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Carbon Nanotubes

Carbon nanotubes (CNTs), especially single-walled CNTs (SWNTs), are a representative nanomaterial which has various unique properties in addition to a diameter as small as 1 nm and a length as long as 1 mm and longer. Various applications have been proposed and extensively researched, however, practical applications are very limited due to the undeveloped fabrication processes. We are trying to fundamentally understand the way how the carbon nanotubes grow, and to fabricate them simply by self-organization processes.

Rapid Growth of Nanotubes. High resolution image is here.

• Original Catalyst Screening Method: Combinatorial Masked Deposition, CMD
  
  In general, CNTs grow by precipitation of carbon from metal nanoparticles of a few nanometers. Nanoparticles can be suspended in the gas-phase or supported on subtrates, and in either case, the key is how to realize a small particle size at the elevated temperatures needed for CNT growth.
  
  Even when supported on substrates, catalyst metals diffuse over the substrate surfaces and nanoparticles get coarsened at high temperatures. Not by suppressing this phenomenon but by utilizing it, particles can be grown up to the desired size and utilized as catalyst. By depositing catalyst metals on substrates by sputtering through a physical filter, gradient thickness profiles of catalyst metals can be simply formed [20]. When CVD is carried out on thse "catalyst libraries", CNTs grow up at the regions where nanoparticles of suitable sizes are spontaneously formed [31]. In addition, orthogonal thickness profiles of binary components, can be formed by using a physical filter with a slit, enables the optimization of the amount and composition of binary components on a single substrate [34].
  
  This method is quite simple but effective, and thus, We hope many people to use it. If you have any questions, please feel free to e-mail me.
  

  Catalyst screening by our CMD method.

  CMD for binary systems.

  Mass Production by Fluidized Bed: Movie

  
  ref.: [20] Appl. Surf. Sci. 225, 372 (2004). [31] Appl. Phys. Lett. 86, 173106 (2005). [34] Carbon 44, 1414 (2006).



• Basics of catalytic growth of carbon nanotubes
  
  By using the CMD method, the optimum nominal thickness was found to be as small as 0.1 nm for Co catalyst on SiO2 in efficiently growing high-quality SWNTs from ethanol [31]. Ultra-thin Co layer below one atomic layer yielded 2-nm-sized Co nanoparticles at an areal density of 1x10¹⁶/ m². SWNTs growth was realized also by Ni catalyst, which had been believed to grow MWNTs rather than SWNTs when supported on substrates [35]. Catalyst metals had been too thick to grow SWNTs in the previous works. Fundamental understanding of phenomena enables us to reduce the amount of catalyst with confidence even when the catalysts do not work well.
  
  Co-Mo/SiO2 catalyst is known effective in growing SWNTs from CO or ethanol, but different values are reported for the optimum Co/Mo composition. The CMD method was applied and the vertically aligned SWNT (VA-SWNT) forests were reproduced [34]. Recently, several conditions for the amount/composition of Co/Mo were found reactive in growing VA-SWNTs. Reproduction of the "supergrowth" method, which a number of research groups all over the world failed, was also realized and Al2O3 support was found to be the key for the rapid growth [37].
  

  High-quality SWNTs by alcohol CVD.

  Rapid growth of SWNTs (2-3 mm/ 30 min)

  
  ref.: [31] Appl. Phys. Lett. 86, 173106 (2005). [34] Carbon 44, 1414 (2006). [35] Chem. Phys. Lett. 428, 381 (2006). [37] Jpn. J. Appl. Phys. 46, L399 (2007) (Express Letter).



• "Custom" Production of Carbon Nanotubes for Practical Device Applications
  
  Candidate applications are mapped degree of structural control and the added value of devices below.
  
  To realize high-end application such as field-effect transistors (FETs) in integrated circuits, nanotubes need to be SWNTs with controlled chirality, and need to be precisely positioned at a large scale. Such precise controls of individual SWNTs are still major challenges. We are making continuous efforts in technological development such as for chirality-selective production of SWNTs.
  
  To realize applications such as polymer composites and electrodes of batteries/capacitors, breakthrough is needed both in the cost and scale of nanotube production. We are now designing and developing a practical reactors based on chemical engineering to "upgrade" the rapid growth explained above to mass production.
  
  Electronic/electric devices such as via-filling in integrated circuits, field-emitters for displays, and transparent electrodes for displays and solar cells can be placed in between. They do not require such precise controls of individual nanotubes and mass production of nanotubes. Instead, how to control their ensemble structure and how to realize them at a large area uniformly at a high throughput become important. The flexibility nature of CNTs with narrow diameters cause bundling of CNTs, and by utilizing this phenomenon rather than suppressing it, various ensemble structures will be simply fabricated through self-organization processes. From the viewpoints of structural control and production cost, these applications should be the nearest target for practical use of SWNTs.
  
  CNTs have a large variety, and thus development of the production process customized for each application should be crucial to realize practical use of CNTs. Collaborative projects with private companies have been started.
  

  Map of applications of CNTs.

  Controlled growth of SWNT networks for transparent electrodes.