Structural and chemical characterisation of soot particles formed in Ar/H2/CH4 microwave discharges during nanocrystalline diamond film synthesis

The structural and chemical analysis of the soot particles produced in a bell jar reactor under Ar/H2/CH4 microwave discharges employed for nanocrystalline diamond film synthesis is carried out. SEM, TEM and HRTEM investigations show that the soot is mainly composed of polyhedral graphite particles of roughly 100 nm in size and of smaller particles of the same carbon structure, embedded in graphite crumpled sheets. Spheroid-shape particles of larger size ( > 1 mum), formed of graphite-like foils, are also observed. Amorphous-like states in the form of polyaromatic and aliphatic units are noticed. IR absorption spectroscopy evidences the presence of sp3 aliphatic compounds and of sp2 alkene groups containing chains (trans-pollyacetylene). Small polycyclic aromatic hydrocarbons (PAH) are also evidenced in the soot internal structure with this technique. This is in good agreement with the gas chromatography analysis of the soluble organic fraction containing soot, which exhibits the presence of PAH molecules with 2, 3 and 4 aromatic rings. Their presence is consistent with thermochemical Ar/H2/CH4 plasma models taking into account the soot formation following homogeneous ways, which partly supports some of the growth mechanisms previously proposed for the soot particles.