Role of thin Fe catalyst in the synthesis of double- and single-wall carbon nanotubes via microwave chemical vapor deposition

Abstract

Synthesis of vertically aligned small diameter (single- and double-wall) carbon nanotube films on thermally oxidized $n^{+}\text{-}\mathrm{Si}\left(001\right)$ wafers, with acetylene diluted with ammonia gas mixture using a microwave plasma-assisted chemical vapor deposition technique, is reported. Experiments show that by continuous reduction in the thickness of the iron catalyst film to $\sim 0.3–0.5\mathrm{nm}$ , or alternately, smaller catalyst particles produces hollow concentric tubes with a fewer number of walls. Double- and single-wall carbon nanotubes with diameters ranging from 1 to $5\mathrm{nm}$ were identified using transmission electron microscopy and Raman spectroscopy. A relatively higher deposition temperature $\left(\sim 850°\mathrm{C}\right)$ in conjunction with a controlled catalyst and rapid growth $\left(<40\mathrm{s}\right)$ allowed for the growth of well-graphitized, high areal density $\left(\sim {10}^{12}\text{-}{10}^{13}∕{\mathrm{cm}}^2\right)$ nanotubes with reduced amorphous carbon and iron. Our results also indicate that the base growth is the most appropriate model to describe the growth mechanism for the nanotube films.