Light of states of exotic particles

A new type of light-emitting diode has been developed at TU Wien. The light is produced from the radiative decay of the exciton complexes in layers of a few atoms of thickness.

When particles bind in a free space, they normally create atoms or molecules. However, much more exotic binding states can be produced inside solid objects.

Researchers at TU Wien have succeeded in using this: "multiparticle exciton complexes" have been produced by applying electrical pulses to extremely thin layers of tungsten and selenium material or sulfur. These exciton clusters are bond states consisting of electrons and "holes" in the material and can be converted to light. The result is an innovative form of light-emitting diode in which the wavelength of the desired light can be controlled with great precision. These results have now been published in the journal Nature Communications.

Electrons and holes

In a semiconductor material, the electric charge can be transported in two different ways. On the one hand, electrons can move directly from one element to the other of the material, in which case they take a negative charge. On the other hand, if an electron is missing somewhere in the semiconductor, this point will be positively charged and referred to as the "hole". If an electron rises from a neighboring atom and fills the hole, it in turn leaves a hole in its previous position. In this way, the holes can move through the material in the same way as the electrons, but in the opposite direction.

"In some circumstances, holes and electrons can bind," says Professor Thomas Mueller of the Photonics Institute (Faculty of Electrical Engineering and Information Technology) at TU Wien. "Similar to the way an electron gravitates around the positively charged atomic nucleus in a hydrogen atom, an electron can gravitate around the positively charged hole in a solid object."

Even more complex binding states are possible: so-called trions, biexcitons, or quintons that involve three, four, or five link partners. "For example, biexciton is the exciton equivalent of the H2 hydrogen molecule," says Mueller.

Two-dimensional layers

In most solids, such bonding states are only possible at extremely low temperatures. However, the situation is different with "two-dimensional materials", consisting only of thin layers such as an atom. The TU Wien team, whose members also included Matthias Paur and Aday Molina-Mendoza, created a cleverly designed sandwich structure in which a thin layer of tungsten diselenide or tungsten disulfide is enclosed between two layers of boron nitride . An electrical charge can be applied to this ultra-thin layer system with the help of graphene electrodes.

"Excitons have a much higher binding energy in two-dimensional layer systems than in conventional solids and are therefore considerably more stable at low temperatures," says Mueller. Different exciton complexes can be produced depending on how the system is powered by electrical power using short voltage pulses. When these complexes disintegrate, they release energy in the form of light. This is how the newly developed layer system works as a light-emitting diode.

"Our system of light layers not only represents an excellent opportunity to study excitons, but also provides an innovative light source," said Matthias Paur, lead author of the study. "So we now have a light-emitting diode whose wavelength can be specifically influenced – and very easily, simply by changing the shape of the applied electrical pulse."