Vertically oriented two-dimensional carbon sheets called carbon nanowalls (CNWs) have been fabricated on some substrates by dc plasma-enhanced chemical vapor deposition (dc-PECVD). Each CNW basically consists of several tens of graphene layers. The unique shape and structure can lead to various applications for electronic and energy devices [1]. Furthermore, the doping of nitrogen in CNWs can give rise to the improvement of electrical [2] and electrochemical properties [3]. The understanding of the growth process of CNWs is important for not only the growth control but also the technical applications. Especially, the understanding of the interface between CNW and substrate is important for the construction of electronic circuit and electrodes for devices. Grazing incidence X-ray diffraction studies on CNW growth have showed that graphene layers parallel to the substrate surface are formed before the growth of graphene layers vertical to the substrate, i.e. CNW [4,5]. In this paper, we report the atomic force microscopy (AFM) and Raman spectroscopy study on the early stage of CNW growth by dc-PECVD.

CNWs were fabricated on Si (100) substrate by dc-PECVD with a gas mixture of CH₄, H₂ and Ar. The substrate was heated to a temperature of 973 K. The flow rates of CH₄, H₂ and Ar were 10, 10 and 80 sccm, respectively. In the case of the doping of nitrogen, N₂gas was introduced with gases as mentioned above. The dc plasma power was 3.0 kW. During the deposition reaction, the pressure of dc plasma reactor was kept at 1.0 Pa with a turbomolecular pump. Samples were prepared under various reaction times below several minutes. The surface morphologies of the samples were observed by AFM in dynamic mode. The Raman spectra were obtained using a micro-Raman system, equipped a single spectrometer (1800 g/mm grating) with an optical microscope (100 x objective), a holographic notch filter, and an air-cooled charge coupled device (CCD) detector.

 From AFM and Raman analysis for samples with various reaction times, it is suggested that the growth as mentioned below occur in the early stage of the growth of CNW. First nanodiamond particles with highly defective graphene layers are formed over the substrate. Subsequently nanographite grains are grown on the nanodiamond film. The density of nanographite grains increases with increasing deposition time, and they coalesce to form a continuous graphite film. Finally, CNW is shown to grow vertically on the graphite film. In addition, the effect of nitrogen doping on CNW growth was also investigated by AFM and Raman spectroscopy. In this presentation, the growth process with nitrogen will be discussed compared with CNW growth without nitrogen.

[1] S. C. Shin et al., J. Appl. Phys. 110, 104308-1-4 (2011)

[2] W. Takeuchi et al. Appl. Phys. Lett. 92, 213103 (2008)

[3] J. P. Mcclure et al. J. Electrochem. Soc. 159, F733-F742 (2012)

[4] H. Yoshimura et al. Chem. Phys. Lett. 482, 125-128 (2009)

[5] A. Yoshimura et al. Carbon 50, 2698-2702 (2012)

Corresponding author: Masaru Tachibana, YokohamaCity University, 22-2 Seto, Kanazawa-ku,Yokohama 2360027, Japan, Tel &Fax: +81-45-787-2360, E-mail: [email protected]

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