Insights into the behaviour of excitons in 2D semiconductors

To better understand the potential of two-dimensional semiconductors for future computer and photovoltaic technologies, researchers from the Universities of Göttingen, Marburg and Cambridge investigated the bond that builds between the electrons and holes contained in these materials. The researchers used a special method to break up the bond between electrons and holes; as a result, they were able to gain a microscopic insight into charge transfer processes across the semiconductor interface.

When light shines on a semiconductor, its energy is absorbed. As a result, negatively charged electrons and positively charged holes combine in the semiconductor to form pairs, known as excitons. In modern two-dimensional semiconductors, these excitons have a high binding energy. The researchers set themselves the goal of examining the hole of the exciton.

First author Jan Philipp Bange from the University of Göttingen said the researchers used photoemission spectroscopy to investigate how the absorption of light in quantum materials leads to charge transfer processes. “So far, we have concentrated on the electrons that are part of the electron-hole pair, which we can measure using an electron analyser. Up to now, we didn’t have any way to directly access the holes themselves. So, we were interested in the question of how we could characterise not just the electron of the exciton but also its hole,” Bange said.

The researchers, led by Dr Marcel Reutzel, used a special microscope for photoelectrons and a high-intensity laser. In the process, the breaking up of an exciton led to a loss of energy in the electron measured in the experiment. Reutzel said this energy loss is characteristic for different excitons, depending on the environment in which the electron and the hole interact with each other.

The researchers used a structure consisting of two different atomically thin semiconductors to show that the hole of the exciton transfers from one semiconductor layer to the other, similar to a solar cell. The research team used a model to explain this charge transfer process and what happens at a microscopic level. Going forward, the researchers want to use the spectroscopic signature of the interaction between electrons and holes to study novel phases in quantum materials at ultrashort time and length scales.

The research findings were published in the journal Science Advances.

Image caption: An ultrashort flash of light breaks the bond between the electron (red) and the hole (blue), enabling research on charge-transfer processes in atomically thin semiconductors. Image credit: Lukas Kroll, Jan Philipp Bange, Marcel Reutzel, Stefan Mathias, Science Advances.