During the past couple of decades since the Kraetschmer-Huffman breakthrough in the macroscopic production of fullerenes in 1990, Dr. Shinohara and his research team have been in the forefront of the nano-carbon research, which includes fullerenes, metallofullerenes, carbon nanotubes and nano-peapods. The Shinohara research team has achieved many significant contributions in the area of fabrication and characterization of endohedral metallofullerenes and nano-peapods, in particular. These nano-carbon materials possess not only the fundamental interests in their novel structures and solid-state properties but more importantly in practical applications to MRI contrast agents, channels for nano-electronic devices and even in quantum computing. Some of these Shinohara's fundamental studies can be summarized as follows:
1. Arc-discharge synthesis of metallofullerenes encapsulating a metal cluster
Dr. Shinohara and his research team found in 1992 in arc-processed soot the metallofullerene encapsulating three scandium (Sc) atoms (i.e., scandium trimer), Sc3C2@C80, just after the research team synthesized and characterized yttrium (Y) metallofullerenes such as Y@C82 and Y2@C82. The observed 22-symmetric EPR hyperfine structure indicated the presence of a geometrically equivalent triangular shape of Sc3 cluster in C80 fullerene at least within the time scale of the EPR measurements. Interestingly, an IBM Almaden research group published (only a week later of Shinohara's corresponding paper) almost exactly the same result on Sc3C2@C80, reporting the observation of 22-symmetric EPR hyper fine structure. However, the detailed X-ray structural study on Sc3C2@C80 was reported only recently in 2006. A synchrotron X-ray diffraction study revealed that the three Sc atoms indeed form a triangle shape together with C2 in its center. This is the first and real example that a metal trimer (Sc trimer in this case) can form a triangle structure. The present study initiated a field of metallofullerenes encapsulating metal clusters and showed that these otherwise unstable metal clusters can be trapped and stabilized significantly in the hollow space of fullerenes.
2. The first isolation and X-ray structural confirmation of the endohedral metallofullerenes
In 1993, the Shinohara research team accomplished the first isolation of metallofullerenes incorporating the multi-stage HPLC (high-performance liquid chromatography) method. In fact, the HPLC isolation of metallofullerenes triggered a numerous number of metallofullerene studies of pure forms after that. One of the most important such studies is an X-ray structural determination and eludication of detailed endohedral structure of metallofullerenes. The Shinohara research team showed in 1995 in collaboration with Drs. Masaki Takata and Makoto Sakata that a metal atom is indeed encapsulated inside a fullerene as had been theoretically predicted. The first X-ray evidence of the endohedral nature of a metallofullerene, Y@C82, was provided by a synchrotron X-ray powder diffraction incorporating the so-called MEM (maximum entropy method)/Rietveld method. A highly purified (> 99.9 %) powder sample of Y@C82 was subjected to synchrotron X-ray diffraction at Photon Factory BL-6A2 beamline with Imaging Plates as detectors. Importantly, the Y atom is not in the center of C82 fullerene but close to one of the hexagons of the cage due to an intra-fullerene charge transfer from the Y atom to the C82 cage, Y3+@C823-. The X-ray study revealed that Y@C82 possesses C2v molecular symmetry and that the Y atom is situated on C2 molecular axis. This structural analysis also revealed the super-atom character of a metallofullerene, where a positively charged metal atom is located within a negatively charged fullerene cage. The confirmation of the presence of metallofullerenes together with elucidation of the detailed endohedral nature has been regarded as a milestone in the metallofullerene study.
3. The first synthesis and structural analysis of non-IPR fullerenes
IPR (isolated-pentagon rule), which states that all pentagons are surrounded by five hexagons, is one of the most important and fundamental geometrical constrictions for determining a fullerene structure that had never been violated until the present study was published in 2000. All of the fullerenes so far produced and structurally characterized by that time were known to strictly obey IPR. The Shinohara research team synthesized the first non-IPR metallofullerene, Sc2@C66, in which two Sc atoms are entrapped within a non-IPR C66 fullerene. The presence of two fused-pentagons (pentagons are adjacent to one another) on the C66 cage was elucidated by synchrotron X-ray powder diffraction incorporating the MEM/Rietveld analysis. Out of 4,478 theoretically possible non-IPR C66 fullerene isomers, the present X-ray study identified a single unique structure having C2v molecular symmetry. Even though Sc2@C66 metallofullerene possesses otherwise very unstable fused-pentagons on the cage, the metallofullerene was turned out to be very stable under ambient conditions. This unexpected structural stability was due to an electron transfer from encaged Sc atoms to the fused-pentagons, which significantly reduces the strain energies. IPR is not necessarily a test for stable geometry in endohedral metallofullerenes. After this discovery of the presence of non-IPR metallofullerenes, a variety of no-IPR metallofullerenes have been produced and characterized by Japanese, US, German and Chinese research groups.
4. Discovery of the band-gap modulation in metallofullerene nano-peapods
Nano-peapods (carbon nanotubes encapsulating various types of fullerenes and metallofullerenes) are an important class of carbon nanotube materials, in which electronic properties of the nanotubes are often modulated significantly due to electron/hole doping to the nanotubes. The first metallofullerenes-peapods was fabricated and characterized by the Shinohara research team in collaboration with Dr. Sumio Iijima's group in 2000. In the ensuing year of 2002, by using ultra-high vacuum STM/STS (scanning tunneling microscopy/spectroscopy) Dr. Shinohara and Dr. Young Kuk at Seoul National University jointly discovered the so-called "bandgap modulation of carbon nanotubes" by the presence of the encapsulated metallofullerenes. The research team found that the peapods encapsulating Gd@C82 metallofullerenes show a characteristic spatial modulation of the nanotube bandgap. An intrinsic nanotube bandgap ca. 0.5 eV was narrowed down to ca. 0.1 eV at sites where metallofullerenes were inserted. The observed change of in bandgap is very unique to the metallofullerenes peapod and can be explained by local elastic strain and charge transfer at metallofullerene sites. The present band structure represents one-dimensional multiple quantum dots, analogous to a multiple quantum well in a three-dimensional superlattice. By using many identical quantum dots (i.e., metallofullerenes), the present peapod may also be applied to a model system of quantum computing.
Because of these fundamental and significant contributions to the research and development in the emerging area of metallofullerenes and nano-peapods, Dr. Shinohara's achievements have been recognized as being worthy of The Japan Chemical Society Award.