A century after being discovered, humans have captured the electron orbital image of excitons for the first time

Researchers at the Okinawa Institute of Science and Technology (OIST) measured the momentum distribution of photoelectrons emitted by excitons in a single layer of tungsten diselenide, and captured images showing the internal orbits or spatial distribution of particles in excitons—this is this It is a goal that scientists have been unable to achieve since the exciton was discovered nearly a century ago.

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," Common Said Dr. Michael Man, the first author and scientist in the Femtosecond Spectroscopy Group 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 to 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 be extinguished 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 group. "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 derived from Describe theoretically."

However, in December 2020, scientists from the OIST Femtosecond Spectroscopy Group published a paper in the journal Science describing a revolutionary technique for measuring the momentum of electrons in excitons. Now, in the April 21 issue of "Science Advances", the team used this technology to capture for the first time images showing the distribution of electrons around holes in excitons.

The researchers first generated excitons by sending laser pulses to a two-dimensional semiconductor-a type of material discovered recently that is only a few atoms thick and contains more powerful excitons. After the excitons are 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 technology has some similarities with the collider experiment in high-energy physics. In the collider, the particles are smashed together by strong energy, breaking them up. By measuring the smaller internal particles produced in the collision Trajectory, scientists can start to piece together the internal structure of the original complete particle," Professor Dani said. "Here, we are doing something similar - we are using extreme ultraviolet light photons to break up excitons, and measuring the trajectories of electrons to describe what's inside."

"This is not a simple feat," Professor Dani continued. "The measurement must be done very carefully-at low temperature and low intensity to avoid heating the excitons. It took a few days to acquire an image. In the end, the team successfully measured the wave function of the excitons, and it gave The probability that the electron may be located around the hole.

"This work is an important advancement in this field," said Dr. Julien Madeo, the first author of the study and a scientist in the Femtosecond Spectroscopy Group 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 ones based on these concepts. The quantum state of matter and technology."