Investigation of SF6 injection during cyclic C2H2/SF6 flow for the formation of geometrically controlled carbon coils.

: Carbon coils could be synthesized using C2H2/H2 as source gases along with SF6 as an incorporated additive gas using a thermal chemical vapor deposition (CVD) system. To obtain geometrically controlled carbon coils, a cyclic process, namely the turning on and off of C2H2 or SF6 flow during the initial reaction stage, was carried out. According to the different reaction processes, different interruption/injection times of C2H2 or SF6 flow and different injection sequences of the gas flow were investigated while maintaining the identical overall injection time of C2H2 and/or SF6 flow. The formation of carbon microcoils (CMCs) is favored by the lowest interruption/injection time ratio of SF6 flow within one cycle. In addition, the injection of SF6 flow prior to the injection of C2H2 flow promotes the formation of CMCs. Based on these results we revealed the role of the SF6 flow injection for the enhanced formation of geometrically controlled CMCs. The etching of materials, thereby promoting an increase in the number of nucleation sites for the survived growth species to form CMCs, by the increased fluorine concentration, originating from the dominant SF6 influx, is understood to be the main cause for the exclusive CMCs formation.