Professor, Department of Chemistry and Institute for Advanced Research, Nagoya University
Making ultimate nanocarbon materials
— Endohedral metallofullerenes and peapods —
The “soccer ball” structure composed of 12 pentagons and 20 hexagons exists in some carbon allotropes. In 1985, R. E. Smalley, professor at Rice University and H. W. Kroto, professor at University of Sussex discovered fullerenes (C60) that have this structure. Researchers around the world were surprised by the discovery, but at that time many researchers had nearly given up on studying fullerenes because mass-production of them was extremely difficult.
Astronomer W. Kraetschmer, professor at the Max Planck Institute, Heidelberg, succeeded in mass-producing C60 by accident in September 1990 while trying to synthesize fine carbon particles which were believed to exist in interstellar space. He reported his success at an international conference, and his achievement swept the world. Prof. Shinohara was one of the attendees at the conference. After returning from the conference to Japan, he stopped all his research in order to concentrate on studying fullerenes. “At that time," he says, "all researchers on fullerenes around the world were unable to get enough sleep to focus on studying the materials all day long, because they were afraid that others might publish the same research results first.”
Prof. Shinohara concluded amid the ongoing fullerene fever that he needed to study what others had not done yet, so that the results of his research would stand out among the several thousand papers issued annually on the material. He recalled one of his studies in which he had tried to put metal atoms into cage-shaped water clusters and then came up with the idea of putting them into fullerenes instead of water clusters. While conducting this research, he received a fax from Prof. Smalley, who said he had synthesized the first fullerenes containing a lanthanum atom (La@C82). However, Prof. Shinohara did not give up easily and tried to be a research leader in the field.
When endohedral metallofullerenes are synthesized, not only C60 but also other types of fullerenes are produced at the same time. For studying the structures and properties of metallofullerenes, these need to be separated completely from the other fullerenes. So, he tried to isolate metallofullerenes from the others by using high-performance liquid chromatography. While searching for a column that has compatibility with fullerenes, he found that W. H. Pirkle, professor at the University of Illinois, had developed a column that could be used to isolate polycyclic aromatic materials. Prof. Shinohara thought that metallofullerenes could be isolated with this column because they are also a polycyclic aromatic material. He isolated metallofullerenes for the first time in history by using the column and was able to conduct x-ray structure analysis of pure metallofullerenes isolated by using high-performance multi-step chromatography and continued clarifying the structures and properties of various types of these materials. He was also the first to synthesize and isolate fullerenes containing multiple metal atoms, and then applied the technique to make a gadolinium-containing-fullerene-based contrast agent for magnetic resonance imaging (MRI) that performed 10 to 20 times better than competing agents.
Fullerene-encapsulated carbon nanotubes are called ‘peapods’ because of their shape. In 2000, Prof. Shinohara synthesized peapods composed of metallofullerenes jointly with Prof. Sumio Iijima’s research group. These peapods show very peculiar properties due to the interaction between metal atoms, fullerenes and carbon nanotubes. Field effect transistors (FETs) based on carbon nanotubes, which are being developed globally, are p-type semiconductors, while peapods composed of metallofullerenes can be used for making ambipolar FETs which show the properties of both p-type and n-type transistors, depending on applied gate voltage. Prof. Shinohara is researching applications of the peapods to develop electronic devices jointly with a private company.
Prof. Shinohara enjoys meeting various types of people through conducting many joint projects with researchers both inside and outside Japan, believing that the key to developing the Japanese nanotechnology is human resources, not just equipment, facilities and funding. He has been lecturing at junior and senior high schools as much as time allows, believing that fostering young people is very important. He shows electron microscope images, molecular models and various experiments to these young students to visually stimulate their curiosity. “The privilege of being children," he says, "is that they are interested in many things. Scientists are grownup versions of these children. I usually tell young researchers that they have to continue not only to motivate themselves but also to have big dreams. As long as you have big dreams and lay the groundwork for your dreams, you will achieve them.”
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| Fig. 4 Large Image |
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| Gd@C82 nano-peapods |
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| Fig. 5 Large Image |
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| STS:2-D Map of (11,9)-SWNT Containing Gd@C82 metallofullerenes |
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| Fig. 6 Large Image |
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| Band Structure Modulation of Metallofullerene-Peapods |
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| Fig. 7 Large Image |
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| Metallofullerene-Peapod-FET (Nagoya Group) |
