Tomohiro Nozaki, Shinpei Yoshida, Takuya Karatsu, Ken Okazaki

Atmospheric-pressure plasma synthesis of carbon nanotubes

Journal of Physics D 44 (2011) 111

An atmospheric-pressure radio-frequency discharge (APRFD) has great advantages over vacuum-oriented plasma-enhanced chemical vapour deposition (PECVD) as well as other types of atmospheric-pressure plasma sources in terms of single-walled carbon nanotube (SWCNT) growth. We first provide an overview on the recent advances in PECVD synthesis of CNTs, ranging from low pressure to atmospheric pressure, and then we present our current work focusing on the analysis of reactive species generated in the cathodic plasma sheath for further understanding of the SWCNT growth mechanism in PECVD. It was found that the plasma-generated C(2)H(2) is the main CNT growth precursor in PECVD. Approximately 30% of the CH(4) (initial feedstock) was converted into C(2)H(6), C(2)H(4) and C(2)H(2). A trace amount of C(2)H(2) enabled the synthesis of SWCNTs in the thermal chemical vapour deposition (CVD) regime. H(2) is necessary to grow SWCNTs using PECVD because H(2) suppresses the formation of excess amount of C(2)H(2); however, H(2) does not eliminate amorphous carbon even at H(2)/C(2)H(2) ratios of 300. PECVD using a binary mixture of C(2)H(2) and isotope-modified (13)CH(4) demonstrated that CH(4) does not contribute to CNT growth in C(2)H(2)-assisted thermal CVD. Atmospheric-pressure PECVD performed with a He/CH(4)/H(2) system is equivalent to C(2)H(2)-assisted thermal CVD without an etching gas. APRFD appears to produce a hidden species, which influences the CNT growth process.

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