Excitons pave the way to higher-performance electronics

January 4, 2019 by Sarah Perrin, Ecole Polytechnique Federale de Lausanne
Credit: Mediacom

After developing a method to control exciton flows at room temperature, EPFL scientists have discovered new properties of these quasiparticles that can lead to more energy-efficient electronic devices.

They were the first to control flows at . And now, the team of scientists from EPFL's Laboratory of Nanoscale Electronics and Structures (LANES) has taken their technology one step further. They have found a way to control some of the properties of excitons and change the polarization of the light they generate. This can lead to a new generation of electronic devices with transistors that undergo less energy loss and heat dissipation. The scientists' discovery forms part of a new field of research called valleytronics and has just been published in Nature Photonics.

Excitons are created when an electron absorbs light and moves into a higher energy level, or "energy band" as they are called in solid quantum physics. This excited electron leaves behind an "electron hole" in its previous band. And because the electron has a and the hole a positive charge, the two are bound together by an electrostatic force called a Coulomb force. It's this electron-electron hole pair that is referred to as an exciton.

Unprecedented quantum properties

Excitons exist only in semiconducting and insulating materials. Their extraordinary properties can be easiliy accessed in 2-D materials, which are materials whose basic structure is just a few atoms thick. The most common examples of such materials are carbon and molybdenite.

When such 2-D materials are combined, they often exhibit quantum properties that neither material possesses on its own. The EPFL scientists thus combined tungsten diselenide (WSe2) with molybdenum diselenide (MoSe2) to reveal new properties with an array of possible high-tech applications. By using a laser to generate light beams with , and slightly shifting the positions of the two 2-D so as to create a moiré pattern, they were able to use excitons to change and regulate the polarization, wavelength and intensity of light.

From one valley to the next

The scientists achieved this by manipulating one of the excitons' properties: their "valley," which is related to the extremes of energies of the electron and the hole . These valleys – which are where the name valleytronics comes from – can be leveraged to code and process information at a nanoscopic level.

"Linking several devices that incorporate this technology would give us a new way to process data," says Andras Kis, who heads LANES. "By changing the polarization of in a given device, we can then select a specific valley in a second device that's connected to it. That's similar to switching from 0 to 1 or 1 to 0, which is the fundamental binary logic used in computing."

Explore further: Research team uses excitons to take electronics into the future

More information: Alberto Ciarrocchi et al. Polarization switching and electrical control of interlayer excitons in two-dimensional van der Waals heterostructures, Nature Photonics (2018). DOI: 10.1038/s41566-018-0325-y

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7 comments

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Protoplasmix
5 / 5 (1) Jan 04, 2019
Interesting application, computing. But if you're going to input analog information and you expect an analog answer, why impose a binary architecture? How many different polarizations of light are there between circular and linear?

Also can't help wondering if quasiparticles (maybe phonons instead of excitons?) could be harnessed as a form of reaction mass in a propulsion system, maybe like the EmDrive...
holoman
not rated yet Jan 04, 2019
photon induced electric field poling in ferroelectrics, old science.
holoman
not rated yet Jan 04, 2019
found this :

The dipoles electrical polarity of the ferroelectric molecule physically changes the transitivity, spintronics , interference, exciton's, diffraction, surface morphology/topography, opacity, fluorescence, iridescence, positive and negative index of refraction, and opalescence.

Sahstar
5 / 5 (1) Jan 05, 2019
Interesting application, computing. But if you're going to input analog information and you expect an analog answer, why impose a binary architecture? How many different polarizations of light are there between circular and linear?

(Assuming the computing scheme is truly binary) none. You can only get combinations of circular and linear polarization of light but nothing in between. So why "impose" a binary, or more generally a *digital* architecture? Because digital is much simpler, faster and much more precise, with a very "clean" signal (very high "SNR", i.e. very low error rate), and binary is the simplest digital scheme possible.

Digital/binary is also highly scalable and easier to program for. Analog computing, in contrast, is slower and so error prone you need to triple check everything. You tend to get "approximations" rather than verified results. The noise (error) can often overwhelm the signal (results).
rrwillsj
not rated yet Jan 05, 2019
My comment probably is irrelevant to this research & the other comments.

But here goes... Analog sound reproducing seems much more "real" then digital sound reproduction. Our hearing is analog after all.

Perhaps I'm revealing my generation gap bit I would much rather listen to an acoustic instrument being played than a digital one.

I miss the human warmth of imperfection. I have read that there has been revivals for vinyl records & film cameras.

The Art of Humanity superseding the Sciences?
Protoplasmix
not rated yet Jan 05, 2019
Digital/binary is also highly scalable and easier to program for. Analog computing, in contrast, is slower and so error prone you need to triple check everything.
Thanks, good and valid points. What made me think of analog was the mention that the excitons regulate the polarization, wavelength, and intensity to produce a moiré pattern, and I think circular and linear polarization are just special cases of elliptical polarization with respect to what's between. Sounds like the excitons allow quite precise control over the light.
hat1208
5 / 5 (1) Jan 07, 2019
My comment probably is irrelevant to this research & the other comments.

But here goes... Analog sound reproducing seems much more "real" then digital sound reproduction. Our hearing is analog after all.

Perhaps I'm revealing my generation gap bit I would much rather listen to an acoustic instrument being played than a digital one.

I miss the human warmth of imperfection. I have read that there has been revivals for vinyl records & film cameras.

The Art of Humanity superseding the Sciences?


@rrwillsj Break the pills in half.

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