SPring-8, the large synchrotron radiation facility

A research group consisting of Professor Hiroshi Sawa, Associate Professor Eiji Nishibori, and Assistant Professor Shinobu Aoyagi of Nagoya University (Michinari Hamaguchi, President), succeeded in the single-crystal structure determination of a spherical molecule of endohedral C₆₀ fullerene containing lithium ion (Li@C₆₀),^*1 which was synthesized in a large quantity (several million times larger than the conventional quantity) and refined (highly purified) by a newly developed method using the Single-Crystal Structure Analysis Beamline (BL02B1) at SPring-8 for the first time in the world. This was achieved through a joint research with groups led by Professor Hisanori Shinohara of Nagoya University and Professor Hiromi Tobita of Tohoku University (Akihisa Inoue, President), and scientists of Ideal Star Inc. (Yasuhiko Kasama, Representative Director), Japan Synchrotron Radiation Research Institute (JASRI; Tetsuhisa Shirakawa, President) and RIKEN (Ryoji Noyori, President). These achievements will accelerate the industrial utilization and application of various endohedral metallofullerenes such as Li@C₆₀.

This study showed the possibility of the stable supply and industrial application of the high-purity sample of endohedral metallofullerene using C₆₀ fullerene as a material, which is synthesized in a large quantity. This is a pioneering report on the research on C₆₀-based metallofullerenes, which will be widely discussed in the future, opening new avenues for the function control and application development of fullerenes.

Expectation for Li@₆₀
C₆₀ fullerene, first discovered in 1985, is a soccer-ball-shaped hollow spherical molecule with a diameter of 1 nm (1 nm is a millionth of a mm) consisting of 60 carbon atoms. Recently, the industrial application of C₆₀ fullerene as a major nanotechnological material has progressed in a wide range of fields such as electronic device manufacturing, energy, environment and medical care. This is because the large-scale synthesis of C₆₀ fullerene has become possible and its unique property has been extensively studied worldwide and nearly clarified. Since the discovery of C₆₀ fullerene, it has been considered that its function and property can be controlled and expanded by incorporating a metal atom into the hollow fullerene molecule. Many researchers all over the world have attempted the incorporation of metals into C₆₀ fullerene. While the existence of C₆₀-based metallofullerenes has been reported from the beginning, the isolation of the molecule and the determination of its molecular structure have not been successful until today, 20 years after the discovery, because the molecular reactivity is increased by the incorporation of metals. The isolation and molecular structure determination of high-order endohedral metallofullerenes such as those of C₈₀ and C₈₂ fullerenes with more than 60 carbon atoms have been reported, but the application development of such molecules is very difficult because they are synthesized in extremely small quantities. However, the research on such high-order endohedral metallofullerenes has suggested the possibility of the property control by the incorporation of metals into fullerenes, raising expectations for the progress of the research on C₆₀-based metallofullerenes.

Quantity synthesis, structure determination and stable supply
Ideal Star Inc. and the research group of Professor Tobita at Tohoku University succeeded in synthesizing Li@C₆₀ with a high yield by an original method called the plasma shower method,^*2 and completely isolated and crystallized Li@C₆₀ molecules. The amount of synthesis per unit time is several million times larger than that by the conventional method. Also, the organization of the supply system will open up the avenue for the industrial application of Li@C₆₀.

To demonstrate that the C₆₀ fullerene molecule in the synthesized crystal contains a lithium ion, the research group of Professor Sawa at Nagoya University conducted a high-resolution single-crystal X-ray diffraction^*4 experiment using a large cylindrical imaging plate (IP) camera^*3 of the BL02B1 Beamline at SPring-8. They demonstrated that Li@C₆₀ contains a lithium ion, and determined the molecular structure of Li@C₆₀. Since lithium is an extremely light element of atomic number 3 and it is easily ionized and electrochemically activated, it is used in various fields of industry such as in ion batteries. However, it is generally difficult to examine the precise spatial state of this element due to its lightness. It is possible only by X-ray diffraction measurement using the high-brilliance X-rays at SPring-8. As a result of a detailed analysis called electron density analysis, a lithium ion incorporated in the C₆₀ fullerene was observed, verifying the success in the isolated synthesis of Li@C₆₀. Moreover, the lithium ion in Li@C₆₀ was located 0.13 nm off the center, showing that Li@C₆₀ is completely different from the inert gas molecules such as H₂ and Ar. The two-dimensional arrangement of the electrically polarized Li@C₆₀ molecules in the crystal strongly suggested the possibility of the application of Li@C₆₀ as a single-molecule switch and a ferroelectric thin film, which has been expected on the basis of many theoretical predictions.

Expectations for industrial application
The global competition for the development of organic solar cells, the next-generation solar cells that will replace silicon (Si) solar cells, is increasing. Considering the considerable flexibility and the various utilization forms of C₆₀ fullerene, it is necessary for the development of organic solar cells to use C₆₀ fullerene that can swiftly remove an electron from an exciton excited by light in the active region and separate the free electron from the hole. The appearance of Li@C₆₀ that is found to pull out an electron with a smaller energy than that with C₆₀ fullerene will open up a new possibility to enhance the performance of organic solar cells. Increases in the speed and stability of switching are also required for organic transistors used in flexible displays such as organic electroluminescence (EL) displays and liquid crystal displays. Moreover, while the fullerenes containing alkali metals are organic molecular materials, they show electron conductivity similar to that of n-type semiconductors. It is expected that Li@C₆₀ will be widely used as a functional nanotechnology material for enhancing the performance of these organic electronic elements.

The use of C₆₀ fullerene in contrast media and physiologically active materials has been conventionally attempted in the field of medical care. In addition to that field, there is a possibility that Li@C₆₀, as a ferroelectric molecule generated by the incorporation of alkali metals, will be used in ultrahigh-density memories of more than 500 Tbit/in² with a molecule of 1 nm diameter as one cell. The research on the application of Li@C₆₀, the structure and electronic property of which were clarified in this study, will be further accelerated in a wide range of fields opened up by nanotechnology.

These research achievements were published in the online version of the British scientific journal Nature Chemistry on 20 June 2010.

Publication:
"A Layered Ionic Crystal of Polar Li@C60 Superatoms"
Shinobu Aoyagi, Eiji Nishibori, Hiroshi Sawa, Kunihisa Sugimoto, Masaki Takata, Yasumitsu Miyata, Ryo Kitaura, Hisanori Shinohara, Hiroshi Okada, Takeshi Sakai, Yoshihiro Ono, Kazuhiko Kawachi, Kuniyoshi Yokoo, Shoichi Ono, Kenji Omote, Yasuhiko Kasama, Shinsuke Ishikawa, Takashi Komuro and Hiromi Tobita
Nature Chemistry 2, 678–683 (2010), published online 20 June 2010