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Austrian Scientists Make New Type Of LED

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Light is produced from the radiative decay of exciton complexes in 2D semiconductor sandwich

Researchers at Vienna University of Technology (TU Wien) have produced an innovative LED based around 'multi-particle exciton complexes' by applying electrical pulses to a sandwich of 2D semiconductors. Their findings were published in Nature Communications.

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 LED in which the wavelength of the desired light can be controlled with high precision.

"Under certain circumstances, holes and electrons can bond to each other", says Thomas Mueller from the Photonics Institute (Faculty of Electrical Engineering and Information Technology) at TU Wien. "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: so-called trions, biexcitons or quintons which involve three, four or five bonding partners. "For example, the biexciton is the exciton equivalent of the hydrogen molecule H2", explains Thomas Mueller.

In most solids, such bonding states are only possible at extremely low temperatures. However the situation is different with 2D materials. The team at TU Wien, whose members also included Matthias Paur and Aday Molina-Mendoza, has created a cleverly designed sandwich structure in which a thin layer of WSe2 or WS2 is locked in between two BN 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 2D 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 LED.

"Our luminous layer system not only represents a great opportunity to study excitons, but is also an innovative light source", says Matthias Paur, lead author of the study. "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."

'Electroluminescence from multi-particle exciton complexes in transition metal dichalcogenide semiconductors' by Matthias Paur et al; Nature Communications 10, Article number: 1709 (2019)


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