Scientists combine existing technologies to build new ultrafast electron microscope

A team of researchers from Nagoya University in Japan analyzed the mechanisms of the light-matter interaction in the nanomaterials at the smallest and fastest levels.

Keywords: Nanomaterials, Nanometrology, photoexcitation, Negative Electron Affinity, plasmons, light-matter interactions

Nanomaterials are materials that are sized at the nanoscale between 1 and 100 nm. They play a necessary role in both industry and everyday living. The small size of nanoparticles gives them unique properties compared to larger materials. The properties of nanoparticles are also specific to the nature and environment of the material.

To measure nanomaterials, nanometrology is used by scientists. Nanometrology is a subfield of metrology. It measures length scales at the nanoscale. When the size of the particles is small, it is essential for scientists to also measure events that occur within a fraction of a second.  For an instance, photoexcitation is a phenomenon that normally takes place in picoseconds or one trillionth of a second. Thus keeping a track of these events is necessary. Specialized devices are necessary to measure these instantaneous events. 

A research group that was led by Nagoya University faculty members, Associate Professor Makoto Kuwahara from the Institute of Materials and Systems for Sustainability (IMaSS), and Lira Mizuno, Rina Yokoi, and Hideo Morishita of the Graduate School of Engineering, investigated on whether it is possible to study such photoexcitation processes occurring on single nanoparticles.

In collaboration with senior researchers at Hitachi Hightech Ltd., the researchers developed an ultrafast electron microscope. This microscope was developed by combining a semiconductor photocathode with a 'Negative Electron Affinity' surface, pioneered by Nagoya University, with a general-purpose electron microscope. Thus by combining these technologies, the researchers were able to observe events at the nanoscale. 

In the context of nanoparticles, the team of researchers used chemically synthesized gold nanotriangles. Gold is stable under a range of conditions. It is considered a noble metal. Thus, it is best suited for such experiments. 'Plasmon resonance' is the phenomenon exhibited by the electrons in gold nanoparticles. When a gold nanoparticle undergoes photo excitation with a specific wavelength of light, the electrons in the nanoparticle experience movements such as moving or oscillating. This turns the gold nanoparticle into a bright antenna. 

Considering this, surface plasmons on gold are regularly used for sensing applications and are of great interest in energy conversion. The team of researchers investigated two different plasmon phenomena. The researchers first observed the relaxation of the plasmons on the surface. The new technique of researchers has revealed the relaxation process of these plasmons inside the gold nanoparticles. It has also revealed important implications for the preparation of light-harvesting materials for energy conversion.  

Researcher Kuwahara said, "By understanding phenomena such as photoexcitation and relaxation processes and energy transport, we can improve photoresponsive properties and increase efficiency. In particular, it can be a powerful tool to capture individual time changes in small structural materials with spatial resolution (such as those that exceed sub-micrometers). This has been difficult to achieve with conventional analytical methods using pulsed lasers as probes. We expect this achievement to enable the analysis of photoelectric and thermoelectric conversion materials and their applied devices that contribute to energy conservation. Our research should be useful for the development of light energy conversion, biosensors, and thermoelectric conversion devices".

This newly developed technique will help in analyzing potential materials by exposing ultrafast light-matter interactions.