Semiconductors are ubiquitous in modern technology, working to either enable or prevent the flow of electricity. With the intention to 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. Through the use of a special method to interrupt up the bond between electrons and holes, they were in a position to gain a microscopic insight into charge transfer processes across a semiconductor interface. The outcomes were published in Science Advances.
When light shines on a semiconductor, its energy is absorbed. Consequently, negatively charged electrons and positively charged holes mix within the semiconductor to form pairs, generally known as excitons. Within the newest two-dimensional semiconductors, these excitons have a very high binding energy. Of their study, the researchers set themselves the challenge of investigating the opening of the exciton. As physicist and first writer Jan Philipp Bange from the University of Göttingen explains: “In our laboratory, we use photoemission spectroscopy to analyze how the absorption of sunshine in quantum materials results in charge transfer processes. Thus far, we’ve targeting the electrons which are a part of the electron-hole pair, which we are able to measure using an electron analyser. So far, we did not have any approach to directly access the holes themselves. So, we were concerned with the query of how we could characterise not only the electron of the exciton but additionally its hole.”
To reply this query, the researchers, led by Dr Marcel Reutzel and Professor Stefan Mathias at Göttingen University’s Faculty of Physics, used a special microscope for photoelectrons together with a high-intensity laser. In the method, the breaking up of an exciton results in a lack of energy within the electron measured within the experiment. Reutzel explains: “This energy loss is characteristic for various excitons, depending on the environment wherein the electron and the opening interact with one another.” In the present study, the researchers used a structure consisting of two different atomically thin semiconductors to point out that the opening of the exciton transfers from one semiconductor layer to the opposite, much like a solar cell. Professor Ermin Malic’s team on the University of Marburg was in a position to explain this charge transfer process with a model to explain what happens at a microscopic level.
Mathias summarises: “In the long run, we would like to make use of the spectroscopic signature of the interaction between electrons and holes to review novel phases in quantum materials at ultrashort time and length scales. Such studies may be the idea for the event of latest technologies and we hope to contribute to this in the long run.”
This research benefited from the German Research Foundation (DFG) funding for the Collaborative Research Centres “Atomic scale control of energy conversion” and “Mathematics of Experiment” in Göttingen and “Structure and Dynamics of Internal Interfaces” in Marburg.
