Forum for Science, Industry and Business

Long-distance relationships of particles: Electron-hole pairs in two-dimensional crystals

When light of specific frequency hits a semiconductor crystal, it is absorbed and produces a excitation, a state of higher energy. In solar cells, this energy can be converted into electricity and used. In two-dimensional crystals, which consist of only a few atomic layers, so called “excitons” are the protagonists of these processes: These excitations from light incidence consist of one particle of positive charge and one of negative charge.

A two-dimensional crystal from molybdenum disulfide (MoS2) and Tungsten diselenide (WSe2) (left: top view, right: side view).

Jens Kunstmann

Yet, two-dimensional crystals host a sheer zoo of excitons, making it hard to tell the kind of excitons dealt with in specific situations. Researchers of TU Dresden, in collaboration with an international team, now identified the nature of interlayer excitons in two-dimensional crystals. Their findings were published in the journal Nature Physics.

The two-dimensional crystals are a kind of “sandwich” made of single layers of molybdenum disulfide and tungsten diselenide. Each layer has a thickness of only three atoms. In the laboratory, the layers are stacked on each other one by one by hand. “What makes interlayer excitons so special is the two charged particles being separated in space. So far, it was assumed that the positive one is located in the Tungsten diselenide and the negative one in the molybdenum disulfide,” says Dr. Jens Kunstmann from the Chair of Theoretical Chemistry of TU Dresden.

“We were now able to clearly show that particles of positive charge can be found in both layers, and thence, the interlayer excitons are bound to each other in a much stronger way than presumed formerly.” Theoretical as well as experimental groups were working hand in hand in the course of this global collaboration.

The Dresden group contributed theoretical calculations and analyses in cooperation with Prof. Andrey Chaves of the Universidade Federal do Ceará in Fortaleza, Brazil, and Prof. David R. Reichman of the renowned Columbia University in New York City, USA. The experiments were conducted by the group of Prof. Tobias Korn of the Universität Regensburg: among them Fabian Mooshammer and Philipp Nagler, who contributed to this research in the course of their master and doctoral theses.

“We are still at the beginning, we still don’t know for sure how interlayer excitons in other two-dimensional crystals look like,” Dr. Kunstmann points out. “But we are fascinated by these excitons anyway. The spatial separation of the charges could enable the condensation of excitons to a macroscopic quantum state, as well as the construction of highly efficient solar cells.”

Complete picture caption: A two-dimensional crystal from molybdenum disulfide (MoS2) and Tungsten diselenide (WSe2) (left: top view, right: side view). Light can produce interlayer excitons in these crystals, which are fascinating excited states, consisting of one particle of positive charge and one of negative charge. The coloured outlines in the right picture represent the probability of the particles‘ places.

Media inquiries:
Dr. Jens Kunstmann
Chair of Theoretical Chemistry, TU Dresden
Tel.: +49 (0) 351 463-33635
E-Mail: jens.kunstmann@tu-dresden.de

Kim-Astrid Magister | idw - Informationsdienst Wissenschaft
Further information:
http://www.tu-dresden.de

Im Focus: Regensburg physicists watch electron transfer in a single molecule

For the first time, an international team of scientists based in Regensburg, Germany, has recorded the orbitals of single molecules in different charge states in a novel type of microscopy. The research findings are published under the title “Mapping orbital changes upon electron transfer with tunneling microscopy on insulators” in the prestigious journal “Nature”.

The building blocks of matter surrounding us are atoms and molecules. The properties of that matter, however, are often not set by these building blocks...

Im Focus: University of Konstanz gains new insights into the recent development of the human immune system

Scientists at the University of Konstanz identify fierce competition between the human immune system and bacterial pathogens

Cell biologists from the University of Konstanz shed light on a recent evolutionary process in the human immune system and publish their findings in the...

Im Focus: Transformation through Light

Laser physicists have taken snapshots of carbon molecules C₆₀ showing how they transform in intense infrared light

When carbon molecules C₆₀ are exposed to an intense infrared light, they change their ball-like structure to a more elongated version. This has now been...

Im Focus: Famous “sandpile model” shown to move like a traveling sand dune

Researchers at IST Austria find new property of important physical model. Results published in PNAS

The so-called Abelian sandpile model has been studied by scientists for more than 30 years to better understand a physical phenomenon called self-organized...

Im Focus: Cryo-force spectroscopy reveals the mechanical properties of DNA components

Physicists from the University of Basel have developed a new method to examine the elasticity and binding properties of DNA molecules on a surface at extremely low temperatures. With a combination of cryo-force spectroscopy and computer simulations, they were able to show that DNA molecules behave like a chain of small coil springs. The researchers reported their findings in Nature Communications.

DNA is not only a popular research topic because it contains the blueprint for life – it can also be used to produce tiny components for technical applications.