Scientists have discovered the secret world of dark excitons: tiny energy-carrying particles that hold the key to the future of solar power and LEDs.
Using advanced microscopy techniques, researchers have mapped their formation in unprecedented detail, opening new doors to improving energy efficiency in advanced materials.
Tracking invisible energy carriers in next-generation technology
How can advanced technologies such as solar cells be made more efficient? A research team led by the University of Göttingen has taken a major step towards answering this question with an innovative technique.
For the first time, scientists have been able to precisely track the movements of dark excitons – tiny, elusive energy-carrying particles – through time and space. These previously undetectable particles could play a key role in the development of future solar cells, LEDs and sensors. The team’s findings were published in the journal Nature Photonics .
What are dark excitons and why are they important?
A dark exciton is formed when an electron is excited and leaves a “hole,” creating a bond pair that carries energy but does not emit light; hence the term “dark.” A useful way to visualize this is to imagine a balloon (electron) floating away, leaving behind empty space (hole) that is still bounded by an invisible force called the Coulomb interaction.
Although these particle states are difficult to detect, they are particularly important in ultra-thin two-dimensional semiconductor materials. Understanding their behavior could open the way to major advances in energy-saving technology.
A breakthrough in understanding dark excitons
In a previous publication, the research team led by Professor Stefan Mathias from the Department of Physics at the University of Göttingen was able to demonstrate how these dark excitons are created in incredibly short times and to describe their dynamics with the help of quantum mechanical theory.
In the current study, the team has developed a new technique called “ultrafast dark-field momentum microscopy” and used it for the first time. The technique allowed them to demonstrate how dark excitons form in a special material made of tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂) – and in an incredible time of just 55 femtoseconds (0.0000000000055 seconds), measured with a precise resolution of 480 nanometers (0.00000048 meters).
Revolutionizing solar cells and materials science
“This method allows us to measure the dynamics of charged particles very precisely,” explains first author Dr David Schmitt, also from the Department of Physics at the University of Göttingen. “The results provide fundamental insight into how sample properties affect the motion of charged particles. This means that the technique could be used in the future to specifically improve the quality and thus also the efficiency of solar cells, for example.”
“This means that the technique can be used not only for these specially designed systems, but also to study new types of materials,” added Dr. Marcel Reutzel, a junior researcher in Mathias’ research group.
Reference: “Ultrafast nanoimaging of dark excitons” by David Schmitt, Jan Philipp Bange, Wiebke Bennecke, Giuseppe Meneghini, AbdulAziz AlMutairi, Marco Merboldt, Jonas Pöhls, Kenji Watanabe, Takashi Taniguchi, Sabine Steil, Daniel Steil, R. Thomas Weitz, Stephan Hofmann, Samuel Brem, Prof. Matthijs Jansen, Ermin Malic, Stefan Mathias and Marcel Reutzel, 3 January 2025, Nature Photonics .
This research was supported by the DFG-funded Collaborative Research Centres “Control of Atomic-Scale Energy Conversions” and “Mathematics of Experiments” in Göttingen and the Collaborative Research Centre “Structure and Dynamics of Internal Interfaces” in Marburg.
