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Abstract In pursuit of diamond nanoparticles, a capacitively-coupled radio frequency flow-through plasma reactor was operated with methane-argon gas mixtures. Signatures of the final product obtained microscopically and spectroscopically indicated that the product was an amorphous form of graphite. This result was consistent irrespective of combinations of the macroscopic reactor settings. To explain the observed synthesis output, measurements of C₂and gas properties were carried out by laser-induced fluorescence and optical emission spectroscopy. Strikingly, the results indicated a strong gas temperature gradient of 100 K per mm from the center of the reactor to the wall. Based on additional plasma imaging, a model of hot constricted region (filamentation region) was then formulated. It illustrated that, while the hot constricted region was present, the bulk of the gas was not hot enough to facilitate diamondsp³formation: characterized by much lower reaction rates, when compared tosp²,sp³formation kinetics are expected to become exponentially slow. This result was further confirmed by experiments under identical conditions but with a H₂/CH₄mixture, where no output material was detected: if graphiticsp²formation was expected as the main output material from the methane feedstock, atomic hydrogen would then be expected to etch it awayin situ, such that the net production of thatsp²-hybridized solid material is nearly a zero. Finally, the crucial importance of gas heating was corroborated by replacing RF with microwave source whereby facilesp³production was attained with H₂/CH₄gas mixture.