Hot Electrons Move Faster Than Expected

Ultrafast Motion of Electrons

A new research has given rise to solid-state devices which tend to utilise excited electrons. Engineers and scientists at Caltech have for the first time, been in a position of observing directly the ultrafast motion of electrons instantly after they have been excited by a laser. It was observed that these electrons tend to diffuse in their surroundings quickly and beyond than earlier anticipated.

This performance called as `super-diffusion has been hypothesized though not seen before. A team headed by Marco Bernardi of Caltech and the late Ahmed Zewail had documented the motion of electrons by utilising microscope which had captured the images with a shutter speed of a trillionth of a second at a nanometer-scale spatial resolution and their discoveries had appeared in a study published on May 11 in Nature Communications.

 The excited electrons had displayed a diffusion rate of 1,000 times higher than earlier excitation. Though the phenomenon had lasted only for a few hundred trillionths of a second, it had provided the possibility for operation of hot electrons in this fast system in transporting energy and charge in novel devices.

Assistant professor of applied physics and materials science in Caltech’s Division of Engineering and Applied Science, Bernardi had informed that their work portrayed the presence of fast transient which tends to last for a few hundred picoseconds at the time when electrons move quicker than their speed of room temperature, indicating that they can cover longer distance in a given period of time when operated with the help of lasers.

Ultrafast Imaging Technology

He further added that this non-equilibrium behaviour could be employed in novel electronic, optoelectronic as well as renewable energy devices together with uncovering new fundamental physics. Nobel Laureate Ahmed Zewail, the Linus Pauling Professor of Chemistry, professor of physics as well as the director of the Physical Biology Centre for Ultrafast Science and Technology at Caltech, colleague of Bernardi had passed away on 2nd August 2016.

The research had been possible by scanning ultrafast electron microscopy, which is an ultrafast imaging technology initiated by Zewail, with the potential of creating images with picosecond time with nanometer spatial resolutions. The theory and computer models had been developed by Bernardi which clarified the tentative results as an indicator of super-diffusion.

Bernandi has plans of continuing the research by trying to answer the fundamental questions regarding the excited electrons, like how they equilibrate among themselves as well as with atomic vibrations in material, together with applied ones like how hot electrons could increase the efficiency of energy conversion devices such as solar cells and LEDs.

Super Diffusion of Excited Carriers in Semiconductors

The paper has been entitled `Super Diffusion of Excited Carriers in Semiconductors’. Co-authors comprise of former postdoc Ebrahim Najafi of Caltech, who is said to be the main author of the paper and a former graduate student, Vsevolod Ivanov. The research has been supported by the National Science foundation, together with the Air Force Office of Scientific Research, the Gordon and Betty Moor Foundation as well as the Caltech-Gwangju Institute of Science and Technology – GIST, program.