Abstract
We have developed the growing carbon nanofibers (CNF) process by plasma enhanced chemical vapor deposition (PECVD) and studied the growth mechanism of carbon nanofibers (CNFs) in the plasma environment and the characterizations of field emission. The vertically aligned CNFs are grown on the Si substrate deposited with nanoparticles of Ni catalyst by the inductively coupled plasma (ICP). A pyrolytic graphite/pyrolytic boron-nitride heater, located beneath the graphite stage, was utilized for substrate heating. A DC electric bias was applied on the graphite stage for conductive substrate. The feed gas was a mixture of acetylene, to provide carbon source for CNFs growth, and hydrogen, to keep the catalyst active by etching away amorphous-carbons. The processing gas was a mixture of acetylene and hydrogen and it was introduced into the process chamber. The inductively coupled plasma was turned on under low gas pressure, ~ 20 mTorr. The CNFs are grown at various process parameters such as the ICP power, the DC bias, the temperature of substrate, the process pressure and the ratio of gas flow. The gas components are monitored in situ by optical emission spectroscopy and quadrupole mass spectrometry. The length, diameter and growth density of CNFs are measured by scanning electron microscopy and the sheets and bamboo-like layers of CNFs are observed by transmission electron microscopy. The graphitized degree of CNFs is characterized by Raman spectroscopy. The field emission of CNFs is also characterized. The growth results show that they are less likely for smaller radius CNFs and longer length CNFs to survive as the DC bias and current applied on substrate are high during the growth process. The electrostatic pressure produced by the plasma sheath electric field is the vertical alignment force on the growth of CNFs, but it is also the force to detach the CNFs from the substrate surface. From the results of theoretical analysis, the long chain hydrocarbon produced with acetylene in the plasma environment deposites on the nanoparticle surface of Ni catalyst to form the amorphous carbon film and prevent from catalytic reaction. Therefore, the amorphous carbon needs to be removed by hydrogen atoms and ions and the single carbon atoms remaining on the surface of Ni catalyst form graphite layer by diffusion process. Although the CNFs growth can be improved by increasing the concentration of hydrogen atoms and the flux of hydrogen ions, the etching rate of CNFs body is also increasing.