Carbon Nanotubes: Multiple Promising Applications and Learning on Fundamental Issues

Scientific research on carbon nanotubes witnessed a large expansion¹. The fact that CNTs can be produced by relative simple synthesis processes and their record breaking properties lead to numerous demonstration of many different types of applications ranging from building fast field effect transistors, flat screens, transparent electrodes, electrodes for rechargeable batteries, conducting polymer composites, bullet proof textiles and transparent loudspeakers.

As a result we have seen enormous progress in controlling growth of CNTs. CNTs with controlled diameter can be grown along a given direction parallel to the surface or perpendicular to the surface. In recent years we have seen narrow diameter CNTs with 2 and more walls to be grown at high yield².

Behind this progress one might forget about the remaining tantalizing challenges. Samples of CNTs still contain a large amount of disordered forms of carbon, catalytic metal particles or parts of the growth support are still a large fraction of the carbon nanotube mass. CNT as produced continue to contain a relative wide dispersion of diameters and lengths. To disperse CNTs and control their distribution in a matrix or on a given surface is still a challenge. There has been enormous progress in size selecting CNTs^3,4. However, the often applied techniques limit application to due to the presence of surfactant molecules or cannot be applied to larger volumes.

Analytical techniques are playing an important role when developing new synthesis, purification and separation processes. Screening of carbon nanotubes is essential for any real world application but is also essential for their fundamental understanding such as the understanding the effect of tube bundling, doping and the role of defects⁵.

At the 'Centre for materials elaboration and structural studies', Professor Wolfgang Bacsa and Pascal Puech and have much focused in screening CNTs with optical methods and developing physical processes for carbon nanotubes working closely with the materials chemists at different local institutions. We have much focused our attention on double wall carbon nanotubes grown form the catalytic chemical vapour deposition technique².

Their small diameter, high electrical conductivity and their large length as well as the fact that the inner wall is protected from the environment by the outer wall, are all good attributes for incorporating them in polymer composites. Depending on the synthesis process used we find the two walls are at times strongly or weakly coupled.

By studying their Raman spectra at high pressure⁶, in acids⁷, strongly photo excited or on individual tubes we can observe the effect on the internal and external walls. A good knowledge of Raman spectra of double wall CNTs gives us the opportunity to map the Raman signal of the ultra thin slices of composites and determine the distribution, agglomeration state and interaction with the matrix.

Working on individual CNTs in collaboration with Anna Swan of Boston University, gave us the opportunity to work on precisely positioned individual suspended CNTs. An individual CNT is an ideal point source and this can be used to map out the focal spot and to learn about the fundamental limitations of high resolution grating spectrometers⁸.

The field of carbon nanotube research has grown enormously during the last decade making it difficult to follow all the new results in this field. It is quite clear that applications where macroscopic amounts of CNTs are needed, standardisation of measurement protocols, classification of CNT samples, combined with new processing techniques to deal with large CNT volumes will be needed. Applications where only minute quantities on a surface are used, suffer from the fact that no parallel processing is still limited. This shows further progress in growing CNT on surfaces is still needed although they have been a recent break through in growing CNT in a parallel fashion and with preferential seminconducting or metallic tubes⁹.

References

1. Ali Javel, ACNano 2 (2008) 1329
2. E. Flahaut, R. Bacsa, A. Peigney, Ch Laurent, Chem. Commun. 12 (2003) 1442
3. M. S. Arnold et al Nature Nanotechnology 1 (2006) 60
4. S. Ghosh, S. M. Bachilo, R. Bruce Weisman, Nature Nanotechnology 9 May 2010, doi:10.1038/nnano.2010.68
5. I. Gerber, P. Puech, A. Gannouni, W. Bacsa, Phys. Rev. B 79 (2009) 075423
6. P. Puech, H. Hubel, D. Dunstan, R. R. Bacsa, Ch. Laurent, and W. S. Bacsa, Phys. Rev. Lett. 93 (2004) 095506
7. P. Puech, E. Flahaut, A. Sapelkin, H. Hubel, D.J. Dunstan, G. Landa, W.S. Bacsa, Phys Rev B 73 (2006) 233408
8. A. G. Walsh, W. S. Bacsa, A. N. Vamivakas, A. K. Swan, Nano Letters Vol. 8 (2008) 2215
9. L Ding et al, Nano Letters 9 (2009)