Physicists shine light in a solid way to extend the life of excitons

Optics researchers at the University of Texas at Dallas have shown for the first time that a new method of fabricating ultrathin semiconductors yields material in which excitons survive up to 100 times longer than in materials made by previous methods.

The findings show that excitons, quasiparticles that transport energy, last long enough for a wide variety of potential applications, including as bits in quantum computers.

dr. Anton Malko, professor of physics at the School of Natural Sciences and Mathematics, is corresponding author of an article published online March 30 in Advanced materials that describes tests on ultra-thin semiconductors made with a recently developed method called laser-assisted synthesis technique (LAST). The findings show new quantum physics at work.

Semiconductors are a class of crystalline solids of which: electrical conduction lies between that of a conductor and an insulator. These conductivity can be controlled externally, either through doping or electric gates, making them essential elements for the diodes and transistors underlying all modern electronic technology.

Two-dimensional transition metal dichalcogenides (TMDs) are a new type of ultrathin semiconductor consisting of a transition metal and a chalcogen element arranged in one atomic layer. Although TMDs have been researched for a decade or so, the 2D shape Malko has explored has advantages in terms of scalability and optoelectronic properties.

“LAST is a very pure method. You take pure molybdenum or tungsten and pure selenium or sulfur and vaporize them under intense laser light,” Malko said. “Those atoms are distributed over a substrate and make the two-dimensional TMD layer less than 1 nanometer thick.”

The optical properties of a material are partly determined by the behavior of excitons, which are quasiparticles that can transport energy while remaining electrically neutral.

“When a semiconductor absorbs a photon, it creates in the semiconductor a negatively charged electron paired with a positive hole, to maintain the neutral charge. This pair is the exciton. The two parts are not completely free from each other – they still have a Coulomb interaction between them,” Malko said.

Malko and his team were surprised to find that excitons in LAST-produced TMDs last up to 100 times longer than those in other TMD materials.

“We quickly found that these 2D samples behave completely differently optically than we’ve seen in 10 years with TMDs,” he said. “As we looked deeper into it, we realized it’s not a fluke; it’s repeatable and dependent on growing conditions.”

This longer life, Malko believes, is caused by indirect excitons, which are optically inactive.

“These excitons are used as a kind of reservoir to slowly feed the optically active excitons,” he said.

Lead study author Dr. Navendu Mondal, a former postdoctoral researcher at UT Dallas and now a Marie Skłodowska-Curie Individual Fellow at Imperial College London, said he believes the indirect excitons exist due to the abnormal amount of tension between the monolayer TMD material. and the substrate on which it grows.

“Strain controlling in atomically thin monolayer of TMDs is an important tool to adjust their optoelectronic properties,” Mondal said. “Their electronic band structure is very sensitive to structural deformations. Under sufficient tension, band-gap modifications cause the formation of several indirect ‘dark’ excitons that are optically inactive. Through this finding, we reveal how the presence of these hidden dark excitons influences those excitons made directly by photons.”

Malko said the built-in stress in 2D TMDs is similar to what would be produced by pressing the material with externally placed micro- or nano-sized pillars, although it’s not a viable technology option for such thin films.

“That strain is crucial for creating these optically inactive, indirect excitons,” he said. “If you remove the substrate, the tension is released and this wonderful optical response is gone.”

Malko said the indirect excitons can be controlled electronically as well as converted to photons, opening a path to the development of new optoelectronic devices.

“This longer life has very interesting potential applications,” he said. “If an exciton has a lifetime of only about 100 picoseconds or less, there’s no time to use it. But in this material we can create a reservoir of inactive excitons that live much longer – a few nanoseconds instead of hundreds of picoseconds. You can do a lot with this.”

Malko said the results of the study are an important proof-of-concept for future quantum-scale devices.

“It’s the first time we know that anyone has made this fundamental observation of such long-lived excitations in TMD materials — long enough to be useful as a quantum bit — much like an electron in a transistor or even just for harvesting light.” in a solar cell,” he said. “Nothing in literature can compare this super-long exciton lives, but we now understand why they have these characteristics.”

The researchers will then attempt to manipulate excitons with an electric field, which is an important step toward creating quantum-level logic elements.

“Classic semiconductors have been miniaturized right to the door before quantum effects completely change the game,” Malko said. “If you can apply gate voltage and show that 2D TMD materials will work for future electronic devices, that’s a huge step. The atomic monolayer in 2D TMD material is 10 times smaller than the maximum size with silicon. But can you make logical elements to that size? We have to find that out.”