Syntesis of Carbon Nanostructures Near Room Temperature Using Microwave PECVD

Carbon nanostructures (nanotubes, nanofibers and nanosponges) were synthesized onto Si (001) substrates using a microwave assisted plasma enhanced chemical vapor deposition (PECVD) from C₂H₂-Ar mixtures at low substrate temperatures (120 °C). Catalytic films (Ni and Cu) 3 nm thick were used. Different structures were formed, depending on the C₂H₂ partial pressure. Atomic force microscopy (AFM) and scanning electron spectroscopy (SEM) were employed for the morphological characterization of the catalytic films and the carbon nanostructures, respectively. Raman spectroscopy was used to identify carbon hybridization states. AFM was used to observe the morphology of the catalytic films. At low C₂H₂ partial pressures, nanotubes with nanospheres in their tips, growing from nanoholes were seen. With increasing C₂H₂ pressures, longer nanotubes were observed, reaching lengths from 300 to 500 nm. In their growth, the nanotubes laterally touch each other, forming nanotube bundles, or nanofibers. For the higher C₂H₂ partial pressures, dense sponge-like (nanosponge) structures, consisting of a large number of bundles, are formed. From the Raman spectra, a mixture of sp2 and sp3 hybridizations were identified. Furthermore, the low substrate temperature at which depositions can be carried out, makes possible growth of carbon nanostructures on materials to which high temperatures would be deleterious.

Keywords:
nanofiber; nanosponge; nanospheres; PECVD; microwave plasma; acetylene; AFM; SEM; Raman spectroscopy; carbon hybridization states