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.
Frenkel exciton, bound electron-hole pair where the hole is localized at a position in the crystal represented by black dots, adopted from (1).
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge.[
An exciton can form when a photon is absorbed by a semiconductor.This excites an electron from the valence band into the conduction band. In turn, this leaves behind a positively charged electron hole (an abstraction for the location from which an electron was moved). The electron in the conduction band is then effectively attracted to this localized hole by the repulsive Coulomb forces from large numbers of electrons surrounding the hole and excited electron. This attraction provides a stabilizing energy balance. Consequently, the exciton has slightly less energy than the unbound electron and hole. The wave function of the bound state is said to be hydrogenic, an exotic atom state akin to that of a hydrogen atom. However, the binding energy is much smaller and the particle’s size much larger than a hydrogen atom. This is because of both the screening of the Coulomb force by other electrons in the semiconductor (i.e., its dielectric constant), and the small effective masses of the excited electron and hole. The recombination of the electron and hole, i.e. the decay of the exciton, is limited by resonance stabilization due to the overlap of the electron and hole wave functions, resulting in an extended lifetime for the exciton (1).
Wannier-Mott exciton, bound electron-hole pair that is not localized at a crystal position. This figure schematically shows diffusion of the exciton across the lattice, adopted from (1).
Excitons exist only in semiconducting and insulating materials. Their extraordinary properties can be easily accessed in 2D materials. The most common examples of such materials are carbon and molybdenite.
When such 2D 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 circular polarization, and slightly shifting the positions of the two 2D materials so as to create a moiré pattern, they were able to use excitons to change and regulate the polarization, wavelength and intensity of light.
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 light 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.” (2)
An emerging class of semiconductor heterostructures involves stacking discrete monolayers such as transition metal dichalcogenides (TMDs), e.g. molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2), adopted from (2).
References:
(1) http://www.wikiwand.com/en/Exciton
(2) https://phys.org/news/2018-11-unique-interlayer-state-bilayer-heterostructure.html
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