A new type of transistor that uses excitons instead of electrons has been developed by researchers at Ecole Polytechnique Fédérale de Lausanne (EPFL), setting the stage for optoelectronic devices that consume less energy and are smaller and faster than current devices. The exciton-based transistor uses 2D materials as semiconductors, enabling it to function effectively at room temperature – a challenge that had not been met by previous research.
Excitons, for the uninitiated, are quasiparticles — a term used to describe the interaction between the particles that comprise a given substance, rather than the substance itself. As shown in the clever comic strip-style illustration, they consist of an electron and an electron hole, bound together. An electron absorbs a photon, giving it a higher level of energy and thus becoming “excited” -- that electron leaves behind a missing electron “hole” in the previous level of energy, which is also referred to as a valence band. Because the electron and the hole have opposite charges, electrostatic force keeps them together in a bond called Coulomb attraction.
Now here’s the interesting part. Ultimately, the electron falls back into the hole, causing it to emit the absorbed photon. The exciton then ceases to exist, but it has done its job: A photon has gone in at one end of a circuit and come out the other.
This chain of events involving the energy of excitons had previously been considered too fragile and short-lived to be of use to electronic circuits -- in addition, it could only be produced and controlled in circuits at temperatures around -173° C. But the EPFL researchers added 2D materials into the mix — tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) — which exhibit a particularly strong electrostatic bond and are not quickly destroyed at room temperature.
The electrons always found their way to the MoS2 while the holes always ended up in the WSe2, delaying the process by which the electron returns to the hole. This allowed the excitons to be controlled and moved around using an electron field. A further refinement of the research protected the 2D materials with boron nitride, which kept the excitons going even longer.
“By manipulating excitons, we had come upon a whole new approach to electronics," said Andras Kis, one of the EPFL researchers and head of the institute’s Laboratory of Nanoscale Electronics and Structures (LANES). "We are witnessing the emergence of a totally new field of study, the full scope of which we don't yet know."
The research was published July 25, 2018, in the journal Nature.