Diagnostics of nonthermal capacitively coupled plasma for carbon nanoparticle synthesis

Nonthermal radiofrequency (RF) capacitively coupled plasma (CCP) has been investigated for the synthesis of nanoparticles. Recently, much interest has been put towards methane mixed plasmas for nanodiamond synthesis [1]. We utilized optical emission spectroscopy (OES), fast-camera imaging, thermocouple, double Langmuir probe, and material analysis to understand the discharge mechanism for nanoparticle synthesis in our Ar-H₂-CH₄ CCP. Our CCP reactor consists of a quartz tube (1 cm OD) surrounded by a set of two parallel copper ring electrodes (1 cm in width) with 4 cm spacing. Discharge was generated by the application of a 200 W RF (13.56 MHz) signal. OES was utilized to measure emission lines from C₂, CH, Ar, H₂, and H to characterize the plasma and presence of those species. Fast-camera was utilized to capture the motion of plasmas, and the thermocouple was used to measure the reactor temperature. A double Langmuir probe was previously applied to a CCP of Ar-SiH₄ [2] to determine electron temperature and ion density, and we attempt to do the same for our plasma. We studied how the probe disturbs the plasma, and how to ensure accurate measurements despite formation of nanoparticles on the reactor and probe. With our diagnostics, we investigate which regimes lead to more or less nanoparticle formation. [1] S.V. Baryshev, Mini course on Enabling Scalable Production and Supply Chain of Diamond using Microwave Plasma, ICOPS2023 (May 21-25, 2023) [2] Z. Xiong et al. Plasma Sources Sci. Technol. 32, 035001 (2023).