"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 electrical properties.
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 quantum dots in graphene by modifying its shape might give scientists the best of both worlds: high speed and the band gap 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 orbitsmimicking 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
|Contact: Mark Esser|
National Institute of Standards and Technology (NIST)