In 2D materials, sunlight is converted into electrical energy by a physical phenomena that has been detected for the first time by an international research team led by the University of Göttingen.
The researchers were successful in making dark Moiré interlayer excitons, a type of quasiparticle, visible and deriving a quantum mechanical explanation for how they form. The researchers demonstrate how femtosecond photoemission momentum microscopy, an experimental method recently developed in Göttingen, offers fundamental microscopic insights that are important for the advancement of future technologies.
Two-dimensional semiconductor materials fabricated into atomically thin structures offer intriguing prospects for future electronic, optoelectronic, and photovoltaic components. Interestingly, the atomically thin layers of these semiconductors may be built on top of one another like Lego bricks to regulate their properties in an unexpected way. However, there is another crucial trick: unlike Lego bricks, which can only be piled on top when they are directly stacked or twisted at a 90-degree angle, semiconductors’ rotational angles can be adjusted. For the creation of novel forms of solar cells, it is specifically this rotational angle that is intriguing.
Although altering this perspective can disclose technological advancements, it also presents difficulties for experimenters. The moiré interlayer excitons are sometimes referred to as “black” excitons since conventional experimental methods only have indirect access to these excitons.
“With the help of femtosecond photoemission momentum microscopy, we actually managed to make these dark excitons visible,” explains Dr. Marcel Reutzel, junior research group leader at the Faculty of Physics at Göttingen University. “This allows us to measure how the excitons are formed at a time scale of a millionth of a millionth of a millisecond. We can describe the dynamics of the formation of these excitons using quantum mechanical theory developed by Professor Ermin Malic’s research group at Marburg.”
“These results not only give us a fundamental insight into the formation of dark Moiré interlayer excitons, but also open up a completely new perspective to enable scientists to study the optoelectronic properties of new and fascinating materials,” says Professor Stefan Mathias, head of the study at Göttingen University’s Faculty of Physics.
“This experiment is ground-breaking because, for the first time, we have detected the signature of the Moiré potential imprinted on the exciton, that is, the impact of the combined properties of the two twisted semiconductor layers. In the future, we will study this specific effect further to learn more about the properties of the resulting materials.”