For the first time in the world, researchers at the Okinawa Institute of Science and Technology’s Graduate University (OIST) have copied an image showing the internal orbits or spatial distribution of particles in an exitone – a goal that scientists have avoided for over a century.
Excitons are the exciting states of matter found in semiconductors – the keys to many modern technological devices, including solar cells, LEDs, lasers, and smartphones.
“Exitons are really unique and interesting particles; they are electrically neutral, which means that they behave very differently from other particles, such as electrons. Their presence can really change the way an object reacts to light,” said Co-First Dr. Michael Mann said. He is the author and staff scientist at the OIST Femtosecond Spectroscopy Unit. “This work brings us closer to a complete understanding of the nature of excitons.”
Exitones are formed when semiconductors absorb photons of light, causing negatively charged electrons to jump from a low energy state to a high energy state. These positively charged voids in the positive energy level are called holes. Inversely charged electrons and holes are attracted and they begin to orbit each other, creating exitones.
Exitons are very important within semiconductors, but so far scientists have only been able to detect and measure them in limited ways. One problem is their weakness – they require relatively little energy to break the exitone into free electrons and holes. Also, they are fleeting in nature – in some materials, exitones are extinguished in billions ofths of a second after they are formed, when the excited electrons fall back into the holes.
“Scientists first discovered exitones 90 years ago,” said Professor Keshav Danny, a senior author and head of the Femtosecond Spectroscopy Unit at OIST. “But until very recently, one could usually only access the optical signatures of exitons – for example, the light emitted by an exitone when extinguished. The speed at which their electrons and holes orbit each can only be described theoretically.
However, in December 2020, scientists at the OIST Fentosecond Spectroscopy Unit published a paper Science Describes a revolutionary technique for measuring the momentum of electrons in excitons.
Now, reporting on April 21st Scientific progress, The team used the technique to capture the first image showing the distribution of an electron around a hole inside an exitone.
Researchers first created excitons by sending a laser pulse of light into a two-dimensional semiconductor – a group of recently discovered materials that contain only a few atoms of thickness and contain more potent excitons.
After the exitones were formed, the team used a laser beam with ultra-high energy photons to vacuum space inside an electron microscope to separate the exitones and expel the electrons from the material itself.
The electron microscope measures the angle and energy of electrons as they fly through the material. From this information, scientists were able to determine the initial velocity of the electron when connected to a hole inside the exitone.
“This technology has some similarities with the cooler experiments in high energy physics, where the pieces are broken up with intense force and opened. Doing something – we use extreme ultraviolet light photons to separate the exitones, and measure the path of the electrons to capture what is inside. “
Professor Danny continued. “Measurements need to be done extremely carefully – at low temperatures and low intensities to prevent the exitones from overheating. It took a few days to achieve a single image.”
Ultimately, the team succeeded in measuring the wave activity of the exciton, which gives the possibility of where the electron may be around the hole.
“This work is a major breakthrough in this field,” said Dr. John H. Snyder, co-author and staff scientist at the OIST Fentosecond Spectroscopy Unit. Said Julian Madio. “As the internal orbits of the particles become larger integrated particles, being able to visualize them will allow us to understand, measure, and ultimately control the integrated particles in an unprecedented way. This will allow us to create new quantum states of matter and technology based on these concepts.”
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