Discharge mode transition due to seeding gas effects of nonthermal capacitively coupled plasmas for nanoparticle sysnthesis

We have performed optical emission spectroscopy and fast-camera imaging analysis to investigate the spectrum transitions of nonthermal capacitively coupled plasmas (CCPs) for mixed seeding gasses of CH₄, Ar, and H₂. Nonthermal CCPs have been investigated for nanoparticle synthesis [1,2,3]. Our CCP reactor consists of a quartz tube (1 cm ID) surrounded by a set of two parallel copper ring electrodes. Discharge was generated by the application of a 200 W radio frequency (13.56 MHz) signal. We examined plasma discharges of mixed gases consisting of CH₄, Ar, and H₂, where the total pressure was 10-30 Torr. Observed emission lines of C₂, CH, Ar, H₂, and H, as well as the continuum emission possibly from synthesized carbon nanoparticles, were found significantly affected by the feeding gas composition. For Ar-enriched plasmas, the increase of C₂ and CH line intensities was observed as increasing CH₄ pressure fraction up to ~1%. However, the decrease of C₂ and CH line intensities and increase of continuum emission, which covers the wavelength range of our spectrometer (400-800 nm), were observed as increasing CH₄ pressure fraction above ~1%. We investigate those discharge mechanisms using measured spectra as well as fast-camera imaging to understand the discharge mechanism and regime control, and nanoparticle synthesis mechanism. [1] L. Mangolini et al., Nano Lett. 5, 655 (2005). [2] A. Ho et al., ACS Appl. Nano Mater. 4, 5624 (2021). [3] S.V. Baryshev, 50^th IEEE International Conference on Plasma Science (ICOPS2023) (2023).