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

Synthesis of vertically aligned small diameter (single- and double-wall) carbon nanotube films on thermally oxidized n + - Si ( 001 ) 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 ∼ 0.3 - 0.5 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 nm were identified using transmission electron microscopy and Raman spectroscopy. A relatively higher deposition temperature ( ∼ 850 ° C ) in conjunction with a controlled catalyst and rapid growth ( < 40 s ) allowed for the growth of well-graphitized, high areal density ( ∼ 10 12 - 10 13 ∕ cm 2 ) 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.