Using a unique combination technology, a research team from Nagoya University in Japan has analyzed the mechanisms of light-matter interaction in nanomaterials at the smallest and fastest level.
Nanomaterials, materials with a size in the nano range between 1 and 100 nm, are becoming increasingly important both in industry and in everyday life. Their extraordinarily small size gives them unique properties not found in larger materials. These properties are also specific to the type and environment of the material. In order to expand the library of nanomaterials that can be used effectively, safely and sustainably in products and manufacturing processes, we need a deeper understanding of even the smallest processes that take place on and in the nanoparticles.
To measure nanomaterials, scientists use a branch of metrology known as nanometrology. Nanometrology measures length scales on the nanoscale. To put this in context, a human hair is about 100,000 times wider. With such small particles, scientists also have to measure events that take place within fractions of a second. For example, a phenomenon called photoexcitation usually takes place in picoseconds, or a trillionth of a second. Therefore, specialized equipment is required to measure these near-instantaneous events.
A research group 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 from the Graduate School of Engineering, investigated whether they study such photoexcitation could be processes that take place on individual nanoparticles. In collaboration with senior researchers from Hitachi Hightech Ltd. They developed an ultra-fast electron microscope by combining a semiconductor photocathode with a “negative electron affinity” surface developed by Nagoya University and a general-purpose electron microscope. With the resulting microscope created by combining these technologies, we can observe nanoscale events. The researchers published their findings in Applied Physics Letters.
For the nanoparticles, the group used chemically synthesized gold nanotriangles. Gold lends itself to such experiments because it is a precious metal. This means it is stable under a range of conditions. Electrons in gold nanoparticles exhibit a phenomenon called “plasmon resonance”. When a gold nanoparticle is subjected to photoexcitation with a specific wavelength of light, the electrons in the nanoparticle begin to move or oscillate. This intensifies the light, turning the gold nanoparticle into a bright antenna. For this reason surface plasmons on gold are regularly used for sensing applications and are of great interest for energy conversion.
The plasmons in gold nanoparticles can be photoexcited with the ultrafast laser in the new purpose-built ultrafast electron microscope, while scientists can simultaneously observe individual gold nanoparticles. The researchers used their new technique to study two different plasmon phenomena. They first observed the relaxation of the plasmons on the surface, a well-studied process. However, their new technique also allowed them to observe the change in plasmons inside the gold nanoparticles, even though the light only reached the surface of the nanoparticles. This is the first time a technique has revealed the relaxation process of these plasmons within gold nanoparticles, which has important implications for the fabrication of light-harvesting materials for energy conversion. The newly developed technique should help to analyze potential materials by revealing ultra-fast light-matter interactions.
“By understanding phenomena such as photoexcitation and relaxation processes and energy transport, we can improve photoresponsive properties and increase efficiency,” explains Kuwahara. “In particular, it can be a powerful tool to capture individual time changes in small structural materials with spatial resolution (e.g. those exceeding submicrometers). This was difficult to achieve with traditional analysis methods using pulsed lasers as probes.” he continued. “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.”
Funding: This research was supported by Grant-in-Aid for Scientific Research, Basic Research (A) (21H04637), which began in FY2021.
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