Synthesis of highly oriented and dense conical carbon nanofibers by a DC bias-enhanced microwave plasma CVD method

Conical carbon nanofibers (CCNFs) have been synthesized on Si substrates coated with a Fe thin film of a few nanometers in thickness by a DC bias-enhanced microwave plasma chemical vapor deposition (CVD) method using H₂ and CH₄ as reactant gases under a chamber pressure of about 230 Pa. Without DC bias, only catalytic nanoparticles could be observed by field emission scanning electron microscopy (FE-SEM) on substrate surface after growth. Highly oriented and dense CCNFs could be deposited when a negative DC bias in the range of 150-230 V was applied to the substrate holder during growth. The average density of CCNFs was measured as being of the order of 10¹⁰ cm^-2, while the average length was of the order of 10² nm, and the conical angle was in the range of 10-15°. The relationships between the density, the root diameter and the length of CCNFs are discussed.