View this article J. Phys. Soc. Jpn. 74 (2005) 777

Quantum Jewels in Carbon Nanotubes [March 15, 2005]
Hiromichi Kataura (Self-Assembled Nano-Electronics Group, NRI, AIST)

In 1991, Iijima found pairs of dark lines accompanying symmetric diffraction patterns on a fluorescent screen in transmission electron microscope[1]. He immediately conceptualized the lines and patterns forming a beautiful tubular structure of a hexagonal network, giving rise to carbon nanotubes (CNTs), which are related to fullerenes. The nanometer-scale tubular structure has naturally attracted much interest of researchers because of the expected quantum effects in a robust one-dimensional system. However, experimentalists had to wait later breakthroughs in the synthesis and purification of CNTs for the measurements of their physical properties. On the other hand, theorists had no reason to delay their research on CNTs except for less information about experimental results. Then theorists have kept their advantage in this field for 14 years.

Now we have developed high-purity single-wall carbon nanotubes (SWCNTs). Since the hexagonal sp2 carbon network is flexible in the rolling-up direction, however, CNT permits many kinds of helical structure. Most of the SWCNT samples are a mixture of structural isomers. Some of them are metallic and others are semiconducting[2]. To date, there is no method that can completely separate them according to structure[3]. Thus quality of SWCNT sample is still poor. Experimentalists have to wait more before they can use their skills in the accurate measurement of physical parameters. Despite these problems, however, we confirmed some interesting and important features of SWCNTs, such as Tomonaga Luttinger liquid behavior [4,5] and Aharonov-Bohm effect [6], predicted by theorists.

Fig. 1: Left Photo: High-purity SWCNTs in water. Mean diameters are 1.2 (left) and 1.4 nm (right). They have different colors. Right Graph: Optical absorption of SWCNT film.

If interactions between one-dimensional systems could not be negligible, as in many cases, it is difficult to determine the one-dimensional quantum effects. Carbon nanotubes are one of the rare cases where we can observe clearly their one-dimensional nature. Optical properties are a typical example. One may be surprised that SWCNTs have their own color even though they are made from graphite (see Fig. 1). SWCNT has strong absorption peaks from the infrared to visible region. Each SWCNT should have narrow peaks in the electronic density of states due to one-dimensional van Hove singularities[2]. Expected optical transitions between the density of state peaks in valence and conduction bands could explain the absorption structures briefly[7]. However, in detail, we have to consider a large binding energy of one-dimensional exciton[8]. A strong optical absorption is probably connected to the one-dimensional exciton state in SWCNTs, which is one of the most exciting issues in CNT research. Unfortunately, we still do not have sufficient experimental data on the basis of which we discuss this problem. We do not have the precise experimental values of C-C bond lengths and band structures because of lack of single crystals of a specific SWCNT. We are still at the beginning of CNT research.

Even from a viewpoint of an experimentalist, it is thought to be very difficult to keep the motivation under such a difficult situation. It is similar to walking in the dark.. As results of great efforts of theorists, fortunately, here we have many interesting theoretical predictions for this one-dimensional quantum system. In this issue, we have a review article by Professor Ando[9], in which many attractive physical properties of CNTs are introduced. It looks like a jewel box of quantum effects that we had never seen before the discovery of CNTs. I believe this article can be used as one of the best textbooks for CNT researchers.

References
[1] S. Iijima: Nature 354 (1991) 56.
[2] R. Saito et al.: Appl. Phys. Lett. 60 (1992) 2204.
[3} M. Zheng et al.: Science 302 (2003) 1545.
[4] M. Bockrath et al.: Nature 397 (1999) 598.
[5] H. Ishii et al.: Nature 426 (2003) 540.
[6] S. Zaric et al.: Science 304 (2004) 1129.
[7] H. Ajiki and T. Ando: Physica B 201 (1994) 349.
[8] T. Ando: J. Phys. Soc. Jpn. 66 (1997) 1066.
[9] T. Ando: J. Phys. Soc. Jpn. 74 (2005) 777.

The above article should be referred as "H. Kataura : JPSJ Online-News and Comments [March 15, 2005]" when citing.

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Last updated 2005-3-15