The role of carbon atoms in a dc plasma-enhanced chemical vapor deposition for carbon nanotubes (CNTs) synthesis was investigated. It was observed that at 1.33 kPa pressure of CH4 gas in plasma, a high value of the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum corresponds to the maximum value of the excited C number density in the vicinity of the Si substrate. It was found that a CH4 gas pressure higher than 1.33 kPa leads to an increase of the relative density of the C-2, C-3 Molecules and the clusters, and to a decrease of the C excited atom number density in plasma. The presence of a high amount of sp(2)-graphite in the composition of CNTs observed in Raman spectrum was also confirmed by the measurement of the IR-active G peak at 1584 cm(-1) in the transmission spectrum. (c) 2006 Elsevier B.V. All rights reserved.