Physicists control electrons at femtosecond timescales | MIT News

If you shine a lightweight on a conducting floor like silicon or graphene, that gentle jump-starts sure electrons into high-energy states and kicks off a cascade of interactions that occurs sooner than the blink of a watch. Inside only a few femtoseconds — a thousand trillionth of a second — these energized electrons can scatter amongst different electrons like balls on a billiard desk, rapidly dissipating vitality in an ultrafast course of often called thermalization.

Now physicists at MIT have provide you with a method to manipulate electrons in graphene inside the first few femtoseconds of photo-excitation. With their approach, the researchers can redirect these high-energy electrons earlier than they work together with different electrons within the materials.

The group’s ultrafast management of high-energy electrons could in the end result in extra environment friendly photovoltaic and energy-harvesting units, which seize photo-excited electrons earlier than they lose their vitality to thermalization.

“We’re intellectually enthusiastic about whether or not it will have technological functions,” says Pablo Jarillo-Herrero, affiliate professor of physics at MIT. “It’s too quickly to know, however there are particular angles of this the place it is clear there may be methods to engineer vitality movement or switch in methods which might be novel. Now we’d like extra individuals fascinated about this.”

The group’s outcomes are printed this week within the journal Nature Physics. Jarillo-Herrero’s co-authors embody lead writer and graduate pupil Qiong Ma, together with Jing Kong, professor {of electrical} engineering and laptop science, and Nuh Gedik, affiliate professor of physics.

Directing vitality movement

In a earlier, unrelated experiment, Jarillo-Herrero and his colleagues fabricated an extremely skinny, sandwich-like machine composed of two sheets of graphene, every a single atom skinny, separated by an insulating layer of boron-nitride. The group had been subjecting the construction to various intensities of voltage and light-weight, and observing the ensuing present, or movement of electrons, from one layer to a different.

They discovered that, at sure voltages and wavelengths of sunshine, they may produce a comparatively robust present throughout the boron-nitride layer — a sign that high-energy electrons have been tunneling from one graphene sheet to the opposite with out dropping a lot vitality.

The researchers adopted up on their observations to see how the movement of present inside their machine modified as they various the voltage and light-weight wavelength they utilized. As they shone gentle onto the highest layer of graphene, they have been in a position to tune the movement of present inside only a few femtoseconds.

Relying on the voltage and light-weight wavelength utilized, the researchers may direct high-energy electrons to both keep and dissipate their vitality inside the high graphene layer, or tunnel throughout the boron-nitride layer and into the underside graphene sheet, the place they may then work together with different electrons and scatter their vitality.

“Sometimes you possibly can solely begin doing issues after perhaps 1,000 femtoseconds, after these ultrafast interactions have already taken place,” Jarillo-Herrero says. “We’re in a position to … determine whether or not the electron goes right here or there earlier than it interacts with some other electron, inside just a few femtoseconds.”

Getting out of graphene

The group’s ultrafast management could stem from the character of graphene itself. As a result of graphene is so exceptionally skinny, electrons don’t have very far to leap in the event that they get the appropriate push.

“This femtosecond response is due to the 2-D construction of graphene,” Ma says. “It’s only one atom thick, and the electrons are already on the floor, so it’s simpler for them to leap out and onto one other materials.”

Because the group quickly discovered, coaxing electrons to leap from one sheet of graphene to a different required the appropriate mixture of voltage and light-weight. Ma and Jarillo-Herrero plotted their experimental outcomes and recognized combos of voltage and light-weight wavelength that might direct high-energy electrons to both keep inside the high graphene layer or soar to the underside layer.

“Say you inform me, ‘I’d prefer to have these electrons leaping from one layer to a different, and I solely have blue photons,’” Jarillo-Herrero says. “I can say that at the least with blue photons, it’s a must to apply this voltage. When you solely have inexperienced photons, then it’s best to apply extra voltage than this. That’s what we’re in a position to map.”

Finally, he says these outcomes could assist to enhance photo voltaic cells and vitality harvesting units by enabling them to seize and use extra photo-excited electrons.

“Trendy photo voltaic cells work such that if a photon comes and could be absorbed by silicon, it contributes to present in your photovoltaic machine,” Jarillo-Herrero says. “If the sunshine consists of lower-energy infrared photons, these should not absorbed by silicon. That limits severely the effectivity of silicon photo voltaic cells. Now with our machine, in precept you possibly can take up many lower-energy photons, people who silicon simply lets via, such that the buildup of that vitality can contribute to a present in your electrical circuit. This can be a mechanism by which you possibly can consider doing it.”

“This very thrilling work from the MIT group demonstrates a brand new method to manipulate photo-excited ‘sizzling’ electrons in graphene throughout atomically skinny obstacles,” says Philip Kim, professor of physics at Harvard College, who was not concerned within the analysis. “Their findings lay an vital step towards the conclusion of novel optoelectronic and energy-harvesting units primarily based on graphene heterostructures.”

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