Fabrication and Characterization of Pure and Well-Aligned Carbon Nanotubes Using Methane/Nitrogen−Ammonia Plasma

Abstract

Well-aligned multiwalled carbon nanotubes (CNTs) were grown by microwave plasma-enhanced chemical vapor deposition using N2 and NH3 as the carrier gases and CH4 as the carbon source. Iron films with 1−5 nm thickness on silicon substrates acted as catalysts. Scanning electron microscopy revealed that the CNTs grew via the base growth mechanism at a rate ∼100 nm/s. Transmission electron microscopy showed that multiwalled CNTs had a “bamboo” structure, and the smallest CNTs of ∼6 nm in diameter were acquired on 1 nm Fe film. These “smallest” CNTs were comprised of amorphous structure, due to the formation of sp3 C−H bonds as proven by Fourier transform infrared spectroscopy and electron energy loss spectroscopy, suggesting hydrogen incorporation during growth of CNTs. Without N2 gas, no CNTs could be grown, while curly CNTs with poor alignment were grown with no NH3. In both cases, high-purity CNTs with no CNx impurities were obtained. Field electron emission revealed that the lowest turn-on and threshold fields were obtained from the CNTs grown on the 1 nm thick iron films on silicon substrate, i.e., 3.5 V/μm and 4.5 V/μm, respectively. Under an applied field of 7.5 V/μm, emission current density of 0.63 A/cm2 can be obtained.