2007 NSTI Nanotechnology Conference and Trade Show - Nanotech 2007 - 10th Annual

Single-/Multiwalled Vertically Aligned Carbon Nanotubes: Insights into Growth Aspects and Applications

S. Gupta, Y.Y. Wang and R.J. Nemanich
University of Missouri, US

Keywords:
Vertically aligned, Microwave CVD, surface energy, catalyst-assisted

Abstract:
Vertically aligned high areal density small and large diameter carbon nanotube films were deposited by microwave plasma chemical vapor deposition technique. The iron catalyst was prepared by E-beam evaporation on thermally grown silicon dioxide n-type Si(100) substrates. Experiments show that by continuous reduction in the thickness of Fe (~ 3-5 ), strictly single-walled carbon nanotubes can be achieved. Scanning electron and high-resolution transmission electron microscopy show that the diameter of carbon nanotubes ranged between 0.96 - 5 nm and the films are comprised of both the single- and double-wall carbon nanotubes with 100:5 ratio. Resonance Raman spectroscopy was used to further verify the presence and diameter of nanotubes. A thick iron film (80 nm) was also used to grow nanotubes for comparison. The results show that the catalyst islands become greater than hundred nanometers with increasing thickness and introduce multi-walled and bamboo-like structures with complicated surface morphology. While for thinner layer of iron films smaller sizes of catalyst particles/droplets produce hollow concentric tubes without bamboo structure and with less number of walls (single-wall and double-wall carbon nanotubes). The base growth was the most appropriate model to describe the growth mechanism for our films. These results will be discussed in terms of bulk versus surface diffusion thus helping to understand the observed internal structure transition. One of the driving forces to utilize these materials is in vacuum microelectronic applications such as electron microscopes, photo multipliers, X-ray generators, lamps, and flat panel displays and microwave cathodes as cold emitters. The electron field emission properties such as field electron emission microscopy (FEEM) in conjunction with the temperature dependence (T-FEEM) were measured to investigate the emission site density and their intensity variation. These findings in terms of the role of adsorption will be briefly discussed besides the development of miniaturized thermionic power generators will be mentioned briefly.

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