Ever since J.J. Thompson’s 1897 discovery of the electron, scientists have attempted to describe the subatomic particle’s motion using a variety of different means. Electrons are far too small and fast to be seen, even with the help of a light microscope. This has made measuring an electron’s movement very difficult for the past century. However, new research from the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), published in Nature Nanotechnology, has made this process much easier.
“I wanted to see the electrons in the material. I wanted to see the electrons move, not just to explain their motion by measuring a change of light transmission and reflection in the material,” said Prof Keshav Dani, leader of Unit. The limiting factor to studying electron movement using previous techniques was that the instrumentation could either provide excellent time resolution or spatial resolution, but not both. Dr. Michael Man, a postdoctoral fellow in Prof. Dani’s Unit, combined the techniques of UV light pulses and electron microscopy in order to see electrons moving inside a solar cell.
As the first discrete bundle of massless energy, or photon, changes the material, by rapidly heating it for example, the reflection of the subsequent photon changes. As the material cools down, the reflection goes back to the original one. These differences tell the scientists the dynamic of the observed phenomenon. “The problem is that you do not actually directly observe the electron dynamics that causes the changes: you measure the reflection and then you try to find an explanation based on the interpretation of your data,” Prof Dani said. “You create a model that explains the results of your experiment. But you do not actually see what is happening.”
Prof. Dani’s team found a way to visualize this phenomenon in a semiconductor device. “When the pulse hits the material, it takes some electrons out, and we use an electron microscope that forms an image of where the displaced electrons came from,” Dr. Man said. “If you do this many times, for many photons, you can slowly build up an image of the distribution of the electrons in the material. So you photo-excite the sample, you wait for a certain time, and then you probe your sample and you repeat this process again and again, keeping the delay between the first pulse of photons and the probing photons always the same.” As a final result, you get an image of the location of most of the electrons in the material at a specific time delay.
“We have made a video of a very fundamental process: for the first time we are not imagining what is happening inside a solar cell, we are actually seeing it. We can now describe what we see in this time-lapse video, we no longer have to interpret data and imagine what might have happened inside a material. This is a new door to understanding the motion of electrons in semiconductors materials.” Prof Dani effused. This research provides a new insight into the movement of electrons that could potentially change the way solar cells and semiconductor devices are built. This new insight brings the technology field one step closer to building better and more efficient electronic devices.