Systematic Characterization of Carbon Nanotubes Functionalized in CF4 Plasma

Abstract

We functionalize the surface of carbon nanotubes (CNTs) with different numbers of graphitic layers in a capacitively driven radio-frequency (RF) CF4 plasma. We investigate the resulting structural and bonding properties by transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). XPS analysis shows that carbon-to-fluorine bonds form on the CNTs and the ratio of the intensity of the F 1s peak to that of the C 1s peak greatly depends on RF power. TEM analysis clearly shows that fluorination is initiated at the outermost graphitic layer and proceeds to the inner layers of CNTs with increasing RF power, whereas the depth of fluorination is limited to the surface area. Therefore, we suggest that fluorination in the CF4 plasma at low RF powers involves attaching fluorine atoms to carbon atoms of graphitic layers. We also discuss the correlation between the number of graphitic layers in a CNT and fluorination resistance, and show that an increase in the number of graphitic layers increases fluorination resistance. By estimating primary plasma parameters with a Langmuir probe, we conclude that ion density, i.e., the collision frequency of plasma ions with the surface of CNTs, is closely related to the rate of plasma fluorination.