Transform sunlight into electrical energy in 2D materials

by the University of Göttingen

Artistic representation showing the twisted layers of tungsten diselenide (top) and molybdenum disulfide (bottom). Following excitation by light, a multitude of optically “dark” excitons are formed between the layers. These “dark” excitons are electron-hole pairs linked by Coulomb interaction (light and dark spheres connected by field lines), which cannot be observed directly in visible light. One of the most interesting quasiparticles is the “moiré interlayer exciton” — shown in the middle of the image — in which the hole is located in one layer and the electron in the other. The formation of these excitons on the femtosecond timescale and the influence of the moire potential (illustrated by peaks and valleys in the layers) were investigated in the current study using photoemission pulse microscopy femtosecond and the theory of quantum mechanics. Credit: Brad Baxley, Part to Whole, LLC

Transforming sunlight into electrical energy in 2D materials: An international research team led by the University of Göttingen has, for the first time, observed the accumulation of a physical phenomenon that plays a role in the conversion of sunlight into electrical energy in 2D materials. Scientists succeeded in making quasi-particles, known as dark moiré interlayer excitons, visible and explaining their formation using quantum mechanics. The researchers show how an experimental technique newly developed in Göttingen, femtosecond photoemission pulse microscopy, provides deep insights at the microscopic level, which will be relevant for the development of future technologies. The results were published in Nature.

Atomically thin structures made of two-dimensional semiconductor materials are promising candidates for future components in electronics, optoelectronics and photovoltaics. Interestingly, the properties of these semiconductors can be controlled in unusual ways: like Lego bricks, the atomically thin layers can be stacked on top of each other.

However, there is another important trick: while Lego bricks can only be stacked on top, either straight or twisted at a 90 degree angle, the angle of rotation in the semiconductor structure can be amended. It is precisely this angle of rotation that is of interest for the production of new types of solar cells. However, while changing this angle can reveal breakthroughs for new technologies, it also leads to experimental challenges.

In fact, typical experimental approaches only have indirect access to moiré interlayer excitons, hence these excitons are commonly referred to as “dark” excitons. “With the help of femtosecond photoemission pulse microscopy, we have actually managed to make these dark excitons visible,” says Dr. Marcel Reutzel, Junior Research Group Leader at the Faculty of Physics at the University of Göttingen. “This allows us to measure how excitons form on a timescale of a millionth of a millionth of a millisecond. We can describe the dynamics of the formation of these excitons using the theory of quantum mechanics developed by the research group of Professor Ermin Malic in Marburg.

“These results not only give us fundamental insight into the formation of dark moiré interlayer excitons, but also open up a whole new perspective for scientists to study the optoelectronic properties of new and fascinating materials,” says Prof. Stefan Mathias, Director of Studies at the Faculty of Physics at the University of Göttingen. “This experiment is revolutionary because, for the first time, we have detected the signature of the Moiré potential imprinted on the exciton, ie the impact of the combined properties of the two twisted semiconductor layers. In the future, we will further investigate this specific effect to learn more about the properties of the resulting materials.

This research was published in Nature.

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