Modulation of electronic structures of Carbon Nanotubes
Young Hee Lee
Department of Physics, Center for Nanotubes and Nanostructured Composites, Sungkyunkwan Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, R. O. Korea
*Corresponding author’s e-mail address: leeyoung@skku.edu
Carbon nanotubes have played an important role in leading the nanoscience and nanotechnology due to their peculiar one-dimensional characteristics and potential applicabilities in various areas. One important feature is its peculiar electronic structure that can be metallic and semiconducting depending on the chirality and diameter of nanotubes.Up to now, it has not been possible to control the chirality of nanotubes systematically by the conventional synthesis approaches such as arc discharge, laser ablation, and (high-pressure) chemical vapor deposition methods. Both metallic and semiconducting nanotubes coexist in the grown sample, which often hinders device applications with high performance. For instance, application to nanoscale transistors and memories requires nanotubes to be semiconducting for clear gate modulation. Therefore, tailoring the metallicity of nanotubes is highly desired.
One approach is to transform the electronic structures by functionalization using gas adsorbates. We will introduce simple gases such as fluorine and hydrogen gases to transform electronic structures from metallic to semiconducting.[1,2]
Another approach is to select either metallic or semiconducting nanotubes from their mixtures. Several methods of selectiing semiconducting nanotubes from metallic ones or vice versa using dielectrophoresis, octadecylamine (ODA), bromination, and DNA have been reported. Our aim is to select semiconducting nanotubes from metallic ones in large quantity with high yield and more importantly without affecting nanotube properties so that the separated nanotubes could be used directly for various applications.
We have found a method for a selective removal of metallic single-walled carbon nanotubes from semiconducting ones by stirring SWNT powder in tetramethylene sulfone (TMS)/chloroform solution with nitronium hexafluoro-antimonate (NO2SbF6: NHFA) and tetrafluoroborate (NO2BF4: NTFB). Positively charged nitronium ions (NO2+) were intercalated into nanotube bundles, where the intercalation was promoted also by the counter ions. Nitronium ions selectively attacked the sidewall of the metallic SWNTs due to the abundant presence of electron density at the Fermi level, thus yielding stronger binding energy compared to the counterpart semiconducting SWNTs. The semiconducting SWNTs were left on the filter after filtration, whereas the metallic SWNTs were disintegrated and drained away as amorphous carbons. The effectiveness of selectivity was confirmed by the resonant Raman spectra and absorption spectra.[3] Some other ideas of removing zigzag nanotubes by carbon dioxide gas will be also discussed.[4]
[1] K. H. An et al., Amer. Chem. Soc. 125, 3507 (2003); K. H. An et al., Appl. Phys. Lett. 80, 4235; K. A. Park et al., Phys. Rev. B 68, 045429 (2003).
[2] K. S. Kim et al. Adv. Mat. 14, 181 (2002); K. A. Park et al., . Phys. Chem.B, to be published.
[3] K. H. An et al. J. Amer. Chem. Soc. 127, 5196 (2005).
[4] K. Y. Seo et al., J. Amer. Chem. Soc. Comm. 125, 13946 (2003).