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In a previous, unrelated experiment, Jarillo-Herrero and his colleagues fabricated an incredibly thin, sandwich-like device composed of two sheets of graphene, each a single atom thin, separated by an insulating layer of boron-nitride. The group had been subjecting the structure to varying intensities of voltage and light, and observing the resulting current, or flow of electrons, from one layer to another.
They found that, at certain voltages and wavelengths of light, they could produce a relatively strong current across the boron-nitride layer — an indication that high-energy electrons were tunneling from one graphene sheet to the other without losing much energy.
The researchers followed up on their observations to see how the flow of current within their device changed as they varied the voltage and light wavelength they applied. As they shone light onto the top layer of graphene, they were able to tune the flow of current within just a few femtoseconds.
Depending on the voltage and light wavelength applied, the researchers could direct high-energy electrons to either stay and dissipate their energy within the top graphene layer, or tunnel across the boron-nitride layer and into the bottom graphene sheet, where they could then interact with other electrons and scatter their energy.
“Typically you can only start doing things after maybe 1,000 femtoseconds, after these ultrafast interactions have already taken place,” Jarillo-Herrero says. “We’re able to … decide whether the electron goes here or there before it interacts with any other electron, within a few femtoseconds.”
Getting out of graphene
The team’s ultrafast control may stem from the nature of graphene itself. Because graphene is so exceptionally thin, electrons don’t have very far to jump if they get the right push.
“This femtosecond response is because of the 2-D structure of graphene,” Ma says. “It’s just one atom thick, and the electrons are already at the surface, so it’s easier for them to jump out and onto another material.”
As the team soon found, coaxing electrons to jump from one sheet of graphene to another required the right combination of voltage and light. Ma and Jarillo-Herrero plotted their experimental results and identified combinations of voltage and light wavelength that would direct high-energy electrons to either stay within the top graphene layer or jump to the bottom layer.
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