Direct identification of diamond growth precursor using almost pure CH₄ or C₂H₂ near growth surface

Diamond growth was studied by injecting thermally decomposed Cl atoms into CH₄/H₂ or C₂H₂TH₂. Owing to the extremely short residence time (25 μs) and low gas temperature (< 1000 °C) in the decomposition system, the gas reaction is insignificant. Therefore, the carbon species near the substrate surface can be nearly identical to the input carbon source. With 0.3% CH₄ being the input carbon source, CH₄ remained the dominant carbon species near the surface (only 2.5% C₂H₂ was formed), and an almost continuous diamond film was deposited after 2 h growth. Raman spectra confirmed the formation of diamond. With 0.15% C₂H₂ being the input carbon source, C₂H₂ remained the dominant carbon species near the surface (10% CH₄ was formed), but only a few very small particles were deposited. Therefore, we conclude that CH₃ radicals seem the only diamond growth precursor under the Cl-rich conditions, whereas C₂H₂ is not efficient to grow diamond.