When particles bond in free space, they normally create atoms or molecules. However, much more exotic bonding states can be produced inside solid objects. Researchers at TU Vienna have now managed to utilise multi-particle exciton complexes. They have been produced by applying electrical pulses to extremely thin layers of material made from tungsten and selenium or sulphur. These exciton clusters are bonding states made up of electrons and holes in the material and can be converted into light. The result is an innovative form of light-emitting diode in which the wavelength of the desired light can be controlled with high precision.
Illustration of an excitonic system: Short electric pulses are sent through ultra thin layers, which then emit light. (Source: TU Wien)
In a semiconductor material, electrical charge can be transported in two different ways by electrons or holes. If an electron moves up from a neighbouring atom and fills the hole, it in turn leaves a hole in its previous position. That way, holes can move through the material in a similar manner to electrons but in the opposite direction. “Under certain circumstances, holes and electrons can bond to each other”, says Thomas Mueller from the Photonics Institute at TU Vienna. “Similar to how an electron orbits the positively charged atomic nucleus in a hydrogen atom, an electron can orbit the positively charged hole in a solid object.”
Even more complex bonding states are possible: trions, biexcitons or quintons which involve three, four or five bonding partners. “For example, the biexciton is the exciton equivalent of the hydrogen molecule”, explains Mueller. In most solids, such bonding states are only possible at extremely low temperatures. However the situation is different with two-dimensional materials, which consist only of atom-thin layers. The team has created a cleverly designed sandwich structure in which a thin layer of tungsten diselenide or tungsten disulphide is locked in between two boron nitride layers. An electrical charge can be applied to this ultra-thin layer system with the help of graphene electrodes.
“The excitons have a much higher bonding energy in two-dimensional layered systems than in conventional solids and are therefore considerably more stable. Simple bonding states consisting of electrons and holes can be demonstrated even at room temperature. Large, exciton complexes can be detected at low temperatures”, reports Mueller. Different excitons complexes can be produced depending on how the system is supplied with electrical energy using short voltage pulses. When these complexes decay, they release energy in the form of light which is how the newly developed layer system works as a light-emitting diode.
“Our luminous layer system not only represents a great opportunity to study excitons, but is also an innovative light source”, says Matthias Paur. “We therefore now have a light-emitting diode whose wavelength can be specifically influenced – and very easily too, simply via changing the shape of the electrical pulse applied.” (Source: TU Vienna)
Link: Institute of Photonics, Vienna University of Technology, Vienna, Austria
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