A look into the dark

Dark excitons are tiny pairs made up of one electron together with the hole it leaves behind when it is excited. They carry energy but cannot emit light (hence the name “dark”). One way to visualise an exciton is to imagine a balloon (representing the electron) that flies away and leaves behind an empty space (the hole) to which it remains connected by a force known as a Coulomb interaction. Researchers talk about “particle states” that are difficult to detect but are particularly important in atomically thin, two-dimensional structures in special semiconductor compounds.

In an earlier publication, the research group led by Professor Stefan Mathias from the Faculty of Physics at the University of Göttingen was able to show how these dark excitons are created in an unimaginably short time and describe their dynamics with the help of quantum mechanical theory. In the current study, the team has now developed a new technique known as “Ultrafast Dark-field Momentum Microscopy,” and used it for the first time. This enabled them to show how dark excitons are formed in a special material made of tungsten diselenide (WSe₂) and molybdenum disulphide (MoS₂) — and in an astonishing time, lasting just 55 femtoseconds (0.000000000000055 seconds) measured with a precise resolution of 480 nanometres (0.00000048 metres).

“This method enabled us to measure the dynamics of charge carriers very precisely,” explains first author Dr David Schmitt, also from Göttingen University’s Faculty of Physics. “The results provide a fundamental insight into how the properties of the sample influence the movement of the charge carriers. This means that this technique can be used in future to specifically improve the quality and therefore also the efficiency of solar cells, for example.” Dr Marcel Reutzel, Junior Research Group Leader in Mathias’ research group, adds: “This means that this technique can be used not only for these specially designed systems, but also for research into new types of materials.”

The research was supported by the DFG-funded Collaborative Research Centres “Control of Energy Conversion on Atomic Scales” and “Mathematics of Experimentation” in Göttingen and the Collaborative Research Centre “Structure and Dynamics of Inner Interfaces” in Marburg.