A look into the dark

A look into the dark
by Robert Schreiber
Berlin, Germany (SPX) Jan 31, 2025

An international team of researchers, led by the University of Gottingen, has introduced a new technique to observe the formation of dark excitons - elusive energy carriers with potential applications in solar cells, LEDs, and detectors. Their findings, published in *Nature Photonics*, offer new insights into these energy states, which had previously been challenging to track in real-time.

Dark excitons are particle pairs formed when an excited electron leaves behind a positively charged vacancy, or "hole," to which it remains bound by Coulomb interaction. Unlike typical excitons, dark excitons do not emit light, making them difficult to detect. These states are particularly significant in ultra-thin, two-dimensional semiconductor materials, where they can influence the efficiency of future optoelectronic devices.

Professor Stefan Mathias and his team at Gottingen University have previously described how dark excitons form and behave using quantum mechanical theory. In their latest study, they have advanced the field further by developing "Ultrafast Dark-field Momentum Microscopy" to directly observe these excitons in real-time. This new approach allowed them to track the formation of dark excitons in tungsten diselenide (WSe2) and molybdenum disulphide (MoS2) with an unprecedented temporal resolution of just 55 femtoseconds (0.000000000000055 seconds) and a spatial resolution of 480 nanometers (0.00000048 meters).

"This method enabled us to measure the dynamics of charge carriers very precisely," stated Dr. David Schmitt, the study's first author from the Faculty of Physics at Gottingen University. "Our results provide fundamental insights into how material properties influence charge carrier movement, which can be leveraged to enhance the efficiency of solar cells."

Dr. Marcel Reutzel, Junior Research Group Leader in Mathias' team, emphasized the broader implications of the technique: "This approach is not limited to the specific systems we studied. It can also be applied to new materials, helping to push the boundaries of material science and nanotechnology."

The findings open the door to optimizing optoelectronic devices by better understanding the behavior of dark excitons. With improved efficiency in solar cells and other applications, this breakthrough offers exciting possibilities for future advancements in renewable energy and semiconductor technology.

Research Report:Ultrafast nano-imaging of dark excitons