Researchers tune the strain in graphene drumheads to create quantum dots

Jun 21, 2012
This is an illustration of the deformation of a micron sized graphene drumhead from the competing forces of an STM probe tip and back gating electrode. The strain in the graphene membrane creates pseudo (not real) magnetic fields that are spatially alternating up and down (red and blue colored graphic) which confines the graphene carriers and creates quantized quantum dot like energy levels. The background is a colored scanning electron microscopy image of the graphene drumheads fabricated from a single layer of graphene exfoliated over an array of one micron-size pits etched in silicon dioxide substrates. Credit: N. Klimov and T. Li, NIST/UMD

Researchers have shown that they can tune the strain in graphene suspended like drumheads over microscopic holes in a substrate of silicon oxide using the tip of an advanced scanning probe microscope and a conducting plate below the substrate. Tuning the strain enabled the group to create areas in the graphene where electrons behaved as though they were confined to quantum dots.

Tightening or relaxing the tension on a drumhead will change the way the drum sounds. The same goes for drumheads made from graphene, only instead of changing the sound, stretching graphene has a profound effect on the material's electrical properties. Researchers working at the National Institute of Standards and Technology and the University of Maryland have shown that subjecting graphene to can mimic the effects of magnetic fields and create a quantum dot, an exotic type of semiconductor with a wide range of potential uses in .

The results were reported in the June 22, 2012, issue of Science.

Graphene is a single layer of arranged in a . Able to conduct electricity with little resistance at room temperature, graphene is a for applications ranging from to high speed transistors.

However, the same lack of that makes graphene attractive for some uses also makes it ill-suited for digital computing applications. Graphene conducts electricity so well because it doesn't have a band gap—an energetic threshold beneath which the material won't conduct electricity. This means that graphene can't be turned "off," and computers need "on" and "off" signals to transmit and process information.

Because substrates slow the speed of electrons moving through graphene, Nikolai Klimov, a University of Maryland postdoctoral researcher working at NIST, suspended the graphene over shallow holes in a substrate of silicon dioxide—essentially making a set of graphene drumheads. To measure the graphene's properties, the team used a unique scanning probe microscope designed and built at NIST.

When they began to probe the drumheads, they found that the graphene rose up to meet the tip of the microscope— a result of the van der Waals force, a weak electrical force that creates attraction between objects that are very close to each other.

"While our instrument was telling us that the graphene was shaped like a bubble clamped at the edges, the simulations run by our colleagues at the University of Maryland showed that we were only detecting the graphene's highest point," says NIST scientist Nikolai Zhitenev. "Their calculations showed that the shape was actually more like the shape you would get if you poke into the surface of an inflated balloon, like a teepee or circus tent."

The researchers discovered that they could tune the strain in the drumhead using the conducting plate upon which the graphene and substrate were mounted to create a countervailing attraction and pull the drumhead down. In this way, they could pull the graphene into or out of the hole below it. And their measurements showed that changing the degree of strain changed the material's .

For instance, the group observed that when they pulled the graphene membrane into the tent-like shape, the region at the apex acted just like a quantum dot, a type of semiconductor in which electrons are confined to a small region of space.

Creating semiconducting regions like in graphene by modifying its shape might give scientists the best of both worlds: high speed and the crucial to computing and other applications.

According to Zhitenev, the electrons flow through graphene by following the segments of the hexagons. Stretching the hexagons lowers the energy near the apex of the tent-like shape and causes the electrons to move in closed, clover-shaped orbits—mimicking nearly exactly how the electrons would move in a vertically varied magnetic field.

"This behavior is really quite remarkable," says Zhitenev. "There is a little bit of electron leakage, but we found that if we complemented the pseudomagnetic field with an actual magnetic field, there was no leakage whatsoever."

"Normally, to make a graphene quantum dot, you would have to cut out a nanosize piece of ," says NIST Fellow Joseph Stroscio. "Our work shows that you can achieve the same thing with strain-induced pseudomagnetic fields. It's a great result, and a significant step toward developing future graphene-based devices."

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More information: N. Klimov, S. Jung, T. Li, C. Wright, S. Solares, D. Newell, N. Zhitenev, and J. Stroscio. Electromechanical Properties of Graphene Drumheads. Science. June 22, 2012.

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dnatwork
4.7 / 5 (3) Jun 21, 2012
I wonder if they could tune the quantum-dot properties of the graphene drumheads to create LEDs that emit different wavelengths of light depending on the underlying magnetic field. Then you wouldn't need multiple subpixels or filters to generate all the colors in the spectrum, and you could have a mighty nice display screen. With micron-sized pixels, would that be about 1 million DPI? The limiting factor would be the resolution and color gamut of your image data, not the capability of your monitor to reproduce them. And the black levels should be very black, too.
PhotonX
not rated yet Jun 21, 2012
I wonder if they could tune the quantum-dot properties of the graphene drumheads to create LEDs that emit different wavelengths of light depending on the underlying magnetic field. Then you wouldn't need multiple subpixels or filters to generate all the colors in the spectrum, and you could have a mighty nice display screen. With micron-sized pixels, would that be about 1 million DPI? The limiting factor would be the resolution and color gamut of your image data, not the capability of your monitor to reproduce them. And the black levels should be very black, too.
I don't understand how that would be made to work, but then that's the point-I don't understand. I also don't understand people who still think that nanotech is not a game changer, and am glad to see you aren't one of them.
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Practical limiting factors, if this could be made to work, will of course be price, as it always is at first, then energy consumption second, at least for portable displays.
Ojorf
not rated yet Jun 22, 2012
I wonder if they could tune the quantum-dot properties of the graphene drumheads to create LEDs that emit different wavelengths of light depending on the underlying magnetic field.


The very first thing I thought as I was reading this, can you make a display. Very cool!