Making dark semiconductors shine

Whether a solid can emit light, for example as a light-emitting diode (LED), depends on the energy levels of the electrons in its crystal lattice. An international team of researchers led by physicists Dr. Hangyong Shan and Prof. Dr. Christian Schneider from the University of Oldenburg has managed to manipulate the energy levels in an ultra-thin sample of the semiconductor tungsten diselenide in such a way that this material, which normally has a low luminescence yield, began to glow. The team has now published an article about its research in the scientific journal nature communication†

According to the researchers, their findings represent a first step towards controlling the properties of matter through light fields. “The idea has been discussed for years, but had not yet been convincingly implemented,” said Schneider. The light effect could be used to optimize the optical properties of semiconductors and thus contribute to the development of innovative LEDs, solar panels, optical components and other applications. In particular, the optical properties of organic semiconductors – plastics with semiconducting properties that are used in flexible displays and solar cells or as sensors in textiles – could be improved in this way.

Tungsten diselenide belongs to an unusual class of semiconductors consisting of a transition metal and one of the three elements sulfur, selenium or tellurium. For their experiments, the researchers used a sample consisting of a single crystalline layer of tungsten and selenium atoms with a sandwich-like structure. In physics, such materials, which are only a few atoms thick, are also called two-dimensional (2D) materials. They often have unusual properties because the load carriers they behave in a very different way from those found in thicker solids and are sometimes referred to as ‘quantum materials’.

The team led by Shan and Schneider placed the tungsten diselenide sample between two specially prepared mirrors and used a laser to excite the material. With this method they were able to make a link between light particles (photons) and excited electrons. “In our study, we show that through this coupling, the structure of the electronic transitions can be rearranged so that a dark material behaves effectively like a bright material,” explains Schneider. “The effect in our experiment is so strong that the lower state of tungsten diselenide becomes optically active.” The team was also able to show that the experimental results were highly consistent with the predictions of a theoretical model.

Provided by the University of Oldenburg