Predicting the end of a decade, in a promising new class of extremely thin two-dimensional semiconductors, scientists first visualized and measured elusive particles called dark excitons that could not be seen by light.
The powerful technique described in a leading magazine Science, can revolutionize the research of two-dimensional semiconductors and excitons, with profound implications for future technological devices, from solar cells and LEDs to smartphones and lasers.
Excitons are excited states of matter found in semiconductors ̵
“Holes are the absence of an electron and therefore carry the reverse charge of the electron,” explained senior author Professor Keshav Dani, who heads the femtosecond spectroscopy department at the Institute of Science and Technology in Okinawa (OIST). “These opposing charges are attracted and the electrons and holes combine together to form excitons, which can then move throughout the material.”
In ordinary semiconductors, excitons die out in less than a few billion seconds after creation. In addition, they can be “fragile”, making them difficult to study and manipulate. But about a decade ago, scientists discovered two-dimensional semiconductors where excitons are stronger.
“Healthy excitons give these materials truly unique and exciting properties, so there have been many intensive studies around the world aimed at using them to create new optoelectronic devices,” says co-author Dr. Julien Madeo, a scientist at OIST Femtosecond Spectroscopy. . “But there is currently a major limitation to the standard experimental technique used to measure excitons.”
Researchers are currently using optical spectroscopy techniques – essentially measuring what wavelengths of light are absorbed, reflected or emitted by semiconductor material – to reveal information about the energy states of excitons. But optical spectroscopy captures only a small part of the picture.
Scientists have long known that only one type of exciton, called bright excitons, can interact with light. But there are other types of excitons, including impulse-forbidden dark excitons. In this type of dark exciton, the electrons have a different momentum from the holes to which they are connected, which prevents them from absorbing light. This also means that the electrons in the dark excitons have a different momentum from the electrons in the bright excitons.
“We know they exist, but we can’t see them directly, we can’t study them directly, and therefore we don’t know how important they are or how much they affect the optoelectronic properties of the material,” Dr. Madeo said.
Luminous light on dark excitons
To visualize dark excitons for the first time, scientists modified a powerful technique that was previously used primarily to study single unconnected electrons.
“It was not clear how this technique would work for excitons, which are the constituent particles in which electrons are bound. There has been a lot of theoretical work in the scientific community discussing the validity of this approach,” said Prof. Danny.
Their method assumes that if a beam of light containing photons of sufficiently high energy is used to hit excitons in the semiconductor material, the energy from the photons will break the excitons and eject the electrons directly from the material.
By measuring the direction in which the electrons fly out of the material, scientists will then be able to determine the initial momentum of the electrons when they were part of the excitons. Therefore, scientists will not only be able to see, but also distinguish bright excitons from dark excitons.
But the application of this new technique required solving huge technical challenges. Scientists must generate light pulses with high-energy extreme ultraviolet photons capable of separating excitons and expelling electrons from the material. Then the instrument had to be able to measure the energy and angle of these electrons. In addition, because the excitons are so short-lived, the instrument had to operate in a time interval of less than a thousand billion seconds. Finally, the instrument also requires a sufficiently high spatial resolution to measure 2-D semiconductor samples, which are usually only available in micron sizes.
“When we solved all the technical issues and turned on the instrument, our screen was mostly excitons – it was really amazing,” said co-author Dr. Michael Mann, also of the OIST Femtosecond Spectroscopy Unit.
The researchers saw that both bright and dark excitons were predicted to be present in the semiconductor material. But to their surprise, the researchers also found that dark excitons dominated the material, exceeding the number of bright excitons. In addition, the team noted that under certain conditions, because excited electrons scatter throughout the material and change inertia, excitons can switch between bright or dark.
“The dominance of dark excitons and the interaction between dark and bright excitons suggests that dark excitons affect this new class of semiconductors even more than expected,” said Dr. Madeo.
This technique is a real breakthrough, “concluded Prof. Danny.” It not only provides the first observation of dark excitons and illuminates their properties, but also introduces a new era in the study of excitons and other excited particles. ”
Dark excitons can contribute significantly to the emission of light from nanotubes
Direct visualization of the dark excitons forbidden by the pulse and their dynamics in atomically thin semiconductors. Science (2020). science.sciencemag.org/cgi/doi… 1126 / science.aba1029
Provided by the Okinawa Institute of Science and Technology
Quote: Researchers pioneer a revolutionary new method for direct observation of dark excitons (2020, December 3), retrieved on December 4, 2020 from https://phys.org/news/2020-12-revolutionary-method-dark -excitons.html
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