Extending excitons’ life

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The electrical conductivity of semiconductors can be externally controlled via doping or electrical gating. This makes them vital elements for the diodes and transistors that underpin all modern electronic technology.

For the first time, optics researchers at The University of Texas at Dallas have demonstrated that a new approach for producing ultrathin semiconductors produces material with excitons that last up to 100 times longer than materials made with earlier methods. As they last long, the excitons could have a broad range of potential applications, including bits in quantum computing devices.

TMDs (two-dimensional transition metal dichalcogenides) are a new type of ultrathin semiconductor that combines a transition metal and a chalcogen element into a single atomic layer. While TMDs have been studied for a decade or more, researchers discovered that the 2D variant possesses scalability and optoelectronic features.

The study describes tests on ultrathin semiconductors made with a recently developed laser-assisted synthesis technique (LAST). It shows novel quantum physics at work.

Dr. Anton Malko, professor of physics in the School of Natural Sciences and Mathematics, said, “LAST is a very pure method. You take pure molybdenum or tungsten and pure selenium or sulfur and evaporate them under intense laser light. Those atoms are distributed onto a substrate, making the two-dimensional TMD layer less than 1 nanometer thick.”

“When a semiconductor absorbs a photon, it creates in the semiconductor a negatively charged electron paired with a positive hole to maintain 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.”

Researchers got surprised to know that the excitons in LAST-produced TMDs lasted up to 100 times longer than those in other TMD materials. They discovered that these 2D samples behave differently from any we’ve seen in 10 years of working with TMDs.

Malko said, “When we started to look deeper at it, we realized it’s not a fluke; it’s repeatable and dependent on growth conditions. These longer lifetimes are caused by indirect excitons, which are optically inactive.”

“These excitons are used as a reservoir to feed the optically active excitons slowly.”

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

“Strain-controlling in atomically thin monolayer TMDs is an important tool to tailor their optoelectronic properties. Their electronic band structure is susceptible to structural deformations. Under enough strain, band-gap modifications cause the formation of various indirect ‘dark’ excitons that are optically inactive. Through this finding, we reveal how the presence of these hidden dark excitons influences those excitons created directly by photons.”

Malko said, “The built-in strain in 2D TMDs is comparable to what would be induced by pressing on the material with externally placed micro- or nanosize pillars, although it is not a viable technological option for such thin layers.”

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

“The indirect excitons can be both electronically controlled and converted into photons, opening a path to developing new optoelectronic devices.”

“This increased lifespan has exciting potential applications. When an exciton has a lifespan of only about 100 picoseconds or less, there is 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.”

“The research results 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-living excitations in TMD materials — long enough to be usable as a quantum bit — just like an electron in a transistor or even just for light-harvesting in a solar cell. Nothing in the literature can explain these superlong exciton lifetimes, but we now understand why they have these characteristics.”

In the future, researchers plan to manipulate excitons with an electric field. This could lead to a way for creating quantum-level logic elements.

Malko said“Classical semiconductors have already been miniaturized down to the doorstep before quantum effects change the game entirely. If you can apply gate voltage and show that 2D TMD materials will work for future electronic devices, it’s a huge step. The atomic monolayer in 2D TMD material is 10 times smaller than the size limit with silicon. But can you create logic elements at that size? That’s what we need to find out.”

Journal Reference:

  1. Navendy Mondal, Nurul Azam, Yuri N. Gartstein, Masoud Mahjouri-Samani, Anton V. Malko. Photoexcitation Dynamics and Long-Lived Excitons in Strain-Engineered Transition Metal Dichalcogenides. DOI: 10.1002/adma.202110568

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