Japanese scientists have developed a method for observing two-dimensional semiconductors called dark excitons.
Excitons are bound pairs of electrons and holes in a solid that, in principle, can be used as information carriers.
Holes are the absence of an electron, and therefore they carry the opposite charge to the electron. Opposite charges attract, and electrons and holes bond together to form excitons, traveling through the material.
Keshav Dani, the first study author, and professor who heads the femtosecond spectroscopy department
In conventional semiconductors, excitons are extinguished in about a few billionths of a second. Also, they can be “fragile,” which makes it difficult to study. But about ten years ago, scientists discovered two-dimensional semiconductors: excitons are more stable in them.
Strong excitons give these materials truly unique properties, which is why a lot of research has been carried out worldwide in which two-dimensional semiconductors have been used to create new optoelectronic devices. However, these works were seriously hampered by insufficient technical equipment.
For a long time, scientists have known that only one type of exciton, called bright excitons, can interact with light. But there are other, so-called dark excitons, which have not yet been seen. To visualize dark excitons, the scientists modified a powerful technique previously mainly used to study single unbound electrons.
The research team suggested that if a beam of light containing photons of high enough energy were used to strike excitons in a semiconductor material, the photon energy would destroy the excitons and knock electrons out of the material. By measuring the direction in which the electrons are ejected from the material, it will be possible to determine the electrons’ initial momentum when they were part of the excitons. Thanks to this method, scientists will be able to see and distinguish bright excitons from dark ones.
As a result, it turned out that there are more dark excitons in a semiconductor than light ones. And under certain conditions, light excitons can become dark and vice versa. Scientists now want to create stable excitons that they believe can give materials unique properties.