon first, and see what’s what.

The Whispering Gallery And Solar Cells

Whispering gallery refers to a room with curved walls that create a strange acoustical effect. If you’re standing in the right spot, you can clearly and distinctly hear someone whispering from all the way across the room, but you can barely hear someone who’s whispering just a few feet away.

It happens because sound travels along a curved surface much farther than it will along a flat one. Statuary Hall in the US Capitol Building is one such example, and there are many others around the world.

Back in 2012, CleanTechnica took note when a team of engineers at Stanford University pursued nanoscale whispering galleries to improve solar cell efficiency.

Inspired by the whispering gallery effect, the research team fabricated thin nanoscale shells of nanocyrstalline silicon to form “optical whispering galleries” that trap light and force it to circulate “round and round” rather than passing straight through.

In the computer simulation below, waves of light (reddish orange) travel from the top to the bottom and strike a layer of nanoshells. The red pattern shows how the light resonates in the nanoshell.

As the light continues to circulate, the silicon absorbs more energy, thereby boosting the cell’s solar conversion efficiency. Here’s one of the co-lead authors of the study waxing enthusiastic about the process:

The nanometer spherical shells really hit a sweet spot and maximize the absorption efficiency of the film. The shells both allow light to enter the film easily and they trap it so as to enhance the absorption in a way larger-scale counterparts cannot.

 

Graphene And Whispering Galleries

To be clear, the NIST team doesn’t seem to have solar cell efficiency in their sights, at least not yet. Their new “whispering gallery” graphene study addresses potential applications in other areas where light wave whispering galleries are already beginning to emerge, such as spectroscopy and communications. However, given the emergence of graphene-enhanced solar cells, you might see this foundational R&D popping up in photovoltaics, too.

The problem that the NIST study addresses is the tendency of electrons in graphene to ignore or “tunnel” straight through a barrier, when they encounter it head on. Normally in a semiconductor material, electrons reflect from a potential barrier.

The tunnel thing means that electrons in graphene are harder to steer than electrons in other semiconductor materials. That presents a serious obstacle to realizing the full potential of graphene for engineering more efficient electronic devices.

The idea behind the study is to create a nanoscale, electronic analogue of the wave effect in graphene, to make the electrons bounce along the curved walls of the barrier, rather than tunnel on through. The result, according to one member of the NIST team, is “unlike anything you see in any other electron based system.

Here’s how they did it:

…the team first enriched the graphene with electrons from a conductive plate mounted below it. With the graphene now crackling with electrons, the research team used the voltage from a scanning tunneling microscope (STM) to push some of them out of a nanoscale-sized area. This created the whispering gallery, which is like a circular wall of mirrors to the electron.

But wait, there’s more:

The team can control the size and strength, i.e., the leakiness, of the electronic whispering gallery by varying the STM tip’s voltage. The probe not only creates whispering gallery modes, but can detect them as well.

The full study is available at the journal Science under the title, “Creating and probing electron whispering-gallery modes in graphene.”

For those of you on the go, the top image in this article shows the voltage from a scanning tunneling microscope (right), which pushes out graphene electrons. The whispering gallery is represented by those funny things sticking out on the left. To the graphene electrons, it all looks like a wall of funhouse mirrors.

If you’re keeping score at home, a team at NIST fabricated the graphene device, and physicists the Massachusetts Institute of Technology provided the whispering gallery theory as applied to graphene.

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Image Credits: Top, graphene “whispering gallery” by Jon Wyrick, CNST/NIST; middle, Statuary Hall courtesy of Architect of the Capitol; bottom, nanocrystalline-silicon nanoshells by Jie Yao.