Scalable production of carbon nanostructures to exploit their extraordinary properties and potential technological applications requires an improved understanding of the chemical environment responsible for their synthesis. In this study the spatial distribution of the rotational temperature of hydrogen is investigated via coherent anti-Stokes Raman scattering spectroscopy in the plasma of a microwave plasma chemical vapor deposition reactor under parametrically controlled conditions. The reactor pressure is varied from 10 to 30 Torr and the plasma generator power from 300 to 700 W, simulating the conditions required for the synthesis of carbon nanotubes, graphene and graphitic nanopetals. Temperature measurements are conducted within the plasma sheath and up to 6 mm away from the puck surface in order to elucidate the spatial distribution of temperature within and around the plasma region. The results indicate a linear increase in rotational temperature of H2 with respect to the distance normal from the puck surface. Temperatures also increase with pressure. At 10 Torr the temperature range is approximately 850–1150 K while at 30 Torr it is approximately 1200–1650 K for a plasma generator power of 500 W. In addition, the temperature increases with plasma generator power and the introduction of other substances such as CH4 and N2. These findings may aid in understanding the function of the chemical composition and reactions in the plasma environment of these reactors which, to date, remains obscure. The spectroscopic techniques applied in this work may prove to be suitable in-situ monitoring methods for the scalable manufacturing of carbon nanomaterials.

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