Structural characteristics of carbon nanorods and nanotubes grown using electron cyclotron resonance chemical vapor deposition

Abstract

Different types of carbonaceous materials were synthesized by electron cyclotron resonance chemical vapor deposition on Ni-coated glass substrates with radio-frequency (rf) self-biasing using a gas mixture of CH4 and Ar. Vertically aligned carbon nanorods and multiwalled carbon nanotubes (MWNTs) were obtained at the rf bias voltages of −100 and −200 V, respectively. High-resolution transmission electron microscopy indicated that the distance between two graphene layers of carbon nanorods is much larger than that of well-graphitized MWNTs having a typical value of graphite. Structural characteristics of carbon nanorods and well-graphitized MWNTs were investigated by electron-energy-loss spectra, in which the energy of π+σ plasmon peak obtained from carbon nanorods shifts to a lower value of 23.8 eV, compared to 25.5 eV from well-graphitized MWNT. Low-energy loss plasmon due to π electrons at 6 eV was not observed in carbon nanorods, but clearly defined for well-graphitized MWNTs. In addition, x-ray photoelectron spectroscopy showed that the delocalization of π electrons becomes more pronounced as the structure of carbonaceous films evolves into more crystalline phase from carbon nanowalls to well-graphitized MWNTs with increasing rf bias voltages. These results can suggest that ionic bombardment energy at high rf bias voltages provides sufficient internal energy for dehydrogenation from hydrocarbon molecules, and thus well-graphitized MWNTs can be synthesized even at low temperatures.