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

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 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."