The Exciton, a Quasi-Particle with High Electronic Potential

electronic devices
After successfully controlling the flow of excitons at room temperature, EPFL researchers are discovering new properties, which could lead to the realization of electronic devices that are more energy efficient.

They were the first to control the flow of excitons at room temperature. Researchers at EPFL’s Laboratory of Electronics and Nanoscale Structures (LANES) have gone one step further: they have managed to control some of their characteristics and change the polarization of the light they generate. They open the door to a new kind of electronic device, which could limit energy losses and heat dissipation experienced by current transistors. Their research, conducted in the field of a new cutting-edge discipline called “valletetronic”, has just been published in Nature Photonics .

An exciton is a form that an electron temporarily takes when a material absorbs light. Thus “excited”, the electron goes into a higher energy degree, leaving in what is called in quantum physics solids the “band of energy” inferior, an empty space, an “electron hole” . Since the electron is negatively charged and its hole correspondingly positive, the two entities remain bound by an electrostatic force called Coulomb attraction. It is this pair that defines the exciton.

Unprecedented quantum properties

This phenomenon only occurs in semiconductor materials and insulators. And their extraordinary electronic potential appears mainly in the case of their use within 2D materials – whose basic structure is only a few atoms thick. Carbon or molybdenite are the best known examples.

When combined, these materials often reveal unprecedented quantum features, none of which are individually present. It is by combining two of them, molybdenum diselenide (MoSe 2 ) and tungsten diselenide (WSe 2 ), that EPFL scientists have discovered new technological paths. They found that by using a circularly polarized light source – emanating from a laser – and by creating a moire structure by a slight shift of the two layers of 2D materials, they could use the excitons to control and modify the polarization. , the wavelength and intensity of light.

From valley to valley

All this is possible by manipulating one of the characteristics of the excitons, namely their “valley”, which corresponds in fact to the particular layout of the energy values ​​that can respectively adopt the electron and the “electron hole”. This notion, which gave its name to the “valléetronique”, offers a huge potential in terms of coding and manipulation of nanoscopic information in matter.

“If we can connect several devices based on this discovery, we will have an additional way to process information electronically,” says Andras Kis, who heads LANES. Because the possibility of modifying the polarization of the light in a device then gives the choice, in a second device of the same type that would be connected to it, to select one or other of the valleys. This could be compared to switching from 0 to 1 or from 1 to 0, which is the most basic logical operation on a computer.