Optical emission spectroscopy diagnosis of energetic Ar ions in synthesis of SiC polytypes by DC arc discharge plasma

Silicon carbides are basilic ceramics with proper bandgaps （2.4-3.3 eV） and unique optical properties. SiC@C monocrystal nanocapsules with different morphologies, sizes, and crystal types were synthesized via the fast and facile direct current （DC） arc discharge plasma method. The influence of Ar atmosphere on the formation of nanocrystal SiC polytypes was investigated by optical emission spectroscopy （OES） diagnoses on the arc discharge plasma. Boltzmann＇s plot was used to estimate the temperatures of plasma containing different Ar concentrations as 10,582 K （in 2 × 10^4 Pa of Ar partial pressure） and 14,523 K （in 4 × 10^4 Pa of Ar partial pressure）. It was found that higher energy state of plasma favors the ionization of carbon atoms and promotes the formation of α-SiC, while β-SiC is generally coexistent. Heat-treatment in air was applied to remove the carbon species in as-prepared SiC nanopowders. Thus, the intrinsic characters of SiC polytypes reappeared in the ultraviolet-visible （UV-vis） light absorbance. It was experimentally revealed that the direct bandgap of SiC is 5.72 eV, the indirect bandgap of β-SiC （3C） is 3.13 eV, and the indirect bandgap of α-SiC （6H） is 3.32 eV; visible quantum confinement effect is predicted for these polytypic SiC nanocrystals.