First I would like to present how the carbon nanotube (CNT) was discovered after my long research carrier in 1991, emphasizing on a scientific and convincing presentation of the finding [1]. It might be interesting to talk about what I have been doing before the discovery, where I should introduce my ample experiences with high resolution electron microscopy (HRTEM) and its use in structure characterizations of various nano-structured materials in atomic detail [2]. Without this tool and technique there will be no chance for finding the carbon nanotube. The carbon nanotubes brought us dual excitements in both academia as condensed matter physics and industrial applications. Such a broad range of the attraction is reflected in an extremely high Google citation, its number becoming over 45,000 only for the first paper reporting CNT in 1991. The number is still increasing even after 27 years of the discovery.

CNT, chrysotile asbestos, imogolite [3], and many structures in biological systems are known to have tubular structures, resulting from anisotropic growth in one particular orientation. In the case of CNT, the presence of catalytic metal particles in the tubule formation appears to control a tubular morphology. In general, the formation of nanometer-scale tubular structures is caused mostly by molecular conformation of a building block unit or a total energy minimization of the tubular structure system.

We have recently examined aluminum oxy-hydroxide gamma-AlOOH, boehmite, which has been known to form into a variety of morphologies from a fibril, low-dimensional sheet, platelets, to bulk crystal, depending on a synthesis process. One of them is a quasi-one-dimensional fibril structure, which grows in an aqueous solution as a sol form. We have studied detailed morphology of this fibril boehmite and found that the fibril grows selectively parallel to the c-axis and does not form in a tubular structure but a nanometer-sized ribbon [4]. Electronic energy band gaps of such a ribbon have been studied and showed interesting size-dependent band gaps [5]. The growth was not promoted by a particular catalytic substance so that such an anisotropic growth should be originated from the boehmite structure itself and surroundings.

• Iijima, Nature, 345, 56-58 (1991).
• Iijima, J. Appl. Phys., 42, 5891-5893 (1971).
• Wada et al., Amer. Mineral., 54, 50 (1969).
• Iijima et al., PNAS, 113, 11759-11764 (2016).
• M. Toyoda & S. Saito,private communication 2017.