Gas temperature measurements in a microwave plasma by optical emission spectroscopy under single-wall carbon nanotube growth conditions

Abstract

Plasma gas temperatures were measured via in situ optical emission spectroscopy in a microwave CH4–H2 plasma under carbon nanotube (CNT) growth conditions. Gas temperature is an important parameter in controlling and optimizing CNT growth. The temperature has a significant impact on chemical kinetic rates, species concentrations and CNT growth rates on the substrate. H2 rotational temperatures were determined from the Q-branch spectrum of the (0) transition. N2 rotational and vibrational temperatures were measured by fitting rovibrational bands from the N2 emission spectrum of the C 3Πu → B 3Πg transition. The N2 rotational temperature, which is assumed to be approximately equal to the translational gas temperature, increases with an increase in input microwave plasma power and substrate temperature. The measured H2 rotational temperatures were not in agreement with the measured N2 rotational temperatures under the CNT growth conditions in this study. The measured N2 rotational temperatures compared with the H2 rotational temperatures suggest the partial equilibration of upper excited state due to higher, 10 Torr, operating pressure. Methane addition in the hydrogen plasma increases the gas temperature slightly for methane concentrations higher than 10% in the feed gas.