Single-walled carbon nanotubes (SWNTs) has been reported to grow in a specific direction on sapphire substrates [1,2]. However, the directional growth mechanism is not fully understood yet. Here, we report careful observations of the SWNT growth on pattern-fabricated sapphire substrates and discuss the alignment mechanism based on the observed behavior.
SWNTs were synthesized on a-face sapphire substrate by CVD using Co nanoparticles derived from ferritins [3] or deposited thin film as catalyst. On some of the substrates, trench structures or partially catalyst-deposited areas were fabricated by photolithography.
It was observed by SEM that the SWNTs grow in random direction on the disordered area, but that they are aligned to specific direction on the well-ordered area. On trench-fabricated substrates, SWNTs grow in the expected direction on flat areas in the top and bottom of the trench structures, but grow randomly or bend suddenly on slanted area between the flat areas. Moreover, in-situ SEM observation showed the bending nodes created in growing SWNTs do not move as the growth proceeds. This growth behavior must be incompatible with the root-growth mechanism, in which growing SWNTs slide on the substrate surface.
Next, SWNTs were grown on sapphire substrates on which Co/SiO2 films were partly deposited to divide the catalyst-deposited areas, where the SWNT growth initiates, from the clean ordered area. No catalyst nanoparticles were found in AFM images around the edges of grown SWNTs on the clean ordered area. This indicates that catalyst nanoparticles remain on the SiO2 surface and SWNTs grown by means of the root-growth mechanism have extended to the clean ordered sapphire area. The growth direction of the SWNTs observed there was random, contrary to the aligned growth of SWNTs observed from the Co catalyst on a clean sapphire substrate. This growth behavior is also incompatible with the root-growth mechanism.
All these results indicates that the tip-growth mechanism must be responsible for the aligned growth.
References:
[1] S. Han et al., J. Am. Chem. Soc. 127(2005)5294.
[2] H. Ago et al., Chem. Phys. Lett. 408(2005)433.
[3] G. -H. Jeong et al., J. Am. Chem. Soc. 127(2005)8238.
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