What is the Goal of Your Research?

Effective utilization of energy is very important for future society. For example, fuel cell vehicles and renewable energy technologies such as the utilization of sunlight are being used more and more. However, the complete utilization of inherent energy in a substance or light is still beyond our reach. We are proceeding with research from the standpoint of chemistry, in order to create a substance system that enables the universal use of energy held by electrons and light in a substance to the extent that exceeds conventional limits.

 

What Kind of Experiment are You Doing?

The ratio between the size of light (wave length) and the size of a molecule is almost
same as the ratio between Mt. Fuji and a human 

When a substance is exposed to light, electrons in the substance absorb the light as energy. In general, the color (wave length) of light that is absorbed is limited, and also the amount is very little. Therefore, current solar cells can only utilize part of the light energy. This is because when light is considered a particle, the size (wave length) of the light is much larger but weaker in comparison to a substance such as a molecule that absorbs the light. This results in limited interaction (refer to the following figure). We focused on and worked on the utilization of the characteristics of metal, such as gold or silver, so when the metal is arranged at a size of approximately 10 nanometers (10 nm = 10-8 m, equal to about 1/10,000 of the thickness of a hair), it holds light energy in its small structure. In the experiment, we controlled the shape of a metal nanoparticle. 

Metal nanobridge structure that absorb photons (top)
Photon scattering from a single molecule at the bridge (bottom)

We used a chemical material synthesis method and semiconductor nanotechnology used for drawing an electronic circuit. Then, we fixed a substance to interact with light on the structure surface by controlling its orientation etc. at the atomic and molecular level using an electrochemical method. As a result, we found that coming up with the shape of a nano level metal structure allowed us not only to select and hold light in various colors including red, green, and yellow, but also to hold light energy in a very small gap of less than 1 nm when structures are placed with their sharp points facing each other. After using a special microscope to observe a substance absorbing light, it was found that substances and molecules in this light holding area had unusual behavior against light: a substance that had been unable to absorb light could now absorb it, and the light color that could be absorbed changed. This experiment showed that controlling a combination of substances on the interface at the monomolecular layer level revealed characteristics that challenged conventional understanding. In addition, further research indicated that the absorbed light energy can be drawn out as electric current and used as the driving energy of oxidation-reduction in a chemical reaction. 

 

What will be Your Next Goal?

As a result of the research described above, we discovered that controlling the structure of a substance system at a nano level can not only change a substance’s ability to absorb light, but it also can convert light energy into electron energy for efficient use. These findings are expected to lead to the creation of a more efficient electrode for solar cells and fuel cells that utilizes a principle completely different from the conventional one.
 We hope to develop a substance system that efficiently captures all colors of light in the nanospace and is able to manipulate the energy of the captured light as electron energy. Efficient use of energy needs to take into consideration not only the efficiency of how an element works, but also all energy required for establishing the element, maintaining the element’s performance and recycling an element that has reached the end of its service life. In addition, we are trying to significantly reduce the overall environmental impact by actively applying ordinary and less expensive metals or nanocarbon materials that have almost infinite characteristics, depending on the structural combination with the system described above. We hope to create a highly functional electrode in the future that has high performance, despite its paper-like thinness, and requires no energy for repair and recycling.

 

What are the Advantages in the Research Above at Hokkaido University?

Until I graduated high school, I had originally preferred physics over chemistry. While studying at university, however, I became interested in the chemistry that dealt with substances. Since then, I studied in and with domestic and overseas research institutes and universities related to the field of chemistry, and now I am conducting research here. I believe that developing new research requires wide knowledge of organic/inorganic synthesis, catalytic reactions, optical devices, theoretical chemistry, industrial chemistry and even artificial photosynthesis and environmental science. Hokkaido University has many excellent laboratories in the field of chemistry that drive some of the world’s leading research in each of the above subjects. They are very interactive with each other, which offers great motivation. There exists no other university in the world with so many cutting edge chemical researchers that have different expertise. I think that Hokkaido University has the best environment to potentially solve difficult problems that were previously out of reach.

 

References

(1)    M. Takase, H. Ajiki, Y. Mizumoto, K. Komeda, M. Nara, H. Nabika, S. Yasuda, H. Ishihara, K. Murakoshi, Nature Photonics, 7, 550-554 (2013).

(2)    F. Nagasawa, M. Takase, H. Nabika, K. Murakoshi, "Depolarization of Surface-Enhanced Raman Scattering Photons from a Small Number of Molecules on Metal Surfaces. ", in Vibrational Spectroscopy at Electrified Interfaces, eds. C. Korzeniewski, B. Braunschweig and A. Wieckowski, Chap. 6, pp. 220 -237, John Wiley & Sons, NY (2013).

M. Takase, F. Nagasawa, H. Nabika, K. Murakoshi, "Single Molecule Surface-Enhanced Raman Scattering as a Probe for Adsorption Dynamics on Metal Surfaces", Chap. 5, in Frontiers of Surface-Enhanced Raman Scattering: Single-Nanoparticles and Single Cells, Eds. Y. Ozaki, K. Kneipp and R. Aroca, John Wiley & Sons, NY (2014).