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Japanese Researchers Produce Smallest Carbon Nanotube

3/3/2008

Single-walled carbon nanotubes (SWCNTs) consist of a monolayer of a graphene sheet rolled up to form a seamless cylinder with axial symmetry and in general exhibiting a spiral conformation called chirality. Chirality is defined by a single vector called the chiral vector (n,m). This chiral vector is dependent on the orientation of the tube axis with respect to the hexagonal lattice. SWCNTs with different chiral vectors have dissimilar properties such as optical activity, mechanical strength and electrical conductivity.

Now, a team of scientists at Japan’s Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan have grown  the smallest (SWCNTs) with a diameter of ~ 0.4 nm inside single-wall carbon nanotubes of 1.0-1.2 nm diameter.

Free-standing SWCNTs with the smallest diameters are intrinsically unstable under electron beam irradiation and are quite sensitive to oxidation from the surrounding atmosphere. Nanotubes that are confined in thick multi-walled carbon nanotubes or lie on supporting amorphous carbon film are difficult to observe. To bypass these problems, the researchers directly synthesized the smallest SWCNTs by pyrolysizing ferrocene molecules (FeCp₂) inside commercially available high-pressure CO conversion (HiPCO) SWCNTs.

With the help of a state-of-art microscope, a field emission transmission electron microscope of high resolution (HR-TEM) equipped with a post-specimen spherical aberration coefficient  corrector, the team was able to assign the chiral indices beyond a doubt as (3,3). They also experimentally addressed and proposed the possible cap structures of the smallest SWCNTs as a half-dome of C₂₀ fullerene, which consists of six pentagons only. The cap structures have never been experimentally determined thus far. Much attention has been paid to the cap structure in an effort to reveal the growth mechanism of SWCNT, since the structure of a cap may determine the chirality of the nanotube.

Because the researchers provided a template method for growing carbon nanotubes where the diameter of the newly formed inner carbon nanotube was exclusively determined by the outer host carbon nanotube, the method could be suitable for the selective growth of SWCNTs in a controlled manner. The successful observation of the unstable smallest SWCNT, even its cap, implies that SWCNTs could also act as sheaths for certain unstable molecules that would otherwise not exist. By realizing a simple chemical reaction inside SWCNTs, the scientists have shown that these structures are ideal reaction cells for HR-TEM observations. This technique could be the basis for a possible future application of directly visualizing chemical reactions of single molecules with atomic resolution inside the microscope in real time. For further information, see www.staff.aist.go.jp/suenaga-kazu/.