Role of carbon atoms in plasma-enhanced chemical vapor deposition for carbon nanotubes synthesis M.A. Bratescu a, ⁎ , Y. Suda b , Y. Sakai b , N. Saito c , O. Takai d a n-Factory Co., Ltd., 5-36-4 Kawana-cho, Showa-ku, Nagoya, 466-0856, Japan b Graduate School of Information Science and Technology, Hokkaido University, North 14 West 9, Sapporo 060-0814, Japan c Department of Molecular Design and Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan d EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan Received 17 October 2005; received in revised form 9 January 2006; accepted 9 March 2006 Available online 19 April 2006 Abstract 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 CH 4 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 CH 4 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. © 2006 Elsevier B.V. All rights reserved. PACS: 81.07.De.; 52.70.Kz.; 81.15.Gh. Keywords: Carbon; Chemical vapor deposition (CVD); Methane; Nanostructure; Optical spectroscopy; Plasma processing and deposition; Clusters; Hydrocarbons 1. Introduction Carbon nanotubes (CNTs) are increasingly becoming one of the most important materials for various applications in electronics, optics, and biophysics. Up to now, several synthesis methods for CNTs growth, such as arc-discharge [1], laser ablation [2], chemical vapor deposition (CVD) [3], and plasma- enhanced CVD (PECVD) [4,5] have been investigated. All of these techniques require high temperatures for the growing process, which were obtained in electric furnaces or with a hot filament. In our experiment we used dc plasma, which provides the advantage that the metal catalyst was deposited by Ar sputtering inside the same vacuum chamber where CNTs were produced on a Si substrate heated by Joule effect. The PECVD arrangement offered the possibility to study plasma composition in the vicinity of the Si substrate by spectroscopic methods in the same time with CNTs deposition process. In the present work, the number density of the C excited atoms near the Si substrate was measured, during CNTs for- mation, by laser absorption spectroscopy (LAS) method [6–8]. The plasma composition and the relative number density of the CH radical, the C 2 ,C 3 molecules and the clusters were measured by optical emission spectroscopy (OES) method [9]. The morphology of the synthesized CNTs was probed with a scanning electron microscope (SEM). Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR) characterized the composition of CNTs. In the present paper, it was found that an increased CH 4 gas pressure leads to an increase of the relative density of the C 2 ,C 3 molecules and the clusters. The C excited atom number density in plasma had a maximum value at a pressure of 1.33 kPa. It was observed that the optimum conditions for CNTs growth were at a CH 4 gas pressure higher than 1.33 kPa, since the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum had high values. Thin Solid Films 515 (2006) 1314 – 1319 www.elsevier.com/locate/tsf ⁎ Corresponding author. Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. Tel.: +81 52 789 3259; fax: +81 52 789 3260. E-mail address: maria@plasma.numse.nagoya-u.ac.jp (M.A. Bratescu). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.03.022