OIST Researchers Capture Electrons Orbit Image Through WSe2

In a world-first, researchers at the Okinawa Institute of Science and Technology (OIST) measured the momentum distribution of electrons emitted by excitons in a single layer of tungsten diselenide (WSe₂), and captured electrons orbit image showing the internal orbits or spatial distribution of particles in the excitons.

Excitons are the excited state of matter found in semiconductors - this type of material is the key to many modern technological devices, such as solar cells, LEDs, lasers, and smartphones.

"Excitons are very unique and interesting particles; they are electrically neutral, which means that they behave in materials very differently from other particles such as electrons. Their presence can really change the way materials react to light," said Dr. Michael Man, the co-first author and scientist in the Femtosecond Spectroscopy Unit of OIST. "This work brings us closer to fully understanding the nature of excitons."

Excitons are formed when a semiconductor absorbs photons, which causes negatively charged electrons to jump from a low energy level into a high energy level. This leaves positively charged vacancies at lower energy levels, called holes. The oppositely charged electrons and holes attract each other, and they begin to orbit each other, which creates excitons.

Excitons are vital in semiconductors, but so far, scientists can only detect and measure them in a limited way. One problem lies in their fragility - it takes relatively little energy to break down excitons into free electrons and holes. In addition, they are fleeting in nature. In some materials, excitons will extinguish within a few thousandths of the time after they are formed, at which time the excited electrons will "fall" back into the hole.

"Scientists first discovered excitons about 90 years ago," said Professor Keshav Dani, senior author and head of OIST's Femtosecond Spectroscopy Unit. "But until recently, people usually only got the optical characteristics of excitons - for example, the light emitted when excitons disappear. Other aspects of their properties, such as their momentum, and how electrons and holes work with each other, can only be described theoretically."

However, in December 2020, scientists from the OIST Femtosecond Spectroscopy Unit published a paper in the journal Science describing a revolutionary technique for measuring the momentum of electrons in excitons. Now, the team used this technology to capture the first image using WSe₂ showing the electrons orbit around holes in excitons.

The researchers first generated excitons by sending laser pulses to a two-dimensional semiconductor-a type of material that has been discovered recently that is only a few atoms thick and harbor more powerful excitons.

After the excitons were formed, the research team used a laser beam with ultra-high energy photons to decompose the excitons and kick the electrons directly out of the material into the vacuum space in the electron microscope. The electron microscope measures the angle and energy of electrons as they fly out of the material. From this information, scientists can determine the initial momentum when the electrons combine with the holes in the excitons.

"This work of capturing electrons orbit image through WSe₂ is an important advancement in this field," said Dr. Julien Madeo, the co-first author of the study and a scientist in the Femtosecond Spectroscopy Unit of OIST. "The ability to visually see the internal orbits of particles, because they form larger composite particles, which allows us to understand, measure and ultimately control composite particles in an unprecedented way. This allows us to create new quantum states of matter and technology based on these concepts."

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